Ligand-Mediated Spin-State Changes in a Cobalt-Dipyrrin-Bisphenol Complex

Article abstract of DOI:10.1021/acs.inorgchem.0c01979

The influence of a redox-active ligand on spin-changing events induced by the coordination of exogenous donors is investigated within the cobalt complex [CoII(DPP·2-)], bearing a redox-active DPP2- ligand (DPP = dipyrrin-bis(o,p-di-tert-butylphenolato) wi

Full text of DOI:10.1021/acs.inorgchem.0c01979

This is an open access article published under a Creative Commons Non-Commercial No
Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and
redistribution of the article, and creation of adaptations, all for non-commercial purposes.
pubs.acs.org/IC
Article
Ligand-Mediated Spin-State Changes in a Cobalt-Dipyrrin-Bisphenol
Complex
Nicolaas P. van Leest, Wowa Stroek, Maxime A. Siegler, Jarl Ivar van der Vlugt, and Bas de Bruin*
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: The influence of a redox-active ligand on spin-changing
events induced by the coordination of exogenous donors is investigated
within the cobalt complex [CoII(DPP·2−)], bearing a redox-active DPP2−
ligand (DPP = dipyrrin-bis(o,p-di-tert-butylphenolato) with a pentafluor-
ophenyl moiety on the meso-position. This square-planar complex was
subjected to the coordination of tetrahydrofuran (THF), pyridine, tBuNH2,
and AdNH2 (Ad = 1-adamantyl), and the resulting complexes were analyzed
with a variety of experimental (X-ray diffraction, NMR, UV−visible, high-
resolution mass spectrometry, superconducting quantum interference device,
Evans’ method) and computational (density functional theory, NEVPT2-
CASSCF) techniques to elucidate the respective structures, spin states, and
orbital compositions of the corresponding octahedral bis-donor adducts,
relative to [CoII(DPP·2−)]. This starting species is best described as an open-shell singlet complex containing a DPP·2− ligand
radical that is antiferromagnetically coupled to a low-spin (S = 1/2) cobalt(II) center. The redox-active DPPn− ligand plays a crucial
role in stabilizing this complex and in its facile conversion to the triplet THF adduct [CoII(DPP·2−)(THF)2] and closed-shell singlet
pyridine and amine adducts [CoIII(DPP3−)(L)2] (L = py, tBuNH2, or AdNH2). Coordination of the weak donor THF to
[CoII(DPP·2−)] changes the orbital overlap between the DPP·2− ligand radical π-orbitals and the cobalt(II) metalloradical d-orbitals,
which results in a spin-flip to the triplet ground state without changing the oxidation states of the metal or DPP·2− ligand. In
contrast, coordination of the stronger donors pyridine, tBuNH2, or AdNH2 induces metal-to-ligand single-electron transfer, resulting
in the formation of low-spin (S = 0) cobalt(III) complexes [CoIII(DPP3−)(L)2] containing a fully reduced DPP3− ligand, thus
explaining their closed-shell singlet electronic ground states.
expand upon these functions by investigating the role of a
redox-active ligand in spin-changing events. Specifically, by
keeping the redox-active ligand and metal center constant we
set out to investigate how the coordination of different
additional redox-innocent donors to the metal center
influences the total spin state of the complex, which is
governed by the interactions of the metal d-orbitals and the
redox-active ligand orbitals of π-symmetry.
In this context, we became interested in the family of
dipyrrin-bisphenol ligands (DPP, Figure 1), known since the
1970s.7 Different substitution patterns on the backbone were
explored since 2009, and complexes of Al,8,9 B,8 Ga,9 In,9 Ti,10
Zr,10 Ge,10 Sn,10 and Mn11 have been reported. The DPP
ligand scaffold was first described as being redox-active in 2012
after the synthesis of cobalt- and nickel-DPP complexes.12
INTRODUCTION
■
Spin-state changes (spin crossover) can play an important role
in chemistry and material research, among others in
biochemistry (respiration, enzymatic conversions),1 develop-
ment of molecular magnets2 and spintronics,3 and as a
potential rate-accelerating process in organometallic chemistry
and catalysis.4 Purely metal-centered spin-state changes of
coordination complexes can be explained in terms of the
coordination and geometry-dependent energy difference
between partially filled and empty d-orbitals, as described by
the ligand-field splitting parameter Δ.5
The respective roles of the metal and traditional (redox-
innocent) ligands are well-understood in these cases. However,
when a redox-active ligand, capable of bearing unpaired
electrons, is present in the coordination sphere of the metal,
the relative contributions and the roles and influence of metal
and ligand to changes in the total spin state of the overall
complex are far less well-understood. The main four modes of
action of redox-active ligands that have been studied
thoroughly can be summarized as (i) changing the Lewis
acidity/basicity of the metal, (ii) acting as an electron
reservoir, (iii) generation of a reactive ligand-centered radical,
and (iv) radical-type activation of a substrate.6 We wish to
Received: July 4, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
centered unpaired electrons) are energetically close (∼1 kcal
mol−1).12,17 Although a BSS (S = 0) spin state was inferred
based on experimental data for a Co-DPP complex, the DFT
calculations indicated that the triplet state was slightly favored
(−1.0 kcal mol−1).18 Furthermore, monocoordination of
benzonitrile, dimethyl sulfoxide (DMSO) and pyridine was
observed, with conversion to the octahedral (bis-axially
coordinated) complexes in neat DMSO and pyridine. Bis-
coordination of two pyridine molecules to afford the
octahedral complex was indicated by UV−visible (UV−vis)
studies, and DFT calculations revealed orbital compositions
expected for a trianionic ligand coordinated to a low-spin
(B3LYP functional) or intermediate-spin (OLYP functional)
cobalt(III) center. However, the exact electronic structure of
the investigated species remains largely unknown at this point.
The aforementioned studies on cobalt-DPP complexes
indicate that intermediate- and low-spin configurations on
cobalt are energetically close and that the DPP ligand is redox-
active on cobalt. Because of these properties we selected the
Co-DPP system as a suitable candidate to evaluate the
influence of the redox-active ligand on the total spin state of
the cobalt complex in the presence and absence of axial redox-
innocent donor ligands. Specifically, in this work we study the
electronic configuration of a neutral [CoII(DPP·2−)] complex
(with Ar = C6F5; R1 = R3 = tBu; R2 = H, Figure 1), bearing a
new DPP ligand derivative, upon coordination of different axial
donor ligands with experimental (X-ray diffraction (XRD), μeff,
NMR, high-resolution mass spectrometry (HRMS), UV−vis)
and computational (DFT, NEVPT2-CASSCF) techniques. We
thereby describe how the molecular orbitals are influenced by
Figure 1. General structure of a dipyrrin-bisphenol (DPP) ligand on a
metal (M).
