The broad field of molecular rotors is highly relevant to several types of molecular machines, and in lieu of an extensive list of primary research articles, readers are referred to three expansive reviews. [1][2][3] We have had an ongoing interest in the subnanoscale miniaturization of various macroscopic devices, and this has driven the synthesis and intensive study of two classes of metal complexes in our group. The first, exemplified by trans-II in Scheme 1, has been termed "gyroscope like" due to the obvious geometric resemblance. 3 Such species can be accessed via three-fold ring closing metathesis/hydrogenation sequences involving complexes with trans phosphine ligands of the formula P((CH 2 ) m CHvCH 2 ) 3 (trans-I). [3][4][5][6][7] This affords adducts of cage like dibridgehead diphosphines with three connecting (CH 2 ) n bridges (n = 2m + 2). Depending upon the lengths of the methylene segments and the sizes of the ligands, the ML y rotators may be capable of rotation, exchanging the positions of the ligands or (equivalently) the (CH 2 ) n bridges.
When such sequences are carried out with analogous cis complexes (e.g., cis-I, Scheme 1), the same diphosphine ligands can be generated, but now with the donor atoms connecting cis positions. 8 When accessed in square planar geometries, one (CH 2 ) n bridge can be visualized as lying in the coordination plane, the second above, and the third below. Depending upon the lengths of the methylene segments and the sizes of the ligands, the bridges may be capable of "jump rope" dynamic processes that exchange their positions. When square planar versions of cis-II are viewed from appropriate perspectives, there is some resemblance to a parachutist, so they are often termed "parachute like".
In analyzing the dynamic behavior of both classes of complexes, it has been important to exclude alternative mechanisms for exchanging the positions of ligands or (CH 2 ) n bridges. 9 Specifically, any facile trans/cis isomerization could open up new pathways. In one embodiment, gyroscope like trans-II might first undergo a hypothetically endergonic iso-merization to parachute like cis-II, where (CH 2 ) n exchange might be more rapid than in trans-II ("jump rope" rate > ML y rotation). Alternatively, one can play the devil's advocate in the opposite direction. Thus, we have sought to probe the relative stabilities of cis/trans-II and their thermal interconversion both experimentally and computationally.
In earlier studies, we examined these issues with a series of platinum dichloride complexes, cis-and trans-Pt (Cl) 2 (P((CH 2 ) n ) 3 P) (cis-and trans-2). 4c,8 As shown in Scheme 2, temperatures of 150-185 °C were required to effect equilibrations on reasonable time scales from either direction. In all cases, the gyroscope like trans isomers were favored. To facilitate comparisons with earlier papers, the same letter indices have been employed for n (n = 10, a; 12, b; 14, c; etc.). Computationally, as one goes from (CH 2 ) 10 (2a) to (CH 2 ) 22 (2g), or 13-to 25-membered macrocycles, the gyroscope like trans-2 ranges from 5.1 to 9.2 kcal mol -1 more stable than the parachute like cis-2. 8,10 The acyclic precursor complexes cis-and trans-Pt (Cl) 2 (P((CH 2 ) m CHvCH 2 ) 3 ) 2 (cis-and trans-1) could also be thermally equilibrated. 4c Here, in line with literature precedent, 11,12 there is a significant effect of solvent polarity that tracks the dipole moments (cis > trans; more polar CH 2 Cl 2 : appreciable amounts of both isomers; less polar toluene: >95% trans). 13 Other reactivity data are relevant. As shown in Scheme 3, the dichloride complexes 2 and Ph 2 Zn react at room temperature to give the diphenyl complexes Pt (Ph) 2 (P((CH 2 ) n ) 3 P) (3). 4 Since cis-2c,d and trans-2c,e afford cis-3c,d and trans-3c,e, respectively, these fit the textbook definition of stereospecific reactions. 14 Furthermore, the stereospecificity excludes cis/trans isomerization at any stage of the reaction coordinates.
In this paper, we report the surprising observation of much lower energy pathways for the interconversion of trans-II and cis-II when the chloride ligands in 2 are replaced by one or more alkyl groups. Since these isomerizations impinge upon a number of mechanistic questions, even beyond those expressed above, it was imperative to bring definition to the phenomena. Accordingly, a variety of synthetic, structural, thermolytic, and computational experiments have been carried out, as detailed in the narrative below. None of these data have been previously communicated.
As shown in Scheme 4, the gyroscope like platinum dichloride complexes trans-2c,e,g 4a were treated with diethyl ether solutions of MeLi (2.1-2.3 equiv.). Workups gave the new parachute like dimethyl complexes cis-Pt(Me) 2 (P((CH 2 ) n ) 3 P) (cis-4c, e,g) in 70-94% yields as air stable white solids. When the first such reaction was conducted, the trans/cis isomerization was considered surprising. Thus, the neutral alumina columns used in initial experiments were omitted. However, identical results were obtained.
These and all other new complexes below were characterized by NMR spectroscopy ( 1 H, 13 C{ 1 H}, 31 P{ 1 H}) and microanalyses, as summarized in the Experimental section. The cis stereochemistry was evidenced by a number of features, such as the lower 1 J PPt values (1853-1886 Hz vs. 2389-2398 Hz for trans-2c,e,g). 15 Each 13 C{ 1 H} NMR spectrum showed a doublet of doublets for the methyl groups ( 2 J CP(trans) = 101.7-101.1 Hz, 2 J CP(cis) = 9.9-10.0 Hz). Each 1 H NMR spectrum also showed a doublet of doublets ( 3 J HP(trans) = 7.3-6.7 Hz, 3 J HP(cis) = 6.1-5.7 Hz), and platinum satellites were usually visible ( 2 J HPt = 66-65 Hz). The 1 H and 31 P NMR properties were very similar to those reported for the related acyclic complexes cis-Pt (Me) 2 (PR 3 ) 2 . 16,17 A reaction of trans-2c and only 1.1 equiv. of MeLi was similarly conducted. A 31 P{ 1 H} NMR spectrum of the crude reaction mixture showed 40-50% conversion to the dimethyl complex cis-4c. The only other signal detected was that of the starting material trans-2c. Thus, it was not possible to interrogate the stereochemistry of the presumed intermediate, the methyl chloride complex Pt(Cl)(Me)(P((CH 2 ) 14 ) 3 P) (5c). When the reaction of trans-2c and 2.2 equiv. of MeLi (Scheme 4) was repeated at 0 °C, the result was the same as at room temperature.
As shown in Scheme 5, CH 2 Cl 2 solutions of cis-4c,e,g were treated with 1.0 equiv. of HCl. In some cases the HCl was generated in situ from acetyl chloride (1.0 equiv.) and excess methanol, and in other cases 2.0 M Et 2 O solutions were used. With cis-4c, a chromatographic workup (neutral alumina) gave parachute like cis-Pt(Cl)(Me)(P((CH 2 ) 14 ) 3 P) (cis-5c) in 83% yield. The lower symmetry was evidenced by doubled 31 P NMR signals, and PtC H 3 1 H and 13 C NMR signals that were now doublets of doublets. Analogous patterns have been noted for the related complexes cis-Pt(Cl)(Me)(PR 3 ) 2 . 17,18 The stereochemical homogeneity of cis-5c was quickly found to be tied to a hair trigger. Either exposure to silica gel (in the form of a column or slurry) or crystallization (Et 2 O/ methanol or Et 2 O/ethanol) afforded gyroscope like trans-5c (≥89%). Even CH 2 Cl 2 solutions of the dimethyl complex cis-4c, when slurried with silica gel, gave trans-5c (70%). The NMR properties of trans-5c were similar to those of the dichloride complex trans-2c, with the 1 H and 13 C NMR spectra showing triplets for the PtC H 3 group.
With cis-4e,g, the analogous methyl chloride complexes cis-5e,g could be detected "on the fly", but isolated samples always contained significant amounts of trans-5e,g. These mix-tures were further slurried with silica gel to give pure trans-5e, g (60-64%). Additional details can be found in the Experimental section. The NMR properties were similar to those of trans-5c, and all three complexes exhibited 1 J PPt values (2824-2796 Hz) that were diagnostic of the trans stereochemistry. 15 Given the modest dependence of the chemistry in Schemes 3-5 upon the lengths of the (CH 2 ) n bridges, subsequent experiments were confined to (CH 2 ) 14 systems ("c series").
