Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes

Article abstract of DOI:10.1021/jacs.0c01073

Valence tautomerism (VT) involves a reversible stimulated intramolecular electron transfer between a redox-active ligand and redox-active metal. Bis(dioxolene)-bridged dinuclear cobalt compounds provide an avenue toward controlled two-step VT interconversions of the form {Co^III-cat-cat-Co^III} ? {Co^III-cat-SQ-Co^II}?{Co^II-SQ-SQ-Co^II} (cat^2- = catecholate, SQ^·- = semiquinonate). Design flexibility for dinuclear VT complexes confers an advantage over two-step spin crossover complexes for future applications in devices or materials. The four dinuclear cobalt complexes in this study are bridged by deprotonated 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-5,5′,6,6′-tetraol (spiroH₄) or 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol (Br₄spiroH₄) with Me_ntpa ancillary ligands (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to methylation of the 6-position of the pyridine rings). Complementary structural, magnetic, spectroscopic, and density functional theory (DFT) computational studies reveal different electronic structures and VT behavior for the four cobalt complexes; one-step one-electron partial VT, two-step VT, incomplete VT, and temperature-invariant {Co^III-cat-cat-Co^III} states are observed. Electrochemistry, DFT calculations, and the study of a mixed-valence {Zn^II-cat-SQ-Zn^II} analog have allowed elucidation of thermodynamic parameters governing the one- and two-step VT behavior. The VT transition profile is rationalized by (1) the degree of electronic communication within the bis(dioxolene) ligand and (2) the matching of cobalt and dioxolene redox potentials. This work establishes a clear path to the next generation of two-step VT complexes through incorporation of mixed-valence class II and class II-III bis(dioxolene) bridging ligands with sufficiently weak intramolecular coupling.

Full text of DOI:10.1021/jacs.0c01073

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Article
Understanding the Origin of One- or Two-Step Valence Tautomeric
Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes
Gemma Kate Gransbury, Brooke N. Livesay, Jett T Janetzki, Moya Anne Hay,
Robert W. Gable, Matthew P. Shores, Alyona Starikova, and Colette Boskovic
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 May 2020
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Understanding the Origin of One- or Two-Step
Valence Tautomeric Transitions in Bis(dioxolene)-
Bridged Dinuclear Cobalt Complexes
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Gemma K. Gransbury,† ‖ Brooke N. Livesay,‡ Jett T. Janetzki,† Moya A. Hay,† Robert W. Gable,†
Matthew P. Shores,‡ Alyona Starikova,§ Colette Boskovic*,†
† School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia
‡ Department of Chemistry, Colorado State University, Fort Collins 80523, USA
§ Institute of Physical and Organic Chemistry, Southern Federal University, 344090 Rostov-on-
Don, Russian Federation
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ABSTRACT
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Valence tautomerism (VT) involves a reversible stimulated intramolecular electron transfer
between a redox-active ligand and redox-active metal. Bis(dioxolene)-bridged dinuclear cobalt
compounds provide an avenue toward controlled two-step VT interconversions of the form {CoIII-
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cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII} ⇌ {CoII-SQ-SQ-CoII} (cat2– = catecholate, SQ•–
=
semiquinonate). Design flexibility for dinuclear VT complexes confers an advantage over two-
step spin crossover complexes for future applications in devices or materials. The four dinuclear
cobalt complexes in this study are bridged by deprotonated 3,3,3′,3′-tetramethyl-1,1′-
spirobi(indan)-5,5′,6,6′-tetraol (spiroH4) or 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-
tetrabromo-5,5′,6,6′-tetraol (Br4spiroH4) with Mentpa ancillary ligands (tpa = tris(2-
pyridylmethyl)amine, n = 0‒3 corresponds to methylations of the 6-position of the pyridine rings).
Complementary structural, magnetic, spectroscopic and DFT computational studies reveal
different electronic structures and VT behavior for the four cobalt complexes; one-step one-
electron partial VT, two-step VT, incomplete VT, and temperature-invariant {CoIII-cat-cat-CoIII}
states are observed. Electrochemistry, DFT calculations and the study of a mixed-valence {ZnII-
cat-SQ-ZnII} analog have allowed elucidation of thermodynamic parameters governing the one-
and two-step VT behavior. The VT transition profile is rationalized by 1) the degree of electronic
communication within the bis(dioxolene) ligand and 2) the matching of cobalt and dioxolene redox
potentials. This work establishes a clear path to the next generation of two-step VT complexes
through incorporation of mixed-valence class II and class II-III bis(dioxolene) ligands with
sufficiently weak intramolecular coupling.
