Silicagel support mediated nonphotolytic cleavage of the rhenium-rhenium bond of [{NC5H4N=NC6H4(R)}(CO) 3Re0]2. Synthesis of the monomeric species ReI{NC5H4N=NC6H4(R)} (CO)3Cl, crystal structure, spectroscopic and electron-transfer propertys

Article abstract of DOI:10.1016/S0022-328X(99)00016-9

The reaction of Re₂(CO)₁₀ with the 2-arylazopyridine ligand, L [L=NC₅H₄-N=N-C₆H₄(R), R=H, o-Me/Cl, m-Me/Cl] in dry THF under a dinitrogen atmosphere afforded a metal-metal bonded product of the type (L)(CO)₃Re⁰-Re⁰ (CO)₃(L) 1. On a silica gel column and in the presence of chlorinated solvents (CHCl₃ or CH₂Cl₂) the complex 1 transformed into a mononuclear complex of composition Re^I(L)(CO)₃Cl 2, where the cleavage of Re-Re bond of 1 and the oxidative addition of chlorine to the rhenium center have taken place consecutively. The molecular structure of the complex 2a (R=H) has been determined by single crystal X-ray diffraction. The crystal lattice consists of two crystallographically independent Re(L¹)(CO)₃Cl molecules which are non-superimposable mirror images; d and 1 enantiomers, exist in a 1:1 ratio. The complexes 2 display irreversible Re^I→Re^II oxidation processes near 1.5 V versus SCE and two quasi-reversible azo (N=N) reductions in the ranges -0.28 to -0.48 V and - 0.83 to -1.06 V versus SCE. The complexes of the type 1 exhibit only four successive azo reductions in the range O to -2 V versus SCE. Both the complexes (1 and 2) display dπ(Re) → π*L metal-to-ligand charge-transfer transitions near 500 nm and intraligand n-π* and π-π* transitions near 400 and 300 nm, respectively. The complexes 2 are susceptible to a spontaneous chloride exchange reaction.

Full text of DOI:10.1016/S0022-328X(99)00016-9

Journal of Organometallic Chemistry 579 (1999) 311–320
Silicagel support mediated nonphotolytic cleavage of the
rhenium–rhenium bond of [{NC5H4NꢀNC6H4(R)}(CO)3Re0]2.
Synthesis of the monomeric species ReI{NC5H4NꢀNC6H4(R)}(CO)3Cl,
crystal structure, spectroscopic and electron-transfer properties
R. Samanta a, P. Munshi a, B.K. Santra a, N.K. Lokanath b, M.A. Sridhar b, J.S. Prasad b,
G.K. Lahiri a,*
a Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai-400076, India
b Department of Physics, Uni6ersity of Mysore, Mysore-670006, India
Received 19 November 1999
Abstract
The reaction of Re2(CO)10 with the 2-arylazopyridine ligand, L [LꢀNC5H4–NꢀN–C6H4(R), RꢀH, o-Me/Cl, m-Me/Cl] in dry
THF under a dinitrogen atmosphere afforded a metal–metal bonded product of the type (L)(CO)3Re0–Re0 (CO)3(L) 1. On a
silica gel column and in the presence of chlorinated solvents (CHCl3 or CH2Cl2) the complex 1 transformed into a mononuclear
complex of composition ReI(L)(CO)3Cl 2, where the cleavage of Re–Re bond of 1 and the oxidative addition of chlorine to the
rhenium center have taken place consecutively. The molecular structure of the complex 2a (RꢀH) has been determined by single
crystal X-ray diffraction. The crystal lattice consists of two crystallographically independent Re(L1)(CO)3Cl molecules which are
non-superimposable mirror images; d and l enantiomers, exist in a 1:1 ratio. The complexes 2 display irreversible ReI“ReII
oxidation processes near 1.5 V versus SCE and two quasi-reversible azo (NꢀN) reductions in the ranges −0.28 to −0.48 V and
−0.83 to −1.06 V versus SCE. The complexes of the type 1 exhibit only four successive azo reductions in the range 0 to −2
V versus SCE. Both the complexes (1 and 2) display dp(Re)“p*L metal-to-ligand charge-transfer transitions near 500 nm and
intraligand n–p* and p–p* transitions near 400 and 300 nm, respectively. The complexes 2 are susceptible to a spontaneous
chloride exchange reaction. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: 2-Arylazopyridine; Rhenium; Cleavage of rhenium–rhenium bond; Electron transfer
thermally initiated substitution of Re2(CO)10. Under
strictly thermal conditions substitution proceeds by a
1. Introduction
dissociative pathway with CO loss, without any Re–Re
bond breaking [2].
The present work originates from our interest in
studying the reactivity pattern of Re2(CO)10 in the
versatile reactive nature in solution [1]. Although
There has been continuous research activity on the
reaction studies of dirhenium decacarbonyl, Re2(CO)10.
This is primarily due to its photo- and thermal-induced
presence of the very strongly p-acidic unsymmetrical
azoimine based arylazopyridine ligand [NC5H4Nꢀ
NC6H4(R), L]. Under thermal conditions the interac-
tion of Re2(CO)10 with the ligand L initially follows
a substitution process with the loss of two CO
molecules from each rhenium center affording a new
photo-initiated reactions of Re2(CO)10 proceed with a
substantial degree of homolytic Re–Re bond scission,
metal–metal bond homolysis is not the primary step for
* Corresponding author. Fax: +91-22-578-3480.
