Bifunctional activation of amine-boranes by the W/Pd bimetallic analogs of "frustrated Lewis pairs"

The reaction between basic [(PCP)Pd(H)] (PCP = 2,6-(CH 2 P( t -C 4 H 9 ) 2 ) 2 C 6 H 4 ) and acidic [LWH(CO) 3 ] (L = Cp ( 1a ), Tp ( 1b ); Cp = η 5 -cyclopentadienyl, Tp = κ 3 -hydridotris(pyrazolyl)borate) leads to the formation of bimolecular complexes [LW(CO) 2 (μ-CO) Pd(PCP)] ( 4a , 4b ), which catalyze amine-borane (Me 2 NHBH 3 , t BuNH 2 BH 3 ) dehydrogenation. The combination of variable-temperature ( 1 H, 31 P{ 1 H}, 11 B NMR and IR) spectroscopies and computational (ωB97XD/def2-TZVP) studies reveal the formation of an η 1 -borane complex [(PCP)Pd(Me 2 NHBH 3 )] + [LW(CO 3 )] − ( 5 ) in the first step, where a BH bond strongly binds palladium and an amine group is hydrogen-bonded to tungsten. The subsequent intracomplex proton transfer is the rate-determining step, followed by an almost barrierless hydride transfer. Bimetallic species 4 are easily regenerated through hydrogen evolution in the reaction between two hydrides. Bimetallic complexes [LW(CO) 2 (μ-CO) Pd(PCP)] cooperatively activate amine-boranes for their dehydrogenation via N-H proton tunneling at RDS and H 2 evolution from two neutral hydrides.