Very Large Difference in Electronic Communication of Dimetal Species with Heterobiphenylene and Heteroanthracene Units

Two neutral compounds having [Mo2] units linked by squarate dianions, [Mo2(DAniF)3]2(μ4-C4O4) (DAniF = N,N‘-di(p-anisyl)formamidinate) (1) and [Mo2(DmCF3F)3]2(μ4-C4O4) (DmCF3F = N,N‘-di(m-trifluoromethylphenyl)formamidinate) (2), as well as the singly oxidized compound {[Mo2(DmCF3F)3]2(μ4-C4O4)}SbF6 (3) and the doubly oxidized
species {[Mo2(DAniF)3]2(μ4-C4O4)}(TFPB)2 (TFPB = [B(3,5-(CF3)2C6H3)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 C6 instead of C4 rings (J. Am. Chem. Soc. 2006, 128, 3281) which led to differences in
comproportionation constants of over 108. In the squarate species, the π 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 Mo2 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).

本研究合成并结构表征了一系列由方酸二阴离子以μ₄桥联方式连接[Mo₂]单元的配合物：包括两种中性化合物[Mo₂(DAniF)₃]₂(μ₄-C₄O₄)（DAniF = N,N'-二(对茴香基)甲脒基）（化合物1）与[Mo₂(DmCF₃F)₃]₂(μ₄-C₄O₄)（DmCF₃F = N,N'-二(间三氟甲基苯基)甲脒基）（化合物2），单氧化产物{[Mo₂(DmCF₃F)₃]₂(μ₄-C₄O₄)}SbF₆（化合物3），以及双氧化产物{[Mo₂(DAniF)₃]₂(μ₄-C₄O₄)}(TFPB)₂（TFPB = [B(3,5-(CF₃)₂C₆H₃)₄]⁻，即四(3,5-二三氟甲基苯基)硼酸根）（化合物4）。
对两种中性配合物的电化学测试显示，方酸阴离子桥联的两个二钼单元间仅存在极弱的电子相互作用；这与以C₆环替代C₄环的二氧杂环戊烯类似物的结果形成鲜明对比（J. Am. Chem. Soc. 2006, 128, 3281），后者的归中反应平衡常数差异超过10⁸。
在方酸类配合物中，π电子定域于异金属六元Mo₂O₂C₂环的羰基与二金属单元内，以此降低中心C₄四元环的反芳香性。
氧化产物3与4在物理测量的时间尺度上呈现电子定域化行为。
密度泛函理论（Density Functional Theory, DFT）计算表明，桥联配体与二钼单元的前线轨道能量失配是导致Mo₂单元间弱电子相互作用的原因。
对于双氧化配合物4，DFT计算得到的交换耦合常数J为-130 cm⁻¹，表明两个未成对电子仅存在弱反铁磁耦合，这与磁学研究测得的J = -121 cm⁻¹结果一致。