2,3,7,8,12,13,17,18-Octafluoro-5,10,15,20-tetraarylporphyrins and Their Zinc Complexes: First Spectroscopic, Electrochemical, and Structural Characterization of a Perfluorinated Tetraarylmetalloporphyrin

Article abstract of DOI:10.1021/jo962159t

We report a convenient and general synthesis of β-octafluoroporphyrins bearing meso-tetraaryl substituents, including the first synthesis and full characterization of a perfluorinated tetraarylporphyrin, perfluoro-5,10,15,20-tetraphenylporphyrin, 2. The structural, spectroscopic, and electrochemical data indicate that β-octafluoro-meso-tetraarylporphyrins are a new class of planar, electron-deficient ligands. Particularly impressive is the 0.5 V window over which the formal oxidation potential can be tuned using only aryl substituents. The invariance of the ligand structure with increasingly positive formal oxidation potential is a key advance; electronic effects have been severed from the nonplanar conformations exhibited by all other highly electron-deficient porphyrins.

Full text of DOI:10.1021/jo962159t

1588
J . Org. Chem. 1997, 62, 1588-1593
Articles
2,3,7,8,12,13,17,18-Octa flu or o-5,10,15,20-tetr a a r ylp or p h yr in s a n d
Th eir Zin c Com p lexes: F ir st Sp ectr oscop ic, Electr och em ica l, a n d
Str u ctu r a l Ch a r a cter iza tion of a P er flu or in a ted
Tetr a a r ylm eta llop or p h yr in
Eric K. Woller and Stephen G. DiMagno*
Department of Chemistry, University of NebraskasLincoln, Lincoln, Nebraska 68588-0304
Received November 19, 1996X
We report a convenient and general synthesis of â-octafluoroporphyrins bearing meso-tetraaryl
substituents, including the first synthesis and full characterization of a perfluorinated tetraarylpor-
phyrin, perfluoro-5,10,15,20-tetraphenylporphyrin, 2. The structural, spectroscopic, and electro-
chemical data indicate that â-octafluoro-meso-tetraarylporphyrins are a new class of planar, electron-
deficient ligands. Particularly impressive is the 0.5 V window over which the formal oxidation
potential can be tuned using only aryl substituents. The invariance of the ligand structure with
increasingly positive formal oxidation potential is a key advance; electronic effects have been severed
from the nonplanar conformations exhibited by all other highly electron-deficient porphyrins.
Halogenated metalloporphyrins are well-established
characteristic of the σ-framework and is quite insensitive
to porphyrin conformation,19,20 the magnitude of the
HOMO-LUMO gap and the redox potentials are inti-
mately tied to the porphyrin core structure. Eduction of
the relative importance of electronic and steric param-
eters requires electron-deficient porphyrins in which the
steric and σ- and π-electronic effects are varied indepen-
dently; this is currently impossible with the limited
number of structurally similar ligands available.
The recent synthesis of 5,10,15,20-tetrakis(heptafluo-
ropropyl)porphyrin (THFPP),21-23 a ruffled porphyrin
displaying an optical spectrum typical of a planar mac-
rocycle, demonstrated that π-electronic properties in
other nonplanar electron-deficient porphyrins are inter-
twined with substituent effects. The spectroscopic fea-
tures of THFPP strongly suggest that increased conju-
gation of aryl groups is responsible for the bathochromic
shifts and concomitant destabilization of the porphyrin
HOMO observed for nonplanar â-octabromo- and â-oc-
tachlorotetraarylporphyrins.23
catalysts for alkane hydroxylation and alkene epoxi-
dation,1-10 and â-octahalo-meso-tetraarylmetalloporphy-
rins show substantial catalytic activity and reasonable
longevity in isobutane hydroxylation.11-13 â-Octahalo-
genation (Cl, Br) of tetraarylporphyrins causes decreased
nitrogen basicity due to σ-electron withdrawal, positive
shifts in the porphyrin ring redox potentials, narrowing
of the porphyrin HOMO-LUMO gap, and highly non-
planar porphyrin core structures arising from steric
encumbrance at the ligand periphery.14-17 In addition
to stabilizing the porphyrin against oxidation, â-halo-
genation tunes the relative and absolute d-orbital ener-
gies of the ligated transition metal (and the complex’s
catalytic competency) through an admixture of σ, π, and
steric effects.18 While nitrogen basicity is essentially a
X Abstract published in Advance ACS Abstracts, February 15, 1997.
(1) Chang, D. K.; Ebina, F. J . Chem. Soc., Chem. Commun. 1981,
778.
(2) Hoffman, P.; Robert, A.; Meunier, B. Bull. Soc. Chim. Fr. 1992,
129, 85.
(3) Traylor, P. S.; Dolphin, D.; Traylor, T. G. J . Chem. Soc., Chem.
Commun. 1984, 279.
(4) Traylor, T. G.; Tsuchiya, S. Inorg. Chem. 1987, 26, 1338.
(5) Traylor, T. G.; Hill, K. W.; Fann, W.-P.; Tsuchiya, S.; Dunlap,
B. E. J . Am. Chem. Soc. 1992, 114, 1308.
The small radius and high electronegativity of fluorine
imply that â-perfluorinated metalloporphyrins should be
the first examples of planar, highly electron-deficient
porphyrins; AM1 semiempirical calculations of
2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylpor-
phyrin, 1(Zn ), predict a nearly planar macrocycle with
essentially orthogonal phenyl rings.24 Such a structure
(6) Gonsalves, A. M. d. A. R.; J ohnstone, R. W. W.; Pereira, M. M.;
Shaw, J .; Sobral, A. J . F. d. N. Tetrahedron Lett. 1991, 32, 1355.
