Abnormal ligand binding and reversible ring hydrogenation in the reaction of imidazolium salts with IrH5(PPh3)2

Article abstract of DOI:10.1021/ja026735g

We show that imidazolium salts do not always give normal or even aromatic carbenes on metalation, and the chemistry of these ligands can be much more complicated than previously thought. N,N′-disubstituted imidazolium salts of type [(2-py)(CH₂)

Full text of DOI:10.1021/ja026735g

Published on Web 08/07/2002
Abnormal Ligand Binding and Reversible Ring Hydrogenation
in the Reaction of Imidazolium Salts with IrH5(PPh3)2
Stephan Gr¨undemann, Anes Kovacevic, Martin Albrecht, Jack W. Faller,* and
Robert H. Crabtree*
Contribution from the Department of Chemistry, 225 Prospect Street, Yale UniVersity,
New HaVen, Connecticut 06520-8107
Received April 30, 2002
Abstract: We show that imidazolium salts do not always give normal or even aromatic carbenes on
metalation, and the chemistry of these ligands can be much more complicated than previously thought.
N,N′-disubstituted imidazolium salts of type [(2-py)(CH2)n(C3H3N2)R]BF4 react with IrH5(PPh3)2 to give N,C-
chelated products (n ) 0, 1; 2-py ) 2-pyridyl; C3H3N2 ) imidazolium; R ) mesityl, n-butyl, i-propyl, methyl).
Depending on the circumstances, three types of kinetic products can be formed: in one, the imidazole
metalation site is the normal C2 as expected; in another, the metalation occurs at the abnormal C4 site;
and in the third, C4 metalation is accompanied by hydrogenation of the imidazolium ring. The bonding
mode is confirmed by structural studies, and spectroscopic criteria can also distinguish the cases. Initial
hydrogen transfer can take place from the metal to the carbene to give the imidazolium ring hydrogenation
product, as shown by isotope labeling; this hydrogen transfer proves reversible on reflux when the abnormal
aromatic carbene is obtained as final product. Care may therefore be needed in the future in verifying the
structure(s) formed in cases where a catalyst is generated in situ from imidazolium salt and metal precursor.
Introduction
rare. Experiment and calculations by Cavell and co-workers
indicate that an oxidative addition of imidazolium CH bonds is
N-heterocyclic carbenes 1 show promise as ligands for the
development of new or improved catalyst systems.1 While the
synthesis of the ligand precursors is fairly simple,2 introduction
of the metal is more challenging and, as shown here, can give
unexpected products. Typical procedures are (i) proton abstrac-
tion with bases (e.g. BuLi) prior to metalation (free carbene
route),3 (ii) metalation with a basic metal precursor such as
Pd(OAc)2 and [Ir(COD)(OEt)]2 (direct metalation),4 (iii) metal
exchange starting from silver carbenes (transmetalation),5 and
(iv) oxidative addition of carbon-halogen bonds to low-valent
metal precursors.6 Direct base-free CH-bond activation, by
analogy with arene mercuration,7 was long elusive and is still
feasible,6b,c and Nolan et al.6d have also proposed an example.
Recently we8 and others9 have shown that low-valent metal
precursors are not essential for this CH-bond activation. In
particular, the reaction between an imidazolium salt and metal
hydride precursors with release of H2 according to eq 1 is
suitable for the synthesis of N-heterocyclic carbene complexes.
Very recently, however, we briefly reported that the facile
(3) For isolated free carbenes, see: (a) Arduengo, A. J., III; Dias, H. V. R.;
Harlow, R. L.; Kline, M J. Am. Chem. Soc. 1992, 114, 5530. (b) Herrmann,
W. A.; Elison, M.; Fischer, J.; Ko¨cher, C.; Artus, G. R. J. Chem. Eur. J.
1996, 2, 773. (c) Douthwaite, R. E.; Hau¨ssinger, D.; Green, M. L. H.;
Silcock, P. J.; Gomes, P. T.; Martins, A. M.; Danopoulos, A. A.
Organometallics 1999, 18, 4584. (d) Jafarpour, L.; Nolan, S. P. Organo-
metallics 2000, 19, 2055. For preparation of free carbenes and in situ
metalation, see: Reference 1j. (e) Kernbach, U.; Ramm, M.; Luger, P.;
Fehlhammer, W. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 310.
(4) (a) Ko¨cher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532, 261.
(b) Chen, J. C. C.; Lin, I. J. B. J. Chem. Soc., Dalton Trans. 2000, 839. (c)
Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R. H. Chem. Commun. 2001,
201.
(1) (a) Hill, J. E.; Nile, T. A, J. Organomet. Chem. 1977, 137, 293. (b) Lappert,
M. F.; Maskell, R. K. J. Organomet. Chem. 1984, 264, 217. (c) Herrmann,
W. A.; Goossen, L. J.; Ko¨cher, C.; Artus, G. R. J. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2805. (d) Gardiner, M. G.; Herrmann, W. A.; Reisinger,
C.-P.; Schwarz, J.; Spiegler, M. J. Organomet. Chem. 1999, 572, 239. (e)
Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953.
(f) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. J. Org. Chem. 1999,
64, 3804. (g) Lee, H. M.; Jiang, T.; Stevens, E. D.; Nolan, S. P.
Organometallics 2001, 20, 1255. (h) Hillier, A. C.; Lee, H. M.; Stevens,
E. D.; Nolan, S. P. Organometallics 2001, 20, 4246. (i) Lee, S.; Beare, N.
A.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 8410. (j) Loch, J. A.;
Albrecht, M.; Mata, J.; Peris, E.; Faller, J. W.; Crabtree, R. H. Organo-
metallics 2002, 21, 700. (k) Albrecht, M.; Crabtree, R. H.; Mata, J.; Peris,
E. Chem. Commun. 2002, 32. (l) Herrmann, W. A. Angew. Chem., Int. Ed.
2002, 41, 1290.
(2) (a) Tulloch, A. D. D.; Danopoulos, A. A.; Winston, S.; Kleinhenz, S.;
Eastham, G. J. Chem. Soc., Dalton Trans. 2000, 4499. (b) Chen, J. C. C.;
Lin, I. J. B. Organometallics 2000, 19, 5113. (c) Wasserscheid, P.; Keim,
W. Angew. Chem., Int. Ed. 2000, 39, 3772. (d) Park, S.; Kazlauskas, J. J.
Org. Chem. 2001, 66, 8395. (e) McGuiness, D. S.; Mueller, W.; Wasser-
scheid, P.; Cavell, K. J.; Selton, B. W.; White, A. H.; Englert, U.
Organometallics 2002, 21, 175.
(5) (a) McGuinness, D. S.; Cavell, K. J. Organometallics 2000, 19, 741. (b)
Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H. Inorg. Chim.
Acta 2002, 327, 116.
(6) (a) Fraser, P. J.; Roper, W. R.; Stone, F. G. A. J. Chem. Soc., Chem.
Commun. 1974, 102. (b) McGuiness, D. S.; Yates, B. F.; Cavell, K. J. J.
Chem. Soc., Chem. Commun. 2001, 355. (c) McGuiness, D. S.; Cavell, K.
J.; Yates, B. F.; Skelton, B. W.; White, A. H. J. Am. Chem. Soc. 2001,
123, 8317. (d) Viciu, M.S.; Grasa, G.A.; Nolan, S.P. Organometallics 2001,
20, 3607.
(7) Albrecht, M.; Van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750, and
references therein.
(8) Gru¨ndemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R.
H. J. Chem. Soc., Dalton Trans. 2002, 2163.
(9) Chaumonnot, A.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B.; Buron,
C.; Bertrand, G.; Metevier, P. Organometallics 2001, 20, 5614.
9
10.1021/ja026735g CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 10473-10481
10473

A R T I C L E S
Gru¨ndemann et al.
Figure 1. Molecular structure of the cations of 12a (left) and 12b (right), showing 50% probability thermal ellipsoids. Only the metal bound hydrogens
(calculated positions) are shown.
reaction between a pyridine substituted imidazolium salt and
IrH5(PPh3)2 (5) gives good yields of unprecedented carbenes
of type 2 in which the N-heterocyclic ring is bound in a novel
way via C4(C5).10 This observation is surprising since not only
is the free carbene 3 much more stable than 4 but theoretical
density functional (DFT; B3PW91) calculations11 predict that
binding to the C4(C5)-position can be thermodynamically much
less favored (23.3 kcal/mol) than C2-binding (e.g., for R ) H,
MLn ) PtCl3-). N-H‚‚‚Cl hydrogen bonding, present in the
[(imidazole)PtCl3]- quantum model system, provides an ad-
ditional stabilizing factor for the C2 bound form in this case,
however.
