Metal Dependence of the Contributions of Low-Frequency Normal Coordinates to the Sterically Induced Distortions of Meso-Dialkyl-Substituted Porphyrins

Article abstract of DOI:10.1021/ic970465z

The influence of central metals of different sizes on the macrocyclic structure of a series of 5,15-disubstituted metalloporphyrins has been investigated with X-ray crystallography, molecular mechanics (MM) calculations, and resonance Raman spectroscopy. MM calculations indicate that the series of porphyrins are in a gabled (gab) conformation consisting of a linear combination of ruf (B_1u) and dom (A_zu) out-of-plane normal-coordinate deformation types. The MM-calculatcd gab structures have been structurally decomposed into equivalent displacements along the lowest-frequency normal coordinate of each symmetry type. The contributions of each normal coordinate to the total distortion agree well with the contributions obtained from normal-coordinate structural decomposition of the X-ray crystal structures. Symmetry considerations show that any relative proportion of the ruf and dom deformations is allowed in the gab distortion. Varying the size of the central metal causes a change in the ratio of the dom/ruf contributions. A large metal like zinc disfavors ruffling over doming, significantly reducing the ruffling contribution and slightly increasing the doming contribution. In addition, the total degree of nonplanar distortion is reduced for large metals. This is confirmed for the series of disubstituted metalloporphyrins by smaller downshifts of the structure-sensitive Raman lines for the larger metals.

Full text of DOI:10.1021/ic970465z

Inorg. Chem. 1998, 37, 2009-2019
2009
Metal Dependence of the Contributions of Low-Frequency Normal Coordinates to the
Sterically Induced Distortions of Meso-Dialkyl-Substituted Porphyrins
Xing-Zhi Song,†,‡ Laurent Jaquinod,§ Walter Jentzen,† Daniel J. Nurco,§ Song-Ling Jia,†,‡
Richard G. Khoury,§ Jian-Guo Ma,†,‡ Craig J. Medforth,§ Kevin M. Smith,§ and
John A. Shelnutt*,†,‡
Materials Theory and Computation Department, Sandia National Laboratories,|
Albuquerque, New Mexico 87185-1349, Department of Chemistry, University of New Mexico,
Albuquerque, New Mexico 87131, and Department of Chemistry, University of California,
Davis, California 95616
ReceiVed April 24, 1997
The influence of central metals of different sizes on the macrocyclic structure of a series of 5,15-disubstituted
metalloporphyrins has been investigated with X-ray crystallography, molecular mechanics (MM) calculations,
and resonance Raman spectroscopy. MM calculations indicate that the series of porphyrins are in a gabled (gab)
conformation consisting of a linear combination of ruf (B1u) and dom (A2u) out-of-plane normal-coordinate
deformation types. The MM-calculated gab structures have been structurally decomposed into equivalent
displacements along the lowest-frequency normal coordinate of each symmetry type. The contributions of each
normal coordinate to the total distortion agree well with the contributions obtained from normal-coordinate structural
decomposition of the X-ray crystal structures. Symmetry considerations show that any relative proportion of the
ruf and dom deformations is allowed in the gab distortion. Varying the size of the central metal causes a change
in the ratio of the dom/ruf contributions. A large metal like zinc disfavors ruffling over doming, significantly
reducing the ruffling contribution and slightly increasing the doming contribution. In addition, the total degree
of nonplanar distortion is reduced for large metals. This is confirmed for the series of disubstituted
metalloporphyrins by smaller downshifts of the structure-sensitive Raman lines for the larger metals.
Introduction
large nonplanar distortion which was subsequently shown to
be conserved in cytochromes c from several species.3,4 The
There is growing acceptance of the possible functional
significance of the nonplanar conformational distortions seen
for tetrapyrroles in biological systems.1 Nonplanar distortions
of the tetrapyrrole macrocycles in heme-containing proteins have
been observed in various X-ray crystal structures.2 For example,
the crystal structures of mitochondrial cytochromes c display a
importance of this observation is enhanced by the discovery
that the heme can only be nonplanar at considerable energy
expense to the protein matrix.5 This work forms part of a
growing body of research in both biological systems and highly
nonplanar model porphyrin systems demonstrating the profound
effect that nonplanar distortions can have on the chemical and
photophysical properties of porphyrins and metalloporphyrins.6
Recently, we have observed that nonplanar distortions for a
symmetrically substituted metalloporphyrin often occur along
only one of the lowest-frequency out-of-plane normal coordi-
nates of the macrocycle (B1u ruf, B2u sad, A2u dom, and Eg waV),
called normal deformations.7 For an asymmetrically substituted
porphyrin or a porphyrin in a protein, the more complicated
nonplanar distortion may be represented in terms of a linear
* To whom correspondence should be addressed.
† Sandia National Laboratories.
‡ University of New Mexico.
§ University of California.
| Sandia is a multiprogram laboratory operated by Sandia Corporation,
a Lockheed Martin Company, for the United States Department of Energy
under Contract DE-AC04-94AL85000 (J.A.S.).
Work performed at the University of California at Davis was supported
by grants from the National Science Foundation (CHE-93-05577) and the
National Institutes of Health (HL-22252) (K.M.S.).
(1) (a) Longa, S. D.; Ascone, I.; Fontaine, A.; Castellano, A. C.; Bianconi,
A. Eur. Biophys. J. 1994, 23, 361. (b) Haggin, J. Chem. Eng. News
1991, Jan 14, 23. (c) Ravikanth, M.; Reddy, D.; Chandrashekar, T.
K. Chem. Phys. Lett. 1994, 222, 563. (d) Maiti, W. C.; Ravikanth, M.
J. Chem. Soc., Faraday Trans. 1996, 92, 1095. (e) Tetreau, C.;
Lavalette, D.; Momenteau, M.; Fisher, J.; Weiss, R. J. Am. Chem.
Soc. 1994, 116, 11840. (f) Gentemann, S.; Medforth, C. J.; Ema, T.;
Nelson, N. Y.; Smith, K. M.; Fajer, J.; Holten, D. Chem. Phys. Lett.
1995, 245, 441. (g) Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.;
Jentzen, W.; Medforth, C. J. Chem. Soc. ReV. 1998, 27, 31.
(2) (a) Kratky, C.; Waditschatka, R.; Angst, C.; Johansen, J.; Plaquevent,
J. C.; Schreiber, J.; Eschenmoser, A. HelV. Chim. Acta 1982, 68, 1312.
(b) Waditschatka, R.; Kratky, C.; Jaun, B.; Heinzer, J.; Eschenmoser,
A. J. Chem. Soc., Chem. Commun. 1985, 1604. (c) Geno, M. K.;
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1634.
(3) (a) Berghuis, A. M.; Brayer, G. D. J. Mol. Biol. 1992, 223, 959. (b)
Martinez, S. E.; Smith, J. L.; Huang, D.; Szczepaniak, A.; Cramer,
W. A. In Reseach in Photosynthesis; Murata, N., Ed.; Proceedings of
the IXth International Congress on Photosynthesis; Kluwer Aca-
demic: Dordrecht, The Netherlands, 1992; Vol. 2, p 495.
(4) Hobbs, J. D.; Shelnutt, J. A. J. Protein Chem. 1995, 14, 19.
(5) Anderson, K. K.; Hobbs, J. D.; Luo, L.; Stanley, K. D.; Quirke, J. M.
E.; Shelnutt J. A. J. Am. Chem. Soc. 1993, 115, 12346.
(6) (a) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J.
Am. Chem. Soc. 1988, 110, 7566. (b) Kadish, K. M.; Van Caemel-
becke, E. V.; D’Souza, F. D.; Medforth, C. J.; Smith, K. M.; Tabard,
A. Organometallics 1993, 12, 2411. (c) Kadish, K. M.; Van Caemel-
becke, E. V.; Boulas, P.; D’Souza, F. D.; Vogel, E.; Kisters, M.;
Medforth, C. J.; Smith, K. M. Inorg. Chem. 1993, 32, 4177. (d)
Gudowska-Nowak, E.; Newton, M. D.; Fajer, J. J. Phys. Chem. 1990,
94, 5795. (e) Medforth, C. J.; Berber, M. D.; Smith, K. M.; Shelnutt,
J. A. Tetrahedron Lett. 1990, 31, 3719.
S0020-1669(97)00465-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/01/1998

2010 Inorganic Chemistry, Vol. 37, No. 8, 1998
Song et al.
combination of displacements along these lowest-frequency
normal deformations.8 To test this notion, the normal-coordinate
structural decomposition method has been used to analyze the
out-of-plane deformations of the hemes contained in X-ray
crystal structures of proteins.9 The decomposition results reveal
previously hidden influences of the protein on the conformation
of the heme. In many cases, the conformations of the hemes
are conserved within proteins that belong to a single functional
group. These include cytochromes c, c3, c′, and P450 and the
peroxidases.10 The influences of the refinement method, the
crystallization conditions, and the research group carrying out
the X-ray structure determination were ascertained for the hemes
contained in hemoglobin. The hemes in the R- and â-chains
of human deoxyhemoglobin A crystal structures show R/â
differences, but are similar within each subunit. Specifically,
the distortion of the R-hemes is primarily dom and ruf, while
the distortion of the hemes in the â-chains is sad and dom.9
In order to investigate the roles of these different types of
nonplanar distortions in biological processes, it is useful to
design and study models of metalloporphyrins that have
conformations similar to those of the proteins. Previous studies
have shown that the type and degree of nonplanar deformation
can be controlled by the peripheral substitution pattern, the steric
bulkiness of substituents, and the size of the central metal of
the macrocycle.8,11 In this vein, the investigation of a series of
nickel(II) meso-tetrasubstituted porphyrins has more fully
characterized the pure ruf distortion as a model for the conserved
ruffling of the heme of cytochromes c.7 We have also shown
that the conformations of a series of nickel(II) 5,15-disubstituted
porphyrins consist of both ruffling and doming. Thus, these
disubstituted porphyrins serve as a model for the ruffling and
doming in the hemes of the R-chains of human deoxyhemo-
globin A.8 The series of nickel(II) 5,15-disubstituted porphyrins
(Figure 1) were found to adopt a gabled (gab) or roof
conformation of the macrocycle.8 In this conformation, the
porphyrin ring is folded about a line through the two opposite
bridging meso carbons that possess the alkyl substituents. This
gab distortion was shown to be composed of a linear combina-
tion of distortions along the lowest-frequency out-of-plane
macrocyclic normal coordinates of B1u (ruf) and A2u (dom)
symmetry types. As the substituents became more bulky [in
the order phenyl (P), propyl (Pr), isopropyl (iPr) and tert-butyl
(tBu)], the degree of gabling distortion increases.
