Reactivity of 2,3-bis(2-pyridyl)pyrazine with [Re2(CO)8(CH3CN)2]: Molecular structures of [Re2(CO)8(C14H10N 4)] and [Re2(CO)8(C14H 10N4)Re2(CO)8]

Article abstract of DOI:10.1016/j.jorganchem.2006.10.042

The reaction of the labile compound [Re₂(CO)₈(CH₃CN)₂] with 2,3-bis(2-pyridyl)pyrazine in dichloromethane solution at reflux temperature afforded the structural dirhenium isomers [Re₂(CO)₈(C₁₄H₁₀N ₄)] (1 and 2), and the complex [Re₂(CO)₈(C₁₄H₁₀N ₄)Re₂(CO)₈] (3). In 1, the ligand is σ,σ′-N,N′-coordinated to a Re(CO)₃ fragment through pyridine and pyrazine to form a five-membered chelate ring. A seven-membered ring is obtained for isomer 2 by N-coordination of the 2-pyridyl groups while the pyrazine ring remains uncoordinated. For 2, isomers 2a and 2b are found in a dynamic equilibrium ratio [2a]/[2b] = 7 in solution, detected by ¹H NMR (-50 °C, CD₃COCD₃), coalescence being observed above room temperature. The ligand in 3 behaves as an 8e-donor bridge bonding two Re(CO)₃ fragments through two (σ,σ′-N,N′) interactions. When the reaction was carried out in refluxing tetrahydrofuran, complex [Re₂(CO)₆(C₁₄H₁₀N ₄)₂] (4) was obtained in addition to compounds 1-3. The dinuclear rhenium derivative 4 contains two units of the organic ligand σ,σ′-N,N′-coordinated in a chelate form to each rhenium core. The X-ray crystal structures for 1 and 3 are reported.

Full text of DOI:10.1016/j.jorganchem.2006.10.042

Journal of Organometallic Chemistry 692 (2007) 894–902
www.elsevier.com/locate/jorganchem
Reactivity of 2,3-bis(2-pyridyl)pyrazine with
[Re2(CO)8(CH3CN)2]: Molecular structures of
[Re2(CO)8(C14H10N4)] and [Re2(CO)8(C14H10N4)Re2(CO)8]
a,
a
a
a
*
´
´
Ruben A. Machado , Marıa Cristina Goite , David Rivillo , Ysaura De Sanctis ,
a
b
c
a
Alejandro J. Arce , Antony J. Deeming , Lindora D’Ornelas , Anibal Sierralta ,
a
a
d
´
Reinaldo Atencio , Teresa Gonzalez , Esperanza Galarza
a
Centro de Quı´mica, Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
b
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
Lab. de Sı´ntesis Organometa´lica, Esc. de Quı´mica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47102, Caracas, Venezuela
c
d
Universidad del Valle, Departamento de Quı´mica, Ciudad Universitaria Mele´ndez AA25360, Cali, Colombia
Received 27 September 2006; received in revised form 19 October 2006; accepted 19 October 2006
Available online 26 October 2006
Abstract
The reaction of the labile compound [Re2(CO)8(CH3CN)2] with 2,3-bis(2-pyridyl)pyrazine in dichloromethane solution at reflux tem-
perature afforded the structural dirhenium isomers [Re2(CO)8(C14H10N4)] (1 and 2), and the complex [Re2(CO)8(C14H10N4)Re2(CO)8]
(3). In 1, the ligand is r,r0-N,N0-coordinated to a Re(CO)3 fragment through pyridine and pyrazine to form a five-membered chelate
ring. A seven-membered ring is obtained for isomer 2 by N-coordination of the 2-pyridyl groups while the pyrazine ring remains unco-
ordinated. For 2, isomers 2a and 2b are found in a dynamic equilibrium ratio [2a]/[2b] = 7 in solution, detected by 1H NMR (ꢀ50 ꢀC,
CD3COCD3), coalescence being observed above room temperature. The ligand in 3 behaves as an 8e-donor bridge bonding two Re(CO)3
fragments through two (r,r0-N,N0) interactions. When the reaction was carried out in refluxing tetrahydrofuran, complex
[Re2(CO)6(C14H10N4)2] (4) was obtained in addition to compounds 1–3. The dinuclear rhenium derivative 4 contains two units of the
organic ligand r,r0-N,N0-coordinated in a chelate form to each rhenium core. The X-ray crystal structures for 1 and 3 are reported.
