Ni(III) Complex of an N-Confused Porphyrin Inner C-Oxide

Article abstract of DOI:10.1021/ic020701s

A novel, structurally characterized Ni(III) complex of an N-confused porphyrin inner C-oxide has been synthesized from the oxidation of a Ni(II) N-confused porphyrin using OsO₄. Crystal data: C₅₃H ₄₀N₅NiO·CH₂Cl₂, monoclinic, space group P2/a (No. 13), a = 21.229(1) A, b = 8.6451(5) A, c = 25.762(2) A, β = 93.004-(3)°, V = 4721.6(5) A³, and Z = 4.

Full text of DOI:10.1021/ic020701s

Inorg. Chem. 2003, 42, 8125−8127
Ni(III) Complex of an N-Confused Porphyrin Inner C-Oxide
Ziwei Xiao, Brian O. Patrick, and David Dolphin*
Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall,
VancouVer, British Columbia, V6T 1Z1 Canada
Received December 3, 2002
Scheme 1
A novel, structurally characterized Ni(III) complex of an N-confused
porphyrin inner C-oxide has been synthesized from the oxidation
of a Ni(II) N-confused porphyrin using OsO . Crystal data:
4
C H N NiO‚CH Cl2, monoclinic, space group P2/a (No. 13), a
53 40
5
2
) 21.229(1) Å, b ) 8.6451(5) Å, c ) 25.762(2) Å, â ) 93.004-
3
(3)°, V ) 4721.6(5) Å , and Z ) 4.
Ni(III) tetrapyrrolic macrocycles have been intensively
studied.1 Wolberg reported on the generation of a Ni(III)
porphyrin via electrochemical oxidation of Ni(II) tetraphen-
ylporphyrin at 77 K.2 This reaction was re-examined by
Dolphin and his colleagues where it was observed that a
Ni(II) porphyrin π-cation radical was formed at room
temperature and a low spin Ni(III) porphyrin was generated
upon cooling to 77 K, showing intramolecular electron
transfer.3,4 N-confused porphyrins are porphyrin isomers with
an inverted pyrrolic ring and were first reported indepen-
dently by Latos-Grazynski5 and Furuta.6 They can stabilize
transition metals in high oxidation states, and some Ni(III)
complexes have been prepared by Latos-Grazynski.7 Factor
430 (F430), a nickel tetrapyrrole, is the cofactor of methyl-
coenzyme M reductase, which catalyzes the final steps of
CO2 conversion to methane by methanogenic Archaea, and
a methyl-Ni(III) transient is suggested to be a key inter-
mediate.8 Ni(III)-alkyl intermediates are also considered to
be involved in the reactions of Ni(I) macrocycles with alkyl
halides.9 However, crystallographically characterized Ni(III)
tetrapyrrolic macrocycles are rare, and to the best of our
knowledge, the only related example is an oxidized nickel
porphyrin [Ni(OETPP)(BTD)]+ (OETPP ) â-octaethyl-
meso-tetraphenylporphyrin, BTD ) 2,1,3-benzothiadiazole)
with an EPR spectrum typical of Ni(III).10 The Ni-N (2.00
Å) distance in this complex is rather long relative to other
Ni(III) complexes, and this complex was better described
1
2
1
as a high spin Ni(II) porphyrin π cation (d1x -y , d , π ) in
2
2
z
2
2
which the electron in the dx -y orbital is antiferromagnetically
coupled to the unpaired electron of the porphyrin π radical,
resulting in a pseudo-Ni(III) species.10
Here, we report the synthesis and structural characteriza-
tion of a Ni(III) complex of an N-confused porphyrin inner
C-oxide (2) prepared by oxidation of Ni(II) N-confused tetra-
(p-tolyl)porphyrin (1) with OsO4 (Scheme 1).
The Ni(II) N-confused porphyrin 1 was prepared using
the method described by Latos-Grazynski.5 A solution of
complex 1 (0.20 mmol) and OsO4 (0.25 mmol) in 15% pyri-
dine/CH2Cl2 (50 mL) was stirred at room temperature
for 24 h and then filtered through a silica gel plug using
10% CH3OH/CH2Cl2. The solvent was removed in vacuo,
and the residue was chromatographed using silica gel
(230-400 mesh, 16 g). Compound 211 was eluted with 1.5%
CH3OH/CH2Cl2 to give a yield of 42% (32% starting material
was recovered). When a large excess of OsO4 (10 equiv)
was used, many polar unidentified compounds were pro-
duced.
* To whom correspondence should be addressed. E-mail: ddolphin@
qltinc.com. Fax: 604-822-9678. Phone: 604-822-4571.
(1) Renner, M. W.; Fajer, J. JBIC, J. Biol. Inorg. Chem. 2001, 6, 823-
830.
(2) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982-2991.
(3) Dolphin, D.; Niem, T.; Felton, R. H.; Fujita, E. J. Am. Chem. Soc.
1975, 97, 5288-5290.
(4) Johnson, E. C.; Niem, T.; Dolphin, D. Can. J. Chem. 1978, 56, 1381-
1388.
(5) Chmielewski, P. J.; Latos-Grazynski, L.; Rachlewicz, K.; Glowiak,
T. Angew. Chem., Int. Ed. Engl. 1994, 33, 779-781.
(6) Furuta, H.; Asano, T.; Ogawa, T. J. Am. Chem. Soc. 1994, 116, 767-
768.
(10) Renner, M. W.; Barkigia, K. M.; Melamed, D.; Smith, K. M.; Fajer,
J. Inorg. Chem. 1996, 35, 5120-5121.
(11) Rf (silica-CH2Cl2/5%CH3OH/2%Et3N) 0.70; UV-vis (CH2Cl2) λmax
/
(7) Chmielewski, P. J.; Latos-Grazynski, L. Inorg. Chem. 1997, 36, 840-
nm (log ꢀ) 382 (4.65), 426 (4.59), 470 (4.56), 646 (sh), 862 (br, 3.26);
MS (-LSIMS) 741 (M, 100%); HRMS (-LSIMS) m/e calcd for
C48H35N4NiO 741.21639, found 741.21609 (M). Anal. Calcd for
C48H35N4NiO‚C5H5N: C, 77.47; H, 4.91; N, 8.52. Found: C, 77.36;
H, 4.97; N, 8.61.
845.
(8) Ermler, U.; Grabarse, W.; Shima, S.; Goubeaud, M.; Thauer, R. K.
Science 1997, 278, 1457-1462.
(9) Lahiri, G. K.; Stolzenberg, A. M. Inorg. Chem. 1993, 32, 4409-4413.
10.1021/ic020701s CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/11/2003
Inorganic Chemistry, Vol. 42, No. 25, 2003 8125

