Nickel(II) and Zinc(II) meso-tetracyclohexylporphyrins. Structural and electronic effects induced by meso-cyclohexyl substitution in metalloporphyrins

Article abstract of DOI:10.1021/ic981233i

The synthesis and X-ray structures of the zinc(II) and nickel(II) complexes of meso-tetracyclohexylporphyrin H₂(TCHP) are described. The nonplanarity of the meso substituents results in steric crowding at the porphyrin periphery. In the solid state, the nickel(II) complex Ni(TCHP) has a ruffled porphyrin conformation while Zn-(TCHP) exhibits a stepped distortion of the macrocycle. In chloroform solution, fast rotation of the cyclohexyl groups on the NMR time scale is observed at room temperature for both complexes. Temperature-dependent ¹H NMR spectra showed that the (-g,g,-g,g) conformer of Zn(TCHP) and Ni(TCHP) is prevalent in solution at low temperatures and gave an estimate for the rotation barrier of the cyclohexyl groups (ΔG_c^? = 10-12 kcal mol^-1)-In both complexes, the porphyrin ring is easier to oxidize and harder to reduce than in their tetraphenylporphyrin M(TPP) congeners, in agreement with the stronger electron-donating effect of the cyclohexyl group. The magnitude of the potential shift is larger for the first oxidation than for the first reduction, reflecting a smaller HOMO-LUMO energy gap and a greater degree of macrocycle distortion than in the M(TPP) derivatives. This information is of importance to understanding the protein regulation of electron-transfer processes by cytochrome c and other redox active proteins. Crystal data: Ni(TCHP)·CHCl₃·CH₃CN, monoclinic, C2/c, a = 27.405(12), b = 10.004-(21), c = 32.877(24) A?, β= 107.71(3)° at 127 K, Z = 8. Zn(TCHP), monoclinic, P2₁/a, a = 11.159(15), b = 11.992(7), c = 13.465(20) A?, β= 102.85(16)° at 127 K, Z = 2.

Full text of DOI:10.1021/ic981233i

1772
Inorg. Chem. 1999, 38, 1772-1779
Nickel(II) and Zinc(II) meso-Tetracyclohexylporphyrins. Structural and Electronic Effects
Induced by meso-Cyclohexyl Substitution in Metalloporphyrins†
Marc Veyrat,‡ Rene´ Ramasseul,‡ Ilona Turowska-Tyrk,§ W. Robert Scheidt,*,§ Marie Autret,|
Karl M. Kadish,*,| and Jean-Claude Marchon*,‡
Laboratoire de Chimie de Coordination (URA CNRS 1194), Service de Chimie Inorganique et
Biologique, De´partement de Recherche Fondamentale sur la Matie`re Condense´e, CEA-Grenoble,
38054 Grenoble, France, Department of Chemistry and Biochemistry, University of Notre Dame,
Notre Dame, Indiana 46556, and Department of Chemistry, University of Houston,
Houston, Texas 77004
ReceiVed October 20, 1998
The synthesis and X-ray structures of the zinc(II) and nickel(II) complexes of meso-tetracyclohexylporphyrin
H2(TCHP) are described. The nonplanarity of the meso substituents results in steric crowding at the porphyrin
periphery. In the solid state, the nickel(II) complex Ni(TCHP) has a ruffled porphyrin conformation while Zn-
(TCHP) exhibits a stepped distortion of the macrocycle. In chloroform solution, fast rotation of the cyclohexyl
1
groups on the NMR time scale is observed at room temperature for both complexes. Temperature-dependent H
NMR spectra showed that the (-g,g,-g,g) conformer of Zn(TCHP) and Ni(TCHP) is prevalent in solution at low
temperatures and gave an estimate for the rotation barrier of the cyclohexyl groups (∆Gcq ) 10-12 kcal mol-1).
In both complexes, the porphyrin ring is easier to oxidize and harder to reduce than in their tetraphenylporphyrin
M(TPP) congeners, in agreement with the stronger electron-donating effect of the cyclohexyl group. The magnitude
of the potential shift is larger for the first oxidation than for the first reduction, reflecting a smaller HOMO-
LUMO energy gap and a greater degree of macrocycle distortion than in the M(TPP) derivatives. This information
is of importance to understanding the protein regulation of electron-transfer processes by cytochrome c and other
redox active proteins. Crystal data: Ni(TCHP)‚CHCl3‚CH3CN, monoclinic, C2/c, a ) 27.405(12), b ) 10.004-
(21), c ) 32.877(24) Å, â ) 107.71(3)° at 127 K, Z ) 8. Zn(TCHP), monoclinic, P21/a, a ) 11.159(15), b )
11.992(7), c ) 13.465(20) Å, â ) 102.85(16)° at 127 K, Z ) 2.
Introduction
chemistry of the two complexes in chloroform solution, as well
as their electrochemistry in dichloromethane and benzonitrile.
In the large family of meso-substituted porphyrins, 5,10,15,-
20-tetracyclohexylporphyrin H2(TCHP) is a recent addition1
which can be viewed as a tetraphenylporphyrin derivative with
fully hydrogenated meso-substituents. This macrocyclic ligand
possesses some unusual steric and electronic features which have
attracted recent interest. The nonplanarity of its bulky meso-
cyclohexyl substituents has a strong influence on the stereo-
chemistry of the porphyrin core in its metal complexes,2,3 and
their aliphatic nature has been shown to stabilize the less com-
mon (dxzdyz)4(dxy)1 spin state in its low-spin ferric complexes.4
In this paper we report in detail the synthesis of H2(TCHP) 1
and the X-ray structures of its nickel(II) and zinc(II) complexes
2 and 3; some of these results have been published in a prelim-
inary communication.2 We also describe the dynamic stereo-
Experimental Section
Materials. Solvents and chemicals used for synthesis were obtained
from Aldrich and used as received. Tetracyclohexylporphyrin was
prepared according to a local modification of a literature procedure
(vide infra).1 Tetra-n-butylammonium perchlorate (TBAP) was pur-
chased from Sigma, recrystallized from ethyl alcohol, and dried under
vacuum at 40 °C for at least one week prior to use. Absolute
dichloromethane dried over molecular sieves, obtained from Fluka, was
used for electrochemical studies without further purification. Benzoni-
trile (PhCN) was purchased from Aldrich and distilled over P2O5 under
vacuum prior to use.