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Rhenium carbonyls; a-diimines; Crystal structures; Pyridine; Pyrazine
1. Introduction
tron transfer, magnetic coupling and intervalence transfer
[2].
Polypyridyl ligands bridging two or more remote metal
centers and containing p-delocalized systems have been the
subject of an enormous amount of work [1]. The interest in
metallic complexes with this type of ligand has been stimu-
lated due to the strong metal-to-ligand charge transfer nor-
mally present in these complexes, which is believed to be
potentially useful in molecular materials to promote elec-
The interaction of p-delocalized organic ligands with
metal clusters is well documented [3]. Our interest in the
combination of metal clusters and polycyclic aromatic
ligands is oriented to produce novel systems with potential
electronic properties. A variety of binuclear transition-
metal complexes containing a-diimine ligands has been
studied extensively within the past several years [4]. In
dimetallic clusters, a-diimine ligands such as R-DBA
(1,4-disubstituted-1,4-diaza-1,3-butadienes), R-Pyca (pyri-
dine-2-carbaldiimines 6-R–C5H3N–2-CH = N–R), bipyri-
dine and phenanthroline, adopt chelating r,r0-N,N0
coordinations to yield complexes formulated as
*
Corresponding author. Tel.: +58 212 5041637; fax: +58 212 5041350.
E-mail address: ysantis@ivic.ve (R.A. Machado).
0022-328X/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.10.042

R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
895
[M2(CO)8(a-diimine) (M = Mn, Re) [5]. In this context,
2.1. Reaction of 2,3-bis(2-pyridyl)pyrazine with
[Re2(CO)8(CH3CN)2]. Synthesis of complexes 1–4
when complexes [LnM0M(CO)3(a-diimine)] (LnM0 =
(CO)5Mn, (CO)5Re, (CO)4Co, Cp(CO)2Fe, PhSn;
M = Mn, Re; a-diimine = bipyridine, phenanthroline) are
irradiated under visible light, the homolysis of the M0–M
bond and/or loss of CO from the M(CO)3(a-diimine) frag-
ment is observed [6]. While the rhenium complex
[LnM0Re(CO)3(a-diimine)] uniquely exhibited M0–Re
bond homolysis, both reactions occurred for some others
[LnM0Mn(CO)3(a-diimine)] derivatives [6].
A solution of [Re2(CO)8(CH3CN)2] (0.100 g, 0.145
mmol) and 2,3-bis(2-pyridyl)pyrazine (0.035 g, 0.149
mmol) in THF (50 ml) was refluxed for 5 h, during which
time the color of the solution changed from pale yellow
to blue. The solvent was removed under reduced pressure
and the residue separated by TLC (dichloromethane:n-hex-
ane, 2:3/vol:vol) to give four compounds: the blue struc-
tural isomers 1 and 2 (0.035 g, 28% and 0.008 g, 7%,
respectively) formulated as [Re2(CO)8(C14H10N4)];
[Re2(CO)8(C14H10N4)Re2(CO)8] (3) (blue, 0.013 g, 6%)
and [Re2(CO)6(C14H10N4)2] (4) (yellow, 0.008 g, 5%). Com-
plexes 1 and 3 were obtained as blue crystals after recrystal-
lization from dichloromethane:cyclohexane (1:3 by
volume). Carrying out the same reaction in refluxing
dichloromethane for 2 h only gave 1, 2 and 3 (40%, 16%,
and 10%, respectively). Anal. Found for 1: C, 31.87; H,
1.16; N, 6.84%. Anal. Found for 2: C, 31.71; H, 1.26; N,
6.62%. Calc. for C22H10N4O8Re2: C, 31.81; H, 1.20; N,
6.75%. Anal. Found for 3: C, 25.31; H, 0.68; N, 3.96%.
