A class of tunable visible and near-infrared donor−acceptor Stenhouse adduct (DASA) photoswitches were efficiently synthesized in two to four steps from commercially available starting materials with minimal purification. Using either Meldrum's or barbituric acid "acceptors" in combination with aniline-based "donors", an absorption range spanning from 450 to 750 nm is obtained. Additionally, photoisomerization results in complete decoloration for all adducts, yielding fully transparent, colorless solutions and films. Detailed investigations using density functional theory, nuclear magnetic resonance, and visible absorption spectroscopies provide valuable insight into the unique structure−property relationships for this novel class of photoswitches. As a final demonstration, selective photochromism is accomplished in a variety of solvents and polymer matrices, a significant advantage for applications of this new generation of DASAs.

The first synthetic manganese tetrazene complexes are described as a redox pair comprising anionic [Mn(N4 Ad2 )2 ](-) (1) and neutral Mn(N4 Ad2 )2 (2) complexes (N4 Ad2 =[Ad-N-N=N-N-Ad](2-) ). Compound 1 is obtained in two forms as lithium salts, one as a cationic Li2 Mn cluster, and one as a Mn-Li 1D ionic polymer. Compound 1 is electronically described as a Mn(III) center with two [N4 Ad2 ](2-) ligands. The one-electron oxidized 2 is crystalized in two morphologies, one as pure 2 and one as an acetonitrile adduct. Despite similar composition, the behavior of 2 differs in the two morphologies. Compound 2-MeCN is relatively air and temperature stable. Crystalline 2, on the other hand, exhibits a compositional, dynamic disorder wherein the tetrazene metallacycle ring-opens into a metal imide/azide complex detectable by X-ray crystallography and FTIR spectroscopy. Electronic structure of 2 was examined by EPR and XPS spectroscopies and DFT calculations, which indicate 2 is best described as a Mn(III) ion with an anion radical delocalized across the two ligands through spin-polarization effects.

Reaction of 1,3-diazidopropane with an electron-rich Mn­(II) precursor results in oxidation of the metal center to a Mn complex with concomitant assembly of the macrocyclic ligand into the 1,2,3,4,­8,9,10,11-octa­aza­cyclo­tetra­deca-2,9-diene-1,4,8,11-tetraido (OIM) ligand. Although describable as a Werner Mn­(V) complex, analysis by X-ray diffraction, magnetic measurements, X-ray photoelectron spectroscopy, cyclic voltammetry, and density functional theory calculations suggest an electronic structure consisting of a Mn­(III) metal center with a noninnocent OIM diradical ligand. The resulting complex, (OIM)­Mn­(NH t Bu), reacts via proton-coupled electron transfer (PCET) with phenols to form phenoxyl radicals, with dihydroanthracene to form anthracene, and with (2,4-ditert-butyltetrazolium-5-yl)­amide to extrude a tetrazyl radical. PCET from the latter generates the isolable corresponding one-electron reduced compound with a neutral, zwitterionic axial 2,4-ditert-butyltetrazolium-5-yl)­amido ligand. Electron paramagnetic resonance and density functional theoretical analyses suggest an electronic structure wherein the manganese atom remains Mn­(III) and the OIM ligand has been reduced by one electron to a monoradical noninnocent ligand. The result indicates PCET processes whereby the proton is transferred to the axial ligand to extrude t BuNH2, the electron is transferred to the equatorial ligand, and the central metal remains relatively unperturbed.

The tetrameric Al(I) cyclopentadienyl compound AlCp* (Cp* = CMe) is a prototypical low-valence Al compound, with delocalized bonding between four Al(I) atoms and η ligands bound to the cluster exterior. The synthesis of new [AlR] (R = CMePr, CMeiPr) tetramers is presented. Though these systems failed to crystallize, comparison of variable-temperature Al NMR data with density functional theory (DFT) calculations indicate that these are AlR tetramers analogous to AlCp* but with increased ligand steric bulk. NMR, DFT, and Atoms in Molecules analyses show that these clusters are enthalpically more stable as tetramers than the Cp* variant, due in part to noncovalent interactions across the bulkier ligand groups. Thermochemistry calculations for the low-valence metal interactions were found to be extremely sensitive to the DFT methodology used; the M06-2X functional with a cc-pVTZ basis set is shown to provide very accurate values for the enthalpy of tetramerization and Al NMR shifts. This computational method is then used to predict geometrical structures, noncovalent ligand interactions, and monomer/tetramer equilibrium in solution for a series of Al(I) cyclopentadienyl compounds of varying steric bulk.

The neutral homoleptic tris-bpy aluminum complexes Al( R bpy) 3 , where R = tBu (1) or Me (2), have been synthesized from reactions between AlX precursors (X = Cl, Br) and neutral R bpy ligands through an aluminum disproportion process. The crystalline compounds have been characterized by single-crystal X-ray diffraction, electrochemical experiments, EPR, magnetic susceptibility, and density functional theory (DFT) studies. The collective data show that 1 and 2 contain Al 3+ metal centers coordinated by three bipyridine (bpy • ) 1− monoanion radicals. Electrochemical studies show that six redox states are accessible from the neutral complexes, three oxidative and three reductive, that involve oxidation or reduction of the coordinated bpy ligands to give neutral R bpy or R bpy 2− dianions, respectively. Magnetic susceptibility measurements (4− 300 K) coupled with DFT studies show strong antiferromagnetic coupling of the three unpaired electrons located on the R bpy ligands to give S = 1 / 2 ground states with low lying S = 3 / 2 excited states that are populated above 110 K (1) and 80 K (2) in the solid-state. Complex 2 shows weak 3D magnetic interactions at 19 K, which is not observed in 1 or the related [Al(bpy) 3 ] complex.