New Ruthenium Nitrosyl Pincer Complexes Bearing an O₂ Ligand. Mono-Oxygen Transfer

We report on Ru^(II)(μ²-O₂) nitrosyl pincer complexes that can return to their original Ru(0) state by reaction with mono-oxygen scavengers. Potential intermediates were calculated by density functional theory (DFT) and a mechanism is proposed, revealing a new type of metal–ligand cooperation consisting of activation of the O₂ moiety by both the metal center and the NO ligand. Reaction of the Ru(0) nitrosyl complex <b>1</b> with O₂ quantitatively yielded the crystallographically characterized Ru^(II) (μ²-O₂) nitrosyl complex <b>2</b>. Reaction of <b>2</b> with the mono-oxygen scavengers phosphines or CO gave the Ru(0) complex <b>1</b> and phosphine oxides, or the carbonyl complex <b>3</b> (<b>1</b> trapped by CO) and CO₂, respectively. Reaction of <b>2</b> with 1 equiv of phosphine at room temperature or −40 °C resulted in immediate formation of half an equivalent of <b>1</b> and 1 equiv of phosphine oxide, while half an equivalent of <b>2</b> remained unchanged. Overnight reaction at room temperature of <b>2</b> with excess CO (≥3 equiv) resulted in <b>3</b> and CO₂ gas as the only products. Reaction of <b>1</b> with 1 equiv of mono-oxygen source (dioxirane) at −78 °C yielded the Ru^(II)(μ²-O₂) complex <b>2</b>. Similarly, reaction of the Ru(0) dearomatized complex <b>4</b> with O₂ led to the crystallographicaly characterized Ru^(II)(μ²-O₂) complex <b>5</b>. Further reaction of <b>5</b> with mono-oxygen scavengers (phosphines or CO) led to the Ru(0) complex <b>4</b> and phosphine oxides or complex <b>6</b> (<b>4</b> trapped by CO) and CO₂. When instead only 1 equiv of <b>5</b> was reacted with 1 equiv of phosphine at room temperature, immediate formation of half an equivalent of <b>4</b> and 1 equiv of phosphine oxide took place, while half an equivalent of <b>5</b> remained unchanged. When <b>5</b> reacted with an excess of CO (≥3 equiv), complex <b>6</b> and CO₂ gas were the only products obtained. DFT studies indicate a new mode of metal–ligand cooperation involving the nitrosyl ligand in the oxygen transfer process