The a-diimine 2,3-bis(2-pyridyl)pyrazine is a very ver-
satile ligand, widely used to construct supramolecular
transition-metal compounds [7]. We have previously stud-
ied the interaction of 2,3-bis(2-pyridyl)pyrazine with the
labile [Os3(CO)10(CH3CN)2] and have found the
formation of two structural isomers [Os3(l-H)
(l,g3-C14H9N4)(CO)9] due to the thermolysis of [Os3(C14-
H10N4)(CO)10] [8]. One isomer shows an orthometallated
2-pyridyl group and a N-coordinated 2-pyridyl, forming
a seven-membered chelate ring with the pyrazine ring
uncoordinated. The other isomer contains a 2-pyridyl-
1,4-pyrazine fragment metallated in the pyrazine ring to
form a five-membered chelate ring, as the second pyridine
ring remains uncoordinated [8].
Table 1
In this paper we describe the isolation and
characterization of new dirhenium complexes showing
r,r0-N,N0-4e donor groups, derived from the reaction of
[Re2(CO)8(CH3CN)2] with 2,3-bis(2-pyridyl)pyrazine.
Crystal data, data collection and refinement parameters for
[Re2(CO)8(C14H10N4)] (1) and [Re2(CO)8(C14H10N4)Re2(CO)8] Æ CH2Cl2
(3)
1
3
Formula
M
Crystal size (mm3)
Colour
C
839.74
0.36 · 0.24 · 0.20
Blue prism
22H10N4O8Re2
C31H12Cl2N4O16Re4
1512.15
0.25 · 0.12 · 0.10
Violet prism
2. Experimental
All reactions were carried out under dried and purified
nitrogen. Solvents were purified by standard procedures
and distilled prior to use [9]. [Re2(CO)8(CH3CN)2] was
prepared by the method previously described [10]. 2,3-
Bis(2-pyridyl)pyrazine was purchased from Aldrich and
used as supplied. Reactions were monitored by FT-IR
in the range 2200–1750 cmꢀ1 and the products were sepa-
rated by TLC (SiO2, Merck 60 HF254). IR spectra were
recorded on a Nicolet 5DXC FT-IR spectrometer, using
0.5 mm calcium fluoride cells. 1H NMR spectra were
obtained on a Bruker 300 MHz spectrometer. Elemental
analysis were performed on a Fisons EA-1108 instrument.
Partial decomposition of the derivatives under sun light
was initially observed, so special care was taken to
exclude it from the samples.
Ab initio quantum chemical calculations were carried
out for compound 2. All geometry optimizations and
energy calculations were performed with GAUSSIAN-98
program [11] at unrestricted DFT level of theory using
the Becke’s three-parameter hybrid functional with Lee,
Yang and Parr correlation functionals (UB3LYP func-
tional). The Stevens, Basch and Krauss relativistic com-
pact effective potentials were employed for N, O, C
and Re atoms. For Re atoms, the basis set and the effec-
tive potential include explicitly the 5s2, 5p6, 5d5 and 6s2
electrons.
Temperature (K)
Crystal system
Space group
Unit cell dimensions
293
Triclinic
2
293
Monoclinic
P21/nꢀ
2
ꢀ
Pð1Þ
˚
a (A)
8.2040(16)
10.474(2)
14.506(3)
99.43(3)
102.39(3)
105.68(3)
1138.9(4)
2
2.422
768
4.16–50.08
10.676
4252
9.994(2)
22.468(5)
17.525(4)
90
102.96(3)
90
3835.2(13)
4
2.619
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
3
˚
V (A )
Z
Dc (g cmꢀ3
F(000)
)
2752
2h Range (ꢀ)
l (mmꢀ1
Reflections measured
3.62–50.02
12.799
7153
)
Independent reflections 4011 (0.0558)
(Rint
6741 (0.0646)
)
Data/restraints/
parameters
GOF on F2
Final R index [F2 > 2r] R1 = 0.0696,
wR2 = 0.2218
4011/0/325
6741/6/507
1.070
1.019
R1 = 0.0615,
wR2 = 0.1147
R1 = 0.1423,
wR2 = 0.0914
h 0/11, k 0/26,
l ꢀ20/20
R index (all data)
R1 = 0.0808,
wR2 = 0.2039
h ꢀ9/9, k 0/12,
l ꢀ17/17
Index ranges
Maximꢀu3m peak/hole
˚
3.084 and ꢀ5.941
1.528 and ꢀ1.262
(e A
)

896
R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
Calc. for C30H10N4O18Re4: C, 25.25; H, 0.70; N, 3.93%.