H2(TCHP). In a 5-L three-neck round-bottom flask was poured 4 L
of dichloromethane containing 0.1% ethanol (Note: the presence of
ethanol in the solvent dichloromethane is essential; no tetracyclohexy-
lporphyrin was obtained if the solvent was ethanol-free, or if boron
trifluoride was used as a catalyst instead of trifluoroacetic acid). Argon
was bubbled for 45 min, and 4.50 g of cyclohexanecarboxaldehyde
(40.1 mmol) was added, followed by 2.69 g of pyrrole (40.1 mmol).
The mixture was stirred for 5 min, and a solution of 4.57 g of
trifluoroacetic acid (40.1 mmol) in 10 mL of dichloromethane was
added dropwise. The mixture was stirred for 5 h, 6.83 g of 2,3-dichloro-
5,6-dicyanobenzoquinone (30.1 mmol) was added, and the solution was
brought to reflux for 1.5 h. After cooling, the solvent volume was
reduced to about 2.5 L, and the mixture was filtered on neutral alumina
(activity I). To the filtrate was added 20 g of silica gel, and the solvent
was evaporated to dryness. The resulting solid was placed on top of a
column containing 250 g of silica gel, and the product was eluted with
† Abbreviations: TCHP ) dianion of tetracyclohexylporphyrin; TPP )
dianion of tetraphenylporphyrin; TBAP ) tetra-n-butylammonium perchlo-
rate; PhCN ) benzonitrile.
‡ CEA-Grenoble.
§ University of Notre Dame.
| University of Houston.
(1) Onaka, M.; Shinoda, T.; Izumi, Y.; Nolen, E. Chem. Lett. 1993, 117.
(2) Veyrat, M.; Ramasseul, R.; Marchon, J. C.; Turowska-Tyrk, I.; Scheidt,
W. R. New J. Chem. 1995, 19, 1199.
(3) Jentzen, W.; Hobbs, J. D.; Song, X.; Simpson, M. C.; Ema, T.; Nelson,
N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.;
Ramasseul, R.; Marchon, J. C.; Takeuchi, T.; Goddard, W. A., III;
Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085.
(4) Wolowiec, S.; Latos-Grazynski, L.; Toronto, D.; Marchon, J. C. Inorg.
Chem. 1998, 37, 724. Erratum. Ibid. 1998, 37, 2096.
10.1021/ic981233i CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/31/1999

Nickel(II) and Zinc(II) meso-Tetracyclohexylporphyrins
Inorganic Chemistry, Vol. 38, No. 8, 1999 1773
hexane/dichloromethane mixtures of increasing polarities (90/10, 85/
15, and finally 65/35). The various pure fractions were combined, and
the free base porphyrin was recrystallized from a dichloromethane-
acetonitrile mixture. Yield: 1.49 g of tetracyclohexylporphyrin, 1,
(23%): MS-FAB+ m/z 639, MH+; UV-vis (CH2Cl2) λmax/nm 422,
1
525, 562, 603, 660; H NMR, 200 MHz, CDCl3, 20 °C, δ -1.60 (s,
2H, N-H), 1.83 (m, 12H, H4a, H3a), 2.14 (m, 12H, H4e, H3e), 2.58 (m,
8H, H2e), 2.96 (m, 8H, H2a), 4.76 (tt, 4H, H1, J 12.5 Hz, J′ 3.5 Hz),
9.46 (s, 8H, Hâ); 13C NMR, 75 MHz, CDCl3, 20 °C, δ 26.7 (C4), 28.5
(C3), 38.7 (C2), 46.9 (C1), 122.5 (Cmeso), 129.1(Câ), 143.7 (CR).
Ni(TCHP). A 100 mg amount of 1 (0.16 mmol) and 390 mg of
nickel acetate (1.6 mmol) were dissolved in 20 mL of a 1/1 chloroform/
ethanol mixture. The resulting solution was brought to reflux for 1 h,
and then evaporated to dryness. The solid product was purified by
column chromatography on silica gel (50 g) using a 1/1 dichlo-
romethane/cyclohexane mixture as eluent, and recrystallized from
chloroform-acetonitrile. Yield: 100 mg of nickel(II) tetracyclohexyl-
Figure 1. A schematic view of the (-g,g,-g,g) conformer of a M(II)-
tetracyclohexylporphyrin complex. One cyclohexyl group has been
artificially enlarged for better visibility of the stereochemistry and of
the atom labels.
porphyrin, 2, (90%): MS-FAB+ m/z 694, M+; UV-vis (CH2Cl2):
1
λ
max/nm 425, 508, 550, 586; H NMR, 200 MHz, CDCl3, 20 °C, δ:
1.70 (m, 12H, H4a, H3a), 2.09 (m, 12H, H4e, H3e), 2.42 (m, 8H, H2e),
2.73 (m, 8H, H2a), 4.21 (tt, 4H, H1, J 12.5 Hz, J′ 3.4 Hz), 9.21 (s, 8H,
Hâ); 13C NMR, 75 MHz, CDCl3, 20 °C, δ: 26.6 (C4), 28.2 (C3), 38.1
(C2), 45.6 (C1), 121.2 (Cmeso), 130.6 (Câ), 139.1 (CR).
ylsilane. Cyclic voltammetry was carried out with an EG&G Model
173 potentiostat or an IBM Model EC 225 Voltammetric Analyzer.
