Computational discovery of stable transition-metal vinylidene complexes

Article abstract of DOI:10.1021/om500114u

Experimental results have long suggested that catalyst optimization is an inherently multivariate process, requiring the screening of reaction conditions (temperature, pressure, solvents, precursors, etc.), catalyst structure (metal and ligands), and substrate scope. With a view to demonstrating the feasibility and utility of multivariate computational screening of organometallic catalysts, we have investigated the structural and electronic properties of a library of transition-metal-coordinated alkyne and vinylidene tautomers in different coordination environments. By varying the substituents on the organic moiety of 60 alkyne/vinylidene pairs we were able to capture and quantify the key structural and electronic effects on tautomer preference. For a carefully selected subset of substituents, the effects of metal and ancillary ligands were then explored. We have been able to formulate a protocol for assessing the stabilization of vinylidenes in transition-metal complexes, suggesting that the d⁶ square-based-pyramidal metal fragment [RuCl₂(PR ²₃)(=C=CHR¹)], combined with electron-withdrawing substituents R¹ and electron-rich groups R ², would provide the ideal conditions favoring the vinylidene form thermodynamically.

Full text of DOI:10.1021/om500114u

Article
pubs.acs.org/Organometallics
Computational Discovery of Stable Transition-Metal Vinylidene
Complexes
Oliver J. S. Pickup, Iman Khazal, Elizabeth J. Smith, Adrian C. Whitwood, and Jason M. Lynam*
Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K.
Keshan Bolaky, Timothy C. King, Benjamin W. Rawe, and Natalie Fey*
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
S
* Supporting Information
ABSTRACT: Experimental results have long suggested that catalyst optimization
is an inherently multivariate process, requiring the screening of reaction
conditions (temperature, pressure, solvents, precursors, etc.), catalyst structure
(metal and ligands), and substrate scope. With a view to demonstrating the
feasibility and utility of multivariate computational screening of organometallic
catalysts, we have investigated the structural and electronic properties of a library
of transition-metal-coordinated alkyne and vinylidene tautomers in different
coordination environments. By varying the substituents on the organic moiety of
60 alkyne/vinylidene pairs we were able to capture and quantify the key structural
and electronic effects on tautomer preference. For a carefully selected subset of
substituents, the effects of metal and ancillary ligands were then explored. We have
been able to formulate a protocol for assessing the stabilization of vinylidenes in
transition-metal complexes, suggesting that the d6 square-based-pyramidal metal
fragment [RuCl2(PR2 )(CCHR1)], combined with electron-withdrawing substituents R1 and electron-rich groups R2, would
3
provide the ideal conditions favoring the vinylidene form thermodynamically.
and accurate computational studies.10 However, high-through-
put computation combined with rigorous statistical data
analysis can be used to screen catalyst designs efficiently (see
INTRODUCTION
■
The syntheses of novel organic compounds and natural
products increasingly rely on transformations catalyzed by
for example ref 6b), potentially accelerating the discovery and
transition-metal complexes to access elaborate and valuable
ultimately the (rational) design of novel catalysts on the basis of
structural motifs.1 Key transformations lead to the formation of
computational prediction.10 To date, such computational
carbon−carbon and carbon−heteroatom single and multiple
studies have focused on a single variable, usually the ligand,9
bonds through atom-economical2 and highly selective addition
but catalyst optimization is inherently multivariate.11 While
and cyclization reactions. Metal coordination of substrates
reaction conditions are perhaps best varied experimentally, the
facilitates their activation, while suitable ancillary ligands on the
interplay among metal, substrate, and ancillary ligands in
metal center can be used to fine tune the activity and selectivity
of catalysts.3
determining the properties of organometallic complexes can be
captured computationally; with a view to demonstrating the
The Nobel Prize winning, metal-catalyzed metathesis
(2005)4 and cross-coupling (2010)5 reactions are widely
utility of multivariate computational screening, we have
evaluated the effect of these three variables on the
used, and their ongoing development is underpinned by
catalyst screening6 and detailed mechanistic studies.7 For less
tautomerization (Scheme 1) between alkynes and their
vinylidene analogues.
