Reversible, Metal-Free Hydrogen Activation

Abstract

Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100 degrees C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25 degrees C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.

Dihydrogen Activation by Antiaromatic Pentaarylboroles

“…19 Intermediate II could be deprotonated directly, or proceed to III via heterolytic addition of H2 across a B-C bond, an intermediate analogous to protonated fluorobenzenes. [20][21] This has been proposed as the initial step in the addition of H2 to Stephan's seminal phosphinoborane hydrogen activation system, 4 and supported computationally. 22 This mechanism is conceptually related to that developed for the B(C6F5)3 catalyzed hydrosilation of carbonyl [23][24][25] and imine 26 functions and the dehydrosilation of alcohols.…”

“…1 Recently, interest in more environmentally benign, transition metal-free systems for activation of dihydrogen [2][3][4] has spiked, 5 primarily spurred by the development of the "Frustrated Lewis Pairs" (FLPs) concept. [6][7][8] In FLPs, Lewis acid/base combinations sterically prevented from forming strong classical adducts are capable of heterolytically activating H2.…”

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…There has therefore been significant interest in developing highly active, selective catalysts based on more economical and less harmful elements such as first row transition metals 2 or main group elements. 3,4 With regard to the latter class of "metal-free" catalysts, the primary challenge lies in coaxing main group element compounds into mediating the critical bond activation step necessary for catalytic turnover. 5 In this context, our observation in the mid 1990's that the strongly Lewis acidic organoborane tris-pentafluorophenyl borane, B(C6F5)3, 6 was a highly active catalyst for the hydrosilation of carbonyl 7 and imine 8 functions has proven to be a key discovery in the development of metalfree catalyst systems for the addition of Si-H 9-13 and subsequently H-H [14][15][16] across a number of unsaturated bonds.…”

Synthesis and reactivity of di(silylamido)cyclopentadienyl titanium and zirconium complexes

“…8) provides additional evidence to this reaction pathway as it shows the presence of triphenylphosphonium and methyl-bis(pentafluorophenyl)borate fragments located at the para-positions of the resulting tetrafluorophenyl activated ring. A similar example of an aryl phosphoniumborate R 3 P-C 6 X 4 -BR 3 structure has been recently reported [52].…”