Coordination Compounds of Niobium(IV) Oxide Dihalides Including the Synthesis and the Crystallographic Characterization of NHC Complexes

Article abstract of DOI:10.1021/acs.inorgchem.5b02888

The 1:1 molar reactions of NbOX₃ with SnBu₃H, in toluene at 0 °C in the presence of oxygen/nitrogen donors, resulted in the formation of NbOX₂L₂ (X = Cl, L₂ = dme, 2a; X = Br, L₂ = dme, 2b; X = Cl, L = thf, 2c; X = Cl, L = NCMe, 2d; dme = 1,2-dimethoxyethane, thf = tetrahydrofuran), in good yields. The 1:2 reactions of freshly prepared 2d and 2b with the bulky NHC ligands 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, Imes, and 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene, Ixyl, respectively, afforded the complexes NbOCl₂(Imes)₂, 3, and NbOBr₂(Ixyl)₂, 4, in 50-60% yields. The reactions of 2b with NaOR, in tetrahydrofuran, gave NbOCl(OR) (R = Ph, 5; R = Me, 6) in about 60% yields. All the products were characterized by analytical and spectroscopic techniques; moreover DFT calculations were carried out in order to shed light on synthetic and structural features. Compounds 3 and 4, whose molecular structures have been ascertained by X-ray diffraction, represent very rare examples of crystallographically characterized niobium-NHC systems.

Full text of DOI:10.1021/acs.inorgchem.5b02888

Article
pubs.acs.org/IC
Coordination Compounds of Niobium(IV) Oxide Dihalides Including
the Synthesis and the Crystallographic Characterization of NHC
Complexes
Marco Bortoluzzi,† Eleonora Ferretti,‡,⊥ Fabio Marchetti,*,‡ Guido Pampaloni,‡ Calogero Pinzino,§
and Stefano Zacchini∥
†Dipartimento di Scienze Molecolari e Nanosistemi, Universita
‡Dipartimento di Chimica e Chimica Industriale, Universita
di Pisa, Via G. Moruzzi 13, I-56124 Pisa, Italy
̀
di Venezia Ca’ Foscari, Via Torino 155, I-30170 Mestre, Venice, Italy
̀
§ICCOM-CNR UOS Pisa, Research Area, Via Moruzzi 1, I-56124 Pisa, Italy
∥Dipartimento di Chimica Industriale “Toso Montanari”, Universita
̀
di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
S
* Supporting Information
ABSTRACT: The 1:1 molar reactions of NbOX3 with
SnBu3H, in toluene at 0 °C in the presence of oxygen/
nitrogen donors, resulted in the formation of NbOX2L2 (X =
Cl, L2 = dme, 2a; X = Br, L2 = dme, 2b; X = Cl, L = thf, 2c; X
= Cl, L = NCMe, 2d; dme = 1,2-dimethoxyethane, thf =
tetrahydrofuran), in good yields. The 1:2 reactions of freshly
prepared 2d and 2b with the bulky NHC ligands 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene, Imes, and 1,3-bis(2,6-
dimethylphenyl)imidazol-2-ylidene, Ixyl, respectively, afforded the complexes NbOCl2(Imes)2, 3, and NbOBr2(Ixyl)2, 4, in
50−60% yields. The reactions of 2b with NaOR, in tetrahydrofuran, gave NbOCl(OR) (R = Ph, 5; R = Me, 6) in about 60%
yields. All the products were characterized by analytical and spectroscopic techniques; moreover DFT calculations were carried
out in order to shed light on synthetic and structural features. Compounds 3 and 4, whose molecular structures have been
ascertained by X-ray diffraction, represent very rare examples of crystallographically characterized niobium−NHC systems.
applications in synthetic organic chemistry.5b,7 Also niobium-
INTRODUCTION
■
(V) oxide trichlorides and tribromides are rather easily
accessible compounds,8 whose coordination chemistry has
been investigated to some extent.9 On the other hand, the
synthesis of reduced niobium oxide halides, which are materials
of potential interest also in view of their solid-state properties,10
is a more difficult task. More in detail, the reported synthetic
procedures to afford NbOX2 (X = Cl, Br) require prohibitive
experimental conditions,11 thus determining an important
drawback to the acquisition of the knowledge of the reactivity
of these compounds. As a matter of fact, the coordination
chemistry of NbOX2 (X = Cl, Br) has been almost unexplored
heretofore. The only coordination adducts were claimed to be
prepared by hydrolysis of niobium tetrachloride compounds12
or reduction of NbOCl3/phosphine systems with Na/Hg;
however the products were not unambiguously character-
ized.12a
Since Arduengo’s isolation of the first crystalline N-heterocyclic
carbene (NHC) in 1991, the use of NHC ligands in
organometallic chemistry has seen a tremendous advance,
with applications ranging from medicinal chemistry to
homogeneous catalysis and material science.1 The chemistry
of NHC ligands has been developed especially with reference to
low- and medium-valent metals, belonging to middle to late
transition groups. Otherwise NHC systems based on oxophilic,
early transition metals have been much less investigated, and
one of the reasons may be attributed to the possible moisture
sensitivity affecting the reaction systems.2 In particular, niobium
represents a surprising exception in the crowded scenario of
NHC chemistry:3 indeed very few Nb-NHC systems have
appeared in the literature so far,3,4 and we have recently
contributed with the unique structurally characterized niobium
complex containing a monodentate NHC ligand.3
Herein we present a convenient room-temperature proce-
dure to afford complexes of NbOCl2 and NbOBr2 with simple
oxygen and nitrogen donor ligands, starting from the
corresponding niobium pentahalides. These Nb(IV) products
undergo substitution reactions with phenoxide and methoxide
Niobium pentahalides are among the most easily available
and cost-effective niobium species: their chemistry has
undergone significant progress in the past decade, encouraged
by the low toxicity associated with the metal element and the
outstanding reactivity patterns observed in some NbX5-directed
organic transformations.5 Niobium(V) pentahalides have been
employed as convenient starting reagents to access both
Nb(IV) and Nb(III) halides,6 which in turn have found
Received: December 14, 2015
© XXXX American Chemical Society
A
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Article
anions and with bulky, monodentate NHC ligands (Chart 1),
affording fairly air-stable NbOX2(NHC)2 complexes. The
Scheme 1. Synthesis of NbOX2L2 Complexes
Chart 1. NHC Ligands Used in the Present Work
analogous procedures, also the complexes NbOCl2(thf)2, 2c,
and NbOCl2(NCMe)2, 2d, were obtained (Scheme 1).
