Paramagnetic Rhenium Iodide Cluster with N-Heterocyclic Carbene

Article abstract of DOI:10.1021/acs.inorgchem.1c00568

triangulo-Trirhenium nonaiodide Re3I9 reacts with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) to produce the novel 13-electron paramagnetic cluster Re3I8(IMes)2, which was characterized by means of X-ray diffraction analysis, ESR spectroscopy,

Full text of DOI:10.1021/acs.inorgchem.1c00568

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Paramagnetic Rhenium Iodide Cluster with N‑Heterocyclic Carbene
Pavel A. Petrov,* Taisiya S. Sukhikh, Vladimir A. Nadolinny, Maxim A. Mikhaylov, Alexander N. Lavrov,
Alexey A. Dmitriev, Nina P. Gritsan, and Maxim N. Sokolov
Cite This: Inorg. Chem. 2021, 60, 6746−6752
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ABSTRACT: triangulo-Trirhenium nonaiodide Re3I9 reacts with 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) to produce the
novel 13-electron paramagnetic cluster Re3I8(IMes)2, which was
characterized by means of X-ray diffraction analysis, ESR spectros-
copy, magnetometry, and quantum chemistry.
INTRODUCTION
EXPERIMENTAL SECTION
■
■
General Remarks. Unless otherwise stated, all reagents were
obtained from commercial sources and used without additional
purification. The starting IMes carbene (1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene) was synthesized according to
the published procedure.9 IR spectra were recorded on SCIMITAR
FTS 2000 (4000−400 cm−1) and VERTEX 80 (600−100 cm−1)
Fourier spectrometers. Energy-dispersive X-ray analysis (EDX) was
performed on a Hitachi Tabletop Microscope TM-3000 equipped
with the EDS System QUANTAX 70. The powder diffraction data
were collected on a Shimadzu XRD-7000 diffractometer (Cu−Kα
radiation, OneSight linear detector, 2θ range = 3−60° with steps =
0.0143°, 2θ and 2 s accumulation at the point at room temperature,
the thickness of a thin even layer of the sample is 100 μm). The
reported Re3I9 structure was used to calculate the theoretical powder
diffraction pattern.1c Thermogravimetric analysis was performed on a
NETZSCH STA 449 F1 Jupiter thermal analyzer. The measurements
were made in a helium−hydrogen mixture (10.0 vol % H2) in the
temperature range of 30−800 °C, with a flow rate of 30 mL/min, in
an open Al2O3 crucible, and with a heating rate of 10 °C/min. The
treatment of experimental results was performed using standard
Proteus Analysis software.10 The EPR spectra of 1 were recorded with
a Varian E-109 spectrometer in the X-frequency band in continuous
wave (CW) mode at 300 and 77 K; 2,2-diphenyl-1-picrylhydrazyl
(DPPH, g = 2.0036) was used as a standard for the g-factor.
Parameters of the EPR spectra were calculated and simulated with
WinEPR and Simfonia programs.
