Metal-Stabilized Phenoxonium Cation

Article abstract of DOI:10.1021/ja0385639

An unprecedented metal-stabilized phenoxonium cation was prepared by a process involving dearomatization of a phenoxy complex. The unique η² C=O-metal (iridium) coordination mode leaves the positive charge delocalized within the former aromatic ring. The X-ray structure and conversion into η² C=O-coordinated metal-quinone complex are described. Copyright

Full text of DOI:10.1021/ja0385639

Published on Web 11/27/2003
Metal-Stabilized Phenoxonium Cation
Arkadi Vigalok,† Boris Rybtchinski,† Yael Gozin,† Tehila S. Koblenz,† Yehoshoa Ben-David,†
Haim Rozenberg,‡ and David Milstein*,†
Department of Organic Chemistry and Unit of Chemical Research Support, The Weizmann Institute of Science,
RehoVot 76100, Israel
Received September 17, 2003; E-mail: david.milstein@weizmann.ac.il
Scheme 1
Phenoxonium cations (A, Scheme 1) play important roles in
chemical and biochemical processes. They have been postulated
as key intermediates in the oxidation of phenols and in biological
oxidative coupling reactions, which lead to a variety of natural
products and chemicals.1 Ab initio calculations predicted a singlet
ground state for the phenoxonium cation,2 implying effective
overlap between an empty oxygen p orbital and the arene π system,
resulting in positive charge delocalization into the ring (As), whereas
Scheme 2
in the triplet ground state the positive charge resides only on the
oxygen atom. Although too unstable to be isolated, they have been
generated as transients by various methods3 and their trapping
reactions have led to synthetic methodologies.3a-c,4 One of the most
Scheme 3
significant reactions thought to involve phenoxonium intermediates
is the oxidative coupling of phenols. It is catalyzed by various metal
(e.g., Cu, Co, Fe, Mn) complexes and metalloenzymes1 and is
involved in the biosynthesis of lignin and melanin.1a,5 The industri-
ally important Cu(II) complex-catalyzed oxidative polymerization
of 2,6-dimethylphenol produces the major engineering plastics poly-
(phenylene ether).1a,b Mechanistic and theoretical studies of the
oxidative coupling and polymerization of phenols have invoked a
two-electron transfer from a phenolate ligand to the metal (dinuclear
Cu(II)) to form an oxo-coordinated singlet phenoxonium cation (i.e.,
with a positively charged ring) and a reduced metal center.1a,6
However, such a process (BfC, Scheme 1) was never observed,
and a phenoxonium cation, complexed via the C-O group, or free,
has never been isolated, although η5-oxocyclohexadienyl complexes
and the CH3CN adduct 4 was quantitatively formed (Scheme 3).
are well-known.7 We report here on such a process, including
Formation of 4 causes a further upfield shift of the 31P{1H} NMR
isolation, crystallographic characterization, and reactivity of a
phenoxonium cation stabilized by metal coordination to the CdO
bond.
Reaction of the phenolic ligand 1 with [Ir(cyclooctene)2Cl]2 in
hot benzene afforded the fully characterized8 phenoxy hydride
complex 2 in 82% isolated yield, by an O-H activation process.
signal, which now appears as a singlet at -13.71 ppm. The mild
electron-donating properties of the acetonitrile ligand slightly
decrease the positive charge in the aromatic ring, as evident from
the upfield shifts of the para- and ortho-carbon atom signals in
the 13C{1H} NMR spectrum as compared with those of 3.
Crystals of 4 were obtained from a CH2Cl2 solution layered with
When a red CH2Cl2 solution of 2 was treated with an equivalent
hexane. The X-ray structure of 4 (Scheme 3)10 reveals a unique η2
amount of [Ph3C]+ BF4-, a color change to green took place and
coordination mode of the metal center to the CdO double bond in
a quinonoid compound. As we are unaware of other examples of
crystallographically characterized metal complexes of η2 bound
quinones, we compare the relevant distances around the third row
metal center with those in η2-coordinated ketones and ketenes (sp-
hybridized system). The M-C bond distance of 2.189(5) Å in 4 is
formation of the phenoxonuim complex 3 was observed by 31P
NMR spectroscopy; 3 was then isolated as a green solid in 69%
yield (Scheme 2). The 31P{1H} NMR of 3 shows a high-field singlet
at 6.46 ppm.8 The presence of positive charge at the phenoxonuim
ring causes dramatic downfield shifts of the aromatic carbon atoms
in the 13C{1H} NMR spectrum. As most of the charge is localized
longer than that in Ir ketene complexes11 (1.971(4)-1.982(12) Å)
at the ortho and para positions, the corresponding carbon atoms
and slightly longer than that in the pentaammineosmium(II) acetone
show a 20-40 ppm downfield shift compared with the neutral 2.
complex (2.126(11) Å).12 The M-O bond length of 2.061(3) Å is
The ipso-carbon atom gives rise to a triplet at 83.27 ppm, indicating
slightly shorter than that in coordinated ketenes (2.090(3)-2.099-
coordination to the iridium center. A similar upfield shift was
(8) Å) and compares well with the one in the Os acetone complex
observed for the ipso-carbon atom in the structurally related
(2.057(7) Å). The C-O bond length of 1.356(5) Å indicates strong
methylene arenium complexes.9 Addition of acetonitrile to a solution
coordination of this bond to the iridium center.13 This distance is
