Synthesis and structural characterization of zwitterionic oxorhenium(V) complexes with 2-hydroxypyridine

Article abstract of DOI:10.1016/j.ica.2005.11.029

[ReOX₃(PPh₃)₂] complexes (X = Cl and Br) react with equivalent amounts of 2-hydroxypyridine (Hhp) under formation of the mono-substituted, zwitterionic complexes mer-[ReOCl₃(Hhp)(PPh ₃)] (1) and mer-[ReOBr₃(Hhp)(PPh₃)] (2). Crystal structure determinations of 1·CH₂Cl₂ and 2 revealed the Cl and Br ligands adopt a mer arrangement. The Hhp ligands coordinate neutral and monodentate via their exocyclic oxygen atoms in axial positions, trans to the oxo groups. The distorted octahedral coordination sphere of the rhenium(V) complexes is completed by the phosphorus atom of the remaining PPh₃ ligand.

Full text of DOI:10.1016/j.ica.2005.11.029

Inorganica Chimica Acta 359 (2006) 1513–1518
www.elsevier.com/locate/ica
Synthesis and structural characterization of zwitterionic
oxorhenium(V) complexes with 2-hydroxypyridine
a
a,
a
a
*
´
Elizeu J. de Souza , Victor M. Deflon , Andre G. de A. Fernandes , Sebastiao S. Lemos ,
˜
b
b
Adelheid Hagenbach , Ulrich Abram
a
Instituto de Quı´mica, Universidade de Brası´lia, 70919-970 Brası´lia DF, Brazil
b
Institut fu¨r Chemie, Freie Universita¨t Berlin, D-14195 Berlin, Germany
Received 22 September 2005; received in revised form 10 November 2005; accepted 23 November 2005
Available online 5 January 2006
Abstract
[ReOX3(PPh3)2] complexes (X = Cl and Br) react with equivalent amounts of 2-hydroxypyridine (Hhp) under formation of the mono-
substituted, zwitterionic complexes mer-[ReOCl3(Hhp)(PPh3)] (1) and mer-[ReOBr3(Hhp)(PPh3)] (2). Crystal structure determinations of
1 Æ CH2Cl2 and 2 revealed the Cl and Br ligands adopt a mer arrangement. The Hhp ligands coordinate neutral and monodentate via their
exocyclic oxygen atoms in axial positions, trans to the oxo groups. The distorted octahedral coordination sphere of the rhenium(V) com-
plexes is completed by the phosphorus atom of the remaining PPh3 ligand.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Rhenium zwitterionic complexes; 2-Hydroxypyridine; X-ray crystal structures
1. Introduction
and related ligands has been found in some other rhenium
complexes involving the oxidation states of the metal from
‘‘+2’’ to ‘‘+4’’ [1–9]. Complexes with the composition
Re2(hp)2X4 Æ Hhp Æ solvent (X = Cl, Br) are supposed to
be intermediates in some reactions during the preparation
of [Re2(hp)2X4]. The isolation of a complex with the stoi-
chiometry Re2(hp)2Cl4 Æ Hhp Æ THF is described [10].
This work describes the preparation and structural char-
acterization of new oxorhenium(V) complexes containing
Hhp ligands in a zwitterionic form. The complexes mer-
[ReOCl3(Hhp)(PPh3)] (1) and mer-[ReOBr3(Hhp)(PPh3)]
(2) contain monodentate Hhp ligands in their protonated
neutral form, which is novel for rhenium coordination
compounds with this type of ligand.
