An unusally diverse array of products formed upon carbonylation of a dialkylniobium complex

Article abstract of DOI:10.1021/ja803729k

The reaction of carbon monoxide with a β-diketiminato dimethyl niobium complex (BDI)Me₂Nb(N^tBu) is shown to lead to a variety of products whose distribution displays a remarkable dependence onthe reaction conditions. Among these, the products of metal reduction, enediolate formation, and intramolecular C-H activation have been fully characterized. An investigation into the individual steps leading to these products points to a transient initial monoacyl complex, whose fate may be perturbed via reaction conditions to allow for control over the product distribution. Furthermore, the reaction of (BDI)Me₂Nb(NCMe₃) with XylNC (Xyl = 2,6-Me₂C₆H₃) yields the η²-ketimine complex (BDI)(Me₂C=NXyl)Nb(NCMe₃), whose characterization and reactivity enhance our understanding of the sequences involving CO. Copyright

Full text of DOI:10.1021/ja803729k

Published on Web 07/31/2008
An Unusally Diverse Array of Products Formed upon Carbonylation of a
Dialkylniobium Complex
Neil C. Tomson, Andrew Yan, John Arnold,* and Robert G. Bergman*
Department of Chemistry, UniVersity of California, Berkeley, California 94720
Received May 19, 2008; E-mail: arnold@berkeley.edu; rbergman@berkeley.edu
Scheme 1
The clean, efficient utilization of carbon monoxide as a C1 source
continues to represent a significant chemical challenge. Many
homogeneous late metal-mediated industrial processes have been
developed which center on the utility of well-studied transition metal
acyl intermediates.1 Stoichiometric reactions of carbon monoxide
with simple alkyl and dialkylmetal complexes usually lead cleanly
to carbonylation products. This is true irrespective of the precise
conditions employed, although some substrates may require more
forcing conditions (e.g., high temperature or pressure) than others.
Through this stoichiometric chemistry, many of the classic primary
steps in homogeneous late metal-mediated catalytic organometallic
reactions, such as migratory CO insertion, have been identified.
Herein we describe the carbonylation reaction of the dimethylnio-
bium complex (BDI)Me2Nb(NtBu) (1, BDI ) ꢀ-diketiminate),2 in
which the relative amounts of an unusually wide range of products
change dramatically in response to very modest changes in reaction
conditions. Four different stable products, in which CO insertion
takes place coupled with metal reduction, enediolate formation and
C-H activation, are formed in these transformations. By careful
adjustment of reaction conditions, the majority of these materials
have been isolated and fully characterized.
Scheme 2
On adding 3.0 equiv of CO to a solution of 1 in THF at -72 °C,
stirring rapidly for 5 min, then warming directly to room temper-
ature, a brown solution was obtained from which a green product
was isolated by crystallization. The products were identified as
acetone and the Nb(III) dicarbonyl adduct (BDI)(CO)2Nb(NtBu)
(2, 42% isolated yield, Scheme 1), which can be considered to result
from transfer of both methyl groups to one CO, followed by the
displacement of acetone by two molecules of CO (νCO (2) ) 1988,
1893 cm-1). The formation of acetone was confirmed by both
GC-MS (20% yield)3 and isolation of the 2,4-dinitrophenyl-
hydrazone derivative (43% yield). The release of a ketone and
formation of a dicarbonyl adduct suggest analogies with previous
work on dialkyl metallocenes of the group 4 triad4 and on
η2-acetone complexes in group 5 systems.5,6
The outcome of the reaction of 1 with CO may be altered
dramatically by using pentane as the solvent in place of THF and
by allowing the reaction mixture to remain at -72 °C for 2 h prior
to warming to room temperature. In so doing, a second product
may be isolated by crystallization, whose analytical data are
consistent with its formulation as the enediolate species
(BDIiPr)(Me2C2O2)Nb(NtBu) (3, 54% yield, Scheme 1), presumably
formed via the intramolecular coupling of two Nb-acyl fragments,7-10
though other mechanisms cannot be ruled out. NMR spectral
analysis of the crude reaction mixture indicates that the product
distribution has inverted from 4:1 mixtures of 2:3 obtained when
THF is used as the solvent to 1:4 mixtures of 2:3 obtained when
pentane is used and longer reaction times are employed.
immediately allowing the system to warm to room temperature.
After stirring the resulting mixture for 36 h and removing the
volatile materials, a new product may be crystallized from pentane,
yielding yellow crystals of {ArNC(Me)CHC(Me)N[2-(CHMeCH2)-
6-iPr-C6H3]}(Me2HCO)Nb(NtBu) in 37% yield (4, Scheme 1). The
structure of 4, determined crystallographically, indicates that the
product is formed in a net process of transfer of both methyl groups
to the CO carbon and the formal addition of a ligand C-H bond
across the Nb-O-C linkage.11
The utility of the CO reaction scheme presented here lies in the
ability to perturb the ultimate fate of common intermediates that
appear along the course of the reaction. Compounds 2-4 presum-
ably originate from an initial monoacyl complex (A, Scheme 1).
Indeed, NMR studies on a mixture of 13CO and 1 at -80 °C allowed
us to observe a Nb monoacyl species (BDI)Me(Me13CO)Nb(NtBu)
(5, Scheme 2), displaying a strong 13C NMR signal at 299 ppm.
Still, while the signal lies within a region characteristic of early
metal acyl complexes,12 no 1H coupling information could be
discerned, presumably owing to a complicated coupling pattern.
However, support for the formulation of both 5 and A as
monoacyls is provided by treating the analogous monoalkylated
complex (BDI)MeBrNb(NtBu) (6) with 13CO. In this case a single
product is observed with a 13C NMR resonance at 292 ppm,
