Solid-state valence tautomeric octahedral {CoII[(BQ-N-Cat)]2}0 complex formation via ligand-centered phenolic C-O bond breaking and Co-O bond making

Article abstract of DOI:10.1039/c6cc06433g

Ligand H₄L^O(AP/AP) underwent ligand-centered C-O bond cleavage during a complexation reaction with Co(ii)-salt. The thus formed octahedral {Co^II[(BQ-N-Cat)]₂}⁰ complex showed valence tautomerization in the solid state. While the process was triggered by the presence of lattice solvent, the nature of the solvent molecule has less effect on the process.

Full text of DOI:10.1039/c6cc06433g

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COMMUNICATION
II
0
A Solid-State Valence Tautomeric Octahedral {Co [(BQ-N-Cat)] }
2
Complex Formation via Ligand-Centered Phenolic C-O Bond
breaking and Co-O Bond making
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Manas Kumar Mondal, Archana Tiwari and Chandan Mukherjee*
O(AP/AP)
Ligand H
4
L
underwent ligand-centered C-O bond cleavage form. To initiate an oxidative C-C cross coupling reaction via the
during complexation reaction with Co(II)-salt. Thus formed oxidative addition of both alkyl and halide units to the Co center in
II
0
}
octahedral {Co [(BQ-N-Cat)]
2
complex showed valence a Co complex requires cis- orientation between the alkyl and the
tautomerization in solid state. While, the process was triggered by halide units. Therefore, cis- orientation is necessary between the
the presence of lattice solvent, the nature of the solvent molecule two coordinated non-innocent ligands to create a vacant site for cis-
III
2-
-
has less effect to the process.
2
coordination of alkyl halide. In square planar [Co ([AP] ) ] complex
the two coordinating-ligands orient in trans- fashion. Thus,
oxidative addition of both alkyl and halide units was not possible in
the system.
Transition metal complexes, which are coordinated to redox-active
ligands, have achieved great importance in the field of biomimetic
catalysis, C-H bond activation, C-C cross coupling, C-N bond
activation, bistable as well as valence tautometic complex
1
-3
formation, etc. The redox-active ligands, which are coordinated
to metal ion, may exist in their various oxidation states.
a
Furthermore, a redox-active ligand coordinated to a metal ion in
multiple numbers can exist in different oxidation states. Soper et al.
have reported that the electrophilic and nucleophilic behaviours of
square planar Co(III) complexes depend on the redox state of the
2
coordinated redox-active ligands. Redox-active ligand-coordinated
cobalt complexes have recently been reported as catalysts for C-C
1
g,2
coupling reactions.
Apart from the catalytic reactivity, valence
tautomerization between Co(III)-semiquinone to Co(II)-quinone
electronic isomers are under progressive investigation for the
demand of logic gates, bistable, and spin-switch materials
3
developments. Thus, the study of cobalt complexes that are
O(AP/AP)
coordinated to redox-active ligands has drawn special attention.
Scheme 1. Schematic representation of ligand
1
H
formation. X-ray molecular structure of complex 1CHCl .
4
L
and complex 1
•
3
2
-Anilino-4,6-di-tert-butylphenol (H
2
AP) is an established
4
bidentate non-innocent ligand. The square planar Co(III) complex
As a foremost step to impose non-coplanar arrangement
between two radical-generation units, two H AP ligands were
2
-
that formed by the coordination of two equivalents of [AP] ligand,
III
2-
-
2
[
Co ([AP] )
2
] , is known to behave as a strong nucleophile and
combined by a bridging O atom that connects two ligands via
-
undergoes 2e oxidative addition with alkyl halides. In this process,
the ortho- carbon atom of the two aniline moieties. Thus
O(AP/AP)
formed new ligand will be designated here as H L
4
2
-
each [AP]
ligand undergoes one-electron oxidation and
consequently, alkyl-coordinated square pyramidal complexes
(
Scheme 1; Figure S8, S12). Because of the O-bridge the angle
around the O atom would be 105 , hence, two aniline rings
of the two H AP units in ligand H
III
•1-
having general formula of [Co E([SQ]
2 2
) ] (where E = CH Cl, alkyl)
∼
°
O(AP/AP)
2
4
L
could not situate
a.
parallel to each other in coordination complexes. Hence,
coplanar and trans- alignment of the radical-generating 3,5-di-
tert-butyl amidophenolate units could be restricted and the
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati,
81039, Assam, India
7
b.
Department of Physics, School of Physical Sciences, Sikkim University, Gangtok–
37102, Sikkim, India.
