Technetium and rhenium heterocomplexes containing the diphenylphosphinoferrocenyl fragment

Article abstract of DOI:10.1016/S0022-328X(01)00917-2

Reactions of dppf with labile technetium and rhenium precursors such as [M(N)Cl₄]^- and [M(NPh)Cl₃(PPh₃)₂] yield the monomeric mono-substituted [M(N)Cl₂(dppf)] and [M(NPh)Cl₃(dppf)] complexes, where the diphosphinoferrocenyl fragment coordinates on the equatorial plane of a distorted square pyramid or of a distorted octahedron, respectively.

Full text of DOI:10.1016/S0022-328X(01)00917-2

Journal of Organometallic Chemistry 637–639 (2001) 772–776
www.elsevier.com/locate/jorganchem
Note
Technetium and rhenium heterocomplexes containing the
diphenylphosphinoferrocenyl fragment
Francesco Tisato a,*, Fiorenzo Refosco a, Marina Porchia a, Giuliano Bandoli b,
Giuseppe Pilloni c, Licia Uccelli d, Alessandra Boschi d, Adriano Duatti d
a Istituto di Chimica e Tecnologie Inorganiche e dei Materiali A6anzati, Consiglio Nazionale delle Ricerche, Corso Stati Uniti 4,
35127 Padua, Italy
b Dipartimento di Scienze Farmaceutiche, Uni6ersita` di Pado6a, Via Marzolo, 5, 35131 Padua, Italy
c Dipartimento Chimica Inorganica, Metallorganica e Analitica, Uni6ersita` di Pado6a, Via Marzolo, 1, 35131 Padua, Italy
d Laboratorio di Medicina Nucleare, Dipartimento di Medicina Clinica e Sperimentale, Uni6ersita` di Ferrara, Via Borsari, 46, 44100 Ferrara, Italy
Received 2 January 2001; received in revised form 15 March 2001; accepted 19 March 2001
Abstract
Reactions of dppf with labile technetium and rhenium precursors such as [M(N)Cl4]− and [M(NPh)Cl3(PPh3)2] yield the
monomeric mono-substituted [M(N)Cl2(dppf)] and [M(NPh)Cl3(dppf)] complexes, where the diphosphinoferrocenyl fragment
coordinates on the equatorial plane of a distorted square pyramid or of a distorted octahedron, respectively. © 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Technetium; Rhenium; Nitrido complexes; Mixed ligand complexes; Diphenylphosphinoferrocene; Crystal structures
1. Introduction
microscopic level (nanomolar concentrations or ‘non-
carrier added’ NCA), are performed by utilizing the
99gTc isotope (a soft b-emitter with t1/2=2.12×105
year, Eb=292 keV) and the ‘cold’ natural isotopic
mixture of 185Re and 187Re.
Radiopharmaceuticals containing the g-emitting iso-
tope 99mTc continue to occupy a prominent role in
diagnostic nuclear medicine owing to the favorable
nuclear properties (Eg=142 keV, t1/2=6.02 h) and the
availability of this radionuclide. In the last two decades,
dramatic advancements of the studies of the basic
chemistry of technetium complexes have led to the
introduction of the clinical application of useful tracers
for heart, brain and kidneys [1–3]. More recently, the
advent of the two b-emitters 186Re (t1/2=3.8 day,
In the 1990s a new concept has emerged in the design
of substitution-inert technetium and rhenium agents.
Thus, an essential requirement is generating a robust
‘metal-fragment’, to which small bifunctional ligands
bearing specific pharmacophores can be eventually co-
ordinated. The metal-fragment contains the ion stabi-
lized in an appropriate oxidation state by means of a
suitable ligand framework, which only partially fills the
coordination sphere. Examples of application of this
strategy include the recently developed low-valent
Tc-tricarbonyl [TcI(CO)3]+ [6] and the high-valent ‘su-
per-nitrido’ [TcV(N)(PNP)]2+ [7] (PNP=tridentate
aminediphosphine ligand) fragments. On the opposite
side of the molecule, these substitution-inert moieties
accommodate labile ligands, i.e. water molecules and/or
related hydroxyl functions or halide groups. These
Ebmax=1.07 MeV) and 188Re (t1/2=0.7 day, Ebmax
=
2.11 MeV) in the therapeutic field [4,5] has made the
third-row congener rhenium studies attractive. Synthe-
sis at the macroscopic level (millimolar concentration
or ‘carrier added’, CA), devoted at the elucidation of
the molecular structure of the agents injected in vivo at
* Corresponding author. Tel.: +39-049-8295958; fax: +39-049-
8702911.