2-Hydroxypyridine (Hhp) has interesting ligand proper-
ties, being able to act as a mono-, bi- or tri-nucleating
agent, with different possible coordination modes [1]. The
known examples of rhenium coordination compounds with
6þ
Hhp involve mainly a series of Re2 compounds and
mononuclear complexes prepared from the polyhydride
complex [ReH7(PPh3)2]. In these compounds, 2-hydroxy-
pyridine is singly deprotonated and coordinates N,O-
bidentate as 2-pyridonate, hpꢀ, bridging two transition
element centers, or forms a four-membered chelate ring
bonded to only one metal ion [1]. More recently, the com-
plex [Re2(g2-hp)Cl3(l-dppm)2] was prepared from
[Re2Cl4(l-dppm)2] (dppm = Ph2PCH2PPh2). Its X-ray
crystal structure was determined and confirmed the 2-
pyridonate ligand to act as a chelating agent, being coordi-
O
X
X
Ph3P
nated to only one ReII center in a Re2 core [2]. The same
N,O-chelating coordination mode of 2-hydroxypyridine
4þ
Re
X = Cl
X = Br
1
2
X
O
H
N
*
Corresponding author. Tel.: +5561 3307 2181; fax: +5561 3273 4149.
E-mail address: deflon@unb.br (V.M. Deflon).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.11.029

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E.J. de Souza et al. / Inorganica Chimica Acta 359 (2006) 1513–1518
2. Experimental
2.1. Materials and methods
IR (cmꢀ1, KBr pellet): 3231(m-s), 3158(m), 3091(m-w),
3051(w), 1637(m-s), 1629(m-s), 1587(vs), 1538(vs),
1471(m), 1456(m), 1431(m), 1387(vs), 1274(m), 1213(m),
1160(m-w), 1093(ms), 995(vs, Re@O), 888(s), 867(m-w),
771(s), 760(m-s), 747(m-s), 707(m-s), 693(m-s), 612(s),
526(m), 508(w-m), 494(w-m), 450(m), 279(w).
The starting complexes [ReOX3(PPh3)2] (X = Cl, Br)
were prepared as previously described [11]. All other
reagents were of analytical grade and were used as
purchased, without further purification. NMR spectra were
recorded on a Varian Mercury Plus spectrometer, 7.05 T,
1H NMR (ppm, CD2Cl2) 5.216 (d, J = 9 Hz, 1H, Hhp),
6.763 (dd, J = 6.8 Hz, 1H, Hhp), 7.350–7.910 (m, 17H,
Hhp and Ph of PPh3), 11,470 (br, 1H, NH).
operating at 300.07 MHz for H and 121.47 MHz for 31P.
31P{1H} NMR (ppm, CD2Cl2): ꢀ15.65 (s, Re–PPh3).
1
They were run at room temperature in solutions of
degassed CD2Cl2 by purging it with argon for about
15 min. The 1H spectra were internally referenced to
TMS and 31P{1H} spectra were externally referenced to
H3PO4 (85%, d 0) with positive chemical shifts downfield
the standard. The IR spectra (KBr pellets, 4000–
200 cmꢀ1) were obtained on a Bomem Michelson FT
model MB 102 spectrometer. A Carlo Erba 1104 elemental
analyzer was used for the microanalyses.
2.4. X-ray structure determinations
Single crystals of the complexes suitable for X-ray dif-
fraction were obtained by slow evaporation of CH2Cl2
solutions.
The data collections were performed with Mo Ka radi-
ation (k = 71.073 pm) on a BRUKER SMART CCD for
1 Æ CH2Cl2 and on an Enraf-Nonius CAD-4 diffractometer
for 2 applying standard procedures.
The structures were solved by the heavy atom method
with SHELXS-97 [12] and refined with SHELXL-97 [12]. Hydro-
gen atom positions were calculated at idealized positions
using the riding model option of SHELXL-97 [12]. Additional
crystal data and more information about the X-ray struc-
tural analyses are shown in Table 1.
2.2. Preparation of [ReOCl3 (Hhp)(PPh3)] (1)
About 0.024 g (0.25 mmol) 2-hydroxypyridine (Hhp)
was added to a suspension of 0.208 g (0.25 mmol) [ReO-
Cl3(PPh3)2] in 15 mL of degassed dichloromethane. The
mixture was refluxed for 3 h and after cooling to room tem-
perature, 10 mL of hexane was added to the resulting green
solution. A green precipitate was formed after storing at
ꢀ15 ꢁC for 24 h. The product was filtered off, washed with
cold dichloromethane and hexane and dried in vacuum.
