Synthesis and characterization of tetrahedral and square planar bis(iminopyrrolyl) complexes of cobalt(II)

Article abstract of DOI:10.1021/ic062125w

A series of 2-iminopyrrole ligand precursors with increasing bulkiness [HNC₄H₃C(R)=N-2,6-R′₂C₆H ₃] (R = R′ = H, 1a; R = Me, R′= H, 1b; R = H, R′ = Me, 1c; R = R′ = Me, 1d; R = H, R′ = ⁱPr, 1e; R = Me, R′ = ⁱPr, 1f) were synthesized and deprotonated with NaH to give the corresponding iminopyrrolyl sodium salts 2a-f. A set of homoleptic bis-ligand Co(II) complexes of the type [Co(κ²N,N′- NC₄H₃C(R)=N-2,6-R′₂C₆H ₃)₂] (R = R′= H, 3a; R = Me, R′= H, 3b; R = H, R′ = Me, 3c; R = R′ = Me, 3d; R = H, R′ = ⁱPr, 3e; R = Me, R′ = Pr, 3f) was prepared by reaction of CoCl₂ with the corresponding iminopyrrolyl sodium salts 2a-f. The new complexes were characterized by elemental analysis, magnetic susceptibility measurements, in powder and in solution, UV/vis/NIR, and, in some cases, X-ray crystallography. According to X-ray diffraction and magnetic measurements, the Co complexes 3a-e proved to be tetrahedral, which is the preferred geometry for Co(II) compounds. However, a square planar geometry is observed in the case of 3f, as determined by several characterization techniques. In this case, DFT calculations suggest the square planar geometry is slightly more stable than the tetrahedral one probably due to a combination of steric and electronic reasons.

Full text of DOI:10.1021/ic062125w

Inorg. Chem. 2007, 46, 6880−6890
Synthesis and Characterization of Tetrahedral and Square Planar
Bis(iminopyrrolyl) Complexes of Cobalt(II)
So´nia A. Carabineiro,† Leonel C. Silva,† Pedro T. Gomes,*,† Laura C. J. Pereira,‡ Lu´ıs F. Veiros,†
Sofia I. Pascu,§ M. Teresa Duarte,† So´nia Namorado,† and Rui T. Henriques|
Centro de Qu´ımica Estrutural, Departamento de Engenharia Qu´ımica e Biolo´gica, Instituto
Superior Te´cnico, AV. RoVisco Pais, 1049-001 Lisboa, Portugal, Departamento de Qu´ımica,
Instituto Tecnolo´gico e Nuclear/CFMCUL, Estrada Nacional 10, 2686-953 SacaVe´m, Portugal,
Chemistry Research Laboratory, UniVersity of Oxford, OX1 2TA Oxford, U.K., and Instituto de
Telecomunicac¸o˜es, Instituto Superior Te´cnico, AV. RoVisco Pais, 1049-001 Lisboa, Portugal
Received November 8, 2006
A series of 2-iminopyrrole ligand precursors with increasing bulkiness [HNC4H3C(R)
H, 1a; R Me, R′) H, 1b; R H, R′ ) Me, 1c; R ′ ) Me, 1d; R
H, R′ ) iPr, 1e; R
1f) were synthesized and deprotonated with NaH to give the corresponding iminopyrrolyl sodium salts 2a
of homoleptic bis-ligand Co(II) complexes of the type [Co( -NC4H3C(R) N-2,6-R 2C6H3)2] (R ′) H, 3a;
2N,N
Me, R′) H, 3b; R H, R′ ) Me, 3c; R ′ ) Me, 3d; R
H, R′ ) iPr, 3e; R Me, R′ ) iPr, 3f) was
prepared by reaction of CoCl2 with the corresponding iminopyrrolyl sodium salts 2a f. The new complexes were
d
N-2,6-R
′
2C6H3] (R
Me, R′ ) iPr,
f. A set
) R′ )
)
)
)
R
)
)
−
κ
′
d
′
) R
R
)
)
)
R
)
)
−
characterized by elemental analysis, magnetic susceptibility measurements, in powder and in solution, UV/vis/NIR,
and, in some cases, X-ray crystallography. According to X-ray diffraction and magnetic measurements, the Co
complexes 3a−e proved to be tetrahedral, which is the preferred geometry for Co(II) compounds. However, a
square planar geometry is observed in the case of 3f, as determined by several characterization techniques. In this
case, DFT calculations suggest the square planar geometry is slightly more stable than the tetrahedral one probably
due to a combination of steric and electronic reasons.
Introduction
the characterization data for this compound, including the
X-ray diffraction study,3 have shown a tetrahedral structure
around the Co atom (Chart 1, B). Very recently, the same
type of geometry was found in a Co(II) dinuclear [4 + 4]
helical complex of a diiminodipyrrolyl ligand containing also
tert-butyl substituents.4
In recent times, R-diimine ligands with bulky aryl sub-
stituents gave rise to group 10 metal complexes which are
very active R-olefin polymerization catalysts.5,6 This boosted
the search for new sterically tunable nitrogen-based poly-
dentate ligands.6-8 It is known that the stereochemical control
of the metal centers provides efficient steric shielding of a
The use of iminopyrrolyl ligands in the synthesis of
transition-metal compounds is known, but the first rational
synthetic reports on the subject were presented by Holm et
al. in the 1960s, for compounds of the type [M(iminopyr-
rolyl)2] (M(II) ) Co, Ni, Pd, Cu, Zn).1 For cobalt, due to
the use of aerobic conditions, less bulky R groups gave rise
to oxidized octahedral complexes of the type [CoIII(iminopy-
rrolyl)3]2 (Chart 1, A), and only the use of tert-butyl as
substituent of the imine group provided an efficient sto-
ichiometric control and enabled the synthesis and charac-
terization of the corresponding [CoII(iminopyrrolyl)2].1,2 All
(3) Wei, C. H. Inorg. Chem. 1972, 11, 1100-1105.
(4) Reid, S. D.; Blake, A. J.; Wilson, C.; Love, J. B. Inorg. Chem. 2006,
45, 636-643.
* To whom correspondence should be addressed. E-mail:
pedro.t.gomes@ist.utl.pt. Phone: +351 218419612. Fax: +351 218419612.
† Instituto Superior Te´cnico (CQE).
(5) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414-6415.
‡ Instituto Tecnolo´gico e Nuclear/CFMCUL.
§ University of Oxford.
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| Instituto Superior Te´cnico (IT).
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6880 Inorganic Chemistry, Vol. 46, No. 17, 2007
10.1021/ic062125w CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/21/2007

Tetrahedral and Square Planar Complexes of Co(II)
Chart 1
but in which the imine substituent R is the sterically
demanding 2,6-diisopropylphenyl group. The latter Co
compound, which is the second known example of a
mononuclear Co(II) bis(iminopyrrolyl) complex reported so
far, was only characterized by elemental analysis and mass
spectrometry, but no other information about its molecular
structure was reported.13
It is known that tetracoordinated Co(II) compounds
containing bidentate nitrogen-based chelating ligands show
a clear preference for high-spin tetrahedral geometries (S )
3/2).23,24 The majority of low-spin Co(II) (S ) 1/2) complexes
are square planar chelates of tetradentate macrocycles, such
as porphyrins or Schiff bases derivatives, which, due to their
intrinsic structural features, impose this type of geometry
around the metal center.25 However, there are a few cases
of N,O26-28 or S,S29-31 bidentate chelating ligands which give
rise to square planar Co(II) complexes or compounds which
exhibit tetrahedral-square planar equilibria.