E-mail address: francesco.tisato@ictr.pd.cnr.it (F. Tisato).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00917-2

F. Tisato et al. / Journal of Organometallic Chemistry 637–639 (2001) 772–776
773
monodentate ligands can be readily replaced by the
incoming small chelate ligands, either bidentate or tri-
dentate to remain with the examples detailed above,
which may balance the positive charge of the moiety
and fill the resulting coordination sphere completely.
Looking for additional stable metal-fragments, we
have been exploring new chelates capable of stabilizing
the nitrido and the imido groups. Phosphorus contain-
ing ligands are among the best donors for technetium
and rhenium species characterized by the presence of
multiple metalꢀnitrogen bonds, as demonstrated earlier
in the case of the above-mentioned ‘super-nitrido’ frag-
ment, or in the so-called ‘HYNIC’ compounds
(HYNIC means hydrazinonicotinamine) [8,9], contain-
ing the [Tc=NNR] diazenido core.
According to the sufficiently large bite generated by
the potentially bidentate diphenylphosphine-ferrocene
(dppf) ligand [10], we thought of testing the reactivity
of dppf towards substitution-labile precursors bearing
the [MꢁN]2+ or [MꢂNR]3+ groups (M=Tc, Re). In
this study we report the synthesis and characterization
of four novel hetero bi-metallic complexes, including
the X-ray molecular structure of the representative
compound [Re(N)Cl2(dppf)], 3, which combine distinc-
tive technetium and rhenium cores with the ferrocenyl
diphosphine fragment.
nificant problem caused by the low-energy of the
b-particles. However, normal radiation safety proce-
dures must be used at all times, especially with solid
samples, to prevent contamination and inhalation.
2.2. Synthesis of technetium complexes
2.2.1. [Tc(N)Cl2(dppf)] (1)
To a suspension containing [Ph4As][Tc(N)Cl4] (40.0
mg, 0.063 mmol) in benzene (5 ml) solid dppf (35.0 mg,
0.063 mmol) was added with continuous stirring. The
orange mixture was then refluxed under a dinitrogen
atmosphere. After 2 h the color turned yellow–orange
with an appearance of a solid. The mixture was filtered,
and the pale-orange solid was washed with benzene
(3×1 ml), diethylether (10 ml) and dried under a
dinitrogen stream (Yield: 24 mg (65%)). The solid is
soluble in MeCN, benzene and chlorinated solvents,
insoluble in alcohols and Et2O. Anal. Found: C, 55.45;
H, 3.67; N, 2.01. Calc. for C34H28NP2Cl2FeTc: C,
1
55.31; H, 3.82; N, 1.90%. H-NMR (300 MHz, CDCl3,
l ppm): 4.38, 4.56, 4.64, 4.85 (m, 2H each, CpH),
7.15–7.97 (20H, PPh). 31P-NMR (300 MHz, CDCl3, l
ppm): 47.2 (bs, k1/2=340 Hz).