As depicted in Scheme 6, a parallel sequence was carried out with trans-2c and EtLi in place of MeLi. The parachute like diethyl complex cis-Pt(Et) 2 (P((CH 2 ) 14 ) 3 P) (cis-6c) was isolated in 85% yield. NMR spectra showed a 1 J PPt value of 1746 Hz, and PtCH 2 CH 3 1 H signal patterns similar to those of related acyclic complexes cis-Pt(Et) 2 (PR 3 ) 2 . 16 Next, cis-6c was treated with HCl analogously to the dimethyl complex cis-4c in Scheme 5. However, the same chromatographic workup (neutral alumina) afforded only gyroscope like trans-7c (45%). Accordingly, the Scheme 5 Introduction of halide ligands and attendant isomerization.
Scheme 6 Substitution and isomerization reactions involving ethyl ligands.
NMR spectra now showed a 1 J PPt value of 3002 Hz, and PtCH 2 CH 3 1 H signal patterns similar to those of other complexes of the type trans-Pt(Cl)(Et)(PR 3 ) 2 . 19 No significant amounts of byproducts were noted when crude reaction mixtures were assayed by 31 P NMR.
The main difference between the substitution reactions of the gyroscope like dichloride complex trans-2c in Schemes 4 and 6 and that in Scheme 3 is the use of alkyl lithium versus aryl zinc nucleophiles. Thus, as shown in Scheme 7 (top), trans-2c was treated with PhLi in a procedure analogous to that in Scheme 6. Workup gave the gyroscope like diphenyl complex trans-3c in 85% yield. All of the syntheses in this report have been reproduced several times, often by different coworkers. However, in one case this same reaction gave, under essentially identical conditions, trans-3c and cis-3c in 50% and 29% isolated yields, respectively. Regardless, the chloride/phenyl ligand substitution is not accompanied by the same degree of isomerization as chloride/alkyl ligand substitution.
In previous work, the gyroscope like diphenyl complexes trans-Pt(Ph) 2 (P((CH 2 ) n ) 3 P) (trans-3c,e) had been shown to react with 1.0 equiv. of HCl to give the gyroscope like phenyl chloride complexes trans-Pt(Cl)(Ph)(P((CH 2 ) n ) 3 P) (trans-8c,e) in 93-98% yields after workup. This is exemplified with trans-3c in Scheme 7 (top). However, analogous substitution reactions with parachute like cis-3 had not been investigated. Thus, cis-3c and HCl were similarly combined (Scheme 7, bottom). A chromatographic workup (neutral alumina) gave the easily separated products cis-8c (44%) and trans-8c (29%). The structure of the former, a new compound, followed readily from the types of NMR properties detailed above.
In connection with thermolyses described below, authentic samples of platinum bromide and iodide complexes became of interest. Thus, as shown in Scheme 8, the gyroscope like chloride complex trans-5c was treated (in separate reactions) with LiBr or NaI. Workups gave the gyroscope like halide complexes trans-Pt(X)(Me)(P((CH 2 ) 14 ) 3 P) (X = Br, trans-9c; Cl, trans-10c) in 87-88% yields. The structure and stereochemistry followed readily from the NMR properties, as well as a crystal structure in the following section.
Due to the varied stereochemical issues involving the preceding complexes, crystal structures were sought wherever poss-Scheme 7 Substitution and isomerization reactions involving phenyl ligands. ible. Single crystals of cis-4c, trans-5c, trans-7c, and trans-10c could be obtained, and the X-ray structures were determined as described in the Experimental section and Table 1. Key metrical parameters are summarized in Table 2. Thermal ellipsoid plots are depicted in Fig. 1234. All of the structures exhibited disorder that could easily be modeled (Me/X positions in trans-5c and trans-10c; certain (CH 2 ) n segments in cis-4c, trans-5c, and trans-7c). Data are given only for the dominant conformation.
As shown in Fig. 1, the dimethyl complex cis-4c exhibits, as with other parachute like adducts structurally characterized earlier, 8 one macrocycle in which the proximal CH 2 groups lie close to the coordination plane, and two in which the proximal CH 2 groups lie in roughly perpendicular planes. With the gyroscope like ethyl chloride complex trans-7c, three independent molecules were found in the unit cell. Since these involved rather minor conformational differences, only one is depicted in Fig. 3, but all are illustrated in the ESI. †
The views of the iodide complex trans-10c in Fig. 4 and the analogous chloride complex trans-5c in Fig. 2 have been arranged as closely as possible. While these adducts clearly pass the "eye test" as isostructural, the space groups are not identical (Pbca vs. P12 1 /n1) and the crystal lattices differ.
Attention was turned to addressing certain gaps in the preceding preparative data. Although the reactions in Scheme 5 convincingly show that for the methyl chloride complexes 5c,e,g, the trans/gyroscope isomers are more stable than the cis/parachute isomers, purely thermal conversions of purified samples were lacking. Accordingly, when o-C 6 H 4 Cl 2 or C 6 D 5 Br solutions of cis-5c were kept at 100-110 °C for 3-7 h, or toluene solutions kept at 80 °C for 72 h, or mesitylene solutions kept at 140 °C for 0.5 h, >99% conversions to trans-5c were observed, as assayed by 31 P{ 1 H} NMR. These data are from entries 9-13 of Table 3.
The previously reported thermolyses in Scheme 2 unambiguously show that the gyroscope like dichloride complexes trans-2 are more stable than the parachute like cis-2. Since the procedures for isolating the latter did not involve silica gel (found to promote cis/trans isomerization in Scheme 5), it was of interest to see if conversion to trans-2c might be effected in slurries. However, no reaction occurred when cis-2c was combined with silica gel for 24 h at room temperature in toluened 8 or CDCl 3 (entry 1, Table 3).
The workups for isolating the analogous diphenyl complexes cis-3c,d (Scheme 3) did involve silica gel. 8 Nonetheless, cis-3c was slurried with the same batch used in Scheme 5. No reaction was observed after 5 h at room temperature or 24 h at 80 °C (entry 4, Table 3). When a mesitylene solution of cis-3c was kept at 140 °C for 30 h (entry 3, Table 3), 31 P{ 1 H} NMR spectra showed the gradual conversion to two new species, neither of which was trans-3c (δ/ppm 7.4 (s, 84%) and 4.3 (s, 16%)). 20 When a toluene-d 8 solution of the parachute like dimethyl complex cis-4c was kept at 80 °C for 10 h (entry 5, Table 3) or slurried at room temperature with silica gel for 24 h (entry 7, Table 3), no reaction was observed. However, when slurried at 80 °C for 24 h (entry 8, Table 3), complete conversion to two unknown species occurred (δ/ppm 11.4 (s, 76%) and 6.2 (s, a Values for the dominant X-Pt-CH 3 orientation. b Values for the three independent molecules in the unit cell.
. 20 When a mesitylene solution of cis-4c was kept at 140 °C for 30 h, there was ca. 16% conversion to an unknown new species.
When a mesitylene solution of the gyroscope like methyl chloride complex trans-5c was kept at 140 °C for 30 h (entry 14, Table 3), most of the trans-5c remained, with 13% conversion to an unknown species (7.3 ppm; no detectable cis-5c). However, an analogous experiment with the corresponding ethyl chloride complex trans-7c (entry 15, Table 3) showed the gradual conversion to an unknown product ( 31 P{foot_2 H} NMR: 7.4 ppm, s, 1 J PPt = 2447 Hz), 20 with one main intermediate (18.3 ppm), the NMR properties of which (Experimental section) indicated a platinum hydride complex ( 1 H NMR: -16.78 ppm, t, 2 J HP = 13.8 Hz).
Next, o-C 6 H 4 Cl 2 solutions of the new parachute like phenyl chloride complex cis-8c (Scheme 7) were kept at 100 °C for 6 h or 140 °C for 0.5 h (entries 16 and 17, Table 3). In both cases, >99% conversion to gyroscope like trans-8c was observed. Thus, the relative thermodynamic stabilities of the phenyl chloride complexes are analogous to those of the alkyl chloride complexes.