The energy balance between alkynes and vinylidenes was
selected for this study, as alkynes have been demonstrated to
well understood transformations, catalyst development can be
more difficult; in such cases, computational studies can be used
to test and refine mechanistic proposals7a,8 and to explore the
bind to nearly all transition metals, whereas there are relatively
thermodynamic and kinetic consequences of modifications to
fewer examples of stable vinylidene complexes. In addition,
both catalyst and substrate on the mechanism and performance
while the reactivity of organometallic η2-coordinated alkyne
complexes can resemble that of alkenes, e.g. in metathesis,1 the
reactivity of vinylidene complexes is different from that of either
of catalysis.9
Such computational studies of reaction mechanisms continue
to pose notable challenges, as capturing mechanistic manifolds
and accommodating the structural and conformational
variability of complexes with sufficient accuracy to reproduce
and then predict experimental observations require extensive
Received: January 31, 2014
Published: March 18, 2014
© 2014 American Chemical Society
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Scheme 1. Alkyne/Vinylidene Tautomerization and
Difference in Reactivity on Nucleophilic Attack
Scheme 3. Summary of Gold-Catalyzed Reactions of
Terminal Alkynes Where Gold Vinylidene Intermediates
Have Been Postulated
the free or coordinated alkyne,12 which has been exploited in
carbon−carbon and carbon−heteroatom bond formation.13
The binding of both alkynes and vinylidenes to an electrophilic
metal center increases their reactivity toward nucleophiles, with
nucleophilic attack of alkynes giving Markovnikov addition,14
whereas vinylidenes give rise to net anti-Markovnikov products
(Scheme 1).13c,15
This behavior may be rationalized on the basis of the MO
overlap diagrams for the respective metal-coordinated
tautomers (see the Supporting Information, Figure S1). In
the case of an alkyne, the LUMO is centered on the ligand π*-
orbital and nucleophilic attack may be assisted by slippage of
the metal.16 For a vinylidene, the LUMO is primarily localized
on the α-carbon, thus promoting nucleophilic attack at this
site.17
Computational studies to date suggest that the mechanism of
interconversion may vary with coordination environment,12a,c,24
making a large-scale mechanistic study of the rearrangement
kinetics for multiple tautomeric pairs in different environments
computationally challenging. However, determining the
thermodynamic tautomer preference is less expensive computa-
tionally and could provide insights which will allow synthetic
efforts to be targeted, as well as providing a quantitative guide
for complex/catalyst screening and optimization, similar to the
ligand knowledge bases previously developed in Bristol.25
Here we have investigated the structural and electronic
properties of a library of alkyne and vinylidene tautomer pairs
coordinated to a variety of transition-metal complexes
computationally, with density functional theory (DFT). Our
goal was to explore how substituents on the organic moiety and
changes to the coordination environment afforded by different
transition-metal complexes and ligands would attenuate the
thermodynamic energy balance between the two tautomeric
forms. We note that kinetic effects, not considered here, will
also affect whether the vinylidene tautomer can be accessed
experimentally; in addition, the alkynyl hydride form may be
accessible for monosubstituted pairs in some coordination
environments,26 potentially complicating computational pre-
diction further. We also discuss how high-throughput multi-
variate computation can be used to propose strategies for the
stabilization of the vinylidene tautomer by coordination of
transition-metal complexes.
A number of recent developments have widened the
synthetic scope of these tautomeric pairs. In 2008 Ishii and
co-workers demonstrated that half-sandwich ruthenium com-
plexes promote the formation of disubstituted vinylidene
complexes from internal alkynes (Scheme 2).18 The migration
Scheme 218
RESULTS AND DISCUSSION
■
of keto substituents within alkynes PhCCCOR (R = Me, Ph)
to form vinylidene complexes MCCPh(COR) has also
been observed, a process which proceeds via an η1-OCR-
bound intermediate.19
Experimental Survey. An examination of the available
experimental data for transition-metal vinylidene complexes
indicates that both the metal and the substituent on the
vinylidene affect the nature of their interaction. Table 1
Transition-metal vinylidene complexes have been invoked in
a number of gold-catalyzed reactions of terminal alkynes
(Scheme 3).20 To date, no gold vinylidene complexes have
been directly observed, although a gold(I) complex with a
related allenylidene ligand has been reported,21 as have other
group 1022 and group 1123 species. In these cases the
allenylidene ligand is supported by strong π-donor substituents
and may be better described as a substituted propargyl cation.21
In view of the potential synthetic utility of controlled
activation of alkyne and vinylidene tautomeric pairs from
readily available alkyne precursors, a quantitative understanding
of the different factors which control the tautomer preference
can provide important insights into (1) the regiochemical
outcome of nucleophilic attack, (2) the availability of
disubstituted vinylidene complexes, and (3) how vinylidene
complexes may be supported by late-transition-metal centers.
Table 1. Selected 13C NMR Data for Vinylidene Complexes
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Figure 1. Correlation between the changes in chemical shift of the α-C and the relevant Hammett substituent parameter30 from systematic studies of
transition-metal compounds: [(p-cymene)Ru(μ-Cl)3RuCl(CCHAr)(PCy3)] (1, where Ar = C6H5 (1a), C6H4-4-OMe (1b), C6H4-4-Me (1c),
C6H4-4-Cl (1d), C6H4-4-CF3 (1e), C6H3-3,5-CF3 (1f));31 [Ru(η5-C9H7)(PPh3)2(CCHR)]+ (2, where R = H (2a), C6H4-4-OMe (2b), C6H4-4-
Me (2c), C6H4-4-Ph (2d), C6H4-4-F (2e), C6H4-4-Cl (2f), C6H4-4-I (2g), C6H4-4-MeCO (2h), C6H4-4-NO2 (2j));24d trans-[Ru(κ2-O2CR1)(κ1-
O2CR1)(PPh3)2(CCHAr2)] (5, where R1 = Me and Ar2 with R3 = H (5a), 4-NMe2 (5b), 4-OMe (5c), 4-Me (5d), 4-F (5e), 4-CF3 (5f), 3-Me
(5g); 6, where Ar2 = Ph and Ar1 with R2 = H (6b), 4-NMe2 (6c), 4-OMe (6d), 4-Me (6e), 4-F (6f), 3-F (6g), 3-Me (6h)) (Scheme 4). Trendline:
R2 = 0.998 (1), 0.745 (2), 0.942 (5), 0.820 (6).
