A four-coordinate Fe(III) porphyrin cation

Article abstract of DOI:10.1021/ja078061l

The free four-coordinate Fe(III) porphyrin cation has not been previously observed due to its extreme electrophilicity. In order to isolate this very reactive species, we have synthesized a highly sterically hindered bis-pocket siloxyl porphyrin which has a top opening of only 2 A. NMR studies of this Fe(III) porphyrin cation with different weakly coordinating anions all give the same porphyrin proton chemical shifts in CD₂Cl₂, suggesting that no anion coordination to the iron occurs in solution. In the solid state, the four-coordinate Fe(III) bis-pocket porphyrin can be isolated with the bulky and weakly coordinating hexabromocarborane as the counterion: the single-crystal X-ray structure confirms the absence of axial ligation to the iron. In contrast, with smaller anions (e.g., triflate), the Fe(III) bis-pocket porphyrin complexes are five-coordinate in the solid state, due probably to the smaller size and loss in the solid state of stabilization from anion solvation. Full characterization (EPR, Moessbauer, SSNMR, Evans Method, and SQUID magnetometry) shows that the four-coordinate Fe(III) bis-pocket porphyrin cation has a 3/2 spin state, in agreement with theoretical predictions. Copyright

Full text of DOI:10.1021/ja078061l

Published on Web 01/03/2008
A Four-Coordinate Fe(III) Porphyrin Cation
Ming Fang, Scott R. Wilson, and Kenneth S. Suslick*
School of Chemical Sciences, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Received October 21, 2007; E-mail: ksuslick@uiuc.edu
Scheme 1. Synthesis of a Bis-pocket Siloxylporphyrin,
Few families of compounds have been as intensely studied as
iron porphyrin complexes. In spite of this, a truly four-coordinate
Fe(III) porphyrin cation has never before been reported.1 The
exceptional electrophilicity of the four-coordinate [FeIII(Porph)]+
cation will even coordinate arene solvent upon crystallization if
the counteranion is extremely non-coordinating, as in the work of
Reed with silver dihexabromocarborane,2,3 and in solution, a Br
atom of the hexabromocarborane anion binds to the Fe.4
FeIII(TipsiPP)(Cl)
Here we present the first purely four-coordinate Fe(III) porphyrin
both in the solid state and in solution. To do so, we have combined
both steric and electronic factors by creating a very sterically
hindered bis-pocket siloxyl porphyrin in conjunction with a bulky
and very weakly coordinating anion.
The solvent plays a central role in stabilization of the four-
coordinate Fe(III) cation. The four-coordinate species is only stable
in halogenated solvents (i.e., CH2Cl2, CHCl3, CH2Br2). In aromatic
solvents, such as toluene and benzene, it converts to an admixed
spin state, probably due to arene coordination to the iron center.3
The â-pyrrole chemical shift of [FeIII(TipsiPP)]+ is the same in
either CH2Cl2 or CH2Br2, which shows that the halocarbons do not
perturb the iron center. The compound is not soluble in aliphatic
solvents.
We have synthesized an extremely hindered bis-pocket siloxyl
porphyrin, 5,10,15,20-tetrakis(2′,6′-bis(triisopropylsiloxy)phenyl)-
porphyrin (H2TipsiPP) by the reaction of the octahydroxyl iron
porphyrin5 (5,10,15,20-tetrakis(2′,6′-dihydroxyphenyl)porphyrin with
triisopropyl chlorosilane using imidazole as a catalyst (Scheme 1).
In spite of the driving force of strong Si-O bond formation, high
temperatures (250 °C) are required to overcome steric hindrance
and fully silylate all eight sites. The resulting porphyrin is extremely
sterically hindered with a pocket opening of only 2 Å. Figure 1
compares the space-filling models of the simple FeTPP(Cl) and
the bis-pocket siloxylporphyrin FeIII(TipsiPP)(Cl). The steric hin-
drance prevents anion coordination by reducing the thermodynamic
binding constant.
Red-brown needle-shaped crystals of [FeIII(TipsiPP)]+[CB11H6Br6]-
were grown by slow evaporation of a 1:1 mixture of dichloro-
To replace the sterically undemanding chloro ligand, a dichlo-
romethane solution of FeIII(TipsiPP)(Cl) was mixed with 1 equiv
of AgCB11H6Br6 or AgCF3SO3, which produces a color change from
yellow to red (Figure 2). NMR, MALDI-TOF, and elemental
analysis were used to confirm the purity of the final products. Both
complexes react with water readily to form a five-coordinate spin-
admixed Fe(III) complex, which has diagnostic UV-vis and NMR
features (Figures S1 and S2 in Supporting Information (SI)).
