Functionalization of hafnium oxamidide complexes prepared from CO-induced N2 cleavage

Article abstract of DOI:10.1021/ja106848y

Functionalization of the nitrogen atoms in the hafnocene oxamidide complexes [Me₂Si(η⁵-C₅Me ₄)(η⁵-C₅H₃-3- ^tBu)Hf]₂(N₂C₂O₂) and [(η⁵-C₅Me₄H)₂Hf] ₂(N₂C₂O₂), prepared from CO-induced N₂ bond cleavage, was explored by cycloaddition and by formal 1,2-addition chemistry. The ansa-hafnocene variant, [Me₂Si(η ⁵-C₅Me₄)(η⁵-C₅H ₃-3-^tBu)Hf]₂(N₂C₂O ₂), undergoes facile cycloaddition with heterocumulenes such as ^tBuNCO and CO₂ to form new N-C and Hf-O bonds. Both products were crystallographically characterized, and the latter reaction demonstrates that an organic ligand can be synthesized from three abundant and often inert small molecules: N₂, CO, and CO₂. Treatment of [Me₂Si(η⁵-C₅Me₄) (η⁵-C₅H₃-3-^tBu)Hf] ₂(N₂C₂O₂) with I₂ yielded the monomeric iodohafnocene isocyanate, Me₂Si(η⁵- C₅Me₄)(η⁵-C₅H ₃-3-^tBu)Hf(I)(NCO), demonstrating that C-C bond formation is reversible. Alkylation of the oxamidide ligand in [(η⁵-C ₅Me₄H)₂Hf]₂(N₂C ₂O₂) was explored due to the high symmetry of the complex. A host of sequential 1,2-addition reactions with various alkyl halides was discovered and both N- and N,N′-alkylated products were obtained. Treatment with Bronsted acids such as HCl or ethanol liberates the free oxamides, H(R¹)NC(O)C(O)N(R²)H, which are useful precursors for N,N′-diamines, N-heterocyclic carbenes, and other heterocycles. Oxamidide functionalization in [(η⁵-C ₅Me₄H)₂Hf]₂(N₂C ₂O₂) was also accomplished with silanes and terminal alkynes, resulting in additional N-Si and N-H bond formation, respectively.

Full text of DOI:10.1021/ja106848y

Published on Web 10/11/2010
Functionalization of Hafnium Oxamidide Complexes Prepared
from CO-Induced N2 Cleavage
Donald J. Knobloch, Emil Lobkovsky, and Paul J. Chirik*
Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell UniVersity,
Ithaca, New York 14853
Received July 31, 2010; E-mail: pc92@cornell.edu
Abstract: Functionalization of the nitrogen atoms in the hafnocene oxamidide complexes [Me2Si(η5-
C5Me4)(η5-C5H3-3-tBu)Hf]2(N2C2O2) and [(η5-C5Me4H)2Hf]2(N2C2O2), prepared from CO-induced N2 bond
cleavage, was explored by cycloaddition and by formal 1,2-addition chemistry. The ansa-hafnocene variant,
[Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)Hf]2(N2C2O2), undergoes facile cycloaddition with heterocumulenes such
t
as BuNCO and CO2 to form new N-C and Hf-O bonds. Both products were crystallographically
characterized, and the latter reaction demonstrates that an organic ligand can be synthesized from three
abundant and often inert small molecules: N2, CO, and CO2. Treatment of [Me2Si(η5-C5Me4)(η5-C5H3-3-
tBu)Hf]2(N2C2O2) with I2 yielded the monomeric iodohafnocene isocyanate, Me2Si(η5-C5Me4)(η5-C5H3-3-
tBu)Hf(I)(NCO), demonstrating that C-C bond formation is reversible. Alkylation of the oxamidide ligand in
[(η5-C5Me4H)2Hf]2(N2C2O2) was explored due to the high symmetry of the complex. A host of sequential
1,2-addition reactions with various alkyl halides was discovered and both N- and N,N′-alkylated products
were obtained. Treatment with Brønsted acids such as HCl or ethanol liberates the free oxamides,
H(R1)NC(O)C(O)N(R2)H, which are useful precursors for N,N′-diamines, N-heterocyclic carbenes, and other
heterocycles. Oxamidide functionalization in [(η5-C5Me4H)2Hf]2(N2C2O2) was also accomplished with silanes
and terminal alkynes, resulting in additional N-Si and N-H bond formation, respectively.
incorporated into a cycle for the synthesis of organic nitriles.8
Sita and co-workers have since reported a dinuclear tantalum
Introduction
Coupling the cleavage of the strong NtN triple bond of
atmospheric dinitrogen with nitrogen-carbon bond-forming
reactions is an attractive synthetic route to nitrogen-containing
organic molecules,1-3 as the high energy and fossil fuels costs
of the Haber-Bosch industrial ammonia synthesis would be
avoided.4-6 One approach to achieving this goal is to use a
soluble transition metal complex to promote the N2 cleavage
reaction, followed by elaboration of the resulting metal nitrido
species in subsequent steps with the appropriate reagents to
effect N-C bond formation.7-9 The three-coordinate molyb-
denum(III) compound, Mo(N(tBu)-3,5-Me2-C6H3)3, has been a
focal point of such studies, as this molecule is isolobal with a
nitrogen atom and promotes N2 cleavage at low temperature.10-12
The resulting molybdenum nitride, NMo(N(tBu)-3,5-Me2-
C6H3)3, has served as a nitrogen source for organic molecules
from treatment with trifluoroacetic anhydride7 and has been
end-on dinitrogen complex that promotes the cleavage of N2 at
low temperatures (<0 °C) and undergoes subsequent hydrosi-
lylation to form a silylimido upon treatment with PhSiH3.13
Kawaguchi and co-workers have described the formation of a
diniobium-bridging nitrido species following reductive elimina-
tion of H2 from the corresponding tetrahydride species. Addition
of CH3I resulted in successive alkylation of the nitrogen atoms.14
An alternative strategy is ligand-induced N2 bond cleavage
where NtN scission and NsC bond formation occur in a single
transformation. This approach is potentially broader in scope,
as the transition metal complex does not need to supply all six
electrons for N2 cleavage and a host of reagents to promote the
desired chemistry are plausible. Sobota and Janas provided early
experimental precedent for the viability of this route with the
discovery of N,N-dimethylformamide formation following treat-
ment of an ill-defined titanium dinitrogen complex with carbon
monoxide and subsequently CH3I.15 More recently, Fryzuk and
co-workers have reported a rich N2 cleavage and functional-
ization chemistry to form tantalum imides and nitrides from
(1) MacKay, B. A.; Fryzuk, M. D. Chem. ReV. 2004, 104, 385.
(2) Chatt, J. Philos. Trans. R. Soc. London, Ser B. 1977, 281, 243.
(3) Gambarotta, S. J. Organomet. Chem. 1995, 500, 117.