a
contains selected 13C NMR data for a number of representative
Scheme 4. Compounds Reported in This Work
vinylidene complexes from groups 6−9. Although care must be
taken in the interpretation of the absolute chemical shift of
metal-bound carbon atoms,12b,27 it is clear that the wide
variation (a range of 137.4 ppm in the examples below) in the
chemical shift of the metal-bound carbon atom indicates that
the nature of the metal complex is profoundly affecting the
interaction with the vinylidene ligand.
The substituent on the vinylidene also appears to affect the
nature of the interaction between metal and ligand, which can
be probed via the chemical shift of the carbon atom bound to
the metal. Delaude has shown that in a series of compounds
[(p-cymene)Ru(μ-Cl)3RuCl(CCHAr)(PCy3)] (Ar =
C6H4-R, 1b−f; Figure 1) it is possible to correlate the
Hammett substituent parameter30 with the chemical shift of
the α-carbon atom in comparison to compound 1a (Ar =
C6H5).31 In addition, data reported by Bassetti24d show a
similar correlation for complexes based on [Ru(η5-C9H7)-
a
(PPh3)2(CCHAr)]+ (Ar = C6H4-R, 2b−j; Figure 1), in
Legend: (i) CH2Cl2, room temperature.
this case relative to compound 2a, [Ru(η5-C9H7)(PPh3)2(
CCH2)]+.
However, in the cases where the electronic demand of the
In order to investigate this correlation further, we have
prepared a series of compounds based on trans-[Ru(κ2-
O2CR1)(κ1-O2CR1)(PPh3)2(CCHR2)] (5) by reaction of
the carboxylate complexes cis-[Ru(κ2-O2CR1)2(PPh3)2], (3)
with appropriate alkynes HCCR2 (4) (Scheme 4, see the
Supporting Information for experimental details).8b,32 The 13C
chemical shifts of the α-carbon atom for complexes trans-
[Ru(κ2-O2CMe)(κ1-O2CMe)(PPh3)2(CCHR2)] (5b−f)
relative to the parent phenyl complex 5a exhibit the same
trend as the data reported by Delaude and Bassetti (Figure 1),
showing a negative correlation with the Hammett parameter.
carboxylate complexes is systematically altered (6b−h), the
opposite trend in 13C NMR chemical shifts is observed, giving a
positive correlation. In addition, the chemical shift of the metal-
bound carbon in trans-[Ru(κ2-O2CMe)(κ1-O2CMe)(PiPr3)2(
CCHPh)] (5aiPr), prepared from the reaction of cis-[Ru(κ2-
O2CMe)2(PiPr3)2] with HC2Ph, lies 2.8 ppm to higher field
than the corresponding resonance in 5a.33
Assuming that the chemical shift of the α-carbon of the
vinylidene ligands is broadly indicative of the nature of the
interaction between the metal and the ligand, a number of
trends are evident. Notably, it appears that metal complexes
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which contain more electron-withdrawing substituents exhibit
lower field chemical shifts in comparison to those that are
electron rich (compare, for example, chromium and iridium
vinylidene complexes (Table 1), and the positive slope of the
Hammett plot for compounds 6b−g). The opposite trend is
observed when electron-withdrawing groups are present within
the vinylidene ligand (note the negative slope of the Hammett
plot for compounds 1 and 2 and the difference in Δδ between
5a and 5aiPr).
effects of substituents, metal centers, and ligands iteratively
(Figure 2).
These experimental data confirm that metal−ligand inter-
actions may be affected by both metal fragment and
substituents on the vinylidene ligand and these interactions
are, presumably, also affecting the difference in energy between
vinylidene and alkyne tautomers.
Design of Computational Database. As outlined above,
this work describes a large-scale computational study of the
factors controlling the difference in energy between tautomers;
interconversion pathways and barriers were not considered but
also affect experimental access to tautomers. The general
requirements for such a study, with a view to achieving both
interpretation and prediction of ligand effects, have been
discussed extensively elsewhere25b and can be summarized in
brief as follows: the chosen computational approach (see the
Supporting Information for details) should perform consis-
tently across the complexes studied; quantitative agreement
with e.g. binding energies is less important than the correct
order of variable effects, at least when undertaking statistical
analysis (Table S1 in the Supporting Information summarizes
an evaluation of functional effects on tautomer energy
differences). In addition, the substitution patterns studied
should seek to capture as wide a chemical range as possible,
thus ensuring that chemical space is sampled well. Likewise,
transition-metal complexes should be chosen with a view to
maximizing transferability to varied chemistry and the
parameters used for analysis should be responsive to complex
modifications.
