New mononuclear manganese(III) complexes with hexadentate (N4O2) Schiff base ligands: Synthesis, crystal structures, electrochemistry, and electron-transfer reactivity towards hydroxylamine

Article abstract of DOI:10.1002/ejic.200390201

Four new manganese(III) complexes of hexadentate Schiff base ligand obtained from N,N'-bis(3-aminopropyl)ethylenediamine and salicylaldehyde, or salicylaldehyde derivatives having N₄O₂ donor sets - [Mn(sal-N-1,5,8,12)] ClO₄ (1), [Mn(5-Cl-sal-N-1,5,8,12)]ClO₄ (2), [Mn(5-Br-sal-N-1,5,8,12)]-ClO₄ (3), and [Mn(3-methoxy-sal-N-1,5,8,12)]ClO₄·H₂O (4) have been synthesized and characterized. The crystal structures of 1 and 4 have been determined, and the kinetics of the electron-transfer reactions between these complexes and hydroxylamine hydrochloride have been followed spectrophotometrically in the 5.5-8.0 pH range. Similar kinetic reactivities of all the complexes towards a particular species of the reductant were noted. Hydrogen-bonded adduct formation between the reactants prior to electron transfer seems to be a possible reaction pathway, according to analysis of the experimental data. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejic.200390201

FULL PAPER
New Mononuclear Manganese(III) Complexes with Hexadentate (N4O2) Schiff
Base Ligands: Synthesis, Crystal Structures, Electrochemistry, and Electron-
Transfer Reactivity towards Hydroxylamine
Anangamohan Panja,[a] Nizamuddin Shaikh,[a] Shankareswar Gupta,[a] Ray J. Butcher,[b] and
Pradyot Banerjee*[a]
Keywords: Cyclic voltammetry / Electron transfer / Kinetics / Manganese
Four new manganese(III) complexes of hexadentate Schiff
hydroxylamine hydrochloride have been followed spectro-
base ligand obtained from N,NЈ-bis(3-aminopropyl)ethylene- photometrically in the 5.5−8.0 pH range. Similar kinetic reac-
diamine and salicylaldehyde, or salicylaldehyde derivatives
having N4O2 donor sets − [Mn(sal-N-1,5,8,12)]ClO4 (1),
[Mn(5-Cl-sal-N-1,5,8,12)]ClO4 (2), [Mn(5-Br-sal-N-1,5,8,12)]-
ClO4 (3), and [Mn(3-methoxy-sal-N-1,5,8,12)]ClO4·H2O (4) −
have been synthesized and characterized. The crystal struc-
tivities of all the complexes towards a particular species of
the reductant were noted. Hydrogen-bonded adduct forma-
tion between the reactants prior to electron transfer seems to
be a possible reaction pathway, according to analysis of the
experimental data.
tures of 1 and 4 have been determined, and the kinetics of ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
the electron-transfer reactions between these complexes and
Germany, 2003)
Introduction
An important area of investigation in the bioinorganic
chemistry of manganese is the study of the structures and
reactivities of manganese() complexes incorporating bio-
logically relevant heteroatom donor ligands. In this regard,
syntheses and characterization of manganese() complexes
with polydentate Schiff base ligands[1Ϫ6] have been an on-
going theme. In this communication we report the synthesis
of a series of manganese() complexes with hexadentate
Schiff base ligands possessing N4O2 donor sets (Scheme 1):
[Mn(sal-N-1,5,8,12)]ClO4 (1), [Mn(5-Cl-sal-N-1,5,8,12)]-
ClO4 (2), [Mn(5-Br-sal-N-1,5,8,12)]ClO4 (3), and [Mn(3-
methoxy-sal-N-1,5,8,12)]ClO4·H2O (4). Of these com-
pounds, 1 and 4 have been characterized by X-ray crystal-
lography. A kinetic study of these complexes with hy-
droxylamine in aqueous solution has been carried out in
order to determine the electron-transfer reactivity of these
complexes. The mechanistic versatility noted in the reaction
behavior of hydroxylamine is attractive in that it can act
both as an oxidant or a reductant,[7] and can coordinate
through either the N- or the O-end.[8] The estimated bond
Scheme 1
dissociation energy for the NϪH moiety[9] in hydroxyl-
amine (92 kcal·molϪ1) is similar to that of the OϪH group
of tyrosine (85.5 kcal·molϪ1),[10,11] and so proton movement
Ϫ as observed with tyrosine in PSII Ϫ may have some rel-
evance to the observed reactions with hydroxylamine.