An example of the geometry and magnetic properties
tuning of a Co(II) center is that imposed by the geometric
constraints of a macrocycle ligand, as reported by Lippard
et al.32 In this work, where a series of Co(II) complexes of
tropocoronand macrocycles was synthesized, it was shown
that as the macrocycle is expanded by the introduction of
an increasing number of methylene groups, the geometry
around the Co(II) center switches abruptly from square planar
(S ) 1/2) to tetrahedral geometry (S ) 3/2). These geo-
metrical changes may cause different reactivity patterns
toward the activation of small molecules.25,32
large sector of the coordination sphere and kinetic stabiliza-
tion, leading to modified reactivities and increased stability
of the complexes. With this perspective in view, several
mono- or bis(iminopyrrolyl) early-transition-metal complexes
have been reported in the literature, in which the imine
substituents were bulky aryl groups.9 Gibson et al. reported
bis(iminopyrrolyl) complexes of Cr(II) and Cr(III).10,11 Bis-
(iminopyrrolyl) complexes of group 4 metals were prepared
by Mashima et al. (Zr-diamido complexes),9,12 Bochmann
et al. (Zr-dichloro and -diamido complexes),13 Fujita et
al. (Ti-dichloro complexes),14-17 and Okuda et al. (Zr- and
Hf-dibenzyl complexes).18,19 Bis(iminopyrrolyl) chloro,
amido, or alkyl complexes of rare earth metals have also
been reported.20,21
We have been interested in the chemistry of bulky
iminopyrrolyl ligands and their role in ethylene oligo/
polymerization catalysis when coordinated to late transition
metals.33 Here we describe the synthesis and characterization
of a series of [CoII(iminopyrrolyl)2] complexes in which the
bidentate chelating ligands encompass increasing bulkiness.
The ligands used derive either from known formiminopyr-
roles with variable size aryl substituents of the imine nitrogen
(Ph, 2,6-Me2Ph and 2,6-iPr2Ph) or from new acetiminopyr-
roles, synthesized as described below (Scheme 1). This study
aims to determine whether the Co(II) coordination sphere
Other authors have prepared homoleptic transition-metal
complexes of the type [M(iminopyrrolyl)2] (M ) Co,13 Ni,13
Zn22), similar to those isolated by Holm et al. (see above)
(9) Mashima, K.; Tsurugi, H. J. Organomet. Chem. 2005, 690, 4414-
4423.
(10) Gibson, V. C.; Maddox, P. J.; Newton, C.; Redshaw, C.; Solan, G.
A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 1651-
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(12) Matsuo, Y.; Mashima, K.; Tani, K. Chem. Lett. 2000, 1114-1115.
(13) Dawson, D. M.; Walker, D. A.; Thornton-Pett, M.; Bochman, M. J.
Chem. Soc., Dalton Trans. 2000, 459-466.
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T.; Tanaka, H.; Fujita, T. Chem. Lett. 2000, 1270-1271.
(15) Yoshida, Y.; Matsiu, S.; Takagi, Y.; Mitani, M.; Nakano, T.; Tanaka,
H.; Kashiwa, N.; Fujita, T. Organometallics 2001, 20, 4793-4799.
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Nakano, T.; Kashiwa, N.; Fujita, T. Chem. Commun. 2002, 1298-
1299.
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H. G.; Noltemeyer, M.; Cui, C. Eur. J. Inorg. Chem. 2002, 1060-
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(17) Yoshida, Y.; Mohri, J.; Ishii, S.; Mitani, M.; Saito, J.; Matsui, S.;
Makio, H.; Nakano, T.; Tanaka, H.; Onda, M.; Yamamoto, Y.; Mizuno,
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Organomet. Chem. 2004, 689, 1155-1164.
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Inorganic Chemistry, Vol. 46, No. 17, 2007 6881

Carabineiro et al.
Scheme 1. Synthesis of Ligand Precursors 1a-f
Electronic spectra were recorded in a Perkin-Elmer Lambda 9
UV/vis/NIR spectrophotometer, between 200 and 1500 nm, at room
temperature. Solution samples were prepared in a glovebox, by
dissolving the complexes (or the ligand precursors) in freshly
distilled toluene, to give rigorous solutions (in most cases 10-3 and
10-5 mol L-1), and the corresponding spectra were obtained in
quartz cells with optical lengths of 1 cm, under inert atmosphere.
For comparison, solid-state samples were also measured in Nujol
mulls, which were deposited on the inner surfaces of quartz cells.
In the spectra obtained in solution, molar extinction coefficients
(ꢀ) were calculated for each absorption maximum.
Syntheses of 2-Formiminopyrrole Ligand Precursors (1a,c,e).
A similar procedure used in a previous publication of our group
(where mesitylaniline was employed)33 was followed. This general
procedure was adapted from the literature for 1a34,35 and 1e:13,36,37
2-Formylpyrrole (10.0 mmol), the appropriate aniline (10 mmol),
and a catalytic amount of p-toluenesulfonic acid were suspended
in absolute ethanol (5 mL) in a 50 mL round-bottom flask fitted
with a condenser and a CaCl2 guard tube. The mixture was heated
to reflux for 3 days (over the weekend). NaHCO3 was added (while
still hot) to neutralize the acid. The mixture was allowed to cool,
CH2Cl2 was added, and the suspension was filtered through Celite
and washed through with more CH2Cl2. After removal of all
volatiles (in the rotary evaporator), the product was dissolved in
refluxing hexane. A dark brown oil was decanted from the
supernatant orange solution, which was stored at -20 °C to yield
1.04 g (61%) of a dark yellow microcrystalline solid of 1a, 1.27 g
(64%) of yellow-orange crystals of 1c, or 1.62 g (70%) of orange-
reddish crystals of 1e.
can be tuned from tetrahedral to the rarely observed square
planar geometry using this type of bis-chelating ligand. For
this purpose, structural and spectroscopic investigations have
been carried out on the compounds synthesized, such as
single-crystal X-ray diffraction, magnetic susceptibility
measurements in solid and in solution, UV/vis/NIR spec-
troscopy, and DFT calculations.
Experimental Section
Data for 1a. Anal. Found (calcd): C, 77.38 (77.62); H, 6.49
(5.92); N, 16.27 (16.46). NMR [δH (CDCl3)]: 9.71 (1H, br s, pyrrole
NH), 8.25 (1H, s, NdCH), 7.37 (2H, m, aryl o-H), 7.23-7.16 (3H,
m, aryl m- and p-H), 6.83 (1H, m, pyrrole H3), 6.68 (1H, m, pyrrole
H1), 6.27 (1H, m, pyrrole H2). NMR [δC (CDCl3)]: 151.7 (aryl
ipso-C), 150.0 (NdCH), 130.7 (pyrrole C4), 129.2 (aryl m-C), 125.5
(aryl p-C), 123.4 (pyrrole C1), 120.9 (aryl o-C), 116.8 (pyrrole C3),
110.4 (pyrrole C2).
Data for 1c. Anal. Found (calcd): C, 78.68 (78.75); H, 7.37
(7.12); N, 13.97 (14.13). NMR [δH (CDCl3)]: 10.43 (1H, br, pyrrole
NH), 7.95 (1H, s, NdCH), 7.06 (2H, d, JHH ) 7.5 Hz, aryl m-H),
6.95 (1H, t, JHH ) 7.5 Hz, aryl p-H), 6.59 (1H, m, pyrrole H3),
6.55 (1H, m, pyrrole H1), 6.21 (1H, m, pyrrole H2), 2.14 (6H, s,
aryl o-CH3). NMR [δC (CDCl3)]: 153.2 (NdCH), 150.7 (aryl ipso-
C), 130.0 (pyrrole C4), 128.3 (aryl o-C), 128.1 (aryl m-C), 123.9
(aryl p-C), 123.4 (pyrrole C1), 116.8-116.7 (pyrrole C3 and C1),
109.9 (pyrrole C2), 18.4 (aryl o-CH3).
