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 R 2 NBR′ 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 R 2 N 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(BR 2 ) 3 , and the (4c,6e) π-systems of trisaminoboranes, B(NR 2 ) 3 . The structures of HN(BMe 2 ) 2 , MeN-(BMe 2 ) 2 , and MeB(NMe 2 ) 2 determined by gas electron diffraction were in good agreement with those determined by calculations. The mean N-B π-bond rupture energies, 〈D π 〉, in the prototypical compounds H 2 NBH 2 , HN(BH 2 ) 2 , HB(NH 2 ) 2 , N(BH 2 ) 3 , and B(NH 2 ) 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, 〈D π 〉 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 〈D π 〉 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 ) 〈D π 〉 and a correlation coefficient of F ) 0.97.

A 1,3-Dihydro-1,3-azaborine Debuts

“…0.056 Å). 16b Overall, the observed bonding in 4 indicates delocalized features and is consistent with computationally predicated values. 3 …”

“…1.47 Å). 16 Compared to known boron- and/or nitrogen-containing heteroaromatic compounds, all the intra-ring bond distances in 4 are consistent with electron delocalization. For example, the B-C(2) and B-C(4) distances in 4 (1.525(2) and 1.526(2) Å, respectively) are significantly shorter than the sum of B( sp 3 )–C( sp 3 ) single-bond covalent radii of 1.6 Å, 15,17 but significantly longer than a B( sp 2 )=C( sp 2 ) double bond (ca.…”

A Frustrated and Confused Lewis Pair

“…The X‐ray crystal structure of 3 (Scheme b) displays an alternative coordination motif for this FLP with a covalent B−N bond (N1–B1=1.4124(19) Å) and a free formamide moiety (C1–O1=1.2007(17) Å) that conceptually results from hydroboration of the hindered N=C bond of the isocyanate. This B−N bond is typical of borylamides R 2 B‐NR 2 . The B‐N‐C(H)=O atoms essentially lie in one plane (B1‐N1‐C1‐O1=170.6°) consistent with a delocalized π system involving the carbonyl C=O double bond in conjugation with the N→B π interaction.…”

“…The N−B rotational barrier in 3 is considerably lower than alkyl or arylsubstituted borylamines (ca. 25 kcal mol −1 ) due to competition with the formyl group for the N lone pair. Rather, it is similar to the decreased N−B rotational barriers observed in early transition metal borylamides (Δ G ≠ =15.2–17.4 kcal mol −1 ) in which vacant metal d orbitals can compete with B for acceptance of the N lone pair …”

High‐Pressure Hydrogen Interactions with Polyaminoborane and Polyiminoborane

“…[2][3][4] The structure and dynamics of AB, PAB and PIB is influenced by dihydrogen bonding (DHB) [5,6] arising from the electrostatic interactions between protonic (H d + ) and hydridic (H dÀ ) H atoms and dative bonding due to donation of non-bonding lone pair on nitrogen to the empty p-orbital on boron. [7] Recently, it has been discovered that the presence of organic ligands on Lewis bases and/or donors creates steric hindrance resulting in a frustration of the dative bonding and termed as "frustrated Lewis pairs" (FLPs). [8] Early reports of phosphine-borane FLPs revealed a striking reactivity towards H 2 activation [9,10] and now seen in amineborane FLPs as well.…”