Synthesis, Structure, Reactivity, and Catalytic Activity of Cyclometalated (Phosphine)- and (Phosphinite)ruthenium Complexes

Article abstract of DOI:10.1002/ejic.201700479

Reactions of naphthyl- and o-methylphenyl-substituted phosphines with [RuCl₂(p-cymene)]₂ resulted in the corresponding phosphine-substituted ruthenium dichlorides (1a,b and 3). When the reactions of aryl-substituted phosphines or pho

Full text of DOI:10.1002/ejic.201700479

A Journal of
Accepted Article
Title: Synthesis, structures, reactivity and catalytic activity of
cyclometalated ruthenium phosphine and phosphinite complexes
Authors: Bolin Zhu, Ruichen Sun, Xiaodan Chu, Shaowei Zhang,
Tongyu Li, and Zhuo Wang
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700479
Link to VoR: http://dx.doi.org/10.1002/ejic.201700479

10.1002/ejic.201700479
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis, structure, reactivity and catalytic activity of cyclometalated
ruthenium phosphine and phosphinite complexes
Ruichen Sun,[a] Xiaodan Chu,[a] Shaowei Zhang,[a] Tongyu Li,[a] Zhuo Wang,[a] and
Bolin Zhu*[a]
Keywords: Ruthenium complexes / Phosphine and phosphinite ligands / Cyclometalation / C(sp2)−H
and C(sp3)−H activation / Catalytic reduction of ketones
___________________________________________________________________________________
Reactions of naphthyl and o-methylphenyl-substituted phosphines with [RuCl2(p-cymene)]2 resulted in
corresponding phosphine-substituted ruthenium dichlorides (1a,b, and 3). When the reactions of
aryl-substituted phosphines or phosphinite with [RuCl2(p-cymene)]2 were carried out in the presence of
sodium acetate, aryl-substituted phosphines or phosphinite are cyclometalated to give corresponding
five-membered metallacycle complexes (2a,b, and 4-6) via an intramolecular activation of C(sp2)−H or
C(sp3)−H bond. Competition reaction indicates that the aromatic C(sp2)−H bond is more likely to be
activated than C(sp3)−H bond under the same conditions, and five-membered ring is formed in
preference to six-membered ring. Chemical reactivity of 2a,b, two representatives of cycloruthenated
complexes, were explored, including conversion of 1a,b to 2a,b, substitution reaction of η6 arene ligand
in 2a, synthesis of bis-phosphine complex starting from 2b, and reaction of 2a with diphenylacetylene.
Besides, their catalytic activity on reduction of ketones by transfer hydrogenation was also tested.
Molecular structures of nine compounds 1b, 2a, 4, 5, 6, 7, 8, 9, and 10 were determined by X-ray
diffraction.
___________________________________________________________________________________
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European Journal of Inorganic Chemistry
[a] Tianjin Key Laboratory of Structure and Performance for Functional Molecules; Key Laboratory of
Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education; College of
Chemistry, Tianjin Normal University, Tianjin 300387, People’s Republic of China
http://59.67.75.245/college/hxxy/skin/one/index.asp
Fax: +86-22-23766532; Tel: +86-22-23766515
E-mail: hxxyzbl@gmail.com
2
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Introduction
Cyclometalation is currently one of the most popular organometallic reactions, which refers to the
transition metal-mediated activation of a C−H bond (occasionally C−C, C−O, or C−Si bond) to form a
cyclometalated complex containing a new M−C σ bond.[1] Generally, the reaction consists of two steps:
Metal center is firstly coordinated by a donor atom (such as N, O, and P) from organic ligand, then a
geometrically accessible C−H bond is activated by the metal to give a metallacycle. Of all
cyclometalations using transition metals, cycloruthenation plays an indispensable role,[2] which is
mainly due to the following facts: (i) Cycloruthenated complexes have unique photophysical and
electrochemical characteristics, which may have potential applications as photosensitizers for solar
cells or intervalence electron-transfer systems.[3] (ii) Ruthenacycles can be used as catalysts for a wide
range of reactions, including metathesis reactions,[4] and transfer hydrogenation reactions.[5] (iii) The
great diversity of available ruthenium precursors result in the versatility of cycloruthenation. (iv) The
interesting reactivity of the cycloruthenated complexes, like insertion reactions of unsaturated
molecules into Ru−C bond.[6] (v) And in particular, the cycloruthenated complex has become an
integral part in many catalytic cycles, including ruthenium-catalyzed arylation, or alkenylation of
2-phenyl substituted pyridines or other N-heterocycles.[7] In all above five aspects, most
cyclometalation reactions are with N-donor ligands. But so far, there is less study on phosphine ligands
involved cyclometalation with [RuCl2(arene)]2.[8] In this paper, we aimed to expand the
cyclometalation reactions with phosphorous ligands and add a new dimension to understanding
cyclometalation reactions, and reported synthesis and reactivity of a series of cyclometalated ruthenium
phosphine and phosphinite complexes, as well as their catalytic activity on reduction of ketones by
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transfer hydrogenation.
Results and Discussion
Reactions of [RuCl2(p-cymene)]2 with (1-naphthyl)diphenyl phosphine or diisopropyl(1-naphthyl)
phosphine
First, compounds (1-naphthyl)diphenyl phosphine or diisopropyl(1-naphthyl) phosphine were
chosen as ligands for two reasons: First, α-naphthyl group could provide a C(sp2)−H bond at the
appropriate position for potential C−H bond activation by metal center; and second, phenyl and
isopropyl groups show close steric hindrance, which might promote the cyclometalation reaction.
Reactions of [RuCl2(p-cymene)]2 with (1-naphthyl)diphenyl phosphine or diisopropyl(1-naphthyl)
phosphine in CH2Cl2 at room temperature resulted in corresponding phosphine-substituted ruthenium
dichlorides 1a,b in good yields (Scheme 1). The similar complex (p-cymene)RuCl2(PPh3) has been
reported by several groups, the only difference in their respective synthetic methods is the solvent used
in the reaction, including hexane,[9] CH2Cl2,[10] and toluene.[11] However, when the above reactions were
carried out in methanol in the presence of sodium acetate, the corresponding five-membered
cyclometalated complexes 2a,b were generated in good yields. The formation of complexes 2a,b goes
through an intramolecular activation of C(sp2)−H bond on the adjacent α position of naphthyl ring.
