Desoxo Molybdenum(IV) and Tungsten(IV) Bis(dithiolene) Complexes: Monomer−Dimer Interconversion Involving Reversible Thiol Bridge Formation

Two series of thiol-bridged dimeric desoxo molybdenum(IV) and tungsten(IV) bis(dithiolene) complexes, [Et4N]2[MIV2(SR)2(mnt)4] [M = Mo, R = (1) −Ph, (2) −CH2Ph, (3) −CH2CH3, (4) −CH2CH2OH; M = W, R = (1a) −Ph,
(2a) −CH2Ph, (3a) −CH2CH3, (4a) −CH2CH2OH] and one monomeric desoxo complex, [Et4N]2[WIV(SPh)2(mnt)2]
(5a) are reported. These complexes are diamagnetic, and crystal structures of each of the complex (except 5a)
exhibits a dimeric {MIV2(SR)2} core without any metal−metal bond where each metal atom possesses hexa
coordination. The M−SR distance ranges from 2.437 to 2.484 Å in molybdenum complexes and from 2.418 to
2.469 Å in tungsten complexes. These complexes display Mo−S(R)−Mo angles ranging from 92.84° to 96.20° in
the case of 1−4 and W−S(R)−W angles ranging from 91.20° to 96.25° in the case of 1a−4a. Interestingly, both
the series of Mo(IV) and W(IV) dimeric complexes respond to an unprecedented interconversion between the
dimer and the corresponding hexacoordinated monomer upon change of pH. This pH-dependent interconversion
establishes the fact that even the pentacoordinated Mo(IV) and W(IV) bis(dithiolene) moieties are forced to dimerize;
these can easily be reverted back to the corresponding monomeric complex, reflecting the utility of dithiolene
ligand in stabilizing the Mo(IV)/W(IV) moiety in synthesized complexes similar to the active sites present in native
proteins.