COMMUNICATION
Figure 1. EPR spectrum (X-band, 130 K, toluene) of complex 2.
The EPR spectrum of 2 was measured in frozen toluene
at 130 K (Figure 1), and this spectrum is similar to that of
a Ni(III) complex of N-confused tetraphenylporphyrin con-
taining an axial hydroxyl group.7 The g-factor (g > 2.1)
indicates the unpaired electron is located on the metal center
rather than in the porphyrin π system.3 The EPR signals are
broad, and hyperfine splitting is not explicitly observed. N14
hyperfine splitting was not observed in the previously
reported EPR spectra of Ni(III) N-confused porhyrins.7
The room temperature effective magnetic moment of
complex 2 in CD2Cl2 obtained by Evans’ method12 is 1.87
µB, close to the spin-only value for a low spin d7 Ni(III)
center with one unpaired electron (µeff ) 1.74 µB).
The structure of complex 2 was determined by single
crystal X-ray diffraction analysis13 (Figure 2, Table 1); no
counterion was detected. The parent ion peak of 2 at 741
m/z, detected using LSIMS in the negative ion mode, together
with the absence of the counterion, confirms that 2 is a
Ni(III) complex, since the corresponding Ni(II) complex
would have either N(2) or O(1) protonated to balance the
charge and would have a parent ion peak at 742 m/z. A
Ni(I) complex was ruled out in the same way. Cyclic voltam-
metry (Supporting Information) did not assist in assigning
oxidation states to either the metal or the ligand. The
UV-vis spectrum of 2 is similar to that of C(21)-methylated
Ni(II) N-confused tetraphenylporphyrin,14 suggesting that
the porphyrin ring of 2 is not oxidized and complex 2 is
not a pseudo-Ni(III) species as suggested in the case of
[Ni(OETPP)(BTD)]+.10
Complex 2 is structurally similar to an N-oxide of an iron-
(III) porphyrin, which was proposed as an alternative can-
didate of compound I, as the active intermediate of cyto-
chrome P-450.15 The X-ray crystal structure of a Ni(II)
porphyrin N-oxide has been obtained by Balch.16
In the Ni(II) complex of the porphyrin N-oxide,16 the
nitrogen atom which is bonded to oxygen is not bonded to
the metal ion whereas the nickel ion, in complex 2, is bonded
Figure 2. ORTEP drawings of compound 2 showing atomic labeling and
thermal ellipsoids at 50% probability. Solvent, p-tolyl groups, and H atoms
have been removed for clarity. The major (a) and minor (b) contributions
to the disorder are shown.
Table 1. Selected Bond Lengths (Å) for Complex 2
Ni-C(21)
Ni-N(22)
Ni-N(23)
Ni-N(24)
Ni-N(25)
Ni-O(1)
2.068(4)
2.087(3)
2.007(4)
2.100(3)
2.004(3)
2.037(4)
N(2)-C(1)
N(2)-C(3)
C(3)-C(4)
C(21)-C(1)
C(21)-C(4)
C(21)-O(1)
1.394(5)
1.346(5)
1.409(5)
1.444(5)
1.427(5)
1.376(5)
to both the oxygen atom and C(21). The planarity of the
porphyrin skeleton of 2 is distorted, but not as severe as
that of the C(21)-methylated Ni(II) N-confused porphyrin,
in which the inverted pyrrole plane deviates from the
N(22)N(23)N(24) plane by 42.2°.14 For complex 2, the
dihedral angles between the pyrrole planes and the plane
defined by N(22)N(23)N(24) are as follows: C(21) 20.5-
(2)°, N(22) -17.61(7)°, N(23) 22.5(2)°, N(24) -15.3(2)°.
The Ni-N(C) bond distances (2.007-2.100 Å) in 2 are
longer than those of the high spin Ni(II) complex of N(2),C-
(21)-dimethylated N-confused porphyrin,14 contrary to what
might be expected for the Ni(III) complex.1 The strain asso-
ciated with coordination of Ni to both C(21) and O(1) might
(12) Evans, D. A. J. Am. Chem. Soc. 1959, 81, 2003-2005.
(13) Crystal data for 2: C53H40N5NiO‚CH2Cl2, M ) 906.54, monoclinic,
space group P2/a (No. 13), Dc ) 1.275 g cm-3, a ) 21.229(1) Å, b
) 8.6451(5) Å, c ) 25.762(2) Å, â ) 93.004(3)°, V ) 4721.6(5) Å3,
T ) 173 K, Z ) 4, λ ) 0.71069 Å, µ(Mo KR) ) 5.68 cm-1, R(F) )
0.071 and Rw(F) ) 0.185. The structure displays disorder in the
locations of C(21) pyrrole ring, and the O(1) atom. The structure has
been refined to give 64% of the major and 36% of the minor occupancy
as shown in Figure 2. The unit cell also contains one molecule of
disordered CH2Cl2.
(14) Chmielewski, P. J.; Latos-Grazynski, L.; Glowiak, T. J. Am. Chem.
Soc. 1996, 118, 5690-5701.
(15) Watanabe, Y. JBIC, J. Biol. Inorg. Chem. 2001, 6, 846-856.
(16) Balch, A. L.; Chan, Y. W.; Olmstead, M.; Renner, M. W. J. Am. Chem.
Soc. 1985, 107, 2393-2398. Balch, A. L.; Chan, Y. W.; Olmstead,
M. J. Am. Chem. Soc. 1985, 107, 6510-6514.
8126 Inorganic Chemistry, Vol. 42, No. 25, 2003

