Phosphorus stabilized carbene complexes: bisphosphonate dianion synthesis, reactivity and DFT studies of O∼C∼O zirconium(IV) complexes

The reactivity of the geminal dianion of tetraisopropyl methylenediphosphonate, 3, with [ZrCl4(THF)(2)] affords a trinuclear structure 5 in which an unprecedented triscarbene-Zr dianionic fragment is found. The overall trinuclear arrangement is assembled by PO bridging moieties. The X-ray crystal structure of 5 is presented. The reaction of complex 5 with stoichiometric amounts of pyridine affords a dinuclear complex 6 whose formulation was ascertained by an X-ray crystal structure. Complexes 5 and 6 cleanly react with aldehydes (R-1)(H)C=O to afford the corresponding olefin of general formula [((OiPr)(2)P=O)(2)C=C(R-1)(H)]. DFT calculations which were carried out on the model complex of 5t and a hypothetical monometallic Zr complex [Zr(3)Cl-2(OMe2)(2)] 8t indicate that the Zr-C bond only features a weak pi-interaction between the carbon atom and a vacant orbital at the metal. The polarization of the orbital towards the carbon center is consistent with the observed nucleophilic character of the carbene complexes described here.