Davies et al. have reported that acetate could assist C−H bond activation and promote the
cyclometalation in the reactions of 2-substituted pyridines with [RuCl2(p-cymene)]2,[2c] it is quite clear
that acetate plays a similar role in our case when the donor group is phosphine ligand. The ruthenium
precursors [RuCl2(arene)]2 have been shown to lead to readily cyclometalation reactions with a large
variety of nitrogen-containing ligands, however, the similar cyclometalation reactions with phosphine
ligands are not as common. Only a few cyclometalated arene-ruthenium phosphine complexes have
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been reported in literatures, one of which was formed by base-promoted elimination of HCl from
(C6Me6)RuCl2[PPh2(C6H4Me-o)].[12] Besides, a cyclometalation reaction with the same ligand,
(1-naphthyl)diphenyl phosphine and [CoCH3(PMe3)4] has been reported by Klein et al.[13] Complexes
1a,b and 2a,b were fully characterized by spectroscopic analysis, in which the structures of 1b and 2a
were confirmed by X-ray diffraction analysis.
Scheme 1 Reactions of [RuCl2(p-cymene)]2 with 1-naphthyl-substituted phosphines
The structure of 1b is shown in Figure 1, which reveals a classic three-legged piano stool geometry
with the ruthenium(II) center coordinated by a η6 Cymene, one phosphorus atom P(1), and two chlorine
atoms Cl(1) and Cl(2). The Ru(1)−P(1) bond distance is 2.4034(7) Å, which is longer than that
[14]
(2.3438(6) Å) in (p-cymene)Ru(PPh3)Cl2,
and much longer than that (2.2642(8) Å) in
(p-cymene)Ru[P(OPh)3]Cl2.[9c] This lengthened Ru−P bond in 1b is probably due to the larger steric
bulk of diisopropyl(1-naphthyl) phosphine compared to that of PPh3 and P(OPh)3. The structure of 2a is
shown in Figure 2, which also exhibits a three-legged piano stool geometry with the ruthenium(II)
center coordinated by a η6 cymene, one phosphorus atom P(1), one chlorine atom Cl(1) and one carbon
atom C(19). A five-membered ring (Ru(1)−P(1)−C(13)−C(18)−C(19)) in the structure confirms the
activation of a C(sp2)−H bond on the naphthyl ring. The Ru(1)−P(1) bond distance (2.2961(8) Å) is
much shorter that (2.4034(7) Å) that in 1b, which indicates that cyclometalation may contribute to strain
relief in the molecular structure.
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Figure 1. Thermal ellipsoid drawing of 1b showing the labeling scheme and 50% probability ellipsoids;
hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Ru(1)−P(1) 2.4034(7),
Ru(1)−Cl(1) 2.4247(6), Ru(1)−Cl(2) 2.4209(6), P(1)−C(1) 1.836(2), Ru(1)−Ph(centroid) 1.711,
Ru(1)−P(1)−C(1) 114.80(8).
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Figure 2. Thermal ellipsoid drawing of 2a showing the labeling scheme and 50% probability ellipsoids;
hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Ru(1)−P(1) 2.2961(8),
Ru(1)−Cl(1) 2.4276(8), Ru(1)−C(19) 2.085(3), P(1)−C(13) 1.815(3), C(13)−C(18) 1.422(5),
C(18)−C(19) 1.443(5), Ru(1)−Ph(centroid) 1.741, P(1)−Ru(1)−Cl(1) 92.50(3), P(1)−Ru(1)−C(19)
80.95(9), C(19)−Ru(1)−Cl(1) 83.88(9), Ru(1)−P(1)−C(13) 105.73(11).
Reactions of [RuCl2(p-cymene)]2 with diisopropyl(o-methylphenyl) phosphine
Having demonstrated that phosphine ligand could favor a naphthyl C(sp2)−H bond activation by
bringing the C−H group close to the metal center, we decided to test whether it could also favor a
C(sp3)−H bond activation by the same way. So reaction of [RuCl2(p-cymene)]2 with
diisopropyl(o-methylphenyl) phosphine in the presence of sodium acetate was carried out, which
successfully afforded corresponding five-membered cyclometalated complexes 4 in good yield
(Scheme 2). It is quite clear that the reaction realized the expected C(sp3)−H bond activation. If the
reaction was performed in CH2Cl2 in the absence of sodium acetate, only the normal
phosphine-substituted ruthenium dichlorides 3 was obtained. Complexes 3−4 were fully characterized
by spectroscopic analysis, as well as crystallographic analysis for 4. Complex 4 exhibits a structure
similar to 2a (Figure 3), except that a sp2 hybridized carbon was replaced by a sp3 hybridized one in the
five-membered ring.
Scheme 2 Reactions of [RuCl2(p-cymene)]2 with diisopropyl(o-methylphenyl) phosphine
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Figure 3. Thermal ellipsoid drawing of 4 showing the labeling scheme and 50% probability ellipsoids;
hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Ru(1)−P(1) 2.2982(11),
Ru(1)−Cl(1) 2.4248(10), Ru(1)−C(11) 2.134(4), P(1)−C(17) 1.823(4), C(11)−C(12) 1.515(5),
C(12)−C(17) 1.397(5), Ru(1)−Ph(centroid) 1.741, P(1)−Ru(1)−Cl(1) 90.55(4), P(1)−Ru(1)−C(11)
80.38(11), C(11)−Ru(1)−Cl(1) 83.93(12), Ru(1)−P(1)−C(17) 105.75(13).
Reaction of [RuCl2(p-cymene)]2 with diisopropyl(2-methylnaphthyl) phosphine
Since both C(sp2)−H bond and C(sp3)−H bond could be activated in cyclometalation of
[RuCl2(p-cymene)]2 with phosphines, we were curious about the relative activity between them.
