Ruthenocene 1,2-dicarboxylic acid, carboxylic anhydride, and acid chloride: A facile route to metallocene-fused acenequinones

Article abstract of DOI:10.1021/om2003915

The dehydration of 1′,2′,3′,4′,5′- pentamethylruthenocene-1,2-dicarboxylic acid with acetic anhydride gives 1′,2′,3′,4′,5′-pentamethylruthenocene-1, 2-dicarboxylic anhydride, the first crystallographically characterized, metallocene-fused carboxylic anhyd

Full text of DOI:10.1021/om2003915

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pubs.acs.org/Organometallics
Ruthenocene 1,2-Dicarboxylic Acid, Carboxylic Anhydride, and Acid
Chloride: A Facile Route to Metallocene-Fused Acenequinones
Uttam Raj Pokharel, John P. Selegue,* and Sean Parkin
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States
S Supporting Information
b
ABSTRACT: The dehydration of 10,20,30,40,50-pentamethyl-
ruthenocene-1,2-dicarboxylic acid with acetic anhydride gives
10,20,30,40,50-pentamethylruthenocene-1,2-dicarboxylic anhy-
dride, the first crystallographically characterized, metallocene-
fused carboxylic anhydride. Treatment of the diacid with oxalyl
chloride/DMF produces its 1,2-diacyl chloride, which is an excellent precursor for AlCl3-promoted double FriedelꢀCrafts acylation
reactions with a variety of arenes, including benzene, toluene, o-xylene, p-dimethoxybenzene, and ferrocene. X-ray structural
determinations of an acenequinone and a unique ferrocene/ruthenocene-fused benzoquinone show distortions attributed to strong
electron donation from pentamethylruthenocene.
olycyclic aromatic hydrocarbons are benchmark semiconduct-
with 4 in molten NaCl/AlCl3 resulted in the formation of an
intractable mixture containing a small amount of a monoacyla-
tion product.
P
ing materials for organic field-effect transistors,1 light-emitting
diodes,2 and photovoltaic cells.3,4 Our long-term interest5ꢀ11 in
transition metal complexes of low-band-gap η5-cyclopenta-
[c]thienyl monomers and polymers led us to consider polyacenes
with terminal cyclopentadienyl metal groups, which are unknown
except for simple η5-indenyl complexes and a few benz[f]indenyl
complexes of zirconium12 and ruthenium.13 We report here a
route to metallocene-fused acenequinones, potential precursors
for the desired acenes.
Treatment of 3 with oxalyl chloride/DMF led to its 1,2-diacyl
chloride (5) in 50% isolated yield. Complex 5 is stable under dry
conditions, but is more efficiently generated in situ to acylate
arenes, including benzene, toluene, o-xylene, p-dimethoxyben-
zene, and ferrocene (Scheme 1), giving acenequinone complexes
6aꢀe (42ꢀ58%) and 7 (36%). Complex 7 contains an unusual
μ-[(1,2,3,3a,8a-η:4a,5,6,7,7a-η)-4,8-dihydro-4,8-dioxo-s-inda-
cene-1,4a(1H)-diyl] bridging two different metals, precedented
only by a pair of symmetrical diruthenium compounds.25
1,2-Dicarbophenoxycyclopentadienylsodium (1),14 conveni-
ently prepared by a single-step reaction of cyclopentadienylso-
dium with phenyl chloroformate,11 forms with higher 1,2-
selectivity than alkyl chloroformates, making 1 the preferred
precursor for metallocene-1,2-dicarboxylates. Transmetalation
1
Whereas toluene gives a single product (6b), H NMR shows
that o-xylene gives two isomers (6c,d) in a 1:1 ratio, implying that
the arene undergoes electrophilic substitution first para to an
activating substituent, followed by an indiscriminate second acyla-
tion. Dimethoxyacenequinone 6e undergoes facile demethyla-
tion with anhydrous AlCl3 to give dihydroxyquinone complex 9,
15
of [Ru(μ3-Cl)(Cp*)]4 with 1 gives diester 2 in 78% yield
(Scheme 1). Saponification of the diester16 gives diacid 3 in 95%
yield. Dehydration of 3 with refluxing acetic anhydride produces
1
10,20,30,40,50-pentamethylruthenocene-1,2-dicarboxylic
anhy-
with a H-bonded phenolic H NMR resonance at 12.93 ppm.
The reduction of 6b with LiAlH4 gave a single (by 1H and 13
C
dride (4) in 84% yield. Two strong carbonyl infrared stretching
frequencies at 1820 and 1764 cmꢀ1 support the formation of a
cyclic anhydride. The anhydride is surprisingly stable to water,
