Synthesis, characterization, reactivity, and electrochemical studies of manganese(IV) complexes of bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone

Article abstract of DOI:10.1080/00958972.2010.542238

Manganese(IV) complexes [Mn^IV(npah)(H₂O)₂] (1) and [MnIV(npah)(A)₂] _nH₂O (where A = py (2), 2-pic (3), 3-pic (4), 4-pic (5)) and MnIV(npah)(NN)] (NN = bpy (6) and phen (7)) have been synthesized from bis(2-hydroxy-1-naphthaldehyde) adipoyldihydrazone in methanol. The composition of the complexes has been established by elemental analyses. Complex 3 has been characterized by mass spectral data also. Structural assessment of the complexes has been based on data from molar conductance, magnetic moment, electronic, electron paramagnetic resonance, and infrared (IR) spectral studies. Molar conductances of the complexes in DMSO suggest non-electrolytes. Magnetic moment and EPR studies suggest +4 oxidation state for manganese in these complexes. Electronic spectral studies suggest six-coordinate octahedral geometry around the metal ions. IR spectra reveal that H₄npah coordinates to the metal in enol form. Reaction of the complexes with benzyl alcohol and SO₂ has been investigated. Cyclic voltammetric studies of the complexes have also been carried out.

Full text of DOI:10.1080/00958972.2010.542238

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Synthesis, characterization,
reactivity, and electrochemical
studies of manganese(IV)
complexes of bis(2-hydroxy-1-
naphthaldehyde)adipoyldihydrazone
a
a
a
a
R.A. Lal , D. Basumatary , O.B. Chanu , A. Lemtur , M.
a
b
c
Asthana , A. Kumar & A.K. De
a
Department of Chemistry, North-Eastern Hill University, Shillong
93022, Meghalaya, India
7
b
Department of Chemistry, Faculty of Science and Agriculture,
The University of the West Indies, St. Augustine, Trinidad and
Tobago, West Indies
c
Department of Science and Humanities, Tripura Institute of
Technology, Natsingarh 799009, Tripura, India
Published online: 10 Jan 2011.
To cite this article: R.A. Lal , D. Basumatary , O.B. Chanu , A. Lemtur , M. Asthana , A.
Kumar & A.K. De (2011) Synthesis, characterization, reactivity, and electrochemical studies of
manganese(IV) complexes of bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone, Journal of
Coordination Chemistry, 64:2, 300-313, DOI: 10.1080/00958972.2010.542238
To link to this article: http://dx.doi.org/10.1080/00958972.2010.542238
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Journal of Coordination Chemistry
Vol. 64, No. 2, 20 January 2011, 300–313
Synthesis, characterization, reactivity, and
electrochemical studies of manganese(IV) complexes of
bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone
R.A. LAL*y, D. BASUMATARYy, O.B. CHANUy, A. LEMTURy,
M. ASTHANAy, A. KUMARzand A.K. DEx
yDepartment of Chemistry, North-Eastern Hill University,
Shillong 793022, Meghalaya, India
zDepartment of Chemistry, Faculty of Science and Agriculture,
The University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies
xDepartment of Science and Humanities,
Tripura Institute of Technology, Natsingarh 799009, Tripura, India
(
Received 18 October 2010; in final form 19 October 2010)
IV
Manganese(IV) complexes [Mn (npah)(H
IV
(1) and [Mn (npah)(A)
2
O)
2
]
2
] ꢀnH
2
O (where
IV
A ¼py (2), 2-pic (3), 3-pic (4), 4-pic (5)) and Mn (npah)(NN)] (NN ¼bpy (6) and phen (7))
have been synthesized from bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone in methanol.
The composition of the complexes has been established by elemental analyses. Complex 3 has
been characterized by mass spectral data also. Structural assessment of the complexes has been
based on data from molar conductance, magnetic moment, electronic, electron paramagnetic
resonance, and infrared (IR) spectral studies. Molar conductances of the complexes in DMSO
suggest non-electrolytes. Magnetic moment and EPR studies suggest þ4 oxidation state for
manganese in these complexes. Electronic spectral studies suggest six-coordinate octahedral
geometry around the metal ions. IR spectra reveal that H
form. Reaction of the complexes with benzyl alcohol and SO
4
npah coordinates to the metal in enol
has been investigated. Cyclic
2
voltammetric studies of the complexes have also been carried out.
Keywords: ‘‘EPR of Mn (IV)’’ complexes; Bis(2-hydroxy-1-naphthaldehyde)adipoyldihydra-
zone; Magnetic moment; Electrochemical studies; Spectral studies
1. Introduction
Manganese shares an essential role in electron and oxygen transfer processes in nature
with iron and copper [1]. Manganese-containing enzymes are involved in a number of
fundamental biological processes, such as photosynthesis [2], the regulation of oxidative
stress [3], and bacterial virulence [4] including delignification [2] and superoxide
dismutase [3].
Although manganese occurs in some of the enzymes as Mn(II), in other biological
systems, it occurs in higher oxidation states, such as Mn(III) and Mn(IV) like
*
Corresponding author. Email: ralal@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
DOI: 10.1080/00958972.2010.542238

