The reaction of nitro-1,2-diaminobenzenes with the [ReO]3+ core: Isolation of oxo-free rhenium(V) complexes

Article abstract of DOI:10.1016/S0020-1693(99)00497-1

The reaction of 2 equiv. of 3-nitro-1,2-diaminobenzene (H₂L¹) with trans-ReOCl₃(PPh₃)₂ in ethanol gave the oxo-free rhenium(V) complex [ReCl(PPh₃)(L¹)₂] (1) in good yield. IR, ¹H NMR and X-ray crystallographic results indicate that the diamine ligand L¹ coordinates in a dianionic bidentate diamido form to the metal, and that the complex has the unusual skew-trapezoidal bipyramidal geometry. With 4-nitro-1,2-diaminobenzene (H₂L²) as ligand the complex [Re(L²)Cl₃(PPh₃)₂] (2) was characterized, in which L² coordinates in a dianionic monodentate imido form through one nitrogen only. Crystal data for 1·EtOH: C₃₂H₃₁ClN₆O₅PRe, monoclinic P2₁/c, a = 14.937(11), b = 15.017(8), c = 15.394(12) A?, β = 112.12(6)°, V = 3198.7(38) A?³, Z = 4, D(calc) = 1.728 g cm^-3, structure solution and refinement based on 3151 reflections converged at R = 0.0807. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00497-1

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Inorganica Chimica Acta 303 (2000) 24–29
The reaction of nitro-1,2-diaminobenzenes with the [ReO]3+ core:
isolation of oxo-free rhenium(V) complexes
Giuliano Bandoli a, Alessandro Dolmella a, Thomas I.A. Gerber b,*, Joanne Perils b,
Jan G.H. du Preez b
a Department of Pharmaceutical Sciences, Uni6ersity of Pado6a, 35131 Padua, Italy
b Department of Chemistry, Uni6ersity of Port Elizabeth, PO Box 1600, 6000 Port Elizabeth, South Africa
Received 10 June 1999; accepted 17 September 1999
Abstract
The reaction of 2 equiv. of 3-nitro-1,2-diaminobenzene (H2L1) with trans-ReOCl3(PPh3)2 in ethanol gave the oxo-free
rhenium(V) complex [ReCl(PPh3)(L1)2] (1) in good yield. IR, 1H NMR and X-ray crystallographic results indicate that the
diamine ligand L1 coordinates in a dianionic bidentate diamido form to the metal, and that the complex has the unusual
skew-trapezoidal bipyramidal geometry. With 4-nitro-1,2-diaminobenzene (H2L2) as ligand the complex [Re(L2)Cl3(PPh3)2] (2) was
characterized, in which L2 coordinates in a dianionic monodentate imido form through one nitrogen only. Crystal data for
3
,
,
1·EtOH: C32H31ClN6O5PRe, monoclinic P21/c, a=14.937(11), b=15.017(8), c=15.394(12) A, i=112.12(6)°, V=3198.7(38) A ,
Z=4, Dcalc=1.728 g cm−3, structure solution and refinement based on 3151 reflections converged at R=0.0807. © 2000 Elsevier
Science S.A. All rights reserved.
Keywords: Crystal structures; Rhenium complexes; Oxo-free complexes; Nitro-diamine complexes
1. Introduction
Complexes of Re(V) which lack such multiple bonds
are rare and unusual. The few examples in the literature
(ReH5(PPh3)2(py) and its derivatives, ReCl5, [ReX4-
(diars)2]+ and [Re(HNC6H4S)3]−) [4] were synthesized
by either the reduction of Re(VI) and Re(VII) or the
oxidation of Re(III) and Re(IV) species under extreme
conditions. The only sample that could be found where
the synthesis was via the ligand substitution of a rheni-
um(V) starting complex containing one of the multiply
bonded moieties mentioned above, is [ReH5(PR3)3] [5].
It was made by treating trans-[ReOCl3(PPh3)2] (or
ReCl3(PR3)3) with LiAlH4 or NaBH4 in thf.
