Terminal Alkoxide Research Articles

Synthesis of alternating polyesters using a dual catalytic system of alkali metal alkoxides and crown ether

In this study, a dual catalytic system that merges crown ether with alkali metal alkoxides (AMA) is utilized in the ring-opening alternating polymerization (ROAP) of phthalic anhydride (PA) and propylene oxide (PO). This method streamlines the production of polyesters characterized by a fully alternating sequence, a modifiable molar mass (achieving up to 59.3 kg mol−1), and a narrow molar mass distribution (ÐM < 1.26). Analysis of various commercially available AMAs, notably sodium ethoxide (EtONa), sodium methoxide, and lithium ethoxide, in tandem with crown ethers (15-crown-5 and 18-crown-6), underscores the superior catalytic efficacy of the EtONa and 18-crown-6 pairing. This heightened efficiency is attributed to the coordination of crown ether with alkali metal ions, thereby boosting the nucleophilicity of terminal alkoxide anions. The methodology has been successfully deployed in the ROAP of PA, norbornene anhydride, and multiple epoxides, while also simplifying the creation of aliphatic–aromatic block polyesters from a mixture of PA, PO, and lactide. This strategy presents a straightforward approach to polyester production, thus making significant contributions to the evolution of polyester synthesis technology.

Highly Electrophilic Mononuclear Cationic Aluminum Alkoxide Complexes: Syntheses, Reactivity and Catalytic Applications.

Herein, we report on the synthesis of β-diketiminate supported aluminum complexes bearing terminal alkoxide and mono-thiol functional groups: LAlOMe(Et) (2), LAlOtBu(Et) (3), and LAlSH(Et) (4), (L= [HC{C(Me)N-(2,6-iPr2C6H3)}2]). Complexes 2 and 3 are further utilized as synthons to generate some fascinating cationic aluminum alkoxide complexes, [LAlOMe(µ-OMe)-Al(Et)L][EtB(C6F5)3] (5), [LAlOMe(OEt2)][EtB(C6F5)3] (6), and [LAlOtBu(OEt2)][EtB(C6F5)3] (8). These electrophilic cationic species are well characterized by spectroscopic and crystallographic techniques. The assessment of Lewis acidity by the Gutmann-Beckett method revealed superior Lewis acidity of the cations substituted with electron demanding alkoxy groups in comparison to the known methyl analogue [LAlMe][B(C6F5)4]. This has been further endorsed by computational calculations to determine the NBO charges and hydride ion affinity for complexes 6 and 8. These complexes are also capable of activating triethylsilane in stoichiometric reactions. The applicability of these complexes has been realized in the hydrosilylation of ethers, carbonyls, and olefines. Additionally, the solid state structure of a new THF stabilized aluminum halide cation [LAlCl(THF)][B(C6F5)4] (11) has also been reported.

New insights into pertinent Fe-complexes for the synthesis of iron via the instant polyol process.

Chemically synthesized iron is in demand for biomedical applications due to its large saturation magnetization compared to iron oxides. The polyol process, suitable for obtaining Co and Ni particles and their alloys, is laborious in synthesizing Fe. The reaction yields iron oxides, and the reaction pathway remains unexplored. This study shows that a vicinal polyol, such as 1,2-propanediol, is suitable for obtaining Fe rather than 1,3-propanediol owing to the formation of a reducible Fe intermediate complex. X-ray absorption spectroscopy analysis reveals the ferric octahedral geometry and tetrahedral geometry in the ferrous state of the reaction intermediates in 1,2-propanediol and 1,3-propanediol, respectively. The final product obtained using a vicinal polyol is Fe with a γ-Fe2O3 shell, while the terminal polyol is favourable for Fe3O4. The distinct Fe-Fe and Fe-O bond lengths suggest the presence of a carboxylate group and a terminal alkoxide ligand in the intermediate of 1,2-propanediol. A large Fe-Fe bond distance suggests diiron complexes with bidentate carboxylate bridges. Prominent high-spin and low-spin states indicate the possibility of transition, which favors the reduction of iron ions in the reaction using 1,2-propanediol.

