Crucial Influence of the Intramolecular Hydrogen Bond on the Coordination Mode of RC(S)NHP(S)(OiPr)_2 in Homoleptic Complexes with Ni~(II)

摘要

Reaction of the deprotonated N-thiophosphorylated thioureas RC(S)NHP(S)(OiPr)_2 [R = EtNH (HL~I), iPrNH (HL~(II)), Et_2N (HL~(III)), 2,5-Me_2C_6H_3NH (HL~(IV)), 4-Me_2NC_6H_4NH (HL~V)] with Ni~(II) leads to complexes of the formula [NiL~(I-V)_2]. 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-N_2S_2 ([NiL~(II,IV)_2]) environment formed by the C=S sulfur atoms and the P-N nitrogen atoms, or in a square-planar trans-S_2S'_2 ([NiL~(I,III)_2]) environment formed by the C=S and P=S sulfur atoms of two deprotonated ligands. Reaction of deprotonated N-thiophosphorylated thiourea HLV with NiCl_2 leads to violet [Ni(L-1,3-N,S)_2] or dark violet [Ni(L-1,5-S,S')_2]? (CH_3)_2C=O crystals that were isolated by recrystallization from a mixture of CH_2Cl_2 or acetone, respectively, and nhexane. DFT calculations confirmed that the [Ni(L~(I,II,IV,V)-N,S)_2] conformers are more stable (by 5-7 kcal/mol) than [Ni- (L~(I,II,IV,V)-S,S')_2], whereas [Ni(L~(III)-N,S)_2] is less stable (by 7- 9 kcal/mol) than [Ni(L~(III)-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 [NiL~(III)_2]. In solution, complex [NiL~(III)_2] has revealed an exclusively 1,5-S,S'coordination, whereas compounds [NiL~(I,II,IV,V)_2] reveal at least two isomers in the ~1H and ~(31)P{~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.