H-H and N-H bond cleavage of dihydrogen and ammonia with a bifunctional parent imido (NH)-bridged diiridium complex

Article abstract of DOI:10.1021/ja203538b

Hydrogenation and protonation of parent imido complexes have attracted much attention in relation to industrial and biological nitrogen fixation. The present study reports the structure and properties of the highly unsaturated diiridium parent imido compl

Full text of DOI:10.1021/ja203538b

COMMUNICATION
pubs.acs.org/JACS
HꢀH and NꢀH Bond Cleavage of Dihydrogen and Ammonia with a
Bifunctional Parent Imido (NH)-Bridged Diiridium Complex
Takashi Kimura, Naoyuki Koiso, Koji Ishiwata, Shigeki Kuwata,* and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
S Supporting Information
b
Scheme 1. Metal-Mediated NꢀH Bond Cleavage of
ABSTRACT: Hydrogenation and protonation of parent
imido complexes have attracted much attention in relation
to industrial and biological nitrogen fixation. The present
study reports the structure and properties of the highly
unsaturated diiridium parent imido complex [(Cp*Ir)2-
(μ2-H)(μ2-NH)]þ derived from deprotonation of a parent
amido complex. Because of the Lewis acidꢀBrønsted base
bifunctional nature of the metalꢀNH bond, the parent
imido complex promotes heterolysis of H2 and deprotona-
tive NꢀH cleavage of ammonia to afford the corresponding
parent amido complexes under mild conditions.
Ammonia
Scheme 2. Synthesis of Parent Imido Complex 3a
etalꢀparent imides (MꢀNH) and amides (MꢀNH2) are
M
key intermediates in industrial and biological nitrogen
fixation performed on oxide-supported late transition metals
and a MoFe7S9 cluster in nitrogenase.1 Although the protonation
and hydrogenation of parent imido complexes to form parent
amido and ammine complexes have been extensively studied in
this context,2ꢀ4 most of such reductive transformations are
associated with high-valent early transition metals. This is due
in part to the dπ/pπ repulsion between the low-valent late metal
and nitrogen;5 the resultant Brønsted basic and nucleophilic
character of the nitrogen renders the compact NH ligand
too reactive to be isolated or too much stabilized by triple or
quadruple bridging without lone-pair electrons on the
nitrogen.6,7 Parent amido complexes also have close relevance
to the use of ammonia as a substrate in homogeneous catalysis.8,9
Recent progress in this area has focused on the oxidative
addition of ammonia (Scheme 1a) as a NꢀH bond activation
process.10ꢀ12 An alternative and conventional access to parent
amido complexes has been provided by salt elimination reactions
(Scheme 1b), which in general require external strong bases such
as sodium amides or alkoxides.13 In contrast, deprotonative
NꢀH bond cleavage promoted by an internal Brønsted basic
cooperating ligand (Scheme 1c) remains much less explored. As
an extension of our continuing studies of metalꢀamine/amido
bifunctional catalysts,14ꢀ19 we report here the synthesis of the
highly unsaturated yet isolable diiridium(III) parent imido com-
plex [(Cp*Ir)2(μ2-H)(μ2-NH)][BPh4] (3; Cp* = η5-C5Me5) by
deprotonation of the parent amido complex [(Cp*IrCl)2(μ2-
H)(μ2-NH2)] (1). Facile heterolysis of dihydrogen and depro-
tonative NꢀH bond cleavage of ammonia at the IrꢀNH bond in
a Reagents and Conditions: (a) AgOTf (2 equiv), THF, rt. (b) (i) NEt3
(1 equiv), CH2Cl2, rt; (ii) NaBPh4. OTf = OSO2CF3.
the dinuclear platform, leading to the formation of parent amido
complexes, are described.
