Aryl and NHC compounds of technetium and rhenium

Article abstract of DOI:10.1021/ja3033718

Air- and water-stable phenyl complexes with nitridotechnetium(V) cores can be prepared by straightforward procedures. [TcNPh₂(PPh ₃)₂] is formed by the reaction of [TcNCl ₂(PPh₃)₂] with PhL

Full text of DOI:10.1021/ja3033718

Communication
pubs.acs.org/JACS
Aryl and NHC Compounds of Technetium and Rhenium
Elisabeth Oehlke,† Shushu Kong, Pawel Arciszewski, Swantje Wiebalck, and Ulrich Abram*
Institute of Chemistry and Biochemistry, Freie Universitat Berlin, Fabeckstraße 34/36, D-14195 Berlin, Germany
̈
S
* Supporting Information
Scheme 1. Synthesis of 1 and 2
ABSTRACT: Air- and water-stable phenyl complexes
with nitridotechnetium(V) cores can be prepared by
straightforward procedures. [TcNPh2(PPh3)2] is formed
by the reaction of [TcNCl2(PPh3)2] with PhLi. The
analogous N-heterocyclic carbene (NHC) compound
[TcNPh2(HLPh)2], where HLPh is 1,3,4-triphenyl-1,2,4-
triazol-5-ylidene, is available from (NBu4)[TcNCl4] and
HLPh or its methoxo-protected form. The latter compound
allows the comparison of different Tc−C bonds within one
compound. Surprisingly, the Tc chemistry with such
NHCs does not resemble that of corresponding Re
complexes, where CH activation and orthometalation
dominate.
bipyramidal geometry (Figure 1).8 The Tc−C bond lengths are
in the same range as those in the TcVI dimer [{Tc(O)Me2}2(μ-
he organometallic chemistry of technetium, particularly
T
the structural chemistry of organotechnetium compounds,
is one of the least explored among common transition metals.1
Of the less than 140 crystallographically studied compounds
containing Tc−C bonds (vs ∼4500 entries for the correspond-
ing Re compounds), there are less than 20 that do not contain
carbonyl or isocyanide ligands.2 The focus on the latter
compounds can easily be understood with regard to the use of
the isocyanide complex [99mTc(L)6]+ (R = methoxyisobutyl
isonitrile) as a routine myocardial imaging agent in clinical
nuclear medicine,3 and the fact that with the development of a
kitlike preparation of [99mTc(CO)3(H2O)3]+, carbonyl com-
plexes of technetium have shown their potential for radio-
pharmaceutical imaging.4 We recently introduced another class
of organometallic technetium compounds: complexes with 1,3-
dialkyl-4,5-dimethylimidazol-2-ylidenes.5 The fact that these
compounds as well as their rhenium analogues are water-stable
and can be synthesized by a simple transmetalation approach6
encouraged us to get deeper insights into the organometallic
chemistry of the first artificial element.
Figure 1. Molecular structure of 1. The rhenium complex is virtually
identical and therefore not shown. H atoms and solvent molecules
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg) in 1: Tc−N, 1.628(4); Tc−C1, 2.138(4); Tc−C7, 2.127(4);
P1−Tc−P2, 174.3(1); N−Tc−C1, 118.6(2); C1−Tc−C7, 123.5(2).
In 2: Re−N, 1.660(5); Re−C1, 2.117(4); Re−C7, 2.133(5); P1−Re−
P2, 175.6(1); N−Re−C1, 117.5(2); C1−Re−C7, 124.3(2).
O)2] (Tc−C = 2.08−2.13 Å), the only technetium methyl
complex structurally characterized to date,9 and in the binuclear
carbonyl complex [Tc2(μ-H)(μ-NC5H4)(NC5H5)2(CO)6].10
The rhenium analogue [ReNPh2(PPh3)2] (2)11 is isostruc-
tural to the technetium complex and depicts similar
spectroscopic properties. 1 and 2 are surprisingly inert.
Attempted reactions with PMe2Ph, PMe3, and 1,2-dicyano-
ethene-1,2-dithiolate always resulted in the recovery of the
starting complexes from the reaction solutions.