Hereafter, the redox activity was further studied in Mn,13 Pt,14
Cu,15 and Au16 complexes. Catalytic applications have been
reported for the Ti, Zr, Gn, and Sn complexes (copolymeriza-
tion of epoxides with CO2) and Cu (aerobic alcohol
oxidation).9,15 Contrarily, cobalt(III)-DPP complexes proved
catalytically inactive in the epoxide ring-opening reaction with
alcohols, which was attributed to the low Lewis acidity of the
cobalt center.17
Initial studies on cobalt-DPP complexes were mainly
focused on the comparison of their (redox) properties and
(catalytic) reactivity with cobalt-porphyrin, -salen, and -corrole
analogues. The ligand was predominantly found to coordinate
as a dianionic (radical) ligand to a low-spin cobalt(II) center in
neutral Co-DPP complexes.12 Density functional theory
(DFT) calculations indicated that the triplet and broken-
symmetry open-shell singlet (BSS) ground state (inferring
(anti)ferromagnetic coupling between the metal- and ligand-
a
Scheme 1
a
(A) Synthesis of DDPH3 and [CoII(DPP·2−)]. (B) Displacement ellipsoid plot (50% probability level) of DPPH3. (C) Displacement ellipsoid
plot (50% probability level) of [CoII(DPP·2−)]. H atoms (except NH and OH) and disorder are omitted for clarity. (D) Selection of active orbitals,
occupancies in parentheses, and electronic structure from a NEVPT2-CASSCF(18,14) calculation on [CoII(DPP·2−)]. Isosurface set at 80.
B
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
coordination of tetrahydrofuran (THF), pyridine, and primary
amines and elucidate the exact electronic structures of these
complexes and the influence of the redox-active ligand on the
orbital changes upon coordination of axial donors.
determined by the Evans method,21 also afforded a μeff of 0
μB. The combined XRD, NMR spectroscopic, and magneto-
chemical data thus indicate a diamagnetic ground state,
resulting from the strong antiferromagnetic coupling of the
two unpaired electrons, yielding an overall (open-shell) S = 0
singlet spin state.
To study the electronic structure, we initially performed
DFT calculations at the B3LYP/def2-SVP//B3LYP/def2-
TZVP level of theory, employing an m4 grid and Grimme’s
version 3 dispersion corrections (see the Supporting
Information for more details). The calculated bond metrics
for [CoII(DPP·2−)] in the open-shell singlet state closely
resemble those found in the crystal structure (see Table S1 in
the Supporting Information) and show similar alternating C−
C bond lengths, consistent with oxidation of the ligand to the
DPP·2− redox state. The relative energies of the open-shell
singlet (ΔG°298 K = +1.3 kcal mol−1), triplet (ΔG°298 K = 0.0
kcal mol−1), and closed-shell singlet (ΔG°298 K = +14.8 kcal
mol−1) are consistent with the proposed open-shell (biradical)
character of [CoII(DPP·2−)], but they fail to reproduce the
experimentally observed (open-shell) singlet spin state being
the ground state of the molecule.
Distinguishing between a triplet and a multireference
broken-symmetry singlet (BSS) electronic structure is (nearly)
impossible when relying on single-reference DFT calcula-
tions.22,23 We therefore investigated the electronic structure of
[CoII(DPP·2−)] with multiconfigurational N-electron valence
state perturbation theory (NEVPT2)-corrected complete
active space self-consistent field (CASSCF) calculations (see
the Supporting Information), a method we previously used
successfully to study the orbital compositions of cobalt
complexes bearing a redox-active ligand.24 A NEVPT2-
CASSCF(18,14) calculation, employing 18 electrons in 14
active orbitals on [CoII(DPP·2−)], converged on the singlet
surface and showed a dominant (>96%) contribution from a
multireference open-shell singlet (OSS) electronic configu-
ration of [CoII(DPP·2−)]. A pure triplet spin-state solution
could not be found in this, nor in a reduced, active space. State
averaging of the singlet and triplet state in a 50:50 mixture in
the active space did afford a solution for the triplet spin state,
but this triplet state was found to be +6.5 kcal mol−1 less stable
than the OSS state.
RESULTS AND DISCUSSION
■
Synthesis and Open-Shell Singlet Electronic Ground-
State Configuration of [CoII(DPP·2−)]. The dipyrrin-
bisphenol ligand DPPH3, bearing two tert-butyl groups on
the phenol ring and a pentafluorophenyl substituent on the
meso-position, was obtained via a four-step synthesis in 65%
overall isolated yield according to modified literature
procedures (see Scheme S1 in the Supporting Information
and Scheme 1A).8,12 Coordination of cobalt(II) and in situ
oxidation to the neutral complex was achieved according to an
adapted literature procedure12 by employing Co(OAc)2·
4(H2O) and NEt3 under aerobic conditions to afford
[CoII(DPP·2−)] as a purple powder in 88% isolated yield.
Slow evaporation of a concentrated solution of DPPH3 in
CH2Cl2 afforded single crystals suitable for X-ray structure
determination (Scheme 1B). Single crystals suitable for XRD
analysis of [CoII(DPP·2−)] were also obtained in a similar
manner. The molecular structure of the latter is depicted in
Scheme 1C and shows a slightly distorted square planar
geometry around cobalt. This distortion is most likely caused
by the steric repulsion between the ortho-tert-butyl substituents
on the phenolate rings. Comparison of the bond lengths in
[CoII(DPP·2−)] with those found in the fully aromatic DPPH3
ligand shows alternating elongation and shortening of the C−C
bond lengths (see Table S1 in the Supporting Information),
consistent with the loss of aromaticity due to the oxidation of
the DPP ligand in the complex. The bond lengths are similar
to a previously described DPP ligand in the dianionic (radical)
state on cobalt(II),12 thus supporting the proposed DPP·2−
oxidation state of the ligand. The two 2-pyrrolylphenolato
fragments in [CoII(DPP·2−)] have similar bond metrics,
indicating a fully conjugated ligand and a delocalized ligand-
centered radical coordinated to a cobalt(II) center in the
neutral [CoII(DPP·2−)] complex.19
1H NMR analysis of [CoII(DPP·2−)] in CD2Cl2 showed two
remarkably downfield-shifted resonances at δ = 12.82 (2H)
and 4.29 (18H) ppm. Note that these signals are observed at,
respectively, δ = 7.03 and 1.54 ppm in DPPH3. All other
resonances are shifted ∼1 ppm relative to the free ligand.