E-mail address: lahiri@ether.chem.iitb.ernet.in (G.K. Lahiri)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00016-9

312
R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
class of stable metal–metal bonded product of the type
[(L)(CO)3Re0–Re0(CO)3(L)], 1. However, on a silica gel
column and in the presence of chlorinated solvent,
CHCl3 or CH2Cl2 the complex 1 unexpectedly trans-
eluted in the pure state by acetonitrile. The repetition of
similar chromatographic operation on the unaltered
complexes 1 (recovered from the previous column) re-
sult in a second batch of complexes 2 in the presence of
CHCl3 as eluent. Three or four repetitions of similar
chromatographic cycles are required for the complete
transformation of 1 to 2.
Thus during the chromatographic process the metal–
metal bond of complexes 1 have been cleaved and the
oxidative chlorine addition (via the abstraction of chlo-
rine atom from the chlorinated solvent, CHCl3 or
CH2Cl2) to the newly formed transient monomeric unit
of 1 takes place.
Under dark conditions as well as in the presence of
strong UV light the behavior of the complexes 1 with
respect to the chromatographic operation does not alter
at all as compared to normal laboratory light. Since the
conversion of 1 to 2 does not occur alone in the
chlorinated solvents (CHCl3 or CH2Cl2) even under
heating and the presence of laboratory light does not
alter the rate of the conversion process from 1 to 2, it
can therefore be inferred that the silica gel support here
plays an active role to cleave the Re–Re bond in 1
which leads eventually to the formation of the final
product 2 in the presence of chlorinated solvent.
The reaction of 1 with the silica gel slurry in CHCl3
or CH2Cl2 can also generate the complexes 2 but the
rate of conversion (1“2) is found to be much slower
compared to the silica gel column.
Although the photolytic cleavage of the Re–Re bond
of Re2(CO)10 and its derivatives is a well established
process [2], the nonphotolytic silica gel mediated cleav-
age of the Re–Re bond of dirhenium complexes have
been observed for the first time.
Microanalytical data (C, H, N) of 1 and 2 (Table 1)
are in good agreement with the calculated values. The
complexes (1 and 2) are nonconducting in acetonitrile
solution and diamagnetic (Re0, t26geg1, the unpaired elec-
tron in the eg level of each Re center gets paired up
through the Re0–Re0 bond formation in 1; ReI in 2, t26g,
S=0).
forms into
a monomeric complex of the type
ReI(L)(CO)3Cl 2. Here in the course of chromatogra-
phy the cleavage of the rhenium–rhenium bond of 1
and the oxidative addition of a chlorine atom (gener-
ated by the cleavage of C–Cl bond of the chlorinated
solvent) to the rhenium center have taken place consec-
utively on the silica gel support. To the best of our
knowledge this work demonstrates the first example of
a silica gel mediated rhenium–rhenium bond cleavage
reaction followed by oxidative chlorine addition at the
rhenium center.
Herein we report the synthetic account of the forma-
tion of complexes 1 and 2, their spectroscopic proper-
ties, metal and ligand based electron-transfer properties
and single crystal X-ray structure of a representative
complex 2a.
2. Results and discussion
2.1. Synthesis and characterization of the complexes
The five substituted arylazopyridine ligands (L) used
for the present study are abbreviated as L1–L5. The
reaction of Re2(CO)10 with the ligand L in freshly
distilled dry tetrahydrofuran (THF) solvent under N2
atmosphere results in a red–brown solution, from
which the dark colored metal–metal bonded dimeric
solid product of composition {(CO)3 LRe0–
Re0L(CO)3} 1 is isolated after removal of the solvent
under reduced pressure (Scheme 1).
Although the complexes 1 are stable both in the solid
and solution states, on the silica gel column and in the
presence of chlorinated solvent, CHCl3 or CH2Cl2 as
eluent the red–brown colored complexes 1 have been
transformed partially into a pink-colored complexes of
composition ReI(L)(CO)3Cl 2 (Scheme 1), leaving be-
hind a dark band containing unchanged starting com-
plexes 1 at the top of the column, which can be easily
2.2. Crystal structure of ReI(NC5H4NꢀNC6H5)(CO)3Cl
(2a)
The crystal structure of 2a consists of two crystallo-
graphically independent ReI(NC5H4NꢀNC6H5)(CO)3Cl
molecules (A and B). A view of the molecules is shown
in Fig. 1 and selected bond parameters are given in
Table 2. Although the molecule possesses a pseudo C2
axis, it is chiral since the mirror images are not super-
imposable. Thus the non-superimposable mirror images
(A and B) are d and l enantiomers of the same optically
active complex 2a. Since in the crystal lattice the d and
l enantiomers exist in a 1:1 ratio and since the molecule
2a does not show any optical rotation in solution,
Scheme 1.

