Synthesis, structures, and properties of a series of four-, five-, and six-coordinate cobalt(III) triazacorrole complexes: The first examples of transition metal corrolazines

Article abstract of DOI:10.1021/ic020297x

The syntheses of the first transition metal corrolazine complexes, in which the meso carbon atoms of a corrole framework have been replaced by N atoms, are reported. Metalation of the corrolazine [(TBP)₈CzH₃] (TBP = 4-tert-butylphenyl) (1) with Co-(acac)₂ gives [(TBP)₈CzCO^III] (2) in good yield. Addition of PPh₃ to 2 in pyridine results in the formation of [(TBP)₈CzCo^III(PPh₃)] (3), which was characterized by X-ray crystallography. Likewise, addition of an excess of pyridine to 2 in CH₂Cl₂ followed by slow diffusion of MeOH gives [(TBP)₈CzCO^III(py)₂] (4) as a crystalline solid, which was also characterized by X-ray crystallography. The crystal structures of 3 and 4 reveal that the corrolazine cavity is significantly smaller (～0.1 A) than their regular corrole analogues. Characterization of 2-4 by UV-vis spectroscopy reveals some interesting features in the absorption spectra of these compounds, including a dramatic red-shift of the Soret band. In addition, binding of pyridine to 2 was evaluated quantitatively by UV-vis titration, revealing a formation constant of β₂ = 9.0 × 10⁷ M^-2, which is larger than any of the regular Co^III corrole analogues.

Full text of DOI:10.1021/ic020297x

Inorg. Chem. 2002, 41, 4105−4107
Synthesis, Structures, and Properties of a Series of Four-, Five-, and
Six-Coordinate Cobalt(III) Triazacorrole Complexes: The First Examples
of Transition Metal Corrolazines
Bobby Ramdhanie, Lev N. Zakharov, Arnold L. Rheingold, and David P. Goldberg*
Department of Chemistry, The Johns Hopkins UniVersity, 3400 N. Charles Street, Baltimore,
Maryland 21218, and Department of Chemistry and Biochemistry, UniVersity of Delaware,
Newark, Delaware 19716
Received April 29, 2002
The syntheses of the first transition metal corrolazine complexes,
in which the meso carbon atoms of a corrole framework have
been replaced by N atoms, are reported. Metalation of the
of their relative ease of synthesis4-7 compared to other
metallocorroles. We recently reported the synthesis of the
first triazacorrole (“corrolazine”, Cz) compound, [(TBP)8CzPV-
(OH)](OH) (TBP ) 4-tert-butylphenyl), in which the meso
carbon atoms of a conventional corrole ring have been
replaced with nitrogen atoms. This phosphorus-containing
corrolazine was converted to the free base [(TBP)8CzH3] (1)
by a novel reductive demetalation strategy.8 Although it was
predicted that metal-free 1 would allow entry into a new
family of transition metal corrole-like complexes, to date no
such complexes have been reported in the literature. Herein,
we describe the synthesis and spectroscopic characterization
of [(TBP)8CzCoIII], (2), [(TBP)8CzCoIII(PPh3)] (3), and
[(TBP)8CzCoIII(py)2] (4), starting from the versatile precursor
1. Compounds 2-4 are the first examples of transition metal
corrolazine complexes.
The neutral, four-coordinate CoIII complex [(TBP)8CzCoIII]
was prepared by heating 1 with an excess of CoII(acac)2 in
pyridine (Scheme 1). Removal of the pyridine under vacuum
and purification by column chromatography (silica gel,
hexanes/CH2Cl2 1/1 (v/v)) led to the isolation of the product
in 86% yield. Many of the other conventional cobalt(III)
corroles (i.e., corroles with carbon atoms at the meso
positions) have been isolated and fully characterized only
in the presence of strong axial ligands (e.g., PPh3, pyridine),
affording the five- or six-coordinate complexes.4-7 In addi-
tion, a recent study of (tpfc)CoIII (tpfc ) 5,10,15-tris-
(pentafluorophenyl)corrole) showed that in the absence of
axial ligands this complex was not stable and spontaneously
oxidized to give a dimerized product with a direct Câ-Câ
corrolazine [(TBP)8CzH ] (TBP ) 4-tert-butylphenyl) (1) with Co-
3
III
(acac)2 gives [(TBP)8CzCo ] (2) in good yield. Addition of PPh3 to
III
2 in pyridine results in the formation of [(TBP)8CzCo (PPh3)] (3),
which was characterized by X-ray crystallography. Likewise,
addition of an excess of pyridine to 2 in CH Cl2 followed by slow
2
III
diffusion of MeOH gives [(TBP)8CzCo (py)2] (4) as a crystalline
solid, which was also characterized by X-ray crystallography. The
crystal structures of 3 and 4 reveal that the corrolazine cavity is
significantly smaller (∼0.1 Å) than their regular corrole analogues.
