Synthesis and structure of the hafnium alkylidene complex [P2Cp]Hf=CHPh(Cl) ([P2Cp] = (η5-C5H3-1,3- (SiMe2CH2PPr2i)2))

Article abstract of DOI:10.1021/om000760i

The synthesis, structural characterization, and solution behavior of hafnium complexes stabilized by the potentially tridentate ancillary ligand [P₂Cp] ([P₂Cp] = (η⁵-C₅H₃-1,3- (SiMe₂-CH₂PPr₂ⁱ)₂)) are described. The reaction of [P₂Cp]Li with HfCl₄(THT)₂ produces the hafnium trichloride complex [P₂Cp]HfCl₃ (1), the structure of which was determined by X-ray crystallography. Trichloride 1 is isostructural with the analogous zirconium complex [P₂Cp]ZrCl₃ (2) in the solid state, but in solution 1 exists as an equilibrium mixture of two isomers that interconvert by fluxional phosphine coordination. Treatment of 1 with 2 equiv of KCH₂Ph, followed by thermolysis, yields the first structurally characterized hafnium alkylidene complex, [P₂Cp]Hf=CHPh(Cl) (3). A crystal structure determination obtained for 3 shows this complex to be isostructural with the zirconium analogue [P₂Cp]Zr=CHR(Cl) (4). The primary difference between the Hf systems presented here and the previously studied Zr analogues is that metal-ligand bonding is stronger in the former, which accounts for shorter bond distances, a greater degree of chemically inertness, and the divergent solution behaviors observed for the trichloride derivatives 1 and 2.

Full text of DOI:10.1021/om000760i

1608
Organometallics 2001, 20, 1608-1613
Syn th esis a n d Str u ctu r e of th e Ha fn iu m Alk ylid en e
Com p lex [P 2Cp ]HfdCHP h (Cl) ([P 2Cp ] )
(η5-C5H3-1,3-(SiMe2CH2P P r i ) ))
2 2
Michael D. Fryzuk,* Paul B. Duval, Brian O. Patrick,† and Steven J . Rettig†
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver,
British Columbia, Canada V6T 1Z1
Received September 1, 2000
The synthesis, structural characterization, and solution behavior of hafnium complexes
stabilized by the potentially tridentate ancillary ligand [P2Cp] ([P2Cp] ) (η5-C5H3-1,3-(SiMe2-
CH2PPri ) )) are described. The reaction of [P2Cp]Li with HfCl4(THT)2 produces the hafnium
2 2
trichloride complex [P2Cp]HfCl3 (1), the structure of which was determined by X-ray
crystallography. Trichloride 1 is isostructural with the analogous zirconium complex [P2-
Cp]ZrCl3 (2) in the solid state, but in solution 1 exists as an equilibrium mixture of two
isomers that interconvert by fluxional phosphine coordination. Treatment of 1 with 2 equiv
of KCH2Ph, followed by thermolysis, yields the first structurally characterized hafnium
alkylidene complex, [P2Cp]HfdCHPh(Cl) (3). A crystal structure determination obtained for
3 shows this complex to be isostructural with the zirconium analogue [P2Cp]ZrdCHR(Cl)
(4). The primary difference between the Hf systems presented here and the previously studied
Zr analogues is that metal-ligand bonding is stronger in the former, which accounts for
shorter bond distances, a greater degree of chemically inertness, and the divergent solution
behaviors observed for the trichloride derivatives 1 and 2.
In tr od u ction
conducted on zirconium complexes stabilized by the
ancillary ligand [P2Cp] ([P2Cp] ) (η5-C5H3-1,3-(SiMe2-
Among the transition metals, zirconium and hafnium
comprise two congeners whose chemical properties in
some instances seem virtually indistinguishable. The
similarities are largely attributed to the nearly identical
ionic radii of Zr4+ and Hf4+ (1.45 Å), arising from Hf
being the first element that succeeds the lanthanides
and the associated lanthanide contraction.1 Where
chemical differences have been noted, these usually
relate to stronger metal-ligand σ-bonding in the Hf
complexes due to 6s orbital contraction emanating from
relativistic effects. In general, the resulting trend is for
Hf to be relatively inert in comparison to Zr and for
comparatively shorter bond distances in the Hf com-
plexes. Given the similar size of the two metals, this
latter effect can induce steric crowding in the Hf
coordination sphere when bulky ligands are employed,
leading to structural modifications with respect to the
analogous Zr complex. One example is illustrated by the
homoleptic tetra(cyclopentadienyl) derivatives Cp4M (M
) Zr,2 Hf 3), where only two of the Cp ligands coordinate
with η5 hapticity to Hf, in comparison to three η5-Cp
ligands observed for the Zr species.
