Cationic group 4 metallocene-(o-phosphanylaryl)oxido complexes: Synthetic routes to transition-metal frustrated Lewis Pairs

Article abstract of DOI:10.1002/ejic.201100968

Synthetic routes to cationic group 4 metallocene-(o-phosphanylaryl)oxido compounds of the type [Cp^R₂M(OPR₂)][WCA] (M = Ti, Zr, Hf; WCA = weakly coordinating anion) are described. The neutral mono-methyl complexes [Cp^R

Full text of DOI:10.1002/ejic.201100968

FULL PAPER
DOI: 10.1002/ejic.201100968
Cationic Group 4 Metallocene–(o-Phosphanylaryl)oxido Complexes: Synthetic
Routes to Transition-Metal Frustrated Lewis Pairs
Andy M. Chapman,[a] Mairi F. Haddow,[a] and Duncan F. Wass*[a]
Keywords: Zirconium / Titanium / Hafnium / Frustrated Lewis pairs
Synthetic routes to cationic group 4 metallocene–(o-phos- the case of 2 and 3, this method leads to side reactions. Treat-
∧
phanylaryl)oxido compounds of the type [CpR2M(O PR2)]- ment with B(C6F5)3 yields the desired cations in all cases;
[WCA] (M = Ti, Zr, Hf; WCA = weakly coordinating anion) however, side reactions with the generated [MeB(C6F5)3]
are described. The neutral mono-methyl complexes anion in subsequent reactions leads to problems. Hafnium
∧
∧
[CpR2ZrMe(O PR2)] 1–6 [CpR = Cp (1–3) or Cp* (4); O PR2 = analogues may be synthesised by similar routes. In the case
o-OC6H4(PtBu)2 (1 and 4), OCMe2CH2(PtBu)2 (2) or OC(CF3)2- of titanium, a different method must be adopted: chloride ab-
CH2(PtBu)2 (3)] are prepared by protonolysis of [CpR2ZrMe2] straction using [Et3Si][B(C6F5)4] from the parent complex
∧
by the parent alcohol. The remaining methyl group in such [Cp2TiCl(O PR2)]. Such cationic group 4 metallocene–(o-
complexes is best removed by protonolysis with [DTBP]- phosphanylaryl)oxido compounds exhibit reactivity that is
[B(C6F5)4] (DTBP = 2,6-di-tert-butylpyridinium) to yield the best described by the frustrated Lewis pair concept.
desired cationic complexes 7 and 8 in the case of 1 and 4. In
but also demonstrated additional reactivity, for example,
Introduction
the catalytic dehydrogenation of amine–boranes,[17] a reac-
Solution-phase combinations of sterically hindered Lewis
acid–Lewis base pairs, so-called frustrated Lewis pairs
(FLPs), have been the subject of recent interest, particularly
because of the high latent reactivity of such species in the
activation of small molecules. Initial studies have focused
on the reversible heterolytic cleavage of dihydrogen, which
offers the promise of metal-free catalytic hydrogenation.[1–4]
However, the diversity of the reactions reported is now large
and continues to grow.[5] The pioneering bulky phosphane
and fluorinated borane systems [such as PtBu3/B(C6F5)3]
first reported by Stephan and Ménard have been modified
so that the specific reactivity of FLP systems can be con-
trolled by subtle steric and electronic alterations to either
the Lewis acidic or basic components.[6,7] A great deal of
work has also focused on extending the range of main-
group FLPs to other main-group Lewis acids (e.g. simple
tion only demonstrated in a stoichiometric sense with main-
group FLP systems.[18,19] It is our view that combining the
ability of transition-metal complexes in catalysis with the
capability of FLPs to activate substrate molecules by means
of ditopic activation offers exciting possibilities for exploi-
tation in new activation pathways and reactivity patterns.
Establishing clean and robust protocols for the synthesis
of the target cationic group 4 metallocene–(o-phosphanyl-
aryl)oxido complexes was identified as a critical issue in
going on to fully exploit these species. In this article, we
explore several routes that highlight the crucial factors in
isolating these novel complexes.
Results and Discussion
Our general synthetic strategy was to access neutral pre-
R
cursor compounds of the type [CpR2Zr(Me)(O PR2)] [Cp
∧
alkyl boranes, allanes,[8,9] allenes[10]
) or bases (e.g.,
= cyclopentadienyl (Cp) or pentamethylcyclopentadienyl
amines,[11] carbenes[12] and sulfides[13]). Linking the two
components of the FLP into a single amphoteric molecule
has also led to interesting results.[14,15]
∧
(Cp*)] using the range of phosphanyl alcohols HO PR2 we
reported recently as synthons for linked phosphanyl borin-
ate ester frustrated Lewis pairs.[20] There then exists the
plethora of synthetic methods developed for cationic group
We have been exploring the chemistry of cationic zir-
conocene–(o-phosphanylaryl)oxido complexes as analogues
of linked main-group frustrated Lewis pairs in which the
Lewis acidic borane component is replaced with an electro-
philic transition-metal centre. Our initial results have estab-
lished the analogy with main-group frustrated pairs,[16,17]
4 metallocene polymerisation catalysis to abstract the
+
methyl ligand and yield the target [CpR2Zr(O PR2)] frag-
∧
ments.
Synthesis of Neutral Complexes [CpR2Zr(Me)(O PR2)]
∧
(1–6)
[a] School of Chemistry, University of Bristol,
Cantock’s Close, Bristol, BS8 1TS, United Kingdom
E-mail: duncan.wass@bristol.ac.uk
The protonolysis of [Cp2ZrMe2] by alcohols,[21–24] even
phosphanyl alcohols,[25] has already been reported. Using
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Cationic Metallocene–(o-Phosphanylaryl)oxido Complexes
this approach, the neutral precursors 1–3 were prepared in tures are best explained by the varying steric environments
good yield (Scheme 1). In all three cases, reaction of the between these different derivatives. The NMR spectra of
appropriate phosphanyl alcohol with [Cp2ZrMe2] in 1:1 these compounds supports the expectation that the phos-
stoichiometry proceeded at ambient temperature to selec- phanes are not coordinated to the zirconium centres to any
tively give the mixed alkyl/alkoxy zirconocene species.
extent in solution. In spite of this, for compound 4 detect-
able JH,P and JC,P couplings were observed in the Zr–Me
and Cp–Me groups by 13C{1H} and 1H NMR spec-
troscopy; these couplings are best explained by through-
space couplings that result from the relatively short contacts
(Figure 3), as opposed to through-bond couplings that have
been noted in similar cases where Zr–P is known to be pres-
ent both in solution and in the solid state.[27]
Scheme 1. Synthesis of 1–6.
