Terphenyl Complexes of Molybdenum and Tungsten with Quadruple Metal–Metal Bonds and Bridging Carboxylate Ligands

Mono-
and bis-terphenyl complexes of molybdenum and tungsten with
general composition M2(Ar′)­(O2CR)3 and M2(Ar′)2(O2CR)2, respectively (Ar′ = terphenyl ligand), that contain
carboxylate groups bridging the quadruply bonded metal atoms, have
been prepared and structurally characterized. The new compounds stem
from the reactions of the dimetal tetracarboxylates, M2(O2CR)4 (M = Mo, R = H, Me, CF3;
M = W, R = CF3) with the lithium salts of the appropriate
terphenyl groups (Ar′ = ArXyl2, ArMes2, ArDipp2, and ArTrip2). Substitution of one bidentate carboxylate
by a monodentate terphenyl forms a M–C σ bond and creates
a coordination unsaturation at the other metal atom. Hence in M2(Ar′)2(O2CR)2 complexes
the two metal atoms have formally a low coordination number and an
also low electron count. However, the unsaturation seems to be compensated
by a weak M–Carene bonding interaction that implicates
one of the aryl substituents of the terphenyl central aryl ring, as
revealed by X-ray studies performed with some of these complexes and
by theoretical calculations.