Imidazol-2-yl complexes of Cp*Ir as bifunctional ambident reactants

Article abstract of DOI:10.1021/ja804713u

Loss of chloride ion from imidazol-2-yl complex 4a activates the H-H bond of dihydrogen or the C-H bond of acetylene, forming an Ir(III) N-heterocyclic carbene (NHC) complex (3b or 9). Deprotonation of Ir(III) hydride 4b gives one new species, formulated

Full text of DOI:10.1021/ja804713u

Published on Web 09/10/2008
Imidazol-2-yl Complexes of Cp*Ir as Bifunctional Ambident Reactants
Valent´ın Miranda-Soto,† Douglas B. Grotjahn,*,† Antonio G. DiPasquale,‡ and Arnold L. Rheingold‡
Department of Chemistry and Biochemistry, 5550 Campanile DriVe, San Diego State UniVersity, San Diego,
California 92182-1030, and Department of Chemistry and Biochemistry, 9500 Gilman DriVe, UniVersity of
California at San Diego, La Jolla, California 92093-0385
Received June 19, 2008; E-mail: grotjahn@chemistry.sdsu.edu
Scheme 1. Synthesis of Imidazol-2-ylidene and Imidazol-2-yl
Exploring the chemistry and uses of N-heterocyclic carbenes
(NHCs) has become one of the most active and exciting areas of
organometallic chemistry, because of the useful reactivity which
NHC ligands impart on catalysts.1 The vast majority of reported
NHC ligands are substituted at nitrogen(s) by alkyl, aryl, or other
groups (e.g., structure A with X ) R ) alkyl or aryl). There are a
few but growing number of imidazole-derived NHC complexes
bearing an NH-wingtip (A, X ) H),2 and a few cases of related
NH-bearing carbenes derived from pyridines.3
Complexesa
a Conditions: (a) CD2Cl2, 70 °C, 1 h; (b) EtOH, room temp, 1 d; (c)
THF, 70 °C, 1 d.
However, relatively little is known about the reactivity of NHC
complexes containing an unusual free NH. Our interest in bifunc-
tional catalysis involving proton transfer or hydrogen bonding4 led
us to consider not only structure A (X ) H), but its tautomer B
with higher formal oxidation state at the metal,5 and C, with a
vacant coordination site at the metal and a basic nitrogen6 in close
proximity, which could facilitate breaking a bond, forming D. Here
we report the synthesis of a new series of imidazol-2-yl complexes
of Cp*Ir and their bifunctional behavior as ambident reactants
consistent with A-D.
Reaction of 17 (Scheme 1) with [Cp*IrCl2]2 rapidly afforded 2
at room temperature; heating led to complete tautomerization of
the imidazole2a,f,g and yellow carbene complex 3a. The presence
1
of a low field H NMR signal (δ 10.13, NH), together with the
appearance of a doublet in the 13C {1H} NMR spectrum (δ 140.3,
2JCP ) 21.3 Hz, C-2) are consistent with structure 3a. The NH
group could be deprotonated to give 4a. X-ray structures of 3a
(Figure 1) and 4a (Figure S1)7 confirm the presence of an N-H
group in the acid 3a and its absence in conjugate base 4a.
Interestingly, 3a crystallizes with two H2O molecules, where one
oxygen accepts a hydrogen bond from the N-H of the carbene
[H2O · · ·H ) 1.949(12) Å].7,8 The structure of 4a showed a
Figure 1. Molecular structure of N-heterocyclic carbene complex 3a. Of
the two water molecules, only the one involved in hydrogen bonding is
shown.
spectrum of 3b showed the presence of an N-H (δ 10.53, br) and
a hydride (-16.0 ppm, d, 2JHP ) 28.4 Hz); the 13C spectrum showed
2
a doublet for the carbene carbon (139.2 ppm, JCP ) 14.6 Hz).
Deprotonation of 3b using NaOMe gave 4b, with a hydride
2
resonance at -17.01 ppm (d, JHP ) 32.4 Hz) and absence of a
Cimidazolyl–Ir distance of 2.059(3) Å, whereas the Ccarbene–Ir distance
downfield NH signal. Solutions of 4b in CDCl3 or CD2Cl2 at room
temperature led to 4a and CDHCl2 or CD2HCl.
in 3a is shorter [2.026(2) Å], likely a reflection of the single Ir-C
bond in 4a and the stronger Ir-carbene bond in 3a.
Treatment of a pale yellow solution of 4b (Scheme 2) with 1
equiv of BuLi at room temperature gives a deep red solution. The
single organometallic product is formulated as Ir(I) species 5 with
a plane of symmetry because its 1H and 13C NMR spectra showed
resonances for equivalent phenyl rings as well as only two signals
for the (CH2)2 protons, even at -10 °C,9 whereas pseudotetrahedral
complexes 3 and 4 showed more complex spectra. In comparison
with 3a,b, the carbene resonance is shifted to lower field [sl br s at
Turning toward reactivity studies relevant to B, C, and D, 4a
dehydrogenates primary and secondary alcohols giving aldehydes
and ketones, respectively. In particular, the stoichiometric reaction
of 4a with ethanol in CD2Cl2 gives acetaldehyde and 3b, possibly
through C and D (Y ) OEt), followed by ꢀ-hydride elimination.
The colorless carbene hydride 3b could be better obtained in a one-
