Intramolecular amide → imine reduction in higher valent rhenium complexes stabilized by arylimido coligand: Rate studies and effect of reductants

Article abstract of DOI:10.1016/j.poly.2009.04.040

The present work deals with the imidorhenium(V) complexes of type [ReCl₃(NC₆H₄Y-p)(L)], with Y = OCH₃(1d), CH₃(1c), H(1b), Cl(1a) where L is the pyridylimine Schiff base ligand obtained from facile co

Full text of DOI:10.1016/j.poly.2009.04.040

Polyhedron 28 (2009) 2503–2509
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Intramolecular amide ? imine reduction in higher valent rhenium complexes
stabilized by arylimido coligand: Rate studies and effect of reductants
b
c
c
Jaydip Gangopadhyay a, , Suparna Banerjee , Jiu-Tong Chen , Can-Zhong Lu
*
a Department of Chemistry, St. Paul’s C.M. College, University of Calcutta, 33/1 Raja Rammohan Roy Sarani, Kolkata 700009, India
b Department of Chemistry, Uluberia College, University of Calcutta, Howrah 711315, India
c The State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Suzhou, Fujian 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The present work deals with the imidorhenium(V) complexes of type [ReCl3(NC6H4Y-p)(L)], with
Y = OCH3(1d), CH3(1c), H(1b), Cl(1a) where L is the pyridylimine Schiff base ligand obtained from facile
condensation of pyridine-2-carboxaldehyde and p-phenylenediamine. Structural authentication of one
representative (1d) reveals meridional disposition of three Cl atoms around the metal center in a dis-
torted octahedral ReCl3N3 coordination environment. Re–Npyridine bond lying trans to Re„NC6H4Y-p
motif is lengthened by ꢀ0.2 Å compared to Re–Nimine bond and is attributed to trans influence of imide
nitrogen. The complexes, 1 are reactive towards dilute aqueous nitric acid furnishing amide bound
hexavalent rhenium complexes, 2. Six lines EPR spectra have been recorded for 2 in solution phase
at ambient condition (giso ꢀ 1.945, Aav ꢀ 493 G) and magnetic susceptibility measurement indicates
Received 12 February 2009
Accepted 30 April 2009
Available online 8 May 2009
Keywords:
Amide complex
Outward OAT
Reduction
Imine derivative
Kinetics
strong orbital coupling consistent with one electron paramagnetic nature (ꢀ1.45
l
B). ReVI/ReV responses
for 1 appear at higher potential (ꢀ0.95 V) that those observed for 2 (ꢀ0.12 V). Type 2 complexes are
reduced (low ReVI/ReV reduction potential, +0.15 V) by N2H5þ and NH3OH+ species under mild condition
to regenerate 1. The reduction with N2Hþ is nearly five times faster than NH3OH+. Rate study suggests
5
an associative pathway
(
D
H– = 11.61 kcal molꢁ1
,
D
S– = ꢁ31.22 eu using N2Hþ and
D =
H–
5
10.74 kcal molꢁ1
,
D
S– = ꢁ37.30 eu using NH3OH+) for amide ? imine transformation. No such analo-
gous amide ? imine conversion has yet been achieved in metal free environment, accentuating the
exclusive electronic role of variable metal valence. Further, the oxo complex, 6 does not exhibit intra-
molecular ligand oxidation suggesting decisive electronic role of the coligands (oxo/arylimido) in stabi-
lizing higher metal valence.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
overcome the kinetic sluggishness, middle and late transition met-
als multiply bonded to heteroatom coligands have long been suc-
Higher valent rhenium complexes have spurred interest owing
to active participation as catalysts in various molecular transfor-
mations featuring key changes like functionality conversion [1],
derivatization [2], cyclisation [3,4], oxygen atom transfer [5–7] in
the products. Most of the oxygen atom transfer reactions between
organic molecules characterizing the red-ox functional pairs like
sulphide–sulphoxide; sulphoxide–sulphone; phosphine–phos-
phine oxide do not proceed at reasonable rates at normal temper-
ature even recognized as thermodynamically favourable. To
cessfully employed [8–10] for oxidation of small molecules.
Metal bound oxo ? dioxo, phosphine ? phosphine oxide transfor-
mations and metal catalysed sulphide ? sulphoxide; pyridine
oxide ? pyridine; phosphine ? phosphine oxide conversions
mediated through oxygen atom transfer phenomenon have been
authenticated in rhenium chemistry [11–21]. Moreover, abiologi-
cal molecular oxotransferase incorporating ReVO motif has been
established as useful catalyst to successfully accomplish reductions
of stable inorganic moieties like ClOꢁ4 , NO3ꢁ, NO [22].
Structure-reactivity correlation reveals the prevalence of multi-
ply bonded oxo (O2ꢁ) coligand to the higher valent metal in the
promising catalysts. Our programme focuses on the study of red-
ox related organic functionality pair conversion assisted by imido
(NR2ꢁ) bonded higher valent rhenium complexes. Herein, we re-
port ligand centered oxygen atom abstraction followed by reduc-
tion providing access to route of
* Corresponding author.
