1-Pyridine- and 1-Quinuclidine-1-boraadamantane as Models for Derivatives of 1-Borabicyclo[2.2.2]octane. Experimental and Theoretical Evaluation of the B−N Fragment as a Polar Isosteric Substitution for the C−C Group in Liquid Crystal Compounds†

The suitability of 1-borabicyclo[2.2.2]octane (1) as a structural element for liquid crystals was evaluated using computational methods and experimental studies of two complexes of its close analogue 1-boraadamantane (2). The molecular and crystal structures for 1-pyridine-1-boraadamantane [2-P, C14H20BN, P21/m, a = 8.4404(13) A, b = 6.8469(10) A, c = 10.5269(16) A, β = 104.712(3)°, Z = 2], 1-quinuclidine-1-boraadamantane [2-Q, C16H28BN, P21/n, a = 6.6529(3) A, b = 10.6665(6) A, c = 19.3817(10) A, β = 94.689(3)°, Z = 4], and 1-pyridine-trimethylborane [3-P, C8H14BN, Ccma, a = 6.9875(10) A, b = 15.011(2) A, c = 16.556(2) A, Z = 8] were determined by X-ray crystallography and compared with the results of DFT and MP2 calculations. Gas-phase thermodynamic stabilities of complexes 1-P, 1-Q, 2-P, and 2-Q were estimated using a correlation between theoretical (MP2/6-31+G(d)//MP2/6-31G(d) with B3LYP/6-31G(p) thermodynamic corrections) and experimental data for complexes of BMe3 (3) with amines lacking N−H bonds. ...