Figure 2. Schematic illustrating the exploration of different variables in
the present database.
Substituent Effects. Substituent effects have been probed
by calculating property descriptors for 32 monosubstituted and
28 disubstituted tautomer pairs of alkynes and vinylidenes
(Scheme 5; see the Supporting Information, Table S2, for
Scheme 5. Alkyne and Vinylidene Tautomer Pairs
These criteria still apply, but in the present work additional
considerations arise. Rather than considering the effect of
alkynes and vinylidenes as ligands, i.e. focusing on the
modification of the properties of transition-metal centers, the
tautomeric pairs considered here are of greater interest as
substrates in further transformation and the motivation for the
screening of multiple substituents has changed: here the
structure and form of the substrate itself are of greater interest
than the effect of its coordination on the rest of a complex. This
affects which structural and energetic parameters are most
responsive to modifications and hence useful in statistical
analysis. In addition, treating vinylidenes and alkynes as a pair
highlights substituent effects on the energetic balance between
them, as well as structural effects arising from substituents and
coordination environment (see the Supporting Information for
a more detailed discussion). Finally, π-coordinated ligands have
not previously been included in ligand knowledge bases and
have necessitated minor modifications to both calculation
protocols and the data extracted; these have been summarized
in the Supporting Information.
detailed numbering); in all subsequent discussion, the binding
modes of substrates will be denoted by “a” and “v”, respectively,
to denote the different tautomers.
For monosubstituted ligands, a broad range of alkyl
substituents allows the consideration of steric effects, while
aryl groups with different para substituents facilitate the
consideration of electronic effects free of steric variation,
suitable for comparison with experimental data18a,d,24d and in
particular Hammett parameters.24d In addition, a range of
ester34 and keto19a groups, as well as representative iodine,35
silicon,36 tin,37 and sulfur38 substituents, for which migration
from an alkyne precursor to the vinylidene tautomer have been
reported, have been included. The tautomers have been
considered both as free ligands and in a range of representative
coordination environments (see the Supporting Information for
further information on computational approaches and settings
used).
Two of the complexes (Au-1 and Ru-1-H; Scheme 6) used
for descriptor calculations have been described previously for
the ligand knowledge base for carbenes and other C-donor
ligands (LKB-C),25a but the third complex considered in that
work, [PdCl3L]− (Pd; Scheme 7) gave a strong predicted
preference for the η2-alkyne tautomer for most substituents, of
less interest in the present study.
In their uncoordinated form, alkynes are between 140 and
260 kJ mol−1 more stable than the corresponding vinylidenes,
but a careful choice of substituents, transition metal, and
ancillary ligands can be used to favor the vinylidene tautomer
on coordination. With a view to separating these effects, the
present database consists of three branches, capturing the
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Scheme 6. Metal Complexes Considered in Substituent
Effect Branch
Scheme 7. Metal Complexes Explored in Metal Effect Branch
a
for Pairs 2, 15, 36, 42, 45, and 60
Figure 3. Scatter plot of tautomer energy differences (ΔEv‑a, kJ mol−1)
for metal complexes considered in the substituent effect branch.
Trendline: R2 = 0.709 (Ru-1-H), 0.679 (Rh).
more electron withdrawing groups generally make the vinyl-
idene tautomer more favorable, as does monosubstitution. In
addition, for the late-transition-metal fragment Au-1, most
substituents favor the η2-alkyne, while the vinylidene form is
clearly favored for Ru-1-H throughout. Substrates complexed
to the Rh fragment are closer to the energetic borderline, and
both tautomers may be energetically accessible, provided a
suitable selection of substituents can be accessed. This is
consistent with the observation that both tautomers may be
observed when coordinated to trans-[RhCl(PiPr3)2] frag-
ments.26b Substituents 60 (F/F) give rise to the only
tautomeric pair which favors the vinylidene form for all metal
complexes (Table S4, Supporting Information),42 and for the
other monosubstituted halides (pairs 26−29) only the Au-1
complex is predicted to favor the η2-alkyne form, while the
other complexes are also found to stabilize the vinylidene
tautomer. The substituents also affect the geometries of each
substrate, showing high correlations between the CC/CC
bond lengths of tautomer pairs, and these effects are
transmitted to the metal fragment, affecting fragment geo-
metries, such as metal−ligand bond lengths and angles (see
Table S5 (Supporting Information) for details).
