Synthesis, structure and magnetic behavior of five-coordinate bis(iminopyrrolyl) complexes of cobalt(II) containing PMe3 and THF ligands

Article abstract of DOI:10.1021/ic800992f

The new bis-iminopyrrolyl five-coordinate Co(II) complexes [Co(κ²N,N′-NC₄H₃C(R)=N-2,6- ⁱPr₂C₆H₃)₂(PMe ₃)] (R = H 3a; Me 3b) were synthesized in high yields (ca. 80-90%), using THF and diethyl ether as solvents, respectively, by (a) treatment of CoCl₂(PMe₃)₂ with the corresponding iminopyrrolyl Na salts (Ie or If) or (b) reaction of anhydrous CoCl₂ and PMe₃ with Ie or If. A third route was tested, involving the addition of excesses of PMe₃ to the complexes [Co(κ ²N,N′-NC₄H₃C(R)=N-2,6-ⁱPr ₂C₆H₃)₂] (R = H 1e; Me 1f), which was only successful for the synthesis of 3a, in lower yields (ca. 30%). The synthesis of 3b in THF was unfruitful because of the kinetic competition of the solvent, giving rise to mixtures of 1f and its coordinated THF adduct 4b. The synthesis of the new bis-iminopyrrolyl five-coordinate Co(II) complexes [Co(κ²N,N′-NC₄H₃C(R)=N-2,6- ⁱPr₂C₆H₃)₂(THF)] (R = H 4a; Me 4b) were carried out in high yields (ca. 80-90%) by the reaction of CoCl₂(THF)_1.5 with the corresponding iminopyrrolyl Na salt. All the compounds have been characterized by X-ray diffraction, with 3a and 3b showing axially compressed trigonal bipyramidal geometry (with the PMe₃ ligand lying on the equatorial plane), whereas complexes 4a and 4b exhibit distorted square pyramidal geometries with the THF molecule occupying the axial position. Complex 4a shows clearly a compressed geometry, but for complex 4b, two polymorphs were characterized, exhibiting molecules with different Co-O (THF) bond lengths, one of them being compatible with an elongated form. Magnetic measurements either in the solid or in the liquid phases indicate that complexes 3a and 3b have low-spin ground states (S = 1/2). In toluene solution, the geometry is fully confirmed by EPR data, which further indicates a d_x2-y2/d_xy, ground state. However, compounds 4a and 4b behave unusually because they show magnetic moments that are compatible with high-spin ground states (S = 3/2) in the solid state, but conform to low-spin ground states (S = 1/2) when both complexes are dissolved in toluene solutions. The low-spin ground states in toluene solution are confirmed by EPR spectroscopy, which further supports, for complexes 4a and 4b, an axially elongated square pyramidal geometry and a d_z2 ground state. Thus the change in the ground-state and, consequently, in the geometry of complexes 4a and 4b from solid state to toluene solution might be a consequence of the elongation of the Co-O(THF) bond length. DFT studies performed on complexes 3 and 4 corroborate their different structure and magnetic behaviors and verify, for the latter complexes, the structural differences observed in the solid state and in toluene solution.

Full text of DOI:10.1021/ic800992f

Inorg. Chem. 2008, 47, 8896-8911
Synthesis, Structure and Magnetic Behavior of Five-Coordinate
Bis(iminopyrrolyl) Complexes of Cobalt(II) containing PMe3 and THF Ligands
So´nia A. Carabineiro,† Ronan M. Bellabarba,† Pedro T. Gomes,*,† Sofia I. Pascu,‡ Lu´ıs F. Veiros,†
Cristina Freire,§ Laura C. J. Pereira,| Rui T. Henriques,¶ M. Conceic¸a˜o Oliveira,† and John E. Warren¥
Centro de Qu´ımica Estrutural, Departamento de Engenharia Qu´ımica e Biolo´gica, Instituto
Superior Te´cnico, AVenida RoVisco Pais, 1049-001 Lisboa, Portugal, Department of Chemistry,
UniVersity of Bath, Bath BA2 7AY, United Kingdom; REQUIMTE/Departamento de Qu´ımica,
Faculdade de Cieˆncias, UniVersidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal,
Departamento de Qu´ımica, Instituto Tecnolo´gico e Nuclear/CFMCUL, Estrada Nacional 10,
2686-953 SacaVe´m, Portugal, Instituto de Telecomunicac¸o˜es, Instituto Superior Te´cnico, AVenida
RoVisco Pais, 1049-001 Lisboa, Portugal, and Science & Technology Facilities Council Daresbury
Laboratory, Warrington, Cheshire, WA4 4AD, United Kingdom
Received May 30, 2008
The new bis-iminopyrrolyl five-coordinate Co(II) complexes [Co(κ2N,N′-NC4H3C(R)dN-2,6-iPr2C6H3)2(PMe3)] (R )
H 3a; Me 3b) were synthesized in high yields (ca. 80-90%), using THF and diethyl ether as solvents, respectively,
by (a) treatment of CoCl2(PMe3)2 with the corresponding iminopyrrolyl Na salts (Ie or If) or (b) reaction of anhydrous
CoCl2 and PMe3 with Ie or If. A third route was tested, involving the addition of excesses of PMe3 to the complexes
[Co(κ2N,N′-NC4H3C(R)dN-2,6-iPr2C6H3)2] (R ) H 1e; Me 1f), which was only successful for the synthesis of 3a,
in lower yields (ca. 30%). The synthesis of 3b in THF was unfruitful because of the kinetic competition of the
solvent, giving rise to mixtures of 1f and its coordinated THF adduct 4b. The synthesis of the new bis-iminopyrrolyl
five-coordinate Co(II) complexes [Co(κ2N,N′-NC4H3C(R)dN-2,6-iPr2C6H3)2(THF)] (R ) H 4a; Me 4b) were carried
out in high yields (ca. 80-90%) by the reaction of CoCl2(THF)1.5 with the corresponding iminopyrrolyl Na salt. All
the compounds have been characterized by X-ray diffraction, with 3a and 3b showing axially compressed trigonal
bipyramidal geometry (with the PMe3 ligand lying on the equatorial plane), whereas complexes 4a and 4b exhibit
distorted square pyramidal geometries with the THF molecule occupying the axial position. Complex 4a shows
clearly a compressed geometry, but for complex 4b, two polymorphs were characterized, exhibiting molecules with
different Co-O (THF) bond lengths, one of them being compatible with an elongated form. Magnetic measurements
either in the solid or in the liquid phases indicate that complexes 3a and 3b have low-spin ground states (S ) 1/2).
2
2
In toluene solution, the geometry is fully confirmed by EPR data, which further indicates a dx -y /dxy ground state.
However, compounds 4a and 4b behave unusually because they show magnetic moments that are compatible
with high-spin ground states (S ) 3/2) in the solid state, but conform to low-spin ground states (S ) 1/2) when
both complexes are dissolved in toluene solutions. The low-spin ground states in toluene solution are confirmed by
EPR spectroscopy, which further supports, for complexes 4a and 4b, an axially elongated square pyramidal geometry
2
and a dz ground state. Thus the change in the ground-state and, consequently, in the geometry of complexes 4a
and 4b from solid state to toluene solution might be a consequence of the elongation of the Co-O(THF) bond
length. DFT studies performed on complexes 3 and 4 corroborate their different structure and magnetic behaviors
and verify, for the latter complexes, the structural differences observed in the solid state and in toluene solution.
Introduction
with various applications in research fields, such as materials
science or homogeneous catalysis. However, metal com-
plexes of bidentate ligands containing both an imine and a
pyrrolyl moiety are still scarce. Despite 2-iminopyrrolyl
Schiff base-derived ligands have been widely used as
efficient frameworks for the preparation of metal complexes
* 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).
Instituto Tecnolo´gico e Nuclear/CFMCUL.
Instituto Superior Te´cnico (IT).
Daresbury Laboratory.
‡
¶
University of Bath.
§
¥
University of Porto.
8896 Inorganic Chemistry, Vol. 47, No. 19, 2008
10.1021/ic800992f CCC: $40.75  2008 American Chemical Society
Published on Web 08/20/2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
Chart 1
when stabilized by neutral donor ligands such as PMe3,
affording six-coordinate octahedral 19-electron complexes
2
in which the Co(II) center is in a low-spin ground state (dz ,
S ) 1/2).9
Similarly, complexes 1, although existing as stable entities,
are unsaturated (formally 15-electron compounds) and are
sensitive to small neutral donor molecules such as O2 or H2O.
In the present work, we describe the preparation and
characterizations of PMe3 and THF adducts of 1e and 1f,
using different chemical routes. The compounds obtained
were fully characterized by several techniques such as
elemental analysis, X-ray diffraction, magnetic susceptibility
measurements, EPR spectroscopy, and DFT calculations.
They showed structures varying from the unusual axially
compressed trigonal bipyramidal (TBP) to distorted square
pyramidal (SP) geometries and different magnetic properties.
In fact, the magnetic behavior of cobalt(II) complexes, in
particular that of five-coordinate complexes, is not straight-
forward and dramatic changes may occur because of subtle
distortions in their geometries.10
Results and Discussion
Synthesis of Complexes. The reaction of a solution of
CoCl2(PMe3)2, in THF, with 2 equiv of the bidentate ligand
precursor Ie, obtained by in situ deprotonation of the
corresponding 2-formiminopyrrole with NaH,6,7 and further
extraction of the reaction products in n-hexane, afforded an
almost quantitative amount of the five-coordinate complex
[Co(κ2N,N′-NC4H3C(H)dN-2,6-iPr2C6H3)2(PMe3)](3a)(Scheme
1, route a). This compound can be formulated as a 17-
electron species and was obtained as a dark red crystalline
paramagnetic material, its solutions being particularly sensitive
to air. The corresponding molecular structure was confirmed
by X-ray diffraction and EPR spectroscopy (see below). Unlike
the reaction of CoCl2(PMe3)2 with 2-ketopyrrolyl sodium salts,
where octahedral complexes of [Co(κ2N,O-NC4H3C-
(R)dO)2(PMe3)2] (R ) H, Me) were obtained,9 a PMe3
ligands having been used for a long time,1 interest in them
has increased only more recently when employed as building
blocks of R-olefin polymerization metal catalysts.2-4 In view
of the fact that bulky aryl substituents in the imine group
provide steric tuning and shielding of the metal centers,
leading to modified reactivities and increased stability of the
catalytic species, several mono- or bis(2-iminopyrrolyl) early
and, to a smaller extent, late transition-metal complexes have
been reported in the literature, which are efficient R-olefin
oligo/polymerization catalysts.2 These bulky ligands have
also been used in the synthesis of homoleptic transition-metal
complexes of the type [M(2-iminopyrrolyl)2] (M) Co,5,6
Ni,5,7 Zn8).