Figure 2. POV-ray representation of the molecular structure of 3.
All hydrogen atoms and the borate anion have been omitted for
clarity. The thermal ellipsoids are drawn at the 50% probability
level.
The reaction times varied slightly and reflect the acidity
of the parent alcohol but were generally complete within
hours at ambient temperature. The same methodology was
applied to pentamethylcyclopentadienyl analogues 4–6
using [Cp*2ZrMe2]. In the case of 4, the significantly more
crowded and less electrophilic metal centre required longer
reaction times and heating to drive the reaction to comple-
tion. Compounds 5 and 6 could not be isolated cleanly, be-
ing accompanied by several side- and decomposition prod-
ucts. All of the isolated compounds are highly air- and
moisture-sensitive crystalline solids but showed no signs of
decomposition in the solid state or in solution when kept
under an inert atmosphere. Single crystals of 2–4 were ob-
tained; the X-ray crystal structures (see Figures 1, 2, and 3)
are all very similar (Table 1) and in each case the com-
pounds adopt the expected pseudotetrahedral geometry
with respect to the central Zr atom. All the Zr1–C1 dis-
tances, C1–Zr1–O1 and Cp–Zr1–Cp tilt angles (represented
by φ and θ, respectively) are within the normal range for d0
metallocenes.[26] The subtle variations between the struc-
Figure 3. POV-ray representation of the molecular structure of 4.
All hydrogen atoms and the borate anion have been omitted for
clarity. The thermal ellipsoids are drawn at the 50% probability
level.
Table 1. Comparison of some relevant structural parameters for 2–
5.
Zr1–O1 Cp–Zr1 Zr1–C1 Zr1–O1–C
[Å] [Å] [°]
[Å][a]
θ
[°]
φ
[°]
2
3
4
1.919(2) 2.245
1.9923(9) 2.300
1.955(1) 2.299
2.300(3) 169.6(2)
2.280(2) 165.78(8) 130.36 98.01(5)
2.296(2) 172.9(1) 132.74 92.16(6)
129.41 94.02(9)
Figure 1. POV-ray representation of the molecular structure of 2.
All hydrogen atoms and the borate anion have been omitted for
clarity. The thermal ellipsoids are drawn at the 50% probability
level. Selected bonds lengths and angles are given in Table 1.
[a] Mean of the two Cp–Zr1 centroid distances.
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A. M. Chapman, M. F. Haddow, D. F. Wass
FULL PAPER
+
Synthesis of the Cationic Species [CpR2Zr(O PR2)]
mixture. An analogous reaction has been noted to occur
between [CPh3]+ and related tertiary phosphanes, and it ap-
pears that when adduct formation is precluded by steric
hindrance, rapid SNAr occurs at the para position to give
zwitterionic products (Scheme 2).[40]
∧
Thanks to the ubiquity of zirconocene cations in olefin
polymerization chemistry, there are many synthetic method-
ologies available to prepare such compounds. In every case,
the cationic metals are stabilized by the inclusion of a
weakly coordinating anion (WCA). In general, this class of
anions has proved essential in the isolation of many highly
[28,29]
electrophilic main-group
and transition-metal com-
pounds.[30,31] The most common approach involves removal
of the R ligand from a [Cp2ZrR2] species. Protonolysis of
the kinetically labile Zr–alkyl bond by weak ammonium ac-
ids[32] is widely used but is reliant on the poor donor quali-
ties of the amine liberated upon protonolysis. These seem-
ingly innocent byproducts are often overlooked, but re-
cently have been shown to have a profound influence on the
reactivity of these cations.[33,34] The presence of potentially
reactive byproducts post-activation can be avoided by em-
ploying the powerful carbocationic Lewis acid [CPh3]+ to
abstract methide or hydride from [Cp2ZrR2]. The compara-
tively inert HCPh3 or MeCPh3 byproducts are not known
to coordinate to the cationic metal and are easily re-
moved.[35] In a similar way, neutral Lewis acids such as
B(C6F5)3 may be used to abstract hydride or methide. The
resulting ion pairs [Cp2ZrR][RB(C6F5)3] are often in equi-
librium with the zwitterionic complexes, [Cp2ZrR(μ-R)B-
(C6F5)3], in which the alkyl (or hydride) ligand is only par-
tially dissociated. The position of this equilibrium depends
on the ligand set but is also shown to be highly solvent-
dependent.[36] We have previously reported the viability of
the protonolysis route with our species,[17] although in place
of the standard ammonium acid [PhNMe2H][B(C6F5)4], the
novel reagent [DTBP(H)][B(C6F5)4] (DTBP = 2,6-di-tert-
butylpyridinium) was prepared to avoid the issue of pos-
sible coordination of PhNMe2.[37] Although this worked
well for the zirconocene derivatives tested to date, it has
drawbacks as a methodology since the synthesis of
[DTBP(H)][B(C6F5)4] starting from C6F5Br is tedious and
potentially dangerous.[38,39] For this chemistry, the reagent
must also be obtained in impeccable purity, which was only
possible by multiple precipitations from compatible solvents
(fluorobenzene). Another inherent problem is the very sim-
ilar solubility of the reagent to the products, making purifi-
cation challenging in the event of incomplete reaction. We
were therefore interested in exploring the use of the more
convenient, commercially available reagents for generating
such cations.
