Ambiphilic reactivity of a ruthenaphosphaalkenyl: Synthesis of P -pyrazolylphosphaalkene complexes of ruthenium(0)

Article abstract of DOI:10.1021/om4001988

The novel ruthenaphosphaalkenyl complex [Ru{P-CH(SiMe₃)}Cl(CO) (PPh₃)₂], prepared from [RuHCl(CO)(PPh₃) ₃] and Me₃SiC-P, exhibits ambiphilic behavior, reacting at phosphorus with both nucleophiles and electrophiles. Its reaction with Li(pz′) or K[HB(pz′)₃] (pz′ = pz, pz*) affords [Ru{η¹-N:η²-P,C-P(pz′)-CH(SiMe ₃)}(CO)(PPh₃)₂], a rare example of a ruthenium(0) η²-phosphaalkene complex and the first example of a P-pyrazolylphosphaalkene. Conversely, reaction with the electrophilic PhHgCl leads to metalation at phosphorus, affording [Ru{η¹-P(HgPh)- CH(SiMe₃)}Cl₂(CO)(PPh₃)₂].

Full text of DOI:10.1021/om4001988

Communication
pubs.acs.org/Organometallics
Ambiphilic Reactivity of a Ruthenaphosphaalkenyl: Synthesis of
P‑Pyrazolylphosphaalkene Complexes of Ruthenium(0)
Nicola Trathen, Victoria K. Greenacre, Ian R. Crossley,* and S. Mark Roe
Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.
S
* Supporting Information
ABSTRACT: The novel ruthenaphosphaalkenyl complex [Ru{PCH(SiMe3)}Cl-
(CO)(PPh3)2], prepared from [RuHCl(CO)(PPh3)3] and Me3SiCP, exhibits
ambiphilic behavior, reacting at phosphorus with both nucleophiles and electrophiles.
Its reaction with Li(pz′) or K[HB(pz′)3] (pz′ = pz, pz*) affords [Ru{η1-N:η2-P,C-
P(pz′)CH(SiMe3)}(CO)(PPh3)2], a rare example of a ruthenium(0) η2-
phosphaalkene complex and the first example of a P-pyrazolylphosphaalkene.
Conversely, reaction with the electrophilic PhHgCl leads to metalation at
phosphorus, affording [Ru{η1-P(HgPh)CH(SiMe3)}Cl2(CO)(PPh3)2].
he chemistry of low-coordinate phosphorus1 continues to
develop, with an ever-increasing focus on organometallic
Ph)) react with 1 to afford the respective η1-phosphaalkene
T
complexes [R2CP(E)→RuCl(X)(PPh3)2(CO)].19
systems;2 among these, interest in the metallaphosphaalkenes,
which embody at least one metal substituent on the “PC”
unit, has grown with notable rapidity.3,4 Within this class of
compounds, particularly intriguing are the P-metallaphosphaal-
kenes (M−PCRR′; Chart 1). First described in 19855 and
As part of an extended investigation into unsaturated
phosphacarbon complexes and their derivatization, we were
interested in expanding the range of known P-metallaphos-
phaalkenyls and further exploring their chemistry. Herein, we
report the synthesis of [Ru{PCH(SiMe3)}Cl(CO)(PPh3)2]
and its unprecedented ambiphilic nature, facilitating P-
functionalization by both electrophilic and nucleophilic
reagents; the latter affords the first examples of P-
pyrazolylphosphaalkenes, which are held within the metal
coordination sphere.
Chart 1. Representative P-Metallaphosphaalkenyls
The novel ruthenaphosphaalkenyl compound [Ru{PCH-
(SiMe3)}Cl(CO)(PPh3)2] (3; Scheme 1) is conveniently
Scheme 1. Synthesis of Phosphaalkene Complexes 4 and 5
the subject of continued interest and study,6−8 these remain
relatively rare. As with their organic counterparts, such
compounds traditionally relied upon sterically encumbering
or π-dative substituents (CR2 = C(SiMe3)2, C(NMe2)2; CRR′,
R = SiMe3, R′ = Ph, R = OSiMe3, R′ = Mes, Ph, tBu) to confer
stability; however, with Hill and Jones’ report of [Ru(P
Reagents and conditions: (i) [RuHCl(CO)(PPh3)3], CH2Cl2/
toluene, 2 h; (ii) Li(pz′), thf, 1 h.
