Synthesis, characterization, and reactivity of ruthenium hydride complexes of N-centered triphosphine ligands

Article abstract of DOI:10.1021/ic500030k

The reactivity of the novel tridentate phosphine ligand N(CH ₂PCyp₂)₃ (N-triphos^Cyp, 2; Cyp = cyclopentyl) with various ruthenium complexes was investigated and compared that of to the less sterically bulky and less electron donating phenyl derivative N(CH₂PPh₂)₃ (N-triphos^Ph, 1). One of these complexes was subsequently investigated for reactivity toward levulinic acid, a potentially important biorenewable feedstock. Reaction of ligands 1 and 2 with the precursors [Ru(COD)(methylallyl)₂] (COD = 1,5-cycloocatadiene) and [RuH₂(PPh₃)₄] gave the tridentate coordination complexes [Ru(tmm){N(CH₂PR₂) ₃-δ³P}] (R = Ph (3), Cyp (4); tmm = trimethylenemethane) and [RuH₂(PPh₃){N(CH ₂PR₂)₃-δ³P}] (R = Ph (5), Cyp (6)), respectively. Ligands 1 and 2 displayed different reactivities with [Ru₃(CO)₁₂]. Ligand 1 gave the tridentate dicarbonyl complex [Ru(CO)₂{N(CH₂PPh₂)₃- δ³P}] (7), while 2 gave the bidentate, tricarbonyl [Ru(CO) ₃{N(CH₂PCyp₂)₃-δ²P} ] (8). This was attributed to the greater electron-donating characteristics of 2, requiring further stabilization on coordination to the electron-rich Ru(0) center by more CO ligands. Complex 7 was activated via oxidation using AgOTf and O₂, giving the Ru(II) complexes [Ru(CO)₂(OTf){N(CH ₂PPh₂)₃-δ³P}](OTf) (9) and [Ru(CO₃)(CO){N(CH₂PPh₂)₃- δ³P}] (11), respectively. Hydrogenation of these complexes under hydrogen pressures of 3-15 bar gave the monohydride and dihydride complexes [RuH(CO)₂{N(CH₂PPh₂) ₃-δ³P}] (10) and [RuH₂(CO){N(CH ₂PPh₂)₃-δ³P}] (12), respectively. Complex 12 was found to be unreactive toward levulinic acid (LA) unless activated by reaction with NH₄PF₆ in acetonitrile, forming [RuH(CO)(MeCN){N(CH₂PPh₂)₃- δ³P}](PF₆) (13), which reacted cleanly with LA to form [Ru(CO){N(CH₂PPh₂)₃-δ³P} {CH₃CO(CH₂)₂CO₂H- δ²O}](PF₆) (14). Complexes 3, 5, 7, 8, 11, and 12 were characterized by single-crystal X-ray crystallography.

Full text of DOI:10.1021/ic500030k

Article
pubs.acs.org/IC
Synthesis, Characterization, and Reactivity of Ruthenium Hydride
Complexes of N‑Centered Triphosphine Ligands
Andreas Phanopoulos, Neil J. Brown, Andrew J. P. White, Nicholas J. Long,* and Philip W. Miller*
Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, U.K.
S
* Supporting Information
ABSTRACT: The reactivity of the novel tridentate phosphine ligand N(CH2PCyp2)3 (N-triphosCyp, 2; Cyp = cyclopentyl) with
various ruthenium complexes was investigated and compared that of to the less sterically bulky and less electron donating phenyl
derivative N(CH2PPh2)3 (N-triphosPh, 1). One of these complexes was subsequently investigated for reactivity toward levulinic
acid, a potentially important biorenewable feedstock. Reaction of ligands 1 and 2 with the precursors [Ru(COD)(methylallyl)2]
(COD = 1,5-cycloocatadiene) and [RuH2(PPh3)4] gave the tridentate coordination complexes [Ru(tmm){N(CH2PR2)3-κ3P}]
(R = Ph (3), Cyp (4); tmm = trimethylenemethane) and [RuH2(PPh3){N(CH2PR2)3-κ3P}] (R = Ph (5), Cyp (6)), respectively.
Ligands 1 and 2 displayed different reactivities with [Ru3(CO)12]. Ligand 1 gave the tridentate dicarbonyl complex
[Ru(CO)2{N(CH2PPh2)3-κ3P}] (7), while 2 gave the bidentate, tricarbonyl [Ru(CO)3{N(CH2PCyp2)3-κ2P}] (8). This was
attributed to the greater electron-donating characteristics of 2, requiring further stabilization on coordination to the electron-rich
Ru(0) center by more CO ligands. Complex 7 was activated via oxidation using AgOTf and O2, giving the Ru(II) complexes
[Ru(CO)2(OTf){N(CH2PPh2)3-κ3P}](OTf) (9) and [Ru(CO3)(CO){N(CH2PPh2)3-κ3P}] (11), respectively. Hydrogenation
of these complexes under hydrogen pressures of 3−15 bar gave the monohydride and dihydride complexes [RuH(CO)2{N-
(CH2PPh2)3-κ3P}] (10) and [RuH2(CO){N(CH2PPh2)3-κ3P}] (12), respectively. Complex 12 was found to be unreactive
toward levulinic acid (LA) unless activated by reaction with NH4PF6 in acetonitrile, forming [RuH(CO)(MeCN){N-
(CH2PPh2)3-κ3P}](PF6) (13), which reacted cleanly with LA to form [Ru(CO){N(CH2PPh2)3-κ3P}{CH3CO(CH2)2CO2H-
κ2O}](PF6) (14). Complexes 3, 5, 7, 8, 11, and 12 were characterized by single-crystal X-ray crystallography.
phosphine or variance of the phosphine itself. Triphos-type
ligands have been shown to coordinate to a wide range of early
and late transition metals,17−21 some of which have been
evaluated for catalytic activity. Early examples include Rh-
triphosPh complexes used for hydrogenation and hydro-
formylation of various alkenes22 or the hydrogenation of
quinoline, an important impurity found in fossil fuels that
requires degradation.23 More recently, Mo-triphosPh complexes
were shown to coordinate and activate N2, ultimately for
conversion to ammonia or other high-value nitrogen-containing
compounds,24 while Ru-triphos complexes have been employed
in formic acid dehydrogenation25 and direct methylation of
primary and secondary aromatic amines using carbon dioxide
and molecular hydrogen.26 Futhermore, Leitner et al. have
reported the use of Ru-triphosPh systems in several highly
desirable “green chemistry” reactions. A [Ru(acac)3]/triphos
system was used for the stepwise and tunable hydrogenation of
two biomass-derived carboxylic acids (levulinic and itaconic
acid) ultimately to 2- and 3-methyltetrahydrofuran, respec-
INTRODUCTION
■
The vast body of academic literature and industrially applied
ruthenium-catalyzed reactions1−9 is testament to the versatility
and importance of ruthenium-based catalysts. The types of Ru
catalytic transformations are diverse, varying from C−C cross-
coupling reactions10 and activation of inert C−H bonds11 to
the better-known metathesis12 and hydrogenation reactions.13
The majority of these ruthenium catalysts incorporate either
mono- or bidentate phosphine ligands, which are ubiquitous in
homogeneous catalysis owing to their highly tunable electronic
and steric properties. Although less commonly used in catalytic
applications than mono- or bidentate analogues, tridentate
phosphines offer well-defined coordination geometries that can
affect the stability of the catalytically active metal center, in
some cases producing more active and robust catalysts.
Consequently, tridentate ligands are receiving renewed
academic and industrial interest for both coordination
chemistry and catalysis.14−16
The most commonly studied tripodal phosphines are 1,1,1-
tris(diphenylphosphinomethyl)ethane (CH3C(CH2PPh2)3, tri-
phosPh) and its derivatives; these analogues mostly feature
changes to the arm length between the apical carbon and
Received: January 8, 2014
Published: March 26, 2014
© 2014 American Chemical Society
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tively; in both cases greater activity and scope was
demonstrated over the use of analogous mono- or bidentate
phosphine ligands.27 This is significant, as methyltetrahydrofur-
an (MTHF) has been promoted as a greener ethereal solvent
due to its production from renewable resources (such as corn
starch), as well as its relative immiscibility with aqueous
solutions in comparison to tetrahydrofuran (THF), facilitating
workup and recovery.28−30 A [Ru(COD)(methylallyl)2]/
triphos mixture was used for the selective hydrogenolysis via
C−O bond cleavage of lignin model compounds, a vital step in
the valorization process that aims to mildly depolymerize this
natural product, while maintaining the desired functionality.31
Finally, a preformed ruthenium complex, [Ru(tmm){CH3C-
(CH2PPh2)3-κ3P}] (tmm = trimethylenemethane), was shown
to give moderate turnover numbers for the hydrogenation of
CO2 to methanol,32 a vital step for the recycling of carbon that
may eventually lead to the realization of a sustainable carbon
economy.
