Effect of ring size on the properties of μ3-Cycloalkyne complexes: Synthesis of triruthenium complexes containing a perpendicularly coordinated μ3-Allenyl ligand

Article abstract of DOI:10.1016/j.jorganchem.2019.01.020

A series of triruthenium complexes, [{Cp*Ru(μ-H)}₃(μ₃-η²:η²-C_nH_2n-4)] (2a, n = 6; 2b, n = 7; 2c, n = 8), containing a triply bridgin cycloalkyne ligand was synthesized. Although no noticeable differences were detected in the ground state, the rate of dynamic motion of the cycloalkyne ligand increased remarkably as the ring size increased. Owing to the flexibility of the larger rings, an allylic C–H bond can interact with one of the metal centers, stabilizing the transition state of the dynamic process. This interaction also caused allylic C–H bond scission upon chemical oxidation. In the case of 2b and 2c, a cationic μ₃-η²:η²:η²-allenyl complex, [{Cp*Ru(μ-H)}₃(μ₃-η²:η²:η²-C_nH_2n-5)]⁺ (3b, n = 7; 3c, n = 8), was obtained. X-ray diffraction studies of 3 clearly show that the allenyl group is coordinated to one of the Ru-Ru bonds in a perpendicular fashion, unlike the known “parallel” μ₃-allenyl complexes. In contrast to the reactions of 2b and 2c, chemical oxidation of 2a occurred via the incorporation of adventitious H₂O rather than intramolecular C-H bond cleavage. Consequently, cationic μ₃-cyclohexyne-μ-hydroxo complex, [{Cp*Ru(μ-H)}₃(μ-OH)(μ₃-η²-C₆H₈)]⁺ (5a) was obtained in 38% yield by the reaction with 2 equiv [Cp₂Fe]⁺ in the presence of excess water. These results clearly show that the reaction pathway of the μ₃-cycloalkyne complex varies with the ring size.

