Manganese(IV) complex with a polydentate Schiff base ligand: synthesis, crystal structure, TDDFT calculation, electronic absorption and EPR spectral study

Article abstract of DOI:10.1080/00958972.2016.1248949

A six-coordinate Mn(IV) complex, [Mn(H₂L)₂] (1) [H₄L?=?2-methoxy-6-(tris(hydroxymethyl)methyliminomethyl)phenol], was synthesized and characterized by elemental analysis, single-crystal X-ray crystallography, FTIR, UV–Vis

Full text of DOI:10.1080/00958972.2016.1248949

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Manganese(IV) complex with a polydentate Schiff
base ligand: synthesis, crystal structure, TDDFT
calculation, electronic absorption and EPR spectral
study
Soumen Manna, Soumen Mistri, Apurba Bhunia, Aparup Paul, Ennio
Zangrando & Subal Chandra Manna
To cite this article: Soumen Manna, Soumen Mistri, Apurba Bhunia, Aparup Paul, Ennio
Zangrando & Subal Chandra Manna (2016): Manganese(IV) complex with a polydentate Schiff
base ligand: synthesis, crystal structure, TDDFT calculation, electronic absorption and EPR
spectral study, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2016.1248949
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1248949
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Date: 11 November 2016, At: 07:44

Journal of Coordination Chemistry, 2016
ꢁꢁp://ꢂx.ꢂꢃꢄ.ꢃꢅg/10.1080/00958972.2016.1248949
ꢀ
Manganese(IV) complex with a polydentate Schiff base
ligand: synthesis, crystal structure, TDDFT calculation,
electronic absorption and EPR spectral study
a
a
a
a
b
Soumen Manna , Soumen Mistri , Apurba Bhunia , Aparup Paul , Ennio Zangrando
a
and Subal Chandra Manna
ꢆ
b
dꢇpꢆꢅꢁꢈꢇꢉꢁ ꢃꢊ Cꢀꢇꢈꢄꢋꢁꢅꢌ ꢆꢉꢂ Cꢀꢇꢈꢄcꢆꢍ tꢇcꢀꢉꢃꢍꢃgꢌ, Vꢄꢂꢌꢆꢋꢆgꢆꢅ uꢉꢄvꢇꢅꢋꢄꢁꢌ, mꢄꢂꢉꢆpꢃꢅꢇ, iꢉꢂꢄꢆ; dꢇpꢆꢅꢁꢈꢇꢉꢁ
ꢃꢊ Pꢀꢆꢅꢈꢆcꢇꢎꢁꢄcꢆꢍ ꢆꢉꢂ Cꢀꢇꢈꢄcꢆꢍ scꢄꢇꢉcꢇꢋ, uꢉꢄvꢇꢅꢋꢄꢁꢌ ꢃꢊ tꢅꢄꢇꢋꢁꢇ, tꢅꢄꢇꢋꢁꢇ, iꢁꢆꢍꢌ
ABSTRACT
ARTICLE HISTORY
A six-coordinate Mn(IV) complex, [Mn(H L) ] (1) [H L = 2-methoxy-
rꢇcꢇꢄvꢇꢂ 18 fꢇbꢅꢎꢆꢅꢌ 2016
accꢇpꢁꢇꢂ 29 sꢇpꢁꢇꢈbꢇꢅ 2016
2
2
4
6
-(tris(hydroxymethyl)methyliminomethyl)phenol], was synthesized
and characterized by elemental analysis, single-crystal X-ray
crystallography, FTIR, UV–Vis electronic absorption, and EPR
spectroscopy. The single-crystal structural determination reveals
KEYWORDS
mꢆꢉgꢆꢉꢇꢋꢇ(iV) cꢃꢈpꢍꢇx;
ꢋꢎpꢅꢆꢈꢃꢍꢇcꢎꢍꢇ; cꢅꢌꢋꢁꢆꢍ
ꢋꢁꢅꢎcꢁꢎꢅꢇ; ꢇꢍꢇcꢁꢅꢃꢉꢄc ꢋpꢇcꢁꢅꢆ;
dft
that the complex crystallizes in tetragonal space group P4 2 2 and
1
1
the tridentate Schiff base ligands are arranged in mer configuration
chelating manganese, which exhibits as slightly distorted octahedral
coordination sphere with a N O chromophore. Weak hydrogen
2
4
bonding interactions of uncoordinated hydroxyl groups result in a
2-D supramolecular structure. The UV–Vis electronic absorption and
IR spectral data of 1 have been compared with the results obtained
by employing DFT and time-dependent density functional theory
calculation using B3LYP, B3PW91, and MPW1PW91 functionals, with
6
-31G (d-p) and LanL2MB basis sets. The results of these calculations
are functional-dependent and, among those used, B3PW91 proved
to better reproduce the experimental results.
CONTACT sꢎbꢆꢍ Cꢀꢆꢉꢂꢅꢆ mꢆꢉꢉꢆ
ꢋcꢈꢆꢉꢉꢆ@ꢈꢆꢄꢍ.vꢄꢂꢌꢆꢋꢆgꢆꢅ.ꢆc.ꢄꢉ
sꢎppꢍꢇꢈꢇꢉꢁꢆꢍ ꢂꢆꢁꢆ ꢊꢃꢅ ꢁꢀꢄꢋ ꢆꢅꢁꢄcꢍꢇ cꢆꢉ bꢇ ꢆccꢇꢋꢋꢇꢂ ꢆꢁ ꢀꢁꢁp://ꢂx.ꢂꢃꢄ.ꢃꢅg/10.1080/00958972.2016.1248949.
©
2016 iꢉꢊꢃꢅꢈꢆ uK lꢄꢈꢄꢁꢇꢂ, ꢁꢅꢆꢂꢄꢉg ꢆꢋ tꢆꢌꢍꢃꢅ & fꢅꢆꢉcꢄꢋ Gꢅꢃꢎp

