Twists and Turns of Platinum-Allene Complexes: NMR Techniques for the Study of the Dynamic Behavior in Solution

Classic (dynamic exchange line-shape analysis) and novel (SSTD NMR) NMR techniques have been applied in order to obtain the kinetic and thermodynamic parameters of the three main processes occurring in the fluxional behavior of Pt-allene complexes with N-containing ligands, in four and five coordination mode, in solution. Our results show intramolecular helical and rotational movements closely related to each other, confirming η-staggered structures as possible intermediates. The ligand exchange in these complexes seems to occur via a ligand-independent dissociative mechanism, where coordinating solvents might be involved in the stabilization of the intermediates. The differences observed in the interaction of allenes with other metals could be the basis to explain the divergent reactivity observed in platinum-catalyzed processes. ■ INTRODUCTION The mode of coordination in metal-allene complexes is crucial for the understanding of the reactivity of these important systems. η-Coordination of the metal with one of the double bonds is commonly proposed as the ground state (Scheme 1), with different slippage structures toward the central or terminal carbons involved depending on the metal, the ligands, and the substitution on the allenes. More recently, η-metal-allene complexes have been proposed as high energy intermediates or transition states in metal-catalyzed reactions involving allenes. In this mode, the metal coordinates exclusively to the central carbon of the allene, and several structures have been proposed, detected, and even isolated: zwitterionic carbene, σ-allyl cation, or η-bent allenes (Scheme 1). Although a lot of attention has been paid to Au-allene complexes, studies on Pt-allene complexes are still rare. Although platinum exhibits similar reactivity to gold in general, there are increasing examples where, under similar conditions, these two metals get involved in different reaction pathways and give different products. For example, we have reported the Pt-catalyzed dihydroalkoxylation and bisindolylation of allenes, which give acetals or bisindolyl alkanes with double addition of the nucleophile to the terminal or less substituted double bond of the allene, and complete saturation of the second double bond, instead of the most common allyl derivatives obtained under gold catalysis. To explain the different reactivity of the two metals, different mechanisms and metallic intermediates have to be proposed for the two processes. As part of our mechanistic studies on these reactions, we sought to gather experimental evidence on the type of coordination of the platinum and the allenes, in order to understand the subtle differences in reactivities observed in platinum catalysis, and to ascertain the involvement of ηcoordinated allene complexes as reactive species in our catalytic cycle. As the isolation and analysis of PtCl2-allene complexes without stabilizing ligands is very challenging, for this study, we have selected a family of Pt-allene complexes with ancillary nitrogen-containing ligands in a fouror five-coordinated arrangement, active catalysts in our dihydroalkoxylation reaction, and we have studied them in solid state and in solution using different NMR techniques, including our recently reported SSTD NMR method. Three processes are proposed for the behavior of metalallene complexes in solution: the ligand exchange, the rotational movement (π-face exchange, rotation of the metal around the allene), and the helical movement (the movement of the metal Received: October 10, 2016 Scheme 1. Metal-Allene Coordination Modes Article pubs.acs.org/Organometallics © XXXX American Chemical Society A DOI: 10.1021/acs.organomet.6b00778 Organometallics XXXX, XXX, XXX−XXX This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.