Reversible α-elimination in the conversion of N-CH3 to N-CH=Ir by double C-H activation

Article abstract of DOI:10.1039/b007679l

2-Dimethylaminopyridine (pyNMe₂; py = 2-pyridyl) reacts with [H₂Ir(OCMe₂)₂L₂]+ (L = PPh₃) to give acyclic carbene complex [H₂Ir(=CHN(Me)py)L₂]+ via an oxidative additi

Full text of DOI:10.1039/b007679l

Reversible a-elimination in the conversion of N–CH3 to N–CHNIr by double
C–H activation
Dong-Heon Lee,*a Junyi Chen,b Jack W. Faller*b and Robert H. Crabtree*b
a Department of Chemistry, Chonbuk National University, Chonju, 561-756, Korea. E-mail: dhl@moak.chonbuk.ac.kr
b Yale Chemistry Laboratory, 225 Prospect St., New Haven, CT 06520-8107, USA. E-mail: jack.faller@yale.edu;
robert.crabtree@yale.edu
Received (in Irvine, CA, USA) 18th September 2000, Accepted 20th November 2000
First published as an Advance Article on the web
2-Dimethylaminopyridine (pyNMe2; py = 2-pyridyl) reacts
with [H2Ir(OCMe2)2L2]+ (L = PPh3) to give a cyclic carbene
complex [H2Ir(NCHN(Me)py)L2]+ via an oxidative addition,
reversible a-elimination sequence.
character as expected from the resonance form 3b generally
preferred by Fischer carbenes. Even with b-hydrogen atoms
available, we see only the a-elimination product 3. Preferred a-
vs. b-elimination in metal alkyls has been reported.6,7
The evidence discussed below suggests formation of 2
proceeds by initial C–H bond activation, perhaps to form an
intermediate H2 complex 5. Loss of H2 leads to an alkyl, 6,
which then rearranges to the observed carbene 2, by a-
elimination (Scheme 1).
The conceptual conversion of eqn. (1) is highly endothermic
(DH = 111 kcal mol21, triplet; 119 kcal mol21, singlet). If the
carbene is sufficiently stabilized by a metal fragment and by p-
donor heteroatom substitution in the a-position (Fischer
carbene), however, such a conversion becomes thermodynam-
ically allowed. For example, both alkene to carbene1–3 and THF
to carbene4,5 rearrangements of this sort are known, but elevated
temperatures may be required for satisfactory rates.4–5
Formation of an agostic C–H…M bond is typical in
complexes related to 1. For example, the known 2,6-di-
(1)
CH4 ? :CH2 + H2
We now report a room-temperature route for the title reaction
[eqn. (2)] via the activation of two geminal C–H bonds and
liberation of H2.
(2)
We find that 2-dimethylaminopyridine (pyNMe2, 1 equiv.)
reacts with [IrH2(OCMe2)2L2]BF4 1 in CH2Cl2 at 25 °C to give
the cyclic heteroatom-stabilized carbene complex 2 (25 °C, 15
min, 78%), by a net reaction that resembles eqn. (1).† The
identity of the product follows from the 1H NMR spectrum, in
particular the appearance of a low-field singlet of intensity 1H
at d 11.71 assigned to the CHNIr proton. The NMe group of
intensity 3H appears as a singlet at d 3.50. The two inequivalent
hydrides (1H each) resonate at d 29.98 and d 217.78 as triplets
Fig.
1 An ORTEP view of the cation of [Ir(PPh3)2(pyNEtC-
MeH2]BF4·CH2Cl2 3. Selected bond lengths (Å): Ir(1)–C(6) 2.018(5),
Ir(1)–N(1) 2.142(4), N(2)–C(1) 1.410(6), N(2)–C(6), 1.332(7). Only the
ipso carbons of PPh3 are shown for clarity. The ligand H atoms are in
calculated positions. The hydrides were refined with Ir(1)–H(1) 1.43(5) Å
and Ir(1)–H(2) 1.47(5) Å, no doubt shortened by systematic error.
2
of doublets (2JPH 16, JHHA 4 Hz), indicating each is cis to the
two phosphines. The 13C NMR shows a low-field resonance at
d 155.9 characteristic of Fischer carbenes.
The reaction is also possible for 2-diethylaminopyridine
(pyNEt2, 1 equiv.) to give the analogous product 3 (25 °C, 15
min, 80%), shown in eqn. (3). The spectral data for 3 is similar
(3)
1
to that for 2, in particular, the H NMR spectrum shows high-
field resonances at d 210.80 and d 217.92, characteristic of
terminal iridium(III) hydrides. The two hydrides (2JPH 17 Hz,
2JHHA 4 Hz) are mutually coupled and coupled to the PPh3
ligands. The formation of 3 was also confirmed by a crystal
structure determination (Fig. 1).‡ The Ir–C distance of 2.018 Å
is consistent with predominant single bond character while the
C–N bond distance of 1.322 Å indicates some multiple bond
Scheme 1
DOI: 10.1039/b007679l
Chem. Commun., 2001, 213–214
This journal is © The Royal Society of Chemistry 2001
213

