1,2-addition versus σ-bond metathesis reactions in transient bis(cyclopentadienyl)zirconium imides: Evidence for a d0 dihydrogen complex

Article abstract of DOI:10.1021/om7010485

Exposure of a series of zirconocene amido and hydrazido hydride complexes, (η⁵-C₅Me₄)₂Zr(NHR)H (R = ¹Bu, NMe₂, Me, H), to 4 atm of D₂ gas at 56 °C produced isotopic exchange in both the N-H and Zr-H positions. In general, the relative rates of 1,2-elimination can be rationalized on the basis of groundstate effects, whereby amido compounds with the strongest N-H bonds, as judged by the corresponding free amine, undergo the slowest isotopic exchange. For the compound with the strongest N-H bond in the series, (η⁵- C₅Me₄H)₂Zr(NH₂)H, the barrier for 1,2-elimination is sufficiently high such that σ-bond metathesis becomes the dominant intermolecular exchange pathway. For the other amido zirconocene hydrides, the rate constants for deuterium exchange into the N-H position are faster than for the Zr-H position. This behavior is a result of a faster intramolecular isomerization process driven by an equilibrium isotope effect favoring N-D over Zr-D bond formation. Computational studies on a related model compound, (η⁵-C₅H₅)₂Zr(NH ^tBu)H, successfully reproduce these observations and support a pathway involving the formation of rare d⁰ dihydrogen complexes.

Full text of DOI:10.1021/om7010485

872
Organometallics 2008, 27, 872–879
1,2-Addition versus σ-Bond Metathesis Reactions in Transient
Bis(cyclopentadienyl)zirconium Imides: Evidence for a d0
Dihydrogen Complex
Hannah E. Toomey,† Doris Pun,† Luis F. Veiros,‡ and Paul J. Chirik*,†
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853, and Centro de Quimica Estrutural, Complexo I, Instituto Superior Técnico,
AVenida RoVisco Pais 1, 1049-001 Lisbon, Portugal
ReceiVed October 18, 2007
Exposure of a series of zirconocene amido and hydrazido hydride complexes, (η5-C5Me4H)2Zr(NHR)H
(R ) tBu, NMe2, Me, H), to 4 atm of D2 gas at 56 °C produced isotopic exchange in both the N-H and
Zr-H positions. In general, the relative rates of 1,2-elimination can be rationalized on the basis of ground-
state effects, whereby amido compounds with the strongest N-H bonds, as judged by the corresponding
free amine, undergo the slowest isotopic exchange. For the compound with the strongest N-H bond in
the series, (η5-C5Me4H)2Zr(NH2)H, the barrier for 1,2-elimination is sufficiently high such that σ-bond
metathesis becomes the dominant intermolecular exchange pathway. For the other amido zirconocene
hydrides, the rate constants for deuterium exchange into the N-H position are faster than for the Zr-H
position. This behavior is a result of a faster intramolecular isomerization process driven by an equilibrium
isotope effect favoring N-D over Zr-D bond formation. Computational studies on a related model
compound, (η5-C5H5)2Zr(NHtBu)H, successfully reproduce these observations and support a pathway
involving the formation of rare d0 dihydrogen complexes.
dinitrogen with nonpolar reagents. Specifically, our laboratory
has reported a family of bis(cyclopentadienyl)zirconium9–11 and
hafnium12 compounds with strongly activated, four-electron-
reduced dinitrogen ligands that exhibit significant imido char-
acter in the metal-nitrogen bonds (Figure 2).13 These com-
pounds undergo the facile 1,2-addition14 of dihydrogen and
C-H bonds,10,15 resulting in a mild method for assembly of
N-H bonds in solution.
Introduction
1,2-Addition and its microscopic reverse, 1,2-elimination, are
fundamental transformations in organometallic chemistry and
have found widespread application in the activation of
carbon-hydrogen bonds.1 Imido titanium and zirconium com-
plexes, X2TidNR (X ) amide, alkoxide, cyclopentadienyl), are
exemplary; a range of compounds that induce C-H bond
cleavage in methane and higher alkanes have been reported.2–4
This mechanistic paradigm has also been applied to isolobal
and isoelectronic group 4 alkylidene complexes5,6 and more
recently titanium alkylidynes.7 For the imido compounds,
mechanistic2 and computational8 studies support a planar, four-
centered “kite-shaped” transition structure with considerable
C-H bond breaking and N-H bond forming in the hydrocarbon
activation step (Figure 1).
Investigations into the mechanism of N2 hydrogenation10,14
and subsequent diazene dehydrogenation with an ansa-zir-
conocene complex11 have relied on deuterium labeling experi-
ments to gain insight into the 1,2-addition/elimination event.
However, these studies have been complicated by competing
σ-bond metathesis reactions and cyclopentadienyl substituent
cyclometalation.16,17 In principle, the competition between 1,2-
elimination and σ-bond metathesis can be delineated with an
isotopic labeling study (Figure 3). The case where D2 is added
to the perprotio isotopologue is shown in Figure 3. For the
extreme where 1,2-elimination/addition is exclusive, the rate
of disappearance for the two positions will be equal. In the limit
where σ-bond metathesis is exclusive, isotopic exchange occurs
These fundamental studies in C-H activation have served
as the foundation for the functionalization of coordinated
* Corresponding author. E-mail: pc92@cornell.edu.