The cluster nature of rhenium “trihalides”, ReX3 (= Re3X9, X =
Cl, Br, I), was recognized more than half a century ago.1
Interpretation of the bonding situation in these clusters as
involving three Re−Re double bonds in the Re3 triangle was
crucial for the development of the whole concept of metal−
metal bonding. From the structural point of view, crystalline
rhenium trihalides are built of Re3X9 units, rather loosely (via
bridging halides with long Re−X distances) arranged either in
layers (X = Cl, Br) or zigzag chains (Re3I9) (Chart 1). This
means that free Re3X9 molecules would behave as Lewis acids,
and indeed, cluster excision reactions between Re3Cl9 or
Re3Br9 and neutral ligands give expected [Re3X9L3] molecular
complexes. Many examples of such complexes are summarized,
for X = Cl and Br, in the monograph by Cotton and Walton.2
With Re3I9 the situation is different, and to the best of our
knowledge, the only reported adducts of this type are
[Re3I9(RNC)3], for which structural information is lacking.3
The paucity of the available data is partially explained by the
apparently high propensity of Re3I9 to undergo radical
+
transformations, i.e., oxidation to ReO2 in reactions with
pyridine, γ-picoline, and pyrazine, or reductive cluster
degradation into [Re2I4(PR3)4] with phosphines.4 This
contrasting behavior of Re3Cl9 versus Re3I9 was highlighted
in a recent publication by Krawczyk et al.5 Reaction of Re3I9
with water and an excess of 3-methylpyridine (3-Mepy) leads
to the formation of the octahedral cluster [Re6O12(3-
Mepy)6]+.6 Last, but not the least, high-temperature reactions
between Re3I9, KCN, and P or As were used for preparation of
novel, unprecedented metal-pnictogenide clusters
[{Re4(PO)3(PO2)}(CN)12]8− and [{Re4As2(AsO)2}-
(CN)12]8−, respectively.7
Preparation of Re3I9. NaReO4 (0.80 g, 2.92 mmol) was placed
into a 32 mL PTFE reactor, and 55% HI (20 mL) was added. The
reactor was tightly closed by a Teflon cap and closely fitted into a
Received: February 25, 2021
Published: April 20, 2021
In this work, we address the problem of the Lewis acidity of
Re3I9 by reacting it with N-heterocyclic carbenes, which are
among the strongest σ-donor ligands known.8
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and was refined with the ShelXL13 refinement package using the
Olex2 GUI.14 The positions of hydrogen atoms were calculated
corresponding to their geometrical conditions and were refined by
applying a riding model. Atomic thermal displacement parameters for
non-hydrogen atoms were refined anisotropically with the exception
of the solvate THF molecule.
Chart 1. Structures of Polymeric Re3X9 (X = Cl, Br (a) and I
(b)) and Schematic View of Monomeric Adduct [Re3X9L3]
(c)
a
Crystal data for 1•THF. C46H56I8N4ORe3 (M = 2254.74 g/mol),
orthorhombic, space group Pna21 (no. 33), a = 16.6723(5) Å, b =
14.7405(6) Å, c = 23.3366(8) Å, V = 5735.2(4) Å3, Z = 4, T = 150(2)
K, μ(Mo−Kα) = 10.658 mm−1, dcalc = 2.611 g/cm3, 19282 reflections
measured (1.746° ≤ 2Θ ≤ 52.806°), 8985 unique (Rint = 0.0258,
Rsigma = 0.0534), which were used in all calculations. The final R1 was
0.0221 (I > 2σ(I)), and wR2 was 0.0462 (all data). CCDC no.
1970476.
Magnetic Measurements. Magnetic properties of a polycrystal-
line sample of 1 were studied on a Quantum Design MPMS-XL
SQUID magnetometer in the temperature range of 1.77−320 K in
magnetic fields of 0−10 kOe. In order to determine the paramagnetic
component of molar magnetic susceptibility χp(T) we subtracted the
contributions of Larmor diamagnetism χd and ferromagnetism of trace
impurities χF from the measured values of the total molar
susceptibility χ = M/H (where M is the magnetization). The
temperature-independent contribution of χd was calculated according
to the additive Pascal scheme, whereas in order to evaluate the
ferromagnetic contribution χF, we measured field dependences M(H)
and temperature dependences M(T) at different magnetic fields. To
determine the effective magnetic moment μeff, temperature depend-
ences χp(T) were analyzed using the Curie−Weiss dependence χp(T)
= NAμ2eff/3kB(T − θ), where NA and kB are the Avogadro number and
the Boltzmann constant, respectively.
