Bidentate phosphinophenol ligand: Synthesis and characterization of oxo and phenylimido rhenium(V) complexes of 2-diisopropylphosphinophenol

Article abstract of DOI:10.1016/S0277-5387(00)00360-0

Six-coordinate, monosubstituted and disubstituted phosphinophenol complexes ReYX₂ (PPh₃) (P ~ O) and ReYX (P ~ O)₂ (Y = O, NPh; X = Cl, Br, I) were obtained by reacting 2-diisopropylphosphinophenol (P ~ OH) with ReOX₃(PPh₃)₂ and Re(NPh)X₃(PPh₃)₂ in toluene. A mixture of the monosubstituted and disubstituted complexes ReYX₂(PPh₃) (P ~ O) and ReYX(P ~ O)₂ was precipitated when the reactions were performed with a 2:1 ligand to metal ratio. Excess ligand (4 equiv.) was needed to obtain quantitatively the disubstituted complexes. ¹H and ³¹P{¹H} NMR show that all monosubstituted and disubstituted complexes exist as single isomers with a trans-P,P 'twisted' octahedral geometry, the site trans to the multiple bond being filled by the oxygen donor of one P ~ O ligand. The compounds were also characterized by microanalysis, IR and mass spectrometry. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00360-0

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1111–1116
Bidentate phosphinophenol ligand: synthesis and characterization
of oxo and phenylimido rhenium(V) complexes
of 2-diisopropylphosphinophenol
a
a
a,
b
´ ´
`
*
Frederique Loiseau , Yolande Lucchese , Michele Dartiguenave , Yvon Coulais
a
´
Laboratoire de Chimie Inorganique, Universite Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France
Laboratoire Traceurs et Traitement de l’Image, Hopital Purpan, Universite Paul Sabatier, Place du Dr Baylac,
31059 Toulouse, France
b
ˆ
´
Received 20 January 2000; accepted 8 March 2000
Abstract
Six-coordinate, monosubstituted and disubstituted phosphinophenol complexes ReYX2(PPh3)(P;O) and ReYX(P;O)2 (YsO, NPh;
XsCl, Br, I) were obtained by reacting 2-diisopropylphosphinophenol (P;OH) with ReOX3(PPh3)2 and Re(NPh)X3(PPh3)2 in toluene.
A mixture of the monosubstituted and disubstituted complexes ReYX2(PPh3)(P;O) and ReYX(P;O)2 was precipitated when thereactions
were performed with a 2:1 ligand to metal ratio. Excess ligand (4 equiv.) was needed to obtain quantitatively the disubstituted complexes.
1H and 31P{1H} NMR show that all monosubstituted and disubstituted complexes exist as single isomers with a trans-P,P ‘twisted’ octahedral
geometry, the site trans to the multiple bond being filled by the oxygen donor of one P;O ligand. The compounds were also characterized
by microanalysis, IR and mass spectrometry.
q2000 Elsevier Science Ltd All rights reserved.
Keywords: Phosphinophenols; Rhenium
1. Introduction
Peq,Pax ‘twisted’ octahedral arrangement A with an O_Re–
O linkage (Scheme 1), while the trans-Peq,Pax arrangement
B with a PhN_Re–O linkage was observed for Re-imido
species. Thus these ligands favour the formation of a single
isomer, which is essential for radiopharmaceutical
applications.
In contrast to the relatively abundant literature on the reac-
tions of Ph2PC6H4OH ligands on rhenium precursors [4–8],
to our knowledge the reactivity of dialkylphosphinophenols
has not yet been investigated. This may be due in part to the
increased reductive character of the alkylphosphines which
are known to stabilize Re(III) complexes. In order to deter-
mine the influence of an alkyl substituent on the phosphino-
phenol reactivity, we have studied the reaction of
2-(diisopropylphosphino)phenol (P;OH) on ReOX3-
Because of their chemical and structural diversity and their
good coordinative properties, functionalized phosphines
have attracted considerable attention in strategies aimed at
developing new chelatesfor metallicradioisotopestodevelop
imaging (technetium) and therapeutic (rhenium) agents in
nuclear medicine.
