Control of structure in lead(II) complexes using poly(pyrazolyl)borate ligands. Stereochemically inactive lone pair in octahedral [HB(3,5-Me2pz)3]2Pb (pz = pyrazolyl ring)

The reaction of 2 equiv of K[HB(pz)4-n] (n = 0-2; pz = pyrazolyl ring) or K[HB(3,5-Me2pz)3] With PbCl2 produces the respective bis[poly(pyrazolyl)borato]lead complex in good yield. The complex [HB(PZ)3]PbCl is formed in a similar reaction using 1 equiv each of the ligand and PbCl2. The structure of [B(pz)4]2Pb (1) has been determined crystallographically: monoclinic, Pc, a = 8.889 (7) angstrom, b = 8.022 (6) angstrom, c = 20.901 (18) angstrom, beta = 97.70 (6)-degrees, V = 1447.0 (21) angstrom3, Z = 2, R(F) = 5.62%. Complex 1 has a pseudo-trigonal-bipyramidal geometry with each ligand having bidentate coordination and spanning an axial and equatorial site. The lead lone pair presumably occupies the remaining equatorial vertex. The H-1 NMR spectra show that 1 is dynamic in solution. All of the pyrazolyl rings are equivalent at ambient temperatures with a 3/1 pattern observed at -89-degrees-C. The low-temperature spectrum can be explained by a low-energy process that equilibrates in each ligand the three pyrazolyl rings close to lead, but the fourth ring remains unique. At higher temperatures, the fourth ring also equilibrates by a boat-boat flip of the six-membered PbN4B ring (9.1 kcal/mol barrier). The structure of [HB(PZ)3]2Pb (2) has also been determined crystallographically: monoclinic, P2(1)/c, a = 16.600 (9) angstrom, b = 8.040 (5) angstrom, c = 17.214 (9) angstrom, beta = 90.77 (4)-degrees, V = 2297.4 (23) angstrom3, Z = 4, R(F) = 5.79%. In the structure of 2, both ligands are tridentate, forming a six-coordinate, monomeric structure. The geometry about the lead atom is best described as based on a capped octahedron with the lone pair on the metal located in the capping position. The structure of [HB(3,5-Me2pz)3]2Pb (4) has also been determined crystallographically: trigonal, R3BAR (No. 148), a = 11.152 (3) angstrom, c = 25.301(7) angstrom, V = 2725.8 (10) angstrom3, Z = 3, R(F) = 4.6%. The structure is six-coordinate, and the geometry about the lead atom is a trigonally distorted octahedron. The lead atom sits on a center of inversion, and the planes formed by the three nitrogen donor atoms of each ligand are parallel. The lone pair on lead is clearly stereochemically inactive. Lead-207 NMR studies indicate that these structures are the same in solution. The complex [H2B(pz)2]2Pb (3) shows two types of dynamic processes in solution as observed by H-1 NMR spectroscopy. The axial and equatorial pyrazolyl rings in the pseudo-trigonal-bipyramidal struct