Dinitrogen functionalization with terminal alkynes, amines, and hydrazines promoted by [(η5-C5Me4H) 2Zr]2(μ2,η2, η2-N2): Observation of side-on and end-on diazenido complexes in the reduction of N2 to hydrazine

Article abstract of DOI:10.1021/ja050387b

Functionalization of the N₂ ligand in the side-on bound dinitrogen complex, [(η⁵-C₅Me₄H) ₂Zr]₂-(μ₂,η², η²-N₂), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene diazenido complexes, [(η⁵-C₅Me₄H)₂Zr(C≡CR)] ₂(μ₂,η²,η²-N ₂H₂) (R = ⁿBu, ^tBu, Ph). Characterization of [(η⁵- C₅Me₄H) ₂Zr(C≡CCMe₃)]₂(μ₂, η²,η²-N₂H₂) by X-ray diffraction revealed a side-on bound diazenido ligand in the solid state, while variable-temperature ¹H and ¹⁵N NMR studies established rapid interconversion between η¹,η¹ and η²,η² hapticity of the [N₂H ₂]^2- ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene diazenido complexes, [(η⁵-C ₅Me₄H)₂ZrX]₂(μ₂, η¹,η¹-N₂H₂) (X = Cl, I, OTf, CH₂Ph, CH₂SiMe₃), afforded η¹,η¹ coordination of the [N₂H ₂]^2- fragment both in the solid state and in solution, demonstrating that sterically demanding, in some cases π-donating, ligands can overcome the electronically preferred side-on bonding mode. Unlike [(η⁵-C₅Me₄H)₂ZrH] ₂(μ₂,η²,η²-N ₂H₂), the acetylide and alkyl zirconocene diazenido complexes are thermally robust, resisting α-migration and N₂ cleavage up to temperatures of 115 °C. Dinitrogen functionalization with [(η⁵-C₅Me₄H)₂Zr] ₂(μ₂,η²,η²-N ₂) was also accomplished by addition of proton donors. Weak Bronsted acids such as water and ethanol yield hydrazine and (η⁵-C ₅Me₄H)₂Zr(OH)₂ and (η⁵-C₅Me₄H)₂-Zr(OEt) ₂, respectively. Treatment of [(η⁵-C ₅Me₄H)₂Zr]₂(μ₂, η²,η²-N₂) with HNMe₂ or H₂NNMe₂ furnished amido or hydrazido zirconocene diazenido complexes that ultimately produce hydrazine upon protonation with ethanol. These results contrast previous observations with [(η⁵-C ₅Me₅)₂Zr(η¹-N₂)] ₂(μ₂,η¹,η¹-N ₂) where loss of free dinitrogen is observed upon treatment with weak acids. These studies highlight the importance of cyclopentadienyl substituents on transformations involving coordinated dinitrogen.

Full text of DOI:10.1021/ja050387b

Published on Web 05/07/2005
Dinitrogen Functionalization with Terminal Alkynes, Amines,
and Hydrazines Promoted by [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2):
Observation of Side-On and End-On Diazenido Complexes in
the Reduction of N2 to Hydrazine
Wesley H. Bernskoetter, Jaime A. Pool,† Emil Lobkovsky, and Paul J. Chirik*
Contribution from the Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell UniVersity, Ithaca, New York 14853
Received January 20, 2005; E-mail: pc92@cornell.edu
Abstract: Functionalization of the N2 ligand in the side-on bound dinitrogen complex, [(η5-C5Me4H)2Zr]2-
(µ2,η2,η2-N2), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene
diazenido complexes, [(η5-C5Me4H)2Zr(CtCR)]2(µ2,η2,η2-N2H2) (R ) nBu, tBu, Ph). Characterization of [(η5-
C5Me4H)2Zr(CtCCMe3)]2(µ2,η2,η2-N2H2) by X-ray diffraction revealed a side-on bound diazenido ligand in
the solid state, while variable-temperature 1H and 15N NMR studies established rapid interconversion between
η1,η1 and η2,η2 hapticity of the [N2H2]2- ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene
diazenido complexes, [(η5-C5Me4H)2ZrX]2(µ2,η1,η1-N2H2) (X ) Cl, I, OTf, CH2Ph, CH2SiMe3), afforded η1,η1
coordination of the [N2H2]2- fragment both in the solid state and in solution, demonstrating that sterically
demanding, in some cases π-donating, ligands can overcome the electronically preferred side-on bonding
mode. Unlike [(η5-C5Me4H)2ZrH]2(µ2,η2,η2-N2H2), the acetylide and alkyl zirconocene diazenido complexes
are thermally robust, resisting R-migration and N2 cleavage up to temperatures of 115 °C. Dinitrogen
functionalization with [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) was also accomplished by addition of proton donors.
Weak Brønsted acids such as water and ethanol yield hydrazine and (η5-C5Me4H)2Zr(OH)2 and (η5-C5Me4H)2-
Zr(OEt)2, respectively. Treatment of [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) with HNMe2 or H2NNMe2 furnished amido
or hydrazido zirconocene diazenido complexes that ultimately produce hydrazine upon protonation with
ethanol. These results contrast previous observations with [(η5-C5Me5)2Zr(η1-N2)]2(µ2,η1,η1-N2) where loss
of free dinitrogen is observed upon treatment with weak acids. These studies highlight the importance of
cyclopentadienyl substituents on transformations involving coordinated dinitrogen.
Introduction
has allowed observation of many intermediates in the reduction
cycle,6 ultimately resulting in complexes that catalytically fix
Dinitrogen functionalization promoted by soluble transition
metal complexes continues to be actively investigated.1 Para-
mount to this effort is the discovery of mild, energy-efficient
methods for the construction of nitrogen-hydrogen bonds given
both the industrial2 and the biological3 significance of ammonia.
For well-defined, homogeneous metal dinitrogen complexes, the
so-called “Chatt-cycle” is the most common and well-established
method for N2 reduction.4 Classically, a weakly activated,
zerovalent dinitrogen complex of molybdenum or tungsten,
[(R3P)4M(N2)L] (L ) N2 or other neutral ligand), is treated with
a mineral acid to yield N2H4 or NH3.5 Introduction of tri-
amidoamine ligands with sterically demanding aryl substituents
N2 to ammonia by a series of successive proton and electron-
transfer reactions.7 Recently, this approach has been extended
to cobalt and iron-dinitrogen complexes to affect C-N and
C-Si bond formation.8
Electrophilic functionalization of strongly activated early
transition metal complexes has also been used to prepare
ammonia,9 hydrazine,10 and heterocycles11 directly from N2.
Significantly, the reduced dinitrogen ligands in this class of
molecule also open new pathways for nitrogen-element bond
formation using nonpolar reagents.12 Dinuclear tantalum and
zirconium dinitrogen complexes supported by macrocyclic
phosphine diamide ligands13 have proven particularly interesting
for this method of functionalization, resulting in the hydrosilation
† Current address: Chemistry Division, Los Alamos National Laboratory,
Los Alamos, NM 87545.