(1) Holm, R. H.; Chakravorty, A.; Theriot, L. J. Inorg. Chem. 1966, 5,
625–635, and references cited therein.
(2) Mashima, K.; Tsurugi, H. J. Organomet. Chem. 2005, 690, 4414–
4423, and references cited therein.
(3) Britovsek, G; J, P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int.
Ed. 1999, 38, 428–447, and references cited therein.
(4) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283–315,
and references cited therein.
(5) Dawson, D. M.; Walker, D. A.; Thornton-Pett, M.; Bochmann, M.
J. Chem. Soc., Dalton Trans. 2000, 459–466.
(6) Carabineiro, S. A.; Silva, L. C.; Gomes, P. T.; Pereira, L. C. J.; Veiros,
L. F.; Pascu, S. I.; Duarte, M. T.; Namorado, S.; Henriques, R. T.
Inorg. Chem. 2007, 46, 6880–6890.
(7) Bellabarba, R. M.; Gomes, P. T.; Pascu, S. I. Dalton Trans. 2003,
4431–4436.
(8) Hao, H.; Bhandari, S.; Ding, Y.; Roesky, H. W.; Magull, J.; Schmidt,
H. G.; Noltemeyer, M.; Cui, C. Eur. J. Inorg. Chem. 2002, 1060–
1065.
(9) Carabineiro, S. A.; Gomes, P. T.; Veiros, L. F.; Freire, C.; Pereira,
L. C. J.; Henriques, R. T.; Warren, J. E.; Pascu, S. I. Dalton Trans.
2007, 5460–5470.
In a previous work, we have synthesized and characterized
a series of four-coordinate Co(II) bis-iminopyrrolyl com-
plexes [Co(κ2-N,N′-NC4H3C(R)dN-2,6-R′2C6H3)2],6 most of
them showing a high-spin (S ) 3/2) tetrahedral geometry
(Chart 1, 1a-e). However, when a combination of high
stereochemical interligand repulsion and electronic donation
of the imino group was employed, such as in the case of R
i
) Me and R′ ) Pr, the complex structure switched to a
low-spin (S ) 1/2) square planar geometry (Chart 1, 1f).
Recently, we have also reported on the preparation and
characterization of Co(II) complexes containing bidentate
N,O 2-formyl- and 2-acetylpyrrolyl ligands (Chart 1, 2a, b).9
The ketopyrrolyl ligands used in that work are the parent
compounds of the 2-iminopyrrolyl ligands referred above and
are simultaneously less sterically demanding and weaker
electronic donors than their corresponding isoelectronic N,N
counterparts. Therefore, species analogous to 1 ([Co(κ2-N,O-
2-ketopyrrolyl)2]) are not stable and can only be isolated
(10) For instance, see: (a) Berry, J. F.; Cotton, F. A.; Liu, C. Y.; Lu, T.;
Murillo, C. A.; Tsukerblat, B. S.; Villagra´n, D.; Wang, X. J. Am. Chem.
Soc. 2005, 127, 4895–4902, and references cited therein. (b) Galet,
A.; Gaspar, A. B.; Mun˜oz, M. C.; Real, J. A. Inorg. Chem. 2006, 45,
4413–4422. (c) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.;
Bochmann, M. AdVanced Inorganic Chemistry, 6th ed.; John Wiley
& Sons: New York, 1999; p. 822.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8897

Carabineiro et al.
Scheme 1. Syntheses of Complex 3a in THF
Scheme 2. Syntheses of Complex 3b in Et2O
ligand was expelled from the Co(II) coordination sphere,
giving rise to a distorted trigonal bipyramidal (TBP) geom-
etry (see below). This is a consequence of the bulkiness of
the 2,6-diisopropylphenyl substituents of the iminic nitrogens.
If a stoichiometric amount of ligand precursor is used, the
same compound is obtained with 40% yield and not the
expected monosubstituted [Co(κ2N,N′-NC4H3C(H)dN-2,6-
iPr2C6H3)2(PMe3)2-nCl] (n ) 0 or 1) species.
Following the syntheses of 3a, we prepared the analogous
dark red paramagnetic compound [Co(κ2N,N′-NC4H3C-
(Me)dN-2,6-iPr2C6H3)2(PMe3)] (3b), containing a methyl
substituent on the iminic carbon, using exactly the same
methodologies (Scheme 2), but using Et2O as solvent. Indeed,
in both synthetic routes a and b, high yields (87% and 86%,
respectively) of complex 3b were obtained. Its molecular
structure was determined by X-ray diffraction, EPR spec-
troscopy, and ESI-MS (see below). However, attempts to
synthesize compound 3b by route c, in Et2O, did not succeed
using excess amounts (1:5) of PMe3 and reaction times up
to 7 days, highlighting the stereochemically hindered nature
of complex 1f.
The same synthetic procedures for the preparation of 3b
were also attempted in THF. In fact, the reaction of
CoCl2(PMe3)2 with 2 equiv of the sodium salt If (Scheme 3,
route a) gave, after 2 h of reaction, a first crop of crystals
(39%) of the desired compound 3b. However, a second crop
(55%) of square planar complex 1f6 was also obtained, which
was characterized by elemental analysis and EPR (see
below). When the same reaction was carried out for 12 h, a
second crop of 1f was similarly obtained, but this time
compound 3b was not present in the first crop. In fact, the
latter contained more than a single species, which could not
be clearly identified either by EPR spectroscopy or elemental
analyses. However, ESI-MS experiments showed the absence
of 3b, and the presence of the square planar complex 1f and
a different complex [Co(κ2N,N′-NC4H3C(Me)dN-2,6-iPr2-
C6H3)2(THF)] (4b), where a molecule of the solvent was
coordinated to the Co(κ2N,N′-NC4H3C(Me)dN-2,6-iPr2C6-
H3)2 fragment, instead of a PMe3 ligand. Some single crystals
could also be isolated from the bulk product, their X-ray
diffraction analysis confirming the molecular structure of
compound 4b (polymorph 4b_II, see X-ray diffraction). The
formation of 1f and 4b possibly results from the kinetic
instability of 3b in the THF reaction mixture, leading to the
replacement of PMe3 by the solvent. Alternatively, they can
arise from the presence of [CoCl2(THF)x(PMe3)2-x] species
likely originated, either by coordination to or exchange of
solvent molecules with the starting reagent CoCl2(PMe3)2.
The weaker donor capacity and lower steric influence of the
THF ligand in complex 4b (cone angles (equivalent) of 106°
and 88°, respectively, taken along and perpendicularly to the
C-O-C plane of THF11), in comparison with PMe3 (cone
angle (equivalent) ) 111° 11), together with a higher steric
hindrance and electron donation of the If ligand framework,
in comparison with Ie,6 may facilitate the release of the THF
molecule from the Co coordination sphere, giving rise to
the square planar complex 1f. In fact, the formation of the
formimino analogue of 4b (4a, see below) was not observed
in the preparation of 3a in THF.
The reaction of the sodium salt If with a mixture of
anhydrous CoCl2 and 2 equiv of PMe3 (Scheme 3, route b),
in THF, for 2 or 12 h, gave the square planar complex 1f as
the only product obtained. No traces of 3b or 4b could be
detected. In fact, the anhydrous CoCl2, virtually insoluble
in THF, reacts faster with the sodium salt If than with PMe3.
The nonformation of 3b or 4b is a consequence of the high
(11) Marc¸alo, J. Private communication. The cone angle (equivalent) values
were calculated on the basis of solid angles measurements according
to Marc¸alo, J.; de Matos, A. P. Polyhedron 1989, 8, 2431–2437. For
cone angle definition and calculation, see, for example: (a) Tolman,
C. A. Chem. ReV. 1977, 77, 313–348. (b) Dias, P. B.; da Piedade,
M. E. M.; Simo˜es, J. A. M. Coord. Chem. ReV. 1994, 135, 737–807.
8898 Inorganic Chemistry, Vol. 47, No. 19, 2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
Scheme 3. Synthesis Attempts of Complex 3b in THF
Scheme 4. Syntheses of Complexes 4a and 4b in THF
stereochemical hindrance observed in that square planar
complex because of the presence of the methyl substituents
of the iminic carbons6 that subsequently prevent the coor-
dination of either the PMe3 ligand or the THF solvent
molecules to 1f. For the same reasons, several attempts to
prepare compound 3b by addition of an excess of PMe3 to
1f (Scheme 3, route c), in THF, varying the temperature
(room temperature and 50 °C) and reaction time (up to 5
days, at both temperatures), were unfruitful. This behavior
shows a marked difference in the reactivities of 1e and 1f
toward PMe3 and THF ligands.
As in the case of 3a, the attempts to prepare 3b starting
from CoCl(PMe3)3 (instead of [CoCl2(PMe3)2]), in THF, were
unsuccessful.
To isolate complex 4b in a pure state and perform its full
characterization, we have used [Co4Cl8(THF)6] (CoCl2-
(THF)1.5)12 as starting material, as shown in Scheme 4.
Compound 4b, as well as its formimino analogue [Co(κ2N,N′-
NC4H3C(H)dN-2,6-iPr2C6H3)2(OC4H8)] (4a), were obtained
in high yields (84 and 92%, respectively), as paramagnetic
dark red crystals, that were characterized by elemental
analysis, X-ray diffraction, magnetic susceptibility measure-
ments, and EPR spectroscopy. Another crystalline polymorph
of 4b (4b_I) was obtained in the crystallization of this
reaction product (see X-ray diffraction).
structures (two polymorphs of the latter compound were
measured: 4b_I and 4b_II). Selected bond distances (Å) and
angles (°) are listed in Table 1.