Current-voltage curves were recorded on an EG&G Princeton Applied
Research Model RE-0151 X-Y recorder. A three-electrode system was
used and consisted of a glassy carbon or platinum button working
electrode, a platinum wire counter electrode, and a saturated calomel
reference electrode (SCE). This reference electrode was separated from
the bulk of the solution by a fritted-glass bridge filled with the solvent/
supporting electrolyte mixture. Ferrocene was used as an internal
standard, but all potentials are referenced to the SCE. UV-visible
spectroelectrochemical experiments were performed with a home-built
platinum thin-layer electrode of the type described in the literature.10
Potentials were applied and monitored with an EG&G Model 173
potentiostat. Time-resolved UV-visible spectra were recorded with a
Princeton Instrument PDA-1024 diode array and ST 1000 detector
controller. Data acquisition and processing were performed using
OSMA and PSMA software.
Zn(TCHP). This complex was prepared from 1 and zinc acetate by
a procedure entirely similar to that used for Ni(TCHP). Yield: 95 mg
(85%) of zinc(II) tetracyclohexylporphyrin, 3, after recrystallization
from chloroform-pentane: MS-FAB+ m/z 700, M+; UV-vis (CH2-
1
Cl2) λmax/nm 423, 524, 562, 624; H NMR, 200 MHz, CDCl3, 20 °C,
δ 1.90 (m, 12H, H4a, H3a), 2.17 (m, 12H, H4e, H3e), 2.62 (m, 8H, H2e),
3.10 (m, 8H, H2a), 5.06 (tt, 4H, H1, J 12.5 Hz, J′ 3.5 Hz), 9.75 (s, 8H,
Hâ); 13C NMR, 75 MHz, CDCl3, 20 °C, δ 26.8 (C4), 28.7 (C3), 38.8
(C2), 47.2 (C1), 124.1 (Cmeso), 129.6 (Câ), 143.7 (CR).
X-ray Structure Determinations of Ni(TCHP) and Zn(TCHP).
Red, single crystals suitable for X-ray structure determinations were
obtained by recrystallization from chloroform-acetonitrile. Unit cell
parameter determination with an area detector have been described
earlier.5 Data collection was processed with an Enraf-Nonius FAST
area detector diffractometer with a FR-588 low-temperature device and
graphite-monochromated Mo KR radiation. For 2, a total of 9857
collected reflections were considered as observed (Fo g 1.9σ(Fo)), of
which 3726 were unique; for 3: 7701 and 2799 (Fo g 1.7σ(Fo)),
respectively. Intensities of all reflections were reduced using Lorentz
and polarization corrections. Both structures were solved by Patterson
methods from the SHELXS86 program.6 For 2, the position of Ni was
used in the DIRDIF program that enabled us to find all atoms. For 3,
positions of all atoms were revealed. For all nonhydrogen atoms,
anisotropic least-squares refinement was used. Hydrogen atoms were
included as fixed, idealized contributions. The refinement converged
to a value of R ) 0.092, wR ) 0.103 and R ) 0.072, wR ) 0.087 for
2 and 3, respectively. The maximum electron density on a final
difference Fourier map was 1.02 e/Å3 (in a distance 2.30 Å from C(3))
and the minimum was -0.65 e/Å3 for 2 and 1.05 e/Å3 (1.35 Å from
N(2)) and -0.80 e/Å3 for 3, respectively. Other programs used in this
study included local modifications of Beurskens, Bosman, Doesburg,
Gould, van den Hark, Prick, Noordik, Parthasarathi, Bruins, Slot,
Haltiwanger, Strumpel, and Smits’s DIRDIF, Busing and Levy’s
ORFFE and ORFLS, Jacobson’s ALLS, Zalkin’s FORDAP, and
Johnson’s ORTEP2. Atomic form factors were from Cromer and Mann.7
Real and imaginary corrections for anomalous dispersion in the form
factor of the Ni and Zn atoms were from Cromer and Liberman.8
Scattering factors for hydrogen were from Stewart et al.9
Results
Synthesis and Spectral Characterization. We find that 1
can be obtained in 23% yield by conventional Lindsey con-
densation11 of pyrrole and cyclohexanecarboxaldehyde in
dichloromethane containing 0.1% ethanol, using trifluoroacetic
acid as the catalyst. As explained below for 2 and 3, the highly
1
symmetric H and 13C NMR spectra of 1 are consistent with
fast averaging of equatorially bound cyclohexyl groups in a chair
conformation shown schematically in Figure 1.
X-ray Structures. A summary of crystallographic data is
shown in Table 1. The structures of 2 and 3, which are shown,
respectively, in Figures 2 and 3, confirm the equatorial bonding
of the meso-cyclohexyl groups, consistent with the greater
stability of equatorially substituted cyclohexane isomers.12
Conformations in which the mean plane of the cyclohexyl
groups would be coplanar with that of the porphyrin obviously
are restricted by steric crowding between H2e and Hâ. In the
observed geometry, these planes are approximately orthogonal,
resulting in gauche conformations ((g, see below) of the CR-
Cmeso-C1-C2 fragments. Bumping interactions between hy-
drogen atoms in axial position on C1 and on the neighboring
â-pyrrolic position are effectively relieved by minute rotations
of the cyclohexyl groups off the strictly orthogonal orientation.
A relative g or -g conformation can be defined for each of the
four cyclohexyl substituents,13 depending on the clockwise or
Instrumentation. UV-vis spectra were recorded on a Perkin-Elmer
1
Lambda 9. H NMR spectra were recorded on a Bruker 200 or 300
MHz NMR spectrometer. Chemical shifts are referenced to tetrameth-
(5) Scheidt, W. R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314.
(6) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(7) Cromer, D. T.; Mann, J. B. Acta Crystallogr., Sect. A 1968, 24, 321.
(8) Cromer, D. T.; Liberman, D. J. J. Chem. Phys. 1970, 53, 1891.
(9) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys. 1965,
42, 3175.
(10) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498.
(11) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828.
(12) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; Wiley: New York, 1994; Chapter 11, p 694.

1774 Inorganic Chemistry, Vol. 38, No. 8, 1999
Veyrat et al.