Data for 1e. Anal. Found (calcd): C, 80.50 (80.27); H, 8.41
(8.72); N, 10.35 (11.01). NMR [δH (CDCl3)]: 10.97 (1H, br, pyrrole
NH), 7.97 (1H, s, NdCH), 7.23-7.15 (3H, m, aryl m- and p-H),
6.60 (1H, m, pyrrole H1), 6.20 (1H, m, pyrrole H3), 6.15 (1H, m,
pyrrole H2), 3.10 (2H, h, JHH ) 6.6 Hz, CH(CH3)2), 1.12 (12H, d,
JHH ) 6.6 Hz, CH(CH3)2). NMR [δC (CDCl3)]: 152.7 (NdCH),
148.5 (aryl ipso-C), 138.9 (aryl o-C), 129.9 (pyrrole C4), 124.5
(aryl p-C), 124.1 (pyrrole C1), 123.2 (aryl m-C), 116.6 (pyrrole
C3), 109.9 (pyrrole C2), 27.9 (CH(CH3)2), 23.6 (CH(CH3)2).
Syntheses of 2-Acetiminopyrrole Ligand Precursors (1b,d,f).
A similar procedure used in a previous publication of our group
(where mesitylaniline employed)33 was followed: 2-Acetylpyrrole
General Considerations. All experiments dealing with air- and/
or moisture-sensitive materials were carried out under inert
atmosphere using a dual vacuum/nitrogen line and standard Schlenk
techniques. Nitrogen gas was supplied in cylinders by specialized
companies (e.g., Air Liquide, etc.) and purified by passage through
4 Å molecular sieves. Unless otherwise stated, all reagents were
purchased from commercial suppliers (e.g., Acros, Aldrich, Fluka)
and used without further purification. All solvents to be used under
inert atmosphere were thoroughly deoxygenated and dehydrated
before use. They were dried and purified by refluxing over a suitable
drying agent followed by distillation under nitrogen. The following
drying agents were used: sodium (for toluene, diethyl ether, and
tetrahydrofuran); calcium hydride (for hexane and dichloromethane).
Deuterated solvents were dried by storage over 4 Å molecular sieves
and degassed by the freeze-pump-thaw method. Solvents and
solutions were transferred using a positive pressure of nitrogen
through stainless steel cannulas, and mixtures were filtered in a
similar way using modified cannulas that could be fitted with glass
fiber filter disks.
Nuclear magnetic resonance (NMR) spectra were recorded on a
Varian Unity 300 MHz spectrometer at the following frequencies:
1H at 299.995 MHz; 13C at 75.4296 MHz. Spectra were referenced
internally using the residual protio solvent resonance relative to
tetramethylsilane (δ ) 0). All chemical shifts are quoted in δ (ppm),
and coupling constants are given in Hz. Multiplicities were
abbreviated as follows: broad (br); singlet (s); doublet (d); triplet
(t); quartet (q); heptet (h); multiplet (m). For air- and/or moisture-
stable compounds, samples were dissolved in CDCl3 and prepared
in common NMR tubes. The NMR assignments of the pyrrole ring
were made according to the X-ray labeling (see ligand precursor
syntheses below). For air- and/or moisture-sensitive materials,
samples were prepared in J. Young tubes, using a glovebox.
Elemental analyses were obtained from the IST or ITN elemental
analysis services.
(34) Yeh, K.-N.; Barker, R. H. Inorg. Chem. 1967, 6, 830-833.
(35) Yoshida, Y.; Matsui, S.; Takagi, Y.; Mitani, M.; Nakano, T.; Tanaka,
H.; Kashiwa, N.; Fujita, T. Organometallics 2001, 20, 4793-4799.
(36) Li, Y.-S.; Li, Y.-R.; Li, X.-F. J. Organomet. Chem. 2003, 667, 185-
191.
6882 Inorganic Chemistry, Vol. 46, No. 17, 2007

Tetrahedral and Square Planar Complexes of Co(II)
Table 1. Crystal and Structure Refinement Data for Compounds 1f and
3e,f
(10.0 mmol) and the appropriate aniline (10 mmol) were placed in
a 50 mL round-bottom flask with a catalytic amount of p-
toluenesulfonic acid, under nitrogen. A CaCl2 guard tube was fitted
on top of the flask, and the whole flask was immersed in an oil
bath and heated to 80 °C (1b), 120 °C (1d), or 140 °C (1f), for 5
days. To neutralize the acid, NaHCO3 (powder) was added under
nitrogen flow and while the mixture was still hot. The mixture was
allowed to cool, and the remaining aniline was eliminated by trap-
to-trap distillation. The resulting iminopyrrole compound was
vacuum sublimed at ≈80 °C (10-2 mbar), further redissolved in
boiling hexane, and filtered. The product was stored at -20 °C to
yield 0.56 g (30%) of a dark yellow microcrystalline solid of 1b,
0.47 g (22%) of a dark yellow microcrystalline powder of 1d, or
0.43 g (16%) of bright yellow crystals of 1f. In the case of the
latter compound, crystals suitable for X-ray diffraction were
obtained.
param
formula
M
λ/Å
T/K
1f
3e
3f
C18H24N2
268.40
0.710 73
180
C34H42CoN4
565.65
0.710 69
100
C36H46CoN4
593.72
0.681 40
150
cryst system
space group
a/Å
b/Å
c/Å
triclinic
P1h
triclinic
P1h
triclinic
P1h
8.7397(2)
11.5140(2)
17.0729(4)
84.1779(8)
77.7817(9)
71.7589(15)
1593.59(6)
4
10.952(3)
16.279(5)
18.283(4)
84.472(9)
80.667(10)
80.271(8)
3162.3(14)
2
8.1966(14)
9.0582(16)
11.615(2)
69.060(2)
79.503(2)
80.932(2)
787.9(2)
1
R/deg
â/deg
γ/deg
V/Å3
Z
F
calc/g cm-3
1.119
0.066
27
13 198
7275
1.188
0.570
19.90
15 647
5439
0.1347
0.0770
0.1713
0.906
-0.596
0.450
1.251
0.575
32
9484
5023
0.023
µ/mm-1
max/deg
Data for 1b. Anal. Found (calcd): C, 78.28 (78.23); H, 6.40
(6.56); N, 15.36 (15.20). NMR [δH (CDCl3)]: 9.55 (1H, br, pyrrole
NH), 7.31 (2H, t, JHH ) 7.5 Hz, aryl m-H), 7.06 (1H, t, JHH ) 7.5
Hz, aryl p-H), 6.89 (1H, m, pyrrole H1), 6.78 (2H, d, JHH ) 7.5
Hz, aryl o-H), 6.65 (1H, m, pyrrole H3), 6.25 (1H, m, pyrrole H2),
2.11 (3H, s, NdC(CH3)). NMR [δC (CDCl3)]: 157.6 (NdC), 150.8
(aryl ipso-C), 132.5 (pyrrole C4), 128.9 (aryl m-C), 123.3 (aryl
p-C), 121.8 (pyrrole C1), 120.4 (aryl o-C), 112.3 (pyrrole C3), 109.7
(pyrrole C2), 16.4 (NdC(CH3)).
