Agostic versus Terminal Ethyl Rhodium Complexes: A Combined Experimental and Theoretical Study

Article abstract of DOI:10.1021/acs.organomet.6b00036

Ethylene insertion into Rh-H bonds in complexes bearing an anionic fac-triphosphane ligand gives hydrido complexes, β-agostic species, or noninteracting ethyl derivatives depending on the reaction conditions. Several chemical equilibria between these species have been analyzed by NMR and DFT calculations, which revealed that they are mainly controlled by the entropy. Moreover, β-agostic species were found to be lower in enthalpy than the corresponding hydride-ethylene complexes, probably due to the steric pressure exerted by the bulky fac-triphosphane ligand.

Full text of DOI:10.1021/acs.organomet.6b00036

Article
pubs.acs.org/Organometallics
Agostic versus Terminal Ethyl Rhodium Complexes: A Combined
Experimental and Theoretical Study
Ana M. Geer,† Jose A. Lopez, Miguel A. Ciriano, and Cristina Tejel*
́
́
́ ́ ́
Departamento Química Inorganica, Instituto de Síntesis Química y Catalisis Homogenea (ISQCH), CSIC-Universidad de Zaragoza,
Pedro Cerbuna 12, 50009-Zaragoza, Spain
S
* Supporting Information
ABSTRACT: Ethylene insertion into Rh−H bonds in complexes
bearing an anionic fac-triphosphane ligand gives hydrido
complexes, β-agostic species, or noninteracting ethyl derivatives
depending on the reaction conditions. Several chemical equilibria
between these species have been analyzed by NMR and DFT
calculations, which revealed that they are mainly controlled by the
entropy. Moreover, β-agostic species were found to be lower in
enthalpy than the corresponding hydride−ethylene complexes,
probably due to the steric pressure exerted by the bulky fac-
triphosphane ligand.
A key feature of these particular M−alkyl moieties can be
related to their lability, providing (or not) a vacant site on the
coordination sphere of the metal, which, in turn, significantly
impacts the reactivity of the complex. Consequently, the study
of the dynamics of such species has been the focus of much
attention from both experimental14 and theoretical ap-
proaches,15 most of them related to the estimation of the
migratory insertion barriers in the context of the polymerization
of olefins.16
The subtle balance between geometric and electronic effects
on the strength of such weak interactions is not evident for late
transition metal complexes yet.4e Among others, some relevant
factors would include electronic characteristics of the metals,17
steric requirements such as the size of substituents on ligands,18
trans influence,19 or solvent effects.20
INTRODUCTION
■
Weak interactions between nonpolar C−H bonds and
transition metals have been proved to be of paramount
importance in organometallic chemistry and catalysis, con-
tributing to major developments in the field of C−H bond
activation and/or functionalization processes,1 as well as in
dehydrogenation reactions.2 Both versions, intermolecular (σ-
complexes) and intramolecular (agostic3 complexes) are
known, the latter being important intermediates connecting
two fundamental reactions in organometallic chemistry, namely,
the insertion of olefins into M−H bonds and the reverse one,
the β-hydrogen elimination. Nowadays, the significance of
agostic species becomes evident from the large body of
literature in which the multiple roles they can play is
enlightened.4 As a way of example, α-agostic interactions
have been proved to have a strong impact on the stereo-
specificity and rate of olefin insertion in polymerization;5 β-
agostic species often lead to C−H activation reactions,6 while
the γ-agostic ones have been reported to be crucial in the
stabilization of the propagating species in vinyl norbornene
polymerization.7 Moreover, a delicate balance between
electronic versus steric factors can tip the stability of α- versus
β-agostic compounds,8 or even between β- and γ-agostomers.9
Furthermore, longer range interactions such as the rare δ- and
ε-agostic ones are involved in uncommon intramolecular 1,4-,
metal migration or 1,5-σ bond metathesis, respectively.10 In
other instances, they are valuable intermediates connecting the
transition states that lead to C−H versus C−C activation
reactions,11 and also documented is their participation in the
stabilization of highly unsaturated intermediatessuch as T-
shaped d8-ML3 complexes12requiring two agostic interac-
tions in some cases.13
A survey of the literature revealed that rhodium complexes
bearing simultaneously hydride and olefin ligandsor their
isomeric alkyl-agostic structuresare quite scarce, being
limited to [Rh(C2H4)(H)(PiPr3)2],21 cyclopentadienylrhodium
complexes,22 rhodacarborane species,23 and the more recently
reported with pincer type ligands.14b,24
−
−
Anionic P-based tripodal ligands such as PhBP3 (PhBP3
=
−
PhB(CH2PPh2)3 ) have not been explored before in this field,
while they show the additional attractive of labeling three
positions in octahedral complexes (by means of the NMR
active phosphorus nuclei). Consequently, the “Rh(PhBP3)”
scaffold seems to be particularly adequate for the study of
dynamics undergone by Rh-agostic species. Additionally,
PhBP3− binds strongly to rhodium,25 allowing the development
Received: January 18, 2016
© XXXX American Chemical Society
A
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of a rich chemistry including oxygen activation,26 stabilization
of unusual tetrahedral environments for rhodium(I),27 multiple
RhN bonds with imido ligands,28 and catalysis such as the
selective hydrogenation of CC bonds in α,β-unsaturated
substrates,29 and coupling of aldehydes to esters,30 both
catalyzed by the highly reactive bis(hydride) complex [Rh-
(PhBP3)(H)2(NCMe)]. While relevant for catalysis, the easy
and fast hydrogen transfer of the hydride ligands in
[Rh(PhBP3)(H)2(NCMe)] to olefins prevents its use as
reagent for the stepwise study of olefin insertion reactions.