2.2.2. [Tc(NPh)Cl3(dppf)] (2)
To a suspension of [Tc(NPh)Cl3(PPh3)2] (27.0 mg,
0.032 mmol) in benzene–CH2Cl2 (4:1, ml/ml) solid dppf
(18.0 mg, 0.032 mmol) was added with continuous
stirring. The light-green mixture was then refluxed for 2
h under a nitrogen atmosphere. The color turned
green–brown and a solid appeared with time. The
mixture was filtered, and the green solid was washed
with MeOH (3×1 ml), Et2O (10 ml) and dried under a
dinitrogen stream (Yield: 19 mg (85%)). The solid is
soluble in chlorinated solvents, insoluble in MeCN,
alcohols and Et2O. Anal. Found: C, 56.72; H, 3.77; N,
1.75. Calc. for C40H33NP2Cl3FeTc: C, 56.46; H, 3.90;
2. Experimental
2.1. General
All the chemicals were of reagent grade and used as
such without prior purification. The synthesis of 1,1%-
bis(diphenylphosphino)ferrocene (dppf) was carried out
according to the protocols published in Ref. [10]. Rhe-
nium was purchased from Aldrich as KReO4 and con-
verted first to the labile precursors [n-Bu4N][Re(N)Cl4]
[11] and [Re(NPh)Cl3(PPh3)2] [12] according to the pro-
tocols published. Technetium starting complexes
[Ph4As][Tc(N)Cl4] [13] and [Tc(NPh)Cl3(PPh3)2] [14]
were prepared by following the methods outlined in the
literature. Proton and 31P-NMR spectra were recorded
on a Bruker AMX-300 instrument, using SiMe4 as the
1
N, 1.64%. H-NMR (300 MHz, CDCl3, l ppm): 4.61
(m, 2H), 4.66 (m, 4H), 5.35 (m, 2H) [CpH], 6.78 (t,
2H-b), 7.14 (dd, 2H-a), 7.42 (t, 1H-g) [NPh], 7.02 (m,
6H-a/g), 7.83 (t, 4H-b) [PPh-anti ], 7.32 (m, 6H-a/g),
8.03 (t, 4H-b) [PPh-syn]. 31P-NMR (300 MHz, CDCl3,
l ppm): 22.4 (bs, k1/2=1100 Hz).
1
internal reference (for H) and 85% aq. H3PO4 as the
external reference (for 31P). Samples were dissolved in
CDCl3 at a concentration of ca. 1–2%. Elemental
analyses for C, H, and N were carried out on a Fisons
EA1108 elemental analyzer.
2.3. Synthesis of rhenium complexes
Rhenium compounds were prepared as detailed
above for the technetium analogs starting from [n-
Bu4N][Re(N)Cl4] and [Re(NPh)Cl3(PPh3)2], respec-
tively. Spectroscopic data are reported below.
Caution! 99gTc is a weak b-emitter (Eb=0.292 MeV,
t
1/2=2.12×105 year). All manipulations were carried
out in laboratories approved for low-level radioactivity
using monitored hoods and gloveboxes. When handled
in milligram amounts, 99gTc does not present a serious
health hazard as common laboratory glassware pro-
vides adequate shielding. Bremsstrahlung is not a sig-
2.3.1. [Re(N)Cl2(dppf)] (3)
Anal. Found: C, 49.05; H, 3.76; N, 1.81. Calc. for
C34H28NP2Cl2FeRe: C, 49.47; H, 3.82; N, 1.70%. H-
NMR (300 MHz, CDCl3, l ppm): 4.40 (m, 2H), 4.57
1

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F. Tisato et al. / Journal of Organometallic Chemistry 637–639 (2001) 772–776
(m, 2H), 4.80 (m, 4H) [CpH], 7.18–7.86 (20H, PPh).
31P-NMR (300 MHz, CDCl3, l ppm): 27.7 (s).
2.4. Crystallographic structure determination of
[Re(N)Cl2(dppf)] (3)
2.3.2. [Re(NPh)Cl3(dppf)] (4)
Anal. Found: C, 52.01; H, 3.45; N, 1.50. Calc. for
C28H25NP2Cl3FeRe: C, 51.21; H, 3.54; N, 1.49%. H-
NMR (300 MHz, CDCl3, l ppm): 4.59, 4.64, 4.68, 5.27
(m, 2H each; CpH), 6.79 (t, 2H-b), 7.11 (dd, 2H-a),
7.40 (t, 1H-g) [NPh], 6.68 (d, 2H-a%), 7.02 (m, 4H-a/g),
7.83 (t, 4H-b) [PPh-anti ], 7.33 (m, 6H-a/g), 8.00 (t,
4H-b) [PPh-syn]. 31P-NMR (300 MHz, CDCl3, l ppm):
−17.1 (s).
Bright orange crystals of 3 were grown from
CH2Cl2–MeOH solutions. Data were collected by using
a Bruker Siemens R3m/V diffractometer. Crystal data
and refinement parameters are summarized in Table 1.
