Synthesis and characterization of a binuclear rhenium nitropyrazole complex [Re2O3Cl2(PPh3)2(C 3H2N3O2)2]

Article abstract of DOI:10.1016/S0020-1693(96)05537-5

The binuclear nitropyrazole compound [Re₂O₃Cl₂(PPh₃)₂(C ₃H₂N₃O₂)₂] (3) was synthesized by refluxing [ReOCl₂(OEt) (PPh₃)₂] and 2-nitropyrazole in ethanol for 12 h. The octahedral {ReO₂N₂ClP} sites are linked by two bridging nitropyrazoles and an oxo group, with an Re-O-Re angle of 124.7(6)^o. Compound 3 is electrochemically active, displaying a reversible one electron reduction at - 0.55 V with respect to Ag/Ag⁺. Crystal data: C₃₆H₃₄Cl₂N₆O₇P ₂Re₂ (3), monoclinic C2/c, a = 32.569(7), b = 9.624(2), c = 29.151(6) A, β = 109.33(3)^o, V = 8622(3) A³, Z = 8, D_calc = 1.799 g cm^-3, structure solution and refinement based on 3364 reflections converged at R = 0.056.

Full text of DOI:10.1016/S0020-1693(96)05537-5

Inorganica Chimica Acta 260 (1997) 83–88
Note
Synthesis and characterization of a binuclear rhenium nitropyrazole
complex [Re2O3Cl2(PPh3)2(C3H2N3O2)2]
Kevin P. Maresca, David J. Rose, Jon Zubieta U
Department of Chemistry, Syracuse University, Syracuse, NY 13244, USA
Received 27 July 1996; accepted 18 October 1996
Abstract
The binuclear nitropyrazole compound [Re2O3Cl2(PPh3)2(C3H2N3O2)2]( 3) was synthesized by refluxing [ReOCl2(OEt)(PPh3)2] and
2-nitropyrazole in ethanol for 12 h. The octahedral {ReO2N2ClP} sites are linked by two bridging nitropyrazoles and an oxo group, with an
Re–O–Re angle of 124.7(6)8. Compound 3 is electrochemically active, displaying a reversible one electron reduction at y0.55 V with respect
q
˚
to Ag/Ag . Crystal data: C36H34Cl2N6O7P2Re2 (3), monoclinic C2/c, as32.569(7), bs9.624(2), cs29.151(6) A, bs109.33(3)8,
Vs8622(3) A ,Z s8, Dcalcs1.799 g cmy3, structure solution and refinement based on 3364 reflections converged at Rs0.056.
3
˚
Keywords: Crystal structures; Rhenium complexes; Nitropyrazole complexes; Binuclear complexes
1. Introduction
[9,10]. Previously reported Pt(II) and Rh(II) nitroimida-
zole and related compounds have shown some success in
these applications [11,12]. In this study, we report the
synthesis and characterization of a binuclear rhenium nitro-
pyrazole species, [Re2O3Cl2(PPh3)2(C3H2N3O2)2].
Recently there has been expanded interest in metal-based
nitroimidazole-related compounds [1–4]. The importance of
these compounds is demonstrated by their ability to mark
hypoxic (oxygen deficient) cell regions and radiosensitize
tumor cells [5,6]. Such hypoxic regions are characteristic of
solid tumors, which often contain poorly oxygenated regions
due to the rapid cell growth that outpaces the developing
blood vessels, causing the reduction of the supply of nutrients
to the cells, particularly oxygen. The slow metabolism of the
nitro compounds and their ability to diffuse through nonvas-
cular cell masses allow deep penetration, making them effec-
tive tools for marking the hypoxic cells present in the tumor.
Radiosensitization refers to the enhancement of radiation
induced therapy with certain drugs [7], a requirement for
tumors containing hypoxic regions as these cells are resistant
to radiotherapy. Since the role of the radiosensitizers is to act
similarly to dioxygen in these hypoxic regions, the electron
affinic nitro group is an essential component thatallowsdrugs
requiring dioxygen to be more effective [8].