Therefore, attention was focused on the monohydride version
[Rh(PhBP3)(H)(NCMe)2]+ ([1]+), straightforwardly prepared
by protonation of one of the hydride ligands of [Rh(PhBP3)-
(H)2(NCMe)] in acetonitrile.25a Complex [1]+ combines a
single hydride ligand with two labile acetonitrile ligands,
making it a valuable precursor for the study of olefin
coordination to Rh(III) species and further insertion reactions.
Herein, we report a combined experimental and theoretical
study on this topic including a full picture of the fluxional
behavior undergone by the β-agostic complex [Rh(PhBP3)-
(CH2CH2-μ-H)(NCMe)]+ ([3]+), which in turn, sheds light on
the potential dynamics of such species. In addition, the
determining role of entropy and steric effects in the migratory
insertion of olefins into M−H bonds is also reported.
models. Stationary points for both complexes were located. The
calculated structure for the hydride bis(acetonitrile) complex,
[Rh(PhBP3)(H)(NCMe)2]+ ([1a]+), shows rhodium in an
octahedral geometry while rhodium displays a square pyramid
geometry in [2a]+, as commonly observed for d6-RhL5
compounds. Two protons of the phenyl groups in [2a]+ were
found to be placed in close proximity to rhodium (3.339 and
2.907 Å) providing, probably, some stabilization to this
intermediate.
Acetonitrile dissociation from [1a]+ was analyzed by
modeling the structures obtained from the separation of one
NCMe ligand from rhodium up to 7 Å. However, the transition
state TS-1 (Figure 1) could not be found since a continuum of
energy was obtained with no clear maximum. Nonetheless, the
difference in enthalpy between [1a]+ and [2a]+ from DFT was
found to be 17.8 kcal mol−1, a value that nicely fits to that
experimentally calculated (ΔH⧧ = 16.3
1 kcal mol−1).
Therefore, extrusion of acetonitrile from [1]+ is a feasible
process expected to occur at room temperature.
Reactions of Complex [Rh(PhBP3)(H)(NCMe)2]+ ([1]+)
with Ethylene. Saturation of a CD2Cl2 solution of [1]+ with
ethylene at −30 °C causes ethylene insertion into the Rh−H
bond to produce an equilibrium with the β-agostic complex
[Rh(PhBP3)(CH2CH2-μ-H)(NCMe)]+ ([3]+) and acetonitrile
(Figure 2). The reaction was found to be reversible, and
RESULTS AND DISCUSSION
■
Energetics for Dissociation of Acetonitrile in Complex
[Rh(PhBP3)(H)(NCMe)2]+ ([1]+). The feasibility for the
dissociation of the acetonitrile in complex [Rh(PhBP3)(H)-
(NCMe)2]+ ([1]+)to eventually produce the species [Rh-
(PhBP3)(H)(NCMe)]+ ([2]+), Figure 1was experimentally
Figure 1. Acetonitrile dissociation from [1]+ and DFT modeled
−
structures for complexes [1a]+ and [2a]+. Only the P atoms of PhBP3
(purple) and N atoms of acetonitrile (blue) are shown for clarity.
Figure 2. VT-31P{1H} NMR spectra of the reaction mixture of the
hydride complex [Rh(PhBP3)(H)(NCMe)2]+ ([1]+) with ethylene in
CD2Cl2 (in black). The 31P{1H} of pure samples of the β-agostic
complex [3]+ is shown in red (bottom trace) for comparative
purposes.
evaluated from the VT-1H NMR spectra in d8-toluene, a solvent
in which [1]+ is slightly soluble. On heating, broad-line effects
on the signal corresponding to acetonitrile were clearly
1
observable in the H NMR spectra. Simulation of the spectra
and fitting the chemical exchange rate constants (k) into the
Eyring equation gave the activation parameters ΔH⧧ = 16.3
1
addition of acetonitrile to the reaction mixture fully shifts the
equilibrium toward [1]+. On the contrary, no new species were
observed after pressurizing the NMR tube with ethylene (3
bar). Moreover, no experimental evidence for the hydride
ethylene complex [Rh(PhBP3)(C2H4)(H)(NCMe)]+ was
found even carrying out the reaction at −90 °C. On the
contrary, the related ethylbis(acetonitrile) complex [Rh-
(PhBP3)(CH2CH3)(NCMe)2]+ ([4]+) was clearly detected in
the low temperature region (Figure 2).
kcal mol−1 and ΔS⧧ = 3 2 cal mol−1 K−1 (see Supporting
Information). In separate experiments, the dependence of k
with the concentration of acetonitrile was examined. Values of k
were found to be independent of the concentration of MeCN,
indicating that they correspond to the rates of acetonitrile
dissociation from [1]+. Moreover, the low value for the
activation entropy (3
2 kcal mol−1 K−1) agrees with a
transition state like TS-1, in which the departing acetonitrile is
still close to rhodium (Figure 1).
