Synthesis, crystal, molecular and electronic structure of rhenium nitrosyl with pyrazole in the coordination sphere

Article abstract of DOI:10.1016/j.inoche.2005.06.019

The reaction of [ReBr₃(NO)(PPh₃)₂] complex with pyrazole (pzH) has been examined and a new rhenium nitrosyl - [ReBr ₃(NO)(PPh₃)(pzH)] - has been obtained. The X-ray structure of the [ReBr₃(NO)(PPh₃)(pzH)] complex has been determined and its electronic structure has been calculated with the density functional theory (DFT) method with the use of B3LYP functional. Additional information about binding in the [ReBr₃(NO)(PPh₃)(pzH)] complex has been obtained by NBO analysis. The time-dependent DFT method (TDDFT) has been used for calculations of spin-allowed doublet-doublet electronic transitions of [ReBr₃(NO)(PPh₃)(pzH)], and on this basis the UV-Vis spectrum has been discussed.

Full text of DOI:10.1016/j.inoche.2005.06.019

Inorganic Chemistry Communications 8 (2005) 960–965
www.elsevier.com/locate/inoche
Synthesis, crystal, molecular and electronic structure of
rhenium nitrosyl with pyrazole in the coordination sphere
a,
b
c
B. Machura *, R. Kruszynski , M. Jaworska
a
Department of Inorganic and Radiation Chemistry, Institute of Chemistry, University of Silesia, 9th Szkolna Street, 40-006 Katowice, Poland
b
X-ray Crystallography Laboratory, Institute of General and Ecological Chemistry, Technical University of Ło´dz´,
_
116 Zeromski Street, 90-924 Ło´dz´, Poland
Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, 9th Szkolna Street, 40-006 Katowice, Poland
c
Received 14 April 2005; accepted 25 June 2005
Available online 31 August 2005
Abstract
The reaction of [ReBr3(NO)(PPh3)2] complex with pyrazole (pzH) has been examined and a new rhenium nitrosyl –
[ReBr3(NO)(PPh3)(pzH)] – has been obtained. The X-ray structure of the [ReBr3(NO)(PPh3)(pzH)] complex has been determined
and its electronic structure has been calculated with the density functional theory (DFT) method with the use of B3LYP functional.
Additional information about binding in the [ReBr3(NO)(PPh3)(pzH)] complex has been obtained by NBO analysis. The time-
dependent DFT method (TDDFT) has been used for calculations of spin-allowed doublet–doublet electronic transitions of
[ReBr3(NO)(PPh3)(pzH)], and on this basis the UV–Vis spectrum has been discussed.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Rhenium nitrosyl complexes; Pyrazole; Phosphine ligands; X-ray structure; DFT calculations
For many years, nitrosyl transition metal complexes
have been the centre of interest to those scientists
engaged in basic research and to those trying to employ
these compounds in catalysis, production of organoni-
trogen compounds and pollutant control (reduction of
NO in exhaust fumes). The attention of scientists
concentrates on synthetic aspects, structural and spec-
troscopic properties of transition metal nitrosyl com-
plexes as well as the reactivity of the coordinated NO
group. The recent discovery of a key role of nitric oxide
in human cardiovascular, nervous systems and in
immune response to pathogen invasion has resulted in
an added increase in the interest of nitrosyl transition
metal complexes [1–14]. Due to the favourable nuclear
properties of 186Re and 188Re nuclides in diagnostic nu-
clear medicine and radioimmunotherapy, the chemistry
of nitrosyl rhenium complexes arouses particular inter-
est [15,16].
