Alkyne gem-Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character

Article abstract of DOI:10.1002/anie.201904255

Parahydrogen (p-H₂) induced polarization (PHIP) NMR spectroscopy showed that [Cp^XRu] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem-hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich Cp^X rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT-assisted analysis of the chemical shift tensors determined by solid-state ¹³C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis-active Grubbs-type catalysts.

Full text of DOI:10.1002/anie.201904255

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Alkyne gem-Hydrogenation: Formation of Pianostool Ruthenium
Carbene Complexes and Analysis of Their Chemical Character
Authors: Alois Fürstner, Tobias Biberger, Christopher P. Gordon,
Markus Leutzsch, Sebastian Peil, Alexandre Guthertz, and
Christophe Copéret
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904255
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10.1002/anie.201904255
Angewandte Chemie International Edition
Alkyne gem-Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis
of Their Chemical Character
Tobias Biberger, Christopher P. Gordon, Markus Leutzsch, Sebastian Peil, Alexandre Guthertz,
Christophe Copéret, and Alois Fürstner*
Abstract: Parahydrogen (p-H2) induced polarization (PHIP) NMR spectroscopy showed that [CpXRu]
complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives
into gem-hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron
rich CpX rings lower the barriers, stabilize the resulting complexes and hence provide opportunities
for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium
complexes was studied by DFT-assisted analysis of the chemical shift tensors determined by solid-
state 13C NMR spectroscopy. The combined experimental and computational data draw the portrait
of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics
more befitting metathesis-active Grubbs-type catalysts.
Transition metal complexes comprising a formal metal-carbon double bond are commonly classified
as either “Fischer carbenes” or “Schrock alkylidenes”.1 The former are electrophilic entities which
typically incorporate low-valent metal centers with strong-field ancillary ligands; their carbene unit
itself usually bears a heteroelement substituent.2,3 “Schrock alkylidenes”, in contrast, feature a high
valent metal center bound to a nucleophilic carbon substituent, often comprised within a weaker-
field ligand environment.4,5 It is important to recognize, however, that this terminology denotes only
*
T. Biberger, Dr. M. Leutzsch, S. Peil, Dr. A. Guthertz, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
45470 Mülheim/Ruhr (Germany)
Email: fuerstner@kofo.mpg.de
C. P. Gordon, Prof. C. Copéret
Department of Chemistry and Applied Biosciences
ETH Zürich
Vladimir-Prelog-Weg 1-5
8093 Zürich (Switzerland)
Supporting Information and the ORCID identification numbers for the authors of this article can be found under
###########
1
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10.1002/anie.201904255
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the extremes of a continuum: it is not necessarily intuitive by looking at the molecular structure to
which category a given metal carbene complex may belong. The famous Grubbs catalysts, which have
revolutionized the practice of olefin metathesis, may perhaps be cited in this context.6,7,8,9,10
Scheme 1. The interwoven pathways of trans- and gem-hydrogenation of alkynes; Cp* =
pentamethylcyclopentadienyl
Pianostool ruthenium complexes of type D are isoelectronic with Grubbs carbenes. Although such
species had been invoked as reactive intermediates in a number of different transformations
including cyclopropanation,11,12 little was known about structure and bonding until very recently. Our
current interest was invigorated by the finding that carbenes of this type can be generated by gem-
hydrogenation of ordinary alkynes using [Cp*RuCl]4 as the catalyst.13,14 This unorthodox
transformation, in which both H-atoms of H2 are delivered to one and the same C-atom of the triple
bond, has no precedent in the classical canon of homogeneous catalysis; it constitutes the early
phase of a reaction pathway that allows alkynes to be converted into E-alkenes H by trans-selective
2
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semi-reduction (Scheme 1).13-15 It is important to note that this stepwise process leading from C via D
to competes with a concerted mechanism, which bifurcates at the stage of the
H
metallacyclopropene intermediate C initially formed upon transfer of the first H-atom to the alkyne
ligand in -complex B. For unbiased substrates, the barriers for either route downstream of C were
computed to be very similar;13 propargylic –OR substituents, however, which are able to engage with
an emerging carbene via hydrogen bonding or a Lewis acid/Lewis base interaction, foster the
stepwise path by stabilizing the intermediates and exert a pronounced directing effect; in favorable
cases have the resulting pianostool ruthenium carbenes of type D been isolated in analytically pure
form and were fully characterized by spectroscopic and crystallographic means.13,14
This involved overall manifold is of fundamental interest but renders reaction optimization difficult:
The concerted step from C to G is intrinsically selective for the E-alkene H, whereas a transient
carbene D opens a gateway to side reactions such as positional alkene isomerization and/or
overreduction, which often plaque trans-hydrogenation.15 Experimental and computational data
concur in that the conversion of D to product as well as to the side products proceeds via an
associative mechanism, in which a second H2 molecule needs to be ligated to the ruthenium center
to lower the barriers.13 On the other hand, one can envisage that purposeful carbene formation by
gem-hydrogenation allows alkynes to be used as surrogates for hazardous carbene sources such as
diazo derivatives or cyclopropenes. The possible outlook of harnessing genuine carbene chemistry
downstream from D, however, is contingent on the ability to either fully stop or outperform the
evolution of D into the trans-alkene H and/or side products via F and E, respectively. Moreover, it
mandates an answer to the question whether ruthenium complexes of type D fall into the realm of
Fischer carbenes or Schrock alkylidenes.
