Synthesis of hafnium and zirconium imino - Amido complexes from bis-imine ligands. A new family of olefin polymerization catalysts

Article abstract of DOI:10.1021/om7004723

The reaction of 2,6-diisopropylaniline-based bisimine ligands (4, 9) with M(CH₂Ph)₄ (M = Hf, Zr) led to formation of novel imino - amido tribenzyl complexes via migratory insertion of a benzyl group into a C=N bond. Imino - amido complexes were found to undergo unprecedented dibenzyl elimination to form ene - diamido complexes. Imino - amido complexes were found to be active ethylene polymerization catalysts.

Full text of DOI:10.1021/om7004723

3896
Organometallics 2007, 26, 3896-3899
Synthesis of Hafnium and Zirconium Imino-Amido Complexes
from Bis-imine Ligands. A New Family of Olefin Polymerization
Catalysts
Philip De Waele, Brian A. Jazdzewski, Jerzy Klosin,* Rex E. Murray,†
Curt N. Theriault, and Paul C. Vosejpka
Corporate R&D, The Dow Chemical Company, 1776 Building, Midland, Michigan 48674
Jeffrey L. Petersen
C. Eugene Bennett Department of Chemistry, West Virginia UniVersity,
Morgantown, West Virginia 26506-6045
ReceiVed May 13, 2007
Summary: The reaction of 2,6-diisopropylaniline-based bis-
imine ligands (4, 9) with M(CH2Ph)4 (M ) Hf, Zr) led to
formation of noVel imino-amido tribenzyl complexes Via
migratory insertion of a benzyl group into a CdN bond. Imino-
amido complexes were found to undergo unprecedented dibenzyl
elimination to form ene-diamido complexes. Imino-amido
complexes were found to be actiVe ethylene polymerization
catalysts.
to novel zirconium bis-phenoxy-amido and pyrrolo-amido
complexes.8 We wondered if an analogous reaction could occur
between the neutral bis-imine ligands 1 and transition-metal
Metallocene-based catalysis dominated homogeneous olefin
polymerization in the late 1980s and 1990s.1 More recently,
efforts have focused on the development of non-Cp-based
catalysts,2 giving rise to previously unknown polymerization
behaviors3 and new polymeric materials.3a,4 Rapid synthesis and
evaluation of novel non-Cp catalysts is, therefore, highly desired.
We have previously shown5 that the reaction between pyridyl-
imine and tetrabenzylzirconium leads, after selective benzyl
transfer, to the formation of novel catalysts for olefin polym-
erization reactions. Analogous reactions have also been reported
by the groups of Scott6 and Mashima,7 which showed that
reaction between bis(phenol-imine) and pyrrole-imine ligands
with tetrabenzylzirconium leads, after selective benzyl transfer,
alkyl precursors.9,10,18 Such a transformation might lead to the
rapid synthesis of a new family of either imino-amido (2) or/
and bis-amido (3) complexes, as illustrated. Bis-imines have
been extensively studied as supporting ligands in late-transition-
metal olefin polymerization catalysts3a,11 but have not been
directly utilized as ligand precursors for early transition metal
catalysts.
* Towhomcorrespondenceshouldbeaddressed.E-mail: jklosin@dow.com.
† Current address: Chevron Phillips Chemical Company, West Highway
60, Bartlesville, OK 74004.
(6) (a) Woodman, P. R.; Alcock, N. W.; Munslow, I. J.; Sanders, C. J.;
Scott, P. Dalton Trans. 2000, 3340-3346. (b) Knight, P. D.; O’Shaughnessy,
P. N.; Munslow, I. J.; Kimberley, B. S.; Scott, P. J. Organomet. Chem.
2003, 683, 103-113. (c) Tsurugi, H.; Yamagata, T.; Tani, K.; Mashima,
K. Chem. Lett. 2003, 32, 756-757. (d) Tsurugi, H.; Matsuo, Y.; Yamagata,
T.; Mashima, K. Organometallics 2004, 23, 2797-2805.
