A.I. Yanovsky

Bimetallic alkoxides of niobium

Abstract The formation of the I–III group metal alkoxoniobates has been studied using the physicochemical analysis method, namely the investigation of solubility isotherms for the M(OR) n -Nb(OR) 5 -ROH systems (M = Li, Ba, Sc, La; R = 2Et, Pr i ). The crystallization of the following complexes has been observed: LiNb(OEt) 6 ( I ), [LiNbO(OEt) 4 (EtOH)] 2 ( II ), [BaNbO(OEt) 5 (EtOH) 1,5 ] 4 ( III ), BaNb 2 (OPr i ) 12 (Pr i OH) 2 ( IV ) and LaNb 2 (OPr i ) 13 ( V ). It has been found that the bimetallic isopropoxide of Sc and Nb does not exist in the solid state. The oxo complexes II and III are formed on storage (or more rapidly on refluxing) of the solutions of non-oxo compounds in alcohols in the presence of dry air and are, in fact, the products of these compounds oxidation by oxygen. The structure of II is built of dimeric molecules with the metal-oxygen coer analogous to that of Ti 4 (OMe) 16 . The molecule of III is constructed around the [Ba 4 O 4 ] cubane-like unit, whose oxygen vertices are occupied by the oxo atoms of NbO(OEt) 5 octahedra. The structures of the molecules of IV, V and recently studied Nb(OPr i ) 5 are based on the common motif featuring the M 2 O 2 four-membered cycles in edge sharing octahedra. All the observed [NbMNb] units are non-linear, the molecule of V being asymmetric, for the central [LaO 6 ] octahedron is sharing simultaneously an edge with one [NbO 6 ] octahedron and a face with the other. The conditions leading to the formation of oxo groups and the influence of the latter on the complexation process have been discussed. The literature data on the existence of 3 d transition metals alkoxoniobates have not been confirmed. It has been found that interaction of Nb and Co or Ni isopropoxides takes place only due to preliminary microhydrolysis or pyrolysis and leads to formation of oxoalkoxoniobates.

Crystal and molecular structure of the salt [C5Me5RuC5Me4︹CH2]+BPh4−·CH2CI2

The Ru...CH2 distance of 2.270Ain the cation of the salt [C5Me5RuC5Me4CH2]+BPh−4·CH2Cl2 investigated by means of an X-ray diffraction study, is slightly longer than the Os...CH2 distance of 2.244Ain the osmium analogue. The inclination angle of the exocyclic bond in the ruthenium complex (40.3°) is somewhat smaller than in the osmium compound (41.8°). The results reported, and those obtained previously, support the suggestion that the stabilization of the α-carbocationic centre increases along the series Fe « Ru < Os.

Ligand effects in the hydrogenation of methacycline to doxycycline and epi-doxycycline catalysed by rhodium complexes. Molecular structure of the key catalyst [closo-3,3-(η2,3-C7H7CH2)-3,1,2-Rh C2B9H11]

Abstract The catalytic reduction of the exocyclic methylene group of methacycline (A) leads to the formation of two diastereoisomers, doxycycline (B, the α-epimer) and 6-epi-doxycycline (C, the β-epimer), with a selectivity which markedly depends on the nature of hydrocarbon and carborane ligands of closo-(π-cyclodienyl)rhodacarborane catalysts. Neutral norbornadienyl complexes with unsubstituted carborane ligands [closo-3,3-(η2,3-C7H7CH2)-3,1,2-RhC2B9H11] (1) and [closo-2,2-(η2,3-C7H7CH2)-2,1,7-RhC2B9H11] (7) are more active and afford higher selectivity in the formation of doxycycline than those having mono- or di-substituents at the carborane cage, [closo-3,3-(cyclodienyl)-1-R-2-R′-3,1,2-RhC2B9H9] (R = H, R′ = Me, PhCH2; R = R′ = Me; cyclodienyl = η2,3-C7H7CH2 or η-C10H13) as well as those from the closely related series of η5-cyclopentadienyl complexes [(η2,3-C7H7CH2)Rh(η5-C5Rn)]+PF6− (Rn = H5, Me5, or H2-1,2,4-Ph3). Mechanistic aspects of the hydrogenation reaction of methacycline are sketched. The results of the X-ray diffraction study of the best catalyst 1 are reported.

