Direct synthesis of ligand-based radicals by the addition of bipyridine to chromium(II) compounds

Article abstract of DOI:10.1021/ic302590g

The reaction of 2,2′-bipyridine (bpy) with monomeric chromium(II) precursors was used to prepare the S = 1 complexes Cr(tBu-acac)₂(bpy) (1) and (η⁵-Cp)(η¹-Cp)Cr(bpy) (3), as well as the S = 2 compound Cr[N(SiMe₃

Full text of DOI:10.1021/ic302590g

Communication
pubs.acs.org/IC
Direct Synthesis of Ligand-Based Radicals by the Addition of
Bipyridine to Chromium(II) Compounds
Wen Zhou,† Addison N. Desnoyer,† James A. Bailey,† Brian O. Patrick,‡ and Kevin M. Smith*,†
†Department of Chemistry, University of British Columbia Okanagan, 3333 University Way, Kelowna, British Columbia, Canada V1V
1V7
‡Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
S
* Supporting Information
Scheme 1. Synthesis of a Neutral tBu-acac Complex 1
ABSTRACT: The reaction of 2,2′-bipyridine (bpy) with
monomeric chromium(II) precursors was used to prepare
the S = 1 complexes Cr(tBu-acac)2(bpy) (1) and (η5-
Cp)(η1-Cp)Cr(bpy) (3), as well as the S = 2 compound
Cr[N(SiMe3)2]2(bpy) (4). The crystallographically deter-
mined bond lengths indicate that the bpy ligands in 1 and
3 are best regarded as radical anions, while 4 shows no
structural evidence for electron transfer from CrII to the
neutral bpy ligand.
rapid color change from light orange-brown to dark green.
Isolated Cr(tBu-acac)2(bpy) (1) has μeff = 2.74 μB in solution
(Evans, C6D6), consistent with an electronic structure with S = 1.
ver the past 15 years, several remarkably effective first-row
Otransition-metal catalysts have employed chelating ancil-
lary ligands with conjugated pyridine and/or imine donors.1
The UV−vis spectrum of 1 has multiple intense features between
700 and 400 nm, with ε values ranging from 2000 to 4500 M−1
While the ability of diimine-based ligands to accept an electron to
form anionic ligand radicals has long been recognized,2 the
connection between the electronic structure and reactivity
remains to be fully understood.3 Substituted 2,2′-bipyridine
(bpy) ligands have been used in situ to generate first-row metal
catalysts for organic synthesis.4 However, compared to
pyridinediimine and related tridentate ligands,5 general synthetic
methods to prepare well-defined complexes with only a single
bidentate redox-active diimine ligand have yet to be developed.
In this paper, we report new chromium(III) complexes with
radical-anionic ligands prepared by the addition of neutral bpy to
known monomeric chromium(II) precursors. This synthetic
strategy, previously employed by Theopold, Wieghardt, and Mu
with diimine ligands and CrCl2,6−8 relies on the characteristic
single-electron-transfer reactivity9 of CrII to produce inert
octahedral chromium(III) complexes antiferromagnetically
coupled to the ligand-based radical. Recent studies by Wieghardt
and co-workers have demonstrated the ubiquity of these
interactions in what were previously considered to be low-spin
chromium(II) complexes7 and the remarkable correlation
between the crystallographically determined Cpy−Cpy bond
length and the bpy oxidation level revealed through detailed
spectroscopic, magnetic, and computational studies.10 Interest-
ingly, we have found that electron transfer to the bpy lowest
unoccupied molecular orbital (LUMO) π* is dictated not only
by the reducing ability of the CrII d4 precursor but also by the
geometry of the product.
cm−1. The corresponding Cr(tBu-acac)2(tBu-bpy) complex, 1a,
was also prepared by the same procedure using 4,4′-di-tert-butyl-
2,2′-bipyridine (tBu-bpy). A related Cr(acac)2(diimine) complex
was reported by Wieghardt and co-workers.13
Both 1 and 1a have been characterized by single-crystal X-ray
diffraction (Figure 1). In both structures, the Cr center displays a
Figure 1. Thermal ellipsoid diagrams (50%) of 1 (left) and 1a (right).
All H atoms have been omitted for clarity.
relatively undistorted geometry, with Cr−O bond lengths
between 1.962 and 1.976 Å and Cr−N bond lengths of
2.0015(10) Å for 1 and 1.9883(14) and 1.9927(13) Å for 1a.