The slow rotation of the phenyl groups on the PhBP3− ligand
in [4]+ at −90 °C breaks the symmetry plane that relates the
two halves of the molecule, so that two resonances were
observed for the phosphorus nuclei trans to the acetonitrile
Complex [Rh(PhBP3)(H)(NCMe)2]+ ([1a]+) and the
pentacoordinated species [Rh(PhBP3)(H)(NCMe)]+ ([2a]+)
have been studied by DFT methods using the full molecules as
B
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ligands (PA). In fact the very likely, regular propeller-like
orientation of the Ph groups results in a chiral structure where
the PA atoms are diastereotopic. A similar effect, although less
pronounced, was observed for complex [1]+ (Figure 2).
However, signals corresponding to the phosphorus nuclei
trans to the hydrido and ethyl ligands (PB and PM, respectively)
remained almost unaltered. They were found to be high field
shifted, as generally observed for phosphorus placed trans to
ligands with a strong trans influence.31 The rest of the
resonances for complex [4]+ agree with the proposed structure
(see Supporting Information).
According to its formula, the agostic complex [Rh(PhBP3)-
(CH2CH2-μ-H)(NCMe)]+ ([3]+, Figure 2) should be the
major species at low acetonitrile concentration. Moreover, it is
the product from the direct protonation of the rhodium(I)
complex [Rh(PhBP3)(CH2CH2)(NCMe)] (5). At this
point, it should be indicated that repeated preparations of 5
lead to orange crystalline solids of composition 5·2NCMe.25a
Therefore, further treatment of these solids with HBF4
produces similar equilibria to those obtained from the reaction
of [1]+ with ethylene commented above.
Complex 5 is quite unstable in solution but stable enough for
a fast recrystallization by solution in toluene and precipitation
with hexane. This methodology produces a yellow solid, poorly
soluble in toluene, which contains less than 1 mol % (per mol
of 5) of acetonitrile of crystallization (1H NMR evidence).
Reaction of these solids with HBF4 in dichloromethane gave
[Rh(PhBP3)(CH2CH2-μ-H)(NCMe)]+ ([3]+) in almost quan-
titative yield (Figure 2, red trace). The reaction was found to be
instantaneous, as indicated by a color change from yellow to
pale yellow. Attempts to isolate [3]+ as a solid systematically
produced mixtures of unidentified complexes and therefore it
was characterized “in situ”. Thus, three well-defined resonances
were observed in the 31P{1H} NMR spectrum at −89.3 °C,
(Figure 2, red trace), while the ethyl group produces three
signals in a 1:1:3 ratio in the 1H NMR spectrum that
correspond to the two diastereotopic methylene protons and
the methyl group, respectively (see Supporting Information).
Equilibria between Complexes [1]+, [3]+, and [4]+.
Detection of the equilibria between the title complexes was
achieved by VT-1H selective NOE NMR spectra (selnOe); two
of them are shown in Figure 3. The equilibrium between [3]+
and [4]+ is clearly evidenced in the selnOe spectrum at −21.6
°C upon irradiation of the methyl group of the β-agostic
species, which produces an exchange peak with the methyl
group in the ethyl complex [4]+. No exchange signal was
observed for the hydride resonance of [1]+, suggesting that
equilibrium involving [1]+ is a higher energy process.
At 28.5 °C, the participation of [1]+ is clearly detected from
the exchange peaks observed upon irradiation of the signal
corresponding to free ethylene. Notice that the two methylenic
protons of the β-agostic complex (H2a and H2b) are chemically
equivalent at this temperature. These NMR short-time-
consuming experiments allow the different dynamic processes
1
observed by H NMR to be organized in a qualitative, but
precise, way.
The van’t Hoff plot, obtained from the integral data of the
1
VT- H NMR spectra, gave the thermodynamic parameters
listed in Table 1 (see Supporting Information for details). In
a
Table 1. Experimental Thermodynamic Data for Reactions
Involving Complexes [1]+, [3]+, and [4]+
reaction
[3]+ + NCMe ⇆ [1]+ + C2H4 (1) [3]+ + NCMe ⇆ [4]+ (2)
ΔH°
−2.6 0.4 −6.3 0.2
ΔS°
−5
−1.1
−1.61
1
−23
0.4
1
ΔG°298.15
ΔG°198.15
T range
−1.86
−90 to −30 °C
−30 to 30 °C
a
ΔH° and ΔG° in kcal mol−1, ΔS° in cal mol−1 K−1.
both reactions, a negative value of enthalpy was found, which
indicates that both reactions are exothermic, with the ethyl
complex [4]+ and the hydride compound [1]+ being lower in
enthalpy than the β-agostic species [3]+.
