Effect of the spin-orbit interaction of ligands on the parameters of EPR spectra for a series of niobium(IV) complexes of trans-[NbX4(OPPh3)2] (X = Cl, Br, I)

Article abstract of DOI:10.1016/j.ica.2020.120056

Two crystal modifications of new molecular niobium(IV) complex trans-NbI₄(OPPh₃)₂ were obtained by ampoule synthesis, and their X-ray crystal structures were solved (P-1, a = 9.5795 ?, b = 9.7287 ?, c = 11.2337 ?, α = 107.984°, β = 104.061°, γ = 95.500°, V = 949.24 ?³; P2₁, a = 10.204 ?, b = 16.039 ?, c = 11.857 ?, β = 102.663°, V = 1893.3 ?³). For the series of molecular complexes trans-NbX₄(OPPh₃)₂ (X = Cl, Br, I) magnetic properties were studied and systematized using EPR, magnetic susceptibility measurements, and theory calculations with DFT approximation. Theory calculations of EPR parameters have shown good accordance with experimental data and allowed to establish a negative sign of the principal hyperfine tensor values.

Full text of DOI:10.1016/j.ica.2020.120056

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Research paper
Effect of the spin-orbit interaction of ligands on the parameters of EPR spec‐
tra for a series of niobium(IV) complexes of trans-[NbX (OPPh ) ] (X = Cl,
4
3 2
Br, I)
Vladimir A. Nadolinny, Pavel A. Poltarak, Andrey Yu. Komarovskikh,
Sergey V. Tumanov, Denis G. Samsonenko, Vladislav Yu. Komarov, Mikhail
M. Syrokvashin, Pavel V. Dorovatovskii, Vladimir A. Lazarenko, Sofya B.
Artemkina, Vladimir E. Fedorov
PII:
DOI:
Reference:
S0020-1693(20)31256-1
https://doi.org/10.1016/j.ica.2020.120056
ICA 120056
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
28 July 2020
30 September 2020
2 October 2020
Please cite this article as: V.A. Nadolinny, P.A. Poltarak, A.Y. Komarovskikh, S.V. Tumanov, D.G.
Samsonenko, V.Y. Komarov, M.M. Syrokvashin, P.V. Dorovatovskii, V.A. Lazarenko, S.B. Artemkina, V.E.
Fedorov, Effect of the spin-orbit interaction of ligands on the parameters of EPR spectra for a series of
niobium(IV) complexes of trans-[NbX (OPPh ) ] (X = Cl, Br, I), Inorganica Chimica Acta (2020), doi: https://
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3 2
doi.org/10.1016/j.ica.2020.120056
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Effect of the spin-orbit interaction of ligands on the parameters of EPR spectra for a series of
niobium(IV) complexes of trans-[NbX (OPPh ) ] (X = Cl, Br, I)
4
3 2
1
1,3
1
2
Vladimir A. Nadolinny , Pavel A. Poltarak , Andrey Yu. Komarovskikh , Sergey V. Tumanov ,
1
,3
1
1
Denis G. Samsonenko , Vladislav Yu. Komarov , Mikhail M. Syrokvashin , Pavel V. Dorovatovskii
4,
4
1,3
1
Vladimir A. Lazarenko , Sofya B. Artemkina , Vladimir E. Fedorov
1
Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3,
Akad. Lavrentiev Ave, 630090 Novosibirsk, Russia.
2
International Tomography Center, Siberian Branch of Russian Academy of Sciences, 3a,
Institutskaya st., 630090 Novosibirsk, Russia.
3
Novosibirsk State University, 1, Pirogova str., 630090 Novosibirsk, Russia.
4
National Research Centre "Kurchatov Institute", 1, Akademika Kurchatova pl., Moscow, 123182,
Russia
E-mail: sofya.artemkina@gmail.com
Keywords: Niobium(IV) complex, d1 configuration, spin-orbital interaction, X-ray crystal structure,
EPR, magnetic susceptibility, DFT calculations
Abstract: Two crystal modifications of new molecular niobium(IV) complex trans-NbI (OPPh3)2
4
were obtained by ampoule synthesis, and their X-ray crystal structures were solved (P-1, a=9.5795
3
Å, b=9.7287 Å, c=11.2337 Å, α=107.984°, β=104.061°, γ=95.500°, V= 949.24Å ; P2 , a=10.204
1
3
Å, b=16.039 Å, c=11.857 Å, β=102.663°, V= 1893.3Å ). For the series of molecular complexes
trans-NbX (OPPh ) (X=Cl, Br, I) magnetic properties were studied and systematized using EPR,
4
3 2
magnetic susceptibility measurements, and theory calculations with DFT approximation. Theory
calculations of EPR parameters have shown good accordance with experimental data and allowed to
establish a negative sign of the principal hyperfine tensor values.
1
. Introduction
The study of the magnetic properties of coordination compounds has a rich history, which
includes the development of methods for measuring, synthetic methods of new complexes of metals
of interest, and of course, the discussion related to this, including theoretical modelling and
quantum calculations [1,2].
1
Nb(IV) molecular complexes are one of group d complexes interesting from the point of
studying magnetic properties. Among the works presenting EPR data of niobium(IV) compounds,
products of interaction of NbCl with substituted pyridines [3], and products of electrochemical
4
reduction of NbCl5 in HCl saturated ethanol solution [4] are mentioned. Niobium(IV)

