SYNTHESIS AND FLUXIONAL BEHAVIOR OF ALLYLPALLADIUM COMPLEXES WITH POLY(PYRAZOL-1-YL)METHANE LIGANDS

The reaction of the solvento-complex [(eta(3)-2-MeC(3)H(4))Pd(S)(2)]X (S = Me(2)CO) with the stoichiometric amounts of the following poly(py razol-1-yl)methanes: bis(pyrazol-1-yl)methane (bpzm), s(3,5-dimethylpy razol-1-yl)methane(3,5-Me(2)bpzm), tris(pyrazol-1-yl)methane (tpzm) an d tris(3,5dimethyipyrazol-1-yl) methane (3,5-Me(2)-tpzm) leads to the cationic complexes [(eta(3)-C4H7)Pd(bpzm)]BF4, (1), [(eta(3)-C4H7)Pd(b pzm)]PF6, (2), [(eta(3)-C4H7)Pd(tpzm)]BF4, (3), [(eta(3)-C4H7)Pd(3,5-M e(2)bpzm)]PF6, (4), and [(eta(3)-C4H7)Pd(3,5Me(2)tpzm)]PF6, (5). Reson ances in the H-1 NMR spectra have been assigned by considering the NOE effects between the methylene or methyne protons and the H(5) or Me(5 ) groups. NOE effects have also been observed between the H(3) or Me(3 ) and the H(syn) allylic protons. the C-13 NMR resonances have been as signed using H-1-C-13 HETCOR experiments. The fluxional behaviour of 4 and 5 has been studied by H-1 NMR spectroscopy. Two conformers of 4 a re discernible at low temperature, and they interchange when the tempe rature is increased. The AB systems corresponding to methylene groups of both conformers coalesce to a single A(2) system. A mechanism is pr oposed on the basis of this observation and the activation free energi es at the coalescence temperature calculated. The H-1 NMR spectrum of 5 shows the equivalence of the coordinated and uncoordinated pyrazole rings. The energy barrier of this phenomenon is too low to be determin ed by NMR spectroscopy. A tumbling motion, as proposed for similar obs ervations, seems a likely pathway of exchange.