Results and Discussion
Crystal Structures of [MnIIIsal-N(1,5,8,12)]ClO4 (1) and
[Mn(3-methoxy-sal-N(1,5,8,12)]ClO4·H2O (4)
[a]
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
Kolkata 700 032, India
Atom-labeling schemes and molecular views of complex
1 and 4 are shown in Figures 1 and 2, respectively, while
selected bond lengths and angles are given in Tables 1 and
2, respectively. In each case the metal is in a hexadentate
distorted MnN4O2 environment, the two phenolic oxygen
Fax: (internat.) ϩ91Ϫ33/2473 2805
E-mail: icpb@mahendra.iacs.res.in
[b]
Department of Chemistry, Howard University,
2400 Sixth Street, NW, Washington, DC 20059, USA
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
1540  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ1948/03/0408Ϫ1540 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2003, 1540Ϫ1547

Manganese(III) Complexes with Hexadentate (N4O2) Schiff Base Ligands
FULL PAPER
˚
2.102(4) in 1 and 2.054(19) and 2.104(2) A in 4, and the
MnϪN(amine) lengths being 2.207(4)/2.239(5) and
˚
2.163(2)/2.174(2) A, respectively. These are consistent with
those
reported
for
manganese()
Schiff
base
complexes.[1,3Ϫ5] Both the lattices are hydrogen bonded, al-
though the hydrogen bonding is less extensive in 1. The in-
tramolecular hydrogen bonding here involves the NϪH
moiety and the phenolic oxygen of the ligand and O of the
perchlorate anion. The NϪH···O(phenolic) distances are
˚
3.199(12) and 3.387(12) A, whereas NϪH···O(perchlorate)
˚
is 3.098(18) A. In 4, on the other hand, the coordinated
water molecule, together with the phenolic oxygen, amine
nitrogen (NϪH), and perchlorate anion, constitute a net-
work (Figure 3). Both intra- and intermolecular hydrogen
bonding are observed, the O(1W)ϪH···O(phenolic) dis-
˚
tances being 2.858(3) and 2.837(7) A, whereas the
O(1W)ϪH···O(perchlorate) distances are 2.638(10),
˚
3.196(8), and 3.227(7) A. The amine hydrogen also enters
into intramolecular hydrogen bonding with the oxygen of
ϪOCH3 in the aromatic ring, as well as with the H2O mol-
ecule.
˚
Table 1. Selected bond lengths [A] and angles [°] for complex 1
Figure 1. Molecular structure of complex 1 showing atom-
labeling scheme
MnϪO(1A)
MnϪN(1A)
MnϪN(2B)
1.869(4)
2.098(4)
2.207(4)
MnϪO(1B)
MnϪN(1B)
MnϪN(2A)
1.878(4)
2.102(4)
2.239(5)
O(1A)ϪMnϪO(1B)
O(1B)ϪMnϪN(1A)
O(1B)ϪMnϪN(1B)
O(1A)ϪMnϪN(2B)
N(1A)ϪMnϪN(2B)
O(1A)ϪMnϪN(2A)
N(1A)ϪMnϪN(2A)
N(2B)ϪMnϪN(2A)
175.50(15)
90.99(17)
86.39(16)
90.37(16)
165.30(17)
95.25(18)
85.77(18)
80.05(18)
O(1A)ϪMnϪN(1A)
O(1A)ϪMnϪN(1B)
N(1A)ϪMnϪN(1B)
O(1B)ϪMnϪN(2B)
N(1B)ϪMnϪN(2B)
O(1B)ϪMnϪN(2A)
N(1B)ϪMnϪN(2A)
87.06(16)
90.41(17)
109.09(17)
92.52(16)
85.38(17)
88.65(18)
164.39(17)
˚
Table 2. Selected bond lengths [A] and angles [°] for complex 4
MnϪO(1B)
MnϪN(1A)
MnϪN(2B)
1.8692(15) MnϪO(1A)
2.0544(19) MnϪN(1B)
1.8733(15)
2.104(2)
2.174(2)
2.163(2)
MnϪN(2A)
O(1B)ϪMnϪO(1A)
O(1A)ϪMnϪN(1A)
O(1A)ϪMnϪN(1B)
O(1B)ϪMnϪN(2B)
N(1A)ϪMnϪN(2B)
O(1B)ϪMnϪN(2A)
N(1A)ϪMnϪN(2A)
N(2B)ϪMnϪN(2A)
178.49(7)
86.07(7)
92.98(8)
93.08(7)
169.31(9)
87.56(7)
89.59(8)
81.49(8)
O(1B)ϪMnϪN(1A)
O(1B)ϪMnϪN(1B)
N(1A)ϪMnϪN(1B) 104.02(8)
O(1A)ϪMnϪN(2B)
N(1B)ϪMnϪN(2B)
O(1A)ϪMnϪN(2A)
92.42(7)
87.44(8)
88.40(8)
85.39(9)
92.36(8)
N(1B)ϪMnϪN(2A) 165.69(8)
Figure 2. Molecular structure of complex 4 showing atom-
labeling scheme
UV/Visible and IR Spectroscopy
The electronic absorption spectra for all the complexes
atoms are in a trans configuration, and the two coordinat-
ing ONN ligand halves are facial. The mean deviations of 1؊4 were recorded in acetonitrile, and data are given in
˚
the manganese atom from the equatorial plane are 0.10 A Table 3. All the complexes exhibit similar spectral features.
˚
in 1 and 0.08 A in 4. The bond lengths are quite similar There are four important regions in the UV/Vis spectra for
(Tables 1 and 2), the MnϪO(phenolic) lengths being this set of complexes. The least intense bands appear in the
˚
˚
1.869(4) and 1.878(4) A in 1 and 1.869(15) and 1.873(15) A 630Ϫ660 nm range, while another band of higher intensity
in 4, the MnϪN(imine) bond lengths being 2.098(4) and is observed in the 510Ϫ540 nm range. The UV/Vis spectra
Eur. J. Inorg. Chem. 2003, 1540Ϫ1547
1541

A. Panja, N. Shaikh, S. Gupta, R. J. Butcher, P. Banerjee
FULL PAPER
(∆E ϭ 0.036 eV) HOMO-1 also has the same character, but
with less contribution from the metal orbitals (72% metal
and 17% ligand). In 4, the LUMO is also strongly delo-
calized, with a little more contribution from the ligand
(58% metal and 33% ligand) relative to 1. Two closely
spaced (∆E ϭ 0.071 eV) HOMO (83% metal and 7% li-
gand) and HOMO-1 (64% metal and 30% ligand) orbitals
are also delocalized, with a greater contribution from the
metal in the former than in the latter.
