Polynuclear Cu4L4 Copper(II) Aminyl Radical Coordination Complexes

Article abstract of DOI:10.1021/acs.inorgchem.8b00785

We describe the structural features and magnetic properties of two polynuclear copper(II) complexes containing a redox-active ligand. These neutral complexes each bear the formula RL₄Cu₄ (R = ^tBu, Me) with the ligand in a dianion-aminyl radical oxidation state. X-ray data and density functional theory calculations support an aminyl-type radical character in these complexes, making these the first polynuclear metal aminyl radical complexes.

Full text of DOI:10.1021/acs.inorgchem.8b00785

Communication
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Polynuclear Cu4L4 Copper(II) Aminyl Radical Coordination Complexes
Nico M. Bonanno,† Alan J. Lough,‡ and Martin T. Lemaire*,†
†Department of Chemistry, Brock University, St. Catharines, Ontario L2S 3A1, Canada
‡Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
S
* Supporting Information
crystal structures and magnetic properties of 1 and 2 are
ABSTRACT: We describe the structural features and
magnetic properties of two polynuclear copper(II)
complexes containing a redox-active ligand. These neutral
described, and these experimental results are supported by
density functional theory (DFT) calculations. There are very few
reported examples of metal aminyl radical complexes20−30 and,
t
complexes each bear the formula RL4Cu4 (R = Bu, Me)
to our knowledge, none that are polynuclear.
t
with the ligand in a dianion-aminyl radical oxidation state.
X-ray data and density functional theory calculations
support an aminyl-type radical character in these
complexes, making these the first polynuclear metal aminyl
radical complexes.
Ligand BuH3L was prepared as described previously,19 and
MeH3L was prepared similarly (Scheme S1 and Figures S4 and
S5). Complexes 1 and 2 were prepared by reacting equimolar
amounts of the ligands with Cu(ClO4)2 in methanol in the
absence of air (the synthetic details for all new compounds are
given in the Supporting Information). Maroon precipitates were
immediately formed, and single crystals suitable for X-ray
diffraction were grown from tetrahydrofuran/toluene solutions
of the precipitates. The spectroscopic properties of the methanol
precipitates are nearly identical with those of the single crystals
(Figures S8 and S9). Powder X-ray diffraction (pXRD) patterns
of the powders match well to the calculated pXRD patterns
(Figures S10−S13). The electrospray ionization mass spectrom-
etry (ESI-MS) spectrum of complex 1 exhibited a peak at m/z
that corresponds to [M + H]+ with the correct isotope
he coordination chemistry of free-radical ligands is a well-
T
developed branch of inorganic chemistry.1−9 Strong direct
magnetic exchange coupling between paramagnetic transition-
metal ions with open-shell ligands is an avenue toward bulk
magnetic materials comprised of molecular precursors rather
than traditional atom-based materials.1 Spectacular examples of
molecule-based magnets using this “metal-radical” approach have
been reported using different free-radical types.2,10,11 Strong
magnetic exchange coupling in metal-free radical complexes has
also been exploited to produce single-molecule-magnet behavior.5
Some of these reports have shown that strong exchange coupling
in tandem with large magnetoanisotropy can result in large
magnetization relaxation barriers and open thermal hysteresis
loops up to reasonably high temperatures.12−18
tBu
distribution for the L4Cu4 formulation (Figure S6). Complex
2 is less stable than 1, and we identified a peak at m/z
corresponding to [M + 4H]+ in the ESI-MS spectrum of 2 (in
protic solvents, it is likely that these complexes decompose by H-
atom abstraction reactions). There is no perchlorate evident in
the ESI-MS spectrum of 1 or 2 or in the Fourier transform
infrared (FT-IR) spectrum of the complexes.
The molecular structures of 1 and 2 are shown in Figures 1 and
S1, respectively (Tables S1−S4). An alternative view of the
molecular structure of 1 is shown in Figure 2 (Figure S2 for 2),
and the Cu4O4 core structure is illustrated in Figure 2 (Figure S3
for 2).
Recently, we reported the synthesis and properties of an
unusually stable phenoxyl radical, 2,6-bis(8-quinolylamino)-4-
tert-butylphenoxyl (baqp; Chart 1).19 The baqp radical is
Chart 1. Structures of Ligands Used in This Work
1 and 2 are neutral complexes of the form tBuL4Cu4 or MeL4Cu4.