In the recent past, studies have been conducted on the
reaction of ReOCl3(PPh3)2, Re(NPh)Cl3(PPh3)2 and
[TcO4]y with various diphenylphosphines functionalized
with nucleophilic substituents such as Ph2P–Z–CO2H
(ZsCH2, CH2–CH2, o-C6H4) [1,2], Ph2PCH_CPh(OH)
[3], Ph2PC6H4OH [4–8], Ph2PCH2C6H4OH [9] and
Ph2PC6H4OSiMe3 [10], i.e. bidentate ligands with soft phos-
phorus(III) donor and hard carboxo, hydroxo or phenoxo
groups. Although several configurations arepossibleforocta-
hedral complexes bearing three different ligands, most of the
previously reported oxo and imido complexes adopt the cis-
* Corresponding author. Tel.: q33-561-556 121; fax: q33-561-556 118;
e-mail: dartigue@iris.ups-tlse.fr
Scheme 1.
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved.
PII S0277-5387(00)00360-0
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3
(PPh3)2 and Re(NPh)X3(PPh3)2 (XsCl, Br, I) by com-
parison with Ph2P;OH. Isopropyl substituents increase the
steric hindrance of the phosphine (cone angle of 1378 for
iPr2P;OH compared to 1278 for Ph2P;OH) [11] and its
basicity (pKa(PPh3)s2.73; pKa(PiPr3)s9.30)) [12]. We
observetheformationofRe(V)complexeswithoutreduction
of the metal centre whose structures are controlled by the
steric hindrance of the ligand.
Ph, JHHs7 Hz); 7.03 (m, 9H, Ph); 8.04 (ddd, 6H, Ph,
3JHHs8 Hz, 4JHHs1 Hz, 3JHPs11 Hz).
2. Anal. Calc. for C24H36ClO3P2Re: C, 43.93; H, 5.53; Cl,
5.40; P, 9.44; Re, 28.38. Found: C, 48.85; H, 5.56; Cl, 6.55;
P, 9.19; Re, 26.13%. IR (CsI): 962 cmy1, n(Re_O). MS
(DCI/NH3): m/z 621 [MyHClqH]q. 31P{1H} NMR
(CDCl3, 161.98 MHz, ppm): 34.0; 41.6 (dd, 2JPPs213 Hz).
1H NMR (CDCl3, 400.14 MHz, ppm): 1.4 (dd, 12H, CH3,
3JHHs7 Hz, 3JHPs17 Hz); 1.7 (dd, 12H, CH3, 3JHHs7 Hz,
3JHPs17 Hz); 2.8 (d of septets, 1H, CH, JHHs7 Hz,
3
2JHPs1 Hz); 3.0 (d of septets, 2H, CH, 3JHHs7 Hz, 2JHPs1
2. Experimental
3
2
Hz); 3.4 (d of septets, 1H, CH, JHHs7 Hz, JHPs1 Hz);
6.3–7.5 (m, 8H, Ph).
2.1. Reagents and physical measurements
ReOBr2(PPh3)(P;O) (3) and ReOBr(P;O)2 (4). The
same method was used as for 1 and 2, starting from 0.380 g
(1.8 mmol) of P;OH and 0.873 g (0.9 mmol) of
ReOBr3(PPh3)2, stirred in 30 ml of refluxing toluene under
N2 for 46 h.
All reactions were carried out in a nitrogen atmosphere
using standard Schlenk techniques. Ether and toluene were
dried over sodium benzophenone ketyl and distilled prior to
use. Ethanol and acetonitrile were distilled on molecular
sieves (3 A). ReOX3(PPh3)2 [13,14], Re(NPh)X3(PPh3)2
[15], and 2-diisopropylphosphinophenol [16] were pre-
pared by literature methods.