The [ReOX3L2] compounds (X = Cl and Br, L =
PPh3, AsPh3, 1/2 dppe) easily reacts with gaseous nitric
oxide to give a variety of products depending on the
reaction conditions: [ReX3(NO)(OPPh3)2], [ReX3(NO)
(PPh3)2], [ReX2(NO)2(PPh3)2], [ReBr3(NO)(MeCN)
(PPh3)], [ReCl3(NO)(PPh3)(OPPh3)], [ReBr3(NO)(dppe)]
[ReBr4(dppe)], [ReX3(NO)(OAsPh3)2], [ReBr3(NO)
(AsPh3)2] and [ReCl3(NO)(AsPh3)2] [ReCl4(AsPh3)2]
[17–25].
Here, we present the synthesis, spectroscopic charac-
terization, crystal, molecular and electronic structure of
a new rhenium nitrosyl with pyrazole (pzH) in a coordina-
tion sphere – [ReBr3(NO)(PPh3)(pzH)]. The electronic
*
Corresponding author. Tel.: +48322582441; fax: +48322599978.
E-mail addresses: bmachura@poczta.onet.pl(B. Machura), kruszyna
@p.lodz.pl (R. Kruszynski), mj@tc3.ich.us.edu.pl (M. Jaworska).
1387-7003/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.06.019

B. Machura et al. / Inorganic Chemistry Communications 8 (2005) 960–965
961
Table 1
structure of [ReBr3(NO)(PPh3)(pzH)] has been deter-
Crystal data and structure refinement for [ReBr3(NO)(PPh3)(pzH)]
mined based on the results ofthe densityfunctional theory
(DFT) calculations, and additional information about
binding has been obtained by NBO analysis. The spectro-
scopic characterization of [ReBr3(NO)(PPh3)(pzH)] com-
prises IR and UV–Vis studies. The electronic absorption
bands of [ReBr3 (NO)(PPh3)(pzH)] may result from d–
d, metal–ligand charge transfer, ligand–metal charge
transfer and inter- or intra-ligand transitions. In order
to make detailed assignments of the electronic transitions
to the experimental absorption bands, we have calculated
the spin-allowed doublet–doublet electronic transitions
of [ReBr3(NO)(PPh3)(pzH)] with the time-dependent
DFT method.
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C21H19Br3N3OPRe
786.29
292(2) K
˚
0.71073 A
Monoclinic
P21/n
˚
Unit cell dimensions
a = 11.6905(9) A
˚
b = 17.8638(14) A,
b = 103.832(8)ꢁ
˚
c = 11.7262(13) A
3
˚
Volume
2377.8(4) A
Z
4
Density (calculated)
Absorption coefficient
F(0 0 0)
2.196 Mg/m3
10.237 mmꢀ1
1476
All the reagents used in the synthesis of the complex
are commercially available and were used without fur-
ther purification. The [ReBr3(NO)(PPh3)2] complex
was prepared according to the literature method [24].
[ReBr3(NO)(PPh3)2] (1 g, 1 mmol) and pzH (0.27 g,
4 mmol) in chloroform (100 cm3) were refluxed for 5 h
under argon atmosphere. The volume was condensed
to 20 cm3 and dark red microcrystalline solid was
formed by an addition of 100 cm3 of diethyl ether. The
product was washed with EtOH and cold ether, and
dried in vacuo. The crystals suitable for X-ray investiga-
tion were obtained by recrystallization from CHCl3–
EtOH with yield equal to 70%.
IR spectrum was recorded on a Nicolet Magna 560
spectrophotometer in the spectral range 4000–
400 cmꢀ1 with the samples in the form of KBr pellets.
Electronic spectrum was measured on a spectrophotom-
eter Lab Alliance UV–Vis 8500 in the range 900–180 nm
in deoxygenated dichloromethane solution. Magnetic
susceptibilities were measured at 296 K by the Faraday
method. Elemental analyses (C H N) were performed
on a Perkin–Elmer CHN-2400 analyzer.