Figure 1. Molecular structures of the 5,15-disubstituted porphyrins
(hydrogen atoms not shown).
the ruf and dom deformations in the crystal structures of several
metal complexes of the 5,15-disubstituted porphyrins are found
to be metal-dependent in a fashion that is accurately predicted
by molecular modeling. The results obtained also demonstrate
the quantitatiVe validity of normal-coordinate structural decom-
position when only the lowest-frequency coordinate of each
symmetry type is used to describe the structure.
Materials and Methods
Synthesis of the 5,15-Disubstituted Metalloporphyrins. The
synthesis of the free bases and nickel(II) complexes of the 5,15-
disubstituted porphyrins has been described in our earlier paper.8
Cobalt(II), copper(II), and zinc(II) complexes were prepared and
characterized as described below.
Co(dPP). To a solution of free base H2dPP (40 mg) in degassed
CHCl3 (20 mL) was added 300 mg of Co(Acac)2. The reaction mixture
was refluxed for 4 h under argon and filtered through a short silica gel
column, and the porphyrin was eluted with CH2Cl2. The solvent was
then removed under vacuum, and the cobalt complex was recrystallized
from CH2Cl2. Yield ) 35 mg (78%). Mp: >300 °C. NMR: δH (ppm)
(C6D5CH3) 28.1 (br, 2H, Hmeso), 16.9 and 15.4 (br, 4H each, â-H), 12.2
(br, 4H, Hortho), 9.43 (br, 4H, Hmeta), 9.26 (br, 2H, Hpara). Anal. Calcd
for C32H20N4Co: C, 73.99; H, 3.88; N, 10.79. Found C, 73.85; H,
3.92; N, 10.77.
Cu(dPP). H2dPP (100 mg) dissolved in CHCl3/THF/MeOH (10 mL/
10 mL/5 mL) was treated with Cu(OAc)2‚H2O (240 mg) and refluxed
for 2 days. The reaction mixture was passed through a bed of Al2O3
(grade III), rinsed thoroughly with THF, and evaporated to dryness.
The residue was recrystallized from CH2Cl2/MeOH to yield 92 mg of
the title compound (yield ) 81%). Mp: >300 °C. Anal. Calcd for
C32H20N4Cu‚(H2O)0.25: C, 72.71; H, 3.91; N, 10.60. Found: C, 72.92;
H, 3.91; N, 10.68.
Zn(dPP). To a solution of H2dPP (60 mg) in THF (20 mL) was
added a saturated solution of Zn(OAc)2 in MeOH (4 mL). The mixture
was refluxed overnight, cooled to room temperature, and filtered through
a short column of Al2O3 (grade III), and the porphyrin was eluted with
CH2Cl2. The residue was recrystallized from methylene chloride to
yield 60 mg (88%) of the title compound. Mp: >300 °C. NMR: δH
(ppm) (CDCl3) 10.24 (s, 2H, Hmeso), 9.36 (d, 4H, â-H), 9.06 (d, 4H,
â-H), 8.17 (m, 4H, Hortho), 7.68 (m, 6H, Hmeta and Hpara). Anal. Calcd
for C32H20N4Zn‚H2O: C, 70.60; H, 4.08; N, 10.30. Found: C, 70.60;
H, 3.80; N, 10.11.
We now extend our systematic study of the 5,15-disubstituted
porphyrins to an investigation of the metal dependence of these
nonplanar deformations. The series of metal porphyrins for
which the ionic radius of the metal and the optimum metal-
nitrogen bond length increase within the metal series [nickel-
(II), cobalt(II), copper(II), and zinc(II)] is investigated using
X-ray crystallography, molecular mechanics calculations, and
resonance Raman spectroscopy. The relative contributions of
(7) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.-Z.; 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.
(8) Song, X.-Z.; Jentzen, W.; Jia, S.-L.; Jaquinod, L.; Nurco, D. J.;
Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1996,
118, 12975.
(9) Jentzen, W.; Song, X.-Z.; Shelnutt, J. A. J. Phys. Chem. B 1997, 101,
1684.
Co(dPrP) was prepared using the same procedure as described for
Co(dPP) and recrystallized from CH2Cl2. Mp: >300 °C. Anal. Calcd
for C26H24N4Co: C, 69.48; H, 5.36; N, 12.41. Found: C, 69.21; H,
5.45; N, 12.33. Co(dPrP) was not sufficiently soluble in CDCl3, CD2-
Cl2, or C6D5CD3 to allow the measurement of a proton NMR spectrum.
Cu(dPrP) was prepared using the same procedure as described for
Cu(dPP) and recrystallized from CH2Cl2. Mp: >300 °C. Anal. Calcd
(10) Jentzen, W.; Ma, J.-G.; Shelnutt, J. A. Biophys. J. 1998, 74, 753.
(11) (a) Sparks, L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.;
Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am.
Chem. Soc. 1993, 115, 581. (b) Medforth, C. J.; Berber, M. D.; Smith,
K. M.; Shelnutt, J. A. Tetrahedron Lett. 1990, 31, 3719. (c) Medforth,
C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt, J. A. J.
Am. Chem. Soc. 1992, 114, 9859.

Distortions of Meso-Dialkyl-Substituted Porphyrins
Inorganic Chemistry, Vol. 37, No. 8, 1998 2011
for C26H24N4Cu‚(H2O)0.25: C, 67.81; H, 5.36; N, 12.16. Found: C,
67.67; H, 5.03; N, 12.01.
Table 1. Data Acquisition and Structure Refinement Parameters
for Cu(dtBuP), Ni(diPrP), and Zn(diPrP)(py)
Zn(dPrP) was prepared using the same procedure as described for
Zn(dPP) and recrystallized from CH2Cl2/cyclohexane. Mp: >300 °C.
NMR: δH (ppm) (CDCl3 + 10 equiv of pyridine) 10.02 (s, 2H, Hmeso),
9.64 (d, 4H, â-H), 9.37 (d, 4H, â-H), 5.08 (t, 4H, R-CH2), 2.58 (m,
4H, â-CH2), 1.31 (t, 6H, CH3). Anal. Calcd for C26H24N4Zn‚-
(H2O)0.5: C, 66.89; H, 5.40; N, 12.00. Found: C, 67.10; H, 5.23; N,
11.73.
Co(diPrP) was prepared using the same procedure as described for
Co(dPP) and recrystallized from CH2Cl2/cyclohexane. Mp: >300 °C.
NMR: δH (ppm) (CDCl3) 27.5 (br, 2H, Hmeso), 16.48 (br, 4H, â-H),
15.32 (br, 4H, â-H), 12.99 (br, 2H, CH), 6.26 (br, 12H, CH3). Anal.
Calcd for C26H24N4Co: C, 69.48; H, 5.36; N, 12.41. Found: C, 69.01;
H, 5.48; N, 12.34.
Cu(dtBuP)
Ni(diPrP)
Zn(diPrP)(py)
empirical formula C28H28N4Cu C26H24N4Ni
C31H29N5Zn
537.0
red
formula wt
cryst color
cryst syst
space group
a (Å)
b (Å)
c (Å)
484.1
451.2
red
purple
monoclinic
P21/n
8.026(2)
25.745(5)
10.808(2)
90
92.39(3)
90
2231.4(8)
4
orthorhombic triclinic
Pbca
18.466(4)
12.120(2)
18.493(4)
90
P1h
10.198(3)
15.605(4)
16.815(3)
89.34(2)
79.84(2)
75.85(2)
2552.7(11)
4
R (deg)
â (deg)
90
90
γ (deg)
V (Å3)
4138.9(14)
8
0.046
0.122
0.914
Z
Cu(diPrP) was prepared using the same procedure as described for
Cu(dPP) and recrystallized from CH2Cl2/methanol. Mp: >300 °C.
Anal. Calcd for C26H24N4Cu‚(H2O)0.5: C, 67.15; H, 5.42; N, 12.05.
Found: C, 66.92; H, 5.12; N, 11.80.
R1 (obs. data)
wR2 (all data)
GOF (F2)
0.035
0.084
1.050
0.049
0.126
0.979
Zn(diPrP) was prepared using the same procedure as described for
Zn(dPP) and recrystallized from CHCl3/cyclohexane. Mp: >300 °C.
NMR: δH (ppm) (CDCl3 + 20 equiv of pyridine) 10.03 (s, 2H, Hmeso),
9.82 (d, 4H, â-H), 9.36 (d, 4H, â-H), 5.83 (m, 2H, CH), 2.50 (d, 12H,
CH3). Anal. Calcd for C26H24N4Zn: C, 68.20; H, 5.28; N, 12.24.
