A sulfonylimido-bridged coordinatively unsaturated diiridium complex: Intramolecular C-H bond activation promoted by a weak acid

Article abstract of DOI:10.1021/om060777m

The sulfonylimido-bridged diiridium complex [Cp*Ir-(μ₂- NTs)₂IrCp*] (1; Cp* = η⁵-C ₅(CH₃)₅, Ts = SO₂C₆H ₄CH₃-p), readily accessible from the reaction of [Cp*IrCl₂]₂ with TsNH₂, reacted with P(CH₃)₃ and HOTf(Tf= SO₂CF₃) to afford the adduct [Cp*Ir{P(CH₃)₃}(μ₂- NTs)₂IrCp*] and cationic amiaoimido complex [Cp*Ir(μ₂-NHTs)(μ₂-NTs)IrCp*][OTf], respectively. On the other hand, the reaction of 1 with benzoic acid resulted in intramolecular C-H bond activation, giving the cyclometalated complex [Cp*Ir{μ₂-NHSO₂C₆H₃(CH ₃)-κ²N,C}₂-IrCp*].

Full text of DOI:10.1021/om060777m

Organometallics 2006, 25, 5847-5849
5847
A Sulfonylimido-Bridged Coordinatively Unsaturated Diiridium
Complex: Intramolecular C-H Bond Activation Promoted by a
Weak Acid
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
ReceiVed August 25, 2006
Chart 1
Summary: The sulfonylimido-bridged diiridium complex [Cp*Ir-
(µ2-NTs)2IrCp*] (1; Cp* ) η5-C5(CH3)5, Ts ) SO2C6H4CH3-
p), readily accessible from the reaction of [Cp*IrCl2]2 with
TsNH2, reacted with P(CH3)3 and HOTf (Tf ) SO2CF3) to afford
the adduct [Cp*Ir{P(CH3)3}(µ2-NTs)2IrCp*] and cationic amido-
imido complex [Cp*Ir(µ2-NHTs)(µ2-NTs)IrCp*][OTf], respec-
tiVely. On the other hand, the reaction of 1 with benzoic acid
resulted in intramolecular C-H bond actiVation, giVing the
cyclometalated complex [Cp*Ir{µ2-NHSO2C6H3(CH3)-κ2N,C}2-
IrCp*].
platform by forming ionic and strong M-N bonds.5 Herein we
report the synthesis of the novel coordinatively unsaturated
diiridium complex [Cp*Ir(µ2-NTs)2IrCp*] (1; Ts ) SO2C6H4-
CH3-p) with bridging sulfonylimido ligands and its Lewis acid/
Brønsted base bifunctional properties.
The reaction of [Cp*IrCl2]2 with 2 equiv of p-toluenesulfona-
mide in the presence of KOH cleanly afforded the sulfo-
nylimido-bridged diiridium complex 1, as shown in eq 1.6 The
We have recently developed a series of mononuclear half-
sandwich ruthenium, rhodium, and iridium catalysts bearing a
metal/NH bifunctionality for asymmetric hydrogen transfer
reduction and C-C bond formation.1 The high efficiency of
these catalysts at least partly relies on the nature of the chiral
amido intermediates having both a Brønsted basic nitrogen-
donor ligand2 and the Lewis acidic, coordinatively unsaturated
late metal center. We envisioned that this bifunctional effect of
the amido complex would also operate in “doubly unsaturated”
imido-bridged dinuclear half-sandwich complexes3 such as
[Cp*Ir(µ2-NC6H5)2IrCp*] (Cp* ) η5-C5(CH3)5)3a because they
have the same M-N structural motif (Chart 1). However,
exploration into the catalysis of these complexes has been
hampered by their various decomposition processes including
substitution of the imido ligand and imide transfer reactions.