Proton NMR is a very sensitive probe for the spin state of iron.4,6,7
On the basis of the â-pyrrole proton NMR isomer shift, Reed has
established a magnetochemical series for different anions.8 Surpris-
Figure 1. Space-filling model of FeIIITPP(Cl) (left) and FeIII(TipsiPP)-
(Cl) (right); side view, same scale.
ingly, solutions of [FeIII(TipsiPP)]+ with a variety of weakly
coordinating anions (i.e., CB11H6Br6-, SbF6-, ClO4-, and CF3SO3
)
-
all have exactly the same porphyrin 1H NMR chemical shifts with
â-pyrrole proton chemical shift at -81 ppm in CD2Cl2 at 290 K
(Figure S3 in SI), which corresponds to the Fe(III) intermediate
3
spin state (S ) /2).8 Because these anions are very different both
in nucleophilicity and size, the porphyrin NMR spectra would be
different if the anions were bound to the iron. This is the case for
FeIIITPP+: â-pyrrole proton chemical shifts for FeTPP(X) are -60
-
ppm for CB11H6Br6-, -31.5 ppm for SbF6-, 13 ppm for ClO4
,
- 8,9
and 39.3 ppm for CF3SO3
.
Possible assignments to either
FeIII(TipsiPP•+) porphyrin cation radicals or FeII(TipsiPP) were
excluded by independent synthesis of the pure complexes, which
show very different chemical shifts and have different spin states.
Figure 2. UV-visible spectrum of five-coordinate starting material FeIII
(TipsiPP)(Cl) (dotted line) and four-coordinate [FeIII(TipsiPP)]+[CB11H6Br6]-
(solid line). Inset shows an expanded absorbance scale.
9
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J. AM. CHEM. SOC. 2008, 130, 1134-1135
10.1021/ja078061l CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
measured by Evan’s method from room temperature to 190 K
(Figure S5 in SI); SQUID data show no change in the magnetic
moment between 300 and 10 K. EPR data of both frozen halocarbon
solutions and the crystalline [FeIII(TipsiPP)]+[CB11H6Br6]- have a
g ) 4.2 signal (Figure S4 in SI). Mo¨ssbauer spectra were obtained
with the 57Fe-enriched sample at 6 K, with an isomer shift of 0.33
mm/s (a typical value for Fe(III)), but with a very large quadrupole
splitting value of 5.16 mm/s (Figure S6 in SI). Solid-state NMR
also gives an upfield signal at -80 ppm for the â-pyrrole hydrogens,
confirming that the four-coordinate Fe(III) porphyrin is stable both
in solid and solution phases.
Although four-coordinate Fe(III) heme is unlikely in any heme
protein, the intermediate spin state of Fe(III) porphyrins has an
interesting counterpart in nature.11-17 In cytochrome c′, the heme
5
3
is believed to be a quantum admixed S ) /2 and S ) /2 spin
state.18-20 Depending on the field strength of the ligand coordinated
to the iron, the ratio of S ) /2 to S ) /2 character varies: as the
5
3
ligand strength becomes weaker, the spin state of iron approaches
3
the pure intermediate spin state of /2.
In summary, the first four-coordinate Fe(III) porphyrin complex
has been isolated and fully characterized in both solution and solid
3
state and shows a pure S )
/ intermediate spin state. The
2
combination of steric hindrance of a bis-pocket porphyrin with
weakly coordinating anions has proved essential in isolation of this
highly elusive species, whose reactivity we are still exploring.
Figure 3. X-ray single-crystal structure of (a) [FeIII(TipsiPP)]+[CB11H6Br6]-
and (b) FeIII(TipsiPP)(CF3SO3). Figure S9 in SI provides a crystal packing
diagram for [FeIII(TipsiPP)]+[CB11H6Br6]- showing a layered salt structure
with alternating planes of positive and negative charges.
Acknowledgment. We gratefully acknowledge the assistance
of Alexandre Anasta´cio and Prof. J. W. Stucki (Mo¨ssbauer), Prof.