The EHMO calculations indicate that the frontier molec-
ular orbitals are predominantly metal-based, but with sig-
nificant contributions from the ligands. The bands in the
630Ϫ660 nm range can therefore be assigned as predomi-
nantly d-d with some admixed MLCT character. These
bands are similar to those reported previously for
manganese()ϪSchiff base complexes.[6,12,13,17] The bands
in the 510Ϫ540 nm range have previously been attributed
to phenolato-to-manganese() LMCT for similar MnIII
complexes.[8,14,15,17] The EHMO calculations, however, indi-
Figure 3. Molecular packing of 4 showing both intra- and inter- cate that all the frontier MOs are predominantly metal-
molecular hydrogen bonding
based with significant contribution from the ligands, so the
above assignment is not unambiguous. From the X-ray
crystal data, the observed MnIIIϪO(phenolate) average
Table 3. UV/Visible spectroscopic data for the complexes 1؊4
˚
˚
bond lengths are 1.873 A for 1 and 1.871 A for 4, which
is quite short in comparison with the reported values for
manganese() complexes.[5,14,15,18] The shift of the
510Ϫ540 nm bands to much lower energy compared to
those (ca. 480Ϫ490 nm) reported earlier indicates stronger
interaction between the phenolato(O) and manganese()
center[12] and supports the assignment of the 510Ϫ540 nm
Complex
λmax/nm (ε/dm3 molϪ1 cmϪ1
)
1
2
3
4
640 sh (245), 512 (881), 342 sh (5670),
271 (22390), 226 (36180)
653 sh (209), 516 (773), 332 sh (5670),
268 sh (17250), 223 (43580)
656 sh (212), 517 (820), 335 sh (6090),
269 sh (17970), 226 (43440)
660 sh (403), 534 (1270), 364 sh (5670),
284 (21640), 231 (46260)
bands as LMCT from phenolato-to-MnIII
All the complexes show similar IR spectral features, each
exhibiting a strong band attributable to ν(CN) between
1620Ϫ1627 cmϪ1. All of them also show broad unsplit
.
of all the complexes show ligand to metal charge-transfer bands at 1081Ϫ1095 cmϪ1, together with bands at 626Ϫ627
bands in the 330Ϫ370 nm region. These bands are similar cmϪ1 indicative of the presence of a noncoordinated per-
to those reported for hexacoordinate [MnIII(BBPEN)](PF6) chlorate counterion. As well as these, 4 exhibits a broad
[H2BBPEN ϭ N,NЈ-bis(2-hydroxybenzyl)-N,NЈ-bis(2-meth- band at 3550 cmϪ1 due to the presence of a non-coordi-
ylpyridyl)ethylenediamine] with N4O2 donor sets.[12] The in- nated water molecule in the crystal, which is consistent with
tense higher-energy bands in the region 220Ϫ280 nm are the molecular structure.
usually attributed to intraligand πǞπ* transitions, as ob-
Electrochemical Studies
Cyclic voltammograms of all the MnIII complexes were
served previously for manganese() ions in octahedral
environments.[6,12Ϫ15] It is interesting to note that all the
bands are affected by the benzene ring substitution, which recorded in acetonitrile solution with TEAP as supporting
indicates that the frontier molecular orbitals of the high- electrolyte in the potential range from Ϫ1.2 to ϩ1.2 V vs.
spin manganese() complexes possess significant contri- SCE. The results are summarized in Table 4. As the cyclic
bution from the ligands.
voltammetric features of all the complexes are similar, a
In order to provide some insight into the nature of redox representative profile for 3 is shown in Figure 4. A notable
and spectroscopically relevant orbitals, extended Hückel feature for all the complexes is that an electrochemically
MO calculations, using the crystallographic parameters of quasi-reversible anodic wave and two well defined irrevers-
1 and 4, were performed with the aid of the CACAO pro- ible cathodic waves are detected. The anodic process corre-
gram ( Figure S1 and Figure S2 in the Supplementary Ma- sponds to the MnIV/MnIII couple, and the peak-to-peak
terial, see footnote on the first page of this article).[16] The separation lies between 80 mV to 120 mV, slightly above the
lowest unoccupied molecular orbital (LUMO) of 1 was value expected for a one-electron electrochemically revers-
found to be strongly delocalized, with 61% metal contri- ible process.[19] The first irreversible cathodic process can be
bution and 29% ligand contribution. The HOMO, on the attributed to the MnIII/MnII couple, in line with previously
other hand, is predominantly metal-based (84%), with little reported complexes of similar ligand environment.[1,13] The
contribution from the ligand (6%). The closely spaced irreversibility of the MnIII/MnII couple might be due to the
1542
Eur. J. Inorg. Chem. 2003, 1540Ϫ1547

Manganese(III) Complexes with Hexadentate (N4O2) Schiff Base Ligands
FULL PAPER
Table 4. Cyclic voltammetric results[a] for manganese() complexes
at 298 K
as the reducing agent. The redox chemistry of hydroxylam-
ine in solution is very complicated, due to its oxidation to
various products depending upon the nature of the oxi-
dants. It has been observed that the most common oxi-
dation products of hydroxylamine are N2O and N2,[8,22Ϫ24]
oxidation of hydroxylamine to NO2Ϫ, NO3Ϫ, and NO[22Ϫ24]
being relatively rare.
