Indenyl zirconium dinitrogen chemistry: N2 coordination to an isolated zirconium sandwich and synthesis of side-on, end-on dinitrogen compounds

Article abstract of DOI:10.1021/ja801021w

Exposure of the isolable zirconocene sandwich compounds, (η⁵-C₅Me₅)(η⁵-C ₉H₅-1-R¹-3-R²)Zr (R¹ = Me, ⁱPr, ^tBu; R² = Me) to one atmosphere of dinitrogen resulted in N₂ coordination. X-ray diffraction and NMR spectroscopy establish that the resulting dimeric dinitrogen compounds contain an unusual μ₂,η²-bridging indenyl ring and a weakly activated N₂ ligand. N₂ coordination from the isolable zirconium sandwich compounds is extremely sensitive to the number and size of the indenyl subsituents. Compounds bearing two [ⁱPr] or three methyl substituents are stable as η⁹ sandwich compounds for weeks under dinitrogen likely due to the inability to dimerize through a two-atom N ₂ bridge. Performing the reduction of (η⁵-C ₅Me₅)(η⁵-C₉H₅-1- R¹-3-R²)ZrCl₂ (R¹ = ⁱPr, _tBu; R₂ = Me; R¹ = R₂ = SiMe ₃) under an N₂ atmosphere produced a different outcome; rare examples of side-on, end-on zirconium dinitrogen compounds were isolated and in one case, crystallographically characterized. Protonolysis studies with weak Bronsted acids were used to evaluate the relative activation of the bridging dinitrogen ligands.

Full text of DOI:10.1021/ja801021w

Published on Web 04/10/2008
Indenyl Zirconium Dinitrogen Chemistry: N2 Coordination to
an Isolated Zirconium Sandwich and Synthesis of Side-on,
End-on Dinitrogen Compounds
Doris Pun, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York, 14853
Received February 13, 2008; E-mail: pc92@cornell.edu
Abstract: Exposure of the isolable zirconocene sandwich compounds, (η5-C5Me5)(η5-C9H5-1-R1-3-R2)Zr
(R1 ) Me, iPr, tBu; R2 ) Me) to one atmosphere of dinitrogen resulted in N2 coordination. X-ray diffraction
and NMR spectroscopy establish that the resulting dimeric dinitrogen compounds contain an unusual µ2,η2-
bridging indenyl ring and a weakly activated N2 ligand. N2 coordination from the isolable zirconium sandwich
compounds is extremely sensitive to the number and size of the indenyl subsituents. Compounds bearing
two [iPr] or three methyl substituents are stable as η9 sandwich compounds for weeks under dinitrogen
likely due to the inability to dimerize through a two-atom N2 bridge. Performing the reduction of (η5-C5Me5)(η5-
i
t
C9H5-1-R1-3-R2)ZrCl2 (R1 ) Pr, Bu; R2 ) Me; R1 ) R2 ) SiMe3) under an N2 atmosphere produced a
different outcome; rare examples of side-on, end-on zirconium dinitrogen compounds were isolated and in
one case, crystallographically characterized. Protonolysis studies with weak Brønsted acids were used to
evaluate the relative activation of the bridging dinitrogen ligands.
Introduction
Dinitrogen complexes of the group 4 transition metals, while
now known for over three decades, have received renewed
interest due, in part, to the number of transformations available
for elaboration of the N-N bond.1–4 The majority of isolable
compounds are dimeric and contain a bridging dinitrogen ligand
in one of two limiting hapticities (Figure 1). Seminal examples
relied on sterically crowded cyclopentadienyl ligands (e.g., Cp*
) η5-C5Me5) for stabilization and contained weakly activated
µ2,η1,η1 (“end-on”) dinitrogen ligands.5–7 Since that time, the
other limiting hapticity, µ2,η2,η2 (“side-on”) has emerged as the
dominant coordination mode1 and remains a prominent synthetic
target due to the prevalence of 4-electron-reduced bridging N2
ligands with elongated N-N bonds.8 A third possibility, µ2,η2,η1
(“side-on, end-on”), is a combination of these two extremes.
To date, the only example of this hapticity in group 4 transition
Figure 1. N2 hapticity in dimeric group 4 transition metal dinitrogen
compounds.
metal dinitrogen chemistry is an unusual titanocene cluster.9
Expanding the number of compounds with this hapticity into
zirconium and hafnium N2 chemistry and understanding the
principles underlying its formation may ultimately lead to new
reactions for elaboration of the N-N bond.
One notable example of side-on, end-on dinitrogen coordina-
tion is Fryzuk’s ditantalum dihydride compound bearing a
tridentate phosphine diamido supporting ligand.10,11 In addition
to the unusual N2 hapticity, this compound is remarkable both
for how it is synthesized and for its rich dinitrogen function-
alization chemistry.12–17 Most early transition metal dinitrogen
(1) MacLachlan, E. A; Fryzuk, M. D Organometallics 2006, 25, 1530.
(2) Chirik, P. J. Dalton Trans. 2007, 16.
(3) van Tamelen, E. E. Acc. Chem. Res. 1970, 3, 361.
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J. E. J. Am. Chem. Soc. 1976, 98, 8358.
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Bercaw, J. E. Organometallics 1985, 4, 97.
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S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960.
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J. Chem. 2005, 83, 315.