2. Experimental
2.1. General considerations
NMR spectra were recorded on a General Electric QE 300
(1H 300.10 MHz) spectrometer in CD2Cl2 (d 5.32), D2O( d
4.75). IR spectra were recorded as KBr pellets with a Perkin-
Elmer Series 1600 FTIR. Electrochemical experiments were
performed by using a BAS CV-27 Cyclic Voltammograph
and a Bas Model RXY recorder. A three-compartment, gas-
adapted cell was used with a glassycarbonworkingelectrode,
a platinum wire auxiliary electrode and a silver/silver chlo-
ride reference electrode. Cyclic voltammograms were
recordedunderN2 indryDMFwhichwas0.2 Minsupporting
electrolyte (tetrabutylammonium hexafluorophosphate
[TBAP]) and sufficient sample for reasonable current
observations. Elemental analysis for carbon, hydrogen and
nitrogen was carried out by Oneida Research Services,
Whitesboro, NY.
We are interested in the synthesis of new rhenium contain-
ing organo-nitro compounds for the potential treatment of
cancer induced hypoxia. By complexing these organo-nitro
ligands to Tc and Re, there exists the possibility to image and
treat these hypoxic regions with the appropriate radionuclide
All synthetic manipulations were carried out open to the
atmosphere. All solvents were of reagent grade and wereused
U Corresponding author.
0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved
PII S0020-1693(96)05537-5

84
K.P. Maresca et al. / Inorganica Chimica Acta 260 (1997) 83–88
Table 1
as received. Ammonium perrhenate and pyrazole were
purchased from Aldrich chemical and used as received.
Summary of crystal data for the structure of [Re2O3Cl2(PPh3)2-
(C3H2N3O2)2]( 3)
2.2. Syntheses
Formula
C42H34Cl2N6O7P2Re2
1240.0
Formula weight
˚
a (A)
32.569(7)
9.624(2)
29.151(6)
109.33(3)
8622(3)
8
1.910
C2/c
5.940
3364
2.2.1. Preparation of C3H3N3O2 (1)
˚
b (A)
All steps were performed open to the atmosphere. In a
100 ml Schlenk flask placed in an ice–water bath was mixed
pyrazole (10.0 g, 147.0 mmol), 95% H2SO4 (20 ml) and
concentrated HNO3 (20 ml). Concentrated H2SO4 (40 ml)
was added to the reaction mixture. The reaction was stirred
and heated at 1108C for 2 days. The solution was poured into
400 ml of ice–water forcing the product 1 to precipitate as a
white solid, which was collected by filtration and washed
with 1.0 l distilled H2O. The filtrate was neutralized to pH
7.0 with NaOH/NaHCO3, yielding more precipitate (10.7 g,
64%). IR (KBr pellet, cmy1): 3176 (s), 3133 (s), 2890
(m), 1505 (s), 1410 (s), 1358 (s), 1288 (s), 1190 (m),
1159 (m), 1029 (m), 997 (s), 938 (s), 893 (m), 850 (m),
817 (s), 755 (s), 592 (s), 546 (s). 1H NMR (D2O, 295 K):
˚
c (A)
b (8)
3
˚
V (A )
Z
Dcalc (g cmy3
Space group
)
Absorption coefficient (Mo Ka) (mmy1
)
No. of reflections (F)6.00s(F))
R a
0.0560
0.0786
Rw b
a 8NNFocNyNF NN/8NF N.
o
b [8w(NFocNyNF N)2/8wNF N ]1/2
.
2
o
pellet, cmy1): 1508 (s), 1435 (m), 1406 (m), 1294 (s),
1189 (w), 1096 (m), 1046 (m), 999 (w), 972 (m), 867
(w), 818 (m), 748 (s), 710 (m), 693 (s), 629 (s), 526 (s).
1H NMR (CD2Cl2, 295 K, d (ppm)): 7.20 (s), 7.34 (m),
7.46 (m), 8.03 (s). Cyclic voltammmetry (E1/2, DMF):
y0.547 V (DEps61 mV).
d
(8.37 s, 2H). Cyclic voltammetry (E1/2, DMF):
y1.086 V, y2.066 V (DEps159 mV).
