(α-diimine)chromium complexes: Molecular and electronic structures; a combined experimental and density functional theoretical study

Article abstract of DOI:10.1021/ic800290s

Dark brown crystals of [Cr(¹L)₂] (1) were obtained from the reaction of [Cr^III(acac)₃] (acac^- = 2,4-pentanedionate) with 2 equiv of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4- diaza-1,3-butadiene (¹L) and 3 equiv of sodium in tetrahydrofuran (thf) under an Ar atmosphere. Complex 1 possesses an S = 1 ground state, which is attained via intramolecular antiferromagnetic coupling between a high-spin Cr^II ion (S_Cr = 2) and two anionic α-diiminato(1-) ligand π radicals (¹L^?)^1-. The molecular structure of 1 exhibits a distorted tetrahedral, nearly square-planar geometry. The average C-N_imine bond length at 1.346 A is characteristic for the π radical anion (¹L^?) ^1-, and therefore, the electronic structure of 1 is best described as [Cr^II(¹L^?)₂]. This has been confirmed by broken symmetry density functional theoretical calculations BS(4,2) (DFT) at the B3LYP level. The reaction of [Cr^III(acac)₃] with 1 equiv of 2,3-dimethyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3- butadiene (²L) and 1 equiv of Na in thf under Ar yields red-brown crystals of [Cr^III(²L^?)(acac)₂] (2) (S = 1). The oxidation of 2 with 1 equiv of Fc(PF₆) (Fc ⁺ = ferrocenium) in CH₂Cl₂ affords crystals of [Cr^III(²L^ox)(acac)₂](PF ₆) (3) (S = 3/2). The crystal structure determinations of 2 and 3 revealed that 2 contains a neutral, octahedral Cr^III species [Cr ^III(²L^?)(acac)₂], whereas in 3 the ligand is oxidized, yielding an octahedral monocation [Cr ^III(²L^ox)(acac)₂]⁺. These electronic structures have been confirmed by DFT calculations.

Full text of DOI:10.1021/ic800290s

Inorg. Chem. 2008, 47, 5963-5970
(r-Diimine)chromium Complexes: Molecular and Electronic Structures;
A Combined Experimental and Density Functional Theoretical Study
Meenakshi Ghosh, Stephen Sproules, Thomas Weyhermu¨ller, and Karl Wieghardt*
Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36,
D-45470 Mu¨lheim an der Ruhr, Germany
Received February 14, 2008
Dark brown crystals of [Cr(1L)2] (1) were obtained from the reaction of [CrIII(acac)3] (acac- ) 2,4-pentanedionate)
with 2 equiv of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (1L) and 3 equiv of sodium in
tetrahydrofuran (thf) under an Ar atmosphere. Complex 1 possesses an S ) 1 ground state, which is attained via
intramolecular antiferromagnetic coupling between a high-spin CrII ion (SCr ) 2) and two anionic R-diiminato(1-)
ligand π radicals (1L•)1-. The molecular structure of 1 exhibits a distorted tetrahedral, nearly square-planar geometry.
The average C-Nimine bond length at 1.346 Å is characteristic for the π radical anion (1L•)1-, and therefore, the
electronic structure of 1 is best described as [CrII(1L•)2]. This has been confirmed by broken symmetry density
functional theoretical calculations BS(4,2) (DFT) at the B3LYP level. The reaction of [CrIII(acac)3] with 1 equiv of
2,3-dimethyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (2L) and 1 equiv of Na in thf under Ar yields
red-brown crystals of [CrIII(2L•)(acac)2] (2) (S ) 1). The oxidation of 2 with 1 equiv of Fc(PF6) (Fc+ ) ferrocenium)
3
in CH2Cl2 affords crystals of [CrIII(2Lox)(acac)2](PF6) (3) (S ) /2). The crystal structure determinations of 2 and 3
revealed that 2 contains a neutral, octahedral CrIII species [CrIII(2L•)(acac)2], whereas in 3 the ligand is oxidized,
yielding an octahedral monocation [CrIII(2Lox)(acac)2]+. These electronic structures have been confirmed by DFT
calculations.
Introduction
oxidation levels have been identified, namely, the neutral
R-diimines Lox, their π radical monoanions (L•)1-, and the
two-electron reduced enediamide form (LRed)2-. It has been
established by Raston et al.7 and van Koten et al.,8 in a series
of complexes containing Li+ and Zn2+ ions, that the C-Nimine
bond distances vary in a characteristic fashion on going from
Lox via (L•)1- to (LRed)2-. This is schematically shown in
Chart 1. These experimental results have recently been nicely
reproduced by DFT calculations.13–15
Careful structural, spectroscopic characterizations and
density functional theoretical (DFT) calculations of main
group element compounds containing R-diimine-type ligands
have unequivocally shown that this type of ligand belongs
to the growing group of redox noninnocent ligands.1–14 Three
* To whom correspondence should be addressed. Phone: ++49-208-
306 3609. Fax: ++49-208-306 3952. E-mail: wieghardt@
mpi-muelheim.mpg.de.
It is therefore rather surprising that tetrahedral bis(R-
diimine)metal complexes of transition metal ions (M ) Ni,
Fe) have in the past in the organometallic literature always
(1) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151.
(2) Richter, S.; Daul, C.; von Zelewsky, A. Inorg. Chem. 1976, 15, 943.
(3) Corvaja, C.; Pasimeni, L. Chem. Phys. Lett. 1976, 261.
(4) Cloke, F. G. N.; Hanson, G. R.; Henderson, M. J.; Hitchcock, P. B.;
Raston, C. L. J. Chem. Soc., Chem. Commun. 1989, 1002.
