Electronic structure of the glyoxalbis(2-hydroxyanil) (gha) ligand in [CoIII(gha)(PPh3)2]+: Radical vs. non-radical states

Article abstract of DOI:10.1039/b802063a

The synthesis, structure and spectroscopic properties of a complex salt [Co^III(gha)(PPh₃)₂][Co^IICl ₃(PPh₃)]?C₂H₅OH (1) are reported; gha = glyoxalbis(2-hydroxyanil). This is the first single crystal X-ray structure of a (gha)^2- complex with a transition element. Though the determined bond parameters and UV-Vis spectroscopic data correlate well with a diradical description for the cation in 1, detailed electronic structure calculations using density functional theory confirm that [Co(gha)(PPh₃)₂]⁺ can be described as a closed shell singlet species which nevertheless displays an interesting electronic structure with significant electron transfer to the formally unoccupied LUMO of the square planar [Co(gha)]⁺ fragment. It was found that without the phosphine coligands, the [Co(gha)]⁺ unit has a triplet ground state with the lowest energy singlet diradical state lying only 1 kcal mol ^-1 higher in energy. The chemistry of the gha ligand is of interest as a spin diverse redox active system.

Full text of DOI:10.1039/b802063a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Electronic structure of the glyoxalbis(2-hydroxyanil) (gha) ligand in
[CoIII(gha)(PPh3)2]+: radical vs. non-radical states†
Amit Saha Roy,a Nicoleta Muresan,b Heikki M. Tuononen,c Sankar P. Rathd and Prasanta Ghosh*a
Received 5th February 2008, Accepted 20th May 2008
First published as an Advance Article on the web 9th June 2008
DOI: 10.1039/b802063a
The synthesis, structure and spectroscopic properties of a complex salt
[CoIII(gha)(PPh3)2][CoIICl3(PPh3)]·C2H5OH (1) are reported; gha = glyoxalbis(2-hydroxyanil). This is
the first single crystal X-ray structure of a (gha)2− complex with a transition element. Though the
determined bond parameters and UV-Vis spectroscopic data correlate well with a diradical description
for the cation in 1, detailed electronic structure calculations using density functional theory confirm
that [Co(gha)(PPh3)2]+ can be described as a closed shell singlet species which nevertheless displays an
interesting electronic structure with significant electron transfer to the formally unoccupied LUMO of
the square planar [Co(gha)]+ fragment. It was found that without the phosphine coligands, the
[Co(gha)]+ unit has a triplet ground state with the lowest energy singlet diradical state lying only 1 kcal
mol−1 higher in energy. The chemistry of the gha ligand is of interest as a spin diverse redox active
system.
diiminobenzosemiquinone,12 iminothiobenzosemiquinone13 and
the more reactive phenoxyl,14 anilino,15 aminyl,16 thiyl17 and azo-
Introduction
anion18 radicals are now well established. To stabilize new organic
mono- and polyradicals as coordination complexes with one
or multiple paramagnetic sites in one molecule is an ongoing
challenge in modern radicals research.
Ever since Gomberg’s discovery of the triphenylmethyl radical
at the turn of the 20th century,1 stable radicals have fascinated
researchers. This has led to isolation and characterization of
an ever-increasing number of stable and persistent odd-electron
species of the main group elements.2 Quintessential examples are
the thiazyl,2c,3 verdazyl2c,4 and nitroxide radical families,2c,5 to name
a few. From a fundamental perspective, the study of these systems
is of importance because they not only challenge our conventional
chemical thinking, but also develop our understanding of molecu-
lar structure and bonding. In addition, the paramagnetic nature of
stable radicals renders them useful in many applications: among
other things, radicals can be used as polymerization catalysis,6
reagents in redox chemistry7 and as building blocks for molecule-
based materials.8
As can be expected, not all radicals are stable in their own
right. Fortunately stabilization can in many cases be achieved
via coordination of the paramagnetic species to metal cen-
ters. The spectroscopic and structural studies of the properties
and reactivities of such systems have become particularly im-
portant in exploring many chemical and biological reactions
in detail.9 For example, the bond parameters and the spec-
tral features of benzosemiquinone,9d,10 iminobenzosemiquinone,11
Recently it has been recognized that the dianion of cis-
glyoxalbis(2-mercaptoanil) (cis-gma), a tetradentate ligand, is
redox active. Its diimine fragment can be reduced by one electron
to form a monoradical trianionic ligand, (cis-gma•)3−, which has
been experimentally characterized as an iron coordinated species.19
Coordination of the novel excited triplet state of (cis-gma)2− has
also been evidenced in [Fe(gma)(X)]n (X = pyridine, n = 1+; X =
CN−, I−, n = 0).19 As a possibility to extend the chemistry of stable
organic radical ligands further, the oxygen analogue of cis-gma,
the cis-glyoxalbis(2-hydroxyanil) (cis-gha) ligand, is of immediate
interest.20 The coordination chemistry of cis-gha has so far been
established in tetradentate dianionic form, (cis-gha)2−;21 in trans
conformation, cis-gha functions as a noninnocent bis-bidentate
bridging ligand.22 However, cis-gha can potentially bind a metal
centre also as an open shell trianionic p-radical, (gha•)3−, in which
the unpaired electron becomes localized on the diimine moiety
(Scheme 1). Parallel behavior has been observed for the (trans-
gha)2− ligand in its dirutheniumIII complex.22
aDepartment of Chemistry, R. K. Mission Residential College, Narendrapur,
Kolkata, 103, India. E-mail: ghoshp_chem@yahoo.co.in; Fax: 091 33 2477
3597; Tel: 091 33 2428 7017
bMax-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-
45470, Mu¨lheim an der Ruhr, Germany
cDepartment of Chemistry, University of Jyva¨skyla¨, P.O. Box 35, FI-40014,
Jyva¨skyla¨, Finland
dDepartment of Chemistry, Indian Institute of Technology, Kanpur, 208016,
India
Scheme 1 Deprotonated dianionic (gha)2− and trianionic (gha•)3− forms
of metal coordinated glyoxalbis(2-hydroxyanil).
