Five-coordinate MII-semiquinonate (M = Fe, Mn, Co) complexes: Reactivity models of the catechol dioxygenases

Article abstract of DOI:10.1039/c3cc49143a

A series of five-coordinate M^II-semiquinonate (M = Fe, Mn, Co) complexes were synthesized and characterized, including the first example of a mononuclear Fe^II-semiquinonate. Intermediates were observed in the reactions of M^II-phenSQ (M = Fe, Co) with O₂. Evidence for the relevance of these intermediates to the intradiol catechol dioxygenases was obtained by characterization of the oxidized semiquinone-derived product, muconic anhydride, resulting from the reaction of [PhTt^tBu]Co ^II(3,5-DBSQ) with O₂. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc49143a

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Five-coordinate MII-semiquinonate (M = Fe, Mn, Co)
complexes: reactivity models of the catechol
dioxygenases†
Cite this: Chem. Commun., 2014,
50, 5871
Received 1st December 2013,
Accepted 14th April 2014
Peng Wang, Glenn P. A. Yap and Charles G. Riordan*
DOI: 10.1039/c3cc49143a
www.rsc.org/chemcomm
A series of five-coordinate MII-semiquinonate (M = Fe, Mn, Co) by Fiedler and co-workers, including a mononuclear FeII-(imino)-
complexes were synthesized and characterized, including the first semiquinonate complex7 and a semiquinonate-bridged diiron(II)
example of a mononuclear FeII-semiquinonate. Intermediates were complex.8 Herein, we report the first well characterized mononuclear
observed in the reactions of MII-phenSQ (M = Fe, Co) with O2. FeII-semiquinonate complex and its MnII and CoII analogues –
Evidence for the relevance of these intermediates to the intradiol [PhTttBu]M(phenSQ) (M = Fe, Mn, Co, PhTttBu = phenyltris-
catechol dioxygenases was obtained by characterization of the ((tert-butylthio)methyl)borate, phenSQ = 9,10-phenanthrene-
oxidized semiquinone-derived product, muconic anhydride, resulting semiquinonate). The suitability of these complexes in modelling
from the reaction of [PhTttBu]CoII(3,5-DBSQ) with O2.
catalytic intermediates of the intradiol dioxygenases was evaluated
by O2 reactivity studies. A related complex, [PhTttBu]Co(3,5-DBSQ)
Intradiol catechol dioxygenases are non-heme iron enzymes that (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate) exhibited the
catalyze the oxidative cleavage of the C1–C2 bond of catechols.1 The intradiol reactivity, suggesting the relevance of the observed
state of the enzyme that reacts with O2 contains a five-coordinate intermediates to the intradiol catechol dioxygenases.
metal site. The activity of the enzyme and its synthetic analogs has
[PhTttBu]M(phenSQ) were synthesized using two complemen-
been attributed to the partial FeII-semiquinonate (SQ) character tary preparative routes, Scheme 1. Metathesis of [PhTttBu]MI
within the FeIII-catecholate species,2 which results in the formation (M = Fe, Mn, Co) with Tl(phenSQ) yielded [PhTttBu]M(phenSQ)
of a FeIII-alkylperoxy intermediate upon addition of O2. The extradiol in excellent yields (89–95%). A similar method was applied to the
catechol dioxygenases contain iron or manganese active sites and synthesis of [PhTttBu]Co(3,5-DBSQ) by replacing Tl(phenSQ) with
catalyze the oxidative cleavage of C2–C3 bond of catechols.1,3 During Tl(3,5-DBSQ). Alternatively, the oxidative addition of phenQ to
catalytic turnover, superoxo-FeII-semiquinonate and FeII-alkylperoxy monovalent metal precursors, [PhTttBu]M(PMe3) (M = Fe, Co)9
intermediates have been detected.4 Interestingly, comparable afforded [PhTttBu]M(phenSQ) (M = Fe, Co) in good yields
extradiol-cleaving activities were obtained by substituting the native (65–70%). High resolution mass spectroscopy (HRMS) data
iron with manganese or cobalt suggesting the intermediacy of combined with 1H NMR spectral analyses confirmed the
superoxo-MII-semiquinonate species (M = Fe, Mn, Co) in the catalytic composition and purity of the [PhTttBu]MII-(SQ) complexes
cycles.5 Moreover, reactions of redox-active ligand complexes with (Fig. S4–S7, S11–S14, ESI†).
dioxygen have received recent attention due to their potential utility
in stoichiometric and catalytic transformations.6
In spite of the implications of the FeII-semiquinonate species in
both intradiol and extradiol dioxygenases, to the best of our
knowledge, well characterized mononuclear FeII-semiquinonate
complexes are unknown. Related complexes were reported recently
Department of Chemistry & Biochemistry, University of Delaware, Newark,
DE 19716, USA. E-mail: riordan@udel.edu; Tel: +1 302 831 1073
† Electronic supplementary information (ESI) available: Experimental details, new
compound characterization data and crystallographic data, etc. CCDC 969713
([PhTttBu]Fe(phenSQ)), 969714 ([PhTttBu]Co(phenSQ)), 969715 ([PhTttBu]CoI), 969716
([PhTttBu]MnI), 969717 ([PhTttBu]Mn(phenSQ)) and 992704 ([PhTttBu]Co(3,5-DBSQ)).
