Insight into the mechanism of decarbonylation of methanol by ruthenium complexes; a deuterium labelling study

Article abstract of DOI:10.1039/c8dt01733f

In the reaction of [RuHClP₃] (P = PPh₃) with NaOMe in methanol, the product is [RuH₂(CO)P₃]. Short reaction times show that the final product is formed through [RuH₄P₃] as the major interme

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Insight into the mechanism of decarbonylation of methanol by
ruthenium complexes; a deuterium labelling study
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
Patrizia Lorusso, Graham R. Eastham and David J. Cole-Hamilton*
DOI: 10.1039/x0xx00000x
In memory of Roberto Sánchez-Delgado a good scientist and great friend.
www.rsc.org/
In the reaction of [RuHClP
that the final product is formed through [RuH
product is [RuH P' ] (P' is PPh partially deuterated in the ortho positions of the aromatic rings). Further reaction leads to a
mixture of [RuH ] (n = 0, 22 %; n = 1, 2 isomers each 28 %; n = 2, 22 %). Mechanistic aspects of both steps of the
3
] (P = PPh
3
) with NaOMe in methanol, the product is [RuH
2
(CO)P
3
]. Short reaction times show
4
P
3
] as the major intermediate. Using NaOCD
3
in CD OD, the first formed
3
4
3
3
n 3
D2-n(CO)P
reaction are explored and, together with previously published calculations, they provide definitive mechanisms for both
dehydrogenation and decarbonylation in these interesting systems.
2
N gas when not specified (dried through a Cr(II)/silica packed
Introduction
glass column) using different techniques including a standard
Schlenk vacuum line and a glove box. Solvents were degassed
prior to use and dried when required.
As part of our studies on the production of hydrogen from
1
-5
4, 6, 7
alcohols and the related decarbonylation
catalysed by [RuH ] (P = PPh )‡ and related complexes, we
have recently reported some DFT calculations on the
of alcohols
3 2
RuCl ∙3H O was purchased from Alfa Aesar. All solvents were
4
P
3
3
purchased from Sigma-Aldrich. Toluene was dried using a
Braun Solvent Purification System. Methanol was dried and
degassed by distillation from magnesium methoxide under
7
mechanism of alcohol decarbonylation to give [RuH
especially when using methanol as the substrate. A series of
routes was investigated and kinetic isotope effects (k /k )
2 3
(CO)P ],
8
9-11
12, 13
nitrogen. [RuCl
and [RuH (PPh
and all observations and NMR data were in accordance with
those reported in the literature, except for [RuH (PPh
where no NMR data were available. [RuH (CO)(PPh ] was
2
(PPh
3
)
3
] , [RuHCl(PPh
3
)
3
]
4 3 3
, [RuH (PPh ) ]
H
D
13
2
3 4
) ]
were prepared by published procedures
calculated for each of them. We then wished to compare these
kinetic isotope effects with experimental values. We originally
expected to do this by carrying out reactions using a mixture of
2
3 4
) ]
2
3 3
)
CH OH and CD OD, but complications arose because of H/D
3
3
6
prepared by an adaptation of a literature method. Sodium
methoxide was prepared by reacting sodium metal (supplied
by Lancaster Synthesis in mineral oil; the small pieces were
exchange. In the end we obtained the experimental value
1
3
7
using mixtures of CH OH and CD OD. Even then, our
3
3
1
anticipated use of H NMR spectroscopy of the hydrido signals
was complicated by isotope shifts and D broadening so we
washed with hexane) with degassed CH
3
3
OH, CD
3
OD or
1
13
CH OH. Triethylamine was purchased from Fischer Scientific
3
used C NMR of the carbonyl resonance for the quantitative
analysis. Some unexpected observations of the H/D labelling
pattern of the products suggested that the mechanism might
not be exactly as we expected so we investigated these
reactions further.
and degassed before use. All gases were purchased from BOC
gases. All solvents and deuterated solvents, which were not
previously dried, were only degassed prior to use.
NMR spectra were recorded on Bruker Avance II 400 and 500
1
MHz spectrometers ( H NMR at 400 MHz and C NMR at 100
13
1 13
MHz or H NMR at 500 MHz and C NMR at 125 MHz
respectively) at room temperature.
1
Experimental
H NMR solvent residual signals: CD
at 2.08 ppm; THF-d at 1.72 ppm. C{ H} NMR
solvent signals: CD Cl set at 53.84 ppm; toluene-d at 20.43
ppm; THF-d at 25.31 ppm.
