Determination of the field strengths of weak neutral ligands having oxygen as coordination atom

Article abstract of DOI:10.1246/cl.2004.1234

Weakness of various neutral ligands having oxygen as coordination atom was ranked on the basis of the pyrrole proton chemical shifts of planar [Fe(TPP)L₂]⁺. Furthermore, weakness of the ligands, which is difficult to determine from t

Full text of DOI:10.1246/cl.2004.1234

1234
Chemistry Letters Vol.33, No.10 (2004)
Determination of the Field Strengths of Weak Neutral Ligands Having Oxygen
as Coordination Atom
Akito Hoshinoyy and Mikio Nakamuraꢀy;yy
yDepartment of Chemistry, School of Medicine, Toho University, Ota-ku, Tokyo 143-8540
yyDivision of Biomolecular Science, Graduate School of Science, Toho University, Funabashi 274-8510
(Received May 31, 2004; CL-040613)
Weakness of various neutral ligands having oxygen as coor-
dination atom was ranked on the basis of the pyrrole proton
chemical shifts of planar [Fe(TPP)L2]þ. Furthermore, weakness
of the ligands, which is difficult to determine from the NMR data
of planar [Fe(TPP)L2]þ, was successfully differentiated by the
use of highly ruffled [Fe(TiPrP)L2]þ. Among the 10 ligands ex-
amined, THF turned out to be the weakest ligand.
Table 1. 1H NMR Chemical Shifts (CD2Cl2, 298 K, ꢂppm)
Ligands
(L)
o
m
p
Int
/%
Entries
Pyrrole
(H Þ (H )
ꢃ
ꢁ
þ
a) Fe(TPP)L2
1a
2a
3a
4a
5a
6a
7a
8a
Ph3PO
73.2
71.0
70.7
68.9
67.2
67.2
60.2
48.1
33.1
3.1
12.3
12.8
12.7
13.3
13.2
13.0
13.5
9.4 9.6
9.4 9.7
9.4 9.7
9.5 9.8
9.7 9.9
9.6 9.7
0
2
2
4
6
6
3,5-Me2PyNO
4-MePyNO
PyNO
4-ClPyNO
DMSO
DMF
Oxygen-containing neutral compounds can coordinate vari-
ous metal ions and regulate the physicochemical properties of a
wide variety of synthetic complexes as well as naturally occur-
ring metalloenzymes. In general, the field strength of these li-
gands is difficult to be determined because of their weak coordi-
nation ability. Several years ago, Reed and Guiset ranked the li-
gand field strength of weakly binding anion (X) on the basis of
the magnetic properties of [Fe(TPP)X] and called the hierarchy
as magnetochemical series.1,2 We have applied this method to
determine the weakness of neutral ligands having oxygen as co-
ordination atom.
The CD2Cl2 solution of oxygen-containing neutral ligand
(L) was added to [Fe(TPP)]ClO4 placed in an NMR sample tube.
In each case, the pyrrole-H signal shifted downfield and ap-
proached to the constant value, which was taken as the pyr-
role-H chemical shift of [Fe(TPP)L2]þ. Table 1 lists the chemi-
cal shifts of the pyrrole and phenyl protons of all the complexes
examined in this study. Figure 1 shows the Curie plots of the pyr-
role signals of some selected complexes.
9.9 9.9 12
CH3OH
Et2N–NO
THF
13.8 10.1 10.0 23
14.1 10.0 10.1 37
14.1 10.1 10.3 64
9a
10a
þ
b) Fe(TiPrP)L2
1b
2b
3b
4b
5b
6b
7b
8b
9b
10b
Ph3PO
37.2 (12.9) (6.1)
—
—
—
—
—
—
—
—
—
—
33
14
14
30
35
74
87
93
3,5-Me2PyNO
4-MePyNO
PyNO
4-ClPyNO
DMSO
DMF
CH3OH
Et2N–NO
THF
57.6 (10.9) (6.3)
58.5 (10.8) (6.2)
41.7 (9.9) (6.1)
35.2 (10.7) (5.8)
ꢁ7:7 (8.0) (4.6)
ꢁ21:5 (7.3) (4.2)
ꢁ28:3 (7.3) n.d.
ꢁ33:9 (9.6) (3.8)
ꢁ35:5 (5.7) n.d.
99
100
n.d.: not determined.
iPr
iPr
because the d orbital maintains the unpaired electron. The un-
ꢀ
Ph
Ph
paired electron in the d orbital delocalizes to the ꢁ-pyrrole car-
N
Fe
N
N
Fe
N
ꢀ
bon atoms by the iron (d )–porphyrin (3eg) interaction and indu-
L
L
ꢀ
N
N
N
N
ces the upfield shift of the pyrrole-H signal by the spin polariza-
tion mechanism. The data in Table 1 clearly indicate that the
bis(thf) complex 10a has the largest S ¼ 3=2 character among
the [Fe(TPP)L2]þ complexes, which indicates that the field
strength of THF is much weaker than any other ligands exam-
ined in this study. It is difficult, however, to rank the field
strengths of the ligands in 1a–6a because these complexes ex-
hibit the pyrrole signals at nearly the same positions. Thus,
TPP is not a suitable porphyrin to differentiate the field strengths
of these ligands.