surviving an aqueous workup. The only previous metallocene-
fused carboxylic anhydride is 1,2-ferrocenedicarboxylic anhy-
dride, prepared in very low yields by N,N0-dicyclohexylcarbodii-
mide (DCC) dehydration of 1,2-ferrocenedicarboxylic acid,17,18
but properties (e.g., light sensitivity) reported by two groups are
not entirely consistent. In contrast, DCC treatment of 3 gives a
stable N,N0-dicyclohexyl-N-ruthenocenoylurea rather than the
expected anhydride. Compounds 3 and 4 were characterized by
their spectroscopic properties and X-ray structural determina-
tions (vide infra).
NMR) diol complex (8, 68%), presumably the syn isomer resulting
from nucleophilic hydride addition to the side of the acenequi-
none ligand opposite the large Cp* group.
X-ray crystal structures of 3, 4, 6a, and 7 (Figure 1) all exhibit
typical ruthenocene geometry with nearly linear (Cp centroid)ꢀ
Ruꢀ(Cp* centroid) angles ranging from 177.38(13)° (3) to
175.60(6)° (7). In all structures the Ru atom is situated closer to
the two ring-fusion carbons than to the other three carbons of the
Cp ring, with the RuꢀC shift ranging from 0.026(3) Å in 3 to
0.099(16) Å in 7. Similar asymmetry was observed in [(Ru-
(Cp*)}2{μ-η5:η5-C5H3(CO)2C5H3}],25 [Ru(η5-C5H4COMe)-
(Cp*)],26 and [Ru{η5-1,2-C5H3(COPh)2}(Cp*)].12 Curiously,
Organic carboxylic anhydrides react with electron-rich aromatics
under FriedelꢀCrafts conditions to give acenequinones.19ꢀ24
However, our attempts to diacylate 1,4-dimethoxybenzene
Received: May 11, 2011
Published: May 24, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om2003915 Organometallics 2011, 30, 3254–3256
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Organometallics
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Scheme 1. Synthesis of Pentamethylruthenocene-1,2-dicarboxylic Acid and Its Derivatives
Figure 1. Thermal ellipsoid plots of the molecular structures of [Ru{η5-C5H3(CO2H)-1,2}(Cp*)] (3), [Ru{η5-C5H3(CO)2O-1,2}(Cp*)] (4),
[Ru{η5-C5H3(CO)2C6H4}(Cp*)] (6a), and [Ru(Cp*){μ2-η5:η5-1,2-C5H3(CO)2-1,2-C5H3)}Fe(Cp)] (7).
the Fe atom in 7 shows a classic indenyl effect,27ꢀ30 shifting
0.082(16) Å away from the two ring-fusion carbon atoms. In 6a,
the CpꢀCO bonds, C1ꢀC13 and C5ꢀC6, are 0.044(4) Å
shorter than the PhꢀCO bonds, C12ꢀC13 and C6ꢀC7.
Although the disubstituted cyclopentadienyl ligands of 3, 4, 6a,
and 7 are nearly planar, there are some subtle deviations. The two
carboxylic acid moieties of 3 lie in the cyclopentadienyl plane
(interplanar angles 3.2ꢀ4.4°), held together by a network of
intra- and intermolecular hydrogen bonds with carboxylate
OꢀHꢀO distances ranging from 2.532(4) to 2.592(4) Å and a
methanol oxygen atom 2.859(5) Å from O3A. The cyclopentadienyl
Chart 1. Structural Distortions in Compounds 6a and 7
anhydride ring of 4 is essentially planar, with angles of 3.2° and
5.3° between the Cp (C1ꢀC5) and anhydride (C1, C5ꢀC7,
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Organometallics
COMMUNICATION
Chart 2. Resonance Description of Compound 7
O1ꢀO3) planes in two independent molecules. The bending of
the quinone ligands of 6a and 7 is illustrated in Chart 1. In both
complexes, the quinone portion of the ligand is inclined slightly
toward [Ru(Cp*)], but in 7 the quinone is inclined slightly away
from [Fe(Cp)]. Much of the bending is due to a slight folding of
the quinone ring along the OꢀCꢀCꢀO axis, by 4.1° in 6a and
11.5° in 7.
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avenues in organometallic FriedelꢀCrafts chemistry. The result-
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’ ASSOCIATED CONTENT
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S
Supporting Information. Syntheses and data for 1ꢀ9;
b
tables of crystallographic details, atomic coordinates, displace-
ment parameters, bond distances, and angles; and CIFs for 3, 4,
6a, and 7. This material is available free of charge via the Internet
at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: selegue@uky.edu.
’ ACKNOWLEDGMENT
We thank the Research Corporation for support of this research.
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