Crystal size
h Range for data collection
Index ranges
0.28 · 0.17 · 0.04 mm
3.18–25.12ꢁ
ꢀ13 6 h 6 13
ꢀ21 6 k 6 21
ꢀ13 6 l 6 13
24315
4217
96.2%
0.3254 and 0.0722
4217/0/271
1.171
Reflections collected
Independent reflections
Completeness to 2h
Maximum and minimum transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R1 = 0.0687
wR2 = 0.1569
R1 = 0.0728
wR2 = 0.1596
R indices (all data)
3
˚
Largest difference peak and hole
1.062 and –1.287 e/A
the calculations. Atomic scattering factors were those
incorporated in the computer programs.
Gaussian03 program [30] was used in the calculations.
The geometry optimization was carried out with the DFT
method with the use of B3LYP functional [31,32]. The
electronic spectrum was calculated with the TDDFT
method [33]. The N–Re–N linkage defines the z molecular
axis and the x molecular axis goes along the Br(1)–Re–
Br(2) moiety. LANL2DZ basis set [34] with additional
d (with exponent a = 0.3811) and f (with exponent a =
2.033) functions was used on the rhenium atom,
6-31G(d) on the bromine, nitrogen, oxygen, phosphorous
and carbon atoms, and 6-31G basis on the hydrogen
atoms in the calculations. Natural bond orbital (NBO)
calculations were performed with the NBO code [35]
included in Gaussian03.
The complex [ReBr3(NO)(PPh3)(pzH)] crystallizes in
the monoclinic space group P21/n and its structure con-
sists of discrete and well-separated monomers. A
perspective drawing of the [ReBr3(NO)(PPh3)(pzH)]
molecule is shown in Fig. 1. The rhenium atom is in a
distorted octahedral environment with the bromine
ligands in meridional geometry and the linear N–O
group is trans to the pyrazole molecule and cis to the
PPh3 ligand. The nitrosyl group of the starting complex
occupies the trans position to one of the bromine
The X-ray intensity data of [ReBr3(NO)(PPh3)
(pzH)] collected on a KM-4-CCD automatic diffrac-
tometer equipped with CCD detector with 34 s expo-
sure time were used and all reflections inside the
Ewald sphere up to 2h = 50ꢁ were collected. The unit
cell parameters were determined from least-squares
refinement of the setting angles of 6772 strongest
reflections. Details concerning crystal data and refine-
ment are given in Table 1. Lorentz, polarization and
numerical absorption corrections [26] were applied.
The structure was solved by the Patterson method
and subsequently completed by the difference Fourier
recycling. All the non-hydrogen atoms were refined
anisotropically using full-matrix least-squares tech-
nique. The hydrogen atoms were treated as ‘‘riding’’
˚
on their parent carbon atoms [d(C–H)=0.96 A] and
assigned isotropic temperature factors equal to 1.2
times the value of equivalent temperature factor of
the parent carbon atom. SHELXS97 [27], SHELXL97
[28] and SHELXTL [29] programs were used for all

962
B. Machura et al. / Inorganic Chemistry Communications 8 (2005) 960–965
Table 2
˚
The experimental and optimized bond lengths [A] and angles [ꢁ] for
[ReBr3(NO)(PPh3)(pzH)]
Experimental
Optimized
Bond lengths
Re(1)–N(1)
Re(1)–N(2)
Re(1)–Br(1)
Re(1)–Br(2)
Re(1)–Br(3)
Re(1)–P(1)
N(1)–O(1)
1.751(11)
2.199(10)
2.5276(13)
2.5074(12)
2.5461(14)
2.508(3)
1.759
2.238
2.578
2.635
2.577
2.596
1.174
1.180(14)
Bond angles
N(1)–Re(1)–N(2)
N(1)–Re(1)–Br(1)
N(2)–Re(1)–Br(1)
N(1)–Re(1)–Br(2)
N(2)–Re(1)–Br(2)
Br(1)–Re(1)–Br(3)
N(1)–Re(1)–Br(3)
N(2)–Re(1)–Br(3)
Br(1)–Re(1)–Br(2)
Br(3)–Re(1)–Br(2)
N(1)–Re(1)–P(1)
N(2)–Re(1)–P(1)
Br(1)–Re(1)–P(1)
Br(2)–Re(1)–P(1)
Br(3)–Re(1)–P(1)
O(1)–N(1)–Re(1)
177.6(4)
94.4(3)
86.5(3)
94.2(3)
85.0(3)
91.04(5)
92.8(4)
85.0(3)
170.51(5)
92.38(5)
92.1(4)
176.2
92.9
86.0
95.3
86.2
94.3
92.9
83.6
169.6
91.8
92.7
90.9
88.6
84.6
173.6
177.8
90.1(3)
87.90(7)
87.93(7)
175.08(7)
177.9(10)
Fig. 1. Thermal ellipsoid plot of [ReBr3(NO)(PPh3)(pzH)] showing the
atom numbering scheme (50% probability ellipsoids).
