Multiply-bonded dimetal fluoroalkoxides—II. The disordered structure of Mo 2(OC(CF 3) 3) 4(NMe 2) 2 and its implications as to rearrangement by an internal flip

Abstract

The existence of the title compound, which was tentatively reported in 1977, has been confirmed and its structure studied crystallographically. It crystallizes in space group P2 1/ n with a = 10.571(3), b = 14.927(3), c = 12.099(3) Å, β = 103.21(3)°, V = 1859(2) Å 3, Z = 2. The centrosymmetric molecules have an anti, staggered rotational conformation, with the C 2NMoMoNC 2 group of atoms being essentially planar. There is a disorder such that 69% of the Mo 2 units lie in one direction and 31% in a nearly perpendicular direction. This is presumed to indicate that there are entire molecules that adopt these two different orientations. Although no more than one of each ligand atom could be discerned, all of them, especially those in the (CF 3) 3C groups refined with unusually high thermal displacement parameters. The two MoMo distances, 2.216(2) Å (69%) and 2.201(3) Å (31%), while different in a statistical sense (0.015(4) Å) are nearly equal and of the appropriate magnitude for a triple bond between the molybdenum atoms. The observed disorder leads to the concept that the Mo 2 unit might undergo internal flips from one orientation to the other within a ligand cage that need relax only slightly from one orientation to the other. This would provide a simple unimolecular mechanism for interchanging proximal and distal R groups on NR 2 ligands in this and related molecules.