2.2.2. Preparation of [ReOCl2(OEt)(PPh3)2] (2)
All steps were performed open to the atmosphere. To a
solution of PPh3 (5 g, 19 mmol) in EtOH (30 ml) was added
a solution of 37% HCl (3.5 ml, 30 mmol) containing
NH4ReO4 (1 g, 3.73 mmol). The solution was refluxed in a
50 ml schlenk flask for 10 min, whereupon the color of the
solution changed from milky white to olive. The reaction was
cooled to room temperature and filtered. The precipitate was
washed with two 10 ml portions of EtOH and two portions
of Et2O to yield [ReOCl2(OEt)(PPh3)2] in quantitative
yield (3 g) [13].
2.3. X-ray crystallography
Compound 3 was studied using a Rigaku AFC5S diffrac-
tometer, equipped with a low temperature device. To mini-
mize crystal degradation, data collection was collected at
253 K. Crystal stability was monitored using 3 intense reflec-
tions in each case and no significant changes in the intensities
of these standards were observed over the course of the data
collection. An empirical absorption correction based on psi
scans was applied to the data, resultingintransmissionfactors
ranging from 0.72 to 1.00. The data was corrected forLorentz
and polarization affects. The structure was solved by the
Patterson method. All calculations were performed using the
SHELXTL crystallographic software package [14], as
described previously [15]. Due to the relatively low data to
parameter ratio, the phenyl groups were treated as idealized
rings. The crystal parameters and other experimental details
of the data collection are summarized in Table 1. Atomic
positional parameters are listed in Table 2 and bond lengths
and angles in Table 3. A complete description of the details
of the crystallographic methods is given in the supplementary
materials. An ORTEP view of 3 is presented in Fig. 1 and a
schematic view of the bridge geometry in Fig. 2.
2.2.3. Preparation of [Re2O3Cl2(PPh3)2(C3H2N3O2)2] (3)
2.2.3.1. Method 1
All steps were performed open to the atmosphere. In a
50 ml schlenk flask, NH4ReO4 (0.05 g, 0.186 mmol), 2-
nitropyrazole (0.083 g, 0.746 mmol), and PPh3 (0.098 g,
0.372 mmol) were added in 10 ml methanol containing 37%
HCl (0.275 g, 7.463 mmol). The reaction was refluxed for
2 h. The green product 3 was collected by filtration (0.055 g,
24%)
2.2.3.2. Method 2
All steps were performed open to the atmosphere. In a
50 ml Schlenk flask, [ReOCl2(OEt)(PPh3)2] (0.20 g,
0.241 mmol) was added to 2-nitropyrazole (0.113 g,
0.990 mmol) in ethanol (25 ml). The reaction was refluxed
overnight. The green product 3 was collected by filtration
(0.12 g, 39%). A small sample was dissolved in CH2Cl2 and
layered with pentane to yield dark green crystals. Anal. Calc.
