Metal-controlled reactivity of a pincer-type, σ-coordinated naphthyl radical anion

Article abstract of DOI:10.1021/ja0615066

Reduction of the new, naphthalene-based pincer complex [(C₁₀H₅(CH₂PⁱPr₂)₂)Rh(η¹-N₂)] with potassium metal gave the corresponding σ-coordinated naphthyl radical anion complex, with a ring-centered radical and no change in the formal metal oxidation state. This paramagnetic complex can be reoxidized to the diamagnetic one with [Cp₂Fe][BF₄], the structural integrity being retained. Unexpectedly, treatment of a THF solution of the reduced complex with water leads to the immediate evolution of dihydrogen, with reoxidation to the starting complex. This is in striking contrast with the well-known reactivity of the naphthide radical anion, which undergoes ring protonation by water to form 1,4-dihydronaphthalene and naphthalene in a 1:1 ratio. Copyright

Full text of DOI:10.1021/ja0615066

Published on Web 05/16/2006
Metal-Controlled Reactivity of a Pincer-type, σ-Coordinated Naphthyl Radical
Anion
Christian M. Frech,† Yehoshoa Ben-David,‡ Lev Weiner,§ and David Milstein*,‡
Department of Organic Chemistry and Unit of Chemical Research Support, Weizmann Institute of Science,
76100 RehoVot, Israel, and Department of Inorganic Chemistry, UniVersity of Zurich, 8057 Zurich, Switzerland
Received March 3, 2006; E-mail: david.milstein@weizmann.ac.il
Aromatic radical anions have been extensively studied, including
structural, physical, theoretical, and reactivity aspects, and have
found many applications.1,2 They show interesting reactivities
toward a variety of substrates, such as alkanes, alkenes, and alkynes,
aliphatic and aromatic halides, silyl halides, aldehydes and ketones,
sulfur dioxide, sulfonates and sulfonamides, carbon dioxide, dihy-
drogen, dinitrogen, as well as aliphatic and aromatic esters. An
example for a synthetic use is the reductive metalation of phenyl
thioethers,3,4 providing a useful route to the preparation of organo-
alkali compounds, such as tertiary organolithium compounds, that
are difficult to prepare by other routes.5 In contrast to alkali metals,
aromatic radical anions are soluble in polar nonprotic solvents, such
as THF and DME, allowing fast and facile stoichiometric electron
transfers in the homogeneous phase.
Pincer-type complexes constitute a family of compounds that
have attracted much recent interest. They play important roles in
organometallic reactions and mechanisms, catalysis, and the design
of new materials.6 The high thermal stability of such complexes,
particularly those based on an aromatic backbone, permits their
use as catalysts at elevated temperatures in various catalytic
applications.6
We were intrigued by the possibility of the synthesis of
organometallic radical anions of aromatic pincer-type systems, in
which the metal center is σ-bound to the aromatic core. Such novel
systems might show unique physical properties and reactivity
modes, combining metal- and arene-based properties.
We have recently observed that reduction of a phenyl-based PCP
Pd(II) complex with sodium metal led to collapse of the pincer
system to form a binuclear complex.7 Since the electron affinity of
naphthalene is higher than that of benzene, a new, naphthyl- (rather
than phenyl) based pincer ligand, 1,3-(CH2PiPr2)2C10H5 1, was
prepared in order to facilitate the generation of a stable radical anion
pincer complex.1
Ligand 1 was synthesized by treatment of 1,3-dibromomethyl-
naphthalene with diisopropylphosphine in the presence of triethyl-
amine.8 When a THF solution of [Rh(COE)2Cl]2 (COE ) cy-
clooctene) and 2 equiv of 1 was stirred under N2 for 1 h at room
temperature followed by the addition of an excess of tBuOK, facile
cyclometalation took place, yielding the Rh(I) mononuclear and
binuclear dinitrogen complexes [(C10H5(CH2PiPr2)2)Rh(η1-N2)] and
2 in a ∼2:8 ratio, at room temperature, respectively. The 31P{1H}
NMR spectrum of the solution exhibited two doublets at δ ) 65.33
ppm (1JRhP ) 155.9 Hz) and at δ ) 64.23 ppm (1JRhP ) 155.8 Hz).
Bubbling with argon or freeze-pump-thaw cycles accompanied
with argon refill resulted in almost exclusive formation of the
dinuclear complex 2. Similar dinuclear-mononuclear equilibria of
dinitrogen PCP complexes were reported.9 The 13C{1H} NMR
spectrum of 2 exhibits a signal at δ ) 173.15 ppm (dt,1JRhC
)
40.2 Hz and 2JPC ) 5.7 Hz) assigned to the Rh-Cipso carbon atom,
confirming the presence of a cyclometalated rhodium core. The IR
spectrum of [(C10H5(CH2PiPr2)2)Rh(η1-N2)] gave rise to an absorp-
tion at 2144 cm-1, indicating slightly more back-bonding to the
i
dinitrogen ligand as compared with [(C6H3(CH2 Pr2)2)Rh(η1-N2)],9a
which exhibits an absorption at 2165 cm-1. The Raman spectrum
of 2 revealed an intense band at 2068 cm-1 due to the Rh-Nt
N-Rh core of the dinuclear species, indicating a significantly higher
back-bonding to the bridging N2 unit as compared to the terminal
one.
The measured reduction potential of complex 2 in THF is -1.63
V versus FeCp2 (0/+1), which is in the general range observed for
polycyclic aromatic hydrocarbons. The reduction is quasi-reversible
(∼25%).
Treatment of 2 with potassium metal in THF at room temperature
(Scheme 1) resulted in a color change from brown to dark red within
hours, and the reduction was complete after 2 days. The 31P{1H}
NMR spectrum of a THF-d8 solution of the new product 3 exhibited
no signal, compatible with formation of a paramagnetic radical
anion. Indeed, EPR measurements of 3 gave rise to a strong,
isotropic signal with a resolved hyperfine structure.8 This can be
rationalized by coupling of the unpaired electron with the five
hydrogen atoms of the naphthalene unit. Further interactions with
