Metal oxidation promoted C-H activation in manganese complexes of N-confused porphyrin

Article abstract of DOI:10.1021/ic025573m

The N-confused porphyrin complex Mn^II(NCHPP)Br exhibits a nonplanar porphyrin ring with an inner core C-H on the inverted pyrrole ring. The aerobic metal oxidation promotes the dissociation of an inner-core proton on the inverted pyrrole ring and changes the N-confused porphyrin conformation to a planar porphyrin ring to form Mn^II(NCPP)Br. The conjugate systems and metal oxidation states are confirmed by crystal structures as well as spectroscopic data. The reverse reaction can be achieved by treating the Mn^II(NCPP)Br with p-toluenesulfonhydrazide.

Full text of DOI:10.1021/ic025573m

Inorg. Chem. 2002, 41, 3334−3336
Metal Oxidation Promoted C−H Activation in Manganese Complexes of
N-Confused Porphyrin
,‡
D. Scott Bohle,† Wan-Chin Chen,‡ and Chen-Hsiung Hung*
Departments of Chemistry, National Changhua UniVersity of Education,
Changhua 50058, Taiwan, and UniVersity of Wyoming, Laramie, Wyoming 82071
Received March 6, 2002
II
The N-confused porphyrin complex Mn (NCHPP)Br exhibits a
nonplanar porphyrin ring with an inner core C−H on the inverted
pyrrole ring. The aerobic metal oxidation promotes the dissociation
of an inner-core proton on the inverted pyrrole ring and changes
the N-confused porphyrin conformation to a planar porphyrin ring
III
to form Mn (NCPP)Br. The conjugate systems and metal oxidation
states are confirmed by crystal structures as well as spectroscopic
data. The reverse reaction can be achieved by treating the
III
Mn (NCPP)Br with p-toluenesulfonhydrazide.
N-Confused porphyrin (NCP), first isolated in 1994
independently by Furuta1 and Latos-Grazynski,2 has an
inverted pyrrole ring jointed to the porphyrinic conjugated
system with a â-carbon. A recent review3 by Latos-Grazynski
demonstrated that N-confused porphyrin exhibits unusual
physical properties as well as coordination chemistry. Ac-
cording to the theoretical calculations,4 N-confused porphyrin
can exist in different tautomeric forms. The free base NCP
adopts the most stable tautomeric form, 1, with an inner C-H
and two trans-positioned inner core NHs in CH2Cl2. There
was not much information available for the alternations
between different tautomeric forms until recently Furuta and
co-workers5 observed through NMR spectroscopy in DMF
that NCP adopts the tautomeric form 2. This is also the case
in the solid-state structure as well.5 In terms of the metal
complexes, the Ag(III) complex6 of NCP adopts tautomeric
form 1 with a 3- charged deprotonated NCP ring system.
Other N-confused porphyrin complexes7 adopt the higher
energy tautomeric form 2 with one peripheral N-H and a
2- charged NCP core. Interestingly, there is still little
discussion on the processes of the C-H activation during
metalation in both tautomeric forms.
C-H activation is one of the most topical research areas
in chemistry. In addition to applications on catalysis, the
biological systems rely on the C-H activation to carry out
further enzymatic hydroxylation.8 Several models have been
proposed to elucidate the mechanism of enzymatic C-H
activation.9,10 The unusual porphyrin core geometry of
N-confused porphyrin with an inner core carbon atom makes
it an ideal compound for the study of C-H activation within
a large conjugated ring system. Recently, we have isolated
two five-coordinated N-confused porphyrin iron complexes11
which exhibit nonplanar NCP core geometry with a tilted
inverted pyrrole ring. The relatively short distances between
iron and inner core C-H suggest that an agostic interaction
* Author to whom correspondence should be addressed. E-mail: chhung@
cc.nceu.edu.tw.
† University of Wyoming.
‡ National Changhua University of Education.
(1) Furuta, H.; Asano, T.; Ogawa, T. J. Am. Chem. Soc. 1994, 116, 767.
(2) Chmielewski, P. J.; Latos-Grazynski, L.; Rachlewiwicz, K.; Glowiak,
T. Angew. Chem., Int. Ed. Engl. 1994, 33, 779.
(3) Latos-Grazynski, L. In Core Modified Heteroanalogues of Porphyrins;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 2.
