Group 11 Metal Complexes of N-Heterocyclic Carbene Ligands:  Nature of the MetalCarbene Bond

Abstract

The silver complex of the tripodal N-heterocyclic carbene ligand TIME Me , [(TIME Me ) 2 Ag 3 ]-(PF 6 ) 3 (3), reacts with copper(I) bromide and (dimethyl sulfide)gold(I) chloride to yield the corresponding D 3 -symmetrical copper(I) and gold(I) complexes [(TIME Me ) 2 Cu 3 ](PF 6 ) 3 (4) and [(TIME Me ) 2 Au 3 ](PF 6 ) 3 (5). Single-crystal X-ray diffraction, spectroscopic, and computational studies of this series of metal NHC complexes are described. The group 11 metal complexes of the TIME Me ligand exhibit isostructural geometries, with three metal ions bridging two of the TIME Me ligands. Each metal ion is linearly coordinated to two carbene centers, with each of the carbenoid carbons stemming from a different ligand. Overall, the molecules possess D 3 symmetry. The electronic structure of these newly synthesized compounds was elucidated with the aid of DFT calculations. In contrast to the common assumption that NHCs are pure σ-donor ligands, our calculations reveal the existence of both σ-and π-type interactions between the metal ions and the carbenoid carbons. A study of the closely related D 2d -symmetrical species Pd(CN 2 Bu t 2 C 2 H 2 ) 2 (6) and the simplified D 2h -symmetrical model complexes M(IM Me C:) 2 (8-10; M ) Ag, Cu, Au) allowed for quantitative comparison of the two different types of bonding interactions. It was found that π-back-bonding interactions in these diaminocarbene model species contribute to approximately 15-30% of the complexes' overall orbital interaction energies.

Palladium Complexes of N‐Heterocyclic Carbenes as Catalysts for Cross‐Coupling Reactions—A Synthetic Chemist's Perspective

“…[9,16,18] Whereas the p-acceptor properties of phosphanes are wellestablished, [58,59] there is a consensus that p-back donation from Pd to the NHC p* orbital is negligible. [60,61] However, some recent computational and experimental studies challenge such views [62] -the NHCs can use different orbitals for bonding to match the complementary metal orbitals. This bonding versatility is illustrated by the following two examples: Abernety et al found significant p-back bonding from the chloro ligands in the cis position to the NHC in the isolated [(IMes)VOCl 3 ] complex.…”

Aus der Sicht des Synthetikers: Palladiumkomplexe N‐heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

“…[9,16,18] Die Phosphane sind auch ausgeprägte pElektronenakzeptoren, [58,59] nach allgemeiner Auffassung gibt es dagegen keine merkliche p-Rückbindung von Pd in das p*-Orbital von NHCs. [60,61] Kürzlich stellten jedoch theoretische und experimentelle Ergebnisse diese Vorstellung infrage [62] die NHCs können nämlich verschiedene Orbitale für eine Bindung mit den jeweils komplementären Metallorbitalen verwenden. Zwei Beispiele sollen diese variable Bindung illustrieren: Im isolierbaren Komplex [(IMes)VOCl 3 ] entdeckten Abernety et al eine signifikante p-Rückbindung von den Chloroliganden in cis-Stellung zum NHC.…”

Structures, Energies, and Bonding Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues

“…Although the first stable transition metal carbene complex was investigated in 1964 (Fischer & Maasbӧl 1964), but after a long time, the metal-ligand bonding in complexes of mixed carbene-halogen complexes (NHC-TMX with TM = Cu, Ag, Au and X = F -I) was published for using a charge decomposition analysis which was noticed for the first time by Frenking and Boehme (1998) and group 11 elements (Cu, Ag, Au) called as coinage metals, have aroused intense interest . The chemical bonding between NHCs and group 11 metals have been investigated theoretically (Nemcsok et al 2004;Hu et al 2004). Moreover, theoretical studies of the electronic structure of transition metal complexes with NHCs ligand have been recently carried out by other group (Schwarz et al 2000;Weskamp et al 1999;Lee et al 2004).…”