Structure and dynamics of neutral β-H agostic nickel alkyls: A combined experimental and theoretical study

Article abstract of DOI:10.1021/ja0477221

Addition of BF₃-OEt₂ to ethereal solutions of the Ni(II) β-diketiminates [Me₂NN]Ni(R)(2,4-lutidine) (R = Et (1), Pr (2)) allows the isolation of the neutral β-H agostic monoalkyls [Me ₂NN]Ni(R) (R = Et (3), Pr (4)). X-ray studies of primary alkyls 3 and 4a reveal acute Ni-Cα-Cβ angles with short Ni-Cβ distances, indicating structures along the β-H elimination pathway. Positional disorder of the alkyl group in the X-ray structure of 4 corresponds to partial (22%) occupancy by the secondary alkyl [Me₂NN]Ni(CHMe₂) (4b). Variable-temperature NMR spectra of 3 and 4 reveal fluxional behavior that result from β-H elimination, in-plane rotation of the β-CH ₃ group, and a tetrahedral triplet structure for 3 that were investigated by density functional theory calculations at the Becke3LYP/6-31G* level of theory without simplifications on the β-diketiminate ancillary ligand. Calculations support low temperature NMR studies that identify the linear β-H agostic propyl isomer 4a as the ground state with the branched β-H agostic isomer 4b slightly higher in energy. NMR studies and calculations show that the β-agostic 3 reluctantly coordinates ethene and that 3 is the ground state for this ethylene oligomerization catalyst. The thermodynamic isotope effect K_H/K _D= 1.3(2) measured for the loss of 2,4-lutidine from 1 to form β-agostic 3 was also examined by DFT calculations.

Full text of DOI:10.1021/ja0477221

Published on Web 09/04/2004
Structure and Dynamics of Neutral â-H Agostic Nickel Alkyls:
A Combined Experimental and Theoretical Study
Elzbieta Kogut,† Alexander Zeller,‡ Timothy H. Warren,*,† and Thomas Strassner*,‡
Contribution from the Department of Chemistry, Georgetown UniVersity, Box 571227,
Washington, D.C. 20057-1227 and Department of Inorganic Chemistry,
Technical UniVersity Munich, Lichtenbergstrasse 4, D-85747 Garching, Germany
Received April 20, 2004; E-mail: thw@georgetown.edu; thomas.strassner@chemie.tu-dresden.de
Abstract: Addition of BF3‚OEt2 to ethereal solutions of the Ni(II) â-diketiminates [Me2NN]Ni(R)(2,4-lutidine)
(R ) Et (1), Pr (2)) allows the isolation of the neutral â-H agostic monoalkyls [Me2NN]Ni(R) (R ) Et (3), Pr
(4)). X-ray studies of primary alkyls 3 and 4a reveal acute Ni-CR-Câ angles with short Ni-Câ distances,
indicating structures along the â-H elimination pathway. Positional disorder of the alkyl group in the X-ray
structure of 4 corresponds to partial (22%) occupancy by the secondary alkyl [Me2NN]Ni(CHMe2) (4b).
Variable-temperature NMR spectra of 3 and 4 reveal fluxional behavior that result from â-H elimination,
in-plane rotation of the â-CH3 group, and a tetrahedral triplet structure for 3 that were investigated by density
functional theory calculations at the Becke3LYP/6-31G* level of theory without simplifications on the
â-diketiminate ancillary ligand. Calculations support low temperature NMR studies that identify the linear
â-H agostic propyl isomer 4a as the ground state with the branched â-H agostic isomer 4b slightly higher
in energy. NMR studies and calculations show that the â-agostic 3 reluctantly coordinates ethene and that
3 is the ground state for this ethylene oligomerization catalyst. The thermodynamic isotope effect KH/KD
)
1.3(2) measured for the loss of 2,4-lutidine from 1 to form â-agostic 3 was also examined by DFT calculations.
Introduction
ing the phosphine in SHOP-like catalysts with more weakly
coordinating neutral ligands led to similarly heteroatom-tolerant
catalysts capable of producing linear, high molecular weight
polyethylene.
Seminal work by Brookhart has shown that cationic monoalkyl
complexes of nickel and palladium supported by diimine ligands
bearing bulky N-aryl substituents can lead to very active R-olefin
polymerization systems.16-27 Both the polymerization activity
as well as the degree of branching can be tuned by varying the
bulk of the N-aryl substituents as well as the monomer
concentration.16,19,21,22,28 While associative chain transfer leading
Later transition-metal catalysts for ethylene oligomerization
and polymerization have attracted a great deal of attention
because of their lower electrophilicity and resulting greater
heteroatom tolerance as compared to that of early metal
systems.1-5 An early example is the Shell higher olefin process
(SHOP)6-8 developed by Keim et al., which employs mono-
anionic O∩P ligands in [Ph2PC(R)dC(R)O]Ni(Ph)L (L )
phosphine) complexes that catalyze the formation of linear
R-olefins (C6-C20) from ethylene in 1,4-butanediol.9 Later work
by Ostoja-Starzewski10-13 and Klabunde14,15 showed that replac-
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(15) Klabunde, U.; Mu¨lhaupt, R.; Herskovitz, T.; Janowicz, A. H.; Calabrese,
J.; Ittel, S. D. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 1989.
(16) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414.
† Georgetown University.
‡ Technical University Munich. Current address: Technical University
Dresden, Institute of Organic Chemistry, D-01062 Dresden, Germany.
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11984
J. AM. CHEM. SOC. 2004, 126, 11984-11994
10.1021/ja0477221 CCC: $27.50 © 2004 American Chemical Society

Neutral
â
-H Agostic Nickel Alkyls
A R T I C L E S
Scheme 1. Reversible Loss of Lutidine from 1
to low molecular weight polymers is significantly attenuated
by bulky N-aryl substituents, facile â-H elimination/reinsertion
leads to branched polymers and competes with ethylene insertion
into the M-C bond by “chain-walking”.
Spectroscopic16,20,23-26,29 and computational30-34 studies have
shown that the resting state of the cationic Ni- and Pd-based
diimine monoalkyl catalysts is either an alkyl-olefin complex
or a â-H agostic species, but that these are in equilibrium with
one another. In the polymerization of ethylene, Brookhart has
shown that the resting state is an alkyl-ethylene complex while
in the case of the more sterically demanding propylene
monomer, a pseudo three-coordinate â-agostic structure is
favored.20 The dominance of a â-H agostic resting state with
R-olefins requires that chain growth becomes first-order in
olefin, resulting in significantly lower TOFs than those observed
with ethylene. A â-H agostic resting state can also lead to a
significant inverse isotope effect in the ethylene polymerization
because of a smaller zero-point energy difference in the ground
state. This was proposed on the basis of labeling studies in a
cationic Co(III) system (kH/kD ) 0.48)35 and could be confirmed
by DFT calculations.36
While a great deal of structural and solution dynamic
information has been collected for cationic diimine systems,
corresponding neutral Ni-monoalkyls have been much less
studied, despite the development of several productive catalyst
systems with monoanionic O∩N and N∩N donors.37-45 We
recently reported the synthesis of four-coordinate nickel-
monoalkyl complexes [Me2NN]Ni(R)(2,4-lutidine) (R ) Me,
Et (1), Pr (2)) supported by the â-diketiminate ligand [Me2NN]-
derived from 2,4-bis(2,6-dimethylphenylimido)pentane.46 Vari-
able temperature 1H NMR spectra of these square-planar
monoalkyls in toluene-d8 indicated that the 2,4-lutidine base
was labile and, in the case of the â-H containing species,
identified an equilibrium with the lutidine-free species [Me2-
NN]Ni(R) (R ) Et (3), Pr (4)). The upfield 1H NMR shifts for
the â-H atoms in 3 at δ -5.3 ppm as well as two broad signals
at δ -2.7 and -7.3 ppm for two isomers of 4 strongly suggested
â-H agostic interactions in these species. While a few late metal
cationic â-H agostic alkyl complexes have been structurally
characterized, such as Spencer’s [(But2PCH2CH2PBut2)Ni(CH2-
CH3)]+ and [Cp*Co(P(o-tolyl)3)(CH2CH3)]+,47,48 structural ex-
amples of such neutral complexes are limited to early metal
systems such as TiCl3Et(dmpe)49,50 and [P2N2]Ta(CH2CH2)Et
([P2N2] ) PhP(CH2SiMe2NSiMe2CH2)2PPh).51 Herein we report
our successful efforts at isolating the lutidine-free species 3 and
4, their structural and spectroscopic characterization, as well as
high-level density functional theory calculations that provide
insight into their structure and reactivity.