(7) Mansuy, D. Coord. Chem. Rev. 1993, 125, 129.
(8) Meunier, B. Chem. Rev. 1992, 92, 1411.
(9) Fujii, H. J . Am. Chem. Soc. 1993, 115, 4641.
(10) Wijesekera, T. P.; Lyons, J . E.; Ellis, P. E., J r. Catal. Lett. 1996,
36, 69.
(11) Ellis, P. E., J r.; Lyons, J . W. Coord. Chem. Rev. 1990, 105, 181.
(12) Ellis, P. E., J r.; Lyons, J . E. Catal. Lett. 1989, 3, 389.
(13) Grinstaff, M. W.; Hill, M. G.; Labinger, J . A.; Gray, H. B. Science
1994, 264, 1311.
(14) Ochsenbein, P.; Ayougou, K.; Mandon, D.; Fischer, J .; Weiss,
R.; Austin, R. N.; J ayaraj, K.; Gold, A.; Terner, J .; Fajer, J . Angew.
Chem., Int. Ed. Engl. 1994, 33, 348.
(19) Gassman, P. G.; Ghosh, A.; Almlof, J . J . Am. Chem. Soc. 1992,
114, 9990.
(20) Ghosh, A. J . Am. Chem. Soc. 1995, 117, 4691.
(21) DiMagno, S. G.; Williams, R. A.; Therien, M. J . J . Org. Chem.
1994, 59, 6943.
(22) Goll, J . G., Moore, K. T., Ghosh, A.; Therien, M. J . J . Am. Chem.
Soc. 1996, 118, 8344.
(23) DiMagno, S. G.; Wertsching, A. K.; Ross, C. R., II. J . Am. Chem.
Soc. 1995, 117, 8279.
(15) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239.
(16) Wijeskera, T.; Matsumoto, A.; Dolphin, D.; Lexa, D. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1028.
(17) Birnbaum, E. R.; Schaefer, W. P.; Labinger, J . A.; Bercaw, J .
E.; Gray, H. B. Inorg. Chem. 1995, 34, 1751.
(18) Grinstaff, M. W.; Hill, M. F.; Birnbaum, E. R.; Schaefer, W. P.;
Labinger, J . A.; Gray, H. B. Inorg. Chem. 1995, 34, 4896.
(24) Results from the AM1 semiempirical calculations (Dewar, M.
J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P. J . Am. Chem. Soc.
1985, 107, 3902) are included in the Supporting Information. These
calculations predict a planar macrocycle for 1(Zn ). The free base is
predicted to have a shallow potential energy surface on which the
planar structure is a local minimum approximately 3 kcal/mol above
the global minimum.
S0022-3263(96)02159-7 CCC: $14.00 © 1997 American Chemical Society

Characterization of a Perfluorinated Tetraarylmetalloporphyrin
J . Org. Chem., Vol. 62, No. 6, 1997 1589
Ta ble 1. Sp ectr oscop y of F r ee Ba se P or p h yr in s
Soret bandb
λmax (nm)
B(0,0)-Q(0,0)
NH Shiftd
(ppm)
compda
Q bands λmax (nm)
splittingc (cm-1)
ref
1
2
3
P
402
392
406
394
377
418
412
404.5
436
454
499, 532, 581, 637
493, 579, 632
7630
8210
7390
7590
9000
7080
7300f
7820
7070f
6610f
-4.18
-4.23
-4.14
-3.93
-3.76
-2.79
-2.91
-2.30
-1.0
this work
this work
this work
this work
38
this work
30
22
500, 533, 581, 636
489, 519, 561, 613
494, 530, 564, 618
514, 549, 590, 646
506, 584, 638
510, 545, 594, 647
536, 622
552, 636
OMPe
TPP
TPFPP
THFPP
OCTPFPP
OBTPFPP
30
30
-0.5
a
Abbreviations: P, porphine; OMP, 2,3,7,8,12,13,17,18-octamethoxyporphyrin; TPP, 5,10,15,20-tetraphenylporphyrin; TPFPP, 5,10,15,20-
tetrakis(perfluorophenyl)porphyrin; THFPP, 5,10,15,20-tetrakis(heptafluoropropyl)porphyrin; OCTPFPP, 2,3,7,8,12,13,17,18-octachloro-
5,10,15,20-tetrakis(perfluorophenyl)porphyrin; OBTPFPP, 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(perfluorophenyl)porphyrin.
b
Absorption spectra of the free bases were recorded in CH2Cl2 unless otherwise noted. c Calculated from B(0,0) - [(Qx(0,0) + Qy(0,0))/2].
d
NMR spectra were recorded in CDCl3. e The optical spectrum was recorded in toluene. f The energies of the unreported Q bands were
estimated using a vibrational spacing of 1500 cm-1
.
is consistent with an optical spectrum hypsochromically
shifted in comparison to TPP. In stark contrast to this
prediction, a putative synthesis of 2,3,7,8,12,13,17,18-
octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)por-
phyrin, 2, by direct fluorination produced a compound
possessing a distinctly red-shifted absorption spectrum
(Soret band at 430 nm) characteristic of a highly non-
planar macrocycle.25 Several reports of metalated per-
fluorinated porphyrins appear in the patent literature;
however, no characterization data for these compounds
have been published.26
Herein, we report a convenient and general synthesis
of â-octafluoroporphyrins bearing meso tetraaryl substit-
uents, including the synthesis, characterization, and
X-ray structure determination of a perfluorinated met-
alloporphyrin, 2(Zn ). The spectroscopic data for the
newly synthesized compounds indicate that the previous
report was erroneous and that the structures of â-oc-
tafluoroporphyrins are correctly predicted by semiem-
pirical molecular orbital theory.