CH2PPri ) )). Starting from the precursor trichloride
2 2
complex [P2Cp]ZrCl3 (2), a variety of alkyl complexes
of the general formula [P2Cp]ZrCl3-xRx (x ) 1, 2, 3; R )
Me, CH2Ph, CH2SiMe3) have been prepared.4,5 The
dialkyl derivatives [P2Cp]ZrClR2 are unique in that, in
certain cases, they yield rare Zr alkylidene complexes
in high yield upon thermolysis.5,6 Given that stable
hafnium alkylidene complexes are currently unknown,
we considered whether the generality of the zirconium
alkylidene syntheses with this ancillary ligand system
could be extended to include new Hf derivatives. Herein
we report the synthesis and structural characterization
of the Hf complex [P2Cp]HfCl3 (1), which serves to
generate alkyl derivatives and the first example of a
stable Hf alkylidene complex, [P2Cp]HfdCHPh(Cl) (3).
Although much of the reactivity and structural features
of the Hf complexes mimic those of the Zr systems, an
intriguing difference in solution behavior between the
isostructural trichloride derivatives 1 and 2 is observed.
A combination of steric factors associated with the bulky
ancillary ligand and stronger metal-ligand binding in
1, together with the option of fluxional sidearm phos-
phine coordination, are likely responsible for these
observed differences.
Our current interest in hafnium coordination chem-
istry derives from previous work from our laboratory
† Professional Officers: UBC X-ray Structural Laboratory; S.J .R.
deceased October 27, 1998.
(1) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; p 878.
(2) Rogers, R. D.; Bynum, R. V.; Atwood, J . L. J . Am. Chem. Soc.
1978, 100, 5238.
(3) Rogers, R. D.; Bynum, R. V.; Atwood, J . L. J . Am. Chem. Soc.
1981, 103, 692.
(4) Fryzuk, M. D.; Mao, S. S. H.; Duval, P. B.; Rettig, S. J .
Polyhedron 1995, 14, 11.
(5) Fryzuk, M. D.; Duval, P. B.; Mao, S. S. H.; Zaworotko, M. J .;
MacGillivray, L. R. J . Am. Chem. Soc. 1999, 121, 2478.
(6) Fryzuk, M. D.; Mao, S. S. H.; Zaworotko, M. J .; MacGillivray, L.
R. J . Am. Chem. Soc. 1993, 115, 5336.
10.1021/om000760i CCC: $20.00 © 2001 American Chemical Society
Publication on Web 03/14/2001

Hafnium Alkylidene Complex
Organometallics, Vol. 20, No. 8, 2001 1609
Resu lts a n d Discu ssion
Syn th esis a n d Str u ctu r e of [P 2Cp ]HfCl3 (1). The
stoichiometric reaction of the ligand [P2Cp]Li with
HfCl4(THT)2 (THT ) tetrahydrothiophene) in toluene
at 65 °C for 24 h produces the Hf trichloride complex
[P2Cp]HfCl3 (1) in high yield as colorless air- and
moisture-sensitive crystals (eq 1). In contrast to this
sluggish reaction, formation of the Zr analogue [P2Cp]-
ZrCl3 (2) is complete within 12 h at room temperature.4
F igu r e 1. ORTEP view of 1, showing 33% probability
thermal ellipsoids for the non-hydrogen atoms.
Ta ble 1. Cr ysta llogr a p h ic Da ta a
1
3
formula
fw
C23H47Cl3HfP2Si2 C33.50H57ClHfP2Si2
726.59
791.88
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
orthorhombic
P212121 (No. 19)
13.760(4)
15.8367(12)
14.274(2)
90
triclinic
P1h (No. 2)
11.7554(11)
12.488(2)
13.9584(10)
98.3839(7)
95.0101(9)
111.100(3)
1869.5(3)
2
An X-ray crystal structure determination was ob-
tained for the Hf trichloride derivative 1. The molecular
structure is shown in Figure 1, and a summary of the
crystallographic data is given in Table 1. The Hf
complex 1 is isostructural in the solid state with the
previously characterized Zr derivative 2; therefore,
selected bond lengths and bond angles are presented
for both 1 and 2 in Table 2 for comparison. The angular
parameters for 1 and 2 are indeed very similar. How-
ever, the corresponding metal-ligand bond distances
are approximately 0.02 Å shorter for the hafnium
derivative, a feature that reflects a general trend of