R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
313
Table 1
Microanalytical, selected infrared and electronic spectral data
Compound
Elemental analysis (%)a
IRb, wmax (cm−1
)
Electronic spectral datac
C
H
N
CO
NꢀN
Re–Cl
–
umax (nm) (m=dm3 mol−1 cm−1
)
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
3
37.01
(37.07)
2.03
(1.99)
9.35
(9.27)
2027
1924
1884
1376
507(4079), 372(6996), 300(12140)
510(3695), 365(6500), 295(11100)
514(4395), 378(7200), 305(12920)
507(3342), 382(6375), 310(11772)
517(4412), 381(7050), 298(12830)
38.62
(38.52)
2.41
(2.35)
(8.90)
(8.98)
2041
1935
1920
1372
1375
1370
1370
–
–
–
–
34.53
(34.45)
1.67
(1.64)
8.53
(8.61)
2034
1936
1927
38.58
(38.52)
2.29
(2.35)
8.91
(8.98)
2034
1930
1905
34.54
(34.45)
1.60
(1.64)
8.69
(8.61)
2028
1925
1892
34.29
(34.37)
1.80
(1.84)
8.67
(8.59)
2022
1920
1881
1380
1387
1384
1374
1374
1382
325
322
325
327
328
–
523(4660), 395(8940)d, 337(11340)
517(5090), 385(4700)d, 304(8270)
529(6670), 379(3620)d, 304(10810)
517(4570), 408(8320)d, 381(10900)
533(3730), 374(8720)d, 318(17600)
447(6590), 404(6700), 336(7520)
35.71
(35.80)
2.23
(2.18)
8.44
(8.35)
2038
1940
1910
32.22
(32.11)
1.50
(1.53)
8.10
(8.03)
2038
1942
1920
35.69
(35.80)
2.15
(2.18)
8.27
(8.35)
2030
1927
1890
32.24
(32.11)
1.48
(1.53)
8.14
(8.03)
2032
1933
1886
2038
1938
1920
33.64
(33.57)
2.27
(2.30)
9.14
(9.21)
a Calculated values are in parentheses.
b In KBr disk.
c In dichloromethane solution.
d Shoulder.
bered chelate ring is planar. The acute chelate bite
angles 72.4 (12)° and 71.1(11)° for A and B molecules,
respectively are the major distorting factors in the
structures. The dimensions of the chelate ring compare
well with those found in the ruthenium [3], osmium [4]
and other type of rhenium [5] chelates of L.
therefore the complex 2a also represents the racemic
mixture in the crystal. The bond distances and angles in
the two molecules A and B are found to be slightly
different (Table 2), which is presumably due to the
effect of crystal packing forces.
The Re(N)2(CO)3Cl coordination sphere is distorted
from regular octahedral geometry as can be seen from
the angles subtended at the metal. The coordinated
carbonyl groups are disposed facially. The five-mem-
Reported ReI–Cl distances lie in the range 2.49–2.52
,
A [6]. The average Re–Cl length in the molecule 2a
,
[2.47(8) A] is slightly shorter than the reported dis-

314
R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
,
tances. The average Re–CO distance [1.87(4) A] in 2a is
spectra of complexes 1 and 2 reveal the presence of
common (L)(CO)3Re unit in both the complexes.
The Raman spectra of complexes 1 exhibit a band
near 130 cm−1 corresponding to the Re–Re bond [9].
The complexes 1 and 2 display two major intense
bands near 500 and 300 nm (Table 1). The higher
energy band near 300 nm is associated with a shoulder
at the lower energy part near 400 nm. The band near
500 nm is assigned to dp(Re)“L(p*) (where L(p*) is
believed to be dominated primarily by the LUMO of
the azoimine chromophore [8]) MLCT transition. A
transition of this type is well documented in low-spin
rhenium (I) [10], ruthenium (II) [11], osmium (II) [12],
cobalt(III) [13] complexes of unsaturated ligands.
The higher energy bands near 400 and 300 nm are
presumably due to intra-ligand n–p* and p–p* transi-
tions, respectively [14].
compared well with the reported ReI–CO distances
,
(1.85–1.88 A) [6]. The average Re–N(pyridine), Re–
N(azo) and NꢀN bond lengths in 2a are, respectively
2.14(3), 2.15(5) and 1.25(5) A, which fall within the
,
limit of reported distances [5,7].
2.3. IR,UV-Vis,1H-NMR and FAB mass spectra
The complexes of both the type (1 and 2) exhibit
similar IR spectra except in the metal halide region.
Selected IR frequencies are listed in Table 1 and the
carbonyl stretching frequencies are shown in Fig. 2.
There are three notable features of the IR spectra: (i)
Three distinct CꢁO stretching vibrations in the range
2000–1850 cm−1 in each case. If the local symmetry of
the complexes 2 is C36, two bands are expected with the
irreducible representations A1 and E. In the present
cases, however, the doubly degenerated E vibrations are
somewhat split, because the symmetry is lowered by the
presence of unsymmetrical ligand L and Cl. A careful
inspection of the carbonyl stretching frequencies of the
dinuclear complexes 1 (Fig. 2(b)) indicate the presence
of more than three bands possibly due to further reduc-
tion of symmetry in the complexes 1. (ii) The azo
stretching frequency of the coordinated ligand L is
appreciably lowered (ꢀ1380 cm−1) compared to that
in the free L (ꢀ1425 cm−1), evidently due to the dp
Re“p*(L) back-bonding effect [8]. (iii) One sharp
band near 325 cm−1 due to w(Re–Cl) in each case of 2
has been observed, which is absent in the spectra of the
complexes 1 as expected. A direct comparison of the IR
The complexes of both the type 1 and 2 exhibit
1
similar H-NMR spectra corresponding to one ligand L
as the presence of mirror plane through the Re–Re
bond in 1 makes each half of the molecule 1 equivalent.
Spectral data for 2 are listed in Table 3. The pyridine
proton signals appear in an isolated region except for
the 5-H proton, and are clearly identified. The 8-H and
12-H protons appear as coincident doublets in complex
2a whereas 8-H signal appears as a separate singlet for
the meta substituted complexes 2d and 2e. Further, the
8-H singlet is significantly shifted to higher and lower
fields in the complexes 2d and 2e respectively compared
to 2a (Table 3) based on the electron releasing and
withdrawing properties of the methyl and chloride sub-
stituents, respectively. Ortho substituted complexes (2b
Fig. 1. Perspective view for two independent [ReI(NC5H4NꢀNC6H5)(CO)3Cl] molecules 2a.

