Synthesis, magnetic properties, and phosphoesterase activity of dinuclear cobalt(II) complexes

Article abstract of DOI:10.1021/ic302418x

A series of dinuclear cobalt(II) complexes has been prepared and characterized to generate functional and spectroscopic models for cobalt(II) substituted phosphoesterase enzymes such as the potential bioremediator GpdQ. Reaction of ligands based on 2,2'-(((2-hydroxy-5-methyl-1,3-phenylene) bis(methylene))bis((pyridin-2-ylmethyl)azanediyl)))diethanol (L1) and 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (L2) with cobalt(II) salts afforded [Co₂(CO₂EtH ₂L1)(CH₃COO)₂](PF₆), [Co ₂(CO₂EtL2)(CH₃COO)₂](PF ₆), [Co₂(CH₃L2)(CH₃COO) ₂](PF₆), [Co₂(BrL2)(CH₃COO) ₂](PF₆), and [Co₂(NO₂L2)(CH ₃COO)₂](PF₆). Complexes of the L2 ligands contain a coordinated methyl-ether, whereas the L1 ligand contains a coordinated alcohol. The complexes were characterized using mass spectrometry, microanalysis, X-ray crystallography, UV-vis-NIR diffuse reflectance spectroscopy, IR absorption spectroscopy, solid state magnetic susceptibility measurements, and variable-temperature variable-field magnetic circular dichroism (VTVH MCD) spectroscopy. Susceptibility studies show that [Co ₂(CO₂EtH₂L1)(CH₃COO) ₂](PF₆), [Co₂(CO₂EtL2)(CH ₃COO)₂](PF₆), and [Co₂(CH ₃L2)(CH₃COO)₂](PF₆) are weakly antiferromagnetically coupled, whereas [Co₂(BrL2)(CH ₃COO)₂](PF₆) and [Co₂(NO ₂L2)(CH₃COO)₂](PF₆) are weakly ferromagnetically coupled. The susceptibility results are confirmed by the VTVH MCD studies. Density functional theory calculations revealed that magnetic exchange coupling occurs mainly through the phenolic oxygen bridge. Implications of geometry and ligand design on the magnetic exchange coupling will be discussed. Functional studies of the complexes with the substrate bis(2,4-dinitrophenyl) phosphate showed them to be active towards hydrolysis of phosphoester substrates.

Full text of DOI:10.1021/ic302418x

Article
pubs.acs.org/IC
Synthesis, Magnetic Properties, and Phosphoesterase Activity of
Dinuclear Cobalt(II) Complexes
Lena J. Daumann,† Peter Comba,‡ James A. Larrabee,∥ Gerhard Schenk,†,⊥ Robert Stranger,§
German Cavigliasso,§ and Lawrence R. Gahan*,†
†School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane QLD 4072, Australia
‡Anorganisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
§Research School of Chemistry, Australian National University, Canberra 0200, Australia
∥Department of Chemistry and Biochemistry, Middlebury College, Middlebury, Vermont 05753, United States
⊥Department of Chemistry, National University of IrelandMaynooth, Maynooth, Co. Kildare, Ireland
S
* Supporting Information
ABSTRACT: A series of dinuclear cobalt(II) complexes has been prepared and characterized to generate functional and
spectroscopic models for cobalt(II) substituted phosphoesterase enzymes such as the potential bioremediator GpdQ. Reaction of
ligands based on 2,2′-(((2-hydroxy-5-methyl-1,3-phenylene)bis(methylene))bis((pyridin-2-ylmethyl)azanediyl)))diethanol (L1)
and 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (L2) with cobalt(II) salts afforded
[Co2(CO2EtH2L1)(CH3COO)2](PF6), [Co2(CO2EtL2)(CH3COO)2](PF6), [Co2(CH3L2)(CH3COO)2](PF6), [Co2(BrL2)-
(CH3COO)2](PF6), and [Co2(NO2L2)(CH3COO)2](PF6). Complexes of the L2 ligands contain a coordinated methyl-ether,
whereas the L1 ligand contains a coordinated alcohol. The complexes were characterized using mass spectrometry, microanalysis,
X-ray crystallography, UV−vis−NIR diffuse reflectance spectroscopy, IR absorption spectroscopy, solid state magnetic
susceptibility measurements, and variable-temperature variable-field magnetic circular dichroism (VTVH MCD) spectroscopy.
Susceptibility studies show that [Co2(CO2EtH2L1)(CH3COO)2](PF6), [Co2(CO2EtL2)(CH3COO)2](PF6), and
[Co2(CH3L2)(CH3COO)2](PF6) are weakly antiferromagnetically coupled, whereas [Co2(BrL2)(CH3COO)2](PF6) and
[Co2(NO2L2)(CH3COO)2](PF6) are weakly ferromagnetically coupled. The susceptibility results are confirmed by the VTVH
MCD studies. Density functional theory calculations revealed that magnetic exchange coupling occurs mainly through the
phenolic oxygen bridge. Implications of geometry and ligand design on the magnetic exchange coupling will be discussed.
Functional studies of the complexes with the substrate bis(2,4-dinitrophenyl) phosphate showed them to be active towards
hydrolysis of phosphoester substrates.
scopic and magnetic properties of cobalt(II) mimics, compared
to those of cobalt(III) systems, are enhanced by the fact that
cobalt(II) is frequently used to reconstitute enzyme activity,
and that the vast majority of Co(II) substituted Zn(II) enzymes
are hyperactive compared to the Zn(II)-form.16 Of the available
probes for cobalt(II), magnetic susceptibility and magnetic
circular dichroism (MCD) spectroscopy have been shown to be
INTRODUCTION
■
The role of cobalt biomimetics in the elucidation of structure,
spectroscopy, and mechanism of metalloenzymes, particularly
in hydrolytic systems, is an active area of study. In early studies
cobalt(III) complexes with ammine, macrocyclic, and tripodal
amine ligands were employed as both structural and functional
models of metallophosphatases.1−7 The role of a coordinated
hydroxide ion in these cobalt(III) systems was also investigated
extensively.1−9 There are examples of cobalt(II) complexes as
phosphoesterase mimics.10−15 The advantages of the spectro-
Received: November 6, 2012
Published: February 1, 2013
© 2013 American Chemical Society
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Chart 1. Ligands Used To Mimic the Active Site Residues of GpdQ
within error as a 1:1 mixture of buffer and acetonitrile.31 Assays for pH
dependence were 40 μM in complex and 5 mM in BDNPP; for
substrate dependence they were 40 μM in complex and 1−11.5 mM in
BDNPP. pH-dependence data for monoprotic events were fit to the
following equation.32
useful to investigate the magnetic, geometric, and electronic
structure of biologically relevant metal complexes.17−20
Herein, we report a continuation of our previous studies with
zinc(II) and cadmium(II) biomimetic systems21−23 as models
of a glycerophosphodiesterase enzyme (GpdQ) from Entero-
bacter aerogenes.18,24−28 We report a series of cobalt(II)
complexes prepared from ligands based on the 2,2′-(((2-
hydroxy-5-methyl-1,3-phenylene)bis(methylene))bis((pyridin-
2-ylmethyl)azanediyl))diethanol29 and 2,6-bis(((2-
methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methyl-
phenol21 frameworks with a range of substituents (CO2Et-,
NO2-, Br-) replacing the CH3- para- to the phenolic −OH in
the parent ligands (Chart 1). The cobalt(II) complexes have
been structurally characterized with X-ray crystallography, and
the magnetic and spectroscopic properties have been
investigated using variable-temperature variable-field (VTVH)
MCD spectroscopy and solid-state magnetic susceptibility
measurements. Density functional theory (DFT) calculations
(broken symmetry approach) have been conducted to confirm
the interpretation of the magnetic coupling constants obtained.
In addition, functional studies have been undertaken with the
substrate bis(2,4-dinitrophenyl)phosphate.
Vmax
v0 =
[H+]
1 +
Ka
(1)
The data derived from substrate dependence were fit to the
Michaelis−Menten equation.32
V
max[S]
v =
Km + [S]
(2)
Here, v is the initial rate, Vmax is the maximum rate, Km is the
Michaelis constant, and [S] is the substrate concentration. Complex
dependence was measured with a fixed substrate concentration at 5
mM and complex concentrations ranging from 20 to 120 μM.
Background assays were conducted by measuring the autohydrolysis
and hydrolysis by 2 equiv of cobalt(II) acetate and were subtracted
from the data.