Manganese(IV) hydrazone complexes
301
oxygen-evolving complex [2, 3] and as Mn(III) in manganese superoxide dismutase [4].
Further, manganese is found in a variety of enzymes as mononuclear [5], binuclear [6],
or multinuclear [7]. High oxidation state manganese moieties have long been accepted
as the reactive intermediates in reactions, such as water oxidation for oxygen evolution.
Oxomanganese(V) reactive units are believed to serve as key intermediates in
oxygenation mediated by synthetic manganese complexes [8]. The oxygen-evolving
center of photosystem II, where water is split into dioxygen and protons, remains a
mysterious reaction [9, 10]. Its active site is proposed to consist of four manganese ions
in a ‘‘3þ1’’ tetranuclear complex fashion in which a trimer is linked to a monomer [11].
X-ray absorption and electron paramagnetic resonance (EPR) spectroscopy have
established that the Mn ions are present in the þ2, þ3, and þ4 oxidation states [12, 13].
Polynuclear Mn-oxo complexes have been studied as models for this active site [14].
Further, higher oxidation state manganese complexes serve as oxidizing agents,
catalysts, and electrocatalysts for oxidation of alcohols, ethers, and water [15]. A
number of reports [16] deal with Mn(III) and Mn(IV) complexes as oxidizing agents.
Higher oxidation oxo-bridged dinuclear manganese complexes have been reported in
aliphatic and aromatic C–H activation and oxygen insertion reactions [11]. This has
attracted attention to trinuclear, binuclear, and mononuclear complexes of manganese
in þ3 and þ4 oxidation states as structural and functional models for enzymes as well
as for a variety of catalytic reactions.
Schiff base and oxo-bridged manganese complexes have been of particular interest
because they seem to mimic the coordination environment of Mn in PSII which does
not contain a porphyrinic system and is thought to be made up of O and N donors only
[
17, 18]. An interesting feature associated with ligands containing phenolate/naptholate
donors is their ability to stabilize higher oxidation state and their tendency to form
phenyl radical. Dihydrazones derived from condensation of acyldihydrazines,
aroyldihydrazines, and pyridoyldihydrazines with o-hydroxy aromatic aldehydes and
ketones are potential polyfunctional ligands possessing as many as eight or nine
bonding sites, such as phenolic/naptholic–OH, azomethine, secondary-NH, and
carbonyl oxygen in duplicate and pyridyl nitrogen in their molecular skeleton [19,
2
0]. They are capable of giving mononuclear and polynuclear complexes [20].
A survey of literature reveals that although work on metal complexes of
monohydrazide-based ligands and their Schiff bases has been carried out in some
detail [21–25], metal complexes of dihydrazones are quite meager [19, 20, 26–31].
Although some Cu(II), Ni(II), Co(II) [19, 28, 29], uranyl complexes [30], and a few zinc-
uranyl heterobimetallic complexes [31] of disalicylaldehyde adipoyldihydrazone have
been synthesized and characterized, those of bis(2-hydroxy-1-naphthaldehyde)adipoyl-
dihydrazone are absent. Moreover, the metal complexes of dihydrazones in higher
oxidation state are almost non-existent. In view of the significant role played by
manganese in biological systems and its compounds in higher oxidation states in
catalysis and the meager amount of work done on its metal complexes with
dihydrazones containing bulky fragments, we have set a goal of synthesizing
manganese(IV) and manganese(V) complexes either from preformed dihydrazones
containing bulky fragments in their molecular skeleton or in situ [27]. The ligand bis(2-
hydroxy-1-naphthaldehyde)adipoyldihydrazone (H npah) is a multidentate ligand
4
containing two hydrazones joined together through four intervening methylene
groups (figure 1). Further, it contains bulky naphthyl fragment in its molecular
skeleton and is capable of giving complexes possessing discrete molecularity.