Despite the great interest in triphos complexes, the synthesis
of triphos ligands is not always straightforward. Typically,
highly air and moisture sensitive metal phosphide reagents are
required that can result in low yields owing to incomplete
substitution reactions. It would be desirable to obtain a more
facile synthesis of triphos analogues that impart the same
coordination chemistry features and catalytic activity. The
previously reported N-centered triphos ligand N,N,N-tris-
(diphenylmethyl)amine, where the central bridgehead CH3−
C moiety in triphosPh is replaced by a single nitrogen atom to
give N(CH2PPh2)3 (N-triphosPh, 1), is conveniently synthe-
sized via a phosphorus variation of the Mannich reaction from
the corresponding secondary phosphine.33 Despite this facile
synthesis, complexes with N-triphos ligands remain rare with
only, to the best of our knowledge, a select few having been
reported in the literature.17,33−37 These ligands have previously
been targeted within our group, as their facial coordination
simplifies complexation by reducing the number of potential
isomers, as well as resulting in a cis conformation of any other
coordinated groups, facilitating reductive elimination during
catalysis. It was for these primary reasons that mer-coordinating
analogues were not investigated, despite many such ligands
being studied for both complexation and catalysis.38−40
Examples of such coordination geometries can be found in
the vast body of work by Milstein et al., who have studied many
pincer-type complexes, including the coordination and
reactivity of PONOP ligands,41 as well as CNN-, PNN-, and
PNP-type ligands for activity toward hydrogenation of straight
and cyclic esters,42,43 and coupling reactions to form imines and
pyrroles.44,45 Other examples include POP- and PSP-type
ligands that have similarly been investigated.46,47
improve catalytic activity. Herein, we report a series of new
ruthenium N-triphos complexes, including three with the novel
dicyclopentyl N-triphos ligand (N-triphosCyp, 2). The coordi-
nation chemistry of 2 is compared to that of the less bulky
phenyl derivative 1, using three different ruthenium precursors.
The reactivity of one of these complexes toward levulinic acid
(LA) was assessed after initial activation by oxidation of the
Ru(0) center, subsequent hydride formation under H2 pressure,
and a final activation step involving reaction with a proton
donor.
RESULTS AND DISCUSSION
■
Ligand Synthesis. It has been previously shown by us that
the coordination mode of N-triphos derivatives is controlled by
the steric constraints imposed by the phosphine arms;
consequently, the bulky di-tert-butyl and dicyclohexyl deriva-
tives were found to exclusively form κ2 coordination complexes,
resulting in one free pendant arm.17 The novel dicyclopentyl
derivative N-triphosCyp (2) was therefore selected, as its
reduction in steric bulk from the previously studied derivatives
should enable the desired tridentate coordination. A switch to
diarylphosphines was not explored, as dialkylphosphines
possess the desired electron-donating ability required to
adequately promote oxidative addition during catalytic cycles.
Ligand 1 was prepared as previously reported, via a
conveniently air-stable phosphonium salt intermediate (Scheme
1a). Isolation and reaction of the phosphonium salt precursor
for ligand 2 was found to give reduced yields; hence, ligand 2
was prepared via a one-pot in situ method (Scheme 1b).
Scheme 1. Synthesis of (a) N-triphosPh via a Phosphonium
Salt Intermediate and (b) N-triphosCyp via a
Dialkyl(hydroxymethyl)phosphine Intermediate
N-triphosCyp was prepared in moderate yield (59%) via the
reaction of ammonia and in situ generated dicyclopentyl-
(hydroxymethyl)phosphine. Cooling the reaction mixture to
−35 °C overnight resulted in the formation of a white
crystalline solid that was isolated in high purity simply by
filtration. The 31P{1H} NMR spectrum showed a single
resonance at δ −18.4, and subsequent NMR spectra analysis
revealed it does not exhibit strong J coupling between
phosphorus and other spin-active nuclei. The reason for these
unknown, however similar, spectral characteristics were
observed for the ethyl, isopropyl, tert-butyl, and cyclohexyl
derivatives, suggesting that this can be considered characteristic
of this kind of ligand.17,37,48
Our previous investigations of N-triphos ligands have shown
that steric encumberment of the phosphine arms determines
whether a κ3 or κ2 coordination geometry is achieved.17 Bulky
phosphino-alkyl groups (dicyclohexyl- or di-tert-butylphos-
phine) only coordinate through two arms, while the third
remains pendant; in addition, Gade et al. have recently shown
that the smaller isopropyl derivative can bind through all three
arms.36 Since triphos complexes have previously been
demonstrated to be highly effective catalysts, it would be of
significant interest to expand the family of known κ3-
coordinating triphosphine ligands. Investigation into a wider
range of sterically larger or smaller phosphines, in addition to
exploring various phosphine substituents in order to tune
electronic properties, may help elucidate critical aspects to
Comparison of Coordination Chemistry between N-
triphosPh and N-triphosCyp. Recent papers by the groups of
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Scheme 2. Summary of N-triphosPh and N-triphosCyp Ru Complexes Prepared during This Study
Leitner and Gade have shown [Ru(COD)(methylallyl)2] to be
a versatile starting material for producing κ3 triphosphine
ruthenium complexes.31,36,37 When ligands 1 and 2 were
reacted with 1 equiv of [Ru(COD)(methylallyl)2] in toluene
under reflux, all three phosphine arms were found to coordinate
to the Ru center, displacing the COD ligand and one
methylallyl moiety, generating facially capping κ3 complexes
(Scheme 2). Thus, ligands 1 and 2 afford the complexes
[Ru(tmm){N(CH2PPh2)3-κ3P}] (3) and [Ru(tmm){N-
(CH2PPh2)3-κ3P}] (4), respectively. The symmetric nature of
complexes 3 and 4 is evident from the 31P{1H} NMR spectra,
which each display singlets at δ 18.2 and 14.2, respectively.
Similar to the compound [Ru(tmm){N(CH2PiPr2)3-κ3P}]
reported by Gade et al.,36 a singlet is observed for the signals
assigned to the methylene protons of the tmm moieties in
complex 4, while the quaternary carbon (13C{1H} δ 102.8) is
Figure 1. Crystal structure of the C3-symmetric complex 3 (50%
probability ellipsoids). Selected bond lengths (Å) and angles (deg) for
3: Ru(1)−P(3), 2.2828(3); Ru(1)−C(21), 2.085(2); Ru(1)−C(22),
2.2518(13); P(3)−Ru(1)−C(21), 125.820(8); P(3)−Ru(1)−C(22),
103.89(4); P(3)−Ru(1)−P(3A), 89.211(12); P(3)−Ru(1)−C(22A),
164.01(4); P(3)−Ru(1)−P(3B), 89.211(12); P(3)−Ru−C(22B),
99.93(4); C(21)−Ru(1)−C(22), 38.51(4); C(21)−Ru(1)−P(3A),
125.820(8); C(21)−Ru(1)−C(22A), 38.51(4); C(21)−Ru(1)−P-
(3B), 125.820(8); C(21)−Ru(1)−C(22B), 38.51(4); C(22)−Ru−
P(3A), 99.93(4); C(22)−Ru(1)−C(22A), 65.26(6); C(22)−Ru(1)−
P(3B), 164.01(4); C(22)−Ru(1)−C(22B), 65.26(6); P(3A)−Ru(1)−
P(3B), 89.211(12); P(3A)−Ru(1)−C(22A), 103.89(4); P(3A)−
Ru(1)−C(22B), 164.01(4); P(3B)−Ru(1)−C(22A), 99.93(4); P-
(3B)−Ru(1)−C(22B), 103.89(4); C(22A)−Ru(1)−C(22B),
65.26(6).
highly shifted, indicating η4 coordination. Characterization of
complex 3 by NMR proved difficult due to solubility issues,
allowing only a weak 31P{1H} NMR spectrum to be recorded.
High-resolution mass spectrometry (HRMS) showed only one
peak cluster displaying the expected isotope pattern for
ruthenium, strongly suggesting a monometallic structure.