However, the entropy change is also negative in both cases.
For equilibrium 2, the value of −23 1 cal mol K−1 mainly
corresponds to coordination of acetonitrile to rhodium, while
for the first one, the smaller value of −5 1 cal mol K−1 mainly
reflects the difference between acetonitrile versus ethylene (as
an ethyl group) coordination. Looking at the reactions in the
opposite sense, formation of the β-agostic complex [3]+ is
endothermic, but in both cases, the entropy change is positive.
Therefore, the formation of the β-agostic complex [3]+ from
[1]+ or [4]+ is an entropy-driven reaction, in which acetonitrile
dissociation from both the ethyl and the hydride complexes is
the driving force.
Data in Table 1 also account for the experimental
observation that the addition of acetonitrile to the β-agostic
complex quantitatively produces the hydride complex [1]+
rather than the expected ethyl complex [4]+. Indeed, a value
of ΔG°298.15 of −1.5 kcal mol−1 can be estimated for the
reaction [4]+ ⇆ [1]+ + C2H4 from data in Table 1.
DFT Studies on the β-Agostic Complex [3a]+ and the
Related Ethyl Rotamers [3b]+ and [3c]+. Complex [3]+ was
first examined by a DFT study (b3-lyp, LanL2DZ, and 6-
31G**) using the full complex [Rh{PhB(CH2PPh2)3}(C2H4-μ-
−
H)(NCMe)] ([3a]+) as a model. The BF4 counteranion was
not included, since evidence for its noncoordination to
rhodium was obtained from dosy experiments. Thus, a value
for the diffusion coefficient of 19.04 m2 s−1 corresponding to a
hydrodynamic radius of 2.89 Å was obtained from 19F-dosy
NMR spectra of [3]BF4 in CD2Cl2. These values correspond
−
well with that reported for the BF4 anion in inorganic salts
Figure 3. Selective NOE (selnOe) spectra upon irradiation of selected
resonances (indicated with a ray) of the reaction mixture from the
hydride complex [Rh(PhBP3)(H)(NCMe)2]+ ([1]+) and ethylene in
CD2Cl2.
such as LiBF4.32
An energy minimum was found for the β-agostic complex
[3a]+ with a Rh···H3a distance of 2.278 Å, which lies in the
C
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observed range (1.69−2.52 Å) for Rh···HC interac-
tions.6c,10,12f,23b,33 Nonetheless, a more precise structure was
obtained with LanL2TZ(f), which uses the f-polarization
functions developed by Frenking’s group,34 allowing a better
representation of the secondary interactions. With this basis set,
the Rh···H3a distance reduces to 2.083 Å, a value that matches
much better with that expected. Therefore, all the rest of the
intermediates and transition states described below (as well as
the above commented complexes [1a]+ and [2a]+) have been
calculated at the same level of theory for comparative purposes.
The β-agostic interaction in [3a]+ is also associated with an
elongation of the corresponding Cβ−H3a bond distance (1.143
Å) if compared with the other two Cβ−H3b/3c bond distances
(Figure 4, left). The carbon−carbon distance in the ethyl group
ethyl group from eclipsed to staggered. Since this difference for
ethane is about 2.8 kcal mol−1, the β-agostic interaction in [3a]+
can be estimated as ca. 2.8 + 0.6 = 3.5 kcal mol−1.36
The three isomers easily interconvert through transition
states TS-2 and TS-3 (Figure 5). The gray path relates [3a]+
Figure 5. Energy profile for transformations between [3a]+, [3b]+,
[3c]+, and the ethyl bis(acetonitrile) complex [4a]+. Values of ΔH are
−
given in kcal mol−1. Only the P atoms of PhBP3 (purple) and N
atoms of acetonitrile (blue) are shown for clarity.
with [3b]+, and corresponds to the “in place rotation”,
responsible for the chemical equivalence of the methyl protons
H3a, H3b, and H3c. The process has practically no energy barrier,
and accordingly, the three methyl protons are chemically
Figure 4. DFT calculated structures for complexes [3a]+, [3b]+, and
[3c]+. Values of energy are given in kcal mol−1 and C−H bond
distances in Å. Only Cipso of the phenyl groups and protons of the ethyl
group are shown for clarity.
1
equivalent in the H NMR spectrum of [3]+ at −90 °C.
The path in green connects [3a]+ to [3c]+ and then to the
ethyl bis(acetonitrile) complex [4a]+ after acetonitrile coordi-
nation to the vacant site in [3c]+. The enthalpy value for the
reaction [3a]+ + NCMe ⇆ [4a]+ has been estimated as −8.3
kcal mol−1, in good agreement with the experimental value of
−6.3 kcal mol−1 measured experimentally (Table 1).