for C42H34N6O7Re2Cl2P2 (mol. wt. 1239.68): C, 40.7; H,
2.74; N, 6.77. Found: C, 40.7; H, 2.61; N, 6.94. IR (KBr
3. Results and discussion
3.1. Synthesis and chemical properties
The 2-nitropyrazole was prepared in moderate yield by
heating pyrazole in a combination of H2SO4 and HNO3 for

K.P. Maresca et al. / Inorganica Chimica Acta 260 (1997) 83–88
85
Table 2
Table 3
˚
Bond lengths (A) and angles (8)
Atomic coordinates (=104) and equivalent isotropic displacement coeffi-
cients a (A=10 )
3
˚
Re(1)–Cl(1)
Re(1)–O(1)
Re(1)–N(1)
Re(2)–Cl(2)
Re(2)–O(1)
Re(2)–N(2)
P(1)–C(25)
P(1)–C(37)
P(2)–C(13)
O(4)–N(3)
O(6)–N(6)
N(1)–N(2)
N(2)–C(3)
N(4)–N(5)
N(5)–C(6)
C(1)–C(2)
C(4)–C(5)
2.352 (6)
1.912 (15) Re(1)–O(2)
2.120 (17) Re(1)–N(4)
Re(1)–P(1)
2.477 (6)
1.696 (17)
2.116 (17)
2.476 (8)
Re(1)
Re(2)
Cl(1)
Cl(2)
P(1)
P(2)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
N(1)
N(2)
N(3)
N(4)
N(5)
N(6)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
903(1)
1068(1)
1418(2)
1366(3)
1166(2)
1747(2)
1262(4)
473(5)
799(5)
y378(7)
y578(7)
377(6)
298(6)
462(6)
550(5)
y350(8)
748(5)
785(5)
390(7)
118(7)
y21(7)
268(8)
597(7)
532(7)
662(7)
1848(8)
1521(8)
1585(8)
1978(10)
2285(9)
2234(9)
1711(7)
1631(8)
1556(8)
1547(8)
1623(8)
1695(8)
2251(9)
2623(9)
3005(10)
3041(11)
2672(12)
2281(12)
2041(8)
2487
1638(1)
3749(1)
y159(7)
5914(7)
2763(7)
3085(7)
2811(14)
884(16)
4158(16)
7071(21)
5461(24)
y1121(19)
y2507(18)
3285(19)
4211(16)
5887(26)
854(17)
1740(17)
y1393(22)
3797(23)
4993(23)
5202(26)
y361(24)
y295(23)
1055(23)
4210(25)
4402(26)
5278(26)
5897(34)
5768(29)
4943(27)
1322(22)
960(27)
y452(28)
y1466(28)
y1049(26)
301(26)
3074(27)
2401(31)
2424(30)
2997(34)
3556(38)
3652(35)
2021(25)
2075
1537(1)
663(1)
1786(2)
973(3)
2346(2)
513(2)
1299(5)
1643(5)
67(5)
950(7)
1352(9)
y646(6)
y89(6)
1242(7)
913(6)
1129(9)
821(6)
464(6)
y230(8)
1335(8)
1084(8)
835(9)
628(8)
137(8)
55(8)
26(1)
31(1)
48(3)
64(4)
37(3)
36(3)
24(6)
39(7)
38(7)
69(10)
90(12)
62(9)
58(8)
32(5)
19(4)
57(6)
23(4)
24(4)
46(6)
36(6)
32(6)
45(7)
33(6)
30(6)
32(6)
41(6)
45(7)
49(7)
77(10)
58(8)
52(7)
27(5)
47(7)
54(7)
49(7)
46(7)
47(7)
49(7)
64(8)
65(9)
82(10)
100(12)
88(11)
73(18)
67(17)
19(10)
58(16)
53(15)
43(7)
54(15)
88(22)
66(9)
76(19)
37(12)
32(6)
63(17)
81(20)
93(23)
69(9)
53(7)
71(9)
2.350 (7)
Re(2)–P(2)
1.969 (14) Re(2)–O(3)
2.093 (18) Re(2)–N(5)
1.844 (24) P(1)–C(31)
1.824 (28) P(2)–C(7)
1.812 (22) P(2)–C(19)
1.245 (32) O(5)–N(3)
1.228 (30) O(7)–N(6)
1.406 (26) N(1)–C(1)
1.293 (30) N(3)–C(2)
1.382 (25) N(4)–C(4)
1.304 (28) N(6)–C(5)
1.358 (31) C(2)–C(3)
1.378 (33) C(5)–C(6)
1.713 (14)
2.138 (16)
1.832 (27)
1.834 (28)
1.815 (23)
1.208 (39)
1.220 (29)
1.333 (33)
1.414 (36)
1.322 (25)
1.465 (30)