attributable to the species (BDI)Br(Me13CO)Nb(NtBu) (7, Scheme
1
2). The H-coupled 13C NMR spectrum reveals the signal to be a
2
A third product from the reaction of CO with 1 is obtained by
slowly introducing 1.0 equiv of CO into the headspace above a
solution of 1 in THF in a flask cooled to -72 °C, followed by
pseudoquartet (93Nb, 100%, I ) 9/2) with JC-H ) 6.5 Hz,
corresponding to a doublet with the same coupling constant in the
1H NMR spectrum at 2.56 ppm. This complex is stable in solution
9
11262 J. AM. CHEM. SOC. 2008, 130, 11262–11263
10.1021/ja803729k CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
Scheme 4
B may also be described as a Nb(V) oxaniobacyclopropane, which,
in the absence of excess CO, would ring-open (due to strain within
in the three-membered metallacycle) by way of an intramolecular
C-H bond activation to yield 4. This behavior would reflect the
dual Nb(III)/Nb(V) character exhibited by the electronically and
structurally analogous ketimine complex 8.
These results indicate an ability to dial in a specific product
mixture based on very subtle modifications to the reaction
conditionssa technique particularly useful in transforming CO into
various organic species. The close energetic proximity of diverse
reaction pathways for second-row transition metals14 often impedes
progress in these areas. However, these initial results demonstrate
an ability to make use of these small energy differences for
accessing separate product distributions. Further research will be
directed toward uncovering factors that would allow us to control
the product distribution with higher precision.
at room temperature for ca. 30 min before transforming into an
unidentified species.
Assuming that products 2 and 3 arise from a common acyl
intermediate A and that sufficient amounts of CO are present in
both systems to allow for either reaction pathway to occur, then
the factor determining the product distribution appears to be the
coordinating ability of the solvent. In pentane, free CO (a weak
ligand for d0 metal systems) coordinates to the metal center and
undergoes insertion into the remaining Nb-Me bond to form a
diacyl complex. The diacyl then undergoes intramolecular C-C
coupling to form 3. In THF, however, the solvent occupies a
coordination site preferentially over CO, allowing methyl transfer
to the acyl group to occur on warming. This second methyl group
transfer amounts to a formal reduction of Nb(V) to Nb(III).
Displacement of the coordinated acetone with 2 equiv of CO then
leads to the formation of 2. These hypotheses, which exclude control
by differences in solvent polarity, are supported by two key
observations: (i) that higher concentrations of 1 in either solvent
lead to greater 3:2 ratios and (ii) that performing the synthesis of
2 with 2,5-Me2THF as the solvent leads to a 1.1:1 ratio of 3:2.
The formation of 4 could conceivably proceed through a number
of pathways. The formation of 4-d6 from (BDI)(CD3)2Nb(NtBu)
(1-d6) revealed that the two diastereotopic deuterium-labeled methyl
groups remained intact. This excludes the possibility of pathways
involving an initial C-H(D) activation at one of the methyl groups
and suggests that an intermediate acetone adduct B may be involved
in this mechanism.
Acknowledgment. This work was supported by the American
Chemical Society Petroleum Research Fund (ACS-47249AC3), the
National Science Foundation (CHE-0416309), and the National
Institutes of Health (R01-GM025459-29).
Supporting Information Available: Experimental procedures and
characterization data for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) Kuhlmann, E. J.; Alexander, J. J. Coord. Chem. ReV. 1980, 33, 195–225.
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B, the reaction of 1 with isocyanides, an isoelectronic variant of
CO, was investigated. The reaction of 1 with 1.0 equiv of XylNC
(Xyl ) 2,6-Me2C6H3) in Et2O at room temperature gave a deep
red solution that, after workup, yielded (BDI)(η2-XylNCMe2)-
Nb(NtBu) (8, Scheme 3) as a dark red crystalline solid in 48%
yield. While an X-ray diffraction study indicates the product of
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and chemical observations imply assignment of the oxidation state
to be more complicated than the crystallographic data suggest.
(3) The low yield obtained by GC-MS is attributed to low reaction concentra-
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(4) Erker, G.; Dorf, U.; Czisch, P.; Petersen, J. L. Organometallics 1986, 5,
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1
The room temperature H NMR spectrum of 8 displays broad
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(ArO)2TiMe2 (Ar ) 2,6-Ph2-C6H3). The latter reaction is believed to proceed
through an intermediate ketimine complex, although no direct evidence
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resonances, indicating either general intramolecular rearrangement
processes or rotation about a ketimine-Nb(III) vector. A more
concrete demonstration of the ability of 8 to serve as a source of
Nb(III) was provided by treating 8 with 2.0 equiv of CO in C6D6
at room temperature. Over the course of three hours, 2 grows in
cleanly and quantitatively with concomitant formation of free
ketimine XylN)CMe2 (Scheme 4).
The contrast between the oxidation states assigned to reactions
involving 8 and the ground-state of 8 (XRD) allow for informed
speculation as to the mechanism leading to 4. While intermediate
B would yield 2 by displacement of acetone with 2.0 equiv of CO,
(12) Gomez, M. Eur. J. Inorg. Chem. 2003, 368, 1–3697.
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Nb-N, and N-C single bonds: Nb(1)-C(1) ) 2.197(4) Å, Nb(1)-N(1)
) 1.964(3) Å, N(1)-C(1) ) 1.477(4) Å.
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