Alkoxo Ligands Research Articles

Well-Defined Ti Surface Sites in Ziegler-Natta Pre-Catalysts from 47/49Ti Solid-State Nuclear Magnetic Resonance Spectroscopy.

Treatment of Ziegler-Natta (ZN) catalysts with BCl3 improves their activity by increasing the number of active sites. Here we show how 47/49Ti solid-state nuclear magnetic resonance (NMR) spectroscopy enables us to understand the electronic structure of the Ti surface sites present in such treated ZN pre-catalysts, prior to activation with alkyl aluminum. High-field (21.1 T) and low-temperature (∼100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of 47/49Ti NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit 49Ti NMR signatures (δiso,exp = -170 ppm; CQ,exp = 9.3 MHz; κ = 0.05) corresponding to well-defined fully chlorinated hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl2 support, formed upon post-treatment with BCl3 and removal of the alkoxo ligands, paralleling the increased polymerization activity.

Versatile Organic Chemistry on Vanadium-Based Multi-Electron Reservoirs.

We report the synthesis, post-functionalization, and redox behavior of two organically functionalized aggregates, [V6 O7 (OMe)9 {(OCH2 )3 C-CH2 N3 }] and [V6 O7 (OMe)9 {(OCH2 )3 C-NH2 }]. All twelve μ2 -oxo groups on the edges of the Lindqvist-type {V6 O19 } core were replaced by alkoxo ligands. The absence of a negative charge and the closed organic shell make these neutral mixed-valence compounds very stable towards hydrolysis and well soluble in almost all common organic solvents. These are important advantages over classical POMs. By post-functionalization through copper(I)-catalyzed Huisgen cycloaddition or imine formation, various organic moieties could be introduced. Even a well-soluble trimer composed of three hexanuclear vanadium units connected through an aromatic triimino core was synthesized and studied. The diverse redox behavior, the versatile reactivity, the good stability, and the excellent solubility make our vanadium compounds highly interesting for applications as building blocks in macromolecular chemistry as well as redox labels in biochemistry.

Nickel Pincer Complexes with Frequent Aliphatic Alkoxo Ligands [(iPrPCP)Ni-OR] (R = Et, nBu, iPr, 2-hydroxyethyl). An Assessment of the Hydrolytic Stability of Nickel and Palladium Alkoxides.

A series of nickel pincer complexes with terminal alkoxo ligands [(iPrPCP)Ni-OR] (R = Et, nBu, iPr, CH2CH2OH; iPrPCP is the 2,6-bis(diisopropylphosphinomethyl)phenyl pincer ligand) was synthesized and fully characterized. Together with the previously reported methoxo analogues of Ni and Pd, these complexes constitute a unique series of isostructural late transition-metal alkoxides. Spectroscopic and X-ray diffraction data provide direct indications of the strong polarization of their covalent Ni-OR bonds. One of the most salient features of this class of compounds is their facile hydrolysis with traces of moisture, leading to equilibrium mixtures with the corresponding hydroxides [(iPrPCP)M-OH] (M = Ni or Pd) and alcohols, ROH. To compare the hydrolytic stability of nickel and palladium alkoxides, we performed NMR titrations of both hydroxides with several alcohols and determined the corresponding equilibrium constants. In general, these constants are ca. 1 order of magnitude smaller for M = Ni than Pd, indicating that Ni alkoxide complexes are more readily hydrolyzed than their Pd counterparts. For alkoxide complexes containing heteroatom-free R groups, the tendency to hydrolyze decreases as the parent alcohol ROH becomes more acidic, that is, R = Me > Et > iPr. This intuitive trend is broken for 2-methoxyethanol, the most acidic alcohol investigated. The hydroxo/2-methoxyethanol exchange equilibrium constants are comparable to those of ethanol (M = Ni) or methanol (M = Pd), showing that the corresponding 2-methoxyethoxide complexes are more prone to hydrolysis than anticipated. These experimental observations were rationalized in the light of density functional theory calculations.

