Synthesis and coordination chemistry of sterically hindered cobalt(II) β-ketoiminate complexes

Article abstract of DOI:10.1016/j.ica.2018.10.051

The reactions of cobalt(II) chloride with α,β-unsaturated-β-ketoamines, CH₃C([dbnd]O)CH[dbnd]C(NHR)CH₃, where R = -phenyl, HL^Ph; R = -mesityl, HL^mes; R = -2,6-diisopropylphenyl, HL^dipp, and sodium hyd

Full text of DOI:10.1016/j.ica.2018.10.051

InorganicaChimicaActa486(2019)441–448
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis and coordination chemistry of sterically hindered cobalt(II) β-
ketoiminate complexes
Suman Debnath⁎, Navamoney Arulsamy, Mark P. Mehn
Department of Chemistry, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, United States
A R T I C L E I N F O
A B S T R A C T
Keywords:
The reactions of cobalt(II) chloride with α,β-unsaturated-β-ketoamines, CH3C(]O)CH]C(NHR)CH3, where
R = -phenyl, HLPh; R = -mesityl, HLmes; R = -2,6-diisopropylphenyl, HLdipp, and sodium hydride in THF solvent
β-Ketoiminate ligand
Cobalt(II) complex
X-ray crystallography
Magnetic susceptibility
under anaerobic conditions affords brown or orange crystals of Co(Lph)2, Co(Lmes
)
2 and Co(Ldipp)2, respectively.
In THF/acetonitrile solvent mixture, the complexation reactions are not successful as unreactive salts containing
networks of acetonitrile-coordinated sodium cations and the respective anions are formed. One of them, the
sodium salt of HLdipp, is isolated and characterized as [Na(Ldipp)(CH3CN)]4. Single-crystal X-ray diffraction data
for the cobalt(II) complexes reveal that the metal centers assume distorted tetrahedral geometries. The degree of
distortion increases with the size of the substituents present in the phenyl groups: Co(Lph
)
< Co(Lmes
) < Co
2
(Ldipp)2. Crystallographic data measured at 296, 200, 150 and 100 K for Co(Ldipp
)
reveal that it exists2as two
2
geometrical forms, namely, distorted tetrahedral and distorted square-planar. The data also indicate that the
pseudo-tetrahedral to distorted square-planar transition is favored at lower temperatures. Electrochemical data
measured in THF and DCM solvents reveal the likely presence of Co(II)-solvent interaction in Co(Lph
) and Co
in the 4–3020 K tem-
(Lmes
)
and no such interaction in Co(Ldipp)2. Magnetic susceptibility data for Co(Ldipp
)
2
perature region reveal spin-crossover at ca. 130 K. Qualitatively, the magnetic and cryst2allographic data com-
plement each other with respect to the presence of the high spin pseudo-tetrahedral and low spin pseudo-square
planar geometries.
1. Introduction
The β-ketoaminate bidentate ligand has many similarities to both
the β-diketiminate and acetylacetonate ligands [1]. A tautomeric
structure for β-ketoiminate can be drawn for the ligand as shown below
more accurately describing the bonding and charge distribution present
in it. Even with several similarities, metal complexes of the β-ketoi-
minate ligand type have not been investigated in detail. With a pre-
ference to form four-coordinate metal compounds [1,2], a resistance to
undergo polymerization [2–3], an easy route for changing steric in-
teractions (by varying R and Rα,β,γ) [1,2], and conformational flex-
ibility which allows the metal complexes to undergo planar to tetra-
hedral conversions [4–6], the monoanionic β-ketoiminate ligand is
unique and has experienced a resurgence in inorganic and organome-
tallic chemistry as a scaffold for metal catalyzed reactions [7]. Parti-
cular attention has recently been focused on the roles of β-ketoiminate
complexes as active site models for metalloenzymes [8], catalysts in
polymerization [9], and other CeC bond forming reactions [10].
In comparison to the β-ketoiminate complexes of many other tran-
sition metals [1,2], cobalt(II) β-ketoiminate complexes have received
relatively little attention, and the effect of electronic and steric effects
present in ligand systems on their Co(II) complexes remains poorly
understood. In one of the earlier studies, Holm and coworkers have
investigated a series of cobalt(II) complexes of bidentate β-ketoiminate
ligands HLR, CH3C(]O)CH]C(NHR)CH3, where R = –H, –CH3, –iPr, n-
C3H7 and those of closely related tetradentate schiff base ligands [6].
This study has shown that the Co(II) complexes adopt pseudo-tetra-
hedral geometries and are high-spin. After Holm’s study, research
pertaining to Co(II) β-ketoiminates has remained dormant for over
40 years. Recently, Robson et al have studied a related series of aryl-
substituted β-ketoamines, PhC(]O)CH]C(NHR)CH3, where R = aryl,
⁎ Corresponding author.
