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A Ketimide-Stabilized Palladium Nanocluster with a Hexagonal Aromatic Pd7 Core

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posted on 08.01.2020, 19:36 by Andrew W. Cook, Peter Hrobárik, Peter L. Damon, Guang Wu, Trevor W. Hayton
Herein, we report the synthesis and characterization of the mixed-valent, ketimide-stabilized Pd7 nanosheet, [Pd7(NCtBu2)6] (1), via reaction of PdCl2(PhCN)2 and Li­(NCtBu2). tBuCN, isobutylene, and isobutane are also formed in the reaction. The presence of these products suggests that Li­(NCtBu2) acts as a reducing agent in the transformation, converting the Pd­(II) starting material into the mixed-valent Pd­(I)/Pd(0) product. Complex 1 features a hexagonal planar [Pd7]6+ core stabilized by six ketimide ligands, which surround the [Pd7]6+ center in an alternating up/down fashion. In situ NMR spectroscopic studies, as well as density functional theory (DFT) calculations, suggest that 1 is formed via the intermediacy of the bimetallic Pd­(II) ketimide complex, [(tBu2CN)­Pd­(μ-N,C–NC­(tBu)­C­(Me)2CH2)­Pd­(NCtBu2)] (2). DFT calculations also reveal that 1 is a rare example of an all-metal aromatic nanocluster with hexagonal symmetry, sustaining a net diatropic ring-current of 10.6 nA/T, which is similar to that of benzene (11.8 nA/T) or other well-established transition-metal aromatic systems. Finally, we have found that 1 reacts with Ph3P, cleanly forming the tris-ligated 16-electron Pd(0) phosphine complex, Pd­(PPh3)3 (3), suggesting that 1 could be a useful precatalyst for a variety of cross-coupling reactions.