7
†
Electronic Supplementary Information (ESI) available: Synthesis, characterization required vacant site for cis- coordination might be availed.
and molecular structures of the ligand and the complexes, proposed mechanism,
crystallographic data. See DOI: 10.1039/x0xx00000x
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Journal Name
O(AP/AP)
Ligand H
4
L
was synthesized in 64% yield by reacting underwent valence tautomerization; at low temperature (77 K)
2
-
1
:2 2,2′-oxodianiline (
hexane under air in the presence of triethyl amine (Et
Scheme 1]. The ligand reacted with equivalent amount of ligands were in (BQ-N-Cat) form and the central metal was
Co(ClO 6H O or CoCl 6H O in CH CN under air for 7 hours Co(II). Vanishing of X-band EPR signal at 295 K was due to zero-
in the presence of Et N and provided blue-green solid field splitting (> 0.3 cm ) that resulted by high spin-orbit
complex ). The solid upon recrystallization from a 5:2 coupling at Co(II). Herein, it is noteworthy that valence
CHCl :CH
crystals (
during X-ray diffraction measurement (Table S2, S16) even at however, the phenomenon is absent in solid state within 2-295
I) and 3,5-di-tert-butyl catechol (II) in the composition of the complex was a Co(III), a (SQ-N-Cat)
1
-
3
N) and a (BQ-N-Cat) , while, at 295 K both the coordinated
1
-
[
4
)
2
•
2
2
•
2
3
-
1
3
(
1
III
3
3
CN solvent mixture appeared as rectangular-shaped tautomerization between octahedral {Co [(SQ-N-Cat)(BQ-N-
0
II
0
3 2
1•1CHCl ). However, the crystals were not stable Cat)]} and {Co [(BQ-N-Cat)] } is well known in solution,
7
1
00 K. Hence, the molecular structure analysis of the formed K temperature range.
complex 1CHCl [Scheme 1; Figure S8, S12; Table S1-S2, S15-
3
1•
S16] was not worthy enough for the definite assignment of
oxidation state of the central metal ion as well as the
coordinated ligands. Nevertheless, from the crystallographic
analysis it was evident that the complex was neutral,
mononuclear-octahedral (mer), and the ligand backbone was
different from that of the initially employed ligand for the
complexation reaction, i.e., new tridentate ligand was formed
during the reaction via ligand-centered phenolic C-O bond
cleavage.
Scheme 3. Valence tautomeric species.
3
-
Scheme 2. Various possible oxidation states of (Cat-N-Cat) species.
Two analogous tridentate ligands, as present in complex
5
, are already available in literatures and it has been
1•
1CHCl
3
demonstrated that the ligands may exist in their three
different stable oxidation states in its corresponding transition
metal complexes (Scheme 2). Therefore, the composition of
the neutral complex
1 can be either, (a) a Co(II) (SCo(II) = 3/2,
1
-
high-spin) ion with two coordinated diamagnetic (BQ-N-Cat)
0
II
ligands, i.e., {Co [(BQ-N-Cat)]
2
} or (b) a Co(III) (SCo(III) = 0, low-
2
spin) with coordinated paramagnetic (SQ-N-Cat) [S
-
L
= 1/2]
1
and (BQ-N-Cat) (SL' = 0) ligands, i.e., {Co [(SQ-N-Cat)(BQ-N-
-
III
0
Cat)]} (Scheme 3).
In order to distinguish between the two possible
6
compositions, X-band EPR measurement on
performed at 77 K in a CH Cl
A). A ligand-centered S = 1/2 signal with a reasonable Co (I =
/2) super-hyperfine interaction was established by the
1
•
0.3CHCl
3
was
2
2
/toluene solvent mixture (Figure
5
9
1
7
Figure 1. X–band EPR spectra measured at 77 and 295 K; (A) Complex 1•0.3CHCl
3
2 2
in a CH Cl /toluene (2:1) solvent mixture: microwave frequency (GHz): 9.444;
modulation frequency (KHz): 100; amplitude = 1.0. (B) Complex
solid state: microwave frequency (GHz): 9.142; modulation frequency (KHz): 100;
amplitude (G) = 0.3; (C) Complex 0.8CH CN in solid state, microwave frequency
GHz): 9.138; modulation frequency (KHz): 100; amplitude (G) = 1.0, and (D)
3
1•0.3CHCl in
simulation of the experimental spectrum. The parameters
1
•
3
(
obtained by the simulation were; g
1
= 2.007, g
2 3
= 2.007, and g
Complex
1 in solid state: X–band microwave frequency (GHz): 9.143; modulation
frequency (KHz): 100; amplitude (G) = 1.0.