The parent imido complex 3 was obtained by stepwise
dehydrochlorination of the parent amido complex 117 through
the intermediary triflate complex 2 (Scheme 2). X-ray analysis of
2, which was generated by chloride abstraction of 1 in 61%
isolated yield, revealed the dinuclear structure with a bridging
triflate ligand (Figure S1 in the Supporting Information). The two
amido protons as well as the two CF3 groups in 2 are equivalent
even at ꢀ40 °C in the 1H and 19F NMR spectra, indicating facile
dissociation of the triflate ligand. Unlike the chloro complex 1, the
cationic triflate complex 2 smoothly reacted with an equimolar
amount of triethylamine to afford the parent imido complex 3 in
24% overall yield after anion exchange with tetraphenylborate.
The 1H NMR spectrum of 3 exhibited a characteristic singlet due
to the NH proton at δ 15.78, which was split into a doublet
(1JNH = 81.6 Hz) upon 15N labeling, along with a hydrido
resonance at δ ꢀ1.76. The NH stretching band appeared at
Received: April 18, 2011
Published: May 17, 2011
r
2011 American Chemical Society
8880
dx.doi.org/10.1021/ja203538b J. Am. Chem. Soc. 2011, 133, 8880–8883
|

Journal of the American Chemical Society
COMMUNICATION
Figure 1. Structure of 3. Hydrogen atoms except for the imido
hydrogen have been omitted for clarity.
3272 cmꢀ1 in the IR spectrum. The detailed structure of 3 was
determined by X-ray crystallography, as shown in Figure 1. The
IrꢀN distancesof 1.891(6) and 1.893(8) Å are much shorter than
those in 1 and 2 [2.054(4)ꢀ2.069(6) Å], suggesting sizable π
donation from the parent imido ligand to the unsaturated iridium
centers.18 The IrꢀIr distance of 2.6708(5) Å is also shorter than
that in 2 [2.8188(3) Å]. The NH group points almost perpen-
dicular to one of the phenyl rings in the tetraphenylborate
Figure 2. Structure of the cationic part of 4 2THF. Hydrogen atoms
except for the imido hydrogens have been omitted for clarity.
3
Scheme 3. HꢀH and NꢀH Cleavage with Parent Imido
Complex 3
anion, with a close NH
indicating the presence of an NH
30-electron, bulky imido-bridged dirhodium(II) complex without
bridging hydride, [(Cp*Rh)2(μ2-NC6H3Pri2-2,6)], was recently
isolated.21
(Ph centroid) contact of 2.27 Å,
3 3 3
π interaction.20 A related
3 3 3
The Ir2(μ2-NH) core in 3 features the potentially Lewis acidic
iridium centers and the adjacent Brønsted basic imido nitrogen.
This bifunctional nature led to base-assisted dimerization of 3,
possibly triggered by deprotonation of the imido ligand, to
generate the bis(μ3-NH)-bridged tetranuclear cluster 4 as the
acidꢀbase adduct in 72% yield (eq 1):
Figure 3. Structure of 5. Hydrogen atoms except for the amido
Figure 2 depicts the crystal structure of dicationic cluster 4. The
four iridium atoms form a butterfly-type core with five IrꢀIr
contacts ranging from 2.9097(4) to 2.9484(4) Å, suggesting
delocalized metalꢀmetal bonding interactions in the 64-electron
cluster, for which only four metalꢀmetal bonds would be
expected on the basis of the effective atomic number rule.
