New rhenium(I) and rhenium(II) species assembled by stereospecific azopyridine chelation

Article abstract of DOI:10.1002/ejic.200390147

The spontaneous reaction of [ReO(OEt)I₂(PPh₃)₂] with 2,2′-azobipyridine (L) in acetonitrile, and of [ReOCl₃(PPh₃)₂] with L in toluene, proceeds stereospecifically furnishing the green tris-chelate [Re^IL₃]I (1) and the red bis-chelate [Re^IICl₂L₂] (2), respectively. A structure determination reveals that the coordination geometry of 1 is facial and that of 2 is cis(Cl,Cl)- trans(N^p,N^p)-cis(N^a,N^a) (N^p is a pyridine nitrogen and N^a is an azo nitrogen). The isomer preference is exclusive in both cases and no other isomer has been observed. The average N-N distance in both compounds is about 1.35 A, signifying the presence of strong d(Re)-π*(azo) back-bonding which plays a crucial role in controlling isomer specificity. The bischelate 2 is a one-electron paramagnet and displays a six-line EPR spectrum in fluid solution with g = 2.080 and A = 320 G. The ¹H NMR spectrum of diamagnetic 1 is consistent with the facial geometry. In acetonitrile solution 1 displays an Re^II/Re^I couple and 2 an Re^III/Re^II couple, the reduction potentials being 0.60 and 0.50V vs. SCE respectively. The violet-coloured imide complex [Re^VCl₃(NC₆H₄Cl-p)L] (3) is also reported; it displays an Re^VI/Re^V redox wave at 1.33 V vs. SCE. The stabilization of the lower rhenium oxidation states in 1 and 2 is consistent with the decrease of metal reduction potential with progressive L chelation. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejic.200390147

FULL PAPER
New Rhenium(
I
) and Rhenium(II) Species Assembled by Stereospecific
Azopyridine Chelation
Suman Sengupta,[a] Indranil Chakraborty,[a] and Animesh Chakravorty*[a,b]
Keywords: Rhenium / N ligands / Chelates / Back-bonding / Isomer specificity
The spontaneous reaction of [ReO(OEt)I2(PPh3)2] with 2,2Ј-
azobipyridine (L) in acetonitrile, and of [ReOCl3(PPh3)2] with
L in toluene, proceeds stereospecifically furnishing the green
tris-chelate [ReIL3]I (1) and the red bis-chelate [ReIICl2L2] (2),
line EPR spectrum in fluid solution with g = 2.080 and A =
320 G. The H NMR spectrum of diamagnetic 1 is consistent
with the facial geometry. In acetonitrile solution 1 displays
an ReII/ReI couple and 2 an ReIII/ReII couple, the reduction
1
respectively. A structure determination reveals that the coor- potentials being 0.60 and 0.50V vs. SCE respectively. The
dination geometry of 1 is facial and that of 2 is cis(Cl,Cl)-
violet-coloured imide complex [ReVCl3(NC6H4Cl-p)L] (3) is
trans(Np,Np)-cis(Na,Na) (Np is a pyridine nitrogen and Na is also reported; it displays an ReVI/ReV redox wave at 1.33 V
an azo nitrogen). The isomer preference is exclusive in both
cases and no other isomer has been observed. The average
N−N distance in both compounds is about 1.35 A, signifying
vs. SCE. The stabilization of the lower rhenium oxidation
states in 1 and 2 is consistent with the decrease of metal re-
duction potential with progressive L chelation.
˚
the presence of strong d(Re)-π*(azo) back-bonding which
plays a crucial role in controlling isomer specificity. The bis- ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
chelate 2 is a one-electron paramagnet and displays a six-
Germany, 2003)
Introduction
In the design of potential radiopharmaceuticals based on
rhenium the higher oxidation states of the metal (especially
ϩ5) have commonly been employed, although lower states
like ϩ1 and ϩ2 are now receiving increasing attention.[1,2]
This has prompted us to investigate the synthesis and char-
acterization of new rhenium() and rhenium() species.
