Synthesis of a base-free hafnium nitride from N2 cleavage: A versatile platform for dinitrogen functionalization

Article abstract of DOI:10.1021/ja405477m

The synthesis and characterization of a metastable, base-free isocyanato dihafnocene μ-nitrido complex from CO-induced dinitrogen cleavage is described. The open coordination site at hafnium suggested the possibility of functionalization of the nitrogen atom by cycloaddition and insertion chemistry. Addition of the strained, activated alkyne, cyclooctyne, resulted in N-C bond formation by cycloaddition. The alkyne product is kinetically unstable engaging the terminal hafnocene isocyanate and promoting deoxygenation and additional N-C bond formation resulting in a substituted cyanamide ligand. Group transfer between hafnium centers was observed upon treatment with Me₃SiCl resulting in bridging carbodiimidyl ligands. Amidinato-type ligands, [NC(R)N]^3- were prepared by addition of either cyclohexyl or isobutyronitrile to the base free dihafnocene μ-nitrido complex, which also engages in additional N-C bond formation with the terminal isocyanate to form bridging ureate-type ligands. Heterocummulenes also proved reactive as exposure of the nitride complex to CO₂ resulted in deoxygenation and N-C bond formation to form isocyanate ligands. With substituted isocyanates, cycloaddition to the dihafnocene μ-nitrido was observed forming ureate ligands, which upon thermolysis isomerize to bridging carbodiimides. Taken together, these results establish the base free dihafnocene μ-nitrido as a versatile platform to synthesize organic molecules from N₂ and carbon monoxide.

Full text of DOI:10.1021/ja405477m

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Article
Synthesis of a Base-Free Hafnium Nitride from N2 Cleavage:
A Versatile Platform for Dinitrogen Functionalization.
Scott P. Semproni, and Paul James Chirik
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja405477m • Publication Date (Web): 05 Jul 2013
Downloaded from http://pubs.acs.org on July 16, 2013
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Synthesis of a Base-Free Hafnium Nitride from N2 Cleavage: A
Versatile Platform for Dinitrogen Functionalization.
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Scott P. Semproni and Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey, 08544
pchirik@princeton.edu
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Abstract
The synthesis and characterization of a metastable, base-free isocyanato dihafnocene µ-
nitrido complex from CO-induced dinitrogen cleavage is described. The open
coordination site at hafnium suggested the possibility of functionalization of the nitrogen
atom by cycloaddition and insertion chemistry. Addition of the strained, activated alkyne,
cyclooctyne, resulted in N-C bond formation by cycloaddition. The alkyne product is
kinetically unstable engaging the terminal hafnocene isocyanate and promoting
deoxygenation and additional N-C bond formation resulting in a substituted cyanamide
ligand. Group transfer between hafnium centers was observed upon treatment with
Me3SiCl resulting in bridging carbodiimidyl ligands. Amidinato-type ligands, [NC(R)N]3-
were prepared by addition of either cyclohexyl or isobutyronitrile to the base free
dihafnocene µ-nitrido complex, which also engages in additional N-C bond formation
with the terminal isocyanate to form bridging ureate-type ligands. Heterocummulenes
also proved reactive as exposure of the nitride complex to CO2 resulted in deoxygenation
and N-C bond formation to form isocyanate ligands. With substituted isocyanates,
cycloaddition to the dihafnocene µ-nitrido was observed forming ureate ligands, which
upon thermolysis isomerize to bridging carbodiimides. Taken together, these results
establish the base free dihafnocene µ-nitrido as a versatile platform to synthesize organic
molecules from N2 and carbon monoxide.
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Introduction
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The functionalization and cleavage of molecular nitrogen is a long-standing
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challenge in synthesis.1,23,4 While the Haber-Bosch process5,6 and the nitrogenase family
of enzymes7 , 8 , 9 provide industrial and biological routes to ammonia,10 respectively,
catalytic methods for the synthesis of more complex organic nitrogen compounds directly
from N2 are less common.11 Methods for the catalytic silylation of dinitrogen have been
reported using titanium,12 chromium,13 molybdenum,14 tungsten14 and most recently iron
precursors.15 In the titanium examples, the method was adapted for the synthesis of
heterocycles16 and natural products including monomorine I, pumiliotoxin C, and
lycopodine.17
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Despite these advances in N-H and N-Si bond formation, methods for nitrogen-
carbon bond formation, particularly coupled to N2 cleavage, have proven to be a more
formidable challenge. Selected examples of alkylation18 and acylation19,20 of N2-derived
metal nitrides are known but remain rare. Ligand-induced N2 cleavage, whereby an
incoming ligand is responsible for both delivering reducing equivalents and serving as a
reagent for forming new bonds to nitrogen, has emerged as promising strategy for N-C
bond formation. Pioneering work by Sobota and coworkers reported the synthesis of
titanium isocyanates following the carbonylation of N2 in the presence of TiCl4-
magnesium mixtures. 21 More recently, Fryzuk and coworkers have extended this
approach to tantalum dinitrogen chemistry and established that hydride sources such as
alanes,22 silanes23 and boranes24,25 are effective ligands for cleavage of the side-on, end-
on coordinated N2 ligand. The resulting putative µ-nitrido ditantalum compound has
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proven quite reactive, often engaging in P-N bond forming chemistry with the supporting
amide-phosphine ligand. 26
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Our laboratory has applied this concept to strongly activated, side-on bound
dinitrogen complexes of zirconium and hafnium.27,28 Carbonylation of the ansa-hafnocene
dinitrogen compound, [Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)Hf]2(µ2,η2,η2-N2) resulted in
rapid cleavage of the N-N bond with concomitant formation of two N-C bonds and one
C-C bond to generate a unique example of a bridging oxamidide ligand. 29 This
transformation has proven general among zirconium and hafnium complexes with
strongly activated dinitrogen ligands30,31 and has also provided a versatile platform for the
synthesis of various N-containing organic molecules including substituted oxamides32 and
formamides.33,34 A combination of pressure studies, kinetic analysis, isotopic labeling35
and computational efforts 36 , 37 support the intermediacy of a bimetallic µ-nitrido
intermediate following initial carbonylation and N-N cleavage (Scheme 1).
