Subnanometer-Sized Copper Clusters: A Critical Re-evaluation of the Synthesis and Characterization of Cu₈(MPP)₄ (HMPP = 2-Mercapto-5-<i>n</i>-propylpyrimidine)

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-n-propylpyrimidine (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^-, NO₃^-, 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.