Computational Details. Quantum chemical calculations were
performed for a truncated crystallographic structure of complex 1 with
the mesityl substituents replaced by methyl groups (1m). The atomic
charges, spin populations, and molecular orbitals (MOs) were
calculated at the UB3LYP level15 with the scalar-relativistic second-
order Douglas−Kroll−Hess (DKH2) Hamiltonian.16 The full-
electron relativistic basis sets (WTBS for Re,17 ANO-RCC-TZP for
I,18 and def2-TZVP for all other atoms19) were employed. These
calculations were performed with the ORCA 4.2.1 suit of programs.20
The g-tensor for 1m and hyperfine coupling (HFC) tensors and
constants (aiso) with rhenium and iodine nuclei were calculated using
the exact two-component relativistic X2C method21 with a self-
consistent account of spin−orbit coupling (SOC), implemented in
the ADF2019 software package.22 These calculations were performed
with the B3LYP functional and the STO-type basis set (TZ2P-J for Re
and DZP for all other atoms)23 and the Gaussian model of nonpoint
nuclei.24 A collinear approximation of the spin polarization was used
in the case of the spin−orbit coupling.25 A spherical Gaussian nuclear
charge distribution model26 was used for accurate predictions of HFC
constants. The good quality of the Becke integration grid (“excellent”
for Re) was chosen.27
a
Legend: Re, dark grey; X, green; L, red.
metal cylinder with a metal screw cap. The assemblage was kept in an
oven at 200 °C for 10 h. After it was cooled, the reactor was opened,
and the reaction mixture was filtered through a G4 glass frit. A black
crystalline powder was collected on the filter and washed with ethanol
until the washings become colorless and then the powder was washed
with diethyl ether. Yield: 1.47 g (89%). EDX: Re/I = 1/3. IR
(polyethylene, cm−1): 225 m, 186 m, 174 w, 163 w.
Preparation of Re3I8(IMes)2 (1). Re3I9 (110 mg, 0.064 mmol)
was predried at 110 °C overnight, placed into a Schlenk tube
equipped with a J. Young PTFE valve, and dried in a dynamic vacuum
at 100 °C for 1 h. In a glovebox, IMes (65 mg, 0.214 mmol, 3.3 equiv)
was loaded into the Schlenk tube. The tube was evacuated, and 20 mL
of dry and degassed THF was vacuum-transferred into it at −196 °C.
After spontaneous warming, the vessel was stirred at room
temperature, and the solution gradually turned emerald-green. After
72 h, the suspension was heated with stirring at 60 °C for 24 h and
cooled to room temperature. The dark precipitate was filtered off
through a G4 glass frit, and the dark brownish-green solution was
placed into an L-shaped pyrex tube and fire-sealed. Slow evaporation
of the solution into one knee afforded almost black crystals of 1•THF,
suitable for X-ray analysis, along with some large crystals of a colorless
admixture. Yield: 75 mg, 51%. Found: C, 25.30; H, 2.75; N, 2.60.
Calcd for C46H56I8N4ORe3 (%): C, 25.50; H, 2.50; N, 2.48. IR (KBr,
cm−1): 1608 m, 1564 m, 1542 m, 1482 s, 1457 s, 1419 m, 1394 s,
1267 s, 1244 s, 1218 m, 1161 w, 1128 m, 1101 m, 1075 m, 1047 m,
1016 w, 978 m, 961 m, 930 m, 885 m, 736 m, 696 m, 679 w, 644 w,
587 m, 563 s.
RESULTS AND DISCUSSION
■
For preparation of Re3I9 we used the reaction recommended
by Malatesta, namely, reduction of perrhenate in concentrated
HI.28 We have simplified it by using NaReO4 instead of
HReO4 and by employing HI both as the iodine source and
reductant (instead of ethanol). Under hydrothermal con-
ditions, crystalline, phase-pure, water-free Re3I9 is obtained in
nearly quantitative yield. Powder diffraction data for the
product (Figure S1) matched the theoretical one calculated for
the reported single-crystal diffraction data for Re3I9.1c
Thermogravimetric analysis confirms the composition of
Re3I9, leaving only metal Re as the nonvolatile product after
heating above 300 °C in reducing (H2/He) atmosphere
(Figure S2).