The molecular structure of complex 3a is shown in Figure
1a.13 This Co(II) complex has an unusual coordination
geometryaroundthecobaltatom,generatedbyN1-N2-N3-N4-
P1 atoms, which can be described as axially compressed
trigonal bipyramidal. In fact, the axial positions are occupied
by the anionic pyrrolyl rings of both bidentate ligands, which
are trans to each other, with an almost linear N3-Co1-N1
axis (176.35(7)°), being Co1-N1 and Co1-N3 (1.9153(16)
X-ray Diffraction. Crystals suitable for X-ray diffraction
were obtained for all compounds, 3a, 3b, 4a, and 4b,
enabling the determination of their crystal and molecular
(13) In the first attempt to synthesise 3a by route b (Scheme 1), almost
certainly carried out in sub-optimal experimental conditions, a material
was isolated with CHN elemental analysis values higher than expected.
X-ray diffraction showed a crystal structure containing 3a co-
crystallized with the neutral acetiminopyrrole ligand precursor in a
ratio of 1:1 (3a·L). The molecular features of the Co complex of 3a·L
are very similar to those of 3a (see Table S1 and Figures S2 and S3
of Supporting Information).
(12) (a) Kern, R. J. J. Inorg. Nucl. Chem. 1962, 24, 1105–1109. (b) Fowles,
G. W. A.; Rice, D. A.; Walton, R. A. J. Inorg. Nucl. Chem. 1969, 31,
3119–3131. (c) Sobota, P.; Olejnik, Z.; Utko, J.; Lis, T. Polyhedron
1993, 12, 613–616.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8899

Carabineiro et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) for Compounds 3a, 3b, 4a, and 4b (Polymorphs 4b_I and 4b_II)
3a
3b
4a
4b_I
4b_II
Co(1)-P(1)
2.2580(5)
2.2387(15)
Co(1)-O(1)
2.0484(17)
2.0820(18)
2.0899(17)
2.0814(18)
2.0942(17)
2.103(2)
2.014(2)
2.112(2)
2.020(2)
2.110(2)
2.182(12)
2.055(4)
2.073(4)
Co(1)-N(1)
1.9153(16)
2.0483(16)
1.9214(16)
2.0481(16)
1.824(2)
1.820(2)
1.821(2)
1.382(2)
1.351(3)
1.307(2)
1.434(2)
1.304(3)
1.434(3)
1.385(3)
1.348(3)
93.78(5)
112.47(5)
106.27(5)
89.90(5)
1.903(2)
2.059(2)
Co(1)-N(2)
Co(1)-N(3)
Co(1)-N(4)
P(1)-C(39)
1.835(5)
1.787(4)
1.803(4)
1.377(4)
1.357(4)
1.305(4)
1.432(4)
P(1)-C(38)
P(1)-C(37)
N(1)-C(4)
1.371(3)
1.353(3)
1.298(3)
1.429(2)
1.295(3)
1.435(3)
1.370(3)
1.348(3)
1.381(3)
1.354(3)
1.299(3)
1.431(3)
1.302(3)
1.437(3)
1.381(3)
1.350(3)
1.375(6)
1.359(6)
1.294(6)
1.438(6)
N(1)-C(1)
N(2)-C(5)
N(2)-C(13)
N(4)-C(11)
N(4)-C(19)
N(3)-C(10)
N(3)-C(7)
P(1)-Co(1)-N(1)
P(1)-Co(1)-N(2)
P(1)-Co(1)-N(4)
P(1)-Co(1)-N(3)
O(1)-Co(1)-N(1)
O(1)-Co(1)-N(2)
N(1)-Co(1)-N(4)
N(2)-Co(1)-N(4)
N(2)-Co(1)-N(2_1)
N(1)-Co(1)-N(3)
N(1)-Co(1)-N(1_1)
N(2)-Co(1)-N(3)
N(2)-Co(1)-N(1_1)
N(1)-Co(1)-N(2)
N(3)-Co(1)-N(4)
Co(1)-P(1)-C(37)
Co(1)-P(1)-C(38)
Co(1)-P(1)-C(39)
C(37)-P(1)-C(38)
C(38)-P(1)-C(39)
Co(1)-N(1)-C(4)
Co(1)-N(1)-C(1)
τ parametera
91.69(9)
107.81(8)
97.02(8)
105.04(8)
98.53(8)
94.40(9)
100.56(8)
96.81(7)
141.24(7)
95.10(7)
145.64(6)
160.45(8)
144.38(15)
176.62(17)
144.2(2)
172.0(2)
176.35(7)
96.49(7)
165.38(7)
95.93(7)
171.28(9)
98.51(8)
96.96(10)
81.99(10)
95.42(15)
82.10(15)
82.07(7)
82.17(7)
80.15(7)
80.14(7)
80.05(8)
79.92(8)
118.88(8)
115.32(7)
115.45(8)
101.52(11)
100.23(10)
113.85(13)
139.40(14)
0.59
108.0(2)
123.8(2)
120.6(2)
99.88(18)
100.78(18)
113.74(18)
139.9(2)
0.54
110.59(13)
143.39(15)
0.33
112.09(16)
141.07(18)
0.18
108.3(3)
143.6(4)
0.46
a See the definition of τ in ref 17 or in Figure S1 of Supporting Information.
and 1.9214(16) Å) the shortest distances to the metal. In the
single case found in the literature of a TBP Co fragment
containing two pyrrole units bound to the metal, which is
part of a bimetallic Co(III)/U(V) complex,14 the X-ray
structure does not present an axially compressed geometry
because the axial pyrrole ligand shows a longer Co-N
distance of 2.002 Å and the equatorially bound pyrrole has
a Co-N distance of 1.973 Å. The Co1-N1 and Co1-N3
bond distances of complex 3a are somewhat smaller than
those observed in the “parent” tetrahedral S ) 3/2 complex
[Co(κ2N,N′-NC4H3C(H)dN-2,6-iPr2C6H3)2] 1e (1.952(8)
-1.985(9) Å), in other [Co(κ2N,N′-iminopyrrolyl)2] analogue
complexes (1.980-1.989 Å),15 or even in the square planar
[Co(κ2N,N′-NC4H3C(Me)dN-2,6-iPr2C6H3)2] 1f (1.9388(13)
Å).6 However, they are within the range of Co-N bonds
found in the literature for other Co–pyrrolyl bonds, which
are essentially square pyramidal Co(III) porphirinates and
corrolatesthatcontainpyrrolylunitsboundtoCo(1.791-1.979
Å).16 The remaining three equatorial coordination positions
of 3a are occupied by the two imine nitrogen atoms (N2
and N4) and the phosphorus P1 atom of the PMe3 ligand,
which form an equatorial plane containing the cobalt atom
(∆) 0.016 Å), the sum of the angles around the Co center
being virtually 360° (359.98°). However, the N2-Co1-N4
angle(141.24°)ismuchlargerthantheothertwo(P1-Co1-N2,
112.47°; P1-Co1-N4, 106.27°), which is likely a conse-
quence of the repulsion between the bulky imine 2,6-
diisopropylphenyl substituents. This leads to a certain extent
of distortion of the TBP toward a square pyramidal (SP)
geometry, which is pointed out by a τ value of 0.59. The
parameter τ17 (see also definition in Figure S1 of Supporting
Information) is used to describe the degree of distortion of
penta-coordinated complexes (τ ) 1 for an ideal trigonal
bipyramid and τ ) 0 for an ideal square pyramid).
Higher values of Co-N bond distances are found for the
neutral imine nitrogens (Co1-N2 ) 2.0483(16) and Co1-N4
(16) Results from a search carried out in the Cambridge Structural Database
using the program ConQuest, developed by the CCDC.
(17) (a) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349–1356. (b) O’Sullivan,
C.; Murphy, G.; Murphy, B.; Hathaway, B. J. Chem. Soc., Dalton
Trans. 1999, 1835–1844.
(14) Arnold, P. L.; Patel, D.; Blake, A. J.; Wilson, C.; Love, J. B. J. Am.
Chem. Soc. 2006, 128, 9610–9611.
(15) (a) Wei, C. H. Inorg. Chem. 1972, 11, 1100–1105. (b) Reid, S. D.;
Blake, A. J.; Wilson, C.; Love, J. B. Inorg. Chem. 2006, 45, 636–
643.
8900 Inorganic Chemistry, Vol. 47, No. 19, 2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
Figure 2. ORTEP III diagram of complexes (a) 4a and (b) 4b (from
polymorph 4b_I) using 50% probability level ellipsoids. Hydrogens have
been omitted for clarity.
Figure 1. ORTEP III diagram of complexes (a) 3a and (b) 3b using 50%
probability level ellipsoids. Hydrogens have been omitted for clarity.
distances and angles observed in the chelating ligands of 3a
and their corresponding bite angles to the Co atom are similar
to those found for the tetrahedral 1e or square planar 1f
complexes.6
) 2.0481(16) Å) in comparison with the pyrrolyl rings
Co-N bond distances. This could be caused by both the
anionic nature of the pyrrolyl nitrogen and the steric bulk of
the 2,6-diisopropyl substituents of the phenyl rings. Never-
theless, the other examples known in the literature of TBP
complexes of Co(II) containing two axial anionic and two
equatorial neutral N atoms show longer axial bond lengths.18
The Co1-N2 and Co1-N4 distances are similar to those
found for the imine groups in the tetrahedral complex 1e
(2.034(7)-2.060(8) Å) but considerably larger than those
found in the square planar complex 1f (1.9528(14) Å).6 The
Co1-P1 distance (2.2580(5) Å) lies in the normal range of
Co(II)-PMe3 bond distances.19,20 Both chelating ligands can
be considered planar since the planes defined by N2-C5-C4-
N1 and N3-C7-C8-N4, and the respective pyrrolyl rings
form angles of 1.29 and 3.41°, respectively. The internal
For complex 3b, crystals suitable for X-ray diffraction
were grown from n-hexane solutions. The corresponding
molecular structure (Figure 1b) shows the central atom bound
to two acetiminopyrrolyl ligands and a disordered PMe3
coordinated to the fifth coordination site of the metal.
The asymmetric unit consists of only half of the complex;
the other half was generated by symmetry. As in 3a, the
molecule shows an axially compressed TBP geometry around
the cobalt atom, slightly more distorted (τ) 0.54) than 3a.