1.378 (40)
1.412 (32)
Cl(1)–Re(1)–P(1)
P(1)–Re(1)–O(1)
P(1)–Re(1)–O(2)
92.3(2)
91.5(4)
93.3(5)
Cl(1)–Re(1)–O(1)
Cl(1)–Re(1)–O(2) 100.2(5)
94.9(4)
O(1)–Re(1)–O(2)
P(1)–Re(1)–N(1)
O(2)–Re(1)–N(1)
P(1)–Re(1)–N(4)
O(2)–Re(1)–N(4)
Cl(2)–Re(2)–P(2)
P(2)–Re(2)–O(1)
P(2)–Re(2)–O(3)
Cl(2)–Re(2)–N(2)
O(1)–Re(2)–N(2)
Cl(2)–Re(2)–N(5) 173.1(6)
O(1)–Re(2)–N(5)
N(2)–Re(2)–N(5)
Re(1)–P(1)–C(31) 109.2(7)
Re(2)–P(2)–C(7) 111.3(8)
Re(2)–P(2)–C(19) 118.4(11)
163.9(6)
91.9(5)
84.6(7)
171.6(5)
95.0(7)
91.0(3)
91.2(5)
93.8(6)
87.5(5)
79.4(6)
Cl(1)–Re(1)–N(1) 173.4(6)
O(1)–Re(1)–N(1)
Cl(1)–Re(1)–N(4)
O(1)–Re(1)–N(4)
N(1)–Re(1)–N(4)
Cl(2)–Re(2)–O(1)
79.9(7)
87.5(5)
80.2(6)
87.6(7)
94.7(4)
C(7)
C(8)
C(9)
54(9)
Cl(2)–Re(2)–O(3) 101.2(5)
y386(9)
y742(10)
y633(12)
y226(10)
138(10)
285(8)
y210(9)
y334(10)
y1(9)
461(9)
620(10)
1023(10)
997(11)
1373(11)
1829(13)
1858(15)
1488(13)
2529(9)
2784
3155
3271
3015
2644
2675(8)
2996
3413
3510
3189
O(1)–Re(2)–O(3)
P(2)–Re(2)–N(2)
O(3)–Re(2)–N(2)
P(2)–Re(2)–N(5)
O(3)–Re(2)–N(5)
Re(1)–P(1)–C(25) 117.9(9)
Re(1)–P(1)–C(37) 110.9(8)
Re(2)–P(2)–C(13) 110.7(8)
Re(1)–O(1)–Re(2) 124.5(6)
Re(1)–N(1)–C(1)
Re(2)–N(2)–N(1)
Re(1)–N(4)–N(5)
Re(2)–N(5)–N(4)
163.3(6)
170.2(4)
95.9(7)
92.7(5)
84.4(7)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
C(26)
C(27)
C(28)
C(29)
C(30)
C(25)
C(32)
C(33)
C(34)
C(35)
C(36)
C(31)
C(38)
C(39)
C(40)
C(41)
C(42)
C(37)
79.4(6)
87.9(7)
Re(1)–N(1)–N(2)
119.2(14)
106.2(17)
134.8(17)
132.0(16)
132.9(16)
134.1(16) N(2)–N(1)–C(1)
118.1(12) Re(2)–N(2)–C(3)
118.1(12) Re(1)–N(4)–C(4)
119.4(12) Re(2)–N(5)–C(6)
two days. The white product was precipitated by pouring the
solution into ice–water.
The binuclear complex [Re2O3Cl2(PPh3)2(C3H2N3O2)2]
(3) was obtained in good yield from the rhenium starting
material [ReOCl2(OEt)(PPh3)2] and 2-nitropyrazole,
which were refluxed 12 h in ethanol in the absence of a depro-
tonating agent. The product 3 precipitates from this solution
as a pale green solid, which was recrystallized by slow
diffusion of pentane into a CH2Cl2 solution of 3.
2649
2367
1921
1759
865(9)
719
684
795
942
977
597(9)
391
524
862
3047
3965
3911
2939
369(21)
y401
224
1619
2388
1764
4802(28)
6092
7081
6779
The IR spectrum for 3 does not exhibit strong bands in the
800–1000 cmy1 region, assignable to n(Re_O); however,
medium intensity bands do appear at 972 and 818 cmy1. The
presence of the ligand is demonstrated by bands at 1508 and
2772
2500(10)
2406
2134
1956
1294 cmy1. The H NMR spectrum at room temperature
1
1068
936
5488
4500
2051
2323
exhibits a complex aromatic region, while the free ligand
gives only one peak at 8.37 ppm.
a EquivalentisotropicUdefinedasone thirdofthetraceoftheorthogonalized
Uij tensor.