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E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.
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10.1021/ja106848y  2010 American Chemical Society

Functionalization of Hafnium Oxamidide Complexes
A R T I C L E S
Scheme 1. CO-Induced N2 Cleavage by Hafnocene Dinitrogen Complexes
addition of simple hydride reagents such as boranes,16,17
silanes,18,19 alanes,20 and zirconium hydrides21 to ([NPN]Ta)2(µ-
H)2(µ2,η1,η2-N2) (NPN ) PhP(CH2SiMe2NPh)2).22
Zirconocene and hafnocene dinitrogen complexes with strongly
activated, side-on-bound N2 ligands offer rich functionalization
chemistry,23 including hydrogenation to ammonia24-26 and N-C
bond formation via isocyanate27 and CO2 insertion28,29 as well
as stepwise addition of electrophiles such as CH3OTf to form
metal-bound diazenide and hydrazonato compounds.30 The ansa-
hafnocene derivative, [Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)-
Hf]2(µ2, η2, η2-N2), exhibits one of the most activated N-N
bonds in this family of compounds, as evidenced by an elongated
N-N distance of 1.457(5) Å.31 Carbonylation with 1-4 atm
of CO at 23 °C resulted in facile N-N bond cleavage, with
concomitant formation of two N-C bonds and one new C-C
bond (Scheme 1).31 The resulting bridging, tetraanionic ligand,
[N2C2O2]4-, was termed “oxamidide”, as it is a conjugate base
of oxamide, H2NC(O)C(O)NH2. Performing the carbonylation
with only 1 equiv of carbon monoxide also resulted in CO-
induced bond cleavage, forming terminal isocyanate and bridg-
ing imido ligands (Scheme 1). The latter likely result from C-H
activation by a transient µ-nitrido dihafnium species. The
generality of the carbonylation reaction was recently explored
within a series of zirconocene and hafnocene complexes with
side-on-bound N2 ligands.32 In all four cases examined, oxa-
(16) MacKay, B. A.; Johnson, S. A.; Patrick, B. O.; Fryzuk, M. D. Can.
J. Chem. 2005, 83, 315.
(17) Fryzuk, M. D.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O. Angew.
Chem., Int. Ed. 2002, 41, 3709.
(18) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc.
2003, 125, 3234.
(19) MacKay, B. A.; Munha, R. F.; Fryzuk, M. D. J. Am. Chem. Soc. 2006,
128, 9472.
(20) MacKay, B. A.; Patrick, B. O.; Fryzuk, M. D. Organometallics 2005,
24, 3836.
(21) Spencer, L. P.; MacKay, B. A.; Patrick, B. O.; Fryzuk, M. D. Proc.
Natl. Acad. Sci. U.S.A. 2006, 103, 17094.
(22) Fryzuk, M. D. Acc. Chem. Res. 2009, 42, 127.
(23) Chirik, P. J. Dalton Trans. 2007, 16.
(24) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527.
(25) Pool, J. A.; Bernskoetter, W. H.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 14326.
(29) Knobloch, D. J.; Toomey, H. E.; Chirik, P. J. J. Am. Chem. Soc. 2008,
130, 4248.
(30) Knobloch, D. J.; Benito-Garagorri, D.; Bernskoetter, W. H.; Keresztes,
(26) Bernskoetter, W. H.; Olmos, A. V.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2006, 25, 1021.
I.; Lobkovsky, E.; Toomey, H.; Chirik, P. J. J. Am. Chem. Soc. 2009,
131, 14903.
(27) Bernskoetter, W. H.; Olmos, A. V.; Pool, J. A.; Lobkovsky, E.; Chirik,
P. J. J. Am. Chem. Soc. 2006, 128, 10696.
(31) Knobloch, D. J.; Lobkovsky, E.; Chirik, P. J. Nature Chem. 2010, 2,
30.
(28) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Angew. Chem., Int.
Ed. 2007, 46, 2858.
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132, 10553.
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A R T I C L E S
Knobloch et al.
Figure 1. DFT-computed (B3LYP functional) Mayer bond orders and the highest occupied molecular orbital of 2.
midide formation was observed. For the Zr congeners, dinitrogen
activation is typically less pronounced, and zirconocene dicar-
bonyl complexes were also formed in small amounts, demon-
strating that N2 loss was competitive with CO-induced cleavage.
that these atoms may be further elaborated by serving as
nucleophiles or as bases.
Oxamidide Functionalization via Cycloaddition of Heter-
ocumulenes. The possibility of cycloaddition-type reactivity with
the metallocene oxamidides was explored by the addition of
heterocumulenes. Only hafnocene oxamidide complexes 1 and
2 were studied, as the zirconium congeners suffer from
chemistry derived from N2 loss that is competitive with
dinitrogen carbonylation.
Our laboratory previously reported that addition of isocyanates
to [(η5-C5Me4H)Hf]2(µ2,η2,η2-N2) resulted in formation of
nitrogen-carbon bonds arising from both insertion and cy-
cloaddition reactions with the hafnium-nitrogen bonds in the
dinitrogen complex with the heterocumulene.27 Inspired by this
result, similar reactivity was explored with the ansa-hafnocene
oxamidide complex, 1. As described previously,31 this complex
can be synthesized as either the C1- or C2-symmetric isomer,
depending on the pressure of CO used for dinitrogen carbony-
lation and cleavage. Due to its relative ease of isolation, the
C2-symmetric isomer, 1-C2 (as a racemic mixture), was used
for these studies. Addition of an excess (∼5 equiv) of tert-butyl
isocyanate to a toluene solution of 1-C2, followed by removal
of the volatiles and recrystallization from diethyl ether at -35
°C, furnished a yellow solid identified as 1-(tBuNCO)2, arising
from double isocyanate cycloaddition to the oxamidide, in 56%
yield (eq 1).
The oxamidide ligands formed from CO-induced N2 bond
cleavage present a unique opportunity to assemble N-containing
organic molecules from atmospheric nitrogen. One encouraging
feature of this approach is that the NtN bond cleavage event,
likely one of the most difficult transformations in the sequence,
has been accomplished under mild conditions. The remaining
challenges are to further elaborate the [N2C2O2]4- core into
value-added products and to release it from the metal. Here we
describe additional N-C bond-forming reactions in hafnocene
oxamidide complexes by cycloaddition of heterocumulenes and
by 1,2-addition of various alkyl halides, silanes, and alkynyl
C-H bonds. Facile cycloaddition of carbon dioxide was
observed and demonstrates that organic ligands can be synthe-
sized from three abundant and often inert small molecules: N2,
CO, and CO2. In the cases of alkyl halide addition, subsequent
treatment with Brønsted acids released free N-alkyl and N,N′-
dialkyl oxamides, which are useful synthons for diamines,
N-heterocyclic carbenes, and other heterocycles.