Computational Methods. All calculations were performed with
the Amsterdam Density Functional (ADF) package,33−35and the
results reported were obtained using the 2010 version. Functionals
based on the Generalized Gradient Approximation (GGA)36,37and
hybrid methodology,38−40 and basis sets of the all-electron type,41
were used. In the present work we have carried out full geometry
optimizations using pure density functional theory, and we have also
performed additional calculations using hybrid methods. Relativistic
effects were included by means of the Zero Order Regular
Approximation (ZORA)42−44 and the Conductor-like Screening
Model (COSMO) was used for the treatment of solvation effects,45
with ethanol as the solvent.
Crystallographic Measurements. Crystallographic data
for the complexes were collected unless otherwise stated at
293(2) K with an Oxford Diffraction Gemini Ultra dual source
(Mo and Cu) CCD diffractometer with Mo (λKα = 0.710 73 Å)
or Cu (λKα = 1.5418 Å) radiation. The structures were solved
by direct methods (SIR-92) and refined (SHELXL 97) by full
matrix least-squares methods based on F2.46 These programs
were accessed through the WINGX 1.70.01 crystallographic
collective package.47 All non-hydrogen atoms were refined
anisotropically unless they were disordered. Hydrogen atoms
were fixed geometrically and were not refined. X-ray data of the
published structures were deposited with the Cambridge
Crystallographic Data Centre, CCDC903307−903310.
Magnetic Susceptibility Measurements. Solid state
susceptibility data were collected with a MPMS-XL 5T
superconducting quantum interference device (SQUID) from
Quantum Design at the University of Heidelberg, Germany,
MATERIALS AND METHODS
■
General Methods. Positive-ion electrospray mass spectrometry
was carried out with a Q-Star time-of-flight mass spectrometer, and the
data were processed with Bruker Compass Data Analysis 4.0 software.
FT-IR spectroscopy was carried out with a Perkin-Elmer FT-IR
spectrometer SPECTRUM 2000 with a Smiths DuraSamplIR II ATR
diamond window. Elemental microanalyses (C, H, N) were performed
with a Carlo Erba elemental analyzer, model NA1500, by Mr. George
Blazak at the University of Queensland. Phosphatase activity
measurements were conducted with bis(2,4-dinitrophenol)phosphate
(BDNPP) as substrate using a Varian Cary50 Bio UV−vis
spectrophotometer with a Peltier temperature controller and 10-mm
quartz cuvettes. The initial-rate method was employed, and assays
were measured such that the initial linear portion of the data was used
for analysis. Product formation was determined at 25 °C by
monitoring the formation of 2,4-dinitrophenol. Throughout the pH
range studied (4.75−11), the extinction coefficient of this product at
400 nm is 12 100 M−1 cm−1.30 All assays were measured in 50/50
acetonitrile/buffer with the substrate and complex initially dissolved in
acetonitrile. The aqueous multicomponent buffer was composed of 50
mM 2-(N-morpholino)ethanesulfonic acid (MES, pH 5.50−6.50), 4-
(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES, pH 7.00−
8.50), 2-(N-cyclohexylamino)ethane sulfonic acid (CHES, pH 9.00−
10.00), and N-cyclohexyl-3-aminopropanesulfonic acid (CAPS, pH
10.5−11) with controlled ionic strength (LiClO4) 250 mM. The pH
values reported are those of the aqueous component; it should
however be noted that the pH of a solution of the buffer was the same
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and recorded as a function of the applied field (0−5 T), and at
temperatures ranging from 2 to 300 K (zero field cooled
method). The powdered samples were pressed into a PTFE
band to avoid field-induced orientation of the powder and
incorporated into two plastic straws as sample holder. The data
were corrected for the diamagnetism of the PTFE band and
sample holder; Pascal constant corrections for each sample
were applied.48 The program MagSaki was used for the analysis
of magnetic susceptibility data.49
Diffuse Reflectance Spectroscopy. Spectra were meas-
ured at Middlebury College, VT, using a Varian Cary 6000i and
a Purged Praying Mantis Diffuse Reflection Attachment
(Harrick). Diffuse reflectance spectra were measured with
magnesium oxide as blank and carrier material. The finely
powdered samples were mixed (∼1:1) with magnesium oxide
and placed in the sample holder.
bromo-2,6-bis(hydroxymethyl)phenol, 2,6-bis(chloromethyl)-
4-methylphenol, 2-methoxy-N-(pyridin-2-ylmethyl)-
aminoethanol, N-(2-pyridylmethyl)-2-aminoethanol, ethyl-4-
hydroxy-3,5-bis(((2-hydroxyethyl)(pyridin-2-ylmethyl)amino)-
methyl)benzoate (CO2EtH3L1), ethyl-4-hydroxy-3,5-bis(((2-
methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate
(CO2EtHL2), 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)-
amino)methyl)-4-methylphenol (CH3HL2), 4-bromo-2,6-bis-
(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)phenol
(BrHL2), and 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)-
amino)methyl)-4-nitrophenol (NO2HL2) were synthesized as
described previously.21−23
Synthesis of [Co2(CO2EtH2L1)(CH3COO)2](PF6). A meth-
anol solution of CO2EtH3L1 (0.1 M, 1 mL) was combined with
a solution of cobalt(II) acetate tetrahydrate (0.1 M, 2 mL) in
methanol. Subsequently, solid sodium hexafluorophosphate (50
mg, 0.3 mmol) was added. The pink solution was filtered and
left to evaporate in air for 3 days to yield pink needles that were
suitable for X-ray crystallography (88% yield). Microanalysis
C31H39Co2N4O9PF6 requires: C, 42.58; H, 4.50; N, 6.41%.
Found: C, 42.16; H, 4.40; N, 6.39%. ESI mass spectrometry
(methanol) m/z: 729.2, [C31H29Co2N4O9]+; 669.1,
[C29H35Co2N4O7]+; 609.1, [C27H31Co2N4O5]+; 305.1,
[C27H31Co2N4O5]2+. For (acetonitrile) m/z: 729.0
[C31H29Co2N4O9]+; 669.1, [C29H35Co2N4O7]+. FT-IR spec-
troscopy (v, cm−1) 1605.0 (m, CO asym str, acetate); 1442.0
(s, CO sym str, acetate); 1022.4 (m, O−H); 832.9 (s, P−F
str); 767.0 (m, Py-H def); 555.43 (m, P−F). UV−vis
spectroscopy (CH3CN; 0.02 M) λmax = 493 nm (ε = 63.2 L
mol−1 cm−1). Initially the complex syntheses were conducted
under inert atmosphere; however, the complexes were found to
be stable in air, and syntheses undertaken under aerobic
conditions produced the identical product. Therefore, the
syntheses of all cobalt(II) complexes were achieved under
aerobic conditions following the same general procedure.
Synthesis of [Co2(CO2EtL2)(CH3COO)2](PF6). There was
70% yield. Microanalysis C33H43Co2N4O9PF6 requires: C,
43.91; H 4.80; N, 6.21%. Found: C, 43.21; H, 4.61; N,
6.10%. ESI mass spectrometry (methanol) m/z: 757.36,
[C33H43Co2N4O9]+; 729.37, [C32H43Co2N4O8]+. For (acetoni-
trile) m/z: 757.1, [C33H43Co2N4O9]+. FT-IR spectroscopy (v,
cm−1) 1601.0 (m, CO asym str, acetate); 1421.0 (s, CO
sym str, acetate); 830.7 (s, P−F str); 762.4 (m, Py-H def);
555.6 (m, P−F). UV−vis spectroscopy (CH3CN, 0.02 M) λmax
= 508 nm (ε = 73.5 L mol−1 cm−1).
Magnetic Circular Dichroism (MCD). MCD measure-
ments were conducted at Middlebury College, VT. Complex
samples were both measured as solid mulls (poly-
(dimethylsiloxane)) and as saturated ethanol solutions. The
MCD system used has a JASCO J815 spectropolarimeter and
an Oxford Instruments SM4000 cryostat/magnet. Data were
collected at increments of 0.5 Tesla (T) from 0 to 7.0 T and at
temperatures of 1.4, 4.2, 11.3, 26, and 50 K. Each spectrum was
corrected for any natural CD by subtracting the zero-field
spectrum of the sample. Even when no sample is present, the
instrument baseline exhibits a small deviation from zero that is
both field- and wavelength-dependent. Therefore, each
spectrum was also corrected by subtracting a spectrum
recorded at the same magnetic field but with no sample
present. The MCD spectra were fitted to the minimum number
of Gaussian peaks using GramsAI 8.0 software after converting
the spectra to wavenumber units.50 In the fitting process the
bandwidths of the d−d transitions were restrained to lie
between 600 and 1000 cm−1, and a minimum of Gaussians
were fitted to achieve a satisfactory composite spectrum. The
fitting of the VTVH MCD data was achieved using a locally
written Fortran program, VTVH 2.1.1.51 The spin Hamiltonian
and additional details of the fitting program have been
described previously.19 The g values obtained by the SQUID
measurements were used and fixed in fitting of the VTVH
MCD data. In general, the initial values for D and J were those
obtained from the SQUID measurements. In a general run,
those values were fixed at first and then allowed to float one
after the other to achieve a satisfactory fit. The fits were also
tested for robustness once a complete set of parameters had
been obtained. To do this, the initial parameters were set to the
best fit parameters and then all allowed to float. Subsequently,
one key parameter such as J, D, Mxy, Mxz, Myz was chosen, its
initial value was set differently, and the fit process was repeated.
