From bis(N-Alkylimidazole) to bis(NH-NHC) in rhenium carbonyl complexes

Article abstract of DOI:10.1002/anie.201002879

Two birds, one stone: A deprotonation-protonation sequence using only one molar equivalent of base and acid transforms two N-alkylimidazole molecules into N-heterocyclic carbene ligands (see scheme; black C, gray H, green F, blue N, red O, cyan Re, yellow S). Copyright

Full text of DOI:10.1002/anie.201002879

Angewandte
Chemie
DOI: 10.1002/anie.201002879
Heterocyclic Carbenes
From Bis(N-Alkylimidazole) to Bis(NH–NHC) in Rhenium Carbonyl
Complexes**
Miguel A. Huertos, Julio Pꢀrez,* Lucꢁa Riera,* Jesffls Dꢁaz, and Ramón López*
The current importance of nitrogen heterocyclic carbene
(NHC) ligands in several areas of coordination chemistry and
catalysis can hardly be overemphasized.[1] The deprotonation
of imidazolium salts is the prime route to NHCs; however, the
study of the transformations relating imidazole and NHC
ligands is still an emerging area of research. The thermody-
namic preference for N- or C-coordination in imidazoles has
been theoretically investigated by Crabtree
et al.[2] Several examples of tautomerization in
pyridinic ligands have been published;[3] how-
ever, apart from the work reported by Sundberg
et al. on RuII NHC complexes that were
obtained in a very low yield,[4] only a similar
tautomerization in a nonchelated imidazole
ligand has been proposed by Bergman et al.[5]
Ruthenium- and iridium-mediated tautomeriza-
tions from NHC to imidazole ligands were
reported by the groups of Whittlesey[6] and
Li,[7] respectively, whereas imidazole to NHC
tautomerizations aided by chelate ring forma-
tion mediated by Ir and Ru were reported by the
groups of Grotjahn[8] and of Kuwata,[9] respec-
tively. Ruiz and Perandones reported the base-
promoted tautomerization of imidazole ligands
to NHCs at a manganese(I) center.[10] Our group
ReI dramatically depends on the nature of the ancillary
ligands and the substituents at the nitrogen atom of the
N-alkylimidazole (N-RIm) ligand (see below).[11]
As previously found by our group and shown in Scheme 1,
imidazol-2-yl complex A, the product of the deprotonation of
[Re(CO)3(N-MeIm)3]OTf (N-MeIm = N-methylimidazole;
Tf = trifluoromethanesulfonyl), can be methylated to afford
Scheme 1. Reactivity of the ReI imidazole compounds studied.
found that the outcome of related reactions at
[*] M. A. Huertos, Dr. J. Pꢀrez
the NHC compound B or protonated to yield C, which
features an NH–NHC ligand. In contrast, employment of
[Re(CO)3(N-MesIm)3]OTf (N-MesIm = N-mesitylimidazole)
as precursor leads to ring opening.[11b] Also, deprotonation of
complexes with one N-RIm ligand and either 2,2’-bipyridine
or 1,10-phenanthroline results in C–C coupling and activation
of one of the pyridine rings of the diimine chelate.[11a] This
wealth of new reactivity patterns prompted us to extend
our studies by investigating the deprotonation of
[Re(OTf)(CO)3(N-RIm)2] complexes.