Full text of DOI:10.1016/j.jorganchem.2019.01.020

Accepted Manuscript
Effect of ring size on the properties of μ -Cycloalkyne complexes: Synthesis of
3
triruthenium complexes containing a perpendicularly coordinated μ -Allenyl ligand
3
Moeko Saito, Shuhei Kojima, Akiko Inagaki, Koichi Seki, Toshiro Takao
PII:
S0022-328X(19)30038-5
DOI:
https://doi.org/10.1016/j.jorganchem.2019.01.020
JOM 20698
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 26 December 2018
Revised Date: 29 January 2019
Accepted Date: 30 January 2019
Please cite this article as: M. Saito, S. Kojima, A. Inagaki, K. Seki, T. Takao, Effect of ring size on
the properties of μ -Cycloalkyne complexes: Synthesis of triruthenium complexes containing a
3
perpendicularly coordinated μ -Allenyl ligand, Journal of Organometallic Chemistry (2019), doi: https://
3
doi.org/10.1016/j.jorganchem.2019.01.020.
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ACCEPTED MANUSCRIPT
Effect of Ring Size on the Properties of µ3-Cycloalkyne Complexes:
Synthesis of Triruthenium Complexes Containing a Perpendicularly
Coordinated µ3-Allenyl Ligand
Moeko Saito, Shuhei Kojima, Akiko Inagaki, Koichi Seki, Toshiro Takao*
Department of Chemical Science and Engineering, School of Materials and Chemical
Technology, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo
152-8552, Japan
*Corresponding author
Phone/fax number: +81-3-5734-2580
E-mail address: takao.t.aa@m.titech.ac.jp (T. Takao)
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1. Introduction
Chemical transformations of alkynes at multimetallic sites have been intensively
studied since the 1950s and various alkyne-derived ligands, such as acetylide, alkenylidene,
alkylidene, and alkylidyne, have been prepared in addition to alkyne supported clusters
[1−3]. Their reactivities have been also intensively studied because they are considered to
be suitable models of chemisorbed species on a metal surface [4,5]. In fact, the series of
transformations
from
[Ru3(H)(µ3-η2:η2-C≡Ct-Bu)(CO)9]
[Ru3(H)2(µ3-η2-HCCt-Bu)(CO)9],
to
[Ru3(H)2(µ3-η2-C=Ct-BuH)(CO)9],
and
[Ru3(H)3(µ3-CCH2t-Bu)(CO)9] accompanied by the incorporation of dihydrogen, is
regarded as a key step in the hydrogenation of alkynes at multimetallic sites [6].
Scheme 1. Plausible mechanism for the transformation of a linear alkane at the Ru3 plane of 1.
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We have shown that the reaction of the triruthenium pentahydrido complex,
[{Cp*Ru(µ-H)}3(µ3-H)2] (1) (Cp* = η5-C5Me5) with a linear alkane yields a
closo-ruthenacyclopentadiene complex [7] and revealed the whole reaction mechanism,
which consists of two reversible and four irreversible processes eliminating dihydrogen
(Scheme 1) [8]. During the reaction of 1 with hexane, the formation of a
µ3-η2:η2( )-1-hexyne complex was observed in the initial stage [7]. The
µ3-η2:η2( )-1-hexyne complex has been shown to equilibrate with a µ3-η2-hexenylidene
complex via the 1,2-shift of the acetylenic proton [9]. In contrast, an internal alkyne ligand
placed at the multimetallic site cannot undergo this transformation. Thus, it is anticipated
that an alternative transformation involving C−H bond cleavage at the next position of the
acetylenic carbon occurs. The reaction of [Ru3(CO)12] with 4-methylpent-2-yne yields a
mixture of µ3-allenyl complexes via the initial formation of an µ3-alkyne intermediate [10].
The formation of the µ3-allenyl complex demonstrates the importance of the close
proximity of the allylic C−H bond of the alkyne ligand to a one of the metal centers.
The reduction in the valence electrons at the metal center induced by dehydrogenation
from the cluster core or chemical oxidation is compensated by a change in the coordination
mode of the hydrocarbyl ligand to allow more electron donation [11]. This sometimes
causes a skeletal rearrangement of hydrocarbyl ligands at multimetallic sites. Thus, the
flexibility of the hydrocarbyl ligand affects the skeletal rearrangement of the hydrocarbyl
ligand. However, the flexibility of the alkyl group is reduced significantly by cyclization. In
this regard, we previously reported the preparation of triruthenium complexes containing a
µ3-cycloalkyne ligand [12]. In this paper, we report the effect of the ring size on the
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reactivity of µ3-cycloalkyne complexes. The flexibility of the cycloalkyne ligand varies
greatly depending on the ring size, and a larger cycloalkyne ligand has been shown to
undergo skeletal rearrangement to a novel perpendicularly coordinated µ3-η2:η2:η2-allenyl
ligand.
2. Results and discussion
2.1. Properties of µ3-cycloheptyne and cyclooctyne complexes
Similar
to
the
preparation
of
[12],
a
µ3-cyclohexyne
µ3-cycloheptyne
complex,
complex,
[{Cp*Ru(µ-H)}3(µ3-η2:η2( )-C6H8)]
(2a)
a
[{Cp*Ru(µ-H)}3(µ3-η2:η2( )-C7H10)] (2b), was exclusively obtained by the reaction of 1
with cycloheptene (Eq. (1)). The formation of the µ3-η2:η2( )-cycloheptyne ligand in 2b
was unambiguously confirmed by X-ray diffraction (XRD) as shown in Fig. 1. However, a
precise discussions of the structural parameters is hampered by the disorder in the structure.
There is a crystallographic mirror plane bisecting the Ru3 triangle passing through the
Ru(2) and C(5) atoms; hence, the µ3-cycloheptyne ligand is disordered over two positions
(see, Fig. S2b in the Supplementary Material). For clarity, the µ3-cycloheptyne ligand
bonded to the Ru(1)−Ru(2) bond is omitted in Fig. 1.
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Fig. 1. Molecular structure and labeling scheme of 2b with thermal ellipsoids at 30% probability. All of
the hydrogen atoms could not be located due to the disordered structure and the disordered atoms were
omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.8136(6), Ru(1)−Ru(1A)
2.7663(7), Ru(1)−C(2) 2.047(3), Ru(2)−C(1) 1.916(3), Ru(2)−C(2) 2.340(3), Ru(1A)−C(1) 2.142(3),
Ru(1A)−C(2) 2.355(4), C(1)−C(2) 1.4340(13), C(1)−C(7) 1.4953(13), C(2)−C(3) 1.4971(13), C(3)−C(4)
1.5576(14), C(4)−C(5) 1.491(6), C(5)−C(6) 1.552(7) , C(6)−C(7) 1.5687(14), Ru(2)−Ru(1)−Ru(1A)
60.55(1), Ru(1)−Ru(2)−Ru(1A) 58.890(18), C(1)−C(2)−C(3) 117.1(3).
The cycloheptyne ligand is π-bonded to the Ru3 plane through the C(1) and C(2) atoms
in a perpendicular fashion with respect to the Ru(1A)−Ru(2) vector. In contrast to the
µ3-cyclohexyne ligand in 2a shown in Fig. 2, the cycloheptyne ligand is bent at the
C(3)−C(7) axis and inclined greatly toward the Ru(2) atom, which means that the three
ruthenium centers are non-equivalent.
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Fig. 2. Molecular structure and labeling scheme of 2a with thermal ellipsoids at 30% probability. A
solvent molecule (THF) was omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ru(1)−Ru(2) 2.7805(11), Ru(1)−Ru(3) 2.7815(10), Ru(2)−Ru(3) 2.8392(11), Ru(1)−C(2) 2.139(8),
Ru(2)−C(1) 2.034(8), Ru(2)−C(2) 2.293(8), Ru(3)−C(1) 2.055(8), Ru(3)−C(2) 2.297(7), C(1)−C(2)
1.394(11), C(1)−C(6) 1.527(11), C(2)−C(3) 1.533(11), C(3)−C(4) 1.54(2), C(3)−C(4A) 1.51(2),
C(4)−C(5) 1.51(3), C(4A)−C(5A) 1.51(3), C(5)−C(6) 1.533(19)
,
C(5A)−C(6) 1.57(2),
Ru(2)−Ru(1)−Ru(3) 61.39(3), Ru(1)−Ru(2)−Ru(3) 59.32(3), Ru(1)−Ru(3)−Ru(2) 59.29(3),
C(2)−C(1)−C(6) 125.3(7), C(1)−C(2)−C(3) 119.4(7).
In the 1H NMR spectrum recorded at 25 °C, the three Cp* groups were observed to be
equivalent and resonated at δ 1.84 ppm (Fig. 3b). This is due to the switchback motion of
the µ3-cycloheptyne ligand over the three Ru−Ru edges [12]. The switchback motion also
renders the inner- and outer methylene signals at the C(3) and C(7) atoms equivalent, as
well as those at the C(4) and C(6) atoms. Consequently, three complexly coupled signals
were observed at δ 2.69 (C3H2 and C7H2), 1.73 (C5H2), and 1.38 ppm (C4H2 and C6H2) with
an intensity ratio of 2:1:2.
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1
Fig. 3. Variable-temperature H NMR spectra of (a) 2a, (b) 2b, and (c) 2c showing Cp* regions (400
MHz, (a) toluene-d8, (b), (c) toluene-d8:THF-d8= 1:4).
The Cp* signal broadened with decreasing temperature, and decoalesced into three
broad signals below −90 °C. The hydrido signal resonating at δ −12.90 ppm as a singlet at
25 °C also divided into three signals at δ −7.55, −7.91, and −25.54 ppm at −110 °C (see,
Fig. S8b in the Supplementary Material). These spectral features are consistent with the
XRD structure of 2b, but different from the low-temperature limiting spectra of 2a recorded
at −70 °C, which contained two signals for Cp* with an intensity ratio of 2:1 (Fig. 3a).
Although µ3-cyclohexyne complex 2a is also intrinsically unsymmetrical, it displayed a
time-averaged CS spectrum owing to the rapid wagging motion of the C(4) and C(5) atoms.
Α µ3-cyclooctyne complex, [{Cp*Ru(µ-H)}3(µ3-η2:η2( )-C8H12)] (2c), was also
obtained in the reaction of 1 with cyclooctene, but considerable amounts of unidentified
byproducts were also formed during the reaction, unlike the reaction with cyclohexene and
cycloheptene. In contrast, 2c was cleanly formed by reaction with 1,3-cyclooctadiene,
which proceeded at low temperature (60 °C) (Eq. (2)). Although the reaction of 1 with
1,3-cyclohexadiene
yielded
a
face-capped
benzene
complex,
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[{Cp*Ru(µ-H)}3(µ3-η2:η2:η2-C6H6)] [13], µ-η2:η2-coordination of 1,3-cyclooctadiene
seems to be unfavorable because of the increased steric repulsion with the Cp* groups.
Thus, 1,3-cyclooctadiene should react as an internal alkene along with the hydrogenation of
the remaining C=C double bond. The reaction of 1 with internal alkenes showed that two
hydrides were removed from 1 via hydrogenation [12]. Although we did not detect the
formation of cyclooctene, it is inferred that hydrogenation of 1,3-cyclooctadiene occurs at
the initial stage of the reaction.
Although the XRD structure of 2c could not be refined because of the presence of
disorder in both the Cp* and cyclooctyne moieties, the preliminary result suggests that the
structure of 2c is similar to 2b; that is, it possesses a cyclooctyne ligand inclined toward
one side of the Ru−Ru bond where the cyclooctyne ligand is perpendicularly coordinated
1
[14]. The H NMR spectrum of 2c recorded at −110 °C contains three Cp* and three
1
hydrido signals like those observed in the H NMR spectrum of 2b recorded at −110 °C
(Fig. 3c).
Because the ring inversion of the cycloalkyne ligand was not completely frozen in 2b
and 2c even at −110 °C, kinetic analysis using curve fitting of the spectra could not be
performed. Thus, the mobility of these cycloalkyne ligands could not be compared
quantitatively. Nevertheless, Fig. 3 shows that the rate of the switchback motion of the
cycloalkyne ligand notably increased as the ring size increased. The Cp* signal of 2a
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significantly broadened at −10 °C, whereas that of 2b broadened only slightly at −50 °C.
Furthermore, the Cp* signal of 2c even remained sharp even at −50 °C (Fig.3).
If the magnitude of back donation to the cycloalkyne moiety changed depending on the
ring size, in particular because of the steric repulsion, the mobility of the cycloalkyne
ligand at the Ru3 site should be affected. The difference in π-interactions would also affect
the highest occupied molecular orbital (HOMO) energy. However, these µ3-cycloalkyne
complexes displayed similar redox potentials for the 0/+1 process: reversible redox waves
at half-wave potentials (E1/2) of −734, −735, and −723 mV for 2a, 2b, and 2c, respectively,
