Janus-type organopotassium chemistry observed in deprotonation of mesoionic imidazolium aminides and amino N-heterocyclic carbenes: Coordination and organometallic polymers

Article abstract of DOI:10.1039/c3cc49517e

The first direct observation of the tautomeric equilibrium between mesoionic imidazolium aminides (IA) and amino NHCs (AC) shows its dependence on the aminide substituent. A potassium (imidazol-2-ylidenyl)(anilide), an ion-pair with the 'free' anionic NHC, and potassium organometallic polymers, with the repeat unit exhibiting 'Janus-type' coordination, are reported. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc49517e

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‘Janus-type’ organopotassium chemistry observed
in deprotonation of mesoionic imidazolium
aminides and amino N-heterocyclic carbenes:
coordination and organometallic polymers†
Cite this: Chem. Commun., 2014,
50, 3055
Received 16th December 2013,
Accepted 24th January 2014
Andreas A. Danopoulos*a and Pierre Braunstein*b
DOI: 10.1039/c3cc49517e
www.rsc.org/chemcomm
The first direct observation of the tautomeric equilibrium between
mesoionic imidazolium aminides (IA) and amino NHCs (AC) shows its
dependence on the aminide substituent. A potassium (imidazol-2-
ylidenyl)(anilide), an ion-pair with the ‘free’ anionic NHC, and potassium
organometallic polymers, with the repeat unit exhibiting ‘Janus-type’
coordination, are reported.
Scheme 1 Equilibrium between (IA) and (AC) tautomers. In the text
a tautomer is designated as n(IA) or n(AC) and an equilibrium mixture as
There is considerable and general interest in the study of novel, n(IA # AC), n = 1–6.
‘unconventional’ carbene- and carbenoid- species, in which the
heteroatom stabilisation of the divalent carbon atom partici-
pating in the heterocyclic ring is diminished, resulting in
more reactive chemical entities.1 The objective is to uncover
novel bonding patterns and reactivity with applications in
coordination chemistry and in catalysis with metal-based or
metal-free systems. Mesoionic carbenes (MICs) have emerged
as a new, broad class of N-heterocyclic carbenes (NHCs).2–4
They are reactive, usually transient, species generated in situ in
the presence of metals.
Tautomeric equilibria between mesoionic azolium compounds
(a sub-class of mesomeric betaines)5 bearing a remote aminide
(i.e. amido) substituent and (non-mesoionic) remote amino NHCs
have been recently postulated in two cases.6 This was based on the
‘carbene-like’ reactivity of the mesoionic azolium compound, albeit
the corresponding carbene partner has only been implicated. We
have now directly observed the equilibrium between imidazolium
aminides (IA), imidazolium-4-aminides, and their tautomeric
amino-NHCs (AC), 4-aminoimidazol-2-ylidenes (Scheme 1) and
isolated key derivatives of general relevance. Deprotonation of
any of the tautomers in this equilibrium would give an overall
anionic NHC (Scheme 2).
Recently, we obtained computational evidence that the nature of
the R group at the exocyclic aminide N atom (Nexo) influences the
position of the equilibrium (Scheme 1) and the coordination
behaviour of the anionic NHC (Scheme 2).7 In view of the more
pronounced ionic character of the potassium organometallics,
compared e.g. to the lithium analogues,8a and the widely recognised
properties of potassium not to be involved in adduct formation
with NHCs,2,3,8b–d we used benzyl potassium (KBz) as a base to
access anionic NHCs.
We now demonstrate experimentally that by selecting a
suitable R group we can: (i) observe the equilibrium shown in
Scheme 1 by NMR spectroscopy; (ii) isolate and characterise
one monomeric potassium (imidazol-2-ylidenyl)(anilide), and
one ‘free’ anionic NHC (accompanied by the non-interacting
a Institute for Advanced Study (USIAS), and Laboratoire de Chimie de Coordination,
´
Institut de Chimie (UMR 7177 CNRS), Universite de Strasbourg, 4 rue Blaise
Pascal, 67081 Strasbourg Cedex, France. E-mail: danopoulos@unistra.fr
b Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS),
´
Universite de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France.
E-mail: braunstein@unistra.fr
† Electronic supplementary information (ESI) available: Experimental procedures,
characterisation data and X-ray crystallographic data for all new ligands and
complexes. CCDC 965579–965585 for 5(IA), 6(IA), 7, 8, 9, 10, 12. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc49517e
Scheme 2 Resonance forms of (IA) (top) and of the anionic NHC derived
thereof (bottom).
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Fig. 1 Structure of 7.
the CNHC. The deprotonation was evidenced by the disappearance of
the two doublets due to the two ring Hs of 1(IA) at d 6.00 and 4.92
and the appearance of a singlet at d 4.76, due to the only H left on
the heterocycle. In solution in THF the K+–p-aromatic system
interaction is non-rigid as evidenced by the broadness of the i-Pr
and the aromatic proton signals associated with this particular ring,
and by variable-temperature NMR spectroscopy. Deprotonation with
KN(SiMe3)2 gave the same product less cleanly.