Results and Discussion
To further probe the â-agostic nature of lutidine-free 3, we
prepared [Me2NN]Ni(CD2CD3)(2,4-lutidine) (1-d5) employing
CD3CD2MgI, which allowed us to determine the thermodynamic
isotope effect KH/KD ) 1.3(2) at 25 °C in toluene-d8 by
comparing lutidine dissociation from 1 and 1-d5 (Scheme 1). A
similar value (KH/KD ) 1.7) has previously been reported for
the equilibrium between the cationic cobalt(III) alkyl [Cp*Co-
(P(OMe3)3(CH2CH3)(2-fluoropyridine)]+ that dissociates 2-fluo-
ropyridine to give the spectroscopically characterized â-H
agostic [Cp*Co(P(OMe)3)(CH2CH3)]+.35 Such thermodynamic
isotope effects are expected in agostic systems because of the
lower C-H(D) vibrational frequencies of the agostic bond
relative to the terminal bond leading to the preference of a C-H
bond over a C-D bond in an agostic position.36
Chemical removal of lutidine from red-orange 1 and 2 by
addition of ca. 1.5 equiv BF3(etherate) in ether results in a cloudy
solution that fades to light yellow. Removal of solvent and
extraction with pentane allows the isolation of yellow crystals
of the base-free â-agostic complexes [Me2NN]Ni(R) (R ) C2H5
(3); C3H7 (4)) in about 50% yield, their isolation somewhat
hampered by their extreme solubility in hydrocarbons (Scheme
2).52,53
(28) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. F. Science 1999, 283,
2059.
(29) Tempel, D. J.; Brookhart, M. Organometallics 1998, 17, 2290.
(30) Deng, L.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 1094.
(31) Stromberg, S.; Zetterberg, K.; Siegbahn, P. E. M. J. Chem. Soc., Dalton
Trans. 1997, 4147.
(32) Musaev, D. G.; Froese, R. D. J.; Morokuma, K. Organometallics 1998,
17, 1850.
The X-ray structures of 3 and 4 reveal square-planar Ni
centers with Ni-alkyl groups for which two sets of positions
each could be refined. The major site occupancies 3a (83%)
and 4a (78%) contain primary â-H agostic alkyl groups,
illustrated in Figures 1 and 2. The Ni-C23A (Câ) distance of
(33) Woo, T. K.; Ziegler, T. J. Organomet. Chem. 1999, 591, 204.
(34) Tomita, T.; Takahama, T.; Sugimoto, M.; Sakaki, S. Organometallics 2002,
21, 4138.
(35) Tanner, M. J.; Brookhart, M.; DeSimone, J. M. J. Am. Chem. Soc. 1997,
119, 7617.
(36) Zeller, A.; Strassner, T. Organometallics 2002, 21, 4950.
(37) Johnson, L. K.; Bennett, A. M. A.; Ittel, S. D.; Wang, L.; Parthasarathy,
A.; Hauptman, E.; Simpson, R. D.; Feldman, J.; Coughlin, E. B. WO Patent
Application 9830609 to DuPont, priority date Jan 4, 1997.
(38) Bansleben, D. A.; Friedrich, S. K.; Younkin, T. R.; Grubbs, R. H.; Wang,
C.; Li, R. T. WO Patent Application 9842664 to W. R. Grace & Co., priority
date March 24, 1997.
(47) Conroy-Lewis, F. M.; Mole, L.; Redhouse, A. D.; Litsler, S. A.; Spencer,
J. L. Chem. Commun. 1991, 1601.
(48) Cracknell, R. B.; Orpen, A. G.; Spencer, J. L. Chem. Commun. 1984, 326.
(49) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. Chem. Commun.
1982, 802.
(39) Wang, C.; Freidrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.; Bansleben,
D. A.; Day, M. W. Organometallics 1998, 17, 314.
(40) Domho¨ver, B.; Kla¨ui, W.; Kremer-Aach, A.; Bell, R.; Mootz, D. Angew.
Chem., Int. Ed. 1998, 37, 3050.
(50) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K.; Shultz, A. J.;
Williams, J. M.; Koetzle, T. F. J. Chem. Soc., Dalton Trans. 1986, 1629.
(51) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc. 2001, 123,
1602.
(52) The synthesis and X-ray structure of the neutral â-agostic species 3 and
4a have been presented: Warren, T. H.; Wiencko, H. L.; Kogut, E.; Dai,
X.; McDermott, J. International Symposium on Olefin Metathesis, Cam-
bridge, MA, August 2001.
(53) After this methodology was initially presented, Brookhart used a similar
approach to generate solutions of neutral â-agostic Ni-Et and Ni-Pr
complexes supported by anilinotroponate ligands. We thank the authors
for communication of their work prior to publication: Jenkins, J. C.;
Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827.
(41) Kla¨ui, W.; Bongards, J.; Reiss, G. J. Angew. Chem., Int. Ed. 2000, 39,
3894.
(42) Hicks, F. A.; Brookhart, M. Organometallics 2001, 20, 3217.
(43) Hicks, F. A.; Jenkins, J. C.; Brookhart, M. Organometallics 2003, 22, 3533.
(44) Jenkins, J. C.; Brookhart, M. Organometallics 2003, 22, 250.
(45) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Waltman, A. W.; Grubbs,
R. H. Chem. Commun. 2003, 2272.
(46) Wiencko, H. L.; Kogut, E.; Warren, T. H. Inorg. Chim. Acta 2003, 345,
199.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11985

A R T I C L E S
Kogut et al.
Figure 1. ORTEP diagram of the solid-state structure of 3 (ethyl group in
site of 83% occupancy). Selected bond distances (Å) and angles (deg): Ni-
N1 1.892(3), Ni-N2 1.859(3), Ni-C22A 1.890(5), Ni-C23A 2.110(6),
Ni-H23A 1.66*, C22A-C23A 1.489(6), N1-Ni-N2 97.44(12), Ni-
C22A-C23A 76.3(3). (* â-CH3 H-atoms placed in idealized positions based
on riding model.)
Figure 3. ORTEP diagram of the solid-state structure of 4b (propyl group
in site of 22% occupancy; [Me2NN]Ni framework unchanged). Selected
bond distances (Å) and angles (deg): Ni-C22B 1.865(18), Ni‚‚‚C23B 2.66,
Ni‚‚‚C24B 2.77.
disorder within the Ni-propyl group (see below). Contracted
CR-Câ distances indicate progress along a pathway toward
complete â-hydride elimination, which was investigated by DFT
calculations discussed later.