† Cornell University.
‡ Instituto Superior Técnico.
(1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507.
(2) (a) Wolczanski, P. T.; Bennett, J. L. J. Am. Chem. Soc. 1997, 119,
10696. (b) Wolczanski, P. T.; Bennett, J. L. J. Am. Chem. Soc. 1994, 116,
2179. (c) Schaller, C. P.; Cummins, C. C.; Wolczanski, P. T. J. Am. Chem.
Soc. 1996, 118, 591.
(9) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527.
(10) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2005, 127, 14051.
(11) Hanna, T. E.; Keresztes, I.; Lobkovsky, E.; Chirik, P. J. Inorg.
Chem. 2007, 46, 1675.
(3) (a) Duncan, A. P.; Bergman, R. G. Chem. Rec. 2002, 2, 431. (b)
Hoyt, H. E.; Michael, F. E.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126,
1018.
(12) Bernskoetter, W. H.; Olmos, A. V.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2006, 25, 1021.
(4) Hazari, N.; Mountford, P. Acc. Chem. Res. 2005, 38, 839.
(5) Mindiola, D. J. Acc. Chem. Res. 2006, 39, 813.
(6) Hanna, T. E.; Keresztes, I.; Lobkovsky, E.; Bernskoetter, W. H.;
Chirik, P. J. Organometallics 2004, 23, 3448.
(13) Chirik, P. J. Dalton Trans. 2007, 16.
(14) Pool, J. A.; Bernskoetter, W. H.; Chirik, P. J. J. Am. Chem. Soc.
2004, 126, 14326.
(15) Bernskoetter, W. H.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am.
Chem. Soc. 2005, 127, 7901.
(7) Bailey, B. C.; Fan, H. J.; Baum, E. W.; Huffman, J. C.; Baik, M. H.;
Mindiola, D. J. J. Am. Chem. Soc. 2005, 127, 16016.
(8) (a) Cundari, T. R. Organometallics 1993, 12, 1998. (b) Cundari,
T. R.; Klinckman, T. R.; Wolczanski, P. T. J. Am. Chem. Soc. 2002, 124,
1481.
(16) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203.
(17) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491.
10.1021/om7010485 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/07/2008

Transient Bis(cyclopentadienyl)zirconium Imides
Organometallics, Vol. 27, No. 5, 2008 873
Figure 1. 1,2-Addition and elimination reactions with group 4 imido complexes and application to C-H bond activation.
or hydrazine to [(η5-C5Me4H)2ZrH2]219 (eq 1). Each compound
1
exhibited the number of H and 13C NMR peaks expected for
a Cs symmetric bent zirconocene derivative. The complete
spectroscopic assignment for each compound is reported in the
Experimental Section. Diagnostic N-H and Zr-H resonances
were observed in the vicinity of 4.5 – 6.0 ppm and were assigned
based on selective isotopic labeling studies (Vide infra).
(1)
Figure 2. Application of 1,2-addition to the functionalization of
coordinated dinitrogen.
Exposure of each of the zirconocene amido hydride com-
pounds to four atmospheres of deuterium gas at 56 °C resulted
in the gradual disappearance of the N-H and Zr-H resonances.
1
Conversions were determined by H NMR spectroscopy, and
2
the final products were assayed by H NMR spectroscopy to
evaluate the possibility of side reactions. The qualitatiVe oVerall
rates of isotopic exchange (into both the N-H and Zr-H
positions) as a function of amido substituent are presented in
Figure 4. More detailed kinetic analyses will be presented later
in the article.
The overall, qualitative rate of isotopic exchange correlates
Figure 3. Isotopic exchange reactions to probe the relative rate of
with the N-H bond strength of the free amine, where weaker
N-H bonds correspond to faster intermolecular isotopic ex-
change rates. For example, zirconocene anilido hydride, 1-(N-
HPh)H (BDE(N-H, PhNH2) ) 92 kcal/mol),20 undergoes
exchange over the course of 11 h (56 °C), while the parent
zirconocene amide hydride, 1-(NH2)H (BDE(N-H, NH3) ) 107
kcal/mol),21 undergoes exchange over several weeks (56 °C).
Analysis of the deutero isotopologues of 1-(NH2)H,
1-(NHtBu)H, and 1-(NHNMe2)H by 2H NMR spectroscopy
following treatment with D2 established clean isotopic
exchange exclusively into the N-H and Zr-H positions,
thereby ruling out competing cyclometalation processes. In
contrast, the 2H NMR spectra of 1-(NHPh)H and 1-(N-
HMe)H exhibit features indicative of competing side reac-
tions involving the amido substituent. For the anilido
complex, 1-(NHPh)H, a peak centered at 6.35 ppm, corre-
sponding to the ortho phenyl position, was observed after
36 h at 56 °C (eq 2). By comparison, deuterium exchange in
the Zr-H and N-H positions occurred over the course of
1,2-addition and σ-bond metathesis reactions.
solely in the Zr-H position (Figure 3).18 Deviations from these
two extremes provide the relative rates for the two competing
processes.