Structural details of monomeric octahedral d 2-rhenium(V) oxo complexes with oxygen atoms in the trans-positions to oxo ligands. Complexes with the anionic ligands OR n− (n = 1, 2) in trans-positions to O(oxo)

The paper considers details of the structures of mononuclear octahedral rhenium(V) oxo complexes with oxygen atoms of aromatic (phenolic) and other similar cyclic ligands OR− (R = Ph, Cy, C6H4R′ (R′ = OH, OMe), C10H6OH) and with other structurally studied anionic ligands ORn− (n = 1, 2) (R = P(O)(OMe)2, C(O)(CF3), OC(Me(CF3)2, BF3) located in the trans-position to the multiply bonded oxo ligands. The complexes contain halide ligands and organic mono-, bi-, and tetradentate ligands in the equatorial plane. The Re-O(OR)trans bonds as well as the bonds with aliphatic alkoxo ligands (ORalk)trans (R = Me, Et, Pr) are not elongated due to the trans-effect of doubly bonded oxo groups but are substantially shortened (with one exception). This is attributable to the fact that these Re-O(OR) bonds, like Re-O(oxo) bonds, have a higher order, i.e., d2-rhenium(V) complexes of this type should be treated as pseudo-dioxo compounds [Re(=O)(=OR)]3 − n (n = 1, 2).

(Arylimido)vanadium(V)-alkylidene complexes containing fluorinated aryloxo and alkoxo ligands for fast living ring-opening metathesis polymerization (ROMP) and highly cis-specific ROMP.

(Arylimido)vanadium(V)-alkylidene complexes, V(CHSiMe3)(N-2,6-X2C6H3)(OC6F5)(PMe3)2 [X = Me (2), Cl (4)], exhibited remarkable catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene, and the ROMP by 2 proceeded in a living manner, affording ultrahigh molecular weight polymers. Cis-specific ROMP was achieved with the alkoxo analogues, V(CHSiMe3)(N-2,6-X2C6H3)[OC(CH3)(CF3)2]-(PMe3)2 [X = Me (5), Cl (6)]. Both the activity and the selectivity increased upon addition of PMe3, even at 50 °C.

Structural features of monomeric octahedral d 2 rhenium(V) oxo complexes with oxygen atoms in trans-positions to oxo ligands: Complexes with propoxo ligands in trans-positions to O(oxo)

Some structural features of mononuclear octahedral rhenium(V) oxo complexes with oxygen atoms of propoxo ligands O(Pr-iso)− in trans-positions to multiply bonded oxo ligands are considered. The complexes contain monodentate inorganic or organic ligands or bi- and tetradentate organic ligands in the equatorial plane. Re-O(OPr) trans bonds, as well as the other bonds with (ORalkoxo) trans alkoxo ligands (R = Me, Et), are considerably shortened, instead of being elongated, due to the structural manifestation of the trans-effect of multiply bonded oxo ligands. This is explained by that these Re-O(ORalkoxo) bonds also have an increased multiplicity; that is, d 2 rhenium(V) monooxomonoalkoxo complexes should be considered as pseudo-dioxo Re(=O)(=ORalkoxo) compounds.

Ti8O10(OOCR)12] [R = CH(CH3)2 and CCl3] Carboxylate Titanium Oxo‐Clusters: Potential SBUs for the Synthesis of Metal‐Organic Frameworks

AbstractThe synthesis and the characterization of a new octanuclear carboxylated titanium oxo‐cluster [Ti8O10(OOCR)12] [R = CH(CH3)2 and CCl3] is reported. The structure was characterized by single‐crystal X‐ray diffraction. It consists in four vertices‐linked pairs of edge‐sharing TiO6 octahedra, presenting μ2 and μ3 oxygen atoms. 17O and 13C liquid state NMR was performed with results in full accordance with the structure determined by X‐ray diffraction. During synthesis, solvothermal conditions and a large excess of carboxylic acid allow to remove all alkoxo ligands of the titanium alkoxide precursor and lead to stable purely carboxylate clusters. This family of clusters can therefore be considered as interesting SBUs to elaborate new metal‐organic frameworks.