(1) (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. 1995, 34, 1143-1170. (b) Janiak,
C. In Metallocenes; Togni, A., Haltermann, R. L., Eds.; Wiley-VCH:
Weinheim, Germany, 1998; Vols. 1 and 2. (c) Resconi, L.; Cavallo, L.;
Fait, A.; Piemontesi, F. Chem. ReV. 2000, 100, 1253-1345.
(2) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428-447. (b) Gibson, V. C.; Spitzmesser, S. K. Chem.
ReV. 2003, 103, 283-316. (c) Makio, H.; Kashiwa, N.; Fujita, T. AdV. Synth.
Catal. 2002, 344, 477-493. (d) Park, S.; Han, Y.; Kim, S. K.; Lee, J.;
Kim, H. K.; Do, Y. J. Organomet. Chem. 2004, 689, 4263-4276.
(3) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169-1203. (b) Tian, J.; Hustad, P. D.; Coates, G. W. J. Am. Chem. Soc.
2001, 123, 5134-5135. (c) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F.
M.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling,
A.; Otto, S.; Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann,
S. J. Am. Chem. Soc. 2004, 126, 14712-14713. (d) Boussie, T. R.; Diamond,
G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M. K.; Murphy, V.;
Shoemaker, J. A. W.; Turner, H.; Rosen, R. K.; Stevens, J. C.; Alfano, F.;
Busico, V.; Cipullo, R.; Talarico, G. Angew. Chem., Int. Ed. 2006, 45,
3278-3283.
(7) (a) Tsurugi, H.; Yamagata, T.; Tani, K.; Mashima, K. Chem. Lett.
2003, 32, 756-757. (b) Tsurugi, H.; Matsuo, Y.; Yamagata, T.; Mashima,
K. Organometallics 2004, 23, 2797-2805.
(8) This insertion chemistry is reminiscent of metal-alkyl insertion into
the CdN bond of carbodiimides to produce transition-metal amidinate
complexes: Sita, L. R.; Babcock, J. R. Organometallics 1998, 17, 5228-
5230 and references therein.
(9) Reactions between bis-imines and main-group alkyl species (Zn, Mg,
Al) are well-known: (a) Klerks, J. M.; Stufkens, D. J.; van Koten, G.;
Vrieze, K. J. Organomet. Chem. 1979, 181, 271-283. (b) Jastrzebski, J.
T. B. H.; Klerks, J. M.; van Koten, G.; Vrieze, K. J. Organomet. Chem.
1981, 210, C49-C53. (c) Kaupp, M.; Stoll, H.; Preuss, H.; Kaim, W.; Stahl,
T.; van Koten, G.; Wissing, E.; Smeets, W. J. J.; Spek, A. L. J. Am. Chem.
Soc. 1991, 113, 5606-5618. (d) Wissing, E.; Jastrzebski, J. B. H.; Boersma,
J.; van Koten, G. J. Organomet. Chem. 1993, 459, 11-16. (e) Wissing, E.;
van Gorp, K.; Boersma, J.; van Koten, G. Inorg. Chim. Acta 1994, 220,
55-61. (f) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White,
A. J. P.; Williams, D. J. Chem. Commun. 1998, 2523-2524.
(4) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.;
Wenzel, T. T. Science 2006, 312, 714-719.
(5) (a) Murray, R. E.; George, V. M.; Nowlin, D. L.; Schultz, C. C.;
Petersen, J. L. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2002,
43, 294-295. (b) Murray, R. E. U.S. Patent 6,103,657 (Union Carbide
Corp.), 2000.
(10) There is only one example of a reaction between transition-metal
alkyl complexes and bis-imines: Riollet, V.; Coperet, C.; Basset, J.-M.;
Rousset, L.; Bouchu, D.; Grosvalet, L.; Perrin, M. Angew. Chem., Int. Ed.
2002, 41, 3025-3027.
