Synthesis and coordination chemistry of new asymmetric donor/acceptor pincer ligands, 1,3-C6H4(CH2PtBu(Rf))2 (Rf = CF3, C2F5)

Article abstract of DOI:10.1039/c8dt02738b

Syntheses of new asymmetric pincer precursors 1,3-C₆H₄{CH₂P(^tBu,X)}₂ (^tBu,XPCPH; X = Cl, SiMe₃, OPh) and a new class of hybrid donor/acceptor pincer ligands 1,3-C₆H₄{CH₂P(^tBu,R_f)}₂ (^tBu,RfPCPH; R_f = CF₃, C₂F₅) are reported. All ^tBu,XPCPH compounds are obtained as mixtures of meso and rac diastereomers in varying ratios (meso?:?rac ～ 4?:?1 to 3?:?2) which were used without separation. Treatment of Ru(cot)(cod) with ^tBu,CF₃PCPH under 1 atm H₂ in acetone at 20 °C produced the hydride solvate (^tBu,CF₃PCP)Ru(acetone)_xH which was not isolated, but could be trapped as stable diene complexes (^tBu,CF₃PCP)Ru(L)₂H (L₂ = cod (1), nbd (2)). Catalytic cyclooctane dehydrogenation studies demonstrate that 2 has ～50% the activity of (^CF₃PCP)Ru(cod)(H), but significantly higher catalyst stability and is able to operate at higher catalyst loading concentrations without deactivation via bimolecular decomposition.

Full text of DOI:10.1039/c8dt02738b

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Synthesis and Coordination Chemistry of New Asymmetric Donor/Acceptor
Pincer Ligands, 1,3-C6H4(CH2PtBu(Rf))2 (Rf = CF3, C2F5)
Contribution from the Department of Chemistry, Box 3838
University of Wyoming, Laramie, Wyoming 82071
Suman Debnath, Gabriel Venegas, Navamoney Arulsamy, and Dean M. Roddick*
1

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Abstract
Syntheses of new asymmetric pincer precursors 1,3ꢀC6H4{CH2P(tBu,X)}2 (tBu,XPCPH; X = Cl,
SiMe3, OPh) and a new class of hybrid donor/acceptor pincer ligands 1,3ꢀC6H4{CH2Pꢀ(tBu,Rf)}2
(
tBu,RfPCPH; Rf = CF3, C2F5) are reported. All tBu,XPCPH compounds are obtained as mixtures of
meso and rac diastereomers in varying ratios (meso:rac ~ 4:1 to 3:2) which were used without
separation. Treatment of Ru(cot)(cod) with tBu,CF3PCPH under 1 atm H2 in acetone at 20 °C
produced the hydride solvate (tBu,CF3PCP)Ru(acetone)xH which was not isolated, but could be
trapped as stable diene complexes (tBu,CF3PCP)Ru(L)2H (L2 = cod (1), nbd (2)). Catalytic
cyclooctane dehydrogenation studies demonstrate that 2 has ~50% the activity of
(
CF3PCP)Ru(cod)(H), but significantly higher catalyst stability and is able to operate at higher
catalyst loading concentrations without deactivation via bimolecular decomposition.
†Submitted in honor of Richard A. Andersen, on the occasion of his 75th birthday
2

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Introduction.
Since the original 1976 report by Moulton and Shaw,1 the chemistry of pincerꢀligated
transitionꢀmetal complexes has experienced substantial development and expanded applications.2
This class of ligands is generally regarded as promoting a stable meridional tridentate
coordination environment with exceptional thermal stability.3 The “PCP” subclass of ligands,
1,3ꢀC6H4(CH2PR2)2 (unmetallated) and 2,6ꢀC6H3(CH2PR2)2 (metallated; generally abbreviated as
RPCP ligands) gained prominence with Jensen’s 1996 report of thermally stable alkane
dehydrogenation catalysis using [tBuPCP]IrHx precatalysts.4 This initial work was subsequently
expanded upon by the Goldman and Brookhart research groups, which largely employed both
[RPCP]IrHx and [RPOCOP]IrHx precursors with stericallyꢀdemanding donor phosphine pendant
alkyl groups R = tBu or iPr.5,6 Some significant limitations of alkylphosphineꢀbased pincer alkane
dehydrogenation systems are catalyst sensitivity to nitrogen and air,l as well as strong product
alkene inhibition.4,7,8
In 2007 our research group introduced perfluoroalkylꢀsubstituted PCP pincer ligands9 and
subsequently demonstrated their applicability to alkane dehydrogenation chemistry. Group 8
dehydrogenation catalysts employing the CF3ꢀsubstituted pincer, (CF3PCP)M(cod)H (M = Ru,
Os),10 along with the corresponding iridium catalyst (CF3PCP)Ir(cod)11 are all stable to air and
moisture and relatively insensitive to alkene product inhibition. While the initial cyclooctane
(coa) dehydrogenation activity of (CF3PCP)Ir(cod) was modest (40 TO hrꢀ1) in 1:1 coa/tbe (tbe =
tbutylethylene) mixtures at 200 °C, high total TON’s (~3000) and complete conversion of coa to
cyclooctene (coe) was observed in more dilute 5:1 coa/tbe solutions. (CF3PCP)Ir(cod) is moreover
quite insensitive to product inhibition: in the presence of 800 equiv. coe, the initial
CF
dehydrogenation activity is only reduced by 33%. Group 8 3PCPꢀbased catalysts are much
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more reactive, yet significantly less stable: thermolysis of (CF3PCP)Ru(cod)H in 1:1 coa/tbe at
200 °C gave an initial activity of 1,000 turnovers hrꢀ1 for the formation of coe. The hydrogen
transfer activity of the corresponding osmium catalyst was slightly less (75%), but activity was
maintained ~6 times longer.
A decrease in catalyst activity over time for (CF3PCP)Ru(cod)H is due to thermal catalyst
decomposition as well as the reversible formation of an unusual [ꢀꢀη1,η6,κ3ꢀ(CF3PCP)]ꢀbridged
ruthenium dimer under higher catalyst loadings;10a a similar bimetallic product, the unusual arylꢀ
bridged [(ꢁꢀ1κ2(P,C),2κ2(P′,C)ꢀCF3PCP)Ir(H)2]2(ꢁꢀCF3PCPH)(ꢁꢀH) complex, was also observed in
(
CF3PCP)Ir(cod) chemistry (Figure 1).11 The tendency of (CF3PCP)M systems to participate in
CF
bimolecular chemistry reflects the tendency of the less stericallyꢀencumbered 3PCP ligand to
form nonꢀmeridional pincer complexes.12,13
H
P
H
(F3C)2P
Ir
Ir
(CF3)2
P
P
H
Ru
H
H
(F3C)2P
-H2
-H2
H
H
P
Ru
P(CF3)2
P
H
P(CF3)2
Ru
P(CF3)2
P(CF3)2
Ir
H
P(CF3)2
Ru
-cod
P(CF3)2
Ir
P(CF3)2
-cod
H
P(CF3)2
P(CF3)2
Figure 1. Reversible formation of catalytically inactive dimers from (CF3PCP)Ru(cod)H and
(
CF3PCP)Ir(cod) pincer systems
The role of pincer sterics for donor PCP pincer cyclooctane dehydrogenation systems has
been investigated by Goldman et al.14 Replacing one of the four tBu groups on (tBuPCP)IrH4 with
a methyl group to form (tBu MePCP)IrH4 resulted in a modest increase (~5 fold) in catalytic
3
activity; calculations indicated that this increase resulted from lowered steric congestion for
4

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R
t
4
rateꢀlimiting βꢀH elimination from ( PCP)Ir(nꢀalkyl)(H). Replacement of a second Bu group
on the opposite pincer phosphine arm had a negative effect on catalytic rates: (tBu Me PCP)IrH4
was less active and decomposed to a bimetallic product with a bridging metallated PꢀMe group.
