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)](2)P(O)Ph} (1) reacts with [Ni(cod)(2)] (cod = 1,4-cyclooctadiene) to give the diphosphine-phosphide oxide kappa(P,P(O),P) pincer complex 3. According to DFT calculations, the Ph-P(O) bond activation involves a three-center P,C-ipso,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 kappa(P,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 [Pd-2(dba)(3)], the palladium hydride kappa(P,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 kappa(P,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 kappa(P,P(OH),P) pincer complex 9.