E-mail address: sdebnath@uwyo.edu (S. Debnath).
https://doi.org/10.1016/j.ica.2018.10.051
Received 15 July 2018; Received in revised form 23 October 2018; Accepted 26 October 2018
Availableonline05November2018
0020-1693/©2018ElsevierB.V.Allrightsreserved.

S. Debnath et al.
InorganicaChimicaActa486(2019)441–448
and note that the ligands also form tetrahedral Co(II) complexes [11].
Significantly, the redox properties of the new series of complexes are
found to be sensitive to the presence of sterically-demanding aryl
substituents.
couple which is reported to be +0.53 V vs SCE in [nBu4N](ClO4) in THF
[13]. The peak separations were reported with a scan rate of 200 mV/s
(the observed Fc+/Fc0 peak separation was 90 mV under these condi-
tions).
As part our research efforts toward the synthesis of catalysts of earth
abundant metals for small molecule activation, we have identified β-
ketoiminates as a suitable ligand system for the modulation of the
geometrical and electronic attributes of transition metal complexes. In
this work, we employ three β-ketoiminates with varying extents of
steric encumbrance and donor strengths. As shown in Eqs. 1 and 2, the
ligands encompass both steric and electronic variation at the N-aryl
group allowing us to study the influence of both of these factors in the
reactivity of the Co(II) center. We have previously demonstrated that
while the β-ketoiminate ligand systems are capable of enforcing strictly
four-coordinate geometry around Fe(II) and Zn(II) ions, the highly
sterically encumbered Ldipp ligand is also capable of forming a five-
coordinate Fe(II) complex [12]. The unexpected coordination proper-
ties of Ldipp appears to result from the unique steric and electronic ef-
fects induced by the isopropyl substituents present in the N-aryl group.
Hoping that the flexible protonation/deprotonation behavior of the li-
gand can further be exploited in small molecule activation, we have
extended our investigation to include Co(II). For comparison, we have
also studied the Co(II) complexes of the closely related LPh and Lmes
ligands. As will be discussed in the later sections, the Co(II) complex of
Ldipp exhibits yet another form of geometrical flexibility.
Low resolution electrospray ionization mass spectral data (ESI-MS)
for the complexes were obtained using an LCQ mass spectrometer
(Finnigan MAT) for THF solutions that were directly infused into the
spectrometer via a syringe pump. The heated capillary was set at
150 °C. Magnetic measurements were recorded for 45–55 mg crystalline
samples using a Quantum Design Corp. SQUID variable-temperature
susceptometer. The field dependence was measured and found to be
linear from 5 to 25 kg. Sample measurements were made from 5 to
300 K at a magnetic field strength of 1000 Oe in a Lilly Gel Cap. Molar
susceptibilities for the complex were calculated following corrections
for bucket paramagnetism and underlying atomic diamagnetism.
Ligands, HLph (R = -phenyl), HLmes (R = -mesityl) and HLdipp (R = -
diisopropylphenyl) were prepared following literature methods [7,12].
Attempted complexation reactions between the deprotonated ligands
and CoCl2 in THF/acetonitrile solvents were unsuccessful. In one of the
syntheses with HLdipp, the sodium salt of the ligand was isolated instead
of the desired Co(II) complex. Subsequently, the sodium salt was pre-
pared as follows. NaH (0.048 g, 2.00 mmol) was added to a solution of
HLdipp (0.519 g, 2.01 mmol) in THF and stirred until the evolution of
hydrogen ceased. The mixture was evaporated to dryness, and the solid
obtained was dissolved in acetonitrile and layered with diethyl ether.
Colorless crystals of [Na(Ldipp)(CH3CN)]4 formed were filtered and
dried. Yield: 0.124 g (0.278 mmol, 27.8%). Anal Calcd. C19H27N2NaO:
C, 70.78; H, 8.44; N, 8.69. Found: C, 70.76; H, 8.56; N, 8.40. ESI/MS
(THF, 150 °C): m/z = 282.1 ([M]+ = [Na(HLdipp)]+). When the reac-
tions were carried out exclusively in THF, the respective Co(II) com-
plexes were isolated.
(1)
Co(Lph)2. NaH (0.048 g, 2.00 mmol) was added to a solution of HLph
(0.351 g, 2.00 mmol) in THF and stirred until the evolution of hydrogen
ceased. CoCl2 (0.128 g, 0.992 mmol) was added and the mixture was
stirred vigorously for 12 h. The reaction mixture was filtered to remove
the inorganic salts. The filtrate was allowed to evaporate slowly when
dark brown blocks formed. Yield: 0.285 g (0.70 mmol, 69.2%). Anal
Calcd. C22H24CoN2O2; C, 64.86; H, 5.94; N, 6.88. Found: C, 64.58; H,
5.82; N, 6.54. UV–vis, λmax, nm (ε, M−1cm−1) in THF: 260 (9,500), 326
(42,000), 410 (924), 520 (62), 925 (55). ESI/MS (THF, 150° C): m/
z = 407.13 ([M]+ = Co(Lph)2]+).
