The binuclear iridium (II) hydride complex [(C5ME5)Ir(μ-H)]2: A novel base for reversible deprotonation acidic organic compounds and a unique catalyst for C-C bond cleavage of aromatic 1,2-diols and Michael additions [2]

Full text of DOI:10.1021/ja010186h

5812
J. Am. Chem. Soc. 2001, 123, 5812-5813
Scheme 1
The Binuclear Iridium(II) Hydride Complex
[(C5Me5)Ir(µ-H)]2: A Novel Base for Reversible
Deprotonation of Acidic Organic Compounds and a
Unique Catalyst for C-C Bond Cleavage of
Aromatic 1,2-Diols and Michael Additions
Zhaomin Hou,*,† Take-aki Koizumi,†,§ Akira Fujita,‡
Hiroshi Yamazaki,‡ and Yasuo Wakatsuki*,†
Organometallic Chemistry Laboratory
RIKEN (The Institute of Physical and Chemical Research)
Hirosawa 2-1, Wako, Saitama 351-0198, Japan
Department of Applied Chemistry, Chuo UniVersity
Kasuga 1-13-27, Bunkyo, Tokyo 112-0003, Japan
new signals could be compared with those previously reported
for the dimeric trihydridoiridium cation species {[(C5Me5)Ir]2(µ-
H)3}+(PF6)- (2‚PF6) (δ 2.09, -15.33 in CDCl3),2a,5 suggesting
that a similar cationic species such as {[(C5Me5)Ir]2(µ-H)3}+-
[(OMe)(HOMe)n]- (2‚OMe) might be formed in the present
reaction.6 The new signals assignable to the cationic species
became stronger as the temperature was decreased, but disap-
peared upon warming up to room temperature, which suggests
that the formation of the cationic species such as 2‚OMe must
be reversible. Suppose that the methoxide anion in 2‚OMe was
stabilized by one molecule of MeOH (n ) 1, vide infra), an
equilibrium constant of Keq ) 1.02 × 10-2 for the reaction of 1
with MeOH in toluene-d8 at 25 °C (eq 1) could be estimated from
the van’t Hoff equation (∆H ) -18.26 kJ mol-1, ∆S ) -99.35
J mol-1 K-1).
ReceiVed January 23, 2001
Transition-metal hydride complexes bearing the pentamethyl-
cyclopentadienyl (C5Me5) ligand are of particular interest and
importance in organometallic chemistry and homogeneous ca-
talysis. For iridium hydride complexes bearing the C5Me5 ligand,
a number of complexes in the oxidation states of +3, +4, and
+5 have been synthesized and have had their reactivity studied
in the past three decades.1,2 However, this type of complex in the
+2 oxidation state, the lowest accessible and possibly the most
reactive one in this series of complexes, has hardly been explored.1
During our recent studies on transition-metal ketyl complexes,
we serendipitously isolated and structurally characterized a
binuclear iridium(II) dihydride complex, [(C5Me5)Ir(µ-H)]2 (1),3
which, as far as we are aware, represents the first example of a
well-defined (pentamethylcyclopentadienyl)iridium(II) hydride
complex. We have now found that this Ir(II) complex 1 is a unique
metal hydride complex which shows an unprecedented versatile
reactivity toward a variety of substrates. Among its most
remarkable reactions is proton abstraction from acidic organic
compounds to yield reVersibly the corresponding metal-protonated
cationic species {[(C5Me5)Ir]2(µ-H)3}+ (2). The reVersible pro-
tonation/deprotonation character makes 1 a unique catalyst for
C-C bond cleavage of aromatic 1,2-diols and Michael additions.
Described herein are some representative results.