when unsubstituted pyrrole was condensed with the
same aldehydes; 33%, 21%, and 20% for 1, 2, and 3,
respectively.29 The preparation of 1 could also be scaled
up (10 mmol each of pyrrole and benzaldehyde, 500 mL
CH2Cl2) with no loss in yield. The use of trifluoroacetic
acid as the catalyst resulted in precipitously reduced
yields of porphyrin and slower reaction rates. Sparingly
soluble 1 was isolated by recrystallization from toluene/
pentane, while chromatographic workup was performed
for the fairly soluble compounds 2 and 3. Interestingly,
the perfluorinated derivative, 2, demonstrated enhanced
volatility in comparison to TPP or 1, commensurate with
the reduced London dispersion forces characteristic of
perfluorocarbons. Sublimation of 2 to yield thin films
could be performed under moderate vacuum.
Compound 3 was synthesized to examine the extent
of atropisomerism and the rate of aryl rotation in
octafluorotetraarylporphyrins. Three isomers of 3 with
identical optical spectra were chromatographically sepa-
rated. No interconversion was observed, signaling very
slow aryl rotation in these compounds. This steric effect
mirrors that in metalated tetrakis(perfluorophenyl)por-
phyrin, a compound in which slow aryl rotation has been
demonstrated.30 Surprisingly, one isomer of 3 accounted
for more than 95% of the isolated product. Coordination
of the methoxy groups to the excess BF3 etherate at the
porphyrinogen stage may be the basis for this unusual
selectivity.
Deprotonation of 1 and 2 with an excess of the
hindered, non-nucleophilic base 1,8-diazabicyclo[5.4.0]-
undec-7-ene (DBU) yields bluish-green monoanions. Spec-
trophotometric determinations in CH2Cl2 show that the
NH in 1 is less acidic than the protonated DBU by 3.9
pKa units. In contrast, no deprotonation of TPP was
observed with DBU up to the highest base concentration
examined, 0.3 M; thus, 1 is at least 1000 times more
acidic than TPP in CH2Cl2. The dramatic σ-electron-
withdrawing effect of pentafluorophenyl substituents is
evident in the pKa of 2. The porphyrin NH is more acidic
than protonated DBU by 0.2 pKa units; even treatment
of 2 with triethylamine results in a visible color change.
1H NMR spectroscopy of the free bases reveals that the
NH resonances of 1-3 are shifted upfield in comparison
to other benchmark porphyrins (Table 1). This chemical
Resu lts a n d Discu ssion
Octafluoroporphyrins were prepared by the acid-
catalyzed condensation of 3,4-difluoropyrrole27 and the
corresponding benzaldehyde, followed by oxidation. The
synthetic procedures generally followed those outlined by
Lindsey,28 except that a greater than stoichiometric
amount of BF3 etherate “catalyst” was employed and the
reactions were quenched with DDQ after only 0.5-1 h.
The yields of porphyrins were similar to those obtained
(25) Tsuchiya, S.; Seno, M. Chem. Lett. 1989, 263.
(26) Lyons, J . E., Ellis, P. E., J r. U.S. Patent 5,120,886, 1992. Ellis,
P. E., J r.; Lyons, J . E. U.S. Patent 4,970,348, 1990.
(29) Typical isolated yields for TPP and TPFTPP are 36% and 15%,
respectively, in our hands.
(27) Leroy, J .; Wakselman, C. Tetrahedron Lett. 1992, 35, 8605.
(28) Lindsey, J . S.; Schrieman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J . Org. Chem. 1987, 52, 827.
(30) Birnbaum, E. R.; Hodge, J . A.; Grinstaff, M. W.; Schaefer, W.
P.; Henling, L.; Labinger, J . A.; Gray, H. B. Inorg. Chem. 1995, 34,
3625.

1590 J . Org. Chem., Vol. 62, No. 6, 1997
Woller and DiMagno
Ta ble 2. Aver a ge Bon d Len gth s (Å) a n d Bon d An gles
(d eg) for 2(Zn ) a n d Selected Meta llop or p h yr in s
ref core
of least
parameter strain 2(Zn ) Zn(TPP)(THF)2 Zn(TPyP)(py) Zn(TPP)
N-Ct
2.010 2.051
2.010 2.059
1.384 1.382
1.446 1.439
1.355 1.337
1.395 1.397
105.4 107.4
110.3 108.3
2.057
2.057
1.371
1.444
1.349
1.403
107.0
109.3
125.8
107.2
125.0
42
2.047
2.073
1.369
1.447
1.355
1.406
106.6
109.8
125.7
106.9
125.2
43
2.036
2.036
1.375
1.443
1.351
1.399
106.5
109.6
125.7
107.2
125.0
44
N-Zn
N-Ca
Ca-Cb
Cb-Cb
Ca-Cm
Ca-N-Ca
N-Ca-Cb
N-Ca-Cm 125.4 125.5
Ca-Cb-Cb 107.0 108.0
Ca-Cm-Ca 124.6 126.0
ref
41
this
work
F igu r e 1. Perspective drawing of 2(Zn ) showing numbering
scheme. The axial ligand in the five-coordinate structure has
been removed for clarity.