Some attempted thermal equilibrations gave well defined alternative reactions. As shown in Scheme 9, a C 6 D 5 Br solution of the parachute like dimethyl complex cis-4c was kept at 110 °C for 7 h (little change) and then 140 °C for 6 h. The higher temperature gave complete conversion to the gyroscope like methyl bromide complex trans-9c, as confirmed by 31 P and of the diethyl complex cis-6c (140 °C) initially gave what was assigned as the ethyl bromide complex trans-Pt(Br)(Et) (P((CH 2 ) 14 ) 3 P) based upon the similar 1 J PPt value (2771 vs. 2806 Hz). With time, this further converted to the previously isolated dibromide complex trans-Pt(Br) 2 (P((CH 2 ) 14 ) 3 P) ( 1 J PPt 2347 vs. 2344 4a Hz). In another example, an o-C 6 H 4 Cl 2 solution of cis-4c was kept at 140 °C for 12 h. Complete conversion to the gyroscope like methyl chloride complex trans-5c occurred. In this case, an unknown intermediate could be seen by 31 P NMR (8.9 ppm, s).
In order to better interpret the preceding isomerizations, molecular dynamics simulations followed by DFT optimizations were carried out for all complexes with (CH 2 ) 14 bridges ("c series") along the lines reported earlier. 8 The molecular dynamics annealing simulations were run to maximize the likelihood of correctly identifying the lowest energy conformer. 21 The ten lowest conformations from each trajectory were extracted and "cleaned up" using a semi-empirical method (PM7) followed by DFT optimizations (in the gas phase at 25 °C or 298 K). 22 The computed relative energies of these species are shown in Fig. 5. It was critical to incorporate dispersion corrections in order to obtain appropriately folded (CH 2 ) 14 bridges. Subsequently, single point corrections were performed using an implicit solvation model for two typical a Assayed by 31 P NMR as detailed in the Experimental section. b Chemical shifts are provided in the Experimental section; when only 13-16% conversion is noted, the remaining material is the educt. c A platinum hydride intermediate can be observed by 1 H NMR; see text and Experimental section.
Scheme 9 Thermolyses that afford well defined complex/solvent reactions.
This solvents from Table 3 (o-C 6 H 4 Cl 2 , toluene). 23 Frequency calculations were also performed to confirm that minimum structures were obtained and further to afford temperature corrections to the Gibbs energy at a representative temperature from Table 3 (140 °C or 414 K).
It was also sought to compare the relative energies of acyclic model complexes, which would not be subject to possible ring strain effects derived from the dibridgehead diphosphine ligands. For this purpose, PEt 3 as opposed to PMe 3 was selected for the phosphine ligands, as the former better mimics the electronic effect exerted by the three (CH 2 ) 14 bridges. 24 These species are designated with double primes; for example trans-2″ represents trans-Pt(Cl) 2 (PEt 3 ) 2 , the analog of the gyroscope like dichloride complex trans-2c.
In Fig. 5 and6, the energies of the trans isomers are displayed relative to those of the cis isomers symbolized in the columns at the far left. In Fig. 5, most of the gyroscope isomers are 11.5-19.2 kcal mol -1 more stable than the parachute isomers in the gas phase. However, the energies of the two dialkyl complexes cis/trans-4c and cis/trans-6c are closer, with the trans isomers 6.7-7.4 kcal mol -1 more stable. Fig. 5 also shows the energy differences for the dichloride complexes cis/trans-2c. This system was also examined by DFT earlier, 8 with the important result that the trans isomers were more stable for all the (CH 2 ) n bridge lengths examined (n = 10, 12, 14, 16, 18, 20, 22). 10 All bond lengths and angles about platinum nicely model those determined crystallographically (Table 2 and earlier papers for trans-2c, 4a cis-2c, 8 trans-3c, 4a cis-3c, 8 and trans-8c 4b ), and some are depicted in the graphics below.
In Fig. 6, the results for the PEt 3 substituted acyclic model compounds are presented. Here, the gas phase energy differences closely (albeit not perfectly) follow the pattern in Fig. 5, but average 6.6 kcal mol -1 lower. This suggests that the cis/ parachute isomers in Fig. 5 are ca. 6.6 kcal mol -1 destabilized relative to the trans/gyroscope isomers (and/or that the latter are in some manner stabilized). In any case, the cis and trans dialkyl complexes Pt(R) 2 (PEt 3 ) 2 are rendered approximately equal in energy.
In response to a reviewer inquiry regarding the origin of the 6.6 kcal mol -1 difference, the PtX 2 or PtXX′ fragments were excised from the structures of all cis/parachute and trans/gyroscope complexes in Fig. 5, in each case taking the most favorable of the ten conformations analyzed. Single point calculations were carried out on the remaining dibridgehead diphosphines, a process that freezes the phosphorus-phosphorus distances (3.4-3.5 vs. 4.5-4.6 Å) and all other atoms. In all cases, the gas phase energies of the gyroscope derived diphosphines were lower (2.8-7.3 kcal mol -1 ). Next, the diprimary and disecondary diphosphines H 2 P(CH 2 ) 14 PH 2 and HP ((CH 2 ) 14 ) 2 PH, derived by deleting two and one (CH 2 ) 14 bridges, respectively, were similarly examined. A detailed analysis of the results (see highlights in the ESI †) suggests that bridge/ bridge interactions provide stabilization in the trans/gyroscope systems. Torsional interactions and angle strain (e.g., at phosphorus) seemingly have subordinate roles in the energy differences. Further analysis is provided below and in the ESI. †
Schemes 3-8 diagram a number of substitution reactions involving gyroscope and parachute like complexes. Although this study is concerned with general isomerization rates and equilibria as opposed to mechanism, it should be noted that substitution mechanisms of square planar platinum(II) complexes have been extensively studied, going back more than 60 years. 25,26 However, the gyroscope like complexes may have pathways available that lack counterparts in classical complexes. For example, equilibria exist that allow the ML y rotators to "escape" from their cages. 4a,5c,27,28 A possible initial step is shown in Scheme 10 (trans-II → V). This entails the cleavage of Scheme 10 Potential equilibria that may play a role in substitution reactions. one metal-phosphorus bond and isomerization of the dibridgehead diphosphine ligand from an "in/in" to an "out/out" geometry. 29 Such isomerizations are facile with the free diphosphines. 28 Thus, there exists the possibility that substitution occurs external to the cage, perhaps involving V directly, or another transient species. Furthermore, as shown in Scheme 10, either gyroscope (trans-VII) or parachute (cis-VII) products could be preferred kinetically. Also, per studies of related acyclic platinum(II) complexes, mechanisms would be expected to differ depending upon the attacking group, especially for nucleophilic or organometallic agents such as RLi versus electrophilic agents such as HCl. 17 Finally, although purely thermal cis/ trans isomerizations of platinum(II) complexes have been established, a wide variety of catalysts have been documented. 11,[30][31][32] To sum, this digression conveys the diversity of phenomena that might play roles in Schemes 3-8, but are beyond the scope of the present investigation.
One starting point for analysis involves the gyroscope and parachute like dichloride complexes trans-2 and cis-2 (Scheme 2). As reported earlier, 8 both DFT and thermolysis data (Scheme 2) show the former to be more stable, at least for (CH 2 ) n bridges with n ≥ 10. In syntheses where the educts cannot bias the stereochemical outcomefor example, 27 PtCl 2 and the dibridgehead diphosphine P((CH 2 ) 14 ) 3 Ptrans-2 is always the exclusive product. The new computational data in Fig. 5 and 6 establish an intrinsic energetic preference for the gyroscope like isomers (ca. 6.6 kcal mol -1 with (CH 2 ) 14 bridges). The single point calculations with P((CH 2 ) 14 ) 3 P localize the principal origin of the energy differences within the diphosphine ligands. The additional calculations with HP ((CH 2 ) 14 ) 2 PH and H 2 P((CH 2 ) 14 )PH 2 point to non-covalent chain/chain interactions, and any attendant dispersion forces, as stabilizing factors in the gyroscope systems. Efforts to identify destabilizing steric features unique to the parachute systems have so far not been successful.
Computations have previously been carried out with related phosphite complexes, for which only parachute like cis isomers have been synthetically accessed. 8 Here the DFT data show the cis isomers to be considerably more stable. Since phosphite ligands are stronger π acceptors than phosphine ligands, 33,34 geometries in which the π donor chloride ligands 35 are trans to the phosphorus donor atoms become preferred. In other words, this favorable electronic interaction dominates over any steric factors that may prefer gyroscope like dibridgehead diphosphite complexes.