(5) Cloke, F. G. N.; Dalby, C. I.; Henderson, M. J.; Hitchcock, P. B.;
Kennard, C. H. L.; Lamb, R. N.; Raston, C. L. J. Chem. Soc., Chem.
Commun. 1990, 1394.
(10) Baker, R. J.; Farley, R. D.; Jones, C.; Mills, D. P.; Kloth, M.; Murphy,
D. M. Chem.sEur. J. 2005, 11, 2972.
(11) Pott, T.; Jutzi, P.; Kaim, W.; Schoeller, W. W.; Neumann, B.;
Stammler, A.; Stammler, H.-G.; Wanner, M. Organometallics 2002,
21, 3169.
(6) Thiele, K.-H.; Lorenz, V.; Thiele, G.; Zo¨nnchen, P.; Scholz, J. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1372.
(12) (a) Baker, R. J.; Farley, R. D.; Jones, C.; Kloth, M.; Murphy, D. M.
Chem. Commun. 2002, 1196. (b) Baker, R. J.; Farley, R. D.; Jones,
C.; Kloth, M.; Murphy, D. M. Dalton Trans. 2002, 3844.
(13) Tuononen, H. M.; Armstrong, A. F. Dalton Trans. 2006, 1885.
(14) Tuononen, H. M.; Armstrong, A. F. Inorg. Chem. 2005, 44, 8277.
(15) Muresan, N.; Chlopek, K.; Weyhermu¨ller, T.; Neese, F.; Wieghardt,
K. Inorg. Chem. 2007, 46, 5327.
(7) Gardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee, F. C.; Raston,
C. L. Inorg. Chem. 1994, 33, 2456.
(8) Rijnberg, E.; Richter, B.; Thiele, K.-H.; Boersma, J.; Veldman, N.;
Spek, A. L.; van Koten, G. Inorg. Chem. 1998, 37, 56.
(9) Pott, T.; Jutzi, P.; Neumann, B.; Stammler, H.-G. Organometallics
2001, 20, 1965.
10.1021/ic800290s CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 13, 2008 5963
Published on Web 05/28/2008

Ghosh et al.
structure, as in [(L•)CrII(µ-Cl)2CrII(L•)2]0, where the high-
Chart 1
spin CrII ions (d4, SCr ) 2) are strongly antiferromagnetically
3
coupled to a ligand radical (S* ) /2), two of which couple
to yield the observed diamagnetic ground state.
In order to shed more light on the electronic structures of
coordination complexes of chromium-containing R-diimine-
type ligands, we have synthesized complexes 1, 2, and 3,
shown in Chart 1; determined their crystal structures; and
now report their magnetochemistry and electronic spectra.
We will show that it is unambiguously possible to determine
the oxidation state (level) of the central chromium ion and
the ligands. These experimental results have been supported
by broken symmetry (BS) DFT calculations by using the
B3LYP functional.
Experimental Section
All air-sensitive materials were manipulated by using standard
Schlenk line techniques or a glovebox. [CrIII(acac)3], where (acac)1-
represents the 2,4-pentanedionate(1-) anion, and ferrocenium hexafluo-
rophosphate were purchased from Aldrich. The ligands 2-methyl-1,4-
bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene, (1Lox)0, and 2,3-
dimethyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene, (2Lox),
were synthesized according to published procedures.21
[CrII(1L•)2] (1). To a solution of the ligand (1Lox; 0.56 g, 2.0
mmol) in dry tetrahydrofuran (thf; 20 mL) under an argon
blanketing atmosphere was added sodium (0.07 g, 3.0 mmol) with
stirring at 20 °C. After 20 min, [CrIII(acac)3] (0.35 g, 1.0 mmol)
was added to the solution, whereupon the red color of the solution
turned dark brown. After being stirred for 24 h at 20 °C, the solvent
was removed by evaporation under reduced pressure. The dry
residue was extracted with n-hexane and filtered through celite.
Upon partial removal of the solvent under reduced pressure, dark-
brown crystals of 1 were obtained at -20 °C. Yield: 0.51 g (84%).
Anal. calcd for C38H44N4Cr: C, 75.0; H, 7.3; N, 9.2; Cr, 8.6. Found:
C, 75.1; H, 7.4; N, 9.1; Cr, 8.6.
been described as [M0(Lox)2] despite the fact that the
corresponding paramagnetic zinc complex has unambigu-
ously been identified as [ZnII(L•)2].7,8 Only recently, it has
been established that the electronic structures are correctly
designated as [NiII(L•)2]0,15–17 and [FeII(L•)2]0,18 where high-
spin ions NiII (SNi ) 1) and FeII (SFe ) 2) are intramolecularly
antiferromagnetically coupled to two ligand π radical anions
(L•)1- (Srad ) 1/2), yielding the observed singlet for the nickel
and the triplet ground state for the iron complexes.
Interestingly, tom Dieck and Kinzel19 reported in 1979
the synthesis of a similar bis(R-diimine)chromium complex,
to which they assigned an electronic structure of [Cr0(Lox)2],
where Lox represents N,N′-ethanediylidene-bis(2,4-dimethyl-
3-pentaneamine) (eq 1). This complex was shown to dimerize
isoprene catalytically in the presence of Et2AlOEt.