Results and Discussion
The electronic structure of the hafnocene oxamidide com-
plexes was determined using full-molecule DFT calculations
to provide guiding principles for subsequent reactivity studies.
Due to its higher symmetry and relative simplicity, calculations
were carried out with 2 using the B3LYP functional. Figure 1
presents the Mayer bond orders determined from the calculations
as well as the HOMO of the molecule. The Hf-N bond order
of 1.231 indicates multiple bond character, arising from π-dona-
tion of the oxamidide nitrogen, and suggests that cycloaddition
reactivity33-35 to elaborate the nitrogen may be possible. The
HOMO of the molecule is composed principally of oxamidide
nitrogen lone-pair character in the metallocene wedge, indicating
(33) Duncan, A. P.; Bergman, R. G. Chem. Rec. 2002, 2, 431.
(34) Hazari, N.; Mountford, P. Acc. Chem. Res. 2005, 38, 839.
(35) (a) Guiducci, A. E.; Boyd, C. L.; Mountford, P. Organometallics 2006,
25, 1167. (b) Dunn, S. C.; Hazari, N.; Cowley, A. R.; Green, J. C.;
Mountford, P. Organometallics 2006, 25, 1755. (c) Zuckerman, R. L.;
Bergman, R. G. Organometallics 2001, 20, 1792. (d) Zuckerman,
R. L.; Bergman, R. G. Organometallics 2000, 19, 4795. (e) Thorman,
J. E.; Guzei, I. A.; Young, V. G.; Woo, L. K. Inorg. Chem. 1999, 38,
3814. (f) Polse, J. L.; Andersen, R. A.; Bergman, R. G. J. Am. Chem.
Soc. 1998, 120, 13405.
The benzene-d6 1H and 13C NMR spectra of 1-(tBuNCO)2
exhibit the number of peaks consistent with formation of a
single, C2-symmetric product. Complete peak assignments are
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Functionalization of Hafnium Oxamidide Complexes
A R T I C L E S
right). The solid-state structure was determined by X-ray
diffraction, and a representation of the (S,S)-enantiomer is
presented in Figure 3 (left). Similar to 1-(tBuNCO)2, the
crystallographic data establish cycloaddition of two molecules
of CO2 to each of the hafnium-nitrogen bonds with concomitant
formation of new N-C and Hf-O bonds. The metrical
parameters for 1-(CO2)2 demonstrate that in the solid state the
compound exhibits C-N elongation and CdO contraction
similar to that observed with 1-(tBuNCO)2. The core is nearly
planar and the dihedral angle between the hafnocene subunits
is 3.0°.
The cycloaddition reactivity of the hafnocene oxamidide
complex with carbon dioxide contrasts the CO2 insertion
chemistry observed from carbonylation of the hafnocene and
zirconocene N2 compounds. Perhaps most notably, the synthesis
of 1-(CO2)2 demonstrates that an organic ligand can be
synthesized from three typically inert and abundant small
molecules: N2, CO, and CO2.
Figure 2. Representation of the solid-state structure of (S,S)-1-(tBuNCO)2
at 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
reported in the Experimental Section. Diagnostic bands at 1673
and 1647 cm-1 were observed in the solid-state (KBr) infrared
spectrum. The identity of the preferred diastereomer was
established by X-ray diffraction. A representation of the (S,S)
enantiomer of 1-(tBuNCO)2 is presented in Figure 2; the (R,R)
enantiomer is reported in the Supporting Information. Selected
bond distances and angles of all crystallographically character-
ized compounds in this work as well as those for the hafnocene
oxamidides 1 and 2 are presented in Table 1. The crystal-
lographic data are consistent with the solution NMR spectro-
scopic studies and establish formation of a C2-symmetric product
arising from cycloaddition of the CdO portion of the heterocu-
mulene. Key features are the elongated C-N bonds and
contracted C-O bonds in the oxamidate core in comparison to
the parent oxamidide 1-C2, consistent with increased C-N single
bond and CdO double bond character. The stereochemistry of
the starting hafnocene is preserved in the cycloaddition product,
placing the tert-butyl groups of the isocyanate groups syn to
the tert-butyl substituents on the cyclopentadienyl ligand. The
newly formed core maintains a nearly planar configuration, and
the hafnocene subunits are slightly twisted with respect to each
other with a dihedral angle of 1.7°.
Oxidative C-C Cleavage of Hafnocene Oxamidides with
Iodine. In addition to heterocumulenes, the reactivity of the
ansa-hafnocene oxamidide 1 with iodine was also explored.
Addition of 1 equiv of I2 to a toluene solution of 1 (as a mixture
of C1 and C2 isomers) yielded a single new C1-symmetric
product, identified as the monomeric iodo ansa-hafnocene
isocyanate, 1-(I)(NCO), as a pale yellow solid (eq 3).
t
The clean and facile cycloaddition of BuNCO to 1-C2
prompted exploration of related chemistry with carbon dioxide.
Our laboratory has previously reported dinitrogen functional-
ization with CO2 to yield, following treatment with an electro-
phile, various dicarboxylated hydrazines. For [(η5-C5Me4H)2-
Hf]2(µ2,η2,η2-N2),28 an 85:15 mixture of N,N and N,N′ insertion
products was obtained, while for the ansa-zirconocene, [Me2Si(η5-
C5Me4)(η5-C5H3-3-tBu)Zr]2(µ2,η2,η2-N2), exclusive N,N′ inser-
tion was observed.29 In both compounds, only insertion, not
cycloaddition, of the two CO2 molecules into the metal-nitrogen
bonds was observed.
Addition of 2 equiv of CO2 gas to a frozen toluene solution
of 1-C2, followed by warming to ambient temperature and
stirring for 1 h, produced a yellow solution from which a yellow
solid identified as 1-(CO2)2 was isolated in 88% yield (eq 2).
The benzene-d6 1H and 13C NMR spectra exhibit the number
of peaks consistent with formation of a single diastereomer of
a C2-symmetric product. Isotopic labeling with both 13CO and
13CO2 gases produced multiplets arising from an AA′XX′ spin
2
system centered at 163.18 and 150.54 ppm with with JC-C
)
The presence of a terminal isocyanate ligand formed from
oxidative C-C cleavage of the oxamidide core was established
by a combination of IR spectroscopy, multinuclear NMR
experiments, and X-ray diffraction. The solid-state infrared
3JC-C ) 1.5 Hz,36 confirming CO2 incorporation (Figure 3,
(36) Values reported as an average for the AA′XX′ spin system. See the
Supporting Information for details.
