Bis(indenyl)hafnium chemistry: ligand-induced haptotropic rearrangement and fundamental reactivity studies at a reduced hafnium center

Article abstract of DOI:10.1021/om900046e

chemistry of reduced bis(indenyl)hafnium compounds has been explored. Sodium amalgam reduction of 075-C9H5-1,3-(SiMe3)2)2HfCl2 was studied in several different solvents. While reduction in toluene resulted in decomposition, performing the reaction in the

Full text of DOI:10.1021/om900046e

Organometallics 2009, 28, 2471–2484
2471
Bis(indenyl)hafnium Chemistry: Ligand-Induced Haptotropic
Rearrangement and Fundamental Reactivity Studies at a Reduced
Hafnium Center
Doris Pun, Scott M. Leopold, Christopher A. Bradley, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Cornell UniVersity, Ithaca, New York 14853
ReceiVed January 20, 2009
The chemistry of reduced bis(indenyl)hafnium compounds has been explored. Sodium amalgam
reduction of (η5-C9H5-1,3-(SiMe3)2)2HfCl2 was studied in several different solvents. While reduction in
toluene resulted in decomposition, performing the reaction in the presence of THF or 1,2-dimethoxyethane
(DME) yielded η6,η5-bis(indenyl)hafnium solvent (THF or DME) complexes arising from ligand-in-
duced haptotropic rearrangement. Using less coordinating solvents such as diethyl ether or 2,5-dimethyl
THF furnished a rare example of a structurally characterized hafnocene(III) complex, (η5-C9H5-1,3-
(SiMe3)2)2HfCl. An unusual sodium chloride complex of η6,η5-bis(indenyl)hafnium, [(η6-C9H5-1,3-
(SiMe3)2)(η5-C9H5-1,3,-(SiMe3)2)HfCl]2Na[Na(DME)3], was also isolated and crystallographically char-
acterized as a byproduct from synthesis of the DME compound. The reactivity of the η6,η5-
bis(indenyl)hafnium THF compound as an isolable source of Hf(II) was also evaluated. Addition of H2
or N,N-dimethylaminopyridine (DMAP) resulted in facile oxidative addition, while C-O cleavage of
THF was observed upon mild heating. Preparation of the corresponding hafnocene dicarbonyl derivatives
allowed comparison of the electronic properties of the new complexes as compared to their zirconocene
congeners.
wich complex has proven more challenging, as competing
reactions such as cyclopentadienyl substituent cyclo-
metalation15 are rapid and provide kinetically facile routes to
the thermodynamically preferred highest oxidation state.
Replacing the cyclopentadienyl rings with 1,3-disubstituted
indenyl ligands has provided a wealth of reduced zirconium
chemistry. For example, sodium amalgam reduction of (η5-C9H5-
1,3-R2)2ZrCl2 complexes bearing relatively large substituents
Introduction
The chemistry of reduced bis(cyclopentadienyl) group 4
transition metal complexes has attracted continued attention
since Wilkinson’s seminal studies of metal sandwich complexes.1,2
Applications range from two-electron reducing agents for
organic synthesis3 to N2-fixing complexes that yield ammonia
upon hydrogenation4 and nitrogen-containing organic molecules
when treated with the appropriate carbon sources.5-9
Within the group 4 triad, divalent bis(cyclopentadienyl)tita-
nium compounds are the most prevalent.10 Seminal reports from
Bercaw and Brintzinger described the solution characterization
of (η5-C5Me5)2Ti,11 a molecule that has thus far eluded isolation
in the solid state. Increasing the size of one of the cyclopentadienyl
substituents has produced an array of isolable titanocenes and
resulted in the structural characterization of a family of (η5-
C5Me4R)2Ti (R ) silyl, alkyl) complexes.12-14 Observation or
isolation of a bis(cyclopentadienyl)zirconium or hafnium sand-
t
(e.g., R ) CHMe2, SiMe3, SiMe2 Bu) has resulted in isolation
of the bis(indenyl)zirconium sandwich complexes. Instead of a
traditional η5,η5 linear metallocene, one of the indenyl ligands
is significantly buckled due to η9-coordination, where all nine
carbons are bound to the metal center.16 Both NMR spectro-
scopic16 and computational studies17 established facile and
reversible dissociation of the coordinated benzo rings in benzene
solution at 23 °C, providing access to the putative η5,η5-
bis(indenyl)zirconium sandwich, the indenyl version of the long
sought after “zirconocene”.
Reducing the size of the indenyl substituents to a combination
of methyl and isopropyl groups has resulted in the synthesis of
activated, end-on dinitrogen complexes with inclusion of NaCl
(Figure 1).18 Similar substituent effects were observed in mixed
pentamethylcyclopentadienyl (Cp*), indenyl complexes where
* Corresponding author. E-mail: pc92@cornell.edu.
(1) Cotton, F. A. J. Organomet. Chem. 2001, 637, 18.
(2) Wilkinson, G.; Birmingham, J. M. J. Am. Chem. Soc. 1954, 76, 4281.
(3) Titanium and Zirconium in Organic Synthesis; Marek, I., Ed.; Wiley-
VCH: Weinheim, 2002, and references therein.
(4) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527.
(5) Mori, M. J. Organomet. Chem. 2004, 689, 4210.
(6) Morello, L.; Love, J. B.; Patrick, B. O.; Fryzuk, M. D. J. Am. Chem.
Soc. 2004, 126, 9480.
(13) (a) Lukesova´, L.; Hora´cek, M.; Stepnicka, P.; Fejfarova´, K.; Gyepes,
R.; Cisorova´, I.; Kubista, J.; Mach, K. J. Organomet. Chem. 2002, 663,
134. (b) Horacek, M.; Kupfer, V.; Thewalt, U.; Stepnicka, P.; Polasek, M.;
Mach, K. Organometallics 1999, 18, 3572.
(7) Knobloch, D. J.; Toomey, H. E.; Chirik, P. J. J. Am. Chem. Soc.
2008, 130, 4248.
(8) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Angew. Chem.,
Int. Ed. 2007, 46, 2858.
(9) Bernskoetter, W. H.; Olmos, A. V.; Pool, J. A.; Lobkovsky, E.;
Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 10696.
(10) Chirik, P. J.; Bouwkamp, M. W. ComprehensiVe Organometallic
Chemistry III.
(11) Bercaw, J. E.; Brintzinger, H. H. J. Am. Chem. Soc. 1971, 93, 2045.
(12) Hitchcock, P. B.; Kerton, F. M.; Lawless, G. A. J. Am. Chem. Soc.
1998, 120, 10264.
(14) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 14688.
(15) Hora´cek, M.; Stepnicka, P.; Kubista, J.; Fejfarova´, K.; Gyepes, R.;
Mach, K. Organometallics 2003, 22, 861.
(16) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik,
P. J. J. Am. Chem. Soc. 2004, 126, 16937.
(17) Veiros, L. F. Chem.-Eur. J. 2005, 11, 2505.
(18) Pun, D.; Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J.
J. Am. Chem. Soc. 2008, 130, 14046.
10.1021/om900046e CCC: $40.75
2009 American Chemical Society
Publication on Web 03/23/2009

2472 Organometallics, Vol. 28, No. 8, 2009
Pun et al.
Figure 1. Synthesis and dinitrogen chemistry of bis(indenyl)zirconium complexes.
the sterically hindered complex, Cp*(η5-C9H5-1,3-(CHMe2)2)Zr,
adopts a sandwich structure, while the more open Cp*(η5-C9H5-
1-(CHMe2)-3-Me)Zr complex coordinates N2 to form a weakly
activated, end-on dinitrogen compound (Figure 1).19
of the C2V symmetric, 1,3-[SiMe3]- and [CHMe2]-disubstituted
hafnocene dichloride complexes 1-Cl2 and 2-Cl2 was initially
targeted. Straightforward ligand metalation was accomplished
by stirring 2 equiv of the appropriate lithium indenide with HfCl4
in diethyl ether at 23 °C. Subsequent filtration, extraction into
pentane or toluene, and recrystallization from pentane yielded
the desired hafnocene dichloride complexes, 1-Cl2 and 2-Cl2,
in 42% and 67% yields, respectively (eq 1).
Given the challenges associated with the synthesis and
isolation of reduced “zirconocenes”, it is not surprising that
hafnium sandwich chemistry is even less developed. Alkali
20
metal reduction of either (η5-C5Me5)2HfI2 or (η5-C5Me4H)2-
21
HfI2 yielded the corresponding “end-on” and “side-on”
dinitrogen complexes, respectively. The latter compound exhibits
rich N2 functionalization chemistry including dinitrogen hydro-
genation,21 carboxylation,8 and isocyanate cycloaddition.9 Here
we describe the study of the reduction chemistry of bis(in-
denyl)hafnium compounds and report the synthesis and structural
characterization of a hafnocene(III) monochloride as well as
unusual η6,η5-bis(indenyl)hafnium THF and DME (DME ) 1,
2-dimethoxyethane) compounds by ligand-induced haptotropic
rearrangement. Evaluation of the thermal stability and additional
reactivity studies establish that both the THF and DME
derivatives serve as isolable sources of hafnium(II).
Because of the dinitrogen coordination observed with bis-
(indenyl)zirconium complexes with smaller ring substituents,
(η5-C9H5-1-Me-3-(CHMe2))2HfCl2 (3-Cl2) was also synthesized.
Using a method identical to that used to prepare 1-Cl2 and 2-Cl2,
3-Cl2 was obtained as an equimolar mixture of rac- and meso-
diasteromers in 72% yield. No attempts were made to separate
the isomers.
Sodium amalgam reduction of each of the hafnocene dichlo-
ride complexes in various solvents was explored in an attempt
to prepare the corresponding sandwich or dinitrogen derivatives.
For all three compounds, reduction in toluene yielded a complex
Results and Discussion
Synthesis and Characterization of η6,η5-Bis(indenyl)
Hafnium THF and DME Compounds. Based on the results
of our studies in bis(indenyl)zirconium chemistry,16 the synthesis
(19) Pun, D.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2008, 130,
6047.
(20) Roddick, D. M.; Fryzuk, M. D.; Seidler, P. F.; Hillhouse, G. L.;
Bercaw, J. E. Organometallics 1985, 4, 97.
(21) Bernskoetter, W. H.; Olmos, A. V.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2006, 25, 1021.