Herein we report the unprecedented rhenium coordination
types ReIN6 and ReIICl2N4 in the form of 1 and 2, respect-
ively, assembled by 2,2Ј-azobipyridine (L) chelation. The
compounds occur exclusively in a single geometrical form:
facial in the case of 1 and cis(Cl,Cl)-trans(Np,Np)-cis-
(Na,Na) in the case of 2.
320, 330 cmϪ1) in the IR spectrum corresponding to the
cis-ReCl2 moiety and is paramagnetic (1.95 µB; t52g, S ϭ
1/2) displaying a six-line EPR spectrum (185Re and 187Re:
I ϭ 5/2)[3] in dichloromethane solution (300 K) with g ϭ
2.080 and A ϭ 320 G. The 1H NMR spectrum of dia-
magnetic (t62g) 1 is consistent with a facial geometry in
which the three chelate rings are equivalent in solution.
The geometries of 1 and 2 have been authenticated by an
X-ray structure determination. Molecular views are shown
in Figure 1 and 2.
Careful chromatographic workup during the syntheses
did not reveal the presence of the meridional isomer of 1
or of any of the other theoretically possible isomers (cis-
cis-cis, trans-cis-cis, cis-cis-trans, and trans-trans-trans) of 2.
Bond length data provide a clue to this geometrical specifi-
Results and Discussion
The reaction of [ReO(OEt)I2(PPh3)2] with excess L in
boiling acetonitrile afforded the green tris-complex [ReL3]I
(1). On the other hand a 1:2.5 mixture of [ReOCl3(PPh3)2]
and L reacted in boiling toluene furnishing red coloured
˜
[ReCl2L2] (2). The latter displays two ReϪCl stretches (ν ϭ
[a]
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
Kolkata 700 032, India
Fax: (internat.) ϩ 91-33/2473-2805
E-mail: icac@mahendra.iacs.res.in
[b]
Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore 560 064, India
Eur. J. Inorg. Chem. 2003, 1157Ϫ1160  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ1948/03/0306Ϫ1157 $ 20.00ϩ.50/0 1157

S. Sengupta, I. Chakraborty, A. Chakravorty
FULL PAPER
derline the presence of strong d(Re)-π*(azo) back-bonding
in 1 and 2. In osmium() bis-chelates[5,6] of azoheterocyclic
ligands related to L, the NϪN length is shorter by about
II
˚
0.04 A than that in 2, signifying the π-basicity order Re
Ͼ OsII.
The cis (as opposed to trans) disposition of the two Na
donor sites ensures lack of competition between them for
the same metal d-orbitals, thus maximizing back-bonding.
In facial [ReL3]ϩ three cis ReNa pairs are present, repres-
enting a model situation for back-bonding. This geometry
is, however, more crowded (aryl groups pendant from the
same face) than the unobserved meridional form which has
only two cis ReNa pairs. It is proposed that the stronger
back-bonding more than offsets the steric disadvantage of
the facial geometry which alone becomes observable. As a
π-base rhenium() is superior to isoelectronic osmium()
and this has a dramatic geometrical consequence. Os-
mium() tris-chelates of 2-arylazopyridines (pendant pyrid-
ine ring of L replaced by, for example, phenyl) occur prim-
arily in the meridional form.[5]
In the case of [ReCl2L2] the cis-trans-cis form is sterically
more favourable than the other two possible but unob-
served isomers — cis-cis-cis and trans-cis-cis — with a cis
ReNa pair. In the case of azoheterocyclic bis-chelates of
MIIX2 (M ϭ Ru,[7] Os;[5,6] X ϭ Cl, Br) isomers have been
isolated and these undergo spontaneous thermal conversion
into the stable cis-trans-cis form.[8] The stability is more crit-
ically poised for [ReCl2L2], which occurs only in the cis-
trans-cis form 2.