spectrum (KBr) exhibits a strong band centered at 2229 cm-1
,
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A R T I C L E S
Knobloch et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) for 1,31 2,32 and Compounds Crystallographically Characterized in This Work
1-C2
1-(tBuNCO)2
1-(CO2)2
2c
2-(PhSiH3)2d
2-(CH3I)2d
d(Hf-N)
d(Hf-O)
2.062(2)
2.0557(19)
2.0570(16)
2.0584(17)
1.534(3)
1.270(3)
1.275(3)
1.341(3)
1.339(3)
2.165(2)
2.175(2)
2.2440(18)
2.2115(18)
1.506(4)
1.295(3)
1.302(3)
1.267(3)
1.269(3)
1.417(3)
1.407(3)
2.2054(18)
2.1970(19)
2.174(2)
2.186(2)
2.2528(18)
2.2432(17)
1.524(4)
1.299(3)
1.300(3)
1.258(3)
1.259(3)
1.420(3)
1.429(3)
2.1880(18)
2.2081(19)
2.043(5) (E3)
2.057(4) (E1)
2.058(5) (E4)
2.062(4) (E2)
1.531(8)
1.299(7) (E3)
1.320(8) (E1)
1.303(7) (E4)
1.297(7) (E2)
2.322(4)
2.358(6)
2.253(3)
2.211(4)
d(C-C)
d(C-N)
1.499(8)
1.317(5)
1.468(9)
1.295(9)
d(C-O)
1.273(5)
1.786(4)
1.91(4)
1.277(9)
1.47(1)
d(N-E)a
d(Hf-X)b
2.9788(7)
a E denotes heteroatom in the newly formed bond to nitrogen (e.g., C, Si). b X denotes heteroatom in the newly formed bond to hafnium (e.g., I, H,
O). c E1/E3 and E2/E4 are indistinguishable by crystallography. d Centrosymmetry relates both halves of the molecule.
Figure 3. (Left) Molecular structure of (S,S)-1-(CO2)2 at 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. (Right) Benzene-d6 13C NMR
spectrum of 1-13C2-(13CO2)2 prepared from 13CO and 13CO2.
diagnostic of a hafnocene terminal isocyanate.31 The 13C NMR
spectrum of 1-(I)(N13CO) exhibits a broad peak centered at
136.81 ppm due to broadening from the quadrupolar 14N
nucleus (Figure 4, right). In the 15N-labeled isotopologue,
1-(I)(15N13CO), this peak splits into a well-resolved doublet
(1JN-C ) 33.0 Hz), confirming the presence of an N-C bond
assembled from N2 and CO.
Preliminary investigations into the cycloaddition chemistry
of the unlinked hafnocene oxamidide 2 with organic isocyanates
and carbon dioxide as well as C-C cleavage chemistry with I2
were also explored. Initial results and analysis by NMR
spectroscopy indicated that the expected oxamidate and io-
dohafnium isocyanate products are formed, as in the case of
the ansa-hafnocene. However, these compounds were formed
in low yield and accompanied by considerable amounts of
unidentified insoluble, intractable byproducts.
Functionalization Attempts with Terminal and Internal
Alkynes. Alkynes have proven effective substrates for cycload-
ditions across early transition metal compounds with multiple
bond character to nitrogen,35,41d For terminal alkynes, this
approach has been extended to dinitrogen functionalization.37,38
These results inspired the addition of terminal and internal
alkynes to the hafnocene oxamidide complex 2. Addition of a
10-fold excess of either diphenylacetylene or 2-butyne to 2
produced no reaction at ambient temperature and gradual
The identity of the observed diastereomer was confirmed by
single-crystal X-ray diffraction. A representation of the solid-
state structure is presented in Figure 4 (left) and confirms the
formation of the terminal isocyanate syn to the tert-butyl ligand.
Independent synthesis of 1-(I)(NCO) was also achieved by
stirring a THF slurry of Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)HfI2
and AgNCO at 80 °C for 3 days. Analysis of the product by 1H
and 13C NMR spectroscopy established formation of the same
diastereomer as from C-C bond cleavage of the C1 -and C2-
symmetric ansa-hafnocene oxamidides. The distances and angle
for the terminal hafnocene isocyanate ligand are statistically
indistinguishable from the corresponding values in the bimetallic
hafnocene µ-imide isocyanate compound reported previously31
and are also comparable to those of the recently reported
hafnium isocyanate compounds.32
(37) Morello, L.; Love, J. B.; Patrick, B. O.; Fryzuk, M. D. J. Am. Chem.
Soc. 2004, 126, 9480.
(38) Bernskoetter, W. H.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am.
Chem. Soc. 2005, 127, 7901.
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Functionalization of Hafnium Oxamidide Complexes
A R T I C L E S
Figure 4. (Left) Molecular structure of (S)-1-(I)(NCO). Hydrogen atoms are omitted for clarity. (Right) Isocyanate region of the 13C NMR spectrum of
1-(I)(N13CO) (top) and 1-(I)(15N13CO) (bottom) in benzene-d6.
decomposition upon heating to 100 °C for 24 h. In light of these
unsuccessful attempts at cycloaddition, terminal alkynes were
targeted in hopes of accessing the products of carbon-hydrogen
bond activation. The addition of a slight excess of phenylacety-
lene to 2 resulted in complete conversion, after 1 h at 65 °C, to
the dimeric oxamidato hafnium acetylide product, 2-(PhCCH)2
(eq 4).
bonds.43 Our attention focused on the alkylation and silylation
of the hafnocene oxamidide complexes due to the utility of
substituted oxamides as slow-release fertilizers and as synthons
for various nitrogen-containing organic molecules such as
diamines, N-heterocyclic carbenes, and N-heterocycles. The
tetramethylated hafnocene oxamidide complex 2 was used for
these studies due to its high symmetry and ease of synthesis
compared to the isomers of 1. Furthermore, the DFT results
with 2 suggested nucleophilic and basic oxamidide nitrogens.
The zirconocene oxamidide complexes reported previously were
not studied due to N2 loss chemistry that is competitive with
dinitrogen carbonylation.32
Addition of 1 equiv of CH3I to a toluene solution of 2 at 23
°C, followed by solvent removal and washing with cold pentane,
furnished a yellow solid, identified as 2-(CH3I), arising from a
single 1,2-addition of the alkyl halide across a Hf-N bond
(Scheme 2). Formation of a new nitrogen-carbon bond was
confirmed by preparation of various combinations of 13C and
15N isotopologues from 13CO, 13CH3I, and 15N2, for which 13C
and 15N NMR spectra were obtained (N-CH3, 1JC-N ) 5.3 Hz,
3
1
2JC-C ) 3.0 Hz, JC-C ) 1.8 Hz; C-C, JC-C ) 75.3 Hz).
Representative spectra are presented in the Supporting Informa-
tion (Figure S1). The benzene-d6 1H NMR spectrum is also
diagnostic. The C2h-symmetric starting material, 2, was cleanly
and quantitatively converted to a Cs-symmetric molecule with
inequivalent hafnocene subunits.