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Scheme 1. CO-induced N2 cleavage in an ansa-hafnocene complex that proceeds through
an unobserved µ-nitrido intermediate.
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O
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N
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Hf
N
Hf
Me2Si
SiMe2
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H
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O
O
C
C
N
N
N
N
H2
CO
Hf
Hf
Me2Si
Hf
Hf
N
Hf
Hf
N
SiMe2
Me2Si
SiMe2
Me2Si
SiMe2
H
H
Not Observed
xs CO
O
N
C
C
Hf
Hf
Me2Si
SiMe2
N
O
+ Isomer
Because C-H activation of a tert-butyl cyclopentadienyl substituent in the ansa
compound prohibits observation of the dihafnocene µ-nitrido intermediate, a less
sterically protected hafnium N2 complex was studied for dinitrogen carbonylation.
Addition of approximately one equivalent of CO to [(η5-C5H2-1,2,4-Me3)2Hf]2(µ2, η2, η2-
N2) in the presence of substituted pyridines such as 4-(dimethylamino)pyridine (DMAP)
resulted in isolation and crystallographic characterization of the base-stabilized nitride
complex, [(η5-C5H2-1,2,4-Me3)2Hf]2(µ2-N)(NCO)(DMAP) (Scheme 2).38 This compound,
prepared from N2 cleavage, is a rare example of a molecular hafnium nitride39 and serves
as a competent intermediate for subsequent carbonylation chemistry to form [(η5-C5H2-
1,2,4-Me3)2Hf]2(μ2-O)(NCO)(CN). Thus, isocyanate and cyanide ligands can readily be
accessed from CO-induced N2 bond cleavage through a bridging nitride intermediate
(Scheme 2).
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Scheme 2. Synthesis of a pyridine-stabilized µ-nitrido dihafnocene complex prepared
from N2 cleavage.
O
O
C
C
N
X
N
N
N
N
1-4 atm CO
18 hours
Hf
Hf
O
Hf
N
Hf
Hf
Hf
1-4 atm CO
C
N
seconds
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-
N
X
N
X = OMe, NMe2
X
alkynes
nitriles
CO2
No Reaction
No Reaction
No Reaction
Analysis of the bonding in [(η5-C5H2-1,2,4-Me3)2Hf]2(μ2-N)(NCO)(DMAP) in
conjunction with computational predictions36 and experimental observations in ansa-
hafnocene dinitrogen chemistry, suggested that hafnocene nitrides prepared from N2
cleavage should exhibit a rich reaction chemistry. Transformations such as
hydrogenation, C-H activation, alkylation, arylation and cycloaddition are key and
necessary components of synthetic and ideally catalytic schemes for the functionalization
of N2. Unfortunately, the base stabilized compound, [(η5-C5H2-1,2,4-Me3)2Hf]2(μ2-
N)(NCO)(DMAP), has proven unreactive in many of these processes likely a result of the
strong coordination of the substituted pyridine ligand which inhibits access to the
hafnocene nitrido core (Scheme 2). As a result, the corresponding base free dihafnocene
µ-nitrido complex was targeted with the goal of increasing the reactivity of the nitrogen
atom relevant to N2 functionalization. Here we describe the successful synthesis of such a
compound and its diverse N-C bond forming chemistry derived from cycloaddition of
activated π-systems of an alkyne and heterocummulenes including CO2 as well as
insertion of organic nitriles. In several of these processes, the N-C bond formation at the
hafnium-nitride triggers a cascade reaction involving the terminal isocyanate, opening
new avenues for functionalization of N-atoms derived from molecular nitrogen. This
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unique reactivity is likely derived from the appropriate cyclopentadienyl profile, which
allows cooperativity between the hafnocene subunits and enables formation of bridging
ligands.
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Results and Discussion
Synthesis of a Base-Free Hafnocene Nitride from N2 Cleavage. The synthesis of target
base-free isocyanato dihafnocene µ-nitrido complex, [(η5-C5H2-1,2,4-Me3)2Hf]2(µ2-
N)(NCO) (1) was accomplished by exposure of a frozen benzene-d6 solution of [(η5-
C5H2-1,2,4-Me3)2Hf]2(µ2, η2, η2-N2) to one equivalent of carbon monoxide followed by
warming to ambient temperature (eq 1).
O
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N
N
CO
Hf
(1)
Hf
N
Hf
Hf
seconds
N
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Compound 1 proved to be metastable in benzene-d6 solution decomposing to an
intractable mixture of organometallic products over the course of hours (t1/2 ~ 30 minutes
at 23 ºC). Characterization by NMR spectroscopy was conducted immediately following
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generation of the compound. The benzene-d6 H NMR spectrum of 1 at 23 ºC exhibits
four broad resonances assigned to the cyclopentadienyl hydrogens and six peaks for the
ring methyl groups, consistent with a Cs symmetric compound. The observed broadness
in the signals at 23 ºC likely arises from restricted cyclopentadienyl ring rotation. Similar
behavior has been observed with bimetallic complexes with this cyclopentadienyl ligand
array.34 Cooling a toluene-d8 solution of 1 to -78 ºC resulted in decoalescence of the
signals but baseline resolution was not achieved.
