Reactions of the dirhenium(II) complex Re2Cl4(μ -dppm)2 with pyridinecarboxylic acids. Examples of bidentate O,O versus N,O coordination and tridentate O,N,O coordination and structural isomers that contain the pyridine-2,

Article abstract of DOI:10.1021/ic030153y

Pyridine-2-carboxylic acid, pyridine-2,3-dicarboxylic acid, and pyridine-2,4-dicarboxylic acid or their [(Ph₃P)₂N] ⁺ salts react with the triply bonded dirhenium(II) complex Re ₂Cl₄(μ-dppm)₂

Full text of DOI:10.1021/ic030153y

Inorg. Chem. 2003, 42, 5924−5931
Reactions of the Dirhenium(II) Complex Re2Cl4(µ-dppm)2 with
Pyridinecarboxylic Acids. Examples of Bidentate O,O versus N,O
Coordination and Tridentate O,N,O Coordination and Structural Isomers
That Contain the Pyridine-2,6-dicarboxylate Ligand
Swarup Chattopadhyay, Phillip E. Fanwick, and Richard A. Walton*
Brown Laboratory of Chemistry, Purdue UniVersity, 425 Central DriVe,
West Lafayette, Indiana 47907-2018
Received May 8, 2003
Pyridine-2-carboxylic acid, pyridine-2,3-dicarboxylic acid, and pyridine-2,4-dicarboxylic acid or their [(Ph3P)2N]+ salts
react with the triply bonded dirhenium(II) complex Re2Cl4(µ-dppm)2 (dppm ) Ph2PCH PPh2) in refluxing ethanol to
2
afford unsymmetrical substitution products of the type Re2(η2-N,O)Cl3(µ-dppm)2, where N,O represents a chelating
pyridine-2-carboxylate ligand (N,O ) O C-2-C H N(1), O C-2-C H N(-3-CO Et) (3), or O C-2-C H N(-4-CO H) (4)).
2
5
4
2
5
3
2
2
5
3
2
The carboxylate groups in the 3- and 4- positions are not bound to the metal centers; in the case of 3 this group
undergoes esterification in the refluxing ethanol solvent. Structure determinations have shown that 1, 3, and 4
possess similar structures in which there is an axial Re−O (carboxylate) bond (collinear with the RetRe bond)
and the µ-dppm ligands are bound in a trans,cis fashion to the two Re atoms which have the ligand atom arrangement
[P NOClReReCl2P ]. The tridentate dianionic pyridine-2,6-dicarboxylate ligand (dipic) reacts with Re2Cl4(µ-dppm)2
2
2
in ethanol at room temperature to give a compound Re2(dipic)Cl2(µ-dppm)2 (6) in which the dipic ligand is bound
in a symmetrical η3-(O,N,O) fashion to one Re atom, with the N atom in an axial position (collinear with the
RetRe bond) and with preservation of the same trans,trans coordination of the µ-dppm ligands that is present in
Re2Cl4(µ-dppm)2. Under reflux conditions, this kinetic product isomerizes to the thermodynamically favored isomer
5 with an unsymmetrical structure in which the dipic ligand chelates to one Re atom (as in 1, 3, and 4) and uses
its other carboxylate group to bridge to the second Re atom. The isomerization of 6 to 5, which also results in a
change in the coordination of the pair of µ-dppm ligand to trans,cis, is believed to occur by a partial “merry-go-
round” process, a mechanism that probably explains the structures of the thermodynamic products 1, 3, and 4.
The reaction of Re2Cl4(µ-dppm)2 with pyridine-3-carboxylate gives the trans isomer of Re2(µ:η2-O C-3-C H N)2-
2
5 4
Cl2(µ-dppm)2 (2) in which a pair of carboxylate bridges are present and the pyridine N atom is not coordinated.
Single-crystal X-ray structural details are reported for 1−6.
Introduction
platinum(II) units at the corners of the square.2 The key to
this chemistry was the isolation of the synthon cis-Re2(µ-
O2C-4-C5H4N)2Cl2(µ-dppm)2 (dppm ) Ph2PCH2PPh2) in
which the pyridyl nitrogen atoms are not coordinated to the
dirhenium unit.2 Although a couple of other dirhenium
complexes that contain the isonicotinate ligand are known,
specifically the trans isomer of Re2(µ-O2C-4-C5H4N)2Cl2(µ-
dppm)23 and the paramagnetic dirhenium(III,II) complex Re2-
Studies that involve the incorporation of multiply bonded
dimetal units into supramolecular assemblies1 have recently
included the use of the 4-pyridinecarboxylate (isonicotinate)
ligand to prepare a hybrid molecular square that is composed
of alternating triply bonded dirhenium(II) and mononuclear
* To whom correspondence should be addressed. E-mail: rawalton@
purdue.edu.
(1) (a) Chisholm, M. H. Acc. Chem. Res. 2000, 33, 53-61. (b) Cotton,
F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res. 2001, 34, 759-771.
(c) Chisholm, M. H. J. Organomet. Chem. 2002, 641, 15-25. (d)
Cotton, F. A.; Lin, C.; Murillo, C. A. Proc. Nat. Acad. Sci. U.S.A.
2002, 99, 4810-4813.
(2) Bera, J. K.; Smucker, B. W.; Walton, R. A.; Dunbar, K. R. Chem.
Commun. 2001, 2562-2563.
(3) Derringer, D. R.; Buck, E. A.; Esjornson, S. M. V.; Fanwick, P. E.;
Walton, R. A. Polyhedron 1990, 9, 743-750.
5924 Inorganic Chemistry, Vol. 42, No. 19, 2003
10.1021/ic030153y CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/22/2003

Reactions of Re2Cl4(µ-dppm)2
(µ-O2C-4-C5H4N)Cl4(µ-dppm)2,4 both of which are poten-
tially capable of participating in the formation of mixed-
metal assemblies, no other examples of pyridinecarboxylates
of dirhenium are known. Since such compounds might
exhibit interesting structural chemistry, and also be incor-
porated into homo- and heteronuclear assemblies in which
diamagnetic and paramagnetic dirhenium cores are present,
we have set out to examine the reactions of the triply bonded
mg (74%). Recrystallization from dichloromethane/hexane gave
single crystals of 1 without the presence of lattice solvent (as
established by X-ray crystallography).