† Electronic supplementary information (ESI) available: Full crystallo-
graphic tables of 1 as well as optimized coordinates for [Co(gha)]+ and
[Co(gha)(PH3)]+. CCDC reference number 621602. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b802063a
In order to correctly describe the electronic properties of the
gha ligand, or for that matter any other kind of ligand, in its
3438 | Dalton Trans., 2008, 3438–3446
This journal is
The Royal Society of Chemistry 2008
©

coordination complexes, it is of critical importance to obtain
structural and spectroscopic data of the systems under study.
Unfortunately the low solubility of the metal complexes of gha
in common non-coordinating solvents is a problem which to
some extent thwarts both crystallization and collection of different
physiochemical data. In addition, it is in general difficult to explore
the coordination chemistry of the gha ligand because it forms
an equilibrium with 2,2ꢀ-bisbenzooxazoline,20e,23 the solubility of
which is low and is therefore easily separated out from the
reaction mixture. The only published single crystal X-ray structure
of a gha complex is for a uranium species aquaglyoxalbis(2-
hydroxyanil)dioxouranium, (H2O)UO2(gha), and the reported
bond parameters do not allow elucidation of the details of
the electronic structure within the dianionic ligand.21a,21c The
scarcity of fully characterized (including single crystal X-ray data)
transition metal compounds containing the gha ligand has not
allowed the detailed determination of its electronic structure till
date though the ligand itself has been reported as early as 1950s.20
The electronic structure of the (cis-gha)2− ligand can be ade-
quately described using the resonance structure I containing two
phenolato-type ions (see Scheme 2). However, depending on the
metal fragment to which the ligand binds to, the significance
of other resonance structures can increase considerably. The
inclusion of closed shell resonance structures II and III to the
description in Scheme 2 emphasizes this by incorporating one
neutral quinone-type ring and another dianionic catecholato-type
ring in one molecule, thus, delocalizing the formal charge from
oxygen to nitrogen atoms. However, since the (gha)2− ligand is
known to be redox active, the description should be augmented
also with radical-type resonance structures alike IV as their
importance cannot be neglected a priori.
We have now synthesized a (cis-gha)2− complex in which the
tetradentate dianionic ligand is coordinated to an octahedral
cobalt(III) ion in a salt [CoIII(gha)(PPh3)2][CoIICl3(PPh3)]·C2H5OH
(1). The structural and electronic properties of the complex were
studied via different experimental methods i.e. X-ray crystal-
lography, spectroscopy, electrochemistry and magnetochemistry.
In addition, we have used density functional theory (DFT) in
order to rationalize the bonding and spectroscopic properties of
[Co(gha)(PPh3)2]+. Compound 1 represents the first single crystal
X-ray structure of a (gha)2− complex with a d-block element.
The collected experimental and computational data indicates
that in the cation of 1, the (cis-gha)2− ligand adopts a different
electronic configuration depending on the coordination sphere of
the CoIII center: a truncated model system [Co(gha)]+ represents
a chemical species in which the gha ligand coordinates to a metal
using an open shell diradical singlet state (IV in Scheme 2),
whereas a closed shell singlet configuration is observed for the
ligand in [Co(gha)(PPh3)2]+. However, the structural and spectral
data determined for the cation of 1 indicates that its electronic
structure is phenoxyl rather than phenolato-type, reminiscent of
the diradical character calculated for [Co(gha)]+.
Experimental
Synthesis
[CoIII(gha)(PPh3)2][CoIICl3(PPh3)]·C2H5OH (1). To a 30%
aqueous solution of glyoxal (1 mmol) in absolute ethanol (30 ml)
and acetonitrile (5 ml), 2-hydroxyaniline (220 mg, 2 mmol) was
added and the reaction mixture refluxed for 40 min after which the
solution turned yellowish-brown. To this hot solution, anhydrous
CoCl2 (260 mg, 2 mmol) and triphenylphosphine (1050 mg,
4 mmol) were added successively and the mixture refluxed gently
for further 60 min. The dark green solution was filtered hot and
the colored filtrate was cooled at room temperature and allowed
to evaporate slowly in air. Within two days, dark green needles of 1
separated out. Yield: 700 mg (56% with respect to Co). Elemental
anal. (%) for C68H55N2O2P3Cl3Co2·C2H5OH: Calcd. C 64.91, H
4.67, N 2.16; Found C 64.60, H 4.45, N 2.05. ESI MS: m/z,
821.2 [Co(gha)(PPh3)2]+, 425.5 [CoCl3(PPh3)]−. IR(KBr) (cm−1):
1585(w), 1509(s), 1435(vs), 1093(s), 694(vs), 519(vs).
