Synthesis, Structure, and Properties of Biimidazole‐Chelated Arylruthenium Complexes

By reacting Ru(η 2 ‐RL)(PPh 3 ) 2 (CO)Cl ( 1 ) with excess 4,5‐dimethyl‐2,2′‐biimidazole (dmbi), organometallics of the type [Ru(η 1 ‐RL)(PPh 3 ) 2 (CO)(dmbi)](PF 6 ) ( 2 ) have been isolated in excellent yield (η 2 ‐RL is C 6 H 2 O‐2‐CHNHC 6 H 4 R( p )‐3‐Me‐5, η 1 ‐RL is C 6 H 2 OH‐2‐CHNC 6 H 4 R( p )‐3‐Me‐5 and R is Me, OMe and Cl). In this process the dmbi ligand undergoes five‐membered chelation, the iminium‐phenolato function tautomerizes to the imine‐phenol function and the Schiff base performs a large rotation around the Ru−C bond. The crystal and molecular structure of [Ru(η 1 ‐MeL)(PPh 3 ) 2 (CO)(dmbi)](PF 6 )·CH 2 Cl 2 is reported. For steric reasons the carbon monoxide ligand is located syn to the phenolic oxygen atom as opposed to anti in the precursor 1 . In the hydrogen bonded imine‐phenol function the O−H, H ··· N, and N ··· O distances are 0.83(8), 1.77(8), and 2.552(6) Å, respectively, the O−H ··· N angle being 156(2)°. The Ru(dmbi) chelate along with the metallated carbon atom and the CO ligand define an equatorial plane from which the metallated aldimine fragment is rotated by 34.5° due to interligand repulsion. This results in two unequal [2.348(2) and 2.411(2) Å] Ru−P bonds and an apparent suppression of the trans influence of the metallated carbanionic site. The cation and anion in the complex are engaged in N−H ··· F and C−H ··· F hydrogen bonding, generating a centrosymmetric dimer in the lattice. One of the hydrogen bonds, characterized by H ··· F, 2.03(8) Å and N−H ··· F, 166(2)°, is particularly strong. Solutions of 2 absorb near 400 nm and emit near 460 nm with quantum yields and lifetimes of ca. 10 −3 and ⩽ 20 ns, respectively. The emission, which probably involves the 3 MLCT state incorporating a π*(dmbi) contribution, is peculiar to the coligation of η 1 ‐RL and dmbi to bivalent ruthenium. In dichloromethane solution 2 displays a quasireversible 2 + / 2 couple near 0.70 V vs. SCE [ 2 + is the ruthenium( III ) analogue of 2 ]. Coulometrically generated solutions of 2 + display a strong absorption near 400 nm and rhombic EPR spectra with g values near 2.55, 2.15, and 1.83. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)