Additionally, crystals of 3 suitable for X-ray diffraction analysis
were grown by allowing a dilute toluene solution to stand at
room temperature for 5 days (Figure 1). The molecular
structure revealed the complex has crystallographic C3
symmetry about an axis that passes through N(1) and the
ruthenium center, as well as confirming the expected distorted-
octahedral structure. The short Ru−Ctmm distance observed
(2.085 Å) also supports an η4 coordination mode. Unlike
complex 3, the cyclopentyl derivative complex 4 is remarkably
soluble in many solvents, and it is due to this, as well as its air
and moisture sensitivity, that it was only possible to be isolated
in relatively low yields (20%). Attempts to further extract or
precipitate the desired complex invariably resulted in
decomposition to “ruthenium black” and free ligand.
2). Both complexes show a characteristic 31P{1H} NMR AM2X
splitting pattern, manifesting as two doublets of triplets and an
apparent triplet. The two phosphine arms of the NP3 ligands
trans to the hydrides are easily identified from the relative 2:1
intensity in comparison to the other signals. The remaining two
phosphorus signals were assigned on the basis of 2D
1H/31P{1H} and 13C{1H}/31P{1H} NMR correlation experi-
ments, indicating the remaining arms on the NP3 ligands have
δP values of 26.9 and 25.6 for complexes 5 and 6, respectively.
Reaction of ligands 1 and 2 in toluene at 50 °C with
[Ru(H)2(PPh3)4] results in ligand substitution of three PPh3
units, resulting in the tridentate complexes [RuH2(PPh3){N-
(CH2PR2)3-κ3P}] (R = phenyl (5), cyclopentyl (6)) (Scheme
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The 1H NMR spectra show distinctive hydride resonances that
appear as complex multiplets centered around δ −7.73 and
1
−9.72 for complexes 5 and 6, respectively. When H{31P}
spectra were recorded, these signals were found to simplify to
singlets (Figure S7, Supporting Information). Crystals of 5
suitable for X-ray diffraction analysis were grown from a
concentrated ether solution, and the structure shows a
distorted-octahedral coordination geometry (Figure 2). Ini-
Figure 3. Crystal structure of 7 (50% probability ellipsoids). Selected
bond lengths (Å) and angles (deg) for 7: Ru−P(3), 2.3460(4); Ru−
P(5), 2.3515(4); Ru−P(7), 2.3601(4); Ru−C(50), 1.8839(17); Ru−
C(60), 1.8950(15); P(3)−Ru−P(5), 93.667(13); P(3)−Ru−P(7),
86.450(13); P(3)−Ru−C(50), 147.51(5); P(3)−Ru−C(60),
87.27(5); P(5)−Ru−P(7), 86.265(13); P(5)−Ru−C(50), 118.55(5);
P(5)−Ru−C(60), 109.37(5); P(7)−Ru−C(50), 91.27(5); P(7)−Ru−
C(60), 163.49(5); C(50)−Ru−C(60), 85.88(7).
Unlike [Ru(COD)(methylallyl)2] and [RuH2(PPh3)4], for
which ligands 1 and 2 showed identical reactivity, reaction of 3
equiv of ligand 2 with [Ru3(CO)12] in toluene under reflux
overnight did not lead to tridentate coordination; instead, the
κ2 complex [Ru(CO)3{N(CH2PCyp2)3-κ2P}] (8) was formed.
Crystals suitable for X-ray diffraction analysis were grown by
layering a CH2Cl2 solution of 8 with methanol, confirming the
bidentate coordination of 2, which coordinates in a distorted-
trigonal-bipyramidal geometry (Figure 4 and Table 1). The
Figure 2. Crystal structure of 5 (50% probability ellipsoids). Selected
bond lengths (Å) and angles (deg) for 5: Ru−P(3), 2.2696(5); Ru−
P(5), 2.3311(6); Ru−P(7), 2.3358(6); Ru−P(50), 2.3197(5); P(3)−
Ru−P(5), 95.54(2); P(3)−Ru−P(7), 88.06(2); P(3)−Ru−P(50),
149.45(2); P(5)−Ru−P(7), 86.37(2); P(5)−Ru−P(50), 107.80(2);
P(7)−Ru−P(50), 112.31(2).
tially, synthesis of complex 5 was performed under reflux in
diglyme; however, the inherent instability of the precursor
[RuH2(PPh3)4] under these reaction conditions resulted in
significantly lower yields. These lower yields were presumably
due to a significant amount of decomposition occurring in
tandem with product formation, as the precursor is highly
fluxional in solution due to the disassociation/association of
PPh3. Changing the solvent to toluene and gently heating to 50
°C resulted in a yield increase from 19% to 54%.
[Ru(CO)2{N(CH2PPh2)3-κ3P}] (7), previously reported by
us but not structurally characterized,48 can be formed in high
yields via the reaction of 3 equiv s of 1 with [Ru3(CO)12] in
toluene under reflux. The 31P{1H} NMR spectrum of 7 shows a
single phosphorus resonance at δ 8.95 that remains unaltered
when the temperature is lowered to −50 °C, demonstrating the
highly fluxional nature of the complex in solution. X-ray
diffraction analysis of crystals grown from a concentrated
toluene solution show that, when crystalline, this complex
adopts a distorted-square-pyramidal structure (for the square-
based-pyramidal parameter τ of 7 see Table S2 in the
Supporting Information) (Figure 3). Complex 7 is valuable as
a precursor complex, from which a wide variety of potentially
catalytically interesting complexes can be synthesized. It can be
prepared in high yield and is a robust and stable complex that
can be handled in air in the solid state, proving to be stable for
days and only slowly oxidizing after several weeks to the
carbonate complex [Ru(CO3)(CO){N(CH2PPh2)3-κ3P}] (11).
This transformation can be prevented by storing complex 7
under nitrogen.
Figure 4. Structure of one (8-A) of the eight independent complexes
present in the crystals of 8 (50% probability ellipsoids).
crystals of 8 were found to contain eight crystallographically
independent complexes (8-A−8-H). Unsurprisingly, the
N(1)···Ru separation seen in 8 is much larger (by ca. 0.46 Å)
than those seen in the complexes with a tridentate N-triphos
ligand (Table S3, Supporting Information). Neither sustained
further heating to 165 °C in diglyme nor irradiation with a 400
W UV light resulted in ejection of a CO ligand and subsequent
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Table 1. Selected Bond Lengths (Å) for the Eight Independent Complexes (A−H) Present in the Crystals of 8
Ru(1)−P(3)
Ru(1)−P(5)
Ru(1)−C(38)
Ru(1)−C(39)
Ru(1)−C(40)
A
B
C
D
E
2.3861(14)
2.3669(14)
2.3785(14)
2.3800(14)
2.3685(16)
2.3672(14)
2.3937(14)
2.3591(14)
2.3710(15)
2.3778(15)
2.3732(13)
2.3837(14)
2.3846(15)
2.3866(14)
2.3806(14)
2.3845(14)
1.902(6)
1.881(7)
1.895(6)
1.898(7)
1.895(6)
1.881(6)
1.904(6)
1.908(6)
1.918(6)
1.899(6)
1.915(6)
1.907(7)
1.900(8)
1.907(7)
1.895(6)
1.912(6)
1.904(6)
1.908(7)
1.894(6)
1.874(7)
1.904(8)
1.895(7)
1.915(7)
1.902(6)
F
G
H
Scheme 3. Activation of Complex 7 by Oxidation using Two Different Oxidants
chelation of the pendant arm. Heating only resulted in
decomposition of the complex into uncharacterizable products.
Complex 8 remained completely unreactive under UV
irradiation, with only starting material being observed, even
after several hours, suggesting that the carbonyl ligands are
highly stabilizing for the electron-rich ruthenium center. The
increased electron-donating ability of the dicyclopentylphos-
phine arms (cf. diphenylphosphine) must require the
ruthenium to be further stabilized by additional carbonyl
ligands in comparison to the corresponding κ3 N-triphosPh
complex 7.
Synthesis of Ruthenium Hydride Complexes from
[Ru(CO)2{N(CH2PPh2)3-κ3P}] (7) and Subsequent Reac-
tivity. In comparison to [Ru(CO)2(PPh3)3], which forms the
dihydride complex [RuH2(CO)2(PPh3)2] at room temperature
under 1 bar of H2,49 complex 7 is unreactive toward molecular
hydrogen even under forcing conditions of 50 bar and 100 °C.
This is due to the added stability of the chelating NP3 tridentate
ligand and the reduced Ru(0) center, which is further stabilized
by electron-withdrawing CO ligands. In order to produce a
more reactive 16-electron complex, oxidation of 7 was
undertaken using two different oxidizing agents (Scheme 3).