X-Ray crystallographic data collection and refinement
of the structure
Single crystal X-ray data† were collected at −173 ◦C on a
Bruker SMART APEX CCD diffractometer using graphite-
˚
monochromatic Mo Ka radiation (k = 0.71073 A). The linear
absorption coefficients, scattering factors for the atoms and the
anomalous dispersion corrections were taken from International
Tables for X-Ray Crystallography. The data integration and
reduction were processed with SAINT software.24 An absorption
correction was applied.25 The structure was solved by direct
methods using SHELXS-97 and was refined on F2 by full-
matrix least-squares technique using the SHELXL-97 program
package.26 All the non-hydrogen atoms of the molecule were
Scheme 2 Possible resonance structures for the (gha)2− ligand.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3438–3446 | 3439
©

Table 1 Crystallographic data for [Co(gha)(PPh3)2][CoCl3(PPh3)]·
C2H5OH (1)
Chemical formula
C68H55N2O2P3Cl3Co2·C2H5OH
FW
1295.32
Monoclinic
P21/n
15.4277(15)
11.4944(11)
34.924(4)
101.703(2)
6064.5(10)
4
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
T/K
100(2)
1.419
Scheme 3 Synthesis of 1.
qcalcd/g cm−3
Reflections collected
R(int)
14481
0.1126
To slightly modify the synthetic approach in Scheme 3, we
decided to see what effect, if any, the introduction of coligands
(L) to the reaction mixture would have. It was hoped that by
approaching a higher coordination at the metal centre, the desired
metal complex would crystallize out from the reaction mixture. So
far the chosen approach has been the most successful when M =
Co and when PPh3 acts as a coligand, separating out the complex
salt 1. We note that similar reactions employing various other 3d
metal ions did not yield any crystalline products.
Unique reflections; I > 2r(I)
14481; 6618
749
0.71073; 0.809
0.0718; 0.874
0.1306
No. of parameters
−1
˚
k/A; l(Mo Ka)/mm
R1a; goodness of fitb
wR2c [I > 2r(I)]
Residual density/e A
−3
˚
0.914
ꢀ
ꢀ
a Observation criterion: I > 2r(I). R1 = ꢁFo| − |Fcꢁ/ |Fo|. b GOF =
ꢀ
ꢀ
ꢀ
[w(Fo − Fc2)2]/(n − p)]1/2
.
c wR2 = [ [w(Fo − Fc2)2]/ [w(Fo2)2]]1/2
2
2
[
where w = 1/r2(Fo2) + (ap)2 + bP, P = (Fo + 2Fc2)/3.
2
refined anisotropically. The C–O bond length and C–C–O angle
of the ethanol are restrained to sensible values. In the refinement,
hydrogen atoms were treated as riding atoms using SHELXL
default parameters. Crystallographic data of compound 1 are
listed in Table 1.
Density functional theory calculations
Interestingly, the reaction of 2-mercaptoaniline and glyoxal with
Co under similar reaction conditions does not yield a cation
of the type 3 containing the (gma)2− ligand but furnishes only
2,2ꢀ-bisbenzothiooxazoline (4). The increased reactivity of the
dioxygen species in contrast to its heavier sulfur analogue is
encouraging butthe degradation of 1 to 2 in solution is nevertheless
found to be rapid. Hence, the transformation of gha and gma to
the more stable oxazoline 2 and thiooxazoline 4, respectively, is the
limiting factor for the development of coordination chemistry of
these ligands. Only a very specific combination of transition metal
cations and coligands seems to be effective in stabilizing 1 so that
it can be crystallized.
We performed the theoretical calculations of this paper using the
Turbomole and ORCA program packages.27 The geometry opti-
mizations were carried out at the BP86 level.28 The Gaussian basis
sets were those reported by the Ahlrichs group.29 Triple-f quality
basis sets with one set of polarization functions on the cobalt,
oxygen, nitrogen and phosphorus atoms were used (TZVP).29b
The carbon and hydrogen atoms were described by slightly smaller
polarized split-valence SV(P) basis sets that is double-f quality in
the valence region and contains a polarizing set of d-functions on
the non-hydrogen atoms.29a The auxiliary basis sets for all com-
plexes used to expand the electron density in the calculations were
chosen to match the orbital basis. Fragment orbital analysis for
[Co(gha)(PH3)2]+ was performed with the ADF program using the
PB86 density functional with TZP quality STO basis sets.30 Orbital
and density plots were obtained by the program gOpenMol.31
X-Ray structure
The compound 1 has been isolated as dark green crystals from
which single crystals for X-ray structure determination have been
collected. The complex 1 crystallizes in P21/n space group. Fig. 1
displays the structure of 1 determined at 100 K.