R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
315
Table 2
metal as well as the ligand (L) centers and display the
same three redox processes in the potential range92
V versus SCE at 298 K, whereas the complexes 1
exhibit only the ligand reductions within the potential
,
Selected bond distances (A) and angles (°) in 2a (with estimated
standard deviations in parentheses)
Distances
Re(1)–C(15A)
Re(1)–C(17A)
Re(1)–C(19A)
Re(1)–N(7A)
Re(1)–N(14A)
Re(1)–Cl(1)
1.85(3) Re(2)–C(15B)
1.88(4) Re(2)–C(17B)
1.82(7) Re(2)–C(19B)
2.12(5) Re(2)–N(7B)
2.15(3) Re(2)–N(14B)
2.452(8) Re(2)–Cl(2)
1.28(5) N(7B)–N(8B)
1.90(3)
1.88(3)
1.96(4)
2.19(5)
2.13(3)
2.499(7)
1.19(5)
range92 V versus SCE. Representative voltam-
mograms are shown in Fig. 3 and the reduction po-
tentials are listed in Table 4. Complexes 2 show one
irreversible oxidation process, Epa near 1.5 V versus
SCE. The one-electron nature of this oxidation pro-
cess is confirmed by direct comparison of its current
height (ipa) with that of the experimentally established
one-electron reversible reduction process observed at
the negative side of the SCE (see later) (Fig. 3). This
oxidation process is assigned to the rhenium(I) to
rhenium(II) oxidation, Eq. (1):
N(7A)–N(8A)
Angles
C(15A)–Re(1)–C(17A) 85.1(16) C(17B)–Re(2)–C(15B) 93.3(11)
C(15A)–Re(1)–C(19A) 91(2)
C(17A)–Re(1)–C(19A) 81(2)
C(15A)–Re(1)–N(7A)
C(17A)–Re(1)–N(7A) 104.9(14) C(17B)–Re(2)–N(7B) 101.0(15)
C(19A)–Re(1)–N(7A) 171.3(19) C(19B)–Re(2)–N(7B) 168.2(15)
C(15A)–Re(1)–N(14A) 93.9(16) C(15B)–Re(2)–N(14B) 94.5(10)
C(17A)–Re(1)–N(14A) 177.1(17) C(17B)–Re(2)–N(14B) 169.1(14)
C(19B)–Re(2)–C(15B) 87.4(19)
C(17B)–Re(2)–C(19B) 90.8(17)
95.8(18) C(15B)–Re(2)–N(7B) 92.4(17)
[ReI(L)(CO)3Cl] −e−
“
[ReII(L)(CO)3Cl]+
(1)
2
2+
C(19A)–Re(1)–N(14A) 102(2)
N(7A)–Re(1)–N(14A) 72.4(12) N(14B)–Re(2)–N(7B)
C(19B)–Re(2)–N(14B) 97.2(13)
Since in the complexes 2 except for chloride all the
other coordinated ligands (CO and L) are well-known
strong p-acidic centers, therefore, the presence of
strong p-acidic ligands around the metal ion in the
coordination sphere stabilizes the ReI state (t26g
configuration) to a great extent and this is reflected in
the high and irreversible ReI“ReII oxidation poten-
tial (ca. 1.5 V) [15]. The formal potential of Re(I)“
Re(II) oxidation process (Eq. (1)) varies depending on
the nature and the position of the substituents present
in the ligand framework and it follows the order:
2dB2bB2aB2cB2e.
71.1(11)
176.0(17)
89.0(9)
95.9(13)
83.9(7)
C(15A)–Re(1)–Cl(1)
C(17A)–Re(1)–Cl(1)
C(19A)–Re(1)–Cl(1)
N(7A)–Re(1)–Cl(1)
N(14A)–Re(1)–Cl(1)
178.9(16) C(15B)–Re(2)–Cl(2)
95.9(10) C(17B)–Re(2)–Cl(2)
89.9(16) C(19B)–Re(2)–Cl(2)
83.5(7) N(7B)–Re(2)–Cl(2)
85.1(9) N(14B)–Re(2)–Cl(2)
82.9(6)
and 2c) also exhibit the expected changes in the dis-
tribution of the phenyl ring proton signals with re-
spect to the unsubstituted complex (2a). The chemical
shift of the methyl group follows the order: o-Me
(2b), 2.53 ppm\m-Me (2d), 2.37 ppm, indicating the
effect of nearby electron withdrawing azo group on
the ortho methyl group is more pronounced compared
to the meta methyl group.
FAB mass spectra of the two representative com-
plexes 1a and 2a were recorded. The molecular ion
peaks are observed at m/z, 906.6 and 489 for 1a and
2a, respectively which are in good agreement with the
calculated mass of the complexes 1a (formula weight,
906.4) and 2a (formula weight, 488.7), respectively.
All our attempts to grow suitable single crystals for
X-ray characterization of the complexes of type 1
have failed altogether. However, elemental analysis,
conductivity, magnetic moment, FAB mass and spec-
tral data collectively are in agreement with the com-
position and structure of 1.
2.4. Electron-transfer properties
2.4.1. Metal redox
Electron-transfer properties of the complexes 1 and
2 have been studied by cyclic voltammetry in acetoni-
trile solvent using a platinum working electrode. The
complexes 2 are electroactive with respect to the
Fig. 2. Infrared spectra of (a) [ReI{NC5H4NꢀNC6H4(o-Me)}(CO)3Cl]
2b; (b) [{NC5H4NꢀNC6H4(o-Me)}(CO)3Re0]2 1b; (c) [ReI{NC5H4Nꢀ
NC6H4(o-Me)}(CO)3(CH3CN)]ClO4 3 in KBr disks.