(1) (a) MacKay, B. A.; Fryzuk, M. D. Chem. ReV. 2004, 104, 381. (b)
Gambarotta, S.; Scott, J. Angew. Chem., Int. Ed. 2004, 43, 5298.
(2) Smil, V. Enriching the Earth: Fritz Haber, Carl Bosch, and the
Transformation of World Food Production; MIT Press: Cambridge, MA,
2001.
(3) (a) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.;
Howard, J. B.; Rees, D. C. Science 2002, 297, 1696. (b) Burgess, B. K.;
Lowe, D. J. Chem. ReV. 1996, 96, 2983.
(4) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. ReV. 1978, 78, 589.
(5) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc., Dalton Trans.
1977, 1852.
(6) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6262.
(7) (a) Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock, A.
S.; Davis, W. M. J. Am. Chem. Soc. 2004, 126, 6150. (b) Yandulov, D.
V.; Schrock, R. R. Science 2003, 76, 301.
(8) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782.
(9) (a) Vol’pin, M. E.; Shur, V. B. Nature 1966, 209, 1236. (b) Luneva, N. P.;
Nikonova, L. A.; Shilov, A. E. React. Kinet. Catal. Lett. 1976, 5, 149. (c)
Shestakov, A. F.; Shilov, A. E. Kinet. Catal. 2001, 42, 653.
(10) Manriquez, J. M.; Bercaw, J. E. J. Am. Chem. Soc. 1974, 96, 6229.
(11) Mori, M.; Akashi, M.; Hori, M.; Hori, K.; Nishida, M.; Sato, Y. Bull. Chem.
Soc. Jpn. 2004, 77, 1655.
9
10.1021/ja050387b CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 7901-7911
7901

A R T I C L E S
Bernskoetter et al.
(Zr,14 Ta15), hydroboration (Ta),16 and even partial hydrogena-
tion13 of coordinated dinitrogen. Most recently, nitrogen-carbon
bond formation by addition of terminal alkynes to the zirconium
dinitrogen complex has been described, forming a bridging
acetylide and a carbon-substituted hydrazido ligand.17
Our laboratory has been exploring the effect of cyclopenta-
dienyl ligand substitution on the hapticity and reactivity of
coordinated dinitrogen in low-valent group 4 metallocene
complexes.18 Whereas [(η5-C5Me5)2Zr(η1-N2)]2(µ2,η1,η1-N2)10
and [(η5-C5Me5)(η5-C5Me4H)Zr(η1-N2)]2(µ2,η1,η1-N2)19 contain
weakly activated “end-on” coordinated dinitrogen ligands that
are readily displaced by dihydrogen, the octamethyl-substituted
zirconocene dinitrogen complex, [(η5-C5Me4H)2Zr]2(µ2,η2,η2-
N2) (1), has a side-on bound N2 ligand with an elongated N-N
bond of 1.377(3) Å. Exposure of 1 to a dihydrogen atmosphere
resulted in nitrogen-hydrogen bond formation to afford [(η5-
C5Me4H)2ZrH]2(µ2,η2,η2-N2H2) (2), which upon warming to 85
°C under additional H2 produced small quantities of free
ammonia (eq 1).20
addition of dihydrogen to afford the side-on bound hydrido
zirconocene diazenido complex, [(η5-C5Me4H)2ZrH]2(µ2,η2,η2-
N2H2) (2), dinitrogen functionalization with other molecules that
could possibly undergo similar cycloaddition-type chemistry was
investigated. Alkynes are attractive candidates given their well-
documented reaction chemistry with group 4 imido com-
plexes.21,22 While cycloaddition to yield azametallacycles is
frequently observed,23,24 formal C-H activation of terminal
alkynes by the base-free titanocene imido, (η5-C5Me5)2TidNPh,
affords the corresponding anilido acetylide species, (η5-C5Me5)2-
Ti(N(Ph)H)CtCR (R ) Ph, SiMe3),25 demonstrating the
potential utility of these substrates to promote N-H bond
formation from coordinated dinitrogen.
Addition of 2 equiv of a terminal acetylene to 1 at ambient
temperature furnished the acetylide zirconocene diazenido
compounds, [(η5-C5Me4H)2Zr(CtCR)]2(µ2,η2,η2-N2H2) (R )
(CH2)3CH3, 3; CMe3, 4; C6H5, 5), arising from formal activation
of the alkyne C-H bond by the dinitrogen complex (eq 2).
Complete conversion to products is observed immediately upon
thawing frozen benzene-d6 solutions. Formation of two new
N-H bonds from alkyne addition to a dinitrogen complex is,
to our knowledge, unprecedented and contrasts the reactivity
of a bis(phosphine)diamide-ligated zirconium dinitrogen com-
plex where cycloaddition yields a new nitrogen-carbon bond
and a bridging acetylide.17 The different products observed from
the two types of zirconium-N2 complexes are most likely steric
in origin, where the more hindered zirconocene system favors
C-H activation over cycloaddition. Similar observations have
been made in permethyltitanocene oxo chemistry, where
equilibration of oxometallocyclobutanes to the thermodynami-
cally favored and sterically less congested hydroxide acetylide
complexes occurs.26 This rationale has also been invoked to
explain the observed anilido acetylide products formed from
addition of terminal alkynes to the permethyltitanocene imido
complex mentioned above.25
Subsequent isotopic labeling and computational studies
established that the unique hydrogenation reactivity of 1 stems
from the “twisted” ground-state structure of the dinuclear
zirconium complex.19 As the dihedral angle between zirconocene
wedges approaches orthogonality, the 1a1 orbitals of the two
metals can effectively overlap with the perpendicular lobes of
the π* molecular orbitals of the η2-N2 ligand, thereby facilitating
the addition of dihydrogen.18 In light of these observations, we
became interested in the possibility of dinitrogen functional-
ization by both electrophilic and cycloaddition pathways. Here,
we describe formation of N-H bonds by addition of terminal
alkynes, dimethylamine, and N,N-dimethylhydrazine to 1 and
explore the mechanism of N2 functionalization. The solution
dynamics, molecular structures, and coordination preferences
of the resulting zirconocene diazenido complexes are also
reported. These studies underscore the significance of both
cyclopentadienyl ligand substituents and N2 hapticity on N-H
bond forming reactions involving coordinated dinitrogen.
The functionalized products, 3-5, were characterized by a
combination of multinuclear (1H, 13C, 15N) NMR experiments,
IR spectroscopy, elemental analysis, and, in one case, 4, X-ray
Results and Discussion
Dinitrogen Functionalization with Terminal Alkynes.
Because [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) (1) undergoes facile
1
diffraction. At 23 °C in benzene-d6, the H NMR spectra for
3-5 display sharp peaks for the acetylide resonances and one
sharp cyclopentadienyl methyl group. The other cyclopenta-
dienyl methyl peaks as well as the cyclopentadienyl hydrogen
(12) Protonation of group 6 dinitrogen complexes using H2 as the hydrogen
source has been accomplished by pre-coordination to ruthenium or metal
sulfide complexes. See: (a) Ru: Nishibayashi, Y.; Iwai, S.; Hidai, M.
