Dative σ- and π-bonding in boron–nitrogen compounds: molecular structures of (CH3)2NB(CH3)N(CH3)B(CH3)2 and [(CH3)2B]2NN(CH3)2 determined by gas electron diffraction and quantum chemical calculations

Abstract

The molecular structures of dimethylamino[(dimethylboryl)methylamino]methylborane, Me₂NBMeNMeBMe₂ (1) and 1,1-bis(dimethylboryl)-2,2-dimethylhydrazine, (Me₂B)₂NNMe₂ (2) have been determined by gas electron diffraction (GED), density functional theory calculations at the B3PW91/6-311++G** level and ab initio calculations at the MP2/6-311++G** level. 1 adopts an open structure similar to that of the isoelectronic hydrocarbon molecule permethylbutadiene; the central B–N bond distance at 148.0/149.3(7) pm (MP2/GED) corresponds to a single covalent N–B bond distance, the two terminal distances, 140.9/140.5(4) pm and 141.8/141.3(4) pm, correspond to the distance between N and B atoms joined by a covalent σ-bond and a dative π-bond. A closed form where the establishment of a dative bond between the terminal N and B atoms has led to the formation of a four-membered ring also corresponds to a minimum on the potential energy surface, but the energy is calculated to be 14.3 kJ mol⁻¹ higher at the MP2 level. This structure is also incompatible with the GED data. 2 adopts a structure in which a dative σ-bond between the dimethylamino N atom and one of the boron atoms has led to the formation of a three-membered N₂B ring. The dative σ-bond distance is 165.5/164.0(13) pm, the two other bond distances in the ring are N–B = 150.6/148.9(9) pm corresponding to a covalent σ-bond and N–N = 145.1/145.4(3) pm. The terminal B–N distance 139.6/138.9(9) pm is consistent with a covalent σ-bond augmented by a dative π-bond. An open Y-shaped structure also corresponds to a minimum on the potential energy surface, but the energy is 18.7 kJ mol⁻¹ higher (MP2) and it is incompatible with the GED data.