10.1021/om7004723 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/07/2007

Communications
Organometallics, Vol. 26, No. 16, 2007 3897
Scheme 1. Reactions of the Bis-imine 4 with M(CH2Ph)4 (M
) Hf, Zr)
Figure 1. Molecular structure of 5. Hydrogen atoms and phenyls
of the Hf benzyl groups were removed for clarity. Thermal ellipsoids
are shown at the 40% probability level. Selected bonds (Å) and
angles (deg): Hf-N1 ) 2.070(3), Hf-N2 ) 2.423(3), C2-N2 )
1.278(5), C1-C2 ) 1.527(5), C1-N1 ) 1.503(4); N2-Hf-N1
) 70.8(1).
groups indicates rapid benzyl group exchange on the NMR time
scale. The X-ray crystal structure of 5 (Figure 1) reveals a
significantly longer Hf-N(imino) bond (2.423(3) Å) as com-
pared to the Hf-N(amido) (2.070(3) Å) bond. The two carbon
and two nitrogen atoms of the metallacycle are almost planar,
with the Hf atom located 0.64 Å above this plane. In an attempt
to produce a bis-amido complex (such as 3), by transfer of a
second benzyl group, a toluene solution of 5 was heated to
70 °C overnight.
In this communication we report our preliminary results on
the reaction between two different bis-imine ligands (4, 9) and
M(CH2Ph)4 (M ) Hf, Zr). These transformations produce
imino-amido tribenzyl complexes that display good activity
as catalysts in ethylene/octene (EO) copolymerization reactions.
Additionally, the unprecedented formation of ene-diamido
complexes has been observed.
Thermolysis of 5 did indeed produce a new diamido complex;
however, its structure was different from that expected. Instead
of a second benzyl transfer, elimination of dibenzyl from 5 was
exclusively observed, yielding the ene-diamido13 complex 7
(Scheme 1) quantitatively. While many ene-diamido complexes
are known,14 conversion of 5 to 7 via elimination of a dialkyl
species is unprecedented. Decomposition of 5 follows first-order
Reaction of the bis-imine 4 with tetrabenzylhafnium at
ambient temperature quantitatively yields the chiral imino-
amido complex 5 within 24 h, upon benzyl transfer from Hf to
the bis-imine ligand.12 The most notable feature in the 1H NMR
spectrum of 5 is the presence of two sets of two mutually
coupled doublets in a 3:1 ratio, corresponding to the diaste-
reotopic methylene protons of the benzyl groups. The observa-
tion of only one pair of doublets for the three Hf-bound benzyl
kinetics with a kobs value of 1.1 × 10-4 s-1 at 70.5 °C (t1/2
)
105 min) and with ∆Hq and ∆Sq values of 33.3(6) kcal/mol
and 20(2) eu, respectively. The X-ray crystal structure of 7
(Figure 2) shows the ene-diamido fragment coordinated to Hf
via both nitrogen atoms and the double bond. The dihedral angle
between planes defined by N1-Hf-N2 and N1-C1-C2-N2
is 122.9°. The Hf atom is positioned above the ene-diamido
(11) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414-6415.
(12) Preparation of 5. To a 40 mL vial was added 2.00 g (3.68 mmol)
of Hf(CH2Ph)4 and 1.49 g (3.68 mmol) of N,N′-bis(2,6-diisopropylphenyl)-
2,3-butanediimine followed by 8 mL of toluene to dissolve all reagents.
