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) Å, b = 6.8469(10) Å, c = 10.5269(16) Å, β = 104.712(3)°, Z = 2], 1-quinuclidine-1-boraadamantane [2-Q, C₁₆H₂₈BN, P2₁/n, a = 6.6529(3) Å, b = 10.6665(6) Å, c = 19.3817(10) Å, β = 94.689(3)°, Z = 4], and 1-pyridine-trimethylborane [3-P, C₈H₁₄BN, C_cma, a = 6.9875(10) Å, b = 15.011(2) Å, c = 16.556(2) Å, 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. The analysis showed the generally higher thermodynamic stability for the quinuclidine (Q) complexes compared to that of the pyridine (P) analogues in the gas phase and an overall order of stability of 1 > 2 > 3. This order is paralleled by high ring strain energy of 1 (SE = 27 kcal/mol) as compared to that of 1-boraadamantane (2, SE = 16.5 kcal/mol). The chemical stability of 2-P and 2-Q, with respect to hydrolytic and oxidative reagents, is high for the pyridine derivative and satisfactory for the quinuclidine complex at ambient temperature, which implies sufficiently high stability of 1-borabicyclo[2.2.2]octane complexes for materials applications. Molecular dipole moments of 6.2 ± 0.1 and 6.0 ± 0.15 D were measured for 2-Q and 2-P, respectively.