Alkyl ligands are considered to be strong σ donorsindeed, the strongest among the ligands in this study 34 and in the context of the trans influence (vide infra), trans geometries are considered to be particularly unfavorable. From this perspective, the formation of the parachute like dialkyl complexes cis-4c,e,g and cis-6c in Schemes 4 and 6 is less surprising. However, we were unable to access any of the corresponding trans isomers, precluding an experimental confirmation of the relative stabilities. Thus, two scenarios remain in play: (a) cis-4c,e,g and cis-6c represent kinetically but not thermodynamically favored substitution products and conditions for isomerizations to trans-4c,e,g and trans-6c remain to be developed, or (b) cis-4c,e,g and cis-6c represent thermodynamically favored substitution products, either produced directly (kinetic products) or via transient trans isomers.
In our opinion, the computational data fit best with the latter. Although parachute like cis-4c and cis-6c remain 7.4-6.7 kcal mol -1 less stable than gyroscope like trans-4c and trans-6c in the gas phase (Fig. 5), these differences narrow to 5.7-2.9 kcal mol -1 in toluene or o-C 6 H 4 Cl 2 , with the diminution always more pronounced in the more polar dichlorinated solvent. For the acyclic model compounds 4″ and 6″ (Fig. 6), the cis and trans isomers have essentially equal energies. In addition, ethyl is a stronger σ donor than methyl, 36 further destabilizing the trans isomers. In any case, Fig. 5 reproduces the relative trends in our data, although trans-4c and trans-6c appear overstabilized compared to their cis isomers. 37 Consider next the diphenyl complexes 3 introduced in Scheme 3. The computations in Fig. 5 indicate much greater stabilities for the trans/gyroscope versus cis/parachute isomers as compared to the dialkyl analogs. The same trend is seen with the acyclic analogs in Fig. 6. The ligating sp 2 carbon atom of a phenyl ligand constitutes a weaker σ donor than the sp 3 carbon atom of an alkyl ligand, 36 attenuating the electronic mismatch in the trans isomers. 38 However, numerous experimental attempts to establish equilibria (Table 3) have been unsuccessful. Regardless, it can be concluded that the reaction of cis-2c and Ph 2 Zn to give cis-3c (Scheme 3) is under kinetic control.
The remaining equilibrium issues in Schemes 3-8 are quite clear cut. In all cases, it can be shown experimentally that the trans/gyroscope methyl or phenyl chloride complexes 5c,e,g and 8c are much more stable than the cis/parachute analogs (Schemes 5 and 7), and the same can be presumed for the ethyl chloride complex 7c (Scheme 6). These trends are fully supported by the computations. In all cases the weak σ donor chloride ligand is preferentially directed opposite to the stronger alkyl or phenyl σ donor. 34 There still remain various open questions. For example, a reviewer inquired regarding the mechanism of the cis/trans isomerizations that take place over silica gel in Scheme 5. Silica gel promotes or catalyzes quite a broad spectrum of organic reactions, 39 but to the authors' knowledge has not been previously observed to effect geometric isomerizations of metal complexes. Nonetheless, given the plethora of cis/trans equilibration mechanisms established for square planar platinum(II) complexes, 11,[30][31][32] many possible roles are easily envisioned. Since isomerization also occurs upon attempted Et 2 O/ROH crystallization, perhaps the hydroxyl groups associated with silica are involved.
The preceding analysis made little reference to the "trans influence", which is considered the thermodynamic counterpart of the "trans effect". 25,26,40,41 There have been extensive studies of these phenomena in platinum(II) chemistry. For the ligands of interest in this paper, the trans influence increases in the order Cl -< I -< Ph -< PR 3 < Me -< Et -. 36,41c However, these concepts are often applied rather simplistically, although even in early studies it was appreciated that the trends represent a complex mash-up of σ and π donor/acceptor effects, the relative importance of which can vary from substrate to substrate. 42 Accordingly, correlations with the equilibria established above are not attempted.
Much work involving the trans influence has been focused upon structural trendsspecifically, the effect of a series of trans ligands upon a metal-ligand bond distance. A longer bond trans to the ligand of interest is taken as evidence of destabilization, and a shorter bond is taken as evidence for stabilization. 36,40,41 One question is whether our computed structures, excerpted in Fig. 7, conform to these expectations.
For example, with the chloride substituted gyroscope like complexes trans-7c, trans-5c, trans-8c, and trans-2c, the platinum-chlorine bond lengths (2.47, 2.46, 2.44, 2.35 Å) contract as the trans influences of the ethyl, methyl, phenyl, and chloride ligands decrease. The same trend holds for the platinumcarbon bond lengths in the diethyl, dimethyl, and diphenyl complexes trans-6c, trans-4c, and trans-3c (2.16, 2.15, 2.09 Å). With the cis/parachute complexes, the platinum-phosphorus bond lengths do not change as much as the trans ligand is varied, except in the case of the dichloride complex cis-2c (2.28 Å vs. 2.38-2.39 Å for trans ethyl, methyl, and phenyl ligands). However, the platinum-carbon bond lengths follow the same trends as in the trans complexes. They are (as compared to trans complexes) uniformly longer in cis-5c, cis-7c, and cis-8c (all bonds cis to chloride), and shorter in cis-4c, cis-6c, and cis-3c (all bonds cis to alkyl).
The experimental structural data for cis-4c, trans-5c, and trans-7c (Table 2) are in excellent agreement with the bond lengths in Fig. 7 and other computed metrical parameters. All values are in the normal ranges for square planar platinum(II) complexes. [43][44][45] The analogous methyl and ethyl chloride complexes trans-5c and trans-7c exhibit bond lengths and angles about platinum that are within experimental error. In the methyl chloride and iodide complexes trans-5c and trans-10c, the platinum-carbon bonds lengthen from 2.074(7) Å to 2.179 (5) Å. This can be attributed to the greater trans influence of the iodide ligand noted above. 25,41c Additional properties of new complexes Although this study is not concerned with the dynamic properties of the complexes in Schemes 3-8, certain features are apparent from the 13 C NMR spectra. Specifically, all of the trans/gyroscope complexes in the "c series" exhibit only seven signals. This indicates that rapid ML y rotation (or (CH 2 ) 14 bridge exchange) takes place on the NMR time scale at room temperature (with the number of signals, 14/2, reflecting the formal horizontal mirror plane). Similar behavior has been observed with other methyl substituted rotors, 5c,7 but trans-7c represents the first case with a larger ethyl ligand. The analogs with longer (CH 2 ) n bridges, trans-4e,g, also gave the minimum number of 13 C NMR signals. These processes have been analyzed in depth in a recent review that collects all available activation parameters. 3 A key consideration involves the van der Waals radii of the ML y rotators versus the clearance allowed by the (CH 2 ) n bridges.
In contrast, the parachute like complex cis-3c exhibits eleven CH 2 13 C NMR signals, indicating that the "jump rope" process is slow on the NMR time scale at room temperature. In theory, two sets of seven signals with a ca. 2 : 1 intensity ratio should be observable, but sufficient resolution was not available at 126 MHz. As reported earlier, the corresponding dichloride complex cis-2c exhibits twelve signals. 8 The higher homologs cis-3e,g give the minimum number of signals (nine and eleven), indicative of rapid jump rope processes. The thermolyses in Scheme 9 convert the parachute like dimethyl complex cis-4c to the gyroscope like methyl halide complexes trans-5c and trans-9c. This suggests an initial oxidative addition of the high boiling haloarene solvent to give a platinum(IV) species, a type of reaction with significant precedent. 46 Indeed, toluene-d 5 can be identified as a product in the reaction with bromobenzene-d 5 . Evidence has also been obtained from related reactions for the formation of species with new platinum-carbon bonds. Another fascinating lead only briefly mentioned above is the apparent formation of a platinum bis( phosphine) hydride intermediate upon attempted isomerization of the ethyl chloride complex trans-5c. All of these transformations and the mesitylene-derived products in Table 3 remain under active investigation.