Anal. Found for 4: C, 40.51; H, 1.93; N, 11.06%. Calc.
for C34H20N8O6Re2: C, 40.48; H, 1.98; N, 11.11%.
520 nm) for 4 h, during which time the color of the solution
changed from blue to orange. The solvent was removed
under reduced pressure and TLC of the residue (eluant:
dichloromethane:n-hexane, 7:3 vol:vol) gave [Re2(CO)10]
and [Re2(CO)6(C14H10N4)2] (4) (29% and 26%,
respectively).
2.2. Reaction of [Re2(CO)8(C14H10N4)] (1) with
[Re2(CO)8(CH3CN)2]
A solution of compound 1 (0.035 g, 0.031 mmol) and
[Re2(CO)8(CH3CN)2] (0.021 g, 0.031 mmol) in dichloro-
methane (50 ml) was refluxed for 1 h. The solvent was
removed under reduced pressure and the residue separated
by TLC eluting with dichloromethane:n-hexane (2:3 vol:
vol) to give 2 and 3 (6% and 14%, respectively).
2.5. X-ray crystal structure determinations for compounds 1
and 3
Data for compounds 1 and 3 were collected using a
Rigaku AFC7S diffractometer, provided with a graphite
˚
monochromated Mo Ka radiation (k = 0.71073 A).
Details of the crystal data and structure refinement are
shown in Table 1. A semi-empirical absorption correction
[12] was applied to both set of data. Crystal structures
were solved by direct methods, and the final models were
reached by Fourier techniques. The crystals structure of 3
shows a disordered CH2Cl2 molecule, which was modeled
in two different orientations. All non-H atoms were
refined with anisotropic displacement parameters. The
H-atoms were included in the last cycle in their calculated
positions, except those of the disordered solvent molecule
found in 3 that were not considered, and refined using a
riding model. The isotropic displacement parameters for
the H-atoms were considered according to their parent
atoms. The final cycle of full-matrix least-squares was
based on F2. The data reduction were carried out using
the TEXSAN program [13]. All structure solutions and
refinements were made using the SHELXTL-PLUS package
[14].
2.3. Thermal treatment of complex
[Re2(CO)8(C14H10N4)] (1) in THF
A solution of compound 1 (0.035 g, 0.031 mmol) in
THF was heated under reflux for 4 h. The solvent was
removed under reduced pressure and TLC of the residue
(eluant: dichloromethane:n-hexane, 7:3 vol:vol) gave
[Re2(CO)10] and [Re2(CO)6(C14H10N4)2] (4) (10% and 4%
respectively). When the reaction was carried out in the
darkness, only a black residue was obtained, probably
due to decomposition of 1.
2.4. Irradiation of complex [Re2(CO)8(C14H10N4)] (1) in
dichloromethane
A solution of compound 1 (0.035 g, 0.052 mmol) in
dichloromethane was irradiated under visible light (380–
Scheme 1.