(2)
2. Experimental
All manipulations were carried out using standard glove box tech-
niques under a dinitrogen atmosphere. All reagents and solvents were
obtained from commercial vendors and used as received.
Tetahydrofuran (THF), toluene, and diethyl ether were distilled under
nitrogen gas from Na/benzophenone and subsequently stored over ac-
tivated alumina. Acetonitrile and dichloromethane were distilled from
calcium hydride under N2. Non-halogenated solvents were typically
tested with a standard purple solution of sodium benzophenone ketyl in
tetrahydrofuran to confirm effective oxygen and moisture removal.
Chemical reactions were all performed at high altitude (∼7200 feet or
∼2200 m).
Co(Lmes)2. NaH (0.049 g, 2.04 mmol) was added to a solution of
HLmes (0.435 g, 2.00 mmol) in THF and stirred until the evolution of
hydrogen ceased. CoCl2 (0.128 g, 0.992 mmol) was added and the
mixture was stirred overnight. The solvent was removed from the re-
sultant dark brown solution, and the residue dissolved in toluene.
Filtration yielded a brown solution which was evaporated to dryness.
Upon layering a concentrated solution of the residue with acetonitrile
brown crystals formed. Yield: 0.278 g (0.56 mmol, 47.4%). Anal Calcd.
C
28H36CoN2O2; C, 68.42; H, 7.38; N, 5.70. Found: C, 68.30; H, 7.30; N,
5.48. UV–vis, λmax, nm (ε, M−1cm−1) in THF: 300 (23,000), 510 (120),
405 (976), 945 (62), 1236 (58). ESI/MS (THF, 150 °C): m/z = 491.13
([M]+ = [Co(Lmes)2]+).
Elemental Analyses were carried out at Columbia Analytical
Services, Inc., Tucson, AZ. Electronic spectra (UV–Vis) were recorded
on an Agilent 8453 diode-array spectrophotometer (250–1100 nm)
which was equipped with a liquid nitrogen cryostat (Unisoku). Near IR
UV–Vis absorption spectra were measured with a Cary 500 Scan
UV–Vis-NIR spectrophotometer (250–3300 nm) (Varian Instruments). A
quartz cell with 10 mm optical path length was used. The IR spectra
were recorded as KBr pellets at room temperature on a Varian 800 FTIR
(Scimitar Series) set to 1 cm−1 resolution. Electrochemical measure-
ments were carried out in a dry box under N2 in THF solution with
0.4 M (nBu4N)(ClO4) as the supporting electrolyte using a model ED-
401 computer controlled potentiostat (eDAQ). A three-electrode con-
figuration with a glassy carbon working electrode, a Ag wire reference
electrode, and a platinum wire auxiliary electrode was used. The redox
potential values were referenced to an internal ferrocenium/ferrocene
Co(Ldipp)2. NaH (0.047 g, 1.99 mmol) was added to a solution of
HLdipp (0.517 g, 1.99 mmol) in THF and stirred until the evolution of
hydrogen ceased. CoCl2 (0.128 g, 0.98 mmol) was added and the mix-
ture was stirred overnight. The solvent was removed from the resultant
dark brown solution and the residue dissolved in toluene. A brown
solution was isolated via filtration of the inorganic salts. After evapor-
ating toluene under N2 atmosphere, the solid was dissolved in THF and
layered with acetonitrile. Orange brown crystals were obtained by slow
evaporation of the THF/acetonitrile solution. Yield: 0.348 g
(0.60 mmol, 60%); Anal Calcd. C34H48CoN2O2; C, 70.87; H, 8.40; N,
4.86. Found: C, 70.68; H, 8.43; N, 5.16. UV–vis, λmax, nm (ε, M−1cm−1
)
in THF: 310 (22,000), 500 (88), 542 (1 0 2), 973 (57), 1280 (50). ESI/
MS (THF, 150 0C): m/z = 575.1 ([M]+ = [Co(Ldipp)2]+). IR (KBr,
442

S. Debnath et al.
InorganicaChimicaActa486(2019)441–448
cm−1): 3080 ѵ(NeH), 2959 ѵ(CeH), 1564 (ѵ(C]C), 1506 ѵ(C]N).
X-ray diffraction analysis. [Na(Ldipp)(CH3CN)]4, Co(Lph)2, Co(Lmes)2,
and Co(Ldipp)2 and were characterized by single crystal X-ray diffraction
data. Crystals of the compounds were glued to MiTeGen micro-mounts
using Paratone N oil and mounted on a Bruker SMART APEX II CCD
area detector system equipped with a graphite monochromator and a
Mo Kα fine-focus sealed tube operated at 1.5 kW power (50 kV, 30 mA).
the structures from the 200 and 296 K data sets also progress well,
however, the free variables refine to 0.66 and 0.76 indicating occur-
rence of the two isomers in 66:34 and 76:24 ratios at 200 and 296 K,
respectively.
