Reactions of the triply-bonded complex cis-Re2(μ-O 2CCH3)2 Cl2-(μ-dppm)2 with pyridine carboxylic acids

Article abstract of DOI:10.1039/b306331n

The diamagnetic dirhenium(II) complexes Re₂(pic)Cl ₃(μ-dppm)₂ (2), Re₂(dipic)Cl ₂(μ-dppm)₂ (3), Re₂(HnicO) ₂Cl₂(μ-dppm)₂ (4) and Re ₂(picO)₂(μ-dppm)₂ (5) that are formed by the reactions of cis-Re₂(μ-O₂CCH₃) ₂Cl₂(μ-dppm)₂ (1) with picolinic acid (Hpic), dipicolinic acid (H₂dipic), 2-hydroxynicotinic acid (HnicOH) and 6-hydroxypicolinic acid (HpicOH) represent four different types of reaction products from the displacement of the cis acetate groups of 1. All four compounds have been characterized by X-ray crystallography. Compound 2 can be prepared more logically and in higher yield from Re₂Cl ₄(μ-dppm)₂. Compound 3, which retains the same cis,cis coordination of μ-dppm ligands that is present in 1, is the third structural isomer of Re₂(dipic)Cl₂(μ-dppm)₂; the other two, which are prepared from Re₂Cl₄(μ-dppm) ₂, contain trans,trans and transmis ligation by the pair of μ-dppm ligands. Compounds 4 and 5 are the first dirhenium(II) compounds that contain bridging 2-pyridonate ligands; in 4 the carboxylic acid group of μ-HnicO does not coordinate, whereas the μ-picO ligands in 5 are bound in a tridentate fashion and, consequently, the two axial Re-Cl bonds of 1 have been replaced by axial Re-O (carboxylate) coordination. Complexes 3-5 have quite accessible oxidation chemistry as shown by cyclic voltammetric measurements.

Full text of DOI:10.1039/b306331n

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Reactions of the triply-bonded complex cis-Re (ꢀ-O CCH ) Cl -
2
2
3 2
2
(
ꢀ-dppm) with pyridine carboxylic acids. The isolation and
2
structural characterization of a third structural isomer of
Re (dipic)Cl (ꢀ-dppm) (dipic ؍ pyridine-2,6-dicarboxylate)
2
2
2
Swarup Chattopadhyay, Phillip E. Fanwick and Richard A. Walton*
Brown Laboratory of Chemistry, Purdue University, 425 Central Drive, West Lafayette,
Indiana 47907-2018, USA. E-mail: rawalton@purdue.edu
Received 5th June 2003, Accepted 28th July 2003
First published as an Advance Article on the web 14th August 2003
The diamagnetic dirhenium() complexes Re (pic)Cl (µ-dppm) (2), Re (dipic)Cl (µ-dppm) (3), Re (HnicO) Cl -
2
3
2
2
2
2
2
2
2
(
µ-dppm) (4) and Re (picO) (µ-dppm) (5) that are formed by the reactions of cis-Re (µ-O CCH ) Cl (µ-dppm) (1)
2 2 2 2 2 2 3 2 2 2
with picolinic acid (Hpic), dipicolinic acid (H dipic), 2-hydroxynicotinic acid (HnicOH) and 6-hydroxypicolinic acid
2
(
HpicOH) represent four different types of reaction products from the displacement of the cis acetate groups of 1. All
four compounds have been characterized by X-ray crystallography. Compound 2 can be prepared more logically and
in higher yield from Re Cl (µ-dppm) . Compound 3, which retains the same cis,cis coordination of µ-dppm ligands
2
4
2
that is present in 1, is the third structural isomer of Re (dipic)Cl (µ-dppm) ; the other two, which are prepared from
2
2
2
Re Cl (µ-dppm) , contain trans,trans and trans,cis ligation by the pair of µ-dppm ligands. Compounds 4 and 5 are the
2
4
2
first dirhenium() compounds that contain bridging 2-pyridonate ligands; in 4 the carboxylic acid group of µ-HnicO
does not coordinate, whereas the µ-picO ligands in 5 are bound in a tridentate fashion and, consequently, the two
axial Re–Cl bonds of 1 have been replaced by axial Re–O (carboxylate) coordination. Complexes 3–5 have quite
accessible oxidation chemistry as shown by cyclic voltammetric measurements.