Therefore, diisopropyl(2-methylnaphthyl) phosphine was chosen as the ligand, which provides both
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C(sp2)−H and C(sp3)−H bonds at the appropriate positions, to test the competition reaction. Therefore
the reaction of [RuCl2(p-cymene)]2 with diisopropyl(2-methylnaphthyl) phosphine was performed under
the above-mentioned conditions, which selectively afforded five-membered cyclometalated complex 5,
the C(sp2)−H bond activated products (Scheme 3). This result indicated that aromatic C(sp2)−H bond
activation is favored over C(sp3)−H bond activation, which is probably owing to the higher thermal
stability of aryl hydrogen activated product when compared to alkyl hydrogen activated product.[15] The
similar result has been observed by Corriu et al. in the reactions of (η5-C5H5)Fe(CO)2SiR3 with
phosphorus ligands upon UV irradiation.[15b] Complex 5 was fully characterized by spectroscopic
analysis (for the single crystal structure and crystallographic data of 5, see the Supporting Information ).
Scheme 3 Reaction of [RuCl2(p-cymene)]2 with diisopropyl(2-methylnaphthyl) phosphine
Reaction of [RuCl2(p-cymene)]2 with (1-naphthyl)diisopropyl phosphinite
In order to explore applicable scope of this type of cyclometalation, ligand was extended to
phosphinite. Similar reaction of [RuCl2(p-cymene)]2 with (1-naphthyl)diisopropyl phosphinite was
examined, which produced corresponding five-membered cyclometalated complexes 6 and a
phosphinite-substituted ruthenium dichloride 7 (Scheme 4), the ratio of them is about 5:3. Theoretically,
another C(sp2)−H bond at an adjacent α position of naphthyl ring could also be activated, however, no
corresponding six-membered cyclometalated complex was detected. This regioselectivity of C−H bond
activation should arise from the stability of five-membered ring compounds as compared with
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six-membered ring compounds. The selectivity of five-membered ring compounds is recognizable in all
cyclometalation reactions, and according to some statistics by Omae, the mean selectivity ratio may
reach to 78% among all four, five and six-membered ring compounds.[1b] It is interesting to note that the
reactions also afforded methyldiisopropylphosphinite-substituted ruthenium dichloride 7. To track the
source of methoxy in 7, the reaction mentioned above was performed in methanol-d4, which gave
corresponding product 7-d3. This result indicated that methoxy group comes from the solvent methanol,
and ligand-exchange between methoxy and 1-naphthoxy groups occurred in the reaction. Besides, the
ligand-exchange reaction was not observed in the absence of [RuCl2(p-cymene)]2, which suggested that
metal coordination to the phosphinite might assist this exchange process. Recently we reported similar
reactions of [Cp*MCl2]2 (M = Ir and Rh) with (1-naphthyl)diisopropyl phosphinite, which also
produced
methyldiisopropylphosphinite-substituted metal dichlorides.[16]
Scheme 4 Reaction of [RuCl2(p-cymene)]2 with (1-naphthyl)diisopropyl phosphinite
corresponding
five-membered
cyclometalated
complexes
and
The structure of 6 is shown in Figure 4, which also exhibits a three-legged piano stool geometry
with the ruthenium(II) center coordinated by a η6 cymene, one phosphorus atom P(1), one chlorine atom
Cl(1) and one carbon atom C(1). A five-membered ring (Ru(1)−P(1)−O(1)−C(2)−C(1)) in its structure
confirms the activation of a C(sp2)−H bond at an adjacent β position of naphthyl ring. The Ru(1)−P(1)
bond distance is 2.2697(7) Å, slightly shorter than those (2.2961(8) and 2.2982(11) Å) in
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five-membered cyclometalated complexes 2a and 4. The structure of 7 is depicted in Figure 5, it
exhibits a structure similar to 1b. The Ru(1)−P(1) bond distance is 2.3427(7) Å, which is in good
agreement with that (2.3438(6) Å) in (p-cymene)Ru(PPh3)Cl2,[14] and a little shorter than that (2.4034(7)
Å) in 1b.
Figure 4. Thermal ellipsoid drawing of 6 showing the labeling scheme and 50% probability ellipsoids;
hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Ru(1)−P(1) 2.2697(7),
Ru(1)−Cl(1) 2.4167(7), Ru(1)−C(1) 2.073(3), P(1)−O(1) 1.650(2), O(1)−C(2) 1.394(3), C(1)−C(2)
1.373(4), Ru(1)−Ph(centroid) 1.756, P(1)−Ru(1)−Cl(1) 92.94(2), P(1)−Ru(1)−C(1) 77.25(8),
C(1)−Ru(1)−Cl(1) 86.42(8), Ru(1)−P(1)−O(1) 107.35(8), P(1)−O(1)−C(2) 112.71(18).
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Figure 5. Thermal ellipsoid drawing of 7 showing the labeling scheme and 50% probability ellipsoids;
hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Ru(1)−P(1) 2.3427(7),
Ru(1)−Cl(1) 2.4059(6), Ru(1)−Cl(2) 2.4229(7), P(1)−O(1) 1.6246(17), Ru(1)−Ph(centroid) 1.706,
Cl(1)−Ru(1)−Cl(2) 86.68(2), Cl(1)−Ru(1)−P(1) 87.44(2), Cl(2)−Ru(1)−P(1) 94.58(2),
Ru(1)−P(1)−O(1) 105.99(7).
Five-membered cyclometalated complexes have rich reactivity and application, including insertion
of unsaturated molecules into M−C bond,[6] decyclometalation, and catalytic reduction of ketones by
transfer hydrogenation.[5] In order to develop a general understanding of this type of cyclometalated
ruthenium phosphine complexes, we have chosen two representatives (1a,b) and explored their
reactivity and catalytic application below.