Complexes 2c,d are exceedingly air sensitive products, and
especially 2d is so intractable that a satisfying characterization
has not been possible. Attempts to prepare acetonitrile and
tetrahydrofuran adducts of NbOBr2 failed.
synthetic aspects and the structural characterization of the
new complexes (including X-ray characterization of the NHC
complexes) will be discussed, with the assistance of DFT
results.
Compounds 2a−c represent rare examples of well-defined
coordination complexes of NbOX2. They were characterized by
elemental and magnetic analysis and IR and UV−vis spectros-
copy. The magnetic susceptivity values of 2a,b are lower than
the spin-only value (1.73 μB) for a d1-species and fall within the
interval 1.40−1.65 μB, resembling previous findings on mono-
and dinuclear Nb(IV) complexes.15 The IR spectra of 2a,b
display a strong band at ca. 1020 cm−1, attributed to a terminal
NbO moiety. In the UV−vis spectra, absorptions in the
region 450−750 nm feature low molar extinction coefficients
and are probably due to d−d transitions within the metal ion.
According to the experimental evidence, we suggest that 2a,b
RESULTS AND DISCUSSION
■
We prepared NbOX3 (X = Cl, Br) in ca. 80% yields by allowing
the respective precursors NbX5 to react with one equivalent of
hexamethyldisiloxane, in dichloromethane at room temper-
8c
ature, eq 1. This method was previously reported for NbOCl3
and has been employed here also to obtain NbOBr3.13
NbX5 + O(SiMe3)2 → NbOX3 + 2SiMe3X
16
have a dinuclear structure similar to that adopted by NbX5
X = Cl, Br
(1)
and MoCl517 in the solid state, with the hexacoordinated metal
centers sharing two bridging halide ligands. It should be noted
that the d1 metal based dimeric MoCl5 displays a magnetic
susceptivity corresponding to a magnetic moment of 1.6 μB.18
Furthermore, the only two examples of crystallographically
characterized vanadium(IV) complexes based on the core
VOCl2L2 and containing terminal oxide ligands, i.e.,
VOCl2(NCMe)2 and [VOCl3(OH2)]−, possess a dinuclear
structure.19
The complexes NbOX3(dme) (X = Cl,14 Br; dme = 1,2-
dimethoxyethane), 1a,b, were afforded in good yields upon
addition of dme to NbOX3 in toluene, eq 2.
NbOX3 + dme → NbOX3(dme)
X = Cl, 1a
X = Br, 1b
(2)
According to NMR evidence, the unprecedented 1b exists in
dichloromethane solution as a single isomeric form (93Nb
resonance at −205 ppm), presumably ascribable to a
(distorted) octahedral geometry with mer configuration (13C
NMR resonances due to the nonequivalent methyl groups
occur at 70.3 and 63.6 ppm). This result is consistent with DFT
EDF2 calculations, the mer isomer being more stable than the
fac one by about 8.9 kcal mol−1 (Gibbs free energy). Figure S1
and Table S1 within the Supporting Information show the
calculated structures of the possible isomers of 1b and the
bonding parameters related to the most stable isomer.
It should be remarked that niobium chlorides and bromides
bearing niobium in a reduced oxidation state can be afforded by
the straightforward reactions of NbX5 with the appropriate
amount of SnnBu3H.6 For instance, NbX3(dme) can be
generated in high yields from NbX5 and two equivalents of
SnnBu3H in dme. Therefore, with the aim of obtaining
niobium(IV) oxide dihalides, we studied the possible reduction
process of 1a,b with tributyltin hydride. These reactions
successfully led to the formation of NbOX2(dme) (X = Cl,
2a; Br, 2b). Optimal reaction conditions imply the addition of
tributyltin hydride to toluene solutions of in situ generated 1a,b
(Scheme 1). By using this method, 2a,b were isolated after
workup in 60−65% yields as dark blue-violet solids. Following
On theoretical grounds, among the possible dinuclear
structures of 2a (Figure S2, Table S2), that containing two
bridging chlorines and the oxide ligands in axial positions (2aA)
resulted to be the slightly most stable one (EDF2 calculations).
Instead 2aA is largely more stable compared to the
corresponding monomeric form.20 Similar conclusions were
achieved by DFT analysis (EDF2 level) on 2b (Figure S3,
Table S3).20 The geometry of the most stable form of 2b, i.e.,
2bA (Figure S3), was optimized also by using the ωB97X DFT
functional in combination with a polarized split-valence basis
set and the C-PCM solvation model for dichloromethane: the
result is drawn in Figure 1, with the caption reporting selected
bonding parameters.
Even though it was not possible to isolate and characterize
the acetonitrile adduct 2d, we studied by DFT the possible
dinuclear structures for this compound. The computer
outcomes matched the results achieved for 2a,b. Thus, EDF2
calculations indicated 2dC, containing bridging Cl ligands, as
the most stable dinuclear structure for 2d (Figure S5, Table
S5). A view of 2dC, optimized at the C-PCM/ωB97X DFT
level, is provided as Figure 2.
In contrast with the computational findings related to 2a,b,d,
major stability of μ-O-containing structures is predictable for
the tetrahydrofuran adduct 2c. The calculated dinuclear
B
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Figure 1. DFT-optimized structure of 2bA (C-PCM/ωB97X,
dichloromethane as implicit solvent). Selected bond lengths (Å):
NbO 1.681, 1.682; Nb−O 2.275, 2.436, 2.277, 2.431; Nb−
Br(terminal) 2.633, 2.631; Nb−Br(bridging) 2.726, 2.793, 2.718,
2.783; Nb---Nb 4.045. Selected angles (deg): ONb−O 91.4, 160.4,
92.3, 161.0; O−Nb−O 69.5, 69.3; ONb−Br(terminal) 102.3, 103.1;
ONb−Br(bridging) 95.9, 106.3, 95.1, 103.1; Br(bridging)−Nb−
Br(bridging) 84.6, 84.9.
