An anion-dependent switch in selectivity results from a change of C-H activation mechanism in the reaction of an imidazolium salt with IrH 5(PPh3)2

Article abstract of DOI:10.1021/ja055317j

Changing the counteranion along the series Br, Bp₄, PF ₆, SbF₆ in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH₅(PPh ₃)₂ to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir ^III to give Ir^V with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir^III, not Ir^V, because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. ¹⁹F,¹H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).

Full text of DOI:10.1021/ja055317j

Published on Web 10/28/2005
An Anion-Dependent Switch in Selectivity Results from a
Change of C-H Activation Mechanism in the Reaction of an
Imidazolium Salt with IrH5(PPh3)2
Leah N. Appelhans,† Daniele Zuccaccia,‡ Anes Kovacevic,† Anthony R. Chianese,†
John R. Miecznikowski,† Alceo Macchioni,‡ Eric Clot,§ Odile Eisenstein,*,§ and
Robert H. Crabtree*,†
Contribution from the Department of Chemistry, 225 Prospect Street, Yale UniVersity,
New HaVen, Connecticut 06520-8107, Dipartimento di Chimica, UniVersita` di Perugia,
Via Elce di Sotto 8, 06123, Perugia, Italy, and LSDSMS (UMR 5636 CNRS), cc 14,
Institut Gerhardt, UniVersite´ Montpellier 2, F-34095 Montpellier Cedex 5, France
Received August 4, 2005; E-mail: robert.crabtree@yale.edu; eisenst@univ-montp2.fr
Abstract: Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl
imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic
carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT)
suggests that the AN path involves C-H oxidative addition to IrIII to give IrV with little anion dependence.
The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride.
The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer,
leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via
IrIII, not IrV, because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE
NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY
NMR experiments indicate an ion-pair structure for the products that is consistent with the computational
prediction (ONIOM(B3PW91/UFF)).
reported.7,8 In this context, Lacour has seen IP effects in
asymmetric anions,9 and the importance of ion-pair formation
Introduction
Counterion effects resulting from ion-pairing (IP) have
recently taken a more prominent role in organometallic chem-
istry.1 Some of us have applied 1H NOESY and 19F,1H-HOESY
NMR spectroscopy to the investigation of predominant ion-pair
solution structures of organometallic complexes by the detection
of interionic contacts.2-4 We have shown that the solution ion-
pair structures can be predicted by ONIOM (B3PW91/UFF)
calculations of the ion-pair geometry.5,6 The results suggest that
these ion-pairs have surprisingly well-defined structures. Al-
though absolute IP energies can be large, only differences in
this energy with change of anion can lead to any experimentally
observable counterion-dependent switching of reaction pathway
in solution. Even then, the two transition states (TS) involved
in any two such competing reactions need to be close in energy.
For asymmetric catalysis, this condition is automatically fulfilled
and several cases of anion-dependent effects on ee have been
in the kinetics of deprotonation of dihydrogen complexes has
been studied by calculations.10
In this paper, we report in full on strong counterion effects
in the reaction of 2-pyridylmethyl imidazolium salts with IrH5-
(PPh3)2 to give chelating11,12 NHC13-18 complexes; depending
on the anion, the NHCs can be bound either at C2 or at C5.19
With anions such as bromide, capable of strong IP by
C-H‚‚‚Br hydrogen bonding at C2,20 the normal C2 NHC is
(7) Pfaltz, A.; Blankenstein, J.; Hilgraf, R.; Ho¨rmann, E.; McIntrye, S.; Menges,
F.; Scho¨nleber, M.; Smidt, S. P.; Wu¨stenberg, B.; Zimmermann, N. AdV.
Synth. Catal. 2003, 345, 33.
(8) Llewellyn, D. B.; Adamson, D.; Arndtsen, B. A. Org. Lett. 2000, 2, 4165.
(9) Lacour, J.; Hebbe-Viton, V. Chem. Soc. ReV. 2003, 373.
(10) Basallote, M. G.; Besora, M.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Lledo´s,
A.; Ma´n˜ez, M. A.; Maseras, F. J. Am. Chem. Soc. 2004, 126, 2320.
(11) Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R. H. Chem. Commun. 2001,
201.
(12) Peris, E.; Crabtree, R. H. Coord. Chem. ReV. 2004, 248, 2239.
(13) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39.
(14) Herrmann, W. A.; Reisinger, C. P.; Spiegler, M. J. Organomet. Chem. 1998,
557, 93.
† Yale University.
‡ Universita` di Perugia.
§ Universite´ Montpellier 2.
(1) Macchioni, A. Chem. ReV. 2005, 105, 2039 and references therein.
(2) Bellachioma, G.; Cardaci, G.; Macchioni, A.; Reichenbach, G.; Terenzi,
S. Organometallics 1996, 15, 4349.
(3) Macchioni, A.; Bellachioma, G.; Cardaci, G.; Gramlich, V.; Ru¨egger, H.;
Terenzi, S.; Venanzi, L. M. Organometallics 1997, 16, 2139.
(4) Macchioni, A. Eur. J. Inorg. Chem. 2003, 195.
(5) Macchioni, A.; Zuccaccia, C.; Clot, E.; Gruet, K.; Crabtree, R. H.
Organometallics 2001, 20, 2367.
(6) Gruet, K.; Clot, E.; Eisenstein, O.; Lee, D.-H.; Patel, B.; Macchioni, A.;
Crabtree, R. H. New J. Chem. 2003, 27, 80.
(15) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1291.
(16) Ho, V. M.; Watson, L. A.; Huffman, J. C.; Caulton, K. G. New J. Chem.
2003, 27, 1446.
(17) Grudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247.
(18) Cavell, K. J.; McGuinness, D. S. Coord. Chem. ReV. 2004, 248, 671.
(19) Kovacevic, A.; Gru¨ndemann, S.; Miecznikowski, J. R.; Clot, E.; Eisenstein,
O.; Crabtree, R. H. Chem. Commun. 2002, 2580.
(20) Sambrook, M. R.; Beer, P. D.; Wisner, J. A.; Paul, R. L.; Cowley, A. R.;
Szemes, F.; Drew, M. G. B. J. Am. Chem. Soc. 2005, 127, 2292 and
references therein.
9
10.1021/ja055317j CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 16299-16311
16299

A R T I C L E S
Appelhans et al.
Table 1. Ratio of 2 to 3 for Various R and A in Eq 1
the normal C2 NHC, however, the abnormal C5 isomer was
obtained. The normal isomer could still be obtained if the C4
and C5 positions were blocked, as in the benzimidazolium salts.
For small wingtip groups at N3, such as methyl, a mixture of
normal (2) and abnormal (3) NHCs was seen (Table 1). For
example, the reaction of eq 1 (R ) Me, A ) BF4) gave a 45:55
ratio of products 2d and 3d (R ) Me, A ) BF4).
R
A
2 (normal)
3 (abnormal)
a
b
c
d
e
f
g
h
i
Me
Me
Me
Me
Me
Me
Me
i-Pr
i-Pr
n-Bu
Br
91
80
70
45
50
50
11
84
0
9
20
30
55
50
50
89
16
100
95
OAc
OCOAra
BF4
PF6
OCOPh
SbF6
Br
BF4
BF4
j
5
a O2CAr ) 2,4,6-triisopropylbenzoate.
obtained (N path), while changing the counterions along the
series Br, BF4, PF6, SbF6 progressively switches the kinetic
product to the abnormal C5 species (AN path). Such C5 NHCs
are still rare, but several other examples have now been
reported.21-23
Formation of NHCs by C-H oxidative addition was proposed
some years ago,24 but this pathway was only authenticated re-
cently by Peris et al. who observed the oxidative addition pro-
duct directly.25 Previous computational (DFT) work has analyzed
the bonding associated with the NHC-metal interaction.26-32
Few examples have yet been reported on the pathways by which
NHC complexes are formed by reaction of parent imidazolium
salts with metal complexes.25,33,34 We now find that such
calculations have allowed us to understand the anion effect as
a result of the N and AN paths having different mechanisms
with different sensitivities to the nature of the anion.
The product ratio is kinetic because the 2/3 ratio is invariant
on subsequent change of the anion. Thermal interconversion of
2 and 3 was never seen even after prolonged reactions times.
For example, the 45:55 mixture from 1d (R ) Me, A ) BF4)
can be converted either to the Br or to the SbF6 salt by ion
exchange. The products retain the 45:55 ratio of the starting
material even on heating (5 h, refluxing thf). The ion present
during the synthesis therefore decides the ratio, not any anion
subsequently introduced.
The abnormal isomer was obtained in pure form by synthesis
with the BF4 salt, followed by recrystallization, and fully
characterized. The normal isomer was obtained in pure form
from the imidazolium salt and [IrH2(Me2CO)2L2]BF4 (see
Experimental Section). The very different spectroscopic char-
acteristics of the two types of NHC, fully discussed previously
by comparison with crystallographically authenticated examples,
allows secure identification even in mixtures.36 In particular,
the adjacent imidazole C4 and C5 aromatic protons of normal
isomer 2d are relatively close in proton NMR chemical shift
(CDCl3), δ 7.44 and 6.31, as expected on the basis of prior
work, while the nonadjacent aromatic protons of abnormal 3d,
C2 and C4, are more separated, δ 8.66 and 4.88, again as
expected. The particularly downfield shift of the C2 proton is
consistent with its acidic character. Also in line with previous
data, the C2 carbene carbon has a lower field 13C NMR chemical
shift, (CHCl3) 169.9 ppm (BF4 salt), than the C5 carbene, 142.0
ppm (BF4 salt).
Results and Discussion
(1) Experimental and Theoretical Studies of the Ion-Pair
Structures. NHCs are attracting attention in organometallic
chemistry,13,14 but improved synthetic routes are sought for
metallating the precursor imidazolium salts. The harsh conditions
of the early work (e.g., BuLi) are not compatible with the
functionality-rich NHCs that are increasingly employed.35 We
therefore attempted metalation via reaction of a metal hydride,
[IrH5(PPh3)2], with the imidazolium salt 1i (R ) i-Pr, A ) BF4;
see Table 1 for the numbering scheme) with the result that the
NHC was obtained, accompanied by loss of H2.36 Instead of
(21) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc.