COMMUNICATION
well prevent the porphyrin from adopting a structure with
shorter Ni-N(C) bond lengths. Elongation of Ni(III)-C
bond lengths has also been observed in the case of [NiIII-
(C6Cl5)4]-.17 The bond distances of N(2)-C(1), N(2)-C(3),
and C(3)-C(4) are shorter than those of C(21)-C(1) and
C(21)-C(4) (Table 1), suggesting that the C(21) atom
approaches sp3 hybridization. The Ni-O(1)-C(21) ring
distorts the octahedral coordination geometry of the center
metal. While the pyridine ring is almost perpendicular to
the N(22)N(23)N(24) plane with a dihedral angle of 83.3-
(2)°, the O-Ni-C(21) angle is only 39.15(16)°.
In summary, a Ni(III) complex of an N-confused porphyrin
inner C-oxide has been synthesized and characterized. The
ability of N-confused porphyrin inner C-oxide to stabilize
other high oxidation state metals is currently under investiga-
tion.
Acknowledgment. This work was supported by NSERC
of Canada. We thank Dr. Barry Liboiron for his help with
the EPR spectroscopy.
Supporting Information Available: X-ray crystallographic file
(CIF) and cyclic voltammogram for compound 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
(17) Alonso, P. J.; Falvello, L. R.; Fornies, J.; Martin, A.; Menjon, B.;
Rodriguez, G. Chem. Commun. 1997, 503-504.
IC020701S
Inorganic Chemistry, Vol. 42, No. 25, 2003 8127