Table 1. Crystallographic Data for Ni(TCHP) and Zn(TCHP)
Ni(TCHP)‚CHCl3‚CH3CN
Zn(TCHP)
formula
FW
crystal system
a, Å
b, Å
c, Å
C44H52N4Ni‚CHCl3‚CH3CN
856.04
C44H52N4Zn
702.31
monoclinic
11.159(15)
11.992(7)
13.465(20)
102.85(16)
1757(7)
P21/a
monoclinic
27.405(12)
10.004(21)
32.877(24)
107.71(3)
8586(30)
C2/c
â, deg
V, Å3
space group
Z
8
2
Dc, g/cm3
radiation, Å
µ, mm-1
temperature, °C
R1a
1.318
0.71073
0.676
-146(1)
0.092
1.322
0.71073
0.748
-146(1)
0.072
wR2
0.103
0.087
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo| and wR2 ) {∑[w(Fo - Fc2)2]/
4
∑[wFo ]}1/2. The conventional R-factors R1 are based on F, with F set
to zero for negative F2. The criterion of F2 < 2σ(F2) was used only for
calculating R1. R-factors based on F2 (wR2) are statistically about twice
as large as those based on F, and R-factors based on all data will be
even larger.
Figure 2. (a) The structure of Ni(TCHP) 2 in the crystal, showing the
ruffled distortion of the porphyrin. Thermal ellipsoids of the H atoms
are reduced for clarity. Selected bond lengths (Å) and angles (deg):
Ni-N(1) 1.892(8), Ni-N(2) 1.897(9), Ni-N(3) 1.882(9), Ni-N(4)
1.881(9), N(1)-Ni-N(2) 90.7(4), N(1)-Ni-N(3) 178.9(4), N(1)-Ni-
N(4) 89.7(4), N(2)-Ni-N(3) 89.9(4), N(2)-Ni-N(4) 178.1(4), N(3)-
Ni-N(4) 89.7(4). (b) Top view showing the (-g,g,g,g) conformation
of the cyclohexyl groups.
anticlockwise orientation of the C1-H1 vector around the
porphyrin ring. Thus, four conformers can be expected for a
metal(II)-tetracyclohexylporphyrin complex, i.e. (g,g,g,g),
(-g,g,g,g), (-g,-g,g,g), and (-g,g,-g,g)sor the equivalent
descriptors obtained by permutation of g for -g (Figure 4). The
unique solid state conformation found for Ni(TCHP) 2 is
(-g,g,g,g), while that for Zn(TCHP) 3 is (-g,g,-g,g).
The porphyrin ring of 2 has a strongly ruffled conformation14
as in many other nickel porphyrin complexes. In 2, this ruffling
Figure 3. (a) The structure of Zn(TCHP) 3 in the crystal, showing
the step-shaped distortion of the porphyrin. Selected bond lengths (Å)
and angles (deg): Zn-N(1) 2.040(5), Zn-N(2) 2.018(5), N(1)-Zn-
N(2) 90.24(21), N(2)-Zn-N(1) 89.76(21). (b) Top view showing the
(-g,g,-g,g) conformation of the cyclohexyl groups.
(13) (a) Colombus, I.; Biali, S. J. Org. Chem. 1993, 58, 7029. (b) Colombus,
I.; Biali, S. J. Org. Chem. 1994, 59, 3402. (c) Colombus, I.; Cohen,
S.; Biali, S. J. Am. Chem. Soc. 1994, 116, 10306. (d) Colombus, I.;
Biali, S. J. Org. Chem. 1994, 59, 8132.
results from the small size of the nickel(II) ion concurrent with
steric crowding at the macrocycle periphery,3 and it leads to
(14) Scheidt, W. R.; Lee, Y. J. Struct. Bond. (Berlin) 1987, 64, 1.

Nickel(II) and Zinc(II) meso-Tetracyclohexylporphyrins
Inorganic Chemistry, Vol. 38, No. 8, 1999 1775
1
Figure 5. Temperature-dependent 300 MHz H NMR spectra of Zn-
(TCHP) in CD2Cl2.
Figure 4. The four possible conformers of a M(II)-tetracyclohexyl-
porphyrin complex.
the shortest values observed for the nickel-porphyrin nitrogen
distances (Ni-NPav ) 1.888(9) Å). The combination of a
(-g,g,g,g) conformation and of a ruffled distortion in 2 results
in a chiral Ni(TCHP) molecule; both enantiomers are present
in the crystal lattice since the space group is nonchiral.
The porphyrin core conformation in 3 is also decidedly
nonplanar. The Zn atom sits on a crystallographically required
inversion center, and the porphyrin core has an approximate
C2h symmetry; it exhibits a step-shaped distortion14 in which
two neighboring pyrrole rings are tilted below the mean N4 plane
while the other two are tilted above. This type of nonplanar
distortion has been found in a number of complexes that have
a metal at the center of the ring,15 as well as in some out-of-
plane complexes.16 The two conformations reflect the nonpla-
narity required by the peripheral substituents17 and the differing
size of the two metal ions.
Dynamic NMR Spectroscopy. The 1H and 13C NMR spectra
of 2 were assigned using a combination of 2D 1H-1H and 13C-
1H correlation.2 At room temperature a single peak is observed
in the 13C and 1H spectra for the resonances of positions 2 and
2′, and for those of positions 3 and 3′ of the cyclohexyl groups,
and for the pyrrole â position as well. The resonance of H1 is
a triplet of a triplet, and the values of the two coupling constants
are in the range expected for axial-axial (J ) 12.5 Hz) and
axial-equatorial (J′ ) 3.4 Hz) interactions,12 in agreement with
1
Figure 6. Temperature-dependent 300 MHz H NMR spectra of Ni-
(TCHP) in CD2Cl2.
the equatorial bonding of the cyclohexyl substituents. Very
similar 1H and 13C spectra were obtained for 3 at room
temperature.