Energy Profile for Equilibrium between Complexes
[1a]+ and [3a]+. The reaction of the hydride [Rh(PhBP3)-
(H)(NCMe)2]+ ([1]+) with ethylene leading to [Rh(PhBP3)-
(CH2CH2-μ-H)(NCMe)]+ ([3]+) would require the three
elementary steps depicted in Figure 6: (i) acetonitrile
elongates from 1.443(7) Å observed in [Rh(PhBP3)(CH2
CH2)(NCMe)] (5) to 1.508 Å according to the presence of a
single C−C bond. Moreover, the coordination polyhedron of
the metal is close to the octahedron with Cα trans to PM, N
trans to PA, and the methyl group approaching to the position
trans to PB. Furthermore, the eclipsed conformation of the ethyl
group also supports the β-agostic interaction, since otherwise a
staggered conformation should be expected.
Two related minima close in energy, [3b]+ and [3c]+, were
also found, and their structures are shown in Figure 4. Both
isomers are better described as nonagostic ethyl complexes,
which is remarkable since such type of isomers have been
considered high energy species.14b,15b,f [3b]+ and [3c]+ show a
staggered conformation for the ethyl group in contrast to the
eclipsed conformation found for [3a]+ (Figure 4).
In addition, isomer [3b]+ shows very long Rh···H3a/3b
distances while the Cβ−H3a/3b/3c bond distances are almost
identical. Concerning isomer [3c]+, it could be considered as an
α-agostic species, but the H2a proton is far away from the
coordination vacancyrepresented with a square in Figure 4
(right)leading to a quite long Rh···H2a distance. A major
difference between the ethyl complexes [3b]+ and [3c]+ comes
from the orientation of the ethyl group in such a way that the
methyl group is placed in the region corresponding to the sixth
position of the octahedron in [3b]+, while it is fully eclipsed to
the acetonitrile ligand in [3c]+. This orientation is associated
with an opening of the angle Rh−Cα−Cβ, in [3c]+ relative to
[3b]+, and, most probably, it is the origin of the higher energy
found for [3c]+.35
Figure 6. Energy profile for the reaction of [Rh(PhBP3)(H)-
(NCMe)2]+ ([1a]+) with ethylene to give the β-agostic complex
[Rh(PhBP3)(CH2CH2-μ-H)(NCMe)]+ ([3a]+). Values of ΔH are
A comparison between isomers [3a]+ and [3b]+ indicates
that the small difference in energy between them represents the
balance of cleaving the agostic interaction versus the
stabilization provided by the conformational change of the
−
given in kcal mol−1. Only the P atoms (purple) of PhBP3 and N
atoms (blue) of acetonitrile are shown for clarity.
D
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dissociation, (ii) ethylene coordination, and (iii) ethylene
insertion into the Rh−H bond. The first step gives the square-
pyramidal complex [Rh(PhBP3)(H)(NCMe)]+ ([2a]+) as
commented above. Ethylene coordination renders the hydride
ethylene complex [Rh(PhBP3)(C2H4)(H)(NCMe)]+ ([6a]+)
and takes place through a transition state TS-4 whose enthalpy
is similar to that of the intermediate [2a]+. Ethylene insertion in
[3a]+ occurs with a low energy barrier (2.5 kcal mol−1) through
the transition state TS-5, which was also found to possess an
agostic interaction (see Supporting Information).
kcal mol−1, ΔS2⧧ = −0.5 1 cal mol−1 K−1 for the second one
(see Supporting Information for details).
The simplest explanation for both fluxional processes is
illustrated in Scheme 1. Starting from isomer [3b]+, a shift of
Scheme 1. Dynamic Processes Undergone by Complex [3]+
in Solution Detected by 31P{1H} NMR
From a thermodynamic point of view, the calculated
enthalpy for the equilibrium [3a]+ + NCMe ⇆ [1a]+ + C2H4
was estimated as −4.7 kcal mol−1, in good agreement with that
experimentally measured (−2.6 kcal mol−1, Table 1).
Under a kinetic perspective, the activation barrier for the
transformation of the hydride complex [1a]+ into the β-agostic
species [3a]+ is mainly determined by the acetonitrile extrusion
(ΔH⧧ = 16.3 1.0 kcal mol−1 by NMR and ca. 17.8 kcal mol−1
by DFT) or the ethylene coordination through TS-4 (ΔH⧧ =
18.3 by DFT), which prevents direct measurements of the
barrier for the ethylene insertion by NMR.
acetonitrile to the coordination vacancy equilibrates PA with PB,
while moving the ethyl group to the coordination vacancy
equilibrates PB with PM. Both processes would take place
through the corresponding TBPY geometries, so that the small
difference in the enthalpy measured experimentally would
represent the difference between the geometries TBPY-5-15
and TBPY-5-14.
Figure 6 also includes the isomer of the hydrido−ethylene
complex [6b]+, in which the ethylene ligand is rotated 90° and
lies higher in enthalpy by 8.0 kcal mol−1 relative to [6a]+.
Ethylene rotation (path in purple) takes place through the
accessible transition state TS-6 in which ethylene is twisted in
around 23°.