Crystal structures and thermal properties of some rare earth alkoxides with tertiary alcohols

Rare earth (RE) alkoxides of Y, La, Pr, and Gd were synthesized using three tertiary alcohols namely 2,3-dimethyl-2-butanol, 2,2-dimethyl-3-ethyl-3-pentanol, and 3-isopropyl-2,4-dimethyl-pentan-3-ol. 1H and 13C NMR, elemental analysis (CHN) and MS were used to characterize the complexes. New crystal structures of dimeric [Y(OCEt2 t Bu)3]2, [La(OCEt2 t Bu)3]2, [Gd(OC i Pr3)3]2, and [La(OC i Pr3)3]2 were solved. Thermal properties of these compounds were studied with TG/SDTA and vacuum sublimation experiments. With all the cations Y, La, Pr, and Gd, the compounds with smallest ligand i PrMe2CO− seem to be the most volatile. The compounds with larger alkoxo ligands have lower volatility but their thermal stability is better. Atomic layer deposition experiments showed that RE oxide thin films could be grown from these alkoxides using water as the oxygen precursor.

Ti8O8(OOCR)16, a New Family of Titanium–Oxo Clusters: Potential NBUs for Reticular Chemistry

AbstractThe reactions of titanium alkoxides with a large excess of different carboxylic acids under nonhydrolytic conditions leads to the reproducible formation of well‐defined nano‐building units (NBUs) with the formula [Ti8O8(OOCR)16] [R = C6H5, C(CH3)3, CH3]. The structures of these titanium–oxo–carboxylate clusters have been determined by crossingdifferent characterization techniques and methodologies (single‐crystal X‐ray diffraction, 13C and 1H NMR spectroscopy, and FTIR spectroscopy). These NBUs are obtained in high yields and, since all the alkoxo ligands have been removed by using solvothermal‐synthesis conditions, they present better stability upon hydrolysis than the often reported alkoxo–carboxylate–titanium–oxo clusters [TinO2n–x/2–y/2(OR′)x(OOCR)y] (n ≥ 2; x ≥ 1; y ≥ 1). In addition, the solubility and transferability of these clusters in common solvents can be tuned by selecting the nature of the organic ligand. Moreover, we also report for the first time, a robust post‐modification of the carboxylate ligands by transesterification reactions on the titanium–oxo clusters. These reactions keep the integrity of the octameric titanium–oxo core intact, while completely exchanging the organic shell of the cluster. This family of [Ti8O8(OOCR)16] clusters, which present 16 points of extension, a symmetric shape, and the ability to be post‐modified with conservation of the core structure, can therefore be considered as interesting NBUs to form new metal–organic frameworks.

Deperoxidation of cyclohexyl hydroperoxide by silica-supported alkoxo-tantalum complexes

Silica-supported alkoxo-tantalum complexes were used in the deperoxidation of cyclohexyl hydroperoxide into cyclohexanol and cyclohexanone. The effects of the number of bonds with the silica support and of the nature of the alkoxo ligands were studied. In all cases the activity was quite the same and was relatively good for a heterogeneous catalyst. The main interest of these systems is that there is no leaching of tantalum in the reaction medium, leading to an easy recycling of the catalyst.

Modification of alkoxo ligands of BINOL–Ti ladder: Isolation and X-ray crystallographic analysis

The stable binaphthol–titanium ladder complexes have been successfully prepared by using bulky alkoxo ligands. From the secondary OR ligand (cyclohexyloxo, 2,4-dimethyl-3-pentyloxo or 2-adamantyloxo) and terially OR ligand ( tert-butyloxo, 1-adamantyloxo), partial hydrolysis proceeded to give the μ 3-oxo titanium complexes. The use of [Ti(BINOLato)(OEt) 2] n made it possible to prepare the Ti(BINOLato)(OR) 2 complexes using alcohols (ROH) of high boiling point (R = cyclohexyl, 2-adamantyl, 1-adamantyl). X-ray analyses of [( R)-1,1′-bi-2-naphtholato]bis(O-2,4-dimethyl-3-pentyloxo)titanium and [( R)-3,3′-dimethyl-1,1′-bi-2-naphtholato]bis(2-adamantyloxo)titanium showed a good agreement with the estimated ladder complexes. The catalytic activity of BINOL–Ti catalyst analogues, obtained by partial hydrolysis of Ti(BINOLato)(OR) 2 with wet MS 4A was studied in asymmetric glyoxylate-ene reaction by two methods. Moderate to good chemical yields and enantioselectivities were obtained.

