Cooperation between metal and ligand in oxygen atom transport by N-confused porphyrin oxorhenium(V) complexes

Article abstract of DOI:10.1039/c2dt30885a

N-Confused porphyrin oxorhenium(v) complexes were prepared and their X-ray structures were elucidated. The oxorhenium(v) complexes can transfer oxygen atom from pyridine N-oxide to triphenylphosphine, in which unique cooperation between metal and ligand was observed.

Full text of DOI:10.1039/c2dt30885a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 9154
www.rsc.org/dalton
COMMUNICATION
Cooperation between metal and ligand in oxygen atom transport by
N-confused porphyrin oxorhenium(V) complexes†
Takaaki Yamamoto,a Motoki Toganoha and Hiroyuki Furuta*a,b
Received 24th April 2012, Accepted 25th May 2012
DOI: 10.1039/c2dt30885a
N-Confused porphyrin oxorhenium(V) complexes were
prepared and their X-ray structures were elucidated. The
oxorhenium(V) complexes can transfer oxygen atom from
pyridine N-oxide to triphenylphosphine, in which unique
cooperation between metal and ligand was observed.
The properties of a transition metal complex are governed by the
interaction between the metal centre and its surrounding ligands.
When a metal complex operates a substrate, the role of surround-
ing ligands is to tune electronic state of the centre metal as well
as the steric environment, in which the ligand itself does not
participate in substrate or atom transport.1 Representative
examples of atom transport by the ligand are found in nature.
The galactose oxidase converts primary alcohol into aldehyde
Scheme 1 Preparation of NCP rhenium complexes.
efficiently, in which abstraction of a hydrogen atom by a tyrosyl
radical ligand and electron abstraction by copper(II) metal play a
critical role.2 Artificial examples on such cooperation between
metal and ligand have been developed in the last decade.3 Never-
theless, in most cases, only hydrogen atoms are transported by
the ligands and transport of heavy atoms has been seldom
reported. This time we have succeeded to transport an oxygen
atom of pyridine N-oxide to PPh3 by the N-confused porphyrin
rhenium(V) complexes, where unique cooperation between metal
and ligand was observed.
The rhenium(V) complexes of N-confused porphyrin were pre-
pared by the oxidation of the corresponding rhenium(I) complex
Fig. 1 The ORTEP drawings of (a) 5 and (b) 6.
(Scheme 1). When the parent N-confused porphyrin (NCP, 1)4,5
was treated with Re2(CO)10 or Re(CO)5Br, a skeletal rearrange-
the presence of K2CO3.9 Alternatively, the oxorhenium(V)
ment readily occurred to give the N-fused porphyrin rhenium(I)
complex (2).6 Thus, N-methyl-NCP (3) was used to prepare the
complexes 5 and 6 could be prepared directly from 3 in compar-
NCP rhenium(I) tricarbonyl complex (4),7,8 which was oxidized
able yields. This is the first example of N-confused porphyrin
by t-BuOOH. After purification, the oxorhenium(V) complex (5)
was obtained in 7% yield and the oxorhenium(V) complex (6)
was obtained in trace yield. Significant improvement of the
product yields (5: 48%, 6: 2%) was achieved when 4 was
oxidized by 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in
oxometal complexes as well as rhenium(V) complexes.
The structures of 5 and 6 were elucidated by X-ray crystallo-
graphic analysis (Fig. 1 and Table 1).‡ The N-confused pyrrole
ring (gray-colored) was oxidized and became the amide-like
form. In 6, the rhenium metal was placed inside the NNNC core
and was further coordinated by the oxo ligand, forming a typical
square pyramidal geometry. The structure of 5 was similar to that
of 6 except for the carbon atom of the NNNC core being
oxidized to form a unique Re–O3–C1 metalaoxirane. The
Re–O2 lengths for 5 and 6 are around 1.67 Å, which is typical
for oxo-rhenium(V) complexes.10 The Re–C1 length of 6 is
2.070(4) Å, which is comparable to the Re–N bond lengths (avg.
2.073 Å). Meanwhile, the Re–C1 lengths of 5 are 2.241 Å
(avg.), which are significantly longer than that of 6. The C1–O3
aDepartment of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, Fukuoka 819-0395, Japan.