Conversion of 1a,b to 2a,b
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Originally, in an attempt to purify the products 1a,b by column chromatography on silica gel,
depicted in Scheme 1, we observed a yellow band when the column was first washed with petroleum
ether. The yellow solid obtained from solvent evaporation was identified as the cyclometalated
complexes 2a,b. We found longer contact time of complexes 1a,b with silica gel (acidic, pH = 6−7)
increased the proportion of the yellow band. Therefore, we directly mixed a concentrated solution of
complexes 1a,b in CH2Cl2 with silica gel together and exposed to air at room temperature, conversion
of 1a,b into 2a,b was similarly observed, especially for 1b, almost quantitative conversion was obtained
after 24 hours. The formation of 2a,b presumably resulted from HCl elimination of 1a,b, which is
absorbed by silica gel. Some phosphine-substituted metal dichlorides have been shown to lead to
successful orthometalation reactions with the help of base. The ruthenium analogue
(C6Me6)RuCl2[PPh2(C6H4Me-o)] undergoes orthometalation in the presence of Na2CO3.[12] The iridium
and rhodium analogues Cp*MCl2PMe2(Xyl) (M = Ir, Rh) generally undergo orthometalation to give
corresponding cyclometalated compounds by treatment with base, like 2,2,6,6-tetramethylpiperidine, or
LinBu.[17] When we performed the similar reaction of 1a in methanol in the presence of base, like
sodium hydroxide, or sodium acetate, the expected cyclometalated ruthenium complex 2a was observed
but in low yield (about 20%), at the same time, we also obtained some free ligand, (1-naphthyl)diphenyl
phosphine. In order to avoid the loss of 1a,b during column chromatography, the products were directly
precipitated at the end of the reaction by addition of excess Et2O, which were recrystallized from
CH2Cl2/n-hexane to give red crystals. They are fairly stable in solid state or in CHCl3.
Exchange reaction of η6 arene ligand in 2a
When a solution of complex 2a in o-xylene was irradiated by high-pressure Hg lamp for 8 h or
heated to reflux for 20 h, we observed that η6 cymene ligand in 2a is substituted by o-xylene to give
product 8 (Scheme 5). Complex 8 was fully characterized by spectroscopic and crystallographic
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analysis (for crystal structure of 8, see the Supporting Information ). When the reaction was carried out
in toluene under the above conditions, substitution reaction of arene ligand similarly occurred to afford
an analogue 8’. The substitution of p-cymene by toluene is rather unusual since the replacement of the
arene by another less substituted (and therefore less electron-donating) is not favored in intermolecular
processes. A similar intramolecular arene ligand substitution was reported by Fürstner and Dixneuf that
complex [(p-cymene)RuCl2PCy2-(CH2)3C6H5)] undergoes a substitution of the cymene ligand with the
tethered phenyl ring at 140°C in chlorobenzene to give the chelate complex
[(η6-C6H5-(CH2)3-PCy2)RuCl2].[18]
Scheme 5 Chemical reactivity of complexes 2a,b
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Synthesis of bis-phosphine complex 9
Bis-phosphine ruthenium(II)-arene complexes have potential application as good catalyst
precursors for hydrogenation.[19] With cyclometalated ruthenium complex 2b in hand, we used it as a
starting material to react with PPh3 in EtOH under reflux followed by metathesis using [NH4]PF6, and
obtained the corresponding bis-phosphine ruthenium (II) complex 9 (Scheme 5). Dyson has previously
reported synthesis and catalytic application of a similar bis-phosphine ruthenium complex
[Ru(PPh3)(η2-PPh2(C6H4O))(p-cymene)]PF6.[19b]
Figure 6. Thermal ellipsoid drawing of 9 showing the labeling scheme and 50% probability ellipsoids;
anion part and hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Ru(1)−P(1)
2.3415(5), Ru(1)−P(2) 2.3328(5), Ru(1)−C(11) 2.093(2), P(2)−C(19) 1.813(2), C(19)−C(20) 1.421(3),
C(11)−C(20) 1.439(3), Ru(1)−Ph(centroid) 1.840, P(1)−Ru(1)−P(2) 93.223(19), C(11)−Ru(1)−P(1)
83.58(5), C(11)−Ru(1)−P(2) 79.83(6), Ru(1)−P(2)−C(19) 104.87(7).
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The structure of 9 was confirmed by X-ray diffraction analysis (Figure 6), which reveals a
three-legged piano stool geometry with the ruthenium(II) center coordinated by a η6 cymene, two
phosphorus atoms P(1) and P(2), and one carbon atom C(11).
Reaction of complex 2a with diphenylacetylene
Recently, we studied insertion reactions of a series of alkynes in to M−C bonds of similar
five-membered cycloiridated and cyclorhodated complexes, which presented various insertion modes,
including 1,2-insertion, 1,1-insertion after the initial vinylidene rearrangement, and 1,1-insertion
followed by a novel structural rearrangement or another 1,1-insertion.[16] Besides, there were several
reported cases about insertion reactions of alkynes in to Ru−C bonds of similar five-membered
cycloruthenated complexes, which generally afforded corresponding seven-membered cycloruthenated
complexes by means of 1,2-insertion of alkyne.[6,20] Therefore, we tried the reaction of cycloruthenated
complex 2a with diphenylacetylene in methanol or 1,2-dichloroethane, and expected the similar product
containing seven-membered ring. However, the reaction surprisingly generated an organic product 10,
no ruthenium-containing products were isolated (Scheme 5). Subsequently, we also tested the reaction
of 2b with diphenylacetylene under the same conditions, however, no product 10 was obtained. So the
mechanism for the formation of 10 remains unclear, it may go through a seven-membered intermediate
followed by the cleavage of Ru−C and P−C bonds, and then form the compound
4,5-diphenylacephenanthrylene (10).
10 is a known compound,[21] but its structure was first confirmed by X-ray diffraction. The structure
of 10 is shown in Figure 7. By analysis of C−C bond lengths of five-membered ring
C(7)−C(8)−C(15)−C(20)−C(21), C(7)−C(21) (1.482(2) Å) and C(8)−C(15) (1.474(2) Å) are assigned as
single bonds, and C(7)−C(8) (1.374(2) Å) is assigned as double bond. The dihedral angle between two
phenyl rings is 53.2°.