Complex[a] MnIV/MnIII MnIII/MnII
Epa/V
Epc/V
∆Ep/V (E1/2)/V Epc/V
1
2
3
4
0.66
0.78
0.78
0.64
0.54
0.68
0.70
0.55
0.12
0.10
0.08
0.09
0.60
0.67
0.70
0.59
Ϫ0.60
Ϫ0.52
Ϫ0.54
Ϫ0.58
Stoichiometry
[a] Solvent: acetonitrile; working electrode: platinum; reference elec-
trode: SCE; Epa and Epc are anodic and cathodic peak potentials
respectively; E1/2 ϭ 0.5(Epa ϩ Epc); ∆Ep ϭ Epa Ϫ Epc; scan rate:
Because of the formation of various products and the
complicated redox chemistry involved, variable stoichi-
ometry has usually been encountered for the oxidation of
hydroxylamine by various oxidants.[8,22,23] The stoichi-
ometry was therefore examined very carefully in the pres-
ence both of excess reductant and of excess oxidant. In the
presence of excess hydroxylamine over the complexes, the
stoichiometry was determined by bromatometric estimation
of the unchanged reductant,[25] whereas in the presence of
excess oxidant, the stoichiometry was measured spectro-
photometrically with the aid of the known extinction coef-
ficient of the complex (λmax ϭ 517 nm, ε ϭ 820 dm3
molϪ1·cmϪ1 for complex 3). The results in Table 5 also im-
ply a variable stoichiometry in our system. Thus, under the
kinetic reaction conditions, the ‘‘putative first step’’ of the
reaction may be given as shown in the following equations.
50 mV·sϪ1
.
Table 5. Stoichiometry of the reduction of complex 3 by hy-
droxylamine hydrochloride (all concentrations are in mmol·dmϪ3
at 30 °C
)
Figure 4. Cyclic voltammogram (scan rate 50 mV·sϪ1) of complex
3 in acetonitrile at a platinum electrode with ferrocene as internal
standard and TEAP as supporting electrolyte
[Complex]
pH
[NH2OH]
∆[complex]/∆[NH2OH]
instability of the formally MnII species.[20] The second ca-
thodic process is very difficult to assign. It has been ob-
served that the cyclic voltammogram of the free ligands fea-
tures an irreversible reduction at about Ϫ1.0 V. The more
negative (ca. Ϫ0.88 V) cathodic process in the complex is
close to that observed for the free ligand. This chemical
environment is not suitable to stabilize the Mn() state, and
MnII/MnI couples are not usually observed for this type of
complexes.[14] Because of the instability of the formally
MnII species, there might be free ligand (probably pro-
tonated) available in the solution, resulting in the second
irreversible cathodic wave.[20] The electrochemical results
are consistent with those observed previously for manga-
nese() complexes.[1,12,21] In the group of complexes at
hand, both the Epc and E1/2 values are sensitive to substi-
tution, both increasing with increasing electron-with-
drawing power of the substituents. The standard extended
Hückel MO calculation indicates that the frontier molecular
orbitals are predominantly metal-based with significant
contribution from the ligands. The ligand contribution both
to the HOMO and to the LUMO is reflected in the change
in the redox potentials with substitution in the benzene
ring.
3.00
3.00
20.0
10.0
3.00
3.00
20.0
5.50
5.50
5.50
5.50
8.00
8.00
8.00
30.0
15.0
2.00
2.00
30.0
12.0
2.00
1.05
0.97
1.95
1.75
1.03
1.20
1.98
MnIIIL ϩ NH2OH Ǟ MnIIL ϩ 1/2 N2 ϩ Hϩ ϩ H2O
(1)
(2)
and with an excess of MnIII
2 MnIIIL ϩ NH2OH Ǟ 2 MnIIL ϩ 1/2 N2O ϩ
1/2 H2O ϩ 2 Hϩ
The appearance of a six-line EPR signal in the g ϭ 2.0
region indicates that at least some complex is in the ϩ2
state in the final form, thus supporting the above mecha-
nism. The electrochemical study shows that the formally
MnIIL species formed upon reduction of the parent com-
plexes are unstable and readily decompose. The decom-
posed species may be hydrated Mn2ϩ ion in aqueous solu-
tion, together with free ligand. The formulation of the re-
Electron-Transfer Reactivity
The electron-transfer reactivities of all four complexes duced product as MnIIL is therefore not unambiguous.
were studied in the 5.5Ϫ8.0 pH range, with hydroxylamine However, externally added Mn2ϩ ion in the form of
Eur. J. Inorg. Chem. 2003, 1540Ϫ1547
1543

A. Panja, N. Shaikh, S. Gupta, R. J. Butcher, P. Banerjee
FULL PAPER
Mn(ClO4)2·6H2O was found to exert no effect on the rate complexes 1؊4 are octahedral, with all the coordination
of the reactions, indicating that the reduced product of the sites occupied by the four N atoms and the two O atoms of
complexes has no influence on the electron transfer rate.