This paper describes the products of the reactions of
3-nitro-1,2-diaminobenzene (H2L1) and 4-nitro-1,2-di-
aminobenzene (H2L2) (see Scheme 1) with trans-
ReOCl3(PPh3)2 in ethanol. With H2L1 the oxo-free
rhenium(V) complex [ReCl(PPh3)(L1)2] was isolated, in
which L1 acts as a bidentate dianionic diamido chelate.
With H2L2 the imido rhenium(V) complex [Re-
(L2)Cl3(PPh3)2] was formed, with L2 coordinated in a
dianionic monodentate mode.
The success of therapeutic rhenium radiopharmaceu-
ticals in the palliation of osseous metastases in cancer
patients [1] has rekindled interest in the coordination
chemistry of this metal. These research efforts concen-
trate mainly on the V oxidation state, since it is easily
obtained from the reduction of perrhenate and easily
stabilized by a large variety of ligands [2]. However, the
most distinctive feature of Re(V) chemistry is the exis-
tence of a large number of stable complexes in which
the metal forms multiple bonds to oxygen, nitrogen or
sulfur [3]. These complexes are mostly octahedral and
contain one of the [ReO]3+, [ReO2]+, [Re2O3]4+
,
[ReꢀN]2+, [ReꢁNR]3+ or [ReꢁS]3+ moieties. These
multiply bonded donor atoms severely restrict the struc-
ture, geometry, configuration, reactivity and magnetic
properties of Re(V) complexes.
* Corresponding author. Tel.: +27-41-504 2274; fax: +27-41-504
2573.
E-mail address: chatig@upe.ac.za (T.I.A. Gerber)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00497-1

G. Bandoli et al. / Inorganica Chimica Acta 303 (2000) 24–29
25
Table 1
Crystal data and structure refinement for 1
Empirical formula
Formula weight
Temperature (K)
C32H31ClN6O5PRe
832.25
293(2)
,
Wavelength (A)
0.71073
monoclinic
P21/c
Crystal system
Space group
Scheme 1.
Unit cell dimensions
,
a (A)
14.937(11)
15.017(8)
15.394(12)
90
112.12(6)
90
3199(4),
4
We have previously reported the products isolated
from the reactions of the aromatic diamines 1,2-di-
aminobenzene, 2,3-diaminophenol and 2,3-diamino-
toluene (H2N–R–NH2) with trans-ReOCl3(PPh3)2 in
ethanol. In all these cases trans-[Re(N–R–NH2)Cl3-
(PPh3)2] was isolated, in which the diamine ligands
coordinate to rhenium(V) in the dianionic monodentate
imido form [6].
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
Volume (A ),
Z
Dcalc (Mg m−3
)
1.728
3.984
1648
Absorption coefficient (mm−1
F(000)
)
Crystal size (mm)
q Range for data collection (°)
0.10×0.08×0.05
2.68–20.05
2. Experimental
Limiting indices
05h514, 05k514,
−145l513
3151
2.1. Reagents
Reflections collected
Independent reflections
Observed reflections (I\2|(I))
Absorption correction
Refinement method
3010 (Rint=0.0998)
1739
No
trans-ReOCl3(PPh3)2 was synthesized by a published
procedure [7]. Solvents were reagent grade, and were
purified, dried and deoxygenated before use. All other
reagents were obtained commercially (Aldrich), and their
purity was checked by H NMR and melting point. All
reactions were performed under a nitrogen atmosphere,
using standard techniques.
full-matrix least-squares on
F2
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
3008/0/221
1.069
R1=0.0807, wR2=0.1917
R1=0.1466, wR2=0.2519
0.0000(4)
1
Extinction coefficient
Largest difference peak and hole
0.989 and −0.833
2.2. Synthesis
−3
,
(e A
)
2.2.1. [ReCl(PPh3)(L1)2] (1)
A
mixture of 106 mg (127 mmol) of trans-
ReOCl3(PPh3)2 and 39 mg (255 mmol) of H2L1 in 30 cm3
of ethanol was heated under reflux conditions for 3 h.