Reaction of 1(IA) with one equiv. of KBz in the presence of
18-crown-6 afforded the extremely air-sensitive 8 which was
structurally characterised (Fig. 2). It comprises a pair of well-
separated discrete ions: the cation [K(18-crown-6)]+ and the ‘free’
deprotonated NHC. The heterocyclic ring, the Nexo and the Cipso
atoms of all three DiPP substituents are coplanar, in support of
extensive electronic delocalisation (ESI†). The reduction of the
angle at the CNHC points to increased s-character in the s-lone
pair compared to the C–H s-bonding orbital of the imidazolium.
This is a recurring observation with other imidazolium–NHC
conjugate acid–base pairs. Spectroscopically, the deprotonation
was evidenced by the disappearance of the two doublets associated
with the two ring protons of 1(IA) at d 6.00 and 4.92 and the
appearance of a singlet at d 4.76 (see above for 7); however, in 8
there is no evidence of fluxionality at room temperature.
Scheme 3 Synthesis of potassium derivatives (localised K–C, K–N and K–O
bonds are represented as solid, cation–p interactions as dotted, bonding
interactions of neighboring polymer repeat units as dashed lines); Ar = 2,6-i-
Pr2-C6H4, DiPP: (i) 1.25 equiv. of KBz, in THF; (ii) 1.25 equiv. of KBz, 1.20 equiv.
of 18-crown-6 in THF; (iii) 1.20 equiv. of KBz in THF and (iv) 1.70 equiv. of
KBz in THF.
cation [K(18-crown-6)]+), and (iii) obtain (co)polymeric organo-
metallics in which anionic NHC repeat units act as ambidentate,
‘Janus-type’ ligands9 (Scheme 3). The only related anionic NHCs
are with B-, P- or (t-Bu)2Zn-remote substitutions and the anionic
dicarbenes (carbanionic carbenes).10
Even though the (IA) form was exclusively present in solution
and the solid state when R = mesityl and 4-t-Bu-phenyl, the position
of the tautomeric equilibrium was shifted toward the (AC) form for
R = t-Bu, especially in aromatic solvents at room temperature. The
1H- and 13C-NMR data (see ESI†) provided unequivocal evidence for
an equilibrium in favor of the (AC) form (KRT = 1.5). Lowering the
temperature of toluene to À60 1C shifted the equilibrium to K = 0.9,
while in THF there was no significant temperature dependence.
Importantly, attempts to crystallise 6(AC) from pentane at À40 1C
gave only crystals of 6(IA) (see crystal structure in the ESI†), which
upon redissolving in toluene restored the original equilibrium
mixture. For n(IA # AC), n = 4, 5, KRT E 0.25 in toluene.
The mechanism by which the t-Bu influences the position of the
equilibrium is not clear. Plausible explanations may invoke the
reduced electronegativity of t-Bu compared to i-Pr and Cy11
leading to destabilisation of 6(IA). However, comparison of the
solid state structures of 5(IA) and 6(IA) (ESI†) does not reveal any
significant differences. DFT calculations are in progress.
Deprotonation of the less bulky 3(IA) and 4(IA # AC) with
1.2 equiv. of KBz gave good yields of the polymeric complexes 9
and 10 shown in Fig. 3 and 4, respectively.
Unexpected results were obtained upon attempted second
deprotonation from n(IA), n = 1–3. Thus, reaction of 1(IA) with
Deprotonation of 1(IA) with KBz in slight excess (1.2 fold) in
THF gave highly air-sensitive 7 after crystallisation at À40 1C.
Complex 7 (Fig. 1) can be considered as formally arising from
deprotonation of 1(AC) by KBz; the K+ coordination sphere com-
prises one aminide group bound via the planar Nexo and three THF
molecules. Additional stabilisation is provided by interaction with
the p-system of the proximal DiPP; there is no close contact involving
Fig. 2 Structure of the anion in 8; the [K(18-crown-6)]+ ion is omitted.
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Fig. 5 Structure of the copolymer 12; one repeat unit is shown in red; due
Fig. 3 Structure of 9; four consecutive repeat units (one in red) are shown.
to symmetry the KCH2C6H5 (C51) has half-occupancy.
Urbaine de Strasbourg for the award of fellowships and a
Gutenberg Excellence Chair (2010–11), respectively.
Notes and references
1 (a) O. Schuster, L. Yang, H. G. Raubenheimer and M. Albrecht,
Chem. Rev., 2009, 109, 3445; (b) R. H. Crabtree, Coord. Chem. Rev.,
2013, 257, 755; (c) A. Schmidt, S. Wiechmann and T. Freese,
ARKIVOC, 2013, 424.
2 G. Guisado-Barrios, J. Bouffard, B. Donnadieu and G. Bertrand,
Angew. Chem., Int. Ed., 2010, 49, 4759.
3 (a) E. Aldeco-Perez, A. J. Rosenthal, B. Donnadieu, P. Parameswaran,
G. Frenking and G. Bertrand, Science, 2009, 326, 556;
Fig. 4 Structure of 10; four consecutive repeat units (one in red) are shown.
