Carbene complexes. Part 17. Crystal structure of trans-tetracarbonylbis(1,3-dimethylimidazolidin-2-ylidene)molybdenum(0), trans-[Mo(CO)4{CN(Me)CH2CH2NMe}2], structural comparison with the cis isomer, and a kinetic study of the trans→cis isomerisation

Abstract

Crystals of the title compound are monoclinic, space group C2/m, with a= 14.706(5), b= 7.866(3), c= 8.132(3)Å, and β= 114.40(2). The molecule lies on a site of 2/m symmetry with both the carbene ligands in the crystallographic mirror plane (i.e they are not orthogonal). The structure was refined by full-matrix least squares to an R value of 0.027 for 87 variables (including isotropic hydrogen atoms) and 1 661 data. The Mo–C_caro, distance of 2.232(2)Å is ca. 0.06 Å shorter than in the cis isomer. The Mo–CO length of 2.001(2)Å compares with the Mo–CO (trans CO) and Mo–CO (trans carbene) lengths of 2.028(3) and 1.980(3)Å respectively in the cis isomer. The activation energy (70.4 ± 2 kJ mol^–1, or 74.7 ± 0.7 kJ mol^–1 for the ethyl homologues) for the trans→cis isomerisation was measured in diethyl phthalate by differential scanning calorimetry, using digital data logging equipment and a computer program. The reaction is first order and the negative activation entropy is attributed to interaction with solvent molecules in the transition state.