New Insight into a Deceptively Simple Reaction: The Coordination of bpy to Ru II –Carbonyl Precursors – The Central Role of the fac ‐[Ru(bpy)Cl(CO) 3 ] + Intermediate and the Chloride Rebound Mechanism

This work demonstrates how a careful reexamination of well‐trodden fields can fill conceptual gaps that previously escaped full understanding. The coordination of 2,2′‐bipyridine (bpy) to the known Ru II –chlorido–carbonyl precursors – the dinuclear [RuCl 2 (CO) 3 ] 2 ( P1 ) and the polymeric [RuCl 2 (CO) 2 ] n ( P2 ) – has been investigated by several groups in the past, and a remarkably large number of ruthenium–mono(bpy)–carbonyl compounds were identified and fully characterized. Many were investigated as catalysts or key intermediates for the photochemical, electrochemical, and photo‐electrochemical reduction of CO 2 , and for the water–gas shift reaction. Nevertheless, even though most – if not all – of the reaction products are known already, a careful examination of the literature led us to believe that a convincing general scheme interconnecting them all was still missing and important questions remained unanswered. For this reason, we investigated the reactivity of two mononuclear Ru II –carbonyl–dmso precursors, trans , cis , cis ‐[RuCl 2 (CO) 2 (dmso‐O) 2 ] ( P3 ) and fac ‐[RuCl 2 (CO) 3 (dmso‐O)] ( P4 ) – which can be considered as “activated forms” of P2 and P1 , respectively – towards the coordination of bpy. Compounds P3 and P4 allowed us to gain new mechanistic insight and a deeper level of understanding. In particular, we found that coordination of bpy to P4 (or P1 ) generates first the tricarbonyl cation fac ‐[Ru(bpy)Cl(CO) 3 ] + . This key intermediate undergoes the facile and selective nucleophilic attack on the CO trans to Cl (by RO – in alcoholic solvents or OH – from adventitious water in other solvents), leading to all other species. We also demonstrated that Cl – – even when in large excess – is unable to replace a carbonyl on fac ‐[Ru(bpy)Cl(CO) 3 ] + . However, the chloride set free from the precursor competes efficiently with bpy for the coordination to Ru II ( chloride rebound mechanism).