Introduction
picolinic acid (Hpic, py-2-CO
H), dipicolinic acid (H dipic,
2 2
py-2,6-(CO H) ), 2-hydroxypyridine, 2-hydroxynicotinic acid
2
2
The substitutional lability of the pair of acetate groups in
the complex cis-Re (µ-O CCH ) Cl (µ-dppm) (1) towards
monoanionic bridging ligands, especially other carboxylates,
provides a convenient strategy for incorporating the triply
(
HnicOH) and 6-hydroxypicolinic acid (HpicOH) were obtained
1
2
2
3
2
2
2
from Aldrich Chemical Co. All other reagents and organic
solvents were purchased from commercial sources and were
used without further purification. Solvents were deoxygenated
by purging with dinitrogen prior to use and all reactions were
carried out under an atmosphere of dinitrogen. NMR spectra
and cyclic voltammograms were determined as described pre-
4ϩ
bonded [Re᎐Re] unit into homometallic and hetereometallic
᎐
2
–4
assemblies. The presence of a pendant coordinating group
attached to the carboxylate moiety, as present in the iso-
nicotinate and 4-(diphenylphosphino)benzoate ligands, has
6
viously. Elemental microanalyses were done by Dr H. D. Lee
3
4
4
been used to prepare mixed Re Pt , Re Pd2 and Re Au2
4
2
2
2
of the Purdue University Microanalytical Laboratory.
assemblies, while bridging dicarboxylate ligands, such as
terephthalate and trans-1,4-cyclohexanedicarboxylate, can be
used to prepare either supramolecular triangles or “dimers-
of-dimers”, the latter being linked through intermolecular
B
Reactions of cis-Re2
(ꢀ-O CCH ) Cl (ꢀ-dppm) (1) with
2
3
2
2
2
pyridine carboxylic acids
2
(i) Synthesis of Re (pic)Cl (ꢀ-dppm) (2). A mixture of pico-
2
3
2
hydrogen-bonds into zigzag chains. These studies are of
linic acid (46 mg, 0.374 mmol) and 1 (100 mg, 0.075 mmol) was
stirred in 30 mL of ethanol and then slowly brought to reflux.
After one day, the mixture was cooled to r.t. and the insoluble
green solid was filtered off and washed with cold ethanol (3 × 5
mL) and diethyl ether (3 × 5 mL). Yield of 2: 48 mg (47%).
Recrystallization from dichloromethane–hexanes gave single
crystals of 2 which were shown by X-ray crystallography to be
identical to the product formed by the reaction of Re Cl -
relevance to the growing body of work that has been focused on
the incorporation of multiply bonded dimetal units into
supramolecular assemblies.
5
While the displacement of the acetate groups in 1 by iso-
nicotinate to form cis-Re (µ-O C-4-C H N) Cl (µ-dppm) leaves
2
2
5
4
2
2
2
a pair of nitrogen atoms available for coordination to other
3
metal units, studies have not yet been carried out of the
2
4
reactions between 1 and pyridine-2-carboxylate ligands in
which the nitrogen can also bind to the same dirhenium unit
that is complexed by the carboxylate group. To ascertain
whether this is the case, we have examined the reactions of 1
with picolinic acid, dipicolinic acid, 6-hydroxypicolinic acid
and 2-hydroxynicotinic acid. The reaction course is different in
each case, and the pyridine carboxylate ligands are found to
bridge the dirhenium() unit and/or chelate to one of the metal
centers.
7
(
µ-dppm) with picolinic acid or the salt [(Ph P) N]pic.
2
3
2
(
ii) Synthesis of Re (dipic)Cl (ꢀ-dppm) (3). A mixture of 1
2 2 2
(
100 mg, 0.075 mmol) and dipicolinic acid (63 mg, 0.377 mmol)
in ethanol (30 mL) was refluxed for one day. Work-up as in
section B(i) gave 3 as a red solid. Yield of 3: 75 mg (76%). This
product was recrystallized from dichloromethane–hexanes.
