Synthesis and Characterization of a Novel Class of Dicopper(I) Bis(carboxylate)-Bridged Complexes

The syntheses, spectroscopic properties, crystal and
molecular structures, and bonding of several dicopper(I)
bis(carboxylate)-bridged complexes are described in which the bridging
dicarboxylate ligand is the dianion of
m-xylylenediamine bis(Kemp's triacid imide)
(H2XDK, 1). A sterically demanding
benzyl derivative of H2XDK
was prepared, H2BXDK (6). Reaction of
these two ligands as well as the propyl analog, H2PXDK
(2), with
thallium ethoxide provided dithallium salts Tl2L (L =
XDK, 7; PXDK, 8; and BXDK, 9).
Reaction of 7, 8, or
9 with excess CuBr(Me2S) in
CH2Cl2/MeCN afforded dinuclear
acetonitrile adducts [Cu2L(MeCN)] (L =
XDK,
10; PXDK, 11; and BXDK, 12) in high
yield and multigram quantities. The coordination chemistry of the
Cu2(XDK) platform was explored with a range of ancillary ligands.
Treatment of 10, 11, or 12 with
an excess of the
specified neutral donor ligand provided complexes
[Cu2(XDK)(PPh3)2]
(13),
[Cu2(XDK)(2,6-Me2C6H3NC)3]
(14a),
[Cu2(XDK)(μ-2,6-Me2C6H3NC)(2,6-Me2C6H3NC)2]
(14b), [Cu2(XDK)(NB)2]
(15), [Cu2(XDK)(tmeda)]
(17), [Cu2(PXDK)(tmeda)] (18), [Cu2(BXDK)(tmeda)]
(19), and
[Cu(4,4‘-Me2bpy)2][Cu(XDK)]
(20). Reaction of 10 with
an excess of cyclohexene resulted in the loss of the acetonitrile
ligand, affording the parent unsubstituted complex
[Cu2(XDK)] (16). Attempts to prepare
anionic carbon-bridged dicopper(I) complexes with alkyl- or
aryllithium
compounds or cyanide reagents resulted instead in extraction of one of
the Cu(I) ions, affording
(Et4N)[Cu(PXDK)] (21) and [CuLi(XDK)(THF)2]
(22). Crystallographic chemical analysis of the
complexes revealed linear
two-coordinate, trigonal three-coordinate, and pseudotetrahedral
four-coordinate copper(I), depending upon the
composition, and variable degrees of Cu−Cu bonding
(dCu-Cu range,
2.5697(8)−3.4211(6) Å).