stronger ligand bonding to Hf versus Zr. The monomer-
ic, quasi-octahedral geometry (given the cyclopentadi-
enyl donor as occupying one coordination site) has been
similarly observed in other structurally characterized
adducts of the general formula CpMCl3(L)2 (M ) Zr,
Hf),7,8 including examples in which one9 or both10 of the
donor atoms are appended to the cyclopentadienyl ring
via a sidearm tether. The equatorial M-Cl and M-P
bonds in 1 are directed away from the Cp ring toward
the axial chloride, so that the P(1)-M(1)-P(1) and Cl-
(1)-M(1)-Cl(2) angles are distorted from idealized
octahedral geometry by approximately 20°, a structural
mode that is prevalent for mono(cyclopentadienyl) com-
plexes.11
90
90
3110.6(10)
4
1.551
Z
D
calc, g/cm3
1.407
F(000)
1464
3.772
810
30.26
µ, cm-1
cryst size, Å
transmn factors
scan type
scan range, deg in ω
data collected
2θmax, deg
4.0 × 3.5 × 1.5
0.686-1.000
ω-2θ
0.95+0.35 tan θ
(h, (k, +l
32.5
6212
6212
2494
280
4.5 × 3.5 × 1.5
0.92-1.00
ω-2θ
1.00+0.35 tan θ
(h, (k, (l
60.1
15 781
8376
6427
374
0.069
0.072
1.82
-2.97 to 2.47
total reflections
independent reflns
no. with I g 3σ(I)
no. of variables
R1 (F2) (all data)
Rw (F2) (all data)
gof
0.0415
0.0343
1.40
residual density, e/Å3 -1.45 to 0.92
a
Temperature 294 K, Rigaku AFC6S diffractometer, Mo KR
radiation (λ ) 0.71069 Å), graphite monochromator, takeoff angle
6.0°, aperture 6.0 × 6.0 mm at a distance of 285 mm from the
crystal, stationary background counts at each end of the scan (scan/
background time ratio 2:1), σ2(F2) ) [S2(C + 4B)]/Lp2 (S ) scan
rate, C ) scan count, B ) normalized background count), function
minimized ∑w(|Fo| - |Fc|)2 where w ) 4Fo2/σ2(Fo2), R(F) ) ∑||Fo|
- |Fc||/∑|Fo|, Rw(F) ) (∑w(|Fo| - |Fc|)2/∑w|Fo| )1/2, and gof(F) )
2
[∑w(|Fo| - |Fc|)2/(m - n)]1/2
.
Solu tion Beh a vior of [P 2Cp ]HfCl3 (1). Although
the trichloride complexes 1 and 2 are isostructural in
the solid state, the solution behavior of these species
as measured by multinuclear NMR spectroscopy differs
(7) Lund, E. C.; Livinghouse, T. Organometallics 1990, 9, 2426.
(8) Erker, G.; Sarter, C.; Albrecht, M.; Dehnicke, S.; Kru¨ger, C.;
Raabe, E.; Schlund, R.; Benn, R.; Ruf´ınska, A.; Mynott, R. J . Orga-
nomet. Chem. 1990, 382, 89.
(9) Zeijden, A. A. H.; Mattheis, C.; Fro¨hlich, R.; Zippel, F. Inorg.
Chem. 1997, 36, 4444.
(10) Mu, Y.; Piers, W. E.; MacGillivray, L. R.; Zaworotko, M. J .
Polyhedron 1995, 14, 1.
(11) Winter, C. H.; Zhou, X.-X.; Dobbs, D. A.; Heeg, M. J . Organo-
metallics 1991, 10, 210.

1610 Organometallics, Vol. 20, No. 8, 2001
Fryzuk et al.
Ta ble 2. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) for [P 2Cp ]MCl3 (1: M ) Hf, 2:
M ) Zr )a
below in B (eq 2) is consistent with the data. Variable-
temperature H NMR spectroscopy also supports this
1
proposed structure, as above -65 °C the emergence of
new resonances consistent with the lower symmetry
presented by B can be discerned. For example, separate
singlets are observed for four inequivalent silylmethyl
groups, and the three cyclopentadienyl protons are
inequivalent. Assignment of the remaining resonances
1
2
1
2
M-Cl(1) 2.466(3) 2.489(1) Cl(1)-M-Cl(2) 160.6(1) 160.61(5)
M-Cl(2) 2.476(3) 2.490(1) Cl(1)-M-Cl(3)
M-Cl(3) 2.514(3) 2.529(1) Cl(2)-M-Cl(3)
M-P(1) 2.876(4) 2.897(1) P(1)-M-P(2)
M-P(2) 2.847(4) 2.871(1) Cl(3)-M-P(1)
80.7(1)
81.1(1)
159.3(1) 159.53(4)
78.1(1)
81.4(1)
177.4
80.79(3)
81.19(5)
78.32(4)
81.43(4)
177.50
1
M-Cp′
2.25
2.260(5) Cl(3)-M-P(2)
Cp′-M-Cl(3)
for B in the H NMR spectrum at these lower temper-
atures is complicated by the coexistence of A, and
analysis is hampered further as the temperature is
raised due to the associated increase in line broadening.
a
Numbering scheme is identical for each.
