Linking metal centres with diimido ligands: synthesis, electronic and molecular structure and electrochemistry of organometallic ditungsten complexes [{WCl2(Ph2PMe)2(CO)}2(N–X–N)] (X = π-conjugated organic)

Abstract

Tungsten(IV) diimido-bridged complexes [{WCl₂(Ph₂PMe)₂(CO)}₂(µ-N–X–N)] have been prepared via oxidative addition of diisocyanates to two equivalents of [WCl₂(Ph₂PMe)₄]. para-Substituted monoimido complexes [WCl₂(Ph₂PMe)₂(CO)(NC₆H₄X-p)] (X = I, Br or CCPh) have also been prepared but attempts to couple the X = I complex as a route to diimido-bridged complexes were unsuccessful. All complexes are air-stable crystalline solids and five diimido (N–X–N = p-NC₆H₄N, p-N-o-MeC₆H₃N, p-N(o-MeOC₆H₃C₆H₃OMe-o)N, 1,5-NC₁₀H₆N or m-NC₆H₄N) and one monoimido complex (X = I) have been characterised crystallographically. All show the same gross structural features, namely a trans arrangement of phosphines and cis chlorides. The central aryl ring generally lies approximately in the Cl₂(CO) plane (torsional angles 4.1–26.1°) except for one complex in which the ring lies almost perpendicular to this (torsional angle 80.2°). A series of density functional calculations conducted on model mono- and di-imido tungsten-(VI) and -(V) compounds indicated that the most stable aryl ring orientation is perpendicular to the plane containing the trans phosphines, i.e. as found in all cases except one (N–X–N = p-NC₆H₄N). The anomaly in the latter may be due to cocrystallisation with chlorobenzene. In order to assess the degree of communication between the tungsten(IV) centres through the highly π-conjugated diimido linkages, electrochemical studies have been carried out. All diimido-bridged complexes show two closely spaced oxidative processes at low temperature indicative of weak electronic communication. The reductive chemistry of the para-phenylene bridged complexes is different from other diimido complexes, displaying two closely spaced reductive processes. Spectroelectrochemical studies have also been carried out on N–X–N = p-NC₆H₄N, oxidation at +1.2 V leading to CO loss. In order to gain further insight into the nature of the electronic communication between metal centres density functional calculations were carried out and were generally in agreement with the electrochemical results, suggesting that there is at best a weak interaction between the metal centres in these π-conjugated diimido-bridged complexes.