X-ray Crystallography. A single-crystal XRD data set for 1•THF
was collected on a Bruker Apex DUO diffractometer equipped with a
4K CCD area detector using graphite-monochromated Mo−Kα
radiation (λ = 0.71073 Å). The φ- and ω-scan techniques were
employed to measure intensities. Absorption corrections were applied
with the use of the SADABS program.11 The structure was solved
with the ShelXT12 structure solution program using Intrinsic Phasing
This easy preparation of Re3I9 indicates that at higher
temperatures (and at least in the acidic media) I− per se is a
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sufficiently strong reductant to reduce Re(VII) into the Re(III)
state, while at the same time promoting the formation of the
Re3 clusters by acting as bridging ligands. This is corroborated
by a report of Kochel, where Re(V) (taken as [Re2OCl10]4−)
was reduced to Re(III) with iodide under similar conditions
(200 °C, 20 h). Due to the presence of HCl, the final product
was isolated as (4,4′-bipyH2)[Re3I3Cl8(H2O)]·2H2O, but the
yield was not reported.29 In our case, I− acts both as the
reductant and promoter of cluster formation, building the
{Re3(μ2-I)3}6+ core. In fact, mixed-halide complexes (4,4′-
bipyH2)[Re3(μ-I)3Cl6(H2O)3],29 [Re3(μ-I)3Cl6(H2O)3],30 and
Cs1.5[Re3(μ-I)3Cl7.5(H2O)1.5]31 remain which are, to the best
of our knowledge, the only structurally characterized
complexes with the {Re3(μ-I)3}6+ core.
Ligand substitution is the common synthetic route to the
complexes of NHCs as they readily substitute weaker donor
ligands. Initially we attempted to prepare [Re3I9(thf)3] as a
starting material, by treatment of Re3I9 with dry THF as it was
described for [Re3Cl9(thf)3].32 However, unlike its chloride
congener, Re3I9 was inert toward THF even after a week-long
extraction in a Soxhlet-like apparatus. Therefore, direct
reaction of Re3I9 with 3.3 equiv of IMes (= 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene) in dry THF was chosen as
a possible pathway to the desired Re3I9(IMes)3. Already at
room temperature the reaction mixture gradually turned
brownish-green, albeit some amount of almost black
precipitate (presumably starting Re3I9) remained even after
heating. The filtration of the THF solution and crystallization
in a sealed tube afforded dark crystals suitable for the XRD
experiment, which turned out to be novel complex
[Re3I8(IMes)2]·THF (1·THF). As the Re/I ratio in 1 is
below 3, it is reasonable to assume that one equivalent of IMes
is consumed to reduce Re from the +3 to +2.67 state by
abstracting iodine. The reducing properties of NHCs are well
known, since they possess a formally divalent carbon center.33
Although no products of direct IMes iodination were isolated
in our case, 2-iodoimidazilium salts (iodide or triiodide) might
be plausible candidates. They were isolated previously by the
reaction of the corresponding NHC with I2. Unfortunately,
Figure 1. Molecular structure of 1 with ellipsoids of 30% probability.
The hydrogen atoms are omitted for clarity. Selected bond lengths
(Å): Re1−Re2 2.5594(6), Re1−Re3 2.5052(4), Re2−Re3 2.5120(5),
Re1−C1 2.312(9), Re2−C22 2.321(9), Re1−I1 2.8207(7), Re1−I2
2.7165(7), Re1−I3 2.7186(7), Re1−I5 2.6065(7), Re2−I1
2.8225(7), Re2−I2 2.7159(6), Re2−I4 2.7048(6), Re2−I6
2.5908(7), Re3−I1 2.8015(7), Re3−I3 2.7078(8), Re3−I4
2.6968(8), Re3−I7 2.6183(7), Re3−I8 2.8384(7).