While the minor occupancy observed in the X-ray structure
of 3 is also a primary â-agostic ethyl group in which the CR is
on the opposite side of the â-diketiminato coordination wedge
(i.e., trans to N2 instead of N1), the minor occupancy refined
in the X-ray structure of 4 is actually the secondary alkyl [Me2-
NN]Ni(CHMe2) (4b) (Figure 3). Although bond distances within
the Ni-CHMe2 moiety of 4b should be viewed with caution
because of its low site occupancy (22%), the Ni-C22B distance
of 1.865(18) Å is comparable to the Ni-C22A distances in both
3a and 4a. The geometry of 4b is “bent trigonal” for which the
N1-Ni-C22B angle of 154.1(6)° is conspicuously more obtuse
than the N2-Ni-C22B angle of 104.0(6)°, a distortion also
observed in the related three-coordinate d9 and d10 lutidine
adducts [Me2NN]M(2,4-lutidine) (M ) Ni,54 Cu55). In the solid
state the isopropyl group of 4b is not agostic, however, as the
long Ni‚‚‚C23B and Ni‚‚‚C24B separations (2.66 and 2.77 Å)
as well as the closest Ni‚‚‚HR (2.36 Å) and Ni‚‚‚Hâ (2.50 Å)
contacts for H-atoms placed in idealized positions are well
outside of the sum of the corresponding van der Waals radii
(Ni-C 2.35 Å; Ni-H 1.85 Å).
Figure 2. ORTEP diagram of the solid-state structure of 4a (propyl group
in site of 78% occupancy). Selected bond distances (Å) and angles (deg):
Ni-N1 1.882(2), Ni-N2 1.870(2), Ni-C22A 1.900(4), Ni-C23A
2.111(5), Ni-H23A 1.61*, C22A-C23A 1.387(6), C23A-C24A
1.544(6), N1-Ni-N2 97.49(10), Ni-C22A-C23A 78.1(3). (* â-CH2
H-atoms placed in idealized positions.)
Scheme 2. Synthesis of â-Agostic Alkyls [Me2NN]Ni(R) (R ) C2H5
(3), C3H7 (4))
Low temperature 1H NMR spectra (500 MHz, -80 °C) of 3
in toluene-d8 are consistent with its solid-state structure. Two
signals are observed for each â-diketiminato N-aryl o-Me and
backbone Me groups, indicating one mirror plane. While the
Ni-ethyl R-CH2 appears as a sharp quartet at δ 0.02 ppm, two
broad resonances at δ -1.0 and -14.1 ppm of respective relative
areas 2 and 1 are observed for the nonagostic and agostic
hydrogen atoms of the â-CH3 group, which indicates that C-C
rotation is almost frozen out. The coalescence temperature of
these two upfield resonances of -50(5) °C allows us to estimate
the activation barrier to methyl group rotation as ∆Gq ) 8.5(5)
kcal/mol at this temperature, similar to the value of 8.7 kcal/
mol reported at -85 °C for a cationic diimine-based â-H agostic
Ni-ethyl complex.26 In addition, a separate fluxional process
is observed that interconverts each set of N-aryl o-Me and
backbone Me signals with an activation barrier ∆Gq ) 14.9(2)
kcal/mol at 15 °C. The coalescence temperature for this fluxional
process in 3-d5 is unaffected by isotopic substitution and thus
possesses a neglible kinetic isotope effect (kH/kD). Furthermore,
both the R-CH2 and â-CH3 1H resonances begin to broaden
2.110(6) Å in 3a [2.111(5) Å in 4a] is only ca. 0.2 Å longer
than the Ni-C22A (CR) distance of 1.890(5) Å in 3a [1.900(4)
Å 4a], which results in distinctly acute Ni-C22A-C23A angles
(3a: 76.3(3), 4a: 78.1(3)). The â-diketiminato Ni-N1 and Ni-
N2 distances (3a: 1.892(3) and 1.859(3)Å, 4a: 1.882(2) and
1.870(2) Å) are also contracted relative to those in the
corresponding alkyl-lutidine complexes (1: 1.915(3),
1.987(3) Å; 2: 1.919(2), 1.972(2) Å), consistent with a lower
coordination number at nickel. Fitting the â-H atoms of 3a
(riding model) and 4a to idealized positions results in short Ni-
H23A distances of 1.66 and 1.61 Å, respectively, consistent
with â-H agostic interactions. While not crystallographically
observed for 3a, a shortened CR-Câ distance of 1.387(6) Å in
4a relative to the CR-Câ distance in nonagostic 2 (1.518(6) Å)
suggests a buildup of sp2 character in the CR and Câ atoms of
the â-agostic alkyl group. Similar Ni-CR (1.940 and 2.078 Å)
and CR-Câ distances (1.435 Å) are found in Spencer’s â-agostic
cation [(But2PCH2CH2PBut2)Ni(CH2CH3)]+,47 though the un-
usually short CR-Câ distance in 4a may be an artifact of the
(54) Wiencko, H. L.; Warren, T. H. 2001, Unpublished observations.
(55) Amisial, L. D. Masters Thesis, Georgetown University, 2003.
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11986 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Neutral
â
-H Agostic Nickel Alkyls
A R T I C L E S
Scheme 3. â-H Elimination/Reinsertion Reaction (via 5TS) and Symmetrization (via 7TS) of 3 (B3LYP/6-31G(d) Free-Energy Values at 298.15
K/1 atm in Kilocalories per Mole)
above 15 °C, suggesting an exchange of H atoms between these
two sites.
R-CH2 unit of 3, and the single â-CH3 coupling constant likely
results from an average of two “alkene-like” and one agostic
1JCH values.56 Broadening of the CR and Câ 13C{1H} resonances
of 3 begins to occur at 20 °C, and these two signals finally
coalesce at 75(3) °C (Figure 4). We ascribe this behavior to
reversible â-H elimination/reinsertion for which an activation
barrier ∆Gq ) 15.1(3) kcal/mol was determined, just slightly
higher than that determined in a related cationic diimine system
(14.0 kcal/mol at 6 °C).26
Use of the R-labeled CH313CH2MgI in the synthesis of
[Me2NN]Ni(CH2CH3)(2,4-lutidine) (1) leads to equal incorpora-
tion of the 13C-label into both positions of the Ni-ethyl group,
demonstrating that â-H elimination/reinsertion occurs on the
chemical time scale. At -40 °C, these 13C NMR signals appear
at δ 8.84 (CR) and 13.84 (Câ) ppm with coupling constants 1JCH
of 128.1 and 124.7 Hz, respectively. Preparing 3 from this
labeled material allows the clear assignment of the 13C NMR
signals (-40 °C) of the ethyl CR and Câ atoms at δ 16.10 and
1.40 ppm that have coupling constants 1JCH ) 152.7 and 121.4
Hz, respectively. Compared to those in 1, these coupling
constants suggest considerable sp2 character in the
1
The very similar barriers to symmetrization in the H and
13C NMR experiments suggested the possibility of a common
intermediate responsible for the observed spectroscopic behav-
ior. A negligible kinetic isotope effect kH/kD was identified by
1H NMR coalescence temperatures of the â-diketiminato N-aryl
groups, which are identical for both 3 and 3-d5. On the other
hand, HR and â-exchange via Hâ elimination/reinsertion should
show a significant KIE. This contradiction initiated the inves-
tigation by DFT calculations. These were performed on the
Becke3LYP/6-31G(d) level of theory without simplifications
to the ancillary â-diketiminate ligand.
First the â-H elimination/reinsertion reaction of the â-agostic
complex 3 was studied as shown in Scheme 3. Important
geometrical parameters for the structures are given in Table 1;
three-dimensional representations appear in Figure 5.