In this study we describe a family of zirconocene amido
hydride complexes, (η5-C5Me4H)2Zr(NHR)H (R ) H, Me,
NMe2, tBu, Ph), and explore the relative rate of 1,2-elimination/
addition versus σ-bond metathesis as a function of amido
substituent. The strength of the N-H bond in the corresponding
free amine allows estimation of the rate of 1,2-elimination, while
intramolecular isotopic exchange experiments augmented by
computational studies have provided evidence for rare d0
dihydrogen complexes.
Results and Discussion
Deuterium Exchange Studies. The series of zirconocene
amido hydride complexes, (η5-C5Me4H)2Zr(NHR)H, used in this
study was prepared by addition of the appropriate primary amine
(19) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999,
18, 1873.
(20) Zhao, Y.; Bordwell, F. G.; Cheng, J.-P.; Wang, D. J. Am. Chem.
Soc. 1997, 119, 9125.
(18) The idealized case assumes no complication from an intramolecular
1,2-elimination event.
(21) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255.

874 Organometallics, Vol. 27, No. 5, 2008
Toomey et al.
Figure 4. Relative rate of isotopic exchange into the Zr-H and N-H position as a function of amido substituent. Values for the bond
dissociation of the corresponding free amine are also presented.20,21
1 h at this temperature, allowing kinetic evaluation of 1,2-
elimination/addition versus σ-bond metathesis (Vide infra).
(4)
(2)
loss and formation of a zirconaaziridine intermediate.22 For
bis(cyclopentadienyl)zirconium, a number of ligand-stabilized
zirconaaziridines have been prepared and are readily accessed
by methane23 or benzene24 elimination from the corresponding
Monitoring deuterium addition to 1-(NHMe)H by 2H NMR
zirconocene methyl or phenyl amide complex, respectively. Both
spectroscopy revealed a competing process that occurs on the
time scale of exchange into the N-H and Zr-H positions. At
56 °C, exchange into the amido and hydride position was clearly
observed after 24 h, and a new peak, centered at 3.15 ppm,
appeared after 36 h. This resonance corresponds to deuterium
incorporation into the amido methyl group (eq 3). Prolonged
heating of the sample for 3.5 days resulted in continued
deuteration of these three positions with no evidence for
cyclopentadienyl group cyclometalation.
linear free energy relationships and observation of large normal,
primary kinetic isotope effects are consistent with a σ-bond
metathesis process that proceeds through an ordered four-
centered transition structure.25 For 1-(NHMe)H and 1-(NMe2)H,
σ-bond metathesis to access the zirconaaziridine intermediate
is likely, although ꢀ-hydrogen elimination from the amide
methyl group26 followed by H2 loss cannot be ruled out
on the basis of our data. Addition of D2 gas to the zirconaaziri-
dine accounts for the isotopic exchange process (Figure 5). A
similar deuteration step from the zirconaaziridine intermediate
is also likely operative for 1-(NHPh)H and accounts for the
selective isotopic exchange observed in the ortho position of
the phenyl substituent.
For 1-(NMe2)H an additional process with a higher barrier
is also operative. Addition of a cyclopentadienyl methyl C-H
to the zirconaaziridine intermediate followed by deuterolysis
also occurs and accounts for the observation of isotopic
exchange in these positions at longer reaction times.
(3)
Kinetics of 1,2-Elimination/Addition versus σ-Bond Meta-
thesis. Rate constants for the isotopic exchange of deuterium
into the Zr-H and N-H positions were measured for a series
of the zirconocene amido hydride compounds with the goal of
gauging the relative rates of 1,2-elimination/addition versus
σ-bond metathesis. Each isotopic exchange reaction was con-
ducted at 56 °C, and conversions were determined by integrating
Isotopic exchange in the N-CH3 position is consistent with
metalation of the methyl substituent by either ꢀ-hydrogen
elimination or σ-bond metathesis to form the zirconaaziridine
intermediate. Subsequent deuteration with D2 gas yields the
experimentally observed isotopologue. Because this process
occurs on the time scale of N-H and Zr-H exchange, accurate
determination of the rate constants for these processes is
complicated.
The scope of deuterium exchange into the methyl amido
position was also explored for (η5-C5Me4H)2Zr(NMe2)H (1-
(NMe2)H). This compound was prepared in a straightforward
manner by treating 1-H2 with HNMe2. Exposure of a benzene
solution of 1-(NMe2)H to 4 atm of D2 gas at 56 °C for 1 h
produced isotopic exchange into the zirconium hydride position
and in the amido methyl group (eq 4). While no evidence for
competing cyclopentadienyl methyl group cyclometalation was
obtained at this temperature, heating to 75 °C resulted in
deuterium incorporation into the cyclopentadienyl methyl groups
after several days.