L
L
iPr
iPr
Ph
Ph
[Fe(TPP)L2]+
[Fe(TiPrP)L2]+
The chemical shift of the pyrrole-H is known to be a good
probe to determine the spin state of iron(III) porphyrin com-
plexes.3 The high-spin (S ¼ 5=2) complexes always show the
pyrrole signal fairly downfield because of the presence of the un-
paired electron in the dx2ꢁy2 orbital. Thus, the complexes 1a–6a
are in a quite pure high-spin state because the pyrrole signals of
these complexes appeared at 67.2–73.2 ppm.4,5 The high-spin
state of these complexes was also confirmed by the EPR spectra
taken in frozen CH2Cl2 solution at 4.2 K; the g and g values of
In the previous papers, we and others have reported that the
deformation of the porphyrin core stabilizes the S ¼ 3=2 spin
state.6 This is because, the deformation always contracts the
Fe–NP bonds and raises the energy level of the dx2ꢁy2 orbital.7
Thus, highly ruffled complexes with weak axial ligands such
as 10b adopt an essentially pure S ¼ 3=2 spin state;6a the aver-
?
k
4a were 5.95 and 2.00, respectively. In contrast, the intermedi-
ate-spin (S ¼ 3=2) complexes show the pyrrole signal fairly up-
field because the dx2ꢁy2 orbital has no unpaired electron and also
age Fe–NP bonds in nearly planar 10a and highly ruffled 10b
8,9
ꢀ
are reported to be 2.016 and 1.967 A, respectively. We have
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.10 (2004)
1235
Scientific Research from Ministry of Education, Culture, Sports,
Science and Technology, Japan. Thanks are due to the Research
Center for Molecular-Scale Nanoscience, the Institute for
Molecular Science (IMS).
150
4a
6a
8a
100
50
4b
5b
References and Notes
1
C. A. Reed and F. Guiset, J. Am. Chem. Soc., 118, 3281
(1996).
2
Abbreviation: TPP and TiPrP, dianions of 5,10,15,20-tetra-
phenylporphyrin and 5,10,15,20-tetraisopropylporphyrin.
Ph3PO, 3,5-Me2PyNO, 4-MePyNO, PyNO, and 4-ClPyNO:
triphenylphosphine oxide, 3,5-dimethylpyridine N-oxide, 4-
methylpyridine N-oxide, pyridine N-oxide, and 4-chloropyr-
idine N-oxide.
F. A. Walker in ‘‘The Porphyrin Handbook,’’ ed. by K. M.
Kadish, K. M. Smith, and R. Guilard, Academic Press, San
Diego (2000), Vol. 5, Chap. 36, p 81.
a) T. Mashiko, M. E. Kastner, K. Spartalian, W. R. Scheidt,
and C. A. Reed, J. Am. Chem. Soc., 100, 6354 (1978). b)
D. L. Budd, G. N. La Mar, K. C. Langry, K. M. Smith,
and R. Nayyir-Mazhir, J. Am. Chem. Soc., 101, 6091 (1979).
M. Nakamura, A. Hoshino, A. Ikezaki, and T. Ikeue, Chem.
Commun., 2003, 1862.
0
–50
–100
6b
8b
10b
3
4
0.003
0.004
0.005
–1
0.006
–1
T
/ K
Figure 1. Curie plots of the pyrrole signals of [Fe(TPP)L2]þ
and [Fe(TiPrP)L2]þ. Each line is signified by the entry number
given in Table 1.
5
6
then expected that the S ¼ 3=2 character should increase on go-
ing from [Fe(TPP)L2]þ to [Fe(TiPrP)L2]þ. Consequently, the
chemical shifts of the pyrrole-H in [Fe(TiPrP)L2]þ could be dif-
ferent even among the complexes 1b–6b. Table 1 also lists the
a) T. Ikeue, T. Saitoh, T. Yamaguchi, Y. Ohgo, M.