Benzene-d6 1H and 13C NMR spectroscopic characterization
established the C2h symmetry of the complex and C-H
activation of the acetylenic C-H bond. The concomitant
formation of new N-H bonds was confirmed by the observed
splitting of the N-H proton (1H NMR) and the N213C2O2 carbon
(13C NMR) in the AA′XX′ non-first-order spin system39 upon
isotopic labeling of the oxamidate core with 13CO, as well as
by multinuclear HSQC experiments. Infrared spectroscopy also
supports the formation of N-H and acetylide functionalities,
with stretching frequencies of 3371 and 2087 cm-1, respectively.
Functionalization of Hafnocene Oxamidides with Alkyl
Halides and Silanes. The cycloaddition reactivity and C-H bond
activation chemistry observed with the hafnium-nitrogen bonds
in 1 and heterocumulenes and with 2 and terminal alkynes
prompted study into other classes of functionalization reactions.
The 1,2-additions of carbon-hydrogen bonds,33,40,41 dihydro-
gen,42 and alkyl halides33,34 are all well-established reactions
for group 4 transition metal compounds with metal-nitrogen
Other products of a single hafnocene oxamidide alkylation
were also prepared. Addition of 1 equiv of ethyl iodide to a
benzene-d6 solution of 2 resulted in rapid 1,2-addition to form
the N-ethylated product, 2-(CH3CH2I), in quantitative yield as
judged by 1H and 13C NMR spectroscopy (eq 5). Similar results
were obtained with PhCH2Br. A single N-benzylation of 2
occurred over the course of 1 h at 23 °C in benzene-d6 (eq 5).
(41) (a) Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119,
10696. (b) Schaller, C. P.; Cummins, C. C.; Wolczanski, P. T. J. Am.
Chem. Soc. 1996, 118, 591. (c) Hoyt, H. M.; Michael, F. E.; Bergman,
R. G. J. Am. Chem. Soc. 2004, 126, 1018. (d) Walsh, P. J.; Hollander,
F. J.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110, 8729.
(42) Toomey, H. E.; Pun, D.; Veiros, L. F.; Chirik, P. J. Organometallics
2008, 27, 872.
(39) Coupling constants could not be determined because the simulated
AA′XX′ spectrum generated infinite solutions that fit the experimental
data.
(40) Mindiola, D. J. Acc. Chem. Res. 2006, 39, 813.
(43) (a) Howard, W. A.; Trnka, T. M.; Waters, M.; Parkin, G. J. Organomet.
Chem. 1997, 528, 95. (b) Howard, W. A.; Waters, M.; Parkin, G.
J. Am. Chem. Soc. 1993, 115, 4917.
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A R T I C L E S
Knobloch et al.
Scheme 2. Methylation and Subsequent Alkylation of 2
Complete assignment of the NMR data for both 2-(CH2CH3I)
and 2-(BnBr) is reported in the Supporting Information.
2-(CH3I) (Scheme 2) or by direct treatment of 2 with 2 equiv
of CH3I (eq 6). The latter procedure proved more convenient
and yielded analytically pure iodohafnocene dimethyl oxamidate,
2-(CH3I)2, in 92% yield. The formation of two N-CH3 bonds
via methylation of the hafnocene oxamidide was readily
established by 1H and 13C NMR spectroscopy as the number of
resonances consistent with a C2h-symmetric dimeric hafnocene
oxamidate complex was observed. Synthesis of 2-15N2-(13CH3I)2
was accomplished with 15N2 gas and 13CH3I, and analysis of
1
the product by 13C NMR spectroscopy established a JC-N of
4.3 Hz, consistent with N-C bond formation from the added
methyl iodide. The solid-state structure of 2-(CH3I)2 was
established by X-ray diffraction and a representation of the
molecule is presented in Figure 5. Selected metrical parameters
are presented in Table 1. Important features include the
elongation of the Hf-N and Hf-O bonds, consistent with a
change in the oxidation state of the core from [N2C2O2]4- to
The synthesis of monomethylated 2-(CH3I) prompted further
alkylation studies, with the ultimate goal of synthesizing the
N-monoalkyl- and N,N′-dialkyl-substituted oxamides in their free
forms. Examples with equivalent and inequivalent alkyl sub-
stituents were targeted. Treatment of a toluene solution of
2-(CH3I) with either ethyl bromide or benzyl bromide resulted in
alkylation of the remaining oxamidide nitrogen atom to furnish
2-(CH3I)(CH3CH2Br) or 2-(CH3I)(BnBr), respectively (Scheme
2). In the case of benzyl bromide addition, heating the reac-
tion mixture to 65 °C for approximately 16 h was required for
complete conversion. NMR spectroscopic data for both
2-(CH3I)(CH3CH2Br) and 2-(CH3I)(BnBr) are reported in the
Supporting Information and are fully consistent with a Cs dihafnium
complex with inequivalent metallocene subunits arising from the
formal 1,2-addition of the carbon-halogen bond.
Dimethylation of the oxamidide ligand was accomplished
either by addition of a second equivalent of methyl iodide to
Figure 5. Representation of the solid-state structure of 2-(CH3I)2 at 30%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
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Functionalization of Hafnium Oxamidide Complexes
A R T I C L E S
[(NCH3)2C2O2]2-. Notably, the hafnium centers retain contact
to both the oxygen and nitrogen and the C-N and C-O
distances in the core are comparable to the values in the parent
oxamidide 2, suggesting only subtle geometrical changes in the
overall transformation of 2 to 2-(CH3I)2. The core of the
molecule is essentially planar with a dihedral angle of 0.0°.
Similar dialkylations of 2 were accomplished by addition of ethyl
bromide. In this manner, 2-(CH3CH2Br)2 was synthesized in nearly
quantitative yield after several days at room temperature or after
16 h at 65 °C (eq 6). As with the other halohafnocene oxamidate
complexes, the NMR spectroscopic data establish two N-C bond-
forming events and are reported in the Supporting Information. In
contrast, addition of excess benzyl bromide to 2-(BnBr) resulted
in no further reaction up to 100 °C.
of free cyanosilanes.32 Addition of a slight excess (2.2 equiv)
of PhSiH3 to a toluene solution of 2 and stirring the resulting
mixture for 1 h at 23 °C, furnished a yellow solution from which
a pale yellow solid identified as 2-(PhSiH3)2 was isolated in
86% yield (eq 7). At shorter reaction times, 2-(PhSiH3)2 was
also the only new product observed by 1H NMR spectroscopy.
The benzene-d6 1H NMR spectrum exhibits the number of peaks
consistent with a C2h-symmetric molecule arising from two 1,2-
addition reactions of Si-H bonds across the hafnium-nitrogen
bonds. A diagnostic hafnium hydride resonance was observed
at 9.54 ppm. Similar idealized C2h-symmetric silylated hafnocene
oxamidate complexes were synthesized from addition of
nHexylSiH3 and Ph2SiH2 to 2 (eq 7). Complete spectroscopic
details for both of these compounds are reported in the
Supporting Information.