Re−Re bond is bridged by an iodide ligand, with a rather
narrow distribution of the Re−I bond lengths. The Re3 and
(μ2-I)3 planes are almost coplanar with the dihedral angle of
only 1.5°. Finally, the coordination environment of each Re
atom is completed with two terminal ligands. Three terminal
iodides are arranged on the side of the Re3 plane, which is
opposed to the μ3 cap, and the fourth I atom, namely, I8, lies
on the opposite side, at a longer distance than other terminal
iodides (2.8384(7) Å). Two carbene ligands coordinate the Re
atoms united by the longest metal−metal bond, while the Re−
C distances (2.312(9) and 2.321(9) Å) are not exceptional
among other carbene complexes of Re.37 The planes of the
azole cycles of both carbenes are almost perpendicular to the
Re3 plane. Of the Re−Re bonds in 1, one is ca. 0.05 Å longer
than two others. It can be argued, as the simplest explanation,
that this distortion comes from the odd number of cluster
skeleton electrons of 1 (13); this, however, may not be
necessarily the case. For instance, the parent Re3I9 structure
features two even more distinct Re−Re bonds of 2.440(3) and
2.507(4) Å, but this difference comes from the crystallographic
nonequivalence of two Re atoms. Nonequivalency in the
coordination number of Re may also result in a difference in
metal−metal bond lengths as happens in [Re3X11]2− anions.38
Thus, the asymmetry of the M3 triangle is not indicative of a
change in the oxidation state; however, the net average bond
length is. The mean Re−Re distance of complex 1 is 2.526 Å,
that is, significantly (by ca. 0.06 Å) longer than that in Re3I9
and other structurally characterized complexes possessing the
{Re3(μ-I)3}6+ core (Table 1). Systematic elongation of the
same order of magnitude was observed earlier upon one-
electron reduction of the related chalcogen-bridged cluster
34
neither [IMes-I]I33 nor [IMes-I]I3 show the characteristic
resonance of a C-2 atom, which handicaps the application of
13C NMR spectroscopy as a tool for their identification.
Nevertheless, we were able to identify the colorless byproduct
crystallized along with 1·THF. Albeit no full diffraction data set
for this crystal was collected, the compound turned out to be
the novel solvent-free 1,3-bis(2,4,6-trimethylphenyl)-
imidazolium iodide [IMesH]I (Figure S3), which was not
isostructural with its chloride congener.35 The geometries of
two independent imidazolium cations in [IMesH]I are close to
those found in chloroform solvate [IMesH]I·2CHCl3.36
Especially, the hydrogen bond C−H···I (C2···I1, 3.641(12)Å;
C22···I2, 3.647(11) Å), characteristic of imidazolium salts, is
to be mentioned. Its formation is obviously a consequence of
the hydrolysis of IMes with traces of water, which, despite all
precautions, commonly occurs in the synthesis of NHC
complexes, especially with early transition metals.
Three Re atoms in 1 constitute a slightly distorted
equilateral triangle with Re−Re bond lengths of 2.5594(6),
2.5052(4), and 2.5120(5) Å (Figure 1). The corresponding
Re−Re−Re angles vary within the range of 59.197(14)−
61.344(14)°. The Re3 triangle is capped with one μ3-iodide,
coordinated in a rather symmetrical fashion, with Re−I
distances falling in the 2.8015(7)−2.8225(7) Å range. Each
39
40
41
systems, {Re3S4}4+/5+
,
{Mo3S4}4+/3+/2+
,
and {Mo3S5}0/1+
.