The calculated structure of 3 (Figure 5) is in good agreement
with the structure obtained from X-ray structural analysis
confirming that the chosen level of theory is appropriate. In
addition, the position of the â-agostic H-atom could be
determined by calculation for which large uncertainties are
present when determined by X-ray diffraction. Further illustrat-
ing its â-H agostic nature, the LUMO+1 calculated for 3
contains an antibonding interaction between a â-C-H bond and
the Ni d-orbital destabilized by σ-donation by the â-diketiminate
N-donors.57 Accordingly, the C23-H23A bond is stretched to
1.16 Å, and the Ni-H23A distance is 1.68 Å. During the course
of the â-H elimination reaction, the C22-C23 fragment bends
out of the Ni-ligand plane. In the transition state 5TS, there is a
Figure 4. Variable temperature 13C{1H} NMR spectra (75 MHz) of the
Ni-CH2CH3 region of
3
prepared using the 13C-labeled reagent
(56) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem. 1988,
CH313CH2MgI. The coalescence temperature for the two resonances is
36, 1.
75(3) °C. (* denotes a minor impurity appearing at higher temperatures.)
(57) Contour plots of the frontier molecular orbitals may be found in the
Supporting Information.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11987

A R T I C L E S
Kogut et al.
Table 1. Geometric Parameters (Å and deg) for Calculated Structures 3-9a
3
5TS
6
7TS
8TS
9
Ni-N1
1.91 [1.892(3)]
1.84 [1.859(3)]
1.87 [1.890(5)]
2.12 [2.110(6)]
1.68 [1.66]
1.90
1.92
1.96
1.98
1.42
1.91
1.41
96.7
69.6
155.9
1.88
1.96
2.00
2.00
1.44
2.30
1.39
95.7
69.5
94.6
1.92
1.92
1.93
1.93
1.41
2.29
1.43
96.6
68.3
166.2
1.88
1.82
1.87
2.39
2.52
1.10
1.54
99.4
88.8
180.0
1.92
1.91
1.96
3.03
-
Ni-N2
Ni-C22
Ni-C23
Ni-H23A
C23-H23A
C22-C23
N1-Ni-N2
Ni-C22-C23
N2-Ni-C22-C23
1.16 [0.98]
-
1.50 [1.489(6)]
98.3 [97.44(12)]
77.3 [76.3(3)]
-180.0 [-177.2]
1.53
97.6
119.8
89.2
a B3LYP/6-31G(d). Results from X-ray analysis of 3 (ethyl group in site of major occupancy) are given in brackets.
Figure 5. Three-dimensional representations of calculated structures 3-9.
slight twist of about 5° between the N-Ni-N and C-Ni-C
planes and the H23A atom is moved in the opposite direction,
indicating only a weak â-C-H interaction (C23-H23A ) 1.91
Å). In the resulting ethene-hydride complex 6a, ethene is bound
perpendicular to the plane containing the hydride and ancillary
â-diketiminate ligands, the favored coordination mode for ethene
in square-planar d8 complexes.58 The C22-C23 bond (1.39 Å)
is somewhat longer than a regular CdC double bond, a result
of π-back-bonding with the metal center. Now a nickel-hydride,
the H23A atom is located in the square plane with a Ni-H
bond distance of 1.44 Å. No structure of 6a with an in-plane
coordination of ethene30,34 could be optimized. The transition
state for the exchange of the ethene and the hydride ligand (7TS)
could also be located. In the tetrahedral 7TS, a significantly longer
C23-H23A separation (2.29 Å) similar to that in the ethene-
hydride ground state 6 (2.30 Å) was identified. We could also
determine the barrier for the C-C bond rotation as ∆Gq ) 7.5
(58) Mingos, D. M. P. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982;
Vol. 3, p 1.
9
11988 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Neutral
â
-H Agostic Nickel Alkyls
A R T I C L E S
Scheme 4. In-Plane Rotation of the â-CH3 Group of 3 (B3LYP/
6-31G (d) Free-Energy Values at 298.15 K/1 atm in Kilocalories
per Mole)
Figure 6. Calculated structure of tetrahedrally biased triplet [Me2NN]Ni-
(CH2CH3) (9). (B3LYP/6-31G (d) free-energy values at 298.15 K/1 atm in
kilocalories per mole)
and backbone substituents on the â-diketiminate ancillary
ligand.65 The paramagnetically shifted 1H NMR resonances
attributed to the d8 LNiMe suggest that it is a triplet. The DFT-
optimized open-shell structure 9 (Figures 5 and 6) is somewhat
pyramidal at Ni, possessing a tetrahedrally biased open coor-
dination site (geometrical parameters in Table 1).
kcal/mol at 298.15 K (Ni-C 2.39 Å; Ni-H 2.52 Å in the TS)
(Scheme 4), which is in general agreement with the value of
8.5(5) kcal/mol estimated by VT NMR for 3.
The free-energy barrier of the elimination calculated by DFT
is 17.8 kcal/mol, a little higher than the experimentally
determined value of 15.1(3) kcal/mol (from VT 13C{1H} NMR).
The ethene-hydride ground-state species 6 is 14.4 kcal/mol
higher in free energy compared to the â-agostic structure 3. The
reverse reaction (ethene insertion into the Ni-H bond) requires
a free activation energy of only 3.4 kcal/mol. These computa-
tional results support the assumption of a â-H elimination/
reinsertion mechanism for the symmetrization of the CR and
Câ atoms in variable temperature 13C{1H} NMR spectra of 3.
The observed exchange of both sets of HR and Hâ as well as
CR and Câ atoms in 3 is thus due to the perpendicular orientation
of ethene relative to the Ni-H23A bond in the square plane of
6 so that reinsertion to reform 3 may occur in either direction
with equal likelihood (C22-H23A and C23-H23A ) 2.30 Å
in 6). According to our DFT calculations, the symmetrization
process observed by 1H NMR that interconverts the â-diketimi-
nato methyl groups on opposite sides of the ancillary ligand in
3 could proceed via the tetrahedral 7TS for which an activation
energy of 21.1 kcal/mol was determined. Thus, the activation
energy for this symmetrization should be 3.3 kcal/mol higher
in energy than that for HR and Hâ exchange. Despite many
attempts, no symmetrization transition state for structure 3 which
contains an orientation of the â-H agostic alkyl group perpen-
dicular to that of the ground state could be optimized.
Extrapolated from a related nickel complex with an anionic
salicylaldimine ligand, where the activation energy of such a
transition state was determined by 32.7 kcal/mol, the energy in
our case should also be higher than 30 kcal/mol.59 This pathway
would therefore not be able to compete with those involving
Coupled with the facile C-C bond rotation expected in a
non-â-agostic ethyl group, movement of the Ni-bound ethyl
group in either direction out of the square plane of 3 to form 9
is equally likely and thus could explain the symmetrization of
1
each side of the â-diketiminato ligand observed in H NMR
spectra of 3. We calculate almost no energy difference between
the singlet state 3 and triplet state 9. This small energetic
difference suggests that interconversion of these electronic
configurations should be facile. The barrier of the intersystem
crossing for a simplified system was calculated to be ∆E )
12.3 kcal/mol, identified by the crossing point between the
singlet and triplet surfaces. An Evans method measurement of
3 in benzene-d6 shows that µeff < 0.1 µB, suggesting that this
excited state would be at least 2 kcal/mol higher in energy
assuming the spin-only expected magnetic moment for the d8
triplet state of ca. 2.8 BM.
