Intriguing chemistry of molybdenum corroles

Article abstract of DOI:10.1021/ic400116u

The development of new methodologies for gaining access to low-valent molybdenum complexes led to spectroscopic identification of mononuclear (oxo)molybdenum(IV) corroles, as well as the full characterization of a binuclear molybdenum(IV) corrole that is bridged through axial O atoms by a Mg(THF)₄ moiety.

Full text of DOI:10.1021/ic400116u

Communication
pubs.acs.org/IC
Intriguing Chemistry of Molybdenum Corroles
Izana Nigel-Etinger,† Israel Goldberg,*,‡ and Zeev Gross*,†
†Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
‡Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 6997, Israel
S
* Supporting Information
Applied reducing agents were NaBH4, cobaltocene (CoCp2),15a
bis(pentamethylcyclopentadienyl)cobalt(II) (CoCp*2),15b and
trimesitylvanadium(III) [VMes3(THF)].15−17 Employment of
the latter led to (tpfc)CrIII, i.e., reductive deoxygenation, while
the one-electron-reduction product [(tpfc)CrIV(O]− was
obtained with the other reagents. The in situ conversion of
[(tpfc)CrIV(O]− to (tpfc)CrIII was enabled by either silylation
with chlorotrimethylsilane (TMSCl) or protonation (by
MeOH) of the former. The such-formed [(tpfc)CrIV(OSiTMS)]
and [(tpfc)CrIV(OH)] are apparently easier to reduce, not at
least because of the presence of reasonable leaving groups.
The MoV/MoIV redox potential of (tpfc)MoV(O) is much
more negative than the CrV/CrIV couple in (tpfc)CrV(O) (−0.35
and 0.11 V, respectively, vs Ag/AgCl in CH2Cl2).9,13 This
explains the inactivity of (tpfc)MoV(O) as an oxygen-transfer
reagent, exemplified by the inertness to triphenylphosphine, a
reaction that was used to convert (tpfc)CrV(O) to (Ph)3PO-
ligated CrIIItpfc.13 (tpfc)MoV(O) exhibits an intense Soret-like
band centered at 434 nm and Q bands centered at 546 and 580
nm (Figure 1, black line), as well as a well-resolved EPR spectrum
ABSTRACT: The development of new methodologies
for gaining access to low-valent molybdenum complexes
led to spectroscopic identification of mononuclear (oxo)-
molybdenum(IV) corroles, as well as the full character-
ization of a binuclear molybdenum(IV) corrole that is
bridged through axial O atoms by a Mg(THF)4 moiety.
hile the capability of corroles (in their trianionic form) to
W
stabilize high-valent metals has and continues to be the
traditional focus in metalloporphyrin chemistry,1−3 much less
research activity has been devoted to the consequential large
activity that corroles may impose on low-valent metals. Examples
that testify for the above are the aerobic oxidation of
chromium(III),4a,12 manganese(III),4b and iron(III) corro-
les,4c−f a phenomenon not shared by the same metal ions in
other coordination environments where only the corresponding
divalent oxidation states bind and/or activate molecular
oxygen.5−7 Considering the strong reducing power of early
transition metals in general, it is quite surprising that the
chemistry of the corresponding corrole chelates remains quite
unexplored. Sporadic reports on titanium and vanadium corroles
are limited to their most stable oxidation states, (oxo)titanium-
(IV) and (oxo)vanadium(IV).8 The same limitation holds for the
heavier congeners of group 6: there are several publications on
(oxo)molybdenum(V) corrole,9 and a trioxo-bridged binuclear
tungsten(VI) corrole has been reported most recently.10 Only
the chemistry of chromium corroles has been developed beyond
its most stable (oxo)chromium(V) state.11 This includes
crystallographic characterization and reactivity studies of both
chromium(III) and (oxo)chromium(V) corroles,12 as well as
spectroscopic investigations of the one-electron-reduction and
-oxidation products of the latter: (oxo)chromium(IV) corrole
and corrole-oxidized (oxo)chromium(V), respectively.13 Con-
sidering the very important chemistry of low-valent molybdenum
complexes in general,14 we have now decided to explore that of
molybdenum corroles. This was done by first exploring a variety
of methodologies for reduction of the (oxo)chromium(V)
complex (tpfc)CrV(O) [tpfc = trianion of tpfc], followed by
looking into their possible adoption for the (oxo)molybdenum-
(V) corrole (tpfc)MoV(O).