(ii) trans-Re2[µ:η2(O,O)-O2C-3-C5H4N]2Cl2(µ-dppm)2 (2). The
reaction between Re2Cl4(µ-dppm)2 (100 mg, 0.078 mmol) and
[(Ph3P)2N](O2C-3-C5H4N) (155 mg, 0.235 mmol) in refluxing
ethanol (30 mL) for 3 h gave this orange product with a workup
procedure similar to that used in section B(i); yield 82 mg (72%).
Recrystallization was carried out from hexane/dichloromethane.
Anal. Calcd for C62H52Cl2N2O4P4Re2: C, 51.13; H, 3.60; N, 1.92.
Found: C, 50.73; H, 3.73; N, 1.90.
(iii) Re2[η2(N,O)-O2C-2-C5H3N(-3-COOEt)]Cl3(µ-dppm)2 (3).
A mixture of Re2Cl4(µ-dppm)2 (100 mg, 0.078 mmol) and pyridine-
2,3-dicarboxylic acid (196 mg, 1.17 mmol) was refluxed in ethanol
(30 mL) for 24 h and filtered, and the green solid was filtered off
and worked-up as in section B(i); yield 81 mg (72%). Single crystals
were obtained as for 1 and 2 by the slow diffusion of hexane into
a dichloromethane solution of the complex under N2(g). Anal. Calcd
for C60H54Cl5NO4P4Re2 (i.e., 3‚CH2Cl2): C, 47.20; H, 3.57; N, 0.92.
Found: C, 46.37; H, 3.37; N, 0.99.
5-7
dirhenium(II) synthon Re2Cl4(µ-dppm)2 with a selection
of pyridinemono- and dicarboxylate ligands. We have found
that the substitution of one or two chloride ligands occurs
and the pyridine carboxylate ligands are involved in a variety
of chelating and/or bridging coordination modes to the
dirhenium core. In addition, a pair of unusual structural
isomers that contain the pyridine-2,6-dicarboxylate ligand
have been isolated and characterized. The synthetic proce-
dures and structures of the resulting complexes are reported.
Experimental Section
(iv) Re2[η2(N,O)-O2C-2-C5H3N(-4-CO2H)]Cl3(µ-dppm)2 (4).
The title complex was obtained by the reaction of Re2Cl4(µ-dppm)2
(100 mg, 0.078 mmol) with pyridine-2,4-dicarboxylic acid (65 mg,
0.39 mmol) in ethanol (30 mL) for 24 h. Workup as in section
B(i) afforded 4 as a purple solid; yield 85 mg (77%). Recrystalli-
zation from benzene/dichloromethane gave single crystals. Anal.
Calcd for C64H56Cl5NO4P4Re2 (i.e.4‚CH2Cl2‚C6H6): C, 48.75; H,
3.58; N, 0.89. Found: C, 49.57; H, 3.67; N, 1.01.
(v) Re2[µ:η3(O,N,O)-(O2C)2-2,6-C5H3N]Cl2(µ-dppm)2 (5) and
Re2[η3(O,N,O)-(O2C)2-2,6-C5H3N]Cl2(µ-dppm)2 (6). A mixture of
Re2Cl4(µ-dppm)2 (100 mg, 0.078 mmol) and [(Ph3P)2N]2[(O2C)2-
2,6-C5H3N] (193 mg, 0.155 mmol) in ethanol (30 mL) was refluxed
for 24 h. Workup as in section B(i) gave 5 as a green solid; yield
67 mg (62%). This product was recrystallized from hexane/
dichloromethane to give X-ray-quality crystals. Anal. Calcd for
A. Starting Materials, General Procedures, and Physical
Measurements. The complexes Re2Cl4(µ-dppm)2 (dppm ) Ph2-
PCH2PPh2) and cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 were prepared
by the usual methods.7 Samples of the pyridinecarboxylic acids
and bis(triphenylphosphine)iminium chloride ([PPN]Cl) were pur-
chased from Aldrich Chemical Co. The compound [PPN]Cl was
used to prepare the [PPN]+ salts of acetic acid, pyridine-2-carboxylic
acid, pyridine-3-carboxylic acid, and pyridine-2,6-dicarboxylic acid
by use of the procedure we have described previously.3 In the case
of [PPN]2dipic, X-ray crystallography was used to confirm the
identity of this compound (vide infra). 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.
C
57.50H48Cl3NO4P4Re2 (i.e. 5‚0.5CH2Cl2): C, 48.65; H, 3.41; N,
Infrared spectra, NMR spectra, and cyclic voltammetric measure-
ments were carried out as described previously.8 Elemental mi-
croanalyses were done by Dr. H. D. Lee of the Purdue University
Microanalytical Laboratory.
0.99. Found: C, 48.92; H, 3.58; N, 0.98.
This same product (5) was obtained, admixed with considerable
quantities of unreacted Re2Cl4(µ-dppm)2, when the dipicH2 was used
in place of [PPN]2dipic; the reaction was carried out in refluxing
ethanol for 3 days, but the conversion of Re2Cl4(µ-dppm)2 to 5 by
this method is quite low.
B. Synthesis of Pyridinecarboxylate Complexes of Dirhenium-
(II). (i) Re2[(η2(N,O)-O2C-2-C5H4N)]Cl3(µ-dppm)2 (1). A mixture
of Re2Cl4(µ-dppm)2 (100 mg, 0.078 mmol) and [(Ph3P)2N](O2C-
2-C5H4N) (103 mg, 0.156 mmol) in ethanol (30 mL) was refluxed
for 3 h. A green solid separated, and the mixture was then cooled
and filtered. The solid residue was washed with ethanol (3 × 5
mL) and diethyl ether (3 × 5 mL) and dried in vacuo; yield 65 mg
(61%). Recrystallization was carried out by the slow diffusion of
hexane into a dichloromethane solution of 1 under N2(g). Anal.
Calcd for C57H50Cl5NO2P4Re2 (i.e., 1‚CH2Cl2): C, 47.07; H, 3.46;
N, 0.96. Found: C, 47.17; H, 3.39; N, 0.97.