CDCl3 as a result of strong ion pairing of bromide with C2-H
of the imidazolium cation.12
Nonclassical Carbene Formation in the Large Bite-Angle
Case. The reaction of methylene linked salts 9a-c with
IrH5(PPh3)2 in refluxing tetrahydrofuran (THF) for 2 h gives
the iridium(III) complexes 12a-c in virtually quantitative yields
(eq 2). Air- or moisture-free conditions are unnecessary for this
reaction. After recrystallization from THF/pentane, 12a-c are
obtained as colorless solids that are stable toward air and
moisture. Carbene coordination via the C4-position was un-
equivocally confirmed by X-ray structure determinations of
12a13a and 12b,13b the latter result having been communicated
previously.10 The structures (Figure 1) show an octahedral
iridium center with two trans phosphines. The positions of the
In this paper we look at the factors that favor normal versus
abnormal binding, such as variation of wingtip groups and ligand
bite angles. We also look at the unexpected reversible hydrogen
transfer from metal to ligand and suggest a mechanism based
on labeling experiments. The transformations and intermediates
reported here appear to be unprecedented but could occur quite
generally in other systems where they could escape detection,
especially in cases where catalysts are formed in situ by heating
imidazolium precursors and metal salts.
(10) Gru¨ndemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R.
H. Chem. Commun. 2001, 2274.
(11) Sini, G.; Eisenstein, O.; Crabtree, R. H. Inorg. Chem. 2002, 41, 602.
(12) Dupont, J.; Suarez, P. A. Z.; De Souza, R. F.; Burrow, R. A.; Kintzinger,
J.-P. Chem. Eur. J. 2000, 6, 2377.
(13) (a) Crystal data for 12a‚CHCl3: triclinic, space group P1h (No. 2), a )
14.1010(6) Å, b ) 14.2829(7) Å, c ) 14.4166(5) Å, R ) 79.144(3)°, â )
64.676(3)°, γ ) 88.336(3)°, V ) 2573.2(2) Å3, Z ) 2, T ) 183.2 K; full
matrix least squares refinement of F (622 parameters) was based on 5911
observed reflections (I > 5.00σ(I)) and converged to R ) 0.038, Rw
)
0.046 (GOF ) 1.19). (b) Crystal data for 12b: monoclinic, space group
P21/c (No. 14), a ) 9.4493(2) Å, b ) 20.6687(6) Å, c ) 22.0865(6) Å, â
) 90.160(2)°, V ) 4313.6(2) Å3, Z ) 4, T ) 183.2 K; full matrix least
squares refinement of F (532 parameters) was based on 4789 observed
reflections (I > 3.00σ(I)) and converged to R ) 0.033, Rw ) 0.035 (GOF.)
0.88). (c) Crystal data for 15‚CHCl3: monoclinic, space group P21/m (No.
11), a ) 10.7855(4) Å, b ) 22.7449(11) Å, c ) 11.3115(4) Å, â )
101.571(3)°, V ) 2718.5(2) Å3, Z ) 2, T ) 183.2 K; full matrix least
squares refinement of F (354 parameters) was based on 3416 observed
reflections (I > 3.00σ(I)) and converged to R ) 0.039, Rw ) 0.041 (GOF.)
1.01). The anion and cation were on a plane of symmetry, and the CHCl3
was disordered. (d) Crystal data for 16a: monoclinic, space group P21/c
(No. 14), a ) 11.6490(6) Å, b ) 26.0118(11)) Å, c ) 16.281(9) Å, â )
105.335(3)°, V ) 4757.6(16) Å3, Z ) 4, T ) 183.2 K; full matrix least
squares refinement of F (577 parameters) was based on 3056 observed
reflections (I > 3.00σ(I)) and converged to R ) 0.033, Rw ) 0.033 (GOF.)
0.75).
Results
Synthesis of the Ligands. The new pyridine functionalized
imidazolium bromide salts 6b,c, 7d, and 8 having different
wingtip groups, R, have been synthesized according to known
procedures.2a Anion exchange from Br- to BF4 was ac-
-
complished with stoichiometric AgBF4 or excess NaBF4 to give
the ligand fluoroborates, 9-11. Successful anion exchange is
indicated by a diagnostic high-field shift of the 1H NMR signal
of the C2-H proton from 10 to 11 ppm to about 9 ppm in
9
10474 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

Reaction of Imidazolium Salts with IrH (PPh3)2
A R T I C L E S
5
Table 1. Selected Bond Lengths and Angles for 12a and 12b
and C5 in normal carbenes) and (ii) a difference in chemical
shift of the metal bound carbon (δ ∼ 140 ppm in abnormal
versus δ ∼ 170 ppm in normal cases, vide infra). Large wingtip
groups such as i-Pr at N(1) lead to exclusive formation of the
abnormal C4 isomer. For smaller R, we expected that the normal
C2 carbene might be formed. Indeed, when R ) Me, both
isomers are formed: normal 13 and abnormal 12d. Integration
of the NMR spectra gives a 12d/13 ratio of ca. 55:45. The
spectrum of 12d was identified by analogy with 12a-c, but a
full assignment for C2 bound 13 was only achieved by 2D NMR
spectroscopy. Characteristically, no proton was seen in the low-
field region near 8.7 ppm; instead the imidazole protons show
resonances at 7.44 and 6.31 ppm. In the 13C{1H} NMR
spectrum, the metal bound carbon appears at 169.9 ppm (JPC
) 7.0 Hz). These observations are fully consistent with a C2
bound carbene 13 (eq 3). The 31P{1H} NMR spectrum has a
single peak at 17.1 ppm. Due to the similar solubilities of 12d
and 13, we have not yet been able to separate the two species.
As isomeric mixtures, however, correct microanalytical data
were obtained (see Experimental Section).
12a
12b
Bond Lengths (Å)
2.297(2)
2.284(2)
2.197(6)
2.092(6)
1.478(9)
1.400(8)
1.331(8)
1.392(8)
1.315(9)
1.447(9)
1.373(9)
Ir1-P1
Ir1-P2
Ir1-N3
Ir1-C7
N1-C6
N1-C7
N1-C9
N2-C8
N2-C9
N2-C10
C7-C8
2.287(2)
2.301(2)
2.193(5)
2.100(6)
1.461(7)
1.404(7)
1.335(8)
1.390(8)
1.327(8)
1.459(8)
1.380(8)
Bond Angles (deg)
88.9(2)
N3-Ir1-C7
P1-Ir1-P2
89.3(2)
163.86(6)
158.59(6)
Dihedral Angles (deg)
178.3(6)
N1-C9-N2-C10
C7-Ir1-N3-C1
N3-Ir1-C7-N1
182.0(6)
18.2(5)
17.6(5)
22.3(6)
22.6(5)
benzylic substituent and the R group (R ) mesityl, i-Pr) define
the positions of the imidazole nitrogen atoms. In both cases the
imidazole ring is undoubtedly attached via the C4-carbon (C7
in the crystallographic numbering), a previously unknown type
of carbene binding. Both molecules are chiral owing to non-
coplanarity of the imidazole with the pyridine. The NMR data
(see below) show equivalent isopropyl methyl and methylene
protons, so the molecule is fluxional on the NMR time scale at
298 K probably via a boat-boat six-membered ring-flip.
Once the normal or abnormal compounds have formed, they
do not easily interconvert. Attempted interconversion by heating
the mixture of 12d and 13 in dimethyl sulfoxide (DMSO)
solution at 100 °C failed. After several hours the initial ratio is
unchanged, and even on prolonged heating (3 days) only
decomposition products were observed. The same result was
obtained when a sample was heated to 50 °C for 18 h in the
presence of 5 equiv of CF3COOH in DMSO.
Nonclassical Carbene Formation in the Small Bite-Angle
Case. The ligand precursors 10b-d also react with IrH5(PPh3)2
(eq 4) like their methylene-linked analogues. Under identical
reaction conditions the products are the abnormal carbene
complexes 14b-d. The reaction of the very bulky mesitylene
substituted 10a with IrH5(PPh3)2 is substantially slower, and
we did not see full conversion to 14a. After recrystallization
from CHCl3/pentane or THF/pentane, 14b-d were obtained as
colorless, high-melting (>200 °C) solids. Notably, for R ) Me,
where a mixture was seen previously (12d/13), only the C4
carbene was now observed. The spectroscopic data (CDCl3, 298
K) for 14 are very similar to that for 12 and indicate C4 carbene
binding. For example, in the case of 14b the two imidazole
protons appear to be well-separated at 9.28 and 5.97 ppm, and
in the 13C{1H} NMR C2 appears at 149.0 ppm (t, JPC ) 4.7
Hz).