Although the hydrido ligands could not be located in the X-ray
hydrogens have been omitted for clarity. The NH (Ph centroid)
3 3 3
distances are 2.60 and 2.69 Å.
consistent with the addition of 1 mol of H2 across the unsaturated
IrꢀNH bond in 3 and the C2v-symmetric structure of the product
5. The crystal structure of 5 shown in Figure 3 again suggests the
1
analysis, the H NMR spectrum of 4 was consistent with an
presence of NH π interactions between the amido protons and
3 3 3
approximately C2v-symmetric structure with two μ3-bridging
hydrido ligands, as in the isoelectronic sulfido clusters.22 The
broad signal due to the NH protons at δ 2.87 disappeared upon
treatment with D2O. It is to be noted that few examples of
isolable and fully characterized pairs of monomeric and dimeric
imido complexes with the same empirical formula such as 3 and 4
have been reported to date.23
More importantly, hydrogenation of the IrꢀNH group in 3
took place under mild conditions to produce the corresponding
IrH(NH2) complex. When a solution of 3 was stirred under a H2
atmosphere (1 atm) at room temperature, the parent amido
complex 5 was isolated in 29% yield, as shown in Scheme 3. The
1H NMR spectrum of 5 exhibited signals ascribed to the Cp*,
hydrido, and amido protons in an intensity ratio of 15:1:1,
the tetraphenylborate anion. The two crystallographically unlo-
cated hydrido ligands most probably bridge the two iridium atoms,
judging from the relative orientation of the two Cp* ligands with
respectto the Ir2N triangle. The IrꢀNH2 bondlengths of 2.063(9)
and 2.114(10) Å, which are slightly longer than those in 1 and 2,
and the short IrꢀIr distance of 2.6138(5) Å might be explained by
the reduction in electron donation from the nitrogen due to the
IrꢀIr multiple-bond character. The reaction could be regarded as a
dinuclear variant of H2 heterolysis at mononuclear late transition
metalꢀamido complexes, some of which have been used as
effective hydrogenation catalysts.15,19,24 The closest example in
the literature is the hydrogenation of the saturated Ruꢀ(μ3-NH)
bond in [(Cp*Ru)3(μ2-H)3(μ3-NH)] to give the parent amido
complex [(Cp*Ru)3(μ2-H)4(μ2-NH2)] through oxidative addition
8881
dx.doi.org/10.1021/ja203538b |J. Am. Chem. Soc. 2011, 133, 8880–8883

Journal of the American Chemical Society
COMMUNICATION
’ AUTHOR INFORMATION
Corresponding Author
skuwata@apc.titech.ac.jp; tikariya@apc.titech.ac.jp
’ ACKNOWLEDGMENT
This paper is dedicated to the memory of Professor Yasushi
Mizobe. This work was supported by Grants-in-Aid for Scientific
Research on Priority Areas (18065007, “Chemistry of Concerto
Catalysis”) and Scientific Research (S) (22225004) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan, and the JGC-S Scholarship Foundation (S.K.).
Figure 4. Structure of the cationic part of 6. Hydrogen atoms in the Cp*
ligands have been omitted for clarity.
of H2,4 which is less probable for the iridium(III) complex 3. Such
heterolytic cleavage of dihydrogen in dinuclear complexes is also
attractive in relation to hydrogenase enzymes.16
’ REFERENCES
(1) (a) Ertl, G. Angew. Chem., Int. Ed. 2008, 47, 3524. (b) Hoffman,
B. M.; Dean, D. R.; Seefeldt, L. C. Acc. Chem. Res. 2009, 42, 609.
(c) Dance, I. Dalton Trans. 2010, 39, 2972.
(2) (a) Hidai, M.; Mizobe, Y. Chem. Rev. 1995, 95, 1115.(b) Peters,
J. C.; Mehn, M. P. In Activation of Small Molecules; Tolman, W. B., Ed.;
Wiley-VCH: Weinheim, Germany, 2006; pp 81ꢀ119. (c) Schrock, R. R.
Angew. Chem., Int. Ed. 2008, 47, 5512. (d) Ballmann, J.; Munhꢀa, R. F.;
Fryzuk, M. D. Chem. Commun. 2010, 46, 1013.
