Two-Center, Three-Center, and Four-Center Dative π-Bonding Systems in Boron−Nitrogen Compounds Studied by Density Functional Theory Calculations: The Molecular Structures of Bis(dimethylboryl)amine, Bis(dimethylboryl)methylamine and Bis(dimethylamino)methylborane Determined by Gas Electron Diffraction

Abstract

The nature of the two-center, two-electron (2c,2e) dative π-bonding system in aminoboranes has been explored by optimizing the equilibrium structures of R2NBR‘2 (R, R‘ = H or Me) by DFT calculations at the B3PW91/6-311++G** level. The π-bond rupture energies were determined by optimizing models in which the relative orientation of the R2N and BR‘2 fragments was fixed in such a manner that the lone-pair atomic orbital on N was orthogonal to the vacant 2p-orbital on the B atom. Similar calculations were carried out on the (3c,2e) dative π-bonding systems of bisborylamines, RN(BR‘2)2; the (3c,4e) π-system of bisaminoboranes, RB(NR‘2)2; the trigonal (4c,2e) π-bonding systems of trisborylamines, N(BR2)3, and the (4c,6e) π-systems of trisaminoboranes, B(NR2)3. The structures of HN(BMe2)2, MeN(BMe2)2, and MeB(NMe2)2 determined by gas electron diffraction were in good agreement with those determined by calculations. The mean N−B π-bond rupture energies, 〈Dπ〉, in the prototypical compounds H2NBH2, HN(BH2)2, HB(NH2)2, N(BH2)3, and B(NH2)3 were found to depend not on the number of π-electrons, but on the number of centers. The mean π-bond rupture energies of the prototypical 2c, 3c, and 4c compounds were found to vary in the order 4.0:3.0:2.0. When H atoms at an acceptor atom (B) are replaced by more electron releasing methyl groups, 〈Dπ〉 is significantly reduced due to a synergetic combination of destabilizing inductive effects and steric strain. When H atoms at the donor atom (N) are replaced by Me groups, the effect on 〈Dπ〉 is determined by the balance of stabilizing inductive and destabilizing steric effects. The N−B bond distances in the 14 molecules tend to decrease with increasing mean π-bond rupture energy. Linear correlation analysis yields R(N−B) = 149.3 pm − (0.075 pm mol kJ-1) 〈Dπ〉 and a correlation coefficient of ρ = 0.97.