θ
tot. data
unique data
Rint
R [I > 3σ(I)]
wR
0.023
0.0503
0.0635
1.0091
-0.28
0.0393
0.0419
1.1132
-0.33
goodness of fit
Fmin, Fmax
0.35
0.42
All volatiles were evaporated in vacuum, and the residue was
extracted with hexane until extracts were colorless. The resulting
orange-brown solution was concentrated and cooled to -20 °C to
yield 0.32 g (80%) of a dark brown powder. Anal. Found (calcd):
C, 66.45 (66.50); H, 4.83 (4.57); N, 14.05 (14.10).
Synthesis of [Co(κ2N,N′-NC4H3C(Me)dNC6H5)2] (3b). The
same procedure described for 3a was followed, but ligand sodium
salt 2b (184.2 mg, 2 mmol) was employed, to give 0.34 g (81%)
of a brownish powder. Anal. Found (calcd): C, 67.56 (67.76); H,
5.75 (5.21); N, 13.14 (13.17).
Synthesis [Co(κ2N,N′-NC4H3C(H)dN-2,6-Me2C6H3)2] (3c).
The same procedure described for 3a was followed, but ligand
sodium salt 2c (242.4 mg, 2 mmol) was employed, yielding 0.36 g
(82%) of a red brown powder. Anal. Found (calcd): C, 68.72
(68.87); H, 6.21 (5.78); N, 11.98 (12.36).
Synthesis of [Co(κ2N,N′-NC4H3C(Me)dN-2,6-Me2C6H3)2] (3d).
The same procedure described for 3a was followed, but ligand
sodium salt 2d (424.6 mg, 2 mmol) was employed, yielding 0.42
g (87%) of dark red small crystals. Anal. Found (calcd): C, 69.57
(69.84); H, 6.03 (6.28); N, 11.27 (11.64).
Synthesis of [Co(κ2N,N′-NC4H3C(H)dN-2,6-iPr2C6H3)2] (3e).
The same procedure described for 3a was followed, but ligand
sodium salt 2e (506 mg, 2 mmol) was employed, yielding 0.46 g
(82%) of dark red crystals. Crystals suitable for X-ray diffraction
were obtained. Anal. Found (calcd): C, 71.94 (72.19); H, 7.72
(7.48); N, 9.65 (9.90).
Synthesis of [Co(κ2N,N′-NC4H3C(Me)dN-2,6-iPr2C6H3)2] (3f).
The same procedure described for 3a was followed, but ligand
sodium salt 2f (536.8 mg, 2 mmol) was employed, yielding 0.54 g
(91%) of bright red crystals. Crystals suitable for X-ray diffraction
were obtained. Anal. Found (calcd): C, 73.02 (72.83); H, 8.10
(7.81); N, 9.12 (9.44).
X-ray Experimental Data. Crystallographic and experimental
details of crystal structure determinations are given in Table 1.
Crystals of compounds 1f and 3e,f were selected under an inert
atmosphere, covered with polyfluoroether oil, and mounted on a
nylon loop. Ligand precursor 1f yielded weakly diffracting crystals.
Data was collected at 180 K on a Nonius KappaCCD with graphite-
Data for 1d. Anal. Found (calcd): C, 78.93 (79.21); H, 7.51
(7.60); N, 12.99 (13.20). NMR [δH (CDCl3)]: 10.01 (1H, br, pyrrole
NH), 7.06 (2H, d, JHH ) 7.5 Hz, aryl m-H), 6.93 (1H, t, JHH ) 7.5
Hz, aryl p-H), 6.75-6.6 (2H, m, pyrrole H1 and H3), 6.22 (1H, m,
pyrrole H2), 2.03 (6H, s, aryl o-CH3), 1.95 (3H, s, NdC(CH3)).
NMR [δC (CDCl3)]: 157.7 (NdC), 148.2 (aryl ipso-C), 132.0
(pyrrole C4), 127.8 (aryl m-C), 127.0 (aryl o-C), 122.9 (pyrrole
C1), 121.9 (aryl p-C), 112.0 (pyrrole C3), 109.5 (pyrrole C2), 18.0
(aryl o-CH3), 16.7 (NdC(CH3)).
Data for 1f. Anal. Found (calcd): C, 80.60 (80.55); H, 10.58
(9.01); N, 10.33 (10.44). NMR [δH (CDCl3)]: 9.72 (1H, br, pyrrole
NH), 7.15-7.04 (3H, m, aryl m- and p-H), 6.69 (1H, m, pyrrole
H1), 6.64 (1H, m, pyrrole H3), 6.24 (1H, m, pyrrole H2), 2.80
(2H, h, JHH ) 6.6 Hz, CH(CH3)2), 1.97 (3H, s, NdC(CH3)), 1.11
(6H, d, JHH ) 6.6 Hz, CH(CH3)(CH3)), 1.08 (6H, d, JHH ) 6.6 Hz,
CH(CH3)(CH3)). NMR [δC (CDCl3)]: 157.5 (NdCH), 145.7 (aryl
ipso-C), 137.3 (aryl o-C), 132.2 (pyrrole C4), 123.5 (aryl p-C), 123.0
(aryl m-C), 121.7 (pyrrole C1), 111.7 (pyrrole C3), 109.6 (pyrrole
C2), 28.1 (CH(CH3)2), 23.3 (CH(CH3)(CH3)), 23.1 (CH(CH3)-
(CH3)), 17.2 (NdC(CH3)).
In Situ Synthesis of the Sodium Salts of Compounds 1a-f
(2a-f). The same procedure followed in a previous publication of
the group was used:33 In a typical experiment, NaH (47.8 mg, 2
mmol) was suspended in tetrahydrofuran and 2 mmol of a neutral
ligand precursor (1a-f) was slowly added as a solid under a
counterflow of nitrogen. An immediate evolution of hydrogen
occurred, and after some minutes, a solution of the sodium ligand
salt was obtained and stirred for 90 min.
Synthesis of [Co(κ2N,N′-NC4H3C(H)dNC6H5)2] (3a). Anhy-
drous CoCl2 (129.9 mg, 1 mmol) was suspended in tetrahydrofuran
and cooled to -78 °C. The ligand sodium salt 2a (186.3 mg, 2
mmol) was filtered and directly added dropwise to the CoCl2
suspension, which was then stirred for 1 h. The mixture was allowed
to warm to room temperature and further stirred for another 2 h.
(37) Iverson, C. N.; Carter, C. A. S. G.; Baker, R. T.; Scollard, J. D.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2003, 125, 12674-
12675.
Inorganic Chemistry, Vol. 46, No. 17, 2007 6883

Carabineiro et al.
monochromated Mo KR radiation. Images were processed with
DENZO and SCALEPACK programs.38 Since crystals of 3f were
extremely small and weakly diffracting, synchrotron radiation was
used to collect diffraction data on this compound. Data was collected
at Station 9.8, Daresbury SRS, Warrington, Cheshire, U.K., using
a Bruker SMART CCD diffractometer at 150K. For both 1f and
3f, structures were solved by direct methods using the program
SIR92,39 and refinements (on F) and graphical calculations were
performed using the CRYSTALS40 program suite.
For both 1f and 3f the non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were located
in Fourier maps and their positions adjusted geometrically (after
each cycle of refinement) with isotropic thermal parameters.