1
markedly. The 31P{1H}, 13C{1H}, and H NMR spectra
of the Zr derivative 2 are consistent with the solid state
structure and remain unchanged over the temperature
range studied (-90 to 95 °C).4 For example, the 31P-
{1H} NMR spectrum of 2 shows a singlet at 10.7 ppm
for two equivalent phosphines coordinated to the metal
1
center. In the H NMR spectrum there are two cyclo-
pentadienyl proton resonances in a 2:1 ratio due to the
Cs symmetry of the complex, along with two peaks for
the silylmethyl (Si(CH3)2) protons, two for the methylene
(PCH2Si) protons, and four peaks for the isopropyl
(PCH(CH3)2) protons of the ligand backbone.
In contrast to the straightforward solution behavior
observed for 2, the temperature-dependent NMR spec-
troscopic features of the Hf derivative 1 are more
complex. For example, the room-temperature 31P{1H}-
NMR spectrum of 1 shows three broad resonances at
-4.9, 14.9, and 24.9 ppm, in an approximate 1:2:1 ratio,
respectively, in comparison to the sharp singlet seen for
1
2. The peaks in the room-temperature H NMR spec-
It is apparent from the NMR spectral data that the
two isomers A and B are in equilibrium (eq 2). The
former is exclusively favored at low temperatures, while
the relative concentration of the latter increases as the
temperature is raised. The broadened resonances in the
31P{1H} and 1H NMR spectra for this system arise
because the fluxional process that interconverts these
species occurs at a rate that is comparable to the NMR
time scale. A similar equilibrium process has been noted
for the zirconium trimethyl complex [P2Cp]ZrMe3.4 To
evaluate the equilibrium in eq 2, the concentration of
each species was measured at separate temperatures
in the range between -65 and 20 °C, for which reliable
measurements could be made from integration of the
31P{1H} NMR spectra. Equilibrium constants were
calculated according to the equation K ) [B]/[A] and
plotted as a function of temperature. The resulting van’t
Hoff plot is shown in Figure 2, from which the thermo-
dynamic parameters ∆H° ) 4.9(5) kcal mol-1 and ∆S°
) 17(1) cal mol-1 K-1 were obtained. The errors were
estimated from separate experimental runs and indicate
reproducibility. The positive entropy term is in accord
with a less-ordered structure for B, represented by the
dangling pendant sidearm and the overall lower sym-
metry of this molecule.
trum for 1 are extremely broad and do not permit
structural assignments. One possibility to account for
these observations is that more than one isomer is
present in solution at this temperature, with the broad
resonances ascribed to exchange processes that inter-
convert these species.12
1
31P{1H} and H NMR spectra were obtained for 1 at
various temperatures to probe any temperature-de-
pendent solution behavior associated with chemical
exchange. As the temperature is lowered below room
temperature, the peaks in the 31P NMR spectrum
gradually sharpen, and the two resonances at -4.9 and
24.9 ppm decrease in intensity relative to the other peak
at 14.9 ppm. This trend continues until only the sharp
singlet at 14.9 ppm is detected below -65 °C. This
singlet can be assigned to a molecular structure as
1
depicted in Figure 1, as the H NMR spectrum for 1 at
this temperature shows well-defined peaks in an overall
1
pattern that resembles closely the room-temperature H
NMR spectrum of the Zr derivative 2. For the following
discussion this isomer will be designated as A.
The other two peaks at -4.9 and 24.9 ppm in the
room-temperature 31P{1H} NMR spectrum for 1 evi-
dently belong to another single isomer, as they are
related by equivalent line shapes and integration in the
NMR spectra obtained at various temperatures. The
downfield peak at 24.9 ppm suggests a strongly coor-
dinating phosphine donor, while the other peak at -4.9
ppm is indicative of an unbound phosphine for this
ligand system. A five-coordinate structure as shown
The different solution behavior exhibited by the
isostructural derivatives 1 and 2 offers a unique ex-
ample of the subtle electronic differences between Zr
and Hf, the two elements that are considered to be the
most similar in chemical behavior among the transition
metals. A comparison of the solid state structures of
these trichloride complexes shows that the correspond-
ing bond distances are shorter in Hf 1, presumably due
(12) Sandstrom, J . Dynamic NMR Spectroscopy; Academic Press:
New York, 1982.

Hafnium Alkylidene Complex
Organometallics, Vol. 20, No. 8, 2001 1611
Sch em e 1
F igu r e 2. van’t Hoff plot for the equilibrium shown in eq
2, based on the 31P{1H} NMR spectral data.
to stronger ligand binding. The more open coordination
sphere in B may therefore relieve steric pressures
arising from the close Hf-ligand contacts in the quasi-
octahedral geometry present in A, a process that is
absent in the less sterically encumbered Zr derivative
2. The observed equilibrium mixture of A and B in
solution for 1 reflects the fact that the two geometries
are energetically comparable (∆G° ) -0.1(5) kcal mol-1).