Calc. for C H Cl NO P Re (i.e. 3ؒCH Cl ): C, 47.64; H, 3.38;
5
8
49
4
4
4
2
2
2
1
N, 0.96. Found: C, 47.09; H, 3.65; N, 0.95%. H NMR spectrum
(
4
δ in CDCl ): 9.0–6.5 (m, 43 H, dipic and Ph of dppm), 5.98 and
3
31
1
.50 (m, 4H, CH of dppm). P{ H} NMR spectrum (δ in
2
Experimental
CDCl ): ϩ5.48 (d), ϩ1.78 (d), Ϫ4.43 (d), Ϫ9.43 (d). Cyclic
3
n
voltammogram (0.1 M Bu NPF –CH Cl at 25 ЊC, Pt-bead
A
Starting materials, reaction procedures, and physical
4
6
2
2
Ϫ1
electrode, v = 200 mV s ): E (ox) = ϩ0.73 V vs. Ag/AgCl.
measurements
1/2
The dirhenium() complex cis-Re (µ-O CCH ) Cl (µ-dppm)
(iii) Synthesis of Re (HnicO) Cl (ꢀ-dppm) (4). A mixture
of 1 (100 mg, 0.075 mmol) and 2-hydroxynicotinic acid (53 mg,
2
2
3
1
2
2
2
2
2
2
2
(
1) was prepared by the literature method. The ligands
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 3 6 1 7 – 3 6 2 1
3617

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0
.381 mmol) was refluxed in ethanol (30 mL) for 3 days.
Work-up as in section B(i) gave 4 as an orange solid. Yield of 4:
3 mg (83%). This product was recrystallized from dichloro-
methane–hexanes. Calc. for C H Cl N O P Re : C, 50.04; H,
the refinement but constrained to ride on the atom to which
they are bonded. An empirical absorption correction using
10
9
SCALEPACꢀ was applied in all cases. The final refinements
11
were performed by the use of the program SHELXL-97. All
non-hydrogen atoms were refined with anisotropic thermal
parameters unless indicated otherwise. Crystallographic draw-
62
52
2
2
6
4
2
1
3
.52; N, 1.88. Found: C, 49.11; H, 3.63; N, 1.66%. H NMR
spectrum (δ in CDCl ): 12.73 (s, 2H, CO H of HnicO), 8.8–6.4
3
2
12
(
m, 46H, HnicO and Ph of dppm), 5.02 (m, 4H, CH of dppm).
ings were done using the program ORTEP.
2
3
1
1
P{ H} NMR spectrum (δ in CDCl ): Ϫ12.58, Ϫ13.20, Ϫ14.43
The structure solutions and refinements of all three
compounds proceeded without significant problem. The crystal
of 3 contained well behaved dichloromethane solvent, the
non-hydrogen atoms of which were refined with anisotropic
thermal parameters. During the structure refinement of 5, a
small amount of residual electron density, probably associated
with disordered dichloromethane solvent, was adjusted using
3
and Ϫ15.05 (components of an AAЈBBЈ pattern). Cyclic
voltammogram (0.1 M Bu NPF –CH Cl at 25 ЊC, Pt-bead
electrode, v = 200 mV s ): E (ox)(1) = ϩ0.53 V and E (ox)(2)
=
n
4
6
2
2
Ϫ1
1/2
1/2
ϩ1.30 V vs. Ag/AgCl.
(
iv) Synthesis of Re (picO) (ꢀ-dppm) (5). A mixture of 1
2
2
2
13
the SQUEEZE option in PLATON. The largest peaks remain-
(
0
100 mg, 0.075 mmol) and 6-hydroxypicolinic acid (26 mg,
.187 mmol) in 30 mL of ethanol was refluxed for 2 days,
ing in the final difference maps of 3ؒ2CH Cl , 4 and 5 were 1.79,
2
2
Ϫ3
1
.82 and 1.26 e Å , respectively.
then cooled to r.t., and filtered. The dark red filtrate was evap-
orated to dryness under a reduced pressure and the residue
extracted into 10 mL of dichloromethane. The dichloro-
methane extracted was allowed to evaporate under a reduced
pressure to afford red microcrystals. Yield of 5: 96 mg (90%).
This product was recrystallized from dichloromethane–
hexanes. Calc. for C H N O P Re (i.e. 5ؒH O): C, 51.95; H,
CCDC reference numbers 212135–212138.
See http://www.rsc.org/suppdata/dt/b3/b306331n/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
6
2
52
2
7
4
2
2
1
3
.66; N, 1.95. Found: C, 51.51; H, 3.70; N, 1.84%. H NMR
The reactions of cis-Re (µ-O CCH ) Cl (µ-dppm) (1) with
2
2
3
2
2
2
spectrum (δ in CDCl ): 8.0–6.7 (m, br, 46H, picO and Ph of
picolinic acid (Hpic), dipicolinic acid (H dipic), 2-hydroxynico-
3
2
31
1
dppm), 6.08 and 5.28 (m, 4H, CH of dppm). P{ H} NMR
tinic acid (HnicOH) and 6-hydroxypicolinic acid (HpicOH)
give four different types of products as shown in Scheme 1.
