Research –
N–carbenes (NHCs) have emerged as versatile ligands in organometallic chemistry, and offer a useful alternative to the ubiquitous phosphine ligands. Although these carbene ligands are considered to have bonding properties similar to those of trialkylphosphines their steric properties differ significantly. Considering this comparison, we have initiated a study investigating the reactivity of complexes where the phosphine moieties in our commonly–used bisdiphenylphosphinomethane (dppm) ligands (FIgure 1a) are replaced with linked NHC groups as di-N-Heterocyclic Carbenes (Figure 1b).
Di-NHC-Bridged Species
Di–NHC–Bridged Species
Swapping phosphines for Carbenes
These linked dicarbenes can be accessed through the linking of two substituted imidazole rings, then subsequent deprotonation of the analogous diimidazolium salts (Scheme 1). Using
[Rh(μ-OAc)(COD)]2 as a source of both base and rhodium, we have prepared several homobimetallic complexes of dirhodium linked by the di-NHCs(Scheme 2). Owing to the rotational freedom in the linking groups between the dicarbene ends, complexes bridged by only one of these groups tend to adopt a skewed arrangement in which the metals are separated by more than 6 Å. In order to bring the metals together in a bid to optimize metal-metal interactions and their probability of interacting in a cooperative manner, a second bridging group is needed. We have recently been able to lock these metals in close proximity using dppm as the second bridch, in which the Rh···Rh separation has reduced to 3.4 Å (Scheme 3). For more information see Kyle’s work.
a)
b)
Figure 1 – Phosphines vs di-NHCs
Scheme 1 – Synthesis of Diimidazolium Salts
Scheme 2 – Di-NHC-Bridged Dirhodium Complexes
Scheme 3 – Dirhodium Hybrid “A-Frame” Systems
We have also extended our investigation to complexes in which di–NHC ligands could be used as bridging groups connecting different pairs of metals. For the rational generation of mixed–metal species it appeared that stepwise incorporation of the different metals was the most straightforward strategy. A series of carbene–anchored/pendent–imidazolium salts appeared ideal for this purpose (Scheme 4). For more information, see Matt’s work.
Scheme 4 – Stepwise Strategy to Mixed-Metal Systems