Figure 1. Perspective view and atom labeling scheme of [ReL3]ϩ;
all non-hydrogen atoms are represented by 30% thermal probability
ellipsoids (hydrogen atoms are omitted for clarity); selected bond
˚
lengths [A]: Re1ϪN3 1.965(9), Re1ϪN7 1.999(9), Re1ϪN5
2.111(8), Re1ϪN11 1.982(7), Re1ϪN1 2.110(8), Re1ϪN9 2.115(8),
N2ϪN3 1.360(12), N10ϪN11 1.333(10), N6ϪN7 1.312(11); se-
lected bond angles [°]: N3ϪRe1ϪN11 96.1(3), N11ϪRe1ϪN7
96.0(3),
N11ϪRe1ϪN1
97.0(3),
N3ϪRe1ϪN5
96.7(3),
N7ϪRe1ϪN5 73.4(4), N3ϪRe1ϪN9 164.9(3), N7ϪRe1ϪN9
97.1(3), N5ϪRe1ϪN9 95.3(3), N3ϪRe1ϪN7 95.1(4),
N3ϪRe1ϪN1 74.5(3), N7ϪRe1ϪN1 164.1(3), N11ϪRe1ϪN5
164.0(3), N1ϪRe1ϪN5 95.5(3), N11ϪRe1ϪN9 73.8(3),
N1ϪRe1ϪN9 95.2(3)
A comment on the redox stabilization of rhenium oxida-
tion states by L is in order here. In acetonitrile solution
the nearly reversible (peak-to-peak separation 70Ϫ80 mV)
Figure 2. Perspective view and atom labeling scheme of [ReL2Cl2];
all non-hydrogen atoms are represented by 30% thermal probability
ellipsoids (hydrogen atoms are omitted for clarity); selected bond
˚
lengths [A]: Re1ϪN3 2.001(13), Re1ϪN5 2.043(10), Re1ϪN7
2.005(11), Re1ϪN1 2.080(13), Re1ϪCl2 2.374(4), Re1ϪCl1
2.384(4), N2ϪN3 1.37(2), N6ϪN7 1.32(2); selected bond angles [°]:
N3ϪRe1ϪN7 91.8(5), N7ϪRe1ϪN5 74.4(4), N7ϪRe1ϪN1
108.3(5),
N3ϪRe1ϪCl2
93.4(4),
N5ϪRe1ϪCl2
90.3(3),
N3ϪRe1ϪCl1 163.4(4), N5ϪRe1ϪCl1 88.6(3), Cl2ϪRe1ϪCl1
92.25(14), N3ϪRe1ϪN5 106.9(5), N3ϪRe1ϪN1 75.0(6),
N5ϪRe1ϪN1 176.8(4), N7ϪRe1ϪCl2 164.7(3), N1ϪRe1ϪCl2
87.0(3), N7ϪRe1ϪCl1 86.8(4), N1ϪRe1ϪCl1 89.7(4)
Figure 3. Perspective view and atom labeling scheme of
[ReCl3(NC6H4Cl-p)L]; all non-hydrogen atoms are represented by
30% thermal probability ellipsoids (hydrogen atoms are omitted for
city. The average ReϪNa and ReϪNp lengths are 1.98 and
˚
˚
2.11 A in 1 and 2.00 and 2.06 A in 2. The average NϪN
˚
clarity) selected bond lengths [A]: Re1ϪN5 1.72(2), Re1ϪN1
˚
length in both compounds is about 0.1 A longer than the
2.18(3), Re1ϪN3 2.00(2), Re1ϪCl1 2.354(9), Re1ϪCl2 2.386(9),
uncoordinated azo length (1.25 A).[4] These data clearly un- Re1ϪCl3 2.335(9), N2ϪN3 1.30(4)
˚
1158
Eur. J. Inorg. Chem. 2003, 1157Ϫ1160

New Rhenium(I) and Rhenium(II) Species
FULL PAPER
51 mg, 60% yield based on [ReO(OEt)I2(PPh3)2]}.1H NMR
(300 MHz, CDCl3, 298 K): δ ϭ 8.12 (d, J ϭ 3.7 Hz, H1), 7.85
(complex multiplet, H2), 7.85 (complex multiplet, H3), 8.00 (d, J ϭ
6.0, H4), 6.99 (d, J ϭ 6.0, H7), 7.47 (t, J ϭ 6.0, H8), 7.59 (t, J ϭ
8.7, H9), 7.20 (d, J ϭ 5.6, H10)
cyclic voltammetric metal redox couples ReII/ReI in 1 and
ReIII/ReII in 2 have E1/2 values of 0.60 and 0.50 V versus
SCE respectively. No other metal redox couples are ob-
served in the accessible potential window. On the other
hand the rhenium() state has been realized in the violet-
coloured imide complex [ReCl3(NC6H4Cl-p)L] (3; Figure 3)
synthesized by reacting [ReOCl3(PPh3)2] with L in 1:1.5 ra-
tio in the presence of excess p-ClC6H4NH2.