Unlike the situation with rhenium, where reactions of various
nitridorhenium precursors with 1,3,4-triphenyl-1,2,4-triazol-5-
ylidene (HLPh) always give [ReNCl(HLPh)(LPh)] (3) with one
orthometalated NHC ligand,12 reactions of (NBu4)[TcNCl4]
with HLPh result in the formation of a five-coordinate
Here we present the syntheses and reactivities of the first
phenyl and triazolylidene technetium complexes. The lack of
knowledge about aryltechnetium complexes is surprising, given
the fact that aryl complexes of all other transition metals are
well-known.2,7
The synthesis of the first phenyl complex of technetium was
straightforward.8 Treatment of [TcNCl2(PPh3)2] with 3 equiv
of PhLi in dry THF afforded orange-red crystals having the
composition [TcNPh2(PPh3)2] (1) (Scheme 1). The IR
spectrum of the complex shows a distinct band at 1092 cm−1
for the TcN vibration. The 13C NMR spectrum exhibits a
triplet at 174.4 ppm that can be assigned to the carbon atoms
bonded directly to the technetium atom. The crystal structure
of 1 shows a five-coordinate complex with distorted trigonal-
Received: April 14, 2012
Published: May 17, 2012
© 2012 American Chemical Society
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Communication
technetium(V) compound having the composition
Scheme 3. Syntheses and Reactions of Re Triazolylidenes
[TcNCl2(HLPh)2] (4) (Scheme 2). Reduction of the metal in
Scheme 2. Syntheses and Reactions of Tc Triazolylidenes
be synthesized from [RuCl2(p-cymene)]2.13a In contrast, the
benzylidene compound [RuCl2{P(C6H11)3}(CHPh)(HLPh)]
contains the carbene only as a monodentate ligand.14
Consequently, rhenium compound 3, which was prepared
following the procedure given in ref 12a, shows another
reaction pattern with PhLi and forms the mixed aryl/carbene
complex [ReNPh(HLPh)(LPh)] (6). The 13C NMR spectrum of
6 exhibits two carbene signals at 191.6 and 186.7 ppm as a
result of the nonequivalence of the carbene carbon atoms. Two
signals at 154.3 and 152.5 ppm correspond to the phenyl
carbon atoms directly bonded to the rhenium atom. Similar
chemical shifts were reported for 3, with the signal of the
carbene carbon atom trans to the chloro ligand shifted slightly
downfield (200.1 ppm).12a X-ray diffraction showed that the
coordination mode in 6 is similar to those in the previously
reported triazolylidene rhenium complexes [ReNX(HLPh)-
(LPh)] (X = halide, pseudohalide, thiolate).12 As in the case
of the technetium complex 5, two different metal−carbon bond
lengths were observed, with the bonds to the carbene carbon
atoms (Re−CNHC = 2.102(6) and 2.128(7) Å) being slightly
such reactions is common, and in cases where the ligands are
not reducing and no additional reducing agent is added, Cl−
ions may act as such. The two remaining chloro ligands of 4 can
be substituted by phenyl ligands, resulting in the mixed aryl/
carbene complex [TcNPh2(HLPh)2] (5).
The 13C NMR spectra of the two complexes exhibit signals at
154.1 and 153.7 ppm, respectively, which can be assigned to the
carbene carbon atoms. The spectrum of 5 contains an
additional signal at 137.8 ppm corresponding to the phenyl
carbon atoms bonded directly to the technetium atom. Crystal
structure analysis of 5 (Figure 2) showed a five-coordinate
shorter than the bonds to the phenyl carbon atoms (Re−CPh
=
2.160(7) and 2.169(7) Å) (Figure 3).
Figure 2. Molecular structure of 5. H atoms and solvent molecules
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Tc−N, 1.606(4); Tc−C1, 2.168(3); Tc−C11, 2.129(3); N−
Tc−C1, 104.5(1); N−Tc−C11, 101.2(1); C1−Tc−C11, 87.7(1);
C1−Tc−C1′, 157.6(2); C11−Tc−C11′, 151.0(2).
