[Me2NN]Co(η6-toluene): O=O, N=N, and O=N Bond Cleavage Provides β-Diketiminato Cobalt μ-Oxo and Imido Complexes

Article abstract of DOI:10.1021/ja036308i

The synthesis and structure of a novel β-diketiminato Co(I) arene adduct [Me₂NN]Co(η⁶-toluene) (2) are described, that serves as a synthon to the reactive, "naked" 12-electron [Me₂NN]Co fragment via loss of toluene in its

Full text of DOI:10.1021/ja036308i

Published on Web 03/25/2004
[Me2NN]Co(η6-toluene): OdO, NdN, and OdN Bond Cleavage Provides
â-Diketiminato Cobalt µ-Oxo and Imido Complexes
Xuliang Dai, Pooja Kapoor, and Timothy H. Warren*
Department of Chemistry, Georgetown UniVersity, Box 571227, Washington, D.C. 20057-1227
Received May 23, 2003; E-mail: thw@georgetown.edu
Oxo (O2-) and imido (NR2-) functionalities bound to later first
Scheme 1. Synthesis, Structure, and Reactivity of 2
row transition metals attract interest as active agents in atom1 and
group transfer2 reactions to alkenes as well as in C-H bond
functionalization.3 Due to their important biological roles (especially
for Fe),4 such oxo complexes have commanded considerably more
attention than isoelectronic imido counterparts. Despite cobalt’s
seminal role in synthetic metal-dioxygen chemistry,5 species
bearing oxo functionalities have been established only recently. The
bulky tris(pyrazolyl)borates {Me3TpCo}2(µ-OH)2 react with H2O2
to provide {Me3TpCo}2(µ-O)2 species that are susceptible to
intramolecular C-H bond abstraction by the oxo ligand.6,7
{
i-PrMeTpCo}2(µ-N2) directly reduces O2 by 4 electrons to produce
a related, but especially thermally sensitive, bis(µ-oxo)dicobalt-
(III) complex.7,8 Furthermore, a square-planar [CoIII2(µ-O)2]2-
species was isolated in the disproportionation of the Co(II) complex
[Co(H2L)2]2- (H2L ) bis[(tert-butyl)aminocarbonyl]-1,2-diamido-
ethane) in the presence of O2.9 Imido functionalities bound to later
first row metals are beginning to exhibit reactivity patterns
reminiscent of oxo species. For instance, Theopold observed
intramolecular C-H bond abstraction by inferred Tp′′CodNTMS
intermediates in the reaction of Tp′′Co(N2) with N3TMS.10 Hillhouse
showed that the terminal (dtbpe)NidNAr undergoes imido group
transfer to CO to provide an isocyanate,11 a reaction also observed
by Peters with tris(phosphino)borate complexes [P3B]MtN(p-tolyl)
(M ) Fe,12a Co12b). Exploring relationships between oxo and imido
functionalities in later first row chemistry, we describe herein the
synthesis of a â-diketiminato Co(I) arene adduct and its reactivity
with dioxygen, organoazides, and a nitrosobenzene to provide a
family of structurally diverse µ-oxo and imido complexes.13
The paramagnetic, tetrahedral Co(II) â-diketiminate14 [Me2NN]-
Co(I)(2,4-lutidine) (1) may be prepared in 90% yield from reaction
of Tl[Me2NN]15 with CoI2(2,4-lutidine)2 as teal crystals. Reduction
of this species with Mg powder in toluene provides air-sensitive,
red crystals of [Me2NN]Co(η6-toluene) (2) in 60-70% yield on a
gram scale (Scheme 1). This d8 Co(I)-arene adduct is high-spin as
evidenced by its room-temperature magnetic moment of 2.7 µB in
toluene-d8. While diamagnetic [Me2NN]Rh(arene) and {[Me2NN]-
Rh}2(arene) complexes favor η4-arene coordination,16 the X-ray
structure of 2 reveals an η6-toluene ligand (Co-C ) 2.207(6)-
2.288(5) Å) bound to the [Me2NN]Co fragment (Scheme 1).
Addition of several equivalents of dry oxygen to 2 in ether at
room temperature results in an immediate color change from red
to violet, signaling the formation of {[Me2NN]Co}2(µ-O)2 (3) that
may be isolated in 75% yield as maroon crystals (Scheme 1). The
X-ray structure of 3 obtained at -90 °C consists of two [Me2NN]-
Co fragments related by inversion separated by 2.716(4) Å. Final
refinement suggested positional disorder for the bridging oxygen
atoms. The predominant orientation (86% occupancy; Figure 1)
consists of oxo atoms related by inversion that appear in roughly
square-planar sites9 (22.2° and 23.0° twist angles between the
N-Co-N and O1-Co-O1′ planes) with nearly identical Co-O1
and Co-O1′ bond distances of 1.784(3) and 1.793(4) Å. The minor
orientation consists of two inequivalent oxo atoms that sym-
metrically bridge in a tetrahedral disposition and exhibit somewhat