For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc49143a
Scheme 1 Synthetic routes to [PhTttBu]M(SQ) complexes (M = Fe, Mn, Co)
and O2 reactivity of [PhTttBu]Co(3,5-DBSQ).
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Table 1 Molecular structure of [PhTttBu]Fe(phenSQ) and selected metric
parameters of the [PhTttBu]M(phenSQ) complexes. Hydrogen atoms are omitted
for clarity. (Structures of the other new complexes are available in the ESI)
Fe(phenSQ)
Mn(phenSQ)
Co(phenSQ)
Fig. 1 Electronic spectra of the [PhTttBu]M(SQ) complexes. The insert
highlights ligand field transitions.
M1–O1
M1–O2
O1–C22
O2–C35
C35–C22
t-value
2.064(2)
2.015(2)
1.285(2)
1.285(2)
1.435(3)
0.14
2.109(2)
2.080(2)
1.282(4)
1.280(4)
1.436(5)
0.58
1.922(1)
1.912(1)
1.301(2)
1.297(2)
1.411(2)
0.01
The electronic spectra of the [PhTttBu]M(SQ) complexes are
contained in Fig. 1. [PhTttBu]Fe(phenSQ) shows two features
of low intensity at 600 nm (e = 868 MÀ1 cmÀ1) and 935 nm
(e = 539 MÀ1 cmÀ1), the latter being consistent with a typical description. All complexes are air-sensitive in solution as
ligand field transition for five-coordinate, high-spin FeII com- demonstrated by O2 reactivity studies, vide infra, which can be
plexes.10 No apparent ligand field transition was observed for rationalized by the MII-SQ charge distribution.2a–c Space filling
[PhTttBu]Mn(phenSQ), indicating a high-spin MnII center. models (Fig. S21, ESI†) indicate O2 accessibility to both the metal
[PhTttBu]Co(phenSQ) exhibits two features at 683 (e
1310 MÀ1 cmÀ1) and 803 (e = 1290 MÀ1 cmÀ1), both of which
=
centers and the semiquinonate ligands.
The MII-SQ complexes showed paramagnetic H NMR spectra.
1
agree well with the ligand field transitions of five-coordinate, Their effective magnetic moments measured in solution by the
high-spin CoII.11 Unlike the previously reported [TpCum,Me]- Evans method are [PhTttBu]Mn(phenSQ) and [PhTttBu]Co(3,5-DBSQ)
Co(3,5-DBSQ),12 the ligand field transitions of [PhTttBu]Co(3,5- meff = 5.01(6) mB and 2.91(2) mB, respectively. These values are very
DBSQ) were not observed in the spectrum presumably due to the close to the spin-only values for S = 2 and S = 1 systems, indicating
overlap with the intense band at 784 nm (e = 5470 MÀ1 cmÀ1).13 strong antiferromagnetic coupling between high-spin divalent
The description of the metal complex electronic structures metal centers and the SQ radicals. [PhTttBu]Fe(phenSQ) and
as MII-SQ is supported by their infrared spectra. In contrast to [PhTttBu]Co(phenSQ) display meff = 4.65(2) mB and 3.43(3) mB,
the IR spectra of the respective [PhTttBu]MI complexes, respectively. These values are higher than the spin-only values
[PhTttBu]M(phenSQ) exhibit intense bands between 1500 cmÀ1 for S = 3/2 and S = 1 systems, but lower than expected for non-spin
and 1600 cmÀ1, which are consistent with the CQO stretches of coupled systems,12 suggesting either weaker antiferromagnetic
phenSQ (Fig. S15–S17, ESI†).14 Although [PhTttBu]CoI also shows coupling or more likely, non-negligible spin–orbit coupling.
two bands in the 1400–1450 cmÀ1 range, the much more intense
band at 1462 cmÀ1 of [PhTttBu]Co(3,5-DBSQ) is tentatively to evaluate the redox characteristics of the MII-SQ complexes
assigned to the n(CQO) of 3,5-DBSQ (Fig. S18, ESI†).15 (Fig. S22, ESI†). [PhTttBu]Co(phenSQ) and [PhTttBu]Co(3,5-DBSQ)
The cyclic voltammograms (CV) were measured in THF
All the complexes are five-coordinate as deduced by X-ray exhibit reversible reduction events at À0.97 V and À0.82 V (vs.