E-mail address: gjaydip@rediffmail.com (J. Gangopadhyay).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.04.040

2504
J. Gangopadhyay et al. / Polyhedron 28 (2009) 2503–2509
½ReVIðNC6H4YÞCl3ðC12H10N3OÞꢂ þ 3N2Hþ
5
Y
N
Y
3
! ½ReVðNC6H4YÞCl3ðC12H11N3Þꢂ þ N2 þ 3NHþ þ H2O
ð2Þ
ð3Þ
4
2
4½ReVIðNC6H4YÞCl3ðC12H10N3OÞꢂ þ 6NH3OHþ
! 4½ReVðNC6H4YÞCl3ðC12H11N3Þꢂ þ 3N2O þ 6Hþ þ 7H2O
N
Cl
Cl
Cl
Cl
Cl
Cl
NH2
V
NH2
VI
Re
Re
2.2. Magnetic moment, NMR and EPR spectra
N
N
N
N
H
The type 1 complexes are diamagnetic corresponding to low
spin 5d2 electronic configuration and the effective magnetic mo-
ment data of complexes 2 confirm the presence of one unpaired
O
1
2
electron (5d1, s = 1/2) in
a
pseudo-octahedral environment
although the magnetic moments are significantly lower (2a, 1.42
B; 2b, 1.45 B; 2c, 1.46 B; 2d, 1.47 B) than the spin only value.
l
l
l
l
facile amide ? imine conversion in secondary metal coordination
sphere. The amide chelated imidorhenium(VI) complexes (2) were
prepared by oxidation of imine bound imidorhenium(V) complexes
(1) with dilute HNO3. Complexes of type 2, on reaction with either
The lowering could be attributed to appreciable orbital coupling
[24] as encountered in other hexavalent rhenium species.
The type 1 complexes uniformly exhibit well separated NMR sig-
nals in solution consistent with diamagnetism. However, amide
complexe (2) display paramagnetically shifted relatively broad
peaks lacking well resolved spin-spin structures. The type 2 com-
plexes are EPR active (Fig. 1, Table 1) in fluid solution at room tem-
perature and display well resolved spectra consisting of six isotropic
hyperfine lines due to the interaction of the unpaired electron with
the nuclear spin of both rhenium isotopes (I = 5/2; 185Re, 37.07%;
187Re, 62.93%). The isotopic fine structure has not been resolved be-
cause of the small difference (1%) in the two nuclear quadrupole mo-
ments. There is a systematic increase in the separation between the
adjacent hyperfine lines on going from lower to higher fields due to
the second order effects [25]. The range of this variable hyperfine
N2Hþ or NH3OH+ undergoes outward oxygen atom transfer and re-
5
duced to the imine complex 1. This observation is unprecedented in
the sense of mobilizing a metal bound amide function (most inac-
tive carboxylic acid derivative) to participate in electronic and geo-
metrical changes resulting in the generation of reduced imine form.
The syntheses, representative structure determination of 1, me-
tal redox and spectroscopic properties of complexes are reported.
The kinetic behaviour of outward oxygen atom transfer has been
studied and a viable mechanism consistent with the experimental
findings and kinetics is proposed.
2. Results and discussion
2.1. Synthesis
Facile condensation of pyridine-2-carboxaldehyde and p-phen-
ylenediamine in a 1:1 molar ratio respectively, afforded a neutral
condensate, N-p-aminophenyl-2-pyridinecarboxaldimine L, which
has been employed as a bidentate ligand in the present
NH2
N
N
H
, C H11N3
L
12
work. The violet coloured complexes 1, [Re(NC6H4Y-p)Cl3L]
(Y = Cl(1a), H(1b), CH3(1c), OCH3(1d)) are formed in good yields
upon reacting [Re(NC6H4Y-p)Cl3(PPh3)2] with L in 1:1.5 molar ratio
in benzene under reflux followed by chromatographic work up of
the reaction mixture. In the product, two trans oriented PPh3 groups
are being substituted by the ligand L, Eq. (1).
Fig. 1. EPR diagram of Complex 1d in solution phase at 9.1 GHz and 298 K.
½ReVðNC6H4YÞCl3ðPPh3Þ ꢂ þ L ! ½ReVðNC6H4YÞCl3ðLÞꢂ þ 2PPh3 ð1Þ
2
Table 1
1 reacts smoothly with aqueous nitric acid [23] in acetonitrile solu-
tion at room temperature under stirring condition resulting yellow
picolinamide complexe (2). The feasibility of the reverse reaction
2 ? 1 has been scrutinized with different reducing agents and suc-
cessfully carried out using nitrogenous reagents like hydrazine sul-
phate (Eq. (2)) and hydroxylamine hydrochloride (Eq. (3)) at mild
condition in acetonitrile medium.