3. Greensolid, yield9%. Anal. Calc.forC30H33Br2O2P2Re:
C, 43.23; H, 3.99. Found: C, 44.27; H, 3.58%. IR (CsI): 969
cmy1, n(Re_O). MS (DCI/NH3): m/z 835 [MqH]q, 755
[MyHBrqH]q. 31P{1H} NMR (CDCl3, 161.98 MHz,
˚
Infrared spectra (4000–200 cmy1) were recorded as CsI
pellets on a Bruker Vector 22 spectrometer. 1H, 13C{1H} and
31P{1H} NMR spectra were recorded at room temperature in
CD2Cl2, CDCl3 or C6D6 on Bruker ARX 400 and AMX 300
WB spectrometers, using SiMe4 as internal reference (1H,
13C) and H3PO4 (82%/D2O) as external reference (31P).
All chemical shifts are reported in ppm. Desorption chemical
ionization using NH3 (DCI/NH3) was recorded on a NER-
MAG R 1010 mass spectrometer. Elemental analyses were
run at the Laboratoire de Chimie de Coordination du CNRS,
Toulouse, France.
2
1
ppm): 8.0; 26.1 (dd, JPPs260 Hz). H NMR (CDCl3,
3
400.14 MHz, ppm): 1.55 (dd, 6H, CH3, JHHs7 Hz,
3JHPs16 Hz); 1.64 (dd, 6H, CH3, JHHs7 Hz, JHPs16
Hz); 3.62 (d of septets, 2H, CH, 3JHHs7 Hz, 2JHPs2 Hz);
5.9–7.8 (m, 19H, Ph). L(CH3CN): 30.3 Vy1 cm2 My1
(molecular).
3
3
4. Green solid, yield 50%. Anal. Calc. for
C24H36BrO3P2Re: C, 41.15; H, 5.18. Found: C, 41.40; H,
4.92%. IR (CsI): 964 cmy1, n(Re_O). MS (DCI/NH3):
m/z 701 [MqH]q, 621 [MyHBrqH]q. 31P{1H} NMR
(CDCl3, 161.98 MHz, ppm): 34.1, 41.7 (dd, 2JPPs212 Hz).
1H NMR (CDCl3, 400.14 MHz, ppm): 1.1–1.7 (dd, 24H,
CH3, 3JHHs7 Hz, 3JHPs16 Hz); 2.8–3.1 (m, 4H, CH); 6.3–
7.5 (m, 8H, Ph). L(CH3CN): 15.1 Vy1 cm2 My1
(molecular).
ReOI2(PPh3)(P;O) (5) and ReOI(P;O)2 (6). The
same method was used as for 1 and 2, starting from the
reaction of 0.45 g (2.14 mmol) of P;OH and 1.185 g (1.07
mol) of ReOI3(PPh3)2 in refluxing toluene under N2 for 46
h. Concentration of the solution gave an oily solid. Work-up
with diethyl ether and cooling the solution gave a mixture of
5 (yield 27%) and 6 (yield 9%) as a dark brown solid, which
was filtered off and dried in vacuo. Purification by column
chromatography as for the Cl and Br analogues gave 5 and 6
as pure compounds.
2.2. Synthetic work
ReOCl2(PPh3)(P;O) (1) and ReOCl(P;O)2 (2). The
two complexes were obtained from the reaction of P;OH
(0.373 g, 1.77 mmol) and ReOCl3(PPh3)2 (0.739 g, 0.89
mmol) in refluxing toluene under N2 for 35 h. Concentration
of the solution in vacuo produced a green solid. Work-up
with diethyl ether and cooling of the solution gave a mixture
of 1 and 2. Purification by column chromatography using
silica as support gave 1 with chloroform and 2 with diethyl
ether as eluents. Recrystallization from diethyl ether at
y208C gave 1 (yield 10%) and 2 (yield 43%) as green
crystals.
1. Anal. Calc. for C30H33Cl2O2P2Re: C, 48.39; H, 4.47.
Found: C, 47.82; H, 4.78%. IR (CsI): 970 cmy1, n(Re_O).