the ligand. An effort was made to examine the electrochem-
istry of the complex at different pH values in an aceto-
nitrile/water medium, but no variance was seen. There was
also no noticeable change in the electronic spectra of the
complexes when recorded at different pH values. The con-
clusion that can be drawn from these observations is that
either only one species of the MnIII complexes is present,
or that the reactivity of the protonated species is identical
to that of the parent species, if one exists, over the pH range
studied. The pH variation results are also inconsistent with
those expected from the thermodynamic potential values of
the reductants. It is well documented that the deprotonated
forms of the reductants are more reactive than the pro-
tonated species, due to the changed oxidation potential.[28]
On protonation, the oxidation potential of the reductant
decreases significantly from ϩ3.04 V for NH2OH to ϩ1.87
V for NH3OHϩ, according to[29]
Kinetics and Mechanism
The kinetics of the oxidation of NH2OH were observed
under pseudo first-order conditions, with the reductant be-
ing maintained in large excess over the complex. At a fixed
hydrogen ion concentration and ionic strength (pH ϭ 6.0,
I ϭ 0.2 mol·dmϪ3 (NaCl), [phosphate] ϭ 0.05 mol·dmϪ3
,
temperature ϭ 30 °C and λ ϭ 360 nm), the observed rate
constants (kobsd./sϪ1) [Table 6] follow rate saturation kine-
tics at higher values of [NH2OH] for all the complexes 1؊4.
These observations are consistent with the theoretical rate
law in Equation (3).
Table 6. Pseudo-first order rate constants (kobsd./sϪ1) for the oxi-
dation of NH2OH by the complexes 1؊4 at a fixed pH of 6.0
{kinetic conditions: [complex] ϭ 5Ϫ6 ϫ10Ϫ5 mol·dmϪ3, I ϭ 0.20
mol·dmϪ3 (NaCl), [buffer] ϭ 0.05 mol·dmϪ3 (phosphate), 30 °C}
Table 7. Pseudo-first order rate constants (kobsd./sϪ1) for the oxi-
dation of NH2OH by the complexes 1؊4 at different pH values
(kinetic conditions: [NH2OH] ϭ 6 ϫ 10Ϫ4 mol·dmϪ3, [complex] ϭ
5Ϫ6 ϫ10Ϫ5 mol·dmϪ3, I ϭ 0.20 mol·dmϪ3 (NaCl), [buffer] ϭ 0.05
mol·dmϪ3 (phosphate), 30 °C)
103 [NH2OH] [NaCl][a]
mol %
D2O
103 kobsd. [sϪ1
]
[mol·dmϪ3
]
[mol·dmϪ3
]
1
2
3
4
0.6
0.6
0.6
0.6
0.6
0.6
0.8
1.0
3.0
5.0
7.0
10
0
50
90
2.86 3.88 4.01 1.36
2.85
2.82
3.22
3.20
3.20
pH
104 kobsd./sϪ1
0.2
0.4
0.8
1
2
3
4
5.50
5.75
6.00
6.25
6.50
7.00
7.50
8.00
61.3
41.7
28.6
23.5
19.0
9.80
5.50
3.40
64.2
41.1
40.0
32.3
27.9
14.4
69.9
55.3
42.7
31.6
25.6
16.7
28.8
19.0
13.6
11.2
3.65 4.19 4.60 1.48
4.28 4.65 5.00 1.68
6.63 6.61 7.68 3.25
7.68 7.99 9.29 3.88
7.88 8.93 9.57 4.05
8.31 9.68 11.1 4.33
9.10
6.50
3.00
1.20
9.00
5.00
4.50
2.50
[a]
I ϭ 1.0 mol·dmϪ3 (NaNO3).
kobsd. ϭ (k Q [NH2OH])/(1 ϩ Q [NH2OH])
(3)
where Q is the association constant between the complex
and the reductant prior to the electron-transfer reaction,
and the parameters derived by fitting the experimental data
to above equation at pH 6.0 are:
(i) k ϭ (9.4Ϯ0.1) ϫ10Ϫ3 sϪ1 and Q ϭ (791Ϯ37) for 1
(ii) k ϭ (1.03Ϯ0.05) ϫ10Ϫ2 sϪ1 and Q ϭ (820Ϯ130) for 2
(iii) k ϭ (1.20Ϯ0.05) ϫ10Ϫ2 sϪ1 and Q ϭ (792Ϯ98) for 3
(iv) k ϭ (5.20Ϯ0.1) ϫ10Ϫ3 sϪ1 and Q ϭ (527Ϯ39.8) for 4
The effect of pH on the rate of reaction was studied at a
constant concentration of NH2OH (6·10Ϫ4 mol·dmϪ3) at
which a strict first-order dependence is valid. The observed
rate constants decrease with increasing pH (Table 7). Fig-
ure 5 shows the plot of kox vs. [Hϩ]. The observations are
not unprecedented, similar findings having been made in
the reaction between this reductant and several mononu-
clear and polynuclear manganese complexes reported pre-
viously,[9,26,27] and have been attributed to the generation
Figure 5. Plot of kox vs. [Hϩ] for the oxidation of hydroxylamine
by 1 at 30 °C, I ϭ 0.2 mol·dmϪ3 (NaCl), 0.05 mol·dmϪ3 phos-
phate buffer
2 NH2OH N2 ϩ 2 Hϩ ϩ 2 H2O ϩ 2 e; E0 ϭ ϩ3.04 V
Ǟ
ǟ
of more reactive aquated derivatives of the complexes. The 2 NH3OHϩ Ǟ N2 ϩ 4 Hϩ ϩ 2 H2O ϩ 2 e; E0 ϭ ϩ1.87 V
ǟ
1544
Eur. J. Inorg. Chem. 2003, 1540Ϫ1547

Manganese(III) Complexes with Hexadentate (N4O2) Schiff Base Ligands
FULL PAPER
It is therefore unlikely that the observed decrease in kobsd. (1.18Ϯ0.45) ϫ10Ϫ6 for 3; kЈ1 ϭ 10.00Ϯ3.4 dm3·molϪ1·sϪ1
,
at higher pH values is due to the greater kinetic activity of kЈ2 ϭ 0.56Ϯ0.18 dm3·molϪ1·sϪ1, and Ka ϭ (4.14Ϯ0.9)
NH3OHϩ over NH2OH. Rate saturation at higher ϫ10Ϫ6 for 4.