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INTRODUCTION
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Molecular materials that can be switched between two or more physically distinguishable states
by external stimuli have far-reaching future prospects in materials with applications ranging from
sensing and display devices to quantum computing, molecular electronics and molecular
spintronics.1-3 Physical distinction of states enables colorimetric readout as a diode, sensor or
display, and is implemented in devices by attaching molecules to surfaces or incorporating them
into thin films.2-5 Recently, switchable molecules have been used to selectively address spins for
initialization, manipulation and readout in quantum information processing.6-7 Multifunctional
molecules are also under consideration for spin-switching and spin-filtering applications in
spintronics, and could be used for data storage as a molecular metal spin can be manipulated and
readout at the nanoscale.1,8 Of interest are species with three or more accessible states that enable
more complex logic processes in molecular electronics and spintronics1,9-10 and potential high
density ternary data storage which scales as 3n rather than 2n for binary data.11-13
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Multiswitchable materials manifest transitions with two or more steps. Multistep
transitions of molecular origin may be accessed with molecules on surfaces, in solution or as non-
crystalline materials, and are therefore more versatile than traditional multistep switchable
materials that depend on crystallographically-unique complexes or interactions in extended
solids.6,14-15 A simple example of a two-step interconversion in a molecular species is a dinuclear
metal complex that exhibits spin crossover (SCO) at each of the metal centers.16 The origin of the
two-step transitions in dinuclear Fe(II) has been investigated extensively and has been linked to
the use of constrained ligands to increase electronic communication,17 intermolecular
interactions18-21 and the stability of the mixed low spin-high spin state.22 The current understanding
is that small changes in one metal site upon SCO at the other site are insufficient to give rise to the
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two-step interconversion,19,22 and instead the transition profile is directed by local electronic
effects, which are difficult to control.22
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Alternative switchable molecules include those that display valence tautomerism (VT): the
reversible stimulated intramolecular electron transfer between a redox-active metal and a redox-
active ligand.23 The most common VT transition occurs with cobalt and 1,2-dioxolene ligands
where cobalt may exist in the Co(II) or Co(III) oxidation state and the dioxolene is typically in the
catecholate (cat2–) or semiquinonate (SQ•–) redox forms. In these systems the octahedral low-spin
(LS) Co(III) center undergoes a spin transition upon reduction to give high-spin (HS) Co(II) and
so there exists an VT equilibrium between the {CoIII-cat} (CoIII = LS-Co(III)) and {CoII-SQ} (CoII
= HS-Co(II)) states. The cobalt-dioxolene valence tautomeric transition is entropy-driven and has
been induced by stimuli including temperature, light and pressure, resulting in changes in color,
magnetic moment and polarization.24-27 Valence tautomeric molecules have been attached to a
metal surface and incorporated into thin films while retaining their switchable properties.6,28-29
Dinuclear VT complexes are candidates for two-step molecular transitions, and theoretical
studies indicate these could be used as a two-qubit quantum gates in quantum information
processing,30 or as molecular switches in molecular electronics or spintronics due to their state-
dependent conduction properties.31 The profile of the magnetic transition (Figure 1) is expected to
be affected by the electronic communication between the two Co-dioxolene units, which can be
controlled for dinuclear bis(dioxolene)-bridged VT systems. This contrasts with the lack of control
in Fe(II) SCO systems. One such example is [{Co(Me2tpa)}2(diox-S-diox)]2+, (diox-S-diox = 6,6ʹ-
((1,4-phenylenebis(methylene))bis(sulfanediyl))bis(3,5-di-tert-butyl-benzene-1,2-diol; Chart S1),
where the absence of electronic communication across the bis(dioxolene)-bridging ligand results
in a concerted one-step two-electron VT transition (Figure 1).32 A second influencing factor is the
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matching of the cobalt and dioxolene redox potentials, which must also be considered for a VT
transition to occur.33-34 The cobalt redox potential is modulated by the ancillary ligand – for
example, successive methylation in the 6-positions of the pyridyl rings of tris(pyridylmethyl)amine
(Mentpa, n = 0‒3) will increase the Co(III/II) reduction potential.33,35 The dinuclear complex
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[{Co(bpy)2)}2(thM)]2+ (bpy
=
2,2′-bipyridine; thMH4
=
3,3ʹ,4,4ʹ-tetrahydroxy-5,5ʹ-
dimethoxybenzaldazine; Chart S1) provides one example of the importance of the ancillary ligand:
here the excessive stabilization of the Co(III) state by the bpy ligand affords the onset of a transition
only evident at the highest measured temperatures.36
Figure 1. Schematic of valence tautomeric transitions in dinuclear cobalt-dioxolene complexes
showing (left) the electronic states associated with a two-step transition, (center) the profiles of
complete one-step, complete two-step, partial and incomplete transitions, and (right) the aim to
relate the transition profile of a complex to its electronic communication and redox properties.
The target two-step VT transition in a dinuclear bis(dioxolene)-bridged complex occurs via
a single-molecule mixed-valence {CoIII-cat-SQ-CoII} intermediate (Figure 1, left).18 A two-step
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transition could also be observed for a mixture of complexes in the {CoIII-cat-cat-CoIII} and {CoII-
SQ-SQ-CoII} states (usually 1:1).37 In addition, partial, one-electron-one-step VT transitions are
possible (Figure 1), in which a transition is observed between the {CoII-SQ-SQ-CoII} state at high
temperature and either the {CoIII-cat-SQ-CoII} state or a 1:1 mixture of {CoIII-cat-cat-CoIII}:{CoII-
SQ-SQ-CoII} at low temperature.
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The only example of a two-step VT transition for a dinuclear cobalt complex with a
bridging bis(dioxolene) ligand was reported by some of us in 2012.38 The complex
[{Co(Me2tpa)}2(spiro)]2+ (12+; spiroH4 = 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-5,5′,6,6′-tetraol;
Chart S1) shows two-step VT in both solution and solid states as measured by variable temperature
UV-Vis spectroscopy and magnetic susceptibility, respectively.35 The observation of VT in
solution confirms that for 12+ the VT interconversion is molecular in origin, corresponding to
individual transitions at the Co-dioxolene moieties: {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII} ⇌
{CoII-SQ-SQ-CoII}. It was suggested that the two-step behavior of 12+ could be the result of the
temperature-dependent delocalization of an unpaired electron over the spiro ligand which is linked
to an unidentified vibronic mode.35
In this work, we aimed to synthesize dinuclear cobalt bis(dioxolene) complexes to elucidate
the parameters that influence the system to display one- or two-step VT transitions. For the
synthesis of new dinuclear VT compounds, we selected the proligand 3,3,3′,3′-tetramethyl-1,1′-
spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol (Br4spiroH4). The Br4spiro ligand presents
shifted redox behavior with respect to the parent spiro ligand, affording useful insights into how
redox properties effect the VT transition. The Mentpa ancillary ligands were chosen to provide a
direct comparison with 12+ and to allow access to an isostructural family in which the redox
properties of cobalt can be matched to Br4spiro. This gives rise to the target complexes
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[{Co(Mentpa)}2(Br4spiro)]2+ (Chart 1) where n = 0, 2, 3 (22+, 32+ and 42+, respectively), which are
potential VT complexes. The hexafluorophosphate salts (2a, 3a and 4a) were synthesized in
addition to the perchlorate salt, 2b. The original two-step VT complex was isolated as the
perchlorate salt (1b); for comparison to 2a‒4a, we report in this work the hexafluorophosphate
salt, [{Co(Me2tpa)}2(spiro)](PF6)2 (1a). Zinc(II) analogs [{ZnII(Me3tpa)}2(Br4spirocat-SQ)](PF6)
(5a) and [{ZnII(Me3tpa)}2(Br4spiroSQ-SQ)](PF6)2 (6a) were also been synthesized to facilitate the
exploration of electronic communication properties of Br4spiro without the complication of redox-
activity in the metal centers.