Figure 3. Lowest energy DFT EDF2 calculated structure of 2c (2cE,
see Figure S4). Selected bond lengths (Å): Nb−Cl(trans thf) 2.434;
Nb−Cl(trans μ-O) 2.450, 2.452; Nb−O(thf, trans Cl) 2.239, 2.248;
Nb−O(thf, trans μ-O) 2.331, 2.329; Nb−O(μ) 1.894, 2.030, 1.892,
2.021. Selected angles (deg): Cl−Nb−Cl 91.2, 90.8; O(thf)−Nb−
O(thf) 88.7, 89.1; O(μ)−Nb−O(μ) 79.5, 79.8.
the air inertness of the Nb(IV) compounds. The NHC
molecules 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene,
Imes, and 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene, Ixyl,
were selected for the present study (Chart 1).
According to DFT, the substitution reactions of the organic
ligands in 2a−d with NHC units appear energetically favorable
(see for instance eqs 3 and 4).
1
2
[NbOCl2(NCMe)2]2 + 2Imes → NbOCl2(Imes)2
2d
3
+ 2NCMe
ΔG(C−PCM/ωB97X) = −33.2 kcal mol−1
Figure 2. DFT-optimized structure of 2dC (C-PCM/ωB97X,
dichloromethane as implicit solvent). Selected bond lengths (Å):
NbO 1.681, 1.681; Nb−N 2.235, 2.249, 2.235, 2.249; Nb−
Cl(terminal) 2.476, 2.477; Nb−Cl(bridging) 2.535, 2.945, 2.535,
2.956; Nb---Nb 4.202. Selected angles (deg): ONb−N 95.1, 97.6,
95.1, 97.6; O−Nb−Cl(terminal) 103.1, 103.1; ONb−Cl(bridging)
103.5, 169.7, 103.5, 169.7; Cl(bridging)−Nb−Cl(bridging) 80.2, 80.2.
(3)
(4)
1
2
[NbOBr (dme)]2 + 2Ixyl → NbOBr (Ixyl)2 + dme
2
2b
2
4
ΔG(C−PCM/ωB97X) = −26.9 kcal mol−1
The NHC complex 4 could actually be afforded from 2b/
Ixyl, while freshly prepared 2d was revealed to be the
preferential starting material (rather than 2a and 2c) for the
synthesis of 3. These reactions were performed in toluene at ca.
50 °C; 3 and 4 were isolated as brown solids in 50−60% yields
(Scheme 2).
Compounds 3 and 4 are fairly air stable (decomposition
completed within 5−15 min). The IR spectra (in the solid
state) contain the absorption related to the NbO moiety at
1035 cm−1. Magnetic susceptibilities were as expected for
mononuclear Nb(IV) complexes15 and comparable to the value
reported for VOCl2(Imes)2.2d In the UV−vis spectra, the
absorptions in the region 300−400 nm are ascribable to the
NHC ligands. Crystals of both 3 and 4 suitable for X-ray
diffraction could be collected after several attempts. The
molecular structures of 3 and 4 are shown, respectively, in
Figures 4 and 5, with relevant bonding parameters reported in
Table 1. Compounds 3 and 4 represent the first examples of
structurally characterized Nb(IV) complexes of the type
NbOX2L2 (X = anionic ligand; L = neutral ligand). From the
structures of 2c are shown in Figure S4, and a view of the
lowest energy structure (2cE) is given also as Figure 3. The
computer outcome of 2c is consistent with the experimental
data. Indeed, no intense IR absorption was detected in the
1010−1030 cm−1 region (terminal NbO, see above);
otherwise a band at 672 cm−1 was attributed to vibration of
the Nb−O−Nb moieties. In addition, the magnetic measure-
ment on 2c furnished a lower value (1.19 μB) than what was
expected for isolated Nb(IV) centers, in agreement with the
presence of bridging oxide groups allowing some communica-
tion between the niobium centers.
2a−d are highly air sensitive compounds (see above), their
decomposition being completed within a few seconds if allowed
contact with air. On account of the fact that simple
mononuclear coordination adducts of NbCl5 with bulky
NHC ligands have exhibited superior resistance to hydrolysis,
with respect to the precursor NbCl5,3 we supposed that the
introduction of a bulky NHC ligand could significantly improve
C
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Scheme 2. Preparation of Niobium(IV) NHC Complexes
Table 1. Selected Bond Distances (Å) and Angles (deg) for 3
and 4
3
4
Nb(1)−O(1)
Nb(1)−X(1)
Nb(1)−X(2)
Nb(1)−C(1)
Nb(1)−C(31)
C(1)−N(1)
C(1)−N(2)
N(1)−C(2)
N(2)−C(3)
C(2)−C(3)
C(31)−N(3)
C(31)−N(4)
N(3)−C(32)
1.699(4)
2.4425(15)
2.4348(16)
2.322(6)
2.332(5)
1.343(7)
1.375(7)
1.397(7)
1.395(7)
1.334(9)
1.349(7)
1.369(7)
1.384(7)
1.391(7)
1.351(9)
156.22(19)
144.99(6)
102.1(2)
101.66(19)
106.85(16)
108.15(16)
1.688(5)
2.5604(13)
2.5334(13)
2.337(7)
2.305(8)
1.353(10)
1.367(10)
1.399(11)
1.393(10)
1.331(12)
1.354(10)
1.371(10)
1.410(10)
1.374(11)
1.343(13)
152.7(3)
N(4)−C(33)
C(32)−C(33)
C(1)−Nb(1)−C(31)
X(1)−Nb(1)−X(2)
C(1)−Nb(1)−O(1)
C(31)−Nb(1)−O(1)
O(1)−Nb(1)−X(1)
O(1)−Nb(1)−X(2)
154.45(5)
105.2(3)
102.1(3)
102.5(2)
103.0(2)
crystallographically characterized. In [(C−N−C′)NbCl3(thf)],
C−N−C′ = 2,6-bis(imidazolylidene)pyridine, in which the
binding of the NHC ligand is reinforced by multidentate
coordination, the Nb(III)−C distance values are around 2.20
Å.4a On the other hand, in the complex NbCl5(Ipr), Ipr = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene, in which the
NHC binding is affected by steric factors, the Nb(V)−C
distance is 2.396(12) Å.3 In 3 and 4, an intermediate situation
is observed [Nb(1)−C(1): 2.322(6) Å for 3, 2.337(7) for 4;
Nb(1)−C(31): 2.332(5) Å for 3, 2.305(8) for 4]. However, the
Nb−C bonds in 3 and 4 are significantly longer than both
Nb(IV)−alkylidene moieties22 and even classical Nb(IV)−alkyl
σ-bonds.23 The carbene atoms of 3 and 4 display C···X [3.185−
3.362 Å for 3; 3.323−3.409 Å for 4] and C···O [3.150−3.151 Å
for 3; 3.128−3.220 Å for 4] contacts with the terminal X and O
ligands, respectively, within the sum of the van der Waals radii
of the respective atoms [sum = 3.45 Å for C···Cl, 3.55 Å for C···
Br, and 3.22 Å for C···O].24 Similar interactions have been
observed in a variety of high-valent metal halide NHC
complexes2b−d,f−h and have been associated with a form of π
back-donation from the halide ligands to the carbenic carbons
(see below for the DFT discussion).