316
R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
Table 3
1H-NMR spectral data in CDCl3
l (J)a
Compound
H(6)
H(5)
H(4)
H(3)
H(8)
H(9)
H(10)
H(11)
H(12)
2a
2b
2c
2d
2e
9.04
(5.5)b
9.04
7.68
(6.2)c
7.71
8.26
(7.4)c
8.27
8.61
(7.9)b
8.59
7.99
(7.2)b
Me
2.53
–
7.60
(7.1)c
7.4d
–
7.59
(6.8)b
Me
7.64
(6.2)c
7.4d
7.60
(7.1)c
7.4d
7.99
(7.2)b
7.8
(5.4)b
9.04
(6.7)c
7.74
(7.8)c
8.28
(7.6)b
8.67
–
–
(7.4)b
7.8
7.45
7.50
(7.3)c
7.67
(7.0)c
7.45
(6.8)c
(5.3)b
9.04
(6.0)c
7.69
(7.3)c
8.26
(7.6)b
8.6
(7.4)c
7.48
(7.6)b
7.82
7.46e
8.04e
(5.3)b
9.05
(6.6)c
7.71
(8.0)c
8.28
(7.6)b
8.64
2.37
–
(7.7)b
7.98
(7.5)b
7.93
(5.5)b
(6.9)c
(8.3)c
(7.8)b
(7.51)b
(7.6)b
a Tetramethylsilane is the internal standard.
b Doublet.
c Triplet.
d Complex multiplet pattern centered at l=7.4.
e Singlet.
Under identical experimental conditions the com-
plexes of type 1 on the other hand do not show any
metal oxidation within +2 V versus SCE potential
limit which indicates that under the metal–metal
bonded dimeric environment as in 1 the rhenium ion
gets preferentially stabilized in the zero oxidation state,
which is certainly due to the perfect synchronization of
the metal-oxidation state and the back-bonding effect
under an environment of having only strong p-acidic
ligands (CO and L) around the metal ion.
of solute) were unstable. However, we have succeeded
in recording the EPR spectrum for the reduced complex
2b− by quick freezing the electrolyzed solution (liquid
nitrogen, 77 K). The reduced complex 2b− displays an
EPR signal with center field g value 2.0029. The hy-
perfine splittings due to nitrogen nuclear spin (I=1)
have been detected on both sides of the central intense
line. This suggests that the unpaired electron in the
reduced product 2− is predominantly localized in a
ligand character orbital [17]. Coulometric reduction of
the second couple (Fig. 3) generated reduced species
2.4.2. Ligand reduction
On the negative side of SCE two successive reversible
reductions (the anodic and cathodic peak heights are
equal and vary as the square root of the scan rate) are
observed for all the complexes of type 2 (Fig. 3; Table
4). The observed two successive one-electron reductions
for each of the complexes 2 are believed to be the
reductions of azo group of the coordinated ligand L,
Eqs. (2) and (3) [16].
I
[ReI(CO)3Cl(L)]+e− X [Re (CO)3Cl(L −)]−
(2)
’
I
I
[Re (CO)3Cl(L −)]− +e− X [Re (CO)3Cl(L −)]2−
’
’’
(3)
The one-electron nature of the first reduction couple,
Eq. (2) is confirmed by constant potential coulometry
in acetonitrile solvent at 263 K. The coulometrically
produced reduced brown solutions (2−) (obtained by
reducing the complexes 2 at a potential 100 mV nega-
tive to the corresponding Epc of the first reduction
couple (Fig. 3); n values: 2a, 1.04; 2b, 1.06; 2c, 0.97;
2d,1.04; 2e, 1.07; n=Q/Q% where Q% is the calculated
Coulomb count for a one-electron transfer and Q is
that found after exhaustive electrolysis of 10−2 mmol
Fig. 3. Cyclic voltammograms (scan rate 50 mV s−1) of a 10−3 mol
dm−3 solution of [{NC5H4NꢀNC6H4(o-Me)}(CO)3Re0]2 1b (–·–·–),
[ReI{NC5H4NꢀNC6H4(o-Me)}(CO)3Cl]
2b
(—)
and
[ReI{NC5H4NꢀNC6H4(o-Me)}(CO)3 (CH3CN)]ClO4 3 (-------) in ace-
tonitrile at 298 K.

R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
317
Table 4
The diamagnetic complex 3 exhibits 1:1 conductivity
in acetonitrile solution (\M, 145 V−1 cm2 mol−1). The
IR spectrum of 3 shows three CO stretching frequencies
(Table 1) similar to the parent complex 2b (Fig. 2(c))
which supports the stereoretentive substitution reaction.
A strong and broad vibration at 1097 cm−1 and a
strong and sharp band at 624 cm−1 are observed due to
the presence of ionic perchlorate. It exhibits MLCT
transitions at 447 and 404 nm and a ligand based
transition at 336 nm (Table 1).The complex 3 displays
an irreversible ReI“ReII oxidation process (anodic
peak, Epa) at 1.78 V versus SCE (Eq. (5); Fig. 3).