2004, 126, 5046.
(22) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E. Organome-
tallics 2004, 23, 166.
(23) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003, 22, 3016.
(24) (a) McGuinness, D. S.; Cavell, K. J.; Yates, B. F.; Skelton, B. W.; White,
A. H. J. Am. Chem. Soc. 2001, 123, 8317. (b) McGuinness, D. S.; Saendig,
N.; Yates, B. F.; Cavell, K. J. J. Am. Chem. Soc. 2001, 123, 4029. (c)
Gru¨ndemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R.
H. Dalton Trans. 2002, 2163.
(25) Viciano, M.; Mas-Marza, E.; Poyatos, M.; Sanau, M.; Crabtree, R. H.; Peris,
E. Angew. Chem., Int. Ed. 2005, 44, 444.
(26) Frenking, G.; Frohlich, N. Chem. ReV. 2000, 100, 717.
(27) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.; Hoff,
C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485.
(28) Perrin, L.; Clot, E.; Eisenstein, O.; Loch, J.; Crabtree, R. H. Inorg. Chem.
2001, 40, 5806.
(29) Baba, E.; Cundari, T. R.; Firkin, I. Inorg. Chim. Acta 2005, 358.
(30) Mungur, S. A.; Liddle, S. T.; Wilson, C.; Sarsfield, M. J.; Arnold, P. J.
Chem. Commun. 2004, 2738.
(31) Hu, X. L.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics
2004, 23, 755.
While complex 2 was not converted to 3 nor vice versa
thermally under neutral conditions, even after anion exchange,
HBF4/CH2Cl2 treatment at room temperature completely con-
verts 3 to 2, so 2 is the thermodynamically more stable product.
This confirms that, although the energies of 2 and 3 may be
close, any abnormal product 3 formed in eq 1 is entirely a kinetic
product. This acid treatment is also a method for obtaining the
normal isomers in pure form.
We looked at the effect on eq 1 of variation of the anion, A,
prompted by computational work, detailed below, that suggested
that the energetics of the system 2/3 are dependent on the anion.
The results of the reaction of eq 1 for a series of anions and
wingtip groups are shown in Table 1. Moving to less strongly
ion-pairing and bulkier counterions along the IP effect series
Br > OAc > BF4 > PF6 > SbF6 progressively switches the
kinetic product from the normal C2 to the abnormal C5 species.
(32) Green, J. C.; Herbert, B. J. Dalton Trans. 2005, 1214.
(33) Clement, N. D.; Cavell, K. J.; Jones, C.; Elsevier, C. J. Angew. Chem., Int.
Ed. 2004, 43, 1277.
(34) McGuiness, D. S.; Cavell, K. J.; Yates, B. F. Chem. Commun. 2001, 355.
(35) Rivera, G.; Crabtree, R. H. J. Mol. Catal. A 2004, 222, 59.
(36) Gru¨ndemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R.
H. J. Am. Chem. Soc. 2002, 124, 10473.
i
For a fixed anion, the more bulky R group, Pr, leads to the
less hindered abnormal NHC being relatively favored versus
9
16300 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Anion-Dependent Switch in Selectivity
A R T I C L E S
the R ) Me case (Table 1, entries a vs h, and d vs i,
respectively). The magnitude of the effects seen here goes
beyond the modest ones usual in organometallic chemistry,
although larger anion effects have come to light more recently.37
When equimolar quantities of the bromide (1a) and SbF6 (1g)
salts were employed, a 55:45 ratio of normal:abnormal resulted
(R ) Me). This suggests that each ion-pair reacts independently
at similar rates.
in pseudo-trans position with respect to the anion. This apparent
predisposition toward the formation of the abnormal carbene is
not, however, the key factor (vide infra). The same prevalent
orientation is observed in THF-d8 for 1i (R ) i-Pr and A )
BF4) as that of the solvent used in the syntheses (Figure 1, left).
A confirmation of the preferred orientation of the pyridine ring
comes from the observation of 11/2 NOE intensity that is about
3 times that of 11/5 in the 1H-NOESY NMR spectrum recorded
for 5 (R ) i-Pr and A ) BF4) in CD2Cl2 (Figure S2 in
Supporting Information). The anion has a key role in determin-
ing the relative orientation of the pyridine to the imidazolium:
in CD3NO2, where PGSE (pulsed field gradient spin-echo)39,40
NMR experiments suggest the prevalence of free ions (Table
S1 in Supporting Information), the intensities of the 11/2-11/5
NOEs almost invert (Figure S3 in Supporting Information), so
there is now an apparent predisposition toward normal carbene.
The anion hydrogen bonded as C-H‚‚‚A to the C2 position
of the imidazolium reactant 1 might have been expected to
sterically block reaction at that site. In the series Br > OAc >
BF4 > PF6 > SbF6, the steric and electronic factors reinforce
each other: SbF6 is both bulkier and less strongly ion-pairing
than Br. Steric effects do operate in this system. If the methyl
i
wingtip group is replaced by Pr, the abnormal product is
obtained with the weakly coordinating anion BF4 (Table 1, entry
i), an anion that previously gave 45:55 mixtures (Table 1, entry
d). The larger Pr group causes the reaction to favor the less
NMR Studies of the Iridium Ion-Pair Solution Structure.
The ion-pair structure of complexes 2i, 2j, 3i, and 3j was
investigated in CD2Cl2 and THF-d8 solution, where PGSE
measurements show that the complexes are mainly present as
intimate ion-pairs (Table S1 in Supporting Information). The
greater solubility of the n-Bu and i-Pr analogues led to the choice
of these derivatives for this part of the study. Dipolar interactions
i
hindered abnormal NHC. Only with Br is the normal compound
seen, and then only in an 84:16 ratio (Table 1, entry h). The
results for the three carboxylates (b, c, and f), particularly the
contrast between PhCO2 and the bulky ArCO2, suggest that we
have a combination of steric and electronic effects in this case.
NMR Studies of the Imidazolium Ion-Pair Solution
Structure. The starting imidazolium salt has an ion-pair
structure (see 1) in which the anion is hydrogen bonded to the
C2 proton, consistent with the present computational data (vide
infra) and with literature expectations.38 This is perhaps most
easily shown by the sensitivity of the proton NMR shift of
C2-H to the nature of the anion. For example, for the model
imidazolium salt 4, the resonance shifts (CDCl3) are as
follows: A ) Br, δ 10.45; BF4, 9.85; PF6, 8.79; SbF6, 8.50.
The same interaction is detected in the abnormal carbene 3,
where the acidic CH is directed away from the metal. The range
of shifts seen is very similar: A ) Br, δ 9.71 (3a); BF4, 8.66
(3d); PF6, 8.44 (3e); SbF6, 8.38 (3g). Bromide can readily
replace SbF6 (or BF4) as ion-pairing partner in 4, as shown by
the change in chemical shift of the C2-H proton from that of
the SbF6 salt to that of the Br salt on addition of NBu4Br in
CDCl3. The titration curve suggests Keq is 1.6 in favor of Br.
Similar ion-pairing effects in imidazolium salts have been
reported.38
1
between anion 19F and cation H nuclei are detected in the
19F,1H-HOESY spectra. As shown from the NMR data in Figure
1 (right) obtained in exactly the same conditions as the synthesis,
the F nuclei of the counteranion of complexes 3i interact with
2, 12, 6, 14, m, and, likely, o (overlapped with 12) protons. No
contact is observed with the two hydrides. The same interactions
are also observed for both abnormal and normal complexes in
CD2Cl2 (Figure S4 in Supporting Information). A response was
also detected at the R position of the n-Bu wingtip group only
in the abnormal case, where the R group projects into the region
where the anion binds. A more detailed discussion of this NMR
experiment is available for a closely related complex.5
Ion-Pair Structure-Computational. From the NMR studies
on the products, the anion is essentially located in the vicinity
of the most protonic C-H bond: C2-H for the abnormal and
C5-H for the normal isomer. However, IP contacts were also
seen with the protons of the methylene linker and with H12 on
the pyridine ring. To better characterize the network of H-bonds
operating in these systems, ONIOM (B3PW91/UFF) calculations
were carried out. In the system chosen, the wingtip group is iPr
treated at the MM level (UFF) together with the phenyl groups
on PPh3, whereas the rest of the molecule, including the anion,
is treated at the DFT (B3PW91) level. The anions considered
are BF4, Br, and OAc, and Figure 2 shows the optimized
geometries for the normal (N) and abnormal (AN) carbenes,
respectively. Selected geometrical parameters are given in Table
2.
19F,1H-HOESY NMR spectra of 1i (R ) i-Pr and A ) BF4)
and 5 (R ) i-Pr and A ) BF4) pyridyl-imidazolium salts
recorded in CD2Cl2 show strong dipolar interactions between
the anion 19F and cation 2, 13, 12, CH2 (in the case of 1i), and
11 1H nuclei (Figure S1 in Supporting Information). This
indicates that the pyridine nitrogen is predominantly oriented
In the absence of anion, the geometries of the normal (N)
and abnormal (AN) isomers are very similar. The only
(39) (a) Hahn, E. L. Phys. ReV. 1950, 80, 580. (b) Stejskal, E. O.; Tanner, J. E.
J. Chem. Phys. 1965, 42, 288. (c) Stilbs, P. Prog. Nucl. Magn. Reson.
Spectrosc. 1987, 19, 1. (d) Price, W. S. Concepts Magn. Reson. 1997, 9,
299. (e) Price, W. S. Concepts Magn. Reson. 1998, 10, 197. (f) Johnson,
C. S., Jr. Prog. Nucl. Magn. Reson. Spectrosc. 1999, 34, 203.
(40) For reviews on applications to organometallic chemistry, see: (a) Valentini,
M.; Ru¨egger, H.; Pregosin, P. S. HelV. Chim. Acta 2001, 84, 2833. (b)
Binotti, B.; Macchioni, A.; Zuccaccia, C.; Zuccaccia, D. Comments Inorg.