R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
897
3. Results and discussion
group between cluster units, the reaction of 1 with an
excess of [Re2(CO)8(CH3CN)2] in dichloromethane at
reflux temperature was carried out, giving compounds 2
and 3. In a separate experiment, 1 was thermally and pho-
tochemically transformed into 4. The general synthetic
route to the rhenium derivatives is depicted in Scheme 1,
Reaction of [Re2(CO)8(CH3CN)2] with 2,3-bis(2-pyr-
idyl)pyrazine in dichloromethane solution at refluxing
temperature, affords the structural isomers [Re2(CO)8(C14-
H10N4)] (1 and 2) and the complex [Re2(CO)8(C14H10N4)-
Re2(CO)8] (3). However, if the same reaction is carried
out under refluxing THF for 5 h, in addition to complexes
1–3, compound [Re2(CO)6(C14H10N4)2] (4) is obtained.
Taking into account the fact that the symmetrical 2,3-
bis(2-pyridyl)pyrazine ligand could serve as a versatile
bridging ligand and has the potential to act as a linking
1
and their H NMR and IR data is shown in Table 2.
3.1. Complex [Re2(CO)8(C14H10N4)] (1)
The IR spectrum of 1 in the carbonyl stretching region
resembles that for [(CO)5Re–Mn(CO)3(i-Pr-DAB)], whose
Table 2
IR and 1H NMR data for compounds 1–4
Compound
m(CO) (cmꢀ1 a
)
1H NMR (d)b
JH–H (Hz)
[Re2(CO)8(C14H10N4)] (1)
2073 s
2025 w
1992 vs
1977 s
1907 m
9.34 (d, Ha)
8.72 (d, Hb)
ab 3.3
cd 4.8
ce 1.7
cf 1.1
de 7.7
df 1.2
ef 7.7
gh 5.7
gi 1.6
gj 0.9
hi 7.5
hj 1.3
ij 8.5
9.23 (ddd, Hg)
7.77 (ddd, Hi)
7.55 (ddd, Hh)
7.29 (ddd, Hj)
8.69 (ddd, Hc)
8.16 (m, He)
7.90 (m, Hf)
7.67 (ddd, Hd)
[Re2(CO)8(C14H10N4)] (2)
2073 s
1992 vs
1976 s
1905 m
9.32 (ddd, Ha)
9.03 (bs, He)
8.93 (bd, Hd)
8.06 (bt, Hc)
7.76 (bt, Hb)
9.33 (ddd, HaH0a)c
9.12 (s, H0e)
ab 5.8
ac 1.4
ad 0.8
bc 7.5
bd 1.3
cd 8.5
9.08 (d, H0d)
9.03 (s, He)
8.97 (d, Hd)
8.12 (m, Hc)
8.06 (m, H0c)
7.79 (m, Hb)
7.68 (m, H0b)
[Re2(CO)8(C14H10N4)Re2(CO)8] (3)
2082 m
2072 m
1999 vs
1989 vs
1905 m
9.09 (d, Ha)
8.91 (d, Hb)
ab 4.2
cd 5.7
ce 1.5
de 7.5
df 1.2
ef 8.6
gh 5.6
gi 1.5
hi 7.2
hj 1.1
ij 8.2
9.35 (m, Hc, Hg)
7.69 (ddd, Hd)
8.02 (ddd, He)
8.82 (bd, Hf)
7.75 (ddd, Hh)
8.06 (ddd, Hi)
9.05 (ddd, Hj)
[Re2(CO)6(C14H10N4)2] (4)
2026 vs
1926 s
1905 s
9.02 (d, Ha)
8.67 (d, Hb)
ab 3.0
cd 5.3
ce 1.8
cf 0.7
de 7.0
df 1.5
ef 8.3
gh 4.9
gi 1.7
gj 1.0
hi 6.0
hj 1.0
ij 7.7
9.04 (ddd, Hc)
7.45 (ddd, Hd)
7.64 (ddd, He)
7.12 (ddd, Hf)
8.67 (ddd, Hg)
7.54 (ddd, Hh)
8:00 (m, Hi)
7.90 (m, Hj)
a
CH2Cl2.
CDCl3.
(CD3)2CO, 248 K.
b
c

898
R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
structure was determined by X-ray diffraction analysis [15].