No perchlorate was found in the lattice, but solvent molecules are
present. The regular M4L4 “square” or [2 × 2] grid
structures31−35 that we initially anticipated were not formed as
a result of an unusual ligand conformation. Each Cu ion is five-
coordinate with a distorted square-pyramidal coordination
geometry (Figure 2, right). The Cu ion is coordinated to three
donors (O, Namido, and Nquin) from the same ligand as well as O
and Naminyl donors from a different ligand. The three donors from
one ligand and the O donor from the other ligand comprise the
basal plane. These coordinate bonds are short and suggest a
divalent oxidation state assignment for the Cu ion. An alternative
produced from air or chemical oxidation of the phenol precursor
(tBuH3L; Chart 1) and has a ditopic structure that looks amenable
to building polynuclear coordination complexes with the
advantage of a bridging O atom that is spin-density-rich. Herein,
we describe the synthesis of the 4-methyl-substituted analogue to
t
BuH3L (MeH3L; Chart 1) and the first coordination complexes
t
with both ligands. To our surprise, coordination of BuH3L and
MeH3L to Cu(ClO4)2·6H2O resulted in the formation of RL4Cu4
t
[R = Bu (1), Me (2)] clusters, where the ligand can only be
Received: March 27, 2018
described as an aminyl radical, not a phenoxyl radical. The X-ray
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00785
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Communication
Figure 3. Variable-temperature magnetic susceptibility data for 1 at an
external field of 5000 Oe. Red squares and blue crosses are the data
points, and the black line represents the best fit to the experimental data
using the model described in the text.
of the ligand form the basal coordination plane of the Cu ions.
The plane containing the other quinoline ring is nearly
perpendicular to the plane containing the rest of the ligand
atoms, and this quinoline ring is therefore unconjugated from the
rest of the ligand. The C−N bond joining this rotated quinoline
ring to the ligand is long [ranging from 1.438(7) to 1.461(6) Å],
significantly longer than the other C−N bond joining the
coplanar quinoline ring to the central ring [1.375(6)−1.392(7)
Å]. The central ring C−C bond distances reveal a regular
aromatic ring structure [C−C bond distances for the four rings
are similar and range from 1.376(7) to 1.430(6) Å in one of the
four rings], and the long C−O bond distances [range from
1.354(6) to 1.381(6) Å] are indicative of a bound phenolate. In
uncoordinated baqp, the structural features of the central ring
and the short C−O distance [1.255(2) Å] are typical for a
phenoxyl radical.19 In a handful of structurally characterized
metal phenoxyl radical complexes, the C−O bond distance is
typically much shorter, generally under 1.3 Å in length.36−38
These observations indicate that the ligand dianion radical in
complexes 1 and 2 is not a phenoxyl radical. Our structural data
and DFT calculations point to a structure with Namido and O
anionic donor atoms from the coplanar part of the ligand, and the
other rotated quinoline ring is joined to the ligand via a localized
aminyl radical (Figure 2).
Figure 1. Molecular structure of 1 (displacement ellipsoids at 30%
probability). H atoms and solvent molecules removed for clarity.
oxidation state of 3+ for Cu with the ligand binding as a closed-
shell trianion with two amido and one phenolate donors would
generate a diamagnetic complex, which is not observed
experimentally. The coordinate bond distances in the basal
plane range from 1.905(4) to 1.973(3) Å for Cu(1) and are very
similar for all four Cu ions. The other Naminyl donor provides the
axial coordinate bond, which is very long; the average axial
coordinate bond length is 2.588 Å. These binding features
produce an irregular bridging Cu4O4 core structure (Figure 3)
with a Cu−O−Cu bond angle that ranges from 120.34(18) to
127.12(18)°.
The Cu2+ oxidation state assignment in 1 requires that the
coordinated ligand is a dianion to satisfy charge balance. One
possibility is that the coordinated ligand is not fully
deprotonated; however, in the FT-IR spectra of 1 and 2, there
is no evidence of an O−H or N−H stretch, and no H atoms could
be located on the N or O atoms in the X-ray structure. Inspection
of the coordinated ligand in the structure of 1 and 2 reveals an
irregular conformation: One quinoline ring is coplanar with the
central ring, and the N, N, and O donor atoms from this portion
Figure 2. Left: View of the molecular structure of 1 highlighting the Cu4O4 core and the long axial Cu−N coordinate bonds. Right: Closeup of one of the
Cu2L units.
