Rhenium(V) complexes with pentadentate P,N,O ligands

Article abstract of DOI:10.1021/ic901152y

Novel rhenium(V) complexes were isolated from reactions of [NBu ₄][ReOCI₄] with the potentially pentadentate Schiff base PhP{C₆H₄-2-(HC=N(C₆H₄-2-OH))}2, H₂L¹, and the

Full text of DOI:10.1021/ic901152y

8072 Inorg. Chem. 2009, 48, 8072–8074
DOI: 10.1021/ic901152y
Rhenium(V) Complexes with Pentadentate P,N,O Ligands
Ali Barandov and Ulrich Abram*
::
Freie Universitat Berlin, Institute of Chemistry and Biochemistry, Fabeckstr. 34-36, D-14195 Berlin, Germany
Received June 15, 2009
Novel rhenium(V) complexes were isolated from reactions of
[NBu4][ReOCl4] with the potentially pentadentate Schiff base
PhP{C6H4-2-(HCdN(C6H4-2-OH))}2, H2L1, and the correspond-
ing amine PhP{C6H4-2-(CH2-NH(C6H4-2-OH))}2, H4L2. While
the Schiff base undergoes partial hydrolytic decomposition and a
redox reaction, the amine remains intact and acts as a pentaden-
tate ligand, which encapsulates the metal atom of the {ReVO}3þ
core or stabilizes a {ReVCl}4þ center by the formation of an imido-
type ligand system.
through a standard imine condensation between bis(2-for-
mylphenyl)phenylphosphine and 2 equiv of 2-aminophenol.6
The product was isolated as a yellow solid in a high yield. The
31P-{1H} spectrum of the product shows a single resonance at
1
-18 ppm. A doublet at 9.1 ppm (JPH = 4.9 H) in the H
NMR spectrum of H2L1 suggests the presence of the HCdN
unit.
H2L1 reacts with an excess of NaBH4 in boiling ethanol
under reduction of the imine function and formation of the
related secondary amine H4L2.6 The 31P-{1H} NMR spec-
trum presents a single resonance at -27.4 ppm. The 9 ppm
low-field shift with respect to H2L1 is consistent with the loss
Despite the fact that pentadentate ligand systems would be
perfectly suitable for the stabilization of {ReVO}3þ
,
{ReVNPh]3þ, or {ReVN}2þ cores, which are under discussion
for applications in nuclear-medical therapy with the β--
emitting isotopes 186Re and 188Re,1 there are only a few
examples of structurally well-characterized complexes.2 De-
composition or incomplete coordination of potentially pen-
tadentate ligands was observed instead in a number of
additional examples.3
The ready availability of formylphosphines of the compo-
sition Ph3-nP(2-C6H4CHO)n (n = 1-3)4 gives access to
Schiff bases, which may contain additional donor atoms
such as from hydroxylic, thiol, or carboxylic groups and
their derived amines. Recently, we reported about reactions
of rhenium complexes with Ph2P(C6H4-2-HCdN(C6H4-2-
OH) (HL0) and its amine derivative.5
(3) (a) Chen, X.; Femia, F. J.; Babich, J. W.; Zubieta, J. Inorg. Chim. Acta
2000, 310, 237. (b) Banbery, H. J.; McQuillan, F.; Hamor, T. A.; Jones, C. J.;
McCleverty, J. A. Polyhedron 1989, 8, 559.
(4) Del Zotto, A.; Baratta, W.; Ballico, M.; Herdtweck, E.; Rigo, P.
Organometallics 2007, 26, 5636.
(5) Barandov, A.; Abram, U. Polyhedron 2009, 28, 1155.
(6) Synthesis of H2L1. Bis(2-formylphenyl)phenylphosphine (1 g, 3.1
mmol) and 2-aminophenol (0.7 g, 6.2 mmol) were suspended in 10 mL of
ethanol and heated under reflux for 1.5 h. A yellow solid precipitated upon
cooling. The product was filtered off and washed with cold ethanol. Yield:
1.3 g (82%). For analytical and spectroscopic data, see the Supporting
Information. Synthesis of H4L2. H2L1 (1.5 g, 3 mmol) and NaBH4 (1.7 g, 48
mmol) were suspended in 20 mL of EtOH and heated. The color of the
reaction mixture turned to colorless within 10 min. Subsequently, the solvent
was evaporated to dryness under a vacuum and the product extracted by 50
mL of a THF/toluene mixture. The organic phase was washed with water
and dried over MgSO4. H4L2 was isolated as a colorless solid after removal of
all volatiles under a vacuum. Yield: 1.3 g (86%). For analytical and
spectroscopic data, see the Supporting Information.
