Zerovalent Nickel Compounds Supported by 1,2-Bis(diphenylphosphino)benzene: Synthesis, Structures, and Catalytic Properties.

Zerovalent nickel compounds which feature 1,2-bis(diphenylphosphino)benzene (dppbz) were obtained via the reactivity of dppbz towards Ni(PMe3 )4 , which affords sequentially (dppbz)Ni(PMe3 )2 and Ni(dppbz)2 . Furthermore, the carbonyl derivatives (dppbz)Ni(PMe3 )(CO) and (dppbz)Ni(CO)2 may be obtained via the reaction of CO with (dppbz)Ni(PMe3 )2 . Other methods for the synthesis of these carbonyl compounds include (i) the formation of (dppbz)Ni(CO)2 by the reaction of Ni(PPh3 )2 (CO)2 with dppbz and (ii) the formation of (dppbz)Ni(PMe3 )(CO) by the reaction of (dppbz)Ni(CO)2 with PMe3 . Comparison of the ν(CO) IR spectroscopic data for (dppbz)Ni(CO)2 with other (diphosphine)Ni(CO)2 compounds provides a means to evaluate the electronic nature of dppbz. Specifically, comparison with (dppe)Ni(CO)2 indicates that the o-phenylene linker creates a slightly less electron donating ligand than does an ethylene linker. The steric impact of the dppbz ligand in relation to other diphosphine ligands has also been evaluated in terms of its buried volume (%Vbur ) and steric maps. The nickel center of (dppbz)Ni(PMe3 )2 may be protonated by formic acid at room temperature to afford [(dppbz)Ni(PMe3 )2 H]+ , but at elevated temperatures, effects catalytic release of H2 from formic acid. Analogous studies with Ni(dppbz)2 and Ni(PMe3 )4 indicate that the ability to protonate the nickel centers in these compounds increases in the sequence Ni(dppbz)2 < (dppbz)Ni(PMe3 )2 < Ni(PMe3 )4 ; correspondingly, the pKa values of the protonated derivatives increase in the sequence [Ni(dppbz)2 H]+ < [(dppbz)Ni(PMe3 )2 H]+ < [Ni(PMe3 )4 H]+ . (dppbz)Ni(PMe3 )2 and Ni(PMe3 )4 also serve as catalysts for the formation of alkoxysilanes by (i) hydrosilylation of PhCHO by PhSiH3 and Ph2 SiH2 and (ii) dehydrocoupling of PhCH2 OH with PhSiH3 and Ph2 SiH2 .