Chem. 2002, 23, 417. (c) Pregosin, P. S.; Martinez-Viviente, E.; Kumar,
P. G. A. Dalton Trans. 2003, 4007.
(37) Smidt, S. P.; Zimmermann, N.; Studer, M.; Pfaltz, A. Chem.-Eur. J. 2004,
10, 4685.
(38) Cowan, J. A.; Clyburne, J. A. C.; Davidson, M. G.; Harris, R. L. W.;
Howard, J. A. K.; Ku¨pper, P.; Leech, M. A.; Richards, S. P. Angew. Chem.,
Int. Ed. 2002, 41, 1432.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16301

A R T I C L E S
Appelhans et al.
Figure 1. 19F,1H-HOESY NMR spectra (376.65 MHz, 323 K, THF-d8) of 1i (left, R ) i-Pr and A ) BF4) and 3i (right, R ) i-Pr and A ) BF4) showing
the selective interactions of the anion with the indicated protons. “*” denotes THF and water resonances.
41
significant difference is the longer Ir-HC bond with the
hydride trans to C5 in AN (1.623 Å) than trans to C2 in N
(1.600 Å). The higher electron-donating power of the AN form42
leads to a higher trans effect, also illustrated by the shorter Ir-C
bond in AN (2.098 Å) than in N (2.115 Å). The C-H bond
that is not activated has the same value in both isomers (1.079
Å), thus giving a reference for systems with anion interactions
at this position.
ligand that can form H-bonds with several centers at once. These
data are in full agreement with the NMR studies.
The higher acidity of the H atom bound to C2 in AN-BF4
induces a shift in the position of the BF4 anion resulting in a
shortened contact (1.820 Å) with C2-H and an elongated
contact with the pyridine (2.147 Å). This shift is best illustrated
by the increase in the Ir-C‚‚‚B angle from 111.8° in N-BF4 to
123.7° in AN-BF4.
Upon interaction with BF4, the geometry of the two carbene
isomers, N-BF4 and AN-BF4, is slightly altered. The most
significant change is the elongation of the C-H bond close to
the anion (C5-H ) 1.082 Å, N-BF4; C2-H ) 1.092 Å, AN-
BF4), clearly indicating the presence of H-bonding interactions
with the latter. As expected, the effect is more pronounced for
the more acidic C2-H bond. The interaction associated with
BF4 in AN-BF4 is also reflected by the elongation of the Ir-
C5 bond (2.098 Å, AN; 2.107, AN-BF4), whereas the Ir-N
bond remains unchanged. This situation is not observed in
N-BF4 where the Ir-C2 bond is left unchanged while the Ir-N
bond shortens (2.239 Å, N; 2.219, N-BF4). Several short contacts
are observed between the anion and the cation (Table 2). For
the normal isomer, the shortest contacts are with the hydrogen
atom on the pyridine ring (2.109 Å) and bound to C5 (2.129
Å) consistent with both protons having similar acidity. A short
contact is also obtained with the bridging methylene protons
(2.053 Å). The BF4 anion thus acts as a chelating, outer sphere
We have also compared the contacts in AN-BF4 to those
observed in the X-ray structure.36,43 Because of the uncertainty
in the location of the H atoms in the X-ray structure, we have
considered F‚‚‚C distances in the comparison. In the crystallo-
graphically characterized structure, the contacts with the pyridine
and C2 are 3.189 and 3.507 Å, respectively. In the calculated
structure AN-BF4, the corresponding values are 2.895 and 3.133
Å, respectively. There is thus a qualitative agreement, even
though the experimental values are longer, perhaps due to
interaction of the anion with molecules of other unit cells. These
results clearly indicate that the anion position, conserved both
in the solid and in solution, is associated with the presence of
a network of hydrogen bonds.
The nonchelating anion, Br, is less apt to form this network
of hydrogen bonds because Br can at most only be involved in
bifurcated H-bonds. The optimal geometries for N-Br and AN-
Br (Figure 2) therefore reflect the best compromise between
the various possible interactions. In the normal isomer N-Br,
the pyridine and C5-H hydrogen atoms, having similar acidity,
(41) Hc refers to the hydride trans to the carbene ligand, and HN refers to the
hydride trans to pyridine in the product of C-H activation; see Table 2.
(42) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R.
H. Organometallics 2004, 23, 2461.
(43) Gru¨ndemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R.
H. Chem. Commun. 2001, 2274.
9
16302 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Anion-Dependent Switch in Selectivity
A R T I C L E S
is significantly elongated (1.097 vs 1.085 Å) as a result of
stronger H-bonding. As observed with BF4, the counteranion
shifts closer to the imidazolium ion no doubt to enhance the
preferred H-bonding interaction (Ir-C2‚‚‚Br ) 121.5°, N-Br;
Ir-C5‚‚‚Br ) 124.5°, AN-Br). However, the Br anion still
interacts with multiple Hs, not just with C2-H in a linear C2-
H‚‚‚Br hydrogen bond.
Moving to a chelating anion, acetate CH3COO (OAc), allows
interaction with a greater number of H atoms. In the normal
isomer N-OAc, the very short contact with H-C5 (1.842 Å)
indicates formation of a strong hydrogen bond. The C5-H bond
is significantly more elongated (1.106 Å) than in the case of
BF4 (1.082 Å) or Br (1.085 Å). Also, the O‚‚‚H-C5 atoms are
almost collinear (170°) as expected for a classical hydrogen
bond. Nevertheless, the chelating ability of OAc means that the
formation of this strong H-bond is not made at the expense of
the interaction with the equally acidic pyridine proton and the
contact with pyridine is also the shortest among the series of
normal isomers (2.109 Å, N-BF4; 2.740 Å, N-Br; 2.019 Å,
N-OAc).
In the abnormal isomer AN-OAc, the more acidic H-C2
center attracts the OAc so that the O‚‚‚H-C2 contact is very
short (1.659 Å) and the elongation of the H-C2 bond is very
large (1.135 Å). In this case, almost perfect linearity is achieved
(172.3°) as expected for a strong H-bond. A contact with the
pyridine ring is still possible via the second oxygen atom with
comparable distances (2.147 Å, AN-BF4; 2.170 Å, AN-OAc).
This argument means that the intrinsic basicity of the anion
is not the sole factor. The NMR data on 4 clearly demonstrate
that Br is a better H-bond acceptor for H-C2 than BF4, but in
this system there was no competition with other protons of
similar acidity (the butyl hydrogens are not so acidic). Therefore,
the sole possibility is for the anion to engage in a hydrogen
bond with the sole acidic proton, H-C2. For the pyridine-
substituted ligand, the situation is more complex and the
presence of the acidic pyridine protons allows for several
H-bonding interactions with formation of a network of hydrogen
bonds that can only be fully exploited by anions with several
H-bonding sites (BF4, OAc). Depending upon the IP architec-
ture, this possibility for extra stabilization in chelating anions
can lead to an inversion in the expected IP stabilization along
a series versus that expected from anion basicity.
Ion-Pair Energetics-Computational. ONIOM(B3PW91/
UFF) calculations show that the thermodynamic preference is
always for the normal carbene isomer. The free normal NHC
1-N proved to be 14.5 kcal mol-1 more stable than the abnormal
isomer 1-AN. Coordination to the Ir fragment reduced the
difference to 10.1 kcal mol-1, no doubt because the abnormal
isomer is a stronger σ-donor as shown experimentally by
reduced ν(CO) frequencies.42 In full agreement with the creation
of the network of H-bonding interactions discussed above,
introduction of the anion causes a much larger reduction to 3.8
kcal mol-1 for Br, 1.6 kcal mol-1 for BF4, and 0.9 kcal mol-1
for acetate (Table 3). The values reflect the efficiency with
which the anion can interact with the full set of protons available
in the imidazolium and pyridine rings as described previously.
Thus, the nonchelating anion (Br) affords less extra stabilization
for the abnormal isomer when compared to the strongly
chelating H-bond acceptor, acetate. Modeling the CH2Cl2 solvent
(CPCM continuum model) slightly increases the preference for
Figure 2. Optimized QM/MM geometries for normal (N) and abnormal
(AN) carbene complexes in their ion-pairs with BF4 (N-BF4 and AN-BF4),
Br (N-Br and AN-Br), and OAc (N-OAc and AN-OAc). For clarity, only
the ipso PPh3 carbons are shown.
adopt an intermediate geometry with long Br‚‚‚H contacts,
automatically giving a shorter contact with the linker CH2
protons (Table 2). The C5-H elongation is slightly greater than
that with BF4 (1.085 Å, N-Br; 1.082 Å, N-BF4), showing that,
despite its remote position, Br is a better H-bond acceptor.
The greater acidity of C2-H in AN results in a shortening
of the contact with the C2-H proton (2.422 Å) and a
lengthening of that with the pyridine (3.059 Å). The C2-H bond
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16303

A R T I C L E S
Appelhans et al.
Table 2. Selected Geometric Parameters (Å) for the Various Complexes Optimized (HN and HC Refer to the Hydrides Trans to Pyridine and
to the Carbene Carbon, Respectively)
N
N-BF4
N-Br
N-OAc
AN
AN-BF4
AN-Br
AN-OAc
Ir-HN
Ir-HC
Ir-N
Ir-C2
Ir-C5
Ir-P
1.573
1.581
1.578
1.579
1.577
1.623
2.230
1.582
1.630
2.230
1.582
1.630
2.234
1.582
1.633
2.230
1.600
2.239
2.115
1.608
2.219
2.114
1.608
2.236
2.121
1.610
2.232
2.121
2.098
2.326/2.367
1.079
2.107
2.314/2.360
1.092
2.109
2.316/2.359
1.097
2.109
2.313/2.357
1.135
2.327/2.393
1.079
2.331/2.371
1.082
2.316/2.386
1.085
2.316/2.380
1.106
C2-H
C5-H
A‚‚‚H2a
A‚‚‚H5
A‚‚‚CH2
1.820
2.422
1.659
2.129
2.053
2.109
2.771
2.372
2.740
1.842
1.851
2.019
b
2.059
2.147
2.451
3.059
1.875
2.170
c
A‚‚‚CHpy
a These contacts between the anion and the cation correspond to the shortest distance in each case. b CH2 denotes the methylene linker between pyridine
and imidazole. c CHpy corresponds to the pyridine proton H12.
