Di- and tetrametallic hafnocene oxamidides prepared from CO-induced N 2 bond cleavage and thermal rearrangement to hafnocene cyanide derivatives

Article abstract of DOI:10.1021/om3005542

Carbonylation of the hafnocene dinitrogen complex [(η⁵- C₅H₂-1,2,4-Me₃)₂Hf] ₂(μ₂,η²:η²-N ₂) with 4 atm of carbon monoxide yielded the tetrametall

Full text of DOI:10.1021/om3005542

Article
pubs.acs.org/Organometallics
Di- and Tetrametallic Hafnocene Oxamidides Prepared from CO-
Induced N2 Bond Cleavage and Thermal Rearrangement to
Hafnocene Cyanide Derivatives
Scott P. Semproni, Grant W. Margulieux, and Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: Carbonylation of the hafnocene dinitrogen complex [(η5-C5H2-1,2,4-Me3)2Hf]2(μ2,η2:η2-N2) with 4 atm of
carbon monoxide yielded the tetrametallic hafnocene oxamidide complex [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4, a new structural
motif arising from CO-induced N2 cleavage. The more commonly observed dimeric hafnocene oxamidide [(η5-C5H2-1,2,4-
Me3)2Hf]2(N2C2O2) was observed by multinuclear NMR spectroscopy when the carbonylation was performed at lower (∼1
atm) CO pressure. Over the course of 1 h at 23 °C, the dimeric hafnocene oxamidide undergoes dimerization to the tetrametallic
compound, establishing its intermediacy for synthesis of the latter. Additional functionalization of the hafnium−nitrogen bonds
t
in the tetrametallic complex was accomplished by cycloaddition of BuNCO or 1,2-addition of CySiH3. The former example
maintains a tetrametallic hafnocene where only two of the four Hf−N bonds have undergone [CO] cycloaddition of the
heterocumulene. In contrast, the primary silane yielded a dimeric hafnocene product where all of the hafnium−nitrogen linkages
have undergone 1,2-addition. Thermolysis of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4 at 110 °C provided a route to a new μ-oxo
hafnocene complex with both terminal isocyanate and cyanide ligands. This process is general among hafnocene oxamidides and
provides a route to rare hafnium cyanide complexes that undergo preferential [CN] rather than [NCO] group transfer.
zirconocene dinitrogen complex.16,17 This approach is in
contrast with the seminal work of van Tamelen and co-
INTRODUCTION
■
The synthesis of nitrogen-containing organic molecules from
workers, where reduction of titanocene chloride with excess
dinitrogen is a long-standing challenge owing to the kinetic and
magnesium powder followed by treatment with ketones or acid
thermodynamic stability of the N2 molecule.1 Biological2 and
chlorides yields amines or nitriles, respectively.18 It was
industrial nitrogen fixation3,4 produces ammonia on an
postulated that the combination of the transition metal and
enormous scale, and the resulting NH3 is used to prepare
most synthetic nitrogen compounds.5 Methods that directly
the alkaline-earth-metal reductant supplied the necessary
electrons to promote N2 cleavage.19,20
assemble nitrogen−carbon bonds from N2 are attractive to
circumvent the energy requirements for ammonia synthesis.6−8
Carbon monoxide induced N2 cleavage has emerged as a
powerful method for N2 cleavage, with concomitant assembly
Ligand-induced dinitrogen cleavage, where an incoming
reagent simultaneously supplies reducing equivalents and forms
a new nitrogen-element bond, has evolved into an effective
of N−C and C−C bonds.21−23 First observed by Sobota in a
TiCl4/Mg mixture,24 our laboratory has since developed this
reaction as a general transformation among zirconocene and
strategy for elaboration of N2. This approach is particularly
hafnocene compounds with strongly activated N2 ligands25 and
useful with early-transition-metal compounds that do not have
as a method to synthesize substituted oxamides.26 CO-induced
the necessary reducing equivalents to promote NN scission.
dinitrogen cleavage has also been applied to the synthesis of a
Fryzuk and co-workers have pioneered this strategy in tantalum
hafnocene μ-formamidide ([NC(H)O]2−) ligand and ulti-
mately free formamide following silylation of [(η5-C5H2-1,2,4-
Me3)2Hf]2(μ2,η2:η2-N2) (1-N2).27 Experimental18,28 and com-
putational investigations29 into the mechanism of oxamidide
formation from carbonylation of zirconocene and hafnocene
dinitrogen compounds support the formation of intermediate
dinitrogen chemistry9 and have demonstrated that addition of
alanes,10 silanes,11 boranes,12 and zirconocene hydrides13
induce N−N cleavage with concomitant formation of new
nitrogen-element bonds. Our laboratory has extended this
approach to group 4 metallocene dinitrogen compounds where
the reduced zirconium or hafnium centers supply only four of
the six electrons required for N2 splitting.14,15 It is likely that
ligand-induced N2 cleavage plays a role in the liberation of
ammonia from hydrogenation of a strongly activated
Received: June 18, 2012
Published: August 21, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Pyridine-Stabilized Hafnocene μ-Nitrido Complexes by CO-Induced N2 Bond Cleavage
μ-nitrido complexes, and pyridine-stabilized examples have
recently been isolated and crystallographically characterized
(Scheme 1).30
Here we describe additional studies into the base-free
carbonylation of 1-N2 and report the isolation and character-
ization of an unusual tetrametallic oxamidide compound
prepared from dinitrogen cleavage. The cyclo- and 1,2-addition
reactivity of this species is reported and compared to more well
established dinuclear hafnocene oxamidides. A new thermally
induced deoxygenation and isomerization of hafnocene
oxamidides has also been discovered and applied to the
synthesis of rare hafnium cyanide complexes and ultimately
organic nitriles.
Figure 1. Representation of the solid-state structure of [1-(NCO)]4
with 30% probability ellipsoids. Hydrogen atoms, cyclopentadienyl
methyl groups, and one molecule of tetrahydrofuran are omitted for
clarity. Half of the molecule has been generated by the symmetry
operator −x, −y, −z.
RESULTS AND DISCUSSION
■
Carbonylation of [(η5-C5H2-1,2,4-Me3)2Hf]2(μ2,η2:η2-N2).
Exposure of a THF solution of 1-N2 to 4 atm of carbon
monoxide followed by recrystallization at −35 °C furnished
yellow crystals identified as the tetrametallic hafnocene
oxamidide complex [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4 ([1-
NCO]4) in 51% yield (eq 1). The benzene-d6 13C NMR
C distances of 1.544(7) and 1.301(6)/1.260(6) Å are typical of
the values previously observed with dimeric zirconocene and
hafnocene oxamidides prepared from CO-induced N2 bond
cleavage.18,20
The observation of a new tetrametallic structural motif
arising from dinitrogen carbonylation raised the question as to
whether the more commonly observed dimeric hafnocene
oxamidide is synthetically accessible and if it is an intermediate
in the formation of [1-(NCO)]4. Performing the carbonylation
of 1-N2 with 1 atm of CO at 23 °C and monitoring the progress
of the reaction by NMR spectroscopy in benzene-d6 allowed
identification of [(η5-C5H2-1,2,4-Me3)2Hf]2(N2C2O2) ([1-
(NCO)]2). The 13C, 15N isotopologue [(η5-C5H2-1,2,4-
Me3)2Hf]2(15N213C2O2) was prepared from 13CO and 15N2
gas and exhibited a triplet in the 13C NMR spectrum centered
at 158.9 ppm with 1JCN and 2JCN couplings of 3.9 Hz, consistent
with N−C bond formation and similar to values for other
oxamidide complexes of this type.18,20 Accordingly, the
benzene-d6 15N NMR spectrum exhibited a triplet (1JC−N
=
2JC−N = 3.9 Hz) also confirming formation of N−C and C−C
bonds from CO and N2. The solid-state (KBr) infrared
spectrum of [1-(NCO)]2 exhibits a band at 1625 cm−1, distinct
from the band at 1570 cm−1 for [1-(NCO)]4, allowing clear
identification of the bimetallic and tetrametallic oxamidides.
