Template condensation of 2,2‘-diaminobiphenyl,
1,4-bis(2-formylphenyl)-1,4-dithiabutane, and
copper(II) tetrafluoroborate yields the new macrocyclic compound
[CuI(bite)](BF4) (bite =
biphenyldiimino dithioether).
[CuI(bite)]BF4 crystallizes in
the orthorhombic space group
P212121 with
a = 14.379(3) Å, b = 21.370(3) Å,
c =
8.046(2) Å, V = 2534.7(7) Å3,
Z = 4, R
1 = 0.045, and
R
2 = 0.048. The X-ray structure of
[CuI(bite)](BF4)
reveals
distorted tetrahedral N2S2 coordination about
copper, with one unusually short Cu−S(thioether) bond of
2.194(2) Å.
Oxidation of [CuI(bite)](BF4)
with nitrosyl tetrafluoroborate gives
[CuII(bite)](BF4)2.
[CuII(bite)](BF4)2
crystallizes
in the tetragonal space group I41/a
with a = 11.640(2) Å, c =
39.527(3) Å, V = 5355.6(7) Å3,
Z = 8, R
1 = 0.061,
and R
2 = 0.063. X-ray crystallography of
[CuII(bite)](BF4)2
reveals an approximately square planar CuN2S2
structure
with two distant axial BF4
- anions (Cu−F
2.546(4) Å) completing a “pseudo-octahedral” coordination
sphere.
Comparative EXAFS studies of solid samples and acetonitrile
solutions of [CuI(bite)](BF4) and
[CuII(bite)](BF4)2
demonstrate that the primary coordination environments of both species
are the same in solution as in the solid.
Copper(I/II) electron self-exchange kinetics measured by
1H NMR line broadening of
[CuI(bite)]+ in the presence
of
[CuII(bite)]2+ reveal an overall
first-order process with a rate constant of 21.7(1.9)
s-1 at 295 K in
acetone-d
6. This
result represents the first example of fully-gated electron transfer by
small-molecule copper(I). The gating process
likely involves inversion at sulfur and the tetrahedral → square
planar structural change coincident with electron
transfer. Variable-temperature 1H NMR coalescence
temperatures for methylene ligand protons of
[CuI(bite)](BF4)
(287 K) demonstrate possible correlation of fast electron transfer with
high ligand mobility for this and related
small-molecule copper(I/II) couples. Comparison with other
small-molecule copper systems also reveals that fast
electron transfer is not always observed with
coordination-number-invariance and conserved geometry during
redox
turnover, contrary to popular interpretations of the entatic state
hypothesis for blue-copper protein active sites.