Science 1998, 279, 540. (b) Molybdenum-sulfides: Nishibayashi, Y.;
Wakiji, I.; Hirata, K.; DuBois, M. R.; Hidai, M. Inorg. Chem. 2001, 40,
578. (c) Iron-sulfides: Nishibayashi, Y.; Iwai, S.; Hidai, M. J. Am. Chem.
Soc. 1998, 120, 10559.
(21) (a) Mountford, P. Chem. Commun. 1997, 2127. (b) Duncan, A. P.; Bergman,
R. G. Chem. Rec. 2002, 2, 431.
(22) Schaller, C. P.; Cummins, C. C.; Wolczanski, P. T. J. Am. Chem. Soc.
1996, 118, 591.
(13) Fryzuk, M. D. Chem. Rec. 2003, 3, 2.
(14) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. Science 1997, 275, 1445.
(15) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc. 2003,
125, 3234.
(23) (a) Harlan, C. J.; Tunge, J. A.; Bridgewater, B. M.; Norton, J. R.
Organometallics 2000, 19, 2365. (b) Harlan, C. J.; Hascall, T.; Fujita, E.;
Norton, J. R. J. Am. Chem. Soc. 1999, 121, 7274. (c) Harlan, C. J.; Bridge-
water, B. M.; Hascall, T.; Norton, J. R. Organometallics 1999, 18, 3827.
(24) (a) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. Organometallics 1993,
12, 3705. (b) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem.
Soc. 1988, 110, 8729.
(16) Fryzuk, M. D.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O. Angew.
Chem., Int. Ed. 2002, 41, 3709.
(17) Morello, L.; Love, J. B.; Patrick, B. O.; Fryzuk, M. D. J. Am. Chem. Soc.
2004, 126, 9480.
(18) Pool, J. A.; Chirik, P. J. Can. J. Chem. 2005, 83, 286.
(19) Pool, J. A.; Bernskoetter, W. H.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 14326.
(25) Polse, J. L.; Anderson, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1998,
120, 13405.
(26) Polse, J. L.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 5393.
(20) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527.
9
7902 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005

Dinitrogen Functionalization
A R T I C L E S
Table 1. Selected Infrared Spectroscopic Data for 3-5 Recorded
in KBr
1
1
compound
R
ν
N−
H (cm-
)
ν Ct )
C (cm-
3
4
5
CH2(CH2)2CH3
CMe3
C6H5
3453
3582
3297
2093
2079
2079
are significantly broadened. Likewise, a resonance attributable
to the hydrogen attached to the nitrogen is not observed at this
temperature. These data, in combination with more detailed
variable-temperature studies (vide infra), are consistent with a
dynamic process where the diazenido ligand is interchanging
between η1,η1 and η2,η2 coordination on the time scale of the
NMR experiment.
Verification of N-H bond formation was provided by solid-
state (KBr) infrared spectroscopy. Spectra of 3-5 exhibited
broad peaks between 3297 and 3582 cm-1 (Table 1) attributed
to N-H stretches. Confirmation of these assignments was
obtained by preparation of [(η5-C5Me4H)2Zr(CtCnBu)]2(µ2,η2,η2-
N2D2) (3-d2), in which the N-D stretch shifts to 2441 cm-1, in
excellent agreement with the prediction made by the harmonic
oscillator model. In addition to the N-H peaks, diagnostic
bands26,27 centered between 2079 and 2093 cm-1 are observed
for the terminal acetylide ligands (Table 1).
The solid-state structure of 4 was elucidated by single crystal
X-ray diffraction, and a representation of the molecule is shown
in Figure 1. Selected bond distances and angles are reported in
Table 2. The diffraction data were of sufficient quality such
that all of the hydrogens, including those attached to the
nitrogens, were located and freely refined, thus providing
unambiguous assignment of the product. In the solid state, the
core of the molecule contains a side-on bound diazenido ligand
that is puckered by 45.54(5)° due to the pyramidalization of
the nitrogens, similar to the geometry of 2.20 The acetylide
ligands are oriented in a transoid fashion to minimize steric
interactions between the tert-butyl substituents. While a steric
argument seems reasonable, it should be noted that a similar
transoid arrangement is also observed in 2,20 where small
hydride ligands are present in the metallocene wedge.
Figure 1. (a) Molecular structure of 4 at 30% probability ellipsoids.
Hydrogen atoms, except for those attached to nitrogen, are omitted for
clarity. (b) View of the core of the molecule.
Table 2. Selected Bond Distances (Å) and Angles (°) for 4
Zr(1)-N(1)
Zr(1)-N(2)
Zr(2)-N(1)
Zr(2)-N(2)
2.190(2)
2.337(2)
2.328(2)
2.192(2)
Zr(1)-C(21)
Zr(2)-C(27)
2.251(2)
2.267(2)
Also in agreement with the solid-state structure of 1 and 220
is the observation of a significant twisting of the dimer in 4 as
evidenced by a dihedral angle of 73.7° formed between the
planes defined by the zirconium and the cyclopentadienyl
centroids. The N-N bond length of 1.454(2) Å is consistent
with reduction of coordinated dinitrogen to a diazenido ligand
and is comparable to the bond lengths of 1.46 and 1.457(3) Å
observed in hydrazine28 and 2,20 respectively. Also consistent
with a [N2H2]2- ligand29 is the observation of an asymmetric
core with short Zr-N bonds of 2.190(2) and 2.192(2) Å for
the anionic, “X-type” interactions and longer 2.337(2) and
2.328(2) Å distances for the neutral, “L-type” Zr-N bonds.
The hydrido zirconocene diazenido, 2, is known to undergo
R-hydrogen migration concomitant with H2 loss and N2 cleavage
upon warming to 45 °C.19,20 This unusual behavior prompted
investigation of the thermal stability of 3-5 with the goal of
forming a nitrogen-carbon bond and comparing the energetic
requirements of the two processes. All three acetylide-substituted
N(1)-N(2)
1.454(2)
1.216(2)
1.215(2)
C(21)-C(22)
C(27)-C(28)
Zr(1)-N(1)-Zr(2)
Zr(1)-N(2)-Zr(2)
N(1)-N(2)-Zr(1)
N(1)-N(2)-Zr(2)
N(1)-Zr(2)-N(2)
Zr(1)-C(21)-C(22)
Zr(2)-C(27)-C(28)
122.93(7)
122.39(7)
65.84(8)
76.39(8)
37.29(5)
177.66(16)
176.31(16)
zirconocene diazenido complexes are thermally robust; heating
sealed NMR tubes containing benzene-d6 solutions of 3-5 to
115 °C for several days produced no observable change in the
1H NMR spectra. Decomposition by loss of free alkyne and
formation of unidentified zirconium compounds is observed
upon heating to 160 °C. Although an sp hybridized acetylide
(29) For examples of complexes with [N2H2]2- ligands: (a) Churchill, M. R.;
Li, Y.-J.; Blum, L. Organometallics 1984, 3, 109. (b) Blum, L.; Williams,
I. D.; Schrock, R. R. J. Am. Chem. Soc. 1984, 106, 8316. (c) Gonza´lez-
Maupoey, M.; Rodr´ıguez, G. M.; Cuenca, T. Eur. J. Inorg. Chem. 2002,
2057.