[CrIII(2L•)(acac)2]0 (2). To a solution of the ligand (2Lox) (0.81
g, 2.0 mmol) in tetrahydrofuran (20 mL) under an Ar blanketing
atmosphere was added sodium (0.05 g, 2.0 mmol) and [CrIII(acac)3]0
(0.70 g, 2.0 mmol) with stirring at 20 °C for ∼20 h. After removal
of the solvent by evaporation under reduced pressure, the dry residue
was extracted with n-hexane and filtered through celite. X-ray-
quality crystals were obtained from a concentrated n-hexane solution
at -20 °C. Yield: 1.05 g (80%). Anal. calcd for C38H54N2O4Cr: C,
69.7; H, 8.3; N, 4.3; Cr, 7.9. Found: C, 69.8; H, 8.4; N, 4.2; Cr,
7.9.
At that time, only the electronic spectrum and its mass
spectrum were reported; the spin ground state of this
molecule is unknown.
More recently, Theopold et al.20 reported the synthesis and
crystal structure of a planar, dinuclear species, eq 2:
[CrIII(2Lox)(acac)2](PF6)·thf (3). To a solution of 2 (0.26 g, 0.40
mmol) in CH2Cl2 (10 mL) was added 1 equiv of ferrocenium
(16) Muresan, N.; Weyhermu¨ller, T.; Wieghardt, K. Dalton Trans. 2007,
4390.
(17) Khusniyarov, M. M.; Harms, K.; Burhaus, O.; Sundermeyer, J. Eur.
J. Inorg. Chem. 2006, 2985.
(18) Muresan, N.; Lu, C. C.; Ghosh, M.; Henling, L. E.; Peters, J.;
Weyhermüller, T.; Bill, E.; Wieghardt, K. Inorg. Chem. In print.
(19) tom Dieck, H.; Kinzel, A. Angew. Chem., Int. Ed. Engl. 1979, 18,
324.
The structural formula in eq 2 appears to invoke two
(20) Kreisel, K. A.; Yap, G. P. A.; Dmitrenko, O.; Landis, C. R.; Theopold,
K. H. J. Am. Chem. Soc. 2007, 129, 14162.
3
antiferromagnetically coupled CrI ions (d5, S ) /2) in a
(21) Druzhkov, N. O.; Teplova, I. A.; Glushakova, V. V.; Skorodumova,
V. A.; Abakumov, G. A.; Zhao, B.; Berluche, E.; Schulz, D. N.; Baugh,
L. S.; Ballinger, C. A.; Squire, K. R.; Canich, J.-A. M.; Bubnov, M. P.
Patent PCT/VS 2003/022356, WO2004/007509, 2004.
(22) ShelXTL, v.5; Siemens Analytical X-Ray Instruments, Inc.:, Madison,
WI, 1994.
square-planar environment. In contrast, the reported C-Nimine
distance at 1.340 Å and the corresponding average C-C
distance within the two five-membered chelate rings at 1.388
Å point to the presence of two ligand π radicals (L•)1-.
Therefore, we suggest that this species has an electronic
(23) Sheldrick, G. M. ShelXL97; Universita¨t Go¨ttingen: Germany, 1997.
5964 Inorganic Chemistry, Vol. 47, No. 13, 2008

(r-Diimine)chromium Complexes
Table 1. Crystallographic Data for 1, 2, and 3·thf
1
2
3 ·thf
chem. formula
fw
C38H44CrN4
608.77
C38H54CrN2O4
654.83
C42H62CrF6N2O5P
871.91
space group
a, Å
b, Å
c, Å
ꢁ, deg
V, Å
C2/c, No. 15
10.7469(6)
13.0959(6)
23.2424(12)
99.348(3)
3227.7(3)
4
C2/c, No. 15
21.835(2)
12.5567(10)
27.350(2)
90.743(7)
7498.1(11)
8
P21/n, No. 14
17.2368(3)
12.4547(2)
20.9158(4)
90.085(3)
4490.2(2)
4
Z
T, K
100(2)
100(2)
100(2)
F calcd, g cm-3
reflns collected/2Θmax
unique reflns/I > 2σ(I)
no. of params/restr.
λ, Å/µ(KR), cm-1
R1a/goodness of fitb
wR2c (I > 2σ (I))
residual density, eÅ-3
1.253
1.160
1.290
19891/66.48
6084/4758
205/0
0.71073/3.87
0.0487/ 1.041
0.1209
55245/55.00
8549/6022
420/0
0.71073/3.44
0.0562/1.022
0.0950
110846/65.00
16062/13116
561/31
0.71073/3.58
0.0460/ 1.030
0.1178
+0.30/-0.60
+0.30/-0.46
+0.71/-0.71
2
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|. b GOF ) [∑[w(Fo - Fc )2]/(n - p)]1/2 c wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2, where
.
2
2
2
2
2
2
2
w ) 1/σ2(Fo ) + (aP)2+ bP, P ) (Fo + 2Fc )/3.
hexafluorophosphate (0.13 g, 0.40 mmol) with stirring for 1.5 h at
20 °C. The original dark-brown color of the solution changed to
green-brown. The resulting solution was filtered and the solvent
removed by evaporation under reduced pressure. The residue was
washed 3-4 times with n-hexane in order to remove ferrocene.
The crude compound was dissolved in tetrahydrofuran and filtered
through celite. After slow evaporation of the solvent, single crystals
of X-ray quality were obtained. Yield: 0.48 g (92%). ESI mass
spectrometry (pos. ion; CH2Cl2): m/z 654 {3-PF6}+. Anal. calcd
for C42H62N2O5F6PCr: C, 57.8; H, 7.1; N, 3.2; Cr, 6.0. Found: C,
57.9; H, 7.2; N, 3.2; Cr, 6.1.