However, in all cases, the two cis acetato groups of 1 are dis-
placed in refluxing ethanol. Complex 2, which is formed in
2
spectrum (δ in CDCl ): ϩ19.8 and ϩ1.2 (m, br). Cyclic vol-
3
n
4
tammogram (0.1 M Bu NPF –CH Cl at 25 ЊC, Pt-bead
electrode, v = 200 mV s ): E (ox)(1) = ϩ0.15 V and E (ox)(2)
6
2
2
Ϫ1
1/2
1/2
7
=
ϩ1.13 V vs. Ag/AgCl.
Compound 5 is also obtained when the complex Re Cl -
<50% yield from picolinic acid, has been prepared previously
by the reaction of Re Cl (µ-dppm) with Hpic or [PPN]pic
2
4
2
4
ϩ
2
(
µ-dppm) is reacted with 6-hydroxypicolinic acid in refluxing
(where PPN = [(Ph P) N] ). Its formation from 1 attests to
2
3 2
ethanol with use of these same reaction conditions and
work-up. This sample was recrystallized from dichloro-
methane–hexanes. Yield: 32%. The identities of single crystals
its stability, since the reaction mechanism must involve the
sacrifice of a portion of the starting material 1 in order to
provide the extra chloride ligand that is incorporated into the
product. The structure of 2, which was established by X-ray
crystallography, shows that there has been a switch from a
cis,cis coordination of the µ-dppm ligands in 1 to a trans,cis
arrangement in 2 (see Scheme 1). This compound is not
discussed further (see ref. 7 for additional details). In the case of
3–5, a cis,cis coordination of the dppm ligands is retained as
confirmed by X-ray crystal structure determinations of all three
complexes (Figs. 1–3).
8
selected from samples of both 5 and 5ؒH O were established by
2
X-ray crystallography and these were shown to be essentially
identical with the exception of the presence of a water molecule
in the latter crystal. Only structural data for 5 are reported
and discussed herein. Full crystallographic data for 5ؒH O are
2
available in the CIF file.
(
v) Reaction of 1 with 2-hydroxypyridine. The reaction of
these reagents in refluxing ethanol for 3 days left an orange
31
1
solid that was shown by P{ H} NMR spectroscopy and cyclic
voltammetry to be unreacted 1. Based upon the amount
recovered (>90%), 2-hydroxypyridine does not appear to react
under these conditions.
C
X-Ray crystallography
Single crystals of composition Re (dipic)Cl (µ-dppm) ؒ
2
2
2
2
CH Cl2 (3ؒ2CH Cl ), Re (HnicO) Cl (µ-dppm) (4), Re2-
2 2 2 2 2 2 2
(
picO) (µ-dppm)2 (5) and Re (picO) (µ-dppm) ؒH O (5ؒH O)
2 2 2 2 2 2
were obtained by slow diffusion of hexanes into dichloro-
methane solutions of the complexes 3–5 under a dinitrogen
atmosphere.
The crystals were mounted on glass fibers in random
orientations. The data collections were carried out at 150(±1) ꢀ
with graphite-monochromated Mo-ꢀα radiation (λ
=
0
.71073 Å) on a Nonius ꢀappaCCD diffractometer. Lorentz
and polarization corrections were applied to all data sets. The
key crystallographic data for 3ؒ2CH Cl , 4 and 5 are given in
2
2
Table 1.
The structures were solved using the structure solution
9
program PATTY in DIRDIF-99. The remaining atoms were
located in succeeding difference Fourier syntheses. Hydrogen
atoms were placed in calculated positions according to ideal-
Scheme 1 Products formed by the reactions of cis-Re (µ-O CCH ) -
2 2
2
2
3 2
ized geometries with U(H) = 1.3U (C). They were included in
Cl (µ-dppm) (4) with pyridine carboxylic acids in refluxing ethanol.