lengthened Co-O distances.17 The short metal-metal and metal-
oxo distances found in 3 compare favorably to those in related
“diamond core” M2(µ-O)2 (M ) Fe-Cu) structures.3c,9
In contrast to Borovik’s square-planar [CoIII2(µ-O)2]2- complex
that exhibits strong antiferromagnetic coupling, solid-state magnetic
susceptibility data for 3 follow Curie-Weiss behavior from 50 to
200 K for which an average µeff ) 3.5 ( 0.2 µB was determined.17
A plot of 1/ø versus T reveals a deviation from 200 to 300 K,
mirrored by the reversible increase of the solution magnetic moment
from 3.9 to 4.3 µB (toluene-d8) over the range 200-350 K. Coupled
with an upward inflection at ca. 300 K in both solid state and
solution µeff versus T plots, these observations suggest population
of a higher spin state at elevated temperatures.
Aiming to prepare related imido complexes, we explored the
reactivity of 2 with organoazides (Scheme 1). Reaction of 2 with
N3Ar (Ar ) 3,5-Me2C6H3) in ether results in rapid effervescence
and formation of the tetrahedral Co(III)-imido bridged dimer
{[Me2NN]Co}2(µ-NAr)2 (4). Its solution magnetic moment of 8.8
µB at room temperature (benzene-d6) is consistent with two
noninteracting, high-spin d6 centers. In addition, solid-state magnetic
susceptibility data reveal antiferromagnetic coupling with a Neel
temperature of 25 K.17 The X-ray structure of 4 (Figure 1) exhibits
a considerably longer Co-Co separation (3.067(3) Å) than found
in 3 with Co-N(imido) bond distances of 1.983(3) and 1.988(3)
Å. In contrast, reaction of 2 with the more sterically demanding
N3Ad (Ad ) 1-adamantyl) leads to the formation of the three-
coordinate terminal imide [Me2NN]CotNAd (5) that may be
isolated in 50% yield as red crystals. The X-ray structure of 5
(Figure 1) reveals a Co-N(imide) bond distance of 1.624(4) Å
9
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J. AM. CHEM. SOC. 2004, 126, 4798-4799
10.1021/ja036308i CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. X-ray structures of 3-6.
which is at the short end of the range 1.64-1.70 Å observed in
sparse examples of Fe,12a,18 Co,12b and Ni11a,19 terminal imides.
Although in the solid state the imido substituent is somewhat bent
(Co-N3-C22 ) 161.5(3)°), diamagnetic 5 exhibits C2V-symmetric
1H and 13C NMR spectra down to -70 °C (toluene-d8). DFT
calculations support the presence of a low-spin d6 Co(III) center in
5, stabilized by 1σ,2π-donation from the imido ligand which leads
to considerable multiple bond character in this 16-electron, three-
coordinate complex.
To explore the generality of 2 as a precursor to oxo and imido
species via reductive cleavage of double bonds to O and N, we
exposed 2 to 0.5 equiv of OdNAr (Ar ) 3,5-Me2C6H3) in ether
which results in the formation of the binuclear {[Me2NN]Co}2(µ-
O)(µ-NAr) (6) in 33% isolated yield (Scheme 1). This 4-electron
reduction of a nitrosobenzene stands in contrast to the reaction of
the related CpCo(C2H4)2 with OdNPh which leads to [CpCo]2(µ-
η2:η1-PhNO)2.20 The X-ray structure of 6 (Figure 1) is intermediate
between the structures observed for the square-planar bis(µ-oxo) 3
and tetrahedral bis(µ-imido) 4 with opposing [Me2NN]Co fragments
that are nearly orthogonal (89.2° twist angle). While the Co-Co
separation (2.7420(6) Å) and Co-O distances (1.783(2) and 1.786-
(2) Å) in 6 are similar to those found in the square-planar bis(µ-
oxo) 3, the Co-N distances (1.821(3) and 1.823(3) Å) lie between
those in the tetrahedral bis(µ-imido) 4 and terminal imido 5. In the
solid state, 5 exhibits Curie-Weiss behavior over 50-300 K with
an average µeff ) 4.4 ( 0.4 µB with a solution magnetic moment
(benzene-d6) of 4.9 µB at room temperature, the spin-only value
predicted for a S ) 2 system.
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Acknowledgment. T.H.W. thanks Georgetown University, the
ACS PRF (Type-G), and the NSF CAREER program (#0135057)
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Supporting Information Available: Experimental procedures with
characterization data and DFT calculation details (PDF) as well as
crystallographic details (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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