A tetracoordinated phosphasalen nickel(III) complex

Article abstract of DOI:10.1002/anie.201309222

The oxidation of a NiII complex bearing a tetradentate phosphasalen ligand, which differs from salen by the presence of an iminophosphorane (P=N) in place of an imine unit, was easily achieved by addition of a silver salt. The site of this oxidation was investigated with a combination of techniques (NMR, EPR, UV/Vis spectroscopy, X-ray diffraction, magnetic measurements) as well as DFT calculations. All data are in agreement with a high-valent NiIII center concentrating the spin density. This markedly differs from precedents in the salen series for which oxidation on the metal was only observed at low temperature or in the presence of additional ligands or anions. Therefore, thanks to the good electron-donating properties of the phosphasalen ligand, [Ni(Psalen)]+ represents a rare example of a tetracoordinated high-valent nickel complex in presence of a phenoxide ligand.

Full text of DOI:10.1002/anie.201309222

.
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DOI: 10.1002/anie.201309222
Nickel(III) Complexes
A Tetracoordinated Phosphasalen Nickel(III) Complex**
Thi-Phuong-Anh Cao, Grꢀgory Nocton, Louis Ricard, Xavier F. Le Goff, and Audrey Auffrant*
Abstract: The oxidation of a NiII complex bearing a tetraden-
tate phosphasalen ligand, which differs from salen by the
over the salen moiety with a small contribution of the nickel
orbitals.[3b,4] However, in coordinating solvents, or in the
presence of exogenous ligands such as pyridine or coordinat-
ing anions, the geometry of the complex is modified and so is
the site of the oxidation, allowing the formation of NiIII
complexes.[4a,b,5] Temperature-dependent behavior, that is,
valence tautomerism, was also described in a few exam-
ples.[5b,6] Moreover, the overall electronic distribution may be
finely tuned by the substituents of the phenoxide rings. They
influence not only the site of the oxidation (which takes place
at the most electron-rich phenoxide ring) as evidenced with
complexes featuring unsymmetrical salen,[3c,7] but also the
extent of radical delocalization.[3b,8] The hybridization of the
nitrogen atoms is also of prime importance since it may
influence the oxidation site, the spin state of the metal, and
the magnetic interactions within the molecule.[5b,9]
=
presence of an iminophosphorane (P N) in place of an imine
unit, was easily achieved by addition of a silver salt. The site of
this oxidation was investigated with a combination of tech-
niques (NMR, EPR, UV/Vis spectroscopy, X-ray diffraction,
magnetic measurements) as well as DFT calculations. All data
are in agreement with a high-valent NiIII center concentrating
the spin density. This markedly differs from precedents in the
salen series for which oxidation on the metal was only observed
at low temperature or in the presence of additional ligands or
anions. Therefore, thanks to the good electron-donating
properties of the phosphasalen ligand, [Ni(Psalen)]+ repre-
sents a rare example of a tetracoordinated high-valent nickel
complex in presence of a phenoxide ligand.
N
,N’-ethylenebis(salicylimine) (salen) derivatives consti-
Astonishingly, despite numerous structural variations
proposed for salen ligands, the introduction of heteroatoms
into their backbone was rarely investigated.[10] As we are
tute a very successful class of ligands that have found
numerous applications in inorganic chemistry and catalysis.[1]
Their extraordinary popularity comes from their easy syn-
thesis, allowing a variety of skeleton variations, and their
ability to coordinate to a great number of different metal ions.
Moreover the presence of two phenoxide and imine functions
is reminiscent of the coordination environment generated by
two histidine and two tyrosine residues, encountered in
several enzymes. Thus, in 1998, the one-electron oxidation
of a (salen)copper(II) complex was described, and the
resulting persistent phenoxy radical was established as
a biomimetic functional model of galactose oxidase.[2] Since
then, one-electron-oxidized metal salen complexes of first-
row transition metals have received considerable attention.[3]
Depending on the nature of the redox-active orbitals,
oxidation of salen complexes leads either to high-valent
metal centers (metal-centered oxidation) or to species where
the ligand is oxidized to a radical (ligand-centered oxidation).