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Chart 1. Dinuclear metal complexes based on Br4spiro and numbering scheme. R = H or Me for
Mentpa; n = 0, 2 or 3.
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The detailed multi-technique experimental and DFT studies of the Co compounds and their
Zn analogues presented herein have allowed attribution of the VT profile (two-step, concerted one-
step, partial or incomplete) to a function of the bis(dioxolene) intraligand electronic
communication and cobalt-dioxolene redox separation (Figure 1, right) and determination of the
origin of two-step versus partial VT. We have quantified the ideal degree of electronic
communication in the bis(dioxolene) bridging ligands by studying the mixed valence (MV) class
of the (cat-SQ)3- ligand state. Finally, we propose simple guidelines as a basis for future efforts to
achieve two-step VT compounds.
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SYNTHESIS
The dinuclear cobalt compounds 1a, 2a, 3a and 4a were prepared under a nitrogen atmosphere by
combining two equivalents of cobalt(II) chloride and the ancillary ligand in methanolic solution
with one equivalent of bis(dioxolene) ligand and four equivalents of triethylamine dissolved in
methanol. The resulting solution was concentrated under reduced pressure and then bubbled with
compressed air for a minimum of 30 mins (Br4spiro complexes) or until no further color change
was observed. The cobalt complexes were precipitated by addition of a saturated aqueous
potassium hexafluorophosphate solution. Bulk samples of grey-green 1a·CH2Cl2 and tan-colored
3a were obtained by layering of a dichloromethane solution with diethyl ether, while single crystals
of 3a were grown from dichloromethane/1,4-dioxane layering. Compounds 2a·4H2O and 4a·H2O
were obtained by recrystallization from hot methanol and ethanol to give light and dark green
microcrystalline solids, respectively. Single crystals were grown from the same solvents.
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The perchlorate salt of 22+ was also targeted due to the straightforward synthesis of
(H3tpa)(ClO4)3. The synthesis of compound 2b was adapted from the synthesis of
[{Co(tpa)}2(spiro)](ClO4)2·6H2O, using Br4spiroH4 in the place of spiroH4 and 2.2 equivalents of
triethylamine to deprotonate Br4spiroH4.35 The green compound 2b·3H2O was recrystallized from
acetone/hexane vapor diffusion, while diamond- (2bpp·7acetone) and hexagonal-
(2bpd·3.9acetone) shaped green crystals of diffraction quality were obtained from acetone/hexane
or acetone/cyclohexane vapor diffusion. Subscripts pp and pd label the geometric isomer present
(detailed in SI). Low resolution PXRD indicate 2a·4H2O and 2a·2.5MeOH have the same
structure, while the bulk sample of 2b·3H2O is a mixture of 2bpp and 2bpd (Figures S1–S2).
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Synthesis of zinc compound 5a followed a similar procedure to compound 4a above, with
zinc(II) acetate used in the place of the cobalt salt and the reaction performed in aerobic conditions.
Bubbling with compressed air was performed for a minimum of 1 h and the resultant suspension
was filtered before precipitation of 5a with aqueous potassium hexafluorophosphate. The crude
product was recrystallized twice from acetone/diethyl ether layering to give 5a·4.4H2O a bright
green microcrystalline solid, while single crystals were obtained by slowly evaporating an
acetone/toluene solution. Compound 6a was synthesized by sonicating compound 5a with exactly
one equivalent of ferrocenium hexafluorophosphate suspended in toluene.39 Compound 6a·0.5tol
was obtained as brown-green powder from acetone/toluene layering, and single crystals of
diffraction quality were obtained by layering a dichloromethane solution with toluene. Solvation
of all compounds was confirmed by elemental analysis and thermogravimetric analysis (Figure
S4).
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INFRARED ABSORPTION SPECTROSCOPY
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Infrared (IR) absorption spectra of 1a–6a and 1b–2b are reported in Figures S5–S7, with tabulated
data and assignments in Table S4. The IR spectra confirm that complex 12+ in 1a is isostructural
with the complex in the previously reported perchlorate analog, 1b (Figure S5);35 likewise the
cobalt complexes in 2a·4H2O and 2b·3H2O are isostructural (Figure S6). Previous combined DFT
and transient IR studies enable the elucidation of ligand charge distributions from IR spectra.40-41
The presence of catecholate bands (~1235, 1270 and 1329 cm–1) in the IR spectra of 2a·4H2O,
2b·3H2O and 3a (Figures S6–S7, Table S4) suggest a ligand charge distribution of (cat-cat)4– at
room temperature. Compounds 4a·H2O and 6a·0.5tol display bands characteristic of
semiquinonate moieties (1455 cm–1) at room temperature, consistent with a (SQ-SQ)2– ligand
charge distribution. Bulk samples of compounds 1a·CH2Cl2 and 5a·4.4H2O contain signature IR
bands of both the catecholate and semiquinonate moieties (Figures S5 and S7, Table S4), indicative
of an active VT interconversion in 1a·CH2Cl2 at room temperature and a mixed-valence (cat-SQ)3–
ligand charge distribution in 5a·4.4H2O. Compound 5a therefore consists of a monocationic
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–
–
complex balanced by a single PF6 anion; this is confirmed by less intense PF6 bands (845 and
557 cm–1) in the IR spectrum of 5a compared to 2a–4a and 6a (Figure S7).
