Synthesis, characterization and reactivity of a η1- methylphosphaalkyne complex, [RuH(dppe)2(η1- P≡CMe)][CF3SO3]

Article abstract of DOI:10.1002/ejic.200701330

The synthesis of the first example of a complex in which methylphosphaalkyne solely η¹-coordinates a metal center, [RuH(dppe)2(η¹-P≡CMe)][CF₃SO₃] [dppe = 1,2-bis(diphenylphosphanyl) ethane], is reported. Treatment of this compound with HBF₄ (acting as a source of HF) has led to the reduction of the phosphaalkyne and the formation of a rare PF₂Et complex, [RuH(dppe)₂(η¹-PF²Et)][BF⁴]. Both complexes have been crystallographically characterized. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701330

SHORT COMMUNICATION
DOI: 10.1002/ejic.200701330
Synthesis, Characterization and Reactivity of a η1-Methylphosphaalkyne
Complex, [RuH(dppe)2(η1-PϵCMe)][CF3SO3]
Cameron Jones,*[a] Christian Schulten,[a,b] and Andreas Stasch[a]
Keywords: Phosphaalkynes / Main-group elements / Low coordination / Coordination complex
The synthesis of the first example of a complex in which
methylphosphaalkyne solely η1-coordinates a metal center,
[RuH(dppe)2(η1-PϵCMe)][CF3SO3] [dppe = 1,2-bis(diphenyl-
phosphanyl)ethane], is reported. Treatment of this compound
with HBF4 (acting as a source of HF) has led to the reduction
of the phosphaalkyne and the formation of a rare PF2Et com-
plex, [RuH(dppe)2(η1-PF2Et)][BF4]. Both complexes have
been crystallographically characterized.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
This was seen as of fundamental interest because the prop-
erties of such complexes could be compared with those in-
corporating bulkier phosphaalkynes. In addition, we wished
to explore the reactivity of PϵCMe towards nucleophiles
and electrophiles within the metal coordination sphere, as
very novel results have been achieved with bulkier phos-
phaalkynes in this respect. The preliminary results of our
endeavors in this area are reported herein.
Introduction
The coordination chemistry of sterically stabilized phos-
phaalkynes, e.g. PϵCtBu, has been extensively developed
over the past two decades.[1] These compounds can ligate
mono-, di- and polymetallic fragments in a number of
modes involving electron donation from the P-lone pair
and/or the PϵC bond of the phosphaalkyne. However,
complexes which encompass solely η1-P-ligating phospha-
alkynes are very rare.[2] This results from their P–C bond
polarization, δ+PϵCδ–, and their relatively high-energy π-
orbital HOMOs which lead to them being more alkyne than
nitrile like in their coordination chemistry. Generally, η1-P-
coordination of phosphaalkynes to metal fragments only
occurs if η2-PϵC coordination is precluded by the steric
properties of the metal fragment. Such a situation occurs
in, for example, trans-[W(dppe)2(η1-PϵCtBu)2] [dppe = 1,2-
bis(diphenylphosphanyl)ethane],[2f] which was prepared by
displacement of the labile dinitrogen ligands from trans-
[W(dppe)2(η1-N2)2] upon its treatment with PϵCtBu.
In the past two years we have begun to study the further
chemistry of the sterically unhindered phosphaalkyne,
PϵCMe, the phosphorus analogue of acetonitrile and pro-
pyne. This study has shown that PϵCMe is, in general, sig-
nificantly more reactive than its hindered counterparts. Its
propensity to ligate metal centers in an η2-fashion has been
demonstrated,[3] it has been shown to readily cyclodimerize
within the coordination sphere of transition metals,[3] and
a number of novel heterocyclic and cage products have re-
sulted from its cycloaddition reactions with a variety of un-
saturated substrates.[4,5]
Results and Discussion
As the dinitrogen ligands of trans-[W(dppe)2(η1-N2)2]
have been shown to be readily displaced by PϵCtBu, we
believed its treatment with the less hindered phosphaalkyne,
PϵCMe,[6] would lead to a similar result. Surprisingly, how-
ever, no reaction occurred. This is perhaps because the
lesser electron donating properties of the methyl group rela-
tive to tert-butyl lead to PϵCMe being a weaker Lewis base
than PϵCtBu. As a result, bulky, coordinatively unsatu-
rated metal fragments were sought which could potentially
coordinate PϵCMe without the need for it to displace other
ligands. The cationic complexes [MH(dppe)2]+ (M = Ru[2a]
or Fe[7]) were chosen because bulky phosphaalkynes
(PϵCR, R = tBu, SiPh3, CPh3) are known to readily ligate
them in an η1-fashion.