While the analysis of bivariate correlations and scatter plots
can help to explore substituent effects for individual descriptors,
consideration of the whole database is more difficult and
multivariate approaches need to be employed. Principal
component analysis (PCA) is a statistical projection method43
which can be used to process a multivariate and correlated data
set to fewer orthogonal, i.e. uncorrelated, variables (principal
components, PCs). The resulting PCs are linear combinations
of the original variables, derived to maximize the variance/
information content in the first few PCs. This gives rise to a set
of principal component scores for each tautomeric pair, which
can be plotted to produce “maps” of chemical space where the
proximity of scores indicates similarity.
a
See Table S2 in the Supporting Information for substituent
numbering.
For the substituent screen, this has been replaced by a
square-planar rhodium complex, trans-[RhCl(PMe3)2L] (Rh),
which provides a similar coordination geometry and allows the
extraction of trans influences but has been found to support the
vinylidene form.
Experimentally, the vinylidene tautomers have been observed
for complexes related to the fragments [RuCl2(PR3)2] (Ru-1-R;
R = H, Me)39 and [RhCl(PMe3)2] (Rh),26b,36a,40 albeit with
larger substituents on the phosphine ligands. In contrast, η2-
alkyne complexes have been observed for the gold fragment
[AuCl] (Au-1).41 Scheme 6 shows the metal complexes
considered in this branch, and a summary of the parameters
extracted from these calculations may be found in Table S3
(Supporting Information); these include CC/CC and M−
C bond lengths, tautomer energy differences, bond energies,
and structural parameters for the metal fragments.
The energy differences between vinylidene and alkyne pairs
have been calculated for the complexes considered in the
substituent effect branch of the database. For each of the metal
complexes considered, these energy differences span consid-
erable ranges (between 67.8 and 103.5 kJ mol−1; Table S4
(Supporting Information)) and are reasonably highly correlated
(Figure 3; correlation coefficients R = 0.842 (Au1/Ru-1-H),
0.824 (Au1/Rh), 0.923 (Ru-1-H/Rh)).
Here we have performed PCA of the correlation matrix on
the 35 variables in Table S3 (Supporting Information). While
these 35 initial descriptors will give rise to the same number of
derived variables (PCs), the first few are sufficient to capture
most of the information content/variation, and Figure 4 shows
such a map for the first two PCs, in this case already capturing
about 67% of the variation in the data set, while Table S6
(Supporting Information) collects descriptor loadings and
Table S7 (Supporting Information) includes the principal
These results suggest that substituent effects are broadly
consistent across different coordination environments and that
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Figure 4. Principal component score plot (PC1 and PC2) for all tautomeric pairs (Table 1), capturing 67% of variation in the data. Pairs selected for
exploration of metal and ligand effects are shown in red squares.
component scores for each tautomeric pair. The Supporting
Information includes a larger version of the PCA map discussed
here (Figure S2) with all ligand numbers as shown in Table S2,
as well as further details of the PCA.
This map of chemical space confirms expected chemical
similarities, e.g. for the alkyl- and aryl-substituted pairs (black
and blue colors in Figure 4), PC scores cluster, but also
highlights outliers, such as pairs 60 (F/F) and 39 (CF3/CF3),
both with very electron-withdrawing substituents, pairs 31 and
58 (H/SnMe3 and Ph/SnMe3; see discussion below), and to a
lesser extent the diester-substituted systems 40 and 41
(CO2Me/CO2Me, CO2Et/CO2Et).
suggesting that the main difference between these two subsets
has been captured.
Inspection of trends across the map in terms of likely
substituent electronic effects suggests that tautomeric pairs with
more electron withdrawing substituents appear at lower/more
negative values of PC1. As indicated above, PC2 expresses a
difference between mono- and disubstitution, in line with the
greater relative stabilization of vinylidenes in the monosub-
stituted pairs. Figure 5 highlights the effect on tautomer energy
preference for ΔEv‑a(Ru-1-H).
The fact that the tin-containing complexes appear as outliers
prompted us to explore their properties in more detail. In both
pairs 31 and 58 the vinylidene complexes have very short CC
distances and the optimized geometries show a distortion
around the β carbon (cf. other alkyl substituents). The
difference in behavior highlighted by PCA is supported by
experimental data. For complexes based on the [Mn(η5-
C5H4R1)(R2 PCH2CH2PR2 )(CCR3R4)] framework, the
2
2
13C NMR spectra for the carbon atoms exhibit resonances at δ
320−330 when a tin substituent is present, whereas when alkyl-
and aryl-containing groups are present, this resonance is at δ
>340.37b In addition, the crystal structure of [Mn(η5-C5H5)-
(Me2PCH2CH2PMe2)(CC(SnMe3)Ph)] shows a signifi-
cant distortion at the β-carbon atom, with the CαCβ−Sn
angle (115.2(4)°) much smaller than CαCβ−Ph (123.8(5)°).
This indicates that the tin atom is closer to the metal-bound
carbon, which, taken in conjunction with the conclusion from
Ozawa36c that the addition of SiMe3 groups increases the
stability of the vinylidene complexes through (σ−π)p hyper-
conjugation, may be related to the β-silicon effect.
