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Synthesis and Characterization of Atomically Precise Copper Nanoclusters


The reactivity of MCl3(TEMPO) (M = Fe, Al; TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) with a variety of lignin models, including 3,4-dimethoxybenzyl alcohol, 1-phenyl-2-phenoxyethanol and 1,2-diphenyl-2-methoxyethanol is investigated. FeCl3(TEMPO) is effective in cleanly converting these substrates to the corresponding aldehyde or ketone. AlCl3(TEMPO) is also able to oxidize these substrates, however in a few instances the products of over-oxidation are also observed. In contrast, 2-phenoxyethanol is not oxidized by MCl3(TEMPO); instead it likely coordinates to the metal center, forming a 2-phenoxyethoxide complex. Oxidation of activated alkanes by MCl3(TEMPO) suggests that the reactions proceed via an initial 1-electron concerted proton-electron transfer (CPET) event. Finally, reaction of TEMPO with FeBr3 in Et2O results in oxidation of the solvent.

The copper hydride clusters [Cu14H12(phen)6(PPh3)4][X]2 (X = Cl, OTf) are obtained in good yields by reaction of [(Ph3P)CuH]6 with 1,10-phenanthroline, in the presence of a halide or pseudohalide source. [Cu14H12(phen)6(PPh3)4][Cl]2 reacts with CO2 in CH2Cl2, in the presence of excess Ph3P, to form the formate complex, [(Ph3P)2Cu(κ2-O2CH)], along with [(phen)(Ph3P)CuCl].

[Cu25H22(PPh3)12]Cl and [Cu18H17(PPh3)10]Cl, are isolated from the reaction of Cu(OAc) and CuCl with Ph2SiH2, in the presence of PPh3. [Cu25H22(PPh3)12]Cl formally features partial Cu(0) character. Subsequent reaction with Ph2phen resulted in the isolation of [Cu29Cl4H22(Ph2phen)12]Cl (Ph2phen = 4,7-diphenyl-1,10-phenanthroline), in good yields. A time-resolved kinetic evaluation of the formation of [Cu29Cl4H22(Ph2phen)12]Cl reveals that the mechanism of cluster growth is initiated by rapid ligand exchange, followed by slower extrusion of CuCl monomer, transport, and subsequent capture by intact clusters.

Two Cu26 nanoclusters, tentatively formulated as [Cu26H17(PPh3)9(OAc)3] and [Cu26H22(PPh3)10(OAc)2], are isolated from the reaction of Cu(OAc) with Ph2SiH2, in the presence of PPh3. As formulated, [Cu26H17(PPh3)9(OAc)3] features a magic number N* = 6, which is unprecedented for a copper nanocluster. XANES supports an assignment of more Cu(0) character than [Cu25H22(PPh3)12]Cl (N* = 2) for this complex.

A critical reevaluation of the synthesis and characterization of Cu8(MPP)4 is reported. This product was reportedly formed by reaction of Cu(NO3)2 with 2-mercapto-5-n-propylpyrimidine (HMPP) and NaBH4, in ethanol, in the presence of [N(C8H17)4][Br]. However, upon reevaluation, no experimental evidence to support the existence of Cu8(MPP)4 was found. Instead, the material isolated from this reaction is a complex mixture containing [N(C8H17)4]+, Br-, NO3-, 2-mercapto-5-n-propyl-1,6-dihydropyrimidine (H2MPP*), along with the Cu(I) coordination polymer, [Cu(MPP)]n. H2MPP* and [Cu(MPP)]n, as well as the related Cu(I) coordination complexes, [Cu(HMPP*)]n and [Cu2(MPP*)]n are independently synthesized to support these conclusions.

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