The pertinent NMR spectroscopic data of the complexes
12a-c are similar and consistent with rigid abnormal carbene
1
binding in solution. The H NMR features of 12b are repre-
sentative: a low-field peak at 8.72 ppm is assigned to the acidic
C2 proton; the proton at C5 is high field shifted to 5.17 ppm,
and the CH2 linker group appears as a singlet at 4.70 ppm,
consistent with a nonrigid ligand conformation and an effective
Cs symmetry in solution. The two inequivalent hydrides have
high-field resonances at -10.83 ppm (probably trans to N) and
-21.49 ppm (trans to C)14 both split into doublets of triplets
by mutual coupling (2JHH ) 4.9 Hz, typical for cis hydrides15)
and coupling to apparently equivalent phosphorus nuclei (2JPH
) 19.6 and 18.6 Hz, respectively). The singlet at 21.4 ppm in
the 31P{1H} NMR spectrum is consistent with magnetically
equivalent phosphorus nuclei and hence with Cs symmetry. The
13C{1H} NMR spectrum confirms C4-carbene binding because
the C4 resonance at 141.1 ppm is a triplet by coupling to
phosphorus (JPC ) 7.1 Hz).
Two 1H NMR spectroscopic characteristics appear to be
diagnostic for abnormal C4 carbene binding: (i) a large shift
difference between the two heterocyclic protons (>3 ppm for
C2 and C5 in abnormal carbenes compared to <1 ppm for C4
Normal Carbene Formation in the Small-Bite-Angle Case.
If abnormal binding is blocked, however, we find that normal
carbene binding can occur, so this is not forbidden for the small-
(14) Goldman, A. S.; Halpern, J. J. Organomet. Chem. 1990, 382, 237.
(15) Gru¨ndemann, S.; Limbach, H.-H.; Buntkowsky, G.; Sabo-Etienne, S.;
Chaudret, B. J. Phys. Chem. A 1999, 103, 4752.
9
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A R T I C L E S
Gru¨ndemann et al.
Figure 2. Molecular structure of the cation of 15, showing 50% probability
thermal ellipsoids. Only the metal bound hydrogens (calculated positions)
are shown.
Figure 3. Molecular structure of the cation of 16a, showing 50% probability
thermal ellipsoids. Only the metal bound hydrogens (calculated positions)
are shown.
Table 2. Selected Bond Lengths and Angles for 15
bound carbon appears at δ 192.5 (t, JPC ) 6.3 Hz) closer to the
value obtained for 13.
Bond Lengths (Å)
Ir1-P1
Ir1-P1a
Ir1-N3
Ir1-C9
N1-C1
N1-C7
N1-C9
N2-C8
N2-C9
N2-C14
C7-C8
2.312(2)
2.312(2)
2.167(7)
2.027(9)
1.410(10)
1.413(11)
1.384(10)
1.401(11)
1.339(10)
1.475(11)
1.418(13)
Hydrogen Transfer to and from the Imidazole Ring. A
very unexpected hydrogenated intermediate, 16, was seen at
early reaction times in the synthesis of 14. This proved to result
from hydrogen transfer from the metal to the imidazolium ring
with formation of a hydrogenated abnormal carbene, 16. This
type of reaction seems to be quite unknown for any previous
metal σ-aryl or heterocycle complexes, although hydrogenation
of arenes via π-bonding is of course known.16 When the reaction
of 10 with IrH5(PPh3)2 was quenched after 10-15 min by rapid
cooling of the reaction mixture followed by addition of pentane,
16 was precipitated along with starting material and small
quantities of the final product 14 (eq 6).
Bond Angles (deg)
N3-Ir1-C9
P1-Ir1-P2
76.0(3)
163.77(8)
Dihedral Angle (deg)
N1-C9-N2-C14
180.0
bite-angle ligands. The reaction of IrH5(PPh3)2 with the benz-
imidazolium salt 11 under the same conditions (refluxing THF,
2 h) gave the C2 carbene complex 15 (eq 5).
It was hard to isolate 16 because it usually rapidly converted
to 14, so we turned our attention to more slowly reacting
imidazolium salts. Particularly attractive is the mesitylene-
substituted ligand, 10a, which allowed isolation and full
characterization of the corresponding intermediate 16a. Recrys-
tallization from THF/pentane yielded crystals suitable for
diffraction.13d Like 14, the structure of 16a (Figure 3) shows
an octahedral iridium(III) center. Remarkably, the imidazolyl
heterocycle is fully hydrogenated and best described as an
imidazolidinium system. This is reflected, for example, in the
bonding situation around N2 (N2 denotes crystallographic
numbering). Bond lengths and angles clearly reveal sp3 hybrid-
ization at N2, which requires saturation of the C8-N2-C9
fragment. Furthermore the C-N bonds around C9 have single
Recrystallization from CHCl3/pentane yielded crystals suitable
for single-crystal diffraction.13c The structure (Figure 2, Table
2) unambiguously proved normal carbene binding. The molecule
is positioned on a crystallographic mirror plane, defined by the
pyridine, the Ir-C9 bond to benzimidazole, and the IrH2
moieties. The carbene Ir-C distance (2.027(9) Å) in 15 is
significantly shorter than in the abnormal carbenes 12a and 12b
(Ir1-C7 ) 2.09 Å (12a) and 2.10 Å (12b), respectively). Similar
but less pronounced trends are seen with the Ir-N bond lengths,
which are slightly shorter than in C4 carbenes. The length of
the C7-C8 bond (1.418 Å (15)) is consistent with a delocalized
aromatic ring. The NMR spectroscopic data are consistent with
the X-ray structure. In the 13C{1H} NMR spectrum, the metal
(16) (a) Muetterties, E. L.; Hirsekorn, F. J. J. Am. Chem. Soc. 1974, 96, 4063.
(b) Hirsekorn, F. J.; Rakowski, M. C.; Muetterties, E. L. J. Am. Chem.
Soc. 1975, 97, 237. (c) Muetterties, E. L.; Rakowski, M. C.; Hirsekorn, F.
J.; Larson, W. D.; Basus, V. J.; Anet, F. A. L. J. Am. Chem. Soc. 1975, 97,
1266
9
10476 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

Reaction of Imidazolium Salts with IrH (PPh3)2
A R T I C L E S
5
Table 3. Selected Bond Lengths and Angles for 16a
with water, we performed the reaction with 10b under anhydrous
conditions. Careful integration of the H NMR spectra of the
1
Bond Lengths (Å)
Ir1-P1
Ir1-P2
Ir1-N3
Ir1-C7
N1-C1
N1-C7
N1-C9
N2-C8
N2-C9
N2-C10
C7-C8
2.293(2)
2.304(3)
2.132(7)
2.004(9)
1.413(11)
1.335(10)
1.494(11)
1.462(10)
1.428(11)
1.428(10)
1.484(12)
intermediate, 16b, and final product, 14b, gave the deuteration
levels shown in eq 7. Deuterium appeared at C2, but the original
deuterium level in the hydride positions was diluted by
approximately 25%. No deuterium incorporation was ever seen
into C5. Experiments for various reaction times, from a few
minutes up to 2 h, always gave the same deuteration levels.
Additionally, starting IrD5(PPh3)2 with 44% deuterium content
resulted in a qualitatively similar deuterium distribution (Ir-
H, 33% each; C2, 33%). Reaction of 9b with IrD5(PPh3)2 gave
no deuterium incorporation into the unreacted free ligand. A
control experiment in which undeuterated 12b was monitored
in the presence of D2O over a period of 2 h showed that no
exchange takes place with aromatic hydrogens (eq 8).
Bond Angles (deg)
N3-Ir1-C7
P1-Ir1-P2
77.8(3)
158.97(8)
Dihedral Angles (deg)
N1-C9-N2-C10
142.7(8)
bond character because they are significantly stretched, 1.43
and 1.49 Å, versus 1.33 Å in 12. A remarkably short N1-C7
bond length indicates predominant double bond character, as
does the Ir1-C7 distance (2.004 Å).
More definitively, the NMR data (298 K, CDCl3) is also
consistent with the hydrogenated structure. The methylene
proton signals for C(2) and C(5) at 5.13 and 3.98 ppm are
considerably highfield shifted relative to 14. Each integrates
4
for two protons and shows mutual coupling (triplets, JHH
)
4.1 Hz) which is confirmed by decoupling of 31P. Further
evidence comes from the 13C{1H} NMR spectroscopy. The
metal bound carbon appears at 247.7 ppm, a value typical for
Fischer carbenes of the type LnMCR(NR2).17 In addition, C2
and C5 at 73.0 and 71.8 ppm, respectively, are shifted into the
aliphatic region. Compounds 16b-d are metastable and at 298
K slowly convert to the final product 14. Hence attempts to
crystallize them failed, and their characterization was limited
to spectroscopic techniques. However, the spectroscopic proper-
ties of 16b-d are very similar to those of the fully characterized
example, 16a, which suggests a similar structure for all four
compounds. Independent support for C4 binding in 16b comes
from NOESY experiments: significant NOE’s were detected
between the methyl protons of the isopropyl group and both
the C2 and C5 protons. In control experiments it was shown
that recrystallized 16a can also be converted to 14a in vigorously
refluxing THF. We find that the rate of this transformation is
strongly dependent on the vigor of the reflux, probably resulting
from the varying rate of removal of H2 from the system. This
has so far prevented the extraction of reliable kinetic data. No
analogues of 16 have been observed for the reactions between
the methylene linked ligand, 9, and IrH5(PPh3)2.