When dichloromethane (10 mL) was used as the solvent for the
reaction between Re2Cl4(µ-dppm)2 (100 mg, 0.078 mmol) and
[PPN]2dipicolinate (136 mg, 0.109 mmol), an orange colored
solution was formed when this mixture was stirred for 10 min at
room temperature. The mixture was filtered and evaporated to
dryness, and 31P{1H} NMR spectroscopy of the orange residue
showed the presence of a single dirhenium product (6) (AA′BB′
multiplets centered at δ ) -4.3 and δ ) -6.4 with the most intense
inner components at δ ) -4.70 and δ -5.9, respectively) in
addition to [PPN]+ salts (Cl- and dipic2-). We have not yet been
able to purify 6 from this particular reaction mixture. However,
we find that when Re2Cl4(µ-dppm)2 (100 mg, 0.078 mmol) is
reacted with [PPN]2dipic (387 mg, 0.312 mmol) in 25 mL of ethanol
at room temperature for a period of 6 h, then pure 6 is formed in
high yield; yield 80 mg (75%). Anal. Calcd for C57H47Cl2NO4P4-
Re2: C, 49.71; H, 3.44; N, 1.02; Cl, 5.15. Found: C, 49.07; H,
3.80; N, 0.96; Cl, 4.89.
This same compound can be obtained by refluxing a mixture of
Re2Cl4(µ-dppm)2 (100 mg, 0.078 mmol) and pyridine-2-carboxylic
acid (48 mg, 0.39 mmol) in ethanol (30 mL) for 1 day; yield 79
(4) Bera, J. K.; Cle´rac, R.; Fanwick, P. E.; Walton, R. A. J. Chem. Soc.,
Dalton Trans. 2002, 2168-2172.
(5) Ebner, J. R.; Tyler, D. R.; Walton, R. A. Inorg. Chem. 1976, 15, 833-
840.
(6) Barder, T. J.; Cotton, F. A.; Dunbar, K. R.; Powell, G. L.; Schwotzer,
W.; Walton, R. A. Inorg. Chem. 1985, 24, 2550-2554.
(7) Cutler, A. R.; Derringer, D. R.; Fanwick, P. E.; Walton, R. A. J. Am.
Chem. Soc. 1988, 110, 5024-5034.
(8) Ganesan, M.; Shih, K.-Y.; Fanwick, P. E.; Walton, R. A. Inorg. Chem.
2003, 42, 1241-1247.
When the insoluble red orange product 6 is suspended in fresh
ethanol and the mixture refluxed for 24 h, it is converted
quantitatively into green 5 (as shown by 31P NMR spectroscopy).
Inorganic Chemistry, Vol. 42, No. 19, 2003 5925

Chattopadhyay et al.
Table 1. Crystallographic Data for the Salt [PPN]2dipic‚H2O and Dirhenium(II) Complexes That Contain Pyridinecarboxylate Ligands
[PPN]2dipic‚
4‚CH2Cl2‚
C6H6
H2O
1
1‚CH2Cl2
2
3‚CH2Cl2
5‚0.5CH2Cl2
6‚3C6H6
empirical
formula
fw
space group Pnma (No. 62) P1h (No. 2)
a, Å
C85H77N3O7P4 C56H48Cl3NO2- C57H50Cl5NO2- C62H52Cl2N2O4- C60H54Cl5NO4- C64H56Cl5NO4-
C
57.50H48Cl3NO4- C75H65Cl2NO4-
P4Re2
1369.67
P4Re2
1454.60
P1h (No. 2)
12.0594(3)
14.3049(4)
15.5468(5)
86.2266(13)
89.7152(13)
83.5631(12)
2659.26(13)
2
P4Re2
1456.32
P21/n (No. 14)
11.5457(2)
18.2181(4)
13.8189(3)
90
106.8691(11)
90
2781.60(19)
2
P4Re2
1526.66
P1h (No. 2)
12.7682(2)
14.8301(2)
15.9949(2)
72.9221(10)
85.2756(10)
77.3973(10)
2824.85(7)
2
P4Re2
1576.73
P21/c (No. 14) C2/c (No. 15)
16.3898(3)
13.6250(3)
26.8924(7)
90
101.4007(8)
90
P4Re2
1419.68
P4Re2
1611.56
1376.47
P21/c (No. 14)
17.4698(9)
20.5735(14)
21.2477(13)
90
102.921(5)
90
7443.4(8)
4
1.438
3.495
0.064
0.141
21.2803(5)
23.5958(4)
14.4185(3)
90
90
90
7239.9(3)
4
11.0355(2)
12.1148(3)
21.2878(6)
73.7737(9)
78.0557(9)
67.0559(17)
2500.95(10)
2
22.9639(7)
27.4965(9)
19.2811(5)
90
119.2191(18)
90
10625.5(5)
8
1.775
4.933
0.040
0.079
0.946
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
5886.9(2)
4
F
calcd, g/cm-3 1.263
1.819
1.816
1.739
1.795
1.779
µ, mm-1
R(Fo)a
0.157
0.046
0.117
1.088
5.233
0.046
0.087
0.910
5.026
0.050
0.104
0.985
4.667
0.035
0.075
0.952
4.738
0.030
0.062
1.004
4.550
0.048
0.112
1.195
Rw (Fo )b
2
GOF
0.922
2
2
2
2
2 2
a R ) ∑||Fo| - |Fc||/∑|Fo| with Fo > 2σ(Fo ). b Rw ) [∑w(|Fo | - |Fc |)2/∑w|Fo | ]1/2
.
(vi) Reaction of 1 with [PPN]O2CCH3. A mixture of 1 (50
mg, 0.037 mmol) and [(Ph3P)2N]O2CCH3 (44 mg, 0.074 mmol)
was refluxed in ethanol (15 mL) for 24 h. The insoluble orange
product cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 was filtered off, washed
with ethanol (3 × 5 mL) and diethyl ether (3 × 5 mL), and dried
in vacuo; yield 39 mg (80%). The identity of this product was
established by a comparison of its spectroscopic and electrochemical
properties with the literature data.7
In most instances, the crystals were found to contain identifiable
solvent molecules (see Table 1), the non-hydrogen atoms of which
refined satisfactorily with anisotropic thermal parameters except
in the case of 6‚3C6H6 where the three benzene molecules were
refined isotropically. For compound 1, structures were determined
for crystals with and without lattice CH2Cl2. The salt of composition
[PPN]2dipic‚H2O contained a water molecule that was weakly
H-bonded in a symmetrical fashion to the oxygen atoms O(21) and
O(62) of the two carboxylate groups with distances O(1w)‚‚‚O(21)
and O(1w)‚‚‚O(62) of 2.82 and 2.81 Å, respectively. The dirhenium
unit in 2 was located about a center of inversion that was coincident
with the center of the Re-Re bond. Consequently, atoms C(13)
and N(13) of the pyridyl rings were modeled as a disorder involving
half-atoms. In the structure of 3‚CH2Cl2, the dichloromethane
molecule was disordered such that there were two half-carbon atoms
(C(91A) and C(91B)) associated with two full chlorine atoms (Cl-
(91) and Cl(92)). The structure of 5‚0.5CH2Cl2 contained a well-
behaved half molecule of dichloromethane that was located in the
vicinity of a phenyl ring of one of the PPh2 groups of a dppm ligand
(atom P(3)). Each of the phenyl rings of this particular PPh2 group
were disordered such that there were two orientations for each ring.