Notably, allowing red benzene-d6 solutions of [1-(NCO)]2 to
stand for approximately 1 h at 23 °C (with or without a CO
atmosphere) resulted in precipitation of a yellow solid
identified as [1-(NCO)]4, demonstrating the intermediacy of
the dimeric compound (Scheme 2).
spectrum of the 13C-labeled isotopologue exhibits a singlet
centered at 163.2 ppm, consistent with oxamidide formation
and a symmetric [N2C2O2]4− core of the molecule. The solid-
state (KBr) infrared stretching frequency of the oxamidides was
located at 1570 cm−1. The tetrametallic hafnocene oxamidide is
a new structural motif that is likely a consequence of the less
sterically encumbered cyclopentadienyl rings that allow
assembly of four hafnocene subunits.
Formation of the tetrametallic hafnocene oxamidide [1-
(NCO)]4 was also confirmed by single-crystal X-ray diffraction.
A representation of the molecular structure is presented in
Figure 1. While a definitive distinction of O and N atoms by X-
ray diffraction is tenuous, the best fit of the data is the transoid
disposition of the like atoms consistent with all zirconocene and
hafnocene complexes prepared to date.18,20 The C−C and N−
Reactivity of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4. The re-
activity of [1-(NCO)]4 was explored with the goal of
elaborating the oxamidide core and comparing it with the
established chemistry of analogous dimeric hafnocene oxami-
dides.18,21 We were particularly interested in exploring the
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Scheme 2. Synthesis of [1-(NCO)]2 and Its Conversion to [1-(NCO)]4
t
Scheme 3. Cycloaddition of Excess BuNCO to [1-(NCO)]4 at 65 and 130 °C
number of hafnium−nitrogen bonds that are available for
subsequent functionalization chemistry. Previous studies have
demonstrated that the cycloaddition of heterocumulenes to
dimeric hafnocene oxamidides is an effective strategy for
elaboration of both of the Hf−N bonds of the [N2C2O2]4−
ligand.21 Heating a THF solution of [1-(NCO)]4 to 65 °C in
(NCO)]2(tBuNCO)2 were also characterized by X-ray
diffraction, and representations of the molecular structures
are given in Figures 2 and 3, respectively.
The 1,2-addition of primary silanes has also proven to be an
effective means of functionalizing hafnocene oxamidide cores.21
To evaluate whether all four Hf−N bonds could be
functionalized in this manner and to explore the molecularity
of the resulting complex, the reactivity of [1-(NCO)]4 with
primary silanes was studied. Addition of 4 equiv of CySiH3 to a
THF solution of [1-(NCO)]4 at 23 °C for 2 h followed by
recrystallization from a fluorobenzene−pentane mixture fur-
nished colorless crystals identified as [1-(NCO)]2(CySiH3)2 in
70% yield (eq 2). The identity of the product was established
by X-ray diffraction (Figure 4), and the structure was of
sufficient quality that the hydrogen atoms, including the
hafnium hydrides, were located and refined. Performing the
reaction with only 1 or 2 equiv of the silane produced a mixture
of products containing [1-(NCO)]4 and [1-
(NCO)]2(CySiH3)2.
t
the presence of excess (>4 equiv) BuNCO for 18 h followed
by recrystallization from toluene furnished yellow blocks
identified as [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4(tBuNCO)2
([1-(NCO)]4(tBuNCO)2) in 55% yield. The C2h symmetric
product is a result of the [CO] cycloaddition of the
heterocumulene to two transoid disposed Hf−N bonds
(Scheme 3).
Performing the cycloaddition reaction at 130 °C also resulted
in additional N−C bond formation but instead yielded the
bimetallic hafnocene [(η5-C5H2-1,2,4-Me3)2Hf]2(N2C2O2)-
(tBuNCO)2 ([1-(NCO)]2(tBuNCO)2), where each Hf−N
bond has been functionalized. Presumably cycloaddition across
all four hafnium−nitrogen bonds disrupts the tetrametallic core
to produce dimeric hafnocene complexes. To further probe this
possibility, [1-(NCO)]4(tBuNCO)2 was heated with excess
tBuNCO over the course of 18 h at 130 °C. Complete
conversion to 1-(NCO)]2(tBuNCO)2 was established by NMR
spectroscopy. Both [1-(NCO)]4(tBuNCO)2 and [1-
Thermolysis of Hafnocene Oxamidides for the Syn-
thesis of Cyanide Complexes. The thermal stability of the
tetrametallic hafnocene oxamidide was also assayed. Heating a
benzene solution of [1-NCO]4 to 110 °C for 2 days followed
by cooling to room temperature deposited colorless crystals of
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Figure 2. Representation of the solid-state structure of [1-
(NCO)]4(tBuNCO)2 with 30% probability ellipsoids. One molecule
of toluene, hydrogen atoms, and cyclopentadienyl methyl groups are
omitted for clarity. Half of the molecule has been generated by the
symmetry operator −x, −y, −z.
Figure 4. Representation of the solid-state structure of [1-
(NCO)]2(CySiH3)2 with 30% probability ellipsoids. Hydrogen
atoms, except those attached to Hf and Si, are omitted for clarity.
Half of the molecule has been generated by the symmetry operator −x,
−y, −z.
([1-(O)(NCO)CN]), arising from partial deoxygenation and
isomerization of the oxamidide ligand (eq 3).
Figure 3. Representation of the solid-state structure of [1-
(NCO)]2(tBuNCO)2 with 30% probability ellipsoids. Hydrogen
atoms are omitted for clarity.
The molecular structure of [1-(O)(NCO)CN] was con-
firmed by single-crystal X-ray diffraction (Figure 5) and is to
our knowledge the first example of a structurally characterized
hafnocene cyanide complex. Thewalt and co-workers have
reported the synthesis and solid-state structures of [(η5-
C5H5)2Ti(CN)]4,31 [(η5-C5H5)2Ti(CN)(OMe)], and [(η5-
C5H5)2Ti(CN)]2(O).32 In the context of N2 functionalization,
Cummins and co-workers have reported the synthesis of a
terminal molybdenum cyanide from the corresponding N2-
derived metal nitride compound33 and have also used the same
platform to develop a synthetic cycle for the synthesis of free
organic nitriles.34 In the structure of [1-(O)(NCO)CN],
typical bent metallocene coordination geometries are observed
with Hf(1)−C(34) and C(34)−N(2) distances of 2.236(9) and
1.166(12) Å, respectively, for the terminal cyanide ligand. In
solid-state structures of this type, the isocyanate and cyanide
fragments are disordered over both metal centers. The ligands
were modeled over both positions and refined freely.
the μ-oxo dihafnocene isocyanate cyanide complex [(η5-C5H2-
1,2,4-Me3)2Hf(NCO)](μ-O)[(η5-C5H2-1,2,4-Me3)2Hf(CN)]
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at 23 °C, [1-(NCO)]4 slowly converted to [1-(O)(NCO)CN]
with no evidence for the formation of [1-NCO]2 or any other
intermediates. While direct observation of interconversion of
both constituents of the equilibrium was not achieved by NMR
spectroscopy, the reactivity of [1-NCO]4 toward silanes and
tert-butyl isocyanate suggests that such an equilibrium is likely
operative.