(7) Chmielewski, P. J.; Latos-Grazynski, L.; Glowiak, T. J. Am. Chem.
Soc. 1996, 118, 5690.
(4) Ghosh, A.; Wondimagegn, T.; Nilsen, H. J. J. Phys. Chem. B 1998,
102, 10459.
(5) Furuta, H.; Ishizuka, T.; Osuka, A.; Dejima, H.; Nakagawa, H.;
Ishikawa, Y. J. Am. Chem. Soc. 2001, 123, 6207.
(6) Furuta, H.; Ogawa, T.; Uwatoko, Y.; Araki, K. Inorg. Chem. 1999,
38, 2676.
(8) Meunier, B. Chem. ReV. 1992, 92, 1411.
(9) Basch, H.; Musaev, D. G.; Mogi, K.; Morokuma, K. J. Phys. Chem.
A 2001, 105, 3615.
(10) Collman, J. P.; Chien, A. S.; Eberspacher, T. A.; Brauman, J. L. J.
Am. Chem. Soc. 1998, 120, 425.
(11) Chen, W.-C.; Hung, C.-H. Inorg. Chem. 2001, 40, 5070.
3334 Inorganic Chemistry, Vol. 41, No. 13, 2002
10.1021/ic025573m CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/30/2002

COMMUNICATION
between the iron and inverted pyrrolic C-H bond is likely.
The axial ligand dependent core geometries are controlled
by the basicity of ligands consistent with having an agostic
interaction.12 Still, little information is available for the
mechanism of conformational change from a nonplanar NCP
core to a planar NCP geometry in NCP metal complexes. In
this paper we report that in the Mn(II) oxidation state, the
NCP adopted the tilted geometry similar to the iron
complexes; however, the inverted pyrrolic C-H bond can
be activated when Mn(II) is oxidized to Mn(III) to form a
planar NCP manganese complex.
Figure 1. Molecular structure of MnII(NCHPP)Br (3). Important bond
lengths (Å) and angles (deg): Mn(1)-C(3) 2.437(7); Mn(1)-N(2) 2.164-
(5); Mn(1)-N(3) 2.085(4); Mn(1)-N(4) 2.151(5); C(3)-C(4) 1.383(9);
C(4)-N(1) 1.377(7); N(3)-Mn(1)-C(3) 140.9(2); N(3)-Mn(1)-Br(1)
115.58(13).
When the free base NCP was treated with MnBr2 in
the mixed solvent of 2:1 CH3CN/THF in the presence of a
few drops of lutidine under anaerobic conditions,
MnII(NCHPP)Br (3)13 was isolated in 82% yield. The
electronic spectrum of 3 in CH2Cl2 gives a Soret type
absorption band at 462 nm while the Q bands are located at
715 and 782 nm. The pattern of the electronic spectrum is
distinct from the free base NCP but resembles other
nonplanar N-confused porphyrin complexes such as methy-
lated (NCP)Ni7 and Fe(NCP)Br.11 The similar electronic
spectra suggest that these compounds exhibit identical ring
conformations. MnII(NCHPP)Br has a magnetic moment of
5.50 µB at room temperature from the SQUID magnetic
susceptibility measurement. The plot of 1/ø vs T is linear
between 5 and 300 K. The room temperature magnetic
5
moment suggested a high-spin Mn(II) (s ) /2) state in 3.
Figure 2. The time trace of absorption spectra for the oxidation reaction
The ESR spectrum of the MnII(NCHPP)Br in toluene glass
at 6.8 K gives an s ) 5/2 Mn(II) type high-spin axial spectrum
with g at 5.65 and g| at 2.01.
of MnII(NCHPP)Br in the presence of O2.
MnII(NCHPP)Br is stable in the solid state as well as in
solution under anaerobic conditions. Interestingly, the elec-
tronic spectrum changed immediately when 3 was exposed
to air. A time trace of the electronic spectra is shown in
Figure 2. The intensity of the Soret band at 462 nm for
MnII(NCHPP)Br decreases and is accompanied by an in-
crease in a Soret type peak at 507 nm. The final product,
after 24 h, gives the electronic spectrum with the Soret band
at 507 nm and the Q band at 754 and 825 nm. The isolated
product gives an effective magnetic moment of 4.87 µB at
room temperature from SQUID and suggests a one-electron
oxidation of the MnII(NCHPP)Br to give MnIII(NCPP)Br
(4).17 4 is stable in the solid state as well as in solution under
aerobic conditions. No further variation was observed when
the absorption spectra were traced under aerobic conditions
for 72 h.