phthalocyanine oxocomplexes [5], as well as N,N-diethylcarbamato and hexafluoroacetylacetonato
derivatives of niobium(IV) (Nb(O CNEt ) and Nb(hfacac) ) [6] were isolated as crystalline
2
2 4
4
samples, and charge state of niobium(IV) was confirmed by EPR. A series of works are devoted to
halogenide niobium(IV) complexes with different phosphines [7-9], at that synthesis and their
crystal structures are discussed. In the case of phosphines without voluminous substituents,
formation of niobium dimers with Nb–Nb bond are observed (for example, Nb Cl (PMe ) ).
2
8
3 4
Mononuclear niobium(IV) complexes are usually synthesized from tetra- or
pentahalogenides by solution chemistry methods. Recently [10] we proposed procedure for
preparation of niobium(IV) complexes trans-NbX (OPPh ) (X = Cl (1), Br (2)) using one step
4
3 2
synthesis in an evacuated ampoule. To complement the present data on magnetic properties of
niobium(IV) compounds, we synthesized new complex trans-NbI (OPPh3)2 (3a, 3b), and
4
systematically studied magnetic properties of trans-NbX (OPPh ) (X = Cl, Br, I) series. For the
4
3 2
complexes of the same structure and symmetry, we investigated the effect of halogens on the EPR
spectra.
2
2
. Experimental section
.1.
Materials
Niobium powder, and iodine (99%, “Iodobrom” Russia), triphenylphosphine (Acros, 99%),
triphenylphosphine oxide (Sigma Aldrich, 98%), and acetonitrile (Reakhim, analytical grade) were
used without pre-purification.
2
.2.
Synthetic procedures
2
.2.1. Synthesis of NbI . NbI was obtained by the reaction of the elements with a 10%
5
5
excess of iodine in evacuated two-zone quartz ampoule. Nb powder (0.639g, 6.88 mmol) and iodine
4.8g, 19 mmol) were put into a quartz ampoule with 50 ml total volume. The end of the ampoule
(
containing iodine was maintained at room temperature. The other end containing niobium was
heated (400°С, 2h) until most of the iodine is reacted. During this time the ampoule was carefully
pushed into the furnace in order to accelerate the evaporation of iodine. Then ampoule was heated
in a muffle furnace (400°С, 40h) to complete the reaction. Excess of iodine was condensed in the
one end by heating the other end to 120°С, and then the ampoule part with the product was soldered
apart.
2
.2.2. Synthesis of Nb I . Nb I was obtained by the reaction of niobium metal with
3 8 3 8
stoichiometric amount of iodine in evacuated two zone quartz ampoule. Nb powder (0.645 g,
.94 mmol) and iodine (2.4 g 9.5 mmol) put into a quartz ampoule of volume 50 ml. The one end of
the ampoule containing iodine was maintained at room temperature. The other end containing
6