The observation of oxamidide isomerization and partial
deoxygenation upon thermolysis of [1-NCO]4 raised the
question of the generality of the transformation. Heating a
benzene-d6 solution of [(η5-C5Me4H)2Hf]2(N2C2O2) ([2-
NCO]2) to 110 °C for 1 week resulted in clean formation of
a new C1-symmetric hafnocene product identified as the μ-oxo
dihafnocene isocyanato cyanide complex [(η5-C5Me4H)2Hf-
(NCO)](μ-O)[(η5-C5Me4H)2Hf(CN)] ([2-(O)(NCO)CN];
Scheme 4). Similar chemistry was observed with the C2-
symmetric isomer of the ansa-hafnocene oxamidide complex
[Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)Hf]2(N2C2O2) ([3-NCO]2-
C2). Clean conversion to the isomerized product [Me2Si(η5-
C5Me4)(η5-C5H3-3-tBu)Hf(NCO)](μ-O)[Me2Si(η5-C5Me4)-
(η5-C5H3-3-tBu)Hf(CN)] ([3-(O)(NCO)CN]) was observed
upon heating to 110 °C for 18 h in benzene. Both [2-
(O)(NCO)CN] and [3-(O)(NCO)CN] were characterized by
X-ray diffraction, and representations of the molecular
structures are reported in Figures 6 and 7, respectively. [3-
(O)(NCO)CN] contained a crystallographically imposed
center of symmetry, with only half of the dimer being present
in the asymmetric unit. Consequently, the cyanide and
isocyanate ligands were disordered over the same metal center.
The disorder was treated through the use of a PART command,
which allowed both fragments to freely and separately refine.
Additionally, because of the proximity of the two ligands and
their similar electron densities, the final refinement was
completed using the EADP command, giving each ligand
atom similar anisotropic displacement parameters. The related
compound [2-(O)(NCO)CN] was not generated by symmetry
but displayed a similar disorder of the isocyanate and cyanide
ligands. However, attempting to treat the disorder similarly
through the use of PART commands was unsuccessful, and the
final crystallographic model therefore displays larger than usual
anisotropic displacement parameters for the disordered ligands.
The 13C NMR spectra of both compounds exhibit diagnostic
peaks centered at 162.7 and 164.4 ppm for the terminal cyanide
ligands. The 13C NMR spectra of both compounds exhibit
diagnostic peaks centered at 162.7 and 164.4 ppm for the
terminal cyanide ligands.
Figure 5. Representation of the solid-state structure of [1-(O)-
(NCO)CN] with 30% probability ellipsoids. Hydrogen atoms are
omitted for clarity.
The partial deoxygenation and isomerization of the
oxamidide ligand in [1-NCO]4 was also confirmed by
multinuclear NMR spectroscopy. The benzene-d6 13C NMR
spectrum of [1-(O)(15N13CO)13C15N] exhibits a doublet
centered at 135.4 ppm (1JC−N = 32.8 Hz), diagnostic for a
terminal hafnocene isocyanate, and a second doublet centered
at 164.1 ppm (1JC−N = 8.3 Hz), assigned to the cyanide ligand.
Accordingly, the 15N NMR spectrum exhibits two doublets at
94.15 and 324.15 ppm for the [NCO] and [CN] ligands,
respectively. The benzene solution infrared spectrum of [1-
(O)(NCO)CN] exhibits strong bands at 2223 and 2203 cm−1
for the isocyanate and cyanide ligands, respectively. These
peaks shift appropriately to 2148 and 2138 cm−1 for [1-
(O)(15N13CO)13C15N]. The spectroscopic data for [1-(O)-
(NCO)CN] are identical with those previously reported for the
hafnocene bis(isocyanate) complex [(η5-C5H2-1,2,4-
Me3)2Hf]2(NCO)(μ2-NCO),25 establishing the correct identity
of the compound as the isomeric μ-oxo dihafnium isocyanate
cyanide. Thus, carbonylation of the pyridine-stabilized
dihafnium nitride complexes provides a more facile entry
point to [1-(O)(NCO)CN].
The possibility that the dimeric and tetrametallic oxamidide
complexes are in equilibrium was explored. A recrystallized
sample of pure [1-(NCO)]4 was dissolved in benzene-d6 and
1
monitored by H NMR spectroscopy. Over the course of 24 h
Scheme 4. Thermal Isomerization of Hafnocene Oxamidide Complexes
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apparent trend as a function of molecularity. The ansa-
hafnocene oxamidide [3-(NCO)]2 undergoes the fastest
rearrangement in the series (18 h at 110 °C), while the time
scale for the tetrametallic compound [1-(NCO)]4 is longer but
comparable (48 h at 110 °C). Introduction of a methyl
substituent on each cyclopentadienyl ring furnishes a dimeric
hafnocene oxamidide compound that exhibits markedly
improved stability toward isomerization, as complete con-
version with [2-NCO]2 requires thermolysis at 110 °C for 1
week. The origin of this effect is not understood at the present
time.
The formation of a terminal cyanide ligand from thermal
isomerization of hafnocene oxamidide complexes prompted
examination of the reactivity of these compounds. We were
particularly interested in determining whether the terminal
isocyananate or cyanide is more labile and which more readily
undergoes group transfer processes. Treatment of a benzene-d6
solution of [1-(O)(NCO)CN] with Me3SiI followed by heating
to 65 °C for 3 days furnished a new C2-symmetric hafnocene
complex, identified as [(η5-C5H2-1,2,4-Me3)2Hf(I)](μ-O)[(η5-
C5H2-1,2,4-Me3)2Hf(NCO)] ([1-(O)(NCO)I]) and isolated
in 81% yield (Scheme 5). Analysis of the volatiles from the
reaction mixture by NMR spectroscopy allowed identification
of Me3SiCN as the byproduct of the reaction, confirming
preferential group transfer of the terminal cyanide ligand. The
benzene-d6 13C NMR spectrum of [1-(O)(N13CO)I] exhibited
a diagnostic terminal isocyanate resonance at 135.5 ppm, and
the solid-state infrared (KBr) displayed a strong band centered
at 2221 cm−1 for the [NCO] ligand, which shifts appropriately
to 2161 cm−1 upon 13C labeling. The identity of the
organometallic compound was also confirmed by single-crystal
X-ray diffraction (Figure 8). The unit cell contained two
crystallographically independent molecules with nearly identical
structural parameters. For each of these molecules, the iodide
ligand was disordered over both metal centers. The halide
fragments were refined to 80/20 occupancy, and the isocyanate
fragment was treated in a similar manner. The benzene-d6 1H
NMR spectrum of Me3Si13CN (Figure 9), prepared from [1-
(O)(N13CO)13CN] and Me3SiI, exhibits a doublet centered at
Figure 6. Representation of the solid-state structure of [2-(O)-
(NCO)CN] with 30% probability ellipsoids. Hydrogen atoms are
omitted for clarity. Only one of the two crystallographic independent
molecules is shown. The isocyanate and cyanide ligands are disordered
over two positions (see text).