were observed in the cyclic voltammograms (Fig. 4). In addition, these complexes also
show similar UV-vis spectra (Fig. S13 in the Supplementary Material). These results imply
that the ring size has little influence on the electronic structure of the µ3-cycloalkyne
complex.
Fig. 4. Cyclic voltammograms of (a) 2a, (b) 2b, and (c) 2c measured in THF at 25 °C. The red dotted
lines are the results of the scans ranging from −1150 mV to −450 mV. Conditions: electrolyte, 0.1M
[NBu4](PF6); working electrode, Pt; counter electrode, Pt, scan rate 100 mV/s. Potentials are referenced
to Cp2Fe/[Cp2Fe]+.
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Because the XRD data of 2b and 2c were of poor quality owing to the presence of
disorder, density functional theory (DFT) calculations were performed for 2a, 2b, and 2c to
assess the interaction of the Ru centers with the cycloalkyne moiety. Because optimized
structure 2a′ is consistent with the XRD structure, the same basis sets were used for the
optimization of 2b and 2c. The optimized structures, 2a′, 2b′, and 2c′, are shown in Fig. 5
and relevant bond lengths are listed in Table 1, together with the observed values for 2a.
Fig. 5. Central parts of the optimized structures of (a) 2a′, (b) 2b′, and (c) 2c′. Cp* moieties were
omitted for clarity; (above) Top view, (below) side view. Gaussian 09 at the ωB97XD level with triple-ζ
SDD basis sets for Ru and 6-31G(d) basis sets for H and C.
2a (XRD)
2a′
2b′
2c′
2.7805(11)
2.7815(10)
2.8392(11)
2.139(8)
2.794
2.796
2.829
2.142
2.039
2.259
2.797
2.791
2.831
2.164
2.042
2.260
2.799
2.802
2.842
2.158
2.037
2.343
Ru(1)−Ru(2)
Ru(1)−Ru(3)
Ru(2)−Ru(3)
Ru(1)−C(2)
Ru(2)−C(1)
Ru(2)−C(2)
2.034(8)
2.293(8)
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Table 1. Selected bond lengths (Å) of the
optimized structures 2a , 2b , and 2c and
corresponding lengths observed in 2a.
2.055(8)
2.297(7)
1.394(11)
2.023
2.288
1.397
2.031
2.305
1.392
2.046
2.229
1.396
Ru(3)−C(1)
Ru(3)−C(2)
C(1)−C(2)
′
′
′
The Ru−Ru lengths increased, although only slightly (< 0.013 Å), with increasing ring
size. The differences in Ru−C bond lengths also lie within 0.1 Å. In particular, the C1−C2
bond lengths are almost the same in these complexes. These results strongly suggest that
the strength of the π-interactions of these cycloalkyne ligands are not different, at least in
the range of 6-, 7-, and 8-membered rings. Thus, the increased mobility of the cycloheptyne
and cyclooctyne ligands does not arise from the difference in the ground state but from the
transition state of the switchback motion. The barrier of the switching motion would be
lowered owing to the larger cycloalkyne ligand.
Owing to their coordinatively unsaturated nature, the hydrides in µ3-η2:η2( )-alkyne
complexes undergo H/D exchange between C6D6 above 80 °C [15]. Although the
thermolysis of 2a and 2b in C6D6 at 120 °C only resulted in the scrambling of deuterium at
the hydrido sites, 2c underwent additional H/D exchange in the cyclooctyne moiety.
Deuterium was incorporated at all the methylene positons, in addition to the hydrido
ligands, and the ratio of deuteration reached ca. 50% after 7 days. During this period, a
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considerable amount of 2c (ca. 20%) decomposed. Although the products have not been
characterized at present, this thermal instability of 2c may be the reason why 2c was not
cleanly obtained by the reaction with cyclooctene unlike 2a and 2b.
The deuteration of the cyclooctyne ligand most likely proceeds via the formation of
µ3-allenyl intermediate B-1, as shown in Scheme 2. The flexible cyclooctyne ligand in 2c
allows the oxidative addition of the C−H bond at the allylic position. Because 2c is
coordinatively unsaturated, the endo C−H bond at the C3 position can interact with a Ru
center prior to the C−H bond scission and form a saturated intermediate A. On adopting a
saturated 48-electron configuration, the orientation of the alkyne ligand changes from
perpendicular to parallel with respect to the Ru−Ru bond. A µ3-η2:η2-allenyl intermediate,
B-1, is formed by the following C−H bond scission. The formations of the µ3-η2:η2-allenyl
complex from a µ3-η2(||)-alkyne complex has been reported previously [16−19], as well as
its reverse process upon hydrogenation [20,21].
Scheme 2. Plausible mechanism of the H/D exchange in 2c.
H
D
D
C3
C2
H
C1
C1
D
C3
C2
C1
Ru
D
C3
C2
Ru
Ru
Ru
Ru
Ru
Ru
D
Ru
switchback
motion
Ru
H
D
D
D
H
H
D
H
2c-1
2c-2
2c-3
C6D6
CHnD2-n
D2-nHn
D2-nHn
D2-nHn
C
H
H
C3
C2
CHnD2-n
CHnD2-n
H
C3
C2
C
C1
C3
H
C1
H
C2
D
C1
C
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
D
D
D
D
D
D
D
D
H
HmD3-m
Ru
windshield
wiper
A
B-1
B-2
motion
Ru = (C5Me5)Ru
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Although we could not observe B-1 directly, Braga et al. isolated a triruthenium cluster
containing an eight-membered ring bonded in a µ3-η2:η2-allenic mode [22]. The
µ3-η2:η2-allenyl ligand in B exhibited a windshield-wiper-like motion on the Ru3 plane via
C−H bond cleavage at the C3 position accompanied by C−D bond formation at the C1
position, leading to the formation of B-2. This gives rise to the incorporation of deuterium
into the cyclooctyne moiety and also causes the site-exchange between the C1 and C3 atoms.
Although such motion of a µ3-η2:η2-allenyl complex has never been observed, it has been
inferred for [CpMo{P(OMe)3}2(η2-PhC=C=CPhH)] (Cp
=
η5-C5H5) from
variable-temperature 31P NMR measurements [23]. Through these processes, the
cyclooctyne ligand rotates vertically on the Ru3 plane like a wheel, and deuterium becomes
incorporated into all the methylene positions by linking with the switchback motion.
It is reasonable to assume that the coordination mode of the alkyne ligand transforms
into a parallel mode in the transition state during the switchback motion of the alkyne
ligand. The parallel coordination mode is characteristic of coordinatively saturated
µ3-alkyne complexes [2]. In this regard, additional σ-coordination of the allylic C−H bond,
as seen in intermediate A in Scheme 1, is expected to stabilize the transition state
effectively. This would accelerate the switchback motion of the µ3-alkyne ligand. As
mentioned above, pronounced acceleration of the switchback motion was also observed for
2b. Although 2b did not show H/D exchange of the cycloheptyne moiety, the cycloheptyne
ligand seems to be sufficiently flexible to form an agostic interaction at the allylic C−H
position.
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2.2. Preparation of cationic µ3- : :η
η2 η2 2-allenyl complexes
As mentioned above, the formation of the µ3-η2:η2-allenyl complex is implied during
the H/D exchange reaction of 2c. Because 2c gradually decomposed upon thermolysis,
chemical oxidation of 2c was examined to obtain the skeletal rearrangement of the
cycloalkyne moiety. A decrease in the number of valence electrons at the metal center
would be compensated by the skeletal rearrangement of the hydrocarbyl ligand [11]. The
weak interaction of the allylic C−H bond with a metal center in 2c should promote C−H
bond scission, leading to the formation of an allylic ligand.
As shown in the cyclic voltammogram of 2c (Fig. 4c), the second anodic current was
observed around 0 mV. Thus, two-electron oxidation of 2c using [Cp2Fe]+ in toluene was
1
carried out, which immediately afforded a dark brown precipitate (Eq. (3)). The H NMR
spectrum of the crude mixture indicates the disappearance of 2c, but the signals were
severely broad. This broadening is likely ascribed to equilibrium between the cationic
µ3-η2:η2:η2-allenyl complex [{Cp*Ru(µ-H)}3(µ3-η2:η2( )-C8H11)]+ (3c) and its protonated
species. Spontaneous deprotonation from a transient dicationic species was also observed in
the two-electron oxidation of [(Cp*Ru)3(µ3-CH)(µ3-η3-CHCMeCH)(µ-H)], yielding
[(Cp*Ru)3(µ3-CH)(µ3-η3-C3MeH2)]+ [24]. Although the dicationic complex could not be
isolated from the mixture,
a
monocationic µ3-η2:η2:η2-allenyl complex,
[{Cp*Ru(µ-H)}3(µ3-η2:η2:η2-C8H11)]+ (3c), was isolated in 21% yield by column
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chromatography on alumina.
Judging from the reversibility of the first redox process in the cyclic voltammogram, the
45-electron intermediate C shown in Scheme 3 seems to be similar to 2c. The oxidation of
2c likely proceeds consecutively, but C was not observed in the reaction conducted with an
equimolar amount of [Cp2Fe]+. Instead, 3c was obtained nearly quantitatively. This result is
rationalized as shown in Scheme 3. Because the second oxidation step smoothly occurred,
half of 2c underwent two-electron oxidation, leading to the formation of dicationic
intermediate D. Allylic C−H bond scission forming a 46-electron intermediate E would be
promoted because of the highly unsaturated nature of D. Complex 3c is formed by the
subsequent deprotonation from E. The H+ liberated from E then reacted with the remaining
2c to yield cationic µ3-η2:η2( )-cyclooctyne complex 4c. Although 4c has not been isolated
in an analytically pure form yet, it was prepared by the reaction of 2c with HBF4 [25].
Complex 4c gradually transformed to 3c at ambient temperature via dehydrogenation, and
3c was finally obtained in 88% yield using an equimolar amount of [Cp2Fe]+.
Scheme 3. Plausible mechanism of the formation of µ3- : :η
η2 η2 2-allenyl complex 3c.
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The seven-membered ring analog [{Cp*Ru(µ-H)}3(µ3-η2:η2( )-C7H9)]+ (3b) was also
obtained by the reaction with 2 equiv [Cp2Fe]+ in 9% yield. As seen for 2c, a mixture of 3b
and 4b was obtained upon treatment with 1 equiv [Cp2Fe]+. However, 4b was not
transformed into 3b unlike 4c. Instead, 4b decomposed into an unidentified paramagnetic
species. Thus, 3b could not be synthesized in high yield. As for µ3-cyclohexyl complex 2a,
the µ3-allenyl complex was not obtained at all upon oxidation, but it gave a different
cationic complex, as mentioned later. These facts strongly imply that the flexibility of the
cycloalkyne ligand, that is, the accessibility of the allylic C−H bond to one of the metal
centers, is crucial for the formation of the µ3-allenyl complex.
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Fig. 6. Molecular structure and labeling scheme of 3b with thermal ellipsoids at 30% probability.
−
Anionic part of the salt (PF6 ) and solvent molecule (THF) were omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ru(1)
Ru(1) C(2) 2.041(3), Ru(1) C(3) 2.225(3), Ru(2)
2.062(3), Ru(3) C(2) 2.099(3), C(1) C(2) 1.392(4), C(2)
Ru(1) Ru(2) Ru(3) 61.493(10), Ru(1) Ru(3) Ru(2) 59.887(9), C(1)
−
Ru(2) 2.8378(4), Ru(1)
C(1) 2.034(3), Ru(2)
C(3) 1.395(4), Ru(2)
C(2) C(3) 137.4(3).
−
Ru(3) 2.8828(4), Ru(2)
C(2) 2.278(3), Ru(3)
Ru(1) Ru(3) 58.620(8),
−
Ru(3) 2.8009(3),
−
−
−
−
−C(1)
−
−
−
−
−
−
−
−
−
−
−
Fig. 7. Molecular structure and labeling scheme of 3c with thermal ellipsoids at 30% probability.
−
Disordered atoms (C(1A), C(3A), C(4A), and C(8A)) and anionic part of the salt (BPh4 ) were omitted
for clarity. The hydrogen atom attached to C(3) could not be located owing to the disorder. Selected
bond lengths (Å) and angles (deg): Ru(1)
2.8206(4), Ru(1) C(2) 2.085(4), Ru(1) C(3) 2.208(8), Ru(2)
C(1) 2.056(7), Ru(3) C(2) 2.136(4), C(1) C(2) 1.360(9), C(2)
Ru(1) Ru(3) 59.502(11), Ru(1) Ru(2) Ru(3) 60.772(11), Ru(1) Ru(3) Ru(1) 59.727(11),
−
Ru(2) 2.8271(5), Ru(1)
C(1) 2.041(8), Ru(2)
C(3) 1.439(9),
−
Ru(3) 2.8567(4), Ru(2)
−Ru(3)
−
−
−
−C(2) 2.285(5),
Ru(3)
Ru(2)
−
−
−
−
−
−
−
−
−
−
17