Figure 4. Molecular structure of NbOCl2(Imes), 3, with key atoms
labeled. Displacement ellipsoids are at the 50% probability level.
The X(1)−Nb(1)−X(2) angle is considerably larger in 4
[154.45(5)°] compared to 3 [144.99(6)°], in view of the larger
size of Br vs Cl. In fact, VOCl2(IMes)2 displays a Cl−V−Cl
angle [146.55°] very close to that in 3,2d irrespective of the
different nature of the central metal atom.
The dihedral angles between the two least-squares planes
comprising the two NHC rings are different in 3 [49.41°] and 4
[27.60°], leading to different spatial arrangements of the NHC
ligands. It is plausible that this feature is the result of different
packing effects in the two structures (see below for DFT
discussion). It is noteworthy that the corresponding dihedral
angle in VOCl2(IMes)2 [28.15°] is very close to that found in
4, although the V(IV) complex contains the same IMes and Cl
ligands as 3.
Figure 5. Molecular structure of NbOBr2(Ixyl), 4, with key atoms
labeled. Displacement ellipsoids are at the 50% probability level.
other side, they are isostructural with VOCl2(IMes)2,2d thus
displaying a distorted trigonal bipyramidal structure with the
oxido and the halide ligands in equatorial positions, whereas the
two NHC ligands occupy the two axial positions. The probable
cleavage of the dinuclear structures of 2d and 2b (see above),
upon addition of two NHCs, seems to be a consequence of the
encumbrance of the latter.
The Nb(1)−O(1) distances [1.699(4) Å for 3; 1.688(5) for
4] resemble other reported NbO double-bond distan-
ces.9,14,21 Only two Nb-NHC systems were previously
D
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Figure 6. DFT-optimized structure of NbOCl2(IMes)2 and NbOBr2(Ixyl)2 (C-PCM/ωB97X, dichloromethane as implicit solvent). Hydrogen atoms
have been omitted for clarity. Selected bond lengths for NbOCl2(IMes)2 (Å): NbO 1.682; Nb−Cl 2.519, 2.519; Nb−C 2.366, 2.366. Selected
angles for NbOCl2(IMes)2 (deg): ONb−Cl 100.5, 100.5; Cl−Nb−Cl 158.9; ONb−C 99.5, 99.5; C−Nb−C 161.0. Selected bond lengths for
NbOBr2(Ixyl)2 (Å): NbO 1.678; Nb−Br 2.697, 2.698; Nb−C 2.371, 2.372. Selected angles for NbOBr2(Ixyl)2 (deg): ONb−Br 99.8, 99.8; Br−
Nb−Br 160.3; ONb−C 99.9, 99.9; C−Nb−C 160.2.
In order to shed light on the structural and bonding features
of 3 and 4, we performed DFT calculations. The calculated
structures of 3 and 4 are shown in Figure 6, the caption
reporting the calculated values of the main bonding parameters.
The most important difference with respect to the X-ray data
concerns the dihedral angles between the two least-squares
planes comprising the two NHC rings. These angles have
similar moduli on comparing the computed ground-state
geometries of 3 and 4 [X = Cl (3): experimental 49.41,
calculated 31.2°; X = Br (4): experimental 27.60, calculated
38.3°]. This result corroborates the hypothesis that the
discrepancy between 3 and 4, experimentally observed, is due
to packing forces operative in the solid state (see above for X-
ray discussion).
The metal−ligand σ-interactions in 3 are as expected for a
distorted trigonal bipyramid. Niobium mainly overlaps with
chloro and carbene ligands with two perpendicular p-type
orbitals (α-HOMO−11/β-HOMO−10, α-HOMO−14/β-
HOMO−14) and one d-type orbital (α-HOMO−19/β-
HOMO−18). Other MOs, where the metal center participates
with s-, p-, and d-type functions, account at the same time for
Nb−O, Nb−C, and Nb−Cl σ-bonding (α-HOMO−21/β-
HOMO−19, α-HOMO−35/β-HOMO−34). The Nb−O bond
is strongly strengthened by π-type interactions. For what
concerns the Nb−C bonds, a scarce π-overlap can be detected
in α-HOMO−17/β-HOMO−13, which are orbitals mainly
describing π-donation from the chloro ligands to a d-type
orbital of niobium. On considering the π-combinations of the
carbene ligands involved, we can conclude that the σ-Nb−C
bonds are supported by a weak NHC to Nb π-donation (see
the X-ray discussion). α-HOMO is the antibonding counterpart
of α-HOMO−17 and has π* Nb−Cl character. The
contribution of the donor carbon atoms to this last MO is
negligible. Figure 7 shows the α-MOs discussed in this section.
The metal−ligand σ-type interactions found for 4 are
Figure 7. Selected occupied α-MOs computed for NbOCl2(IMes)2, 3.
qualitatively comparable to those in 3. The π-bonds of niobium
C-PCM/ωB97X calculations, surface isovalue = 0.025 au. Hydrogen
with the two carbenes are mainly accounted for by the α-
HOMO−15/β-HOMO−13 orbitals. In these MOs, a strong
contribution from the two halogens can be observed, as
atoms have been omitted for clarity.
described for 3. The additional strength of the Nb−C bonds
supplied by π-contribution, as ascertained by computer
outcomes, is in alignment with the experimental detection of
C(carbene)−X interactions in the X-ray structures of 3 and 4.
The comparison of the α-HOMO−17/β-HOMO−13 orbitals
of 3 and the α-HOMO−15/β-HOMO−13 orbitals of 4 (Figure
E
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8) evidences a greater Nb−C π-overlap in the case of the
bromo derivative 4. It is, however, to be highlighted that the
Figure 9. DFT-optimized structure (ωB97X functional) of [NbOCl-
(OMe)(μ-Cl)(μ-OMe)NbO(μ-Cl)(μ-OMe)NbO(μ-Cl)(μ-OMe)-
NbO(Cl)(OMe)]2− and spin density surface (isovalue = 0.01 au).