Figure 3. Molecular structure of 6. H atoms have been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Re−N, 1.618(6);
Re−C1, 2.102(6); Re−C3, 2.128(7); Re−C56, 2.169(7); Re−C71,
2.160(7); N−Re−C1, 102.4(3); C3−Re−C56, 75.4(3); C3−Re−C71,
150.1(3); C1−Re−C71, 89.3(3).
complex with square-pyramidal geometry. Interestingly, the
bonds of the technetium atom to the NHC ligands (Tc−CNHC
= 2.129(3) Å) are slightly shorter than the bonds to the phenyl
carbon atoms (Tc−CPh = 2.168(3) Å).
The exclusively monodentate coordination of the triazolyli-
dene ligands in the Tc complexes is in contrast to the
coordination mode of triazolylidenes in the already known
nitridorhenium complexes, which include one orthometalated
ligand (e.g., 3 in Scheme 3). Orthometalation is not without
precedent in the chemistry of HLPh but can hardly be
predicted.13 It has been observed, for instance, in the
ruthenium(II) complex [RuCl2(p-cymene)(HLPh)], which can
The different coordination modes of the triazolylidene
ligands in rhenium complex 3 and technetium complex 4
result in different reactivities, as demonstrated by reactions with
a more basic imidazolylidene. The technetium complex reacts
with 1,3,4,5-tetramethylimidazol-2-ylidene (LMe) via exchange
of all of the equatorial ligands to form [TcNCl(LMe)4]Cl (7).
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The product has an octahedral structure with the four NHC
ligands arranged in a pinwheel-like fashion in the equatorial
plane (Figure 4), a coordination mode that has been observed
Figure 5. Molecular structure of 8. H atoms have been omitted for
clarity. Selected bond lengths (Å): Re−C1, 2.07(3); Re−C3, 2.15(4);
Re−C72, 2.24(3); Re−C56, 2.19(3); Re−C26, 2.45(4).
Figure 4. Molecular structure of 7. H atoms have been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Tc−C1, 2.207(4);
N−Tc−Cl, 180.0; C1−Tc−C1′, 88.8(1).
In conclusion, previously unreported air-stable technetium
complexes with phenyl and triazolylidene ligands have been
synthesized, closing the last gap in the aryl chemistry of
transition metals. Both the aryl and triazolylidene complexes
show interesting reactivities and open new perspectives for the
synthesis of stable organometallic technetium complexes. The
chemistry of technetium with triazolylidenes does not resemble
that of rhenium, which should be subject of further studies.
previously for other technetium imidazolylidene complexes.5
The occupation of the axial coordination positions (i.e., the
{NTc−Cl}+ unit) is disordered along the fourfold axis of the
tetragonal structure (space group P4nc). This required a
refinement of the Cl and N atoms on shared positions, making
the bond lengths on this axis less reliable. Details of the
refinement model are given in the Supporting Information.
Another type of disorder concerns the Cl− counterions, which
are situated in channels formed by the large complex molecules
on another fourfold axis parallel to the metal core. The Tc−C
bond lengths are unexceptional with respect to other
imidazolylidene technetium complexes.5
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data, and crystallo-
graphic details for compounds 1, 2, 4, 5, 7, and 8 (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
Treatment of 3 with LMe in benzene results in the exclusive
substitution of the chloro ligand and the formation of the first
transition-metal complex containing imidazolylidene together
with triazolylidene ligands. NMR or mass spectra of [ReN-
(LMe)(LPh)2] (8) could not be recorded because of the low
solubility of 8 in inert solvents. In CDCl3, in which the
compound is readily soluble, it rapidly decomposes via
formation of a paramagnetic species, most probably a ReIV
complex, which exhibits a broad EPR line at a g value of 2.031
without any hyperfine structure. Single crystals of 8 could be
obtained by crystallization from a large amount of MeCN.
Unfortunately, even the best data set obtained from our X-ray
diffraction studies was of low quality (R value of 14.4%). Thus,
the resulting bond lengths and angles are accordingly
characterized by comparably large standard deviations and
should not be overvalued. Only a structural sketch without
ellipsoids is given in Figure 5. All of the main structural features
of the compound, however, can certainly be derived from the
structural analysis. The most interesting feature of 8 is the
orthometalation of the second HLPh ligand (which was
monodentate in 3) trans to the nitrido ligand. Such a structural
feature is without precedent and indirectly confirms the
presence of agostic interactions in [ReNX(HLPh)(LPh)]
complexes (X = halide, pseudohalide, thiolate), as recently
postulated on the basis of crystal structure data.12b
AUTHOR INFORMATION
Corresponding Author
Ulrich.abram@fu-berlin.de
■
Present Address
†ANSTO Life Sciences, Locked Bag 2001, Kirrawee DC, NSW
2232, Australia (E.O.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. Adelheid Hagenbach for valuable help with
crystallographic problems.
