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

Abstract

AbstractBy reacting Ru(η2‐RL)(PPh3)2(CO)Cl (1) with excess 4,5‐dimethyl‐2,2′‐biimidazole (dmbi), organometallics of the type [Ru(η1‐RL)(PPh3)2(CO)(dmbi)](PF6) (2) have been isolated in excellent yield (η2‐RL is C6H2O‐2‐CHNHC6H4R(p)‐3‐Me‐5, η1‐RL is C6H2OH‐2‐CHNC6H4R(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(η1‐MeL)(PPh3)2(CO)(dmbi)](PF6)·CH2Cl2 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 3MLCT state incorporating a π*(dmbi) contribution, is peculiar to the coligation of η1‐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)