Chebychev weighting schemes and empirical absorption corrections
were applied in each case.41
Crystallographic data for complex 3e was collected using
graphite-monochromated Mo KR radiation (λ ) 0.710 69 Å) on a
Bruker AXS-KAPPA APEX II diffractometer equipped with an
Oxford Cryosystems open-flow nitrogen cryostat, at 150 K. Cell
parameters were retrieved using Bruker42 software and refined using
Bruker SAINT43 on all observed reflections. Absorption corrections
were applied using SADABS.44 Structure solution and refinement
were performed using direct methods with the programs SIR9745
and SHELXL46 both included in the package of programs WINGX-
version 1.70.01.47 The poor diffracting power (θmax ) 20°), size,
and crystal quality (Rint ) 0.1347) prevented the anisotropic
refinement of all the non-hydrogen atoms; only Co atoms were
refined with anisotropic displacement parameters. Due to the
presence of two independent molecules in the asymmetric unit, the
anisotropic refinement of the two molecules would lead to a very
poor ratio of refined parameters/number of reflections that could
prevent a good final and reliable refinement. All hydrogen atoms
were inserted in idealized positions and allowed to refine riding in
the parent carbon atom.
purpose characterization system, MagLab 2000 (Oxford Instru-
ments). The temperature dependence of the magnetic susceptibility
was measured under magnetic fields of 5 T in the temperature range
1.7-300 K. The isothermal magnetization measurements up to 5
T and taken at different temperatures (1.7, 10, and 25 K) were also
performed. The diamagnetism correction for the experimental data
was estimated using the Pascal constants.51
Computational Details. All calculations were performed using
the Gaussian 98 software package52 and the B3LYP hybrid
functional. That functional includes a mixture of Hartree-Fock53
exchange with DFT54 exchange correlation, given by Becke’s three
parameter functional55 with the Lee, Yang, and Parr correlation
functional, which includes both local and nonlocal terms.56,57 The
geometry optimizations were accomplished without symmetry
constraints using a LanL2DZ basis set58 for all atoms (B1). Single
point energy calculations were performed on the B3LYP/B1
geometries, using the same functional and a standard 6-311G(d,p)
basis set59 (B2) for all elements. Spin contamination was carefully
monitored for all calculations. The values of S2 indicate minor
spin contamination and are presented in the Supporting Information.
Given the known dependence of relative spin-state energies on
the amount of exact exchange included in the functional,60-62 energy
calculations were repeated with a modified B3LYP functional
(B3LYP*) with only 15% exact (Hartree-Fock) exchange and, also,
with BP86, a “pure” DFT functional.63 Low-spin species are
increasingly favored with the amount of the exact exchange included
in the functional, as expected,60 but the general trend is maintained.
The energy values discussed in the text (B3LYP/B2//B3LYP/B1)
are the ones that fit better the experimental results.
(50) Sur, S. K. J. Magn. Res. 1989, 82, 169-173.
(51) Carlin, R. L. In Magnetochemistry: Springer-Verlag: Berlin, 1986.
(52) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.:
Pittsburgh, PA, 1998.
Figures were generated using ORTEP3.48 Data was deposited
in CCDC under the deposit nos. 624571 for 1f, 624572 for 3e, and
624573 for 3f.
Magnetic Measurements. Magnetic susceptibility measurements
in solution were carried out by the Evans method,49,50 using a 3%
solution of hexamethyldisiloxane in deuterated toluene as reference.
Magnetic measurements by the dc extraction method were
performed on polycrystalline samples (10-20 mg) using a multi-
(38) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter, C. N.,
Jr., Sweet, R. M., Eds.; Academic Press: New York, 1996; p 276.
(39) SIR92: Altomare, A.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi,
A. J. Appl. Crystallogr. 1993, 26, 343-350.
(40) (a) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.
CRYSTALS; Oxford University: Oxford, U.K., 1996. (b) Betteridge,
P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J.
Appl. Crystallogr. 2003, 36, 1487. (c) Watkin, D. J.; Prout, C. K.;
Pearce, L. J. CAMERON; Oxford University: Oxford, U.K., 1996.
(41) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, 39, 158-166.
(42) SMART Software for the CCD detector System, version 5.625; Bruker
AXS Inc.: Madison, WI, 2001.
(43) SAINT Software for the CCD detector System, version 7.03; Bruker
AXS Inc.: Madison, WI, 2004.
(44) Sheldrick, G. M. SADABS, version 2.10; Bruker AXS Inc.: Madison,
WI, 2003.
(45) SIR97: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115-119.
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release 97-2; Institu¨t fu¨r Anorganische Chemie der Universita¨t:
Tammanstrasse 4, D-3400 Go¨ttingen, Germany, 1998.
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Molecular Orbital Theory; John Wiley & Sons: New York, 1986.
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157, 200-206.
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Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1. (b)
Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) Wadt, W.
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Phys. 1980, 72, 650-654. (c) Wachters, A. J. H. J. Chem. Phys. 1970,
52, 1033-1036. (d) Hay, P. J. J. Chem. Phys. 1977, 66, 4377-4384.
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P. Phys. ReV. B 1986, 33, 8822-8824.
(47) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
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6884 Inorganic Chemistry, Vol. 46, No. 17, 2007

Tetrahedral and Square Planar Complexes of Co(II)
Scheme 2. Synthesis of Complexes 3a-f
Tables of atomic coordinates for all the optimized species are
available in the Supporting Information.
Results and Discussion
Synthesis of Complexes. The six iminopyrrole ligand
precursors 1a-f used in this work (Scheme 1) were prepared
by condensation of 2-formylpyrrole or 2-acetylpyrrole with
different arylamines (aniline, 2,6-dimethylaniline, and 2,6-
diisopropylaniline), following the methods described in an
earlier publication of our group.33 The synthesis of formimi-
nopyrrole derivatives is relatively straightforward, and
standard conditions were employed.33-37 However, the
preparation of the new acetiminopyrrole derivatives required
forcing conditions (no solvent and longer reflux) and resulted
in lower yields. The yields in formimines vary from 60 to
70%, and those of acetimines are in the range 15-30%,
decreasing with increasing R′ size. The crystal structure of
the bulkier ligand precursor 1f was determined (see below).
Deprotonation of the ligand precursors with NaH, in THF,
followed by addition of the resulting solution to a THF
suspension of anhydrous CoCl2, in a molar ratio of 2:1
(ligand precursor:CoCl2), gave rise to complexes 3a-f
(Scheme 2). The resulting unsaturated [Co(iminopyrrolyl)2]
complexes were extracted in hexane and precipitated by
storage at -20 °C, yielding brown (3a-c), dark red (3d,e),
and red (3f) powders or crystals, in good yields. All the
syntheses have been carried out under rigorous inert atmo-
sphere conditions since all compounds have shown to be very
sensitive to air, especially when in solution. The synthesis
of compound 3e was already reported in the literature by
Bochmann et al.,13 but it was only characterized by elemental
analysis and mass spectrometry. This preparation employed
a slightly different procedure, using the corresponding Li
salt of the ligand precursor and diethyl ether as solvent,
leading however to a much lower yield (25%).
X-ray Diffraction. Crystals suitable for X-ray diffraction
were obtained in the cases of compounds 1f and 3e,f enabling
the determination of their crystal structures.
The molecular structure of the ligand precursor 1f is shown
in Figure 1. The asymmetric unit of the ligand precursor 1f
contains two independent molecules. Selected bond distances
(Å) and angles (deg) are listed in the Figure 1 caption (for
molecule 1). The pyrrole ring shows bond distances within
1.36-1.38 Å and a longer value of 1.408(2) Å for C2-C3.