With respect to the gyroscope like dichloride complexes trans-2c,e,g, this study has established that isomerization normally accompanies reactions with alkyl lithium reagents to give the dialkyl complexes cis-4c,e,g and cis-6c (Schemes 4 and 6). In contrast, phenyl nucleophiles normally afford trans diphenyl complexes, although lesser amounts of cis adducts are sometimes observed in reactions with PhLi. Subsequent reactions of cis-4c,e,g and HCl (1.0 equiv.) give detectable quantities of cis alkyl chloride complexes that easily convert (over silica gel or at ∼100 °C) to the gyroscope like isomers trans-5c,e,g. The cis phenyl chloride complex cis-8c undergoes an analogous thermal isomerization to trans-8c. DFT calculations indicate that the trans/gyroscope complexes always have lower energies than the cis/parachute complexes in the gas phase. However, isomeric dimethyl and diethyl complexes have nearly the same energies in o-C 6 H 4 Cl 2 . 37 With acyclic model compounds the energy differences are much less, and the isomeric dimethyl and diethyl complexes exhibit essentially equal energies.
Overall, this work has bought several concepts from traditional platinum(II) coordination chemistry into the modern context of the chelating and potentially trans spanning dibridgehead diphosphine ligands P((CH 2 ) n ) 3 P. These data greatly enhance the confidence with which the NMR data associated with various dynamic processes can be interpreted. These include ML y rotation in the gyroscope like complexes and the "jump rope" (CH 2 ) n exchange in the parachute like complexes, both of which will be topics of future reports.
Reactions were conducted under nitrogen atmospheres unless noted. Materials were utilized as follows: Et 2 O, CH 2 Cl 2 , and THF, purified by a Glass Contour system; hexanes (98.5%, Sigma-Aldrich), EtOAc (99.5%, Sigma-Aldrich), methanol (99.8%, Sigma-Aldrich), benzene (99.88%, Sigma-Aldrich), mesitylene (97%, TCI), o-C 6 H 4 Cl 2 (99%, Sigma-Aldrich), CDCl 3 , C 6 D 6 , toluene-d 8 , C 6 D 5 Br (4 × Cambridge Isotopes), MeLi (1.6 M in Et 2 O, Sigma-Aldrich), EtLi (0.50 M in benzene, Sigma-Aldrich), PhLi (1.8 M in Bu 2 O, Sigma-Aldrich), CH 3 COCl (98%, Alfa-Aesar), HCl (2.0 M in Et 2 O, Alfa-Aesar), Et 3 N (99%, Alfa-Aesar), LiBr (99.99%, Alfa-Aesar), NaI (99%, EMD), celite (EMD), silica gel (40-63 µm, 230-400 mesh, Silicycle), and neutral alumina (Brockmann I, for chromatography, 40-300 µm, 60A, Acros), used as received.
NMR spectra were recorded on standard 500 MHz FT spectrometers at ambient probe temperatures and referenced as follows (δ, ppm): 1 H: residual internal CHCl 3 (7.26) or C 6 D 5 H (7.16); 13 3 J HP(trans) = 7.1 Hz, 3 J HP(cis) = 6.1 Hz, 2 J HPt = 66 Hz, 48 PtCH 3 ); 13 2 J CP(trans) = 101.7 Hz, 2 J CP(cis) = 9.9 Hz, PtC H 3 ); 31 P{ 1 H} (202 MHz) 6.9 (s, 1 J PPt = 1886 Hz). 48 cis-Pt(Me) 2 (P((CH 2 ) 18 ) 3 P) (cis-4e). A. A Schlenk flask was charged with trans-Pt(Cl) 2 (P((CH 2 ) 18 ) 3 P) (trans-2e; 4a 0.1239 g, 0.1142 mmol) and Et 2 O (20 mL). Then MeLi (1.6 M in Et 2 O, 0.160 mL, 0.256 mmol) was added with stirring (rt). After 12 h, the mixture was exposed to air. After 1 h, the sample was filtered through celite, which was washed with diethyl ether (10-20 mL). The solvent was removed from the filtrate by rotary evaporation. The residue was chromatographed on neutral alumina (1 : 0.005 v/v hexanes/Et 3 N). The solvent was removed from the product containing fractions (TLC monitoring) by rotary evaporation to give cis-4e as a colorless oil (0.0838 g, 0.0802 mmol, 70%). B. A Schlenk flask was charged with trans-2e (0.1436 g, 0.1323 mmol) and Et 2 O (25 mL). Then MeLi (1.6 M in Et 2 O, 0.210 mL, 0.336 mmol) was added with stirring (rt). After 12 h, the mixture was exposed to air. After 1 h, the sample was filtered through celite, which was washed with Et 2 O (10-20 mL). The solvent was removed from the filtrate by rotary evaporation to give cis-4e as a beige oil (0.1298 3 J HP(trans) = 6.7 Hz, 3 J HP(cis) = 5.7 Hz, 2 J HPt = 65 Hz, 48 PtCH 3 ); 13 = 10.0 Hz, PtC H 3 ); 31 P{ 1 H} (202 MHz) 4.5 (s, 1 J PPt = 1860 Hz). 48 cis-Pt(Me) 2 (P((CH 2 ) 22 ) 3 P) (cis-4g). A Schlenk flask was charged with trans-Pt(Cl) 2 (P((CH 2 ) 22 ) 3 P) (trans-2g; 4c 0.1469 g, 0.1172 mmol) and Et 2 O (20 mL). Then MeLi (1.6 M in Et 2 O, 0.170 mL, 0.272 mmol) was added with stirring (rt). After 12 h, the mixture was exposed to air. After 1 h, the sample was filtered through celite, which was washed with Et 2 O (10-20 mL). The solvent was removed from the filtrate by rotary evaporation. The residue was dissolved in benzene and sample freeze dried to give cis-4g as a white powder (0.1294 3 J HP(trans) = 7.3 Hz, 3 J HP(cis) = 6.1 Hz, 2 J HPt = 66 Hz, 48 PtCH 3 ); 13 C{ 1 H} MHz) 32.0 (virtual t, 49 49 J CP = 9.3 Hz, PC H 2 ), 3.9 (dd, 2 J CP(trans) = 101.7 Hz, 2 J CP(cis) = 10.0 Hz, PtC H 3 ); 31 P{ 1 H} (202 MHz) 4.6 (s, 1 J PPt = 1853 Hz). 48 cis-Pt(Cl)(Me)(P((CH 2 ) 14 ) 3 P) (cis-5c). Under air, a round bottom flask was charged with cis-3c (0.4007 g, 0.4573 mmol), CH 2 Cl 2 (10 mL), and methanol (1.0 mL). Then CH 3 COCl (0.0344 mL, 0.4865 mmol) was added with stirring. After 0.5 h, the solvent was removed by rotary evaporation. The white solid was chromatographed on neutral alumina (25 : 1 : 0.2 v/v/v hexanes/EtOAc/Et 3 N). The solvent was removed from the product containing fractions (TLC monitoring) by rotary evaporation to give cis-5c as a white solid (0.3610 g, 0.4026 mmol, 83%), mp (open capillary) 122-127 °C. Anal. Calcd for C 43 H 87 ClP 2 Pt (896.63): C, 57.60; H, 9.78. Found: C, 57.80; H, 10.00. NMR (CDCl 3 , δ/ppm): 47 1 H (500 MHz) 2.42-2.33 (br m, 2H, PCH 2 ), 1.92-1.83 (br m, 2H, CH 2 ), 1.82-1.68 (br m, 6H, CH 2 ), 1.68-1.53 (br m, 6H, CH 2 ), 1.50-1.21 (br m, 68H, remaining CH 2 ), 0.58 (dd, 3H, 3 J HP(trans) = 7.0 Hz, 3 J HP(cis) = 4.0 Hz, 2 J HPt = 50 Hz, 48 PtCH 3 ); 13 C { 1 H} (126 MHz) 30.9 (virtual t, 49 49 J CP = 7.3 Hz, PC H 2 ), 4.5 (dd, 2 J CP(trans) = 93.1 Hz, 2 J CP(cis) = 8.0 Hz, PtC H 3 ); 31 P{ 1 H} (202 MHz) 11.6 (d, 2 J PP = 12.1 Hz, 1 J PPt = 1765 Hz, 48 trans to CH 3 ), 5.0 (d, 2 J PP = 12.1 Hz, 1 J PPt = 4278 Hz, 48 cis to CH 3 ).
trans-Pt(Cl)(Me)(P((CH 2 ) 14 ) 3 P) (trans-5c). A. Under air, a round bottom flask was charged with cis-4c (0.0245 g, 0.0273 mmol), silica gel (0.2577 g), and CH 2 Cl 2 (2.5 mL). The mixture was stirred (rt). After 12 h, the sample was filtered and the silica gel further washed with CH 2 Cl 2 (25 mL). The solvent was removed from the filtrate by rotary evaporation to yield a colorless oil, which solidified after 4 h to give trans-5c (0.0174 g, 0.0192 mmol, 70%) as a white solid, mp (open capillary) 123-127 °C. Anal. Calcd for C 43 H 87 ClP 2 Pt (896.63): C, 57.60; H, 9.78. Found: C, 57.85; H, 10.05. B. Under air, a vial was charged with cis-5c (0.0255 g, 0.0284 mmol), silica gel (0.9528 g) and CH 2 Cl 2 (10.0 mL). The mixture was stirred (rt). After 12 h, the sample was filtered and the silica gel further washed with CH 2 Cl 2 (50 mL). The solvent was removed from the filtrate by rotary evaporation to yield a colorless oil, which solidified after 1 d to give trans-5c (0.0227 g, 0.0253 mmol, 89%) as a white solid. C. As described in the Crystallography section below, trans-5c was also obtained upon attempted crystallization of cis-5c.
