Search

Since the discovery of a formal quintuple bond in Ar'CrCrAr' (CrCr = 1.835 A) by Power and co-workers in 2005, many efforts have been dedicated to isolating dichromium species featuting quintuple-bond character. In the present study we investigate the electronic configuration of several, recently synthesized dichromium species with tigands using nitrogen to coordinate the metal centers. The bimetallic bond distances of Power's compound and Cr2-diazadiene (1) (CrCr = 1.803 A) are compared to those found for Cr2(4)-ArxylNC(R)NArxyl), (2) (CrCr = 1.746 Å; R = H, Ar = 2,6- Et2C6H3), Cr2 μ2-ArxylIC(H)NArxyl)3 (3) (CrCr = 1.740r0 reduced/1.817neutral Å; Ar = 2,6-C6H3-(CH3)2), Cr2( μ- η2- TippPyNMes)2 (4) (CrCr = 1.749 A; TippPyNMes = 6-(2,4,6-triisopropyfphenyl)pyridin-2-yt (2,4,6-trimethytphenyl)- amide), and Cr2(μ- η2-DippNC(NMe2)N-Dipp)2 (5) (CrCr = 1.729 A, Dipp = 2,6-i-Pr2C6H3). We show that the correlation between the CrCr bond length and the effective bond order (EBO) is strongly affected by the nature of the ligand, as well as by the steric hindrance due to the ligand structure (e.g., the nature of the coordinating nitrogen). A linear correlation between the EBO and CrCr bond distance is established within the same group of tigands. As a result, the CiCr species based on the amidinate, aminopyddinate, and guanidinate ligands have bond patterns similar lathe Ar'CrCrAr' compound. Unlike these latter species, the dichromiurn diazadiene complex is characterized by a different bonding pattern invoMng Cr-Nл interactions, resulting in a lower bond order associated with the short metal-metal bond distance. In this case the short CrCr distance is most probably the result of the constraints imposed by the diazadiene tigand, implying a Cr2N4 core with a closer CrCr interaction. [ABSTRACT FROM AUTHOR]

This review shortly discusses recent results obtained by the application of density functional theory for the calculations of the adsorption and diffusion properties of small molecules and their reactivity on heterogenous catalytic systems, in the ambit of the Nanocat project. Particular focus has been devoted to palladium catalysts, either in atomic or small cluster form. Some protocols have been tested to obtain efficient ways able to treat the electronic and geometric influence of supports like zeolites and carbon nanotubes on the catalytic properties of palladium. The hydroisomerization of cis-but-2-ene is discussed as model reaction on supported and unsupported Pd clusters. Some preliminary results on the structural investigation of systems formed by a palladium clusters and block copolymers are also presented. [ABSTRACT FROM AUTHOR]

Monte Carlo algorithms and codes, used to study heterogeneous catalytic systems in the frame of the computational section of the NANOCAT project, are presented along with some exemplifying applications and results. In particular, time dependent Monte Carlo methods supported by high level quantum chemical information employed in the field of heterogeneous catalysis are focused. Technical details of the present algorithmic Monte Carlo development as well as possible evolution aimed at a deeper interrelationship of quantum and stochastic methods are discussed, pointing to two different aspects: the thermal-effect involvement and the three-dimensional catalytic matrix simulation. As topical applications, (i) the isothermal and isobaric adsorption of CO on Group 10 metal surfaces, (ii) the hydrogenation on metal supported catalysts of organic substrates in two-phase and three-phase reactors, and (iii) the isomerization of but-2-ene species in three-dimensional supported and unsupported zeolite models are presented. [ABSTRACT FROM AUTHOR]