STRUCTURE DESCRIPTION
The solid-state structures of Br4spiroH4·2Et2O, 2a, 2b, 3a, 4a, 5a and 6a were determined by single
crystal X-ray diffraction, (42+ is shown in Figure 2; crystallographic data, additional images and
further discussion are provided in the Supporting Information). It was not possible to obtain single
crystals of diffraction quality for any salt of 12+.35 The cationic complexes 22+–62+ consist of two
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metal atoms bridged by the Br4spiro bis(dioxolene) ligand, with each metal center capped by a the
tripodal tetradentate Mentpa (n = 0, 2, 3) ligand. Due to the inequivalence of the O1 and O2 oxygen
atoms of Br4spiro (Figure 2), geometric isomerization is possible.35 Compounds 2a–4a crystallize
with a C2 axis running through the central spirocyclic carbon (C11) and a proximal geometry at
the two crystallographically equivalent metal centers (pp isomer; see Supporting Information for
naming convention). For compound 2b we obtained crystal structures of two geometric isomers,
pp and pd (d = distal), while 5a and 6a crystallize as pp and pd isomers respectively. Compounds
2b, 5a and 6a do not possess any molecular symmetry. In compound 3a the two methyl groups of
Me2tpa were approximately evenly disordered over three sites.
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Figure 2. Complex cation in 4a·2EtOH at 100 K. Color code: carbon on Me3tpa, black; carbon on
Br4spiro, orange; oxygen, red; nitrogen, blue; cobalt, aqua green; bromine, brown. Hydrogen
atoms have been omitted for clarity.
The presence of counterions and solvent in the crystal structures are consistent with the
formulas 2a·2.5MeOH, 2bpp·7acetone, 2bpd·3.9acetone, 3a·4.5dioxane, 4a·xEtOH (x = 2–3),
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5a·2tol and 6a·tol (Chart 1). The crystal structure formulae differ from the bulk recrystallized
compounds which analyzed as 2a·4H2O, 2b·3H2O, 3a, 4a·H2O, 5a·4.4H2O and 6a·0.5tol; this is
attributed to different solvent combinations required to obtain homogeneous bulk samples and the
exchange of crystallization solvent with water on standing. Low resolution PXRD (Figure S3)
shows 4a·H2O has the same packing as 4a·xEtOH but with ethanol solvate in the channels having
been exchanged for water.
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To determine the cobalt oxidation state, we used the Co-O, Co-Namine (N1 = Namine) and
Co-Npy (N2–N4 = Npy) bond lengths and octahedral distortion parameters (Σ, Θ and octahedral
SHAPE index).42-43 The SHAPE index calculated in SHAPE 2.1 represents the distortion of a
coordination environment from an ideal polyhedron.44-45 Cobalt(III) has short metal-ligand bonds
and highly octahedral geometries compared to Co(II).34,46 Typical parameters for CoIII/CoII with
Mentpa ligands are 1.85–1.91/1.99–2.11 Å for Co-O, 1.91–1.97/2.09–2.13 Å for Co-Namine, 1.87–
2.03/2.14–2.29 Å for Co-Npy and 0.1–0.5/1.4–1.8 for the octahedral SHAPE index.33-35 The
dioxolene C-C and C-O bond lengths are correlated to the ligand oxidation state (Carugo et al.),47
such that a least-squares fit enables assignment of an apparent metrical oxidation state (MOS,
Brown et al.).48 Catecholate ligands have a MOS of around –2 (c.f. SQ = –1), longer C-O bonds
and shorter C1-C2 bonds. Based on the parameters given in Tables S6‒S7, 2a, 2b and 3a are
assigned the charge distribution {CoIII-cat-cat-CoIII} in the solid state at low temperature (100 and
130 K). Compound 6a has a {ZnII-SQ-SQ-ZnII} charge distribution, and compound 5a is assigned
a {ZnII-cat-SQ-ZnII} charge distribution at 100 K based on intermediate MOS values and bond
lengths between those of 3a and 6a (Table S6). The dioxolene sites in 5a are indistinguishable; the
SQ/cat redox states are crystallographically disordered or electronically delocalized. The low
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temperature crystallographic charge distributions of 2a, 2b, 3a, 5a and 6a are consistent with the
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room temperature IR data.
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At 300 K, the Co-O/N bond lengths and distortion parameters for 4a·xEtOH unequivocally
confirm a {CoII-SQ-SQ-CoII} charge distribution, consistent with the room temperature IR
spectrum (Figure 3, Table S8).35,46 On cooling, there is a contraction of Co-O/N bond lengths by
0.07–0.14 Å, which indicates VT transition. The unit cell parameters and cell volume slightly
contract, which is reproduced in variable-temperature PXRD of the bulk sample (Figure S3 and
S13). At 100 K, the bond lengths and octahedral distortion parameters at the cobalt site are
intermediate between HS-Co(II) and LS-Co(III) values. The C-O bonds, C1-C2 bond and MOS
value of –1.4(1) are also halfway between catecholate and semiquinonate values. Thus, single
crystals of 4a·xEtOH undergo a partial temperature-induced VT transition (Figure 1) between a
{CoIII-cat-SQ-CoII} charge distribution at 100 K, in which the two cobalt centers are
crystallographically equivalent, and a {CoII-SQ-SQ-CoII} tautomer at 300 K. Crystallographic
symmetry is maintained across the temperature range, suggesting a {CoIII-cat-SQ-CoII} ⇌ {CoII-
SQ-SQ-CoII} equilibrium rather than a {CoIII-cat-cat-CoIII} ⇌ {CoII-SQ-SQ-CoII} transition in
50% of molecules.20 The partial VT transition in 4a·xEtOH does not result in a dramatic change
in the geometry of the complex (Figure S14).
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Figure 3. Temperature dependence of the Co-O (red) and Co-N (blue) bond lengths for 4a·xEtOH
(left axis) overlaid with the magnetic susceptibility data (MT) measured for 4a·H2O (grey, right
axis) from Figure 4.
MAGNETIC MEASUREMENTS
Variable-temperature magnetic susceptibility data for compounds 1a–4a are plotted as χMT vs T in
Figure 4, where χM is the molar magnetic susceptibility. Compound 2a·4H2O shows temperature-
independent paramagnetism typical for Co(III) ions49 and a temperature-independent {CoIII-cat-
cat-CoIII} charge distribution that agrees with crystallographic studies.