2
S. MANNA ET AL.
1
. Introduction
Polydentate Schiff base ligands with 3d metals allow synthesis of coordination compounds
which are widely investigated not only for their interesting structures and stereochemistry
[
1], but also for the formation of polynuclear derivatives [2]. In this respect, 2-meth-
oxy-6-(tris(hydroxymethyl)methyliminomethyl)phenol (H L) is used as a potential multiden-
4
tate Schiff base ligand for its different hapticity also modulated by deprotonation of hydroxyl
groups. Using this ligand, different transition metal complexes, such as [VO(HL)] [3, 4],
2
[
Mn(H L) ] [5], [Ni(H L) ] [6], and [Ni(H L) ] [7], have been reported.
3 2 3 2 2 4
Many 3d metal Schiff base compounds are reported to model biologically important
molecules, and among these are specific manganese enzymes [8]. Study of coordination
chemistry of manganese is dominated by stable manganese(II) and manganese(III) ions,
while the chemistry at higher oxidation states is still less known. The chemistry of Mn(IV)
coordination compounds is significant as many biological systems, with different mecha-
nisms of action, contain manganese in this oxidation state, for example the OEC (oxygen
evolving complex) of photosynthesis II [9]. In addition to the biological importance, Mn(IV)
compounds are also used as asymmetric catalysts [10, 11].
No Mn(IV) compound with H L has been reported. The role of reaction condition in design-
4
ing the Mn(IV) compound is critical in this study. Hu et al. reported a mononuclear manga-
nese compound with this ligand [5], where the metal has +2 oxidation state. We have
performed the same reaction at room temperature adopting a higher pH of the reaction
medium (pH 9.28) and isolated [Mn(H L) ] (1), where the oxidation state of manganese is
2
2
+4.
To provide more insight into the properties and behavior of 3d metal coordination com-
pounds, quantum chemical calculations using density functional theory (DFT) and time-de-
pendent density functional theory (TDDFT) have been useful tools to reproduce electronic
and spectroscopic properties [12]. Literature survey shows a limited number of reports on
the DFT/TDDFT calculation to explain the structure and spectroscopic properties of man-
ganese(IV) coordination compounds [13].
Currently we are engaged in synthesis of 3d metal coordination compounds [14, 15] and
study of their spectroscopic properties with the help of DFT/TDDFT calculations [16]. The
present contribution reports on the synthesis, crystal structure, electronic absorption, and
EPR spectroscopic study of 1. IR and electronic absorption spectral properties of 1 were
explained by DFT/TDDFT computation using different functions.
2
. Experimental
.1. Materials and physical measurements
High purity manganese acetate tetrahydrate [Mn(OAc) ∙4H O] was purchased from Merck
2
2
2
(India) and all other chemicals used were of analytical grade. Solvents used for spectroscopic
studies were purified and dried by standard procedures before use [17].
Elemental analyses (carbon, hydrogen and nitrogen) were performed using a Perkin–
Elmer 240C elemental analyzer. IR spectra were recorded as KBr pellets on a Bruker Vector
−1
2
2 FTIR spectrophotometer operating from 400 to 4000 cm . ESI-MS spectra of the com-
pounds in methanol were recorded on an Agilent Q-TOF 6500 mass spectrometer using the