EPR data for 2 at 298 K in dichloromethane-benzene (1:1) solution.
Complexes
Center field g
A (G, average)
2a
2b
2c
2d
1.939
1.945
1.948
1.957
488
492
498
496

J. Gangopadhyay et al. / Polyhedron 28 (2009) 2503–2509
2505
spacing spans the domain 360-640 G, consistent with very high oxi-
dation state (+VI) of the metal. Centre field g values and average
hyperfine splitting A for the complexes fall in the ranges 1.939–
1.957 and 488–498 G, respectively. Well resolved sextet EPR spectra
in solution at ambient temperature are relatively rare for Re(VI) ow-
ing to the comparatively large line width followed by significant
overlapping and only broad signals [26–28] are usually observed.
However, Re(VI) complexes of type [ReNX4]ꢁ (X = Cl, Br and NCS)
follow similar spectral characteristics [29] at room temperature.
The spectra of 2 in a frozen (77 K) dichloromethane/toluene
glass become axial and are characterized with individual g-tensors
ꢀ1.92 (g||) and ꢀ1.98 (g\). The observed g inequality is consistent
with a tetragonally distorted octahedral geometry. The hyperfine
components due to the g|| resonance (reference axis, Re„NAr),
especially the outer components have been resolved to a satisfac-
tory extent whereas the components corresponding to g\ absorp-
tions overlap considerably. This observation corroborates that the
rhombic field is not strong enough to split the perpendicular
absorption into individual tensors (gxx and gyy). The separations
between g|| hyperfine lines are unequal and gradually increases to-
wards higher field. It is noteworthy that the separations between
g|| hyperfine structures are larger (350–800 G) than those in isotro-
pic solution (360–640 G) and the inter spacing between g\ compo-
nents are correspondingly smaller.
2.3. UV–visible and IR spectra
The electronic spectra for type 1 and 2 complexes were re-
corded in dichloromethane solution at room temperature. The
ReV chelates (1) display a relatively weak transition near 750 nm
related to 5dxy ? dyz, dxz excitation. Well separated moderately in-
tense second band appeared at about 540 nm, presumably LMCT
(p
Cl ? ReV) in origin. Another highly intense band near 350 nm
could be ascribed to bound aldimine based internal CT transition.
Spectra of 2 are characterized with two peaks in higher energy
(535 and 350 nm) and disappearance of lowest energy peak.
All IR spectra of 1 and 2 exhibit two primary N–H stretching
modes near 3350 and 3280 cmꢁ1 as sharp doublets for the asym-
metric and symmetric vibrations, respectively. The strong bands
observed in the range 1590–1610 cmꢁ1 are assigned to C@N
stretch and red shift of this azomethine absorption by ꢀ25 cmꢁ1
Fig. 2. A perspective view (25% probability thermal ellipsoids) and atom labeling
scheme of 1d. Hydrogen atoms have been omitted for clarity.
Table 2
Selected bond distances (Å) and angles (°) for 1d.
Distances
Re–N(4)
Re–N(1)
Re–Cl(1)
1.714(3)
2.236(3)
2.3826(9)
Re–N(2)
Re–Cl(2)
Re–Cl(3)
2.045(2)
2.3813(10)
2.3856(9)
from free ligand to the complexes suggests weak
p–accepting
property of the coordinated ligand. Two Re–Cl stretches were ob-
served in the region 320–340 cmꢁ1, consistent with mer-ReCl3 mo-
tif for both types of complexes. Picolinamide bound complexes (2)
uniformly exhibit another characteristic band near 1640 cmꢁ1 for
C@O function of the ligand.
Angles
N(4)–Re–N(2)
N(2)–Re–N(1)
N(2)–Re–Cl(2)
N(4)–Re–Cl(1)
N(1)–Re–Cl(1)
N(4)–Re–Cl(3)
N(1)–Re–Cl(3)
Cl(1)–Re–Cl(3)
98.63(11)
74.85(10)
87.34(7)
96.51(8)
90.09(7)
95.56(9)
82.29(7)
88.12(3)
N(4)–Re–N(1)
N(4)–Re–Cl(2)
N(1)–Re–Cl(2)
N(2)–Re–Cl(1)
Cl(2)–Re–Cl(1)
N(2)–Re–Cl(3)
Cl(2)–Re–Cl(3)
173.01(10)
99.72(9)
82.69(7)
164.81(7)
88.83(4)
91.71(7)
164.66(4)
2.4. Structure
The X-ray structure of 1d has been determined. Molecular view,
atom numbering scheme are presented in Fig. 2 and selected bond
parameters are collected in Table 2.
Molecular packing reveals that one Cl atom of a molecule
underwent two weak Clꢃ ꢃ ꢃH contacts (2.829 and 2.898 Å) with
two other proton donor molecules oriented in a nearly parallel
manner to the acceptor. The donor fragments are linked through
Oꢃ ꢃ ꢃH interaction (2.461 Å) to form a trimeric assembly and the
pattern is extended inside the lattice. Apart from the said interac-
tions, the stability of the trimer is partly imparted from p–p stack-
ing force operating between arylimido and aminophenyl groups
(centroidꢃ ꢃ ꢃcentroid distance – 3.885 Å) of two donor molecules.
