π-face donor properties of N-heterocyclic carbenes

Article abstract of DOI:10.1039/b512008j

The donor properties of aryl substituted N-heterocyclic carbenes are characterized by lone pair donation from the carbene carbon and, as is shown here, by donation of electron density of the aromatic π-face of the NHC aryl groups towards the metal. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b512008j

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COMMUNICATION
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p-Face donor properties of N-heterocyclic carbenes{
Marcus Su¨ßner and Herbert Plenio*
Received (in Cambridge, UK) 24th August 2005, Accepted 9th September 2005
First published as an Advance Article on the web 30th September 2005
DOI: 10.1039/b512008j
11, which turned out to be inactive in the cyclopolymerization of
1,6-heptadyines.19
The donor properties of aryl substituted N-heterocyclic
carbenes are characterized by lone pair donation from the
carbene carbon and, as is shown here, by donation of electron
density of the aromatic p-face of the NHC aryl groups towards
the metal.
The redox potentials of various Ru(II)/(III) complexes deter-
mined by cyclic voltammetry are presented in Table 1. The Ru
electrochemistry is close to reversible up to scan rates of
2000 mV s21 as evidenced by the small differences of the anodic
and cathodic peak potential of between 60 and 91 mV. At least
on this time scale there is no evidence for EC-processes. The
redox potentials within the two pairs of Grubbs II complexes
(Cl2Ru(CHPh)(IXylR)(PCy3) 2/Cl2Ru(CHPh)(SIXylR)(PCy3) 3
and Cl2Ru(CHPh)(IXylR)(PCy3) 4/Cl2Ru(CHPh)(SIXylR)(PCy3)
N-Heterocyclic carbenes (NHC) turn out to be a increasingly
important class of ligands, which has found numerous applications
in homogeneous catalysis;1 especially in various cross coupling
reactions (Heck, Sonogashira, Suzuki–Miyaura, Buchwald–
Hartwig amination),2–6 hydrosilylation7 and in the Ru-mediated
olefin metathesis.8,9 For the latter reactions utilizing Ru-based
Grubbs II10 and Grubbs–Hoveyda type catalysts,11 NHC ligands
have demonstrated their superiority to the more traditional
phosphines, owing to their special donor properties.12
Consequently, it is highly desirable to better understand the
precise nature of NHC as a ligand. The importance of precisely
controlling electronic and steric properties of olefin metathesis
catalysts was demonstrated recently by Grela and co-workers, who
systematically modified the 2-isoproxybenzylidene fragment in
olefin metathesis catalysts of the Grubbs–Hoveyda type, to
generate significantly more active catalysts.13,14
Various attempts have been made to quantify the electronic
properties of NHC ligands, notable in this respect is work from
Nolan and co-workers.15–17 Typically the n(CO) of selected NHC
metal carbonyls is monitored,17,18 to indirectly study the electron
density transferred from the NHC ligand via the metal. Probing
the Ru(II)/Ru(III) redox potential by cyclic voltammetry should
also be a useful tool to gain information on the electronic situation
at the metal center. Consequently, we have synthesized a number
of Grubbs II and Grubbs–Hoveyda type complexes with modified
NHC ligands and have determined their redox potentials.
The synthesis of the various Grubbs–Hoveyda type catalysts
follows the general scheme outlined in Fig. 1; omitting the
reduction of the diimine, leads to the corresponding ruthenium
complexes with unsaturated NHC ligands. However, while the
various complexes Cl2Ru(CHC6H4OiPr)(SIXylR) with saturated
ligands are highly stable green powders, the related unsaturated
complexes Cl2Ru(CHC6H4OiPr)(IXylR) form brownish-green
materials, which slowly decompose during chromatography and
display only limited stability in solution. Surprisingly the imidazo-
lium based Grubbs–Hoveyda complexes are virtually unknown in
the literature and we are only aware of work from Buchmeiser,
Nuyken and co-workers who describe catalytic tests with complex
Anorganische Chemie im Zintl-Institut des FB Chemie, TU Darmstadt,
Petersenstr. 18, 64287, Darmstadt, Germany.