Attempts to reverse this dehydrogenation by hydrogenation
of 14 failed. When H2 was bubbled through a THF solution of
14b at 298 K for 18 h, traces of IrH5(PPh3)2 along with some
unidentified decomposition products were formed, but no 16
was detected.
Mechanistic Aspects. The mechanism seems to be complex,
and we can do no more than give initial indications here; a full
study, now in progress, will be reported in future. IrD5(PPh3)2
(d-5), obtained from D2 and [Ir(cod)(PPh3)2]PF6, had a deu-
teration level of 70%. Since the hydrides in 5 undergo exchange
Discussion
The formation of abnormal C4 carbenes, so far unique to
this system, contrasts with theoretical calculations and also with
previous studies on C-H bond activation in imidazolium
systems which have so far always predicted or generated normal
C2 carbenes.1 Steric congestion favors abnormal carbene
binding, while the sterically least demanding Me group pro-
moted C2-H bond activation and formation of normal carbenes.
Neither thermal nor acid-mediated interconversion of the C4
carbene into the C2 form or vice versa was ever observed. Due
to the kinetic stability of the products, it has not been possible
to determine the thermodynamic product distribution experi-
mentally. The fact that carbene formation is irreversible,
however, points to kinetic control of product formation.
The observed hydrogenation and dehydrogenation in the
reaction of 10a-d with IrH5(PPh3)2 are unprecedented. It is
not yet clear why the hydrogenated intermediate is only seen
for the smaller bite-angle ligand, however. We can only
speculate about the explanation, but it seems possible that the
smaller bite angle could align the imidazole ring to favor H
transfer.
The deuteriation study shows that we do not have a simple
C-H activation with a choice between two possible C-H
bonds. Branching presumably occurs between two independent
pathways, one that gives normal metalation and the other that
gives hydrogenation and then the abnormal aromatic carbene.
At the request of a reviewer we provide a working mechanistic
hypothesis (Figure 4) that fits the facts currently known, but
we hope to obtain a better picture in future experimental work
and theoretical work in collaboration with O. Eisenstein and E.
Clot. In this proposal, the H/D exchange at C2 is accomplished
by reversible CH oxidative addition to an [IrD3L2] intermediate;
(17) Luecke, H.; Arndtsen, B. A.; Burger, P.; Bergman, R. G. J. Am. Chem.
Soc. 1996, 118, 2517.
9
J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10477

A R T I C L E S
Gru¨ndemann et al.
possible, so these possibilities also need to be considered in
future mechanistic work.
Experimental Section
General Methods. IrH5(PPh3)2 (5),18 N-mesityl-N′-(2-pyridylmeth-
yl)imidazolium bromide (6a),2a N-methyl-N′-(2-pyridylmethyl)imid-
azolium bromide (6d),5a N-mesityl-N′-2-pyridylimidazolium bromide
(7a),8 N-butyl-N′-2-pyridylimidazolium bromide (7c),1j and N-isopropyl-
N′-2-pyridylimidazolium bromide (7b)8 were prepared according to
literature methods; all other reagents are commercially available and
were used as received. All NMR spectra were recorded at room
temperature on Bruker spectrometers operating at 400 or 500 MHz
(1H NMR) and 100 or 125 MHz (13C NMR), respectively, and
referenced to SiMe4 (δ in parts per million, J in hertz). Assignments
are based on COSY and HMQC spectroscopies. Melting points are
uncorrected. Elemental analyses were performed by Atlantic Microlab,
Inc; residual solvent molecules have been identified by 1H NMR
spectroscopy.
N-Butyl-N′-(2-pyridylmethyl)imidazolium Bromide (6c). A modi-
fication of the procedure of Tulloch et al. was used.2a 2-(Bromo-
methyl)pyridine hydrobromide (4 g, 16 mmol) was neutralized using a
saturated aqueous solution of sodium carbonate. The liberated 2-(bromo-
methyl)pyridine was extracted into diethyl ether (3 × 30 mL) at 0 °C,
dried with magnesium sulfate, and filtered. 1-Butylimidazole (1.95 g,
16 mmol) in methanol (100 mL) was added at 0 °C to the filtrate, the
ether removed under reduced pressure, and the solution stirred at room
temperature for 12 h. The methanol is evaporated under reduced
pressure, and the formed oil was purified by repetitive precipitation
from CH2Cl2/Et2O mixtures. The resulting oily product was dried in
vacuo. Yield: 4.52 g (95%).1H NMR (CHCl3, 298 K): δ 10.44 (s,
1H, NCHN), 8.49 (d, 1H, 3JHH ) 5.0 Hz, Hpy), 7.80 (d, 1H, 3JHH ) 7.7
Hz, Hpy), 7.74-7.69 (m, 1H, Hpy), 7.67 (s, 1H, Him), 7.42 (s, 1H, Him),
Figure 4. Current working hypothesis for the mechanism (d denotes lower
deuteration level).
in this process the initial 70% deuteration level, represented as
D, is diluted to the final lower level, d, by exchange of three D
with one H. Subsequent insertion of the imidazolium NdC bond
into an Ir-H bond leads to hydrogenation of the C2 position
with the adjacent N3 bound to Ir. The metal can now bind to
the C4 position, followed by a 1,2 hydride shift to give the
hydrogenated product. The ring dehydrogenation is proposed
to proceed by â-elimination, followed by a 1,3-H shift and loss
of H2. The feasibility of these steps is under active study.
Complications of the sort described here, abnormal C4
binding and ring hydrogenation, may be most likely in reactions
with hydride complexes, but the generality of this chemistry
has not yet been assessed. The use of benzimidazole prevents
abnormal C4 binding as no doubt would use of other 4,5-
substituted imidazoles. Large wingtip R groups commonly used
in the area should favor abnormal binding. Other abnormal
binding modes may be possible with heterocyclic ligands in
general.
3
7.28-7.24 (m, 1H, Hpy), 5.77 (s, 2H, CH2), 4.27 (t, 2H, JHH ) 7.5
Hz, CH2), 1.91-1.82 (m, 2H, CH2), 1.39-1.29 (m, 2H, CH2), 0.91 (t,
3
3H, JHH ) 7.5 Hz, CH3). 13C{1H} NMR (CDCl3, 298 K): δ 152.32
(Cpy), 149.52 (Cpy), 137.80 (Cpy), 136.98 (NCN), 124.08 (Cpy), 123.99
(Cpy), 122.80 (Cim), 121.56 (Cim), 53.61 (CH2), 49.86 (CH2), 31.92
(CH2), 19.36 (CH2), 13.33 (CH3); Anal. Calcd for C13H18BrN3 (296.21)‚
1.5H2O: C, 48.31; H, 6.55; N, 13.00. Found: C, 47.85; H, 6.54; N,
12.96.
N-Isopropyl-N′-(2-pyridylmethyl)imidazolium Bromide (6b). 2-
(Bromomethyl)pyridine hydrobromide (1.01 g, 4 mmol) was neutralized
using a saturated aqueous solution of sodium carbonate. The liberated
2-(bromomethyl)pyridine was extracted into diethyl ether (3 × 30 mL)
at 0 °C, dried with magnesium sulfate, and filtered into a solution of
1-isopropylimidazole (0.44 g, 4 mmol) in 1,4-dioxane (30 mL). The
ether was removed under reduced pressure and the solution refluxed
for 12 h. The volatiles were removed in vacuo, and the formed oil was
purified by repetitive precipitation from MeOH/Et2O and finally
recrystallized from CH2Cl2/Et2O to give colorless crystals Yield: 0.68
g (60%).1H NMR (CHCl3, 298 K): δ 10.97 (s, 1H, NCHN), 8.56-
8.52 (m, 1H, Hpy), 7.93 (d, 1H, 3JHH ) 7.8 Hz, Hpy), 7.75 (dt, 1H, 4JHH
) 1.7 Hz, 3JHH ) 7.5 Hz, Hpy), 7.65 (t, 1H, 3JHH ) 1.5 Hz, Him), 7.31-
7.24 (m, 1H, Hpy), 7.22 (t, 1H, 3JHH ) 1.5 Hz, Him), 5.81 (s, 2H, CH2),
Conclusions
This work has established that imidazolium salts do not
always give normal carbenes on metalation, implying that the
chemistry of this ligand system can be more complicated than
currently thought. Binding can occur at C2 or C4, and we give
spectroscopic criteria for distinguishing the two cases. Hydrogen
transfer from the metal to the imidazolium-derived carbene is
possible in some cases to give a fully hydrogenated ring, but
this proved reversible on reflux and the abnormal aromatic
carbene was obtained as final product. Abnormal binding is
blocked in benzimidazole. This work shows that a surprising
variety of structures are available from imidazolium salts. Care
must be taken to verify what structure has been formed in any
given case, especially when a catalyst is generated in situ from
imidazolium salt and metal precursor. Transformations between
the various structures could allow reactions to occur during
catalysis that could not occur if normal binding were alone
3
3
4.74 (septet, 1H, JHH ) 6.7 Hz, CH), 1.63 (d, 6H, JHH ) 6.7 Hz,
CH3). 13C{1H} NMR (CDCl3, 298 K): δ 152.57 (Cpy), 149.72 (Cpy),
137.80 (Cpy), 136.80 (NCN), 124.60 (Cpy), 124.06 (Cpy), 122.78 (Cim),
118.65 (Cim), 53.86 (CH2), 53.43 (CH), 23.09 (2C, CH3). Mp ) 85-
86 °C. Anal. Calcd for C12H16BrN3 (282.18)‚H2O: C, 48.01; H, 6.04;
N, 14.00. Found: C, 48.33; H, 5.81; N, 14.10.