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Preparation of the C, N isotopologue, 1-13C, N was achieved using readily
available 13CO and 15N2 gas. The benzene-d6 13C NMR spectrum exhibits a broad doublet
(1JCN = 35.4 Hz) centered at 131.0 ppm, diagnostic for a terminal isocyanate ligand.29-35
The 15N NMR spectrum contains two peaks, a doublet centered at 98.2 ppm for the
terminal isocyanate and a singlet at 589.9 ppm for the µ-nitrido. These values are similar
to the previously reported pyridine stabilized complex, [(η5-C5H2-1,2,4-
Me3)2Hf]2(NCO)(µ2-N)(4-OMe-pyridine) (2).38 Formation of a terminal isocyanate ligand
was also confirmed by solid-state (KBr) infrared spectroscopy, which features an intense
band at 2221 cm-1. Additional evidence for synthesis of the base free nitride was obtained
from the clean and quantitative generation of 2 from treatment of a benzene-d6 solution of
1 with 4-methoxypyridine. Likewise, exposure of a benzene-d6 solution of 1 to 1-4 atm of
CO furnished the µ-oxo cyano hafnocene isocyanate described previously.31
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The bonding in the base-free isocyanato dihafnocene µ-nitrido was analyzed with
the assistance of full molecule DFT calculations. The coordinates for the
crystallographically characterized, pyridine-stabilized derivative, 238 served as the
starting point following deletion of the heterocycle. Attempted geometry optimization
resulted in C-H activation of a cyclopentadienyl methyl group in silico, reactivity
previously observed with an ansa-hafnocene variant.29 As a result, single point
calculations were conducted using the geometry observed in the crystal structure of the
pyridine-stabilized complex.
The DFT computed HOMO-1, HOMO and LUMO for 1 are presented in Figure
1. The HOMO-1 is principally comprised of a nitrogen p-orbital in the metallocene
wedge interacting weakly with a hafnocene 1a1 orbital. Analogous to 2, the HOMO
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consists of an orthogonal nitrogen p-orbital perpendicular to the hafnocene wedge.
Accordingly this orbital has little interaction with either hafnium. Unlike the pyridine
stabilized complex 2, where the LUMO is centered on the π-system of the aryl ring, the
LUMO of the base-free compound is a hafnium 1a1 orbital with little to no ligand
contribution. The availability of this orbital to interact with incoming ligands accounts for
the vastly expanded reactivity of the base-free complex in further N-atom
functionalization.
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Figure 1. DFT computed HOMO-1, HOMO and LUMO for 1.
Cycloaddition of Cyclooctyne with 1. The isolation of a base free isocyanate
dihafnocene µ-nitrido complex presents new opportunities for N2 functionalization
following cleavage of the N≡N bond. In one limit, the nitrogen atom in 1 can be viewed
as an imido and may offer functionalization reactivity by cycloaddition. Imido character
in zirconocene and hafnocene complexes bearing strongly activated dinitrogen ligands
has also proven useful for preparing N-C bonds by cycloaddition of various
heterocummulenes to coordinated N2.40,41,42 Despite the synthesis of numerous zirconium
and hafnium complexes with strongly activated N2 ligands, dinitrogen functionalization
and N-C bond formation by alkyne cycloaddition remains rare. Typically, N2 dissociation
and metallocycle formation are observed.43 One notable exception was reported by
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Fryzuk,44 who described addition of terminal, aryl-substituted alkynes to a bis(phosphine)
diamido-ligated zirconium dinitrogen complex. CO-induced N2 cleavage offers an
alternative and promising strategy for N-C bond formation by cycloaddition as dinitrogen
dissociation pathways are eliminated.
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Unactivated olefins and alkynes such as ethylene, butadiene and 2-butyne proved
unreactive toward 1. However, addition of the strained and activated alkyne, cyclooctyne
to a benzene-d6 solution of 1 furnished a new Cs symmetric dihafnocene product, [(η5-
C5H2-1,2,4-Me3)2Hf]2(NCO)(µ2,κ2-N-(cyclooctenyl)) (3) in 59% yield following
1
recrystallization (Scheme 3). The benzene-d6 H NMR spectrum of 3 exhibits diagnostic
pseudo triplets at 2.36 and 3.03 ppm for the allylic methylene hydrogens from the
azametallocycle, consistent with N-C bond formation from alkyne cycloaddition.
Additional evidence for formation of an azametallocycle was provided by 15N NMR
spectroscopy. The labeled isotopologue, 3-15N/13C exhibits a doublet (1JCN = 31.9 Hz)
centered at 87.9 ppm for the terminal isocyanate and a singlet, centered at 286.6 ppm
from the µ2,κ2-cyclooctenyl nitrogen (Figure 2). Formation of a terminal isocyanate
ligand was also confirmed by the 13C NMR spectrum of 3-15N/13C, which exhibited a
doublet centered at 135.9 ppm.
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Scheme 3. Cycloaddition of cyclooctyne to 1 and subsequent rearrangement chemistry.
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O
O
C
C
N
N
O
65 ºC
8
+
Hf
N
Hf
Hf
Hf
Hf
N
Hf
9
N
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C
N
1
3
4
Via:
O
1.5 Me3SiCl
O
C
N
C
N
N
Hf
Hf
Hf
N
Hf
N
C
C
N
Hf
1/2
Hf
N
N
4
5
+
OSiMe3
Cl
Hf
Figure 2. Benzene-d6 15N NMR spectrum of 3-15N/13C at 298 K.
