Tris(2-(arylamido)-4,6-di- tert -butylphenoxo)molybdenum( vi ) complexes ( R ap) 3 Mo can be prepared either from (cycloheptatriene)Mo(CO) 3 and the N -aryliminoquinone, or from MoO 2 (acac) 2 and the aminophenol. In contrast to all other reported unconstrained transition metal tris(amidophenoxide) complexes, the molybdenum complexes show a facial geometry in the solid state. In solution, the fac isomer predominates, though a small amount of mer isomer is detectable at room temperature. At elevated temperature the two species interconvert through Rây-Dutt trigonal twists, which are faster than Bailar twists in this system, presumably because of steric effects of the N -aryl groups. Substituents on the N -aryl ring shift the fac / mer equilibrium of the complex, with more electron-withdrawing substituents generally increasing the proportion of the mer isomer. The preference for fac over mer geometry is thus suggested to be due to enhanced π bonding in the fac isomer. In contrast to analogous catecholate complexes, the tris(amidophenoxide) complexes are not Lewis acidic and are inert to nucleophilic oxidants such as amine- N -oxides.
more »
« less
η2 Coordination of Electron-Deficient Arenes with Group 6 Dearomatization Agents
The exceptionally π-basic metal fragments {MoTp(NO)(DMAP)} and {WTp(NO)(PMe3)} (Tp = tris(pyrazolyl)borate; DMAP = 4-(N,N-dimethylamino)pyridine) form thermally stable η2-coordinated complexes with a variety of electron-deficient arenes. The tolerance of substituted arenes with fluorine-containing electron withdrawing groups (EWG; −F, −CF3, −SF5) is examined for both the molybdenum and tungsten systems. When the EWG contains a π bond (nitriles, aldehydes, ketones, ester), η2 coordination occurs predominantly on the nonaromatic functional group. However, complexation of the tungsten complex with trimethyl orthobenzoate (PhC(OMe)3) followed by hydrolysis allows access to an η2-coordinated arene with an ester substituent. In general, the tungsten system tolerates sulfur-based withdrawing groups well (e.g., PhSO2Ph, MeSO2Ph), and the integration of multiple electron-withdrawing groups on a benzene ring further enhances the π-back-bonding interaction between the metal and aromatic ligand. While the molybdenum system did not form stable η2-arene complexes with the sulfones or ortho esters, it was capable of forming rare examples of stable η2-coordinated arene complexes with a range of fluorinated benzenes (e.g., fluorobenzene, difluorobenzenes). In contrast to what has been observed for the tungsten system, these complexes formed without interference of C–H or C–F insertion.
more »
« less
- Award ID(s):
- 1800051
- NSF-PAR ID:
- 10283143
- Date Published:
- Journal Name:
- Organometallics
- Volume:
- 39
- Issue:
- 13
- ISSN:
- 1520-6041
- Page Range / eLocation ID:
- 2493-2510
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The importance of electron deficient Tp ligands motivates the introduction of electron-withdrawing substituents into the scorpionate framework. Since perfluorophenyltris(pyrazol-1-yl)borate affects significant anodic shifts in half-cell potentials in their metal complexes relative those of phenyltris(pyrazol-1-yl)borate analogues, the tuning opportunities achieved using 3,4,5-trifluorophenyl- and 3,5-bis(trifluoromethyl)phenyl(pyrazol-1-yl)borates were explored. Bis(amino)boranes ((3,4,5-F)C 6 H 2 )B(NMe 2 ) 2 and ((3,5-CF 3 )C 6 H 3 )B(NMe 2 ) 2 are precursors to fluorinated tris(pyrazol-1-yl)phenylborates. Thallium salts of these scorpionates exhibit bridging asymmetric κ 3 - N , N , N coordination modes consistent with the reduced π-basicity of the fluorinated phenyl substituents relative those of other structurally characterized tris(pyrazol-1-yl)phenylborates. While a comparative analysis of the spectral and X-ray crystallographic data for classical Mo(0), Mo( ii ), Mn( i ), Fe( ii ) and Cu( ii ) complexes of [((3,4,5-F)C 6 H 2 )Bpz 3 ] − and [((3,5-CF 3 )C 6 H 3 )Bpz 3 ] − could not differentiate these ligands with respect to their metal-based electronic impacts, cyclic voltammetry suggests that 3,4,5-trifluorophenyl- and 3,5-bis(trifluoromethyl)phenyl(pyrazol-1-yl)borates affect similar anodic shifts within their metal complexes, with coordination of [((3,5-CF 3 )C 6 H 3 )Bpz 3 ] − rendering metal centers more difficult to oxidize, and sometimes even more difficult to oxidize than their [C 6 F 5 Bpz 3 ] − analogues. These data suggest that the extent of phenyl substituent fluorination necessary to minimize metal center electron-richness in phenyltris(pyrazol-1-yl)borate complexes cannot be confidently predicted.more » « less
-
The conformational preferences of 28 sterically and electronically diverse N-aryl amides were detd. using d. functional theory (DFT), using the B3LYP functional and 6-31G(d) basis set. For each compd., both the cis and trans conformers were optimized, and the difference in ground state energy calcd. For six of the compds., the potential energy surface was detd. as a function of rotation about the N-aryl bond (by 5° increments) for both cis and trans conformers. A natural bond orbital (NBO) deletion strategy was also employed to det. the extent of the contribution of conjugation to the energies of each of the conformers. By comparing these computational results with previously reported exptl. data, an explanation for the divergent conformational preferences of 2° N-aryl amides and 3° N-alkyl-N-aryl amides was formulated. This explanation accounts for the obsd. relationships of both steric and electronic