Frustrated Lewis Pair Chelation and Reactivity of Complexed Parent Iminoborane and Aminoborane

An intramolecular phosphine–borane frustrated
Lewis pair
(FLP) chelate, iPr2P(C6H4)BCy2 or PB (Cy = cyclohexyl),
was used to coordinate aminoborane (H2BNH2)
and iminoborane (HBNH) units via donor–acceptor
stabilization. Attempts to induce dehydrogenation from these B–N
adducts with known metal catalysts (or pre-catalysts) have been unsuccessful
thus far, and related observations were noted with an H2BNH2 complex supported by a modified FLP chelate bearing
a geometrically constrained bicyclic 9-borabicyclo(3.3.1)nonane (BBN)
unit. Treatment of the iminoborane adduct [PB{HBNH}] with a chlorinating
agent led to ligand activation via B–C bond
cleavage instead of the expected H/Cl exchange at boron to give [PB{ClBNH}].
Nucleophilic attack at the boron center in [PB{HBNH}] was observed
upon addition of BnK (Bn = benzyl), yielding the amidoborate complex
[PB{H(Bn)BNH}{K(THF)2}].