eq
3
618
D a l t o n T r a n s . , 2 0 0 3 , 3 6 1 7 – 3 6 2 1

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Table 1 Crystallographic data for the dirhenium() complexes Re (dipic)Cl (µ-dppm) ؒ2CH Cl (3ؒ2CH Cl ), Re (HnicO) Cl (µ-dppm) (4), and
2
2
2
2
2
2
2
2
2
2
2
Re (picO) (µ-dppm) (5)
2
2
2
3
ؒ2CH Cl
4
5
2
2
Formula
Formula weight
Crystal system
C H Cl NO P Re
1547.08
Monoclinic
C H Cl N O P Re
1488.32
Monoclinic
C H N O P Re
62 50 2 6 4 2
1415.40
Monoclinic
5
9
51
6
4
4
2
62 52
2
2
6
4
2
Space group
P2 /n (no. 14)
P2 /c (no. 14)
P2 /n (no. 14)
1
1
1
a/Å
b/Å
c/Å
β/Њ
19.4967(4)
15.3146(3)
19.7094(4)
93.0862(8)
4
5876.4(2)
1.749
13.0405(3)
22.9844(5)
19.7779(3)
107.5243(15)
4
5652.9(2)
1.749
12.1326(3)
21.4366(5)
23.4676(6)
104.9374(11)
4
5897.2(3)
1.594
Z
V/Å
3
Ϫ3
D /g cm
c
Ϫ1
µ(Mo-ꢀα)/mm
Reflections:
collected
4.601
4.597
4.313
45465
13990
10360
685
0.072
0.044
0.104
0.982
43144
13025
6795
703
0.090
0.045
0.117
1.031
24531
13040
8688
685
0.047
0.041
0.076
0.913
independent
observed [I > 2σ(I )]
No. of variables
Rint
a
R(Fo)
w
2
b
R (F )
o
GOF
a
2
2
b
2
2
c
2
2 2 1/2
o
R = Σ||F | Ϫ |F ||/Σ|F | with F > 2σ(F ). R = [Σw(|F | Ϫ |F |) /Σw|F | ]
.
o
c
o
o
o
w
o
12
Fig. 1 ORTEP representation of the structure of the dirhenium()
complex Re (dipic)Cl (µ-dppm) as present in 3ؒ2CH Cl . Thermal
12
Fig. 2 ORTEP representation of the structure of the dirhenium()
complex Re (HnicO) Cl (µ-dppm) (4). The thermal ellipsoids are
2
2
2
2
2
2
2
2
2
ellipsoids are drawn at the 50% probability level. The phenyl carbon
atoms of the dppm ligands have been omitted for clarity except for the
carbon atoms that are bound to phosphorus (these are shown as circles
of arbitrary radius). Selected bond distances (Å) and bond angles (Њ):
Re(1)–Re(2) 2.2583(3), Re(1)–Cl(1) 2.5753(13), Re(1)–N(1) 2.185(4),
Re(1)–O(71) 2.061(4), Re(1)–P(1) 2.4596(13), Re(1)–P(3) 2.4430(14),
Re(2)–Cl(2) 2.3672(14), Re(2)–O(81) 2.107(4), Re(2)–P(2) 2.3642(14),
Re(2)–P(4) 2.3630(14), C(7)–O(71) 1.307(6), C(7)–O(72) 1.211(6), C(8)–
O(81) 1.281(7), C(8)–O(82) 1.228(7); Re(2)–Re(1)–Cl(1) 168.15(3),
N(1)–Re(1)–P(1) 159.40(12), O(71)–Re(1)–P(3) 158.51(11), N(1)–
Re(1)–O(71) 75.91(15), Cl(2)–Re(2)–P(4) 145.42(5), O(81)–Re(2)–P(2)
drawn at the 50% probability level. The phenyl carbon atoms of the
dppm ligands have been omitted for clarity except for the carbon atoms
that are bound to phosphorus (these are shown as circles of arbitrary
radius). Selected bond distances (Å) and bond angles (Њ): Re(1)–Re(2)
2.3035(6), Re(1)–Cl(1) 2.562(3), Re(1)–N(11) 2.231(9), Re(1)–O(22)
2.154(8), Re(1)–P(1) 2.413(3), Re(1)–P(3) 2.411(3), Re(2)–Cl(2)
2.539(3), Re(2)–N(21) 2.214(9), Re(2)–O(12) 2.184(8), Re(2)–P(2)
2.409(3), Re(2)–P(4) 2.418(3), C(12)–O(12) 1.296(15), C(22)–O(22)
1.299(15), C(17)–O(171) 1.317(19), C(17)–O(172) 1.260(18), C(27)–
O(217) 1.351(18), C(27)–O(272) 1.209(15); Re(2)–Re(1)–Cl(1)
70.29(17), Re(1)–Re(2)–Cl(2) 171.67(8), N(11)–Re(1)–P(3) 170.9(3),
1
1
66.10(11). The torsion angles P(1)–Re(1)–Re(2)–P(2), O(71)–Re(1)–
Re(2)–Cl(2), N(1)–Re(1)–Re(2)–O(81) and P(3)–Re(1)–Re(2)–P(4) are
1.29(5), 30.69(10), 43.47(14) and 24.12(5)Њ, respectively.