2 2
Cl set at 5.32 ppm;
All manipulations and reactions were carried out under Ar or
13
1
toluene-d
8
8
2
2
8
1
4
8
Synthesis of known complexes for comparative NMR data
RuH (CO)P ] (P = PPh ) was prepared by an adaptation of a
literature method. Sodium methoxide was prepared by
reacting sodium metal (0.02 g, 0.87 mmol) with CH OH (0.4
[
2
3
3
6
3
mL, 9.8 mmol), under an inert atmosphere of dinitrogen and
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the formed solution was subsequently added to a suspension washed once with dry and degassed appropriately labelled
1
3
of [RuHClP
3
] (0.015 g, 0.016 mmol) in toluene (3 mL). After methanol (not for
3
CH OH), in order to remove the
refluxing the reaction mixture for 1 hour at 100 °C, the methoxide, filtered, dried in vacuo and analysed by NMR
8
resulting orange solution was cooled to room temperature and spectroscopy (toluene-d ; see Figure 1).
evaporated to dryness. The recovered solid was washed once
with dry, degassed methanol in order to remove the CD
3
OD:
H NMR (400 MHz; toluene-d
). The H and and JH-Pa = 30.9 Hz, H trans to CO), -8.41 (dt, 1H, JH-Pb = 28.78
P{ H} spectral data are identical to those described in the and JH-Pa = 73.9 Hz, H cis to CO) ppm. [RuH (CO)P ] is also
): δ = 57.1 (br, 4P,
1
methoxide and dried in vacuo. The collected yellow solid was
8
): δ = -6.58 (td, 1H, JH-Pb = 15.11
1
analysed via NMR spectroscopy (toluene-d
8
3
1
1
2
3
1
5
31
1
literature.
present. P{ H} NMR (400 MHz, toluene-d
mutually trans P), 44.8 (br, 2P, unique P) ppm. H{ H} NMR
8
2
1
1
2
H NMR (400 MHz; toluene-d
8
): δ = -6.5 ppm (tdd, JHa-Pa (cis)
=
2 2
(500 MHz, CH Cl ): 7.7, 7.30, 7.2 ppm ( H, phenyl, ortho). A full
3
=
0.8 Hz; JHa-Pb (cis) = 15.2 Hz; JHa-Hb = 6.4 Hz), -8.4 (dtd, JHb-Pb (trans) analysis of the products from this reaction is provided in the
3
1
1
74.2 Hz; JHb-Pa (cis) = 28.6 Hz; JHa-Hb = 6.4 Hz). P{ H} (400 MHz; ESI, Sections 2 and 3.
): δ = 57.4 (d, 2P, JP-P = 17.7, mutually trans P), 45.3
t, 1P, JP-P = 17.7, unique P) ppm.
toluene-d
8
1
3
(
CH
3
OH
H NMR (400 MHz; toluene-d
] The dinitrogen compound was identified by 18.7 Hz, H trans to CO), -8.4 (dtt, JHb-C = JH-H = 6.4 Hz, H
1
2
8
Ha-C
): δ = -6.5 ppm (tddd, J =
[
RuH
comparing the NMR spectra collected when the reaction was CO) ppm. P{ H} (400 MHz; toluene-d
carried out under N (see time and temperature dependent 17.5 and JP-C = 7.5 Hz, P ), 45.3 (td, 1P, JP-P = 17.5 and JP-C = 9
experiments) and under Ar (current experiment). The Hz, P ) ppm. C{ H} NMR (400 MHz, toluene-d δ = 207.3 (dt,
C-Pa = 7.5, JPb = 9 Hz, CO), 140.4 (vt, JCP+CP’ = 40.8 Hz, Cipso,
2
(N
2
)P
3
a
b
cis to
3
1
1
8
): δ = 57.4 (dd, 2P, JP-P
=
2
a
1
3
1
b
8
):
1
disappearance of two hydrido signals in the H NMR spectrum
J
of the experiment described below is attributable to the mutually trans phosphines), 138.6 (d, JC-P = 28.4 Hz, Cipso
missing dinitrogen compound under the current reaction unique phosphine), 134.4-133.9 (m, Ar), 128.2 (s, Ar), 127.3-
conditions. [RuHClP
toluene (2 mL) and reacted with NaOCD
of sodium metal (0.01 g, 0.43 mmol) with CD
mmol), at RT for 5 min under Ar. The reaction was stopped as A solution of CD
soon as the mixture turned from purple to red/orange and the metal (0.02 g, 0.87 mmol) to form the methoxide, was added
flask placed in a cold bath (dry ice/acetone). The mixture was to a suspension of [RuHClP ] (0.06 g, 0.065 mmol) in toluene-
then reduced to dryness affording an orange solid which was (3 mL). The experiment was performed using a ramping
subsequently washed with CD OD, filtered using a cannula temperature (20 min at RT, heated under reflux to 55 °C for 40
3
] (0.03 g, 0.032 mmol) was suspended in 127.1 (m, Ar) ppm.