For one of the rings, two carbon atoms (C(311) and C(313)) were
shared by the two orientations leaving the other four atoms of each
ring as half-atoms (i.e. a total of eight atoms). The two orientations
for the other disordered phenyl ring shared only one carbon atom
(i.e., C(321) at full occupancy), leaving a total of 10 half-carbon
atoms. All these carbon atoms refined satisfactorily with anisotropic
thermal parameters. On the basis of chemically meaningful
intermolecular distances involving atom Cl(92) of the half-CH2Cl2
solvent molecule, we conclude that it is associated with the half-
phenyl ring C(311), C(31f), C(313), C(31h), C(31j), and C(31k),
with nonbonding distances between these ring carbons and Cl(92)
being in the range 3.71-4.92 Å. During the structure refinements
of both [PPN]2dipic‚H2O and 5‚0.5CH2Cl2, small amounts of badly
disordered solvent molecules that could not be modeled satisfac-
torily were removed with use of the squeeze option in PLATON.14
The largest peaks remaining in the final difference maps of
[PPN]2dipic‚H2O and 1-6 (in the order given in Table 1) were
0.30, 1.74, 1.67, 1.51, 1.12, 1.17, 1.34, and 2.04 e/Å3, respectively.
C. X-ray Crystallography. Single crystals of composition
[(Ph3P)2N]2[(O2C)2-2,6-C5H3N]‚H2O, hereafter referred to as
[PPN]2dipic‚H2O, were grown by the slow diffusion of diethyl ether
into an acetone solution of the compound, while X-ray-quality
crystals of complexes of composition 1, 1‚CH2Cl2, 2, 3‚CH2Cl2,
4‚CH2Cl2‚C6H6, and 5‚0.5CH2Cl2 were obtained as described in
the individual synthetic procedures in section B. Crystals of
composition 6‚3C6H6 were obtained by recrystallization of 6 from
dichloromethane/benzene. In all instances, data collections were
carried out at 150((1) K with graphite-monochromated Mo KR
radiation (λ ) 0.710 73 Å) on a Nonius KappaCCD diffractometer.
Lorentz and polarization corrections were applied to the data sets.
The key crystallographic data are given in Table 1.
The structure of [PPN]2dipic‚H2O was solved by direct methods
with the use of SIR 2002,9 while for the structures of 1-6 the
structure solution program PATTY in DIRDIF9910 was used. The
remaining atoms were located in succeeding difference Fourier
syntheses. Hydrogen atoms were placed in calculated positions
according to idealized geometries with C-H ) 0.95 Å and
U(H) ) 1.3Ueq(C). They were included in the refinement but
constrained to ride on the atom to which they are bonded. An
empirical absorption correction using SCALEPACK11 was applied.
The final refinements were performed by the use of the program
SHELXL-97.12 All non-hydrogen atoms were refined with aniso-
tropic thermal parameters unless indicated otherwise. Crystal-
lographic drawings were done using the program ORTEP.13
(9) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giaco-
vazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2003, to be
published.
(10) Beurskens, P. T.; Beurskens, G.; deGelder, R.; Garcia-Granda, S.;
Gould, R. O.; Israel, R.; Smits, J. M. M. The DIRDIF99 Program
System; Technical Report; Crystallography Laboratory, University of
Nijmegen: Nijmegen, The Netherlands, 1999.
(11) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276, 307-326.
(12) Sheldrick, G. M. SHELXL97. A Program for Crystal Structure
Refinement; University of Gottingen: Gottingen, Germany, 1997.
(13) Johnson, C. K. ORTEP II; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
(14) Sluis, P. V. D.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194-
201.
5926 Inorganic Chemistry, Vol. 42, No. 19, 2003

Reactions of Re2Cl4(µ-dppm)2
Scheme 1. Products Formed by the Reactions of Re2Cl4(µ-dppm)2
with Pyridinecarboxylic Acids (or Their [PPN]+ Salts) in Refluxing
EtOH: [py-2-CO2]- (1); [py-3-CO2]- (2); [py(-3-CO2Et)-2-CO2]- (3);
[py(-4-CO2H)-2-CO2]- (4); [py-2,6-(CO2)2]2- (5)
Figure 1. ORTEP13 representation of the structure of the complex Re2-
(pic)Cl3(µ-dppm)2 as present in 1‚CH2Cl2. Thermal ellipsoids are drawn at
the 50% probability level except for the phenyl carbon atoms of the dppm
ligands which are circles of arbitrary radius. Selected bond distances (Å)
and angles (deg) are as follows: Re(1)-Re(2) 2.2937(4), Re(1)-N(1) 2.129-
(6), Re(1)-Cl(11) 2.3863(19), Re(1)-O(71) 2.234(4), Re(1)-P(2) 2.4638-
(18), Re(1)-P(4) 2.4816(18), Re(2)-P(1) 2.3836(19), Re(2)-P(3) 2.3890-
(19), Re(2)-Cl(21) 2.4283(18), Re(2)-Cl(22) 2.4003(18), O(71)-C(7)
1.278(9), O(72)-C(7) 1.217(9); N(1)-Re(1)-Cl(11) 149.21(15), P(2)-
Re(1)-P(4) 172.23(6), P(1)-Re(2)-Cl(21) 157.32(7), P(3)-Re(2)-Cl(22)
145.99(6), P(1)-Re(2)-P(3) 109.30(7), O(71)-Re(1)-Re(2) 168.58(12).
Results and Discussion
In all instances, the reactions of Re2Cl4(µ-dppm)2 with
pyridinecarboxylic acids, or their [PPN]+ salts, afford the
unsymmetrical dirhenium(II) complexes 1-5 that are shown
in Scheme 1. These reactions were carried out under similar
conditions, namely, reflux in ethanol for periods of 3-24 h;
the longer reaction times were typically used with the
dicarboxylic acids. Only one of these products is of a type
that has been obtained previously, namely, the nicotinate
complex trans-Re2(µ-O2C-3-C5H4N)2Cl2(µ-dppm)2 (2), which
is similar to the analogous isonicotinate complex trans-Re2-
(µ-O2C-4-C5H4N)2Cl2(µ-dppm)2.3 The cis isomer of the
isonicotinate has also been prepared and structurally char-
acterized,2 as have both the cis and trans isomers of the
acetate complex Re2(µ-O2CCH3)2Cl2(µ-dppm)2.3,7 While the
trans isomer of the acetate (and propionate) complex readily
isomerizes to the more thermodynamically stable cis isomer,
the trans isomer of the nicotionate (reported here) and the
isonicotinate3 are much more stable to isomerization and,
therefore, could be useful synthons for preparing mixed-metal
assemblies based upon a trans-Re2(µ-O2CR)2Cl2(µ-dppm)2
template. Such studies are now underway.