The rhombic zero field splitting parameter E was shown to have
no effect on the VTVH fits, so E/D was set to zero in all fits
(general convention 0 ≤ E/D ≤ 1/3). Percent polarization for a
given fitted band was calculated from Mxy, Myz, and Mxz using %
Mx = (Mxy·Mxz)2/[(Mxy·Mxz)2 + (Mxy·Myz)2 + (Mxz·Myz)2]. My
and Mz were calculated correspondingly.52 Finally, the VTVH
MCD data fitting program used the Hamiltonian H = −2JS1S2,
so the J values from VTVH MCD data fitting were converted to
be comparable with those obtained by DFT calculations and
magnetic susceptibility measurements.
Synthesis of [Co2(CH3L2)(CH3COO)2](PF6). There was
71% yield. Microanalysis C31H41Co2N4O7PF6 requires: C,
44.09; H, 4.89; N, 6.63%. Found: C, 44.09; H, 4.96; N,
6.58%. ESI mass spectrometry (methanol) m/z: 699.2,
[C31H41Co2N4O7]+. For (acetonitrile) m/z: 699.1,
[C31H41Co2N4O7]+. FT-IR spectroscopy (ν, cm−1): 2925.1
(w, C−H str), 1595.6 (m, CO asym str, acetate); 1474.7 (m,
CO sym str, acetate); 1422.9 (m, C−H def); 1085.1 (w, C−
O str), 829.2 (s P−F str), 555.3 (s, P−F str). UV−vis
spectroscopy (CH3CN, 0.02 M) λmax = 471 nm (ε = 46.2 L
mol−1 cm−1), λmax = 519 nm (ε = 50.5 L mol−1 cm−1).
Synthesis of [Co2(BrL2)(CH3COO)2](PF6). There was 32%
yield. Microanalysis C30H38Co2BrN4O7PF6 requires: C, 39.62;
H, 4.21; N, 6.16%. Found: C, 39.35; H, 4.20; N, 6.18%. ESI
mass spectrometry (methanol) m/z: 734. 99,
[C29H38BrCo2N4O6]+. For (acetonitrile) m/z: 765.0
[C30H38BrCo2N4O7]+. FT-IR spectroscopy (v, cm−1) 2930.7
(m, C−H); 1599.0 (m, CO asym str, acetate); 1421.3 (s,
Ligand Syntheses. Ethyl-4-hydroxy-3,5-bis-
(hydroxymethyl)benzoate, ethyl-3,5-bis(bromomethyl)-4-hy-
droxybenzoate, 2,6-bis(bromomethyl)-4-nitrophenol, 4-
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Figure 1. Structures of (a) [Co2(CO2EtH2L1)(CH3COO)2](PF6), (b) [Co2(CO2EtL2)(CH3COO)2](PF6), (c) [Co2(CH3L2)(CH3COO)2](PF6),
and (d) [Co2(BrL2)(CH3COO)2](PF6) and views of the respective first coordination spheres. Counter ions and hydrogen atoms have been omitted
for clarity (25% ellipsoid probability in all ORTEP plots).
CO sym str, acetate); 831.2 (s, P−F str); 758.8 (m, Py-H
def); 555.9 (m, P−F). UV−vis spectroscopy (CH3CN, 0.01 M)
λmax = 460 nm (ε = 4.0 L mol−1 cm−1), λmax = 509 nm (ε = 4.1
L mol−1 cm−1).
Synthesis of [Co2(NO2L2)(CH3COO)2](PF6).H2O. There
was 66% yield. Microanalysis C30H40Co2N5O10PF6 requires: C,
40.33; H, 4.51; N, 7.84%. Found: C, 40.41; H, 4.42; N, 7.92%.
ESI mass spectrometry (methanol) m/z: 784.1,
[C30H44Co2N5O12]+. For (acetonitrile) m/z: 730.1
[C30H38Co2N5O9]+. FT-IR spectroscopy (ν, cm−1): 2932.7
(w, C−H str); 1594.6 (s, acetate asym str); 1505.7 (w, NO2
asym str); 1428.6 (s, acetate sym str); 1317.2 (s, NO2 sym str);
1085.9 (m, C−O str); 829.1 (s, P−F str); 752.3 (m, py C−H
def); 657.8 (w, Ar−H def); 555.5 (s, P−F). UV−vis
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spectroscopy (CH3CN, 0.02 M) λmax = 466 nm (ε = 77.7 L
(Co(1)−O(2) 2.235(3), 2.294(3), 2.223(8) Å; Co(2)−O(3)
2.221(3), 2.261(2), 2.266(9) Å), respectively, in addition to the
bridging acetates for all complexes. The six coordinate
geometry is completed by the bridging oxygen from the
phenolate (Co(1)−O(1) 1.992(8), 2.016(3), 2.014(2),
2.014(7) Å; Co(2)−O(1) 2.041(8), 2.044(3), 2.0026(19),
2.009(7) Å) with Co(1)−O(1)−Co(2) 114.6(3)°,
112.25(14)°, 112.81(10)°, and 113.4(3)° and Co(1)−Co(2)
distances of 3.394, 3.372, 3.346, and 3.363 Å, respectively. For
the complexes with Co(II)−OR ether donors, the bond lengths
are in general longer (av 2.253 Å) than for the complexes with
Co(II)−O(alkoxide) donors (av 2.195 Å). This finding is in
accord with previous studies using a similar complex
[Co2(bomp)(CH3COO)2](BPh4) (Hbomp = 2,6-bis(bis(2-
methoxyethyl)aminomethyl)-4-methylphenol), with Co(II)−
OR ether donor bonds (av 2.186 Å).55 For the analogous
complex [Co2(bhmp)(CH3COO)2](BPh4) (Hbhmp = 2,6bis-
(bis(2-hydroxyethyl)aminoethyl)aminomethyl)-4-methylphe-
nol), the Co(II)−O alcohol donor bond lengths (av 2.120 Å)
are similar to those found for [Co2(CO2EtH2L1)-
(CH3COO)2](PF6) (av 2.146 Å).56 A significant distortion of
the coordination sphere around the cobalt centers is apparent
in all structures, with the bond distances to the six donor atoms
in, for example, [Co2(CO2EtH2L1)(CH3COO)2](PF6) ranging
from 1.989 Å (Co(1)−O(1)) to 2.187 Å (Co(1)−O(6)). The
Co(II)−Npy distances are similar to those reported for the
[Co2(L2)(CH3COO)2(MeCN)2](BPh4) (HL2 = 2,6-bis(3-
pyridin-2-yl)pyrazole-1-ylmethyl]-4-methylphenol) (2.156(5)
and 2.178(5) Å).15 The Co···Co separations are similar to
those found for [Co2(bhmp)(CH3COO)2](BPh4) and
[Co2(bomp)(CH3COO)2](BPh4) (3.356 and 3.336 Å, respec-
tively) and for [Co2(L2)(CH3COO)2(MeCN)2](BPh4)
(3.378(5) Å).15,55,56 The Co(1)−O(1)−Co(2) angles, span-
ning the phenoxo bridge, do not vary significantly
(112.25(14)−114.6(3)°).
A crystal structure of the complex with the ligand NO2HL2
could not be obtained despite several attempts. The structure of
the complex was subsequently determined from DFT
calculations (vide infra). The starting parameters for the
calculation were those from [Co2(CH3L2)(CH3COO)2](PF6).
Geometry optimizations were also conducted for all structures
using the Amsterdam Density Functional program and the
Becke−Perdew functional.35 Interestingly, the calculated
separation of the cobalt atoms in the complex [Co2(NO2L2)-
(CH3COO)2](PF6) (3.455 Å) differs significantly from that in
the other structures. This value is, however, still in the range
reported for the triply bridged μ-oxo-bis(μ2-CH3COO-
κ2O:O′)cobalt(II) cores.20 In general, calculated bond lengths
and angles are in good agreement with the experimental values
(Table S2).
mol−1 cm−1).