It is evident that steric tuning of pincer catalyst activity requires a delicate balance between
catalyst metal site protection, substrate/product binding, and bimolecular decomposition
pathways.
2
2
Noting the above considerations, we have developed new hybrid perfluoroalkyl/tbutyl
t
functionalized PCP ligands Bu,RfPCPH (Rf = CF3, C2F5) which might combine the desirable
CF
N2/air/moisture stability and lower alkene inhibition properties of electronꢀpoor 3PCP systems
t
t
with the steric protection and stability of established BuPCPꢀbased iridium catalysts. Bu,RfPCP
pincers with two asymmetric phosphine centers lead to diastereomeric metal coordination with
either a bisecting mirror plane (“meso”) or a C2 axis along the metalꢀaryl bond (“rac”):
Figure 2. Possible stereoisomers for pincer ligands with P(tBu)(Rf) substitution. The solid and
dashed lines to the metal center depict alternate conformations of the central aryl group and
benzylic CH2 arms.
These stereoisomers should have very similar net phosphine donor properties but distinctly
different pincer “quadrant” steric properties. In particular, the meso stereoisomer has an
t
intrinsically unfavorable syn BuꢀtBu interaction that hinders bending toward this side of the
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t
pincer plane while the rac stereoisomer, with less hindered syn BuꢀRf interactions, allows for
nonꢀmeridional pincer bending. Differing “quadrant” sterics can potentially alter the chemistry
and corresponding catalytic chemistry of meso and rac stereoisomers. The results presented in
this paper sought to test this hypothesis.
Results and Discussion
There are relatively few reports of asymmetrical PCP pincer compounds in the literature.
Introduction of chirality in pincer ligands was first achieved by installation of chiral substituents
at the benzylic positions.15,16 PCP pincer ligands with cyclic17 and acyclic18,19 stereogenic Pꢀ
centers were subsequently developed and applied to enantioselective catalysis. Of particular note
is Jensen’s report of the mixed phenyl/tꢀbutyl ligand R,Rꢀ{C6H4ꢀ2,6ꢀ(CH2P*PhBut )2}, which
was shown to be a transfer dehydrogenation catalyst with turnover numbers similar to those
observed for (tBuPCP)IrH2.18b
Synthesis of 1,3-C6H4{CH2P(tBu,Rf)}2 (tBu,RfPCPH) (Rf = CF3, C2F5). We initially examined
asymmetric silylated pincers tBu,TMSPCPH as precursors to the desired tBu,RfPCPH systems, since
PꢀSiMe3 groups may be readily converted to PꢀH and PꢀCl precursors, or directly to PꢀRf groups.
Racemic PtBu(TMS)H was prepared by the reaction of 1 equiv. of methanol with PtBu(TMS)2 in
t
THF; a small amount (ca 10%) of BuPH2 side product was also formed. Reaction of
PtBu(TMS)H with α,α′ꢀdibromoꢀmꢀxylene gave a mixture of products. The major product (ca.
30ꢀ35%) was assigned as tBu,TMSPCPH. The sensitivity of the product PꢀTMS bonds toward the
released HBr side product likely accounts for the poor selectivity and low observed yield. Use of
the corresponding phosphide proved more effective: deprotonation of PtBu(TMS)H with nꢀBuLi
in THF at −30 °C followed by warming to ambient temperature and addition of α,α′ꢀdibromoꢀmꢀ
xylene afforded the desired asymmetric pincer tBu,TMSPCPH (Equation 1). A single 31P
6

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resonance for tBu,TMSPCPH appears as a broad singlet at −41.7 ppm; however, a ~2:1 mixture of
1
mesoꢀ and racꢀdiastereomers is evident in H NMR spectra, which show clearly separated tꢀBu
and TMS resonances associated with each diastereomer. The assignment of the major
stereoisomer as meso is supported by XꢀRay diffraction data for the related borane adduct
tBu,ClPCPH(BH3)2 (see Supplementary Material).
Brisdon has reported perfluoroalkylꢀcontaining phosphines are obtained by treatment of
the trimethylsilylꢀsubstituted phosphines with perfluoroalkyl iodides (RfI).20 Accordingly, NMRꢀ
scale reactions with tBu,TMSPCPH and RfI (Rf = CF3, C2F5, iꢀC3F7) were examined (Scheme 1).
Treatment of tBu,TMSPCPH with 4 equiv. CF3I resulted in clean conversion (~95%) to a single
31
19
product, as indicated by the appearance of a single P NMR resonance at 61.0 ppm. F spectra
exhibit two doublets of doublets at −32.6 (2JFF = 186 Hz, 2JPF = 60 Hz) and −35.7 ppm (2JFF
=
2
186 Hz, JPF = 71 Hz), characteristic of a ABX splitting pattern for diastereotopic YꢀCF2ꢀX
1
fluorines, along with characteristic H and 19F resonances due to trimethylsilyl fluoride. An
additional set of ABX resonances were observed for the minor rac stereoisomer (meso:rac ~
3:1). Based on the 19F data and the unexpected formation of Me3SiF rather than Me3SiI, we
tentatively assign this complex as the unusual P(CF2I)ꢀsubstituted compound tBu,CF2IPCPH. In
contrast, treatment of tBu,TMSPCPH with iꢀC3F7I in benzeneꢀd6 at ambient temperature produced
i
tBu, C F7PCPH as the major product (ca 65%) after 2 hours with the accompanying release of
3
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19
trimethylsilyl iodide. In addition to a single broad 31P NMR resonance at 41.5 ppm, F NMR
i
spectra of tBu, C F7PCPH show a perfluoroisopropyl CF3 doublet resonance −72.9 ppm with an
accompanying doublet of septet CꢀF resonance appearing at −179.8 ppm. The benzylic CH2
groups appear as a complex multiplet at δ 3.26 in the 1H NMR spectrum. The observation of two
resolved tertꢀbutyl doublets at δ 1.08 and 1.05 in 3:2 ratio confirm the presence of meso and rac
stereoisomers. In contrast to the reactions of tBu,TMSPCPH with CF3I and (CF3)2CFI, the reaction
of tBu,TMSPCPH with C2F5I failed to produce a major identifiable product. The formation of a Pꢀ
CF2I substituted pincer rather than the expected PꢀCF3 product indicates surprisingly different
regioselectivities for the reactions of CF3I and iꢀC3F7I with tBu,TMSPCPH. Both steric and
electronic factors can influence fourꢀcenter transition states involved in CꢀF versus CꢀI addition
across PꢀTMS bonds, and the nonꢀspecific nature of C2F5I addition appears to reflect a crossover
in the underlying bond metathesis preferences.