At low temperatures, compound 3a is diamagnetic, with a χMT value of less than 0.06 cm3
K mol–1 between 2 and 275 K. The low temperature charge distribution is therefore {CoIII-cat-cat-
CoIII}, also consistent with the crystal structure of 3a·4.5dioxane at 100 K. On the first heating
cycle, the value of χMT for 3a increases above 275 K, first gradually and then more rapidly above
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310 K, to reach a maximum value of 1.19 cm3 K mol–1 at 360 K. The increase in χMT between 275
and 360 K is consistent with the onset of a VT transition that is far from complete at 360 K, with
~40% of the molecules attaining the {CoIII-cat-SQ-CoII} charge distribution. Repeated heating-
cooling cycles on 3a reveal a more gradual VT transition (Figure S16), which is commonly
observed in VT compounds and may be associated with a structural change.34,50
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Figure 4. Plots of χMT vs T for the first heating of 1a·CH2Cl2 (black); 2a·4H2O (red), 3a (blue)
and 4a·H2O (green) and corresponding equations.
Compound 4a·H2O exhibits a χMT value of 6.23 cm3 K mol–1 between 300 and 360 K,
which is similar to other {CoII-SQ-SQ-CoII} complexes (6.15–7.37 cm3 K mol-1)32,35 and confirms
the charge distribution observed in room temperature crystallography. The room temperature χMT
value is within the range expected for two isolated or weakly-interacting spin-orbit coupled HS-
Co(II)-SQ moieties (2.47–3.75 cm3 K mol–1 each),33,51 and is congruous with DFT calculations
predicting weak coupling over the Br4spiro bridging ligand (Table S17).
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Compound 4a·H2O displays a single step transition between 140 and 300 K, which is more
abrupt on the low-temperature side (Figure 4). The χMT value plateaus around 80–140 K at 3.1–
3.4 cm3 K mol-1, which is approximately half of the high temperature value, before decreasing
again to reach 2.21 cm3 K mol–1 at 2 K. Repeated heating-cooling cycles are superimposable on
the original heating curve (Figure S16). The magnetic data are consistent with a {CoIII-cat-SQ-
CoII} ⇌ {CoII-SQ-SQ-CoII} partial VT interconversion involving all molecules and centered at Tc
= 190 K. The VT transition profile closely follows the variable temperature crystallography bond
lengths (Figure 3) and the assignment of a partial transition is consistent with crystallography. The
decrease in χMT at low temperature is due to the depopulation of the HS-Co(II)-SQ spin-orbit states
and is commonly observed in HS-Co(II)-SQ complexes.35,46,52-53
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Compound 1a·CH2Cl2 displays a two-step VT transition, with a gradual first step in the
range 70–300 K (χMT 1.93–4.57 cm3 K mol-1), and a more abrupt second step above 300 K reaching
5.87 cm3 K mol-1 at 360 K (Figure 4). The χMT value at 70 K and the signature decrease on lowering
temperature (χMT 0.82 cm3 K mol-1 at 2 K) indicates that there is a trapped fraction of HS-Co(II)-
SQ. The trapped fraction is estimated as ~25% {CoII-SQ-SQ-CoII} based on a maximum attainable
-
1
χMT value of 7.37 cm3 K mol as measured for [{CoII(Me3tpa)}2(spiroSQ-SQ)](ClO4)2.35 The
maximum attainable χMT values are significantly different in 1a and 4a, but this is compatible with
both the cobalt single-ion contribution and the exchange interaction in the HS-Co(II)-SQ unit being
highly sensitive to molecular geometry.46 The magnetic data for 1a are consistent with the
remaining ~75% of molecules undergoing the first {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII} step
VT interconversion between 70 and 300 K. The molecules then undergo an incomplete second
{CoIII-cat-SQ-CoII} ⇌ {CoII-SQ-SQ-CoII} step above 300 K such that ~60% of molecules reach
the {CoII-SQ-SQ-CoII} state. Further cooling-heating cycles show a more gradual transition
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(Figure S16) which is consistent with a structural change upon potential solvent loss.35,54 The
overall magnetic behavior of 1a is similar to that of the previously reported perchlorate analog,
1b, with both datasets showing a two-step VT transition but with compound 1a displaying a larger
trapped HS-Co(II)-SQ fraction than compound 1b.35 The thermal VT behavior is summarized in
Figure 4.
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ELECTRONIC ABSORPTION SPECTROSCOPY
Electronic absorption spectra were recorded for acetonitrile (MeCN) solutions of compounds 1a–
6a and are presented in Figure 5 with absorption bands and assignments tabulated in Table S11.
Solution stability results are shown in Figures S17–S18: 1a is stable in MeCN solution in a nitrogen
atmosphere and 2a–6a are stable in aerobic MeCN solution. The visible absorption spectrum of
1a appears identical to the one reported for 1b previously, indicating the counterion does not
significantly affect the spin state in solution.35 The charge distribution of 1a is therefore
predominately {CoII-SQ-SQ-CoII} at 298 K in MeCN.
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Figure 5. UV-Vis (top) and NIR (bottom) absorption spectra for acetonitrile solutions of 1a
(black), 2a (red), 3a (blue), 4a (green), 5a (purple) and 6a (brown) at 298 K. Red dashed line
indicates fitting of IVCT as described in the Supporting Information.
The visible absorption spectra of 2a and 3a are characteristic of LS-Co(III)-cat species and
confirm the solid state {CoIII-cat-cat-CoIII} charge distribution is retained in solution.34 The spectra
of 2a and 3a both display a ligand-to-metal charge transfer (LMCT) band at 674 nm and 809 nm,
respectively.55 The LMCT band is shifted to lower energies in 3a due to the smaller separation of
{CoIII(Mentpa)-cat} and {CoII(Mentpa)-SQ} states for n = 2 than n = 0; this is consistent with the
observation of solid-state VT in 3a but not 2a.