RESULTS AND DISCUSSION
■
Ligand Nomenclature and Syntheses. The syntheses of
the ligands in this study and the nomenclature employed have
been described previously and are discussed only briefly here
(ligands are shown in Chart 1).21−23 CO2EtH3L1 denotes that
the ligand has an ethyl ester (CO2Et-) at the position para- to
the phenolic oxygen and that the ligand has potentially three
sites for deprotonation, the phenol and two pendant alcohol
donors. Two pyridines and two alcohol arms make up the L1
donor atom set. The second ligand class, L2, offers a direct
comparison of methyl ether donors with the alkoxide donor in
CO2EtH3L1 and denotes symmetric ligands with two pendant
pyridines and two methyl ethers. As only one proton can be
potentially subtracted from the L2 ligands upon deprotonation,
the nomenclature is adjusted accordingly. The cobalt(II)
complexes were synthesized by adding 2 equiv of cobalt(II)
acetate to a methanol solution of the respective ligand
(CO2EtH3L1, CO2EtHL2, CH3HL2, NO2HL2, and BrHL2),
followed by addition of solid sodium hexafluorophosphate.
Crystal Structures. The complexes [Co2(CO2EtH2L1)-
(CH3COO)2](PF6), [Co2(CO2EtL2)(CH3COO)2](PF6),
[Co2(CH3L2)(CH3COO)2](PF6), and [Co2(BrL2)-
(CH3COO)2](PF6) were characterized structurally. Selected
crystallographic data are shown in Table S1; selected bond
lengths and angles are displayed in Table S2. ORTEP plots are
shown in Figure 1.53 The structures comprise the ligand
monoanion, two cobalt(II) ions, and two bridging acetates
completing the hexacoordinate coordination sphere, with the
charge on each complex balanced by a hexafluorophosphate
−
ion. In each case there was disorder around the PF6 anion; in
addition, [Co2(CO2EtL2)(CH3COO)2](PF6) was found to
have significant disorder around the ethyl ester, so the ethyl
group was refined in two separate positions. The complexes
crystallized in different isomeric forms. Considering that the
phenol ring backbone and the two Co(II) ions are in the plane
of the page, the ligand arms can be either “in front” of or
“behind” the plane. For [Co2(CO2EtH2L1)(CH3COO)2](PF6)
the pyridine nitrogen donors are both syn54 to the phenolic O
atom, with pseudo-C2 symmetry; one pyridine nitrogen donor
is in front of, and one is behind, the plane. For
[Co2(CO2EtH2L2)(CH3COO)2](PF6) one pyridine nitrogen
donor is syn and the other is anti with respect to the phenolic O
atom. Both [Co2(CH3L2)(CH3COO)2](PF6) and
[Co2(BrL2)(CH3COO)2](PF6) employ the same coordination
geometry as found in the respective zinc complexes, with the
pyridine arms anti, and an overall pseudo-C2 symmetry.54 The
overall charge on the complex and the lengths of the metal-to-
ligand oxygen donor bonds for [Co2(CO2EtH2L1)-
(CH3COO)2](PF6) suggest that the alcohol donors remain
protonated. For [Co2(CO2EtH2L1)(CH3COO)2](PF6),
[Co2(CO2EtL2)(CH3COO)2](PF6), [Co2(CH3L2)-
(CH3COO)2](PF6), and [Co2(BrL2)(CH3COO)2](PF6), the
coordination environment of both metal ions is composed of
the tertiary amine (Co(1)−N(1) 2.168(9), 2.175(4), 2.161(2),
2.172(9) Å; Co(2)−N(3) 2.120(10), 2.154(3), 2.149(3),
2.142(8) Å), the pyridine nitrogen (Co(1)−N(2) 2.145(10),
2.114(4), 2.142(3), 2.111(9) Å; Co(2)−N(4) 2.070(11),
2.182(4), 2.118(2), 2.113(9) Å), the ether oxygens, for
[Co2(CO2EtL2)(CH3COO)2](PF6), [Co2(CH3L2)-
(CH3COO)2](PF6), and [Co2(BrL2)(CH3COO)2](PF6)
Infrared Spectra. The infrared spectra of all complexes
showed a shift of the asymmetric and symmetric acetate stretch
to higher frequencies with respect to free acetate (νasym = 1578
cm−1, νsym = 1414 cm−1) indicating a bridging bidentate
binding mode for the acetate ligands ([Co2(CO2EtH2L1)-
(CH3COO)2](PF6), [Co2(CO2EtL2)(CH3COO)2](PF6),
[Co2(CH3L2)(CH3COO)2](PF6), [Co2(BrL2)(CH3COO)2]-
(PF6), and [Co2(NO2L2)(CH3COO)2](PF6) νasym = 1605,
1601, 1596, 1599, and 1595 cm−1 and νsym = 1442, 1421, 1424,
1421, and 1429 cm−1, respectively). The magnitude of the
separation Δνasym‑sym confirms a symmetric bidentate coordi-
nation for all cobalt complexes in the solid state.57 The
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hexafluorophosphate stretch and deformation bands were
found in all spectra around 830 and 555 cm−1, respectively.
Mass Spectral Characterization. Mass spectra were
recorded in methanol and acetonitrile solution for all
complexes. The spectra measured in methanol revealed the
presence of a negatively charged ligand, two cobalt(II) ions, and
two acetate ligands for all complexes; fragmentation involving
partial loss of the acetate ligands and coordination of methanol
solvent molecules was also observed. The mass spectrum of
[Co2(CO2EtH2L1)(CH3COO)2](PF6) showed a number of
peaks (m/z 729.16, [Co2(CO2EtH2L1)(CH3COO)2]+ calcd
m/z 729.14; m/z 669.1, [Co2(CO2EtHL1)(CH3COO)]+ calcd
m/z 669.12; m/z 609.08, [Co2(CO2EtL1)]+ calcd m/z 609.10)
in addition to a doubly charged ion (m/z 305.05,
[Co2(CO2EtHL1)]2+ calcd m/z 305.05). The mass spectrum
of [Co2(CO2EtL2)(CH3COO)2](PF6) showed two fragments
with m/z 757.36 and m/z 729.37 assigned to [Co2(CO2EtL2)-
(CH3COO)2]+ (calcd m/z 757.17) and [Co2(CO2EtL2)-
(CH3COO)(CH3O)]+ (calcd m/z 729.17). The spectrum of
[Co2(CH3L2)(CH3COO)2](PF6) indicated the formation of
[Co2(CH3L2)(CH3COO)2]+ (found m/z 699.17; calcd m/z
699.16), whereas that of [Co2(BrL2)(CH3COO)2](PF6)
showed a species at m/z 734.99, attributed to [Co2(BrL2)-
(CH3COO)(CH3O)]+ (calcd m/z 735.06). The mass spectrum
of one complex suggested the presence of water
([Co2(NO2L2)(CH3COO)2(H2O)3]+, m/z 784.1; calcd m/z
784.17). The spectra recorded in acetonitrile are shown for all
complexes in the Supporting Information (S1−S5). The mass
spectrum of [Co2(CO2EtH2L1)(CH3COO)2](PF6) showed
similar peaks (m/z = 729.0, 669.1) as those observed in the
methanol spectrum in addition to a new doubly charged ion
(m/z 335.1, calcd m/z 335.06 [Co2(CO2EtH2L1)-
(CH3COO)]2+). For [Co2(CO2EtL2)(CH3COO)2](PF6)
peaks were found at m/z 757.1 and 349.0 (calcd m/z 757.17
and 349.08 for [Co2(CO2EtL2)(CH3COO)2]+ and
[Co2(CO2EtL2)(CH3COO)]2+, respectively). Similar species
were also found for [Co2(CH3L2)(CH3COO)2](PF6) (m/z
699.1 and 320.0, calcd m/z 699.16 and 320.07 for
[Co2 (CH3 L2)(CH3 COO)2 ]+ and [Co2 (CH3 L2)-
(CH3COO)]2+, respectively) and [Co2(BrL2)(CH3COO)2]-
(PF6) (m/z 765.0 and 352.9, calcd m/z 763.06 (100%), 765.06
(99.2%) and 352.02 (100%), 353.02 (97.3%) for [Co2(BrL2)-
(CH3COO)2]+ and [Co2(BrL2)(CH3COO)]2+). The latter
complex also showed a species at m/z 847.1, which is proposed
to arise from additional coordination of two acetonitrile
molecules (calcd m/z 845.11 (100%), 847.11 (98.1%)
[Co2(BrL2)(CH3COO)2(MeCN)2]+. The complex
[Co2(NO2L2)(CH3COO)2](PF6) showed one distinct peak
at m/z 730.1 (calcd m/z 730.13, [Co2(NO2L2)-
(CH3COO)2]+).
