Phenylimido complexes of technetium and rhenium with maleonitriledithiolate

Article abstract of DOI:10.1002/zaac.201000359

[Tc(NPh)Cl₃(PPh₃)₂] or [Re(NPh)Cl ₃(PPh₃)₂] react with two equivalents of Na ₂mnt (mnt^2- = 1,2-dicyanoethene-1,2-dithiolate) with formation of anionic complexes of the

Full text of DOI:10.1002/zaac.201000359

ARTICLE
DOI: 10.1002/zaac.201000359
Phenylimido Complexes of Technetium and Rhenium with
Maleonitriledithiolate
Bernd Kuhn[a] and Ulrich Abram*[a]
Keywords: Rhenium; Technetium; Crystal structure; Phenylimido complexes; Biomolecules
Abstract. [Tc(NPh)Cl3(PPh3)2] or [Re(NPh)Cl3(PPh3)2] react with two bonds are almost linear. A similar phenylimido complex with an addi-
equivalents of Na2mnt (mnt2– = 1,2-dicyanoethene-1,2-dithiolate) with tional amino group was synthesized from [Re(NC6H4-4-
formation of anionic complexes of the composition [M(NPh)(mnt)2]–. NH2)Cl3(PPh3)2]. The presence of such substituents may allow coup-
+
The products can be isolated as large red blocks of their AsPh4 salts. ling of the metal complexes to biomolecules such as peptides, proteins,
The complex anions contain square-pyramidal coordinated metal at- or sugars, provided the M=N bonds are sufficiently stable against hy-
oms with the phenylimido ligands in apical positions. The M–N–C drolysis.
Introduction
The use of 99mTc compounds in diagnostic nuclear medicine
is well-established[1] and 186Re and 188Re are β-emitting iso-
topes with radiation properties, which make them interesting
for therapeutic purposes.[2] For both applications, an exact
knowledge of the coordination chemistry of the elements is
Results and Discussion
Reactions of the sparingly soluble phenylimido complexes
[M(NPh)Cl3(PPh3)2] (M = Tc and Re) with Na2mnt in metha-
nol give dark red solutions, from which the tetraphenylarso-
nium salts of the [M(NPh)(mnt)2]– complexes could be isolated
as dark red crystals (Scheme 1).
required. Of particular interest is the stability of cores (MO3+,
MO2 , MN2+, MNPh3+), which are characteristic for techne-
+
tium and rhenium complexes in higher oxidation states. Fre-
quently, hydrolysis of the nitrogen-containing compounds and
their conversion into oxido complexes is observed, which re-
stricts their suitability as potential radiopharmaceuticals.[1–4]
Chemical studies with technetium are commonly done with
the long-lived isotope 99Tc, which is a weak β-emitter with a
half-life of about 2 × 105 years. This isotope is available in
macroscopic amounts and allows conventional chemical stud-
ies provided that some elemental rules of radiation protection
are respected.
Herein, we report the synthesis and structural characteriza-
tion of the first phenylimidotechnetium(V) and phenylimido-
rhenium(V) complexes with 1,2-dicyanoethene-1,2-dithiolate
(mnt2–) (1). Pentacoordinate technetium and rhenium com-
plexes with this dithiolene ligand are known with the oxido
(2) and nitrido cores (3).[5–7] Additionally, one technetium(IV)
tris(dithiolene) complex[8] and two mixed-ligand species are
known.[9,10]
Scheme 1. Syntheses of the complexes.
The products are stable in air and readily soluble in CH2Cl2
or acetone. Expectedly, their IR spectra show the characteristic
C≡N stretches at 2205 cm–1. The M=NPh stretches appear at
1020 cm–1 for the technetium complex and 1030 cm–1 for its
1
rhenium analogue. The H NMR signals of the phenylimido
ligands appear upfield-shifted with respect to the protons of
the tetraphenylarsoniun counterion. The FAB– MS spectrum of
the rhenium compound gives evidence for the molecular anion
at m/z = 558.