3
02
R.A. Lal et al.
O
H
C
H2
C
H
N
C
N
H
H C
2
O
O
H
H C
2
H
N
H C
C
N
2
C
H
O
4
Figure 1. Structure of bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone (H npah).
This article reports the synthesis, characterization, reactivity, and electrochemical
studies of manganese(IV) complexes derived from reaction of bis(2-hydroxy-1-
naphthaldehyde)adipoyldihydrazone in alkaline methanol.
2
2
. Experimental
.1. Materials and methods
The metal salts Mn(OAc) ꢀ4H O, diethyladipate [(CH ) (COOEt) ], hydrazine hydrate
2
2
2 4
2
(
N H ꢀH O), 2-hydroxy-1-naphthaldehyde, and KOH were E-Merck or equivalent
2
4
2
grade reagents. Adipoyldihydrazone was prepared by reacting diethyl adipate (10.0 g,
9.50 mmol) with hydrazine hydrate (4.95 g, 99.00 mmol) and crystallizing from
4
methanol. Bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone was prepared by
reacting a solution of adipoyldihydrazone (10.37 mmol) in ethanol with 2-hydroxy-1-
naphthaldehyde (25.93 mmol) in hot ethanol. The yellow precipitate obtained on
ꢁ
cooling the solution was thoroughly washed with ethanol and air dried (m.p. 4300 C).
2
2
.2. Preparation of the complexes
IV
.2.1. Preparation of [Mn (naph)(H O) ] (1). A methanol solution (20 mL) of
2
2
Mn(OAc) ꢀ4H O (0.51 g, 2.08 mmol) was added to a suspension of dihydrazone
2
2
(
5
1.00 g, 2.07 mmol) in methanol (20 mL) accompanied by vigorous stirring for 10 min at
ꢁ
0 C. To the resulting yellow solution, KOH (0.47 g, 8.21 mmol) in methanol (20 mL)
was added accompanied by gentle stirring. The resulting solution was stirred for
another half an hour. The solution was then kept for 2 days, which precipitated a
shiny black compound. This was suction filtered, washed with methanol, and air dried.
ꢁ
The complex was finally dried in an electric oven maintained at 90 C (yield: 60%).
IV
IV
2
.2.2. Preparation of [Mn (naph)(A) ] nH O and [Mn (naph)(NN)] (where
2 2
.
A^pyridine (py, 2); 2-picoline (2-pic, 3); 3-picoline (3-pic, 4); 4-picoline (4-pic, 5);
0
NN^2,2 -bipyridine (bpy, 6); 1-10 phenanthroline (phen), (7); (n^0,1)). These
complexes were also prepared by following essentially the above method and adding