(27) Pellny, P.-M.; Kirchbauer, F. G.; Burlakov, V. V.; Baumann, W.;
Spannenberg, A.; Rosenthal, U. Chem.-Eur. J. 2000, 6, 81.
(28) Fryzuk, M. D.; Johnson, S. A. Coord. Chem. ReV. 2000, 200, 379.
9
J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005 7903

A R T I C L E S
Bernskoetter et al.
Figure 2. Thermal stability of acetylide zirconocene diazenido complexes.
ligand would require the least orbital reorganizational energy
of any hydrocarbyl for R-migration, the barrier is significantly
higher than the corresponding hydrogen migration and is not
observed under typical thermolysis conditions. A combination
of experimental and computational studies from Wolczanski
have shown that 1,2-elimination of alkyne from (tBu3SiNH)3Zr-
(CtCR) was inhibited by a high kinetic barrier and that [2+2]
cycloaddition and reversion pathways were energetically com-
petent, accounting for associative exchange of the acetylide
ligand with free alkyne.22 For zirconocene diazenido compounds,
previous work from our laboratory with isotopologues of 2 have
shown the 1,2-elimination of dihydrogen precedes R-migration,
suggesting that the lack of reactivity observed with 3-5 may
be a result of a high kinetic barrier associated with elimination
of alkyne.
Examination of potential acetylide exchange reactions also
supports these observations. Thermolysis of 3 in benzene-d6 at
115 °C in the presence of 5 equiv of HCtCCMe3 produced no
reaction over the course of days (Figure 2). Likewise, heating
4 to 115 °C in the presence of 1-hexyne also did not yield new
products, further demonstrating the thermal stability of these
molecules. Importantly, these results establish that a competitive
[2+2] cycloreversion/addition pathway that can interchange
alkynes is also not operative as is the case with other group 4
metallacyclobutenes21,26 and (tBu3SiNH)3Zr(CtCR) com-
pounds.22
Preparation of Halide and Alkyl Zirconocene Diazenido
Complexes and Coordination Preferences of the [N2H2]2-
Core. Because of the importance of the diazenido complexes
in dinitrogen functionalization and cleavage processes, the
coordination preferences of the [N2H2]2- ligand were explored
in more detail. Ideally, establishing structure-reactivity relation-
ships would provide new opportunities for dinitrogen function-
alization. We hypothesized that introduction of sterically
demanding or π-donating (or both) “spectator” ligands in the
zirconocene wedge would disfavor side-on coordination of the
[N2H2]2- fragment and allow isolation of η1,η1 diazenido
complexes. Treatment of 2 with a slight excess of MeI induced
loss of 2 equiv of methane and furnished dark purple crystals
of [(η5-C5Me4H)2ZrI]2(µ2,η1,η1-N2H2) (6) (eq 4). Preparation
of the isotopologues, [(η5-C5Me4H)2ZrI]2(µ2,η1,η1-N2D2) (6-d2)
and [(η5-C5Me4H)2ZrI]2(µ2,η1,η1-15N2H2) (6-15N), was accom-
plished by addition of CH3I to 2-d4 and 2-15N,20 respectively.
The 1H NMR spectrum of 6 recorded in benzene-d6 at 23 °C
exhibits the expected number of cyclopentadienyl ligand
resonances for a static molecule where the two halves of the
dimer are related by symmetry. The N-H resonance is shifted
substantially downfield to 8.90 ppm, in contrast to 2, where
this peak is centered at 0.83 ppm. This suggests a different
magnetic environment and hence a different hapticity for the
diazenido ligand. Spectra of two isotopologues confirmed this
assignment. The 2H NMR spectrum of 6-d2 in benzene exhibits
Evidence that terminal alkynes can participate in protonolysis/
σ-bond metathesis pathways has been obtained by addition of
terminal alkynes to [(η5-C5Me4H)2ZrD]2(µ2,η2,η2-N2D2) (2-d4).20
Gentle warming of a benzene-d6 solution of 2-d4 to 50 °C in
the presence of an excess of HCtCCMe3 resulted in loss of
HD and formation of [(η5-C5Me4H)2Zr(CtCCMe3)]2(µ2,η2,η2-
N2D2) (4-d2) (eq 3). Importantly, no evidence for the perproteo
isotopologue, 4, was obtained. Thus, protonolysis/σ-bond me-
tathesis is favored over the 1,2-elimination/addition sequence.
Significantly, previous studies with 2 and 2-d4 have shown that
1,2-elimination of dihydrogen is facile at 50 °C.19
1
a single peak centered at 8.90 ppm, while in the H NMR
spectrum of 6-15N this resonance appears as an AA′XX′ spin
system. Simulation of the data30 (Figure S1, Supporting
3
Information) provided the following coupling constants: JH-H
:
1
2
1
9.4 Hz, JN-H: 72.6 Hz, JN-H: -3.1 Hz, and JN-N: 21.6
Hz. Infrared spectra of benzene-d6 solutions also supported the
(30) Marat, K. Spinworks; University of Manitoba: Manitoba, Canada, 2004.
9
7904 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005

Dinitrogen Functionalization
A R T I C L E S
Table 3. 1H and 15N NMR Chemical Shifts of Selected
Zirconocene Diazenido Complexes in Benzene-d6 at 22 °C
Table 4. Selected Bond Distances (Å) and Angles (°) for 6
and 7
δ
N−H
δ
15N
6
7
compound
(ppm)
(ppm)
Zr(1)-N(1)
2.085(2)
2.083(2)
2.8871(5)
2.8712(6)
2.0721(12)
2.0645(12)
2.4731(3)
2.4906(4)
[(η5-C5Me4H)2ZrH]2(µ2,η2,η2-N2H2) (2)
0.83
74.6c
78.7b
Zr(1)-N(2)
Zr(1)-X(1)a
Zr(2)-X(2)
[(η5-C5Me4H)2Zr(CtCnBu)]2(µ2,η2,η2-N2H2) (3)
[(η5-C5Me4H)2Zr(CtCtBu)]2(µ2,η2,η2-N2H2) (4)
2.65a
5.00b
4.80b
N(1)-N(2)
N(1)-H(1N)
N(2)-H(2N)
1.414(3)
0.89(3)
0.88(3)
1.4173(15)
0.90(2)
0.86(2)
86.0c
224.4d
285.9
[(η5-C5Me4H)2ZrI]2(µ2,η1,η1-N2H2) (6)
8.90
8.91
8.14
7.93
8.21
7.03
3.58e
[(η5-C5Me4H)2ZrCl]2(µ2,η1,η1-N2H2) (7)
N(1)-Zr(1)-X(1)
N(2)-Zr(2)-X(2)
N(1)-N(2)-Zr(2)
N(2)-N(1)-Zr(1)
H(1N)-N(1)-N(2)
H(2N)-N(2)-N(1)
H(1N)-N(1)-Zr(1)
H(2N)-N(2)-Zr(2)
90.61(6)
93.23(6)
132.35(16)
131.94(15)
104.1(18)
106(2)
93.27(3)
96.94(3)
[(η5-C5Me4H)2Zr(CH2Ph)]2(µ2,η1,η1-N2H2) (9)
[(η5-C5Me4H)2Zr(CH2SiMe3)]2(µ2,η1,η1-N2H2) (11)
[(η5-C5Me4H)2Zr(NMe2)]2(µ2,η1,η1-N2H2) (14)
[(η5-C5Me4H)2Zr(NHNMe2)]2(µ2,η1,η1-N2H2) (15)
147.17(9)
126.53(9)
108.0(11)
102.9(13)
125.2(11)
108.3(13)
121.9(18)
121(2)
2
a Determined from H NMR in benzene. b Spectrum recorded at 75 °C
in toluene-d8. c Spectrum recorded at -78 °C in toluene-d8. d Spectrum
recorded at 115 °C in toluene-d8. e N-H on the terminal hydrazido ligand.
a X ) I (6); Cl (7).