, obtained by reaction
OD (0.2 mL, 4.9
3
Time and temperature dependent NMR studies with NaOCD
3
3
3
OD (0.4 mL, 9.8 mmol) containing sodium
3
d
8
3
filtration and dried again under vacuum. The collected solid min and subsequently to 100 °C for 30 min) with continuously
was analysed by NMR spectroscopy and the resonances sampling at each ramping temperature step with immediate
missing from these spectra when compared with the spectra cooling to room temperature and collection of the NMR
of a sample obtained from an identical reaction performed spectrum.
1
1
2 2 2 3 4 3
under N were assigned to [RuH (N )P ]. The H NMR data Three intermediates were identified via H NMR: [RuH P ],
1
6, 17
1
12
were in agreement with those reported in the literature.
[RuH
2
(N
2
)P
3
] and [RuH
2
P
4
]. The H spectral data of [RuH
were identical to those described in the
] were as described above.
6 Hz; JHb-Pa (cis) = 30 Hz; JHa-Hb = 6 Hz), -12.8 (tdd, 1H, JHa-Pa = 27; An experiment using identical conditions but under argon gave
4 3
P ]
1
6, 17
and [RuH
2
2 3
(N )P ]
1
H NMR (400 MHz; toluene-d
8
): δ = -8.6 (dtd, 1H, JHb-Pb (trans)
=
2 4
literature. Spectral data of [RuH P
7
3
1
1
JHa-Pb = 14 Hz) ppm. P{ H} (400 MHz; toluene-d
8 2 2 3
): δ = 56.3 (d, the same products without [RuH (N )P ].
2
P, JP-P = 16.4 Hz), 43.9 (t, 1P, JP-P = 16.4 Hz) ppm.
Time and temperature dependent NMR studies with NaOCH
] was synthesised under argon according to A solution of CH OH (0.4 mL, 9.8 mmol) containing sodium
metal (0.02 g, 0.87 mmol) to form the methoxide, was added
to a suspension of [RuHClP ] (0.06 g, 0.065 mmol) in toluene-
(3 mL). The experiment was performed using a ramping
spin system, see Fig 3 and ESI Figure S29). temperature (15 min at RT, heated under reflux to 55 °C for 30
3
[RuH
2
(PPh
3
)
4
3
1
3
literature procedure.
3
1
H NMR (400 MHz, toluene-d
8
): δ = -10.2 ppm (m, 2H, AA’
d
8
signal, AA’MM’X
2
3
1
1
P{ H} NMR (400 MHz, toluene-d
Hz), 41 (t, 2P, JP-P = 11.9 Hz) ppm.
8
): δ = 49.14 (t, 2P, JP-P = 13.8 min and subsequently to 100 °C for 30 min) with continuously
sampling at each ramping temperature step with immediate
cooling to room temperature and collection of the NMR
Reaction of [RuHClP
As above for the preparation of [RuH
methoxide was obtained by reacting sodium (0.01 g, 0.43
3
] with labelled methoxide
spectrum.
2
(CO)P ] except that the
3
1
3
mmol) with CD
3 3 3
OD, CH OH or a mixture (1:1) of CH OH and
CD OD (0.2 mL, 4.94 mmol). The recovered yellow solid was
3
2
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Finally, a 1:1 mixture of CH
RuH (CO)P’
3
OH/CD
3
OD gives a mixture of
Results
[
2
31
3
] and both isomers of [RuHD(CO)P’
The P{ H} NMR spectrum of [RuH (CO)P’ ] (Figure 2a) gives a
sharp doublet ( 57.4, P , JP-P = 17.7 Hz) and triplet (45.3 P ).
These are broadened significantly if deuterium is present
3
] (Figure 1d).