Manganese(IV) hydrazone complexes
303
pyridine bases to the reaction mixtures obtained by mixing Mn(OAc) ꢀH O and
2
2
H naph maintaining Mn(OAc) ꢀ4H O : H naph : KOH molar ratio at 1 : 1 : 4 : 10 in
4
2
2
4
0
case of pyridine bases and 1 :1 : 4 : 3 in case of 2,2 -bipyridine and 1,10-phenanthroline.
Yield: 70% (2), (3); 67% (4); 58% (5); 50% (6); and 48% (7)).
(
2
.3. Elemental analyses
Determination of manganese was done following the standard procedure [32]. C, H,
and N were determined microanalytically. The molar conductances of the complexes
ꢂ3 ꢂ1
(
10 mol L in DMSO) were measured on a Systronics Direct Reading Conductivity
meter-303 with dip-type conductivity cell at room temperature. Room temperature
magnetic susceptibility measurements were carried out on a Sherwood Scientific
Magnetic Susceptibility Balance. Experimental magnetic susceptibility values have been
corrected for diamagnetism by the procedures given by Figgis and Lewis [33]. Infrared
(
4
IR) spectra were recorded on a Bomen DA-8FT-IR spectrophotometer from 450 to
ꢂ1
000 cm in KBr disks. Electronic spectra were recorded from 200 to 1000 nm in
DMSO on a Perkin-Elmer Lambda 25 UV-Vis spectrophotometer. EPR spectra of the
complexes as powders as well as in solution were recorded at X-band frequency on a
Varian E-112 E-line century series ESR spectrometer using TCNQ (g ¼2.0027) as an
internal field marker. A FAB mass spectrum of one of the complexes was recorded on a
JEOLSX102/DA-6000 mass spectrometer/data systems using Argon/Xenon (6 kV,
1
0 mA) as FAB gas. Nitrobenzyl alcohol was used as the matrix. Cyclic voltammetric
measurements of the compounds in DMSO were done using a CH Instruments’
Electrochemical analyzer under nitrogen. The electrolytic cell comprised of three
electrodes, the working electrode was a Pt disk, while the reference and auxiliary
electrodes were Ag/AgCl separated from the sample solution by a salt bridge;
ꢂ1
0
.1 mol L TBAP was used as the supporting electrolyte.
3
. Results and discussion
The complexes with their color, decomposition point, and analytical and magnetic
IV
moment data given in table 1 are of composition [Mn (naph)(A) ] ꢀnH O and
2
2
IV
Mn (naph)(NN)] (where A ¼H O (1); py (2); 2-pic (3); 3-pic (4); 4-pic (5); NN ¼bpy
[
(
2
6); phen (7); (n ¼0, 1)). All the complexes are shiny black, air stable, and decompose
ꢁ
above 300 C without melting, indicating that the metal–ligand bonds have ionic
character. The complexes were insoluble in water and common organic solvents such as
ethanol, acetone, chloroform, carbon tetrachloride, ether, and benzene, sparingly
soluble in methanol, acetonitrile, and dichloromethane, and completely soluble in
DMSO and DMF. The solutions are black brown in color. Complexes 2, 4, and 5 show
ꢁ
weight loss corresponding to one water molecule at 110 C suggesting water in the lattice
ꢁ
structure, while 1 shows loss corresponding to two water molecules at 180 C, indicating
ꢁ
they are coordinated to the metal. Complexes 2–5 show weight loss at 220 C
corresponding to two pyridine or substituted pyridine molecules; 6 and 7 show weight
loss corresponding to one bipyridine and one 1,10-phenanthroline, respectively.

3
04
R.A. Lal et al.

Manganese(IV) hydrazone complexes
305
An effort was undertaken to crystallize the complexes in various solvent systems
under different experimental conditions to establish their molecularity and structure by
X-ray crystallography. Both saturated and dilute solutions of the complexes in various
solvent systems such as DMSO, DMF, DMSO–CH CN, DMSO–CH Cl , and DMF–
3
2
2
CH Cl each were kept for half, 1, and 2 months at ambient temperature to grow
2
2
ꢁ
ꢁ
ꢁ
crystals. Also, solutions were gently evaporated at 40 C, 50 C, and 60 C in an electric
oven to promote crystal growth. Solvent diffusion methods were also used to grow
crystals. Unfortunately, in all our efforts, only amorphous compounds precipitated
preventing analysis by X-ray crystallography.
3
.1. Mass spectra
Complex 3 was characterized by FAB mass spectroscopy as a representative sample.
The mass spectrum shows a signal at m/z 536, probably from the formation of
þ
þ
[Mn(npah)] . [Mn(npah)] results from loss of both 2-picoline molecules. The mass
spectrum of the complex shows its monomeric nature.
3
.2. Molar conductance
ꢂ1
2
The complexes have molar conductance values in the range 1.9–3.7 ꢀ cm mol in
DMSO at 10 mol L , indicating non-electrolytes [34].
ꢂ1
ꢂ3
ꢂ1
3
.3. Magnetic moment
The ꢀB values for the complexes lie in the range 3.83–4.30 BM, consistent with
3
manganese(IV) complexes in a d high-spin electronic configuration [17]. The ꢀ values
B
rule out the possibility of spin–spin coupling in the solid state between unpaired
electrons belonging to different Mn(IV) centers in the structural unit of the complexes.
3
.4. Electronic spectra
The free ligand shows three bands at 290 (2041), 313 (2800), and 363 (1352) in DMSO,
assigned to intra-ligand n !ꢂ* and ꢂ !ꢂ* transitions. The band at 363 nm is
characteristic of naphthyl of dihydrazone as reported in several monoacylhydrazones
[
25]. Bands at 290 and 313 nm in the free ligand are shifted by 20–25 and 45–55 nm in
the complexes, 305–315 and 356–368 nm, respectively. Such a big red-shift of the ligand
bands in spectra of the complexes indicates strong bonding between metal and ligand.
The ligand band at 363 nm remains almost unshifted in the complexes and is merged
with the band in the region 356–368 nm. In addition to the intra-ligand bands, the
complexes show new bands at 430–470 and 610–630 nm, respectively. For octahedral
4
4
manganese(IV) complexes, three spin-allowed d–d transitions
4
A !T ,
2
g
2g
4
4
4
A !T (F), and A !T g(P) are expected. The manganese(IV) complex,
2
g
1g
2g
1
2
ꢂ
4
4
[
MnF ] , shows absorptions at 360 and 450 nm assignable to A !T (F) and
6
2g
1g
4
4
A !T (F) transitions, respectively [35, 36], while the third band arising from
2
g
2g
4
4
A !T (F) is obscured either by charge transfer or ligand bands. Sawyer et al. [37]
2
g
1g
and Pecoraro et al. [38] reported a band at 545–575 nm in mononuclear tris(glucarato),

