Calculations of thermodynamic stability of CpCoC<Subscript>2</Subscript>B<Subscript>9</Subscript>H<Subscript>11</Subscript> cobaltacarborane isomers

Quantum-chemical calculations of the geometry and energies of nine possible isomers of 12-vertex cobaltacarborane CpCoC₂B₉H₁₁ (1) were carried out by the DFT method (PBEPBE/DGDZVP/DGA1). Thermodynamic stability of the isomers increases with increasing distance between the carbon atoms in the cage and is virtually independent of the position of the CpCo vertex. The relative stabilities of the 1,2,3-(17.57 kcal mol⁻¹), 1,2,4-(3.72 kcal mol⁻¹), and 1,2,9-isomers of 1 (0 kcal mol⁻¹) are similar to the corresponding values for the ortho (17.61 kcal mol⁻¹), meta (3.21 kcal mol⁻¹), and para isomers (0 kcal mol⁻¹) of carborane C₂B₁₀H₁₂. The results of the present study confirm a close similarity of the CpCo and BH fragments in metallacarborane chemistry. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1557–1559, July, 2005.     

Ligand Design for Site-Selective Metal Coordination: Synthesis of Transition-Metal Complexes with η6-Coordination of the Central Ring of Anthracene

A polycyclic aromatic ligand for site-selective metal coordination was designed by using DFT calculations. The computational prediction was confirmed by experiments: 2,3,6,7-tetramethoxy-9,10-dimethylanthracene initially reacts with [(C₅H₅)Ru(MeCN)₃]BF₄ to give the kinetic product with a [(C₅H₅)Ru]⁺ fragment coordinated at the terminal ring, which is then transformed into the thermodynamic product with coordination through the central ring. These isomeric complexes have markedly different UV/Vis spectra, which was explained by analysis of the frontier orbitals. At the same time, the calculations suggest that electrostatic interactions are mainly responsible for the site selectivity of the coordination.     

Synthesis of homologs of 2-aminoethylphosphonic acid

Conclusions Some homologs of 2-aminoethylphosphonic acid were synthesized by the addition of dialkyl phosphites to nitroolefins and the subsequent reduction of the thus obtained esters of 2-nitroethylphosphonic acid to the amino esters. Hydrolysis of the latter gave the free acids.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1382–1385, June, 1973.     

(Mesitylene)ruthenium π-complexes with benzo-15-crown-5 and dibenzo-18-crown-6

The reactions of benzo-15-crown-5 and dibenzo-18-crown-6 with 1 equiv. of [(mes)Ru(MeNO₂)₃]²⁺ (mes = 1,3,5-C₆H₃Me₃) give the mononuclear complexes [(mes)Ru(η⁶-benzo-15-crown-5)]²⁺ (1) and [(mes)Ru(η⁶-dibenzo-18-crown-6)]²⁺ (2) in 50% yield. Similar reaction with 2 equiv. of [(mes)Ru(MeNO₂)₃]²⁺ produces the dinuclear complex [(μ-η⁶:η⁶-dibenzo-18-crown-6)Ru₂(mes)₂]⁴⁺ (3) in 96% yield as a 2:3 mixture of cis- and trans-isomers. Structures of 2(OTf)₂ and trans-3(OTf)₄ were confirmed by X-ray diffraction. The NMR titration showed that mononuclear dications 1 and 2 bind Na⁺ ion less effective (K_a = 600 and 250 M^-1) than free benzo-15-crown-5 and dibenzo-18-crown-6 (K_a = 2 × 10⁵ and 5 × 10⁶ M⁻¹). The dinuclear tetracation 3 does not bind Na⁺ within measurable limits of NMR titration method. The electrochemical behaviour of complexes 1-3 was studied in propylene carbonate solution. They exhibit a partially chemically reversible Ru(II)/Ru(I) reduction, which in the case of the dinuclear complex 3 proceeds through two slightly separated steps. The redox activity of the complexes is substantially unaffected by the presence of sodium ion.     

Metal complexes with an aminosubstituted tricarbollide ligand

The reaction of the tricarbollide salt Tl[7-tBuNH-7,8,9-C₃B₈H₁₀] (Tl1) with [(cod)Rh(THF)_x]⁺ gives the rhodium complex [1-(cod)-12-tBuNH-1,2,4,12-RhC₃B₈H₁₀] in almost quantitative yield. Analogous reactions of Tl1 with [(ring)M(THF)_x]²⁺ ((ring)M = Cp*Rh and (1,3,5-C₆H₃Me₃)Ru) afford the corresponding metallatricarbollides [1-(ring)-12-tBuNH-1,2,4,12-MC₃B₈H₁₀] in ca. 50% yield. Refluxing Tl1 with [Mn(CO)₃(MeCN)₃]⁺ in THF give the tricarbollide analogue of cymantrene, [1,1,1-(CO)₃-12-tBuNH-1,2,4,12-MnC₃B₈H₁₀], the structure of which was determined by single-crystal X-ray diffraction analysis. In all cases, the formation of the metallatricarbollide complexes is accompanied by polyhedral rearrangement leading to the maximum separation of the cage carbon atoms.     

Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

A Planar‐Chiral Rhodium(III) Catalyst with a Sterically Demanding Cyclopentadienyl Ligand and Its Application in the Enantioselective Synthesis of Dihydroisoquinolones

The rapid development of enantioselective C?H activation reactions has created a demand for new types of catalysts. Herein, we report the synthesis of a novel planar‐chiral rhodium catalyst [(C₅H₂^tBu₂CH₂^tBu)RhI₂]₂ in two steps from commercially available [(cod)RhCl]₂ and tert‐butylacetylene. Pure enantiomers of the catalyst were obtained through separation of its diastereomeric adducts with natural (S)‐proline. The catalyst promoted enantioselective reactions of aryl hydroxamic acids with strained alkenes to give dihydroisoquinolones in high yields (up to 97 %) and with good stereoselectivity (up to 95 % ee).     

Planar Chiral Rhodium Complexes of 1,4-Benzoquinones

Diene rhodium complexes are important catalysts in modern organic synthesis. Herein, we report a new approach to such complexes with the uncommon planar chirality. The synthesis is achieved by face-selective coordination of the prochiral 2,5-disubstituted-1,4-benzoquinones (R₂-Q) with rhodium precursors containing the chiral auxiliary ligand S-salicyl-oxazoline (S-Salox). Such coordination leads to the formation of (R,R-R₂-Q)Rh(S-Salox) complexes in high yields and with exceptional diastereoselectivity (d. r.>20 : 1). Subsequent replacement of the auxiliary ligand provides various benzoquinone rhodium complexes with retention of the planar chirality. Combined theoretical and experimental studies show that due to their electron-withdrawing nature benzoquinones bind metals stronger than the related 1,4-cyclohexadiene, but weaker than other common diene ligands, such as cyclooctadiene.