5
1.998; giso = 2.004; Co(A
9
-4
-1
=
1
, A
2
, A
3
) = (7, 7, 27) × 10 cm .
Interestingly, it was observed that the signal vanished at 295 K.
The temperature-dependent signal was also hold true for
solid-state X-band EPR measurements of the complex (Figure
Inspired by X-band EPR results, variable-temperature
magnetic susceptibility measurement was carried out on solid
6
0.3CHCl3 at 1 T external magnetic field
sample of complex
1•
1B). The intensity of the signal decreases drastically at 295 K.
to reinforce solid-state valence tautomeric nature of the
In fact, the nature of the observed EPR signal was different
than that of ligand-centered S =1/2 and indeed, very weak in
intensity, i.e., almost X-band EPR silent. Therefore, it can be
argued that in solution as well as in solid-state the complex
complex. The χMT vs T plot is depicted in Figure 2. At 10 K, the
-1
χMT = 0.44 emu K mol was observed. This value was closely
commensurate with the system having one-unpaired electron
2
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(
an S = 1/2 spin). The value remained almost unchanged up to lowering in temperature,
χ
18 K. Interestingly, in between 118 to 125 K temperature 1.71 emu K mol at 5 K. Herein, the decrease in
M
T value decreased and reached at
-1
1
χ
T = 2.34 due to zero-field splitting (D = 13.4 cm , Figure 2B) that arose
M
T value was
-1
range a sharp transition in magnetic moment to
χ
M
-1
emu K mol was observed. The magnetic moment attained at because of strong spin-orbit coupling at Co(II) center. Thus, it
25 K remained almost constant up to 300 K. This feature was apparently evident that in the absence of lattice solvent
1
II
0
supported a valence tautomeric phenomenon in the system, the complex remained in {Co [(BQ-N-Cat)]
where, low-spin diamagnetic Co(III) was reduced to high-spin valence tautomerization took place.
Co(II) by ligand-centric SQ unit that underwent then to its one-
2
} form and no
electron oxidized BQ form (Scheme 3). Therefore, it can be
III
argued that till 118 K the complex was existing in {Co [(SQ-N-
0
Cat)(BQ-N-Cat)]} form, while, above the 125 K the complex
II
0
was continued to exist in {Co [(BQ-N-Cat)]
temperature (T ), which is defined as the temperature at which
the free energy difference ( G) between the two inter-
convertible species is nil, was 122 K.
2
} form. The critical
c
ꢀ
8
Figure 2.
measurement of
χ
M
T vs T plots: (A)
0.3CHCl
1
•
0.3CHCl
3
and
(D) hysteresis measurement of
1
•
0.8CH
3
CN, (B) 1, (C) hysteresis
0.8CHCl .
3
1
•
3
1
•
In order to understand the origin of the tautomerization,
and such a rare and abrupt transition, initially we have tried to
Figure 3. H-honds between the two adjacent molecules: (A) complex
was complex
1CHCl , and (C) complex 1CH CN.
1, (B)
understand the solvent effect. Hence, the complex
recrystallized from; (a) a 3:1 Et O:CH CN solvent mixture, and
b) toluene. Crystals, which were obtained from the two
crystallization processes, were measured by X-ray single crystal
diffraction methods. Indeed, 1 equivalent of CH CN was found of lattice solvent (Table S2, S16). Therefore, it was evident that
to exist in the crystal lattice of the crystal ( 1CH CN) that the presence of lattice solvent molecules within the crystal
obtained from the 3:1 Et O:CH CN solvent mixture (Figure S8D, lattice promoted flexibility or softness
S12). Existence of no solvent molecule was confirmed in the (Figure S9-S10, S13-S14) and favored the required-structural
crystal (complex ) that obtained from toluene (Figure S8C, changes in valence tautomeric transition. Crystal-structure
S12). Noteworthy, the quality of crystals were not good packing analyses revealed that in complex two adjacent
enough for the assignment of acute and appropriate oxidation molecules were firmly held by strong intermolecular H-bonds,
state of ligands as well as metal center (Table S1-S2, S15-S16). while, in complex 1CHCl and complex 1CH CN the
Nevertheless, variable-temperature magnetic susceptibility intermolecular H-bonds were weak in nature (Figure 3). The
1
1•
1•
3
3
2
3
(
The single-crystal volume of
1 increased with incorporation
3
1•
3
3
a,3n
2
3
in the systems
1
1
1•
3
1•
3
6
6
measurements for
solid samples at 1 T external magnetic field. The
are depicted in Figure 2. The temperature-dependent change structural change for valence tautomerization. The abrupt
in T for 0.8CH CN was very similar as that of 0.3CHCl transition within very small (7 K) temperature range (Figure 2)
However, the T for 0.8CH CN was slightly higher ( 4.4 K) in 0.3CHCl and 0.8CH CN reinforced a strong cooperative
compared to 0.3CHCl . Unlike solvated crystals, in case of effect that was propagated through lattice solvent network
complex T = 0.25 emu K mol value remained almost (Figure S9-S10, S13-S14). Although, the crystallographic phases
constant in the temperature range 50-300 K. Upon further and the nature of the lattice solvent molecule were different
1
•
0.8CH
3
CN and
1
were carried out on strong H-bonds, which can be consider as stabilizing factor,
χ
M
T vs T plots presumably prevented complex in undergoing any required
1
χ
M
1
•
3
1•
3
.