However, the bond distances and angles are very similar to
those found for 3a despite the C6 and C12 extra methyl
groups substituting now the imine C5 and C11 quaternary
carbons.
The crystallographic structure of complex 4a consists of
two formiminopyrrolyl ligands coordinated to the Co central
atom, which is also coordinated to a molecule of (disordered)
THF (Figure 2a and Table 1). The geometry may be
interpreted as distorted square pyramidal (τ ) 0.33) rather
than TBP. The pyrrolyl rings are placed trans to each other
(18) (a) Patra, A. K.; Ray, M.; Mukherjee, R. J. Chem. Soc., Dalton Trans.
1999, 2461–2466. (b) Vickovic, I.; Suste, A.; Falk, H.; Hulita, N. K.;
Tonejic, A. M. Monatsh. Chem. 1995, 126, 971–982.
(19) Alnaji, O.; Peres, Y.; Dartiguenave, M.; Dahan, F.; Dartiguenave, Y.
Inorg. Chim. Acta 1986, 114, 151–158.
(20) Klein, H. F.; Beck, R.; Florke, U.; Haupt, H. J. Eur. J. Inorg. Chem.
2003, 240–248.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8901

Carabineiro et al.
Table 2. Effective Magnetic Moments µeff (µB) for Complexes 3a, b
and 4a,b, Measured in Powder (DC Extraction and SQUID Methods)
and in Toluene Solution (Evans Method-NMR), at 300 K
with a N1-Co1-N3 axis (165.38(7)°) considerably more
distorted from linearity than in 3a or 3b (Table 1). The
distances Co1-N1 and Co1-N3 of 2.082(18) and 2.0814(18)
Å are much larger than those found in complexes 3a or 3b
(∆ ≈ 0.17 Å) or in the related tetrahedral [Co(κ2N,N′-
NC4H3C(H)dN-2,6-iPr2C6H3)2] 1e (∆ ≈ 0.10 - 0.12 Å)6
or square planar [Co(κ2N,N′-NC4H3C(Me)dN-2,6-iPr2C6H3)2]
1f (∆ ≈ 0.15 Å).6 The remaining two coordination positions
in the distorted square plane are occupied by the two imine
nitrogen atoms (N2 and N4). The oxygen O1 atom of the
THF ligand occupies the apical position of the pyramid, and
the cobalt atom is situated above (∆ ) 0.441 Å) the basal
square plane. Larger Co-N bond distances are found for
the imine nitrogens (Co1-N2 ) 2.0899(17) Å and Co1-N4
) 2.0942(17) Å) in relation to those of 3a (∆ ) 0.0272 Å),
3b (∆ ) 0.0275 Å), 1f (∆ ) 0.1215 Å),6 and 1e (∆)
0.0155-0.0215 Å).6 The Co1-O1 distance is 2.0487(17)
Å, which lies in the normal range of Co(II)-THF bond
distances (1.876-2.225 Å; average ) 2.09)16 but is lower
than the other bonds in the equatorial plane, indicating a
compressed SP-geometry. The chelating ligands can be con-
sidered planar because the angle defined by (N1-C4-C5-N2)
and the pyrrolyl ring is 0.82° (and by N3-C10-C11-N4
and the other pyrrolyl ring is 1.35°). The internal dis-
tances and angles observed in the chelating ligands of 4a
and their corresponding bite angles to the Co atom are similar
to those found for the tetrahedral complexes 3a, 3b, 1e,6
and 1f.6 The fact that all the Co-N distances have expanded
in relation to complex 3a corroborates the results obtained
by magnetic moment measurements in the solid state (see
below) that indicate the complex 4a has a high-spin ground
state (S ) 3/2), rather than a low-spin ground state (S)1/2),
found for complex 3a.
solid state
solution
complex
µeff (µB)
µeff (µB)
3a
2.03b
1.98b
4.87c
4.87c
2.31
2.61
1.76
1.78
3b
4a
4ba
a The sample measured corresponds to polymorph 4b_I. b DC extraction
method. c SQUID method.
Figure 3. Temperature dependences of the effective magnetic moment µeff
of complexes 3a, 3b, 4a, and 4b (from polymorph 4b_I), taken in the solid
state, at 5 T (white experimental points) and taken in toluene solutions of
compounds 4a and 4b, by the Evans method (black experimental points).
S4 Supporting Information and Table 1). The geometry of
4b in polymorph 4b_II can be interpreted as a considerably
distorted SP geometry (τ ) 0.46). However, because the
Co1-O1 bond length (2.182(12) Å) is much longer than
those found in 4a (∆ ) 0.134 Å) or 4b_I (∆ ) 0.079 Å),
the molecules of polymorph 4b_II can be considered to be
in an elongated square pyramidal geometry, with the
distances and angles in the equatorial plane relatively similar
to those of 4a; this Co1-O1 bond length still lies in the
normal range of Co(II)-THF bond distances (1.876-2.225
Å; average ) 2.09).16
Magnetic Measurements. The results obtained for the
magnetic susceptibility measurements of the different com-
plexes, taken both in toluene solution (by Evans method-
NMR) and in powder (DC extraction method for 3a, b and
SQUID magnetometer for 4a, b), at 300 K, are summarized
in Table 2.
The temperature dependence of the effective magnetic
moment, µeff, obtained for polycrystalline powder samples
of complexes 3a, 3b, 4a, and 4b (from polymorph 4b_I) is
presented in Figure 3. For samples 3a and 3b, the decrease
of the magnetic moment at lower temperatures (T < 10 K)
is a consequence of the high field employed in the measure-
ments (5 T).
The µeff data taken at 300 K and presented in Table 2 show
that the magnetic moments of compounds 3a and 3b,
obtained in solid and solution phases, are close to the
expected spin-only value for low-spin S ) 1/2 complexes
(1.73 µB), in agreement with the EPR data obtained in toluene
presented below. The small differences on the values
Crystals of compound 4b suitable for X-ray diffraction
were obtained either in the synthesis of 4b via the synthetic
method indicated in Scheme 4 (monoclinic polymorph 4b_I)
or, as a byproduct, in the synthesis attempts of 3b in THF
(tetragonal polymorph 4b_II). The molecular structure of
the monoclinic polymorph 4b_I consists of two acetimi-
nopyrrolyl ligands coordinated to a Co central atom, which
is also coordinated to a molecule of THF (Figure 2b and
Table 1). Its geometry can also be interpreted as distorted
square pyramidal (τ ) 0.18), but some of the structural data,
mainly related to the metal atom, are considerably different
from those of 4a. As the Co1-O1 bond of 4b_I (2.014(2)
Å) is longer (∆ ) 0.055 Å), the Co1-N1 and Co1-N2
belonging to the same bidentate ligand have, respectively, a
significant increase (∆ ) 0.0546 Å) or decrease (∆) 0.068
Å) in relation to 4a. The angle formed between the Co atom
and the imine nitrogens, N2-Co1-N4 (160.45(8)°), increase
by 14.81°, whereas that involving the pyrrolyl nitrogens,
N1-Co1-N3 (171.28(8)°), increase by ∼6°.
The molecular structure of the polymorph 4b_II shows
the cobalt atom coordinated to two acetiminopyrrolyl ligands
in a distorted square pyramidal arrangement, with a THF
molecule coordinated to the fifth coordination site of the
metal. The asymmetric unit consists of only half of the com-
plex; the other half-was generated by symmetry (see Figure
8902 Inorganic Chemistry, Vol. 47, No. 19, 2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
obtained by these two methods are very common, normally
caused by several factors, such as the local geometry or the
different calculation methods inherent to each technique.21
In this case, the µeff obtained by the Evans method were 2.3
and 2.6 µB for 3a and 3b, respectively, which still lie in the
typical range observed for trigonal bipyramidal Co(II)
complexes with low-spin ground states with S ) 1/2 (2.1-2.9
µB).22,23 The high temperature range data (100-300 K)
obtained for solid samples of 3a and 3b (DC extraction
method) fit well to the Curie-Weiss law, giving negative
values for the Weiss constants, θ ) -16.9 and -22.3 K,
respectively. This indicates the presence of antiferromagnetic
exchange interactions in the solid state between adjacent
Co(II) ions (see Figure S5, Supporting Information).
For both complexes 4a and 4b (polymorph 4b_I), the
magnetic moments measured in the solid state (SQUID
method) were 4.87 µB, at room temperature, (Table 2 and
Figure 3). This value lies in the range observed for Co(II)
complexes with high-spin ground state,22 although it is much
larger than the spin-only magnetic moment value of 3.87 µB
(S ) 3/2), which indicates that some significant orbital
contribution from low energy excited states can be involved.
The fitting of the high temperature range magnetic suscep-
tibility data of 4a (150-300 K) and 4b (100-300 K) to the
Curie-Weiss law (see Figure S6, Supporting Information)
also reveals the presence of antiferromagnetic interactions
between adjacent Co(II) ions, the values for the Weiss
constants (θ ) -41.6 and -59.8 K, respectively) being more
negative than those of 3a, b. Concerning complex 4b, it
should be emphasized that the only sample analyzed was
polymorph 4b_I because crystals of polymorph 4b_II have
only been obtained once, but in a mixture with compound
3b (see Synthesis of Complexes and X-ray Diffraction).
Therefore, the magnetic moment of polymorph 4b_II has
not been measured, and the spin state of its molecules is
unknown. The exchange pathway for these antiferromagnetic
interactions can be inferred from the short contacts found in
the molecular packing. In fact, for 4a, the distance between
the outmost carbons of two phenyl rings in neighboring
molecules is about 3.34 Å, favoring some π· · ·π interactions,
and for 4b (in polymorph 4b_I), in spite of the intrinsic
disorder, several short contacts can be noticed between
pyrrole rings of neighboring units (C-H · · · π interactions,
∼2.62 Å) and between THF methylene hydrogens with
pyrrole rings (C-H· · ·π interactions, ∼2.89 Å) or isopropyl
methyl groups of (C-H · · · C interactions, ∼2.87 Å) of
adjacent molecules (Figures S7 and S8, Supporting Informa-
tion).