These unusual shifts are suggestive of (minor) paramagnetic
A selection of the active orbitals and their occupations
derived from the NEVPT2-CASSCF(18,14) calculation on
[CoII(DPP·2−)] is depicted in Scheme 1D. The dxy, dyz, dxz,
and a ligand (L) orbital of π symmetry are doubly filled,
1
contributions to the observed chemical shift in the H NMR
2
2
whereas the dx −y orbital is empty. The two main contributors
spectrum, which seems to correlate with the experimentally
determined bond lengths from XRD that suggest a ligand-
centered radical (DDP·2−) and consequently a cobalt(II)
(radical) center. However, whether these apparent para-
magnetic contributions are best explained by an open-shell
singlet ground state (with temperature-independent para-
magnetism (TIP)) or as the result of the population of an
excited higher spin-multiplicity state (either thermally or
induced by weak and dynamic coordination of CD2Cl220) is
unclear at this stage. Nonetheless, these shifts are noteworthy.
Measurement of the effective magnetic moment (μeff) of
[CoII(DPP·2−)] in the solid state as a function of the
temperature with a superconducting quantum interference
device (SQUID), to investigate the coupling of the two
unpaired electrons, showed no significant magnetization in the
5−290 K range (see Figure S1 in the Supporting Information).
The effective magnetic moment in CD2Cl2 solution, as
2
to the multireference OSS solution are described by the dz
orbital, which has a bonding and antibonding combination
2
with the π-frame of the ligand (Lπ+dz −L′π) or is nonbonding
2
(dz ). Specifically, 50.6% of the total wave function is described
2
2
by a doubly filled Lπ+dz −L′π orbital (and empty dz ), while
2
45.5% of the wave function is described by a doubly filled dz
2
(and empty Lπ+dz −L′π orbital). The electronic structure of
[CoII(DPP·2−)] is thus best described as an open-shell singlet
1
based on the combined experimental (XRD, H NMR, μeff)
and computational (NEVPT2-CASSCF) studies. Effectively,
2
one unpaired electron resides in the dz orbital on cobalt, and
another unpaired electron is fully delocalized over the ligand
2
with a small contribution from the dz orbital on cobalt. As
such, this complex is best described as a system containing
antiferromagnetically coupled cobalt(II)- and DPP·2− ligand-
centered unpaired electrons.
C
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 2. (left) UV−Vis spectra of [CoII(DPP·2−)] in pure THF ([CoII(DPP·2−)(THF)2], 32.27 μM, green solid line), in CH2Cl2 (32.27 μM,
1
purple solid line), and titration of THF to this CH2Cl2 solution (dashed lines and solid light green, orange and brown line). (right) H NMR
spectra of [CoII(DPP·2−)] dissolved in pure CD2Cl2 (5.89 mM, (A, F), purple), CD2Cl2 with 1.1 M THF ((B), THF/THF-d8 = 1:9), 2.0 M THF
((C), THF/THF-d8 = 1:20), 3.8 M THF ((D), THF/THF-d8 = 1:45) and pure THF-d8 ((E), green). The sample was concentrated and
thoroughly dried between the measurements (E, F).
Scheme 2. Formation of [CoII(DPP·2−)(THF)2] via
Spin-Flip to a Triplet State upon Coordination of THF
a
on [CoII(DPP·2−)] to Generate [CoII(DPP·2−)(THF)2].
Whereas [CoII(DPP·2−)] is purple in non-coordinating
solvents (CH2Cl2, toluene) we noticed a distinct color change
to green upon solvation of the complex in coordinating
solvents (THF, MeOH, MeCN), indicative of solvent
coordination. The UV−vis spectra of [CoII(DPP·2−)] in
THF (solid green line, λmax = 318, 409, 423, 474, 632, and
833 nm) and CH2Cl2 (solid purple line, λmax = 326, 374, 556,
and 755 nm) are shown in Figure 2 left. Titration of THF
(guest) to a CH2Cl2 solution of [CoII(DPP·2−)] (host)
afforded spectral changes in the UV−vis spectra characteristic
for multiple binding events (see Figure 2 left and Figure S4 in
the Supporting Information). Isosbestic points are found in
two regimes; between 0 and 1.3 × 104 equiv of THF (solid
purple to solid light green line, λ = 413, 488, 600, 667, 771
nm) and between 5.2 × 104 and 1.4 × 105 equiv of THF (solid
orange to solid brown line, λ = 393, 489, 593, 679, 776 nm).
Between these two regimes the spectral crossing points are
found between the isosbestic points, suggestive of the
simultaneous presence of three species.
Coordination of THF to [CoII(DPP·2−)]
a
Ar = C6F5.
NMR spectrum upon addition of THF to a 5.89 mM CD2Cl2
solution of [CoII(DPP·2−)] (Figure 2 right). The presence of
1.1, 2.0, or 3.8 M THF (spectra B, C, D) led to signal
broadening and gradual downshifting of one resonance
(labeled as a red circle), while three other resonances (labeled
as a blue square, orange triangle, and yellow diamond) are
strongly shifted upfield, approaching the shifts observed in neat
THF-d8 (spectrum E). The observed sharp paramagnetically
shifted resonances in the −65 to +45 ppm region in neat THF-
d8 clearly indicate conversion towards a new open-shell species.
Interestingly, concentrating and thoroughly drying the sample
obtained in neat THF-d8, followed by dissolution in CD2Cl2,
afforded a purple solution for which the spectral data (1H
NMR, UV−vis) exactly matched that of [CoII(DPP·2−)]
(spectrum F). These combined data thus point to weak and
reversible binding of THF to the square planar complex,
consistent with the low Kass as derived from the UV−vis
spectroscopic titration study.
No THF adducts were observed by HRMS, presumably due
to the reversible weak binding and low boiling point of THF,
which is likely easily lost in the ionization process. Attempts to
crystallize a THF adduct of [CoII(DPP·2−)] were unfortu-
nately unsuccessful. Determination of the effective magnetic
moment of [CoII(DPP·2−)(THF)2] in THF-d8 by the Evans
method21 afforded μeff = 2.91 μB, indicating the formation of a
triplet (S = 1) complex.
The titration data could be fitted25 to weak noncooperative
host−guest−guest binding with an overall association constant
(Kass) of 1.2 M−1 for binding of two THF molecules (see
Figure S2 in the Supporting Information). The data are
therefore consistent with the initial (predominant) formation
of a mono-THF adduct (first regime), followed by the
formation of a bis-THF adduct (second regime, see Figure
S4 in the Supporting Information). Full conversion to this
latter species is not reached at the end of the depicted titration
curve (brown solid line, Figure 2 left) and is only observed in
neat THF (solid green line, Figure 2 left), as indicated by the
increased absorbance of various spectral bands and the
shoulder at 474 nm. Similar mono- and bis-coordination of
solvent has been described in the literature for related Co-DPP
complexes,18 and in combination with the titration data in
Figure 2 we thus propose the formation of a bis-THF adduct
([CoII(DPP·2−)(THF)2]) in neat THF (Scheme 2).