Figure 7. Representation of the solid-state structure of (S,S)-[3-
(O)(NCO)CN] with 30% probability ellipsoids. Solvent, hydrogen
atoms, and disordered cyanide/isocyanate groups are omitted for
clarity. Half of the dimer has been generated by symmetry (see text).
3
−0.22 ppm with a JC−H coupling of 2.7 Hz. The 13C NMR
spectrum of the same material exhibits a doublet at −2.1 ppm
(2JC−C = 5.0 Hz) for the [Me3Si] group and an intense
resonance at 126.9 ppm for the isotopically labeled nitrile
carbon.
Comparing the relative rates of partial deoxygenation−
isomerization among the various metallocenes reveals no
Scheme 5. Treatment of [1-(O)(NCO)CN] with Electrophiles and Synthesis of Me3SiCN and CH3CN from CO and N2
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CONCLUDING REMARKS
■
Carbonylation of the hafnocene dinitrogen complex [(η5-C5H2-
1,2,4-Me3)2Hf]2(μ2,η2:η2-N2) with 4 atm of CO resulted in
formation of a tetrametallic hafnocene oxamidide complex, a
new structural motif arising from carbon monoxide induced N2
cleavage. The more common dimeric hafnocene oxamidide
[(η5-C5H2-1,2,4-Me3)2Hf]2(N2C2O2) was observed by NMR
spectroscopy when the carbonylation reaction was performed at
slightly lower carbon monoxide pressure. The dimeric
compound converts to the tetrametallic species over time in
benzene-d6 solution. Treatment of the tetrametallic hafnocene
oxamidide with tBuNCO resulted in functionalization of two of
the four Hf−N bonds via [CO] cycloaddition. In contrast,
addition of CySiH3 resulted in 1,2-addition to all four
hafnium−nitrogen bonds and furnished a dimeric hafnocene
product. Thermolysis of the tetrametallic or previously reported
dimeric hafnocene oxamidides resulted in partial deoxygenation
and isomerization to μ-oxo dihafnocene complexes with
terminal isocyanate and cyanide ligands. For the [(η5-C5H2-
1,2,4-Me3)2Hf] derivative, addition of electrophiles such as
Me3SiI and CH3OTf resulted in preferential group transfer of
the cyano ligand, establishing a method to prepare organo-
nitriles, albeit stoichiometrically, from CO and N2.
Figure 8. Solid-state structure of [1-(O)(NCO)I] with 30%
probability ellipsoids. Hydrogen atoms are omitted for clarity. One
of the two crystallographically independent molecules is shown; one
disordered iodine atom is omitted for clarity.
Performing a similar experiment with CH3OTf at 23 °C and
[1-(O)(NCO)CN] also furnished [(η5-C5H2-1,2,4-Me3)2Hf-
(OTf)](μ-O)[(η5-C5H2-1,2,4-Me3)2Hf(NCO)] ([1-(O)-
(NCO)OTf]) as a white solid in 91% yield (Scheme 5). As
with the related iodo product, [1-(O)(NCO)OTf] exhibits a
diagnostic 13C NMR resonance at 135.9 ppm for the terminal
isocyanate ligand and a strong band at 2221 cm−1 in the
benzene-d6 solution infrared spectrum. Analysis of the volatile
reaction products by 1H NMR spectroscopy following
treatment of [1-(O)(N13CO)13CN] with CH3OTf confirmed
formation of CH313CN, as evidenced by a doublet (2JC−H = 9.9
Hz) centered at 0.57 ppm (Figure 9). Accordingly, one doublet
(1JC−C = 57.9 Hz) was observed at 0.40 ppm and an intense
singlet was observed at 116.3 ppm in the 13C NMR spectrum
for the methyl and 13C-labeled cyano groups, respectively.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were carried out using standard high-vacuum-line, Schlenk, or
cannula techniques or in an M. Braun inert atmosphere drybox
containing an atmosphere of purified nitrogen. The M. Braun drybox
was equipped with a cold well designed for freezing samples in liquid
nitrogen. Solvents for air- and moisture-sensitive manipulations were
dried and deoxygenated using literature procedures.35 Deuterated
solvents for NMR spectroscopy were distilled from sodium metal
under an atmosphere of argon and stored over 4 Å molecular sieves.
Carbon monoxide gas was passed through a column containing 4 Å
molecular sieves before use. The hafnocene dinitrogen compound
[(η5-C5H2-1,2,4-Me3)2Hf]2(μ2,η2:η2-N2)27 and the hafnocene oxami-
Figure 9. Benzene-d6 1H NMR spectra of Me3Si13CN (left) and CH313CN (right) at 23 °C.
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dide complexes [(η5-C5Me4H)2Hf]2(N2C2O2)25 and [Me2Si(η5-
C5Me4)(η5-C5H3-3-tBu)Hf]2(N2C2O2)21 were prepared according to
literature procedures.
°C): δ 371.07 (t, JCN, JCN = 3.9, N2C2O2). IR (KBr): 1625 cm−1
(CN).
1
2
Preparation of [(η5-C5H2-1,2,4-Me3)2Hf]4(N2C2O2)2(tBuNCO)2.
A thick-walled glass vessel was charged with a stirbar, 0.035 g (0.021
mmol) of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4, and 5 mL of THF. To
1H NMR spectra were recorded on a Varian Inova 400
spectrometer operating at 399.860 MHz. All chemical shifts are
t
1
this solution was added 0.010 g (0.082 mmol) of BuNCO via
reported relative to SiMe4 using H (residual) chemical shifts of the
solvent as a secondary standard. 13C NMR spectra were recorded on a
Bruker 500 spectrometer operating at 125.71 MHz. 13C chemical shifts
are reported relative to SiMe4 using chemical shifts of the solvent as a
secondary standard where applicable. 15N NMR spectra were recorded
on a Bruker 500 spectrometer operating at 50.663 MHz, and 15N
chemical shifts are reported relative to liquid NH3 using an external
standard. Infrared spectroscopy was conducted on a Thermo-Nicolet
iS10 FT-IR spectrometer calibrated with a polystyrene standard.
Elemental analyses were performed at Robertson Microlit Laborato-
ries, Inc., in Ledgewood, NJ.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and then
quickly transferred to the goniometer head of a Bruker X8 APEX2
diffractometer equipped with molybdenum and copper X-ray tubes (λ
= 0.710 73 and 1.541 84 Å respectively). Preliminary data revealed the
crystal system. The collection routine was optimized using the Bruker
COSMO software suite. The space group was identified, and the data
were processed using the Bruker SAINT+ program and corrected for
absorption using SADABS. The structures were solved using direct
methods (SHELXS) completed by subsequent Fourier synthesis and
refined by full-matrix least-squares procedures.
microsyringe. The vessel was removed from the box, and the solution
was stirred for 18 h at 65 °C. The solvent was removed in vacuo and
the yellow residue recrystallized from toluene at −35 °C to give yellow
blocks identified as [(η5-C5H2-1,2,4-Me3)2Hf]4(N2C2O2)2(tBuNCO)2
in 55% yield (22 mg, 0.012 mmol). Anal. Calcd for C78H106Hf4N6O6:
1
C, 48.35; H, 5.51; N, 4.34. Found: C, 48.59; H, 5.78; N, 3.98. H
NMR (benzene-d6, 23 °C): δ 1.78 (s, 18H, NCMe3), 2.07 (s, 12H,
C5H2-1,2,4-Me3), 2.09 (s, 12H, C5H2-1,2,4-Me3), 2.21 (s, 12H, C5H2-
1,2,4-Me3), 2.26 (s, 12H, C5H2-1,2,4-Me3), 2.30 (s, 12H, C5H2-1,2,4-
4
Me3), 2.34 (s, 12H, C5H2-1,2,4-Me3), 5.71 (d, JHH = 2.4, 4H, C5H2-
4
4
1,2,4-Me3), 5.79 (d, JHH = 2.4, 4H, C5H2-1,2,4-Me3), 5.93 (d, JHH
=
4
2.4, 4H, C5H2-1,2,4-Me3), 6.24 (d, JHH = 2.4, 4H, C5H2-1,2,4-Me3).