Figure 4. Molecular structure of 7 at 30% probability ellipsoids. Hydrogen
atoms, except for those attached to nitrogen, are omitted for clarity.
Figure 3. Molecular structure of 6 at 50% probability ellipsoids. Hydrogen
atoms, except for those attached to nitrogen, are omitted for clarity.
η2,η2 bound [N2H2]2- ligands. The two zirconocene portions
of the molecule are twisted with respect to one another, forming
a dihedral angle of 76.7° between the planes formed from the
metal and the two centroids of the cyclopentadienyl ligands.
The change in hapticity of the diazenido ligand in 6 could be a
consequence of the sterically demanding iodide ligand, inhibiting
η2,η2 coordination of the [N2H2]2- fragment. It should also be
noted that iodide could also serve as a π-donor and a combina-
tion of the effects is most likely responsible for the observed
hapticity change.
To further explore the origin of the change in coordination
mode of the diazenido ligand, synthesis of the corresponding
dichloride complex, [(η5-C5Me4H)2ZrCl]2(N2H2), was targeted.
Treatment of 2 with allyl chloride, benzyl chloride, trityl
chloride, and methyl chloride all produced no reaction, while
addition of protic reagents such as pyridinium hydrogen chloride
or triethylammonium chloride resulted in conversion to the
zirconocene dichloride, (η5-C5Me4H)2ZrCl2, with loss of N2H4
(vide infra). However, simply stirring 2 in dichloromethane at
23 °C resulted in chlorine atom abstraction and furnished [(η5-
C5Me4H)2ZrCl]2(µ2,η1,η1-N2H2) (7) in high yield (eq 4). As with
6, the crystal structure (Figure 4, Table 4) and the observation
of a downfield N-H resonance at 8.91 ppm (Table 3) establish
end-on coordination of the diazenido ligand for 7 both in the
solid state and in solution.
presence of N-H bonds, exhibiting a broad N-H stretch
centered at 3237 cm-1 which shifts appropriately to 2313 cm-1
for 6-d2.
The 15N NMR spectrum of 6-15N contains a single resonance
1
centered at 285.9 ppm. A collection of H and 15N chemical
shifts of the diazenido ligands are reported in Table 3 and
provide a convenient diagnostic tool for identification of the
hapticity of the [N2H2]2- core. In general, η1,η1-[N2H2]2- ligands
exhibit a downfield N-H resonance in the 1H NMR spectrum,
in the region of 7-9 ppm, whereas those which are side-on
coordinated appear upfield around 1 ppm. While the data are
limited, a similar trend is observed by 15N NMR spectroscopy,
where end-on diazenido ligands have downfield chemical shifts
as compared to their side-on counterparts.
Confirmation of the η1,η1 hapticity of the diazenido ligand
in 6 was provided by X-ray diffraction. Single crystals were
obtained from slow evaporation of a benzene solution of 6, and
the solid-state structure is shown in Figure 3 with selected bond
distances and angles reported in Table 4. The data were of
sufficient quality such that all of the hydrogens, including those
attached to nitrogen, were located and freely refined. Despite
the change in hapticity, the N-N bond length of 1.414(3) Å in
the diazenido ligand is comparable to the distances of 1.457(3)
and 1.454(2) Å found in 220 and 4, respectively, that contain
9
J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005 7905

A R T I C L E S
Bernskoetter et al.
Figure 6. Molecular structure of 9 at 50% probability ellipsoids. Hydrogen
atoms, except for those attached to nitrogen, are omitted for clarity.
Figure 5. Molecular structure of 8 at 30% probability ellipsoids. Hydrogen
atoms, except for those attached to nitrogen, are omitted for clarity.
Table 6. Selected Bond Distances (Å) and Angles (°) for 9
Table 5. Selected Bond Distances (Å) and Angles (°) for 8
Zr(1)-N(1)
Zr(1)-C(11)
N(1)-N(1A)
2.0975(14)
2.3700(19)
1.420(3)
Zr(1)-N(1)
Zr(1)-O(1)
N(1)-N(1A)
2.086(3)
2.213(2)
1.413(7)
N(1)-Zr(1)-C(11)
N(1A)-N(1)-Zr(1)
94.25(6)
137.62(15)
N(1)-Zr(1)-O(1)
N(1A)-N(1)-Zr(1)
97.24(11)
137.2(3)
160 °C liberates alkane and forms unidentified zirconium(IV)
products, which yield ammonium chloride upon treatment with
excess HCl.
Synthesis of a similar triflato zirconocene diazenido complex,
[(η5-C5Me4H)2ZrOTf]2(µ2,η1,η1-N2H2) (8), was accomplished
by addition of 2 equiv of methyl triflate to 2 (eq 4). Unlike 6
and 7, this complex is insoluble in common organic solvents
prohibiting characterization by NMR spectroscopy. However,
the solid-state (KBr) infrared spectrum of 8 contains a broad
N-H stretch centered at 3351 cm-1, which shifts appropriately
to 2482 cm-1 upon preparation of 8-d2. In addition, a band
centered at 1336 cm-1 for the triflato ligand was also observed.
Slow diffusion of methyl triflate into a benzene solution of
2 produced single crystals of 8 suitable for X-ray diffraction.
The molecular structure of the compound is presented in Figure
5, and selected bond distances and angles are reported in Table
5. In the crystal structure, the molecule contains an inversion
center that relates the two halves of the dimer and the transoid
triflato ligands. As with 6, an end-on diazenido ligand is
observed with a typical N-N bond length of 1.413(7) Å.
Confirmation of the diazenido ligand hapticity was provided
by X-ray diffraction. The molecular structure of 9 is presented
in Figure 6 with selected metrical parameters reported in Table
6. In the solid state, the molecule contains a center of inversion,
relating the cyclopentadienyl ligands on the two ends of the
dimer as well as the transoid benzyl groups. In agreement with
the solution 1H NMR chemical shift, the diazenido ligand in 9
is end-on coordinated with a typical N-N bond length of
1.420(3) Å.
