Monoradical-containing four-coordinate Co(III) complexes: Homolytic S-S and Se-Se bond cleavage and catalytic isocyanate to urea conversion under sunlight

Article abstract of DOI:10.1039/c7cc03486e

Four-coordinate, monoradical-containing Co(iii) complexes participated in the non-innocent ligand driven homolytic cleavage of S-S and Se-Se bonds and catalyzed the conversion of RNCO (R = phenyl and naphthyl) to the corresponding urea derivatives (TON 480) in dry CH₂Cl₂ under sunlight stimulus.

Full text of DOI:10.1039/c7cc03486e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Monoradical-Containing Four-Coordinate Co(III) Complexes:
Homolytic S-S, Se-Se Bond Cleavage and Catalytic Isocyanate to
Urea Conversion Under Sunlight
Received 00th January 20xx,
Accepted 00th January 20xx
a†
a†
a
DOI: 10.1039/x0xx00000x
Ganesh Chandra Paul, Samir Ghorai and Chandan Mukherjee *
www.rsc.org/
Four-coordinate, monoradical-containing Co(III) complexes position of the aniline part. We envisaged that the presence of an
participated in non-innocent ligand driven homolytic cleavage of ortho- substituent would exert steric crowding and consequently,
S-S and Se-Se bonds and catalyzed the conversion of RNCO (R = the possibility of four-coordinate cobalt complex formation over six-
phenyl and naphthyl) to the corresponding urea derivatives (TON coordinate would be favorable. Noteworthy, cobalt complexes with
480) in dry CH
Cl
2 2
under sunlight stimulus.
unsaturated-coordination environment are essential for the
substrate activation and catalysis.
Since last two decades, metal-coordinated redox active organic
ligands, known as non-innocent ligands, are under continuous
investigation as electron acceptor and/or electron donor for
1
,2
catalytic oxidation and reduction reactions.
For instances:
Chaudhuri, Wieghardt, and coworkers have studied Cu(II)-
bis(iminosemiquinone) complexes extensively as functional models
of Galactose Oxidase for the two-electron oxidation of primary
1
h
alcohols to their corresponding aldehydes ; a four-coordinate
Co(III)-bis(amidophenolate) complex has been successfully
employed as a catalyst for C-C bond formation reactions by Soper
1
a
and coworkers. Recently, Sarkar and coworkers have described an
electrocatalytic C-C bond formation reaction employing
electrochemically in situ generated a four-coordinate Co(II)-bis(1,2-
diamide) complex as the catalyst and benzylbromide as the
1
e
substrate. In 2015, van der Vlugt and coworkers have utilized the
redox active nature of a coordinated-2-amidophenoate derivative in
a four-coordinate Pd(II) complex for the one-electron homolytic S-S
1
i
bond cleavage of diphenyl disulfide. Recently, we have Scheme 1. (A) Cartoons of the ligands employed in this study. (B) Three possible
oxidation states of the ligands. Subscriptions AP, ISQ and IBQ stand for
gas can be generated employing Cu(II)- amidophenol, iminosemiquinone, and iminobenzoquinone forms of the ligands,
respectively.
demonstrated that H
bis(iminoquinone) complex and NaBH
2
1
g
4
in dry acetonitrile.
To continue our study on the development of ligand-radical-
containing transition metal complexes for diatomic (homo and/or
Herein, we report on two four coordinate, monoradical-
hetero) bond scission/formation reactions, we have investigated on
III AP(R)
ISQ(R) 0
containing Co(III) complexes (1 and 2; [Co (L )(L
)] ), which
•6H
AP(R)
few Co(III) complexes based on the non-innocent ligands H
2
L
(R
were synthesized by reacting the ligands with Co(ClO
4
)
2
2
O
=
-Me and -Ph), shown in Scheme 1. The ligand scaffolds were
under air. Thus formed complexes reduced diphenyl disulfide and
diphenyl diselenide by an electron and provided the corresponding
five-coordinate, diradical-containing complexes where the axial
position was occupied by a –XPh group (X = S(1a/2a) and Se(2b);
primarily based on bidentate 2-anilino-3,5-di-tert-butylphenol
AP
2
(H L ) backbone with methyl and phenyl substituents at the ortho-
III ISQ(R)
0
a.
