Facile Addition of B–H and B–B Bonds to an Iron(IV) Nitride Complex

The
nitride ligand in the iron(IV) complex PhB(iPr2Im)3FeN reacts with boron
hydrides to afford PhB(iPr2Im)3FeN(B)H (B = 9-BBN
(1), Bpin (2)) and with (Bpin)2 to afford PhB(iPr2Im)3FeN(Bpin)2 (3). The iron(II) borylamido
products have all been structurally and spectroscopically characterized,
demonstrating facile insertion into B–H and B–B bonds
by PhB(iPr2Im)3FeN.
Density functional theory (DFT) calculations reveal that the quintet
state (S = 2) is significantly lower in energy than
the singlet (S = 0) and triplet (S = 1) states for all products. Stoichiometric reaction with (Bpin)2 does not produce the mono-borylated iron imido species PhB(iPr2Im)3FeN(Bpin). DFT
calculations suggest that this is because PhB(iPr2Im)3FeN(Bpin) is unstable toward disproportionation
to the starting iron(IV) nitride and PhB(iPr2Im)3FeN(Bpin)2. Attempts at B–C
bond insertion using phenyl- and benzyl-pinacol borane were unsuccessful,
which we attribute to unfavorable kinetics.