To further study the coordination of THF to
1
[CoII(DPP·2−)] we followed the spectral changes in the H
D
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
DFT calculations (B3LYP/def2-SVP//B3LYP/def2-TZVP,
m4 grid, Grimme’s version 3 dispersion corrections) indicated
that both the square pyramidal mono-THF adduct
([CoII(DPP·2−)(THF)]) and the octahedral bis-THF complex
([CoII(DPP·2−)(THF)2]) have a triplet spin (S = 1) ground
state, consistent with the experimentally determined spin
state.26 To obtain more insight in the electronic structure of
[CoII(DPP·2−)(THF)2] and to investigate possible multi-
reference contributions to the ground-state wave function, we
performed NEVPT2-CASSCF(18,15) calculations on the
singlet, triplet, and quintet spin surfaces. The triplet state
was again found to be the lowest in energy, with the (open-
shell) singlet and quintet states being disfavored by +32.2 and
+33.0 kcal mol−1, respectively. Dominant multireference
character was observed in the triplet spin state, leading to an
interesting electronic structure wherein cobalt retains the +II
oxidation state and is ferromagnetically coupled to a ligand-
centered radical on the DDP·2− ligand (Figure 3).
orbital in comparison to the more localized (less diffuse)
dyz−PyrLπ orbital. Consequently, in the multiconfigurational
description of the total triplet spin state wave function, the
ligand-centered unpaired electron is mainly (46.1%) localized
on the least-diffuse orbital (dyz−PyrLπ).
The spin−flip in the transition from [CoII(DPP·2−)] (OSS)
to [CoII(DPP·2−)(THF)2] (triplet) can be understood by
looking at the composition of the SOMOs. The α- and β-spin
II
2−
2
electrons in [Co (DPP· )] are located in the dz -based
2
2
orbitals (dz and Lπ+dz −L′π). A triplet state would lead to
severe (α) spin−spin repulsion in this cobalt-centered orbital,
and consequently an OSS solution is favored. This is not the
case for [CoII(DPP·2−)(THF)2], wherein the two unpaired
2
electrons reside in spatially different orbitals (dz and dyz-
based). In this case Hund’s rule28 applies, which states that the
maximization of the total spin is favored for a given electronic
configuration, thus leading to the observed triplet spin state.
Closed-Shell Singlet Spin State via Metal-to-Ligand
Single-Electron Transfer Induced by Coordination of
Stronger Donors. We next set out to explore the influence of
replacing the weak-field ligand THF with the stronger-field
ligand pyridine. Addition of excess (100 equiv) pyridine to
[CoII(DPP·2−)] in CH2Cl2 afforded quantitative formation of
the bis-pyridine adduct [CoIII(DPP3−)(Py)2] as a green
powder after workup (Scheme 3A). The six-coordinate
complex was characterized inter alia by 1H NMR spectroscopy
and positive mode cold-spray ionization (CSI) HRMS. Single
crystals suitable for X-ray structure determination were grown
by slow evaporation of a concentrated solution of the complex
in a 5:1 mixture of CH2Cl2 and MeOH. Three octahedral
[CoIII(DPP3−)(Py)2] complexes are present in the unit cell
(see Figure S6 in the Supporting Information), one of which
(the left structure) is depicted in Scheme 3B. The bond
metrics of all three [CoIII(DPP3−)(Py)2] molecules are similar,
although the relative rotation of the pyridine ligands differs
from nearly parallel to perpendicular (see Table S2 in the
Supporting Information).
The experimentally determined C−C bond lengths of the
DPP moiety (see the Supporting Information) in the crystal
structure of [CoIII(DPP3−)(Py)2] resemble the aromaticity
that is also observed in the free DPPH3 ligand, thus suggesting
a fully reduced trianionic DPP3− redox state for the ligand and
consequently a cobalt(III) center in the neutral complex. The
1H NMR resonances of [CoIII(DPP3−)(Py)2] do not show any
paramagnetic shifts and are found entirely in the diamagnetic
region (δ = 8.04−1.25 ppm), suggesting a closed-shell singlet
electronic configuration, that is, a low-spin CoIII center. The
SQUID analysis of solid [CoIII(DPP3−)(Py)2] did not show
significant magnetization in the 4−290 K range, again
consistent with a singlet ground state (see Figure S7 in the
Supporting Information).
Figure 3. Selected active orbitals, occupancies in parentheses, and
electronic structure of cobalt and the ligand from a NEVPT2-
CASSCF(18,15) calculation on [CoII(DPP·2−)(THF)2]. Isosurface
set at 80.
DFT calculations (B3LYP/def2-SVP//B3LYP/def2-TZVP,
m4 grid, Grimme’s version 3, dispersion corrections) indicated
that formation of the closed-shell singlet octahedral bis-
pyridine [CoIII(DPP3−)(Py)2] complex is more exergonic
(ΔG°298 K = −14.2 kcal mol−1) than the formation of the
square pyramidal monopyridine adduct [CoII(DPP·2−)(Py)]
(ΔG°298 K = −9.1 kcal mol−1, S = 1). Orbital analysis clearly
showed that coordination of pyridine in [CoIII(DPP3−)(Py)2]
(Scheme 3C) leads to a strongly destabilized dz orbital (dz −
2NPy,σ), resulting in a quite large gap between the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of 0.10346 Eh and,
With the dxy, dxz, and dyz orbitals on cobalt doubly filled, the
unpaired (and uncorrelated) α-spin electron on cobalt resides
in the dz orbital (Figure 3). The other α-spin electron mainly
2
resides in the strongly correlated antibonding combinations of
the dyz orbital with the ligand pyrrole π-framework (dyz−PyrLπ,
−0.262 54 Eh, 46.1% only α-spin, 1.51 total electron
occupation) and the complete ligand π-system (Lπ−dyz,
−0.208 87 Eh, 34.9% only α-spin, 1.59 total electron
occupation).27 The energetically slightly higher lying Lπ−dyz
orbital is more diffuse over the ligand π-system, thus leading to
a smaller electron−electron repulsive interaction (i.e., pairing
energy of the α- and β-spin electrons) upon filling of this
2
2
E
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
a
Scheme 3
a
(A) Formation of [CoIII(DPP3−)(Py)2]. (B) Displacement ellipsoid plot (50% probability level) of one [CoIII(DPP3−)(Py)2] molecule. H atoms
and disorder are omitted for clarity. (C) Selection of DFT-calculated orbitals and the electronic structure of cobalt and the ligand in
[CoIII(DPP3−)(Py)2]. Isosurface set at 80.