Figure 1. Electronic spectrum of (tpfc)MoV(O) (black) and changes
upon the addition of CoCp*2 (red) and [VMes3(THF)] (blue).
at room temperature.13 The addition of either NaBH4 or CoCp2
to a N2-purged THF solution of (tpfc)MoV(O) led to the
appearance of a sharper and blue-shifted Soret band (from 434 to
426 nm; Figures S6 and S7). The use of the more powerful
reducing reagent CoCp*2 revealed a much more prominent
intensity increase of the Soret band and also a new Q band with
λmax of 590 nm (Figure 1, red line). These results, together with
the disappearance of the EPR signal and appearance of a well-
resolved NMR spectrum (Figure 2c, vide infra), are fully
consistent with the full and clean formation of [(tpfc)-
MoIV(O)]−. Identical spectral changes were obtained at longer
The first step was a thorough investigation of the reduction of
(tpfc)CrV(O), in tetrahydrofuran (THF) under a N2 atmosphere
(Supporting Information, SI). Reaction progress was traced by
relying on the fact that the spectroscopic features [UV−vis,
NMR, and electron paramagnetic resonance (EPR)] of the
5,10,15-tris(pentafluorophenyl)corrole chromium complexes
are well established in all oxidation and coordination states.11−13
Received: January 16, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic400116u | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
pronounced difference is the separation of ortho-19F chemical
shifts (Δδ). The Δδ value of 1 ppm in [(tpfc)MoV(O)]− (Figure
2c) reflects the differences in the magnetic environments of the o-
F atoms with syn and anti relationships relative to the Mo−O
moiety. The much larger Δδ of 2 ppm in the new complex
(Figure 2b) must, however, be of different origin. Δδ values that
exceed 1 ppm are characteristic of binuclear corroles where one
of the two o-F atoms in each C6F5 group experiences the
diamagnetic ring current effect of the other corrole. Representa-
tive examples are the μ-oxo-bridged iron(IV) corrole
[FeIV(tpfc)]2O and the μ-trioxo-bridged tungsten(VI) corrole
[WVI(tpfc)]2O3.19,4b These spectroscopy-based hints about the
formation of a binuclear complex in the reaction between
(tpfc)MoV(O) and [VMes3(THF)] were verified when high-
quality crystals were obtained via crystallization from cold ether
(4 °C) in the N2-purged glovebox (Figure 3).
Figure 2. Projection of [Mo(tpfc)]2O2Mg(THF)4: (A) top view; (B)
side view.
reaction times with CoCp2, thus indicating that it only reacts
slower than CoCp*2 because of a smaller thermodynamic driving
force. Spectroelectrochemical investigation of the reduction
process confirmed this proposal and the spectroscopic character-
istics of [(tpfc)MoIV(O)]−. Holding the applied potential at −0.5
V (E1/2 = −0.96 vs Ag/AgCl for CoCp2/[CoCp2]+) led to an
intensity increase and a 10 nm blue shift (from 430 to 420 nm) of
the Soret band, and the visible band at 580 nm also increased and
shifted to 590 nm (Figure S8).
The effect of TMSCl was found to depend on the reagent
applied for the (tpfc)MoV(O) to [(tpfc)MoIV(O)]− conversion.
With NaBH4 as the reducing agent, the addition of TMSCl
initiated a color change from red to orange, a red shift of the Soret
band from 420 to 430 nm, and the appearance of two Q bands at
542 and 578 nm. The anaerobically measured 1H NMR spectrum
pointed toward the formation of a diamagnetic complex that is
consistent with (tpfc)MoIVOSi(CH3)3: sharp β-pyrrole CH
doublets at 9.2−9.7 ppm and a singlet at −1.8 ppm due to
coordinated TMS (Figure S14). The conclusion is that applying
this reaction sequence on (tpfc)MoV(O), in contrast to the
results with (tpfc)Cr(O), did not lead to the formation of
trivalent molybdenum. The addition of TMSCl to a solution of
[(tpfc)Mo(O)]−, obtained via the reduction of (tpfc)MoV(O) by
excess CoCp*2, induced the formation of a dark-green (Soret
band at 432 nm and Q bands at 550 and 585 nm) complex. While
this might indicate a further reduction of the initially formed
(tpfc)MoIVOSi(CH3)3, the NMR spectra of that product could
not be confidently analyzed. All crystallization attempts of the
new complexes did not yield X-ray-quality crystals, and their
exposure to air led to the reappearance of (tpfc)MoV(O).
Because of the ambiguity in the results obtained by the
stepwise reduction/deoxygenation of (tpfc)MoV(O), the ex-
tremely oxophilic [VMes3(THF)] was applied next. The
addition of [VMes3(THF)] to an anaerobic solution of
(tpfc)MoV(O) in THF induced immediate changes in the
electronic spectra: all bands became more intense and the Soret
band maximum shifted from 432 to 428 nm (Figure 1, blue line).
More detailed information was obtained from the very
informative 19F NMR spectrum of the easily purified product.18
It revealed very sharp resonances, thus indicative of a new
diamagnetic complex rather than the paramagnetic (tpfc)-
MoV(O) starting material (Figure 2a) and the expected
MoIIItpfc. Another surprising phenomenon is the similarity of
the complexes formed by the treatment of (tpfc)MoV(O) with
[VMes3(THF)] (Figure 2b) and CoCp*2 (Figure 2c), by virtue
of their almost identical electronic spectra (Figure 1) and similar
19F NMR spectra.