of 3 in CH2Cl2 resulted in a color change from green to brown,
close to the corresponding one in the Os acetone complex (1.322-
(13) Å). These observations point toward the η2 coordination
character for the two neighboring third row metals. For comparison,
† Department of Organic Chemistry.
‡ Unit of Chemical Research Support.
9
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10.1021/ja0385639 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
metal-oxygen and carbon-oxygen bond lengths in mid-row
tungsten η2 ketone complexes show values between 1.933(4)-
1.970(6) Å and 1.379(8)-1.397(11) Å, respectively, indicating a
metallacycle coordination mode in these complexes.14 The carbon-
carbon bond alternation in the aromatic ring in 4 is typical for metal-
stabilized arenium compounds.9 Most of the positive charge is
localized at the ortho- and para-carbon atoms as evidenced by the
C(11)-C(12) and C(14)-C(15) bond distances of 1.362(6) and
1.319(6) Å, respectively, being shorter than the rest of the distances
in the ring. The bond angles inside the ring are nearly equal to
120°, suggesting that the structure of 4 is better described in terms
of the CdO double-bond coordination to the Ir(I) center rather than
in terms of a three-centered Ir(III) metallacycle. The X-ray structure
also shows that all six carbon atoms are located in the same plane
with the coordinated oxygen atom pushed out of the plane by
approximately 10°. In the η5-oxocyclohexadienyl complexes of Ir,
the carbon atom itself is removed out of plane, indicating
nonaromaticity.7c A slight deviation of the oxygen atom with respect
to the ring plane has also been observed in chelated η1-coordinated
Pd(II) anthraquinone complexes.15 Due to steric constrains, the
acetonitrile ligand occupies the position trans to the coordinated
CdO bond.
While phenoxonium cations are implicated in chemical and
biochemical processes,1 they have never been isolated. As previ-
ously demonstrated with the elusive methylene arenium species,
metal complexation to the exocyclic double bond provides a useful
tool for stabilizing such a species without direct metal interaction
with the aromatic π-system.16 However, while η2 metal complex-
ation to a CdC double bond is well-known, no examples of such
coordination to CdO bonds in quinonoid compounds have been
reported. Complexes 3 and 4 are, therefore, the first examples of
metal-stabilized phenoxonium compounds, which also involve the
unprecedented η2 CdO quinonoid coordination mode. Various
coordination modes in quinonoid compounds have been explored.
Metal complexes of bidentate o-quinone derivatives have received
considerable attention due to their unique electronic and magnetic
properties.17 In addition, π-metal complexes of p-quinones were
studied with regard to their application in new materials design
and in catalysis.18,19 Recently, metal coordination to a quinone
methide-type oxygen atom was demonstrated to trigger DNA
alkylation.20
the aromatic ring. The phenoxonium cation undergoes facile Me-O
bond cleavage, resulting in the corresponding p-quinone complex
while maintaining the unprecedented η2 coordination mode of a
quinonoid CdO bond.
Acknowledgment. This work was supported by the Israel
Science Foundation, by the Minerva Foundation, and by the Helen
and Martin Kimmel Center for Molecular Design. D.M. holds the
Israel Matz Professorial Chair of Organic Chemistry
Supporting Information Available: Experimental details for
compounds 1-5 and crystal data for complex 4 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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The positive charge delocalization within the aromatic ring is
further evidenced by the chemical reactivity of the phenoxonium
complex 3. Thus, addition of water to a solution of 3 in dioxane
results in quantitative hydrolysis of the methoxy group in the para
position within a few minutes at room temperature. For comparison,
removal of the methoxy group in anisol requires activation by strong
Lewis acids.21,22 Addition of NEt3 results in instantaneous depro-
tonation of the phenolic proton to give the iridium quinone complex
5 (Scheme 3). The 31P{1H} NMR spectrum of 5 exhibits a singlet
at 23.72 ppm. The 13C{1H} NMR spectrum shows the ipso-carbon
signal as a triplet at 71.86 ppm, indicating an η2 metal coordination
mode and disappearance of the positive charge at the former
phenoxonium ring. The quinonoid para-carbon atom appears as a
triplet at 188.35 ppm, which corresponds well with organic carbonyl
signals in quinones. The strong IR band at 1644 cm-1 further
confirms the formation of a neutral quinonoid moiety.
(21) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic Synthesis,
3rd ed.; John Wiley & Sons: New York, 1999; Chapter 3.
In summary, we have reported the first example of a phenoxo-
nium cation stabilized by an unprecedented η2 metal coordination
to the quinonoid CdO double bond. The X-ray structure and the
spectroscopic data confirm the positive charge delocalization within
(22) Me-O bond cleavage in a η5-oxocyclohexadienyl-Ir crown ether complex
with suggested participation of the crown ether chain: Amouri, H.;
Vaissermann, J.; Rager, M. N.; Resace, Y. Inorg. Chem. 1999, 38, 1211.
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