Single-crystal X-ray structure determinations on 1-5 have
confirmed the structures that are represented in Scheme 1.
The ORTEP13 representations of the structures are shown in
Figures 1-5; in all cases the important bond distances and
angles are given in the captions to the figures. In addition,
the structural identity of [PPN]2dipic, which was used in the
synthesis of compound 5, was confirmed by X-ray crystal-
lography. Crystallographic details for this crystal are given
in Table 1, and full structural data are provided as Supporting
Information.
Figure 2. ORTEP13 representation of the structure of the dirhenium
complex Re2(µ-nic)2Cl2(µ-dppm)2 (2). Thermal ellipsoids are drawn at the
50% probability level except for the phenyl carbon atoms of the dppm
ligands which are circles of arbitrary radius. Selected bond distances (Å)
and angles (deg) are as follows: Re-Re(a) 2.2931(3), Re-O(11) 2.134-
(3), Re-O(12) 2.108(3), Re-P(1) 2.4252(12), Re-P(2) 2.4358(12), Re-
Cl 2.5993(11); O(11)-Re-O(12) 177.93(11), P(1)-Re-P(2) 161.20(4),
Cl-Re-Re(a) 178.83(3).
structure is noteworthy in that the trans sets of Re-P bonds
that are present in Re2Cl4(µ-dppm)2 have become a trans and
cis set in Re2(pic)Cl3(µ-dppm)2. The Re-Re distances in 1
and 1‚CH2Cl2 differ by about 0.012 Å, the values being
2.2816(4) Å for 1 and 2.2937(4) Å for 1‚CH2Cl2, while the
axial Re-O bond distance (Re(1)-O(71)) is the same in
these two structures (2.232(4) and 2.234(4) Å, respectively).
Accordingly, the variation in Re-Re distances does not
reflect any significant difference in the Re-O (carboxylate)
binding.
For triply bonded [Re2]4+ complexes there is no electronic
barrier to rotation about the Re-Re bond15 and, as expected,
there is a pronounced staggering of the two [ReL4] units both
The structure of the dirhenium units in the crystals of 1
and 1‚CH2Cl2 are essentially identical; the structure shown
in Figure 1 is of the molecule Re2(pic)Cl3(µ-dppm)2 (pic )
pyridine-2-carboxylate) that is present in 1‚CH2Cl2. The
(15) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
2nd ed.; Oxford University Press: Oxford, U.K., 1993; and references
therein.
Inorganic Chemistry, Vol. 42, No. 19, 2003 5927

Chattopadhyay et al.
Figure 3. ORTEP13 representation of the structure of the dirhenium
complex Re2[(O2C-2-(EtO2C-3-)py]Cl3(µ-dppm)2 as present in 3‚CH2Cl2.
Thermal ellipsoids are drawn at the 50% probability level except for the
phenyl carbon atoms of the dppm ligands and of the ethyl ester groups
which are circles of arbitrary radius. Selected bond distances (Å) and angles
(deg) are as follows: Re(1)-Re(2) 2.2752(2), Re(1)-N(1) 2.143(3), Re-
(1)-Cl(1) 2.3931(9), Re(1)-O(61) 2.221(2), Re(1)-P(1) 2.4625(9), Re-
(1)-P(3) 2.4635(9), Re(2)-P(2) 2.3851(10), Re(2)-P(4) 2.4032(9), Re(2)-
Cl(3) 2.4127(9), Re(2)-Cl(2) 2.4036(9), O(61)-C(61) 1.267(5), O(62)-
C(61) 1.221(5); N(1)-Re(1)-Cl(1) 155.85(8), P(1)-Re(1)-P(3) 171.69(3),
P(2)-Re(2)-Cl(3) 156.10(3), P(4)-Re(2)-Cl(2) 152.01(3), P(2)-Re(2)-
P(4) 108.27(3), O(61)-Re(1)-Re(2) 175.83(6).
Figure 5. ORTEP13 representation of the structure of the dirhenium
complex Re2(dipic)Cl2(µ-dppm)2 as present in 5‚0.5CH2Cl2. Thermal
ellipsoids are drawn at the 50% probability level except for the phenyl
carbon atoms of the dppm ligands which are circles of arbitrary radius.
Selected bond distances (Å) and angles (deg) are as follows: Re(1)-Re(2)
2.2512(3), Re(2)-N(1) 2.188(4), Re(2)-Cl(2) 2.3941(13), Re(2)-O(21)
2.197(3), Re(2)-P(2) 2.4623(14), Re(2)-P(4) 2.4775(14), Re(1)-P(1)
2.3836(14), Re(1)-P(3) 2.3516(14), Re(1)-Cl(1) 2.4128(15), Re(1)-O(11)
2.103(3), O(11)-C(7) 1.300(7), O(12)-C(7) 1.229(7), O(21)-C(8) 1.290-
(6), O(22)-C(8) 1.225(7); N(1)-Re(2)-Cl(2) 160.20(12), P(2)-Re(2)-
P(4) 173.34(5), P(1)-Re(1)-O(11) 162.33(11), P(3)-Re(1)-Cl(1) 145.59-
(6), P(1)-Re(1)-P(3) 107.45(5), O(21)-Re(2)-Re(1) 168.09(9).
CH2Cl2 presumably reflect small differences in intermolecu-
lar packing forces. The fact that the Re-Re distance in 1 is
a little shorter than in 1‚CH2Cl2 could in part be a
consequence of a slightly more favorable conformation for
the fused five-membered RePCH2PRe rings in the former
crystal as well as a minimization of intramolecular repulsion
forces. Of further note are differences in some of the bond
angles associated with the coordinatiVely unsaturated atom
Re(2) in these two structures. These differences, which reflect
the ease of distorting the pyramidal geometry16 about Re-
(2), could also contribute to a slight difference in the Re-
Re bond distances.