Complex [Re(OTf)(CO)3(N-MesIm)2] (1a), prepared by
reaction of [ReBr(CO)5] and N-MesIm followed by AgOTf,
reacted instantaneously with an equimolar amount of
KN(SiMe3)2 in THF (see Scheme 2). Compound 2a could
be isolated in 38% yield from the crude reaction mixture and
was characterized by IR spectroscopy, NMR spectroscopy,
and X-ray diffraction (Figure 1a).[12,13] It was found to contain
a fac-{Re(CO)3} fragment (nCO bands at 1995 and 1872 cmÀ1
in the IR spectrum) bonded to one N-MesIm ligand, one
imidazol-2-yl ligand, and one NH–NHC ligand, the latter two
Departamento de Quꢁmica Orgꢂnica e Inorgꢂnica-IUQOEM
Universidad de Oviedo–CSIC
C/Juliꢂn Claverꢁa 8, 33006 Oviedo (Spain)
E-mail: japm@uniovi.es
Dr. L. Riera
Instituto de Ciencia de Materiales de Aragꢃn (ICMA)
Universidad de Zaragoza–CSIC
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
E-mail: riera@unizar.es
Dr. J. Dꢁaz
Departamento de Quꢁmica Orgꢂnica e Inorgꢂnica
Universidad de Extremadura
Avda de la Universidad s/n, 10071 Cꢂceres (Spain)
Dr. R. Lꢃpez
Departamento de Quꢁmica Fꢁsica y Analꢁtica
Universidad de Oviedo
C/Juliꢂn Claverꢁa 8, 33006 Oviedo (Spain)
E-mail: rlopez@uniovi.es
[**] Financial support from the Ministerio de Ciencia e Innovaciꢃn
(MICINN, No. CTQ2009-12366) and Principado de Asturias
(No. IB08-104) is gratefully acknowledged. NHC=N-heterocyclic
carbene.
À
À
resulting from N Re to C Re change in the coordination
mode of two N-MesIm ligands. The large shift to lower-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002879.
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evident from the 1H and 13C NMR spectra of 2a, thus
indicating the fast (even at low temperature) H+ transfer
between the two nitrogen atoms, that is, the complex can be
described as featuring two imidazol-2-yl ligands that share a
proton. The Re-bonded carbon atom of these ligands occurs
at d = 180.7 ppm in the 13C NMR spectrum, and the two
À
À
Re C bond lengths are undistinguishable (Re C2 2.214(10)
À
and Re C22 2.207(9) ꢀ), which shows the close similarity
between imidazol-2-yl and NH–NHC ligands. It could be
argued that their similarity is the result of an intermediate
imidazolyl carbene character resulting from the proton being
equally shared between both ligands. However, it should be
noted that similar Re–C distances and 13C chemical shifts
were also found in compounds A and C in Scheme 1.[11b,c] In a
few instances imidazol-2-yl complexes were found to be stable
enough so that they could be isolated, and these species and
NH–NHC complexes can interconvert by H+-transfer reac-
tions.[8,9]
Scheme 2. Reactivity of [Re(OTf)(CO)3(N-RIm)2] compounds 1a and
1b.
The yield of 2a increased to 86% when its preparation
was conducted in the presence of an equimolar amount of N-
MesIm, as expected since, in its absence, part of the
bis(imidazole) precursor 1a must have acted as a sacrificial
source of N-MesIm.[14]
The deprotonation of the cationic tris(N-mesitylimida-
zole) rhenium tricarbonyl complex under the same conditions
led to a completely different product (compound D in
Scheme 1). This proves that it is the neutral complex 1a,
rather than the [Re(CO)3(N-MesIm)3]OTf species (the likely
product of the reaction of 1a with N-MesIm) that undergoes
deprotonation. This is not surprising because the formation of
[Re(CO)3(N-MesIm)3]OTf from 1a and N-MesIm could not
be spectroscopically detected after two days in THF at room
temperature.[15]
The reaction of [Re(OTf)(CO)3(N-MeIm)2] (1b) with
KN(SiMe3)3 and N-MeIm afforded the N-methyl analogue of
2a, which was spectroscopically characterized. To investigate
the exact fate of the entering imidazole in the product, the
reactions of a) 1a with base and
N-MeIm, and b) 1b with base and N-MesIm were conducted.
The products, the mixed complexes 3a and 3b, respectively,
were fully characterized by spectroscopic means, and the
solid-state structure of 3a, determined by X-ray diffrac-
tion,[12,16] was found to be similar to that of 2a (Figure 1b).
The externally added imidazole was found to be N-coordi-
nated in these complexes, which suggests that the change in
À
À
the N Re to C Re coordination mode precedes the coordi-
nation of the entering substituted imidazole. Indeed, this
reaction seemed to be quite general, and the addition of 4-
dimethylaminopyridine (py*) as external ligand similarly
afforded [Re(CO)3(C-MeIm)2(py*)]OTf (4).[14]
Figure 1. Molecular structures of a) 2a and b) 3a (thermal ellipsoids
set at 30% probability).