MS (DCI/NH3): m/z 745 [MqH]q, 709 [MyHClqH]q.
31P{1H} NMR (C6D6, 161.98 Hz, ppm): 7.1; 24.7 (dd,
5. Brown solid, yield 27%. Anal. Calc. for C30H33I2-
O2P2Re: C, 38.85; H, 3.59; I, 27.36. Found: C, 38.07; H,
3.28; I, 26.94%. 31P NMR (CDCl3, 161.98 MHz, ppm): 6.9;
2
1
2JPPs262 Hz). H NMR (C6D6, 400.14 MHz, ppm): 1.36
17.4 (dd, JPPs257 Hz). MS (DCI/NH3): m/z 929
[MqH]q.
3
3
(dd, 6H, CH3, JHHs7 Hz, JHPs15 Hz); 1.50 (dd, 6H,
CH3, 3JHHs7 Hz, 3JHPs17 Hz); 3.33 (d of septets, 2H, CH,
6. Brown solid, yield 9%. Anal. Calc. for C24H36IO3P2Re:
C, 38.56; H, 4.85; I, 16.97. Found: C, 38.17; H, 4.60; I,
16.12%. 31P{1H} NMR (CDCl3, 161.98 MHz, ppm): 33.1;
2
3
3JHHs7 Hz, JHPs2 Hz); 6.22 (dd, 1H, Ph, JHHs7 Hz,
4JHPs4 Hz); 6.56 (dd, 1H, Ph, 3JHHs7 Hz); 6.93 (dd, 2H,
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2
42.4 (dd, JPPs210 Hz). MS (DCI/NH3): m/z 749
[MyHIqH]q. 31P{1H} NMR (CDCl3, 161.98 MHz,
ppm): 31.8; 33.3 (dd, 2JPPs217 Hz).
[MqH]q.
Re(NPh)Cl2(PPh3)(P;O) (7) and Re(NPh)Cl-
(P;O)2 (8). The same method was used as for 1 and 2,
starting from the reaction of P;OH (0.47 g, 2.24 mmol)
and Re(NPh)Cl3(PPh3)2 (1.02 g, 1.12 mmol) in refluxing
toluene (40 ml) under N2 for 37 h.
3. Results and discussion
3.1. Synthesis
7. Beige solid, yield 45%. Anal. Calc. for C36H38Cl2-
NOP2Re: C, 52.75; H, 4.67; N, 1.71. Found: C, 51.91; H,
4.73; N, 1.78%. 31P{1H} NMR (CD2Cl2, 161.98MHz,ppm):
y5.2; 27.7 (dd, 2JPPs260 Hz). MS (DCI/NH3): m/z 820
[MqH]q.
iPr2PC6H4OH (P;OH) reacts readily, in refluxing tolu-
ene, on ReOX3(PPh3)2 and Re(NPh)X3(PPh3)2 but the
reactions are sensitive to the ligand:metal ratio and care must
be taken to use carefully dried and degassed solvents to pre-
vent phosphine oxidation.
A 4:1 ligand to metal ratio gives quantitatively the disub-
stituted ReYX(P;O)2 complexes (YsO, NPh; XsCl, Br,
I). However, when the reaction is performed in a stoichio-
metric ratio (L:Ms2:1), a mixture of monosubstituted and
disubstituted ReYX2(PPh3)(P;O) and ReYX(P;O)2
complexes is obtained; the ratio ReYX(P;O)2/ReYX2-
(PPh3)(P;O) decreases as the size of the halide increases.
Such a mixture of complexes was not observed with diphen-
ylphosphinophenol whose reactions were stoichiometric,
indicating that the basicity of the phosphine is not the main
factor governing the reaction.
In an effort to follow the reaction, to detect intermediate
compounds and to confirm that chelation of the phosphine
ligand occurs with the concomitant release of both PPh3
and Cl, we monitored the reaction of P;OH with
ReOCl3(PPh3)2 by 31P NMR (Fig. 1). The spectrum after 5
h reveals the presence of all of the expected species. Besides
free PPh3 (singlet at y4 ppm), the major species is
ReOCl2(PPh3)(P;O) (1) (pair of doublets at 7 and 25
ppm, 2JPPs262 Hz), followed by ReOCl(P;O)2 (2) (pair
of doublets at 34 and 42 ppm, 2JPPs213 Hz) (vide infra).