13C{1H} NMR (benzene-d6, 23 °C): δ 13.5, 13.5, 13.9, 14.7, 15.3, 16.7
(C5H2-1,2,4-Me3), 32.2 (NCMe3), 53.1 (NCMe3), 111.5, 111.5, 112.1,
114.3, 117.4, 118.6, 118.8, 120.0, 122.4, 122.6 (C5H2-1,2,4-Me3), 160.5
(tBuNCO), 161.0 (d, JCC = 77.6, N2C2O2), 162.3 (d, JCC = 77.6,
1
1
N2C2O2). IR (C6H6): 1637, 1597 cm−1 (CN).
Preparation of [(η5-C5H2-1,2,4-Me3)2Hf]2(N2C2O2)(tBuNCO)2. A
thick-walled glass vessel was charged with a stirbar, 0.070 g (0.042
mmol) of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4, and 5 mL of benzene.
t
Via microsyringe, 0.019 g (0.160 mmol) of BuNCO was added, the
Preparation of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4 ([1-(NCO)]4). A
thick-walled glass vessel was charged with 0.100 g (0.123 mmol) of
[(η5-C5H2-1,2,4-Me3)2Hf]2(η2,η2-N2) and 10 mL of THF. On a high-
vacuum line, the contents of the vessel were frozen and evacuated.
Carbon monoxide gas (1 atm at 77 K) was then added. The reaction
mixture was thawed and shaken vigorously for approximately 1 min,
and a color change to light red was observed. The vessel was degassed
and brought into the glovebox, and the solution was concentrated to
approximately 2 mL in vacuo. Storage at −35 °C for 18 h resulted in
deposition of 55 mg (0.032 mmol, 51%) of analytically pure bright
yellow crystals identified as [(C5H2-1,2,4-Me3)2Hf(NCO)]4. Anal.
Calcd for C68H88Hf4N4O4: C, 46.95; H, 5.10; N, 3.22. Found: C,
46.70; H, 5.18; N, 2.91. 1H NMR (benzene-d6, 23 °C): δ 2.11 (s, 12H,
C5H2-1,2,4-Me3), 2.12 (s, 12H, C5H2-1,2,4-Me3), 2.14 (s, 12H, C5H2-
1,2,4-Me3), 2.31 (s, 12H, C5H2-1,2,4-Me3), 2.38 (s, 12H, C5H2-1,2,4-
vessel was removed from the box, and the solution was stirred for 18 h
at 130 °C. Slowly cooling the solution to room temperature deposited
colorless crystals that were washed with pentane to furnish [(η5-C5H2-
1,2,4-Me3)2Hf]2(N2C2O2)(tBuNCO)2 in 45% yield (40 mg, 0.038
mmol). Anal. Calcd for C44H62N4O4Hf2: C, 49.48; H, 5.85; N, 5.25.
1
Found: C, 49.73; H, 5.96; N, 5.14. H NMR (benzene-d6, 23 °C): δ
1.66 (s, 18H, OCNCMe3), 1.85 (s, 12H, C5H2-1,2,4-Me3), 1.97 (s,
12H, C5H2-1,2,4-Me3), 2.14 (s, 12H, C5H2-1,2,4-Me3), 5.41 (d, 4H,
4JHH = 2.2, C5H2-1,2,4-Me3), 5.52 (d, 4H, 4JHH = 2.2, C5H2-1,2,4-Me3).
13C{1H} NMR (benzene-d6, 23 °C): δ 12.8, 12.9, 14.8 (C5H2-1,2,4-
Me3), 31.7 (OCNCMe3), 52.0 (OCNCMe3), 115.2, 115.3, 116.7,
1
121.4, 122.8 (C5H2-1,2,4-Me3), 147.9 (OCNCMe3), 160.8 (t, JCN
,
2JCN = 13.4, N2C2O2). 15N{1H} NMR (benzene-d6, 23 °C): δ 191.82
(t, JCN, 2JCN = 13.4, N2C2O2). IR (C6H6): 1651, 1562 cm−1 (CN).
1
Preparation of [(η5-C5H2-1,2,4-Me3)2Hf(H)](N2C2O2)(SiH2Cy)2.
A scintillation vial was charged with a stirbar, 0.035 g (0.021 mmol) of
[(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4, and 5 mL of THF. To this
solution was added 0.010 g (0.082 mmol) of cyclohexylsilane via
microsyringe, and the contents of the vial were stirred at room
temperature for 2 h. The solvent and volatiles were removed in vacuo,
and the colorless residue was recrystallized from fluorobenzene−
pentane to give colorless crystals identified as [(η5-C5H2-1,2,4-
Me3)2Hf(H)]2(N2C2O2)(SiH2Cy)2 in 70% yield (33 mg, 0.029
mmol). Anal. Calcd for C46H72N2O2Si2Hf2: C, 50.31; H, 6.61; N,
4
Me3), 2.49 (s, 12H, C5H2-1,2,4-Me3), 5.75 (d, JHH = 2.2, 4H, C5H2-
4
4
1,2,4-Me3), 5.80 (d, JHH = 2.2, 4H, C5H2-1,2,4-Me3), 5.88 (d, JHH
=
4
2.2, 4H, C5H2-1,2,4-Me3), 5.96 (d, JHH = 2.2, 4H, C5H2-1,2,4-Me3).
1H NMR (THF-d8, 23 °C): δ 2.10 (s, 12H, C5H2-1,2,4-Me3), 2.12 (s,
12H, C5H2-1,2,4-Me3), 2.13 (s, 12H, C5H2-1,2,4-Me3), 2.16 (s, 12H,
C5H2-1,2,4-Me3), 2.18 (s, 12H, C5H2-1,2,4-Me3), 2.31 (s, 12H, C5H2-
4
1,2,4-Me3), 5.69 (d, JHH = 2.2, 4H, C5H2-1,2,4-Me3), 5.81 (d
4
4
overlapped, JHH = 2.2, 4H, C5H2-1,2,4-Me3), 5.88 (d, JHH = 2.2, 4H,
C5H2-1,2,4-Me3), 5.91 (d, JHH = 2.2, 4H, C5H2-1,2,4-Me3). 13C{1H}
4
1
NMR (benzene-d6, 23 °C): δ 13.7, 13.7, 14.3, 14.4, 15.1, 15.5 (C5H2-
1,2,4-Me3), 112.3, 114.0, 114.5, 114.6, 115.1, 118.6, 119.1, 120.8,
121.5, 122.4 (C5H2-1,2,4-Me3), 163.2 (C2O2N2). 13C{1H} NMR
(THF-d8, 23 °C): δ 13.5, 13.6, 14.3, 14.9, 15.6, 16.0 (C5H2-1,2,4-Me3),
112.3, 114.0, 114.4, 114.7, 115.2, 118.5, 119.4, 120.8, 121.7, 122.5
(C5H2-1,2,4-Me3), 163.2 (C2O2N2). IR (KBr): 1570 cm−1 (CN).