(8) For examples see:(a) Hirotsu, M.; Fontaine, P. P.; Zavalij, P. Y.; Sita,
L. R. J. Am. Chem. Soc. 2007, 129, 12690. (b) MacLachlan, E. A.;
Hess, F. M.; Patrick, B. O.; Fryzuk, M. D. J. Am. Chem. Soc. 2007,
129, 10895. (c) Hanna, T. E.; Keresztes, I.; Lobkovsky, E.; Chirik,
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Loehr, T. M.; Mylvaganam, M.; Rettig, S. J. Inorg. Chem. 1998, 37,
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D. H.; Rettig, S. J. J. Am. Chem. Soc. 1993, 115, 2782. (f) Fryzuk,
M. D.; Haddad, T. S.; Rettig, S. J. J. Am. Chem. Soc. 1990, 112, 8185.
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Chem., Int. Ed. 2002, 41, 3709.
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Nat. Acad. Sci. U.S.A. 2006, 103, 17094.
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128, 9472.
9
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2008 American Chemical Society
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A R T I C L E S
Pun et al.
NMR spectroscopic studies28 augmented by computational
results29 support rapid interconversion of the η9 and η5-bound
rings in solution and the intermediacy of an η5,η5-zirconocene
complex (Figure 2). The observed dynamic behavior raised the
question as to whether the putative η5,η5-intermediate could be
trapped with N2 to form rare examples of indenyl zirconium
dinitrogen compounds. Previous studies from our group30 and
from Veiros31 have demonstrated the influence of the 1,3-indenyl
substituents on the stability of the η9 interaction. While 1,3-
disilylsubstituted zirconocenes undergo rapid haptotropic rear-
rangement upon addition of THF and 1,2-dimethoxyethane, the
corresponding alkylated compounds are unreactive.30 In this
contribution, we explore the effect of introducing smaller 1,3-
dialkyl substituents with the goal of trapping an indenyl
dinitrogen complex with η5,η5 ring coordination through dimer-
ization. These studies have resulted in the first example of
dinitrogen coordination by an isolated zirconium sandwich as
well as yielded rare examples of side-on, end-on N2 compounds
of zirconium. Both of these classes of compounds exploit the
haptotropic flexibility of the indenyl ring for stabilization.
Figure 2. η9,η5-Bis(indenyl)zirconium sandwich complexes.
compounds with strongly activated N2 ligands are prepared by
alkali metal reduction of a dihalide precursor.18 In contrast, the
ditantalum dihydride compound with the side-on, end-on
dinitrogen ligand is obtained by H2 reductive elimination from
the corresponding ditantalum tetrahydride.10 Alkali metal-free
preparative routes for activated dinitrogen compounds may
ultimately prove important in the development of catalytically
compatible N2 functionalization schemes.
Results and Discussion
Our laboratory has been actively exploring the hydrogenation
and related chemistry of zirconium dinitrogen complexes with
side-on bound, 4-electron reduced N2 ligands.2 One aspect of
this program has been focused on developing new methods for
the synthesis of group 4 metallocene dinitrogen compounds that
do not rely on alkali metal reduction for the N2 coordination
step. One approach, similar to the Fryzuk tantalum chemistry,
is the reductive elimination of H2 from a metallocene mono-19,20
or dihydride, and examples are known in both titanocene20 and
zirconocene chemistry.21,22 An alternative strategy is direct N2
coordination to an isolated, divalent metallocene sandwich.23,24
Whilesuchcompoundsarenowwell-precedentedfortitanium,24–26
η5,η5-bis(cyclopentadienyl)zirconium and hafnium examples
have not been prepared. Masked forms of zirconocene have been
synthesized by replacing the cyclopentadienyl rings with 1,3-
disubstituted indenyl ligands27 where coordination of the benzo
ring to the metal center has resulted in isolation of unusual η9,η5-
bis(indenyl) metal sandwich compounds (Figure 2).28
Dinitrogen Activation by Mixed-Ring Zirconium Sandwich
Complexes. To systematically explore the stability of the η9
interaction relative to dinitrogen coordination in bis(indenyl)
zirconium sandwich compounds, sodium amalgam reduction of
a family of mixed ring, pentamethylcyclopentadienyl-indenyl
zirconocene dihalide complexes was explored (eq 1).
Initial studies focused on compounds with 1,3-disubstituted
indenyl rings as these are known to form isolable η9 sandwich
compounds.28 While [iPr] and [SiMe3] substituted compounds
were prepared for comparison, particular attention was devoted
to smaller alkyl substituents with the goal of trapping dinitrogen
compounds through dimerization. Each zirconium sandwich
compound prepared in this study is presented in Figure 3 along
with its shorthand designation.
(16) MacKay, B. A.; Patrick, B. O.; Fryzuk, M. D. Organometallics 2005,
24, 3836.
(17) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc.
2003, 125, 3234.
Our studies commenced with the previously reported isopro-
pyl-substituted compound, 1.32 Both the reduction of the dihalide
and the isolated sandwich are not sensitive to dinitrogen,
suggesting that this indenyl substitution pattern is sufficiently
large to inhibit dimerization. During the course of our investiga-
tions, we obtained X-ray quality crystals of 1 and a depiction
of the solid state structure is presented in Figure 4. Selected
metrical parameters and a comparison to the previously reported
bis(indenyl) zirconium sandwich compound are reported in
Table 1. The X-ray structure of 1 confirms the zirconocene
sandwich structure and the η9 hapticity of the indenyl ligand,
which was previously assigned by NMR spectroscopy.32 As is
typical for this coordination environment, the indenyl ring is
bent with a fold angle of 37.0°, similar to the value found in
(18) Oshita, H.; Mizobe, Y.; Hidai, M. J. Organomet. Chem. 1993, 456,
213.
