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

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.

In an effort to address the synthetic challenge of preparing Pd nanoclusters, we have been searching for new ligands that can stabilize low-valent Pd. In this regard, the ketimide ligand (R2C=N -) appears to be a suitable choice. The ketimide ligand is known to be a good acceptor, 30,31 and thus should be able to stabilize low-valent metal clusters. Moreover, it is known to promote metal-metal bonding, as evidenced by the isolation of the bimetallic complexes   30,31,[33][34][35][36] Moreover, ketimides are useful co-ligands for olefin polymerization catalysts, [37][38][39]40,41 and they can serve as "masked" nitrenes, 42 which can mediate C-H bond activation chemistry when unmasked. They have also appeared as intermediates in C-N bond forming reactions. 43,44 Yet, despite these diverse roles, ketimides have not been previously used to stabilize APNCs.
Herein, we report the synthesis and characterization of the ketimide-stabilized Pd7 nanosheet, [Pd7(N=C t Bu2)6], demonstrating for the first time that ketimides can promote the formation of low-valent palladium APNCs.

Results and Discussion
Reaction of PdCl2(PhCN)2 with 2 equiv of Li(N=C t Bu2) in THF results in the formation of a dark green solution. Work-up of the reaction mixture results in the isolation of [Pd7(N=C t Bu2)6] (1), in 40% yield as deep green blocks (Scheme 1     45 These resonances are observed in a 2:9:9:9:6:9:9 ratio.
Both the chemical shifts and relative intensities are essentially identical to those observed for the Pt analogue, supporting our assignment. Experimentally determined bond lengths are indicated in blue.
Given the distribution of products in the reaction mixture, we hypothesize that the formation of 1 proceeds via the intermediacy of 2. Specifically, we propose that reaction of PdCl2(PhCN)2 with Li(N=C t Bu2) results in initial formation of 2 and HN=C t Bu2. Complex 2 then converts to low-valent 1 via oxidation of its ketimide ligands, concomitant with formation of t BuCN, isobutylene, and isobutane (Scheme 1). In situ NMR spectroscopic monitoring appears to confirm this hypothesis ( Figure S5). Thermochemical calculations at the DFT level are also consistent with this proposal, as the formation of Pd7 (2) was found to be highly exergonic (Scheme 2).
Moreover, we have previously demonstrated that thermolysis of M(N=C t Bu2)4 (M = Mn, Fe) results in ketimide oxidation and formation of Mn3(N=C t Bu2)6 and Fe2(N=C t Bu2)5, respectively, along with t BuCN, isobutane, and isobutylene. 34,35 Incidentally, our calculations also explain why the Pt analogue of 1, [Pt7(N=C t Bu2)6], is not experimentally observed during the reaction of PtCl2(1,5-COD) with Li(N=C t Bu2). 45 Its formation from the [( t Bu2C=N)Pt(-N,C-   Table 1 as well as   Table S4 and Figure S14 together with a short discussion in Supporting Information). Hence, complex 1 sustains a net diatropic ring-current of 10.6 nA/T (see also Figure 5 for the current density), which is comparable with that of benzene (11.      in C6D6 match closely to those previously reported for this material in toluene-d8 (Figures S6 and   S7). 67,68 To account for the reduction of the Pd centers in 1 upon formation of 3, we propose the ketimide ligands are oxidized, forming t BuCN, isobutane, and isobutylene as by-products. A similar process is occurring during the initial formation of 1 and further demonstrates the redox activity of the ketimide ligand. Significantly, the facile formation of Pd(0) upon decomposition of 1 suggests that it could be a useful catalyst precursor for a variety of cross coupling reactions. [69][70][71] Indeed, Pd nanoclusters and nanoparticles have previously been shown to act as reservoirs for catalytically-competent mono-metallic Pd(0) species. 72

Conclusions
The isolation of the Pd7 nanosheet, Pd7(N=C t Bu2)6 (1), via reaction of PdCl2(PhCN)2 and Li(N=C t Bu2), demonstrates that ketimides are effective at stabilizing low-valent group 10 nanoclusters, thereby broadening the scope of ligands that are known to promote the formation of this class of materials. Complex 1 is built around a [Pd7] 6+ core and its central Pd atom features a rare hexagonal planar coordination geometry. According to DFT calculations, 1 features a similar level of aromaticity as other previously reported 4d transition metal aromatic clusters, such as [Mo3O9] 2or [(Ar3P)3Pd3(SR)3] + , but 1 is unique because it is the first all-metal system with hexagonal (i.e., benzene-like) symmetry. Preliminary reactivity studies suggest that 1 can act as a source of Pd(0), a consequence of the facile oxidative decomposition of the ketimide ligands. Going forward, we plan to explore the small molecule reactivity of Pd7(N=C t Bu2)6, which, given the unique coordination environment of the central Pd(0) atom, could result in new modes of reactivity. In addition, we will explore the ability of the ketimide ligand to stabilize higher nuclearity Pd clusters, which could potentially be achieved by changing the alkyl substituents on the ketimide ligand. which was subsequently cooled to -25 ºC. Concurrently, 2 equiv of Li(N=C t Bu2) (82 mg, 0.56 mmol) was dissolved in THF (2 mL) to give a colorless solution, which was also cooled to -25 ºC.
Over the course of 1 min, the Li(N=C t Bu2) solution was added dropwise to a stirring solution of PdCl2(PhCN)2. The reaction mixture was then allowed to warm to room temeprature with sitrring.
The resulting mixture slowly turned to a dark forest green color. After 5h, the volatiles were removed in vacuo to give a dark green oily solid. This solid was dissolved in pentane (2 mL) and filtered through a Celite column supported on glass wool (0.5 cm × 2 cm) to give a clear dark green filtrate, while leaving a white precipitate on the Celite. The filter pad was washed with pentane (2

Reaction of 1 with PPh 3 :
To a stirring, deep green solution of 1 (24 mg, 0.016 mmol) in THF (2 mL) was added PPh3 (87 mg, 0.33 mmol) as a clear colorless THF solution (2 mL). The reaction mixture was stirred at room temperature for 48 h, whereupon the mixture lightened to pale green.
The volatiles were removed in vacuo to give a pale green solid. The solid was rinsed with hexanes (2 mL) to afford a yellow solid. The solid was then dissolved in toluene (5 mL) and the resulting solution was filtered through a Celite column supported on glass wool (0.5 cm × 2 cm) to give a clear yellow filtrate. The filtrate was concentrated to ca. 2 mL and layered with hexanes (5 mL).
Storage of this solution at -25 °C for 24 h resulted in the deposition of yellow blocks of Pd(PPh3)3 (3), which were isolated by decanting the supernatant (79 mg, 82%). The 1 H and 31 P NMR spectra of 3 were in good agreement with the reported spectra. 67

ASSOCIATED CONTENT
 Experimental and computational details and additional data/analysis for complexes 1 and