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The spectra of 4a, 5a and 6a exhibit ligand-centered Br4spiro semiquinonate transitions at
420 nm and 650–1000 nm, which are more intense for 4a and 6a than for 5a. The semiquinonate
bands confirm the presence of two semiquinonate radicals in 4a and 6a but one radical in 5a. Thus,
5a and 6a have solution charge distributions of {ZnII-cat-SQ-ZnII} and {ZnII-SQ-SQ-ZnII},
respectively. The spectrum of 4a also displays a strong metal-to-ligand charge transfer band at 614
nm; this spectrum is characteristic of HS-Co(II)-SQ,34,46 and indicates 4a exists in the {CoII-SQ-
SQ-CoII} state in MeCN solution at room temperature. The room temperature charge distributions
of compounds 2a–6a are therefore maintained between solid and solution states. Variable-solvent
measurements indicate the charge distributions are unchanged for 1a, 2a and 4a in tetrahydrofuran
(THF) and 5a in 1,2-dichloroethane and chlorobenzene (Figures S19–S21).
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Inspection of the near infrared (NIR) absorption spectra of 1a–6a (Figure 5) reveals that
only complex 5a exhibits a broad NIR absorption at 1494 nm. The NIR absorption is assigned to
an intervalence charge transfer (IVCT) between the catecholate and semiquinonate moieties of 5a.
Fitting and analysis of the IVCT band of 5a in MeCN (dielectric constant, κ = 37.5) and 1,2-
dichloroethane (κ = 10.36) revealed a very weak solvent-dependence (detailed in the Supporting
Information). This indicates a class II-III MV system that shows solvent averaging but electronic
localization,56-57 consistent with the observation of both catecholate and semiquinonate IR bands
(Table S4). The electronic coupling parameter, HAB, was estimated as 1990 cm–1 in MeCN
(Equation S5),58 with 2HAB/νmax = 0.71, which is on the border of class II-III and class II
localization (Table S13), consistent with the low intensity broad IVCT band.59 The IVCT analysis
suggests the Br4spiro ligand in 5a exhibits stronger electronic coupling than the unsubstituted spiro
ligand, which has previously been reported to have both class I and class II mixed-valence
behavior.60-62
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ELECTROCHEMISTRY
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The redox and electronic communication properties of the spiro and Br4spiro complexes were
investigated by electrochemistry. Cyclic and rotating disk electrode (RDE) voltammograms were
recorded for MeCN solutions of 1a–6a (Figure 6 and S24) and THF solutions of 1a, 2a and 4a
(Figure S25). Midpoint potentials (Em), peak-to-peak separations (∆Ep), half-wave potentials (E1/2)
and limiting currents (iL) are reported in Table 1 (MeCN) and Table S14 (THF). We report peak
potentials (Ep) in the case of irreversible processes, which are identified as having ∆Ep larger than
ferrocene under the same conditions (68‒81 mV in MeCN, 80‒98 mV in THF).
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Figure 6. (Left) Cyclic voltammograms of acetonitrile solutions of compounds 1a–6a (1.0 mM
with 0.25 M Bu4NPF6) obtained with a scan rate of 100 mV s–1. (Right) Corresponding RDE
voltammograms at a scan rate of 50 mV s–1 and a rotation rate of 500 rotations min–1. Arrows
indicate the starting potential and direction of the scan.
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Table 1. Cyclic voltammetry and rotating disk electrode voltammetry data for compounds 1a–6a
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Cyclic Voltammetry Data
Rotating Disk Electrode Voltammetry
Em or Ep / V (ΔEp / mV)
E1/2 / V (iL / μA)
I/I′
II/II′
III
IV
V
I/I′
II/II′
III
IV
V
–0.857 –0.739 –0.084
1.139 b
n/a
–0.875 –0.713
–0.011
(50.9)
-
n/a
1a
2a
3a
4a
5a
6a
(88)
(70)
(155)
n/a
(27.1)
(27.2)
0.166
(69)
0.303
(74)
1.091 b –0.993
(259)
0.178
(22.9)
0.328
(22.8)
n/a
n/a
1.100
(41.2)
–1.116
(48.1)
0.149 b 0.307 b n/a
1.159 b –0.640
(92)
0.072
(23.7)
0.246
(23.6)
0.995
(22.5)
–0.660
(45.0)
–0.583 –0.419 0.286 b
1.208 b
n/a
n/a
n/a
–0.614 –0.429
(26.2) (26.3)
0.236
(49.3)
-
n/a
n/a
n/a
(67)
(65)
–0.558 –0.383 0.250 b
(70) (70)
n/a
–0.539 –0.368
(28.5) (29.9)
0.255
(58.2)
n/a
n/a
–0.533 –0.387 0.318 b
(90) (74)
n/a
–0.560 –0.389
(21.4) (22.5)
0.283
(44.4)
a 1.0 mM in acetonitrile with 0.25 M Bu4NPF6, scan rate 100 mV s–1. Potentials reported versus Fc/Fc+ couple. Error in potentials
is ±5 mV. b Ep rather than Em.
The cyclic voltammograms of 1a, 4a, 5a and 6a display three redox events in the central
potential region (Figure 6, Figure S25): two closely-spaced one-electron processes (I and II) and
a two-electron process (III). The RDE position of zero current identifies I and II as reductions and
III as an oxidation for compounds 1a, 4a and 6a, which compares well with the previously reported
electrochemistry of 1b.35 For compound 5a, the position of zero current has shifted relative to 6a
so that process I is a reduction and II and III are oxidations. Processes I and II are chemically-
reversible and diffusion-controlled in all four complexes, except for process I for 1a and 6a that is
quasi-reversible. Process III is irreversible for 4a–6a, quasi-reversible for 1a in MeCN (∆Ep = 155
mV) and reversible for 1a in THF (∆Ep = 129 mV).
