Facile O-H bond activation in alcohols by [Cp*RuCl( iPr2PSX)] (X = pyridyl, quinolyl): A route to ruthenium(IV) hydrido(alkoxo) derivatives

Article abstract of DOI:10.1021/ic201912b

The complexes [Cp*RuCl(ⁱPr₂PSX)] (X = pyridyl, quinolyl) react directly with alcohols ROH (R = Me, Et, iPr, nPr) and NaBPh4, affording the novel cationic hydrido(alkoxo) derivatives [Cp*RuCl( ⁱPr₂PSX)]-[BPh₄]. These ruthenium(IV) compounds result from the formal oxidative addition of the alcohol to the 16-electron fragment {[Cp*Ru(ⁱPr₂PSX)]⁺}, generated in situ upon chloride dissociation. The hydrido(alkoxo) complexes are reversibly deprotonated by a strong base such as KOBu^t, yielding the neutral alkoxides [Cp*Ru(OR)-(ⁱPr₂PSX)], which are remarkably stable toward β elimination and do not generate the corresponding hydrides. The hydrido(alkoxo) complexes undergo a slow electron-transfer process, releasing H2 and generating the dinuclear ruthenium(III) complex [{Cp*Ru(κ²-N, S-μ S-SC ₅H₄N)}₂][BPh₄]₂. In this species, the Ru-Ru separation is very short and consistent with what is expected for a Ru≡Ru triple bond.

Full text of DOI:10.1021/ic201912b

Communication
pubs.acs.org/IC
Facile O−H Bond Activation in Alcohols by [Cp*RuCl(iPr2PSX)] (X =
Pyridyl, Quinolyl): a Route to Ruthenium(IV) Hydrido(alkoxo)
Derivatives
Manuel Jimenez-Tenorio,* M. Carmen Puerta,* and Pedro Valerga
́
́
́ ́
Departamento de Ciencia de Materiales e Ingeniería Metalurgica y Química Inorganica, Facultad de Ciencias, Universidad de Cadiz,
11510 Puerto Real, Cad
́
iz, Spain
S
* Supporting Information
addition of the alcohol to the 16-electron fragment {[Cp*Ru-
(iPr2PSX)]+}, generated in situ upon chloride dissociation.
Although monomeric hydrido(hydroxo) or hydrido(aryloxido)
complexes of ruthenium have been isolated as result of the
oxidative addition of either water6 or p-cresol7 to ruthenium(0)
complexes, this is the first case in which the formal oxidative
addition of alcohols to a ruthenium(II) complex has been
observed. Other hydrido(alkoxo) complexes of ruthenium(II)
have been obtained or postulated as intermediate products in
hydrogen-transfer reactions to ketones.4,8 In our case, the
hydrido(alkoxo) derivatives 3a, 3b, and 4a−4c were charac-
terized by NMR spectroscopy. These complexes exhibit one
outlying methyl resonance of one of the phosphine isopropyl
substituents in the range 0.3−0.6 ppm, whereas the other
methyl groups resonate around 1.1 ppm. This upfield shift is
most likely attributed to ring magnetic current effects from
pyridine or quinoline groups rather than to an unlikely agostic
interaction. We have previously noted a similar upfield shift
affecting phosphine isopropyl substituents in the case of TpRu
complexes, also attributable to ring-current effects.9 The
ABSTRACT: The complexes [Cp*RuCl(iPr2PSX)] (X =
pyridyl, quinolyl) react directly with alcohols ROH (R =
i
n
Me, Et, Pr, Pr) and NaBPh4, affording the novel cationic
hydrido(alkoxo) derivatives [Cp*RuH(OR)(iPr2PSX)]-
[BPh4]. These ruthenium(IV) compounds result from
the formal oxidative addition of the alcohol to the 16-
electron fragment {[Cp*Ru(iPr2PSX)]+}, generated in situ
upon chloride dissociation. The hydrido(alkoxo) com-
plexes are reversibly deprotonated by a strong base such as
KOBut, yielding the neutral alkoxides [Cp*Ru(OR)-
(iPr2PSX)], which are remarkably stable toward β
elimination and do not generate the corresponding
hydrides. The hydrido(alkoxo) complexes undergo a
slow electron-transfer process, releasing H2 and generating
the dinuclear ruthenium(III) complex [{Cp*Ru(κ2-N,S-μ
S-SC5H4N)}2][BPh4]2. In this species, the Ru−Ru
separation is very short and consistent with what is
expected for a RuRu triple bond.
