Inverted-sandwich dichromium(I) complexes supported by two β-diketiminates: A multielectron reductant and syntheses of chromium dioxo and imido

Article abstract of DOI:10.1021/ja072003i

Treatment of [Cr(μ-Cl)(Nacnac)]₂ (Nacnac = HC(C(Me)NC₆H₃-i-Pr₂)₂) with 2.5 equiv of KC₈ in toluene allows the isolation of an inverted-sandwich toluene-bridged complex, (μ-η⁶:η⁶-C₇H₈)[Cr(Nacnac)]₂ (1), in 88.2% yield as a dark-purple solid. The valency of each Cr is determined to be I by X-ray absorption spectroscopy. The twelve Cr-C distances are moderately long and 1 readily undergoes an arene exchange reaction with benzene. As a result, 1 behaves as a two-Cr(Nacnac) synthetic equivalents upon reaction with appropriate substrates. For example, treatment of 1 with 2 equiv of O₂, 4 equiv of MesN₃ (Mes = 2,4,6-C₆H₂Me₃), and Ph₂N₂ engenders the formation of a monomeric Cr(O)₂(Nacnac) (2), Cr(NMes)₂(Nacnac) (3), and (μ-NPh)(Nacnac)]₂ (4), respectively. Complex 1 is therefore an 8-electron reductant. Copyright