Therefore, various factors may affect the oxidation site, as it
was nicely illustrated in the case of nickel salen complexes. In
noncoordinating solvents, one-electron oxidation occurs on
the ligand, and the unpaired electron is highly delocalized
=
interested in iminophosphorane (P N) based ligands as an
alternative to conventional imine-based ligands,[11] we devel-
oped a new class of ligands featuring iminophosphorane units,
which we termed phosphasalen (or Psalen). We have recently
demonstrated that these ligands are more flexible and
electron-donating than their carbon analogues[12] and that
they provide efficient and selective initiators for rac-lactide
polymerization.[13] In the present work, we wish to report the
synthesis and characterization of a NiII phosphasalen complex
and its one-electron oxidation which leads to a temperature-
persistent high-valent tetracoordinated NiIII complex.
With the objective of studying the oxidation of a [Ni-
(Psalen)] complex, we chose a ligand featuring substituents on
the ortho and para positions of the phenoxide rings to
increase its stability, and preferred tert-butyl groups for
solubility reasons. The required phosphasalen ligand 1 was
prepared following a procedure we recently published
(Scheme 1):[13b] Reaction of 1 with [NiBr2(dme)] in THF led,
after removal of the potassium salts, to formation of the NiII
complex 2 as a purple solid in 92% yield. In the 31P{1H} NMR
spectrum the coordination is indicated by a deshielding of the
phosphorus nuclei appearing as a singlet at d(CD2Cl2) =
36.0 ppm. This complex was characterized by multinuclear
NMR spectroscopy, elemental analysis, and X-ray diffraction
analysis. As expected for a d8 complex, 2 adopts a square-
planar geometry around the metal center with a slight
deviation from planarity (8.58, see the Supporting Informa-
tion).
[*] Dr. T.-P.-A. Cao, Dr. G. Nocton, Dr. L. Ricard, Dr. X. F. Le Goff,
Dr. A. Auffrant
Laboratoire Hꢀtꢀroꢀlꢀments et Coordination, CNRS
ꢁcole Polytechnique
Route de Saclay, 91128 Palaiseau (France)
E-mail: audrey.auffrant@polytechnique.edu
Homepage: http://www.dcph.polytechnique.fr
[**] CNRS and ꢁcole Polytechnique are thanked for financial support.
We are grateful to ICMMO (Paris-sud Orsay) for the opportunity to
measure solid-state EPR spectra on its platform and to Prof. Y. Jean
for fruitful discussions concerning calculations.
The cyclic voltammogram of 2 exhibits three oxidation
waves in CH2Cl2 at + 0.16, + 0.96, and + 1.22 V versus
ferrocene/ferrocenium (Fc/Fc+; Table 1). The first oxidation
is much easier than the one reported for the related complex
[Ni(tBu-salen)] (DE ꢀ 420 mV, Table 1),[5b,14] in agreement
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309222.
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1.887(2) ꢀ and 1.881(2) ꢀ, respectively, in the
neutral complex). Interestingly, the contraction
of the coordination sphere is not symmetrical,
and is stronger along O1-Ni-N2 than along O2-
ꢁ
Ni-N1. Noteworthy, the shortening of the Ni N
ꢁ
bond is accompanied by an elongation of the P
N bond from 1.619(2) ꢀ in 2 to 1.639(3) ꢀ and
ꢁ
ꢁ
1.650(3) ꢀ for P1 N1 and P2 N2, respectively,
again N2 experiences the largest variation. This
may indicate a lower stabilization of the nitrogen
lone pairs by phosphorus.
Scheme 1. Synthesis of nickel complexes 2 and 2+ SbF6
.
ꢁ
Table 1: Electrochemical data for 2 (3.0 mmolLꢁ1 in CH2Cl2; [Bu4N][BF4]:
0.12mꢁ1; 50 mVsꢁ1) and [Ni(tBu-salen)].[5b,14] Potential values given
relative to E1/2(Fc+/Fc).
Complex
E
1/2,1 [V]
E1/2,2 [V]
E1/2,3 [V]
2
0.16
0.58
0.96
1.04
1.22
[Ni(tBu-salen)]
with the better electron-donating ability of the phosphasalen.
Moreover the difference between the first and second
oxidation waves of 2 is large (DE1/2 = 0.8 V) compared to
that observed for the salen analogue (DE1/2 = 0.46 V).[5b,14] In
nickel salen complexes, the two first oxidations are known to
occur on both phenoxide rings. Within the series of NiII salen
complexes, the rather large separation between the two first
half-wave potentials observed for [Ni(tBu-salen)] was inter-
preted as a result of the strong electronic communication
between the two phenoxide rings.[5b,14] Therefore the even
larger peak separation observed for 2 may indicate that the
first oxidation in this complex does not occur on the ligand
but might be metal centered.
