Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex
In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O -activating iron enzymes. This work details an electronic-structure investigation of [Fe (O)(L )(NCMe)] (L = 3,9,14,20-tetraaza-1,6,12,1...
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| Published in: | Journal of the American Chemical Society Vol. 138; no. 43; p. 14312 |
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| Main Authors: | , , , , , , , , , , , |
| Format: | Journal Article |
| Language: | English |
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02.11.2016
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| ISSN: | 1520-5126, 1520-5126 |
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| Abstract | In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O
-activating iron enzymes. This work details an electronic-structure investigation of [Fe
(O)(L
)(NCMe)]
(L
= 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm
. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm
in the E(d
→ d
) excited state. Both values are similar to those measured for [Fe
(O)(TMC)(NCMe)]
(TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm
. We can unequivocally assign them to the ligand field transitions of d
→ d
, d
→ d
, and d
→ d
, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe
(O) (SR-TPA)(NCMe)]
(SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)
core of complex 1 have similar transition energies, whereas the excitation energy for d
→ d
is significantly higher (∼12 000 cm
for [Fe
(O)(SR-TPA)(NCMe)]
). Our results thus substantiate that the tetracarbene ligand (L
) of complex 1 does not significantly affect the bonding in the (FeO)
unit but strongly destabilizes the d
orbital to eventually lift it above d
. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed. |
|---|---|
| AbstractList | In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O2-activating iron enzymes. This work details an electronic-structure investigation of [FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm-1 in the E(dxy → dxz,yz) excited state. Both values are similar to those measured for [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm-1. We can unequivocally assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation energy for dxz,yz → dx2-y2 is significantly higher (∼12 000 cm-1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2-y2 orbital to eventually lift it above dz2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O2-activating iron enzymes. This work details an electronic-structure investigation of [FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm-1 in the E(dxy → dxz,yz) excited state. Both values are similar to those measured for [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm-1. We can unequivocally assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation energy for dxz,yz → dx2-y2 is significantly higher (∼12 000 cm-1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2-y2 orbital to eventually lift it above dz2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed. In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O -activating iron enzymes. This work details an electronic-structure investigation of [Fe (O)(L )(NCMe)] (L = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm . By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm in the E(d → d ) excited state. Both values are similar to those measured for [Fe (O)(TMC)(NCMe)] (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm . We can unequivocally assign them to the ligand field transitions of d → d , d → d , and d → d , respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe (O) (SR-TPA)(NCMe)] (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO) core of complex 1 have similar transition energies, whereas the excitation energy for d → d is significantly higher (∼12 000 cm for [Fe (O)(SR-TPA)(NCMe)] ). Our results thus substantiate that the tetracarbene ligand (L ) of complex 1 does not significantly affect the bonding in the (FeO) unit but strongly destabilizes the d orbital to eventually lift it above d . As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed. |
| Author | Ye, Shengfa Andris, Erik Roithová, Jana Navrátil, Rafael Kupper, Claudia Atanasov, Mihail Meyer, Steffen Neese, Frank Meyer, Franc Krahe, Oliver Mondal, Bhaskar Bill, Eckhard |
| Author_xml | – sequence: 1 givenname: Shengfa surname: Ye fullname: Ye, Shengfa organization: Max-Planck Institut für Chemische Energiekonversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany – sequence: 2 givenname: Claudia surname: Kupper fullname: Kupper, Claudia organization: Institut für Anorganische Chemie, Georg-August-Universität Göttingen , Tammannstr. 4, D-37077 Göttingen, Germany – sequence: 3 givenname: Steffen surname: Meyer fullname: Meyer, Steffen organization: Institut für Anorganische Chemie, Georg-August-Universität Göttingen , Tammannstr. 4, D-37077 Göttingen, Germany – sequence: 4 givenname: Erik surname: Andris fullname: Andris, Erik organization: Department of Organic Chemistry, Faculty of Science, Charles University in Prague , Hlavova 8, 128 43 Praha 2, Czech Republic – sequence: 5 givenname: Rafael surname: Navrátil fullname: Navrátil, Rafael organization: Department of Organic Chemistry, Faculty of Science, Charles University in Prague , Hlavova 8, 128 43 Praha 2, Czech Republic – sequence: 6 givenname: Oliver surname: Krahe fullname: Krahe, Oliver organization: Max-Planck Institut für Chemische Energiekonversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany – sequence: 7 givenname: Bhaskar surname: Mondal fullname: Mondal, Bhaskar organization: Max-Planck Institut für Chemische Energiekonversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany – sequence: 8 givenname: Mihail surname: Atanasov fullname: Atanasov, Mihail organization: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences , 1113 Sofia, Bulgaria – sequence: 9 givenname: Eckhard surname: Bill fullname: Bill, Eckhard organization: Max-Planck Institut für Chemische Energiekonversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany – sequence: 10 givenname: Jana surname: Roithová fullname: Roithová, Jana organization: Department of Organic Chemistry, Faculty of Science, Charles University in Prague , Hlavova 8, 128 43 Praha 2, Czech Republic – sequence: 11 givenname: Franc surname: Meyer fullname: Meyer, Franc organization: Institut für Anorganische Chemie, Georg-August-Universität Göttingen , Tammannstr. 4, D-37077 Göttingen, Germany – sequence: 12 givenname: Frank surname: Neese fullname: Neese, Frank organization: Max-Planck Institut für Chemische Energiekonversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany |
| BackLink | https://www.ncbi.nlm.nih.gov/pubmed/27682505$$D View this record in MEDLINE/PubMed |
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| Title | Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex |
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