The bifunctional metalꢀNH moiety in the dinuclear parent
imido complex 3 also facilitates NꢀH bond cleavage of ammonia
in the manner illustrated in Scheme 1c. As shown in Scheme 3,
treatment of 3 with an equimolar amount of ammonia at room
temperature afforded the parent bis(amido) complex 6 in 44%
isolated yield. The 1H NMR spectrum of 6 displayed two broad
signals at δ ꢀ0.50 and 0.64 with a 2H intensity, which are ascribed
to the exo and endo amido protons. The structure of 6 was deter-
mined by X-ray crystallography (Figure 4). The IrꢀN distances
[2.077(2)ꢀ2.104(2) Å] are consistent with coordinative satura-
tion of the metal centers. Related deprotonative NꢀH cleavage of
ammonia without formal redox of the metal has been docu-
mented for some hydroxo,11 oxo,12 and alkyl25 complexes; how-
ever, the subsequent dissociation of the protonated cooperating
ligands (LH in Scheme 1c) has prevented the catalytic application of
such metalꢀligand bifunctional activation. Quite recently, Milstein
and co-workers26 demonstrated the deprotonative and reversible
NꢀH cleavage of ammonia through metalꢀligand cooperation,
wherein the parent amido intermediate was not directly ob-
served. In solid-surface organometallic chemistry, formation of a
parent amido species from a parent imido ligand and ammonia
on a silica-supported tantalum complex has been reported.27 The
formation of 6 represents the first example of metalꢀligand
bifunctional activation of ammonia in which the produced parent
amido complex has been unambiguously characterized and the
cooperating ligand stays in the coordination sphere throughout
the transformation.
In conclusion, the parent imido-bridged diiridium complex 3
featuring the Lewis acidꢀBrønsted base bifunctionality of the
sterically encumbered MꢀNH bonds promotes heterolysis of
dihydrogen and NꢀH bond cleavage of ammonia to give the
corresponding parent amido complexes under mild conditions.
The facile proton shifts between the reduced nitrogenous ligands
and the dihydrogen molecule observed for the late metal com-
plexes provide insights into the mechanism of nitrogen fixation as
well as the organometallic transformation of ammonia. Further
studies aimed at catalytic reactions of dihydrogen and ammonia
on multimetallic centers furnished with metalꢀligand bifunc-
tionality are in progress.
(3) (a) Cabon, J.-Y.; Le Roy, C.; Muir, K. W.; Pꢀetillon, F. Y.; Quentel,
F.; Schollhammar, P.; Talarmin, J. Chem.—Eur. J. 2000, 6, 3033.
(b) Meyer, T. J.; Huynh, M. H. V. Inorg. Chem. 2003, 42, 8140.
(c) Scepaniak, J. J.; Young, J. A.; Bontchev, R. P.; Smith, J. M. Angew.
Chem., Int. Ed. 2009, 48, 3158. (d) Sch€offel, J.; Rogachev, A. Y.; DeBeer
George, S.; Burger, P. Angew. Chem., Int. Ed. 2009, 48, 4737.
(e) Bowman, A. C.; Bart, S. C.; Heinemann, F. W.; Meyer, K.; Chirik,
P. J. Inorg. Chem. 2009, 48, 5587. (f) Nikiforov, G. B.; Vidyaratne, I.;
Gambarotta, S.; Korobkov, I. Angew. Chem., Int. Ed. 2009, 48, 7415.
(g) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. Nat. Chem. 2010, 2, 30.
ꢁ
(h) Sch€offel, J.; Suꢁsnjar, N.; N€uckel, S.; Sieh, D.; Burger, P. Eur. J. Inorg.
Chem. 2010, 4911. (i) Scepaniak, J. J.; Vogel, C. S.; Khusniyarov, M. M.;
Heinemann, F. W.; Meyer, K.; Smith, J. M. Science 2011, 331, 1049.
(j) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2011, 3, 120.
(4) Kameo, H.; Nakajima, Y.; Suzuki, H. Eur. J. Inorg. Chem.
2007, 1793.
(5) Caulton, K. G. New J. Chem. 1994, 18, 25.
