A Dimetalloxycarbene Bonding Mode and Reductive Coupling Mechanism for Oxalate Formation from CO2

We describe the stable and isolable dimetalloxycarbene [(TiX3)2(μ2‐CO2‐κ2C,O:κO′)] 5, where X=N‐(tert‐butyl)‐3,5‐dimethylanilide, which is stabilized by fluctuating μ2‐κ2C,O:κ1O′ coordination of the carbene carbon to both titanium centers of the dinuclear complex 5, as shown by variable‐temperature NMR studies. Quantum chemical calculations on the unmodified molecule indicated a higher energy of only +10.5 kJ mol−1 for the μ2‐κ1O:κ1O′ bonding mode of the free dimetalloxycarbene compared to the μ2‐κ2C,O:κ1O′ bonding mode of the masked dimetalloxycarbene. The parent cationic bridging formate complex [(TiX3)2(μ2‐OCHO‐κO:κO′)][B(C6F5)4], 4[B(C6F5)4], was simply deprotonated with the strong base K(N(SiMe3)2) to give 5. Complex 5 reacts smoothly with CO2 to generate the bridging oxalate complex [(TiX3)2(μ2‐C2O4‐κO:κO′′)], 6, in a CC bond formation reaction commonly anticipated for oxalate formation by reductive coupling of CO2 on low‐valent transition‐metal complexes. Between a rock and a hard place: A structurally characterized dimetalloxycarbene complex, which is stabilized by coordination of the carbenoid carbon atom on two metal centers (see picture: C gray, N blue, O red, Ti magenta), reacts with CO2 to form an oxalate complex. DFT calculations show that nucleophilic attack of a free carbene species occurs on CO2.