Anomalous 13C NMR chemical shifts of high-spin saddle shaped manganese(III) octaethyltetraphenylporphyrin complexes

Article abstract of DOI:10.1246/cl.2006.156

NMR spectra of highly saddled Mn(III) porphyrinates such as Mn(OETPP)(CD₃OD)₂⁺ have been measured. The extraordinarily upfield shifted meso-¹³C signals, which reach as much as -492 ppm at 298 K, are ascribed to

Full text of DOI:10.1246/cl.2006.156

156
Chemistry Letters Vol.35, No.2 (2006)
13
Anomalous C NMR Chemical Shifts of High-spin Saddle Shaped Manganese(III)
Octaethyltetraphenylporphyrin Complexes
Akira Ikezaki,1 Mikio Nakamura,ꢀ1;2;3 and Ru-Jen Cheng4
1Department of Chemistry, School of Medicine, Toho University, Ohta-ku, Tokyo 143-8540
2Division of Chemistry, Graduate School of Science, Toho University, Funabashi 274-8510
3Research Center for Materials with Integrated Properties, Toho University, Funabashi 274-8510
4Department of Chemistry, National Chung-Hsing University, Taichung, Taiwan 402, Taiwan
(Received October 11, 2005; CL-051294; E-mail: mnakamu@med.toho-u.ac.jp)
NMR spectra of highlþy saddled Mn(III) porphyrinates such
as Mn(OETPP)(CD3OD)2 have been measured. The extraordi-
narily upfield shifted meso-13C signals, which reach as much as
ꢁ492 ppm at 298 K, are ascribed to the dx2ꢁy2 –a2u, dxy–a1u, and
p
m
o
(c)
(b)
d –3eg interactions, the former two of which occur only when
ꢀ
the porphyrin ring is saddled.
-CH3
-CH2
-CH2
-CH2
-CH2
Electronic structures of metal porphyrinates are controlled
not only by the number and nature of the axial ligands but also
by the nonplanarity of porphyrin ring.1 This is because some
metal–porphyrin interactions, which are symmetry forbidden
in planar complexes, become possible by the ring deforma-
tion.2–4 In the process of studying the physicochemical proper-
ties of deformed Mn(III) porphyrinates, we found that þhighly
saddled Mn(OETPP)Cl (1a) and Mn(OETPP)(CD3OD)2 (1b)
exhibit quite unusual 13C NMR spectra. In this paper, we report
the anomalous 13C NMR chemical shifts of 1 and explain the
reasons for the anomaly.
(a)
50
40
30
20
ppm
10
0
-10
Figure 1. 1H NMR spectra of (a) 1a and (b) phenyl-deuterated
1a-d20 taken in CD2Cl2 at 298 K. (c) H NMR spectrum of 1a-
d20 determined in CHCl3 at 298 K.
2
the high-spin five-coordinate complexes (1a–5a), the unpaired
electrons delocalize through the d –3eg and d –a2u interactions
Five-coordinate 1a was prepared from (OETPP)H2 and
MnCl2 in refluxed DMF solution,5,6 which was converted to
six-coordinate 1b in CD3OD solution.7 These complexes are
z2
ꢀ
regardless of the porphyrin structures.2,13 The former induces the
upfield shift of the Py–H and the downfield shifts of the CH2 sig-
nals while the latter causes the downfield shift of the meso-C and
m-H signals; note that the 3eg orbital has relatively large coeffi-
cient at the ꢁ-C while the a2u orbital has large coefficient at the
meso-C as shown in Scheme 2. In the six-coordinate complexes
(1b–4b), the Mn(III) ion is in the center of the N4 plane. Thus,
1
high-spin (S ¼ 2) with (dxy)1(dyz)1(dxz)1(dz2
)
in solution as
revealed by the effective magnetic moments, 5.1 mB for both
complexes, determined by the Evans method.8 The 1H and
13C NMR chemical shifts of the analogous OEP (2), TPP (3),
TiPrP (4), and TtBuP (5) were also examined for comparison
(Scheme 1).9–12
z2
the d –3eg interaction is strengthened while the d –a2u interac-
ꢀ
Figures 1 and 2 show the 1H, 2H, and 13C NMR spectra.