(7) Synthesis of [ReOCl(L1a)] (1): Solid H2L1 (50 mg, 0.1 mmol) was
added to a solution of [NBu4][ReOCl4] (58 mg, 0.1 mmol) in 10 mL of
MeOH, which was not dried previously. The mixture was stirred at room
temperature for 30 min. During this time, a clear brown solution was formed,
from which a brown solid precipitated. The product was filtered off and
washed with cold methanol. Yield: 28 mg (41%). For analytical and
spectroscopic data, see the Supporting Information. Synthesis of
[ReCl(L2)] (2): H4L2 (50 mg, 0.1 mmol) was added to a solution of
[NBu4][ReOCl4] (58 mg, 0.1 mmol) in 5 mL of MeOH. The color of the
solution turned immediately to cherry red, and after the reaction solution
was stirred at 50 °C for 2 h, a microcrystalline solid was obtained. The solid
was filtered off and washed with MeOH. The product was recrystallized from
a CH2Cl2/MeOH mixture. Yield: 50 mg (68%). For analytical and spectro-
scopic data, see the Supporting Information. Synthesis of [ReO(HL2)] (3):
Solid H4L2 (50 mg, 0.1 mmol) was added to a solution of [NBu4][ReOCl4] (58
mg, 0.1 mmol) in 10 mL of MeOH and stirred for 30 min. Thereafter, NEt3
(0.3 mL) was added, and the reaction mixture was stirred at ambient
temperature for another 1 h. The product, which precipitated directly from
the reaction mixture, was filtered off and washed with MeOH. Recrystalliza-
tion from THF gave red crystals of [ReO(HL2)]. Yield: 62 mg (81%). For
analytical and spectroscopic data, see the Supporting Information.
Here, we extend this work to the potentially pentadentate
ligand systems H2L1 and H4L2. H2L1 was synthesized
*To whom correspondence should be addressed. E-mail: abram@chemie.
fu-berlin.de.
(1) (a) Deutsch, E.; Libson, K.; Vanderheyden, J.-L.; Ketring, A.; Maxon,
H. R. Nucl. Med. Biol. 1986, 13, 465. (b) Jurisson, S.; Berning, D.; Jia, W.; Ma,
D. Chem. Rev. 1993, 93, 1137. (c) Abram, U.; Alberto, R. J. Braz. Chem. Soc.
2006, 17, 1486.
(2) (a) Tisato, F.; Refosco, F.; Mazzi, U.; Bandoli, G.; Nicolini, M. Inorg.
Chim. Acta 1991, 189, 97. (b) Kramer, A.; Alberto, R.; Egli, A.; Novak-Hofer, I.;
Hegetschweiler, K.; Abram, U.; Bernhardt, P. V.; Schubiger, P. A. Bioconjugate
Chem. 1998, 9, 691. (c) Luo, H.; Liu, S.; Rettig, S. J.; Orvig, C. Can. J. Chem.
1995, 73, 2272. (d) Abrahams, A.; Gerber, T. I. A.; Luzipo, D. R; Mayer, P. S. J.
Coord. Chem. 2007, 60, 2213.
r
pubs.acs.org/IC
Published on Web 07/27/2009
2009 American Chemical Society

Communication
Inorganic Chemistry, Vol. 48, No. 17, 2009 8073
Scheme 1. Synthesis of [ReOCl(L1a)]
of the electron-withdrawing imine groups and the presence of
the electron-donating methylene components. A less-re-
solved doublet at 4.4 ppm and a broad signal at 5.4 ppm
are associated with the methylene and amine protons. The
stretching vibrations of the NH and OH components were
detected as broad bands at 3514 and 3425 cm-1 in the IR
spectrum of H4L2.
Figure 1. Molecular structure of 1.12 Hydrogen atoms are omitted for
clarity. Selected bond lengths: Re-O10, 1.675(7); Re-P, 2.394(2); Re-
N1, 1.967(8); Re-O1, 1.973(8); Re-Cl, 2.435(2); Re-O2, 2.414(7); N1-
ꢀ
˚
C11, 1.43(1); N1-C7, 1.44(1); C27-O2, 1.22(1); C27-O3, 1.31(1) A.
The reaction of H2L1 with [NBu4][ReOCl4] gives the
unexpected product [ReOCl(L1a)] (1), where H2L1a is PhP-
(C6H4-2-COOMe){C6H4-2-(CH2-NH(C6H4-2-OH))}.7 One
of the imine groups of the initially used Schiff base ligand
undergoes successive hydrolysis/solvolysis reactions, which
are accompanied by the oxidation of the carbon atom and the
formation of an ester. The two released electrons are con-
sumed in the reduction of the second imine group to the
related amine (Scheme 1). Decomposition of Schiff bases
during reactions with rhenium complexes is not without
precedent, and several complexes with coordinated frag-
ments, such as amine, arylimido, alcoholato, or hemiacetals,
are described.2c,3a,8,9 Oxidative hydrolysis of an imine group
was observed for Re(V) complexes with diazabutadiens, and
Re(IV) and Re(III) products were isolated as products.10
Although the mechanism of the present redox process is not
yet fully clear, it is most probable that the reaction is metal-
induced or at least metal-mediated. This is strongly suggested
by the coordinative bonding of the central phosphorus atom
(no formation of phosphine oxide or rhenium complexes with
the metal in other oxidation states was detected during the
entire reaction) and the fact that no evidence for the formation
of H2L1a was found during a treatment of H2L1 with
methanol. Thus, an intramolecular redox process is probable,
despite the fact that the yield of the isolated compound 1 was
less than 50%.