■
REFERENCES
■
(1) Sattelberger, A. P.; Scott, B. L. Technetium. In Comprehensive
Organometallic Chemistry III; Crabtree, R., Mingos, D. M. P., Eds.;
Elsevier: Oxford, U.K., 2007; Vol. 5, p 833.
(2) Cambridge Crystal Structure Database, version 5.30; Cambridge
Crystallographic Data Centre: Cambridge, U.K., 2008.
(3) Jones, A. G.; Abrams, M. J.; Davison, A.; Brodack, J. W.;
Toothaker, A. K.; Adelstein, S. J.; Kassis, A. I. Nucl. Med. Biol. 1984, 11,
225.
(4) (a) Alberto, R.; Schibli, R.; Schubiger, P. A. Polyhedron 1996, 15,
1079. (b) Alberto, R.; Schibli, R.; Egli, A.; Abram, U.; Kaden, T. A.;
Schubiger, P. A. J. Am. Chem. Soc. 1998, 120, 7987. (c) Alberto, R.;
9120
dx.doi.org/10.1021/ja3033718 | J. Am. Chem. Soc. 2012, 134, 9118−9121

Journal of the American Chemical Society
Communication
Ortner, K.; Wheatley, N.; Schibli, R.; Schubiger, A. P. J. Am. Chem. Soc.
2001, 123, 3135.
(5) (a) Braband, H.; Kuckmann, T. I.; Abram, U. Inorg. Chem. 2003,
̈
42, 6160. (b) Braband, H.; Abram, U. Organometallics 2005, 24, 3362.
(6) Oehlke, E.; Kuckmann, T. I.; Abram, U. Z. Anorg. Allg. Chem.
̈
2007, 633, 830.
(7) Comprehensive Organometallic Chemistry III; Crabtree, R., Mingos,
D. M. P., Eds.; Elsevier: Oxford, U.K., 2007; Vol. 5.
(8) CCDC 860229−860233 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
(9) Herrmann, W. A.; Alberto, R.; Kiprof, P.; Baumgartner, F. Angew.
̈
Chem. 1990, 120, 208; Angew. Chem., Int. Ed. Engl. 1990, 29, 189.
(10) Zuhayra, M.; Lutzen, U.; Lutzen, A.; Papp, L.; Henze, E.;
̈
̈
Friedrichs, G.; Oberdorfer, F. Inorg. Chem. 2008, 47, 10177.
(11) (a) Chatt, J.; Garforth, J. D.; Rowe, G. A. J. Chem. Soc. A 1966,
12, 1834. (b) Yam, V. W.-W.; Tam, K.-K.; Lai, T.-F. J. Chem. Soc.,
Dalton Trans. 1993, 651.
(12) (a) Braband, H.; Abram, U. Chem. Commun. 2003, 2436.
(b) Braband, H.; Neubacher, S.; Grosskopf, S.; Abram, U. Z. Anorg.
Allg. Chem. 2005, 631, 1645.
(13) (a) Simal, F.; Ian, D.; Delaude, L.; Demonceau, A.; Spirlet, M.-
R.; Noels, A. F. Can. J. Chem. 2001, 79, 529. (b) Baratta, W.; Schultz,
J.; Herdtweck, E.; Herrmann, W. A.; Rigo, P. J. Organomet. Chem.
2005, 690, 5570.
(14) Furstner, A.; Ackermann, L.; Gabor, B.; Goddard, R.; Lehmann,
̈
C. W.; Mynott, R.; Stelzer, F.; Thiel, O. R. Chem.Eur. J. 2001, 7,
3236.
9121
dx.doi.org/10.1021/ja3033718 | J. Am. Chem. Soc. 2012, 134, 9118−9121