N2 hydrogenation from activated end-on bis(indenyl) zirconium dinitrogen complexes

Article abstract of DOI:10.1021/ja8048332

Sodium amalgam reduction of the bis(indenyl)zirconium dihalide complexes, (η⁵-C₉H₅-1-ⁱPr-3-Me)₂ZrX₂ (X = Cl, Br, I), yielded the corresponding end-on dinitrogen complexes, [(η⁵-C₉H₅-1-ⁱPr-3-Me)₂Zr(NaX)]₂(μ₂, η¹, η¹-N₂), with inclusion of 1 equiv of salt per zirconocene. The solid state structures of the chloro and iodo congeners establish short Zr=N and elongated N-N bonds, consistent with modest to strong activation of the coordinated dinitrogen molecule. Exposure of the N₂ compounds to 1 atm of dihydrogenresulted in rapid N-H bond formation to yield a hydrido zirconocene hydrazido compound concomitant with salt elimination. These studies establish a new structural type of zirconocene dinitrogen complex and demonstrate that side-on coordination of the N₂ ligand in the ground state is not a prerequisite for dinitrogen hydrogenation. Copyright

Full text of DOI:10.1021/ja8048332

Published on Web 10/01/2008
N2 Hydrogenation from Activated End-On Bis(indenyl) Zirconium Dinitrogen
Complexes
Doris Pun, Christopher A. Bradley, Emil Lobkovsky, Ivan Keresztes, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received June 24, 2008; E-mail: pc92@cornell.edu
The synthesis of N-H bonds from molecular hydrogen and
atmospheric nitrogen (N2) is a challenge that has confronted
chemists for the past century.1 The Haber-Bosch ammonia
synthesis has revolutionized global food production,2 and the
mechanism of the industrially implemented Fe-Ru catalysts has
been the subject of intense investigation.3 In contrast, the hydro-
genation of coordinated N2 with soluble transition metal complexes
Figure 1. Synthesis of 1 and [1-NaX]2N2 compounds.
has only been known for a little over a decade.4,5 This chemistry
is dominated by “side-on” bound Zr and Hf complexes where the
mixed Cp*, indenyl dinitrogen compounds with weakly activated
µ2, η2, η2-N2 hapticity serves to exclude additional π-acid coordina-
end-on or side-on, end-on N2 ligands.9 In these cases, the weak
tion, increase metal-ligand backdonation, and generate strongly
activation is a consequence of η2 or η4 benzo ring coordination.
For [1-NaCl]2N2, the ZrdN bond lengths of 1.878(3) Å are only
activated [N2]4- ligands. Subsequent computational investigations
have posited that µ2,η2,η2-N2 (side-on) coordination is a “necessary
slightly longer than those reported for crystallographically characterized
condition” for successful dinitrogen hydrogenation6 despite the well-
zirconium imido10 compounds, such as (η5-C5H5)2Zr(NtBu)THF (dZrdN
established precedent for 1,2-addition to terminal group 4 imido
) 1.826(4) Å).11 The N(1)sN(1A) distance of 1.352(5) Å approaches
compounds.7 Here, we report the synthesis and characterization of
the value 1.377(3) Å found in [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2).4c The
bis(indenyl)zirconium dinitrogen compounds with strongly activated
increased reduction of the bridging N2 ligand is likely a result of
µ2,η1,η1 N2 (end-on) ligands that are readily hydrogenated.
the coordination of the benzo rings to the Na rather than the Zr,
Our laboratory has been exploring the chemistry of reduced
allowing increased metal-ligand backbonding.