A third pair of doublets at 20.5 and 31 ppm (2JPPs220 Hz)
could be assigned to the trans-P,P-trans-O,O complex
ReOCl2(P;OH)(P;O) containing a unidentate P;OH
ligand. The remaining signals, a pair of unresolved peaks at
21.2 and 23.2 ppm and a singlet at 27.2 ppm, which disappear
quickly, could correspond to the thermodynamicallyunstable
cis-P,P ReOCl(P;O)2 and the trans-P,P ReOCl3(P;OH)2
species, respectively. After an extra 10 h under reflux, the
spectrum shows free PPh3, ReOCl2(PPh3)(P;O) (1),
ReOCl(P;O)2 (2) and ReOCl2(P;OH)(P;O). 20 h
later, the spectrum has become remarkably simple: besides
free PPh3, only ReOCl2(PPh3)(P;O) (minor component)
and ReOCl(P;O)2 (major component) are observed. Thus,
ReOCl(P;O)2 is the thermodynamic product, since no fur-
ther changes occur as heating is continued. Interestingly,
under the same experimental conditions, using Ph2PC6H4OH
as ligand produced quantitatively ReOCl(P;O)2.
8. Beige solid, yield 10%. Anal. Calc. for C30H41Cl-
NO2P2Re: C, 49.27; H, 5.65; N, 1.92. Found: C, 48.95; H,
5.90; N, 1.72%. 31P{1H} NMR (CD2Cl2, 161.98MHz,ppm):
2
30.7; 33.3 (dd, JPPs224 Hz). MS (DCI/NH3): m/z 732
[MqH]q.
Re(NPh)Br2(PPh3)(P;O) (9) and Re(NPh)Br-
(P;O)2 (10). The same method was used as for 7 and 8,
starting from 0.55 g (2.6 mmol) of P;OH and 1.362 g (1.3
mmol) of Re(NPh)Br3(PPh3)2 in 35 ml of toluene. 9 yield
56%, and 10 yield 14%.
9. Anal. Calc. for C36H38Cl2NOP2Re: C, 47.59; H, 4.22;
N, 1.54. Found: C, 46.95; H, 4.08; N, 1.42%. MS (DCI/
NH3): m/z 910 [MqH]q. 31P{1H} NMR (C6D6, 161.98
MHz, ppm): y6.3; 24.1 (dd, 2JPPs246 Hz).
10 was obtained in 30% yield by refluxing a mixture of
0.355 g (1.7 mmol) of P;OH and 0.44 g (0.42 mmol) of
Re(NPh)Br3(PPh3)2 in 20 ml of toluene for 46 h. Concen-
tration of the solution gave a brown precipitate, which was
recrystallized from diethyl ether (20 ml). Cooling the solu-
tion at y208C gave 10 as a brown solid which was filtered
off, washed with diethyl ether and dried in vacuo. Anal. Calc.
for C30H41BrNO2P2Re: C, 46.45; H, 5.33; N, 1.81. Found: C,
46.50; H, 4.77; N, 1.64%. IR (CsI): 1026 cmy1, n(Re_O).
MS (DCI/NH3): m/z 776 [MqH]q. 31P{1H} NMR (C6D6,
161.98 MHz, ppm): 30.8; 33.1 (dd, 2JPPs222 Hz). 1H NMR
(C6D6, 400.14 MHz, ppm): 0.7–1.6 (dd, 24H, CH3, 3JHHs7
3
3
Hz, JHPs15 Hz); 2.5–3.4 (d of septets, 4H, CH, JHHs7
Hz, 2JHPs1 Hz); 6.6–7.8 (m, 13H, Ph).
