Bent vs. linear imido ligation at the octahedral molybdenum(VI) dithiocarbamate stabilised centre

Abstract

Molybdenum(VI) bis(imido) complexes [Mo(NR)₂(S₂CNEt₂)₂] 3a–3p have been prepared via two synthetic routes and their structures investigated. Thermolysis of [MoO₂(S₂CNEt₂)₂] 1 with 2,6-disubstituted aryl isocyanates affords air-stable bis(imido) complexes 3i–3m in high yields after column chromatography, a route which fails for sterically less demanding isocyanates. A more general preparation involves room temperature addition of two equivalents of [S₂CNEt₂]^– to the labile bis(imido) complexes [Mo(NR)₂Cl₂(dme)] 2a–2p (dme = 1,2-dimethoxyethane) to afford 3a–3p in high yields. In this manner related complexes [Mo(NR)₂(S₂CNMe₂)₂] 4 (R = Ph or Bu^t) and [Mo(NPh)₂(S₂PPh₂)₂] 5a have also been prepared. Structural and spectroscopic studies have been carried out in order to determine the degree of bending of the imido ligands. Six complexes have been characterised by X-ray crystallography. In all cases one of the imido ligands is linear (Mo–N–C ≥ 160°), while the angle at the second is dependent upon the nature of R and ranges from 180 to 139°. Solid-state ¹³C CPMAS spectroscopy reveals that the difference in bond angles at the two nitrogen centres (Δ°) correlates closely with the difference in chemical shifts of the ipso-carbon atoms (Δδ). This then has been used to probe the relative bond angles at nitrogen for non-crystallographically characterised complexes. In solution even at low temperatures both imido ligands are equivalent indicating a rapid interconversion between linear and bent bonding modes.