niobium was heated (400°С, 2h) until most of the iodine is reacted. During this time the ampoule
was carefully pushed into the furnace in order to accelerate the evaporation of iodine. Then ampoule
was heated in muffle furnace (700°С, 80h) in order to achieve completion of the reaction.
The purity of NbI and Nb I was confirmed by XRPD (Fig. S1 and S2, respectively).
5
3 8
2
.2.3. Synthesis of trans-[NbCl (OPPh ) ] (1) and trans-[NbBr (OPPh ) ] (2). The
4 3 2 4 3 2
crystalline powders 1 and 2 were synthesized according to the procedure in [10]. Generally, NbCl5
(
0.54g, 2.0 mmol) or NbBr (0.98g, 2.0 mmol) were reacted with a mixture of PPh (0.50g
5
3
1
.9 mmol) and OPPh (1.30g, 4.7 mmol). The samples were placed in sealed glass ampoules filled
3
with argon; the ampoules were heated at 250°С. The ampoules were opened in air, and the product
mixtures were washed with acetonitrile 3×20ml. The yields were 80% (1) and 94% (2). The purity
of 1 and 2 was confirmed by XRPD (Figs. S3 and S4, respectively).
2
.2.4. Synthesis of crystals trans-[NbI (OPPh ) ] (3а). Well-faceted brown-orange
4 3 2
crystals of irregular habitus were obtained in the reaction of Nb I (~50 mg, 0.04 mmol) and melt of
3
8
PPh (100 mg, 0.38 mmol) within an evacuated glass ampoule. The ampoule was heated 250°С
3
during 35 h. The product of this reaction consisted of unreacted Nb I , 3a and 3b, according to
3
8
XRPD (Fig. S5). The yield of crystals 3a was not higher then 5-10%. EDX: Nb:P:I=1:1.9:3.9.
.2.5. Synthesis of trans-[NbI (OPPh ) ] (3b). NbI (1.45 g, 2.0 mmol), OPPh (1.30 g, 4.7
2
4
3 2
5
3
mmol) and PPh (0.50 g, 1.9 mmol) were put into a glass ampoule; these manipulations were
3
carried out in argon atmosphere using a dry box. The ampoule was sealed and heated up to 250ºC,
and kept at this temperature for 15 h. The ampoule was opened in air and the product mixture was
washed with acetonitrile 3×20ml. The product 3b was brown-orange fine crystalline powder, yield
2
,31 g (98% referred to Nb). Crystal for X-ray diffraction was selected from the product manually.
The product purity confirmed by X-ray powder diffractometry (Fig. S6). Analysis for light
elements: C 37.3 % (clcd 37.4); H, 2.7 (clcd 2,6%). IR-spectrum of 3b, as well as ones for 1 and 2,
is similar to the OPPh spectrum (Fig.S7-10).
3
2
.3.
Methods
Diffraction patterns of powder samples were recorded using Philips PW 1830/1710
diffractometer (CuK radiation, graphite monochromator, silicon plate as the external standard).
α
CHNS analyses were performed with Vario MICRO Cube CHNS analyser. The Nb:X:I element
ratios were determined by a desktop scanning electron microscope with a Hitachi TM-3000 energy
dispersive analyzer of the chemical composition. IR spectra were recorded on a SCIMITAR FTS
2
000 instrument. EPR spectra were obtained on Varian Е-109 and Bruker Elexsys E580
spectrometers in X and Q frequency ranges in the temperature range 50 - 300 K. The EPR spectra
were modeled using the EasySpin toolbox in the MatLab software. Powdered samples of the