Bis(indenyl)hafnium Chemistry
Organometallics, Vol. 28, No. 8, 2009 2473
Figure 2. Solid state structure of 1-THF with 30% probability ellipsoids and hydrogen atoms omitted for clarity. Partially labeled view
(left) and top view (right).
mixture of unidentified products. These results are similar to
those established in bis(cyclopentadienyl) hafnium chemistry,
where sodium amalgam reduction of (η5-C5Me4H)2HfCl2 yielded
an intractable mixture of products. For the Cp compounds, this
obstacle was overcome with preparation of the corresponding
diiodide complex (η5-C5Me4H)2HfI2.21
of the zirconium congener (η6-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-
(SiMe3)2) Zr(THF) (5-THF), the compound is more crystalline
and is easier to isolate in pure form than the corresponding
sandwich (η9-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3-(SiMe3)2)Zr (5).22
Reduction of a diethyl ether solution of 1-Cl2 with excess 0.5%
sodium amalgam in the presence of 20 equiv of THF furnished
a dark green solid identified as the η6,η5-bis(indenyl)hafnium
THF compound 1-THF (eq 3). Attempts to prepare 2-THF from
2-Cl2 using a similar protocol were unsuccessful, resulting in a
mixture of unidentified products. It should be noted that the
isolated zirconium sandwich (η9-C9H5-1,3-(CHMe2)2)(η5-C9H5-
1,3-(CHMe2)2)Zr does not coordinate THF even at very high
concentrations,22 a consequence of the reluctance of electron-
rich sandwiches to undergo ligand-induced haptotropic rear-
rangement.
Inspired by these observations, the synthesis of (η5-C9H5-1-
Me-3-(CHMe2))2HfI2 was pursued. Treatment of 3-Cl2 with BI3
in toluene did not furnish the desired diiodide complex, 3-I2,
but rather the hafnocene piano stool compound (η5-C9H5-1-Me-
3-(CHMe2))HfI3 (4-I3) was isolated as a brown powder in 63%
yield (eq 2). Monitoring the reaction by 1H NMR spectroscopy
established that fulvene compounds, along with other unidenti-
fied organics, accompanied formation of 4-I3 and account for
the fate of the ejected indenyl ring. Synthesis of the desired
bis(indenyl)hafnocene dichloride, 3-I2, was ultimately ac-
complished by treatment of 4-I3 with 1 equiv of the lithium
indenide Li[C9H5-1-Me-3-(CHMe2)] in diethyl ether. Yellow
3-I2 was isolated as an equimolar mixture of rac- and meso-
diastereomers in 91% yield (eq 2).
At 23 °C, the benzene-d6 1H and 13C NMR spectra of 1-THF
exhibit the number of peaks consistent with a Cs symmetric
compound with inequivalent indenyl rings. Two 1H NMR
resonances are shifted upfield to 3.62 and 3.69 ppm and are
assigned to the benzo hydrogens distal to the five-membered
ring and the cyclopentadienyl hydrogen on the η6-indenyl ring,
respectively. The upfield shifting of the peaks is likely due to
transannular ring currents rather than a direct consequence of
the unusual hapticity. Similar effects have been observed in
iridium compounds lacking η6-indenyl coordination and in the
zirconium congener 5-THF.22
The solid state structure of 1-THF was determined by X-ray
diffraction (Figure 2). Two independent molecules were present
in the asymmetric unit, and selected bond distances and angles
for both are reported in Table 1. Also contained in Table 1 are
the corresponding metrical parameters for the zirconium con-
With the desired hafnocene diiodide in hand, reduction
chemistry was explored. Stirring a toluene slurry of 3-I2 with
excess 0.5% sodium amalgam under vacuum or a dinitrogen
atmosphere furnished a mixture of products from which variable
amounts of rac/meso-(η5-C9H5-1-Me-3-(CHMe2))2Hf(H)I (3-
(I)H) and rac/meso-(η5-C9H5-1-Me-3-(CHMe2))2HfH2 (3-H2)
were identified. The source of the hydrogen atoms in these
reductions is unknown; however observation of hydride com-
pounds from sodium amalgam reduction of group 4 metallocene
dihalides has been observed previously.11,21 Attempts to prepare
(η5-C9H5-1,3-(SiMe3)2)2HfI2 (1-I2) from analogous routes met
with limited success. Importantly, no tractable η9,η5-bis(in-
denyl)hafnium sandwich compound was observed from sodium
amalgam reduction using various reaction conditions.
Because the sodium amalgam reduction of 1-Cl2 in toluene
produced a mixture of unidentified products, focus shifted to
the synthesis of the corresponding THF complex. In the case
(22) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am.
Chem. Soc. 2005, 127, 10291.

2474 Organometallics, Vol. 28, No. 8, 2009
Pun et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) for 1-THF
and 5-THF
Hf(1)-C(1) and Hf(1)-C(4) distances of 2.284(2)/2.299(2) and
2.294(2)/2.283(2) Å. All of the distortions to the six-membered
ring in the hafnocene congener are slightly more pronounced
than in the zirconium compound 5-THF, consistent with the
third-row metal being more reducing than the second.
1-THF
1-THF
5-THFc
M(1)-O(1)
M(1)-C(1)
M(1)-C(2)
M(1)-C(3)
M(1)-C(4)
M(1)-C(5)
M(1)-C(6)
M(1)-C(10)
M(1)-C(11)
M(1)-C(12)
M(1)-C(13)
M(1)-C(14)
C(1)-C(2)
2.1977(16)
2.284(2)
2.459(2)
2.455(2)
2.294(2)
2.475(2)
2.448(2)
2.470(2)
2.410(2)
2.445(2)
2.575(2)
2.602(2)
1.427(3)
1.463(3)
1.378(4)
1.433(4)
1.464(3)
1.432(3)
22.95(11)
178.5(3)
2.1888(15)
2.299(2)
2.462(2)
2.460(2)
2.283(2)
2.441(2)
2.469(2)
2.473(2)
2.434(2)
2.483(2)
2.600(2)
2.590(2)
1.443(3)
1.464(3)
1.385(3)
1.419(3)
1.469(3)
1.434(3)
22.44(18)
170.6(1)
2.254(2)
2.340(2)
2.472(3)
2.493(2)
2.355(3)
2.468(3)
2.487(3)
2.623(3)
2.501(3)
2.454(3)
2.494(2)
2.608(3)
1.425(4)
1.458(4)
1.394(4)
1.414(4)
1.466(3)
1.444(3)
19.0(1)
The scope of the ligand-induced haptotropic rearrangement
in reduced hafnium chemistry was further explored with 1,2-
dimethoxyethane (DME). In analogous zirconium chemistry,
DME proved to be an effective ligand for inducing haptotropic
rearrangement and yielding a structurally characterized η6,η5-
bis(indenyl)zirconium DME complex.22,23 The synthesis of
1-DME was initially accomplished by reduction of a diethyl
ether solution of 1-Cl2 with excess sodium amalgam in the
presence of 5 equiv of DME. Extraction followed by recrys-
tallization from pentane yielded a green solid identified as [(η6-
C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3,-(SiMe3)2)Hf-(C4H10O)] (1-
DME) (eq 4). Additional experimentation established that
1-DME was more reliably synthesized by addition of an excess,
typically 2.2 equiv, of 1,2-dimethoxyethane to a benzene
solution of 1-THF (eq 4).
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
dihedral anglea
rotational angleb
177.8(1)
a Defined as the deviation from planarity of the benzo ring, which is
the angle between the planes formed by C(1)-C(2)-C(3)-C(4) and
C(4)-C(5)-C(6)-C(1). b Defined as the angle formed between the
plane defined by Hf(1), C(8), and the centroid between C(5)-C(6) and
the plane defined by Hf(1), C(11), and the centroid between C(13)-
C(14) in molecule 1. c Data for 5-THF taken from ref 22.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
{[1-Cl]2Na}-
The benzene-d6 1H and 13C NMR spectra of 1-DME at 23
°C exhibit the number of peaks consistent with inequivalent
indenyl rings and overall Cs molecular symmetry. As with
1-THF and the zirconium examples,22 an upfield-shifted
benzo resonance was located at 3.91 ppm, consistent with
η6,η5 indenyl hapticity.
Hf(1)-Cl(1)
Hf(1)-C(1)
Hf(1)-C(2)
Hf(1)-C(3)
Hf(1)-C(4)
Hf(1)-C(5)
Hf(1)-C(6)
Hf(1)-C(10)
Hf(1)-C(11)
Hf(1)-C(12)
Hf(1)-C(13)
Hf(1)-C(14)
C(1)-C(2)
2.4235(9)
2.284(3)
2.479(4)
2.473(4)
2.291(3)
2.499(4)
2.478(3)
2.477(3)
2.413(3)
2.484(3)
2.612(3)
2.617(3)
1.408(5)
1.4580(5)
1.370(6)
1.437(5)
1.455(5)
1.436(5)
2.722(3)
25.37(28)
174.1(1)
During attempts to recrystallize 1-DME faint yellow
crystals were isolated. Single-crystal X-ray diffraction es-
tablished the molecular structure not as the expected 1-DME
compound but as an unusual sodium chloride compound, {[1-
1
Cl]2Na}{Na(DME)3}. The H NMR spectrum of the single
C(1)-C(6)
crystals established a diamagnetic compound with idealized
Cs symmetry and inequivalent indenyl rings. A diagnostic
upfield-shifted benzo resonance was observed at 3.64 ppm,
diagnostic of η6-hapticity. The sodium-coordinated DME was
confirmed with the observation of broad resonances centered
at 1.27 (Me) and 1.61 (CH2) ppm, likely due to rapid and
reversible dissociation-coordination in benzene-d6 solution.
The solid state structure of the anion is presented in Figure
3 and establishes the identity of the molecule. The cation,
Na(DME)3, was severely disordered, and the SQUEEZE pro-
gram was used to remove it from the final solution of the anion.
This approach includes only the sodium cation and dramatically
improved the R-factor of the final solution. For the anion, a
dimeric structure is observed with a bridging sodium atom that
is bound to the hafnium chlorides (dNa-Cl ) 2.5860(10) Å) and
the unsubstituted cyclopentadienyl carbon (dNa-C ) 2.722(3)
Å) of the η6-coordinated indenyl ligand. The geometry of the
sodium atom is best described as idealized square planar with
the sum of the angles being 360.00(16)°.
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
Na(1)-C(8)
dihedral anglea
rotational angleb
a Defined as the deviation from planarity of the benzo ring, which is
the angle between the planes formed by C(1)-C(2)-C(3)-C(4) and
C(4)-C(5)-C(6)-C(1). b Defined as the angle formed between the
plane defined by Hf(1), C(8), and the centroid between C(5)-C(6) and
the plane defined by Hf(1), C(11), and the centroid between
C(13)-C(14) in molecule 1.
gener 5-THF for comparison. As was observed with the
zirconium compound, 1-THF adopts an idealized Cs symmetric
structure where the mirror plane is perpendicular to the
metallocene wedge and contains the Hf(1)-O(1) vector.
The solid state structure clearly confirms the η6,η5 hapticity of
the indenyl ligands that was established by NMR spectroscopy.
The coordinated benzo ring exhibits distortions consistent
with two-electron reduction, a typical feature of zirconium
compounds containing η6-indenyl ligands.22 For example, the
six-membered rings in 1-THF are puckered by 22.95(11)/
22.44(18)° as defined by the dihedral angle formed between
the planes of C(1)-C(2)-C(3)-C(4) and C(1)-C(6)-C(5)-
C(4). These buckles are accompanied by relatively short
As is typical for zirconium and hafnium complexes with
η6,η5-indenyl ligands, the rings are anti with a rotational angle
of 174.1(1)°, likely to minimize transannular interactions
between the bulky indenyl substituents. The distortions in the
(23) Bradley, C. A.; Veiros, L. F.; Chirik, P. J. Organometallics 2007,
26, 3191.