The structure of the crystal solvate of magnesium methoxide with methanol, Mg(OMe)2·3.5MeOH

Abstract The X-ray diffraction study of the crystals deposited from the solution obtained by the reaction of magnesium with methanol allowed the formulation of the product as Mg(OMe)2·3.5MeOH (I). Its structure is built of the residues of four types, namely cubane [Mg4(μ3-OR)4(OR)4(ROH)8] neutral molecules, [Mg4(μ3-OR)4 (OR)2(ROH)10]2+ cations, [(RO)2H]− anions (R = Me) and eight crystallographically independent non-coordinated solvating methanol molecules. All residues are linked into a three-dimensional framework by means of a complicated hydrogen-bonding system. The crystal structure provides the basis for discussion of the possible mechanism of desolvation of I.

Reactions of diacetylene ligands with trinuclear clusters. I Reactions of 2,4-hexadiyne-1,6-diol and its dicobalthexacarbonyl derivatives with H2Os3(CO)10

Abstract Reactions of H 2 Os 3 (CO) 10 with the diyne ligand HOCH 2 C 2 C 2 CH 2 OH and its dicobalthexacarbonyl derivatives {CO 2 (CO) 6 }(μ 2 ,η 2 -HOCH 2 C 2 C 2 CH 2 OH) and {Co 2 (CO) 6 } 2 (μ 2 ,η 2 : μ 2 ,η 2 -HOCH 2 C 2 C 2 CH 2 OH) have been studied. The reaction of the uncomplexed ligand yields the cluster with the completely rearranged starting ligand. The structure of this compound was determined by a single-crystal X-ray study. The rearranged ligand forms a pseudo-furan ring with the C CH 3 substituent in the α-position. The reactions of H 2 Os 3 (CO) 10 with the both dicobalthexacarbonyl derivatives yield the (μ-H)(μ-OH)Os 3 (CO) 10 cluster ar the main osmium-containing product. The structure of this compound was also established by a single-crystal X-ray study.

The synthesis and X-ray crystal structure of molybdenum oxomethoxide [MoO(OMe)4]2

MoO(OMe) 4 has been prepared by anodic oxidation of molybdenum metal in anhydrous methanol containing LiCl as an electrolyte. The X-ray single crystal study showed the complex to consist of pseudo-centrosymmetric dimeric molecules, [MoO(OMe) 3 (μ-OMe)] 2 . The general effect of the M(μ-OR) 2 M ring upon the arrangement of bonds with increased multiplicity is discussed in this and related molecules.

Reaction of Ru3(CO)12 with ferrocenylacetylene. Crystal and molecular structure of Ru2(CO)6[C4H2(C5H4FeC5H5)2]

The reaction of Ru3(CO)12 with ferrocenylacetylene FcCCH in refluxing hexane gives Ru3(μ-H)(CO)9(μ3-η2-CCFc) I, Ru2(CO)6[C4H2(Fc)2] II and Ru2(CO)6[μ-C(H)C(Fc)COC(H)C(Fc)] III. Complexes I–III were characterized by IR and NMR spectroscopy, and II also by X-ray diffraction. Complex II crystallizes in the space group Cmc21 with a = 20.072(4), b = 14.322(2), c = 9.703(2) A, and Z = 4. The structure was solved by the heavy-atom method and refined by least-squares analysis to R = 0.031 and Rw = 0.037 for 1677 unique observed reflexions. Molecule II contains a ruthenacyclopentadiene ring, π-bonded to a Ru(CO)3 group. The Ru2(CO)6 moiety adopts a “sawhorse” geometry, in contrast to the known structures of ruthenium complexes of this type, which have a “non-sawhorse” structure. Compound III possesses a nonsymmetrical diruthenacycloheptadienone moiety. The 1H NMR spectroscopy of III did not reveal any fluxionality of the “flyover” bridge leading to the exchange of σ- and π-bonds at two ruthenium atoms.

