Linear and cyclic tetranuclear copperi complexes containing anions of N,N′-bispyrimidine-2-ylformamidine

The reaction of Kpmf pmf anion of N,N′-bispyrimidyl-2-ylformamidine, Hpmf with CuSCN afforded the complexes KCu4pmF3SCN2, 1, and Cu4pmf4, 2. Reaction of 1with n-Bu4NPF6in THF gave the complex n-Bu4NCu4pmf3SCN2, 3. Their structures were characterized by X-ray crystallography. Complexes 1and 3are the first linear tetranuclear complexes containing only Cui atoms, while complex 2is cyclic. The four Cui atoms of complexes 1and 3are helically bridged by three tetradentate pmf−ligands. The Cu4pmf3SCN2−anions of 1show weak interactions with adjacent KTHF5cations through the sulfur atoms, forming infinite chains which are subjected to a series of intermolecular π–π interactions. In complex 2, the pmf−ligands are coordinated to the copper atoms in bidentate fashion through the two central amine nitrogen atoms, leaving the pyrimidine nitrogen atoms uncoordinated. Unexpected fluxional behaviors were observed for complexes 1and 3in solution. By the DNMR analysis, the free energy of activation ΔGc≠ for the exchange is 12.8 kcal mol−1at 278 K Tc, and the rate constant of exchange Kc is 470 s−1for 1. The ΔGc≠and Kcare 12.6 kcal mol−1at 273 K and 433 s−1, respectively, for 3.