O(22)–Re(1)–P(1) 170.6(2), N(21)–Re(2)–P(2) 170.1(3), O(12)–Re(2)–
P(4) 173.4(2). The torsion angles P(1)–Re(1)–Re(2)–P(2), N(11)–Re(1)–
Re(2)–O(12), O(22)–Re(1)–Re(2)–N(21) and P(3)–Re(1)–Re(2)–P(4)
are 21.32(10), 20.92(35), 19.63(35) and 20.40(10)Њ, respectively.
2
3
The structure of 3 (Fig. 1) shows an unsymmetrical η -
7
O,N,OЈ coordination of the dipic ligand which bridges the two
Re atoms. This stable compound is the third isomeric form of
Re (dipic)Cl (µ-dppm) ; two other isomers (6 and 7) have
and 7 (2.2512(3) Å). All three complexes possess staggered
rotational geometries, the χ values for the usual torsion
av
1
4
angles being 29.9, 45.8 and 42.8Њ, respectively. Since there is
no electronic barrier to rotation in triply bonded [Re ] com-
plexes, these variations in χav are to be expected since they
2
2
2
4ϩ
previously been prepared by the reaction of Re Cl (µ-dppm)
2
4
2
2
7
14
with [N(PPh ) ] dipic in ethanol. At room temperature the
3
2 2
kinetic isomer 6 is isolated, but it converts quantitatively to the
thermodynamically stable isomer 7 when refluxed in ethanol.
We have found no conditions that lead to the interconversion of
largely reflect the minimization of intramolecular non-bonded
repulsions, and the various conformation demands of the
different five- and six-membered rings that are contained within
these complexes. The small differences in Re–Re distances are
most likely a result of the differences in the axial ligation that
3
and 7. The Re–Re triple bond distance in 3 is 2.2583(3) Å, a
value that is very similar to the distances in 6 (2.2750(10) Å)
7
D a l t o n T r a n s . , 2 0 0 3 , 3 6 1 7 – 3 6 2 1
3619

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of each HnicO ligand is not involved in coordination to the
dirhenium() unit. Since the distances between the oxygen
atoms O(171) and O(271) of the two –CO H groups and the
2
corresponding atoms O(12) and O(22) of the 2-pyridonate
moieties are quite short (2.540 and 2.586 Å, respectively),
this implies that intramolecular O–H ؒ ؒ ؒ O hydrogen bonds
are participating in the formation of planar six-membered rings
1
O–H ؒ ؒ ؒ O–C–C–C (see Fig. 2). In the H NMR spectrum of 4
(
recorded in CDCl ) the ‘free’ acid groups appear as a singlet at
3
δ ϩ12.73. The structure of 5 resembles that of 4 except that the
two terminal Re–Cl bonds are replaced by Re–O bonds formed
by the carboxylic acid groups of the bridging tridentate
6
-hydroxypicolinic acid ligand. The Re–Re distances in 4 and 5
are very similar (2.3035(6) and 2.3194(3) Å, respectively) but
are longer than the distance in 3 (2.2583(3) Å). This reflects
the presence of two axial Re–Cl or Re–O bonds in 4 and 5 in
contrast to 3 which has only one Re–Cl axial bond. While 4 and
5
both have staggered rotation geometries, the value of χav for
1
2
Fig. 3 ORTEP representation of the structure of the dirhenium()
complex Re (picO) (µ-dppm) (5). The thermal ellipsoids are drawn at
4 (20.6Њ) is close to twice that in 5 (12.0Њ), reflecting the
much more rigid nature of the tridentate µ-picO ligands com-
pared to bidentate µ-HnicO. Compounds 4 and 5 are the first
examples of complexes of dirhenium() that contain 2-pyrido-
nate ligands, although a variety of dirhenium() complexes are
2
2
2
the 50% probability level. The phenyl carbon atoms of the dppm
ligands have been omitted for clarity except for the carbon atoms that
are bound to phosphorus (these are shown as circles of arbitrary
radius). Selected bond distances (Å) and bond angles (Њ): Re(1)–Re(2)
.3194(3), Re(1)–O(171) 2.175(3), Re(1)–N(1) 2.154(4), Re(1)–O(26)
.096(3), Re(1)–P(1) 2.3873(13), Re(1)–P(3) 2.4091(15), Re(2)–O(271)
.171(3), Re(2)–N(21) 2.157(4), Re(2)–O(16) 2.113(4), Re(2)–P(2)
.3998(13), Re(2)–P(4) 2.3820(15), C(17)–O(171) 1.287(6), C(17)–
15–17
2
2
2
2
known.
