A new supramolecular bonding motif involving NH bonds of ammonium salts and macrocycles derived from platinum corners and butadiynediyl linkers

Supramolecular chemistry features two ubiquitous compositional elements that can come into play both independently and in concert: macrocyclic hosts and hydrogen bonding. 1 Some of the wide range of hydrogen bond donors and acceptors have biological roots 2 or are bio-inspired, 3 and others result from molecular design. 4,5 However, it is very unusual these days for new and effective hydrogen bonding partners to emerge "out of the blue". In this communication, an unexpected new family of hydrogen bond acceptors, four-sided Pt 4 C 16 macrocycles I (Scheme 1) that feature platinum corners and butadiynediyl edges, is disclosed.

We and three other groups (Youngs, Bruce, Anderson) [6][7][8][9] have reported syntheses of I by the route shown in Scheme 1, which involves a cis monoplatinum bis(butadiynyl) complex, a cis monoplatinum dichloride complex, a CuI catalyst, and a secondary amine cosolvent such as HNEt 2 . For heretofore obscure reasons, in nearly all cases the ammonium salt byproduct H 2 NEt 2 + Cl -has been very difficult to remove, with the solid materials initially isolated best represented as I•(H 2 NEt 2 + Cl -) n . [7][8][9] Other secondary amines as well as tertiary amines gave similar results.

Although we investigated reactions with six different 1,3diphosphine ligands and numerous workups, analytically pure ammonium-salt-free samples were never obtained. 9 NMR spectra typically showed 1-2 equivalents of the salt, four of which are generated per macrocycle. Nonetheless, by difficultly reproducible procedures, salt-free single crystals could be grown in three cases. Three further crystalline samples, but none that incorporated ammonium salts, were obtained by Bruce. 8 The six X-ray structures showed both planar and folded Pt 4 conformations, as quantified by the angle between the two Pt-Pt-Pt planes that constitute the "hinge leaves" (0°for planar macrocycle). DFT computations suggested these to be very close in energies, separated by a miniscule barrier, and thereby dictated by packing forces. 9 In our previous report 9 we computed electrostatic potential energy maps, as exemplified by II-IV in Scheme 1. The first shows a marked negatively charged region associated with the Pt 4 C 16 core. The analogous Pt 5 C 20 or triangular Pt 3 C 12 species exhibit the same charge density pattern, suggesting that this entire series of compounds could be effective hosts for ammonium cations, per the typical electrostatic energy map V (Scheme 1). With related cis-L 2 Pt(CuC) n H species (n = 1, 2), for which various types of π adducts of the CuC linkages have been characterized, 10 no markedly negative region is apparent (III, n = 2). The same holds for complexes with Pt(CuC) n Pt linkages (e.g., IV). Thus, the high negative charge density is not an embodiment of the Pt y C 4y corners or linkers, but rather a characteristic realized upon generation of the macrocycle.

Accordingly, we sought to physically substantiate specific I/ ammonium salt bonding interactions. After repeated efforts to obtain X-ray structures of marginal crystals, the dichloride building blocks in Scheme 1 were changed to the corresponding diiodides to match the copper halide catalyst used (CuI). Reactions were set up as summarized in Scheme 2 using two chelating 1,3-diphosphines (Ph 2 PCH 2 CR 2 CH 2 PPh 2 ; R = Me, Et) that lead to Pt 4 C 16 macrocycles 1 and 2, and three secondary amine cosolvents (a, HNEt 2 ; b, HN(CH 2 CHvCH 2 ) 2 , c, morpholine). An analogous reaction was carried out with the tertiary amine NEt 3 (d). The Pt 4 C 16 /ammonium iodide adducts 1•Ha + I -, 1•(Hc + I -) 3 , 2•Ha + I -, 2•Hb + I -, and 2•(Hd + I -) 3 precipitated as yellow solids in 92-42% yields. The NMR properties closely matched those of diethyl-or triethylammonium chloride species isolated earlier. 9 The stoichiometry with respect to the ammonium iodide salts (1-3 equiv.) was established by 1 H NMR and microanalyses.

With effort, crystals of solvates of the preceding adducts could be reproducibly obtained. Structural characterization was hampered by rapid partial desolvation upon removal from the mother liquor, solvate leaching into the oils commonly used during data collection, disorder, and other problems. 11 Nonetheless, three structures could be solved, all featuring a single ammonium halide per macrocycle: 1•Ha + I -•1.62CH 2 Cl 2 , 2•Hb + I -•3.75CHCl 3 •2.83C 7 H 8 , and 1•Hc + Cl -. The chloride anion in the last salt is believed to arise from the CH 2 Cl 2 solvent, perhaps by reaction with the original iodide anion during the crystallization period. 12 As shown in Fig. 1, the ammonium cations were strongly associated with the Pt 4 C 16 macrocycles, consistent with expectations from electrostatic effects alone. The macrocycles were moderately to sharply folded, with the hinge planes (Pt1-Pt2-Pt4 and Pt2-Pt3-Pt4) defining angles of 144°, 135°, and 96°, respectively. A representative overall structure is depicted in Fig. 2. The crystal structures of ammonium-salt-free 1 and 2 determined earlier (see above) 9 gave hinge plane/hinge plane angles of 111°and 138-135°(two solvates). Interestingly, 1 opens from 111°to 144°to accommodate the guest in 1•Ha + I -•1.62CH 2 Cl 2 , but "bites down" to 96°in 1•Hc + Cl -.