The solution was stirred for 42 h at ambient temperature, yielding a dark
orange solution. The solvent amount was reduced under reduced pressure
to 4 mL. During solvent removal a yellow solid appeared. Hexane (25 mL)
was added to this suspension to induce further precipitation. After the
mixture was stirred for 10 min, the solid was collected on a frit, washed
with 20 mL of hexanes, and dried under reduced pressure to give 2.55 g of
product. The filtrate was stored overnight at -30 °C, causing the
precipitation of additional solid. Upon workup (as above) a second crop of
0.3 g was isolated. Combined yield: 2.85 g, 82%. 1H NMR (C6D6, 300
Hz, 23 °C): δ 7.29 (dd, 1H, 3JH-H ) 7.8 Hz, 4JH-H ) 2.1 Hz), 7.24 (t, 1H,
3JH-H ) 7.5 Hz), 7.16 (dd, 1H, 3JH-H ) 7.2 Hz, 4JH-H ) 1.8 Hz), 6.80 (t,
3
3
3H, JH-H ) 7.5 Hz, p-CH2Ph), 6.94-7.14 (m, 14H), 6.62 (d, 6H, JH-H
) 7.5 Hz, o-CH2Ph), 3.83 (d, 1H, 2JH-H ) 13.5 Hz, C-CH2Ph), 3.75 (septet,
3
3
1H, JH-H ) 6.6 Hz, CH(CH3)3), 3.53 (septet, 1H, JH-H ) 6.6 Hz,
2
CH(CH3)3), 3.42 (d, 1H, JH-H ) 13.5 Hz, C-CH2Ph), 3.04 (septet, 2H,
3JH-H ) 6.6 Hz, CH(CH3)2), 2.34 (d, 3H, JH-H ) 11.7 Hz, HfCH2Ph),
2
2
3
2.21 (d, 3H, JH-H ) 11.7 Hz, HfCH2Ph), 1.33 (d, 3H, JH-H ) 6.9 Hz,
3
3
CH(CH3)2), 1.32 (d, 6H, JH-H ) 6.9 Hz, CH(CH3)2), 1.23 (d, 3H, JH-H
3
) 6.9 Hz, CH(CH3)2), 1.18 (d, 3H, JH-H ) 6.6 Hz, CH(CH3)2), 1.11 (d,
3H, 3JH-H ) 6.6 Hz, CH(CH3)2), 1.06 (d, 3H, 3JH-H ) 6.9 Hz, CH(CH3)2),
3
1.01 (s, 3H, CH3), 0.85 (s, 3H, CH3), 0.84 (d, 3H, JH-H ) 6.6 Hz, CH-
(CH3)2). 13C{1H} NMR (C6D6, 75 Hz, 23 °C): δ 198.25 (NdC), 148.11
(quat), 148.04 (quat), 147.89 (quat), 146.11 (quat), 143.17 (quat), 140.64
(quat), 140.24 (quat), 138.13 (quat), 130.90 (CH), 128.54 (CH), 128.41
(CH), 127.83 (CH), 127.21 (CH), 126.48 (CH), 126.21 (CH), 125.60 (CH),
125.56 (CH), 124.73 (CH), 89.20 (br s, Hf(CH2Ph)3), 78.21 (quat), 54.45
(C-CH2Ph), 29.92 (CH), 28.79 (2 carbons, CH), 28.02 (CH), 27.67 (CH3),
26.51 (CH3), 26.35 (CH3), 26.14 (CH3), 25.77 (CH3), 24.99 (CH3), 24.70
(CH3), 24.23 (CH3), 23.15 (CH3), 22.62 (CH3). NMR spectra are given in
the Supporting Information. Anal. Calcd for C56H68HfN2: C, 70.98; H, 7.23;
N, 2.96. Found: C, 70.50; H, 7.14; N, 2.84.
Figure 2. Molecular structure of 7. Hydrogen atoms were removed
for clarity. Thermal ellipsoids are shown at the 40% probability
level. Selected bonds (Å) and angles (deg): Hf-N1 ) 2.005(3),
Hf-N2 ) 2.028(3), Hf-C1 ) 2.505(3), Hf-C2 ) 2.533(3), C2-
N2 ) 1.417(4), C1-C2 ) 1.368(5), C1-N1 ) 1.422(4); N2-
Hf-N1 ) 87.2(1).

3898 Organometallics, Vol. 26, No. 16, 2007
Communications
Scheme 2. Reactions of 5 with 6 equiv of TEMPO
Figure 3. Molecular structure of 13. Hydrogen atoms and phenyls
of the Zr benzyl groups were removed for clarity. Thermal ellipsoids
are shown at the 40% probability level. Selected bonds (Å) and
angles (deg): Zr-N1 ) 2.429(2), Zr-N2 ) 2.121(2), C2-N2 )
1.453(2), C1-C2 ) 1.488(2), C1-N1 ) 1.286(2); N2-Zr-N1 )
70.05(5).