Coordination of mono- and diamines to titanium and zirconium alkoxides

The structural chemistry of crystalline adducts M2(OR)8(amine)2 (M = Ti: OR = OiPr, amine = NH2CH2Ph, NH2Bu; M = Ti: OR = OEt, amine = piperidine; M = Zr: OR = OiPr, amine = NH2CH2Ph, NH2C6H11) is discussed. The adducts were obtained by reaction of Ti(OEt)4 or M(OiPr)4 with primary or secondary amines. The monoamine adducts are centrosymmetric dimers in which the amines are coordinated axially to the M2μ-OR)2 ring and hydrogen-bonded to neighboring alkoxo ligands. The adducts are sufficiently stable if the hydrogen bond is strong. 15N NMR studies revealed that the amines are also coordinated in solution. Coordination polymers were obtained when diamines with two terminal NH2 groups are reacted with Ti(OiPr)4. The general structure is the same as that of Ti2(OiPr)8(RNH2)2. However, the diamines bridge the Ti2(OiPr)8 units. When Zr(OiPr)4 was reacted with NH2CH2CH2NHMe, the adduct Zr2(OiPr)8[NH2CH2CH2NHMe]2 was obtained where only the NH2 group is coordinated.

A General Study of Aryloxo and Alkoxo Ligands in the Titanium‐Catalyzed Intermolecular Hydroamination of Terminal Alkynes.

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A General Study of Aryloxo and Alkoxo Ligands in the Titanium‐Catalyzed Intermolecular Hydroamination of Terminal Alkynes

AbstractA general study of the regioselective hydroamination of terminal alkynes in the presence of Ti(NEt2)4 and different aryloxo and alkoxo ligands is presented. Depending on the ligand the regioselectivity towards the Markovnikov and the anti‐Markovnikov addition product can be controlled. The experimentally observed isomer distribution is explained perfectly by detailed theoretical investigations which demonstrate that the regioselectivity is determined by the relative stability of the corresponding alkynetitanium π complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Synthesis and characterization of cyclopentadienyl/alkoxo titanium dichlorides: structural analysis of monocyclopentadienyl titanium dichlorides with ligands derived from menthol and borneol

A variety of monocyclopentadienyl alkoxo titanium dichloride and bisalkoxo titanium dichloride complexes have been prepared and characterized by spectroscopic techniques. The titanium derivatives containing both cyclopentadienyl and various alkoxo ligands [Ti(η 5-C 5H 5)(OR)Cl 2] ( 1– 5) have been synthesized from the reaction of [Ti(η 5-C 5H 5)Cl 3] with 1 equivalent of the corresponding alcohol in THF in the presence of triethylamine (ROH = Adamantanol, 1 R,2 S,5 R-(−)-menthol, 1 S-endo-(−)-borneol, cis-1,3-(−)-benzylideneglycerol, 1,2:3,4-di- O-isopropylidene-α- d-galactopyranose). The bisalkoxo titanium dichloride derivatives [TiCl 2(OR) 2] ( 6– 10) have been prepared by a redistribution reaction between Ti(OR) 4 and TiCl 4 compounds 6– 8 (OR = Adamantanoxy, (1 R,2 S,5 R)-(−)menthoxy, (1 S-endo)-(−)-borneoxy) and by reaction of [Ti(OR) 2(OPr i ) 2] 2 with CH 3COCl compounds 9 and 10 (OR = 1,2:3,4-di- O-isopropylidene-α- d-galactopyranoxy, and 1,2:5,6-di- O-isopropylidene-α- d-glucofuranoxy). The molecular structures of 2 and 3 have been determined by single crystal X-ray diffraction studies.