Table 3. Relative ONIOM(B3PW91/UFF) and CPCM(B3PW91/
will be fully expressed in the transition state for C-H activation,
CH2Cl2) Energies (kcal mol-1) of the Abnormal (AN, AN-BF4,
and the outcome will result from the competition between (i)
AN-Br, AN-Ac) with Respect to the normal (N, N-BF4, N-Br,
N-Ac) Carbene Complexa
maximum H-bonding stabilizing interaction versus (ii) minimal
steric impact from the anion.
b
Eb
(2) Computational Studies of the Reaction Mechanism.
The computational study of the reaction pathways leading to
the normal and abnormal carbene was carried out on a model
system consisting of IrH3(PMe3)2 and the N,N′-dimethyl-
imidazolium cation in the presence of A- where A ) BF4 or
Br. This modeling is considered appropriate because prior
mechanistic work36 shows that (i) the reactions of IrH5L2
proceed via initial H2 loss and (ii) very similar results are found
whether we use the chelate imidazolium salt or a mixture of
N,N′-dialkyl imidazolium salt and free pyridine (eq 2).42 The
reaction is proposed to start by loss of H2 to generate IrH3L2
followed by a C-H bond activation of the imidazolium salt
with formation of an H2 complex,44 in turn followed by the
replacement of the H2 ligand by the pyridine (see Figure 3).
The pathways corresponding to the activation of C2-H and
C5-H are named N and AN, respectively. The activated C-H
is named C-Hac, and the uncleaved C-H bond is named
C-Hun.
anion
ONIOM
CPCM
normal
abnormal
none
BF4
10.1
1.6
3.8
0.9
7.7
1.9
5.7
4.5
-8.4
-0.8
-11.5
-12.3
-3.6
-15.9
Br
CH3COO
a The normal is always more stable. b The ion-pair binding energies Eb
(kcal mol-1) are evaluated for each isomer at the CPCM(B3PW91/CH2Cl2)
level.
the normal isomer: 5.7 kcal mol-1 (Br), 4.5 kcal mol-1 (OAc),
and 1.9 kcal mol-1 (BF4). Nevertheless, inclusion of the solvent
does not qualitatively alter the stabilizing effect of the anion.
We also estimated the ion-pair interaction energy with CPCM
calculations. The ion-pair ONIOM geometries were used in three
different CPCM calculations: ion-pair, cation, and anion. From
the three energies obtained at the B3PW91 level within the
CPCM scheme, the interaction energy Eb was computed as
E(ion-pair) - {E(cation) + E(anion)}. The negative values
obtained (Table 3) indicate a stabilizing tendency for the ion-
pair in agreement with NOE and PGSE NMR observations. The
interaction energies are smaller when the anion interacts with
the less acidic H-C5. Interestingly, as anticipated from the
analysis of the network of H-bonds with BF4 as compared to
the Br case, the interaction energy Eb with BF4 is much larger
than that with Br (Table 3). As a chelating anion and a good
H-bond acceptor, acetate gives a large interaction energy for
both isomers. These ion-pair formation energies are comparable
to binding energies of a weak ligand like H2 and should thus
be considered more carefully in any theoretical reactivity study
of organometallic salts.
Our results suggest that when several protons of similar
acidity are close, a multidentate anion such as BF4 will search
for the most efficient network of interactions, whereas an anion
like Br will prefer to create one strong interaction with the most
acidic center. In any intramolecular proton transfer, the BF4
anion would be expected to find it hard to “follow” the migrating
proton, while at the same time trying to maintain its network
of H-bonds. In contrast, if the transferring proton is the most
acidic, then the Br anion would gain in “following” the
transferring hydrogen at the expense of less acidic protons in
an anion-coupled proton transfer. The difference in behavior
Anion-free reaction pathways were examined first, but,
although the transition state for activation of C5-H was found,
no transition state could be located for C2-H and therefore
only reaction pathways in the presence of anion are discussed.
The structures of all extrema are shown in Figure 4 for BF4
and in Figure 5 for Br.
Reactions in the Presence of BF4. IrH3(PMe3)2 is a square-
based pyramid with an apical hydride and a pair of mutually
2
trans basal hydrides. The LUMO is the well-known dz -based
orbital (z apical). Interestingly, the HOMO is not a metal d
orbital but the out-of-phase combination of the two basal
hydrides. Thus, in addition to the usual Lewis acid property
(44) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure, Theory
and ReactiVity; Kluwer Academic Publishers: New York, 2001.
9
16304 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Anion-Dependent Switch in Selectivity
A R T I C L E S
Figure 3. The proposed normal (N) and abnormal (AN) pathways (L ) PMe3).
associated with the LUMO, the metal fragment has some Lewis
base activity associated with the basal hydrides.
The C-H bond cleavage is more advanced in the N pathway
(C2-H ) 1.429 Å) than in the AN path (C5-H ) 1.384 Å),
and the Ir-C bond is accordingly shorter in the N case (Ir-C2
) 2.157 Å) than in the AN case (Ir-C5 ) 2.188 Å). The
interaction between BF4 and the imidazolium ring is based on
principles similar to those in the reactants: Fa is close to Hac
and Fb is close to Hun. The F‚‚‚H distance is the shortest for
C2-Hac (N path), where the Fa‚‚‚Hac-C2 is 1.984 Å but Fb‚‚
‚Hun-C5 is 3.114 Å. In the AN case, the Fa‚‚‚Hac-C5 distance
is 2.40 Å, and Fb is only 2.789 Å from H-C2.
In the AN case, the transition state leads to a pentagonal
bipyramidal IrV intermediate with four equatorial H ligands and
an equatorial carbene. The Ir-C5 distance of 2.128 Å indicates
a single Ir-C bond. The BF4 lies above the imidazolium ring,
near the C-Hun, with Hun‚‚‚Fb being 2.130 Å. In an important
difference between the two paths, the N path transition state
leads not to an IrV species but to an IrIII dihydrogen complex
with H2 trans to hydride and cis to the NHC. The H2 ligand
and the NHC are twisted relative to the IrH3 plane. The BF4 is
close to the protonic dihydrogen ligand with an H‚‚‚Fa distance
of 2.036 Å.
The reaction starts with formation of a σ-complex of the C-H
bond to be cleaved, C-Hac, which binds trans to the apical
hydride. In both the AN and the N paths, the coordinated C-H
bond is oriented perpendicular to the IrH3 plane and the
imidazolium ring plane is perpendicular to the IrL2(C-Hac)
plane. The metric data for the σ-complex differ slightly for the
two cases (Ir-C ) 2.334 Å, Ir‚‚‚Hac ) 1.821 Å for AN; Ir-C
) 2.360 Å, Ir‚‚‚Hac ) 1.900 Å for N). The BF4 anion is located
above the imidazolium ring with one Fa pointing toward C-Hac,
and another, Fb, toward the other C-H bond, C-Hun (both
shown with dotted lines in Figure 4). The BF4 is much closer
to the azolium ring in the AN pathway: the Fa‚‚‚Hac distance
being 2.306 Å for AN but 3.895 Å for N. The anion therefore
interacts more strongly with Hac, and the distance is even shorter
when C-Hac is C2-H (N path). Any H‚‚‚F distance longer than
2.3 Å can be considered a weak interaction, as is the case for
all except the C2-Hac/BF4 combination. In all cases, a large
number of weak interactions are also present involving the IrH3-
(PMe3)2, the imidazolium ion, and the BF4; in particular, the
acidic N-methyl groups are close to the basic basal hydrides
and the BF4 is not far from two methyl groups of the PMe3
ligand. The details of these interactions will no doubt be different
in the real experimental systems, which contain PPh3 and not
PMe3, but in what follows, we will concentrate on the main
interactions that do not involve PR3 and are expected to be
common for both.
Reactions in the Presence of Br. The nonchelating Br anion
cannot optimally simultaneously interact with two different H
atoms of the imidazolium group but prefers to interact strongly
with one H, the choice being either Hac or Hun. The basic features
of the minima and transition states are similar for the two anions.
The reactant is a σ-complex with the C-H bond in the plane
perpendicular to the IrH3 plane. For the AN path, Br is far from
Hac but close to Hun (Br‚‚‚Hun ) 2.766 Å). For the N path, Br
is close to Hac with the angle Cl‚‚‚Hac‚‚‚Ir being 158°. At the
transition state for the AN path, Br is close to Hun (2.876 Å)
and far from Hac (>4 Å). In contrast, for the N path, Br is very
close to Hac (Hac‚‚‚Br ) 2.403 Å) with a Br-Hac-Ir angle of
164.9°. In the tetrahydride intermediate formed from the AN
The metal remains octahedral in the transition state, the NHC
carbon being one of the ligands. The imidazolium rotates from
the position it had in the σ-complex, so as to bring the C-H
proton closer to the hydride to which proton transfer is to occur,
and the NHC ligand follows this motion so that it is no longer
perpendicular to the plane containing the phosphine ligands.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16305

A R T I C L E S
Appelhans et al.
Figure 4. Optimized geometries for the σ-complex, transition state, and intermediate located along the N and AN pathways with BF4 as counteranion. In
an important difference between the two pathways, the N intermediate is an IrIII complex of H2 and the AN intermediate is an IrV polyhydride. Energy
profiles (kcal mol-1) relative to the σ-complex: N pathway, 2.6 (TS), -17.5 (intermediate); AN pathway, 2.0 (TS), -19.9 (intermediate).
transition state, the Br anion is near Hun (Br‚‚‚Hun ) 3.00 Å).