Electrochemical data in acetonitrile solvent at 298 Ka
Compound Epa (V)b E° (V) (DEp (mV))
298
ReI–ReII Ligand reductions
1a
1b
1c
1d
1e
–
–
–
–
–
–0.45 (90), −0.95(110), −1.21(100),
−1.65(120)
−0.48(100), −1.01(110), −1.27(120),
−1.73(120)
−0.41(90), −0.89(110), −1.19(120),
−1.62(110)
−0.50(90), −1.04(100), −1.31(130),
−1.77(100)
−0.38(80), −0.84(120), −1.15(120),
−1.59(120)
[ReI(L2)(CO)3(CH3CN)]+ −e−
“
[ReII(L2)(CO)3(CH3CN)2+
]
3
3+
2a
2b
2c
2d
2e
3
1.54
1.52
1.55
1.51
1.58
1.78
−0.39(60), −0.85(120)
−0.48(70), −1.06(120)
−0.37(70), −0.84(110)
−0.42(70), −0.90(110)
−0.28(70), −0.83(120)
−0.29(60), −0.91(130)
(5)
Under identical experimental conditions the Epa of the
ReI“ReII process of the corresponding chloro complex
Re(L2)(CO)3Cl, 2b appears at 1.52 V. The observed
0.26 V positive shift of the rhenium(I)–rhenium(II)
oxidation potential on moving from complex 2b to 3
reveals that the acetonitrile complex has superior redox
stability for rhenium(I) as compared to the chloro-com-
plex. It exhibits two quasi-reversible reductions at
−0.29 and −0.91 V versus SCE (Fig. 3) due to
successive two reductions of the azo group of coordi-
nated ligand L2.
a Conditions: solvent, acetonitrile; supporting electrolyte,
NBu4ClO4 (0.1 M); reference electrode, SCE; solute concentration,
10−3M; working electrode, platinum; scan rate, 100 mV s−1; E°
=
298
0.5(Epc+Epa) where Epc and Epa are the cathodic and anodic peak
potentials, respectively.
b Epa values are considered due to irreversible nature of the voltam-
mograms.
unstable even at 258 K and this has precluded its
further characterization. The one-electron nature of the
second reduction process (Eq. (3)) is confirmed from
the current height considerations. The formal potentials
of the couples (Eqs. (2) and (3)) vary systematically
depending on the nature of the substituents present on
the ligand (Table 4).
The acetonitrile group of complex 3 can be replaced
readily further by other monodentate ligands keeping
the overall geometry of the complex intact.
The mechanism along with the general applicability
of the silica gel mediated metal–metal bond cleavage
reaction in other systems are under scrutiny.
The complexes 1 display four successive quasi-re-
versible ligand reductions (Fig. 3; Table 4) due to the
presence of two ligands (L) in the complex.
3. Conclusion
Although the metal–metal bond of dirhenium car-
bonyl species, Re2(CO)10 and its derivatives are suscep-
tible to photolytic Re–Re bond cleavage reaction, the
interaction of arylazopyridine ligand (L) with the
Re2(CO)10 results in stable metal–metal bonded com-
plexes of the type (CO)3LRe0–Re0L(CO)3 1 via the
partial replacement of CO group from each of the
rhenium centers of Re2(CO)10. However, on the silica
gel column and in presence of chlorinated solvent
(CHCl3 or CH2Cl2) the nonphotolytic cleavage of the
Re–Re bond of 1 and the oxidative chlorine addition
to the rhenium center have taken place consecutively,
2.5. Chloride exchange reaction
In acetonitrile solvent the complexes 2 are susceptible
to a chloride exchange in the presence of AgClO4 at
room temperature which results in an orange-colored
acetonitrile complex of the type [ReI(L)(CO)3(CH3-
CN)]ClO4 3, Eq. (4):
AgClO
4
ReI(L)(CO)3Cl ꢂꢂꢂꢂꢂꢂꢂꢃ [ReI(L)(CO)3(CH3CN)] ClO4
CH CN, stirring
3
+AgCl
(4)
Here the exchange of a monoanionic chloride ligand by
the neutral acetonitrile group takes place without any
change in metal oxidation state or spin state. Although
all the complexes (2a–2e) react similarly with AgClO4
in presence of acetonitrile solvent and affording the
species 3 readily, only the complex 2b has been isolated
in pure solid state.
affording
a
monomeric complex of the type
ReI(L)(CO)3Cl 2. The effect of the presence of strong
p-acidic ligands (L and CO) in the coordination envi-
ronment of 2 have been reflected in the high metal
oxidation potentials. The chloride group of 2 is suscep-
tible to facile chloride exchange reaction.

318
R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
4. Experimental
4.1.1. Hexacarbonyl bis-(2-phenylazopyridine)-
dirhenium(0),{(L1)(CO)3Re0}2 (1a) and
Re2(CO)10 was obtained from Aldrich, USA. The
ligands L1–L5 were prepared by condensing 2-aminopy-
ridine with the appropriate nitrosobenzene according to
the reported procedure [14]. For spectroscopic and
electrochemical studies HPLC grade solvents were used.
Silica gel (60–120 mesh) used for chromatography was
of BDH quality. Commercial tetrabutyl ammonium
bromide was converted into pure tetrabutyl ammonium
perchlorate by following an available procedure [18].
UV–visible spectra were recorded by using a Shi-
madzu-160 spectrophotometer. FT-IR spectra were
taken on a Nicolet spectrophotometer with samples
prepared as KBr pellets. Laser Raman spectra were
taken in CHCl3 using an Ar-ion laser system, Coherent,
USA, attached to a Jobin Yvon Romonor HG2S spec-
trometer, France. Solution electrical conductivity was
checked using a Systronic 305 conductivity bridge.
