Dicopper(II) complexes of a 28-membered N8 macrocycle. Evidence for proton transfer from co-ordinated secondary amine to thiocyanate, leading to formation of an lsothiocyanic acid complex

Abstract

Schiff-base template condensation of 2,6-diacetylpyridine with the diamine 3,3′-diaminodipropylamine on Ag⁺, Pb²⁺, Ca²⁺, or Sr²⁺ led to binuclear Ag⁺ and Pb²⁺ and mononuclear Ca²⁺ and Sr²⁺ complexes of the 28-membered ligand H₂L. A series of binuclear Cu^II complexes of this macrocycle, including [Cu₂(H₂L)(NCS)₄], were prepared by transmetallation in acetonitrile solution. A complex of formula Cu₂(HL)(BPh₄)₃·2MeCN was prepared from [Sr(H₂L)][BPh₄]₂ and Cu(O₂CMe)₂ in the presence of excess NaBPh₄. This cation was characterised by e.s.r. as a dicopper(II) complex of the singly deprotonated ligand H L^–. The complex [Cu₂(L)(HNCS)₂][BPh₄]₂·1.5MeCN was prepared, which showed ν_asym(NCS) at 1 992 cm^–1, characteristic of bridging thiocyanate. A single-crystal X-ray structure determination was carried out. Crystals were triclinic, space group P with a= 14.695(11), b= 14.051(11), c= 20.721(11)Å, α=111.5(1), β= 98.8(1), γ= 100.3(1)°, and Z= 2.3 969 Independent reflections above background have been measured on a diffractometer and the structure refined to R= 0.089. The unit cell contains two independent cations both with imposed symmetry, four BPh₄^– anions and solvent molecules. In both cations the Cu ⋯ Cu distance is 7.25 Å. Each copper atom is bonded to four nitrogen atoms of the macrocycle and nitrogen of a terminal thiocyanate. The Cu–N–C(S) angles were unusually small at 100(2) and 109(2)°. We believe that the explanation for the correlated phenomenon of small CU–N–C angle and a low value for the ν_asym(NCS) absorption is that proton transfer has occurred from the co-ordinated NH group to the NCS ion, making this a complex of thiocyanic acid with the macrocyclic ligand being in the doubly deprotonated form L^2–.