Bis(indenyl)hafnium Chemistry
Organometallics, Vol. 28, No. 8, 2009 2475
The molecular structure of 1-Cl was determined by single-
crystal X-ray diffraction and is presented in Figure 4. Two
independent molecules were located in the asymmetric unit, and
data from both compounds are presented in Table 3. Both rings
are η5-coordinated and exhibit metrical parameters consistent
with typical bent metallocenes of hafnium. The two indenyl rings
are nearly eclipsed with rotational angles of 31.8(1)° and
27.2(1)°. The [SiMe3] substituents are geared in a manner to
avoid transannular interactions. As expected for a bent metal-
locene, the chloride ligand is located in the center of the
metallocene wedge with Hf-Cl bond distances of 2.3807(7)
and 2.3855(8) Å.
The structural data establish a rare example of a crystallo-
graphically characterized, monomeric hafnium(III) complex.
Several bis(cyclopentadienyl)zirconium(III) monohalide com-
pounds have been synthesized and structurally characterized.
Depending on the specific cyclopentadienyl substitution pattern,
these compounds can be either diamagnetic or paramagnetic.
24
For example, [(η5-C5H5)2ZrCl]2 and [(η5-C5H4-SiMe3)2ZrX]2
(X ) Cl, I)25 are both dimeric and diamagnetic, owing to
antiferromagnetic coupling between the two Zr(III) centers.26
More substituted zirconocenes such as [(η5-C5H3-1,3-(SiMe3)2)2-
ZrCl]2 are dimeric but have weak interactions between the metal
centers and are hence paramagnetic.25 In one case, a monomeric,
paramagnetic compound, (η5-C5H3-1,3-tBu)2ZrCl, was structur-
ally characterized.27
By comparison, there is a paucity of well-defined, structurally
authenticated hafnium(III) compounds. Reduction of HfI4 with
metallic aluminum was reported to yield HfI3.28 Cotton29 and
Girolami30,31 have obtained the solid state structures of dia-
magnetic, dimeric hafnium(III) phosphine compounds purported
to have metal-metal bonds. In metallocene chemistry, bis-
(cyclopentadienyl)hafnium(III) monohalide compounds are
rare27 and, to our knowledge, lack structural characterization.
Reduction of (η5-C5H5)2HfCl2 with sodium amalgam report-
edly yields [(η5-C5H5)2HfCl]2, which was formulated as a
dimer based on limited spectroscopic and other analytical
data.32The hafnium(III) anion of (η5-C5H5)2HfCl2 was generated
Figure 3. Solid state structure of the anion {[1-Cl]2Na}- with 30%
probability ellipsoids and hydrogen atoms omitted for clarity.
bound benzo ring are consistent with two-electron reduction with
a relatively large dihedral angle of 25.37(28)°, comparable to
1-THF.
Interestingly, treatment of benzene-d6 solutions of {[1-
Cl]2Na}{Na(DME)3} (these solutions also contain variable
amounts of 1-DME) with 1 atm of CO resulted in precipitation
of sodium chloride and formation of the hafnocene dicarbonyl
electrochemically33 and observed by EPR spectroscopy (gav
)
1
complex 1-(CO)2. No 1-Cl2 was detected by H NMR spec-
1.9839). Lappert has reported the EPR spectrum (gav ) 1.9887)
of the hafnium(III) alkyl (η5-C5H4-iPr)2HfCH2SiMe3,34 as well
as a hafnocene dialkyl anion (gav ) 1.987).35
troscopy, suggesting that {[1-Cl]2Na}{Na(DME)3} behaves more
like a sodium chloride complex of Hf(II) rather than a Hf(III)
monochloride compound (Vide infra).
In addition to X-ray diffraction, 1-Cl was also characterized
by solution spectroscopic techniques. The benzene-d6 1H NMR
spectrum of 1-Cl at 23 °C exhibits a single broad (∆ν1/2 ) 287
Hz) resonance centered at 5.72 ppm likely due to the [SiMe3]
substituents. Toluene solution and glass EPR measurements were
Additional solvents were screened for the sodium amalgam
reduction of 1-Cl2. More hindered THF ligands were of
particular interest, as it was hoped that less donating cyclic ethers
would stabilize reactive intermediates and prevent decomposition
but not bind as strongly as the THF and DME ligands in 1-THF
and 1-DME, respectively. Reduction of a diethyl ether solution
of 1-Cl2 with excess 0.5% sodium amalgam in the presence of
excess 2,5-dimethyl or 2-methyl THF resulted in isolation of a
green solid identified as the bis(indenyl)hafnocene chloride
complex 1-Cl (eq 5). Performing the reduction reaction in pure
diethyl ether also produced the same product and proved a more
reproducible and convenient protocol.
(24) Wielstra, Y.; Gambarotta, S.; Meetsma, A.; De Boer, J. L.
Organometallics 1989, 8, 250.
(25) Pool, J. A.; Chirik, P. J. Can. J. Chem. 2005, 83, 286.
(26) (a) DeKock, R. L.; Peterson, M. A.; Reynolds, L. E. L.; Chen,
L.-H.; Baerends, E. J.; Vernooijs, P. Organometallics 1993, 12, 2794. (b)
Be´nard, M.; Rohmer, M.-M. J. Am. Chem. Soc. 1992, 114, 4785.
(27) Urazowski, I. F.; Ponomaryev, V. I.; Nifant’ev, I. E.; Lemenovskii,
D. A. J. Organomet. Chem. 1989, 368, 287.
(28) Baker, W. A.; Janus, A. R. J. Inorg. Nucl. Chem. 1964, 26, 2087.
(29) Cotton, F. A.; Kibla, P. A.; Wojtczak, W. A. Inorg. Chim. Acta
1990, 177, 1.
(30) Morse, P. M.; Wilson, S. R.; Girolami, G. S. Inorg. Chem. 1990,
29, 3200.
(31) Riehl, M. E.; Wilson, S. R.; Girolami, G. S. Inorg. Chem. 1993,
32, 218.
(32) Cuenca, T.; Royo, P. J. Organomet. Chem. 1985, 293, 61.
(33) Fakhr, A.; Mugnier, Y.; Gautheron, B.; Laviron, E. J. Organomet.
Chem. 1986, 302, C7.
(34) Lappert, M. F.; Pickett, C. J.; Riley, P. I.; Yarrow, P. I. W. J. Chem.
Soc. Dalton 1981, 805.
(35) Lappert, M. F.; Raston, C. L. Chem. Commun. 1980, 1284.

2476 Organometallics, Vol. 28, No. 8, 2009
Pun et al.
Figure 4. Solid state structure of 1-Cl with 30% probability ellipsoids. Only one of the independent molecules found in the asymmetric unit
is shown.
Table 3. Selected Bond Distances (Å) and Angles (deg) for the Two
Independent Molecules of 1-Cl in the Asymmetric Unit
1-Cl
2.3807(7)
1-Cl
2.3855(8)
Hf(1)-Cl(1)
Hf(1)-C(1)
Hf(1)-C(2)
Hf(1)-C(3)
Hf(1)-C(4)
Hf(1)-C(9)
Hf(1)-C(10)
Hf(1)-C(11)
Hf(1)-C(12)
Hf(1)-C(13)
Hf(1)-C(18)
C(5)-C(6)
Hf(2)-Cl(2)
Hf(2)-C(31)
Hf(2)-C(32)
Hf(2)-C(33)
Hf(2)-C(34)
Hf(2)-C(39)
Hf(2)-C(40)
Hf(2)-C(41)
Hf(2)-C(42)
Hf(2)-C(43)
Hf(2)-C(48)
C(35)-C(36)
C(36)-C(37)
C(37)-C(38)
C(44)-C(45)
C(45)-C(46)
C(46)-C(47)
2.451(2)
2.417(2)
2.481(3)
2.580(3)
2.574(2)
2.436(2)
2.438(2)
2.492(3)
2.556(2)
2.543(2)
1.359(4)
1.401(4)
1.360(4)
1.368(4)
1.400(4)
1.367(4)
2.436(3)
2.433(3)
2.489(3)
2.556(3)
2.539(3)
2.447(3)
2.423(3)
2.481(3)
2.592(3)
2.592(3)
1.350(4)
1.420(4)
1.353(4)
1.353(4)
1.404(4)
1.350(4)
C(6)-C(7)
C(7)-C(8)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
indenyl fold anglesa 8.79(9)/8.21(15) indenyl fold angles 7.96(16)/9.63(8)
dihedral angleb
rotational anglec
2.63(13)/2.59(28) dihedral angles
31.8(1) rotational angle
3.02(25)/2.64(19)
27.2(1)
Figure 5. Toluene glass EPR spectrum of 1-Cl recorded at 77 K.
a Defined as the angle between the planes formed by the five-
membered cyclopentadienyl and the six-membered benzo rings of the
indenyl. b Defined as the deviation from planarity of the benzo ring,
which is the angle between the planes formed by C(5)-C(4)-C(9)-
C(8) and C(5)-C(6)-C(7)-C(8) in molecule 1. c Defined as the angle
formed between the plane defined by Hf(1), C(2), and the centroid
between C(4)-C(9) and the plane defined by Hf(1), C(11), and the
centroid between C(13)-C(18) in molecule 1.
mine whether 1-THF and 1-DME could serve as convenient
and isolable sources of divalent bis(indenyl)hafnium. The lability
of the coordinated THF was evaluated with an isotopic labeling
experiment. Addition of excess THF-d8 to a benzene-d6 solution
of 1-THF yielded a mixture of 1-THF and 1-THF-d8 over the
course of minutes at 23 °C. The isolation of 1-DME and
previous kinetic studies with the zirconium congener 5-THF22
suggest that THF exchange is likely associative.
made over a range of temperatures, and a representative
spectrum collected at 77 K is presented in Figure 5. Spectra
recorded at 5, 26, 50, and 293 K are reported in the Supporting
Information. At 77 K, a rhombic signal is observed for 1-Cl
with gx) 1.98, gy ) 1.96, and gz ) 1.70. Hyperfine coupling to
both spin-active hafnium nuclei, 179Hf (S ) 9/2) and 177Hf (S
) 7/2), is observed.
The thermal stability of 1-THF was also evaluated. Thermolysis
of the zirconium compound 5-THF at 85 °C resulted in C-O bond
cleavage and yielded the ring-opened zirconium(IV) product
(η5-C9H5-1,3-(SiMe3)2)2Zr(cyclo-O(CH2)3CH2).36,37 For the hafno-
cene complex, warming a benzene-d6 solution of 1-THF to 85
°C produced a mixture of products, depending on the concentra-
A preliminary survey of the reactivity of 1-Cl was conducted.
Addition of 1 atm of carbon monoxide to a benzene-d6 solution
of 1-Cl induced disproportionation and yielded a near equimolar
mixture of the hafnocene dichloride 1-Cl2 and the dicarbonyl
1-(CO)2. This reaction is typical of zirconocene(III) monohalide
compounds.24,25 In one-electron chemistry, 1-Cl is an efficient
chlorine atom acceptor, as addition of PbCl2 cleanly furnished
the hafnocene dichloride compound 1-Cl2.
(36) Bradley, C. A.; Veiros, L. F.; Pun, D.; Lobkovsky, E.; Keresztes,
I.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 16600.
(37) For related C-O bond cleavage chemistry with reduced group 4
metallocenes see: (a) Beweries, T.; Burlakov, V. V.; Bach, M. A.; Baumann,
W.; Spannenberg, A.; Rosenthal, U. Organometallics 2007, 26, 247. (b)
Beweries, T.; Ja¨ger-Fiedler, U.; Bach, M. A.; Burlakov, V. V.; Arndt, P.;
Baumann, W.; Spannenberg, A.; Rosenthal, U. Organometallics 2007, 26,
3000.
Reactivity of 1-THF and 1-DME. The reactivity of the η6,η5-
bis(indenyl)hafnium ligand complexes was examined to deter-

Bis(indenyl)hafnium Chemistry
Organometallics, Vol. 28, No. 8, 2009 2477
Scheme 1
tion of THF. When the thermolysis was conducted in the
presence of 5 equiv of THF, the predominant product was that
arising from C-O bond cleavage, (η5-C9H5-1,3-(SiMe3)2)2Hf-
(cyclo-O(CH2)3CH2) (1-cyclo-O(CH2)3CH2) Scheme 1).
In contrast, allowing a benzene-d6 solution of 1-THF to stand
at ambient temperature under vacuum produced a 2.7:1 mixture
of 1-cyclo-O(CH2)3CH2 and a new C1 symmetric compound,
identified as the hafnium cyclometalated hydride 1-CMH
(Scheme 1). Repeating the reaction at 85 °C resulted in a 4:1
mixture of 1-cyclo-O(CH2)3CH2 to 1-CMH. The observation
of 1-CMH following THF loss suggests that C-H oxidative
addition to the putative η5,η5-bis(indenyl)hafnium is facile, more
facile than the benzo ring coordination; thus the desired η9,η5-
bis(indenyl)hafnium sandwich complex was not observed.
For 1-DME, C-O bond cleavage was more rapid. Allowing a
benzene-d6 solution of the compound to stand at 23 °C resulted in
smooth conversion to the bis(indenyl)hafnocene bis(methoxide)
1-(OMe)2 and metallocyclopentane 1-C4H8 in an approximate 2:1
ratio (Scheme 1). The latter derives from the reductive coupling
of 2 equiv of ethylene, unlike the zirconium congener, where the
ethylene complex was isolated from DME cleavage. 1-C4H8 was
independently synthesized by addition of ethylene to 1-THF (eq
6). Attempts to isolate the bis(indenyl)hafnocene ethylene com-
pound by addition of substoichiometric amounts of olefin resulted
in partial conversion to 1-C4H8.
Comparison of the Electronic Structures of Bis(indenyl)-
zirconium and Hafnium Complexes. Bis(indenyl)hafnium
dicarbonyl compounds were synthesized to evaluate the elec-
tronics of the metal center as compared to the zirconium
congeners.18,38 Exposure of a pentane solution of 1-THF to 4
atm of carbon monoxide for two days at 23 °C followed by
solvent removal and recrystallization from pentane at -35 °C
furnished a dark solid identified as 1-(CO)2 (Figure 6). Because
THF and DME complexes of the alkylated bis(indenyl)hafnium
compounds were not isolated, the corresponding hafnocene
dicarbonyl complexes were prepared by alkali metal reduction
of the dichloride compounds under a CO atmosphere.
With a family of dicarbonyl complexes in hand, the infrared
spectrum of each complex in pentane was recorded. The
silylated hafnocene dicarbonyl 1-(CO)2 exhibits six CO bands,
similar to the zirconium congener. Two bands are expected for
a bent metallocene dicarbonyl. The additional bands were
attributed to the presence of different rotamers that have different
carbonyl stretching frequencies. It is remarkable that the relative
orientation of the indenyl rings has such a measurable impact
on the carbonyl stretching frequencies. The zirconocene and
hafnocene dicarbonyl complexes with smaller indenyl substit-
uents exhibit fewer carbonyl bands due to either population of
fewer rotamers or the rotamers having indistinguishable CO
stretching frequencies.
The general trend established for bis(indenyl)zirconium
dicarbonyl compounds also holds for the corresponding hafnium
compounds. The silylated hafnocene 1-(CO)2 is the least electron
rich in the series, while introduction of alkyl substituents on
the indenyl ring produces more reducing hafnium centers (Figure
(38) Bradley, C. A.; Flores-Torres, S.; Lobkovsky, E.; Abrun˜a, H. D.;
Chirik, P. J. Organometallics 2004, 23, 5332.