Found: C, 68.20; H, 5.60; N, 11.84.
Co(dtBuP) was prepared using the same procedure as described for
Co(dPP) and recrystallized from CH2Cl2/methanol. Mp: >300 °C.
NMR: δH (ppm) (CDCl3) 23.6 (br, 2H, Hmeso), 15.46 (br, 8H, â-H),
5.61 (br, 18H, CH3). Anal. Calcd for C28H28N4NCo: C, 70.14; H,
5.89; N, 11.68. Found: C, 69.35; H, 5.68; N, 11.43.
Cu(dtBuP) was prepared using the same procedure as described
for Cu(dPP) and recrystallized from CHCl3/toluene. Mp: >300 °C.
Anal. Calcd for C28H28N4Cu‚H2O: C, 66.98; H, 6.02; N, 11.16.
Found: C, 66.97; H, 5.68; N, 11.06.
Zn(dtBuP) was prepared using the same procedure as described for
Zn(dPP) and recrystallized from CHCl3/cyclohexane. Mp: >300 °C.
NMR: δH (ppm) 9.62 (d, 4H, â-H), 9.53 (s, 2H, Hmeso), 8.89 (d, 4H,
â-H), 2.43 (s, 18H, CH3). Anal. Calcd for C28H28N4Zn: C, 69.21; H,
5.81; N, 11.53. Found: C, 69.06; H, 5.42; N, 11.80.
Molecular Modeling Studies. Classical molecular mechanics
calculations have previously been used for predicting porphyrin
structures by our group,7,8,11a,13 Munro,14 Marques,15 and Kollman16 et
al. Molecular mechanics calculations were carried out using POLYGRAF
software (Molecular Simulations, Inc.). The porphyrin force field13a
was developed on the basis of the DREIDING II force field17 and the
normal coordinate analysis of NiOEP.18 The unconstrained equilibrium
bond length for the nickel-nitrogen(pyrrole) bond was set to 1.855 Å,
and the other equilibrium bond distances and angles were adjusted so
that the energy-minimized conformation of NiOEP matched the planar
crystal structure of NiOEP19 as closely as possible. This original force
field has been modified as follows: (1) Parameters for additional metals
were obtained by changing the equilibrium M-N bond distance Req
[1.855 Å for Ni(II), 1.930 Å for Co(II), 1.970 Å for Cu(II), and 2.070
Å for Zn(II)], the homonuclear nonbond separation Rnb [2.27 Å for
Ni(II), 3.40 Å for Co(II), 3.40 Å for Cu(II), and 4.54 Å for Zn(II)],
and the atomic mass [58.700 for Ni(II), 58.933 for Co(II), 63.546 for
Cu(II), and 65.380 for Zn(II)].11a Req of the metals is obtained by fitting
planar metalloporphyrin X-ray structures.20 (2) Electrostatic terms were
included in calculating the total energy.21 (3) The van der Waals
potential energy term for hydrogen atoms was changed from a Lennard-
Jones 6-12 to an exponential-6 functional form.22 (4) A cutoff distance
of 50 Å instead of 9 Å was employed when electrostatic and van der
Waals energy terms were calculated.22 (5) DREIDING II parameters
X-ray Crystallography. Crystals were transferred directly from
crystallization tubes to a light hydrocarbon oil (Paratone N), in which
they were examined and cut as necessary. Crystals were mounted on
glass fibers and placed on a Siemens R3 m/V diffractometer equipped
with graphite-monochromated Mo KR radiation from a normal-focus
sealed tube operating at 2.0 kW [Cu(dtBuP) and Zn(diPrP)(py)] or on
a Syntex P21 diffractometer equipped with graphite-monochromated
Cu KR radiation from a normal-focus sealed tube operating at 2.0 kWa
[Ni(diPrP)]. The crystals were cooled to 130(2) K using a stream of
anhydrous nitrogen supplied from a locally modified Nonius low-
temperature apparatus (Siemens R3 m/V) or a Syntex LT-1 system
(Syntex P21). Data for Cu(dtBuP) were collected to 2θmax ) 45.0°
with ω scans in the index ranges 0 ) h ) 8, 0 ) k ) 27, -11 ) l )
11. A total of 3253 reflections were collected including 2927
independent reflections (Rint ) 0.023). Data for Zn(diPrP)(py) were
collected to 2θmax of 55.1° with ω scans in the index ranges 0 ) h )
13, -20 ) k ) 20, -21 ) l ) 21, for a total of 11 745 independent
reflections. Data for Ni(diPrP) were collected to 2θmax of 114.1° with
θ/2θ scans in the index ranges 0 ) h ) 14, 0 ) k ) 20, 0 ) l ) 21,
(13) (a) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barkigia, K. M.;
Smith, K. M. J. Am. Chem. Soc. 1991, 113, 4077. (b) Shelnutt, J. A.;
Majumder, S. A.; Sparks, L. D.; Hobbs, J. D.; Medforth, C. J.; Senge,
M. O.; Smith, K. M.; Miura, M.; Luo L.; Quirke, J. M. E. J. Raman
Spectrosc. 1992, 23, 523. (c) Anderson, K. K.; Hobbs, J. D.; Luo, L.;
Stanley, K. D.; Quirke, J. M. E.; Shelnutt, J. A. J. Am. Chem. Soc.
1993, 115, 12346. (d) Sparks, L. D.; Anderson, K. K.; Medforth, C.
J.; Smith, K. M.; Shelnutt, J. A. Inorg. Chem. 1994, 33, 2297.
(14) (a) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935. (b) Munro, O. Q.;
Bradley, J. C.; Hancock, R. D.; Marques, H. M.; Marsicana, F. J.
Am. Chem. Soc. 1992, 114, 7218.
(15) (a) Marques H. M.; Munro, O. Q.; Grimmer, N. E.; Levendis, D. C.;
Marsicano, F.; Pattrick, G.; Markonlides, T. J. Chem. Soc., Faraday
Trans. 1995, 91, 1741. (b) Hancock, R. D.; Weaving, F. S.; Marques,
H. M. J. Chem. Soc., Chem. Commun. 1989, 1177.
(16) (a) Kollman, P. A.; Grootenhuis, D. D. J.; Lopez, M. A. Pure Appl.
Chem. 1989, 61, 593. (b) Lopez, M. A.; Kollman, P. A. J. Am. Chem.
Soc. 1989, 111, 6212.
for a total of 3187 reflections and 2790 independent reflections (Rint
)
0.008). Structures were solved with direct methods and refined with
a full-matrix least-squares method (based on |F2| and all independent
reflections) using Siemens SHELXTL version 5.03 software. Hydrogen
atom positions were generated using idealized geometries and refined
using riding models. Empirical absorption corrections were applied
(XABS2).12 All non-hydrogen atoms were refined with anisotropic
thermal parameters. Additional experimental details are presented in
Table 1.
(17) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III, J. Phys. Chem.
1990, 94, 8897.
(18) (a) Li, X.-Y.; Czernuszewics, R. S.; Kincaid, J. R.; Spiro, T. G. J.
Am. Chem. Soc. 1989, 111, 7012. (b) Li, X.-Y.; Czernuszewics, R.
S.; R. S.; Kincaid, J. R.; Su, Y. O.; Spiro, T. G. J. Phys. Chem. 1990,
94, 31. (c) Li, X.-Y.; Czernuszewics, R. S.; Kincaid, J. R.; Stein, P.;
Spiro, T. G. J. Phys. Chem. 1990, 94, 47. (d) Kitagawa, T.; Abe; M.;
Ogoshi, H. J. Chem. Phys. 1978, 69, 4516. (e) Abe, M.; Kitagawa,
T.; Kyogoku, Y. J. Chem. Phys. 1978, 69, 4526.
(12) Parkin, S. R.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995, 28,
(19) Brennan, T. D.; Scheidt, W. R.; Shelnutt, J. A. J. Am. Chem. Soc.
1988, 110, 3919.
53.

2012 Inorganic Chemistry, Vol. 37, No. 8, 1998
Song et al.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) and Other Structural Parameters Obtained from Crystal Structures of Metal
Di-tert-butylporphyrins
Ni(dtBuP)
Cu(dtBuP)
10,20
trans-Ni(diPrP)
10,20 5,15
trans-Zn(diPrP)(py)
10,20 5,15
10,20
5,15
5,15
Displacements (Å)a
mean
M
N
CR
Câ
Cm
0.40b
0.17
0.12
0.31
0.21
0.16
0.38
-0.03
0.72
0.26
0.07
0.05
0.05
0.42
0.06
-0.40
-0.44
-0.70
0.47
-0.14
0.89
-0.30
-0.42
-0.53
-0.28
-0.26
-0.49
0.32
0.15
0.56
0.04
-0.05
0.05
0.00
-0.08
-0.02
Bond Lengths (Å)
1.973(3)
M-N
1.897(2)
1.930(2)
2.075(2)
1.377(4)
1.452(5)
1.401(6)
1.331(6)
N-CR
CR-Câ
CR-Cm
Câ-Câ
1.380(4)
1.429(4)
1.381(4)
1.386(3)
1.446(4)
1.403(3)
1.374(4)
1.432(5)
1.385(5)
1.381(4)
1.446(5)
1.383(4)
1.428(5)
1.372(4)
1.388(4)
1.444(5)
1.391(5)
1.368(4)
1.437(5)
1.384(5)
1.409(5)
1.352(4)
1.346(5)
1.349(5)
Bond Angles (deg)
87.3(1)
N-M-N adj
N-M-N opp
M-N-CR
N-CR-Cm
N-CR-Câ
CR-N-CR
CR-Cm-CR
CR-CR-Câ
Cm-CR-Câ
92.3(1)
87.7(1)
177.0(1)
92.7(1)
91.8(1)
88.2(1)
179.2(1)
90.7(1)
86.3(1)
161.2(1)
177.2(1)
124.7(2)
124.2(2)
110.3(2)
128.9(2)
124.2(2)
108.7(2)
123.5(2)
124.9(3)
109.7(3)
129.2(2)
125.6(2)
124.7(3)
110.6(3)
129.0(2)
125.6(3)
109.4(3)
123.9(2)
125.6(3)
109.4(3)
128.5(2)
125.8(3)
108.0(3)
124.7(3)
108.3(3)
106.2(2)
106.9(3)
105.4(2)
107.3(3)
123.7(3)
107.3(2)
125.3(3)
117.3(2)
107.9(3)
126.4(3)
126.3(2)
107.1(3)
125.4(3)
119.1(3)
108.0(3)
126.5(3)
124.6(3)
107.1(3)
124.5(3)
119.6(3)
107.5(3)
124.7(3)
128.2(3)
107.4(3)
125.0(3)
126.3(3)
108.0(3)
126.2(3)
a From the least-squares plane of the 24 atoms of the porphyrin core. b A single value indicates only one displacement, bond length, or bond
angle for the molecule.
