Reaction medium pH dependent existence of MnII bound [ON] donor zwitterionic chelating ligand and self-assembly of hydroxido-bridged Mn 4IIcluster

Article abstract of DOI:10.1002/ejic.201000170

A mononuclear MnN₄O₂ complex with two zwitterionic ligands and a centrosymmetric tetranuclear μ₃-OH-bridged self-assembled Mn₄ cluster were synthesized and characterized by using the [NO(H)N] ligand Hophbip [2,6-bis(phenylmethyliminomethyl)-4- methylphenol], The reaction, in MeOH in the presence of NH₄SCN and a lower stoichiometry of Mn(OAc)₂H₂O without any added base produces [Mn^II(H_Nbip)₂(NCS)₂] ·CH₃CN (1·CH3CN). In the presence of NaOH and NH ₄SCN in MeOH, the cluster [Mn₄^II(μ-bip) ₂(μ₃-OH)₂(NCS)₄] (2), featuring a stepped-cubane, was obtained through, hydroxido-brid.ge-driven hydroxido-bridge-driven dimerization of two μ₃-OHbridged [Mn ₂] fragments derived from. 1-CH₃CN.

Full text of DOI:10.1002/ejic.201000170

FULL PAPER
DOI: 10.1002/ejic.201000170
Reaction Medium pH Dependent Existence of MnII Bound [ON] Donor
Zwitterionic Chelating Ligand and Self-Assembly of Hydroxido-Bridged MnII
Cluster
4
Mrinal Sarkar,[a] Valerio Bertolasi,[b] and Debashis Ray*[a]
Keywords: N,O ligands / Bridging ligands / Schiff bases / Manganese / Zwitterions
A mononuclear MnN4O2 complex with two zwitterionic li-
gands and a centrosymmetric tetranuclear µ3-OH-bridged
self-assembled Mn4 cluster were synthesized and charac-
terized by using the [NO(H)N] ligand HOPhbip [2,6-bis(phen-
ylmethyliminomethyl)-4-methylphenol]. The reaction in
MeOH in the presence of NH4SCN and a lower stoichiometry
of Mn(OAc)2·4H2O without any added base produces
[MnII(HNbip)2(NCS)2]·CH3CN (1·CH3CN). In the presence of
NaOH and NH4SCN in MeOH, the cluster [MnII4(µ-bip)2(µ3-
OH)2(NCS)4] (2), featuring a stepped-cubane, was obtained
through hydroxido-bridge-driven dimerization of two µ3-OH-
bridged [Mn2] fragments derived from 1·CH3CN.
species for enhancing our understanding of SMM behavior.
Creation of mononuclear manganese(II) and its self-as-
sembled tetranuclear congener of the same ligand system is
of great synthetic value in understanding the formation and
growth of the different manganese active sites in biology
and magnetic materials.
In this background we have focused our attention to the
[NON] donor Schiff base ligand 2,6-bis(phenylmethylimino-
methyl)-4-methylphenol (HOPhbip, HOPh is the proton at-
tached to phenolate oxygen atom; Scheme 1).
Introduction
Manganese is known to be present in the active sites of
several metalloenzymes, such as manganese catalase, super-
oxide dismutase (SOD), extradiol dioxygenase, arginase, ri-
bonucleotide reductase, the oxygen-evolving complex
(OEC) of photosystem II (PS II), and lipoxygenase.[1] Both
mononuclear and tetranuclear manganese complexes are of
considerable interest as structural models following the
identification of the mononuclear centers in oxalate decarb-
oxylases (OxDC)[2] and oxalate oxidases (OxOx)[3] and
Mn4 assembly in PS II.[4] X-ray crystallographic studies
have shown the presence of distorted octahedral MnII ions
coordinated to three N atoms of histidine and one O atom
of glutamate residues, and two O atoms of water molecules
in the OxOx enzyme.[5] The coordination of water or its
derivatives (HO– or O2–) in the formation of aggregates of
transition-metal ions is of great significance. Many Mn4
complexes containing hydroxido, oxido, and peroxido brid-
ges have been synthesized and characterized, where the
bridging potential of these coupling groups are utilized for
dimerization and self-assembly.[6] Bridges like µ3-O and µ3-
OMe are more frequently found.[7] However, only two µ3-
hydroxido-bridged tetranuclear manganese complexes that
include MnIIMnIII(µ3-OH)[8] and MnII(µ3-OH)[6c] bonds
are known. As of now, manganese-based small clusters,
which act as single molecule magnets (SMMs),[9] are useful
Scheme 1.