Nakamura, M. Takahashi, and M. Takeda, Chem. Commun.,
´
2000, 1989. b) J.-P. Simonato, J. Pecaut, L. Le Pape, J.-L.
chemical shifts of the pyrrole, methine(H ), and methyl(H )
ꢃ
ꢁ
protons of a series of [Fe(TiPrP)L2]þ. As expected, the pyrrole
signals of 1b–6b shifted upfield in a different degree; they ap-
peared at 37.2, 57.6, 58.5, 41.7, 35.2, and ꢁ7:7 ppm, respective-
ly. The reversal of the order was observed in 1b, which should be
ascribed to the steric repulsion between the Ph3PO ligand and the
meso-isopropyl groups; the repulsion weakens the coordination
of Ph3PO. Thus, the field strength of the ligands, which was
difficult to determine in [Fe(TPP)L2]þ, is now ranked as given
below:
Oddou, C. Jeandey, M. Shang, W. R. Scheidt, J. Wojacynski,
S. Wolowiec, L. Latos-Grazynski, and J.-C. Marchon, Inorg.
Chem., 39, 3978 (2000). c) K. M. Barkigia, M. W. Renner,
and J. Fajer, J. Porph. Phtha., 5, 415 (2001). d) T. Ikeue,
Y. Ohgo, T. Yamaguchi, M. Takahashi, M. Takeda, and
M. Nakamura, Angew. Chem., Int. Ed., 40, 2617 (2001).
e) M. Nakamura, T. Ikeue, Y. Ohgo, M. Takahashi, and
M. Takeda, Chem. Commun., 2002, 1198. f) T. Sakai, Y.
Ohgo, T. Ikeue, M. Takahashi, M. Takeda, and M.
Nakamura, J. Am. Chem. Soc., 125, 13028 (2003).
W. R. Scheidt in ‘‘The Porphyrin Handbook,’’ ed. by
K. M. Kadish, K. M. Smith, and R. Guilard, Academic Press,
San Diego (2000), Vol. 3, Chap. 16, p. 49.
3,5-Me2PyNO ꢂ 4-MePyNO > PyNO > 4-ClPyNO > DMSO:
7
8
9
By measuring the chemical shifts of the pyrrole signals in both
[Fe(TPP)L2]þ and [Fe(TiPrP)L2]þ, we have determined the rel-
ative field strengths of a wide variety of oxygen-containing li-
gands and arranged them in Table 1 in descending order. Clearly,
the ligands with formally charged oxygen atom such as Ph3PO
and PyNO are stronger than those with uncharged oxygen such
as THF. Substituent effect on the field strengths is also seen in
the substituted pyridine N-oxides. If we assume that the chemical
shifts of the pyrrole signals in the pure intermediate- and
high-spin complexes are ꢁ35:5 and +73.2 ppm, respectively,
then the contribution of the S ¼ 3=2 spin state, Int (%), can be
estimated by Eq 1, where ꢂL is the chemical shift of the pyrrole
signal in [Fe(TPP)L2]þ or [Fe(TiPrP)L2]þ.10 The Int (%) values
determined by Eq 1 are also listed in Table 1.
L. Chen, G.-B. Yi, L.-S. Wang, U. R. Dharmawardana,
A. C. Dart, M. A. Khan, and G. B. Richter-Ado, Inorg.
Chem., 37, 4677 (1998).
Y. Ohgo, T. Saitoh, and M. Nakamura, Acta Crystallogr.,
C57, 233 (2001).
10 Strictly speaking, the Int (%) values of [Fe(TPP)L2]þ and
[Fe(TiPrP)L2]þ should be estimated independently. This is
because, the chemical shifts of the pyrrole signals in the pure
S ¼ 5=2 and S ¼ 3=2 complexes should be different be-
tween [Fe(TPP)L2]þ and [Fe(TiPrP)L2]þ. Because there is
no example of [Fe(TPP)L2]þ and [Fe(TiPrP)L2]þ showing
an essentially pure S ¼ 3=2 and S ¼ 5=2, respectively, we
have applied Eq 1 to both types of complexes. Since the
chemical shift of the pyrrole signal in analogous [Fe(TArP)-
(THF)2]þ (Ar = 2,4,6-trimethoxyphenyl), which is reported
to adopt a quite pure intermediate-spin state, is ꢁ28:0 ppm in
CDCl3 at 302 K,11 the Int (%) values of [Fe(TPP)L2]þ listed
in Table 1 could be slightly underestimated.
Int (%) ¼ ½ð73:2 ꢁ ꢂLÞ=108:7ꢃ ꢄ 100
ð1Þ
In conclusion, we were able to rank the weakness of
oxygen containing ligands on the basis of the pyrrole proton
chemical shifts of the planar [Fe(TPP)L2]þ and highly ruffled
[Fe(TiPrP)L2]þ complexes.
11 G. E. Toney, L. W. TerHaar, J. E. Savrin, A. Gold, W. E.
Hatfield, and R. Sangaiah, Inorg. Chem., 23, 2561 (1984).
This work was supported by a Grant in Aid (1454021) for
Published on the web (Advance View) August 28, 2004; DOI 10.1246/cl.2004.1234