Results and discussion
In the cation of 1, the Co(1) center is octahedrally coordinated
to a tetradentate dianionic gha ligand and to two monodentate
triphenylphosphine coligands. In the anion, the Co(2) center
is tetrahedrally coordinated by three chloride ions and one
triphenylphosphine ligand. The average Co(1)–P bond distance
Synthesis
We have been interested in isolating complexes containing the
(gha)2− ligand for physiochemical measurements in order to
establish the potential of this system to act as a spin diverse ligand
in its coordination compounds. However, all our previous attempts
failed to afford a binary compound of gha with a 3d metal. It
has been established that in every case 2,2ꢀ-bisbenzooxazoline (2)
separates out from the reaction mixture (see Scheme 3).
˚
˚
(2.308 A) is shorter than the Co(2)–P(3) distance (2.401 A) by
˚
approximately 0.10 A reflecting the different geometry and formal
oxidation state of the two cobalt centers in 1 (+III and +II in the
cation and the anion, respectively).
3440 | Dalton Trans., 2008, 3438–3446
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©

˚
the C–C bond, the mean C–N bond length in 1, 1.314 A, is
intermediate of C(sp2)–N(sp2) single and double bonds which
further emphasizes the diminished role played by the resonance
structure I to the description of bonding in 1.
The metrical parameters of the dianionic gha ligand show a
quinoid-type distortion in the two aromatic phenolic rings A
and B (see Fig. 2). In the ring A, the average of C(2)–C(3) and
˚
C(4)–C(5) bond distances, 1.372 A, is shorter than the average
bond distance of C(1)–C(2), C(3)–C(4), C(5)–C(6) and C(1)–
˚
C(6), 1.412 A. The same trend is also evident in the ring B in
which both the C(10)–C(11) and C(12)–C(13) bonds are shorter
than any of the other ones. The C–O distances also correlate
well with this description: the C(1)–O(1), and C(14)–O(2) bond
lengths are shorter than C–O bonds in reported cobalt phenolato
complexes.34 It is also noteworthy that both the Co(1)–O(1) and
˚
Co(1)–O(2) bond lengths are ca. 0.05 A longer than the average
34
˚
Co–O distance (1.89 0.01 A) in the reported phenolato systems.
A similar length trend has already been established in a structurally
characterized phenoxyl radical complex of chromium:35 the Cr–O
Fig. 1 X-Ray single crystal structure of 1. The solvent molecule
˚
distance in the phenoxyl system was found to be 1.943(2) A while
C2H5OH and hydrogen◦ atoms are omitted for clarity. Key bond
˚
˚
the same bond length is only 1.920(2) A in the phenolato analogue.
lengths (A) and angles ( ): Co(1)–O(1) 1.934(3), Co(1)–O(2) 1.948(3),
Co(1)–N(1) 1.852(4), Co(1)–N(2) 1.855(4), Co(1)–P(1) 2.2737(15), Co(1)–
P(2) 2.3435(15), O(1)–C(1) 1.314(6), O(2)–C(14) 1.333(5), N(1)–C(6)
1.382(6), N(1)–C(7) 1.306(5), N(2)–C(8) 1.321(6), N(2)–C(9) 1.386(6),
C(7)–C(8) 1.417(6); N(1)–Co(1)–N(2) 83.58(18), N(1)–Co(1)–O(1)
85.31(16), N(2)–Co(1)–O(1), 168.88(17), N(1)–Co(1)–O(2) 167.69(15),
N(2)–Co(1)–O(2) 85.20(16), O(1)–Co(1)–O(2) 105.91(14), P(1)–Co(1)–
P(2) 169.93(6); Co(2)–Cl(1) 2.2677(17), Co(2)–Cl(2) 2.2262(17),
Co(2)–Cl(3) 2.2313(15), Co(2)–P(3) 2.4016(16), Cl(2)–Co(2)–Cl(3)
110.48(7), Cl(2)–Co(2)–Cl(1) 114.20(7), Cl(3)–Co(2)–Cl(1) 109.45(6),
Cl(3)–Co(2)–P(3) 108.00(6), Cl(2)–Co(2)–P(3) 108.48(6).
Taken as a whole, the above structural features lend strong support
for the importance of resonance structures II, III and, possibly,
the radical-type structure IV over I to describe the bonding in the
cation of 1.
The bond distances in the glyoxal bridging unit of the (gha)2−
ligand in 1 are significant in establishing its electronic config-
uration. In general, the C–C and C–N distances in a neutral
diimine are sharply different from those in a corresponding anionic
radical which arises from the occupation of the CC-bonding CN-
antibonding LUMO by an unpaired electron in the latter species.