JOURNAL OF COORDINATION CHEMISTRY
3
Mass hunter software for their analysis. Electronic absorption spectra were obtained with a
Shimadzu UV-1601 UV–Vis spectrophotometer at room temperature. Quartz cuvettes of
3
1
cm path length and 3 cm volume were used for all measurements. EPR spectrum of the
1
complex was obtained on an X-band Microwave Unit, JES-FA200 ESR spectrometer. H and
C NMR spectra were recorded on a Varian 400 MHz instrument.
13
2
.2. Synthesis of 2-methoxy-6-(tris(hydroxymethyl)methyliminomethyl)phenol
(
H L)
4
The ligand was prepared by condensation of 3-methoxysalicylaldehyde and tris(hydrox-
ylmethyl)amino methane at room temperature. A methanolic solution (10 mL) of tris(hy-
droxylmethyl)amino methane (10 mmol, 1.21 g) was added dropwise to a methanolic
solution (10 mL) of 3-methoxysalicylaldehyde (10 mmol, 1.52 g) with constant stirring. The
resulting yellow reaction mixture was stirred for 2 h and a yellow compound was separated
out and filtered. The crude product was washed with ethanol and diethyl ether, dried and
collected. The yellow product was re-dissolved in methanol and filtered and the filtrate
kept for crystallization under slow evaporation conditions at room temperature. The crys-
talline material was collected by filtration and dried in air to afford H L as deep yellow solid.
4
Yield: 1.786 g (70%). C H NO (255.27): C, 56.46; H, 6.71; N, 5.48%. Found: C, 56.44; H, 6.68;
1
2
17
5
+
−1
N, 5.50%. HRMS: [M + H] , m/z, 256.118 Calcd: m/z, 255.27 (100%). FTIR (KBr), cm : 414 (vs),
28 (s), 613 (w), 662 (w), 727 (w), 851 (s), 877 (w), 913 (m), 962 (m), 1023 (s), 1072 (s), 1167
5
(
1
s), 1232 (vs), 1300 (s), 1372 (vs), 1414 (vs), 1467 (s), 1545 (vs), 1646 (vs), 1818 (w), 1880 (w),
1
952 (w), 2606 (w), 2832 (w), 2916 (w), 2949 (w), 2988 (w), 3425 (br). H NMR (DMSO-d ,
6
1
4
00 MHz, δ, ppm): 4.907 (s, H, phenolic–OH), 8.456 (s, 1H, imine), 6.944, 6.924 (doublet,
H, Ar), 6.890, 6.871 (doublet, H, Ar), 6.581, 6.563, 6.542 (m, 1H, Ar), 3.611 (s, 6H, CH ), 3.718
s, 3H, OMe), 3.525 (s, 2H, H O). C NMR (DMSO-d , 400 MHz, δ, ppm): 164.266 (−CH=N−),
1
1
2
1
3
(
2
6
1
6
59 (Ar-C-OMe), 149.754 (Ar-C-OH), 124.343 (Ar-C-imine), 114.551-117.174 (Ar-C),
6.520 (-CH2-OH), 61.311 (tertiary carbon), 55.881 (-OCH ). UV–Vis spectra [λ , nm (ε, L
3
max
−
1
−1
4
4
4
mol cm )] (CH OH solution): 202 (8.82 × 10 ), 241 (7.97 × 10 ), 293 (5.78 × 10 ), 420
3
(
2.53 × 104).
2
.3. Synthesis of [Mn(H L) ] (1)
2
2
A methanolic solution (5 mL) of triethylamine (4 mmol, 0.404 g) was added dropwise to a
methanolic solution (10 mL) of H L (2 mmol, 0.510 g) with constant stirring at 27 °C. To this
4
mixture, dropwise addition of methanolic solution (10 mL) of Mn(OAc) ∙4H O (1 mmol,
2
2
0
.245 g) results in a deep brown solution. The whole reaction mixture was stirred for 2 h and
filtered. The filtrate was kept at room temperature. Slow evaporation of the filtrate produces
brown single crystals after a few days. Yield: 0.422 g (75%). Anal. Calcd for C H MnN O
2
4
30
2
10
+
(
561.44): C, 51.29; H, 5.34; N, 4.98%. Found: C, 51.32; H, 5.36; N, 5.33. HRMS: [M + H] , m/z,
−1
5
7
1
62.139 Calcd: m/z, 561.44 (100%). FTIR (KBr), cm : 564 (w), 597 (w), 636 (w), 665 (w), 749 (s),
84 (w), 814 (w), 988 (vs), 1047 (s), 1105 (w), 1137 (w), 1173 (w), 1221 (s), 1251 (vs), 1306 (vs),
374 (w), 1416 (vs), 1467 (s), 1548 (vs), 1623 (w), 2852 (w), 2943 (w), 2985 (w), 3422 (br). UV–Vis
spectra [λ , nm (ε, L mol− cm )] (CH OH solution): 202 (3.33 × 10 ), 276 (4.33 × 10 ), 420
1
−1
5
4
max
3
3
(
6.71 × 10 ).

4
S. MANNA ET AL.
Table 1. Cꢅꢌꢋꢁꢆꢍ ꢂꢆꢁꢆ ꢆꢉꢂ ꢂꢇꢁꢆꢄꢍꢋ ꢃꢊ ꢋꢁꢅꢎcꢁꢎꢅꢇ ꢅꢇfiꢉꢇꢈꢇꢉꢁ ꢃꢊ 1.
Empirical formula
C H MnN O
24 30 2 10
−1
fꢃꢅꢈꢎꢍꢆ ꢈꢆꢋꢋ (g ꢈꢃꢍ )
Cꢅꢌꢋꢁꢆꢍ ꢋꢌꢋꢁꢇꢈ
spꢆcꢇ gꢅꢃꢎp
a (Å)
561.44
tꢇꢁꢅꢆgꢃꢉꢆꢍ
P 4 2 2
1
1
8.1952(10)
37.640(5)
2528.0(7)
4
1.475
0.582
c (Å)
3
V (Å )
Z
−3
D(Cꢆꢍcꢂ) (g cꢈ )
−1
μ (mꢃ-Kα) (ꢈꢈ )
F(0 0 0)
1172
tꢀꢇꢁꢆ ꢅꢆꢉgꢇ (°)
nꢃ. ꢃꢊ cꢃꢍꢍꢇcꢁꢇꢂ ꢂꢆꢁꢆ
nꢃ. ꢃꢊ ꢎꢉꢄqꢎꢇ ꢂꢆꢁꢆ
Rꢄꢉꢁ
3.30–25.34
4885
2303
0.1040
1689
1.056
obꢋ ꢅꢇflꢇcꢁꢋ [I > 2σ(I)]
2
Gꢃꢃꢂꢉꢇꢋꢋ ꢃꢊ fiꢁ (F )
Pꢆꢅꢆꢈꢇꢁꢇꢅꢋ ꢅꢇfiꢉꢇꢂ
169
ꢆ
R , wR (I > 2σ(I))
0.0856, 0.1922
0.1106, 0.2149
0.817, –0.507
1
2
R , wR (ꢆꢍꢍ ꢂꢆꢁꢆ)
1
2
−3
rꢇꢋꢄꢂꢎꢆꢍꢋ, ꢇ Å
ꢆ
2
2
2 2
c
2 2 ½
R (F ) = Σ∣∣F ∣ – ∣F ∣∣/Σ∣F ∣, wR (F ) = [Σw (F – F ) /Σw (F ) ] .
1
ꢃ
ꢃ
c
ꢃ
2
ꢃ
ꢃ
ꢃ
2
.4. Crystallographic data collection and refinement
Data collection for 1 was carried out at room temperature with Mo-Kαradiation (λ = 0.71073 Å)
on a Xcalibur Sapphire3 diffractometer equipped with CCD. Cell refinement, indexing, and
scaling of the data-sets were done using the CrysAlis PRO package [18]. The choice of space
group P4 2 2 (with respect to the enantiomorphous P4 2 2) was confirmed by successful
1
1
3 1
final refinement with Flack parameter of 0.02 (5). The structure was solved by direct methods
2
and subsequent Fourier analyses [19] and refined by full-matrix least squares on F with all
observed reflections [20]. Hydrogens were placed at calculated positions. Publication mate-
rial and calculations were completed using WingX program Version 2013.3 [21]. Crystal data
and details of the structure refinement are given in table 1.
2
.5. Theory and computational methods
All computations were performed using the Gaussian 09 (G09) software package [22] using
Becke’s three-parameter hybrid exchange and the Lee–Yang–Parr non-local correlation func-
tionals (B3LYP) [23], the Becke’s three-parameter Perdew–Wang 1991 functional (B3PW91)
[
24], the Barone’s Modified Perdew–Wang 1991 exchange and Perdew and Wang’s 1991
correlation functionals (MPW1PW91) [25]. In the calculation, the 6-31G (d-p) basis set was
assigned to all elements with the exception of manganese, for which the Los Alamos ECP
plus MBS (LanL2MB) [26] basis set was employed. The geometric structures of the complex
in the ground state (quartet) were fully optimized at the B3LYP, B3PW91 and MPW1PW91
levels. Vibration frequency calculations were performed to ensure that the optimized geom-
etries represent local minima associated with positive eigen values only. To compare the
ground state (S ) stability of keto and enol tautomeric forms of H L, a potential energy scan
0
4
was performed indicating the reaction coordinates corresponding to the change of location
of the proton from Ndonor to Oacceptor. During potential energy scan, all other degrees of free-
dom remain undisturbed without imposing any symmetry limit for the ground S state [27].
0