N-Methyl-N′-2-pyridylimidazolium Bromide (7d). A mixture of
2-bromopyridine (3.16 g, 20.0 mmol) and 1-methylimidazole (1.64 g,
20.0 mmol) was kept neat at 160 °C for 48 h. After cooling, the formed
(18) Crabtree, R. H.; Felkin, H.; Morris, G. E. J. Organomet. Chem. 1977, 141,
205.
9
10478 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

Reaction of Imidazolium Salts with IrH (PPh3)2
A R T I C L E S
5
3
oily mixture was purified by repetitive precipitation from CHCl3/Et2O
mixtures and the resulting brownish oil precipitated and dried in vacuo.
Yield: 2.74 g (56%). An analytically pure sample (white needles) was
8.23 (d, 1H, JHH ) 5.5 Hz, Hpy), 7.37-7.14 (m, 32H, Hpy, Hph), 6.07
(t, 3JHH ) 6.3 Hz, Hpy), 5.17 (s, 1H, Him), 4.70 (s, 2H, CH2), 4.25 (septet,
3
3
1H, JHH ) 6.5 Hz, CH), 1.19 (d, 6H, JHH ) 6.5 Hz, CH3), -10.83
1
2
3
2
obtained from CH2Cl2/Et2O. H NMR (CHCl3, 298 K): δ 11.84 (s,
(dt, JPH ) 19.6 Hz, JHH ) 4.9 Hz, Ir-H), -21.49 (dt, JPH ) 18.6
1H, NCHN), 8.54-8.48 (m, 2H, Hpy), 8.29 (t, 1H, 3JHH ) 1.8 Hz, Him),
Hz, JHH ) 4.9 Hz, Ir-H). 13C{1H} NMR (CDCl3, 298 K): δ 161.78
3
4
3
8.06 (dt, 1H, JHH ) 1.8 Hz, JHH ) 7.9 Hz, Hpy), 7.49-7.44 (m, 2H,
Hpy, Him), 4.29 (s, 3H, CH3). 13C{1H} NMR (CDCl3, 298 K): δ 148.97
(Cpy), 145.82 (Cpy), 140.74 (Cpy), 136.54 (NCN), 125.22 (Cpy), 123.21
(Cim), 118.75 (Cim), 115.00 (Cpy), 37.17 (CH3). Mp ) 148-149 °C
(dec). Anal. Calcd for C9H10BrN3 (240.01)‚H2O: C, 41.88; H, 4.69; N,
16.28. Found: C, 41.45; H, 4.65; N, 15.96.
(Cpy), 153.10 (Cpy), 141.07 (t, JPC ) 7.1 Hz, Ccarbene), 137.04 (Cpy),
134.93 (t, JPC ) 26.3, Cph), 133.55 (t, JPC ) 6.0, Cph), 132.40 (NCN),
129.55 (Cph), 127.84 (t, JPC ) 5.8, Cph), 125.87 (Cpy), 124.15 (Cpy),
123.56 (Cim), 55.03 (CH2), 50.47 (CH), 22.94 (CH3). 31P{1H} NMR
(CDCl3, 298 K): δ 21.36. Mp ) 246-248 °C (dec). Anal. Calcd for
C48H47BF4IrN3P2 (1006.88)‚THF: C, 57.88; H, 5.14; N, 3.89. Found:
C, 57.84; H, 5.32; N, 3.74.
(η2-C,N)(N-Butyl-N′-(2-pyridylmethyl)imidazole-4-ylidene)bis-
(hydrido)bis(triphenylphosphine)iridium(III) Tetrafluoroborate (12c).
A mixture of 9c (54 mg, 0.18 mmol) and 5 (129 mg, 0.18 mmol) in
THF (8 mL) was refluxed in air. After 20 min a clear solution was
obtained. Refluxing was continued for 2 h more. After the reaction
mixture had cooled to room temperature, it was layered with 10 mL of
heptane. Over a period of 12 h crystals formed which were filtered off
and dried in vacuo. Yield: 124 mg (68%). The complex can be
N-Isopropyl-N′-2-pyridylbenzimidazolium Bromide (8). A mixture
of 2-bromopyridine (986 mg, 6.2 mmol) and 1-isopropylbenzimidazole
(1.0 g, 6.2 mmol) was kept neat at 185 °C for 20 h. After cooling, the
formed oily mixture was purified by repetitive precipitation from
CH2Cl2/Et2O mixtures, and the resulting brownish oil was precipitated,
washed with small amounts of acetone, and dried in vacuo. An
analytically pure sample (colorless needles) was obtained from CHCl3/
1
pentane.Yield: 0.15 g (8%). H NMR (CHCl3, 298 K): δ 11.99 (s,
3
1H, NCHN), 9.02 (d, 1H, JHH ) 8.3 Hz, Hpy), 8.78-8.71 (m, 1H,
Har), 8.67-8.62 (m, 1H, Hpy), 8.15 (dt, 1H, 4JHH ) 1.8 Hz, 3JHH ) 7.8
Hz, Hpy), 8.87-7.80 (m, 1H, Har), 7.74-7.67 (m, 2H, Har), 7.53-7.47
recrystallized from THF/pentane. H NMR (CDCl3, 298 K): δ 8.71
1
(s, 1H, NCHN), 8.19 (d, 1H, 3JHH ) 5.1 Hz, Hpy), 7.37-7.15 (m, 32H,
3
3
(m, 1H, Hpy), 5.37 (septet, 1H, 3JHH ) 6.8 Hz, CH), 1.99 (d, 6H, JHH
Hpy, Hph), 6.07 (t, JHH ) 5.9 Hz, Hpy), 5.03 (s, 1H, Him), 4.72 (s, 2H,
) 6.8 Hz, CH3). 13C{1H} NMR (CDCl3, 298 K): δ 148.60 (Cpy), 148.00
(Cpy), 140.65 (Cpy), 140.29 (NCN), 130.68 (Car), 130.44 (Car), 128.0
(Car), 127.52 (Car), 124.73 (Cpy), 118.00 (2C, Cpy, Car), 113.27 (Car),
53.14 (CH), 22.08 (2C, CH3). Mp ) 219-221 °C (dec). Anal. Calcd
for C15H16BrN3 (318.21): C, 56.62; H, 5.07; N, 13.21. Found: C, 56.50;
H, 5.12; N, 13.13.
CH2), 1.47 (m, 2H, CH2), 1.17 (m, 2H, CH2), 0.90 (t, 3H, JHH ) 7.7
3
Hz, CH3), -10.89 (dt, 2JPH ) 19.6 Hz, 3JHH ) 5.1 Hz, Ir-H), -19.61
(dt, JPH ) 18.4 Hz, JHH ) 5.1 Hz, Ir-H). 13C{1H} NMR (CDCl3,
298 K): δ 161.77 (Cpy), 153.07 (Cpy), 141.30 (t, JPC ) 6.9 Hz, Ccarbene),
137.09 (Cpy), 134.96 (t, JPC ) 26.3, Cph), 133.79 (NCN), 133.61 (t, JPC
) 6.0, Cph), 129.59 (Cph), 127.86 (t, JPC ) 4.7, Cph), 126.13 (Cim), 125.90
(Cpy), 124.18 (Cpy), 55.08 (CH2), 47.89 (CH2), 31.96 (CH2), 19.36 (CH2),
13.37 (CH3). 31P{1H} NMR (CDCl3, 298 K): δ 21.39. Mp ) 229 °C
(dec). Anal. Calcd for C49H49BF4IrN3P2 (1020.98)‚THF: C, 58.24; H,
5.26; N, 3.84. Found: C, 58.23; H, 5.29 N, 3.90.
2
3
Protocol for Anion Exchange of the Ligands 6-8 To Give 9-11.
A 1:1 mixture of the halide ligand and AgBF4 was stirred in CH2Cl2
or acetone in the dark for 12 h. The reaction mixture was filtered
through Celite and the solvent of the filtrate removed in vacuo. The
ligands were used without further purification.