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Figure 7. Thermal ellipsoid drawing of 10 showing the labeling scheme and 50% probability ellipsoids;
hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are C(7)−C(8) 1.374(2),
C(8)−C(15) 1.474(2), C(15)−C(20) 1.409(2), C(20)−C(21) 1.411(2), C(7)−C(21) 1.482(2),
C(7)−C(8)−C(15) 108.97(15), C(7)−C(8)−C(9) 127.49(16), C(9)−C(8)−C(15) 123.44(16),
C(8)−C(15)−C(20) 106.25(15).
Reduction of ketones by transfer hydrogenation
Ruthenacyclic complexes and ruthenium complexes of the type [(η6-arene)RuCl2(PR3)] have been
found widespread catalytic application in the reduction of ketones by hydrogen transfer to the
corresponding alcohols.[5,22] Since a series of new ruthenacyclic complexes 2a,b, 4, 5, and 6 and
phosphine-substituted ruthenium dichlorides 1a,b and 3 were synthesized in this work, we are eager to
explore their catalytic activities in the reduction of benzophenone. First, ruthenium-catalyzed reduction
of benzophenone under various conditions were investigated. The results are summarized in Table 1.
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Treatment of 1a (0.01 mmol), KOH (20.0 equiv), and benzophenone (200 equiv) in isopropanol (75 mL)
t
at 82°C for 48 h gave benzhydrol in 98% yield (entry 1 of Table 1). Use of other bases, like BuOK,
NaOAc, and K2CO3, resulted in lower yields (72−91%, entries 2−4). Lowering the quantity of base
KOH to 5.0 and 2.0 equiv, led to a decrease in yields of 92 and 87% respectively (entries 5 and 6).
When the same reaction was performed in the absence of KOH, still 54% of conversion was obtained
(entry 7). No reaction occurred at room temperature (entry 8). Other new ruthenium complexes,
including phosphine-substituted ruthenium dichlorides 1b and 3, and ruthenacyclic complexes 2a,b, 4, 5,
and 6 as catalysts were also screened under the same conditions mentioned in entry 1, which afforded
lower or comparable yields (58−98%, entries 9−15). Based on the considerations of economic, the
amount of catalyst 1a was decreased to 0.05 mol% or 0.01 mol%, the corresponding reaction still gave
the yield of 92% or 89% (entries 16 and 17, respectively). Typical experiments have been carried out
with model substrate benzophenone, but a larger diversity of ketone substrates have also been used
including diaryl ketone, alkyl aryl ketones, and cyclic dialkyl ketones, which gave moderate to good
yields (45−81%) when catalyzed by 1a under the optimized conditions. (entries 18−21).
Scheme 6
Table 1. Reduction of ketones catalyzed by phosphine-substituted cymene ruthenium complexes.[a]
Entry Cat. R1, R2
Cat. amount [mol%] Base
Yield % [b]
>98
1
R1 = R2 = Ph
0.5
0.5
0.5
0.5
0.5
0.5
KOH
1a
1a
1a
1a
1a
1a
2
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
NaOAc 91
K2CO3 72
tBuOK 89
3
4
5[c]
6[d]
KOH
KOH
92
87
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10.1002/ejic.201700479
European Journal of Inorganic Chemistry
7
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = Ph
R1 = R2 = o-ClC6H4
R1 = Ph, R2 = Me
0.5
−
54
0
1a
1a
1b
3
8[e]
0.5
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
9
0.5
96
77
90
70
96
98
58
92
89
81
70
52
45
10
11
12
13
14
15
16
17
18
19
20
21
0.5
0.5
2a
2b
4
0.5
0.5
0.5
5
0.5
6
0.05
0.01
0.01
0.01
1a
1a
1a
1a
1a
1a
R1 = o-ClC6H4, R2 = Me 0.01
R1 + R2 = (CH2)4
0.01
[a] Reactions were carried out with 1−6 (0.01 mmol), base (20.0 equiv), and ketone (200−10000 equiv)
in isopropanol (75 mL) at 82°C for 48 h. [b] Isolated yield. [c] base (5.0 equiv). [d] base (2.0 equiv). [e]
rt.
Conclusions
A series of five-membered cycloruthenated complexes (2a,b, 4, 5, and 6) and
phosphine-substituted ruthenium dichlorides (1a,b and 3) were synthesized by reactions of
aryl-substituted phosphines or phosphinite with [RuCl2(p-cymene)]2 in the presence and absence of
sodium acetate. The formation of cycloruthenated complexes goes through an intramolecular C−H bond
activation step. Competition reaction indicates that the aromatic C(sp2)−H bond is more likely to be
activated than C(sp3)−H bond under the same conditions, and five-membered ring is formed with more
facility than six-membered ring. The reactivity of two representatives of cycloruthenated complexes
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European Journal of Inorganic Chemistry
2a,b has been investigated, including conversion of 2a,b to 1a,b, exchange reaction of η6 arene ligand
in 2a, synthesis of bis-phosphine complex starting from 2b, and reaction of 2a with diphenylacetylene,
which shows different reactivity from those of N-containing cycloruthenated complexes, such as
[(p-cymene)RuCl(C^N)] (HC^N = 2-phenylpyrazole).[6] Besides, under optimized conditions,
complexes 1−6 exhibited moderate to good catalytic activity on reduction of ketones by transfer
hydrogenation, especially for diaryl ketones. Complex 1a provided full conversion at a [Ru]:substrate
ratio of 1:200 at 82°C..
Experimental
General Considerations
All reactions were carried out under nitrogen using standard Schlenk and vacuum line techniques;
however the workup was carried out in air unless stated otherwise. All solvents were distilled from
appropriate drying agents under nitrogen prior to use. 1H (400 MHz), 13C{1H} (100 MHz), and 31P{1H}
(162 MHz) NMR spectra were recorded on a Bruker AV400 instrument at room temperature with
1
13
CDCl3 as solvent. Chemical shifts were recorded in ppm, referenced to residual H and C signals of
31
the non-deuterated CDCl3 (δ 7.26 and 77.16) as internal standards or to the P signal of PPh3 (δ −5.65)
as an external standard. Elemental analyses were performed on a Perkin−Elmer 240C analyzer. IR
spectra were recorded as KBr disks on a Nicolet 560 ESP FTIR spectrometer. The starting materials
[RuCl2(p-cymene)]2, and diphenylacetylene were purchased from Strem without further purification; the
ligands
diisopropyl(1-naphthyl)
phosphine,[23]
(1-naphthyl)diphenyl
phosphine,[23]
diisopropyl(o-methylphenyl) phosphine,[24] diisopropyl(2-methylnaphthyl) phosphine,[24] and
(1-naphthyl)diisopropylphosphinite[25] were prepared by literature methods.