* Prof. Dr. U. Abram
Fax: +49-30-838-52676
X-ray diffraction studies on 4a and 4b confirm the spectro-
scopic results. The complex ions each contain five-coordinate
metal atoms with square-pyramidal coordination spheres. Fig-
ure 1 illustrates the molecular structure of the
E-Mail: ulrich.abram@fu-berlin.de
[a] Institute of Chemistry and Biochemistry
Freie Universität Berlin
Fabeckstrasse 34–36
14195 Berlin, Germany
242
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 637, 242–245

Phenylimido Complexes of Tc and Re with Maleonitriledithiolate
[Tc(NPh)(mnt)2]– ion. The basal plane of the square pyramid ation is found in corresponding oxido and nitrido complexes
is formed by the sulfur atoms of two mnt2– ligands, and the and commonly discussed as a consequence of the steric bulk
nitrogen atom of the phenylimido unit forms its apex. The of the multiply bonded ligands. With the compounds described
structure of the [Re(NPh)(mnt)2]– ion is virtually identical with in this paper, there exists a consistent set of pentacoordinate,
unexceptional thermal ellipsoids. Thus, there is no extra figure anionic technetium(V) and rhenium(V) oxido, nitrido, and phe-
for this compound. Typical bond lengths and angles of both nylimido complexes with each two mnt2– co-ligands.
complexes are compared in Table 1. The atomic labeling
Principal structural data of the compounds are summarized
scheme of Figure 1 has also been applied for the rhenium com- in Table 2. They suggest a larger extent of steric effects in the
pound.
oxido and phenylimido complexes compared with the nitrido
compounds. This is indicated by the generally larger X–M–S
angles and the fact that the metal atoms are more displaced
from the basal planes of the square-planar complexes than in
the nitrido analogue. On the other hand, a higher degree of π
bonding is expected for the nitrido ligand, which is best re-
flected by the significantly longer M–S bonds.[7] The bonding
parameters of the phenylimido complexes under study suggest
a bonding situation between those of the oxido and the nitrido
complexes. This is evident by all listed parameters in Table 2.
Table 2. Selected bonding parameters in [M(NPh)(mnt)2]–,
[MO(mnt)2]– and [MN(mnt)2]2– (M = Tc and Re).
Figure 1. Ellipsoid representation[11] of the complex anion of
(AsPh4)[Tc(NPh)(mnt)2] with 50 % probability. Hydrogen atoms have
been omitted for clarity.
Table 1. Selected bond lengths /Å and angles /° in the structures of
the [M(NPh)(mnt)2]– anions (M = Tc, Re).
4a
4b
M–N10
1.675(5)
1.40(1)
1.706(2)
1.396(4)
2.336(1)
2.324(1)
2.333(1)
2.324(1)
1.746(3)
1.747(3)
1.356(4)
1.746(3)
1.747(3)
1.349(5)
169.2(2)
108.8(1)
106.3(1)
107.6(1)
105.8(1)
145.4(1)
146.1(1)
85.3(1)
N10–C11
M–S1
[M(NPh)(mnt)2]– [MO(mnt)2]–[5,6] [MN(mnt)2]2–[6,7]
2.335(2)
2.329(2)
2.323(2)
2.330(2)
1.744(6)
1.746(7)
1.36(1)
M–S2
M–X /Å
Tc 1.68
Tc 1.66
Tc 1.59
M–S3
Re 1.71
Tc 2.329
Re 2.329
Re 1.67
Tc 2.315
Re 2.314
Tc 108.7
Re 108,2
Tc 0.74
Re 1.65
Tc 2.391
Re 2.357
Tc 104.3
Re 105.8
Tc 0.59
M–S4
M–Smean /Å
S1–C1
S2–C2
X–M–Smean /° Tc 107.0
C1–C2
Re 107.1
Tc 0.68
Re 0.69
S3–C5
1.731(7)
1.742(7)
1.36(1)
∆ /Å
S4–C6
Re 0.72
Re 0.64
C5–C6
M–N10–C11
N10–M–S1
N10–M–S2
N10–MS3
N10–M–S4
S1–M–S4
S2–M–S3
S1–M–S2
S1–M–S3
S2–M–S4
S3–M–S4
168.2(5)
109.2(2)
106.1(2)
107.8(2)
104.9(2)
145.9(1)
146.1(1)
85.4(1)
The results may support (also with respect to the almost lin-
ear M–N–C bonds) a treatment of NPh2– ligands in complexes
with metals in high formal oxidation states as “nitrenes” with
formal metal–nitrogen triple bonds rather than as “imides”
with formal double bond character. Comprehensive discussions
about the nature of various types of metal–nitrogen multiple
bonds are contained in early reviews[12–15] and have been ex-
tensively reconsidered several years ago.[16] The suggested
strong relationship between nitrido and imido compounds has
84.6(1)
84.6(1)
84.6(1)
84.7(1)
85.5(1)
85.8(1)
The Tc–N and Re–N bond lengths are in the range of double finally also been experimentally proven by the synthesis of
bonds and the M–N–C angles are almost linear with values of imido compounds starting from nitrido complexes by electro-
168.2(5) and 169.2(2)°.
