Although cation-π interactions commonly involve aromatic or heteroaromatic rings as the source of π-electrons, isolated and nonconjugated olefins are equally effective donors of π-electron density. Previous comparisons of these π-electron sources have indicated that the net energy of the binding interactions is not a simple additive function of the number of π-bonds involved. For instance, the enthalpy of binding (ΔH°) of Li(+), Na(+), or K(+) cations to two ethylene molecules or to one benzene molecule is approximately the same, despite the 4:6 ratio of π-electrons involved. This present density functional theory study indicates that geometric factors can partially account for the proportionally greater interaction energies of olefins, but whether they are symmetrically placed around the cation or grouped on one hemisphere has little effect on the binding energy. Instead, flexible ligands that permit olefinic π-electrons to be oriented more favorably toward the metal than those in rigid aromatic rings can be correlated with greater bonding. For Li(+) complexes, this appears to be an appreciable factor, although it is less significant with Na(+) and K(+) complexes. For all three cations, stronger polarization interactions with olefins compared to arenes contribute to the strength of cation-π interactions involving olefinic π-bonds.

Reaction of two equivalents of K­[1,3-(SiMe3)2C3H3] (= K­[A′]) with MnCl2 in THF produces the allyl complex A′2Mn­(thf)2; if the reaction is conducted in ether, the solvent-free heterometallic manganate species K2MnA′4 is isolated instead. With the related allyl K­[1,1′,3-(SiMe3)3C3H2] (= K­[A″]), reaction with MnCl2 in THF/TMEDA produces the corresponding adduct A″2Mn­(tmeda). In the solid state, both A′2Mn­(thf)2 and A″2Mn­(tmeda) are monomeric complexes with σ-bonded allyl ligands (Mn–C = 2.174(2) and 2.189(2) Å, respectively). In contrast, K2MnA′4 is a two-dimensional coordination polymer, in which two of the allyl ligands on the Mn cation are σ-bonded (Mn–C = 2.197(6), 2.232(7) Å) and the third is π-bonded (Mn–C = 2.342(7)–2.477(7) Å). Both σ-allyls are π-coordinated to potassium cations, promoting the polymer in two directions; the π-allyl ligand is terminal. Density functional theory (DFT) calculations indicate that isolated high-spin (C3R2H3)2Mn (R = H, SiMe3) complexes would possess π-bound ligands. A mixed hapticity (π-allyl)­(σ-allyl)­MnE structure would result with the addition of either a neutral ligand (e.g., THF, MeCN) or one that is charged (Cl, H). Both allyl ligands in a bis­(allyl)manganese complex are expected to adopt a σ-bonded mode if two THF ligands are added, as is experimentally observed in A′2Mn­(thf)2. The geometry of allyl–Mn­(II) bonding is readily modified; DFT results predict that [(C3H5)­Mn]+ and (C3H5)­MnCl should be σ-bonded, but the allyl in (C3H5)­MnH is found to exhibit a symmetrical π-bonded arrangement. Some of this behavior is reminiscent of that found in bis­(allyl)magnesium chemistry.

Two nanoporous metal-organic framework materials incorporating the exotridentate bridging ligand 3-amino-1,2,4triazolate (AmTAZ) have been synthesized through variation of secondary bridging anions: [Zn 3 (AmTAZ) 3 S](NO 3 )) crystallizes in the cubic space group I23 and is constructed from triangular Zn 3 S units that are bridged through AmTAZ ligands into a cationic three-dimensional (3D) network with nitrate and water molecules residing in the cavities. 2 • 2(EtOH) crystallizes in the monoclinic space group C2/c and shows a complex 3D network constructed from seven crystallographically unique zinc centers bridged by AmTAZ, carbonate, and hydroxide anions. The porous nature of both materials has been explored by thermogravimetric analysis, nitrogen sorption, and in situ synchrotronbased powder X-ray diffraction.

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