Reaction of [σ:η5-(C9H6)C 2B9H10]Zr(NMe2)(DME) with guanidines: Metallacarborane-mediated C-N bond cleavage and 1,5-sigmatropic rearrangement

Article abstract of DOI:10.1021/ja0754498

Treatment of [σ:η⁵-(C₉H₆)C₂B₉H₁₀]Zr(NMe₂)(DME) with 2 equiv of guanidines ⁱPrNHC(NR₂)=NⁱPr (R = Me, Et) in refluxing toluene gave unprecedented c

Full text of DOI:10.1021/ja0754498

Published on Web 10/05/2007
Reaction of [σ:η5-(C9H6)C2B9H10]Zr(NMe2)(DME) with Guanidines:
Metallacarborane-Mediated C-N Bond Cleavage and 1,5-Sigmatropic
Rearrangement
Hao Shen, Hoi-Shan Chan, and Zuowei Xie*
Department of Chemistry and Center of NoVel Functional Molecules, The Chinese UniVersity of Hong Kong,
Shatin, New Territories, Hong Kong, China
Received July 21, 2007; E-mail: zxie@cuhk.edu.hk
Scheme 1. Reaction of 1 with Guanidines
Guanidine derivatives, as electronically and sterically flexible
ligands, have received increasing attention. They are capable of
exhibiting a variety of coordination modes and a range of donor
properties leading to compatibility with a wide range of metal ions
from all parts of the periodic table.1,2 The inert nature of guanidi-
nates as suitable spectator ligands supporting organometallic
fragments is well demonstrated in various types of reactions of [η2-
(iPrN)2C(NR2)]ZrCl2.2d In view of these, we would like to incor-
porate guanidinates into metallacarboranes3 to study the effects of
metal/charge combinations on the reactivity of group 4 metal
complexes. From the reaction of 1 with 2 equiv of guanidines, we
isolated unprecedented zirconacarboranes 6.4 This result shows
clearly that C-N bond-breaking and C-C bond-forming reactions
are involved in the process, and the Zr-indenyl σ-bond remains
surprisingly in the product. This communication describes the
mechanism by which they are formed.
Scheme 2. Transformations among Complexes 1-6
Treatment of 1 with 2 equiv of guanidines iPrNHC(NR2)dNiPr
(R ) Me (2a), Et (2b)) in refluxing toluene gave [η1:σ:η5-{2-[CdNi-
Pr(NHiPr)]C9H5}C2B9H10]Zr[η2-(iPrN)2C(NR2)] (R ) Me (6a), Et
(6b)) in 30-47% isolated yields (Scheme 1).4 The molecular
structures of both 6a,b were confirmed by single-crystal X-ray
analyses. The key structural data indicate that the interactions
between the Zr atom and the indenyl-dicarbollyl unit in 6a,b are
very similar to those observed in 1.4
The 1H NMR spectra showed that the molar ratio of R2N/iPr )
1/4 in 6 rather than the expected 1/2, suggesting that one of R2N
groups was dissociated from the guanidinate unit during the reaction.
The NMR experiments indicated the formation of both Me2NH and
Et2NH in the reaction mixture of 1 with 2b, which was further
confirmed by GC/MS. This result may suggest that (1) 1 equiv of
2b reacts with 1 via amine exchange to give Me2NH and the
guanidinate ligand which is η2-bound to the Zr atom,5 and (2) the
second equivalent of 2b undergoes presumably a C-NEt2 bond
cleavage, generating Et2NH and the amidine. To gain some insight
into these reactions, a stepwise reaction of 1 with carbodiimides
R′NdCdNR′ was performed.
indicates that the indenyl group may be η3-bound to the Zr atom
in the solution.6
Treatment of 3a,c with 1 equiv of R′NdCdNR′ in THF at room
temperature generated another type of insertion products [σ:η5-{3-
i
[C(dNR′)-NR′]-1-C9H6}C2B9H10]Zr[η2-(R′N)2C(NMe2)] (R′ ) -
Pr (5a), Cy (5c)) in >90% isolated yields (Scheme 2).4 Both
structures were confirmed by single-crystal X-ray analyses.4 It is
very clear that the R′NdCdNR′ inserted into the 3-position, rather
than 1-position, of the indenyl ring. This result supported the
argument of an η3-bonding mode between the indenyl and Zr atom
in the solution of 3a,c.
Complexes 5a,c were quantitatively converted to [η1:σ:η5-{2-
An equimolar reaction of 1 with R′NdCdNR′ in THF at room
temperature afforded monoinsertion products [σ:η5-(C9H6)C2B9H10]-
i
[CdNR′(NHR′)]C9H5}C2B9H10]Zr[η2-(R′N)2C(NMe2)] (R ) Pr
i
Zr[η2-(R′N)2C(NMe2)](THF) (R′ ) Pr (3a), Cy (cyclohexyl; 3c))