(19) deWolf, J. M.; Blaauw, R.; Meetsma, A.; Teuben, J. H.; Gyepes, R.;
Varga, V.; Mach, K.; Veldman, N.; Spek, A. L. Organometallics 1996,
15, 4977.
(20) Hanna, T. E.; Bernskoetter, W. H.; Bouwkamp, M. W.; Lobkovsky,
E.; Chirik, P.J. Organometallics 2007, 26, 2431.
(21) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003,
125, 2241.
(22) Chirik, P. J.; Henling, L. M.; Bercaw, J. E. Organometallics 2001,
20, 534.
(23) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 6018.
(24) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 14688.
(25) Hitchcock, P. B.; Kerton, F. M.; Lawless, G. A. J. Am. Chem. Soc.
1998, 120, 10264.
(26) (a) Lukesová, L.; Horácek, M.; Stepnicka, P.; Fejfarová, K.; Gyepes,
R.; Cisorová, I.; Kubista, J.; Mach, K. J. Organomet. Chem. 2002,
663, 134. (b) Horácek, M.; Kupfer, V. ; Thewalt, U.; Stepnicka, P.;
Polasek, M; Mach, K. Organometallics 1999, 18, 3572.
(27) Bradley, C. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003,
125, 8110.
(29) Veiros, L. F. Chem. Eur. J. 2005, 11, 2505.
(30) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am. Chem.
Soc. 2005, 127, 10291.
(31) Veiros, L. F. Organometallics 2006, 25, 4698.
(32) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Chirik, P. J. Organo-
metallics 2006, 25, 2080.
(28) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik,
P. J. J. Am. Chem. Soc. 2004, 126, 16937.
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Indenyl Zirconium Dinitrogen Chemistry
A R T I C L E S
Table 1. Selected Bond Distances and Angles for 1 and
(η9-C9H5-1,3-(CHMe2)2)(η5-C9H5-1,3-(CHMe2)2) Zr28
1
(η9-C9H5-1,3-(CHMe2)2)(η5-C9H5-1,3-(CHMe2)2)Zr
C(1)-C(2) Å
C(1)-C(1a) Å
C(1a)-C(2a) Å
1.423(2)
1.390(3)
1.423(2)
1.418(3)
1.395(3)
1.426(2)
37.6
η9 fold angle (deg)a 37.0
buckle (deg)b
8.2
6.3
a Defined as the angle between the planes formed by the
five-membered cyclopentadienyl and the six-membered benzo rings of
the indenyl. b Defined as the deviation from planarity of the benzo ring,
which is the angle between the planes formed by C(2)-C(1)-C(1a)-
C(2a) and C(2)-C(3)-C(3a)-C(2a). Indenyl labeling scheme based on
the structure in Figure 4.
benzene-d6 solutions of 2 to stand at 23 °C increased the amount
of cyclometalation. Only one diastereomer of 2-CMH has been
detected by 1H and 13C NMR spectroscopy and has been
assigned by NOESY NMR experiments as the isomer depicted
in equation 2. This diastereomer is likely preferred because of
the sterically demanding [SiMe3] substituent in the more open
lateral position of the metallocene wedge unlike the alternative
where this group would be in the narrow, “back” portion of the
molecule.
Figure 3. Zirconocene compounds prepared in this study.
Figure 4. Molecular structure of 1 at 30% probability ellipsoids. Hydrogen
atoms omitted for clarity.
Each of the new zirconium sandwich compounds, 2-6
(Figure 3), was characterized by 1H and 13C NMR spectroscopy.
Upfield-shifted benzo resonances were observed in the benzene-
d6 1H and 13C NMR spectra, diagnostic of η9 coordination.28
The dimethyl-substituted compound, 3, has proven challenging
to synthesize, and the standard sodium amalgam reduction
method used to prepare related compounds28 has been irrepro-
ducible as oftentimes no clean product was obtained. Neverthe-
the corresponding bis(indenyl)sandwich.28 Accordingly, the
carbon-carbon bond distances in the coordinated benzo ring
are elongated to 1.390(3) and 1.423(2) Å, consistent with
reduction by the formally low-valent zirconium center. Thus,
replacing the indenyl ligand with a permethylcyclopentadienyl
ring has little impact on the metrical parameters of the η9 indenyl
(Table 1).
1
less, the compound was obtained and observed by H NMR
spectroscopy. Attempts to prepare sandwich compounds bearing
less substituted indenyl ligands were unsuccessful (Figure 3).
Sodium amalgam reduction of Cp*(Ind)ZrCl2, Cp*(Ind-2-Me)-
ZrCl2, and Cp*(Ind-1-iPr)ZrCl2, produced unidentified products.
However, magnesium or sodium amalgam reduction under a
CO atmosphere yielded the corresponding zirconocene dicar-
bonyl compounds, suggesting that the presence of strong field
ligands facilitates reduction and stabilizes the formally low-
The [SiMe3]-substituted zirconium sandwich, 2, was also
prepared by reduction of the corresponding dihalide, 2-Cl2,
under an argon atmosphere (Figure 3). This compound has
proven challenging to obtain in pure form due to competing
formation of the zirconocene cyclometalated hydride, 2-CMH
(eq 5). At shorter reaction times small amounts (∼10%) of
1
2-CMH were observed by H NMR spectroscopy; allowing
9
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A R T I C L E S
Pun et al.