Single crystal X-ray structure of the paramagnetic spirocyclic
Li(1,4-di-tert-butyl-1,4-diazabutadiene)2 (S = 1/2) has established
the feature unequivocally:32 the C–N and C–C distances in the
2−
˚
Fig. 2 C–C bond distances (A) in the (gha) ligand in 1: C(1)–C(2)
1.410(7), C(2)–C(3) 1.364(7), C(3)–C(4) 1.417(7), C(4)–C(5) 1.379(6),
C(5)–C(6) 1.406(7), C(1)–C(6) 1.414(7), C(9)–C(10) 1.393(7), C(10)–C(11)
1.377(7), C(11)–C(12) 1.395(7), C(12)–C(13) 1.368(7), C(13)–C(14)
1.410(7), C(9)–C(14) 1.396(7).
Electronic spectra
˚
neutral diimine are 1.24 and 1.49 A respectively, whereas the
The electronic (UV-Vis) spectra of phenoxyl radicals are highly
characteristic. It has been established that when coordinated to a
metal ions, the phenoxyl radicals absorb strongly in the ultraviolet
part of the spectrum (p–p* transitions around 400 and <300 nm)
and only a weak broad transition maximum (ligand-to-metal
charge transfer) is observed at around 600–700 nm.14,36 For the
free phenoxyl radical, the origin of the p–p* transitions involving
low-lying excited electronic states (2A2, 2B1, 2A2 and 2B1) has been
determined using a combination of spectroscopic methods and
time-dependent density functional theory.37 Most importantly,
the intense band at around 400 20 nm is highly indicative of
phenoxyl radical formation as analogous phenolato complexes
show no absorption at that region.14,37
corresponding bond lengths in the diimine anion radical are
˚
1.32 and 1.40 A. A similar trend has also been established in
the Fe(gma•) species.19 Interestingly, the bond parameters of the
glyoxal fragment in 1 do not support the presence of a diimine
(resonance structure I in Scheme 2). We also note that the complex
1 exhibits a –C N– stretching frequency at 1508 cm which is
much less than the stretching frequency of a coordinated aldimine
function (>1600 cm−1)33 favouring lower bond order of the –C N–
−1
=
=
group.
2
˚
In 1, the C(7)–C(8) length, 1.417(6) A, is the average of C(sp )–
2
˚
˚
C(sp ) single (1.49 A) and double (1.33 A) bonds, and, thus,
indicates that the bonding description is either in between the
two extremes shown in resonance structures I and II–IV, or the
[Co(gha)(PPh3)2]+ cation contains a diimine anion radical moiety.
The latter possibility would imply the presence of a CoIV center
which would be atypical oxidation state for cobalt, and is shown to
be incorrect description by both structural data as well as results
from electronic structure calculations (see below). Similarly to
Fig. 3 shows the electronic spectrum of 1 in CH2Cl2 at 20 ◦C. The
absorption maxima, kmax (e/M−1 cm−1), occur at 720 (8320), 605
(3790), 410 (19890), 348 (26830) and 250 nm (47410), which
indicates that the electronic structure of the ligand in the cation of
1 is closer to phenoxyl rather than phenolato in agreement with
the conclusions drawn from structural parameters.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3438–3446 | 3441
©

Fig. 4 Temperature dependence of the magnetic moment of a solid sample
of 1. The solid line represents the best fit to the data obtained by using S =
Fig. 3 Electronic spectra of 1 in CH2Cl2 at 20 ◦C.
3/2 spin state, g = 2.238 and D = 13.5 cm−1
.
critical importance to connect the experimental data to theoretical
predictions. In the next two sections, a detailed picture of the
electronic structure of [Co(gha)(PPh3)2]+ is given on the basis of
results from comprehensive DFT calculations.
Magnetic measurements
The compound 1 is paramagnetic with S = 3/2 ground state
as established via temperature-dependent (4–290 K) magnetic
susceptibility measurements (Fig. 4). The cobaltII ion in the
tetrahedral [CoCl3(PPh3)]− anion is high-spin with three un-
paired electrons (S = 3/2 spin state). Consequently, the cation
[Co(gha)(PPh3)2]+ is necessarily diamagnetic which at first glance
implies the coordination of singlet state dianionic gha ligand
The calculated geometries† of the monocations [Co(gha)(PH3)2]+
and [Co(gha)]+
Calculations were first performed on a model system in which
all phenyl groups in [Co(gha)(PPh3)2]+ were replaced with
hydrogen atoms. The optimized structural parameters of the
[Co(gha)(PH3)2]+ complex in closed shell singlet state are given in
Table 2 along with the corresponding experimental values (atom
labeling follows that in the crystal structure, see Fig. 1). It is seen
that the calculated bond lengths are in reasonable agreement with
the experimental values. Though the trends in C–C bond lengths
are well described by the chosen level of theory, the Co–N and
(S = 0) to a low-spin octahedral cobaltIII ion (t2g configuration).
6
However, both the metrical parameters as well as the UV-Vis
spectroscopic data indicate that the electronic structure of the
(gha)2− ligand in 1 is of phenoxyl-type i.e. it shows diradical-like
behavior in which the “unpaired” electron density is not localized
on the diimine moiety but on the OCCN chelating fragments
(structure IV in Scheme 2). In order to correctly identify the
complex electronic features of the cation in 1, it is therefore of
+
˚
Table 2 Experimental bond distances (A) of octahedral [Co(gha)(PPh3)2] in comparison with the calculated values for the model systems
[Co(gha)(PH3)2]+ and [Co(gha)]+
[Co(gha)(PPh3)2]+ Expt.