In the neutral pH range (pH ≈ 7) and in the metal bound
form it may exist as zwitterion HNbip with nonadjacent
positive and negative charges. This ampholyte form can
function as a [NO] donor bidentate neutral ligand. At
higher pH values, deprotonation of HOPhbip affords the po-
tentially binucleating bis-bidentate [NON] donor ligand
bip– possessing the potential to act in both chelating and
bridging capacities. In this pH range, the availability of HO–
groups leads to zwitterionic proton abstraction, coordina-
tion, bridge, and self-condensation to a Mn4 complex. The
intention to use this type of ligand is to generate coordina-
tively unsaturated manganese sites, which is favored by the
low denticity of the ligand. This could then simultaneously
bind more than one thiocyanate anion through the forma-
[a] Department of Chemistry, Indian Institute of Technology,
Kharagpur 721302, India
Fax: +91-3222-82252
E-mail: dray@chem.iitkgp.ernet.in
[b] Dipartimento di Chimica e Centro di Strutturistica
Diffrattometica, Università di Ferrara,
via Borsari 46, 44100 Ferrara, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000170.
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pH Dependent Existence of MnII Bound Donor Zwitterionic Chelating Ligands
tion of mononuclear and hydroxido-bridge-driven self-as- Mn2 product such as 4 (Scheme 2) was observed, perhaps
sembled clusters occupying the vacant positions in the coor- because of the additional stability of 1, which crystallizes
dination spheres.
as a bis-chelated mononuclear transition metal complex of
The central phenolate group bearing a [NON] donor li- any binucleating ligand assembled through H-bonding. A
gand in transition-metal assemblies is relatively new, al- proposal for the formation of the Mn2 complex comes from
though some notable results have been reported, consisting our endeavors reported recently on the cobalt(II) chemistry
of a [Zn6][10] and a [Cu4] species.[11] Numerous Mn4 clusters with this ligand.[15] The use of NaOH in MeOH in the
have been reported with the core structure varying from above-mentioned reaction inhibits the formation of the li-
butterfly, cubane, double cubane, adamantane, square, bas- gand zwitterion and results in the formation of complex 2
ket, and linear to “pair-of-dimer”, among many in a 69% yield [Equation (2), Scheme 2]. Interestingly, the
others.[6a,6b,9c,12] There have been many concepts and ideas reaction of 1·CH3CN with Mn(OAc)2·4H2O and NH4SCN
to predict which species will self-assemble in solution, and in methanolic NaOH resulted in the quantitative transfor-
these considerations are mainly based on the information mation of 1·CH3CN into 2 in a 63% yield [Equation (3)].
stored in the coordinatively unsaturated dinuclear building The formation of both complexes is consistent with elemen-
units, their positioning in space, and metal-ion electronic tal analysis and electrical conductivity data in CH3CN.
configurations. Surprisingly,
a Mn4 cluster having a
2HOPhbip + Mn(OAc)2·4H2O + 2NH4SCN Ǟ
stepped-cubane core structure and different coordination
geometries has not been reported, although the topology is
rather simple. The tendency of manganese ions to accept a
fifth ligand at the apical coordination site extends the bridg-
ing ability of HO– group in a µ3 fashion and is responsible
for associating two dinuclear units to a stepped-cubane
structure. The stepped-cubane geometry is unusual in man-
ganese coordination chemistry due to its restriction in coor-
dination geometry, but it is known in copper complexes as a
consequence of the “plasticity” of the copper coordination
spheres.[13]
Herein we have investigated the reaction medium pH de-
pendent binding behavior of bip– toward manganese(II)
and isolated the complexes resulting from the zwitterionic
form of the ligand at neutral pH and the hydroxido-bridge-
driven aggregation at high pH values. Reported here are
the stable mononuclear and tetranuclear MnII complexes of
[Mn(HNbip)2(NCS)2] + 2NH4OAc + 4H2O
(1)
2HOPhbip +4Mn(OAc)2·4H2O + 4NH4SCN + 4NaOH Ǟ
[Mn4(µ-bip)2(µ3-OH)2(NCS)4] + 4NH4OAc + 4NaOAc + 18H2O
(2)
[Mn(HNbip)2(NCS)2] + 3Mn(OAc)2·4H2O + 4NaOH + 2NH4SCN
Ǟ
[Mn4(µ-bip)2(µ3-OH)2(NCS)4]
+
4NaOAc
+
2NH4OAc
+
(3)
14H2O
We failed to prepare 3 in the absence of NaOH by follow-
ing an attempted procedure of Equation (2) with an excess
amount of NH4SCN and a stoichiometric amount of NEt3
(Scheme 2). Use of an excess amount of NH4SCN even up
to a molar ratio of 6:1 with respect to the metal salt could
not substitute the bridging HO– groups by NCS– in 2 and
afforded 1.