Gas phase oxidation of alkoxo ligands in bis(peroxo)[MO(O2)2(OR)]? and trisoxo [MO3(OR)]? anions (M = Cr, Mo, W)

The anions [M(VI)O(O(2))(2)(OR)](-) and [M(VI)O(3)(OR)](-)(M = Cr, Mo, W; R = H, Me, Et, (n)Pr, (i)Pr) were transferred to the gas phase by the electrospray process. Their decomposition was examined by multistage mass spectrometry and collisional activation experiments. The molybdate and tungstate anions [M(VI)O(O(2))(2)(OR)](-) underwent parallel elimination of aldehyde (ketone) and dioxygen while the equivalent chromate underwent loss of dioxygen only. The peroxo ligands were the source of oxidising equivalents in both reactions. For each alkoxo ligand, the total yield of aldehyde for the tungstate system exceeded that for the molybdate system. Collisional activation of [M(VI)O(3)(OMe)](-) led to clean elimination of formaldehyde with the metal centre supplying the oxidising equivalents. For larger alkoxo ligands, only the chromate centre eliminated aldehyde, while the molybdate and tungstate centres underwent clean loss of alkene. Threshold activation voltages indicated that the peroxo ligands of [W(VI)O(O(2))(2)(OMe)](-) are more oxidising than the tungstate centre of [W(VI)O(3)(OMe)](-). (2)H and (18)O isotope tracing experiments were consistent with a formal hydride transfer mechanism operating for oxidation of alkoxo ligand in each system. In the solid state, anions [M(VI)O(O(2))(2)(OR)](-) are typically pentagonal pyramidal (oxo in apical site) while [M(VI)O(3)(OR)](-) are tetrahedral. The data indicate that an equatorial ligand position is the site of alkoxo oxidation in [M(VI)O(O(2))(2)(OR)](-) anions. Comparisons of the gas phase data with those for a solution phase system are made.

Vanadium(V) complexes of a chelating dianionic [ONNO]-type amine bis(phenolate) ligand: synthesis and solid state and solution structures.

The reaction between VO(OR)(3) (R = (i)()Pr, (t)()Bu, CH(2)CF(3)) and the chelating dianionic bis(phenoxy)amine ligand [ONNO]H(2) affords a mixture of two isomers (A and B in a ratio A:B approximately 3:1) formulated as VO(OR)[ONNO] (1a-c) (R = (i)()Pr (1a), (t)Bu (1b), CH(2)CF(3) (1c)). Multinuclear and NOESY NMR spectroscopy experiments were able to determine the structure in solution of the complexes. Both isomers have the symmetry-related phenolate groups in a trans configuration, the difference arising from the different configuration of the oxo and alkoxo ligands being located either cis (in isomer A) or trans (in isomer B) to the tripodal amino nitrogen donor atom and the (dimethylamino)ethyl sidearm respectively for the oxo and the alkoxo ligands. Crystals of isomer A (cis-1a) were obtained, and the structure determination confirms the arrangement of the ligands around the vanadium center. Analogue complexes VO(X)[ONNO] (X = Cl (2); X = N(3) (3)) were prepared by reacting equimolar amount of [ONNO]H(2) and VO(X)(n)(OR)(3-n) (X = Cl, R = Et, n = 1; X = N(3), R = (i)Pr, n = 2) at ambient temperature. Compounds 2 and 3 were further characterized by NMR spectroscopy experiments and X-ray structure determination. For both 2 and 3, a single isomer is obtained, having a trans-(O,O) configuration for the phenolate groups and a trans configuration of the oxo ligand in respect to the tripodal amino nitrogen donor atom. Finally, complex 2 could also be obtained by chlorination of 1a or 3 using a large excess of ClSiMe(3) in refluxing toluene.

