The synthesis of isostructural Mo2+ porphyrin and N-confused porphyrin complexes

Article abstract of DOI:10.1039/b508913a

We have synthesized the first early transition metal N-confused porphyrin complex Mo(CNCTPP)(pip)₂; this species is isostructural to its normal porphyrin analog Mo(TPP)(pip)₂ but exhibits significant electronic differences arising from the inversion of a single pyrrolic group. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b508913a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
The synthesis of isostructural Mo2+ porphyrin and N-confused
porphyrin complexes
John D. Harvey,a Janet L. Shaw,a Richard S. Herrickb and Christopher J. Ziegler*a
Received (in Berkeley, CA, USA) 22nd June 2005, Accepted 29th July 2005
First published as an Advance Article on the web 23rd August 2005
DOI: 10.1039/b508913a
[Rh(CO)2Cl]2 for the synthesis of a bis-Rh(I) NCP complex,
Mn2(CO)10 for the synthesis of the dimer [Mn(NCTPP)]2, and of
Re2(CO)10 leading to the formation of a N-fused tetraphenylpor-
phyrin variant Re(NFTPP)(CO)3.7,9 In the case of [Mn(NCTPP)]2
the lack of coordinating anions was essential to the isolation of the
dimer complex which was shown to break apart in coordinating
solvents such as pyridine. Comparing the above metallations with
their normal porphyrin counterparts provides evidence of the
structural differences that often result in the metallation of
porphyrin analogs and isomers. Reactions of [Rh(CO)2Cl]2 or
Re2(CO)10 with TPP generates a porphyrin with two metals bound
to the interior, while the reaction of Mn2(CO)10 with TPP places
the metal in the macrocycle in a square planar configuration. The
differences in reactivity of the porphyrinoids result from the
presence of a nitrogen at the periphery, and the lack of acidity of
the internal C–H group.10 The structural diversity observed in
metallo-N-confused porphyrins often makes direct comparisons to
their normal porphyrin analogs difficult.7,11
We have synthesized the first early transition metal N-confused
porphyrin complex Mo(NCTPP)(pip)2; this species is isostruc-
tural to its normal porphyrin analog Mo(TPP)(pip)2 but
exhibits significant electronic differences arising from the
inversion of a single pyrrolic group.
Porphyrins and related complexes continue to be active foci of
research owing to their biological relevance and their applications
to materials such as catalysts and photonic arrays.1 The porphyrin
isomer N-confused porphyrin2 (NCP) has been vigorously
explored since the synthesis presented by Geier et al. allowed for
the high yield production of this macrocycle.3 NCP differs
structurally from normal porphyrin in that it has a confused, or
inverted, pyrrole ring which results in the placement of a nitrogen
atom on the periphery of the ring and a carbon atom at an interior
position. It has been shown that NCP often forms new, unique
binding modes not observed in normal porphyrin due to the
presence of this external nitrogen.4 This structural change in NCP
also leads to interesting properties not observed in normal
porphyrin, such as tautomerization and abrogated aromaticity.5
However, exploring the electronics of NCP metal complexes in
relation to normal porphyrin metal complexes has been difficult
due to the above structural differences. In this paper, we present
the metallation of 5,10,15,20-tetraphenyl-N-confused porphyrin
We are currently investigating the early transition metal
chemistry of N-confused porphyrin, which remains largely
unexplored. We examined the molybdenum chemistry via the
reaction of an equimolar ratio of cis-Mo(NHC5H10)2(CO)4 with
H2NCTPP in refluxing toluene under anaerobic conditions. After
three hours, the solution changes in colour from brown to green,
and affords the species Mo(NCTPP)(pip)2 (1). The absorption
spectrum of this compound has a blue shifted Soret band at
420.5 nm with a shoulder at 481 nm and a Q band at 686 nm with
a shoulder at 633 nm (Fig. 2). Upon cooling of the reaction
solution, large single crystals form in the reaction flask. The
structure of the product was elucidated by single crystal X-ray
diffraction.{ As seen in Fig. 3, the Mo atom lies in the core of the
(H2NCTPP) with
a group VI metal using the activated
molybdenum carbonyl cis-Mo(NHC5H10)2(CO)4 and its anala-
gous porphyrin complex. The piperidine carbonyl reagent6 allows
the reactions to be performed in lower boiling solvents, such as
toluene. In addition, unlike other reactions of NCP with metal
carbonyls, the resulting Mo-TPP and Mo-NCTPP complexes are
isostructural. This feature allows for an unusual look into the
factors governing the electronics of NCP–metal complexes.