3
06
R.A. Lal et al.
tris(gluconato), tris(sorbitolato) manganese(IV) complexes and manganese(IV) com-
plexes of 1-hydroxy-2-(salicylideneamino)ethane and related hydroxyl ligand with
3
ꢂ1
ꢂ1
molar extinction coefficients in the 1508–2380 dm mol cm region. They assigned
this band to the d–d transition. The higher value of molar extinction coefficient was
attributed to the presence of adjacent charge-transfer band which contributed to its
intensity. Cooper et al. [36] reported an intense band at 585 nm with molar extinction of
3
ꢂ1
ꢂ1
ꢃ4500 dm mol cm in manganese(IV) complexes derived from catecholato ligands.
They assigned this band to a catechol !Mn(IV) LMCT band on the basis of its high
molar extinction coefficient. Okawa et al. [39] reported a band at ꢃ650 nm with molar
3
ꢂ1
ꢂ1
extinction coefficient of 2090–3020 dm mol cm in manganese(IV) complexes
derived from 2-substituted (salicylideneamino) phenol, assigned to charge-transfer
transition from phenolic oxygen to manganese(IV). The molar extinction coefficient of
3
the band at 430–470 nm is 458–3280 dm mol cm . These values are in the same
ꢂ1
ꢂ1
4
region in which the d–d transition T
4
A2g in manganese(IV) complexes has been
assigned albeit with some charge-transfer contribution from ligand-to-metal [37–41].
2
g
4
Hence, this band is assigned to arise from T
4
A2g transition for manganese(IV).
2
g
However, slightly higher values of molar extinction coefficients of this band than those
expected for d–d bands in the manganese(IV) complexes may be attributed to arise from
contribution due to LMCT bands. The band observed at 610–630 nm has molar
3
ꢂ1
ꢂ1
extinction coefficient of 3200–5000 dm mol cm ; almost the same ꢃmax values have
been reported by Cooper et al. [36] and Okawa et al. [39] in manganese(IV) complexes
and have been attributed to LMCT transitions. Hence, it is suggested that the band in
the region 610–630 nm in the present complexes arises from LMCT transitions, most
probably, from naphtholate oxygen to the metal.
3
.5. Electron paramagnetic resonance
Only 1 and 5–7 have been analyzed by EPR spectroscopy. These complexes exhibit
highly anisotropic EPR spectra in polycrystalline state as well as in the glassy state at
LNT. EPR spectra for 1 in the powder form and for 6 in powder as well as in solution at
LNT are shown in Supplementary material as representative examples. Complexes 1
and 5 show two resonances with g ꢄ4 and g ꢄ2.0 in the solid state and in solution.
r
k
The former resonance is more intense in the solid state than the latter, but in the glassy
state both resonances are almost of the same intensity. Complexes 6 and 7 also show
resonances at g ꢄ5.0 and g ꢄ2.0. The signal at g ꢄ5.0 is of low intensity compared
r
k
r
to the signal at g in both crystalline and glassy states. For both complexes, the
k
polycrystalline phase and magnetically dilute glassy state spectra are well-resolved, with
5
5
the hyperfine coupling (S ¼3/2) of Mn nucleus well-resolved in the glassy state [41, 42]
with six clean hyperfine lines at g ꢄ2, suggesting that they retain the same structures in
5
5
solution and solid state. The EPR signal at g ꢄ2 shows Mn hyperfine splitting equal
to 80 G in 6 and 7 nearer to Mn(IV) hyperfine splitting constant of 77 G than Mn(II)
hyperfine coupling constant of 87 G.
3
4
In a crystal field of strict octahedral symmetry, a d ion has A ground state leading
2
g
to an isotropic resonance at g ꢄ2.0. Distortion and spin-orbit coupling split the ground
2
2
state quartet into two Kramer’s doublets separated by 2(D þE ), where D and E are,
respectively, axial and rhombic zero field splitting parameters [43]. The complexity of
3
EPR spectra of a d ion in an axial field (E/D ¼0) is dependent on the magnitude of