Figure 5. Molecular structure of 42-N2 (a) side view and (b) top view at 30% probability ellipsoids. Hydrogen atoms omitted for clarity.
valent species.33 Complete experimental details and spectro-
scopic characterization of the all of the zirconocene dicarbonyl
compounds prepared in this work are reported in the Supporting
Information.
η9 benzo resonances at 5.13, 5.02, 3.84, and 3.68 ppm to new
upfield peaks at 6.41, 6.33, 3.30, and 2.63 ppm, signaling a
change in indenyl hapticity. The chemical shift range of the
new peaks indicates that coordination to the zirconium is
maintained. This spectral change occurred over the course of
seven days and was only observed under N2; no change was
observed under vacuum. The 15N NMR spectrum of 42-15N2,
prepared by addition of 15N2 gas to a benzene-d6 solution of
42-N2, exhibits a single resonance centered at 598.7 ppm,
indicating formation of a single isomer with equivalent nitrogen
atoms. A medium intensity pentane solution IR band was
observed at 1563 cm-1. These data are consistent with a weakly
activated, µ2,η1,η1-N2 ligand.34
Allowing pentane solutions of 4 to stand at -35 °C under a
dinitrogen atmosphere deposited crystals of 42-N2. The solid-
state structure of 42-N2 was determined by X-ray diffraction
and confirms the formation of an end-on dinitrogen complex
with µ2,η2-bridging indenyl ligands (Figure 5).35 Consistent with
benzene-d6 solution NOESY NMR data, the observed isomer
is the one in which the indenyl rings are trans across the dimer
with the isopropyl groups in a syn arrangement. The bridging,
end-on dinitrogen ligand is weakly activated with an Nt N bond
distance of 1.197(3) Å, in agreement with the frequency of the
band observed by IR spectroscopy. The bond distances
(d(C(7)-C(8), C(7a)-C(8a)) ) 1.429(3) Å) for the µ2,η2-
indenyl fragment are consistent with 2-electron reduction and
significant dialkyl character in the zirconium carbon bonds. This
interaction is also the origin of the upfield chemical shifts
observed in the 1H NMR spectrum and is also likely responsible
for the weak activation of the bridging N2 ligand. Thus, these
complexes are conceptually similar to Bercaw’s [(η5-C5Me5)2-
Zr(N2)]2(µ2,η1,η1-N2),34 where the presence of additional π-acids
results in weak activation of the bridging dinitrogen ligand.
Accordingly, each metal in the indenyl zirconium dinitrogen
complexes is best described as a coordinatively saturated, 18-
electron Zr(IV) center with a 2-electron-reduced η2 alkene and
an [N2]0 ligand completing the coordination sphere. An alterna-
Mixed-ring sandwich compounds 1, 2, and 4 were treated
with large excesses of THF to explore the possibility of ligand-
induced haptotropic rearrangement.30 No evidence for THF
1
coordination or indenyl migration was obtained by H NMR
spectroscopy. This lack of reactivity is likely due to the presence
of the electron-donating pentamethylcyclopentadienyl rings.
Both experimental30 and computational31 studies have estab-
lished that electron-releasing indenyl (or in this case cyclopen-
tadienyl rings) disfavor ligand induced haptrotropic rearrangement.
Allowing benzene-d6 or pentane solutions of 3, 4, or 5 to
stand under 1 atm of N2 at 23 or -35 °C, resulted in color
changes from burgundy to deep purple, signaling formation of
new zirconocene dinitrogen compounds (eq 3). For 4, N2
coordination at 23 °C occurred over the course of 7 days;
conversion of 3 and 5 took place over one month. Exposure of
the dinitrogen compounds to vacuum and heating to 45 °C
resulted in N2 dissociation and reformation of the sandwich.
However, repeated cycles of heating the samples induced
decomposition to unidentified zirconium compounds. Introduc-
tion of a third indenyl methyl substituent inhibited N2 coordina-
tion, as no change was observed upon letting solutions of 6
stand for 7 months under a dinitrogen atmosphere. For both
42-N2 and 52-N2, only one isomer was observed by NMR
spectroscopy. Because the resulting N2 compounds are similar
in each case, only the chemistry and spectroscopic characteriza-
tion of 42-N2 will be described in detail. Complete characteriza-
tion data for the other compounds are reported in experimental
details contained in the Supporting Information.
(33) Pool, J. A.; Chirik, P. J. Can. J. Chem. 2005, 83, 286.
(34) Manriquez, J. M.; Bercaw, J. E. J. Am. Chem. Soc. 1974, 96, 6229.
(35) For other early bimetallic early transition metal compounds with
bridging indenyl ligands see: V: (a) Jonas, K.; Ru¨ssler, W.; Krüger,
C.; Raabe, R Angew. Chem., Int. Ed. 1986, 25, 928. (b) Gauvin, F.;
Britten, J.; Samuel, E.; Harrod, J. F. J. Am. Chem. Soc. 1992, 114,
1489. (c) Ti: Gyepes, R.; Cisarova, I.; Horacek, M.; Cejka, J;
Petrusova, L.; Mach, K. Collect. Czech. Chem. Commun. 2000, 65,
1248.