Figure 4. EPR spectra of complexes 3a and 3b, in toluene, at 77 K.
value for low-spin complexes with S ) 1/2. Furthermore,
variable-temperature Evans method measurements on these
compounds reveal that their µeff is approximately invariant
with temperature (Figure 3), suggesting that the observed
numerical change in the µeff from the solid state to solution
could not be associated with any equilibrium between species
with different spin ground states. In addition, the aforemen-
tioned antiferromagnetic interactions (rather than ferromag-
netic ones) observed in 4a and 4b rule out the presence of
µeffmagnifying effects caused by solid state cooperative effects.
The influence of different environments, such as solution and
solid state, may strongly influence the stereoelectronic properties
of the ligands, either because of polar effects or, indirectly, by
controlling the ligands’ conformation.21,24
To obtain further information on the electronic structure
of all Co(II) complexes, including an explanation for the
abrupt change in the spin ground-state of complexes 4a
and 4b from the solid state to solution, EPR spectra of
all complexes, were obtained in frozen toluene solu-
tion.
EPR Spectra. The experimental EPR spectra of complexes
3a and 3b and 4a and 4b, obtained at 77 K, are depicted in
Figures 4 and 5, respectively. Complexes of types a and b
show almost identical EPR spectra, indicating that the two
different substituents (R ) H and Me) of the imine bond in
the bidentate ligand do not have a significant influence in
the electronic structure of the cobalt center. In frozen toluene
solution, the spectra are of axial (complexes 3) and rhombic
(complexes 4) types and with large tensor anisotropy, gav in
the range of 2.2-2.4, ([gav ) (2g1,2 + g3)/3], where g1,2 and
g3 refer to the lowest and highest magnetic field g values,
respectively), and typical of low-spin cobalt(II) complexes
Unexpectedly, the magnetic moments of complexes 4a and
4b (polymorph 4b_I) measured in solution by the Evans
method, at room temperature, 1.76 and 1.78 µB, respectively
(Table 2), are much lower than those obtained in the solid
state, being very close to the spin-only magnetic moment
(21) de Buysser, K.; Herman, G. G.; Bruneel, E.; Hoste, S.; van Driessche,
I. Meas. Sci. ReV. 2005, 5, 30–33.
(22) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Amster-
dam, 1968.
(23) Kennedy, B. J.; Fallon, G. D.; Gatehouse, B. M. K. C.; Murray, K. S.
Inorg. Chem. 1984, 23, 580–588.
(24) Wolny, J. A.; Rudolf, M. F.; Ciunik, Z.; Gatner, K.; Wolowiec, S.
J. Chem. Soc., Dalton Trans. 1993, 1611–1622.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8903

Carabineiro et al.
observed superhyperfine splittings are compatible with an
interaction of the unpaired electron with the phosphorus
nucleus of the coordinated PMe3 (|a⊥|P ) 7.0 - 8.5 mT, I(P)
) 1/2), and further splitting of these signals is caused by
the presence of two equivalent imine nitrogen atoms of the
bidentate ligands located in the equatorial plane; a nice well
resolved quintuplet is clearly seen in the perpendicular region
of complex 3b spectrum (|a⊥|N) 2.0 - 2.7 mT; I(N) ) 1).25
The EPR results also confirm that not only the geometry of
complexes 3a and 3b have been kept in toluene solution,
but also that no ligand intramolecular fluxionality is occurring
because the coordinated atoms kept their positions within
the trigonal bipyramid.
For complexes 4a and 4b, the observations of gxy > gz
and |Axy|Co > |Az|Co, indicate the complexes have a different
ground-state and geometry of those of complexes 3a and
3b. These previous observations, along with the absolute
values obtained for g and A parameters, are consistent with
a square pyramidal geometry, although with axial elongation,
for both complexes 4a and 4b, which lead to dz2 ground
states. Although the spectra do not have very good resolution,
in part because of the extensive overlap between the xy- and
z-regions, it is possible to see in the gz region the majority
of the eight predicted cobalt hyperfine splittings caused by
the interaction of the unpaired spin and the metal center
(|Az|Co ) 10.5 - 11.0 mT; I(Co) )7/2). The absence of any
additional superhyperfine splittings in the previous cobalt
features further indicates that the axial position of the square
pyramid has to be occupied by the THF molecule, which
coordinates to Co through the oxygen atom (I ) 0 for the
isotope with higher natural abundance). In this context, this
Co-O bond will have a larger bond length compared to any
of the Co-N bonds within the equatorial plane. In the xy-
region, the observed nonresolved hyperfine couplings are
caused by the interaction of the unpaired spin with the cobalt
atom but no superhyperfine splittings because the coordinat-
ing atoms from the equatorial ligands are usually seen for
this type of ground state (dz2) since the spin interaction
mechanisms would be of indirect nature (polarization), thus
being of very small magnitude.25b,d
The EPR results for complexes 4a and 4b are partially in
disagreement with the structural data from X-ray diffraction
because they suggest, for both complexes, axially elongated
square pyramidal geometries, with consequent low-spin
ground states. As seen in the last section, both complexes
exhibit, in the solid state, magnetic moments compatible with
high-spin ground states which are in accordance with the
X-ray data. Nevertheless, in toluene solution the magnetic
moments suggest a low-spin ground state, but do not pro-
vide a structural rationalization for the change in the spin
state of the complex. Herein, EPR data definitely confirms
the change in the spin ground-state to low-spin in toluene
solution, further indicating the same square pyramidal
geometry for complexes 4a and 4b, although with different
axial distortions. Therefore, the magnetic behavior exhibited
by complexes 4a and 4b, reflecting different spin ground
states in solid state and solution, is attributed to a relaxation
of the compressed geometry of complexes 4, upon dissolution
Figure 5. EPR spectra of complexes 4a and 4b (polymorph 4b_I), in
toluene, at 77 K.
(d7) with S ) 1/2.25 However, complexes 3 and 4 exhibit
EPR spectra with different g patterns and hyperfine/super-
hyperfine splittings in the high and low magnetic field
regions, anticipating different geometries and, consequently,
different electronic ground states for complexes 3 and 4. The
first important conclusion obtained from the EPR data is their
full agreement with the results obtained by the magnetic
moment measurements in solution (by the Evans method)
for all the compounds, in particular, for complexes 4a and
4b.
The observed g patterns and hyperfine/superhyperfine
splittings are similar to other low-spin pentacoordinated
cobalt(II) complexes,25 and in the absence of EPR crystal
data for complexes 3 and 4, these similarities can be further
extended to support the following orientation scheme for the
tensor axis in these cobalt complexes: for complex 3a and
3b, EPR parameters in the high and low field regions
correspond to parallel and perpendicular parameters, respec-
tively, whereas for complexes 4a and 4b the assignment of
the EPR parameters is the opposite. In this context, for
complexes 3a and 3b, g1,2 ) g| > g3 ) g⊥, and |A1,2|Co
|A||Co > |A3|Co ) |A⊥|Co, but for complexes 4a and 4b, g1,2
)
)
gxy > g3 ) gz, and |A1,2|Co ) |Axy|Co > |A3|Co ) |Az|Co. The
EPR parameters obtained from spectra simulation are sum-
marized in Table 3.
The g and A Co patterns observed in the spectra of
complexes 3a and 3b, namely, g| > g⊥ |A||Co > |A⊥|Co, as
well as their values, are typical of cobalt(II) complexes with
2
2
compressed trigonal bipyramidal geometry with a dx -y /dxy
ground state,25 indicating that the solid state geometry,
obtained by X-ray diffraction, has been kept in toluene
solution. In the parallel region (higher magnetic field region),
the hyperfine splittings caused by the interaction between
the metal unpaired electron and the cobalt center (|A||Co
)
6.2 - 6.5 mT; I(Co) ) 7/2) are clearly seen. The perpen-
dicular region is of more difficult interpretation, but the
(25) (a) Daul, C.; Schla¨pfer, C. W.; von Zelewsky, A. Struct. Bonding
(Berlin) 1979, 36, 129–171. (b) McGarvey, B. R. Can. J. Chem. 1975,
53, 2498–2511. (c) Malatesta, V.; McGarvey, B. R. Can. J. Chem.
1975, 53, 3791–3800. (d) Hitchman, M. A. Inorg. Chem. 1977, 16,
1985–1993. (e) Nishida, Y.; Kida, S. Bull. Chem. Soc. Jpn. 1972, 45,
461–465. (f) Nishida, Y.; Ida, K.; Kida, S. Inorg. Chim. Acta 1980,
38, 113–116.
8904 Inorganic Chemistry, Vol. 47, No. 19, 2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
Table 3. EPR Parameters for Complexes 3a, 3b, 4a, and 4b (Polymorph 4b_I)
a
complex
g|
g⊥
|A||Co (mT)
|A⊥|Co (mT)a
|a||P (mT)
|a⊥|P (mT)a
|a||N (mT)
|a⊥|N (mT)a
3a
3b
2.38
2.41
2.11
2.13
6.2
6.5
0.5
0.5
1.5
1.5
8.5
7.0
0.5
0.5
2.7
2.0
a
a
complex
gx
gy
gz
|Ax|Co (mT)a
|Ay|Co (mT)a
|Az|Co (mT)
4a
4b
2.60
2.65
2.45
2.35
2.00
2.05
7.0
6.5
6.0
6.0
11.0
10.5
a The values are not accurate. See experimental section for details.
distorted and axially compressed trigonal bipyramidal ge-
ometries, in accordance with the X-ray results. The pyrrolyl
nitrogen atoms occupy the axial positions, while the phos-
phorus and the two imine nitrogen atoms occupy the
equatorial plane. The agreement between calculated and
experimental structures can be evaluated by the mean (δ)
and maximum (∆) absolute deviations obtained for the
coordination distances (Co-X). For the low-spin form of
complex 3a, values of δ ) 0.08 Å and ∆ ) 0.18 Å indicate
a poor agreement. The geometry optimized for complex 3b
(S ) 1/2) present similar values: δ ) 0.09 Å and ∆ ) 0.19
Å. In both cases, the Co-P bond is grossly overestimated,
corresponding to the maximum deviation referred above (∆).
The values obtained for the Co-N distances are compara-
tively better, with overestimations of about 0.02 and 0.09
Å, for the pyrrolyl and the imine nitrogen atoms, respectively.