ACCEPTED MANUSCRIPT
C(1)−C(2)−C(3) 145.2(5).
The molecular structures of 3b and 3c are shown in Figs. 6 and 7, respectively. The
cyclic allenyl ligand in 3c is disordered, and the disordered carbon atoms are omitted for
clarity. Although the hydrido positions could not be determined for 3c, three µ-hydrides
were successfully located at each Ru−Ru edge in 3b. The Ru(1)−C(3) distances (3b,
2.225(3) Å; 3c, 2.208(8) Å) are significantly shortened from that in 2b (3.04 Å) upon the
formation of a µ3-η2:η2:η2-allenyl ligand and lie in the typical range of π-coordinated
Ru−C bond lengths. Multiple bonding between C(2) and C(3) is also inferred by the
shortening in the C(2)−C(3) distances (3b, 1.395(4) Å; 3c, 1.439(9) Å) compared to the
C(2)−C(3) length in 2b (1.4971(13) Å). In contrast, the C(1)−C(2) distances (3b, 1.392(4)
Å; 3c, 1.360(9) Å) are not significantly different from those of the perpendicularly
coordinated alkyne ligands.
The most salient feature of 3b and 3c is the coordination mode of the allenyl ligand,
which is coordinated to the Ru(2)−Ru(3) edge in a perpendicular fashion through the C(1)
and C(2) atoms. This coordination is unlike that of known µ3-η2:η2-allenyl complexes. In
Scheme 4, the molecular structure of [Ru3(µ-H)(µ3-η2:η2-C8H11)(CO)9] [22] is shown for
comparison. In this structure, the allenyl ligand is π-bonded to Ru(3) and is nearly parallel
to the Ru(1)−Ru(2) vector. According to the effective atomic number (EAN) rule, a
µ3-allenyl ligand acts as a five-electron donor. Although all of the reported µ3-allenyl
complexes are saturated 48-electron complexes, both 3b and 3c adopt an unsaturated
46-electron configuration. Thus, the µ3-allenyl ligand adopts a perpendicular coordination
mode to compensate for the unsaturated nature of the Ru3 core, which is similar to the
18