Scheme 2. Side-product formation.
Activation by Protonolysis
The apparent incompatibility of [CPh3][B(C6F5)4] and
phosphane components in these systems promoted further
investigation of routes based on protonolysis. Attempts
were made to generate the desired complexes by means of
a one-pot procedure from the appropriate [(C5R5)2ZrMe2]
reagent and the pre-formed phosphonium salt of the phos-
phanyl alcohol (Scheme 3). This route is particularly at-
tractive since only the phosphane Lewis base is present at
the end of the reaction and the byproduct is methane. Un-
fortunately, this method did not give clean reactions and
was abandoned in favour of more traditional, controlled
approaches based on protonation of the remaining alkyl
group of the pre-formed mixed alkyl/alkoxide complexes. In
chlorobenzene solvent, activation with [DTBP(H)][B-
(C6F5)4] furnishes the desired cations from 1 and 4 almost
instantaneously and cleanly; as previously reported, the Cp
compound 7 contains a Zr–P bond, whereas the Cp* ana-
logue 8 has no Zr/P interaction and is isolated as the choro-
benzene adduct. With a view to generating solvent-ligand-
free cations, a brief survey of the activation of 4 in other
solvents was carried out (Scheme 3). The protonolysis of 4
proceeded smoothly in PhF and PhBr by 31P{1H} NMR
spectroscopy resulting in the appropriate halobenzene ad-
ducts 8. However, no reaction in benzene or toluene solvent
even at 100 °C was apparent. Although solubility issues
may also be important, this suggests that the loss of meth-
ane by protonolysis is an associative process and that an
incoming ligand, even a very weak halobenzene donor, is
essential for reactivity. In this regard, it is noteworthy that
Activation with [CPh3][B(C6F5)4]
The initial investigations into the generation of the cat- compound 1 reacts smoothly to generate 7, in which the
ionic species focused on methide abstraction with [CPh3]- internal phosphane donor can coordinate, even in benzene,
[B(C6F5)4]. Unfortunately, a persistent side reaction ac- in which the resulting product is only sparingly soluble. In
companied the formation of the desired metal cations, all successful cases, it seems likely that this reaction pro-
which proved difficult to purify. We suggest this competing ceeds through the initial protonation of the phosphane-
reaction occurs between the pendant phosphane and moiety to generate a phosphonium intermediate, which
[CPh3]+, the product of which was assigned on the basis of then acts as an internal acid for the associative methane-
the 31P{1H} NMR spectrum and ESI-MS of the reaction loss step.
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Cationic Metallocene–(o-Phosphanylaryl)oxido Complexes
31P{1H} NMR spectra were identical to those of com-
pounds 7 and 8 obtained by protonolysis; this suggests that
the desired cations [Cp2Zr(OC6H4PtBu2)][MeB(C6F5)3] (7a)
and [Cp*2Zr(OC6H4PtBu2)(ClPh)][MeB(C6F5)3] (8a) have
been formed. The same methodology shows some promise
for 2 and 3, for which NMR spectroscopy signals are con-
sistent with the desired cations 9 and 10, albeit alongside
several side products that persisted during purification. We
suggest that a weak interaction with an only partially disso-
ciated methyl group bridging between Zr and B may aid
stability compared with the rapid decomposition seen after
protonation (Scheme 5).
Scheme 3. Attempted (left) and successful (right) synthesis of 7 and
8 by protonolysis of 1 and 4. Reagents and conditions: (a) [tBu2-
∧
PH OH][B(C6F5)4] (0.98 equiv.), PhF, 25–100 °C, 1–16 h; (b)
[DTBP(H)][B(C6F5)4] (0.98 equiv.), PhX (X = F, Cl or Br), 25–
100 °C, 1–16 h.
Activation of 2 and 3 by means of the same procedure
was not straightforward. In both cases, it was possible to
tentatively assign species as the anticipated cationic ana-
logues of 7. However, these compounds contain persistent
impurities that could not be removed by standard purifica-
tion methods. Jordon et al. have noted the decomposition
of the related species [Cp2ZrOtBu][B(C6F5)4] in chloroben-
zene to give [{Cp2Zr(μ-O)2}2] and poly(isobutylene).[22] We
propose a similar reaction pathway in this case (Scheme 4).
Clearly, the nature of the linker is very important in ac-
cessing stable isolable compounds, and our original o-phen-
ylene plays a critical role in this regard.
Scheme 5. Proposed heterolytic activation of H2 by 9 and 10 and
further reaction with [MeB(C6F5)3] anion.
Scheme 4. Possible routes for the decomposition of 2 (top) and 3
(bottom).
With this evidence for the cations derived from 2 and 3
in hand, we were interested to explore the reactivity of these
species with hydrogen. In many ways, the heterolytic cleav-
Activation with B(C6F5)3
As well as being commercially available and easily puri- age of hydrogen is the standard reaction for frustrated
fied by sublimation, B(C6F5)3 (and the neutral zirconocene Lewis pair-type reactivity, and we have previously reported
precursors) is highly soluble in hydrocarbon solvents which, that whilst 7 is apparently inert towards hydrogen at tem-
combined with the insolubility of the ion-pair products, peratures up to 80 °C, 8 rapidly and irreversibly cleaves the
provides an effective purification route. Addition of solu- H–H bond. With both 9 and 10 the reaction with hydrogen
tions of 1 or 4 in toluene or hexane to B(C6F5)3 resulted in is not straightforward or clean; however, NMR spec-
the immediate precipitation of a bright yellow oil that was troscopy data is consistent with quantitative formation of
isolated by washing with hexane and drying under vacuum the known anion [HB(C6F5)3]. One possibility is a reaction
to yield a light yellow powder. When redissolved in PhCl or sequence involving methane loss by proton transfer to
PhF, the 11B and 19F NMR spectra were consistent with [MeB(C6F5)3] from a postulated phosphonium cation, fol-
the clean formation of the [MeB(C6F5)3] anion, and lowed by hydride abstraction from the Zr–H fragment by
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A. M. Chapman, M. F. Haddow, D. F. Wass
FULL PAPER
+
the generated B(C6F5)3 (Scheme 5). This unexpected reac- Synthesis of [CpR2Ti(O PR2)] (14)
∧
tivity highlights the crucial role of the anion as well as the
zirconocene fragment, but unfortunately rules out the use
of B(C6F5)3 as an activating agent with these species.