The reaction of [RuH(dppe)2][CF3SO3] with PϵCMe in
dichloromethane led to a high yield of the thermally stable
target complex, 1, after recrystallization from a dichloro-
methane/hexane mixture (Scheme 1). In contrast, the corre-
sponding reaction with [FeH(dppe)2][BPh4] was not clean
and although the 31P{1H} NMR spectrum of the reaction
mixture suggested the presence of [FeH(dppe)2-
(PϵCMe)][BPh4], this compound could not be isolated
upon work-up and only an intractable mixture of products
was obtained. The spectroscopic data for 1 are fully consis-
tent with its proposed formulation. Of most note is its
We wished to extend the chemistry of PϵCMe to the
formation of complexes in which it acts as a η1-P ligand.
[a] School of Chemistry, Monash University,
P. O. Box 23, Melbourne, VIC, 3800, Australia
[b] School of Chemistry, Main Building, Cardiff University,
CF10 3AT, UK
E-mail: cameron.jones@sci.monash.edu.au
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C. Jones, C. Schulten, A. Stasch
SHORT COMMUNICATION
31P{1H} NMR spectrum which displays a doublet signal
2
for the dppe ligands (δ = 61.5 ppm, JPP = 30 Hz) and a
quintet for the phosphaalkyne at a chemical shift (δ
–38.7 ppm) significantly downfield from that of the free
phosphaalkyne (δ –61.0 ppm).[6] The hydride signal in the
1H NMR spectrum of the compound appears as a doublet
2
of quintets (δ –9.60 ppm, JPH = 127 and 17 Hz). Similar
spectral patterns have been observed for the complexes,
[2b]
[RuH(dppe)2(PϵCR)][CF3SO3] (R = SiPh3
or CPh3[2a]).
Figure 1. Structure of the cationic component of 1 (25% thermal
ellipsoids, dppe hydrogen atoms omitted for sake of clarity). Se-
lected bond lengths [Å] and angles [°]: Ru(1)–P(1) 2.3148(13),
Ru(1)–P(5) 2.3453(11), Ru(1)–P(2) 2.3592(11), Ru(1)–P(4)
2.3682(11), Ru(1)–P(3) 2.3786(11), P(1)–C(1) 1.535(6), C(1)–C(2)
1.474(8), C(1)–P(1)–Ru(1) 153.7(2), C(2)–C(1)–P(1) 174.7(5), P(2)–
Ru(1)–P(3) 80.82(3), P(5)–Ru(1)–P(4) 82.94(4).
2
down field (δ = 165.6 ppm, quint., JPP = 28 Hz, 1 P; δ =
2
65.2 ppm, d, JPP = 28 Hz, 4 P). This observation suggests
the product contains a P-coordinated phosphaalkene or
phosphaalkenyl fragment, though its structure cannot be
certain.[10] Upon warming the reaction mixture past –10 °C,
the product appeared to decompose to an unidentifiable
mixture of phosphorus containing products which pro-
hibited isolation and further characterization of the com-
Scheme 1. Synthesis of complexes 1 and 2.
The X-ray crystal structure of 1 was determined and its pound.
cationic component is depicted in Figure 1. Its P–C triple
Previous studies have shown that bulky η1-P-coordinated
bond length [1.535(6) Å] is close to those in the few struc- phosphaalkynes can be transformed to, for example, phos-
turally characterized free phosphaalkynes [e.g., 1.538(2) Å phaalkenes,[2c] phosphanes[2c] and phosphorus heterocy-
[2a]
in PϵCCPh3
and 1.532 Å (mean) in the diphosphaal- cles[2a] upon treatment with proton sources. In a similar
kyne, PϵCC(C6H4)3CCϵP],[8] but significantly shorter vain, we examined the reaction of 1 with an excess of a
than the P–C bonds in η2-complexes of methylphosphaal- diethyl ether solution of HBF4 which led to a moderate
kyne {e.g. 1.617 Å in [Pt(PCy3)2(η2-PϵCMe)]3}. Although yield of the difluorophosphane complex, 2 (Scheme 1), after
the phosphaalkyne in 1 is close to linear, its coordination recrystallization from a hexane/dichloromethane solution.
to the distorted octahedral ruthenium center deviates signif- In this reaction the HBF4 is presumably acting as a source
icantly from linear [Ru–P–C 153.7(2)°] because of interac- of HF which doubly reduces the coordinated phosphaal-
tions with the surrounding phenyl groups.