Although isolation of the cationic species 2‚OMe from 1 and
MeOH was difficult, an analogous reaction of 1 with 2 equiv of
2,2′-biphenol in toluene afforded the corresponding cationic
complex 2‚OAr (OAr ) 2-(2′-hydroxyphenyl)phenoxide) in 85%
isolated yield, in which the aryloxide counteranion was stabilized
by interaction with one molecule of 2,2′-biphenol as confirmed
by an X-ray diffraction study (Scheme 1).7 When 2‚OAr was
treated with 4 equiv of tBuOK in CH2Cl2, the neutral complex 1
was recovered almost quantitatively, which clearly shows that
the cationic unit {[(C5Me5)Ir]2(µ-H)3}+ in 2‚OAr is protonic and
can be deprotonated by an appropriate base.
Although an apparent reaction between 1 and MeOH was not
1
observed at 25 °C in toluene-d8 or CD2Cl2 by H NMR, the
reaction of 1 with MeOD (ca. 200 equiv) under similar conditions
yielded almost quantitatively the corresponding deuteride complex
3 (Scheme 1). Treatment of 3 with MeOH or H2 (1 atm) at room-
temperature regenerated 1 almost quantitatively (Scheme 1).4
When the mixture of 1 and MeOH was cooled to -30 °C, two
new signals with the proton ratio of 30 to 3 appeared at δ 1.66
(C5Me5) and -15.38 (Ir-H) in toluene-d8 or δ 2.01 and -15.55
in CD2Cl2, respectively, in addition to the peaks for 1.3 These
(4) Monitoring of the reaction of 1 with MeOD and that of 3 with MeOH
or H2 by 1H or 2H NMR suggested the formation of a monohydride/
monodeuteride species such as [(C5Me5)Ir]2(µ-H)(µ-D) (δ -13.42, Ir-H/D)
at the early stage of the reaction. This is the only observable intermediate at
room temperature. A longer-time (>10 h) reaction of 3 with H2 afforded the
2c,e
known Ir(IV) complex [(C5Me5)IrH3]2
hydride species.
together with some unidentified
† RIKEN.
‡ Chuo University.
(5) For structural characterization of {[(C5Me5)Ir]2(µ-H)3}+(A)- (A ) BF4
or ClO4), see: (a) Bau, R.; Teller, R. G.; Kirtley, S. W.; Koetzle, T. F. Acc.
Chem. Res. 1979, 12, 176. (b) Stevens, R. C.; Mclean, M. R.; Wen, T.;
Carpenter, J. D.; Bau, R.; Koetzle, T. F. Inorg. Chim. Acta 1989, 161, 223.
(6) For previous examples of protonation of transition-metal hydride
complexes, see: (a) Kristja´nsdo´ttir, S. S.; Norton, J. R. Acidity of Hydrido
Transition Metal Complexes in Solution. In Transition Metal Hydrides; Dedieu,
A. Ed.; VCH: New York, 1991; pp 309-359. (b) Jessop, P. G.; Morris, R.
H. Coord. Chem. ReV. 1992, 121, 155. (c) Heinekey, D. M.; Oldham, W. J.,
Jr. Chem. ReV. 1993, 93, 913. (d) Crabtree, R. H. Acc. Chem. Res. 1990, 23,
95. (e) Kubas G. J. Acc. Chem. Res. 1988, 21, 120. (f) Papish, E. T.; Rix, F.
C.; Spetseris, N.; Norton, J. R.; Williams, R. D. J. Am. Chem. Soc. 2000,
122, 12235 and references therein.
§ T.K. is a Special Postdoctoral Researcher under the Basic Science Program
of RIKEN.
(1) Reviews: (a) Maitlis, P. M. Acc. Chem. Res. 1978, 11, 301. (b) Leigh,
G. J.; Richards, R. L. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 5, p
541. (c) Graham, W. A. G. J. Organomet. Chem. 1986, 300, 81. (d) Bergman,
R. G. J. Organomet. Chem. 1990, 400, 273. (e) Atwood, J. D. In Compre-
hensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson,
G., Atwood, J. D., Eds.; Pergamon: Oxford, 1995; Vol. 8, p 303. (f) Bianchini,
C. Meli, A. Acc. Chem. Res. 1998, 31, 109.
(2) For examples, see: (a) White, C.; Oliver, A. J., Maitlis, P. M. J. Chem.