9
10
t
CHR)Cl(CO)(PPh3)2] (R = Bu (1), Ad (2)), obtained by
hydroruthenation of the respective phosphaalkynes, access to
monosubstituted and unencumbered P-metallaphosphaalkenes
was finally established. Nonetheless, few further analogues have
been reported.11−13
prepared by a modification of Hill’s method, from [RuHCl-
(CO)(PPh3)3] and excess Me3SiCP,20−22 as an air-sensitive
yellow solid.23 While 3 has not yet ceded to the growth of
crystals, its identity follows from analytical and spectroscopic
data.24 These include a characteristic 31P NMR spectrum,
Investigation of the chemistry of P-metallaphosphaalkenes
has included demonstration of their cycloadditions with
alkenes,14 alkynes,15 and azo and diazo compounds;16,17
additionally, the Pδ−Cδ+ polarization achieved within C-
amino derivatives has been exploited in protonation and
alkylation reactions.18 Significantly, comparable reactivity
toward electrophilic species has been demonstrated for 1,
which is devoid of additional π-donor substituents. Thus, the
electrophiles “EX” (MeI, AuCl(PPh3) and RHgCl (R = Fc,
which indicates retention of two PPh3 ligands (d, δP 34.6, JPP
=
8 Hz, 2P) and also exhibits a heavily deshielded resonance (dt,
δP 548.5, JPP = 8 Hz, JPH = 21 Hz, 1P), attributed to the
Received: March 11, 2013
Published: April 24, 2013
© 2013 American Chemical Society
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Organometallics
Communication
phosphaalkenyl center, which collapses to a triplet in the
31P{1H} NMR spectrum, consistent with loss of the scalar
interaction to the vinylic CH proton. The latter is observed at
7.28 ppm in the 1H NMR spectrum and identified on the basis
of its correlation with the phosphorus (δP 548.5), carbon (δC
168.0), and silicon (δSi −9.40) centers of the phosphaalkenyl
moiety. Retention of the carbonyl ligand is apparent from
infrared data (νCO 1920 cm−1), while the gross composition
was verified by elemental analysis.
phosphiranes (1.8−1.9 Å). Moreover, while the acute angles
about phosphorus (∠Ru−P−C = 62.4°, ∠N−P−C = 94.4°,
∠N−P−Ru = 81.9°) might reasonably be attributed to the
geometric constraints of the pyrazolyl bridge, those about
carbon are more clearly indicative of a geometry intermediate
between sp2 and sp3, albeit that spectroscopic data (δC 47.5; δH
1.59, 1JCH ≈ 123 Hz; cf. CH4 125 Hz30) more strongly support
an sp3 model.
The 31P NMR data are not conclusively consistent with the
ruthenaphosphirane model, since the intrinsic metal−phos-
phide linkage ought to induce appreciable deshielding of the
phosphorus center, albeit that phosphiranes typically exhibit
appreciably shielded phosphorus centers.31−33 The infrared
data are more informative, suggesting a ruthenium(0) center
(νCO ∼1900 cm−1) rather than Ru(II).34−36 We thus reason
that 5 is better formulated as an η1:η2-coordinated P-
pyrazolylphosphaalkene complex, the observed spectroscopic
data reflecting a dominant contribution from dπ → π*PC
retro-donation in phosphaalkene binding.37 Indeed, Cowley
described a similar situation for [Ni{η2-(Me3Si)2CPCH-
(SiMe3)2}(PMe3)] (dPC = 1.773(8) Å),38 the first reported
η2-phosphaalkene complex, which also exhibited an unusually
low frequency resonance for the alkenic phosphorus center (δP
23). A comparable scenario was recently noted for the only
other ruthenium η2-phosphaalkene complex, A (Chart 2; dPC
In THF solution, compound 3 reacts readily with the lithium
pyrazolates Li(pz′) (pz′ = pz*, pz) over 1 h to afford in each
case high yields (>70%) of a single phosphorus-containing
product (4 and 5, respectively), obtained in analytical purity by
recrystallization from CH2Cl2.25,26 Both 4 and 5 exhibit
characteristic 31P NMR spectra, which indicate loss of the
phosphaalkenic center, its resonance apparently being replaced
by one in the saturated region of the spectrum (60−30 ppm)
that couples to each of the, now inequivalent, PPh3 centers.