kyne. The HBF4 is also the source of the counter anion in
It seemed that compound 1 could prove useful as a plat- 2. It is noteworthy that PϵCtBu {within the complex trans-
form to test the further reactivity of PϵCMe towards elec- [FeH(dppe)2(η1-PϵCtBu)]+} has been similarly reduced to
trophiles and nucleophiles. A prior theoretical study on F2PCH2tBu by treatment with HBF4.[2c] In that reaction,
PϵCMe concluded that its methyl protons are quite acidic the stepwise nature of the reduction was confirmed by the
and that it should be more easily deprotonated than, for isolation of an intermediate containing a P-coordinated
example, NϵCMe.[9] Accordingly, we treated 1 with a vari- fluorophosphaalkene, FP = CHtBu. No similar intermedi-
ety of bases (e.g. NaOH, KOtBu, NaOPh and LiNiPr) but ate (viz. trans-[RuH(dppe)2{η1-P(F)=C(H)Me}]+) was ob-
all reactions led to intractable mixtures of products. It is served in the current reaction, which is perhaps in line with
noteworthy that treatment of the related complex, the previously demonstrated greater reactivity of PϵCMe
[RuH(dppe)2(PϵCSiPh3)]+, with NaOPh has recently been over PϵCtBu. Indeed, treating 1 with one equivalent of
reported to give the first terminal “cyaphide” complex, HBF4 led only to a mixture of 2 and unreacted 1.
[RuH(dppe)2(CϵP)].[2b] Attention then turned to the reac-
The spectroscopic data for 2 are compatible with its so-
tion of the electrophilic reagent, MeI, with 1 in CH2Cl2. lid-state structure. In its 31P{1H} NMR spectrum the fluo-
Monitoring this reaction by 31P NMR spectroscopy re- rophosphane signal appears as a triplet of quintets at low
vealed that a compound was formed at ca. –50 °C with a field (δ = 244.4 ppm) displaying characteristic 1JPF and 2JPP
spectral pattern similar to that of 1, but with the signal couplings (1094 and 30 Hz, respectively). The low field po-
derived from the phosphaalkyne shifted by ca. 200 ppm sition of this signal is not surprising in light of the electron-
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Eur. J. Inorg. Chem. 2008, 1555–1558

η1-Methylphosphaalkyne-Ru Complex
C6D6, 298 K): δ = –38.7 (quin, 2JPPЈ = 30 Hz, PCMe), 61.5 (d, 2JPPЈ
= 30 Hz, dppe) ppm. 19F{1H} NMR (281.3 MHz, C6D6, 298 K): δ
withdrawing nature of the fluorine substituents and it can
be compared to a chemical shift of δ = 279.5 ppm for the
corresponding signal in the spectrum of trans-[FeH(dppe)2-
{η1-P(F)2C(H)2tBu}]+.[2d] The structure of the cationic
component of 2 (Figure 2) reveals its ruthenium center to
have a similar octahedral geometry to that of 1, while the
geometry of the PF2Et ligand is unremarkable. Saying this,
there has been no previous crystallographic elucidation of
this phosphane.
= –78.5 (s, CF SO ) ppm. IR (Nujol): ν = 1560 [w (PϵC)], 1458
˜
3
3
(m), 1376 (m), 1309 (m), 1272 (m), 1187 (m), 1053 (m), 998 (m)
cm–1. (MS/EI): m/z (%) = 958 (3) [RuH(dppe)2(PCMe)+], 899 (32)
[RuH(dppe)2+], 398 (100) [dppe+].
Synthesis of [RuH(dppe)2(η1-PF2Et)][BF4] (2): HBF4 (0.18 mL of a
54% solution in Et2O, 0.135 mmol) was added to a solution of 1
(50 mg, 0.045 mmol) in dichloromethane (5 mL) at 20 °C. After
12 h volatiles were removed in vacuo and the residue dissolved in
dichloromethane (1 mL). Layering this with hexane (10 mL)
yielded 2 as yellow crystals overnight (yield 20 mg, 40%). M.p.
1
176–182 °C. H NMR (500 MHz, CD2Cl2, 298 K): δ = –7.9 (d of
quin, 2JPH = 115 and 21 Hz, 1 H, RuH), 2.06–2.50 (m, 8 H, PCH2
and 3 H, CH3), 2.80 (m, 2 H, PCH2), 7.11–7.33 (m, 40 H, Ar-H)
ppm. 31P{1H} NMR (121.6 MHz, CD2Cl2, 298 K): δ = 62.5 (d,
2JPPЈ = 30 Hz, dppe), 244.4 (tr. of quin, 2JPPЈ = 30, 1JPFЈ = 1094 Hz,
PF2) ppm. 19F{1H} NMR (281.3 MHz, CD2Cl2, 298 K): δ = –153.2
1
(4 F, BF4), –56.2 (d, JPF = 1094 Hz, 2 F) ppm. IR ν (Nujol): ν =
˜
˜
1376 (m), 1261 (m), 1225 (m), 1029 (m), 890 (m) cm–1. (MS/EI):
m/z (%) = 1000 (83) [RuH(dppe)2(PF2Et)+], 899 (100) [RuH-
(dppe)2+].