Complex 2: L (55.3 mg, 0.300 mmol) was added to a suspension
of [ReOCl3(PPh3)2] (100 mg, 0.120 mmol) in toluene (25 mL). The
resulting mixture was refluxed for 20 min affording a reddish solu-
tion. The solvent was then quickly removed under reduced pressure
and the solid mass thus obtained was dissolved in a small amount
of dichloromethane and subjected to chromatography in the same
manner as mentioned above. A reddish orange band was eluted out
with a toluene/acetonitrile (25:2) mixture. Solvent removal under
reduced pressure afforded [ReCl2L2] (2) in pure form which was
dried under vacuo over fused calcium chloride {2: 49 mg, 65% yield
based on [ReOCl3(PPh3)2]}.
It displays a single metal redox couple ReVI/ReV having
an E1/2 of 1.33V. Evidently the E1/2 of a given Renϩ1/Ren
couple increases dramatically as the number of L chelate
rings increase. The variation of this number along with col-
igand control provides an excellent handle for stabilizing
various oxidation states of rhenium under ambient condi-
tions. Monochelation stabilizes rhenium() as has also been
documented for related azoheterocyclic ligands.[9,10] Tris-
and bis-chelation are now shown to make the mono- and
divalent states accessible in the form of 1 and 2, respectively.
Complex 3: L (33 mg, 0.180 mmol) and p-chloroaniline (76 mg,
0.600 mmol) were added to a suspension of [ReOCl3(PPh3)2]
(100 mg, 0.120 mmol) in toluene (25 mL). The mixture was heated
to reflux for 2.5 h, affording a violet solution. The solvent was then
removed under reduced pressure, and the mass thus obtained was
subjected to chromatographic workup as before. Excess amine (p-
ClC6H4NH2) was eluted with toluene. A violet band was eluted
with a toluene/acetonitrile (25:1) mixture. Solvent removal from the
eluate under reduced pressure afforded [ReCl3(NC6H4Cl-p)L] (3)
as a dark solid {3: 43 mg, 60% yield based on [ReOCl3(PPh3)2]}.
Conclusion
So far rhenium() compounds have been primarily known
to involve[11,12] carbon monoxide, isocyanide and tertiary
phosphane coordination. On the other hand, authentic
mononuclear rhenium() compounds generally incorpor-
ate[11,13] nitrosyl/thionitrosyl and phosphane binding, with
a few exceptions.[1,14] The [ReL3]I and [ReCl2L2] chelates
Crystal Structure Determination: Single crystals of complexes 1, 2
and 3 were grown by slow diffusion of hexane into dichlorome-
add a new dimension to compound types that can span the thane solutions of the respective compounds. Data were collected
on a Nicolet R3m/V four-circle diffractometer with graphite-mon-
ochromated Mo-Kα radiation (λ ϭ 0.71073 A) by the ω-scan tech-
ϩ1 and ϩ2 oxidation states of rhenium.
˚
nique in the range 3° Յ 2θ Յ 47° for complexes 1 and 3 and 3° Յ
2θ Յ 50° for 2. All data were corrected for Lorentz-polarization
and absorption.[18] The metal atoms were located from Patterson
maps and the rest of the non-hydrogen atoms emerged from suc-
cessive Fourier syntheses. The structures were then refined by a
full-matrix least-squares procedure on F2. All non-hydrogen atoms
were refined anisotropically. All hydrogen atoms were included in
calculated positions. Calculations were performed using the
SHELXTLTM V 5.03 Program package.[19] In the structure of 3
there are two very similar but crystallographically independent mo-
lecules in the asymmetric unit. Figure 3 refers to molecule 1.