The aforementioned observations regarding the structures
of 1 and 1‚CH2Cl2 are supported by a structure determination
we carried out on a crystal of composition 2Re2(pic)Cl3(µ-
dppm)2‚Re2Cl6(µ-dppm)2‚2.172CH2Cl2 that was obtained on
one occasion when we recrystallized a sample of 1 from
dichloromethane/benzene. The presence of Re2Cl6(µ-dppm)2,
whose structure has been reported previously,17 presumably
arose from the oxidation of a small amount of the Re2Cl4-
(µ-dppm)2 starting material. This is a well-known transfor-
mation,17 but it is unclear why it occurs in the present
instance. The crystal structure was determined and refined
satisfactorily; full details are provided as Supporting Infor-
mation. The Re2(µ-Cl)2Cl4(µ-dppm)2 molecule is located
about an inversion center (the Re-Re distance is 2.7101(7)
Å) while the molecule of Re2(pic)Cl3(µ-dppm)2 in the
asymmetric unit is similar to that present in the crystals of
1 and 1‚CH2Cl2 (Figure 1). The value for the Re-Re distance
(2.2841(5) Å) is between that found in 1 and 1‚CH2Cl2, and
Figure 4. ORTEP13 representation of the structure of the dirhenium
complex Re2[(O2C-2-(HO2C-4-)py]Cl3(µ-dppm)2 as present in 4‚CH2Cl2‚
C6H6. Thermal ellipsoids are drawn at the 50% probability level except for
the phenyl carbon atoms of the dppm ligands which are circles of arbitrary
radius. Selected bond distances (Å) and angles (deg) are as follows: Re-
(1)-Re(2) 2.2713(5), Re(1)-N(1) 2.147(7), Re(1)-Cl(11) 2.403(2), Re-
(1)-O(11) 2.240(6), Re(1)-P(1) 2.481(2), Re(1)-P(3) 2.443(2), Re(2)-
P(2) 2.397(3), Re(2)-P(4) 2.401(2), Re(2)-Cl(21) 2.387(2), Re(2)-Cl(22)
2.428(2), O(11)-C(11) 1.284(11), O(12)-C(11) 1.242(11); N(1)-Re(1)-
Cl(11) 155.78(18), P(1)-Re(1)-P(3) 169.40(8), P(2)-Re(2)-Cl(22) 153.71-
(8), P(4)-Re(2)-Cl(21) 155.34(9), P(2)-Re(2)-P(4) 106.00(8), O(11)-
Re(1)-Re(2) 175.26(15).
in 1 and 1‚CH2Cl2. This can be gauged by considering the
four torsion angles P(2)-Re(1)-Re(2)-P(1), Cl(11)-Re-
(1)-Re(2)-P(3), P(4)-Re(1)-Re(2)-Cl(21), and N(1)-Re-
(1)-Re(2)-Cl(22) (see Figure 1); the average of these
torsion angles (øav) in 1 and 1‚CH2Cl2 is 33.4 and 53.8°,
respectively, signifying that the cis-ReP2Cl2 units in these
two structures are similarly twisted away from a fully
eclipsed conformation relative to the trans-ReP2NCl unit at
the coordinatively saturated Re center. Variations in the
torsion angles are the consequence of a low barrier to rotation
about the RetRe bond and the actual values for 1 and 1‚
(16) Mota, F.; Novoa, J. J.; Losada, J.; Alvarez, S.; Hoffmann, R.; Silvestre,
J. J. Am. Chem. Soc. 1993, 115, 6216-6229.
(17) Barder, T. J.; Cotton, F. A.; Lewis, D.; Schwotzer, W.; Tetrick, S.
M.; Walton, R. A. J. Am. Chem. Soc. 1984, 106, 2882-2891.
5928 Inorganic Chemistry, Vol. 42, No. 19, 2003

Reactions of Re2Cl4(µ-dppm)2
Table 2. Selected Spectroscopic and Electrochemical Data for Dirhenium(II) Complexes Containing Pyridinecarboxylate Ligands
chem shift, δa
CV half-wave potentials , Vb
compd no.
31P{1H}NMRc
1H NMRd
Ep,a
E
1/2(ox)
E1/2(red)
1
2
3
+7.7 (m), -6.1 (m)
+4.4 (s)g
+6.17 (m, 2H, dppm),e +6.02 (m, 2H, dppm)e
+4.93 (br, 4H, dppm)
+1.30
+0.64 (75)f
+1.11 (90)
+0.78 (100)f
-0.12 (80)
+7.7 (m), -6.2 (m)
+6.23 (m, 2H, dppm),e +6.07 (m, 2H, dppm),e
+4.18 (q, 2H, OCH2CH3), +1.21 (t, 3H, OCH2CH3)
+6.17 (m, 2H, dppm),e +6.04 (m, 2H, dppm)e
+5.73 (m, 4H, dppm)
+1.45
4
5
6
+7.8 (m), -5.7 (m)
+13.5 (m), -3.15 (m)h
-4.3 (m), -6.4 (m)
+1.30
+1.50
+0.66 (60)f
+0.69 (130)f
+0.32 (160)f
+5.16 (m, br, 4H, dppm)
∼+1.2i
a NMR spectra recorded on CDCl3 solutions unless otherwise noted. Abbreviations: s ) singlet; t ) triplet; q ) quartet; m ) multiplet. b Data are given
for dirhenium-centered processes and are based upon single scan cyclic voltammograms (scan rate (ν) ) 200 mV/s) measured on 0.1 M TBAH/CH2Cl2
solutions at a Pt-bead electrode and referenced to the Ag/AgCl electrode. Under our experimental conditions E1/2 ) +0.47 V for the ferrocenium/ferrocene
couple. E1/2 values are for one-electron processes with ip,a ) ip,c, and numbers in parentheses are the approximate values of ∆Ep ()Ep,a - Ep,c) for the
c
reversible processes. The spectra of compounds 1, 3, 4, and 6 appear as simple AA′BB′ patterns; the approximate centers of the two multiplets are given.
d Resonances for the phenyl groups of dppm and pyridyl ring protons are not given. e Multiplets of an ABX4 pattern. f This E1/2 value was determined from
a single-scan CV with a switching potential of ∼+0.9 V, i.e., less than the onset of the irreversible oxidation (Ep,a). g Spectrum recorded in CD2Cl2. h The
multiplet centered about δ ) +13.5 has the appearance of a doublet-of-doublets while that at δ ) -3.15 approximates to a quartet-of-doublets. i This
oxidation is actually two closely spaced processes with Ep,a values of =1.01 and =1.35 V; there is a weak coupled reduction process at Ep,c = +0.8 V and
a chemical product wave at Ep,c = -0.02 V.
so is the value of øav (42.7°) for the four torsion angles
mentioned above.