The presence of two closely spaced but distinct â-diketiminato
backbone C-H resonances in low temperature NMR spectra
of the â-agostic propyl complex 4 indicates the presence of two
isomers. At -79 °C in toluene-d8 (500 MHz), the major isomer
observed is the primary n-propyl 4a that exhibits diastereotopic
1H NMR signals at δ 0.22 and 0.00 ppm for the Ni-CH2 group
as a consequence of the γ-CH3 group being held out of the
square plane because of the â-H agostic interaction. The â-CH2
resonances are broad but well-separated, appearing at δ -0.40
and -14.43 ppm corresponding to the nonagostic and agostic
sites, respectively, while the γ-CH3 group appears as a triplet
at δ 0.59 ppm. Each set of R-CH2 and â-CH2 resonances
coalesces with increasing temperature and gives rise to a triplet
at δ 0.11 ppm and a broad signal at δ -7.4 (each of area 2),
consistent with rapid interchange of the â-H atoms into and
out of the agostic site. An activation barrier of 9.9(3) kcal/mol
was calculated for this process at -60 °C.
the lower-energy transition states 5TS and 7TS
.
Furthermore, the observation of no KIE in the symmetrization
1
observed by H NMR suggests a mechanism for this process
other than reversible â-hydride elimination/reinsertion since the
latter should show a significant kH/kD. The theoretically
determined KIE at 298.15 K for 5TS is kH/kD ) 2.40 (∆GH )
17.81 vs ∆GD ) 18.33), while the corresponding value for 7TS
is kH/kD ) 3.04 (∆GH ) 21.07 vs ∆GD ) 21.73).
Two resonances attributable to the Ni-bound isopropyl group
in [Me2NN]Ni(CHMe2) (4b) are also observed at -79 °C in
1
the H NMR spectrum (500 MHz) of 4. A multiplet at δ 0.62
ppm and a broad resonance centered at δ -4.8 ppm are assigned
to the R-CH and one â-Me group, respectively. We attribute
We considered an additional pathway for the symmetrization
1
observed by H NMR that involves a change from the singlet
ground state to a triplet excited state. Spin crossing processes
are not appreciably affected by isotopic substitution60-64 and
therefore could lead to an indistinguishable KIE in this case. In
a recent article, Holland et al. described some evidence for the
low temperature formation of the related, three-coordinate
nickel(II)-methyl complex LNiMe that employs bulkier N-aryl
(60) Schro¨der, D.; Fiedler, A.; Ryan, M. F.; Schwarz, H. J. Phys. Chem. 1994,
98, 68.
(61) Fiedler, A.; Schro¨der, D.; Shaik, S.; Schwarz, H. J. Am. Chem. Soc. 1994,
116, 10734.
(62) Schro¨der, D.; Schwarz, H.; Clemmer, D. E.; Chen, Y.; Armentrout, P. B.;
Baranov, V. I.; Bo¨hme, D. K. Int. J. Mass Spectrom. Ion Processes 1997,
161, 175.
(63) Filatov, M.; Shaik, S. J. Phys. Chem. A 1998, 102, 3835.
(64) Schro¨der, D.; Shaik, S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139.
(65) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte, R.
T. J. Am. Chem. Soc. 2002, 124, 14416.
(59) Deubel, D. V.; Ziegler, T. Organometallics 2002, 21, 1603.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11989

A R T I C L E S
Kogut et al.
Scheme 5. â-H Elimination/Reinsertion Reaction of 4a (via 10TS) to Give an Equilibrium with Branched Isomer 4b (B3LYP/6-31G(d)
Free-Energy Values at 298.15 K/1 atm in Kilocalories per Mole)
Table 2. Geometric Parameters (Å and deg) for Calculated Structures 4a, 4b, 10-12a
4a
4b
10TS
11
12
Ni-N1
1.91 [1.882(2)]
1.85 [1.870(2)]
1.88 [1.900(4)]
2.14 [2.111(5)]
1.66 [1.61]
1.91 [1.882(2)]
1.85 [1.870(2)]
1.89 [1.865(18)]
2.11 [2.66]
1.68
1.15
1.50
1.52 (C22-C24)
98.0
1.91
1.95
1.97
2.01
1.42
2.09
1.41
1.51
95.9
71.0
146.8
1.89
1.97
2.00
2.02
1.43
2.25
1.40
1.51
2.01
1.96
1.94
3.07
-
Ni-N2
Ni-C22
Ni-C23
Ni-H23A
C23-H23A
C22-C23
C23-C24
N1-Ni-N2
Ni-C22-C23
N2-Ni-C22-C23
1.16 [0.98]
-
1.49 [1.387(6)]
1.53 [1.544 (6)]
98.4 [97.49(10)]
77.8 [78.1(3)]
-179.8 [-176.0]
1.52
1.39 (C24-C25)
93.4
124.0
-100.9
95.5
70.3
117.5
76.3
-167.8
a
B3LYP/6-31G(d). Results from X-ray analysis of 4a and 4b are given in brackets. The X-ray structure of 4b is not â-H agostic.
the upfield shift of the broad resonance to the rapid exchange
of all H atoms of this â-Me group through an agostic site, though
no specific â-H interaction was identified in the minor oc-
cupancy attributed to 4b in the X-ray structure of 4. Above 0°
C, the broad resonance at δ -4.8 shifts to δ -2.7 ppm,
indicating rapid rotation of all â-H atoms through the agostic
site. Above 0 °C, interconversion between the primary and
secondary alkyls 4a and 4b via reversible â-H elimination to
form the propene-hydride intermediate 11 becomes significant
on the NMR time scale (Scheme 5).
In contrast to that observed in related cationic Ni-diimine26
and neutral anilinotroponate systems,53 the secondary propyl 4b
lies somewhat higher in energy than the linear n-propyl ground
state 4a (Scheme 5). An equilibrium constant Keq ) [4b]/[4a]
may be determined over the temperature range 0-40 °C to give
∆H ) +1.7(4) and ∆S ) 5(1). Above 30 °C, the averaged
â-agostic resonances in 4a and 4b at δ -2.7 and -7.4 ppm
begin to broaden more appreciably and the R-CH2 and γ-CH3
resonances of 4a coalesce, signaling rapid isomerization between
4a and 4b on the NMR time scale (Scheme 5). Curiously, the
predicted equilibrium constant [secondary]/[primary] of 0.25 at
the temperature at which 4 was crystallized (-35 °C) is very
similar to the secondary/primary occupancy ratio of 0.22/0.78
) 0.28 observed in the X-ray structure of 4.
Accordingly, calculations on the â-H elimination/reinsertion
reaction were repeated for the linear â-agostic propyl complex
4a (Scheme 5, geometric parameters in Table 2, three-
dimensional respresentations in Figure 7). During the â-H
elimination process involving 4a, the bond lengths in the
transition state 10TS are only slightly affected by the substitution
by a methyl group as compared to that observed in 5TS for the
corresponding ethyl analogue 3. Steric interactions of the
additional methyl group lengthen the C23-H23A bond distance
in 10TS to 2.09 Å as compared to 1.91 Å in 5TS, signifying that
the transition state is located later on the reaction coordinate
than that for 3. The propene ligand in the â-H elimination
product 11 is twisted from being perpendicular to the square
plane by ca. 30°, a result of steric interactions with the
â-diketiminato N-aryl groups. It should be noted that in an earlier
DFT study of a cationic nickel diimine propene-hydride
complex with less bulky N-substituents, the propene ligand was
oriented perpendicular to the square plane.32
Consistent with the solution NMR characterization of 4a and
4b, we calculate the â-agostic branched propyl complex [Me2-
NN]Ni(CHMe2) (4b) to be somewhat higher in energy than the
â-agostic linear complex [Me2NN]Ni(CH2CH2CH3) (4a). The
barrier for the â-H elimination transition state 10TS (16.3 kcal/
mol) is comparable to that found for 3 (17.8 kcal/mol), although
9
11990 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Neutral
â
-H Agostic Nickel Alkyls
A R T I C L E S
Figure 7. Three-dimensional representations of the calculated structures for 4a, 4b, and 10-12.