were used consistently instead of a hybrid of DREIDING I for the
macrocycle and DREIDING II for the substituents.7 (6) Torsions for
resonance atom types exocyclic to aromatic ring systems were reduced
to 40% of the value internal to the ring, consistent with DREIDING
II.17 (7) A minor error in the counting of the nonbond interactions of
the atoms bonded to the metal was corrected in the POLYGRAF code.8
(8) The r-dependent dielectric constant of 2.64 for CS2 was used instead
of 1.00 for vacuum in the calculations of electrostatic energy terms,
mimicking the experimental solvent environment.8 These modifications
have only a minor influence on the calculated porphyrin structures but
affect the relative energies of conformers. After these changes, the
force field bond distances and angles for the macrocycle were
reoptimized using a least-squares method.23
The most important change in the force field is a reduction in the
out-of-plane force constants by 50%,8 which approximately corrects
for the addition of the out-of-plane and in-plane force constants
independently obtained from separate normal coordinate analyses. This
revision improves the ability of the force field to predict the structures
of nonplanar porphyrins and gives more accurate relative energies of
the stable porphyrin conformers.
excitation in the B band region. The scattered light was collected in
the 90° scattering geometry, and the Raman cell was rotated at 50 Hz
to prevent local heating of the sample and to alternately probe the
sample and reference solutions. Typical conditions for the spectra of
the nickel(II), cobalt(II), and copper(II) porphyrins were 50-60-mW
laser power, approximately 2.7-cm-1 spectral slit width, 4-6 scans with
0.3-cm-1 step increment, and 1-s integration time. Since the zinc(II)
porphyrins decomposed during the measurements, lower laser power
(20 mW), fewer scans (2-3), larger step increments (0.5), and shorter
integration times (0.5 s) were used. The frequencies in the regions
above and below 900 cm-1 were calibrated with the 1373.3- (ν4) and
391.9-cm-1 (ν8) lines of NiTPP.25 In addition, all the spectra were
corrected for the nonlinearity of the spectrometer to obtain the absolute
frequencies of the lines to an accuracy of (1 cm-1 25
.
Results
Crystal Structures of Metal Complexes of the 5,15-
Dialkylporphyrins. In an earlier study, we reported preliminary
data from the crystal structures of Ni(dPP), Ni(dPrP), and Ni-
(diPrP) and full details of the crystal structure of Ni(dtBuP).8
To investigate the metal dependence of the nonplanar distortions
of the 5,15-dialkylporphyrins in this work, X-ray crystal
structures were determined for complexes with other metals,
namely, the copper(II) complex of dtBuP (Cu(dtBuP)) and the
monopyridine zinc(II) complex of diPrP [Zn(diPrP)(py)]. Full
details of the crystal structure of Ni(diPrP) are presented
elsewhere.25 Table 1 gives details of the data acquisition and
structure refinement parameters for Cu(dtBuP), Ni(diPrP), and
Zn(diPrP)(py). Out-of-plane displacements, bond lengths, and
bond angles for Ni(dtBuP), Cu(dtBuP), Ni(diPrP), and Zn-
(diPrP)(py) are summarized in Table 2. Note for the diPrP
complex that either the methyl groups of both isopropyl groups
can point toward C10 or C20 (cis conformation) or the methyl
groups of one isopropyl can point toward C10 and the methyls
Resonance Raman Spectroscopy. Resonance Raman spectra were
obtained using a partitioned Raman cell and a dual-channel spectrometer
described previously.24 Approximately 0.1 mM CS2 solutions of the
sample and a reference porphyrin [nickel(II) tetraphenylporphyrin
(NiTPP)] were placed in a dual-compartment cell. The 413.1-nm line
from a krypton ion laser (Coherent, INNOVA 20) was used for
(20) (a) Dewyer, P. N.; Madura, P.; Scheidt, W. R. J. Am. Chem. Soc.
1974, 96, 4815. (b) Pak, R.; Scheidt, W. R. Acta Crystallogr. 1991,
c47, 431. (c) Collins, D. M.; Hoard, J. L. J. Am. Chem. Soc. 1970,
92, 3761. (d) Scheidt, W. R.; Cunningham, J. A.; Hoard, J. L. J. Am.
Chem. Soc. 1973, 95, 8289. (e) Reed, C. A.; Mashiko, T.; Bentley, S.
P.; Kastner, M. E.; Scheidt, W. R.; Spartalian, K.; Lang, G. J. Am.
Chem. Soc. 1979, 101, 581.
(21) Sparks, L. D.; Chamberlain, J. R.; Hsu, P.; Ondrias, M. R.; Swanson,
B. A.; Oritiz de Montellano, P. R.; Shelnutt, J. A. Inorg. Chem. 1993,
32, 3153.
(22) Hobbs, J. D.; Majumder, S. A.; Luo, L.; Sickelsmith, G. A.; Quirke,
J. M. E.; Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem.
Soc. 1994, 116, 3261.
(23) Dasgupta, S.; Goddard, W. A., III. J. Chem. Phys. 1989, 90, 207.
(24) Shelnutt, J. A. J. Phys. Chem. 1983, 87, 605.
(25) Jentzen, W.; Turowska-Tyrk, I.; Scheidt, W. R.; Shelnutt, J. A. Inorg.
Chem. 1996, 35, 3559.
(26) Nurco, D. J.; Medforth, C. J.; Smith, K. M. Unpublished results.

Distortions of Meso-Dialkyl-Substituted Porphyrins
Inorganic Chemistry, Vol. 37, No. 8, 1998 2013
the Cs1 atoms of the two substituents on opposite sides of the
macrocycle. When the Cs1 of the substituent is asymmetrically
substituted (like propyl and isopropyl), there are secondary
conformers resulting from the different possible orientations of
Cs2 (the second substituent carbon from the ring). These
secondary substituent configurations are labeled “cis” when the
Cs2 atoms are on the same side of the vertical plane through
the 5 and 15 positions and “trans” when the Cs2 atoms are on
opposite sides of this plane.8 Table 3 summarizes the structural
parameters calculated for the most stable planar and RR
conformers for each 5,15-disubstituted metalloporphyrin. The
structural parameters for the higher-energy Râ conformers are
given in Table S1 of the Supporting Information. The calculated
energies for all of the stable conformations of the 5,15-
disubstituted metalloporphyrin are given in Table S2 of the
Supporting Information.
A planar conformer is predicted for each metal complex of
dPP; the only stable nonplanar conformation is the RR con-
former of Ni(dPP). The energies of the planar and RR
conformers of Ni(dPP) are almost the same. There is no Râ
conformer predicted for the diphenylporphyrins regardless of
the metal in the macrocycle.
For dPrP, six stable conformers (RR, trans-RR, cis-RR, Râ,
trans-Râ, and cis-Râ) are predicted for each metalloporphyrin.
The RR conformer is the most stable one. The conformational
energy difference (∆E ) ERâ - ERR) decreases as the metal
size increases (Ni < Co < Cu < Zn), and the differences are
all within 3 kcal/mol. Because of the latter, it is possible that
many of these conformers coexist in solution. Only the
structural parameters and energies of representative RR and Râ
conformers are tabulated in Table 3 and Tables S1 and S2 of
the Supporting Information.
For diPrP, each metalloporphyrin has four stable conformers
(trans-RR, cis-RR, trans-Râ, and cis-Râ). The averaged
structural parameters of the RR or the Râ conformers are almost
the same, so only the structural parameters of the trans
conformers are given in Table 3 and Tables S1 and S2. ∆E
again decreases as the metal size increases. For Ni(diPrP), ∆E
is large (4.59 kcal/mol), so only the RR conformers exist in
solution. ∆E decreases dramatically with the increasing size
of the central metal and falls to within 3 kcal/mol for Co(diPrP),
Cu(diPrP), and Zn(diPrP), indicating that all four conformers
may coexist in solution for these metals.
For dtBuP, the symmetry of the tert-butyl substituent allows
only RR and Râ conformers. ∆E between these two conformers
is large (over 6 kcal/mol) for every porphyrin, although here
too ∆E decreases as the metal size increases. The large ∆E
means that only the RR conformer is present in solution.
