Synthesis and second-order nonlinear optical properties of three coordinate organoboranes with diphenylphosphino and ferrocenyl groups as electron donors: crystal and molecular structures of (E)-D-CH=CH-B(mes)2 and D-CC-B(mes)2

Article abstract of DOI:10.1016/0022-328X(93)80103-I

We report the synthesis and characterization of five ?-electron conjugated diphenylphosphino and ferrocenyl dimesitylboranes (E)-Ph2P-CH=CH-B(mes)2 (3a) crystallizes in space group P2₁2₁2₁ and exhibits a powder second harmonic generation (SHG) efficiency, x⁽²⁾, approximately equal to that of urea.First molecular hyperpolarizabilities, β, of 3a, Ph2P-C<*>C-B(mes)2 (5a), (E)-(η-C5H5)Fe(η-C5H4)CH=CH-B(mes)2 (3b) and (η-C5H5)Fe(η-C5H4)C<*>C-B(mes)2 (5b) have been measured and the values are comparable to those for small organic compounds.Ground-state charge-transfer interactions have been examined for compounds 3a,b, 5a,b via analysis of their crystal and molecular structures.It is significant that the vinyl group rotates out of the BC3 plane by 16.8 deg for 3a and 12.0 deg for 5a, whereas mesityl groups are typically rotated by 50-70 deg with respect to the boron valence plane.This indicates that the best ?-conjugation is along the desired pathway, i.e. from ?-donor through the vinyl and ethynyl group to the vacant p-orbital on boron.In addition, the B-C(vinyl) bond distance in 3b is somewhat shorter than the average B-C(mes) bond length, and the P-C(vinyl) bond length is shorter than the average P-C(phenyl) bond distance in 3a, also supporting a certain degree of ?-conjugation in these systems.For the phosphine derivatives, the sum of angles about phosphorus is 308-309 deg, typical of a pyramidal configuration.As a result, ?-electronic interaction between boron and phosphorus is probably weak and a large angle exists between the ground-state and excited-state dipole moments.Related arguments also apply to the ferrocenyl boranes.The large β value (- 24 * 1-^-30 esu) measured for 3b is probably the result of a low-lying charge transfer from electron-rich iron to electron-deficient boron.The effect of crystal packing on the observed structures is discussed.