Channel Inclusion Complexation of Organometallics: Dipolar Alignment for Second Harmonic Generation

Guest-host channel inclusion complexation has been found to be a general method of forming acentric crystals capable of second harmonic generation (SHG) from 1.06-mu laser light. Formation of 30 inclusion complexes between hosts thiourea or tris-o-thymotide (TOT) and various organometallic (28) and organic (2) guests is reported. None of the hosts or guests are capable of significant SHG themselves. Of this total studied, 19 (63%) were SHG active. Five of the SHG-active compounds (all examined by X-ray crystallography) crystallize in polar space groups. For thiourea complexation of organometallic guests, 11 of 16 (69%) inclusion complexes were active. For TOT-organometallic complexation, only 6 of 12 (50%) were active. This difference is ascribed to the strong tendency of dipoles to dimerize in an antiparallel (centrosymmetric) orientation. Tight control of channel dimensions is required to prevent this electrostatic interaction which leads to zero SHG. If channel dimensions can be controlled and maintained small relative to guest dimensions, then head-to-tail dipolar orientation becomes electrostatically preferred. The structures of thiourea inclusion complexes of (eta(6)-benzene)Cr(CO)(3) (1), (eta(4)-l,3-cyclohexadiene)Fe(CO)(3) (2), (eta(4)-tri-methylenemethane)Fe(CO)(3) (3), and (eta(5)-cyclohexadienyl)Mn(CO)(3) (4) have been determined by X-ray diffraction studies. Complexes 1, 3, and 4 are isomorphous and crystallize in the space group R3c while 2 crystallizes in the space group Pna2(1). TOT-(eta(6)-tetralin)Cr(CO)(3) (5) crystallizes in the space group Pca2(1). In the structures of 1, 3, 4, and 5, the guests are aligned head-to-tail along the channel direction. Channel inclusion complexation cannot guarantee an acentric structure for cases where the channels are large enough to accommodate a head-to-tail antiparallel pair. This situation is observed for TOT-W(CO)(5)(4-X-pyridine) (X = H, NH2; space group I2/alpha). The head-to-tail dipolar alignment may also not result if other strong packing forces (e.g., hydrogen bonding and pi-pi interaction of aromatic rings) can favor alternative arrangements. Such interactions are postulated for 2 and for TOT-W(CO)(5)(4-Y-pyridine) (Y = Me, Et).