involved.37 If the frontier orbitals are delocalized onto
peripheral substituents, the overall electron interaction
is smaller. Compounds 1 and 3 possess extremely large
B(0,0)-Q(0,0) energy differences for meso-tetrasubsti-
tuted porphyrins, rivaled only by a comparable separa-
tion in meso-(perfluoroalkyl)porphyrins. The energy
difference in 2 is the largest ever reported for a meso
tetrasubstituted porphyrin and is surpassed only by the
extremely hypsochromically shifted â-octaalkoxyporphy-
rins.38,39 Consistent with the 1H NMR data, the aryl
groups in 1-3 are predicted to be nearly orthogonal in
this analysis because delocalization of electron density
onto the meso-substituents would decrease the B(0,0)-
Q(0,0) separation. The absorption spectrum of 2 also
shows vanishingly small Qx(0,0) and Qy(0,0) bands,
characteristic of a free base porphyrin with energetically
degenerate HOMOs.36
A perspective drawing from the X-ray crystal structure
determination of 2(Zn ) is shown in Figure 1.40 Average
bond lengths and angles for 2(Zn ) and selected porphy-
rins are given in Table 2.41-44 The zinc complex is five
coordinate with split occupancy (acetonitrile (70%) or
THF (30%)) of the axial site in the crystal examined.
Mixed solvation appears to be a common, albeit curious,
structural motif for zinc porphyrins bearing pentafluo-
rophenyl groups.45 The four nitrogen atoms form a plane
(0.006 Å average deviation) from which the zinc atom is
displaced 0.173 Å toward the pendant axial ligand. This
displacement is somewhat smaller than is typical for five-
coordinate zinc porphyrins. The zinc-nitrogen bond
lengths average 2.059 Å and are within the reported
range (2.052-2.076 Å).42 Thus, 2(Zn ) possesses a mod-
estly expanded porphyrin core (2.052 Å N-Ct distance)
for a five coordinate zinc complex, but it is smaller than
that observed for six coordinate TPP(Zn)‚(THF)2.42 The
core expansion results from widening of the Ca-CN-Cb,
shift is typical of highly symmetrical, planar porphyrins
without conjugating meso substituents, indicating a
solution structure in which the aryl groups are essentially
orthogonal. Also evident from Table 1 is that nonplanar
distortions decrease the ring current; this effect is ampli-
fied in â-octahalogenated (Cl and Br) tetraarylporphyrins
because increased conjugation of the phenyl groups with
the central ring is possible in the twisted conformation,
thereby making the porphyrin π-orbitals more diffuse.
Variable-temperature 19F spectroscopy in CDCl3 shows
two well-resolved porphyrin signals at low temperature
(-50 °C, 470 MHz) for 1-3. These signals coalesce at
approximately 40 °C; thus, the NH proton exchange is
occurring in each case with an approximate ∆Gq ) 13
kcal/mol. Proton transfer is a degenerate process in
symmetrical porphyrins; thus, the rate should be halved
in order to obtain the true barrier height for the process.31
However, we have reported the observed (raw) free
energy of activation to simplify direct comparison with
the earlier literature. The larger free energy of activation
compared to the analogous process in TPP32 is consonant
with decreased basicity of the nitrogen sp2 orbitals and
a contraction of the NH bond in this innersphere proton
transfer. (Expansion of the porphyrin core is also a
plausible explanation for the higher activation barrier.)
1
Variable-temperature H (500 MHz) and 19F (470 MHz)
spectroscopy of 3 revealed no additional exchange pro-
cesses over the temperature range -50 to 50 °C.
The absorption spectra of 1-3 show large hypsochro-
mic shifts in comparison to other tetraarylporphyrins,
particularly in the Soret (B) band region of the spectra
(Table 1). Also cataloged in the table are B(0,0)-Q(0,0)
energy differences for selected porphyrins. The B and Q
bands arise from the electron interaction between two
pairs of nearly degenerate lowest excited singlets: the
1
1
x-polarized (b1fc2) and (b2fc1) states and the y-polar-
ized (b1fc1) and (b2fc2) states.33,34 Two factors deter-
mine the extent of the electron interaction and, thus, the
magnitude of the B(0,0)-Q(0,0) splitting: the degeneracy
of the states35,36 and the compactness of the orbitals
1
1
(37) The interaction between any two excited singlet configurations,
1
1øik|H| øjl ) 2 ΨjΨk|g|ΨlΨi
-
ΨjΨk|g|ΨiΨl , depends upon the
interelectronic distances (g ) 1/r). Mataga, N.; Kubota, T. Molecular
Interactions and Electronic Spectra; Marcel Dekker: New York, 1970.
(38) Merz, A.; Schropp, R.; Lex, J . Angew. Chem., Int. Ed. Engl.
1993, 32, 291.
(39) Merz, A.; Schropp, R.; Do¨tterl, E. Synthesis 1995, 795.
(40) The atomic coordinates for this structure have been deposited
with the Cambridge Crystallographic Data Centre. The coordinates
can be obtained, on request, from the Director, Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.
(41) Hoard, J . L. Ann. N. Y. Acad. Sci. 1973, 206, 18.
(42) Schauer, C. K.; Anderson, O. P.; Eaton, S. R.; Eaton, G. R. Inorg.
Chem. 1985, 24, 4082.
(31) Crossley, M. J .; Field, L. D.; Harding, M. M.; Sternhell, S. J .
