Unimolecular Dissociation of Hydrogen Squarate (HC4O4-) and the Squarate Radical Anion (C4O4•-) in the Gas Phase and the Relationship to CO Cyclooligomerization

Electronic Instability and Solvation Stabilization of Oxocarbon Dianions (CnOn)2- (n = 4-6)

Oxocarbon dianions (CnOn)2- have been recently found to be promising candidates in the design of high-capacity and fast rechargeable batteries but are intrinsically unstable in the isolated form. Fundamental understandings of their electronic structures, solvent stabilization, and interactions with solvents and counterions are crucial in comprehending their electron transfer reactions occurring in batteries. In this article, we employed microsolvated dianionic clusters as models and combined negative ion photoelectron spectroscopy (NIPES) and theoretical computations to probe the electronic instability and solvation stabilization of (CnOn)2- (n = 4-6) dianions. Through the smallest observable members in each series of microhydrated dianions and their recorded adiabatic and vertical detachment energies (ADEs and VDEs), the minimum numbers of H2O molecules required to stabilize (CnOn)2- dianions are determined to be 4, 3, and 2 for n = 4, 5, and 6, respectively, while 3 and 2 water molecules can make (C4O4)2- and (C5O5)2- metastable and detectable. Using theoretical calculations, we determined the lowest energy structures of each complex. The first few H2O molecules prefer to be directly hydrogen bonded to two adjacent O atoms around the oxocarbon ring. The water binding strengths are generally comparable when each H2O molecule is bound at a separate binding pocket, but the binding strengths decrease when all binding pockets are occupied, in parallel with the observed ADE and VDE shift trends. Moreover, hydrated (C4O4)2- dianions are found to possess distinct electronic structures compared to its (C5O5)2- and (C6O6)2- analogues due to its near-degenerate HOMO and HOMO-1, while there exists a larger gap for the latter two dianions. Upon hydration, the overall electronic structure patterns are maintained without much distortion, but fine changes are noticeable, which warrant future studies.

CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization

The divalent thulium complex [Tm(Cpttt)2] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C2) and ketenecarboxylate (C3) complexes, respectively. DFT calculations were performed to shed light on the elementary steps of CO homologation and support a stepwise chain growth. The attempted decoordination of the ethynediolate fragment by treatment with Me3SiI led to dimerization and rearrangement into a 3,4-dihydroxyfuran-2-one complex. Investigation of the reactivity of the C2 and C3 complexes towards other electrophiles led to unusual functionalization reactions: while the reaction of the ketenecarboxylate C3 complex with electrophiles yielded new multicarbon oxygenated complexes, the addition of CO2 to the ethynediolate C2 complex resulted in the formation of a very reactive intermediate, allowing C-H activation of aromatic solvents. This original intermolecular reactivity corresponds to an unprecedented functionalization of CO-derived ligands, which is induced by CO2.

The unimolecular dissociation of magnesium chloride squarate (ClMgC4O4−) and reductive cyclooligomerisation of CO on magnesium

Cyclooligomerisation to squarate is initiated by the coupling of two CO units in MgCl⁻.