The copper(i)/copper(ii) transition in complexes with 8-alkylthioquinoline based multidentate ligands

Abstract

Three linear multidentate ligands incorporating terminal 8-thioquinoline rings and flexible polyethylene glycol or semi-rigid o-xylylene spacers have been synthesised: pentadentate 1,5-bis(8-quinolylsulfanyl)-3-oxapentane (OESQ), hexadentate 1,8-bis(8-quinolylsulfanyl)-3,6-dioxaoctane (ODSQ) and tetradentate 1,2-bis(quinolin-8-ylsulfanylmethyl)benzene (OBSQ). Reaction of these ligands with various copper salts afforded three families of copper(I) and copper(II) complexes with the general formula [Cu(L¹)(L²)]X_m: L¹ = OESQ and L²/X_m = H₂O/(BF₄⁻)₂4, DMF/(BF₄⁻)₂5, H₂O/(ClO₄⁻)₂6, NO₃⁻/NO₃⁻7, NO₃⁻/ClO₄⁻8, none/ BF₄⁻9; L¹ = ODSQ and L²/X_m = none/(BF₄⁻)₂10, none/(ClO₄⁻)₂11, none/(NO₃⁻)₂12, none/Cu(NO₃)₄²⁻13, none/ClO₄⁻14; L¹ = OBSQ and L²/X_m = ClO₄⁻/ClO₄⁻15, H₂O/(BF₄⁻)₂16, DMF/(ClO₄⁻)₂17, DMF/(BF₄⁻)₂18, NO₃⁻/ NO₃⁻19, H₂O/( NO₃⁻)₂20, DMSO/(ClO₄⁻)₂21, NO₃⁻/BF₄⁻22, none/BF₄⁻23. For comparison, three copper complexes [Cu(EtSQ)₂(ClO₄)₂] 1, [Cu(EtSQ)₂](ClO₄) 2 and [Cu(MeSQ)₂](ClO₄) 3 with the bidentate ligands 8-ethylsulfanylquinoline (EtSQ) and 8-methylsulfanylquinoline (MeSQ) have also been prepared and characterised. The solid-state structures of 1, 3, 7–9, 11, 13, 18·CH₃CN, 20, 21·CH₃CN, 22 and 23 were determined by X-ray diffraction. Compound 1 shows a distorted octahedral structure with two EtSQ ligands chelating Cu²⁺ in the equatorial plane and two ClO₄⁻ ions interacting in the axial positions. Compounds 7 and 8 exhibit a similarly distorted coordination environment except that one axial oxygen atom comes from the ligand OESQ and the other from the nitrate anion. The hexadentate ligand ODSQ in compounds 11 and 13 wraps around the central Cu²⁺ ion to form an approximately octahedral geometry resembling that in 1, however, one sulfur atom must move to the axial position and one ether oxygen atom binds in the equatorial plane. In the four compounds 18·CH₃CN, 20, 21·CH₃CN and 22, very similar square pyramidal geometries are observed around the central Cu²⁺ ions with four donor atoms from OBSQ occupying three corners of the basal plane and the apical position, the fourth corner being completed by a DMF, H₂O, DMSO or NO₃⁻ ligand, respectively. In the copper(I) complexes 3, 9 and 23 the metal centre is distorted tetrahedrally coordinated with two sulfur and two nitrogen donor atoms. The solution structures have been explored by UV–vis and EPR spectroscopy, the copper(II)/copper(I) transition and further reduction processes have been investigated by cyclic voltammetry and UV–vis spectroelectrochemistry. In contrast to the simple 8-alkylthioquinolines MeSQ and EtSQ the bis(N^S) ligands allow for a more reversible Cu^I/Cu^II transition due to the various effects of steric constraint and additional coordination as provided by the spacers.