Am. Chem. Soc. 1987, 109, 2335.
(32) Eaton, S. S.; Eaton, G. R. J . Am. Chem. Soc. 1977, 99, 1601.
Values for proton tautomerism in tetraaryl porphyrins are in the range
11.9-12.5 kcal/mol.
(33) Gouterman, M. J . Mol. Spectrosc. 1961, 6, 138.
(34) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. III, p 1.
(43) Collins, D. M.; Hoard, J . L. J . Am. Chem. Soc. 1970, 92, 3761.
(44) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978,
17, 706.
(35) Binstead, R. A.; Crossley, M. J .; Hush, N. S. Inorg. Chem. 1991,
30, 1259.
(36) Spellane, P. J .; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C.
Inorg. Chem. 1980, 19, 386.
(45) Marsh, R. E.; Schaefer, W. P.; Hodge, J . A.; Hughes, M. E.; Gray,
H. B. Acta Crystallogr. 1993, C49, 1339.

Characterization of a Perfluorinated Tetraarylmetalloporphyrin
J . Org. Chem., Vol. 62, No. 6, 1997 1591
Ta ble 3. Sp ectr oscop ic a n d Electr och em ica l Da ta a for Zin c P or p h yr in s in CH2Cl2
B and Q bands
λmax (nm)
B(0,0)-Q(0,0)
splittingc (cm-1
compdb
1(Zn)
)
E+2/+1
E+1/0
E0/-1
E-1/-2
406, 502, 536, 573
407, 496, 537, 568(s)
419, 548, 586
412, 543, 578
442, 576
7180
6960
6800
6970
6560
6180
1.36
d
1.16
1.58
1.57
1.53
1.26
1.70e
0.80
1.37
1.63
1.57
-1.03
-0.63
-1.33
-0.95
-0.47
-0.48
d
2(Zn)
-1.04
-1.66
-1.37
-0.75
-0.76
TPP(Zn)f,g
TPFPP(Zn)g
OCTPFPP(Zn)g
OBTPFPP(Zn)g
455, 585
a
Cyclic voltammetry conditions: porphyrin concentration, 1 mmol; sweep rate, 50 mV/s; 0.1 M TBAPF6; fc/fc+ internal standard (0.48
V); Ag/AgCl reference. b Abbreviations are the same as in Table 1. c Calculated from B(0,0)-Q(0,0). Unreported Q(0,0) bands were estimated
by assuming a vibrational spacing of 1300 cm-1
.
Irreversible process. e Oxidation of the porphyrin occurs at the limit of the solvent.
d
f Our measurements are in accord with the literature data (ref 52). Data from the last four table entries are from ref 52.
g
Ca-Cm-Ca, and Cb-Ca-Cm bond angles and compression
of the N-Ca-Cb bond angle, in accord with the general
trends delineated by Hoard.41 Although the bond angles
vary in a typical manner for radially expanded metal-
loporphyrins, the alterations in pyrrole bond lengths are
reversed. Whereas the Cb-Cb bond is normally observed
to lengthen as result of radial core expansion, for 2(Zn )
the bond length is quite short. Similarly, the Ca-Cb bond
is compressed in 2(Zn ) while the N-Ca bond length
resembles that of Hoard’s reference porphyrin of least
strain.41 The strong σ-electron-withdrawing effect of
fluorine is responsible for the localized contraction of the
Cb-Cb and Ca-Cb bonds, while lone pair donation into
the delocalized porphyrin π* orbitals partially compen-
sates for this effect. Thus, bond lengths approach the
normal values distant from the site of fluorine substitu-
tion.
nonbonded distances less than 2.8 Å; thus, the contact
distances in this case are short but not exceptional.
Finally, it should be noted that the orthogonality of the
aryl rings in the solid-state structure is consistent with
the solution-phase spectroscopic measurements.
The electrochemical and optical properties of the zinc
complexes and reference compounds are summarized in
Table 3.52 The metalated porphyrins exhibit many of the
same optical features found for the free base: a blue shift
in the Soret absorption compared to TPP(Zn), a large
splitting between the B(0,0)-Q(0,0) band, and a small
molar extinction coefficient for Q(0,0). Metalation raises
the a2u orbital energy, lifting the near degeneracy of the
HOMOs observed for free base. The Q(0,0) band is
clearly observable in the zinc complexes 1(Zn ) and 2(Zn ),
but the Q(0,0)/Q(1,0) ratio remains small.36 The optical
absorption data strongly suggest that no gross change
in macrocycle structure occurs upon metalation.
The porphyrin ring is slightly ruffled with the major
deviations from the porphyrin mean plane occurring at
the meso-positions; these displacements are -0.174 Å
(C5), 0.148 Å (C10), -0.110 Å (C15), and 0.203 Å (C20)
for the indicated atoms. Given that crystal packing forces
are sufficient to cause this degree of nonplanar distortion
for even sterically unencumbered porphyrins,46,47 the
origins of the ruffling cannot be determined with cer-
tainty. The slight ruffling in the structure of 2(Zn )
contrasts strongly in type and magnitude with the drastic
nonplanar saddle distortions observed for metal com-
plexes of â-octabrominated (OBTPFP) and â-octachlori-
nated (OCTPFP) analogs.30,45,48-50 Due to the short C-F
bond lengths, porphyrin ring fluorine atoms would be
collinear with the first carbon atoms in the aryl groups
if 2(Zn ) were planar. For example, in Figure 1 a line
drawn between F2 and F18 would intercept C39. A pure
S4 saddle distortion of the macrocycle cannot alleviate
this unfavorable collinear arrangement at the porphyrin
periphery; such a conformational change can only in-
crease the distance along the F-C-F line of sight. In
contrast, the slight vertical displacement of the phenyl
group in the ruffled conformation obviates this steric
interaction while causing a minimal nonplanar distortion
of the macrocycle.