In our previous work, we have chosen to explore the use of
metal carbonyl starting compounds in the metallation of NCP.7
Carbonyl-based starting materials are very common in the realm
of organometallic chemistry, and they were first used in porphyrin
metallations by Buchler.8 The strictly carbonyl-based methods are
useful because they do not introduce additional coordinating
species during the synthetic protocol. The use of transition metal
carbonyls in the synthesis of metallated NCPs has been relatively
limited (Fig. 1). Examples in the literature include the use of
aDepartment of Chemistry, University of Akron, Akron, OH 44325-
3601, USA. E-mail: ziegler@uakron.edu; Fax: +1-(330)972-7370;
Tel: +1-(330)972-2531
Fig. 1 Top: structures of metalloporphyrins using the carbonyl metallat-
ing reagents [Rh(CO)2Cl]2 (left), Re2(CO)10 (middle) and Mn2(CO)10
(right); bottom: structures of the N-confused porphyrin compounds using
identical reaction conditions.
bDepartment of Chemistry, College of Holy Cross, Box C, Worcester,
MA 01610-2395, USA. E-mail: rherrick@holycross.edu;
Fax: +1-(508)793-3530; Tel: +1-(508)793-2490
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 4663–4665 | 4663

View Article Online
N-confused porphyrin. In normal porphyrin, the symmetry results
in pseudoparity forbidden Q band transitions resulting from
alternancy symmetry. In addition, the Soret band is enhanced in
normal porphyrins because the transition dipoles reinforce,
resulting in a larger oscillator strength. In N-confused porphyrin,
the symmetry is broken and the effect of alternancy symmetry is
significantly less, resulting in a ratio of Soret to Q band closer to
unity. The second significant change to the spectrum upon
inversion of a pyrrole ring is the red shift of the Q band
absorption. This change results from the reduced HOMO–LUMO
gap in N-confused porphyrin relative to normal porphyrin. In
NCPs, the HOMO and HOMO-1 orbitals are destabilized
and inverted, and combined with the lack of degeneracy in the
LUMO and LUMO-1 orbitals, the transition energy is signifi-
cantly lowered relative to normal porphyrin.13 Interestingly, an
opposite trend is observed in the shift of the Soret band versus the
free base upon metallation. This indicates that the hypso/hyper
classification of Gouterman does not readily apply to NCP metal
complexes.14
Fig. 2 Absorption spectra of MoNCTPP(pip)2 1 (—) and MoTPP(pip)2
2 (---). Both complexes were measured at 1.40 6 1025 M in toluene.
porphyrin with two axial piperidines. The internal carbon atom is
deprotonated and is coordinating the metal center in a manner
similar to that observed in nickel and cobalt complexes of NCP.2,7
An identical reaction carried out with H2TPP results in a colour
change from reddish brown to dark brown and affords
Mo(TPP)(pip)2 (2). In the absorption spectrum, the Soret band
red shifts to 429 nm, with a primary Q band appearing at 554 nm
(Fig. 2). Crystallization also occurs upon cooling of the reaction
mixture, and the structure can be elucidated by single crystal X-ray
diffraction.{ The normal porphyrin complex (Fig. 3) is structurally
identical to that observed in compound 1, with the exception of the
pyrrolic inversion. All of the bond angles and lengths for the two
complexes are within experimental error of each other with the
largest deviation being the Mo–N(3) bond length to the axial
The changes to the porphyrin electronic structure have a
significant effect on the energy level of the d orbitals in transition
metal NCP complexes. We recently presented a report on the high
field EPR of Mn(NCTPP)(py)2,15 which exhibits a highly rhombic
zfs in contrast to the axial systems found in normal porphyrins. It
appears that the Mo complexes in this report are similarly affected;
the magnetic susceptibility differs significantly between compounds
1 and 2. For 2, magnetic measurements showed a meff 5 2.79,
corresponding to a spin state of S 5 1. This is similar to the
meff 5 2.83 for the Mo(TTP)(py)2 complex, with a S 5 1 state and
also similar to the Mn(NCTPP)(py)2 complex value with
meff 5 2.83.7,12 However, 1 exhibits a magnetic susceptibility of
meff
5
1.68, much lower than 2, Mo(TTP)py2, and
Mn(NCTPP)py2.7,12 Thus, the switching of two atoms in the
porphyrin backbone has a demonstrable effect on the metal d
orbital energy levels. This decrease in paramagnetism was also
˚
˚
piperidine molecule (1: 2.248(2) A and 2: 2.2620(17) A). Since no
other anions are present in either structure and upon inspection of
the axial bond lengths, we can assign the metal oxidation state as
1
+2. The axial Mo–N bond lengths are also similar to the lengths
˚
found in the molybdenum porphyrin pyridine complex (2.216 A).