Soc., Dalton Trans. 1973, 1901. (b) Gilbert, T. M.; Bergman, R. G. J. Am.
Chem. Soc. 1985, 107, 3502. (c) Gilbert, T. M.; Hollander, F. J.; Bergman,
R. G. J. Am. Chem. Soc. 1985, 107, 3508. (d) Grushin, V. V.; Vymenits, A.
B.; Yanovsky, A. I.; Struchkov, Y. T.; Vol’pin, M. E. Organometallics 1991,
10, 48. (e) Jones, W. D.; Chin, R. M. J. Am. Chem. Soc. 1994, 116, 198. (f)
Vicic, D. A.; Jones, W. D. Organometallics 1997, 16, 1912.
(3) Hou, Z.; Fujita, A.; Koizumi, T.; Yamazaki, H.; Wakatsuki, Y.
Organometallics 1999, 18, 1979.
(7) Crystallographic data for 2‚OAr: orthorhombic, space group Pca21 (No.
29), a ) 20.165(7) Å, b ) 8.580(3) Å, c ) 22.964(8) Å, V ) 3973(2) Å3,
Z ) 4, Dc ) 1.711 g cm-3, µ(Mo KR) ) 67.31 cm-1, R (Rw) ) 0.0513 (0.0982)
1
for 3974 unique data with I > 2σ(I) and 451 parameters. H NMR (CD2Cl2,
25 °C): δ 12.82 (br s, 3H, OH), 6.77-7.35 (m, 16 H, aromatic), 2.01 (s,
30H, C5Me5), -15.57 (s, 3 H, Ir-H). Anal. Calcd for C44H44Ir2O4: C, 51.34;
H, 5.09. Found: C, 51.15; H, 5.05.
10.1021/ja010186h CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/24/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 24, 2001 5813
Scheme 2
Scheme 3
More remarkably, when 1 was stirred with 1 equiv of
benzopinacol in toluene-d8 at room temperature for 6 h, ben-
zophenone and benzhydrol, that is, the C-C bond cleavage
products of benzopinacol, were obtained almost quantitatively,
while 1 remained unchanged as confirmed by the 1H NMR
spectrum. This reaction could be achieved catalytically. In the
presence of 1 mol % of 1, benzopinacol (4a) was transformed
almost quantitatively to benzophenone (5a) and benzhydrol (6a)
at 65 °C within 18 h (Scheme 2). The use of 1,2-bis(biphenyl-
2,2′-diyl)ethane-1,2-diol (4b) in place of benzopinacol afforded
fluorenone (5b) and fluorenol (6b) analogously. Although forma-
tion of a ketyl species by deprotonation and C-C bond cleavage
of a 1,2-diol has been previously reported,8a-c,e the catalytic
separation of a 1,2-diol into a pair of ketone and alcohol is, to
the best of our knowledge, unprecedented.9 These reactions could
be explained by the mechanism shown in Scheme 3, in which
the protonation of 1 by the 1,2-diol 4 to give the 1,2-diolate 7
and the intramolecular deprotonation of the cationic unit in 9 to
release 1 and the alcohol 6 play a crucially important role in the
catalytic cycle.8,10
Scheme 4
acrylnitrile at room temperature afforded almost quantitatively
the addition product 11a or 11b, respectively (Scheme 4). These
reactions represent a rare example of catalytic Michael addition
under neutral conditions.12
In summary, we have found that the binuclear iridium(II)
hydride complex 1 is a novel base for reVersible deprotonation
of acidic organic compounds, leading to unprecedented catalytic
C-C bond cleavage of aromatic 1,2-diols and catalytic Michael
addition under neutral conditions. Further studies on the reactivity
of 1 are under progress.