a
Scheme 4
a
(A) Formation of [CoIII(DPP3−)(NH2tBu)2] and [CoIII(DPP3−)(NH2Ad)2]. Ad = 1-adamantyl. (B) Displacement ellipsoid plot (50%
probability level) of [CoIII(DPP3−)(NH2Ad)2]. (C) Displacement ellipsoid plot (50% probability level) of [CoIII(DPP3−)(NH2tBu)2]. H atoms
and disorder are omitted for clarity.
therefore a low-spin (CSS) configuration. The Lπ−dyz
(HOMO) is doubly filled, reflecting the reduction of the
ligand to the DPP3− state. Cobalt adopts the +III oxidation
state in [CoIII(DPP3−)(Py)2] with doubly filled dxy, dxz, and
F
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
dyz orbitals, which are stabilized due to the higher oxidation
state of cobalt in comparison to [CoII(DPP·2−)]. Thus,
pyridine coordination effectively results in ligand reduction
via metal-to-ligand single-electron transfer. Interestingly,
interaction of the pyridine-π* system with the dyz orbital is
observed in the dyz−2NPy,π* orbital, reflecting at least some π-
back-donation from cobalt to the pyridine ligands.
Coordination of pure σ-donors was achieved via addition of
the primary amines tert-butyl amine and 1-adamantyl amine.
The complex [CoIII(DPP3−)(NH2tBu)2] was obtained in
quantitative yield as a green powder through addition of 100
equiv of tBuNH2 to [CoII(DPP·2−)] and subsequent
concentration and drying under reduced pressure (Scheme
4A). [CoIII(DPP3−)(NH2Ad)2] (Ad = 1-adamantyl) was
obtained in 43% yield as green crystals after addition of 2
equiv of AdNH2 to [CoII(DPP·2−)] and subsequent
crystallization.
The 1H NMR resonances of [CoIII(DPP3−)(NH2tBu)2] and
[CoIII(DPP3−)(NH2Ad)2] are similar to the bis-pyridine
adduct, found within the diamagnetic region, suggesting that
both complexes are most stable in the CSS spin state. Crystals
suitable for X-ray structure determination of both complexes
were obtained by slow evaporation of concentrated solutions in
CH2Cl2 and MeOH (5:1) at room temperature. The C−C and
Co−DPP bond lengths obtained from the crystal structure of
[CoIII(DPP3−)(NH2Ad)2] (Scheme 4B) closely resemble
those found in [CoIII(DPP3−)(Py)2] and are consistent with
a fully reduced (3−) redox state of the DPP ligand and
consequently a cobalt(III) center. The crystallographically
independent molecules of [CoIII(DPP3−)(NH2tBu)2] found in
the asymmetric unit (see Figure S9 in the Supporting
Information) have mutually similar bond metrics, which are
also comparable to those observed in [CoIII(DPP3−)-
(NH2Ad)2]. One molecule found in the crystal structure of
[CoIII(DPP3−)(NH2tBu)2] is depicted in Scheme 4C.
Figure 4. Selection of DFT calculated orbitals and electronic structure
of cobalt and the ligand in [CoIII(DPP3−)(NH2tBu)2]. Isosurface set
at 70.
The DFT (B3LYP/def2-SVP//B3LYP/def2-TZVP, m4 grid,
Grimme’s version 3, dispersion corrections) calculated bond
lengths of [CoIII(DPP3−)(NH2tBu)2] in the CSS spin state are
consistent with the experimentally determined bond metrics
(see Table S3 in the Supporting Information). Moreover, the
corresponding triplet spin state was found to be +4.0 kcal
mol−1 less stable. The DFT orbital analysis of [CoIII(DPP3−)-
pyridine (σ-donor, weak π-acceptor), tBuNH2, or AdNH2 (σ-
donors) afforded the closed-shell singlet octahedral complexes
via metal-to-ligand single-electron transfer. The redox-active
DPP ligand is reduced to the trianionic redox state, and cobalt
adopts a low-spin +III oxidation state.
Concluding, we have described that a redox-active DPP
ligand on cobalt can accommodate three different spin states of
the complex within an integer spin system. The spin-state
changes are induced via coordination of axial ligands to the
square-planar complex, but the relative energy and overlap of
the ligand- and cobalt-centered orbitals determines the most
stable spin state. The capability of the redox-active ligand to
stabilize unpaired electrons and accommodate intramolecular
electron transfer was found to be crucial in this context.
2
(NH2tBu)2] (Figure 4) shows a destabilized empty dz orbital
due to coordination of the tBuNH2 lone σ-pair (NA,σ).
However, the bonding interactions of these lone pairs with the
dyz and dxz orbitals are observed in the doubly filled
dyz+2NA,σ−Lπ and PyrLπ−dxz+2NA,σ orbitals. The dxy, dyz, and
dxz orbitals are all doubly filled, consistent with a low-spin
cobalt(III) electronic configuration formed after metal-to-
ligand single−electron transfer.
CONCLUSIONS
■
The electronic ground state of [CoII(DPP·2−)] is characterized
as a multiconfigurational open-shell singlet, which is best
described as a system containing antiferromagnetically coupled
cobalt(II)- and ligand-centered unpaired electrons. Solvation
of this complex in THF (sp3-hybrid donor) affords the clean
formation of a THF-adduct, [CoII(DPP·2−)(THF)2], which
resides in the triplet spin state. The origin of this spin flip is the
orbital overlap of the redox-active ligand π-framework with the
cobalt d-orbitals, which leads to the population of two ligand-
dyz orbital combinations in a multiconfigurational triplet
solution to reduce spin−spin repulsion. Coordination of
EXPERIMENTAL SECTION
■
General Considerations. All reagents were of commercial grade
and used without further purification, unless noted otherwise. All
reactions were performed under an inert atmosphere in a N2-filled
glovebox or by using standard Schlenk techniques (under Ar or N2),
unless noted otherwise. CH2Cl2 and MeOH were distilled from
CaH2; toluene was distilled from sodium, and THF was distilled from
sodium benzophenone ketyl. Detailed information regarding the
NMR, HRMS, UV−vis, SQUID, and XRD measurements is included
in the Supporting Information. XRD- and DFT-derived bond lengths
are also included in the Supporting Information. The magnetic
moments in solution were determined via the Evans method.21
G
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Synthesis and Characterization. DPPH3. Synthesized in four
steps (overall isolated yield 65%) according to adapted literature
procedures.8,10 Characterized by 1H and 19F NMR, HRMS-FD+,
elemental analysis, and XRD (see the Supporting Information).
