Orbital compatibility in the condensation of polyhedral boranes

Little and large is preferred to little and little or large and large in the condensation of polyhedral boranes due to better orbital compatibility. The stability of the condensed structure increases with increasing difference in the number of vertices of the individual polyhedra of a macropolyhedron. The relative energies of isomers of macropolyhedral structures can be explained by this concept

Orbital Compatibility in the Condensation of Polyhedral Boranes

The Wade n+1 rule$^{[1]}$ and the mno rule$^{[2]}$ describe the electronic requirements for the stability of polyhedral boranes (e.g., 1) and macropolyhedral boranes (e.g., 2 and 3), respectively (see Figure 1). Though useful in explaining and designing structures, electron-counting rules provide a yes or no answer; not all molecules having the stipulated numbers of electrons are equally stable. It is desirable to have a qualitative understanding, wherever possible, of the factors that control the relative stability of a family of molecules. We formulated the concept of compatibility of orbitals to explain the relative stability of polyhedral boranes, carboranes, and metallaboranes.$^{[3]}$ Herein we demonstrate that similar concepts can be used to determine the best combination of polyhedra for the formation of condensed macropolyhedral boranes

Closo versus Hypercloso Metallaboranes: A DFT Study

The structures and electronic relationship of 9-, 10-, 11-, and 12-vertex closo and hypercloso (isocloso) etallaboranes are explored using OFT calculations. The role of the transition metal in stabilizing the hypercloso borane structures is explained using the concept of orbital compatibility. The hypercloso structures, C6H6MBn-1Hn-1 (n = 9-12; M = Fe, Ru, and Os) are taken as model complexes. Calculations on metal free polyhedral borane BnHn suggest that n vertex hypercloso structures need only n skeleton electron pairs (SEPs), but the structure will have one or more six-degree vertices, whereas the corresponding closo structures with n + 1 SEPs have only four- and five-degree vertices. This high-degree vertex of hypercloso structures can be effectively occupied by transition metal fragments with their highly diffused orbitals. Calculations further show that a heavy transition metal with more diffused orbitals prefers over a light transition metal to form hypercloso geometry, This is in accordance with the fact that there are more experimentally characterized hypercloso structures with the heavy transition metals. The size of the exohedral ligands attached to the metal atom also plays a role in deciding the stability of the hypercloso structure. The interaction between the borane and the metal fragments in the hypercloso geometry is analyzed using the fragment molecular orbital approach. The interconversion of the closo and hypercloso structures by the addition and removal of the electrons is also discussed in terms of the correlation diagrams

Relative Stability of closo-closo, closo-nido, and nido-nido Macropolyhedral Boranes: The Role of Orbital Compatibility

The concept of orbital compatibility is used to explain the relative energies of different macropolyhedral structural patterns such as closo-closo, closo-nido, and nido-nido. A large polyhedral borane condenses preferentially with a smaller polyhedron owing to orbital compatibility. Calculations carried out at the B3LYP/6-31G* level show that the macropolyhedron closo(12)-closo(6) is the most preferred structural pattern among the face-sharing closo-closo systems. The relative stabilities of four-shared-atom closo-closo, three-shared-atom closo-closo, three-shared-atom closo-nido, edge-sharing closo-nido, and edge-sharing nido-nido structures are in accordance with the difference in the number of vertices of the individual polyhedra of the macropolyhedra. When the difference in the number of vertices of the individual polyhedra is large, the stability of the macropolyhedra is also large. Calculations further show that the orbital compatibility plays an important role in deciding the stability of the macropolyhedral boranes with more than two polyhedral units. The dependence of the orbital compatibility on the relative stability of the macropolyhedron varies with other factors such as inherent stability of the individual polyhedron and steric factors

Theoretical study of the reaction of B<SUB>20</SUB>H<SUB>16</SUB> with MeCN: Closo/Closo to Closo/Nido conversion

We propose a mechanism for the cage-opening reaction of a four atoms shared closo/closo B20H16 (1) with MeCN to a face shared closo/nido macropolyhedron B20H16(MeCN)2 (4) through a diamond-square-diamond rearrangement. Even though only one isomer of the product has been reported experimentally, our computational studies at the B3LYP/6-31G&#8727; level predict the possibility of the formation of the other isomers. Depending upon the position of the attack of the MeCN ligand on the polyhedral skeleton, different products are formed. The energetics of the reactions of B20H16 with Me2S and H2O are comparable

Theoretical Study of the Reaction of B20H16B_20H_{16} with MeCN: Closo/Closo to Closo/Nido Conversion

We propose a mechanism for the cage-opening reaction of a four atoms shared closo/closo $B_{20}H_{16}$ (1) with MeCN to a face shared closo/nido macropolyhedron $B_{20}H_{16}(MeCN)_2$ (4) through a diamond-square-diamond rearrangement. Even though only one isomer of the product has been reported experimentally, our computational studies at the B3LYP/6-31G* level predict the possibility of the formation of the other isomers. Depending upon the position of the attack of the MeCN ligand on the polyhedral skeleton, different products are formed. The energetics of the reactions of $B_{20}H_{16}$ with $Me_2S$ and $H_2O$ are comparable

Anaerobic Photocleavage of DNA in Red Light by Dicopper(II) Complexes of 3,3'-Dithiodipropionic Acid

Binuclear copper(II) complexes [{(phen)Cu-II)2(mu-dtdp)(2)] (1), [{(dpq)Cu-II}(2)(mu-dtdp)(2)] (2), [{(phen)Cu-II}(2)(mu-az)(2)] (3), and [{(dpq)Cu-II}(2)(mu-az)(2)] (4) and a zinc(II) complex [{(phen)Zn-II}(2)(mu-dtdp)(2)] (5), having 3,3'-dithiodipropionic acid (H(2)dtdp), azelaic acid (nonanedioic acid), 1,10-phenanthroline (phen), and dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), were prepared and characterized by physicochemical methods. Complex I has been structurally characterized by X-ray crystallography. The complexes have each metal center bound to a chelating phenanthroline base and two bridging carboxylate ligands giving a square-planar MN2O2 coordination geometry. The molecular structure of complex 1 shows two sterically constrained disulfide moieties of the dtdp ligands. The complexes show good binding propensity to calf thymus DNA in the major groove. The photoinduced DNA cleavage activity of the complexes has been studied using 365 nm UV light and 647.1 nm and >750 nm red light under both aerobic and anaerobic conditions. The phen complex 1, having dtdp ligand, cleaves supercoiled (SC) DNA to its nicked circular (NC) form. The dpq analogue 2 shows formation of a significant quantity of linear DNA resulting from double-strand breaks (dsb) in air. Mechanistic studies reveal the involvement of HO center dot and O-1(2) as the reactive species under an aerobic medium. The dsb of DNA is rationalized from the docking studies on 2, showing a close proximity of two photosensitizers, namely, the disulfide moiety of dtdp and the quinoxaline ring of dpq to the complementary strands of DNA. The copper(II) complexes of the dtdp ligand cleave SC DNA to its NC form upon exposure to UV or red light under an argon atmosphere. An enhancement of the DNA cleavage activity under argon has been observed upon increasing the concentration of the DMF solvent in the DMF-Tris buffer medium. Theoretical studies suggest the possibility of sulfide anion radical formation from a copper(II)-bound dtdp ligand in >750 nm red light, which further cleaves the DNA. The copper(II) azelate complexes are inactive under similar reaction conditions. The azelate complex of the dpq ligand cleaves DNA in air following the 102 pathway. The zinc(II) complex of the dtdp ligand (5) does not show any photoinduced DNA cleavage activity in red ligh