1
The H NMR spectrum exhibits two doublets (d = 9.34
and 8.72) assigned to pyrazinic protons, and eight different
signals in the aromatic region associated to two different
pyridine rings. This suggests that the structure of 1 involves
a 4e-donating r,r0-N,N0-bonded 2,3-bis(2-pyridyl)pyrazine
ligand. By analogy to the known structure of [(CO)5Re–
Mn(CO)3(i-Pr-DAB)] [15], 1 may contain a coordinated
2-pyridyl-1,4-pyrazine fragment forming a five-membered
chelate ring. The X-ray single-crystal study of 1 (Fig. 1,
selected bond lengths and angles are in Table 3) confirmed
that the organic ligand is coordinated in a similar way to
those observed in the crystal structures of [Os3(C14-
H10N4)(CO)10] [8] and [Re(CO)3Cl(C14H10N4)] [16].
Therefore, one nitrogen atom of a pyridine ring is axially
˚
coordinated [Re1–N3 = 2.174(12) A] and an equatorial site
is occupied by a pyrazine nitrogen atom [Re1–N1 =
˚
2.159(12) A]. The second pyridine moiety remains uncoor-
dinated. The bidentate ligand in 1 adopts a conformation
statistically equal to that observed for the chloride rhenium
complex [Re(CO)3Cl(C14H10N4)] [16], even though in this
compound the Cl ligand is considerable less voluminous
than the Re(CO)5 fragment in 1. For the complex
[Re(CO)3Cl(C14H10N4)] the dihedral angles between the
pyrazine entity and the coordinated and non-coordinated
pyridine rings are 15.3(4)ꢀ and 45.8(3)ꢀ, respectively; for
1, the corresponding values are 17.7(8)ꢀ and 42.4(7)ꢀ. As
for [Re(CO)3Cl(C14H10N4)] [16], the pyrazine ring in 1 is
Fig. 1. ORTEP view (30% probability. Ha atoms omitted for clarity) of
the molecular structure of 1 in the crystal.
late ring similar to the coordination found for complexes
[Os3(C14H10N4)(CO)9] [8], {[Pd(dpq)2](PF6)2}, [PdCl2-
(dpq)] and [ReCl(CO)3(tppz)] Æ 2H2O [dpq = 2,3-bis(dipyr-
idyl)quinoxaline; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine]
˚
1
also slightly twisted (rms of the mean plane: 0.039 A) in
[18]. The variable temperature H NMR spectrum of 2 in
comparison to the uncoordinated 2,3-bis(2-pyridyl)pyra-
˚
zine (rms of the mean plane: 0,007 A) [17].
Table 3
˚
Selected bond lengths (A) and angles (ꢀ) for compound 1
3.2. Complex [Re2(CO)8(C14H10N4)] (2)
Re1–Re2
Re1–N3
Re1–N1
3.084(1)
2.17(1)
2.16(1)
N1–Re1–Re2
N3–Re1–Re2
94.0(3)
88.7(3)
Spectroscopic data (Table 2) suggest for 2 the presence
of a metallocyclic system showing a seven-membered che-
Fig. 2. Variable temperature 1H NMR spectra of 2 in CD3COCD3.

R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
899
CD3COCD3 (Fig. 2) shows broad resonances at 35 ꢀC, but
at ꢀ50 ꢀC sharp resonances are assigned to two highly sym-
metric isomeric compounds, 2a and 2b, where [2a]/[2b] = 7.
The major isomer 2a shows five 1H NMR resonances
(Table 2) assigned to pyrazinic protons (d 9.03) and four
C–H signals for the pyridinic protons (d 9.33, 8.97, 8.12
and 7.79). Likewise, the minor isomer 2b shows a set of res-
onances that can be assigned to pyrazinic protons (d 9.12)
and four C–H signals for the pyridinic protons (d 9.33,
9.08, 8.06 and 7.68). Above 35 ꢀC coalescence is observed
(except for the ortho protons Ha in the pyridine rings) to
give four signals. The dynamic behavior between 2a and
2b could be derived from the different conformations
adopted by twisting the seven-membered metallocycle
(Fig. 2), so the Re(CO)5 moiety can be located in an equa-
torial position relative to the metallocycle mean plane (2a),
or in an axial position (2b). The equatorial orientation of
the Re(CO)5 fragment leads to the diminishing of the steric
repulsion between the carbonyl groups of the Re(CO)5
entity and the pyrazine ring, so this conformation is
assigned to the major isomer 2a.