[NH2OH], however, with high association constant values,
The pKa values obtained from kinetic experiments are in
and also the leveling off of rates at higher [Hϩ], are clear good agreement with that reported for NH2OH (pKa
ϭ
indications of some sort of association between the complex 6.0).[30] The composite parameters obtained from the above
and the reductant prior to the electron-transfer act. The rate equation are indicative of the similar kinetic reactivity
self-exchange rate constant of the NH2OH/NH2OHϩ cou- of all the complexes towards a particular species of the re-
ple is very low (5 ϫ 10Ϫ13 Ϫ1·sϪ1), with E0 ϭ ϩ0.42 V.[9,22] ductant. This is also reflected in the potential values of the
In the absence of any accurately determined potential value complexes, which fall in a narrow range. The hydrogen-
for the NH3OHϩ/NH2OHϩ couple the self-exchange rate bonding association between the complex and the reductant
constant may be taken roughly as that for the NH2OH/ species probably occurs through the phenoxo-O of the
NH2OHϩ couple.[9] The very low self-exchange rate con- Schiff base and the hydrogen atom of the reductant. The
stant value for the NH3OHϩ/NH2OHϩ couple and the low association constant (Q ϭ 5.27 ϫ 102) encountered in
poor reducing character of the protonated reductant disfa- complex 4 is probably a reflection of hindered hydrogen
vors the outer sphere electron-transfer path. The structure bonding involving the phenoxo-O and the reductant species.
of the complex also discounts the possibility of the forma-
Effect of Chloride Ions
tion of a seven-coordinated adduct with the MnIII center,
so formation of a hydrogen-bonded adduct (HA) is more
Salem[27] observed that rate of the reaction between hy-
droxylamine and MnIII-salen complex decreases consider-
reasonable to conceive. The EHMO calculations indicate
ably with increasing chloride ion concentration. It has been
claimed that this is due to the lowering of redox potential
as well as to the common ion effect, both of which shift the
equilibrium according to Equation (9) to the left-hand side.
that the lowest unoccupied molecular orbitals in all the
complexes are predominantly metal-based, with significant
contribution from the phenoxo ligand. Initial association
between the reductant and the complex could involve an
interaction with the metal center in addition to hydrogen
bonding with the ligand prior to the electron transfer, and
would be conducive for an inner sphere mechanism. The
following sequence of reactions may be proposed to explain
the observed kinetics.
NH3OHϩClϪ ǟǞ NH3OHϩ ϩ ClϪ
(9)
To examine any influence of chloride ions in our systems,
we varied its concentration at a fixed ionic strength but no
[ClϪ] effect was observed (Table 6). We also carried out
some kinetic experiments with hydroxylamine nitrate in
place of hydroxylamine hydrochloride; the rate constant
values were found to be unaffected. The lowering of redox
potential is quite understandable for MnIII-salen complex,
since the ClϪ ion can occupy the axial position of MnIII
and produce an effect on redox potential, but this is imposs-
ible in the hexacoordinate complexes under study.
(4)
(5)
(6)
Solvent Isotope Effect
The rate saturation kinetics and pH dependence on the
rate of the reaction indicate the formation of hydrogen-
bonded precursor complexes between the MnIII complexes
and the reductant prior to the electron-transfer process.
These facts prompted us to examine whether the electron-
transfer path is coupled with proton movement, as in the
case of tyrosine in PS II. We measured the rate constants in
aqueous medium partially substituted with D2O at a fixed
concentration of NH2OH and pH {[NH2OH] ϭ 6·10Ϫ4
mol·dmϪ3, pH ϭ 6.0, [buffer] ϭ 0.05 mol·dmϪ3 (phos-
phate), I ϭ 0.2 mol·dmϪ3 (NaCl) and temp. 25 °C}. The
observed rate constants in pure water are almost the same
as those in a 90% D2O/water mixture (Table 6), which rules
out proton-coupled electron-transfer paths in this study.
The termination of the free radicals for the consumption
of a 1:1 ratio was reported to occur as in Equation (7).