philic attacks at the nitrido ligands. This has successfully been
The metal atoms are situated by 0.681(1) Å (technetium demonstrated with triphenylcarbenium salts[17,18] and in situ
complex) and 0.686(1) Å (rhenium complex) above the basal produced carbocations, e.g. with the electrophilic intermediates
plane formed by the four sulfur atoms. A similar bonding situ- of acidic acetone condensation.[19]
Z. Anorg. Allg. Chem. 2011, 242–245
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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243

B. Kuhn, U. Abram
ARTICLE
Na2mnt[23] were prepared following literature procedures. The synthe-
sis of [Re(NPh-4-NH2)Cl3(PPh3)2] was slightly modified to improve
the yield and the purity of the product. Infrared spectra were measured
as KBr pellets with a Shimadzu FT IR spectrometer between 400 and
4000 cm–1. Elemental analyses were determined with a Heraeus vario
EL elemental analyzer. NMR spectra were taken with a JEOL
400 MHz multinuclear spectrometer.
It is noteworthy, that, in contrast to previous reports on imido-
rhenium(V) compounds,[3,4] no hydrolytic decomposition has
been observed for the phenylimido complexes reported in this
paper. This recommends them for considerations with respect to
nuclear medical applications. Thus, we prepared an analogous
complex to compound 4b, which carries an additional amino
group at the phenyl ring of the imido ligand. With this modifi-
cation, coupling reactions with biorelevant molecules such as
peptides are possible. An appropriate starting material,
[Re(NC6H4-4-NH2)Cl3(PPh3)2], has previously been published
by Machura and co-workers.[20]
Syntheses
[Re(NC6H4-4-NH2)Cl3(PPh3)2]:
[ReOCl3(PPh3)2]
(450 mg,
The reaction of [Re(NC6H4-4-NH2)Cl3(PPh3)2] with Na2mnt
in methanol results in a rapid dissolution of the green solid
and the formation of a red solution. Addition of (AsPh4)Cl
results in a red solid of (AsPh4)[Re(NC6H4-4-NH2)(mnt)2] (5).
Recrystallization from CH2Cl2/isopropanol gives long, red
needles. The IR spectrum of the compound is similar to that of
4b with an intense νC≡N vibration at 2200 cm–1. Additionally,
medium bands at 3450 and 3360 cm–1 can be assigned to the
NH frequencies of the amino group.
Figure 2 shows a structural plot of the complex anion of
compound 5, which has also been studied by X-ray diffraction.
The compound crystallizes in the orthorhombic space group
Pna21 with a unit cell of the dimensions a = 25.881(2) Å, b =
6.492(1) Å and c = 22.589(2) Å. The obtained data set with
wR2 = 0.1062 was not of sufficient quality to justify a compre-
0.54 mmol) was suspended in acetone (15 mL). PPh3 (2.5 g,
9.54 mmol) and 4-NH2-aniline (100 mg, 0.92 mmol) were added in
acetone (5 mL). The mixture was heated under reflux for 3 h. The
resulting olive green precipitate was filtered off and washed with ethyl
ether. Yield 350 mg (75 %). Elemental analysis calcd.
C42H36Cl3N2P4Re: C 54.7; H 3.9; N 3.0 %; found: C 54.8; H 3.9; N
2.8 %. IR (KBr): ν = 3460 (m), 3335 (st), 3195 (w), 3055 (w), 1615
˜
(st), 1585 (st), 1480 (m), 1435 (m), 1370 (st), 1325 (m), 1190 (w),
1165 (st), 1090 (m), 1030 (w), 1005 (w), 925 (w), 845 (w), 745 (m),
695 (st), 600 (w), 565 (w), 520 (st), 515 (st), 450 (w), 425 (w) cm–1.
4a: A suspension of [Tc(NPh)Cl3(PPh3)2] (82 mg, 0.1 mmol) and
Na2mnt (51 mg, 0.4 mmol) in methanol (15 mL) was heated under
reflux for 1 hour. After cooling to room temperature, (AsPh4)Cl·2H2O
(42 mg, 0.1 mmol) in methanol (1 mL) was added. The solvent was
removed in vacuo and the resulting red-brown residue was washed
hensive discussion of the crystallographic data. The results of with ethyl ether and recrystallized from methanol. Yield 45 mg (55 %).