Our initial observations of the hydride resonance of
1
2
3
[
RuH
RuHClP
formed by dissolving sodium in CH
n
(D)2-n(CO)P
3 2
] obtained from reactions under N of
δ
a
b
[
3
] in toluene with labelled or unlabelled NaOMe,
1
OD, CH
3
3
OH. CD
3
3
OH or a
13
13
(
Figure 2b) and split by C from CO (Figure 2c). The solution
obtained from the reaction using NaOMe/NaOCD shows
sharp resonances from [RuH (CO)P’ ] superimposed on broad
resonances from the isomers of [RuHD(CO)P’ ] (both isomers,
Figure 2d). There is a slight D isotope shift to lower field for
each resonance in solutions where CD OD has been used.
1:1 mixture of CH
3
OH/CD OD, are shown in Figure 1.
3
3
2
3
3
3
1
Fig. 1 Hydride region of H NMR spectrum of the Ru final species arising by reaction at
1
3
1
00 °C of [RuHClP
3 3 3 3
] in a) CH OH, b) CD OD, c) CH OH or d) a 1:1 mixture of
CH OH/CD OD each containing dissolved sodium to form the methoxide.
3
3
3
1
1
Using CH
3
OH, the hydride resonances are as expected: δ −6.5 Fig. 2 P{ H} NMR spectra of the Ru final species arising by reaction at 100 °C of
13
ppm (ddt, JHa-Pa = 30.8 Hz, cis; JHa-Pb = 15.2 Hz, cis; JHa-Hb = 6.4 Hz, [RuHClP
3
] (in toluene) in a) CH
3 3 3
OH, b) CD OD, c) CH OH or d) a 1:1 mixture of
CH
3
OH/CD OD each containing dissolved sodium to form the methoxide.
3
H
a
) and
δ
-8.4 ppm (JHb-Pb =74.2 Hz, trans; JHb-Pa = 28.6 Hz, cis;
Ha-Hb = 6.4 Hz, H ) (Figure 1a).
When the reaction was performed using only CD
studies showed both isomers of [RuHD(CO)P’ ] (P’ is PPh
J
b
1
OD, H NMR In order to try to explain the products from the reaction using
only CD OD, further studies were carried out over shorter
3
3
3
3
partially deuterated in the ortho positions of the aromatic reaction times and at lower temperatures.
rings, see later) were obtained in equal proportions together An experiment was carried out in toluene-d
8
under N
2
with
o
with a small amount of [RuH
2
(CO)P’
3
] (Figure 1b). We note that slow heating to 100 C, sampling the solution after various
1
there is a small D isotope shift in each case so that each time intervals. The samples were cooled and analysed by H
resonance of the hydride signal of the two isomers of NMR spectroscopy (Figure 3). The initially formed [RuH P’ ],
RuHD(CO)P’ ] is coincident with the low frequency signal of [RuH (N )P’ ] and [RuH P’ ] (Figure 3a) all transformed into the
each H-H doublet in the spectrum of [RuH (CO)P’ ]. Integration mixture of [RuH ] (n = 0-2) once the temperature
of individual peaks in the multiplets against the meta-protons reached 100 C together with small amounts of unidentified
4
3
[
2
2
3
2
4
3
n 3
D2-n(CO)P’
2
3
o
of the aromatic rings (ESI Section 3) shows that there is the species giving doublets at
equivalent of 1 H and 1 D in the hydridic positions and so there
δ
-6.7, -7.5 and -8.6 ppm (Figure 3c).
must be equal amounts of [RuH (CO)P’ ] and [RuD (CO)P’ ].
2
3
2
3
Integration of the individual peaks within the hydride
resonances (ESI Section 3) shows that this mixture is made up
of [RuH (CO)P’ ] (22 %) the two isomers of [RuHD(CO)P’ ]
2
3
3
(
each 28 %) and [RuD (CO)P’ ] (22 %). Unfortunately, we did
2 3
2
not observe Ru-D resonances in the H NMR spectrum (Figure
S5), despite observing D in the ortho positions of the phenyl
rings (see below). This may be because the resonances will be
very weak (1/12 of the intensity of the resonances from the o-
D atoms) not superimposed and extensively broadened.
1
3
13
When using CH OH coupling to CO splits each line of the
3
hydride resonances into a further doublet. Since JHb-C = JH-H the
different resonances in the multiplet from H all appear as
b
triplets (Figure 1c).
