Dinuclear Ti Research Articles

Olefin Epoxidation Catalyzed by Titanium–Salalen Complexes: Synergistic H2O2 Activation by Dinuclear Ti Sites, Ligand H-Bonding, and π-Acidity

Titanium–salalen complexes have recently solved a long-standing problem in homogeneous epoxidation catalysis by enabling the selective catalytic epoxidation of terminal, nonconjugated olefins with ...

Group 4 complexes of salicylbenzoxazole ligands as effective catalysts for the ring-opening polymerization of lactides, epoxides and copolymerization of ε-caprolactone with L-lactide

A series of salicylbenzoxazole pro-ligand 2-(5-X-benzoxazol-2-yl)-6-R1-4-R2-phenol, L1H-L4H were subjected to reaction with titanium, zirconium and hafnium alkoxides. The substituents (R1, R2, X) present in the pro-ligand back bone are (H, H, H) for L1H; (H, H, Cl) for L2H; (Br, Br, H) for L3H; and (Cl, Cl, H) for L4H. The mononuclear Ti complexes, 1a-4a of [M(L)2(OiPr)2] formulation were prepared by combining the pro-ligands L1H-L4H with Ti(OiPr)4 in dry toluene. However, with the addition of H2O (0.5 M equiv.), the controlled hydrolysis of Ti complex 4a in a mixed solvent of THF/toluene produced oxo-bridged dinuclear Ti complex 5a having [M(μ-O)(L)2(OiPr)]2 formulation in good yield (≥75%) using L4H as the pro-ligand. The composition of the mononuclear Zr and Hf complexes of L1H-L4H are found to be quite similar to that of the Ti complexes. Additionally, mononuclear Zr complexes 1b-4b and Hf complexes 1c-4c were prepared. The oxo-bridged dinuclear Zr and Hf complexes 5b and 5c were isolated from the controlled hydrolysis reactions of mononuclear Zr and Hf complexes 4b and 4c. All these complexes were well characterized and crystal structure of six of the fifteen complexes were determined. These complexes were used as initiators for the ring-opening polymerization (ROP) of rac-LA, copolymerization of L-LA with ε-CL and homopolymerization of rac-epoxides. In particular, the dinuclear Zr complex (5b) was found to exhibit extremely high activity in these polymerization studies. Iso-enriched polymers were observed upon using the Zr and Hf complexes of the pro-ligands L3H or L4H as the initiator. DFT studies have been carried out on the ROP of lactides initiated by the Ti, Zr and Hf complexes 1a, 1b and 1c. The results indicated that the activation barrier to reach the transition state using Zr complexes is lower and hence facile as compared to the Ti and Hf complexes.

Improvement in Titanium Complexes Bearing Schiff Base Ligands in the Ring-Opening Polymerization of L-Lactide: A Dinuclear System with Hydrazine-Bridging Schiff Base Ligands.

A series of titanium (Ti) complexes bearing hydrazine-bridging Schiff base ligands were synthesized and investigated as catalysts for the ring-opening polymerization (ROP) of L-lactide (LA). Complexes with electron withdrawing or steric bulky groups reduced the catalytic activity. In addition, the steric bulky substituent on the imine groups reduced the space around the Ti atom and then reduced LA coordination with Ti atom, thereby reducing catalytic activity. All the dinuclear Ti complexes exhibited higher catalytic activity (approximately 10-60-fold) than mononuclear L(Cl-H)-TiOPr2 did. The strategy of bridging dinuclear Ti complexes with isopropoxide groups in the ROP of LA was successful, and adjusting the crowded heptacoordinated transition state by the bridging isopropoxide groups may be the key to our successful strategy.

Chemoselective Claisen–Schmidt bis-substitutional condensation catalyzed by an alkoxy-bridged dinuclear Ti(IV) cluster

The highly efficient and chemoselective α,α′-bis-substitution of alkanones is important in organic synthesis. Herein, a dimeric titanium cluster, Ti2Cl2(OPri)6·2HOPri (Ti2), is used in the Claisen–Schmidt condensation reaction, for the selectively activation of symmetrical ketones containing α,α′-methylene groups and production of α,α′-bis-substituted alkanones in high efficiency and chemoselectivity. The high efficiency and chemoselectivity can be extended to a variety of typical alkanones and aromatic aldehydes. Both of the oxo-bridged dimeric motif of Ti2 and the ionic Ti–Cl bond are responsible for the high efficiency and chemoselectivity.

Electronic Structure and Reactivity of a Well-Defined Mononuclear Complex of Ti(II).