Characterization of 2−4 by UV−vis spectroscopy reveals some
interesting features in the absorption spectra of these compounds,
including a dramatic red-shift of the Soret band. In addition, binding
of pyridine to 2 was evaluated quantitatively by UV−vis titration,
revealing a formation constant of â2 ) 9.0 × 107 M-2 , which is
III
larger than any of the regular Co corrole analogues.
Transition metal corrole complexes have been under
intense investigation recently. Corroles are tetrapyrrolic
macrocycles that have the same CR-CR linkage as the cobalt-
containing nonaromatic corrin ring of vitamin B-12, yet they
maintain the 18 πe- aromatic core found in porphyrins and
phthalocyanines.1-3 Considerable efforts have been expended
on the syntheses and characterization of cobalt corroles in
particular because of their biological relevance, and because
(4) Guilard, R.; Je´roˆme, F.; Barbe, J.-M.; Gros, C. P.; Ou, Z.; Shao, J.;
Fischer, J.; Weiss, R.; Kadish, K. M. Inorg. Chem. 2001, 40, 4856-
4865.
(5) Guilard, R.; Gros, C. P.; Bolze, F.; Je´roˆme, F.; Ou, Z.; Shao, J.; Fischer,
J.; Weiss, R.; Kadish, K. M. Inorg. Chem. 2001, 40, 4845-4855.
(6) Mahammed, A.; Giladi, I.; Goldberg, I.; Gross, Z. Chem.sEur. J. 2001,
7, 4259-4265.
(7) Paolesse, R.; Licoccia, S.; Bandoli, G.; Dolmella, A.; Boschi, T. Inorg.
Chem. 1994, 33, 1171-1176.
(8) Ramdhanie, B.; Stern, C. L.; Goldberg, D. P. J. Am. Chem. Soc. 2001,
123, 9447-9448.
* Author to whom correspondence should be addressed. E-mail:
dpg@jhu.edu.
(1) Paolesse, R. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 2, pp
201-232.
(2) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 2, pp 233-300.
(3) Sessler, J. L.; Weghorn, S. J. Expanded, Contracted, & Isomeric
Porphyrins; Elsevier Science Inc.: New York, 1997; Vol. 15.
10.1021/ic020297x CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/17/2002
Inorganic Chemistry, Vol. 41, No. 16, 2002 4105

COMMUNICATION
a
Scheme 1
a Reagents and conditions: (a) Co(acac)2, pyridine, 115 °C, 1 h; (b) 5
equiv of PPh3, pyridine, 115 °C, 1 h; (c) 2% pyridine in CH2Cl2.
bond.6 The stability of 2 may be due to a combination of
the oxidatively resistant meso nitrogen atoms and phenyl
substituents that block all eight of the â positions in the Cz
ring.
The 1H NMR spectrum of 2 in CDCl3 exhibits sharp peaks
that are shifted over a wide range (+27 to -23 ppm). The
well-resolved pattern is easily interpreted according to the
C2V symmetry of 2, with 4 peaks appearing between 1.6 and
5.1 ppm for the tert-butyl protons and 8 resonances between
26.4 and -21.9 ppm for the unique sets of phenyl ring
protons. The large chemical shifts indicate that the cobalt-
(III) ion is paramagnetic. Recent DFT calculations have
shown that the ground spin state for square planar CoIII-
corrole should be S ) 1.9 The magnitude and alternating
sign of the isotropic shifts for the phenyl ring protons suggest
that they are dominated by contact contributions arising from
significant π-delocalization of the unpaired spin density on
the metal.10,11 This delocalization brings to light the possible
“noninnocent” nature of the corrolazine ring in 2, which is
of some interest because of the recent debate concerning the
noninnocence of conventional corrole rings in various high-
Figure 1. (a) ORTEP diagram of [(TBP)8CzCoIII(PPh3)] (3). Selected bond
lengths [Å]: Co1-N1 1.838(3), Co1-N2 1.852(3), Co1-N3 1.826(3),
Co1-N4 1.834(3), Co1-P1 2.175(1). (b) ORTEP diagram of [(TBP)8CzCoIII-
(py)2] (4). Selected bond lengths [Å]: Co1-N1 1.838(3), Co1-N2 1.850-
(4), Co1-N3 1.830(3), Co1-N4 1.834(3), Co1-N8 1.970(4), Co1-N9
2.000(4). Thermal ellipsoids are drawn at a 50% probability level. Hydrogen
atoms have been omitted for clarity.