Re(NPh)I2(PPh3)(P;O) (11) and Re(NPh)I(P;O)2
(12). The complexes were synthesized as for 9 and 10, with
Re(NPh)I3(PPh3)2 (1.237 g, 1.05 mmol) and P;OH (0.44
g, 2.09 mmol) in toluene (35 ml) for 47 h. Work-up with
ether and purification by column chromatography with
CHCl3 and ether as eluents and recrystallization from diethyl
ether at y208C gave 11 (yield 54%) and 12 (yield 13%) as
brown microcrystalline compounds.
11. Anal. Calc. for C36H38I2NOP2Re: C, 43.12; H, 3.82;
N, 1.40; I, 25.31. Found: C, 41.80; H, 3.75; N, 1.35; I,
25.05%. MS (DCI/NH3): m/z 1004 [MqH]q. 31P{1H}
NMR (CDCl3, 161.98 MHz, ppm): y6.3; 24.1 (dd,
2JPPs246 Hz).
12. Anal. Calc. for C30H41INO2P2Re: C, 43.80; H, 5.02;
N, 1.70; I, 15.43. Found: C, 41.92; H, 4.93; N, 1.65; I,
15.28%. MS (DCI/NH3): m/z 824 [MqH]q; 696
The corresponding bromo and iodo complexes give the
same final products when the reaction is run under identical
conditions, starting from ReOBr3(PPh3)2 and ReOI3-
(PPh3)2, respectively. ReOBr2(PPh3)(P;O) (3) and
ReOBr(P;O)2 (4) form in roughly equal amounts, whereas
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Table 1
31P{1H} NMR data
Complex
Solvent d (ppm)
JPP (Hz)
ReOCl2(PPh3)(P;O) (1)
ReOCl(P;O)2 (2)
ReOBr2(PPh3)(P;O) (3)
ReOBr(P;O)2 (4)
ReOI2(PPh3)(P;O) (5)
ReOI(P;O)2 (6)
C6D6
7.1, 24.7 dd 262
34.0, 41.6 dd 213
8.0, 26.1 dd 260
34.1, 41.7 dd 212
6.9, 17.4 dd 257
33.1, 42.4 dd 210
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
Re(NPh)Cl2(PPh3)(P;O) (7) CD2Cl2 y5.2, 27.7 dd 260
Re(NPh)Cl(P;O)2 (8) CD2Cl2 30.7, 33.3 dd 224
Re(NPh)Br2(PPh3)(P;O) (9) C6D6
Re(NPh)Br(P;O)2 (10) C6D6
y5.8, 26.3 dd 254
30.8, 33.1 dd 222
Re(NPh)I2(PPh3)(P;O) (11) CDCl3 y6.3, 24.1 dd 246
Re(NPh)I(P;O)2 (12)
CDCl3
31.8, 33.3 dd 217
Scheme 2.
monodentate halide. Besides the main ligand vibrations, all
complexes exhibit a Re_O stretching band in the 953–970
cmy1 range, typical of multiply bonded Re_O complexes
[17].
31P NMR is the most convenient method to determine the
geometry of these complexes. The 31P{1H} NMR data are
collected in Table 1. The ligand signals have moved down-
field appreciably from the free ligand values (P;OH, y24
ppm) upon coordination, owing to the strong acidic character
of the Re(V) centres and the downfield shifts associatedwith
the presence of phosphine in five-membered chelate rings
[18].
Fig. 1. Monitoring by 31P{1H} NMR of the reaction of P;OH on
ReOCl3(PPh3)2 (2:1 ratio) in refluxing toluene. Spectra at ts5 h (top)
(Bruker AMX 300WB), 15 h (middle) and 35 h (bottom) (Bruker ARX
400). (l) free PPh3;q, trans-P,P ReOCl2(PPh3)(P;O) (1); (a) trans-
P,P ReOCl(P;O)2 (2); (*), trans-P,PReOCl2(P;OH)(P;O);(s)cis-
P,P ReOCl(P;O)2; (S), trans-P,P ReOCl3(P;OH)2).
