Article pubs.acs.org/IC
Subnanometer-Sized Copper Clusters: A Critical Re-evaluation of the Synthesis and Characterization of Cu8(MPP)4 (HMPP = 2‑Mercapto5‑n‑propylpyrimidine) Thuy-Ai D. Nguyen, Andrew W. Cook, Guang Wu, and Trevor W. Hayton* Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States S Supporting Information *
ABSTRACT: We report a critical re-evaluation of the synthesis and characterization of Cu8(MPP)4. This product was reportedly formed by the reaction of Cu(NO3)2 with 2-mercapto-5-npropylpyrimidine (HMPP) and NaBH4, in ethanol, in the presence of [N(C8H17)4][Br]. In our hands, we found no experimental evidence to support the existence of Cu8(MPP)4 in the reaction mixture. Instead, we demonstrate that the material isolated from this reaction is a complex mixture containing [N(C8H17)4]+, Br−, NO 3 − , and 2-mercapto-5-n-propyl-1,6-dihydropyrimidine (H2MPP*), along with the Cu(I) coordination polymer, [Cu(MPP)]n. To support our conclusions, we have independently synthesized H2MPP* and [Cu(MPP)]n, as well as the related Cu(I) coordination complexes, [Cu(HMPP*)] n and [Cu2(MPP*)]n. All new materials were characterized by NMR spectroscopy and mass spectrometry, while H2MPP*, [Cu(HMPP*)]n (n = 4), and [Cu(MPP)]n (n = 6) were also characterized by X-ray crystallography.
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INTRODUCTION Atomically precise nanoclusters have recently received considerable attention for their unique properties and potential use in a wide variety of applications.1−5 Due to their monodispersity and solubility in a wide range of solvents, complete characterization of these materials is often possible, including analysis by X-ray crystallography.5 That said, nanoclusters have often proven harder to characterize than typical coordination complexes, on account of their large sizes and low yielding syntheses, which leads to challenges in generating analytically pure material.5 In this regard, examples abound in the chemical literature of mischaracterized nanoclusters.5 For example, the gold nanocluster, [Au36(SCH2CH2Ph)24], was reformulated to [Au25(SCH2CH2Ph)18]−, on the basis of ESI-MS data.6 Similarly, [Ag28(SG)16] (HSG = glutathione) was reformulated to [Au25(SG)18]− after further characterization.7 In other examples, Au144(SR)59 was reformulated to Au144(SR)60,8 while Au38(SR)22 was later reformulated to Au38(SR)24.9 In each of these examples, mass spectrometry was the critical technique used to establish the correct formula, demonstrating its importance for the characterization of nanoclusters. In 2011, Chen and co-workers reported the synthesis of “stable Cun (n ≤ 8) nanoclusters by a simple one-pot chemical reduction” in the Journal of the American Chemical Society.10 These clusters were stabilized by ligation to 2-mercapto-5-npropylpyrimidine (HMPP). Within these samples, Cu8(MPP)4 was claimed to be the “dominant Cu-containing component.”10 This claim was made on the basis of ESI-MS characterization. © 2017 American Chemical Society
We found the formulation of this complex to be particularly intriguing. First, it represents an exceptionally rare example of a low-valent (Cu oxidation state