JOURNAL OF COORDINATION CHEMISTRY
5
Vertical electronic excitations based on B3LYP, B3PW91 and MPW1PW91 functionals were
obtained with time-dependent density functional theory (TDDFT) formalism [28] in meth-
anol using the conductor-like polarizable continuum model (CPCM) [29]. GaussSum [30] was
used to calculate the fractional contributions of various groups to each molecular orbital.
3
. Results and discussion
3
.1. Synthetic aspects
The polydentate ligand H L, prepared by a one pot synthesis employing condensation of
4
the corresponding amine and aldehyde in methanol at room temperature, was characterized
by NMR (figure 1), ESI mass (figure 2), FTIR (figure 1S) and electronic absorption spectra. H4L
is an efficient ligand for formation of 3d metal coordination compounds with versatile
1
13
Figure 1. h (ꢆ) ꢆꢉꢂ C (b) nmr ꢋpꢇcꢁꢅꢆ ꢃꢊ h l.
4

6
S. MANNA ET AL.
Figure 2. esi ꢈꢆꢋꢋ ꢋpꢇcꢁꢅꢆ ꢃꢊ (ꢆ) h l ꢆꢉꢂ (b) 1 ꢅꢇcꢃꢅꢂꢇꢂ ꢄꢉ ꢈꢇꢁꢀꢆꢉꢃꢍ.
4
coordination. Table 1S lists 3d metal complexes reported with this ligand. Using H L, 1 was
4
synthesized at room temperature.
3
.2. Molecular structure of [Mn(H L) ] (1)
2 2
The molecular structure of 1 is illustrated in figure 3 and a selection of bond lengths and
angles is reported in table 2. The complex crystallizes in the non-centrosymmetric space
group P4 2 2 and manganese exhibits a slightly distorted octahedral coordination sphere
1
1
with a N O chromophore. Two tridentate ligands are arranged in mer configuration chelating
2
4
the manganese which is located on a twofold axis bisecting the O1–Mn–O1′ and O3–Mn–O3′
bond angles, and only one ligand is crystallographic-independent. Coordination is achieved
via the phenoxo O(1), the imine N(1), and a deprotonated alcoholic O(3), confirming the
Mn(IV) oxidation state of the metal. The Mn–O bond length of the alkoxy oxygen donor is