The interpyridine C−C distances of 1.427(2) Å for 1 and
1.419(2) Å for 1a lie in the range exhibited for bpy ligands that
have been reduced by a single electron.10 In the absence of
As shown in Scheme 1, our synthesis employed Cr(tBu-acac)2
(tBu-acac = 2,2,6,6-tetramethylheptane-3,5-dionate) because it is
soluble in Et2O or hydrocarbon solvents,11 unlike the polymeric
Cr(acac)2.12 The reaction of Cr(tBu-acac)2 with bpy results in a
Received: November 27, 2012
Published: February 18, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
detailed spectroscopic, magnetic, and computational studies, the
precise electronic structures of 1 and 1a cannot be definitively
assigned. Nevertheless, the crystallographic data suggest that the
bpy ligands in both 1 and 1a can be considered as radical anions,
with the observed S = 1 spin state being attributable to
antiferromagnetic coupling between the ligand-based radical and
the three unpaired electrons of the CrIII center.
3 has an (η5-Cp)(η1-Cp)Cr(bpy) structure17 with short Cpy−Cpy
[1.425(2) Å] and Cr−N [1.9825(13) Å and 1.9684(12) Å] bond
lengths consistent with the bpy ligand in the radical-anionic
oxidation level.10 The structure of 3 is similar to that of
Cp*Cr(bpy)(CH2Ph), an S = 1 complex prepared by Theopold
and co-workers by bpy-induced alkyl radical loss from
Cp*CrIII(benzyl)2 precursors.18 Although Cp2Cr typically
requires elevated temperatures for protonolysis reactions with
alcohols,19 3 reacts readily under ambient conditions with 2 equiv
of (tBu-acac)H to generate 1, as identified by UV−vis
spectroscopy.
Single-electron oxidation of 1 with iodine followed by anion
metathesis with NaBPh4 gives the cationic complex [Cr(tBu-
acac)2(bpy)][BPh4] (2). The UV−vis spectrum of 2 displays
only a single band at 550 nm in EtOH with a greatly reduced
extinction coefficient of 90 M−1 cm−1, and its μef3f = 3.88 μB in
solution (Evans, C6D6) is consistent with a S = /2 spin state.
Complex 2 can also be independently synthesized via [Cr(tBu-
acac)2(H2O)2]+ following the procedure of Kaizaki and co-
workers.14 The molecular structure of 2 was determined
crystallographically (see the Supporting Information, SI). The
coordination geometry at Cr remains octahedral, with the
somewhat shorter Cr−O bond lengths between 1.925 and 1.948
Å being attributable to a change in the overall charge from neutral
1 to cationic 2. Significantly, the Cr−N bond lengths of
2.0683(10) and 2.0664(11) Å in 2 are longer than those in 1, and
the Cpy−Cpy bond length has extended by over 0.05 Å to
1.4787(17) Å, consistent with oxidation of the bpy ligand from a
radical anion to neutral.10
The reaction of Cr[N(SiMe3)2]2(THF)2 with bpy results in
the dark-purple complex Cr[N(SiMe3)2]2(bpy) (4; Scheme 2),
Scheme 2. Synthesis of 4
which has two intense absorbances at 509 and 372 nm in Et2O,
each with ε ≈ 1300 M−1 cm−1.20 In contrast to the S = 1 spin state
observed for 1, 1a, and 3, complex 4 has μeff = 4.62 μB in solution
(Evans, C6D6) consistent with an S = 2 spin state. As shown in
Figure 2, 4 exhibits a distorted square-planar geometry in the
solid state, with Npy−Npy−NSi−NSi dihedral angles of 20.0° and
20.7° for the two independent molecules of 4 in the unit cell. The
relatively long Cpy−Cpy [1.481(3) Å and 1.485(3) Å] and Cr−
Npy (between 2.14 and 2.16 Å) distances are consistent with a
neutral bpy ligand.10 Like 3, the bis(amido)bipyridine complex 4
is also converted to 2 when treated with 2 equiv of (tBu-acac)H,
as demonstrated by UV−vis spectroscopy and CV (see the SI).
For 4, the absence of single electron transfer from CrII upon
bpy coordination can be accounted for using concepts from
ligand-field theory. Because of the steric bulk of the N(SiMe3)2
ligands, 4 is stable as a monomeric complex with a coordination
number of 4. This allows the compound to adopt the
electronically favorable (albeit sterically distorted) square-planar
geometry shown in Figure 2 while retaining a high-spin CrII S = 2
spin state.21 The same electronic structure is observed for
Cr(Mes)2(bpy), where Mes (mesityl) = 2,4,6-Me3C6H2, which
also has a slightly distorted square-planar geometry, an S = 2 spin
state, and a neutral bpy ligand.22 This is in contrast to octahedral
complexes such as 1, where electron transfer to the bpy LUMO
π* is energetically favorable compared to either the high-spin (S
= 2, t2g3eg1) or low-spin (S = 1, t2g4eg0) electron configurations for
CrII.6−8,23 Although both 4 and Cr(Mes)2(bpy) would be
expected to have very electron-rich CrII centers, neither complex
exhibits any structural evidence for bpy acting as a π-acceptor
ligand.