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Attempts to obtain single crystals of 3 proved challenging as toluene solutions of
proved thermally unstable even at -35 ºC, converting to a new, Cs symmetric dihafnocene
compound, [(η5-C5H2-1,2,4-Me3)2Hf]2(µ2-O)(µ2,κ2-(cyclooctenyl)N-C=N) (4), over the
course of days. Complete conversion of 3 to 4 was also achieved in two hours upon
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heating to 65 ºC. The H NMR spectrum of 4 signals Cs symmetry and importantly,
contains all the resonances expected for an intact cyclooctenyl fragment. Notably, the 15N
NMR spectrum of 4-15N/13C revealed modification of the hafnium terminal isocyanate,
indicating that additional N-C bond forming chemistry had occurred. A new doublet with
C-N coupling (1JCN = 13.5 Hz) was observed centered at 208.1 ppm along with a second
broad resonance centered at 149.8 ppm. These spectral features are consistent with
deoxygenation of the isocyanate ligand and formation of a new N-cyanamide-type moiety
(Scheme 3). Accordingly, the 13C NMR spectrum of 4-15N/13C exhibited a broad (Δν1/2
=
79 Hz) resonance at 129.0 ppm while the solid state (KBr) infrared spectrum contained a
broad, intense band at 2094 cm-1, assigned to a stretch of a newly formed N-cyanamide
ligand.
Single crystals of 4 for suitable for X-ray diffraction were obtained from a
toluene/pentane mixture at -35 °C and confirmed isocyanate deoxygenation and
additional N-C bond formation (Figure 3). The core of the molecule is a six-membered
ring containing two metal centers, a bridging oxide and the C=C-N portion of the newly
formed N-cyanamide ligand. The C(34)-C(35) distance of 1.322(9) Å is consistent with
retention of double bond character in the cyclooctenyl fragment while the C(34)-N(2)
distance of 1.489(8) Å is indicative of a carbon-nitrogen single bond. The C(33)-N(3)
bond length of 1.152(8) Å is consistent with carbon-nitrogen triple bond character while
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the C(33)-N(2) bond length of 1.313(8) Å is indicative of partial double bond character,
likely a result of a delocalized π-system that extends over the cyanamide and
cyclooctenyl fragments. The thermal instability of 3 is notable as deoxygenation of a
terminal isocyanate ligand triggers a second N-C bond forming reaction and demonstrates
that terminal hafnocene isocyanates, prepared from N2 and CO, may also serve as
building blocks for new nitrogen-containing organic ligands.
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Figure 3. Molecular structure of 4 at 30% probability ellipsoids. Hydrogen atoms and
one molecule of toluene omitted for clarity. Selected bond distances (Å): N(2)-C(33) =
1.152(8), N(3)-C(33) = 1.313(8), N(2)-C(34) = 1.489(8), C(34)-C(35) = 1.322(9).
The conversion of 3 to 4 likely occurs by nucleophilic attack of the nitrogen in the
azametallocycle on the carbon of the terminal hafnium isocyanate (Scheme 3). It is
interesting to note that cycloaddition of the alkyne maintains nucleophilicity of the
nitrogen, allowing it to engage in additional N-C bond forming chemistry.
Dexoygenation of the putative ureate-type intermediate to form the observed µ-oxide
compound with concomitant formation of the bridging cyanamide ligand is likely driven
by release of strain imposed by the metallocycles.
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The synthesis of an alkenyl-substituted, N-bound cyanamide ligand from CO-
induced N2 cleavage prompted exploration of the reactivity of this fragment with the goal
of releasing substituted cyanamides from the coordination sphere of the hafnium.
Addition of Me3SiCl to a benzene-d6 solution of 4 produced a 2:1 mixture of
monometallic and bimetallic products. The monometallic product was identified as (η5-
C5H2-1,2,4-Me3)2Hf(OSiMe3)Cl based on comparison to an authentic sample
independently prepared by treatment of (η5-C5H2-1,2,4-Me3)2HfCl2 with one equivalent
of NaOSiMe3. The bimetallic product was identified as the dimeric dihafnocene
compound, [(η5-C5H2-1,2,4-Me3)2Hf(µ2-N=C=N(cyclooctenyl)]2 (5), arising from group
transfer of the [N=C=N(cyclooctenyl)]2- ligands between metal centers.
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Pure samples of 5 were obtained by fractional crystallization from the reaction
mixture using fluorobenzene. Analysis of single crystals by X-ray diffraction established
the molecular structure (Figure 4). The asymmetric unit contains one half of the molecule
with an inversion center generating the remainder of the structure. The cyanamide ligand
bridges the two hafnium centers and is κ1 N-bound to one metal and κ2-C, N bound to the
other. The Hf(1)-C(17) and Hf(1)-N(1) bond lengths of 2.342(3) Å and 2.268(2) Å are
slightly elongated for X-type ligands. The C(17)-C(24) distance in the cyclooctenyl ring
of 1.342(4) Å is consistent with retention of double bond character. The carbodiimidyl
fragment exhibits disparate C-N bond distances of 1.281(4) and 1.163(4) Å for C(25)-
N(1) and C(25)-N(2) respectively, indicating delocalization of electron density in the
azacyclooctenyl fragment. Based on the observed metrical parameters, the C(25)-N(2)
fragment can be viewed as an L type cyanamide ligand interacting with one hafnium with
the azacyclooctenyl fragment acting as an X2 type donor to the other metal center.
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Figure 4. Molecular structure of 5 at 30% probability ellipsoids. Hydrogen atoms and
disordered orientation of cyclooctyl ring have been omitted for clarity. One half of the
molecule has been generated by symmetry. Selected bond distances (Å): C(17)-C(24) =
1.342(4), N(1)-C(24) = 1.435(4), N(1)-C(25) = 1.281(4), N(2)-C(25) = 1.163(4).
