Ligand Oxidations in High-Spin Nickel Thiolate Complexes and Zinc Analogues

Oxidations of a trigonal-bipyramidal, high-spin Ni(II) dithiolate complex of a pentadentate, N3S2-donor ligand,
N1,N9-bis(imino-2-mercaptopropane)-1,5,9-triazanonane) nickel(II), and the structurally analogous Zn(II) complex,
lead to oxidations of the ligand. Oxidation of the Ni(II) complex with I2 produces a novel Ni(II) macrocyclic cationic
complex containing a monodentate disulfide ligand (2). Crystals of the I3- salt of the complex form in the triclinic
space group P1 with cell dimensions a = 8.508(3) Å, b = 9.681(2) Å, c = 14.066(4) Å, angles α = 90.97(2)°,
β = 91.61(3)°, γ = 90.83(2)°, and Z = 2. The structure was refined to R = 6.31% and Rw = 16.63% (I > 2σ(I)).
Oxidation of the Ni(II) complex with O2 leads to the formation of a novel pentadentate bis-iminothiocarboxylate
complex with trigonal-bipyramidal geometry (3). This neutral product crystallizes in the monoclinic space group
P21/c with cell dimensions a = 13.625(3) Å, b = 7.605(5) Å, c = 14.902(4) Å, angles α = γ = 90°, β =
102.81(2)°, and Z = 4. The structure was refined to R = 7.18% and Rw = 17.86% (I > 2σ(I)). Oxidation of the
Zn(II) dithiolate analogue with O2 leads to the formation of the Zn(II) complex of the pentadentate bis-iminothiocarboxylate ligand. The neutral complex is isomorphous with the Ni(II) complex and crystallizes in the
monoclinic space group P21/c with cell dimensions a = 13.8465(4) Å, b = 7.6453(2) Å, c = 15.0165(6) Å, angles
α = γ = 90°, β = 103.2140(11)°, and Z = 4. The structure was refined to R = 3.96% and Rw = 9.45% (I >
2σ(I)). Details of the crystal structures are reported. Kinetics of the O2 reactions show that the reactions of the
Ni(II) and Zn(II) dithiolates follow the rate law, Rate = k2[1][O2], with k2 = 1.81 M-1 s-1 for the Ni(II) complex and
k2 = 1.93 × 10-2 M-1 s-1 for the Zn(II) complex. The O2 oxidation of the high-spin Ni(II) thiolate complex was
found to follow a similar oxidation mechanism to those of low-spin Ni(II) complexes, which form transient persulfoxide
intermediates that yield S-oxidation products. In the case of the high-spin system reported here, the transient
persulfoxide intermediate gives rise to an alternative ligand oxidation product, a bis-iminothiocarboxylate complex,
because of the reactivity of the ligand, which contains a methylene with acidic H atoms α to the thiolate sulfur. The
proposed mechanism is supported by studies of the analogous Zn dithiolate complex, which gives rise to the
analogous bis-iminothiocarboxylate product (5).