Divalent dirhodium imido complexes: Formation, structure, and alkyne cycloaddition reactivity

Article abstract of DOI:10.1021/ja8033922

Dirhodium amido complexes [(Cp*Rh)₂(μ₂-NHPh)(μ₂-X)] (X = NHPh (2), Cl (3), OMe (4); Cp* = η⁵-C₅Me₅) were prepared by chloride displacement of [Cp*Rh(μ₂-Cl)]₂ (1) and have been used as precursors to a dirhodium imido species [Cp*Rh(μ₂-NPh)RhCp*]. The imido species can be trapped by PMe₃ to give the adduct [Cp*Rh(μ₂-NPh)Rh(PMe₃)Cp*] (5) and undergoes a formal [2 + 2] cycloaddition reaction with unactivated alkynes to give the azametallacycles [Cp*Rh(μ₂-η²:η³-R¹CCR²NPh)RhCp*] (R¹ = R² = Ph (6a), R¹ = H, R² = t-Bu (6b), R¹ = H, R² = p-tol (6c)). Isolation of a relevant unsaturated imido complex [Cp*Rh(μ₂-NAr)RhCp*] (7) was achieved by the use of a sterically hindered LiNHAr (Ar = 2,6-diisopropylphenyl) reagent in a metathesis reaction with 1. X-ray structures of 2, 6a, 7 and the terminal isocyanide adduct [Cp*Rh(μ₂-NAr)Rh(t-BuNC)Cp*] (8) are reported. Copyright