synthesized compounds were used as the objects of EPR study. The EPR spectrum of 2,2-diphenyl-
-picrylhydrazyl (DPPH) with g = 2.0036 was used as the reference. The EPR spectra were
1
recorded in the Q-band (35 GHz) at 20 mW microwave power, 100 kHz of modulation frequency,
and 1 gauss of modulation amplitude. The EPR spectra of 1 and 2 samples were recorded at a
temperature of 50 K because at temperatures above 77 K the lines of the EPR spectra broadened.
For the sample 3b, no broadening of the EPR spectrum lines was observed in the temperature range
5
0 - 300 K.
The DFT calculations of g- and A-tensors were performed for 1, 2,3b, using the ADF
019.3 program package [11,12] with a hybrid functional B3LYP [13]. Triple zeta basis sets and
no frozen core” approximations were used in all the calculations [14]. The relativistic effects were
2
“
calculated at the Spin-Orbit ZORA level [15]. In such a calculation of g- and A-tensors a degenerate
perturbation theory is used with the external magnetic field or nuclear magnetic field as perturbation
[
16,17]. The structures of complex molecules were taken from the structural data and were not
optimized.
Magnetic susceptibilities of 1, 2, 3b complexes were measured by Faraday method. The data
was obtained using quartz torque microbalance and recorded high precision digital voltmeter
Keysight 34465A. The temperature stabilization was controlled using Delta DTB9696 temperature
controller. The sample was placed in the open quartz ampoule and vacuumed to 0.01 Torr
pressure. The measurements were carried out in the helium atmosphere with a pressure of 5 Torr.
The magnetic field strength was 7.3 kOe. The measurements were carried out in two cycles
(
cooling 300-80K and heating 80-300K). The obtained values of magnetic susceptibilities were
averaged.
X-ray single crystal diffraction of compound 3a was performed at 150 K on an automatic
Bruker APEX Duo diffractometer (MoK radiation, λ = 0.71073 Å, graphite monochromator, two-
α
dimensional detector) by the standard procedure [18]. The crystal structure was solved by a direct
method and refined by the full-matrix least squares technique using the SHELXTL program
package [19] with an Olex2 graphical interface [20]. Atomic displacement parameters of non-
hydrogen atoms were refined in the anisotropic approximation. Positions of all H atoms were
found from the difference Fourier maps and included in the refinement in the riding model.
Diffraction data for single-crystal 3b were collected at 100 K on the ‘Belok’ beamline
(λ = 0.79272 Å, φ-scans with a step of 1.0°) of the National Research Center ‘Kurchatov Institute’
(Moscow, Russian Federation) using a Rayonix SX165 CCD detector. The data were indexed,
integrated and scaled, absorption correction was applied using the XDS program package [21]. The
structure was solved by dual space algorithm (SHELXT [22]) and refined by the full-matrix least
squares technique (SHELXL [19]) in the anisotropic approximation (except hydrogen atoms).

Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the
riding model. The crystallographic data and details of the diffraction experiments for compounds 3a
and 3b are summarized in Table 1.
Full tables of interatomic distances and bond angles, atomic coordinates and atomic
displacement parameters for structures 3a and 3b have been deposited with the Cambridge
Crystallography
Data
Center
(CCDC
2013822
(3a)
and
2012709
(3b)
https://www.ccdc.cam.ac.uk/structures/).
Table 1. Experimental X-ray single crystal data for compounds 3a and 3b.
NbI (OPPh ) (3а)
NbI (OPPh ) (3b)
4 3 2
4
3 2
Parameter
М, g/mol
1157.05
1157.05
a, Å
b, Å
c, Å
α, °
β, °
9.5795(8)
9.7287(8)
11.2337(9)
107.984(2)
104.061(3)
95.500(3)
P-1
10.204(2)
16.039(2)
11.857(2)
90
102.663(9)
90
γ, °
Space group
Z
P21
2
1
Crystal size, mm
Temperature, K
0.28 × 0.24 × 0.16
150(2)
0.03 × 0.03 × 0.02
100(2)
3
V, Å
Ρ , g/cm
calc
949.24(14)
2.024
1893.3(5)
2.030
4.969
3
–
1
μ, mm
3.684
Reflections measured/
independent
R /R
1
2918/5311
15285/8633
0.0221/0.0286
1.033
0.0608/0.0782
1.024
int
σ
GooF
R /wR for I > 2σ(I)
0.0202/0.0416
0.0243/0.0432
0.0525/0.1210
0.0720/0.1321
1
2
R /wR for all data
1
2
Residual electron density
0
.525 / -0.553
1.172 / −2.014
–
3
(peak/hole), e Å
3
. Results and Discussion
3
.1. Synthesis
Compounds 3а and 3b were prepared by the method of ampoule synthesis, using niobium
iodides Nb I (for 3a) and NbI (for 3b) as starting compounds. In the case of 3a, we didn’t add an
3
8
5
oxygen source on purpose; we consider that product of the partial hydrolysis of the starting Nb3I8
presumably, niobium oxohalides) presented as an oxygen source in the reaction. Using of Nb I in
(
3
8