The alkylation of 2 with a series of alkyl halides prompted
studies into releasing the functionalized core from the resulting
hafnocene oxamidate compounds. We have previously reported
that treatment of 1 (mixture of C1 and C2 isomers)31 or 232 with
excess ethanol or anhydrous HCl released free oxamide,
H2NC(O)-C(O)NH2, in nearly quantitative yield. On the basis
of these results, the reactivity of each of the newly synthesized
hafnocene oxamidate complexes with Brønsted acids was
studied. A summary of these results is presented in Scheme 3.
In each case, free N-monoalkyl or N,N′-dialkyl oxamides were
liberated and their identities confirmed by 1H NMR, 13C NMR,
and IR spectroscopies. The products obtained from the dinitro-
gen functionalization studies were compared to materials
prepared by independent synthesis or to previously reported data
where possible (see Supporting Information).44,45
The solid-state structure of 2-(PhSiH3)2 was determined by
X-ray diffraction. A representation of the molecule is presented
in Figure 6 and selected bond distances are reported in Table
1. The data were of sufficient quality that the hafnium and silyl
hydrides were located. The crystallographic data confirm Si-N
bond formation and a disilylated oxamidate core. Similar to
2-(CH3I)2, the Hf-O and Hf-N bonds in 2-(PhSiH3)2 are
slightly elongated with respect to the parent oxamidide, 2,
consistent with oxamidate ([(NSiH2Ph)2C2O2]2-) formation. The
Hf-H bond is comparable with those reported previously in
related compounds.32 The oxamidate core is essentially planar,
and the dihedral angle between hafnocene subunits is 24.1°.
These N-monoalkyl- and N,N′- dialkyloxamides are known
synthons for a variety of important small molecules and
heterocycles (Scheme 3).46-48 Importantly, the ease of 15N and
13C isotopic labeling into these complexes provides a convenient
route to the small-scale synthesis of specifically labeled isoto-
pologues of various N-containing small molecules and hetero-
cycles. There is also considerable interest in the synthesis of
substituted oxamides for their coordination properties, in
particular their interesting electronic properties, when bound to
late transition metals.49
Functionalization of Oxamidides with Silanes. The 1,2-
addition reactivity of 2 with alkyl halides and the cycloaddition
chemistry observed with 1 and heterocumulenes suggested that
oxamidide functionalization may be possible with primary and
secondary silanes. Additional motivation for these studies
derives from Fryzuk’s observation of dinitrogen functionaliza-
tion50 and cleavage upon addition of silanes.18,19 Additional
impetus was provided by our laboratory with the observation
that dinitrogen carbonylation in the presence of primary silanes
resulted in cleavage of the NtN and CtO bonds with release
Figure 6. Representation of the solid-state structure of 2-(PhSiH3)2 at 30%
probability ellipsoids. Hydrogen atoms, except for the hafnium and silyl
hydrides, are omitted for clarity.
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A R T I C L E S
Knobloch et al.
Scheme 3. Synthesis of Free N-Monosubstituted and N,N′-Disubstituted Oxamides
Functionalization of the oxamidide core in 2 with both alkyl
halides and silanes prompted exploration of a combined strategy
where additional nitrogen-element bond formation was ac-
complished with both reagent types. The monomethylated
complex, 2-(CH3I), was prepared in situ in benzene-d6 followed
by addition of 1.2 equiv of PhSiH3. Complete conversion to
2-(CH3I)(PhSiH3) was observed immediately (eq 8). Charac-
discovered that exploit the nucleophilicity and basicity of the
hafnium-nitrogen bonds. For the ansa-hafnocene, 1, cycload-
t
dition of heterocumulenes such as BuNCO and CO2 resulted
in additional N-C bond assembly with concomitant formation
of hafnium-oxygen bonds. The latter functionalization reaction
demonstrates that organic molecules, in the form of a coordi-
nated ligand, can be synthesized from three abundant and often
inert small molecules: N2, CO, and CO2. For 2, sequential 1,2-
addition of alkyl halides was observed resulting in the synthesis
of, following protonolysis, free N,N′-disubstituted oxamides that
are useful precursors for the preparation of N,N′-diamines,
N-heterocylic carbenes, and various heterocycles. Given the
availability of 13CO and 15N2, isotopically labeled variants of
these molecules are also synthetically accessible. In addition to
alkyl halides, elaboration of the hafnocene oxamidides was also
accomplished with primary and secondary silanes and terminal
alkynes. DFT studies established Hf-N multiple bond character
in the hafnium oxamidide core, with nucleophilic and basic
nitrogen atoms that facilitate alkylation with alkyl halides,
silylation with organosilanes, and C-H bond activation of
terminal alkynes.
1
terization of 2-(CH3I)(PhSiH3) by H and 13C NMR spectros-
copy was diagnostic of both methyl iodide and silane addition
to the oxamidide core. The benzene-d6 1H NMR spectrum
exhibited the number of peaks consistent with formation of two
inequivalent metallocene subunits along with a diagnostic Hf-H
resonance at 9.38 ppm. The new N-CH3 group was located at
3.67 ppm (3JC-H ) 4.4 Hz), while the Si-H resonance for the
newly formed N-Si bond was assigned at 4.89 ppm. The
benzene-d6 13C NMR spectrum also exhibited diagnostic peaks
for an oxamidate core functionalized with two different groups.
Inequivalent carbons were observed at 170.33 and 173.05 ppm
(1JC-C ) 72.5 Hz). In addition, CdN and Si-H bands were
observed at 1558 and 2119 cm-1, respectively, in the solid-
state (KBr) infrared spectrum.
Experimental Section51
Preparation of ([Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)]Hf)2(tBu-
NCO)2(N2C2O2) (1-(tBuNCO)2). In a drybox, a 20 mL scintillation
vial was charged with 0.090 g (0.087 mmol) of 1-C2 in ap-
proximately 10 mL of toluene. An excess of tert-butylisocyanate
(44) Desseyn, H. O.; Perlepes, S. P.; Clou, K.; Blaton, N.; Van der Veken,
B. J.; Dommisse, R.; Hansen, P. E. J. Phys. Chem. A 2004, 108, 5175.
(45) Pfoertner, K.; Daly, J. J. HelV. Chim. Acta 1987, 70, 171.
(46) Blicke, F. F.; Godt, H. C. J. Am. Chem. Soc. 1954, 76, 3653.
(47) Denk, M. K.; Hezarkhani, A.; Zheng, F. Eur. J. Inorg. Chem. 2007,
3527.
(48) Lambert, J. B.; Huseland, D. E.; Wang, G. Synthesis 1986, 657.
(49) Ruiz, R.; Faus, J.; Lloret, F.; Julve, M.; Journaux, Y. Coord. Chem.
ReV. 1999, 193, 1069.
Concluding Remarks
Methods for the functionalization of hafnocene oxamidide
cores, prepared from CO-induced N2 bond cleavage, were
(50) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997,
275, 1445.
