Chelating Assistance of P–C and P–H Bond Activation at Palladium and Nickel: Straightforward Access to Diverse Pincer Complexes from a Diphosphine–Phosphine Oxide

The diphosphine–phosphine oxide (DPPO) {[o-i-Pr2P-(C6H4)]2P­(O)­Ph} (1) reacts with [Ni­(cod)2] (cod = 1,4-cyclooctadiene) to give the diphosphine–phosphide oxide κP,P(O),P pincer complex 3. According to DFT calculations, the Ph–P­(O) bond activation involves a three-center P,Cipso,Ni transition state. Reaction of the DPPO ligand 1 with [(nbd)­Pd­(ma)] (nbd = 2,5-norbornadiene and ma = maleic anhydride) affords the [(DPPO)­Pd­(ma)] complex 4. Upon heating, the ma coligand is displaced and the κP,P(O),P palladium pincer complex 2 is obtained. The dinuclear complex {(DPPO)­[Pd­(ma)]2} (6) has also been authenticated. X-ray diffraction analysis showed an original situation in which the oxygen atom of the central phosphine oxide moiety bridges the two palladium centers. Addition of trifluoromethanesulfonic acid to DPPO 1 affords the trifunctional phosphine–phosphine oxide–phosphonium derivative 7. Upon reaction with [Pd2(dba)3], the palladium hydride κP,O(P),P pincer complex 8 is cleanly formed as the result of P+–H bond activation. Complex 8 is readily deprotonated by DBU (DBU = 1,8-diazabicycloundec-7-ene), and spontaneous oxidative addition of the Ph–P­(O) bond gives the diphosphine–phosphide oxide κP,P(O),P pincer complex 2. Conversely, addition of trifluoromethanesulfonic acid on 2 does not give back the palladium hydride 8 but leads to the diphosphine–hydroxy phosphine κP,P(OH),P pincer complex 9.