Manganese(IV) hydrazone complexes
307
zero-field splitting parameters [44]. Two limiting cases are when the value of axial zero-
field splitting parameter 2D is either much larger than or much smaller than hꢄ
ꢂ1
(
0.31 cm X-band frequencies). In the first case, when 2D is much larger than hꢄ, the
feature at low field is strong and g ꢄ2 component is weak. This is the observation for
tris(catecholato) manganese(IV) [45] and tris(sorbitolato) manganese(IV) complexes
[
46]. In the second case, when D is small, the g ꢄ2 component should dominate with
relatively weak signals in the low-field region. This is observed for tris(thiohydrox-
amato) manganese(IV) [47] and thiocarbamato complexes [48]. Spectra for 1 and 5 do
not fit either limit. Instead, they exhibit marked rhombic distortion. Complexes 6 and 7
spectra correspond to a Mn(IV) in octahedral field with axial distortion 2D ꢅhꢄ. EPR
spectra reveal that the ligand provides effective electronic environment for the metal
that is highly distorted in the ground state in 1 and 5 but only slightly distorted in the
ground state in 6 and 7. The distortion is sensitive to geometrical orientation of the
donors. The effective environment of Mn(IV) lies close to ideally pseudo-octahedral
environment.
The EPR spectra of 1, 6, and 7 in solution at LNT remain essentially the same as in
the solid state, but 5 is entirely changed on dissolution in DMSO, attributed to
replacement of water and 4-picoline by DMSO. While water and DMSO provide the
same donor atom (oxygen) in bonding to the metal center which leads to almost
unchanged EPR spectral features of 1 in solution state as compared to that in the solid
state. The replacement of 4-picoline by DMSO molecules in 5 significantly changes the
coordination environment around the metal which changes the spectral features
significantly. Essential EPR spectral features of 6 and 7 remain the same in the solid
state and DMSO solution, indicating that the solvent molecules are unable to replace
coordinated bipyridine and 1,10-phenanthroline. The contrasting EPR spectral features
of 1 and 5 as compared to those of 6 and 7 suggest that nitrogen donors from bipyridine
and 1,10-phenanthroline in 6 and 7 occupy cis-position while donors from co-ligands in
1
and 5 occupy trans-positions.
3
.6. IR spectra
Some structurally significant IR bands along with the assignments for free dihydrazone
and complexes are given in table 2. Comparison of IR spectra of the complexes with
that of the free ligand suggests that the dihydrazone is coordinated to the metal in enol
form in all complexes.
ꢂ1
The uncoordinated ligand shows a strong band at 1666 cm assignable to ꢄ(C¼O)
which disappears in the complexes, indicating collapse of amide structure and
coordination of the ligand in enol form. This is also corroborated from observation
ꢂ1
that ꢄ(NH) at 3224 cm in uncoordinated dihydrazone is absent in IR spectra of the
ꢂ1
complexes. The ligand shows strong to medium intensity bands at 1630 and 1593 cm
assigned to stretching vibration of azomethine [19]. On an average, these bands shift to
ꢂ1
lower frequency by 3–11 cm in complexes. The free dihydrazone shows a weak band
ꢂ1
at 1530 cm from mixed contributions of amide II and ꢄ(C–O) (naphtholate). In IR
ꢂ1
spectra of the complexes, this band is replaced by a strong band at 1503–1560 cm
ꢂ
assigned to stretching vibration of newly created NCO group produced as a result of
deprotonation of the enolized carbonyl. In the dihydrazone, a medium intensity band
ꢂ1
at 1281 cm
is similar to other ligands containing phenol [49] and assigned to

3
08
R.A. Lal et al.