the potassium cation are most likely.10 The EPR signal of 3 has a
g value of 2.0035, typical for aromatic hydrocarbon radicals. For
comparison, ligand 1 was also reduced with potassium in THF.
The EPR spectrum of the reduced ligand is almost identical to the
one obtained for 3 and exhibits a close g value, strongly indicating
that the single electron is delocalized in the naphthalene unit of 3,
and thus formally there is no change in the Rh(I) oxidation state.11
The IR spectrum of 3 did not exhibit any absorption in the region
of ∼1800-2200 cm-1. Due to intense fluorescence in the region
where the vibration of bridging N2 is expected, it was impossible
to determine the presence or absence of this ligand by Raman
spectroscopy. Further information regarding the structure of 3 was
obtained from elemental analysis, which indicated the absence of
nitrogen or the solvent THF, suggesting that only the (PCP)Rh unit
is present in the solid state.
Using an Evans balance, the magnetic susceptibilities of complex
3 (+1201 × 10-6 cm3‚mol-1),12 the reduced ligand (+1207 × 10-6
cm3‚mol-1), and potassium naphthalide (+1212 × 10-6 cm3‚mol-1
)
were measured in THF solution at 298 K. The latter value compares
well with the one reported for sodium naphthalide (+1239(10) ×
10-6 cm3‚mol-1 measured in solution at 298 K by the Gouy
method).13 The magnetic susceptibility of 3 was also measured by
the Evans NMR method14 at 298 K in THF, giving a magnetic
moment of 1.68 µB and confirming a one-electron reduction of 3.15,16
While our results indicate that the electron is localized in the
naphthalenic ring, the reactivity of complex 3 is strikingly different
† University of Zurich.
‡ Department of Organic Chemistry, Weizmann Institute of Science.
§ Unit of Chemical Research Support, Weizmann Institute of Science.
9
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J. AM. CHEM. SOC. 2006, 128, 7128-7129
10.1021/ja0615066 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
studies on the properties and reactivity of this unique system and
extension to other σ-aryl transition metal systems are underway.
Acknowledgment. This research was supported by the Israel
Science Foundation, the Helen and Martin Kimmel Center for
Molecular Design, and the Swiss National Science Foundation.
D.M. thanks the Miller Institute for Basic Research in Science at
UC-Berkeley for a Visiting Professorship during the writing of this
paper.
Supporting Information Available: Experimental procedures and
characterization for compounds 1-5 and the EPR of 3 and of the
reduced ligand. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
from that of the naphthalide radical anion. Remarkably, when a
THF solution of complex 3 was treated with water under N2,
immediate evolution of dihydrogen, with almost quantitative
reoxidation to yield the starting complex 2, took place (Scheme
1). A small amount (∼10%) of the dihydrogen complex (C10H5-
(CH2PiPr2)2)Rh(η2-H2) (4) was also formed, as a result of the
reaction of 2 with H2, as confirmed by the independent synthesis
of 4 by passing H2 through a benzene solution of 2 for several
minutes. The resulting solution of the reaction of 3 with water was
basic, in contrast to an aqueous solution of 2, compatible with
formation of potassium hydroxide. The observed reactivity is very
different from that of the naphthalide radical anion, which undergoes
protonation of the ring upon treatment with water, to form 1,4-
dihydronaphthalene and naphthalene in a 1:1 ratio.17 No ligand
protonation of 3 was observed. Apparently, the metal center
dramatically influences the reactivity of the organic unit. This result
is of significant interest regarding catalytic electron transfer
reactions with the possibility to store electrons in the aromatic
ligand, followed by metal-centered reactions.
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Oxidation of 3 readily regenerates the diamagnetic 2. Thus, upon
treatment of 3 with [Fe(Cp)2][BF4], almost quantitative (by NMR)
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to rhodium or to potassium cation) is used for the calculation of the
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organic halide. Thus, treatment of 3 with 2 equiv (or excess) of
benzyl chloride led to the immediate precipitation of potassium
chloride and the formation of a mixture of several (unidentified)
compounds containing the RhIII benzyl chloride adduct [(C10H5-
i
(CH2 Pr2)2Rh(CH2Ph)(Cl)] (5) in 28% yield (Scheme 1). 5 was
independently synthesized in quantitative yield by the treatment of
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Chloride loss followed by coupling of the resulting benzyl radical
would yield dibenzyl, which was detected in the reaction mixture
by GC/MS (Scheme 1). Reaction of 2 with excess benzyl chloride
yields the observed complex 5.
In conclusion, we have prepared an aryl radical anion σ-bound
to a transition metal. While the electron has been proven to be
localized in the naphthyl unit, reactivity is strikingly different from
that of the naphthide radical anion, as exemplified by water
reduction to dihydrogen, rather than the expected ring protonation.
The radical anion complex can be quantitatively oxidized to the
diamagnetic complex, with no loss of structural integrity. Further
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respectively.
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10-6 cm3‚mol-1, respectively. The latter value compares well with the
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Lide, D. R., Ed. Handbook of Chemistry and Physics, 74th ed.; CRC
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