As mentioned earlier, other examples have been noted
where structural differences exist between Zr and Hf
analogues of the same general formula. For the tetra-
(cyclopentadienyl) complexes Cp4M (M ) Zr,2 Hf 3), as
with 1 and 2, adopting a less crowded geometry may be
a similar steric response to stronger metal-ligand
bonding in the Hf derivative.
Syn th esis a n d Molecu la r Str u ctu r e of [P 2Cp ]-
HfdCHP h (Cl) (3). We have previously reported the
synthesis and molecular structure of the first structur-
ally characterized zirconium alkylidene complexes [P2-
Cp]ZrdCHR(Cl) (R ) Ph, SiMe3).5,6 As described below,
these systems have now been extended to include the
first structurally characterized hafnium alkylidene
complex [P2Cp]HfdCHPh(Cl) (3). The reaction of trichlo-
ride 1 with 2 equiv of KCH2Ph generates a complex
mixture of hafnium alkyl species, the nature of which
will be discussed later. Thermolysis of this reaction
mixture in toluene for 6 days at 95 °C produces the
alkylidene complex 3 in high yield (Scheme 1), which
can be isolated as air-sensitive crystals from cold
hexanes. As observed in comparing the formation of the
trichloride species 1 and 2, generation of the Hf ben-
zylidene derivative is considerably slower in comparison
to the analogous Zr complex [P2Cp]ZrdCHPh(Cl) (4),6
where the reaction is complete within 12 h at 65 °C.
Single crystals of Hf benzylidene 3 suitable for X-ray
diffraction were obtained by slow cooling of a hexanes
solution. The data collection and crystallographic pa-
rameters are summarized in Table 1, and selected bond
lengths and bond angles are given in Table 3 along with
those of the Zr benzylidene complex 4 for comparison.
The molecular structure of 3 is depicted in Figure 3. As
with the trichlorides 1 and 2, the Hf benzylidene 3 is
isostructural with the Zr derivative 4, and again the
corresponding bond lengths are shorter for the Hf
complex. The alkylidene unit in 3 is directed syn to the
unique cyclopentadienyl carbon; this stereochemical
preference has been noted before with the [P2Cp] ligand
set.13 A very short Hf(1)-C(24) bond distance of 1.994-
(4) Å is indicative of metal-ligand multiple bonding in
the alkylidene moiety. For comparison, a typical Hf-C
single bond averages approximately 2.30 Å.1 A charac-
teristic feature of metal alkylidene complexes is an
R-agostic C-H interaction to the metal center.14 For
both derivatives the position of the R-hydrogen atom
was located and refined (for 3, this atom was refined
with restraints). The short Hf(1)‚‚‚H(48) contact of 1.93-
(3) Å for 3 is characteristic for an R-agostic bond, which
is also reflected in an open Hf(1)-C(24)-C(25) bond
angle of nearly 170°. The agostic interaction orients the
benzylidene unit such that the phenyl ring points
toward the cyclopentadienyl portion of the ancillary
ligand. The phenyl ring is aligned perpendicular to the
Cp ring, both to minimize steric interactions with the
bulky isopropyl groups on the sidearm phosphines and
to maximize orbital overlap with the metal. In compari-
son to the trichloride derivative 1, the absence of an
axial ligand trans to the cyclopentadienyl donor in 3
(compensated somewhat by the agostic C-H interaction
at this site) results in a modified geometry approaching
that of a typical four-legged piano stool. Therefore the
angular distortion of the equatorial bonds away from
the Cp ring is increased in 3 relative to trichloride 1
and is particularly evident with the trans chloride and
alkylidene ligands, where the Cl(1)-Hf(1)-C(24) bond
angle is less than 110°.
The NMR spectroscopic data for benzylidene 3 are
consistent with the solid state structure. A singlet is
seen in the 31P{1H} NMR spectrum at 23.4 ppm for a
1
pair of equivalent metal-bound phosphines, and the H
NMR spectrum exhibits peaks for the ancillary ligand
as expected for a molecule with Cs symmetry. Diagnostic
of the alkylidene unit are the downfield resonances for
both the R-carbon atom (210 ppm) and R-hydrogen atom
1
(7.33 ppm) in the respective 13C and H NMR spectra.
(13) Fryzuk, M. D.; Duval, P. B.; Mao, S. S. H.; Rettig, S. J .;
Zaworotko, M. J .; MacGillivray, L. R. J . Am. Chem. Soc. 1999, 121,
1707.
(14) Wilkinson, G.; Stone, F. G. A.; Abel, E. W. Comprehensive
Organometallic Chemistry, 2nd ed.; Pergamon Press: Oxford, 1995;
Vol. 5, Chapter 2.