3
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Figure 1. Panel of cationic [CpXRu] complexes with largely different electronic properties
To address these central issues, a library of [CpXRu(MeCN)3]PF6 precatalysts (CpX = substituted
cyclopentadienyl)16 was prepared and their ability to form carbene complexes by gem-hydrogenation
studied using propargylic ether 10 as the model substrate (Figure 1). Cyclic voltammetry confirmed
that all complexes undergo reversible one-electron oxidation at potentials (E1/2) ranging from 0.322 V
to 0.903 V. This favorably large span of almost 0.6 V should help to unveil if CpX ligands of different
donor capacity exert any noticeable effects on carbene formation and stability.17,18,19
Figure 2 captures the behavior of ten different [CpXRu] complexes in the hydrogenation of substrate
10. The reactions were performed with parahydrogen (p-H2) in order to take advantage of the
1
sensitivity of PHIP-enhanced H NMR spectroscopy (PHIP = p-H2-induced polarization), which allows
fleeting intermediates to be monitored even if present in low concentration.20,21,22 The characteristic
spectral fingerprint of the CH2-group formed by gem-hydrogenation of the triple bond proves the
intervention of carbenes 11 in all cases investigated. Moreover, the observed coupling pattern shows
that these carbenes invariably reside distal to the steering OMe group; only in the spectrum derived
from complex 3 is the regioisomeric carbene present in appreciable concentration (proximal:distal,
3:7). The observation that the most crowded CpX ligand entails appreciable carbene formation at the
more hindered site is counterintuitive and deserves closer inspection.
4
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Figure 2. Catalyst-dependent OPSY-NMR spectra recorded during the hydrogenation of 10
Of arguably greater significance in the present context is the fact that PHIP-enhanced signals of the E-
alkene 12 are detected only in the reactions catalyzed by 1 and 3, which carry the most electron-
donating CpX ligands. It is important to note that the olefin detectable in the OPSY experiment must
have been formed by the concerted mechanism in which both alkene H-atoms derive from the same
H2 molecule.23 The observation that electron rich cationic catalysts up-regulate this intrinsically
selective pathway provides valuable guidance for future studies aiming at an improvement of trans-
hydrogenation by ligand tuning.
The comparison with the neutral complexes [Cp*RuCl]4 and [CpERuCl] (E = COOMe)16,24 is equally
informative: the former is the only catalyst that leads to notable overreduction with formation of 13,
whereas neither [CpERuCl] nor any of the cationic species does. Since overreduction is known to
5
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proceed only if the carbene initially formed ligates and activates a second H2 molecule,13 the spectra
shown in Figure 2 provide yet another relevant information for reaction optimization. In any case,
one can conclude that less electron-rich [CpXRu] fragments disfavor associative downstream
processes leading to E-alkene formation, isomerization and/or overreduction.