The 100 K data set does not solve well in C2/c, and despite our best
efforts no satisfactory refinement could be achieved. However, the data
set solves very well in the less-symmetric C2 space group. The asym-
The data were measured for [Na(Ldipp)(CH3CN)]4, Co(Lmes
)
and Co
metric unit consists of two well-ordered halves of Co(Ldipp
) molecules
2
(Ldipp
)
at 150 K. Co(Lph
)
undergoes phase transition at low2tempera-
with the cobalt centers being located on inversion centers. The two
four-coordinate Co(II) complexes differ from each other due to the
varying levels of distortion from tetrahedral geometry.
2
2
tures, and no satisfactory unit cell could be determined at 150 K.
Fortunately, a data set measured at room temperature solves in a
straightforward fashion. X-ray diffraction data were also measured at
100, 200 and 296 K for Co(Ldipp)2.
One crystal specimen was used for the measurement of the four data
sets at 296, 200, 150 and 100 K. The transition between space groups is
reversible, as another data set measured at 296 again solves in C2/c
giving identical refinement parameters.
A series of narrow frames of data were collected with a scan width
of 0.5° in ω or Ф and an exposure time of 10 s per frame. For each of the
data sets, optimized data collection strategies were defined using the
COSMO software program (APEX2 Software Suite v. 2.1–0, Bruker,
AXS: Madison, WI, 2005). The data were corrected for absorption ef-
fects by the multi-scan method (SADABS). A summary of the crystal-
lographic details is given in Table S1. The structures were solved by
direct methods using the Bruker SHELXTL (v. 6.14) software program
included in the Bruker Apex 2 software package. Patterson methods
were used to solve the structures. In all of the structures, the non-hy-
drogen atoms were located in successive Fourier maps and refined
anisotropically. The H atoms in [Na(Ldipp)(CH3CN)]4 were placed in
calculated positions and refined isotropically adapting a riding model.
The hydrogen atoms of the acetonitrile solvate were neither located nor
placed in calculated positions. In Co(Lph)2, the two methine H atom of
the chelate rings were located in the Fourier maps and refined iso-
tropically. In Co(Lmes)2, the H atoms of the aromatic and chelate ring
CeH were located in the Fourier maps and refined isotropically. The
rest of the hydrogen atoms in both of the structures were placed in
calculated positions and refined isotropically adapting a riding model.
All of the H atoms in the four structure of Co(Ldipp)2 determined at 100,
150, 200 and 296 K were placed in calculated positions, and refined
isotropically as above.
3. Results and discussion
3.1. Syntheses
Ligand and Complex Syntheses. The ligands employed in this in-
vestigation were synthesized by condensing the appropriate β-diketone
with a primary amine in presence of catalytic amount of p-toluenesul-
phonic acid under reflux (eq (1)). The resulting α, β-unsaturated-β-
ketoamines were purified by distillation or recrystallization in hexane.
(1)
An attempted synthesis of the Co(II) complexes of the ligands by the
reaction between freshly deprotonated ligands in THF using NaH and
CoCl2 dissolved in acetonitrile were unsuccessful. The syntheses in-
variably led to the isolation of the sodium salt of the ligands. One of
them, namely, the sodium salt of HLdipp, was isolated as a crystalline
solid. The product was characterized as [Na(Ldipp)(MeCN)]4 by crys-
tallographic data. The structure reveals a network of 4 pairs of strongly
interacting Na+ and [Ldipp]- ions forming a distorted-cubic core. We
assume that the formation of network is likely facilitated by acetonitrile
and that the network formation precludes complexation of the ligand
with Co(II). When the syntheses were carried out in THF solvent ex-
clusively, the Co(II) complexes are formed in moderate yield (Eq. 2).
[Na(Ldipp)(CH3CN)]4 crystallizes in the chiral tetragonal space
¯
group P421c. The asymmetric unit of consists of a sodium cation, an
Ldipp anion and a solvated acetonitrile molecule. The cation and anion
are located on general positions and are well ordered, whereas acet-
onitrile molecule is disordered with located on a two-fold symmetry
axis as the methyl carbon atom is located on the axis. The acetonitrile is
also distorted occupying two positions. A free variable refinement set-
tles at to ca. 50% occupancy for the two sites. Attempts to determine
the absolute structure were unsuccessful as the crystal exhibits racemic
twinning [14].
Crystals of Co(Lph
) are obtained from the slow evaporation of the re-
2
action mixture in THF during the synthesis of the complex in a glo-
Crystals of Co(Lph
)
undergo crystalline phase transitions at low
vebox, whereas those of Co(Lmes
)
and Co(Ldipp
) were grown by the
2
2
temperatures, and no satisfactory unit cell could be determined from
data measured at 150 K. Therefore, a data set was measured at room
temperature (296 K). The crystal belongs to the chiral orthorhombic
space group Pna21. The absolute structure was satisfactorily refined by
refining the Flack parameter to 0.010(16).
slow diffusion of acetonitrile into 2toluene solution of the complexes in
the glove box. In the presence of air, the solutions change in color from
brown or orange brown to blue or dark green within 2–3 h, and form a
dark brown powder. As solids, the complexes are stable under ambient
conditions for several days, but they begin to decompose after 1 week
turning blue and losing crystallinity.