The coplanarity of the pyrrole ring with the acetimine group
(torsion angle N2-C5-C4-N1 of 1.29°) and a distance
C4-C5 of 1.456(2) Å, a value shorter than normal for a
typical C-C single bond, points to an extension of the
pyrrole ring π-electronic delocalization toward its acetimino
substituent. The steric hindrance produced by the two
isopropyl substituents, at the 2 and 6 positions of the phenyl
ring, makes it nearly perpendicular (87.8°) to the acetimi-
nopyrrole plane defined by atoms N2-C5-C4-N1. These
features only change slightly upon coordination as it can be
seen in Table 2 and are comparable to those observed for a
similar compound with a mesityl substituent.33
around the Co center, respectively. Table 2 also lists selected
bond distances and angles for 3e,f.
The crystallographic structure of compound 3e consists
of an asymmetric unit containing two independent tetrahedral
molecules. The planes defined by the five-membered rings
Co1-N1-C4-C5-N2 and Co1-N3-C10-C11-N4 are
more deviated from perpendicular (dihedral angles ) 80.3-
82.5°) than those reported in the literature (89.7° 3 and 85.1-
86.2° 4) for the two reported crystallographic structures of
similar tetrahedral Co(II) iminopyrrolyl complexes. These
compounds contain tert-butyl groups as substituents of the
imine nitrogen, which are less sterically demanding than the
2,6-diisopropylphenyl groups used in the present work. The
chelate bite angles N1-Co1-N2 and N3-Co1-N4 are
respectively 82.5-83.0° (molecule 1) and 82.8-83.4° (mol-
ecule 2), while the angles N1-Co1-N3 and N2-Co1-N4
are 120.2-125.2 and 114.6-119.1°, showing a distorted
tetrahedral geometry. The Co-Npyrrolyl bonds (1.952(8)-
The molecular structures of complexes 3e (Figure 2) and
3f (Figure 3) show tetrahedral and square-planar geometries
Inorganic Chemistry, Vol. 46, No. 17, 2007 6885

Carabineiro et al.
Figure 1. ORTEP III diagram of the ligand precursor 1f (molecule 1 of
the asymmetric unit) using 30% probability level ellipsoids. Hydrogens were
omitted for clarity. Selected bond distances (Å) and angles (deg) for
molecule 1: N1-C4 1.3729(19), N1-C1 1.3604(19), C1-C2 1.369(2),
C2-C3 1.408(2), C3-C4 1.381(2), C4-C5 1.456(2), C5-C6 1.507(2),
N2-C5 1.2835(19), N2-C12 1.4295(18); C5-N2-C12 120.03(12), C6-
C5-N2 124.51(13), N2-C5-C4 118.88(13), C5-C4-N1 121.82(13), N1-
C4-C3 107.25(13), C4-N1-C1 109.42(13), C2-C1-N1 108.51(14), C3-
C2-C1 107.18(13), C4-C3-C2 107.64(14).
Figure 2. ORTEP III diagram of complex 3e (molecule 1 of the
asymmetric unit) using 30% probability level ellipsoids. Hydrogens were
omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes
3e and 3f
3e
param
Co1-N1
molecule 1
molecule 2
3f
Distances (Å)
1.985(9)
2.060(8)
1.971(8)
1.961(8)
2.054(8)
1.952(8)
2.034(7)
1.9388(13)
1.9528(14)
Co1-N2
Co1-N3
Co1-N4
N1-C4
N1-C1
C1-C2
2.055(8)
1.358(11)
1.369(12)
1.381(14)
1.364(13)
1.429(14)
1.332(12)
1.470(12)
1.360(12)
1.397(11)
1.297(11)
1.427(11)
1.383(12)
1.373(12)
1.385(13)
1.402(15)
1.384(13)
1.300(10)
1.455(12)
1.355(12)
1.377(11)
1.308(11)
1.447(12)
1.382(2)
1.356(2)
1.387(2)
1.394(3)
1.393(2)
1.307(2)
1.431(2)
Figure 3. ORTEP III diagram of complex 3f using 30% probability level
ellipsoids. Hydrogens were omitted for clarity.
C2-C3
C3-C4
N2-C5
N2-C12
N3-C7
N3-C10
N4-C11
N4-C18
C5-C6
phenyl ring. These distances are very similar to those reported
for a similar Co(tert-butylformiminopyrrolyl)2 (1.981(7) and
2.066(8) Å, respectively).3 The recently reported binuclear
Co(II) complex containing two fragments of Co(tert-butyl-
formiminopyrrolyl)2,4 bridged by a CMe2 group bound to
the pyrrolyl carbons adjacent to the N atoms, also shows
similar distances (1.980(3)-1.989(3) and 2.047(3)-2.055-
(3) Å, respectively). The angle between the planes defined
by the two aryl rings of each molecule of the asymmetric
unit of 3e is 21.3 and 33.8°, respectively.
The crystallographic structure of compound 3f shows an
asymmetric unit formed by a half-molecule of [Co((2,6-
diisopropylphenyl)acetiminopyrrolyl)2] with the Co atom
occupying a special position (0, 0, 0). The corresponding
molecular structure is generated by symmetry, giving rise
to a rare square planar geometry around the Co atom, which
is located in an inversion center (Figure 3). Consequently,
the Co atom is perfectly located in the plane formed by the
four coordinating nitrogen atoms, meaning that the sum of
angles around the Co atom is 360° (being N1-Co-N2 of
82.15(6)° and N1-Co-N2_2 of 97.85(6)°). The pyrrolyl
rings, and thus the imine groups, lie trans to each other. The
corresponding distances Co1-N1 (1.9390(14) Å) and Co1-
N2 (1.9530(14) Å) are smaller than those found in the
1.492(2)
1.414(2)
1.415(2)
1.521(2)
C4-C5
C12-C13
C13-C26
1.387(13)
1.401(13)
1.502(13)
1.416(13)
1.425(13)
1.487(13)
Angles (deg)
N1-Co1-N2
Co1-N1-C4
Co1-N1-C1
C4-N1-C1
Co1-N2-C5
Co1-N2-C12
C5-N2-C12
C6-C5-N2
C6-C5-C4
N2-C5-C4
C5-C4-N1
C5-C4-C3
N1-C4-C3
C4-C3-C2
C3-C2-C1
C2-C1-N1
82.5(3)
83.0(3)
82.15(6)
110.1(7)
143.1(7)
106.6(9)
109.9(7)
132.1(6)
117.8(8)
112.0(7)
143.4(7)
104.4(8)
109.5(7)
130.2(6)
120.3(8)
112.21(11)
142.42(13)
105.37(14)
115.31(11)
125.74(11)
118.70(14)
124.45(15)
120.77(15)
114.71(15)
115.50(15)
133.47(16)
110.87(15)
105.65(15)
107.04(15)
111.06(16)
117.3(9)
120.2(10)
115.2(9)
119.5(10)
130.9(10)
109.6(10)
105.5(10)
108.5(11)
109.7(10)
132.6(10)
112.2(10)
105.0(10)
107.5(10)
110.8(10)
1.985(9) Å) are shorter than the Co-Nimine ones (2.034(7)-
2.055(8) Å) due to the anionic nature of the pyrrolyl nitrogen
and the steric bulk of the 2,6-diisopropyl substituents of the
6886 Inorganic Chemistry, Vol. 46, No. 17, 2007

Tetrahedral and Square Planar Complexes of Co(II)
Table 3. Effective Magnetic Moments µeff (µB) for Complexes 3a-f,
Measured in Toluene Solution (Evans Method) and in Powder (Dc
Extraction Method), at 300 K
complex
Evans µeff (µB)
dc extractn µeff (µB)
3a
3b
3c
3d
3e
3f
4.26
4.29
4.21
4.24
4.24
2.40
4.85
3.77
4.85
3.90
3.85
2.85
tetrahedral complex 3e, particularly that of the imine Co1-
N2 distance that is significantly shorter (ca. 0.1 Å) than in
3e. This may indicate a stronger σ-donor character of N2
atom in 3f induced by the methyl (C6) substituent of the
iminic carbon (C5), which may also cause a higher degree
of steric congestion around the Co atom (see below, in
Molecular Orbital calculations). In fact, the 2,6-diisopropyl-
phenyl groups are nearly perpendicular to the CoN4 square
plane (82.3°) and parallel to each other (0°). Interestingly,
an edge-to-face interaction between C1-H and the aryl
centroid of 3f (C1-H‚‚‚aryl centroid ) 2.376 Å) seems to
take place, although it may be a consequence of this
congestion. These features prevent the most favorable
tetrahedral arrangement to take place, forcing a different low-
spin (S ) 1/2) electronic configuration of the Co atom.