IR (KBr): ν = 3060 (w), 2960 (w), 2915 (w), 2205 (st), 1500 (m),
˜
the study, however, unambiguously confirm the composition
of the complex cation and all main structural features of com-
pound 4b do also apply for 5.
1480 (m), 1465 (m), 1440 (st), 1310 (m), 1185 (w), 1150 (m), 1110
(w), 1080 (m), 1020 (m), 995 (m), 940 (w), 900 (w), 840 (w), 740
(st), 685 (st), 505 (m), 465 (st) cm–1.
4b: [Re(NPh)Cl3(PPh3)2] (91 mg, 0.1 mmol) was suspended in metha-
nol (30 mL) and treated with Na2mnt (43 mg, 0.23 mmol). After heat-
ing to reflux for 10 min,
a clear red solution was obtained.
(AsPh4)Cl·2H2O (42 mg, 0.1 mmol) was added. Deep red crystals de-
posited upon slow evaporation. Yield 54 mg (57 %). Elemental analy-
sis: C38H25AsN5ReS4: calcd. C 48.5; H 2.7; N 7.4; S 13.6 %; found:
C 48.4; H 2.4; N 7.4; S 13.6 %. IR (KBr): ν = 3060 (w), 2205 (st),
˜
1580 (w), 1510 (st), 1475 (st), 1440 (st), 1355 (st), 1185 (w), 1155
(m), 1080 (m), 1030 (w), 995 (m), 740 (st), 685 (st), 505 (m), 465 (m)
1
cm–1. H NMR (CDCl3): δ = 7.9–7.6 (m, 20 H, As-Ph), 7.3–6.9 ppm
(m, 5 H, N-Ph). FAB–-MS: m/z 558 M–, 482 M– - (C–CN)2, 374
[Re(NPh)S3]–.
Figure 2. Space-filling representation of the complex anion of 5 with
a terminal NH2 group being far away from the coordination sphere.
5: [Re(NC6H4-4-NH2)Cl3(PPh3)2] (93 mg, 0.1 mmol) was suspended
in methanol (50 mL) and treated with Na2mnt (51 mg, 0.40 mmol).
After heating to reflux for 10 min, a clear red solution was obtained.
(AsPh4)Cl·2H2O (42 mg, 0.1 mmol) was added and the solvent was
removed in vacuo. The residue was washed with water and ethyl ether.
Recrystallization from CH2Cl2/isopropanol gave long, red needles.
Yield 22 mg (23 %). Elemental analysis: calcd. C38H26AsN6ReS4: C
47.7; H 2.7; N 8.8; S 13.4 %; found: C 48.4; H 2.3; N 8.5; S 13.1 %.
The phenyl ring of the imido ligand in complex 5 acts as a
spacer and, thus, the terminal NH2 group is readily accessible
for coupling reactions with activated peptides and proteins.
This holds also true for phenylimido complexes with other,
more bulky, chelating ligands, which might be more appropri-
ate for biological applications than maleonitriledithiolate.
IR (KBr): ν = 3450 (st), 3360 (st), 3060 (w), 2270 (w), 2200 (st), 1615
˜
(st), 1590 (st), 1505 (m), 1490 (st), 1440 (st), 1340 (w), 1290 (m),
1185 (w), 1165 (m), 1150 (m), 1115 (m), 1080 (m), 1020 (m), 995
(m), 920 (w), 820 (m), 745 (st), 685 (st), 600 (w), 505 (m), 465 (m)
cm–1.1H NMR (CDCl3): δ = 7.8–7.6 (m, 20 H, As–Ph), 6.6–6.4 ppm
Experimental Section
Materials and Measurements
The
starting
complexes
[Re(NC6H4-4-NH2)Cl3(PPh3)2],[20]
[Tc(NPh)Cl3(PPh3)2],[21] (m, 5 H, N–Ph). FAB–-MS: m/z 573 M–, 481 M– - (C–CN)2 -NH2, 389
[Re(NPh)Cl3(PPh3)2],[22]
and [Re{NPh(p-NH2)}S3]–.
244
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Z. Anorg. Allg. Chem. 2011, 242–245

Phenylimido Complexes of Tc and Re with Maleonitriledithiolate
X-ray Crystallography
References
The intensities for the X-ray determinations were collected with a
STOE IPDS 2T instrument with Mo-Kα radiation (λ = 0.71073 Å).