Calculations. All calculations were performed by using the
ORCA program package.24 The geometry optimizations were
carried out at the BP86 level25a,b of DFT. The all-electron
Gaussian basis sets were those reported by the Ahlrichs
group.26,27 Triple-ꢀ-quality basis sets with one set of polarization
functions on the chromium, oxygen, and nitrogen were used
(TZVP). The carbon and hydrogen atoms were described by
slightly smaller polarized split-valence SV(P) basis sets that are
double-ꢀ-quality in the valence region and contain a polarizing
set of d functions on the non-hydrogen atoms.26 The auxiliary
basis sets for all complexes used to expand the electron density
in the calculations were chosen to match the orbital basis.
Electronic energies and properties were calculated at the
optimized geometries with the B3LYP functional.25c,d In this
case, the same basis sets were used. The self-consistent field
calculations were tightly converged (1 × 10-8 Eh in energy, 1
× 10-7 Eh in the density change, and 1 × 10-7 in the maximum
element of the DIIS25e,f error vector). The geometry search for
all complexes was carried out in redundant internal coordinates
without imposing geometry constraints. Corresponding orbitals28
and density plots were obtained by the program Molekel.29 We
describe our computational results for 1 and 2 containing
noninnocent ligands using the BS approach.30–32
Since, for complexes 1 and 2 studied here, one can obtain
broken-symmetry solutions to the spin-unrestricted Kohn-Sham
equations,30–32 we will adopt the following notation: The system
is divided into two fragments. The notation BS(m,n) refers to a
broken symmetry state with m unpaired spin-up electrons on
fragment 1 and n unpaired spin-down electrons essentially
localized on fragment 2. In most cases, fragments 1 and 2
correspond to the metal and the ligand, respectively. Note that
in this notation a standard high-spin open-shell solution would
be written down as BS(m+n,0). In general, the BS(m,n) notation
refers to the initial guess to the wave function. The variational
process does, however, have the freedom to converge to a
solution of the form BS(m-n,0), where effectively the n-spin-
down electrons pair with n < m spin-up electrons on the partner
fragment. Such a solution is then a standard MS ) (m - n)/2
unrestricted Kohn-Sham solution. As explained elsewhere,28
the nature of the solution is investigated via the corresponding
orbital transformation, which via the corresponding overlaps
displays whether the system is to be described as a spin-coupled
or a closed-shell solution.
X-Ray Crystallographic Data Collection and Refinement
of the Structures. A black single crystal of 1 and orange-brown
crystals of 2 and 3· thf were coated with perfluoropolyether,
picked up with nylon loops, and mounted in the nitrogen cold
stream of the diffractometer. A Bruker-Nonius KappaCCD
diffractometer equipped with a Mo-target rotating-anode X-ray
source and a graphite monochromator (Mo KR, λ ) 0.71073 Å)
was used throughout. Final cell constants were obtained from
least-squares fits of all measured reflections. Intensity data were
corrected for absorption using intensities of redundant reflections.
The structures were readily solved by Patterson methods and
subsequent difference Fourier techniques. The Bruker ShelXTL22
software package was used for the solution, refinement, and
artwork rendering of the structures. All non-hydrogen atoms were
anisotropically refined, and hydrogen atoms were placed at
(24) Neese, F. ORCA, version 2.6, revision 4; Max Planck Institute for
Bioinorganic Chemistry: Mu¨lheim, Germany, 2007.
(25) (a) Becke, A. D. J. Chem. Phys. 1988, 84, 4524. (b) Perdew, J. P.
Phys. ReV. B 1986, 33, 8822. (c) Lee, C.; Yang, W.; Parr, R. G. Phys.
ReV. B: Condens. Matter Mater. Phys. 1988, 37, 785. (d) Becke, A. D.
J. Chem. Phys. 1993, 98, 5648. (e) Pulay, P. Chem. Phys. Lett. 1980,
73, 393. (f) Pulay, P. J. Comput. Chem. 1992, 3, 556.
(30) Noodleman, L.; Peng, C. Y.; Case, D. A.; Monesca, J. M. Coord.
Chem. ReV. 1995, 144, 199.
(31) (a) Noodleman, L.; Case, D. A.; Aizman, A. J. Am. Chem. Soc. 1988,
110, 1001. (b) Noodleman, L.; Davidson, E. R. Chem. Phys. 1986,
109, 131. (c) Noodleman, L.; Norman, J. G.; Osborne, J. H.; Aizman,
C.; Case, D. A. J. Am. Chem. Soc. 1985, 107, 3418. (d) Noodleman,
L. J. Chem. Phys. 1981, 74, 5737.
(26) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
(27) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
(28) Neese, F. J. Phys. Chem. Solids 2004, 65, 781.
(29) Molekel I, AdVances InteractiVe 3-Graphics for Molecular Sciences.
http://www.cscs.ch/molekel/ (accessed Apr 2008).
(32) Kapre, R. R.; Bothe, E.; Weyhermu¨ller, T.; De Beer George, S.;
Muresan, N.; Wieghardt, K. Inorg. Chem. 2007, 46, 7827.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5965

Ghosh et al.
Figure 1. Temperature dependence of the magnetic moment, µeff, µB, of
complexes 1, 2, and 3. The black circles represent experimental data,
whereas the solid lines represent simulations (see text).
calculated positions and refined as riding atoms with isotropic
displacement parameters. Crystallographic data of the compounds
are listed in Table 1.
A methyl group (C17) in complex 1 was found to be disordered
over two sites. The split atom model with restrained thermal
displacement parameters (EADP) yielded occupation factors of
about 0.5 for each carbon position.
The thf solvent molecule in compound 3·thf was found to be
severely disordered. A split atom model with three positions was
refined, giving occupation factors of about 0.63, 0.23, and 0.14.
Equal thermal displacement parameters were used using the EADP
instruction, and the SAME instruction of ShelXL9723 was used to
restrain C-C and C-O bond distances of thf.