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The cyclic voltammograms of 2a and 3a exhibit three distinct redox events in the central
potential region: two one-electron oxidations (processes I′ and II′) and a 2e-reduction (process V),
as confirmed by RDE voltammetry. For compound 2a, oxidations I′ and II′ are diffusion-
controlled and chemically reversible, while process V is quasi-reversible. For compound 3a,
oxidations I′ and II′ are irreversible and process V is diffusion-controlled and chemically
reversible, with a peak-to-peak separation of 92 mV. Cobalt compounds 1a–4a also exhibit an
irreversible or quasi-reversible 2e-oxidation (process IV) at highly positive potentials.
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Process I/I′ and II/II′ are defined as spiro/Br4spiro ligand-based processes and process V
is defined as cobalt-based as shown in Scheme 1.35 Compound 5a is again confirmed to have (cat-
SQ)3‒ mixed-valence by the position of zero current. Going from 1a to 4a, process I and II are
shifted by approximately +300 mV as the electron-withdrawing substituents make Br4spiro easier
to reduce than spiro.60 The irreversibility of processes I′ and II′ for compound 3a, suggest that the
{CoIII(Me2tpa)-SQ} species is unstable compared to {CoIII(tpa)-SQ}.34 Process III appears at the
same potential (±35 mV) in cobalt and zinc compounds 4a–6a in MeCN and so is assigned as a
ligand-based oxidation of the bis(semiquinone) ligand to the bis(quinone) for a dicationic complex
(Scheme 1). Quinones are weakly coordinating, so the irreversibility of process III for 4a–6a is
consistent with dissociation of the {CoII-Q-Q-CoII}4+ species formed (Q = quinone). It appears that
the bis(quinone) oxidation state is more stable for the unsubstituted spiro ligand than with the
electron withdrawing substituents on Br4spiro, as process III shows some reversibility for 1a. The
assignment of process IV is discussed in the Supporting Information and is shown in Scheme 1.
The identity of processes I/I′, II/II′, IV and V are consistent with DFT calculations, as detailed in
the Supporting Information (Figures S26–S29).
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Scheme 1. Assigned redox processes for compounds 1a–6a (M = Co, Zn).
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The mixed valence properties of bis(dioxolene) ligands and their HAB values are roughly
correlated with the separation between ligand redox processes I and II, termed ∆diox (Table 2).63-
64 Previously, ∆diox was measured for [{Co(Mentpa)}2(spiro)](ClO4)2 (MeCN, 0.1 M Bu4NPF6),
giving 143 mV for 1b (n = 2) and 158–173 mV for n = 0, 1 and 3.35 We find the class II-III MV
compound 5a to have the largest ∆diox value of 175 mV in MeCN (κ = 37.5, Table 2), followed
by the partial VT compound 4a and other Br4spiro compounds, and finally the two-step VT
compound 1a. As the separation of redox potentials is solvent dependent;65 we repeated the
measurement for 1a, 2a and 4a in non-polar THF (κ = 7.58) and found increased separations but
the same ordering (1a < 2a < 4a; Table 2). We can conclude that the bridging ligand electronic
communication in the bridging ligand increases 1a < 2a < 4a. The MV class of 4a is estimated as
the upper end of class II or lower end of class II-III because of the similarity in ∆diox value to 5a
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and by comparison to ∆diox in bis(dioxolene) literature compounds (Table S13).63-64 Without a
spectroscopic handle, we cannot unequivocally assign the MV class for (cat-SQ)3– in 1a–4a;
however, the ∆diox value of 1a is tentatively assigned as class II MV by comparison to literature
values.64 This is consistent with the rate of electron transfer in [{CoIII(tpa)}2(spirocat-SQ)]3+ being
on the EPR timescale (~1010 s–1), slower than typical solvent reorientations (~1012–1013 s–1).35,57
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a
Table 2. Separation of electrochemical processes for compounds 1a–6a
Acetonitrile
Tetrahydrofuran
Δdiox
120
Δox-red
Δdiox
140
Δox-red
~810 b
1230
1a
2a
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6a
~655 b
1160
790 c
~705 b,c
-
135
180
160 c
165
240
~850 b,c
175
145
-
a Calculated from CV potentials. Values reported in mV to the nearest 5 mV, error in parameters is ±10 mV. b An estimation of
Δox-red as discussed in the text. c Ep used in calculation instead of Em as oxidative processes are irreversible.
The parameter ∆ox-red is defined as the separation between the midpoint potentials of the
(reversible) first 1e-oxidation and first 1e-reduction, in the case when one process is metal-based
and the other ligand-based. In our previous work, we established the approximate rule ∆ox-red <
740 mV for VT to occur in a thermally accessible range for mononuclear VT compounds.34 The
value of ∆ox-red is well defined for {CoIII-cat} species that undergo a cobalt reduction process
(V).39 Values of ∆ox-red in MeCN for 2a and 3a (Table 2) are consistent with the absence of VT
in 2a (∆ox-red >> 740 mV) and the presence of an incomplete VT transition at high temperature
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in 3a (∆ox-red ~ 740 mV). In compounds 1a and 4a, both first oxidation and reduction potentials
are ligand-based and so ∆ox-red is not defined. In this case, we give the separation of ligand-based
redox potentials for II and III as a rough estimate of ∆ox-red, given that {CoII-Q-Q-CoII} species
may rearrange to form {CoIII-SQ-SQ-CoIII} (Table S15). The estimated ∆ox-red values in MeCN
are consistent with both 1a and 4a displaying VT in the solid state (Table 2). Use of less polar
THF results in increased potential separations but maintained trends: estimates of ∆ox-red for VT
complexes 1a and 4a are still significantly less than for the non-VT complex 2a. However, it is
clear that the ∆ox-red criterion is solvent-dependent, and care must be taken to only compare
measurements performed in the same solvent.