E-mail: hfuruta@cstf.kyushu-u.ac.jp; Fax: (+81)92-802-2865
bInternational Research Center for Molecular Systems, Kyushu
University, Fukuoka 819-0395, Japan
†Electronic supplementary information (ESI) available: All the exper-
imental and calculation details. CCDC 867528 and 867529. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30885a
9154 | Dalton Trans., 2012, 41, 9154–9157
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 1 Selected bond lengths in Å for 5 and 6
5-1a
5-2a
6
Re–C1
Re–N2
Re–N3
Re–N4
Re–O2
Re–O3
C1–O3
C2–O1
2.245(9)
2.113(5)
2.032(5)
2.110(3)
1.671(5)
2.031(5)
1.415(11)
1.234(11)
2.237(19)
2.161(6)
2.073(5)
2.05(4)
1.684(16)
2.014(15)
1.423(19)
1.229(19)
2.070(4)
2.076(4)
2.080(4)
2.064(4)
1.668(4)
—
—
1.256(6)
Fig. 2 Absorption spectra of (a) 5 and (b) 6.
a Two disordered structures were observed in the solid state.
Scheme 2 Coordination environment of NCP derivatives. The number
and type of detachable hydrogen atoms inside the macrocycle are shown
in the brackets.
lengths are 1.419 Å (avg.), which is similar to paraffinic carbon–
oxygen single bonds (1.43 Å) but shorter than oxirane carbon–
oxygen single bonds (1.45 Å).11
Scheme 3 Interconversion between 5 and 6.
pyridine to the vacant site of rhenium metal, forming the
hexacoordinate rhenium(V) complex 7 (Scheme 3). Correlation
between solvent polarity and absorption wavelengths is low,
which would negate the possibility of intramolecular charge
transfer for solvatochromism (Fig. S4 and S5†).
Before discussing the valency of the rhenium metals in 5 and
6, coordination environment of NCP ligands should be men-
tioned (Scheme 2). The N-methyl-NCP has one CH and one NH
inside the core and can be mono-negative or di-negative. When
the α-position of N-confused pyrrole is oxidized, an amide-type
N-methyl-NCP ligand (NCPO) is obtained. The NCPO ligand
has one CH and two NHs inside the core and can be di-negative
or tri-negative. Importantly, NCPO can be further oxidized at the
inner CH moiety to give an oxo-NCPO ligand. The oxo-NCPO
ligand would be under equilibrium between the enol and keto
forms, both of which can be tri-negative.
According to the coordination environment of NCPO and
oxo-NCPO ligands, the formal charge of rhenium metal in 5 and
6 is determined to be +5. The rhenium metal of 6 is surrounded
by one tri-negative NCPO ligand and one oxo ligand. Then it
can be described as ReVO(NCPO). In an analogous fashion, the
complex 5 can be described as ReVO(oxo-NCPO). The 1H NMR
spectra in CDCl3 illustrated diamagnetic character of rhenium
complexes 5 and 6, which is consistent with their oxidation state
of +5. The β-pyrrolic protons appeared in the region of δ 8.6–9.0
and 8.9–9.1 ppm for 5 and 6, respectively. Observation of those
signals in the typical aromatic region suggests negligible contri-
bution of intramolecular charge separation and strong aromaticity
both in 5 and 6.
Interestingly, interconversion between 5 and 6 could be
achieved with PPh3 as reductant and pyridine N-oxide as oxidant
(Scheme 3). Thus, treatment of 5 with PPh3 afforded 6 in 81%
yield and oxidation of 6 with pyridine N-oxide afforded 5 in
90% yield. The presumable dioxorhenium(VII) complex 8 was
not observed. Importantly, the formal charge of rhenium metal
does not change in this oxidation/reduction process, indicating
the unique non-innocent character of NCPO ligands.12 In turn,
the NCPO ligand of 5 was oxidized by pyridine N-oxide and the
oxygen atom of the oxo-NCPO ligand in 6 was taken by PPh3,
formally. Thus, this interconversion can be regarded as oxygen
atom transport from pyridine N-oxide to PPh3 by the NCPO
ligand. These conversion processes were carried out under inert
atmosphere and thus molecular oxygen did not participate in any
reactions.
A possible oxygen atom transfer (OAT) reaction pathway for
the interconversion between 5 and 6 is proposed as shown in
Scheme 4. First, coordination of pyridine N-oxide to 6 gives 9.
