Addition-elimination reactions across the M-C bond of metal N-heterocyclic carbenes

Article abstract of DOI:10.1021/ja910673q

Silyl, phosphinyl, stannyl, and boryl reagents can be added across the neutral metal-carbon dative bond in d⁰ f-block metal N-heterocyclic carbene complexes in a reversible manner, allowing additional functional groups to be incorporated into redox-inactive organo-f-block compounds. Copyright

Full text of DOI:10.1021/ja910673q

Published on Web 03/04/2010
Addition-Elimination Reactions across the M-C Bond of Metal N-Heterocyclic
Carbenes
Zoe¨ R. Turner,† Ronan Bellabarba,‡ Robert P. Tooze,‡ and Polly L. Arnold*,†
School of Chemistry, UniVersity of Edinburgh, Edinburgh, EH9 3JJ, U.K. and Sasol Technology U.K.,
Purdie Building, North Haugh, St. Andrews, KY16 9SR, U.K.
Received December 19, 2009; E-mail: Polly.Arnold@ed.ac.uk
In homogeneous catalysis the lanthanides and actinides are
characterized by their facile bond activation reactions that proceed
through a four-centered σ-bond metathesis mechanism rather than
a conventional two-electron oxidative addition-reductive elimina-
tion pathway.1 Indeed, the first example of methane activation was
observed in the reaction between Cp*2YCH3 (Cp* ) C5Me5) and
13CH4 which results in the interchange of labeled Y-13CH3 and
unlabeled Y-12CH3 groups.2 Attractive transformations based on
the σ-bond metathesis reactivity can be envisaged if turnover could
be achieved, but these systems are limited by the absence of further
functionality or valence electrons.3 Organo-f-block systems (i.e.,
alkyls, hydrides, and amido compounds) are excellent catalysts for
the hydroamination, phosphination, alkoxylation, and silylation of
alkenes4 but rely on σ-bond metathesis mechanisms solely to
exchange substrate and product.
In contrast, the ready availability of controlled two-electron redox
processes in the platinum group metals provides a platform for a
vast array of catalyzed reactions. N-Heterocyclic carbenes (NHCs)
are now used widely as ancillary ligands for supporting these metal
cations;5 while generally innocent, alkylated compounds can be
destroyed by reductive elimination processes, for example cis-
PdMe(NHC)L2 complexes can eliminate 2-alkyl imidazolium salts,
Figure 1. Thermal displacement drawings (50% probability ellipsoids) of
forming palladium black.6
the molecular structures of (a) U(LD)N′′2, (b) U(LD,Si′)N′′2I, (c) U(LD)2I2,
and (d) Ce(LD,AdN )N′′2.
3
Herein, we show how NHCs can be used as reactive ligands to
allow further chemistry to be accessed for redox inactive metals:
The addition of reagents across the neutral metal-carbon dative
bond in d0 Group 3 and f-block metal NHC complexes, in a
reversible manner, allows additional functional groups such as
silanes, phosphines, stannanes and boranes to be incorporated into
compounds that are active for σ-bond metathesis chemistry.
We recently described the synthesis of d0 LnIII Y(L)N′′2 and UVI
UO2(L)2 complexes of robust alkoxy-N-heterocyclic carbene
ligands L in which the carbene backbone is saturated (N′′ )
diffraction study of the N-Dipp complex U(LD)N′′2, Figure 1a,
confirms the molecular structure. Distances and angles are within
literature ranges for low-coordinate UIII complexes.
i
N(SiMe3)2, L ) OCMe2CH2{C(NCH2CH2NR)}; R ) Pr, Mes
() 2,4,6-Me3-C6H2), Dipp () 2,6-iPr2C6H3)).7 This obviates the
potential for “abnormal”-type reactions of the backbone C-H
protons, which are relatively acidic in the unsaturated carbene.8
Furthermore, the tethering of the carbene to the anionic group
prevents its loss, allowing the reactivity of this nucleophilic
heterocycle to be incorporated into the reaction chemistry of the
f-element complex.
Treatment of a solution of any of the complexes M(LD)N′′2 (M
) Y, Ce, U) with 1 equiv of Me3SiI in benzene or toluene at room
temperature immediately produces a colorless (Y), pale yellow (Ce),
or dark brown (U) solution. In each case, a complex formu-
lated as [Me3Si{C(NDippCH2CH2N)}CH2CMe2O]MN′′2I, denoted
M(LD)N′′2I, is isolated in good yield. This reaction can formally
be seen as the addition of the silyl halide across the metal-carbene