1
the signals for the Zr-H and N-H position in the H NMR
spectra versus an internal ferrocene standard as a function of
time. The first compound to be examined was 1-(NHtBu)H.
A sample plot for the disappearance of the Zr-H and N-H
resonances as a function of time is presented in Figure 6.
(22) Cummings, S. A.; Tunge, J. A.; Norton, J. R. Top. Organomet.
Chem. 2005, 10, 1.
(23) Buchwald, S. L.; Wanamaker, M. W.; Watson, B. T.; Dewan, J. C.
J. Am. Chem. Soc. 1989, 111, 4486.
(24) Harris, M. C. J.; Whitby, R. J.; Blagg, J. Tetrahedron Lett. 1994,
35, 2431.
(25) Coles, N.; Harris, M. C. J.; Whitby, R. J.; Blagg, J. Organometallics
1994, 13, 190.
Deuterium exchange in the amido methyl positions of
1-(NHMe)H and 1-(NMe2)H most likely occurs via dihydrogen
(26) Tsai, Y. C.; Johnson, M. J. A.; Mindiola, D. J.; Cummins, C. C.;
Klooster, W. T.; Koetzle, T. F. J. Am. Chem. Soc. 1999, 121, 10426.

Transient Bis(cyclopentadienyl)zirconium Imides
Organometallics, Vol. 27, No. 5, 2008 875
Figure 5. Proposed mechanism to account for deuterium exchange into (a) the amido methyl groups in 1-(NHMe)H and 1-(NMe2)H and
(b) the cyclopentadienyl methyl groups of 1-(NMe2)H.
isotopologues of the tert-butylamido zirconocene hydride
complex was targeted. The isotopologue with deuterium in the
amido position, 1-(NDtBu)H, was synthesized by addition of
27
tBuND2 to 1-H2. Its isotopomer, 1-(NHtBu)D, was obtained
t
by treatment of 1-D2 with BuNH2. In the latter procedure,
special care was taken to add the amine almost immediately
(<1 h) after preparation of 1-D2 to avoid complications
associated with competing cyclometalation of the cyclopenta-
dienyl methyl substituents.19 These selective labeling experi-
ments also allowed definitive assignment of the N-H and Zr-H
resonances. Isotopic exchange was not observed at ambient
temperature.
Figure 6. Sample first-order plot for deuterium incorporation in
the Zr-H (boxes) and N-H (diamonds) positions of 1-(NHtBu)H
With both mono(deutero) isotopomers in hand, the equilibra-
tion between them was studied at 56 °C (eq 5). Monitoring the
at 56 °C.
2H NMR spectrum of 1-(NHtBu)D at 56 °C demonstrated steady
Table 1. Rate Constants Determined at 56 °C for Isotopic Exchange
growth of a resonance at 5.23 ppm over the course of 4 h,
corresponding to deuterium incorporation in the N-H(D)
position of the amido group. In the converse experiment,
into the Zr-H and N-H Positions of (η5-C5Me4)2Zr(NHR)H
Complexes
compound
k(Zr-H) × 106 s-1
k(N-H) × 106 s-1
warming a benzene solution of 1-(NDtBu)H under identical
conditions also produced intramolecular isotopic exchange and
yielded the same equilibrium mixture. Importantly, both isoto-
pomers reach equilibrium faster than the time required for
significant intermolecular deuterium incorporation, consistent
with an intramolecular exchange event driven by an equilibrium
isotope effect, favoring N-D over Zr-D formation, as the origin
of the anomalous kinetic behavior.
1-(NHtBu)H
1-(NHNMe2)H
1-(NHPh)H
1-(NH2)H
1.8(4)
0.40(2)
42.6(6)
0.30(1)
3.4(7)
1.1(1)
77.7(4)
0.16(2)
The average of three independent trials yielded observed first-
order rate constants of 3.4(7) and 1.8(4) × 10-6 s-1 at 56 °C
for N-H and Zr-H disappearance, respectively. Although clean
first-order behavior was obtained, the observed rate constant
for isotopic exchange into the N-H position is nearly twice
that for the Zr-H. Rate constants for deuterium exchange with
other (η5-C5Me4)2Zr(NHR)H complexes were also determined
at 56 °C and are compiled in Table 1. With the exception of
1-(NH2)H, the observed rate constants for deuterium incorpora-
tion into the Zr-H position are smaller than those for the N-H
of the amido group.
(5)
With the exception of 1-(NH2)H, the experimentally observed
rate constants demonstrate that an intramolecular isotopic
exchange event occurs faster than the intermolecular σ-bond
metathesis and 1,2-elimination/addition processes presented in
Figure 3. For 1-(NH2)H the ground-state stabilization arising
from the strong N-H bond raises the barrier for 1,2-elimination
and makes intermolecular σ-bond metathesis dominant over the
intramolecular process.
