Mono- and Binuclear Copper(I) Complexes of Thionucleotide Analogues and Their Catalytic Activity on the Synthesis of Dihydrofurans

The reaction of copper­(I) halides with 2-thiouracil (TUC), 6-methyl-2-thiouacil (MTUC), and 4-methyl-2-mercaptopyrimidine (MPMTH) in the presence of triphenylphosphine (tpp) in a 1:1:2 molar ratio results in a mixed-ligand copper­(I) complex with the formulas [Cu₂(tpp)₄(TUC)­Cl] (<b>1</b>), [Cu₂(tpp)₄(MTUC)­Cl] (<b>2</b>), [Cu­(tpp)₂(MPMTH)­Cl]·¹/₂CH₃OH (<b>3</b>), [Cu­(tpp)₂(MTUC)­Br] (<b>4</b>), and [Cu­(tpp)₂(MTUC)­I]·¹/₂CH₃CN (<b>5</b>). The complexes have been characterized by FT-IR, ¹H NMR, and UV–vis spectroscopic techniques and single-crystal X-ray crystallography. Complexes <b>1</b> and <b>2</b> are binuclear copper­(I) complexes. Two phosphorus atoms from tpp ligands are coordinated to the copper­(I) ions, forming two units that are linked to each other by a deprotonated TUC or MTUC chelating ligand through a sulfur bridge. A linear Cu–S–Cu moiety is formed. The tetrahedral geometry around the metal centers is completed by the nitrogen-donor atom from the TUC or MTUC ligand for the one unit, while for the other one, it is completed by the chloride anion. Two phosphorus atoms from two tpp ligands, one sulfur atom from MPMTH or MTUC ligand, and one halide anion (Cl, Br, and I) form a tetrahedron around the copper ion in <b>3</b>–<b>5</b> and two polymorphic forms of <b>4</b> (<b>4a</b> and <b>4b</b>). In all of the complexes, either mono- or binuclear intramolecular O–H···X hydrogen bonds enhance the stability of the structures. On the other hand, in almost all cases of mononuclear complexes (with the exception of a symmetry-independent molecule in <b>4a</b>), intermolecular NH···O hydrogen-bonding interactions lead to dimerization. Complexes <b>1</b>–<b>5</b> were studied for their catalytic activity for the intermolecular cycloaddition of iodonium ylides toward dihydrofuran formation by HPLC, ¹H NMR, and LC-HRMS spectroscopic techniques. The results show that the geometry and halogen and ligand types have a strong effect on the catalytic properties of the complexes. The highest yield of dihydrofurans was obtained when “linear” complexes <b>1</b> and <b>2</b> were used as the catalysts. The activity of the metal complexes on the copper­(I)-catalyzed and uncatalyzed intramolecular cycloaddition of iodonium ylide is rationalized through electronic structure calculation methods, and the results are compared with the experimental ones