factors detg. the geometry of the optimum conformation, and the magnitude of the energetic difference between cis and trans conformers: except under the most extreme scenarios, 2° amides maintain a trans conformation, and the N-bound arene lies in the same plane as the amide unless it has ortho substituents; for 3° N-alkyl-N-aryl amides in which the alkyl and aryl substituents are connected in a small ring, trans conformations are also favored, for most cases other than formamides, and the arene and amide remain in conjugation; and for 3° N-alkyl-N-aryl amides in which the alkyl and aryl substituents are not connected in a small ring, allylic strain between the two N-bound substituents forces the aryl substituent to rotate out of the plane of the amide, and the trans conformation is destabilized with respect to the cis conformation due to repulsion between the π system of the arene and the lone pairs on the oxygen atom of the carbonyl. The cis conformation is increasingly more stable than the trans conformation as electron d. is increased on the arene because the more electron-rich arenes adopt a more orthogonal arrangement, increasing the interaction with the carbonyl oxygen, while simultaneously increasing the magnitude of the repulsion due to the increased electron d. in the π system. The trans conformation is favored for 2° amides even when the arene is orthogonal to the amide, in nearly all cases, because the C-N-C bond angle can expend at the expense of the C-N-H bond angles, while this is not favorable for 3° amides.more » « less
-
more » « less
Abstract We report that the direct macrocyclization of naphthalene monomers bearing ethyl ester functional groups delivers prism[5]arene derivatives, which can be deprotected to yield water‐soluble prism[5]arenes (
H1 andH3 ).1H NMR spectroscopy showed that dicationic guests bind with the hydrophobic cores buried inside the anisotropic magnetically shielding cavity. Isothermal titration calorimetry measurements showed thatH1 andH3 are high‐affinity hosts in PBS‐buffered water with Kavalues exceeding 109 M−1for a select guest. The complexation events are driven by the non‐classical hydrophobic effect, CH⋅⋅⋅π interactions, and electrostatic interactions. HostH1 displays somewhat higher affinity toward a common guest than pillar[6]arene bearing carboxylic acid functional groups but is significantly less potent than pillar[6]arene bearing sulfate groups.H1 andH3 should be considered alongside other high affinity hosts for a variety of chemical and biological applications. -
Palladium-catalyzed fluoroalkylations of aryl halides are valuable reactions for the synthesis of fluorinated, biologically active molecules. Reductive elimination from an intermediate Pd(aryl)(fluoroalkyl) complex is the step that forms the C(aryl)–C(fluoroalkyl) bond, and this step typically requires higher temperatures and proceeds with slower rates than the reductive elimination of nonfluorinated alkylarenes from the analogous Pd(aryl)(alkyl) complexes. The experimental rates of this step correlate poorly with common parameters, such as the steric property or the electron-withdrawing ability of the fluoroalkyl ligand, making the prediction of rates and the rational design of Pd-catalyzed fluoroalkylations difficult. Therefore, a systematic study of the features of fluoroalkyl ligands that affect the barrier to this key step, including steric properties, electron-withdrawing properties, and secondary interactions, is necessary for the future development of fluoroalkylation reactions that occur under milder conditions and that tolerate additional types of fluoroalkyl reagents. We report computational studies of the effect of the fluoroalkyl (RF) ligand on the barriers to reductive elimination from Pd(aryl)(RF) complexes (RF = CF2CN, CF2C(O)Me, etc.) containing the bidentate ligand di-tert-butyl(2-methoxyphenyl)phosphine (L). The computed Gibbs free-energy barriers to reductive elimination from these complexes suggest that fluoroalkylarenes should form quickly at room temperature for the fluoroalkyl ligands we studied, excluding RF = CF3, CF2Me, C2F5, CF2CFMe2, CF2Et, CF2iPr, or CF2tBu. Analyses of the transition-state structures by natural bond orbital (NBO) and independent gradient model (IGMH) approaches reveal that orbital interactions between the Pd center and a hydrogen atom or π-acid bonded to the α-carbon atom of the RF ligand stabilize the lowest-energy transition states of Pd(aryl)(RF) complexes. Comparisons between conformers of transition-state structures suggest that the magnitude of such stabilizations is 4.7–9.9 kcal/mol. In the absence of these secondary orbital interactions, a more electron-withdrawing fluoroalkyl ligand leads to a higher barrier to reductive elimination than a less electron-withdrawing fluoroalkyl ligand. Computations on the reductive elimination from complexes containing para-substituted aryl groups on palladium reveal that the barriers to reductive elimination from complexes containing more electron-rich aryl ligands tend to be lower than those to reductive elimination from complexes containing less electron-rich aryl ligands when the fluoroalkyl ligands of these complexes can engage in secondary orbital interactions with the metal center. However, the computed barriers to reductive elimination do not depend on the electronic properties of the aryl ligand when the fluoroalkyl ligands do not engage in secondary orbital interactions with the metal center.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.organomet.0c00277