+
For the titanium analogue [CpR2Ti(O PR2)] (14; R =
H), the route analogous to that used for the Zr or Hf deriv-
atives was complicated by the very different reactivity of
[Cp2TiMe2] (Petasis’s reagent).[43] Critically, despite being
thermodynamically much less stable than their heavier
homologues (particularly in the solid state),[42] the Ti–C
bonds are kinetically very robust and do not readily un-
dergo protonolysis. The bonds are so stable that the stan-
dard protocol used to synthesize [Cp2TiMe2] involves an
aerobic, aqueous workup. Only upon extended thermolysis
in the presence of alcohols can mixed alkyl/alkoxide com-
pounds be accessed.[44]
∧
+
Synthesis of [CpR2Hf(O PR2)] (12)
∧
To extend this chemistry to the other group 4 elements,
the hafnium compound 12 was synthesized in an identical
fashion to 7 via the neutral intermediate 11 (Scheme 6).[16]
Although both steps proceeded smoothly, extended reaction
times were required. Both the solid-state structure and the
solution 31P{1H} NMR spectra confirmed the presence of
the anticipated Hf–P bond. The X-ray structure of 12 (Fig-
ure 4) is very similar to that of 7; it shows the expected
pseudotetrahedral geometry at Hf and a puckered five-
membered ring with almost identical distances and angles
to those in 7. Of interest is the marginally (ca. 2.5%) con-
tracted Hf–P distance relative to the Zr–P distance in 7. In
light of their similar size, electronegativities and virtually
identical Cp–M distances and Cp–M–Cp tilt angles, it is
not clear why the Hf–P bond is shorter, although shorter
Hf–L bonds are often observed in isostructural Zr and Hf
complexes.[41,42]
In an effort to circumvent these problems, the mixed alk-
oxy/chloride compound 13 was prepared as an alternative
synthon. Complex 13 is very stable and was isolated in high
(82%) yield and purity by means of a modification of a
published protocol.[25] Halide abstraction using [Ag(C6H6)3]-
[B(C6F5)4] was then attempted (Scheme 7). Upon mixing
solutions of 13 and [Ag(C6H6)3][B(C6F5)4] in chloro-
benzene, an encouraging silvery-grey solid was immediately
precipitated; however, analysis of the reaction mixture by
31P NMR spectroscopy and ESI-MS revealed an almost
quantitative conversion to the water addition product 15 (δ
1
= 21 ppm, d, JP,H = 469 Hz, m/z = 433.37) rather than 14,
presumably caused by traces of water carried through from
the synthesis of [Ag(C6H6)3][B(C6F5)4]. This reagent is pre-
pared in aqueous media and only dried by means of K2SO4
and under vacuum.[45] The requirement for stringently an-
hydrous halide-abstraction reagents led us to the cation
[Et3Si][B(C6F5)4], prepared by reaction with [CPh3]-
[B(C6F5)4] in neat Et3SiH.[28] Addition of this reagent to 13
in PhCl resulted in an immediate colour change from deep
red to almost black and the 31P NMR spectrum revealed
80% conversion to 14. The removal of side products neces-
sitated repeated recrystallisations, thereby resulting in a
moderate 30% isolated yield. Crystals suitable for X-ray
Scheme 6. Synthesis of 11 and 12. Reagents and conditions: (a)
[Cp2HfMe2] (1 equiv.), hexanes, 25 °C, 1 d; (b) [DTBP(H)][B-
(C6F5)4] (0.98 equiv.), PhCl, 25 °C, 2 d.
Figure 4. POV-ray drawing of the molecular structure of 12.[16] Hy-
drogen atoms and the borate anion have been omitted for clarity.
The thermal ellipsoids are drawn at the 50% probability level. Se-
Scheme 7. Synthetic routes employed during the synthesis of 14.
Reagents and conditions. (a) [Cp2TiCl2] (1 equiv.), Et2NH
lected bond lengths [Å]: Hf1–O1 1.989(2), Hf1–P1 2.8209(6), C16– (10 equiv.), THF; (b) [Ag(C6H6)3][B(C6F5)4] (1 equiv.), trace H2O,
P1 1.824(3), O1–C11 1.366(3), Cp–Hf1 2.185. Angles [°]: O1–Hf1–
C11 121.7(2), O1–Hf1–P1 71.16(5), Cp–Hf–Cp 128.07.
PhCl, 25 °C, 5 min; (c) [Et3Si][B(C6F5)4] (0.98 equiv.), PhCl, 25 °C,
5 min.
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Cationic Metallocene–(o-Phosphanylaryl)oxido Complexes
diffraction were obtained and the identity of 14 was unam- side reactions. With reliable synthetic protocols in hand,
biguously confirmed (Figure 5). The expected Ti–P interac- studies are ongoing to explore the reactivity of these species
tion is also implied by the significant 31P{1H} NMR spec- as transition-metal-containing frustrated Lewis pairs.
troscopy shift relative to 13 that was observed upon form-
ing the cation. At 2.752 Å, the Ti–P distance is shorter than
Experimental Section
its second-row cousin 4 by approximately 4%, which is mir-
rored by the significantly larger down-field shift of the
phosphane in the 31P{1H} NMR spectrum relative to this
starting material. A comparison of key structural param-
eters for the Ti, Zr and Hf compounds 14, 7 and 12 is given
in Table 2.