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A R T I C L E S
(50 µL, 0.44 mmol) was added via syringe, and the reaction was
stirred for 2 h at ambient temperature. After this time, the solvent
and excess isocyanate were removed in Vacuo, and the resulting
orange oil was washed with pentane to yield a pale yellow solid
which was purified via recrystallization from a minimal amount of
diethyl ether, furnishing pale yellow crystals of 1-(tBuNCO)2 in
excess of phenylacetylene (2.2 equiv, 0.035 mmol) was added via
syringe, and the resulting reaction mixture was shaken and heated
at 65 °C for 1 h. After this time, complete conversion to
1
2-(PhCCH)2 was observed, as judged by H NMR spectroscopy.
1H NMR (benzene-d6): δ ) 1.75 (s, 12H, CpMe4H), 2.06 (s, 12H,
CpMe4H), 2.10 (s, 12H, CpMe4H), 2.12 (s, 12H, CpMe4H), 5.09
(s, 4H, CpMe4H), 6.66 (s, 2H, NH), 7.10-7.15 (m, 6H, CCPh),
7.55-7.60 (m, 4H, CCPh). {1H}13C NMR (benzene-d6): δ ) 12.38,
13.14, 13.94, 14.48 (CpMe), 95.01 (CCPh), 110.29, 114.47, 114.90,
117.42, 120.75, 122.12, 126.25, 128.27, 128.88, 131.49, 132.73 (Cp
and CCPh), 171.34 ((NH)2C2O2), 1 CCPh not found. IR (KBr): ν
) 2087 cm-1 (CC), 3371 cm-1 (NH).
1
56% yield. H NMR (benzene-d6): δ ) 0.52 (s, 6H, SiMe2), 0.58
(s, 6H, SiMe2), 1.41 (s, 18H, C5H3CMe3), 1.68 (s, 18H, Me3CNCO),
1.85 (s, 6H, C5Me4), 1.88 (s, 6H, C5Me4), 2.07 (s, 6H, C5Me4),
2.31 (s, 6H, C5Me4), 5.35 (m, 2H, C5H3CMe3), 5.62 (m, 2H,
C5H3CMe3), 6.27 (m, 2H, C5H3CMe3). {1H}13C NMR (benzene-
d6): δ ) -0.65 (SiMe2), -0.83 (SiMe2), 11.21 (CpMe), 11.89
(CpMe), 14.10 (CpMe), 14.30 (CpMe), 31.85 (CpCMe3), 32.63
(Me3CNCO), 33.55 (CpCMe3), 55.54 (Me3CNCO), 103.28, 109.62,
109.91, 117.86, 121.42, 122.09, 122.90, 129.67, 133.41, 147.32
(Cp), 147.55 (Me3CNCO), N2(tBuNCO)2C2O2 not located. IR (KBr):
ν ) 1647, 1673 cm-1 (CdN).
Preparation of [(η5-C5Me4H)2Hf]2(I)(N(CH3)NC2O2) (2-(CH3I)).
In a drybox, a 100 mL round-bottom flask was charged with 0.100
g (0.11 mmol) of 2 and 20 mL of toluene. A 180° needle valve
was attached, and the contents of the assembly were degassed on
a high-vacuum line. While the contents were frozen at liquid
nitrogen temperature, 23.9 Torr of CH3I was added via a 100.1
mL calibrated gas bulb. The contents of the flask were warmed to
ambient temperature and stirred for 1 h. The excess methyl iodide
and solvent were removed in Vacuo, and the flask was transferred
into the drybox. The resulting yellow oil was washed with cold
pentane and furnished 0.093 g (81%) of a yellow solid, identified
Preparation of ([Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)]Hf)2(CO2)2-
(N2C2O2) (1-(CO2)2). In a drybox, a thick-walled glass vessel was
charged with 0.130 g (0.13 mmol) of 1-C2 and approximately 10
mL of toluene. On a high-vacuum line, the vessel was submerged
in liquid nitrogen and degassed, and 2.2 equiv of carbon dioxide
were added to the reaction vessel via a calibrated gas bulb (50.8
Torr, 100.1 mL). The contents of the vessel were thawed and stirred
for 1 h, after which time excess carbon dioxide and solvent were
removed in Vacuo. The vessel was transferred into a drybox, and
the resulting yellow oil was washed with cold pentane, furnishing
0.124 g (88%) of a yellow solid identified as 1-(CO2)2. Anal. Calcd
for C44H60O6N2Si2Hf2: C, 49.76; H, 5.69; N, 2.64. Found: C, 50.06;
H, 5.67; N, 2.28. 1H NMR (benzene-d6): δ ) 0.48 (s, 6H, SiMe2),
0.51 (s, 6H, SiMe2), 1.37 (s, 18H, C5H3CMe3), 1.62 (s, 6H, C5Me4),
1.76 (s, 6H, C5Me4), 1.97 (s, 6H, C5Me4), 2.01 (s, 6H, C5Me4),
5.38 (m, 2H, C5H3CMe3), 5.57 (m, 2H, C5H3CMe3), 6.25 (m, 2H,
C5H3CMe3). {1H}13C NMR (benzene-d6): δ ) -0.54 (SiMe2), 0.17
(SiMe2), 10.69 (CpMe), 11.64 (CpMe), 13.90 (CpMe), 14.06
(CpMe), 31.31 (CMe3), 34.09 (CMe3), 102.94, 108.61, 110.13,
111.70, 119.16, 119.29, 119.45, 122.64, 128.68, 129.95 (Cp), 150.54
(N2(CO2)2C2O2) (2JCC ) 3JCC ) 1.5 Hz, reported as an average36),
163.18 (N2(CO2)2C2O2). IR (KBr): ν ) 1637 cm-1 (CdN), 1734
cm-1 (CdO).
1
as 2-(CH3I). H NMR (benzene-d6): δ ) 1.66 (s, 6H, CpMe4H),
1.67 (s, 6H, CpMe4H), 1.73 (s, 6H, CpMe4H), 1.95 (s, 6H, CpMe4H),
2.06 (s, 6H, CpMe4H), 2.22 (s, 6H, CpMe4H), 2.44 (s, 6H, CpMe4H),
2.48 (s, 6H, CpMe4H), 2.49 (s, 3H, NCH3), 5.50 (s, 2H, CpMe4H),
5.55 (s, 2H, CpMe4H). {1H}13C NMR (benzene-d6): 11.24 (CpMe),
12.18 (CpMe), 13.01 (CpMe), 13.26 (CpMe), 13.36 (CpMe), 14.75
(CpMe), 14.88 (CpMe), 15.96 (CpMe), 31.58 (NCH3) (1JCN ) 5.3
Hz, 2JCC ) 3.0 Hz, 3JCC ) 1.8 Hz), 109.60, 112.29, 114.61, 117.37,
118.19, 120.85, 123.07, 123.56, 126.95 (Cp), 161.12
(N(CH3)(CO)NCO), 173.31 (N(CH3)(CO)NCO) (1JCC ) 75.3 Hz),
1 Cp resonance not located. IR (KBr): ν ) 1618 cm-1 (CdN).
Preparation of ((η5-C5Me4H)2Hf)2(I)2(N2(CH3)2C2O2) (2-(CH3I)2).