The redox potentials of 1(Zn ) and 2(Zn ) were mea-
sured by square wave and cyclic voltammetry in CH2Cl2.
â-Octafluorination of TPP results in 480 and 300 mV
positive shifts in the formal potentials of the first
oxidation (+1.26) and first reduction (-1.03), respec-
tively, while perfluorination results in 900 and 700 mV
positive shifts. From the standpoint of porphyrin ring
oxidation, 2(Zn ) is the most electron-deficient zinc por-
phyrin ever reported. The onset of irreversible oxidation
of 2(Zn ) occurs at the solvent breakdown of CH2Cl2, so
the apparent potential (+1.70 V) must be regarded as
approximate. In view of the positive shift in the formal
potential for the first reduction (-0.63) and the blue shift
in the Q(0,0) optical absorption of 2(Zn ) compared to
1(Zn ), the reported value for the ring oxidation should
be quite close to the actual formal potential. Finally, the
electron-withdrawing effects of â- and meso-substituents
are nearly additive for 2(Zn ), in contrast to other
â-octahalogenated porphyrins. The constancy of sub-
stituent effects is consistent with planar solution struc-
tures for both 1(Zn ) and 2(Zn ).
Con clu sion
The structural, spectroscopic, and electrochemical data
The pentafluorophenyl groups in 2(Zn ) are at an
average dihedral angle of 83° to the mean porphyrin
plane. The probable origin for the near perpendicularity
of the aryl substituents lies in the close contacts between
the â-fluorine substituents and the aryl ring carbon
atoms. These contacts average 2.77 Å and fall within a
narrow range; the minimum distance is 2.742 Å for C27-
F12, and the maximum distance is 2.785 Å for C39-F2.
The sum of the van der Waals radii is (1.47 Å for fluorine
and 1.77 Å for aromatic carbon)51 is 3.24 Å. A search of
the data base from the Cambridge Crystallographic Data
Centre revealed more than 100 structures with C-F
indicate that â-octafluoro-meso-tetraarylporphyrins are
(46) Scheidt, W. R.; Lee, Y. J . Struct. Bonding 1987, 64, 1.
(47) Ravikanth, M.; Chandrashekar, T. K. Struct. Bonding 1995,
82, 105.
(48) Mandon, D.; Ochsenbein, P.; Fischer, J .; Weiss, R.; J ayaraj, K.;
Austin, R. N.; Gold, A.; White, P. S.; Brigaud, O.; Battioni, P.; Mansuy,
D. Inorg. Chem. 1992, 31, 2044.
(49) Henling, L. M.; Schaefer, W. P.; Hodge, J . A.; Hughes, M. E.;
Gray, H. B. Acta Crystallogr. 1993, C49, 1743.
(50) Schaefer, W. P.; Hodge, J . A.; Hughes, M. E.; Gray, H. B. Acta
Crystallogr. 1993, C49, 1342.
(51) Bondi, A. J . Phys. Chem. 1964, 68, 441.
(52) Hodge, J . A.; Hill, M. G.; Gray, H. B. Inorg. Chem. 1995, 34,
809.

1592 J . Org. Chem., Vol. 62, No. 6, 1997
Woller and DiMagno
lost solvent upon standing. This material was dried under
vacuum to yield 129 mg of pure 2: 1H NMR (360 MHz, CDCl3)
δ -4.22 (s, 2 H); 19F δ (500 MHz, CDCl3, CFCl3 internal
standard, -50 °C) -161.0 (t, 8 F, J ) 20.9 Hz), -149.6 (t, 4 F,
J ) 20.8 Hz), -147.9 (s, 4 F), -142.9 (s, 4 F), -138 (d, 8 F, J
) 17.8 Hz); UV-vis (CH2Cl2) 392 (5.24), 493 (4.29), 579 (3.71);
low-resolution FAB MS 1118 (calcd 1118). Anal. Calcd for
a new class of nearly planar, electron-deficient ligands.
Particularly impressive is the 0.5 V window over which
the formal oxidation potential can be tuned using only
substituents on orthogonal aryl rings. The invariance
of the ligand structure with increasingly positive formal
oxidation potential is a key advance; electronic effects
have been severed from the nonplanar conformations
exhibited by all other highly electron-deficient porphy-
rins. Future studies of structure-function relationships
in metalloporphyrin-catalyzed reactions will benefit from
the extended range of accessible ligand formal potential
and nitrogen basicity offered by â-octafluorinated por-
phyrins.
C
44H2F28N4: C, 47.25; H, 0.18; N, 5.01. Found: C, 47.25; H,
0.05; N, 4.86.