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 3. orteP ꢂꢅꢆwꢄꢉg (ꢇꢍꢍꢄpꢋꢃꢄꢂ pꢅꢃbꢆbꢄꢍꢄꢁꢌ ꢆꢁ 30%) ꢃꢊ 1 (ꢋꢌꢈꢈꢇꢁꢅꢌ cꢃꢂꢇ ꢊꢃꢅ pꢅꢄꢈꢇꢂ ꢆꢁꢃꢈꢋ: 1 − y,
− x, 1/2 − z).
1
Table 2. expꢇꢅꢄꢈꢇꢉꢁꢆꢍ ꢆꢉꢂ cꢆꢍcꢎꢍꢆꢁꢇꢂ (ꢎꢋꢄꢉg B3lyP, B3PW91, ꢆꢉꢂ mPW1PW91 ꢊꢎꢉcꢁꢄꢃꢉꢆꢍꢋ) cꢃꢃꢅꢂꢄꢉꢆꢁꢄꢃꢉ
bꢃꢉꢂ ꢍꢇꢉgꢁꢀꢋ (Å) ꢆꢉꢂ ꢆꢉgꢍꢇꢋ (°) ꢊꢃꢅ 1 (ꢄꢉ ꢁꢀꢇ cꢅꢌꢋꢁꢆꢍ ꢁꢀꢇ ꢈꢇꢁꢆꢍ ꢄꢋ ꢍꢃcꢆꢁꢇꢂ ꢃꢉ ꢆ ꢁwꢃꢊꢃꢍꢂ ꢆxꢄꢋ).
Calc
Bond distances
Exp
B3LYP
B3PW91
MPW1PW91
mꢉ(1)–o(1)
mꢉ(1)–o(3)
mꢉ(1)–n(1)
mꢉ(1)–o(1′)
mꢉ(1)–o(3′)
mꢉ(1)–n(1′)
1.941(5)
1.865(6)
2.003(7)
1.941(5)
1.865(6)
2.003(7)
1.853
1.786
1.963
1.853
1.786
1.963
1.846
1.778
1.950
1.846
1.778
1.950
1.840
1.772
1.946
1.840
1.772
1.946
Bond angles
o(1)–mꢉ(1)–o(3)
o(1)–mꢉ(1)–n(1)
o(1)–mꢉ(1)–o(1′)
o(1)–mꢉ(1)–o(3′)
o(1)–mꢉ(1)–n(1′)
o(3)–mꢉ(1)–n(1)
o(3)–mꢉ(1)–o(1′)
o(3)–mꢉ(1)–o(3′)
o(3)–mꢉ(1)–n(1′)
n(1)–mꢉ(1)–o(1′)
n(1)–mꢉ(1)–o(3′)
n(1)–mꢉ(1)–n(1′)
o(1′)–mꢉ(1)–o(3′)
o(1′)–mꢉ(1)–n(1′)
o(3′)–mꢉ(1)–n(1′)
172.0(3)
174.043
90.046
88.256
88.797
92.164
84.891
88.797
94.591
93.015
92.164
93.015
176.921
174.043
90.046
84.891
174.261
90.239
88.054
88.972
91.838
84.950
88.972
94.399
93.081
91.838
93.081
177.109
174.261
90.239
84.950
174.210
90.171
87.969
89.097
92.128
84.948
89.097
94.249
92.871
92.128
92.871
176.803
174.210
90.171
84.948
89.7(3)
86.7(3)
89.6(2)
98.5(3)
83.8(3)
89.6(2)
95.0(3)
88.6(3)
98.5(3)
88.6(3)
168.8(4)
172.0(3)
89.7(3)
83.8(3)
Pꢅꢄꢈꢇꢂ ꢆꢁꢃꢈꢋ ꢆꢁ −y + 1, −x + 1, −z + 1/2.
slightly shorter (1.865(5) Å) in comparison to the value measured for the phenolato oxygen
1.941(5) Å). The two Mn–N bond distances in trans positions are 2.003(7) Å. The formation
of the five- and six-membered chelating rings leads to coordination bond angles slightly
(

8
S. MANNA ET AL.
away from ideal octahedral values and the N–Mn–N bond angle of 168.8(4)° deviates most.
Usually this hydroxyl-rich ligand gives polynuclear species [31] and only in one case the
II
same ligand afforded a mononuclear [Mn (H L) ]∙(MeOH)∙(H O) complex crystallizing in a
3
2
2
different space group [5]. While the Mn-O(phenol) and Mn–N bond lengths (mean value
.011 and 2.027 Å, respectively) have comparable values to those here, the Mn–OH bonds
mean length 2.134 Å) are significantly longer than the value of 1.867(6) Å measured in 1
2
(
involving an alkoxide group. Mn–O and Mn–N distances are comparable to those of various
reported Mn(IV) complexes containing similar ligands [32]. The uncoordinated hydroxyl
groups have high thermal motion indicating positional disorder. The crystal packing shows
a bifurcated hydrogen bond between O4–H with O1 and O2 of an adjacent complex (at x − 1,
y, z) with O⋯O distances of 2.908(11) and 2.840(13) Å, respectively. O4 behaves as an accep-
tor with respect to O5–H at –y, 1 − x, 0.5 − z (O5⋯O4 = 3.047(16) Å). Although these inter-
actions are rather weak with possibly disordered H atoms, they form a layered arrangement
of complexes parallel to the ab plane (figure 4).
Figure 4. Pꢆckꢄꢉg ꢂꢄꢆgꢅꢆꢈ ꢃꢊ 1 ꢂꢃwꢉ ꢁꢀꢇ c ꢆxꢄꢋ wꢄꢁꢀ ꢄꢉꢂꢄcꢆꢁꢄꢃꢉ ꢃꢊ o–h⋯o ꢀꢌꢂꢅꢃgꢇꢉ bꢃꢉꢂꢋ wꢀꢄcꢀ ꢊꢃꢅꢈ
ꢆ ꢍꢆꢌꢇꢅꢇꢂ ꢆꢅꢅꢆꢉgꢇꢈꢇꢉꢁ ꢃꢊ cꢃꢈpꢍꢇxꢇꢋ.