Both 6 and 8 have proven to be useful synthons for alkyl
zirconocene diazenido complexes. These molecules were of
interest for structural comparison to the acetylide zirconocene
diazenido complexes as well as for their potential utility to form
nitrogen-carbon bonds by R-migration. Alkylation of 6 with 2
equiv of KCH2Ph in diethyl ether afforded, after filtration and
recrystallization, a red powder identified as [(η5-C5Me4H)2Zr-
(CH2Ph)]2(µ2,η1,η1-N2H2) (9) in moderate yield (eq 5). In
addition to the expected number of cyclopentadienyl ligand
Attempts to prepare [(η5-C5Me4H)2Zr(CH2SiMe3)]2(µ2,η1,η1-
N2H2) by alkylation of 6 with 2 equiv of LiCH2SiMe3 afforded
[(η5-C5Me4H)2ZrI][(η5-C5Me4H)2Zr(CH2SiMe3)](µ2,η1,η1-
N2H2) (10), the product of a single salt metathesis (eq 6). Stirring
either 6 or 10 with excess LiCH2SiMe3 did not result in
substitution of the remaining iodide ligand. Characterization of
10 was accomplished by routine spectroscopic and analytic
1
methods. One notable feature of the H NMR spectrum of 10
1
is the observation of two downfield doublets (3JN-H ) 12 Hz)
centered at 8.46 and 9.10 ppm for the hydrogens of the end-on
bound diazenido ligand. Synthesis of the desired dialkylated
product, [(η5-C5Me4H)2Zr(CH2SiMe3)]2(µ2,η1,η1-N2H2) (11),
was accomplished by addition of 2 equiv of LiCH2SiMe3 to 8
resonances, the H NMR spectrum of 9 displays a downfield
shifted N-H resonance at 8.14 ppm, consistent with end-on
coordination of the diazenido ligand. As with the acetylide
complexes, 9 is thermally robust, resisting decomposition upon
heating to 115 °C for several days. Notably, further heating to
9
7906 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005

Dinitrogen Functionalization
A R T I C L E S
1
Figure 7. (a) Variable-temperature H NMR spectra of 3 recorded in benzene-d6. (b) The N-H region of the spectra.
(eq 6). A downfield N-H resonance is observed at 7.93 ppm,
consistent with η1,η1 coordination of the [N2H2]2- ligand.
only the variable-temperature profile (Figure 7) of 3 will be
presented in detail. From 25 to 75 °C, the hexyl chain on the
acetylide ligand and three cyclopentadienyl methyl groups
appear sharp and are essentially invariant with temperature. The
other methyl group is broad at 25 °C and steadily sharpens upon
warming. Similar behavior is observed with the cyclopentadienyl
hydrogen, where it appears as a very broad resonance at 25 °C
and steadily narrows until a sharp peak is observed at 75 °C.
The most significant feature of the variable-temperature spectra
of 3 is the temperature dependence of the N-H chemical shift.
At 25 °C, this peak appears as a very broad singlet centered at
2.85 ppm, consistent with η2,η2 hapticity of the [N2H2]2- ligand.
This resonance steadily shifts downfield with increased tem-
perature, finally appearing at 4.62 ppm at 75 °C, the highest
temperature examined. The dynamic behavior of 3-5 can
therefore be rationalized by a process whereby the [N2H2]2-
ligand is interconverting between side-on and end-on hapticity.
While the diazenido ligand is dynamic, the progressive down-
field migration of the N-H resonance with increased temper-
ature demonstrates an increased population of the end-on isomer
upon warming (eq 7).
Attempts to introduce a smaller alkyl ligand such as methyl
have not been successful. The hapticity of the diazenido and
the migratory aptitude of the smaller alkyl upon thermolysis of
[(η5-C5Me4H)2Zr(CH3)]2(N2H2) were of interest. Unfortunately,
treatment of 6 or 7 with 2 equiv of MeLi or MeMgBr furnished
the zirconocene dimethyl complex, (η5-C5Me4H)2Zr(CH3)2,31
while the same reaction with 8 produced no reaction. The fate
of the diazenido ligand in the reactions with 6 and 7 is unknown.
Preparation of static zirconocene diazenido complexes such
as 6-11 with end-on [N2H2]2- ligands provided insight into
the solution dynamics of the acetylide zirconocene diazenido
complexes, 3-5. Each of the compounds was examined by
variable-temperature NMR spectroscopy and exhibited similar
1H NMR behavior as a function of temperature. As a result,
Solution 15N NMR data also support the interconversion of
the diazenido ligand as the origin of the dynamic behavior. At
(31) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999, 18, 1873.
9
J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005 7907

A R T I C L E S
Bernskoetter et al.
1
Figure 8. Variable-temperature H NMR spectra of 2 recorded in toluene-d8.
range of temperatures studied. Furthermore, the 15N NMR
spectra are essentially invariant; a singlet is observed at 74.6
ppm at -78 °C that shifts only slightly to 78.7 ppm upon
warming to 75 °C. These data demonstrate that a change in
hapticity of the [N2H2]2- on the NMR time scale is unlikely
and that the dynamic process probably arises from nitrogen
inversion in the diazenido core (eq 8). The presence of small,
σ-only ligands such as hydride shift the equilibrium to favor
the formally 18 electron, side-on isomer. Although the end-on
isomer is not sufficiently populated to be observed by NMR
spectroscopy, it has been implicated in isotopic exchange
reactions involving isotopologues of 2,19,20 suggesting that it is
kinetically accessible.
Figure 9. Hydrolysis of zirconocene dinitrogen complexes as a function
of cyclopentadienyl ligand.
-78 °C, 4-15N exhibits a single 15N resonance centered at 86.0
ppm, similar to the value of 74.6 ppm observed for 2-15N (Table
3), consistent with an η2,η2-[N2H2]2- ligand at this temperature.
Warming the benzene-d6 solution of 4-15N resulted in a gradual
downfield shifting of the 15N resonance, ultimately appearing
at 224.4 ppm at 115 °C. This value is in excellent agreement
with the chemical shift of 285.9 ppm observed for 6-15N, a
complex that contains a static η1,η1 [N2H2]2- core.
Observation of the interconversion of the hapticity of the
diazenido ligands in 3-5 prompted a similar variable-temper-
ature NMR study with 2. We previously reported20 that the 1H
NMR spectrum of 2 recorded at 22 °C was suggestive of
dynamic behavior, exhibiting broadened resonances for the
cyclopentadienyl hydrogen and two of the cyclopentadienyl
methyl groups. At this temperature, only one set of ligand
resonances is observed with the two portions of the dimer being
equivalent. Cooling a toluene-d8 solution of 2 to -70 °C lowers
the symmetry of the complex, producing a spectrum with eight
cyclopentadienyl methyl peaks, two cyclopentadienyl hydrogens,
and singlets for the N-H and Zr-H (Figure 8). Warming the
solution to 60 °C produced a single, slightly broadened
cyclopentadienyl resonance and four cyclopentadienyl methyl
peaks, although two are significantly broadened. Importantly,
both the N-H and the Zr-H resonances are essentially
unchanged, both in peak shape and in chemical shift, over the
Dinitrogen Functionalization by Treatment with Electro-
philes. The reactivity of the dinitrogen ligand in 1 was also
explored with electrophilic reagents with the goal of function-
alizing N2 by proton transfer. Transformations of this type would
complement the cycloaddition pathway observed with H2 and
terminal alkynes. We have previously reported that addition of
HCl to 1 yields hydrazine,20 in analogy to Bercaw’s seminal
N2H4 synthesis from [(η5-C5Me5)2Zr(η1-N2)]2(µ2,η1,η1-N2).10
Significantly, addition of a weaker Brønsted acid such as water
to 1 also produced N2H4 along with (η5-C5Me4H)2Zr(OH)2 (12,
Figure 9). This result contrasts the chemistry observed with [(η5-
C5Me5)2Zr(η1-N2)]2(µ2,η1,η1-N2), where treatment with H2O
furnished [(η5-C5Me5)2ZrH]2(µ2-O) with loss of free N2.32
Attempts to observe a hydroxy zirconocene diazenido inter-
mediate during the aqueous protonation of 1 have been
unsuccessful, as addition of substoichiometric amounts of H2O
resulted in only partial conversion to 12 and N2H4. Formation
of hydrazine from coordinated dinitrogen by addition of H2O
prompted investigation of other proton sources for N-H bond
assembly. It was hypothesized that similar reagents would also
allow observation of diazenido intermediates along the N2
(32) Hillhouse, G. L.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5472.