NMR (CDCl 3 , δ/ppm): 49 J CP = 15.7 Hz, PC H 2 ), -23.6 (t, J CP = 6.4 Hz, PtC H 3 ); 31 P{ 1 H} (202 MHz) 10.8 (s, 1 J PPt = 2824 Hz). 48 cis-Pt(Cl)(Me)(P((CH 2 ) 18 ) 3 P) (cis-5e) and trans-5e. A. Under air, a round bottom flask was charged with cis-4e (0.0838 g, 0.0802 mmol), CH 2 Cl 2 (8 mL), and methanol (0.4 mL). Then HCl (2.0 M in Et 2 O, 0.040 mL, 0.080 mmol) was added with stirring. After 0.5 h, the solvent was removed by rotary evaporation. The white solid was chromatographed on neutral alumina (first 3 : 1 : 0.02 v/v/v hexanes/CH 2 Cl 2 /Et 3 N, then 25 : 1 : 0.2 v/v/v hexanes/EtOAc/Et 3 N). The solvent was removed from the product containing fractions (TLC monitoring) by rotary evaporation. This gave trans-5e (0.0389 g, 0.0365 mmol, 46%), with subsequent fractions affording trans-5e/cis-5e mixtures. B. A vial was charged with a trans-5e/cis-5e mixture (0.0274 g, 0.0257 mmol; obtained from a different run of procedure A, ca. 1 : 2 by 31 P NMR), silica gel (1.2375 g) and CH 2 Cl 2 (10 mL). The mixture was stirred (rt). After 12 h, the sample was filtered and the silica gel further washed with CH 2 Cl 2 (40 mL). The solvent was removed from the filtrate to give trans-5e (0.0165 g, 0.0155 mmol, 60%) as a beige oil.
Data for cis-5e: NMR (CDCl 3 , δ/ppm): 31 P{ 1 H} (202 MHz) 10.3 (d, 2 J PP = 12.3 Hz, 1 J PPt = 1750 Hz, trans to CH 3 ), 3.56 (d, 2 J PP = 12.1 Hz, 1 J PPt = 4362 Hz, cis to CH 3 ).
Data for trans-5e: NMR (CDCl 3 , δ/ppm): 47 1 H (500 MHz) 1.88-1.75 (br m, 12H, PCH 2 ), 1.65-1.50 (br m, 12H, PCH 2 CH 2 ), 1.46-1.38 (br m, 12H, PCH 2 CH 2 CH 2 ), 1.38-1.23 (br m, 72H, remaining CH 2 ), 0.31 (t, 3H, 3 J HP = 6.2 Hz, 2 J HPt = 83 Hz, 48 PtCH 3 ); 13 C{ 1 H} (126 MHz) 30.8 (virtual t, 49 J CP = 6.4 Hz, PCH 2 CH 2 C H 2 ), 28.6 (s, C H 2 ), 28.48 (s, C H 2 ), 28.47 (s, C H 2 ), 28.4 (s, C H 2 ), 28.0 (s, C H 2 ), 27.8 (s, C H 2 ), 23.9 (s, PCH 2 C H 2 ), 21.5 (virtual t, 49 J CP = 15.9 Hz, PC H 2 ), -23.4 (t, J CP = 6.5 Hz, PtC H 3 ); 31 P{ 1 H} (202 MHz) 8.8 (s, 1 J PPt = 2824 Hz). 48 cis-Pt(Cl)(Me)(P((CH 2 ) 22 ) 3 P) (cis-5g). A. Under air, a round bottom flask was charged with cis-4g (0.0664 g, 0.0547 mmol),
This CH 2 Cl 2 (5 mL). Then HCl (2.0 M in Et 2 O, 0.0274 mL, 0.0548 mmol) was added with stirring. After 0.5 h, the solvent was removed by rotary evaporation to give a beige oil containing a cis-2g/4g/5g mixture in an approximate 2 : 3 : 5 ratio by 31 P NMR, along with other unidentified species. Attempted chromatographic purifications were unsuccessful. B. A CH 2 Cl 2 solution of crude cis-4g (0.2852 g, ca. 0.2332 mmol) was chromatographed on neutral alumina (100 : 5 : 1 v/v/v hexanes/ EtOAc/Et 3 N). The solvent was removed from the product containing fractions (TLC monitoring) by rotary evaporation to give a 18 : 1 mixture of cis/trans-5g that was ca. 80% pure by 31 P { 1 H} NMR.
Data for cis-5e: NMR (CDCl 3 , δ/ppm): 1 H (500 MHz) 1.91-1.68 (br m, 12H, PCH 2 , 1.54-1.43 (br m, 12H, PCH 2 CH 2 ), 1.43-1.34 (br m, 12H, PCH 2 CH 2 CH 2 ), 1.34-1.23 (br m, 96H, remaining CH 2 ), 0.59 (dd, 3H, 3 J HP(trans) = 7.0 Hz, 3 J HP(cis) = 4.0 Hz, 2 J HPt = 49 Hz, 48 CH 3 ); 31 P{ 1 H} (202 MHz) 10.1 (d, 2 J PP = 12.7 Hz, 1 J PPt = 1739 Hz, trans to CH 3 ), 3.4 (d, 2 J PP = 12.7 Hz, 1 J PPt = 4260 Hz, cis to CH 3 ).
