Periodic Trends within a Series of Five-Coordinate Thiolate-Ligated [M~(II)(S~(Me2)N4(tren))]~+ (M = Mn, Fe, Co, Ni, Cu, Zn) Complexes, Including a Rare Example of a Stable Cu~(II)-Thiolate

摘要

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~(Me2)N4(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 sigma~* orbitals. The copper complex, [Cu~(II)(S~(Me2)N4(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 M~(2+) (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.