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a-PyrroleЦMetal ComplexesЧThe Missing Coordination Mode for MetalЦPorphyrin Interactions.

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n-Pyrrole -Metal Complexes-The Missing Coordination Mode
for Metal- Porphyrin Interactions**
Mathias 0. Senge"
The ability of tetrapyrrole systems to coordinate metals is one
of the most widely studied areas of porphyrin chemistry. Metalloporphyrins are important natural compounds involved in
electron transfer, respiration, and photosynthesis and are gaining industrial importance as catalysts. For a detailed understanding of the mechanisms of catalytic processes performed by
metalloporphyrins a thorough study of the interactions between
metals and (metal1o)porphyrins is necessary.
In theory, four principle coordination modes are possible for
metal-porphyrin interactions (Scheme 1): metal o bonding by
Scheme 1 , Type\ o f possible metal -porphyrin interactions
either the pyrrole nitrogen atoms (A) or through peripheral
side chain functionalities (B). and TC bonding to metals either
through peripherdi aromatic systems (C) or by direct involvement of the aromatic porphyrin x system (D).
The best-known coordination type is derived from the intrinsic capability of free base porphyrins to complex almost any
known metal ion with the pyrrole nitrogen atoms to yield metalloporphyrins by the formation of N-metal CJ bonds.['] This area
of metalloporphyrin chemistry is well developed and encompasses not only the simple chelation of metal centers but also the
investigation of metal - metal bonding and organometallic systems. In the latter case the M-C bonds are formed to carbon
centers in the axial positions of the metalloporphyrin (type A).
DI- M 0. Scngi.
lnslitut fur Organische Chemie (WE02) der Frek Universitidt
Takusrrasse 3. D-14195 Berlin (Germany)
Fas: Int. code +(30)838-4248
c-mail. mosengcvr
1 thank the the Deutsche Forschungsgemeinschaft and the Fonds der chemischeti Industrie lor their supporr
So far organometallic porphyrin chemistry has been restricted
to this type of "axial coordination chemistry".["
Examples for metal coordination by the porphyrin by using
functionalities other than the central N atoms are much more
scarce. Three different binding motifs have been described for
coordination type B. 1) Suitable peripheral substituents may be
introduced to establish a second metal chelation site in close
vicinity to the metal center of the metall~porphyrin.[~]
2) When
heteroatom functionalities are present at the porphyrin periphery, these may serve as ligands for another metal.14] 3) In a
similar manner, the central metal of one metdlloporphyrin of
type B may form bonds with a second metalloporphyrin by
interaction of the heteroatom and the metal, yielding polymeric[5a1or self-organized s y ~ t e m s . [ ~Typical
~ . ~ ] examples are porphyrins with meso-pyridyl or meso-nitro substituents as well as
chlorophyll-type aggregates.
The structure types A and B are characterized by o bonds to
the metal. In a similar manner. two coordination modes are
conceivable that involve TC bonding to the metal center. The first
example in which the x system of a peripheral substituent of a
porphyrin participated in coordination to metal centers was
described in 1970 by Gogan and Siddiqui (Scheme 1, type C) .Iba1
Reaction of zinc(i1)tetraphenylporphyrin [Zn"(tpp)] with hexacarbonylchromium(0) yielded the tricarbonylchromium arene
complex 1, in which the tricarbonylchromium(o) species is TCbonded to a nzeso-phenyl substituent. The corresponding
[Zn"(tpp)(Cr(CO),j.,] complex was also characterized. The second example for type C are ferrocenyl porphyrins, which were
prepared directly by reacting pyrrole with suitable aldehydes
under the conditions applied by Adler. For instance, use of
ferrocenylaldehyde yielded tetraferrocenylporph yrin 2,[6b1while
4-ferrocenylbenzaldehyde gave tetra(4-ferrocenylphenyl)porphyrin 3.16'' Unfortunately, no definite structural data are available for any of these interesting compounds to date.
Examples for coordination type D (Fig. I ) , in which the aromatic system of the porphyrin forms K bonds to metal centers.
were unknown until recently. In contrast to this the organometallic chemistry of "simple" pyrrole compounds is much better
developed. Although N-heterocycles seem to prefer /I' coordination with metal-nitrogen CJ bonds, there are a number of
compounds with q5-heteroatom-metal x bonds.