Manganese(IV) hydrazone complexes
309
ꢂ1
ꢅ(C–O) (naptholic). This band shifts to higher frequency by 2–18 cm
ꢂ1
in the
complexes, 1283–1299 cm , indicating bonding of naptholate oxygen to the metal.
ꢂ1
Free pyridine bases absorb at ꢃ604 cm due to in-plane ring deformation [50]. In 3–5,
ꢂ1
a new medium to weak band is observed at 650–660 cm assigned to in-plane ring
deformation of pyridine bases indicating their coordination. Complex 7 also shows a
ꢂ1
weak band at ꢃ660 cm assigned to in-plane ring deformation of 1,10-phenanthroline
ꢂ1
indicating coordination. In metal complexes, new bands at 509–526 and 353–375 cm
are assigned to ꢄ(M–O) (naptholate) and ꢄ(M–N) indicating coordination of naptholate
ꢂ1
oxygen and azomethine nitrogen, respectively [50]. New bands at 274–280 cm are
assigned to ꢄ(M–N) due to coordinated pyridine bases/bipyridine/1,10-phenanthroline.
Complexes 2–5 show a new non-ligand, medium to strong band at 430–440 cm
assigned to ꢄ(M–O)(enolate), indicating coordination of carbonyl oxygen to
manganese(IV) [50].
ꢂ1
3
.7. Reactivity
When 1 and 3 were allowed to react with benzyl alcohol either under ambient condition
ꢁ
or at 50 C for 4 h in CH OH under stirring, no reaction was observed. The unreacted
3
components were recovered from the reaction mixture by filtration and fractional
distillation. IR spectra and the magnetic moment of the residue matched with those of
the original complexes, signifying that the complexes did not react. The boiling point of
ꢁ
the last fraction matched with that of the reported value for benzyl alcohol, b.p. 203 C
ꢁ
(202–205 C in the literature) [51]. Dioxygen gas was also passed through the above
reaction mixture without any reaction. When SO2 gas was passed through the
suspension of the complexes in CH OH, a vigorous exothermic reaction occurred. The
3
color of the solution changed from shining black to light brown to colorless, giving a
white precipitate. The precipitate was recovered by filtration and washed with
1
methanol. Its melting point, IR and H-NMR matched with that of the uncoordinated
2
ꢂ
dihydrazone. The presence of SO4 in the filtrate was confirmed by chemical tests. The
2þ
2
4
ꢂ
results indicated that SO was oxidized to SO ion with reduction to Mn , evident
2
from the light pink color of the filtrate. The dihydrazone did not suffer any degradation
with SO2.
3.8. Cyclic voltammetry
Cyclic voltammograms (CV) of 2 mmol solutions of the ligand and complexes in DMSO
ꢂ1
under nitrogen with 0.1 mol L TBAP as supporting electrolyte were run at a scan rate
ꢂ1
of 100 mV s . The free ligand exhibits a single quasi-reversible redox couple at þ0.27 V
(
Supplementary material as representative examples.
E1/2) (DE ¼90 mV) against Ag/AgCl electrode. The CV of 1, 5, and 6 are provided in
All complexes except 6 and 7 show two oxidative and two reductive waves (table 3).
The oxidative and reductive waves are well-separated from those associated with free
dihydrazone. Hence, these waves may be attributed to metal-centered electron-transfer
reactions. Both redox couples in 4 and 6 are reversible as the separation between
oxidative and reductive waves is around 59 mV. However, in 5 and 7, the separation
between the cathodic and anodic peaks is more than 59 mV. Hence, the redox couples in

