Isolated monomeric and dimeric mixed diorganocuprates based on the size-controllable bulky "rind" ligands

Article abstract of DOI:10.1021/ja9071964

(Figure Presented) Isolable mixed diorganocopper(I) complexes have been synthesized and structurally characterized by introducing a variety of bulky "Rind" groups based on a fused-ring octa-α-substituted s-hydrindacene skeleton. Treatment of RindLi with C

Full text of DOI:10.1021/ja9071964

Published on Web 11/30/2009
Isolated Monomeric and Dimeric Mixed Diorganocuprates Based on the
Size-Controllable Bulky “Rind” Ligands
Mikinao Ito,† Daisuke Hashizume,‡ Takeo Fukunaga,‡ Tsukasa Matsuo,*,† and Kohei Tamao*,†
Functional Elemento-Organic Chemistry Unit and AdVanced Technology Support DiVision, RIKEN AdVanced
Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Received August 26, 2009; E-mail: matsuo@riken.jp; tamao@riken.jp
Scheme 1
Organocopper compounds have been among the most widely
used and powerful tools in synthetic organic chemistry;1 these
2
include organocopper(I) oligomers (CuR)n and a large variety of
3
organocuprates such as diorganocuprates MCuR2 and hetero-
organocuprates MCuRX.4 The structural elucidation of the actual
species of organocopper reagents has met with difficulties because
of their continuous aggregation-dissociation processes in solution.5
For example, mixed diorganocuprates MCuRR′, which were
introduced mainly on the basis of the so-called “dummy ligand”
concept6 and for selective ligand cross-coupling on copper,7
generally come to equilibrium with two homoleptic species MCuR2
and MCuR′2 in solution, which sometimes causes undesirable side
reactions. Thermally stable mixed diorganocuprates are quite
rare.3b,e,4 In this paper, we present the selective synthesis and
isolation of monomeric and dimeric mixed diorganocuprates by
introduction of the size-controllable “Rind” ligands based on a rigid
fused-ring s-hydrindacenyl skeleton (Schemes 1 and 2), which we
have recently developed8 by the modified Kennedy’s method.9 The
new bulky Rind groups have several advantages such as ease of
preparation, high chemical stability, and size-controllability of steric
bulkiness by the proximate substituents.
through a short σ bond [Cu1-C29 ) 1.878(2) Å] and a longer π
coordination [Cu2-C29 ) 2.013(2) and Cu2-C30 ) 2.144(2) Å],
thus adopting the rare case of the µ2-η1,η2 bridging mode.12 The
CtC bond [1.236(3) Å] is slightly elongated in comparison to a
normal triple bond (1.20 Å); accordingly, the IR spectrum of 2
exhibits a ν(CtC) stretching band at 1955 cm-1, which is lower
than that of Me3SiCtCH (2036 cm-1).
RindLi (Rind ) EMind and Eind), which had been prepared from
RindBr, was allowed to react with CuBr to afford neutral tetra-
nuclear Cu(I) complexes 1 with a 2:1 Cu/Rind ratio (Scheme 1).
Although we examined this reaction with various RindLi/CuBr
molar ratios, no other organocopper species except 1 were observed
because of the steric hindrance of the Rind ligands. Complexes 1
were isolated as air- and moisture-sensitive colorless crystals, and
the molecular structure of 1b was determined by X-ray analysis
(Figure 1a). The four copper atoms form a planar and nearly
rectangular structure alternately bridged by a bromine atom and a
Figure 1. Molecular structures of (a) 1b and (b) 2. Hydrogen atoms have
been omitted for clarity.
Cipso atom of the Eind ligand. The side Cu-Cu distances, 2.4274(10)
and 2.7080(10) Å, are indicative of d10 metal-metal interactions.10
The perpendicularly bridged Eind ligands are linked to the two Cu
atoms in a symmetrical three-center-two-electron (3c-2e) bonding
mode2 with Cipso-Cu distances of 2.000(4) and 1.981(4) Å. While
compounds 1 are among the well-characterized RCu/CuX ag-
gregated species,11 1 can also be regarded as the net dimer of two