Complex 1 could also reversibly abstract a proton from active
methylene compounds and act as a catalyst for Michael addition
reactions. The reaction of 1 with 1 equiv of CH2(COCF3)2 in THF
gave the structurally characterizable cationic complex {[(C5Me5)-
Ir]2(µ-H)3}+[CH(COCF3)2]- (2‚CH(COCF3)2) in 78% isolated
yield.11 In the presence of 3 mol % of 1, the reaction of ethyl
acetoacetate (10a) or ethyl cyanoacetate (10b) with 2 equiv of
Acknowledgment. This work was partly supported by a Grant-in-
aid from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
(8) The elementary steps, which correspond to the C-C bond cleavage of
the 1,2-diolate 7 to give the ketyl species 8,8a-c,e the hydrogen radical (H•)
abstraction of the ketyl radical 8 from the 1,2-diol 4 to yield 9 and 11,8a,b,d
and the homolytic cleavage of the central C-C bond in 11 to give the ketone
58b,d in Scheme 3, have been described previously. (a) Hou, Z.; Jia, X.; Fujita,
A.; Tezuka, H.; Yamazaki, H.; Wakatsuki, Y. Chem. Eur. J., 2000, 6, 2994.
(b) Hou, Z.; Fujita, A.; Zhang, Y.; Miyano, T.; Yamazaki, H.; Wakatsuki, Y.
J. Am. Chem. Soc. 1998, 120, 754. (c) Hou, Z.; Fujita, A.; Yamazaki, H.;
Wakatsuki, Y. J. Am. Chem. Soc. 1996, 118, 2503. (d) Hou, Z.; Fujita, A.;
Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc. 1996, 118, 7843. (e) Hou,
Z.; Miyano, T.; Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc. 1995, 117,
4421.
(9) Oxidation of aromatic 1,2-diols to give the corresponding ketones has
been previously reported. For examples, see: (a) Bockman, T. M.; Hubig, S.
M.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6542. (b) Han, D. S.; Shine,
H. J. J. Org. Chem. 1996, 61, 3977. (c) Penn, J. H.; Duncan, J. H. J. Org.
Chem. 1993, 58, 2003. (d) Perrier, S.; Sankararaman, S.; Kochi, J. K. J. Chem.
Soc., Perkin Trans. 2 1993, 825.
Supporting Information Available: A listing of atomic coordinates,
thermal parameters, bond distances and angles, and ORTEP drawings
for 2‚OAr and 2‚CH(COCF3)2 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA010186H
(11) Crystallographic data for {[(C5Me5)Ir]2(µ-H)3}+[CH(COCF3)2]-‚THF:
monoclinic, space group C2/c (No. 15), a ) 16.551(5) Å, b ) 17.307(4) Å,
c ) 13.311(8) Å, â ) 118.116(8)°, V ) 3363(2) Å3, Z ) 4, Dc ) 1.851 g
cm-3, µ(Mo KR) ) 79.87 cm-1, R (Rw) ) 0.0552 (0.0867) for 2635 unique
data with I > 3σ(I) and 182 parameters.1H NMR (C6D6, 25 °C): δ 6.23 (s,
1 H, CH(COCF3)2), 1.60 (s, 30 H, C5Me5), -15.37 (s, 3 H, Ir-H). Anal.
Calcd for C25H34Ir2O2F6: C, 34.71; H, 3.96. Found: C, 35.01; H, 4.03.
(12) For examples of catalytic Michael reactions under neutral conditions,
see: (a) Murahashi, S.-I.; Naota, T.; Taki, H.; Mizuno, M.; Takaya, H.;
Komiya, S.; Mizuho, Y.; Oyasato, N.; Hiraoka, M.; Hirano, M.; Fukuoka, A.
J. Am. Chem. Soc. 1995, 117, 12436. (b) Gomez-Bengoa, E.; Cuerva, J. M.;
Mateo, C.; Echavarren, A. M. J. Am. Chem. Soc. 1996, 118, 8553 and
references therein.
(10) Some processes such deprotonation and the C-C bond cleavage of
the 1,2-diolate 7 to give 8 in Scheme 3 are reversible (see also ref 8). However,
the reverse steps in Scheme 3 were omitted for clarity.