(μ-η²:η²-Disulfido)dinickel(II) complexes supported by 6-methyl-TPA ligands

A series of (μ-η²:η²-disulfido)dinickel(II) complexes 2ⁿ have been synthesized by the reaction of Na₂S₂ and nickel(II) complexes 1ⁿ supported by tris[(pyridin-2-yl)methyl]amine (TPA; 1⁰) [(6-methylpyridin-2-yl)methyl]bis[(pyridin-2-yl)methyl]amine (Me₁TPA; 1¹), bis[(6-methylpyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine (Me₂TPA; 1²) and tris[(6-methylpyridin-2-yl)methyl]amine (Me₃TPA; 1³), respectively, and characterised by UV-vis and resonance Raman spectroscopy. X-ray crystallographic analyses on 2² and 2³ supported by Me₂TPA and Me₃TPA, respectively, have revealed that the Ni₂S₂ core is largely bent (30°) along the S–S axis, being in sharp contrast to the planar Ni₂S₂ core structure of the (μ-η²:η²-disulfido)dinickel(II) complexes reported so far. The UV-vis spectra of 2⁰ and 2¹ supported by TPA and Me₁TPA, respectively, exhibiting an intense absorption band at 360 nm together with a shoulder around 400 nm and a weak and broad absorption band around 450–600 nm, are very close to those of 2² and 2³, suggesting that 2⁰ and 2¹ also exhibit a similar distorted Ni₂S₂ core structure. Resonance Raman spectra of 2ⁿ showed a characteristic S–S stretching vibration mode at 450 cm⁻¹ with an isotope shift Δν(³²S–³⁴S) = 10–15 cm⁻¹. The reaction of 2ⁿ with (p-Me-C₆H₄)₃P gave (p-Me-C₆H₄)₃PS quantitatively based on 2ⁿ. Hammett analysis on the sulfur atom transfer process from 2ⁿ to the phosphine derivatives [(p-X-C₆H₄)P; X = OMe, Me, H and Cl] has indicated that the reaction involves an electrophilic ionic mechanism. Moreover, the order of reactivity of 2ⁿ toward PPh₃ has been found as 2⁰ > 2³ > 2¹ > 2². On the basis of these results, the ligand effects of Me_nTPA on the structure and reactivity of the nickel(II) complexes have been discussed.