Upon lowering the temperature, the 1H NMR spectrum of 3
shows no change in the cyclohexyl resonances, while the peak
of the pyrrolic protons broadens and splits in a pair of equivalent
signals below 250 K (Figure 5).
1
(15) See, for example: (a) Kutzler, F. W.; Swepston, P. N.; Berkovitch-
Yellin, Z.; Ellis, D. E.; Ibers, J. A. J. Am. Chem. Soc. 1983, 105,
2996. (b) Jia, S. L.; Jentzen, W.; Shang, M.; Song, X. Z.; Ma, J. G.;
Scheidt, W. R.; Shelnutt, J. A. Inorg. Chem. 1998, 37, 4402.
(16) See, for example: Belcher, W. J.; Boyd, P. D. W.; Brothers, P. J.;
Liddell, M. J.; Rickard, C. E. F. J. Am. Chem. Soc. 1994, 116,
8416.
The temperature dependence of the H NMR spectrum of 2
is more complex (Figure 6), and the underlying conformational
changes are not immediately apparent (vide infra). At 183 K,
the lowest temperature which could be attained in these
experiments, the resonance of the pyrrole protons begins to split
into two lines. The resonance of H1 remains unchanged
throughout the temperature range from 303 to 183 K, while
(17) Shelnutt, J. A.; Song, X. Z.; Ma, J. G.; Jia, S. L.; Jentzen, W.; Medforth,
C. J. Chem. Soc. ReV. 1998, 27, 31.

1776 Inorganic Chemistry, Vol. 38, No. 8, 1999
Veyrat et al.
Zn(TPP) and other zinc porphyrins have been extensively
studied.19 All of the previously studied complexes show two
reductions leading to a π-anion radical and dianion. A π-cation
radical and the dication are also formed during the oxidation.
Unexpectedly, four oxidation steps are seen in the case of Zn-
(TCHP) but one, in addition to the two expected, may be due
to isoporphyrin formation. In benzonitrile, the cyclic voltam-
mogram shows three oxidations which are at potentials close
to those measured in CH2Cl2. Only the first two are completely
reversible. Compared to Zn(TPP), the first oxidation and
reduction potentials of Zn(TCHP) are shifted toward negative
values, by 210 mV for the first process and by 140 mV for the
last one.
Ni(TCHP). A single reversible reduction is measured in CH2-
Cl2 at E1/2 ) -1.49 V when scanning the potentials toward
cathodic values. However, three oxidations are observed. The
first, at E1/2 ) +0.81 V, is reversible. The second, at Epa
+1.17 V, is coupled with a rereduction peak at Epc ) +0.35 V
and likely gives an isoporphyrin. The third oxidation, at E1/2
+1.56 V, is not completely reversible. The cyclic voltammogram
in benzonitrile shows three oxidations waves, but the ratios ipa/
ipc is higher than 1.0, suggesting an EC mechanism.
)
)
Figure 7. Cyclic voltammograms of tetracyclohexylporphyrin deriva-
tives in dichloromethane containing 0.1 M TBAP.
Table 2. Half Wave Potentials (V vs SCE) for Oxidation and
Reduction of TCHP Derivatives in CH2Cl2, Containing 0.1 M
TBAP
A number of nickel porphyrins shows two one-electron
reductions, the first of which leads to a π-anion radical.19 The
separation between the two reduction waves is 440 mV in the
case of Ni(TCHP). This may be compared to a 420 ( 50 mV
separation generally observed for porphyrins which undergo
electron addition at the conjugated π-ring system to give
porphyrin π-anion radicals. In the case of Ni(TPP) the first
oxidation and the first reduction are ring centered, but the
electron-transfer site can be shifted toward the metal by changing
different factors such as the temperature or the solvent.20
By comparing the measured redox potentials of Ni(TCHP)
with those of Ni(TPP), it appears that the values of the former
complex are shifted toward negative potentials. The potential
difference is 210 mV for the first reduction and 240 mV for
the first oxidation, in CH2Cl2.
Spectroelectrochemistry. Zn(TCHP). In thin-layer time
scale, the voltammogram of the zinc compound shows only two
reversible oxidations in PhCN, 0.2 M TBAP. The third oxidation
and the reduction previously observed by cyclic voltammetry
are not reversible in these experimental conditions. The UV-
visible spectra recorded during thin-layer controlled-potential
oxidations are presented in Figure 8, and the data are sum-
marized in Table 3. The electronic absorption spectrum of Zn-
(TCHP) shows one Soret band, at λ ) 422 nm, and two visible
bands located at λ ) 559 and 602 nm.
During the first oxidation, when the controlled-potential is
set to +0.7 V, the Soret intensity decreases, but no significant
shift is observed (λ ) 420 nm for the oxidized compound). In
the same time, the visible bands decrease and widen, and a weak
broad absorption band is then observed in this region. These
spectral modifications are reversible when the controlled
potential is set back to 0.0 V.
The spectral changes observed upon the second controlled-
potential oxidation, at +0.95 V, indicate that the Soret band
dramatically decreases and broadens in the same time (Figure
8). No visible band is seen in the spectrum of the doubly
oxidized species, [Zn(TCHP)]2+. Contrary to what was observed
during the first oxidation, the UV-visible spectral modifications
are not reversible, and the spectrum of the singly oxidized form
cannot be recovered when the potential is set back to +0.7 V.
potentials
oxidation
reduction
compound solvent
H2(TCHP) CH2Cl2
+1.44a +0.83a -1.30 -1.67
Zn(TCHP) CH2Cl2 +1.41 +1.25 +0.80 +0.57 -1.52
Ni(TCHP) CH2Cl2
+1.56 +1.17a +0.81 -1.49
a Value given is Epa, at 100 mV/s.
those of H2a and H2e show extensive broadening and splitting.
Broadening of the peaks of H3a and H3e is observed below 193
K.
Electrochemistry. The cyclic voltammograms of the tetra-
cyclohexylporphyrin free base and of its nickel and zinc
complexes in CH2Cl2 containing 0.1 M TBAP are presented in
Figure 7, and the redox potentials are summarized in Table 2.