Full text of DOI:10.1021/ja072003i

Published on Web 06/13/2007
Inverted-Sandwich Dichromium(I) Complexes Supported by Two
â-Diketiminates: A Multielectron Reductant and Syntheses of Chromium
Dioxo and Imido
Yi-Chou Tsai,*,† Po-Yang Wang,† Shin-An Chen,‡ and Jin-Ming Chen‡
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan, and National Synchrotron
Radiation Research Center, Hsinchu 30077, Taiwan, R.O.C.
Received March 21, 2007; E-mail: yictsai@mx.nthu.edu.tw
Low-coordinate mononuclear transition-metal complexes are
interesting not only in the structural motifs, but also in the reactivity
toward small molecules.1 For instance, Cummins et al. have
demonstrated the remarkable three-coordinate complex Mo(N[R]-
Ar)3 capable of splitting dinitrogen and thus forming the nitride
Mo(N)(N[R]Ar)3 under mild conditions.2 Recently, â-diketiminates
have received considerable attention as ancillary ligands for metal
complexes.3 Of particular interest is their ability in stabilizing
unusual low-coordinate univalent transition metals.4-6 Our interest
in â-diketiminato complexes focuses on the synthesis and reactivity
of Cr(I) complexes. In this communication, we describe the
reduction of a â-diketiminato-Cr(II) complex in toluene. The
Figure 1. Synthesis and molecular structure of 1 with thermal ellipsoids
resulting product has been characterized and its reactivity examined
with dioxygen, organoazide, and azobenzene to provide a family
of structurally diverse Cr(V) dioxo and imido complexes. It thus
reveals that this compound is a useful reagent for the generation
of the “Cr(Nacnac)” species, which acts as a four-electron reductant.
Reduction of [Cr(µ-Cl)(Nacnac)]2 (Nacnac ) HC(C(Me)NC6H3-
i-Pr2)2)7 with KC8 in toluene affords an inverted sandwich
compound in which a toluene molecule bridges two Cr(Nacnac)
fragments in an η6,η6 fashion.8 Compound 1, (µ-η6:η6-C7H8)[Cr-
(Nacnac)]2, is obtained in 88.2% yield as a dark-purple crystalline
substance. The compound is paramagnetic between 50 and 300 K
and exhibits a solid-state magnetic moment of ca. 6.75 µB, as
measured by SQUID magnetometry, consistent with an S ) 3
ground state. To facilitate assignment of the NMR spectrum of 1,
the deuterated variant 1-d8 was prepared by carrying out the
synthesis in toluene-d8. The four resonances for the bound toluene
were thereby identified at 17.8, 14.2, 12.9, and 12.4 ppm in the 2H
NMR spectrum of the compound; the high-field resonances are
assigned to the aryl deuterons, and the downfield signal signifies
the deuteriomethyl group. It was found also that benzene-bridged
dichromium compound could be obtained by carrying out the KC8
reaction in C6D6, and a single resonance peak was observed at 13.7
ppm in the 2H NMR spectrum of (µ-C6D6)[Cr(Nacnac)]2, in accord
with the chemical shift assignments for 1.
X-ray crystallography was used to confirm the dinuclear nature
of compound 1 as shown in Figure 1. Interestingly, the two
(Nacnac)Cr six-member rings are arranged in an orientation with
a N(1)-Cr(1)-Cr(2)-N(4) torsion angle of 57.3°. Whereas Cr-
(2)-C distances to the toluene ring are approximately equal and
average to 2.264(4) Å, corresponding Cr(1)-C distances range from
2.221(4) to 2.471(3) Å, the longest distance being to one of the
methyl group substituted carbon atoms of the ring. Both chromium
atoms tend toward the same edge, C(62)-C(63), and as a result
the whole molecule is distorted. The average C-C distance for
the bridging toluene molecule was determined to be 1.431(5) Å.
at the 30% probability level and a plot of X-ray absorption edge energy E0
vs oxidation states of Cr.
The six-membered rings of 1 adopt a planar conformation. The
arene thus undergoes a slight (ca. 0.04 Å) increase in dC-C upon
complexation, relative to free toluene.9
Formulation of compound 1 as (µ-C7H8)[Cr(Nacnac)]2 implies
various possibilities for the chromium valency. One extreme
requires a formally univalent chromium center. To clarify the
detailed electronic structure of the chromium ions in 1, the X-ray
absorption measurements were undertaken. The Cr K-edge absorp-
tion spectra of Cr foil, 1, CrCl2, and CrCl3 are displayed in Figure
S2 (Supporting Information). A plot of X-ray absorption edge E0
versus various oxidation states of Cr displays a linear relationship
(Figure 1), suggesting an oxidation state of I for Cr in 1. The
chemical reactivity of compound 1 is also consistent with the
formality of univalent chromium, inasmuch as 1 behaves as an
8-electron reductant, giving rise to chromium(V) derivatives and
extruding neutral toluene upon reaction with appropriate substrates.
Metal complexes in which benzene or toluene bridges two metal
centers in an η6,η6 fashion are rare.10-13 Inspired by Cummins and
Evans’ arene-bridged diuranium complexes displaying remarkable
reactivity,11,12 we were also interested in exploring the reactivity
of 1. Noteworthy is that 1 readily undergoes arene exchange
reactions with retention of the inverted sandwich structure. The
dissolution of 1 in C6D6 results in the formation of (µ-C6D6)[Cr-
(Nacnac)]2 as well, and 1 is thus believed to act as two Cr(Nacnac)
synthetic equivalents. Addition of several equivalents of dry oxygen
to 1 in ether at room-temperature elicits a rapid color change from
purple to orange-brown, signaling the formation of the mononuclear
dioxo complex Cr(O)2(Nacnac) (2) that may be isolated in 53.4%
yield as dark-brown crystals. The X-ray structure of 2 consists of
two terminal oxo atoms attached to a tetrahedral chromium center
(Figure 2).
Compound 2 is subjected to a detailed EPR study as a result of
its solution magnetic moment of 1.71 µB at room temperature. The
room-temperature EPR spectrum of 2 in toluene displays a sharp
isotropic signal at giso ) 1.9779. It features an intense central
† National Tsing Hua University.
‡ National Synchrotron Radiation Research Center.
9
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J. AM. CHEM. SOC. 2007, 129, 8066-8067
10.1021/ja072003i CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Reactivity of 1 and X-ray molecular structures of 2-4.
resonance of five lines attributable to coupling of the unpaired
electron to two magnetically equivalent 14N nuclei of the Nacnac
ligand. Successful simulation of the observed spectrum is achieved
by specifying two hyperfine couplings: Aiso[53Cr, 9.54%] ) 1.55
G and Aiso[14N, 99.63%] ) 2.25 G.
versity) is acknowledged for help with crystallographic details. We
also thank Professor Daniel J. Mindiola for insightful comments.
Supporting Information Available: Experimental details for the
synthesis, X-ray crystallographic data of 1, 2, 3, and 4 including tables
and CIF files, X-ray absorption spectrum of 1, and SQUID data of 1
and 4. This material is available free of charge via the Internet at http://
pubs.acs.org.
Reaction of 1 with four equivalents of N3Mes (Mes ) 2,4,6-
Me3C6H2) in ether leads to rapid effervescence and formation of
the tetrahedral Cr(NMes)2(Nacnac) (3) which is isolated in 32.6%
yield as dark green crystals. Its solution magnetic moment of 2.02
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In conclusion, a remarkable inverted-sandwich dichromium
complex 1 is successfully synthesized and fully characterized.
Preliminary reactivity studies of 1 establish that the “Cr(Nacnac)”
platform will at least support three formal oxidation states (CrI,
CrIII, CrV). Reactions of 1 and other organic functionalities are
ongoing and will be reported in due course.
Acknowledgment. We are indebted to the Taiwan, R.O.C.
National Science Council for support under Grant NSC 94-2113-
M-007-033. Mr. Ting-Shen Kuo (National Taiwan Normal Uni-
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