H
OPhbip resulting from the tuning of the reaction medium
pH and from varying the metal/ligand stoichiometry. These
two compounds were isolated and crystallographically char-
acterized. The aggregating ability of bip–, together with the
added HO– in alkaline medium and its µ3 binding proper-
ties, was exploited here in the preparation of 2.
Spontaneous Dimerization Reaction for Self-Assembly
The direct reaction of Hbip with Mn(OAc)2·4H2O,
NH4SCN, and NaOH yielded 2. The N,O chelation of one
bip– around two MnII followed by binding of one NCS– to
each metal ion and one hydroxido bridge yielded the elec-
troneutral intermediate I (Scheme S2, Supporting Infor-
mation). Interaction toward dimerization of two such spe-
cies takes place through two apical Mn–O interactions from
hydroxido bridges to produce intermediate II, which finally
separates as 2 from the reaction mixture.
Results and Discussion
Synthesis and Reactivity
Formation of Mn4 Stepped-Cubane by Uphill
Transformation of Bis-thiocyanato-Coordinated 1
The Schiff base 4-methyl-2,6-bis(phenylmethylimino-
methyl)phenol (Hbip) was prepared (Scheme S1, Support-
ing Information) by following a literature procedure,[14] and
In the absence of any hydroxido bases and in the pres-
its reaction with manganese(II) salts was investigated under ence of a large excess of thiocyanate anions the only prod-
different reaction conditions (Scheme 2). We carried out the uct obtained was mononuclear manganese(II) complex 1,
reaction of Mn(OAc)2·4H2O with Hbip [designated as in which the zwitterionic binucleating ligands function as
HOPhbip in Equation (1)] and NH4SCN in MeOH and in neutral bidentate N,O ligands. The protonated imine func-
the absence of any externally added base and obtained tions remain H-bonded to phenolate oxygen atoms. In
[MnII(HNbip)2(NCS)2] (1) in 74% yield (Scheme 2), which MeOH and in the presence NaOH, 1 undergoes a self-as-
shows that the formation of the zwitterion of HNbip, in the sembly reaction [Equation (3), vide infra] with extra
presence of AcO– ions, prevents full utilization of its metal Mn(OAc)2·4H2O and NH4SCN to yield 2, spontaneously
ion binding ability. However, no sign of formation of any and quantitatively.
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M. Sarkar, V. Bertolasi, D. Ray
FULL PAPER
Scheme 2. Schematic representation of the ligand and thiocyanate assembly for MnII assembly against other unknown species.
FTIR Spectra
13970 –1 cm–1) for 1 and 2, respectively, possibly originate
from the πǞπ* transition associated with the azomethine
group.[18] Weak transitions at 410 (ε, 13170 –1 cm–1) and
450 nm (ε, 4780 –1 cm–1) for 1 and 2, respectively, may be
referred to the combination of metal-to-ligand charge
transfer (MLCT) and d–d transitions.
The sharp peak in the FTIR spectra of complexes 1 and
2 at 1647 and 1637 cm–1, respectively, are characteristic of
the C=N functionality of bip–. For 2, a broad and medium
band at 3600 cm–1 is observed, which corresponds to the
νOH stretching mode of the bridging HO– groups.[16] The
mode of binding of the thiocyanate anions in 1 and 2 was
identified from the νCN stretching frequencies at 2060 and
2059 cm–1 for 1 and 2, respectively, whereas the C–S stretch-
ing (νCS) and bending (δNCS) modes in the 700–825 cm–1
region could not be identified with certainty in the presence
of strong absorptions by bip– in this region.