Mechanistic Proposals and Computational Studies. Two
mechanisms were considered for the intramolecular isotopic
exchange process and are presented in Figure 7. The first
pathway (Path A) begins with 1,2-elimination from 1-(NHtBu)D
to form an imido zirconocene η2 dihydrogen complex as an
intermediate. Rotation of the η2 H-D ligand (without dissocia-
tion) followed by 1,2-addition yields the experimentally ob-
To further explore the mechanism of the intramolecular
isotopic exchange process, the preparation of mono(deutero)
(27) Wang, J.; Dash, A. K.; Kapon, M.; Berthet, J.-C.; Ephritikhine,
M.; Eisen, M. S. Chem.-Eur. J. 2002, 8, 5384.

876 Organometallics, Vol. 27, No. 5, 2008
Toomey et al.
Figure 7. Proposed mechanisms for intramolecular isotopic exchange with 1-(NDtBu)H.
served 1-(NDtBu)H. In the second pathway (Path B),
1-(NHtBu)D undergoes reductive coupling to form a transient
amine complex, which then undergoes oxidative addition to
yield the observed product.
Because distinguishing these two pathways is difficult
experimentally, computational studies were conducted. The
model complex (η5-C5H5)2Zr(NHtBu)H (A) was used for
computational expediency. Free energy profiles evaluating each
mechanistic possibility are presented in Figure 8.
reverse of the first, regenerating the original reactant (A) by
oxidative addition.
In summary, the results of the computational study clearly
support 1,2-elimination followed by formation and rotation of
an intermediate η2-dihydrogen complex. Thus, the observation
of intramolecular isotopic exchange between 1-(NDtBu)H and
1-(NHtBu)D at 56 °C at a rate faster than intermolecular isotopic
exchange between 1-(NHtBu)H and D2 gas provides kinetic
evidence for an intermediate imido zirconocene dihydrogen
complex (Figure 9).
For the 1,2-elimination pathway, an accessible barrier of 26
kcal/mol was calculated for the initial, rate-determining H2
elimination step followed by a very shallow surface for rotation
of the η2-dihydrogen ligand. The transition state calculated for
H2 elimination (TSAB) is a rather late one. The formation of
the H-H bond is well advanced, with a distance (0.972 Å) only
0.2 Å longer than the one in the resulting dihydrogen complex,
B. The H-H Wiberg index (WI)28 corroborates the intermediacy
of an η2-dihydrogen complex: 0.515 in TSAB and 0.783 in B.
Rotation of the η2-dihydrogen ligand in B is facile, with an
energy barrier of only 2 kcal/mol. In the corresponding 180°
movement, the conformation of dihydrogen goes from coplanar
to the metallocene wedge, in B, to perpendicular to this plane
in the transition state, TSBB, and finally back to planar. The
third step is the microscopic reverse of the first one and
regenerates the amido zirconocene hydride complex A.
By comparison, the alternative reductive coupling pathway
proceeds with a much higher barrier of 43 kcal/mol to form the
amine complex. When the transition state (TSAC) is reached, a
strong Zr-H bond persists (d ) 1.950 Å, WI ) 0.489), while
the formation of the new N-H bond is well advanced (d )
1.351 Å, WI ) 0.382). In the second step, subsequent Zr-N
rotation on intermediate C, the amine complex, proceeds on a
fairly shallow surface, with an energy barrier of only 2 kcal/
mol, going through TSCC. The final step is the microscopic
The computational studies also successfully reproduce the
equilibrium isotope effect measured for the intramolecular
rearrangement of 1-(NDtBu)H (eq 5). Recall, an experimental
Keq of 0.41 was measured (favoring the N-D over Zr-D
isotopomer), corresponding to a difference in free energy (56
°C) of 0.6 kcal/mol. Using the model complex, (η5-C5H5)2-
Zr(NDtBu)H, an equilibrium constant of 0.26 was computed,
corresponding to a free energy difference of 0.9 kcal/mol.
Transition metal dihydrogen complexes, first discovered
by Kubas in 1984,29 are ubiquitous for d6 and d8 metal ions.30
Because it is now well established that η2-H2 complexes are
stabilized by back-donation from a filled metal orbital into
the dihydrogen σ*,31 it is not surprising that d0 σ-complexes
are rare. Computational studies have implicated η2-H2
intermediates in both σ-bond metathesis32 and 1,2-addition
reactions8b,33–35 involving d0 group 4 transition metal com-
pounds, although experimental evidence is limited. Ziegler
has also calculated that a constrained geometry titanocene
cation prefers an η2-H2 allyl tautomer over the olefin hydride
alternative.36 From these studies, the binding energy of the
dihydrogen ligand has been estimated between 4 and 11 kcal/
mol depending on the ancillary ligation. In the case of η2-
(29) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451.
(30) Kubas, G. J. Metal Dihydrogen and σ-Bonded Complexes; Kluwer:
New York, 2001.
(31) Saillard, J. Y.; Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 2006.
(32) Ziegler, T.; Folga, E.; Berces, A. J. Am. Chem. 1993, 115, 636.
(33) Rappe, A. K. Organometallics 1987, 6, 354.