[Co(gha)H3)2]+ Calcd. RKS DFT
[Co(gha)]+ Calcd. RKS DFT
[Co(gha)]+ Calcd. BS-UKS DFT
Co(1)–N(1)
Co(1)–N(2)
Co(1)–O(1)
Co(1)–O(2)
Co(1)–P(1)
Co(1)–P(2)
O(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(1)–C(6)
N(1)–C(6)
N(1)–C(7)
C(7)–C(8)
N(2)–C(8)
N(2)–C(9)
C(9)–C(10)
C(10)–C(11)
C(11)–C(12)
C(12)–C(13)
C(13)–C(14)
C(14)–C(9)
O(2)–C(14)
1.852(4)
1.855(4)
1.934(3)
1.948(3)
2.2737(15)
2.3435(15)
1.314(6)
1.410(7)
1.364(7)
1.417(7)
1.379(7)
1.406(7)
1.414(7)
1.382(6)
1.306(5)
1.417(6)
1.321(6)
1.386(6)
1.393(7)
1.377(7)
1.395(7)
1.368(7)
1.410(7)
1.396(7)
1.333(5)
1.881
1.881
1.967
1.967
2.234
2.234
1.336
1.418
1.397
1.422
1.393
1.418
1.442
1.387
1.335
1.428
1.335
1.387
1.418
1.393
1.422
1.396
1.418
1.442
1.336
1.823
1.823
1.871
1.871
—
1.842
1.842
1.867
1.867
—
—
—
1.316
1.420
1.394
1.430
1.394
1.414
1.458
1.376
1.366
1.396
1.366
1.376
1.414
1.394
1.430
1.394
1.420
1.458
1.316
1.319
1.420
1.394
1.429
1.396
1.413
1.458
1.379
1.354
1.409
1.354
1.379
1.413
1.396
1.429
1.394
1.420
1.458
1.319
3442 | Dalton Trans., 2008, 3438–3446
This journal is
The Royal Society of Chemistry 2008
©

Co–O bond lengths are nevertheless both overestimated by
function. Recently the BS approach has successfully been applied
to analyze the bonding in transition metal complexes in the groups
of Wieghardt–Neese38 and others.39
˚
0.03 A. The biggest discrepancies are observed for Co–P bond
lengths which are predicted to be significantly shorter than the
experimental values most likely due to the lesser steric bulk of the
hydrogen atoms compared to the phenyl groups.
The BS(1,1) calculation for the truncated [Co(gha)]+ cation
converged to the Ms = 0 solution indicating the presence of two
antiferromagnetically coupled electrons in the ligand. The closed
shell singlet state for the truncated model [Co(gha)]+ is higher in
energy by 1.2 kcal mol−1 compared to the BS(1,1) Ms = 0 state.
The lowest energy triplet solution is energetically on par with the
broken symmetry singlet and is found to be 2.0 kcal mol−1 lower
than the closed shell singlet state. Hence, at the BP86 level of
theory, the ground state of the [Co(gha)]+ cation is a high-spin
triplet which offers some explanation why the synthetic approach
depicted in Scheme 3 was not successful without the introduction
of coligands.
The stability of the singlet self consistent field (SCF) Kohn–
Sham (KS) solution for the [Co(gha)(PH3)2]+ cation in its opti-
mized geometry was tested with respect to triplet perturbations
whose presence is indicative of singlet diradical character. The
closed shell solution was found to be fully stable under all
perturbations considered. However, the lowest energy Hessian
eigenvalue is only 0.025 with a dominant (96%) contribution
from the highest occupied molecular orbital (HOMO) to lowest
unoccupied molecular orbital (LUMO) excitation. Since both of
the active orbitals are in this case centered on the (gha)2− ligand and
have no contributions from the phosphine coligands, we decided to
test what effect, if any, the removal of the PH3 groups would have
to the molecular structure as well as to the stability of the SCF KS
solution. This is very informative considering the experimental
results i.e. without coligands the synthetic approach failed to
produce any crystalline products.
The electronic structure of the singlet [Co(gha)]+ cation can
be examined in detail via analyzing the MOs of the broken
symmetry KS solution. The square-planar structure features three
predominantly metal-type d orbitals which are all doubly occupied
and found in the spin-up as well in the spin-down manifold. This
orbital occupation pattern defines a low-spin CoIII (d6, S = 0)
configuration for the metal center. In addition, singly occupied
ligand-based orbitals are identified which possess antiparallel spin
orientation and give rise to the total spin density distribution
depicted in Fig. 5. The analysis of the calculated Mulliken spin
populations reveals that the spin density is polarized strictly in
the ligand manifold (0.6 a-spin on one side and 0.6 b-spin on the
other side of the ligand) and essentially no net spin density is
localized on the central cobalt ion. Hence, this pattern sustains
the representation of a singlet [Co(gha)]+ as a low-spin cobalt(III)
coordinated to a singlet diradical dianionic gha ligand, resulting
in an overall diamagnetic state.