[CoII(DPP·2−)]. Prepared in 88% isolated yield according to a
literature procedure for the insertion of cobalt in a DPP ligand.12
AUTHOR INFORMATION
Corresponding Author
■
Bas de Bruin − Homogeneous, Supramolecular and Bio-Inspired
Catalysis Group, van’t Hoff Institute for Molecular Sciences,
University of Amsterdam, Amsterdam 1098 XH, The
Netherlands; orcid.org/0000-0002-3482-7669;
Email: b.debruin@uva.nl
1
Characterized by H and 19F NMR, HRMS-FD+, UV−vis, elemental
analysis, μeff (Evans method and SQUID), and XRD (see the
Supporting Information).
[CoII(DPP·2−)(THF)2]. Quantitatively prepared by solvation of
[CoII(DPP·2−)] in THF. Characterized by 1H NMR, μeff (Evans
method), and UV−vis (see the Supporting Information).
[CoIII(DPP3−)(Py)2]. Obtained in quantitative isolated yield by the
addition of pyridine (100 equiv) to a solution of [CoII(DPP·2−)] in
Authors
Nicolaas P. van Leest − Homogeneous, Supramolecular and
Bio-Inspired Catalysis Group, van’t Hoff Institute for Molecular
Sciences, University of Amsterdam, Amsterdam 1098 XH, The
Netherlands
Wowa Stroek − Homogeneous, Supramolecular and Bio-Inspired
Catalysis Group, van’t Hoff Institute for Molecular Sciences,
University of Amsterdam, Amsterdam 1098 XH, The
Netherlands
Maxime A. Siegler − Department of Chemistry, John Hopkins
University, Baltimore 21218, Maryland, United States;
orcid.org/0000-0003-4165-7810
Jarl Ivar van der Vlugt − Homogeneous, Supramolecular and
Bio-Inspired Catalysis Group, van’t Hoff Institute for Molecular
Sciences, University of Amsterdam, Amsterdam 1098 XH, The
Netherlands
1
CH2Cl2. Characterized by H and 19F NMR, HRMS-CSI+, UV−vis,
elemental analysis, μeff (SQUID), and XRD (see the Supporting
Information).
[CoIII(DPP3−)(NH2tBu)2]. Obtained in quantitative isolated yield
by the addition of tBuNH2 (100 equiv) to a solution of
[CoII(DPP·2−)] in CH2Cl2. Characterized by 1H and 19F NMR,
HRMS-CSI+, UV−vis, and XRD (see the Supporting Information).
[CoIII(DPP3−)(NH2Ad)2]. Obtained in 43% isolated yield by
addition of AdNH2 (2.0 equiv) to a solution of [CoII(DPP·2−)] in
1
CH2Cl2. Characterized by H and 19F NMR, HRMS-CSI+, and XRD
(see the Supporting Information).
Computational Studies. DFT. Calculations were conducted on
full atomic models at the B3LYP29/def2-SVP30//B3LYP/def2-
TZVP30 level of theory on an m4 grid with Grimme’s version 331
(“zero-damping”) dispersion corrections with the TURBOMOLE
7.332 software package coupled to the PQS Baker optimizer33 via the
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01979
BOpt package.34 Orbital interpretation was performed by Lowdin
̈
Author Contributions
population analysis of quasi-restricted orbitals (QRO) generated with
the ORCA 4.135 software package at the B3LYP/def2-TZVP level,
using the coordinates from the structures optimized in TURBO-
MOLE as the input and using the UNO keyword. Graphical
representations of orbitals were generated using IboView.36 Energies,
xyz coordinates, and more details on the calculations are included in
the Supporting Information.
NEVPT2-CASSCF. The NEVPT2-corrected CASSCF calculations
were performed with the ORCA 4.135 software package on the
geometries optimized in TURBOMOLE. The def2-TZVP30 basis set
was used together with the RIJCOSX37 approximation in conjunction
with the def2-TZVP/C fitting the basis set to reduce computational
cost. The single-root spin state was calculated. The NEVPT238
calculations using the RI approximation were performed on
converged CASSCF wave functions. Energies, contributions to the
wave functions, full representations of the active spaces, and more
details on the calculations are included in the Supporting Information.
All authors have given approval to the final version of the
manuscript.
Funding
This research was funded by The Netherlands Organization for
Scientific Research TOP-Grant 716.015.001 to B.d.B.
Notes
The authors declare no competing financial interest.
CCDC 2012086 (DPPH3), 2012087 ([CoII(DPP·2−)]),
2012088 ([CoIII(DPP3−)(Py)2]), 2012089 ([CoIII(DPP3−)-
(NH2tBu)2]), and 2012090 ([CoIII(DPP3−)(NH2Ad)2]).
ACKNOWLEDGMENTS
■
Financial support from The Netherlands Organization for
Scientific Research (NWO TOP-Grant 716.015.001) to B.d.B.
and the research area Sustainable Chemistry of the University
of Amsterdam (RPA SuSChem, UvA) is gratefully acknowl-
edged. Dr. E. Bill (MPI CEC) is acknowledged for the SQUID
measurements and related discussions. E. Zuidinga is thanked
for the HRMS measurements, and T. Bouwens is thanked for
the valuable discussions regarding the UV−vis titration data.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01979.
Experimental details, synthetic procedures, NMR,
HRMS, and UV−vis spectra, crystallographic refinement
details, geometries (xyz coordinates) and energies of
DFT-calculated structures, description of the NEVPT2-
CASSCF calculations (PDF)
REFERENCES
■
(1) Likhtenshtein, G. I. Fundamentals, Methods, Reactions
Mechanisms, Magnetic Phenomena, Structure Investigation. In
Electron Spin Interactions in Chemistry and Biology; Springer Interna-
tional Publishing, 2016.
(2) Molecular Magnets Recent Highlights; Linert, W., Verdaguer, M.,
Eds.; Springer International Publishing, 2003.
(3) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.;
Accession Codes
CCDC 2012086−2012090 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
́
von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M.
Spintronics: A Spin-Based Electronics Vision for the Future. Science
2001, 5546, 1488−1495.
̈
(4) Dzik, W. I.; Bohmer, W.; de Bruin, B. In Spin States in
Biochemistry and Inorganic Chemistry: Influence on Structure and
Reactivity; Swart, M., Costas, M., Eds.; Wiley, 2015; pp 103−130.