Synthesis and self-assembly of new fluorescent cholesteryloxy-substituted fluorinated terphenyls with gel formation and mesogenic phases

Fluorescent mesogenic compounds have been reported as suitable materials for electronic displays with low energy consumption. Aromatic nucleophilic substitution (SNAr) reactions have been extremely para selective on various pentafluorophenyl derivatives, which is ideal to build up fluorescent calamitic (rod-like) liquid crystals. In the current study, we report the use of SNAr strategy toward the synthesis of emissive low molecular mass organogels (LMMOs) with mesogenic phases. The synthesis of these new cholesteryloxy-substituted fluorinated terphenyls was carried out through decarboxylative cross-coupling reaction catalyzed by copper iodide, followed by the application of SNAr chemistry. The produced organogelator was obtained in excellent purity and high yield starting from aryl iodides and pentafluorobenzoate. A series of asymmetric fluorinated terphenyls were synthesized with different terminal alkoxides. The molecular structures were studied by elemental analysis, 1H, 13C and 19F NMR, and infrared spectroscopic methods. The liquid crystalline phases were explored by differential scan calorimeter (DSC) and optical polarizing microscopy (POM) demonstrating at least one mesogenic phase. The absorbance and fluorescence spectral curves exhibited solvatochromic properties. The morphological properties were studied by scan electron microscope (SEM), which showed nanofibrous scaffolds generated from the assembly of cholesteryloxy-substituted terphenyls. This self-assembly process was supported by π-stack packing and van der Waals forces.

Understanding cyclic by-products and ether linkage formation pathways in the transesterification synthesis of aliphatic polycarbonates

An overall picture of the reaction pathways towards the side products and final polymers during the transesterification synthesis of biodegradable aliphatic carbonates (APCs) were delineated for the first time. Catalysed by sodium ethoxide, the normal reaction between α,ω-diols (carbon number n=2–6) and diethyl carbonate generates APC oligomers with propagating terminal alkoxide anions. These anions can backbite at the nearest carbonate carbons or the nearest carbonate α-CH2 carbons in the same APC chains, eliminating corresponding cyclic carbonates or cyclic ethers. Also, each alkoxide anion can randomly attack a carbonate α-CH2 carbon in another APC chain, generating ether linkages in the final polymers whose percentages are about 100%, 1.35%, 0.08% and 0% for n=2, 3, 4 and 5, respectively. The content of cyclic carbonates shows a similar decreasing trend; ethylene carbonate (n=2) even predominates over the final polymer while trimethylene carbonate (n=3) and 1,4-butylene carbonate (n=4) are only minor by-products. Moreover, the latter two cyclic carbonates are not stable enough and can further decarboxylate, forming relatively high amount of allyl alcohol and tetrahydrofuran, respectively. However, trimethylene oxide (n=3) and tetrahydropyran (n=5) are minor cyclic ethers. The thorough understanding of the chemical process is essential to improve the APC synthesis.

Experimental and theoretical studies of hydrolysis of nerve agent sarin by binuclear zinc biomimetic catalysts

A complex (ZnL1) of 2,2-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis ((pyridin-2-ylmethyl)azanediyl)diethanol (this ligand is named by L1) functionalized with two Zn(II) centers, has been previously suggested to be a structural model for binuclear zinc phosphotriesterases (PTEs) and proven to be an effective catalyst for the hydrolysis of bis(2,4-dinitrophenyl)phosphate (BDNPP). In this paper, ZnL1 was further found to have a high catalytic activity for the hydrolysis of isopropyl methylphosphonofluoridate (sarin, GB) with kcat/Km=0.051s−1M−1 at 303K, examined by on-site NMR analysis. The subsequent density functional theory (DFT) calculations indicate that a terminal alkoxide (Ot) bound by Zn may work as a general base to activate a water molecule and then a hydroxide derived from the latter performs the initial nucleophilic attack on the phosphor in GB. Inspired by this mechanism, a new biomimetic catalyst was designed and synthesized by replacing the two pyridines of ZnL1 by hydroxyls, i.e. a complex of two Zn(II) with 2,6-bis((bis(2-hydroxyethyl)amino)methyl)-4-methylphenol (the ligand is named by L2). This replacement was expected to increase the Ot basicity, thereby facilitating the nucleophilic attack and the overall hydrolysis of GB. It was shown that ZnL2 had a very high catalytic efficiency for the hydrolysis of GB, with kcat/Km=0.11s−1M−1 at 303K and about 90% conversion in 30min. The following DFT calculations proposed a detailed reaction mechanism of ZnL2 and gave an energy barrier (5.8kcalM−1) very close to the experimental activation energy (5.6kcalM−1). In this study, a mechanism-inspired design strategy has been demonstrated to be successful in developing biomimetic catalyst.