Standard procedures were applied for data reduction and absorption
correction. Structure solution and refinement were performed with
SHELXS-97 and SHELXL-97.[24] Hydrogen atom positions were cal-
culated for idealized positions and treated with the “riding model” op-
tion of SHELXL. Numerical absorption correction was applied for
compound 4b, whereas such a procedure did not improve the results
of the refinement of the technetium compound with a linear absorption
coefficient of just 1.559. More details on data collections and structure
calculations are summarized in Table 3.
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2004,vol. 5, pp. 127,and references cited therein.
[2] U. Abram, Rhenium in Comprehensive Coordination Chemistry
II (Eds.: J. A. McCleverty, T. J. Meyer), Elsevier: Amsterdam,
2004,vol. 5, pp. 271,and references cited therein.
[3] J. B. Arterburn, I. M. Fogarty, K. A. Hall, K. C. Ott, J. C. Bryan,
Angew. Chem. 1996, 108, 3039; Angew. Chem. Int. Ed. Engl.
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[6] U. Abram, M. Braun, S. Abram, R. Kirmse, A. Voigt, J. Chem.
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[8] S. F. Colmanet, M. F. Mackay, Aust. J. Chem. 1988, 41, 1127.
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[10] U. Abram, S. Ritter, Inorg. Chim. Acta 1994, 216, 31.
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30, 565.
[12] K. Dehnicke, J. Strähle, Angew. Chem. 1981, 93, 451; Angew.
Chem. Int. Ed. Engl. 1981, 20, 413.
[13] W. A. Nugent, B. L. Haymore, Coord. Chem. Rev. 1980, 31, 123.
[14] W. A. Nugent, J. M. Mayer, Metal-Ligand Multiple Bonds, Wiley,
New York, 1988.
[15] K. Dehnicke, J. Strähle, Angew. Chem. 1992, 104, 978; Angew.
Chem. Int. Ed. Engl. 1992, 31, 955.
[16] Nitrido Bridges Between Transition Metals and Main Group Ele-
ments , Final Report of the DFG Priority Program (Eds.: K. Deh-
nicke, R. Kniep), Z. Anorg. Allg. Chem. 2003, 629, 727–927.
[17] U. Abram, A. Voigt, R. Kirmse, Polyhedron 2000, 19, 1741.
[18] U. Abram, R. Kirmse, A. Voigt, Inorg. Chem. Commun. 1998, 1,
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[19] S. Ritter, U. Abram, Z. Anorg. Allg. Chem. 1994, 620, 1786.
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[24] G. M. Sheldrick, SHELXS-97 and SHELXL-97, Programs for the
Solution and refinement of Crystal Structures, University of Gött-
ingen, Germany, 1997.
CCDC-795336 (4a) and CCDC-795337 (4b) contain the supplemen-
tary crystallographic data for the structures in this paper. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Centre,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: 44-1223-336-
033; or E-Mail: deposit@ccdc.cam.ac.uk).
Table 3. Crystal data and refinement results.
4a
4b
Empirical formula
C38H25AsN5S4Tc
852.79
C38H25AsN5S4Re
940.99
Formula weight
Temperature /K
Crystal system
200(2)
200(2)
Triclinic
Triclinic
¯
¯
Space group
a /Å
b /Å
c /Å
P1
P1
12.232(2)
12.772(2)
13.035(2)
64.46(1)
83.23(1)
88.67(1)
1823.6(4)
2
12.340(1)
12.733(1)
13.010(1)
64.19(1)
83.50(1)
89.01(1)
1827.4(2)
2
α /°
β /°
γ /°
V /Å3
Z
D
calcd. /g·cm–3
1.553
1.710
μ /mm–1
F(000)
1.559
4.487
856
920
Absorption correction None
Integration
0.5379 / 0.3496
19530
T
max/Tmin
–
Collected reflections
11629
Unique reflections / Rint 6236 / 0.1004
9754 / 0.0429
9754 / 442
0.0288
0.0590
0.910
Data / Parameters
6236 / 442
R1 = 0.0551
wR2 = 0.0959
0.944
R values [I > 2σ(I)]
Received: October 5, 2010
Published Online: December 27, 2010
Goodness of-fit, S
Z. Anorg. Allg. Chem. 2011, 242–245
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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245