Co(Lmes
)
crystallizes in the centrosymmetric monoclinic space
2
group P21/n. The asymmetric unit consists of a half of the molecule as
the metal center is situated on an inversion center, and the molecule is
well ordered.
Three of the data sets measured for Co(Ldipp)2 at 150, 200 and 296 K
belong to the same centrosymmetric monoclinic space group C2/c
(Table S2). The structures are solved and refined similarly. The asym-
metric unit in each of the structures consists of a half of the complex
molecule with the metal center being situated on an inversion center.
All atoms except for the ligating O atom are well ordered. The O atom
was assigned two sites, and their occupancies refined by applying a free
variable refinement. The free variable refines to 0.57 for the data sets
measured at 150 K. The disordered O atom reflects the presence two
(2)
3.2. Description of structures
[Na(Ldipp)(MeCN)]4. A view of the salt is shown in Fig. 1. The
structure contains a cubane-like core featuring alternating sodium and
oxygen atoms. Such cores are known in the structures of sodium salts of
tetradentate Schiff base ligands [15]. The ligand anion in the present
salt binds the sodium cation through its enolate O atom and imine N
isomers of Co(Ldipp
) in 57:43 ratio at 150 K. The overall refinement of
2
443

S. Debnath et al.
InorganicaChimicaActa486(2019)441–448
with each of the Co(II) centers being coordinated to two ligand anions
through their enolate O and imine N atom pairs. Views of the complexes
are given in Fig. 2, and selected structural parameters are listed in
Table 1. Both Co(Lph)2 and Co(Lmes
)
2 are well ordered. In Co(Ldipp)2, all
atoms except for the ligating O atom are well ordered. The O is assigned
two sites, and their occupancies refined applying a free variable. The
free variable refines to 0.76 for the data set measured at 296 K. The
disordered O atom reflects the presence of two isomers of Co(Ldipp
) in
2
76:24 ratio. A summary of crystallographic data for the low tempera-
ture structures are given in Table S2.
In Co(Lph
)
2 and Co(Lmes
)2 and in that of the larger component of Co
(Ldipp)2, the metal centers are in pseudo tetrahedral geometry. The
cobalt (II) centers are strictly tetra-coordinated as the closest Co·HeC
distances in the complexes are > 3.15 Å ruling out any Co(II)⋯H
agostic interactions. The bond angles at the cobalt center significantly
deviate from 109° in all of the complexes revealing the distortion. The
extent of distortion is quantified by calculating their τ4 values (Table 1)
as described by Houser et al [16], and the distortion is found to increase
in the following order: Co(Lph
)
< Co(Lmes
)
< Co(Ldipp)2. In all of the
complexes, the OC3N chelate2rings formed2 by the ligands are nearly
coplanar with the associated CeO, CeC and CeN bond distances as-
sociated being in the range of 1.279(3)–1.413(1) Å consistent with the
delocalization of two double bonds throughout the chelate ring. The
dihedral angles between the two chelate rings are close to 90° for Co
Fig. 1. View of the cubane-like structure in [Na(Ldipp)(CH3CN)]4. Only one of
the two positions for the disordered acetonitrile molecules is shown. The
thermal ellipsoids are drawn at 30% probability, H atoms are omitted and the
symmetric equivalents are unlabeled in this and following Figure.
(Lph)2 and Co(Lmes)2, but the angles in the two components of Co(Ldipp
)
are significantly smaller. The large isopropyl substituents on the li2-
gand’s phenyl group clearly disfavor the near orthogonal configuration
seen in the other two complexes. The dihedral angles between the
chelate and phenyl ring planes also reflect the impact of the steric effect
(Table 1); the angles are significantly small in Co(Lph)2 in comparison to
atom with the Na⋯O and Na⋯N bond distances of 2.255(2) and
2.466(7) Å, respectively. A set of 12 symmetrically equivalent Na⋯O
bonds for the 12 edges of the Na4O4 pseudo-cubane were shown. The
necessarily equivalent 8 faces of the core are planar with the diagonal
Na⋯Na and O⋯O distances being 3.381(8) and 3.187(8) Å. Whereas
the imine N atom is monodentate, the enolate oxygen atom acts as a
tridentate ligand as it also binds two other sodium atoms with slightly
longer Na⋯O bond distances, 2.390(2) and 2.420(2) Å. Therefore, the
sodium atom interacts with three Ldipp ligands forming a rectangular
prismatic Na4O4 unit-containing cluster. Each of the sodium centers
also interact with a half of the disordered acetonitrile solvate with the
associate Na⋯N bond distance being 2.466(7) Å. The corresponding
distance with the other half acetonitrile is considerably longer at
2.608(8) Å. The OC3N chelate ring is nearly coplanar. The mean planes
passing through the chelate ring and the phenyl ring of the ligand
dissect each other at 86.98(11)°.