The optimized geometry of the truncated [Co(gha)]+ model
system is given in Table 2. The calculated data set can be compared
to the optimized structure of the full cation which shows that
the cobalt–oxygen bond lengths are the structural parameters
most significantly affected by the removal of the PH3 groups and
˚
at 1.87 A are much shorter than in the X-ray crystal structure
despite the decrease of the coordination number from six to four
in the calculated model. Interestingly, the closed shell singlet SCF
KS solution for [Co(gha)]+ is unstable with respect to triplet
perturbations as the lowest Hessian eigenvalue is negative, −0.070
(89% contribution from the HOMO−1 to LUMO excitation). This
indicates that the removal of the phosphine coligands alters the
electronic structure of the (gha)2− ligand, changing its ground state
from a closed shell singlet ([Co(gha)(PH3)2]+) to a singlet diradical
([Co(gha)]+).
The geometry optimization for [Co(gha)]+ was performed for a
second time using a broken symmetry (BS) approach to account
for the singlet diradical nature of the system; the calculated bond
lengths are listed in Table 2. For the calculated geometry of BS
[Co(gha)]+, the predicted distances within the ligand backbone are
in good agreement in comparison to those found for the analogous
Fig. 5 Spin density plot of a square-planar monocation [Co(gha)]+ with
calculated Mulliken spin density values derived from BS DFT calculations.
closed shell structure. Most notably, the cobalt–nitrogen bond
The broken symmetry KS solution for [Co(gha)]+ localizes
spin density mostly on the two oxygen atoms. The stability
analysis indicated that the active orbitals in the closed shell singlet
determinant of [Co(gha)]+ are the HOMO−1 and LUMO which
+
˚
lengths are elongated by 0.02 A with respect to RKS [Co(gha)]
and are now in excellent agreement with the experimental values.
2
are depicted in Fig. 6. The HOMO−1 corresponds to cobalt dz
Electronic structures of the monocations [Co(gha)(PH3)2]+ and
[Co(gha)]+
orbital (83%), whereas the LUMO is a linear combination of metal
dxz orbital (36%) with pz orbitals of oxygen (11%), carbon (2–5%)
and nitrogen (2%).
Since we used broken symmetry calculations in the remaining part
of the computational work, we will adopt the following notation.
The system is divided into two fragments and 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 would correspond to the metal and the ligand, respectively.
In our case, however, both the fragments 1 and 2 correspond to
the diradical ligand. In general, the BS(m, n) notation refers to
the initial guess of the symmetry of the KS determinant or a wave
As expected, the shape of the LUMO resembles the spatial
features in the spin density distribution calculated from the
broken symmetry KS determinant (cf . Fig. 5). Considering the
symmetry characteristics of the active orbitals, one expects that the
HOMO−1 is (one of the MOs) able to mix with the frontier orbitals
of the coligands when the two PH3 groups bind to the square
planar complex [Co(gha)]+. A fragment MO analysis conducted
for [Co(gha)(PH3)2]+ confirms this notion. However, since the
HOMO−1 of [Co(gha)]+ and the HOMO of PH3 are both fully
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Dalton Trans., 2008, 3438–3446 | 3443
©

Fig. 6 HOMO−1 (top) and LUMO (bottom) of [Co(gha)]+ (isosurface
value 0.05).
occupied in the respective fragments and their PCoP antibonding
combination is unoccupied in [Co(gha)(PH3)2]+ (LUMO+1), some
other linear combination of the initially unoccupied fragment
orbitals needs to take electron density when the octahedral
complex forms. Indeed, this orbital is found to be the HOMO
of the full complex which corresponds to an almost pure LUMO
(92%) of the [Co(gha)]+ fragment.
As elucidated above, the addition of two PH3 (or, in general,
PR3) groups to [Co(gha)]+ causes significant changes in the
orbital framework which leads to transfer of electron density to
the LUMO of the metal–gha fragment. In essence, this effect,
though mediated by the two PH3 groups, is analogous to what
takes place in the broken symmetry treatment of singlet state
[Co(gha)]+. Hence, it can be readily understood why the full
complex featuring an octahedral ligand field does not display
SCF instability although the square planar truncated model is
best viewed as containing a diradical ligand. We conclude this
section by noting that despite the fact that the ground state of the
[Co(gha)(PH3)2]+ cation can be described as a closed shell singlet,40
the occupation of the LUMO of the [Co(gha)]+ fragment in
[Co(gha)(PH3)2]+ leads to an electronic structure which is phenoxyl
rather than phenolato-type.
Fig. 7 Cyclic voltammogram of 1 in CH2Cl2 at the scan rate of 0.80 V s−1
at −10 ◦C.
electrode. The measured data can be compared to the cyclic
voltammograms measured for different metal complexes of the
(gha)2− ligand. However, only two studies, both conducted on
ruthenium coordinated species, have been reported.22 We note that
the difference between the first reduction and oxidation potentials
in both cases is around 1.50 V. In the current case, the two electron
oxidation at +0.650 V vs. Fc+/Fc and the second reduction at
−0.880 V vs. Fc+/Fc are both assigned as gha centered (Scheme 4).