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1
Fig. 3 Time and temperature dependent H NMR study of the reaction of [RuHClP
3
] with
NaOCD : hydride region of the reaction mixture.
3
A similar experiment (15 min at RT, heated to 55 °C for 30 min,
and to 100 °C for 30 min) was conducted in the presence of
3 3
NaOCH (obtained by reaction of sodium metal with CH OH),
with the only difference being that the first two ramping
temperature steps were shortened in an attempt to observe
the formation of similar intermediates (ESI Section 1.8), based
Sch. 2 Proposed mechanism for the formation of [RuHD
NaOCD in the presence of CD OD.
3 3 3
P ] from [RuHClP ] and
2 3 3
on the evidence that formation of [RuH (CO)(PPh ) ] already
3
3
occurs at RT in the presence of non-deuterated methoxide.
1
The first two H NMR spectra collected after 15 min (RT) and
A plausible mechanism for the formation of the tetrahydrido
species is shown in Scheme 2. However, this mechanism shows
that the product should have 3 D atoms distributed amongst
the hydrido and dihydrogen ligands, which are known to be
exchanging, whereas the observed product appears to have
4
5 min (55 °C) exhibited the same resonances observed when
using CD OD along with those corresponding to
RuH (CO)(PPh ]. Based on the intensity of the signals from
3
[
2
3 3
)
the intermediates, it is evident that the decarbonylation
2
reaction is much faster when no H is involved, but the hydride
none. The only source of H in the system is PPh
that there is fast exchange between the hydrido/dihydrogen
ligands and the ortho H atoms on PPh
3
so we propose
signals of the intermediates are the same.
Summarising, the key observations are: after short reaction
3
.
times, the first formed species from [RuHClP
RuH P’ with all the hydrides apparently being H. This is in
equilibrium with [RuH P’ ] and, when under nitrogen, with
RuH (N )P’ ]. The same complexes are also formed when
using NaOCH but [RuH (CO)P ] is also formed. Over longer
times and at higher temperatures, all the intermediates
transform into [RuH (CO)P ] from NaOMe or a mixture of
RuH ] (n = 0-2) with the H/D ratio in the hydrides
3 3
] with NaOCD is
Linn and Halpern, in a paper concerning the coordination
chemistry of ruthenium polyhydrides in catalytic
hydrogenation of ketones and arenes and related kinetic
4
3
2
4
[
2
2
3
studies, described that, when a solution of [RuH
THF (prepared by reaction of [RuH (N )(PPh ] with D
heated to 30 °C, intramolecular exchange with the ortho
2
D
2
(PPh
3
)
3
] in
3
2
3
2
2
3
)
3
2
) was
2
16
2
3
phenyl hydrogens was observed by H NMR studies. In a
complementary work, Gusev et al. reported that complex
[
n 3
D2-n(CO)P’
being 1 and in the ratios shown in Scheme 1.
4 3 8 2
[RuH P ] in toluene-d under a D atmosphere undergoes
isotopic substitution of the hydride ligands and incorporation
of deuterium into the ortho positions of the phenyl rings of the
1
12
PPh
our hypothesis that the H atoms in [RuH
RuHClP ] and NaOCD come from the ortho positions of the
phenyl rings. H NMR studies of a dried and CD OD washed
3
ligands ( H NMR evidence). These two studies support
4
3
P’ ] formed from
[
3
3
2
3
o
Sch. 1 Labelling of products obtained in the two steps of the reaction of [RuHClP
3
] with
final product ([RuHClP
the presence of D in the aromatic region of the NMR spectrum.
Figure 4a). The resonances at 7.30 and 7.24 ppm may be
attributable to D in the ortho positions of the phenyl rings of
the phosphine ligands of and respectively of
RuHX(CO)P ] (X = H or D) whilst the resonance at 7.7 is from
D in the ortho positions of the phenyl rings in Ph PO.
Furthermore, the small amounts of PPh and Ph PO present in
3 3
]/Na/CD OD, 1.25 h, 100 C) confirm
-
CD
3
OD/CD
3
O . P’ is PPh
3
with significant incorporation of D into the o-positions of the
aromatic rings.
(
δ
6
In a previous study some of us have presented IR evidence for
formaldehyde and formyl intermediates during the reaction of
P
a
P
b
[
3
δ
3 3
[RuHClP ] with NaOCH , but we have not observed these
complexes during this NMR study.