Usually, Co(II) tetracoordinated with nitrogen-donor chelat-
ing ligands show a clear preference for high-spin tetrahedral
geometry (S ) 3/2).
For both 3e,f, the imine double bond C5-N2 distances
(1.312(10)-1.306(2) Å) are longer than that of the free ligand
(namely 1.283(2) Å for 1f), showing some degree of π-π*
back-bonding from the metal to the imine fragment.
Magnetic Measurements. The results obtained for the
magnetic susceptibility measurements of the different com-
plexes, taken both in solution (Evans method) and in powder
(dc extraction method), are shown in Table 3.
In solution measurements, at 300 K, the values of µeff
found for complexes 3a-e are very similar (ca. 4.2-4.3 µB)
and lie in the range typical of known tetrahedral Co(II)
complexes (4.2-5.1 µB) with S ) 3/2.4,27,28,32,64 These values
are higher than the expected spin-only value of 3.87 µB,
suggesting the presence of spin-orbit coupling effects.4,32,64
On the other hand, complex 3f is the only one in the series
which µeff value (2.4 µB) fall within the typical range of low-
spin S ) 1/2 square-planar Co(II) complexes (2.0-2.7
µB),28,32,64,65 although values such as 1.75 µB were also
reported in the literature,26 which are very similar to the
calculated S ) 1/2 spin-only value of 1.73 µB. The µeff value
obtained for complex 3f, containing the bulkiest iminopyr-
rolyl in the ligand series, shows also significant spin-orbit
coupling effects but points clearly to a square-planar
structure, which was also confirmed by X-ray diffraction (see
above).
Figure 4. Temperature dependence of the effective magnetic moment,
eff, of the complexes 3a-f, in the range 5-300 K (dc extraction method).
µ
polycrystalline powder samples of complexes 3a-f, using
the dc extraction method.
In general, the values are similar and show the same trend
observed for the magnetic moments determined in toluene
solution, although values obtained for complexes 3a,c
(formimino derivatives) are slightly higher and those of
complexes 3b,d,e are somewhat lower, the latter lying close
to the spin-only value. The slight increase of the magnetic
moments of 3a,c, as temperature decreases to 50 K, also
reinforces the presence of significant spin-orbit coupling
effects. Below 10 K the decrease in µeff suggests the presence
of very weak intermolecular magnetic interactions. At the
lower temperature, 1.7 K, the values of the magnetic field
dependence of the magnetization, M(B) (see Supporting
Information), show some tendency to saturate at higher fields,
and the linearity of the M(B) curves obtained for higher
temperatures confirms their paramagnetic behavior.
For all the complexes, cooling-warming cycles of mag-
netization versus temperature, in the temperature range
300-5 K, give exactly the same values. This behavior
indicates that no spin-crossover equilibria occur between S
) 3/2 and S ) 1/2 ground states, which may be associated
with a high-energy barrier of the tetrahedral to square planar
interconversion.
Electronic Spectra. The spectra of the ligand precursors
1a-f and of complexes 3a-f were recorded in the range
200-1500 nm, in toluene solutions, under nitrogen atmo-
sphere. Their comparison enabled assignment of all bands
observed in the range 270-350 nm to intraligand (IL)
transitions. The toluene cutoff prevented the spectra visual-
ization below 270 nm, being that the spectra of the ligand
precursors were also recorded in Nujol mulls. According to
the literature,32,66 the bands observed above 400 nm may be
assigned to transitions involving Co(II) ions. Table 4
summarizes the results obtained for the complexes studied
above 400 nm. All the compounds 3a-f show bands in the
range 450-640 nm. On the basis of their molar extinction
coefficients (10 < ꢀ < 850 L mol-1 cm-1), these electronic
absorptions can be assigned to d-d transitions.32,66 In
particular, for the square-planar complex 3f, it can also be
Figure 4 shows the temperature dependence of the effec-
tive magnetic moment (µeff ) 2.83(øT)1/2 µB) obtained for
(64) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemisty, 5th ed.;
John Wiley and Sons: New York, 1988.
(65) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon
Press: London, 1984.
(66) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Am-
sterdam, 1968.
Inorganic Chemistry, Vol. 46, No. 17, 2007 6887

Carabineiro et al.
Table 4. Absorption Maxima Values (nm) and, in Parentheses, Molar Extinction Coefficients (L mol-1 cm-1) Taken from the Electronic Spectra
(above 400 nm) of Complexes 3a-f in Toluene at Room Temperaturea
3a
3b
3c
3d
3e
3f
obsdb
410 (sh, 1500)
440 (sh, 270)
410 (sh, 850)
435 (sh, 490)
410 (sh, 7500)
400 (sh, 16 400)
410 (sh, 600)
CT or dd
dd
dd
dd
dd
dd
490 (sh, 247)
535 (sh, 225)
630 (sh, 33)
490 (sh, 124)
535 (sh, 113)
630 (sh, 17)
484 (sh, 549)
523 (sh, 462)
631 (sh, 39)
460 (sh, 1000)
544 (sh, 500)
635 (sh, 35)
640 (sh, 54)
1152 (8)
a sh ) shoulder. b Tentative band assignment: CT ) charge transfer; d-d ) metal d orbital internal transitions.
Chart 2 Four Structures Optimized by DFT (R ) H or Me)
four Co-N distances are well within bonding values (1.95
and 1.98 Å).
The geometry calculated for the tetrahedral species with
R ) H (bottom of Figure 5) compares fairly well with the
corresponding X-ray structure (Figure 2). For example, the
calculated Co-N bonding distances are in the range 2.01-
2.09 Å, being close to the experimental values: 1.95-2.06
Å (there are two molecules in the asymmetric unit, in the
corresponding X-ray crystal structure). A reasonable agree-
ment is also observed for the relevant angles. The angle
between five-membered rings is 79° (calcd) compared with
80-83° (expt), revealing a somewhat distorted structure with
respect to an ideal tetrahedral where the corresponding angle
would be 90°. The orientation of the 2,6-diisopropylphenyl
groups with respect to the five-membered rings is not
perpendicular as in the square planar molecule, being instead
tilted: the angles between the aryl ring and the five-
membered ring are 67° (calcd) and 58-75° (expt) (as there
are two molecules in the asymmetric unit, in the correspond-
ing X-ray crystal structure). The reason for this distortion,
that is, the deviation of those angles from 90°, is evident in
the optimized geometries represented in Figure 5. In fact, a
tetrahedral coordination forces the proximity of the 2,6-
diisopropylphenyl substituents of the two different N-N
seen an extra band in the near-infrared region (1152 nm),
which is predicted in the literature.66 A corresponding band
is not present in the spectra of the tetrahedral 3a-e
complexes. Such a difference between the electronic spectra
of square-planar and tetrahedral d7 Co(II) complexes was
also noticed by Lippard et al.32
Molecular Orbital Calculations. The structural charac-
terization of the complexes clearly shows 3a-e as tetrahedral
species whereas 3f is square planar. To better understand
this structural preference, DFT calculations54 were performed
on complexes 3e,f, which are those containing the bulkier
ortho aryl substituents (iPr) and the only ones with available
X-ray structures. Their chemical difference lies only in the
iminic carbon substituent of the iminopyrrolyl ligand, R )
H (3e) or Me (3f) (see Chart 2). Thus, for each R group (H
or Me), two coordination geometries were tested, square
planar and tetrahedral, representing a total of four different
species.