diffraction analyses (Fig. S20, ESI†). The coordination geometry Fc+/0) which are assigned as ligand-centered reductions, phenSQ/
of the [PhTttBu]Mn(phenSQ) lies between trigonal-bipyramidal phenCat and 3,5-DBSQ/DBCat, respectively. These redox potentials
and square pyramidal (t5 = 0.58),16 whereas the geometries of match very well with the values reported for Me4cyclam supported
[PhTttBu]Fe(phenSQ) (t5 = 0.14), [PhTttBu]Co(phenSQ) (t5 = 0.01) CoII-semiquinonate complexes (À1.00 V for phenSQ/phenCat and
and [PhTttBu]Co(3,5-DBSQ) (t5 = 0.17) are best described as À0.85 V for 3,5-DBSQ/DBCat).19 The redox events for [PhTttBu]M-
distorted square pyramids. Key bond lengths support the redox (phenSQ) (M = Mn, Fe) are irreversible on the electrochemical
state assignment of the bidentate ligand as semiquinonate, timescale, exhibiting Ec values of À1.17 V for [PhTttBu]Fe(phenSQ)
Table 1 and Fig. S20 (ESI†). For the MII-phenSQ complexes, the and À1.11 V for [PhTttBu]Mn(phenSQ), assigned as phenSQ/
C–O distances are in the range of 1.28–1.30 Å and C–C distances are phenCat reductions. The trend in reduction potentials among
in the range of 1.41–1.44 Å. These bond distances are characteristic the [PhTttBu]M(phenSQ) species, Fe o Mn o Co, indicates the
of a bound phenSQ ligand.14a,17 For [PhTttBu]Co(3,5-DBSQ), the Co complex is most readily reduced. The oxidations of [PhTttBu]-
average C–O distance is 1.314(2) Å. This distance is certainly among Co(phenSQ) and [PhTttBu]Co(3,5-DBSQ) at 0.26 V and 0.47 V,
the longest C–O distances for 3,5-DBSQ, but is not unprecedented.18 respectively are also irreversible.
Furthermore, the ‘‘four long/two short’’ quinoid distortion in
the semiquinonate ring further supports its electronic structure model putative intermediates in catechol dioxygenase catalysis,
To evaluate the utility of five-coordinate MII-SQ complexes to
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Notes and references
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Fig. 2 Electronic spectral changes during the oxygenation of [PhTttBu]-
Fe(phenSQ) at À90 1C in toluene. (a) Spectral changes after exposing
[PhTttBu]Fe(phenSQ) to O2. Intermediate growth indicated by red arrow.
Spectra were recorded at 2 min intervals. (b) Spectra of intermediate and
product collected at À90 1C. The spectrum of the product was obtained
by warming the sample to 0 1C for 5 min to allow the decay of the
intermediate and cooling back to À90 1C for 15 min.
electronic spectroscopy was employed to monitor the reaction
of [PhTttBu]M(phenSQ) with O2.20 Even at very low temperature
(À90 1C) rapid spectroscopic changes were observed upon
addition of O2 to [PhTttBu]M(phenSQ) (M = Fe, Co), producing
intermediates which decay to the thermodynamic products in
5–10 minutes upon warming to higher temperatures (Fig. 2 and
Fig. S28, ESI†).21 Indirect evidence for the relevance of these
synthetic intermediates to the intradiol dioxygenases was
obtained using [PhTttBu]Co(3,5-DBSQ). Upon reaction with O2 in
THF for 16 hours, [PhTttBu]Co(3,5-DBSQ) produced the intradiol
cleavage product, muconic anhydride in 16% yield, Scheme 1 (see
ESI† for details). This result, together with a previous discovery that
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13 Analogous to [PhTttBu]Co(3,5-DBSQ), an intense band at 818 nm was
observed for [PhTttBu]Co(phenSQ) at À90 1C.
the five coordinate complex [TpiPr ]Mn(3,5-DBSQ) also produces the
2
intradiol product,22 suggests that the semiquinonate character of
the ligand may contribute to the intradiol cleaving reactivity, even
for different metals. To the best of our knowledge, this is the first
example of intradiol reactivity of a CoII(3,5-DBSQ) complex.23
In summary, a series of five-coordinate MII-semiquinonate
(M = Fe, Mn, Co) complexes supported by the tris(thioether)
ligand [PhTttBu] were synthesized and characterized, including
the first example of a mononuclear FeII-semiquinonate complex.
While [PhTttBu]Co(3,5-DBSQ) was found to be a reactivity model
for the intradiol catechol dioxygenases, [PhTttBu]M(phenSQ) (M =
Fe, Co) serve as potential precursors to model the putative
intermediates in intradiol dioxygenase catalysis. Our current
efforts are focused on additional spectroscopic and structural
characterization of the intermediates produced from the low
temperature reactions of [PhTttBu]M(phenSQ) with O2. Also
under investigation are the reactions of [PhTttBu]M(phenSQ) with
superoxide to model intermediate(s) of relevance in extradiol
dioxygenase catalysis.
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intermediate was formed from the reaction of [(triphos)Ir(phenCat)]-
BPh4 with O2. See: P. Barbaro, C. Bianchini, C. Mealli and A. Meli,
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(phenSQ) with O2 at À90 1C.
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ring cleavage of the catechol. However, the organic products were
not unambiguously characterized as intradiol or extradiol products.
See: S. Nakashima, H. Ohya-Nishiguchi, N. Hirota, S. Tsuboyama
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The US National Science Foundation supported this research
program via CHE-1112035 to CGR. The X-ray diffractometer
(CHE-1048367) and LIFDI mass spectrometer (CHE-1229234)
acquisitions were supported by NSF.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 5871--5873 | 5873