The electrochemistry of 2-nitropyrazole displays an irre-
versible reduction at y1.086 V and a reversible reduction at

86
K.P. Maresca et al. / Inorganica Chimica Acta 260 (1997) 83–88
Fig. 1. A view of the structure [Re2O3Cl2(PPh3)2(C3H2N3O2)2]( 3) showing the atom-labeling scheme.
oxo group. The familiar {Re2O3} core has a m-oxo bridge
with a Re–O–Re angle of 124.7(6)8, most probably due to
the geometric constraints of the two bridging nitropyrazole
ligands. The overall structure of 3 consists of two corner-
sharing octahedra with each rhenium having a terminal oxo-
group, two nitrogen donors, chlorine donor, them-oxobridge,
and the phosphorous donor of a triphenylphosphine defining
the coordination sites. The structure resembles a ‘butterfly
complex’ with the two nitropyrazoles as the ‘wings’.
The bond lengths and angles observed for the {Re2O3}
core are compared to a variety of binuclear m-oxo rhenium
complexes in Table 4. In compounds bridged only by a
single m-oxo group such as [Re2O3Cl4(C5H5N)4] and
[Re2O3(CN)8], the Re–O–Re angle is essentially linear. In
contrast the angle decreases as the geometric constraints of
other bridging ligands are introduced, as illustrated by
[Re2O3(OMe)6], [Re2OCl5(C2H5CO2)(PPh3)2], andcom-
pound 3. The Re–O distance for the m-oxo bridge is approx-
Fig. 2. Schematic representation of the Re-coordination and of the bridge
geometry of 3. The carbon atoms of the Ph3P groups have been omitted for
clarity.
y2.066 V. The reduction process at y1.086 V is typical of
other nitro-containing ligands with potential radiosensitizing
properties including the substituted nitroimidazoles, fluori-
nated nitroazoles, and the extensively studied misonidazole
[16]. The electrochemistry of 3 exhibits a reversible one-
electron reduction at y0.547 V with DEps61 mV. The shift
to more positive potential observed for the metal-bound pyr-
azole in 3 compared to the free ligand is not unprecedented
[9]. The electrochemistry observed for nitro-containing
complexes has been shown to be strongly influenced by sol-
vent, nature of the supporting electrolyte, pH, and concentra-
tion. These factors make any direct comparisons between
reduction potentials for this class of compounds tenuous at
best.
˚
imately 1.90–2.00 A, while rhenium terminal oxo-group
˚
distances range from 1.65–1.75 A, as illustrated by the iodo
complexes [Re2O5I2(PPh3)2] and [Re2O5I2(PPh3)-
(OPPh3)]. The structural diversity of the dimers is reflected
by the numerous possible bridging coligands.
Asnoted, the terminal Re–Olengthsin3areunexceptional,
˚
˚
exhibiting distances of 1.69(2) A and 1.70(1) A for Re1 and
˚
Re2, respectively. The Re–N distances range from2.08(2) A
to 2.14(2) A, which is well within the range of distances
˚
observed for other examples of Re(V) complexes, as listed
in Table 5.
4. Conclusions
3.2. Description of structure
The structure of 3 reveals the presence of two rhenium(V)
The ease of preparation, high purity, reasonable yield, and
centers linked through two bridging nitropyrazoles and an
the presence of labile chlorine donors in 3 lead us to believe

K.P. Maresca et al. / Inorganica Chimica Acta 260 (1997) 83–88
87
Table 4
a
˚
Selected bond distances (A) and angles (8) for representative oxo-bridged Re complexes
b
Complex
Re–Ob
Re_Ot
Re–Ob–Re
X c
Ot_Re–Ob
Ref.