Yield: 80% (0.134 g). Anal. Calc. for C23H20Cl3NO2-
PRe: C, 41.50; H, 3.03; N, 2.11. Found: C, 41.22; H,
3.06; N, 1.96%.
3. Results and discussion
3.1. Synthesis of the complexes
The complexes 1 and 2 were prepared in satisfactory
yields by ligand exchange reactions starting from
[ReOX3(PPh3)2] complexes (X = Cl in 1 and Br in 2) and
2-hydroxypyridine in dichloromethane (Eq. (1)). The
chloro derivative is rapidly formed, while the synthesis of
2 requires prolonged heating on reflux.
IR (cmꢀ1, KBr pellet): 3253(m), 3052(m-w), 1634(s),
1588(vs), 1538(s), 1481(m), 1433(m), 1388(s), 1262(m-w),
1213(w), 1160(w), 1093(m), 995(s, Re@O), 886(m), 772(m-
s), 759(w), 751(m), 707(w-m), 693(m), 626(w), 611(w-m),
591(w-m), 527(m), 508(w-m), 495(w), 474(w), 452(w),
333(vw), 307(w), 280(vw).
½ReOX3ðPPh3Þ ꢁ þ Hhp ! ½ReOX3ðHhpÞðPPh3Þꢁ þ PPh3
ðX ¼ Cl; BrÞ2
ð1Þ
1H NMR (ppm, CD2Cl2) 5.350 (Hhp, overlapped with
the triplet of residual CDHCl2), 6.755 (dd, J = 6.7 Hz,
1H, Hhp), 7.030–8.025 (m, 17H, Hhp and Ph of PPh3),
11.712 (br, 1H, NH).
Both complexes precipitate as green solids upon addi-
tion of n-hexane to the reaction solutions and by keeping
them at a low temperature. The solids are nearly insoluble
in MeOH, toluene and hydrocarbons. They are sparingly
soluble in CHCl3 and moderately soluble in CH2Cl2 or
CH3CN. The compounds are air-stable as solids. In solu-
tion, however, oxidation of the PPh3 ligand to OPPh3 takes
place which can be seen in the 31P NMR spectra as small
signals at 30.9 ppm (free OPPh3) and 49.8 ppm (coordi-
nated OPPh3) [13]. This process could be efficiently
eliminated if degassed solvents were employed in 31P
NMR measurements. After becoming conscious of that,
all preparations were conducted on previously degassed
solvents in order to avoid the formation of side products.
The new complexes were characterized by IR and NMR
spectroscopy, elemental analysis and X-ray structure deter-
31P{1H} NMR (ppm, CD2Cl2): ꢀ18.35 (s, Re–PPh3).
2.3. Preparation of [ReOBr3 (Hhp)(PPh3)] (2)
A mixture of 0.242 g (0.25 mmol) [ReOBr3(PPh3)2] and
0.024 g (0.25 mmol) Hhp in 15 mL of degassed dichloro-
methane was heated under reflux for 10 h. After cooling to
room temperature, 10 mL hexane was added and the mixture
was left at ꢀ15 ꢁC for 24 h to complete the precipitation. The
green solid was isolated by filtration, washed with dichloro-
methane and hexane and dried in vacuum. Yield: 60%
(0.127 g). Anal. Calc. for C23H20Br3NO2PRe: C, 34.64; H,
2.53; N, 1.76. Found: C, 34.36; H, 2.36; N, 1.73%.