´
(b) I. Fernandez, C. A. Dyker, A. De Hope, B. Donnadieu,
G. Frenking and G. Bertrand, J. Am. Chem. Soc., 2009, 131, 11875;
(c) J. Bouffard, B. K. Keitz, R. Tonner, G. Guisado-Barrios,
G. Frenking, R. H. Grubbs and G. Bertrand, Organometallics, 2011,
30, 2617.
4 J. C. Bernhammer, G. Frison and H. V. Huynh, Chem.–Eur. J., 2013,
19, 12892 and references therein.
5 (a) W. D. Ollis, S. P. Stanforth and C. A. Ramsden, Tetrahedron, 1985,
41, 2239; (b) N. Pidlypnyi, J. C. Namyslo, M. H. H. Drafz, M. Nieger
and A. Schmidt, J. Org. Chem., 2013, 78, 1070.
at least 1.70 equiv. of KBz in THF afforded the new organo-
metallic species 12 as orange, extremely air-sensitive crystals
with limited solution stability at ambient temperature (see ESI†).
The structure of 12 (Fig. 5) revealed the formation of an
organometallic copolymer, the repeat unit of which comprised
two metallated anionic NHCs bound through their Nexo to one
potassium cation and through the CNHC to another potassium
cation; the latter is also interacting with a bridging m-Z6-anionic
benzylic group bound to a third potassium cation; the coordi-
nation sphere of the two potassium cations bound to CNHC is
completed by THF molecules.
The K2–C1 bond length falls into the short-end of the range
for the number of K–C bonds that have been structurally
characterised (average 3.183 Å) but are typical of other K–CNHC
bonds (see above).
In conclusion, the choice of the Nexo substituent allows
tuning of the (IA) vs. (AC) equilibrium and influences the nature
of the anionic products obtained by deprotonation of the
equilibrium partners. The study of these parameters for the
synthesis of metal complexes is in progress. Exploration of any
effect that the organic substituent at the Nexo has on the
coordination modes adopted by the anionic NHC could lead
to novel organometallics.
6 (a) C. Farber, M. Leibold, C. Bruhn, M. Maurer and U. Siemeling,
´
Chem. Commun., 2012, 48, 227; (b) V. Cesar, J.-C. Tourneux,
N. Vujkovic, R. Brousses, N. Lugan and G. Lavigne, Chem. Commun.,
2012, 48, 2349.
7 A. A. Danopoulos, K. Y. Monakhov and P. Braunstein, Chem.–Eur. J.,
2013, 19, 450.
8 (a) F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry,
Wiley-Interscience, New York, 5th edn, 1988, pp. 139–142;
(b) R. W. Alder, M. E. Blake, C. Bortolotti, S. Bufali, C. P. Butts,
E. Linehan, J. M. Oliva, A. G. Orpen and M. J. Quayle, Chem.
Commun., 1999, 241; (c) A. A. Danopoulos, S. Winston, T. Gelbrich,
M. B. Hursthouse and R. P. Tooze, Chem. Commun., 2002, 482;
(d) R. W. Alder, M. E. Blake, L. Chaker, J. N. Harvey, F. Paolini and
J. Schu¨tz, Angew. Chem., Int. Ed., 2004, 43, 5896.
9 (a) F. Baier, Z. Fei, H. Gornitzka, A. Murso, S. Neufeld, M. Pfeiffer,
I. Ru¨denauer, A. Steiner, T. Stey and D. Stalke, J. Organomet. Chem.,
2002, 661, 111; (b) D. M. Khramov, A. J. Boydston and
C. W. Bielawski, Angew. Chem., Int. Ed., 2006, 45, 6186.
10 (a) S. Kronig, E. Theuergarten, C. G. Daniliuc, P. G. Jones and
M. Tamm, Angew. Chem., Int. Ed., 2012, 51, 3240; (b) Y. Wang,
´
Y. Xie, M. Y. Abraham, P. Wei, H. F. Schaefer, P. von Rague Schleyer
and G. H. Robinson, J. Am. Chem. Soc., 2010, 132, 14370; (c) P. K.
Majhi, G. Schnakenburg, Z. Kelemen, L. Nyulaszi, D. P. Gates and
R. Streubel, Angew. Chem., Int. Ed., 2013, 52, 10080; (d) D. R.
Armstrong, S. E. Baillie, V. L. Blair, N. G. Chabloz, J. Diez,
J. Garcia-Alvarez, A. R. Kennedy, S. D. Robertson and E. Hevia,
Chem. Sci., 2013, 4, 4259.
We thank Prof. W. B. Motherwell for helpful discussions,
´
the CNRS, the MESR (Paris), the Universite de Strasbourg
and the ucFRC, (www.icfrc.fr) for support and the Service de
Radiocristallographie (UdS) for the crystal structures. A.A.D. thanks
11 M. B. Smith and J. March, March’s Advanced Organic Chemistry,
Wiley-Interscience, New Jersey, 6th edn, 2007, p. 14.
´
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the USIAS, the CNRS and the Region Alsace, the Communaute
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Chem. Commun., 2014, 50, 3055--3057 | 3057