Spectroscopic Characterization of [(η5-C5H2-1,2,4-
Me3)2Hf]2(N2C2O2). A J. Young NMR tube was charged with 0.015
g (0.018 mmol) of [(η5-C5H2-1,2,4-Me3)2Hf]2(η2:η2-N2) and 0.5 g of
benzene-d6. The tube was removed from the glovebox, and 1 atm of
CO gas was added at ambient temperature. The solution was thawed
and shaken until a color change to light red was observed. The
resulting mixture was immediately analyzed by multinuclear NMR and
IR spectroscopy. 1H NMR (benzene-d6, 23 °C): δ 2.06 (s overlapped,
12H, C5H2-1,2,4-Me3), 2.07 (s overlapped, 24H, C5H2-1,2,4-Me3), 5.63
2.55. Found: C, 50.53; H, 6.70; N, 2.27. H NMR (benzene-d6, 23
°C): δ 1.05−1.68 (m overlapped, 10H, cyclohexyl CH2), 1.78 (s, 12H,
C5H2-1,2,4-Me3), 1.85 (m, 5H, cyclohexyl CH2), 2.14 (s, 12H, C5H2-
1,2,4-Me3), 2.21 (m, 7H, cyclohexyl CH2), 2.26 (s, 12H, C5H2-1,2,4-
Me3), 4.94 (s, 4H, C5H2-1,2,4-Me3), 5.10 (s, 4H, SiH2Cy), 5.62 (s, 4H,
C5H2-1,2,4-Me3), 9.37 (s, 2H, Hf-H). 13C{1H} NMR (benzene-d6, 23
°C): δ 13.1 (C5H2-1,2,4-Me3), 13.6 (C5H2-1,2,4-Me3), 16.3 (C5H2-
1,2,4-Me3), 24.9 (cyclohexyl CH), 26.1, 27.6, 28.6, 29.5 (cyclohexyl
CH2), 109.0, 109.3, 114.2, 114.8, 120.2 (C5H2-1,2,4-Me3), 174.6
(N2C2O2). IR (C6D6): 1581 (CN), 2082, 2112, 2160 cm−1 (Si-H).
Preparation of [(η5-C5H2-1,2,4-Me3)2Hf(CN)](μ-O)[(η5-C5H2-
1,2,4-Me3)2Hf(NCO)]. Method 1. A J. Young NMR tube was charged
with 0.059 g (0.034 mmol) of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4 and
1 mL of benzene. The suspension was heated for 2 days at 110 °C.
Upon slow cooling of the solution to room temperature, [(η5-C5H2-
1,2,4-Me3)2Hf(NCO)](μ-O)[(η5-C5H2-1,2,4-Me3)2Hf(CN)] crystal-
lized from the reaction mixture as colorless needles suitable for X-
ray diffraction. The supernatant was removed, and the remaining
crystals were washed with pentane to afford 0.037 g (0.043 mmol,
4
4
(d, JHH = 2.6, 4H, C5H2-1,2,4-Me3), 5.72 (d, JHH = 2.6, 4H, C5H2-
1,2,4-Me3). 13C{1H} NMR (benzene-d6, 23 °C): δ 13.5, 13.6, 15.2
(C5H2-1,2,4-Me3), 114.7, 115.0, 120.2, 122.7, 123.0 (C5H2-1,2,4-Me3),
158.9 (t, JCN, JCN = 3.9, N2C2O2). 15N{1H} NMR (benzene-d6, 23
1
2
6285
dx.doi.org/10.1021/om3005542 | Organometallics 2012, 31, 6278−6287

Organometallics
Article
63%) of white crystals identified as [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]-
(μ-O)[(η5-C5H2-1,2,4-Me3)2Hf(CN)].
C5H2-1,2,4-Me3)2Hf(CN)], 5.00 μL (0.046 mmol) of methane
trifluoromethylsulfonate, and 0.5 mL of toluene. The solution was
stirred for 18 h at 23 °C, after which time the volatiles were removed
in vacuo and the residue was washed with cold diethyl ether to afford
an analytically pure white solid identified as [(η5-C5H2-1,2,4-
Me3)2Hf(OTf)](μ-O)[(η5-C5H2-1,2,4-Me3)2Hf(NCO)] in 90%
(0.037 mmol, 37 mg) yield. Anal. Calcd for C34H44F3Hf2NO5S: C,
Method 2. A thick-walled glass vessel was charged with a stirbar,
0.049 g (0.060 mmol) of [(η5-C5H2-1,2,4-Me3)2Hf]2(η2:η2-N2), and
0.007 g (0.064 mmol) of 4-methoxypyridine. Toluene (10 mL) was
added, and a color change to blue-green was observed. On a high-
vacuum line, the contents of the vessel were frozen, the vessel was
evacuated, and CO gas (1 atm at 77 K) was admitted. The vessel was
warmed to room temperature and shaken vigorously for 1 min,
whereupon a color change to light red was observed. The solution was
stirred at room temperature for 18 h, over which time a color change
to dark brown was observed. The solvent was removed in vacuo, and
the brown oil was washed with 10 mL of cold pentane and dried in
vacuo to furnish 0.040 g (0.046 mmol, 77%) of an analytically pure
white powder. Anal. Calcd for C34H44Hf2N2O2: C, 46.95; H, 5.10; N,
1
41.13; H, 4.47; N, 1.41. Found: C, 41.01. H, 4.97; N, 1.54. H NMR
(benzene-d6, 23 °C): δ 1.74 (s, 6H, C5H2-1,2,4-Me3), 1.94 (s, 6H,
C5H2-1,2,4-Me3), 2.05 (s, 6H, C5H2-1,2,4-Me3), 2.12 (s, 6H, C5H2-
1,2,4-Me3), 2.16 (s, 6H, C5H2-1,2,4-Me3), 2.17 (s, 6H, C5H2-1,2,4-
4
4
Me3), 5.24 (d, 2H, JHH = 2.5, C5H2-1,2,4-Me3), 5.78 (d, 2H, JHH
=
4
2.5, C5H2-1,2,4-Me3), 5.87 (d, 2H, JHH = 2.5, C5H2-1,2,4-Me3), 6.15
(d, 2H, JHH = 2.5, C5H2-1,2,4-Me3). 19F NMR (benzene-d6): −76.1
4
(CF3). 13C{1H} NMR (benzene-d6, 23 °C): δ 13.0, 13.2, 13.3, 13.8,
14.9, 15.3 (C5H2-1,2,4-Me3), 113.8, 114.6, 116.7, 117.1, 118.5, 119.7,
120.1, 122.9, 123.4, 126.5 (C5H2-1,2,4-Me3), 135.9 (Hf-NCO). IR
(C6D6): 2221 (NCO), 2161 cm−1 (N13CO). Data for free MeCN are
1
3.22. Found: C, 46.76; H, 4.84; N, 3.13. H NMR (benzene-d6, 23
°C): δ 1.72 (s, 6H, C5H2-1,2,4-Me3), 1.92 (s, 6H, C5H2-1,2,4-Me3),
2.14 (s, 6H, C5H2-1,2,4-Me3), 2.16 (s, 6H, C5H2-1,2,4-Me3), 2.18 (s,
4
1
2
6H, C5H2-1,2,4-Me3), 2.52 (s, 6H, C5H2-1,2,4-Me3), 5.06 (d, 2H, JHH
as follows. H NMR (benzene-d6, 23 °C): δ 0.57 (d, 3H, JCH = 9.9,
CH3CN). 13C{1H} NMR (benzene-d6, 23 °C): δ 0.40 (d, 1JCC = 57.9,
CH3CN), 116.3 (CH3CN).