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati,
81039, Assam, India
GCP and SG contributed equally.
parenthesis indicates the complex; [Co (L
2
) XPh] ). Hence, less
7
explored one-electron reduction of S-S and Se-Se bond has been
investigated. Interestingly, the four- and five-coordinate complexes
†
Electronic Supplementary Information (ESI) available: Experimental details; IR,
NMR, mass, UV-vis-NIR spectra of ligands and complexes; bond distances and bond under sunlight in a close-vessel in dry CH Cl catalyzed the
angles tables for the complexes. See DOI: 10.1039/x0xx00000x
2
2
conversion of phenyl isocyanate and naphthyl isocyanate to the
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corresponding urea derivatives. A maximum TON 480 in 6 hours has mixture. Noteworthy, complex 2 could not be obtained as single
been achieved.
crystal due to solubility limitation. Furthermore, the complex
abstract chlorine atom from chlorinated solvents (e.g. CH Cl
CHCl ) and provided the corresponding five-coordinated, diradical-
2
2
,
3
containing Co(III) complex 2c (Figure S22). Similar reaction has also
been observed for complex 1, which provided complex 1b (Figure
S21). Nevertheless, X-band EPR and UV-vis-NIR spectral pattern
analyses (vide infra) consolidated that 1 and 2 have similar-type of
geometry and coordination environment.
3
Complex 1 acquired square planar geometry (
τ
= 0.0). The
coordination sphere was comprised of two nitrogen atoms and two
oxygen atoms form two ligands (Figure 1A). The Co–O = 1.830(3)
and Co–N = 1.832(4) (Table S2) bonds were in accord with the
4
previously reported Co(III) square planar complexes. Hence, the
formal oxidation state of the Co atom in the complex was assigned
to +III. It is to note that complex 1 was neutral in charge. Therefore,
in order to maintain the neutrality, the two coordinated non-
innocent ligands must be present in the complex in two different
oxidation states, i.e. one-electron oxidized iminosemiquinone and
4
,5
fully reduced amidophenolate states. However, CPh–OPh = 1.323(5)
and CPh–NPh = 1.371(5) Å (CPh, NPh, and OPh stand for the C, N, and O
atoms attached or belong to phenyl ring, respectively) bond
distances (Table S2) in the two coordinated-ligands were same and
corresponded to neither iminosemiquinone (C–N = 1.35 Å; C–O =
1
.30 Å) form nor amidophenolate (C–N = 1.38 Å; C–O = 1.35 Å)
form (Scheme 1). These features, as previously reported by
4
Wieghardt, Chaudhuri and coworkers, implied the delocalization of
the negative charge and the radical (hole) over the two
coordinated-ligand units.
Scheme 2: Schematic representation for the syntheses.
The condensation between 1:1 2-R-aniline (R = -Me and -Ph)
and 3,5-di-tert-butylcatechol in the presence of Et
3
N and air
AP(R)
generated H
atmosphere H
with 0.5 equivalent of Co(ClO
presence of Et N for an hour to synthesize the corresponding four-
coordinate, monoradical-containing Co(III) complexes (1; 83% yield
2
and 2; 53% yield). Noteworthy, the employment of CoCl •6H O
2
L
ligands in good yields [Scheme 2(A)]. Under aerial
AP(Me)
AP(Ph)
2
L
, and H
2
L
ligands were reacted individually
•6H O in acetonitrile in the
4
)
2
2
3
2
during the complex synthesis should be restricted as the salt assists
the formation of five-coordinate diradical-containing Co(III)
complexes by the one-electron oxidation of the initially formed
four-coordinate, monoradical-containing Co(III) complexes through
putative inner sphere electron transfer mechanism (Scheme S2).
The addition of either diphenyl disulfide (Ph
2 2
S ) or diphenyl
diselenide (Ph Se ) during the synthesis of 1 and 2 provided the Figure 1. ORTEP plots of: (A)
2
2
1, (B) 1a, (C) 2a and (D) 2b. Thermal ellipsoids were
drawn at 50% probability level. H atoms and methyl groups of the tert-butyl
corresponding five-coordinate diradical-containing 1a/2a and 2b groups (for 1a
2a and 2b) were omitted for the sake of clarity. In , C13 atom is
disordered in 65:35 ratio between C13A and C13B atoms.
,
1
complexes. The complexes could also be synthesized by reacting
Ph or Ph Se to the isolated complexes 1 and 2 (Scheme 2B). X-
2
S
2
2
2
All the five-coordinate neutral complexes were almost square
band EPR measurements during the conversions showed an
instantaneous disappearance of X-band EPR signal of complexes 1
and 2 (vide infra) with concomitant generation of an isotropic signal
at g ∼ 2.00 (Figure S33). This feature implied the formation of PhX•
3
pyramidal (
τ = 0.07[1a], 0.08[2a], and 0.0[2b]). In the complexes,
the central cobalt atom (Co1) was situated above the N2O2 basal
plane, and towards the apical –XPh group (Figure 1). The Co–O =
1
.858-to-1.881 Å, and Co–N = 1.845-to-1.871 Å bond distances
(X = S and Se) species along with diamagnetic 1a/2a and 2b.