Selected bond lengths for the central subunits (Å): NbO 1.683,
1.684; Nb−(μ-Cl) 2.584, 2.584; Nb−(μ-O) 2.153, 2.155; Nb---Nb
3.666.
Figure 8. Comparison of selected occupied MOs computed for
NbOCl2(IMes)2 and NbOBr2(Ixyl)2. C-PCM/ωB97X calculations,
surface isovalue = 0.025 au. Hydrogen atoms have been omitted for
clarity.
calculated dissociation of one carbene moiety, according to the
general reaction NbOX2(NHC)2 → NbOX2(NHC) + NHC,
has comparable Gibbs energy variations for 3 (11.8 kcal mol−1)
and 4 (12.6 kcal mol−1), respectively.
We attempted a series of nucleophilic substitution reactions
on the Nb(IV) dichloride complex 2b. We found that one
chloride ligand could be displaced by an alkoxide (aryloxide)
group. Thus, the novel compounds NbOCl(OR) (R = Ph, 5;
Me, 6) were obtained from 2b and NaOR (1:1 molar ratio) in
tetrahydrofuran and isolated after workup in 53−65% yields
(Scheme 3). The reaction of 2b with even an excess of NaOPh
cleanly led to 5, while the reaction of 2b with two equivalents of
NaOMe presumably afforded a mixture of 6 and NbO(OMe)2.
Figure 10. DFT-optimized structure (ωB97X functional) of [NbOCl-
(OPh)(μ-Cl)(μ-OPh)NbO(μ-Cl)(μ-OPh)NbO(μ-Cl)(μ-OPh)NbO-
(Cl)(OPh)]2− and spin density surface (isovalue = 0.01 au). Selected
bond lengths for the central subunits (Å): NbO 1.678, 1.678; Nb−
(μ-Cl) 2.566, 2.569; Nb−(μ-O) 2.170, 2.170; Nb---Nb 3.675.
Scheme 3. Preparation of Methoxide and Phenoxide
Derivatives of Nb(IV)
As expected, the unpaired electrons were localized on d-type
orbitals of the metal centers. The computed Nb···Nb distances
between the two central units are 3.666 and 3.675 Å for R = Me
and R = Ph, respectively. The corresponding average Nb···Nb
distances in the two models are 3.678 and 3.687 Å. These
values are comparable to those reported for similar niobium-
(IV) coordination polymers in the absence of direct metal−
metal interaction.10c
In agreement with the DFT calculations, 5 and 6 are
paramagnetic compounds, the magnetic susceptibility values
being consistent with the presence of isolated Nb(IV)
centers.15 The IR absorption due to the NbO moiety has
been detected around 800 cm−1, in analogy with what was
previously found for NbOCl2.27
5 and 6 are neutral compounds that, according to the
elemental analyses and the NMR spectra recorded on the
respective samples treated with water, do not contain dme or
thf units (see Experimental Section for details).25 We propose
that the structures of 5 and 6 resemble that of NbOCl2, with
one chloride ligand per metal center replaced by one OR group.
The structure of NbOCl2 consists of chains of NbOCl4
pyramids sharing their opposite edges, the distinct chains
being linked by NbO---Nb contacts.10c,11a Analogously to the
case of NbCl4, each chain shows an alternation of two different
Nb−Nb distances, resulting in properties such as semi-
conduction and diamagnetisms.26 Following the hypothesis of
a NbOCl2-type structure, we optimized by DFT the geometry
of a four-chain unit for both 5 and 6. Views of these structures,
obtained considering four unpaired electrons, are reported in
Figures 9 and 10, together with the respective spin density
surfaces. Attempts to optimize these compounds with lower
multiplicity afforded only distorted and high-energy geometries.
CONCLUSIONS
■
The coordination chemistry of NHC ligands to oxophilic, early
transition metal complexes has received less attention than the
corresponding chemistry with late transition metals. In
particular, very few niobium−NHC complexes have been
F
DOI: 10.1021/acs.inorgchem.5b02888
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
solid was isolated and dried under vacuum. Yield: 198 mg, 70%. Anal.
Calcd for C4H10Br3NbO3: C, 10.95; H, 2.30; Nb, 21.18; Br, 54.64.
Found: C, 11.07; H, 2.21; Nb, 21.03; Br, 54.45. IR (solid state): ν =
2943w, 1464w, 1456w, 1446w-m, 1276w, 1240w, 1185w, 1075m,
structurally characterized up to now. In the present paper, we
have described a route to the synthesis of niobium(IV) oxide
dihalide complexes, which have been almost unknown so far.
These products may represent starting materials for the
preparation of a variety of Nb(IV) compounds containing an
oxide ligand. In particular, fairly air stable Nb(IV)-NHC
coordination adducts have been obtained and crystallo-
graphically characterized. According to X-ray and DFT
outcomes, the Nb−C(carbene) bonds consist of a σ NHC to
metal donation, reinforced by a π metal to NHC contribution,
involving the halide ligands.
1
1020vs (NbO), 989m, 959s, 859s, 824w, 759vs cm−1. H NMR
(CD2Cl2): δ = 4.15 (s, 2H, CH2), 4.14 ppm (s, 3H, CH3). 13C{1H}
NMR (CD2Cl2): δ = 75.6, 70.5 (CH2); 70.3, 63.6 ppm (CH3). 93Nb
NMR (CD2Cl2): δ = −205 ppm (Δν1/2 = 1.5 × 103 Hz). UV−vis
(CH2Cl2): λmax/nm (ε/M−1 cm−1) = 335(10), 387(7), 444(8).
Synthesis and Isolation of NbOX2(dme) (X = Cl, 2a; X = Br,
2b). A suspension of NbOCl3 (380 mg, 1.77 mmol) in toluene (20
mL) was treated with dme (0.21 mL, 2.0 mmol). A light yellow
solution was obtained in 5 min. This solution was cooled with an ice
bath; then SnBu3H (0.48 mL, 1.8 mmol) was added dropwise. The
mixture was allowed to warm to room temperature and stirred for an
additional 18 h. A dark blue precipitate was formed below a green-blue
solution. The solid (2a) was isolated, washed with pentane (15 mL),
and then dried under vacuum. Yield: 291 mg, 61%. Anal. Calcd for
C4H10Cl2NbO3: C, 17.80; H, 3.73; Nb, 34.42; Cl, 26.27. Found: C,
17.69; H, 3.75; Nb, 34.20; Cl, 26.12. IR (solid state): ν = 2949w,
2847vw, 1454m, 1242w, 1191w, 1076m, 1025s (NbO), 957m,
866m-sh, 838s, 804vs, 771s, 701m cm−1. Magnetic measurement:
χMcorr = 7.94 × 10−4 cgsu, μeff = 1.40 μB. UV−vis (THF): λmax/nm (ε/
M−1 cm−1) = 278(6 × 102), 632(9), 741(10).