H2(TCHP). Two reductions are observed in CH2Cl2. The first,
at E1/2 ) -1.30 V, is reversible. The second, at E1/2 ) -1.67
V, is not completely reversible and gives rise to a new
reoxidation peak at Epa ) -0.61 V. Two oxidations are also
seen. The first, at E1/2 ) +0.83 V, is not completely reversible
and give rise to a new rereduction peak at Epc ) +0.06 V. This
irreversible process is probably due to the formation of an
isoporphyrin. The intensity of this wave increases when holding
the potential after the first oxidation. A second irreversible wave,
at Epa ) +1.44 V, is also observed.
In the case of H2(TPP), four reversible redox processes are
observed.18 Comparing the data of H2(TCHP) with that of H2-
(TPP) indicates that the processes are all shifted toward negative
potentials for the TCHP derivative. The difference is 100 and
120 mV for the two reductions and 190 mV for the first
oxidation of H2(TCHP) in CH2Cl2.
Zn(TCHP). Only one reduction is observed at E1/2 ) -1.52
V in CH2Cl2. Four oxidation processes are observed. The first
at E1/2 ) +0.57 V is quasireversible (|Epa - Epc| ) 120 mV)
and so is the second which is located at E1/2 ) +0.8 V (|Epa
-
Epc| ) 120 mV). The last two processes have a smaller current
and are located at E1/2 ) +1.25 V and E1/2 ) +1.41 V.
(18) Davis, D. G. Porphyrins; Dolphin, D., Ed.; Academic: New York,
(19) Kadish, K. M.; Davis, D. G. Ann. N.Y. Acad. Sci. 1973, 206, 495.
(20) Kadish, K. M. Progr. Inorg. Chem. 1986, 34, 435.
1978; Vol. 5 (Part C), p 127.

Nickel(II) and Zinc(II) meso-Tetracyclohexylporphyrins
Table 3. Spectral Data for Ni(TCHP) and Zn(TCHP) and Their Reduction and Oxidation Products, in PhCN Containing 0.2 M TBAP
max (nm) (absorption)a
Inorganic Chemistry, Vol. 38, No. 8, 1999 1777
λ
compound
Zn(TCHP)
reaction
Eappl (V)
Soret
visible
neutral
none
422 (1.00)
559 (0.08), 602 (0.06)
546 (0.11), 585 (0.03)
552 (0.06), 604 (0.04)
1st oxidation
2nd oxidation
neutral
1st oxidation
2nd oxidation
1st reduction
+0.70
+0.95
none
+1.00
+1.25
-1.60
420 (0.56)
420 (0.21)
422 (1.00)
422 (0.71)
428 (0.28)
Ni(TCHP)
424 (0.60), 451 (sh)
a Arbitrary units.
Figure 8. UV-visible spectral changes obtained during the controlled-
potential oxidations of Zn(TCHP) in benzonitrile containing 0.2 M
TBAP.
Ni(TCHP). Three processes, the reduction and the first two
oxidations, were studied by thin-layer spectroelectrochemistry,
although the second oxidation is not completely reversible. The
UV-visible spectra obtained are presented in Figure 9. The
electronic absorption spectrum of the neutral species shows a
Soret band located at λ ) 422 nm, and two bands at 546 and
585 nm in the visible region.
During the controlled-potential reduction at -1.60 V, the
Soret band decreases in intensity. No significant shift is observed
(λ ) 424 nm for the reduced species), but a shoulder at λ )
451 nm rises. In the visible region, the initial band at 546 nm
decreases and slightly shifts to 552 nm. A weak band at 604
nm is also present in the spectrum of the singly reduced species.
The changes are reversible and the initial spectrum is totally
recovered by applying the controlled potential back to 0.0 V
after the electroreduction.
Figure 9. UV-visible spectral changes obtained during the controlled-
potential oxidations and reduction of Ni(TCHP) in benzonitrile contain-
ing 0.2 M TBAP.
Similar UV-visible spectral modifications are observed for
Ni(TCHP) and Zn(TCHP) during the first, as well as during
the second, oxidation process. The fact that, in all cases, the
Soret band strongly decreases but does not shift suggests that
these oxidations are ring centered, leading for both compounds
to the cation radical and dication species.
Moreover, for both compounds, the first two oxidations are
separated by the same potential value, ∆E1/2 ) 250 mV. This
corroborates the hypothesis that the same reactions occur during
the oxidations of these two porphyrins. By comparison, the E1/2
between two ring-centered oxidations, in the case of almost
planar porphyrins, is generally around 400 mV.
Upon the controlled potential oxidation at +1.00 V, the Soret
band decreases without shifting, and the initial visible band is
replaced by two less intense absorption bands at 551 and 605
nm. Comparatively to the controlled-potential reduction, these
spectral changes are fully reversible.
During the oxidation of [Ni(TCHP)]+, a large decrease and
a small red shift (from 421 to 428 nm) of the Soret band are
observed (Figure 9). In the spectrum of the double-oxidized
species, no absorption bands are seen in the visible region. The
spectral changes observed during this controlled-potential oxida-
tion are not reversible, since the UV-visible spectrum of the
singly oxidized form cannot be recovered.
Discussion
Dynamic Stereochemistry in Solution. Zn(TCHP). The
high symmetry observed at room temperature in the 13C and
1H spectra of 3 imply fast rotation of the cyclohexyl substituents.
The splitting of the pyrrole proton resonance below 250 K is

1778 Inorganic Chemistry, Vol. 38, No. 8, 1999
Veyrat et al.
Table 4. Half Wave Potentials (V vs SCE) and HOMO-LUMO
consistent with a slower cyclohexyl exchange on the NMR time
scale. From the coalescence temperature Tc ) 250 ( 5 K and
frequency difference ∆ν ) 45 ( 1 Hz at 223 K, the energy
Energy Gap (V) for TCHP and TPP Derivatives in CH2Cl2
1st
1st
ox.