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General procedures for the reactions of [RuCl2(p-cymene)]2 with phosphines in CH2Cl2
A mixture of [RuCl2(p-cymene)]2 (0.05 mmol) and phosphine (2.0 equiv) in CH2Cl2 (10 mL) was
stirred for 12 h at room temperature and then concentrated under vacuum; the products were
precipitated by addition of 10 mL of Et2O, which were recrystallized from n-hexane/CH2Cl2 (1:1) at
−10 °C to afford 1a,b or 3 as red crystals.
1a, (81% yield). 1H NMR: δ 8.57 (d, J = 8.8 Hz, 1H), 8.03 (d, J = 8.0 Hz, 1H), 7.94 (d, J = 8.0 Hz,
1H), 7.61 (m, 4H), 7.55 (d, J = 7.6 Hz, 1H), 7.45 (t, J = 7.6 Hz, 2H), 7.37−7.25 (m, 7H), 5.20 (m, 2H,
Cym-H), 4.50 (m, 2H, Cym-H), 2.98 (m, 1H, CH), 1.66 (s, 3H, CH3), 1.24 (d, J = 7.2 Hz, 6H, CH3).
13C{1H} NMR: δ 135.7, 134.1, 133.8, 133.7, 133.0, 132.3, 130.7, 130.6, 130.4, 128.9, 127.8, 126.9,
124.6, 113.1, 98.4, 88.4, 86.7, 30.5, 22.3, 18.1. 31P{1H} NMR: δ 31.5. Anal. Calcd for C32H31Cl2PRu: C,
62.14; H, 5.05. Found: C, 62.36; H, 5.17.
1b, (80% yield). 1H NMR: δ 8.57 (d, J = 8.0 Hz, 1H), 8.06 (t, J = 8.0 Hz, 1H), 7.99 (d, J = 8.4 Hz,
1H), 7.93 (d, J = 8.0 Hz, 1H), 7.58 (m, 3H), 5.46 (br, 1H), 4.83 (br, 1H), 4.69 (br, 1H), 4.23 (br, 1H),
3.75 (m, 1H, CH), 3.31 (m, 1H, CH), 2.88 (m, 1H, CH), 1.71−0.66 (br, 18H, CH3), 1.49 (s, 3H, CH3).
31P{1H} NMR: δ 35.7. Anal. Calcd for C26H35Cl2PRu: C, 56.73; H, 6.41. Found: C, 56.64; H, 6.58.
3, (73% yield). 1H NMR: δ 7.80 (t, J = 8.0 Hz, 1H), 7.36 (m, 3H), 5.21 (br, 2H), 4.69 (m, 2H), 3.25
(br, 2H, CH), 2.91 (m, 1H, CH), 2.51 (s, 3H, CH3), 1.68 (s, 3H, CH3), 1.28 (d, J = 6.8 Hz, 6H, CH3),
31
1.60−0.80 (br, 12H, CH3). P{1H} NMR: δ 38.3. Anal. Calcd for C23H35Cl2PRu: C, 53.70; H, 6.86.
Found: C, 53.82; H, 6.73.
General procedures for the reactions of [RuCl2(p-cymene)]2 with phosphines or phosphinite in the
presence of sodium acetate
A mixture of [RuCl2(p-cymene)]2 (0.05 mmol), phosphine or phosphinite (2.0 equiv), and sodium
acetate (4.0 equiv) in methanol (10 mL) was stirred for 2−4 h at room temperature. The solvent was
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European Journal of Inorganic Chemistry
then removed under vacuum and the residue was chromatographed on a silica gel column with a
mixture of petrol ether/Et2O as the eluent. The products were recrystallized from n-hexane/CH2Cl2 (1:1)
at −10 °C to afford 2a,b, 4−7 as yellow, orange, or dark purple crystals.
1
2a, reaction time 4 h, pale yellow crystals (83% yield). H NMR: δ 8.30 (d, J = 6.8 Hz, 1H), 8.10
(m, 2H), 7.85 (d, J = 8.0 Hz, 1H), 7.65 (m, 1H), 7.48−7.29 (m, 9H), 7.19 (m, 2H), 5.81 (t, J = 6.4 Hz,
2H, Cym-H), 4.55 (d, J = 6.4 Hz, 1H, Cym-H), 4.47 (d, J = 6.0 Hz, 1H, Cym-H), 2.42 (m, 1H, CH),
1.93 (s, 3H, CH3), 1.05 (d, J = 6.8 Hz, 3H, CH3), 0.75 (d, J = 7.2 Hz, 3H, CH3). 13C{1H} NMR: δ 140.3,
135.0, 131.5, 131.3, 130.4, 129.8, 129.6, 128.3, 128.0, 127.6, 124.6, 121.8, 108.7, 99.5, 95.1, 92.9, 91.6,
87.3, 31.1, 22.6, 16.3. 31P{1H} NMR: δ 66.3. Anal. Calcd for C32H30ClPRu: C, 66.03; H, 5.20. Found: C,
65.87; H, 5.24.
