Very large difference in electronic communication of dimetal species with heterobiphenylene and heteroanthracene units

Two neutral compounds having [Mo-2]units linked by squarate dianions, [Mo-2(DAniF)(3)](2)(mu(4)-C4O4) (DAniF = N,N'-di(p-anisyl)formamidinate) (1) and [Mo-2(DmCF(3)F)(3)](2)(mu(4)-C4O4) (DmCF(3)F = N,N'-di(m-trifluoromethylphenyl)formamidinate) (2), as well as the singly oxidized compound 1[Mo-2(DmCF(3)F)(3)](2)(mu(4)-C4O4)}SbF6 (3) and the doubly oxidized species {[Mo-2(DAniF)(3)](2)(mu(4)-C4O4)}(TFPB)(2) (TFPB = [B(3,5-(CF3)(2)C6H3)4](-)) (4), were synthesized and structurally characterized. Electrochemical measurements of the two neutral species showed only very weak electronic interactions between the two dimolybdenum units linked by the squarate anion in contrast to what was observed in dioxolene analogues having C-6 instead Of C-4 rings (J. Am. Chem. Soc. 2006, 128, 3281) which led to differences in comproportionation constants of over 10(8). In the squarate species, the pi electrons are localized within the carbonyl and dimetal units in the heterometallic six-membered Mo2O2C2 rings to minimize the antiaromaticity in the central C4 square. The oxidized species 3 and 4 are electronically localized in the time scale of the physical measurements. Calculations at the DFT level suggested that the energy mismatch of the frontier orbitals of the linker and dimetal units contributes to the weak communication between the Mo-2 units. For the doubly oxidized complex 4, DFT calculations gave a J value of -130 cm(-1) which suggests that the two unpaired electrons are only weakly antiferromagnetically coupled, as shown by magnetic studies (J = -121 cm(-1)).