Scheme 6. Reversible Ethene Coordination to 3 (B3LYP/6-31G*
resonances at -2.7 and -7.3 ppm. These new signals are
assigned to primary and secondary Ni-alkyls that interconvert
via â-H elimination/reinsertion and result in the formation of
branched ethylene oligomers.46
Free Energy at 298.15 K/1 atm in Kilocalories per Mole)
DFT calculations on the ethene coordination reaction lead to
an interesting result (Scheme 6, geometric parameters in Table
2; three-dimensional representation in Figure 7). Ethene coor-
dination to 3 is slightly exothermic, but endergonic. As in
ethene-hydride 6, the ethene ligand in 12 adopts a perpendicular
orientation relative to the square plane in a tetrahedrally distorted
complex with the C24-C25 bond lengthened to 1.39 Å by
π-back-bonding. The coordination of ethene to 3 is exothermic
in electronic energy. From DFT calculations, ∆H remains
approximately constant at -2.4 kcal/mol over the temperature
range -50 to 25 °C. As expected, the coordination of ethene
leads to a negative ∆S and results in a positive ∆Grxn of +13.2
kcal/mol at 298.15 K (+10.4 kcal/mol at 243.15 K). Thus, 3 is
the resting state of the oligomerization catalyst, and only low
concentrations of the ethene-ethyl complex 12 should be
present at room temperature with moderate ethene concentra-
tions.
the corresponding propene-hydride 11 (+15.3 kcal/mol) is
destabilized relative to the ethene-hydride 6 (+14.4 kcal/mol).
This is likely a result of the distorted propene orientation, which
leads to a poorer overlap of the ligand π-orbitals with the metal
d-orbitals, weakening π-back-bonding.
Addition of 6 equiv of ethylene to â-agostic 3 at -50 °C
results in the immediate formation of a new species in low
concentration assigned as the ethylene adduct [Me2NN]Ni(CH2-
CH3)(η2-CH2CH2) (12) (Scheme 6). The Ni-ethyl group in 12
1
is not â-agostic, shown by H NMR resonances at δ 0.25 (R-
CH2) and δ 0.95 ppm (â-CH3). Further warming results in an
increase in the concentration of 12 at the expense of 3. Although
slow below 0 °C, ethylene insertion into the Ni-alkyl bond
does occur in 12 to provide new species with broad, upfield 1H
The dissociation reaction of lutidine in the ethyl-lutidine
complex 1 and the corresponding thermodynamic isotope effect
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11991

A R T I C L E S
Kogut et al.
Table 3. Geometric Parameters of the Calculated Structure 1
(B3LYP/6-31G(d)) and from the X-ray Analysis of 1
sparged with nitrogen and then dried by passage through activated
alumina columns. Pentane was first washed to remove olefins, stored
over CaCl2, and then distilled before use from sodium/benzophenone.
All deuterated solvents were sparged with nitrogen, dried over activated
4A molecular sieves, and stored under nitrogen. 1H and 13C NMR
spectra were recorded on either a Mercury Varian 300 MHz spectrom-
eter (300 and 75.4 MHz, respectively) or a Unity Varian 500 MHz
spectrometer (500 and 125 MHz, respectively). All NMR spectra were
indirectly referenced to TMS using residual solvent signals as internal
standards. Elemental analyses were performed on a Perkin-Elmer
PE2400 microanalyzer in our laboratories (Georgetown).
1 (calculated)
1 (from X-ray)46
Ni-N1
2.00
1.94
1.96
1.53
1.91
93.4
87.3
124.0
1.987(3)
1.915(3)
1.965(4)
1.497(7)
1.916(3)
93.16(12)
86.74(14)
119.9(3)
Ni-N2
Ni-C1
C1-C2
Ni-N3
N1-Ni-N2
C1-Ni-N3
Ni-C1-C2
Unlabeled Grignard reagents and BF3(etherate) were obtained from
Aldrich and used as received. CD3CD2I and CH313CH2I used in the
preparation of labeled Grignard reagents were used as received from
Cambridge Isotope Laboratories. [Me2NN]Ni(CH2CH3)(2,4-lutidine) (1)
and [Me2NN]Ni(CH2CH2CH3)(2,4-lutidine) (2) were prepared according
to literature procedures.46 [Me2NN]Ni(CD2CD3)(2,4-lutidine) (1-d5) was
prepared in a similar procedure as used for 1, employing an ether
solution of the labeled Grignard reagent CD3CD2MgI.
KH/KD of 1.3(2) was investigated by DFT methods (Scheme
1). Again the complex 1 was optimized without any simplifica-
tions (Becke3LYP/6-31G(d)). Important geometric parameters
are given in Table 3. The DFT structure again shows good
agreement with the solid-state structure. The calculated free
energies at 298.15 K are -7.05 kcal/mol for 1 and -6.91 kcal/
mol for 1-d5. This results in a theoretically determined KH/KD
of 1.25, consistent with the experimental finding. The reason
for this secondary kinetic isotope effect is the conversion of
the terminal C-H bond to a â-agostic C-H bond. Because of
interactions with the metal, the â-agostic C-H vibrational
frequency is lower than that of the â-C-H bond in the open
structure. This leads to a smaller zero-point energy difference
of the isotopomers in structure 3 and, accordingly, to a smaller
free-energy difference between nonagostic 1 and â-agostic 3.
Computational Details. The DFT calculations were performed with
Gaussian98.66 The density functional hybrid model Becke3LYP67-70
was used together with the valence double-ú basis set 6-31G(d). No
symmetry or internal coordinate constraints were applied during
optimizations. All reported intermediates were verified as true minima
by the absence of negative eigenvalues in the vibrational frequency
analysis. Transition-state structures (indicated by TS) were located using
the Berny algorithm71 until the Hessian matrix had only one imaginary
eigenvalue. The identity of all transition states was confirmed by
animating the negative eigenvector coordinate with MOLDEN.72 The
crossing point between the singlet and triplet state of 9 was determined
by a published procedure.73
Conclusions
Chemical removal of 2,4-lutidine from solutions of [Me2-
NN]Ni(R)(2,4-lutidine) (R ) Et (1), Pr (2)) allows for the
isolation of the base-free, neutral â-agostic species [Me2NN]-
Ni(R) (R ) Et (3), Pr (4)) that have been characterized by single-
crystal X-ray as well as detailed solution and computational
studies. Intermediate in structure between nonagostic monoalkyls
and alkene-hydride complexes, the â-agostic complexes
undergo facile â-H elimination/reinsertion for which an activa-
tion barrier ∆Gq ) 15.1 (3) kcal/mol was measured by VT 13C-
{1H} NMR for 3. Both primary and secondary Ni-propyl
groups are observed in the solid state and in solution for 4,
with the primary â-agostic Ni-CH2CH2CH3 representing the
ground state by a narrow margin. This observation stands in
contrast to the secondary â-agostic Ni-CH(CH3)2 ground state
identified for related cationic R-diimine and neutral anilino-
troponate N∩O systems. This difference may be a consequence
of the more compressed coordination wedge in the â-diketimi-
nate system due to its larger bite angle (as compared to
R-diimines) that brings the two bulky N-aryl substituents closer
together. Furthermore, the tighter coordination wedge apparently
coupled with strong donation by the monoanionic N∩N ancillary
ligand favors pseudo three-coordinate â-agostic alkyls over four-
coordinate alkyl-ethylene complexes. This leads to a â-H
agostic monoalkyl resting state under modest ethylene pressures
with a corresponding low TOF for ethylene oligomerization/
polymerization than that observed in more active cationic and
neutral systems that rest as alkyl-ethylene species.