Magnetic susceptibility was checked with a PAR vibrat-
ing sample magnetometer. NMR spectra were obtained
with a 300 MHz Varian Fourier transform spectrome-
ter. Cyclic voltammetric measurements were carried out
using a PAR model 362 scanning-potentiostat electro-
chemistry system. Platinum wire working and auxiliary
electrodes and an aqueous saturated calomel reference
electrode (SCE) were used in a three electrode configu-
ration. The supporting electrolyte was [NBu4]ClO4 and
the solute concentration was ꢀ10−3 M. The half-wave
tricarbonyl(chloro)(2-phenylazopyridine)rhenium(I),
ReI(L1)(CO)3(Cl) (2a)
Re2(CO)10 (100 mg, 0.153 mmol) was dissolved in 15
ml of freshly distilled tetrahydrofuran (THF) under N2
atmosphere. To this solution 2-phenylazopyridine lig-
and L1 (75 mg, 0.41 mmol) in 5 ml of freshly distilled
dry THF was added. The resulting mixture was heated
to reflux for 3 h under a dinitrogen atmosphere. The
initial red color of L1 gradually changed to a red–
brown color. The solvent was then removed under
reduced pressure. The solid red–brown product thus
obtained was washed thoroughly with hexane to re-
move the excess ligand and finally recrystallization
from 1:1 acetonitrile–benzene yielded the complex 1a.
Yield 73%.
The complex 1a dissolved in minimum volume of
CHCl3 and was subjected to chromatography on a
silica gel (60–120 mesh) column. On elution with chlo-
roform a deep pink colored band was separated leaving
behind a dark band corresponding to the starting com-
plex 1a at the top of the column. The pink colored
band was collected and evaporation of the solvent
under reduced pressure afforded a crystalline solid
product 2a. The dark brown band was then eluted by
acetonitrile. The repetition of similar chromatographic
operation on the unaltered complex 1a (recovered from
the previous column) resulted in a second batch of
complex 2a in presence of CHCl3 as eluent. Four
similar chromatographic cycles were required for the
complete transformation of 1a to 2a. Finally the
product was recrystallized from dichloromethane-hex-
ane (1:4). Yield 67%.
potential E° was set equal to 0.5 (Epa+Epc), where
298
Epa and Epc are anodic and cathodic cyclic voltammet-
ric peak potentials, respectively. Coulometric experi-
ments were done with
a
PAR model 370-4
electrochemistry apparatus incorporating a 179 digital
coulometer. A platinum wire-gauze electrode was used.
All experiments were carried out under a dinitrogen
atmosphere and were uncorrected for junction poten-
tials. The FAB mass spectra at 298 K were recorded on
a JEOL SX 102/DA-6000 mass spectrometer. The ele-
mental analyses were carried out with Carlo Erba
(Italy) elemental analyzer. Optical rotation was checked
by using JASCO DIP-370 digital polarimeter. The EPR
measurements were made with a Varian model 109C
E-line X-band spectrometer fitted with a quartz Dewar
for measurements at 77 K (liquid nitrogen). The spectra
were calibrated by using tetracyanoethylene (TCNE)
(g=2.0037).
4.1.2. Tricarbonyl (acetonitrile){2-(ortho tolylazo)py-
ridine} rhenium(I) perchlorate [ReI(L2)(CO)3(CH3CN)]-
ClO4 (3)
Re(L2)(CO)3Cl, 2b (100 mg, 0.199 mmol) was dis-
solved in 10 ml of acetonitrile. To this solution AgClO4
(95 mg, 0.46 mmol) was added with stirring. The initial
pink color of 2b immediately changed to orange. The
stirring was continued for 2 h. The solution was then
filtered to remove AgCl precipitate and the solvent was
removed from the filtrate under reduced pressure. The
solid mass was then washed several times with ice-cold
water to remove excess AgClO4. The solid product was
dried in vacuo over P4O10. Yield: quantitative.
4.1. Synthesis
4.2. Crystallography
The syntheses of the complexes (1 and 2) were carried
out using the following general procedures. Yields
varied in the ranges 70–75%. and 65–70% for 1 and 2,
respectively. Details are given for one representative
case.
Single crystals of complex 2a were grown by slow
diffusion of a dichloromethane solution of it in hexane.
A dark cubic crystal of dimension 0.1×0.1×0.1 mm3
was selected for X-ray data collection. All measure-
ments were made on a Rigaku AFC7S diffractometer

R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
319
Table 6
with graphite monochromated Cu–Ka radiation. The
data were collected at a temperature of 293K using the
ꢀ–2q scan technique. Cell constants and an orientation
matrix for the data collected were determined from a
least-squares refinement in the range 29.50B2qB
37.01° which corresponded to a primitive monoclinic
Atomic coordinates (×104) and equivalent isotropic displacement
2
3
a
,
parameters (A ×10 ) for 2a
x
y
z
Ueq
Re(1)
Cl(1)
0.48688(11)
0.4109(6)
0.733(2)
0.823(2)
0.828(2)
0.726(2)
0.621(3)
0.620(3)
0.5270(19)
0.446(3)
0.351(3)
0.44413(8) 0.73099(10)
0.4155(15) 0.8796(6)
0.0489(8)
0.063(3)
0.053(9)
0.061(9)
0.069(9)
0.062(9)
0.063(8)
0.044(10)
0.046(7)
0.062(8)
0.052(8)
0.048(7)
0.038(7)
0.039(7)
0.048(7)
0.070(8)
0.084(12)
0.090(11)
0.058(9)
0.090(9)
0.100(18)
0.093(11)
0.0498(8)
0.069(3)
0.060(8)
0.060(8)
0.061(8)
0.067(9)
0.069(10)
0.040(7)
0.051(8)
0.049(8)
0.054(9)
0.067(10)
0.076(12)
0.098(12)
0.085(11)
0.035(6)
0.049(8)
0.063(6)
0.056(9)
0.087(9)
0.050(11)
0.114(15)
,
,
cell with dimensions a=12.410(4) A, b=9.080(2) A,
C(1A)
C(2A)
C(3A)
C(4A)
C(5A)
C(6A)
N(7A)
N(8A)
C(9A)
C(10A) 0.275(2)
C(11A) 0.171(3)
C(12A) 0.157(2)
C(13A) 0.242(2)
N(14A) 0.346(3)
C(15A) 0.543(3)
O(16A) 0.5781(18)
C(17A) 0.615(3)
O(18A) 0.691(2)
C(19A) 0.433(6)
O(20A) 0.387(3)
0.231(4)
0.174(6)
0.052(4)
0.822(2)
0.873(2)
0.921(2)
0.926(2)
0.863(2)
0.817(3)
0.7550(17)
0.721(2)
0.672(2)
0.628(2)
0.570(3)
0.570(2)
0.610(2)
0.658(2)
0.618(2)
0.5470(17)
0.794(3)
0.847(2)
0.716(4)
0.670(3)
3
,
,
c=13.804(6) A, i=102.60(3)°, V=1518.0(9) A , Z=
4, zcald=2.139 g cm−3, F000=920, space group P21.
