DYNAMIC BEHAVIOR AND CRYSTAL-STRUCTURES OF PD2(MU-NS,ETA-2-L)2CL2(PME3)2 (L = PYRIMIDINE-2-THIOLATE, 4-METHYLPYRIMIDINE-2-THIOLATE, AND METHYLIMIDAZOLE-2-THIOLATE) - INFLUENCE OF THE N-C-S BOND ANGLE ON THE STABILITY OF (HETEROCYCLIC 2-THIOLATO)DIPALLADIUM COMPLEXES

Dipalladium complexes Pd2(mu(NS),eta2-L)2Cl2(PMe3)2 (L = pyrimidine-2-thiolato, 2; 4-methylpyrimidine-2-thiolato, 3; and 1-methylimidazole-2-thiolato, 4) were prepared by reaction of [PdCl(mu-Cl)(PMe3)]2 with the corresponding sodium heterocyclic thiolate. The molecular structures of 2-4 have been determined by X-ray diffraction. Crystallographic data: for 2 monoclinic C2/c, Z = 4, a = 19.857 (4) angstrom, b = 11.124 (2) angstrom, c = 12.054 (2) angstrom, beta = 117.12 (1)-degrees, V = 2369.9 (8) angstrom; for 3 monoclinic c2/c, Z = 4, a = 19.616 (7) angstrom, b = 12.343 (4) angstrom, c = 11.886 (4) angstrom, beta = 118.25 (3)-degrees, V = 2535 (2) angstrom3; for 4-(acetone) orthorhombic Pbca, Z = 8, a = 10.460 (3) angstrom, b 15.801 (4) angstrom, c = 32.17 (2) angstrom, V = 5316 (4) angstrom3. Variable-temperature H-1 NMR spectra of 2 indicate a temperature-dependent equilibrium is established with a second species in solution. From the changing ratios of the integrated intensities of the signals as associated with the two species over the range -40 to +50-degrees-C, values of DELTAH = 29 kJ mol-1 and DELTAS = 110 J K-1 mol-1 can be estimated for the equilibrium process. Carbon tetrachloride boiling point elevation experiments indicated that the predominant species in solutions of 2 and 3 at high temperatures have molecular weights of 340 +/- 30 and 370 +/- 30, respectively, and are monomeric species resulting from the cleavage of the parent dimeric complexes. The P-31{H-1} and H-1 NMR spectra of 3 dissolved in toluene-d8 are much more complicated than those of 1 and indicate that not only are both dimeric and monomeric species present in solution but also additional isomeric species are present. Variable-temperature H-1 NMR studies of 3 indicate interconversion of the isomers occurs in concert with the dimer-monomer interconversion and suggests that the dimer-monomer interconversion process involves Pd-N bond rupture. The variable-temperature H-1 NMR spectra of complex 4 over the -50 to +100-degrees-C range are static, indicating the absence of any rapid equilibrium process on the NMR time scale. The molecular structures of 2 and 3 as well as that previously reported for Pd2(mu-pyS)2Cl2(PMe3)2(1) indicate no significant variation in the coordinative interaction of the mu(NS)eta2 bridging heterocyclic 2-thiolate ligands despite the differences in the basicities of the ligands. This suggests that the geometrical constraints of the N-C-S bond angles of the rigid six-membered heterocyclic restricts coordinative interaction and is responsible for the similar dynamic behavior observed for 1-3. The five-membered heterocycle ring of 4 has larger N-C-S bond angles and contains Pd-N bond distances which are significantly shorter than those of 1-3, demonstrating an enhanced Pd-N interaction and accounting for the static variable-temperature H-1 NMR spectra obtained for 4.