HOMO-LUMO
compound red. ∆E(red.)a
∆E(ox.)a
-190
gap
q
barrier ∆Gc for cyclohexyl group rotation was calculated by
the Eyring equation: ∆Gc ) 12 ( 0.5 kcal mol-1. The fact
q
H2(TCHP) -1.30
-100
+0.83
+1.02
+0.57
+0.78
+0.81
+1.05
2.13
2.22
2.09
2.16
2.30
2.33
that the pyrrole â-H peak splits into a pair of singlets indicates
that, among the four possible conformers, only the (-g,g,-
g,g) species with D2h symmetry is present in solution at low
temperatures. Since this species is also the only one found in
the crystal structure, it appears that it has the lowest energy of
the four conformers.
H2(TPP)
-1.20
Zn(TCHP) -1.52
Zn(TPP) -1.38
Ni(TCHP) -1.49
Ni(TPP) -1.28
-140
-210
-210
-240
a ∆E (mV) ) [E1/2(H2(TCHP))] - [E1/2(H2(TPP))].
Ni(TCHP). Similarly, the observed symmetry of the 13C and
1H spectra of 2 at room temperature indicates that rotation of
the cyclohexyl substituents is fast on the NMR time scale. Since
the ruffled porphyrin which is observed in the crystal structure
of 2 is probably also present in solution, inversion of the
macrocycle is probably fast too.21 Here again, the fact that the
pyrrole peak splits into two lines below 183 K indicates that
the (-g,g,-g,g) species is prevalent in solution at low temper-
atures; however, the ruffled distortion of this species has an
important consequence. Inversion of a D2-symmetric ruffled
(-g,g,-g,g) species produces the D2-symmetric enantiomorph
which is undistinguishable by NMR. In the fast exchange
regime, an average peak is obtained for each conformer of the
inverting porphyrin, while in the slow exchange regime a single
set of resonances is also observed since the two ruffled
conformers are an enantiomeric pair. Thus, the energy barrier
for porphyrin inversion cannot be obtained in this NMR
experiment, and a single dynamic process, i.e., cyclohexyl group
rotation, is responsible for the temperature-dependent changes
in the spectra of 2.
In addition to the splitting of the pyrrole proton peak, the
resonances of H2a and of H2e exhibit a complex behavior as the
solution of 2 is cooled to low temperatures. A tentative
interpretation of these splittings is proposed in Figure 6. The
peak of H2e first broadens to the point of complete vanishing at
213 K, while at that temperature the peak of H2a is broad but
visible. At 183 K, each resonance is split into two peaks,
indicating inequivalence of the two faces of the ruffled porphyrin
in the slow exchange limit (see Figure 1). With the assumption
that cyclohexyl group rotation is the common origin of the
observed spectral changes for the three types of protons, the
energy barrier ∆Gcq for this process can be calculated for each
type, with the following results: Hâ (∆ν ) 20 ( 2 Hz at 183
K): ∆Gcq ) 9.5 ( 0.5 kcal mol-1 at Tc ) 190 ( 5 K; H2a (∆ν
) 240 ( 10 Hz at 183 K): ∆Gcq ) 9.0 ( 0.5 kcal mol-1 at Tc
) 200 ( 5 K; H2e (∆ν ) 560 ( 20 Hz at 183 K): ∆Gcq ) 9.6
( 0.5 kcal mol-1 at Tc ) 220 ( 5 K. The good agreement
between the three calculated values of the energy barrier
supports the starting hypothesis.
substituents in TCHP complexes is probably related to their
greater flexibility and to the higher degree of nonplanar
distortion of their porphyrin cores.
Electrochemistry. The redox potentials of the tetracyclo-
hexylporphyrin complexes obtained by cyclic voltammetry in
dichloromethane are summarized in Table 2. None of the
investigated complexes are stable upon oxidation, and an
isoporphyrin is probably formed in CH2Cl2 after the second
oxidation of Ni(TCHP) and H2(TCHP). Evidence for this
assignment is given by the rereduction process observed at peak
potentials between +0.6 V and +0.2 V on the reverse potential
scan.
Thin-layer, time-dependent electronic absorption spectra were
taken during the first two oxidations of Zn(TCHP) and Ni-
(TCHP) in PhCN containing 0.2 M TBAP (see Figures 8 and 9
and Table 3). Similar spectral changes are observed for both
compounds. The intensity of the Soret band decreases upon the
first oxidation while the visible bands are weak and ill-defined
in the final spectra. Both of the doubly oxidized porphyrins have
a very weak Soret band suggesting that porphyrin π-cation
radicals followed by dications are obtained upon oxidation. The
UV-visible changes obtained upon the thin-layer electroreduc-
tion of Ni(TCHP) indicate generation of a porphyrin π-anion
radical species.
Table 4 summarizes potentials for the first reduction and first
oxidation of the investigated TCHP derivatives and the corre-
sponding TPP species. The introduction of cyclohexyl groups
onto the meso positions of the porphyrin macrocycle leads to
complexes which are easier to oxidize and harder to reduce than
the corresponding tetraphenylporphyrins. The effect of electron-
donating substituents onto the porphyrin ring is to raise the
energy of both the HOMO and the LUMO, which results in an
anodic potential shift when going from the TPP to the TCHP
species. This is consistent with the electron-donating effect of
the cyclohexyl groups as compared to the phenyl groups.