= 2.2, C5H2-1,2,4-Me3), 5.24 (d, 2H, 4JHH = 2.2, C5H2-1,2,4-Me3), 5.72
4
4
(d, 2H, JHH = 2.2, C5H2-1,2,4-Me3), 5.99 (d, 2H, JHH = 2.2, C5H2-
1,2,4-Me3). 13C{1H} NMR (benzene-d6, 23 °C): δ 13.0, 13.8, 14.9,
15.2, 15.2, 15.4 (C5H2-1,2,4-Me3), 112.8, 113.3, 114.9, 115.3, 117.6,
118.9, 118.9, 119.4, 124.6, 126.0 (C5H2-1,2,4-Me3), 134.6 (Hf-NCO),
164.2 (Hf-CN). 1H NMR (dichloromethane-d2, 23 °C): δ 1.92 (s, 6H,
C5H2-1,2,4-Me3), 1.99 (s, 6H, C5H2-1,2,4-Me3), 2.16 (s, 6H, C5H2-
1,2,4-Me3), 2.33 (s, 6H, C5H2-1,2,4-Me3), 2.38 (s overlapped, 12H,
Preparation of [(η5-C5Me4H)2Hf(NCO)](μ-O)[(η5-C5Me4H)2Hf-
(CN)]. A J. Young NMR tube was charged with 0.100 g (0.107 mmol)
of [(η5-C5Me4H)2Hf]2(N2C2O2) and 1 mL of benzene. The
suspension was heated at 110 °C for 8 days before the volatiles
were removed in vacuo, and [(η5-C5Me4H)2Hf(NCO)](μ-O)[(η5-
C5Me4H)2Hf(CN)] was recrystallized from the crude reaction mixture
as colorless blocks by the slow evaporation of a toluene solution at 23
°C. The supernatant was removed, and the remaining crystals were
washed with pentane to furnish 0.054 g (0.058 mmol, 54%) of a light
yellow solid identified as [(η5-C5Me4H)2Hf(CN)](μ-O)[(η5-
C5Me4H)2Hf(NCO)]. Anal. Calcd for C38H52Hf21N2O2: C, 49.30; H,
5.66; N, 3.03. Found: C, 49.02; H, 5.48; N, 2.77. H NMR (benzene-
d6, 23 °C): δ 1.60 (s, 3H, C5Me4H), 1.68 (s, 3H, C5Me4H), 1.78 (s,
3H, C5Me4H), 1.82 (s, 3H, C5Me4H), 1.91 (s, 3H, C5Me4H), 1.95 (s,
3H, C5Me4H), 1.96 (s, 3H, C5Me4H), 1.97 (s, 3H, C5Me4H), 2.00 (s,
6H, C5Me4H), 2.10 (s, 3H, C5Me4H), 2.16 (s, 3H, C5Me4H), 2.23 (s,
3H, C5Me4H), 2.46 (s, 3H C5Me4H), 2.52 (s, 3H, C5Me4H), 2.71 (s,
3H, C5Me4H), 5.21 (s, 1H, C5Me4H), 5.33 (s, 1H, C5Me4H), 5.54 (s,
1H, C5Me4H), 6.07 (s, 1H, C5Me4H). 13C{1H} NMR (benzene-d6, 23
°C): δ 11.43, 11.73, 11.82, 12.21, 12.29, 12.69, 12.86, 12.93, 13.23,
13.28, 13.36, 13.48, 13.52, 13.61, 13.73, 13.80 (C5Me4H), 112.5, 113.9,
114.8, 115.2, 115.3, 115.4, 115.5, 115.6, 116.6, 118.5, 121.0, 121.5,
122.1, 122.5, 122.8, 122.9, 123.2, 124.9, 125.7, 125.8 (C5Me4H), 135.6
(Hf-NCO), 164.4 (Hf-CN). IR (KBr): 2223 cm−1 (NCO).
4
C5H2-1,2,4-Me3), 5.56 (d, 2H, JHH = 2.2, C5H2-1,2,4-Me3), 5.68 (d,
2H, 4JHH = 2.2, C5H2-1,2,4-Me3), 5.70 (d, 2H, 4JHH = 2.2, C5H2-1,2,4-
4
Me3), 5.85 (d, 2H, JHH = 2.2, C5H2-1,2,4-Me3). 13C{1H} NMR
(benzene-d6, 23 °C): δ 12.9, 13.6, 13.6, 15.2, 15.3, 15.4 (C5H2-1,2,4-
Me3), 112.7, 113.2, 115.5, 115.6, 117.3, 118.4, 119.0, 119.4, 124.2,
126.0 (C5H2-1,2,4-Me3), 135.4 (d, 1JCN = 32.8, NCO), 164.1 (d, 1JCN
=
8.3, CN). 15N{1H} NMR (benzene-d6, 23 °C): δ 94.15 (d, 1JCN = 32.8,
1
NCO), 324.15 (d, JCN = 8.3, CN). IR (benzene-d6): 2223 (NCO),
2203 (CN), 2163 (13CN), 2148 (15N13CO), 2138 cm−1 (13C15N).