In a drybox, a thick-walled glass vessel was charged with 0.120 g
(0.13 mmol) of 2 and approximately 20 mL of toluene. On a high-
vacuum line, the vessel was degassed, and 59.8 Torr of methyl
iodide was added via a 100.1 mL calibrated gas bulb. The contents
of the vessel were thawed and stirred at 65 °C for 16 h. After this
time, the excess methyl iodide and solvent were removed in Vacuo.
The vessel was transferred into the drybox and the resulting yellow
oil washed with cold pentane to furnish 0.144 g (92%) of a yellow
solid, identified as 2-(CH3I)2. Anal. Calcd for C40H58O2N2I2Hf2:
Preparation of [Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)]Hf(I)(NCO)
(1-(I)(NCO)). In a drybox, a 20 mL scintillation vial was charged
with 0.057 g (0.055 mmol) of 1 (mixture of C1 and C2 isomers)
dissolved in approximately 10 mL of toluene. A solution containing
0.015 g (0.059 mmol) of iodine in approximately 5 mL of toluene
was added dropwise over about 1 min with stirring. After 1 h, the
solvent and the unreacted iodine were removed in Vacuo. The
remaining yellow residue was washed with pentane to furnish 0.065
g (92%) of a pale yellow solid identified as 1-(I)(NCO). The
reaction was conveniently performed on the NMR scale, enabling
the synthesis and characterization of the 15N and 13C isotopolo-
gues, (1-(I)(N13CO) and 1-(I)(15N13CO)). Anal. Calcd for
C19H26N1O1I1Hf1: C, 38.69; H, 4.44; N, 2.38. Found: C, 38.92; H,
1
C, 39.71; H, 4.83; N, 2.32. Found: C, 39.98; H, 4.90; N, 2.29. H
NMR (benzene-d6): δ ) 1.55 (s, 12H, CpMe4H), 1.86 (s, 12H,
CpMe4H), 2.12 (s, 12H, CpMe4H), 2.15 (s, 12H, CpMe4H), 3.58
(s, 6H, NCH3), 5.40 (s, 4H, CpMe4H). {1H}13C NMR (benzene-
d6): δ ) 12.14 (CpMe), 14.52 (CpMe), 15.12 (CpMe), 16.32
(CpMe), 45.50 (NCH3) (1JCN ) 4.3 Hz), 112.62, 116.07, 120.08,
122.43, 127.50 (Cp), 167.94 (N2(CH3)2C2O2). IR (KBr): ν ) 1606
cm-1 (CdN).
Preparation of [(η5-C5Me4H)2Hf]2(H)2((NSiH2Ph)2C2O2) (2-(Ph-
SiH3)2). In a drybox, a 20 mL scintillation vial was charged with
0.100 g (0.11 mmol) of 2 and approximately 10 mL of toluene.
Via syringe, 2.2 equiv (0.029 mL, 0.24 mmol) of phenylsilane was
added with stirring. Once the addition was complete, the resulting
reaction mixture was stirred for 1 h at 23 °C, after which time
excess phenylsilane and solvent were removed in Vacuo. The
resulting yellow oil was washed with cold pentane, furnishing 0.106
g (86%) of a pale yellow solid, identified as 2-(PhSiH3)2. The
product was recrystallized from toluene at -35 °C and isolated as
pale yellow crystals. Anal. Calcd for C50H68O2N2Si2Hf2: C, 52.57;
1
4.45; N, 2.24. H NMR (benzene-d6): δ ) 0.31 (s, 3H, SiMe2),
0.36 (s, 3H, SiMe2), 1.31 (s, 9H, C5H3CMe3), 1.58 (s, 3H, C5Me4),
1.64 (s, 3H, C5Me4), 1.84 (s, 3H, C5Me4), 2.31 (s, 3H, C5Me4),
5.10 (m, 1H, C5H3CMe3), 5.37 (m, 1H, C5H3CMe3), 7.25 (m, 1H,
C5H3CMe3). {1H}13C NMR (benzene-d6): δ ) -0.43 (SiMe2),
-0.25 (SiMe2), 11.89 (CpMe), 15.08 (CpMe), 15.54 (CpMe), 15.90
(CpMe), 31.20 (CMe3), 33.42 (CMe3), 98.39, 104.58, 105.39,
108.12, 111.81, 121.95, 123.39, 125.60, 133.51, 149.73 (Cp), 136.81
(NCO) (1JNC ) 33.0 Hz). IR (KBr): ν ) 2229 cm-1 (NCO).
Preparation of [(η5-C5Me4H)2Hf]2(CCPh)2((NH)2C2O2) (2-
(PhCCH)2). A J. Young NMR tube was charged with 0.015 g (0.016
mmol) of 2 and approximately 0.5 mL of benzene-d6. A slight
1
H, 6.00; N, 2.45. Found: C, 52.36; H, 5.80; N, 2.40. H NMR
(benzene-d6): δ ) 1.72 (s, 12H, CpMe4H), 1.84 (s, 12H, CpMe4H),
2.09 (s, 24H, CpMe4H, 2 coincident), 4.93 (s, 4H, NSiCH2Ph), 5.81
(s, 4H, CpMe4H), 7.22-7.33 (m, 6H, NSiCH2Ph), 8.09 (d, 4H,
NSiCH2Ph), 9.54 (s, 2H, HfH). {1H}13C NMR (benzene-d6): δ )
(51) General considerations and additional experimental details are reported
in the Supporting Information.
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J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010 15349

A R T I C L E S
Knobloch et al.
12.25 (CpMe), 12.72 (CpMe), 13.74 (CpMe), 15.19 (CpMe), 107.69,
111.78, 115.57, 118.17, 120.50 (Cp), 128.26, 130.49, 136.65, 137.95
(SiH2Ph), 175.37 (N2(SiCH2Ph)2C2O2). IR (KBr): ν ) 1552 cm-1
(CdN), 2157 cm-1 (SiH2).
Supporting Information Available: General experimental
considerations, additional procedures, computational details, and
representative NMR spectra; crystallographic data for 1-
(tBuNCO)2, 1-(CO2)2, 1-(I)(NCO), 2-(CH3I)2, and 2-(PhSiH3)2
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. We thank the Director, Office of Basic Energy
Sciences, Chemical Sciences Division, of the U.S. Department of
Energy (DE-FG02-05ER15659) and the Frasch Foundation, admin-
istered by the American Chemical Society, for financial support.
JA106848Y
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15350 J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010