Table 1. Polymerization Data for Complexes 5, 7, and 12a
Scheme 3. Reactions of the Bis-imine 9 with M(CH2Ph)4 (M
cat. (amt,
polymer
yield (g)
cat.
octene
) Hf, Zr)
µmol)
activityb
Mw (K)/PDIc
content (mol %)
5 (0.25)
7 (10)
12 (5)
29.5
3.3
26.8
661 000
3 617
30 030
238/7.4
175/25
362/8
13.1
9.2
10.2
a Polymerization conditions: 533 mL of Isopar-E; 250 g of octene;
temperature 120 °C; ethylene pressure 460 psi; procatalyst:activator ) 1:1.1;
activator [HNMe(C18H37)2][B(C6F5)4]; 10 µmol of MMAO; reaction time
15 min. b Activity in units of g of polymer/g of metal. c Polydispersity index
(Mw/Mn).
only a trace of dibenzyl formation observed (Scheme 2). This
reaction follows a first-order decomposition with a rate only
slightly faster (1.4 × 10-4 s-1 at 70.5 °C) than that observed
for the conversion of 5 to 7. Ene-dienyl-based radicals have
been proposed as intermediates in the formation of zinc imino-
amido compounds.9c
Reaction of the bis-imine 4 with Zr(CH2Ph)4 proceeds more
slowly than that observed with Hf(CH2Ph)4 and is only 57%
complete after 4 d at ambient temperature, yielding a 1:1 mixture
of 6 and 8 (Scheme 1). Complex 8 can be obtained in pure
form when the reaction is conducted at higher temperature
(e.g. 65 °C), to allow for complete decomposition of 6. Complex
8 is also fluxional, but the barrier for this process (∆Gq ) 16.9
kcal/mol at 71 °C) is higher by about 3 kcal/mol compared to
that for 7. The X-ray crystal structure of 8 is analogous to that
of 7.15
plane. Complex 7 is fluxional at ambient temperature (∆Gq )
14.1 kcal/mol at 51 °C), due to rapid dynamic equilibrium
between two nonequivalent folded metallacycle conformations.
There are two potential pathways for dibenzyl elimination
from 5. One involves reductive elimination of dibenzyl groups
from Hf with concomitant formation of a Hf(II) species,
followed by benzyl group transfer from the ligand back to
hafnium. The second possible mechanism involves homolytic
cleavage and subsequent recombination of one benzyl group
attached to Hf and the one bonded to the ligand. Preliminary
results point to the homolytic, radical pathway for this decom-
position, as heating of 5 at 70.5 °C in the presence of 6 equiv
of TEMPO produced TEMPO-CH2Ph nearly quantitatively, with
Reaction of the bis-imine 9 with either Hf(CH2Ph)4 or Zr-
(CH2Ph)4 at ambient temperature gave mixtures of the imino-
amido and ene-diamido complexes 12/14 (3:1 ratio) and 13/
15 (1:1.5 ratio), respectively (Scheme 3). In this case, however,
the derived imino-amido complexes are achiral, as indicated
by the appearance of only two singlets for the benzyl group
1
methylene protons in the H NMR spectra. Inspection of the
reaction mixtures in the early stages of the reactions revealed
the presence of the chiral intermediate 10/11, which within
minutes underwent competitive transformation into 12/13 and
14/15, presumably by a 1,2-hydrogen shift and dibenzyl
elimination, respectively. Significant solubility differences
between imino-amido and ene-diamido complexes allowed
for their easy separation. In addition to characterization by NMR
and EA, the identity of 13 was confirmed by X-ray analysis
(Figure 3).
(13) Formally, these ligands are called diazabutadienes. For early-TM
complexes, however, these ligands coordinate in their dianionic (ene-
diamido) form (σ2-N,N′,π).
(14) Scholz, J.; Hadi, G. A.; Thiele, K.-H.; Gorls, H.; Weimann, R.;
Schumann, H.; Sieler, J. J. Organomet. Chem. 2001, 626, 243-259.
(15) See the Supporting Information for details.