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with ΔH = 134.1 mT and g = 2.115 (Figure 2). It should be
Table 1. Complexes with the Re3(μ-I)3 Core Known to Date
and Some of Their Analogues
noted that there are two stable isotopes of rhenium, i.e., 187Re
Re−Re bond lengths
complex
(averaged value), Å
ref
this
work
1
2.5594(6), 2.5052(4), 2.5120(5)
(av. 2.526)
Re3I9
2.440(3), 2.507(4) (av. 2.462)
1c
[Re3(μ-I)3Cl6(H2O)3]·
2.433(1)−2.459(1) (av. 2.449)
30
30
31
3H2O
[Re3(μ-I)3Cl6(H2O)3]·
2.5H2O
2.452(1)−2.460(1) (av. 2.456)
Cs1.5[Re3(μ-
2.466(1)
I)3Cl7.5(H2O)1.5
]
[Re3(μ-I)3Cl6(H2O)3]2−
2.4707(14)−2.4772(11) (av. 2.475) 29
[Re3(μ-Cl)3Cl6(μ3-
2.4667(8), 2.4657(11) (av. 2.466)
45
SO4)]3−
[Re3(μ-Cl)3Cl6(μ3-
2.478(1), 2.485(1), 2.452(1)
(av. 2.472)
46
SO4)]2−
Direct comparison is, of course, liable to caution, because
cluster cores in starting Re3I9 and 1 belong to different
structural types. To the best of our knowledge, there are no
direct structural analogues of 1, at least among halide or
chalcogenide clusters. Among tungsten alkoxides, however,
there are two clusters, [W3(μ3-CCH3)(μ-OiPr)3(OiPr)6]42 and
Figure 2. EPR spectrum of powder sample of complex 1 at 77 K in
the X-frequency band (a, experimental spectrum; b, simulation).
43
t
t
[W3(μ3-P)(μ-OCH2 Bu)3(OCH2 Bu)6], that have the same
ligand distribution around the triangular metal core. Never-
theless, they are not isoelectronic (the W3 clusters have only
six cluster skeleton electrons, which form long (ca. 2.75 Å)
W−W bonds of order 1), and, what is more important from
the structural point of view, the oxygen atoms of the bridging
alkoxide ligands are not coplanar with the W3 plane but are
rather directed to the side opposite to the capping ligands; it is
the oxygen atoms from the terminal ligands that are nearly
coplanar to the W3 plane. Whether this “inversion” in the
orientation of the bridging and terminal ligands is due to
different electronic population of the cluster remains to be
seen. The family of bicapped 10-electron clusters [Mo3(μ3-
I)(μ3-H)(μ-I)3I6L3] (L = thf, RCN, PR3) with rather short
metal−metal bonds (2.49−2.59 Å) is also known.44
and 185Re, with the same spin I = 5/2 and natural abundance of
62.6% and 37.4%, respectively. Because the ratio of nuclear g-
factors for them is 1.01,48 the difference between their HFC
tensors and constants can be neglected. Since the spectrum is
one symmetrical line, it can be assumed that the unpaired
electron is delocalized over three rhenium atoms with the same
spin population.
The broadness of the EPR signal of 1 is in good agreement
with the shape of the hyperfine structure envelope expected for
three Re atoms with the aiso(Re) = 18.5 mT (Figure 2). It is
worth noting that typical aiso(Re) constants vary in the range of
40−70 mT, regardless of the oxidation state (+2 or +6) of the
metal.49 If the electron is delocalized over three rhenium
atoms, one could expect one-third of these values for each Re
atom (13−23 mT), which agrees with the observed HFC
constant of 18.5 mT. Moreover, these assumptions and
estimates are in good agreement with the results of relativistic
quantum chemical calculations, which we shall discuss in the
next section of this paper.