Zwitterionic Ring-Opening Polymerization: Models for Kinetics of Cyclic Poly(caprolactone) Synthesis

Investigations of the kinetics of zwitterionic ring-opening polymerization of e-caprolactone by N-heterocyclic carbenes (NHC) were carried out to illuminate the key reaction steps responsible for the formation of high molecular weight cyclic poly(caprolactones). Modeling of both the decay in monomer concentration as well as the evolution of molecular weights and polydispersities were necessary to identify the key reaction steps responsible for initiation, propagation, cyclization and chain-transfer. Nucleophilic attack of the NHC on e-caprolactone to generate reactive zwitterions is slow and reversible. The modeling indicates that less than 60% of the carbenes are transformed to active zwitterions, but that these zwitterions rapidly add monomer and cyclize by intramolecular backbiting of the terminal alkoxides on internal esters of the zwitterions. This cyclization event maintains the concentration of active zwitterions. The reactivation of cyclized chains by active zwitterions is a key step that leads to...

Role of ReOx in Re-modified Rh/ZrO2 and Ir/ZrO2 catalysts in glycerol hydrogenolysis: Insights from first-principles study

The thermodynamics of glycerol hydrogenolysis to produce 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) over Ru/ZrO 2 , Rh/ZrO 2 , ReO x -Rh/ZrO 2 , and ReO x -Ir/ZrO 2 were studied using density functional theory calculations, with a special focus on the mechanism controlling the activity and selectivity of the reactions. It is found that the decomposition of glycerol on Ru/ZrO 2 and Rh/ZrO 2 proceeds through a dehydration-hydrogenation mechanism. The formation of 1,2-PDO is thermodynamically favored, and the activity of the Ru-based catalyst is higher than that of the Rh-based one. In contrast, a direct hydrogenolysis mechanism is proposed for the Re-modified Rh and Ir catalysts, in which a dissociated H atom on the Rh(Ir) metal surface attacks the C–O bond neighboring the alkoxide species on the ReO x cluster. In the presence of ReO x -Rh/ZrO 2 , the modified catalyst favors the production of 1,2-PDO, and 1,3-PDO production becomes competitive. However, the ReO x -Ir/ZrO 2 catalyst significantly improves 1,3-PDO selectivity. The direct hydrogenolysis pathway, as opposed to the indirect hydrogenolysis mechanism for monometallic catalysts, may be the key to the high 1,3-PDO selectivity on the modified catalysts, where the hydroxylated Re group facilitates the formation of terminal alkoxide species rather than secondary alkoxides. Steric effects are important in preferential terminal alkoxide formation on the ReO x -Ir/ZrO 2 catalysts because of the growth of large Ir-Re clusters, resulting in high selectivity for 1,3-PDO. The decisive role of ReO x present in Re-modified bifunctional catalysts in promoting the activity and 1,3-propanediol selectivity, compared with monometallic clusters, is highlighted.