and 1H NMR spectroscopy revealed a diamagnetic spectrum,
indicating that the CoIII ion has been converted from a high-
spin to a low-spin configuration. The molecular structure of
3 is shown in Figure 1a.15 Despite the steric crowding that
might be expected from the 8 peripheral phenyl substituents,
the 23-atom corrole core exhibits substantial planarity, with
the maximum displacement from the average plane being
-0.19(1) Å for C(11) (R-C atom adjacent to N(3)) and
+0.22(1) Å for N(4), and the mean displacement from the
plane being 0.10(1) Å. The cavity size of 3, as measured by
the distances between trans pyrrole nitrogen atoms (Np-Np
) 3.638 and 3.612 Å), is significantly smaller than the cavity
size of conventional CoIII-PPh3 corroles (d(Np-Np(av)) )
3.72 Å).7,16-18 There is also a larger displacement of the CoIII
ion out of the plane of the corrolazine, compare Co-Cz-
(plane) ) 0.44(1) Å versus Co-corrole(plane) ) 0.29-0.39
1
valent complexes.12-14 However, the H NMR spectrum of
2 does not by itself point to complete charge transfer; that
is, it does not prove that a (Cz2-)CoII description is more
appropriate than a (Cz3-)CoIII assignment. Further spectro-
scopic analysis is needed to provide a definitive description.
A solution of 5 equiv of PPh3 and 2 in pyridine was
refluxed for 1 h, and then, the pyridine was removed under
vacuum to give [(TBP)8CzCoIII(PPh3)] (3) as a crude green-
brown solid (Scheme 1). Purification by chromatography
(silica gel, hexanes/CH2Cl2 1/1 (v/v)) gave 3 in 83% yield,
(9) Rovira, C.; Kunc, K.; Hutter, J.; Parrinello, M. Inorg. Chem. 2001,
40, 11-17.
(10) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 5, pp
81-183.
(11) Drago, R. S. In Physical Methods for Chemists, 2nd ed.; Saunders
College Publishing: Philadelphia, 1992; pp 514-518.
(12) Ghosh, A.; Steene, E. J. Biol. Inorg. Chem. 2001, 6, 739-752.
(13) Gross, Z. J. Biol. Inorg. Chem. 2001, 6, 733-738.
(14) Cai, S.; Walker, F. A.; Licoccia, S. Inorg. Chem. 2000, 39, 3466-
3478.
(15) Disordered solvent molecules were processed by the program SQUEEZE,
as described in the Supporting Information.
(16) Paolesse, R.; Nardis, S.; Sagone, F.; Khoury, R. G. J. Org. Chem.
2001, 66, 550-556.
(17) Paolesse, R.; Jaquinod, L.; Nurco, D. J.; Mini, S.; Sagone, F.; Boschi,
T.; Smith, K. M. J. Chem. Soc., Chem. Commun. 1999, 1307-1308.
(18) Hitchcock, P. B.; McCaughlin, G. M. J. Chem. Soc., Dalton Trans.
1976, 1927.
4106 Inorganic Chemistry, Vol. 41, No. 16, 2002

COMMUNICATION
spectrum upon addition of pyridine to obtain the formation
constant for 4 via a spectrophotometric titration.
The inset of Figure 2 shows that upon successive addition
of pyridine in CH2Cl2, the initial spectrum of 2 is converted
to a spectrum that matches that of 4. Clean isosbestic
behavior is present throughout the titration, indicating that
the monopyridine intermediate, which would be a third
absorbing species, is not observed and the overall equilibrium
is best described by the direct formation of 4 from 2.
Quantitative evaluation of the formation constant (â2) for
this reaction was obtained by a Hill plot,21 which exhibits a
slope of ∼2 throughout the pyridine concentration range,
confirming the presumed 2:1 binding stoichiometry. The
value of â2 ) 9.0 × 107 M-2 was determined from this
analysis and indicates very strong overall binding of the two
axial pyridines, although the individual 1:1 binding events
(represented by the binding constants K1 and K2) cannot be
distinguished. It is clear, however, that K2 must be signifi-
cantly larger than K1 to account for the lack of any observable
monopyridine intermediate. Iron(III) porphyrinates21 show
similar behavior in the binding of pyridine with K2 . K1,
and only â2 can be extrapolated. In contrast, a series of CoIII
corroles with both alkyl and aryl â-carbon substituents
exhibits successive 1:1 binding of pyridine with K1 ) 6.0 ×
103-3.0 × 105 M-1 and K2 ) 38-240 M-1, giving formation
constants (â2 ) K1K2) of 6 × 105-5 × 107 M-2.5 The
formation constant found for 4 is larger than those found
for any of the latter CoIII corrole complexes.