E-mail: Plenio@tu-darmstadt.de
{ Electronic supplementary information (ESI) available: NMR data and
CV traces of 1–13. See DOI: 10.1039/b512008j
Fig. 1 Synthesis of substituted Grubbs II and Grubbs–Hoveyda type
complexes. Reagents: (a) glyoxal, HCOOH; (b) LiAlH4, thf; (c) HC(OEt)3,
HCOOH; (d) KOtBu, Grubbs I, toluene–thf; (e) CuCl, 2-isopropoxy-
styrene; (f) HCHO, HCl, dioxane.
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(a) Why have substituents on the phenyl ring in the position
Table 1 Redox potentials of ruthenium complexes studied by cyclic
voltammetry (scan rate 100 mV s21 in CH2Cl2, 0.1 M TBAPF6.
para to the nitrogen atom such a profound influence on the redox
potential of the remote ruthenium center?
(Ea 2 Ec)/
Compound
R
DE1/2/V mV
A quick answer would be to assume the transfer of electronic
information from the para-substituents across the system of
conjugated double bonds to the ruthenium center. However, on
closer inspection of the complexes, one immediately realizes that
this can hardly be the case. First of all, the shortest bond path
between the ruthenium metal and the para-substituents entails
seven covalent bonds, furthermore, the planes of the six-membered
ring and that of the five-membered ring are orthogonal. Given
these restraints a through bond mechanism appears to be
extremely unlikely. In order to come up with an explanation for
the remote substituents effect it is useful to analyze the X-ray
crystal structures of a Grubbs II,10 a Grubbs–Hoveyda type
catalyst,20 and a related NHC–Ru complex.21 In these structures
the planes of the arene rings are almost coplanar with the plane
formed by Cl2RuLC unit. However, for the Grubbs II complex the
RuLCH unit is in the same plane resulting in a face-to-face
orientation of the respective p-orbitals, while in the Grubbs–
Hoveyda catalyst the respective C–H unit is pointing towards the
center of the six-membered ring, resulting in an even more intimate
contact of the two units. Common to both structures are the rather
short distances of two planes, which for the phenyl carbon bonded
to nitrogen (i.e. in the para-position to the variable group R) and
the benzylidene carbon are close to 305 pm. Consequently, we
believe that the transfer of the electronic information from the
aromatic ring into the transition metal center occurs through
this pathway.
Cl2Ru(CHPh)(PCy3)2
1
2
3
4
5
6
7
8
9
—
Me
Me
Br
Br
H
0.585
0.455
0.447
0.536
0.537
0.870
0.895
87
82
77
75
74
90
76
87
84
91
75
82
74
Cl2Ru(CHPh)(IXylR)(PCy3)
Cl2Ru(CHPh)(SIXylR)(PCy3)
Cl2Ru(CHPh)(IXylR)(PCy3)
Cl2Ru(CHPh)(SIXylR)(PCy3)
Cl2Ru(CHC6H4OiPr)(SIXylR)
Cl2Ru(CHC6H4OiPr)(SIPr)
Cl2Ru(CHC6H4OiPr)(SIXylR)
Cl2Ru(CHC6H4OiPr)(IXylR)
Cl2Ru(CHC6H4OiPr)(SIXylR) 10 Me
Cl2Ru(CHC6H4OiPr)(IXylR) 11 Me
Cl2Ru(CHC6H4OiPr)(SIXylR) 12 Br, H
Cl2Ru(CHC6H4OiPr)(SIXylR) 13 Br
H
OC12H25 0.836
OC12H25 0.756
0.850
0.765
0.905
0.935
5 are almost identical, while the two bromine substituted
complexes 4 and 5 are anodically shifted by ca. +85 mV with
respect to 2 and 3. Obviously, but somewhat unexpectedly the
variation of the R group in the periphery of the complex has a
profound influence on the redox potential Ru(II)/(III).
