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

New metal(II)−thiolate complexes supported by the tetradentate ligand 1,5-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L8py2) have been synthesized and subjected to physical, spectroscopic, structural,
and computational characterization. The X-ray crystal structures of these complexes, [L8py2M(S-C6H4-p-CH3)]BPh4 (M = Co, Ni, Zn), reveal distorted square-pyramidal divalent metal ions with four equatorial
nitrogen donors from L8py2 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 (ν = k2[L8py2M(SAr)+][PhCH2Br])
for all complexes (where M = Fe, Co, Ni, or Zn). Of particular significance is the disparity between k2 for
M = Fe and Co versus k2 for M = Ni and Zn, in that k2 for M = Ni and Zn is ca. 10 times larger (faster) than
k2 for M = Fe and Co. An Eyring analysis of k2 for [L8py2Co(SAr)]+ and [L8py2Ni(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:  ΔH⧧ = 45(2) kJ mol-1, ΔS⧧ =
−144(6) J mol-1 K-1; M = Ni:  ΔH⧧ = 43(3) kJ mol-1, ΔS⧧ = −134(8) J mol-1 K-1). Electronic structure
calculations using density functional theory (DFT) reveal that the enhanced reaction rate for [L8py2Ni(SAr)]+ is rooted in a four-electron repulsion (or a “filled/filled interaction”) between a completely filled nickel(II) dπ 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 [L8py2Zn(SAr)]+ relative to that of [L8py2Ni(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.