The similarity in the coordination geometry at CrIII between
neutral 1 and cationic 2 should facilitate outer-sphere single-
electron-transfer reactions. Consistent with this expectation,
cyclic voltammetry (CV) of cationic 2 in tetrahydrofuran
(THF)/0.1 M [NBu4][PF6] shows a reversible reduction at
−1.53 V versus [Cp2Fe]+/0. UV−vis spectroelectrochemistry
demonstrates that the reduction of 2 results in the clean
formation of 1. A second reversible reduction at −2.63 V versus
[Cp2Fe]+/0 may be due to a further reduction of 1 to anionic
[Cr(tBu-acac)2(bpy)]−, although we have not yet been able to
prepare this complex synthetically. Complex 1 also reacts rapidly
with Ph3CBr to give [Cr(tBu-acac)2(bpy)]+ and trityl radical,15
as identified by UV−vis spectroscopy (see the SI).
The direct addition of bpy to chromocene at room
temperature generates Cp2Cr(bpy) (3). Unlike its highly reactive
heavier group 6 congeners, chromocene typically only forms
weak bonds with neutral donor ligands because of the relative
stability of the S = 1 state for Cp2Cr.16 The stability of 3 can be
attributed to single electron transfer from CrII to bpy to generate
an inert chromium(III) complex antiferromagnetically coupled
to a bpy radical anion while retaining the S = 1 state with μeff
=
2.92 μB in solution (Evans, C6D6). As shown in Figure 2, complex
New paramagnetic chromium bipyridine complexes have been
prepared by the addition of the neutral ligand to well-defined
monomeric chromium(II) precursors. For Cr(tBu-acac)2 and
Cp2Cr, the S = 1 products 1 and 3 are best regarded not as low-
spin CrII but as CrIII antiferromagnetically coupled to an unpaired
electron on the bpy ligand.7,10 The geometry of the octahedral or
three-legged-piano-stool products appears to play a significant
role in electron transfer from CrII to bpy LUMO π*, as the four-
coordinate complex 4 remains S = 2, and displays the bond
lengths expected for a neutral bpy ligand. The protonolysis
Figure 2. Thermal ellipsoid diagrams (50%) of 3 (left) and 4 (right). All
H atoms have been omitted for clarity, and only one of the two
independent molecules present in the unit cell is shown for 4.
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Inorganic Chemistry
Communication
Wieghardt, K. Inorg. Chem. 2012, 51, 3718−3732. (c) Scarborough, C.
reactions of 3, 4, and related complexes, as well as their outer-
sphere reactivity with organic halides,24 are currently under
investigation.
C.; Sproules, S.; Doonan, C. J.; Hagen, K. S.; Weyhermuller, T.;
̈
Wieghardt, K. Inorg. Chem. 2012, 51, 6969−6982.
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ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files, tables and text giving crystallographic data for 1, 1a, 2,
3, and 4, complete experimental details, and UV−vis spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(11) Gerlach, D. H.; Holm, R. H. Inorg. Chem. 1969, 8, 2292−2297.
(12) Cotton, F. A.; Rice, C. E.; Rice, G. W. Inorg. Chim. Acta 1977, 24,
231−234.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kevin.m.smith@ubc.ca.
■
(13) Ghosh, M.; Sproules, S.; Weyhermuller, T.; Wieghardt, K. Inorg.
̈
Chem. 2008, 47, 5963−5970.
(14) Tsukahara, Y.; Iino, A.; Yoshida, T.; Suzuki, T.; Kaizaki, S. J. Chem.
Soc., Dalton Trans. 2002, 181−187.
Notes
The authors declare no competing financial interest.
(15) Colle, T. H.; Glaspie, P. S.; Lewis, E. S. J. Org. Chem. 1978, 43,
2722−2725.
ACKNOWLEDGMENTS
■
(16) (a) Tang Wong, K. L.; Brintzinger, H. H. J. Am. Chem. Soc. 1975,
97, 5143−5146. (b) Green, J. C.; Jardine, C. N. J. Chem. Soc., Dalton
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canadian
Foundation for Innovation, and the University of British
Columbia for financial support. K.M.S. thanks Anita Lam for
collecting X-ray diffraction data for complexes 1, 1a, 2, and 4.
Trans. 1999, 3767−3770. (c) Schaper, F.; Wrobel, O.; Schworer, R.;
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(17) (η5-Cp)(η1-Cp)Cr(NO)2 was reported over 50 years ago and was
one of the first organometallic complexes proposed to undergo rapid
intramolecular rearrangement processes. For studies of (η5-Cp)(η1-Cp)
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DEDICATION
■
This paper is dedicated to Prof. Peter Legzdins on the occasion of
his 70th birthday.
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