Coordination versus Solvation in Al+(benzene)n Complexes Studied with Infrared Spectroscopy

Photodissociation Spectroscopy and Photofragment Imaging to Probe Fe+(Benzene)1,2 Dissociation Energies

Tunable laser photodissociation spectroscopy measurements and photofragment imaging experiments are employed to investigate the dissociation energy of the Fe+(benzene) ion-molecule complex. Additional spectroscopy measurements determine the dissociation energy of Fe+(benzene)2. The dissociation energies for Fe+(benzene) determined from the threshold for the appearance of the Fe+ fragment (48.4 ± 0.2 kcal/mol) and photofragment imaging (≤49.3 ± 3.2 kcal/mol) agree nicely with each other and with the value determined previously by collision-induced dissociation (49.5 ± 2.9 kcal/mol), but they are lower than the values produced by computational chemistry at the density functional theory level using different functionals recommended for transition-metal chemistry. The threshold measurement for Fe+(benzene)2 (43.0 ± 0.2 kcal/mol) likewise agrees with the value (44.7 ± 3.8 kcal/mol) from previous collision-induced dissociation measurements.

Fabrication of Swellable Organic-Inorganic Hybrid Polymers for CO2-Assisted Hydration of Propylene Epoxide

Swelling is a very common phenomenon in organic substances. However, the swelling behaviors of inorganic substances had rarely been reported. In this study, a new type of swellable organic-inorganic hybrid polymer (PIL@CHT) was designed and successfully synthesized through free-radical copolymerization of polymerizable phosphonium ionic liquid monomer and vinyl-functionalized hydrotalcite (CHT). The swelling behaviors of PIL@CHT in various solvents with a wide range of Hansen solubility parameters (δ_T) were investigated, and PIL@CHT exhibited excellent swellable capacity in the solvents with δ_T > 24.4 MPa^1/2. The swollen state of the hybrid PIL@CHT in water presented a network structure with a diameter of approximately 8-12 μm, and CHT particles were well dispersed to the channel of PIL. PIL@CHT was applied to catalyze the CO₂-assisted hydration of propylene oxide (PO), in which a cascade reaction including the cycloaddition of CO₂ and PO and the subsequent hydrolysis of propylene carbonate (PC) occurred. PIL@CHT, combining the active sites of PIL and CHT, synergistically catalyzed this cascade reaction and achieved a high yield (93.0%) and selectivity (98.2%) of 1,2-propanediol (1,2-MPG) under a low H₂O/PO ratio of 1.5/1. Moreover, the catalyst could be recycled seven times without any significant loss of catalytic activities and had good substrate generality.

Transition-Metal Chemistry of Alkaline-Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr, Ba)

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)3 (M=Sr, Ba) in low-temperature Ne matrix. Both complexes are characterized by a D3 symmetric structure involving three equivalent η6 -bound benzene ligands and a closed-shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal-ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n-1)d AOs of M and strong backdonation from the occupied (n-1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20-electron complexes have 18 effective valence electrons, and, thus, fulfill the 18-electron rule if only the metal-ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.

Low-Energy Photoelectron Imaging Spectroscopy of Lan(benzene) (n = 1 and 2)

La_n(benzene) (n = 1 and 2) are formed in a pulsed laser-ablation molecular beam source and characterized by low-energy photoelectron imaging spectroscopy. The photoelectron spectrum of La₂(benzene) displays a strong origin band, a short metal-ligand stretching progression, and a weak ring deformation band. Four isomers are considered for La₂(benzene), and the preferred structure is an inverse sandwich with two La atoms residing on the opposite sides of the benzene ring. The ground electronic state of the inverse sandwich is ¹A_1g (D_3d) with (5d_xy,x²-y² + π*)⁴6s² electron configuration. Ionization removes a La-based 6s electron and yields a ²A_1g ion. The spectrum of La(benzene) is similar to the zero-electron kinetic energy spectrum reported previously by our group, although the spectral line width is somewhat broader. The measurement of the photoelectron angular distribution of La(benzene) confirms that the ejected electron has largely a p wave character. The metal-ligand bonding of La₂(benzene) is considerably stronger than that of La(benzene) due to the threefold binding of each La atom in the dilanthanum species and the twofold binding in the monolanthanum complex.

Effects of electronic structure on the hydration of PbNO3(+) and SrNO3(+) ion pairs

Hydration of PbNO3(+) and SrNO3(+) with up to 30 water molecules was investigated with infrared photodissociation (IRPD) spectroscopy and with theory. These ions are the same size, yet the IRPD spectra of these ion pairs for n = 2-8 are significantly different. Bands in the bonded O-H region (∼3000-3550 cm(-1)) indicate that the onset of a second hydration shell begins at n = 5 for PbNO3(+) and n = 6 for SrNO3(+). Spectra for [PbNO3](+)(H2O)2-5 and [SrNO3](+)(H2O)3-6 indicate that the structures of clusters with Pb(ii) are hemidirected with a void in the coordinate sphere. A natural bond orbital analysis of [PbNO3](+)(H2O)5 indicates that the anisotropic solvation of the ion is due to a region of asymmetric electron density on Pb(ii) that can be explained by charge transfer from the nitrate and water ligands into unoccupied p-orbitals on Pb(ii). There are differences in the IRPD spectra of PbNO3(+) and SrNO3(+) with up to 25 water molecules attached. IR intensity in the bonded O-H region is blue-shifted by ∼50 cm(-1) in nanodrops containing SrNO3(+) compared to those containing PbNO3(+), indicative of a greater perturbation of the water H-bond network by strontium. The free O-H stretches of surface water molecules in nanodrops containing 10, 15, 20, and 25 water molecules are red-shifted by ∼3-8 cm(-1) for PbNO3(+) compared to those for SrNO3(+), consistent with more charge transfer between water molecules and Pb(ii). These results demonstrate that the different electronic structure of these ions significantly influences how they are solvated.