trans-Pt(Cl)(Me)(P((CH 2 ) 22 ) 3 P) (trans-5g). A. Under air, a vial was charged with cis-4g (0.0146 g, 0.0120 mmol), silica gel (0.2572 g), and CH 2 Cl 2 (2.5 mL). The mixture was stirred (rt). After 12 h, the sample was filtered and the silica gel further washed with CH 2 Cl 2 (25 mL). The solvent was removed from the filtrate by rotary evaporation to give trans-5g (0.0118 g, 0.0096 mmol, 80%) as a colorless oil. B. A vial was charged with a ca. 18 : 1 cis/trans-5g mixture (0.0350 g, 0.0284 mmol; see preceding paragraph), silica gel (0.9325 g), and CH 2 Cl 2 (10 mL). The mixture was stirred (rt). After 12 h, the sample was filtered and the silica gel further washed with CH 2 Cl 2 (40 mL). The solvent was removed from the filtrate to give trans-5g (0.0224 g, 0.0182 mmol, 64%) as a beige oil. Anal. 48 trans-Pt(Br)(Me)(P((CH 2 ) 14 ) 3 P) (trans-9c). A Schlenk flask was charged with trans-5c (0.0981 g, 0.1094 mmol), LiBr (0.0488 g, 0.562 mmol) and THF (10 mL). The mixture was stirred (rt). After 12 h, the sample was concentrated and chromatographed on silica gel (3 : 2 v/v hexanes/CH 2 Cl 2 ). The solvent was removed from the product containing fractions (TLC monitoring) to give trans-9c as a colorless oil, which solidified after 1 h (0.0903 g, 0.0960 mmol, 88% 49 J CP = 16.0 Hz, PC H 2 ), -20.2 (t, J CP = 6.4 Hz, PtC H 3 ); 31 P{ 1 H} (202 MHz) 8.5 (s, 1 J PPt = 2804 Hz). 48 trans-Pt(I)(Me)(P((CH 2 ) 14 ) 3 P) (trans-10c). Under air, a vial was charged with trans-5c (0.0599 g, 0.0668 mmol), NaI (0.0557 g, 0.372 mmol), THF (4 mL) and acetone (4 mL). The mixture was stirred (rt). After 2 d, the sample was concentrated and chromatographed on silica gel (hexanes, then CH 2 Cl 2 ). The solvent was removed from two sets of product fractions (TLC monitoring) to give trans-Pt(I) 2 (P((CH 2 ) 14 ) 3 P) (0.0077 g, 0.007 mmol, 10%) 4a 48 cis-Pt(Et) 2 (P((CH 2 ) 14 ) 3 P) (cis-6c). A Schlenk flask was charged with trans-2c (0.3018 g, 0.3291 mmol) and Et 2 O (15 mL). Then a benzene solution of EtLi (0.5 M, 0.200 mL, 0.100 mmol) was added with stirring (rt). After 12 h, the mixture was exposed to air. After 1 h, the sample was filtered through celite, which was washed with Et 2 O (10-20 mL). The solvent was removed from the filtrate by rotary evaporation. The residue was chromatographed on neutral alumina (1 : 0.005 v/v hexanes/Et 3 N). The solvent was removed from the product containing fractions (TLC monitoring) by rotary evaporation. The sample was kept at 4 °C for 2 d but did not solidify, affording cis-6c as a colorless oil (0.2524 g, 0.2791 mmol, 85% 16.4 (apparent t, J CP = 2.1 Hz, PtCH 2 C H 3 ), 14.8 (dd, 2 J CP(trans) = 99.8 Hz, 2 J CP(cis) = 9.1 Hz, PtC H 2 CH 3 ); 31 P{ 1 H} (202 MHz) 6.6 (s, 1 J PPt = 1746 Hz). 48 trans-Pt(Cl)(Et)(P((CH 2 ) 14 ) 3 P) (trans-7c). Under air, a roundbottom flask was charged with cis-6c (0.3781 g, 0.4181 mmol), CH 2 Cl 2 (15 mL), and methanol (1.0 mL). Then CH 3 COCl (0.0298 mL, 0.4214 mmol) was added with stirring. After 2 h, the solvent was removed by rotary evaporation. The residue was chromatographed on neutral alumina (5 : 1 : 0.02 v/v/v hexanes/Cl 2 Cl 2 /Et 3 N). The solvent was removed from the product containing fractions (TLC monitoring) by rotary evap- 48 cis-Pt(Cl)(Ph)(P((CH 2 ) 14 ) 3 P) (cis-8c) and trans-8c. Under air, a round bottom flask was charged with cis-Pt (Ph) 2 (P((CH 2 ) 14 ) 3 P) (cis-3c; 8 0.0598 g, 0.0598 mmol) and CH 2 Cl 2 (6 mL). Then HCl (2.0 M in Et 2 O, 0.030 mL, 0.060 mmol) was added with stirring (rt). After 1 h (based upon TLC monitoring), another charge of HCl was added (0.012 mL, 0.024 mmol). After 0.5 h, the solvent was removed by rotary evaporation to give a white solid, which was chromatographed on neutral alumina (first 5 : 1 v/v hexanes/ CH 2 Cl 2 , then 20 : 1 v/v hexanes/EtOAc). Two sets of fractions were collected (TLC monitoring). The solvents were removed by rotary evaporation, benzene was added, and the two samples were freeze dried. The first gave trans-8c (0.0167 g, 0.0174 mmol, 29%) 4b as a white powder; the second gave cis-8c 48 trans to Ph), 0.2 (d, 2 J PP = 14.0 Hz, 1 J PPt = 4228 Hz, 48 cis to Ph).
cis-Pt(Ph) 2 (P((CH 2 ) 14 ) 3 P) (cis-3c) and trans-3c. A. A Schlenk flask was charged with trans-2c (0.1915 g, 0.2088 mmol) 4a and Et 2 O (20.0 mL). Then PhLi (1.8 M in Bu 2 O, 0.235 mL, 0.423 mmol) was added dropwise with stirring (rt). After 12 h, the mixture was exposed to air. After 1 h, the sample was filtered through celite, which was washed with diethyl ether (20 mL). The solvent was removed from the filtrate by rotary evaporation. The residue was chromatographed on silica gel (first 8 : 1 v/v hexanes/CH 2 Cl 2 , then 2 : 1 v/v hexanes/CH 2 Cl 2 ). The solvent was removed from two sets of product containing fractions (TLC monitoring) by rotary evaporation to give trans-3c (0.1036 g, 0.1036 mmol, 50%) and cis-3c (0.0602 g, 0.0602 mmol, 29%) as white solids. The NMR data for each product agreed well with those published earlier. 4a,8 B. A Schlenk flask charged with trans-2c (0.2232 g, 0.2434 mmol) 4a and Et 2 O (20.0 mL). Then PhLi (1.8 M in Bu 2 O, 0.300 mL, 0.540 mmol) was added dropwise with stirring (rt). After 12 h, the mixture was exposed to air. After 1 h, the sample was filtered through celite, which was washed with diethyl ether (20 mL). The solvent was removed from the filtrate by rotary evaporation. The residue was chromatographed on silica gel (8 : 1 v/v hexanes/CH 2 Cl 2 ). The solvent was removed from product containing fraction (TLC monitoring) by rotary evaporation to give trans-3c (0.2077 g, 0.2076 mmol, 85%).
NMR tubes were charged with the following materials and kept at the indicated temperature except for brief intervals when spectra (δ/ppm) were recorded.
Entry 1. cis-2c (0.0086 g, 0.0094 mmol or 0.0096 g, 0.0104 mmol), silica gel (0.0925 g), toluene-d 8 or CDCl 3 (0.6 mL), 25 °C, 24 h. 31 P{ 1 H} NMR 4.5 or 4.8 (s, 1 J PPt = 3498 or 3548 Hz, 48 cis-2c, >99%).
Entry 2. cis-3c (0.0070 g, 0.0070 mmol), toluene-d 8 (0.7 mL), 80 °C, 10 h. 31 P{ 1 H} NMR 4.9 (s, 1 J PPt = 1762 Hz, 48 cis-3c, >99%).
Entry 3. cis-3c (0.0090 g, 0.0090 mmol), mesitylene (0.6 mL), 140 °C, 30 h. 31 P{ 1 H} NMR 7.4 (s, unknown, 84%), 4.3 (s, unknown, 16%). 20 Spectra taken after 1.5-12 h showed additional minor signals.
Entry 4. cis-3c (0.0059 g, 0.0059 mmol), silica gel (0.0927 g), toluene-d 8 (0.6 mL), 25 °C, 5 h and then 80 °C, 24 h. 31 P{ 1 H} NMR 0.5 (s, 1 J PPt = 1760 Hz, 48 cis-3c, >99%).
Entry 5. cis-4c (0.0062 g, 0.0068 mmol), toluene-d 8 (0.7 mL), 80 °C, 10 h. 31 P{ 1 H} NMR 7.3 (s, 1 J PPt = 1868 Hz, 48 cis-4c, >99%). Entry 6. cis-4c (0.0081 g, 0.0092 mmol), mesitylene (0.6 mL), 140 °C, 30 h. 31 P{ 1 H} NMR 13.6 (s, unknown, 8%), 10.8 (unknown, 2%), 9.3 (s, unknown, 6%), 7.3 (s, 1 J PPt = 1865 Hz, 48 cis-4c, 84%.
Entry 7. cis-4c (0.0071 g, 0.0081 mmol), silica gel (0.1096 g), toluene-d 8 (0.6 mL), 25 °C, 24 h. 31 P{ 1 H} NMR 7.3 (s, 1 J PPt = 1870 Hz, 48 cis-4c, >99%).
Entry 8. cis-4c (0.0072 g, 0.0082 mmol), silica gel (0.0927 g), toluene-d 8 (0.6 mL), 25 °C, 5 h (little conversion) and then 80 °C, 24 h. 31 P{ 1 H} NMR 11.4 (s, unknown, 76%), 6.2 (s, unknown, 24%). 20 Entry 9. cis-5c (0.0061 g, 0.0068 mmol), toluene-d 8 (0.7 mL), 80 °C, 10 h. 31 P{ 1 H} NMR 11.3 and 5.0 (2 d, 2 J PP = 11.5 Hz/ 1 J PPt = 1736 Hz 48 and 2 J PP = 11.5 Hz/ 1 J PPt = 4167 Hz, 48 cis-5c, 86%), 11.1 (s, trans-5c, 14%). The 72 h experiment was similarly carried out in toluene-d 0 .