After cooling the solution to room temperature (r.t.), a
dark reddish–brown crystalline precipitate was removed
by filtration, washed with ethanol and diethyl ether, and
dried under vacuum. Recrystallization was from acetone;
yield 65.5 mg (62%), m.p. \300°C. Anal. Calc. for
C30H25ClN6O4PRe: C, 45.83; H, 3.21; N, 10.69. Found:
C, 45.99; H, 3.24; N, 10.78%. IR (KBr): w(NH) 3332(m),
from acetonitrile gave orange prisms; yield 66 mg (52%),
m.p. 159°C. Anal. Calc. for C42H35Cl3N3O2P2Re: C,
52.10; H, 3.64; N, 4.34. Found: C, 51.81; H, 3.66; N,
4.35%. IR (KBr): w(NH2) 3390(m), 3195(m); l(NH2)
1631(m); ws(NO2) 1330(vs); w(ReꢀN–) 1092(m); w(Re–
Cl) 312(m), 295(m). 1H NMR (294 K) ppm: 8.10 (d, 1H,
H5); 7.91 (m, 14H, NH2, PPh3); 7.44 (m, 18H, PPh3); 6.86
(s, 1H, H3); 6.38 (d, 1H, H6). UV–Vis: 458 (8800), 363
(11 000), 332 (10 400), 259 (42 100).
1
3284(m); ws(NO2) 1339(s); w(Re–Cl) 301(m). H NMR
(294 K) ppm: 13.59 (s, 2H, N2H); 13.21 (s, 2H, N1H);
7.69 (d, 2H, H4); 7.64 (d, 2H, H6); 7.37–7.49 (m, 9H,
PPh3); 7.12 (m, 6H, PPh3); 6.96 (t, 2H, H5). UV–Vis: 482
(5900), 380 (14 700), 329 (15 400), 228 (56 200).
2.3. X-ray data collection, structure solution and
refinement of 1
Crystals of the formulation [ReCl(PPh3)(L1)2]·EtOH
were obtained from a 1:1 mixture of ethanol and
dichloromethane solvents. The crystal data and details
of data collection are given in Table 1. The crystal data
and intensities were measured on a Nicolet Siemens
R3m/V four-circle diffractometer (Mo Ka, u=0.71073
2.2.2. [Re(L2)Cl3(PPh3)2] (2)
To 109 mg (130 mmol) of trans-ReOCl3(PPh3)2 in 20
cm3 of ethanol was added 40 mg (261 mmol) of H2L2, and
the resultant solution was heated under reflux conditions
for 2 h. After cooling to r.t., the solution was filtered to
give an orange precipitate, which was washed with
ethanol and diethyl ether, and dried. Recrystallization
,
A), using the ꢀ–2q technique. Unfortunately, all the
crystals were of a small size and the only remotely
suitable sample for a single-crystal structure determina-

26
G. Bandoli et al. / Inorganica Chimica Acta 303 (2000) 24–29
tion was substandard. Poor quality was quickly mani-
fested in the broad scan widths during early search
routines on the diffractometer. In addition, the diffract-
ing ability of the sample fell off rapidly with increasing
Bragg angle and the data collection was restricted to
2q=40°. Because of this, much of the higher angle data
collected were weak and bore negative intensities. As a
consequence, refinement of the crystal structure was not
satisfactory, and moreover, it was hampered by the
disorder of the ethanol molecule. The latter was found
to be severely disordered into a number of partially
occupied sites, but attempts to model the disorder did not
produce a reliable structure, and so refinement was
completed without any disorder model yielding passable
residuals. Refinement on F2 (anisotropy only to the three
heavy atoms) converged rather smoothly to the fractional
atomic coordinates. The structure was solved by Patter-
son methods using SHELXTL/PC [8] and refined by the
full-matrix least-squares method using the SHELXL-93 [9]
package.
