Kinetics and mechanism of halogen-bridge cleavage in dimethylaminomethylphenyl-C1,N pallada- and platina-cycles by pyridines. pressure effects, and crystal structures of the N,N-cis reaction product, its N,N-trans orthometallated analogue and a dimer of similar reactivity

Abstract

An ambient and high-pressure stopped-flow kinetic study of the halogen-bridge cleavage reaction in the pallada- and platina-cycles [{M(o-C₆H₃RCH₂NMe₂)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 (ΔH^‡, ΔS^‡ and ΔV^‡) were in full accord with the proposed associative mechanism. The Pd dimers transformed into N,N-trans monomers of the type [Pd(o-C₆H₃RCH₂NMe₂)X(py)](py = pyridine). In contrast, the Pt counterparts afford N,N-cis species [Pt(o-C₆H₃RCH₂NMe₂)X(py)] under the same conditions. The geometry of the N,N-cis complex [Pt{o-C₆H₃(4-MeO)CH₂NMe₂}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)Å, respectively]. The crystal structure of trans-[(Buⁿ₃P)IPd-(µ-I)₂PdI(PBuⁿ₃)] has also been determined and used to account for its similar reactivity to [{Pd(o-C₆H₄CH₂NMe₂)I}₂] in the bridge-splitting reaction.