Walking metals in d ~8?d ~8 hetero-bimetallic complexes: An original dynamic phenomenon

摘要

In the course of our investigations on polymetallic complexes derived from 1,3-bis(thiophosphinoyl)indene (Ind(Ph _2P=S) _2), we observed original fluxional behavior and report herein a joint experimental/ computational study of this dynamic process. Starting from the indenylidene chloropalladate species [Pd{Ind(Ph _2P=S) _2}Cl] ~- (1), the new Pd ~II?Rh ~I hetero-bimetallic pincer complex [PdCl{Ind(Ph _2P=S) _2}Rh(nbd)] (2; nbd=2,5-norbornadiene) was prepared. X-ray crystallography and DFT calculations substantiate the presence of a d ~8?d ~8 interaction. According to multinuclear variable-temperature NMR spectroscopic experiments, the pendant {Rh(nbd)} fragment of 2 readily shifts in solution at room temperature between the two edges of the SCS tridentate ligand. To assess the role of the pincer-based polymetallic structure on this fluxional behavior, the related monometallic Rh complex [Rh{IndH(Ph _2P=S) _2}(nbd)] (3) was prepared. No evidence for a metal shift was observed in that case, even at high temperature, thus indicating that inplane pincer coordination to the Pd center plays a crucial role. The previously described Pd ~II? Ir ~I bimetallic complex 4 exhibited fluxional behavior in solution, but with a significantly higher activation barrier than 2. This finding demonstrates the generality of this metal-shift process and the strong influence of the involved metal centers on the associated activation barrier. DFT calculations were performed to shed light onto the mechanism of such metal-shift processes and to identify the factors that influence the associated activation barriers. Significantly different pathways were found for bimetallic complexes 2 and 4 on one hand and the monometallic complex 3 on the other hand. The corresponding activation barriers predicted computationally are in very good agreement with the experimental observations. A wandering metal: An original metal walk has been evidenced and analyzed. Bimetallic d ~8? d ~8 complexes [ML _n=Rh(nbd) or Ir(coe) _2] (nbd=2,5-norbornadiene, coe=cyclooctene) derived from 1,3-bis(thiophosphinoyl) indene are fluxional in solution (see scheme). The pendant metal center shifts between the two edges of the pincer ligand, as substantiated by variable-temperature NMR experiments and DFT calculations. This dynamic process appears to be quite general and it is facilitated by the inplane coordination to Pd.