1
hydrido ligand appears in the H NMR spectrum of complexes
ydrido(alkoxo) complexes of transition metals produced
3a, 3b, and 4a−4c as one high-field doublet in the range −6 to
2
H
by OH bond activation are known to be involved in
various metal-mediated catalytic transformations.1 Very re-
cently, Milstein and co-workers have demonstrated the
involvement of hydrido(alkoxo) complexes of ruthenium in
the environmentally benign dehydrogenative coupling of
alcohols to esters with liberation of H2 under neutral
conditions.2 This and other related processes are catalyzed by
pyridine-based PNP and PNN pincer complexes of ruthenium
and are possible through remarkable metal−ligand coopera-
tion.3 Ruthenium catalysts capable of oxidizing alcohols to
ketones also feature OH activation, and the OH bond
activation in the alcohol by the metal complex remains one
of the key steps in such a process.4
−8 ppm in all cases. The values of the coupling constants JHP
for the hydride resonance are between 27 and 31 Hz. These
2
values for JHP compare well with those found in transoid
dihydride complexes of ruthenium, which exhibit cisoid
arrangement of the hydride and phosphine atoms as
2
determined by X-ray crystallography, i.e., JHP = 28.4 Hz in
[Cp*RuH2(PPhiPr2)2][BAr′4] [Ar′ = 3,5-(CF3)2C6H3)]10 or
2JHP = 24 Hz in [Cp*RuH2(dippae)][BPh4] [dippae = 1,2-
bis(diisopropylphosphinoamino)ethane].11 In the trihydride
complexes [Cp*RuH3(iPr2PCH2X) (X = pyridyl, quinolyl),12
2
the values of JHP between H and P in cisoid positions are 31
Hz but are ca. 0 Hz for H and P in transoid positions. These
data suggest that the hydride and the phosphorus atom in
complexes 3a, 3b, and 4a−4c are most likely in mutually cisoid
positions. Consistent with this, NOE NMR experiments
performed on solutions of 3a and 3b with irradiation of the
hydride signals revealed no through-space interaction with
OCH3 or OCH(CH3)2 groups. Reciprocally, irradiation of the
OCH3 (3a) or OCH(CH3)2 (3b) resonances did not reveal
through-space interactions with the hydride. If the hydride and
We have recently reported the preparation of the half-
sandwich complex [Cp*RuCl(iPr2PSPy)] (1).5 We now show
that this complex and the related derivative [Cp*RuCl-
(iPr2PSQuin)] (2; prepared by the reaction of [{Cp*RuCl}4]
i
with Pr2PSQuin in petroleum ether) react directly with
i
n
alcohols ROH (R = Me, Pr, Pr) and NaBPh4 over a period
of 6−12 h at room temperature, affording the corresponding
cationic hydrido(alkoxo) derivatives [Cp*RuH(OR)-
i
(iPr2PSX)][BPh4] [X = Py and R = Me (3a), Pr (3b); X =
Quin and R = Me (4a), iPr (4b), nPr (4c)]. These
Received: September 1, 2011
Published: November 16, 2011
ruthenium(IV) compounds result formally from the oxidative
© 2011 American Chemical Society
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Inorganic Chemistry
Communication
a
alkoxide moieties were arranged in a cisoid manner, a through-
space interaction between the hydride and alkoxide groups
should be observed, but this is not the case. The 13C{1H} NMR
resonances for the oxygen-bound carbon atom of the alkoxide
ligands show coupling with phosphorus (3JCP = 12−15 Hz).
Likewise, the protons attached to these carbon atoms also
display coupling to phosphorus, in addition to coupling with
other hydrogen atoms eventually present in the alkoxide
moieties. These spectral data are consistent with a four-legged
piano-stool structure, with a transoid arrangement of hydride
and alkoxide ligands. This description is particularly relevant
because a “classic” oxidative addition of the alcohol would
produce complexes with hydride and alkoxide in mutually
cisoid positions. With a transoid stereochemistry, we should
consider an abnormal oxidative addition mechanism, which
might be related to the one recently proposed by Brookhart
and co-workers13 for the formation of a trans-iridium(III)
dihydride via proton-catalyzed hydrogenation. This possibility
is currently under study.
Scheme 1
The use of the mercaptopyridyl- or mercaptoquinolylphos-
phine ligands iPr2PSX seems crucial for achieving OH
activation because this process has not been observed in the
case of homologous complexes containing pyridyl- or
quinolylphosphines with spacer groups other than S, such as
a
(i) NaBPh4, ROH (R = Me, iPr, nPr), 6−12 h; (ii) KOBut, THF; (iii)
HBF4·OEt2, Et2O, −80 °C; (iv) MeCN or CO/CH2Cl2.
i
iPr2PNHPy or Pr2PCH2X (X = Py, Quin). Thus, no reaction
between [Cp*RuCl(iPr2PCH2X)] and NaBPh4 in MeOH is
observed, whereas in the case of [Cp*RuCl(iPr2PNHPy)], the
reaction with NaBPh4 in MeOH leads to [Cp*RuCl(κ1-
P-iPr2PCH2Py)(κ2-P,N-iPr2PCH2Py)][BPh4].14 No MeOH ac-
tivation products have been detected.
The ruthenium(IV) compounds herein described undergo
reductive elimination readily. Thus, the hydrido(alkoxo)
complexes 3a and 3b react with ligands such as CO or
MeCN, releasing the alcohol and furnishing the corresponding
ruthenium(II) species [Cp*Ru(L)(iPr2PSPy)][BPh4] (L = CO,
MeCN). 3a, 3b, 4a, and 4b are deprotonated by a strong base
such as KOBut in tetrahydrofuran (THF), yielding the neutral
alkoxides [Cp*Ru(OR)(iPr2PSX)] [X = Py and R = Me (5a),
i
iPr (5b); X = Quin and R = Me (6a), Pr (6b)]. These species
are stable toward β elimination and do not generate the
corresponding hydrides even when heated at 70 °C in C6D6.
