2-Benzoylpyridine thiosemicarbazone as a novel reagent for the single pot synthesis of dinuclear CuI–CuII complexes: formation of stable copper(ii)–iodide bonds

Abstract

2-Benzoylpyridine thiosemicarbazone {R¹R²C²N²·N³H–C¹(S)–N⁴H₂, R¹ = py–N¹, R² = Ph; Hbpytsc} with copper(I) iodide in acetonitrile–dichloromethane mixture has formed stable Cu^II–I bonds in a dark green Cu^II iodo-bridged dimer, [Cu₂^II(μ-I)₂(η³-N¹,N²,S-bpytsc)₂] 1. Copper(I) bromide also formed similar Cu^II–Br bonds in a dark green Cu^II bromo-bridged dimer, [Cu₂^II(μ-Br)₂(η³-N¹,N²,S-bpytsc)₂] 3. The formation of dimers 1 and 3 appears to be due to a proton coupled electron transfer (PCET) process wherein copper(I) loses an electron to form copper(II), and this is accompanied by a loss of –N³H proton of Hbpytsc ligand resulting in the formation of anionic bpytsc⁻. When copper(I) iodide was reacted with triphenylphosphine (PPh₃) in acetonitrile followed by the addition of 2-benzoylpyridine thiosemicarbazone in dichloromethane (Cu : PPh₃ : Hbpytsc in the molar ratio 1 : 1 : 1), both Cu^II dimer 1 and an orange Cu^I sulfur-bridged dimer, [Cu₂^II₂(μ-S-Hbpytsc)₂(PPh₃)₂] 2 were formed. Copper(I) bromide with PPh₃ and Hbpytsc also formed Cu^II dimer 3 and an orange Cu^I sulfur-bridged dimer, [Cu₂^IBr₂(μ-S-Hbpytsc)₂(PPh₃)₂] 4. While complexes 2 and 4 exist as sulfur-bridged Cu^I dimers, 1 and 3 are halogen-bridged. The central Cu₂S₂ cores of 2 and 4 as well as Cu₂X₂ of 1 (X = I) and 3 (X = Br) are parallelograms. One set of Cu^II–I and Cu^II–Br bonds are short, while the second set is very long {1, Cu–I, 2.565(1), 3.313(1) Å; 3, Cu–Br, 2.391(1), 3.111(1) Å}. The Cu⋯Cu separations are long in all four complexes {1, 4.126(1); 2, 3.857(1); 3, 3.227(1); 4, 3.285(1) Å}, more than twice the van der Waals radius of a Cu atom, 2.80 Å. The pyridyl group appears to be necessary for stabilizing the Cu^II–I bond, as this group can accept π-electrons from the metal.