2478 Organometallics, Vol. 28, No. 8, 2009
Pun et al.
Figure 6. Carbonyl stretching frequencies for bis(indenyl)zirconium and hafnium complexes.
6). Comparison of zirconium and hafnium congeners establishes
that the third-row metal, as expected, is more reducing.
Oxidative Addition Chemistry: H2 Addition and C-H
Bond Activation Studies. Bis(indenyl)zirconium and hafnium
hydride complexes are of interest due to their potential applica-
tion as initiators in olefin polymerization catalysis39 as well as
in catalytic bond-forming reactions used in small-molecule
acterized.42 The syn isomer was obtained from hydrogen
addition to the sandwich, while the anti was observed from
treatment of the THF complex with H2. Over time, the anti
zirconium DHID complex isomerized to the syn isomer. In
hafnocene chemistry, the hydrogenation of 1-THF yielded
only the syn diastereomer, the stereochemistry of which was
established by NOESY NMR spectroscopy (Figure 7).
The protons of the six-membered ring of the dihydroindene-
diyl ligand were the first resonances assigned on the basis of
both NOE correlations and chemical shifts. Using this approach,
the resonances at 2.32 and 2.51 ppm were assigned as the
methylene protons. Cross-peaks between the hafnium hydride
and the latter methylene proton (the endo position) and the
proton on the adjacent allylic carbon indicate that this compound
is in the syn conformation. The absence of correlations between
the hafnium hydride and other allylic protons further cor-
roborates this assignment.
catalysis such as olefin and imine hydrogenation.40 The putative
monomeric hafnocene dihydride, 1-H2, could in principle
undergo H2 reductive elimination and provide a mild synthetic
route to the desired η9,η5-bis(indenyl)hafnium sandwich. Fun-
damental studies of oxidative addition reactions at a reduced
hafnium center are also rare owing to the paucity of isolable
Hf(II) complexes. For these reasons, the oxidative addition of
H2 and C-H bonds to 1-THF was studied.
Exposure of a pentane solution of 1-THF to 4 atm of
dihydrogen resulted in a rapid color change from dark green to
1
yellow. Analysis of the isolated tan solid by H and 13C NMR
spectroscopy established formation of a C1 symmetric hafnocene
product. The benzene-d6 1H NMR spectrum exhibits a hafnium
hydride peak centered at 3.03 ppm and methylene multiplets
centered at 2.32 and 2.51 ppm. These spectral features are
consistent with formation of (η5-C9H5-1,3(SiMe3)2)(η5,η3- C9H6-
1,3(SiMe3)2)HfH (1-DHID-H), where a hydrogen has migrated
to one of the indenyl rings to form an unusual η5,η3-4,5-
dihydroindenediyl (DHID) ligand (eq 7).
A deuterium labeling study was also conducted to provide
additional support for the NMR assignments and the observed
reaction chemistry. Exposure of a benzene-d6 solution of 1-THF
to 4 atm of D2 gas yielded 1-DHID-d1-D and established
exclusive deuterium incorporation into the hafnium-hydride
(deuteride) and the endo methylene position on the dihydroin-
denediyl ring (eq 8).
There are two possible diastereomers of 1-DHID-H,
defined by the relative orientation of the hafnium hydride
and the newly formed methylene carbon. The case where
these two groups are in proximity has been designated syn,
where they are opposite, anti. In zirconium chemistry, both
diastereomers were isolated and crystallographically char-
As in the zirconium chemistry,42 1-DHID-H is an intermedi-
ate during benzo ring hydrogenation and formation of the
corresponding tetrahydroindenyl compound, 1-THI-H2. How-
ever, complete conversion of 1-DHID-H to 1-THI-H2 required
heating at 65 °C for one week under 4 atm of dihydrogen. The
(39) (a) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV.
2000, 100, 1253. (b) Coates, G. W. Chem. ReV. 2000, 100, 1223. (c)
Brintzinger, H. H.; Fisher, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M.
Angew. Chem., Int. Ed. 1995, 34, 1143.
(40) (a) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1992,
114, 7562. (b) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994,
116, 8952. (c) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994,
116, 11703. (d) Troutman, M. V.; Apella, D. H.; Buchwald, S. L. J. Am.
Chem. Soc. 1999, 121, 4916.
1
final product was characterized by the appearance of H NMR
resonances in the vicinity of 2 ppm and the appearance of a
(41) Chirik, P. J.; Bercaw, J. E. Organometallics 2005, 24, 5407.
(42) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am.
Chem. Soc. 2006, 128, 6454.

Bis(indenyl)hafnium Chemistry
Organometallics, Vol. 28, No. 8, 2009 2479
Figure 7. NOESY NMR spectrum of syn-1-DHID-H. Circles and ovals depict assignment of the dihydroindenediyl ring, while bold squares
the orientation of the Hf-H over H1 and H3.
Isolation of 1-H2 allowed evaluation of its potential inter-
hafnium hydride at 14.95 ppm, consistent with a monomer in
benzene-d6.20,41
mediacy in the isomerization to 1-DHID-H. Allowing a
benzene-d6 solution of 1-H2 to stand at 23 °C resulted in only
trace amount of the isomerization product after several days.
Heating the sample to 65 °C did not induce additional
conversion. Because these results demonstrate that THF likely
plays a role in formation of 1-DHID-H, various amounts of
THF were added to a benzene-d6 solution of 1-H2. The results
of these studies are summarized in Table 4.
Hydrogenation of the corresponding zirconium compound,
5-THF, also yielded a zirconium hydride complex with an η5,η3-
4,5-dihydroindenediyl (DHID) ligand.42 As in the hafnium case,
no intermediate zirconocene dihydride (or THF adduct) was
observed by 1H NMR spectroscopy. For a family of zirconium
dihydride complexes, THF was shown to accelerate the rate of
rearrangement.
Increased amounts of THF favor the isomerization of 1-H2
to 1-DHID-H. Because the reactions occurred immediately after
mixing and do not change over the course of days at 23 °C, the
conversions in Table 4 represent equilibria. Interestingly,
complete conversion of 1-H2 to 1-DHID-H was never observed
in contrast to the hydrogenation of 1-THF (with only 1 equiv
of THF), where no 1-H2 was detected by 1H NMR spectroscopy.
These studies establish that the hydrogenation of 1-THF does
not produce 1-H2 as an intermediate on the isomerization
pathway and that H2 oxidative addition likely occurs directly
at the η6 compound. Similar results were obtained with
deuterium labeling as addition of excess THF to a benzene-d6
solution of 1-D2 resulted in exclusive isotopic incorporation into
the hafnium-hydride (deuteride) position along with the endo
methylene in the dihydroindenediyl ring of 1-DHID-D.
Because dihydrogen addition to 1-THF did not yield an
observable hafnocene dihydride prior to rearrangement to
1-DHID-H, alternative routes for the synthesis of 1-H2 were
explored. Treatment of 1-Cl2 with 2 equiv of a toluene solution
of NaBEt3H followed by filtration and recrystallization from
pentane furnished the desired hafnocene dihydride complex,
1-H2 (eq 9). The benzene-d6 1H NMR spectrum of white 1-H2
exhibits the number of peaks consistent with a C2V symmetric
hafnocene. The hafnium hydride peak appears downfield at
14.85 ppm, consistent with a monomer in solution.41
Upon addition of excess H2, the hafnium hydride resonance
of 1-H2 broadens, a result of fast exchange between free
dihydrogen and the Hf-H positions. Addition of 4 atm of D2 to
a benzene-d6 solution of 1-H2 at 23 °C immediately formed
1-D2 upon mixing.

2480 Organometallics, Vol. 28, No. 8, 2009
Pun et al.
Table 4. Conversion of 1-H2 to 1-DHID-H as a Function of Added
THF
Only one diasteromer was observed by NMR spectroscopy but
was not assigned.
equiv of added THF
% conversion
1
10
50
66
84
94
The oxidative addition of aromatic C-H bonds was also
explored. For reduced complexes of both bis(indenyl)43 and
bis(cyclopentadienyl)zirconium,44 N,N-dimethylaminopyridine
(DMAP) has proven to be a useful substrate, participating in
facile C-H bond activation under relatively mild conditions.
Stirring a pentane solution of 1-THF with 1 equiv of DMAP
for 16 h at 23 °C resulted in clean C-H activation chemistry
to yield the ortho-metalated product, 1-DMAP, as a fluffy white
Based on these results, similar chemistry was explored with
the zirconium congener, 5-THF. Addition of N2CPh2 to 5-THF
also furnished the zirconocene diazoalkane complex with a
molecule of coordinated THF. Similar to its hafnocene congener,
5-N2CPh2(THF) undergoes rapid hydrogenation to form Zr-H
and N-H bonds via 1,2-addition (eq 11).
Because N2 loss from the diazoalkane derivatives of the
silylated metallocenes was not observed, similar chemistry with
a smaller, more electron-rich bis(indenyl)zirconium sandwich
was explored. Addition of N2CPh2 to a pentane solution of the
base-free zirconocene sandwich, 6, furnished a Cs symmetric
product identified as the zirconocene η2-diazoalkane complex
(eq 13). Attempts to induce N2 loss by thermolysis were
unsuccessful. Warming a benzene-d6 solution of the compound
to 65 °C for two days produced no change; additional heating
to 95 °C resulted in decomposition by fulvene loss. As with
the silyl-substituted metallocene diazoalkane complexes,
6-N2CPh2 undergoes rapid hydrogenation (eq 13). Diagnostic
N-H and Zr-H resonances were observed by 1H NMR
spectroscopy.
1
powder (eq 10). The H and 13C NMR spectra recorded in
benzene-d6 exhibit the number of peaks consistent with Cs
symmetry. One notable feature is the observation of the hafnium
hydride centered at 6.93 ppm.
Reactions with N2CPh2. With potential divalent hafnocene
sources in hand, the synthesis of hafnium alkylidene complexes
by N2 displacement from diazoalkanes was explored. Hafnium
alkylidenes are known,45,46 but the lone isolable example was
prepared by R-hydrogen abstraction of a dialkyl derivative.
Treatment of 1-THF with N2CPh2 in pentane solution furnished
an orange, Cs symmetric compound identified as the hafnocene
diazoalkane complex with a molecule of coordinated THF,
1-N2CPh2(THF) (eq 11).
Concluding Remarks
The chemistry of reduced bis(indenyl)hafnium compounds
has been explored. While the η9,η5-bis(indenyl)hafnium sand-
wich has thus far eluded isolation, ligand-induced haptotropic
rearrangements with THF and DME have proven to be a
successful strategy for the synthesis of η6,η5-bis(indenyl)hafnium
ligand compounds. Changing the medium of the reduction
reaction to diethyl ether resulted in isolation and crystallographic
characterization of a rare bis(indenyl)hafnium(III) complex.
Crystallographic characterization of the THF derivative as well
as infrared spectroscopy on dicarbonyl complexes establishes
that the hafnium is more reducing than zirconium in analogous
compounds.
The increased reduction potential of the hafnocene congeners
manifests in the reactivity of the compounds. In the absence of
excess THF, the η6,η5-bis(indenyl)hafnium THF compound
undergoes competitive cyclometalation of an indenyl [SiMe3]
substituent along with C-O bond cleavage. The observation
of facile C-H activation demonstrates that if formed, the η5,η5-
bis(indenyl)hafnium undergoes oxidative addition rather than
benzo ring coordination.
The reactivity of 1-N2CPh2(THF) was briefly explored.
Previously our laboratory reported the 1,2-addition of dihydro-
gen to the bis(cyclopentadienyl) titanium diazoalkane complex
(η5-C5H3-1,3-(SiMe3)2)2Ti(N2CPh2).14 Addition of 1 atm of
dihydrogen to a benzene-d6 solution to 1-N2CPh2(THF) for 2 h
at 23 °C produced an orange solid identified as the product of
H2 1,2-addition, 1-NHNCPh2(H), with loss of free THF (eq
11). Notable 1H NMR spectroscopic features include observation
of an N-H peak centered at 6.22 ppm and the hafnium hydride
at 9.31 ppm. Attempts to induce N2 loss by heating the sample
to 65 °C for 24 h resulted in 1,2-addition of a SiMe3 C-H across
the hafnium imido bond to form 1-CM(NHNdCPh2) (eq 12).
(43) Bradley, C. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
2003, 125, 8110.
(44) Bernskoetter, W. H.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2006, 25, 1092.
(45) Fryzuk, M. D.; Duval, P. B.; Patrick, B. O.; Rettig, S. J.
Organometallics 2001, 20, 1608.
(46) For examples of hafnium alkylidenes prepared by laser ablation: (a)
Cho, H. G.; Andrews, L. Organometallics 2004, 23, 4357. (b) Cho, H. G.;
Wang, X. F.; Andrews, L. Organometallics 2005, 24, 2854.
Other observed reactivity demonstrates the utility of the THF
complex as an isolable hafnium(II) source. Facile oxidative
addition of dihydrogen and an aromatic C-H bond of N,N-
dimethylaminopyridine were observed. In the former case, the
presence of THF facilitated isomerization to a dihydroindenyl