Monitoring the conversion of 4 to 42-N2 in benzene-d6 by
1H NMR spectroscopy revealed smooth disappearance of the
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Indenyl Zirconium Dinitrogen Chemistry
A R T I C L E S
tive description would be an [N2]4- hydrazido complex with
neutral η2-alkene ligands, although this view is inconsistent with
the metrical parameters.
consistent with a weakly activated dinitrogen ligand. This band
shifts to 1533 cm-1 (KBr, calculated ) 1529 cm-1) for the
15N isotopologue, prepared by reduction of 4-Cl2 under 15N2
gas. The NdN band in the side-on, end-on dinitrogen complex,
7, appears at much higher frequency than the value of 1165
cm-1 reported by Fryzuk and co-workers for the tantalum
compound with side-on, end-on N2 hapticity.37 Two inequivalent
nitrogen atoms were also observed by 15N NMR spectroscopy,
consistent with side-on, end-on N2 coordination. For the major
isomer, peaks are observed at 490.0 and 599.9 ppm with a 1JN-N
) 20 Hz. Similar chemical shifts of 494.9 and 610.1 ppm with
1JN-N coupling constants of 21 Hz were observed for the minor.
These values compare to the 15N NMR shifts of 360.0 and 544.0
ppm (1JN-N ) 21.5 Hz) for the ditantalum compound.11 All
15N values are referenced to liquid ammonia.
Notably the transformations of the isolable zirconium sand-
wich compounds, 3-5, to the corresponding dinitrogen com-
plexes are rare examples of N2 coordination from isolated
zirconium sandwich complexes. Typically, to obtain these
compounds, a group 4 transition metal dihalide complex is
treated with a strong reducing agent under a dinitrogen
atmosphere, and the outcome of these reactions is often
complex.36 In addition, these results clearly demonstrate the
importance of indenyl steric effects in dinitrogen complex
formation. The presence of two large 1,3 substituents (e.g., the
size of isopropyl or larger) inhibits dimerization through a
µ2,η1,η1-N2 ligand. Likewise, three methyl groups produce the
same outcome. One methyl substituent and a second methyl,
isopropyl, or tert-butyl group allows dimerization, and hence,
isolable dinitrogen complexes are obtained. The 1,3-indenyl
disubstitution pattern appears essential as reduction of complexes
bearing only one substituent did not provide identifiable
sandwich or dinitrogen compounds. Once again, these results
highlight the importance of reversible benzo coordination to
allow access to Zr(II) chemistry and provide rare examples of
indenyl dinitrogen compounds.
Two other examples of zirconium side-on, end-on dinitrogen
compounds, 8 and 9, were synthesized by sodium amalgam
reduction of 2-Cl2 and 5-Cl2 under an N2 atmosphere, respec-
1
tively (eq 4). The H NMR spectral features of 8 and 9 are
similar to those observed for 7 and clearly establish one indenyl
ring per dimer. Again, free indene was observed during each
synthesis and accounts for the ejected ligand during the
reduction. For 9, a 1.5:1 ratio of two isomers was observed by
NMR spectroscopy. Obtaining silylated dinitrogen complex, 8,
in pure form proved challenging. Approximately 10–15% of
the material was a bridging dichloride complex with a bridging
indenyl ligand. Halide contamination is frequently observed
during reductions of silyl-substituted zirconocenes as the
electron-withdrawing properties of the ligand render reduction
to low oxidation states more challenging.21 Attempts to prepare
7 in higher yield by reduction of 4-Cl2 in the presence of (η5-
C5Me5)ZrCl3 were unsuccessful as the latter compound was
completely consumed prior to reaction of 4-Cl2 with Na(Hg).
X-ray diffraction studies on single crystals of 7 (Figure 6)
contained both isomers of the product in the unit cell and
established the identity of the molecule as the “side-on, end-
on” zirconium dinitrogen compound with bridging chloride and
indenyl ligands. The difference between the two isomers is the
relative orientation of the isopropyl (or methyl) indenyl sub-
stituent relative to the bridging chloride ligand. The major isomer
has these two ligands syn, while the minor isomer has the
opposite configuration (Figure 6). Consistent with the solution
NMR data, there is only one indenyl ring per two zirconium
atoms. The N-N bond distances of 1.196(4) (major) and
1.220(10) (minor) Å are indicative of weak to modest activation
and are significantly shorter than the value of 1.319(4) Å
reported by Fryzuk and co-workers for the tantalum N2
compound with a similar hapticity.11
Synthesis of Side-On, End-On Zirconium Dinitrogen Com-
pounds. Attempts to prepare 42-N2 by direct sodium amalgam
reduction of 4-Cl2 under a dinitrogen rather than argon
atmosphere produced a different outcome, highlighting the
importance of isolating the sandwich for direct N2 coordinaton.
The product of the reduction reaction was neither 4 nor 42-N2
but rather a new compound, 7, identified as a side-on, end-on
zirconium complex with a µ2,η5,η4-indenyl ligand (eq 4).
The bond lengths and angles of the benzo portion of the µ2,η4-
bridging indenyl ligand are also consistent with 2-electron
reduction (Table 2).38 In the major isomer, elongated C(5)-C(6)
and C(7)-C(8) distances of 1.418(3) and 1.402(4) Å, respec-
tively, are observed with concomitant contraction of the
C(6)-C(7) distance to 1.390(3) Å. Likewise, the benzo portion
of the ring is buckled by 23.2° also consistent with dialkyl
character and reduction of the six-membered ring. Thus, the
Zr(1) metallocene can be described as an 18-electron compound
The benzene-d6 1H NMR spectrum of 7 established a 3:1
mixture of isomeric products, each with two inequivalent Cp*
environments and only one indenyl ring per dimer.35 Free indene
1
was detected by H NMR spectroscopy and accounts for the
ejected ligand. Upfield-shifted benzo resonances are observed
between 2 and 5 ppm, diagnostic of coordination to zirconium.28
A medium intensity NdN stretch was observed at 1583 cm-1
(νN2 ) 1584 cm-1 pentane) in the solid state (KBr) IR spectrum,
(37) Studt, F.; MacKay, B. A.; Fryzuk, M. D.; Tuczek, F. J. Am. Chem.
