Electronic Structure Control of the Nucleophilicity of Transition Metal-Thiolate Complexes: An Experimental and Theoretical Study
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- 作者：Derek C. Fox ; Adam T. Fiedler ; Heather L. Halfen ; Thomas C. Brunold ; and Jason A. Halfen
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：June 23, 2004
- 年：2004
- 卷：126
- 期：24
- 页码：7627 - 7638
- 全文大小：267K
- 年卷期：v.126,no.24(June 23, 2004)
- ISSN：1520-5126
文摘
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 equatorialnitrogen donors from L8py2 and axial p-toluenethiolate ligands. The reactions of the complexes with benzylbromide 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 (
= k2[L8py2M(SAr)+][PhCH2Br])for all complexes (where M = Fe, Co, Ni, or Zn). Of particular significance is the disparity between k2 forM = Fe and Co versus k2 for M = Ni and Zn, in that k2 for M = Ni and Zn is ca. 10 times larger (faster) thank2 for M = Fe and Co. An Eyring analysis of k2 for [L8py2Co(SAr)]+ and [L8py2Ni(SAr)]+ reveals that thereaction rate differences are not rooted in a change in mechanism, as the reactions of these complexeswith benzyl bromide exhibit comparable activation parameters (M = Co: 
H
= 45(2) kJ mol-1, S =-144(6) J mol-1 K-1; M = Ni: H = 43(3) kJ mol-1, S = -134(8) J mol-1 K-1). Electronic structurecalculations 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) d
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 ahighly ionic zinc(II)-thiolate bond that enhances the negative charge density on the thiolate sulfur. DFTcalculations on putative thioether-coordinated intermediates reveal that the Co(II)- and Zn(II)-thioethersexhibit weaker M-S bonding than Ni(II). These combined results suggest that while Ni(II) may serve as acompetent replacement for Zn(II) in alkyl group transfer enzymes, turnover may be limited by slow productrelease from the Ni(II) center.
= k2[L8py2M(SAr)+][PhCH2Br])for all complexes (where M = Fe, Co, Ni, or Zn). Of particular significance is the disparity between k2 forM = Fe and Co versus k2 for M = Ni and Zn, in that k2 for M = Ni and Zn is ca. 10 times larger (faster) thank2 for M = Fe and Co. An Eyring analysis of k2 for [L8py2Co(SAr)]+ and [L8py2Ni(SAr)]+ reveals that thereaction rate differences are not rooted in a change in mechanism, as the reactions of these complexeswith benzyl bromide exhibit comparable activation parameters (M = Co: H = 45(2) kJ mol-1, S =-144(6) J mol-1 K-1; M = Ni: H = 43(3) kJ mol-1, S = -134(8) J mol-1 K-1). Electronic structurecalculations 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) d 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 ahighly ionic zinc(II)-thiolate bond that enhances the negative charge density on the thiolate sulfur. DFTcalculations on putative thioether-coordinated intermediates reveal that the Co(II)- and Zn(II)-thioethersexhibit weaker M-S bonding than Ni(II). These combined results suggest that while Ni(II) may serve as acompetent replacement for Zn(II) in alkyl group transfer enzymes, turnover may be limited by slow productrelease from the Ni(II) center.