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

Abstract

A series of oligothiophene bis(dioxolene) complexes, SQ-Thn-SQ (SQ = S = ½ TpCum,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...

Metal–Ligand Exchange Coupling Alters the Open-Shell Ligand Electronic Structure in a Bis(semiquinone) Complex

“…Furthermore, we showed that (ZnSQ) 2 Th 3 possesses an open-shell biradical ground state (Figure D), indicating (NiSQ) 2 Th 2 may also possess an open-shell singlet ground state. In fact, the dioxolene bond lengths of Ni(SQ) 2 Th 2 more closely resemble those of 3,5-di- tert -butyl-semiquinone (having no attached π-system for delocalization) than the dioxolene bond lengths of Zn(SQ) 2 Th 2 .…”

“…X-ray-quality crystals were grown for (NiSQ) 2 Th 2 , and the structure obtained by X-ray diffraction is shown in Figure A. Bond lengths for the bithiophene bridge unit of (NiSQ) 2 Th 2 are shown in Figure B and are compared to the corresponding bond lengths of (ZnSQ) 2 Th 2 in Figure C . One can clearly see that the bithiophene bridge in (NiSQ) 2 Th 2 exhibits a bond length pattern that is less quinoidal than that observed for the dinuclear Zn complex (ZnSQ) 2 Th 2 .…”

“…Similarly, we have used the crystallographic results for (ZnSQ) 2 Th 2 and (ZnSQ) 2 Th 3 to determine a Δ r Zn3–Zn2 of 3.87 Å. Figure shows the SQ–SQ distances for (ZnSQ) 2 Th 2 and (ZnSQ) 2 Th 3 , along with the effective SQ–SQ distance for (NiSQ) 2 Th 2 ( r sQ–SQ exchange = 9.20 Å + 2.08 Å = 11.28 Å) as a function of the experimentally determined J SQ–SQ values.…”

“…While our J Ni−SQ of −291 cm −1 falls within the range of values reported for (five-coordinate) Ni−SQ complexes (J Ni−SQ ≈ −244 to −525 cm −1 ), 34−37 the J SQ−SQ value determined for the nickel complex is 37% less than the corresponding J SQ−SQ value we reported for (ZnSQ) 2 Th 2 (J SQ−SQ = −321 cm −1 ) at parity of the Th 2 bridge. 22 In contrast to our analysis of the (NiSQ) 2 Th 2 data, the J SQ−SQ value for (ZnSQ) 2 Th 2 was obtained by fitting a three-state model (yellow box in Figure 2C), which includes the closedshell quinoidal ground state, to magnetic susceptibility data of a crystalline (ZnSQ) 2 Th 2 sample. Our ability to obtain an excellent fit to the (NiSQ) 2 Th 2 magnetic data without including a closed-shell singlet state, coupled with our structural data, provides strong experimental support for the idea that the quinoidal singlet state is not thermally populated in (NiSQ) 2 Th 2 .…”

“…In contrast, the closed-shell 1 A 1 CS ground state of (ZnSQ) 2 Th 2 was described within this expanded model as an admixture of the closed-shell quinoidal 1 A 1 CS and open-shell 1 A 1 OS electron configurations (Figure 2C). 22 In addition to the 1 A 1 CS ground state, the low-lying 1 B 1 OS and 1 B 1 OS states in (ZnSQ) 2 Th 2 are thermally populated. These 1 B 1 OS and 3 B 1 OS states are the openshell singlet and triplet states, respectively, that derive from the two exchange-coupled S = 1 / 2 SQ spins.…”