(6) Li, Y.; Wong, W.-T. Coord. Chem. Rev. 2003, 243, 191.
(7) In the seminal contributions by Peters and co-workers, μ2-NH
diiron complexes supported by bulky tris(phosphino)borate ligands
were isolated from hydrogenation of bridging nitride or NꢀN bond
cleavage of a μ2,η2:η2-N2H2 ligand, although subsequent hydrogenation
and protonation of the parent imido ligands was not achieved. See:
(a) Brown, S. D.; Mehn, M. P.; Peters, J. C. J. Am. Chem. Soc. 2005,
127, 13146. (b) Saouma, C. T.; M€uller, P.; Peters, J. C. J. Am. Chem. Soc.
2009, 131, 10358. For additional examples of terminal and μ2-NH
complexes of late transition metals, see:(c) Lutz, C. M.; Wilson, S. R.;
Shapley, P. A. Organometallics 2005, 24, 3350. (d) Yi, X.-Y.; Ng, H.-Y.;
Williams, I. D.; Leung, W.-H. Inorg. Chem. 2011, 50, 1161.
(8) (a) Klinkenberg, J. L.; Hartwig, J. F. Angew. Chem., Int. Ed. 2011,
50, 86. (b) Aubin, Y.; Fischmeister, C.; Thomas, C. M.; Renaud, J.-L.
Chem. Soc. Rev. 2010, 39, 4130. (c) van der Vlugt, J. I. Chem. Soc. Rev.
2010, 39, 2302.
(9) For related hydrazido(1ꢀ) complexes generated by NꢀH bond
cleavage of hydrazine, see: (a) Huang, Z.; Zhou, J.; Hartwig, J. F. J. Am.
Chem. Soc. 2010, 132, 11458. (b) Takei, I.; Dohki, K.; Kobayashi, K.;
Suzuki, T.; Hidai, M. Inorg. Chem. 2005, 44, 3768. (c) Vela, J.; Stoian, S.;
Flaschenriem, C. J.; M€unck, E.; Holland, P. L. J. Am. Chem. Soc. 2004,
126, 4522. (d) Jahncke, M.; Neels, A.; Stoeckli-Evans, H.; S€uss-Fink, G.
J. Organomet. Chem. 1998, 565, 97.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
X-ray crystallographic data for 2, 3 CH2Cl2, 4 2THF, 5, and 6
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(10) (a) Zhao, J.; Goldman, A. S.; Hartwig, J. F. Science 2005,
307, 1080. (b) Braun, T. Angew. Chem., Int. Ed. 2005, 44, 5012.
(c) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. Eur. J. Inorg. Chem.
3
3
8882
dx.doi.org/10.1021/ja203538b |J. Am. Chem. Soc. 2011, 133, 8880–8883

Journal of the American Chemical Society
COMMUNICATION
2007, 2677. (d) Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola,
D. J.; Ozerov, O. V. J. Am. Chem. Soc. 2007, 129, 10318. (e) Morgan, E.;
MacLean, D. F.; McDonald, R.; Turculet, L. J. Am. Chem. Soc. 2009,
131, 14234. (f) Peng, Y.; Guo, J.-D.; Ellis, B. D.; Zhu, Z.; Fettinger, J. C.;
Nagase, S.; Power, P. P. J. Am. Chem. Soc. 2009, 131, 16272. (g) Ahijado
Salomon, M.; Jungton, A.-K.; Braun, T. Dalton Trans. 2009, 7669.
(11) Hillhouse, G. L.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5472.
(12) Nakajima, Y.; Kameo, H.; Suzuki, H. Angew. Chem., Int. Ed.
2006, 45, 950.
(13) (a) Klinkenberg, J. L.; Hartwig, J. F. J. Am. Chem. Soc. 2010,
132, 11830. (b) Fox, D. J.; Bergman, R. G. Organometallics 2004,
23, 1656. (c) Bai, G.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.