Evidently the bridging ligand must be carefully designed to
increase the stability of the dinuclear complex without losing
the Lewis acid/Brønsted base bifunctionality. The electron-
withdrawing sulfonyl group, which has rarely been used in the
chemistry of nitrogen-bridged polynuclear complexes,4 is an
attractive candidate to stabilize the imido-bridged dinuclear
notable stability of 1 with respect to moisture as well as oxygen
enables us to use water as a cosolvent to dissolve KOH and
column chromatography under air for purification. The 1H NMR
spectrum of 1 indicates the presence of the Cp* and the tolyl
group in a ratio of 1:1. The dinuclear structure of 1 has been
confirmed by X-ray analysis (Figure 1).7 The sulfonyl oxygen
atoms in the imido ligands do not participate in coordination;
consequently, the two iridium centers in 1 are coordinatively
unsaturated. The metrical parameters in 1 fall in the range of
those found in the related bis(imido)-bridged diiridium com-
* Corresponding
authors.
E-mail:
skuwata@apc.titech.ac.jp;
tikariya@apc.titech.ac.jp.
(1) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393.
(2) Selected references of amido and imido complexes: (a) Fulton, J.
R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem. Res. 2002, 35,
44. (b) Gade, L. H. J. Organomet. Chem. 2002, 661, 85. (c) Kempe, R.
Angew. Chem., Int. Ed. 2000, 39, 468. (d) Li, Y.; Wong, W.-T. Coord.
Chem. ReV. 2003, 243, 191. (e) Sharp, P. R. J. Chem. Soc., Dalton Trans.
2000, 2647. (f) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. ReV.
1989, 95, 1.
(3) For Ir: (a) Dobbs, D. A.; Bergman, R. G. Organometallics 1994,
13, 4594. (b) Danopoulos, A. A.; Wilkinson, G.; Sweet, T. K. N.;
Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1996, 3771. For Ru: (c)
Burrell, A. K.; Steedman, A. J. Organometallics 1997, 16, 1203. (d) Kee,
T. P.; Park, L. Y.; Robbins, J.; Schrock, R. R. J. Chem. Soc., Chem.
Commun. 1991, 121. (e) Takemoto, S.; Kobayashi, T.; Matsuzaka, H. J.
Am. Chem. Soc. 2004, 126, 10802. For Fe: (f) Takemoto, S.; Ogura, S.;
Yo, H.; Hosokoshi, Y.; Kamikawa, K.; Matsuzaka, H. Inorg. Chem. 2006,
45, 4871. (g) Ohki, Y.; Takikawa, Y.; Hatanaka, T.; Tatsumi, K. Organo-
metallics 2006, 25, 3111.
(5) (a) Fujita, K.; Yamashita, M.; Puschmann, F.; Alvarez-Falcon, M.
M.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 9044. (b)
Leung, W.-H.; Wu, M.-C.; Chim, J. L. C.; Wong, W.-T. Inorg. Chem. 1996,
35, 4801.
(6) Synthesis of 1. A mixture of [Cp*IrCl2]2 (2.0002 g, 2.51 mmol),
TsNH2 (0.8749 g, 5.11 mmol), and KOH (1.1680 g, 20.8 mmol) in CH2-
Cl2/H2O (20 mL/20 mL) was stirred for 8 h at room temperature. The
resultant organic layer was washed with H2O (10 mL × 5) and chromato-
graphed on alumina under air. A red band eluted with CH2Cl2 was collected
and evaporated to dryness. The resultant solid was recrystallized from CH2-
Cl2/diethyl ether (10 mL/100 mL) to yield 1 as dark red crystals (2.4022 g,
2.42 mmol, 96%). 1H NMR (C6D6): δ 8.10, 6.85 (d, 4H each, J ) 8.3 Hz,
SO2C6H4CH3), 1.97 (s, 6H, SO2C6H4CH3), 1.58 (s, 30H, Cp*). Anal. Calcd
for C34H44Ir2N2O4S2: C, 41.11; H, 4.46; N, 2.82. Found: C, 40.73; H, 4.57;
N, 2.73.
(7) Crystal data for 1: monoclinic, P21/c, a ) 14.195(9) Å, b ) 14.417-
(8) Å, c ) 17.929(11) Å, â ) 113.210(8)°, V ) 3372.4(35) Å3, Z ) 4, Fcalc
) 1.956 g cm-3, 7690 unique reflections (2θ < 55°), 442 parameters, R1
) 0.040 [I > 2σ(I)], wR2 ) 0.105 (all data), GOF ) 1.008.