Synthesis of polysiloxaneurethane polymers by the aromatic isocyanate-activated opening of the octamethyltetrasiloxane ring via an anionic mechanism

Here, we present the results of the study of the interaction in the octamethylcyclotetrasiloxane-aromatic isocyanate-open coupling analogues system of crown ethers containing terminal hydroxyl and potassium alkoxide groups. It was found that the involvement of the organosiloxane cycle in the polymerization process precedes the opening of the isocyanate groups on the carbonyl component. It is shown that the formation of transverse cross-linking bonds in the polymer formation system is caused by the parallel reaction of cyclotrimerization. It is found that the ratio of the reaction rate constants of the copolymerization and the parallel reactions of cyclotrimerization and urethane formation depend on the synthesis temperature.

Fast Access to Dendrimer-like Poly(ethylene oxide)s through Anionic Ring-Opening Polymerization of Ethylene Oxide and Use of Nonprotected Glycidol as Branching Agent

Dendrimer-like poly(ethylene oxide)s (PEOs) were synthesized through a semicontinuous process based on the anionic ring-opening polymerization (AROP) of ethylene oxide (EO), followed by AROP of a mixture of glycidol (G) and propylene oxide (PO). Glycidol was used as branching agent generating two hydroxyl groups after ring-opening, whereas propylene oxide served to prevent the aggregation of the generated terminal alkoxides. A three-armed PEO star was first prepared through AROP of EO from 1,1,1-tris(hydroxymethyl)ethane as trifunctional precursor using dimethyl sulfoxide (DMSO) as solvent. After completion of EO polymerization and without isolating the PEO star precursor, G and PO (molar ratio 1:3) were added in the same batch to be polymerized either sequentially or randomly. This led to a three-armed PEO star with an average number of terminal hydroxyls per arm which depended on the number of G units inserted at PEO chain ends, as determined by 1H NMR spectroscopy. Growth of the second and the third ge...

Synthesis and characterization of a large heterometallic alkoxide cation: Self-assembly and rational route to [formula omitted

Ionic or ionogenic alkoxides are attractive potential catalysts for polymerization reactions. Direct interaction between CoCl 2 and [Ta(OMe) 5] produced, by self-assembly, a small amount of ionic aggregate clathrate with acetonitrile, [ Co 2 Ta 2 ( OMe ) 12 ( MeCN ) 4 ] 2 + [ CoCl 3 ( MeCN ) ] 2 - · 2 MeCN ( 1 ) . The structure of 1 contains a planar M 4( μ 3-OR) 2(μ-OR) 4-core that can be considered as a fragment of hexagonal packing of metal and ligand atoms completed by 6 (3 + 3) terminal alkoxide ligands at the tantalum and 4 (2 + 2) terminal acetonitrile molecules at the cobalt atoms that are bearing the (+1) positive charge each. A rational synthesis of this compound was achieved by reaction between [NaTa(OMe) 6] with four equivalents of CoCl 2 in toluene/acetonitrile to give a new crystallographic form [Co 2Ta 2(OMe) 12(MeCN) 4] [CoCl 3(MeCN)] 2 ( 2), not containing interstitial solvent molecules.

Synthesis and Intramolecular Cyclization of Diethylphosphono-Substituted Allenic Glycols

This paper describes a convenient and efficient synthesis of new (2R)-5-substituted-5-(diethylphosphono)-penta-3,4-dien-1,2-diols. Phosphorylated allenic glycols are stable enough compounds, but under basic conditions they cyclized to (3R)-5-(3-hydroxy-2,3-dihydrofuryl)(diethylphosphono)alkanes through intramolecular nucleophilic addition of the terminal alkoxide to the central carbon atom of the allene system. Treatment of (3R)-5-(3-hydroxy-2,3-dihydrofuryl)(diethylphosphono)alkanes with a catalytic amount of p-toluenesulfonic acid in chloroform at 40-45 °C for 0.5 hours afforded diethylphosphono(2-furyl)alkanes.