Entry 10. cis-5c (0.0143 g, 0.0159 mmol), o-C 6 H 4 Cl 2 (0.6 mL), 100 °C, 3 h. 31 P{ 1 H} NMR 10.8 (s, 1 J PPt = 2834 Hz, 48 trans-5c, >99%).
This Entry 11. cis-5c (0.0064 g, 0.0071 mmol), o-C 6 H 4 Cl 2 (0.7 mL), 140 °C, 0.5 h. 31 P{ 1 H} NMR 10.8 (s, 1 J PPt = 2834 Hz, 48 trans-5c, >99%).
Entry 12. cis-5c (0.0091 g, 0.0100 mmol), C 6 D 5 Br (0.6 mL), 110 °C, 7 h. 31 P{ 1 H} NMR 11.1 (s, 1 J PPt = 2828 Hz, 48 trans-5c, >99%). 1 H NMR 2.00-1.84 (m, 12H, PCH 2 ), 1.84-1.69 (m, 12H, PCH 2 CH 2 ), 1.61-1.49 (m, 12H, PCH 2 CH 2 CH 2 ), 1.49-1.32 (m, 48H, remaining CH 2 ), 0.54 (t, 3H, 3 J HP = 5.5 Hz, 2 J HPt = 80 Hz, 48 PtCH 3 ).
Entry 13. cis-5c (0.0059 g, 0.0066 mmol), mesitylene (0.6 mL), 140 °C, 0.5 h. 31 P{ 1 H} NMR 10.4 (s, 1 J PPt = 2841 Hz, 48 trans-5c, >99%).
Entry 14. trans-5c (0.0072 g, 0.0080 mmol), mesitylene (0.6 mL), 140 °C, 30 h. 31 P { 1 H} NMR 10.8 (s, 1 J PPt = 2844 Hz, 48 trans-5c, 87%), 7.3 (s, unknown, 13%).
Entry 15. trans-7c (0.0089 g, 0.0098 mmol), mesitylene (0.6 mL), 140 °C, h. 31 P { 1 H} NMR 7.4 (s, 1 J PPt = 2447 Hz, 48 unknown, >99%). 20 Spectra taken after 1.5-18 h showed one main intermediate: 31 P{ 1 H} NMR 18.3 (s, 1 J PPt = 2756 Hz 48 ); 31 P NMR (undecoupled): 18.3 (br s, w 1/2 = 27 Hz, 1 J PPt = 2754 Hz 48 ); 1 H NMR -16.78 (t, 2 J HP = 13.8 Hz, 1 J HPt = 1254 Hz, 48 referenced to C 6 H 3 (CH 3 ) 3 at δ 6.78 ppm).
Entry 16. cis-8c (0.0075 g, 0.0078 mmol), o-C 6 H 4 Cl 2 (0.6 mL), 100 °C, 6 h. 31 P{ 1 H} NMR 7.7 (s, 1 J PPt = 2806 Hz, 48 trans-8c, >99%).
Entry 17. cis-8c (0.0053 g, 0.0055 mmol), o-C 6 H 4 Cl 2 (0.6 mL), 140 °C, 0.5 h. 31 P{ 1 H} NMR 7.7 (s, 1 J PPt = 2806 Hz, 48 trans-8c, >99%).
Entry 18. trans-8c (0.0098 g, 0.0100 mmol), mesitylene (0.6 mL), 140 °C, 30 h. 31 P { 1 H} NMR 7.7 (s, 1 J PPt = 2806 Hz, 48 trans-8c, >99%).
Scheme 9 (right). cis-4c (0.0134 g, 0.0153 mmol), C 6 D 5 Br (0.6 mL), 110 °C, 7 h and then 140 °C, 6 h. 31 P{ 1 H} NMR 8.9 (s, 1 J PPt = 2806 Hz, trans-9c, >99%). 1 H NMR 2.28 (s, 1H, C 6 D 5 CH 3 ), 51 2.03-1.92 (m, 12H, PCH 2 ), 1.79-1.68 (m, 12H, PCH 2 CH 2 ), 1.59-1.49 (m, 12H, PCH 2 CH 2 CH 2 ), 1.49-1.37 (m, 48H, remaining CH 2 ), 0.61 (t, 3H, 3 J HP = 6.0 Hz, 2 J HPt = 80 Hz, 48 PtCH 3 ).
Scheme 9 (left). cis-4c (0.0062 g, 0.0071 mmol), o-C 6 H 4 Cl 2 (0.6 mL), 140 °C, 12 h. 31 P{ 1 H} NMR 10.8 (s, 1 J PPt = 2834 Hz, 48 trans-5c, >99%). A spectrum taken after 1 h showed an additional minor signal at 8.9 (s, unknown).
A. Methanol was slowly added dropwise to an Et 2 O solution of cis-4c. After each drop, the mixture was shaken to redissolve the white precipitate formed. When the precipitate persisted, Et 2 O was added to regenerate a homogeneous solution, which was kept at 4 °C. After 3 d, colorless crystals were obtained, and data were collected as outlined in Table 1. Cell parameters were obtained from 45 frames using a 1°scan and refined with 102 005 reflections. Integrated intensity information for each reflection was obtained by reduction of the data frames with the program APEX3. 52 Lorentz and polarization corrections were applied. Data were scaled, and absorption correction were applied using the program SADABS. 53 The space group was determined from systematic reflection conditions and statistical tests. The structure was solved using XT/XS in APEX3. 52,54 The structure was refined (weighted least squares refinement on F 2 ) to convergence. 54,55 All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were placed in idealized positions using a riding model. The atoms C29 through C42 and C23 to C24 exhibited elongated thermal ellipsoids and/or nearby residual electron density peaks. These were successfully modeled by disorder between two positions, with occupancy ratios of 0.64 : 0.36, and 0.81 : 0.19 respectively. Appropriate restraints and/or constraints were applied to keep the bond distances, angles, and thermal ellipsoids meaningful. The absence of additional symmetry and voids were confirmed using PLATON (ADDSYM). 56 B. Colorless crystals of trans-5c were obtained in a procedure analogous to that for cis-4c, but starting from the isomer cis-5c. Either methanol or ethanol could be used. Data were collected on a crystal from each solvent system and both gave the same structure, which was solved as in A. Two pairs of atoms, Cl1/C1 and C39/ C40 exhibited abnormal thermal ellipsoids. These were successfully modeled by disorder between two positions, with occupancy ratios of 0.89 : 0.11 and 0.88 : 0.12 respectively. C. An Et 2 O solution of trans-10c was layered with methanol. After 7 d, data were collected on the yellow blocks as outlined in Table 1. The structure was solved and refined in a manner parallel to that in A. The iodide and methyl ligands (I1, C1) were disordered, but this could be modeled and refined to a 86 : 14 occupancy ratio (I1 and I1a as well as C1 and C1a were constrained to have the same thermal ellipsoids). D. Colorless crystals of trans-7c were obtained in a procedure analogous to that for cis-4c. Data were collected and the structure was solved in a manner parallel to that in A. Three independent molecules were found in the asymmetric unit. For one, the carbon atoms of the methylene chains exhibited elongated thermal ellipsoids (C91-C132), suggesting disorder. This could be modeled between two positions with an occupancy ratio of 0.33 : 0.67. Appropriate restraints were used to keep bond distances, angles, and thermal ellipsoids meaningful. However, some thermal ellipsoids associated with the modeled positions remained elongated, suggesting further disorder.
Scheme 1 Generalized syntheses of gyroscope like (trans-II) and parachute like (cis-II) metal complexes.Scheme 2 Previously studied isomerizations of parachute and gyroscope like complexes.
Scheme 3 Stereospecific substitution of chloride ligands by phenyl ligands in trans and cis platinum complexes. Scheme 4 Substitution of the chloride ligands in trans-2c,e,g by methyl ligands with isomerization.
H NMR. Toluene-d 5 was also detected. An analogous reaction
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