observed in the H NMR spectra. In 1, the pyrrole and phenyl
12
protons exhibited paramagnetic shifts, but were shifted far less
than those observed in complex 2.
While the two porphyrinoid complexes are structurally similar,
the UV-visible spectra exhibit significant differences. First, the
relative magnitude of the extinction coefficients of the Soret band
and the Q bands is much larger in normal porphyrin than in
In conclusion, we have synthesized the first early transition
metal N-confused porphyrin complex Mo(NCTPP)(pip)2. Unlike
most N-confused porphyrin metal complexes, this species is
˚
Fig. 3 Molecular structure of Mo(NCTPP)(pip)2 (1, left) and Mo(TPP)(pip)2 (2, right) with 50% thermal ellipsoids. Selected bond lengths (A) for: 1: Mo–
N(1) 2.071(2), Mo–C(8) 2.0814(19), Mo–N(3) 2.248(2); 2: Mo–N(1) 2.0726(16), Mo–N(2) 2.0826(16), Mo–N(3) 2.2620(17). Due to disorder of the nitrogen
atom at the confused pyrrole ring and high symmetry of the crystal, one of eight possible orientations is shown for 1.
4664 | Chem. Commun., 2005, 4663–4665
This journal is ß The Royal Society of Chemistry 2005

View Article Online
1 E. Kim, E. Chufa´n, K. Kamaraj and K. Karlin, Chem. Rev., 2004, 104,
1077; B. Meunier, A. Robert, G. Pratviel and J. Bernadou, The
Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith, R. Guilard,
Academic Press, New York, 2000, vol. 4, p. 119; D. Kim and A. Osuka,
Acc. Chem. Res., 2004, 37, 735.
2 H. Furuta, T. Asano and T. Ogawa, J. Am. Chem. Soc., 1994, 116, 767;
P. J. Chmielewski, L. Latos-Grazynski, K. Rachlewicz and T. Glowiak,
Angew. Chem., Int. Ed. Engl., 1994, 33, 779.
isostructural to its normal porphyrin analog, Mo(TPP)(pip)2 and
allows for investigation of the electronic changes induced by
inversion of a single pyrrolic ring. Changes can be readily observed
in the p system via UV-visible spectroscopy, and in the metal d
orbitals via magnetic susceptibility. We are continuing this work
and will investigate the EPR spectroscopy of these compounds,
which have the same d electron counts as our recently reported
Mn(NCTPP)(py)2 species.15
3 G. R. Geier, III, D. M. Haynes and J. S. Lindsey, Org. Lett., 1999, 1,
1455.
4 H. Furuta, H. Maeda and A. Osuka, Chem. Commun., 2002, 1795;
J. D. Harvey and C. J. Ziegler, Coord. Chem. Rev., 2003, 247, 1.
5 H. Furuta, T. Ishizuka, A. Osuka, H. Dejima, H. Nakagawa and
Y. Ishikawa, J. Am. Chem. Soc., 2001, 123, 6207; A. Ghosh,
T. Wondimagegn and H. J. Nilsen, J. Phys. Chem. B, 1998, 102, 10459.
6 D. J. Darensbourg and R. L. Kump, Inorg. Chem., 1978, 17, 2680.
7 J. D. Harvey and C. J. Ziegler, Chem. Commun., 2002, 1942; J. D. Harvey
and C. J. Ziegler, Chem. Commun., 2003, 2890; J. D. Harvey and
C. J. Ziegler, Chem. Commun., 2004, 1666.
8 J. W. Buchler, 10 Synthesis and Properties of Metalloporphyrins, in The
Porphyrins, ed. D. Dolphin, Academic Press, New York, 1978, vol. I,
pp. 389–481.