Reproducible microanalyses could not be obtained on both com-
pounds due to the presence of variable amounts of dichlorometh-
ane of crystallization.
Crystal Data. 1·(CH2Cl2): C56H54Cl2F3O3P5RuS, M = 1190.87, or-
thorhombic, space group Pna21, a = 16.611(3), b = 27.891(6), c =
11.840(2) Å, V = 5485.3(19) Å3, Z = 4, Dc = 1.442 gcm–3, F(000) =
2440, µ(Mo-Kα) = 0.620 mm–1, 150(2) K, 11326 unique reflections
[R(int) = 0.0845], R (on F) = 0.0471, wR (on F2) = 0.1154 (I Ͼ
2σI). 2·(CH2Cl2): C55H56BCl2F6P5Ru, M = 1168.63, monoclinic,
space group P21/c, a = 13.224(3), b = 23.865(5), c = 18.624(4) Å,
β = 101.08(3)°, V = 5768(2) Å3, Z = 4, Dc = 1.346 gcm–3, F(000) =
2392, µ(Mo-Kα) = 0.557 mm–1, 150(2) K, 10139 unique reflections
[R(int) = 0.0538], R (on F) = 0.0535, wR (on F2) = 0.0538 (I Ͼ
2σI).
Figure 2. Structure of the cationic component of 2 (25% thermal
ellipsoids, dppe hydrogen atoms omitted for sake of clarity). Se-
lected bond lengths [Å] and angles [°]: Ru(1)–P(1) 2.2941(13),
Ru(1)–P(5) 2.3394(12), Ru(1)–P(3) 2.3607(13), Ru(1)–P(4)
2.3684(13), Ru(1)–P(2) 2.3783(13), P(1)–F(2) 1.583(3), P(1)–F(1)
1.610(3), P(1)–C(1) 1.811(4), C(1)–C(2) 1.518(7), F(2)–P(1)–F(1)
98.66(15), F(2)–P(1)–C(1) 103.21(19), F(1)–P(1)–C(1) 96.86(18),
P(3)–Ru(1)–P(2) 78.79(4), P(5)–Ru(1)–P(4) 84.40(4), C(2)–C(1)–
P(1) 115.6(3).
Conclusions
CCDC-670214 (for 1) and -670215 (for 2) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
We have reported the synthesis and characterization of
the first example of a complex in which methylphospha-
alkyne solely η1-coordinates a metal center. Preliminary re-
activity studies have shown that the phosphaalkyne can be
reduced to difluoroethylphosphane within the coordination
sphere of this complex. This result lays the ground-work
for further transformations of this, and other, unhindered,
metal-coordinated phosphaalkynes. Work in this area is on-
going in our laboratory.
Acknowledgments
We gratefully acknowledge financial support from the Australian
Research Council (fellowships for CJ and AS) and the EPSRC
(partial studentship for CS). Thanks also go to the EPSRC Mass
Spectrometry Service.
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Experimental Section
Synthesis of [RuH(dppe)2(η1-PϵCMe)][CF3SO3] (1): PϵCMe
(0.56 mL of a 0.34  solution in diethyl ether, 0.190 mmol) was
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a
solution of [RuH(dppe)2][CF3SO3] (100 mg,
0.101 mmol) in dichloromethane (10 mL) at 20 °C to give a yellow
solution. After 3 h volatiles were removed in vacuo and the residue
dissolved in dichloromethane (1 mL). Layering this with hexane
(10 mL) yielded 1 as yellow crystals overnight (yield 90 mg, 75%).
1
M.p. 188–190 °C. H NMR (500 MHz, C6D6, 298 K): δ = –9.6 [d
2
2
of quin, JP(dppe)H = 17, JP(PCMe)H = 127 Hz, 1 H, RuH], 2.02 (d,
3JPH = 14 Hz, 3 H, CH3) 2.10 (br., 4 H, CH2), 2.52 (br., 4 H,
CH2), 7.01–7.32 (m, 40 H, Ar-H) ppm. 31P{1H} NMR (121.6 MHz,
Eur. J. Inorg. Chem. 2008, 1555–1558
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SHORT COMMUNICATION
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Published Online: February 21, 2008
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