Soc. 2004, 126, 280.
(38) Chirik, P. J.; Bradley, C. A. Complexes of Zirconium and Hafnium in
Oxidation States 0 to II; Bochmann, M., Ed.;In Comprehensive
Organometallic Chemistry III, Vol. 4; Elsevier: the Netherlands, 2007;
p 697.
(36) Morello, L.; Ferreira, M. J.; Patrick, B. O.; Fryzuk, M. D. Inorg. Chem.
2008, 47, 1319.
9
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A R T I C L E S
Pun et al.
Figure 6. Molecular structure of the (a) major and (b) minor isomers of 7 at 30% probability ellipsoids. Hydrogen atoms omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 42-N2
and the Isomers of 7
42-N2
7
C(4)-C(5) (Å)
C(5)-C(6) (Å)
C(6)-C(7) (Å)
C(7)-C(8) (Å)
C(8)-C(9) (Å)
η9 fold angle (deg)
buckle
1.435(3)
1.350(3)
1.433(3)
1.429(3)
1.435(3)
10.2
1.445(3)
1.418(3)
1.390(3)
1.402(4)
1.458(3)
1.6
Figure 7. Preparation and attempted alkali metal reduction of 4-THI-I2.
1.0
23.2
dihedral angle (deg)
57.9
1.2
with excess Me3SiI resulted in smooth conversion to the desired
zirconocene diiodide. All of the compounds in this sequence
were readily identified by routine NMR spectroscopic experi-
ments and by comparison to related compounds.39
with a side-on bound [N2]2- ligand completing the coordination
sphere. In this view, the other zirconium, Zr(2), is a formally
16-electron metal center with a dative bond to the end-on
coordinated N2 ligand and a 2-electron reduced, “butadiene-
like” η4 benzo ring.
For the tantalum compound, Fryzuk and co-workers have
provided compelling evidence that the bridging hydrides force
the two metal centers so close together that alternative N2
hapticities become disfavored.11 To determine whether the µ2,η4-
hapticity of the indenyl ligand was responsible for the observa-
tion of side-on, end-on N2 coordination in 7-9, a related
zirconium tetrahydroindenyl diiodide complex was prepared and
reduced under dinitrogen (Figure 7).
Exposure of benzene-d6 or toluene solution of 4 to 4 atm of
dihydrogen resulted in rapid formation of the corresponding
zirconocene dihydride, 4-H2, where the indenyl ring remains
intact. Prolonged exposure to H2 gas resulted in complete
hydrogenation of the benzo ring to the tetrahydroindenyl
derivative, 4-THI-H2.39,40 Treatment of the dihydride complex
Sodium amalgam reduction of 4-THI-I2 yielded a 5:1 mixture
of two zirconocene compounds, identified as two isomers of
the zirconocene hydrido iodide. It is likely that the major isomer
is the one where the hydride is syn to the isopropyl group and
the minor the one where it is anti. Diagnostic, downfield hydride
peaks are observed in the benzene-d6 1H NMR spectrum at 7.96
(major) and 8.08 (minor) ppm. The source of the hydrogen atom
during the reduction is unknown, but minor amounts of a
cyclometalated zirconocene iodide were also observed, sug-
gesting a ligand C-H bond activation is likely. It should also
be noted that hydrido chloride compounds are frequently
observed as byproducts in the reductions of zirconocene and
hafnocene dihalide compounds with sodium amalgam.41,45
Notably, no dinitrogen compound was observed from these
experiments.
Protonolysis Studies. With new structural types of zirconocene
dinitrogen compounds in hand, the activation of the dinitrogen
(39) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999, 18,
1873.
(41) Bernskoetter, W. H.; Olmos, A. V.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2006, 25, 1021.
(40) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am. Chem.
Soc. 2006, 128, 6454.
(42) Hillhouse, G. L.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5472.
9
6052 J. AM. CHEM. SOC. VOL. 130, NO. 18, 2008

Indenyl Zirconium Dinitrogen Chemistry
A R T I C L E S
Table 3. Protonolysis of Indenyl-Substituted Zirconium Dinitrogen
Compounds to Hydrazine; Protonolysis of [(η5-C5Me4H)2Zr]2-
(µ2,η2,η2-N2) Included for Comparison
activated end-on compounds only yield hydrazine when treated
with mineral acid.
Experimental Section46
protonolysis reaction
% N2H4
42-N2+ xs HCl
45
0
70
57
73
66
66
>95
84
Preparation of (η5-C5Me5)(η5-C9H5-1-(iPr)-3-(Me))ZrCl2 (4-
Cl2). A 500 mL round-bottom flask was charged with 2.21 g (6.64
mmol) of (η5-C5Me5)ZrCl3 and approximately 100 mL of diethyl
ether was added. The pale yellow solution was chilled in a liquid
nitrogen cooled cold well for 20 min and 1.18 g (6.64 mmol) of
Li[C9H5-1-(iPr)-3-(Me)] was added to the solution and the resulting
mixture stirred for two days. The solvent was removed in Vacuo
and the yellow solid washed with pentane and subsequently
extracted into toluene and filtered through Celite. Removal of the
toluene in Vacuo yielded 2.56 g (82%) of 4-Cl2 as a yellow powder.