3
3
3
3
1
the product of a similar reaction give P resonances (Figure
4b) which show a signal in the position expected for unlabelled
compounds along with several resonances at lower frequency
Discussion
The most surprising observation in this study is that the major
(
unresolved for Ph
different numbers of ortho-D atoms, which give rise to isotope
shifts of the P resonance. We note that, in [RuHD ] there are
8 ortho H atoms so under fast Ru-H/ortho-H exchange
conditions the equilibrium composition should have 3/22 =
3.6 % D atoms on the Ru. The hydride resonances of [RuH
(N )P’ ] (P’ is PPh containing significant D in the ortho
3
PO) attributable to compounds bearing
species formed on reaction of [RuHClP
room temperature is [RuH P’ ] and that the hydride, not
deuteride, is formed even when X = D.
3 3
] with Na/CX OX at
4
3
3 3
P
1
1
n 2-
D
n
2
3
3
positions of the rings) clearly shows H-H coupling with no
enhancement of the lower field signals of each doublet
suggesting that all the hydrides in [RuH
Integration of the ortho relative to the meta protons In the
mixture of [RuHX(CO)P’ ] (X = H or D) suggest 11-12 D atoms
4 3
P’ ] are H not D.
3
4
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are incorporated per complex molecule, much more than the Na/MeOH/CD
3
OD. At that time, we were unaware of the
involvement of [RuH
RuHClP ] reacted to give [RuH(OMe)P
neutral mechanism of that paper, which has a computed
of 1.63. However, given the presence of [RuH ] and
the basic nature of the medium, it is probably that the first
step in the decarbonylation reaction is attack of methoxide
4
P
3
] in this reaction and assumed that
expected 3. This observation is discussed in more detail in the
ESI Section 2, but means that more than enough H atoms have
been exchanged to account for the 4 hydrides observed. The
incorporation of D into the ortho positions of the aromatic
[
3
3
] and hence initiated
E
7
k
H
/k
D
4 3
P
3
1
rings also accounts for the broadening of the P resonances
shown in Figures 2b and d. [RuH YP’ ] (Y = N or P’) will form
by displacement of H by Y, as previously reported.
2
3
2
onto the ruthenium centre, replacing
H
2
to give
] and hence leading into the anionic mechanism
of our theoretical paper, partially reproduced in Scheme 3
1
6, 17
-
2
2 3
[RuH (OMe)P
G
and more fully in Scheme 4. This mechanism has the lowest
activation energy of all the mechanisms evaluated by DFT
7
= 1.88. A mechanism such as
calculations and k
H
/k
D
G
is also
supported by our experimental studies on the
dehydrogenation of 2-propanol catalysed by [RuH ] under
basic conditions, in which we isolated intermediate, [RuH
4 3
P
-
2
3 3
P ] ,
consistent with the cycle being initiated by attack of alkoxide
to give anionic intermediates.
The products from the reaction using CD
ratio among the hydrides = 1. Since the second step of the
reaction starts with [RuH P’ ] containing 4 hydridic/H H atoms
and since 4 D are removed from CD OD in forming the CO
3
OD/Na have an H/D
4
3
2
3
ligand, it is tempting to think that these 8 H/D atoms scramble
to give 1H and 1D on the metal together with 3 molecules of
hydrogen with an overall H/D ratio also of 1. However, this is
not possible because the first step of the reaction leads to
-
O and this H
displacement specifically of H
2
by CD
3
2
will be lost
because the reaction is carried out under reflux. In addition,
most of the reactions are carried out under N where H in
RuH P’ ] is replaced by N or PPh and again will be lost.
2
2
[
4
3
2
3
A possible explanation of the overall 1:1 ratio of H/D in the
product mixture of [RuHnD2-n(CO)P’ ] (n = 0-2), of the
3
equivalent amounts of [RuH (CO)P’ ] and [RuD (CO)P’ ] as well
3
2
3
2
as of the equivalent amounts of the two isomers of
RuHD(CO)P’ ] and of the enhanced amounts of the two
[
3
isomers of [RuHD(CO)P’ ] is shown in Scheme 4, which is based
3
7
G of our previous theoretical paper. In this
on Mechanism
scheme, D transfers from the CD OD derived ligands directly
3
to coordinated H or D are shown so as to reduce the number
of intermediates in the figure, but calculations show that they
7
2
1
occur via transfer to the metal and then assembly of D or HD.