Further salient spectroscopic data include (i) 1H NMR
resonances associated with a single pz′ unit, integrating
1
consistently against PPh3, (ii) appreciably shifted H and 13C
resonances associated with the P−CH unit (vide infra), (iii) 1H
and 29Si signals indicative of the retained SiMe3 group, and (iv)
a single carbonyl stretching mode (νCO 1900 cm−1). The
identities of 4 and 5 were ultimately established from X-ray
diffraction studies, single crystals being obtained by slow
cooling of saturated CH2Cl2 solutions. While the data for 4 are
of low quality, they confirm connectivity comparable to that
adopted by 5 (Scheme 1, Figure 1).27
Chart 2. The First (Thus Far Only) η2-Phosphaalkene
Complex of Ruthenium
= 1.775(3) Å), which provides the sole structural comparison
for 5.39 With respect to the phosphaalkene unit, we note
contracted Ru−P distances for 5 (2.377(2), 2.308(6) Å)
relative to A (2.437(1) Å), while the Ru−C separations lie
within the limits of statistical comparability (2.215(6), 2.214(5)
Å; cf. A 2.182(3) Å). This can presumably be attributed to the
pyrazolyl bridge constraining the proximity of metal and
phosphorus centers, whereas A lacks any such constraint.
Interestingly, we first encountered 4 and 5 unexpectedly, as
the major products (inter alia intractable trace materials) from
the reaction of 3 with KTp′ (Tp′ = Tp, Tp*), which failed to
afford the anticipated pyrazolylborate complexes. Given the
absence of detectable levels of free pz′H within the KTp′ salts,
one must presume fragmentation of the borates, a process that
has been documented to complicate the pursuit of “Tp*Ru”
complexes.40 However, Tp is typically installed without
difficulty. Indeed, [TpRu(PCHtBu)(CO)(PPh3)] can be
obtained cleanly from 1 and KTp;41 thus, it would seem
reasonable to conclude that the alkenyl substituent (SiMe3 vs
tBu) exerts a significant influence upon reactivity, at least in the
interaction with azolylborate salts.
Figure 1. Molecular structure of 5 (50% thermal ellipsoids). The
phenyl groups are reduced and hydrogen atoms omitted for clarity.
The asymmetric unit contains two unique molecules of 5; numbering
of the second molecule (shown in the Supporting Information) is
comparable (i.e., P11 vs P21). Selected bond lengths (Å) and angles
(deg): P21−C21 = 1.793(6), P21−N21 = 1.809(5), Ru21−P21 =
2.377(2), Ru21−C21 = 2.215(6), Ru21−C22 = 1.835(5), Ru21−N22
= 2.190(4), C22−O21 = 1.154(6); P21−C21−Si21 = 116.7(3), P21−
C21−H21 = 112.8, Si21−C21−H21 = 112.8.