Selected interatomic distances and angles are cumulated
in Table 2. The structure was solved by heavy-atom
methods, completed by subsequent Fourier syntheses,
and refined on F2 with full-matrix least-squares meth-
ods. All non-hydrogen atoms were refined with an-
isotropic displacement coefficients and all hydrogen
atoms were treated as idealized contributions. Features
of the final difference map showed peaks (up to 1.01
1
Table 1
Crystal data for compound 3
e A−3) in chemically unreasonable positions and were
,
considered to be noise. All scattering factors and
anomalous dispersion coefficients are contained in the
SHELXTL-NT 5.10 program library [15].
Empirical formula
Formula weight
Temperature (K)
C34H28NP2Cl2FeRe
825.46
293(2)
,
Wavelength (A)
0.71073
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/c
3. Results and discussion
,
a (A)
9.885(4)
18.347(7)
17.321(7)
97.38
3115(2)
4
1.760
4.65
1616
,
b (A)
Despite the rich chemistry exhibited by most transi-
tion metals with ferrocenyl ligands incorporating a
variety of donor atoms [16], mixed technetium–iron
complexes have appeared only recently in the literature
[17]. According to the well-established ferrocene/fer-
rocenium reversible one-electron couple, the incorpora-
tion of such moiety through appropriate coordinating
group onto Tc and Re centers would produce species
showing single or multiple electron exchange at suitable
potentials. Redox active complexes of technetium have
been demonstrated to show specific in vivo biodistribu-
tion [18] because of their participation in some biologi-
cal processes involving the transfer of one or more
electrons. Therefore, studies aiming at the synthesis of
mixed technetium–iron or rhenium–iron compounds
may be relevant to nuclear medicine, both for diagnos-
tic (Tc) and therapeutic (Re) purposes. In this connec-
tion, we have recently prepared the di-substituted
nitrido technetium complex [Tc(N)(FcCS2)2] [17] out-
lined in Scheme 1. Although this species exhibits an
interesting redox behavior, no significant biological
data were recovered when this agent, prepared at NCA
level, was injected in vivo [19].
These results prompted us to introduce the ferrocene
unit into technetium and rhenium complexes by using a
different donor group like phosphine, as in dppf. Ex-
amples of low-valent mixed rhenium carbonyl–dppf
complexes appeared in the literature mostly in the
beginning of the 1990s [20–25]. In these compounds,
the diphosphine ligand acts as mono-dentate, m-bridg-
ing and/or bidentate ligand. The large bite generated
in the latter coordination mode could promote the
formation of novel metal-fragments of the type
,
c (A)
i (°)
3
,
V (A )
Z
zcalcd (g cm−3
)
Absorption coefficient (mm−1
F(000)
)
Max/min transmission
0.714, 0.563
2.2–25.1
4142
Theta range for data collection, [ (°)
Observed reflections [I]2|(I)]
Independent reflections
4538
Final R indices [I]2|(I)]
R1a=0.038
wR2b=0.094
1.056
Goodness-of-fit on F2
a R1=ꢀꢁ ꢁFoꢁ−ꢁFcꢁ ꢁ/ꢀꢁFoꢁ.
b wR2={ꢀ[w(Fo2−Fc2)2]/ꢀ[w(F2o)2}1/2
.
Table 2
,
Selected bond lengths (A) and bond angles (°) for compound 3
Bond lengths
ReꢀN
ReꢀP(1)
ReꢀP(2)
P(1)ꢀC(1)
P(1)ꢀC(1A)
P(1)ꢀC(1B)
1.603(6)
2.433(2)
2.422(2)
1.803(6)
1.835(8)
1.842(6)
ReꢀCl(1)
ReꢀCl(2)
FeꢀCaverage
P(2)ꢀC(6)
P(2)ꢀC(1C)
P(2)ꢀC(1D)
2.417(2)
2.409(2)
2.071(5)
1.822(6)
1.832(6)
1.858(6)
Bond angles
NꢀReꢀCl(1)
NꢀReꢀCl(2)
NꢀReꢀP(1)
NꢀReꢀP(2)
Cl(1)ꢀReꢀCl(2)
Cl(1)ꢀReꢀP(1)
110.6(2)
Cl(1)ꢀReꢀP(2)
Cl(2)ꢀReꢀP(1)
Cl(2)ꢀReꢀP(2)
P(1)ꢀReꢀP(2)
ReꢀP(1)ꢀC(1)
ReꢀP(2)ꢀC(6)
149.6(1)
151.6(1)
83.2(1)
97.1(1)
106.3(2)
107.1(2)
111.3(2)
96.7(2)
99.4(2)
81.7(1)
84.4(1)

F. Tisato et al. / Journal of Organometallic Chemistry 637–639 (2001) 772–776
775
Scheme 1.