3
The chlorophosphine derivative, 1,3ꢀC6H4{CH2P(tBu,Cl)}2 (tBu,ClPCPH), serves as a
versatile precursor to asymmetric tBu,RPCPH pincer ligands (Scheme 2). Treatment of
tBu,TMSPCPH with hexachloroethane afforded tBu,ClPCPH in high yield. An alternate, more
8

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efficient tBu,ClPCPH synthesis was also developed based on Hayes’ procedure:21 Reaction of α,α′ꢀ
t
dichloroꢀmꢀxylene with 3 equiv. of magnesium followed by 2 equiv. of BuPCl2 directly
produced tBu,ClPCPH in moderate yield.
The perfluoroalkylꢀsubstituted ligand tBu,C F5PCPH was prepared from tBu,ClPCPH in a
2
manner analogous to other perfluoroethylꢀsubstituted phosphines in our laboratory (Scheme 3):22
addition of tBu,ClPCPH to in situ prepared C2F5Li at −80 °C gave tBu,C F5PCPH as a viscous oil.
2
The CF3ꢀsubstituted pincer derivative was obtained following Caffyn’s procedure:23 Addition of
Me3SiCF3 to an ether suspension of tBu,OPhPCPH (prepared by metathesis of tBu,ClPCPH with
phenol) and excess CsF produced a dark brown solution. Distillation under reduced pressure
afforded 1,3ꢀC6H4(CH2P{tBu,CF3})2 (tBu,CF3PCPH) as a viscous brown oil.
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Direct monotrifluoromethylation of secondary and primary arylꢀ and alkylphosphines
using hypervalent iodine (III)ꢀCF3 reagents was reported by Togni in 2008.24 The required
secondary phosphine tBu,HPCPH was readily prepared from tBu,TMSPCPH by treatment with
methanol. Reaction of tBu,HPCPH with either stoichiometric or excess 1ꢀtrifluoromethylꢀ1,2ꢀ
benziodoxolꢀ3(1H)ꢀone (Togni Reagent II), however, led to the formation of only ~20% of
expected tBu,CF3PCPH ligand along with other uncharacterized products. Alternatively, reaction of
Togni reagent II directly with the TMS pincer precursor tBu,TMSPCPH at room temperature for 4
hours resulted in 50ꢀ55% formation of tBu,CF3PCPH, however attempts to isolate pure tBu,CF3PCPH
ligand from this reaction mixture were unsuccessful.
NMR data for all compounds obtained in Scheme 3 indicated ca. 2:1 mixtures of meso
and rac diastereomers. Attempts to separate diastereomers by fractional distillation or
recrystallization were unsuccessful. Reaction of tBu,ClPCPH with two equiv. of BH3·THF resulted
in the clean formation of the diborane adduct in moderate yield (Equation 2). Xꢀray diffraction
10

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data for [tBu,ClPCPH](BH3)2 was obtained for the preferentially crystallized the meso isomer (see
ESI).
Imamoto has reported an efficient method for phosphineꢀborane deprotection with an excess of
various types of amines to form the free phosphine ligand;25 however, our attempts to isolate
pure mesoꢀtBu,ClPCPH following this approach were unsuccessful. Accordingly, distereomeric
(
tBu,RfPCPH) mixtures were used for all our metallation studies.
Metallation Studies with tBu,CF3PCPH and tBu,C F5PCPH. Since Ru(cod)(η3ꢀ2ꢀmethylallyl)2 has
been employed as a ruthenium pincer complex precursor in previous studies,10a,26 metallation
experiments were performed using similar conditions to those employed for the reactions of
CF3PCPH with Ru(cod)(η3ꢀ2ꢀmethylallyl)2. Thermolysis of 1:1 mixtures of tBu,CF3PCPH and
Ru(cod)(η3ꢀ2ꢀmethylallyl)2 in toluene at 130 °C in the presence of 3ꢀ10 equiv. of 1,5ꢀ
cyclooctadiene under an H2 atmosphere gave only 10% conversion to the expected metallated
2
product after 1 hr, and tBu,C F5PCPH was found to be completely unreactive with Ru(cod)(η3ꢀ2ꢀ
2
methylallyl)2 under analogous metallation conditions.
In view of these limited results, we next examined Ru(0) complex (ƞ4ꢀ1,5ꢀ
cyclooctadiene)(ƞ6ꢀ1,3,5ꢀcyclooctatriene)ruthenium(0) (Ru(cod)(cot)) as a potential metallation
precursor. Forcing conditions used in previous metallation studies were first examined: Warming
equimolar mixtures of Ru(cod)(cot) and tBu,CF3PCPH in benzene to 100 °C under 3 atm of H2 gas
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in the presence of 3 equiv. of cod gave an uncharacterized mixture of products. Repeating this
metallation reaction at room temperature for 24 hrs, however, gave a major product with a
31
hydride triplet at δ −14.42 as well as a 106.8 ppm P resonance that was previously observed in
Ru(cod)(η3ꢀ2ꢀmethylallyl)2 metallation attempts. Integration of unreacted tBu,CF3PCPH versus the
product peak showed that product conversion was ~50%. No further spectroscopic changes were
observed after 24 hours. After warming to 100 °C for 2 hours, the 106.8 ppm product had
disappeared and no pincer product was observed; from these observations we conclude that the
asymmetric pincer product (tBu,CF3PCP)Ru(cod)H is much less thermally stable under metallation
conditions relative to (CF3PCP)Ru(cod)(H).
We have optimized conditions to afford
tBu,CF3PCP)Ru(L)2H derivatives more selectively and in higher yield (Scheme 4). A 1.1:1
(
tBu,CF3PCP)Ru(cod)H and related
(
mixture of tBu,CF3PCPH and Ru(cod)(cot) stirred in acetone for 20 hours under 1 atm hydrogen
31
resulted in P product resonances at 99.4 and 93.9 ppm in a 2:1 ratio. A single hydride triplet
was observed at δ −19.57 which was significantly downfield from the hydride triplet reported for
(
CF3PCP)Ru(cod)H (−11.94 ppm). This suggests the initial formation of a solvated pincer hydride
adduct (tBu,CF3PCP)Ru(acetone)xH. This solvated hydride complex has limited stability: removal
of the solvent before the addition of a trapping diene ligand resulted in decomposition and 31P
NMR spectra showed significant amounts of free tBu,CF3PCPH. Subsequent reaction of in situꢀ
formed (tBu,CF3PCP)Ru(acetone)2H solutions with 1,5ꢀcyclooctadiene and norbornadiene resulted
in the formation of stable (tBu,CF3PCP)Ru(diene)H products: addition of cod (10 equiv.) gave
(
tBu,CF3PCP)Ru(cod)H, a brown oil, as the major product (~80%), which could be only partially
purified by extraction and cold precipitation from petroleum ether. Two triplet hydride
resonances appear at −14.96 (2JHP = 24.5 Hz) and −14.74 ppm (2JHP = 24.3 Hz) in a 55:45 ratio
12

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which are assigned to meso and racꢀ(tBu,CF3PCP)Ru(cod)H (1) diastereomers. In all obtained
tBu,CF3PCP)Ru(L)H products we have observed additional unresolved hydride fine structure for
(
the downfield rac resonance due to pincer asymmetry and the chemical inequivalence of the
pincer P centers. The cod adduct is moderately stable: no decomposition is observed after
repeated addition/removal of solvent at ambient temperatures.