The odd number of electrons and the doublet ground state
of complex 1 were also proven using magnetometry. The
behavior of the inverse magnetic susceptibility can be well
described in the entire available temperature range of 1.77−
320 K by the Curie−Weiss law with μeff ≈ 1.62 μB and θ ≈ 0
(Figure S4). The obtained value of μeff is rather close to the
theoretical value of 1.73 μB for one unpaired electron (S = 1/2,
g = 2) per formula unit. This conclusion is also supported by
the magnetization curve M(H) taken at T = 1.77 K, whose
shape and parameters match the expected behavior for
noninteracting electrons with S = 1/2 and the density being
slightly below one electron per formula unit. The observed
deviation in the number of electrons below one per formula
unit can be accounted for by the remaining nonmagnetic
impurity (likely [IMesH]I).
The complex Cs3[Re3(μ-Cl)3Cl6(μ3-SO4)] is also worth
8+
mentioning as it had been the only example of the Re3 core
characterized by means of single-crystal X-ray diffraction.45 It
was obtained by electrochemical reduction of a Re3Cl9 solution
in 6 M aqueous HCl followed by addition of H2SO4. The
analytical data for the reduction product are in agreement with
the crystal structure, and the UV−vis spectrum differs from
that for the initial Re3Cl9 solution. However, the Re−Re bond
lengths and virtually all geometrical parameters of this
presumably 13-electron anion [Re3(μ-Cl)3Cl6(μ3-SO4)]3−
match those for the 12-electron congener [Re3(μ-Cl)3Cl6(μ3-
SO4)]2− (characterized as NMe4 salt)46 suspiciously well.
+
Specifically, the averaged Re−Re bond lengths of these clusters
differ only by 0.006 Å. This implies that reoxidation of the 13-
electron cluster upon crystallization might have occurred.
Attempted chemical reduction of [Re3Cl12]3− resulted in
extremely air-sensitive products, none of which were
structurally characterized.47 In our case, by contrary, the
increased mean Re−Re bond length agrees well with the 13-
electron nature of cluster 1.
The odd number of electrons in complex 1 make possible
the use of ESR techniques. While both the powder and THF
solution of 1 were EPR silent at room temperature, the
spectrum of powder sample at 77 K yielded a single broad line
To clarify the electronic structure and predict the spin-
Hamiltonian parameters of complex 1, we performed quantum
chemical calculations for the simplified structure obtained by
replacement of the mesityl substituents by methyl groups
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(1m). The electronic structure was analyzed using the results
of the full-electron DFT calculations with the scalar relativistic
DKH2 Hamiltonian. These calculations predict the ground
doublet state of 1m with a spin density localized mainly on
three Re atoms, with Mulliken spin populations of 0.31, 0.29,
and 0.32. Moreover, three of the terminal iodine atoms (I5, I6,
and I7, Figure 1) are also characterized by a non-negligible
spin population (0.04, 0.04, and 0.06). Figure 3 shows that the
I6−I8, they are negative and are in the range of −0.33 to
−0.72. Therefore, the value of 18.5 mT, estimated from the
experiment and neglecting the hyperfine interaction with
iodine atoms, is actually significantly higher than the aiso(Re)
value averaged for three rhenium atoms. Thus, the calculated
parameters of the spin-Hamiltonian describe well the
experimental EPR spectrum.
CONCLUSIONS
■
Re3I9 reacts with N-heterocyclic carbene IMes to produce the
13-electron paramagnetic cluster [Re3I8(IMes)2] which has a
doublet ground state. The reaction is accompanied by
reduction of the cluster {Re3}9+ core into the {Re3}8+. The
extra electron occupies an antibonding orbital with equal
contribution from each of the three Re atoms, which accounts
for the considerable lengthening of the Re−Re bonds. A
redistribution of the bridging and terminal ligands also takes
place upon formation of [Re3I8(IMes)2] from Re3I9. To the
best of our knowledge, there are no direct structural analogues
of 1, at least among halide or chalcogenide clusters.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00568.
Figure 3. SOMO isosurface of complex 1m calculated at the DKH2-
UB3LYP level.