those in Co(Lmes
)
and Co(Ldipp)2. The observed near orthogonality of
the aromatic π-sy2stems relative to the respective chelate rings in these
complexes negates any electron withdrawing resonance contribution
from the phenyl rings. However, we expect that the +I effect due to the
alkyl substituents will still operate, and the large isopropyl groups in
the Ldipp anion clearly effects distortion from the ideal tetrahedral to
square planar geometry in the solid state. The geometry of the second
component of Co(Ldipp
) can be better described as a highly distorted
2
square-planar. The corresponding dihedral angles in the closely related
[Fe(Ldipp)2] and [Zn(Ldipp)2] are 59.41(3)° and 88.90(3)°, respectively
[12]. Both these complexes show four-coordinate metal centers with
bidentate β-ketoiminates and similar degrees of distortion on the basis
of continuous symmetry measurements. [Fe(Ldipp)2] exhibits a seesaw
or sawhorse geometry with a τ4 value of 0.53.
The strong macromolecular unit formed by 4 pairs of the sodium
anion and Ldipp ligand anion leads to the association of the polar che-
lating ends of the ligand with the nonpolar framework forming a hy-
drophobic shell around the core. Interestingly, despite the presence of
the sterically imposing isopropyl groups, the [Na(Ldipp)(MeCN)]4 mac-
romolecules are tightly packed with no voids being present for even
probe radius values as small as 0.9 Å. The absence of complexation with
cobalt(II) salts, therefore, can be attributed to the tight packing.
The 150 and 200 K data sets measured for Co(Ldipp
) also solve in
the same monoclinic space group, C2/c, and the overall2refinement of
the structures progress well. However, the free variable refinement of
the disordered O atom refines to 0.57 and 0.66 indicating the occur-
rence of the two isomers in 57:43 and 66:34 ratios at 150 and 200 K,
respectively. The 100 K data set solves in another monoclinic space
group, C2, and contains two unique Co(Ldipp
) molecules. One of them
2
Co(Lph)2, Co(Lmes
)
and Co(Ldipp)2. Crystals of Co(Lph
) are found to
2
2
is similar to the larger component structure determined from the 150,
200 and 296 K data sets, and the other is similar to the respective lower
component structure. This can be interpreted to indicate that the ratio
of the pseudo-tetrahedral and distorted square-planar molecules is
50:50. The τ4 values calculated for the larger component molecules at
150, 200 and 296 K are 0.62, 0.61 and 0.62, respectively, and the τ4
values for the smaller component are 0.27, 0.28 and 0.27, respectively.
The τ4 values for the two components in equal amounts at 200 are 0.64
and 0.26. These data reveal that essentially there are two geometrical
undergo crystalline phase transition at low temperatures and Co(Ldipp
)
to exhibit temperature dependent geometrical isomerization, wherea2s
those of Co(Lmes)2 are well behaved with so such behavior. Fortunately,
a good quality diffraction data set could be measured for Co(Lph
) at
2
room temperature (296 K). A data set for Co(Lmes
) collected at 150 K
2
was solved and refined in a straightforward manner. In contrast, a data
set measured for Co(Ldipp
) at 296 K solves well, but the structure re-
2
veals the presence of two kinds of geometry for the Co(II) center.
Therefore, three other data sets were also collected at 100, 150 and
isomers of Co(Ldipp
) existing at the various temperatures. The struc-
2
200 K for Co(Ldipp)2. The molecular structures of Co(Lph
)
and Co
2
tures are interconvertible and the structure close to square planar
geometry is preferred at lower temperatures. A sterically-driven square-
planar, low-spin S = 1/2 Co(II) configuration was recently observed in a
bis(iminopyrollyl)cobalt(II) species [17]. Square-planar cobalt(III)
complexes bearing redox non-innocent aminophenolate or o-
(Ldipp
)
at room temperature and that of Co(Lmes
) at 150 K are dis-
cussed2here first, and the structures of Co(Ldipp
peratures are compared later.
) at the various tem-
2
2
The coordination environment in the three complexes is similar
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S. Debnath et al.
InorganicaChimicaActa486(2019)441–448
Fig. 2. Views of Co(Lph
)
2
(a), Co(Lmes
)
2
(b), and the major (c) and minor isomers of Co(Ldipp
)
at 296 K (d).
2
phenylenediamine groups are also known in the literature [18]. How-
ever, unlike the known complexes, [Co(Ldipp)2] exhibits fluxional geo-
metries demonstrating the unique combination of steric and electronic
effects in the Ldipp anion.
The UV–vis and NIR spectra for Co(Ldipp
) exhibit a shoulder centered
at ca. 540 nm (ε = 102 M−1 cm−1) and2two weak bands at 10,300
(971 nm, 50 M−1 cm−1) and 7,800 cm−1 (1,280 nm, 57 M−1 cm−1).
