Periodic Trends within a Series of Five-Coordinate Thiolate-Ligated [M^II(S^Me2N₄(tren))]⁺ (M = Mn, Fe, Co, Ni, Cu, Zn) Complexes, Including a Rare Example of a Stable Cu^II−Thiolate

A series of five-coordinate thiolate-ligated complexes [M^II(tren)N₄S^Me2]⁺ (M = Mn, Fe, Co, Ni, Cu, Zn; tren = tris(2-aminoethyl)amine) are reported, and their structural, electronic, and magnetic properties are compared. Isolation of dimeric [Ni^II(SN₄(tren)-RS^dang)]₂ (“dang”= dangling, uncoordinated thiolate supported by H bonds), using the less bulky [(tren)N₄S]^1- ligand, pointed to the need for gem-dimethyls adjacent to the sulfur to sterically prevent dimerization. All of the gem-dimethyl derivatized complexes are monomeric and, with the exception of [Ni^II(S^Me2N₄(tren)]⁺, are isostructural and adopt a tetragonally distorted trigonal bipyramidal geometry favored by ligand constraints. The nickel complex uniquely adopts an approximately ideal square pyramidal geometry and resembles the active site of Ni-superoxide dismutase (Ni-SOD). Even in coordinating solvents such as MeCN, only five-coordinate structures are observed. The M^II−S thiolate bonds systematically decrease in length across the series (Mn−S > Fe−S > Co−S > Ni−S ∼ Cu−S < Zn−S) with exceptions occurring upon the occupation of σ* orbitals. The copper complex, [Cu^II(S^Me2N₄(tren)]⁺, represents a rare example of a stable Cu^II−thiolate, and models the perturbed “green” copper site of nitrite reductase. In contrast to the intensely colored, low−spin Fe(III)−thiolates, the M(II)−thiolates described herein are colorless to moderately colored and high−spin (in cases where more than one spin-state is possible), reflecting the poorer energy match between the metal d- and sulfur orbitals upon reduction of the metal ion. As the d-orbitals drop in energy proceeding across the across the series M²⁺ (M= Mn, Fe, Co, Ni, Cu), the sulfur-to-metal charge-transfer transition moves into the visible region, and the redox potentials cathodically shift. The reduced M⁺¹ oxidation state is only accessible with copper, and the more oxidized M⁺⁴ oxidation state is only accessible for manganese.