pot synthesis from 3a using NaOMe in ethanol. The 1H NMR
2
δ 149.2 (C6D6), a sharp doublet δ 150.0, JCP ) 16.6 Hz (THF-
d8)]. Complex 5 is highly moisture sensitive, reforming 4b on
† San Diego State University.
‡ University of California at San Diego.
9
13200 J. AM. CHEM. SOC. 2008, 130, 13200–13201
10.1021/ja804713u CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Interconversions of Ir(III) and Ir(I) Speciesa
) 131.8 Hz. Similarly, ionization of 4a in the presence of hydrogen
led to heterolysis of the latter and formation of 3b-B(C6F5)4.
In summary, fundamental reactivity of NH-bearing NHC com-
plex 3a, its conjugate base 4a, and related species provide evidence
for structures A-D. Notably, the relationship of B and A with
different formal oxidation states is highlighted in Scheme 2.
Reactivity of 4a perhaps via C with acetylene or hydrogen leads
to D by bond activation prompted by the basic heterocyclic nitrogen.
Taken together, these results highlight new transformations made
possible in an NHC complex by the presence of an NH group, as
well as reactivity of the conjugate base at the free heterocyclic
nitrogen. Both types of secondary interaction are expected to expand
the possibilities of NHC complexes in organometallic chemistry
and catalysis.
a Conditions: (a) C6D6, toluene-d8, or THF-d8, room temp, <10 min;
(b) CH3OTf, C6D6, -40 °C to room temp; (c) CH3CH2CH2CH2-I, THF-
d8, -78 °C to room temp.
Scheme 3. Reactions Using Ionizing Reagentsa
Acknowledgment. We thank the NSF for partial support of this
work.
Supporting Information Available: Details of compound prepara-
tion and characterization, and CIF files for structures of 3a and 4a.
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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a Conditions: (a) C6D6, -40 °C to room temp, 1-2 h; (b) KB(C6F5)4,
THF-d8, 70 °C, 16 h (9a), 21 h (9b); (c) KB(C6F5)4, THF-d8, room temp,
67 h (3b), 4 d (9).
´
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contact with water. Conversion of 4b to 5 would be an example of
transforming B to A, perhaps driven by the highly electropositive
nature of X ) Li in the latter species.
Thus far, alkylation reactions of 5 have occurred at the metal
1
(Scheme 2), giving 6a or 6b. In 6a, the Ir-CH3 unit showed H
3
and 13C NMR resonances at -0.03 ppm (d, JHP ) 5.2 Hz) and
2
-21.1 (d, JCP ) 8.2 Hz), respectively.
(4) Grotjahn, D. B.; Miranda-Soto, V.; Kragulj, E. J.; Lev, D. A.; Erdogan, G.;
Zeng, X.; Cooksy, A. L. J. Am. Chem. Soc. 2008, 130, 20 and references
therein.
Ionization of the chloride ligand in 4a could allow formation of
structure C, at least transiently. Adding CH3OTf to 4a,b gave
carbenes 7a,b (Scheme 3), rather than formation of either CH4 or
CH3Cl. The N-CH3 protons of 7a and 7b appeared as singlets
near 4 ppm, whereas the hydride in 7b was revealed by a doublet
(5) For observation of a mixture of (hydrido)acyl and hydroxycarbene complexes
and ambident reactivity, see: Casey, C. P.; Czerwinski, C. J.; Hayashi, R. K.
J. Am. Chem. Soc. 1995, 117, 4189.
(6) Enhanced basicity and nucleophilicity of 2-pyridyl metal complexes has been
noted.3f-i
(7) See Supporting Information for full details.
2
(8) See for example, ref 3d, with a hydrogen bond between an NHC NH and
metal-bound OH ligand.
(-15.90, JHP ) 27.6 Hz). In contrast, chloride abstraction from
4a by KB(C6F5)4 was observed, though not until other reactants
were added. Propene or 1-methylimidazole coordinated to the metal,
giving 8a or 8b. In principle, two diastereomers of 8a are possible,
but the ratio must be >10:1 because only one major isomer could
be seen in 81% yield, along with two minor species. In the case of
acetylene, the heterocyclic nitrogen acts as a base, forming acetylide
carbene complex 9, an example of B f C f D. The use of
H13C13CH allowed full identification of the structure shown, 1JCC
(9) As a solution made in toluene-d8 is cooled below-10 °C, decoalescence of
the peaks for the CH2CH2PPh2 unit begins, plausibly consistent with slowing
of the flipping of the metallacycle ring in 5 or with slowing of interconversion
of Ir(III) valence tautomer 6c (R ) Li) and its enantiomer. If indeed 6c is
the ground state structure, in order for the Li to move from one (hindered)
face of the complex to the other, it would seem that 5 is a likely intermediate.
For whatever site exchange process is occurring, VT NMR data show Ea )
11.8 kcal mol-1. 7Li NMR chemical shift data were not diagnostic. Attempts
to crystallize 5 for X-ray diffraction continue.
JA804713U
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J. AM. CHEM. SOC. VOL. 130, NO. 40, 2008 13201