The size and complexity of the molecules prevent the use
of higher theory level methods and bigger basis sets for the
geometry optimizations (see Computational Details). Since
similar deviations are observed for the Co-X coordination
distances in both complexes, 3a and 3b, the results obtained
may be used in the comparative study of these two molecules
presented below.
The complexes with the low-spin ground state (S ) 1/2)
correspond to the more stable species for complexes 3a and
3b, in good agreement with the solution EPR spectra, the
experimental magnetic susceptibility measurements in both
solid state and solution (see Tables 2 and 3), and the X-ray
structural data (e.g., τ values, see Table 4). A significant
expansion of about 0.1 Å is observed for all coordinative
distances, Co-X, going from the low- to the high-spin (S )
3/2) complexes, similar to what has been observed in related
Co(II) complexes.6,9
Figure 6. EPR spectrum of complex 1f, in toluene, at 77 K.
in toluene, through an elongation of the Co-O bond length.
This increase leads to a modification in the ratio between
the equatorial and axial ligand fields, ultimately chang-
ing the spin ground-state of the complexes. Further insights
into the different available geometries and ground states for
complexes 3 and 4 were also analyzed by DFT calculations
(see below).
Compound 1f, obtained as a byproduct on the attempted
synthesis of 3b in THF, and already structurally characterized
in a previous work as a complex with a low-spin (S ) 1/2)
ground state,6 was also characterized by EPR spectroscopy in
frozen toluene solution (Figure 6). The spectrum is of the
rhombic type with a very large tensor anisotropy, g1 ) 4.60, g2
) 2.32, and g3 ) 1.97 (gav ) 2.96), with g1 and g3 correspond-
ing to the lowest and highest magnetic field g values, respec-
tively, which are typical of Co(II) low-spin square planar
complexes with a dx -y /dxy ground state,25a,d confirming the
results described in ref 6. The spectrum also shows in all g
regions well resolved hyperfine splittings because of the
interaction between the unpaired spin and the cobalt center
(I(Co) ) 7/2), whose coupling constants corroborate the square
planar geometry: |A1|Co ) 24 mT, |A2|Co ) 7.8 mT, and |A3|Co
) 7.8 mT. Although no well-resolved superhyperfine splittings
resulting from the four equivalent nitrogens from the equatorial
ligands could be detected in any of the g regions, the simulation
of the spectrum was made taking with account the following
nitrogen coupling constants: |a1|N ) 2.2 mT and |a2,3|N ) 1.0
mT; I(N) )1.
2
2
The different behavior of the two complexes, 3a and 3b,
in the presence of THF was studied through the calculation
of the energy balance for the addition of PMe3 to the parent
[Co(2-iminopyrrolyl)2] complexes, 1e and 1f. The free energy
values presented in Scheme 5 include solvation effects
(THF), taken into account by means of the PCM model (see
Computational Details). The values obtained for the forma-
tion of complexes 3a and 3b show a moderately favorable
reaction (∆G ) -1.3 and -2.5 kcal/mol, respectively) and,
being similar for both species, suggest that there is no
thermodynamic reason for the observed differences. Thus,
the instability of 3b in THF is probably a consequence of
kinetic factors.
DFT Calculations. The relative stability of the two
different spin ground states (S ) 3/2 and S ) 1/2) for both
complexes with PMe3 (3a and 3b) and THF (4a and 4b)
was investigated by means of DFT calculations.26
The optimized geometries calculated for complexes 3a and
3b are presented in Figure 7. Their structures correspond to
The THF complexes (4a and 4b) were also the subject of
geometry optimizations, and the calculated structures are
represented in Figure 8.
(26) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8905

Carabineiro et al.
Figure 7. Optimized geometries (B3LYP) for the high-spin (S ) 3/2, top) and the low-spin (S ) 1/2, bottom) ground states of complexes 3a and 3b. The
relative energy (gas phase) of both ground states (kcal/mol), and the coordination distances (Co-X, Å), are presented.
Table 4. Comparison of Experimental (X-ray Diffraction) and
result, obtained from gas phase energy calculations (see
Calculated (DFT) Structural Parameter τ of Complexes 3a, 3b, 4a, and
4ba
Computational Details), is in agreement with the solid state
magnetic susceptibility measurements (see Table 2), sug-
gesting that the X-ray structures determined for 4a and 4b
should correspond to the complexes with high-spin ground
states. It should be noticed that the relative stability calculated
in the gas phase for the two spin ground states of complexes
4a, b (see Figure 8) is maintained if dielectric solvent effects
are taken into consideration (toluene, PCM model). On the
other hand, the Co-O distances calculated for the low-spin
ground-state complexes 4a and 4b are ∼0.2 Å longer than
the corresponding molecules with high-spin ground states,
representing the larger difference in coordination distances
between spin ground states. The resulting geometry of the
low-spin complexes 4a and 4b correspond to a square
pyramid axially elongated along the Co-O bond, in excellent
agreement with the EPR results (see above).
τcalcd
complex
τexp
S ) 1/2
S ) 3/2
3a
3b
4a
4b
0.59
0.54
0.33
0.49
0.53
0.25
0.30
0.46
0.44
0.31
0.41
0.18 (4b_I), 0.46 (4b_II)
a See the definition of τ in ref 17 or in Figure S1 of Supporting
Information.
Scheme 5
The reason for the reversed stability calculated in the gas
phase for the complexes 4a and 4b with different spin ground
states, when compared with complexes 3a and 3b, can be
traced to the nature of the corresponding ligands, THF and
PMe3, respectively. In fact, the phosphine is a much better
σ donor and, thus, a stronger ligand than THF.10c The Co-P
bonds in the low-spin complexes 3a, b are longer than the
Co-O bond distances in 4a, b (S ) 3/2), but the former
correspond to stronger bonds, as shown by the corresponding
Wiberg indices27 (WI): 0.332(3a)/0.342(3b) and 0.080(4a)/
0.081(4b), respectively.28 The charge on the metal29 shows
The geometries calculated for the THF complexes (4a and
4b) agree with the corresponding X-ray determined struc-
tures, being closer to square pyramidal arrangements dis-
cussed above (see τ values in Table 4). The agreement
between experimental and calculated Co-X bond distances
is better for the THF species (S ) 3/2) than for the PMe3
complexes, as demonstrated by smaller values of the
maximum and mean absolute deviations: ∆ ) 0.10 and δ )
0.06 Å for 4a, and ∆ ) 0.09 and δ ) 0.06 Å for 4b
(comparing with the X-ray structure of polymorph 4b_I).
For the THF complexes the high-spin ground state (S )
3/2) is more stable than the low-spin ground state (S ) 1/2),
showing a reversed stability order, compared with the
phosphine derivative complexes, 3a and 3b. Curiously, this
(27) (a) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096. (b) Wiberg
indices are electronic parameters related to the electron density between
atoms. They can be obtained from a Natural Population Analysis and
provide an indication of the bond strength.
(28) The comparison was made between the more stable spin isomers of
complexes 3a-b and 4a-b, that is, the S ) 1/2 and 3/2 species,
respectively. However, the trend is maintained if the other isomers
are considered: WICo-P ) 0.231(3a, S ) 3/2)/0.224(3b, S ) 3/2),
WICo-O ) 0.040(4a, S ) 1/2)/0.056 (4b, S ) 1/2).
(29) Obtained by means of a Natural Population Analysis (NPA).
8906 Inorganic Chemistry, Vol. 47, No. 19, 2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
Figure 8. Optimized geometries (B3LYP) of the low- (S ) 1/2, top) and the high-spin (S ) 3/2, bottom) ground states of complexes 4a and 4b. The relative
energy (gas phase) of both ground states (kcal/mol) and the coordination distances (Co-X, Å) are presented.
the same trend: a stronger bond and a better donor in 3a, b
is associated with an electron richer metal. This trend is
verified for both the high- and the low-spin complexes.30
The spin density calculated for the low-spin (S ) 1/2)
complex 3a is centered on the bipyramidal trigonal xy
equatorial plane (Figure 9a), while that of the low-spin (S
) 1/2) complex 4a is distributed along the square pyramidal
Co-O apical axis (Figure 9b), being their shapes reminiscent
2
2
2
of the dx -y /dxy and dz orbitals, respectively. This result is
in excellent agreement with the frozen solution EPR spectra
2
2
2
(see above) that revealed dx -y /dxy and dz low-spin ground
states for 3a and 4a, respectively. The spin densities
calculated for the low-spin complexes 3b and 4b (see Figure
S12, Supporting Information) are almost identical to those
of the corresponding 3a and 4a complexes, also in agreement
with the EPR data.
Conclusions
The new bis-iminopyrrolyl five-coordinate Co(II) com-
plexes [Co(κ2N,N′-NC4H3C(R)dN-2,6-iPr2C6H3)2(PMe3)] (R)
H 3a; Me 3b) may be synthesized in high yields, using THF
and diethyl ether as solvents, respectively, by the (a)
treatment of CoCl2(PMe3)2 with the corresponding iminopy-
rrolyl Na salts (Ie or If) or (b) reaction of anhydrous CoCl2
and PMe3 with Ie or If. A third route was attempted,
involving the addition of excesses of PMe3 to the tetrahedral
and square planar complexes 1e and 1f, respectively,
[Co(κ2N,N′-NC4H3C(R)dN-2,6-iPr2C6H3)2] (R ) H 1e; Me
1f), being only successful for the low yield synthesis of 3a,
because of the difference in hindrance of the formimino and
acetiminopyrrolyl ligands. The synthesis of 3b in THF is
unfruitful because of the kinetic competition of the solvent,
giving rise to mixtures of 1f and its coordinated THF adduct
4b.
The synthesis of the new bis-iminopyrrolyl five-coordi-
nate Co(II) complexes [Co(κ2N,N′-NC4H3C(RH)dN-2,6-
iPr2C6H3)2(THF)] (R ) H 4a; Me 4b) was carried out in
high yields by the reaction of CoCl2(THF)1.5 with the
corresponding iminopyrrolyl Na salt.
Figure 9. Spin densities calculated (B3LYP) for the low-spin ground states
(S ) 1/2) of complexes (a) 3a and (b) 4a.