9 A. Srinivasan, H. Furuta and A. Osuka, Chem. Commun., 2001, 1666;
M. Toganoh, T. Ishizuka and H. Furuta, Chem. Commun., 2004, 2464.
C.J.Z. and R.S.H. acknowledge the Petroleum Research Fund
(PRF# 39625-G5M). C.J.Z. acknowledges the University of
Akron for faculty research grant (FRG-1565). We also wish to
acknowledge NSF grant CHE-0116041 for funds used to purchase
the Bruker-Nonius diffractometer.
Notes and references
{ Crystallographic summary for 1: data was collected at 100 K (Bruker
KRYO-FLEX) on a Bruker SMART APEX CCD-based X-ray
diffractometer system. MoC61H58N6 1; M 5 971.07, black plate 0.20 6
0.10 6 0.03 mm, triclinic, space group P-1, Z 5 1 in a cell of dimensions
´
´
10 M. Stepien and L. Latos-Graz˙ynski, Acc. Chem. Res., 2005, 38, 88.
11 W. Chen and C. Hung, Inorg. Chem., 2001, 40, 5070; H. Furuta,
T. Ishizuka and A. Osuka, J. Am. Chem. Soc., 2002, 124, 5622;
D. S. Bohle, W.-C. Chen and C.-H. Hung, Inorg. Chem., 2002, 41, 3334;
C.-H. Hung, W.-C. Chen, G.-H. Lee and S.-M. Peng, Chem. Commun.,
2002, 1516.
˚
a 5 9.8887(18), b 5 11.630(2), c 5 11.910(2) A, a 5 112.359(3)u,
3
23
˚
b 5 94.035(3)u, c 5 103.615(3)u, V 5 1211.5(4) A , Dc 5 1.331 Mg m
,
m(Mo-Ka) 5 0.318 mm21, F(000) 5 508, final R indices on 5580
independent reflections [I . 2s(I)]: R1 5 0.0447, wR2 5 0.1108.
Crystallographic summary for 2: data was collected at 100 K (Bruker
KRYO-FLEX) on a Bruker SMART APEX CCD-based X-ray
diffractometer system. MoC61H58N6 2; M 5 971.07, black plate 0.26 6
0.19 6 0.04 mm, triclinic, space group P-1, Z 5 1 in a cell of dimensions
12 J. Colin, A. Strich, M. Schappacher, B. Chevrier, A. Veillard and
R. Weiss, Nouv. J. Chim., 1984, 8, 55.
13 J. P. Belair, C. J. Ziegler, C. S. Rajesh and D. A. Modarelli, J. Phys.
Chem. A, 2002, 106, 6445.
14 P. G. Seybold and M. J. Gouterman, J. Mol. Spectrosc., 1969, 31, 1;
M. J. Gouterman, in The Porphyrins, ed.D. Dolphin, vol. III, Academic
Press, New York, 1978, pp. 1–165.
15 J. D. Harvey, C. J. Ziegler, J. Telser, A. Ozarowski and J. Krzystek,
Inorg. Chem., 2005, 44, 4451.
˚
a 5 9.9529(9), b 5 11.6864(10), c 5 11.9876(11) A, a 5 112.4130(10)u,
3
23
˚
b 5 93.904(2)u, c 5 103.4010(10)u, V 5 1234.77(19) A , Dc 5 1.306 Mg m
,
m(Mo-Ka) 5 0.312 mm21, F(000) 5 508, final R indices on 5671
independent reflections [I . 2s(I)]: R1 5 0.0378, wR2 5 0.0875. CCDC
273837 and 273838. See http://dx.doi.org/10.1039/b508913a for crystal-
lographic data in CIF or other electronic format.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 4663–4665 | 4665