The 31P NMR spectrum (in C6D6) of ReOCl2(PPh3)-
(P;O) (1) consists of two doublets centred at 24.7 and 7.1
ppm, respectively, with a coupling constant of 262 Hz typical
of a trans-P,P arrangement. In the structure proposed
(Scheme 2), the very stable trans-[O_Re–O–] core is
assumed to be retained. The downfield signal corresponds to
the ring Pax atom as confirmed by the exchange of PPh3 with
pyridine. When pyridine is added to a C6D6 solution of 1 in
an NMR tube, the AB signal of 1 disappears within 2 h and
the trans-P,P coupling is lost: new singlets appear at y4 ppm
for free PPh3 and at 19 ppm for a new pyridine complex.
Thus, the doublet at 7.1 ppm for 1 corresponds to the labile
PPh3 ligand, which is stereoselectively substituted by pyri-
dine. The upfield shift from 24.7 to 19 ppm experienced by
the ring P atom is consistent with the substitution of PPh3 by
pyridine, a stronger s donor and weaker p acceptor. The
stereochemistry of 1 is confirmed by the 1H NMR spectrum.
The two pairs of CH3 groups appear as doublets of doublets
ReOI2(PPh3)(P;O) (5) is predominant over ReOI-
(P;O)2 (6) (;4:1 ratio). This can be ascribed to the
increasing size of the halogen.
The reaction of P;OH with Re(NPh)X3(PPh3)2 follows
a similar pattern: a 2:1 ratio leads to a mixture of the mono-
substituted and disubstituted species, whereas a 4:1 ratio is
needed to obtain Re(NPh)X(P;O)2.
In all cases, the monosubstituted and disubstituted com-
plexes are successfully separated by column chromatography
on silica, using CHCl3 as eluent in one case and ether in the
other, and recrystallized from ether.
3.2. Characterization of the compounds
3.2.1. Rhenium(V) oxo complexes
Mass spectrometry confirms the mononuclear formulation
based on elemental analysis: the DCI/NH3 spectra all exhibit
the [MqH]q parent peak. In severalcases,[MyHXqH]q
peaks due to halide loss are also observed, suggesting that
the bidentate P;O ligand is more tightly bound than the
3
at 1.36 ppm (3JHHs7 Hz, JHPs15 Hz) and 1.50 ppm
3
(3JHHs7 Hz, JHPs17 Hz) respectively, shifted slightly
from the free ligand value. The two methine protons give a
doublet of septets at 3.33 ppm (3JHHs7 Hz, 2JHPs2 Hz). A
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1115
similar multiplet is present at 2.14 ppm (3JHHs7 Hz, 2JHPs2
Hz) in the free ligand indicating that the signal has moved
downfield on coordination, but that the magnetic equivalence
of the two protons is not affected by the bonding to rhenium.
This is consistent with the isomer containing the trans-
[O_Re–O] core, since the other isomers would contain non-
equivalent isopropyl groups and, as a consequence, probably
give split methine signals. The aromatic protons give rise to
five multiplets in the 6.2–8.0 ppm range.
replaced by two singlets, one at y4 ppm for free PPh3 and
the other at 22 ppm for the new pyridine complex.
4. Concluding remarks
All Re(V) complexes prepared in this study contain phos-
phinophenolato ligands in the bidentate anionic P;Oy form,
with one phenolato oxygen atom trans to the multiple bond.