[Pt(NH3)4]2[Re2O3(CN)8][Re2O3(OMe)6]
1.9149(4)
1.917(13)
1.916(13)
2.031(6)
1.797(6)
2.079(9)
1.78(1)
1.896(11)
1.903(16)
1.943(16)
1.922(14)
1.955(13)
1.698(7)
1.690(11)
1.703(12)
1.670(7)
180.0
83.8(5)
O
179.5(8)
99.4(6)
99.8(6)
179.9(5)
104.5–117.8
175.9(4)
109.4–111.1
[17]
[18]
(OMe)
[Re2O5I2(PPh3)2]
164.3(4)
153.5(5)
O
O
[19]
[19]
[Re2O5I2(PPh3)(OPPh3)]
[Re2OCl(C2H5CO2){P(C6H5)3}2]
1.639(9)
83.8(5)
174.5(9)
Cl
O
[20]
[21]
1.764(16)
1.715(16)
1.688(16)
1.704(13)
165.5(7)
172.8(7)
163.4(6)
163.8(6)
[Re2O3Cl2(PPh3)2(C3H2N3O2)2]
124.7(6)
O
This work
a E.s.d.s in parentheses.
b Abbreviations: Ob'bridging oxo-group; Ot'terminal oxo-group.
c X'ligand trans to bridging oxo-group.
Table 5
a
˚
Selected bond distances (A) for representative Re(V)–oxo complexes
Complex
Re_O
Re–Cl
X b
Re–P
Y c
O
Ref.
[ReOCl2(PPh3) (acac)]
[Re2O2Cl4(PPh3)(salen)]
[ReOCl2(PPh3){Ph(O)–CNNCMe3}]
trans-[ReOCl2(PPh3)(Me–sal)]
cis-[ReOCl2(PPh3)(Me–sal)]
trans-[ReOCl3(PEt2Ph)2]
1.69(1)
2.376(7)
2.339(7)
2.367(3)
2.365(3)
2.344(4)
2.388(3)
2.375(2)
2.430(2)
2.355(3)
2.397(3)
2.41
Cl
Cl
N
N
N
P
Cl
Cl
N
P
Cl
O
Cl
O
O
O
O
N
N
2.431(4)
[22]
[23]
[24]
[25]
[25]
[26]
1.68(1)
1.68(1)
1.685(8)
2.466(4)
2.472(4)
2.472(2)
Cl
Cl
Cl
1.701(5)
1.660(8)
1.60
2.471(2)
2.485(3)
N
Cl
P
2.45
2.48
2.47
2.43
[Re2OCl5(C2H5CO2)(PPh3)2]
[Re2O3Cl2(PPh3)(C3H2N3O2)2]
2.318(4)
2.302(5)
2.340(5)
2.371(5)
2.349(6)
2.350(6)
2.474(4)
2.506(4)
Cl
Cl
[20]
1.688(16)
1.704(13)
2.476(6)
2.475(7)
N
N
This work
a E.s.d.s in parentheses.
b X'donor group trans to Cl.
c Y'donor group trans to P.
that 3 is a useful binuclear rhenium starting material in devel-
oping a system of rhenium organo-nitro derivatives for
medicinal applications. The potential for exploiting186Reand
188Re in nuclear medical applications is particularlyattractive
in view of its half-life (90 h) and strong b emission
(bmax.s1070 keV), which enable this isotope to deliver
high radiation doses to tissues [27]. The chlorine ligands
should allow for facile substitution to occur, although early
attempts to crystallize derivatives have been unsuccessful to
date. Ligand substitution for chlorine provides the means to
study influence of coligands on the electrochemical behavior
and biodistribution patterns of the derivatives [28].
ples employ later transition elements [29,30]. This may
allow for the development of early transitionelementhypoxic
marker derivatives. Moreover, the combination of a radio-
sensitizing agent plus a radioactive element in the same mol-
ecule could allow for improved hypoxic tumor treatment.
Acknowledgements
The research was funded by a grant from the Department
of Energy Office of Health and Environmental Research
under grant DE-FG02-93ERG1571.
We have successfully incorporated a nitropyrazole into a
Group 7 transition metal complex whereas all previousexam-

88
K.P. Maresca et al. / Inorganica Chimica Acta 260 (1997) 83–88
[16] Y. Shibamoto, S. Nishimoto, K. Shimokawa, Y. Hisanaga, B. Zhou, J.
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