Exceptionally Facile CO Addition to a Saturated Ruthenium Complex

Synthesis and characterization of Cp*Ru[η3-HC(PPh2NPh)2], 1, reveals it to have a “piano stool” structure with the ligand bound to Ru(II) via two N and the unique, sp3 hybridized carbon. While the analogous (cymene) Ru[η3-HC(PPh2NPh)2]+ does not react with CO, under the same conditions, 1 adds one CO rapidly (25 °C, 1 atm CO). Characterization, including an X-ray structure determination, shows that CO has displaced one chelate ligand nitrogen, which then hangs off the molecule, free of Ru. DFT calculations reveal a possible mechanism via a remarkably low energy (+9.3 kcal/mol) intermediate, pendant N, but with one phenyl on phosphorus stabilizing Ru via donation from a C(ipso)C(ortho) bond. DFT calculations show that the electronic energy change for binding CO is over 20 kcal/mol less favorable for cymene than for C5Me5 - as ligand; the reactivity difference is thus thermodynamic in origin.