The ReCl3 fragment is uniformly disposed in a meridional fash-
ion in the distorted octahedral complex. The Cl1, Cl2, Cl3 and N2
atoms define a good equatorial plane (mean deviation ꢀ0.01 Å)
from which the metal atom is displaced towards the imide nitro-
gen N4 by 0.30 Å. The five membered chelate ring (Re, N1, C5, C6
and N2) along with the pyridine moiety constitute a satisfactory
plane (md < 0.02 Å) with which the p-aminophenyl group of the li-
gand L is obliquely oriented (dihedral angle ꢀ70°). Trans influence
of imido nitrogen (N4) is reflected in the Re–N1 bond length
(2.236 Å), longer by about 0.19 Å compared to normal Re–N2
length (2.045 Å). The Re–N4 length, 1.714(3) Å and Re–N4–C13 an-
2.5. Electrochemistry
gle, 173.9(2)° are consistent with the triply bonded (r2
p
4), more
All the two families are electroactive in acetonitrile solution at
platinum electrode. The reduction potential data are collected in
or less linear arylimidorhenium (ReV„NC6H4Y) motif [30,31].

2506
J. Gangopadhyay et al. / Polyhedron 28 (2009) 2503–2509
Table 3
Cyclic voltammetric formal potentials at 298 K in acetonitrile solvent (TEAP, supporting electrolyte) at a platinum working electrode.a,b,c,d
Complex 1
E
½
(ReVI/ReV),V (
D
Ep, mV)
Complex 2
E ,V (DEp, mV)
½
ReVI/ReV
ReVII/ReVI
1a
1b
1c
1d
1.02 (80)
0.97(85)
0.95(80)
0.91(80)
2a
2b
2c
2d
0.16(80)
0.13(80)
0.11(85)
0.08(80)
1.58(80)
1.54(85)
1.52(85)
1.48(80)
a
Scan rate 50 mVsꢁ1
.
b
c
E
½
= ½ (Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials, respectively.
D
Ep = EpaꢁEpc
.
d
Reference electrode SCE.
Table 5
Table 4
Rate constants and and Activation parametersa for the conversion 2!1 in acetonitrile
Rate constants and activation parametersa for the conversion 2!1 in acetonitrile
using [NH3OH+].b,c
using [N2Hþ5 ].b,c
[N2Hþ5 ], M
103 kobsd, sꢁ1
103k2, Mꢁ1sꢁ1
T (K)
[NH3OH+], M
103 kobsd, sꢁ1
103k2, Mꢁ1sꢁ1
T (K)
299
1.11
1.67
2.22
1.11
1.67
2.22
1.11
1.67
2.22
1.11
1.67
2.22
1.11
1.67
2.22
0.54
0.89
1.13
0.50
0.71
0.99
0.42
0.61
0.83
0.25
0.37
0.54
0.15
0.24
0.35
0.53(0.04)
299
1.11
1.67
2.22
1.11
1.67
2.22
1.11
1.67
2.22
1.11
1.67
2.22
1.11
1.67
2.22
2.84
4.33
5.74
2.30
3.49
4.68
2.04
3.06
4.02
1.19
1.88
2.54
0.72
1.21
1.64
2.63(0.02)
296
293
288
283
0.44(0.03)
0.37(0.01)
0.26(0.02)
0.18(0.03)
296
293
288
283
2.13(0.03)
1.77(0.04)
1.21(0.04)
0.82(0.03)
a
H– = 10.74 (0.62) kcal molꢁ1 and
D
S– = ꢁ37.30 (2.1) eu.
a
D
H– = 11.61 (0.59) kcal molꢁ1 and
D
S– = ꢁ31.22 (2.4) eu.
D
b
c
The initial concentration of 2 is 2.5 ꢄ 10ꢁ4 M.
b
c
The initial concentration of 2 is 2.5 ꢄ 10ꢁ4 M.
Least squares deviations are given in parentheses.
Least squares deviations are given in parentheses.
dition (excess hydrazine), the rates are found to be proportional to
the concentration of amide complex 2 (Rate = kobsd [2]), as evident
Table 3. The type 1 complexes display well-defined quasi-revers-
ible anodic peak (peak to peak separation ꢀ 80 mV) near 0.95 V
versus SCE. The response is tentatively assigned to the oxidation
of ReV to ReVI and relevant to the redox reaction 1+ + e ?1. The po-
tential increases with the increase in electron withdrawing power
of Y (OCH3 < CH3 < H < Cl) and the observed trend follows usual
Hammett order. The picolinamide complexes of type 2 exhibit
two successive quasi-reversible (peak to peak separation ꢀ 80 mV)
one electron signals near 0.15 and 1.50 V for Re(VI)/Re(V) and
Re(VII)/Re(VI) couples respectively (Eq. (4)).