General Remarks: Unless otherwise stated, all manipulations were
carried out under an inert atmosphere of argon using standard
Schlenk-line and glovebox (M-Braun, O2 Ͻ 0.1 ppm, H2O Ͻ
0.1 ppm) techniques, and all glassware was oven-dried (200 °C)
overnight and allowed to cool under vacuum prior to use.
tBu2P(C6H4)OH, tBu2P[CH2C(Me)2]OH, tBu2P[CH2C(CF3)2]OH
and [DTBP][B(C6F5)4] were synthesized as described previously by
us.[16,20] The standard reagents [Ag(C6H6)3][B(C6F5)4] and [Et3Si]-
[B(C6F5)4] were synthesized by published methods.[46,28] Com-
pounds 1, 4, 7 and 8 have been previously reported. Solvents were
purified and pre-dried using an Anhydrous Engineering column
purification system then vacuum-transferred from the appropriate
drying agent (K/benzophenone for aromatics, ethers; CaH2 for hy-
drocarbons and chlorinated solvents) prior to use. NMR spectra
were recorded using a JEOL ECP 300 spectrometer at 300 MHz,
and Varian 400 and 500 spectrometers at 400 and 500 MHz, respec-
tively (using the appropriate deuterated solvent, purchased from
Cambridge Isotope Labs or Sigma–Aldrich and purified by vac-
uum-transfer from the appropriate desiccant) and referenced to an
internal standard (residual solvent signal for 1H; BF3·OEt2 for 11B;
85% H3PO4 for 31P; and FCCl3 for 19F NMR spectroscopy). Spec-
tra of air- and moisture-sensitive compounds were recorded using
sealable J-Youngs tap NMR spectroscopy tubes. Microanalysis was
carried out by the Microanalytical Laboratory, University of Bris-
tol, using a Carlo–Elba spectrometer.
Figure 5. POV-ray drawing of the molecular structure of 14. All
hydrogen atoms and the borate anion have been omitted for clarity.
The thermal ellipsoids are drawn at the 50% probability level. Se-
lected bond lengths [Å]: Ti1–P1 2.785(2), Ti1–O1 1.901(5), O1–C11
1.358(7), C16–P1 1.812(7), Cp–Ti1 2.056. Angles [°]: P1–Ti1–O1
71.5(1), Ti1–O1–C11 122.5(4), O1–C11–C16 119.6(5), C11–C16–
P1 113.0(4), C16–P1–Ti1 89.3(2), Cp–Ti–Cp 129.28.
Synthesis of 2: [Cp2ZrMe2] (157.4 mg, 0.63 mmol) and
tBu2P[CH2C(Me)2]OH (136.7 mg, 0.63 mmol) were weighed out
into two small vials, and each compound was dissolved in hexane
(4 mL). The solutions were mixed in a Schlenk flask, including
small hexane washes of the vials, which resulted in vigorous gas
evolution. The flask was allowed to stand for 16 h and 31P{1H}
NMR spectra of the solution showed Ͼ99% conversion to 2. The
flask was removed from the glovebox and cooled in stages to
–78 °C, thereby resulting in the precipitation of colourless crystals.
The supernatant was removed by using a cannula and the solids
were dried under high vacuum for approximately 1 h before being
isolated inside the glovebox as colourless crystals; yield 255 mg,
0.56 mmol, 89%. 1H NMR (300 MHz, [D8]toluene): δ = 5.82 (s, 10
Table 2. Comparison of some relevant structural parameters for 14,
7 and 12.
M–O
[Å]
Cp–M
[Å][a]
M–O–C
[°]
θ
[°]
φ
[°]
14 (Ti)
7 (Zr)
12 (Hf)
1.901(5)
1.997(2)
1.989(2)
2.06
2.20
2.19
122.5(4)
124.51
121.7(2)
129.3
128.2
128.1
71.5(1)
70.18(4)
71.16(5)
[a] Mean of the two M–Cp centroid distances.
2
H, C5H5), 1.55 (d, JH,P = 5.1 Hz, 2 H, CH2), 1.22 [s, 6 H,
3
C(CH3)2], 1.11 [d, JH,P = 10.6 Hz, 18 H, C(CH3)3], 0.27 (s, 1 H,
Conclusion
ZrCH3) ppm. 13C{1H} NMR (100 Hz, [D8]toluene): δ = 110.8 (s,
C5H5), 80.9 [d, 2JC,P = 25.7 Hz, C(CH3)2], 37.4 (d, 1JC,P = 26.5 Hz,
A series of neutral and cationic group 4 phosphanylaryl-
oxy–metallocene complexes have been prepared. The meth-
odology chosen to form the cation is critical in obtaining a
clean product. In general, protonation of [CpR2Zr(Me)-
3
2 H, CH2), 32.0 (d, JC,P = 4.6 Hz, CH3), 31.9 [partial d,
2
C(CH3)3], 29.6 [d, JC,P = 13.3 Hz, C(CH3)3], 18.81 (s, ZrCH3)
ppm. 31P{1H} NMR (161 MHz, [D8]toluene): δ = 22.0 (s) ppm.
ESI-MS: 453.15 [M + H]+. Elemental analysis: calcd. C 60.88, H
8.66; found C 60.58, H 8.73.
∧
(O PR2)] by [DTBP][B(C6F5)4] gives the best results. Anal-
ogous hafnium compounds are accessed in the same way.
B(C6F5)3 may also be used but this leads to complications
in subsequent reactivity through exchange of the methyl
group on the [MeB(C6F5)3] anion formed post-activation.
Titanium derivatives require a different route: chloride ab-
Synthesis of 3: Prepared in an analogous fashion to 2 using
[Cp2ZrMe2] (297.8 mg, 1.19 mmol) and tBu2P[CH2C(CF3)2]OH
(387.5 mg, 1.19 mmol). Colourless crystals; yield 470 mg,
1
0.84 mmol, 71%. H NMR (300 Hz, [D8]toluene): δ = 5.92 (s, 10
2
3
H, C5H5), 2.02 (d, JH,P = 3.3 Hz, 2 H, CH2), 1.04 [d, JH,P
=
straction from [CpR2Ti(Cl)(O PR2)] by [Et3Si][B(C6F5)4].