Bis(indenyl)hafnium Chemistry
Organometallics, Vol. 28, No. 8, 2009 2481
Preparation of (η5-C9H5-1,3-(CHMe2)2)2HfCl2 (2-Cl2). This
molecule was prepared in a similar manner to 1-Cl2 with 2.05 g
(9.94 mmol) of Li[C9H5-1,3-(CHMe2)2] and 1.60 g (5.00 mmol) of
HfCl4, yielding 2.16 g (67%) of 2-Cl2. Anal. Calcd for C30H38HfCl2:
hafnium hydride derivative, which is an intermediate to the
corresponding tetrahydroindenyl hafnium dihydride. Reactivity
studies with diphenyldiazomethane were also conducted, and
hydrogenation of the coordinated diazoalkane by 1,2-addition
was observed.
1
C, 55.60; H, 5.91. Found: C, 56.56; H, 3.78. H NMR (benzene-
d6): δ 0.94 (d, 8 Hz, 12H, CHMe2), 1.24 (d, 8 Hz, 12H, CHMe2),
2.98 (m, 4H, CHMe2), 6.76 (s, 2H, Cp), 6.91 (m, 4H, Benzo), 7.24
(m, 4H, Benzo). 13C NMR (benzene-d6): δ 20.74, 25.42, 25.62
(CHMe2), 123.69, 125.04, 125.85, 125.95 (Cp/Benzo). One Cp/
Benzo resonance not located.
Experimental Section
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out using standard vacuum line, Schlenk,
or cannula techniques or in an M. Braun inert atmosphere drybox
containing an atmosphere of purified nitrogen. Solvents for air- and
moisture-sensitive manipulations were initially dried and deoxy-
genated using literature procedures.47 Benzene-d6 and toluene-d8
for NMR spectroscopy were distilled from sodium metal under an
atmosphere of argon and stored over 4 Å molecular sieves or sodium
metal. Dichloromethane-d2 was distilled from CaH2 under vacuum
prior to use. Chlorotrimethylsilane was purchased from Acros
Organics and dried over CaH2 before use. Carbon monoxide was
Preparation of rac/meso-(η5-C9H5-1-(iPr)-3-(Me))2HfCl2 (rac/
meso-3-Cl2). This molecule was prepared in a similar manner to
1-Cl2 in a 100 mL round-bottom flask with 2.00 g (11.22 mmol)
of Li[C9H5-1-iPr-3-Me], 1.80 g (5.61 mmol) of HfCl4, and ap-
proximately 160 mL of diethyl ether. This yielded 2.34 g (72%) of
yellow 3-Cl2 as a mixture of rac and meso isomers. Anal. Calcd
1
for C26H30HfCl2: C, 52.76; H, 5.11. Found: C, 52.80; H, 4.77. H
NMR (benzene-d6): δ 0.99 (d, 7 Hz, 6H, CHMe2), 1.06 (d, 7 Hz,
6H, CHMe2), 1.14 (d, 7 Hz, 12H, CHMe2), 2.20 (s, 6H, Me), 2.21
(s, 6H, Me), 3.31 (m, 2H, CHMe2) 3.52 (m, 2H, CHMe2), 5.59 (s,
2H, CpH), 5.67 (s, 2H, CpH), 6.92 (m, 6H, Benzo), 7.01 (m, 2H,
Benzo), 7.17 (m, 2H, Benzo), 7.23 (m, 2H, Benzo), 7.37 (m, 2H,
Benzo), 7.44 (m, 2H, Benzo). 13C{1H} NMR (benzene-d6): δ 12.47,
12.75 (Me), 21.26, 22.21, 24.91, 24.93, 27.25, 27.84 (CHMe2),
100.22, 105.31, 109.49, 113.21, 117.95, 119.34, 121.11, 122.99,
123.60, 124.55, 124.77, 124.87, 124.99, 125.59, 126.75, 129.98 (Cp/
Benzo). Two Cp/Benzo resonances not located.
purchased from Aldrich and was passed through a liquid nitrogen
48,49
cooled trap before use. Both C9H6-1,3-(SiMe3)2
and Li[C9H5-
1,3-(SiMe3)2]49 were prepared according to literature procedures.
1H and 13C NMR spectra were recorded on a Varian Inova 400
spectrometer operating at 399.779 MHz (1H) and 110.524 MHz
(13C), respectively. Two-dimensional NMR spectra were recorded
on a Varian Inova 500 spectrometer operating at 500.62 Hz. All
1
chemical shifts are reported relative to SiMe4 using H (residual)
Preparation of (η5-C9H5-1-(iPr)-3-(Me))HfI3 (4-I3). A 100 mL
round-bottom flask was charged with 1.70 g (2.87 mmol) of 2-Cl2,
and approximately 60 mL of toluene was added. BI3 (1.41 g, 3.59
mmol) was added slowly to the yellow solution, and the resulting
mixture stirred for two days. The solvent was removed in Vacuo,
and the resulting brown solid washed with cold pentane, yielding
1.32 g (63%) of 4-I3 instead of the desired diiodide species (4-I2)..
1H NMR (benzene-d6): δ 0.93 (d, 7 Hz, 3H, CHMe2), 1.29 (d, 7
Hz, 3H, CHMe2), 2.30 (s, 3H, Me), 3.70 (m, 1H, CHMe2), 6.48 (s,
1H, CpH), 6.75 (m, 2H, Benzo), 7.34 (m, 2H, Benzo).
Preparation of rac/meso-(η5-C9H5-1-(iPr)-3-(Me))2HfI2 (rac/
meso-3-I2). A 100 mL round-bottom flask was charge with 0.921
g (1.26 mmol) of 4-I3, and approximately 30 mL of diethyl ether
was added. The pale yellow solution was chilled in a liquid nitrogen
cooled cold well for 20 min, 0.225 g (1.26 mmol) of Li[C9H5-1-
(iPr)-3-(Me)] was added to the solution, and the resulting mixture
was stirred for two days. The solvent was removed in Vacuo, and
the orange solid washed with pentane and subsequently extracted
into toluene and filtered through Celite. Removal of the toluene in
Vacuo yielded 0.888 g (91%) of 3-I2 as a yellow powder. Anal.
Calcd for C26H30HfI2: C, 40.30; H, 3.90. Found: C, 39.63; H, 3.40.
1H NMR (benzene-d6): δ 0.97 (d, 7 Hz, 6H, CHMe2), 0.98 (d, 7
Hz, 6H, CHMe2), 1.12 (d, 7 Hz, 6H, CHMe2), 1.19 (d, 7 Hz, 6H,
CHMe2), 2.08 (s, 6H, Me), 2.28 (s, 6H, Me), 3.32 (m, 2H, CHMe2)
3.95 (m, 2H, CHMe2), 6.01 (s, 2H, CpH), 6.42 (s, 2H, CpH), 6.86
(m, 4H, Benzo), 6.92 (m, 4H, Benzo), 7.17 (m, 2H, Benzo), 7.28
(m, 2H, Benzo), 7.33 (m, 2H, Benzo), 7.43 (m, 2H, Benzo). 13C{1H}
NMR (benzene-d6): δ 12.73, 14.21 (Me), 21.28, 22.05, 25.53, 25.84,
27.73, 30.16 (CHMe2), 106.85, 110.17, 122.91, 123.83, 124.15,
124.25, 124.99, 125.20, 125.50, 125.79, 126.48, 126.64, 126.92,
127.04, 127.29, 128.59 (Cp/Benzo). Two Cp/Benzo resonances not
located.
or 13C NMR chemical shifts of the solvent as a secondary standard.
Infrared spectroscopy was conducted on a Thermo Nicolet spec-
trometer. X-Band ESR spectra were recorded on a Bruker EMX
spectrometer at a frequency of 9.55 GHz under standard conditions
in 4 mm i.d. quartz tubes. The tubes were sealed under vacuum at
77 K. Spectra at temperatures 4.2-77 K were recorded using a
liquid helium cryostat, ESR-10 (Oxford Instruments Ltd., England).
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox and were quickly transferred to
the goniometer head of a Siemens SMART CCD area detector
system equipped with a molybdenum X-ray tube (λ ) 0.71073 Å).
Preliminary data revealed the crystal system. A hemisphere routine
was used for data collection and determination of lattice constants.
The space group was identified and the data were processed using
the Bruker SAINT program and corrected for absorption using
SADABS. The structures were solved using direct methods
(SHELXS) completed by subsequent Fourier synthesis and refined
by full-matrix least-squares procedures. Elemental analyses were
performed at Robertson Microlit Laboratories, Inc., Madison, NJ.
Preparation of (η5-C9H5-1,3(SiMe3)2)2HfCl2 (1-Cl2). A 500 mL
round-bottomed flask was charged with 3.65 g (13.7 mmol) of
Li[C9H5-1,3(SiMe3)2] and approximately 150 mL of diethyl ether.
The resulting yellow solution was chilled in a cold well cooled to
liquid nitrogen temperature, and 2.19 g (6.85 mmol) of HfCl4 was
added. The reaction mixture was stirred for one day, and the solvent
was removed in Vacuo. The resulting solid was dissolved in
dichloromethane and filtered through a pad of Celite. Removal of
dichloromethane in Vacuo yielded a brown oil that was recrystallized
from diethyl ether at -35 °C to afford 2.21 g (42%) of 1-Cl2 as a
yellow solid. Anal. Calcd for C30H46Si4HfCl2: C, 46.89; H, 6.03.
1
Found: C, 46.75; H, 6.23. H NMR (benzene-d6): δ 0.37 (s, 36H,
SiMe3), 6.93 (s, 2H, Cp), 7.00 (m, 4H, Benzo), 7.74 (m, 4H, Benzo).
13C NMR (benzene-d6): δ 1.73 (SiMe3), 119.31, 126.49, 133.18,
138.29 (Cp/Benzo). One Cp/Benzo resonance not located.
Preparation of (η6-C9H5-1,3-(SiMe3)2)(η5-C9H5-1,3(SiMe3)2)-
Hf(C4H8O) (1-THF). A 20 mL scintillation vial was charged with
3.56 g of mercury and approximately 3 mL of diethyl ether. To
the stirring mixture was added 0.020 g (0.875 mmol) of sodium,
and the slurry was stirred for 10 min to ensure amalgamation. A
diethyl ether slurry (∼8 mL) containing 0.100 g (0.130 mmol) of
1-Cl2 was then added followed by 0.250 g of THF. The resulting
reaction mixture was stirred vigorously for 1 h. Filtration of the
(47) Pangborn, A.; Giardello, M.; Grubbs, R. H.; Rosen, R.; Timmers,
F. Organometallics 1996, 15, 1518.
(48) Davison, A.; Rakita, P. E. J. Organomet. Chem. 1970, 23, 407.
(49) Brady, E. D.; Overby, J. S.; Meredith, M. B.; Mussman, A. B.;
Cohn, M. A.; Hanusa, T. P.; Yee, G. T.; Pink, M. J. Am. Chem. Soc. 2002,
124, 9556.