from the linear plots of ꢁln(A ꢁAt) versus time t at different hydra-
a
zine concentrations under variable temperatures. The plot be-
tween any set of kobsd values and used hydrazine concentrations
at any recorded temperature is linear in nature and the corre-
sponding slope gives the second order rate constant k2 as stated
in Eq. (5). First order dependence of 2 and
Rate ¼ k2½2ꢂ½N2Hþꢂðkobsd ¼ k2½N2HþꢂÞ
ð5Þ
5
5
hydrazine on rate implies that the oxygen atom transfer reaction
proceeds through the rate determining step involving close associ-
2 þ e ! 2ꢁ
2
ð4aÞ
ð4bÞ
þ þ e ! 2
ation (D
S– = ꢁ31.22 eu) of hexavalent metal amide substrate and
the reducing agent.
Lowering of reduction potential for ReVI/ReV couple by a sub-
The rate of reduction of 2 by N2Hþ was found to be nearly five
5
stantial margin of ꢀ0.8 V for type 2 complexes compared to that
of 1 indicates superior stabilization of amide bound hexavalent
rhenium in the former species [32]. Like 1, type 2 complexes also
exhibit substitution effect at Y of arylimido moiety in a usual
manner.
times faster than that measured with NH3OH+ (Table 5) at all re-
corded temperatures. The difference in rates can be reconciled by
considering higher reducing ability of N2Hþ (higher oxidation po-
5
tential) over NH3OH+ (lower oxidation potential).
2.7. Reaction model
2.6. Reduction kinetics
N2H4 has the unique ability to act as either one electron or multi
electron (two and four) donor reducing agent depending on the
reaction condition and nature of the oxidant. The number of proton
In order to understand the kinetic behaviour of amide ? imine
reduction, the progress of the reaction was monitored by spectro-
photometry in acetonitrile solution. The conversion 2 ? 1 was
characterized by the increase in absorbance at 535 nm in the tem-
perature interval 283–299 K. The second order rate constants at
different temperatures and activation parameters for the reaction
using N2H5+ are collected in Table 4. Under pseudo-first order con-
and electron equivalents transferred from each equivalent of N2Hþ
5
is a requisite for the proposition of reduction mechanism. Isolation
of ammonium salt from the products of 2 ? 1 conversion is consis-
tent with one electron and one proton donor function of N2Hþ, rel-
5
evant to half cell reaction, N2Hþ ? ½N2 + NHþ + H+ + e.
5
4

J. Gangopadhyay et al. / Polyhedron 28 (2009) 2503–2509
2507
_
VI
X Re
V
Ar
..
Ar
..
N
VI
Re
Ar
.
X
Re
amd
amd
..
N
N
X
amd
+e
+ H+
.
N
N
N
O
O
H
O
-
3
2
2
+2H+
+e
Ar = -C6H4NH2
+
3N2H5
X = Cl3(NC6H4Y)
+
V
V
XRe
VI
amn
amn
Ar
H
H
OH
Ar
H
H
OH
Re
X
XRe
N imn
N
N
Ar
N
-H2O
N
+e
N
H
1
5
4
Scheme 1.
2 ? 1 conversion does not involve nucleophilic attack of hydra-
zine instead outward transfer of oxygen atom from the bonded li-
gand occurs through proton assisted amide C@O labilisation. In
attempt to extend the reaction model (Scheme 1), the initiation
is thought to occur via fast metal reduction (low ReVI/ReV reduction
2.8. Coligand effect on intramolecular oxidation
-acidic ReV ion can be stabilized with the help of
p
p-donor oxo
and arylimido coligands. The scrutiny of imine ? amide conversion
in the ReV-oxo complex (6), [ReVOCl3L] can help us to understand
role of electronic effect of coligands exerted on oxidation process.
With the aim to investigate, we prepared the concerned oxo com-
plex and allowed to react with aqueous HNO3 under the same con-
dition. The reaction was only ended up with no observable colour
change and no isolation of the corresponding amide product. The
observation implies that the ReVI ion is better stabilized by arylimido
potential, ꢀ0.15 V) by N2Hþ to form the anionic complex 2ꢁ. Lower
5
metal valence (+V) in 2ꢁ considerably decreases the Lewis acidity
of the metal and expansion of radius upon reduction substantially
weakens Re-amido nitrogen bonding.