ꢁ
Figure 1. ORTEP view of the cation 2+. The SbF6 anion, solvent
molecules, and hydrogen atoms have been omitted for clarity. Thermal
ellipsoids drawn at the 50% probability level. Selected distances[ꢂ]
and angles[8]: Ni1–O1 1.828(2), Ni1–O2 1.844(2), Ni1–N1 1.875(3),
Ni1–N2 1.842(3), P1–N1 1.639(3), P2–N2 1.650(3), O1–C3 1.327(4),
O2–C30 1.326(5), N1–C1 1.486(5), N2–C2 1.486(5); N1-Ni1-O1
94.9(1), O2-Ni1-N2 95.3(1), N1-Ni1-N2 86.8(1), O1-Ni1-O2 84.2(1),
O1-N1-N2-O2 11.06.
This latter result prompted us to investigate the chemical
oxidation of 2. Cation 2+ was synthesized by adding one
equivalent of a silver salt to a CH2Cl2 solution of 2. This
induced a rapid color change from purple to black. Monitor-
ing the reaction with 31P{1H} NMR spectroscopy showed
disappearance of the starting material and formation of
To get insight into the structural features of 2 and 2+, DFT
calculations were carried out on both complexes. For the
oxidized complex, calculations were carried out with and
without the anion, giving comparable results (see the
Supporting Information). The geometry optimizations well
reproduce the experimental structures, including the non-
symmetric contraction of the coordination sphere upon
oxidation (see Table S6). In the neutral complex 2, the two
highest occupied molecular orbitals, which are of similar
energy, result from antibonding interactions between the dxz
or dyz orbital of Ni and pp orbitals of the oxygen atoms (see
the Supporting Information). Upon oxidation this degeneracy
is removed, as one electron is removed from the orbital
featuring an antibonding interaction of the Ni dyz orbital and
the pp orbitals of O1 and N2. This explains why the oxidation
causes a stronger contraction on this particular axis. Impor-
tantly, this observation is in accordance with the highest
occupied quasi-restricted orbital calculated for [Ni(tBu-
Psalen)]+ (2+) as well as the Mulliken spin distribution,
which is mainly concentrated on Ni (0.69), O1 (0.13), and N2
(0.12) (Figure 2). Therefore, from the DFT calculations, and
1
a paramagnetic species, whose H NMR signals were in the
range of chemical shift values already observed for NiIII
complexes.[15] Variable-temperature experiments confirmed
the temperature dependence of these chemical shifts. Various
silver salts were used and led to similar results nevertheless
only 2+ SbF6ꢁ did crystallize, which is why we focused on this
complex. Complex 2+ is stable for months in the solid state
and for days in toluene or CH2Cl2 solution, at room temper-
ature under an inert atmosphere.
The ORTEP plot of 2+ is presented in Figure 1. The nickel
ion adopts a slightly distorted square-planar geometry
(deviation from planarity O1-N1-N2-O2: 11.068). No stacking
ꢁ
interaction or hydrogen bond was observed with the SbF6
ꢁ
anion. The O CAr bonds do not change significantly upon
oxidation (1.327(4) ꢀ and 1.326(5) ꢀ vs 1.317(3) ꢀ), whereas
ꢁ
ꢁ
all Ni O and Ni N bonds shorten (1.875(3) ꢀ and 1.842(3) ꢀ
+
ꢁ
ꢁ
for Ni N and 1.828(2) ꢀ and 1.844(2) ꢀ for Ni O in 2 vs
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over the z axis (g3) but with a significant distortion in the xy
plane (g1 differs from g2). This observation is in good
agreement with the X-ray data that feature two sets of
distances in the xy plane for the axes N1-Ni-O2 and N2-Ni-O1
but, unfortunately, no hyperfine coupling with 14N and/or 31
P
in the x and y axes could be observed to further sharpen this
analysis. The powder EPR spectrum of 2+ SbF6 is also in
ꢁ
agreement with a NiIII complex (see the Supporting Informa-
tion), ruling out an eventual axial solvent coordination in
solution. EPR g-tensor calculations were also performed on
2+ SbF6ꢁ and gave a very similar outcome (g1 = 2.34, g2 = 2.21,
and g3 = 2.09, giso = 2.21; see the Supporting Information).
