Barriers to racemization in C3-symmetric complexes containing the hydrotris(2-mercapto-1-ethylimidazolyl)borate (TmEt) ligand

Philip J. Bailey, Alice Dawson, Chiara McCormack, Stephen A. Moggach, Iain D. H. Oswald, Simon Parsons, David W. H. Rankin, Andrew Turner

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Abstract

The tripodal ligands hydrotris(N-ethyl-2-mercaptoimidazol-1-yl)borate (NaTmEt) (1) and hydrotris(N-benzyl-2-mercaptoimidazol-1-yl)borate (NaTmBn) (2), analogues of the hydrotris(N-methyl-2-mercaptoimidazol- 1-yl)borate ligand (Tm) containing alternative nitrogen substituents, have been employed to examine the racemization of their C3-symmetric complexes with both four- and six-coordinate metals. The ligands react at room temperature with metal halides to provide C3-symmetric metal complexes. The syntheses of the four-coordinate complexes [TmEtZnCl] (3), [Tm EtCdBr] (4), [TmEtHgCl] (5), [TmEtCuPPh 3] (6), [TmEtAgPPh3] (7), and [Tm BnZnCl] (8) are reported. The six-coordinate complexes [Tm EtRu(p-cymene)]Cl (9), [TmEtRu(p-cymene)]PF6 (10), and [TmEtMn(CO)3] (11) were also synthesized. The X-ray crystal structures of 3, 4, 6, and 9 are reported. The diastereotopic nature of the ethyl and benzyl hydrogen atoms in the ligands allows the enantiomeric forms of these complexes to be distinguished by 1H NMR spectroscopy. Variable-temperature (VT) 1H NMR spectra have thus been used to investigate the energies of the racemization processes occurring in these chiral complexes. In solvents the activation energies to racemization for the four-coordinate complexes lay in the range of 53-77 kJ mol-1. In non-donor solvents the energies are reduced and a dissociative mechanism is therefore implicated. No interconversion could be observed by VT NMR for the six-coordinate complexes in any solvent. To further explore the racemization mechanisms ab initio density functional theory calculations have been conducted on the ground- and transition-state structures of representative six-coordinate [Mn(I)] and four-coordinate [Zn(II)] complexes following a proposed nondissociative mechanism of racemization. The calculated energy barriers to racemization are 163 and 121 kJ mol-1, respectively. It is concluded that the low-energy racemization of substitution-labile four-coordinate complexes occurs via a dissociative mechanism, while substitution-inert six-coordinate complexes experience a significantly higher barrier to racemization. Whether this is due to the operation of a dissociative mechanism with a higher activation barrier or to a nondissociative mechanism remains unknown.

LanguageEnglish
Pages8884-8898
Number of pages15
JournalInorganic Chemistry
Volume44
Issue number24
DOIs
Publication statusPublished - 28 Nov 2005

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Borates
borates
Ligands
ligands
Substitution reactions
Nuclear magnetic resonance
Metal halides
Methimazole
Energy barriers
Coordination Complexes
Carbon Monoxide
Temperature
Nuclear magnetic resonance spectroscopy
Density functional theory
Hydrogen
Nitrogen
Activation energy
Crystal structure
Metals
Chemical activation