The core geometry of 5 might reasonably be described as a
ruthenaphosphirane, insofar as significant pyramidalization is
noted at the Calkene center (∠P−C−H = 112.8°, ∠Si−C−H =
112.8°, ∠P−C−Si = 116.7°), while the P−C linkage (1.793(6)
Å; P11−C11 = 1.779(6) Å) is appreciably lengthened from
those previously reported for phosphaalkenyls (1.65−1.75 Å).28
This distance is, however, shorter than a typical P(sp3)−C(sp3)
single bond (1.855(19) Å)29 and, indeed, those of other
Notwithstanding, for the most part, the chemistry of 3 does
seemingly mirror that of 1: in particular, the facility with which
it undergoes addition of electrophiles to afford η1-phosphaal-
kene complexes. Thus, the reaction of 3 with PhHgCl affords
[Ru{P(HgPh)CH(SiMe3)}Cl2(CO)(PPh3)2] (6) exclu-
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Communication
sively.42 The formulation of 6 follows from spectroscopic
data,43 in comparison with related systems; thus, the Palkenyl
center is significantly shifted (with respect to 3) to lower
frequency (δP 406.8) and exhibits satellite coupling to 199Hg
(JHgP = 1792 Hz), alongside a mutual interaction with the
retained PPh3 ligands (JPP = 27 Hz). Proton, carbon, and silicon
spectra confirm retention of the other functionalities, while the
mercury fragment is apparent from (i) a doublet resonance in
metal coordination sphere, viz. [Ru{η1-N:η2-P,C-P(pz′)
CH(SiMe3)}(CO)(PPh3)2] (pz′ = pz* (4), pz (5)), which
represents only the second report of η2-phosphaalkene
coordination to ruthenium(0). Addition of electrophilic
PhHgCl to 3 affords instead the mercurio-phosphaalkene
complex [Ru{P(HgPh)CH(SiMe3)}Cl2(CO)(PPh3)2] (6),
only the second such material to be structurally characterized.
We have further described the unexpected formation of 4 and 5
via fragmentation of Tp′ ligands, in contrast to the facile
generation of [TpRu(PCHtBu)(CO)(PPh3)], hinting at a
1
the 199Hg{1H} spectrum (δHg −845, JHgP = 1796 Hz) and (ii)
the observation of Hg satellites on the 13C resonance associated
with the phenyl unit. The connectivity was ultimately
confirmed by an X-ray diffraction study (Figure 2),44 suitable
t
significant influence of the alkenyl substituent (SiMe3 vs Bu)
upon reactivity, an influence that we continue to explore,
alongside the mechanistic features, implications, and wider
scope of the nucleophilic derivatization of 3.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and CIF files giving full experimental details for all
compounds, structures of both unique molecules of 5 showing
the numbering scheme, and details of the crystal structure
determinations of 5 (CCDC 910162) and 6 (CCDC 927274).
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for I.R.C.: i.crossley@sussex.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 2. Molecular geometry of 6 (50% thermal ellipsoids) in crystals
of the toluene solvate. The phenyl rings are reduced and hydrogen
atoms omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ru1−P1 2.256(2), Ru1−P2 2.399(2), Ru1−P3 = 2.411(2), Ru1−Cl1
= 2.477(2), Ru−Cl2 = 2.469(2), Ru1−C1 = 1.845(5), C1−O1 =
1.139(5), P1−C2 = 1.662(5), P1−Hg1 = 2.390(2), Hg1−C6 =
2.089(5); Ru1−P1−C2 = 126.84(19), Ru1−P1−Hg1 = 117.60(5),
Hg1−P1−C2 = 115.37(19), P1−C2−Si1 = 131.4(3).
We are grateful to one reviewer for drawing our attention to
relevant recent literature. We thank the Royal Society,
Leverhulme Trust (F/00230/AL; studentship to N.T.), and
University of Sussex (studentship to V.K.G.) for financial
support. I.R.C. gratefully acknowledges the award of a Royal
Society University Research Fellowship. We thank Dr. I. J. Day
(Sussex), Dr. M. P. Coles (UVW), and Prof. A. F. Hill (ANU)
for useful discussions.
