Single crystal X-ray structures of 2-pyridinecarboxaldehydeazine and biacetylazine: Implications of the conjugation in systems with carbon-nitrogen double bonds

Crystal structures were determined for two compounds, 2-pyridinecarboxaldeh ydeazine, 1, and biacetylazine, 2. 2-Pyridinecarboxaldehydeazine crystalliz es in space group P2(1)/c with a = 10.0487(7) Angstrom, b = 4.6452(7) Angst rom, c = 11.6700(11) Angstrom, beta = 91.030(6)degrees, V = 544.65(10) Angs trom(3), Z = 2, R = 0.0345. Biacetylazine crystallizes in space group C2/c with a = 9.879(7) Angstrom, b = 12.409(4) Angstrom, c = 7.950(6) Angstrom, beta = 98.44(6)degrees, V = 964.0(11) Angstrom(3), Z = 4, R = 0.0496. Compa rison of the imine bond lengths of these and other azine and diimine system s found in the literature suggests that conjugation of imines is better thr ough the carbon-carbon bond than through the nitrogen-nitrogen bond. Semiem pirical structural calculations demonstrate that the N-N bond in these azin es is rotationally soft, thereby allowing significant twisting at little en ergy cost. This accounts for the observation that 1 is planar and 2 is not.