In the dihydrogen complex formed from the N transition state,
the Br anion is very close to the dihydrogen ligand (shortest
Br‚‚‚H distance ) 2.528 Å). These geometrical features for the
minima and transition state (extrema) on the two paths show
that the Br always prefers to be near H-C2 rather than H-C5,
and this preference is enhanced when H-C2 is the bond being
activated.
Energy Profiles for C-H Bond Activation and Discussion.
After initial H2 loss, the reaction continues with the coordination
of the imidazolium/anion ion-pair to IrH3(PMe3)2 via a C-H
σ-complex. Although the resulting pair of σ complexes could
be calculated for coordination at C2-H (N) and at C5-H (AN),
the relative free energies of these two species versus the
separated imidazolium/anion and IrH3(PMe3)2 fragment are not
easy to evaluate. The anion is paired with the more acidic C2-H
in the imidazolium/anion ion-pair in a C-H‚‚‚A hydrogen bond.
Coordination to C5-H thus does not require much structural
reorganization because this bond is remote from the anion. This
is not the case for coordination at C2-H where the anion has
to be displaced. These changes, which of course occur in a polar
solvent, cannot be easily evaluated with the present level of
theory. Therefore, the relative population of the two σ-com-
plexes cannot be evaluated and is certainly not well represented
by the calculated difference in electronic energies between the
two systems (6 kcal mol-1 in favor of the σ-complex at H-C2
in the gas phase and 2 kcal mol-1 in the THF solvent with BF4
as the counteranion). In the absence of better information, we
have analyzed the computational profiles by focusing on the
energy barriers ∆Eq from each of the σ-complexes (Table 4).
The activation energies ∆Gq, also given in Table 4, show similar
trends, and so we have chosen to discuss the results based on
∆Eq.
The fact that we could not locate a transition state for
activation of the C2-H bond in absence of counteranion is in
line with a previous finding that the activation of the C2-H
bond of an imidazolium salt, in the absence of the counteranion,
by a Pd(0) diphosphine species occurs without any energy
barrier.24a The energy barrier found in the present study for the
activation of the C5-H was 0.2 kcal mol-1. In the presence of
anions, both energy barriers are nonzero but very small. After
passage of the transition state, the energy profile goes sharply
downward. In the case of Br, the energy barrier for the AN
path is 3.8 kcal mol-1 and only 2.5 kcal mol-1 for the N path.
With BF4, the respective values are 2.0 and 2.6 kcal mol-1 (Me/
Me entries in Table 4). These results show that there is no great
kinetic preference for activating either the C2-H or the C5-H
bonds despite the fact that in many decades of work on the
metalation of imidazolium salts up to 2002, only the normal
C2 NHC products were ever observed, presumably because
NHCs were only very rarely synthesized by C-H bond
activation. The results also suggest that the nature of the anion
9
16306 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Anion-Dependent Switch in Selectivity
A R T I C L E S
Figure 5. Optimized geometries for the σ-complex, transition state, and intermediate located along the N and AN pathways with Br as counteranion. The
intermediates are IrIII (N) and IrV (AN) as in the BF4 pathway of Figure 4. Energy profiles (kcal mol-1) relative to the σ-complex: N pathway, 2.5 (TS),
-18.6 (intermediate); AN pathway, 3.8 (TS), -16.5 (intermediate).
Table 4. Energetics (kcal mol-1) Associated with the N and AN
Pathways Calculated for N,N′-Dialkyl-imidazolium Salts of BF4 or
Br Reacting with IrH3(PMe3)2
Scheme 1. The Two Possible Trajectories from the σ-Complex at
C2-H of the N-Methyl-N′-isopropyl Imidazolium Cation with IrH3L2
(L ) PMe3)
a
anion
subst.
path
∆
Eq
∆E
∆
Gq
∆G
BF4
Me/Me
N
AN
N
AN
N
N
AN
N
AN
2.6
2.0
2.3
2.7
4.2
2.5
3.8
2.5
3.5
-17.5
-19.9
-16.7
-18.3
-11.9
-18.6
-16.5
-18.5
-16.4
1.9
1.5
1.7
3.1
3.3
0.3
3.2
0.8
3.6
-17.0
-20.3
-16.0
-16.4
-10.3
-19.3
-16.1
-17.8
-16.5
BF4
Me/iPr
BF4
Br
iPr/Me
Me/Me
Br
Me/iPr
a The notation R/R′ (R ) Me, Pr; R′ ) Me, Pr) indicates a C-H
oxidative process where the migrating proton is adjacent to the R group
(see Scheme 1). The activation barriers (∆Eq and ∆Gq) and reaction energies
(∆E and ∆G) are evaluated from the corresponding σ-complex in each case.
N (respectively AN) refers to the N (respectively AN) pathway.
i
i
Table 4), and in the other, the Me group (Me/iPr in Table 4).
For BF4, the more hindered trajectory is calculated to have a
significantly higher barrier (iPr/Me, 4.2 kcal mol-1; Me/iPr, 2.3
kcal mol-1). For the Br anion, the Me/iPr trajectory is the only
one obtained and has an activation barrier similar to the Me/
Me case (2.5 kcal mol-1). All attempts to find a TS for the
iPr/Me trajectory with Br failed as rotation of the imidazolium
in the optimization procedure switched the pathway to the less
hindered Me/iPr trajectory. Unlike the BF4 anion, the bromide
needs to track the proton and cannot do so efficiently if it has
to approach a bulky group.
tunes the outcome in a subtle way. One can hardly consider
differences in barrier height of less than 1 kcal mol-1 as reliable.
Nevertheless, the lower barrier calculated for the N path in the
case of Br is consistent with the formation of the normal NHC
with Br, and the lower barrier for the AN path in the case of
BF4 is consistent with the formation of the abnormal NHC with
BF4.
To understand the substituents effects, the same calculations
were carried out on the N-methyl-N′-isopropyl imidazolium salt.
Even though the starting σ-complex in the calculations for the
normal pathway had a geometry such that the proton could
migrate to either of the pair of trans hydrides (Scheme 1), when
For the AN reaction, the calculations were carried out with
the C-Hac bond next to the methyl group (Me/iPr in Table 4,
A ) BF4, Br) as this is the major regioisomer observed in the
experiment.42 In the σ-complex, the change of methyl to
isopropyl shifts the anion away from the imidazolium ring to
i
an Pr group is present, the symmetry is broken, and in one
trajectory, the proton needs to pass by the iPr group (iPr/Me in
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16307

A R T I C L E S
Appelhans et al.
minimize repulsion with the i-Pr group. Because BF4 cannot
easily interact with the distant azole C-Hac bonds, it compen-
sates by moving close to C-Hun. Remarkably, this significantly
lengthens the Ir-C bond (2.334 Å, Me/Me; 2.517 Å, Me/iPr).
In the AN transition state for C-H activation, the BF4 remains
quite far from the C-Hac bond and close to H-C2. This
contrasts with the N,N′-dimethyl-imidazolium case where the
BF4 approaches the C5-H being cleaved. This shows that
although BF4, as a chelating anion, is able to interact with two
H atoms of the imidazolium, it does so preferentially with
H-C2, especially when it is the C-H bond being cleaved, and
does not approach H-C5 even if cleaved, plausibly because of
the steric effect of the isopropyl group.
of the geometrical constraints that impose a substantial distance
between the carbon of the NHC and the cis hydride, a
counteranion is necessary to stabilize the migrating proton in
the transition state in an anion-coupled proton transfer. Br is a
much more effective anion for this purpose than BF4. Although
the BF4 is not far from the C-Hac hydrogen in the N transition
state, it is not very effective in stabilizing this transition state
because of the delocalization of the negative anionic charge over
the four fluorine atoms. Thus, the preferred transition state for
BF4 is an oxidative addition of the only moderately polar C5-H
bond, where the BF4 anion stabilizes the unactivated but more
acidic C2-H bond. In contrast, the ion-pairing interaction with
the nonchelating, small Br essentially involves only one
imidazolium H. Bromide is therefore able to approach close to,
and to strongly stabilize, a transferring proton; this favors the
heterolytic cleavage, which is also preferred for the more acidic
C2-H bond.
In the critical finding of this study, the significantly larger
Hac‚‚‚Br Wiberg bond index (0.120) at the transition state for
the N pathway versus that in the σ-complex (0.049) suggests
that Hac has become more protonic to interact with Br. None of
the other pathways presents the same features (Table S2 in the
Supporting Information). Thus, in the presence of the Br anion,
the activation of C2-H is a proton transfer from the imidazo-
lium cation to the hydride on the metal. This nonredox process
naturally leads to the formation of an IrIII-dihydrogen complex
as the intermediate formed immediately after the C-H bond
activation.
The calculations show that the barriers are similar for the
N,N′-dimethyl-imidazolium and N-methyl-N′-isopropyl-imida-
zolium salts in the case of Br, because the proton prefers to
pass by the methyl group and, therefore, never approaches the
more bulky iPr substituent. However, the preference for activa-
tion of C5-H over C2-H is increased with BF4 in the
N-methyl-N′-isopropyl case: BF4 is prevented from approaching
the more acidic H-C2 by the bulky isopropyl ligand in the N
path, whereas it can approach it while staying far from the
isopropyl group in the AN path. These results are consistent
with the experimental data: the AN/N ratio is larger for the
N-methyl-N′-isopropyl imidazolium/BF4 salt than for the N,N′-
dimethyl case (Table 1). Although this should be considered as
a fortuitous agreement, the discussion below suggests that a
deeper analysis of the calculations does allow us to identify
the essential features of the anion effect.
