Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11031
Scheme 1. Peralkylated Diamine Ligands (PDLs)
tically convert alcohols to aldehydes in high yields under
ambient conditions while TMED is among the commonly
employed amines. CuCl/TMED/O2 also oxidatively couples
2,6-disubstituted phenols to form poly-2,6-disubstiuted 1,4-
phenylene ethers, 2,4-disubstituted phenols to bis-phenols,24
and acetylenes to diacetylenes in high yields.25 However,
little information is known about the active Cu/O2 species
that form with TMED and allow the reactions to be
catalytic.
As the simplest PDL, TMED stands as a promising ligand
to stabilize an accessible O species for exogenous substrates
and the C-H groups of the methyl substituents should be the
most oxidatively robust based on simple bond dissociation
energies.16 However, the expected O species with TMED is
conspicuously absent from previous studies because of am-
biguous and ill-defined oxygenation chemistry.11 This study
discusses the challenges and strategies to maximize the forma-
tion of the O species with TMED and investigates its physical
properties and reactivities toward exogenous substrates.
capable of stabilizing a d8 Cu(III) center and sufficient steric
demands to preclude the formation of a nonreactive
[(L)2Cu(I)]1þ complex. Tridentate and tetradentate mononu-
cleating, nitrogen-based ligands can stabilize the O species,
albeit this occurs by a denticity reduction, in which ligating
atoms do not associate closely with the copper centers.1 The
steric demands of the described bidentate ligands tend to
stabilize three-coordinate Cu(I) complexes in which the third
ligand is derived from a weakly associated anion or solvent
molecule such as acetonitrile. While such complexes provide
access of O2 to the copper center and rapid formation of the O
species, this dimer is very compact with a Cu Cu distance of
3 3 3
˚
only 2.73-2.85 A. The consequence is that neither the copper
2. Results
centers nor the oxide ligands are very accessible to exogenous
substrates. Predictably, the larger the steric demands, the more
oxidative damage that is inflicted on the ligands upon warming
these O species to ambient conditions. Intramolecular ligand
hydroxylation is a prevalent decomposition pathway,15 thereby
limiting the thermal stability of most O species. Directing this
oxidative reactivity outward to exogenous substrates requires
minimizing the steric demands of the ligands while retaining
oxidatively robust substituents.
We have explored extensively the peralkylated diamine
ligands (PDLs) in stabilizing O species as simple and versatile
ligands (Scheme 1). Systematic variation of the steric demands
and bite-angle provides an exquisite probe into the nature of the
Cu/O2 species formed.11 The three most extensively explored
families of PDLs, defined by their parent diamine backbone, are
the (R,R)-1,2-cyclohexanediamines (CD),16 1,2-ethylenedia-
mines (ED),7,17 and 1,3-propanediamines (PD).18,19 Notably
in the ED series, simply reducing the steric demands of alkyl
moieties, the predominant Cu/O2 species shifts from a SP
species17 to an O species16,20 finally to a trinuclear species,
[Cu(III)Cu(II)2(μ3-O)2]3þ (T),11 demonstrating the versatility of
ED ligands. The aliphatic substituents create strongly basic, ter-
tiary N-donors, which are able to stabilize the high Cu(III) oxi-
dation state at Cu-N distances longer than secondary amines.
N,N,N0,N0-tetramethylethylenediamine (TMED) is the
most widely used PDL in Cu-catalyzed aerobic organic
transformations.21-23 CuCl/amine/O2 combinations cataly-
Synthesis of [(TMED)Cu(I)]1þ Complex. [(TMED)-
Cu(I)]1þ was synthesized by mixing equimolar amounts of
TMEDand[Cu(I)(MeCN)4](X) (X-=SbF6-, CF3SO3-) in
dry CH2Cl2. Slight deviations from the 1:1 Cu(I)/TMED
ratiooftenleadtoanunstable[(TMED)Cu(I)]1þ complex or
incomplete oxygenation as assessed by optical absorptions.
The[(TMED)Cu(I)]1þ solutions degrade faster compared to
other PDL-Cu(I) complexes in a N2 drybox; a yellow
precipitate forms within 12 h, and over longer periods the
solution turns blue accompanied with a brown precipitate
presumably due to the disproportionation of the Cu(I)
complex. Purification of the Cu(I) complex by a reported
method26 yields a white solid, which can be oxygenated at
low temperature. However, it still turns into a yellow solid
over 24 h, which no longer reacts with dioxygen at low
temperatures. The ligating acetonitrile present in the white
solid is lost in the yellow solid by 1H NMR analysis, leading
to a nonreactive Cu(I) complex toward O2. The instability of
the Cu(I) complex mandates that the Cu(I) solutions are
prepared immediately before oxygenation at low tempera-
tures for reproducible results.
Oxygenation of [(TMED)Cu(I)]1þ Complex. [(TMED)-
Cu(I)](X) (X-=SbF6-, CF3SO3-) solutions were oxygen-
ated by injection into a rapidly stirred O2-saturated acetone
solution at 183 K. The rapid full formation (<5 s) of the O
species is indicated by the yellow color of the initial colorless
solution. The 392 nm ligand to metal charge transfer
(LMCT) band (Figure 1, ε = 18.5 mM-1 cm-1 per dimer)
is consistent with other related O species.1 The intensity of
this optical feature is not altered by purging the solution with
N2 nor by evacuating the reaction vessel, consistent with an
irreversible oxygenation reaction. Titration of the O species
with ferrocene monocarboxylic acid, a technique developed to
quantify the formation of such oxidizing species,14 gives 80%
of the expected yield calibrated against well-established, fully
formed O species (Supporting Information, Figure S1).
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