Description of Structures
Single crystals suitable for X-ray structure determination
were obtained by slow evaporation of a hot saturated
CH3CN solution of 1·CH3CN after 2 h and a CH3CN/
MeOH (1:1) solution of 2 after a week. Selected interatomic
Electronic Spectra
The d–d transition intensities of 1 and 2 are very low, as distances and angles are collected in Tables 1 and 2, and
6
transitions from the A1g state to other higher states are the crystallographic data are summarized in Table 3. The
doubly forbidden.[17] In MeCN solutions, the bands below ORTEP diagrams of the two compounds are shown in Fig-
400 nm viz., at 215 (ε, 22680 –1 cm–1) and 260 nm (ε, ures 1 and 2.
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pH Dependent Existence of MnII Bound Donor Zwitterionic Chelating Ligands
Table 1. Selected interatomic distances [Å] and angles [°] for com- tahedral geometry in Nim/th4O2 (im and th stand for imine
plex 1·CH3CN.
and thiocyanate nitrogen atoms) environments (Figure S1,
Supporting Information). In the basal plane, two phenolate
oxygen atoms (O1: 2.096 Å and O2: 2.117 Å) are in a trans
disposition, and one nitrogen atom of the imine group (N3:
2.316 Å) from the zwitterionic HNbip ligand and one nitro-
gen atom of NCS– (N6: 2.218 Å) are coordinated to the
MnII ion. The apical positions are occupied by the atoms
N1 (2.321 Å) and N5 (2.242 Å) from HNbip and NCS–,
respectively. The angular distortions are also noticeable
with trans and cis angles in the 163–174 and 81–100° ran-
ges, respectively. The two terminal NCS– groups are coordi-
nated differently to MnII, as evidenced from the Mn–N–C
angles (174.15 and 138.94°). The negatively charged NCS–
groups bind more strongly to the MnII center (av. 2.230 Å)
than the neutral imine functionalities (av. 2.319 Å).[20] The
potentially binucleating Hbip ligand functions as a zwitter-
ionic [Nim,O] donor bidentate ligand in 1 and is responsible
for the formation of the ctc (cis–trans–cis orientations of
Nim, OPh, and Nth donor atoms) isomer exclusively
(Scheme 3) most probably due to the steric bulk of the li-
gand, which alters the coordination environment around
the metal ion, and intramolecular H-bonding stabilization
within the molecule.
Distances
Mn1–N5
Mn1–O1
Mn1–O2
Mn1–N6
2.0959(15)
2.1177(15)
2.218(2)
2.242(2)
2.3158(19)
2.3218(19)
Mn1–N3
Mn1–N1
Angles
O1–Mn1–O2
O1–Mn1–N6
O2–Mn1–N6
O1–Mn1–N5
O2–Mn1–N5
N6–Mn1–N5
O1–Mn1–N3
O2–Mn1–N3
174.40(6)
100.28(8)
85.26(8)
82.92(7)
97.66(7)
93.81(9)
93.70(7)
80.80(6)
N6–Mn1–N3
N5–Mn1–N3
O1–Mn1–N1
O2–Mn1–N1
N6–Mn1–N1
N5–Mn1–N1
N3–Mn1–N1
N3–Mn1–N1
165.87(8)
85.84(8)
81.03(6)
98.83(6)
83.94(8)
163.11(7)
100.38(7)
100.38(7)
Table 2. Selected interatomic distances [Å] and angles [°] for com-
plex 2.[a]
Distances
Mn1–N3
Mn1–O2
Mn1–N1
Mn1–O1
Mn1–Mn2
Mn2–N4
1.909(11)
1.925(6)
1.933(8)
1.956(6)
2.9970(16)
1.933(9)
Mn2–O2
Mn2–N2
Mn2–O1
Mn2–O2*
O2–Mn2*
O2–Mn2*
1.958(6)
1.962(8)
1.968(6)
2.451(7)
2.451(7)
2.451(7)
Angles
N3–Mn1–O2
N3–Mn1–N1