Modeling the Metal Dependence of Symmetric Deforma-
tions. The out-of-plane distortions seen for the 5,15-disubsti-
tuted porphyrins are mixtures of symmetric distortions, mainly
ruffling and doming of the macrocycle. To understand the metal
dependence of these basic symmetric deformations, a series of
macrocyclic structures with different degrees of nonplanar
distortion and different symmetric deformation types were
generated from meso-tetra-(tert-butylporphyrin (TtBuP, without
methyl hydrogens) by orienting the substituents. Specifically,
the dom structure was given by the RRRR orientation (all four
substituents above the macrocycle plane), the ruf structure was
given by the RâRâ orientation (substituents alternatively above
and below the porphyrin plane), and the waV structure was given
by the RRââ orientation. The pure sad structures were obtained
from meso-tetraphenylporphyrin (TPP) instead of TtBuP, which
does not form a saddled structure. The purity of the distortion
Figure 2. Comparison of the X-ray crystal structure (heavy) and the
energy-optimized structure (light) for copper(II) di-tert-butylporphyrin.
of the other isopropyl toward C20 (trans conformation). The
crystal structure of Ni(diPrP) shows a trans conformation,
whereas both cis and trans conformers are observed in the unit
cell of Zn(diPrP)(py). For consistency, the data given for Zn-
(diPrP)(py) in Table 2 is for the trans conformer only. Complete
information on the crystal structures is given in the Supporting
Information.
The crystal structure of Cu(dtBuP) is shown in Figure 2 and
is generally similar to that previously reported for NidtBuP.8
The differences between the two structures can be related to
the smaller metal atom in NidtBuP, which favors shorter metal-
nitrogen bond distances and a more nonplanar macrocycle.7,8
For example, the average M-N distance seen for Ni(dtBuP)
[1.897(2) Å] is significantly shorter than that seen for Cu(dtBuP)
[1.973(3) Å], and the mean out-of-plane displacement (0.40 Å)
is greater than that observed for Cu(dtBuP) (0.31 Å). Similar
metal effects are seen for trans-Ni(diPrP) and trans-Zn(diPrP)-
(py), where the Zn-N distance is 2.075(2) Å and the Ni-N
distance 1.930(2) Å, and the mean out-of-plane displacements
are 0.05 and 0.26 Å, respectively.
The bond lengths and bond angles for the diPrP and dtBuP
complexes (Table 2) provide insights into the structural defor-
mations of these porphyrins. Significant differences are ob-
served between the bond lengths and bond angles associated
with the 5,15 axis (bearing the substituents) and those associated
with the 10,20 axis (unsubstituted). The largest differences are
seen for the N-M-Nadj, M-N-CR, and CR-Cm-CR bond
angles (Table 2). Similar differences have recently been seen
in 2,3,5,7,8,12,13,15,17,18-decasubstituted porphyrins26,27 and
were attributed to the need to minimize steric strain between
the 5,15 meso-substituents and the alkyl groups at the pyrrole
positions. It is likely that a related steric interaction, in this
case between the meso substituents and the pyrrole rings, is
present in the 5,15-dialkylporphyrins. Other differences in the
bond angles and bond lengths associated with the 5,15 and 10,-
20 axes are smaller. For example, the 5,15 N-CR, CR-Cm,
and CR-Câ bond lengths are longer than those for the 10,20
axis. Likewise, the N-CR-Câ bond angles are smaller and the
CR-Câ-Câ and Cm-CR-Câ bond angles are larger along the
5,15 axis. Typically, these structural differences are also
observed in the energy-optimized structures of these porphyrins.
Calculation of Minimum-Energy Structures for the Metal
Complexes of the 5,15-Dialkylporphyrins. Molecular me-
chanics calculations generally predict that the 5,15-disubstituted
metalloporphyrins have two types of primary conformations
resulting from different orientations of the first meso-bonded
carbon (Cs1) of the 5 and 15 substituents relative to the porphyrin
macrocycle. The most stable conformer has the RR configu-
ration, with the Cs1 atoms of the two substituents on the same
side of the macrocycle. The higher-energy Râ conformer has
(27) (a) Medforth, C. J.; Senge, M. O.; Forsyth, T. P.; Hobbs, J. D.; Shelnutt,
J. A.; Smith K. M. Inorg. Chem. 1994, 33, 3865. (b) Senge, M. O.;
Medforth, C. J.; Forsyth, T. P.; Lee, D. A.; Olmstead, M. M.; Jentzen,
W.; Pandey, R. K.; Shelnutt, J. A.; Smith K. M. Inorg. Chem., in
press.

2014 Inorganic Chemistry, Vol. 37, No. 8, 1998
Song et al.
Table 3. Selected Structural Parameters Obtained from the Most Stable Calculated Conformations of the Metal 5,15-Disubstituted Porphyrins
Ni-N%
(Å)
dihed angle
(deg)
CR-N-CR
(deg)
N-Ni-N
(deg)
Câ-Câ
(Å)
CR-Cm
(Å)
CR-N
(Å)
NCR-CâCâ
(deg)
NCR-CmCR
(deg)
porphyrin
planar dPP
Ni
Co
Cu
1.949
1.979
1.995
2.036
0.2
0.0
0.1
0.1
104.3
105.3
105.9
107.3
180.0
180.0
180.0
180.0
1.326
1.329
1.331
1.335
1.378
1.383
1.386
1.394
1.382
1.380
1.378
1.376
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
Zn
RR-dPP
Ni
1.944
12.7
104.4
179.8
1.326
1.378
1.382
0.8
4.9
RR-dPrP
Ni
1.929
1.970
1.990
2.033
24.7
15.4
11.6
6.2
104.9
105.6
106.0
107.3
179.2
179.1
179.0
178.7
1.328
1.329
1.331
1.334
1.381
1.385
1.388
1.395
1.381
1.380
1.379
1.376
1.2
0.6
0.4
0.3
10.1
7.2
5.8
3.8
Co
Cu
Zn
trans-RR-diPrP
Ni
Co
1.913
1.956
1.978
2.028
33.2
26.0
22.5
14.8
105.4
106.0
106.4
107.6
178.2
177.7
177.4
176.0
1.330
1.331
1.332
1.335
1.384
1.387
1.390
1.402
1.379
1.378
1.377
1.377
1.1
1.0
1.4
0.8
14.8
13.0
12.0
9.5
Cu
Zn
RR-dtBuP
Ni
1.892
1.935
1.958
2.018
41.4
35.7
32.9
26.3
106.0
106.6
107.0
107.9
175.9
174.6
173.7
168.9
1.334
1.335
1.336
1.338
1.387
1.390
1.392
1.398
1.376
1.375
1.374
1.374
2.0
1.9
1.8
1.3
20.5
19.6
19.2
17.8
Co
Cu
Zn
in each structure was checked by decomposing the structure
into symmetric components (ruf, sad, dom, waV, see below).
The degree of the distortion for each of the structures was altered
by changing the van der Waals distance parameter of the
substituent carbons (from 0.5 to 5 Å in increments of 0.5 Å).
The calculations were carried out for each of the metals. Figure
3 shows the plots of the resulting metal-nitrogen (M-N)
distance as a function of total deformation (obtained from
normal-coordinate structural decomposition) for the ruf, dom,
and waV types of deformation.
Structural Decomposition of the X-ray Crystal Structures
and Calculated Structures. The available X-ray structures and
the calculated stable conformers for the series of 5,15-disub-
stituted porphyrins were structurally decomposed in terms of
displacements along the lowest-frequency out-of-plane normal
coordinates of the macrocycle by using the normal-coordinate
structural decomposition (NSD) method. NSD results are given
for both minimal and extended basis sets of normal-coordinate
eigenvectors (see Discussion).9 Table 4 tabulates the decom-
position results obtained using the minimal basis set for the
X-ray structures and the most stable calculated conformations.
The results obtained using the extended basis for these
conformations are shown in Table S3 of the Supporting
Information. The decomposition results for the Râ conformers
obtained using the minimal and extended basis are given in
Tables S4 and S5 of the Supporting Information, respectively.
Resonance Raman Spectroscopy. The 413.1-nm excited
resonance Raman spectra of dPP, dPrP, diPrP, and dtBuP with
Ni, Co, Cu, and Zn as central metals in high-frequency (1300-
1700 cm-1), low-frequency (200-600 cm-1), and mid-frequency
(900-1300 cm-1) regions are shown in Figures S1-3 of the
Supporting Information. The quality of the spectra of the zinc
porphyrins is generally poor because of their strong fluorescence
and partial decomposition during measurements. The frequen-
cies of some strong Raman lines, ν2, ν4, ν9, ν6, and ν8, are
tabulated in Table 5.
Figure 3. Metal-nitrogen distance as a function of total out-of-plane
displacements of the macrocyclic atoms for pure symmetric distortions
(ruf, dom, and waV). The curves are obtained from molecular mechanics
calculations on tetra-tert-butyl metalloporphyrin (without hydrogen) by
increasing the van der Waals radius of the substituent carbon atoms
from 0.0 at 0.5-Å intervals. The deformation type is determined by the
initial orientation of the substituent groups (ruf; RâRâ; dom, RRRR;
waV; RRââ).
For the same porphyrin with different metals, both ν2 and ν4
downshift with increasing metal size. This relationship is
analogous to the core-size correlation of nearly planar porphy-
rins.28 A subtler trend in the variation of the Raman frequencies
with metal size can also be seen. For ν2, the range of downshifts
for the metal derivatives decreases, as the porphyrin substituents
become more bulky. The trend is less definite for ν4.
The high-frequency region contains most of the structure-
sensitive and oxidation-state marker lines. For porphyrins with
the same central metal but different substituents, the structure-
sensitive line ν2 downshifts with the increasing steric size of
the substituents. On the other hand, the oxidation-state marker
line ν4 varies little with the different substituents.