The observation of NaCl inclusion with [1-NaCl]2N2 prompted
reduction of the bis(indenyl)zirconium dibromide (1-Br2) and
indenyl zirconium derivatives.8 For compounds bearing relatively
i
large 1,3-ring substituents (e.g., SiMe3 or Pr), monomeric bis-
(indenyl)zirconium sandwich complexes with η9,η5-indenyl ligands
were isolated.8 Replacing one of the isopropyl groups with a methyl
substituent in mixed pentamethylcyclopentadienyl (Cp*), indenyl
compounds reduced the steric protection of the Zr, allowed
dimerization, and resulted in isolation of weakly activated end-on
and side-on, end-on dinitrogen complexes that are subject to N2
loss.9 Inspired by these observations, the chemistry of the homo-
leptic bis(indenyl)zirconium complexes was explored.
diiodide (1-I2) compounds. Using a similar synthetic procedure,
two additional dinitrogen compounds, [1-NaBr]2N2 and [1-NaI]2N2,
were isolated (Figure 1). Crystallographic characterization of
[1-NaI]2N2 established a similar core structure to the chloride
congener. Both the ZrdN and NsN distances of 1.901(2) and
1.316(5) Å are consistent with a modestly activated, end-on
dinitrogen compound. These values are slightly less pronounced
than those found in [1-NaCl]2N2, demonstrating the coordinated
halide attenuates the reduction potential of the metal center. The
different radii of the respective halogens may also be important.
Unlike the chloride compound, [1-NaI]2N2 is dimeric and does not
exhibit interactions with neighboring dimers. As presented in the
SI, the metrical parameters of the indenyl rings establish no
reduction unlike in sandwich 1 and other indenyl dinitrogen
compounds with η2 or η4 benzo interactions.8,9
Sodium amalgam reduction of a pentane or toluene solution of
a rac/meso-mixture of (η5-C9H5-1-iPr-3-Me)2ZrCl2 (1-Cl2) under
Ar or vacuum resulted in isolation of the bis(indenyl)zirconium
sandwich compound, 1, as an equimolar mixture of diastereomers
1
(Figure 1). Dark burgundy 1 exhibited H and 13C NMR spectro-
scopic features diagnostic of η9 indenyl hapticity.8 The sandwich
structure was also confirmed by X-ray diffraction (Figure 2).
Performing the Na(Hg) reduction of 1-Cl2 under N2 produced a
different outcome. Recrystallization of the red product from pentane
furnished a crystalline solid identified as the bis(indenyl)zirconium
end-on dinitrogen compound, [1-NaCl]2N2, with inclusion of 1
equiv of NaCl per Zr (Figure 1). Only the meso diastereomer of
the metallocenes was observed in the solid state; similar selectivity
was also observed in the crystal structure of 1. Reactivity studies
establish the presence of rac zirconocene subunits in bulk samples
of the compound (Vide infra).
1
The H NMR spectra of the three activated end-on dinitrogen
compounds are broad and relatively uninformative, likely due to
salt inclusion. EPR spectroscopy and magnetic susceptibility
measurements establish diamagnetic molecules free of paramagnetic
impurities; variable temperature NMR experiments produced no
gross changes.
N2 coordination in bulk samples was confirmed by combustion
analysis and reactivity studies. Treatment of [1-NaCl]2N2 with an
excess of either PbCl2 or I2 furnished the zirconocene dihalide (1-
Cl2 or 1-I2) along with 91 and 95% of the expected N2 gas (Toepler
pump), respectively. Likewise, protonation of [1-NaCl]2N2 with
excess H2O or HCl yielded 96 and 45% of the expected hydrazine
and no NH3. Reduction of N2 to hydrazine with weak Brønsted
acids is typically reserved for strongly activated N2 compounds.12,13
The overall solid state structure of [1-NaCl]2N2 (Figure 2)
consists of four zirconocenes, comprised of two dimers bridged by
interactions between Na atoms with neighboring chlorides (see
Supporting Information (SI) for additional structural details). The
metrical parameters of [1-NaCl]2N2 establish a more strongly
activated N2 ligand than those found in the previously reported
9
14046 J. AM. CHEM. SOC. 2008, 130, 14046–14047
10.1021/ja8048332 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Molecular structures of 1 (left), [1-NaCl]2N2 (middle), and [1-NaI]2N2 (right) at 30% probability ellipsoids.