Figure 2. Plot of χMT versus T for powdered samples of the complexes
●
([Co2(CO2EtH2L1)(CH3COO)2](PF6) =
, [Co2(CO2EtL2)-
□
▲
(CH3COO)2](PF6) = , [Co2(CH3L2)(CH3COO)2](PF6) =
,
⧫
[Co2(BrL2)(CH3COO)2](PF6) = , [Co2(NO2L2)(CH3COO)2]-
(PF6) = Δ). The solid lines represent the best fit obtained with
MagSaki software. Calculated parameters are shown in Table 3. Inset:
low temperature data for the complexes.
3
high spin d7 ion (S = /2, 3.87 μB). Including the term for
orbital momentum (L = 3) results in an expected magnetic
moment of μeff/Co 5.20 μB, which is close to those observed
and suggests some quenching of the magnetic moment. All
complexes showed significant temperature dependence of the
magnetic moment, although upon cooling the behavior of the
complexes differs. For [Co2(BrL2)(CH3COO)2](PF6) and
[Co2(NO2L2)(CH3COO)2](PF6) the magnitude of χMT
decreases smoothly until approximately 12 K and then rises,
whereas for [Co2 (CO2 EtL2)(CH3 COO)2 ](PF6 ),
[Co2(CH3L2)(CH3COO)2](PF6), and [Co2(CO2EtH2L1)-
(CH3COO)2](PF6) χMT decreases smoothly to 2 K with
values from 1.55 to 3.09 cm3 mol−1 K, μeff/Co 2.49−3.51 μB.
The decrease in the magnitude of χMT with decreasing
temperature for these types of complexes has been ascribed
to the effect of three factors: the contribution of the orbital
angular momentum, intramolecular ferromagnetic coupling,
and intermolecular antiferromagnetic coupling.15,56 The
susceptibility data were analyzed using software (MagSaki)
that takes the three factors listed above into account and
calculates the magnetic susceptibility considering axial dis-
tortion, spin−orbit coupling, and anisotropic exchange
interaction of two axially distorted octahedral Co(II)
centers.49,60,64 This approach uses the axial-splitting parameter
Δ (the splitting of the orbital degeneracy of the 4T1 term by the
asymmetric ligand component), an orbital reduction factor κ,
the spin−orbit coupling parameter λ, and the magnetic
exchange coupling parameter J.49,60 These variables allow the
axial zero field splitting (ZFS) parameter D and anisotropic g-
factors to be calculated; the resulting parameters are displayed
in Table 1 along with data for similar complexes.56 Figure 2
shows the experimental data and the fits to the data obtained at
0.5 T. The orbital reduction factor κ for the cobalt(II)
complexes is typical for high spin cobalt(II) ions.62 The κ value
for some of the complexes approaches the free ion value
(∼0.93), and a similar observation is made for the
[Co2(bhmp)(CH3COO)2](BPh4) and [Co2(bomp)-
(CH3COO)2](BPh4) complexes.56 The values of λ are smaller
than that reported for the free ion (λo = −180 cm−1), but in the
range reported for other cobalt(II) complexes; the difference is
Magnetic Susceptibility Measurements. High-spin
4
octahedral cobalt(II) has a T1g ground state. The magnetic
moments of complexes with T ground terms are often found to
show considerable temperature dependence, and interpreting
them is difficult because the occurring angular orbital
momentum is partially quenched.49,55,58−63 Due to large
spin−orbit coupling, these systems exhibit substantial zero
field splitting, which causes high magnetic anisotropy.
In the solid state the susceptibility data were acquired in the
range from 2 to 300 K. Plots of the χMT versus T are illustrated
in Figure 2. At 300 K the values of χMT range from 5.49 to 6.29
cm3 mol−1 K (for the dicobalt(II) complexes) with μeff/Co =
4.68−5.01 μB, larger than the spin-only value expected for a
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Table 1. Magnetic Data for [Co2(CO2EtH2L1)(CH3COO)2](PF6), [Co2(CO2EtL2)(CH3COO)2](PF6),
[Co2(CH3L2)(CH3COO)2](PF6), [Co2(BrL2)(CH3COO)2](PF6), and [Co2(NO2L2)(CH3COO)2](PF6) in Addition to
[Co2(bhmp)(RCOO)2](BPh4) and [Co2(bomp)(RCOO)2](BPh4)55,64
d
a
b
field
(Tesla)
J
λ
Rχ ( ×
Rμ ( ×
c
complex
(cm−1
)
(cm−1
)
κ
ν
Δ
gz
gx
D (cm−1
)
10−3
)
10−4
)
[Co2(CO2EtH2L1)(CH3COO)2](PF6)
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
0.5
0.5
0.5
0.5
−0.66
−0.66
−0.08
−0.10
−0.67
−0.68
+0.42
+0.38
+3.13
+3.19
−0.30
−0.21
−0.38
−0.67
−121
−118
−125
−122
−112
−108
−173
−173
−102
−102
−122
−99
0.72
0.70
0.93
0.93
0.86
0.83
0.75
0.75
0.84
0.84
0.80
0.93
0.93
0.84
−4.4
−3.3
−5.6
−5.6
−5.0
−4.9
−4.4
−5.0
−2.4
−2.3
−6.9
−6.0
−5.0
−3.9
383
347
651
635
482
444
571
649
206
197
673
552
581
461
2.3
4.7
98
2.2
0.52
0.98
0.37
0.57
0.36
0.56
2.4
2.3
4.7
96
1.9
[Co2(CO2EtL2)(CH3COO)2](PF6)
[Co2(CH3L2)(CH3COO)2](PF6)
[Co2(NO2L2)(CH3COO)2](PF6)
[Co2(BrL2)(CH3COO)2](PF6)
2.2
4.9
108
106
98
0.61
0.67
1.3
2.2
4.9
2.2
4.9
2.2
4.8
94
1.2
2.3
4.7
146
133
135
137
74
5.0
2.2
4.7
4.3
3.3
2.9
4.7
0.26
0.26
0.82
0.82
2.9
4.7
[Co2(bhmp)(CH3COO)2](BPh4)56,64
[Co2(bhmp)(C6H5COO)2](PPh4)56,64
[Co2(bomp)(CH3COO)2](BPh4)55,94
2.08
2.15
2.25
2.42
4.74
4.92
4.96
4.85
0.033
0.20
0.11
1.6
0.035
0.15
0.91
1.5
69
−125
−141
120
144
[Co2(bomp)(C6H5COO)2](PPh4)55,64
a
b
c
d
Rχ = Σ(χAcalc − χAobs)2/Σ(χAobc)2. Rμ = Σ(χμeffcalc − χμeffobs)2/Σ(χμeffobs)2.64 Defined as ν = Δ/(κλ).56,64 H = −JexS1·S2.
ascribed to covalency effects.55,56,62,65−67 The ZFS parameter D
is typical for six coordinate cobalt(II) complexes and ranges
from +95 to +146 cm−1.55,56,62,65−67 The positive values for the
axial splitting parameter Δ (205−651 cm−1) suggest that the
axis is elongated and that the orbital singlet is lowest in
energy.68−70 The magnitude of J (H = -JexS1·S2) for
[Co2(CO2EtH2L1)(CH3COO)2](PF6), [Co2(CO2EtL2)-
(CH3COO)2](PF6), [Co2(CH3L2)(CH3COO)2](PF6), and
[Co2(NO2L2)(CH3COO)2](PF6) (J = −0.66, −0.10, −0.67,
and +0.42 cm−1 (0.5 T), respectively) suggests that the
intramolecular coupling between the two cobalt(II) sites is
extremely weak and is similar to those determined for
[Co2(bhmp)(CH3COO)2](BPh4) and [Co2(bomp)-
(CH3COO)2](BPh4) (J = −0.33 to −0.70 cm−1), complexes
with a similar ligand μ-phenoxo-bis(μ-acetato) core.56 For the
[Co2(BrL2)(CH3COO)2](PF6) complex (J = +3.13 cm−1), the
situation is less clear. No obvious correlation exists between the
magnitude or sign of J and the inductive effects of the
differently para (NO2, Br, CO2Et, and CH3) substituted
complexes.71
that seen with the μ-O(phenoxo) analogue. The bis(μ2-RCOO-
κ2O:O′);μ2-O;κ2O,O′-CH3COO core appears to promote
ferromagnetic coupling, and the μ-bipym bridge leads to
antiferromagnetic coupling. The relationship between Co(II)−
X bond distances and the magnitude and sign of J (Figure S6a)
is extremely weak, and the Co(II)···Co(II) distance also has no
bearing on the coupling (Figure S6b). The extent of distortion
around the Co(II) center appears to have little influence;
neither the ferromagnetically coupled [Co2 (μ-
CH3COO)3(urea)(tmen)2](OTf) (Δ = +80 cm−1)20,74 nor
the antiferromagnetically coupled [Co2(tidf)(ClO4)2(H2O)2]69
(Δ = +8.74 cm−1) complexes are substantially distorted from
octahedral geometry around the Co(II) sites. Of the structural
parameters considered, the Co(II)−X−Co(II) bridge angles
appear to have some influence on the sign and magnitude of J
(Figure S6c). This observation agrees with those in earlier
studies that suggested the importance of the Co−X−Co
angle,20,58,67,73 but unlike coupled dicopper(II)75,76 and
dinickel(II)77 complexes, the case for a structural relationship
for dicobalt(II) complexes is not well resolved.
