Variation from closed-shell to open shell electronic structures in oligothiophene bis(dioxolene) complexes

A series of oligothiophene bis(dioxolene) complexes, SQ–Thn–SQ (SQ = S = ½TpCum,MeZnII(3-tert-butyl-orthosemiquinonate); TpCum,Me = tris(5-cumenyl-3-methylpyrazolyl)borate anion) have been synthesized, structurally characterized, and studied as a function of the number of thiophene bridging units, n (n = 0–3) using a combination of variable–temperature (VT) electronic absorption and EPR spectroscopies, and VT magnetic susceptibility measurements. The thiophene bridge bond lengths determined by X-ray crystallography display dramatic differences across the SQ–Thn–SQ series. Bridge bond deviation values (Σ|Δi|) display a progressive change in the nature of the bridge fragment bonding as the number of thiophene groups increases, with quinoidal bridge character for n = 1 (SQ–Th–SQ) and biradical character with “aromatic” bridge bond lengths for n = 3 (SQ–Th3–SQ). Remarkably, for n = 2 (SQ–Th2–SQ) the nature of the bridge fragment is intermediate between quinoid and biradical aromatic, which we describe as having open-shell character as opposed to biradicaloid since the open-shell biradical configuration does not have the correct symmetry to mix with the quinoidal ground-state configuration. This bridge bonding character is reflected in the energies of the lowest lying open-shell states for these three molecules. The SQ–Th–SQ molecule is diamagnetic at all temperatures studied, and we provide evidence for SQ–SQ antiferromagnetic exchange coupling and population of triplet states in SQ–Th2–SQ and SQ–Th3–SQ, with JSQ–SQ(ave) = −279 cm−1 (VT EPR/electronic absorption/magnetic susceptibility) and JSQ–SQ = −117 cm−1 (VT EPR/electronic absorption/magnetic susceptibility), respectively. The results have been interpreted in the context of state configurational mixing within a simplified 4-electron, 3-orbital model that explicitly contains contributions of a bridge fragment. Variable–temperature spectroscopic- and magnetic susceptibility data are consistent with two low-lying open-shell states for SQ–Th3–SQ, but three low-lying states (one closed-shell and two open-shell) for SQ–Th2–SQ. This model provides a simple symmetry-based framework to understand the continuum of electronic and geometric structures of this class of molecules as a function of the number of thiophene units in the bridge.