∧
11.0 Hz, 18 H, C(CH3)3], 0.54 (s, 1 H, ZrCH3) ppm. 13C{1H} NMR
(100 Hz, [D8]toluene): δ = 125.16 (q, 1JC,F = 291.1 Hz, CF3), 112.3
The linker group used in these compounds is critical; o-
phenoxy moieties give highly air/moisture sensitive but
2
2
(s, C5H5), 83.7 [d. sept, JC,P = 27.4 Hz, JC,F = 15.3 Hz,
1
2
otherwise robust complexes, whereas other linkers lead to C(CF3)2], 32.3 [d, JC,P = 23.6 Hz, C(CH3)3], 30.6 [d, JC,P
=
Eur. J. Inorg. Chem. 2012, 1546–1554
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1551

A. M. Chapman, M. F. Haddow, D. F. Wass
FULL PAPER
1
15.0 Hz, C(CH3)3], 25.1 (d, JC,P = 34.8 Hz, CH2), 27.9 (br. s,
PhF/[D8]toluene): δ = –15.1 (s) ppm. 19F NMR (470 Hz, PhF/[D8]-
3
3
ZrCH3) ppm. 19F NMR (376 Hz, [D6]benzene): δ = –74.4 (d, JF,P
toluene): δ = –131.8 (d, JF,F = 19.1 Hz, 6 F, ortho-F), –164.1 (t,
= 14.6 Hz) ppm. 31P{1H} NMR (161 Hz, [D6]benzene): δ = 16.54 3JF,F = 20.8 Hz, 4 F, para-F), –166.6 (m, 6 F, meta-F) ppm.
(septet, JP,F = 16.4 Hz) ppm. ESI-MS: sample decomposed. Ele-
Again for 3, an inseparable mixture of products was obtained albeit
mental analysis: calcd. C 49.18, H 5.92; found C 49.14, H 5.99.
with some data that can be tentatively assigned to 10: 1H NMR
Attempted Activation with [CPh3][B(C6F5)4]: Compound 1 (0.05–
0.1 mmol) and [CPh3][B(C6F5)4] (0.05–0.1 mmol) were weighed out
into two vials, and each compound was dissolved in PhF (1–2 mL).
The [CPh3][B(C6F5)4] solution was added dropwise to the solution
of the complex over 10 min with rapid stirring, including small PhF
washes of the vial. The contents of the vial were transferred to an
NMR spectroscopy tube and analysed by 31P{1H} NMR spec-
troscopy and ESI-MS. The 31P{1H} NMR spectra showed Ͼ99%
conversion of 1 to a new species assigned as the product of SNAr
at the para position in [CPh3]. 31P{1H} NMR (161 Hz, PhF/
[D8]toluene): δ = 42.5 (s) ppm. ESI-MS: 717.24 [M]+.
(300 MHz, PhF/[D8]toluene): δ = 6.02 (s, 10 H, C5H5), 2.28 (br. s,
3
CH2), 1.05 [s, 6 H, C(CH3)2], 1.03 (br. s, BCH3), 0.87 [d, JH,P
=
13.2 Hz, 18 H, C(CH3)3] ppm. 31P{1H} NMR (161 Hz, PhF/[D8]-
toluene 5:1): δ = 45 (v br. s) ppm. 11B{1H} NMR (96 Hz, PhF/[D8]-
toluene): δ = –15.1 (s) ppm. 19F NMR (470 Hz, PhF/[D8]toluene):
3
3
δ = –131.8 (d, JF,F = 19.1 Hz, 6 F, ortho-F), –164.1 (t, JF,F
20.8 Hz, 4 F, para-F), –166.6 (m, 6 F, meta-F) ppm.
=
Attempted Activation of Hydrogen with 9 and 10: Both procedures
were performed in an identical fashion and only that of 9 is re-
ported here. A solution of 9 in PhF/[D8]toluene was prepared as
above in an NMR spectroscopy tube fitted with a Teflon needle
valve. The tube was removed, connected to a Schlenk line and sub-
jected to three freeze–pump–thaw degassing cycles then back-filled
with 1 bar hydrogen at room temperature by means of a liquid-
nitrogen trap. The solution immediately became colourless. The
course of the reaction was monitored by NMR spectroscopy and
revealed that a complex mixture of products was obtained. After
several days a light green oil was observed to separate from solu-
tion. A similar set of observations was made using 10. In both
cases, the anion [MeB(C6F5)3] was fully converted into the anion
[HB(C6F5)3] after standing overnight: 11B NMR (96 Hz, PhF/[D8]-
Reaction of 2 and 3 with [DTBP(H)][B(C6F5)4]: Typically, 2 or 3
(0.05–0.1 mmol) and [DTBP(H)][B(C6F5)4] (0.05–0.1 mmol) were
weighed out into two vials, and each compound was dissolved in
PhF (1–2 mL). The [DTBP(H)][B(C6F5)4] solution was added drop-
wise to the solution of the complex, including small PhF washes
of the vial. The contents of the vial were transferred to an NMR
spectroscopy tube and analysed by 31P{1H} NMR spectroscopy
and ESI-MS.
For compound 2, the 31P{1H} NMR spectra of the reaction mix-
ture indicated approximately 90% conversion to 9 (s, 61.7 ppm)
along with approximately 10% to a unidentified species (δ =
24.7 ppm, br. s). Attempted isolation of the reaction mixture by
repeated precipitation into hexanes or crystallisation at low tem-
1
toluene): δ = –15.1 (d, JB,H = 79.9 Hz) ppm. 19F NMR (470 Hz,
3
PhF/[D8]toluene): δ = –134.2 (d, JF,F = 20.1 Hz, 6 F, ortho-F),
3
–165.0 (t, JF,F = 20.4 Hz, 4 F, para-F), –167.7 (m, 6 F, meta-F)
ppm.