Reaction of 7 with 2 equiv of AgOTf in CH2Cl2 at room
temperature resulted in instant precipitation of metallic silver
and the formation of [Ru(CO)2(OTf){N(CH2PPh2)3-κ3P}]-
(OTf) (9). The slow diffusion of diethyl ether into the CH2Cl2
solution gave a yellow crystalline precipitate that displayed a
sharp doublet and triplet in the 31P{1H} NMR spectrum at δ
−23.9 (2P) and 19.6 (1P), respectively. This implies that
tridentate coordination of 1 persists; however, the complex is
now six-coordinate with two different ancillary ligands. The
presence of two distinct singlets in the 19F{1H} NMR spectrum
at δ −76.8 and −78.5 indicates that one triflate is coordinating
while the other is noncoordinating. This is also inferred from
the FT-IR spectrum of 9, which shows several very broad and
intense peaks in the region 1158−1279 cm−1, suggesting two
different triflate environments. The presence of two carbonyl
ligands is also evident from the FT-IR spectrum as two peaks at
2055 and 2095 cm−1.
Reaction of 9 with molecular hydrogen at 3 bar for 18 h at 70
°C in THF led to high-yielding formation of the monohydride
species [RuH(CO)2{N(CH2PPh2)3-κ3P}](OTf) (10) along
with concomitant formation of 1 equiv of triflic acid. The
31P{1H} NMR spectrum of 10 still displays a doublet and triplet
similar to those seen for complex 9, while the 19F{1H} NMR
spectrum, as expected, displays one resonance corresponding to
1
the noncoordinating triflate anion. The H NMR spectrum of
10 shows a characteristic doublet of triplets hydride signal
centered at δ −6.75.
Oxidation of 7 using molecular oxygen (Scheme 3), by
deliberate exposure of a solid sample to air for several weeks or
by bubbling O2 through a toluene solution of 7 for ca. 20 min,
resulted in conversion to the carbonate complex [Ru(CO3)-
(CO){N(CH2PPh2)3-κ3P}] (11). Reactivity similar to this has
been observed for another ruthenium triphosphine complex
using the mer-coordinating triphosphine PhP-
(CH2CH2CH2PCy2)2.50 The presence of the new carbonate
ligand can clearly be observed in the FT-IR spectrum of 11,
which now displays only a single CO stretch at 1994 cm−1 and
additional stretches typical of bidentate carbonate at 1565,
1434, 1272, and 1092 cm−1.51 Crystals suitable for X-ray
diffraction analysis of complex 11 were grown by layering a
concentrated CH2Cl2 solution with toluene and allowing this
mixture to stand at room temperature overnight (Figure 5),
confirming the bidentate coordination mode of carbonate.
Similar to the crystal structures of 3 and 5, 11 was found to
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Figure 6. Crystal structure of 12 (50% probability ellipsoids). Selected
bond lengths (Å) and angles (deg) for 12: Ru−P(3), 2.3337(6); Ru−
P(5), 2.3359(6); Ru−P(7), 2.3532(6); Ru−C(50), 1.881(3); P(3)−
Ru−P(5), 88.95(2); P(3)−Ru−P(7), 90.48(2); P(3)−Ru−C(50),
99.99(8); P(5)−Ru−P(7), 88.36(2); P(5)−Ru−C(50), 99.47(8);
P(7)−Ru−C(50), 166.97(8).
Figure 5. Structure of one (11-A) of the two independent complexes
present in the crystals of 11 (50% probability ellipsoids).
adopt a distorted-octahedral geometry and crystallized as two
crystallographically independent complexes (11-A and 11-B)
(Table 2).
hydrogenation reactions, as the dihydride complex 12 would
form readily in situ before undergoing any necessary activation
steps prior to catalysis.
Reaction of 11 with a modest pressure of molecular
hydrogen (3 bar) led to the formation of the neutral dihydride
species [Ru(H)2(CO){N(CH2PPh2)3-κ3P}] (12) within 18 h,
though higher pressures (∼15 bar) gave complete conversion
within 2 h; layering a concentrated toluene solution with
methanol gave crystals suitable for X-ray diffraction analysis
(Figure 6). Complex 12 shows an apparent doublet of doublets
Attempts to activate 8 by oxidation and facilitate tandem κ3
coordination were not successful. Oxidation using O2 did not
give the analogous carbonate complex observed with 11 but
instead led to decomposition and oxidation of the ligand.
Presumably the pendant third arm reduces the overall stability
of 8, making it more susceptible to oxidative degradation
processes.
1
splitting pattern for the hydride resonance in the H NMR
spectrum, centered around δ −6.50, instead of the expected
doublet of triplets due to phosphorus coupling. This same
pattern was reported by Leitner et al. for the analogous
TriphosPh complex [RuH2(CO){CH3C(CH2PPh2)3-κ3P}].52
Complex 12 is reactive toward chlorinated solvents, instantly
reacting with CHCl3 and more slowly with CH2Cl2, forming
three separate uncharacterized products, as is evident from
2D-31P{1H} NMR correlation experiments. The previously
mentioned TriphosPh analogue has been implicated in several
important catalytic cycles as a dormant form of the active
catalytic species, hypothesized to be the cationic monohydride
[RuH{CH3C(CH2PPh2)3-κ3P}]+,52 including the hydrogena-
tion of the biomass-derived levulinic and itaconic acids, as well
as CO2 reduction to methanol using hydrogen.27,32
The use of the air-stable carbonate complex 11 as a precursor
for the synthesis of 12 and its analogues offers a route that is
easier and milder than those previously reported for the
equivalent Triphos-Ru dihydride complexes. These previous
synthetic schemes require either harsh reaction conditions (120
bar of H2, 150 °C, 20 h)52 or several highly air-sensitive
steps.53,54 Since 11 is readily converted to 12 under pressures of
hydrogen, 11 may be used as a stable precatalyst for
The transformation of biomass-derived products into
platform chemicals and solvents is of significant industrial
interest, as these can offer green alternatives to products
previously only accessible via petrochemical routes. The triphos
complex [RuH2(CO){CH3C(CH2PPh2)3-κ3P}] has previously
been reported as an active catalyst for the hydrogenation of
biomass-derived acids, such as levulinic acid (LA), to industrial
solvents.27,52 The reactivity of the equivalent N-triphos
complex 12 with LA was found to be markedly different
from that of [RuH2(CO){CH3C(CH2PPh2)3-κ3P}]. When 1
equiv of LA was treated with 12, no reaction was found to
occur in either nonpolar (C6D6) or polar (THF) solvents, even
after heating under reflux overnight. In contrast, [RuH2(CO)-
{CH3C(CH2PPh2)3-κ3P}] was reported to react cleanly with
LA to form the coordinated complex [RuH{CH3C-
(CH2PPh2)3-κ3P}{CH3CO(CH2)2CO2H-κ2O}], with concom-
itant release of H2 and CO.52 The lower reactivity of 12 toward
LA can be overcome by initial in situ activation through
formation of a monohydride cationic species by reaction with
the proton donor NH4PF6, which has previously been shown to
react with dihydride ruthenium complexes in this fashion.55
Table 2. Selected Bond Lengths (Å) for the Two Independent Complexes (A and B) Present in the Crystals of 11
A
B
A
B
Ru(1)−P(3)
Ru(1)−P(5)
Ru(1)−P(7)
Ru(1)−C(50)
2.4018(7)
2.2985(7)
2.3007(7)
2.537(3)
2.4188(7)
2.2872(8)
2.2986(7)
2.537(3)
Ru(1)−O(51)
Ru(1)−O(52)
Ru(1)−C(60)
2.1303(19)
2.132(2)
1.906(3)
2.1265(19)
2.124(2)
1.905(3)
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Scheme 4. Reaction of 12 with NH4PF6 To Form 13 and Subsequent Coordination of Levulinic Acid To Form 14
When 12 is reacted at room temperature with NH4PF6 in a
coordinating solvent such as acetonitrile, it undergoes initial
hydride loss as H2, forming the reactive cationic 16-electron
species [RuH(CO){N(CH2PPh2)3-κ3P}]+ that rapidly stabilizes
through coordination of acetonitrile, forming [RuH(CO)-
(MeCN){N(CH2PPh2)3-κ3P}](PF6) (13) (Scheme 4). This
intermediate species is highly reactive and gives a 31P{1H}
NMR spectrum that shows a multiplet and two doublets of
2
doublets at δ −12.4, 3.9 (2JPP = 31.6 Hz, JPP = 27.9) and 26.5
2
(2JPP = 32.4 Hz, JPP = 22.3), respectively. The characteristic
1
pseudo doublet of doublets in the hydride region of the H
NMR spectrum of 12 has now been replaced by a pseudo
doublet of triplets for 13 at δ −6.3 (2JHP = 77.5 Hz, 2JHP = 16.4
Hz).