Isomeric Mixture of (η2-C,N)(N-Methyl-N′-(2-pyridylmethyl)-
imidazole-4-ylidene)bis(hydrido)bis(triphenylphosphine)-
iridium(III) Tetrafluoroborate (12d) and (η2-C,N)(N-Methyl-N′-
(2-pyridylmethyl)imidazole-2-ylidene)bis(hydrido)bis(triphenyl-
phosphine)iridium(III) Tetrafluoroborate (13). A mixture of 9d (40
mg, 0.15 mmol) and 5 (110 mg, 0.15 mmol) in THF (12 mL) was
refluxed in air for 2 h. After the reaction mixture had cooled to room
temperature, the precipitation was completed by addition of 75 mL of
pentane. The product was filtered to give a yellowish powder. Yield:
110 mg (75%). An analytically pure sample of the mixture (55%
abnormal carbene and 45% normal carbene) can be obtained by
recrystallization from CHCl3/pentane.
(η2-C,N)(N-Mesityl-N′-(2-pyridylmethyl)imidazole-4-ylidene)bis-
(hydrido)bis(triphenylphosphine)iridium(III) Tetrafluoroborate (12a).
A mixture of 9a (50.5 mg, 0.14 mmol) and 5 (100 mg, 0.14 mmol) in
THF (5 mL) was refluxed in air. After 45 min a clear solution was
obtained. Refluxing was continued for 2 h more. After the reaction
mixture had cooled to room temperature, the crude product was
precipitated with 75 mL of pentane. Yield: 120 mg (79%). An
analytically pure sample was obtained from THF/pentane. Crystals
suitable for X-ray analysis were grown from CHCl3/pentane.1H NMR
3
(CDCl3, 298 K): δ 8.75 (s, 1H, NCHN), 8.11 (d, 1H, JHH ) 5.6 Hz,
3
Hpy), 7.37-7.15 (m, 32H, Hpy, Hph), 6.86 (s, 2H, Har), 6.02 (t, JHH
)
6.0 Hz, Hpy), 5.12 (s, 1H, Him), 4.93 (s, 2H, CH2), 2.30 (s, 3H, CH3),
(a) Spectroscopic Data for the Abnormal Carbene 12d. 1H NMR
(CDCl3, 298 K): δ 8.66 (s, 1H, NCHN), 8.19 (d, 1H, JHH ) 5.5 Hz,
1.88 (s, 6H, CH3), -11.10 (dt, 3JHH ) 4.8 Hz, 2JPH ) 20.7 Hz, Ir-H),
3
3
2
-21.23 (dt, JHH ) 4.8 Hz, JPH ) 18.2 Hz, Ir-H). 13C{1H} NMR
(CDCl3, 298 K): δ 161.48 (Cpy), 152.91 (Cpy), 140.53 (t, JPC ) 6.1
Hz, Ccarbene), 139.41 (Car), 137.14 (Cpy), 135.06 (t, JPC ) 26.5, Cph),
134.85 (Car), 134.07 (NCN), 133.61 (t, JPC ) 5.9, Cph), 132.09 (Car),
129.70 (Cph), 129.10 (Car), 127.95 (t, JPC ) 5.0, Cph), 127.61 (Cim),
126.35 (Cpy), 124.17 (Cpy), 55.43 (CH2), 21.03 (CH3), 17.27 (CH3).
31P{1H} NMR (CDCl3, 298 K): δ 21.39. Mp ) 255-257 °C (dec).
Anal. Calcd for C54H51BF4IrN3P2 (1082.97)‚H2O: C, 58.91; H, 4.85;
N, 3.82. Found: C, 58.88; H, 4.78; N, 3.94.
Hpy), 7.35-7.15 (m, 32H, Hpy, Hph), 6.10-6.04 (m, 1H, Hpy), 4.88 (s,
1H, Him), 4.69 (s, 2H, CH2), 3.38 (s, 3H, CH3), -10.89 (dt, 3JHH ) 4.7
2
3
2
Hz, JPH ) 19.8 Hz, Ir-H), -21.53 (dt, JHH ) 4.7 Hz, JPH ) 18.5
Hz, Ir-H). 13C{1H} NMR (CDCl3, 298 K): δ 161.83 (Cpy), 153.71
(Cpy), 141.97 (t, JPC ) 7.6 Hz, Ccarbene), 137.13 (Cpy), 134.92 (t, JPC
26.1, Cph), 134.27 (NCN), 133.62 (t, JPC ) 5.8, Cph), 129.59 (Cph),
127.84 (t, JPC ) 5.0, Cph), 127.48 (Cim), 125.83 (Cpy), 124.21 (Cpy),
55.08 (CH2), 34.56 (CH3). 31P{1H} NMR (CDCl3, 298 K): δ 21.14.
(b) Spectroscopic Data for the Normal Carbene 13. 1H NMR
(CDCl3, 298 K): δ 8.03 (d, 1H, 3JHH ) 5.5 Hz, Hpy), 7.64 (d, 1H, 3JHH
)
(η2-C,N)(N-Isopropyl-N′-(2-pyridylmethyl)imidazole-4-ylidene)-
bis(hydrido)bis(triphenylphosphine)iridium(III) Tetrafluoroborate
(12b). A mixture of 9b (40 mg, 0.14 mmol) and 5 (100 mg, 0.14 mmol)
in THF (5 mL) was refluxed in air. After 15 min a clear solution was
obtained. Refluxing was continued for 2 h more. After the reaction
mixture had cooled to room temperature, the product was precipitated
by addition of 25 mL of pentane. The yellow solid was filtered off and
dried in vacuo. Yield: 120 mg (85%). The complex can be recrystallized
from THF/pentane. Crystals suitable for X-ray analysis can be obtained
from CH2Cl2/Et2O. 1H NMR (CDCl3, 298 K): δ 8.72 (s, 1H, NCHN),
3
3
) 7.4 Hz, Hpy), 7.52 (t, 1H, JHH ) 7.5 Hz, Hpy), 7.44 (d, 1H, JHH
)
3
2.0 Hz, Him), 7.35-7.15 (m, 30H, Hpy, Hph), 6.31 (d, 1H, JHH ) 2.0
Hz, Him), 6.16 (t, 1H, 3JHH ) 7.4 Hz, Hpy), 4.83 (s, 2H, CH2), 2.54 (s,
3H, CH3), -11.04 (dt, 3JHH ) 5.3 Hz, 2JPH ) 19.8 Hz, Ir-H), -21.00
(dt, JHH ) 5.5 Hz, JPH ) 17.1 Hz, Ir-H). 13C{1H} NMR (CDCl3,
298 K): δ 169.90 (t, JPC ) 7.0 Hz, Ccarbene), 160.82 (Cpy), 153.04 (Cpy),
137.93 (Cpy), 134.15 (t, JPC ) 26.7, Cph), 133.28 (t, JPC ) 5.8, Cph),
130.04 (Cph), 128.16 (t, JPC ) 5.0, Cph), 126.29 (Cpy), 123.60 (Cpy),
123.26 (Cim), 120.58 (Cim), 55.08 (CH2), 37.20 (CH3). 31P{1H} NMR
3
2
9
J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10479

A R T I C L E S
Gru¨ndemann et al.
(CDCl3, 298 K): δ 17.14. Anal. Calcd for C46H43BF4IrN3P2 (978.82)‚
2CHCl3: C, 47.35; H, 3.73; N, 3.45. Found: C, 47.35; H, 3.69; N,
3.41.
Mp ) 230-233 °C (dec). Anal. Calcd for C45H41BF4IrN3P2 (964.80)‚
0.5CHCl3: C, 54.20; H, 4.36; N, 3.99. Found: C, 53.96; H, 4.41; N,
4.35.
(η2-C,N)(N-Isopropyl-N′-(2-pyridyl)imidazole-4-ylidene)bis-
(hydrido)bis(triphenylphosphine)iridium(III) Tetrafluoroborate (14b).
A mixture of 10b (32 mg, 0.12 mmol) and 5 (80 mg, 0.12 mmol) in
THF (50 mL) was heated to reflux in air. After 10 min a clear yellow
solution was obtained. After heating was continued for another 15 h,
the reaction mixture was cooled to room temperature and concentrated
to 10 mL of THF. The product was precipitated by addition of 50 mL
of pentane. The product was filtered off and dried in vacuo to give a
yellow solid. Yield: 70 mg (65%). The complex can be crystallized
from CHCl3/pentane. 1H NMR (CDCl3, 298 K): δ 9.28 (s, 1H, NCHN),
7.74 (d, 1H, 3JHH ) 8.2 Hz, Hpy), 7.67 (d, 3JHH ) 5.5 Hz, Hpy), 7.57 (t,
3JHH ) 7.9 Hz Hpy), 7.44-7.17 (m, 30 H, Hph), 6.26 (t, 3JHH ) 6.5 Hz,
Hpy), 5.97 (s, 1H, Him), 4.44 (septet, 3JHH ) 6.9 Hz, CH), 1.25 (d, 3JHH
(η2-C,N)(N-Isopropyl-N′-(2-pyridyl)benzimidazole-2-ylidene)bis-
(hydrido)bis(triphenylphosphine)iridium(III) Tetrafluoroborate (15).