Figure 2. (a) Electronic spectrum of 1 in CH2Cl2 at 22 °C. (b) The
corresponding spectra of 2 (solid line) and 3 (broken line) in CH2Cl2 at
22 °C. The inset shows the spectrum of 3 in the visible region.
Magnetic Measurements. Magnetic susceptibility data were
measured from powder samples of solid material in the temperature
range 2-300 K by using a SQUID susceptometer (MPMS-7,
Quantum Design) with a field of 1.0 T. The experimental data were
corrected for underlying diamagnetism by the use of tabulated
Pascal’s constants, as well as for temperature-independent para-
magnetism. The susceptibility and magnetization data were simu-
lated with our own package julX for exchange coupled systems
written by Dr. E. Bill Powder summations were done by using a
16-point Lebedev grid. Intermolecular interactions were considered
by using a Weiss temperature, ΘW, as perturbation of the temper-
ature scale, kT′ ) k(T - ΘW) for the calculation.
that 1 possesses a triplet ground state (S ) 1). The effective
magnetic moment is constant at 2.90 µB between 30 and
∼200 K but increases slightly to 3.0 µB at 290 K (Figure 1).
This increase was tentatively modeled by assuming a central
high-spin CrII ion (SCr ) 2) coupled antiferromagnetically
to two ligand π radical anions (1L•)1- (Srad ) 1/2). Using the
spin Hamiltonian eq 4, a coupling constant of J ) -215
cm-1 was obtained together with g values of 2.02 for CrII
and 2.0 for the radicals.
ˆ ˆ
F
ˆ ˆ
F F
Results
F
ˆ
[
]
H ) -2J SCr · Srad,1 + SCr · Srad,2
+
a. Syntheses and Characterization. The reaction of
[CrIII(acac)3] with 2 equiv of the ligand (1Lox; Chart 1) in
thf at 20 °C affords, upon the addition of 3 equiv of sodium
under strictly anaerobic conditions, dark brown crystals of
[Cr(1L)2] (1). Note that the generic notation (xL) denotes a
given R-diimine ligand irrespective of its oxidation level
which can be specified as neutral R-diimine (xLox)0 or its
reduced π radical anion (xL•)1-, or its doubly reduced
enediamide dianion (xLRed)2-. The above synthetic procedure,
shown in eq 3, follows the recipe described by tom Dieck
and Kinzel19 for [Cr(L)2], where L represents the ligand
N,N′-ethanediyliden-bis(2,4-dimethyl-3-pentaneamine).
ˆ
ˆ
ˆ
F
µB gCrSCr + grad Srad,1 + Srad,2 · B + DCr S2 - 2 (4)
F
F
F
[
(
)
]
[
]
Cr,z
The increase at low temperatures is due to weak ferromag-
netic intermolecular interaction according to a Weiss tem-
perature ΘWeiss ) +0.7 K. The effect was superimposed by
a decrease of the magnetic moments due to zero-field splitting
from the CrII ion with |D|Cr ) 1.7 ((1.5) cm-1.
This model implies that the electronic structure of 1 should
be described as [CrII(1L•)2] and not as [Cr0(1Lox)2]. Figure
2a shows the electronic spectrum of 1 in CH2Cl2. Three
intense absorption maxima at 730, 500, and 410 nm (ε >
0.2 × 104 M-1 cm-1) are tentatively assigned as intraligand
transitions of the π radical anions.
When a solution of [Cr(acac)3] in thf was treated with 1
equiv of the ligand (2Lox) and 1 equiv of sodium under strictly
anaerobic conditions at 20 °C, red-brown crystals of
[Cr(2L)(acac)2] (2) were obtained in good yields, eq 5.
thf
III
1 ox
Cr acac 3 + 2 L + 3Na
8 Cr(1L) 0 + 3Na(acac)
2
[
]
(
)
(
)
[
]
20 ° C
(3)
From the temperature-dependent magnetic susceptibility
measurements of a powder sample of 1 (1.0 T), it follows
5966 Inorganic Chemistry, Vol. 47, No. 13, 2008

(r-Diimine)chromium Complexes
thf
2
ox
Cr acac 3 + L + Na
8 Cr(2L)(acac) + Na(acac)
2
(
)
[
(
) ]
[
]
20 ° C
(5)
As shown in Figure 1, a temperature-independent magnetic
moment (5-295 K) of 2.75 µB indicates a triplet ground state
(S ) 1) for 2 which is attained via strong intramolecular
antiferromagnetic coupling between a central CrIII ion (SCr
3
1
) /2) and a single π radical anion (2L•)1- (Srad ) /2). A
simulation using eq 6 yields gCr ) 1.94 and DCr ) 2 cm-1,
with grad fixed to 2.
ˆ ˆ
ˆ
ˆ
F
H ) -2JSCr · Srad + µB gCrSCr + gradSrad B + DCr S2
-
F
F
F
F
ˆ
(
)
[
Cr,z
5/4 (6)
]
The lower g value of CrIII, compared to that of CrII found in
1, is the reason that the trace of 2 is lower than that of 1.
For the CrIII-radical coupling in 2, only a lower limit can
be given: -J > 450 cm-1 (as used for the actual result).
The electronic spectrum of 2 shown in Figure 2b displays
intense (ε > 0.1 × 104 M-1 cm-1) absorption maxima in
the visible region at 680, 560, and 460 nm characteristic of
the presence of a ligand π radical anion, as in 1.
We conclude that the electronic structure of octahedral 2
should be described as [CrIII(2L•)(acac)2]0. Interestingly, 2
can be chemically (by ferrocenium hexafluorophosphate) or
electrochemically (reversibly) one-electron oxidized, eq 7.
