Copper(II)-promoted oxidation of mercaptocarboxylic acids. The characterization of disulphide- and oxalate-containing products. Crystal structures of [{Cu(bipy)(2,2′-dtdp)}n]·3nH2O and [{Cu(bipy)(3,3′-dtdp)}n]·2nH2O (bipy = 2,2′-bipyridine, dtdp = dithiodipropanoate)

Abstract

The reaction of 2- and 3-mercaptopropanoic acids with an aqueous solution of 2,2′-bipyridine (bipy) and copper(II) acetate dihydrate (molar ratio 1 : 2 : 1) under aerobic conditions yields the copper(II) disulphide complexes [{Cu(bipy)(2,2′-dtdp)}_n]·3nH₂O (2,2′-dtdp = 2,2′-dithiodipropanoate) and [{Cu(bipy)(3,3′-dtdp)}_n]·2nH₂O (3,3′-dtdp = 3,3′-dithiodipropanoate). Both complexes have been structurally characterized by single-crystal X-ray crystallography. Crystals of the complex [{Cu(bipy)(2,2′-dtdp)}_n]·3_nH₂O are triclinic, space group P, with a= 8.598(5), b= 10.378(6), c= 11.205(6)Å, α= 96.43(5), β= 94.34(4), γ= 91.16(5)°, and Z= 2. The structure consists of crystallographically equivalent Cu(bipy)²⁺ units linked by 2,2′-dtdp ligands into infinite chains. The lattice water molecules participate in a hydrogen-bonding network that connects the 2,2′-dtdp ligands in adjacent chains. Each copper atom has a distorted square-pyramidal co-ordination geometry with the two bipy nitrogen and two carboxylate oxygen atoms in equatorial positions [Cu–N(1) 1.998(6), Cu–N(2) 2.008(6), Cu–O(1) 1.925(5), and Cu–O(3) 1.981 (5)Å]. An apically bound disulphide sulphur completes the co-ordination sphere. The Cu–S distance [2.678(2)Å] is considered to be at the short end of the range expected for axial Cu–S (disulphide) bond lengths. Crystals of the complex [{Cu(bipy)(3,3′-dtdp)}_n]·2nH₂O are monoclinic, space group P2₁/n, with a= 9.686(4), b= 9.409(3), c= 21.231 (4)Å, β= 102.0(2)°, and Z= 4. The structure consists of crystallographically equivalent Cu(bipy)²⁺ centres linked by 3,3′-dtdp ligands into infinite zigzag chains. The chains are connected into sheets via carboxylate oxygen atoms which asymmetrically bridge adjacent copper atoms from neighbouring chains. The sheets are connected via a network of hydrogen bonds involving the lattice water molecules and carboxylate oxygen atoms. The copper atoms are in an irregular six-co-ordinate environment. The bipy nitrogens and two carboxylate oxygens occupy an approximate square plane [Cu–N(1) 1.988(3), Cu–N(2) 1.995(4) Cu–O(1) 1.934(3), and Cu–O(3) 1.961 (3)Å]. Axial bonding is achieved by a bridging carboxylate oxygen [Cu–O(3′) 2.231 (4)Å], which is also linked equatorially to an adjacent copper atom, and a weakly bound carboxylate oxygen (Cu–O(4) 2.826(5)Å]. In contrast to the above results, the reaction of sodium mercaptoacetate with 2,2′-bipyridine and copper(II) acetate dihydrate results in desulphurization of the mercaptoacetate and formation of a ternary oxalate (OX) complex [Cu(bipy)(OX)]·2H₂O.