The cobalt(II) center in tetrahedral geometry possesses two excited
states, namely, 4T2(F) and 4T1(F), derived from the free ion with the 4F
term, and another excited state derived from the 4P term, 4T1(P) in the
UV–vis-NIR region. The three excited states are accessible for the elec-
trons present in the 4A2(F) ground state, and the three possible spin-for-
bidden transitions can be arranged in the following order of decreasing
energy: 4A2(F) → 4T1(P) > 4A2(F) → 4T1(F) > 4A2(F) → 4T2(F) [19].
Therefore, the high energy shoulder and peaks observed at ca. 410, 475
3.3. Description of physical properties
The cobalt(II) β-ketoiminate complexes show no noticeable color
changes between the solid state and solution retaining their orange or
brown color. The UV–vis spectra for Co(Lph)2, Co(Lmes
)
and Co(Ldipp
)
2
exhibit an intense absorption 300–326 nm consistent2with literature
precedent on other divalent β-ketoaminate complexes [12]. Fig. 3
shows the spectra in the UV–vis region. The observed high extinction
coefficients (ε = 22,000–41,000 M−1cm−1) lead us to assign this band
to two overlapping transitions of the ligand based π-π* and metal-to-
ligand charge transfer. In the visible region, a poorly resolved broad
shoulder at ca. 410 nm (ε = 924 M−1cm−1) is observed for Co(Lph)2. In
and 540 nm for Co(Lph)2, Co(Lmes
)
2
and Co(Ldipp)2, respectively are as-
signed to the 4A2(F) → 4T1(P) transition. As can be seen from Fig. 3, the
transition is too broad for Co(Lph)2, whereas it is reasonably well resolved
for Co(Lmes
)
and Co(Ldipp)2. We attribute it to the higher degree of dis-
tortion at th2e Co(II) center for the latter complexes in comparison to the
less distorted tetrahedral (Td) geometry in Co(Lph)2. It is well known that
tetrahedral Co(II) complexes exhibit solvent dependent shifts in their λmax
contrast, the spectra for Co(Lmes
)
and Co(Ldipp
) contain reasonably
2
2
positions [20]. The nearly perfect tetrahedral Co(Lph
) may be less ac-
2
well-defined peaks. Specifically, Co(Lmes)2 shows a shoulder centered at
ca. 475 (120 M−1 cm−1) and two low peaks in the NIR region at 10,500
(952 nm, 62 M−1 cm−1) and 8,070 cm−1 (1,239 nm, 58 M−1 cm−1).
cessible to solvent interaction with its metal center, whereas the metal
centers in Co(Lmes)2 and Co(Ldipp)2 are likely accessible to THF molecules
445

S. Debnath et al.
InorganicaChimicaActa486(2019)441–448
Table 1
Selected structural data for Co(Lph
at 296 K.
)
2 at 296 K, Co(Lmes
)
2 at 150 K and Co(Ldipp
)
2
Co(Lph
)
Co(Lmes
)
Co(Ldipp
)
2
Co(Ldipp
)
2
a
b
2
2
Co1-O1 (Å)
1.926(2)
1.919(2)
1.961(2)
1.964(2)
117.60(7)
96.77(7)
115.63(9)
115.94(9)
96.83(7)
115.42(7)
89.18(5)
1.9250(7)
1.9250(7)
1.9733(8)
1.9733(8)
111.43(5)c
96.54(3)
1.951(3)
1.951(3)
1.812(7)
1.812(7)
Co1-O2 (Å)
Co1-N1 (Å)
1.9708(10)
1.9708(10)
130.5(5)a
93.88(7)
101.1(2)a
101.1(2)
93.88(7)
146.6(7)
63.9(1)
1.9708(10)
1.9708(10)
176(2)d
Co1-N2 (Å)
O1-Co1-O2 (°)
O1-Co1-N1 (°)
92.1(3d
O2-Co1-N1 (°)
111.35(3)c
111.35(3)c
96.54(3)
89.2(4)d
89.2(4)
O1-Co1-N2 (°)
O2-Co1-N2 (°)
92.1(3)
N1-Co1-N2 (°)
129.80(5)c
84.37(2)
143.6(7)
37.9(3)
dihedral angle between
chelate planes
dihedral angle between
chelate and phenyl planes
tetrahedral distortion, τ4
66.67(10),
70.30(11)
0.89
88.22(3)
0.84
80.42(6)
0.62
82.20(7)
0.27
Fig. 4. Cyclic voltammograms for Co(Lph
)
(
), Co(Lmes
)
(
), and Co
2
) in THF solutions (1.0 mM) containing [nBu42N](ClO4) (0.4 M)
a
b
c
(Ldipp)2, (
O2 is the symmetric equivalent of O1.
For the larger component.
as the supporting electrolyte at a scan rate of 200 mV/s.