O2–Mn1–N1
N3–Mn1–O1
O2–Mn1–O1
N1–Mn1–O1
N3–Mn1–Mn2 129.1(3)
O2–Mn1–Mn2 39.88(19)
N1–Mn1–Mn2 133.0(3)
O1–Mn1–Mn2 40.35(17)
N4–Mn2–O2
N4–Mn2–N2
O2–Mn2–N2
N4–Mn2–O1
O2–Mn2–O1
92.3(3)
97.4(4)
166.9(3)
165.7(4)
79.6(3)
92.6(3)
N2–Mn2–O1
91.4(3)
87.4(4)
82.5(3)
100.6(3)
96.5(2)
129.6(3)
39.07(18)
130.7(2)
40.07(17)
94.70(15)
99.6(3)
101.1(3)
105.1(3)
97.5(3)
N4–Mn2–O2*
O2–Mn2–O2*
N2–Mn2–O2*
O1–Mn2–O2*
N4–Mn2–Mn1
O2–Mn2–Mn1
N2–Mn2–Mn1
O1–Mn2–Mn1
O2*–Mn2-Mn1
Mn1–O1–Mn2
Mn1–O2–Mn2
Mn1–O2–Mn2*
Mn2–O2–Mn2
Mn2–O2–Mn2*
92.0(3)
97.9(3)
169.7(3)
169.0(4)
78.5(2)
97.5(3)
[a] Symmetry operation: * –x + 1, –y + 1, –z + 1.
[MnII(HNbip)2(NCS)2]·CH3CN (1·CH3CN)
Complex 1 forms orange crystals belonging to the mono-
clinic crystal system, space group P21/n. The ORTEP[19]
representation of complex 1 (Figure 1) shows a mono-
Figure 1. Labeled ORTEP view of [MnII(NCS)2(Hbip)2]; atom
numbering scheme and thermal ellipsoids are drawn at the 30%
nuclear MnII cation that is six-coordinate in a distorted oc- probability level.
Scheme 3. Four possible isomers in the Nth2O2Nim environment.
2
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M. Sarkar, V. Bertolasi, D. Ray
FULL PAPER
The noncoordinating imine groups of two ligands are cal bridging by µ3-OH– ions (Mn–O av. 2.446 Å; Figure S5,
protonated and H-bonded to the metal-bound phenolate Supporting Information). The long apical Mn–OH bonds
oxygen atoms (N···O av. 2.572 Å, Figure 1). Compared to did not allow two SCN– terminals to establish bridging con-
the distorted N3O3 coordination environments around the nectivity and octahedral coordinations to each manganese
MnII ions in OxOx and OxDC, our complex has a N4O2 ion such as in 5 (Scheme 2). Thus, two four-coordinate
binding site. The average bond lengths around the MnII square planar and two five-coordinate square pyramidal
center observed in 1 (Mn–Nav: 2.274 Å, Mn–Oav.: 2.106 Å) MnII centers in N2O2 and N2O3 coordination environments,
are similar to those reported for the active site of OxDC respectively, are present within the stepped-cubane cluster
(Mn–Nav.: 2.27 Å, Mn–Ocarboxylate: 2.04 Å). This sort of having in-plane and out-of-plane av. Mn···Mn separations
mononuclear binding of any potential binucleating ligand of 2.997 and 3.332 Å, respectively. The number of Mn2O2
is not reported in the literature. One representative diagram rhombi present in complex 2 is three. Within the three
of the crystal packing along the b axis shows side-by-side Mn2O2 rhombi of the stepped-cubane structure, the av.
placement of molecules (Figure S2, Supporting Infor- Mn–O–Mn and O–Mn–O angles are 99.37 and 80.18°,
mation).
respectively. MnII ions sit within perfect square planes (av.
distance of MnII from the centroid of N2O2 squares are
0.011 and 0.059 Å) in SP–SPY (SP = square-planar; SPY
= square-pyramidal) coordination geometries. The two ter-
minal NCS– groups are coordinated differently to MnII, as
evidenced from the Mn–N–C angles (172.91 and 157.44°).