JOURNAL OF COORDINATION CHEMISTRY
9
The +4 oxidation state of manganese in 1 has been confirmed also using Bond Valence
Sum analysis [33], which indicates a value of 3.88.
3
.3. Geometrical optimization and electronic structure
To establish electronic structure and spectral transitions of H L and of 1, DFT/TDDFT com-
4
putations were performed. The geometric structures of the isolated ligand and 1 were fully
optimized using B3LYP, B3PW91, and MPWIPW91 functional in the ground state (singlet for
the ligand and quartet for 1). The optimized structures of the ligand using different func-
tionals are reported in the Supplementary Material (figure 2S). The potential energy scan of
the ligand in the singlet ground state shows that the keto–tautomeric form is more stable
by 1.255 kcal mol− than the corresponding enol (figure 5).
1
The optimized structures of 1 using different functionals are shown in figure 3S.
Comparison of bond lengths and angles between calculated and the X-ray structure is shown
in table 2. Among the different functionals, B3PW91 is the best approach to reproduce the
experimental results. Selected molecular orbitals along with their energies and contributions
from the ligands and metal are given in figures 6 and 4S–6S. For H L, the energies of highest
4
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are
–
6.06 and –1.76 eV, respectively. For 1, HOMO and LUMO energies are –2.87 and 0.62 eV,
respectively. Thus the energy difference of the frontier molecular orbitals HOMO-LUMO is
2
−
4
1
2
.3 eV in H L and 3.49 eV in 1. Contribution to the following orbitals, HOMO-5 [82% H L ,
4
2
2
−
2−
2−
8% Mn], HOMO-4 [86% H L , 14% Mn], HOMO-3 [99% H L , 1% Mn], HOMO-2 [98% H L ,
2
2
2
2
−
2−
2−
% Mn], HOMO-1 [97% H L , 3% Mn], HOMO [79% H L , 21% Mn], LUMO [98 % H L , 2%
2
−
2
2
2
2−
2−
Mn], LUMO + 1 [99% H L , 1% Mn], LUMO + 2 [57% H L , 43% Mn], LUMO + 3 [53% H L ,
2
2
2
2
−
2−
4
7% Mn], LUMO + 4 [98% H L , 2% Mn], and LUMO + 5 [95% H L , 5% Mn] indicates that
2
2
this is greatly larger for the Schiff base ligand than for the manganese ion.
TDDFT calculations in methanol using CPCM were performed and theoretically possible
spin-allowed (singlet–singlet in H L and quartet–quartet in 1) electronic transitions with
4
their assignments are listed in tables 2S and 3S for 1 and H L, respectively. The TDDFT results
4
show that for H L, HOMO → LUMO + 4 and HOMO→LUMO transitions are the possible highest
4
Figure 5. Pꢃꢁꢇꢉꢁꢄꢆꢍ ꢇꢉꢇꢅgꢌ cꢎꢅvꢇ ꢆꢁ ꢁꢀꢇ gꢅꢃꢎꢉꢂ s ꢋꢁꢆꢁꢇ ꢊꢃꢅ h l cꢆꢍcꢎꢍꢆꢁꢇꢂ ꢆꢁ ꢁꢀꢇ dft/B3PW91 ꢍꢇvꢇꢍ.
0
4

10
S. MANNA ET AL.
Figure 6. sꢎꢅꢊꢆcꢇ pꢍꢃꢁ ꢃꢊ ꢁꢀꢇ ꢊꢅꢃꢉꢁꢄꢇꢅ ꢃꢅbꢄꢁꢆꢍꢋ (α moꢋ) ꢃꢊ 1 ꢆꢍꢃꢉg wꢄꢁꢀ ꢁꢀꢇꢄꢅ ꢇꢉꢇꢅgꢄꢇꢋ ꢆꢉꢂ cꢃꢈpꢃꢋꢄꢁꢄꢃꢉꢋ
ꢎꢋꢄꢉg B3PW91 ꢊꢎꢉcꢁꢄꢃꢉꢆꢍ).
(
and lowest energy electronic transitions, respectively, and both these transitions are intra-
ligand π → π* in nature. For 1, HOMO-6(β) → LUMO + 4(β) and HOMO(β) → LUMO + 1 (β) are
the possible highest and lowest energy electronic transitions, respectively. Highest energy
electronic transition is interligand charge transfer (ILCT), whereas the lowest energy one is
a ligand to metal charge transfer (LMCT).
Results of TDDFT computation using B3LYP and MPW1PW91 functionals are given in
tables 4S–7S and figures 7S–10S. Comparison of the data derived fromTDDFT computations,
using B3PW91, B3LYP, and MPW1PW91 functionals, reveals that B3PW91 provided spectro-
scopic data (tables 3, 4, 2S and 3S, figure 11S) that more closely reproduce the experimental