organohalocuprate [RindCuBr]- units sharing two Cu(I) ions.
In fact, the bromine atoms in 1 can be replaced by organic groups
to form a net dimer of a mixed diorganocuprate sharing two Cu(I)
ions, as shown in Scheme 1. Thus, the reaction of 1b with lithium
trimethylsilylacetylide gave 2 as pale-yellow crystals in 54% yield.
According to the X-ray diffraction studies (Figure 1b), the long
Cu-Cu distances in the planar Cu4 core are further extended to
2.9041(5) Å by the ligand substitution, keeping the bridged Rind/
Cu2 core intact. Each alkynyl group is linked to the two Cu atoms
The copper clusters 1b and 2 represent new members of the
luminescent d10 clusters.10 Thus, complex 1b shows a weak green
emission in the solid state with an emission maximum (λem) at
513 nm and a quantum yield (Φ) of 0.03, which might be attributed
to an admixture of halogen to metal charge-transfer (XMCT)
bromide f Cu4 and metal-centered (MC) 3d f 4s/p transitions.10 No
detectable emission was observed in hexane or benzene, suggesting a
weakened Cu-Cu interaction in solution. In contrast, complex 2
exhibits a strong orange emission in both the solid state (λem ) 613
nm, Φ ) 0.28) and solution (λem ) 605 nm, Φ ) 0.50 in hexane).
The tetranuclear structure of 2 found in the crystal appears to
be maintained even in solution because of the unique coordina-
tion of the alkynyl ligands, resulting in the efficient luminescence
of the π(alkynyl) f 4s/p(Cu) transition with LMCT character.10b,13
Treatment of the much bulkier RindLi (Rind ) MPhind and
MsFluind) with CuBr exclusively produced monomeric organo-
bromocuprates 3 with a 1:1 Cu/Rind ratio (Scheme 2). Figure
† Functional Elemento-Organic Chemistry Unit.
‡ Advanced Technology Support Division.
9
18024 J. AM. CHEM. SOC. 2009, 131, 18024–18025
10.1021/ja9071964  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Acknowledgment. We thank the Ministry of Education, Culture,
Sports, Science, and Technology of Japan for the Grant-in-Aid for
Specially Promoted Research (19002008).
Supporting Information Available: Experimental details and
crystallographic data (CIF). This material is available free of charge
via Internet at http://pubs.acs.org.
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The subsequent reaction of 3a with LiCtCSiMe3 produced the
lithium aryl(alkynyl)cuprate 4 as colorless crystals containing two
molecules of THF in 47% yield. Figure 2b shows its discrete
monomeric nature, with the formation of a contact ion pair in which
the lithium atom is bonded to the two carbon atoms of the acetylene
unit. The geometry around the copper atom is slightly bent, having
a C-Cu-C angle of 165.40(11)°, probably as a result of steric
repulsion between the phenyl groups on the MPhind and the
coordinated THF molecules. The Cu-C(sp2) distance [1.942(2) Å]
is longer than that of Cu-C(sp) [1.871(3) Å]. The CtC distance
is 1.222(4) Å, and the ν(CtC) stretching vibration is observed at
1945 cm-1, which is comparable to that of 2. The mixed diorga-
nocuprate 4 was found to undergo the oxidative ligand-coupling
reaction7 by the action of chloranil7d to selectively afford the bulky
alkyne 5. This provides direct experimental evidence for oxidative
cross-coupling via the mixed diorganocuptrate arising from two
different organolithium or magnesium reagents.
We have shown that the size-controllable Rind ligands can
be used to generate isolable mixed diorganocuprates. In par-
ticular, while the chemical formula of MCuRR′ is generally
accepted in organocopper chemistry, complex 4, which was
realized by the significant bulkiness of the MPhind group,
represents to the best of our knowledge the first isolable
monomeric mixed diorganocuprate.
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