The bond lengths around the MnII centers observed in 2
(av. Mn–Nim: 1.947 Å, av. Mn–Nth: 1.920 Å, and av. Mn–
OPh: 1.962 Å) are shorter than those observed in the case
of 1 having wide variations (av. Mn–Nim: 2.319 Å, av. Mn–
Nth: 2.230 Å, and av. Mn–OPh: 2.107 Å). The sum of the
angles around the O atoms of the central phenolate and
hydroxido bridges are 357.9 and 303.7°, respectively, which
clearly demonstrates µ-planar and µ3-pyramidal binding
modes. Two sulfur ends of coordinated thiocyanate anions
make complementary contacts with coordinated phenolate
oxygen atoms with adjacent Mn4 units (av. S···O 3.315 Å)
and lead to a 1D network (Figure S6, Supporting Infor-
mation).[22]
[MnII4(µ-bip)2(µ3-OH)2(NCS)4] (2)
Complex 2 crystallizes as brown crystals that belong to
the triclinic crystal system, space group P1. The tetranu-
¯
clear complex (Figure 2) can be described as [MnII2(µ-
bip)(µ-OH)(NCS)2]2 (Figure S3, Supporting Information),
and it was obtained as a dimerized product of deprotonated
bip– bound binuclear subunits. The Mn4O4 complex (O
atoms are from PhO– and HO–) is centrosymmetric with a
central core of four MnII ions that can be described as a
stepped-cubane (Figure S4, Supporting Information).[21]
The stepped-cubane unit is a charge neutral aggregate of
four MnII ions linked together by the template action of
two HO– groups obtained from water molecules present in
the solvent.
Cyclic Voltammetry
The redox behaviors of 1 and 2 were investigated by cy-
clic voltammetry in acetonitrile, under a nitrogen atmo-
sphere at room temperature, in the potential range from
+1.75 to –1.0 V vs. Ag/AgNO3 at a scan rate of 100 mVs–1.
The data were recorded without stirring during the voltage
sweeps and by using only freshly prepared solutions. A
glassy-carbon working electrode and a Pt wire auxiliary
electrode together with a 0.1  solution of tetrabutylammo-
nium perchlorate (TBAP) as supporting electrolyte were
used. The voltammogram for complex 1·CH3CN is shown
Figure 2. Labeled ORTEP view of [MnII4(NCS)4(µ3-OH)2(µ-bip)2]; in Figure 3, and the data for compound 2 is provided in the
atom numbering scheme and thermal ellipsoids are drawn at the
30% probability level and H atoms are omitted for clarity.
Supporting Information.
Complex 1 shows one quasireversible process at E1/2
=
1.1 V (∆Ep = 0.27 V) attributed to the MnII Ǟ MnIII pro-
Two imine nitrogen atoms of one ligand bound to two cess in the mononuclear species. In both OxDC and OxOx
manganese and phenolate oxygen atom bridges to provide for oxidative oxalate metabolism, the MnII ion in the dis-
[MnII2(µ-bip)(µ-OH)(NCS)2] before the dimerization step. torted octahedral N3O3 coordination domain acts as the
In the next step, two µ-OH– groups of two dimers expand catalytic center in the resting state. The catalysis is sug-
their binding potential by making apical new bonds to two gested to require a MnII Ǟ MnIII oxidation process, where
MnII ions. The structure can be described as two layers of MnIII is assumed to be the active oxidation level. Complex
dinuclear subunits linked by basal bridging of two phenol- 2 displays two quasireversible processes at E1 = 0.93 V
1/2
ate O atoms (Mn–O av. 1.962 Å) of the ligand and two api- (∆Ep = 0.12 V) and E2 = 1.23 V (∆Ep = 0.14 V) and an
1/2
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pH Dependent Existence of MnII Bound Donor Zwitterionic Chelating Ligands
solution of NH4SCN (0.076 g, 1.00 mmol) in methanol (20 mL)
with stirring at ambient temperature in air, and the reaction mix-
ture was stirred for 1 h more, at which point it was deep orange in
color. After evaporation of the reaction mixture an orange solid
was obtained. The solid was isolated, washed with cold methanol,
and dried under vacuum with P4O10. Orange single crystals suitable
for X-ray analysis were obtained from hot MeCN over 2 h. Yield:
0.316 g (74%). C50H47MnN7O2S2 (897.03): calcd. C 66.94, H 5.28,
N 10.93; found C 66.86, H 5.18, N 10.68. FTIR (KBr): ν = 3435
˜
(br.), 2060 (vs), 1647 (vs), 1540 (vs), 1213 (m), 827 (m), 820 (m),
751 (m), 699 (m), 486 (m) cm–1. Molar conductance (CH3CN): ΛM
= 3.0 Ω–1 cm2 mol–1. UV/Vis (CH3CN): λ (ε, –1 cm–1) = 410
(13170), 215 (22680) nm.