ACCEPTED MANUSCRIPT
relationship between the parallel and perpendicular alkyne complexes [2].
Scheme 4. Comparison of coordination modes of the allylic C8 moiety in (a) 3c and (b)
η2 2-C8H11)(CO)9 [22].
Ru3(µ-H)(µ3- :η
In spite of the difference in the coordination mode, the values of C(1)−C(2)−C(3) (3b,
137.5(3)°; 3c, 145.2(5)°) lie in the reported range for “parallel” µ3-allenyl complexes
(134.0−148.3°) [26]. Although the C(2)−C(3) distance in 3c (1.439(9) Å) is slightly longer
than the reported values, the C(1)−C(2) and C(2)−C(3) distances are also comparable to the
reported values for known µ3-η2:η2-allenyl complexes (C(1)−C(2), 1.34−1.48 Å; C(2)−C(3),
1.34−1.40 Å) [26]. Notably, the C(1)−C(2)−C(3) value and the C(2)−C(3) distance of 3c
are considerably larger than those of 3b. These results suggest that smaller ring strain of the
eight-membered ring allows stronger π-interactions with the Ru(1) atom.
1
The H NMR spectrum of 3b recorded at −10 °C contains three sharp Cp* signals at δ
1.99, 1.88, and 1.77 ppm. With increasing temperature, the two low field resonances
broadened and coalesced at 56 °C as shown in Fig. 8a. The high-field Cp* signal remained
unchanged up to 60 °C. This is likely because of the inversion of the µ3-η2:η2:η2-allenyl
ligand, leading the formation of a time averaged CS plane bisecting the Ru3 triangle
(Scheme 5). The activation parameters of the spectral change are estimated to be ∆H‡ =
19