Communications
Organometallics, Vol. 26, No. 16, 2007 3899
Complexes 5, 7, and 12 were evaluated as catalysts in an
ethylene/octene copolymerization process at 120 °C and
460 psi of ethylene pressure. The data presented in Table 1 show
that the imino-amido complexes (5 and 12) led to active
catalysts upon activation, with 5 demonstrating especially high
activity (661 000 g of EO/g of Hf). Complex 5 also is a good
octene incorporator, leading to an octene content of 13.1 mol
%. Substantially different polymerization activities between 5
and 7 suggest that the activated complex 5+ is not converted
(at least not entirely) into 7+ in the polymerization reactor. The
observed broad polydispersities of the polymer samples are
indicative of multicenter behavior during catalysis. The ene-
diamido complex 7 exhibited polymerization activity signifi-
cantly lower than that of the imino-amido complexes. It is
interesting to note that subtle changes in ligand structure led to
catalysts with very different activities and polymer molecular
weight capabilities (5 vs 12, Table 1). This offers a promise
that examination of other bis-imine ligands with transition-metal
alkyl complexes may lead to catalysts with diverse catalytic
behavior.
of the N(imine) donor from the Hf center, rotation along the
Hf-N(amido) bond, and recoordination of the N(imine) frag-
ment to the metal center. The 19F NMR shows clean formation
of a single borate anion with a chemical shift difference between
meta and para fluorine resonances of 2.54 ppm. This indicates
that the benzyl group of the PhCH2-B(C6F5)3 anion does not
coordinate to the Hf cationic center,16 presumably due to steric
congestion at the metal center.17 The high stability of 16 at
ambient temperature suggests that multi-sited behavior of
imino-amido catalysts is most likely derived from their
decomposition at high temperature or side reactions with
alkylaluminum species present during polymerization reactions.
In summary, we have demonstrated that reactions between
bis-imine ligands and tetrabenzylhafnium and -zirconium led
to formation of new imino-amido complexes in good yields.
These complexes were found to be good precatalysts for
ethylene/octene copolymerization at high polymerization tem-
peratures. One-step synthesis of a variety of bis-imine ligands
from commercially available diones and amines offers the
possibility of easy access to diverse families of imino-amido
complexes. We are currently investigating the reactivity of other
bis-imines with transition-metal alkyls, and our research in this
area will be the subject of future reports.
Observation of multicenter behavior suggests that perhaps
the activation chemistry leads to multiple different catalytically
active species. To probe this possibility, 5 was reacted with
1 equiv of B(C6F5)3 in C6D5Cl. This reaction leads to a single
room-temperature-stable ion pair, as shown by multinuclear and
multidimensional NMR spectroscopy.15 Unlike 5, which exhibits
Supporting Information Available: Synthetic procedures,
NMR spectra, kinetic data, polymerization protocol, and X-ray data
including CIF files. This material is available free of charge via
the Internet at http://pubs.acs.org.
1
only one type of Hf-bound benzyl groups in the H NMR
spectrum (two mutually coupled doublets), the ion pair 16 shows
OM7004723
(16) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg, H.
Organometallics 1996, 15, 2672-2674.
(17) For examples of a metal-bound PhCH2-B(C6F5)3 anion, see: (a)
Horton, A. D.; de With, J. Organometallics 1997, 16, 5424-5436. (b) Thorn,
M. G.; Etheridge, Z. C.; Fanwick, P. E.; Rothwell, I. P. Organometallics
1998, 17, 3636-3638. (c) Pindado, G. J.; Thornton-Pett, M.; Hursthouse,
M. B.; Coles, S. J.; Bochmann, M. J. Chem. Soc., Dalton Trans. 1999,
1663-1668. (d) Shafir, A.; Arnold, J. Organometallics 2003, 22, 567-
575. For examples of a metal-unbound PhCH2-B(C6F5)3 anion, see: (e)
Pellecchia, C.; Immirzi, A.; Pappalardo, D.; Peluso, A. Organometallics
1994, 13, 3773-3775. (f) Reference 17c. (g) Axenov, K. V.; Kilpelainen,
I.; Klinga, M.; Leskela, M.; Repo, T. Organometallics 2006, 25, 463-471.
(18) Some of this work has appeared in the patent literature: Murray, R.
E. U.S. Patent 6,096,676 (Union Carbide Corp.), 2000.
two distinct Hf-bound benzyl groups (two sets of two mutually
coupled doublets) at ambient temperature. These two benzyl
groups, however, undergo slow chemical exchange (2.8 s-1 at
28 °C), as shown by magnetization transfer using the DPFGSE-
NOE method. This exchange most likely occurs by dissociation