Attempts have been made to correlate structural parameters
with strength of coupling for dicobalt(II) complexes.20,58,72
Previous suggestions include that the strength of the coupling
varies according to bridging type (O2− > OH− > H2O),72 that
Co(II)−O−Co(II) bond angles around 96° in certain
complexes give rise to ferromagnetic coupling through
orthogonal magnetic orbitals,20,67,73 and that bis(μ2-syn,syn-
CH3COO-κ2O:O′) bond angles are important.15 Tomkowicz et
al.20 reviewed structural parameters for dicobalt(II) complexes
with respect to the magnitude of the observed magnetic
coupling. One conclusion was that, for complexes with the μ-
UV−vis Spectroscopy. UV−vis spectra were recorded for
all complexes in solvents employed in the kinetic and MCD
studies (acetonitrile and ethanol, respectively). The diffuse
reflectance spectra of the complexes as powders were collected
as well, and these spectra are shown in Figure S7. The observed
transitions for the complexes are listed in Table S3. For all
complexes, bands typical for six-coordinate Co(II) were present
in solution.56,78 The UV−vis spectra of [Co2(CO2EtH2L1)-
(CH3COO)2](PF6) and [Co2(CO2EtL2)(CH3COO)2](PF6)
in acetonitrile were analyzed. Three transitions were apparent
at around 1250, 625, and 450 nm. Gaussian curve analysis
suggested that for both complexes the band at around 1250 nm
(ν1; the origin is the 4T1g→4T2g transition in Oh symmetry)78,79
could be simulated by two bands at 1076 and 1257 nm, and at
1073 and 1322 nm, respectively, for [Co2(CO2EtH2L1)-
(CH3COO)2](PF6) and [Co2(CO2EtL2)(CH3COO)2](PF6).
These complexes have an octahedral cis-CoN2O4 geometry at
each cobalt(II) site consistent with the splitting of this band
into two components under an axial distortion.55,66,78 Addi-
tionally, for each complex the envelope between 750 and 450
nm could be simulated by three bands at 584, 517, and 464 nm,
O
bridge/bis(μ2-RCOO-κ2O:O′) core, the variations in magnetic
coupling could be related to the kind of μ-Obridge, the Co−
Obridge−Co angle, and the type of R-group.20 The extent and
variation in factors can be further seen in the examples included
in Table 2 and Figure S6. It is apparent that extremely weak
coupling generally results for complexes with the μ-
O
(phenoxo);bis(μ2-CH3COO-κ2O:O′), either weakly antiferro-
or ferromagnetic, although there are exceptions.15 Complexes
with the μ-H2O;bis(μ2-RCOO-κ2O:O′) core appear to promote
weak antiferromagnetic coupling, although it is stronger than
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and at 570, 515, and 459 nm, respectively, for
[Co2(CO2EtH2L1)(CH3COO)2](PF6) and [Co2(CO2EtL2)-
(CH3COO)2](PF6). The highest energy band is assigned to the
spin-forbidden transition (4T1g →2 T2 g,2T1 g in Oh
symmetry).56,78 The other two are assigned to spin allowed
transitions ν2 (4T1g→4A2g in Oh) and ν3 (4T1g→4T1g(P) in Oh
symmetry), respectively.56,79,80 On the basis of these assign-
ments, the Racah parameters B (830, 820 cm−1) and Dq (910,
930 cm−1) were determined for [Co2(CO2EtH2L1)-
(CH3COO)2](PF6) and [Co2(CO2EtL2)(CH3COO)2](PF6),
respectively.81 The magnitudes of B and Dq appear typical for
these types of complexes.56,78 Full spectral data for all
complexes are reported in Table S3.
Magnetic Circular Dichroism. MCD spectroscopy was
used to determine the nature of the magnetic exchange
coupling and to explore the coordination geometries of the
dicobalt(II) in ethanol solution. The spectra are dominated by
temperature-dependent C-terms (Figure S8). The MCD
intensity above 50 K drops dramatically; thus, no variable-
temperature, variable-field (VTVH) data were collected at
higher temperatures. Figure S9 displays the field dependence of
the spectra for [Co2(CO2EtL2)(CH3COO)2](PF6). It is
apparent that beyond 5 T the C-term intensity is essentially
saturated and only the weak field-dependent B-term remains.
The solid-mull and ethanol-solution MCD spectra for the
complexes are shown in Figures S10−16 along with magnet-
ization curves at 1.5, 4.2, and 11.4 K (for the [Co2(CH3L2)-
(CH3COO)2](PF6) and [Co2(NO2L2)(CH3COO)2](PF6)
complexes, the solid-mull spectra were excessively noisy and
are therefore not included). The MCD spectrum of
[Co2(CH3L2)(CH3COO)2](PF6) is displayed as an example
in Figure 3 along with the magnetization curves at 455 and 495
nm. In ethanol, three to four negative bands (∼450, 490, 515,
540 nm) dominate the MCD spectra. These bands originate
4
from low symmetry splitting of the T1g→4T1g(P) transition
found in high-spin hexacoordinate Co(II). The analysis of
VTVH-MCD intensity behavior of the spectra resulted in a
series of parameters obtained for a selection of transitions
(Table 3). The magnetic data obtained from the MCD suggest
weak antiferromagnetic coupling for the [Co2(CO2EtH2L1)-
(CH3COO)2](PF6), [Co2(CO2EtL2)(CH3COO)2](PF6), and
[Co2(CH3L2)(CH3COO)2](PF6) complexes and ferromag-
netic coupling for [Co2(NO2L2)(CH3COO)2](PF6) and
[Co2(BrL2)(CH3COO)2](PF6), albeit weak for the former
complex, in accord with the results found from the
susceptibility measurements.
Angular Overlap Model Calculations. Spectral simu-
lations were made using the angular overlap model (AOM)
using the program AOMX essentially as described previ-
ously.1 9 , 8 2 , 8 3 Calculations were performed for
[Co2(CO2EtH2L1)(CH3COO)2](PF6) and [Co2(CO2EtL2)-
(CH3COO)2](PF6) on the basis of the transitions obtained
from the diffuse reflectance and MCD data. The coordinates
were generated from the respective crystal structures, and each
cobalt(II) metal site was treated separately. The calculations
effectively simulated the experimental data for these two
complexes; results are listed in Table 4. The Racah parameters
C and B were fitted separately, with n in C = nB varying from 4
to 4.7. The Racah B parameter is similar to that determined
from the solution spectra, and the eσ values are as expected in
the low range of previously observed parameters;84 the
contribution from π bonding was set to be zero to simplify
calculations. The excellent agreement between the AOM
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calculated and experimental band positions (Table 4) gives
confidence that assigning these bands to d−d transitions is
correct and that they are not caused by charge transfer
transitions. This idea is further supported by comparing relative
intensities of the solution spectra (or diffuse reflectance
spectra) with the MCD spectra. Charge transfer bands are
relatively intense in absorption spectra and relatively weak in
MCD spectra, whereas the opposite is true for d−d transitions.
Electronic Structure Calculations. Our previous compu-
tational studies of [Mn(II)Mn(III)] and [Fe(II)Fe(III)]
complexes85,86 have shown that calculations using pure density
functionals are able to predict exchange coupling constant (J)
values that agree qualitatively with experimental results but that
calculated results deviate quantitatively from experimental
observations, usually by approximately 20 cm−1. The agreement
between experiment and calculation was found to improve
significantly, to within approximately 1 cm−1, for calculations
using hybrid methods.85
The exchange coupling constant was calculated by combining
the results for the antiferromagnetic (broken symmetry = BS)
and ferromagnetic (high spin = Smax) ground state energy,
according to eq 387,88
2
E(Smax) − E(BS) = −J·Smax
(3)
where Smax = 3. Results derived from full geometry
optimizations (Table 5) are in satisfactory (qualitative)
agreement with observations, and the magnitude of quantitative
deviation is similar to that reported in previous studies.85,86 The
sole exception is [Co2(BrL2)(CH3COO)2]+, with calculations
predicting antiferromagnetic behavior, contrary to experimental
observations.