E.J. de Souza et al. / Inorganica Chimica Acta 359 (2006) 1513–1518
1515
Table 1
Crystal data and structure refinement parameters for 1 Æ CH2Cl2 and 2
Complex
1 Æ CH2Cl2
2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C
24H22Cl5NO2PRe C23H20Br3NO2PRe
750.85
173(2)
triclinic
799.30
293(2)
monoclinic
P21/n
ꢀ
P1
a (pm)
b (pm)
c (pm)
a (ꢁ)
918.2(1)
945.0(3)
1903.9(3)
1413.3(2)
90
107.61(1)
90
1103.3(1)
1345.0(2)
91.61(1)
98.62(1)
98.94(1)
1.3290(3) · 106
2
b (ꢁ)
c (ꢁ)
V (pm3)
2.4234(8) · 106
4
Z
Absorption coefficient (mmꢀ1
Absorption correction
Tmax/Tmin
)
5.158
10.047
empirical
1.000/0.593
0.4 · 0.04 · 0.02
green needle
18324
9189/0.0194
8600
307
R1a = 0.0174,
wR2b = 0.0415
0.970
empirical
0.4332/0.1077
0.40 · 0.30 · 0.10
green plate
6212
5252/0.0424
3281
280
R1a = 0.0495,
wR2b = 0.0884
1.003
Crystal size (mm)
Crystal shape and color
Collected reflections
Unique reflections/Rint
Observed data [I > 2 r(I)]
Refined parameters
Structure factors [I > 2 r(I)]
Fig. 1. Ellipsoid representation [19] of the complex molecule in 1 Æ CH2Cl2
with the thermal ellipsoids representing 50% probability. The dashed lines
indicate the N–Hꢂ ꢂ ꢂCl hydrogen bonds, with the following distances and
angles: N–H(6) = 86 pm, H(6)ꢂ ꢂ ꢂCl(2) = 261 pm, Nꢂ ꢂ ꢂCl(2) = 319.0(2)
pm, N–H(6)ꢂ ꢂ ꢂCl(2) = 126.2ꢁ, H(6)ꢂ ꢂ ꢂCl(3) = 267 pm, Nꢂ ꢂ ꢂCl(3) =
324.8(2) pm, and N–H(6)ꢂ ꢂ ꢂCl(3) = 125.7ꢁ.
Goodness-of-fit, Sc
CCDC reference No.
CCDC 283846
CCDC 283847
P
kF ojꢀjF c
k
a
b
c
P
R1 ¼
.
jF o
j
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
2
½wðF 2oꢀF 2c Þ ꢁ
P
wR2 ¼
.
2
½wðF 2oÞ ꢁ
O
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
Cl
Cl
O
2
½wðF 2oꢀF 2c Þ ꢁ
Cl
Br
Br
S ¼
.
Br
ðnꢀpÞ
Re
O
Re
O
PhEt2P
PhEt2P
mination. All results agree with the zwitterionic structures
of 1 and 2, in which the Hhp ligand remains protonated in
the complexes.
PEt2Ph
NH
3
4
3.2. Crystal and molecular structures of 1 Æ CH2Cl2 and 2
Both complexes show distorted octahedral coordination
spheres for the Re(V) central atoms, composed by the oxo
ligand, the phosphorus atom of PPh3, the three halogeno
ligands in a mer arrangement and the oxygen atom of
Hhp. The latter one is coordinated in trans position to the
oxo oxygen, as is shown in Figs. 1 and 2. Selected bond
lengths and angles for 1 Æ CH2Cl2 and 2 are given in Table 2.
In both complexes, the Hhp ligands coordinate monod-
entate and are neutral via the exocyclic oxygen atoms. The
protonation of the pyridine nitrogen atoms is supported by
N–Hꢂ ꢂ ꢂX (X = Cl, Br) hydrogen bonds, which are estab-
lished between the pyridine rings and halogeno ligands.