Scheme 2. Synthesis of [ReCl(L2)] (2) and [ReO(HL2)] (3)
Undesired and uncontrollable reactions in the novel multi-
dentate ligand system during reactions with rhenium pre-
cursors can be avoided, when the Schiff base is reduced to the
corresponding secondary amine. The resulting potentially
pentadentate aminophosphine H4L2 reacts with [NBu4]-
[ReOCl4] under the formation of different products depend-
ing on the experimental conditions. Reactions in neat metha-
nol give a product of the composition [ReCl(L2)] (2) in good
yields, while similar reactions in the presence of the support-
ing base NEt3 yield the oxorhenium(V) complex [ReO(HL2)]
(3) (Scheme 2).
The 31P NMR spectrum of 1 shows a single resonance at
21.9 ppm associated with the coordinated phosphine. The 1H
NMR spectrum confirms the presence of the ester function
by a singlet at 3.5 ppm. The electrospray ionization (time of
flight) mass spectrum of 1 supports the molecular structure
by a base peak at 678.1 amu associated with a [M þ H]þ
species. The coordination environment of the rhenium atom
is a distorted octahedron with only weak Re-O2 interactions
[ReCl(L2)] (2) belongs to some rare examples of rhenium(V)
complexes, which do not contain one of the common
ReO3þ, ReN2þ, or Re(NPh)2þ cores. Other representatives
are [ReCl5], [Re(HNC6H4S)3]-, and the mixed-ligand com-
plexes [ReCl(PPh3)(NHC6H4S)2] and [ReCl(PPh3)(L)2] (L =
3-nitro-1,2-diamidobenzene).13
Both compounds 2 and 3 are diamagnetic. Their 31P NMR
spectra show single resonances of the coordinated phos-
phines at þ11.1 ppm (2) and -4.8 ppm (3). The absence of
the NH and OH stretching vibrations in the range of 3100-
3500 cm-1 in the IR spectrum of 2 is consistent with the
deprotonation of both amine groups of the phosphine ligand,
while a medium band at 3178 cm-1 in the IR spectrum of 3
(Figure 1).11 The bond lengths N1-C7 of 1.43(1) A and
˚
˚
C11-N1 of 1.43(1) A are in the range of the single bonds,
which is consistent with the reduction of the imine group to
˚
the related amine function. The O2-C27 bond of 1.22(1) A
and the O2-C27-O3 angle of 121.9(8)° confirm the presence
1
of the ester function.
can be assigned to a NH stretch. The H NMR spectra of
2 and 3 support the complete or partial deprotonation
(8) Abram, U. Rhenium. In Comprehensive Coordination Chemistry II;
McCleverty, J. A., Mayer, T. J., Eds.; Elsevier: Amsterdam, The Netherlands,
(12) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
2003; Vol. 5, p 271.
::
(13) (a) Cotton, F. A.; Robinson, W. R.; Walton, R. A. Inorg. Chem.
1967, 6, 223. (b) Mucker, K.; Smith, G. S.; Johnson, Q. Acta Crystallogr. 1968,
B24, 874. (c) Gardner, J. K.; Pariyadath, N.; Corbin, J. L.; Stiefel, E. I. Inorg.
Chem. 1978, 17, 897. (d) Bandoli, G.; Dolmella, A.; Gerber, T. I. A.; Perils, J.; du
Preez, J. G. H. Inorg. Chim. Acta 2000, 303, 24.
(9) Barandov, A. Ph.D. Thesis, Freie Universitat, Berlin, 2009.
(10) Das, S.; Chakraborty, I.; Chakravorty, A. Inorg. Chem. 2003, 42,
6545.
(11) For details about the structure determinations and bond lengths and
angles, see the Supporting Information.