H
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
1
(5) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 5th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2009.
(6) (a) Lyaskovskyy, V.; de Bruin, B. Redox Non-Innocent Ligands:
Versatile New Tools to Control Catalytic Reactions. ACS Catal. 2012,
2, 270−279. (b) van Leest, N. P.; Epping, R. F. J.; van Vliet, K. M.;
Lankelma, M.; van den Heuvel, E.; Heijtbrink, N.; Broersen, R.; de
oxidation state is also evident from H NMR analysis due to two
downshifted resonances and was also consistent with the NEVPT2-
CASSCF calculations.
(20) (a) Newbound, T. D.; Colsman, M. R.; Miller, M. M.;
Wulfsberg, G. P.; Anderson, O. P.; Strauss, S. H. Dichloromethane is a
coordinating solvent. J. Am. Chem. Soc. 1989, 111, 3762−3764.
(b) Kulawiec, R. J.; Crabtree, R. H. Coordination chemistry of
halocarbons. Coord. Chem. Rev. 1990, 99, 89−115.
́
Bruin, B. In Advances in Organometallic Chemistry; Perez, P. J., Stone,
F. G. A., West, R., Eds.; Elsevier, 2018; Vol. 70, pp 71−180.
(c) Broere, D. L. J.; Plessius, R.; van der Vlugt, J. I. New avenues for
ligand-mediated processes - expanding metal reactivity by the use of
redox-active catechol, o-aminophenol and o-phenylenediamine
ligands. Chem. Soc. Rev. 2015, 44, 6886−6915. (d) van der Vlugt, J.
I. Radical-Type Reactivity and Catalysis by Single-Electron Transfer
to or from Redox-Active Ligands. Chem. - Eur. J. 2019, 25, 2651−
5662.
(21) (a) Evans, D. F. The determination of the paramagnetic
susceptibility of substances in solution by nuclear magnetic resonance.
J. Chem. Soc. 1959, 2003−2005. (b) Deutsch, J. L.; Poling, S. M. The
determination of paramagnetic susceptibility by NMR: A physical
chemistry experiment. J. Chem. Educ. 1969, 46, 167−168. (c) Piguet,
C. Paramagnetic Susceptibility by NMR: The “Solvent Correction”
Removed for Large Paramagnetic Molecules. J. Chem. Educ. 1997, 74,
815−816. (d) Sur, S. K. Measurement of magnetic susceptibility and
magnetic moment of paramagnetic molecules in solution by high-field
fourier transform NMR spectroscopy. J. Magn. Reson. 1989, 82, 169−
173. (e) Grant, D. H. Paramagnetic Susceptibility by NMR: The
“Solvent Correction” Reexamined. J. Chem. Educ. 1995, 72, 39.
(f) Hoppe, J. I. Effective magnetic moment. J. Chem. Educ. 1972, 49,
505. (g) Bain, G. A.; Berry, J. F. Diamagnetic Corrections and Pascal’s
Constants. J. Chem. Educ. 2008, 85, 532−536.
(7) Bloom, S. M.; Garcia, P. P. U.S. Patent 3691161, 1972.
(8) Ikeda, C.; Ueda, S.; Nabeshima, T. Aluminium complexes of
N2O2-type dipyrrins: the first hetero-multinuclear complexes of
metallo-dipyrrins with high fluorescence quantum yields. Chem.
Commun. 2009, 2544−2546.
(9) Sumiyoshi, A.; Chiba, Y.; Matsuoka, R.; Noda, T.; Nabeshima, T.
Efficient luminescent properties and cation recognition ability of
heavy group 13 element complexes of N2O2- and N2O4-type
dipyrrins. Dalton Trans. 2019, 48, 13169−13175.
(10) Nakano, K.; Kobayashi, K.; Nozaki, K. Tetravalent Metal
Complexes as a New Family of Catalysts for Copolymerization of
Epoxides with Carbon Dioxide. J. Am. Chem. Soc. 2011, 133, 10720−
10723.
(22) Cramer, C. J. Essentials of Computational Chemistry: Theories
and Models, 2nd ed; John Wiley & Sons Ltd: West Sussex, U.K., 2004;
pp 205−210.
(23) DFT, being a single reference method, is not suitable to
accurately describe a multireference electronic structure, but it can be
used to accurately calculate the molecular structure with reasonable
computational costs (see the Supporting Information for comparison
with experimentally determined bond lengths and geometries).
Performing these geometry optimizations with multireference
NEVPT2-CASSCF calculations is unfeasible in terms of computa-
tional costs. However, the NEVPT2-CASSCF calculations can be
used to correctly describe the multireference electronic structure, and
therefore the two computational methods are complementary.
(24) (a) van Leest, N. P.; Tepaske, M. A.; Oudsen, J.-P. H.;
Venderbosch, B.; Rietdijk, N. R.; Siegler, M. A.; Tromp, M.; van der
Vlugt, J. I.; de Bruin, B. Ligand Redox Noninnocence in [CoIII-
(TAML)]0/- Complexes Affects Nitrene Formation. J. Am. Chem. Soc.
2020, 142, 552−563. (b) van Leest, N. P.; Tepaske, M. A.;
Venderbosch, B.; Oudsen, J.-P. H.; Tromp, M.; van der Vlugt, J. I.;
de Bruin, B. Electronically Asynchronous Transition States for C-N
Bond Formation by Electrophilic [CoIII(TAML)]-Nitrene Radical
Complexes Involving Substrate-to-Ligand Single-Electron Transfer
and a Cobalt-Centered Spin Shuttle. ACS Catal. 2020, 10, 7449−
7463.
(11) (a) Rausaria, S.; Kamadulski, A.; Rath, N. P.; Bryant, L.; Chen,
Z.; Salvemini, D.; Neumann, W. L. Manganese(III) Complexes of
Bis(hydroxyphenyl)dipyrromethenes Are Potent Orally Active Perox-
ynitrite Scavengers. J. Am. Chem. Soc. 2011, 133, 4200−4203. (b) El
́
Ghachtouli, S.; Wojcik, K.; Copey, L.; Szydlo, F.; Framery, E.; Goux-
Henry, C.; Billon, L.; Charlot, M.-F.; Guillot, R.; Andrioletti, B.;
Aukauloo, A. Dipyrrinphenol-Mn(III) complex: synthesis, electro-
chemistry, spectroscopic characterisation and reactivity. Dalton. Trans.
2011, 40, 9090−9093.
(12) Kochem, A.; Chiang, L.; Baptiste, B.; Philouze, C.; Leconte, N.;
Jarjayes, O.; Storr, T.; Thomas, F. Ligand-Centered Redox Activity in
Cobalt(II) and Nickel(II) Bis(phenolate)-Dipyrrin Complexes. Chem.
