SYNTHESES, STRUCTURES, CD-113 SOLUTION NMR CHEMICAL-SHIFTS, AND INVESTIGATIONS OF FLUXIONAL PROCESSES OF BIS[POLY(PYRAZOLYL)BORATO]CADMIUM COMPLEXES

The reactions of 2 equiv of K[H2B(pz)2], K[HB(pz)3], K[B(pz)4], K[H2B(3,5-Me2pz)2], K[HB(3,5-Me2pz)3], and K[B(3-Mepz)4] with CdCl2 lead to the formation of the respective [poly(pyrazolyl)borato]2Cd complexes in high yields. A similar reaction with a 1/1 mixture of K HB(3,5-Me2pz)3] and either K[H2B(pz)4] yields [HB(3,5-Me2pz)3]Cd[H2B(pz)2] (7) or [HB(3,5-Me2pz)3]Cd[B(pz)4] (8), respectively. The solid-state structures of [B(pz)4]2Cd (3), [HB(3,5-Me2pz)3]2Cd (5), and [B(3-Mepz)4]2Cd.2C7H8 (6) have been determined crystallographically. All three complexes show a distorted octahedral arrangement of the nitrogen donor atoms, with the cadmium atom located on a center of symmetry. For 5, all Cd-N bonding distances are equal, but for 3 and 6, three different Cd-N bond distances, with the differences being as large as 0.103 angstrom for 3, are observed. The solution Cd-113 chemical shifts for the six-coordinate complexes range from 198.3 to 221.1 ppm, the resonance for the five-coordinate complex 7 is at 225.1 ppm, and the resonances for the four-coordinate complexes [H2B(pz)2]2Cd (1) and [H2B(3,5-Me2pz)2]2Cd(4) are at 298.7 and 303.3 ppm, respectively. Variable-temperature H-1 NMR spectra show that complexes 1, 3, 4, and 6-8 are dynamic in solution. For 3, 6, and 8 all of the pyrazolyl rings of the tetrakis-(pyrazolyl)borate ligands are equivalent at high temperatures with a 3/1 pattern observed at low temperatures. A mechanism that exchanges a coordinated with the noncoordinated pyrazolyl ring for the tetrakis(pyrazolyl)borate ligands is presented to explain these data. For the mixed-ligand complex 7, at low temperatures each resonance type for the [HB(3,5-Me2pz)3]- ligand appears as two resonances in a 2/1 ratio, resonances that coalesce at higher temperatures. The pyrazolyl ring resonances for the [H2B(pz)2]- ligand resonances remain equivalent at low temperatures, indicating a five-coordinate, square pyramidal arrangement of the nitrogen donor atoms. For 8, the resonances for the [HB(3,5-Me2pz)3]- ligand remain equivalent at low temperatures, indicating that the complex is six-coordinate. Crystal data: [B(pz)4]2Cd, monoclinic, P2(1)/c, a = 7.744(2) angstrom, b = 10.029(2) angstrom, c = 17.807(5) angstrom, beta = 92.80(2)degrees, V = 1381.4(5) angstrom3, Z = 2, R(F) = 3.79%; [HB(3,5-Me2pz)3]2Cd, rhombohedral, R3BAR, a = 11.003(2) angstrom, c = 24.784(5) angstrom, V = 2598.4(10) angstrom3, Z = 3, R(F) = 5.50%; [B(3-Mepz)4] 2Cd.2C7H8, monoclinic, P2(1)/c, a = 13.925(3) angstrom, b = 12.412(2) angstrom, c = 14.024(3) angstrom, beta = 92.62(2)degrees, V = 2421.3(8) angstrom3, Z = 2, R(F) = 7.72%.