The Bonding Model Of N-Heterocyclic Carbene Cu/Ag/Au Two-Coordinate Complexes

N-heterocyclic carbene Cu/Ag/Au two-coordinate complexes have a wide application in homogeneous catalysis,medicinal chemistry,luminescence functional materials and so on in organic chemistry field.It is very important to research bonding mechanism and the bonding nature in these complexes for developing the catalysts,functional meterials and slow-release drugs.As the development of the technology,it has stimulated extensive attention both on experimental and theoretical studies.The present work focuses on the theoretical research including a bonding mechanism on N-heterocyclic carbene Cu/Ag/Au two-coordinate complexes and the understanding of the bonding nature in these dative bonding.Additionally,we extend the resonance bonding model to understand the bonding mechanism in noble gas-noble metal halides complexes.Early synthetic works with N-heterocyclic carbene(NHC)ligands and Arduengo's successful isolation of stable free NHC have spurred intensive experimental studies to explore the remarkable bonding properties of these ligands in a broad range of transition metal(TM)complexes.NHC-ligated TM species are now recognized to offer unique catalytic functionality and a variety of promising practical applications.Current theoretical understanding of NHC-TM ligation rests mainly on pioneering computational studies of the Frenking group.As early at 1998,Boehme and Frenking carried out extensive density functional theory(DFT)investigations of dative carbene-to-metal bonding in a series of coinage metal complexes NHC-MX(M = Cu,Ag,Au;X = halogen),showing encouraging agreement of computed and experimental properties.However,the electronic features responsible for the remarkable structural and reactive properties of these species remain somewhat obscure.The dispute on the nature of the bonding between metal and the carbene has been debated for many years.In noble gases-noble metal complexes,noble gas and metal are both extremely reluctant to be involved in chemical bonding.Consequently,the preparation of noble gas-noble metal complexes has always been an enormous challenge for experimental chemists.During 2000 to 2006,Gerry's group reported the preparation and characterization of a series of heavier noble gas-noble metal complexes NgMX(Ng = Ar,Kr,Xe;M = Cu,Ag,Au;X = F,Cl,Br)using Fourier transform microwave spectroscopy.Their work opened an important new chapter in noble gas chemistry.Subsequently,some new members of NgMX complexes continue to be discovered.These experimental studies have provided opportunity for theoretical chemists to develop new bonding models in chemical sciences and triggered intensive research on the bonding of NgMX complexes.The main work and conclusions are as follows:(1)We use NBO analyses to analyze the charge transfer and second-order perturbation energy of ?-back donation and ?-donation in M-NHC.We use the “$DEL” in NBO program to study the influence of the ?-back donation on the binding energy of NHC-M and make an improved understanding of the bonding mechanism in NHC-MX complexes.Our results give strong evidence of that ?-back donation is too small to impact the NHC-M bond energy.Additionally,we discuss the charge transfer from ?-back donation which is very large,confirming that NHC is not a purely ?-donor,but has a significant ?-back donation component.However,when transfer the same charge,the impact of ?-back donation and ?-donation in NHC-M bonding is quite different.As a result,we can not use the charge transfer as a descriptor to explain why ?-back donation can be neglected.Overall,we give a clear answer of this debate.(2)We use natural bond orbital(NBO)analyses along with the natural resonance theory(NRT)analyses on nitrogen heterocyclic carbene metal complexes NHC-MX(M = Cu,Ag,Au;X = F,Cl,Br,I,OH,NH2,CH3).Our results show that each complexes should be better described as a hybrid of two resonance structures,NHC: M-X ? NHC+-M :X-,originating in nNHC??*MX hyperconjugation interaction,where NHC: M–X means dative interactions while +NHC–M :X-implies electron-shared bonding.Additionally,we compared the results of Energy Decomposition Analysis(EDA)and NBO/NRT analyses.EDA analysis combining with DCD model is a widely used method to explain the bonding between NHC and MX.This DCD bonding was described as dative(donor-acceptor)bond.Herein,we use NBO/NRT methods and analyze a series of NHC-MX complexes.The analyses show that the NHC-M bond possesses dual character of dative and electron-shared bonds.Combining with technical comparison between NBO/NRT and EDA methods,we find out the reason that the inability of EDA-based methods is due to considering only one aspect(dative)of dual character.Overall,we provide a resonance bonding picture,with a full understanding in dual character of bonding between NHC and MX.We also address the question why it is not the EDA analysis method,whereas it is the NBO/NRT analysis method to understand the DCD bonding between NHC and MX.We believe that these results will aid in the design of medicine and materials of NHC-metal species,and that this NBO/NRT analysis tool will be found a wide range of application in bonding analyses of other transition metal carbene complexes.Furthermore,we display the usefulness of NRT bond orders in correlating and rationalizing observed structural and energetic trends throughout the family of NHC-MX complexes.(3)The bonding between noble gas and noble metal halide like hydrogen bonding(resonance-bonding)motivates us to investigate the bonding mechanism and the bonding covalency in NgMX(Ng = He,Ne,Ar,Kr,Xe,Rn;M = Cu,Ag,Au;X = F,Cl,Br,I)complexes using NBO/NRT analyses.In this study,we introduce the new resonance bonding model in H-bonding into NgMX bonding.We provide strong evidence for resonance bonding involving two important resonance structures: Ng: M-X ? Ng+-M :X-in each of NgMX complexes,originating in the n Ng??*MX hyperconjugation interaction.The covalency of the bonding could be understood by the localized nature of Ng-M bonds in these two resonance structures,and the degree of Ng-M covalency can be quantitatively described by calculated NRT bond orders bNgM.Furthermore,we find that the bond order satisfies conservation of bond orders,bNgM + bMX = 1 for all of the studied complexes.On the basis of the obtained results,we also reveal that the Ng-M bond(except He-Ag and Ne-Ag bonds)can be tuned by changing auxiliary ligands.Overall,the present studies provide new insight into the bonding mechanism and the covalency of the bonding in noble gas-noble metal halides.