Structural and thermal stability studies of Ti(IV) hexanuclear oxo trimethylacetato isopropoxide complex

A new titanium hexanuclear oxo carboxylato alkoxide complex, with the general formula [Ti 6O 6(OPr i ) 6(OOCBu t ) 6] ( 1), has been synthesized in a reaction of titanium isopropoxide (Ti(OPr i ) 4) with Bu t COOH in the ratio 1:1. The structure was analyzed with the elemental analysis, IR and NMR spectroscopy, and X-ray single-crystal diffraction methods. Crystal structure of 1 was solved in the trigonal centrosymmetric system R 3 ̄ r . Titanium atoms form the hexanuclear cluster, where three μ 3-oxo, two μ-carboxylato bridges and the terminal alkoxide ligand, in the distorted octahedron, surround each central cation. Analysis of spectroscopic data revealed a significant nonequivalence of interactions formed by the carboxylate groups, resulting from the asymmetry of Ti-μ-OOC bonds of carboxylate bridges in the complex molecule. TGA and MS studies suggest that volatile, titanium hexanuclear species may be formed between 423 and 553 K.

4,6-Dihydroxypyrimidine: a selective bridging ligand for controlled Group IV metal alkoxide structures

The generation of controlled metal alkoxide structures has been explored using 4,6-dihydroxypyrimidine (DHP-H2) as a selective bridging ligand. DHP-H2 was reacted with Ti(OPri)4 in pyridine (py). Crystallization from this solution led to the isolation of [(OPri)2(μ-OPri)Ti(μ-DHP)Ti(OPri)3(py)]2 (1). The two dinuclear subunits of 1 possess a bridging DHP ligand bound through both the O and N of the ring, forming an M–O–C–N ring on each metal center. The first Ti possesses three terminal OR ligands and one coordinated pyridine solvent molecule. The second Ti is connected to a μ-OPri group from the other subunit, forming a tetranuclear species. In contrast, recrystallization of 1 from toluene provided the solvent-free, cyclic adduct {[(OPri)2(μ-OPri)Ti]2(μ-DHP)}2 (2). An identical reaction using more sterically hindering alkoxy precursors yielded products of the general structure [(py)(RO)3M]2(μ4-DHP), where M = Ti, OR = OCH2CMe3 (3), OPri/OCMe3 (4); M = Zr, OR = OCMe3 (5). Each metal center adopts an octahedral geometry, with one py, three terminal alkoxide ligands, and one bridging DHP. Solid-state 13C MAS NMR studies indicate the bulk materials are consistent with the crystal structures and solution 1H NMR is consistent with retention of the structures in solution for 2–5.

Solution Phase and Solid Supported Syntheses of End-Functionalized Poly(MMA) by Aldol-Type Reaction of Samarium(III) Enolate at the Chain End

Aldol-type reactions of a samarium enolate at the propagating end of living anionic poly(MMA) with aldehydes were carried out. The enolate underwent aldol-type end-capping reaction with 3-phenylpropanal to give the corresponding samarium alkoxide at the polymer terminal. Successive treatment of the alkoxide with benzoyl chloride resulted in its benzoylation. End-capping reaction with 2-phenylpropanal also smoothly proceeded to give the poly(MMA) having the corresponding secondary alkoxide at the terminal. Its benzoylation was not successful; however, an intramolecular reaction of the terminal alkoxide with the methyl ester moiety of the penultimate MMA unit predominantly proceeded to give the corresponding poly(MMA) having a lactone moiety at its terminal. In addition, a terminal samarium enolate of a living poly(MMA), which was supported on polystyrene beads by a benzoyl ester-type linker, reacted with the aldehydes to afford the end-capped poly(MMA), which could be efficiently isolated from the beads by selective cleavage of the linker by trifluoroacetic acid treatment.

Synthesis and structural characterization of 2,6-dimesitylphenyl complexes of scandium, ytterbium, and yttrium.