The relevant bond lengths and angles are listed in
Table 2. Due to the fact that the crystals were substan-
dard and that the refinement was not satisfactory with
a relatively high value for the R indices, only the bond
lengths and angles in the coordination sphere around the
rhenium are given, since the bond lengths and angles in
the ligands L1 and PPh3 would be of very little chemical
significance. The structural analysis did however attain
its objective by providing proof of the oxidation state of
rhenium and the overall stereochemistry of the complex.
3. Results and discussion
3.1. Synthesis
The reaction of trans-ReOCl3(PPh3)2 with 3-nitro-1,2-
diaminobenzene (H2L1) in a 1:2 molar ratio in boiling
ethanol gave the product [ReCl(PPh3)(L1)2] (1) in good
yield:
ReOCl3(PPh3)2+2H2L1E“tOH[ReCl(PPh3)(L1)2]+2HCl
D
+H2O+PPh3
However, when trans-ReOCl3(PPh3)2 was reacted with
4-nitro-1,2-diaminobenzene (H2L2) under the same con-
ditions as for complex 1, the imido complex
[Re(L2)Cl3(PPh3)2] (2) was isolated:
ReOCl3(PPh3)2+H2L2E“tOH[Re(L2)Cl3(PPh3)2]+H2O
D
The formulation and structure of complexes 1 and 2
were verified by 1H NMR spectrometry, IR spec-
troscopy, elemental analyses, and with regard to complex
1, X-ray crystallography.
Both complexes are air stable and diamagnetic, and
they are non-electrolytes in acetonitrile and DMF. Com-
plex 1 is very soluble in a variety of solvents such as
dichloromethane, acetone, acetonitrile, chloroform and
ethyl acetate. The solubility of 2 in most polar solvents
is low, but it can be crystallized from ethyl acetate and
acetonitrile. Both complexes are stable in solution for
days, and for months in the solid state.
2.4. Physical measurements
3.2. Description of the structure of [ReCl(PPh3)(L1)2]
(1)
The scientific instrumentation used in this study is the
same as reported elsewhere [10]. IR spectra were obtained
in KBr discs and 1H NMR spectra were run in (CD3)2SO.
Electronic spectra were all obtained in dichloromethane,
and data are given as umax with extinction coefficients (in
units M−1 cm−1) in parentheses. Elemental analyses for
carbon, hydrogen and nitrogen were carried out by the
Division of Materials Science and Technology of the
CSIR in Pretoria, South Africa.
The crystal structure shows that 1·EtOH (Fig. 1) is
monomeric and neutral, and exhibits the skew-trape-
zoidal bipyramidal geometry. The complex contains two
dinegative L1 ligands symmetrically coordinated to the
Re(V) centre. The coordinating mode of L1 (shown
below) is evidenced by the four Re–NH bond distances
Table 2
,
Relevant bond distances (A) and angles (°) for 1
(all equal within the range of the e.s.d. values; mean value
,
of 1.98(2) A), which are consistent with the values found
for this particular bond in eight Re(V) octahedral com-
Bond distances
Re–Cl
Re–N(1)
Re–N(2)
2.393(9)
1.98(3)
1.96(2)
Re–P(1)
Re–N(1%)
Re–N(2%)
2.431(9)
1.97(3)
1.98(3)
,
plexes (14 entries with a mean value of 2.00(1) A) [11].
The Re–P bond (2.431(10) A) is shorter than in other
Re(V) complexes [12], probably because of the absence
,
Bond angles
N(1)–Re–N(2)
Re–N(1)–C(1)
Re–N(2)–C(6)
Cl–Re–N(1)
P(1)–Re–N(2)
N(1)–Re–N(2%)
77(1)
120(2)
121(2)
139.1(8)
130.2(7)
130(1)
N(1%)–Re–N(2%)
Re–N(1%)–C(1%)
Re–N(2%)–C(6%)
Cl–Re–N(1%)
P(1)–Re–N(2%)
N(1%)–Re–N(2)
79(1)
115(2)
117(2)
129.4(8)
135.3(8)
144(1)
of an atom trans to it. The Re–Cl bond length (2.393(9)
,
A) is within the range observed for a large variety of
Re(V) oxo, nitrido and imido complexes [6,11]. The Re,
Cl and P(1) atoms are above the N(1), N(2), N(1%), N(2%)
,
mean plane by 0.72, 2.51 and 2.57 A, respectively. The
two L1 ligands make a dihedral angle of 60.3°.

