Ligand bending and tilted coordination in the coordinatively unsaturated NHC complex lateral-bis(N,N′-dineopentyl-benzimidazoline-2-ylidene)molybdenumtricarbonyl - Synthesis and structural investigations

Abstract

Reaction of free dineopentyl-benzimidazoline-2-ylidene 1 (ligand: np2bimy) with (η3-cycloheptatriene)Mo(CO)3, molar ratio 3:1, provides the coordinatively unsaturated lateral-(np2bimy)2Mo(CO)3 complex 2. Crystal structure analysis of 2 reveals an almost ideal cis-square pyramidal geometry with tilted coordination of the metal, interplanar angles of the np2bimy ligands to the C4Mo plane of about 70° and significant bending of the N-heterocyclic carbene rings towards each other. Quantum chemical calculations (B97-D/6-31G(d)[Mo:cc-pVTZ-PP] level of theory) reproduce this peculiar structure and show it to be more stable by 36 kJ/mol than the second energy-minimum structure with trigonal bipyramidal geometry and the carbene ligands in the axial positions. Calculations indicated no significant Mo-C(II) back-bonding or interactions of the empty d-orbital with adjacent p-orbitals of the ligands, implying that the ligand distortions, tilted coordination and the occurrence of a free coordination site are caused by the spatial demand of the N-substituents.