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Cu(PPh3)3(TCNE) (TCNE = tetracyanoethylene) and 14 other examples form [TCNE]22- dimers possessing a long 2.89 ± 0.05 Å two-electron four-center (2e-/4c) C-C bond in the solid state. This bond arises from the overlap of the b2g π* singly occupied molecular orbital (SOMO) on each [TCNE]·- fragment, forming a filled bonding orbital of b2u symmetry, and the stabilizing effect of the cation⋯anion interactions in the crystal that exceed the anionic repulsion. In contrast, Mn(C5H5)(CO)2(TCNE) exhibits a related, but different, [TCNE]·-⋯[TCNE]·- motif in the solid state that lacks the 2e-/4c C-C bonding. To better understand the unusual nature of 2e-/4c C-C bonding, the genesis of the differences between their respective π-[TCNE]·-⋯[TCNE]·- interactions was sought. The lack of 2e-/4c C-C bond formation is attributed to the weaker radical character of the [TCNE]·- ligand, which has a total spin population of only 0.5 electron, half of that required for two S = 1/2 [TCNE]·- moieties to form a [TCNE]22- dimer. Hence, the antiferromagnetic MnII-[TCNE]·- intramolecular interaction (between the formally S = 1/2 Mn-bound [TCNE]·- and the paramagnetic Mn(II)) dominates over the intermolecular π-[TCNE]·--[TCNE]·- spin coupling (between two S = 1/2 [TCNE]·- needed to form [TCNE]22-). Therefore, by selecting specific metal ions that can interact with σ-[TCNE]·-, dimerization forming [TCNE]22- can be favored or disfavored. [ABSTRACT FROM AUTHOR]

The structures of three temperature-dependent polymorphs of solvent-free decamethylferrocenium tetracyanoethenide, [FeCp*[sub2]][TCNE], are determined from high-resolution synchrotron powder diffraction data. [FeCp*[sub2]][TCNE] is the first organic-based ferromagnetic material to be synthesized and is known to have two structural phase transitions at 249 and 282 K. The low-temperature phase, which exhibits spontaneous ferromagnetic order below 4.8 K, was determined at 12 K. At that temperature, it has monoclinic space group P2[sub1]/c [a = 9.6637(4) Å, b = 14.1217(5) Å, c = 18.6256(7) A, β = 113.231 (2)°, Z = 4] and consists of parallel chains of alternating [Fe(C[sub5]Me[sub5])[sub2]][sup∙+] and [TCNE][sup∙-] ions, with an intrachain Fe⋯Fe distance of 10.45 Å. Structures of the intermediate and ambient temperature phases, also studied here, are characterized by increasing disorder. At 250 K, the unit cell space group is F2[sub1]/m [a = 9.7100(3) Å, b = 14.4926(4) Å, c = 9.4997(3) Å, β = 113.153(1)°, Z= 2]. At ambient temperature, the lattice, albeit quite disordered, belongs to the orthorhombic space group Cmcm [a = 10.629(1) Å, b = 16.128(1) Å, c 14.593(1) Å, Z = 4]. Nearest-neighbor magnetic interactions were evaluated for the 12 K structure by CASSCF and CASSCF/MCQDPT calculations (a methodology similar to the CASPT2 method). Similar trends are observed in computations with and without inclusion of spin-orbit coupling. The strongest are two intrachain [FeCp*[sub2]][sup&3x2219;-]⋯[TCNEI interactions (ferromagnetic with values of ∼45 and ∼29 cm[sup1]), although weaker, nonnegligible, ferro- or antiferromagnetic interchain interactions of less than ±0.2 cm[sup1] are also present. Magnetic interactions that lead to ordering are therefore three-dimensional, despite the vastly different intra- and interchain coupling strengths. [ABSTRACT FROM AUTHOR]

Two-dimensional conjugated polymers (2DCPs)─organic 2D materials composed of arrays of carbon sp 2 centers connected by π-conjugated linkers─are attracting increasing attention due to their potential applications in device technologies. This interest stems from the ability of 2DCPs to host a range of correlated electronic and magnetic states (e.g., Mott insulators). Substitution of all carbon sp 2 centers in 2DCPs by nitrogen or boron results in diamagnetic insulating states. Partial substitution of C sp 2 centers by B or N atoms has not yet been considered for extended 2DCPs but has been extensively studied in the analogous neutral mixed-valence molecular systems. Here, we employ accurate first-principles calculations to predict the electronic and magnetic properties of a new class of hexagonally connected neutral mixed-valence 2DCPs in which every other C sp 2 nodal center is substituted by either a N or B atom. We show that these neutral mixed-valence 2DCPs significantly energetically favor a state with emergent superexchange-mediated antiferromagnetic (AFM) interactions between C-based spin- 1 / 2 centers on a triangular sublattice. These AFM interactions are surprisingly strong and comparable to those in the parent compounds of cuprate superconductors. The rigid and covalently linked symmetric triangular AFM lattice in these materials thus provides a highly promising and robust basis for 2D spin frustration. As such, extended mixed-valence 2DCPs are a highly attractive platform for the future bottom-up realization of a new class of all-organic quantum materials, which could host exotic correlated electronic states (e.g., unusual magnetic ordering, quantum spin liquids).