The absorption spectrum of 6 changed largely by adding an
excess of pyridine N-oxide, which would support formation of 9
in solution (Fig. S6†). Second, the inner carbon atom of the
NCPO core is oxidized by pyridine N-oxide to give 5 and
pyridine. Third, the oxo-rhenium(V) moiety is reduced by PPh3
to form ReIII(oxo-NCPO) 10. Finally, the oxygen atom is moved
from the oxo-NCPO ligand to the rhenium metal, giving back to
6. Isomerization from 10 to 6 can be regarded as an intramolecu-
lar oxidation reaction of the rhenium metal by the oxo-NCPO
Absorption spectra of 5 and 6 in CH2Cl2 and pyridine are
shown in Fig. 2. In the absorption spectrum of 5 in CH2Cl2, a
sharp Soret-type band was observed at 506 nm. Essentially the
same spectrum was obtained in pyridine, indicating no coordi-
nation site was left in 5. By contrast, absorption spectra of 6 are
strongly dependent on solvents. A Soret-type band of 6 was
observed at 484 nm in CH2Cl2 and 539 nm in pyridine. This
solvatochromism would be explained by the coordination of
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9154–9157 | 9155

View Article Online
Scheme 5 Oxygen atom transport catalyzed by 6.
reaction proceeded in 97% yield with 1 mol% of 5. When the
same reaction was achieved with 1 mol% of triphenylcorrole
oxo-rhenium(V) complex (ReVO(COR)),15 only 9.8% yield was
observed. Accordingly, the existence of Re–C bond like 5 and 6
would be important for efficient oxygen atom transport. It
should be noted that oxygen atom transport without any metal
complexes required elevated temperature above 230 °C.16
Scheme 4 Plausible mechanism for the catalytic OAT reaction.
In conclusion, the N-confused porphyrin oxorhenium(V) com-
plexes 5 and 6 were prepared for the first time and their struc-
tures were elucidated by X-ray crystallographic analysis. These
complexes can transport oxygen atom efficiently from pyridine
N-oxide to PPh3, in which cooperation between the metal centre
and the NCPO ligand would be expected. Further study on the
application of this unique cooperation to catalysis is now under-
way. Development of cooperative ligands based on N-confused
porphyrin would be also expected in the near future.17
The present work was supported by the Grant-in-Aid for
Scientific Research (22350020, 22655044) from the MEXT.
Notes and references
Fig. 3 HOMO and LUMO of 5 and 6.
‡Crystallographic data 5·2MeOH: C47H37N4O5Re, MW 924.04, tricli-
ˉ
nic, space group P1 (No. 2), a = 11.231(3), b = 13.123(4), c = 14.134(5)
Å, V = 1870.9(10) Å3, Z = 2, T = 123 K, R = 0.0646 (I > 2σ(I)), RW
=
ligand. Such cooperation between the rhenium metal and the
NCPO ligand would be a noticeable example among cooperating
ligands.
0.1473 (all data), GOF on F2 = 0.992 (all data), CCDC reference
number 867528; 6·1/2C6H6: C48H32N4O2Re, MW 882.98, triclinic,
ˉ
space group P1 (No. 2), a = 11.841(6), b = 12.438(7), c = 14.373(7) Å,
V = 1795.6(16) Å3, Z = 2, T = 123 K, R = 0.0432 (I > 2σ(I)), RW
=
The above reaction mechanism was supported by DFT calcu-
lations.† All the calculations were achieved on the corresponding
meso-free derivatives. The HOMO and LUMO of 5 and 6 are
shown in Fig. 3. First of all, both the HOMO and LUMO of 5
and 6 are similar to each other. The HOMO of 6 is exclusively
composed of the NCPO π-orbitals and thus oxidation of 6 would
not occur at the rhenium metal but at the NCPO ligand. Contrast-
ingly, significant contribution of the oxo-rhenium moiety is
observed in the LUMO of 5, which indicates that the reduction
would occur at the oxo-rhenium moiety to give 10. Formation of
6 directly from 5 is unlikely to occur, since trivial contribution
at the C–O bond moiety was observed. Besides, no C–O bond
cleavage has been reported in the reported oxo-NCP metal com-
plexes.13 The rhenium(III) complex 10 is energetically less stable
than the isomeric rhenium(V) complex 6 by 69.2 kcal mol−1 and
no transition state was found between 10 and 6 in DFT calcu-
lations. Hence this transformation is supposed to be fast. The
relative stability for the other possible intermediates is also con-
sistent with the plausible reaction mechanism (see ESI†).