9
7908 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005

Dinitrogen Functionalization
A R T I C L E S
Figure 10. Dinitrogen functionalization by proton transfer.
Figure 11. Mechanistic comparison for protonation reactions in zirconocene dinitrogen complexes.
functionalization pathway. Careful addition of 2 equiv of
anhydrous ethanol to 1 induced partial conversion to (η5-
C5Me4H)2Zr(OEt)2 (13) along with free hydrazine. Other
unidentified zirconium compounds are also observed using this
procedure. However, clean conversion of 1 to 13 and N2H4 was
accomplished by addition of a slight excess (∼5 equiv) of
ethanol (Figure 10).
Coordination of a primary or secondary amine to an elec-
trophilic early transition metal center is known to increase the
acidity of the N-H bond and facilitate proton transfer,31
suggesting that amines and hydrazines could protonate coordi-
nated dinitrogen and allow observation of an amido or hydrazido
zirconocene diazenido complex. Given the similar pKa values
of the conjugate acids of the free amines or hydrazines and the
zirconium diazenido ligand, it was anticipated that the reaction
would afford the [N2H2]2- complex and not liberate free
hydrazine. As expected, addition of excess HNMe2 or H2NNMe2
to 1 furnished [(η5-C5Me4H)2Zr(NMe2)]2(µ2,η1,η1-N2H2) (14)
and [(η5-C5Me4H)2Zr(NHNMe2)]2(µ2,η1,η1-N2H2) (15), respec-
tively (Figure 10). In benzene-d6, sharp N-H resonances are
observed at 8.21 (14) and 7.03 ppm (15), respectively, diagnostic
of static, η1,η1-[N2H2]2- ligands. Liberation of hydrazine was
9
J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005 7909

A R T I C L E S
Bernskoetter et al.
Figure 12. Proposed mechanism for hydrazine formation from 1 by treatment with proton donors.
Concluding Remarks
accomplished by treatment of 14 or 15 with excess ethanol,
thus demonstrating the competency of diazenido intermediates
in the formal reduction of N2 to hydrazine (Figure 10).
Functionalization of the side-on bound, activated dinitrogen
ligand in [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) to afford zirconocene
diazenido complexes, [(η5-C5Me4H)2ZrX]2(µ2-N2H2), has been
accomplished by addition of terminal alkynes, dimethylamine
and N,N-dimethylhydrazine. The hapticity of the [N2H2]2- core
depends on the size of the anionic ligand in the zirconocene
wedge - smaller donors such as hydride yield diazenido
complexes that are predominantly side-on bound, whereas
intermediate size acetylide ligands afford compounds where the
interconversion between the two hapticities is facile in solution
and population of the end-on isomer becomes more significant
at higher temperatures. Larger donors such as chloride, iodide,
triflate, or alkyl produce diazenido complexes that are exclu-
sively η1,η1 coordinated in both the solid state and in solution.
It should be noted that in the case of the halides and the triflato
compounds, the influence of π-donation on the preferred
hapticity cannot be discounted. Regardless of coordination mode,
the diazenido compounds are intermediates in the reduction of
dinitrogen to hydrazine, as treatment with ethanol yields N2H4.
The protonation results observed with [(η5-C5Me4H)2Zr]2-
(µ2,η2,η2-N2) contrasts related chemistry with [(η5-C5Me5)2Zr-
(η1-N2)]2(µ2,η1,η1-N2), where the weakly activated, end-on
bound dinitrogen ligands are readily displaced as free N2 upon
treatment with water. These results establish the importance of
side-on coordination of the N2 ligand for the construction of
N-H bonds either by cycloaddition or protonolysis pathways
and further highlight the critical role cyclopentadienyl substit-
uents play in zirconocene dinitrogen chemistry.
The ability of 1 to undergo protonation to yield N2H4 with
weak acids underscores the influence of cyclopentadienyl
substituents on transformations involving coordinated dinitrogen.
The difference in reactivity between 1 and [(η5-C5Me5)2Zr(η1-
N2)]2(µ2,η1,η1-N2) can be traced to the steric environments about
the two zirconium centers. For the more sterically hindered
permethylated zirconocene, Bercaw32 proposed the mechanism
shown in Figure 11 to account for the observed loss of dinitrogen
upon addition of water. Initial substitution of a terminal N2
ligand by H2O induces scission of the dimer to yield a transient,
monomeric bis(dinitrogen) complex, (η5-C5Me5)2Zr(η1-N2)2, and
the zirconium(II) water adduct, (η5-C5Me5)2Zr(OH2). Experi-
mental support for an 18-electron bis(dinitrogen) complex has
recently been provided by our laboratory with the isolation and
crystallographic characterization of the titanium congeners, (η5-
C5Me4R)2Ti(η1-N2)2 (R ) CHMe2, CMe3, SiMe3).33 Formation
of [(η5-C5Me5)2ZrH]2(µ2-O) is then accomplished by R-elimina-
tion in the water adduct followed by trapping by the bis-
(dinitrogen) complex.
Treatment of [(η5-C5Me5)2Zr(η1-N2)]2(µ2,η1,η1-N2) with stron-
ger acids such as HCl does indeed yield hydrazine. Based on
the stoichiometry of the reaction and isotopic labeling studies,
Bercaw34 proposed the pathway shown in Figure 11 involving
protonation of both the terminal and the bridging dinitrogen
ligands. Importantly, both pathways involve the formation of
monomeric intermediates, a consequence of the steric environ-
ment imparted by the bulky pentamethylcyclopentadienyl ligand.
