Ruthenium [NNN] and [NCN]-type pincer complexes with phosphine coligands: synthesis, structures and catalytic applications

A series of ruthenium [NNN]- or [NCN]-type complexes (3–7) bearing PPh3 ancillary ligands have been synthesized from pyridine- or phenylene-bridged bis(triazoles) 1 and 2. In the case of [NNN]-pincer complex 3, an unusual and unexpected cis-orientation adopted by two sterically demanding PPh3 ligands was observed, and such configuration proved to be unchanged in solution for a long time. By contrast and as expected, the two phosphines are found to be trans to each other in the case of [NCN]-type pincer complex 4, but an oxidation of RuII center to RuIII occurred. Complex cis-3 underwent ligand exchanges leading to the formations of diphosphine derivatives 5 and 6. As a representative, cis-3 was treated with the base in isopropanol affording a mixture of Ru–hydrido complexes with various phosphine binding modes, one of which (trans-7) bearing two trans-standing phosphines has been successfully isolated and fully characterized. The catalytic performances of all newly synthesized Ru complexes have been examined and compared in transfer hydrogenations of ketones and enones, in which mono-phosphine complexes proved to be significantly superior to their diphosphine counterparts. The catalytic process proved to involve Ru–H key intermediates, but the trans-oriented Ru–H species is unlikely to be the main catalytic contributor. In particular, the best performer cis-3 exhibits high chemoselectivity in certain cases catalyzing α,β-unsaturated ketones, whose behavior is quite different compared to most precedents.