Keywords

  • C3-symmetric complexes
  • hydrotris(methimazolyl)borate ligands
  • metals

Cite this

Bailey, Philip J. ; Dawson, Alice ; McCormack, Chiara ; Moggach, Stephen A. ; Oswald, Iain D. H. ; Parsons, Simon ; Rankin, David W. H. ; Turner, Andrew. / Barriers to racemization in C3-symmetric complexes containing the hydrotris(2-mercapto-1-ethylimidazolyl)borate (TmEt) ligand. In: Inorganic Chemistry. 2005 ; Vol. 44, No. 24. pp. 8884-8898.
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abstract = "The tripodal ligands hydrotris(N-ethyl-2-mercaptoimidazol-1-yl)borate (NaTmEt) (1) and hydrotris(N-benzyl-2-mercaptoimidazol-1-yl)borate (NaTmBn) (2), analogues of the hydrotris(N-methyl-2-mercaptoimidazol- 1-yl)borate ligand (Tm) containing alternative nitrogen substituents, have been employed to examine the racemization of their C3-symmetric complexes with both four- and six-coordinate metals. The ligands react at room temperature with metal halides to provide C3-symmetric metal complexes. The syntheses of the four-coordinate complexes [TmEtZnCl] (3), [Tm EtCdBr] (4), [TmEtHgCl] (5), [TmEtCuPPh 3] (6), [TmEtAgPPh3] (7), and [Tm BnZnCl] (8) are reported. The six-coordinate complexes [Tm EtRu(p-cymene)]Cl (9), [TmEtRu(p-cymene)]PF6 (10), and [TmEtMn(CO)3] (11) were also synthesized. The X-ray crystal structures of 3, 4, 6, and 9 are reported. The diastereotopic nature of the ethyl and benzyl hydrogen atoms in the ligands allows the enantiomeric forms of these complexes to be distinguished by 1H NMR spectroscopy. Variable-temperature (VT) 1H NMR spectra have thus been used to investigate the energies of the racemization processes occurring in these chiral complexes. In solvents the activation energies to racemization for the four-coordinate complexes lay in the range of 53-77 kJ mol-1. In non-donor solvents the energies are reduced and a dissociative mechanism is therefore implicated. No interconversion could be observed by VT NMR for the six-coordinate complexes in any solvent. To further explore the racemization mechanisms ab initio density functional theory calculations have been conducted on the ground- and transition-state structures of representative six-coordinate [Mn(I)] and four-coordinate [Zn(II)] complexes following a proposed nondissociative mechanism of racemization. The calculated energy barriers to racemization are 163 and 121 kJ mol-1, respectively. It is concluded that the low-energy racemization of substitution-labile four-coordinate complexes occurs via a dissociative mechanism, while substitution-inert six-coordinate complexes experience a significantly higher barrier to racemization. Whether this is due to the operation of a dissociative mechanism with a higher activation barrier or to a nondissociative mechanism remains unknown.",
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Bailey, PJ, Dawson, A, McCormack, C, Moggach, SA, Oswald, IDH, Parsons, S, Rankin, DWH & Turner, A 2005, 'Barriers to racemization in C3-symmetric complexes containing the hydrotris(2-mercapto-1-ethylimidazolyl)borate (TmEt) ligand' Inorganic Chemistry, vol. 44, no. 24, pp. 8884-8898. https://doi.org/10.1021/ic0505293

Barriers to racemization in C3-symmetric complexes containing the hydrotris(2-mercapto-1-ethylimidazolyl)borate (TmEt) ligand. / Bailey, Philip J.; Dawson, Alice; McCormack, Chiara; Moggach, Stephen A.; Oswald, Iain D. H.; Parsons, Simon; Rankin, David W. H.; Turner, Andrew.

In: Inorganic Chemistry, Vol. 44, No. 24, 28.11.2005, p. 8884-8898.

Research output: Contribution to journalArticle

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T1 - Barriers to racemization in C3-symmetric complexes containing the hydrotris(2-mercapto-1-ethylimidazolyl)borate (TmEt) ligand

AU - Bailey, Philip J.

AU - Dawson, Alice

AU - McCormack, Chiara

AU - Moggach, Stephen A.

AU - Oswald, Iain D. H.

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AU - Rankin, David W. H.