2
Fig. 4 a) Part of the H{ H} NMR spectrum (CH
2
Cl
2
) showing the resonances potentially
2
attributable to H in the ortho positions of the phenyl rings of the phosphine ligands of The key to this mechanism is that the H/D atom that is trans to
complex [RuHD(CO)P
and of Ph PO (δ 7.7 ppm); b) P NMR spectrum of the products from reaction of
RuHClP ] with NaOCD . Inset are the expanded resonances from Ph PO and PPh
3 2 3 2 3
] (two isomers), [RuH (CO)P ] and [RuD (CO)P’ ] (δ 7.2 and 7.3)
the formyl group in the formyl complex remains trans to the
31
3
carbon-based ligand throughout the transformation of the
[
3
3
3
3
latter into CO by a series of β-D transfer reactions. We define
the mutually trans H and the C based ligand as axial with the
other ligands being in the equatorial plane until the fluxional 5
coordinate [RuHnD2-n(CO)P’ ] is formed. All assembly of H D
n 2-n
Having established that the first complex to be formed in the
reaction of [RuHClP ] with Na/MeOH is [RuH ], we then
looked at the mechanism for the formation of [RuH (CO)P
from this complex and Na/MeOH. The key observation is that,
when using NaOCD and [RuH P’ ], the major products are a
mixture of [RuH ] (n = 0, 22 %, n = 1, two isomers,
3
4 3
P
2
2
3
]
(
n = 0-2) also occurs in the equatorial plane. Furthermore, it
requires that the hydrides and one deuteride can exchange in
the formaldehyde complex, [RuH D(OCD )P . This is
3
4
3
a
2
2
2
n 3
D2-n(CO)P
reasonable proposition since the rate determining step is the
reorganisation of formaldehyde in this complex (Scheme 3) so
it will have a long lifetime. The transition state for this rate
each 28 % and n = 2, 22 %). With the H:D ratio in the hydrides
being 1:1.
We have recently reported DFT calculations aimed at
18
determining step involves an agostic interaction of one of the
4 3
understanding the mechanism of transformation of [RuH P ]
7
D atoms of coordinated formaldehyde, which might facilitate
exchange of the positions of H and D through the formation of
into [RuH
paper we also reported the experimental isotope effect
isotope effect (k /k = 4) for the reaction of [RuHClP ] with
2 3
(CO)P ] by decarbonylation of methanol. In that
H
D
3
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DOI: 10.1039/C8DT01733F
ARTICLE
coordinated HD. These suggestions are also shown in Scheme
Journal Name
NaOCD3 H2
RuH4P'3]
P'
3.
H
Ru
OCD3
H
[
P'
P'
P'
P'
P' Ru
OCD3
H
H
H
D
D
P'
P'
Ru
H
Ru
P'
H
6
6.7
P'
D2C
33.3
D
H
D2C
O
O
H
Ru
C
H
Ru
C
D2
P'
P'
H
D
P'
P'
H
P'
Ru
P'
HD
33.3
3
3.3
33.3
33.3
HD
C
7
O
D
O
O
Sch. 3 Rate determining step of mechanism G from our theoretical paper together
D
D
with a possible mechanism for scrambling H/D in the formyl complex shown in Scheme
HD
D2
HD
2
.
D
Ru
CO
H
H
H
P'
P'
3
3.3
D
P'
P'
D
P'
33.3
Ru H
Ru D
P'
D
CO
CD OD
CO
CD3OD
CD3O-
CD3OD
CD3OD
CD3O-
3
Calculations suggest that the dehydrogenation and
decarbonylation reactions from pathway of our theoretical
CD3OD
CD3O-
CD3O-
CD3O-
G
H
H
Ru
CO
H
D
D
7
paper proceed via intermediates involving two coordinated
D2
P'
P'
D
P'
D
P'
D2
H
P'
D2
DH
P'
P'
P'
33
16.7
Ru
P'
Ru
CO
16.7
1
6.7
Ru H 16.7
CO
Ru
P'
D
HD
CO
CO
phosphines and this seems to be required to explain the
formation of the two isomers of [RuHD(CO)P’
form in equal amounts by coordination of P’ to the fluxional 5
coordinate [RuHD(CO)P’ ].