(
Table S2-S3, S5-S6) in the complexes corresponded to previously
Single crystal suitable for X-ray diffraction analysis for complex 1
reported square pyramidal cobalt complexes having +III oxidation
was obtained by slow evaporation of a 10:1 Et
2
O:CH
3
CN solvent
4
state. Thus, herein, the oxidation state of the central cobalt atom
2
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has been assigned as +III. All the C−C bonds of the tert-butyl-groups ligand(iminosemiquinone) to ligand(iminosemiquinone) charge
containing C phenyl rings were not within 1.39±0.01 Å, rather, an transfer band was present at around 830 nm (Figure S24, S27-S28).
alternating short-long-short, i.e., a quinoid-type distortion, has Thus, the conversion of four-coordinate [Co (L
6
III AP(R)
ISQ(R) 0
)(L
)] to five-
III ISQ(R)
0
been observed (Table S2-S5). Furthermore, average CPh–OPh
=
=
coordinate [Co (L
) XPh] or vice versa could be monitored by
2
1
.311(4)[1a], 1.307(3)[3a] 1.298(8)[2b]
Å
and
C
Ph–NPh
analyzing the band at 1600 nm.
1.349(4)[1a], 1.351(4)[2a], 1.362(9)[2b] Å bond distances were in
between of their single bond and double bond values and
4,5
emphasized the one-electron oxidized iminosemiquinone form of
the coordinated ligands in the complexes. Hence, X-ray single
crystal analyses suggested that four-coordinate complexe 1 was
monoradical-containing, while, the five-coordinate complexes (1a,
2a, and 1b) were diradical-containing. Noteworthy, better quality
crystal data would have been more convincing, however, diffraction
measurements even at 100 K could not improve the staructural
quality, which is due to the nature of the crystals.
Table 1. Optimization of urea derivatives formation.⊥
#
Substrate
Product
Cat (mol %)
2
Isolatedyield (%)
1
2a
2.5
0
1
72
0
2
2a
76
0
1
2
3
4
5
6
7
III
III
I
IV
NR
II
2.5
0
2.5
0
78
0
2.5
0
2.5
0
2.5
0
80
0
86
0
82
0
I
NR
IV
a
III+Ph
2
S
2
2.5
2.5
1.0
0.1
-
68
75
72
48
-
III
IV
−
−
−
−
−
−
−
−
III
IV
Figure 2. X-band EPR spectra of: (A)
temperature 25 C; microwave frequency (GHz)
modulation frequency (kHz) = 100[ ]; modulation amplitude (G) = 20.0[
0[ ]; and microwave power (mW) = 0.998[ ], 0.995[ ].
1
and (B)
2
in CH
2
Cl
2
=
solution. Conditions:
9.437[ ], 9.442[ ];
],
Absence of sunlight
=
°
1
2
1
1
,
2
8
III
III+Ph
I
IV
IV
II
2.5
2.5
2.5
2.5
2.5
2.5
2.5
-
10
12
12
11
15
-
5
2
1
2
a
9
2
S
2
9
1
0
2.5
10
10
The five-coordinate diradical-containing square pyramidal
⊥
NR stands for no reaction. The temperature of the reaction bath was 36 °C. All the
a
Co(III) [low-spin; SCo = 0] complexes (1a, 2a, and 2b) were reactions were carried out for 6 hours in a closed tube. 1:1 mixture.
diamagnetic owing a strong antiferromagnetic coupling between
the two ligand-centered π-radicals (S
character of the complexes was further supported by H NMR irradiation, the 1600 nm band was started appearing with the
R
= 1/2). The diamagnetic
2 2
When CH Cl solutions of 1a and 2a were subjected to sunlight
1
analysis (Figure S18-S20). In the monoradical-containing four- concomitant shifting of ∼570 nm band to 660 nm (Figure S30-S31).