The smaller cavity size found for corrolazine 4 is expected
to result in stronger σ donation from the pyrrole N atoms
and, hence, a decreased Lewis acidity at the metal center
compared to that of conventional corroles. This hypothesis
is clearly disproven by the â2 value for 4, which implies
that the CoIII ion in 4 is a better Lewis acid than in most
other corroles. A simple explanation comes from a com-
parison of the electronegativity of N versus C at the meso
positions; the more electronegative meso N atoms of a
corrolazine ring result in a CoIII ion that is significantly more
Lewis acidic.22 These findings suggest that further novel
reactivity involving cobalt and other transition metal corro-
lazines can be expected.
Figure 2. UV-vis spectra of 2 (s) and 3 (- -) in CH2Cl2 and 4 (‚‚‚) in
CH2Cl2/pyridine. Inset: changes in the UV-vis spectrum upon titration of
pyridine (0; 6.2 × 10-6-1.5 × 10-3 M) into a solution of 2 (1.43 × 10-4
M) in CH2Cl2.
Å.7,16,17 The average Co-Np distance for 3 (1.838(9) Å) is
also somewhat smaller than that found in the conventional
analogues (1.87-1.92 Å).7,16-18
Addition of an excess of pyridine to 2 in CH2Cl2 (CH2-
Cl2/pyridine, 49/1, (v/v)), followed by the slow diffusion of
MeOH, gave [(TBP)8CzCoIII(py)2] (4) (Scheme 1) as a
crystalline solid. The crystal structure of 4 is shown in Figure
1b.15 As found in 3, the cavity size (Np-Np ) 3.663, 3.678
Å) and the mean Co-Np distance (1.838(9) Å) of 4 are
smaller than those of recently reported analogues, for
example, (Me4Ph4Cor)Co(py)2 ((Co-Np)av ) 1.894(5) Å,
Np-Np ) 3.772, 3.790 Å)5 and [(tpfc)Co(py)2] (Co-Np )
1.873-1.900 Å, Np-Np ) 3.766, 3.774 Å).6 As opposed to
3, the cobalt ion in 4 is seated directly in the plane of the
ring (0.084 Å out-of-plane displacement) in order to accom-
modate the second axial ligand. Curiously, the ∼90° dihedral
angle between the planes of the two axial pyridines is quite
different from the dihedral angle of ∼0° for [(tpfc)Co(py)2],6
yet it matches that for (Me4Ph4Cor)Co(py)2.5
A comparison of the UV-vis spectra of 2-4 (Figure 2)
is enlightening. Although all three complexes exhibit spectra
that are similar to that of tetraazaporphyrins, with intense
Soret and Q-bands, the Soret bands for 2-4 (450, 441, and
445 nm, respectively) as well as for the phosphorus (447
nm) and metal-free (460 nm) Czs reported previously8 are
dramatically red-shifted compared to those of tetraazapor-
phyrins (λmax(Soret) < 400 nm).19,20 The Soret transition in
a tetraazaporphyrin can be ascribed to a porphyrin-like four-
orbital electronic transition a2u (π, second HOMO) f eg (π*,
LUMO) (in D4h symmetry) in which the second HOMO has
large orbital coefficients on the meso positions. We suggest
that the Cz red-shift is caused by a destabilization of this
second HOMO upon removal of an electronegative meso N
atom during ring contraction.20 Additionally, there is a large
red-shift in the Q-band (600 to 670 nm) upon binding of
PPh3 or pyridine to 2. We used this change in the UV-vis
Acknowledgment. We thank the NSF for financial
support (Grants CHE0094095 and CHE0089168 to D.P.G.,
Grant CHE0091968 to A.L.R.) and Mr. J. Spenner of Johns
Hopkins School of Medicine for useful discussions. D.P.G
is also grateful for an Alfred P. Sloan Research Fellowship
(B.R.-4153).
Supporting Information Available: Synthetic details for 2-4
and X-ray crystallographic data for 3 and 4 in CIF format. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
IC020297X
(21) Balke, V. L.; Walker, F. A.; West, J. T. J. Am. Chem. Soc. 1985, 107,
1226-1233 and references therein.
(19) Michel, S. L. J.; Hoffman, B. M.; Baum, S. M.; Barrett, A. G. M.
Prog. Inorg. Chem. 2001, 50, 473-590.
(20) Kobayashi, N. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 2,
pp 301-360.
(22) A recent study of (tpfc)CoIII has indicated tight binding of pyridine,
which was also ascribed to electron-withdrawing groups at the meso
positions, although a direct comparison of the formation constant â2
is not available. See ref 6.
Inorganic Chemistry, Vol. 41, No. 16, 2002 4107