The molecular structures of a number of 2,6-dimesitylphenyl-based (2,6-dimesitylphenyl = Dmp) complexes of the group 3 elements scandium and yttrium as well as of the lanthanide element ytterbium are reported. Reaction of 1 equiv of DmpLi with 1 equiv of MCl(3) (M = Sc, Yb, Y) in tetrahydrofuran at room temperature followed by crystallization from toluene/hexanes at -30 degrees C produces DmpMCl(2)(THF)(2) (M = Sc: 1; M = Yb: 2) and DmpMCl(2)(THF)(3) (M = Y: 3), respectively. The one-pot reaction of DmpLi with 1 equiv of YbCl(3) in tetrahydrofuran at room temperature followed by addition of 1 equiv of KO(t)Bu produces the heterobimetallic monoalkoxide complex DmpYb(THF)(O(t)Bu)(mu-Cl)(2)Li(THF)(2) (4), which was crystallized from toluene/tetrahydrofuran (20:1) at -30 degrees C. Crystal data for 1: monoclinic, P2(1)/n; T = 203 K; a = 10.178(3) A; b = 15.468(3) A; c = 20.132(5) A; beta = 101.85(3) degrees; V = 3102.0(17) A(3); Z' = 4; D(calcd) = 1.228 g cm(-3); R(1) = 5.89%. Crystal data for 2: monoclinic, P2(1)/n; T = 173 K; a = 10.2447(7) A; b = 15.5683(12) A; c = 20.0979(14) A; beta = 101.749(4) degrees; V = 3238.3(5) A(3); Z' = 4; D(calcd) = 1.485 g cm(-3); R(1) = 4.32%. Crystal data for 3: monoclinic, P2(1)/n; T = 203 K; a = 15.950(3) A; b = 11.865(2) A; c = 18.254(3) A; beta = 92.323(3) degrees; V = 3451.9(10) A(3); Z' = 4; D(calcd) = 1.327 g cm(-)(3); R(1) = 4.43%. Crystal data for 4: triclinic, P1; T = 193 K; a = 10.2252(2) A; b = 11.3497(2) A; c = 18.5814(2) A; alpha = 98.7353(6) degrees; beta = 102.8964(6) degrees; gamma = 94.8058(5) degrees; V = 2062.09(5) A(3); Z' = 2; D(calcd) = 1.375 g cm(-3); R(1) = 4.56%. The molecular structures of 1-3 feature monomeric complexes with distorted trigonal-bipyramidal (1 and 2) or octahedral (3) coordination geometry about the metal atom, with the two chlorine atoms occupying the axial positions. 4 represents the first example of an alkoxide derivative of a terphenyl lanthanide complex. The molecular structure of the ate complex 4 exhibits a heavily distorted trigonal-bipyramidal coordination polyhedron about the ytterbium atom, with one of the mu-chlorine atoms and the oxygen atom of the tetrahydrofuran ligand representing the axial positions of the trigonal-bipyramidal arrangement. A terminal alkoxide ligand is another main feature of the molecular structure of complex 4.

Basicity of uranyl oxo ligands upon coordination of alkoxides.

