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Abstract
A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (CArY-MICs) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two cis-positioned open coordination sites, is based on the controlled cyclisation of a P,P′-diisopropyl-substituted 2,2′-diphosphinotolane (1) and leads to chelate complexes coordinated by a phosphine donor and the CArY-MIC carbon atom. The C–P bond formation involved in the former partial cyclisation of 1 proceeds under mild conditions and was shown to be applicable all over the d-block. In the presence of a third fac-positioned open coordination site, the P–C bond formation was found to be reversible, as shown for a series of molybdenum complexes. DFT modelling studies are in line with an interpretation of the target compounds as CArY-MICs.
Highlights
N-Heterocyclic carbenes (NHC),1 mesoionic carbenes (MIC)2 and cyclic(amino) carbenes (CAAC)3 are among the most widely used carbene ligands
Spatial protection of the carbene centres certainly plays a role for isolating the free carbenes, the introduction of steric bulk is occasionally not required as metaltemplated synthetic routes and in situ trapping protocols are well-established nowadays.5. The emergence of these procedures opened-up new possibilities to modulate the electronic properties of N-containing carbene ligands, for example by replacing the alkyl groups in cyclic (alkyl)(amino) carbenes (CAAC) for aryl moieties or by introducing an exocyclic ylide as a replacement for one of the nitrogens in NHCs. This approach led to cyclic(aryl) carbenes (CAArC)6 and cyclic(ylide) carbenes (CAYC)7, respectively, which have been generated in situ and trapped as their metal complexes
By formally combining cyclic (amino)(aryl) carbenes (CAArC) and P-ylidic cyclic (amino)(ylide) carbenes (CAYC), a new motif in carbene chemistry comes into reach, namely cyclic(ylidic) carbenes (CArYCs), which do not contain an endocyclic nitrogen atom
Summary
N-Heterocyclic carbenes (NHC),1 mesoionic carbenes (MIC)2 and cyclic (alkyl)(amino) carbenes (CAAC)3 are among the most widely used carbene ligands. These crystallographic data support a description of complexes 3–12 as CArY-MICs, which may be denoted with a no-bond and a betaine resonance structure
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