Theoretical Study On The3-center-4-electron Hyperbonds In NHC-MX Complexes

NHCs complexes, because of their diverse properties in bonding and structure,have been applied in medicine and organic catalysis. It is important to understand thebonding between NHC and metal. As early as1998, Boehme and Frenking carried outpioneering ab initio investigations of dative metal-carbon bonding in NHC-MCl (M=Cu, Ag, Au) complexes. Their work focused primarily on-donation vs. π-backdonation aspects of M-C (or analogous M-Si, M-Ge) bonding, emphasizing theimportant covalency of-donation and relative weakness of π-back donation, but theirconcluding discussion also included a schematic MO depiction of “the competition forthe empty metal s orbital between donation from Cl-and the carbene lone pair” that isthe focus of the present work. Quite recently, Hashmi et al. synthesized seven differentNHC-gold(I) phenolate complexes. A further analysis by using natural bond orbital(NBO) program automatically identified3c/4e hyperbonds (ω bonds) among thecarbene carbon, the gold atom, and the oxygen atom/chloride atom in gold-hydroxy andgold-chloride complexes. Weinhold put forward the use of the resonance theory tounderstand the3c/4e bonds. The ω bond represents strong nL→σ*ML1delocalization inthe L: M–L1Lewis structure, and vice versa, strong nL1→σ*MLdelocalization in theL–M:L1Lewis structure. Its presence typically implies a preference for lineararrangement of hyperbonded L:-M-:L1triad. Such3c/4e triads are more aptlydescribed as the strong resonance mixing of the form L: M–L1L–M:L1along withnearly equivalent high NRT weightings of both resonance structures. As many exampleshave shown, such ω-bonding is ubiquitous and robust in transition-metal coordinationcompounds. Therefore, the question arises: Could we better understand and quantify thebonding between NHC and MX in terms of a resonance-type ω-bonding description? We will use the resonance theory to understand the NHC-M bonding. In addition, weinvestigate the π-back donation in NHC-M bonding. We also study the structureinfluence on ω-bonding in (NHC)2M+complexes.All the complexes were optimized at the DFT level of theory by using the BP86level, the6-311++G**basis set was adopted for C, N, H, O, F, Cl and Br atoms.Considering the relativistic effects of coinage metal, the effective core potentials basisset aug-cc-pVDZ-PP (LAN2DZ basis set in chapter4is also used). The main work is asfollows:Firstly, the3-center-4-electron (3c/4e) bonding among the carbene carbon, themetal atom and ligand X in NHC-MX (M=Cu, Ag, Au, X=F, Cl, Br, OH, NH2, CH3)complexes has been re-examined by using natural bond orbital (NBO) and naturalresonance theory (NRT) methods. NBO/NRT results indicate that each member of thestudied complexes can be best regarded as a strong resonance hybrid of NHC: M–X NHC+–M:X-, corresponding to strong hyperconjugation interactions and comparableNRT weightings of both resonance structures. Detailed analyses on the NRT descriptorsreveal a complementary relationship between NHC–M and M–X bond orders. Such animproved understanding emphasizes the resonance and competing aspects of thebonding around the coinage metal. On the basis of such a bonding scheme, we showhow a simple estimate of%NHC–Au vs.%Au–Cl character for the NHC:-Au-:Cl3c/4e bonding can provide an explanation why the chloride can be readily substituted inNHC-AuCl. Additionally, the investigation of ligand effects shows that the modifiedNHC has little influence on the bonding, but that the3c/4e bonding in the studiedcomplexes is highly tunable in its dependence on the nature of the auxiliary ligands.Secondly, we use the different approximation methods (different levels of theory)and basis sets to investigate the π-back donation in MHC-MCl complexes. The resultsindicate that the second-order perturbation energies ΔEi-j*(2)is largely influenced bydifferent methods and basis sets. However, different methods and basis sets have littleinfluence on charge transfer result. Thus, the charge transfer result is reliable. Thecharge transfer results suggest that π-back donation can not be negligible in NHC-M complexes.Thirdly, we optimize the (NHC)2M+（M=Cu, Ag, Au）complexes at the BP86level.The result indicates that there are two structures (two NHC rings in plane and out plane)in each Ag or Au complexes. And only an out plane structure for the Cu complex. Inaddition, we find the NHC-M bond dissociation energy in (NHC)2M+complexes isweaken than that in NHC-M+complexes. Finally, we investigate the difference of theorbital interactions, bond length and NRT weightings of C-M-C bonds between thedifferent structures of (NHC)2M+complexes.