(4) Foch, I.; Pa´rka´nyi, L.; Besenyei, G.; Sima´ndi, L. I.; Ka´lma´n, A. J.
Chem. Soc., Dalton Trans. 1999, 293.
10.1021/om060777m CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/09/2006

5848 Organometallics, Vol. 25, No. 25, 2006
Communications
Figure 1. Crystal structure of 1. Hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at 30% probability. Selected
bond distances (Å): Ir1-N1, 2.007(8); Ir1-N2, 2.004(6); Ir2-
N1, 2.021(7); Ir2-N2, 2.016(7).
Figure 2. Crystal structure of 3. One of the two crystallographically
independent units is shown, and hydrogen atoms except for the
amido hydrogen are omitted for clarity. Thermal ellipsoids are
drawn at 30% probability. Selected bond distances (Å): Ir1-Ir2,
2.8345(4); Ir1-N1, 2.114(6); Ir1-N2, 1.969(6); Ir2-N1, 2.131-
(6); Ir2-N2, 1.989(7); Ir3-Ir4, 2.8408(3); Ir3-N3, 2.125(5); Ir3-
N4, 1.970(6); Ir4-N3, 2.109(6); Ir4-N4, 1.970(6).
Table 1. Selected Interatomic Distances (Å) and Angles
(deg) for Imido-Bridged Diiridium Complexes
[Cp*Ir(µ2-NR)2IrCp*]
R
c-C5H11 C6H5 2,6-(CH3)2C6H3
CN
Ts
Ir-Ir (Å)
2.6133(5) 2.778(1)
2.894(1)
1.973
2.8179(9) 2.8343(4)
Ir-N (Å)a
2.034
1.986
2.01
332a
2.012
2.046(8)
sum of the bond 339.8
angles around 351.8
N (deg)
338a
354a
343.8
353.5
bridged dinuclear core in 1. The reactions of the cyanamido-
bridged diiridium complex [Cp*Ir(µ2-NCN)2IrCp*] with phos-
phines are also known to end in the simple Lewis acid-base
reaction without rupture of the M2N2 core.9 Apparently, the
electron-withdrawing sulfonyl groups in 1 increase the strength
of the Ir-N bonds.
ref
3b
3a
3a
9
this work
a Mean values.
Scheme 1
Electron-withdrawing groups on the imido nitrogen should
make the imido ligand less nucleophilic. Nevertheless, the
sulfonylimido-bridged diiridium complex 1 did undergo facile
N-protonation with an equimolar amount of triflic acid to afford
the amido- and imido-bridged cationic complex [Cp*Ir(µ2-
NHTs)(µ2-NTs)IrCp*][OTf] (3; Tf ) SO2CF3).10 The 1H NMR
spectrum of 3 shows two sets of signals for the tolyl groups in
the amido and imido ligands along with one Cp* singlet; the
amido proton is observed at δ 10.11. The protonation of the
imido nitrogen atom is also evident from the IR spectrum, which
exhibits a νNH band at 3108 cm-1. The single-crystal X-ray
analysis of 3 has confirmed its detailed structure (Figure 2).11
The close contact (3.05 Å, mean of the two crystallographically
independent units) between the N(1) atom and the triflate oxygen
strongly suggests the presence of a hydrogen bond between the
NHTs ligand and the triflate ion, although the amido hydrogen
plexes, as summarized in Table 1. Table 1 also shows that slight
steric modification in the imido substituents, e.g., from phenyl
to 2,6-xylyl, renders the geometry at the nitrogen atoms much
closer to planarity.
(9) Kajitani, H.; Tanabe, Y.; Kuwata, S.; Iwasaki, M.; Ishii, Y. Orga-
nometallics 2005, 24, 2251.