Anal. Calcd for C23H30ZrCl2: C, 58.95; H, 6.45. Found: C, 57.93;
H, 6.34. 1H NMR (benzene-d6): δ ) 1.21 (d, 9.2 Hz, 3H, CHMe2),
1.28 (d, 9.2 Hz, 3H, CHMe2), 1.79 (s, 15H, Cp* Me), 2.12 (s, 3H,
Me), 3.26 (m, 1H, CHMe2), 5.77 (s, 1H, CpH), 7.05 (m, 2H, Benzo),
7.49 (d, 8.4 Hz, 1H, Benzo), 7.67 (d, 8.4 Hz, 1H, Benzo). 13C{1H}
NMR (benzene-d6): δ ) 12.42, 12.44 (Cp* Me/Ind Me), 23.47,
24.00, 28.69 (CHMe2), 119.13, 124.40, 124.73, 125.00, 125.14,
125.35 (Cp/Benzo).
42-N2+ xs H2O
52-N2+ xs HCl
7 + xs HCl
7 + xs H2O
7 + xs EtOH
8 + xs HCl
[(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) + HCl
[(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) + H2O
ligand observed in the ground-state was probed by a series of
protonolysis experiments. Previous work from Bercaw’s labora-
tory42 and our research group43 has established that zirconocene
dinitrogen complexes bearing weakly activated, [N2]0-type
ligands undergo loss of dinitrogen when hydrolyzed with EtOH
or water, whereas strongly activated, side-on bound haptomers
with [N2]4- yield hydrazine.44 Both structural types yield
hydrazine upon treatment with mineral acids such as HCl.34,45
The results of the protonolysis of several of the new indenyl
ligated zirconium dinitrogen compounds are summarized in
Table 3. Previous work with the side-on bound bis(cyclopen-
tadienyl)zirconocene dinitrogen compound is also reported for
comparison. The indenyl zirconium dinitrogen compound bear-
ing the weakly activated, end-on coordinated N2 ligand, 42-N2,
produced no N2H4 upon treatment with water. As anticipated,
all of the end-on dinitrogen compounds produce a modest to
good yield of hydrazine upon addition of excess gaseous HCl.
The stronger activation observed in the ground state of the side-
on, end-on dinitrogen compound translates on its protonolysis
chemistry. Unlike the end-on compounds, addition of water or
ethanol to 7 produced 73 and 66% of the expected amount of
hydrazine, respectively.
Preparation of (η5-C5Me5)(η9-C9H5-1-(iPr)-3-(Me))Zr (4). A 100
mL round-bottom flask was charged with 8.84 g (44.05 mmol) of
mercury and approximately 10 mL of pentane in a nitrogen drybox.
With stirring, 0.044 g (1.91 mmol) of sodium metal was added
and the resulting amalgam was stirred for 20 min. A pentane slurry
containing 0.150 g (0.32 mmol) of 4-Cl2 was added. The resulting
reaction mixture was stirred vigorously for 24 h under argon and
the burgundy solution was decanted from the amalgam and filtered
through Celite in a nitrogen filled drybox. Removal of the pentane
1
in Vacuo yielded 0.126 g (99%) of 1 as a burgundy oil. H NMR
(benzene-d6): δ ) 1.10 (d, 6.8 Hz, 3H, CHMe2), 1.12 (d, 6.8 Hz,
3H, CHMe2), 1.78 (s, 15H, Cp* Me), 2.05 (s, 3H, Ind Me), 3.24
(m, 1H, CHMe2), 3.68 (m, 1H, η9-Benzo), 3.84 (m, 1H, η9-Benzo),
5.02 (d, 6 Hz, 1H, η9-Benzo), 5.13 (d, 6 Hz, 1H, η9-Benzo), 5.56
(s, 1H, CpH). 13C {1H} NMR (benzene-d6): δ ) 11.49, 12.06 (Me),
22.13, 27.05, 28.94 (CHMe), 65.47, 66.45, 94.03, 96.19 (η9-Benzo),
112.08, 113.90, 118.51 (Cp). Three Cp resonances not located.
Preparation of [(η5-C5Me5)Zr]2 [µ2-η2,η2-{4,5-(η5-C9H5-1-(iPr)-
3-(Me))}2](µ2-η1,η1-N2) (42-N2). A 20 mL scintillation vial was
charged with 0.116 g of 4 and approximately 3 mL pentane. The
resulting solution was chilled at -35 °C for one week, producing
a purple solution with precipitate. The brown precipitate was
isolated and yielded 20 mg (17%) of a brown solid identified as
42-N2. Anal. Calcd for C46H60Zr2N2: C, 67.10; H, 7.34; N, 3.40.
Found: C, 67.25; H, 7.31; N, 3.03.1H NMR (benzene-d6): δ ) 1.34
(d, 6.5 Hz, 6H, CHMe2), 1.42 (d, 6.5 Hz, 6H, CHMe2), 1.80 (s,
30H, Cp* Me), 2.25 (s, 6H, Ind Me), 2.63 (m, 4H, CHMe2, η2-
Benzo), 3.30 (m, 2H, η2-Benzo), 5.64 (s, 2H, CpH), 6.33 (m, 2H,
Benzo), 6.41 (d, 8 Hz, 2H, Benzo). 13C {1H} NMR (benzene-d6):
δ ) 12.06 (Cp* Me), 13.51 (Ind Me), 24.82, 25.45 (CHMe), 29.56
(CHMe), 66.03, 77.68 (η2-Benzo), 103.97 (Cp), 109.63 (Ind CpH),
112.09 (Cp), 112.54 (Benzo), 124.16 (Cp), 126.05 (Benzo). Two
Cp/Benzo resonances not located. 15N{1H} NMR (benzene-d6): δ
Concluding Remarks
New indenyl-ligated zirconocene dinitrogen complexes have
been synthesized that exploit the haptropic flexibility of the
indenyl ligand and as a result, add to the structural diversity of
known group 4 transition metal N2 complexes. Isolated sandwich
i
compounds bearing one methyl and one other (R ) Me, Pr,
tBu) indenyl substituent reversibly coordinate dinitrogen fol-
lowing dissociation of the benzo ring of the η9 indenyl ligand.