1
The benzene-d6 H NMR spectrum of 5 displays the expected resonances for a C2v
symmetric hafnocene dimer. The observed higher symmetry in solution versus the solid
state structure is likely a result of the fluxionality of the 8-membered rings. Multinuclear
NMR spectroscopy on the 15N/13C isotopologue 5-15N/13C confirmed that the central
carbodiimidyl atom was derived from 13CO. The 13C NMR spectrum of 5-15N/13C in
benzene-d6 contains an isotopically enhanced doublet of doublets, which is partially
obscured by the residual solvent signal (see SI). Preparing the sample in tetrahydrofuran-
d8 allowed clear observation of the resonance and determination of the two coupling
constants to 15N (1JCN = 30.1, 24.3 Hz). The 15N NMR spectrum of 5-15N/13C in benzene-
d6 exhibits an upfield doublet (1JCN = 24.3 Hz) at 70.2 ppm and another doublet (1JCN
=
30.1 Hz) at 161.3 ppm, which were assigned to N(1) and N(2) respectively.
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N-C Bond Formation by Nitrile Insertion. The cycloaddition of the strained alkyne to
the µ-nitrido ligand in 1 followed by isocyanate deoxygenation and additional N-C bond
formation prompted study of other activated π-systems. Nitriles were interesting targets
given their potential for two coordination and hence reactivity modes – one as an L-type
ligand from σ-bonding through the nitrogen lone pair and the other from insertion or
10
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cycloaddition reactivity engaged through the π-system of the C≡N bond. Addition of one
equivalent of pivalonitrile to a benzene-d6 solution of 1 resulted in straightforward
coordination of the nitrile to form the ligand-stabilized dihafnocene µ-nitride complex,
[(η5-C5H2-1,2,4-Me3)2Hf]2(NCO)(tBuCN)(µ2-N) (6) as judged by NMR spectroscopy and
X-ray diffraction. Complete characterization details and a representation of the molecular
structure are reported in the Supporting Information.
Complex 6 proved to be unstable in solution, decomposing in both benzene and
tetrahydrofuran over the course of hours at 23 ºC to unidentified organometallic products
with no evidence for C-N bond formation. Reasoning that the large tert-butyl substituent
inhibited the π-system of the nitrile from engaging in productive chemistry with the µ-
nitride, nitriles with smaller substituents were explored. Addition of benzonitrile to 1
again resulted in straightforward coordination to the hafnium, yielding [(η5-C5H2-1,2,4-
Me3)2Hf]2(NCO)(PhCN)(µ2-N) (7). This compound, like 6 also proved unstable in
solution and no productive N-C bond forming chemistry was observed. Addition of
excess benzonitrile to 1 also produced 7 but did not result in stabilization of the complex.
Addition of cyclohexanecarbonitrile (CyCN) to a benzene-d6 solution of 1
resulted in productive chemistry. As with the other nitriles, coordination occurred
immediately and the expected ligand-stabilized complex, [(η5-C5H2-1,2,4-
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Me3)2Hf]2(NCO)(CyCN)(µ2-N) (8) was observed as judged by multinuclear NMR
spectroscopy. Over the course of approximately 45 minutes at 23 ºC, benzene-d6
solutions of 8 cleanly and quantitatively converted to a new Cs symmetric dihafnocene
complex, [(η5-C5H2-1,2,4-Me3)2Hf]2(NCO)(µ2-NC(Cy)N) (9) identified based on a
combination of multinuclear NMR and IR spectroscopies (Scheme 4). The 13C NMR
spectrum of the doubly labeled isotopologue, 9-13C, 15N exhibited an isotopically
enhanced doublet (1JCN = 29.0 Hz) centered at 129.0 ppm diagnostic for terminal hafnium
isocyanate. A second, natural abundance doublet (1JCN = 7.9 Hz) was located at 182.6
ppm for the carbon atom of the nitrile. The observed coupling to an isotopically labeled
nitrogen confirmed insertion of the unsaturate into the dihafnocene µ-nitrido forming a
[NC(Cy)N]3- amidinato ligand. The term amidinato was coined as this fragment is the
trianionic version of the well-established monoanionic amidinate ligand.
10
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Scheme 4. Reactivity of 1 with various organonitriles.
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H
H
H
O
O
O
N
N
C
C
N
C
2 iPrCN
Me3SiCl
N
Hf
N
Hf
Hf
N
Hf
Hf
Hf
N
N
N
Cl
Cy
N
N
iPr
C
Cy
C
7
13
14
11
+
8
9
CN
Me3Si
10
11
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CyCN
O
O
H
C
C
O
N
N
N
C
iPrCN
RCN
45 min, 23 ºC
R = Cy
1
N
Hf
Hf
N
Hf
Hf
Hf
N
Hf
N
N
C
N
N
Cy
C
Cy
C
12
9
R
6: R = tBu
7: R = Ph
8: R = Cy
XylNC
O
C
C
N
Hf
N
Hf
N
C
Cy
N
Xyl
10
The N NMR spectrum of 9-13C, N exhibits a resonance at 86.7 ppm, (1JCN
=
15
15
29.0 Hz) typical for a terminal hafnocene isocyanate. A second resonance was located at
374.0 ppm, significantly upfield shifted from the value of 601.5 ppm observed in the
nitrile complex of the nitrido, 8, assigned as the imido-type nitrogen atom in the
[NC(Cy)N]3- ligand. Attempts to obtain single crystals of 9 have been unsuccessful as
solutions of 9 underwent decomposition to unidentified products over the course of days
at -35 ºC. To circumvent decomposition pathways, one equivalent of 2,6-dimethylphenyl
isocyanide was added to a benzene-d6 solution of 9 and furnished the ligand-stabilized
hafnocene product, [(η5-C5H2-1,2,4-Me3)2Hf]2(NCO)(CN-2,6-Me2C6H3)(µ2-NC(Cy)N)
(10).