Powder diffraction and thermogravimetric analysis data,
quantum-chemical calculations, and magnetometry
results (PDF)
SOMO of 1m is localized mainly on the 5d-AOs of Re atoms
with some delocalization on the 5p-AOs of I atoms. It is
strongly antibonding, and this accounts for the observed Re−
Accession Codes
9+
8+
Re bond lengthening on going from Re3 to Re3 clusters.
The calculations also predict positively charged rhenium atoms
with Mulliken charges of 0.60−0.65 and negatively charged
iodine atoms (−0.18 to −0.47), in agreement with their
respective electronegativities. According to the calculations, the
Mayer bond orders are equal to 1.10 0.01 for Re−Re bonds
and 0.25 for Re1−C1 and Re2−C22 bonds (Figure 1),
indicating the weakness of the latter. The Mayer bond orders
for Re−I bonds are in the range of 0.51−0.60 for the {Re3(μ-
I)3}6+ core, in the range of 0.33−0.40 for three Re−(μ3-I)
bonds, and in the range of 0.92−0.96 for the shortest Re1−I5,
Re2−I6, and Re3−I7 bonds (Figure 1). As expected, the
Mayer bond orders correlate with the corresponding bond
lengths.
CCDC 1970476−1970477 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.
AUTHOR INFORMATION
■
Corresponding Author
Pavel A. Petrov − Nikolaev Institute of Inorganic Chemistry
SB RAS, 630090 Novosibirsk, Russia; orcid.org/0000-
0002-2521-8984; Email: panah@niic.nsc.ru
Authors
The spin-Hamiltonian parameters were calculated using the
exact two-component relativistic X2C method with a self-
consistent account of spin−orbit coupling (SOC), as described
in the Computational Details section. This approach recently
demonstrated complete agreement with experiment for the
HFS tensor of 125Te in a tellurium-containing radical anion.50
For complex 1, these accurate calculations predicted an
anisotropic g-tensor with giso = 2.135 (see the SI for details)
with good agreement with experiment (giso = 2.115). As
expected from the spin density distribution and the SOMO
shape (Figure 3), the HFS tensors and constants are close for
Taisiya S. Sukhikh − Nikolaev Institute of Inorganic
Chemistry SB RAS, 630090 Novosibirsk, Russia;
orcid.org/0000-0001-5269-9130
Vladimir A. Nadolinny − Nikolaev Institute of Inorganic
Chemistry SB RAS, 630090 Novosibirsk, Russia
Maxim A. Mikhaylov − Nikolaev Institute of Inorganic
Chemistry SB RAS, 630090 Novosibirsk, Russia
Alexander N. Lavrov − Nikolaev Institute of Inorganic
Chemistry SB RAS, 630090 Novosibirsk, Russia
Alexey A. Dmitriev − Novosibirsk State University, 630090
Novosibirsk, Russia; Voevodsky Institute of Chemical Kinetics
and Combustion SB RAS, 630090 Novosibirsk, Russia;
orcid.org/0000-0001-6260-8124
Nina P. Gritsan − Novosibirsk State University, 630090
Novosibirsk, Russia; Voevodsky Institute of Chemical Kinetics
and Combustion SB RAS, 630090 Novosibirsk, Russia;
orcid.org/0000-0002-2263-1300
three rhenium atoms with |aiso (
187Re)| equal to 9.67, 9.91, and
11.74 mT for Re1−Re3, respectively (Table S1). These values
are somewhat lower those proposed for simulating the
experimental EPR spectrum (18.5 mT). However, moderate
values of HFS constants were also predicted for eight iodine
atoms, which also have I = 5/2. For I1−I5 atoms, the aiso(127I)
constants are positive, being in the range of 0.26−1.99 mT. For
6750
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Article
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Maxim N. Sokolov − Nikolaev Institute of Inorganic
Chemistry SB RAS, 630090 Novosibirsk, Russia;
orcid.org/0000-0001-9361-4594
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00568
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Russian Foundation for Basic Research (project 18-33-
20056) is acknowledged for financial support.
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