ligands. Two intramolecular hydrogen bonds [36,37]
appears at significantly lower wavenumber than that
of free NO (1870 cmꢀ1), which indicates a strong back
donation of electron density from the metal centre to
the pꢂNO orbitals. The lowering of the mNO of [ReBr3
(NO)(PPh3)(pzH)] in comparison with the starting
[ReBr3(NO)(PPh3)2] nitrosyl (1776 cmꢀ1) [24] results
from changing of the ligand in trans position to the
NO group – pzH in [ReBr3(NO)(PPh3)(pzH)] and Br
in [ReBr3(NO)(PPh3)2]. The band (3263 cmꢀ1) in the
range assignable to N–H vibrations confirms the coordi-
nation of the pyrazole in a neutral monodentate form.
The geometry of [ReBr3(NO)(PPh3)(pzH)] was opti-
mized in a doublet state by the DFT method with the
B3LYP functional. The doublet state agrees well with
the experimental value of the effective magnetic moment
(1.8 BM) for [ReBr3(NO)(PPh3)(pzH)]. The optimized
geometry parameters for the complex are given in Table
2, and the calculated bond lengths agree with the exper-
imental values.
The calculated charge (from the Natural Population
Analysis) on the nitrogen atom of the nitrosyl group is
positive (0.231), whereas the oxygen atom is negatively
charged (ꢀ0.241). The total charge on the NO group
is close to zero (ꢀ0.01) and it is far from the expected
value +1. The similar charge distribution on the nitrosyl
ligand is observed in other transition metal complexes
with a linear NO+ group [40]. The calculated charge
on the rhenium atom of [ReBr3(NO)(PPh3)(pzH)]
(0.348) is also considerably smaller than the formal
linking N(3)–H(3A)ꢁ ꢁ ꢁBr(2) (Dꢁ ꢁ ꢁA distance 3.206(11)
˚
A
and D–Hꢁ ꢁ ꢁA angle 116.5ꢁ) and C(21)–
˚
H(21)ꢁ ꢁ ꢁBr(1) (Dꢁ ꢁ ꢁA distance 3.292(14) A and D–
Hꢁ ꢁ ꢁA angle 114.2ꢁ) provide additional stabilization of
the [ReBr3 (NO)(PPh3)(pzH)] complex. Except those
mentioned in the structure, any short contacts that can
be classified as intra- or intermolecular hydrogen bonds
cannot be found. Table 1 presents crystal data and struc-
tural refinement for [ReBr3(NO)(PPh3)(pzH)]. The most
important experimental bond lengths and angles for
[ReBr3(NO)(PPh3)(pzH)] are reported in Table 2. The
Re–NO and N–O bond lengths are well compared with
the values found for the other {ReNO}5 complexes [17–
25]. The linearly bonded nitrosyl ligand is a poor r-do-
nor and a very strong p-acceptor. It shows Ôstructural
trans-effectÕ (STE) with respect to p-acceptor ligands
[38]. Consequently, an elongation of the Re–NpzH bond
length [2.199(10)] in comparison with [ReOBr3(pz-
˚
H)(OAsPh3)] – 2.089(18) A [39] is observed. A clear
trans effect of PPh3 ligand is observed in the elongation
˚
of Re–Br(3) [2.5461(14) A], the mutually trans Re–Br
˚
bonds are equal to 2.5276(13) and 2.5074(14) A, respec-
tively. However, some shortening of the mutually trans
Re–Br distances in comparison with the Re–Br(3) bond
can also result from the contribution of the Br(1) and
Br(2) ligands in the hydrogen bond formation.
