Light induced sulfur-dealkylation of phosphino-thioether nickel(0) complexes

Article abstract of DOI:10.1016/s0020-1693(96)05237-1

The observation of homolytic S-CH₃ bond cleavage in (Ph₂P(o-C₆H⁴)SCH₃) ₂Ni⁰ under photochemical conditions has prompted further investigation of nickel(0) complexes and their stability. Tetradentate P₂S′₂ donor ligands (S′ = thioether type S donor) with aromatic rings incorporated into the P to S links, Ph₂P(o-C₆H₄)S(CH₂) ₃S(o-C₆H₄)PPh₂ (arom-PSSP), or the S to S links, Ph₂P(CH₂)₂SCH₂(o-C₆H ₄) CH₂S (CH₂)₂PPh₂ (PS-xy-SP), have been used to form four-coordinate, square planar nickel (II) complexes, [(arom-PSSP)Ni] (BF₄)₂ (2) and [(PS-xy-SP)Ni] (BF₄)₂ (3). The bidentate and tetradentate ligands, Ph₂P(o-C₆H₄)SCH₂CH₃ (arom-PSEt) and Ph₂P(CH₂)₂S(CH₂) ₃S(CH₂)₂PPh₂ (PSSP), give similar complexes, [(arom-PSEt)₂Ni] (BF₄)₂ (1) and [(PSSP)Ni] (BF₄)₂ (4), respectively. Cyclic voltammograms of the Ni^II complexes in CH₃CN show two reversible redox events assigned to Ni^II/I and Ni^I/0. The one-electron reduction product produced by stoichiometric amounts of Cp₂Co can be characterized by EPR. At 100 K rhombic signals show hyperfine coupling to two phosphorus atoms. Complete bulk chemical reduction of complexes 1, 2, 3 and 4 with Na/Hg amalgam provided the corresponding nickel(0) complexes 1^R, 2^R, 3^R and 4^R which were isolated as red solutions or solids and characterized by magnetic resonance properties and reaction products. Photolysis of these nickel(0) complexes leads to S-dealkylation to produce alkyl radicals and dithiolate nickel(II) complexes. Complex 3 crystallized in the monoclinic space group P2₁/c with a = 20.740(5), b = 9.879(3), c = 17.801(4) ?, β = 92.59(2)°, V = 3644(2) ?³ and Z=4; complex 4: P2₁ with a = 13.815(4), b = 13.815(4), c = 15.457(5) ?, β = 104.89(2)°, V = 3365.4(14) ?³ and Z = 4.