Plots showing the lowest unoccupied level and the five
highest occupied levels, with spatial representations of
corresponding molecular orbitals, are included in Figure 4 for
[Co2(NO2L2)(CH3COO)2]+ and Figures S17−20, for
[Co2(CO2EtH2L1)(CH3COO)2]+, [Co2(CO2EtL2)-
(CH3 COO)2 ]+ , [Co2(CH3 L2)(CH3 COO)2 ]+ , and
[Co2(BrL2)(CH3COO)2]+. Bonding analysis suggests that, in
all five cases, the most important orbital mechanism for
interaction between the metal sites involves the phenoxo
bridge. As observed in previous studies of [Mn(II)Mn(III)]
and [Fe(II)Fe(III)] complexes,85,86 orbital interactions with
combined (M−O) σ-antibonding and π-bonding character are
also a common feature of electronic structures across the series
of [Co(II)Co(II)] complexes investigated in this work.
The general similarities in relevant aspects of the electronic
structures, and the small magnitude of the exchange coupling
constants across the series, suggest that relatively small
structural differences may be associated with the prediction of
ferromagnetic behavior for [Co2(NO2L2)(CH3COO)2]+, but
antiferromagnetic behavior for [Co2(CO2EtH2L1)-
(CH3COO)2]+, [Co2(CO2EtL2)(CH3COO)2]+, and
[Co2(CH3L2)(CH3COO)2]+. A comparison of calculated
geometric parameters (which would be most relevant to
structures in solution) for the [M−O−M] bridge indicates that
the [Co2(NO2L2)(CH3COO)2]+ complex exhibits slightly
greater [M−M] and [M−O] interatomic distances and a
slightly smaller value for the [M−O−M] angle (Table S4).
Computational results for [Co2(BrL2)(CH3COO)2]+, ob-
tained using pure density functionals, indicate that the [M−O−
M] bridge structural parameters are, in general, closer to those
calculated for [Co2 (CO2 EtH2 L1)(CH3 COO)2 ]+ ,
[Co2(CO2EtL2)(CH3COO)2]+, and [Co2(CH3L2)-
Figure 3. MCD spectrum of [Co2(CH3L2)(CH3COO)2](PF6) in
ethanol at 1.5 K and 7.0 T (path length 0.62 cm, 34 mM). The
experimental spectrum is shown in blue. Gaussian deconvolved
spectrum is identical with the recorded spectrum except for noise.
Only the Gaussians for the d−d transitions are displayed. On the right:
magnetization plots from VTVH analysis of the bands at 455 and 495
nm.
Table 3. Selected Parameters from VTVH MCD
a
transition [nm]
J
[cm−1
]
D [cm−1
]
%x
%y
%z
[Co2(CO2EtH2L1)(CH3COO)2](PF6)
mull
530
506
335
489
510
−0.33
−0.33
−0.33
−0.34
−0.34
100
100
100
102
102
97
84
80
68
43
2
1
mull
15
6
1
EtOH
EtOH
EtOH
14
5
27
55
2
[Co2(CO2EtL2)(CH3COO)2](PF6)
EtOH
EtOH
mull
465
499
465
530
−0.05
−0.05
−0.10
−0.10
108
108
100
100
1
1
0
0
0
99
85
99
65
15
1
mull
35
[Co2(CH3L2)(CH3COO)2](PF6)
mull
240
322
533
495
455
−0.33
−0.33
−0.74
−0.30
−0.30
98
91
82
100
91
93
6
3
6
0
8
6
mull
98
12
0
mull
98
EtOH
EtOH
100
100
1
1
[Co2(NO2L2)(CH3COO)2](PF6)
EtOH
EtOH
EtOH
480
500
521
0.75
0.45
0.75
100
140
149
83
14
2
3
2
0
96
100
0
[Co2(BrL2)(CH3COO)2](PF6)
1.5 80
EtOH
499
93
0
0
7
8
[Co2(BrL2)(CH3COO)2](PF6) + DPP
497 0.45 80 92
EtOH
a
H = −2JS1S2.
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Table 4. AOMX Calculations for [Co2(CO2EtH2L1)(CH3COO)2](PF6) and [Co2(CO2EtL2)(CH3COO)2](PF6)
transition in Oh
obsd
calcd
[nm]
εσ Ntert
εσ Npy
εσ Ophenol
εσ OAc
εσ Oalcohol/ether
a
b,c
complex
C
B
symmetry
[nm]
[cm−1
4576
]
[cm−1
5002
]
[cm−1
4801
]
[cm−1
3489
]
[cm−1
726
]
Co(1) [Co2(CO2EtH2L1)
(CH3COO)2](PF6)
3390
715
4T1g→4T2g
1304
1323
1046
568
541
510
490
467
1304
992
588
539
503
489
468
1299
4T1g→4A2g
4T1g→4T1g(P)
4T1g→doublet
4T1g→4T2g
Co(2) [Co2(CO2EtH2L1)
(CH3COO)2](PF6)
3237
785
3500
3522
4075
3670
999
1046
568
541
510
490
467
1256
1095
560
532
519
487
468
1279
4T1g→4A2g
4T1g→4T1g(P)
doublet
4T1g→
Co(1) [Co2(CO2EtL2)
3309
3319
819
808
4T1g→4T2g
3480
3895
4539
4773
3686
3998
3475
980
698
(CH3COO)2](PF6)
1053
546
1061
543
4T1g→4A2g
4T1g→4T1g(P)
518
518
500
500
465
466
Co(2) [Co2(CO2EtL2)
(CH3COO)2](PF6)
4T1g→4T2g
1256
1273
4773
1053
546
518
500
465
1047
546
518
498
468
4T1g→4A2g
4T1g→4T1g(P)
a
b
CoL1A denotes Co(1) from the crystal structure of complex CoL1 and CoL1B stands for Co(2) of this complex. All near IR bands are from the
c
diffuse reflectance spectra. All other bands are taken from the MCD.
Table 5. Computational Parameters (ΔE = EBS − ES=3) for
the Calculation of the Magnetic Exchange Coupling via the
a
Broken Symmetry Approach (Equation 3)
complex
ΔE/kJmol−1
J/cm−1
[Co2(CO2EtH2L1)(CH3COO)2]+
[Co2(CO2EtL2)(CH3COO)2]+
[Co2(CH3L2)(CH3COO)2]+
[Co2(NO2L2)(CH3COO)2]+
[Co2(BrL2)(CH3COO)2]+
−1.35
−1.20
−3.38
1.90
−12.5
−11.1
−31.4
17.7
−1.88
−17.5
a
Becke Perdew Functional.
(CH3COO)2]+, and antiferromagnetic behavior is also
predicted in this case, in contrast to the experimental results.
Nevertheless, results derived from single-point calculations
involving hybrid methods do show qualitative agreement with
experimental observations in the [Co2(BrL2)(CH3COO)2]+
case (Table S5).
Binding of Phosphoesters in Solution. The binding of
phosphate esters in solution was investigated by MCD
spectroscopy and mass spectrometry. The mass spectrum of
[Co2(CO2EtH2L1)(CH3COO)2]+ in the presence of 25 equiv
of diphenyl phosphate (DPP) is shown in Figure 5. (DPP is a
substrate analogue that is not hydrolyzed, because it lacks the
activating nitro groups.) The base peak at 1109.2 m/z is
proposed to arise from the species [Co2(CO2EtH2L1)-
(DPP)2]+ (calcd m/z 1109.17 (100.0%), 1110.18 (56.3%);
Figure 4. Energy levels and spatial orbital plots for [Co2(NO2L2)-
(CH3COO)2]+.
found m/z 1109.0). The species at m/z 895.0 is assigned to
[Co2(CO2EtH2L1)(DPP)(H2O)(OH)]+ (calcd m/z 895.16
(100.0%), 896.16 (43.1%)). The mass spectrum of
[Co2(CO2EtL2)(CH3COO)2]+ with DPP is shown in Figure
S21. The spectrum displays a prominent species assigned to
[Co2(CO2EtHL2)(DPP)2]+ (found m/z 1137.0; calcd m/z
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Figure 6. MCD spectra of [Co2(BrL2)(CH3COO)2]+ in ethanol at 1.5
K and 7.0 T (path length 0.62 cm, 10 mM) in the presence and
absence of 25 equiv of diphenyl phosphate (DPP). After addition of
DPP, the solution was left at room temperature for 12 h prior to
recording the spectra. The experimental spectra are shown in blue.