The optimized geometries of the complexes with R) H
(complex 3e) in the two coordination geometries are shown
in Figure 5. The square planar molecule (top of Figure 5)
presents a practically undistorted geometry, close to an ideal
square planar structure. The angle between the planes of the
two five-membered rings CoNCCN is 0°, and the 2,6-
diisopropylphenyl groups are perpendicular to the corre-
sponding five-membered ring, the angle between the plane
of the aryl ring and the five-membered ring being 90°. The
Figure 5. Optimized geometries (B3LYP) for complex 3e (with R ) H)
with square planar (top) and tetrahedral (bottom) geometries. The Co and
the coordinating N atoms are highlighted, and the energy difference (kcal
mol-1) between the two species is presented.
6888 Inorganic Chemistry, Vol. 46, No. 17, 2007

Tetrahedral and Square Planar Complexes of Co(II)
with respect to the five-membered ring planes (73°).
However, the relative stability between the two coordination
geometries is reversed. In the case of the complexes with R
) Me the square planar molecule is favored by 1.2 kcal
mol-1 with respect to the tetrahedral complex. A stereo-
chemical reason can be partially responsible for this change
as highlighted by the space-filling representations of Figure
6. In fact, the presence of the methyl substituents in the
carbon adjacent to the nitrogen atom of the ligand restrains
the tilting of the 2,6-diisopropylphenyl groups as shown by
the corresponding angles: 67 and 73°, for R ) H and Me,
respectively. As a consequence of the repulsion between
methyl and 2,6-diisopropylphenyl, this substituent is pushed
closer to the corresponding group in the opposite iminopyr-
rolyl ligand, originating a more crowded environment around
the metal and causing some destabilization in the molecule.
For example, the closest C-C contact between isopropyl
groups in different iminopyrrolyl ligands is 4.34 Å in the
tetrahedral complex with R) H (3e) and becomes 4.08 Å in
the equivalent molecule with R ) Me (3f).
In addition, electronic factors can also favor the square
planar geometry in 3f. Comparing the two square planar
complexes, the HOMO-LUMO gap calculated for the
complex with acetiminopyrrolyl (R ) Me) is 0.1 eV higher
than that obtained for the complex with formiminopyrrolyl
(R ) H), indicating the former is a higher field ligand and,
thus, expected to favor the low-spin complex.
Figure 6. Optimized geometries (B3LYP) for complex 3f (with R ) Me)
with square planar (bottom) and tetrahedral (top) geometries. The Co and
the coordinating N atoms are highlighted, and the energy difference (kcal
mol-1) between the two species is presented. On the left-hand side, the
structures are represented with the 2,6-diisopropylphenyl groups in dark
gray. On the right-hand side, for each molecule, one 2,6-diisopropylphenyl
and the methyl in the adjacent carbon atom are presented with space-filling
representations.
ligands, while in the square planar environment those
substituents lie in opposite sides of the molecule, in a trans
geometry. As a consequence, a tilting from perpendicularity
of both the angle between the two CoNCCN five-membered
rings and the angle between the phenyl ring and the
respective five-membered ring is required to keep the
substituents away from each other, specially the bulky
isopropyl groups. Despite this geometrical distortion, the
tetrahedral isomer is calculated to be the most stable (by
2.4 kcal mol-1). Thus, the high electron pairing energy
characteristic of first row transition metals such as Co(II)
(d7) favors the high-spin molecule (tetrahedral, S ) 3/2) with
respect to the low-spin species (square planar, S ) 1/2).
The optimized geometries for the complexes with R )
Me (complex 3f) are represented in Figure 6. The structure
calculated for the square planar molecule (bottom of Figure
6) compares well with the experimental one, obtained by
X-ray diffraction (Figure 3), both in what concerns the Co-N
bonding distances (calcd, 1.95-2.00 Å; expt, 1.94-1.95 Å)
as in the relevant angles. The two five-membered rings are
coplanar since the angle between CoNCCN planes is 0° in
the calculated and in the experimental structures. The aryl
rings are essentially perpendicular to the corresponding five-
membered ring with angles close to 90° between the C6 and
the CoNCCN planes (calcd, 88°; expt, 82°).
Conclusions
A new series of Co(II) complexes 3a-f [Co(κ2N,N′-
NC4H3C(R)dN-2,6-R′2C6H3)2] (R ) R′ ) H, 3a; R ) Me,
R′ ) H, 3b; R ) H, R′ ) Me, 3c; R ) R′ ) Me, 3d; R )
i
i
H, R′ ) Pr, 3e; R ) Me, R′ ) Pr, 3f) was prepared. The
complexes contain bidentate iminopyrrolyl ligands with
increasing bulkiness. Magnetic susceptibility measurements,
in powder and in solution, X-ray diffraction, and UV/vis/
NIR proved that complexes 3a-e exhibit tetrahedral geom-
etry, whereas complex 3f showed a rare square planar
geometry. DFT calculations suggest that the preference of a
square planar geometry, in the case of complex 3f, probably
results from a combination of stereochemical interligand
repulsion and electronic factors. On one hand, a square planar
geometry allows larger separations between the substituents
(R and R′) of the iminopyrrolyl ligands, and on the other,
the iminopyrrolyl ligand with R ) Me is a stronger donor,
favoring the low-spin molecule.
Acknowledgment. We wish to thank the Fundac¸a˜o para
a Cieˆncia e Tecnologia for financial support (Projects POCTI/
QUI/42015/2001 and POCI/QUI/59025/2004, cofinanced by
FEDER) and for fellowships to S.A.C. (SFRH/BPD/14902/
2004) and L.C.S. (SFRH/BD/9127/2002). We also thank the
Royal Society for funding to S.I.P. We are grateful to Prof.
Cristina Freire (University of Porto, Porto, Portugal) for
helpful discussions.
The structure calculated for the tetrahedral complex (R )
Me, top of Figure 6) is similar to the one optimized for the
species with R ) H in its main geometrical features. The
four Co-N distances are well within bonding values (2.00-
2.09 Å), the angle between CoNCCN planes (80°) deviates
from 90°, and the 2,6-diisopropylphenyl groups are tilted
Supporting Information Available: CIF files containing X-ray
structural data, including data collection parameters, positional and
thermal parameters, and bond distances and angles, for complexes
Inorganic Chemistry, Vol. 46, No. 17, 2007 6889

Carabineiro et al.
1f and 3e,f, figures containing the magnetic field dependence of
the magnetization, M(B), for all complexes, at 1.7 and 25 K, a table
containing the absorption maxima of the electronic spectra UV
region of complexes 3a-f, and a table of atomic coordinates for
the optimized species (3e,f) obtained with B3LYP/B1. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC062125W
6890 Inorganic Chemistry, Vol. 46, No. 17, 2007