Conclusion
The C-H activation step has an exothermicity of between
16 and 20 kcal mol-1, consistent with the experimental absence
of a back reaction that would allow isomerization of AN to N
product. This result also agrees with the computational studies
of Cavell et al.24a The formation of a tetrahydride NHC
intermediate for the AN path and a dihydrogen-NHC complex
for the N path in the present study is readily rationalized on
the basis that the stronger donor power of the abnormal NHC
favors IrV over IrIII.42 The displacement of the labile H2 ligand
by pyridine can readily occur for the dihydrogen intermediate
on the N path but requires another step for the tetrahydride in
the AN path. The transition state for the formation of the
dihydrogen-NHC complex from the tetrahydride was located
for the case of BF4. The energy barrier from the tetrahydride
complex is only 2.1 kcal mol-1, and the dihydrogen complex
lies 0.9 kcal mol-1 above the tetrahydride complex. In this
dihydrogen complex, BF4 remains near the unactivated H-C2
bond. The transformation of the tetrahydride into the dihydrogen
complex is thus facile, and the replacement of the dihydrogen
by pyridine leading to the final NHC-pyridine complex is thus
facile for both N and AN pathways.
Changing counteranion in 2-pyridylmethyl imidazolium salts
along the series Br, BF4, PF6, SbF6 causes their kinetic reaction
products with IrH5(PPh3)2 to be switched from chelating NHCs
having normal C2 to ones having abnormal C5 binding. Ion
pairing is detected in the products by NMR studies. Both
computational and NMR studies suggest that the anions are
bound near the NHC group. In the abnormal NHC, the C-H
bond at C2 that points away from the metal becomes the site
of ion-pairing. This C-H bond is highly acidic so the ion-pairing
in a C-H‚‚‚A hydrogen bond (A ) anion) is particularly strong
for the abnormal NHC. The computational study suggests that
the relative energetics of the normal versus the abnormal forms
is strongly affected by choice of anion, hence the big anion
effects observed. Despite the abnormal free carbene being nearly
15 kcal mol-1 less stable than the normal one, the two forms
become close to isoenergetic on metal binding and ion-pair
formation because of the strong ion-pairing to the abnormal
NHC.
The apparent complexity of the system in fact reduces to a
few simple points as a result of the computational work. For a
significant anion effect on the regioselectivity, the two compet-
ing paths have to have similar barriers but go by different
mechanisms. For a significant selectivity change, the two
transition states must be differentially stabilized by the anion,
relative to their respective ground states, so that the relative
barrier heights show an anion dependence. In the present case,
the AN path involving oxidative addition is expected to show
little anion dependence, because there are no great changes in
the charge distributions during the reaction. The heterolytic N
path, with its strongly protonic H migration, shows a particularly
The role of the anion in the various extrema can be
rationalized from the NBO charges and Wiberg bond indices
(see Table S2 in the Supporting Information). The azole-BF4
interaction is spread over several H centers and several F atoms
for the two pathways according to the geometric constraints.
Consequently, no one F has any very strong interaction with
any one H as suggested by the Wiberg indices. The activation
of the imidazolium C-H bond by the metal-hydride fragment
can occur via a redox or a heterolytic process with proton
transfer from the imidazolium to the hydride ligand. Because
9
16308 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Anion-Dependent Switch in Selectivity
A R T I C L E S
of ln(I/I0) versus G2 were fitted using a standard linear regression
algorithm; the R factor was always higher than 0.99. Different values
of ∆, “nt” (number of transients), and number of different gradient
strengths (G) were used for different samples. The methodology for
treating the data was described previously.51
strong H‚‚‚Br interaction, as judged by the Wiberg index in the
transition state, leading to a big acceleration by Br. The small
Br anion can readily accommodate itself to the proton motion
from the imidazolium C-H to the adjacent Ir-H in what can
be termed an anion-coupled proton transfer.
N-Isopropyl-N′-(2-pyridylmethyl)imidazolium Bromide (1h). 2-Bro-
momethylpyridine hydrobromide (Aldrich, 1.01 g, 4.0 mmol) was
neutralized using saturated aqueous sodium carbonate. The liberated
2-bromomethylpyridine was extracted into diethyl ether (3 × 30 mL)
at 0 °C, dried with magnesium sulfate, and added to a solution of
1-isopropylimidazole (0.44 g, 4.0 mmol) in 1,4-dioxane (30 mL). The
ether was removed under reduced pressure, and the solution was
refluxed for 12 h. The volatiles were removed in vacuo, and the oil so
formed 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,
The reason for the change of mechanism is the very strong
donor character of the abnormal NHC, which favors attainment
of IrV in the AN path versus the highly protonic character of
the C2-H of an imidazolium salt that favors the heterolytic
C-H activation of the N path. This relatively rare case of an
anion-dependent selectivity change is therefore explained by a
relatively rare coincidence of several criteria. A search for
further examples would help confirm or deny the general
principles proposed here.
The results emphasize a potentially important caveat for
workers in the field not to assume that the choice of counterion
is immaterial for organometallic reactivity. They could also offer
a potentially important opportunity in that change of anion may
be a more generally useful method of tuning selectivity, than
currently believed, especially in asymmetric catalysis where the
two pathways necessarily have similar barriers.
3
NCHN), 8.56-8.52 (m, 1H, Hpy), 7.93 (d, 1H, JHH ) 7.8 Hz, Hpy),
4
3
3
7.75 (dt, 1H, JHH ) 1.7 Hz, JHH ) 7.5 Hz, Hpy), 7.65 (t, 1H, JHH
)
1.5 Hz, Him), 7.31-7.24 (m, 1H, Hpy), 7.22 (t, 1H, 3JHH ) 1.5 Hz, Him),
5.81 (s, 4H, CH2), 4.74 (septet, 1H, 3JHH ) 6.7 Hz, CH), 1.63 (d, 6H,
3JHH ) 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‚H2O (282.18): C, 48.01;
H, 6.04; N, 14.00. Found: C, 48.33; H, 5.81; N, 14.10.
Experimental Section
N-Butyl-N′-(2-pyridylmethyl)imidazolium Bromide (1j). In a
modification of the procedure of Danopoulos et al.,52 2-(bromomethyl)-
pyridine hydrobromide (4.0 g, 16 mmol) was neutralized using a
saturated aqueous sodium carbonate. The liberated 2-bromomethylpy-
ridine was extracted into diethyl ether (3 × 30 mL) at 0 °C, dried with
magnesium sulfate, and filtered. 1-Butylimidazole (1.95 g, 16.0 mmol)
in methanol (100 mL) was added at 0 °C, the ether was removed under
reduced pressure, and the solution was stirred at room temperature for
12 h. The methanol was then evaporated under reduced pressure, and
the oil so formed was purified by repeated precipitation from CH2Cl2/
Et2O. The resulting oily product was dried in vacuo. Yield: 4.52 g
All operations were carried out under argon atmosphere using
standard Schlenk techniques. Solvents were dried over calcium hydride
(CH2Cl2) or sodium/benzophenone (Et2O). Solvents were degassed prior
to use. IR spectra were recorded on a Midac M1200 FT-IR spectrometer.
Microanalyses were carried out by Robertson Microlit Laboratories and
Atlantic Microlabs, Inc., and the presence of the appropriate solvents
of crystallization was verified by proton NMR. NMR spectra were either
recorded on GE Omega 300, GE Omega 500, Bruker 400, or Bruker
500 MHz spectrometers, and chemical shifts were measured by
reference to the residual solvent resonance or by reference to external
TMS (1H and 13C), CFCl3 (19F), and 85% H3PO4 (31P).
1
IrH5(PPh3)2,45 N-methyl-N′-(2-pyridylmethyl)imidazolium bromide
(95%). H 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), 7.28-7.24 (m, 1H,
(2),46 and [H2Ir(OCMe2)(PPh3)2] BF4 were prepared according to
47
literature methods; all other reagents that are commercially available
were used as received. Assignments are based on COSY and HMQC
spectroscopy.
3
Hpy), 5.77 (s, 2H, CH2), 4.27 (t, 2H, JHH ) 7.5 Hz, CH2), 1.91-1.82
3
(m, 2H, CH2), 1.39-1.29 (m, 2H, CH2), 0.91 (t, 2H, 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‚1.5H2O: C, 48.31; H, 6.55;
N, 13.00. Found: C, 47.85; H, 6.54; N, 12.96.
1
NOE NMR Measurements. The H-NOESY NMR experiments48
were acquired by the standard three-pulse sequence or by the PFG
version.49 Two-dimensional 19F,1H-HOESY NMR experiments were
acquired using the standard four-pulse sequence or the modified
version.50 The number of transients and the number of data points were
chosen according to the sample concentration and to the desired final
digital resolution. Semiquantitative spectra were acquired using a 1 s
relaxation delay and 800 ms mixing times.
PGSE NMR Measurements. 1H and 19F PGSE NMR measurements
were performed by using the standard stimulated echo pulse sequence40a
on a Bruker AVANCE DRX 400 spectrometer equipped with a GREAT
1/10 gradient unit and a QNP probe with a Z-gradient coil, at 296 K
without spinning. The shape of the gradients was rectangular, their
duration (δ) was 4-5 ms, and their strength (G) was varied during the
experiments. All of the spectra were acquired using 32K points, a
spectral width of 5000 (1H) and 18 000 (19F) Hz, and processed with a
line broadening of 1.0 (1H) and 1.5 (19F) Hz. The semilogarithmic plots
(η2-C,N)(N-Methyl-N′-(2-pyridylmethyl)imidazole-2-ylidene)bis-
(hydrido)bis(triphenylphosphine)iridium(III) Bromide (2a). N-Methyl-
N′-(2-pyridylmethyl)imidazolium bromide (36 mg, 0.14 mmol) and
IrH5(PPh3)2 (100 mg, 0.140 mmol) in THF (18 mL) were vigorously
refluxed in air for 2 h. After the mixture was cooled to room
temperature, about 10 mL of THF was removed in vacuo, and 60 mL
of pentane was added. A white precipitate was filtered off and dried in
vacuo. Yield: 95 mg (70%). Pure 2a can be obtained by recrystallization
from CHCl3/pentane.