The addition of 1.5 equiv of LA to a solution of 13 resulted
in LA coordination to ruthenium through two oxygen donors
via initial loss of hydride (as H2) and the bound MeCN,
forming [Ru(CO){N(CH2PPh21)3-κ3P}{CH3CO(CH2)2CO2H-
κ2O}](PF6) (14) (Scheme 4). H and 31P{1H} NMR spectra
were recorded hourly for the reaction between 13 and LA at
room temperature. The proton spectra showed the gradual
disappearance of the Ru−H signal (Figure 7), while the
phosphorus signals of 13 in the 31P{1H} NMR spectra also
disappear and are replaced by a new pseudo triplet and doublet
of 14 at δ −16.2 (2JPP = 26.4 Hz) and 19.8 (2JPP = 26.4 Hz),
respectively (Figure 8). The apparent chemical equivalence of
two phosphorus atoms arises from the similarity of the two
coordinating oxygen atoms of LA trans to these phosphorus
atoms, resulting in the observed apparent triplet and doublet
Figure 7. Stacked 1H NMR spectra showing the hydride region for the
conversion of 13 to 14.
instead of the expected three doublets of doublets signals.
Continued NMR analysis revealed that the reaction is complete
after 21 h.
The remarkable difference in reactivity between 12 and 13
with LA has implications for the use of these complexes as
hydrogenation catalysts: namely, that 12 will not be an active
catalyst for the hydrogenation of LA since the two components
do not interact, presumably due to the high stability of 12.
Consequently, a proton source is necessary for the
preactivation of this complex before LA can bind and
subsequently undergo catalytic hydrogenation.
Complex 7 was activated via oxidation using either AgOTf or
oxygen, resulting in the formation of [Ru(CO)2(OTf){N-
(CH2PPh2)3-κ3P}](OTf) (9) and [Ru(CO3)(CO){N-
(CH2PPh2)3-κ3P}] (11). Subsequent hydrogenation of these
complexes resulted in the facile formation of the ruthenium
hydride complexes [RuH(CO)2{N(CH2PPh2)3-κ3P}](OTf)
(10) and [RuH2(CO){N(CH2PPh2)3-κ3P}] (12). The reac-
tivity of 12 toward levulinic acid was investigated and found to
coordinate only after an additional activation step with the
proton donor NH4PF6 to give the solvent-bound [RuH(CO)-
(MeCN){N(CH2PPh2)3-κ3P}](PF6) (13). This intermediary
complex was found to react cleanly with levulinic acid via loss
of hydride (as H2) and acetonitrile. Levulinic acid then
coordinates in a bidentate fashion through two oxygen donors,
affording the cationic complex [Ru(CO){N(CH2PPh2)3-κ3P}-
{CH3CO(CH2)2CO2H-κ2O}](PF6) (14). Catalytic testing of
complexes 3−12 is currently underway.
CONCLUSIONS
■
The coordination chemistry of the novel ligand N(CH2PCyp2)3
(N-triphosCyp) was compared to that of the known N-
(CH2PPh2)3 (N-triphosPh) using three different ruthenium
precursors. In two instances the analogous facially capping
tridentate complexes were formed (complexes 3−6). Reaction
of ligands 1 and 2 with [Ru3(CO)12] resulted in a less bulky
phenyl derivative, forming the tridentate dicarbonyl species
[Ru(CO)2{N(CH2PPh2)3-κ3P}] (7), while the more bulky
cyclopentyl derivative exclusively gave the bidentate tricarbonyl
species [Ru(CO)3{N(CH2PCyp2)3-κ2P}] (8).
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Figure 8. Stacked 31P{1H} NMR spectra showing the conversion of 13 to 14.
13C{1H} NMR (CDCl3, 101 MHz): δ 26.0 (d, 2JCP = 9.6 Hz, CH2Cyp),
EXPERIMENTAL SECTION
■
27.6 (s, CH2Cyp), 27.8 (d, 1JCP = 40.8 Hz, CHCyp), 51.2 (d, 1JCP = 54.0
Hz, PCH2OH). 31P{1H} NMR (CDCl3, 162 MHz): δ 31.8 (s). HRMS
(ESI): m/z calcd for C12H24O2P ([M − Cl]+) 231.1513, found
231.1509. Anal. Calcd for C12H24O2PCl (found): C, 54.03 (54.16); H,
9.07 (8.94).
General Considerations. All preparations were carried out using
standard Schlenk line techniques under an inert atmosphere of N2
unless otherwise stated. Solvents were dried over standard drying
agents and stored over 3 Å molecular sieves. All starting materials were
of reagent grade purchased from either Sigma-Aldrich Chemical Co. or
VWR International and used without further purification. Ligand 1 was
prepared as previously reported.33,48 1H, 13C{1H}, and 31P{1H} NMR
spectra were recorded on Bruker AV-400, AV-500, and DRX-400
spectrometers at 294 K unless otherwise stated. Chemical shifts are
reported in ppm using the residual proton impurities in the solvents.
Pseudo triplets and pseudo doublets of triplets that occur as a result of
identical J value coupling to two or more chemically nonequivalent
nuclei are assigned as dd, dt, or ddd and are recognized by the
inclusion of only one or two J values. 13C NMR spectra were assigned
with the aid of DEPT-135, HSQC, and HMBC correlation
experiments. Mass spectrometry analyses were conducted by the
Mass Spectrometry Service, Imperial College London. Infrared spectra
were recorded on a Perkin-Elmer Spectrum 100 FT-IR spectrometer.
Elemental analyses were carried out by Mr. Stephen Boyer of the
Department of Health and Human Science, London Metropolitan
University. X-ray diffraction analysis was carried out by Dr. Andrew
White of the Department of Chemistry at Imperial College. Details of
the single-crystal X-ray diffraction analysis can be found in the
Supporting Information.
Dicyclopentyl(hydroxymethyl)phosphonium Chloride. In a
Schlenk flask were placed dicyclopentylphosphine (1.00 g, 5.87
mmol), degassed aqueous formaldehyde solution (35 wt %, 1.10 mL),
and degassed concentrated HCl (37 wt %, 0.53 mL). The solution was
stirred for 20 min at room temperature. Solvent was removed to give a
viscous gum that was subsequently crystallized from acetone after
being cooled to −5 °C to initiate crystallization. A white crystalline
solid was obtained that was filtered while cold, washed with diethyl
N,N,N-Tris((dicyclopentylphosphino)methyl)amine (N-tri-
phosCyp, 2). Method A. In a Schlenk flask were placed
dicyclopentylphosphine (1.00 g, 5.87 mmol) and methanol (5 mL).
Degassed aqueous formaldehyde solution (35 wt %, 0.60 mL) was
added, and the solution was stirred for 3 h at room temperature. A
methanolic solution of NH3 (2 M, 0.98 mL) was then added and the
solution brought to reflux for 4 h, forming an opaque emulsion. The
solution was placed in a freezer overnight, which resulted in the
formation of a white crystalline precipitate (651 mg, 1.15 mmol, 59%).
1H NMR (C6D6, 400 MHz): δ 1.42−2.09 (m, 54H, Cyp), 3.19 (s, 6H,
NCH2P). 13C{1H} NMR (C6D6, 101 MHz): δ 26.5 (d, 2JCP = 6.9 Hz,
CH2Cyp), 26.8 (d, JCP = 7.0 Hz, CH2C1 yp), 30.9 (d, JCP = 1.8 Hz,
CH2Cyp), 31.0 (br s, CH2Cyp), 36.1 (d, JCP = 12.5 Hz, CHCyp), 58.4
(m, NCH2P). 31P{1H} NMR (C6D6, 162 MHz): δ −18.5 (s). HRMS
(ES): m/z calcd for C33H60NP3O3 ([M + HO3]+) 612.3864, found
612.3871. Anal. Calcd for C33H60NP3 (found): C, 70.30 (70.25); H,
10.73 (10.67); N, 2.49 (2.54).
2
3
Method B. In a Schlenk flask were placed dicyclopentyl-
(hydroxymethyl)phosphonium chloride (626 mg, 2.35 mmol), NEt3
(0.76 mL), and methanol (10 mL), and the solution was stirred for 10
min at room temperature. Subsequent addition of NH3 (2 M, 0.39
mL) to the solution and stirring under reflux for 2 h resulted in an
opaque emulsion, which when left in the freezer overnight allowed
precipitation of the product as a white powder. The powder was
washed with methanol (3 × 2 mL) and dried in vacuo to afford
analytically pure 2 with characterization data identical with those
produced via method A (98.9 mg, 0.175 mmol, 45%).