Distortions of Meso-Dialkyl-Substituted Porphyrins
Inorganic Chemistry, Vol. 37, No. 8, 1998 2015
Table 4. Displacements (in Å) along the Lowest-Frequency Out-of-Plane Normal Coordinates of the Macrocycle: Normal-Coordinate
Structural Decomposition Results Using the Minimal Basis for the Selected Most Stable Conformers of the Series of 5,15-Disubstituted
Porphyrins and the X-ray Crystal Structures (Bold)
dtot
ruf
B1u
sad
B2u
dom
A2u
waV(x)
Egx
waV(y)
Egy
pro
A1u
porphyrin conformer
sim
calcd
planar dPP
Ni
Co
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Cu
Zn
-0.005
-0.004
0.609
0.736
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.005
0.004
0.610
0.744
0.005
0.004
0.611
0.745
Ni RR
X-ray
RR-dPrP
Ni
Co
Cu
0.015
0.059
-0.075
-0.004
-0.048
0.007
1.186
0.757
0.576
0.319
0
0
0
0
0.083
0.100
0.105
0.114
0
0
0
0
0
0
0
0
0
0
0
0
1.189
0.764
0.585
0.338
1.193
0.769
0.591
0.346
Zn
trans-RR-diPrP
Ni
1.603
1.467
1.280
1.120
0.763
0.052
-0.138
0.047
0.046
0.040
0.227
0.202
0.256
0.271
0.315
0
0
0
1.620
1.488
1.307
1.153
0.827
1.631
1.490
1.322
1.170
0.848
X-ray
Co
Cu
0.011
-0.010
0.022
0
0
0
0
0
0
0
0
0
Zn
Zn(py)
trans
cis
X-ray trans
X-ray cis
RR-dtBuP
Ni
X-ray
Co
Cu
0.860
0.852
0.097
0.523
0.032
0
-0.097
-0.116
0.347
0.175
-0.259
-0.217
0
0
0
0
0.928
0.870
0.305
0.583
0.944
0.888
0.307
0.585
-0.012
0.062
0.024
0.012
0.053
-0.072
-0.013
0
2.013
2.181
1.773
1.656
1.694
1.368
0
0.541
0.488
0.607
0.646
0.637
0.792
0
0
0
2.085
2.239
1.874
1.778
1.811
1.581
2.114
2.249
1.909
1.816
1.826
1.625
-0.021
0
0
-0.008
0
-0.079
0
0
0.055
0
0.078
0
0
-0.005
0
0.007
0
0
-0.009
0
X-ray
Zn
Table 5. Frequencies of Selected Resonance Raman Lines for
Metal Disubstituted Porphyrins in CS2 Solution
ν2 band. For diPrP, ν2 is almost symmetric for Ni(diPrP), but
asymmetric for Co(diPrP) and Cu(diPrP) [and possibly for Zn-
(diPrP)]. For dtBuP, ν2 of Ni(dtBuP) is apparently a doublet
with two almost equally intense sublines; the intensity of the
high-frequency subline gradually decreases in the order Ni >
Co > Cu > Zn.
The Raman lines such as ν9, ν6, and ν8 in the middle- and
low-frequency regions show no obvious trends of the depen-
dence of frequency on the size of the central metal and bulkiness
of the substituents. However, the shape of ν8, which is sensitive
to structural heterogeneity, does show dependence on these
factors. For dPP, ν8 is broad and asymmetric for nickel and
becomes less broad and asymmetric with larger cobalt, and
narrow and symmetric with even larger copper. For dPrP, ν8
is uniformly broad and asymmetric. For diPrP, the somewhat
broad ν8 line is symmetric with nickel, but asymmetric with
larger metals. For dtBuP, ν8 is narrow and symmetric with all
metals.
porphyrin
ν2
ν4
ν9
ν6
ν8
dPP
Ni
Co
Cu
Zn
1573.9
1567.3
1560.8
1547.6
1374.4
1370.9
1368.3
1365.6
1072.9
1071.8
1071.0
1070.3
1001.8
1001.0
1001.8
1003.0
397.5
394.6
394.6
396.8
dPrP
Ni
Co
Cu
Zn
1573.3
1568.5
1562.6
1547.6
1373.0
1369.9
1368.2
1364.9
1076.6
1076.0
1075.4
1075.4
1011.8
1007.3
1009.9
1010.2
382.5
377.3
378.1
378.6
diPrP
Ni
Co
Cu
Zn
1563.4
1561.1
1558.3
1546.0
1374.7
1372.3
1371.7
1366.5
1077.6
1077.5
1079.3
1013.9
1008.2
1008.3
1008.0
386.1
376.6
371.1
369.1
dtBuP
Ni
Co
Cu
Zn
1548.2
1546.0
1542.6
1539.0
1371.2
1367.4
1365.0
1355.0
1079.0
1077.9
1077.9
1077.8
1022.5
1016.8
1014.4
1008.1
398.1
391.6
384.5
378.9
Discussion
Representation of Metalloporphyrin Nonplanar Distor-
tions in Terms of Displacements along the Lowest-Frequency
Normal Coordinates. The nonplanar distortions of metal-
loporphyrins can be classified according to irreducible repre-
sentations of the nominal D4h point group of a square-planar
porphyrin macrocycle.7 The commonly observed ruf and sad
distortions resemble the lowest-frequency normal coordinates
of B1u and B2u symmetry, respectively, and the dom and
occasionally observed waV distortions are similar to the lowest-
frequency normal coordinates of A2u and Eg symmetries. This
suggests that a convenient way to describe these structures is
in terms of the displacements along these lowest-frequency
normal coordinates.
The band shape of the strong structure-sensitive line ν2 also
exhibits systematic changes with the size of the central metal.
For dPP, ν2 of the Ni(II) compound is broad and asymmetric
with an asymmetric tail on the low-frequency side; ν2 of the
Co(II) derivative is less broad and less asymmetric; the ν2 lines
of the Cu(II) and Zn(II) compounds are narrow and symmetric.
For dPrP, every metalloporphyrin has a broad and asymmetric
(28) (a) Parthasarathi, N.; Hanson, C.; Yamayuchi, S.; Spiro, T. G. J. Am.
Chem. Soc. 1987, 109, 3865. (b) Strong, J. D.; Kubaska, R. J.; Shupack,
S. I.; Spiro, T. G. J. Raman Spectrosc. 1980, 9, 312. (c) Ghottard, G.;
Baccioni, P.; Baccioni, J. P.; Lange, M.; Mansuy, D. Inorg. Chem.
1981, 20, 1718.

2016 Inorganic Chemistry, Vol. 37, No. 8, 1998
Song et al.
For symmetrically substituted porphyrins with four identical
meso substituents, nonplanar distortion usually occurs along only
one of the lowest-frequency normal coordinates given a
particular configuration of the substituents. For an asymmetri-
cally substituted porphyrin or a porphyrin in an asymmetrical
environment as in a protein, the macrocycle tends to deform
along two or more of these normal coordinates.8 In this case,
the nonplanar distortion can be represented in terms of displace-
ments along a linear combination of these normal coordinates.
For a given structure, the displacements that best simulate the
distortion of the macrocycle can be determined using a recently
devised computational procedure, called normal-coordinate
structural decomposition.9
Theoretically, the complete set of normal coordinates for the
macrocycle forms a basis for describing any distortion of a
porphyrin macrocycle. For a macrocycle with D4h symmetry,
the distortions can be divided into in-plane and out-of-plane
deformations. Thus, any out-of-plane distortion of the macro-
cycle can be quantitatively represented by a linear combination
of the 21 (N - 3) unit vectors of the out-of-plane normal
coordinates (normal deformations). However, usually only a
few of the lowest-frequency modes are required to adequately
describe the distortion,9 since these are the softest modes of
deformation of the porphyrin. That is, the restoring forces are
the smallest for displacements along the lowest-frequency
normal coordinates, so displacements along the coordinates
require little energy. Since the lowest-frequency B2u and B1u
modes usually appear at lower frequency than the lowest-
frequency A2u mode, the sad and ruf distortions are observed
more frequently than dom distortion. The lowest-frequency Eg
mode is found at even higher frequency than the lowest-
frequency A2u mode, and so, the waV distortion is seen only
occasionally. The lowest-frequency mode of A1u symmetry
(pro, propellering of the pyrroles) is so high in frequency that
the pro deformation is rarely observed. (The calculated
frequencies of the modes forming the minimal basis set in the
NSD procedure are 88 cm-1 for B1u(1), 65 cm-1 for B2u(1), 335
Figure 4. Comparisons of the structures simulated using the minimal
and extended basis with the X-ray crystal structures and calculated trans-
RR conformers of copper di-tert-butylporphyrin.
out-of-plane displacements for the X-ray crystal structures of
Cu(dtBuP), Ni(diPrP), and Zn(diPrP)(py) obtained using the
minimal and extended basis sets. Clearly, the lowest-frequency
normal deformations (minimal basis) alone adequately simulate
the nonplanar distortions. Of course, the accuracy of the
simulated structures improves when the extended basis set is
used (Figure 4).
(1)
cm-1 for A1u(1), 135 cm-1 for A2u(1), and 176 cm-1 for Eg
symmetries,9 where the superscript (1) indicates the lowest-
frequency mode of the indicated symmetry.)