The solid-state structure of 10 was determined by X-ray diffraction and a
representation of the molecule is presented in Figure 5. The crystallographic data
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confirmed insertion of the nitrile into the µ-imido fragment and formation of the
[NC(Cy)N]3- ligand. The C-N distances of 1.311(7) and 1.324(7) Å in the amidinato
ligand are statistically indistinguishable despite the difference in formal bond order,
suggesting a degree of delocalization. Accordingly, the hafnium-nitrogen bonds Hf(2)-
N(1) and Hf(1)-N(3) are also nearly identical at 1.932(4) and 1.936(5) Å. The similarity
in Hf-N bond distances may arise from contributions from a zwitterionic resonance form,
where a formal negative charge is located on N(1) and the corresponding positive charge
is localized on Hf(2) while maintaining a Hf-N single bond. This increased charge on
N(1) may also explain the further reactivity of this complex (vide infra). While
monoanionic amidinate ligands are widely used in coordination and organometallic
chemistry including supporting early transition metals that promote N2 activation and
functionalization,45 this is to the best of our knowledge the first example of such a
trianionic, unsubstituted fragment. It is also notable that the nitrile ligand inserts into the
hafnium imido bond rather than participating in cycloaddition chemistry. Thus, the π-
system of the nitrile is important for promoting reactivity but distinguishes the chemistry
observed with cyclooctyne.
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Figure 5. Molecular structure of 10 at 30% probability ellipsoids. Hydrogen atoms and
one molecule of benzene solvate omitted for clarity. Selected bond distances (Å): Hf(2)-
N(1) = 1.932(4), Hf(1)-N(3) = 1.936(5), N(1)-C(34) = 1.324(7), N(3)-C(34) = 1.311(7).
The similar spectral properties between 9 and structurally characterized 10,
suggest that despite the similarity in bond distances, in one canonical form, one of the
hafnium-nitrogen bonds in 9 may be viewed as having imido-character and offer
associated reactivity. Additional nitrogen-element bond formation was of interest as the
nitrogen is derived from N2. Treatment of a benzene-d6 solution of 9 with a further
equivalent of CyCN furnished a new Cs symmetric hafnocene compound, 11 (Scheme 4).
A combination of multinuclear NMR spectroscopy, infrared spectroscopy and X-ray
diffraction established an additional N-C bond-forming event arising from formal attack
of the “imide-like” amidinato nitrogen on the terminal hafnocene isocyanate. This
cascade is accompanied by deprotonation of the exogeneous nitrile to form a ketimide
ligand with concomitant protonation of the nitrogen from the NCO ligand (Scheme 4).
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The 15N NMR spectrum of the doubly labeled isotopologue 11-15N/13C exhibits a
doublet of doublets (1JNH = 66.2, 1JCN = 10.1 Hz) centered at 135.1 ppm, confirming N-H
bond formation at N(2). The central ureate nitrogen, N(1) was located as a doublet at
392.1 ppm (1JCN = 1.4 Hz). The cause of the large disparity in C-N coupling constants
between the two fragments is unknown at this time. The 13C NMR spectrum of 11-
15N/13C contains an isotopically enhanced doublet of doublets at 168.8 ppm (1JCN = 10.1,
1JCN = 1.4 Hz) supporting N-C bond formation via intramolecular attack on the terminal
isocyanate. The benzene-d6 IR spectrum contains an NH band at 3340 cm-1 in addition to
terminal ketimine and imine C=N stretches at 2056 and 1585 cm-1 respectively.
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The solid-state structure of 11 was confirmed by single crystal X-ray diffraction
and a representation is shown in Figure 6. The core of the molecule contains an eight
membered bicycle comprised of four- and six-membered substructures. The two hafnium
atoms are essentially coplanar within these ring systems. One hafnocene is coordinated
by a formally dianionic N,O-carbodiimidate-type ligand while the other has a
coordination sphere containing imidinato and amide coordination. The hydrogen atom
attached to N(2) was independently located and the deprotonation of the cyclohexyl
methine carbon is clearly indicated by the metrical parameters. The C(41)-C(42) bond
distance of 1.344(9) Å is consistent with a carbon-carbon double bond and the bond
angles about C(42) are consistent with sp2 hybridization. The former amide/imide subunit
is no longer symmetric, with distinct N(3)-C(34) and N(1)-C(34) bond lengths of
1.281(7) and 1.418(7) Å, consistent with C-N double and single bonds, respectively.
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Figure 6. ORTEP plot of 11 at 30% probability ellipsoids. Disordered cyclohexyl
carbons and hydrogen atoms (except those attached to N2 and C35) omitted for clarity.
Selected bond distances (Å): N(1)-C(33) = 1.353(7), N(1)-C(34) = 1.418(7), N(2)-C(33) =
1.325(7), N(3)-C(34) = 1.281(7), C(41)-C(42) = 1.344(9).
The unique cascade chemistry observed upon addition of a second equivalent of
nitrile to 9 prompted exploration of the generality of the transformation. Unlike the other
cascade initiated by cyclooctyne cycloaddition, engagement of the terminal hafnium
isocyanate does not result in net deoxygenation, instead attack on the central carbon atom
results in an anionic oxygen ligand bound to the second hafnium, highlighting the
cooperative nature of each metallocene subunit in the bimetallic structure. Of particular
interest was whether other nitriles with acidic α-hydrogens would promote similar
reactivity. Addition of isobutyronitrile to a benzene-d6 solution of 9 furnished the Cs
symmetric dihafnocene 12 where cyclohexylnitrile was exclusively incorporated into the
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extended ureate core and the isobutyronitrile had undergone deprotonation to form the
terminal ketimido ligand (Scheme 4). Importantly, there was no evidence by NMR
spectroscopy for formation of the isomer derived from cyclohexylnitrile deprotonation,
consistent with either irreversible insertion of the first equivalent of nitrile or rapid
intramolecular attack on the terminal isocyanate.