As the decrease of the EPR signal intensity could be
associated with valence tautomerism,[5b] temperature-depen-
dent magnetic data were recorded in the solid state for
2+ SbF6ꢁ. The data show a Curie behavior over the temper-
ature range 4–300 K (cM = C/T; see the Supporting Informa-
tion). The cM T value (C = 0.46 cm3 Kmolꢁ1) is a little higher
than expected for a d7 low-spin complex but can be fitted with
a giso value of 2.22, which is in very good agreement with the
giso value obtained from EPR (giso = 2.19). No decrease of the
cM T value was observed in the temperature range 4–300 K,
which reinforces that the oxidized nickel phosphasalen
complex features a d7 high-valent NiIII center which persists
upon increasing the temperature.
Figure 2. Spin-density distribution in complex 2+.
Visible spectroscopy was also performed on 2 and
2+ SbF6 at room temperature in CH2Cl2 and toluene. Com-
ꢁ
plex 2 presents two bands at lmax = 565 nm (17700 cmꢁ1) and
lmax = 665 nm (15040 cmꢁ1; see Figure S4) that account for
metal!ligand charge transfer (MLCT), typical in NiII square-
planar complexes.[4a] When one electron is removed by
oxidizing 2 to 2+ SbF6ꢁ, the purple color of 2 changes to
black, which is indicative of high absorption in the visible
Figure 3. a) X-band EPR spectra of 2+ SbF6ꢁ at different temperatures.
2+ SbF6ꢁ (4 mm) in toluene:benzene, frequency 9.3690 GHz,
P=0.1 mW at 5 K, 60 mW at other temperatures. b) Experimental (at
5 K) and simulated spectrum.
range. Two broad bands are centered in toluene at lmax
=
490 nm (20400 cmꢁ1) and lmax = 990 nm (10100 cmꢁ1) with
absorption coefficients over 5000 cmꢁ1mꢁ1 (Figure 4). To
better understand this particular spectrum, a time-dependent
DFTanalysis was performed using the optimized geometry of
contrary to its salen analogue [Ni(tBu-salen)]+, 2+ may be
described as a high-valent NiIII complex rather than a metal
complex with a phenoxy radical ligand.
The X-band EPR spectrum of 2+ SbF6 was recorded in
ꢁ
a toluene/benzene mixture at different temperatures. At 5 K,
2+ SbF6ꢁ displayed a large g-tensor anisotropy (Figure 3) with
g1 = 2.29, g2 = 2.215, and g3 = 2.061 (giso = 2.19), indicative of
a S = 1/2 ground state in a rhombic symmetry centered on the
nickel ion, that is, a NiIII center. When the temperature was
increased, the EPR signal decreased but no change in the
g strengths was noticed up to 100 K. Above this temperature,
the signal was very weak, which prevented reasonable
ꢁ
simulation, and finally disappeared so that 2+ SbF6 is EPR-
silent at room temperature. Large g values account for
significant orbital contributions that allow low-lying states
to mix through spin–orbit coupling. This is rather usual in
d7 complexes, and may also relate to the loss of intensity of the
EPR signal at higher temperatures.[16,17] In a perfect square-
planar geometry the g matrix orientation would be as gx =
gy @ gz.[18] In the case of 2+ SbF6ꢁ, it is as g1 > g2 @ g3 and
indeed indicates a predominance of the xy plane (g1 and g2)
Figure 4. UV/Vis spectra of 2+ SbF6ꢁ in CH2Cl2 (black) and toluene
(gray).
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2+ described above. This analysis predicted three principal
transitions at 11337 cmꢁ1 (I, fosc = 0.066), 11661 cmꢁ1 (II, fosc
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=
0.064), and 22409 cmꢁ1 (III, fosc = 0.094), with energies that
agree to some extent with the experimental transitions
(10100 cmꢁ1 and 20400 cmꢁ1; see Figure S10). All calculated
transitions have a multiconfigurational character that does
not allow sharp conclusions,[19] but this analysis indicates that I
and II are essentially composed of L!M CTs and explains
well the broad and intense transitions and the dark color of
the complex. No other significant transition is predicted at
lower energy which reinforces the picture of a localized NiIII
center versus a delocalized system.[4a]
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In summary, a NiII complex with a novel tert-butyl
substituted phosphasalen ligand was synthesized and charac-
terized. Upon oxidation, this complex led to a high-valent
NiIII[20] complex as demonstrated by X-ray analysis, EPR
spectroscopy, magnetic measurements, and visible spectros-
copy as well as DFT calculations. 2+ SbF6ꢁ is therefore one of
the rare tetracoordinated NiIII complexes,[15,16,18a,21] and to the
best of our knowledge the only one featuring a phenoxide
ligand. This observation markedly differs from precedents in
the salen chemistry where NiIII complexes were observed only
at low temperature, or in the presence of additional ligands or
anions. It can be rationalized by the better electron-donating
ability of iminophosphoranes compared to imines, and by the
decreased electron density at the phenol rings which are
substituted by an electron-demanding phosphorus atom.[12]
Preliminary investigations concerning the reactivity of
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2012, 18, 14117 – 14127.