Uranium(VI) alkoxide complexes are prepared via metathesis reactions of [UO2Cl2(THF)2]2 with potassium alkoxides in nonaqueous media. The dark red compound U[OCH2C(CH3)3]6, 1, results from redistributive exchange of oxo and neopentoxide ligands between more than one uranium species. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by six neopentoxide ligands. Imposition of steric congestion at the metal center prevents oxo-alkoxide ligand exchange in the reactions using more sterically demanding alkoxides. Simple metathesis between uranyl chloride and alkoxide ligands occurs in the synthesis of golden yellow-orange UO2(OCHPh2)2(THF)2, 2, and yellow UO2[OCH(tBu)Ph]2(THF)2, 3. Single-crystal X-ray diffraction analysis of 2 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, two diphenylmethoxide ligands occupying trans positions, and two tetrahydrofuran ligands. Coordination of diisopropylmethoxide allows for synthesis of a more complex binary alkoxide system. Single-crystal X-ray diffraction analysis of watermelon red [UO2(OCH(iPr)2)2]4, 4, reveals a tetramer in which each uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, one terminal alkoxide, two bridging alkoxide ligands, and one bridging oxo ligand from a neighboring uranyl group. These compounds are characterized by elemental analysis, 1H NMR, infrared spectroscopy, and, for 1, 2, and 4, single-crystal X-ray diffraction analysis. Luminescence spectroscopy is employed to evaluate the extent of aggregation of compounds 2-4 in various solvents. Vibrational spectroscopic measurements of 2-4 imply that, in contrast to the case of uranyl complexes prepared in aqueous environments, coordination of relatively strongly donating alkoxide ligands allows for enhancement of electron density on the uranyl groups such that the uranyl U=O bonds are weakened. Crystal data are as follows. 1: monoclinic space group C2/m, a = 10.6192(8) A, b = 18.36(1) A, c = 10.6151(8) A, beta = 109.637(1) degrees, V = 1949.1(3) A3, Z = 2, dcalc = 1.297 g cm-3. Refinement of 2065 reflections gave R1 = 0.045. 2: monoclinic space group P2(1)/c, a = 6.1796(4) A, b = 15.669(1) A, c = 16.169(1) A, beta = 95.380(1) degrees, V = 1558.7(2) A3, Z = 2, dcalc = 1.664 g cm-3. Refinement of 3048 reflections gave R1 = 0.036. 4: tetragonal space group I4, a = 17.8570(6) A, b = 17.8570(6) A, c = 11.4489(6) A, V = 3650.7(3) A3, Z = 2, dcalc = 1.821 g cm-3. Refinement of 1981 reflections gave R1 = 0.020.

Formic Acid Modified Ti(OCHMe(2))(4). Syntheses, Characterization, and X-ray Structures of Ti(4)(&mgr;(4)-O)(&mgr;-O)(OFc)(2)(&mgr;-OR)(4)(OR)(6) and Ti(6)(&mgr;(3)-O)(6)(OFc)(6)(OR)(6) (OFc = O(2)CH; OR = OCHMe(2)).

Novel structural arrangements for carboxylic acid modified titanium alkoxides were obtained from stoichiometric reactions between titanium isopropoxide (Ti(OPr(i))(4)) and formic acid (HOFc). The 1:1 reaction in toluene forms the tetranuclear species Ti(4)O(2)(OFc)(2)(OPr(i))(10), 1. The structure of 1 was solved in the triclinic space group P&onemacr; with a = 13.034(3) Å, b = 13.467(2) Å, c = 13.996(2) Å, alpha = 98.79(1) degrees, beta = 104.68(2) degrees, and gamma = 97.82(2) degrees for Z = 2. The general structure of 1 resembles two face-shared [Ti-O](4) cubes with one set of mirror-related titanium atoms removed. The oxygen atoms are represented by one &mgr;(4)-O, one &mgr;-O, and four &mgr;-OR ligands, with the remaining sites filled by two OFc ligands and six terminal alkoxides. Increasing the stoichiometry to 1:2 (Ti/HOFc) leads to the isolation of Ti(6)O(6)(OFc)(6)(OR)(6), 2. The structure of 2 was solved in the monoclinic space group P2(1)/c with a = 8.968(2) Å, b = 26.520(4) Å, c = 20.046(2) Å, and beta = 93.19(1) degrees for Z = 4. The structure of 2 consists of two offset six-membered [Ti-(&mgr;(3)-O)](3) rings joined through Ti-O bonds. The OFc ligands are arranged externally around the central hexagon-prism, oscillating between the top and bottom rings. Compound 1 adopts a very symmetrical arrangement in solution due to the labile OFc ligands and was found to undergo "aging" by a trans-esterification mechanism, the rate of which is enhanced by heating. Compound 2 maintains its solid-state structure in solution.