(Table 2), similar to the chemical shift reported for trans-
Re2(µ-O2C-4-C5H4N)2Cl2(µ-dppm)2 (δ ) +3.7). In addition,
the CV properties of 2 (Table 2) are typical of those reported
for other carboxylate complexes of the type trans-Re2(µ-O2-
CR)2Cl2(µ-dppm)2,3,7 thereby confirming that the trans
structure is retained in solution.
The complexes 3 and 4, which are formed by reacting Re2-
Cl4(µ-dppm)2 with pyridine-2,3-dicarboxylic acid and pyri-
dine-2,4-dicarboxylic acid, have structures very similar to
that of 1 (see Figures 3 and 4). The carboxylate groups in
the 3 and 4 positions of 3 and 4 are not involved in
coordination to the dirhenium unit. In the case of 3, the
carboxylic acid group in the 3 position of the free ligand is
converted into the ethyl ester under the reflux conditions
used, while the crystals of 4 contain dirhenium units that
are linked into infinite chains through intermolecular hy-
drogen bonds that are formed between the atom O(12) of
the coordinated carboxylato group in the 2 position and the
“free” carboxylic acid group in the 4 position (O(41)) of a
neighboring molecule. The hydrogen-bonded distance
O(12)‚‚‚O(41) is 2.69 Å. The Re-Re bond distance in the
crystals of 3‚CH2Cl2 and 4‚CH2Cl2‚C6H6 are 2.2752(2) and
2.2713(5) Å, respectively, and are similar to the Re-Re triple
bond distances in 1 and 1‚CH2Cl2 albeit a little shorter by
0.01-0.02 Å. Very similar staggered geometries are found
in 1, 3, and 4. The close similarities in the electronic and
molecular structures of these three complexes are also
reflected by the similarity of their 31P{1H} NMR spectra and
cyclic voltammetric properties (Table 2). The latter show
two one-electron oxidations, which is typical of [RetRe]4+
complexes,15 although the [Re2]6+/[Re2]5+ process is clearly
irreversible (Table 2).
Complex 2, which is formed by the reaction between Re2-
Cl4(µ-dppm)2 and (PPN)nic in refluxing ethanol, is similar
to the related compound trans-Re2(µ-O2C-4-C5H4N)2Cl2(µ-
dppm)2 that is prepared by an analogous reaction using the
(PPN)+ salt of isonicotinate.3 The related cis isomer of the
isonicotinate complex can be obtained2 by the reaction
between cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 and isonicotinic
acid. The structure of 2, which is shown in Figure 2, is similar
to that of the analogous µ-acetato complex trans-Re2(µ-O2-
CCH3)2Cl2(µ-dppm)2.3 The Re-Re distance in the latter
complex (2.2763(7) Å)3 is similar to that in 2 (2.2931(3)
Å). The 31P{1H} NMR spectrum shows a singlet at δ ) +4.4
The green colored complex 5, which is formed by the use
of a procedure very similar to that by which Re2Cl4(µ-dppm)2
is converted to 1-4 (i.e. reflux in ethanol), is the most
interesting compound of this group, and its characterization
provides insights into the reason 1 and 3-5 have such
unsymmetrical structures. The use of [PPN]2dipic rather than
dipicH2 is much preferred in the preparation of 5 (see
Experimental Section) since the reaction with the free acid
is very slow. The structure of 5 as present in a crystal of
composition 5‚0.5CH2Cl2 is shown in Figure 5. The Re-Re
bond distance of 2.2512(3) Å accords with the retention of
a RetRe bond and is similar to the distances in 1, 3, and 4
(range 2.27-2.29 Å), albeit a little shorter. The structure of
5 also resembles those of 1, 3, and 4 in the environment
about the coordinatively saturated Re atom, i.e., that with
the atom set [ReClNP2O] in which there is a pair of trans
Re-P bonds and an axially bound O atom from the
carboxylate group in the 2 position of the pyridyl ring. This
structure differs from 1, 3, and 4 in that the second
carboxylate group (that in the 6 position) has displaced one
of the Cl atoms bound to the coordinatively unsaturated Re
atom to form a pyridine carboxylate bridge between the two
metal centers (Figure 5). This leads to the formation of a
stable, puckered, six-membered ReNCC(O)ORe ring and
results in the retention of a staggered rotational geometry
(øav ) 42.8°) and a cis arrangement of Re-P bonds, as is
the case in 1, 3, and 4.
When the reaction between Re2Cl4(µ-dppm)2 and [PPN]2-
dipic is carried out in ethanol or dichloromethane at room
temperature, a single product (6) is isolated as orange
crystals. It converts quantitatively to 5 when heated (as a
suspension in ethanol) for 24 h. Microanalytical data indicate
that it has the same composition as 5, and this has been
Inorganic Chemistry, Vol. 42, No. 19, 2003 5929

Chattopadhyay et al.
Scheme 2. Possible Partial “Merry-Go-Round” Processes in Which
an Intermediate That Contains a trans,trans-[Re2(µ-dppm)2] Structure
Converts to Compound 1, 3, 4, or 5 in Which This Unit Has a Trans,
Cis Stereochemistrya
Figure 6. ORTEP13 representation of the structure of the dirhenium
complex Re2(dipic)Cl2(µ-dppm)2 as present in 6‚3C6H6. Thermal ellipsoids
are drawn at the 50% probability level except for the phenyl carbon atoms
of the dppm ligands which are circles of arbitrary radius. Selected bond
distances (Å) and angles (deg) are as follows: Re(1)-Re(2) 2.2750(10);
Re(1)-N(1) 2.210(13), Re(1)-O(211) 2.085(9), Re(1)-O(611) 2.115(10),
Re(1)-P(1) 2.439(4), Re(1)-P(3) 2.452(4), Re(2)-P(2) 2.413(5), Re(2)-
P(4) 2.418(5), Re(2)-Cl(21) 2.353(4), Re(2)-Cl(22) 2.375(4), O(211)-
C(21) 1.336(19), O(212)-C(21) 1.224(19), O(611)-C(61) 1.304(17),
O(612)-C(61) 1.250(18); O(211)-Re(1)-O(611) 145.5(4), P(1)-Re(1)-
P(3) 177.77(15), Cl(21)-Re(2)-Cl(22) 136.69(15), P(2)-Re(2)-P(4)
173.85(14), N(1)-Re(1)-Re(2) 179.1(3).