such a way does not yield analytically pure material. In this case we obtained only a few crystals 3a
in the mixture of unreacted starting species.
First, we studied the reaction of Nb I with PPh which resulted in crystals 3a. Further, for
3
8
3
obtaining pure product 3a, we used NbI , OPPh and PPh as starting reagents, and this experiment
5
3
3
resulted in 3b, not 3a. Unfortunately, attempts to find experimental conditions for the synthesis of
a failed.
Powder of 3b was obtained in reactions of niobium pentaiodide with the melt of
3
triphenylphosphine and triphenylphosphine oxide with yield of 98% referred to Nb. We assume that
triphenylphosphine reduces niobium to +4 oxidation state:
2
NbI + 4OPPh + PPh = 2NbI (OPPh ) (3b)+ I PPh
5 3 3 4 3 2 2 3
In general, compounds 3a and 3b were obtained in similar way – in ampoule syntheses, as 1
and 2 which were synthesized by us earlier [10].
3
.2. X-ray crystal structures of 3a and 3b
Crystal structures of both compounds 3a and 3b include complexes trans-[NbI (OPPh ) ]
4
3 2
but differ in the packing arrays as well as geometries of the complexes (Fig. 1). Compound 3a
crystallizes in space group P-1, it is isostructural with 1 and 2 [10]. Coordination environments of
Nb atoms are octahedral (4I+2О) with trans-positions of oxygen atoms. In the structures, the
complexes [NbI (OPPh ) ] themselves differ by mutual orientations of coordinated molecules
4
3 2
OPPh . Complex [NbI (OPPh ) ] in 3a is located in a special position with an inversion center on Nb
3
4
3 2
atom, and P-O-Nb-O-P fragment has a zigzag form. Complex [NbI (OPPh ) ] in 3b is located in a
4
3 2
general position, and P-O-Nb-O-P fragment has a bow form.
Selected values of bond lengths and angles in structures 3a and 3b are listed in Tables 2 and
. Bond lengths Nb–I in structures 3a and 3b are in the range 2.77 – 2.83 Å, that is 0.1Å longer than
3
bonds Nb–I (terminal) in the structure of NbI (2.65-2.70Å) [23]. Bond lengths Nb–O in the
4
structures 3a and 3b differ from the analogous Nb–O lengths not more than 0.02 Å. The angles on
Nb atoms in 3a and 3b are very close; they differ by no more than 3°.

Figure 1. Structures of the complexes trans-[NbI (OPPh ) ] in 3а (a) and 3b (c), and their packing
4
3 2
motifs in 3а (b) and 3b (d).
Table 2. Selected bond lengths in X-ray crystal structures of 3a and 3b.
Bond
Length, Å
Bond
Length, Å
Bond
Length, Å
Bond
Length, Å
3
a
Nb1 -- I1 2.8191(2) Nb1 -- I2 2.8110(2) Nb1 -- O1 1.9788(14) O1 -- P1 1.5412(14)
3
b
Nb1 -- I1 2.8305(19) Nb1 -- I2 2.8088(19) Nb1 -- O1 2.003(11) O1 -- P1 1.518(12)
Nb1 -- I3 2.7732(19) Nb1 -- I4 2.8093(19) Nb1 -- O2 1.985(12) O2 -- P2 1.546(12)
Table 3. Selected angle values in X-ray crystal structures of 3a and 3b.
Angle
value , º
89.888(7)
90.15(5)
Angle
value , º
90.09(4)
89.8(3)
Angle
value , º
90.84(4)
91.7(3)
Angle
value , º
165.91(10)
163.5(8)
3
a
I1-Nb1-I2
I1-Nb1-I2
O1-Nb1-I1
O1-Nb1-I1
O1-Nb1-I2
O1-Nb1-I2
P1-O1-Nb1
P1-O1-Nb1
3
b