These hydrogen bonds are intramolecular in 1 Æ CH2Cl2
and intermolecular in 2, as is indicated in the Figs. 1 and 3,
respectively.
mer-[ReOCl3(L)(PPh3)] with L = 2-(diethylaminomethyl)-
4-methylphenol (3) [14]. Complex 2 is the first zwitterionic
complex of the type ReO2Br3P, which was structurally ana-
lyzed, and the only other example of a structurally charac-
terized Re(V) complex of this donor atom constellation is
[ReOBr3(OPEt2Ph)(PEt2Ph)] (4) [15].
The Re–O1 distances of 166.4(1) in 1 Æ CH2Cl2 and
165.9(6) pm in 2 are in the expected range for rhenium(V)
monooxo complexes [16], and can easily be explained by
the multiple bond character of these bonds. The inter-
atomic distances between the rhenium atoms and the
2(1H)-pyridinonate oxygen atoms coordinated in the axial
position trans to the oxo groups (1 Æ CH2Cl2: Re-
O(2) = 206.8(1) pm; 2: Re–O(2) = 203.7(6)) are consider-
ably longer than the corresponding bond to the phenoxy
oxygen atom in 3 (191.5(3) pm [14]) and all examples where
alkoxy donor sites are bonded trans to oxo ligands in
[ReO(OR)Cl2(PPh3)2 complexes (R = Et, iso-Prop), show
Re–O(alkoxy) bond lengths between 188.8 and 192.8 pm
To our knowledge, there is only one other reported
example of a zwitterionic oxorhenium(V) complex of the
ReO2Cl3P type, the structure of which was determined
crystallographically, namely

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E.J. de Souza et al. / Inorganica Chimica Acta 359 (2006) 1513–1518
Re–X bond lengths). A good similarity is also found
between the bond lengths in complexes 2 and 4.
The bond lengths within the ring of the coordinated
Hhp in 1 and 2 (see Table 2) show considerable double
bond character. The ligand can be best described by the
three resonance structures a–c shown below, in which the
negative charge located at the donor oxygen atom, in a
and b, is formally transferred to the rhenium(V) center
via the O–Re bond.
O
O
O
H
H
H
N
N
N
c
a
b
3.3. Spectroscopic characterization
Strong absorptions at 995 cmꢀ1 in the IR spectra of
the products 1 and 2 are assigned to m(Re@O). The pres-
ence of the 2-hydroxypyridine ligand coordinated in the
protonated form is confirmed by the characteristic
m(N–H) bands at 3253 and 3231 cmꢀ1. Its coordination
via the oxygen atom in the products results in a shift
of the m(CO) band from 1653 cmꢀ1 in the uncoordinated
Hhp to 1634 and 1637 cmꢀ1 in 1 and 2, respectively. The
coordination of triphenylphosphine in 1 and 2, is con-
Fig. 2. Ellipsoid representation [19] of 2 with the thermal ellipsoids
representing 50% probability.
[17]. The comparably short Re–O bond length in the latter
compounds is discussed to be a result of delocalization of
p-electron density from the oxo bonds to the trans-situated
ones and has also been found for some chelating ligands
with phenoxy or alkoxy functionalities, which direct their
oxygen donor atoms to the trans-positions of the oxo
ligands [16,18]. Despite the fact that the oxygen atoms of
the Hhp ligands in 1 and 2 are also situated in trans posi-
tions to an oxo group, such a delocalization of electron
density seems not to play a significant role in the rhenium
complexes under study.
firmed by its typical absorptions m(P–C) at 1093 cmꢀ1
,
c(out of plane CH in PPh3) at 751 cmꢀ1 and d(phenyl
ring deformation) at 693 cmꢀ1, without significant chang-
ing when compared to the starting complexes. A band
attributable to m(Re–Cl) in 1 was found at 307 cmꢀ1
.
The corresponding m(Re–Br) band in 2 could not be
clearly identified in its spectrum.