In all three structures, the NH hydrogen atoms (shaded blue) were clearly bonding to the C 4 linkages of the macrocycles. As sketched for the diethylammonium salt 1•Ha + I -•1.62CH 2 Cl 2 in Fig. 1 (top), the two NH atoms exhibited short NH⋯(CuC) centroid contacts of 2.41 and 2.84 Å. The NH⋯C sp distances are tabulated in the ESI. † With the diallylammonium salt 2•Hb + I -•3.75CHCl 3 •2.83C 7 H 8 , only one short NH⋯(CuC) centroid contact was evident (2.70 Å). With the morpholinium salt 1•Hc + Cl -, four were found (2.89, 3.10, 2.48, 2.59 Å). These are well within typical distances for NH hydrogen bonds to heteroatoms bearing lone pairs. 13 The corresponding N⋯(CuC) centroid distances, which are more accurately It is also well established that the NCH hydrogen atoms of ammonium salts can serve as hydrogen bond donors. 14 Similarly close NCH⋯(CuC) centroid and NC⋯(CuC) centroid contacts are evident in the preceding crystal structures (see atoms and distances in red). For example, the shortest NH⋯ (CuC) centroid distances in 1•Ha + I -•1.62CH 2 Cl 2 are 2.61, 2.83, and 2.92 Å. Comparable distributions are found in the other structures, as tabulated in the ESI. † Importantly, attempts were made to crystallize a variety of related adducts. While only the three structures in Fig. 1 could be solved at a sufficient level, others with chelating diphosphines bearing p-C 6 H 4 X in place of phenyl groups unmistakably gave analogous host/guest adducts. 15 Next, attention was turned to probing macrocycle/ ammonium cation interactions in solution, using the samples isolated in Scheme 2. In all cases, non-overlapping 1 H NMR signals were available, facilitating diffusion ordered spectroscopy (DOSY) experiments. 16 The customary magnetic field gradient was applied along the z axis, and a series of spectra were recorded in CDCl 3 . These exhibited the usual decay of signal intensities with increasing gradient strengths, from which the diffusion coefficients in Table S5 † were calculated. S5 †). The 1 : 1 adduct 2•Hb + I -also showed appreciable correlation (Fig. S5 and Table S5 †), although the cation signals gave D values ca. 15% greater than those of the macrocycle. However, with the 1 : 3 adducts 1•(Hc + I -) 3 and 2•(Hd + I -) 3 , the cations and macrocycles exhibi-ted markedly different diffusivities (Fig. S3, S4 and Table S5; † 100% greater for the cations). This trend is not surprising, as all crystallized samples feature only a single ammonium cation, suggesting stronger interactions of the macrocycle with the first cation. Thus, additional ammonium cations become progressively more weakly bound, and since rapid exchange would be expected, their diffusivities show less and less correlation to the macrocycle.

Other potential probes of macrocycle/ammonium cation interactions were investigated. Mass spectra of the preceding samples generally showed signals corresponding to ammonium cation adducts (1•H 2 a 2+ (85%), 2•H 2 a 2+ (100%), 2•Hd + (20%), 1•H 2 c + (100%), and 2•Hb + (7%)). Bruce observed similar H 2 NEt 2 -containing ions with isolated samples of his Pt 4 C 16 adducts (I, L 2 = 2PEt 3 , dppe, dppp). 7 Under carefully controlled conditions in solution, the IR ν NH bands of ammonium salts 17 can be effective probes of hydrogen bonding. 18 However, none of the products in Scheme 2 exhibited detectable absorptions under ATR IR conditions ( powder film), although ν CH bands were plainly visible.

Others have interrogated the hydrogen bond acceptor abilities of CuC units. 18,19 It is not unusual in crystal structures of CuCH species to find CuCH⋯(CuC) centroid interactions involving neighboring molecules. 19 Two AuCuCAu adducts have been crystallized as CHCl 3 solvates, with Cl 3 CH⋯ (CuC) centroid distances of 2.42-2.58 Å that were interpreted as CH/π interactions. 20 In a detailed study of [n-Oct 3 NH] + [B (C 6 F 5 ) 4 ] -in non-polar CCl 4 solutions, 18 3-hexyne was found to be a weak hydrogen bond acceptor, comparable to cycloalkenes (albeit stronger than methylbenzenes C 6 H 6-n Me n (n = 0-6), conjugated dienes and terminal alkynes). In any case, none of these CuC based acceptors come close to the voracities that characterize the Pt 4 C 16 systems 1 and 2.

In integrating the preceding data for Pt 4 C 16 /ammonium cation systems, the relative importance of electrostatic and hydrogen bonding interactions is challenging to assess. Furthermore, a reviewer has noted the possible relevancy of cation/π interactions. 21 The electronic structures of close models for the Pt 4 C 16 macrocycles have been characterized by