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11
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Incorporation of isobutyronitrile into the extended ureate core was accomplished
by straightforward treatment of 1 with two equivalents of nitrile at 23 ºC and resulted in
isolation of 13. Attempts to observe an intermediate insertion product analogous to 9 by
NMR spectroscopy were unsuccessful, demonstrating a rapid deprotonation reaction with
isobutyronitrile likely from the reduced steric profile of the nitrile.
The assembly of a unique ureate-type core from CO-induced N2 splitting followed
by nitrile insertion in 11 prompted attempts at further elaboration. Addition of Me3SiCl to
a benzene-d6 solution of 11 resulted in immediate formation of a new Cs symmetric
1
product as judged by H NMR spectroscopy. The data established disappearance of the
cyclohexyl resonances for the ketimide ligand with concomitant formation of 1-
(trimethylsilyl)cyclohexane-1-carbonitrile, a result of electrophilic attack of the
trimethylsilyl group on the ketimide carbon (Scheme 4). The structure of the
organometallic product was established as [(η5-C5H2-1,2,4-Me3)2Hf]2(Cl)(µ2-
NC(Cy)NCN(H)O) (14) by X-ray diffraction (Figure 7) and confirmed replacement of
the ketimide by chloride. Addition of excess Me3SiCl to 14 produced no reaction.
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Figure 7. Molecular structure of 14 at 30% probability ellipsoids. Two molecules of
tetrahydrofuran solvate and hydrogen atoms (except those attached to N1 and C35)
omitted for clarity.
In an attempt to divert reactivity away from ketimide displacement, carboxylation
reactions were explored. Exposure of a benzene-d6 solution of 11 to two equivalents of
CO2 resulted in rapid formation of a new hafnium product, 15 (Scheme 5). Analysis of
the benzene-d6 1H NMR spectrum revealed a downfield shift of the N-H proton by
approximately 1 ppm, signaling modification of the ureate core. Performing the
carboxylation with 13CO2 furnished 15-13C2 and the resulting 13C NMR spectrum
contained two isotopically enhanced resonances centered at 158.6 and 175.5 ppm. In
addition, the methylene carbons of the former ketimide ligand exhibit C-C coupling (2JCC
= 2.9 Hz) to the signal at 175.5 ppm, indicating CO2 addition occurred at both the ureate
and ketimide portions of the molecule. Attempts to observe intermediates in the reaction
by addition of one equivalent of CO2 gas to 11 were unsuccessful and resulted in an
approximate equimolar mixture of the starting hafnocene and 15.
Scheme 5. Carboxylation of 11.
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N
C
4
5
O
6
7
8
9
H
O
Hf
O
H
N
C
O
2 CO2
N
Hf
Hf
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N
C
N
N
Hf
N
Cy
N
O
C
Cy
11
O
15
The solid-state structure of 15 was confirmed by a single crystal X-ray diffraction
experiment and a representation of the molecule is presented in Figure 8. The structural
data establish addition of one equivalent of CO2 into the ketimide ligand with
concomitant rearrangement to form an unusual carboxylate ligand. A second independent
molecule was located in the asymmetric unit, which differs slightly from the one
presented in Figure 8 in that the acetate ligand adopts κ1 hapticity. In both cases, the
carboxylate ligands contain a new C-C bond between the carbon of the CO2 molecule and
the cyclohexane ring of the former ketimide ligand. The second equivalent of CO2
inserted into the nitrile-derived nitrogen of the ureate core forming a carbamate-type
ligand. The combined carboxylation events trigger a change in hapticity of the bridging
ureate ligand where in 15 a single oxygen atom forms the linkage between the hafnocene
subunits.
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Figure 8. Molecular structure of one of two independent molecules of 15 at 30%
probability ellipsoids. Hydrogen atoms (except that attached to N5) and one molecule of
toluene solvate omitted for clarity. Selected bond distances (Å): N(5)-C(82) = 1.279(8),
N(6)-C(82) = 1.395(7), N(6)-C(83) = 1.327(8), N(7)-C(83) = 1.351(7), N(7)-C(90) = 1.387(8),
Hf(3)-N(5) = 2.235(5), Hf(3)-N(7) = 2.199(5).
Carboxylation of 14 proceeded in a similar manner. Replacement of the ketimide
ligand by chloride confines the CO2 addition to the ureate core and cleanly furnished the
dihafnocene product, [(η5-C5H2-1,2,4-Me3)2Hf]2(Cl)(µ2-O2CNC(Cy)NCN(H)O) (16). The
benzene-d6 13C NMR spectrum of 16-13C, prepared using 13CO2, exhibited an isotopically
enhanced resonance centered at 158.4 ppm. Without structural characterization, definitive
assignment of the hapticity of the ureate core is challenging.
The reactivity of 9 and 10 with electrophiles was also explored in an attempt to
induce group transfer of the ureate-like core. Addition of one equivalent of Me3SiCl to a
benzene-d6 solution of either compound cleanly furnished a new Cs symmetric
1
dihafnocene product, 17. Analysis of the H and 13C NMR data established that the
product arises from delivery of chloride to one hafnocene subunit which induces
intramolecular attack of the imido-like nitrogen on the terminal hafnocene isocyanate. As
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shown in Scheme 6, initial chloride abstraction by the hafnocene generates a nucleophilic
carbodiimidyl nitrogen which attacks the carbon of the terminal isocyanate. Capture of
the silyl group generates the observed product, 17. Notably, additional silylation does not
occur at the nitrile carbon.