Figure 3. Cyclic voltammograms of 1 (top) and 2 (bottom) in CH2Cl2 at
22 °C. Conditions: 0.10 M [N(n-Bu)4]PF6 supporting electrolyte; scan rate,
100 mV s-1; glassy carbon working electrode.
addition, an irreversible reduction peak at -1.9 V which was
not studied in further detail. Both reversible waves involve
oxidations of 1, yielding a monocation and a dication, eq 9.
III 2
•
0
III 2 ox
[
(
)
]
(
)
[
]
Cr L acac 2 + Fc PF f Cr ( L )(acac) PF +
[
]
(
)
6
6
2
+e
+e
II 1
•
0
I 1 ox
1
•
2+
/ Cr ( L )( L ) + / CrII(1Lox)
[
(
)
]
2
[
]
Cr
L
[
]
Fc0 (7)
2
-e
E21⁄2
-e
E11⁄2
(9)
Thus, from a CH2Cl2 solution of 2, greenish-brown crystals
of [CrIII(2Lox)(acac)2](PF6) (3) were obtained upon the
addition of 1 equiv of Fc(PF6).
We propose that both processes are essentially ligand-
based where the coordinated π radical anions (1L•)1- are
successively oxidized to the neutral R-diimines (1Lox)0. Very
similar CVs have been reported for [NiII(L•)2] species,15,16
From temperature-dependent magnetic susceptibility mea-
surements, it was established (Figure 1) that 3 possesses an
S ) 3/2 ground state (µeff ) 3.75 µB, 3-295 K) as is expected
for an octahedral chromium(III) (d3) species with six closed-
shell ligands.
The simulation using eq 8 yields gCr ) 1.94, in nice
agreement with the result found for 2, whereas the zero-
field parameter DCr can only be estimated as |D|Cr < 0.5 cm-1.
1
2
where values for E1/2 of ∼-0.2 V and E1/2 of ∼-0.70 V
have been reported. The monocation [Ni(L)2]+ has been
shown by X-ray crystallography to contain two neutral (Lox)
ligands and a Ni(I) center. Here, we prefer an electronic
structure for the monocation as in [CrII(1Lox)(1L•)]+, but with
the data at hand, this remains speculative.
The CV of 2 displays a single, reversible one-electron
transfer wave at E1/2 ) -0.80 V versus Fc+/Fc according to
eq 10.
ˆ
H ) µBgCrSCr · B + DCr S2 - 5/4
(8)
F
F
ˆ
[
]
Cr,z
Its electronic spectrum lacks the intense (ε > 103 M-1 cm-1)
transitions in the visible (Figure 2b), which are seen in the
spectra of 1 and 2. Instead, a weak d-d transition is observed
at ∼600 nm (ε ∼ 150 M-1 cm-1).
+e
+
CrIII(1L•)(acac) 0. / CrIII(1Lox)(acac)
[
]
[
]
2
2
-e
(10)
E1⁄2
The above data are in accord with the notion that the cation
in 3 has an electronic structure which is best described as
[CrIII(2Lox)(acac)2]+. Thus, the one-electron oxidation of 2
affording 3 is ligand-based.
c. X-Ray Structures. The structures of 1, 2, and 3 have
been determined by single-crystal X-ray crystallography at
100(2) K. Table 1 summarizes crystallographic data, and
Table 2 summarizes important bond lengths and angles.
Figure 4 shows the structure of a neutral molecule in crystals
of 1, and Figure 5 shows that of a molecule in crystals of 2;
Figure 6 displays the structure of a monocation in 3.
The neutral complex [CrII(1L•)2] contains two N,N′-
coordinated π radical anions (1L•)1- as is clearly established
b. Electrochemistry. Figure 3 exhibits the cyclic volta-
mmograms (CVs) of 1 and 2 in CH2Cl2 (0.10 M [N(n-
Bu)4]PF6 supporting electrolyte) recorded at 22 °C.
The CV of 1 displays two reversible one-electron transfer
1
2
waves at E1/2 ) -0.39 V and E1/2 ) -0.78 V and, in
Inorganic Chemistry, Vol. 47, No. 13, 2008 5967

Ghosh et al.
Table 2. Selected Experimental and Calculated Bond Distances [Å] in 1, 2, and 3
1
2
3
exp.
calcd
exp.
calcd
exp.
calcd
Cr1-N1
Cr1-N4
N1-C2
N4-C3
C2-C3
N1-C11
N4-C5
2.019(1)
2.030(1)
1.351(2)
1.344(2)
1.395(2)
1.428(2)
1.428(2)
2.053
2.041
1.356
1.367
1.406
1.428
1.433
Cr1-N1
Cr1-N4
Cr1-O31
Cr1-O35
Cr1-O41
Cr1-O45
N1-C2
1.995(2)
2.014(2)
1.981(1)
1.987(2)
2.006(1)
2.006(2)
1.353(3)
1.357(3)
1.420(2)
1.445(3)
1.442(3)
2.023
2.005
1.989
1.998
2.006
2.009
1.356
1.357
1.424
1.435
1.433
2.099(1)
2.109(1)
1.939(1)
1.953(1)
1.938(1)
1.948(1)
1.289(2)
1.290(2)
1.512(2)
1.451(2)
1.456(2)
2.112
2.112
1.952
1.969
1.952
1.969
1.308
1.308
1.495
1.441
1.441
N4-C3
C2-C3
N1-C11
N4-C5
by the average C-N bond length at 1.348 Å (see Chart 1);
the C-C bond distance in the five-membered chelate ring
at 1.395 Å is also in excellent agreement with this notion.