Owing to the coordinatively unsaturated nature, the sulfo-
nylimido-bridged diiridium complex 1 reacted with trimeth-
ylphosphine at room temperature to give the phosphine adduct
[Cp*Ir{P(CH3)3}(µ2-NTs)2IrCp*] (2) as shown in Scheme 1.
(10) Synthesis of 3. To a solution of 1 (99.7 mg, 0.100 mmol) in CH2-
Cl2 (5 mL) was added TfOH (11 µL, 0.12 mmol), and the mixture was
stirred for 1 h at room temperature. The resultant dark reddish-brown
solution was concentrated to ca. 1 mL. Addition of diethyl ether (10 mL)
gave 3 as dark reddish-brown crystals (79.6 mg, 0.070 mmol, 69%). 1H
NMR (C6D6): δ 10.11 (s, 1H, NH), 8.40, 8.30, 7.09, 6.83 (d, 2H each, J
) 7.8 Hz, SO2C6H4CH3), 1.93, 1.90 (s, 3H each, SO2C6H4CH3), 1.42 (s,
30H, Cp*). Anal. Calcd for C35H45F3Ir2N2O7S3: C, 36.77; H, 3.97; N, 2.45.
Found: C, 36.64; H, 3.90; N, 2.34.
(11) Crystal data for 3: triclinic, P1h, a ) 15.028(3) Å, b ) 15.560(3)
Å, c ) 18.069(3) Å, R ) 69.227(5)°, â ) 86.128(7)°, γ ) 83.760(7)°, V
) 3925.0(12) Å3, Z ) 4, Fcalc ) 1.935 g cm-3, 17 194 unique reflections
(2θ < 55°), 1027 parameters, R1 ) 0.043 [I > 2σ(I)], wR2 ) 0.119 (all
data), GOF ) 1.001.
1
Complex 2 has been characterized by H and 31P{1H} NMR
spectroscopy along with a preliminary X-ray analysis.8 Imide
transfer to give phosphine imide, which is known for the
corresponding phenylimido complex,3a did not occur even in
the presence of an excess of the phosphine and at elevated
temperatures, suggesting the robustness of the bis(imido)-
(8) See Supporting Information.

Communications
Organometallics, Vol. 25, No. 25, 2006 5849
Scheme 2
structural refinement was unable to be completed due to severe
disorder.8,14 We have recently reported acidic alcohol or phenol-
assisted cyclometalation of the coordinatively unsaturated
mononuclear amido complexes [LM(Tsdpen)] (LM ) (p-
cymene)Ru, Cp*Ir; TsdpenH ) TsNHCH(C6H5)CH(C6H5)-
NH2).15 In these reactions, the intermediary alkoxo-amine
complexes such as [LM(OCH2CF3)(TsdpenH)] are observed (for
Ru) and even isolated (for Ir), although the details of the
subsequent reaction pathway leading to the cyclometalation
products remain somewhat obscure. Based on these results as
well as the acid-base bifunctionality of 1 substantiated by the
above experiments, it seems plausible that the first step in the
acid-promoted cyclometalation of 1 involves an equilibrium
between 1 and the amido-imido-benzoato complex A as in
the corresponding mononuclear systems.15,16
The present study clearly demonstrates that the electron-
withdrawing sulfonyl group adjusts the balance of the Brønsted
basicity of the bridging imido ligands and the stability of the
bis(imido)-bridged coordinatively unsaturated dinuclear complex
1. The cyclometalation of 1 promoted by benzoic acid is
particularly noteworthy because it opens the possibility of
cooperative catalysis, using nitrogen-bridged dinuclear com-
plexes based on the basic nitrogen ligand and adjacent coordi-
natively unsaturated metal center, as in the corresponding
mononuclear amido complexes.1,17 Further elucidation of the
mechanism of the cyclometalation and development of new
catalysis using sulfonylimido-bridged multimetallic complexes
are the focus of our ongoing work.
could not be located on the difference Fourier map. The
tetrahedral configuration of the N(1) atom is also indicated by
the sum of the Ir-N-Ir and two Ir-N-S angles (323.3°, mean).