1612 Organometallics, Vol. 20, No. 8, 2001
Fryzuk et al.
Ta ble 3. Selected Bon d Len gth s (Å) a n d Bon d An gles (d eg) for [P 2Cp ]MdCHP h (Cl) (3: M ) Hf, 4: M ) Zr )a
3
4
3
4
M(1)-Cl(1)
M(1)-P(1)
M(1)-P(2)
M(1)-C(24)
C(24)-C(25)
M(1)-Cp′
2.5068(11)
2.8038(14)
2.7986(13)
1.994(4)
1.481(5)
2.22
2.5418(13)
2.8299(16)
2.8425(17)
2.024(4)
1.456(5)
2.234
Cl(1)-M(1)-P(1)
Cl(1)-M(1)-P(2)
P(1)-M(1)-P(2)
Cl(1)-M(1)-C(24)
M(1)-C(24)-C(25)
M(1)-C(24)-H(48)
75.90(4)
76.79(4)
152.68(4)
108.87(13)
167.8(4)
71.6(17)
77.23(4)
76.35(4)
153.58(4)
110.11(11)
169.2(3)
79.8(23)
M(1)‚‚‚H(48)
1.93(3)
2.07(4)
a
Numbering scheme is identical for each.
Con clu d in g Rem a r k s
Utilization of the multidentate ancillary ligand [P2-
Cp] has been extended to include an examination of its
coordination chemistry with hafnium, with the synthe-
sis of trichloride, benzyl, and benzylidene complexes.
Although Hf alkylidene complexes have been inferred
as reactive intermediates,15 benzylidene 3 is the first
example of a structurally characterized Hf alkylidene
complex. The reactions that produce the Hf derivatives
are found to be considerably more sluggish than the
corresponding reactions that generate the Zr analogues,
in accord with the more inert chemical properties of Hf.
X-ray crystallographic determinations revealed that the
hafnium trichloride complex 1 and the benzylidene
complex 3 are isostructural to their zirconium analogues
2 and 4, respectively, in the solid state. However,
shorter metal-ligand bond distances were found for the
Hf species, reflecting a trend that is generally observed
between these two congeners, that ligands form stronger
bonds to Hf relative to Zr. Despite the similar solid state
structures, the complex fluxional solution behavior
exhibited by the Hf trichloride derivative 1 differs
significantly from the Zr analogue 2. The equilibrium
mixture of isomers seen for 1 is likely a response to
alleviate steric congestion arising from the above-noted
closer metal-ligand contacts.
F igu r e 3. ORTEP view of 3, showing 33% probability
thermal ellipsoids for the non-hydrogen atoms and H(48).
Exp er im en ta l Section
A previous examination of the formation of the Zr
alkylidene complexes [P2Cp]ZrdCHR(Cl) (R ) Ph, SiMe3)
revealed a composite mechanism.5 The dialkyl complex
was determined to be the direct precursor that under-
went R-abstraction to give the alkylidene product, but
a complicating preequilibrium involving all three alkyl
species was observed in synthetic attempts to isolate
the dialkyl precursor. Similarly, the reaction of trichlo-
ride 1 with KCH2Ph yields a mixture of alkyl species.
The identity of two of these complexes as mono(benzyl)
[P2Cp]HfC12(CH2Ph) (5) and tris(benzyl) [P2Cp]Hf(CH2-
Ph)3 (7) has been confirmed by independent syntheses
using the appropriate stoichiometry of the alkylating
reagent. Mono(benzyl) 5 is produced as a slightly impure
oil, while tris(benzyl) 7 is obtained in good yield as an
air-sensitive oil. Both of these species resemble the
previously characterized Zr analogues, based on a
comparison of the NMR spectroscopic data. Unfortu-
nately, in contrast to the Zr systems, where the dialkyl
derivatives could be assigned from the NMR spectra,
the analogous hafnium bis(benzyl) complex [P2Cp]HfCl-
(CH2Ph)2 (6) could not be positively identified from the
reaction mixture. However, given the overall similarities
noted in the Zr and Hf systems, it seems likely that
alkylidene formation of 3 follows a course similar to that
depicted for the zirconium analogue.
Gen er a l Con sid er a tion s. All manipulations were per-
formed under an atmosphere of prepurified nitrogen in a
Vacuum Atmospheres HE-553-2 glovebox equipped with a MO-
40-2H purification system or in standard Schlenk-type glass-
ware on
a
dual vacuum/nitrogen line. 1H NMR spectra
(referenced to nondeuterated impurity in the solvent) were
performed on one of the following instruments depending on
the complexity of the particular spectrum: Bruker WH-200,
Varian XL-300, or Bruker AM-500. 13C NMR spectra (refer-
enced to solvent peaks) were run at 75.429 MHz on the XL-
300 instrument, and 31P NMR spectra (referenced to external
P(OMe)3 at 141.0 ppm) were run at 121.421 and 202.33 MHz,
on the XL-300 and Bruker AM-500 instruments, respectively.
