On the chemistry of p-cymene ruthenium iodide complexes: entry into octahedral phenylated ruthenium(II) complexes supported by chelating bidentate N,N'-donor ligands.

Abstract

This study investigates the synthesis and reactivity of (η6-p-cymene)ruthenium(II) iodide complexes supported by the phosphite ligand P(OCH2)3CEt, aiming to better understand the behavior of the Ru-I bond in the context of synthesizing ruthenium(II) complexes featuring bidentate nitrogen-donor ligands. The complex (η6-p-cymene)RuI2(P{OCH2}3CEt) (2) was synthesized and phenylated to produce (η6-p-cymene)RuPh(I)(P{OCH2}3CEt) (6). Both compounds were subjected to halide abstraction reactions with silver tetrakis[3,5-((trifluoromethyl)phenyl)borate], affording their acetonitrile-coordinated, cationic species [(η6-p-cymene)RuI(NCMe)(P{OCH2}3CEt)][BArF'] (4) and [(η6-p-cymene)RuPh(NCMe)(P{OCH2}3CEt)][BArF'] (7) (BArF' = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [B(C6H3-3,5-(CF3)2)4]-), respectively. Complex 4 dimerizes when heated in the absence of acetonitrile to form [(η6-p-cymene)Ru(μ-I)(P{OCH2}3CEt)]2[BArF']2 (5), while complex 7 activates chloroform to produce the isoelectronic chloro analogue of 5, [(η6-p-cymene)Ru(μ-Cl)(P{OCH2}3CEt)]2[BArF']2 (8). Heating 6 in acetonitrile affords the tetra-acetonitrile complex [(NCMe)4RuPh(P{OCH2}3CEt)][I] (9), whose iodide counterion can be exchanged with triflate or BArF' anions to yield complexes 10 and 11, respectively. The tetra(acetonitrile)ruthenium complexes (9, 10, and 11) exhibit differentiated lability among its acetonitrile ligands, enabling selective substitution with bidentate N,N'-donor ligands to give cationic species of the type [(κ2-N,N-L)RuPh(P{OCH2}3CEt)(NCMe)2]+, where L = bis-(2,6-diisopropylphenyl)ethane-1,2-diimine (DAB, 12) or 4,4'-tert-butyl-2,2'-bipyridine (t-bipy, 14). The DAB ligand of complex 12 is highly labile when heated in acetonitrile, while the t-bipy analogue maintains coordination to the metal center at elevated temperatures. Heating complex 14 in benzene under pressurized ethylene resulted in stoichiometric formation of styrene, most likely via olefin insertion followed by β-hydride elimination. Cyclic voltammetry revealed a Ru(III/II) redox potential of +0.53 V for 14, suggesting that the complex may be too electron-rich to serve as an efficient olefin hydroarylation catalyst. All complexes were characterized by multinuclear NMR spectroscopic methods (1H, 13C, 31P, 19F), and several were structurally confirmed by single-crystal X-ray diffraction (2, 5, 6, 9, 11, and 12). The structure of 14 was assigned using advanced 2D NMR techniques (COSY, NOESY, HSQC).