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18.7 ± 0.1 kcal mol−1 and ∆S‡ = 7.3 ± 0.9 cal mol−1 K−1 based on spectral simulations. As
the rate of the inversion increased, the doublet of doublets assignable to the methine proton
at the C3 atom (δ 5.17 ppm; 3JHH = 8.9, 5.2 Hz) observed at −10 °C became a triplet (3JHH
=
7.0 Hz) at 60 °C (Fig. 8b).
1
Fig. 8. Variable-temperature H NMR spectra of 3b showing (a) Cp* signals and (b) methine signal at
the C3 position (400 MHz, THF-d8).
The methine proton did not exchange with the hydrides. Thus, the inversion of the C7
ring occurs without C−H bond scission at the C3 position. It proceeds most likely via
decoordination of the C(2)−C(3) bond from Ru(1) as shown in Scheme 5. The fact that 3c
is rigid at least at 25 °C unlike 3b suggests that weaker π-interaction of the C7 ring in 3b is
responsible for the fluxional process. A related wagging motion of a parallel
µ3-η2:η2-allenyl ligand in [Ru3(µ-H)(µ3-η2:η2-CMe=C=CMe2)(CO)9], which showed the
coalescence of two methyl signals at the CMe2 moiety at −8 °C, has been reported by Aime
et al [27].
20

ACCEPTED MANUSCRIPT
Scheme 5. Inversion of the cyclic allenyl ligand in 3b.
Three hydride signals, which are coupled to each other, are observed at δ −12.26 (dd,
2JHH = 5.8, 1.3 Hz), −15.98 (dd, 2JHH = 4.9, 1.3 Hz), and −25.61 (dd, 2JHH = 5.8, 4.9 Hz) at
−10 °C. The two low-field signals broadened with increasing temperature like the Cp*
signals (Fig. S19 in the Supplementary Material). However, the activation parameters are
1
not identical to those of the inversion of the C7 moiety: ∆H‡ = 19.6 ± 0.1 kcal mol− and
1
1
∆S‡ = 15.8 ± 1.8 cal mol− K− . Site exchange of the two hydrides occurs much faster than
the inversion of the C7 ring. In addition, the high-field hydrido signal resonating at δ −25.61
also started to broaden above 15 °C. The activation parameters of the second hydrido
1
site-exchange were estimated to be ∆H‡ = 21.4 ± 0.1 kcal mol− and ∆S‡ = 15.6 ± 1.3 cal
1
1
mol− K− . Although the mechanisms for the site exchange of the hydrides are not clear at
present, the kinetic analyses strongly indicate that they occur independently from the
motion of the C7 moiety.
1
In contrast to the H NMR spectrum of 3b, well-resolved signals were observed for 3c
1
even at 25 °C. Although we did not measure the H NMR spectrum at elevated
temperatures, the spectrum strongly suggests that the inversion of the C8 ring is very slow.
21

ACCEPTED MANUSCRIPT
This is likely due to the strong π-interaction of the C(2)−C(3) bond with the Ru(1) atom,
which is consistent with the longer C(2)−C(3) distance than that in 3b.
13
The C signals of the allenyl positions were observed at δ 219.8 (s, C1), 124.9 (s, C2),
and 87.9 (d, 1JCH = 166 Hz, C3) for 3b and δ 218.0 (s, C1), 117.7 (s, C2), and 87.3 (d, 1JCH
=
166 Hz, C3) for 3c. Although four 13C NMR spectra of neutral triruthenium µ3-η2:η2-allenyl
complexes have been reported so far [21, 28−30], that of the cationic complex has never
been
[Ru3(µ-H){µ3-η2:η2-C(Me)=C=C(NMe2)H}(CO)9] but found that it gave a µ3-η2-alkyne
complex, [Ru3(µ-H)2{µ3-η2-MeC C−C(=NMe2)H}(CO)9]+, rather than a protonated allenyl
reported.
Aime
et
al.
reported
the
protonation
of
≡
complex because of rapid isomerization [31]. Owing to the cationic charge, the 13C signals
of the allenyl moiety are expected to show a downfield shift. In fact, the C1 and C3 signals
appeared downfield compared to those of neutral triruthenium µ3-η2:η2-allenyl complexes
(C1, δ 172−153 ppm; C3, 58−13 ppm). However, the C2 signal showed a considerable
upfield shift compared to reported values (C2, δ 180−143 ppm). A similar unexpected
upfield shift was also observed for the µ3-η2:η2( )-alkyne complexes, in which the inner
acetylenic carbon signal shifted upfield by ca. 100 ppm compared to that of the outer
carbon signal [32]. The five-coordinate nature of the C(2) atom seems to be responsible for
this upfield shift.
The isomerization of a µ3-η2:η2-allenyl complex possessing an allylic proton at the C3
position into a µ3-η3-dimetalloallyl complex upon heating has been reported several times
[10, 16, 20, 31, 33, 34]. These results indicate that a dimetalloallyl structure is
thermodynamically favorable for trinuclear complexes. We have also reported equilibrium
22