C. M. Rienstra, and Dr. Donghua Zhou (SSNMR), Dr. M. J. Nilges
(EPR), and Jin Ho Bang (SQUID). These studies were supported
by National Science Foundation (CHE03-15494).
methane/heptane solution inside an inert atmosphere box. The single
crystals were redissolved in dry CD2Cl2 and tested by NMR to
confirm no contamination with water during the crystal growing
process. The crystal structure (Figure 3) confirms the absence of
axial ligation to the iron porphyrin. The average Fe-N bond length
is 1.94 ( 0.01 Å with the iron sitting in the porphyrin plane. This
very short bond length is consistent with the removal of the unpaired
Supporting Information Available: Synthesis, characterization,
and X-ray crystallographic details in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
electron from the dx -y orbital.1,10 The porphyrin is slightly ruffled,
with meso-carbons above and below the average nitrogen plane by
0.23 Å. The carborane anion is definitiVely not coordinated to the
iron and is fully outside of the porphyrin pocket. The resulting salt
has a layered structure with alternating planes of positive and
negative charges (a crystal packing diagram is given as Figure S9
in SI). A similar packing structure has been reported for [FeTPP]-
[Ag(CB11H6Br6)2]‚4(p-xylene) with the exception of the coordina-
tion of the xylene to the iron center.2,3
2
2
References
(1) Scheidt, W. R. In Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 3, pp
49-112.
(2) Evans, D. R.; Fackler, N. L. P.; Xie, Z.; Rickard, C. E. F.; Boyd, P. D.
W.; Reed, C. A. J. Am. Chem. Soc. 1999, 121, 8466-8474.
(3) Xie, Z.; Bau, R.; Reed, C. A. Angew. Chem. 1994, 106, 2566-2568; see
also Angew Chem., Int. Ed. Engl. 1994, 2533 (2523/2524), 2433-2564).
(4) Evans, D. R.; Reed, C. A. J. Am. Chem. Soc. 2000, 122, 4660-4667.
(5) Sen, A.; Suslick, K. S. J. Am. Chem. Soc. 2000, 122, 11565-11566.
(6) Walker, F. A. Inorg. Chem. 2003, 42, 4526-4544.
(7) Walker, F. A. In Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 5, pp
81-183.
In the solid state, the four-coordinate iron porphyrin cation can
only be isolated with hexabromocarborane as the counterion. In
crystals of solids with other anions (e.g., CF3SO3-), the anion enters
the porphyrin pocket and coordinates to the iron, presumably driven
by the loss of solvation of the anion. The powder X-band EPR of
FeIII(TipsiPP)(CF3SO3) shows a g )5.6 signal, consistent with an
admixed spin assignment (Figure S4 in SI). The average Fe-N
bond length in FeIII(TipsiPP)(CF3SO3) is 2.053(2) Å (which is 0.1
Å longer than in the true four-coordinate structure), and the iron
atom is out of the plane of the four nitrogens by 0.406 Å. To
accommodate the triflate ion, the phenyl groups rotate ∼12° about
the plane normal to porphyrin, which further demonstrates the
interplay of electronic and steric factors in these highly hindered
systems.
(8) Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282.
(9) Boersma, A. D.; Goff, H. M. Inorg. Chem. 1982, 21, 581-586.
(10) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543-555.
(11) Dolphin, D. H.; Sams, J. R.; Tsin, T. B. Inorg. Chem. 1977, 16, 711-
713.
(12) Summerville, D. A.; Cohen, I. A.; Hatano, K.; Scheidt, W. R. Inorg. Chem.
1978, 17, 2906-2910.
(13) Reed, C. A.; Mashiko, T.; Bentley, S. P.; Kastner, M. E.; Scheidt, W. R.;
Spartalian, K.; Lang, G. J. Am. Chem. Soc. 1979, 101, 2948-2958.
(14) Goff, H.; Shimomura, E. J. Am. Chem. Soc. 1980, 102, 31-37.
(15) Balch, A. L.; Cheng, R. J.; La Mar, G. N.; Latos-Grazynski, L. Inorg.
Chem. 1985, 24, 2651-2656.
(16) Schunemann, V.; Gerdan, M.; Trautwein, A. X.; Haoudi, N.; Mandon,
D.; Fischer, J.; Weiss, R.; Tabard, A.; Guilard, R. Angew. Chem., Int. Ed.
1999, 38, 3181-3183.
(17) Nesset, M. J. M.; Cai, S.; Shokhireva, T. K.; Shokhirev, N. V.; Jacobson,
S. E.; Jayaraj, K.; Gold, A.; Walker, F. A. Inorg. Chem. 2000, 39, 532-
540.
(18) Maltempo, M. M.; Moss, T. H. Q. ReV. Biophys. 1976, 9, 181-215.
(19) Zeng, Y.; Caignan, G. A.; Bunce, R. A.; Rodriguez, J. C.; Wilks, A.;
Rivera, M. J. Am. Chem. Soc. 2005, 127, 9794-9807.
Further characterization data on the four-coordinate [FeIII-
(TipsiPP)]+[CB11H6Br6]- are all consistent with a 3/2 intermediate
spin ground state. The solution magnetic susceptibility of the four-
coordinate complex is 4.1 µB in a dichloromethane solution
(20) Weiss, R.; Gold, A.; Terner, J. Chem. ReV. 2006, 106, 2550-2579.
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