Approximate free energies were obtained through thermochemical
analysis of the frequency calculation, using the thermal correction to
Gibbs free energy as reported by Gaussian98. This takes into account
zero-point effects, thermal enthalpy corrections, and entropy. For the
purpose of comparability all energies reported in this article, unless
otherwise noted, are free energies at 298 K and have not been
recalculated at the actual temperature of the experiment. Frequencies
were scaled by 0.9806.74 All transition states are maxima on the
electronic potential energy surface. These may not correspond to
maxima on the free-energy surface. We did not attempt to correct the
free energy for hindered internal rotations.
Kinetic isotope effects were calculated from free-energy differences
obtained from frequency calculations.
(66) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
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General Consderations. All experiments were carried out in a dry
nitrogen atmosphere using an MBraun glovebox and/or standard
Schlenk techniques. 4A molecular sieves were activated in vacuo at
180 °C for 24 h. Diethyl ether and tetrahydrofuran (THF) were first
(72) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14, 123.
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9
11992 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Neutral
â
-H Agostic Nickel Alkyls
A R T I C L E S
3, NiCH(CH3)agost(CH3). 13C NMR (toluene-d8, -79 °C): δ 158.85,
158.49, 157.58, 155.76, 155.01, 154.16, 152.77, 131.31, 130.99, 130.12,
129.81, 129.74, 129.05, 128.34, 128.22, 128.15, 124.06, 123.74, 123.63,
97.55 (backbone-C-H 4b), 97.23 (backbone-C-H 4a), 24.62, 23.99,
23.17, 22.37, 21.55, 21.17, 19.54, 19.35, 19.18, 19.15, 18.54, 15.67,
14.93. Anal. Calcd for C24H32N2: C, 70.79; H, 7.92; N, 6.88. Found:
C, 70.37; H, 8.31; N, 7.25.
k
∆GD - ∆GH
KIE ) H ) exp
(1)
(2)
[
]
kD
RT
∆GX ) GXq - GXeducts X ) H or D
Determination of Thermodynamic Isotope Effect for 2,4-Lutidine
Dissociation from [Me2NN]Ni(CH2CH3)(2,4-lutidine) (1). A sample
of [Me2NN]Ni(CH2CH3)(2,4-lutidine) (0.050 g, 0.10 mmol) or [Me2-
NN]Ni(CD2CD3)(2,4-lutidine) (0.050 g, 0.10 mmol) was dissolved in
1.0 mL of toluene-d8 in a volumetric flask and transferred into an NMR
tube. Pentane was added as an internal standard. 1H NMR spectra were
acquired every 10 °C over the temperature range -40 to 50 °C. The
integration of resonances corresponding to free (δ 8.38 ppm) and bound
lutidine (δ 7.47 ppm) against the internal standard allowed the
calculation of the equilibrium constant at a given temperature using
the following dependence:
Determination of Barrier to â-H Elimination/Reinsertion for
[Me2NN]Ni(CH2CH3). 13C-labeled Grignard reagent (prepared from
CH313CH2I) was used to prepare mono 13C-labeled [Me2NN]Ni(CH2-
CH3)(2,4-lutidine) (1-13C). 13C{1H} NMR spectra of 1-13C indicated
equal incorporation of the 13C-label into the CR and Câ positions. Mono
13C-labeled [Me2NN]Ni(CH2CH3) (3-13C) was prepared following an
analogous procedure for 3. Coalescence of the 13C{1H} (75.4 MHz)
resonances at δ 16.12 (CR) and 1.32 (Câ) ppm (from the low temperature
limit) at 75(3) °C leads to ∆Gq ) 15.1(3) kcal/mol at this temperature.
Keq ) {[free lutidine]2/[bound lutidine]} ×
[initial concentration of 1 (or 1-d5)]
Determination of ∆H and ∆S for Equilibrium between 4a and
1
4b. H NMR spectra of a toluene-d8 solution of 4 were taken at 10°
intervals between 0 and 40 °C. The unitless equilibrium constant Keq
) 4b/4a was determined by integration of the Ni-CH2CH2CH3 signal
of 4a at δ 0.11 ppm (relative area 2) against the backbone C-H
resonance at δ 5.04 ppm that corresponds to both 4a and 4b (each
relative area 1). The mole fraction ø4a could thus be directly determined,
from which Keq ) 1 - ø4a/ø4a. A van’t Hoff plot (see Supporting
Information) allowed the determination of ∆H ) +1.7(4) kcal/mol and
∆S ) 5(1) cal/mol‚K.
Addition of Ethylene to [Me2NN]Ni(CH2CH3) (3). A sample of
[Me2NN]Ni(CH2CH3) (0.046 g, 0.117 mmol) was dissolved in 1.0 mL
of toluene-d8 in a volumetric flask and transferred into an NMR tube.
Mesitylene (0.007 g, 0.06 mmol) was added as an internal standard.
Approximately 6 equiv of ethylene at 1 atm was added by syringe at
-78 °C, and a 1H NMR spectrum was acquired at -50 °C. In addition
to resonances for 3 and free ethylene at δ 5.28 ppm, a new species in
low concentration with a nonagostic ethyl group was observed at δ
0.95 (t, 3, Ni-CH2CH3) and 0.25 (q, 2, Ni-CH2CH3) ppm assigned to
[Me2NN]Ni(CH2CH3)(η2-CH2CH2) (12). The bound ethylene ligand of
12 is likely obscured by N-aryl Me resonances. An equilibrium constant
could be estimated at this temperature by integration of normalized
resonances corresponding to free ethylene (δ 5.28 ppm), [Me2NN]Ni-
(CH2CH3)(η2-CH2CH2) (δ 0.25 ppm) as well as the â-agostic [Me2-
NN]Ni(CH2CH3) (δ 0.08 ppm) against an internal standard (mesity-
lene): Keq ) {[12]/[3][free ethylene][initial concentration of 3] ) 0.45
M-1 (at -50 °C). Followed by 1H NMR, insertion of ethylene at higher
temperatures led to linear (δ -7.3 ppm) and branched (δ -2.7 ppm)
â-agostic Ni-alkyl groups.
Van’t Hoff plots of ln Keq vs 1/T allowed the calculation of ∆H )
9.4(1.0) kcal/mol and ∆S ) 23(4) cal/mol‚K for lutidine dissociation
from 1 (Figure S7) as well as ∆H ) 8.3(7) and ∆S ) 19(3) cal/mol‚K
for lutidine dissociation from 1-d5 (Figure S8), which leads to the
thermodynamic isotope effect KH/KD ) 1.3(2) at 25 °C.