Figure 3. 19F NMR spectra (under N2, in THF-d8, relative to CFCl3, δ =
0.00) of (tpfc)MoV(O) (a). Complex obtained upon the addition of
[VMes3(THF)] to (tpfc)MoV(O) (b). Complex obtained upon the
addition of CoCp*2 to (tpfc)MoV(O) (c).
The crystal data are as follows (excluding additional severely
disordered diethyl ether solvent in the crystal lattice, eight
molecules per unit cell, which could not be modeled by discrete
atoms): C90H64F30MgMo2N8O6·2C4H8O, M = 2267.76, mono-
clinic, space group P21/n, a = 15.0152(7) Å, b = 15.6890(6) Å, c
= 24.8277(12) Å, β = 102.512(2)°, V = 5709.8(4) Å3, Z = 2, T =
110(2) K, ρcalcd = 1.319 g cm−3, 25490 reflections measured,
10006 unique (Rint = 0.061; 2θmax = 50.1°), final R = 0.079 (Rw =
0.189) for 5984 reflections with I > 2σ(I) and R = 0.126 (Rw =
0.207) for all data.
The dinuclear complex is located on crystallographic centers of
inversion, with two of the four magnesium-bound THF ligands
revealing conformational disorder. The individual (MoIV−
O)(tpfc) units on opposite sites of the inversion have a domed
structure with a square-pyramidal coordination environment
around the Mo center, similar to that observed in the monomeric
MoV(O) complex.9a The Mo ion is located 0.78 Å above the
plane of the four pyrrole N atoms and 1.0 Å above the mean
plane of the 19-atom carbon macrocycle. All of the equatorial
Mo−N(pyrrole) bonds of the five-coordinate Mo ion are within
2.03−2.05 Å. The axial Mo−O bond length is 1.714(4) Å,
significantly longer than that of 1.684(2) Å observed for
(tpfc)MoV(O).9a The Mg ion in the center of the corrole
dimer is six-coordinate, with four THF ligands occupying the
equatorial positions at Mg−OTHF distances of 2.085 and
2.086(4) Å and two Mo-linked O sites at the axial positions
While the spectroscopic features suggest the formation of
molybdenum(IV) corroles (low-spin d2) in both cases, one
B
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Inorganic Chemistry
Communication
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We provide fresh insight into the chemistry of molybdenum
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reduction of the very stable (oxo)molybdenum(V) complex.
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corrole. The latter case is apparently novel, displaying a structure
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corrolate and one axial O atom and these subunits are bridged by
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ASSOCIATED CONTENT
* Supporting Information
■
S
Reduction of (tpfc)CrV(O) by NaBH4, CoCp2, CoCp*2, and
[VMes3(THF)], proposed mechanism of the deoxygenative
reduction of (tpfc)CrV(O), and crystallographic data for
[Mo(tpfc)]2O2Mg(THF)4 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Chem. Soc., Chem. Commun. 1984, 886. (b) Ruiz, J.; Vivanco, M.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: goldberg@post.tau.ac.il (I.G.), chr10zg@tx.technion.
ac.il (Z.G.).
■
Notes
(18) Synthesis of [Mo(tpfc)]2O2Mg(THF)4: A 20 mL THF solution
of (tpfc)MoV(O) (50 mg, 0.125 mmol) was stirred with excess of
[VMes3(THF)] at room temperature in a glovebox under N2. After 10
min, the color of the mixture changed from blood red (reactant) to light
purple (product). THF was evaporated and replaced by diethyl ether, in
which unreacted [VMes3(THF)] was not soluble. The solution was
filtered and evaporated. Crystallization from a minimum amount of
diethyl ether yielded 70 mg (69%) of purple X-ray-quality crystals. 19F
NMR (188 MHz, THF-d8): δ −139.33 (dt, J = 26.59 and 8.08 Hz, 6F; o-
F), −141.69 (dd, J = 24.51 and 7.4 Hz, 4F; o-F), −141.92 (dd, J = 20.98
and 7.6 Hz, 2F; o-F), −157.19 (t, J = 20.73 Hz, 4F; p-F), −157.34 (t, J =
21.02 Hz, 2F; p-F), −164.85 (m, 6F; m-F), −165.28 (m, 6F; m-F). 1H
NMR (200 MHz, THF-d8): δ 9.08 (d, 4H), 8.72 (d, 4H), 8.64 (d, 4H),
8.55(d, 4H). UV−vis (CH2Cl2; λmax [nm] (ε × 10−3 [M−1 cm−1]): 424
(185.3), 577 (37.6).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a grant from the Israel Science
Foundation to Z.G.
■
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