Fluxional Behavior of Complex [3]+ in Solution. The
dynamic processes undergone by the β-agostic complex [3]+
were examined by VT-31P{1H} NMR. Experimental line shapes
were compared to calculated ones by using the gNMR
simulation program,37 and a set of observed and simulated
spectra is shown in Figure 7. Two different rate constants were
Attempts to find these TBPY geometries as transition states
by DFT failed, but it was possible to find the related TBPY-5-14
starting from the simplest species containing a hydride ligand
instead of the ethyl group [Rh(PhBP3)(H)(NCMe)]+ ([2a]+)
(see Supporting Information). Thus, the difference in enthalpy
between SPY-[2a]+ and TBPY-5-14-[2a]+ was found to be 11.0
kcal mol−1, which lies in the range of the measured values for
the acetonitrile shift or ethyl shift in the β-agostic complex [3]+.
Notice that any process shown in Scheme 1 also equilibrates
H2a/H2b (the methylenic protons of the ethyl group), for which
the energy barrier has been rarely measured experimentally.
An intriguing feature of the VT-31P{1H} spectra (Figure 7) is
the “appearance” of the hydride complex [1]+, whose signals
increase in intensity on raising temperature, and certainly, this
is not a problem of solubility of [1]+ in CD2Cl2. Therefore, the
most reasonable proposal to explain this observation is to
consider the existence of an additional source of acetonitrile in
solution. Consequently, dissociation of acetonitrile from both
the β-agostic species and the hydride olefin intermediate
[Rh(PhBP3)(H)(C2H4)(NCMe)]+ ([6a]+) have also been
analyzed by DFT studies.
DFT Studies on the β-Agostic [Rh(PhBP3)(CH2CH2-μ-
H)]+ ([8a]+) and Related Complexes. Figure 8 shows the
energy profile corresponding to the extrusion of acetonitrile
from [6a]+ and [3a]+; both processes converge into the β-
agostic species [Rh(PhBP3)(CH2CH2-μ-H)]+ ([8a]+). Starting
from [6a]+, the dissociation of acetonitrile produces the square-
pyramidal complex [Rh(PhBP3)(H)(C2H4)]+ ([7a]+), and the
insertion of ethylene into the Rh−H bond to give [Rh-
(PhBP3)(CH2CH2-μ-H)]+ ([8a]+) occurs through the tran-
sition state TS-7. Nonetheless, the direct dissociation of
acetonitrile from [Rh(PhBP3)(CH2CH2-μ-H)(NCMe)]+
([3a]+) produces an alternative path lower in enthalpy.
Complex [Rh(PhBP3)(CH2CH2-μ-H)]+ ([8a]+) was found
to be 16.6 kcal mol−1 (in enthalpy) higher than [Rh(PhBP3)-
(CH2CH2-μ-H)(NCMe)]+ ([3a]+), a value slightly smaller
Figure 7. Experimental and calculated (traces in gray) VT-31P{1H}
NMR spectra of [Rh(PhBP3)(CH2CH2-μ-H)(NCMe)]+ ([3]+) in
CD2Cl2. The asterisk denotes signals corresponding to the hydride
complex [1]+.
required to reproduce the experimental spectra. The first one
(kP1) corresponds to the process that equilibrates PA with PB,
while the second one (kP2) is responsible of the equilibration of
PA/PB with PM.
Fitting the data into the corresponding Eyring plots gives the
activation parameters: ΔH1 = 11.4 0.4 kcal mol−1, ΔS1
=
⧧
⧧
−0 1 cal mol−1 K−1 for the first one and ΔH2⧧ = 12.8 0.5
E
DOI: 10.1021/acs.organomet.6b00036
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Organometallics
Article
Figure 9. DFT calculated structures for complexes [8a]+ and [9a]+.
Distances are given in Å. Only Cipso of the phenyl groups and protons
of the ethyl group are shown for clarity.
according to the preceding comments for complexes [3a]+,
[3b]+, and [3c]+ (Figure 4).
Further Comments on β-Agostic Rhodium Species.
The study reported here provides a unique opportunity to
analyze the subtle influence of electronic and steric factors on
the strength of the β-agostic interaction and on the driving
force that favors (or not) the transfer of the hydrogen to the
metal. As nicely depicted in Table 2, the strength of the β-
Figure 8. Energy profile for the extrusion of acetonitrile from [6a]+
and [3a]+. Values of ΔH are given in kcal mol−1. Only the P atoms
−
(purple) of PhBP3 and N atoms (blue) of acetonitrile are shown for
clarity.
than that found for the extrusion of acetonitrile from the
hydride complex [Rh(PhBP3)(H)(NCMe)2]+ ([1a]+) to give
[Rh(PhBP3)(H)(NCMe)]+ ([2a]+, 17.8 kcal mol−1). Since the
entropy change associated with the dissociation of acetonitrile
is expected to be similar for both reactions (from [1a]+ and
from [3a]+), we can conclude than complex [8a]+ is indeed
present in solution. Moreover, the formation of [8a]+ is the
origin of the additional source of acetonitrile in the reaction
medium.