Synthesis and Thermolytic Behavior of Mixed-Valence Homo- and Heterometallic Group 14 Alkoxides

The mixed-valence molecule Sn{sup II}Sn{sup IV} (OPr){sub 6} is conveniently synthesized and is shown to exchange bridging and terminal alkoxides in solution in its Sn({micro}-OPr){sub 3}Sn(OPr){sub 3} structure. Pb{sub 3}SnO(POr){sub 8} is synthesized and shown to undergo an intramolecular fluxional process in solution. Both molecules are sufficiently volatile for CVD study, and comparison of CVD behaviors of these and of M(OPr){sub 2} (M = Sn, Pb), Sn(POr){sub 4}, SnZr(OPr){sub 6}, and Pb{sub 3}-ZrO(OPr){sub 8} shows the reducing capacity of the isopropoxide moiety dominates the thermolytic behavior of all these species, giving metallic Sn or Pb in all cases. Only Zr, when it is present, forms ZrO{sub 2}. The oxide in Pb{sub 3}SnO(POr){sub 8}yields PbO (and 2 Pb{sup 0} and 1 Sn{sup 0}), in contrast to the relative electropositivity of Sn and Pb, a fact attributed to kinetic control of CVD behavior.

Synthesis of Homoleptic Neopentoxide and Hydrido-Neopentoxide Trirhenium(III) Clusters.

The purpose of this study was to synthesize Re(3)(&mgr;-OCH(2)CMe(3))(3)(OCH(2)CMe(3))(6) and to determine if it could serve as a source of Re(3)(&mgr;-OCH(2)CMe(3))(3)(H)(OCH(2)CMe(3))(5) by analogy to Re(3)(&mgr;-O-i-Pr)(3)(O-i-Pr)(6), a previously reported cluster that exists in solution as an equilibrium mixture with Re(3)(&mgr;-O-i-Pr)(3)(H)(O-i-Pr)(5) and acetone. The halide Re(3)(&mgr;-Cl)(3)Cl(6)(THF)(3) reacts at room temperature in THF with 9 equiv of NaOCH(2)CMe(3) to give Re(3)(&mgr;-OCH(2)CMe(3))(3)(OCH(2)CMe(3))(6). An X-ray crystallographic study shows that the core of the homoleptic cluster has virtual D(3h) symmetry, which is consistent with solution NMR data. In refluxing THF, Re(3)(&mgr;-Cl)(3)Cl(6)(THF)(3) reacts with 10 equiv of NaOCH(2)CMe(3) to give [Na(THF)(2)][Re(3)(&mgr;-OCH(2)CMe(3))(3)(H)(OCH(2)CMe(3))(6)]. The same anionic hydride cluster is also produced when Re(3)(&mgr;-OCH(2)CMe(3))(3)(OCH(2)CMe(3))(6) reacts with excess NaOCH(2)CMe(3) in hot THF. Spectroscopic and X-ray crystallographic data show that [Re(3)(&mgr;-OCH(2)CMe(3))(3)(H)(OCH(2)CMe(3))(6)](-) has virtual C(s)() core symmetry with a terminal hydride and two terminal alkoxides located at the unique rhenium atom. The hydride and one of the alkoxides have a trans-H-Re-OR arrangement, and in the solid state structure the terminal alkoxide ligands at the unique rhenium atom interact with [Na(THF)(2)](+). It is proposed [Na(THF)(2)][Re(3)(&mgr;-OCH(2)CMe(3))(3)(H)(OCH(2)CMe(3))(6)] forms when Re(3)(&mgr;-OCH(2)CMe(3))(3)(H)(OCH(2)CMe(3))(5) or its pivaldehyde adduct, which is generated by neopentoxide beta-hydrogen elimination from Re(3)(&mgr;-OCH(2)CMe(3))(3)(OCH(2)CMe(3))(6), is trapped by NaOCH(2)CMe(3). Crystal data are as follows. C(45)H(99)O(9)Re(3) at -50 degrees C: Pbca (orthorhombic); a = 19.570(2), b = 28.192(3), c = 21.203(3) Å; Z = 8. Na(+)[C(45)H(100)O(9)Re(3)](-).2C(4)H(8)O at -50 degrees C: P&onemacr; (triclinic); a = 13.040(2), b = 14.716(2), c = 19.653(4) Å; alpha = 92.38(1), beta = 92.30(1), gamma = 113.42(1) degrees; Z = 2.