[MnII4(µ-bip)2(µ3-OH)2(NCS)4] (2)
Method A – Direct Route: To a yellow solution of Hbip (0.342 g,
1.00 mmol) in methanol (15 mL) was added a solution of Mn(OAc)2·
4H2O (0.490 g, 2.00 mmol) in methanol (20 mL). To the resulting
Figure 3. Cyclic voltammogram (scan rate 100 mVs–1) of 10–3
solutions of 1·CH3CN in CH3CN at a glassy-carbon electrode (T
= 293 K).

orange-colored solution was dropwise added
a solution of
NH4SCN (0.152 g, 2.00 mmol) in methanol (20 mL) with stirring
at ambient temperature in air, and the reaction mixture turned deep
orange in color. After 15 min, a solution of NaOH (0.080 g,
2.00 mmol) in methanol (15 mL) was added to the deep-orange-
colored solution, and the reaction mixture was further stirred for
1 h. The deep-orange solution formed initially changed to brown
in about 5 min. After evaporation of the reaction mixture a brown
solid was obtained. The solid was isolated, washed with cold meth-
anol, and dried under vacuum with P4O10. Brown single crystals
suitable for X-ray analysis were obtained from MeCN over 7 d.
Yield: 0.403 g (69%). C50H44Mn4N8O4S4 (1168.96): calcd. C 51.37,
irreversible anodic peak at Epa = 0.54 V (Figure S7, Sup-
porting Information). The first response E1 can be tenta-
1/2
tively attributed to the MnII Ǟ MnIII processes in the Mn4
complex.[23]
Concluding Remarks
In summary, we have synthesized and characterized one
mononuclear and one tetranuclear complex of manga-
nese(II) with zwitterionic and anionic forms of a binucleat-
ing [NON] donor ligand. We also demonstrated the hy-
droxido-bridge-driven self-assembly and the transformation
of 1·CH3CN into 2. Supply of HO– ions to the former de-
protonates the iminium nitrogen atom of the zwitterionic
ligand and allowed bridging of the two dinuclear units by
two HO– ions, which are found in the unsupported µ3 coor-
dination mode, leading to the formation of the [MnII4] com-
plex. The conversion of 1·CH3CN into 2 also points to the
role of HO– ions in the formation of Mn4 clusters in pres-
ence of extra metal ions and terminal thiocyanate ligands.
We are currently working to exploit the asymmetry induced
by external bridges in this reaction system to induce the
formation of heterometallic high nuclearity assemblies.
H 3.79, N 9.58; found C 51.29, H 3.68, N 9.32. FTIR (KBr): ν =
˜
3432 (br.), 2059 (vs), 1637 (vs), 1542 (vs), 1449 (m), 1217 (m), 1027
(br.), 827 (m), 753 (m), 700 (m), 601 (m), 486 (m) cm–1. Molar
conductance (CH3CN): ΛM = 6 Ω–1 cm2 mol–1. UV/Vis (CH3CN):
λ (ε, –1 cm–1) = 450 (4780), 260 (13970), 200 (21870) nm.
Method B – Uphill Transformation from 1: To an orange solution
of 1·CH3CN (0.224 g, 0.25 mmol) in CH3CN/MeOH (1:1, 30 mL)
was added a solution of Mn(OAc)2·4H2O (0.183 g, 0.75 mmol) in
methanol (10 mL) followed by the dropwise addition of a solution
of NH4SCN (0.04 g, 0.50 mmol) in methanol (10 mL) with stirring
at ambient temperature in air, and the reaction mixture turned deep
orange in color. After 15 min, a solution of NaOH (0.04 g,
1.00 mmol) in methanol (10 mL) was added to the deep-orange-
colored solution, and the reaction mixture was further stirred for
1 h. The deep-orange solution formed initially changed to brown
in about 5 min. After evaporation of the reaction mixture a brown
solid was obtained. The solid was isolated, washed with cold meth-
anol, and dried under vacuum with P4O10. Yield: 0.184 g (63%).