G. Bandoli et al. / Inorganica Chimica Acta 303 (2000) 24–29
27
ture having two identical bidentate ligands, there is a
marked preference for compounds with small normal-
ized bite ligands (B1.30) to form the cis-octahedral
structure. However, the trans structure remains possible
at low b when the effective bond length ratio, R
(unidentate/bidentate)\1.0 [14].
Examples of six-coordinate complexes of the type
[M(bidentate)2(unidentate)2] with the skew-trapezoidal
bipyramidal structure are scarce in the literature, and
the few that were found are the tin(IV) complexes given
in Table 3, together with their values of the normalized
bite b and effective bond length ratio R.
The bidentate dianionic diamido coordination mode
is quite common in the coordination chemistry of 1,2-
diaminobenzene (H2L3) with transition metals, as the
examples [UO2(L3)2](NO3)2 [19], [Tc(L3)3](TcO4) [20]
and [Re(L3)3] [21] verify.
Fig. 1. The molecular structure of [ReCl(PPh3)(L1)2] (1) showing the
atom-numbering scheme and 40% displacement ellipsoids.
3.3. Spectroscopic properties
3.3.1. Complex 1
In the infrared spectrum of complex 1 there is no
band observed in the region 900–1000 cm−1 which can
be ascribed to the ReꢁO stretching frequency, indicat-
ing the absence of the oxo group. Two N–H absorp-
tions occur at 3332 and 3284 cm−1, which is consistent
with the two inequivalent N–H protons of L1. The
ws(NO2) is assigned to a sharp band at 1339 cm−1 (1430
cm−1 in the free ligand H2L1). The Re–N bands occur
at 433 and 463 cm−1, and the Re–Cl stretching fre-
quency appears as a band of medium intensity at 301
cm−1
.
Fig. 2. Skew-trapezoidal bipyramidal arrangement of the donor
The proton NMR spectrum of 1 clearly indicates the
presence of two chemically equivalent molecules of L1
and one triphenylphosphine group. The triplet at 6.96
ppm integrates for two protons, and it is assigned to the
2 equiv. H5 protons on the two phenylene rings. The
doublets at 7.64 and 7.69 ppm integrate for two pro-
tons each, and are assigned to protons H6 and H4,
respectively. The protons of the PPh3 group appear as
two multiplets at 7.12 and in the region 7.37–7.49 ppm,
which integrate for six and nine protons, respectively.
atoms around Re(V) in complex 1.
The two stereochemistries usually envisaged for six-
coordinate complexes of the type [M(bidentate)2-
(unidentate)2] are the cis- and trans-octahedral struc-
tures. However, both these structures become signifi-
cantly distorted for bidentate ligands of small
normalized bite [b=2 sin(i−M−j/2)]. The trans-octa-
hedral structure distorts to form the skew-trapezoidal
bipyramidal structure [13]. Surprisingly, this geometry
occurs in 1 with a rather great value of the normalized
bite [b=2 sin(N(1)–Re–N(2)/2)=1.26].
Bidentate ligands that form the five- or six-membered
chelate rings and have relatively large normalized bites
(bꢀ1.3–1.5) normally form undistorted trans struc-
tures. As the normalized bite is decreased, however,
energy calculations [14] show that the rectangle formed
by the two coplanar bidentate ligands becomes dis-
torted and forms a planar trapezoid. The unidentate
ligands are simultaneously skewed towards, or even
apart, the long edge of the trapezoid (Fig. 2).
Table 3
Complexes of the type [M(bidentate)2(unidentate)2] with the skew–
trapezoidal bipyramidal structure (b=normalized bite; R=effective
bond length ratio)
Complex
b
R
Ref.