Full text of DOI:10.1021/ja8033922

Published on Web 06/18/2008
Divalent Dirhodium Imido Complexes: Formation, Structure, and Alkyne
Cycloaddition Reactivity
Shin Takemoto,* Shohei Otsuki, Yasuhiro Hashimoto, Ken Kamikawa, and Hiroyuki Matsuzaka*
Department of Chemistry, Graduate School of Science, Osaka Prefecture UniVersity, Gakuen-cho 1-1, Naka-ku,
Sakai, Osaka 599-8531, Japan
Received May 7, 2008; E-mail: takemoto@c.s.osakafu-u.ac.jp; matuzaka@c.s.osakafu-u.ac.jp
Scheme 1. Synthesis of [(Cp*Rh)2(µ2-NHPh)(µ2-X)] (2-4)
Utilization of transition metal imido complexes as reagents or
catalysts in organic synthesis is a topic of broad current interest.1–3
Among numerous reactivity patterns of multiple-bonded MdNR
functionality,1 the [2 + 2] cycloaddition with alkynes has received
considerable attention as a crucial C-N bond forming step in the
catalytic hydroamination of alkynes.3–6 While certain monomeric
imido complexes of group 4 and some neighboring metals have
proven effective for this type of transformation,4–6 few late
transition metal imido complexes have exhibited comparable
reactivity.7 We previously described the formation of a dinuclear
azametallacycle from the reaction of the 16-electron ruthenium
amido complex [Cp*Ru(µ2-NHPh)]2 (Cp* ) η5-C5Me5) with
diphenylacetylene, which likely proceeds via alkyne coordination
and aniline elimination followed by imido-alkyne cycloaddition.8
Exploring the scope of late transition metal imido complexes as
reagents for C-N bond formation,9 we here report the chemistry
of corresponding Cp*Rh amido and imido complexes, including
the synthesis of the divalent amido complex [Cp*Rh(µ2-NHPh)]2,
generation and alkyne cycloaddition of a transient imido species
[Cp*Rh(µ2-NPh)RhCp*], and X-ray structure determination of a
sterically protected imido complex [Cp*Rh(µ2-NAr)RhCp*] (Ar )
2,6-diisopropylphenyl).
Scheme 2. Generation, PMe3 Trapping, and Alkyne Cycloaddition
of a Dirhodium Imido Species [Cp*Rh(µ2-NPh)RhCp*]
Amido10 and imido7a,11 derivatives of Cp*M (M ) group 8-10
metals) fragments have been most commonly derived from the
corresponding Cp*M halides by displacement reactions.12 In this
study, we employed the Rh(II) chloride [Cp*RhCl]2 (1), reported
by Sharp and co-workers,13 as a starting material and prepared the
amido complexes 2-4 (Scheme 1) that can be used as precursors
to imido complexes. The violet dimeric amide 2 was obtained in
74% yield upon treatment of 1 with 2 equiv of LiNHPh in THF.
Heating the chloro dimer 1 with excess aniline in THF at 60 °C
resulted in the selective monosubstitution of a chloride ligand to
give the amido chloro complex 3 in 71% yield, which was then
converted to the amide methoxide 4 in 72% yield upon treatment
with NaOMe. Complexes 2-4 were isolated after extraction with
hexanes and identified by standard spectroscopic and analytical
methods; 2 was further defined by an X-ray diffraction which
revealed the nonplanar M2N2 core and equatorial phenyl groups
similar to those reported for [Cp*Ru(µ2-NHPh)]2.10e The Rh-Rh
distance of 2.6097(9) Å is comparable to that of 1 (2.617(1) Å)13
and is consistent with a single bond between the d7 Rh(II) centers.
azametallacycle 6a in 78% yield (Scheme 2). It seems likely that
6a is formed by alkyne cycloaddition to a transiently generated
imido species [Cp*Rh(µ2-NPh)RhCp*], since 3 did not react with
diphenylacetylene in the absence of the base under comparable
reaction conditions (THF, 25 °C, 12 h). With terminal acetylenes,
tert-butylacetylene and para-tolylacetylene, the cycloaddition pro-
ceeded regioselectively to give the Markovnikov adducts 6b (85%
yield) and 6c (91% yield), respectively. The structure of 6a has
been determined by X-ray crystallography. Although terminal CPh
and NPh groups in the bridging azapropenylidene ligand PhCCPh-
NPh are disordered, solved structure clearly shows the unsym-
metrical η2:η3 bonding, which markedly contrasts to the symmetri-
cal η3:η3 bonded structure of the diruthenium complex [(Cp*Ru)2(µ2-
η3:η3-PhCCPhNPh)].8 A fluxional behavior of the azametallacycles
An initial evidence that an imido species can be generated from
the amido complexes 2-4 was obtained by dehydrochlorination
of 3 with NaN(SiMe3)2 in the presence of PMe3 that afforded the
imido complex [Cp*Rh(µ2-NPh)Rh(PMe3)Cp*] (5) in 80% yield
(Scheme 2). This compound is an analogue of the iridium imido
complex [Cp*Ir(µ2-NPh)Ir(PMe3)Cp*] reported by Dobbs and
Bergman as a product of imido transfer reaction from [Cp*Ir(µ2-
NPh)]2 to PMe3.11f Analogous deprotonation of 3 in the presence
of diphenylacetylene resulted in the formation of a dinuclear
9
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10.1021/ja8033922 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
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6a-c was suggested by their H NMR spectra. For example, in
In summary, the amido complexes 2-4 provided a chemistry
attributable to a reactive imido species [Cp*Rh(µ2-NPh)RhCp*]
including a formal [2 + 2] cycloaddition reaction with unactivated
alkynes. With the use of a sterically hindered arylimido ligand, a
relevant coordinatively unsaturated imidodihodium complex was
isolated and structurally characterized. Efforts will be directed
toward detailed investigation of this system including catalytic
alkyne hydroamination by an imido mechanism.4
THF-d8 at -90 °C complex 6a showed two inequivalent Cp* methyl
resonances that coalesced into one sharp singlet as the temperature
was raised. This can be accounted for by assuming a rapid flipping
of the bridging azapropenylidene moiety between the two Rh
centers.
Acknowledgment. This work was supported in part by the
Grant-in-Aid for Scientific Research on Priority Areas (No.
18065007, “Chemistry of Concerto Catalysis”) from the Ministry
of Education, Culture, Sports, Science and Technology, Japan. We
also acknowledge financial support from the Toyota Motor
Corporation.
The azametallacycle 6a was also formed in 47% yield from the
bis-amide 2 with elimination of 1 equiv of aniline when 2 was
heated at 120 °C for 7 days in the presence of 10 equiv of
diphenylacetylene (eq 1). The amide methoxide 4 more smoothly
reacted with the same alkyne (1 equiv, 60 °C 18 h) to give 6a in
98% yield. A preliminary kinetic estimation revealed that the rate
of formation of 6a is first order in the concentration of 4 with little
dependence of kobs values on the concentration of alkyne (4.3 (
1.0 × 10-5 s-1 in the presence of 10-30 equiv of alkyne in C6D6
at 50 °C), which again points to a dissociative pathway involving
the imido intermediate [Cp*Rh(µ2-NPh)RhCp*].
Use of a sterically hindered arylimido ligand allowed isolation
of an unsaturated imido complex relevant to the above-postulated
imido intermediate. Treatment of 1 with 2 equiv of LiNHAr (Ar )
2,6-diisopropylphenyl) afforded the singly bridged imido complex
[Cp*Rh(µ2-NAr)RhCp*] (7), which was isolated in 64% yield and
crystallographically characterized (Scheme 3). The molecule con-
tains a triangular Rh2N core surrounded by the bulky Cp* and Ar
groups. The planar arrangement around nitrogen and the short
Rh-N distances (1.8946(18) and 1.8969(19) Å) indicate delocalized
Rh-N multiple bonding interactions.8,11 The Rh-Rh distance of
2.5190(7) Å is consistent with a Rh-Rh single bond with which
each rhodium center would attain a formal 16-electron configuration.
While 7 did not react with diphenylacetylene or tert-butylacetylene,
it reacts instantaneously with tert-butyl isocyanide to give the adduct
[Cp*Rh(t-BuNC)(µ2-NAr)RhCp*] (8; 63% yield), in which the
terminal isocyanide ligand rapidly migrates between the two Rh
centers as evidenced by a single-crystal X-ray and variable
temperature NMR studies.
Supporting Information Available: Experimental procedure and
crystallographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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