All chemical shifts are reported in ppm, and all coupling
constants are reported in Hz. Elemental analyses were per-
formed by Mr. P. Borda of this department.
Rea gen ts. Hexanes and tetrahydrofuran (THF) were pre-
dried over CaH2 followed by distillation from sodium ben-
zophenone ketyl under argon. Toluene was dried over sodium
under argon, and hexamethyldisiloxane was distilled from
sodium benzophenone ketyl under nitrogen. The deuterated
solvents C6D6 and C6D5CD3 were dried over molten sodium,
vacuum transferred, and degassed by freeze-pump-thaw
(15) Bulls, A. R.; Schaefer, W. P.; Serfas, M.; Bercaw, J . E. Orga-
nometallics 1987, 6, 1219.

Hafnium Alkylidene Complex
Organometallics, Vol. 20, No. 8, 2001 1613
technique before use. KCH2Ph16 and [P2Cp]Li6 were prepared
according to literature methods.
[P 2Cp ]HfCl2(CH2P h ) (5). A solution of KCH2Ph (115 mg,
0.88 mmol) in 30 mL of THF was added dropwise to a cooled
(-78 °C) stirring solution of 1 (650 mg, 0.89 mmol) dissolved
in 60 mL of THF. The colorless Hf solution gradually turned
yellowish-orange. The mixture was allowed to warm to room
temperature and then stirred for 12 h. The volatiles were then
removed under vacuum, and the residue was extracted with
hexanes and filtered. Removal of the solvent under vacuum
yielded an impure yellowish oil, soluble in pentane, consisting
of approximately 80% 5 by NMR spectroscopy. 1H NMR (20
°C, C6D6): δ 0.02 (s, 12H, Si(CH3)2), 0.24 and 0.60 (d, 2H, 2J PH
) 5 Hz, SiCH2P), 0.91 (m, 24H, CH(CH3)2), 1.55 (m, 4H,
CH(CH3)2), 2.42 (s, 2H, CH2Ph), 6.38 (d, 2H, 4J HH ) 2 Hz, Cp-
[P 2Cp ]HfCl3 (1). Toluene (100 mL) was added to an
intimate mixture of [P2Cp]Li (1.82 g, 4.06 mmol) and HfCl4-
(THT)2 (2.02 g, 4.08 mmol) at 0 °C. The mixture was heated
to 65 °C and stirred at that temperature for 24 h, during which
the yellowish color of the ligand gradually diminished to give
a pale buff yellow slurry. The mixture was filtered and the
solvent reduced to 10 mL, from which pale yellow crystals were
obtained at -78 °C. The crystals were washed with cold
hexanes and dried. Yield: (1.99 g, 67%). Anal. Calcd for C23H47
-
Cl3HfP2Si2: C, 38.02; H, 6.52. Found: C, 38.40; H, 6.70. In
solution, 1 exists as an equilibrium mixture consisting of two
isomers (A and B).
4
3
H), 6.74 (t, 1H, J HH ) 1 Hz, Cp-H), 6.79 (t, 1H, J HH ) 7 Hz,
Isom er A. 1H NMR (-40 °C, C7D8): δ 0.12 and 0.38 (s, 6H,
3
3
p-C6H5), 7.23 (t, 2H, J HH ) 7 Hz, m-C6H5), 7.42 (d, 2H, J HH
) 7 Hz, o-C6H5). 31P{1H} NMR (-20 °C, C7D8): δ 0 and 19
(very br).
2
2
Si(CH3)2), 0.84 (dd, 4H, J HH ) 6 Hz, J PH ) 4 Hz, SiCH2P),
0.99 and 1.34 (m, 12H, CH(CH3)2), 2.15 and 2.77 (sept., 2H,
3J HH ) 7 Hz, CH(CH3)2), 6.51 (d, 2H, 4J HH ) 1 Hz, Cp-H), 7.18
[P 2Cp ]Hf(CH2P h )3 (7). A solution of KCH2Ph (84 mg, 0.64
mmol) in 30 mL of THF was added dropwise over a period of
20 min to a cooled (-78 °C) stirring solution of 1 (156 mg, 0.21
mmol) dissolved in 30 mL of THF. The solution was allowed
to slowly warm to room temperature and stirred for 12 h,
during which the colorless Hf solution gradually turned bright
yellow. The volatiles were then removed under vacuum, and
the residue was extracted with hexanes and filtered. Removal
of the solvent under vacuum yielded a hydrocarbon soluble
4
(t, 1H, J HH ) 1 Hz, Cp-H). 31P NMR (-40 °C, C7D8): δ 14.9
(s).
