KINETICS AND MECHANISM OF HALOGEN-BRIDGE CLEAVAGE IN DIMETHYLAMINOMETHYLPHENYL-C-1,N PALLADA-CYCLES AND PLATINA-CYCLES BY PYRIDINES - PRESSURE EFFECTS, AND CRYSTAL-STRUCTURES OF THE N,N-CIS REACTION-PRODUCT, ITS N,N-TRANS ORTHOMETALLATED ANALOG AND A DIMER OF SIMILAR REACTIVITY

An ambient and high-pressure stopped-flow kinetic study of the halogen-bridge cleavage reaction in the pallada- and platina-cycles [{M(o-C(6)H(3)RCH(2)NMe(2))X}(2)] (M = Pd or Pt; R = H, 4-MeO, 5-Me or 5-F; X = Cl or I) by a series of substituted pyridines in chloroform as solvent revealed that it is a fast, associatively driven second-order process, with strong steric rather than electronic demands. Substituent effects and activation parameters (Delta H-double dagger, Delta S-double dagger and Delta V-double dagger) were in full accord with the proposed associative mechanism. The Pd dimers transformed into N,N-trans monomers of the type [Pd(o-C(6)H(3)RCH(2)NMe(2))X(py)] (py = pyridine). In contrast. the Pt counterparts afford N,N-cis species [Pt(o-C(6)H(3)RCH(2)NMe(2))X(py)] under the same conditions. The geometry of the N,N-cis complex [Pt{o-C6H3(4-Meo)CH(2)NMe2}X(py)], as well as of the N,N-trans platinacycle [Pt{o-C6H4C(Me)=NOH}Cl(py)], has been confirmed by X-ray crystallography. The most striking structural differences in the N,N-cis and N,N-trans related platinacycles are the Pt-Cl and Pt-N(py) bond distances [2.300(1) and 2.408(5), 2.138(4) and 2.02(1) Angstrom A, respectively]. The crystal structure of trans-[(Bu(3)(n)P)IPd-(mu-I)(2)Pdl(PBu(3)(n))] has also been determined and used to account for its similar reactivity to[{Pd(o-C(6)H(4)CH(2)NMe(2))I}(2)] in the bridge-splitting reaction.