It has been shown previously that the disubstituted
ReOX(Ph2P;O)2 complexes with a cis-P,P ‘twisted’ octa-
hedral structure were the only productsisolatedwhen2equiv.
of Ph2P;OH were reacted on ReOCl3(PPh3)2 in refluxing
The 31P NMR spectrum of ReOCl(P;O)2 (2) consists
of a pair of doublets (AB spin system) with signals centred
at 34.0 and 41.6 ppm, and a 2JPP value of 213 Hz, consistent
with two equivalent P atoms occupying trans positions in a
‘twisted’ octahedron (B, Scheme 1). The structure is differ-
ent to that obtained for diphenylphosphinophenol complexes
where the 31P NMR showed two non-equivalent P atoms
occupying cis positions in a ‘twisted’ octahedron(A, Scheme
1), as observed in the crystal structure of ReOCl(Ph2P;O)2
[8]. The trans-P,P structure is consistent with the 1H NMR
data which show the inequivalence of all the protons of the
two ligands. The two methyl signals of each ligand, corre-
sponding to 12H, are observed asdoubletsof doubletscentred
i
toluene. However, under the same conditions, Pr2P;OH
leads to monosubstituted ReOX2(PPh3)(P;O) complexes,
along with ReOX(P;O)2. Therefore chelation of one
P;OH ligand takes place easily but binding the second
ligand is more difficult, observations that suggest that
although P;OH is a stronger s donor, its reactivity is ham-
pered by its greater steric demand. The fact that the trans-
P,P ‘twisted’ octahedral structure is favoured for all
ReOX(P;O)2 complexes indicates that the electronic effect
is not the factor determining the stereochemistry of the com-
plexes and, as a consequence, that the steric effect is impor-
tant. The reaction of the phenylimido precursor with P;OH
does not follow the same trends as the oxo precursor since
both ligands give trans-P,P isomers. A possible explanation
is that the imido phenyl group increases steric hindrance as a
whole even for Ph2P;OH.
3
at 1.4 and 1.7 ppm, respectively, with JHPs17 Hz and
3JHHs7 Hz shifted slightly downfield from the values of 0.9
and 1.1 ppm found in the free ligand. Doublets of septets at
2.8 (1H), 3.0 (2H) and 3.4 (1H) ppm, with coupling con-
stants 2JHPs1 Hz and 3JHHs7 Hz, are due to the four non-
equivalent methine hydrogens that have moved downfield
upon coordination, but they are not individually affected by
the low symmetry of the complex. Eight multiplets between
6.3 and 7.5 ppm originate from the aromatic protons.
Thus, the thermodynamic stability and the structural vari-
ety of the Re(V) complexes with P;O ligands, together
with their presence as a single isomer, suggest that it is worth
pursuing the development of these molecules as candidates
for eventual applications as radiopharmaceuticals.
3.2.2. Rhenium(V) imido complexes
The phenylimido complexes 7–12 are stable in the solid
state and in organic solvents where they are soluble (DMSO,
DMF, CH2Cl2, CHCl3, THF). The formula proposed is based
on microanalyses and mass spectra (DCI/NH3), in which
the parent [MqH]q peaks and [MyHXqH]q peaks cor-
responding to halide loss are observed. The Re_NPh stretch
of phenylimido-Re(V) complexes, difficult to detect in the
IR because of overlapping ligand vibrations, is only clearly
identified at 1025 cmy1 for 9, in agreement with the literature
[17].
Acknowledgements
`
¸
This work was supported by The Ministere Francais de
´
l’Enseignement Superieur, de la Recherche et de la Techno-
logie and the Centre National de la Recherche Scientifique
(MD) which are gratefully acknowledged.
The trends observed above for the oxo systems apply to
the imido complexes as well. The 31P NMR spectra of the
Re(NPh)X(P;O)2 complexes (XsCl, (8); XsBr, (10);
XsI, (12)) in CDCl3 all contain doublets at ;31 and ;33
ppm with JPP coupling constants of ;220 Hz, consistentwith
trans-P,P ‘twisted’ octahedral species. The Re(NPh)X2-
(PPh3)(P;O) compounds 7, 9 and 11 also adopt a trans-
P,P structure, since their 31P NMR spectra show doublets
around y6 and 26 ppm (2JPP of 246–260 Hz). By analogy
with ReOCl2(PPh3)(P;O), the peak at ;26 ppm is
assigned to the P;O ligand on the basis of PPh3 exchange.
When pyridine is added to a solution of Re(NPh)-
Br2(PPh3)(P;O), the doublets at y6 and 26 ppm are
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