- Eur. J. 2012, 18, 14590−14593.
(13) Lecarme, L.; Chiang, L.; Moutet, J.; Leconte, N.; Philouze, C.;
Jarjayes, O.; Storr, T.; Thomas, F. The structure of a one-electron
oxidized Mn(III)-bis(phenolate)dipyrrin radical complex and oxida-
tion catalysis control via ligand-centered redox activity. Dalton Trans.
2016, 45, 16325−16334.
(14) Yamamura, M.; Takizawa, H.; Gobo, Y.; Nabeshima, T. Stable
neutral radicals of planar N2O2-type dipyrrin platinum complexes:
hybrid radicals of the delocalized organic π-orbital and platinum d-
orbital. Dalton Trans. 2016, 45, 6834−6838.
(15) Lecarme, L.; Kochem, A.; Chiang, L.; Moutet, J.; Berthiol, F.;
Philouze, C.; Leconte, N.; Storr, T.; Thomas, F. Electronic Structure
and Reactivity of One-Electron-Oxidized Copper(II) Bis(phenolate)-
Dipyrrin Complexes. Inorg. Chem. 2018, 57, 9708−9719.
(16) Thomas, K. E.; Desbois, N.; Conradie, J.; Teat, S. J.; Gros, C.
P.; Ghosh, A. Gold dipyrrin-bisphenolates: a combined experimental
and DFT study of metal-ligand interactions. RSC Adv. 2020, 10, 533−
540.
(17) Feng, Y.; Burns, L. A.; Lee, L.-C.; Sherrill, C. D.; Jones, C. W.;
Murdock, C. Co(III) complexes of tetradentate X3L type ligands:
Synthesis, electronic structure, and reactivity. Inorg. Chim. Acta 2015,
430, 30−35.
(18) Shan, W.; Desbois, N.; Pacquelet, S.; Stephane Brandes;
Rousselin, Y.; Conradie, J.; Ghosh, A.; Gros, C. P.; Kadish, K. M.
Ligand Noninnocence in Cobalt Dipyrrin-Bisphenols: Spectroscopic,
Electrochemical, and Theoretical Insights Indicating an Emerging
Analogy with Corroles. Inorg. Chem. 2019, 58, 7677−7689.
(19) Two-electron oxidation of the DPP ligand to DPP1− is highly
unlikely to be stable on a cobalt(I) center. Furthermore, the DPP·2−
(25) Fitted with http://limhes.net/optim/ (accessed June 2, 2020)
according to an HGG model with a cooperativity parameter of 1.
(26) See the Supporting Information for the calculated exchange
coupling constant for the two unpaired electrons and the NEVPT2-
CASSCF(14,10) calculations on [CoII(DPP·2−)(THF)].
(27) The larger occupation number for the energetically higher
correlated orbital is sometimes described as a HOMO-SOMO
inversion or a non-Aufbau orbital configuration. This is however, at
least in our case, purely caused by the increased electron−electron
repulsion (i.e., destabilization) upon double filling of the orbital. See:
(a) Kumar, A.; Sevilla, M. D. SOMO-HOMO Level Inversion in
Biologically Important Radicals. J. Phys. Chem. B 2018, 122, 98−105.
(b) Gryn’ova, G.; Coote, M. L.; Corminboeuf, C. Theory and practice
of uncommon molecular electronic configurations. WIREs Comput.
Mol. Sci. 2015, 5, 440−459.
(28) Atkins, P.; Overton, T.; Rourke, J.; Weller, M.; Armstrong, F.
Shriver & Atkins’ Inorganic Chemistry, 5th ed.; Oxford University
Press: Oxford, U.K., 2010; pp 18−47.
(29) (a) Becke, A. D. Density-functional thermochemistry. III. The
role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti
I
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
correlation-energy formula into a functional of the electron density.
Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789.
(30) (a) Weigend, F.; Ahlrichs, R. Balanced basis sets of split
valence, triple zeta valence and quadruple zeta valence quality for H to
Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys.
2005, 7, 3297−3305. (b) Weigend, F.; Haser, M.; Patzelt, H.;
Ahlrichs, R. RI-MP2: optimized auxiliary basis sets and demonstration
of efficiency. Chem. Phys. Lett. 1998, 294, 143−152.
(31) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and
accurate ab initio parametrization of density functional dispersion
correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010,
132, 154104−154119.
(32) TURBOMOLE, ver. 7.3; TURBOMOLE Gmbh: Karlsruhe,
Germany, 2018.
(33) (a) PQS, ver. 2.4; Parallel Quantum Solutions: Fayetteville, AR,
2001. (b) Baker, J. An algorithm for the location of transition states. J.
Comput. Chem. 1986, 7, 385−395.
(34) Budzelaar, P. H. M. Geometry optimization using generalized,
chemically meaningful constraints. J. Comput. Chem. 2007, 28, 2226−
2236.
(35) Neese, F. The ORCA program system. Wiley Interdiscip. Rev.:
Comput. Mol. Sci. 2012, 2, 73−78.
(36) Available at (a) Knizia, G. Intrinsic Atomic Orbitals: An
Unbiased Bridge between Quantum Theory and Chemical Concepts.
J. Chem. Theory Comput. 2013, 9, 4834−4843. (b) Knizia, G.; Klein, J.
E. M. N. Electron Flow in Reaction Mechanisms-Revealed from First
Principles. Angew. Chem., Int. Ed. 2015, 54, 5518−5522.
(37) Izsak, R.; Neese, F. An overlap fitted chain of spheres exchange
method. J. Chem. Phys. 2011, 135, 144105−144111.
(38) (a) Angeli, C.; Cimiraglia, R.; Evangelisti, S.; Leininger, T.;
Malrieu, J.-P. Introduction of n-electron valence states for multi-
reference perturbation theory. J. Chem. Phys. 2001, 114, 10252−
10264. (b) Angeli, C.; Cimiraglia, R.; Malrieu, J.-P. N-electron valence
state perturbation theory: a fast implementation of the strongly
contracted variant. Chem. Phys. Lett. 2001, 350, 297−305. (c) Angeli,
C.; Cimiraglia, R.; Malrieu, J.-P. N-electron valence state perturbation
theory: A spinless formulation and an efficient implementation of the
strongly contracted and of the partially contracted variants. J. Chem.
Phys. 2002, 117, 9138−9153.
J
https://dx.doi.org/10.1021/acs.inorgchem.0c01979
Inorg. Chem. XXXX, XXX, XXX−XXX