crystals being obtained by slow cooling of a saturated CH2Cl2/
hexane solution. In common with the limited examples of
structurally characterized heterodinuclear phosphaalkenyl com-
plexes,19b,e the geometry about Ru is distorted octahedral with
interligand angles in the range 84.62(4)−93.43(4)°. The Ru−
Cl distances differ beyond 3σ, indicative of a marginally greater
trans influence for the phosphaalkenyl group (Ru−Cl1 =
2.478(2) Å) vs CO (Ru−Cl2 = 2.469(2) Å); while this is in
contrast with the case of [Ru{P(HgFc)CHtBu}Cl2(CO)-
(PPh3)2] (Fc = ferrocenyl),19e a similar scenario was described
for [Ru{P(AuPPh3)CHtBu}Cl2(CO)(PPh3)2].19b The Ru−
P1 distance (2.256(2) Å) is markedly shorter than those to the
phosphanes (2.411(2), 2.399(2) Å), as was previously observed
and attributed to appreciable π acidity of the phosphaalkene
ligand.19b Finally of note, the Hg−P(sp2) bond (2.390(2) Å) is
somewhat shorter than that reported for the ferrocenyl
derivative (2.402(1) Å),19e the only other such linkage known.
In summary, we have reported the unprecedented ambiphilic
reactivity of the novel ruthenaphosphaalkenyl complex
[Ru{PCH(SiMe3)}Cl(CO)(PPh3)2] (3) toward nucleophilic
and electrophilic reagents, both leading to functionalization of
the phosphorus center. We have thus obtained the first
examples of P-pyrazolylphosphaalkenes, stabilized within the
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Davidson, F.; McEwen, C. N.; Keys, D. E. Organometallics 2007, 26,
5050.
(40) See for example: Onishi, M. Bull. Chem. Soc. Jpn. 1991, 64, 3039.
(41) The original report of [TpRu{PCHtBu}(CO)(PPh3)]
describes it as “difficult to obtain in analytical purity” and presents
no characterization data.4 We have recently obtained this material in
purity, confirming the facility of its synthesis: Trathen, N. Unpublished
results.
(21) Averre, C. E.; Coles, M. P.; Crossley, I. R.; Day, I. J. Dalton
Trans. 2012, 41, 278.
(22) (a) Mansell, S. M.; Green, M.; Kilby, R. J.; Murry, M.; Russell,
C. A. C. R. Chem. 2010, 13, 1073. (b) Mansell, S. M.; Green, M.;
Russell, C. A. Dalton Trans. 2012, 41, 14360.
(23) Synthesis of 3. To [RuHCl(CO)(PPh3)3] (1.00 g, 1.05 mmol)
suspended in CH2Cl2 was added excess (1.2 equiv) Me3SiCP as a
solution in toluene. After 2 h, the solvent was removed under reduced
pressure and the residue thoroughly agitated with hexane. Anaerobic
filtration afforded pure 3, which was dried in vacuo. Yield: 0.8 g, 95%.
1
(24) Compound 3 was characterized by 31P, H, 13C{1H}, 29Si{1H}
(42) Synthesis of 6. Typically, a solution of 3 (200 mg, 0.248 mmol)
in CH2Cl2 (10 cm3) was added to a CH2Cl2 suspension of 1 equiv
PhHgCl. After it was stirred anaerobically for 2 h, the solution was
filtered and solvent removed in vacuo. Yield = 80%.
NMR and infrared spectroscopy, and elemental analysis. See
Supporting Information.
(25) Synthesis of 4 and 5. Typically, to 3 (200 mg, 2.48 × 10−4 mol)
suspended in thf (5 cm3) was added 1 equiv of Li(pz′) (generated in
situ); the mixture was stirred for 1 h at room temperature and the
solvent removed at reduced pressure. The residue was extracted into
CH2Cl2, the extract filtered, and the solvent removed in vacuo. Yield:
70%.
1
(43) Compound 6 was characterized by 31P, H, 13C{1H}, 29Si{1H},
199Hg{1H} NMR and infrared spectroscopy, and elemental analysis.
See Supporting Information.
(44) CCDC 927274. See Supporting Information.
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dx.doi.org/10.1021/om4001988 | Organometallics 2013, 32, 2501−2504