Synthesis and Reactivity of Group Six Metal PCP Pincer Complexes: Reversible CO Addition Across the Metal–C_aryl Bond

In the present study, Cr­(III) complexes of the type trans-[Cr­(PCPNEt-iPr)­(solvent)­Cl2] (solvent = CH3CN, THF), the Cr(0) complex [Cr­(κ3P,CH,P-P­(CH)­PNEt-iPr)­(CO)3] which features an η2-Caryl–H agostic bond as well as seven coordinate cationic chloro carbonyl Mo­(II) and W­(II) complexes of the type [M­(PCPNEt-iPr)­(CO)3Cl] featuring PCP pincer ligands based on a 1,3-diaminobenzene scaffold were prepared and characterized. The seven coordinate chloro tricarbonyl complexes exhibit fluxional behavior in solution due to rapid CO ligand interconversions. Another interesting aspect is a rapid and reversible addition of one CO ligand across the metal–Cipso bond. The mechanism of the dynamic process of the chloro carbonyl complexes was investigated by means of DFT calculations. The Mo­(II) and W­(II) tricarbonyl complexes could be reduced to the respective anionic Mo(0) and W(0) complexes [Mo­(PCPNEt-iPr)­(CO)3]− and [Mo­(PCPNEt-iPr)­(CO)3]−. These air sensitive compounds are readily protonated by MeOH to form agostic and hydride species of the types [Mo­(κ3P,CH,P-P­(CH)­PNEt-iPr)­(CO)3] and [W­(PCPNEt-iPr)­(CO)3H]. Structures of representative complexes were determined by X-ray single crystal analyses