In the mechanism shown in Scheme 3, [RuH
formed from β−D transfer in [RuH (OCD )P’ ] can have various
positions of the H and D, with those in the equatorial plane
being able to be protonated by CD OD. Scheme 4 shows how
these various intermediates formed lead to the observed
isomers of [RuH ]. The numbers shown in bold in
3
], which will
D2
D2
HD
D
HD
D2
P'
P'
P'
P'
D
H
D
H
50
16.7 OC Ru
OC Ru
16.7
1
6.7 OC Ru
OC Ru
H
H
D
P'
P'
2
P'
P'
P'
P'
2
D(CD
2
=O)P’
2
]
P'
P'
P'
P'
D
Ru
H
Ru
CO
D
Ru
2
3
2
H
H
Ru
P'
P'
P'
H
P'
P'
P'
D
P'
P'
P'
D
P'
P'
P'
CO
CO
CO
3
Predicted %
Found
1
6.7
33.3
33.3
28
16.7
%
2
2
28
22
n 3
D2-n(CO)P’
Sch. 4 Proposed mechanism for the formation of [RuH
isomers) and [RuH (CO)P’ from reaction of [RuH
numbers beside each intermediate represent the theoretical percentage of each
2
(CO)P’
3
], [RuHD(CO)P’
3
] (2
OD. The
Scheme 4 indicate the percentage of each intermediate that
would be formed if no isotope effects operate during this part
2
3
]
4
P’ with Na/CD
3
]
3
of the reaction. The predicted percentages of each product isotopomer of that intermediate. The numbers at the bottom represent the
predicted percentage of each isotopomer of the final product compared with the
actual percentages observed using H NMR spectroscopy.
are: [RuH
[RuHD(CO)P’
2
(CO)P’
3
]
and [RuD
2
(CO)P’
3
]
(each 16.7 %).
] (each isomer 33.3 %). These values compare
1
3
quite well with the experimentally determined values of 22 %
and 28 % respectively, shown in italics in Scheme 4. The small
deviations are probably due to experimental error in the
integration of the individual peaks in the hydride signals of the
1
H NMR spectrum. Indeed, if only the largest peaks in the
Conclusions
hydride signals are used for the analysis (better signal to noise
ratio), the observed ratios are 20:30, closer to the predicted
values. It is unlikely that the differences between the observed
and calculated values arise because of small isotope effects in
The decarbonylation of methanol by [RuHClP
3
] under mild
] which is in
] and when under nitrogen with
3
]. This result shows that alcohol dehydrogenation is
basic conditions proceeds first to [RuH
equilibrium with [RuH
RuH (N )P
4 3
P
2 4
P
2
the assembly of D or HD in the equatorial plane of the
[
2
2
relevant intermediates because these processes have been
calculated to have essentially zero activation energy.
7
favoured over decarbonylation at least under mild conditions
and confirms that the rate determining step of methanol
2
dehydrogenation is loss of H
reaction is carried out using CD
intermediate is [RuH P’ ] where P’ is PPh
incorporated into several of the ortho positions of the
aromatic rings. At higher temperatures, [RuH (CO)P ] is
O the products
] (n = 0, 22 %; n = 1, 2 isomers each 28 %; n
2, 22 %). These products confirm that the decarbonylation
2
from [RuH
4
P
3
]. When this
-
3
O /CD
3
OD, the key
4
3
3
with deuterium
2
3
-
-
formed in CH
3 3 3
OH/MeO , whilst in CD OD/CD
are [RuH D2-n(CO)P
n 3
=
-
occurs through the anionic intermediate mer-[RuH
It is proposed that there is free exchange of all the Ru-H/D in
the formaldehyde intermediate, [RuH D(CD =O)P’ ] which is
2 3
(OMe)P ]
2
2
2
present before the rate determining step but that H or D trans
to the carbon based ligand does not participate in exchange
reactions in subsequent intermediates of the reaction
Acknowledgements
6
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Plea sDe a dl t oo nn oT tr aa nd sj ua cs tt i om nasrgins
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DOI: 10.1039/C8DT01733F
Journal Name
ARTICLE
We thank Lucite international for funding and Michael Bühl for
helpful discussions.
Notes and references
‡
coordinated hydrido and dihydrogen ligands exchanging fast on
[RuH (PPh ) ] is known to be [RuH (H )(PPh ) ] with the
4 3 3 2 2 3 3
1
9
the NMR timescale. We use the condensed form to avoid
confusion when referring to partially D-labelled analogues.
1
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P= PPh3
P’ = partially o-deuterated PPh3