2b, reaction time 4 h, red crystals (85% yield). 1H NMR: δ 8.28 (d, J = 6.8 Hz, 1H), 7.73 (b, J = 8.0
Hz, 1H), 7.49 (t, J = 7.2 Hz, 1H), 7.38 (d, J = 8.0 Hz, 1H), 7.34−7.27 (m, 2H), 6.10 (d, J = 6.0 Hz, 1H,
Cym-H), 6.07 (d, J = 6.0 Hz, 1H, Cym-H), 5.30 (d, J = 6.0 Hz, 1H, Cym-H), 4.87 (d, J = 5.6 Hz, 1H,
Cym-H), 2.84 (m, 1H, CH), 2.71 (m, 2H, CH), 1.92 (s, 3H, CH3), 1.48 (dd, J1 = 14.0 Hz, J2 = 7.2 Hz,
3H, CH3), 1.39 (dd, J1 = 16.0 Hz, J2 = 7.2 Hz, 3H, CH3), 1.18 (d, J = 6.8 Hz, 3H, CH3), 1.12 (dd, J1 =
14.4 Hz, J2 = 6.8 Hz, 3H, CH3), 1.08 (d, J = 6.8 Hz, 3H, CH3), 1.05 (dd, J1 = 13.6 Hz, J2 = 7.2 Hz, 3H,
CH3). 31P{1H} NMR: δ 80.4. Anal. Calcd for C26H34ClPRu: C, 60.75; H, 6.67. Found: C, 60.83; H, 6.54.
1
4, reaction time 3 h, orange crystals (36% yield). H NMR: δ 7.34 (d, J = 7.6 Hz, 1H), 7.22 (t, J =
7.2 Hz, 1H), 7.15 (t, J = 7.6 Hz, 1H), 7.01 (t, J = 7.2 Hz, 1H), 5.60 (d, J = 5.6 Hz, 1H, Cym-H), 5.40 (d,
J = 6.0 Hz, 1H, Cym-H), 5.00 (m, 1H), 4.59 (d, J = 5.6Hz, 1H, Cym-H), 3.87−3.77 (m, 2H, CH2), 2.86
(m, 1H, CH), 2.81 (m, 1H, CH), 2.23 (m, 1H, CH), 2.15 (s, 3H, CH3), 1.51 (dd, J1 = 15.2 Hz, J2 = 6.8
Hz, 3H, CH3), 1.30 (m, 6H, CH3), 1.27 (m, 3H, CH3), 1.02 (dd, J1 = 12 Hz, J2 = 7.2 Hz, 3H, CH3), 0.83
(dd, J1 = 14.8 Hz, J2 = 6.8 Hz, 3H, CH3) . 13C{1H} NMR: δ 160.7, 132.2, 129.2, 123.9, 112.6, 97.9, 88.0,
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31
85.0, 82.4, 31.2, 29.7, 27.0, 26.2, 23.3, 20.8, 18.9, 18.5, 18.0. P{1H} NMR: δ 74.1. Anal. Calcd for
C23H34ClPRu: C, 57.79; H, 7.17. Found: C, 57.93; H, 7.25.
5, reaction time 4 h, orange crystals (82% yield). 1H NMR: δ 8.34 (dd, J1 = 7.2 Hz, J2 = 1.2 Hz, 1H),
7.63 (dd, J1 = 8.0 Hz, J2 = 2.0 Hz, 1H), 7.32 (d, J = 8.4 Hz, 1H), 7.22 (t, J = 7.6 Hz, 1H), 7.12 (dd, J1 =
8.0 Hz, J2 = 2.8 Hz, 1H), 6.10 (m, 2H, Cym-H), 5.22 (d, J = 6.0 Hz, 1H, Cym-H), 4.93 (d, J = 6.0 Hz,
1H, Cym-H), 2.93 (m, 2H, CH), 2.73 (m, 1H, CH), 2.63 (d, , J = 0.8 Hz, 3H, CH3), 1.93 (s, 3H, CH3),
1.62 (dd, J1 = 18.4 Hz, J2 = 6.8 Hz, 3H, CH3), 1.25 (dd, J1 = 14.4 Hz, J2 = 7.2 Hz, 3H, CH3), 1.19 (d, J
= 7.2 Hz, 3H, CH3), 1.18 (dd, J1 = 14.0 Hz, J2 = 7.2 Hz, 3H, CH3), 1.07 (d, J = 7.2 Hz, 3H, CH3), 0.95
(dd, J1 = 14.0 Hz, J2 = 7.2 Hz, 3H, CH3). 31P{1H} NMR: δ 92.6. Anal. Calcd for C27H36ClPRu: C, 61.41;
H, 6.87. Found: C, 61.26; H, 6.93.
6, reaction time 2 h, yellow crystals (52% yield). 1H NMR: δ 8.03 (d, J = 8.4 Hz, 1H), 7.89 (d, J =
8.4 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.33 (m, 2H), 7.24 (m, 1H), 6.11 (m, 2H, Cym-H), 5.30 (d, J =
6.0 Hz, 1H, Cym-H), 5.10 (d, J = 6.0 Hz, 1H, Cym-H), 3.04 (m, 1H, CH), 2.76 (m, 1H, CH), 2.64 (m,
1H, CH), 1.88 (s, 3H, CH3), 1.48 (dd, J1 = 14.4 Hz, J2 = 7.2 Hz, 3H, CH3), 1.40−1.27 (m, 9H, CH3),
1.14 (d, J = 6.8 Hz, 3H, CH3), 1.07 (d, J = 6.8 Hz, 3H, CH3). 31P{1H} NMR: δ 193.4. Anal. Calcd for
C26H34ClOPRu: C, 58.92; H, 6.47. Found: C, 59.07; H, 6.42.
7, reaction time 2 h, dark purple crystals (33% yield). 1H NMR: δ 5.51 (d, J = 6.4 Hz, 2H, Cym-H),
5.41 (d, J = 6.0 Hz, 2H, Cym-H), 3.74 (d, J = 9.6 Hz, 3H, OCH3), 2.90 (m, 3H, CH), 2.15 (s, 3H, CH3),
1.36 (dd, J1 = 14.0 Hz, J2 = 7.2 Hz, 6H, CH3), 1.27 (d, J = 6.8 Hz, 6H, CH3), 1.26 (dd, J1 = 14.8 Hz, J2
= 7.2 Hz, 6H, CH3). 31P{1H} NMR: δ 156.0. Anal. Calcd for C17H31Cl2OPRu: C, 44.94; H, 6.88. Found:
C, 44.83; H, 6.72.