(28) (a) Wiberg, K. B. Tetrahedron 1968, 24, 1083. (b) Wiberg indices
are electronic parameters related to the electron density between atoms.
They can be obtained from a natural population analysis and provide an
indication of the bond strength.

Transient Bis(cyclopentadienyl)zirconium Imides
Organometallics, Vol. 27, No. 5, 2008 877
Figure 8. Free energy profiles for evaluating 1,2-elimination (top) versus reductive coupling (bottom) for intramolecular isotopic exchange
t
for (η5-C5H5)2Zr(NHtBu)H. A ) (η5-C5H5)2Zr(NHtBu)H, B ) (η5-C5H5)2Zr(NtBu)(ηH2), C ) (η5-C5H5)2ZrNH2 Bu.
Figure 9. Proposed mechanism for intra- and intermolecular isotopic exchange with the amide hydrogen and the zirconium hydride.
H2 in model complex B, a binding energy of 7.2 kcal/mol
was calculated, well within the expected range.37 Importantly
this value is higher than that (2 kcal/mol) calculated for the
180° rotation.
The relative rates of intra- and intermolecular isotopic
exchange in the zirconocene amido hydride (deuteride) com-
plexes provide rare experimental and computational evidence
for the intermediacy of a d0, η2-H2 zirconocene complex. These

878 Organometallics, Vol. 27, No. 5, 2008
Toomey et al.
1H NMR spectra were recorded on a Varian Inova 400
results demonstrate that following 1,2-elimination, the barrier
for substitution by dissociation of the η2-H2 (or isotopologue)
by free H2 (or isotopologue) is higher than for rotation and
readdition. To our knowledge, the only other experimental
evidence for d0 σ-complexes has been provided by Schafer and
Wolczanski, who reported that alkane compounds precede C-H
activation in tungsten(VI) tris(imido) species.38
2
spectrometer operating at 399.799 MHz (1H), while H and 13C
spectra were collected on a Varian Inova 500 spectrometer operating
at 76.7407 and 125.704 MHz, respectively. Infrared spectra were
collected on a Thermo spectrometer. Elemental analyses were
performed at Robertson Microlit Laboratories, Inc., in Madison,
NJ.
General Procedure for Collecting Kinetic Data. The desired
amido zirconocene sample was prepared from a 0.060 M stock
solution in benzene-d6 and transferred (0.5 mL) into a J. Young
tube along with approximately 3 mg of ferrocene as an internal
standard. On the high-vacuum line, the tube was degassed and 1
atm of dihydrogen was added at -196 °C. The sample was warmed
to room temperature and then placed in a temperature-controlled
silicon oil bath set to 56 °C. The decay of the 1H NMR signal for
the NH and ZrH peaks was monitored as a function of time versus
the ferrocene standard. Rate constants for 1-(NHPh)H were
determined in the NMR probe directly at 56 °C.
Computational Details. All calculations were performed using
the Gaussian 03 software package,41 and the PBE1PBE functional,
without symmetry constraints. That functional uses a hybrid
generalized gradient approximation (GGA), including a 25%
mixture of Hartree–Fock42 exchange with DFT43 exchange-
correlation, given by the Perdew, Burke, and Ernzerhof functional
(PBE).44 The optimized geometries were obtained with the LanL2DZ
basis set45 augmented with an f-polarization function,46 for Zr, and
a standard 6-31G(d,p)47 for the remaining elements. Transition-
state optimizations were performed with the synchronous transit-
guided quasi-Newton method (STQN) developed by Schlegel et
al.48 Frequency calculations were performed to confirm the nature
of the stationary points, yielding one imaginary frequency for the
transition states and none for the minima. Each transition state was
further confirmed by following its vibrational mode downhill on
both sides and obtaining the minima presented on the energy
Concluding Remarks
Deuterium exchange studies with a series of bis(cyclopen-
tadienyl)zirconium amido hydride compounds have revealed a
number of competing pathways. For all compounds studied,
intermolecular isotopic exchange was observed in both the N-H
position of the amido ligand and in the zirconium hydride. For
compounds bearing anilido or methylamido substituents, com-
peting cyclometalation, likely through a zirconaaziridine inter-
mediate, resulted in exchange into the phenyl or methyl group.
In general, the rate of intermolecular isotopic exchange cor-
relates with the N-H bond strength of the free amine, where
stronger N-H bonds result in slower rates. Accordingly, the
parent amido zirconocene hydride undergoes intermolecular
isotopic exchange predominantly by a σ-bond metathesis
pathway. In some instances, the rate constant for the Zr-H
isotopic exchange was slower than that for deuterium incorpora-
tion into the N-H position. This unexpected observation is
readily accommodated by a faster intramolecular process
whereby 1,2-elimination produces a rare example of a d0 η2-
dihydrogen complex that undergoes rotation faster than substitu-
tion. The anomalous rate constants observed for isotopic
exchange are driven by an equilibrium isotope effect where the
deuterium atom prefers the N-H over the Zr-H position.