1
Isom er B. H NMR (-40 °C, C7D8): δ 0.08, 0.30, 0.68, and
0.71 (s, 3H, Si(CH3)2), 1.08 and 1.17 (m, 12H, CH(CH3)2), 1.52
and 1.84 (m, 2H, CH(CH3)2), 6.62, 6.66, and 7.12 (m, 1H, Cp-
H). 31P NMR (-40 °C, C7D8): δ -4.9 and 24.9 (br s).
[P 2Cp ]HfdCHP h (Cl) (3). A solution of KCH2Ph (143 mg,
1.10 mmol) in 30 mL of THF was added dropwise with stirring
to a cooled (-78 °C) solution of 1 (400 mg, 0.55 mmol) dissolved
in 60 mL of THF. The color of the solution slowly changed to
bright yellow. The solution was warmed to room temperature
and then stirred for another 3 h. The solvent was then removed
in vacuo, and the residue extracted with hexanes and filtered.
The filtrate was reduced under vacuum to give a yellow oil,
which was redissolved in 30 mL of toluene. This solution was
placed in an oil bath at 95 °C for 6 days, after which time no
further reaction could be detected by NMR spectroscopy (for
a corresponding NMR scale reaction). The solvent was re-
moved, and the oil extracted with hexanes to give a bright
yellow solid, which yielded crystals suitable for X-ray diffrac-
tion from a toluene/hexanes solution mixture at -40 °C.
1
yellowish oil. Yield: 135 mg, 72%. H NMR (20 °C, C6D6): δ
0.34 (s, 12H, Si(CH3)2), 0.56 (d, 4H, 2J PH ) 5 Hz, SiCH2P), 0.95
2
(m, 24H, CH(CH3)2), 1.50 (sept, 4H, J HH ) 7 Hz, CH(CH3)2),
4
1.90 (s, 6H, CH2Ph), 6.28 (d, 2H, J HH ) 2 Hz, Cp-H), 6.69 (t,
3
3
1H, J HH ) 2 Hz, Cp-H), 6.78 (d, 6H, J HH ) 7 Hz, o-C6H5),
3
3
6.87 (t, 3H, J HH ) 7 Hz, p-C6H5), 7.17 (t, 6H, J HH ) 7 Hz,
m-C6H5). 31P{1H} NMR (20 °C, C6D6): δ -5.6 (s). 13C{1H}
1
NMR (20 °C, C6D6): δ -0.2 and 0.2 (s, Si(CH3)2), 8.1 (d, J PC
) 35 Hz, SiCH2P), 16.5, 17.2, 17.4 and 18.0 (s, CH(CH3)2), 26.0
1
and 26.4 (d, J PC ) 15 Hz, CH(CH3)2), 70.1 (s, CH2Ph), 121.6
(s, Cp), 130.1 (s, o-C6H5), 131.7 (s, m-C6H5), 140.9 (s, p-C6H5).
1
Yield: 305 mg, 70%. H NMR (20 °C, C6D6): δ 0.12 and 0.31
Ack n ow led gm en t. Financial support for this re-
search was provided by NSERC of Canada in the form
of a Research Grant to M.D.F. and a postgraduate
scholarship to P.B.D.
2
2
(s, 6H, Si(CH3)2), 0.42 and 0.49 (dd, 4H, J HH ) 5 Hz, J PH
)
7 Hz, SiCH2P), 0.79, 0.96, 1.33 and 1.14 (dd, 6H, 3J HH ) 7 Hz,
3J PH ) 6 Hz, CH(CH3)2), 1.73 and 2.43 (m, 2H, CH(CH3)2), 6.11
4
3
(t, 1H, J HH ) 1 Hz, Cp-H), 6.67 (t, 1H, J HH ) 7 Hz, p-C6H5),
4
3
7.09 (d, 2H, J HH ) 1 Hz, Cp-H), 7.16 (t, 2H, J HH ) 7 Hz,
4
m-C6H5), 7.33 (s, 1H, CHPh), 7.45 (d, 2H, J HH ) 7 Hz,
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic files, in CIF format, for the structure determination
of [P2Cp]HfCl3 (1) and [P2Cp]HfdCHPh(Cl) (3) are available.
This material is available free of charge via the Internet at
http://pubs.acs.org.
o-C6H5). 31P NMR (20 °C, C6D6): δ 23.4 (s). Anal. Calcd for
C
30H53ClHfP2Si2(C7H8)0.5: C, 50.81; H, 7.25 Found: C, 50.86;
H, 7.51.
(16) Schlosser, M.; Hartmann, J . Angew. Chem., Int. Ed. Engl. 1973,
12, 508.
OM000760I