a Key: (a) conversion of 6 to 5; (b) conversion of an intermediate (not
isolated) in the case of 1, 3, and 4. Note: (1) If a pair of P atoms is trans,
then these atoms are not shown in the representations. (2) Chlorine atom(s)
that transfer from one Re atom to the other are designated as an encircled
Cl.
ligand; (3) the switch from a trans- to a cis-ReP2 geometry
at the coordinatively unsaturated Re center. A similar
intramolecular mechanism could also explain why com-
pounds 1, 3, and 4, all of which involve N,O chelation by a
pyridine-2-carboxylate ligand, have the same type of
trans,cis-[Re2(µ-dppm)2] structure (see Scheme 2b).
confirmed by a single-crystal X-ray structure determination
(see Figure 6), which shows that it is an isomer of 5. Its
clean conversion to 6 implies that it is a kinetic product
isolable with the use of mild reaction conditions. In the
structure of 6, the Re-Re bond is a little longer than that
present in 5 (2.2750(10) Å versus 2.2512(3) Å) and it has a
staggered rotational geometry with a øav value of 45.8°. It
differs from 5 in having an axially bound pyridine N atom
rather than an axial carboxylate O atom and has the pyridine-
2,6-dicarboxylate ligand bound in a O,N,O tridentate fashion
to only one Re atom, thereby giving a more symmetrical
structure. Furthermore, there are two pairs of trans Re-P
bonds rather than a trans and a cis pair as in 5. These
differences are also reflected by differences in the 31P{1H}
NMR spectra and cyclic voltammetric properties of 5 and 6
(see Table 2). Like most [Re2]4+ complexes, 5 and 6 show
two metal-based oxidations, but in both cases only the first
approaches reversibility and is much more accessible in 6
(by almost 0.4 V), implying that the different coordination
modes of the dipic ligand in these two compounds induces
significant differences in the metal-metal bond orbital
energies of 5 and 6 which is in accord with the changes in
the ligand atom sets that are bound to the two Re centers in
these complexes.
Concluding Remarks
In this study, we find that Re2Cl4(µ-dppm)2 reacts with
various pyridine-2-carboxylate ligands (either as the free
acids or their [PPN]+ salts) to give complexes 1, 3, and 4
(Scheme 1) in which the products contain the expected N,O
chelation by the ligand but an unexpected switch in the
coordination of the two dppm ligands from a trans, trans to
a trans, cis stereochemistry. In the case of the reaction of
Re2Cl4(µ-dppm)2 with the pyridine-2,6-dicarboxylate ligand,
both the kinetic product 6 and thermodynamic product 5 have
been isolated and fully characterized. The quantitative
conversion of 6 to 5 provides an explanation of how the trans,
trans coordination of dppm ligands in Re2Cl4(µ-dppm)2 and
Re2[η3(O,N,O)-dipic]Cl2(µ-dppm)2 (6) can convert to the
trans,cis arrangement in Re2[µ:η3(O,N,O)-dipic]Cl2(µ-dppm)2
(5) and, by implication, how 1, 3, and 4 come to have
structures similar to that of 5 (see Scheme 2).
The reaction of 1 with [PPN]O2CCH3 results in the
complete replacement of picolinate by acetate and the
formation of the well-known compound cis-Re2(µ-O2CCH3)2-
Cl2(µ-dppm)27 in high yield (see Experimental Section). We
have not yet been able to prepare mixed acetate/pyridine
carboxylate complexes such as Re2(µ-O2CCH3)(µ-O2C-2-
C5H4N)Cl2(µ-dppm)2 by this strategy.
Any mechanism for the conversion of 6 to the more
thermodynamically stable isomeric form 5 must, among other
things, account for the change from two sets of trans Re-P
bonds in 6 to a trans and cis pair in 5. We believe that this
conversion is readily explainable by the pathway represented
in Scheme 2a, in which a partial “merry-go-round” process,
of a type that is well documented for other dirhenium(II)
complexes which contain the [Re2(µ-dppm)2]4+ unit,8,18,19 can
account for the following:
We also note that attempts to prepare dirhenium(II)
compounds that contain the 3,4- and 3,5-pyridinecarboxylate
ligands were unsuccessful. The reactions of Re2Cl4(µ-dppm)2
with these acids and their [PPN]+ salts in refluxing ethanol
(18) Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Falvello, L. R.; Tetrick,
S. M.; Walton, R. A. J. Am. Chem. Soc. 1985, 107, 3524-3530.
(19) Wu, W.; Fanwick, P. E.; Walton, R. A. J. Am. Chem. Soc. 1996, 118,
13091-13092.
(1) the transfer of a chloride ligand from one metal atom
to the other; (2) the change in coordination mode of the dipic
5930 Inorganic Chemistry, Vol. 42, No. 19, 2003

Reactions of Re2Cl4(µ-dppm)2
led to almost complete recovery of the unreacted starting
material. In contrast, pyridine-3-carboxylate (as its [PPN]+
salt) reacts to give trans-Re2[µ:η2(O,O)-O2C-3-C5H4N]2Cl2-
(µ-dppm)2 (2), which is similar to the isonicotinate complex
trans-Re2[µ:η2(O,O)-O2C-4-C5H4N]2Cl2(µ-dppm)2 and to the
trans isomer of the related acetate complex that have been
isolated previously.3 Like the isonicotinate complex cis-Re2-
(µ-O2C-4-C5H4)2Cl2(µ-dppm)2,2 and other transition metal
complexes that contain isonicotinate or nicotinate ligands and
which can be used to make mixed-metal assemblies (e.g.
those of dirhodium(II)20 and platinum(II)21), compound 2 and
its isonicotinate analogue3 should be capable of fulfilling a
similar role. Attempts to use 2 to prepare small mixed-metal
assemblies or polymers are now underway.
Acknowledgment. R.A.W. thanks the John A. Leighty
Endowment Fund for support of this work.
Supporting Information Available: X-ray crystallographic files
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC030153Y
(20) Schiavo, S. L.; Nicolo`, F.; Tresoldi, G.; Piraino, P. Inorg. Chim. Acta
2003, 343, 351-356.
(21) Song, R.; Kim, M.; Sohn, Y. S. Inorg. Chem. 2003, 42, 821-826.
Inorganic Chemistry, Vol. 42, No. 19, 2003 5931