This observation is not necessarily surprising, given the fact that
quantitative studies performed on the mechanism of β-
hydrogen elimination from square-planar iridium(I) alkoxide
complexes have shown that such species can be quite robust
and require hours to decompose at 80−110 °C.15 The neutral
ruthenium(II) alkoxides are unreactive toward the insertion of
CS2 to yield xanthato complexes and toward primary amines
such as PhNH2. However, and quite remarkably, they are
protonated with HBF4 in Et2O at the metal site, regenerating
the corresponding cationic hydrido(alkoxide) [Cp*RuH(OR)-
(iPr2PSX)]+ in the form of a [BF4]− salt. These reactions are
summarized in Scheme 1.
Attempts made to crystallize any of the hydrido(alkoxo)
derivatives from dichloromethane/petroleum ether mixtures
were unsuccessful. In all cases, the solutions turned deep green
upon standing under dinitrogen or argon. Crystals of the
dinuclear ruthenium(III) complex [{Cp*Ru(κ2-N,S-μ-S-
SC5H4N)}2][BPh4]2 (7) were obtained from the attempted
recrystallization of 3b. An ORTEP view of the cation
[{Cp*Ru(κ2-N,S-μ-S-SC5H4N)}2]2+ is shown in Figure 1,
together with selected bond lengths and angles. The thiolate-
bridged dinuclear Cp*Ru complex 7 is structurally related to
Figure 1. ORTEP drawing (50% thermal ellipsoids) of [{Cp*Ru-
(SC5H4N)}2]2+ in 7. Hydrogen atoms have been omitted. Selected
bond lengths (Å) and angles (deg) with estimated standard deviations
in parentheses: Ru1−Ru1b 2.4964(10), Ru1−S1 2.4218(13), Ru1−
S1b 2.4512(12), Ru1−N1 2.163(4); Ru1−S1−Ru1b 61.63(4), S1−
Ru1−S1b 117.18(4), N1−Ru1−S1b 66.52(11).
the species extensively studied by Nishibayashi and co-workers,
which are particularly relevant in the context of catalytic
propargylation reactions of ketones.16 The structure of the
complex cation in 7 is very similar to that of the homologous
species [{CpRu(κ2-N,S-μ-S-SC5H4N)}2]2+, reported by Kirch-
ner and co-workers.17 The Cp* ligands are mutually cisoid, and
the SC5H4N ligands act simultaneously as chelating and
bridging ligands. The most important difference between the
cations [{(C5R5)Ru(SC5H4N)}2]2+ (R = H, Me) lies in the
separation of the two ruthenium atoms and also in the value of
the angle Ru1−S1−Ru1b. In 7, the Ru1−Ru1b bond length is
2.4964(10) Å, whereas for [{CpRu(SC5H4N)}2]2+, it is
2.789(1) Å. These values are consistent with a RuRu triple
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Inorganic Chemistry
Communication
bond18 and a Ru−Ru single bond, respectively, and with the
diamagnetic character of both cations. Also, the Ru1−S1−Ru1b
angle in 7 is significantly more acute [61.63(4)°] than that in
the Cp analogue [73.9(1)°], suggesting a much stronger Ru−
Ru interaction. The contraction in Ru−Ru bond distances upon
going from Cp to Cp*, which is a stronger donor, is a most
interesting observation. In spite of this, the observed short
distance does not necessarily imply multiple bonding. Density
functional theory calculations are clearly needed here in order
to clarify the status of the metal−metal interactions in this
complex.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, detailed synthetic
procedures, NMR spectral data for the complexes, and
experimental details for the X-ray structure analysis of 7. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: manuel.tenorio@uca.es (M.J.-T.), carmen.puerta@
uca.es (M.C.P.). Fax: (+34) 956 016288.
■
We can tentatively explain the formation of the dinuclear
complex 7 at the expense of the hydrido(alkoxide) complexes
3a and 3b by considering an electron transfer from the hydride
to the metal, leading to an intermediate ruthenium(III)
alkoxide species with concomitant loss of dihydrogen.
Migration of the alkoxide group over the PiPr2 moiety with
subsequent cleavage of the P−S bond would generate
ACKNOWLEDGMENTS
We thank the Spanish Ministerio de Investigacio
■
́
Innovacio
́
́
Andalucıa (Grants PAI FQM188 and P08 FQM 03538) for
financial support and Johnson Matthey plc for generous loans
of ruthenium trichloride.
i
P(OR)iPr2 (R = Me, Pr) plus [(C5Me5)Ru(SC5H4N)]+.
Dimerization of the latter yields 7 (Scheme 2).
REFERENCES
■
Scheme 2. Proposed Reaction Sequence for the Formation
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