To validate the efficiency of interconversion between 5 and 6,
oxygen atom transport from pyridine N-oxide to PPh3 was exam-
ined with a catalytic amount of 5 or 6.14 Satisfactory, the reaction
proceeded quantitatively at 80 °C for 24 h with 1 mol% of 6
(Scheme 5). The reaction proceeded even in the presence of
0.01 mol% of 6 in 55% yield (TON = 5500). Similarly, the
0.0826 (all data), GOF on F2 = 1.030 (all data), CCDC reference
number 867529.
1 H. Grützmacher, Angew. Chem., Int. Ed., 2008, 47, 1814.
2 J. W. Whittaker, Arch. Biochem. Biophys., 2005, 433, 227.
3 (a) R. Noyori, M. Yamakawa and S. Hashiguchi, J. Org. Chem., 2001,
66, 7931; (b) K. Muñiz, Angew. Chem., Int. Ed., 2005, 44, 6622;
(c) J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, J. Am. Chem.
Soc., 2005, 127, 10840; (d) C. Gunanathan, Y. Ben-David and
D. Milstein, Science, 2007, 317, 790.
4 (a) H. Furuta, T. Asano and T. Ogawa, J. Am. Chem. Soc., 1994, 116,
767; (b) P. J. Chmielewski, L. Latos-Grażyński, K. Rachlewicz and
T. Głowiak, Angew. Chem., Int. Ed. Engl., 1994, 33, 779.
5 (a) M. Toganoh and H. Furuta, Chem. Commun., 2012, 48, 937;
(b) P. J. Chmielewski and L. Latos-Grażyński, Coord. Chem. Rev., 2005,
249, 2510.
6 (a) M. Tognaoh, T. Ishizuka and H. Furuta, Chem. Commun., 2004,
2464; (b) M. Toganoh, S. Ikeda and H. Furuta, Inorg. Chem., 2007, 46,
10003.
7 P. J. Chmielewski and L. Latos-Grażyński, J. Chem. Soc., Perkin Trans.
2, 1995, 503.
8 (a) M. Toganoh and H. Furuta, Chem. Lett., 2005, 34, 1034;
(b) M. Toganoh, T. Hihara and H. Furuta, Inorg. Chem., 2010, 49, 8182.
9 C. A. Lippert, S. A. Arnstein, C. D. Sherrill and J. D. Soper, J. Am.
Chem. Soc., 2010, 132, 3879.
10 J. H. Espenson, X. Shan, Y. Wang, R. Huang, W. D. W. Lahti, J. Dixon,
G. Lente, A. Ellern and I. A. Guzei, Inorg. Chem., 2002, 41, 2583.
11 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and
R. Talor, J. Chem. Soc., Perkin Trans. 2, 1987, S1.
12 (a) R. Eisenberg and H. B. Gray, Inorg. Chem., 2011, 50, 9741;
(b) W. Kaim, Inorg. Chem., 2011, 50, 9752.
9156 | Dalton Trans., 2012, 41, 9154–9157
This journal is © The Royal Society of Chemistry 2012

View Article Online
13 (a) Z. Xiao, B. O. Patrick and D. Dolphin, Inorg. Chem., 2003, 42, 8125;
(b) K. Rachlewicz, S. L. Wang, J. L. Ku, C. H. Hung and L. Latos-Gra-
żyński, J. Am. Chem. Soc., 2004, 126, 4420.
14 (a) Y. Wang and J. H. Espenson, Org. Lett., 2000, 2, 3525; (b) R. Sanz,
J. Escribano, Y. Fernández, R. Aguado, M. R. Pedrosa and F. J. Arnáiz,
Synlett, 2005, 1389; (c) M. Toganoh, K. Fujino, S. Ikeda and H. Furuta,
Tetrahedron Lett., 2008, 49, 1488; (d) M. Toganoh, S. Ikeda and
H. Furuta, Chem. Commun., 2005, 4589; (e) T. Higuchi and M. Hirobe,
J. Mol. Catal. A: Chem., 1996, 113, 403.
15 M. K. Tse, Z. Zhang, T. C. W. Mak and K. S. Chan, Chem. Commun.,
1998, 1199.
16 E. Howard Jr. and W. F. Olszewski, J. Am. Chem. Soc., 1959, 81, 1483.
17 W.-M. Ching, C.-H. Chuang, C.-W. Wu, C.-H. Peng and C.-H. Hung,
J. Am. Chem. Soc., 2009, 131, 7952.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9154–9157 | 9157