I3-Nb1-I4
90.75(6)
O1-Nb1-I3
88.9(3)
O1-Nb1-I4
93.2(3)
P2-O2-Nb1
159.6(8)
As a result, we have a series of niobium(IV) complexes with very similar X-ray crystal
structures. Moreover, these complexes [NbX (OPPh ) ] (X=Cl, Br, I), having one unpaired electron
4
3 2
per Nb atom, could serve as convenient objects for the study of magnetic properties. We examined
the magnetic properties of the compounds NbX (OPPh ) by EPR and magnetic susceptibility
4
3 2
measurements.
3
.3. EPR spectra of the complexes 1, 2, 3b
The [NbX (OPPh ) ] (X=Cl, Br, I) complexes were studied by EPR spectroscopy. Q-band
4
3 2
experimental and simulated EPR spectra of these complexes are given in Fig. 2.
The experimental spectra can be well fitted with spin Hamiltonian (1)
H = g βH S + g βH S + g βH S + A(Nb) I S + A(Nb) I S + A(Nb) I S
zz z z
(1)
xx
x
x
yy
y
y
zz
z
z
xx x
x
yy y
y
with S = 1/2, I = 9/2, and the principal values of g- and A-tensors given in Table 4
9
3
(
considering the natural abundance of Nb isotope is 100%). The observed EPR spectra with the
9
3
1
electron spin S = 1/2 and the HFS of one Nb nucleus (I = 9/2) is typical for the 4d electronic
configuration of niobium ion niobium(IV). All spectra were simulated with the linewidths ΔH = 8.0
mT (Н || g ) and ΔH = 19.0 mT (Н g ). The linewidths for parallel and perpendicular directions of
zz
┴ zz
NbX (OPPh ) EPR spectra are different probably due to the unresolved HFS of chlorine (chlorine
4
3 2
3
5
37
isotopes Cl and Cl with natural abundance 75.78% and 24.22%, respectively, with the nuclear
moment I=3/2), bromine (bromine isotopes 79Br and 81Br with natural abundance 50.69% and
4
9.31%, respectively, with the nuclear moment I=3/2), and iodine atoms (iodine isotope 127I with
natural abundance 100% with the nuclear moment I=5/2).
EPR spectra are very similar for 2 and 3b. They were simulated assuming g ≠g for 3b; the
xx
yy
principal values A(Nb) and A(Nb) have been obtained from the wide perpendicular component
xx
yy
of the spectrum. It should be noted that the lines are quite broad and it is impossible to estimate
precisely the principal values A(Nb) and A(Nb) of the niobium A-tensor, as well as the possible
xx
yy
contribution of superhyperfine interaction (SHFI) due to bromine or iodine atoms.

T=300K
T=50K
T=50 K
1
2
1
2
1
2
1000
1200
1400
1600
1800
600
800 1000 1200 1400 1600 1800
1000
1200
1400
1600
1800
Magnetic field, mT
Magnetic field, mT
Magnetic field, mT
a)
b)
c)
Fig. 2. The Q-band EPR spectra of the 1 (a), 2 (b), and 3b (c) complexes: 1 – experimental, 2 –
simulated. EPR spectra of the 1 (a), 2 (b) complexes were recorded at 50K, and 3b (c) complexes at
3
00K.
Table 4. Experimental and simulated EPR parameters of the 1, 2, 3b.
9
3
Compound
g-tensor
A-tensor of
g-tensor
(B3LYP, TZP,
SO ZORA)
A-tensor of
g-tensor
A-tensor of Nb,
mT (B3LYP,
9
3
93
Nb, mT
Nb, mT (B3LYP, TZP
(B3LYP,
TZP, SO
ZORA)
except of Nb
TZP except of
with TZ2P-J, Nb with TZ2P-J,
SO ZORA)
SO ZORA)
[
NbCl
4
(OPPh
3
)
2
]
g
g
g
g
g
g
g
g
xx=1.518,
yy=1.518,
zz=1.813
xx=1.658,
yy=1.658,
zz=1.990
xx=1.540,
yy=1.650,
|Axx|=17.3,
|Ayy|=17.3,
|Azz|=28.5
|Axx|=15.0,
|Ayy|=15.0,
|Azz|=23.0
|Axx|=8.0,
|Ayy|=8.0,
|Azz|=16.0
g
g
g
g
g
g
g
g
g
xx=1.41,
yy=1.44,
zz=1.91
xx=1.51,
yy=1.52,
zz=2.02
xx=1.53,
yy=1.54,
A
A
A
A
A
A
A
A
xx=-15.1,
yy=-14.6,
zz=-21.6
xx=-12.5,
yy=-12.2,
zz=-18.8
xx=-10.5,
yy=-10.3,
zz=-13.8
g
g
g
g
g
g
xx=1.48,
yy=1.50,
zz=1.91
xx=1.55,
yy=1.56,
zz=2.02
xx=1.56,
yy=1.57,
A
xx=-14.3, Ayy=-
14.0, Azz=-21.9
[NbBr
4
(OPPh
3
)
2
]
A
xx=-12.2, Ayy=-
12.0, Azz=-19.0
[
NbI
4
(OPPh
3
)
2
]
g
g
A
xx=-10.3, Ayy=-
10.2, Azz=-14.1
g
zz=2.098
zz=2.18
A
gzz=2.18
3
.4. Theory calculations
Theoretical calculation of the hyperfine interaction of the unpaired electron with the nuclear
9
3
magnetic moment of niobium Nb is discussed in [24]. The unpaired electron localized on the s-
orbital gives A = 235.15 mT, whereas the unpaired electron localized on the d-orbital gives Aaniso
iso
=
6.527 mT. Usually, the absolute values of the HFS tensor are about 10–30 mT for niobium(IV)
compounds [3–6,25], which is consistent with our results.
The absolute values of niobium A-tensor decrease and the g components of g-tensor increase
zz
simultaneously from X = Cl to I. At the same time, the Nb–X bond lengths increase from 2.433 –
2
.421 Å for chlorine to 2.811 – 2.819 Å for iodine. Such change in geometry corresponds to
increasing length of Nb-X bonds due to atomic radius of halogen atoms. But all three complex
compounds have the same symmetry of the coordination environment of niobium atom.The reason
for the changes in the g- and A-tensors may be the effect of spin-orbit interaction which is specified