ReCl(PPh3)(L1)2
Sn(NO3)2Me2
Sn(S2CNMe2)2Me2
Sn{S2P(OEt)2}2Ph2
Sn(ONHCMeO)2Me2
1.26
0.92
1.06
1.14
1.17
1.22
1.20
1.20
1.29
1.12
this work
[15]
[16]
[17]
[18]
In six-coordinate complexes of known crystal struc-

28
G. Bandoli et al. / Inorganica Chimica Acta 303 (2000) 24–29
ence of aqueous HCl and triphenylphosphine gave the
rhenium(V) S,N-chelated thiosemicarbazate(3–) com-
plexes [ReCl2(NNCSNHR)(PPh3)2] [24].
The signals of the four NH protons appear as two
singlets at 13.21 and 13.59 ppm, that each integrates for
two protons. These latter two signals disappear on the
addition of D2O to the sample solution.
The infrared and NMR data unambiguously estab-
lish the fact that the two H2L ligands in 1 are doubly
deprotonated, with the result that the rhenium in 1 is in
the formal oxidation state +V.
4. Concluding remarks
The isolation of complex 1 presents a very interesting
example of an oxo-free complex of rhenium(V), which
was surprisingly easily prepared from the oxo complex
trans-ReOCl3(PPh3)2. The substitution of an oxo group
from rhenium(V) is not often observed, and rhenium(V)
complexes lacking a multiply bonded donor atom as
ligand are unusual. Complex 2 presents another exam-
ple of the unusual monodentate imido formation of
1,2-diaminobenzenes in complexes of rhenium(V).
3.4. Complex 2
The infrared spectrum of complex 2 displays the
ReꢁNR stretching frequency at 1092 cm−1. There is no
band in the region 900–1000 cm−1 which can be
attributed to w(ReꢁO), indicating the replacement of the
oxo oxygen by the imido group. The two medium
intensity bands at 3390 and 3195 cm−1 [w(NH2)], and
the appearance of l(NH2) at 1631 cm−1 indicate the
presence of an uncoordinated amino group in 2. In the
far infrared region the w(Re–Cl) appears as medium
bands at 312 and 295 cm−1, with the lower band
indicative of the chloride coordinated trans to the rhe-
nium imido bond.
5. Supplementary material
Crystallographic data (comprising detailed informa-
tion on structure data collection and structure refine-
ment, final fractional atomic coordinates, thermal
parameters and a full listing of bond lengths/angles) for
the structures reported in this paper have been de-
posited with the Cambridge Crystallographic Data
Centre as supplementary publications. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
1
In the H NMR spectrum the signals of the H6, H3
and H5 protons of L2 are seen clearly at 6.38 (doublet),
6.86 (singlet) and 8.10 ppm (doublet), respectively. The
two multiplets at 7.44 and 7.91 ppm integrate for 18
and 14 protons, respectively, with the latter overlapping
the proton signal of the uncoordinated NH2 group.
This particular signal was previously found [6] in the
range 6.50–7.32 ppm.
These results indicate that L2 coordinates in a dian-
ionic monodentate imido form to rhenium(V). Al-
though it was impossible to establish which nitrogen (1
or 2) is coordinated to the metal, we speculate that due
to the mesomeric effect of the 4-nitro group, the basic-
ity of the 1-amino group will be reduced due to the
increased delocalization of the 1-nitrogen’s lone pair
over the aromatic ring system, with the effect that the
2-nitrogen will be deprotonated and coordinated in the
imido form to the metal.
Acknowledgements
T.I.A.G and J.G.H.dP. are grateful to the University
of Port Elizabeth and the Foundation for Research
Development for financial support.
References
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complex with a bidentate diamido ligand, there are
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bidentate nitrogen-donor ligands, that were synthesized
from trans-ReOCl3(PPh3)2, in the literature. The reac-
tion of ReOCl3(PPh3)2 with (o-aminophenyl)diphenyl-
phosphine (H2L) in toluene yielded the neutral rheni-
um(V) complex [ReCl2(L)(HL)]. Both ligands are coor-
dinated bidentately, with L2− bonded as a P,N-donor
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