To describe the out-of-plane distortions more accurately, the
basis set needs to be extended to include the second-lowest-
frequency normal mode of each symmetry type. This is
especially important for the description of dom and waV
distortions where the second-lowest-frequency modes for the
A2u and Eg symmetries are close in energy to the lowest-
frequency modes. The calculated frequencies of the second-
lowest-frequency modes9 are 516 cm-1 for B1u(2) (5.9B1u(1)), 721
cm-1 for B2u(2) (11.1B2u(1)), 680 cm-1 for A1u(2) (2.0A1u(1)), 359
For the gab conformer, the out-of-plane displacements of the
macrocyclic atoms are represented by a linear combination of
the unit vectors for the lowest-frequency B1u and A2u coordi-
nates. For the minimal basis set, the deviations are mainly due
to the doming components, for which the contribution from the
second-lowest-frequency normal mode is usually 40-80% of
that of the lowest-frequency mode (Table S3 of the Supporting
Information). Second-order ruffling contributions are usually
only 4-14% of the lowest-frequency mode contributions,
reflecting a larger frequency separation between the second-
lowest- and lowest-frequency B1u modes.
The same reasoning explains why the extended basis set is
needed to accurately describe the Râ conformers. In fact, to
accurately describe the distortions of the Râ conformers the basis
set needs to be extended to the third-order normal modes. The
Râ conformer is composed of equal amounts of the waV(x) and
waV(y) Eg deformations only. That higher order normal
coordinates are needed for Eg distortion is not surprising given
that the frequency of the third-lowest-frequency Eg mode
(calculated at 467 cm-1 for the Cu porphyrin macrocycle) is
lower than most of the second-lowest-frequency normal modes
for the other symmetries.9
cm-1 for A2u (2.7A2u(1)) and 238 cm-1 for Eg (1.4Eg(1)).
Obviously, to obtain an exact description of the nonplanar
distortion it would be necessary to include all 21 of the out-
of-plane normal modes in the basis set. However, usually this
is unnecessary because the displacements along the high-
frequency modes are smaller than the experimental error in the
X-ray crystal structures.
(2)
(2)
For the series of 5,15-disubstituted metalloporphyrins, mo-
lecular mechanics calculations predict that there are stable RR
and Râ conformers as shown in Tables 3 and S1. Structural
decomposition of these conformers (Tables 4 and S3-5) verifies
that the distortions of this series of porphyrins occur primarily
along the lowest-frequency normal coordinates for all the metals.
This is clearly shown in Figures 4, S4, and S5. Figures 4, S4,
and S5 show the comparisons of the simulated and experimental
The structural decomposition results also show that the
relative contributions of different normal coordinates to the total

Distortions of Meso-Dialkyl-Substituted Porphyrins
Inorganic Chemistry, Vol. 37, No. 8, 1998 2017
distortion are metal dependent. To understand this result, the
metal dependence of the core size was studied for each normal
coordinate (Figure 3).
conformations result from the perturbations caused by the
different orientations of the substituents. Specifically, RâRâ
orientation (B1u symmetry) leads to the ruf conformation, and
the RRRR orientation (A2u symmetry) results in the dom
conformation. Because these symmetries do not belong to the
same representation in the lower-symmetry point groups of the
molecule, the ruf and dom distortions cannot mix as they do
for the RR 5,15-disubstituted porphyrins.
Dependence of the Metal-Nitrogen Distance on the
Magnitude of the Deformations. The ruf, dom, and waV
deformations have different influences on the core size (M-N
distance). Conversely, incorporation of metals of different size
in the center of the macrocycle will influence the relative
contributions of the nonplanar deformations to the total distor-
tion. The calculated dependence of the M-N distance on the
displacement along the ruf, dom, and waV(x) normal deforma-
tions is shown in Figure 3. The ruf deformation reduces the
average M-N distance, and thus the core-size, the most for a
given amount of distortion and peripheral steric strain (see
caption). The average M-N distance is used because the waV
deformation causes a significant decrease of the M-N distance
along one direction, but the M-N distance perpendicular to that
direction changes little. The sad deformation (not shown) also
reduces the core size but to a lesser degree than ruffling. The
dom deformation does not change the M-N distance ap-
preciably. The observations hold regardless of the metal. When
the equilibrium M-N distance is larger than the optimum core
size of the macrocycle (e.g., Zn), doming even causes the M-N
distance to increase.
Since both the ruf and dom deformations can occur in any
proportion in the RR conformer and since these deformations
are affected by metal size differently, one might expect the
relative amounts of these deformations to be influenced by metal
size. For the RR conformers of the 5,15-disubstituted porphy-
rins, the distortion decreases dramatically along the ruf deforma-
tion and increases along the A2u dom deformation, as the size
of the metal atom increases (Ni < Co < Cu < Zn). This metal
dependence occurs because the ruf deformation reduces the core
size (and M-N distance), while the dom deformation hardly
changes the M-N distance. Accordingly, when the central
metal is small like Ni(II), the porphyrin macrocycle prefers a
significantly shorter metal-nitrogen distance [1.85 Å for Ni-
(II)] than the optimum distance of about 2.00 Å for a planar
porphyrin. The most efficient way to shorten the M-N distance
is to ruffle the porphyrin macrocycle (Figure 3). However, this
preference for ruffling decreases as the metal size increases [Co-
(II) and Cu(II)]. When the metal is so large that it prefers a
metal-nitrogen distance larger than the optimum distance of a
planar macrocycle, for example, Zn(II) (2.07 Å), ruffling is
disfavored and doming is favored. Therefore, for the same 5,-
15-disubstituted porphyrin, the ruf contribution decreases as the
metal changes from Ni(II) to Co(II), Cu(II), and Zn(II). As
the ruf distortion decreases, the distortion along the A2u dom
normal coordinate increases slightly to help relieve the steric
strain of the substituents. The large decrease in the ruf
deformation with increasing metal size also results in an overall
decrease in the total distortion for a given porphyrin (dPrP,
diPrP, or dtBuP). This trend can be seen in Table 4 and Table
S3 of the Supporting Information.
On the basis of these observations, a small metal such as
nickel(II) favors ruffling the most and saddling less and disfavors
doming the most and waving less; a large metal favors doming
the most and disfavors ruffling the most.
Nonplanar Distortions of Metal Complexes of the 5,15-
Disubstituted Metalloporphyrins. The results of normal-
coordinate structural decomposition show that the gabled RR
conformers consist mainly of B1u ruf and A2u dom symmetric
deformations (Table 4 and Table S3 of the Supporting Informa-
tion). The Râ conformers are a mixture of waV(x) and waV(y)
deformations (Tables S4 and S5 of the Supporting Information).
These structural findings can be understood qualitatively in terms
of symmetry, with the aid of a correlation table for the D4h
group. The RR conformer has C2V (C2, σd) symmetry. Among
the five out-of-plane symmetries in the D4h point group, the
A2u and B1u representations are totally symmetric (transform
like A1) in the C2V group. Consequently, the A2u and B1u
symmetries may mix in the lower symmetry in any combination.
Similarly, the Râ conformer has C2h (C′′2) symmetry, and only
the Eg representations are totally symmetric (transform like Ag)
in the C2h group. In this case, the Egx and Egy deformations
must mix in equal proportions.
Additional symmetries may contribute when the symmetry
is further lowered by, for example, the different relative
orientations of asymmetric meso substituents. In particular,
because of the asymmetry of the isopropyl groups, the molecular
symmetry of the trans-RR conformer of diPrP is lowered from
the C2V group to the C2 (C2) group. Moreover, in the C2 group,
in addition to the A2u and B1u, the B2u and A1u symmetries are
also totally symmetric (transform like A). Thus, the sad and
pro distortions are allowed to mix with ruf and dom in the lower
symmetry. This then explains the small sad contributions in
addition to the main ruf and dom contributions for the distortions
of trans-RR-diPrPs as shown in Table 4. The distortion along
the A1u coordinate is zero because the perturbation energy is
too small to produce a measurable distortion of this symmetry
type.
For the metal 5,15-disubstituted porphyrins, the degree of
nonplanarity (total displacements in Table 4 and Table S3 of
the Supporting Information) increases as the substituents become
more bulky (phenyl < propyl < isopropyl < tert-butyl) as a
way of relieving the increasing steric repulsion of the substit-
uents. Furthermore, the contribution from the ruf distortion is
always larger than the contribution of the dom contribution, even
though any degree of mixing is symmetry-allowed. The larger
contribution from the ruf distortion is reasonable because the
lowest-frequency mode of B1u symmetry (at 88 cm-1) is softer
than that of A2u symmetry (135 cm-1).
As noted, the ratio of dom to ruf deformations increases as
the steric bulk of the substituents increases, a trend shown clearly
in Figure 5, which plots the dom/ruf ratio against the total
distortion. This trend can be understood in terms of how easily
and how efficiently that steric interaction between the substit-
uents and the adjacent pyrrole rings can be relieved. For the
RR 5,15-disubstituted porphyrins, the two symmetry-allowed
distortions of the macrocycle, B1u ruf and A2u dom, can mix in
any ratio. For small distortions from planarity, the easiest way
to relieve the steric interactions for a meso-alkyl porphyrin is
to distort along the lowest-frequency normal coordinate of B1u
(ruf) symmetry. It is a more efficient method of relieving the
substituent crowding than doming because it moves the meso
carbons and their substituents out of the plane of the macrocycle
and away from the Câ atoms of the pyrrole. Thus ruffling is
Relative Contributions of Symmetric Distortions for the
5,15-Disubstituted Metalloporphyrins. In the previously
studied meso-tetrasubstituted porphyrins,7 the ruf and dom

2018 Inorganic Chemistry, Vol. 37, No. 8, 1998
Song et al.
Figure 6. Frequency of the structure-sensitive Raman line ν2 as a
function of metal-nitrogen distance for the series of disubstituted
porphyrins.