Experimental Section35
The η2,η2 hapticity of the N2 ligand in 1 coupled with twisting
of the dimer results in increased back-bonding and as a
consequence yields a formal [N2]4- ligand.19 In [(η5-C5Me5)2-
Zr(η1-N2)]2(µ2,η1,η1-N2), the dinitrogen ligands are not signifi-
cantly elongated from free N2, behaving more like neutral donors
than hydrazido fragments. The increased electron density of the
formal [N2]4- core in 1 renders the dinitrogen ligand sufficiently
basic to undergo protonation by weak Brønsted acids. In other
words, the formal four-electron reduction of N2 by the two
zirconocenes, made possible by side-on coordination, allows
metathetical synthesis of N2H4 in contrast to the permethyl-
zirconocene system, where hydrazine formation can be viewed
in analogy to Chatt-type N2 reduction schemes. Furthermore,
the reduced steric bulk imparted by the [C5Me4H] ligand also
allows formation of dimeric zirconocene diazenido compounds,
which are key intermediates on the protonolysis pathway (Figure
12).
Preparation of [(η5-C5Me4H)2Zr(CtCtBu)]2(µ2,η2,η2-N2H2) (4).
A 25 mL round-bottomed flask was charged with 0.080 g (0.115 mmol)
of [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) (1) and approximately 10 mL of
pentane. The dark green solution was frozen at -196 °C, and 21 Torr
(0.23 mmol) of tert-butyl acetylene was admitted via a 100.1 mL
calibrated gas bulb on a high vacuum line. The resulting dark red
solution was stirred at ambient temperature for 2 h. After this time,
the solvent was removed in vacuo leaving a white powder. Recrystal-
lization from diethyl ether layered with pentane at -35 °C afforded
0.085 g (86%) of 4 as colorless crystals. Anal. Calcd for C48H72Zr2N2:
1
C, 67.07; H, 8.44; N, 3.26. Found: C, 66.93; H, 8.72; N, 2.94. H
NMR (benzene-d6, 20 °C): δ ) 1.41 (s, 18H, CMe3), 1.69 (br s, 16H,
C5Me4H), 2.09 (br s, 16H, C5Me4H), 2.18 (br s, 16H, C5Me4H), 5.42
(br s, 4H, C5Me4H). 1H NMR (benzene-d6, 80 °C): δ ) 1.37 (s, 18H,
CMe3), 1.85 (s, 12H, C5Me4H), 2.08 (s, 12H, C5Me4H), 2.10 (s, 12H,
C5Me4H), 2.18 (s, 12H, C5Me4H), 4.80 (br s, 2H, N-H), 5.42 (br s, 4H,
C5Me4H). 13C{1H} NMR (benzene-d6, 23 °C): δ ) 12.65, 13.90, 29.26
(C5Me4H), 32.93 (CMe3), 123.16, 124.08 (Cp). 15N NMR (toluene-d8,
-78 °C): δ ) 86.0. 15N NMR (toluene-d8, 115 °C): δ ) 224.4. IR
(KBr): νN-H ) 3297 cm-1, νC≡C ) 2079 cm-1
.
(33) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126,
14688.
(34) Manriquez, J. M.; Sanner, R. D.; Marsh, R. E.; Bercaw, J. E. J. Am. Chem.
Soc. 1976, 98, 3042.
(35) General considerations and additional experimental procedures are contained
in the Supporting Information.
9
7910 J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005

Dinitrogen Functionalization
A R T I C L E S
Preparation of [(η5-C5Me4H)2ZrI]2(µ2,η1,η1-N2H2) (6). A thick
walled reaction vessel was charged with 0.050 g (0.072 mmol) of [(η5-
C5Me4H)2ZrH]2(µ2,η2,η2-N2H2) (2) and approximately 10 mL of toluene.
On the vacuum line, the vessel was chilled to -196 °C and evacuated.
At this temperature, 70 Torr (0.36 mmol) of CH3I was admitted to the
vessel via a 100.1 mL calibrated gas bulb. The resulting reaction mixture
was warmed to ambient temperature and stirred for 2 h forming a deep
purple solution. The solvent was removed in vacuo, yielding a dark
purple oil which was washed several times with pentane to afford 0.068
g (95%) of a dark purple solid identified as 6. Single crystals suitable
for X-ray diffraction were obtained from a concentrated benzene
solution. Anal. Calcd for C36H54I2N2Zr2: C, 45.46; H, 5.72; N, 2.95.
dark red. After this time, the reaction mixture was filtered through Celite
and the solvent removed in vacuo and washed with pentane and cold
diethyl ether to afford 0.107 g (66%) of a red powder identified as 9.
Crystals suitable for X-ray diffraction were obtained from a concentrated
benzene solution. Anal. Calcd for C50H68N2Zr2: C, 68.28; H, 7.79; N,
1
3.18. Found: C, 68.16; H, 7.64; N, 2.78. H NMR (benzene-d6, 20
°C): δ ) 1.83 (s, 12H, C5Me4H), 1.84 (s, 12H, C5Me4H), 1.88 (s, 12H,
C5Me4H), 1.90 (s, 12H, C5Me4H), 5.31 (s, 4H, C5Me4H), 6.94 (t, 4H,
8 Hz, m-Ar), 7.21 (d, 4H, 5 Hz, o-Ar), 7.30 (t, 4H, 2 Hz, p-Ar), 8.14
(s, 2H, N-H). 13C{1H} NMR (benzene-d6, 20 °C): δ ) 12.21, 12.44,
13.30, 13.72 (C5Me4H), 51.75 (CH2Ar), 109.68, 116.85, 120.11, 121.46
(Cp), 116.06, 121.23, 155.11 (Ar). IR (KBr): νN-H ) 3344 cm-1
.
1
Found: C, 44.97; H, 5.88; N, 2.65. H NMR (benzene-d6): δ ) 1.81
Acknowledgment. We thank the National Science Foundation
(CAREER Award to P.J.C.), Research Corporation (Cottrell
Scholarship), and the Packard Foundation for financial support.
Cambridge Isotope Laboratories is also acknowledged for a
generous gift of 15N2.
(s, 12H, C5Me4H), 1.99 (s, 12H, C5Me4H), 2.13 (s, 12H, C5Me4H), 2.21
(s, 12H, C5Me4H), 5.51 (s, 4H, C5Me4H), 8.90 (s, 2H, N-H). 13C{1H}
NMR (benzene-d6): δ ) 12.87, 13.66, 14.78, 15.65 (C5Me4H), 109.92,
116.45, 118.94, 122.67, 124.19 (Cp). 15N NMR (benzene-d6): δ )
285.9. IR (KBr): νN-H ) 3327 cm-1. IR (benzene-d6): νN-H ) 3237
cm-1. IR (benzene-d6): νN-D ) 2312 cm-1
.
Preparation of [(η5-C5Me4H)2Zr(CH2Ph)]2(µ2,η1,η1-N2H2) (9). A
20 mL scintillation vial was charged with 0.175 g (0.184 mmol) of
[(η5-C5Me4H)2ZrI]2(µ2,η1,η1-N2H2) (6) and approximately 15 mL of
diethyl ether. The resulting dark purple solution was chilled to -32
°C and treated with 0.052 g (0.405 mmol) of KCH2Ph. The reaction
mixture was stirred at ambient temperature for 36 h, slowing turning
Supporting Information Available: Additional experimental
procedures, the spectral simulation of 6-15N, and crystallographic
data for 4, 6, 7, 8, and 9. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA050387B
9
J. AM. CHEM. SOC. VOL. 127, NO. 21, 2005 7911