Organometallics 2001, 20, 2962.
(14) (a) Ikariya, T. Bull. Chem. Soc. Jpn. 2011, 84, 1. (b) Ikariya, T.;
Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (c) Ikariya, T.; Murata, K.;
Noyori, R. Org. Biomol. Chem. 2006, 4, 393. (d) Gridnev, I. D.;
Watanabe, M.; Wang, H.; Ikariya, T. J. Am. Chem. Soc. 2010,
132, 16637. (e) Ito, M.; Shiibashi, A.; Ikariya, T. Chem. Commun.
2011, 47, 2134.
(15) (a) Ito, M.; Kobayashi, C.; Himizu, A.; Ikariya, T. J. Am. Chem.
Soc. 2010, 132, 11414. (b) Ito, M.; Ootsuka, T.; Watari, R.; Shiibashi, A.;
Himizu, A.; Ikariya, T. J. Am. Chem. Soc. 2011, 133, 4240.
(16) Kuwata, S.; Ikariya, T. Dalton Trans. 2010, 39, 2984.
(17) Kimura, T.; Arita, H.; Ishiwata, K.; Kuwata, S.; Ikariya, T. Dalton
Trans. 2009, 2912.
(18) Arita, H.; Ishiwata, K.; Kuwata, S.; Ikariya, T. Organometallics
2008, 27, 493.
(19) Ishiwata, K.; Kuwata, S.; Ikariya, T. J. Am. Chem. Soc. 2009,
131, 5001.
(20) (a) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond;
Oxford University Press: Oxford, U.K., 1999. (b) Watanabe, D.; Gondo,
S.; Seino, H.; Mizobe, Y. Organometallics 2007, 26, 4909.
(21) Takemoto, S.; Otsuki, S.; Hashimoto, Y.; Kamikawa, K.;
Matsuzaka, H. J. Am. Chem. Soc. 2008, 130, 8904.
(22) (a) Seino, H.; Saito, A.; Kajitani, H.; Mizobe, Y. Organometallics
2008, 27, 1275. (b) Chꢀerioux, F.; Therrien, B.; S€uss-Fink, G. Chem.
Commun. 2004, 204.
(23) (a) Dobbs, D. A.; Bergman, R. G. Organometallics 1994,
13, 4594. (b) Kajitani, H.; Tanabe, Y.; Kuwata, S.; Iwasaki, M.; Ishii,
Y. Organometallics 2005, 24, 2251.
(24) (a) Ito, M.; Ikariya, T. Chem. Commun. 2008, 5134. (b) Morris,
R. H. Chem. Soc. Rev. 2009, 38, 2282. (c) Noyori, R.; Ohkuma, T. Angew.
Chem., Int. Ed. 2001, 40, 40. (d) Conner, D.; Jayaprakash, K. N.;
Cundari, T. R.; Gunnoe, T. B. Organometallics 2004, 23, 2724.
(e) Fryzuk, M. D.; MacNeil, P. A. Organometallics 1983, 2, 682.
(25) (a) Ni, C.; Lei, H.; Power, P. P. Organometallics 2010, 29, 1988.
(b) Roesky, H. W.; Bai, Y.; Noltemeyer, M. Angew. Chem., Int. Ed. Engl.
1989, 28, 754.
(26) Khaskin, E.; Iron, M. A.; Shimon, L. J. W.; Zhang, J.; Milstein, D.
J. Am. Chem. Soc. 2010, 132, 8542.
(27) Gourꢀe, E.; Avenier, P.; Solans-Monfort, X.; Veyre, L.;
Baudouin, A.; Kaya, Y.; Taoufik, M.; Basset, J.-M.; Eisenstein, O.;
Quadrelli, E. A. New J. Chem. 2011, 35, 1011.
8883
dx.doi.org/10.1021/ja203538b |J. Am. Chem. Soc. 2011, 133, 8880–8883