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COMPUTATIONAL STUDY
Density functional theory calculations were performed for compounds 1a, 2b, 3a and 4a in the gas
phase to examine their magnetic behavior (optimized structures and energies are given in the
Supporting Information); 2b was selected over 2a to examine the effect of pp and pd geometric
isomers. Valence tautomeric mononuclear complexes have been investigated with DFT in the
past,34,39,66-68 but to date DFT on dinuclear cobalt VT systems has only been used to predict VT
transitions in hypothetical complexes.69-71 To analyze dinuclear VT transitions we draw a parallel
with DFT investigations of two-step SCO systems.22,72 As such, all three charge distributions in a
dinuclear complex can be considered to be in equilibrium. The nature of the transition is then
described by the thermochemical parameter ρ, given by Equation (3) where state energies, E, are
given relative to E{CoIII-cat-cat-CoIII}.
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ꢅꢅꢅ
ꢅꢅ
ꢅꢅ
ꢅꢅ
ꢍ
ꢁꢂꢃꢄ -ꢆꢇꢈ-ꢉꢊ-ꢃꢄ ꢋꢌ ꢏ ꢁꢂꢃꢄ -ꢉꢊ-ꢉꢊ-ꢃꢄ
ꢋ
ꢎ
ꢀ =
(3)
ꢅꢅ
ꢅꢅ
ꢐ
ꢑ
ꢁ ꢃꢄ -ꢉꢊ-ꢉꢊ-ꢃꢄ
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For the case that all three charge distributions are thermally accessible and E{CoIII-cat-cat-
CoIII} < E{CoIII-cat-SQ-CoII} < E{CoII-SQ-SQ-CoII}, ρ > 0 implies a two-electron-one-step VT
transition: {CoIII-cat-cat-CoIII} ⇌ {CoII-SQ-SQ-CoII}. A two-step transition is favored for ρ < 0,
with a more negative ρ value corresponding to a larger separation between the two steps. If {CoIII-
cat-SQ-CoII} is the ground state (ρ < –0.5), the only possible VT transition is a partial transition
{CoIII-cat-SQ-CoII} ⇌ {CoII-SQ-SQ-CoII} (Figure 1).
Density functional theory calculations on compound 1a used the DFT-optimized geometry
of 3a as a starting point in the absence of a crystal structure. Calculations confirmed the {CoIII-cat-
cat-CoIII} charge distribution as the ground state, with both the {CoIII-cat-SQ-CoII} and {CoII-SQ-
SQ-CoII} charge distributions being thermally accessible, at 1.1 and 4.0 kcal mol–1, respectively
(Figure 7). This is consistent with the assignment of the experimentally observed two-step
behavior as a two-step {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII} ⇌ {CoII-SQ-SQ-CoII} VT
transition of molecular origin. The parameter ρ = –0.24 is concordant with two well-separated
interconversion steps.
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Figure 7. Schematic showing the relative energies of different electronic states of 1a, 2bpp, 3a and
4a calculated by the DFT UTPSSh/6-311++G(d,p) method.
Calculations for compound 2b revealed the 2bpp and 2bpd isomers to be approximately
equal in energy (0.2 kcal mol–1, Table S15) in the {CoIII-cat-cat-CoIII} charge distribution, and so
further calculations focused on 2bpp. For 2bpp, the {CoIII-cat-cat-CoIII} charge distribution is
predicted to be the ground state (Figure 7), in agreement with the experimental results. The Co-
O/N and dioxolene bond lengths in 2bpp {CoIII-cat-cat-CoIII} closely reproduce the X-ray structure,
2bpp·7acetone (Figure S31). The {CoIII-cat-SQ-CoII} and {CoII-SQ-SQ-CoII} charge distributions
for 2bpp have relative energies of 12.3 kcal mol–1 and 27.3 kcal mol–1 respectively, and so are
predicted to be thermally inaccessible (Figure 7). The DFT prediction is consistent with the
temperature invariant {CoIII-cat-cat-CoIII} state for 22+.
For compound 3a, the ground state is the {CoIII-cat-cat-CoIII} charge distribution, with the
next highest state being {CoIII-cat-SQ-CoII} at 5.0 kcal mol–1. The DFT calculations predict a
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diamagnetic {CoIII-cat-cat-CoIII} ground state with a thermally-induced VT transition to the {CoIII-
cat-SQ-CoII} state, consistent with experiment. The {CoII-SQ-SQ-CoII} state is predicted to have
a relative energy of 11.4 kcal mol–1 and is therefore unlikely to form in an accessible temperature
range. However, the ρ-value for 3a (and 2bpp) is ‒0.06 (Table S16), which indicates that a two-
step transition is expected at a sufficiently high temperature—the electronic properties of Br4spiro
are such that a two-step transition is possible for 3a but is outside the experimentally-accessible
temperature range.
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Calculations on compound 4a revealed the {CoIII-cat-cat-CoIII} and {CoIII-cat-SQ-CoII}
charge distributions to be isoenergetic (Table S16). However, the zero-point harmonic vibrations
(Table S15) and spin degeneracy result in an entropic term that more favourably stabilizes the
{CoIII-cat-SQ-CoII} state. As thermally-induced VT is an entropy-driven process, no VT transition
is expected between the {CoIII-cat-SQ-CoII} and {CoIII-cat-cat-CoIII} charge distributions. The
{CoII-SQ-SQ-CoII} charge distribution is higher in energy than the {CoIII-cat-SQ-CoII} state by
1.7 kcal mol–1 (Figure 7). Thus, DFT calculations on 4a predict a partial {CoIII-cat-SQ-CoII} ⇌
{CoII-SQ-SQ-CoII} VT transition with ρ = –0.57.
The isotropic exchange coupling constants in the {CoIII-cat-SQ-CoII} and {CoII-SQ-SQ-
CoII} change distributions of 1a, 2bpp, 3a and 4a were calculated by the Broken Symmetry
approximation.73 The exchange interaction between unpaired electrons on the HS-Co(II) ion and
the nearest (coordinating) semiquinone was calculated to be strongly ferromagnetic (Table S17),
consistent with DFT calculations on the mononuclear complexes with Mentpa.74 However, this
interaction is known to be anisotropic with contributions from spin–orbit coupling that are not
represented accurately by DFT calculations.46 The calculated exchange interactions between the
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