Using an analogous procedure to 1, the norbornadiene adduct (tBu,CF3PCP)Ru(cod)H (2) is
readily prepared; cold filtration from petroleum ether resulted in the isolation of an analyticallyꢀ
pure solidified oil that also liquefied upon warming to room temperature. 1H NMR spectra for 2
exhibit two hydride resonances, at δ –13.82 (meso) and –12.68 (rac) in a 57:43 ratio. Unlike the
single unresolved resonance found for 1, 31P NMR spectra for 2 show three clearly
31
distinguishable P resonances: a broad singlet at 116.0 ppm, assigned as the meso isomer, and
two doublets corresponding to a rac AB phosphorus system at 119.8 and 114.1 ppm with a large
2
trans coupling JPP = 175 Hz. Repeated recrystallization attempts to isolate pure mesoꢀ2 and/or
racꢀ2 were unsuccessful.
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In contrast to the moderately clean chemistry observed for tBu,CF3PCPH, tBu,C F5PCPH
2
under identical reaction conditions with Ru(cod)(cot) showed only ~50% conversion to the
31
expected (tBu,C F5PCP)Ru(acetone)2H product, as judged by P NMR. Addition of excess cod or
nbd to the acetone solvate solution gave a complex mixture of products which were not further
characterized.
2
Alkane Dehydrogenation Catalysis by (tBu,CF3PCP)Ru(nbd)H. We have previously reported
that (CF3PCP)Ru(cod)H exhibits strong catalytic activity for dehydrogenation of cycloalkanes at
low catalyst loading and concentration (0.033 mol %, 1.25 mM); however, at higher catalyst
loadings (0.33 mol%, 12.5 mM) conversion to (ꢁꢀCF3PCPH)Ru(H)(ꢁꢀH)(ꢁꢀη6,κ3ꢀCF3PCP)Ru(H)
dominates and minimal turnovers were observed.10a In light of these previous results, catalyst
studies were initially carried out using low 2 catalyst loading in order to obtain useful
comparisons to the original (CF3PCP)Ru(cod)H system. Thermolysis of 0.033 mol % of 2 in (1:1)
mixtures of cyclooctane (coa) and tertꢀbutylethylene (tbe) at 150 °C and 200 °C were monitored
by 1H NMR (Figure 3).
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Figure 3. Comparative catalyst activity plot for (tBu,CF3PCP)Ru(nbd)H (■) and (CF3PCP)Ru(cod)H
(ꢀ) in 1:1 cyclooctane/tꢀbutylethylene at 150 °C and 200 °C (0.033 mol % catalyst loading,
1.25 mmol catalyst).
1
Initial rates of coe production using 2 were taken from H NMR integrations after 10
minutes and estimated to be 78 turnovers hrꢀ1 at 150 °C, compared to 180 turnovers hrꢀ1 initial
activity found for (CF3PCP)Ru(cod)H. Catalyst lifetime was, however, significantly improved:
after 480 min., 214 TO’s were observed for 2 and a TON of 312 was achieved after 4 days. In
contrast, the catalytic activity of the (CF3PCP)Ru(cod)H system ceased due to catalyst
decomposition after 3 h with a TON of 152. 19F NMR spectra of catalyst solution 2 after 4 days
at 150 °C show that some catalyst was still present in solution (~20%), but no further
cyclooctene production was observed.
At 200 °C, 2 produced 460 TO’s after 24 hrs with an initial rate of 504 turnovers hrꢀ1,
approximately 50% that of (CF3PCP)Ru(cod)H, which shows a TON 350 and is completely
deactivated after only 30 min. 19F NMR spectra of (tBu,CF3PCP)Ru(nbd)(H) catalyst solutions
after 24 h at 200 °C show only the presence of unligated tBu,CF3PCPH; the major decomposition
product for the (CF3PCP)Ru(cod)H system has not been identified. The initial turnover rate and
total turnovers for the hybrid system 2 are considerably less than those reported for iridium
phosphinite complexes such as (pꢀArFPCPtBu)IrH2 (8715 turnovers hrꢀ1, 2200 TON at 200 °C)27,28
and the ferroceneꢀbridged pincers (tBuPCPFe)IrH2 (3,300 TON at 180 °C).29 The TON for 2 is,
however, higher than reported for (tBuPCP)IrH2 (230 TON at 200 °C),28 and comparable to the 5d
analogue acceptor system (CF3PCP)Os(cod)H (510 TON at 200 °C).10b
Strong catalytic rate inhibition by alkene product build up is observed for all reported
alkane dehydrogenation catalyst systems. Inhibition for the (tBu,CF3PCP)Ru(nbd)(H) system was
examined in the presence of 0ꢀ800 equiv. added coe (Figure 4). Initial activities in the presence
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of 400 and 800 equiv. coe, were 52% and 32% of the initial activity, respectively, half the
inhibition effect found for (CF3PCP)Ru(cod)H under identical conditions (400 equiv: 29%, 800
equiv: 16%).10a
Figure 4. Product inhibition study: Initial catalyst activity plots for (tBu,CF3PCP)Ru(nbd
Ru(nbd)H (■) and (CF3PCP)Ru(cod)H (ꢀ) in 1:1 coa/tbe at 200 °C (0.033 mol % catalyst
loading) as a function of equivalents added cyclooctene.
To further test catalyst stability, thermolysis of 2 in 1:1 coa:tbe was carried out for 3 hrs
at 200 °C (Figure 5). At this point the catalysis rate had slowed to 20 turnovers hrꢀ1. From
inhibition data in Figure 4, we would estimate that the rate would be slowed to ~290 turnovers
hrꢀ1 in the presence of 339 equiv. coe. if only product inhibition was occurring. All volatiles
were removed, the residue was taken up in a fresh 1:1 coa/tbe mixture, and the thermolysis was
reinitiated. A renewed dehydrogenation activity of 174 turnovers hrꢀ1 was observed, which
indicates the inhibition by coe was contributing significantly to the diminished activity. The
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restart activity is 35% of the initial rate, suggesting that 65% of the activity loss after 180 min. is
due to decomposition of the catalyst at 200 °C.
Figure 5. Catalyst activity plots for fresh (ꢀ, t = 0 min.) and restarted (■, t = 180 min.)
(
tBu,CF3PCP)Ru(nbd)H in 1:1 coa/tbe at 200 °C (0.033 mol % catalyst loading).