The cyclic voltammograms of 4 and 5 (recorded in 0.1 M
n
4
Bu NPF –CH Cl ) each show two one-electron processes that
can be assigned to the [Re ] /[Re ]
couples. These processes are more accessible in 5 than 4 (e.g. a
E (ox)(1) value of ϩ0.15 V in 5 vs. ϩ0.53 V in 4) but the
6
2
2
6ϩ
5ϩ
5ϩ
4ϩ
and [Re ] /[Re ]
2
2
2 2
O(172) 1.247(6), C(27)–O(271) 1.284(6), C(27)–O(272) 1.238(6); Re(2)–
Re(1)–O(171) 157.82(11), Re(1)–Re(2)–O(271) 158.37(10), N(1)–Re(1)–
P(3) 167.41(11), O(26)–Re(1)–P(1) 165.59(11), N(21)–Re(2)–P(2)
1/2
1
7
67.87(12),
O(16)–Re(2)–P(4)
165.84(10),
O(171)–Re(1)–N(1)
unsymmetrical nature of these complexes and delocalization
within the µ-HnicO and µ-picO ligands makes it difficult to
attribute these shifts just to variations in the nature of the
axial interactions (i.e. Re–Cl in 4 vs. Re–OC(O)– in 5). The
coordination of the carboxylate groups in 5 is quite strong as
shown by the Re–O (carboxylate) distances of 2.175(3) and
4.04(14), O(271)–Re(2)–N(21) 73.74(14). The torsion angles N(1)–
Re(1)–Re(2)–O(16), O(26)–Re(1)–Re(2)–N(21), P(1)–Re(1)–Re(2)–P(2)
and P(3)–Re(1)–Re(2)–P(4) are 10.79(14), 9.39(16), 14.08(5) and
1
3.61(5)Њ, respectively.
occurs between 3, 6 and 7. What is very unusual is that 3, 6 and
contain the three possible types of coordination behavior for
2
.171(3) Å. Also, the Re–N and Re–O distances associated with
7
the bridging 2-pyridonate units in this complex are shorter than
the comparable distances in 4 by between 0.04 and 0.09 Å (see
Figs. 2 and 3).
a pair of µ-dppm ligands in a dimetal complex, i.e. cis,cis-,
trans,trans- and trans,cis-, respectively.
Concluding remarks
The isolation and structural characterization of 3 is noteworthy
because it provides the third structural isomer of Re (dipic)-
2
Cl (µ-dppm) . The kinetic isomer 6 has been shown to convert
2
2
quantitatively to the thermodynamically stable form 7. Like
isomer 7, compound 3 contains a similar but not identical
coordination of the tridentate dipic ligand such that it chelates
to one Re center and uses its second carboxylate group to
All three isomeric dipic complexes possess distinctly different
P{ H} NMR spectra as a consequence of differences in P–P
coupling associated with the cis,cis, trans,trans, and trans,cis
coordination of the µ-dppm ligands. In the case of 3, the four
P resonances that are associated with the four chemically and
magnetically inequivalent P atoms have the appearance of
31
1
2Ϫ
bridge to the other Re atom. The flexibility of [dipic] enables
4ϩ
it to bind to the [Re (µ-dppm) ] core such that it can allow for
2 2
either cis,cis-, trans,trans- or trans,cis- arrangements of the
adjacent ReP units. Also of note is our isolation of the first
2
dirhenium() complexes (4 and 5) that contain 2-pyridonate
bridging ligands. In one of these, compound 4, a free carboxylic
acid group is present on each of the monoanionic HnicO
ligands, while in 5 the dianionic picO ligands are coordinated in
a tridentate fashion. Interestingly, we find that 1 does not react
with 2-hydroxypyridine (Hhp) with use of the same reaction
conditions that gave 4 and 5. Apparently, this ligand is not able
to protonate the µ-acetate ligands in refluxing ethanol and
facilitate their loss as acetic acid in order to form Re (µ-hp) -
doublets (see Experimental section); the inner pair at δ ϩ1.78
2
and Ϫ4.43 have J ∼ 77 Hz while those at δ ϩ5.48 and Ϫ9.43
P–P
−
2
have J
∼ 85 Hz. The cyclic voltammograms of the three
−
n
P–P
isomers (recorded in 0.1 M Bu NPF –CH Cl ) each show a
4
6
2
2
quasi reversible one-electron process that is associated with
5ϩ
4ϩ
the [Re ] /[Re ] couple and has an E value of ϩ0.73, ϩ0.32
2
2
1/2
7
7
and ϩ0.69 V vs. Ag/AgCl for 3, 6 and 7, respectively. This
surprisingly large variation must reflect differences in energy of
the HOMO (primarily the metal δ* orbital), which is influenced
by the differences in the ligand sets and coordination numbers
at the two metal centers in each of the complexes. However, it is
not obvious that 3 should be the hardest to oxidize.
