Neutral Paddlewheel Diruthenium Complexes with Tetracarboxylates of Large π-Conjugated Substituents:  Facile One-Pot Synthesis, Crystal Structures, and Electrochemical Studies

A one-pot reaction of a cationic diruthenium complex, [Ru₂^II,III(O₂CCH₃)₄(THF)₂](BF₄), with arylcarboxylic acids, ArCO₂H, (PhCO₂H = benzoic acid, NapCO₂H = 1-naphthoic acid, AntCO₂H = 9-anthracenecarboxylic acid) in NDMA (NDMA = N,N-dimethylaniline) has led to isolation of neutral paddlewheel-type diruthenium complexes, [Ru^II₂(O₂CAr)₄(THF)₂] (Ar = Ph (1), Nap (2), Ant (3)). Paramagnetic variable temperature (VT) ¹H NMR studies and GC-MS studies show that the reaction consists of two steps:  a one-electron reduction of the Ru₂ core by NDMA and a simple carboxylate-exchange reaction. All compounds 1−3 were structurally characterized by X-ray crystallography. While the structural features of the Ru₂ core are very similar in all the compounds, the dihedral angles between the carboxylate plane and the aromatic ring are larger with the expanding of aryl groups from phenyl to anthracene. The effect of π−π stacking leads to the formation of a 1-D chain structure in compound 3, whereas compounds 1 and 2 are fully isolated from each other. The electrochemical measurements show that the quasireversible one-electron oxidation step is observed at +0.06, +0.09, and +0.17 V (vs Ag/Ag⁺) for 1−3, respectively, assigned to the Ru^II₂/Ru^II,III₂ redox couple. These potentials are found to demonstrate a linear relationship with the substituent constants for aryl compounds, .