Acceptorless dehydrogenation by 2 was also examined. Refluxing 2.5 mM catalyst in
cyclooctane (∼150 °C at 590 Torr ambient pressure) for 24 hr produced 30 turnovers of
cyclooctene (Figure 6) with an initial rate of 18 turnovers hrꢀ1. Interestingly, while 2 was still the
major species (~60% of total pincer species observed by 31P NMR) after 24 h, no further
catalytic activity was observed. Under analogous reflux conditions, (CF3PCP)Ru(cod)H produced
19
10 turnovers of cyclooctene after 2 hr with an initial rate of 14 turnovers hrꢀ1, and F showed
complete conversion to the catalytically inactive dimer (ꢁꢀCF3PCPH)Ru(H)(ꢁꢀH)(ꢁꢀη6,κ3ꢀ
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CF3PCP)Ru(H).10a For comparison, 2 mM (tBuPCP)IrH2 is reported to produce 190 turnovers after
120 h, with an initial rate of 11 turnovers hrꢀ1.30
Figure 6. Acceptorless dehydrogenation catalyst activity for (tBu,CF3PCP)Ru(nbd)H (■) and
(
CF3PCP)Ru(cod)H (●) (2.5 mmol catalyst loading) in cyclooctane under reflux conditions (∼150
°C, 590 Torr N2).
Previous alkane dehydrogenation studies of (PCP)Ir systems have been carried out under
argon due to strong inhibition by nitrogen owing to the formation of stable bridged Ir2(ꢁꢀN2)
products.4,7 A key result of the previous work on alkane dehydrogenation by (CF3PCP)Ru(cod)H
was the ability for the catalyst to function in a variety of environments.10a No change in initial
rate or total turnovers was found when catalyst runs with (CF3PCP)Ru(cod)H were performed
under N2, vacuum, air, or in the presence of excess H2O. A series of controls with
(
tBu,CF3PCP)Ru(nbd)H over 24 h at 200 °C under vacuum, N2, or in the presence of 100 equiv.
H2O also showed no notable change in initial rate, total turnovers, or decomposition products.
Dehydrogenation activity in the presence of oxygen was not evaluated.
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We have also examined the cyclooctane (coa) dehydrogenation activity of 2 at higher
catalyst loading. As reported previously,10a thermolysis of 12.5 mM (0.33 mol %)
(
CF3PCP)Ru(cod)H in a 1:1 mixture of coa and tertꢀbutylethylene at 200 °C showed minimal
dehydrogenation activity. After the mixture was cooled to 20 °C, a red brown precipitate was
observed and complete conversion to (ꢁꢀCF3PCPH)Ru(H)(ꢁꢀH)(ꢁꢀη6,κ3ꢀCF3PCP)Ru(H) was
indicated by 31P and 19F NMR. In contrast, thermolysis of 12.5 mM (0.33 mol%) of the
asymmetric hybrid pincer catalyst (tBu,CF3PCP)Ru(nbd)H at 200 °C in 1:1 coa and tertꢀ
butylethylene produced 51 turnovers of cyclooctene after 6 h with an initial rate of 120 turnovers
hꢀ1 (Figure 7). 19F NMR spectra of catalyst solution at this point show significant amount
(~90%) of free tBu,CF3PCPH, but no significant resonances attributable to deactivation by dimer
formation.
Figure 7. Catalyst activity of (tBu,CF3PCP)Ru(nbd)H, at 1.25 mM (●) and 12.5 mM (■) in 1:1
cyclooctane/tertꢀbutylethylene at 200 °C.
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Summary. New hybrid alkyl/perfluoroalkyl phosphine pincer ligands have been
prepared and their ruthenium coordination chemistry has been examined. Unstable
(
tBu,CF3PCP)Ru(solv)2(H) is obtained by reacting tBu,CF3PCPH with the labile Ru(0) precursor
Ru(cod)(cot) in acetone at ambient temperatures under H2. Subsequent addition of excess cod or
nbd afforded (tBu,CF3PCP)Ru(cod)(H) or (tBu,CF3PCP)Ru(nbd)(H) in reasonable yield as the major
products; these oily diastereomeric mixtures have proven difficult to purify further.
(
tBu,CF3PCP)Ru(nbd)(H) (2) shows an initial cyclooctane dehydrogenation activity only ~50% that
of (CF3PCP)Ru(cod)(H), however the catalytic lifetime is significantly increased, coe product
inhibition is reduced, and catalytic activity under acceptorless conditions is slightly improved.
The desirable N2 and moisture stability of the parent acceptor system was effectively conferred
to the donor/acceptor hybrid pincer catalysts. 2 shows reduced yet significant catalytic activity
at higher catalyst loadings (0.33 mol%, 51 turnovers after 6 hours) whereas (CF3PCP)Ru(cod)(H)
is inactive at this catalyst concentration. This suggests that the more stericallyꢀhindered
asymmetric catalyst 2 may prevent the formation of areneꢀbridged dimer which is responsible for
the activity loss in (CF3PCP)Ru(cod)(H) system, but demetallation and loss of free tBu,CF3PCPH at
later stages of catalysis is observed and may be a limiting feature of this asymmetric hybrid
system.
It is unfortunate that we were not able to address the initial guiding concept of preparing
electronically yet stereochemically distinct meso and rac pincer complexes, since the
stereoisomers could not be separated and compared in their pure forms. Nevertheless, while the
stereochemical complexity introduced by the new hybrid pincer ligands in this work was not
specifically exploited, these types of ligands may prove to be advantageous in future
enantioselective chemistry.
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Experimental Section.
General Procedures. All manipulations were conducted under N2 or vacuum using highꢀ
vacuumꢀline and glovebox techniques unless otherwise noted. All ambientꢀpressure chemistry
was carried out at approximately 590 Torr (elevation ∼2195 m). Dry, oxygen free solvents were
prepared using standard procedures and stored under vacuum. Aprotic deuterated solvents used
in NMR experiments were dried over activated 3 Å molecular sieves. NMR spectra were
obtained with a Bruker DRXꢀ400 instrument using 5 mm NMR tubes fitted with Teflon valves
(Chemglass, CGꢀ512 or New Era CAVꢀVBP). 31P spectra were referenced to an 85% H3PO4
external standard. 19F spectra were referenced to a CF3CO2Et (δ −75.32) external standard.
Elemental Analyses were performed using a Perkin Elmer Series II CHNS/O 2400 Analyzer. The
t
compounds BuPCl2,31 (tBu)(TMS)2P,32 1ꢀTrifluoromethylꢀ1,2ꢀbenziodoxolꢀ3(1H)ꢀone (Togni
Reagent II),33 Ru(cod)(ƞ3ꢀ2ꢀmethylallyl)2,34 and Ru(cod)(cot)35 were prepared following
published procedures. Me3SiCF3 and C2F5Cl were purchased from Synquest Labs, Inc., and
RuCl3(H2O)3 from Pressure Chemical Co.; all other reagents were purchased from Aldrich and
were used without further purification.