8074 Inorganic Chemistry, Vol. 48, No. 17, 2009
Barandov and Abram
Figure 2. Structure of complex 2.12 Hydrogen atoms are omitted for
clarity. Selected bond lengths: Re-Cl, 2.379(3); Re-O1, 2.018(9); Re-
O2, 2.048(8); Re-N1, 1.961(9); Re-N2, 1.935(9); Re-P, 2.354(3); N1-
Figure 3. Structure of complex 3.12 Hydrogen atoms bonded to carbon
atoms are omitted for clarity. Selected bond lengths: Re-O10, 1.687(6);
Re-O1, 2.035(6); Re-O2, 2.016(7); Re-N1, 2.035(6); Re-N2, 1.943(8);
Re-P, 2.400(3); N1-C11, 1.45(1); N1-C7, 1.46(2); N2-C31, 1.40(1);
ꢀ
˚
C11, 1.38(1); N1-C7, 1.47(2); N2-C31, 1.42(2); N2-C27, 1.49(2) A.
situation of the pentadentate ligand in both complexes, as
given in Scheme 2. The NH resonance is detected at 8.9 ppm
ꢀ
˚
N2-C27, 1.49(1) A.
1
in the H NMR spectrum of 3, while the spectrum of 2
is underlined by the fact that the oxo complex 3 is readily
converted into 2 by the addition of HCl to a THF solution of
the compound.15 Thus, compound 3 can be regarded as an
intermediate of the formation of 2 from [ReOCl4]-. How-
ever, no evidence was found for the formation of dioxo or μ-
oxo dimers during our studies with the pentadentate ligands.
The chelating phosphine ligand shows a pentadentate
coordination also in the oxo complex 3. One of its nitrogen
atoms, however, remains protonated, which goes along with
a slightly longer Re-N1 bond compared to the Re-N2 bond
and the bonding situation in 2.
presents no signal in the range of 8.1-10.0 ppm.
X-ray diffraction confirms the structure of [ReCl(L2)] (2)
as a distorted octahedral rhenium complex with a 4-fold
deprotonated, pentadentate phosphine ligand (Figure 2).11 A
chloro ligand occupies the sixth position. The bond lengths
˚
˚
between N1-C11 of 1.38(1) A and Re-N1 of 1.96(1) A are
shorter than typical single bonds. This implies a partially
double-bond characteristic resulting from the conjugation of
the lone pair of N1 with the π electrons of the adjacent phenyl
ring [C11 to C16] and unoccupied orbitals of the metal. The
presence of a partial double-bond character is confirmed by
the planar environment of N1 (angles: C11-N1-C7,
117(1)°; C11-N1-Re, 119.4(8)°; Re-N1-C7, 122.1(7))
The second nitrogen atom N(2) shows a similar planar
orientation, despite the fact that the corresponding C-N
bond lengths are in the range of single bonds of rhenium
complexes with secondary amines.14 The Re-N(2) bond
The pentadentate phosphine H4L2 is the first representa-
tive of a novel, versatile class of ligands, which are well-suited
for the formation of stable complexes with rhenium(V) and
also technetium(V) centers. The stability of the resulting
amido/imido complex justifies the use of this new class of
compounds in further experiments, particularly such as the
synthesis of bioconjugates with 186,188Re or 99mTc. For the
synthesis of radioactive bioconjugates with peptides or pro-
teins using this new class of compounds, the attachment of an
anchor group is required. The new ligand systems possess
remarkable synthetic potential in this field. Corresponding
experiments with bioconjugatable derivatives such as H2L3
are being performed in our laboratory, and such with H4L4
are planned for the near future.
˚
length of 1.93(1) A, however, has marked double-bond
character.
The double-bond character of the Re-N bonds in 2
explains the diamagnetism of the formed d2 amido/imido
complex by a similar bonding situation like in rhenium(V)
complexes with oxo, nitrido, or phenylimido cores.8 The
formation ofimido species from oxo compoundsunderacidic
conditions corresponds to the general procedure that is also
applied for the synthesis of phenylimides from oxo com-
plexes.8 Protonation of the oxo core is most likely the first
step of such reactions. This is avoidedwhena supporting base
such as NEt3 is added. Consequently, the oxo core is retained
under such conditions, and compound 3 is formed (Figure 3).
The essential role of the presence of protons in such reactions
Acknowledgment. A.B. gratefully acknowledges a
Ph.D. scholarship from DAAD.
(14) Basak, S.; Rajak, K. K. Inorg. Chem. 2008, 47, 8813.
(15) Compound 3 (0.1 mmol) was dissolved in 3 mL of THF, and 100 μL
of aqueous HCl (36 w%) was added. An immediate color change from brown
to cherry red was observed, and the 31P NMR spectrum of the resulting
solution confirms the complete conversion from 3 into 2 within 5 min.
Expectedly, the reverse reaction, the conversion of the amido/imido complex
2 into the oxo compound 3 by the addition of base Et3N or NaOH/MeOH),
is not observed.
Supporting Information Available: Experimental procedure
descriptions, additional figures and tables, and a crystallo-
graphic information file. This material is available free of charge
via the Internet at http://pubs.acs.org.