JOURNAL OF COORDINATION CHEMISTRY
11
Table 3. sꢇꢍꢇcꢁꢇꢂ uV–Vꢄꢋ ꢇꢉꢇꢅgꢌ ꢁꢅꢆꢉꢋꢄꢁꢄꢃꢉꢋ ꢆꢁ ꢁꢀꢇ tddft/ (B3lyP, B3PW91, mPW1PW91) ꢍꢇvꢇꢍ ꢊꢃꢅ
4
−1
−1
h l. (expꢇꢅꢄꢈꢇꢉꢁꢆꢍ vꢆꢍꢎꢇꢋ ꢆꢅꢇ: λ = 202 ꢉꢈ, ε = 8.82 × 10 m cꢈ , ꢇV = 6.12; λ = 241 ꢉꢈ,
ꢇxp
4
ꢇxp
ꢇxp
ꢇxp
4
−1
−1
4
−1
−1
ε
ε
= 7.97 × 10 m cꢈ , ꢇV = 5.13; λ = 293 ꢉꢈ, εꢇxp = 5.81 × 10 m cꢈ , ꢇV = 4.93 ꢆꢉꢂ λ = 420 ꢉꢈ,
ꢇxp ꢇxp
4 −1 −1
= 2.51 × 10 m cꢈ , ꢇV = 2.95).
ꢇxp
λcal (nm), εcal(M−1 cm ),
−1
Oscillator
Excited
state
(eV)
strength (f)
Key transition
Character
3
B3lyP
s3
s4
s12
s14
s15
s24
s25
428.57, 1.33 × 10 , (2.89)
413.14, 1.42 × 10 , (3.00)
273.07, 3.05 × 10 , (4.54)
238.61, 3.17 × 10 , (5.19)
237.43, 4.35 × 10 , (5.22)
205.97, 1.03 × 10 , (6.01)
203.99, 1.33 × 10 , (6.07)
200.12, 2.15 × 10 , (6.19)
426.26, 1.34 × 10 , (2.90)
414.18, 1.41 × 10 , (2.99)
273.51, 2.79 × 10 , (4.53)
240.35, 1.08 × 10 , (5.15)
234.55, 4.86 × 10 , (5.28)
206.40, 5.48 × 10 , (6.00)
202.37, 9.90 × 10 , (6.12)
425.08, 1.35 × 10 , (2.91)
0.0139
0.0183
0.0603
0.0029
0.0008
0.0046
0.0046
0.0091
0.009
homo-2 → lumo (49%)
homo-2 → lumo+1 (49%)
homo-2 → lumo (79%)
homo-3 → lumo (89%)
homo → lumo+2 (88%)
homo-1 → lumo+2 (63%)
homo → lumo+7 (48%)
homo-1 → lumo+3 (82%)
homo-2 → lumo+1 (48%)
homo-1 → lumo (34%)
homo-2 → lumo (80%)
homo-3 → lumo (86%)
homo-4 → lumo (98%)
homo → lumo+5 (46%)
homo → lumo+4(54%)
homo-2 → lumo (48%)
homo-2 → lumo (81%)
homo-3 → lumo (81%)
homo → lumo+2 (85%)
homo → lumo+4 (75%)
homo-1 → lumo+2 (34%)
homo-5 → lumo (89%)
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
3
4
3
3
4
4
4
2
6
3
B3PW91
s4
s5
3
0.008
4
s12
s14
s15
s21
s22
s3
s12
s14
s15
s21
s22
s23
0.0425
0.0025
0.0036
0.0017
0.0038
0.0058
0.032
0.0142
0.0003
0.0174
0.0116
0.0049
3
3
3
3
3
mPW1PW91
4
266.98, 2.76 × 10 , (4.64)
229.06, 8.55 × 10 , (5.41)
229.01, 8.65 × 10 , (5.41)
204.90, 7.31 × 10 , (6.05)
200.89, 8.49 × 10 , (6.17)
200.41, 9.16 × 10 , (6.18)
3
3
3
3
3
values. Results of geometry optimization among the three functions shows B3LYP provides
closest calculated values of the experimental geometrical parameters (table 2); all calculated
bond lengths of the three functionals are much shorter than the experimental values.
3
.4. ESI mass spectrometry
ESI mass spectra (figure 2) of H L and 1 are recorded in methanol. The ESI mass spectrometric
4
+
data of H L show a peak at m/z = 256.118, which corresponds to the [C H NO + H] mono
4
12 17
5
cation, confirming the chemical composition of ligand. ESI mass spectrometric data of 1
+
show two main peaks at m/z = 256.118 and 562.139, corresponding to [C H NO + H] and
1
2
17
5
+
[
Mn(C H NO ) + H] mono cations, respectively. This observation indicates that the chem-
12 15 5 2
ical composition of 1 in methanol solution is same as detected in the solid state.
3
.5. EPR spectral results
The EPR spectrum of 1 was recorded with X-band frequency (9449 MHz) at liquid nitrogen
temperature. The main characteristic of the spectrum (figure 7) is a strong and broad signal
at g ~4.9 and one weak but resolved resonance at g ~2. Mn hyperfine coupling was not
detected for either signal. The observed ESR spectrum corresponds to the presence of a
Mn(IV) center in 1 [34]. The axially distorted Mn(IV) compound with zero-field splitting
parameter shows one strong resonance at g ~4 and a second weak resonance at g ~2. Thus
low temperature EPR spectral measurement of 1 indicates that manganese is in + 4 oxidation
state with a large axial zero-field splitting parameter i.e. 2-D >> hν [11].

12
S. MANNA ET AL.

JOURNAL OF COORDINATION CHEMISTRY
13

14
S. MANNA ET AL.
3
.6. IR spectral results of H L and 1
4
The most important absorption bands in IR spectra (figure 1S) of ligand and 1 are summarized
in the Experimental section and tabulated in tables 8S and 9S. The spectrum of 1 exhibits a
−1
strong broad band from 3200–3600 cm due to ν(O–H) of free hydroxyl [35] of the ligand.
−
1
−1
Bands at 2985 cm and 2949 cm correspond to the aromatic ν(C–H); bands at 2852 and
−
1
−1
2
832 cm , 1137 and 1167 cm correspond to aliphatic ν(C–H) and ν(O–CH ), respectively.
Stretching vibrations of aliphatic-CH group of 1 are at 859 (ρ ), 1173 (ρ ) and 1306 cm (ρ ),
3
−
1
2
r
t
w
whereas stretching vibrations of aliphatic-CH of the ligand are at 851 (ρ ), 1072 (ρ ) and
2
r
t
−1
1
372 cm (ρ ). Theoretically possible IR spectral bands of optimized structures (using 6-31G
w
(
d-p) and LanL2MB basis sets; CPCM model in methanol) are listed in tables 8S and 9S, and
are in agreement with the experimental results.
3
.7. Electronic absorption spectral results of H L and 1
4
The electronic spectra of H L and 1 were recorded using 1.639 × 10− mM methanolic solu-
3
4
tion. The spectrum of H L (figure 8) shows significant transitions at 202 nm (ε ~
4
4
−1
−1
4
−1
−1
8
.82 × 10 L mole cm ), 241 nm (ε
~
7.97 × 10 L mole cm ), 293 nm (ε
~
Figure 7. X-bꢆꢉꢂ ePr ꢋpꢇcꢁꢅꢎꢈ ꢃꢊ 1 ꢅꢇcꢃꢅꢂꢇꢂ ꢄꢉ ꢈꢇꢁꢀꢆꢉꢃꢍ ꢆꢁ ꢍꢄqꢎꢄꢂ ꢉꢄꢁꢅꢃgꢇꢉ ꢁꢇꢈpꢇꢅꢆꢁꢎꢅꢇ.
Figure 8. tꢀꢇꢃꢅꢇꢁꢄcꢆꢍ (ꢎꢋꢄꢉg B3PW91 ꢊꢎꢉcꢁꢄꢃꢉꢆꢍ) ꢆꢉꢂ ꢇxpꢇꢅꢄꢈꢇꢉꢁꢆꢍ ꢇꢍꢇcꢁꢅꢃꢉꢄc ꢋpꢇcꢁꢅꢆ ꢃꢊ h l. oꢉ ꢁꢀꢇ ꢅꢄgꢀꢁ
4
ꢁꢀꢇ ꢈꢃꢋꢁ pꢅꢃꢈꢄꢉꢇꢉꢁ moꢋ ꢄꢉvꢃꢍvꢇꢂ ꢄꢉ ꢁꢅꢆꢉꢋꢄꢁꢄꢃꢉꢋ ꢆꢉꢂ ꢁꢀꢇꢄꢅ ꢂꢄꢆgꢅꢆꢈꢋ.

