Synthesis, structure, and properties of biimidazole-chelated arylruthenium complexes

Abstract

By 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(ρ)-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) Åand 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.