Compound 2b was obtained by a procedure similar to that
described for 2a, from NbOBr3 (514 mg, 1.47 mmol), dme (0.60 mL,
5.8 mmol), and SnBu3H (0.40 mL, 1.5 mmol). Pentane (30 mL) was
added to the final dark blue solution in toluene in order to obtain 2b as
a dark precipitate. Yield: 343 mg, 65%. Anal. Calcd for C4H10Br2NbO3:
C, 13.39; H, 2.81; Nb, 25.89; Br, 44.54. Found: C, 13.27; H, 2.72; Nb,
25.60; Br, 42.88. IR (solid state): ν = 2943w, 2845w, 1447m, 1258w,
1187w, 1076m, 1018s (NbO), 953m-s, 849s, 799s, 755vs, 695s
cm−1. Magnetic measurement: χMcorr = 1.13 × 10−3 cgsu, μeff = 1.65 μB.
UV−vis (THF): λmax/nm (ε/M−1 cm−1) = 351(5 × 102), 461(20),
609(12), 722(10), 805(3 × 102).
EXPERIMENTAL SECTION
■
General Procedures. Warning! The metal compounds reported in
this paper are highly moisture-sensitive; thus rigorously anhydrous
conditions were required for the reaction, crystallization, and separation
procedures. The reaction vessels were oven-dried at 150 °C prior to
use, evacuated (10−2 mmHg), and then filled with argon. NbCl5 (99+
% purity) was purchased from Strem and stored under an argon
atmosphere as received. NbBr5,28 NbOCl3,8c Imes, and Ixyl29 were
prepared according to the respective literature procedures. All the
organic reactants were commercial products (Sigma-Aldrich or Apollo
Sci.) of the highest purity available. Solvents (Sigma-Aldrich) were
distilled over appropriate drying agents under an argon atmosphere
before use. Infrared spectra were recorded at 298 K on a FT IR-
PerkinElmer spectrometer, equipped with a UATR sampling
accessory. Conductivity measurements were carried out using a
Eutech Con 700 instrument (cell constant = 1.0 cm−1).30 UV−vis
spectra were recorded with an Ultrospec 2100 Pro spectrophotometer.
EPR spectra were recorded at 298 K on a Varian (Palo Alto, CA, USA)
E112 spectrometer operating at X band, equipped with a Varian E257
temperature control unit and interfaced to an IPC 610/P566C
industrial grade Advantech computer, using an acquisition board31 and
software package especially designed for EPR experiments.32
Experimental EPR spectra were simulated by the WINSIM 32
program.33 Magnetic susceptibilities (reported per metal atom) were
measured at 298 K on solid samples with a Magway MSB Mk1
magnetic susceptibility balance (Sherwood Scientific Ltd.). Diamag-
Synthesis and Isolation of NbOCl2(thf)2, 2c. This product was
obtained by the same procedure described for 2b, from NbOCl3 (249
mg, 1.16 mmol), thf (0.21 mL, 2.6 mmol), and SnBu3H (0.31 mL, 1.2
mmol). Dark blue solid, yield: 195 mg (52%). Anal. Calcd for
C8H16Cl2NbO3: C, 29.65; H, 4.98; Nb, 28.67; Cl, 21.88. Found: C,
29.31; H, 4.75; Nb, 28.89; Cl, 21.60. IR (solid state): ν = 2955w,
netic corrections were introduced according to Konig.34 NMR spectra
̈
2926w, 2872s, 1455w-m, 1251w, 1005m, 892s, 812vs, 672vs (Nb−O−
were recorded at 298 K on a Bruker Avance II DRX400 instrument
corr
Nb) cm−1. Magnetic measurement: χM = 5.88 × 10−4 cgsu, μeff
=
1
equipped with a BBFO broadband probe. The chemical shifts for H
1.19 μB. UV−vis (CH2Cl2): λmax/nm (ε/M−1 cm−1) = 270(5 × 102),
716(5), 746(7).
and 13C were referenced to the nondeuterated aliquot of the solvent,
while the chemical shifts for 93Nb were referenced to external
[NEt4][NbCl6]. The 1H and 13C NMR spectra were assigned with the
Synthesis and Isolation of NbOCl2(Imes)2, 3. A suspension of
NbOCl3 (230 mg, 1.07 mmol) in toluene (15 mL) was treated with
MeCN (0.13 mL, 2.5 mmol). The resulting light yellow solution was
cooled with an ice bath; then SnBu3H (0.29 mL, 1.1 mmol) was added
dropwise. The mixture was allowed to warm to room temperature and
stirred for an additional 30 min. The resulting dark blue precipitate
was formed in a green-blue solution, which was eliminated with a
syringe. The solid was washed with toluene (20 mL). Thus, toluene
(15 mL) and Imes (660 mg, 2.17 mmol) were added in the order
given. The mixture was stirred for 2 h at 50 °C; then it was allowed to
cool to room temperature. The final solution was concentrated,
layered with pentane, and set aside at −30 °C; a microcrystalline
brown material was obtained after 24 h. Yield: 515 mg, 61%. Crystals
suitable for X-ray analysis were collected from a toluene/hexane
mixture at −30 °C. Anal. Calcd for C42H48Cl2N4NbO: C, 63.96; H,
6.13; N, 7.10; Nb, 11.78; Cl, 8.99. Found: C, 63.60; H, 6.22; N, 7.03;
Nb, 12.01; Cl, 8.58. IR (solid state): ν = 2920m, 2855w-m, 2760w-br,
1675w, 1605m, 1587w, 1540m, 1483m-s, 1457m, 1380w, 1319w,
1262w, 1230s, 1163w, 1100w, 1035m (NbO), 930m, 851s, 756w,
732m, 695w, 677m cm−1. EPR (solid state): g∥ = 2.214, A∥ = 163.7 G;
g⊥ = 2.412, A⊥ = 60.9 G. Magnetic measurement: χMcorr = 1.21 × 10−3
cgsu, μeff = 1.70 μB. UV−vis (toluene): λmax/nm (ε/M−1 cm−1) = 373
(80), 716 (20).
