Linear and cyclic tetranuclear copper(i) complexes containing anions of N,N′-bis(pyrimidine-2-yl)formamidine

Abstract

The reaction of Kpmf (pmf = anion of N,N′-bis(pyrimidyl-2-yl)formamidine, Hpmf) with CuSCN afforded the complexes K[Cu₄(pmF)₃(SCN)₂], 1, and Cu₄(pmf)₄, 2. Reaction of 1 with [(n-Bu)₄N]PF₆ in THF gave the complex [(n-Bu)₄N][Cu₄(pmf)₃(SCN)₂], 3. Their structures were characterized by X-ray crystallography. Complexes 1 and 3 are the first linear tetranuclear complexes containing only Cu(I) atoms, while complex 2 is cyclic. The four Cu(I) atoms of complexes 1 and 3 are helically bridged by three tetradentate pmf⁻ ligands. The [Cu₄(pmf)₃(SCN)₂]⁻ anions of 1 show weak interactions with adjacent [K(THF)₅]⁺ cations 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 1 and 3 in solution. By the DNMR analysis, the free energy of activation (ΔG_c^≠) for the exchange is 12.8 kcal mol⁻¹ at 278 K (T_c), and the rate constant of exchange (K_c) is 470 s⁻¹ for 1. The ΔG_c^≠ and K_c are 12.6 kcal mol⁻¹ at 273 K and 433 s⁻¹, respectively, for 3.