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 = pi-conjugated organic)

Tungsten(IV) diimido-bridged complexes [{WCl2(Ph2PMe)(2)(CO)}(2)(mu-N-X-N)] have been prepared via oxidative addition of diisocyanates to two equivale nts of [WCl2(Ph2PMe)(4)]. para-Substituted monoimido complexes [WCl2(Ph2PMe )(2)(CO)(NC6H4X-p)] (X = I, Br or C=CPh) have also been prepared but attemp ts to couple the X = I complex as a route to diimido-bridged complexes were unsuccessful. All complexes are air-stable crystalline solids and five dii mido (N-X-N = p-NC6H4N, p-N-o-MeC6H3N, p-N(o-MeOC6H3C6H3OMe-o)N, 1,5-NC10H6 N or m-NC6H4N) and one monoimido complex (X = I) have been characterised cr ystallographically. All show the same gross structural features, namely a t rans arrangement of phosphines and cis chlorides. The central aryl ring gen erally lies approximately in the Cl-2(CO) plane (torsional angles 4.1-26.1 degrees) except for one complex in which the ring lies almost perpendicular to this (torsional angle 80.2 degrees). A series of density functional cal culations conducted on model mono- and di-imido tungsten-(VI) and -(V) comp ounds indicated that the most stable aryl ring orientation is perpendicular to the plane containing the trans phosphines, i.e. as found in all cases e xcept one (N-X-N = p-NC6H4N). The anomaly in the latter may be due to cocry stallisation with chlorobenzene. In order to assess the degree of communica tion between the tungsten(IV) centres through the highly pi-conjugated diim ido linkages, electrochemical studies have been carried out. All diimido-br idged complexes show two closely spaced oxidative processes at low temperat ure indicative of weak electronic communication. The reductive chemistry of the para-phenylene bridged complexes is different from other diimido compl exes, displaying two closely spaced reductive processes. Spectroelectrochem ical 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 ca lculations were carried out and were generally in agreement with the electr ochemical results, suggesting that there is at best a weak interaction betw een the metal centres in these pi-conjugated diimido-bridged complexes.