Experimental Section
Physical Measurements: The elemental analyses (C, H, N) were per-
formed with a Perkin–Elmer model 240C elemental analyzer. FTIR
spectra were recorded with a Perkin–Elmer 883 spectrometer. The
solution electrical conductivity and electronic spectra were ob-
tained by using a Unitech type U131C digital conductivity meter
with a solute concentration of about 10–3  and a Shimadzu UV
3100 UV/Vis/NIR spectrophotometer, respectively.
Materials: Sodium hydroxide, ammonium thiocyanate, and manga-
nese acetate tetrahydrate were obtained from S. D. Fine Chem. (In-
dia), and benzylamine was obtained from SRL (India). All other
chemicals and solvents were reagent-grade materials and were used
as received without further purification.
Hbip Ligand: The Schiff base was prepared from the single-step
condensation of 2,6-diformyl-4-methylphenol (1.640 g, 10 mmol)
and benzylamine (2.18 mL, 20 mmol) in methanol (40 mL) under
reflux for 1 h, as reported previously.[14]
Crystal Data Collection and Refinement for 1·CH3CN and 2: The
intensity data of complex 1·CH3CN was collected with a Bruker-
APEX-2 CCD X-ray diffractometer by using graphite-monochro-
mated Mo-Kα radiation (λ = 0.71073 Å) by the hemisphere method.
[MnII(HNbip)2(NCS)2]·CH3CN (1·CH3CN): To a yellow solution of The intensity data of complex 2 was collected with a Nonius CAD4
Hbip (0.342 g, 1.00 mmol) in methanol (15 mL) was added a solu-
tion of Mn(OAc)2·4H2O (0.122 g, 0.5 mmol) in methanol (20 mL).
To the resulting orange-colored solution was dropwise added a
X-ray diffractometer by using graphite-monochromated Mo-Kα ra-
diation (λ = 0.71073 Å) by the ω-scan method. Data were collected
at 293 K. Information concerning X-ray data collection and struc-
Eur. J. Inorg. Chem. 2010, 2530–2536
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2535

M. Sarkar, V. Bertolasi, D. Ray
FULL PAPER
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Table 3. Crystallographic data for 1·CH3CN and 2.
1·CH3CN
2
Formula
M
Space group
Crystal system
a [Å]
b [Å]
c [Å]
C50H47N7O2S2Mn
897.03
P21/n
monoclinic
16.8499(7)
13.6412(5)
20.6317(8)
90
C50H44N8O4S4Mn4
1168.96
P1
triclinic
9.135(8)
11.687(3)
13.312(5)
64.39(10)
¯
α [°]
β [°]
91.4090(10)
90
4740.8(3)
293
78.57(3)
77.32(3)
1241.7(5)
293
γ [°]
V [Å3]
T [K]
Z
4
1
Dcalcd. [gcm–3]
F(000)
1.257
1876
4.13
63468
10898
0.0559
1.561
594
12.16
4362
4362
0.0000
µ(Mo-Kα) [cm–1]
Measured reflns.
Unique reflns.
Rint
Obs. reflns. IՆ2σ(I) 6421
2418
1.71/24.97
θmin/θmax [°]
1.54/27.62
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R(F2) (obs. reflns.)
wR(F2) (all reflns.)
No. variables
0.0472
0.1346
570
0.0728
0.2359
316
Goodness of fit
1.018
1.074
∆ρmax; ∆ρmin [eÅ–3]
0.341; –0.298
0.985; –0.682
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diagram of complex 1·CH3CN; ORTEP view of the dinuclear seg-
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Acknowledgments
M. S. is thankful to the Council of Scientific and Industrial Re-
search, New Delhi for financial support. V. B. acknowledges the
Italian Ministry of University and Scientific Research (MIUR),
Rome.
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Received: February 11, 2010
Published Online: May 6, 2010
2536
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 2530–2536