A mixture of 11 (20 mg, 0.061 mmol) and 5 (44 mg, 0.061 mmol) in
THF (25 mL) was refluxed in air overnight. It becomes clear after a
few minutes. After the reaction mixture was cooled to room temperature,
it is concentrated to 15 mL and 50 mL of pentane was added to
complete precipitation. The product was filtered off as a yellow solid
which was dried in vacuo. Yield: 51 mg (80%). Crystals suitable for
X-ray analysis can be obtained from CHCl3/pentane.1H NMR (CDCl3,
3
3
298 K): δ 8.17 (d, JHH ) 8.5 Hz, Har), 7.98 (d, JHH ) 8.5 Hz, Hpy),
3
3
7.90 (t, 1H, JHH ) 7.5 Hz, Hpy), 7.73 (d, 1H, JHH ) 5.7 Hz, Hpy),
7.49-7.43 (m, 1H, Har), 7.26-7.22 (m, 2H, Har), 7.19-7.02 (m, 30 H,
3
3
Hph), 6.40 (t, JHH ) 6.5 Hz, Hpy), 4.96 (septet, 1H, JHH ) 7.0 Hz,
3
2
3
3
) 6.9 Hz, CH3), -11.35 (dt, JHH ) 5.5 Hz, JPH ) 17.9 Hz, Ir-H),
CH), 0.55 (d, 6H, JHH ) 7.0 Hz, CH3), -11.42 (dt, JHH ) 5.5 Hz,
3
2
3
2
-19.73 (dt, JHH ) 5.5 Hz, JPH ) 16.8 Hz, Ir-H). 13C{1H} NMR
(CDCl3, 298 K): δ 155.28 (Cpy), 149.0 (t, JPC ) 4.7 Hz), 151.3 (Cpy),
138.4 (Cpy), 133.5 (t, JPC ) 26.5, Cph), 133.4 (t, JPC ) 6.6, Cph), 131.1
(NCN), 129.7 (Cph), 127.8 (t, JPC ) 4.4, Cph), 122.9 (Cpy), 122.6 (Cim),
112.8 (Cpy), 51.6 (CH), 22.8 (CH3). 31P{1H} NMR (CDCl3, 298 K): δ
21.9. Mp ) 250-252 °C (dec). Anal. Calcd for C47H45BF4IrN3P2
(993.27)‚CHCl3: C, 51.83; H, 4.17; N, 3.78. Found: C, 52.27; H, 4.36;
N, 3.82.
2JPH ) 19.4 Hz, Ir-H), -19.12 (dt, JHH ) 5.5 Hz, JPH ) 15.9 Hz,
Ir-H). 13C{1H} NMR (CDCl3, 298 K): δ 192.5 (t, JPC ) 6.3 Hz,
Ccarbene), 154.2 (Cpy), 153.0 (Cpy), 141.0 (Cpy), 133.0 (t, JPC ) 6.0, Cph),
132.6 (t, JPC ) 27.2, Cph), 132.6 (Car), 132.3 (Car), 130.4 (s, Cph), 128.3
(t, JPC ) 5.0, Cph), 125.2 (Car), 124.2 (Car), 122.0 (Cpy), 113.6 (Cpy),
113.5 (Car), 112.3 (Car), 56.3 (CH), 19.4 (CH3). 31P{1H} NMR (CDCl3,
298 K): δ 18.4. Mp ) 263-265 °C (dec). Anal. Calcd for C51H47BF4-
IrN3P2 (1042.91)‚2CHCl3: C, 49.67; H, 3.85; N, 3.28. Found: C, 49.57;
H, 4.00; N, 3.32.
(η2-C,N)(N-Butyl-N′-(2-pyridyl)imidazole-4-ylidene)bis(hydrido)-
bis(triphenylphosphine)iridium(III) Tetrafluoroborate (14c). A
mixture of 10c (57 mg, 0.19 mmol) and 5 (143 mg, 0.19 mmol) in
THF (11 mL) were refluxed in air. After 30 min of refluxing, a clear
yellow solution was obtained. After another 30 min, precipitation
started. Refluxing was then continued for another 2 h. After the reaction
mixture was cooled to room temperature, 100 mL of pentane was added
to complete precipitation. The product was filtered off as a yellow solid
which was dried in vacuo. Yield: 155 mg (80%). The complex can be
(η2-C,N)(N-Mesityl-N′-(2-pyridyl)imidazolidine-4-ylidene)bis(hy-
drido)bis(triphenylphosphine)iridium(III) Tetrafluoroborate (16a).
A mixture of 9a (50 mg, 0.14 mmol) and 5 (100 mg, 0.14 mmol) in
THF (50 mL) was heated to reflux in air. After 15 min a clear yellow
solution was obtained. After heating was continued for another 2 min
the reaction mixture was immersed in an ice bath until it was cooled
to room temperature, when 60 mL of pentane was added to precipitate
the product, which was then filtered off and dried in vacuo. Yield: 111
mg (74%). Crystals suitable for X-ray analysis can be obtained from
THF/pentane.1H NMR (CDCl3, 298 K): δ 7.91 (t, 1H, 3JHH ) 7.9 Hz,
1
recrystallized from THF/pentane. H NMR (CDCl3, 298 K): δ 9.29
(s, 1H, NCHN), 7.71 (d, 1H, 3JHH ) 5.5 Hz, Hpy), 7.65 (d, 3JHH ) 8.5
Hz, Hpy), 7.53 (t, 3JHH ) 8.3 Hz, Hpy), 7.49-7.15 (m, 30 H, Hph), 6.27
Hpy), 7.62 (d, 1H, JHH ) 8.2 Hz, Hpy), 7.53 (d, JHH ) 5.2 Hz, Hpy),
3
3
3
3
3
(t, JHH ) 6.9 Hz, Hpy), 5.92 (s, 1H, Him), 3.91 (t, 1H, JHH ) 6.9 Hz,
CH2), 1.56-1.46 (m, 2H, CH2), 1.07-0.94 (m, 2H, CH2), 0.82 (t, 3JHH
7.44-7.27 (m, 30 H, Hph), 6.71 (s, 2H, Har), 6.41 (t, JHH ) 6.5 Hz,
4
Hpy), 5.13 (t, 2H, JHH ) 4.1 Hz, NCH2N), 3.98 (br s, 2H, Him), 2.20
3
2
3
2
) 7.6 Hz, CH3), -11.44 (dt, JHH ) 5.5 Hz, JPH ) 18.2 Hz, Ir-H),
(s, 3H, CH3), 1.61 (s, 6H, CH3), -10.03 (dt, JHH ) 5.0 Hz, JPH )
-19.65 (dt, JHH ) 5.5 Hz, JPH ) 15.8 Hz, Ir-H). 13C{1H} NMR
20.5 Hz, Ir-H), -18.56 (dt, JHH ) 5.0 Hz, JPH ) 16.15 Hz, Ir-H).
13C{1H} NMR (CDCl3, 298 K): δ 247.7 (s, Ccarbene), 154.1 (Cpy), 152.4
(Cpy), 140.4 (Cpy), 138.1 (2C, Car), 137.1 (Car), 136.8 (Car), 133.0 (t,
JPC ) 6.6, Cph), 132.6 (t, JPC ) 28.75, Cph), 130.6 (Cph), 129.6 (2C,
Car), 128.4 (t, JPC ) 5.5, Cph), 123.3 (Cpy), 115.5 (Cpy), 73.0 (NCN),
71.8 (Cim), 20.8 (CH3), 17.7 (2C, CH3). 31P{1H} NMR (CDCl3, 298
K): δ 19.5. Mp ) 203-205 °C (dec). Anal. Calcd for C54H53BF4-
IrN3P2 (1084.99): C, 59.44; H, 4.80; N, 3.92. Found: C, 59.14; H,
4.94; N, 3.98.
3
2
3
2
(CDCl3, 298 K): δ 155.4 (Cpy), 151.1 (Cpy), 151.0 (t, JPC ) 6.9 Hz,
Ccarbene), 138.3 (Cpy), 133.5 (t, JPC ) 26.5, Cph), 133.4 (t, JPC ) 6.6,
Cph), 132.6 (NCN), 129.7 (Cph), 127.9 (t, JPC ) 5.5, Cph), 125.6 (Cim),
122.9 (Cpy), 112.7 (Cpy), 48.8 (CH2), 32.1 (CH2), 19.2 (CH2), 13.5 (CH3).
31P{1H} NMR (CDCl3, 298 K): δ 22.2. Mp ) 237-239 °C (dec).
Anal. Calcd for C48H47BF4IrN3P2 (1006.88)‚CHCl3: C, 52.26; H, 4.30;
N, 3.73. Found: C, 52.91; H, 4.51; N, 3.70.