(t-butyl)(trimethylsilyl)phosphine. To (tBu)(TMS)2P (24.7 g, 0.105 mol) dissolved in 100 mL
THF at 0 °C was added 4.2 mL (3.2 g, 0.11 mol) of methanol dissolved in 100 mL of THF via
cannula over the course of 1h. After an additional 8 h at ambient temperature the THF,
t
methoxytrimethylsilane, and a small amount (ca 10%) of side product BuPH2 were removed in
vacuo and the crude product was purified by distillation (bp 40 ꢀ 45 °C, ~ 0.01 torr) to give a
colorless liquid (9.24 g, 54.2 % yield). (tBu)(TMS)PH has been prepared previously by an
alternate route.36 1H NMR (C6D6, 400.13 MHz, 25 °C): δ 2.45 (br. d, JPH = 192 Hz, 1H; PH),
1.22 (dd, 3JHP = 12.0 Hz, 4JHP = 2.1 Hz, 9H; C(CH3)3), 0.21 (dd, 3JHP = 4.3 Hz, 4JHP = 2.5 Hz 9H;
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Si(CH3)3). 31P NMR (C6D6, 161.97 MHz, 25 °C): δ −85.5 (d, JPH = 192 Hz). Caution:
(tBu)(TMS)PH is pyrophoric.
1,3-C6H4{CH2P(tBu)(TMS)}2 (tBu,TMSPCPH). To a solution of (tBu)(TMS)PH (7.14 g, 43.8
mmol) in 60 mL of THF was added 17.52 mL of a 2.5 M solution of nꢀBuLi (44 mmol) in
hexanes at −30 °C. The reaction mixture was allowed to warm to ambient temperature and was
stirred for 2 h. and then 5.50 g (20.8 mmol) of α, α′ꢀdibromoꢀmꢀxylene dissolved in 40 mL of
THF was added and the reaction mixture was stirred for 12 h at room temperature. The solvent
and volatiles were removed and replaced by 30 mL of petroleum ether. The resulting precipitate
was removed by filtration and removal of the filtrate solvent afforded tBu,TMSPCPH as an oily
colorless liquid (6.68 g, 75.2% yield), which was judged by NMR to be a 3:2 mixture of meso
and rac diastereomers, respectively. The crude mixture (> 95% pure, by 31P NMR) was not
1
purified further. H NMR (C6D6, 400.13 MHz, 25 °C), meso isomer: δ 7.7 – 7.0 (m, 4H; ArH),
3.05 (d, 2JAB = 14 Hz, 2H; CHAHBP), 2.92 (d, 2JAB = 14 Hz, 2H; CHAHBP), 1.14 (d, 3JPH = 11.6
3
Hz, 18H; C(CH3)3), 0.14 (d, JPH = 3.9 Hz, 18H; Si(CH3)3); rac isomer: δ 7.7 – 7.0 (m, 4H;
ArH), 3.04 (d, 2JAB = 14 Hz, 2H; CHAHBP), 2.91 (d, 2JAB = 14 Hz, 2H; CHAHBP), 1.14 (d, 3JPH
=
3
31
11.6 Hz, 18H; C(CH3)3), 0.12 (d, JPH = 3.8 Hz, 18H; Si(CH3)3). P NMR (C6D6, 161.97 MHz,
25 °C): δ −41.8 (br. s (ν = 80 Hz)); unresolved meso and rac resonances).
½
Reactions of tBu,TMSPCPH with CF3I, i-C3F7I, and C2F5I. To solutions of tBu,TMSPCPH,
prepared as described above, in 0.5 mL benzeneꢀd6 were added excess (4 equiv.) CF3I and C2F5I
by vacuum transfer and 4 equiv. iC3F7I by microliter syringe. NMR spectra after warming to
ambient temperature for 2 h showed complete reaction; a complex product mixture was observed
with C2F5I, but CF3I and C3F7I afforded the major products 1,3ꢀC6H4{CH2P(tBu)(CF2I)}2
tBu,CF2IPCPH, ~90%; ~3:1 meso:rac) and 1,3ꢀC6H4{CH2P(tBu)(iC3F7)}2 (tBu,iC F7PCPH, ~70%;
3
(
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~3:2 meso:rac), respectively. The formation of Me3SiF and Me3SiI metathesis products was
1
confirmed by H and 19F NMR. Isolation of these products on a preparative scale was not
1
attempted. tBu,CF2IPCPH data: H NMR (C6D6, 400.13 MHz, 25 °C), δ 7.4 – 6.8 (m, 4H; ArH),
3.23 (m, 2H; CHAHBP; meso), 2.85 (m, 2H; CHAHBP; meso), 3.4 – 2.7(m, 2H; CH2P; rac), 0.98
3
3
31
(d, JPH = 12 Hz, 18H; C(CH3)3; rac), 0.96 (d, JPH = 12 Hz, 18H; C(CH3)3; meso); P NMR
(C6D6, 161.97 MHz, 25 °C): δ 61.0 (m (ν = 135 Hz)); unresolved meso and rac resonances). 19F
½
2
2
NMR (C6D6, 376.50 MHz, 25 °C): δ ꢀ32.6 (dd, JFF = 186 Hz , JPF = 60 Hz, 2F; P(CFaFb);
meso), ꢀ32.6 (dd, 2JFF = 186 Hz, 2JPF = 19 Hz, 2F; P(CFaFb); rac), ꢀ35.7 (dd, 2JFF = 186 Hz , 2JPF
2
2
= 71 Hz, 2F; P(CFaFb); meso), ꢀ35.8 (dd, JFF = 186 Hz, , JPF = 70 Hz, 2F; P(CFaFb); rac).
1
tBu,iC F7PCPH data: H NMR (C6D6, 400.13 MHz, 25 °C), δ 7.3 – 6.8 (m, 4H; ArH), 3.34 (m,
3
2H; CHAHBP; overlapping meso and rac), 3.20 (m, 2H; CHAHBP; overlapping meso and rac),
3
3
1.09 (d, JPH = 12 Hz, 18H; C(CH3)3; rac), 1.07 (d, JPH = 12 Hz, 18H; C(CH3)3; meso); 31P
NMR (C6D6, 161.97 MHz, 25 °C): δ 41.6 (m (ν = 290 Hz)); unresolved meso and rac
½
resonances). 19F NMR (C6D6, 376.50 MHz, 25 °C): δ ꢀ72.8 (m, 6F; PCF(CF3)a(CF3)b); meso and
rac), ꢀ73.2 (m, 6F; PCF(CF3)a(CF3)b); meso and rac), ꢀ179.9 (overlapping heptets, 3JFF = 12 Hz ,
2F; PCF(CF3)a(CF3)b); meso and rac).
1,3-C6H4{CH2P(tBu)(Cl)}2 (tBu,ClPCPH). To a solution of tBu,TMSPCPH (3.48 g, 8.15 mmol) in
30 mL of toluene was added 4.63 g (19.56 mmol) of C2Cl6 dissolved in 30 mL of toluene at 0 °C.