To gain insight into the mechanism details, a density
functional theory (DFT) study was performed.[17] A schematic
view of the most favorable reaction mechanism is shown in
Scheme 3 (see also the Supporting Information for alternative
reaction mechanisms). The Gibbs free energy in THF solution
(in parentheses) is referred to that of the deprotonated
species [Re(OTf)(CO)3(N-MeIm)2]À (see I in Scheme 3) and
N-MeIm. The reaction starts with the loss of triflate from I to
give intermediate IIa, in which the Re atom is simultaneously
wavenumber values in the nCO bands (those of 1a occur at
2033, 1925, and 1897 cmÀ1) reflects the strong s-donor
character of the C-bound heterocyclic ligands.
The NH group of the NH–NHC ligand acts as a hydrogen-
bond donor towards the uncoordinated nitrogen atom of the
imidazol-2-yl ligand, and contributes to the virtual coplanarity
of the two heterocyclic ligands. A molecular mirror plane is
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Scheme 3. Most favorable mechanism for the evolution of the deprotonation product of [Re(OTf)(CO)3(N-MeIm)2] at the B3LYP/6-31+G(d,p)
(LANL2DZ+f for Re) theoretical level. Relative Gibbs energies in solution [kcalmolÀ1] are given in parentheses.
interacting with the noncoordinated N atom and the C2 atom
of the imidazolyl ligand. All attempts to locate the transition
state (TS) where the triflate anion came out failed, but the
energy of this route has to be really small considering that the
new species is neutral; moreover, potassium triflate is an
insoluble salt that precipitates from the reaction medium.
Intermediate IIa undergoes a rotation of the imidazole ring
À
around the N Re bond through TS-IIa/IIIa to give the
intermediate IIIa. TS-IIIa/IVa connects IIIa with intermedi-
ate IVa wherein the two heterocyclic ligands are C-bound to
the Re atom. Finally, addition of N-MeIm to IVa leads to the
formation of a rhenium imidazol-2-yl (carbene) complex Va
without any TS. The formation of the rhenium imidazolyl
carbene complex would imply a Gibbs energy barrier in
solution of 21.5 kcalmolÀ1, consistent with the fast formation
of the product experimentally observed.
A key feature of the proposed mechanism is the inter-
mediacy of h2-N,C-imidazolyl complexes, which make possi-
ble ligand dissociation without going through high-energy
five-coordinate species. Stable h2-N,C-imidazolyl complexes
have been disclosed by Monreal and Diaconescu in scandium
and uranium chemistry.[18]
The reaction of 2b with trifluoromethanesulfonic acid
(HOTf) afforded 5, the triflate salt of the bis(NH–NHC)
complex resulting from protonation at nitrogen.[19] The shift
to higher frequencies of the IR nCO bands of 5 (2012, 1913,
and 1886 cmÀ1) indicates the formation of a cationic deriva-
tive. The NMR spectra (1H and 13C NMR) showed the Cs
symmetry of the molecule, and the most informative signal at
175.0 ppm is assigned to carbenic C atoms. An X-ray diffrac-
tion analysis (Figure 2) confirmed the proposed structure,[12,20]
Figure 2. Molecular structure of the cation of compound 5 (thermal
ellipsoids set at 30% probability).
isomers are obtained as products. The formation of the ring-
opening product in the former case or the imidazolyl carbene
in the latter shows again the extreme sensitivity of the
reaction course to the exact nature and number of the ligands.
Further studies on the deprotonation of rhenium imidazole
complexes with different sets of ligands are in progress.
Received: May 12, 2010
Published online: July 29, 2010
Keywords: carbene ligands · density functional calculations ·
.
nitrogen heterocycles · protonation · rhenium
À
the Re C carbenic bond distances (2.183(6) and 2.185(5) ꢀ)
being very similar to those discussed above for 2a and those
previously reported.[11b,c]
In summary, the overall formation of 5 from 1b involves,
besides the substitution of OTf by the entering imidazole, the
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109, 3612 – 3676; b) F. E. Hahn, M. C. Jahnke, Angew. Chem.