2
2
18
Cl (µ-dppm) . In a previous study, we found that 6-(diphenyl-
2
2
phosphine)-2-pyridone (pyphosH), which is a phosphine sub-
stituted 2-hydroxypyridine ligand, reacts with the dirhenium()
n
4
complexes (Bu N) Re Cl , Re (µ-O CCH ) Cl or cis-Re (µ-O -
2
2
8
2
2
3
4
2
2
2
Complexes 4 and 5 both contain a pair of cis 2-pyridonate
ligands (Figs. 2 and 3) that bridge in a head-to-tail fashion. In
compound 4, the carboxylic acid group in the 3-position
CCH ) Cl (H O) to afford the reduced dirhenium() complex
3 2 4 2 2
Re Cl (µ-pyphos) (pyphosH). However, a structural study
2
2
2
showed that there is at best only weak Re ؒ ؒ ؒ O bonding (the
3
620
D a l t o n T r a n s . , 2 0 0 3 , 3 6 1 7 – 3 6 2 1

View Article Online
18
Re–O distances are in the range 2.42–2.98 Å in this molecule).
Therefore, 4 and 5 constitute the first bona-fide examples of
-pyridonate N,O bridged dirhenium() complexes. We should
Commun., 2002, 41, 1036; (b) S.-M. ꢀuang, P. E. Fanwick and
R. A. Walton, Inorg. Chim. Acta, 2002, 338, 219.
5
(a) M. H. Chisholm, Acc. Chem. Res., 2000, 33, 53; (b) F. A. Cotton,
C. Lin and C. A. Murillo, Acc. Chem. Res., 2001, 34, 759; (c) M. H.
Chisholm, J. Organomet. Chem., 2002, 641, 15; (d ) F. A. Cotton,
C. Lin and C. A. Murillo, Proc. Natl. Acad. Sci., 2002, 99, 4810.
2
now be in a position to develop this chemistry further by
synthesizing a range of compounds with substituents in the 3-,
4
- and 5- positions of the rings (as we have done in the case of
) which can bind other dimetal or monometal species to form
6 M. Ganesan, ꢀ.-Y. Shih, P. E. Fanwick and R. A. Walton, Inorg.
Chem., 2003, 42, 1241.
4
7
S. ꢀ. Chattopadhyay, P. E. Fanwick and R. A. Walton, Inorg. Chem.,
in press.
supramolecular assemblies. This possibility is attractive in the
4
ϩ
case of Re because of the accessibility of the paramagnetic
Re2 core.
2
8
9
S. ꢀ. Chattopadhyay and R. A. Walton, Inorg. Chim. Acta, in press.
P. T. Beurskens, G. Beurskens, R. deGelder, S. Garcia-Granda,
R. O. Gould, R. Israel and J. M. M. Smits, The DIRDIF-99
Program System, Crystallography Laboratory, University of
Nijmegan, Netherlands, 1998.
5ϩ
2
Acknowledgements
1
1
0 Z. Otwinowski and W. Minor, Methods Enzymol., 1996, 276, 307.
1 G. M. Sheldrick, SHELXL-97. A Program for Crystal Structure
Refinement, University of Gottingen, Germany, 1997.
2 C. ꢀ. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge
National Laboratory, TN, USA, 1976.
3 P. V. D. Sluis and A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46,
194.
R. A. W. thanks the John A. Leighty Endowment Fund for
support of this work.
1
1
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D a l t o n T r a n s . , 2 0 0 3 , 3 6 1 7 – 3 6 2 1
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