Table 2. Selected Parameters for Complexes Shown and
ΔH° for the β-Hydride Elimination Reaction to the
a
Corresponding Hydrido−Ethylene Compound
Agostic complexes with low electron counts are very unusual,
although complexes [Rh(POCOP)(CH2CH2-μ-H)]+ (POCOP
= 2,6-bis(di-tert-butylphosphinito)benzene),14b [Rh(C5Me5)-
(CH2CH2-μ-H)]+,15c and [Rh(PiPr3)2(CH2CH2-μ-H)]+,21
have been studied by DFT in rhodium chemistry.
[3a]+
[8a]+
[9a]+
Rh···H3a (Å)
2.084
86.74
0.09
2.260
90.94
9.63
3.131
106.87
176.64
+1.4
Rh−Cα−Cβ (deg)
H−Cα−Cβ−H (deg)
ΔH°
+3.7
+2.8
From a thermodynamic point of view, a value of ΔG°298
=
−3.9 kcal mol−1 has been calculated for the reaction shown in
Scheme 2, which accounts for the experimental observations
just commented.
a
Values of ΔH° are given in kcal mol−1.
agostic interaction, evidenced by shorter Rh···H3a distances,
more acute Rh−Cα−Cβ angles, and smaller torsion angles for
the protons of the ethyl groups, decreases on going from [3a]+
to [9a]+, complex [9a]+ being a nonagostic species, but [8a]+
and [3a]+ being clearly β-agostic compounds.
Scheme 2. Transformation of β-Agostic Complex [3a]+ into
Complexes [8a]+ and [1a]+
Of particular relevance is the highly unsaturated complex
[8a]+, which lies between [3a]+ and [9a]+ (despite containing
the most electrophilic rhodium center) since it is universally
accepted that shorter M···H3a bond distances (associated with
stronger interactions) come from more electrophilic metal
centers. Moreover, addition of a ligand to [8a]+ can either fully
destroy the β-agostic interaction or reinforce it, as exemplified
by complexes [9a]+ and [3a]+, respectively. In any case, the
beneficial role of the acetonitrile ligand on the stabilization of
the β-agostic interaction is remarkable
Values of enthalpy relative to their corresponding hydrido−
ethylene counterparts follow a similar trend in such a way that
the three ethyl complexes in Table 2 are more stable than the
corresponding hydride−ethylene counterparts. This constitutes
a quite unusual situation in rhodium chemistry,38 since the
general trend is just the opposite (as expected for second row
transition metals). Since other factors such as trans or solvent
effects can be excluded, the steric pressure exerted by the
For comparative purposes, the ethyl complex [Rh(PhBP3)-
(C2H4)(CH2CH3)]+ ([9a]+) and the hydride bis(ethylene)
counterpart [Rh(PhBP3)(C2H4)2(H)]+ ([10a]+) have also
been calculated at the same level of theory. For the addition
of a second molecule of ethylene to [8a]+ (Figure 8) the
enthalpy changes are small but the entropy change is strong
leading thus to large and positive values for ΔG°298. In good
agreement, neither [9a]+ nor [10a]+ was observed in solution.
Figure 9 displays selected structural parameters for
complexes [8a]+ and [9a]+ enlightening their different nature.
While [8a]+ is clearly a β-agostic compound, the related [9a]+
is better described as a terminal nonagostic ethyl complex
F
DOI: 10.1021/acs.organomet.6b00036
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
−
phenyl groups on the PhBP3 ligand should be the key factor
that favors the inserted products versus the hydrido−ethylene
derivatives, since the former are expected to be less constrained
structures than the latter.
A third aspect considered concerns the energy barriers for
two closely related processes: the “in place rotation” (Ei) and
the β-hydride elimination reaction (Eβ). As schematically
depicted in Scheme 3, the “in place rotation” requires the
Scheme 3. Schematic Representation of the Transition States
for the Processes
a
Figure 10. DFT-calculated structure for [11a]+ along with selected
structural parameters (left) and 1H selnOe NMR spectrum of a
solution of [1]+ in the presence of 10 molar equiv of styrene upon
irradiation of the signal corresponding to the hydrido ligand in [1]+
(right).
Structural parameters calculated by DFT for [11a]+ fit well
with those expected for a β-agostic complex (Figure 10, left), in
which the electron withdrawing character of the phenyl group is
reflected in a slightly short Rh···H3a distance of 1.916 Å when
compared to that observed in the ethyl analogue [3a]+.
Accordingly, the energy barrier for the “in place rotation” takes
place through a transition state similar to complex [3b]+ and it
a
(i) “In place rotation” and (ii) β-hydride elimination.
cleavage of the β-agostic interaction through a transition state
in which the M···H distance elongates (di > d). On the
contrary, the transition state for the β-hydride elimination is
associated with a shortening of such distance (de < d). It can
thus be expected that stronger β-agostic interactions would
require higher energy barriers for the “in place rotation”, but
lower ones for the β-hydrido elimination reaction. Indeed, the
stronger a β-agostic interaction is, the closer the structure is to
the corresponding transition state for the β-elimination.
Accordingly, the data available from DFT calculations for the
rhodium complexes fit nicely under this perspective (see Table
3).
is associated with a ΔG⧧
value of 1.8 kcal mol−1 (see
298
Supporting Information), indicating that interaction of the
proton with rhodium is slightly stronger than that observed in
the ethyl derivative [3a]+.