(7)
The rate law derived from the above sequence of reac-
tions can be given as:
k
ox ϭ (kЈ1·[Hϩ] ϩ kЈ2·Ka)/(Ka ϩ [Hϩ])
(8)
where kox ϭ kobsd./[NH2OH], kЈ1 ϭ k1Q1, and kЈ2 ϭ k2Q2.
The derived kinetic parameters for all the complexes are:
kЈ1
dm3·molϪ1·sϪ1 and Ka ϭ (3.61Ϯ0.80) ϫ10Ϫ6 for 1; kЈ1 ϭ
13.0Ϯ1.5 dm3·molϪ1·sϪ1, kЈ2 ϭ 1.16Ϯ0.42 dm3·molϪ1·sϪ1
and Ka ϭ (1.03Ϯ0.42) ϫ10Ϫ6 for 2; kЈ1 ϭ 15.29Ϯ1.5
ϭ , kЈ2 ϭ 0.95Ϯ0.29
20.32Ϯ4.5 dm3·molϪ1·sϪ1
Conclusions
,
The synthesis and characterization of four new manga-
dm3·molϪ1·sϪ1, kЈ2 ϭ 0.76Ϯ0.39 dm3·molϪ1·sϪ1, and Ka ϭ nese() complexes with hexadentate Schiff base ligands
Eur. J. Inorg. Chem. 2003, 1540Ϫ1547
1545

A. Panja, N. Shaikh, S. Gupta, R. J. Butcher, P. Banerjee
[Mn(5-Cl-sal-N-1,5,8,12)]ClO4 (2): Yield: 4.94 g, 82%.
C22H26Cl3MnN4O6 (603.8): calcd. C 43.74, H 4.31, N 9.28; found
C 43.63, H 4.49, N 9.19%. FTIR (KBr, cmϪ1): ν(CϭN) 1624,
ν(CϪO) 1281, ν(ClO4Ϫ) 1088 and 626. µeff 4.89 BM.
FULL PAPER
with N4O2 donor sets Ϫ derived from N,NЈ-bis(3-amino-
propyl)ethylenediamine and salicylaldehyde, or salicylal-
dehyde derivatives Ϫ are reported. Two of them have been
characterized by X-ray crystallography, which shows two
phenolic oxygen atoms in trans configurations and the two
coordinating ONN ligand halves facial. Both the lattices are
stabilized through extended hydrogen bonding. The ligand
contribution both to the HOMO and to the LUMO is re-
flected in the change in the redox potentials as well as the
shifting of UV/Vis spectra with substitution in benzene
ring. The electron-transfer reactivities of all the complexes
towards hydroxylamine in the 5.5Ϫ8.0 pH range are almost
identical, as reflected in the evaluated kinetic parameters. It
is also notable that the hydrogen-bonding association be-
tween the manganese() complexes and the reductant spec-
ies probably occurs through the phenoxo-O of the Schiff
base and the hydrogen atom of hydroxylamine. Chloride ion
has no effect on the electron-transfer act, which contrasts
with Salem’s observations[27] for an almost identical reac-
tion with hydroxylamine. Since the estimated bond dis-
sociation energy for the NϪH of hydroxylamine is similar
to that of the OϪH in tyrosine, an attempt was made to
examine the possibility of proton movement during the
electron-transfer act. The results of solvent isotope effect
studies with variable mixtures of D2O and water do not
indicate any proton-coupled electron-transfer path.
[Mn(5-Br-sal-N-1,5,8,12)]ClO4(3):
Yield:
5.54 g,
80%.
C22H26Br2ClMnN4O6 (692.7): calcd. C 38.13, H 3.75, N 8.09;
found C 38.23, H 3.76, N 7.89%. FTIR (KBr, cmϪ1): ν(CϭN) 1627,
ν(CϪO) 1283, ν(ClO4Ϫ) 1088 and 627. µeff 4.94 BM.
[Mn(3-methoxy-sal-N-1,5,8,12)]ClO4·H2O(4): Yield: 5.08 g, 83%.
C24H34ClMnN4O9 (612.9): calcd. C 47.02, H 5.55, N 9.14; found
C 47.13, H 5.49, N 9.01%. FTIR (KBr, cmϪ1): ν(CϭN) 1621,
ν(CϪO) 1281, ν(ClO4Ϫ) 1082 and 627, ν(OϪH) 3550. µeff 4.92 BM.
Physical Measurements: Microanalysis (CHN) was performed in a
PerkinϪElmer 240C elemental analyzer. Magnetic susceptibility
measurement was carried out on a PAR 155 vibrating sample mag-
netometer. The EPR spectra were obtained on a Varian model 109
E-line X-band spectrometer equipped with a low-temperature
quartz Dewar for measurements at 77 K. pH measurements were
made with a Systronics digital pH meter (model 335, India). The
observed pH was corrected by using the relationship, pH ϭ
Ϫlog[Hϩ] ϭ pHobsd. Ϯ 0.02. IR spectra were obtained on a Nicolet,
MAGNA-IR 750 spectrometer with samples prepared as KBr pel-
lets. Cyclic voltammetry (scan rate of 0.05 V·sϪ1) was performed
at a platinum electrode with an EG&G PARC electrochemical
analysis system (model 250/5/0) in acetonitrile under dry nitrogen
atmosphere in conventional three-electrode configurations.
