TEMPLATE-DIRECTED SYNTHESIS OF 1-1 LAYERED COMPLEXES OF ALPHA,OMEGA-DINITRILES AND UREA - PACKING EFFICIENCY VERSUS SPECIFIC FUNCTIONAL-GROUP INTERACTIONS

When urea is crystallized from MeOH in the presence of alpha,omega-dinitriles of the formula NC-(CH2)(n)CN, it forms layered 1:1 cocrystals when n = 3-5, and channel inclusion compounds (commensurate or incommensurate) when n = 6, 8, 10, and 12. Our analysis suggests that this ''homologous polymorphism'' is the result of a competition between hydrogen bonding (which is favored in the 1:1 complexes) and packing efficiency (which is greater for the channel type inclusion compounds of the long chain guests). One measure of packing efficiency is the relationship between average electron density per unit cell (D(e)) and the average atomic number (z(ave)). For a collection of 338 organic crystal structures, plots of D(e) versus z(ave) give different least-squares lines for crystals with intermolecular hydrogen bonds and those without them. For the 1:1 cocrystals, donor-acceptor complementarity favors an inefficient packing mode, and successive addition of CH2 units decreases D, more rapidly than predicted from the relationships obtained for the other two data sets. The switch from cocrystal to inclusion compound is accompanied by a dramatic increase in packing efficiency. To circumvent the packing efficiency problem and to extend the range of dinitrile structures that form layered complexes, a template effect was employed. Thus, bis(5-cyanopentyl)urea forms 1:1 layered cocrystals with urea either from solution or by grinding. (This differs from the molecular crystal of bis(5-cyanopentyl)-urea, which contains gauche nitriles that pack in a herringbone fashion.) All of the 1:1 cocrystals exhibit favorable interlayer nitrile-nitrile contacts, as supported by atom-atom potential calculations. Crystal data for structures reported in this paper are as follows: pimelonitrile/urea (1:1 cocrystal): monoclinic, space group P2/c, Z = 2, a = 7.196(2), b = 4.583(3), c = 15.973(4) angstrom, beta = 98.679(21)-degrees, R = 0.051 at T = 296 K; glutaronitrile/urea (1:1 cocrystal): orthorhombic, space group Pmmn, Z = 2, a = 6.5020(16), b = 13.712(3), c = 4.5693(10) angstrom, R = 0.058 at 213 K;bis(5-cyanopentyl)urea: orthorhombic, space group P2(1)2(1)2, Z = 2, a = 6.9991(11), b = 21.552(3), c = 4.5747(7) angstrom, R = 0.071 at T = 99 K; bis(5-cyanopentyl)urea/urea (1:1 cocrystal): monoclinic, space group P2/n, Z = 2, a = 13.708(3), b = 4.5879(12), c = 13.964(3) angstrom, beta = 112.335(10)-degrees, R = 0.038 at 111 K.