2,3,7,8,12,13,17,18-Octaflu or o-5,10,15,20-tetr akis(3-m eth -
oxyp h en yl)p or p h yr in (3). m-Anisaldehyde (0.3 mL, 2.5
mmol), 3,4-difluoropyrrole (215 mg, 2.1 mmol), and 100 mL of
dry CH2Cl2 were placed under N2 in a 250 mL round-bottom
flask equipped with a magnetic stir bar. The reaction was
stirred while BF3 etherate (0.9 mL, 7.1 mmol) was added via
syringe. To monitor the reaction, aliquots were periodically
removed from the reaction vessel, oxidized with DDQ, neutral-
ized with pyridine, and chromatographed by silica gel TLC
using CHCl3 as eluent. After 30 min, DDQ (1 g) and pyridine
(3 mL) were added. The reaction was stirred for 12 h and
filtered through a short silica gel column using CH2Cl2 as
eluent, and the solvent was evaporated. A rough purification
was done using a silica gel column with CHCl3 as eluent. The
solvent was removed, and the product was further purified on
silica gel using chloroform/hexane (1:4). The pure fractions
were combined and concentrated to afford 88.8 mg of pure 3
in 20% yield: 1H NMR (360 MHz, CDCl3) δ 7.65 (t, 12 H), 7.30
(m, 4 H), 4.02 (s, 12 H), -4.14 (s, 2 H); 19F δ (500 MHz, CDCl3,
-50 °C) δ -140.9 (s, 4 F), -146.0 (s, 4 F); UV-vis (CH2Cl2)
406 (5.31), 500 (4.22), 533 (3.67), 581 (3.57), 636 (3.56); low-
resolution FAB MS 878 (calcd 878). Anal. Calcd for
Exp er im en ta l Section
In str u m en ta tion a n d Ma ter ia ls. Manipulations of air-
and water-sensitive reagents were carried out either in a
glovebox (Innovative Technologies Labmaster 150) or by
standard Schlenk techniques. THF was distilled from sodium/
benzophenone prior to use. Methylene chloride was distilled
from CaH2. Benzaldehyde and BF3 etherate were distilled
immediately prior to use. The remaining reagents were
obtained from Aldrich or Fisher Chemical Cos. and used as
received. Optical spectra were performed using an OLIS-14
modification of a Carey-14 UV-vis-NIR spectrometer. NMR
spectra were obtained in the instrumentation center at the
University of Nebraska using 300, 360, or 500 MHz spectrom-
eters. Proton NMR spectra were collected in CDCl3 using the
residual protiochloroform as a chemical shift reference (7.24
ppm). 19F NMR was conducted at 470 MHz. Chemical shifts
are given with reference to an added internal standard, CFCl3.
Analyses were conducted by Desert Analytics, Tucson, AZ.
2,3,7,8,12,13,17,18-Octaflu or o-5,10,15,20-tetr aph en ylpor -
p h yr in (1). Benzaldehyde (0.35 mL, 3.5 mmol), 3,4-difluoro-
pyrrole (0.33g, 3.2 mmol), and 250 mL of distilled CH2Cl2 were
placed under N2 in a 500 mL round-bottom flask equipped with
a magnetic stir bar. The reaction was stirred while BF3
etherate (0.5 mL, 3.9 mmol) was added via syringe. To monitor
the reaction, aliquots were periodically removed from the
reaction vessel, oxidized with DDQ, neutralized with pyridine,
and chromatographed by silica gel TLC using CHCl3 as eluent.
After 30 min, DDQ (1 g) and pyridine (5 mL) were added. The
reaction was stirred for 12 h, filtered through a short silica
gel column using CH2Cl2 as eluent, and evaporated. The
resulting solid was washed three times with 10 mL of pentane
followed by three washes with 10 mL of ethanol. The product
was recrystallized from toluene/hexane by layering to yield 127
mg of pure 1 in 21% yield (first crop). A second crystallization
using identical conditions yielded an additional 75 mg for a
total isolated yield of 33%: 1H NMR (360 MHz, CDCl3) δ 8.03
(d, 8 H, J 1 ) 6.7 Hz), 7.74 (m, 12 H), -4.18 (s, 2 H); 19F NMR
(500 MHz, CDCl3, -50 °C) δ -141.1 (s, 4 F), -146.4 (s, 4 F);
UV-vis (CH2Cl2) 402 (5.31), 499 (4.21), 532 (3.71), 581 (3.55),
637 (3.60); low resolution FAB MS 758 (calcd 758). Anal.
Calcd for C44H22F8N4: C, 69.66; H, 2.92; N, 7.38. Found: C,
69.43; H, 2.90; N, 6.95.
2,3,7,8,12,13,17,18-Oct a flu or o-5,10,15,20-t et r a k is(p en -
ta flu or op h en yl)p or p h yr in (2). Pentafluorobenzaldehyde
(0.3 mL, 2.4 mmol), 3,4-difluoropyrrole (226 mg, 2.2 mmol),
and 250 mL of dry CH2Cl2 were placed under N2 in a 500 mL
round-bottom flask equipped with a magnetic stir bar. The
reaction was stirred while the BF3 etherate (1 mL, 7.8 mmol)
was added via syringe. To monitor the reaction, aliquots were
periodically removed from the reaction vessel, oxidized with
DDQ, neutralized with pyridine, and chromatographed by
silica gel TLC using CHCl3 as eluent. After 30 min, DDQ (0.5
g) and pyridine (3 mL) were added. The reaction was stirred
for 12 h and filtered through a short silica gel column using
CH2Cl2 as eluent, and the solvent was evaporated. The
product was purified by silica gel chromatography with pen-
tane. Collection was continued until the eluent was colorless.
The solvent was removed, and the resulting product was
recrystallized from CHCl3 to yield reddish purple crystals that
C
48H30F8N4O4: C, 65.61; H, 3.44; N, 6.38. Found: C, 64.49;
H, 3.46; N, 5.61.