1
assistance of H,13C correlations measured through gs-HSQC and gs-
HMBC experiments.35 Carbon, hydrogen, and nitrogen analyses were
performed on a Carlo Erba model 1106 instrument. The halide
content was determined by the Mohr method36 on solutions prepared
by dissolution of the solid in aqueous KOH at boiling temperature,
followed by cooling to room temperature and addition of HNO3 up to
neutralization. Niobium was analyzed as Nb2O5, obtained by
hydrolysis of the samples followed by calcination in a platinum
crucible.
Synthesis and Isolation of NbOBr3. A suspension of NbBr5
(6.40 g, 0.0130 mol) in CH2Cl2 (50 mL) was treated with O(SiMe3)2
(2.76 mL, 0.0130 mmol). The mixture was stirred at room
temperature for 48 h, during which progressive color changing of
the mixture from brick-red to yellow was noticed. Then the volatile
materials were removed under vacuum, and the residue was washed
with pentane (2 × 30 mL). The product was isolated as a yellow
powder. Yield: 3.67 g, 81%. Anal. Calcd for Br3NbO: Nb, 26.65; Br,
68.76. Found: Nb, 26.30; Br, 69.06. IR (solid state): ν = 740vs (Nb
O) cm−1.
Synthesis and Isolation of NbOBr3(dme), 1b. NbOBr3 (225
mg, 0.645 mmol) was suspended in toluene (10 mL); then dme (0.093
mL, 0.90 mmol) was added. A red solution formed in a few minutes,
and this solution was stirred for an additional 30 min. Addition of
pentane (30 mL) determined the precipitation of a yellow solid. The
Synthesis and Isolation of NbOBr2(Ixyl)2, 4. A suspension of
NbOBr3 (198 mg, 0.568 mmol) in toluene (10 mL) was treated with
G
DOI: 10.1021/acs.inorgchem.5b02888
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
dme (0.062 mL, 0.60 mmol). A light yellow solution was obtained in a
few minutes. This solution was cooled with an ice bath; then SnBu3H
(0.15 mL, 0.57 mmol) was added dropwise. The mixture was allowed
to warm to room temperature and stirred for an additional 18 h.
Pentane (25 mL) was added in order to allow the precipitation of a
dark blue solid from a green-blue solution, which was eliminated with a
syringe. Thus, toluene (15 mL) and Ixyl (330 mg, 1.19 mmol) were
added in the order given. The mixture was stirred for 4 h at 50 °C;
then it was allowed to cool to room temperature and filtrated in order
to remove some solid. Volatiles were eliminated from the filtrated
solution, and crystallization of the residue from toluene/pentane
afforded a dark brown solid, which was washed with pentane (2 × 20
mL) and dried under vacuum. Yield: 233 mg, 50%. Crystals suitable
for X-ray analysis were collected from a toluene/hexane mixture set
aside at −30 °C. Anal. Calcd for C38H40Br2N4NbO: C, 55.56; H, 4.91;
N, 6.82; Nb, 11.31; Br, 19.45. Found: C, 55.63; H, 5.02; N, 6.66; Nb,
11.12; Br, 19.28. IR (solid state): ν = 3163w, 3132w, 3026w, 2917w-m,
1687w, 1603w, 1532w-m, 1472s, 1446m-sh, 1392w-m, 1380m, 1276m,
1215m, 1167w-m, 1100m, 1035m (NbO), 960m, 948s, 923m,
866m, 770vs, 730vs, 695vs cm−1. Magnetic measurement: χMcorr = 9.42
× 10−4 cgsu, μeff = 1.50 μB. EPR (solid state): g∥ = 2.219, A∥ = 176.5 G;
g⊥ = 2.331, A⊥ = 73.8 G. UV−vis (toluene): λmax/nm (ε/M−1 cm−1) =
320 (2.0 × 103), 384 (1.0 × 103), 480 (2.5 × 102).
Table 2. Crystal Data and Experimental Details for 3·
(1/2hexane) and 4·toluene
3·(1/2hexane)
4·toluene
formula
C45H55Cl2N4NbO
831.74
C45H48Br2N4NbO
913.60
fw
T, K
100(2)
100((2)
0.71073
orthorhombic
Pbca
λ, Å
0.71073
monoclinic
C2/c
cryst syst
space group
a, Å
30.131(6)
12.795(2)
24.138(5)
100.206(3)
9159(3)
8
21.228(3)
15.011(2)
26.045(4)
90
b, Å
c, Å
β, deg
cell volume, Å3
8299(2)
8
Z
Dc, g cm−3
μ, mm−1
1.206
1.462
0.415
2.254
F(000)
3488
3720
cryst size, mm
θ limits, deg
reflns collected
indep reflns
data/restraints/params
goodness of fit on F2
R1(I > 2σ(I))
wR2(all data)
0.16 × 0.13 × 0.10 0.18 × 0.16 × 0.14
1.37−26.00
1.56−25.03
43 118
56 936
Synthesis of NbOCl(OR) (R = Ph, 5; Me, 6). A solution of freshly
prepared 2a (from 1.10 mmol of NbOCl3) in tetrahydrofuran (20 mL)
was treated with NaOPh (128 mg, 1.10 mmol). An immediate color
change from deep blue to brown was observed, and the mixture was
left stirring at room temperature for an additional 18 h. Thus, the
volatiles were removed under vacuum, and the resulting residue was
dissolved in toluene (30 mL). The solution was filtered in order to
remove some insoluble material; then the solvent was removed under
vacuum. The residue was washed with pentane (2 × 20 mL); thus
compound 5 was obtained as a light brown powder. Yield: 170 mg
(65%). An aliquot (ca. 20 mg) was dissolved into CDCl3 (0.7 mL),
treated with a large excess of H2O, and allowed to contact air
8988 [Rint = 0.0821] 7308 [Rint = 0.0765]
8988/246/475
1.033
7308/6/478
1.044
0.0742
0.0806
0.2383
0.2651
largest diff peak and hole, e Å−3 1.981/−0.963
2.824/−2.819
in SHELXL; s.u. 0.005) and to the C−C distances (DFIX 1.53 line in
SHELXL; s.u. 0.01).