AN ALKOXO-BRIDGED BINUCLEAR MANGANESE COMPLEX BIS(μ-PROPIONATO-O: O′)-BIS(μ-3-SALICYLIDENE-AMINO-PROPANOLATO-O″,O‴,N:O″) BISMANGANESE(III)

The alkoxo-bridged binuclear complex of MnIII, [Mn2(salpa)2(C2H5—COO)2] (H2salpa = 3-salicylidene-amino-1-propanol), has been prepared and its crystal structure has been determined by X-ray diffraction. The structure of the title complex consists of discrete binuclear manganese units in which the MnIII atoms are bridged by two alkoxo ligands via oxygen atoms and supported by two carboxylato bridging ligands in the syn - syn fashion. The complex lies about a crystallographic inversion center. Each MnIII atom is located in an elongated octahedral environment with one imino N, one phenolic O and two alkoxo O atoms coordinated in the equatorial plane and two O atoms from carboxylato groups at the apical positions. The remarkably longer coordination bond distances in the axial direction are attributed to Jahn-Teller distortion at the d 4 manganese center. The distance between the manganese atoms is 2.8662(9) Å. The asymmetrical and symmetric stretching vibrations for carboxylato groups found at 1550 and 1440 cm−1, respectively, with a separation of less than 200 cm−1 confirmed the bidentate mode of the carboxylato groups.

Syntheses, structure, and reactivity of chiral titanium compounds: procatalysts for olefin polymerization.

Titanium complexes with chelating alkoxo ligands have been synthesised with the aim to investigate titanium active centres in catalytic ethylene polymerisation. The titanium complexes cis-[TiCl2(eta2-maltolato)2] (1, 89%), and cis-[TiCl2(eta2-guaiacolato)2] (2, 80%) were prepared by direct reaction of TiCl4 with maltol and guaiacol in toluene. The addition of maltol to [Ti(OiPr)4] in THF results in the formation of species [Ti(OiPr)2(maltolato)2] (3, 82%). The titanium compound cis-[Ti(OEt)2(eta2-maltolato)2] (4, 74%) was obtained by the transesterification reaction of species 3 with CH3CO2Et. When compound 4 is dissolved in THF a dinuclear species [Ti2(mu-OEt)2(OEt)4-(eta2-maltolato)2] (5, 45%) is formed. Reaction of [Ti(OiPr)4] with crude guaiacol in THF yields a solid, which after recrystallisation from acetonitrile gives [Ti4(mu-O)4(eta2-guaiacolato)] x 4CH3CN (6, 55%). In contrast, reaction of TiCl4 with crude guaiacol in tetrahydrofuran affords [Ti2(mu-O)Cl2(eta2-guaiacolato)4] (7, 82%). Crystallographic and electrochemical analyses of these complexes demonstrate that maltolato and guaiacolato ligands can be used as a valuable alternative for the cyclopentadienyl ring. These complexes have been shown to be active catalysts upon combination with the appropriate activator.

Synthesis, structure and oxo-transfer properties of dioxotungsten(VI) complexes with pyridine-based NO-and NS-bidentate ligands

Treatment of [WO2Cl2(DME)] with 2-pyridylalkoxo NO-bidentate ligands [HLn (n = 1–5)] in the presence of n-BuLi or 2-pyridylthiolato NS-bidentate ligands [LiLn (n = 6, 7)] gave the corresponding dioxotungsten(VI) complexes [WO2(Ln)2] (n = 1–7). The new compounds were spectroscopically characterised and the molecular structures of [WO2(Ln)2] (n = 3–5) were also determined by X-ray diffraction analysis. While the complexes [WO2(Ln)2] (n = 4, 5) adopt a normal distorted octahedral geometry with cis-dioxo ligands, the metal centre in [WO2(L3)2] exhibits an unusual bicapped tetrahedral coordination, which may arise from the strong σ-donating ability of the alkoxo ligands. These high-valent dioxo complexes, in particular [WO2(L7)2], which contains two sulfur-containing ligands, are active towards oxygen atom transfer reactions and can catalyse the oxidation of benzoin with DMSO.