The change from a Grubbs II to a Grubbs–Hoveyda catalyst
and consequently the presence of an ether oxygen donor instead of
PCy3 trans to the NHC, results in a drastic anodic shift of the
redox potential of +400 mV (5A13). Next, the influence of
the remote groups R on the Ru(II)/Ru(III) redox potential in the
Grubbs–Hoveyda complexes was studied in a more systematic
manner. Again we were surprised to observe significant shifts of
the redox potentials, as a function of the nature of the R group
(Table 1, Fig. 2), which are in accord with the electron donating
ability of the remote substituents. This is obvious when comparing
the series of 6, 8, 10, 12 and 13 (Fig. 2). It is also worth noting
that the para-substituents also have an effect on the stability of the
respective ruthenium complexes. In this vein, the more electron
donating substituents, especially –OR, lead to a significant
destabilisation of the respective Grubbs II and Grubbs–Hoveyda
complexes as evidenced by some decomposition during chromato-
graphic purification.
(b) Why is there such a significant difference in the redox
potentials of the saturated and the unsaturated N-heterocyclic
carbene ruthenium complexes of the Grubbs–Hoveyda type.
Several studies in the literature have been devoted to studying
the donor properties of saturated and unsaturated NHC versus
phosphine ligands. As indicated above there can be no doubt that
NHC ligands are significantly more electron-donating than
phosphines. This is confirmed on comparing the Ru(II)/(III) redox
potentials of 1 (+0.585 V), 2 (+0.455 V) and 3 (+0.448 V).
Replacing a single phosphine ligand (PCy3) by a NHC ligand
results in a cathodic shift of the redox potential by ca. 130 mV,
while the redox potentials of 2 and 3 are almost identical. This
situation changes drastically in the Grubbs–Hoveyda type
complexes. Here the Ru(II)/(III) redox potentials of complexes
with saturated NHCs are significantly more anodic (by 75–
100 mV), than those with the unsaturated NHC ligands. Based on
these results alone, it is tempting to propose that the unsaturated
NHC ligands are better donors than their saturated counterparts.
However, obviously the similarity of the redox potentials of 2
and 3 and of 4 and 5 does not support this view. It might be of
significance in this respect, that in the Grubbs II type complexes a
strong s-donor (PCy3) in the trans position is replaced by a weak
ether donor (ether oxygen) in the Grubbs–Hoveyda catalysts.
Since on the other hand the differences in the redox potential are
of the same order as those induced by the various para-
substituents, it can not be excluded that subtle structural
differences between the Grubbs–Hoveyda complexes with
saturated and unsaturated ligands account for the different redox
potentials. It should, however, be noted here that very recently
Nolan and co-workers,17 based on an analysis of the IR spectra of
At least three questions arise from the results of the
experimental studies presented above.
Fig. 2 Redox-potentials of Grubbs–Hoveyda type catalyst complexes
with variable R group.
5418 | Chem. Commun., 2005, 5417–5419
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decoupled from the nitrogen heterocycles provides strong evidence
of the p-face coordination of the Ru–carbene. It will be the subject
of extensive future studies to elucidate the effect of the remote
substituents on the catalytic activity and to understand whether
unsaturated NHC ligands posses an intrinsically superior donor
ability or if other effects, unique to the Grubbs–Hoveyda catalyst,
are responsible for the unexpected redox potential shifts.
This work was supported by the TU Darmstadt and the DFG.
Dipl.-Ing. Steffen Leutha¨ußer is thanked for help with electro-
chemical measurements.
Dedicated to Prof. Dr. H. Vahrenkamp on the occasion of his
65th birthday.
Notes and references
Fig. 3 Product formation in the RCM with electronically modified
catalysts.
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As evidenced by cyclic voltammetry studies of Grubbs–
Hoveyda type complexes, the saturated and the unsaturated
NHC ligands can give rise to significantly different redox
potentials Ru(II)/(III). The systematic changes of the redox
potential according to the electron donating nature of the remote
substituents and the fact that the aryl ring is electronically
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Chem. Commun., 2005, 5417–5419 | 5419