The symbols chosen also illustrate differences between
mono- and disubstitution; monosubstituted pairs generally have
lower/more negative scores on PC2 than the related
disubstituted pairs (e.g., 17 and 47, 25 and 55). This becomes
more pronounced on consideration of the third PC (Figure S3,
Supporting Information); pairs appear in bands across PC2,
Figure 5. Principal component score plot (PC1 and PC2), with
continuous coloration according to ΔEv‑a(Ru-1-H) (kJ mol−1). More
negative values are displayed in shades of red and yellow and less
negative ones in shades of green and blue.
Analysis of experimental data, individual descriptors, and the
data set as a whole thus confirms the importance of substituent
effects, with more electron withdrawing substituents making the
vinylidene tautomer relatively more favorable.
The coordinated metal center also plays a substantial role in
determining the energy difference between tautomers, with the
Au-1 fragment showing a high energy threshold for vinylidene
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Figure 6. Calculated tautomer energy differences, ΔEv‑a (kJ mol−1), for different metal complexes (see Scheme 7 for details): (a) results for pair 15
(H/Ph); (b) mean energy differences ΔEv‑a (kJ mol−1) and their standard deviations for all six substrate pairs considered (2, 15, 36, 42, 45, 60; see
the Supporting Information for full data).
stabilization while this tautomer becomes accessible for the Rh
and Ru-1-H fragments. For the rhodium complex Rh, the
tautomer energy differences are not large and substituent effects
are sufficient to change the calculated preference, with
monosubstituted pairs favoring the vinylidene form while
most of the disubstituted substrates are expected to favor the
alkyne.
Metal Effects. While modification of the substituents in
tautomeric pairs can be used to access a broad range of
substrate properties, the transition-metal center itself appears to
determine the relative threshold for vinylidene stabilization in
each tautomeric pair (Figure 3). In order to explore this more
extensively, a subset of six substrate pairs was selected, chosen
to capture the substituent effects discussed above and to reflect
key experimental data. The pairs selected are highlighted on the
substituent map (Figure 4, red squares); they access different
areas of chemical space and are representative of both mono-
and disubstituted pairs.
The effect of changing from an alkyl to an aryl substituent in
the monosubstituted substrate is captured by including 2 (H/
Me) and 15 (H/Ph) and comparison of 15 with 42 (Ph/Ph)
allows consideration of mono- vs disubstitution effects. Since
keto- and ester-substituted pairs are sensitive to conformational
noise, especially for the disubstituted pairs (see computational
details in the Supporting Information), 36 (Me/CO2Me) was
chosen as a simple example; spontaneous conversion from
alkyne to vinylidene was observed experimentally for the
related pair 40 (CO2Me/CO2Me).18a In addition, 45 (Ph/
C(O)Ph) was included both because spontaneous migration
from alkyne to vinylidene was observed for these substituents in
a RuCp(PPh3)2 fragment19,44 and because we wanted to
explore the effect of keto substitution. Finally, pair 60 (F/F) is
the only example for which the vinylidene has been preferred in
all metal complexes considered so far.
For these pairs, a wider range of metal fragments, shown in
Scheme 7, were considered to probe the effects of the
transition-metal group (e.g., Cr/Mo, Pd/Pt, Ag/Au-1) and
period (e.g., Mn/Ru-2-CO) on the predicted tautomer energy
preferences. Of the fragments employed here, well-defined
vinylidene complexes are known for [Cr(CO)5] (Cr)45 and
[Mn(η5-C5H5)(CO)2] (Mn),46 as well as complexes related to
Rh and Ru-1-H, as discussed above. In contrast, Au-1 is known
to support η2-alkynes (discussed above), and so is [PdCl3]−
(Pd).47 The fragment [AuN((C3F7)C(Dipp)N2)],48 related to
Au-3, is also known to support alkyne ligands bound in an η2
fashion; however, considering the number of times that η2-
alkyne complexes have been proposed as intermediates in gold-
catalyzed reactions, there is still a paucity of isolated examples
of this type of compound.41,48,49 The carbene fragment
[AuIMes]+ (Au-2) has been included here as a further gold(I)
complex representative of common catalyst precursors and is
likely to favor the η2-alkyne tautomer.
The tautomer energy preferences ΔEv‑a for the 6 pairs
selected have been calculated in the 11 complexes shown in
Scheme 7; no further structural or energetic data have been
considered in this part of the study.
Metal effect results for the representative substituents are
summarized in Figure 6, and full data tables have been included
in the Supporting Information. Figure 6a shows the results for
substrate pair 15 (H/Ph) and Figure 6b shows mean energy
differences and their standard deviations for the subset of six
substrate pairs considered here. The standard deviations shown
in Figure 6b are large, primarily a consequence of including pair
60 (F/F), which strongly favors the vinylidene form in all metal
complexes.