Crystal Data for 1: C30H24IN12Re, monoclinic, C2/c (no. 15) a ϭ
Experimental Section
General: [ReOCl3(PPh3)2],[15] [ReO(OEt)I2(PPh3)2],[16] and 2,2Ј-
azobipyridine[4] were prepared by reported methods. For electro-
chemical work HPLC-grade acetonitrile was used. All other chem-
ical and solvents were of reagent grade and were used as received.
Spectral measurements were made with the following equipment:
IR (KBr disk), PerkinϪElmer 783 IR spectrometer; X-band EPR,
Varian E-109C spectrometer. 1H NMR, Bruker 300 MHz FT spec-
trometer (the proton numbering scheme used is the same as in crys-
tallography and the assignments are based on chemical shifts, coup-
ling constants and previous work[5]). The magnetic susceptibility
was measured on a PAR 155 vibrating-sample magnetometer. Elec-
trochemical measurements were performed under a nitrogen atmo-
sphere using a PAR 370Ϫ4 electrochemistry system with platinum
working electrode.[17] The supporting electrolyte was tetraethylam-
monium perchlorate (TEAP), and the potentials are referenced to
the saturated calomel electrode (SCE) without junction correction.
˚
22.914(5), b ϭ 13.236(3), c ϭ 21.722(4) A, β ϭ 110.61(3)°, V ϭ
3
˚
˚
6166(2) A , Z ϭ 8, Mo-Kα (λ ϭ 0.71073 A), 4589 unique reflections
(Rint ϭ 0.0360). Final residuals R1 ϭ 0.0391 and wR2 ϭ 0.0826 [I
Ͼ 2σ(I)], 403 parameters.
Crystal Data for 2: C20H16Cl2N8Re, orthorhombic, Pbcn (no. 60),
3
˚
˚
a ϭ 19.190(9), b ϭ 15.431(9), c ϭ 15.062(14) A, V ϭ 4460(5) A ,
˚
Z ϭ 8, Mo-Kα (λ ϭ 0.71073 A), 3937 unique reflections (Rint
ϭ
0.0338). Final residuals R1 ϭ 0.0603 and wR2 ϭ 0.1356 [I Ͼ 2σ(I)],
Complex 1: L (107 mg, 0.582 mmol) was added to a solution of
[ReO(OEt)I2(PPh3)2] (100 mg, 0.097 mmol) in acetonitrile (15 mL).
The resulting mixture was heated to reflux for 3 h, affording a
brownish green solution. The solvent was then removed under re-
duced pressure. The mass thus obtained was dissolved in a min-
imum amount of dichloromethane and subjected to chromatog-
raphy on a silica gel column (20 ϫ 1 cm, 60Ϫ120 mesh). A deep-
green band was eluted out with a toluene/acetonitrile (1:1) mixture.
Removal of solvent from the eluate gave [ReL3]I (1) in pure form.
It was finally dried under vacuo over fused calcium chloride {1:
280 parameters.
¯
Crystal Data for 3: C16H13Cl3N5Re, triclinic, P1 (no. 2), a ϭ
13.379(5), b ϭ 13.652(5), c ϭ 13.992(6) A, α ϭ 61.90(3), β ϭ
˚
3
˚
65.44(3), γ ϭ 66.12(3)°, V ϭ 1980(1) A , Z ϭ 4, Mo-Kα (λ ϭ
˚
0.71073 A), 5815 unique reflections (Rint ϭ 0.0368). Final residuals
R1 ϭ 0.0981 and wR2 ϭ 0.2232 [I Ͼ 2σ(I)], 464 parameters.
CCDC-192908 (1), -192909 (2) and -192910 (3) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retriev-
ing.html [or from the Cambridge Crystallographic Data Centre, 12,
Eur. J. Inorg. Chem. 2003, 1157Ϫ1160
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S. Sengupta, I. Chakraborty, A. Chakravorty
FULL PAPER
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Received September 17, 2002
[I02515]
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