Structural and Electronic Rearrangements upon the Oxidation of Binuclear (Ru₂) and Trinuclear (MoRu₂) Complexes with Bridging <i>o-</i>Phenylenediamido Ligands^†

The redox capabilities of some binuclear and trinuclear ruthenium complexes with bridging
o-phenylendiamido ligands, C6H4(NH)2-o, have been investigated. The known species [Ru2{μ-C6H4(NH)2-o}(μ-dppm)(CO)2(PPh3)2] (1) and [Ru2Mo{μ-C6H4(NH)2-o}2(CO)6(PPh3)2] (5),
reacted with [Fe(Cp)2](PF6), yield the salts [Ru2{μ-C6H4(NH)2-o}(μ-dppm)(CO)2(PPh3)2](PF6)2
(2) and [Ru2Mo{μ-C6H4(NH)2-o}2(CO)6(PPh3)2](PF6)2·CH2Cl2 (6), respectively (dppm = Ph2PCH2PPh2). Binuclear compounds analogous to 2 were obtained with pairs of PPh3 or CO
ligands in place of the diphosphine dppm. The redox properties of the compounds have been
explored also with electrochemical methods. The structural determinations of 2 and 6 show,
as an evident consequence of the oxidation, the bending of the originally upright C6H4(NH)2-o
bridge toward one metal that is eventually η4-coordinated by the ligand. In the trinuclear
species 6, such a bending is limited to only one bridge and is directed toward the central
molybdenum atom. Another structural consequence, attributable to electronic effects, is the
reciprocal reorientation of two terminal L3M fragments with modes that are different in 2
and 6, respectively. These aspects have been highlighted through a theoretical analysis with
DFT calculations and an extension of the perturbation theory arguments already introduced
for the precursors 1 and 5. There is evidence that, in each case, the oxidation affects mainly
the bridging chelate C6H4(NH)2-o that transforms from diamido to diimino in character. The
activated back-donation from the η4-coordinated metal into the diimino ligand causes also
an evident weakening of the metal−metal bond. Complex 6 represents the first well-characterized case where one o-phenylenediamido and one o-diiminobenzene coexist as
ligands.