phenylpyridine species 7,8 is fluxional [eqn. (4)] via a pathway
analogous to that of Scheme 1. To probe the formation of a
of the hydride peaks). Incremental addition of acetone to the
solution then led to displacement of the equilibrium in favor of
6. For example, when 2 and 3 equivalents of acetone were
added, the mole ratio of 6+2 became 2+1 and 3.3+1, re-
spectively.
Several factors may make the rearrangement of the agostic
species (8) to the carbene (2) thermodynamically favorable, in
contrast with the highly endothermic base case of eqn. (1). The
metal complexation stabilizes the carbene but this is not
sufficient on its own, as shown by the failure of the agostic
8-methylquinoline species 9 to convert to the corresponding
carbene, a species that is as yet unknown. Heteroatom
stabilization by the adjacent amino group is clearly an important
stabilizing factor. In addition, carbene 2 can be considered as a
metallacycle with 10 p-electrons, which could in principle
benefit from aromatic stabilization.
(4)
possible agostic C–H…M intermediate 4 in the present case, we
treated 1 in CD2Cl2 with pyNMe2 at low temperature and
monitored the reaction by 1H NMR. At 280 °C, a new species,
8, is seen that is too reactive to isolate [eqn. (5)]. After 40 min
In summary, we have a rare double C–H activation route that
provides a mild and fast synthetic method to generate chelating
Fischer carbene complexes.
(5)
We thank the NSF (R. H. C, J. W. F), the DOE (J. C.), and the
Korean Research Foundation (D.-H. L., grant KRF-
2000-015-DP0305) for funding.
at 0 °C, it converts to 2. Comparison of 8 with the fully
characterized, stable material 9, made via the route of eqn. (5),9
shows very close H NMR spectral similarities, suggesting 8
1
Notes and references
has the agostic structure, 4, shown in Scheme 1. For example,
the inequivalent hydrides resonate as a pair of signals (8, d
220.69, d 229.84; 9, d 219.20, d 228.60) coupled both to two
cis phosphines (8, 2JPH 17 Hz; 9, 2JPH 15 Hz) and to each other
(8, 2JHHA 7 Hz; 9, 2JHHA 8 Hz).
Net loss of H2 occurs and free H2 was detected (d 4.2) in the
1H NMR spectrum of the reaction mixture. After loss of H2 to
generate a vacant site cis to the newly formed iridium alkyl, an
a-elimination gives the final product.
† Synthesis: 2: the BF4 salt of 1 (280 mg, 0.3 mmol) was dissolved in
degassed CH2Cl2 (4 mL) and 2-dimethylaminopyridine (37 mg, 0.3 mmol)
was added. The resulting clear yellow solution was stirred for 15 min. Slow
addition of diethyl ether (ca. 10 mL) gave a light yellow precipitate. The
solution was then filtered and the light yellow powder was washed with
diethyl ether (15 mL) and dried in vacuo to give pure 2. Yield: 217 mg
(78%). Complex 3 can be prepared similarly by treating 1 with 1 equiv. of