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 9. expꢇꢅꢄꢈꢇꢉꢁꢆꢍ ꢆꢉꢂ cꢆꢍcꢎꢍꢆꢁꢇꢂ (ꢎꢋꢄꢉg B3PW91 ꢊꢎꢉcꢁꢄꢃꢉꢆꢍ) ꢇꢍꢇcꢁꢅꢃꢉꢄc ꢆbꢋꢃꢅpꢁꢄꢃꢉ ꢋpꢇcꢁꢅꢆ ꢃꢊ 1. oꢉ
ꢁꢀꢇ ꢅꢄgꢀꢁ ꢁꢀꢇ ꢈꢃꢋꢁ pꢅꢃꢈꢄꢉꢇꢉꢁ moꢋ ꢄꢉvꢃꢍvꢇꢂ ꢄꢉ ꢁꢅꢆꢉꢋꢄꢁꢄꢃꢉꢋ ꢆꢉꢂ ꢁꢀꢇꢄꢅ ꢂꢄꢆgꢅꢆꢈꢋ.
4
−1
−1
4
−1
−1
5
.81 × 10 L mole cm ) and 420 nm (ε ~ 2.51 × 10 L mole cm ). Results of TDDFT cal-
culations and details of spectral assignments (table 3) indicate that the experimental spectral
transitions at 202, 241, 293, and 420 nm correspond to HOMO → LUMO + 4 (λ , 202 nm),
cal
HOMO-3 → LUMO (λ , 240 nm), HOMO-2 → LUMO (λ , 273 nm), and HOMO-1→LUMO (λ ,
cal
cal
cal
414 nm) transitions, respectively. All of these transitions are intraligand π → π* in nature.
The electronic absorption spectrum of 1 (figure 9) shows transitions at 202 nm
5
−1
−1
4
−1
−1
(
1
ε ~ 3.36 × 10 L mole cm ), 276 nm (ε ~ 4.33 × 10 L mole cm ), 353 nm (ε ~
4
−1
−1
3
−1
−1
.22 × 10 L mole cm ), and 420 nm (ε ~ 7.93 × 10 L mole cm ). TDDFT calculation and
spectral assignments (table 4) indicate that experimental electronic spectral transition at
276 nm corresponds to several theoretically possible transitions in the region 270 to 280 nm
and these transitions are mixture of inter ligand charge transfer (ILCT) and ligand to metal
charge transfer (LMCT). Experimental weak band at 353 nm corroborates several theoretically
possible transitions (ILCT/LMCT) from 350 to 356 nm. Experimentally observed very weak
band at ~420 nm corresponds to theoretically possible transitions from 410 to 430 nm. These
transitions are also mixture of ILCT and LMCT.
Experimental bands are rather weak, and calculated oscillator strengths are small, but
calculated epsilon coefficients are large. This is due to that fact that oscillator strength (f)
and epsilon coefficient (ε) are related with the following relation [36]:
−9
f = 4.33 × 10
휀 d ̄휈
̄휈
ꢀ
−
9
Right side of this equation is multiplied by 10 and hence oscillator strength (f) is very small
even for very high value of epsilon coefficient (ε).
4
. Conclusion
We have presented the synthesis, single-crystal structure and DFT/TDDFT calculation of a
six-coordinate Mn(IV) complex, a rarely investigated species. In the mononuclear complex
two tridentate ligands are arranged mer and O–H⋯O hydrogen bonding interactions of
uncoordinated hydroxyl groups result in formation of a 2-D supramolecular structure in the
solid state. EPR spectral study shows a strong and a weak signal at g ~4.9 and ~2,

16
S. MANNA ET AL.
respectively, which corroborate the presence of +4 oxidation state of manganese. DFT/
TDDFT calculations using different functionals with 6-31G (d-p) and LanL2 MB basis sets
have been carried. DFT calculation on the isolated complex is in agreement with structural
results obtained by X-ray diffraction. TDDFT calculations were also performed and theoret-
ically possible spin-allowed, quartet–quartet electronic transitions along with their assign-
ments have been discussed.
Supplementary material
CCDC 1440349 contains the supplementary crystallographic data for 1. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223–336-033; or E-mail: deposit@
ccdc.cam.ac.uk. IR and electronic absorption spectra; results of DFT/TDDFT calculations.
Acknowledgements
The authors gratefully acknowledge the financial assistance given by the CSIR, Government of India,
to Dr. Subal Chandra Manna [Project No. 01 (2743)/13/EMR-II].
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the CSIR, India [grant number 01 (2743)/13/EMR-II].
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