Preparation of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)](μ-O)[(η5-C5H2-
1,2,4-Me3)2Hf(I)]. A J. Young NMR tube was charged with 12 mg
(0.014 mmol) of [(η5-C5H2-1,2,4-Me3)2Hf(CN)](μ-O)[(η5-C5H2-
1,2,4-Me3)2Hf(NCO)], 2.1 μL (0.015 mmol) of iodotrimethylsilane,
and 0.5 mL of benzene. The solution was warmed to 65 °C for 36 h
before the volatiles were removed in vacuo and the residue was washed
with 2 × 5 mL of cold pentane to furnish 46 mg (0.045 mmol, 81%) of
an analytically pure white solid identified as [(η5-C5H2-1,2,4-
Me3)2Hf(I)](μ-O)[(η5-C5H2-1,2,4-Me3)2Hf(NCO)]. The reported
yield is from four combined NMR tubes. Crystals suitable for X-ray
diffraction were grown by slow evaporation of a concentrated
fluorobenzene solution at −35 °C for 4 days. Scale-up of this reaction
was hampered by the relative insolubility of [1-(O)(CN)(NCO)] in
benzene. Due to this complication, the stoichiometry of the reaction
was more difficult to control on larger scales and resulted in the
formation of unwanted side products. Pure product was obtained most
reliably on an NMR scale. Anal. Calcd for C33H44Hf2INO2: C, 40.84;
H, 4.57; N, 1.44. Found: C, 40.67; H, 4.66; N, 1.66. 1H NMR
(benzene-d6, 23 °C): δ 1.73 (s, 6H, C5H2-1,2,4-Me3), 1.91 (s, 6H,
C5H2-1,2,4-Me3), 2.18 (s, 6H, C5H2-1,2,4-Me3), 2.20 (s, 6H, C5H2-
1,2,4-Me3), 2.22 (s, 6H, C5H2-1,2,4-Me3), 2.45 (s, 6H, C5H2-1,2,4-
Preparation of ([Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)]Hf(NCO))(μ-
O)([Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)]Hf(CN)). A thick-walled glass
vessel was charged with 0.160 g (0.162 mmol) of ([Me2Si(η5-
C5Me4)(η5-C5H3-3-tBu)]Hf)2(μ2-N2) and 10 mL of toluene. The
vessel containing the dark purple solution was removed from the
glovebox and degassed on a high-vacuum line, and 1 atm of CO gas
was admitted at 77 K. The resulting dark red solution was stirred for 2
h before the CO was removed in vacuo and replaced with 1 atm of N2
gas. The dark red solution was then heated to 110 °C with rapid
stirring for 18 h. The volatiles were removed from the resulting brown
solution in vacuo, and the oily brown residue was recrystallized from a
minimum amount of diethyl ether at −35 °C to afford 0.075 g (0.072
mmol, 47% yield) of an analytically pure white solid identified as
([Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)]Hf(CN))(μ-O)([Me2Si(η5-
C5Me4)(η5-C5H3-3-tBu)]Hf(NCO)) from multiple batches. Colorless
crystals suitable for X-ray diffraction were grown by slow evaporation
of a concentrated THF solution stored at −35 °C for 3 days. Anal.
Calcd for C42H60Hf2N2O2Si2: C, 48.59; H, 5.83; N, 2.70. Found: C,
48.51; H, 5.87; N, 2.41. 1H NMR (benzene-d6, 23 °C): δ 0.42 (s, 3H,
SiMe3), 0.49 (s, 3H, SiMe3), 0.58 (s, 3H, SiMe3), 0.59 (s, 3H, SiMe3),
1.39 (s, 9H, C5H3CMe3), 1.61 (s, 9H, C5H3CMe3), 1.79 (s, 3H,
C5Me4), 1.86 (s, 3H, C5Me4), 1.90 (s, 3H, C5Me4), 1.95 (s, 3H,
C5Me4), 2.02 (s, 3H, C5Me4), 2.13 (s, 3H, C5Me4), 2.20 (s, 3H,
C5Me4), 2.39 (s, 3H, C5Me4), 5.27 (m, 1H, C5H3CMe3), 5.42 (m, 1H,
C5H3CMe3), 5.89 (m, 1H, C5H3CMe3), 6.05 (m, 1H, C5H3CMe3),
4
4
Me3), 5.27 (d, 2H, JHH = 1.9, C5H2-1,2,4-Me3), 5.30 (d, 2H, JHH
=
4
1.9, C5H2-1,2,4-Me3), 5.91 (d, 2H, JHH = 1.9, C5H2-1,2,4-Me3), 6.24
(d, 2H, JHH = 1.9, C5H2-1,2,4-Me3). 13C{1H} NMR (benzene-d6, 23
4
°C): δ 13.0, 13.9, 15.1, 15.4, 16.0, 16.5 (C5H2-1,2,4-Me3), 114.0, 114.2,
114.5, 114.6, 117.5, 119.0, 119.4, 120.1, 124.1, 126.3 (C5H2-1,2,4-
Me3), 135.5 (Hf-NCO). IR (C6D6): 2221 cm−1 (NCO), 2161
cm−1(N13CO). Data for free Me3SiCN are as follows. 1H NMR
(benzene-d6, 23 °C): δ −0.22 (d, 9H, 3JCH = 2.7, Me3SiCN). 13C{1H}
2
NMR (benzene-d6, 23 °C): δ −2.1 (d, JCC = 5.0, Me3SiCN), 126.9
(Me3SiCN). IR (C6D6): 2190 cm−1 (CN), 2140 cm−1 (13CN).
Preparation of [(η5-C5H2-1,2,4-Me3)2Hf(OTf)](μ-O)[(η5-C5H2-
1,2,4-Me3)2Hf(NCO)]. A 20 mL scintillation vial was charged with
36 mg (0.041 mmol) of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)](μ-O)[(η5-
6286
dx.doi.org/10.1021/om3005542 | Organometallics 2012, 31, 6278−6287

Organometallics
Article
6.46 (m, 1H, C5H3CMe3), 6.49 (m, 1H, C5H3CMe3). 13C{1H} NMR
(benzene-d6, 23 °C): δ −0.5, −0.4, 0.1, 0.2 (SiMe2), 11.8, 12.1, 13.8,
14.1, 14.4, 14.9, 15.0, 15.6 (C5Me4), 31.3, 31.8 (C5H3CMe3), 33.8, 33.9
(C5H3CMe3), 100.0, 101.4, 106.4, 106.9, 108.0, 108.3, 108.4, 110.4,
112.4, 112.6, 118.5, 118.6, 121.9, 122.3, 126.0, 129.7, 132.9, 133.3,
154.0, 155.0 (Cp C), 135.8 (NCO), 162.7 (CN). IR (C6H6): 2228
cm−1 (NCO).
B.; Katokov, N. A.; Nekaeva, I. M.; Kudryavtsev, V. Chem. Commun.
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(21) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. Nat. Chem. 2010, 2,
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(22) For carbonylation of early-transition-metal nitrides prepared
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D. J. J. Am. Chem. Soc. 2010, 132, 1458. (b) Silvia, J. S.; Cummins, C.
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(23) For CO-induced N2O cleavage see: Reeds, J. P.; Yonke, B. L.;
Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2011, 133, 18602.
(24) Sobota, P.; Janas, Z. J. Organomet. Chem. 1984, 276, 171.
(25) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
2010, 132, 10553.
(26) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
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(27) Semproni, S. P.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
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(28) Knobloch, D. J.; Semproni, S. P.; Lobkovsky, E.; Chirik, P. J. J.
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ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving crystallographic details for [1-(NCO)]4, [1-
(NCO)]4(tBuNCO)2, [1-(NCO)]2(tBuNCO)2, [1-
(NCO)]2(CySiH3)2, [1-(O)(NCO)CN], [2-(O)(NCO)CN],
(S,S)-[3-(O)(NCO)CN], and [1-(O)(NCO)I] and figures
giving representative NMR spectra and a representation of the
solid-state structure of (R,R)-[3-(O)(NCO)CN]. This material
is available free of charge via the Internet at http://pubs.acs.org.
(29) Zhang, X.; Butschke, B.; Schwarz, H. Chem. Eur. J. 2010, 16,
12564.
(30) Semproni, S. P.; Milsmann, C.; Chirik, P. J. Angew. Chem., Int.
Ed. 2012, 51, 5213.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
■
(31) Schinnerling, P.; Thewalt, U. J. Organomet. Chem. 1992, 431, 41.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Director of the Office of Basic Energy Sciences,
Chemical Sciences Division, U.S. Department of Energy (DE-
FG-02-05ER15659), for financial support. S.P.S. thanks the
Natural Sciences and Engineering Research Council of Canada
for a predoctoral fellowship (PGS-D).
(35) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
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