Intramolecular hydrogen bonding controls 1,3-N,S- vs. 1,5-S,S′-coordination in NiII complexes of N-thiophosphorylated thioureas RNHC(S)NHP(S)(OiPr)2

Abstract

Reaction of the deprotonated N-thiophosphorylated thioureas RNHC(S)NHP(S)(OiPr)₂ (R = Ph, HL^I; 2-MeC₆H₄–, HL^II; 2,6-Me₂C₆H₃–, HL^III; 2,4,6-Me₃C₆H₂–, HL^IV; Me–, HL^V) with Ni^II leads to complexes of the formula [NiL^I–V₂]. The molecular structures of the thioureasHL^II–V and the complexes [NiL^II–V₂] in the solid were elucidated by single-crystal X-ray diffraction analysis. In the complexes, the metal is found to be in a square planar trans-N₂S₂ ([NiL^II–IV₂]) environment formed by the CS sulfur atoms and the P–N nitrogen atoms, or in a square planar trans-S₂S′₂ ([NiL^V₂]) environment formed by the CS and PS sulfur atoms of two deprotonated ligands. DFT calculations confirmed that the [Ni(L^II–IV-N,S)₂] isomers are more stable (by 16–21 kcal mol⁻¹) than the corresponding [Ni(L^II–IV-S,S′)₂] conformers. The main reason for higher stability of the 1,3-N,S vs. 1,5-S,S′ isomers is the formation of intramolecular N–H⋯SP hydrogen bonds. In solution the complexes [Ni(L^II–V-N,S)₂] have an exclusive 1,3-N,S coordination, while the compound [Ni(L^I-N,S)₂] exhibits two isomers in the ¹H and ³¹P NMR spectra. The major species is assigned to the 1,3-N,S coordinated isomer, while the minor (∼25%) signals are due to the 1,5-S,S′ isomer. UV-Vis spectroscopic results are in line with this. The electrochemical measurements reveal reversible one-electron reduction and irreversible oxidations, both assigned to ligand-centred processes.