In line with experimental observations, these results confirm
that group 7−9 transition metals in low oxidation states are
most likely to stabilize the vinylidene tautomer, with later
transition metals increasingly favoring the alkyne form. Periodic
trends can be observed, and these are discussed in greater detail
in the Interpretation below.
Ligand Effects. Ligands represent the third possible
variable for the stabilization of the vinylidene tautomer in
different coordination environments. To evaluate whether
tautomer energy differences could be fine-tuned by the choice
of ancillary ligands on different transition-metal centers, ligand
effects within selected ruthenium complexes have been
explored further. Using the same subset of substituents as for
the metal branch, i.e. 2, 15, 36, 42, 45, and 60, the electronic
properties of coordinated ligands in the [RuCl2(PR3)2] (Ru-1-
R) and [Ru(η5-C5H5)(L1)2]+ (Ru-2-L) fragments have been
varied systematically (Scheme 8). Experimentally, a vinylidene
complex has been isolated for [Ru(η5-C5H5)(PMe3)2]+ (Ru-2-
PMe3).50 Again, only the effect on the tautomer energy
difference has been considered; these results have been
summarized in Figure 7, with full data tables included in the
Supporting Information.
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increased stabilization of the vinylidene form relative to the
alkyne in monosubstituted substrates is likely to contribute to
this differentiation. Inspection of a subset of sterically similar
substituents (small alkyl and para-substituted aryl) shows a
reasonably high linear correlation between tautomer energy
differences for mono- and disubstitution with these groups,
suggesting that the observed differences may indeed be
attributed mainly to steric differences. This is further supported
by similar substrate dissociation energies (BE) for mono- and
disubstituted versions of pairs, which are reasonably highly
correlated once steric effects are accounted for (see the
Supporting Information for details).
Scheme 8. Ligands Explored for Pairs 2, 15, 36, 42, 45, and
60 in Ligand Effect Branch for Ruthenium Complexes
The exploration of metal effects suggests that the greatest
preference for the vinylidene tautomer occurs on coordination
to group 8 metals, although both the manganese and rhodium
complexes also significantly stabilize the vinylidene. The
greatest stabilization is present in the d6 Ru-1-R complexes,
which appear to offer an optimal combination of π interactions
and orbital size/availability for the vinylidene form.
Although the chromium and molybdenum complexes also
have a d6 electronic configuration, albeit in an octahedral
geometry, their five carbonyl ligands are very good π acceptors,
reducing the scope for π back-bonding from metal to substrates
and hence relatively disfavoring the vinylidene tautomer. In
molybdenum, where π back-bonding is expected to be less
extensive than in the chromium case,53 the alkyne tautomer
becomes more favorable. When the manganese complex is
compared with the isostructural Ru-2-CO, again in the d6
electronic configuration, the increase of metal charge is likely to
reduce the extent of π back-bonding and thus the stabilization
of the vinylidene tautomer.
The group 10 and 11 complexes Pd, Pt-2, and Ag and all
gold complexes, Au-1, Au-2, and Au-3, favor the alkyne
tautomer for all substituents apart from 60 (F/F), although this
preference is quite small for the d10 complex Pt-2. The increase
in effective nuclear charge is likely to cause a contraction of d
orbitals, which may affect the efficiency of π interactions and
thus potentially reduce the capability for π back-donation.
While this has little effect on the alkyne, the additional
stabilization from this interaction appears to be more important
for the vinylidene tautomer, which becomes energetically
inaccessible. The progression from second- to third-row metals
in the d8 and d10 electronic configurations of isostructural
complexes Pd/Pt-1 and Ag/Au-1 counters the trend observed
for the group 6 metals (although first- to second-row changes
were considered), likely due to the contribution of relativistic
effects in the third transition series,54 which may also be
affected by additional shielding from the lanthanide contraction
on the extent of π interactions.
The magnitudes and ranges of substrate dissociation energies
are summarized in Table S5 (Supporting Information) and
appear to confirm these global observations. The alkyne
binding energies are relatively lower for Ru-1-H and higher
for Au-1, while vinylidene binding energies show the opposite
trend. The rhodium complex Rh, which has a d8 electronic
configuration in a square-planar coordination geometry, lies in
between: the metal center is more electron-rich than Ru-1-H
but, unlike Au-1, an empty d orbital may still be accessible for
ligand to metal π donation, albeit high in energy. This appears
to be reflected in the binding energies (Table S5)the
rhodium−alkyne bond dissociation energies are similar in range
and magnitude to those for gold−alkyne complexes, whereas
Figure 7. Ligand effects on calculated tautomer energy differences,
ΔEv‑a (kJ mol−1), for different ruthenium complexes (Scheme 8) and
substrate pairs 2, 15, 36, 42, 45, and 60 (see the Supporting
Information for data tables).