[Ru(tmm){N(CH2PPh2)3-κ3P}] (3). To a solution of 1 (357 mg,
0.584 mmol) in toluene (5 mL) was added [Ru(COD)(methylallyl)2]
(187 mg, 0.585 mmol), and the solution was stirred under reflux for 90
h. Within minutes the colorless solution turned yellow and
1
ether (3 × 2 mL), and dried in vacuo (683 mg, 2.56 mmol, 44%). H
NMR (CDCl3, 400 MHz): δ 1.60−2.20 (m, 16H, CH2Cyp), 2.49−2.62
(m, 2H, CHCyp), 4.62 (s, 4H, PCH2OH), 5.21 (br s, 2H, OH).
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effervesence was observed. The product was observed to precipitate
during the course of the reaction as an off-white powder that was
highly insoluble in many solvents, hindering characterization and
prohibiting collection of a 13C{1H} NMR spectrum. The precipitate
was isolated via cannula filter, washed with toluene (3 × 3 mL), and
dried in vacuo (259 mg, 0.338 mmol, 58%). Crystals suitable for X-ray
diffraction analysis were grown from a dilute toluene solution at room
2P, Pcis), 25.6 (dt, 2JPPh = 212.4 Hz, 2JPP = 28.2 Hz, 1P, Ptrans), 61.9 (dt,
3
2JPPh P = 213.2 Hz, JPPh P = 22.9 Hz, 1P, PPh3). FT-IR (ν/cm−1):
2
3
3
hydride stretches 1881, 1850. HRMS (ES): m/z found 394.1654
(85%), 751.2789 (60%). Anal. Calcd for C51H77NP4Ru (found): C,
65.92 (65.84); H, 8.35 (8.40); N, 1.51 (1.60).
Synthesis of [Ru(CO)2{N(CH2PPh2)3-κ3P}] (7). This compound
was previously reported.48 It should be noted that the complex slowly
reacts with CDCl3 at room temperature. Crystals suitable for X-ray
diffraction analysis were grown from a concentrated toluene solution
left to stand at room temperature overnight.
1
temperature over 5 days. H NMR (C6D6, 400 MHz, 353 K): δ 2.10
(s, 6H, CH2tmm), 3.92 (s, 6H, NCH2P), 6.75−7.09 (m, 25H, Ph),
7.35−7.49 (m, 5H, Ph). 31P{1H} NMR (C6D6, 162 MHz, 353 K): δ
19.1 (s). HRMS (ES): m/z calcd for C43H43NP3102Ru ([M + H]+)
768.1652; found 768.1723. Anal. Calcd for C43H42NP3Ru (found): C,
67.35 (67.28); H, 5.52 (5.48); N, 1.83 (1.79).
[Ru(CO)3{N(CH2PCyp2)3-κ2P}] (8). To a solution of 2 (386 mg,
0.685 mmol) in toluene (15 mL) was added [Ru3(CO)12] (146 mg,
0.228 mmol), and the solution was stirred under reflux for 18 h,
initially turning very dark before becoming lighter red. Removal of the
solvent in vacuo and treatment of the resultant residue with methanol
(10 mL) caused precipitation of an orange powder, which was isolated,
washed with methanol (2 × 3 mL), and dried in vacuo. Dissolving the
solid in dichloromethane (5 mL) and layering this solution with
methanol (15 mL) and allowing it to stand at room temperature
overnight afforded bright orange crystals suitable for X-ray diffraction
experiments (241 mg, 0.322 mmol, 47%). 1H NMR (C6D6, 400
MHz): δ 1.32−1.92 (m, 50H, CHCyp and CH2Cyp), 2.02−2.14 (m, 4H,
CHCyp), 2.45 (s, 2H, NCH2P), 2.66 (s, 4H, NCH2P). 13C NMR
(C6D6, 101 MHz): δ 26.3 (m, CH2Cyp), 26.6 (t, JCP = 4.6 Hz, CH2Cyp),
26.7 (d, JCP = 6.5 Hz, CH2Cyp), 29.4 (br s, CH2Cyp), 30.7 (t, JCP = 13.2
Hz, CH2Cyp), 35.7 (d, 1JCP = 12.2 Hz, CHCyp-bound phosphorus), 40.7
(t, 1JCP = 12.0 Hz, CHCyp-unbound phosphorus), 58.2 (dt, 1JCP = 17.0
Hz, 3JCP = 8.7 Hz, NCH2P-unbound), 65.2 (q, 1JCP = 9.7 Hz, 3JCP = 7.6
Hz, NCH2P-bound), 215 (t, 2JCP = 8.7 Hz, CO). 31P NMR (C6D6, 162
MHz): δ −19.9 (s, 1P, unbound P), 29.0 (s, 2P, P−Ru). FT-IR (ν/
cm−1): carbonyl stretches 1867, 1902, 1978. HRMS (ES): m/z found
795.3019 (100%), 754.2841 (60%). Anal. Calcd for C36H60NP3O3
(found): C, 57.74 (57.81); H, 8.08 (7.98); N, 1.87 (1.95).
[Ru(tmm){N(CH2PCyp2)3-κ3P}] (4). To a solution of 2 (89.2 mg,
0.158 mmol) in toluene (5 mL) was added [Ru(COD)(methylallyl)2]
(50.4 mg, 0.158 mmol), and the solution was stirred under reflux for
16 h. Cooling the solution to room temperature overnight resulted in
the formation of orange crystals that were isolated via filtration,
washed with methanol (3 × 2 mL), and dried in vacuo (22.3 mg,
1
0.0310 mmol, 20%). H NMR (CDCl3, 400 MHz): δ 1.23 (s, 6H,
CH2tmm), 1.31−1.67 (m, 36H, CH2Cyp), 1.80−1.90 (m, 12H, CH2Cyp),
2.04−2.13 (m, 6H, CHCyp), 3.28 (s, 6H, NCH2P). 13C{1H} NMR
(CDCl3, 101 MHz): δ 25.7−26.0 (m, CH2Cyp), 29.6 (s, CH2Cyp), 31.0
(s, CH2Cyp), 35.6−36.1 (m, CH2tmm), 46.6−46.7 (m, CHCyp), 51.2−
51.6 (m, NCH2P), 102.8 (s, Ctmm). 31P{1H} NMR (CDCl3, 162
MHz): δ 14.2 (s). HRMS (ES): m/z calcd for C37H67NP3102Ru ([M +
H]+) 720.3530, found 720.3569. Anal. Calcd for C37H66NP3Ru
(found): C, 61.81 (61.76); H, 9.25 (9.30); N, 1.95 (1.89).
[Ru(H)2(PPh3){N(CH2PPh2)3-κ3P}] (5). To a solution of 1 (219 mg,
0.358 mmol) in toluene (10 mL) was added [RuH2(PPh3)4] (412 mg,
0.358 mmol), the Schlenk flask was wrapped in silver foil, and the
solution was stirred at room temperature for 1 h, before the
temperature was raised to 50 °C and stirred for a further 11 h. The
solvent was removed in vacuo and the yellow residue extracted with
diethyl ether (3 × 5 mL). Concentrating the ether solution and
allowing it to stand at room temperature overnight gave yellow crystals
[Ru(CO)2(OTf){N(CH2PPh2)3-κ3P}](OTf) (9). To a solution of 7
(124 mg, 0.162 mmol) in CH2Cl2 (5 mL) was added AgOTf (83.2 mg,
0.324 mmol), resulting in instantaneous precipitation of Ag(s). The
resulting suspension was stirred for 1 h at room temperature. A pale
yellow solution was filtered from the suspension via cannula, layered
with diethyl ether (5 mL), and cooled to −20 °C overnight. A yellow
microcrystalline powder was isolated, washed with diethyl ether (3 × 3
1
suitable for X-ray diffraction analysis (189 mg, 0.193 mmol, 54%). H
NMR (C6D6, 400 MHz): δ −7.73 (m, 2H, Ru−H), 3.93−4.10 (m, 6H,
NCH2P), 6.65−7.97 (m, 45H, Ph). 13C{1H} NMR (C6D6, 101 MHz):
δ 52.8 (m, NCH2P), 59.1 (m, NCH2P), 127.2−128.0 (m, CHPh),
128.3 (d, JCP = 6.6 Hz, CHPh), 132.2 (d, JCP = 10.7 Hz, CHPh), 133.1
1
mL), and dried in vacuo overnight (110 mg, 0.103 mmol, 64%). H
(t, JCP = 5.6 Hz, CHPh), 133.9 (t, JCP = 7.1 Hz, CHPh), 134.5 (d, JCP
=
NMR (CD2Cl2, 400 MHz): δ 4.57 (s, 2H, NCH2P), 4.84 (app q, 4H,
11.2 Hz, CHPh), 142.0 (t, JCP = 15.7 Hz, CPh), 143.2 (d, JCP = 38.8 Hz,
CPh), 144.6 (d, JCP = 34.4 Hz, CPh). 31P{1H} NMR (C6D6, 162 MHz):
JHP = 11.6 Hz, NCH2P), 7.04−7.73 (m, 30H, Ph). 13C{1H} NMR
1
3
(CD2Cl2, 101 MHz): δ 49.7 (td, JCP = 12.3 Hz, JCP = 5.7 Hz,
NCH2Pcis), 53.6 (dt, 1JCP = 17.4 Hz, 3JCP = 5.2 Hz, NCH2Ptrans), 119.1
2
2
2
δ 8.6 (dt, JPP = 25.9 Hz, 2P, Pcis), 27.3 (dt, J
= 210.8 Hz, JPP =
PPh3
27.8 Hz, 1P, Ptrans), 57.8 (dt, 2JPPh P = 210.0 Hz, 2JPPh P = 25.3 Hz, 1P,
1
4
1
(qd, JCF = 319.2 Hz, JCP = 2.38 Hz, bound CF3), 121.5 (q, JCF
=
=
3
3
PPh3). FT-IR (ν/cm−1): hydride stretches 1915, 1882. HRMS (ES):
m/z calcd for C57H52NP4102Ru ([M − H]+) 976.2094, found 976.2083.