Most known pyrrole TC complexes are obtained from the corresponding azacyclopentadienylanion, which is isoelectronic to
cyclopentadienyl (Cp).['Sandwich complexes with one
or two (5)[8b,clq5-pyrroIyI groups as well as tricarbonyl($-pyrrolyl) complexes (6)[9"1and carborane-stabilized
$-pyrrolyl double and triple-decker compounds have been prepared.["] In most cases, stable products of the diazametallocene
structure type 5 were obtained only after shielding the nonbonding lone pairs of electrons on the nitrogen atom, for example, by
employing sterically demanding a-substituents or by using Nalkylated derivatives.[8* 'I Nevertheless, some n-pyrrole comas are
plexes with the neutral pyrrole ligand (7) are
species that contain both nitrogen-metal 0 and n bonding
An interesting combination of the structure type A with
an azaferrocene (5) has been achieved in an Fe"-porphyrin,
which is bisaxially coordinated by the pyrrole nitrogen aotms of
azaferrocene.[' '1
An intermediary position between organometallic derivatives
of pyrrole and those of porphyrins is adopted by metal complexes of porphyrinogens. Porphyrinogens are formally hexahydroporphyrins with four isolated pyrrole units separated by sp3hybridized carbon atoms. Such systems have been investigated
in detail by Floriani and c o - ~ o r k e r s . [The
~ ~ 1recurring structural
theme in transition metal complexes of octaethylporphyrinogen
is the formation of both metal-nitrogen 0 bonds and metal($-pyrrolyl) n bonds, which is facilitated by the free rotation of
pyrrole rings around the C,-C, bonds, for example as shown
for the Zr derivative 9.[13]The intrinsic differences between the
porphyrinogen and the porphyrin ligand are also made evident
by a comparison of the respective lithium derivatives 10 and
While the porphyrin derivative 11 consists o f a [Li(thf),]+
VerlagsgesellschaftnibH, 0-69451 Weinheim. 1996
ion and a lithium porphyrin anion, in which the lithium atom is
bonded to the four pyrrole nitrogen a t ~ m s , I ' the
~ ~ porphyrino]
gen complex 10 contains three coordination modes for the four
lithium atoms. One lithium atom is bonded solely by the pyrrole
nitrogen atoms, while the other three are $-bonded to a pyrrole
ring (either to a C,-C, or N-C, unit) and 0-bonded to another
pyrrole nitrogen
The first example of the coordination type D (n bonding
between the aromatic porphyrin system and a metal) has
now been described by Rauchfuss et
Treatment of
[ (2,3,7,8,12,13,17,18-octaethylporphyrinato)nickel(11)] Wi"(oep)]
with the arenophiles [ (cymene)Ru](BF,), or [ (cymene)Ru](F,CSO,), , (cymene = p-isopropyltoluene) in CH,CI, at room
temperature yielded a green complex of the composition
[{(cymene)Ru)Ni"(oep)](BF,), (12). The single-crystal X-ray
analysis clearly showed a sandwich-type structure with (@cymene)Ru($-pyrrole) n bonding involving one of the four
pyrrole subunits. The planes of the arene and the n-bound
pyrrole ring are almost coplanar and show structural characteristics similar to those found for the complex cation [($~ymene)Ru(q~-C,Me,N)]~+
.Ii6] These results were confirmed
by NMR spectroscopy, which also indicated a considerably decreased ring current in 12. This novel type of sandwich complex
is air-stable; however, even weakly coordinating solvents like
acetonitrile dissociate the complex under liberation of
Similar arene complexes were formed by reacting [Ni"(oep)]
with [(cymene)OsI2+ or [(Cp*)IrI2+ (Cp* = C,Me,). Reaction
of [Zn"(oep)] with these reagents gave rise to compounds 13 and
14, respectively. The latter compound contained a [ ($Cp*)Ir(q5-pyrrole)] structural motif, opening a way to combine the
rich organometallic chemistry of cyclopentadienyl derivatives
with that of porphyrins. The results achieved to date indicate
that metalloporphyrins seem to be capable of binding only one
[ (arene)MJ2 unit, since reaction with excess arenophile yielded
only the 1 : 1 adducts.
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Angew. Chem. Int. Ed. Engl. 19%. 35, No. 17
2 F~CSO;
12 M = N i , M ' = Ru
13 M = Z n , M ' = O s
The spectroscopic characteristics of the products are quite
different from those of the porphyrin starting materials. For
instance, the UV/vis spectra showed line broadening and
bathochromic shifts of the absorption bands. Very characteristic
was the formation of a long wavelength band around 680770 nm [depending on the [(arene)MI2' unit] similar to those
found for chlorins. The coordination of [(arene)M]* completely quenched the fluorescence of [Zn"(oep)]. Thus, clearly n complexation has a strong effect on the metalloporphyrin properties. Further investigation of these and related compounds
might help to unravel the mechanism of catalytic processes involving metal -rnetalloporphyrin interactions.
German version: Angew. Chem. 1996, 108, 2051 -2053
Keywords: arene complexes porphyrinoids
plexes sandwich compounds
n-pyrrole com-
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missing, interactions, complexesчthe, mode, coordination, pyrroleцmetal, metalцporphyrin
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