X-ray single-crystal analysis confirms the solid-state
conformation of MnII(NCHPP)Br. As shown in Figure 1, the
N-confused porphyrin ring adopts a nonplanar geometry with
inverted pyrrole ring tilted away from the mean plane de-
fined by the tripyrrole unit. The comparison of the bond
distances and bond angles in the porphyrin ring with the
theoretical values14 agrees that the porphyrin core in the
MnII(NCHPP)Br adopts the higher energy resonance form
as in 2. The distance of 2.437(7) Å between Mn and inner
core carbon in the inverted pyrrolic ring is much longer than
the normal Mn-C bond.15 The Mn atom sits 0.77 Å atop
the mean porphyrin plane, resembling other five-coordinated
porphyrin complexes. Finally, the Mn-Br bond distance of
2.491 Å is almost identical to 2.490 Å in the five-coordinated
Mn(TPP)Br.16
(12) The distance between iron and inner core C-H hydrogen in
Fe(NCTPP)Br is 1.971 Å while in Fe(NCTPP)(S-C7H7) the corre-
sponding bond distance increases to 2.334 Å.
(15) Brookhart, M.; Lamanna, W.; Humphrey, M. B. J. Am. Chem. Soc.
1982, 104, 2117.
(16) Turner, P.; Gunter, M. J.; Skelton, B. W.; White, A. H. Aust. J. Chem.
1998, 51, 835.
(13) MnII(NCHPP)Br: absorption spectrum (CH2Cl2) [λmax, nm (log ꢀ, M-1
cm-1)] 352 (4.69), 462 (5.14), 782 (4.41), 715 (4.06), 598 (3.54), 656
(3.7). Anal. Calcd for C44H29N4BrMn‚0.8CH2Cl2: N, 6.86; C, 65.90;
H, 3.78. Found: N, 6.74; C, 65.88; H, 4.04. MnBrN4C44H29: calcd
mass 748.582, obsd 667 (M - H - Br, FAB). Crystallographic data
for 3: C45.50H30BrCl3MnN4, T ) 295(2) K, MW ) 875.93, monoclinic,
space group P2/c, a )13.2250(11) Å, b ) 13.8871(11) Å, c )
22.5276(18) Å, R ) 90°, â ) 92.651(2)°, γ ) 90°, V ) 4132.9(6)
(17) MnIII(NCPP)Br: absorption spectrum (CH2Cl2) [λmax, nm (log ꢀ, M-1
cm-1)] 340 (4.39), 394 (4.46), 454 (4.31), 507 (4.70), 583 (3.64), 754
(3.66), 825 (3.91). Anal. Calcd for MnBrN4C44H28‚0.5THF‚0.1CH2-
Cl2: N, 7.07; C, 69.90; H, 4.10. Found: N, 7.27; C, 69.87; H, 4.26.
MnBrN4C44H28: calcd mass 747.574, obsd 667 (M - Br, FAB).
Crystallographic data for 4: C44H28BrMnN4, T ) 293(2) K, MW )
747.55, monoclinic, space group P21/n, a ) 10.2386(6) Å, b )
15.8204(9) Å, c ) 20.9468(13) Å, R ) 90°, â ) 90.000(1)°, γ )
90°, V ) 3392.9(3) Å3, Z ) 4, Dc ) 1.463 Mg/m-3, λ ) 0.71073 Å,
µ ) 1.606 mm-1, F(000) ) 1520. Data were collected on a Bruker
Smart 1000 diffractometer for 1.61° < θ < 27.54°. The structure was
solved by direct methods and refined by least-squares against F2 to
R1 ) 0.0753 (wR2 ) 0.1734) and SGOF ) 0.732.