The resulting dinitrogen complexes contain an end-on bound
N2 ligand and exhibit a µ2,η2 indenyl interaction. Related
compounds bearing larger 1,3 indenyl substituents do not form
isolable dinitrogen complexes likely due to the inability to
dimerize through a two atom, N2 bridge. Performing the
reductions of the zirconocene dichloride complexes directly
under an N2 atmosphere produced rare examples of side-on,
end-on dinitrogen compounds, where one indenyl ligand has
been ejected and the remaining ring bridges two zirconium
centers. This µ2,η5,η4-interaction likely brings the two metal
centers in proximity and favors side-on, end-on coordination.
The stronger activation of the dinitrogen ligand in this class of
compounds is translated onto N2 functionalization by weak
Brønsted acids such as ethanol and water. In contrast, the weakly
) 598.69. IR (pentane): ν(NN) ) 1563 cm-1
.
Preparation of [(η5-C5Me5)Zr]2[µ2-η5,η4-(η5-C9H5-1-(iPr)-3-
(Me))](µ2-η2,η1-N2)(µ2-η1,η1-Cl) (7). This compound was prepared
in a similar manner to 4 using 8.84 g (44.05 mmol) of mercury,
0.044 g (1.91 mmol) of sodium metal and 0.150 g (0.32 mmol) of
4-Cl2. Stirring the reaction mixture for 2 days and subsequent
workup in a nitrogen drybox afforded a brown oil. Recrystallization
from pentane afforded 0.079 g (72%) of a brown solid identified
as two isomers of 7 in a 3:1 ratio. Anal. Calcd for C33H45Zr2N2Cl:
1
C, 57.64; H, 6.60; N, 4.07. Found: C, 57.61; H, 6.47; N, 3.87. H
(43) Bernskoetter, W. H.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am.
Chem. Soc. 2005, 127, 7901.
(46) Representative procedures for one specific class of compounds are
reported. Full details on all of the remaining compounds, including
general considerations, are described in the Supporting Information.
(44) Leigh, G. H. Acc. Chem. Res. 1992, 25, 177.
(45) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527.
9
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A R T I C L E S
Pun et al.
NMR (benzene-d6): δ ) 0.58 (d, 8 Hz, 3H, minor CHMe2), 0.63
(d, 8 Hz, 3H, major CHMe2), 0.74 (s, 3H, major Ind Me), 0.91 (d,
8 Hz, 3H, minor CHMe2), 1.04 (s, 3H, minor Ind Me), 1.11 (d, 8
Hz, 3H, major CHMe2), 1.90 (s, 15H, major Cp* Me), 1.91 (s, 15H,
minor Cp* Me), 1.94 (s, 15H, major Cp* Me), 1.95 (s, 15H, minor
Cp* Me), 1.96 (m, 2H, major/minor CHMe2), 2.22 (dd, 8 Hz, 4
Hz, 1H, major Benzo), 2.66 (dd, 8 Hz, 4 Hz, 1H, minor Benzo),
3.49 (m, 1H, minor Benzo), 3.83 (dd, 8 Hz, 4 Hz, 1H, major Benzo),
4.34 (s, 1H, major CpH), 4.42 (s, 1H, minor CpH), 4.73 (m, 1H,
major Benzo), 4.85 (d, 8 Hz, 1H, minor Benzo). 13C{1H} NMR
(benzene-d6): δ ) 9.15 (major Ind Me), 11.86, (major Cp* Me),
11.96 (minor Cp* Me), 11.98 (major Cp* Me), 12.06 (minor Cp*
Me), 22.56 (major CHMe2), 22.90 (minor CHMe2), 24.81 (major
CHMe2), 25.07, 27.14 (minor CHMe2), 27.87 (major CHMe2),
68.24, 68.45, 83.19, 84.08, 87.52, 93.11, 93.79, 94.39, 95.72, 95.88,
100.81, 106.36, 114.95, 115.25, 115.97, 116.02, 116.39, 118.29,
119.15, 119.90, 120.58, 121.87 (Cp/Benzo). Minor Ind Me reso-
nance not located. 15N {1H} NMR (benzene-d6): δ ) 490.0 (d, 20
Hz, major), 494.9 (d, 21 Hz, minor), 599.9 (d, 20 Hz, major), 610.1
(d, 21 Hz, minor). IR(pentane): ν(NN) ) 1584 cm-1
.
Acknowledgment. We thank the Director, Office of Basic
Energy Sciences, Chemical Sciences Division, of the U.S.
Department of Energy (DE-FG02-05ER15659) for financial
support. P.J.C. is a Cottrell Scholar sponsored by the Research
Corporation, a David and Lucile Packard Fellow in science and
engineering, and a Camille Dreyfus Teacher-Scholar.
Supporting Information Available: Experimental procedures
and crystallographic data for 1, 42-N2, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org
JA801021W
9
6054 J. AM. CHEM. SOC. VOL. 130, NO. 18, 2008