1
The bonding situation in the equatorial coordination
spheres of 1 and 2 is very similar (except the different
The H NMR spectra of compounds 1 and 2 in CD2Cl2
at room temperature exhibit the expected signals for the
Table 2
Selected bond distances (pm) and angles (ꢁ) for 1 Æ CH2Cl2 and 2
1 Æ CH2Cl2
2
1 Æ CH2Cl2
2
Re–O(1)
Re–O(2)
Re–X(1)a
Re–X(2)a
Re–X(3)a
Re–P
166.4(1)
206.8(1)
236.07(5)
240.06(5)
239.87(5)
247.77(5)
165.9(6)
203.7(6)
254.4(1)
253.7(1)
252.9(1)
249.9(3)
C(1)–O(2)
C(1)–N
C(5)–N
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
129.1(2)
135.0(2)
136.4(2)
140.8(2)
137.0(3)
140.3(3)
135.5(3)
128.7(10)
133.1(11)
134.2(12)
138.3(13)
137.9(15)
139.5(15)
133.2(15)
O(1)–Re–O(2)
O(1)–Re–P
172.68(6)
90.79(5)
99.37(5)
98.03(5)
92.76(5)
84.16(4)
85.65(4)
87.40(4)
172.9(3)
92.6(2)
93.7(3)
100.6(2)
92.8(3)
81.1(2)
89.6(2)
85.9(2)
P–Re–X(1)a
86.68(2)
170.39(2)
97.59(2)
88.12(2)
167.10(2)
85.84(2)
82.70(4)
90.10(6)
166.41(6)
95.18(6)
85.72(4)
171.45(4)
87.61(4)
84.6(2)
P–Re–X(2)a
O(1)–Re–X(1)a
O(1)–Re–X(2)a
O(1)–Re–X(3)a
O(2)–Re–P
P–Re–X(3)a
X(1)–Re–X(2)a
X(1)–Re–X(3)a
X(2)–Re–X(3)a
O(2)–Re–X(3)a
O(2)–Re–X(1)a
O(2)–Re–X(2)a
a
X = Cl for 1 and Br for 2.

E.J. de Souza et al. / Inorganica Chimica Acta 359 (2006) 1513–1518
1517
Fig. 3. PLUTON view [20] of the unit cell, showing three units of 2 forming a linear chain of hydrogen bonded molecules, which is oriented perpendicular
to the crystallographic b-axis, bisecting the b angle. Except H(6), the hydrogen atoms are omitted for clarity. N–H(6) = 84 pm, H(6)ꢂ ꢂ ꢂBr(2)0 = 261 pm,
Nꢂ ꢂ ꢂBr(2)0 = 341.6(8) pm, N–H(6)ꢂ ꢂ ꢂBr(2)0 = 156.3ꢁ. Symmetry operation used to generate Br(1)0: x ꢀ 1/2, ꢀy + 3/2 , z ꢀ 1/2.
aromatic protons of the PPh3 and Hhp ligands. In addi-
tion, the NH resonance can be seen as broad signals in both
compounds. The 31P{1H} NMR spectra of 1 and 2 show
sharp singlets at ꢀ18.35 (1) and ꢀ15.65 ppm (2). The spec-
troscopic results agree with the coexistence of Hpy and
PPh3 ligands in the complexes and are in accord with the
solid-state structures.
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6þ
In contrast to the reactions between Hhp, and Re2
4þ
and Re2 centres, or [ReH7(PPh3)2] species, which lead
to complexes containing hpꢀ as chelating or bridging
ligands [1–9], zwitterionic complexes of the type
[ReOX3(Hhp)(PPh3)] are obtained from reactions of
Hhp with [ReOX3(PPh3)2] (X = Cl, Br) under mild con-
ditions. The products contain the Hhp ligands in a
monodentate, neutral O-bonded coordination mode.
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hydrogen bonds.
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Acknowledgements
The authors are grateful to FINEP (CT INFRA 0970/
01) and CNPq for the financial support of this work and
to CNPq for providing a scholarship to A.G.A.F.

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