In contrast, the corresponding angle sum for the N(2) atom
(345.9°, mean) is consistent with the sp2-hybridized imido
nitrogens. The Ir-Namido bonds of 2.120 Å (mean) are signifi-
cantly longer than the Ir-Nimido bonds (1.975 Å, mean), as
observed in the N-cyclohexyl congener [Cp*Ir(µ2-NHCy)(µ2-
NCy)IrCp*]Cl (Cy ) cyclohexyl).3b
Notably, protonation of 1 with a weaker acid led to a totally
different result. When 1 was treated with benzoic acid in
refluxing toluene, intramolecular C-H activation of the to-
sylimido ligands occurred to afford the doubly cyclometalated
diiridium complex [Cp*Ir{µ2-NHSO2C6H3(CH3)-κ2N,C}2IrCp*]
(4) in 82% yield (Scheme 2).12,13 Use of benzoic acid proved
to be essential because no thermal reaction took place without
benzoic acid. The 1H NMR spectrum of 4 features singlets at δ
7.81 and 4.39 ascribed to the cyclometalated tolyl groups and
the amido protons, respectively. The presence of the NH protons
is also suggested by the νNH band at 3256 cm-1 in the IR
spectrum. The dimeric structure of 4 with the two Cp* ligands
sitting in mutually syn position with respect to the Ir2N2 ring
has been revealed by single-crystal X-ray analysis, although the
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas (Nos. 14078209
and 18065007) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (T.I.) and Asahi Glass Founda-
tion (S.K.).
Supporting Information Available: Text giving details of the
syntheses and X-ray studies of 1-4, and CIF files giving X-ray
crystallographic data for 1 and 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM060777M
(12) Synthesis of 4. A mixture of 1 (926.0 mg, 0.932 mmol) and C6H5-
COOH (113.8 mg, 0.932 mmol) in toluene (50 mL) was allowed to reflux
for 48 h. After removal of the solvent, the resultant solid was dissolved in
CH2Cl2 (10 mL) and chromatographed on alumina under air. A yellow band
eluted with CH2Cl2 was collected and dried in vacuo to yield 4 as a yellow
(14) The dimeric structure of 4 was also supported by the ESI-MS
measurement, showing the [M + Na]+ peak at m/z 1017.5.
(15) Koike, T.; Ikariya, T. Organometallics 2005, 24, 724.
(16) (a) Davies, D. L.; Donald, S. M. A.; Al-Duaij, O.; Macgregor, S.
A.; Po¨lleth, M. J. Am. Chem. Soc. 2006, 128, 4210. (b) Davies, D. L.; Al-
Duaij, O.; Fawcett, F.; Giardiello, M.; Hilton, S. T.; Russell, D. R. Dalton
Trans. 2003, 4132. (c) Arita, S.; Koike, T.; Ikariya, T., manuscript in
preparation.
(17) (a) Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval,
C.; Noyori, R. J. Am. Chem. Soc. 2006, 128, 8724. (b) Clarke, Z. E.; Maragh,
P. T.; Dasgupta, T. P.; Gusev, D. G.; Lough, A. J.; Abdur-Rashid, K.
Organometallics 2006, 25, 4113. (c) Morris, D. J.; Hayes, A. M.; Wills,
M. J. Org. Chem. 2006, 71, 7035.
1
powder (760.5 mg, 0.766 mmol, 82%). H NMR (C6D6): δ 7.81 (s, 2H,
SO2C6H3CH3), 6.33 (d, 2H, J ) 8.2 Hz, SO2C6H3CH3), 4.39 (s, 2H, NH),
2.17 (s, 6H, SO2C6H3CH3), 1.35 (s, 30H, Cp*). The signal for the rest aryl
proton is overlapped with the solvent signal. Anal. Calcd for C34H44-
Ir2N2O4S2: C, 41.11; H, 4.46; N, 2.82. Found: C, 41.08; H, 4.43; N, 2.72.
(13) We could not observe any intermediates in this reaction, even at
room temperature. On the other hand, the amido-imido complex 3 in
toluene started to decompose at 80 °C to give p-toluenesulfonamide without
formation of 4.