The IR spectrum of [ReBr3(NO)(PPh3)(pzH)] exhib-
its strong band at 1756 cmꢀ1 assignable to m(NO). It

B. Machura et al. / Inorganic Chemistry Communications 8 (2005) 960–965
963
charge +2. It results from charge donation from the bro-
mine ions and triphenylphosphine ligand. There is large
positive charge on the P atom (1.373) and the charges on
the bromine ions are significantly smaller than ꢀ1
(ꢃꢀ0.31 for Br(1) and Br(3) and –0.367 for Br(2)).
The charge on the pyrazole nitrogen atom bonded with
the rhenium is negative (ꢀ0.362).
at N), and the s and p nitrogen orbitals and s and d rhe-
nium orbitals take part in the Re–N bond formation.
The Re–N bond orbitals b spins are polarized towards
the metal atom and only d-orbitals of Re are included
into the bonding. The atomic orbital compositions of
the detected Re(1)–N(1) natural bond orbitals with b
spins indicate their p character. Undetected rRe–NO
bond has a character of predominant Coulomb-type
interactions between the central ion and NO ligand
[42]. It confirms the presence of the lone pair orbital
with b spin on the N(1) atom and strong delocalization
of the electron density from the LPN(1) onto non-Lewis
rhenium orbitals. The stabilization energy DEij (esti-
mated by the second-order perturbative) associated with
delocalization of the electron density from the LPN(1)
onto non-Lewis rhenium orbital is equal to 115.64
kcal/mol.
The electronic configuration of the metal centre in the
investigated complex is as follows: (dxz)2(dyz)2(dxy)1. The
dxy rhenium orbital makes main contributions into the
HOMO with a-spin and LUMO with b-spin. In both
the molecular orbitals, the dxy rhenium orbitals bear
antibonding character towards the p orbitals of the bro-
mine ligands. The occupied dyz and dxz rhenium orbitals
participate in the back-donation from the central ion to
the NO ligand. The dyz/dxz and pꢂNO orbitals are distrib-
uted among several occupied MOs to contribute several
HOMO (H-16, H-15, H-3, H-2 and H-1 with a spin and
H-14, H-13, H-12, H-2 and H-1 with b spin) and LUMO
orbitals (L + 1, L + 2 with a and b spin). The bonding
pRe–NO orbitals are localized mainly on the rhenium d
orbitals, whereas the pꢂRe–NO orbitals have prevalent
NO character. It confirms the nitrosonium (NO+) char-
acter of the NO group. The HOMO with b spin is
mainly localized on the bromine ligands. The LUMO
with a spin and LUMO + 3 with b spin are of
The experimental and calculated electronic spectra of
[ReBr3(NO)(PPh3)(pzH)] are presented in Fig. 2. Each
calculated transition was represented by a gaussian
function with the height equal to the oscillator strength
and width equal to 0.04.