1H NMR (CDCl3, 298 K): δ 8.04 (d, 1H, 3JHH ) 7.5 Hz, Hpy), 7.98
(d, 1H, 3JHH ) 5.5 Hz, Hpy), 7.96 (d, 1H, 3JHH ) 1.5 Hz, Him), 7.51 (t,
3
3
1H, JHH ) 7.0 Hz, Hpy), 7.34-7.18 (m, 30H, Hph), 6.31 (d, 1H, JHH
3
) 1.6 Hz, Him), 6.14 (t, 1H, JHH ) 6.7 Hz, Hpy), 5.33 (s, 2H, CH2),
2.51 (s, 3H, CH3), -11.17 (dt, 3JHH ) 5.1 Hz, 2JPH ) 19.8 Hz, Ir-H),
(45) Crabtree, R. H.; Felkin, H.; Morris, G. E. J. Organomet. Chem. 1977, 141,
205.
(46) McGuiness, D. S.; Cavell, K. J. Organometallics 2000, 19, 741.
(47) Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J. M.; Parnell, C. A.;
Quirk, J. M.; Morris, G. E. J. Am. Chem. Soc. 1982, 104, 6994.
(48) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979,
71, 4546.
-21.00 (dt, JHH ) 5.0 Hz, JPH ) 17.2 Hz, Ir-H). 13C{1H} NMR
(CDCl3, 298 K): δ 169.78 (t, JPC ) 6.7 Hz, Ccarbene), 160.61 (Cpy),
153.06 (Copy), 137.70 (Copy), 134.11 (t, JPC ) 26.9 Hz, Cph), 133.24 (t,
3
2
(49) Wagner, R.; Berger, S. J. Magn. Reson., Ser. A 1996, 123, 119.
(50) Lix, B.; So¨nnichsen, F. D.; Sykes, B. D. J. Magn. Reson., Ser. A 1996,
121, 83.
(51) Zuccaccia, D.; Macchioni, A. Organometallics 2005, 24, 3476.
(52) Tulloch, A. A. D.; Danopoulos, A. A.; Winston, S.; Kleinhenz, S.; Eastham,
G. J. Chem. Soc., Dalton Trans. 2000, 4499.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16309

A R T I C L E S
Appelhans et al.
JPC ) 6.0 Hz, Cph), 129.99 (Cph), 128.11 (t, JPC ) 4.8 Hz, Cph), 126.66
(Cpy), 123.54 (Cpy), 123.41 (Cim), 120.32 (Cim), 54.75 (CH2), 37.08
(CH3). 31P{1H} NMR (CDCl3, 298 K): δ 19.05. Anal. Calcd for C46H43-
BrIrN3P2.2CHCl3: C, 47.62; H, 3.75; N, 3.47. Found: C, 47.95; H,
3.73; N, 3.47.
120.50 (Cim), 56.13 (NCH2Py), 50.88 (NCH2CH2CH2CH3), 31.54
(NCH2CH2CH2CH3), 20.34 (NCH2CH2CH2CH3), 13.84 (NCH2CH2-
CH2CH3). 31P{1H} NMR (CD2Cl2, 298 K): δ 17.43. Anal. Calcd for
C49H49BF4IrN3P2: C, 57.26; N, 4.17; H, 4.71. Found: C, 57.33; N,
4.20; H, 4.78.
(η2-C,N)(N-Methyl-N′-(2-pyridylmethyl)imidazole-5-ylidene)bis-
(hydrido)bis(triphenylphosphine)iridium(III) Hexafluoroantimo-
niate (3g). N-Methyl-N′-(2-pyridylmethyl)imidazolium hexafluoroan-
timoniate (33 mg, 0.080 mmol) and IrH5(PPh3)2 (55 mg, 0.080 mmol)
in THF (5 mL) were vigorously refluxed in air for 2 h. After the mixture
was cooled to room temperature, 50 mL of pentane was added. A
yellowish precipitate was filtered off and dried in vacuo. Yield: 71 mg
(80%). Pure 3g can be obtained by recrystallization from CHCl3/
pentane.
Ion Exchange of Bromide for Tetrafluoroborate. The imidazolium
bromide salt is dissolved in a small quantity of dichloromethane and
loaded onto a silica gel column (prepared with ethyl acetate). Sufficient
ethyl acetate is run through the column to remove any mobile materials
(200-300 mL). The salt is eluted with a solution of 2 equiv of NaBF4
in 500 mL of acetone. In some cases, further elution with pure acetone
was required to complete the elution. Using additional NaBF4 results
in contamination. After collection of the eluate fractions, and removal
of the acetone by rotary evaporator, the residue is dissolved in a large
quantity of dichloromethane, filtered through Celite to remove any
excess NaBF4, and the solvent is then evaporated to yield a brown-
1H NMR (CDCl3, 298 K): δ 8.38 (s, 1H, NCHN), 8.22 (d, 1H, 3JHH
3
) 5.5 Hz, py-H), 7.38-7.18 (m, 32H, py-H, ph-H), 6.10 (t, 1H, JHH
n
i
-
yellow oil (R ) Bu) or yellow crystals (R ) Pr) of the BF4 salt.
Yields vary between 65% and 75%.
) 6.6 Hz, py-H), 4.89 (s, 1H, im-H), 4.58 (s, 2H, CH2), 3.38 (s, 3H,
3
2
CH3), -10.86 (dt, JHH ) 5.0 Hz, JPH ) 19.8 Hz, Ir-H), -21.53 (dt,
3JHH ) 5.0 Hz, 2JPH ) 18.6 Hz, Ir-H). 13C{1H} NMR (CDCl3, 298 K):
δ 161.97 (Cpy), 153.22 (Cpy), 142.43 (t, JPC ) 6.5 Hz, Ccarbene), 137.23
(Cpy), 134.87 (t, JPC ) 26.5, Cph), 133.61 (t, JPC ) 6.3, Cph), 133.39
(NCN), 129.65 (Cph), 128.00 (Cpy), 127.87 (t, JPC ) 4.8, Cph), 125.50
(Cim), 124.21 (Cpy), 55.36 (CH2), 34.57 (CH3). 31P{1H} NMR (CDCl3,
298 K): δ 22.67. Anal. Calcd for C46H43F6IrN3P2Sb‚2CHCl3: C, 45.26;
H, 3.56; N, 3.37. Found: C, 45.55; H, 3.43; N, 3.55.
(η2-C,N)(N-Isopropyl-N′-(2-pyridylmethyl)imidazole-2-ylidene)-
dihydridobis(triphenylphosphine)iridium(III) Tetrafluoroborate (2i).
[H2Ir(OCMe2)2(PPh3)2]BF4 (178 mg, 0.194 mmol) and N-isopropyl-
N′-(2-pyridylmethyl)imidazolium tetrafluoroborate (1i) (56 mg, 0.19
mmol) were combined in dichloromethane (12 mL) and stirred at room
temperature for 1 h. Excess sodium bicarbonate was then added, and
the mixture was stirred for 30 min, during which time the color changed
from red-orange to gray-green. The solution was then filtered through
Celite, and the solvent was removed by rotary evaporation. The crude
product was dissolved in ca. 2 mL of dichloromethane and layered
with pentane. After standing overnight, the mixture separated into a
dark oil and a clear supernatant that was separated, the solvent was
removed in vacuo, and the resulting white powder was recrystallized
from dichloromethane/pentane to yield translucent crystals of the
(η2-C,N)(N-Isopropyl-N′-(2-pyridylmethyl)imidazole-5-ylidene)-
bis(hydrido)-bis(triphenylphosphine)iridium(III) Tetrafluoroborate
(3i). A mixture of N-isopropyl-N′-(2-pyridylmethyl)imidazolium tet-
rafluoroborate (40 mg, 0.140 mmol) and IrH5(PPh3)2 (100 mg, 0.140
mmol) in THF (5 mL) was refluxed in air for 2 h. After 15 min, a
clear solution was obtained. The yellow product precipitated by addition
of 25 mL of pentane to the cooled mixture was filtered off and dried
in vacuo. Yield: 120 mg (85%). The complex can be recrystallized
from THF/pentane. Crystals of a quality suitable for X-ray analysis
product. Yield: 54 mg (28%).
3
1H NMR (CD2Cl2 δ 5.32, 298 K): δ 7.85 (d (broad), 1H, JHH
)
3
4
5.7 Hz, Hpy), 7.55 (td, 1H, JHH ) 7.7 Hz, JHH ) 1.4 Hz, Hpy), 7.33
(m, 8H, HPh), 7.24 (m, 12H, HPh, Him), 7.19 (m, 12H, HPh, Hpy), 6.72
(d, 1H, 3JHH ) 2.0 Hz, Him), 6.28 (ddd, 1H, 3JHH ) 7.4 Hz 4JHH ) 5.7
Hz, 5JHH ) 1.4 Hz, Hpy), 4.72 (s, 2H, CH2), 4.12 (sept, 1H, 3JHH ) 6.8
1
were obtained from CH2Cl2/Et2O. H NMR (CDCl3, 298 K): δ 8.72
(s, 1H, NCHN), 8.23 (d, 1H, 3JHH ) 5.5 Hz, Hpy), 7.37-7.14 (m, 32H,
3
Hpy, Hph), 6.07 (t, JHH ) 6.3 Hz, Hpy), 5.17 (s, 1H, Him), 4.70 (s, 2H,
3
3
CH2), 4.25 (septet, 1H, JHH ) 6.5 Hz, CH), 1.19 (d, 6H, JHH ) 6.5
3
Hz, NCH(CH3)2), 0.465 (d, 6H, JHH ) 6.8 Hz, NCH(CH3)2), -11.27
2
3
Hz, CH3), -10.83 (dt, JPH ) 19.6 Hz, JHH ) 4.9 Hz, Ir-H), -21.49
(dt, 2JPH ) 18.6 Hz, 3JHH ) 4.9 Hz, Ir-H). 13C{1H} NMR (CDCl3, 298
K): δ 161.78 (Cpy), 153.10 (Cpy), 141.07 (t, JPC ) 7.1 Hz, Ccarbene),
137.04 (Cpy), 134.93 (t, JPC ) 26.3 Hz, Cph), 133.55 (t, JPC ) 6.0 Hz,
Cph), 132.40 (NCN), 129.55 (Cph), 127.84 (t, JPC ) 5.8 Hz, 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‚THF: C, 57.88; H, 5.14; N, 3.89.