For the smaller component.
d
O1 = O1A and O2 is the symmetric equivalent of O1B.
Co(Ldipp
) is retained, the oxidation potentials are shifted significantly.
2
The anodic peak for Co(Lph
) and the center of the quasi-reversible
couple for Co(Lmes)2 are observ2ed at +350 and +395 mV, respectively.
A larger shift is observed for Co(Ldipp)2, with its CV profile exhibiting a
reversible couple at +190 mV (Fig. 5).
A comparison of the redox properties of the complexes in the two
solvents reveals that the metal center in all of the complexes readily
interact with THF, which is a better coordinating ligand than DCM. We
propose that Co(II)⋯THF interaction plays a vital role in the overall
electrochemical behavior of the complexes. The more open coordina-
tion environment in Co(Lph
)
and Co(Lmes
) allows for facile solvent
coordination resulting in more positive oxi2dation potentials and less
reversible electron transfer due to a readily available EC mechanism.
Qualitative magnetic susceptibility data were also measured for the
cobalt(II) β-ketoiminate complexes to examine the spin state of the
complexes. Effective magnetic moments of ca. 4.4 BM measured for Co
2
Fig. 3. UV–vis spectra in THF: Co(Lph
)
(
), Co(Lmes
)
(
), and Co
2
2
(Ldipp
)
(
).
2
(Lph
)
and Co(Lmes
)
consistent with high-spin d7 S = 3/2 configuration
2
expected for Co(II)2in a tetrahedral geometry [19]. Owing to the pre-
sence of a mixture geometrical isomers in the solid state, temperature
dependent magnetic susceptibility measurements were carried out for
in solutions. The NIR features observed for Co(Lmes
)
and Co(Ldipp
) are
comparable to those reported for high-spin coba2lt(II) β-ketoiminate
complexes at ca. 950 and 1,180 nm by Holm et al. [6], and to those re-
ported for bis(N-arylsalicylaldimino)cobalt(II) complexes [21], which also
contain Co(II) centers in pseudo-tetrahedral geometry. Therefore, the two
NIR peaks are attributed to 4A2(F) → 4T1(F) and 4A2(F) → 4T2(F), with
the low energy peak being assigned to the latter transition.
2
Co(Ldipp
) in the 5–300 K temperature range. As shown in Fig. 6, the
effective 2magnetic moment (μeff) is dependent on temperature and
varies from 4.38 to 2.95 B. M., and the data also reveal that three
We also observe that all of the complexes are sensitive to the pre-
sence of air. The UV–vis spectra measured in the presence of air show a
shift in the positions of the three peaks towards higher wavelengths. On
standing for several hours, the solutions react with air to yield a green
powder. Despite our best efforts the solids remain uncharacterized.
Cyclic voltammetric (CV) data were measured for THF and DCM
solutions of the complexes in order to gain insight into likely solvent
coordination. The peak potentials are given with respect to the ferro-
cene/ferrocinium, Fc0/+ couple. The CV profiles measured for Co(Lph
)
2
in THF contain an irreversible oxidation wave at +255 mV, while those
of Co(Lmes
)
and Co(Ldipp
) exhibit a quasi-reversible one-electron oxi-
2
2
dation at +320 mV and −70 mV respectively (Fig. 4). We attribute the
redox peaks to the Co(II) → Co(III) redox process. The shift of the Co2+/
3+ redox potentials to less positive values for Co(Ldipp)2 indicates that it
is less likely to get oxidized. We interpret Ldipp being a stronger field
ligand than Lph and Lmes, is capable of stabilizing the analogous Co(III)
complex.
Fig. 5. Cyclic Voltammograms for Co(Lph
)
(
), Co(Lmes
)
(
), and Co
The electrochemical study was repeated using the generally non-
coordinating DCM solvent. Although the general pattern of irreversible
2
(Ldipp)2, (
) in DCM solutions (1.0 mM) containing [nBu42N](ClO4) (0.4 M)
oxidation for Co(Lph
)
and quasi-reversible oxidation of Co(Lmes
) and
as the supporting electrolyte at a scan rate of 200 mV/s.
2
2
446

S. Debnath et al.
InorganicaChimicaActa486(2019)441–448
planned towards examining metal-substrate interactions involving sui-
table small molecules.
Acknowledgments
We thank the Department of Chemistry at the University of
Wyoming and the Univ. of Wyoming School of Energy Research
Graduate Assistantship to support this research. We also thank Prof.
Parkinson for use of the Lambda 950 spectrophotometer. Financial
supports by the NSF (CHE 0619920) for the purchase of the Bruker
Apex II Diffractometer and National Institute of General Medical
Sciences of the National Institutes of Health (from the Institutional
Development Award, Grant no. 2P20GM103432) for the purchase of
the Oxford Cryostream 800 cryogenic system are gratefully acknowl-
edged.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2018.10.051.
Fig. 6. Magnetic susceptibility plot of [Co(Ldipp)2].
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