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DOI: 10.1021/acs.inorgchem.8b00785
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The UV−visible−near-IR (NIR) spectra of complexes 1
(Figure S14) and 2 exhibit a strong visible absorption band (523
and 517 nm for 1 and 2, respectively). Upon exposure to air or
after oxidation with AgPF6, new intense and broad absorptions
appear across the visible and NIR spectrum. These transitions are
assigned as intervalence charge-transfer bands. The oxidation of
1 or 2 is ligand-centered and produces ligand mixed-valent states.
Complexes 1 and 2 are stable under inert conditions in the solid
state and in solution but will decompose over a period of several
hours in the solid state in air and nearly instantly upon exposure
to air in solution.
that we are pursuing with the synthesis of other ligand
derivatives. The strong exchange coupling in 1 and 2 augurs
well for the production of high-spin ground-state complexes with
other metal ions.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00785.
Synthetic details, spectroscopic, X-ray, and variable-
temperature magnetic data for 2, and computational data
(PDF)
The variable-temperature magnetic susceptibility properties of
1 and 2 were recorded at a field of 5000 Oe over a temperature
range of 5−325 K (Figures 3 and S16). The data profiles are very
similar to each other, and the broad maxima in the χm versus T
profile suggest strong intramolecular antiferromagnetic coupling.
The values of χMT for 1 and 2 at 325 K are 1.88 and 1.90 cm3
mol−1 K, respectively. This is less than the theoretical value (3.00
cm3 mol−1 K) for four uncoupled CuII ions and S = 1/2 radicals
with g = 2, pointing to strong antiferromagnetic coupling within
the cluster. With decreasing temperature, there is a steady
decrease in χMT tending toward a value of zero at 5 K, revealing
an S = 0 ground state for complexes 1 and 2. Using the PHI
program,39 we obtained very good fits to the experimental data
using a spin Hamiltonian (Figure S15), incorporating one
exchange coupling constant in our model (J = Cu-radical). On
the basis of the similarities in the coordination environment of
each Cu ion and the coordinated ligand structures, we assumed
that the Cu-radical exchange coupling constants would be
approximately the same. For complex 1, we obtained the
following best-fit parameters: JCu‑rad = −92 cm−1, gav = 2.050, zJ′ =
−0.91 cm−1, temperature-independent paramagnetism (TIP) =
1.19 × 10−4 cm3 mol−1, ρ = 0.029, and residual = 0.013. Another
model that incorporated two different exchange coupling
constants, including also a Cu−Cu exchange coupling in addition
to the Cu-radical exchange, resulted in good fits, but with
unrealistically large antiferromagnetic couplings in excess of
−250 cm−1 in both cases. The best-fit parameters for complex 2
are very similar to those found for 1 (Figure S16). The
antiferromagnetic coupling between the ligand radical and CuII
ion is a direct exchange interaction. Given the very long
coordinate bond between spin-density-rich Naminyl and Cu, the
exchange is likely mediated by bridging O atoms, which bear a
smaller but significant spin density in the complexes.40,41
Using the coordinates from the X-ray structures, we carried out
single-point-energy calculations on complexes 1 and 2 in a
restricted singlet state (B3LYP/def2-svp). The converged wave
functions were not stable and were reoptimized to unrestricted
wave functions. The Mulliken spin densities were calculated for
complexes 1 and 2 (Figures S17 and S18 and Table S5 and S6).
Large alternating α and β spin densities are found on the Cu ions
around the Cu4O4 core ( 0.60); large spin densities are also
found on the axially coordinated Naminyl atoms that are bonded to
the rotated quinoline rings (Mulliken spin densities on Naminyl
atoms are +0.63, −0.66, +0.62, and +0.66 and that on an O atom
is 0.05). To our knowledge, complexes 1 and 2 are the first
examples of polynuclear clusters containing multiple aminyl
radical ligands.
Accession Codes
CCDC 1832268−1832269 contain the supplementary crystallo-
graphic 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.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mlemaire@brocku.ca.
■
ORCID
Martin T. Lemaire: 0000-0002-0452-8238
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.T.L. and N.M.B. acknowledge the NSERC (Grants RGPIN-
2017-05230 and CGS M) and Brock University.
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