Figure 5. The calculated dom/ruf deformation ratio of the RR
conformer as a function of total distortion for the series of disubstituted
porphyrins.
and Zn complexes of the 5,15-disubstituted porphyrins. As the
porphyrin substituents become more bulky, ν2 shifts to lower
frequency, a trend expected for an increase in macrocycle
nonplanarity.7,8,11a,29
It has been previously noted that the slope of the correlation
between frequency and core size decreases as the porphyrin ring
becomes more nonplanar.11a The results for the disubstituted
porphyrins confirm this observation. The slope of the downshift
with metal-nitrogen distance becomes smaller as the bulkiness
of the substituents increases and the porphyrin becomes more
nonplanar (Figure 6).
The correlation between the frequency of ν2 and the total
nonplanar distortion is shown in Figure S6. This nonlinear
dependence has been observed previously.8 The data shows
that the Raman frequencies depend weakly on nonplanar
distortion for small deviations from planarity (<1 Å), but vary
rapidly for large deviations from planarity (>1 Å).
In some cases, the molecular mechanics calculations predict
small energy differences between different conformations of the
same metalloporphyrin. The coexistence of multiple conformers
can be detected by resonance Raman spectroscopy.30 One type
of structural heterogeneity is the coexistence of both planar and
nonplanar forms which occurs for porphyrins with small
substituents and small metals. The heterogeneity is a result of
the tradeoff between enhanced conjugation in a planar macro-
cycle and optimal short M-N bond formation in the nonplanar
porphyrin. This is the situation in Ni(dPP), where a planar and
a gab conformer with similar energies are predicted. However,
heterogeneity is not predicted for the other metal complexes
for which only planar structures are predicted. The resonance
Raman spectra of the dPP complexes (Figures S1a and S2a of
the Supporting Information) are consistent with these calcula-
tions. For Ni(dPP), the ν2 and ν8 lines are broad and
asymmetric, but the lines become less broad and asymmetric
for Co and narrow and symmetric for Cu and Zn.
the predominant contribution to the total distortion for small
distortions. However, as the porphyrin becomes more ruffled
because of the increasing bulkiness of the substituents, the steric
strain in the macrocycle is unnecessarily large. This is because
the meso carbons that lack the tert-butyl groups are also
displaced from the mean plane by the ruf distortion. However,
by doming, this unnecessary steric strain is relieved, because
doming brings the 10- and 20-carbons into a more coplanar
configuration with their adjacent pyrrole rings while moving
the substituted meso carbons further out of plane.
Comparison of Calculated Structures with the Structural
Information Obtained from X-ray Crystallography and
Resonance Raman Spectroscopy. The correctness and ac-
curacy of the conformations predicted by molecular mechanics
calculations for the 5,15-disubstituted porphyrin were confirmed
by crystallographic data. For example, the agreement between
the calculated structure and the crystal structures of Cu(dtBuP)
is excellent (Figure 2). The predicted increases in total distortion
and in the dom/ruf ratio as the substituents become bulkier are
also confirmed by the crystal structures. For example, the
observed dom/ruf ratios for the crystal structures of Ni(dPP)
(0.08), Ni(diPrP) (0.14), and NidtBuP (0.22) are in reasonable
agreement with the calculated ratios of 0.02, 0.14, and 0.27,
respectively. In addition, the predicted trend of large metals
favoring doming over ruffling is also confirmed by the X-ray
structures of Ni(dtBuP) and Cu(dtBuP). The dom/ruf distortion
ratio is 0.22 for Ni(dtBuP) and 0.38 for Cu(dtBuP), while the
corresponding calculated ratios are 0.27 and 0.39 (Table 4). In
addition, the dom/ruf distortion ratio for the crystal structure of
Zn(diPrP)(py) is large [2.7 for trans-Zn(diPrP)(py) and 0.41 for
cis-Zn(diPrP)(py)] in comparison with Ni(diPrP) (0.14). The
large values for Zn are partially due to increased metal size
and partially due to the coordinated fifth ligand for the Zn
complex. Ligation typically favors doming over ruffling for
five-coordinate porphyrins.
(29) Czernuszewicz, R. S.; Li, X.-Y.; Spiro, T. G. J. Am. Chem. Soc. 1989,
111, 7024.
(30) (a) Alden, R. G.; Crawford, B. A.; Doolen, R.; Ondrias, M. R.;
Shelnutt, J. A. J. Am. Chem. Soc. 1989, 111, 2070. (b) Anderson, K.
K.; Hobbs, J. D.; Luo, L.; Stanley, K. D.; Quirke, J. M. E.; Shelnutt,
J. A. J. Am. Chem. Soc. 1993, 115, 12346. (c) Jentzen, W. Unger, E.;
Karvounis, G.; Shelnutt, J. A.; Dreybrodt, W.; Schweitzer-Stenner,
R. J. Phys. Chem. 1996, 100, 14184.
The predicted increase in nonplanarity for bulkier substituents
was further confirmed by shifts to lower frequency for structure-
sensitive Raman lines. Figure 6 shows the dependence of the
structure-sensitive line ν2 on the M-N distance for Ni, Co, Cu,

Distortions of Meso-Dialkyl-Substituted Porphyrins
Inorganic Chemistry, Vol. 37, No. 8, 1998 2019
A second type of structural heterogeneity arises when different
orientations of the peripheral substituents give rise to multiple
stable conformers of the macrocycle with nearly the same
energy. This is the case with the dPrP complexes, for which
six stable conformers with similar energies are predicted. This
prediction is confirmed by the generally broad and asymmetric
shape of ν2 and ν8 lines in the Raman spectra of all the dPrP
metal complexes (Figures S1b and S2b of the Supporting
Information).
metal and substituents. A large metal favors doming over
ruffling, reducing the ruffling contribution significantly and
increasing the doming contribution slightly. This leads to an
overall reduction of degree of nonplanar distortion for large
metals. This is explicitly shown by normal-coordinate structural
decomposition of the calculated and X-ray structures. Increasing
the bulkiness of substituents results in an increasing degree of
nonplanar distortion, causing a proportional decrease in the
frequencies of the structure-sensitive Raman lines.
For the diPrP complexes, there are four stable conformers
(two RR and two Râ) predicted for each metal. However, only
the two structurally similar RR conformers are predicted to
coexist in solution for Ni, whereas all four conformers may exist
in solution for Co, Cu, or Zn (see Results). The Raman results
again agree with these predictions, since the Raman spectra of
Ni(diPrP) show almost symmetric ν2 and ν8 lines, and Co(diPrP),
Cu(diPrP), and possibly Zn(diPrP) have asymmetric ν2 and ν8
lines (Figures S1c and S2c of the Supporting Information). In
the case of the dtBuP complexes, the energies of the calculated
structures suggest that irrespective of the size of the central metal
atom only the RR conformer will be appreciably populated in
solution. In agreement with this prediction, the Raman spectra
of the dtBuP complexes (Figure S2d of the Supporting Informa-
tion) show a narrow and symmetric ν8 for all metals.
Finally, it should be noted that the apparent doublet seen for
the ν2 line of the dtBuP complexes (Figure S1d of the
Supporting Information) is not caused by conformational
heterogeneity but by Fermi resonance of ν2 with another line
in the vicinity of ν2. This was confirmed by preparation of the
NidtBuP in which the â-pyrrole positions were selectively
deuterated.8
Acknowledgment. We thank Dr. J. David Hobbs for helpful
discussions and reading of the manuscript.
Supporting Information Available: Selected structural parameters
for the calculated Râ-conformers of the 5,15-disubstituted porphyrins
(Table S1), the energies of the energy-optimized conformers of the
5,15-disubstituted porphyrins (Table S2), the normal-coordinate struc-
tural decomposition results for the RR-conformers using the extended
basis (Table S3), the normal-coordinate structural decompositions of
the Râ-conformers using minimal and extended basis (Tables S4 and
S5, respectively), the resonance Raman spectra for the series of 5,15-
disubstituted metalloporphyrins in the high-, low-, and middle-frequency
regions (Figures S1-3, respectively), comparison of the best fits of
the out-of-plane displacements obtained from the X-ray crystal structure
and calculated RR-conformer of Ni(diPrP) using the minimal and
extended basis (Figure S4), comparison of the best fits of the out-of-
plane displacements obtained from the X-ray crystal structures of trans-
and cis-Zn(diPrP)(py) (Figure S5), the correlation between the frequency
of ν2 and the total nonplanar distortion for the series of disubstituted
metalloporphyrins (Figure S6), the crystal structure of Ni 5,15-
diisopropylporphyrin (Figure 57) and its X-ray determination, data
collection, and refinement parameters, atomic coordinates, isotropic
displacement parameters, bond lengths and angles, and anisotropic
displacement parameters (Tables S6-13), the crystal structure of Cu
5,15-di-tert-butylporphyrin (Figure S8) and its X-ray determination,
data collection, and refinement parameters, atomic coordinates, isotropic
displacement parameters, bond lengths and angles, and anisotropic
displacement parameters (Tables S1-21), and the two molecules in
the X-ray structure of Zn(py) 5,15-diisopropylporphyrin (Figure S9)
and its X-ray determination, data collection, and refinement parameters,
atomic coordinates, isotropic displacement parameters, bond lengths
and angles, and anisotropic displacement parameters (Tables S22-29)
(54 pages). Ordering information is given on any current masthead
page.
Conclusions
The sterically induced nonplanar gab distortion of the
macrocycle for the series of 5,15-disubstituted porphyrins is a
variable combination of ruf and dom deformations. It can be
accurately represented in terms of displacements along the
lowest-frequency normal coordinates of B1u and A2u symmetry.
The next-higher-frequency normal coordinates of B1u and A2u
symmetry are needed to precisely represent the gab distortion.
The absolute and relative contributions of the normal coordinate
deformations to the gab distortion depend on the sizes of the
IC970465Z