2482 Organometallics, Vol. 28, No. 8, 2009
Pun et al.
green solution through Celite followed by solvent removal in Vacuo
yielded a green foam. Recrystallization from pentane at -35 °C
yielded 0.070 g (70%) of a green solid identified as 1-THF. Anal.
Calcd for C34H54Si4OHf: C, 53.06; H, 7.07. Found: C, 53.12; H,
6.73. 1H NMR (benzene-d6): δ 0.27 (s, 18H, SiMe3), 0.30 (s, 18H,
SiMe3), 0.74 (br, 4H, THF), 2.48 (br, 4H, THF), 3.62 (m, 2H,
Benzo), 3.69 (s, 1H, Cp), 5.98 (s, 1H, Cp), 6.75 (m, 2H, Benzo),
6.86 (m, 2H, Benzo), 7.75 (m, 2H, Benzo). 13C NMR (benzene-
d6): δ 1.38, 1.77 (SiMe3), 24.08, 74.83 (THF), 83.70, 90.28, 103.86,
114.47, 118.44, 122.89, 125.11, 126.19, 135.91, 142.43 (Cp/Benzo).
Characterization of (η5-C9H5-1,3-(SiMe3)2)(η5-C9H5-η1-1-
SiMe3-3-SiMe2CH2)]HfH (1-CMH). 1H NMR (benzene-d6): -2.23
(d, 12 Hz, 1H, Zr-CH2), -1.98 (d, 12 Hz, 1H, Zr-CH2), 0.20 (s,
9H, SiMe3), 0.23 (s, 9H, SiMe3), 0.39 (s, 9H, SiMe3), 0.45 (s, 3H,
SiMe2CH2), 0.62 (s, 3H, SiMe2CH2), 6.76 (s, 1H, CpH), 6.79 (m,
2H, Benzo), 6.84 (m, 2H, Benzo), 7.39 (m, 1H, Benzo), 7.41 (m,
1H, Benzo), 7.71 (m, 2H, Benzo), 7.84 (s, 1H, CpH), 13.26 (s,
1H, Zr-H). 13C{1H} NMR (benzene-d6): δ -2.66, -1.03, -0.79,
1.23, 1.83 (SiMe3/SiMe2), 34.38 (Zr-CH2), 91.22, 113.73, 114.88,
118.07, 121.65, 122.47, 123.04, 123.27, 124.05, 125.04, 125.15,
125.81, 126.48. Five Cp/Benzo resonances not located.
Preparation of (η6-C9H5-1,3(SiMe3)2)(η5-C9H5-1,3(SiMe3)2)-
Hf(C4H10O2) (1-DME). To a 20 mL scintillation vial with 0.119 g
(0.155 mmol) of 1-THF dissolved in approximately 6 mL of
benzene was added dropwise 35 mL (0.031 mg, 0.34 mmol) of
dimethoxyethane. The green solution was stirred for 90 min. Solvent
removal in Vacuo followed by recrystallization from diethyl ether
at -35 °C yielded 0.50 g (48%) of 1-DME as a green solid. Anal.
Calcd for C34H56Si4O2Hf: C, 51.84; H, 7.17. Found: C, 51.39; H,
6.58. 1H NMR (benzene-d6): δ 0.23 (s, 18H, SiMe3), 0.44 (s, 18H,
SiMe3), 1.49 (br s, 4H, OCH2s), 3.53 (s, 6H, OCH3), 3.91 (m, 2H,
Benzo), 4.30 (s, 1H, Cp), 6.20 (m, 2H, Benzo), 6.72 (s, 1H, Cp),
6.79 (m, 2H, Benzo), 7.16 (s, 1H, Cp), 7.53 (m, 2H, Benzo). 13C
NMR (benzene-d6): δ 1.92, 3.11 (SiMe3), 68.19 (OCH2), 69.12
(OCH3), 118.46, 141.38 (Cp), 79.22, 105.02, 123.81, 125.54
(Benzo), 83.86, 107.86, 120.40, 137.28 (Benzo).
Characterization of [(η5-C9H5-1,3-(SiMe3)2)2HfCl]2Na2(DME)3
([1-Cl]2Na2(DME)3). 1H NMR (benzene-d6): δ 0.36 (s, 36H,
SiMe3), 0.46 (s, 36H, SiMe3), 1.27 (br s, 18H, DME Me), 1.61 (br
s, 12H, DME CH2), 3.64 (s, 4H, Benzo), 6.10 (s, 2H, CpH), 6.61
(m, 4H, Benzo), 7.02 (m, 4H, Benzo), 7.73 (m, 4H, Benzo). One
CpH resonance not located.
Preparation of (η5-C9H5-1,3-(SiMe3)2)2Hf(CO)2 (1-(CO)2). A
small thick walled glass vessel was charged with 0.080 g of 1-THF
and approximately 10 mL of pentane. The contents of the vessel
were cooled to -196 °C and evacuated. At this temperature, 1 atm
of CO was admitted. The resulting reaction mixture was warmed
to ambient temperature and stirred for two days, during which time
the dark green solution turned black. The contents of the vessel
were cooled again, and the excess CO and solvent were removed
in Vacuo, yielding a dark foam. Recrystallization from pentane at
-35 °C yielded 0.070 g (89%) of 1-(CO)2 as a forest green solid.
Anal. Calcd for C32H46Si4O2Hf: C, 51.00; H, 6.15. Found: C, 50.69;
1
H, 5.87. H NMR (benzene-d6): δ 0.31 (s, 36 H, SiMe3), 5.69 (s,
2H, Cp), 6.79 (m, 3.2 Hz, 4H, Benzo), 7.24 (m, 3.2 Hz, 4H, Benzo).
13C NMR (benzene-d6): δ 1.16 (SiMe3), 94.24, 103.93, 124.73,
125.61, 126.94 (Cp/Benzo), 258.65 (Hf-CO). ν(CO) (pentane) )
1863, 1871, 1888, 1948, 1956, 1971 cm-1
.
Preparation of (η5-C9H5-1,3-(CHMe2)2Hf(CO)2 (2-(CO)2). A
thick walled glass vessel was charged with 0.264 g (0.407 mmol)
of 2-Cl2 and 0.115 g (4.07 mmol) of activated Mg powder. On the
high-vacuum line, approximately 15 mL of THF was vacuum
transferred onto the solids, and the resulting slurry frozen in liquid
nitrogen. One atmosphere of carbon monoxide was added to the
vessel at -196 °C. The vessel was thawed and then placed in a 45
°C oil bath for one day. The CO atmosphere and the solvent were
then removed in Vacuo, leaving a green residue. The vessel was
taken into the drybox, and the residue extracted into pentane.
Filtration through Celite followed by solvent removal and recrys-
tallization from pentane at -35 °C afforded 2-(CO)2 along with
Preparation of [(η5-C9H5-1,3-(SiMe3)2)2HfCl] (1-Cl). A 50 mL
round-bottom flask was charged with 14.37 g (0.072 mol) of
mercury, and approximately 10 mL of diethyl ether was added. To
the stirring solution, 0.072 g (3.13 mmol) of sodium was added,
and the slurry was stirred for 20 min to ensure amalgamation. A
diethyl ether slurry (∼30 mL) containing 0.400 g (0.521 mmol) of
1-Cl2 was then added. The resulting reaction mixture was stirred
vigorously for 1.25 h. Filtration of the green solution through Celite
followed by solvent removal in Vacuo yielded a brown-green foam.
Recrystallization from pentane at -35 °C yielded 70 mg (18%) of
green crystals identified as 1-Cl. Anal. Calcd for C30H46Si4HfCl:
1
unreacted 2-Cl2. H NMR (benzene-d6): δ 0.95 (d, 7 Hz, 12H,
CHMe2), 1.03 (d, 7 Hz, 12H, CHMe2), 2.32 (m, 4H, CHMe2), 5.18
(s, 2H, CpH), 6.73 (m, 4H, Benzo), 7.00 (m, 4H, Benzo). 13C NMR
(benzene-d6): δ 21.22, 25.73, 25.94 (CHMe2), 96.92, 107.82,
112.46, 122.33, 122.96 (Cp/Benzo), 281.95 (Hf-CO). ν(CO) (pen-
1
C, 49.16; H, 6.33. Found: C, 48.88; H, 6.35. H NMR (benzene-
tane) ) 1849, 1944 cm-1
.
d6): δ 5.72 (∆ν1/2 ) 287 Hz). EPR (toluene): gx ) 1.98, gy ) 1.96,
gz ) 1.70.
Preparation of rac/meso-(η5-C9H5-1-Me-3-CHMe2Hf(CO)2
(rac/meso-3-(CO)2). A thick walled glass bomb was charged with
2.85 g (14.21 mmol) of mercury and approximately 3 mL of toluene
in a nitrogen drybox. While stirring, 14 mg (0.61 mmol) of sodium
metal was added, and the resulting amalgam was stirred for 20
min. A toluene solution of 80 mg (0.103 mmol) of 3-Cl2 was added.
The reaction vessel was attached to the vacuum line, and 1 atm of
CO was added at -196 °C. The vessel was warmed to room
temperature and stirred for 15 h. The excess CO was removed in
Vacuo and brought into the drybox. Filtration through Celite
followed by solvent removal and recrystallization from pentane at
-35 °C yielded 0.045 g (75%) of rac/meso-3-(CO)2 as a forest
green solid in a 2 to 1 ratio. Anal. Calcd for C28H30O2Hf: C, 58.28;
Preparation of (η5-C9H5-1,3(SiMe3)2Hf-cyclo-O(CH2)3CH2
(1-cyclo-O(CH2)3CH2). A flame-dried thick-walled glass vessel
was charged with 0.078 g (0.10 mmol) of 1-THF dissolved in
approximately 5 mL of benzene. On the vacuum line, the reaction
vessel was degassed, and 91 Torr (0.49 mmol) of THF was
admitted via a 100.1 mL calibrated gas bulb. The green solution
was then heated in an oil bath for 3 days at 85 °C. Solvent was
removed in Vacuo, and the resulting yellow oil was identified
as 1-cyclo-O(CH2)3CH2. Attempts to recrystallize the com-
pound as a solid have been unsuccessful for elemental anal-
1
ysis. H NMR (benzene-d6): δ 0.35 (s, 18H, SiMe3), 0.41 (s,
1
18H, SiMe3), 0.74 (m, 2H, OCH2CH2CH2CH2), 0.80 (m, 2H,
OCH2CH2CH2CH2), 1.52 (m, 2H, OCH2CH2CH2CH2), 3.87 (t,
5 Hz, 2H, OCH2CH2CH2CH2), 6.01 (s, 2H, CpH), 6.96 (m, 2H,
Benzo), 7.12 (m, 2H, Benzo), 7.60 (m, 2H, Benzo), 7.81 (m,
2H, Benzo). 13C{1H} NMR (benzene-d6): δ 1.36, 1.58 (SiMe3),
27.49 (OCH2CH2CH2CH2), 32.61 (OCH2CH2CH2CH2), 46.31
(OCH2CH2CH2CH2), 73.63 (OCH2CH2CH2CH2), 83.41, 103.52,
123.81, 124.67, 126.28, 126.82, 132.71, 137.24, 138.31 (Cp/
Benzo).
H, 5.24. Found: C, 57.99; H, 5.46. H NMR (benzene-d6): δ 1.05
(d, 8 Hz, 3H, minor CHMe2), 1.08 (d, 8 Hz, 3H, major CHMe2),
1.12 (d, 8 Hz, 3H, minor CHMe2), 1.13 (d, 8 Hz, 3H, major
CHMe2), 1.90 (s, 3H, major Me), 1.98 (s, 3H, minor Me), 2.48 (m,
1H, minor CHMe2), 2.63 (m, 1H, major CHMe2), 4.55 (s, 2H, major
CpH), 4.79 (s, 2H, minor CpH), 6.71 (m, 8H, major/minor Benzo),
6.94 (m, 2H, major Benzo), 7.01 (m, 2H, minor Benzo), 7.09 (m,
2H, major Benzo), 7.17 (m, 2H, minor Benzo). 13C NMR (benzene-
d6): δ 12.14, 12.17 (Me), 22.41, 22.98, 25.47, 25.74, 26.74, 27.38