Arene ligand exchange reaction
A solution of cycloruthenated complex 2a (0.02 mmol) in o-xylene or toluene (5 mL) was refluxed
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European Journal of Inorganic Chemistry
for 20 h. The solvent was then removed under vacuum and the residue was chromatographed on a silica
gel column with a mixture of petrol ether/Et2O as the eluent. The products were recrystallized from
1
n-hexane/CH2Cl2 at −10 °C to give 8a−b as yellow crystals. 8, 65% yield. H NMR: δ 8.22 (m, 1H),
8.08 (m, 2H), 7.84 (m, 1H), 7.65 (m, 1H), 7.46 (m, 5H), 7.34−7.26 (m, 4H), 7.07 (m, 2H), 5.80 (d, J =
5.2 Hz, 1H), 5.37 (m, 1H), 4.86 (t, J = 5.2 Hz, 1H), 3.49 (m, 1H), 2.23 (s, 3H, CH), 2.21 (d, J = 1.2 Hz,
3H). 31P{1H} NMR: δ 68.4. Anal. Calcd for C30H26ClPRu: C, 65.04; H, 4.73. Found: C, 65.21; H, 4.52.
8’, 55% yield. 1H NMR: δ 8.34 (m, 1H), 8.17 (m, 2H), 7.84 (m, 1H), 7.85 (m, 1H), 7.48 (m, 4H), 7.42
(m, 1H), 7.37−7.25 (m, 4H), 7.07 (m, 2H), 5.64 (m, J = 5.6 Hz, 2H), 5.40 (d, 1H), 4.96 (m, 1H), 4.09
31
(m, 1H), 2.20 (s, 3H, CH3). P{1H} NMR: δ 68.2. Anal. Calcd for C29H24ClPRu: C, 64.50; H, 4.48.
Found: C, 64.75; H, 4.31.
Preparation of bis-phosphine complex 9
A suspension of 2b (0.05 mmol) and PPh3 (0.14 mmol) in EtOH (5 mL) was refluxed for 3 h. The
solution was cooled to RT and a solution of [NH4]PF6 (0.14 mmol) in water (5 mL) added. The solid
was then isolated by decantation and washed with water (3 × 2 mL). The solid was then dissolved in
CH2Cl2 and dried with Na2SO4 and the product 9 precipitated by addition of excess Et2O (68%) as dark
yellow powder. 9, 1H NMR: δ 7.73−7.65 (m, 4H), 7.58−7.42 (m, 12H), 7.28 (m, 2H), 7.14 (m, 1H), 6.38
(br, 1H), 6.21 (d, J = 6.4 Hz, 1H), 6.14 (d, J = 6.4 Hz, 1H), 5.77 (br, 1H), 5.38 (d, J = 6.4 Hz, 1H), 4.98
(d, J = 6.4 Hz, 1H), 2.67 (m, 3H, CH), 2.18 (s, 3H, CH3), 1.61 (m, 3H, CH3), 1.39−1.28 (m, 6H, CH3),
1.36 (d, J = 6.8 Hz, 3H, CH3), 0.77 (d, J = 6.8 Hz, 3H, CH3), 0.15 (dd, J1 = 16.4 Hz, J2 = 6.8 Hz, 3H,
31
CH3). P{1H} NMR: δ 79.9 (d, J = 43.3 Hz), 41.5 (d, J = 43.1 Hz), −144.2 (m, J = 713.0 Hz). Anal.
Calcd for C44H49F6P3Ru: C, 59.66; H, 5.58. Found: C, 59.71; H, 5.37.
Reaction of complex 2a with diphenylacetylene
A solution of complex 2a (0.02 mmol) and diphenylacetylene (3 equiv) in methanol (5 mL) was
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10.1002/ejic.201700479
European Journal of Inorganic Chemistry
stirred at room temperature for 20 h. The solvent was then removed under vacuum and the residue was
chromatographed on a silica gel column with petrol ether as the eluent. The products were recrystallized
from n-hexane at −10 °C to give 10 as yellow crystals (74% yield). 10, 1H NMR: δ 7.87 (d, J = 8.4 Hz,
2H), 7.75 (d, J = 8.4 Hz, 2H), 7.61 (dd, J1 = 8.4 Hz, J2 = 7.2 Hz, 2H), 7.45 (m, 4H), 7.37 (m, 4H), 7.31
(m, 2H). Anal. Calcd for C24H16: C, 94.70; H, 5.30. Found: C, 94.55; H, 5.43.
Transfer hydrogenation of ketones
Typical procedure for the catalytic hydrogen-transfer reaction: a suspension of the ruthenium
complex (1−6) (0.01 mmol), base (20.0 equiv), and the corresponding ketone (200-10000 equiv) in
degassed isopropanol (75 mL) was refluxed for 48 h. Volatiles were removed under reduced pressure,
and the residue was chromatographed on a silica column. Elution with ether acetate/petroleum ether
developed a colorless band. After vacuum removal of the solvents from the above eluate, the residue
was obtained as white solid or colorless oil.
Crystallographic Studies
Single crystals of complexes 1b, 2a, 4, 5, 6, 7, 8, 9, and 10 (CCDC reference numbers
1520714-1520722) suitable for X-ray diffraction were obtained by crystallization from
n-hexane/CH2Cl2 (1:1). Data collection was performed on a Bruker SMART 1000, using
graphite-monochromated Mo Kα radiation (ω−2θ scans, λ = 0.71073 Å). Semiempirical absorption
corrections were applied for all complexes. The structures were solved by direct methods and refined by
full-matrix least squares. All calculations were using the SHELXTL-97 program system.
Supporting Information: Single Crystal Structures of 5 and 8 (including selected bond lengths and
angles), the crystal data and summary of X-ray data collection for 1b, 2a, 4, 5, 6, 7, 8, 9, and 10 are
available.
25
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10.1002/ejic.201700479
European Journal of Inorganic Chemistry
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos.
21002069 and 21572160), the Natural Science Foundation of Tianjin (No. 14JCYBJC20300), and the
Program for Innovative Research Team in University of Tianjin (TD12-5038).
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10.1002/ejic.201700479
European Journal of Inorganic Chemistry
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