With a new class of activated end-on zirconocene dinitrogen
compounds in hand, N2 hydrogenation was explored. Exposure of
a benzene-d6 solution of [1-NaCl]2N2, [1-NaBr]2N2, or [1-NaI]2N2
to 1 atm of H2 yielded the hydrido zirconocene hydrazido compound
[1-H]2(N2H2), concomitant with salt elimination (eq 1). Thus, side-
on coordination, at least in the ground state, is not a necessary
condition for N2 hydrogenation.
upon hydrolysis. For the other products, it is likely that both Na
ions are sequestered, as no N2H4 was formed upon hydrolysis,
suggesting weak N2 activation.
In summary, a new class of activated end-on bis(indenyl)zirco-
nium dinitrogen complexes has been synthesized and structurally
characterized. Interactions between the included Na ions and the
indenyl benzo substituents prevent ring coordination to Zr, increas-
ing metal-N2 backbonding. The successful hydrogenation of these
compounds demonstrates that end-on coordination in the ground
state is a viable pathway for the 1,2-addition of 2 equiv of H2.
These observations raise the possibility of η2,η2 to η1,η1 isomer-
ization as a potential mechanism in the hydrogenation of side-on
zirconocene dinitrogen compounds.5
Acknowledgment. We thank the Director, Office of Basic
Energy Sciences, Chemical Sciences Division, of the U.S. Depart-
ment of Energy (DE-FG02-05ER15659) and the Frasch Foundation
administered by the American Chemical Society for financial
support.
For each compound, the hydrogenation reaction was accompanied
by variable amounts of decomposition, as free indenyl ligand was
observed by H NMR spectroscopy. Pure [1-H]2(N2H2) was best
1
obtained from the hydrogenation of the iodo derivative and the free
indenyl ligand removed by washing with copious amounts of cold
pentane. NMR and IR spectroscopy, combustion analysis (no N2
loss), and reactivity studies confirmed N-H bond formation. For
example, a N-H stretch was observed at 3238 cm-1 in the solid
state IR spectrum of [1-H]2(N2H2). This band shifts appropriately
to 2403 cm-1 upon treatment with D2 to yield [1-D]2(N2D2).
The presence of both rac and meso diastereomers coupled with
Zr-H and N-H bond formation produces additional stereochem-
istry and yields at least 10 isomers of [1-H]2(N2H2).14 As a result,
the NMR spectrum of [1-H]2(N2H2) is exceedingly complex. Both
1H-15N HSQC and HMBC experiments were used to assign the
N-H, Zr-H, and 15N resonances and identified formation of 10
isomers (see SI). The observation of upfield shifted N-H peaks in
the vicinity of -0.81 to -1.44 ppm coupled with the observation
of 15N shifts from ∼92 to 98 ppm signals the presence of η2,η2-
N2H2 ligands; no downfield resonances diagnostic of end-on
haptomers were detected.12 Based on two bond coupling, terminal
Zr-H peaks were assigned between 4.52 and 4.81 ppm.
The crown ethers 18-crown-6 and 15-crown-5 and excess THF
were added to benzene-d6 solutions of [1-NaCl]2N2 in an attempt
to sequester the Na ions. In all cases, mixtures of both soluble and
insoluble products were observed by 1H NMR spectroscopy. In
the case of 18-crown-6, one insoluble product {Na[1-Cl]2N2}Na(18-
c-6)1.5 was characterized by X-ray diffraction and contains a
modestly activated N2 ligand (see SI). Importantly only one Na
ion was removed from the coordination sphere of the Zr. Exposure
of these mixtures to an H2 atmosphere produced no N-H bond
formation. The lack of hydrogenation of {Na[1-Cl]2N2}Na(18-c-
6)1.5 is likely a result of its poor solubility, as N2H4 was observed
Supporting Information Available: Complete experimental pro-
cedures, selected NMR spectra and crystallographic data for 1,
[1-NaCl]2N2, [1-NaI]2N2, and {Na[1-Cl]2N2}Na(18-c-6)1.5 (cif). This
material is available free of charge via the Internet at http://pubs.acs.org.
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