(6a), Cy (6c)) upon refluxing in toluene.4 This can be rationalized
by 1,5-sigmatropic rearrangement (vide infra).7 The molecular
structure of 6c is very similar to that of 6a,b as confirmed by single-
crystal X-ray analyses.4
in 71-74% isolated yields (Scheme 2).4 They were fully character-
ized by 1H, 13C, and 11B NMR spectroscopic techniques and
elemental analyses. The proton chemical shift of the Me2N group
was shifted from 3.11 ppm in 1 to 2.64 ppm in 3a and 2.70 ppm
in 3c, respectively. A characteristic CN3 resonance at 174.4 ppm
in 3a and 174.0 ppm in 3c was observed in their 13C NMR spectra.
The spectroscopic features of the indenyl group are very similar in
both 1 and 3. These data suggested that R′NdCdNR′ inserted into
the Zr-N bond to form a guanidinate unit. The bonding interactions
between the indenyl and Zr atom remained intact, which probably
Reaction of 3a with 1 equiv of 2a in toluene at room temperature
gave proton exchange product [η5-(C9H7)C2B9H10]Zr[η2-(iPrN)2C-
(NMe
2)]2 (4a) in 79% isolated yield.4,8 Its molecular structure shows
that the Zr atom has no bonding interactions with the neutral indenyl
ring. Complex 4a was converted to 6a upon refluxing in toluene,
from which 6a was isolated in 62% yield4 and Me2NH was detected
by GC/MS. Complex 6a was also isolated in 31% yield4 from the
9
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J. AM. CHEM. SOC. 2007, 129, 12934-12935
10.1021/ja0754498 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Proposed Pathway for C-N Bond Cleavage
Scheme 4. Proposed Mechanism for the Formation of 6
evidence for the corresponding reverse reaction. In other words,
guanidinates are not inert ligands in certain cases that can undergo
C-N bond cleavage to form carbodiimides and amido units.
Acknowledgment. This work was supported by grants from
the Research Grants Council of the Hong Kong SAR (Project No.
403805), NSFC/RGC Joint Research Scheme (N_CUHK 446/06),
and a Strategic Investments Scheme administrated by The Chinese
University of Hong Kong.
Supporting Information Available: Detailed experimental pro-
cedures, full characterization data, and X-ray data for 1, 4a, 5a‚C7H8,
5c‚C7H8, 6a, 6b, and 6c‚THF in cif format. This material is available
free of charge via the Internet at http://pubs.acs.org.
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refluxing toluene solution of 3a. The formation of Me2NH was con-
firmed by GC/MS. These transformations are outlined in Scheme 2.
A mechanism for the C-N bond cleavage of a guanidinate unit
is proposed in Scheme 3. This process may be driven by heat and
can be viewed as a deinsertion of a carbodiimide from the
guanidinate ligand.9 This proposal is evident from the following
carbodiimide exchange experiments. Treatment of 3c with 8 equiv
of iPrNdCdNiPr in refluxing toluene afforded 6a in 18% isolated
yield. On the other hand, under the same reaction conditions,
reaction of 4a with 6 equiv of CyNdCdNCy generated 6c in 12%
isolated yield.
On the basis of the aforementioned experimental results, a
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in Scheme 4. Reaction of 1 with carbodiimides or guanidines 2
yields 3 which is able to react reversibly with another equivalent
of 2 to generate 4. Heating of 3/3′ results in the cleavage of the
C-N bond and in situ generation of carbodiimide and amido,
leading to the formation of intermediate A. Interaction of A with
2 or of 3′ with carbodiimide produces B which gives 5 via an
insertion reaction. 1,5-Sigmatropic rearrangement7 over the indenyl
ring in 5 affords C, which undergoes an intramolecular proton-
transfer reaction to generate the final products 6.
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In conclusion, although it is well-established that insertion of
carbodiimides into amides is a very useful method for the
preparation of guanidines,2m-o,10 this work provides experimental
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