Table 1 lists the chemical shifts at 298 K. The four and two meth-
ylene signals in 1a and 1b, respectively, suggest that the saddled
structure is maintained in solution. Several spectroscopic charac-
teristics are clearly seen in the data of Table 1 which can be ex-
plained in terms of the Mn(III)–porphyrin orbital interactions. In
tion is weakened; note that the a2u and dz2 orbitals are orthogonal
in the complexes with D2d symmetry. As a result, both the Py–H
and the meso-C signals exhibit a large upfield shift by ca. 10 and
100 ppm, respectively.
The data in Table 1 also suggest that the chemical shifts of 1
are quite different from those of 2–5. The major differences are
i) the upfield shift of the meta-H, ii) the downfield shifts of the o-
and p-H, and iii) the presence of an extremely upfield shifted
meso-C signal. These NMR characteristics, which suggest the
presence of a large negative spin at the meso-carbon atoms,14–16
Et
Et
Et
Et
R
R
N
N
N
L
L
L
Et
Et
Et
Et
Et
Et
Et
Et
N
Mn
N
N
N
Mn
N
N
N
Mn
N
L'
L'
L'
R
R
Et
Et
Et
Et
R = Ph; TPP (3a, 3b)
R = iPr; TiPrP (4a, 4b)
R = tBu; TtBuP (5a)
OETPP (1a, 1b)
OEP (2a, 2b)
Scheme 1. Mn(III) porphyrinates examined in this study. 1a–5a
are five-coordinate complexes where axial ligand is Clꢁ (1a–4a)
or CH3CO2 (5a). 1b–4b are six-coordinate complexes where
axial ligand is CD3OD.
a1u
a2u
Scheme 2. Frontier orbitals of porphyrin.
3eg(x)
3eg(y)
ꢁ
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.2 (2006)
157
þ
(THF)2 also exhibit the meso signals rather upfield.16,19,20
Thus, the upfield shift of the meso signal must be the phenomen-
on commonly observed in highly saddled S ¼ 2 Mn(III) and S ¼
3=2 Fe(III) complexes.
meso
(a)
(b)
This work was supported by the Grant-in-Aid for Scientific
Research (No. 16550061) from Ministry of Education, Culture,
Sports, Science and Technology, Japan. A. I. thanks Futaba
Electronics Memorial Foundation and the Sasakawa Scientific
Research Grant from The Japan Science Society for financial
support.
meso
meso
(c)
(d)
meso
References and Notes
1
W. R. Scheidt in The Porphyrin Handbook, ed. by K. M.
Kadish, K. M. Smith, R. Guilard, Academic Press, San
Diego, 2000, Vol. 3, Chap. 16, p. 49.
400
200
0
-200
ppm
-400
-600
2
3
R.-J. Cheng, P.-Y. Chen, T. Lovell, T. Liu, L. Noodleman,
D. A. Case, J. Am. Chem. Soc. 2003, 125, 6774.
R.-J. Cheng, Y.-K. Wang, P.-Y. Chen, Y.-P. Han, C.-C.
Chang, Chem. Commun. 2005, 1312.
Figure 2. 13C NMR spectra of meso-13C enriched (a) 1a, (b) 1b,
(c) 3a, and (d) 3b taken at 298 K.
4
5
J. Conradie, A. Ghosh, J. Phys. Chem. B 2003, 107, 6486.
A. D. Adler, F. R. Longo, F. Kampas, J. Kim, J. Inorg. Nucl
Chem. 1970, 32, 2443.
K. M. Barkigia, M. D. Berber, J. Fajer, C. J. Medforth, M. W.
Renner, K. M. Smith, J. Am. Chem. Soc. 1990, 112, 8851.
UV-Vis(ꢃmax/nm): 1a(CH2Cl2) 375, 405, 496, 595, 635;
1b(CH3OH) 391, 410, 484, 579, 616.
D. F. Evans, T. A. James, J. Chem. Soc., Dalton Trans. 1979,
723.