1
Complementary to the information gained from the H PHIP NMR experiments about carbene
formation and stability, solid-state 13C NMR allows the electronic character of these organometallic
intermediates to be assessed.25,26,27 While isotropic chemical shifts (iso = (11 + 22 + 33)/3) are
obtained by solution NMR, the three principal components of the chemical shift tensor (11 ≥ 22 ≥
33) can be determined by measurements in the solid state. They are directly related to the shielding
tensor ii (ii = 푟푒푓  ii) which can be calculated by ab initio methods and de-convoluted into
푖푠표
diamagnetic and paramagnetic terms (ii = dia + para).28 While dia is largely related to core electrons
and leads to shielding, the paramagnetic contributions para cause deshielding and arise from
magnetically induced coupling of the electronic ground state to excited states by action of the
̂
angular momentum operator 퐿i. The paramagnetic deshielding is particularly pronounced for an
energetically high-lying ground state and low-lying excited states, or in other words frontier
molecular orbitals (FMOs). In a pictorial view, a strong deshielding of a given nucleus occurs along a
direction i if a high-lying occupied orbital on this nucleus can be superimposed onto a low-lying
vacant orbital on the same nucleus by rotation along the axis i. For example, in a metal carbene the
large deshielding of the 11 component is mainly caused by a coupling of the high-lying occupied
̂
(M-C) orbital with the low-lying vacant *(M=C) orbital by the 퐿1 operator (Figure 4a). As the extent
of deshielding increases with a decreasing energy gap between these (frontier) orbitals, it is strongly
correlated with the nature of the M=C bond. The principal components of the chemical shift tensor
hence constitute an experimental probe for relative energy and orientation of frontier orbitals of a
given carbene fragment. For instance, the rather low-lying *(M=C) orbital of a Fischer carbene
entails massive deshielding, especially of 11, while nucleophilic Schrock alkylidenes experience a
significant contribution of their high energy (M=C) orbital to 11. Based on such patterns, members
of these families can a priori be distinguished on spectroscopic grounds.25-27
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Figure 3. Complexes subjected to CST analysis
This powerful analytical tool was applied to complexes 14 - 18 (Figure 3), which correspond to the
stationary points A - D of the reaction coordinate of gem-hydrogenation; the computed data were
calibrated with the experimental values available for 14, 17 and 18. It has previously been noted that
the C-atoms of 1-(prop-1-yn-1-yl)cyclohexan-1-ol (iso = 84/80 ppm) experience large but unequal
deshielding upon formation of complex 14 (iso = 159/130 ppm; Figure 4b).29,30 This fact was nicely
reproduced by DFT calculations, which indicate that this effect originates from the low lying LUMO
essentially composed of a vacant metal d-orbital interacting with the (CC) bonding orbital (see the
SI for more details). Upon subsequent binding of H2, the -ligand in the “loaded” catalysts 15 regains
partial alkyne character, however with principal components of the chemical shift tensor that are
indicative of significant metal-alkyne interactions (see the SI). This result is in excellent accord with
previous NMR studies on ruthenium catalyzed trans-addition reactions in solution, which emulated
such loaded species by formal replacement of reactive HE (E = H, SiR3, SnR3) by unreactive 2-
electron donor ligands (MeCN, pyridine, Me3P). In all cases investigated were the recorded 13C shifts
of the coordinated triple bond in complexes analogous to 15 very similar to those of the free
alkyne.29,30
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Figure 4. a) Typical orientation of the chemical shift () and shielding () tensor in metal carbenes
and main orbital couplings leading to the large deshielding of 11/11. b) Calculated chemical shift and
shielding tensors of C1 and C2 of the -complex 14 and corresponding values for the C2 atom of the
cyclopropylidene complex 16. In the 3D plots of the shielding tensors, blue lobes represent shielded
regions ( > 0) while orange lobes represent deshielded regions ( < 0). c) Calculated shielding
tensors, simulated NMR spectrum and orbital analysis of the C1 carbon in the ruthenacyclopropene
16 (top), the carbene-carbon in the neutral carbene 17 (middle) and the carbene-carbon in the
cationic carbene 18 (bottom). Upon going from 16 to 17 to 18, the contributions of the (M–C)
orbital to deshielding increase, while the contributions of the (M=C) orbital decrease, indicating a
more pronounced electrophilic character (due to the lower-lying *(M=C) orbital) along this series.
All chemical shift and shielding values are reported in ppm.
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Transfer of the first H-atom to the -bond evokes a considerable change in electronic structure.
Specifically, the resulting complex 16 shows markedly different CST’s at the two carbon atoms: C2,
which has accepted this first H-atom, becomes notably shielded, rather isotropic and fairly sp3-like
(Figure 4b, right). In stark contrast, C1 is significantly deshielded with a highly anisotropic chemical
shift tensor (Figure 4c, top); it actually resembles a true carbene center, indicating a significant
double bond character in the RuC1 bond (see the SI). This point is nicely illustrated by the
comparison of the computed spectra of 16 and 17 shown in Figure 4c (top and middle). Complex 16
is hence best described as a “1-ruthenacyclopropene” rather than a “2-vinylruthenium complex”,
since the latter term carries the potentially misleading connotation of significant CC double bond
character.31
Transfer of the second hydrogen atom to C2 completes the gem-hydrogenation event and affords
carbene 17.13 Gratifyingly, this complex proved sufficiently stable to allow the low intensity signal of
the carbene center to be detected by solid-state NMR (see the SI); the calculated shifts agree with
the experimentally obtained values.32 CST analysis shows the expected strong deshielding of 11
perpendicular to the Ru=C bond, as characteristic for metal carbenes (Figure 4c, middle). This
deshielding is mainly associated with a strong coupling of the (RuC) bonding orbital with the low-
lying *(RuC) orbital. On top, the (RuC) orbital shows a significant contribution to 11, implying
that this particular orbital is sufficiently high in energy to couple with the vacant *(RuC) orbital.