AU - Turner, Andrew

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N2 - The tripodal ligands hydrotris(N-ethyl-2-mercaptoimidazol-1-yl)borate (NaTmEt) (1) and hydrotris(N-benzyl-2-mercaptoimidazol-1-yl)borate (NaTmBn) (2), analogues of the hydrotris(N-methyl-2-mercaptoimidazol- 1-yl)borate ligand (Tm) containing alternative nitrogen substituents, have been employed to examine the racemization of their C3-symmetric complexes with both four- and six-coordinate metals. The ligands react at room temperature with metal halides to provide C3-symmetric metal complexes. The syntheses of the four-coordinate complexes [TmEtZnCl] (3), [Tm EtCdBr] (4), [TmEtHgCl] (5), [TmEtCuPPh 3] (6), [TmEtAgPPh3] (7), and [Tm BnZnCl] (8) are reported. The six-coordinate complexes [Tm EtRu(p-cymene)]Cl (9), [TmEtRu(p-cymene)]PF6 (10), and [TmEtMn(CO)3] (11) were also synthesized. The X-ray crystal structures of 3, 4, 6, and 9 are reported. The diastereotopic nature of the ethyl and benzyl hydrogen atoms in the ligands allows the enantiomeric forms of these complexes to be distinguished by 1H NMR spectroscopy. Variable-temperature (VT) 1H NMR spectra have thus been used to investigate the energies of the racemization processes occurring in these chiral complexes. In solvents the activation energies to racemization for the four-coordinate complexes lay in the range of 53-77 kJ mol-1. In non-donor solvents the energies are reduced and a dissociative mechanism is therefore implicated. No interconversion could be observed by VT NMR for the six-coordinate complexes in any solvent. To further explore the racemization mechanisms ab initio density functional theory calculations have been conducted on the ground- and transition-state structures of representative six-coordinate [Mn(I)] and four-coordinate [Zn(II)] complexes following a proposed nondissociative mechanism of racemization. The calculated energy barriers to racemization are 163 and 121 kJ mol-1, respectively. It is concluded that the low-energy racemization of substitution-labile four-coordinate complexes occurs via a dissociative mechanism, while substitution-inert six-coordinate complexes experience a significantly higher barrier to racemization. Whether this is due to the operation of a dissociative mechanism with a higher activation barrier or to a nondissociative mechanism remains unknown.

AB - The tripodal ligands hydrotris(N-ethyl-2-mercaptoimidazol-1-yl)borate (NaTmEt) (1) and hydrotris(N-benzyl-2-mercaptoimidazol-1-yl)borate (NaTmBn) (2), analogues of the hydrotris(N-methyl-2-mercaptoimidazol- 1-yl)borate ligand (Tm) containing alternative nitrogen substituents, have been employed to examine the racemization of their C3-symmetric complexes with both four- and six-coordinate metals. The ligands react at room temperature with metal halides to provide C3-symmetric metal complexes. The syntheses of the four-coordinate complexes [TmEtZnCl] (3), [Tm EtCdBr] (4), [TmEtHgCl] (5), [TmEtCuPPh 3] (6), [TmEtAgPPh3] (7), and [Tm BnZnCl] (8) are reported. The six-coordinate complexes [Tm EtRu(p-cymene)]Cl (9), [TmEtRu(p-cymene)]PF6 (10), and [TmEtMn(CO)3] (11) were also synthesized. The X-ray crystal structures of 3, 4, 6, and 9 are reported. The diastereotopic nature of the ethyl and benzyl hydrogen atoms in the ligands allows the enantiomeric forms of these complexes to be distinguished by 1H NMR spectroscopy. Variable-temperature (VT) 1H NMR spectra have thus been used to investigate the energies of the racemization processes occurring in these chiral complexes. In solvents the activation energies to racemization for the four-coordinate complexes lay in the range of 53-77 kJ mol-1. In non-donor solvents the energies are reduced and a dissociative mechanism is therefore implicated. No interconversion could be observed by VT NMR for the six-coordinate complexes in any solvent. To further explore the racemization mechanisms ab initio density functional theory calculations have been conducted on the ground- and transition-state structures of representative six-coordinate [Mn(I)] and four-coordinate [Zn(II)] complexes following a proposed nondissociative mechanism of racemization. The calculated energy barriers to racemization are 163 and 121 kJ mol-1, respectively. It is concluded that the low-energy racemization of substitution-labile four-coordinate complexes occurs via a dissociative mechanism, while substitution-inert six-coordinate complexes experience a significantly higher barrier to racemization. Whether this is due to the operation of a dissociative mechanism with a higher activation barrier or to a nondissociative mechanism remains unknown.

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