In order to evaluate this hypothesis, ab initio quantum
chemical calculations were performed at DFT level [11],
giving that at 298 K isomer 2a is thermodynamically more
Fig. 3. Structure obtained by ab initio quantum chemical calculation: (a) isomer 2a and (b) isomer 2b.

900
R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
Table 4
stable than isomer 2b by DG = ꢀ7.45 kcal/mol. This theo-
˚
1
Selected bond lengths (A) and angles (ꢀ) for compound 3
retical result is in agreement with the H NMR evidence
Re1–Re2
Re3–Re4
Re1–N3
Re1–N1
Re3–N2
Re3–N4
3.081(1)
3.068(1)
2.13(1)
2.13(1)
2.15(1)
2.18(1)
N1–Re1–Re2
N3–Re1–Re2
N2–Re3-Re4
N4–Re3–Re4
C4–N1–Re1
C3–N2–Re3
94.2(4)
94.2(4)
93.4(4)
95.6(4)
124.8(1)
123.7(1)
that in solution the relationship [2a]/[2b] = 7.
3.3. Complex [Re2(CO)8(C14H10N4)Re2(CO)8] (3)
The m(CO) absorption spectrum of 3 revealed a pattern
similar to that observed for 1 and for [(CO)5ReMn(-
CO)3(i-Pr-DAB)] [15]. The 1H NMR spectrum of 3 exhibits
a group of signals corresponding to eight pyridinic protons,
and two doublets for the pyrazinic protons. It is worth
mentioning that 3 can also be obtained from the reaction
of 1 and [Re2(CO)8(CH3CN)2] under refluxing dichloro-
methane. Therefore, the formation of 3 occurs as a conse-
quence of the linkage of a Re2(CO)8 unit to compound 1,
so the organic ligand will be r,r0-N,N0-coordinated in a
chelate form to two Re(CO)3 units through the nitrogen
atoms of pyrazine and pyridine rings. This coordination
mode is also found for the dinuclear complex
[Re(CO)3Cl(C14H10N4)Re(CO)3Cl] [16] (see Fig. 3).
dral angles between the pyrazine ring and those of the pyri-
dines N3–C5–C6–C7–C8–C9 and N4–C101–C111–C121–
C131–C141 are 24.2(6)ꢀ and 22.2(6)ꢀ, respectively. Both
dihedral angles are smaller than those observed in the crys-
tal structure of the complex [Re(CO)3Cl(C14H10N4)Re-
(CO)3Cl] [29.7(4)ꢀ and 31.6(3)ꢀ] [16], possibly due to the
presence of the voluminous Re(CO)5 units in 3. In addi-
tion, the pyrazine ring in 3 suffers a kind of buckling that
leads to deviation from planarity. The average deviation
˚
(0.056 A) from the mean plane of this ring is larger than
˚
the value found for 1 (0.039 A). Maximum deviations are
observed for C1 and C2 atoms, which lie below and above
˚
˚
the pyrazine mean plane, [ꢀ0.111(12) A and 0.081(12) A,
A single-crystal X-ray study of 3 confirms these observa-
tions. Its molecular structure is shown in Fig. 4 and
selected bond lengths and angles are in Table 4. The
respectively].