For each compound, the magnitude of the potential shift (see
Table 4) is larger in the case of the first oxidation than in the
case of the first reduction. This indicates that the HOMO is
more affected by the introduction of the cyclohexyl groups than
is the LUMO. The potential difference between the first
oxidation and the first reduction, E1/2(ox) - E1/2(red), at the
porphyrin macrocycle represents the electrochemical HOMO-
LUMO energy gap. For the case of the free base porphyrin and
the nickel and the zinc complexes investigated in the present
study, the electrochemical HOMO-LUMO gap is slightly
Thus, the rotation barrier of the cyclohexyl group is lower in
the highly ruffled nickel complex (ca. 10 kcal mol-1) than in
the stepped zinc complex (ca. 12 kcal mol-1). For comparison,
rotation barriers of 15-20 kcal mol-1 are typically found for
TPP complexes.22 The lower rotation barriers found in this
investigation (10-12 kcal mol-1) for the more bulky meso
(21) (a) Inversion of enantiomorphically ruffled conformers of nickel(II)
ccccc-octaethylpyrrocorphinate with an inversion barrier ∆Gcq ) 11.3
( 0.1 kcal mol-1 has been reported. See Waditschatka, R.; Kratky,
C.; Jaun, B.; Heinzer, J.; Eschenmoser, A. J. Chem. Soc., Chem.
Commun. 1985, 1604. See also (b) Medforth, C. J.; Senge, M. O.;
Smith, K. M.; Sparks, L. D.; Shelnutt, J. A. J. Am. Chem. Soc. 1992,
114, 9859. (c) Veyrat, M.; Maury, O.; Faverjon, F.; Over, D. E.;
Ramasseul, R.; Marchon, J. C.; Turowska-Tyrk, I.; Scheidt, W. R.
Angew. Chem., Int. Ed. Engl. 1994, 33, 220.
(22) (a) Bonnet, J. J.; Eaton, S. S.; Eaton, G. R.; Holm, R. H.; Ibers, J. A.
J. Am. Chem. Soc. 1973, 95, 2141. (b) Fournari, P.; Guilard, R.;
Fontesse, M.; Latour, J. M.; Marchon, J. C. J. Organomet. Chem. 1976,
110, 205. (c) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1977, 99,
6594. (d) Eaton, S. S.; Fishwild, D. M.; Eaton, G. R. Inorg. Chem.
1978, 17, 1542. (e) Laurie, A.; Shroyer, W.; Lorberau, C.; Eaton, S.
S.; Eaton, G. R. J. Org. Chem. 1980, 45, 4296. See also: Noss, L.;
Liddell, P. A.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem.
B 1997, 101, 458.

Nickel(II) and Zinc(II) meso-Tetracyclohexylporphyrins
Inorganic Chemistry, Vol. 38, No. 8, 1999 1779
Table 5. UV-Visible Spectral Parameters for TCHP and TPP
consequences. Steric crowding at the porphyrin periphery can
be seen to be a quite complex issue. It results in a nonplanar
distortion of the core, and the distortion type depends on the
size of the metal ion. In the nickel complex Ni(TCHP), ruffling
of the core leads to the shortest values observed for the Ni-NP
distances. In contrast, the related zinc derivative Zn(TCHP) has
a stepped core conformation to alleviate the crowding, and the
Zn-NP distances are only slightly shorter than normal. Rotation
of the cyclohexyl groups is fast on the NMR time scale at room
temperature, and a 4-fold pseudosymmetry is observed by
averaging of the signals of the four possible conformers. The
more stable (-g,g,-g,g) conformer is prevalent at low tem-
peratures in solutions of Zn(TCHP) and Ni(TCHP). In the two
complexes, the porphyrin ring is easier to oxidize and harder
to reduce than in their tetraphenylporphyrin congeners, in
agreement with the stronger electron-donating effect of the
cyclohexyl group.
Derivatives in CH2Cl2
λ
max (nm)
visible
compound
Soret
H2(TCHP)
H2(TPP)
Zn(TCHP)
Zn(TPP)
Ni(TCHP)
Ni(TPP)
422
417
423
419
426
414
525 562 603 660
515 552 594 650
562 623
548 589
550 587
525 557
smaller for the TCHP derivatives than for the TPP ones,
reflecting a larger destabilization of the HOMO compared to
LUMO.
The same conclusion is obtained from electronic absorption
spectra of the different neutral compounds. The UV-visible
bands of the TCHP derivatives are red-shifted compared to those
of the corresponding TPP complexes (Table 5). The lowest
energy band in the visible region gives an indication of the
HOMO-LUMO gap and indicates that the energy gap is smaller
in the case of the tetracyclohexylporphyrins. The electronic
effect due to the electron-donating cyclohexyl groups usually
destabilizes both the HOMO and the LUMO. In contrast, a
macrocycle distortion resulting from introduction of bulky
groups should increase the energy of only the HOMO, without
affecting the LUMO energy, and the consequence of this
destabilization should result in a decrease of the HOMO-
LUMO energy gap. This is what is observed by both the
electrochemistry and the UV-visible spectroscopy, which
suggests that macrocycle distortion in solution is more important
in the case of the TCHP derivatives than in the case of the TPP
porphyrins.
Acknowledgment. We thank the CNRS (grant URA-1194
to J.C.M.), the NIH (grants GM-38401 and R.R.-06709 to
W.R.S.), and the Robert A. Welch Foundation (grant E-680 to
K.M.K.) for funding, and Dr. Michel Bardet for helpful
discussions.
Supporting Information Available: Table S1, complete crystal-
lographic details for Zn(TCHP) and Ni(TCHP); Table S2, atomic
coordinates; Table S3, anisotropic displacement parameters; Table S4,
atomic coordinates and isotropic displacement parameters for the fixed
atoms; Table S5, bond lengths; Table S6, bond angles for [Ni(TCHP)‚
CHCl3‚CH3CN]; Table S7, atomic coordinates; Table S8, anisotropic
displacement parameters; Table S9, atomic coordinates and isotropic
displacement parameters for the fixed atoms; Table S10, bond lengths;
Table S11, bond angles for [Zn(TCHP)]; Figures S12 and S13, formal
diagrams of the porphinato cores of Ni(TCHP) and Zn(TCHP) showing
the patterns of displacements of atoms from the least-squares 24-atom
plane in units of 0.01 Å. This material is available free of charge via
the Internet at http://pubs.acs.org.
Conclusion
Substitution by cyclohexyl groups on the meso positions of
a metalloporphyrin has notable stereochemical and electronic
IC981233I