The reaction mixture was stirred for an hour and then warmed up to room temperature and stirred
for additional 1h. The volatiles were removed and the resulting white solid residue was extracted
with petroleum ether and filtered. Removal of volatiles from the filtrate gave tBu,ClPCPH as an
oily white solid (1.70 g, 59.2 %), which was judged by NMR to be a 2:1 mixture of meso and rac
diastereomers, respectively. The crude isomeric mixture (> 95% pure, by 31P NMR) was not
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1
purified further. H NMR (C6D6, 400.13 MHz, 25 °C), meso isomer: δ 7.3 – 7.0 (m, 4H; ArH),
3
3.1 – 2.7 (m, 4H; overlapping meso and rac CHAHBP multiplets), 0.97 (d, JPH = 12.8 Hz, 18H;
C(CH3)3); rac isomer: δ 7.3 – 7.0 (m, 4H; ArH), 3.1 – 2.7 (m, 4H; overlapping meso and rac
3
31
CHAHBP multiplets), 0.99 (d, JPH = 12.5 Hz, 18H; C(CH3)3). P NMR (C6D6, 161.97 MHz, 25
°C): δ 126.3 (br. s (ν = 70 Hz)); unresolved meso and rac resonances).
½
Alternative synthesis of tBu,ClPCPH. Following a modified literature procedure,21 THF (100
mL) was added to a flask containing α,α’ꢀdichloroꢀmꢀxylene (10.0 g, 57.2 mmol) and magnesium
turnings (4.16 g, 172 mmol). The resulting mixture was stirred at room temperature for 5 h, then
at 40 °C for an additional 17 h. The dark yellowꢀgreen solution was cooled to room temperature
and then added dropwise to a solution of tertꢀbutyldichlorophosphine (18.42 g, 115.8 mmol) in
THF (100 mL) at −78 °C. The reaction mixture was allowed to warm to room temperature and
then stirred for an additional 1 h. Volatiles were removed, 50 mL of petroleum ether was added
and the resulting slurry was stirred for an hour. The slurry was allowed to stand undisturbed, and
the separated liquid layer was transferred into a 100 mL roundꢀbottom flask via cannula.
Removal of petroleum ether from the filtrate gave an oily white solid (10.46 g, 52.4%) which
was judged to be > 95% pure by NMR.
1,3-C6H4{CH2P(tBu)(OPh)}2 (tBu,OPhPCPH). To dry phenol (4.50 g, 47.9 mmol) dissolved in
100 mL of THF was added 6.66 mL of Et3N (4.84 g, 47.9 mmol) via syringe, and the solution
was cooled to 0 °C. tBu,ClPCPH (8.02 g, 22.8 mmol) dissolved in 100 mL of THF was added to
the phenol/Et3N/THF solution via syringe and precipitation of Et3NHCl was observed. The
reaction mixture was warmed to room temperature and stirred for additional 4 h. The resulting
ammonium salt was removed by filtration and washed several times with THF. Removal of
volatiles from the filtrate resulted in an oily colorless liquid (7.54 g, 71.4%), which was judged
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by 31P NMR to be a 3:1 mixture of meso and rac diastereomers, respectively. The crude isomeric
mixture (~95% pure, by 31P NMR, contaminated with ~ 0.12 equiv. THF) was not purified
1
further. H NMR (C6D6, 400.13 MHz, 25 °C), meso isomer: δ 7.4 – 6.7 (m, 14H; overlapping
2
OPh and ArH), 3.08 (br. d, JHH = 14 Hz, 2H; meso CHAHBP), 2.65 (m, 2H; overlapping
3
CHAHBP multiplets), 1.03 (dd, JHP = 12 Hz, JHH = 1 Hz, 18H; C(CH3)3, meso isomer); rac
2
isomer: δ 7.4 – 6.7 (m, 14H; overlapping OPh and ArH), 3.16 (dm, JHH = 14 Hz, 2H; rac
CHAHBP), , 2.65 (m, 2H; overlapping CHAHBP multiplets), 1.08 (d, 3JHP = 12 Hz, 18H; C(CH3)3,
rac isomer). 31P NMR (C6D6, 161.97 MHz, 25 °C): δ 145.4 (s; meso isomer), 144.8 (s; rac
isomer).
1,3-C6H4{CH2P(tBu)(CF3)}2 (tBu,CF3PCPH). 3.54 mL CF3SiMe3 (3.40 g, 24.0 mmol) was added
via syringe over the course of 15 min to a mixture of tBu,OPhPCPH (3.72 g, 7.97 mmol) and CsF
(3.63 g, 23.9 mmol) in 30 mL diethyl ether at 0 °C. The reaction mixture was allowed to warm
up to room temperature and stirred for 24 h, during which time the slurry turned from clear to
dark brown with a white CsF suspension. The volatiles were removed and the resulting brown
slurry was extracted with petroleum ether. Removal of volatiles and vacuum distillation to
remove the silyl ether side product (80ꢀ83 °C, 25 Torr) gave >95% pure tBu,CF3PCPH as a brown
viscous oil (2.48 g, 74.1%), which was judged by integration of tBu proton resonances to be a 4:1
mixture of meso and rac diastereomers, respectively. Anal. Calcd for C18H26F6P2: C, 51.68; H,
1
6.26. Found: C, 51.17; H, 6.04. H NMR (C6D6, 400.13 MHz, 25 °C) meso isomer: δ 7.3 – 6.9
(m, 4H; overlapping ArH), 3.03 (dd, 2JAB = 14.5 Hz, 2JHP = 2.8 Hz, 2H; CHAHBP), 2.73 (d, 2JAB
3
= 14.5 Hz, 2H; CHAHBP), 0.96 (d, JHP = 12.4 Hz, 18H; C(CH3)3). rac isomer: δ 7.3 – 6.9 (m,
2
4H; overlapping ArH), 3.08 (dm, JAB = 14.5 Hz, 2H; CHAHBP), 2.76 (m, 2H; CHAHBP), 0.98
3
31
(d, JHP = 12.2 Hz, 18H; C(CH3)3). P NMR (C6D6, 161.97 MHz, 25 °C): δ 21.8 (m; meso and
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2
rac isomers). 19F NMR (C6D6, 376.50 MHz, 25 °C): δ ꢀ51.5 (d, JPF = 54 Hz; meso and rac
P(CF3)).