These electronic changes in 2ꢁ promote internal electron trans-
fer [33] from pentavalent metal to the amide function and aug-
mented oxygen basicity arising there from induces proton capture
[34] to generate the radical based reactive Re(VI) species, 3. Such
electron transfer events are facile in 2ꢁ containing comparatively
electron rich ReV centre rather than in 2 incorporating electron poor
ReVI metal. Subsequent reduction of amido carbon and nitrogen
O
Cl
Cl
Cl
NH2
V
Re
N
N
sites in 3 resulted cationic
a-hydroxyamine intermediate, 4 [35–
H
39]. Finally, 4 undergo metal reduction followed by dehydration
to regenerate 1. Elimination of water from 5 is driven by stronger
ReV-imine bonding, resulting from metal electron density delocal-
6
ization over p
* framework of aldimine ligand. The proposed mech-
group compared to the oxo coligand. This is attributed to superior p-
donor ability of arylimido group (NAr2ꢁ) over oxo (O2ꢁ) to efficiently
stabilize the ReVI centre as attested from the higher oxidation poten-
tial of oxo complex, 6 (1.55 V) than the imido complexes, 1 (ꢀ0.95 V)
and the lower electronegativity of nitrogen than oxygen. Signifi-
cantly high oxidation potential for the oxo complex renders the rhe-
nium metal inactive for initial oxidation to promote imine ? amide
oxidation. Moreover, coulometric oxidation of [ReVOCl3L] in dry ace-
tonitrile solvent afforded no tractable ReVI product which suggests
that the ReVI analogue is only stable in cyclic voltammetric short
time scale.
anism indicates stepwise utilization of three equivalents of proton
and three equivalents of electron derived from three equivalents of
N2Hþ for the reduction of one equivalent of 2. This observed reac-
5
tion stoichiometry is in complete agreement with Eq. (2).
In support of the existence of transient reduced species 2ꢁ, we
performed constant potential exhaustive reduction of 2 at +0.30 V
in dry acetonitrile solvent under deareated condition. The coulomb
count ratio suggests an almost quantitative one electron reduction
of 2 to form stereoretentive ReV analogue (2ꢁ), evident from the vir-
tually superposable cyclic voltammogram of the electrogenerated
species 2ꢁ (anodic scan) with that of 2 (cathodic scan). Attempts
to isolate 2ꢁ did not succeed. Such an analogous transformation
of N-substituted amide to imine has not yet been documented in
3. Conclusion
metal free condition. Appreciable
p-delocalisation in free amide
Reported pyridylimine chelated type 1 imidorhenium(V) com-
plexes are active towards inward oxygen atom transfer reaction
in dilute nitric acid medium affording pyridylamide bound hexava-
lent rhenium analogues, 2. Low ReVI/ReV potential for 2 provides the
opportunity to scrutiny the reductive susceptibility with nitroge-
makes amido oxygen feebly basic hindering the protonation step
and no scope for induced electron transfer halts the successive
stages of the transformation. Hence, the concerned 2 ? 1 conver-
sion is entirely metal regulated in electronic sense via easy acces-
sion of variable metal valence as required in different steps.

2508
J. Gangopadhyay et al. / Polyhedron 28 (2009) 2503–2509
nous reducing agents like hydrazine sulphate and hydroxylamine
hydrochloride. Rate measurements for 2!1 conversion reveal an
associative pathway and on that basis reaction mechanism is pro-
posed. The reduction mode explains the role of variable metal va-
lence in proton mediated structural changes of the bound amide
ligand in 2. The oxo complex (6) does not respond to such imine?
amide oxidation owing to considerably high metal oxidation
potential.
(cmꢁ1): 325, 335 (Re–Cl), 1595 (C@N). 1H NMR [d (J/Hz), CDCl3
solution]: L, 9.40(H(1), d, 5.5); 6.86 (H(2), t, 7.4); 7.96 (H(3), t,
8.2); 7.33 (H(4), d, 7.3); 6.41(H(6), s); 7.46–7.57 (H(8,9,11,12),
m); 16.08 (NH2, s); NC6H4CH3, 7.06 (2H(o), d, 5.5); 7.15 (2H(m),
d, 5.4); 4.04 (CH3, s).
1b: Anal. Calc. for C18H16Cl3N4Re: C, 37.21; H, 2.75; N, 9.65.
Found: C, 37.23; H, 2.78; N, 9.63. UV–vis (kmax, nm (e
, Mꢁ1
cmꢁ1), CH2Cl2 solution): 735 (1500); 540 (7500); 325 (13300). IR
(cmꢁ1): 320, 335 (Re–Cl), 1600 (C@N). 1H NMR [d (J/Hz), CDCl3
solution]: L, 9.42(H(1), d, 5.5); 6.85 (H(2), t, 7.5); 7.99 (H(3), t,
8.1); 7.35 (H(4), d, 7.2); 6.40(H(6), s); 7.45–7.50 (H(8,9,11,12),
m); 16.05 (NH2, s); NC6H5, 7.09 (2H(o), d, 5.9); 7.13 (2H(m), t,
6.6); 7.19 (1H(p), t, 6.3).
Electroactive complexes of type 2 display ReVI/ReV and ReVII
/
ReVI voltammetric signals near ꢀ0.1 and ꢀ1.6 V in contrast to only
one ReVI/ReV couple (ꢀ0.95 V) for 1. The appearance of ReVII/ReVI
response for 2 implies superior stabilizing effect of amide group
for higher valent rhenium species. Type 2 complexes uniformly ex-
hibit one electron paramagnetism and display well-resolved six
lines EPR spectra at room temperature in solution.
