Structural and spectroscopic characterization of 17- and 18-electron piano-stool complexes of chromium. Thermochemical analyses of weak Cr-H bonds

Edwin F. Van Der Eide, Monte Helm, Eric D. Walter, R Morris Bullock

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Abstract

The 17-electron radical CpCr(CO)2(IMe) (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized by the reaction of IMe with [CpCr(CO)3]2, and characterized by single crystal X-ray diffraction and by electron paramagnetic resonance (EPR), IR, and variable temperature 1H NMR spectroscopy. The metal-centered radical is monomeric under all conditions and exhibits Curie paramagnetic behavior in solution. An electrochemically reversible reduction to 18-electron CpCr(CO) 2(IMe)- takes place at E1/2 = -1.89(1) V vs Cp2Fe+•/0 in MeCN, and was accomplished chemically with KC8 in tetrahydrofuran (THF). The salts K+(18-crown- 6)[CpCr(CO)2(IMe)]-·1/2THF and K+[CpCr(CO)2(IMe)]-·3/ 4THF were crystallographically characterized. Monomeric ion pairs are found in the former, whereas the latter has a polymeric structure because of a network of K·O(CO) interactions. Protonation of K +(18-crown-6)[CpCr(CO)2(IMe)] -·1/2THF gives the hydride CpCr(CO) 2(IMe)H, which could not be isolated, but was characterized in solution; a pKa of 27.2(4) was determined in MeCN. A thermochemical analysis provides the Cr-H bond dissociation free energy (BDFE) for CpCr(CO)2(IMe)H in MeCN solution as 47.3(6) kcal mol-1. This value is exceptionally low for a transition metal hydride, and implies that the reaction 2 [Cr-H] → 2 [Cr] + H2 is exergonic (ΔG = -9.0(8) kcal mol-1). This analysis explains the experimental observation that generated solutions of the hydride produce CpCr(CO)2(IMe) (typically on the time scale of days). By contrast, CpCr(CO)2(PCy3)H has a higher Cr-H BDFE (52.9(4) kcal mol-1), is more stable with respect to H 2 loss, and is isolable.

Original languageEnglish
Pages (from-to)1591-1603
Number of pages13
JournalInorganic Chemistry
Volume52
Issue number3
DOIs
Publication statusPublished - Feb 4 2013

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Chromium
Carbon Monoxide
chromium
Electrons
hydrides
electrons
free energy
dissociation
metal hydrides
Hydrides
tetrahydrofuran
electron paramagnetic resonance
transition metals
salts
Free energy
nuclear magnetic resonance
single crystals
diffraction
metals
spectroscopy

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry

Cite this

Structural and spectroscopic characterization of 17- and 18-electron piano-stool complexes of chromium. Thermochemical analyses of weak Cr-H bonds. / Van Der Eide, Edwin F.; Helm, Monte; Walter, Eric D.; Bullock, R Morris.

In: Inorganic Chemistry, Vol. 52, No. 3, 04.02.2013, p. 1591-1603.

Research output: Contribution to journalArticle

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abstract = "The 17-electron radical CpCr(CO)2(IMe)• (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized by the reaction of IMe with [CpCr(CO)3]2, and characterized by single crystal X-ray diffraction and by electron paramagnetic resonance (EPR), IR, and variable temperature 1H NMR spectroscopy. The metal-centered radical is monomeric under all conditions and exhibits Curie paramagnetic behavior in solution. An electrochemically reversible reduction to 18-electron CpCr(CO) 2(IMe)- takes place at E1/2 = -1.89(1) V vs Cp2Fe+•/0 in MeCN, and was accomplished chemically with KC8 in tetrahydrofuran (THF). The salts K+(18-crown- 6)[CpCr(CO)2(IMe)]-·1/2THF and K+[CpCr(CO)2(IMe)]-·3/ 4THF were crystallographically characterized. Monomeric ion pairs are found in the former, whereas the latter has a polymeric structure because of a network of K·O(CO) interactions. Protonation of K +(18-crown-6)[CpCr(CO)2(IMe)] -·1/2THF gives the hydride CpCr(CO) 2(IMe)H, which could not be isolated, but was characterized in solution; a pKa of 27.2(4) was determined in MeCN. A thermochemical analysis provides the Cr-H bond dissociation free energy (BDFE) for CpCr(CO)2(IMe)H in MeCN solution as 47.3(6) kcal mol-1. This value is exceptionally low for a transition metal hydride, and implies that the reaction 2 [Cr-H] → 2 [Cr•] + H2 is exergonic (ΔG = -9.0(8) kcal mol-1). This analysis explains the experimental observation that generated solutions of the hydride produce CpCr(CO)2(IMe)• (typically on the time scale of days). By contrast, CpCr(CO)2(PCy3)H has a higher Cr-H BDFE (52.9(4) kcal mol-1), is more stable with respect to H 2 loss, and is isolable.",
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T1 - Structural and spectroscopic characterization of 17- and 18-electron piano-stool complexes of chromium. Thermochemical analyses of weak Cr-H bonds

AU - Van Der Eide, Edwin F.