Bis(indenyl)hafnium Chemistry
Organometallics, Vol. 28, No. 8, 2009 2483
(CHMe2), 93.81, 99.27, 99.59, 108.24, 112.26, 115.21, 122.06,
122.23, 122.34, 122.64, 122.67, 122.75, 123.16, 123.57, 125.64,
128.51, 129.28 (Cp/Benzo), 271.71, 274.12 (Hf-CO). One Cp/Benzo
and one Hf-CO resonance not located. ν(CO) (pentane) ) 1871,
g (37%) of a white crystalline solid identified as 1-H2. Anal. Calcd
for C30H48Si4Hf: C, 51.51; H, 6.92. Found: C, 51.23; H, 6.96. H
NMR (benzene-d6): δ 0.31 (s, 18H, SiMe3), 6.78 (m, 4H, Benzo),
7.25 (m, 4H, Benzo), 7.53 (s, 2H, CpH), 14.85 (s, 1H, Zr-H).
13C{1H} NMR (benzene-d6): δ 1.31 (SiMe3), 110.36, 124.31,
126.21, 127.52, 131.56 (Cp/Benzo).
1
1864, 1961, 1948 cm-1
.
Dimethoxyethane Cleavage from 1-DME. A J. Young NMR
tube was charged with 0.010 g (0.013 mmol) of 1-DME dissolved
in approximately 0.5 mL of benzene-d6 and was allowed to sit at
room temperature, and the reaction monitored over two days via
NMR spectroscopy. Reaction resulted in cleavage of the dimethoxy-
ethane to give a yellow solution of 1-OMe2 and 1-C4H8.
Preparation of (η5-C9H5-1,3(SiMe3)2)2Hf(H)(C6N2H9) (1-
DMAP). A 20 mL scintillation vial was charged with 0.058 g (0.075
mmol) of 1-THF, 0.009 g (0.074 mmol) of 4-dimethylaminopy-
ridine, and 10 mL pentane. The resulting solution was stirred
overnight and changed color from dark green to white. The volatile
components were removed in Vacuo. Recrystallization of the
resulting solid from pentane at -35 °C yielded 0.055 g (89%) of
a fluffy white solid identified as 1-DMAP. Anal. Calcd for
Characterization of (η5-C9H9-1,3-(SiMe3)2)2Hf(OMe)2 (1-
1
OMe2). H NMR (benzene-d6): δ 0.25 (s, 36H, SiMe3), 3.64 (s,
6H, OMe), 6.71 (s, 2H, Cp), 7.17 (m, 4H, Benzo), 7.72 (m, 4H,
Benzo). 13C NMR (benzene-d6): δ -0.03 (SiMe3), 78.85 (OMe),
114.00, 123.20, 124.64, 141.29, 141.68 (Cp/Benzo).
1
C30H46Si4HfN2: C, 54.21; H, 6.89. Found: C, 54.00; H, 6.35. H
NMR (benzene-d6): δ 0.47 (s, 18H, SiMe3), 0.48 (s, 18H, SiMe3),
2.35 (s, 6H, NMe2), 4.96 (s, 2H, Cp), 5.84 (d, 4.8 Hz, 1H, DMAP),
6.69 (m, 4H, Benzo), 6.93 (s, 1H, Hf-H), 7.09 (s, 1H, DMAP),
7.28 (m, 7.2 Hz, 4H, Benzo), 7.68 (d, 6 Hz, 1H, DMAP). 13C NMR
(benzene-d6): δ 2.00, 2.02 (SiMe3), 39.15 (NMe2), 108.41 (DMAP),
111.44 (DMAP), 123.38, 123.65, 125.09, 125.57 (Benzo), 124.81
(Cp), 144.99 (DMAP).
Preparation of (η5-C9H9-1,3-(SiMe3)2)2Hf(C4H8) (1-cyclo-
C4H8). A J. Young NMR tube was charged with 0.020 g (0.026
mmol) of 1-THF, and approximately 0.5 mL of benzene-d6 was
added. The tube was submerged in a -196 °C bath, and excess
(approximately 250 Torr) ethylene was admitted. A rapid color
change from green to yellow was observed upon thawing. Removal
of the excess ethylene in Vacuo and recrystallization in pentane
afforded 10 mg of a yellow solid identified as 1-cyclo-C4H8. Anal.
Calcd for C34H54Si4Hf: C, 54.19; H, 7.22. Found: C, 53.91; H, 7.20.
1H NMR (benzene-d6): δ 0.33 (s, 36H, SiMe3), 0.39 (m, 4H,
Hf-CH2), 1.63 (m, 4H, Hf-CH2CH2), 6.32 (s, 2H, Cp), 7.17 (m,
4H, Benzo), 7.77 (m, 4H, Benzo). 13C NMR (benzene-d6): δ 1.58
(SiMe3), 28.91 (Hf-CH2CH2), 57.62 (Hf-CH2CH2) 115.80, 122.90,
125.31, 130.64, 136.97 (Cp/Benzo).
Preparation of (η5-C9H5-1,3(SiMe3)2)((η5, η3-C9H6-1,3(SiMe3)2)-
HfH. (1-DHID-H). A thick walled glass vessel was charged with
0.055 g (0.072 mmol) of 1-THF and approximately 10 mL of
pentane. The contents of the vessel were frozen in liquid nitrogen,
the vessel was evacuated, and 1 atm of dihydrogen was added. After
thawing, the solution immediately changed color from a dark forest
green to light yellow. The volatiles were removed in Vacuo, and
the residue was recrystallized from pentane to yield 0.042 g (83%)
of a flaky tan solid identified as 1-DHID-H. Anal. Calcd for
C30H48Si4Hf: C, 51.51; H, 6.92. Found: C, 51.12; H, 6.51. 1H NMR
(benzene-d6): δ 0.29 (s, 9H, SiMe3), 0.31 (s, 9H, SiMe3), 0.32 (s,
9H, SiMe3), 0.44 (s, 9H, SiMe3), 2.32 (d, 12 Hz, 1H, CH2), 2.51
(m, 1H, CH2), 2.91 (br, 1H, allyl CH), 3.03 (s, 1H, Hf-H), 4.68
(m, 1H, allyl CH), 5.24 (d, 1H, 6.4 Hz, allyl CH), 5.84 (s, 1H,
Cp), 6.85 (m, 2H, benzo), 6.95 (s, 1H, Cp), 7.25 (m, 1H, benzo),
7.56 (m, 1H, benzo). 13C{1H} NMR (benzene-d6): δ 1.20, 1.42,
2.14, 2.67 (SiMe3), 22.49 (CH2), 37.61 (allyl CH), 95.44, 99.70,
102.82, 103.46, 105.86, 109.39, 109.45, 116.98, 123.58, 123.74,
124.68, 125.36, 125.97, 126.51, 126.92, 126.98 (Cp/Benzo).
Preparation of (η5-C9H9-1,3-(SiMe3)2)2HfH2 (1-THI). A J.
Young NMR tube was charged with 0.015 g (0.021 mmol) of
1-DHID-H and approximately 0.5 mL of benzene-d6. The contents
of the tube were heated at 65 °C for 1 week. No significant color
change was observed. 1H NMR (benzene-d6): 0.28 (s, 36H, SiMe3),
1.73 (m, 4H, anti to Hf THI), 2.10 (m, 4H, syn THI), 2.75 (m, 4H,
anti THI), 2.84 (m, 4H, syn THI), 5.82 (s, 2H, CpH), 14.95 (s, 2H,
Hf-H). 13C{1H} NMR (benzene-d6): δ 1.36 (SiMe3), 23.95, 27.86
(THI), 111.87, 117.55, 137.10 (Cp).
Preparation of ((η5-C9H5-1,3-(SiMe3)2)2Hf(N2CPh2)(THF)
(1-N2CPh2(THF)). A 20 mL scintillation vial was charged with
0.040 g (0.059 mmol) of 1-THF and 0.012 g (0.062 mmol) of
diphenyldiazomethane, and approximately 5 mL of pentane was
added. The reddish-brown solution was stirred for 3 h, after which
the solvent was removed in Vacuo. Recrystallization in pentane
afforded 0.010 g (19%) of an orange solid identified as
1-N2CPh2(THF). In benzene-d6 solution, the compound is reddish-
orange. Anal. Calcd for C47H64Si4HfN2O: C, 58.57; H, 6.69; N,
1
2.91. Found: C, 58.22; H, 6.35; N, 2.83. H NMR (benzene-d6):
0.42 (s, 18H, SiMe3), 0.53 (s, 18H, SiMe3), 1.10 (m, 2H, THF),
1.94 (m, 4H, THF), 2.99 (m, 2H, THF), 6.85 (m, 4H, Benzo/Ph),
6.99 (m, 2H, Benzo/Ph), 7.11 (m, 2H, Benzo/Ph), 7.27 (m, 4H,
Benzo/Ph), 7.31 (s, 2H, CpH), 7.36 (m, 2H, Benzo/Ph), 7.95 (m,
2H, Benzo/Ph), 8.12 (m, 2H, Benzo/Ph). 13C{1H} NMR (benzene-
d6): δ 0.61, 1.29 (SiMe3), 25.72, 77.71 (Hf-THF), 107.87, 121.13,
121.20, 121.96, 122.72, 126.41, 126.47, 127.02, 127.48, 130.58,
139.59, 140.15, 140.85, 141.47, 142.58, 150.97 (Cp/Benzo). Two
Cp/Benzo resonances not located.
Preparation of ((η5-C9H5-1,3-(SiMe3)2)2Hf(NHNdCPh2)H
(1-(NHNdCPh2)H). A flame-dried thick walled glass reaction
vessel was charged with 0.037 g (0.038 mmol) of 1-N2CPh2(THF),
and approximately 3 mL of pentane was added. While attached to
the high-vacuum line, the contents of the vessel were frozen at
-196 °C and evacuated. At this temperature, 1 atm of dihydrogen
was admitted. After thawing, the resulting orange solution was
stirred vigorously for 2 h, after which time the solvent was removed
in Vacuo, resulting in an orange solid. Recrystallization from pentane
at -35 °C yielded 33 mg (94%) of an orange solid identified as
1-(NHNdCPh2)H. Anal. Calcd for C43H58Si4HfN2: C, 57.78; H,
6.54; N, 3.13. Found: C, 57.53; H, 6.43; N, 2.84. 1H NMR (benzene-
d6): δ 0.42 (s, 18H, SiMe3), 0.45 (s, 18H, SiMe3), 6.22 (br s, 1H,
NH), 6.53 (m, 4H, Benzo/Ph), 6.89 (m, 2H, Benzo/Ph), 7.08 (m,
2H, Benzo/Ph), 7.12 (s, 2H, CpH), 7.18 (m, 2H, Benzo/Ph), 7.26
(m, 4H, Benzo/Ph), 7.52 (m, 2H, Benzo/Ph), 7.68 (m, 2H, Benzo/
Ph), 9.31 (s, 1H, Hf-H). 13C{1H} NMR (benzene-d6): δ 1.95, 1.98
(SiMe3), 93.99, 122.47, 123.77, 123.86, 124.13, 125.15, 125.88,
126.29, 126.72, 126.97, 127.22, 129.11, 130.47, 131.52, 135.71,
139.40, 145.38 (Cp/Benzo). One Cp/Benzo resonance not located.
Preparation of (η5-C9H5-1,3(SiMe3)2)2HfH2. (1-H2). A 20 mL
scintillation vial was charged with 0.240 g (0.312 mmol) of 1-Cl2
and dissolved in approximately 5 mL of toluene. The yellow
solution was chilled at -35 °C for 20 min, and 0.656 mL (0.656
mmol) of a 1.0 molar solution of sodium triethylborylhydride in
toluene was syringed in. The resultant amber solution was stirred
for 3 h, and the solvent removed in Vacuo. The residue was taken
into pentane and extracted through Celite, and the solvent removed
in Vacuo. Recrystallization from pentane at -35 °C afforded 0.081
IR (KBr): ν(NH) ) 3320 cm-1; ν(ND) ) 2467 cm-1
.
Preparation of (η5-C9H5-1,3-(SiMe3)2)(η5-C9H5-η1-1-SiMe3-
3-SiMe2CH2)]Hf(NHNdCPh2) (1-CM(NHNdCPh2)). A J. Young
NMR tube charged with 0.008 g (0.01 mmol) of 1-THF and 0.002
g (0.01 mmol) of diphenyldiazomethane dissolved in approximately
0.5 mL of benzene-d6 was heated at 65 °C for one day, resulting