Gaussian deconvolved spectra are identical with the recorded spectra
except for noise. Left side: [Co2(BrL2)(CH3COO)2]+ in ethanol
including magnetization plot of the band at 499 nm below. Right side:
[Co2(BrL2)(CH3COO)2]+ + 25 equiv of DPP. Below: magnetization
plot from VTVH analysis of the band at 497 nm of the spectrum with
DPP present.
Figure 5. Mass spectrum of [Co2(CO2EtH2L1)(CH3COO)2]+ in the
presence of 25 equiv of diphenyl phosphate measured in MeCN.
[Co2(CO2EtH2L1)(CH3COO)2]+ was added to a solution of
diphenylphosphate in MeCN (0.01 mM final concentration of
complex and 0.25 mM DPP) 10 min prior to spectra recording.
1137.21 (100.0%), 1138.21 (58.5%)). In addition, a small peak
assigned to [Co2(CO2EtHL2)(DPP)(OH)(CH3CN)]+ is
present (found m/z 947.0; calcd m/z 946.20 (100.0%),
947.21 (47.5%)). Figure S22 also shows the mass spectrum
of [Co2(BrL2)(CH3COO)2]+ in the presence of 25 equiv of
diphenyl phosphate. Only one observed species comprises two
cobalt ions, one negatively charged ligand, and two negatively
charged diphenylphosphate molecules (found m/z 1144.9,
calcd 1143.10 (100%), 1145.09 (97.3%).)
The binding of phosphate esters in solution was also
investigated by MCD spectroscopy. Upon addition of 25 equiv
of DPP, the spectrum of the complex [Co2(BrL2)-
(CH3COO)2]+ changes (Figure 6). The intensities and band
positions are shifted. VTVH-MCD analysis of the band at ∼500
nm revealed that the system is still ferromagnetically coupled in
the presence of DPP; however, the coupling constant (0.45
cm−1) drops to one-third of the original value (1.5 cm−1). The
Co(II) ions are still hexacoordinate, and mass spectral analysis
revealed that two DPP molecules are bound simultaneously.
The [Co2(BrL2)(CH3COO)2](PF6) complex was chosen for
the MCD experiments because its spectrum has a better signal-
to-noise ratio in ethanol than the other complexes do.
Phosphoesterase Activity. There are surprisingly few
comparative studies of cobalt(II) complexes as phosphoesterase
mimics.10−15 For the complexes in the present study the activity
toward organophosphoesters using BDNPP, a commonly used
model substrate, was investigated. [Substrate] dependence was
measured at the pH with highest activity for each complex and
followed Michaelis−Menten-type saturation behavior. [Com-
plex] dependence was linear from 0 to 0.12 mM. The data were
fitted to an equation derived for a monoprotic system,32 and
the results are shown in Figures 7, 8, and 9 and Table 6. All
complexes are good functional mimics for phosphodiesterases
and show one relevant pKa. For the complexes with the methyl-
ether donor, the absence of an alkoxide nucleophile and the
kinetically relevant pKa in the range 8.12−8.75 suggests that a
terminal water molecule bound to cobalt(II) is the active
nucleophile.21,22 For [Co2(CO2EtH2L1)(CH3COO)2](PF6),
fitting of the data resulted in a pKa of 10.54. For this complex,
as with previous studies with this type of ligand,21,22,29 the
possibility exists that the alkoxide may be the active
nucleophile. Previous studies have suggested that the
coordinated alcohol is deprotonated at or below the same pH
as a coordinated water molecule89 and that a coordinated
alcohol is a stronger nucleophile than a coordinated
hydroxide.90,91 However, the pKa of 10.5 for this complex is
still in the range of that for Co(II)−OH2,92 and hence the
identity of the nucleophile is uncertain. What is apparent is that
the activity of the dicobalt(II) complexes toward the substrate
BDNPP is similar to that displayed by the analogous dizinc(II)
complexes21 although it does not approach the efficiency of
Co(II)2−GpdQ (kcat = 1.62 s−1, kcat/Km = 1.16 mM−1
s−1).25,27,93
CONCLUSION
■
The cobalt(II) complexes of five previously reported phenol-
based ligands CO2EtH3L1, CO2EtHL2, CH3HL2, BrHL2, and
NO2HL2 have been prepared. Attempts have been made to
correlate structural parameters with the strength of the
magnetic coupling after analyzing the magnetic and spectro-
scopic properties of the complexes. Computational studies have
also been conducted to verify the experimental magnetic
coupling constants. Bonding analysis suggests that for all the
complexes in this study the most important orbital mechanism
for interaction between the metal sites involves the phenoxo
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Table 6. Kinetic Parameters Obtained for the Complexes in BDNPP Hydrolysis
complex
pKa
pH optimum
kcat × 10−3 [s−1
]
Km [mM]
catalytic efficiency kcat/Km [s−1 M−1
]
[Co2(CO2EtH2L1)(CH3COO)2]+
[Co2(CO2EtL2)(CH3COO)2]+
[Co2(CH3L2)(CH3COO)2]+
[Co2(BrL2)(CH3COO)2]+
10.54 0.10
8.34 0.11
8.53 0.11
8.75 0.12
8.12 0.12
10.70
10.40
11.00
10.00
9.55
9.12 0.95
11.40 1.10
5.48 0.63
19.10 4.15
9.23 1.65
3.26 0.80
4.31 0.85
4.48 1.13
4.62 1.62
6.83 2.48
2.79
2.64
1.22
4.13
1.35
[Co2(NO2L2)(CH3COO)2]+
8.12−8.75. For the complex with CO2EtH3L1, however, the
possibility that an alkoxide ligand arm is the active nucleophile
cannot be discounted. The complexes in this study are good
functional models for enzyme systems.
ASSOCIATED CONTENT
■
S
* Supporting Information
Tables S1 and S2, crystallographic data; Table S3, UV−vis and
NIR transitions observed for the complexes; Table S4,
comparison of structural parameters for the metal−phenoxo
bridge from calculations on broken symmetry states; Table S5,
computational parameters for the calculation of the magnetic
exchange coupling via the broken symmetry approach. Figures
S1−S22. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 7. pH dependence profiles for [Co2(CO2EtH2L1)-
(CH3COO)2](PF6) (o) and [Co2(CO2EtL2)(CH3COO)2](PF6) ( ).
●
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gahan@uq.edu.au. Phone: +61 7 3365 3844. Fax: +61
7 3365 4299.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 8. pH dependence profiles for [Co2(CH3L2)(CH3COO)2]-
(PF6) ( ), [Co2(NO2L2)(CH3COO)2](PF6) (Δ), and [Co2(BrL2)-
(CH3COO)2](PF6) (*) shown on the right.
This work was funded by a grant from the Australian Research
Council (DP0986613). L.J.D. was funded by IPRS and UQ
graduate school scholarships. Deutscher Akademischer Aus-
tausch Dienst (DAAD), the M.G. & R.A. Plowman Scholarship
in Inorganic Chemistry (School of Chemistry and Molecular
Biosciences, The University of Queensland), and Royal
Australian Chemical Institute (RACI) grants awarded to
L.J.D. assisted with visits to the University of Heidelberg and
Middlebury College for experiments reported in this paper and
are gratefully acknowledged. Access to the MagSaki software
and advice concerning the fitting of the susceptibility data from
Professor Hiroshi Sakiyama from the Department of Material
and Biological Chemistry, Faculty of Science, Yamagata
University, Kojirakawa, Yamagata, Japan, is greatly appreciated.
J.A.L. wishes to acknowledge The National Science Foundation
(USA) for financial support from Grant CHE0848433 and
Grant CHE0820965 (MCD instrument). The authors are
grateful for access to the Australian National University
supercomputer facilities of the National Computational Infra-
structure.
⧫
Figure 9. Michaelis−Menten plots for the hydrolysis of BDNPP
catalyzed by [Co2(CO2EtH2L1)(CH3COO)2](PF6) (o),
●
[Co2(CO2EtL2)(CH3COO)2](PF6)
( ), [Co2(CH3L2)-
⧫
(CH3COO)2](PF6) ( ), [Co2(NO2L2)(CH3COO)2](PF6) (Δ), and
[Co2(BrL2)(CH3COO)2](PF6) (*).
bridge; the differences in magnetic behavior are most likely
attributable to minor electronic and structural effects. Mass
spectral and MCD studies in the presence of the slow-reacting
substrate DPP showed that the complexes can bind up to two
substrate molecules and that the geometry of the active site, as
well as the magnetic coupling, changes upon substrate binding.
Kinetic analysis with the activated substrate BDNPP suggested
that, for the complexes derived from L2 ligands, terminal water
is the nucleophile with a kinetically relevant pKa in the range
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