coordinate square planar Co(III) complexes (1, and 2), the metal This phenomenon indicated homolytic Co-SPh bond cleavage and
2
z
center possesses two unpaired electrons, which were residing at d
consequent generation of 1 and 2. Similar phenomenon had also
and dxy magnetic orbitals [SCo = 1.0]. An antiferromagnetic coupling been observed for 2b (Figure S32A), where 2 was generated by the
2
z z
among the Co(III) d magnetic orbital, and ligand center p orbital cleavage of Co-SePh bond. The appearance of Co-centered EPR
led to an Stotal = 1/2 ground state and paramagnetism in the signal (Figure S30-S32) in the processes further supported the
complexes, where the unpaired electron resided at the dxy magnetic formation of 1 and 2. As five-coordinate, diradical-containing
orbital. Hence, cobalt (III)-centered X-band EPR spectra for 1 and 2 complexes could be converted to the corresponding four-
were observed. Experimental as well as simulated EPR spectra for 1 coordinate, monoradical-containing complexes under the sunlight
and 2 complexes were shown in Figure 2. For both the complexes, stimulus, and four-coordinate complexes have already been found
the signals were anisotropic in nature. Simulation to the as one-electron transferring agent, we have investigated the
experimental spectra provided the following parameters: g
1
=
catalytic ability of four-coordinate as well as five-coordinate
1
.992[1], 1.980[2]; g = 2.220[1], 2.020[2]; g = 2.750[1], 3.215[2]; complexes (1, 2 and 2a) for the conversion of RNCO (R = phenyl and
2
3
Co
–4
–1
Co
(A
1
, A
2
, A
3
) = (5, 42, 93) × 10 cm , for complex 1; (A
1 2 3
, A , A ) = naphthyl) to the corresponding C-N coupled urea derivatives.
–4
–1
(2, 1, 110) × 10 cm for complex 2. The average g value for the Noteworthy, 1,3-diaryl urea derivatives are in agricultural and
6
complexes 1 and 2 were 2.342 and 2.405, respectively, and medicinal uses. Table 1 contains the reaction details. In the
supported metal-centered unpaired electron.
catalysis, maximum turnover number (TON) 480 has been achieved
UV-vis-NIR spectra of 1 and 2 showed an absorption manifold by employing 0.1 mol% complex 2. To note, the bulkiness of the
centered at around 1600 nm (Figure S23, S26). It has previously substrates and ortho- substituents did not pronouncedly affect the
been established that the band appeared due to an intervalence product yield. This emphasized that substrates approached the
ligand(amidophenolate) to ligand(iminosemiquinone) charge Co(III) centre along the axial position. The essentiality of both the
4
transfer. This experimental fact further consolidated that both the catalysts as well as sunlight was further established by performing
complexes have same electronic structure. In the UV-vis-NIR blank reactions (Table 1). No products formation in the absence of
spectra of five-coordinate, diradical-containing complexes (1a, 2a catalyst refuted the conversion RNCO compounds to the
and 2b), the band at around 1600 nm was absent and a corresponding amines and photo-driven [2+2] cycloaddition
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reaction (Table 1, #2 and #4). The low TON (4-6) in the absence of
sunlight (Table 1, #8 and #10) supported the possibility that during
the catalysis a substrate-bound five-coordinate intermediate was
formed (Figure 3) and a photo stimulus was then essential for the
regeneration of four-coordinate catalysts by the cleavage of Co-
substrate bond in the intermediate. The formation of 1,1,2,2,-
tetrachloroethane was being established by GC and GC-MS (Figure
SX) analyses during the catalysis of III to IV in the presence of
catalyst 2. The compound was formed by the C-C coupling of two
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derivatives. Although further investigation is essential for the better
understanding of the catalysis, depending on the above stated
results a mechanistic proposal for the C-N coupling between two
isocyanate molecules for the formation of urea derivative is given in
Figure 3.
−
Bruin, J. N. H. Reek, M. A. Siegler and J. I. van der Vlugt,
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complexes have been successfully synthesized and employed for
the S-S/Se-Se bond activation and scission by an electron transfer to
the bonds. The solo example is known in the literature for the
ligand-induced S-S bond cleavage reaction is based on air sensitive
Pd(II)-amidophenolate complex. Thus, less explored phenomena
has been studied employing air stable Co(III) complexes. The
resulted diradical-containing square pyramidal Co(III) complexes
with axial -XPh (X = S and Se) ligands experienced homolytic Co-XPh
bond cleavage under the influence of sunlight and produced the
corresponding four-coordinate, monoradical Co(III) complexes
(
3
4
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best of our knowledge, the first examples for the synthesis of urea
from the solo use of isocyanate compounds. Presently, in our
2
9
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•
laboratory sunlight-driven PhX radical addition reactions to various
organic units are ongoing.
6
This project is funded by SERB [EMR/2015/002491], India. GCP and
SG thank Indian Institute of Technology Guwahati (IITG) for their
doctoral fellowship and for the instrumental facility.
5
, 479-483.
4
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