The investigated complex is of large size, the num-
ber of basis functions is equal to 524. Ninety electron
transitions calculated by the TDDFT method do not
comprise all the experimental absorption bands. The
UV–Vis spectrum of [ReBr3(NO)(PPh3)(pzH)] was cal-
culated only to 260 nm, so the shortest wavelength
experimental band cannot be assigned completely to
the calculated transitions. Considering that the solu-
tion spectra of PPh3 and pyrazole exhibit intense
absorption bands in 250–200 nm region, some addi-
tional intraligand and interligand transitions are ex-
pected to be found at higher energies in the
calculation for [ReBr3(NO)(PPh3)(pzH)].
dx2ꢀy2 –rBr–rP character. The d2 makes the largest contri-
z
bution into the L + 3, L + 4 and L + 10 with a spin and
L + 5, L + 11 with b spin.
Table 3 presents the occupancies and hybridization of
the calculated natural bond orbitals (NBOs) in the
{ReNO} unit. As in open-shell systems the density oper-
ator separates into distinct components for a and b spin,
some differences between NBOs with a and b spins [41]
can be noticed. One Re(1)–N(1) and three N(1)–O(1)
natural bond orbitals with a spins and two Re(1)–N(1)
and one N(1)–O(1) natural bond orbitals with b spin
were detected. The Re(1)–N(1) bond orbital with a spin
is strongly polarized towards the nitrogen atom (ca. 87%
The shoulder at 785.0 nm in the experimental spec-
trum of [ReBr3(NO)(PPh3)(pzH)] may be attributed to
the transitions of p(Br)/dxz ! d character with small
oscillator strength calculated at 723.0 nm.
Table 3
The occupancies and hybridization of the calculated natural bond orbitals (NBOs) in the {ReNO}5 for [ReBr3(NO)(PPh3)(pzH)]
a-spin
b-spin
Occupancy
Hybridization of NBO
Occupancy
Hybridization of NBO
BD
Re(1)–N(1)
0.985
0.484(sd2.73
)
Re + 0.875(sp0.57
)
O
0.991
0.987
0.729(d)Re + 0.685(p)N
0.738(d)Re + 0.675(p)N
N(1)–O(1)
0.997
0.999
0.999
0.671(sd1.83
0.579(p)N + 0.816(p)O
0.580(p)N + 0.815(p)O
)
N + 0.742(sp2.23
)
0.997
0.671(sd1.86 N + 0.742(sp2.24
) )
O
O
LP
N(1)
O(1)
0.803
0.989
0.813
0.806
sp0.54
sp0.44
p
0.989
sp0.45
p
BD denotes 2-center bond; LP, lone pair.

964
B. Machura et al. / Inorganic Chemistry Communications 8 (2005) 960–965
Acknowledgement
The Gaussian03 calculations were carried out in the
Wrocław Centre for Networking and Supercomputing,
WCSS, Wrocław, Poland, under calculational Grant
No. 51/96.
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The experimental band at 676.0 nm can be assigned
to the calculated transition at 636.1 nm. The transition
is of LMCT (pBr ! d) character.
The broad experimental band at 482.5 nm is assigned
to the transitions calculated between 536 and 428 nm.
Among these electronic transitions are transitions of
d ! d character (474.9 and 465.2 nm), transition of
p(Br)n(P) ! d origin at 535.8 nm and some transitions
of p(Br)/dxz ! p*(NO) or p(Br)/dyz ! p*(NO) charac-
ter. The oscillator strengths of the last ones are very
small (below 0.005).
The experimental absorption bands at 390 and
331.5 nm correspond mainly to the transitions of LMCT
(pBr ! d, ppzH ! d and pPh ! d) character. The calcu-
lated transitions of dxy ! p*(NO) and p(Br)/dxz
!
p*(NO) or p(Br)/dyz ! p*(NO) origin in the range
415–300 nm have small oscillator strengths and they
do not contribute significantly to the overall shape of
these bands.
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The shortest wavelength experimental bands of [Re-
Br3(NO)(PPh3)(pzH)] (at 229.0 nm) corresponding to
the electronic transitions are of LMCT and LLCT char-
acter. The last ones occurꢂfrom the bromine, phenyl and
pyrazole orbitals to the pNO or pꢂPh orbitals.
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