AU - Helm, Monte

AU - Walter, Eric D.

AU - Bullock, R Morris

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N2 - The 17-electron radical CpCr(CO)2(IMe)• (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized by the reaction of IMe with [CpCr(CO)3]2, and characterized by single crystal X-ray diffraction and by electron paramagnetic resonance (EPR), IR, and variable temperature 1H NMR spectroscopy. The metal-centered radical is monomeric under all conditions and exhibits Curie paramagnetic behavior in solution. An electrochemically reversible reduction to 18-electron CpCr(CO) 2(IMe)- takes place at E1/2 = -1.89(1) V vs Cp2Fe+•/0 in MeCN, and was accomplished chemically with KC8 in tetrahydrofuran (THF). The salts K+(18-crown- 6)[CpCr(CO)2(IMe)]-·1/2THF and K+[CpCr(CO)2(IMe)]-·3/ 4THF were crystallographically characterized. Monomeric ion pairs are found in the former, whereas the latter has a polymeric structure because of a network of K·O(CO) interactions. Protonation of K +(18-crown-6)[CpCr(CO)2(IMe)] -·1/2THF gives the hydride CpCr(CO) 2(IMe)H, which could not be isolated, but was characterized in solution; a pKa of 27.2(4) was determined in MeCN. A thermochemical analysis provides the Cr-H bond dissociation free energy (BDFE) for CpCr(CO)2(IMe)H in MeCN solution as 47.3(6) kcal mol-1. This value is exceptionally low for a transition metal hydride, and implies that the reaction 2 [Cr-H] → 2 [Cr•] + H2 is exergonic (ΔG = -9.0(8) kcal mol-1). This analysis explains the experimental observation that generated solutions of the hydride produce CpCr(CO)2(IMe)• (typically on the time scale of days). By contrast, CpCr(CO)2(PCy3)H has a higher Cr-H BDFE (52.9(4) kcal mol-1), is more stable with respect to H 2 loss, and is isolable.

AB - The 17-electron radical CpCr(CO)2(IMe)• (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized by the reaction of IMe with [CpCr(CO)3]2, and characterized by single crystal X-ray diffraction and by electron paramagnetic resonance (EPR), IR, and variable temperature 1H NMR spectroscopy. The metal-centered radical is monomeric under all conditions and exhibits Curie paramagnetic behavior in solution. An electrochemically reversible reduction to 18-electron CpCr(CO) 2(IMe)- takes place at E1/2 = -1.89(1) V vs Cp2Fe+•/0 in MeCN, and was accomplished chemically with KC8 in tetrahydrofuran (THF). The salts K+(18-crown- 6)[CpCr(CO)2(IMe)]-·1/2THF and K+[CpCr(CO)2(IMe)]-·3/ 4THF were crystallographically characterized. Monomeric ion pairs are found in the former, whereas the latter has a polymeric structure because of a network of K·O(CO) interactions. Protonation of K +(18-crown-6)[CpCr(CO)2(IMe)] -·1/2THF gives the hydride CpCr(CO) 2(IMe)H, which could not be isolated, but was characterized in solution; a pKa of 27.2(4) was determined in MeCN. A thermochemical analysis provides the Cr-H bond dissociation free energy (BDFE) for CpCr(CO)2(IMe)H in MeCN solution as 47.3(6) kcal mol-1. This value is exceptionally low for a transition metal hydride, and implies that the reaction 2 [Cr-H] → 2 [Cr•] + H2 is exergonic (ΔG = -9.0(8) kcal mol-1). This analysis explains the experimental observation that generated solutions of the hydride produce CpCr(CO)2(IMe)• (typically on the time scale of days). By contrast, CpCr(CO)2(PCy3)H has a higher Cr-H BDFE (52.9(4) kcal mol-1), is more stable with respect to H 2 loss, and is isolable.

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