ACCEPTED MANUSCRIPT
between µ3-η2:η2( )-alkyne, µ3-η2-alkenilidene, and µ3-diruthenaallyl complexes in
relation to the reaction of 1 with a linear alkane (Scheme 1) [8]. In this case, however, the
formation of a µ3-dimetalloallyl structure seems to be difficult because of the strain of the
eight-membered ring structure. In contrast, the reactions of [Ru3(CO)12] with
cyclododeca-1,5,9-triene [35, 36] and trans-cyclododecene [37] give a µ3-η3-diruthenaallyl
complex exclusively.
2.3. Chemical oxidation of µ3- :η
η2 2( )-cyclohexyne complex 2a
Although complex 2a displayed similar redox behavior to those of 2b and 2c, a
µ3-η2:η2:η2-allenyl complex was not formed upon chemical oxidation. Instead, a cationic
µ3-cyclohexyne
complex
containing
a
µ-hydroxo
ligand,
[{Cp*Ru(µ-H)}3(µ-OH)(µ3-η2(||)-C6H8)]+ (5a), was formed via the uptake of adventitious
water. Two-electron oxidation in the presence of 100 equiv H2O gave 5a in 38% yield (Eq.
(4)). Because allylic C−H bond cleavage is difficult in the cyclohexyne ligand, the
increased Lewis acidity at the metal centers is stabilized by the incorporation of H2O.
Successive deprotonation from the coordinated H2O molecule would afford µ-hydroxo
complex 5a.
As shown in Fig. 9, the coordination mode of the cyclohexyne ligand changed from
perpendicular to parallel, accompanied by the adoption of a saturated 48-electron
23

ACCEPTED MANUSCRIPT
configuration. The Ru3 core of 5a forms an isosceles triangle with sides of 2.8311(3),
2.8468(3), and 2.6976(3) Å. The µ-hydroxo ligand bridges the Ru(2)−Ru(3) edge, where
the cyclohexyne ligand is σ-bonded. Consequently, the Ru(2)−Ru(3) bond is considerably
shorter than the other Ru−Ru bonds by ca. 0.15 Å. Although we previously reported the
synthesis of a dicationic µ3-diruthenaallyl complex containing a µ-hydroxo ligand,
[(Cp*Ru)3(µ3-η3-CHCMeCH)(µ3-CH)(µ-OH)]2+ [38], a µ3-alkyne complex containing a
µ-hydroxo group has been still limited [39].
Fig. 9. Molecular structure and labeling scheme of a hexafluorophosphate salt of 5a with thermal
ellipsoids at 30% probability. Selected bond lengths (Å) and angles (deg): Ru(1)
−
−
Ru(2) 2.8311(3),
C(2) 2.138(2),
C(2) 2.092(3), C(1) C(2)
Ru(2) Ru(3) 61.928(7),
Ru(1)
Ru(2)
−
−
Ru(3) 2.8468(3), Ru(2)
O(1) 2.0723(19), Ru(2) C(1) 2.112(3), Ru(3)
F(1) 2.21(4), Ru(2) Ru(1)
Ru(1) 61.340(7), Ru(2) O(1) Ru(3) 81.14(7).
−
Ru(3) 2.6976(3), Ru(1)
−
C(1) 2.116(3), Ru(1)
−
−O(1) 2.0755(19), Ru(3)
−
−
1.400(4), H(4)
Ru(1) Ru(3)
−
−
−Ru(3) 556.732(7), Ru(1)
−
−
−
−
−
In the 1H NMR spectrum, a triplet (δ −17.75) and a doublet (δ −15.92) with a 2JHH value
of 5.1 Hz were observed in the hydrido region. In the 13C NMR spectrum, the acetylenic
carbon atoms resonated at remarkably lower magnetic field (δ 194.2) than those of a neutral
µ3-η2(||)-cyclohexyne complex, [(Cp*Ru)3(µ-H)(µ3-η2(||)-C6H8)(µ3-CO)(µ-CO)] (δ 173.9
24

ACCEPTED MANUSCRIPT
and 143.5) [12]. This downfield shift is likely due to the cationic charge on the Ru3 center.
Although the presence of H2O did not affect the reaction of 2c, two-electron oxidation
of 2b in the presence of 100 equiv H2O afforded a µ-hydroxo complex 5b together with 3b.
1
Because 5b was not isolated from the mixture, it was only characterized based on the H
NMR spectrum of the mixture. Nevertheless, the pair of triplet and doublet signals
resonating at δ −17.97 and −15.85 ppm with an intensity ratio of 1:2 strongly suggests the
formation of µ3-cycloheptyne−µ-hydroxo complex 5b. Although small amounts of
unidentified products were formed at the same time, the ratio between 5b and 3b was
estimated to be ca.1:2 based on the 1H NMR spectrum of the mixture.
The difference in the availability of the allylic C−H bond is thought to determine the
reactivity of the dicationic cycloalkyne intermediate. That is, intramolecular C−H bond
scission is strongly preferred for the flexible cyclooctyne complex, whereas uptake of H2O
preferentially occurs in the oxidation of the cyclohexyne complex, whose rigid structure
suppresses allylic C−H bond scission. For the moderately flexible seven-membered ring,
intramolecular C−H bond scission competes with the reaction with 100 equiv H2O. These
results suggest that the conformation of the hydrocarbyl ligand is important in determining
the changes in the skeletal rearrangement at the multimetallic site. For monometallic
complexes, Sabo-Etienne and coworkers elucidated a dramatic difference in the reactivity
of Ru(H)2(η2-H2)2(PCyp3)2 (Cyp = cyclo-C5H9) compared to its cyclohexyl analog,
Ru(H)2(η2-H2)2(PCy3)2 (Cy = cyclo-C6H11), where the conformation of the cycloalkyl
group has been shown to be crucial for the intramolecular C−H bond activation step [41].
25

ACCEPTED MANUSCRIPT
3. Conclusion
In summary, we have shown the influence of ring size on the properties of
µ3-η2:η2( )-cycloalkyne complexes. Although pronounced differences were not detected in
the interaction with the cyclic alkyne moiety as well as in the redox potentials, the mobility
of the cycloalkyne ligand unexpectedly increased as the ring size increased. A weak
interaction of an allylic C−H bond with one of the metal centers occurs for the cyclic C7
and C8 ligands and this could be responsible for the stabilization of the transition state of
the switchback motion. In particular, the more flexible eight-membered ring in 2c brought
about C−H bond scission, leading to the formation of a µ3-allenyl intermediate, which was
unambiguously confirmed by the incorporation of deuterium into the cyclooctyne moiety.
The difference in the availability of the allylic C−H bond also directed different skeletal
rearrangements of the cycloalkyne moiety upon chemical oxidation. Although the
µ3-η2:η2:η2-allenyl complex was not obtained from µ3-cyclohexyne complex 2a, cationic
allenyl complexes 3b and 3c were obtained from 2b and 2c via C−H bond scission at the
allylic position. Notably, the XRD studies of 3b and 3c revealed that the cyclic allenyl
moiety adopts an unprecedented perpendicular coordination mode with respect to the
trinuclear plane, which likely stems from their coordinatively unsaturated nature.
On the other hand, 2a afforded µ3-η2(||)-cyclohexyne−µ-hydroxo complex 5a upon
chemical oxidation. Owing to the difficulty in intramolecular C−H bond cleavage, the
highly unsaturated dicationic species generated by two-electron oxidation causes the uptake
of H2O rather than intramolecular C−H bond scission. This result indicates that subtle
conformational differences can result in different chemical transformations of the
26