Significant crystal data and data collection parameters
are listed in Table 5.
−0.030(5)
0.027(5)
0.143(4)
0.218(5)
0.131(5)
0.173(6)
0.086(4)
0.137(3)
0.286(4)
0.377(4)
0.318(5)
0.463(6)
0.477(4)
0.546(3)
0.614(5)
0.631(7)
0.756(4)
An empirical absorption correction based on az-
imuthal scans [19] of several reflections was applied
which resulted in transmission factors ranging from
0.106 to 1.00. A total of 1776 data were collected in the
range of 3.2852q549.97° of which 1681 were unique
(Rint=0.05). The structure was solved by direct method
(SHELX-97) [20] and expanded using Fourier techniques.
The function minimized throughout refinement was
ꢀ w(ꢁFoꢁ−ꢁFcꢁ)2 with w=1/|2(Fo). Final refinement
wascarried out with anisotropic thermal parameter for
all non-hydrogen atoms. Hydrogen atoms were in-
cluded at the chemical positions and were not refined.
The final cycle of full-matrix refinement (SHELX-97) was
based on 1681 observed reflections (I\2|(I)) and 398
variable parameters. The refinement converged to R1=
0.083, wR2=0.2117 and goodness of fit=2.176.
Atomic coordinates and isotropic thermal parameters
are tabulated in Table 6.
Re(2)
Cl(2)
0.98692(10)
0.9090(6)
1.132(3)
1.221(2)
1.313(3)
1.318(3)
1.216(3)
1.132(3)
1.025(2)
0.964(3)
0.855(3)
0.765(3)
0.674(4)
0.661(4)
0.754(3)
0.18805(8) 0.23042(10)
0.2130(19) 0.3819(5)
C(1B)
C(2B)
C(3B)
C(4B)
C(5B)
C(6B)
N(7B)
N(8B)
C(9B)
C(10B)
C(11B)
C(12B)
C(13B)
0.611(4)
0.665(5)
0.602(5)
0.465(6)
0.408(5)
0.470(4)
0.423(5)
0.523(4)
0.463(6)
0.567(5)
0.506(6)
0.368(7)
0.269(5)
0.314(3)
0.184(6)
0.187(4)
0.102(5)
0.040(4)
−0.007(5)
−0.117(4)
0.374(2)
0.426(2)
0.4281(19)
0.368(3)
0.317(3)
0.315(2)
0.2545(19)
0.228(4)
0.166(3)
0.133(3)
0.093(3)
0.065(4)
0.112(3)
0.1641(19)
0.1136(18)
0.0411(14)
0.309(2)
0.349(2)
0.192(4)
0.197(4)
5. Supplementary material
Crystallographic data for the structural analysis are
available from the authors.
N(14B) 0.8448(19)
C(15B)
O(16B)
C(17B)
O(18B)
C(19B)
O(20B)
1.0450(18)
1.0770(14)
1.114(2)
1.190(2)
0.924(3)
0.893(4)
Table 5
Crystallographic data for 2a
a Ueq is defined as one third of the trace of the orthogonalized Uij
tensor.
Chemical formula
fw
C28H18Cl2N6O6Re2
977.78
Space group
P21
,
a (A)
12.410(4)
9.080(2)
13.804(6)
102.60(3)
1518(9)
4
293
1.54178
2.139
Acknowledgements
,
b (A)
,
c (A)
i (°)
V (A )
Financial support received from the Department of
Science and Technology, New Delhi, India, is gratefully
acknowledged. Special acknowledgment is made to the
Regional Sophisticated Instrumental Center, RSIC, In-
dian Institute of Technology, Bombay, for providing
NMR, EPR and Laser Raman facilities and RSIC,
Central Drug Research Institute, Lucknow, for provid-
ing the FAB mass facility. The referees comments at the
revision stage were very helpful.
3
,
Z
T (K)
,
u (A)
zcald (g cm−3
)
v (mm−1
R1
)
17.428
0.0831
0.2117
wR2

320
R. Samanta et al. / Journal of Organometallic Chemistry 579 (1999) 311–320
[9] (a) T.G. Spiro, Prog. Inorg. Chem. 11 (1970) 1. (b) J. Lewis,
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