Anal. Calcd for C57H53NP4Ru (found): C, 70.07 (69.92); H, 5.47
(5.40); N, 1.43 (1.47).
319.3 Hz, unbound CF3), 129.5 (t, JCP = 5.1 Hz, CPh), 129.9 (d, JCP
10.9 Hz, CPh), 130.4 (t, JCP = 5.1 Hz, CPh), 131.9 (m, CPh), 132.3 (d,
JCP = 9.5 Hz, CPh), 132.4 (s), 132.8 (d, JCP = 2.5 Hz, CPh), 133.2 (d,
JCP = 4.5 Hz, CPh), 189.6 (m, CO). 31P{1H} NMR (CD2Cl2, 162
MHz): δ −23.9 (d, 2P, 2JPP = 19.5 Hz, Pcis), 19.6 (t, 1P, 2JPP = 19.5 Hz
[Ru(H)2(PPh3){N(CH2PCyp2)3-κ3P}] (6). To a solution of 2 (108
mg, 0.192 mmol) in toluene (10 mL) was added [RuH2(PPh3)4] (221
mg, 0.192 mmol), and the solution was stirred at room temperature
for 2 h before being heated to 50 °C for 14 h. The solvent was
removed in vacuo, and the black-brown residue was washed with
acetone (4 × 5 mL), forming an off-white/yellow powder that was
dried in vacuo (61.5 mg, 66.2 μmol, 35%). 1H NMR (C6D6, 500
MHz): δ −9.72 (m, 2H, Ru−H), 0.96−2.66 (m, 54H, Cyp), 3.24 (m,
P
trans). 19F{1H} NMR (CD2Cl2, 376 MHz): δ −78.5 (br s, 3F,
unbound CF3), −76.8 (s, 3F, bound CF3). FT-IR (ν/cm−1): carbonyl
stretches 2055, 2095; trifluoromethyl C−F stretches 1158 (br), 1203
(br), 1228 (br), 1279 (br); others 998, 1028, 1436. HRMS (ES): m/z
calcd for C42H36NP3O5SF3102Ru ([M − OTf]+) 918.0523, found
918.0541. Anal. Calcd for C43H36NP3O8S2F6Ru (found): C, 48.41
(48.54); H, 3.40 (3.35); N, 1.31 (1.29).
4H, NCH2P), 3.32 (br s, 2H, NCH2P), 7.00 (dt, 3JHH = 7.4 Hz, 5JHP
=
[RuH(CO)2{N(CH2PPh2)3-κ3P}](OTf) (10). A solution of 9 (65.0
mg, 60.9 μmol) in THF (5 mL) was placed under 3 bar of H2 in a
sealed ampule, heated to 70 °C, and stirred vigorously for 17 h,
resulting in a color change from yellow to colorless. The solvent was
removed in vacuo, and the resultant residue was washed with diethyl
ether (4 × 5 mL), forming an off-white powder that was dried in vacuo
overnight (39.8 mg, 0.0433 mmol, 71%). 1H NMR (THF-d8, 500
MHz): δ −6.75 (dt, 1H, 2JHPcis = 14.5 Hz, 2JHPtrans = 60.6 Hz, Ru−H),
1.2 Hz, 3H, CHpara‑Ph), 7.10−7.13 (dt, JHH = 7.9 Hz, JPH = 1.5 Hz,
6H, CHortho‑Ph), 8.25−8.29 (m, 6H, CHmeta‑Ph). 13C{1H} NMR (C6D6,
126 MHz): δ 25.0 (s, CH2Cyp), 26.1 (t, JCP = 5.0 Hz, CH2Cyp), 26.2 (d,
JCP = 9.1 Hz, CH2Cyp), 26.8 (s, CH2Cyp), 26.9 (t, JCP = 3.1 Hz, CH2Cyp),
27.5 (d, JCP = 6.4 Hz, CH2Cyp), 28.7 (s, CH2Cyp), 29.4 (s, CH2Cyp), 29.6
(s, CH2Cyp), 29.9 (s, CH2Cyp), 30.7 (s, CH2Cyp), 31.0 (s, CH2Cyp),
51.0−51.1 (m, NCH2P), 45.9 (d, 1JCP = 21.7 Hz, CHCyp), 47.3 (t, 1JCP
3
3
3
2
= 8.7 Hz, CHCyp), 127.0 (d, JCP = 8.1 Hz, CHmeta‑Ph), 127.5 (s,
4.45 (dd, 4H, JHP = 15.4 Hz, J = 71.6 Hz, NCH2P), 4.89 (s, 2H,
2
1
CHpara‑Ph), 134.7 (d, JCP = 11.4 Hz, CHortho‑Ph) 147.1 (d, JCP = 29.6
NCH2P), 6.92−7.66 (m, 30H, Ph). 13C{1H} NMR (THF-d8, 101
MHz): δ 51.2−51.6 (m, NCH2P), 122.2 (q, JCF = 320.4 Hz, CF3),
Hz CPh). 31P{1H} NMR (C6D6, 202 MHz): δ −5.3 (dt, 2JPP = 25.3 Hz,
3750
dx.doi.org/10.1021/ic500030k | Inorg. Chem. 2014, 53, 3742−3752

Inorganic Chemistry
Article
129.6 (t, JCP = 4.8 Hz, CHPh), 130.2 (d, JCP = 10.1 Hz, CHPh), 131.1
was washed through with additional acetonitrile (2 × 2 mL). The
solution was stirred at room temperature for 2 h, before the solvent
was removed in vacuo, affording the intermediate [RuH(CO)-
(MeCN)N(CH2PPh2)3-κ3P}] (13) as a brown powder that was
washed with hexane (3 × 3 mL) and dried in vacuo for 30 min.
Intermediate 13 was dissolved in degassed acetone-d61(0.5 mL), and
(d, JCP = 12.7 Hz, CHPh), 132.5 (t, JCP = 5.0 Hz, CHPh), 132.8 (d, JCP
=
11.3 Hz, CHPh), 133.9 (t, JCP = 5.0 Hz, CHPh), 134.8 (d, JCP = 21.7 Hz,
CPh), 136.8 (d, JCP = 36.6 Hz, CPh), 197.3−198.3 (m, CO). 31P{1H}
2
NMR (THF-d8, 162 MHz): δ −16.8 (t, 1P, JPP = 25.5 Hz, Ptrans),
2
−2.01 (d, 2P, JPP = 25.5 Hz, Pcis). 19F NMR (THF-d8, 470 MHz): δ
1
initial H and 31P{1H} NMR spectra were recorded. H NMR (400
−79.2 (s). FT-IR (ν/cm−1): carbonyl stretches 2004, 2051;
trifluoromethyl C−F stretches 1025 (br), 1090 (br), 1156 (br),
1224 (br), 1259 (br); others 1435, 1485. HRMS (ES): m/z calcd for
C41H37NP3O2S102Ru ([M − OTf]+) 770.1081, found 770.1094. Anal.
Calcd for C43H36NP3O8S2F6Ru (found): C, 54.90 (54.81); H, 4.06
(4.12); N, 1.52 (1.62).