These parameters exclude formulations as [Cr0(1Lox)2] or
[CrIV(1Lred)2]; they render the physical oxidation state of the
central chromium as +II (d4; high-spin).
It is interesting that the CrN4 coordination polyhedron in
1 is nearly square-planar since the dihedral angle θ between
the two Cr-N-C-C-N five-membered chelate rings is
23.7° (in a perfectly square-planar arrangement, θ ) 0, and
in a tetrahedral one, θ ) 90°). This dihedral angle is 48° in
the corresponding complex [NiII(2L•)2].15,16
It is now immediately evident that [CrII(1L•)2] should exist
in two geometrical isomers, namely, in a transoid and in a
cisoid form (see below). Both forms cocrystallize in 1 in a
1:1 ratio; they are statically disordered, which is born out
by the fact that each ligand 1L apparently carries two methyl
carbon atoms C(17) and C(17)*, each with an occupancy of
0.5. Atoms N(1), N(4), C(2), and C(3) have also been refined
in two split positions (occupancy 0.5).
The CrN2O4 coordination polyhedron of neutral
[CrIII(2L•)(acac)2] (2) is slightly distorted octahedral (Figure
5); there are two bidentate 2,4-pentanedionate(1-) ligands
and a single N,N′-coordinated π radical anion (2L•)1-. The
average N-C bond length at 1.355 Å and the C2-C3 bond
distance at 1.420 in (2L•)1- are only compatible with a
formulation as a radical anion (Chart 1). This assignment
renders the oxidation state of the central chromium ion +III
(d3). The metrical details of the CrIII(acac)2 unit in 2 are very
similar to those reported for [CrIII(acac)3]0.33
The structure of the monocation in crystals of 3 is very
similar to that of the neutral molecules in 2. There are
significant differences in the C-N and C-C distances at
1.289 and 1.512 Å, respectively, in the monocation of 3,
which are 1.355 and 1.420 Å, respectively, in neutral 2. This
is exactly the difference one would expect on going from
the π radical anion (2L•)1- in 2 to the oxidized form (2Lox)
in 3. Thus, the redox activity of 2 and 3 is ligand-based.
d. Calculations. A picture of the electronic structures of
complexes 1, 2, and 3 is derived from BS DFT calculations
by using the B3LYP functional for the molecular orbital
(MO) descriptions andsmost significantlysthe spin distribu-
tions in these complexes. In general, we found that the
optimized geometries of all calculated complexes agree very
well with the X-ray structural data (Table 2).
Figure 4. Structure of a neutral molecule in crystals of 1.
The geometry of the monocation in 3 was calculated spin-
unrestricted (S ) 3/2) where the isopropyl substituents were
replaced with methyl groups to decrease computation time.
Figure 5. Structure of a neutral molecule in crystals of 2.
5968 Inorganic Chemistry, Vol. 47, No. 13, 2008
(33) Morosin, B. Acta Crystallogr. 1965, 19, 131.

(r-Diimine)chromium Complexes
Figure 6. Structure of the monocation in crystals of 3.
The calculated C-O, C-N, and C-C distances of the
ligands are in excellent agreement with experimental results
((0.01 Å). Importantly, the calculated C-Nimine and C-C
distances of the five-membered chelate ring imply the
presence of a N,N′-coordinated (2Lox) ligand. In the qualita-
tive MO bonding scheme (not shown) derived from this spin-
unrestricted calculation, five orbitals of predominantly metal
d character (>90%) were identified; three of these are
occupied with a single electron (t2g3). This is the hallmark
of a CrIII (d3) ion. The Mulliken spin density analysis displays
three unpaired electrons at the CrIII ion and no spin density
on any of the three ligands, which are all closed-shell. These
DFT calculations are in full agreement with simple ligand
field theoretical considerations for an octahedral chro-
mium(III) species.
Similarly, the geometry of the neutral species in 2 was
calculated by using a BS(3,1) MS ) 1 state. Again, the
agreement of geometrical features between the calculation
and experimental results is excellent. Most importantly, the
average C-Nimine and C-C bond distances of the five-
membered chelate ring are faithfully reproduced at 1.356 and
1.424 Å, respectively, indicating the presence of a π radical
anion (2L•)1- in 2.
Figure 7. Top: Qualitative MO diagram for 2 of the corresponding orbitals28
of magnetic pairs calculated at the B3LYP/TZV(P) level. Bottom: Spin
density plot of 2 as derived from BS DFT calculations together with values
of the spin density of the Mulliken spin population analysis.
calculated and experimental structure (Table 2) is excellent.
In particular, the average C-Nimine bond distance (1.362 Å
(calcd) and 1.347 Å (exp)) is indicative of the presence of
two π radical anions (2L•)1-. It is also gratifying that the
calculated dihedral angle between the two five-membered
chelate rings at 37.2° is in reasonable agreement with the
experimental one at 23.7°.
Figure 8 exhibits a qualitative MO scheme of the spin-
unrestricted corresponding orbitals of magnetic pairs and
metal-d orbitals of 1. Four singly occupied metal-d orbitals
(spin-up manifold) are identified which define a high-spin
chromium(II) ion. In addition, two singly occupied ligand
orbitals are identified in the spin-down manifold which are
not populated in the spin-up manifold; this leads to the
observed overall MS ) 1 state. Complex 1 features a CrII
ion which is strongly antiferromagnetically coupled to two
ligand π radical anions: [CrII(1L•)2].
Figure 7 shows a qualitative MO scheme of the corre-
sponding orbitals of magnetic pairs and metal-d orbitals of
2 and, at the bottom, a spin density plot of 2 together with
approximate values of the spin density of the Mulliken spin
population analysis. Five metal-d orbitals are identified, of
which three are singly occupied with parallel spins (R-spin).
