Examining Metal-Ligand and Metal-Metal Cooperativity in Ruthenium (II) bis-(Protic N-Heterocyclic Carbene) Phosphine Complexes for CO2 and Related Substrate Transformations

Abstract

A novel bis-(protic N-heterocyclic carbene, PNHC) phosphine ligand platform is explored to understand its role in CO2 hydrogenation catalysis and as a supporting scaffold for bimetallic complexes. Optimized syntheses for the bis-(PNHC) phosphine ligand and its metalation with a Ru(II) metal center, as well as the synthesis and coordination chemistry of a library of complexes with varying ancillary ligands (2,2’-bipyridine, 4,4’-dibromo-2,2’-bipyridine, 4,4’-dimethoxy-2,2’-bipyridine, and bis-(diphenylphosphino)ethane) are shown. We describe the hydrogenation of CO2 to formate catalyzed by a Ru(II) bis-(protic N-heterocyclic carbene, p-NHC) phosphine complex [Ru(bpy)(MeCN)(PPh(p-NHC)2)](PF6)2 (1). Under catalytic conditions (20 µmol catalyst, 20 bar CO2, 60 bar H2, 5 mL THF, 140 °C, 16 hrs) the activity of 1 is limited only by the amount of K3PO4 present in the reaction, yielding a nearly one to one ratio of turn over number (TON) to equivalents of K3PO4 (relative to 1), with the highest TON = 8,040. Additionally, analysis of the reaction solution post-run reveals the catalyst intact with no free ligand observed. Stoichiometric studies, including examination of unique carbamate and hydride complexes as relevant intermediates, were carried out to probe the operative mechanism and understand the importance of metal-ligand cooperativity in this system. We also describe the synthesis and characterization of [Ru(bpy)(solvent)(P(Ph)(NHC)2-κ2-N,N'-M(solvent)2)]2+ (M = Co(II), Zn(II)) and subsequent reactivity with hydroxide and formate. Facile inter- and intramolecular cooperation is observed and the structurally responsive NHC-Cobalt-NHC binding pocket is investigated.