c
1
•
3
∼
1
•
3
1•
3
3
a,3n
1
•
3
-1
1
,
χ
M
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for the different complexes (Figure 4), no consequence on the his doctoral fellowship. CIF and Department of Chemistry, IITG
nature of transition, transition temperature, and hysteresis are thankfully acknowledged for the instrumental facilities.
effect (Figure 2C, and 2D) were discerned. These experimental
facts reinforced that no correlative inference can be
c
formulated amongst T , solvent nature, and crystallographic
Notes and references
phases.
1
(a) D. L. J. Broere, R. Plessius and J. I. van der Vlugt, Chem.
Soc. Rev., 2015, 44, 6886; (b) A. L. Smith, L. A. Clapp, K. I.
Hardcastle and J. D. Soper, Polyhedron, 2010, 29, 164. (c) S.
Ghorai and C. Mukherjee, Chem. Commun., 2012, 48, 10180;
(
d) S. H. A. M. Leenders, R. Gramage-Doria, B. de Bruin and J.
N. H. Reek, Chem. Soc. Rev., 2015, 44, 433; (e) S. Ghorai and
C. Mukherjee, Dalton Trans., 2014, 43 394; (f) V.
Lyaskovskyy and B. de Bruin, ACS Catal., 2012, , 270; (g) M.
van der Meer, Y. Rechkemmer, I. Peremykin, S. Hohloch, J.
van Slageren and B. Sarkar, Chem. Commun., 2014, 50
,
2
,
1
3
2
1104; (h) S. Ghorai and C. Mukherjee, Chem.-Asian J., 2014,
518; (i) S. Ghorai, C. Mukherjee, RSC Advances, 2014, 4
4698; (j) L. G. Ranis, K. Werellapatha, N. J. Pietrini, B. A.
,
Bunker and S. N. Brown, Inorg. Chem., 2014, 53, 10203.
A. L. Smith, K. I. Hardcastle and J. D. Soper, J. Am. Chem. Soc.,
2
3
2
010, 132, 14358 and references therein.
(a) E. Evangelio, C. Rodriguez-Blanco, Y. Coppel, D. N.
Hendrickson, J. P. Sutter, J. Campo and D. Ruiz-Molina, Solid
State Sci., 2009, 11, 793; (b) H. Spiering, T. Kohlhaas, H.
Romstedt, A. Hauser, C. Bruns-Yilmaz, J. Kusz and P. Gütlich,
Coord. Chem. Rev., 1991, 111, 275; (c) P. Gütlich, A. B.
Figure 4. X-ray Powder diffractograms of crystalline samples of the complexes.
Gaspar and Y. Garcia, Beilstein J. Org. Chem., 2013, 9, 342;
To conclude, herein we have presented a new tetradentate
O(AP/AP)
(d) N. Azzaroli, A. Lapini, M. D. Donato, A. Dei and R. Righini,
J. Phys. Chem. B., 2013, 117, 15492; (e) D. M. Adams and D.
N. Hendrickson, J. Am. Chem. Soc., 1996, 118, 11515; (f) F.
Novio, E. Evangelio, N. Vazquez-Mera, P. González- Monje, E.
