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

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.