MHz, acetone-d6): δ −6.30 (ddd, 1H, 2JHP = 77.5 Hz, 2JHP = 16.4 Hz,
Ru−H). 31P{1H} NMR (162 MHz, acetone-d6): δ −144.2 (septet, 1P,
−
1JPF = 708.4 Hz, PF6 ), −12.4 (m), 3.9 (dd, 1P, 2JPP = 31.6 Hz, 2JPP
=
2
2
27.5 Hz), 26.5 (dd, 1P, JPP = 32.4 Hz, JPP = 22.3 Hz). In the NMR
tube was placed a solution of levulinic acid (10.8 mg, 93.0 μmol, 1.43
equiv) in degassed acetone-d6 (0.5 mL), and the solution was stirred
[Ru(CO3)(CO){N(CH2PPh2)3-κ3P}] (11). Method A. A partially
dissolved suspension of 7 (280 mg, 0.364 mmol) in toluene (5 mL)
was bubbled with O2 for 10−15 min. The orange precipitate that
formed was collected by filtration, washed with toluene (2 × 3 mL)
and diethyl ether (2 × 3 mL), and dried in vacuo (204 mg, 0.255
mmol, 70%). Crystals suitable for X-ray diffraction analysis were grown
by layering a concentrated dichloromethane solution of 11 with
toluene. 1H NMR (CDCl3, 400 MHz): δ 4.05−4.17 (m, 6H, NCH2P),
6.92−7.72 (m, 30H, Ph). 13C{1H} NMR (CDCl3, 101 MHz): δ 50.1
for 2 min using a vortex stirrer. H and 31P{1H} NMR spectra were
1
recorded every 1 h for 16 h, with the first being taken approximately
10 min after stirring. The reaction was complete within 21 h. 1H NMR
(400 MHz, acetone-d6): δ 2.23 (s, 3H, CH3LA), 2.67 (t, 2H, 3JHH = 5.8
Hz, CH2LA), 2.84 (t, 2H, JHH = 5.9 Hz, CH2LA), 4.55−4.68 (m, 6H,
3
NCH2P), 6.95−7.68 (m, 30H, Ph). 13C{1H} NMR (101 MHz,
acetone-d6): δ 28.1 (s, CH3LA), 37.5 (CH2LA), 38.2 (s, CH2LA), 50.7
1
3
1
(td, JCP = 13.2 Hz, JCP = 5.4 Hz, NCH2P), 51.7 (dt, JCP = 19.1 Hz,
1
3
1
(dt, JCP = 18.5 Hz, JCP = 6.2 Hz, NCH2Ptrans), 51.1 (td, JCP = 12.5
3JCP = 5.4 Hz, NCH2P), 129.1 (d, JCP = 9.6 Hz, CHPh), 131.0 (d, JCP
=
3
Hz, JCP = 3.2 Hz, NCH2Pcis), 128.2 (d, JCP = 6.8 Hz, CHPh), 128.5−
2.6, CHPh), 131.6 (s, CHPh), 131.9 (s, CHPh), 132.0 (t, JCP = 5.1 Hz,
CHPh), 133.2 (t, JCP = 4.6 Hz, CHPh), 133.7 (d, JCP = 9.5 Hz, CHPh),
174.0 (s, COLA), 192.0 (s, COLA), 206.7 (d, JCP = 3.0 Hz, CO).
128.7 (m, CHPh), 129.4 (br s, CHPh), 129.8 (s, CHPh), 130.1 (s,
CHPh), 130.9 (t, JCP = 5.0 Hz, CHPh), 132.5 (d, JCP = 9.3 Hz, CHPh),
132.8 (t, JCP = 4.9 Hz, CHPh), 133.3−134.0 (m, CPh), 136.2−136.9 (m,
CPh), 167.8 (s, CO3), 194.0 (dt, 2JCPtrans = 106.4 Hz, 2JCPcis = 11.0 Hz,
31P{1H} NMR (162 MHz, acetone-d6): δ −16.2 (dd, JPP = 26.4 Hz,
2
2
P
trans), 19.8 (dd, JPP = 26.4 Hz, Pcis). MS (ES): m/z calcd for
CO). 31P{1H} NMR (CDCl3, 162 MHz): δ −23.5 (t, JPP = 33.8 Hz,
2
C45H43NP3O4102Ru ([M − PF6]+) 856.1, found 856.4.
Pcis), 15.9 (d, 2JPP = 33.3 Hz, Ptrans). FT-IR (ν/cm−1): carbonyl stretch
1994; κ2-carbonate stretches 1565, 1434; κ2-carbonate bends 1272,
1092; others 1485. HRMS (ES): m/z calcd for C41H37NP3O4102Ru
([M + H]+) 802.0979, found 802.1005. Anal. Calcd for
C41H36NP3O4Ru (found): C, 61.50 (61.27); H, 4.53 (4.34); N, 1.75
(1.84).
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving 31P{1H} NMR spectra of
ligand 2, complexes 3−12, and intermediate 13, a comparison
of the hydride region of 1H and 1H{31P} NMR spectra of 5 and
6, and additional crystallographic data, including 50%
probability ellipsoid ORTEP diagrams of all independent
crystal structures. This material is available free of charge via
the Internet at http://pubs.acs.org.
Method B. A sample of 7 (63.4 mg, 0.0825 mmol) was deliberately
exposed to air for 5 weeks. Washing with wet toluene (5 × 1 mL) and
wet diethyl ether (5 × 1 mL) and drying in air afforded a yellow-brown
powder with characterization data identical with those produced via
method A (48.0 mg, 59.9 μmol, 73%).
[Ru(H)2(CO){N(CH2PPh2)3-κ3P}] (12). A solution of 11 (763 mg,
0.953 mmol) in THF (25 mL) was injected into a high-pressure
reactor under a flow of nitrogen. The atmosphere was changed to
hydrogen and pressurized to 15 bar at room temperature before the
mixture was heated to 100 °C (increasing the internal pressure to ca.
20 bar) and stirred for 2 h. After the mixture was cooled to room
temperature, the gas was carefully vented and the atmosphere changed
to nitrogen. The solution was transferred from the reactor to a Schlenk
flask, filtered, and diluted with methanol (20 mL), and the solvent was
removed in vacuo, giving an orange powder that was washed with
methanol (3 × 5 mL) and diethyl ether (3 × 5 mL) and dried in vacuo
(365 mg, 0.492 mmol, 52%). Crystals suitable for X-ray diffraction
analysis were grown from a concentrated toluene solution layered with
AUTHOR INFORMATION
Corresponding Authors
*E-mail for N.J.L.: n.long@imperial.ac.uk.
*E-mail for P.W.M.: philip.miller@imperial.ac.uk.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. N.J.B. assissted in setting up the high-pressure
reactor and initial hydrogenation reactions. A.J.P.W. was
primarily responsible for the X-ray crystallographic analysis.
1
2
methanol. H NMR (C6D6, 400 MHz): δ −6.50 (dd, 2H, JHPtrans
=
Notes
49.3 Hz, 2JHPcis = 18.8 Hz, Ru−H), 3.66 (s, 2H, NCH2P), 3.82 (m, 4H,
NCH2P), three multiplets 6.69−6.87, 7.27−7.34 and 7.51−7.61 (m,
30H, Ph). 13C{1H} NMR (C6D6, 101 MHz): δ 51.8 (m, NCH2P), 53.5
(m, NCH2P), 127.7−128.3 (m, CHPh), 128.5 (d, JCP = 11.2 Hz,
CHPh), 131.8 (t, JCP = 6.2 Hz, CHPh), 132.5 (d, JCP = 12.0 Hz, CHPh),
132.9 (t, JCP = 6.9 Hz, CHPh), 138.6 (dt, JCP = 38.8 Hz, JCP = 3.5 Hz,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Imperial College London for funding, via the
Frankland Chair endowment. Johnson Matthey plc are also
thanked for the loan of the precious-metal salts used in this
work.
CPh), 139.6 (m, CPh) 141.0 (m, CPh), 209.2 (dt, JCPtrans = 73.7 Hz,
2
2JCPcis = 8.3, CO). 31P{1H} NMR (C6D6, 162 MHz): δ 8.5 (d, JPP
=
2
35.7 Hz, 2P), 18.8 (t, JPP = 34.8 Hz, 1P). FT-IR (ν/cm−1): Ru−CO
and Ru−H stretches 1858, 1926, 2190. MS (ES) m/z calcd for
C40H38NP3O102RuK ([M + K]+) 782.1, found 783.1. Anal. Calcd for
C40H38NP3ORu (found): C, 64.68 (64.50); H, 5.16 (4.97); N, 1.89
(1.73).
2
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