Dicopper Double-Strand Helicates Held Together by Additional π–π Interactions

The
bis-bidentate ligand, obtained from Schiff base condensation
of RR-1,2-cyclohexanediamine and 8-naphthylmethoxyquinoline-2-carbaldehyde
(L–L), forms with [CuI(MeCN)4]­ClO4 a double strand helicate complex, made especially
stable by the presence of four definite interstrand π–π
interactions involving a quinoline subunit and a naphthylmethoxy substituent
of the two strands. The [CuI2(L–L)2]2+ complex, which does not decompose even
on excess addition of either L–L or CuI, undergoes a two electron oxidation in MeCN, through two one-electron
fully reversible steps, separated by 260 mV, as shown by cyclic voltammetry
(CV) studies. The high stability of the mixed valence complex [CuICuII(L–L)2]3+ with respect to disproportionation to [CuI2(L–L)2]2+ and [CuII2(L–L)2]4+ is essentially due to a favorable electrostatic term. CuII forms with L–L a stable species, with a 1:1
stoichiometric ratio, but, in the absence of crystallographic data,
it was impossible to assess whether it is of mono- or dinuclear nature.
However, CV studies on an MeCN solution containing equimolar amounts
of CuII and L–L showed the presence
in the reduction scan of two fully reversible waves, separated by
about 250 mV, which indicated the presence in solution of a dicopper­(II)
double strand helicate complex, [CuII2(L–L)2]4+. This work demonstrates
that additional interstrand π–π interactions can
favor the formation of unusually stable dicopper­(I) and dicopper­(II)
helicate complexes.

由RR-1,2-环己二胺与8-萘甲氧基喹啉-2-甲醛经希夫碱缩合反应得到的双双齿配体（L-L），与高氯酸四(乙腈)合铜(I)([CuI(MeCN)4]ClO4)配位形成双链螺旋配合物。该配合物因两条配链上的喹啉亚基与萘甲氧基取代基之间存在四组明确的链间π-π相互作用而具有极高稳定性。
该[CuI2(L-L)2]2+配合物即使在过量加入L-L或CuI时也不会发生分解；在MeCN中，其可通过两步单电子完全可逆的电子转移过程发生双电子氧化，两步电势差为260 mV，该结论经循环伏安法（Cyclic Voltammetry, CV）研究证实。
混合价态配合物[CuICuII(L-L)2]3+相较于歧化为[CuI2(L-L)2]2+与[CuII2(L-L)2]4+的稳定性极高，这主要源于有利的静电作用项。CuII与L-L可形成化学计量比为1:1的稳定物种，但因缺乏晶体学数据，无法确定其为单核还是双核结构。
不过，对含等摩尔CuII与L-L的MeCN溶液进行循环伏安研究发现，还原扫描过程中存在两个电势差约250 mV的完全可逆波，表明溶液中存在双核铜(II)双链螺旋配合物[CuII2(L-L)2]4+。
本研究证实，额外的链间π-π相互作用可助力形成稳定性异乎寻常的双核铜(I)与双核铜(II)螺旋配合物。