Synthesis of [Me2NN]Ni(CH2CH3) (3). A solution of BF3(etherate)
(0.085 g, 0.600 mmol) in 2 mL of Et2O was added with stirring to a
solution of [Me2NN]Ni(CH2CH3)(2,4-lutidine) (0.300 g, 0.600 mmol)
in 10 mL of Et2O. The solution turned yellow immediately and became
cloudy. The volatiles were removed in vacuo, and the yellow residue
was triturated twice with 5 mL of pentane. The solid residue was then
extracted with 10 mL of pentane, filtered through Celite, and
concentrated to dryness. This pentane extraction/concentration cycle
was carried out a total of three times. A final cycle was then performed
in which the solution was concentrated to small volume (ca. 1 mL)
and kept at -35 °C for 5 days. Tan crystals that formed were collected
on a frit and dried in vacuo to afford 0.111 g (47%) of the product. 1H
NMR (toluene-d8, -79 °C): δ 7.14, 7.06, 7.03, 7.01, 6.99, 6.96, 6.94,
6.91, 6.90, 6.87, 6.85, 6.63 (aromatic), 5.02 (s, 1, backbone-C-H), 2.46
(s, 6, o-CH3), 2.41 (s, 6, o-CH3), 1.54 (s, 3, backbone-CH3), 1.51 (s, 3,
backbone-CH3), 0.08 (q, 2, NiCH2CH3), -1.03 (br, 2, NiCH2CH2Hagost),
-14.11 (br, 1, NiCH2CH2Hagost). 13C NMR (toluene-d8, -40 °C): δ
159.01, 158.15, 155.44, 154.62, 130.67, 130.18, 128.50, 124.35, 123.90,
123.45, 97.01 (backbone-C-H), 22.18, 21.26, 19.13, 19.10, 16.12 (JCR-H
) 152.71), 1.32 (JCR-H ) 121.38). Anal. Calcd for C23H30N2: C, 70.25;
H, 7.71; N, 7.13. Found: C, 70.20; H, 7.90; N, 7.35.
Synthesis of [Me2NN]Ni(C3H7) (4). A solution of BF3(etherate)
(0.145 g, 1.02 mmol) in 2 mL of Et2O was added with stirring to a
solution of [Me2NN]Ni(CH2CH2CH3)(2,4-lutidine) (0.526 g, 1.02 mmol)
in 10 mL of Et2O. The solution turned yellow immediately and became
cloudy. The volatiles were removed in vacuo, and the yellow residue
was triturated twice with 5 mL of pentane. The solid residue was then
extracted with 10 mL of pentane, filtered through Celite, and
concentrated to dryness. This pentane extraction/concentration cycle
was carried out a total of three times. A final cycle was then performed
in which the solution was concentrated to small volume (ca. 1 mL)
and kept at -35 °C for several days. Tan crystals that formed were
collected on a frit and dried in vacuo to afford 0.256 g (62%) of the
X-ray Structure Refinement Details. Single crystals of 3 and 4
were mounted under mineral oil on glass fibers and immediately placed
in a cold nitrogen stream at -90(2) °C on a Bruker SMART CCD
system. Full spheres of data were collected (0.3° ω-scans; 2θmax
)
56°; monochromatic Mo KR radiation, λ ) 0.7107 Å) and integrated
with the Bruker SAINT program. Structure solutions were performed
using the SHELXTL/PC suite75,76 and XSEED.77 Intensities were
corrected for Lorentz and polarization effects, and an empirical
absorption correction was applied using Blessing’s method as incor-
porated into the program SADABS.78 With the exception of the Ni-
ethyl and Ni-propyl groups in 3 and 4, respectively, non-hydrogen
atoms were refined with anisotropic thermal parameters and hydrogen
atoms were included in idealized positions. Tables of all atomic
1
product. H NMR (toluene-d8, -79 °C, primary isomer 4a): δ 7.04-
6.90 (aromatic), 5.03 (s, 1, backbone-C-H), 2.48 (s, 6, o-CH3), 2.45 (s,
6, o-CH3), 1.55 (s, 3, backbone-CH3), 1.50 (s, 3, backbone-CH3), 0.59
(br, 3, NiCH2CH2CH3), 0.22 (br, 1, NiCHHCH2CH3), -0.003 (br, 1,
NiCHHCH2CH3), -0.40 (br, 1, NiCH2CHHagostCH3), -14.43 (br, 1,
NiCH2CHHagostCH3). 1H NMR (toluene-d8, -79 °C, secondary isomer
4b): δ 7.04-6.90 (aromatic), 5.04 (s, 1, backbone-C-H), 2.60, 2.45,
2.37, 2.35, (s, 3, o-CH3), 1.55, 1.51 (s, 3, backbone-CH3), 0.62 (br, 1,
NiCH(CH3)agost(CH3), -0.40 (br, 3, NiCH(CH3)agost(CH3)), -4.8 (v br,
(75) SHELXTL-PC, version 5.10; Bruker Analytical X-ray Services: Madison,
WI, 1998.
(76) Sheldrick, G. M. SHELX-97; Universita¨t Go¨ttingen: Go¨ttingen, Germany,
1997.
(77) (a) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189. (b) XSEED Home
Page, http://xseed.net.
(78) (a) Sheldrick, G. M. SADABS, version 2.01; Bruker-Analytical X-ray
Services: Madison, WI, 1999. (b) Based on the method described in:
Blessing, R. H. Acta Crystallogr., Sect A. 1995, 51, 33.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11993

A R T I C L E S
Kogut et al.
coordinates, anisotropic thermal parameters, and complete bond lengths
and bond angles are included as Supporting Information.
not realistic. Nonetheless, this conformation would be nonagostic even
if they assumed a more normal value (e.g., C23A-C24A ) 1.544(6)
In the initial refinement of 3, elongated anisotropic thermal ellipsoids
of C22 and C23 perpendicular to the C-C bond suggested disorder in
the Ni-ethyl group, which was modeled with two sets of positions in
a 83:17 ratio. C22A/C22A and C22B/C23B were refined anisotropically,
though thermal parameters within each pair of C atoms were constrained
to be similar. The initial refinement of 4 similarly suggested disorder
in the Ni-propyl group. After lowering the occupancy of the Ni-
propyl C atoms of the â-agostic linear isomer 4a (C22A-C24A) and
isotropically refining them, we identified three new peaks corresponding
to the C atoms of the isopropyl isomer 4b in the Fourier difference
map. These peaks assigned as C22B-C24B were allowed to isotro-
pically refine against C22A-C24A after constraining their isotropic
thermal parameters to be similar to those of C22A-C24A. Fixing the
occupancy ratio (4a/4b ) 78:22), releasing the thermal parameter
constraint, and anisotropically refining C22A-C24A and C22B-C24B
(anisotropic parameters of sets of atoms C22A-C24A and C22B-
C22B lightly constrained to be similar) led to a reduction of R1
[I > 2σ(I)] from 5.29% to 4.29% with a concomitant reduction in the
maximum absolute peak height from 0.89 e-/Å3 to 0.57 e-/Å3. Because
of the low occupancy of minor isomer 4b (22%), the unconstrained
C-C distances within the isopropyl group (1.39(2) and 1.66(3) Å) are
Å).
Acknowledgment. T.H.W. is grateful to Georgetown Uni-
versity for startup funding as well as the NSF CAREER program
(No. 0135057) for partial support of this work. E.K. thanks Dr.
Angel deDios for assistance in obtaining high field NMR
spectra. T.S. thanks the Leibniz Computer Center, Munich for
computer time.
Supporting Information Available: VT 1H NMR spectra for
3 and 4, van’t Hoff plots for loss of lutidine from 1 and 1-d5 as
well as the equilibrium between 4a and 4b, contour plots of
the frontier molecular orbitals of 3, tables of X-ray refinement
data, bond distances, and bond angles along with fully labeled
ORTEP plots for 3 (major and minor occupancies), 4a, and 4b
(also provided in CIF format). This material is available free
of charge via the Internet at http://pubs.acs.org.
JA0477221
9
11994 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004