Organoimido-complexes of tungsten-(VI)-(V), and -(IV) : crystal and molecular structures of [W(NPh)Cl3(PPh3)2] and [W(NPh)Cl2(PMe3)3]

Abstract

Reaction of RNCO (R = Ph or Et) with WOCl₄ gave the tungsten(VI) imido-compounds [W(NR)Cl₄] which, in further reactions, gave [W(NR)Cl₄(thf)](thf = tetrahydrofuran) and [{W(NPh)(µ-O)Me₂(PMe₃)}₃]. In the presence of phosphine ligands (L = PPh₃ or PMe₂Ph; L₂= Ph₂PCH₂CH₂PPh₂), in boiling benzene, reduction occurred forming the paramagnetic tungsten(V) compounds [W(NPh)Cl₃L₂]. More powerful reduction (sodium amalgam) in the presence of ligands (L = PMe₃, PMe₂Ph, and CNBu^t) gave the tungsten(IV) compounds [W(NR)Cl₂L₃]. Other reactions led to cleavage of the organoimidogroup from the tungsten. An X-ray structure determination of [W(NPh)Cl₃(PPh₃)₂] showed the meridional configuration of chlorines with mutually trans-phosphines. The short W–N bond length (1.742 Å) and practically linear WNC (172.3°) suggest that the imido-group is acting as a four-electron ligand. Structure determination of [W(NPh)Cl₂(PMe₃)₃] shows that the three phosphines are meridional, and all cis to the WN–Ph unit. Although the imido-function seems to exert a small trans-lengthening influence on the relevant W–Cl bond, in the trichloride, this effect does not seem to be present in the dichloride.