Gen er a l Meta la tion P r oced u r e for â-Octa flu or in a ted
P or p h yr in s. The porphyrin (50 mg) was suspended in 20 mL
of 1:1 CH2Cl2:THF solution containing a 10-fold excess of
ZnCl2. Several drops of triethylamine were added to the
solution; metalation commenced immediately and was com-
plete within 5 min. The mixture was partitioned between
CH2Cl2 and water and extracted. The organic layer was
washed, dried over MgSO4, filtered through a short silica gel
column, and evaporated. 1(Zn ) and 2(Zn ) were crystallized
from hot acetonitrile.
[2,3,7,8,12,13,17,18-Octaflu or o-5,10,15,20-tetr aph en ylpor -
p h in a to]zin c, 1(Zn ): 1H NMR (500 MHz, CDCl3) δ 7.99 (dd,
8 H, J 1 ) 6.8 Hz, J 2 ) 1.6 Hz), 7.75 (tt, 4 H, J 1 ) 7.6 Hz, J 2
)
1.2 Hz), 7.69 (t, 8 H, J 1 ) 7.6 Hz); 19F NMR (500 MHz, CDCl3)
δ -143.3 (s); UV-vis (CH2Cl2) 406 (5.60), 502 (3.49), 536 (4.31)
573 (3.62); FAB MS 820 (calcd 820).
[2,3,7,8,12,13,17,18-Octa flu or o-5,10,15,20-tetr a k is(p en -
ta flu or op h en yl)p or p h in a to]zin c, 2(Zn ). The crystalline
compound isolated from the above procedure contained 0.5
equiv of THF and 0.5 equiv of acetonitrile: 19F NMR (500 MHz,
CDCl3) δ -139.6 (dd, 8 F, J 1 ) 23.4 Hz, J 2 ) 7.4 Hz), -145.4
(s, 8 F), -150.7 (t, 4 F, J 1 ) 19.8 Hz), -161.9 (td, 8 F, J 1
)
23.4 Hz, J 2 ) 8.6 Hz); UV-vis (CH2Cl2) 407 (5.45), 537 (4.30);
FAB MS 1180 (calcd 1180).
Str u ctu r e Deter m in a tion of 2(Zn ). The selected crystal
of 2(Zn ) [N4C44F28]Zn[CH3CN)0.69/(THF)0.31] was, at 20 ( 1 °C,
monoclinic, space group P21/n (an alternate setting of P21/c -
C5 (No. 14)) with a ) 16.168(4) Å, b ) 16.856(4) Å, c )
2h
18.301(4) Å, â ) 91.65(2)°, V ) 4985(2) Å3, and Z ) 4 [dcalcd
)
1.642 g/cm , µa(Mo KR) ) 0.64 mm-1]. A total of 8216 independent
3
absorption-corrected reflections having 2θ(Cu KR) < 48.3° (the
equivalent of 0.70 limiting Cu KR spheres) were collected on
a computer-controlled Nicolet autodiffractometer using ω scans
and graphite-monochromated Mo KR radiation. The structure
was solved using “heavy-atom” Patterson techniques with the
Siemens SHELXTL-PC software package as modified at Crys-
talytics Co. The resulting structural parameters have been
refined to convergence [R1 (unweighted, based on F) ) 0.055
for 3638 independent absorption-corrected reflections having
2θ(Mo KR) < 48.3° and I > 3σ(I)] using counter-weighted full-
matrix least-squares techniques and a structural model that
incorporated anisotropic thermal parameters for all nonhy-

Characterization of a Perfluorinated Tetraarylmetalloporphyrin
J . Org. Chem., Vol. 62, No. 6, 1997 1593
drogen atoms except disordered carbon atoms C1s and C3s,
which were included in with isotropic thermal parameters.
The fifth coordination site of the Zn atom is occupied by the
nitrogen of an acetonitrile ligand 69% of the time and the
oxygen of a THF ligand 31% of the time. C1s, C3s, and C43s
were included in the structural model as carbon atoms with
occupancy factors of 0.31 and carbon atom C5s with an
occupancy factor of 0.69. The position occupied by carbon atom
C2s is a methylene carbon 31% of the time and a methyl carbon
69% of the time. The geometry for partial occupancy THF
ligand coordinated to the Zn atom was restrained in the least-
squares refinement cycles. The C-C bond length between
methylene carbons was included in the least-squares refine-
ment as a free variable that refined to a final value of 1.49(2)
Å, and the C1s-C3s and C2s-C4s separations were restrained
to be 1.633 times this value. The lattice also contains disorder
solvent molecules between solvent molecules between porphy-
rin molecules near the crystallographic inversion at the origin
of the unit cell. Carbon atom C6s was included in the
structural model with an occupancy of 0.75.
analysis of 2(Zn ) and Crystalytics Company of Lincoln,
Nebraska for the use of its equipment and supplies for
this structure analysis. We thank Professors J ody
Redepenning and Richard Shoemaker for their as-
sistance and helpful discussions. This work was sup-
ported by the Centers for Materials Research and
Metallobiochemistry at the University of Nebraska, and
NSF/EPSCoR.
Su p p or tin g In for m a tion Ava ila ble: A summary of the
semiempirical AM1 calculations for 1 and 1(Zn ); optical
spectra of 1, 2, 1(Zn ), and 2(Zn ); spectrophotometric pKa
determination for 2; variable-temperature 19F NMR spectra
for 2; and representative mass spectra (18 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
Ack n ow led gm en t. We thank Professor Victor Day
of this department for performing the crystal structure
J O962159T