10
11
12
13
14
15
16
17
18
19
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Scheme 6. Treatment of dihafnocene ureate-type complexes with Me3SiCl.
O
SiMe3
C
O
N
N
C
Me3SiCl
N
Hf
Hf
N
Hf
Hf
N
N
C
Cl
Cy
Cy
9
17
Me3Si+
O
O
C
-
N
C
N
-
Hf
N
Hf
Hf
N
Hf
N
C
Cl
Cy
Me3Si+
N
C
Cl
Cy
Cycloaddition of Heterocummulenes with 1. The rich cycloaddition chemistry of 1
prompted study with heterocummulenes. Previous studies have focused on
functionalization of activated group 4 transition metal dinitrogen compounds with
heterocummulenes.42, 46 Less attention has been devoted to nitride functionalization
following N2 cleavage. Addition of one equivalent of CO2 gas to a benzene-d6 solution of
1 resulted in rapid precipitation of an off-white solid identified as the µ-oxo dihafnocene
1
bis(isocyanate) complex, 18 (Scheme 7). The H NMR spectrum established equivalent
hafnocene subunits while the presence of terminal isocyanate ligands was readily
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identified by IR and C NMR spectroscopies. The solid state infrared spectrum in KBr
exhibits two strong [NCO] bands at 2222 and 2085 cm-1, which shift to 2160 and 2027
cm-1 upon labeling with 13CO and 13CO2 (harmonic oscillator: 2176 and 2041 cm-1). The
13C NMR spectrum of 18-13C exhibits a broad singlet centered at 135.3 ppm.
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Formation of 18 likely proceeds by initial cycloaddition of CO2 across the Hf-N
bond to form a µ2-[NCO2]3- ligand, which undergoes rearrangement to form the observed
µ-oxo core and a terminal isocyanate. Similar deoxygenation reactions have been
observed with (η5-C5H5)2ZrNR complexes upon addition of isocyanates.47 In an attempt
to suppress deoxygenation chemistry and observe heterocummulene cycloaddition to the
dihafnocene imido, 1 was treated with alkyl isocyanates. Addition of one equivalent of
tBuNCO to a benzene-d6 solution of 1 at 23 ºC rapidly and cleanly furnished the desired
cycloaddition product, [(η5-C5H2-1,2,4-Me3)2Hf]2(µ2-NCONtBu)(NCO) (19) in 75 %
yield following washing of the crude material with pentane at -35 ºC (Scheme 7).
Scheme 7. Addition of heterocummulenes to 1.
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O
C
N
CO2
Hf
O
Hf
7
fast
N
8
9
C
O
10
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16
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O
C
N
18
Hf
N
Hf
O
1
C
N
RNCO
fast
Hf
N
Hf
C
N
O
R
19 (R = tBu)
20 (R = 1Ad)
1
Like 4, the benzene-d6 H NMR spectrum of 19 exhibits the number of resonances
expected for a Cs symmetric compound but the 13C NMR spectrum contains both an
isocyanate resonance at 132.0 ppm and a new peak centered at 159.7 ppm. Preparation of
the 15N/13C isotopologue from 15N2 and 13CO gas resulted in splitting of the broad singlet
at 132.0 ppm into a doublet (1JCN = 32.6 Hz). Unfortunately, no C-N coupling was
observed for the resonance at 159.7 ppm but the 15N NMR spectrum of 19 clearly
exhibits a resonance at 90.2 ppm for a terminal isocyanate and a new resonance at 221.8
ppm, shifted significantly upfield from 1 (589.9 ppm) consistent with formation of a
ureate ligand.
The solid-state structure of 19 was determined by X-ray diffraction and
definitively confirmed formation of a ureate ligand from isocyanate cycloaddition (Figure
9). The core of the ureate ligand features indistinguishable C(34)-N(1) and C(34)-N(3)
bond lengths of 1.360(3) and 1.361(3) Å while the C(34)-O(1) distance is significantly
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shorter with a value of 1.298(3) Å. Unlike zirconium imido compounds studied by
Bergman, where cycloaddition of isocyanates leads to formation of [Cp2ZrO]x and
carbodiimides,47 the second hafnium center in 19 likely stabilizes the resultant µ-ureate
core and inhibits immediate deoxygenation. Similar cycloaddition reactivity was
observed with 1-adamantyl isocyanate and yielded the analogous isocyanato dihafnocene
ureate product, 20 in 87% yield following recrystallization (Scheme 7). The
spectroscopic signatures of 20 are similar to 19 and the solid-state structure was also
confirmed by X-ray diffraction and is presented in the Supporting Information. Attempts
to prepare analogous products by cycloaddition of benzyl or p-tolyl isocyanate with 1
were unsuccessful, producing unidentified mixtures of organometallic products.
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Figure 9. Molecular structure of 19 at 30% probability ellipsoids. Hydrogen atoms
omitted for clarity. Selected bond distances (Å): Hf(1)-N(1) = 2.105(2), Hf(2)-N(1) =
2.231(2), Hf(1)-N(3) = 2.144(2), N(1)-C(34) = 1.360(3), N(3)-C(34) = 1.361(3).
The observation of rapid formation of a µ-oxo hafnocene product following CO2
cycloaddition prompted study of the thermal stability of 19 and 20. Heating a benzene-d6
solution of 19 to 110 ºC and monitoring the progress of the reaction by NMR
spectroscopy over the course of 18 hours established gradual rearrangement to the µ-oxo
dihafnocene carbodiimidyl complex, [(η5-C5H2-1,2,4-Me3)2Hf]2(NCO)(N=C=NtBu)(µ2-
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