2+ SbF6 showed it to have a high but difficult to control
ꢁ
oxidation capacity. Our current efforts are devoted to this,
considering a fine tuning of the ligand structure. Further
studies will also focus on the oxidation of phosphasalen
complexes of metals different from nickel.
[9] T. Storr, P. Verma, Y. Shimazaki, E. C. Wasinger, T. D. P. Stack,
Chem. Eur. J. 2010, 16, 8980 – 8983.
[10] E. M. Broderick, N. Guo, C. S. Vogel, C. Xu, J. Sutter, J. T.
Miller, K. Meyer, P. Mehrkhodavandi, P. L. Diaconescu, J. Am.
Chem. Soc. 2011, 133, 9278 – 9281.
Experimental Section
Synthesis of 2+ SbF6ꢁ: Addition of AgSbF6 (34.4 mg, 0.1 mmol) to
a solution of 2 (89.4 mg, 0.1 mmol) in CH2Cl2 (5 mL) induced a rapid
color change from purple to black and precipitation of silver. After
removal of Ag by centrifugation, CH2Cl2 was evaporated and the
residue was dissolved in toluene (1.5 mL), giving a deep red-brown
solution. Dark crystals were obtained after one week of storage at
ꢁ408C, isolated by centrifugation, and dried under reduced pressure
[11] a) A. Buchard, A. Auffrant, C. Klemps, L. Vu-Do, L. Boubekeur,
X. F. Le Goff, P. Le Floch, Chem. Commun. 2007, 1502 – 1504;
b) A. Buchard, B. Komly, A. Auffrant, X. F. Le Goff, P. Le Floch,
Organometallics 2008, 27, 4380 – 4385; c) A. Buchard, A. Auf-
frant, L. Ricard, X. F. Le Goff, R. H. Platel, C. K. Williams, P.
Le Floch, Dalton Trans. 2009, 10219 – 10222; d) T. P. A. Cao, E.
Payet, A. Auffrant, X. F. Le Goff, P. Le Floch, Organometallics
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1
(104 mg, 95%). H NMR: d = 19.10 (s, 2H, CH(OAr), 15.30 (s, 2H,
CH(OAr), 8.31 (s, br, 16H, CH(PPh2)), 7.20 (s, br, 4H, CH(PPh2)),
3.17 (s, 18H, C(CH3)3), 2.50 (s, 18H, C(CH3)3), 1.68 (s, 2H, N CH2
[12] T. P. A. Cao, S. Labouille, A. Auffrant, Y. Jean, X. F. Le Goff, P.
Le Floch, Dalton Trans. 2011, 40, 10029 – 10037.
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
CH2 Nꢁ), 1.63 ppm (s, br, 2H, N CH2 CH2 N). Analysis calcd for
2+ SbF6 (C54H64F6N2NiO2P2Sb): C 57.42; H 5.71; N 2.48; found: C
57.50; H 5.80; N 2.37.
[13] a) C. Bakewell, T. P. A. Cao, N. Long, X. F. Le Goff, A. Auffrant,
C. K. Williams, J. Am. Chem. Soc. 2012, 134, 20577 – 20580;
b) T. P. A. Cao, A. Buchard, X. F. Le Goff, A. Auffrant, C. K.
Williams, Inorg. Chem. 2012, 51, 2157 – 2169; c) C. Bakewell,
T. P. A. Cao, N. Long, X. F. Le Goff, A. Auffrant, C. K. Williams,
Organometallics 2013, 32, 1475 – 1483.
Received: October 22, 2013
Published online: December 16, 2013
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Naruta, O. Yamauchi, J. Am. Chem. Soc. 2007, 129, 2559 – 2568.
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13063.
[16] P. J. Alonso, A. B. Arauzo, M. A. Garcia-Monforte, A. Martin,
B. Menjon, C. Rillo, M. Tomas, Chem. Eur. J. 2009, 15, 11020 –
11030.
Keywords: high-valent metals · iminophosphoranes · nickel ·
one-electron oxidation · phosphasalens
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