Found: C, 57.84; H, 5.32; N, 3.74.
2
2
2
(td, 1H, JPH ) 21.2 Hz, JHH ) 5.1 Hz, Ir-H), -20.92 (td, 1H, JPH
) 16.9 Hz, 2JHH ) 5.1 Hz, Ir-H). 13C{1H} NMR (CD2Cl2 δ 54.0, 298
K): δ 169.79 (t, JPH ) 7.1 Hz, Ccarbene), 161.22 (Cpy), 154.03 (Cpy),
2
138.63 (Cpy), 134.81 (t, JPC ) 26.4 Hz, CPh), 134.04 (t, JPC ) 5.8 Hz,
CPh), 130.80 (CPh), 128.84 (t, JPC ) 4.9 Hz, CPh), 125.75 (Cpy), 124.69
(Cpy), 122.88 (Cim), 117.46 (Cim), 56.40 (CH2), 52.51 (CH(Me)2), 21.90
(CH(CH3)2). 31P{1H} NMR (CD2Cl2, 298 K): δ 17.21. Anal. Calcd
for C48H47BF4IrN3P2‚THF: C, 57.88; N, 3.89; H, 5.14. Found: C, 57.68;
N, 3.82; H, 5.13.
(η2-C,N)(N-Butyl-N′-(2-pyridyl-methyl)imidazole-5-ylidene)bis-
(hydrido)-bis(triphenylphosphine)iridium(III) Tetrafluoroborate
(3j). A mixture of N-butyl-N′-(2-pyridylmethyl)imidazolium tetrafluo-
roborate (54 mg, 0.18 mmol) and IrH5(PPh3)2 (129 mg, 0.18 mmol) in
THF (8 mL) was refluxed in air for 2 h. After 20 min, a clear solution
was obtained. The cooled reaction mixture was layered with heptane
(10 mL). Over 12 h, crystals formed that were filtered off and dried in
vacuo. Yield: 124 mg (68%). The complex can be recrystallized from
THF/pentane. 1H NMR (CDCl3, 298 K): δ 8.71 (s, 1H, NCHN), 8.19
(η2-C,N)(N-(n-Butyl)-N′-(2-pyridylmethyl)imidazole-2-ylidene)-
dihydridobis(triphenylphosphine)iridium(III) Tetrafluoroborate (2j).
[H2Ir(OCMe2)2(PPh3)2]BF4 (155 mg, 0.169 mmol) and N-(n-butyl)-N′-
(2-pyridylmethyl)imidazolium tetrafluoroborate (1j) (51 mg, 0.17 mmol)
were combined in dichloromethane and treated exactly as was the Pr
derivative above. Yield: 98 mg (55%).
i
1H NMR (CD2Cl2 δ 5.32, 298 K): δ 8.00 (d, 1H, JHH ) 5.6 Hz,
Hpy), 7.60 (td, 1H, JHH ) 7.7 Hz, JHH ) 1.2 Hz, Hpy), 7.42 (t, 1H, JHH
) 7.1 Hz, Hpy), 7.38-7.20 (m, 31H, HPh, Him), 6.51 (d, 1H, 3JHH ) 1.8
Hz, Him), 6.30 (ddd, 1H, JHH ) 7.1 Hz, JHH ) 5.6 Hz, JHH ) 1.2 Hz,
3
(d, 1H, JHH ) 5.1 Hz, Hpy), 7.37-7.15 (m, 32H, Hpy, Hph), 6.07 (t,
3JHH ) 5.9 Hz, Hpy), 5.03 (s, 1H, Him), 4.72 (s, 2H, CH2), 3.63 (t, 1H,
3JHH ) 6.9 Hz, CH2), 1.47 (m, 2H, CH2), 1.17 (m, 2H, CH2), 0.90 (t,
3
4
Hpy), 4.69 (s, 2H NCH2Py), 2.96 (dd, 2H, JHH ) 8.2 Hz, JHH ) 7.2
Hz, NCH2CH2CH2CH3), 1.02-0.86 (m, 4H, NCH2CH2CH2CH3), 0.648
2
3
3JHH ) 7.7 Hz, CH3), -10.89 (dt, JPH ) 19.6 Hz, JHH ) 5.1 Hz,
Ir-H), -19.61 (dt, 2JPH ) 18.4 Hz, 3JHH ) 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 Hz, Cph), 133.79 (NCN),
133.61 (t, JPC ) 6.0 Hz, Cph), 129.59 (Cph), 127.86 (t, JPC ) 4.7 Hz,
Cph), 126.13 (Cim), 125.90 (Cpy), 124.18 (Cpy), 55.08 (CH2), 47.89 (CH2),
3
2
(t, 3H, JHH ) 7.2 Hz, NCH2CH2CH2CH3), -11.11 (td, 1H, JPH
)
20.4 Hz, 2JHH ) 5.2 Hz, Ir-H), -20.93 (td, 1H, 2JPH ) 20.4 Hz, 2JHH
) 5.2 Hz, Ir-H). 13C{1H} NMR (CD2Cl2 δ 54.0, 298 K): δ 170.30 (t,
2JPH ) 6.9 Hz, Ccarbene), 161.52 (Cpy), 153.90 (Cpy), 138.62 (Cpy), 134.64
(t, JPC ) 26.8 Hz, CPh), 133.89 (t, JPC ) 5.9 Hz, CPh), 130.66 (CPh),
128.72 (t, JPC ) 4.6 Hz, CPh), 125.84 (Cpy), 124.63 (Cpy), 122.55 (Cim),
9
16310 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

Anion-Dependent Switch in Selectivity
A R T I C L E S
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 C49H49BF4-
IrN3P2.THF: C, 58.24; H, 5.26; N, 3.84. Found: C, 58.23; H, 5.29 N,
3.90.
Anion Exchange: Method 2. This was typically carried out from
the bromide salt with either the silver salt of the desired anion in CH2-
Cl2 or the sodium salt in acetone (1:1 molar ratio, 2 h, room temperature,
stirring). AgBr or NaBr was removed by filtration, and the product
was crystallized by addition of Et2O. The outcome can be monitored
by proton NMR using the CH proton at C2 as an indicator (1 or 3) or
by ESI-MS. Occasionally, more than one exchange cycle was required
for complete exchange.
Computational Details. All of the calculations have been performed
with the Gaussian 98 package of programs.53 The calculations on the
structure and energetic of the ion-pairs were performed with the ONIOM
method.54 The experimental systems 2i and 3i were explicitly considered
where the phenyl groups on PPh3 and the iPr group on N were treated
at the UFF level,55 and the remainder of the ion-pair, including the
counteranion BF4, Br, or OAc, was calculated at the B3PW91 level.56,57
The energies of the ion-pair were calculated within the CPCM
scheme58,59 (CH2Cl2 as a solvent) on the ONIOM geometries, and the
formation energy of the ion-pair was evaluated as Eb ) E(ion-pair) -
{E(cation) + E(anion)} where the energies are calculated at the CPCM
level on frozen geometries for the fragments as obtained in the ONIOM
calculations.
In all of the calculations throughout the paper, Ir, Br, and P were
represented with the small core RECP from the Stuttgart’s group and
the associated basis set,60,61 augmented by a polarization function.62,63
The remaining atoms (C, H, N, B, F, O) were represented by a 6-31G-
(d,p) basis set.64 The nature of all of the extrema located has been
determined after analytic calculations of the vibrational frequen-
cies.
Acknowledgment. We thank the Universite´ Montpellier 2
(O.E., E.C.), the CNRS (O.E., E.C.), the US DOE (A.K., J.R.M.,
A.R.C., R.H.C.), Johnson Mathey (L.N.A.), Ministero dell’Uni-
versita´ e della Ricerca Scientifica e Tecnologica (MURST,
Rome, Italy; A.M.), programma di Rilevante Interesse Nazio-
nale, Cofinanziamento 2004-2005 (A.M.), and the COST D24/
WG 0014/02 action (A.M., E.C.) for funding.
Supporting Information Available: A complete list of authors
for ref 53; 19F,1H-HOESY spectrum of 5 in CD2Cl2 (Figure S1);
1H-NOESY spectra of 5 in CD2Cl2 (Figure S2) and CD3NO2
(Figure S3); 19F,1H-HOESY spectra of 3j and 2j in CD2Cl2
(Figure S4); PGSE NMR data (Table S1); NPA charges and
Wiberg bond indices (Table S2); and a full list of atomic
Cartesian coordinates, energies (E, au), and free energies (G,
au) for all computed systems. This material is available free of
charge via the Internet http://pubs.acs.org.
For the mechanistic study, the system considered was the N,N′-
dialkyl-imidazolium salts (R ) Me, R′ ) Me; R ) Me, R′ ) iPr; A )
BF4, Br) reacting with IrH3(PMe3)2. The calculations have been
performed at the B3PW91 level.
JA055317J
(60) Andrae, D.; Ha¨ussermann, U.; Dolg, M.; Stoll, H.; Preuâ, H. Theor. Chim.
Acta 1990, 77, 123.
(53) Pople, J. A.; et al. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
(54) Svensson, M.; Humbel, S.; Froese, R. D. J.; Matsubara, T.; Sieber, S.;
Morokuma, K. J. Phys. Chem. 1996, 100, 19357.
(61) Bergner, A.; Dolg, M.; Ku¨chle, W.; Stoll, H.; Preuâ, H. Mol. Phys. 1993,
30, 1431.
(55) Rappe´, A. K.; Casewitt, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W.
M. J. Am. Chem. Soc. 1992, 114, 10024.
(62) Ehlers, A. W.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.; Jonas,
V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys.
Lett. 1993, 208, 111.
(56) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(57) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244.
(58) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995.
(59) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003,
24, 669.
(63) Ho¨llwarth, A.; Bo¨hme, H.; Dapprich, S.; Ehlers, A. W.; Gobbi, A.; Jonas,
V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys.
Lett. 1993, 208, 237.
(64) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16311