ACCEPTED MANUSCRIPT
hydrocarbyl ligand at multimetallic sites. In this regard, 5a could be viewed as an
intermediate in the hydration of cyclohexene. Although we have not investigated the
reactivity of 5a yet, we previously reported the transformation of
a
µ3-diruthenaallyl−µ-hydroxo complex, [(Cp*Ru)3(µ3-η3-CHCMeCH)(µ3-CH)(µ-OH)]2+,
into a µ3-oxaruthenacyclopentenyl complex, [(Cp*Ru)3(µ3-CH)(µ3-OCHCMeCH-)]+ via
C−O bond formation at the Ru3 site [38].
These results show that skeletal rearrangement of hydrocarbyl ligands at multimetallic
sites is highly influenced by the availability of a C−H bond at a specific position. We
continue to study the skeletal rearrangements of hydrocarbyl ligands with various shapes to
understand the role of subtle C−H interactions in determining the reaction pathway at
multimetallic sites, which could lead to the development of novel catalysis exploiting
multimetallic interactions.
4. Experimental
4.1. General Procedures
All compounds were manipulated using standard Schlenk and high-vacuum-line
techniques under an atmosphere of argon. Dehydrated toluene, tetrahydrofuran (THF),
acetone, diethyl ether, pentane, and hexane used in this study were purchased from Kanto
Chemicals and stored under an atmosphere of argon. Deionized water was degassed by the
freeze-pump-thaw cycle and stored under an atmosphere of argon. C6D6 and THF-d8 were
dried over sodium−benzophenone ketyl and stored under an atmosphere of argon.
Acetone-d6 was dried over MS-3A and stored under an atmosphere of argon. Other reagents
27

ACCEPTED MANUSCRIPT
were used as received. Triruthenium pentahydrido complex, {Cp*Ru(µ-H)}3(µ3-H)2 (1),
1
was prepared according to the published method [41]. H and 13C NMR spectra were
recorded on Varian INOVA-400 and Varian 400MR spectrometers. 2H NMR spectrum was
recorded on a Varian 400MR spectrometer. 1H NMR spectra were referenced to
tetramethylsilane as an internal standard. 2H NMR spectra were referenced to the deuterium
signal of the solvent employed (C6D6). 13C NMR spectra were referenced to the
natural-abundance carbon signal of the solvent employed. Cyclic voltammogram analyses
were performed using a BAS CV-50W voltammetric analyzer interfaced to a personal
computer. The working electrode was platinum, and the counter electrode was a platinum
wire. The reference electrode was a silver wire housed in a glass tube sealed with a porous
Vycor tip and filled with a 0.1 M solution of AgNO3 in acetonitrile. The data obtained
relative to a reference electrode (Ag/Ag+) were converted to the potential relative to the
redox potential of ferrocene, which was measured under the same conditions at the same
time. Tetrabutylammonium hexafluorophosphate (TBAPF6; Wako) was recrystallized from
THF, dried under vacuum, and stored under an argon atmosphere. A concentration of ∼1
mM of complexes in 0.1 M TBAPF6 in THF was used. UV-vis spectra were recorded on a
Shimadzu UV-2550 spectrophotometer. Elemental analyses were performed on a
PerkinElmer 2400II CHN analyzer.
4.2. Variable temperature NMR spectra and NMR simulations
Variable-temperature NMR studies were performed in NMR tubes equipped with a J.
Young valve using a Varian INOVA-400 Fourier transform spectrometer. The NMR
simulations for the temperature-dependent Cp* and hydrido signals of 3b were performed
28

ACCEPTED MANUSCRIPT
using gNMR v5.0.6.0 (2006, Ivory Soft) via an iterative parameter search upon the
exchange constants k1, k2, and k3, which were the rates for the inversion of the 7-membered
ring in 3b and the two hydride site exchange processes, respectively. The details of the
fitting procedure are shown in Tables S8 and S9 and Fig. S23 in the Supplementary
Material. The rate constants, k1, k2, and k3, that accurately modeled the experimental spectra
at each temperature are also given in the Supplementary material (Figs. S20−22). The
activation parameters ∆H‡ and ∆S‡ were determined from the plot of ln(k/T) versus 1/T.
Estimated standard deviations (σ) in the slope and y intercept of the Eyring plot determined
the error in ∆H‡ and ∆S‡, respectively. The standard deviation in ∆G‡ was determined from
the formula: σ(∆G‡)2 = σ(∆H‡)2 + [T σ(∆S‡)]2 − 2T σ(∆H‡) σ(∆S‡).
4.3. X-ray Diffraction Studies
Single crystals of 2a, 2b, 3b, 3c, and 5a for the X-ray analyses were obtained directly
from the preparations described below and mounted on nylon Cryoloops with Paratone-N
(Hampton Research corp.). Diffraction experiment was performed on a Rigaku AFC-7R
four-circle diffractometer for 2a and Rigaku R-AXIS RAPID imaging plate diffractometer
for 2b, 3b, 3c, and 5a with graphite-monochromated Mo-Kα radiation (λ = 0.71069 Å).
Cell refinement and data reduction were performed using TEXSAN for 2a and the
PROCESS-AUTO program [42] for 2b, 3b, 3c, and 5a. Intensity data were corrected for
Lorentz-polarization effects and for numerical and empirical absorption. The structures
were solved by the direct method using SHELXT-2018/1 and further refined with the
SHELXL-2016/6 program package [43,44]. All non-hydrogen atoms were found by a
difference Fourier synthesis and were refined anisotropically except for the disordered
29