1a: Anal. Calc. for C18H15Cl4N4Re: C, 35.12; H, 2.44; N, 9.10.
Found: C, 35.17; H, 2.41; N, 9.06. UV–vis (kmax, nm (e
, Mꢁ1
cmꢁ1), CH2Cl2 solution): 735 (1500); 540 (7600); 325 (13200). IR
(cmꢁ1): 320, 335 (Re–Cl), 1595 (C@N). 1H NMR [d (J/Hz), CDCl3
solution]: L, 9.39(H(1), d, 5.8); 6.83 (H(2), t, 7.5); 7.97 (H(3), t,
8.2); 7.34 (H(4), d, 7.4); 6.42(H(6), s); 7.44–7.53 (H(8,9,11,12),
m); 16.06 (NH2, s); NC6H4Cl, 7.05 (2H(o), d, 5.8); 7.12 (2H(m), d,
5.6).
4. Experimental
4.1. Materials and physical measurements
The starting materials [Re(NC6H4Y)Cl3(PPh3)2] [40] and [Re-
OCl3(PPh3)2] [41] and the ligand L [42] were synthesized by re-
ported standard methods. All other chemicals were of reagent
grade and used as received. Solvents were dried and distilled prior
to use. The IR spectra were recorded on KBr pellets with a Perkin–
Elmer FT-IR spectrometer. A Perkin–Elmer 2400 II elemental ana-
lyzer was used for microanalysis. The electronic spectra and kinetic
studies were done using Hitachi U-3501 spectrophotometer fitted
with thermostated cell compartments. Electrochemical measure-
ments were performed using a PAR model Versastat-2 electro-
4.2.2. Synthesis of [ReVI(NC6H4Y)Cl3(C12H10N3O)], 2
The same general method was used to prepare the above com-
plexes from [ReV(NC6H4Y)Cl3(C12H11N3)], 1. Procedural details are
given for one representative case (2d). Yields are in the range
75–80%.
One hundred and two milligrams (0.17 mmol) of 1d was dis-
solved in 25 ml acetonitrile, and 4.5 ml 1(N) nitric acid (acidity
ꢀ0.15 N) was added. The violet solution was stirred for 1 h, during
which the colour turned yellowish brown. Solvent evaporation
yields dark brown product. The mass thus obtained was thor-
oughly washed with cold water to remove adherent nitric acid
and finally dried in vacuum over fused CaCl2. Yield: 80 mg (77%).
Anal. Calc. for C19H17Cl3N4O2Re: C, 36.45; H, 2.72; N, 8.95. Found:
chemical analyzer, with
a platinum working electrode. The
supporting electrolyte was tetraethylammonium perchlorate
(TEAP), and the potentials were referenced to saturated calomel
electrode (SCE) without junction correction. X-band EPR spectra
in solution and frozen glass states were recorded using a Bruker
300E spectrometer. Magnetic susceptibilities were measured on a
PAR 155 vibrating sample magnetometer.
C, 36.40; H, 2.75; N, 8.99. UV–vis (kmax, nm (e
, Mꢁ1 cmꢁ1), CH2Cl2
solution): 535 (1600); 355 (12200). IR (cmꢁ1): 320, 335 (Re–Cl),
1595 (C@N), 1635 (C@O).
4.2. Preparation of complexes
2c: Anal. Calc. for C19H17Cl3N4ORe: C, 37.41; H, 2.79; N, 9.19.
Found: C, 37.45; H, 2.75; N, 9.22. UV–vis (kmax, nm (e
, Mꢁ1
4.2.1. Synthesis of [ReV(NC6H4Y)Cl3(C12H11N3)], 1
cmꢁ1), CH2Cl2 solution): 535 (1600); 355 (12000). IR (cmꢁ1): 320,
335 (Re–Cl), 1595 (C@N), 1635 (C@O).
The same general procedure was used to synthesize the above
complexes from [ReV(NC6H4Y)Cl3(PPh3)2]. Procedural details are
given for one representative case (1d). Yields varied in the range
80–85%.
2b: Anal. Calc. for C18H15Cl3N4ORe: C, 36.27; H, 2.52; N, 9.40.
Found: C, 36.30; H, 2.57; N, 9.33. UV–vis (kmax, nm (e
, Mꢁ1
cmꢁ1), CH2Cl2 solution): 535 (1600); 355 (12300). IR (cmꢁ1): 325,
335 (Re–Cl), 1595 (C@N), 1635 (C@O).