The first reduction at 0.0 V vs. Fc+/Fc is due to the CoIII/CoII
couple; the reduction couple is very similar to that in the reported
cobalt(salen) species with a similar coordination sphere.41
Scheme 4 Redox states of the complex and the ligand in 1.
Conclusion
Cyclic voltammetry
We reported the first single crystal X-ray structure of a
The electronic structure of compound 1 was assessed by perform-
ing cyclic voltammetry in dichloromethane solution at −10 ◦C;
the experiments were performed in the scan rate variation from
0.05 to 1.60 V s−1. There is one two electron oxidation and two
one electron reductions discernible in the cyclic voltammogram of
1 (Fig. 7). It is seen that the second reduction at a more negative
potential is irreversible at all scan rates and at +0.200 V possesses
a peak potential of Ep = −0.88 V but the first reduction at 0.0 V vs.
Fc+/Fc is reversible at higher scan rates. The oxidation is almost
irreversible at slow scans (0.05 V s−1) and gains some reversibility
at higher scan rates - in line with an interfering homogeneous
follow-up reaction. At a scan rate of 0.80 or at 1.60 V s−1 the
oxidation looks relativel◦y reversible and one can evaluate a formal
reduction potential of E = +0.650 V vs. Fc+/Fc. Surprisingly, the
reversibility of the oxidation did not improve much upon cooling
the solution even to −30 ◦C. Therefore, coulometric and EPR
spectroscopic studies are not expected to deliver significant new
information.
glyoxalbis(2-hydroxyanil) (gha) coordinated to a transition
metal ion in an unique complex salt [CoIII(gha)(PPh3)2]-
[CoIICl3(PPh3)]·C2H5OH (1). Complex 1 opens up an interesting
opportunity to investigate the electronic properties of the redox
active dianionic gha ligand which has eluded complete structural
characterization for more than 50 years. Crystallographic bond
parameters, spectral and magnetic properties as well as results
from DFT calculations were collectively used to determine the
ground state of the gha ligand in the cation of 1. It was found
that without the phosphine coligands, the Co–gha complex has
a triplet ground state with the lowest energy singlet diradical
state lying only 1 kcal mol−1 higher in energy. The introduction
of phosphine groups alters the electronic structure of the square
planar cation [Co(gha)]+ by transferring electron density from the
metal to the atoms in the ligand framework. This gives rise to an
overall closed shell singlet ground state which nevertheless retains
phenoxyl-type electronic structure with some “radical character”
as evidenced by the determined structural parameters and UV-Vis
spectroscopic data for 1. The possibility of the dianionic gha ligand
to undergo intraligand redox chemistry represents an attractive
feature. It would be interesting to see whether, with appropriate
The peak current of oxidation was found to be about twice
as high as that of reduction. The voltammogram was referenced
to ferrocene which was oxidized at +0.203 V vs. the reference
3444 | Dalton Trans., 2008, 3438–3446
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The Royal Society of Chemistry 2008
©

metal atoms or coligands, it is possible to alter the electronic
structure of the ligand so as to induce a diradical ground state.
Work in this direction is in progress.
18 (a) M. Shivakumar, K. Pramanik, P. Ghosh and A. Chakravorty, Inorg.
Chem., 1998, 37, 5968; (b) M. Shivakumar, K. Pramanik, P. Ghosh and
A. Chakravorty, Chem. Commun., 1998, 2103; (c) K. Pramanik, M.
Shivakumar, P. Ghosh and A. Chakravorty, Inorg. Chem., 2000, 39,
195; (d) M. Schwach, H. D. Hausen and W. Kaim, Inorg. Chem., 1999,
38, 2242; (e) N. Doslik, T. Sixt and W. Kaim, Angew. Chem., Int. Ed.
Engl., 1998, 37, 2403.
Acknowledgements
19 P. Ghosh, E. Bill, E. Bothe, T. Weyhermu¨ller, F. Neese and K.
Wieghardt, J. Am. Chem. Soc., 2003, 125, 1293.
The present work was supported by the Council of Scientific and
Industrial Research, CSIR (01(1957)/04/EMR-II), New Delhi,
India. H.M.T acknowledges the Academy of Finland and the
University of Jyva¨skyla¨ for financial support.
20 (a) E. Bayer, Angew. Chem., 1957, 69, 107; (b) E. Bayer, Angew.
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24 SAINT+, Version 6.02, Bruker-AXS, Madison, WI, 1999.
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broken singlet UKS determinant that is lower in energy than the
corresponding RKS solution can nevertheless be used as a qualitative
indicator of diradical character. However, the wave function can have
an internal instability even if all eigenvalues in the KS SCF stability
analysis are found to be positive.
41 H. Shimakoshi, W. Ninomiya and Y. Hisaeda, J. Chem. Soc., Dalton
Trans., 2001, 1971.
3446 | Dalton Trans., 2008, 3438–3446
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