Experimental Section
(41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out using standard high vacuum line,
Schlenk, or cannula techniques or in an MBraun inert atmosphere
drybox containing an atmosphere of purified dinitrogen. The
MBraun drybox was equipped with a cold well designed for freezing
samples in liquid nitrogen. Solvents for air- and moisture-sensitive
manipulations were dried and deoxygenated using literature pro-
cedures.39 Toluene, benzene, pentane, and heptane were further
dried by distillation from “titanocene”.40 Deuterated solvents for
NMR spectroscopy were dried over 4 Å molecular sieves. Argon,
hydrogen, and deuterium gas were purchased from Airgas Incor-
porated and passed through a column containing manganese oxide
on vermiculite and 4 Å molecular sieves before admission to the
high-vacuum line. All amines and dimethylhydrazine were pur-
chased from Aldrich or Acros and dried over CaH2.
(42) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; John Wiley & Sons: New York, 1986.
(43) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(44) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1997,
(34) (a) Yates, D. F.; Basch, H.; Musaev, D. G.; Morokuma, K. J. Chem.
Theory Comp. 2006, 2, 1298. (b) Bobadova-Parvanova, P.; Wang, Q. F.;
Quinonero-Santiago, D.; Morokuma, K.; Musaev, D. G. J. Am. Chem. Soc.
2006, 128, 11391.
78, 1396. (b) Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
(45) (a) Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical Chemistry;
Schaefer, H. F., III. Plenum: New York, 1976; Vol. 3, p 1. (b) Hay, P. J.;
Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) Wadt, W. R.; Hay, P. J.
J. Chem. Phys. 1985, 82, 284. (d) Hay, P. J.; Wadt, W. R. J. Chem. Phys.
1985, 82, 2299.
(46) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.; Höllwarth,
A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
Chem. Phys. Lett. 1993, 208, 111.
(47) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971,
54, 724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,
56, 2257. (c) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (d)
Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (e) Hariharan, P. C.; Pople,
J. A. Theor. Chim. Acta 1973, 28, 213.
(48) (a) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput.
Chem. 1996, 17, 49. (b) Peng, C.; Schlegel, H. B. Isr. J. Chem. 1994, 33,
449.
(35) Martinez, S.; Morokuma, K.; Musaev, D. G. Organometallics 2007,
26, 5978.
(36) Margl, P. M.; Woo, T. K.; Blochl, P. E.; Ziegler, T. J. Am. Chem.
Soc. 1998, 120, 2174.
(37) Calculated as the difference between the energy of the dihydrogen
complex (B) and the energy of the separated fragments: [E{H2} +
E{ZrCp2(NtBu)}] - E{ZrCp2(H2)(NtBu)}.
(38) Schafer, D. F.; Wolczanski, P. T. J. Am. Chem. Soc. 1998, 120,
4881.
(39) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(40) Marvich, R. H.; Brintzinger, H. H. J. Am. Chem. Soc. 1971, 93,
2046.

Transient Bis(cyclopentadienyl)zirconium Imides
Organometallics, Vol. 27, No. 5, 2008 879
profiles. A natural population analysis (NPA)49 and the resulting
Wiberg indices28 were used to study the electronic structure and
bonding of the optimized species. Free energy values for the reaction
profiles (Figure 8) were obtained at 298.15 K and 1 atm by
conversion of the zero point corrected electronic energies with the
thermal energy corrections based on the calculated structural and
vibrational frequency data. The free energy difference (and equi-
librium constant) for the two isotopomers, (η5-C5H5)2Zr(NHtBu)D
and (η5-C5H5)2Zr(NDtBu)H, was calculated at 329.15 K and 1 atm,
by the same process.
6H, C5Me4H), 2.23 (s, 6H, C5Me4H), 4.79 (s, 1H, ZrH), 5.27 (s,
2H, C5Me4H), 5.34 (s, 1H, NH). 13C{1H} NMR (benzene-d6, 23
°C): δ 11.45, 12.56, 13.57, 13.94 (CpMe), 54.58 (NMe), 108.52,
114.78, 115.51, 119.19, 119.54 (Cp). IR (KBr): νN-H 3178 cm-1
.
Anal. Calcd for C20H34N2Zr1: C, 61.01; H, 8.70; N, 7.11. Found:
C, 60.97; H, 8.45; N, 7.39.
1
Characterization Data for 1-(NHMe)H. H NMR (benzene-
d6, 23 °C): δ 1.84 (s, 6H, C5Me4H), 2.01 (s, 6H, C5Me4H), 2.05 (s,
12H, C5Me4H), 3.20 (d, 3H, NMe2), 4.70 (s, 1H, ZrH), 4.90 (d,
1H, NH), 5.14 (s, 2H, CpH). 13C{1H} NMR (benzene-d6, 23 °C):
δ 12.08, 13.23, 14.12, 14.39 (CpMe), 41.67 (NMe), 108.38, 115.17,
115.52, 119.72, 119.91 (Cp). This compound was isolated as a thick
oil, precluding combustion analysis.