This contribution is small for classical Fischer carbenes but had been observed for Grubbs carbenes
before.26 Likewise, the orbital contributions to the 22/22 components show a pattern reminiscent of
Grubbs catalysts,26 because they mostly involve magnetic coupling of the (CC) and (CC’) orbitals
to the vacant *(RuC) orbital (see the SI).
To probe the influence of positive charge as well as of a less electron rich CpX ligand on the chemical
nature of the resulting carbenes, solid-state NMR spectra of the cationic complex 18 were recorded
and analyzed, even though the short lifetime of this species even at low temperatures rendered data
acquisition difficult (see the SI).32 As expected, all principal components of the chemical shift tensor
are even more deshielded than in 17 (Figure 4c, bottom). For 11, this is mostly due to a more
pronounced coupling of the (RuC) orbital with the *(RuC) orbital, evidencing a low-lying and
largely C-centered LUMO of *(RuC) character, which in turn implies a strongly electrophilic
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character. Along the same line, one can deduce that the contribution of (RuC) to the deshielding of
11 is smaller than in the neutral analogue 17, which again means that this orbital is lower in energy.

Taken together, these features draw the portrait of 18 as a fairly typical Fischer carbene complex,
even though it lacks a heteroatom substituent at the carbene center. Moreover, the comparison of
17 and 18 shows that the character of such pianostool ruthenium carbenes can be fine-tuned by
modulation of the donor properties of the CpX ligands.
We conclude that all tested [CpXRu] catalysts were able to engage propargylic alcohol derivatives in
gem-hydrogenation with formation of ruthenium carbene complexes. Importantly, however, the
barriers for this unorthodox transformation can be lowered and the stability of the resulting
complexes be notably increased by choosing less electron donating CpX rings; this correlation is
arguably relevant for applications of reactive intermediates of this type in catalytic applications other
than hydrogenation. Their spectroscopic signatures indicate a tunable electrophilic character of the
M=C bond with some similarities to classical Grubbs carbenes. One might hence expect such
pianostool carbenes to participate either in cyclopropanation or metathesis reactions. As will be
shown in the accompanying paper, they can do both, even though a somewhat counterintuitive
structure/activity relationship raises interesting mechanistic questions.
Acknowledgements
CPG acknowledges the Scholarship Fund of the Swiss Chemical Industry for funding. Generous
financial support by the MPG is gratefully acknowledged. We thank Mrs. K. Radkowski for valuable
assistance and advise, Dr. C. Werlé, MPI for Chemical Energy Conversion, Mülheim, for allowing us to
record the cyclic voltammograms, Mr. J. Rust and Prof. C. W. Lehmann for solving the X-ray
structures contained in the SI, Dr. D.-A. Rosca for helpful discussions, and the analytical departments
of our Institute for excellent support.
10
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For the Table of Contents
Portrayal: Less electron-donating cyclopentadienyl (CpX) ligands lower the barrier for the perplexing
gem-hydrogenation of alkynes by [CpXRu] complexes and kinetically stabilize the resulting pianostool
ruthenium carbenes. The chemical shift tensors extracted from solid state 13C NMR spectra draw the
portrait of electrophilic carbenes of the “Fischer-type” with a certain flavor reminiscent of Grubbs
catalysts.
Keywords: Carbenes Complexes  Chemical shift tensors  NMR Spectroscopy  parahydrogen 
Ruthenium
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16
The established synthesis routes give only access to cationic complexes; for details concerning
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the structurally-defined, chloride bridged neutral complexes [Cp*RuCl]4 and [Cp^RuCl]2 (Cp^ =
5-1-methoxy-2,4-tert-butyl-3-neopentyl-cyclopentadienyl) do not work well with other CpX
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Sun, Y.-D. Wu, J. Am. Chem. Soc. 2013, 135, 13835-13842.
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19
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20
21
22
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23
24
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Signal enhance by the PHIP effect mandates pairwise transfer of the H-atoms of H2, cf. ref. 20-
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Control experiments showed that it makes no difference whether the active catalyst [Cp*RuCl]
is generated by disassembly of preformed [Cp*RuCl]4 or is formed in situ by mixing
[Cp*Ru(MeCN)3]PF6 and nBu4NCl, see the Supporting Information; [CpERuCl] (E = COOMe) was
generated in situ from [CpERu(MeCN)3]PF6 (9)/nBu4NCl.
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10.1002/anie.201904255
Angewandte Chemie International Edition
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The experimental determination of the components of the chemical shift tensor is complicated
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difficult to fit.
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