Å3, Z ) 4, Dc ) 1.408 Mg/m-3, λ ) 0.71073 Å, µ ) 1.517 mm-1
,
F(000) ) 1776. Data were collected on a Bruker Smart 1000
diffractometer for 1.54° < θ < 27.57°. The structure was solved by
direct methods and refined by least-squares against F2 to R1 ) 0.0545
(wR2 ) 0.1051) and SGOF ) 1.136.
(14) Szterenberg, L.; Latos-Grazynski, L. Inorg. Chem. 1997, 36, 6287.
Inorganic Chemistry, Vol. 41, No. 13, 2002 3335

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Scheme 1
Figure 3. Molecular structure of MnIII(NCPP)Br (4). Important bond
lengths (Å) and angles (deg): Mn(1)-C(1) 2.020(6); Mn(1)-N(2) 2.026-
(5); Mn(1)-N(3) 2.049(5); Mn(1)-N(4) 2.044(5); C(1)-C(20) 1.390(8);
C(20)-N(1) 1.403(7); N(3)-Mn(1)-C(1) 162.5(2); N(3)-Mn(1)-Br(1)
97.10(15).
promoted C-H activation. To confirm the product as well
as to establish a possible catalytic cycle, we have treated
the oxidized MnIII(NCPP)Br with reductants (Scheme 1).
When MnIII(NCPP)Br was treated with sodium borohydride
in THF, the distinctive Soret band for MnIII NCP at 507 nm
disappeared accompanied by an increased Soret band at
457 nm. The fine structure of the Q band with λmax at 722
nm, however, is distinctively different from MnII(NCHPP)-
Br. The preliminary crystal structure demonstrates an elimi-
nation of bromide to give a MnII NCP product with a THF
as axial ligand and a planar 2- charged NCP core.
Interestingly, the p-toluenesulfonhydrazide reacts slowly with
MnIII(NCPP)Br to give the final product with an electronic
spectrum identical to that of the MnII(NCHPP)Br.
These studies for the first time shed light on the C-H
activation processes of N-confused porphyrin complexes.
They also suggest that N-confused porphyrin complexes have
the potential of being used as catalysts, where there is a redox
active metal coupled to a redox-tautomeric chelating ligand.
The detailed mechanisms and ligand effect of the C-H
activation in the systems of iron and manganese N-confused
porphyrins are currently under intense study.
The crystal structure of the oxidative product 4 (Figure
3) clearly demonstrates that the conformation of the N-
confused porphyrin ring changes from a nonplanar geom-
etry in the MnII(NCHPP)Br to a planar conformation in
MnIII(NCPP)Br. The comparable bond distances and bond
angles around the porphyrin cores suggest that 3 and 4 are
under the same resonance forms and the planar porphyrin
ring with a short (2.020(6) Å) Mn-C bond in 4 is gen-
erated from the activation of the inner core C-H bond in 3.
The averaged Mn-N bond distance of 2.040 Å, which is
shorter than the corresponding distance of 2.133 Å for
MnII(NCHPP)Br, is consistent with a metal oxidation of
Mn(II) to Mn(III). A possible mechanism of the conformation
change would be that the aerobic oxidation of the Mn(II)
metal center in the nonplanar N-confused porphyrin core
gives a more electron deficient Mn(III) center and promotes
a direct electrophilic addition18 of Mn(II), acting as a Lewis
acid, into an acidic C-H bond. Alternatively, metal oxidation
would increase the acidity of the C-H bond through a
stronger agostic interaction19 and the electron pair on the
carbon would coordinate to the metal center after the proton
dissociation.
Acknowledgment. We thank Professor Jui-Hsien Huang
for helpful discussions and the National Science Council
(Taiwan) for support of this work.
Supporting Information Available: Experimental procedures,
UV-vis, EPR, and SQUID data, and X-ray crystallographic data
for3 and 4. This material is available free of charge via the Internet
at http://pubs.acs.org.
The reaction of MnII(NCHPP)Br with bases such as
lutidine failed to deprotonate the inner core C-H on the
inverted pyrrole ring consistent with a metal oxidation
IC025573M
(18) Taylor, R. Electrophilic Aromatic Substitution; John Wiley & Sons:
(19) Vigalok, A.; Uzan, O.; Shimon, L. J. W.; Ben-David, Y.; Martin, J.
New York, 1990; Chapter 5.
M. L.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 12539.
3336 Inorganic Chemistry, Vol. 41, No. 13, 2002