General Method for the Preparation of (η5-C5Me4H)2Zr-
(NHR)H. A representative procedure is given for the preparation
of 1-(NHtBu)H. A 25 mL round-bottom flask was charged with
0.100 g (0.30 mmol) of (C5Me4H)2ZrH2 and approximately 10 mL
of pentane. On the high-vacuum line, the flask was degassed at
1
Characterization Data for 1-(NH2)H. H NMR (benzene-d6,
t
23 °C): δ 1.83 (s, 6H, C5Me4H), 1.87 (s, 6H, C5Me4H), 2.03 (s,
6H, C5Me4H), 2.14 (s, 6H, C5Me4H), 4.20 (s, 2H, NH2), 4.85 (s,
1H, ZrH), 5.02 (s, 2H, C5Me4H). 13C{1H} NMR (benzene-d6, 23
°C): δ 11.79, 13.38, 13.75, 15.06 (CpMe), 106.10, 114.35, 116.42,
119.68, 121.19 (Cp). IR (KBr): νN-H 3429, 3353 cm-1. Anal. Calcd
for C18H29N1Zr1: C, 61.65; H, 8.34; N, 3.99. Found: C, 61.74; H,
8.54; N, 3.86.
-196 °C followed by addition of BuNH2 with a 100.1 mL gas
bulb (0.30 mmol, 55 Torr). After stirring overnight, the solvent
was removed in Vacuo, yielding a white solid, which was recrystal-
1
lized from pentane. H NMR (benzene-d6, 23 °C): δ 1.27 (s, 9H,
tBu), 1.81 (s, 6H, C5Me4H), 1.96 (s, 6H, C5Me4H), 2.04 (s, 6H,
C5Me4H), 2.18 (s, 6H, C5Me4H), 4.79 (s, 1H, ZrH), 5.18 (s, 1H,
NH), 5.44 (s, 2H, C5Me4H). 13C{1H} NMR (benzene-d6, 23 °C):
δ 12.35, 13.18, 14.32, 14.72 (CpMe), 36.22 (CMe3), 56.75 (CMe3),
107.99, 114.97, 116.85, 119.12, 120.20 (Cp). IR (KBr): νN-H 3328
Characterization Data for 1-(NMe2)H. 1H NMR (benzene-d6,
23 °C): δ 1.86 (s, 6H, C5Me4H), 2.01 (s, 6H, C5Me4H), 2.04 (s,
6H, C5Me4H), 2.11 (s, 6H, C5Me4H), 2.58 (s, 6H, NMe2), 5.05 (s,
2H, CpH), 5.88 (s, 1H, ZrH). 13C{1H} NMR (benzene-d6, 23 °C):
δ 12.05, 13.42, 13.49, 14.71 (CpMe), 40.79 (NMe), 108.74, 113.81,
116.69, 120.34, 124.63 (Cp). This compound was isolated as a thick
oil, precluding combustion analysis.
cm-1
.
Characterization Data for 1-(NHPh)H. 1H NMR (benzene-
d6, 23 °C): δ 1.72 (s, 6H, C5Me4H), 1.85 (s, 6H, C5Me4H), 2.02 (s,
6H, C5Me4H), 2.06 (s, 6H, C5Me4H), 5.21 (s, 2H, C5Me4H), 5.77
(s, 1H, NH), 5.87 (s, 1H, ZrH), 6.82 (m, 3H, Ph), 7.15 (m, 2H,
Ph). 13C{1H} NMR (benzene-d6, 23 °C): δ 11.91, 13.48, 14.21,
14.29 (CpMe), 108.52, 117.01, 117.69, 120.04 (Cp), 121.47 (Ph),
121.64 (Cp), 129.20, 156.72 (Ph). IR (KBr): νN-H 3340 cm-1. Anal.
Calcd for C24H33N1Zr1: C, 67.55; H, 7.79; N, 3.28. Found: C, 67.14;
H, 7.57; N, 2.90.
Acknowledgment. We thank the Department of Energy,
Office of Basic Energy Sciences (DE-FG02-05-ER659), for
financial support. P.J.C. is a Cottrell Scholar sponsored by
the Research Corporation, a David and Lucille Packard
Fellow in Science and Engineering, and a Camille Dreyfus
Teacher-Scholar.
Characterization Data for 1-(NHNMe2)H. 1H NMR (benzene-
d6, 23 °C): δ 1.92 (s, 6H, C5Me4H), 2.03 (s, 6H, C5Me4H), 2.09 (s,
(49) (a) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM)
1988, 169, 41. (b) Carpenter, J. E. Ph.D. Thesis, University of Wisconsin,
Madison, WI, 1987. (c) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980,
102, 7211. (d) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066.
(e) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 1736. (f) Reed,
A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (g)
Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (h)
Weinhold, F.; Carpenter, J. E. The Structure of Small Molecules and Ions;
Plenum: New York, 1988; p 227.
Supporting Information Available: Select kinetic data, repre-
sentative H NMR spectra of 1-(NMe2)H and 1-(NHMe)H, and
coordinates of all computationally optimized structures in PDF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
1
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