bond, forming the metal iodide bis(amido) alkoxide with a pendant
silylated imidazolium group, eq 1. The addition reaction is equally
facile for Me3SiCl or Me3SiN3 and can be further generalized since
the group added to the carbene carbon can also be varied. The use
Bright yellow Ce(LD)N′′2 and dark blue U(LD)N′′2 can be made
from CeN′′3 or UN′′3 by analogous protonolysis reactions to those
that made Y(L)N′′2 by treatment with an equivalent of HLD (D )
Dipp ) 2,6-iPr2-C6H3) in toluene or hexanes respectively at room
temperature. All three derivatives were isolated in good yield as
hydrocarbon-soluble crystalline solids. A single crystal X-ray
† University of Edinburgh.
‡ Sasol Technology U.K.
9
4050 J. AM. CHEM. SOC. 2010, 132, 4050–4051
10.1021/ja910673q  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Addition-Elimination Cycles of Reactivity for d0 Metal
Carbene Complexes with Me3Si-X (X ) Cl, I, N3), Si′ ) SiMe3
ously into organo-f-block complex σ-alkyl bonds, but not into a
metal-ligand dative bond.9
No evidence of extrusion of N2 (to form the superbasic
imidazolin-iminato group used by Tamm et al.10) was observed
upon heating a solution of Ce(LD,AdN )N′′2 to 80 °C for 24 h.
3
In recent years, the use of transition metal catalysts has opened
up many new possibilities in the area of heteroatom functionaliza-
tion chemistry such as borylation and phosphination reactions; work
is in progress to identify opportunities for these carbene-function-
alized organolanthanide systems to incorporate other functional
groups or unsaturated molecules into their σ-bond metathesis
chemistry, and to develop the potential for a relevant catalytic
cycle.11 The potential for the incorporation of the triazenido, arrested
insertion intermediate in insertion chemistry and heterocycle
syntheses will also be reported on in due course.
of halophosphines, boranes, and stannanes allows the formation of
C-P, C-B, and C-Sn containing substrates, with the metal
pseudohalide bond providing a significant driving force for the
reaction.
Once formed, all these addition complexes are stable at ambient
temperature (for characterization, see Supporting Information). The
complexes are less soluble than the parent amido complexes, but
readily crystallize, allowing the isolation of pure materials and
characterization by X-ray diffraction; the molecular structure of
the trivalent uranium imidazolium iodide U(LD,E)N′′2I is shown in
Figure 1b.
Acknowledgment. We thank Sasol Technology UK (studentship
for ZRT), EaStCHEM, the UK EPSRC (fellowship for PLA), and
the University of Edinburgh for funding. We thank Professor Simon
Parsons for solving the structure of Ce(LD,AdN )N′′2 and Dr. Lam
3
for the use of the GC machine.
Heating a benzene solution of the Y or Ce addition products
M(LD,E)N′′2X (E ) SiMe3) (the metal complexes that are not readily
oxidizable) results in the elimination of tris(trimethylsilyl)amine
N′′′ (identified by GC) and the reformation of the metal carbene
bond in M(LD)XN′′, Scheme 1. This mixed ligand complex
rearranges to a 50:50 mixture of the starting material M(LD)N′′2
and M(LD)X2, which can be converted to the starting material by
a salt metathesis reaction with 1 equiv of KN′′.
Supporting Information Available: Full experimental details and
X-ray crystallographic data (CIFs deposited with the CCDC, codes
759036-759041, and 766364). This material is available free of charge
Via the Internet at http://pubs.acs.org.
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bond is not cleaved, so an unusual insertion product M(LD,AdN )N′′2
3
is isolated instead, eq 2. In this, the formerly neutral azide group
now binds as a κ2N1,3 triazenido anion to the metal, with a covalent
bond formed between the terminal azido nitrogen and the carbene
carbon: Charge compensation is achieved by the associated positive
charge now on the imidazolinium NCN group.
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3
single crystal X-ray diffraction study, Figure 1d, confirms the
connectivity. Similar insertion chemistry has been observed previ-
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