This pattern defines again a CrIII configuration at the metal
center. In addition, a single ligand-centered orbital at the 2L
ligand is identified in the spin-down manifold, which is not
populated in the spin-up manifold, thus leading to the
observed overall MS ) 1 state. Complex 2 therefore features
a CrIII ion which is strongly antiferromagnetically coupled
to a ligand π radical. The Mulliken spin density analysis
confirms that there are three electrons at the CrIII center (R-
spin), and one unpaired electron resides in a π* orbital of
the R-diimine ligand (ꢁ-spin). No significant spin density
has been identified on the two acac- ligands.
Figure 8 (bottom) also displays the spin density from a
Mulliken spin population analysis. Four unpaired electrons
(R-spin) reside on the central chromium(II) ion, and a single
unpaired electron resides on each of the R-diiminate ligands
(1L•)1-.
Discussion
Previously, it has been shown that bis(R-diimine)metal
complexes of Zn,7,8,15 Ni,15–17 and Fe18 possess an electronic
structure which invariably invokes a divalent central transi-
tion metal ion (ZnII, NiII, and FeII) and two π radical
monoanions (L•)1-: [MII(L•)2]. The observed ground states
S ) 0 and 1 for the ZnII complex, S ) 0 for the NiII, and S
) 1 for the FeII are composed of (a) two antiferromagneti-
cally or ferromagnetically coupled ligand π radicals in
[ZnII(L•)2] where both states are isoenergetic, (b) two π
radicals which are intramolecularly antiferromagnetically
coupled to a central nickel(II) ion (SNi ) 1, d8), or, similarly,
The geometry of the neutral complex in 1 was calculated
by using a BS(4,2) MS ) 1 state. Agreement between the
Inorganic Chemistry, Vol. 47, No. 13, 2008 5969

Ghosh et al.
complex35 [Cr(o-C6H4(NH)2)Cl2(PMe2Ph)2] (S ) 1) contains
an o-phenylenediamide(1-) π radical and does not possess a
central CrIV ion (as reported) but rather is a chromium(III)
species with a N,N′-coordinated π radical anion. The reported
geometrical features of the radical ligand are in full agree-
ment with this assignment.36 As pointed out in the Introduc-
tion, Theopold et al.’s20 dinuclear complex should be
formulated as [(L•)CrII(µ-Cl)2CrII(L•)]0 since the reported
dimensions of the R-diimine ligand are those typical for a π
radical anion.
Finally, we have recently shown that R-iminopyridine
ligands are also redox noninnocent. The synthesis and
structural as well as spectroscopic characterization of the five-
coordinate species [CrII(R-iminopyridinate·)2(diethyl)] with
a triplet ground-state also contains two π radical anions.37
Figure 8. Top: Qualitative MO diagram for a 1 of corresponding orbitals28
of magnetic pairs. Bottom: Spin density plot of 1 (red, R spin; yellow, ꢁ
spin) together with values of the spin density of the Mulliken spin population
analysis.
(c) two π radicals antiferromagnetically coupled to a central
high-spin ferrous ion (SFe ) 2, d6). It is therefore gratifying
that the same scheme holds for 1. Clearly, two ligand π
radical anions are antiferromagnetically coupled to a high-
spin chromium(II) ion (SCr ) 2, d4): [CrII(1L•)2].
These results have been corroborated by the common
structural features of the N,N′-coordinated (L•)1- ligands
where the average C-Nimine bond distance is observed at
∼1.34 Å (Chart 1) and, more convincingly, the broken
symmetry calculation on each of these species including the
present 1 displays a spin density of about one electron per
ligand and the correct number of unpaired electrons for a
given divalent first-row transition metal ion.
In summary, we have shown that early transition metal
ions such as CrII and CrIII readily form complexes with N,N′-
coordinated, redox noninnocent R-diimine-type ligands. The
π radical monoanion represents a quite common oxidation
level in this chemistry.
Acknowledgment. We thank the Fonds of the Chemical
Industry for financial support. M.G. and S.S. are grateful
for a Max-Planck postdoctoral fellowship.
In all of these cases, we have not found any evidence for
“low valent” complexes involving Ni(0), Fe(0), or Cr(0) as
described in the previous literature. It is interesting that the
octahedral complex 2 contains one (2L•)1- radical anion and
Supporting Information Available: X-ray crystallographic CIF
files for complexes 1, 2, and 3. This material is free of charge via
the Internet at http://pubs.acs.org.
3
a central chromium(III) ion (SCr ) /2) which are antiferro-
magnetically coupled, yielding the observed triplet ground
state. Similar results have been reported for octahedral CrIII
complexes containing a single O,O′-coordinated o-ben-
zosemiquinone.34 It is therefore very likely that Wilkinson’s
IC800290S
(35) Leung, W.-H.; Danopoulos, A. A.; Wilkinson, G.; Hussain-Bates, B.;
Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1991, 2051.
(36) Chlopek, K.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2006, 45, 6298.
(34) (a) Wheeler, D. E.; McCusker, J. K. Inorg. Chem. 1998, 37, 2296.
(b) Rodriguez, J. H.; Wheeler, D. E.; McCusker, J. K. J. Am. Chem.
Soc. 1998, 120, 12051.
(37) Lu, C. C.; Bill, E.; Weyhermu¨ller, T.; Bothe, E.; Wieghardt, K. J. Am.
Chem. Soc. 2008, 130, 3181.
5970 Inorganic Chemistry, Vol. 47, No. 13, 2008