SUMMARY AND CONCLUSIONS
■
In this paper, we have combined NMR spectroscopy and
computational (DFT) studies to gain information about the
stability, dynamics, and behavior in solution of β-agostic/ethyl
−
species in complexes bearing the tripodal ligand PhBP3 . Two
of them, [Rh(PhBP3)(CH2CH2-μ-H)(NCMe)]+ ([3a]+) and
the highly unsaturated species [Rh(PhBP3)(CH2CH2-μ-H)]+
([8a]+), were found to be real β-agostic complexes, while the
related rotamers, [3b]+ and [3c]+, as well as [Rh(PhBP3)-
(C2H4)(CH2CH3)]+ ([9a]+) are better described as nonagostic
ethyl complexes. This result is remarkable, since such type of
compounds have been considered as high energy species.
Moreover, a comparison of complexes [3a]+ and [9a]+ revealed
the key role of the fifth ligand in stabilizing (acetonitrile) or
destroying (ethylene) the β-agostic interaction in these ethyl
derivatives. Furthermore, starting from [Rh(PhBP3)-
(CH2CH3)(NCMe)2]+ ([4a]+), dissociation of acetonitrile
was found to be the driving force to [Rh(PhBP3)(CH2CH2-
μ-H)(NCMe)]+ ([3a]+); a similar behavior was observed for
[3a]+, which transforms into [Rh(PhBP3)(CH2CH2-μ-H)]+
([8a]+) at higher temperature. Accordingly, NMR-measured
equilibrium constants for both equilibria indicate that they are
entropy-driven reactions.
Table 3. DFT-Calculated Rh−H and Rh···H3a Bond
Distances (Å) in Complexes [Rh(L3)(L)(H)(H2CCH2)]+
and [Rh(L3)(L)(CH2CH3)]+, Respectively, and ΔG⧧ for
298
the Processes “In Place Rotation” (Ei) and β-Elimination
Reactions (Eβ) (kcal mol−1)
a
L3
L
Rh···H3a
TS
Rh−H
Ei
Eβ
ref
C5H5
P(OMe)3
PMe3
1.771
1.773
1.790
1.818
1.823
1.827
2.083
2.260
1.624
1.628
1.594
1.629
1.632
1.599
1.621
1.632
1.567
1.564
1.562
1.466
1.573
1.568
1.561
1.572
10.7
9.5
0
15b
15b
15c
15b
15b
15c
b
C5H5
0
C5H5
C2H4
C5Me5
C5Me5
C5Me5
PhBP3
PhBP3
P(OMe)3
PMe3
6.7
6.0
1.6
0.2
C2H4
NCMe
0.4
0
7.4
6.6
b
a
b
Rh···H distance in the transition state connecting both species. This
work.
Notably, the three related hydride complexes [Rh(PhBP3)-
(C2H4)(H)(NCMe)]+ ([6a]+), [Rh(PhBP3)(C2H4)(H)]+
([7a]+), and [Rh(PhBP3)(C2H4)2(H)]+ ([10a]+) were found
to be higher in enthalpy than the corresponding β-agostic/ethyl
species. This noteworthy result for rhodium chemistry is well
explained when taking into account the steric pressure exerted
Finally, other olefins such as styrene also insert into the Rh−
H bond of complex [1]+, although the expected complex
[Rh(PhBP3)(CHPhCH2-μ-H)(NCMe)]+ ([11]+) could not be
observed by NMR. Nonetheless, it was detected by exchange
spectroscopy experiments. Thus, irradiation of the hydride
signal of complex [Rh(PhBP3)(H)(NCMe)2]+ ([1]+) in the
presence of styrene revealed the exchange with the signals
corresponding to the olefinic protons of styrene (Figure 10,
right).
−
by the bulkier PhBP3 ligand in comparison to other systems
that have been studied. As noted earlier, small variations of
electronic and steric factors can control the strength of the β-
agostic interaction and will therefore determine the stability of
the insertion products relative to the hydrido−ethylene species.
Moreover, the stronger this interaction is, the more easily the
G
DOI: 10.1021/acs.organomet.6b00036
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00036.
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395−408.
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Experimental details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ctejel@unizar.es.
■
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†A.M.G.: School of Chemistry, University of Nottingham
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The generous financial support from MICINN/FEDER
(Project CTQ2011-22516), MINECO/FEDER (Project
■
́
CTQ2014-53033-P), and Gobierno de Aragon/FSE (GA/
FSE, Inorganic Molecular Architecture Group, E70) is
gratefully acknowledged. A.M.G. thanks GA/FSE for a
fellowship. This article is dedicated to the memory of Prof.
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I
DOI: 10.1021/acs.organomet.6b00036
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(38) The sole exceptions to this general trend are the complexes
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Brookhart, M.; Lincoln, D. M. Organometallics 1992, 11, 127−138.
J
DOI: 10.1021/acs.organomet.6b00036
Organometallics XXXX, XXX, XXX−XXX