For the ruthenium complex family Ru-2-L, the tautomer
energy differences show increasing stabilization of the vinyl-
idene tautomer (more negative ΔEv‑a) as the ancillary ligands
become more electron rich. While the carbonyl-substituted Ru-
2-CO favors the alkyne tautomer for all substituents except 60
(F/F), the electron-poor phosphines in Ru-2-PF3 and Ru-2-
PH3 show improved stabilization of the vinylidene. The most
electron rich phosphine in this series, Ru-2-PMe3, achieves the
switch to make the vinylidene the favored form for all pairs
considered here. Likewise, while the vinylidene is favored for
Ru-1-H with all substituents, this preference grows when
replacing PH3 with PMe3 in Ru-1-Me. Considerable
modification/fine tuning of tautomer energy preferences can
thus be achieved by altering the ancillary ligands coordinated to
the transition-metal center, again in good agreement with the
experimental observations discussed above.
Interpretation. These data sets illustrate that the vinylidene
tautomer is favored by the presence of electron-withdrawing
substituents on the alkyne substrate and can be further
supported by the presence of electron-donating ancillary
ligands on the metal; meanwhile, the coordinated metal center
determines the magnitude and sign of the tautomer energy
preference. These results may be rationalized on the basis of the
MO overlap diagrams (Figure S1, Supporting Information),
which illustrate that the interaction between the metal and
vinylidene ligand is likely to be strengthened by increased π
back-bonding.51,52 The presence of electron-withdrawing
groups on the vinylidene and/or electron-rich ligands
coordinated to the metal center will promote metal to ligand
back-donation and hence stabilize the vinylidene tautomer.
This is particularly important, as vinylidenes are believed to be
better π acceptors than the corresponding alkynes.46a
As illustrated by the PCA maps, mono- and disubstituted
substrates occupy different regions of chemical space; the
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rhodium−vinylidene binding resembles the ruthenium−vinyl-
idene interactions more closely.
vinylidene compounds, our observations can perhaps guide
experimental searches in the right direction.
Consideration of the data shown in Figures 3, 6, and 7 also
reveals an important additional facet of these results: the
ordering of the relative stability of the vinylidene complexes in
comparison to that of the alkyne is intrinsically related to the
substituent and is essentially independent of the metal center.
The metal simply moves this sequence to either higher or lower
energy; hence, the correct choice of metal can ensure that the
vinylidene (or alkyne) tautomer is thermodynamically favored.
Indeed, on the basis of these results we can predict that
complexes based on the [RuCl2(PR3)2] fragment will have the
greatest preference for the vinylidene form; this can be further
supported by selecting electron-withdrawing substituents and
electron-donating ancillary ligands PR3.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF and .xyz files giving experimental
and computational details, crystallographic data, databases
evaluating substituent, metal, and ligand effects, further
diagnostic statistics, plots identifying all compounds and
sample input files for all complexes considered, and computed
Cartesian coordinates of representative, as well as unusual,
complexes discussed in this study. This material is available free
of charge via the Internet at http://pubs.acs.org. The .xyz file
may be opened as a text file to read the coordinates, or opened
directly by a molecular modeling program such as Mercury
(version 3.3 or later, http://www.ccdc.cam.ac.uk/pages/Home.
aspx) for visualization and analysis.
CONCLUSION
■
Experimentally, it has been demonstrated that catalyst
optimization is an inherently multivariate process, requiring
the screening of catalyst structure (metal and ligands), substrate
scope, and reaction conditions (temperature, pressure, solvents,
precursors, etc.). We have demonstrated here that computa-
tional chemistry can provide a valuable tool for the in silico
screening and evaluation of at least three of these variables,
exploring the effects of substituents, metal, and ligands on the
thermodynamic tautomer preference for metal-coordinated
alkyne and vinylidene pairs.
AUTHOR INFORMATION
Corresponding Authors
*J.M.L.: e-mail, jason.lynam@york.ac.uk; tel, +44 01904
322534.
*N.F.: e-mail, Natalie.Fey@Bristol.ac.uk; tel, +44 0117
3318260.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This approach can be used to identify not only unusual
substituents, such as the β-heteroatom stabilization observed
for tin (and, to a lesser extent, silicon), but also groups with
similar properties and to explore the range of structural and
electronic effects accessible by substituent variation. Consid-
eration of a small number of metal complexes in this initial
substituent screen has allowed us to identify the interplay
between substituent and metal effects, leading to the selection
of a subset of tautomeric pairs for which metal and ligand
effects were explored more fully. On the basis of the resulting
databases we have been able to formulate a protocol for the
stabilization of vinylidenes in transition-metal complexes,
We thank Dr. John Slattery (York) and Dr. Chris Russell
(Bristol) for helpful discussions of this work. J.M.L. thanks the
EPSRC for the awards of grant EP/H011455/1 and a DTA
studentship to E.J.S.. N.F. thanks the EPSRC for the award of
an Advanced Research Fellowship (EP/E059376/1). K.B.,
T.C.K., and B.W.R. thank the University of Bristol and the
Centre for Computational Chemistry for hosting their research
projects.
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