[M(N)(dppf)]2+ and [M(NR)(dppf)]3+. In fact, reac-
tions of labile precursors such as [M(N)Cl4]− or
[M(NPh)Cl3(PPh3)2] yield the monomeric mono-sub-
stituted complexes [M(N)Cl2(dppf)] (1, 3) and
[M(NPh)Cl3(dppf)] (2, 4). Elemental analyses and spec-
troscopic data, as reported in Section 2, are in agree-
ment with the proposed formulation, which is further
supported by the X-ray structural determination of the
representative complex 3. The diffractometric analysis
shows a five-coordinate structure (Fig. 1) realizing a
square-pyramidal environment (trigonality index of
0.03 [26]), with the nitrido group at the apex and the
remaining P2Cl2 donors at the base of the pyramid. The
Bond lengths and bond angles (Table 2) do not show
any unusual features [27]. In fact, comparison with the
sole monomeric Re–dppf complex previously deter-
mined, i.e. fac-[ReCl(CO)3(dppf)] [21] shows the same
,
P···P separation (3.64 A), the same P···Fe···P angle
(63.2°) and PꢀReꢀP bite angle (97.1° in 3 and 96.9°),
while, as expected, the ReꢀP and ReꢀCl distances
,
lengthen by ca. 0.07 A on going from coordination
number five (in 3) to six.
NMR data confirm the structure of neutral nitrido
complexes 1 and 3 in solution to be identical to that
established in the solid state for the rhenium species.
According to the equatorial cis-P coordination of dppf,
the phenyl substituents at P become magnetically in-
equivalent because of the asymmetry introduced in the
molecule by the nitrido group. Consequently, such
phenyl proton signals are spread over a region (7.1–8.0
ppm) wider than that exhibited by uncoordinated dppf
(7.25–7.35 ppm), and the Cp protons give rise to four
distinct multiplets. A similar pattern is observed for
phenylimido complexes 2 and 4, indicating an equato-
rial dppf coordination as well. In the latter compounds
neutrality is achieved by the coordination of a further
halide group in the position trans with respect to the
imido linkage (Scheme 1) giving rise to a distorted
octahedral geometry. As already shown by similar
phosphine containing nitrido compounds [7,28,29], the
strong acid character of the [M(N)]2+ groups induces a
significant downfield shift of the 31P signal of dppf on
coordination, the pertinent values being l=47.2, 27.7
,
Re atom is displaced by 0.59 A from the mean equato-
rial plane toward the nitrido nitrogen and the four
,
basal donors are displaced by only 90.02 A. The dppf
acts as h2-diphoshine ligand that represents its pre-
ferred coordination mode. In fact, more than 70% of
the mononuclear and polynuclear dppf containing com-
plexes exhibit the h2-coordination mode, as retrieved
from a search through the CCDB [27]. The twist angle
~, defined as the C(1)···XA···XB···C(6) torsion angle (XA
is the Cp ring ‘A’ centroid and XB is the Cp ring ‘B’
centroid), is 3.2°. Thus, this structure shows dppf in the
‘syn-periplanar eclipsed’ conformation with ~ confined
between 0 and 18°, while in the other structures the
‘syn-clinal staggered’ arrangement (~ between 18 and
54°) is largely preferred [16]. The two Cp rings are
virtually parallel (dihedral angle between the mean
planes of 6.2°) and the XA···Fe···XB angle is 174.7°.
Fig. 1. ORTEP view of complex 3. The thermal ellipsoids are drawn at a 40% probability. Hydrogen atoms have been omitted for clarity.

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F. Tisato et al. / Journal of Organometallic Chemistry 637–639 (2001) 772–776
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Crystallographic data for structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 155601 for compound 3.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
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Acknowledgements
Bracco s.p.a is gratefully acknowledged for the finan-
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for the elemental analyses.
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