1
perature was unsuccessful. H NMR (300 MHz, PhF/[D8]toluene):
Synthesis of Compound 13: [Cp2TiCl2] (248.9 mg, 1.0 mmol) and
tBu2P(C6H4)OH (238.3 mg, 1.0 mmol) were each weighed into a
Schlenk flask and suspended in THF (10 mL) and an excess
amount of HNEt2 (ca. 0.5 mL). The suspension was left to stir
overnight. The red-brown suspension was diluted with hexane
(10 mL), then filtered through a frit into a clean Schlenk tube,
along with several hexane washings of the filter cake. The solvent
was concentrated to approximately 5 mL, which caused a red
microcrystalline precipitate to form. The flask was left to stand
at –20 °C overnight to induce further precipitation. The pale red
supernatant was removed by using a cannula and the solids were
washed with hexane before being dried under vacuum. Compound
13 was obtained as a red microcrystalline solid; yield 401.2 mg,
0.89 mmol, 89%. 1H NMR (400 Hz, [D6]benzene): δ = 7.60 (dt,
δ = 5.92 (s, 10 H, C5H5), 1.99 (s, CH2), 1.10 [s, 6 H, C(CH3)2], 0.90
3
[d, JH,P = 13.1 Hz, 18 H, C(CH3)3] ppm. 31P{1H} NMR (161 Hz,
PhF/[D8]toluene): δ = 61.70 (s) ppm. ESI-MS: (accurate mass)
437.1534 [M]+.
For compound 3, the 31P{1H} NMR spectra of the reaction mix-
ture indicated approximately 80% conversion to 10 (δ = 62.7 (s)
ppm) along with numerous other unidentified species [δ = 45.8 (s),
37.3 (s), 33.03 (s), 23.5 (s) ppm]. Attempted isolation of the reaction
mixture by repeated precipitation into hexanes or crystallisation at
low temperature were unsuccessful. 1H NMR (300 MHz, PhF/[D8]-
3
toluene): δ = 6.02 (s, 10 H, C5H5), 2.01 (br. s, CH2), 0.93 [d, JH,P
= 11.6 Hz, 18 H, C(CH3)3] ppm. 31P{1H} NMR (161 Hz, PhF/[D8]-
toluene): δ = 62.7 (s) ppm. 19F NMR (470 Hz, PhF/[D8]toluene): δ
= –75.3 ppm (s, CF3) {data for [B(C6F5)4] anion not reported}.
ESI-MS: not observed.
4
3
3JH,H = 7.6 Hz, JH,H = 1.9 Hz, 1 H, HC6) 7.49 (ddd, JH,H
=
4
4
8.2 Hz, JH,H = 5.0 Hz, JH,H = 1.3 Hz, 1 H, HC4), 7.19 (dt, ddd,
3JH,H = 8.7 Hz, JH,H = 7.2 Hz, JH,H = 1.8 Hz, 1 H, HC5), 6.79
4
4
Activation with B(C6F5)3: The same general method was used for
1, 2, 3 and 4 and is described here for 1. B(C6F5)3 (0.05 mmol)
and 1 (0.05 mmol) were weighed out into two small vials, and each
compound was dissolved in hexane (4 mL). The solutions were
mixed in a Schlenk flask, including small hexane washes of the
vials, thereby resulting in the precipitation of a yellow oil. The su-
pernatant was decanted and the oil was washed with further por-
tions of hexane before being dried under vacuum to give a tacky
yellow solid. NMR spectroscopy revealed both 7a and 8a to have
identical data to the cation component of 7 and 8, respectively.
3
4
4
(overlapping ddd, JH,H = 8.7 Hz, JH,H = 7.5 Hz, JH,H = 1.3 Hz,
1 H, HC3), 6.15 (s, 10 H, C5H5), 1.20 [d, JH,P = 11.5 Hz, 18 H,
3
C(CH3)3] ppm. 13C{1H} NMR (100 Hz, [D6]benzene): δ = 176.7
(d, 3JC,P = 22.6 Hz, C1), 135.4 (d, 3JC,P = 3.4 Hz, C6), 131.2 (s, C4),
123.2 (d, JC,P = 21.8 Hz, C2), 120.0 (s, C5), 119.1 (d, JC,P
=
1
2
3.1 Hz, C3), 118.07 (s, C5H5), 32.9 [d, JC,P = 24.1 Hz, C(CH3)3],
1
31.4 [d, JC,P = 15.6 Hz, C(CH3)3] ppm. 31P{1H} NMR (161 Hz,
2
[D6]benzene): δ = 11.7 (s) ppm. ESI-MS: 451.14 [M + H]+. Elemen-
tal analysis: calcd. C 63.94, H 7.15; found C 64.05, H 7.46.
In the case of 2, an inseparable mixture of products was obtained
albeit with some data that can be tentatively assigned to 9: 1H
Attempted Synthesis of 14 by Halide Abstraction with [Ag-
(C6H6)3][B(C6F5)4]: A sample of 13 (22.6 mg, 0.05 mmol) was
NMR (300 MHz, PhF/[D8]toluene): δ = 5.90 (s, 10 H, C5H5), 1.98 loaded into an NMR spectroscopy tube fitted with a Teflon needle
(br. s, CH2), 1.05 [s, 6 H, C(CH3)2], 0.95 (br. s, BCH3), 0.90 [d,
3JH,P = 12.7 Hz, 18 H, C(CH3)3] ppm. 31P{1H} NMR (161 Hz,
PhF/[D8]toluene 5:1): δ = 54 (v br. s) ppm. 11B{1H} NMR (96 Hz,
valve and dissolved in PhCl (0.7 mL) to give a bright red solution.
Solid [Ag(C6H6)3][B(C6F5)4] was added in one portion, thereby im-
mediately lightening the colour of the solution and producing a
1552
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1546–1554

Cationic Metallocene–(o-Phosphanylaryl)oxido Complexes
silvery-grey precipitate. The 31P{1H} NMR spectra of the solution
revealed approximately 90% conversion to a new species. The sam-
ple was returned to the glovebox and an aliquot was removed for
The structures were solved by direct methods or Patterson methods
in XS[49] and structures were refined against all F2o data with hydro-
gen atoms on carbon atoms riding in calculated positions using
SHELXTL.[50] The crystallographic details for compounds 2, 3, 4
analysis by ESI-MS. 31P{1H} NMR (161 Hz, unlocked, PhCl): δ =
1
21.0 ppm. 31P NMR (161 Hz, unlocked, PhCl): δ = 21.0 (d, JP,H and 14 are given in Tables 3 and 4).