Computational Studies. The computational geometry optimiza-
tions were carried out without symmetry constraints, using the hybrid-
GGA EDF2 functional39 in combination with the 6-31G** basis set
(ECP-based LANL2DZ basis set for elements beyond Kr).40 The
“unrestricted” formalism was applied for compounds with unpaired
electrons, and the lack of spin contamination was verified by
comparing the computed ⟨S2⟩ values with the theoretical ones. The
stationary points were characterized by IR simulations (harmonic
approximation), from which zero-point vibrational energies and
thermal corrections (T = 298.15 K) were obtained.41 Further
optimization of selected geometries was carried out using the range-
separated DFT functional ωB97X,42 in combination with a polarized
basis set composed by the 6-31G(d,p) set on the light atoms and the
ECP-based LANL2TZ(f) and LANL2DZ(d) sets on niobium and
bromine, respectively.43 The C-PCM implicit solvation model (ε =
9.08) was added to ωB97X calculations.44 The software used for C-
PCM/ωB97X calculations was Gaussian’09,45 while EDF2 calculations
were performed with Spartan’08.46
1
overnight; a subsequent H NMR spectrum of the solution pointed
out the presence of PhOH and traces of thf. Anal. Calcd for
C6H5ClNbO2: C, 30.35; H, 2.12; Nb, 39.12; Cl, 14.93. Found: C,
30.16; H, 2.18; Nb, 39.02; Cl, 14.76. IR (solid state): ν = 3061w,
2958w, 1586m, 1477s, 1451w-sh, 1238s, 1160m, 1098w, 1067w,
1020w, 1000w, 884m, 795s (NbO), 749vs, 685vs cm−1. Magnetic
corr
measurement: χM
= 9.69 × 10−4 cgsu, μeff = 1.53 μB. UV−vis
(toluene): λmax/nm (ε/M−1 cm−1) = 361 (40). ΛM (toluene) = 0.1 S
cm2 mol−1. Compound 6 was obtained by a procedure analogous to
that described above, by allowing 2a (freshly prepared from 1.00 mmol
of NbOCl3) to react with NaOMe (54 mg, 1.00 mmol). Complex 6
could be isolated as a sticky, strongly air sensitive light brown solid.
Yield: 93 mg, 53%. NMR analysis on an aliquot of 6 (ca. 20 mg)
dissolved into CDCl3 (0.7 mL) and then treated with a large excess of
H2O pointed out the presence of MeOH. Anal. Calcd for
CH3ClNbO2: C, 6.85; H, 1.72; Nb, 52.97; Cl, 20.21. Found: C,
6.76; H, 1.75; Nb, 52.65; Cl, 19.98. IR (solid state): ν = 2920w,
2821w, 1579w, 1450w, 1373w, 1152m, 1083m, 1014vs, 854m-sh,
ASSOCIATED CONTENT
* Supporting Information
■
corr
816s-sh (NbO), 793vs, 682s cm−1. Magnetic measurement: χM
=
S
8.67 × 10−4 cgsu, μeff = 1.44 μB. UV−vis (CH2Cl2): λmax/nm (ε/M−1
cm−1) = 362 (6 × 102), 438 (70). ΛM (toluene) = 0.1 S cm2 mol−1.
X-ray Crystallography. Crystal data and collection details for 3·
(1/2hexane) and 4·toluene are reported in Table 2. The diffraction
experiments were carried out on a Bruker APEX II diffractometer
equipped with a CCD detector using Mo Kα radiation. Data were
corrected for Lorentz polarization and absorption effects (empirical
absorption correction SADABS).37 Structures were solved by direct
methods and refined by full-matrix least-squares based on all data using
F2.38 Hydrogen atoms were fixed at calculated positions and refined by
a riding model. All non-hydrogen atoms were refined with anisotropic
displacement parameters unless otherwise stated. The hexane molecule
in 3·(1/2hexane) is disordered over two equally populated symmetry-
related (by 2) positions. Its independent image has been refined
isotropically applying restraints to the thermal parameters (SIMU line
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02888. CCDC reference numbers 1438214 (3) and
1438215 (4) contain the supplementary crystallographic data
for the X-ray studies reported in this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(internat.) + 44-1223/336-033; e-mail: deposit@ccdc.cam.ac.
uk).
Figures S1−S5 showing the DFT-calculated structures of
1b and 2a−d; Tables S1−S5 containing the relevant
bonding parameters (PDF)
H
DOI: 10.1021/acs.inorgchem.5b02888
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Crystallographic data for 3 and 4 (CIF)
Article
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(8) (a) Hartwig, S.; Hillebrecht. Z. Anorg. Allg. Chem. 2008, 634,
115−120. (b) Kohler, J.; Simon, A.; van Wullen, L.; Cordier, S.;
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2683−2690. (c) Gibson, V. C.; Kee, T. P.; Shaw, A. Polyhedron 1990,
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(9) Levason, W.; Reid, G.; Trayer, J.; Zhang, W. Dalton Trans. 2014,
43, 3649−3659 and references therein..
(10) (a) Tran, T. T.; Gooch, M.; Lorenz, B.; Litvinchuk, A. P.;
Sorolla, M. G., II; Brgoch, J.; Chu, P. C. W.; Guloy, A. M. J. Am. Chem.
Soc. 2015, 137, 636−639. (b) Cordier, S.; Gulo, F.; Roisnel, T.;
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42, 8320−8327. (c) Whangbo, M. H. Inorg. Chem. 1982, 21, 1721−
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(11) (a) Beck, J.; Kusterer, C. Z. Anorg. Allg. Chem. 2006, 632, 2193−
2194. (b) Fairbrother, F. The Halides of Niobium and Tantalum. In
Halogen Chemistry; Gutmann, V., Ed.; Academic Press: London, 1967;
Cartesian coordinates of all DFT-optimized compounds
(XYZ)
AUTHOR INFORMATION
■
̈
̈
Corresponding Author
*E-mail: fabio.marchetti1974@unipi.it.
Present Address
⊥Institut fur Anorganische Chemie, Georg-August-Universitat
̈
̈
Gottingen, Tammannstrasse 4, D-37077 Gottingen, Germany.
̈
̈
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of Pisa for financial support.
REFERENCES
■
Vol. 3, pp 123−178. (c) Schafer, H.; Sibbing, E.; Gerken, R. Z. Anorg.
̈
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J. Coord. Chem. Rev. 2015, 293−294, 48−79. (b) Nelson, D. J. Eur. J.
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