A possible route for the formation of complexes 1–3 is
given in Scheme 2. It is proposed that [Re2(CO)8-
(CH3CN)2] dissociates into the coordinatively unsaturated
species [Re2(CO)8(CH3CN)], I [19]. 2,3-Bis(2-pyridyl)pyra-
zine (dpp) coordinates via a r-N bond of one pyridyl moi-
ety as shown for intermediate II. Complex 1 could be
formed from II by a concerted process involving CO shift
and elimination of CH3CN (L) from the second rhenium
atom [20], followed by chelation through a pyrazine N-
atom. For the formation of 2, the ligand in II rotates
˚
(CO)5Re–Re(CO)3 fragments [Re1–Re2 = 3.0806(14) A
˚
and Re3–Re4 = 3.0676(14) A] are linked by a doubly-che-
lating bipyridylpyrazine coordinated through all nitrogen
˚
˚
atoms [Re1–N1 = 2.125(13) A, Re1–N3 = 2.129(13) A;
˚
˚
Re3–N2 = 2.148(14) A, Re3–N4 = 2.179(14) A]. There-
fore, two five-membered chelate rings [Re1–N1–C1–C5–
N3 and Re3–N2–C2–C101–N4] are formed. The bridging
coordination to the metal centers requires the organic
ligand to adopt a twisted conformation, in which the dihe-
Fig. 4. ORTEP view (30% probability, H atoms omitted for clarity) of the molecular structure of 3 in the crystal.

R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
901
Scheme 2.
around the single C–C pyrazine-pyridine bonds, with sub-
sequent pyridine N-coordination. The reaction of complex
1 with the coordinatively unsaturated species I gives III,
which could be converted to 3 by a concerted process sim-
ilar to that involved in the formation of 1 from II.
protons, so the ligand symmetry is lost. The small differ-
ences between the chemical shifts observed for 4 compared
to 1, suggest that two organic fragments are coordinated in
a r,r0-N-N0-chelating manner to each Re(CO)3 moiety.
The structure proposed for compound 4 in Scheme 1 is
based on the results obtained when a solution of 1 is ther-
mally treated under visible light. In this case [Re2(CO)10] is
obtained as a by-product, probably due to a homolytic Re–
Re bond cleavage in 1, where [ÅRe(CO)5] radicals so
obtained combine to give [Re2(CO)10], easily identified by
its m(CO) spectrum [25]. Concomitantly [ÅRe(CO)3-
(C14H10N4)] radicals could be formed and self-combined
to yield the complex [Re2(CO)6(C14H10N42)] (4), in which
two equivalent fac-Re(CO)3 subgroups are obtained. The
same behavior is observed for the homolytic metal–metal
bond cleavage of the complexes [(CO)5MnMn(CO)3Phen],
[(CO)5ReRe(CO)3Phen] and [(CO)5ReRe(CO)3Biquin]
(Phen = 1,10-phenanthroline and Biquin = 2,20-biquino-
3.4. Complex [Re2(CO)6(C14H10N4)2] (4)
Unlike 1, the IR carbonyl spectrum of 4 in CH2Cl2
shows a very simple band pattern similar to the one
observed for [Re2(l:g6-C24H18N4)(CO)6] [21], [Re2-
(CO)6(OX)2] (X = 8-quinolinate) [22] and [Re(CO)3-
(C12H8N2)Cl]2 [23], as pseudo-octahedral rhenium
complexes with only facial CO ligands usually display three
bands in the carbonyl stretching region [24]. The 1H NMR
spectrum of 4 in the aromatic region contains eight differ-
ent signals associated to two non-equivalent pyridinic sys-
tems and two different signals for the four pyrazinic

902
R.A. Machado et al. / Journal of Organometallic Chemistry 692 (2007) 894–902
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L. D’Ornelas, D.A. Oliveros, J. Organomet. Chem. 690 (2005)
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J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E.
Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels,
K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M.
Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
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N. Rega, P. Salvador, J.J. Dannenberg, D.K. Malick, A.D. Rabuck,
K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G.
Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A.
Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W.
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line) when irradiated under visible light to form the corre-
sponding species [M2(CO)6(a-diimine2)] [6].
4. Supplementary material
CCDC 285341 and 285342 contain the supplementary
crystallographic data for 1 and 3. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk.
Acknowledgements
We thank FONACIT for projects S1-2001000665 and
´
LAB-97000665, and Consejo de Desarrollo Cientıfico y
´
Humanıstico (UCV) for Project CDCH PG03-12-4866-
2001.
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