= 469 Hz) ppm. ESI-MS: 433.37 [M]+.
Table 4. Crystallographic details for 4 and 14.
Synthesis of 14: Compound 13 (45.1 mg, 0.1 mmol) and [Et3Si]-
4
14
[B(C6F5)4] (71.5 mg, 0.9 mmol) were each weighed into vials and
each compound was dissolved in PhCl (ca. 1 mL). The two solu-
tions were quickly mixed resulting in a dark brown/black solution.
The solution stood for 30 min before being precipitated by being
added dropwise to rapidly stirred hexanes (ca. 5 mL). It was neces-
sary to repeat this procedure four more times to obtain a solid.
The resulting black solid was dissolved in PhCl (ca. 1 mL) and
carefully layered with hexanes (ca. 2 mL). This was allowed to
stand overnight precipitating some black crystals and a black oil.
The crystals were carefully removed from the vial and were washed
with hexane and dried under vacuum. Black/purple needles; yield
Colour, habit
Size [mm]
Empirical formula
Mr
colourless block
0.42ϫ0.18ϫ0.16
C35H55OPZr
613.98
orange needles
0.19ϫ0.06ϫ0.05
C48H32BF20OPTi
1094.42
Crystal system
Space group
a [Å]
b [Å]
c [Å]
monoclinic
P21/c
12.9016(2)
15.6429(3)
16.8153(3)
90.00
monoclinic
P2(1)/c
11.191(2)
17.624(4)
22.999(5)
90.00
α [°]
β [°]
106.9070(10)
90.00
3246.96(10)
4
0.413
100
103.71(3)
90.00
4406.8(15)
4
0.355
100
1
18.4 mg, 0.041 mmol, 45%. H NMR (500 Hz, PhCl/[D6]benzene,
γ [°]
5:1): HC3, HC4 and HC6 aromatic H signals are obscured by PhCl
V [Å3]
Z
signals and could not be unambiguously identified. δ = 6.27 (dd,
μ [mm–1]
T [K]
4
3JH,H = 8.2 Hz, JH,H = 4.4 Hz, 1 H, HC5), 5.87 (s, 5 H, C5H5),
3
0.99 [d, JH,P = 13.6 Hz, 18 H, C(CH3)3] ppm. 13C{1H} (125 Hz,
θmin,max
1.65, 27.52
1.47, 27.54
2
PhCl/[D6]benzene, 5:1): δ = 170.6 (d, JC,P = 15.6 Hz, C1), 133.2
Completeness
0.997 to θ = 27.52° 0.994 to θ = 27.54°
(d, 4JC,P = 2.0 Hz, C5), 132.6 (d, 3JC,P = 2.5 Hz, C4), 124.2 (d, 1JC,P
= 28.4 Hz, C2), 123.0 (d, 3JC,P = 4.4 Hz, C6), 116.6 (s, C5H5), 115.6
Reflections: total/independent 31401/7441
38243/10103
0.0843
Rint
0.0230
2
1
(d, JC,P = 4.9 Hz, C3), 39.4 [d, JC,P = 3.4 Hz, C(CH3)3], 30.2 [d,
2JC,P = 3.9 Hz, C(CH3)3] ppm. 31P{1H} (161 Hz, PhCl/[D6]benz-
ene): δ = 70.2 (s) ppm. ESI-MS: 415.17 [M]+. Elemental analysis:
calcd. C 52.68, H 2.95; found C 52.52, H 3.27.
Final R1 and wR2
Largest peak, hole [eÅ–3]
ρcalcd. [gcm–3]
0.0249, 0.0643
0.424, –0.318
1.256
0.0841, 0.2603
1.865, –0.894
1.650
Crystallographic Information: X-ray diffraction experiments on all
crystals were carried out at 100 K on a Bruker APEX II dif-
fractometer using Mo-Kα radiation (λ = 0.71073 Å). The data col-
lections were performed using a CCD area detector from a single
crystal mounted on a glass fibre. Intensities were integrated[47] from
several series of exposures measuring 0.5° in ω or φ. Absorption
corrections were based on equivalent reflections using SADABS.[48]
CCDC-843725 (for 2), -843726 (for 3), -822763 (for 4), and -843727
(for 14) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Acknowledgments
The University of Bristol is thanked for funding.
Table 3. Crystallographic details for 2 and 3.
2
3
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130, 14117.
Colour, habit
Size [mm]
Empirical formula
Mr
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
colourless block
0.22ϫ0.15ϫ0.10
C23H39OPZr
453.73
monoclinic
P21/c
9.3758(4)
13.7878(5)
36.7863(14)
90.00
90.040(2)
90.00
4755.4(3)
8
0.539
100
1.11, 27.48
colourless block
0.38ϫ0.33ϫ0.22
C23H33F6OPZr
561.68
tetragonal
I4
23.7172(2)
23.7172(2)
8.7436(1)
90.00
90.00
90.00
4918.32(8)
8
0.570
100
1.21, 30.53
¯
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2011, 30, 4497.
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γ [°]
V [Å3]
Z
μ [mm–1]
T [K]
θmin,max
Completeness
0.989 to θ = 27.48° 0.999 to θ = 30.53°
Reflections: total/independent 65530/10780
25509/7462
0.0187
0.0193, 0.0507
0.368, –0.292
1.517
Rint
0.1058
Final R1 and wR2
Largest peak, hole [eÅ–3]
ρcalcd. [gcm–3]
0.0357, 0.0790
1.400, –1.054
1.268
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www.eurjic.org
1553

A. M. Chapman, M. F. Haddow, D. F. Wass
FULL PAPER
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Received: September 12, 2011
Published Online: December 9, 2011
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