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À
formation of two new Re C bonds at the expense of the two
À
Re N bonds. Deprotonation of a coordinated N-MeIm ligand
in [Re(CO)3(N-MeIm)3]OTf followed by protonation of the
resulting imidazol-2-yl ligand affords an NHC complex (see
Scheme 1). The present formation of the bis(carbene) com-
plex from the bis(imidazole) precursor is not just twice that
process, because the addition of only one equivalent of base
À
À
triggers the N Re to C Re rearrangement of two imidazole
ligands. Notably, when the deprotonation reactions of a) the
tris(N-MesIm) compound or b) the triflato complex 1a are
carried out in the presence of free N-MesIm, the components
of the reactant mixture are the same, whereas different
[4] R. J. Sundberg, R. F. Bryan, I. F. Taylor, Jr., H. Taube, J. Am.
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Angew. Chem. Int. Ed. 2010, 49, 6409 –6412
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Communications
[5] a) K. L. Tan, R. G. Bergman, J. A. Ellman, J. Am. Chem. Soc.
[14] See the Supporting Information for further experimental details.
[15] On the other hand, starting materials 1a and 1b are obtained as
pure species that have been fully characterized, including by X-
ray diffraction for compound 1a (see the Supporting Informa-
tion).
[16] Selected crystallographic data for 3a: C35H38N6O3Re, M =
776.91, monoclinic, C2/c, a = 21.6583(6), b = 15.1624(5), c =
21.1537(5) ꢀ, a = 90, b = 106.518(2), g = 908, 150(1) K, V=
6660.0(3) ꢀ3, Z = 8; 28086 reflections measured, 6837 independ-
ent (wRint = 0.0516); R1 = 0.0323, wR2 = 0.0712 (all data).
[17] Quantum chemical computations were carried out with Gaus-
sian 03 (RevisionD.01), M. J. Frisch et al., Gaussian, Inc.,
Pittsburgh, PA, 2004. See the Supporting Information for details
and complete reference.
[18] M. J. Monreal, P. L. Diaconescu, J. Am. Chem. Soc. 2010, 132,
7676 – 7683, and references therein.
[19] An example of a rhenium bis(NHC) complex has been recently
prepared by a different method, see: O. Hiltner, E. Herrdtweck,
M. Drees, W. A. Herrmann, F. E. Kꢅhn, Eur. J. Inorg. Chem.
2009, 1825 – 1831.
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[9] K. Araki, S. Kuwata, T. Ikariya, Organometallics 2008, 27, 2176 –
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[10] J. Ruiz, B. F. Perandones, J. Am. Chem. Soc. 2007, 129, 9298 –
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[11] a) M. A. Huertos, J. Pꢃrez, L. Riera, J. Am. Chem. Soc. 2008, 130,
5662 – 5663; b) M. A. Huertos, J. Pꢃrez, L. Riera, A. Menꢃndez-
Velꢂzquez, J. Am. Chem. Soc. 2008, 130, 13530 – 13531; c) M. A.
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16, 8495 – 8507.
[12] CCDC 776191 (1a), 776192 (2a), 776193 (3a), and 776195 (5)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[20] Selected crystallographic data for 5: C16H18F3N6O6ReS, M =
¯
665.62, triclinic, P1, a = 7.428(5), b = 12.332(5), c = 12.392(5) ꢀ,
[13] Selected crystallographic data for 2a: C46H49N6O3Re, M =
920.11, orthorhombic, Pbca, a = 19.2434(3), b = 15.2881(2), c =
28.6367(5) ꢀ, a = 90, b = 90, g = 908, 150(2) K, V= 8424.8(2) ꢀ3,
a = 94.329(5), b = 95.113(5), g = 98.155(5)8, 150(1) K, V=
1114.7(10) ꢀ3, Z = 2; 16691 reflections measured, 4372 inde-
pendent (wRint = 0.0484); R1 = 0.0331, wR2 = 0.0729 (all data).
Z = 8; 29800 reflections measured, 8171 independent (Rint
0.1176); R1 = 0.0699, wR2 = 0.1435 (all data).
=
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