To a green suspension of [ReV(NC6H4Y)Cl3(PPh3)2] (190 mg,
0.20 mmol) in 30 ml of benzene was added 60 mg (0.30 mmol) of
L in 10 ml of benzene, and the mixture was heated to reflux for
2 h, affording a violet solution. The solvent was then removed un-
der reduced pressure, and the dark mass thus obtained was
subjected to chromatography on a silica gel column (25 ꢄ 1 cm,
60–120 mesh). Excess ligand and phosphine was eluted with ben-
zene. A violet band was then eluted with benzene-acetonitrile
(20:1) mixture. Solvent removal form the eluate under reduced
pressure afforded [ReV(NC6H4OCH3)Cl3(L)], 1d as a violet solid.
Yield: 106 mg (82%). Anal. Calc. for C19H18Cl3N4ORe: C, 37.35; H,
2.95; N, 9.17. Found: C, 37.30; H, 2.98; N, 9.13. UV–vis (kmax, nm
2a: Anal. Calc. for C18H14Cl4N4ORe: C, 34.28; H, 2.22; N, 8.89.
Found: C, 34.32; H, 2.25; N, 8.81. UV–vis (kmax, nm (e
, Mꢁ1
cmꢁ1), CH2Cl2 solution): 535 (1500); 355 (12400). IR (cmꢁ1): 320,
330 (Re–Cl), 1595 (C@N), 1635 (C@O).
4.2.3. Synthesis of 1 from 2
4.2.3.1. Hydrazine method. To
a yellow–brown solution of 2
(125 mg, 0.20 mmol) in acetonitrile (25 ml) was added hydrazine
sulphate (91 mg, 0.70 mmol) in 10 ml solvent. It was then stirred
for 1.5 h at room temperature whereupon the solution gradually
turned violet. The solvent was removed from the resulting solution
under reduced pressure. The obtained violet mass (1) was washed
with cold water to free adhered hydrazine and dried in vacuum
over fused CaCl2. Yield: 100 mg (82%).
(e
, Mꢁ1 cmꢁ1), CH2Cl2 solution): 735 (1500); 540 (7500); 325
(13200). IR (cmꢁ1): 320, 335 (Re–Cl), 1595 (C@N). 1H NMR [d (J/
Hz), CDCl3 solution]: L, 9.41(H(1), d, 5.5); 6.84 (H(2), t, 7.5); 7.98
(H(3), t, 8.4); 7.34 (H(4), d, 7.7); 6.41(H(6), s); 7.45–7.55
(H(8,9,11,12), m); 16.06 (NH2, s); NC6H4OCH3, 7.08 (2H(o), d,
5.3); 7.13 (2H(m), d, 5.4); 3.99 (OCH3, s).
4.2.3.2. Hydroxylamine method. To a yellow–brown solution of 2
(125 mg, 0.20 mmol) in acetonitrile (25 ml) was added hydroxyl-
amine hydrochloride (28 mg, 0.40 mmol) in 5 ml solvent. It was
then stirred for 8 h at room temperature. The solvent was removed
from the resulting solution under reduced pressure. The obtained
1c: Anal. Calc. for C19H18Cl3N4Re: C, 38.35; H, 3.03; N, 9.42.
Found: C, 38.31; H, 2.99; N, 9.37. UV–vis (kmax, nm (
e
, Mꢁ1
cmꢁ1), CH2Cl2 solution): 735 (1500); 540 (7500); 330 (13400). IR

J. Gangopadhyay et al. / Polyhedron 28 (2009) 2503–2509
2509
Table 6
Crystallographic data for 1d.
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Complex
1d
Formula
M
System
Space group
a (Å)
C19H18Cl3N4ORe
610.92
Acknowledgement
monoclinic
P2(1)/c
9.123(2)
15.961(3)
15.333(3)
90
We thank the University Grants Commission (UGC), India for
financial support.
b (Å)
c (Å)
a
(°)
b (°)
106.19(3)
90
2144.1(7)
4
1.893
293(2)
6.058
5954
0.0410
27807
References
c
(°)
V (Å3)
Z
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D (mg m-3
T (K)
l
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(mmꢁ1
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Independent reflections
Rint
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R1, wR2 [I > 2
r
(I)]
0.0251, 0.0516
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To a green solution of [ReOCl3(PPh3)2] (208 mg, 0.25 mmol) in
30 ml of dichloromethane was added 60 mg (0.30 mmol) of L in
10 ml of dichloromethane, and the mixture was stirred for
20 min, affording a pink solution. The volume of the solvent was
then reduced to one fifth under reduced pressure, and excess of
n-hexane was added into it. It causes precipitation of the pink com-
plex and the solution was then filtered. The pink product was col-
lected by filtration and washed twice with diethyl ether. The mass
was finally dried in vacuum over fused CaCl2. The pink mass is
unstable on silica gel column and can not be isolated by chroma-
tography. Yield: 111 mg (88%). Anal. Calc. for C12H11Cl3N3ORe: C,
28.56; H, 2.18; N, 8.33. Found: C, 28.49; H, 2.24; N, 8.26. IR
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to all non-hydrogen atoms. Significant crystal data are listed in
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Supplementary data
CCDC 673975 contains the supplementary crystallographic data
for 1d. These data can be obtained free of charge via http://