PHYSICOCHEMICAL APPROACH TO THE STUDIES OF METAL ALKOXIDES

Abstract The review provides a description of main events in the development of chemistry of metal alkoxides in Russian and summarizes the results obtained by the authors. The studies of solubility and vapor pressure in the M(OR)n-ROH (R  Li, Mg, Ca, Sr, Ba, Tl) systems permitted the determination of the composition of the solvates formed and the optimization of the isolation conditions for distinct derivatives. The electrochemical synthetic approach to the alkoxide and 2-methoxyethoxide derivatives of III–VIII Group elements has been elaborated. Improvement of the MCln with NaOR metathesis conditions led to practically quantitative yields of alkoxides. The examples of the structures of polynuclear oxoalkoxides, earlier erroneously considered to be orthoderivatives, i.e. ‘M(OR)n’, are given. It has been stated that among the alkoxoderivatives of Zr and Hf these are only M(OC2H4OMe)4 and M(OR)4·ROH (RPri, Bui) that exist as individual compounds, while the samples of those with different R contain oxocomplexes of M3O(OR)10 and M4O(OR)14 composition. The questions connected with the origin of oxogroups in the molecules of alkoxides and their influence on the properties of the samples are discussed. The irreproducibility of the physicochemical constants for the samples (physical state, melting points, solubility in alcohols, intensity of coloration) is caused by different ‘chemical’ (synthetic procedures, isolation conditions) and the thermal prehistory and storage times; it originates from the difference in their molecular composition (different fractions of different types of oligomeric and polymeric [M(OR)n]m and MxOy(OR)z aggregates). Formation of bimetallic alkoxides is studied using the plotting to 20°C solubility isotherms in M(OR)n− M′(OR)m-Solvent systems. Different kinds of systems are considered and the composition and structure of bimetallic complexes and oxocomplexes, originating from their decomposition, are described. Decomposition of alkoxomolybdates and -tungstates in solution may result in crystallization of complex oxides as the final products. The example of MTiO3 (MMg, Ba) oxide preparation is discussed as most illustratively demonstrating the need in studies of metal alkoxides interaction in solution for the optimization of synthesis of oxides by the sol-gel technique.

Crown compounds for anions. A spinning top-shaped complex of cyclic pentameric perfluoroisopropylidenemercury with two chloride anions

Abstract It has been found that cyclic pentameric perfluoroisopropylidenemercury, [(CF 3 ) 2 CHg] 5 , is capable of forming complexes with [PPh 4 ] + Cl − and [PPh 4 ] + Br − of the type {[(CF 3 ) 2 CHg] 5 X 2 } 2− [PPh 4 ] 2 + (X = Cl or Br). According to an X-ray diffraction study, the chloride anions in the complex of [(CF 3 ) 2 CHg] 5 with [PPh 4 ] + Cl − are located above and below the mercury-containing metallacycle, each Cl − anion being coordinated with all the five mercury atoms of [(CF 3 ) 2 CHg] 5 .

Complexation of cyclic trimeric perfluoro-o-phenylenemercury with nitriles. A remarkable sensitivity of the composition and structure of the resulting complexes to the nature of a nitrile

Abstract Cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4Hg)3 (I) is capable of reacting with nitriles to form complexes whose composition and structure are dramatically dependent on the nature of the nitrile used. In the case of acrylonitrile, the complex, [(o-C6F4Hg)3(CH2CHCN)], containing one nitrile molecule per one molecule of I is produced. According to an X-ray structure analysis, the acrylonitrile ligand in this complex is bonded to all Hg atoms of the cycle through the nitrogen atom. The reaction of I with acetonitrile gives complex [(o-C6F4Hg)3(MeCN)2] which contains two nitrile ligand per one molecule of the macrocycle. This complex has a shape of a spinning top the equatorial girdle of which is the mercury-containing metallacycle, whilst the axis direction is fixed by the linear MeCN ligands located above and below the metallacycle plane. Finally, the isolated complex of I with benzonitrile, [(o-C6F4Hg)3(PhCN)3], contains three molecules of the coordinated nitrile per one macrocycle molecule. A remarkable structural feature of this complex is that here all the nitrile ligands are located on one side of the metallacycle plane and, in addition, differ from each other in the geometry of coordination with the Hg atoms.