All the compounds have been characterized by X-ray
diffraction, with 3a and 3b showing axially compressed
trigonal bipyramidal geometries, whereas 4a and 4b show
distorted square pyramidal ones, with axially compressed (4a
and polymorph 4b_I) or elongated (polymorph 4b_II) forms.
Magnetic measurements either in the solid or in the liquid
(30) Calculated Co NPA charges: 1.23 (3a S ) 1/2), 1.23 (3b S ) 1/2),
1.30 (4a S ) 1/2), 1.30 (4b S ) 1/2), 1.47 (3a S ) 3/2), 1.49 (3b S
) 3/2), 1.57 (4a S ) 3/2), 1.58 (4b S ) 3/2).
Inorganic Chemistry, Vol. 47, No. 19, 2008 8907

Carabineiro et al.
ER ) 35M. The modulation amplitude was kept well below the
line width and the microwave power well below saturation. A
finger-quartz cryostat with liquid nitrogen was used for acquiring
the spectra at 77 K. Samples were prepared in a glovebox, by
dissolving the complexes in freshly distilled toluene and filtering
the solutions to EPR tubes. The reported EPR parameters were
obtained by simulation of the experimental spectra using the
program WinEPR Simfonia (Bruker) assuming axial spin Hamil-
tonians for complexes 3 and rhombic ones for complexes 4 and 1f.
For all complexes, the g and A parameters in the perpendicular/xy
regions, are less accurate because of their dependence on the line
widths (1.5-2.0 mT for complexes 3 and 2.0-5.0 mT for
complexes 4) used in the simulations and low spectral resolution.
states indicate that complexes 3a and 3b have low-spin
ground states (S ) 1/2). However, compounds 4a and 4b
behave unusually since they show magnetic moments
compatible with high-spin ground-state complexes (S ) 3/2)
in the solid state, but in toluene solutions, they match with
spin-only values for low-spin ground state (S ) 1/2)
complexes. This behavior is confirmed by EPR spectroscopy,
and can be assigned to an elongation of the Co-THF bond
that occurs upon toluene dissolution. Such type of changes
in the metal-ligand bond lengths can easily occur for weak
ligands, such as THF.
Synthesis of [Co(K2N,N′-NC4H3C(H)dN-2,6-iPr2C6H3)2-
(PMe3)] (3a). Method a. NaH (48 mg, 2 mmol) was suspended in
tetrahydrofuran, and 2 mmol (0.508 g) of the neutral formiminopy-
rrole ligand precursor was slowly added as a solid, under a
counterflow of nitrogen. The resulting orange-brown solution was
filtered and added dropwise to a previously cooled (-78 °C)
suspension of CoCl2(PMe3)2 (0.253 g, 1 mmol) in tetrahydrofuran.
This mixture was stirred for an hour and then allowed to cool to
room temperature. After 2 h, all volatiles were evaporated under
vacuum, and the residue was extracted with n-hexane until extracts
remained colorless. The resulting orange-brown solution was
concentrated and cooled to -20 °C to yield 0.627 g (98%) of dark
red crystals. Crystals suitable for X-ray diffraction were obtained.
This reaction was also carried out using a 1:1 stoichiometric ratio
of ligand/metal yielding 0.256 g (40%) of 3a.
Method b. Anhydrous CoCl2 (0.130 g, 1 mmol) was suspended
in THF, and 2 mL of a 1 M solution of P(CH3)3 in toluene (2 mmol)
was added. The color of the solution changed from deep turquoise
blue to dark purple, and the mixture was further stirred for 90 min.
After cooling to -78 °C, a solution of the ligand Na salt (2 mmol)
in THF was filtered and added dropwise. The resulting reaction
mixture was allowed to warm to room temperature and stirred for
2 h. The volatiles were evaporated in vacuum to dryness, and the
residue was extracted with n-hexane until extracts were colorless.
The resulting orange-brown solution was concentrated and cooled
to -20 °C to yield 0.614 g (96%) of dark brown crystals of 3a.
Crystals suitable for X-ray diffraction were obtained.
Experimental Section
General Considerations. All experiments dealing with air- 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 Air Liquide 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
were used under inert atmosphere and thoroughly deoxygenated,
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)
and calcium hydride (for n-hexane). Deuterated solvents were dried
by storage over 4 Å molecular sieves and degassed by the
freeze-pump-thaw method. Solvents and solutions were trans-
ferred using a positive pressure of nitrogen through stainless steel
cannulae and mixtures were filtered in a similar way using modified
cannulae that could be fitted with glass fiber filter disks.
The ligand precursors (2,6-diisopropyl-phenyl-(1H-pyrrol-2-yl-
methylene)-amine) and (2,6,-diisopropyl-phenyl-[1-(1H-pyrrole-2-
yl)-ethylidene]-amine) and their corresponding sodium salts, re-
spectively, Ie and If, were prepared according to our previous
work.6,7 The syntheses of CoCl2(PMe3)2,31 CoCl(PMe3)3,32 and
[Co4Cl8(THF)6] (CoCl2(THF)1.5)12c were adapted from the literature.
The syntheses of complexes [Co(κ2N,N′-NC4H3C(R)dN-2,6-
iPr2C6H3)2] (R ) H 1e, Me 1f) were performed as described in the
literature.6
Method c. The complex [Co(κ2N,N′-NC4H3C(H)dN-2,6-
iPr2C6H3)2] (1e) (0.283 g, 0.5 mmol) was dissolved in tetrahydro-
furan (30 mL), and 2 mL of a 1 M solution of P(CH3)3 in toluene
(2 mmol) were added. The initially red solution turned darker and
was further stirred for 4 days at room temperature. The solvent
was removed under vacuum to dryness, and the residue was
extracted with n-hexane until extracts were colorless. The resulting
solution was concentrated and stored at -20 °C to yield dark red
crystals of 3a. A second crop of crystals were obtained after partial
evaporation of the mother liquor and cooling to -20 °C. Total yield:
0.100 g (31%). Attempts using shorter reaction times (e.g., 12 h,
both at room temperature and 50 °C) did not afford the desired
compound.
Nuclear magnetic resonance (NMR) spectra were recorded on
Varian Unity 300 MHz or Bruker Avance III 300 spectrometers,
at 299.995 and 300.13 MHz (1H), and were referenced internally
using the residual protio solvent resonance relative to tetrameth-
ylsilane (δ)0). For air- and -moisture sensitive materials, samples
were prepared in a glovebox, in J. Young NMR tubes, using
toluene-d8 (magnetic susceptibilities measurements by the Evans
method).
Elemental analyses were obtained from the IST or ITN elemental
analysis service.
Mass spectra were collected on a 500 MS LC Ion Trap (Varian,
Inc., Palo Alto, CA, USA) mass spectrometer equipped with an
ESI source. The spray voltage was set at ( 5 kV and the capillary
voltage was set at 10 V.
ESI-MS in CH2Cl2: m/z 640 for [Co(L1)2PMe3]+ (L1 )
NC4H3C(H)dN-2,6-iPr2C6H3). Anal. Found (calcd) for C37H51-
CoN4P: C 69.19 (69.25), H 8.46 (8.01), N 8.65 (8.73)%.
EPR spectra were obtained using an X-band Bruker ESP300E
spectrometer equipped with a microwave bridge ER033, a rectan-
gular cavity operating in the T102 mode and a NMR gaussmeter
Synthesis of [Co(K2N,N′-NC4H3C(Me)dN-2,6-iPr2C6H3)2-
(PMe3)] (3b). Similar procedures described above for 3a in methods
a-c were employed (including reagent amounts), but the synthesis
was carried out in diethyl ether during 36 h. The acetiminopyrrole
ligand precursor was however deprotonated in a THF solution, as
described for the formiminopyrrole, which was then evaporated to
dryness, and the residue was redissolved in Et2O, being further
(31) Jensen, K. A.; Nielsen, P. H.; Pedersen, C. T. Acta Chem. Scand. 1963,
17, 1115–1125.
(32) (a) Klein, H. F.; Karsch, H. H. Chem. Ber. 1975, 108, 944–955. (b)
Herrman, W. A., Ed. In Synthetic Methods of Organometallic and
Inorganic Chemistry (Herrman/Brauer); Georg Thieme Verlag: Stut-
tgard, Germany, 1997; Vol. 7,Part 1, pp. 70-71.
8908 Inorganic Chemistry, Vol. 47, No. 19, 2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
Table 5. Crystal Data and Structure Refinement for Compounds 3a, 3a·L,13 4a, 4b_I, and 4b_II
complex
3a
3a·L
3b
4a
4b_I
4b_II
formula
M
λ (Å)
T (K)
C37H51CoN4P
641.75
0.71073
150
C54H73CoN6P
896.12
0.71073
180
C39H55CoN4P
669.80
0.71069
150
C38H50CoN4O
637.77
0.6709
150
C40H54CoN4O
665.80
0.71069
150
C40H54CoN4O
665.80
0.69040
150
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
9.31940(10)
40.7694(4)
9.90740(10)
90
110.7118(4)
90
3521.00(6)
4
1.211
0.563
27
15 157
8009
monoclinic
P21/n
10.9676(2)
30.2749(4)
15.4792(3)
90
92.7610(7)
90
5133.79(15)
4
1.159
0.405
25
34 727
9043
monoclinic
C2/c
15.9200(8)
11.1660(7)
21.0200(12)
90
101.147(3)
90
3666.1(4)
4
1.213
0.543
28
6303
6303
0.034
monoclinic
P21/n
11.6637(11)
21.386(2)
14.1464(13)
90
93.3850(10)
90
3522.6(6)
4
1.203
0.521
30
42 070
11 528
0.028
monoclinic
P21/c
21.665(3)
9.4940(12)
18.167(3)
90
91.618(7)
90
3735.2(9)
5
1.079
0.065
25.142
58 945
6572
tetragonal
I41/a
18.208(2)
18.208(2)
21.189(3)
90
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
90
90
7024.6(17)
4
1.135
0.292
25.5
30 447
3564
0.119
Fcalcd (g cm-3
)
µ (mm-1
)
θmax (deg)
total data
unique data
Rint
0.019
0.140
0.047
R [I < 3σ(I)]
reflns R [I < 3σ(I)]
wR
GOF
F min
F max
0.0317
5007
0.0340
3729
0.0383
1.1287
-0.20
0.38
0.0597
3649
0.0563
1.1304
-0.97
1.36
0.0411
5189
0.0441
1.0392
-0.37
0.38
0.0330
3873
0.0383
1.1437
-0.55
0.59
0.0573
1204
0.0659
1.1394
-0.39
0.67
0.0322
1.0729
-0.25
0.23
added to the solution of CoCl2(PMe3)2. Upon cooling of the
n-hexane extracts of the reaction products to -20 °C, 0.586 g (87%)
of dark red crystals suitable for X-ray diffraction were obtained
for method a and 0.578 g (86%) of a similar material for method
b. Shorter reaction times, such as 12 h, showed that reaction was
not complete, and a large portion of the starting material could be
recovered. Method c did not work for a reaction time up to 8 days,
at 25 °C.
added dropwise to a previously cooled (-78 °C) suspension of
[Co4Cl8(THF)6] (CoCl2(THF)1.5) (0.253 g, 1 mmol) in tetrahydro-
furan. The mixture was stirred for an hour and then allowed to
cool to room temperature. After 12 h, all volatiles were evaporated
under vacuum, and the residue was extracted with n-hexane until
extracts remained colorless. The resulting orange-brown solution
was concentrated and cooled to -20 °C to yield 0.587 g (92%) of
dark red crystals. Crystals suitable for X-ray diffraction were
obtained.
