Electronic Structure Control of the Nucleophilicity of Transition Metal−Thiolate Complexes:  An Experimental and Theoretical Study

Abstract

New metal(II)-thiolate complexes supported by the tetradentate ligand 1,5-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)py(2)) have been synthesized and subjected to physical, spectroscopic, structural, and computational characterization. The X-ray crystal structures of these complexes, [L(8)py(2)M(S-C(6)H(4)-p-CH(3))]BPh(4) (M = Co, Ni, Zn), reveal distorted square-pyramidal divalent metal ions with four equatorial nitrogen donors from L(8)py(2) and axial p-toluenethiolate ligands. The reactions of the complexes with benzyl bromide produce isolable metal(II)-bromide complexes (in the cases of Co and Ni) and the thioether benzyl-p-tolylsulfide. This reaction is characterized by a second-order rate law (nu = k(2)[L(8)py(2)M(SAr)(+)][PhCH(2)Br]) for all complexes (where M = Fe, Co, Ni, or Zn). Of particular significance is the disparity between k(2) for M = Fe and Co versus k(2) for M = Ni and Zn, in that k(2) for M = Ni and Zn is ca. 10 times larger (faster) than k(2) for M = Fe and Co. An Eyring analysis of k(2) for [L(8)py(2)Co(SAr)](+) and [L(8)py(2)Ni(SAr)](+) reveals that the reaction rate differences are not rooted in a change in mechanism, as the reactions of these complexes with benzyl bromide exhibit comparable activation parameters (M = Co: DeltaH() = 45(2) kJ mol(-)(1), DeltaS() = -144(6) J mol(-)(1) K(-)(1); M = Ni: DeltaH() = 43(3) kJ mol(-)(1), DeltaS() = -134(8) J mol(-)(1) K(-)(1)). Electronic structure calculations using density functional theory (DFT) reveal that the enhanced reaction rate for [L(8)py(2)Ni(SAr)](+) is rooted in a four-electron repulsion (or a "filled/filled interaction") between a completely filled nickel(II) d(pi) orbital and one of the two thiolate frontier orbitals, a condition that is absent in the Fe(II) and Co(II) complexes. The comparable reactivity of [L(8)py(2)Zn(SAr)](+) relative to that of [L(8)py(2)Ni(SAr)](+) arises from a highly ionic zinc(II)-thiolate bond that enhances the negative charge density on the thiolate sulfur. DFT calculations on putative thioether-coordinated intermediates reveal that the Co(II)- and Zn(II)-thioethers exhibit weaker M-S bonding than Ni(II). These combined results suggest that while Ni(II) may serve as a competent replacement for Zn(II) in alkyl group transfer enzymes, turnover may be limited by slow product release from the Ni(II) center.

Nickel Superoxide Dismutase

“…The resulting structures showed little effect by addition of the proton, and in fact produced shorter Ni-S bond lengths. This is consistent with model studies that involve protonation [84], alkylation [56,[85][86][87], and oxidation [36,37,53,55,[88][89][90][91] of sulfur in nickel thiolate complexes and reveal that little structural change accompanies the chemical modifi cations. The computational results correlate well with the known model complexes and support the model for coordinated thiols in reduced NiSOD.…”

Zinc Thiolate Complexes [ZnLn(SR)]+ with Azamacrocyclic Ligands, Part III: The Influence of the Ligand Ln on the Reactivity of Zinc‐Bound Thiolate

“…A comparable approach has been successfully applied by Halfen, Brunold et al, who investigated the electronic structure of 4-methylphenylthiolate and some transition-metal complexes thereof by density functional calculations in order to obtain an explanation for the observed nucleophilicities. [23] An investigation that provided an accurate description of the observed reactivities would require more than gas-phase optimisations of the starting material complexes. Rather, a calculation of the transition state of the nucleophilic attack, including the modelling of solvent effects, had to be performed for the methylation reaction of every single compound.…”

Nucleophilicity of zinc-bound thiolates

“…[15] The influence of the nature of the metal cation on the nucleophilicity of p-toluenethiolate has been studied both experimentally and by DFT computation, revealing that nickel-and zinc-thiolate complexes are more reactive than ironand cobalt-thiolate complexes. [16] In the case of zinc complexes, both the composition of the ligand set, [17][18][19][20] in terms of donating capability and steric hindrance, and the presence of hydrogen bonding towards the reactive thiolate [21][22][23][24] have been shown to modulate the reactivity against electrophiles.…”