2484 Organometallics, Vol. 28, No. 8, 2009
Pun et al.
in a color change from red-orange to orange and was identified as
1-CM(NHNdCPh2). Attempts to recrystallize as a solid have been
unsuccessful for elemental analysis. 1H NMR (benzene-d6): δ -1.86
(d, 12 Hz, 1H, Zr-CH2), -0.67 (d, 12 Hz, 1H, Zr-CH2), 0.23 (s,
9H, SiMe3), 0.36 (s, 9H, SiMe3), 0.51 (s, 9H, SiMe3), 0.72 (s, 3H,
SiMe2), 0.77 (s, 3H, SiMe2), 4.65 (s, 1H, N-H), 6.46 (m, 1H, Benzo/
Ph), 6.78 (m, 2H, Benzo/Ph), 6.89 (m, 4H, Benzo/Ph), 7.02 (m,
3H, Benzo/Ph), 7.13 (m, 3H, Benzo/Ph), 7.39 (s, 1H, CpH), 7.70
(m, 3H, Benzo/Ph), 7.77 (s, 1H, CpH), 7.78 (m, 1H, Benzo/Ph),
7.95 (m, 1H, Benzo/Ph). 13C{1H} NMR (benzene-d6): δ 1.50, 1.61,
1.83, 1.92, 2.05 (SiMe3/SiMe2), 33.00 (Zr-CH2), 104.63, 105.56,
110.47, 111.81, 117.42, 123.65, 123.69, 124.77, 125.78, 126.46,
128.94, 129.42, 129.44, 129.46, 129.49, 130.56, 130.58, 130.60,
135.57, 136.44, 137.26, 139.72, 144.24 (Benzo/Cp/Ph). Four Benzo/
N2CPh2). This compound was prepared in a similar manner to
1-N2CPh2(THF) with 0.084 g (0.171 mmol) of 6, 0.033 g (0.170
mmol) of diphenyldiazomethane, and approximately 8 mL of
pentane. Recrystallization from pentane at -35 °C yielded 28 mg
(24%) identified as 6-N2CPh2. Anal. Calcd for C43H48N2Zr: C,
1
75.50; H, 7.07; N, 4.09. Found: C, 75.27; H, 6.84; N, 3.58. H
NMR (benzene-d6): δ 0.93 (d, 8 Hz, 6H, CHMe2), 0.98 (d, 8 Hz,
6H, CHMe2), 1.05 (d, 8 Hz, 12H, CHMe2), 2.66 (m, 2H, CHMe2),
2.75 (m, 2H, CHMe2), 6.64 (s, 2H, CpH), 6.83 (m, 2H, Benzo/Ph),
6.92 (m, 2H, Benzo/Ph), 7.12 (m, 6H, Benzo/Ph), 7.23 (m, 6H,
Benzo/Ph), 7.34 (m, 2H, Benzo/Ph), 8.31. 13C{1H} NMR (benzene-
d6): δ 20.93, 21.51, 24.57, 24.89, 25.34, 25.43 (CHMe2), 115.70,
122.70, 122.76, 123.28, 124.07, 124.44, 124.65, 124.76, 125.11,
125.37, 125.88, 126.20, 128.45, 128.48, 128.66, 137.86, 141.01 (Cp/
Benzo/Ph). One Cp/Benzo/Ph resonance not located.
Cp/Ph resonances not located. IR (KBr): ν(NH) ) 3290 cm-1
.
Preparation of ((η5-C9H5-1,3-(SiMe3)2)2Zr(N2CPh2)(THF)
(5-N2CPh2(THF)). This compound was prepared in a similar
manner to 1-N2CPh2(THF) in a J. Young NMR tube with 0.020 g
(0.030 mmol) of 5-THF, 0.006 g (0.031 mmol) of diphenyldiazo-
methane, and approximately 0.5 mL of benzene-d6. The resulting
reddish-brown solution was identified as 5-N2CPh2(THF). This
particular compound is unstable to vacuum, so no elemental analysis
was obtained. 1H NMR (benzene-d6): δ 0.42 (s, 18H, SiMe3), 0.49
(s, 18H, SiMe3), 0.86 (m, 4H, THF), 2.14 (m, 4H, THF), 6.85 (m,
4H, Benzo/Ph), 6.94 (m, 2H, Benzo/Ph), 7.07 (m, 2H, Benzo/Ph),
7.21 (m, 4H, Benzo/Ph), 7.41 (s, 2H, CpH), 7.51 (m, 2H, Benzo/
Ph), 7.57 (m, 2H, Benzo/Ph), 7.83 (m, 2H, Benzo/Ph). 13C{1H}
NMR (benzene-d6): δ 1.22, 1.71 (SiMe3), 26.03, 76.66 (Hf-THF),
111.15, 121.92, 122.95, 123.86, 124.04, 124.20, 125.70, 126.06,
126.67, 126.72, 129.41, 130.95, 133.89, 134.61, 136.63, 138.45 (Cp/
Benzo/Ph). Two Cp/Benzo/Phenyl resonances not located.
Preparation of ((η5-C9H5-1,3-(CHMe2)2Zr(N2CPh2)H) (6-
(NHNCPh2)H). This compound was prepared in a similar manner
to 1-(NHNdCPh2)H using 0.053 g (0.077 mmol) of 6-N2CPh2.
Recrystallization from pentane at -35 °C yielded 33 mg (62%)
identified as 6-(NHNdCPh2)H. Anal. Calcd for C43H50N2Zr: C,
1
75.28; H, 7.35; N, 4.08. Found: C, 75.03; H, 7.61; N, 4.98. H
NMR (benzene-d6): δ 1.25 (d, 8 Hz, 6H, CHMe2), 1.28 (d, 8 Hz,
6H, CHMe2), 1.32 (d, 8 Hz, 6H, CHMe2), 1.34 (d, 8 Hz, 6H,
CHMe2), 3.45 (m, 4H, CHMe2), 4.82 (s, 2H, CpH), 5.58 (br s, 1H,
NH/Zr-H), 5.79 (s, 1H, NH/Zr-H), 6.64 (m, 2H, Benzo/Ph), 6.73
(m, 2H, Benzo/Ph), 7.11 (m, 4H, Benzo/Ph), 7.26 (m, 4H, Benzo/
Ph), 7.32 (m, 2H, Benzo/Ph), 7.38 (m, 2H, Benzo/Ph), 7.40 (m,
2H, Benzo/Ph), 8.05 (m, 2H, Benzo/Ph). 13C{1H} NMR (benzene-
d6): δ 22.78, 23.95, 24.80, 25.72, 27.97, 28.33 (CHMe2), 110.24,
117.35, 118.90, 121.94, 122.31, 122.64, 123.07, 123.98, 126.52,
126.65, 127.83, 129.29, 129.91, 132.50, 138.50 (Cp/Benzo/Ph). One
Cp/Benzo/Ph resonance not located. IR (pentane): ν(NH) ) 3298
Preparation of ((η5-C9H5-1,3-(SiMe3)2)2Zr(N2CPh2)H) (5-
(NHNCPh2)H). A thick walled glass vessel was flame-dried and
charged with 0.156 g (0.230 mmol) of 5-THF, and approximately
10 mL of pentane was added. Also to the vessel was added 0.045
g (0.232 mmol) of diphenyldiazomethane dissolved in 1 mL of
pentane. The resulting reddish-brown solution of 5-N2CPh2(THF)
was stirred for 3 h. On the high-vacuum line, the contents of the
vessel were frozen at -196 °C and then evacuated. At this
temperature, 1 atm of dihydrogen was added. After thawing, the
resulting yellow solution was stirred vigorously for 2 h, after which
time the solvent was removed in Vacuo, resulting in a yellow solid.
Recrystallization from pentane at -35 °C yielded 65 mg (32%) of
an yellow solid identified as 5-(NHNdCPh2)H. Anal. Calcd for
C43H58Si4ZrN2: C, 57.78; H, 6.54; N, 3.13. Found: C, 57.30; H,
7.00; N, 3.34. 1H NMR (benzene-d6): δ 0.39 (s, 18H, SiMe3), 0.50
(s, 18H, SiMe3), 4.61 (s, 2H, CpH), 5.35 (br s, 1H, NH), 5.57 (s,
1H, Zr-H), 6.61 (m, 2H, Benzo/Ph), 6.69 (m, 2H, Benzo/Ph), 7.01
(m, 2H, Benzo/Ph), 7.13 (m, 2H, Benzo/Ph), 7.19 (m, 2H, Benzo/
Ph), 7.33 (m, 4H, Benzo/Ph), 7.64 (m, 2H, Benzo/Ph), 7.86 (m,
2H, Benzo/Ph). 13C{1H} NMR (benzene-d6): δ 1.80, 1.92 (SiMe3),
123.51, 123.55, 123.86, 124.94, 125.66, 126.06, 126.74, 127.22,
127.41, 129.20, 129.33, 129.59, 129.75, 129.91, 131.79, 138.08,
cm-1. ν(ND) ) 2448 cm-1
.
Acknowledgment. We thank the Director, Office of Basic
Energy Sciences, Chemical Sciences Division, of the U.S.
Department of Energy (DE-FG02-05ER15659) and the
Frasch Foundation administered by the American Chemical
Society for financial support. EPR data were collected at the
Advanced Center for ESR Technology (ACERT), Depart-
ment of Chemistry and Chemical Biology, Cornell Univer-
sity, supported by Grant No. 5P41RR016292 from the
National Center for Research Resources (NCRR), a com-
ponent of the National Institutes of Health (NIH), and its
contents are solely the responsibility of the authors and do
not necessarily represent the official view of NCRR or NIH.
We thank Dr. Boris Dzikovski for collecting these data.
Supporting Information Available: Crystallographic data for
1-THF, ([1-Cl]2Na2(DME)2, and 1-Cl in cif format, variable-
temperature EPR data, and additional NMR spectra. This material
may be accessed, free of charge, via the Internet at http://pubs.acs.org.
146.70, 147.76 (Cp/Benzo/Ph). IR (pentane): ν(NH) ) 3303 cm-1
ν(ND) ) 2455 cm-1
Preparation of ((η5-C9H5-1,3-(CHMe2)2Zr(η2-N2CPh2)) (6-
;
.
OM900046E