by the sort of halogen. The spin-orbit coupling constant increases from chlorine to iodine: λ=500
-1
-1
-1
cm (Cl), 2400 cm (Br), and 5000 cm (I) [26].
Only a few works can be found for comparison where a series of complexes with different
halogenide ligands have been studied by EPR spectroscopy. One if not the only work is EPR study
1
2–
of presumably octahedral molybdenum (V) complexes (d ) [MoOX ] (X = Cl, Br) and
5
–
[
MoOI (H O)] , which were obtained in vitro in concentrated HX (X = Cl, Br, I) [27]. The EPR
4
2
spectra of these frozen solutions were measured and the g revealed to increase from chlorine
||
(
1.948) to iodine (2.274) and, starting from the bromide complex (2.093), g was significantly
||
higher than 2.0023. At that, the g value does not change being equal to 1.9375 for chlorine, 1.945
┴
for bromine, and 1.956 for iodine. The g- and A-tensors were calculated within the theory of
molecular orbitals for С symmetry, the spin-orbit coupling both of the ligands and the central ion
4
V
was taken into account [28]. However, in this consideration, the optical absorption data and several
empirical fitting parameters were used to match the calculated and experimental data. This made the
calculations too bulky but in general evaluative. Following the data of Marov [28], the calculated g-
tensor values were in good agreement with the experimental ones.
The molecular orbital approach combined with complete diagonalization (of energy matrix)
method and the perturbation theory method were used to simulate the EPR spectra of niobium(IV)
in a tetrahedrally distorted oxygen octahedron in glasses [29]. In this case, the spin-orbit interaction
due to the niobium and oxygen atoms was taken into account with the covalence effect and
polarization varied. Additionally, the optical transitions data were used. As a result of such
calculations, a good agreement was reached between the calculated (g = 1.88, g = 1.928; A = -
||
┴
||
2
6.8 mT, A = -11.4 mT) and experimental (g = 1.88, g = 1.92; А = 26.2 mT, A = 12.1 mT)
┴ || ┴ || ┴
EPR parameters. The polarization of the niobium electron shell gave the negative values of the
calculated principal values of the A tensor. We should note here that it is hard to determine the sign
of the principal values of A tensor from the EPR experiments.
Niobium complex [NbCl (PEt ) ] was studied by EPR spectroscopy [30]. The EPR
4
3 2
parameters for [NbCl (PEt ) ] (g || = 1.959, g = 1.912; A = 21.85 mT, A = 10.3 mT) differ from
4
3 2
┴
||
┴
that obtained in this work due to difference in donor properties of oxygen and phosphorus, but the
g-tensor (g > g ) and A-tensor (A > A ) are characteristic for octahedral niobium complexes. As
||
┴
||
┴
far as the authors know, the quantum chemical calculations of the EPR parameters of niobium
complexes were not reported previously. Below, the quantum chemical calculations are given for
the series [NbX (OPPh )] (X = Cl, Br, I) performed with a hybrid functional B3LYP [13].
4
3
DFT calculations were performed for compounds 1, 2, 3b. The simulated g- and A-tensor
values are given in Table 4. These are of particular interest considering the specific EPR parameters
discussed above. The g- and A-tensor principal axes are disoriented at an angle of approximately 10