H. M. Goff, A. P. Hansen, Inorg. Chem. 1984, 23, 321.
Table 1. 1H and 13C NMR chemical shiftsa (298 K, ꢄ ppm)
1H NMR
13C NMR
Complexes
o
m
6
7
8
9
Py-H
—
CH2
CH3 meso
p
meso
(CH ) (CH )
ꢂ
ꢁ
1a
38.4 13.7 1.3
—
13.1
5.3 10.5 ꢁ406
4.7
10.2 ꢁ2.2
12.3
13.4
—
1b
2a
—
—
30.3
23.1
3.3 1.1
—
4.3 11.2 ꢁ492
0.5 2.7 52.6
2.3 76.4
—
—
—
—
7.3
7.9
—
—
—
80
ꢁ24
110
ꢁ8
2b
—
14.5
—
—
3ab
3bb
4ab
4bb
5ac
ꢁ21.9
ꢁ30.8
ꢁ19.4
ꢁ31.6
ꢁ2.9, ꢁ28.3
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
8.3
7.3
d
9.2
10 P. Turner, M. J. Gunter, Inorg. Chem. 1994, 33, 1406.
11 A. Ikezaki, M. Nakamura, Inorg. Chem. 2003, 42, 2301.
12 A. Ikezaki, M. Nakamura, Chem. Lett. 2005, 34, 1046.
13 J. Mao, Y. Zhang, E. Oldfield, J. Am. Chem. Soc. 2002, 124,
13911.
(9.5) (3.2)
d
216
87
—
(2.1)
d
(—) (3.2)
—
a1a–5a were taken in CD2Cl2 while 1b–4b were taken in CD3OD. bref 11.
cL ¼ CH3CO2ꢁ, Ref. 12. The pyrrole protons give two signals due to the slow inver-
sion rate of the ruffled porphyrin ring on the 1H NMR timescale. dSignals were
too broad.
14 Presence of a negative spin at the meso carbon was first
proposed by Yatsunyk and Walker15 to explain the 1H NMR
þ
should be ascribed to the specific orbital interactions such as
the dx2ꢁy2 –a2u and dxy–a1u that can occur only when the planar
porphyrin ring is saddled.2–4 Cheng and co-workers recently
proposed on the basis of the DFT calculation that the interaction
between the empty dx2ꢁy2 orbital and the nitrogen lone pairs in
high-spin Mn(III) porphyrinate causes anomalous spin polariza-
tion along the bonding skeleton, inducing the negative and
positive spin at the pyrrole nitrogen and ꢂ carbon atoms, respec-
tively.17 Thus, it is possible that the interaction between the emp-
ty dx2ꢁy2 and the doubly occupied a2u orbital in saddled com-
plexes induces a large negative spin at the meso-carbon and
shifts the meso signal upfield with a considerable broadening.
chemical shifts of the S ¼ 3=2 Fe(OETPP)(4-CNPy)2
reported by Nakamura and co-workers.16
15 L. A. Yatsunyk, F. A. Walker, Inorg. Chem. 2004, 43, 757.
16 T. Ikeue, Y. Ohgo, T. Yamaguchi, M. Takahashi, M. Takeda,
M. Nakamura, Angew. Chem., Int. Ed. 2001, 40, 2617.
17 R.-J. Cheng, S.-H. Chang, K.-C. Hung, Chem. Commun.,
submitted.
18 The d –3eg interaction in saddle shaped low-spin (S ¼ 1=2)
ꢀ
Fe(III) porphyrinate, Fe(OETPP)(imidazole)2þ, induces the
upfield shift of the meso signal by 110 ppm at 298 K. The
upfield shift of the meso signal due to the d –3eg interaction
ꢀ
in 1b could be much larger because the complex has two
unpaired electrons in the d orbitals.
In addition, the dxy–a1u and d –3eg interactions could further
ꢀ
ꢀ
contribute to the upfield shift since both the a1u and 3eg orbitals
have zero coefficient at the meso-carbon atoms.18 We have
reported that the structurally analogous interþmediate-spin Fe(III)
complexes such as Fe(OETPP)(4-CNPy)2 and Fe(OETPP)-
19 T. Sakai, Y. Ohgo, T. Ikeue, M. Takahashi, M. Takeda, M.
Nakamura, J. Am. Chem. Soc. 2003, 125, 13028.
20 A. Hoshino, Y. Ohgo, M. Nakamura, Inorg. Chem. 2005, 44,
7333.
Published on the web (Advance View) December 27, 2005; DOI 10.1246/cl.2006.156