(η2-C,N)(N-Methyl-N′-(2-pyridyl)imidazole-4-ylidene)bis(hydri-
do)bis(triphenylphosphine)iridium(III) Tetrafluoroborate (14d). A
mixture of 10d (34 mg, 0.14 mmol) and 5 (100 mg, 0.14 mmol) in
THF (50 mL) was refluxed in air. The mixture is vigorously refluxed
in air for 9 h. It never became clear. After the reaction mixture was
cooled to room temperature, it was concentrated to 15 mL and 100
mL of pentane was added to complete precipitation. The product was
filtered off as a yellow solid which was dried in vacuo. Yield: 80 mg
(59%). The complex can be recrystallized from CHCl3/pentane.1H NMR
Characterization of (η2-C,N)(N-Isopropyl-N′-(2-pyridyl)imid-
azolidine-4-ylidene)bis(hydrido)bis(triphenylphosphine)irid-
ium(III) Tetrafluoroborate (16b). A mixture of 10b (32 mg, 0.12
mmol) and 5 (80 mg, 0.12 mmol) in THF (10 mL) was heated to reflux
in air. After 15 min a clear yellow solution was obtained. After heating
was continued for another 2 min, the reaction mixture was immersed
in an ice bath until it was cooled to room temperature, when 60 mL of
pentane was added to precipitate the product, which was then filtered
off and dried in vacuo. Yield: 100 mg (86%).1H NMR (CDCl3, 298
3
(CDCl3, 298 K): δ 9.27 (s, 1H, NCHN), 7.78 (d, 1H, JHH ) 5.5 Hz,
Hpy), 7.60 (d, 3JHH ) 8.2 Hz, Hpy), 7.53 (t, 3JHH ) 7.6 Hz, Hpy), 7.48-
K): δ 7.84 (t, 1H, JHH ) 7.9 Hz, Hpy), 7.69 (d, 1H, JHH ) 8.2 Hz,
Hpy), 7.62 (d, 3JHH ) 5.5 Hz, Hpy), 7.40-7.28 (m, 30 H, Hph), 6.45 (t,
3JHH ) 6.5 Hz, Hpy), 4.64 (s, 2H, NCH2N), 3.20 (s, 2H, Him), 2.36
(septet, 3JHH ) 6.2 Hz, CH), 0.78 (d, 3JHH ) 6.2 Hz, CH3), -9.94 (dt,
3
3
3
7.17 (m, 30 H, Hph), 6.31 (t, JHH ) 6.9 Hz, Hpy), 5.84 (s, 1H, Him),
3.62 (s, 3H, CH3), -11.42 (dt, 3JHH ) 5.5 Hz, 2JPH ) 17.9 Hz, Ir-H),
3
2
-19.70 (dt, JHH ) 5.5 Hz, JPH ) 16.5 Hz, Ir-H).13C{1H} NMR
(CDCl3, 298 K): δ 155.4 (Cpy), 151.5 (t, JPC ) 7.3 Hz, Ccarbene), 151.1
(Cpy), 138.3 (Cpy), 133.5 (t, JPC ) 26.4, Cph), 133.4 (t, JPC ) 5.7, Cph),
133.3 (NCN), 129.7 (Cph), 127.9 (t, JPC ) 4.6, Cph), 126.9 (Cim), 122.9
(Cpy), 112.6 (Cpy), 35.4 (CH3). 31P{1H} NMR (CDCl3, 298 K): δ 22.0.
2
3
3JHH ) 4.8 Hz, JPH ) 20.6 Hz, Ir-H), -18.74 (dt, JHH ) 4.8 Hz,
2JPH ) 16.1 Hz, Ir-H). 13C{1H} NMR (CDCl3, 298 K): δ 248.8 (t,
JPC ) 4.0 Hz, Ccarbene), 154.1 (Cpy), 152.9 (Cpy), 140.1 (Cpy), 133.0 (t,
JPC ) 6.1, Cph), 132.6 (t, JPC ) 28.75, Cph), 130.5 (Cph), 128.4 (t, JPC
9
10480 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

Reaction of Imidazolium Salts with IrH (PPh3)2
A R T I C L E S
5
) 5.5, Cph), 122.8 (Cpy), 115.3 (Cpy), 72.7 (NCN), 69.8 (Cim), 52.1
(CH), 20.5 (CH3). 31P{1H} NMR (CDCl3, 298 K): δ 19.3.
immersed in an ice bath until it cooled to room temperature, when a
precipitate formed. After 60 mL of pentane was added to complete
precipitation, the product was then filtered off and dried in vacuo. Yield:
93 mg (87%). 1H NMR (CDCl3, 298 K): δ 7.83 (t, 1H, 3JHH ) 7.1 Hz,
Characterization of (η2-C,N)(N-Butyl-N′-(2-pyridyl)imidazoli-
dine-4-ylidene)bis(hydrido)bis(triphenylphosphine)iridium(III) Tet-
rafluoroborate (16c). A mixture of 10c (35 mg, 0.12 mmol) and 5
(80 mg, 0.12 mmol) in THF (10 mL) was heated to reflux in air. After
15 min a clear yellow solution was obtained. After heating was
continued for another 2 min, the reaction mixture was immersed in an
ice bath until it was cooled to room temperature, when 60 mL of pentane
was added to precipitate the product, which was then filtered off and
dried in vacuo. Yield: 66 mg (58%). 1H NMR (CDCl3, 298 K): δ 7.85
(t, 1H, 3JHH ) 7.9 Hz, Hpy), 7.73 (d, 1H, 3JHH ) 8.3 Hz, Hpy), 7.55 (d,
1H, 3JHH ) 6.2 Hz, Hpy), 7.40-7.15 (m, 30 H, Hph), 6.41 (t, 3JHH ) 6.2
3
3
Hpy), 7.72 (d, 1H, JHH ) 8.2 Hz, Hpy), 7.52 (d, JHH ) 5.5 Hz, Hpy),
3
7.41-7.27 (m, 30 H, Hph), 6.39 (t, JHH ) 6.6 Hz, Hpy), 4.67 (s, 2H,
3
NCH2N), 3.22 (s, 2H, im-H), 1.78 (s, 3H, CH3), -10.01 (dt, JHH
)
)
2
3
2
4.9 Hz, JPH ) 20.3 Hz, Ir-H), -18.66 (dt, JHH ) 4.9 Hz, JPH
15.9 Hz, Ir-H). 13C{1H} NMR (CDCl3, 298 K): δ 248.8 (bs, Ccarbene),
154.1 (Cpy), 152.9 (Cpy), 139.9 (Cpy), 133.0 (t, JPC ) 6.2, Cph), 132.6
(t, JPC ) 28.43, Cph), 130.5 (Cph), 128.4 (t, JPC ) 5.2, Cph), 122.9 (Cpy),
115.6 (Cpy), 77.6 (NCN), 75.2 (Cim), 42.4 (CH3). 31P{1H} NMR (CDCl3,
298 K): δ 19.0.
3
Hz, Hpy), 4.67 (s, 2H, Him), 3.23 (s, 2H, Him), 1.95 (t, 1H, JHH ) 6.9
Hz, CH2), 1.18-1.05 (m, 4H, CH2), 0.82 (t, 3JHH ) 7.6 Hz, CH3), -9.98
(dt, 3JHH ) 4.8 Hz, 2JPH ) 20.6 Hz, Ir-H), -18.69 (dt, 3JHH ) 4.8 Hz,
2JPH ) 16.5 Hz, Ir-H). 13C{1H} NMR (CDCl3, 298 K): δ 248.6 (br s,
Acknowledgment. We gratefully acknowledge financial
support from Deutsche Akademie der Naturforscher Leopoldina
(Grant BMBF-LPD 9901/8-37; S.G.), the Swiss National
Foundation (M.A.), Johnson Matthey, and the U.S. DOE and
NSF (R.H.C., J.W.F.) and helpful suggestions from a reviewer.
Ccarbene), 154.1 (Cpy), 153.0 (Cpy), 140.1 (Cpy), 133.0 (t, JPC ) 5.5, Cph),
132.7 (t, JPC ) 28.75, Cph), 130.5 (Cph), 128.39 (t, JPC ) 5.5, Cph),
122.9 (Cpy), 115.6 (Cpy), 75.9 (NCN), 73.0 (Cim), 54.6 (CH2), 30.6
(CH2), 19.9 (CH2), 13.8 (CH3). 31P{1H} NMR (CDCl3, 298 K): δ 19.1.
Characterization of (η2-C,N)(N-Methyl-N′-(2-pyridyl)imidazoli-
dine-4-ylidene)bis(hydrido)bis(triphenylphosphine)iridium(III) Tet-
rafluoroborate (16d). A mixture of 10d (27 mg, 0.12 mmol) and 5
(80 mg, 0.12 mmol) in THF (10 mL) was heated to reflux in air. After
10 min of vigorous boiling, a clear yellow solution was obtained. After
heating was continued for another 2 min, the reaction mixture was
Supporting Information Available: Detailed crystallographic
data, atomic positional parameters, bond lengths and angles
for12a, 12b, 15, and 16a (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
JA026735G
9
J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10481