Bellido, S. Mendes, N. Kehagias and D. Ruiz-Molina, Sci. Rep.,
non-innocent ligand [H
salt, Et
4
L
) that in the presence of Co(II)-
0
II
3
2
N and air forms {Co [(BQ-N-Cat)] } complex. During
the complex formation, a weak interaction between the Co
center and the bridging O atom is postulated (Scheme S3, S10;
Figure S7, S11). This interaction drives the formation of Co-O
bond via homolytic phenolic C-O bond cleavage. The complex
undergoes valence tautomerization in solution (Figure S11,
S15) as well as in lattice solvent-containing solid phase. From
the solid state X-band EPR measurements, it was confirmed
that at room temperature (295 K), the complex exists as
2
013, 3, 1708; (g) E. Evangelio and D. Ruiz-Molina, Eur. J.
Inorg. Chem., 2005, 15, 2957; (h) P. Gütlich and A. Dei,
Angew. Chem., Int. Ed. Engl., 1997, 36, 2734; (i) T.
Tezgerevska, K. G. Alley and C. Boskovic, Coord. Chem. Rev.,
2
014, 268, 23; (j) A. Cui, k. Takahashi, A. Fujishima and O.
Sato, J. Photochem. Photobiol. A: Chem., 2004, 161, 243; (k)
l) R. M. Buchanan and C. G. Pierpont, J. Am. Chem. Soc.,
(
1980, 102, 4951; (m) C. G. Pierpont, Coord. Chem. Rev., 2001,
217, 99; (n) E. Evangelio and D. Ruiz-Molina, C. R. Chim.,
2008, 11, 1137; (o) D. Kiriya, H. C. Chang, and S. Kitagawa, J.
Am. Chem. Soc., 2008, 130, 5515; (p) D. Kiriya, H. C. Chang, K.
Nakamura, D. Tanaka, K. Yoneda and S. Kitagawa, Chem.
Mater., 2009, 21, 1980; (q) R. D. Schmidt, D. A. Shultz and J.
D. Martin, Inorg. Chem., 2010, 49, 3162; (r) F. Novio, J.
Campo and D. Ruiz-Molina, Inorg. Chem., 2014, 53, 8742; (l)
R. D. Schmidt, D. A. Shultz, J. D. Martin and P. D. Boyle, J. Am.
Chem. Soc., 2010, 132, 6261.
II
0
{
Co [(BQ-N-Cat)]
III
2
} , while, at low temperature (77 K) it exists
0
as {Co [(SQ-N-Cat)(BQ-N-Cat)]} .
Variable-temperature
magnetic
and
susceptibility
0.8CH CN again
measurements on •
substantiate that the tautomerization is triggered by the
1,
1
0.3CHCl
3
1•
3
presence of lattice solvent molecule. Interestingly, the process
c
and the T value are almost independent to the nature of
lattice-solvent and the corresponding crystallographic phase.
Although, it appears that the valence tautomerization in the
present system is governed by the change in temperature-
dependent lattice packing-structure, a good quality crystal-
structure would lead to confirmation. Therefore, in the near
future a further study will be continued to have better quality
crystals by using various others solvents for crystallization.
Additionally, syntheses of other similar-type of ligands, where
various different substituents would be attached at the main
ligand-backbone, and their corresponding Co-complexes will
also be continued to examine packing effect on valence
tautomerization.
4
5
P. Chaudhuri, C. N. Verani, E. Bill, E. Bothe, T. Weyhermüller
and K. Wieghardt, J. Am. Chem. Soc., 2001, 123, 2213.
(a) C. L. Simpson, S. R. Boone and C. G. Pierpont, Inorg.
Chem., 1989, 28, 4379; (b) A. Sasmal, E. Garribba, C. J.
Gómez-García, C. Desplanches and S. Mitra, Dalton Trans.,
2014, 43, 15958.
As obtained by microanalysis (C, H, N values).
(a) S. K. Larsen and C. G. Pierpont, J. Am. Chem. Soc., 1988,
110, 1827; (b) A. Caneschi, A. Cornia and A. Dei, Inorg.
Chem., 1998, 37, 3419; (c) O. Cador, F. Chabre, A. Dei, C.
Sangregorio, J. V. Slageren and M. G. F. Vaz, Inorg. Chem.,
6
7
2
003, 42, 6432.
–1
–1
–1
8
Complex
complex
1•0.3CHCl : ∆H = 101 kJ mol ; ∆S = 825 J K mol ;
3
–1 –1 –1
1•0.8CH CN: ∆H = 79 kJ mol ; ∆S = 618 J K mol ;
3
This project is supported by SERB, India EMR/2015/002491.
MKM thanks Indian Institute of Technology Guwahati (IITG) for
Calculations were performed based on the mole-fraction
formula as described in reference 7c.
4
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