Crucial Influence of the Intramolecular Hydrogen Bond on the Coordination Mode of RC(S)NHP(S)(OiPr)2 in Homoleptic Complexes with NiII



Reaction of the deprotonated N-thiophosphorylated thioureas RC(S)NHP(S)(OiPr)2 [R = EtNH (HLI), iPrNH (HLII), Et2N (HLIII), 2,5-Me2C6H3NH (HLIV), 4-Me2NC6H4NH (HLV)] with NiII leads to complexes of the formula [NiLI–V2]. The molecular structures of the complexes in the solid were elucidated by single-crystal X-ray diffraction analysis. In the complexes, the metal atom is found to be in a square-planar trans-N2S2 ([NiLII,IV2]) environment formed by the C=S sulfur atoms and the P–N nitrogen atoms, or in a square-planar trans-S2S′2 ([NiLI,III2]) environment formed by the C=S and P=S sulfur atoms of two deprotonated ligands. Reaction of deprotonated N-thiophosphorylated thiourea HLV with NiCl2 leads to violet [Ni(L-1,3-N,S)2] or dark violet [Ni(L-1,5-S,S′)2]·(CH3)2C=O crystals that were isolated by recrystallization from a mixture of CH2Cl2 or acetone, respectively, and n-hexane. DFT calculations confirmed that the [Ni(LI,II,IV,V-N,S)2] conformers are more stable (by 5–7 kcal/mol) than [Ni(LI,II,IV,V-S,S′)2], whereas [Ni(LIII-N,S)2] is less stable (by 7–9 kcal/mol) than [Ni(LIII-S,S′)2]. The main reason for higher stability of the 1,3-N,S versus 1,5-S,S′ isomers is the formation of intramolecular N–H···S=P hydrogen bonds. The same hydrogen bonds are impossible in complex [NiLIII2]. In solution, complex [NiLIII2] has revealed an exclusively 1,5-S,S′ coordination, whereas compounds [NiLI,II,IV,V2] reveal at least two isomers in the 1H and 31P{1H} NMR spectra. The major species is assigned to the 1,3-N,S-coordinated isomer, and the minor signals are assigned to the 1,5-S,S′ isomer, which was confirmed by UV/Vis spectroscopic results. The electrochemical measurements reveal reversible one-electron reduction and irreversible oxidations both assigned to ligand-centred processes. Ligand-based oxidation processes agree well with TD-DFT results.