3
10
R.A. Lal et al.
Table 3. Electrochemical parameters for the dihydrazone and its
manganese(IV) complexes (potential vs. Ag/AgCl).
Ligand/complex pa/V
E
E
pc/V
DE
p
(mV)
H
1
4
npah
þ0.32
ꢂ0.06
ꢂ0.65
þ0.70
ꢂ0.50
þ0.65
ꢂ0.58
þ0.25
ꢂ0.56
þ0.15
ꢂ0.24
ꢂ0.60
ꢂ0.79
–
þ0.23
ꢂ0.18
ꢂ1.00
þ0.43
ꢂ0.85
þ0.43
ꢂ0.80
þ0.19
ꢂ0.62
þ0.07
ꢂ0.36
ꢂ0.66
ꢂ0.85
ꢂ0.96
þ0.97
þ0.25
ꢂ0.35
90
120
350
240
350
220
220
60
60
80
120
60
2
3
4
5
6
60
–
130
–
7
þ1.10
–
ꢂ0.20
150
these complexes are quasi-reversible. Thus, the two waves observed in these complexes
may be attributed to the following redox processes (from the positive side).
IV
III
½Mn ðLÞðAÞꢆꢂ1 þe
II
ꢂ2
ꢂꢂꢂꢂ ꢂꢂ *
)ꢂꢂꢂꢂꢂꢂ
ꢂꢂꢂꢂ ꢂꢂ *
)ꢂꢂꢂꢂꢂꢂ
½
Mn ðLÞðH2OÞꢆþe
½Mn ðLÞðAÞꢆ
2
2
2
Separation between oxidative and reductive waves of the redox couples in 1–3 is more
than 200 mV except for the first redox couple (E ¼120 mV) in 1. This separation is
much higher than that for an uncomplicated one-electron redox process (59 mV).
Hence, the redox couples in these complexes are irreversible. The separation between
the oxidative and reductive waves probably originates from a slow heterogeneous
electron exchange rather than intervening homogenous reaction [52]. Complex 6 shows
an irreversible wave at ꢂ0.96 V, attributed to electron transfer centered on bipyridine.
Complex 7 shows an irreversible wave at þ0.25 V in the forward scan which is not
visible in return scans. Evidently, some transient species is produced in the forward scan
which is not stable and reverts back to the original species. This irreversible process may
be attributed to reduction of phenanthroline in the complex. Examination of table 3
shows regular decrease in reduction potential as well as oxidation potential in going
from pyridine to 2–picoline to 3-picoline to 4-picoline complexes for first redox couple
IV
III
Mn /Mn (from positive side). This is attributed to the increasing removal of electron
density toward the metal in going from pyridine to 2-picoline to 3-picoline to 4-picoline.
A necessary criterion for model systems for water oxidation in PSII is that they must
have redox potential more positive than the minimum of 0.6 V versus NHE required to
oxidize water to dioxygen at pH 7 in the biomembrane. Manganese complexes 2 and 3
with their oxidation potentials at þ0.70 and þ0.65 against Ag/AgCl may thus be an
appealing model system to simulate water-oxidizing complex (WOC) of PSII in
plant photosynthesis [53]. Complex 7 also shows very high quasi-reversible redox
behavior at þ0.97 V. Hence, this complex might be another candidate to simulate the
WOC of PSII.

Manganese(IV) hydrazone complexes
311
4. Conclusion
This article reports synthesis and characterization of seven Mn(IV) complexes derived
from bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone. The dihydrazone coordi-
nates to Mn(IV) in enol form through azomethine nitrogen and naptholate oxygen with
manganese occupying the NNOO coordination chamber. The dihydrazone donors are
arranged around manganese in the equatorial plane while the axial positions are
occupied by H O/pyridine/2-picoline/3-picoline/4-picoline in 1–5. In 6 and 7, the
2
nitrogen from azomethine of dihydrazone and bpy and phen are in equatorial positions
HC
N
N
H
C
2
C
H C
A
2
O
O
Mn
A
.nH2O
O
O
H C
2
C
H
C
2
N
N
HC
IV
Figure 2. Suggested structure of the complexes [Mn (naph)(A
2
)] ꢀH
2
2
O (where A ¼H O (1); py (2), 2-pic
(
3); 3-pic (4); and 4-pic (5)).
HC
N
O
C
O
N
N
CH2
CH2
N
H C
2
IV
Mn
H C
2
C
N
N
O
HC
O
Figure 3. Suggested structure of the complexes [Mn(naph)(NN)] (where NN ¼bpy (6) and phen (7)).

3
12
R.A. Lal et al.
and naptholate oxygens are in axial positions. The enolate oxygens remain
uncoordinated. The dihydrazone ligand is suggested to be coordinated to manganese
in the anti-cis configuration in all complexes. This configuration arises due to
coordination of both azomethine nitrogen and naptholate oxygen of the same
dihydrazone to the same metal center which introduces steric crowding. As a result,
one hydrazone arm remains in the equatorial plane while the other hydrazone arm is
axial. In this configuration, axial azomethine absorbs at lower frequency than
equatorial azomethine in IR spectra. The complexes are proposed to have distorted
octahedral stereochemistry. Cyclic voltammetry shows that the metal center cycles
IV
III
II
among the Mn !Mn !Mn oxidation states in all the complexes. Structures of
the complexes have been proposed as shown in figures 2 and 3.
Acknowledgments
The authors are grateful to the Head, SAIF, North-Eastern Hill University, Shillong-
7
93022, Meghalaya, India for elemental analyses and mass spectral studies, the Head,
SAIF, CDRI, Lucknow, Uttar Pradesh, India for FAB mass spectral data, and the
Head, SAIF, IIT Mumbai, India for EPR spectroscopic studies.
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