1,3-C6H4{CH2P(tBu)(C2F5)}2 (tBu,C F5PCPH). 18.56 mL 2.5 M nꢀBuLi in hexanes (46.4 mmol)
2
was transferred into a 500 mL threeꢀneck flask fitted with a vacuum adapter and a digital
thermometer probe. Hexanes were removed, the nꢀBuLi was redissolved in 125 mL of diethyl
ether, and the solution was cooled to −90 °C. A 6.85 mL aliquot of C2F5Cl (bp −39 °C, density
1.568 g/mL, 69.6 mmol) was measured out in a calibrated volume at −80 °C and was slowly
added to the nꢀBuLi solution by vacuum transfer so as to maintain the reaction temperature
below −80 °C. After 30 min the addition was complete and the solution was stirred for an
additional 1h. A solution of tBu,ClPCPH (4.08 g, 11.6 mmol) dissolved in 30 mL of diethyl ether
and 15 mL of THF was added by syringe under nitrogen over a period of ca 30 min to maintain
the temperature below −80 °C. The solution turned brown during this time. After the addition
was complete the mixture was maintained at −80 °C for an additional 1 h and then allowed to
warm to room temperature. After 12 h at room temperature the solution was separated from the
solids via cannula filtration. The solids were washed with diethyl ether (2×50 mL) and combined
with the initial filtrate. Removal of volatiles afforded the crude product. The crude product was
extracted with petroleum ether to give a yellow filtrate. Removal of petroleum ether yielded
1
>95% pure product as a viscous yellow oil (3.92 g, 65.4%). H and 19F NMR indicate a 3:1
mixture of meso and rac diastereomers, respectively, as judged by the C(CH3)3 and CF3
resonances. Anal. Calcd for C20H26F10P2: C, 46.34; H, 5.06. Found: C, 46.67; H, 4.74. 1H NMR
(acetoneꢀd6, 400.13 MHz, 25 °C): δ 7.4 – 6.9 (m, 4H; overlapping ArH; meso and rac), 3.28
3
(overlapping AB patterns, 4H; CH2P; meso and rac), 1.32 (d, JHP = 12.8 Hz, 18H; C(CH3)3;
meso), 1.31 (d, 3JHP = 12.7 Hz, 18H; C(CH3)3; rac). 31P NMR (acetoneꢀd6, 161.97 MHz, 25 °C):
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δ 19.7 (br. s; unresolved meso and rac). 19F NMR (acetoneꢀd6, 376.50 MHz, 25 °C): δ −82.3 (d,
2
2JPF = 16 Hz, 6F; CF3CF2P; rac), −82.4 (d, JPF = 16 Hz, 6F; CF3CF2P; meso), −114.6
(overlapping ABX multiplets, 2JPF = 294 Hz, 4F; CF3CF2P).
t
(
t
Bu,CF
Bu,CF
3PCP)Ru(cod)(H). Ru(cod)(cot) (0.227 g, 0.721 mmol),
3PCPH (0.302 g, 0.721
mmol) and 10 ml acetone were added to a 25 ml round bottom flask and placed under 1 atm of
H2. Stirring for 20 h at room temperature resulted in a deep brown solution. After removal of H2
pressure, 440 ꢁL of cyclooctadiene (0.389 g, 3.60 mmol) was added to the solution and the
mixture was stirred for an additional 24 h under 1 atm of N2. The solvent was removed and the
brown residue was triturated with petroleum ether and stirred for an additional hour. A small
amount of precipitate formed which was filtered off, and the brown filtrate volume was reduced
to 5 mL in volume, cooled to −78 °C, and cold filtered to give initially a brown powder which
melted to become a brown liquid at room temperature. The product purity was ∼80%, along with
t
Bu,CF
31
∼10% free
3PCPH and small amount of other unidentified side products, as judged by P
1
NMR. A second precipitation/cold filtration step gave ∼90% pure product. Integration of H
1
NMR hydride resonances indicated 55:45 mixture of meso and rac stereoisomers. H NMR
(C6D6, 400.13 MHz, 25 °C): δ 7.1 – 6.7 (m, 3H; overlapping meso and rac ArH), 5.85 (br. s, 2H;
2
vinylic cod), 5.83 (br. s, 2H; vinylic cod), 3.72 (dm, JHP = 16 Hz, 2H; CHAHBP; meso), 3.46
2
(dm, JHP = 16 Hz, 2H; CHAHBP; meso), 3.8 – 3.4 (m, 4H; CH2P; rac), 2.09 (m, 8H; aliphatic
cod; overlapping meso and rac), 1.41 (m, 18H; C(CH3)3; overlapping meso and rac), −14.75 (tm,
2
2JHPa ~ 2JHPb = 24 Hz, 1H; RuH; rac), −14.97 (t, JHP = 25 Hz, 1H; RuH; meso). 31P {H} NMR
(C6D6, 161.97 MHz, 25 °C): δ 106.3 (br. s (ν = 70 Hz) unresolved meso and rac resonances).
½
28

Page 29 of 34
Dalton Transactions
View Article Online
DOI: 10.1039/C8DT02738B
2
19F NMR (C6D6, 376.50 MHz, 25 °C): δ −51.1 (m, 6F; CF3; meso), −51.7 (d, JPF = 50 Hz, 3F;
CF3; rac), −55.3 (d, 2JPF = 43 Hz, 3F; CF3; rac).
(
tBu,CF3PCP)Ru(nbd)(H) (2). Ru(cod)(cot) (0.300 g, 0.950 mmol), tBu,CF3PCPH (0.397 g, 0.950
mmol) and 15 ml acetone were added to a 25 mL round bottom flask and placed under 1 atm of
H2. Stirring for 20 h at room temperature resulted in a deep brown solution. After removal of H2
pressure, 488 ꢁL of norbornadiene (0.442 g, 4.80 mmol) was added to the solution and the
mixture was stirred for an additional 6 h under a N2 atmosphere. The solvent was removed and
the remaining brown residue was triturated with benzene and the resulting precipitate was
removed by filtration and removal of benzene gave ∼95% pure crude product along with ∼5%
free tBu,CF3PCPH. The crude product was dissolved in 5 mL of petroleum ether and cooled to −78
C; cold filtration and warming of the isolated solid gave (tBu,CF3PCP)Ru(nbd)(H) as a brown
1
31
19
highly viscous oil in > 95% purity, as judged by NMR. (0.220 g, 38.6%). H, P and F NMR
data indicate a 57:43 mixture of meso and rac stereoisomers. Anal. Calcd for C25H34F6P2Ru: C,
1
49.10; H, 5.60. Found: C, 49.59; H, 5.31. H NMR (C6D6, 400.13 MHz, 25 °C): δ 7.1 – 6.7 (m,
3H; ArH, overlapping meso and rac), 4.07 (br. s, 2H; meso), 5.62 (m, 2H; rac), 4.04 (br. s, 2H;
meso), 3.70 (br. s, 2H; meso), 3.66 (br. s, 2H; meso), 4.0 – 3.0 (m, 10H; overlapping nbd and
rac CH2P), 1.39 (d, 2JHP = 13 Hz, 9H; PC(CH3)3; rac), 1.14 (d, 2JHP = 15 Hz, 9H; PC(CH3)3; rac),
1.05 (pseudo t, JHP = 15 Hz, 18H; PC(CH3)3; meso), 0.96 (m, 2H; nbd CHaHb), −12.68 (tm, 2JHPa
~ 2JHPb = 29 Hz, 1H; RuH; rac), −13.82 (t, 2JHP = 31 Hz, 1H; RuH; meso). 31P{1H} NMR (C6D6,
2
2
161.97 MHz, 25 °C): δ 119.9 (d, JPP = 175 Hz, 1P; rac), 116.0 (s; meso), 114.1 (d, JPP = 175
2
Hz, 1P; rac). 19F NMR (C6D6, 376.50 MHz, 25 °C): δ −51.4 (d, JFP = 35 Hz, 3F; CF3; rac),
−51.5 (pseudo t, JPF = 19 Hz, 6F; CF3; meso), −55.9 (d, 2JFP = 35 Hz, 3F; CF3; rac).
29