Dicopper(II) metallacyclophanes with oligo(p-phenylene-ethynylene) spacers: Experimental foundations and theoretical predictions on potential molecular magnetic wires

摘要

Two novel double-stranded dicopper(II) metallacyclophanes of formula (nBu_4N)_4[Cu_2(dpeba)_2] ·4MeOH·2Et2O (1) and (nBu_4N) 4[Cu_2(tpeba)_2]·12H2O (2) have been prepared by the CuII-mediated self-assembly of the rigid ('rod-like') bridging ligands N,N′-4,4′-diphenylethynebis(oxamate) (dpeba) and N,N′-1,4-di(4-phenylethynyl)phenylenebis(oxamate) (tpeba), respectively. Single crystal X-ray diffraction analysis of 1 confirms the presence of a dicopper(II)tetraaza[3.3]4,4′-diphenylethynophane metallacyclic structure featuring a very long intermetallic distance between the two square planar CuII ions [r = 14.95(1) ?]. The overall parallel-displaced π-stacked conformation of the two nearly planar para substituted diphenylethyne spacers [dihedral angle (ψ) of 7.8(1)] leads to important deviations from the perpendicular orientation of the copper mean basal planes with respect to the facing benzene planes [dihedral angles (π) of 56.4(1) and 58.4(1)]. X-band EPR spectra together with variable-temperature magnetic susceptibility and variable-field magnetization measurements of 1 and 2, both in solution and in the solid state, show the occurrence of a non-negligible, moderate to weak intramolecular antiferromagnetic coupling [-J = 3.9-4.1 (1) and 0.5-0.9 cm~(-1) (2); H = -JS1·S 2 with S_1 = S_2 = S_(Cu) = 1/2]. Density functional calculations on the BS singlet (S = 0) and triplet (S = 1) spin states of the model complexes 1 and 2 with an ideal orthogonal molecular geometry (ψ = 0 and π = 90) support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons occupying the d_(xy) orbital of each square planar Cu~(II) ion through the predominantly π-type orbital pathway of the double p-diphenylethyne (1) and di(phenylethynyl)phenylene spacers (2). Time-dependent density functional calculations reproduce the observed bathochromic shift of the main intraligand (IL) π-π* transition in the electronic absorption spectra of 1 and 2 [λ_1 = 308 (1) and 316 nm (2)]. In the series of orthogonal model complexes 1-5 with linear oligo(p-phenylene-ethynylene) (OPE) spacers, -C_6H _4(C? - CC_6H_4)_n- (n = 1-5), a linear increase of the IL π-π* transition energy with the reciprocal of the intermetallic distance is theoretically predicted [ν_(max) = 1.99 × 10~4 + 2.15 × 10~5 (1/r) (S = 0) or ν = 2.01 × 10~4 + 2.18 × 10~5 (1/r) (S = 1)], which clearly indicates that the effective π-conjugation length increases with the number of phenylethyne repeating units. This is accompanied by an exponential decay of the antiferromagnetic coupling with the intermetallic distance [-J = 1.08 × 10~3 exp(-0.31r)], which supports the ability of the extended π-conjugated OPEs to mediate the exchange interaction between the unpaired electrons of the two Cu~(II) centers with intermetallic distances in the range of 1.5-4.3 nm. Further developments may be then envisaged for this new family of oxamato-based dicopper(II) oligo-p-phenylethynophanes on the basis of the unique ligand capacity to act as a molecular antiferromagnetic wire.