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

Reaction of the deprotonated N-thiophosphorylated thioureas RNHC(S)NHP(S)(OiPr)(2) (R = Ph, HLI; 2-MeC6H4-, HLII; 2,6-Me2C6H3-, HLIII; 2,4,6-Me3C6H2-, HLIV; Me-, HLV) with Ni-II leads to complexes of the formula [NiL2I-V]. The molecular structures of the thioureas HLII-V and the complexes [NiL2II-V] 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 ([NiL2II-IV]) environment formed by the C=S sulfur atoms and the P-N nitrogen atoms, or in a square planar trans-S2S'(2) ([NiL2V]) environment formed by the C=S and P=S sulfur atoms of two deprotonated ligands. DFT calculations confirmed that the [Ni(LII-IV-N,S)(2)] isomers are more stable (by 16-21 kcal mol(-1)) than the corresponding [Ni(LII-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 center dot center dot center dot S=P hydrogen bonds. In solution the complexes [Ni(LII-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 H-1 and P-31 NMR spectra. The major species is assigned to the 1,3-N,S coordinated isomer, while the minor (similar to 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.