2-diethylaminopyridine. Satisfactory analytical and spectroscopic data were
obtained for 2 and 3.
‡ Crystal data: 3: IrCl2P2F4N2C44BH44, pale yellow crystals, M
=
Reversible a-elimination is rare,6 but remarkably, this
process is facile in this system. Dissolving the carbene 2 in
acetone rapidly gives 6 by reversal of the a-elimination step.
The Ir–H of the product 6 resonates as a triplet at d 216.14 (2JPH
14 Hz). This colorless alkyl complex was crystallized from
acetone–diethyl ether and characterized by a crystal structure
(Fig. 2). The equilibration of 2 and 6 was further probed by 1H
NMR. The colorless alkyl complex 6 dissolved in fresh CD2Cl2
at 25 °C with loss of acetone to give the yellow carbene 2 within
seconds. In a typical case, 6+2 occur as a 1+1 ratio (integration
¯
1036.74, triclinic; space group P1 (no. 2), a = 11.8113(4), b = 12.6330(5),
c = 16.5025(7) Å, a = 100.210(2), b = 107.894(2), g = 101.536(2), V =
2219.4(2) Å3, Z = 2; Dc = 1.551 g cm23; T = 183 K, l(Mo-Ka) = 0.71069
Å, Nonius KappaCCD; no. reflections [I > 3.0s(I)] = 6997; R = 0.043, Rw
= 0.042, GOF = 1.34.
6: IrP2F4O2N2C49BH52; colorless crystals, M = 1041.93; monoclinic,
space group P21/n (no. 14), a
= 14.4246(6), b = 14.7760(6), c =
22.8912(7) Å, b = 107.342(2)°, V = 4657.2(3) Å3, Z = 4; Dc = 1.49 g
cm23; T = 183 K, l(Mo-Ka) = 0.71069 Å, Nonius KappaCCD; no.
reflections [I > 3.0s(I)] = 5722; R = 0.032, Rw = 0.034; GOF = 0.81.
CCDC 182/1870. See http://www.rsc.org/suppdata/cc/b0/b007679l/ for
crystallographic files in .cif format.
1 J. N. Coalter, G. J. Spivak, H. Gerard, E. Clot, E. R. Davidson, O.
Eisenstein and K. G. Caulton, J. Am. Chem. Soc., 1998, 120, 9388.
2 M. W. Holtcamp, J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc.,
1997, 119, 848.
3 J. N. Coalter, G. Ferrado and K. G. Caulton, New J. Chem., 2000, 24,
835.
4 H. F. Luecke, B. A. Arndtsen, P. Burger and R. G. Bergman, J. Am.
Chem. Soc., 1996, 118, 2517.
5 C. Slugovc, K. Mereiter, S. Trofimenko and E. Carmona, Angew. Chem.,
Int. Ed., 2000, 39, 2158.
6 R. R. Schrock, S. W. Seidel, N. C. Mosch-Zanetti, K. Y. Shih, M. B.
O’Donoghue and W. M. Davis, J. Am. Chem. Soc., 1997, 119, 11 876 and
references therein.
7 G. Parkin, E. Bunel, B. J. Burger, M. S. Trimmer, A. Vanasselt and J. E.
Bercaw, J. Mol. Catal., 1987, 41, 21.
Fig. 2 An ORTEP view of the cation of [Ir(PPh3)2(pyNMeCH2)(Me2-
CO)(H)]BF4·Me2CO, 6. Selected bond lengths (Å) are: Ir(1)–C(6) 2.072(5),
Ir(1)–N(1), 2.126(4), N(2)–C(1) 1.338(7), N(2)–C(6) 1.480(7). The ligand
H atoms and the hydride are shown in calculated positions.
8 A. C. Albéniz, G. K. Schulte and R. H. Crabtree, Organometallics, 1992,
11, 242.
9 R. H. Crabtree, E. M. Holt, M. Lavin and S. M. Morehouse, Inorg. Chem.,
1985, 24, 1986.
214
Chem. Commun., 2001, 213–214