Intercluster exchanges leading to hydride-centered bimetallic clusters: a multi-NMR, X-ray crystallographic, and DFT study

Encouraged by the successful syntheses of alloy nanoclusters (or nanoparticles) via intercluster (or interparticle) reactions, herein we apply this methodology to prepare a series of bimetallic hydride clusters. Mixing of two clusters, [Ag-7(H){E2P((OPr)-Pr-i)(2)}(6)] (E = S, 1; Se, 3) and [Cu-7(H){E2P((OPr)-Pr-i)(2)}(6)] (E = S, 2; Se, 4), yields two series of hydride-centered bimetallic clusters, [CuxAg7-x(H){E2P((OPr)-Pr-i)(2)}(6)] (x = 0-7; E = S, 5; Se, 6). Their compositions are fully characterized by positive-mode ESI-MS spectrometry, multi-NMR spectroscopy, and the structures of [Cu6Ag(H){S2P((OPr)-Pr-i)(2)}(6)] (5a) and [CuAg6(H){Se2P((OPr)-Pr-i)(2)}(6)] (6a) by single crystal X-ray diffraction. The presence of individual compounds in solution is the result of a (dynamic) chemical equilibrium primarily driven by metal exchanges. In fact, the process of inter-cluster exchange of 1 and 2 leading to hydride-centered bimetallic clusters 5 can be monitored by concentration-dependent P-31 NMR spectroscopy of which the higher concentration of 1 in the reaction, the closer to its resonance will be the distribution, in accord with Le Chatelier's principle. The dynamic equilibrium is further confirmed by 2D exchange spectroscopy that reveals a stepwise process involving one metal exchange at a time. DFT calculations on a model series of clusters 6 show that silver prefers occupying the inner tetrahedral positions, while copper favors capping positions, in full agreement with the crystal structure of 5a and 6a.

Automated summary

Encouraged by successful syntheses of alloy nanoclusters through intercluster reactions, a series of bimetallic hydride clusters were prepared and characterized using ESI-MS, multi-NMR, and X-ray diffraction. Metal exchanges in solution drove a dynamic chemical equilibrium, as indicated by P-31 NMR spectroscopy and 2D exchange spectroscopy. DFT calculations confirmed the preferential metal positions within the clusters, as observed in crystal structures.