degrees. The calculated EPR parameters are reasonably close to the experimental ones considering
the accuracy of the DFT approach [16,17,31,32], the A-values being negative. Previously, the
4
+
8−
negative HFS constants were obtained theoretically for Nb in tetragonally compressed NbO
6
anion in glasses [29]. The negative isotropic HFS constant was reported for six-coordinate
vanadium(IV) tris(maleonitriledithiolene) anion and it was proposed that negative contribution to
the spin density at the nucleus is produced by spin polarization of inner s levels by the unpaired
electron in a 3d level [33,34].
We performed calculations at the same level of theory but taking the TZ2P-J basis set with
additional polarization and five extra tight (mainly 1s) functions for niobium atom in an attempt to
obtain better agreement between the experimental and calculated g- and A-tensor values. Although
the A-tensor values remained almost the same, the g-tensor values obtained in this way had
noticeably better match with the experimental values.
3
.5. Measurements of magnetic susceptibility
Magnetic susceptibility was measured in the temperature range of 80-300 K for the complexes
1
, 2, 3b (Fig. 3). For all examined complexes, the temperature dependence of magnetic
susceptibility could be fitted using the Curie-Weiss law [35]:
C
T− θ
χ (T)=
,
where T is temperature, C is the Curie constant and θ is the Weiss constant. The negative sign of the
Weiss constant indicates a weak antiferromagnetic interaction in the samples at low temperatures.
The
effective
magnetic
moment
value
increases
in
a
series
[
NbCl (OPPh ) ]<[NbBr (OPPh ) ]<[NbI (OPPh ) ]. The effective magnetic moments are close to
4
3 2
4
3 2
4
3 2
that of octahedral niobium(IV) complexes NbX (CH CN) with μ =1.81-1.79 μ (X=Cl), μ=1.59-
4
3
2
B
1
.40 μ (X=Br), and μ=1.45-1.24 μ (X=I) [36,37].
B
B
a)
Figure 3. Magnetic susceptibility dependences on temperature for the complexes 1 (a),
(b), and 3b (c).
b)
c)
2

Table 5. Parameters of magnetic susceptibility for the complexes 1, 2, and 3b.
3
−1
C, cm ·K·mol
μ , μ
θ, K
-2(2)
-7(3)
-9(5)
eff
B
[
NbCl (OPPh ) ]
0.25(1)
0.31(1)
0.35(1)
1.40(1)
1.57(2)
1.68(3)
4
3 2
[
NbBr (OPPh ) ]
4 3 2
[
NbI (OPPh ) ]
4
3 2
4
. Conclusions
In this work, we developed a convenient synthesis method of new niobium(IV) complex
trans-[NbI (OPPh ) ] using ampoule synthesis approach; we studied magnetic properties of the
4
3 2
series of complexes trans-[NbХ (OPPh ) ] (Х=Cl, Br, I). Experimental data and quantum-chemical
4
3 2
calculations have shown that an increase of the spin-orbit coupling constant of the halogen from
chlorine to iodine leads to an increase of the g-tensor values higher than g = 2.0023. Quantum
chemical calculations made it possible to estimate the sign of the hyperfine tensor values for the
niobium ion. The use of the correct values of g-factors in calculating the magnetic susceptibility
showed the practical absence of exchange interactions in the structures of the studied niobium(IV)
compounds.
Declaration of competing interest. The authors declare that they have no known competing
financial interests or personal relationships that could have appeared to influence the work reported
in this paper.
Acknowledgement. This work was supported by the Ministry of Science and Higher Education of
the Russian Federation.
Appendix A. Supplementary data. CCDC 2013822 and 2012709 contains the supplementary
crystallographic data for compound 3a and 3b, respectively. These data can be obtained free of
charge
CambridgeCrystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
223336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data to this article can be found
via
http://www.ccdc.cam.ac.uk/conts/retrieving/html,
or
from
the
1
online at
Appendix B. Supplementary material. Supplementary material contains X-ray powder
diffraction patterns and FTIR-spectra of compounds mentioned in the text.
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