ESI-MS in CH2Cl2: m/z ) 637 for [Co(L1)2THF]+ (L1 )
NC4H3C(H)dN-2,6-iPr2C6H3). Anal. Found (calcd) for C38H50-
CoN4O: C 71.49 (71.56), H 7.78 (7.90), N 8.77 (8.79)%.
Synthesis of [Co(K2N,N′-NC4H3C(Me)dN-2,6-iPr2C6H3)2(OC4-
H8)] (4b). A similar procedure to the described above for compound
4a was employed (including reagent amounts), but using the
acetiminopyrrole ligand precursor instead. Yield: 84% (0.560 g).
Crystals suitable for X-ray diffraction were obtained.
ESI-MS in CH2Cl2: m/z ) 665 for [Co(L2)2THF]+ (L2 )
NC4H3C(Me)dN-2,6-iPr2C6H3). Anal. Found (calcd) for
C40H54CoN4O: C 71.87 (72.16), H 8.23 (8.17), N 8.25 (8.17)%.
Magnetic Measurements. Magnetic susceptibility measurements
in solution were carried out by the Evans method33,34 using a 3%
solution of hexamethyldisiloxane (reference) in toluene-d6.
Static DC magnetic measurements were performed on polycrys-
talline samples (10-20 mg) of 3a, b using a multipurpose
characterization system MagLab 2000 (Oxford Instruments) and
4a, b using a SQUID magnetometer (Cryogenic Ltd.). The
temperature dependence of the magnetization was measured under
magnetic fields of 1 and 5 T in the temperature range of 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
(not shown). The diamagnetism correction for the experimental data
was estimated using the Pascal constants.35
ESI-MS in CH2Cl2: m/z ) 669 for [Co(L2)2PMe3]-, (L2 )
NC4H3C(Me)dN-2,6-iPr2C6H3). Anal. Found (calcd) for C39H55-
CoN4P: C 69.74 (69.94), H 8.64 (8.28), N 8.41 (8.36)%.
Attempts to Synthesize [Co(K2N,N′-NC4H3C(Me)dN-2,6-
iPr2C6H3)2(PMe3)] (3b) in THF. Similar procedures described
above in methods a-c for the synthesis of compound 3a in THF
were employed (including reagent amounts), but using the ace-
timinopyrrole ligand precursor instead. However, when cooling the
n-hexane extracts of the reaction products to -20 °C different
results were obtained.
Method a. When the reaction was carried out for 2 h, 0.262 g
(39%) of 3b were obtained in the first crop and 0.367 g (55%) of
the square planar 1f6 were obtained in the second crop. When a
reaction time of 12 h was employed, 0.429 g of deep red
microcrystals were obtained in the first fraction, which were showed
to be a mixture of 1f and [Co(κ2N,N′-NC4H3C(Me)dN-2,6-
iPr2C6H3)2(OC4H8)] 4b. Crystals of 4b suitable for X-ray diffraction
were obtained. The ESI-MS spectrum in THF showed two peaks
at m/z 594 and 665, for [Co(L2)2 + H]+ and [Co(L2)2(THF)]+,
respectively (L2 ) NC4H3C(Me)dN-2,6-iPr2C6H3).
Method b. Complex 3b was not obtained by this route; instead
compound 1f was obtained, in 91% yield (0.544 g). ESI-MS: m/z
594 for [Co(L2)2 + H]+.
Method c. Method c resulted in recovery of 0.468 g (78%) of
the starting reagent, the square planar 1f.
X-ray Experimental Data. Crystallographic and experimental
details of crystal structure determinations are given in Table 5.
Crystals of 3a, 3a·L,13 3b, 4a, and 4b_I and 4b_II (polymorphs)
Synthesis of [Co(K2N,N′-NC4H3C(H)dN-2,6-iPr2C6H3)2(OC4-
H8)] (4a). NaH (48 mg, 2 mmol) was suspended in tetrahydrofuran
and 2 mmol (0.508 g) of the neutral formiminopyrrole ligand
precursor was slowly added as a solid, under a counterflow of
nitrogen. The resulting orange-brown solution was filtered and
(33) Evans, D. F. J. Chem. Soc. 1959, 2003–2005.
(34) Sur, S. K. J. Magnet. Res. 1989, 82, 169–173.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8909

Carabineiro et al.
were isolated by filtration, and in each case, a specimen crystal
selected under an inert atmosphere, covered with polyfluoroether,
and mounted on the end of a nylon loop. Data for compounds 3a
and 3a·L were collected at 150 and 180 K, respectively, on a
Nonius KappaCCD with graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å). Crystallographic data for complex 3b and
polymorph 4b_II were collected using graphite-monochromated Mo
KR radiation (λ ) 0.71069Å) 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
Bruker36 software and refined using Bruker SAINT37 on all
observed reflections. Absorption corrections were applied using
SADABS.38 A synchrotron radiation source was used to collect
diffraction data for 4a and polymorph 4b_I, at 150 K. Data were
collected at Station 9.8, Daresbury SRS, U.K., using a Bruker
SMART CCD diffractometer. The images were processed with the
DENZO and SCALEPACK programs.39 The structures were solved
by direct methods using the program SIR92.40 The refinement (on
F) and graphical calculations were performed for all compounds
using the CRYSTALS program suite.41 For all compounds, the non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. 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.42
Figures were generated using ORTEP3.43
exchange with DFT26 exchange-correlation, given by Becke’s three
parameter functional46 with the Lee, Yang, and Parr correlation
functional, which includes both local and nonlocal terms.47,48
Geometry optimizations were performed without symmetry con-
straints and a basis set (b1) consisting of a standard LanL2DZ basis
set49 for Co, C, and H, with an f-polarization function added for
Co,50 while the coordinated atoms, N, O, and P were described by
a standard 6-31G(d,p).51 Spin contamination was carefully moni-
tored for all calculations and the values of <S2> indicate minor
spin contamination. The energy values referred along the text (gas
phase) were obtained through single-point energy calculations using
the B3LYP/b1 geometries and a 6-311G(d,p) basis set52 (b2) for
all atoms. A natural population analysis (NPA)53 was performed
with the B3LYP/b2//B3LYP/b1 density to evaluate the charge
distribution on the complexes, and the Wiberg indexes obtained
are used as a measure of bond strength. Spin density representations
were obtained using the program Molekel 4.0.54
This general theoretical method has been tested before for related
Co(II) complexes, yielding reasonable results.6,9
When mentioned in the text, solvent effects were considered in
the PBE1PBE/b2//PBE1PBE/b1 energy calculations using the
Polarizable Continuum Model (PCM) initially devised by Tomasi
and co-workers55 as implemented on Gaussian 03.56 In these cases,
the energy values can be taken as free energy.57 The molecular
cavity was based on the united atom topological model applied on
UAHF radii, optimized for the HF/6-31G(d) level.
Data was deposited in CCDC under the deposit numbers 652807
(3a), 652808 (3a·L), 678406 (3b), 678407 (4a), 678408 (4b_I),
and 652809 (4b_II).
Computational Details. All calculations were performed with
the Gaussian 03 software package,44 using the B3LYP hybrid
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8910 Inorganic Chemistry, Vol. 47, No. 19, 2008

FiWe-Coordinate Bis-iminopyrrolyl Co(II) Complexes
figures containing the definition of parameter τ (Figure S1), the
X-ray structures of compounds 3a · L and 4b (polymorph 4b_II)
(Figures S2-S4), the Curie-Weiss plots of the magnetic
susceptibility data of compounds 3a,b and 4a,b (Figures S5 and
S6), views of the crystal packings of complexes 4a and 4b_I
(Figures S7 and S8), the optimized geometries and relative
energy difference of the high-spin and low-spin ground states
of complexes 3a and 3b (Figure S9) and 4a and 4b (Figure S10),
and the spin density for the low-spin isomers of complexes 3a
and 4a (Figure S11) and 3b and 4b (Figure S12), and tables of
atomic coordinates for all optimized species (Table S2).This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank the Fundac¸a˜o para a Cieˆncia
e Tecnologia for financial support (Project POCI/QUI/59025/
2004, cofinanced by FEDER) and for fellowships to S.A.C.
(SFRH/BPD/14902/2004 and SFRH/BPD/34974/2007) and
R.M.B. (SFRH/BPD/3574/2000), respectively. We also thank
the Royal Society for funding to S.I.P. Thanks are also due
to Dr. Joaquim Marc¸alo, ITN, Sacave´m, Portugal, for the
calculation of cone angles (equivalent) of PMe3 and THF
and for helpful discussions.
Supporting Information Available: CIF files containing X-ray
structural data, including data collection parameters, positional
and thermal parameters, and bond distances and angles, for all
compounds of this work, table containing selected bond distances
and angles for all crystal structures of this work (Table S1),
IC800992F
Inorganic Chemistry, Vol. 47, No. 19, 2008 8911