Kinetic Measurements: Spectral and kinetic measurements were
performed in an UV/Vis spectrophotometer (UV-2100, Shimadzu,
Japan) with thermostatted cell compartments. The disappearance
of the complex peak was monitored at 360 nm as a function of
time. All the measurements were made under pseudo first-order
conditions with use of an excess of hydroxylamine and a constant
ionic strength of 0.2 mol·dmϪ3 (NaCl) with 0.05 mol·dmϪ3 phos-
phate buffer at 30 °C. The corresponding rate constants were evalu-
ated by means of a suitable nonlinear curve fit program with data
taken for at least three half-lives of the reactions. Every rate con-
stant reported here represents the mean value of at least three de-
terminations that fall within Ϯ5%.
Experimental Section
Materials: Hydroxylamine hydrochloride, N,NЈ-bis(3-amino-
propyl)ethylenediamine, salicylaldehyde, 5-bromosalicylaldehyde,
5-chlorosalicylaldehyde, and o-methoxysalicylaldehyde were of
analytical reagent grade (Aldrich) and were used without further
purification. All other chemicals Ϫ manganese perchlorate hexa-
hydrate, NaCl, NaH2PO4, acetonitrile, methanol, sodium
hydroxide, and hydrochloric acid Ϫ were of reagent grade. Solvent
isotope effects were examined with deuterium oxide, 99 atom% D
(Aldrich).
Crystal Structure Determination and Structural Refinements of
Complexes 1 and 4: Single crystals of 1 were grown from aceto-
nitrile/ethanol solution, and single crystals of 4 were obtained from
acetonitrile/methanol solution. Each crystal was then mounted on
an automatic Bruker P4 diffractometer equipped with graphite
Syntheses: All the complexes reported here were synthesized by air
oxidation of solutions made up from Mn(ClO4)2·6H2O and the
Schiff base, by the same general procedure. A typical preparation
is outlined below. (Caution! Although no problems have been enco-
untered in this work, perchlorates are potentially explosive and
should be handled with care and only in small quantities).
˚
monochromated Mo-Kα radiation (λ ϭ 0.71073 A). Unit cell di-
mensions and intensity data were measured at 296 K. The structure
was solved by direct methods and refined by full-matrix, least-
squares based on F2 with anisotropic thermal parameters for non-
hydrogen atoms by use of Bruker SHELXTL (data reduction),
SHELXS[31] (structure solution), and SHELXL[32] (structure re-
finement). The hydrogen atoms were included in structure factor
calculations in their idealized positions. Information concerning
crystallographic data collection and refinement of the structures is
compiled in Table 8.
[Mn(sal-N-1,5,8,12)]ClO4 (1): N,NЈ-Bis(3-aminopropyl)ethylenedi-
amine (1.74 g, 0.01 mol) was added to an ethanol (30 cm3) solution
of salicylaldehyde (2.44 g, 0.02 mol). The yellowish orange solution
was brought to reflux for 1 h and then allowed to cool to room
temperature, which was followed by the dropwise addition of
Mn(ClO4)2·6H2O (3.61 g, 0.01 mol) dissolved in ethanol (20 cm3).
The resulting dark brown solution was heated at reflux for 30 min.
On subsequent cooling to room temperature, a microcrystalline CCDC-191275 and -191276 contain the supplementary crystallo-
compound separated out and was collected by filtration, washed
with cold ethanol and diethyl ether, and dried under vacuum. Yield:
graphic data for this paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
4.54 g, 85%. C22H28ClMnN4O6 (534.9): calcd. C 49.35, H 5.23, N Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
10.46; found C 49.21, H 5.26, N 10.28%. FTIR (KBr, cmϪ1): ν(Cϭ
N) 1625, ν(CϪO) 1280, ν(ClO4Ϫ) 1095 and 627. µeff 4.91 BM.
bridge CB2 1EZ, UK; Fax: (internat.) ϩ44Ϫ1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
1546
Eur. J. Inorg. Chem. 2003, 1540Ϫ1547

Manganese(III) Complexes with Hexadentate (N4O2) Schiff Base Ligands
FULL PAPER
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[8]
Table 8. Crystal data and structure refinement for complexes 1
and 4
[9]
Complex
1
4
[10]
[11]
Empirical formula
Molecular mass
Crystal system
Space group
C22H28ClMnN4O6
534.87
orthorhombic
P212121
7.800(3)
17.359(8)
17.891(6)
90.00
2422.5(17)
4
296(2)
0.700
3957
3755 (0.0565)
C24H34ClMnN4O9
612.94
monoclinic
P21/c
8.056(6)
20.501(10)
16.737(9)
101.831(6)
2705.5(3)
4
296(2)
0.646
6871
6222(0.013)
[12]
˚
a [A]
˚
b [A]
˚
[13]
[14]
c [A]
β [°]
3
˚
V [A ]
Z
[15]
[16]
[17]
T [K]
µ(Mo-Kα) [mmϪ1
]
Reflections collected
Independent
reflections (Rint
Final R1 wR2
[I Ͼ 2σ(I)]
[18]
[19]
)
0.051 0.121
0.046 0.116
[20]
[21]
[22]
[23]
[24]
[25]
[26]
Acknowledgments
A. P. and S. G. gratefully acknowledge financial support from the
Council of Scientific and Industrial Research (New Delhi) in the
form of research fellowships. P. B. also acknowledges the Council
for a research project.
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[I02605]
Eur. J. Inorg. Chem. 2003, 1540Ϫ1547
1547