A facile and high-yielding protocol to the known Ti(II) complex trans-[(py)4TiCl2] (py = pyridine) has been developed. Its electronic structure has been probed experimentally using magnetic susceptibility, magnetic circular dichroism, and high-frequency and high-field electron paramagnetic resonance spectroscopies in conjunction with ligand-field theory and computational methods (density functional theory and ab initio methods). These studies demonstrated that trans-[(py)4TiCl2] has a (3)Eg ground state (dxy(1)dxz,yz(1) orbital occupancy), which, as a result of spin–orbit coupling, yields a ground-state spinor doublet that is EPR active, a first excited-state doublet at ∼60 cm(–1), and two next excited states at ∼120 cm(–1). Reactivity studies with various unsaturated substrates are also presented in this study, which show that the Ti(II) center allows oxidative addition likely via formation of [Ti(η(2)-R2E2)Cl2(py)n] E = C, N intermediates. A new Ti(IV) compound, mer-[(py)3(η(2)-Ph2C2)TiCl2], was prepared by reaction with Ph2C2, along with the previously reported complex trans-(py)3Ti═NPh(Cl)2, from reaction with Ph2N2. Reaction with Ph2CN2 also yielded a new dinuclear Ti(IV) complex, [(py)2(Cl)2Ti(μ2:η(2)-N2CPh2)2Ti(Cl)2], in which the two Ti(IV) ions are inequivalently coordinated. Reaction with cyclooctatetraene (COT) yielded a new Ti(III) complex, [(py)2Ti(η(8)-COT)Cl], which is a rare example of a mononuclear “piano-stool” titanium complex. The complex trans-[(py)4TiCl2] has thus been shown to be synthetically accessible, have an interesting electronic structure, and be reactive toward oxidation chemistry.

Tetravalent Titanium, Zirconium, and Cerium Oxo and Peroxo Complexes Containing an Imidodiphosphinate Ligand

Dinuclear Ti(IV), Zr(IV), and Ce(IV) oxo and peroxo complexes containing the imidodiphosphinate ligand [N(i-Pr(2)PO)(2)](-) have been synthesized and structurally characterized. Treatment of Ti(O-i-Pr)(2)Cl(2) with KN(i-Pr(2)PO)(2) afforded the Ti(IV) di-μ-oxo complex [Ti{N(i-Pr(2)PO)(2)}(2)](2)(μ-O)(2) (1) that reacted with 35% H(2)O(2) to give the peroxo complex Ti[N(i-Pr(2)PO)(2)](2)(η(2)-O(2)) (2). Treatment of HN(i-Pr(2)PO)(2) with Zr(O-t-Bu)(4) and Ce(2)(O-i-Pr)(8)(i-PrOH)(2) afforded the di-μ-peroxo-bridged dimers [M{N(i-Pr(2)PO)(2)}(2)](2)(μ-O(2))(2) [M = Zr (3), Ce (4)]. 4 was also obtained from the reaction of Ce[N(i-Pr(2)PO)(2)](3) with 35% H(2)O(2). Treatment of (Et(4)N)(2)[CeCl(6)] with 3 equiv of KN(i-Pr(2)PO)(2) afforded Ce[N(i-Pr(2)PO)(2)](3)Cl (5). Reaction of (Et(4)N)(2)[CeCl(6)] with 2 equiv of KN(i-Pr(2)PO)(2) in acetonitrile, followed by treatment with Ag(2)O, afforded the μ-oxo-bridged complex [Ce{N(i-Pr(2)PO)(2)}Cl](2)[μ-N(i-Pr(2)PO)(2)](2)(μ-O) (6). 6 undergoes ligand redistribution in CH(2)Cl(2) in air to give 5. The solid-state structures of [K(2){N(i-Pr(2)PO)(2)}(2)(H(2)O)(8)](n) and complexes 1-6 have been determined.

Major impact of N-methylation on cytotoxicity and hydrolysis of salan Ti(IV) complexes: sterics and electronics are intertwined

A series of Ti(IV) complexes containing diamino bis(phenolato) "salan" type ligands with NH coordination were prepared, and their hydrolysis and cytotoxicity were analyzed and compared to the N-methylated analogues. Substituting methyl groups on the coordinative nitrogen donor of highly active and stable Ti(IV) salan complexes with H atoms has two main consequences: the hydrolysis rate increases and the cytotoxic activity diminishes. In addition, the small modification of a single replacement of Me with H leads to a different major hydrolysis product, where a dinuclear Ti(IV) complex with two bridging oxo ligands is obtained, as characterized by X-ray crystallography, rather than a trinuclear cluster. A partial hydrolysis product containing a single oxo bridge was also crystallographically analyzed. Investigation of a series of complexes with NH donors of different steric and electronic effects revealed that cytotoxicity may be restored by fine tuning these parameters even for complexes of low stability.

Surface Species of Titanium(IV) and Titanium(III) in MgCl2-Supported Ziegler−Natta Catalysts. A Periodic Density Functional Theory Study

A systematic consideration of different Ti(IV) and Ti(III) species on the (104) and (110) MgCl2 surfaces has been implemented within DFT using cyclic boundary conditions. Some new mononuclear and dinuclear surface complexes of Ti(IV) and Ti(III) were obtained due to implication of zip coordination mode. A possible spin state of dinuclear Ti(III) species was thoroughly studied: antiferromagnetic (ESR silent) state proved to be the most preferable in a number of cases. The zip antiferromagnetic Ti2Cl6 complexes residing on the dominant (104) MgCl2 surface make it possible to rationalize the fact that the most part of Ti(III) incorporated in activated MgCl2 is ESR silent. Besides, these species produce aspecific active sites, thus explaining that aspecific centers significantly prevail over stereospecific one according to kinetic data on the simplest TiCl4/MgCl2 + AlR3 system.

Pamoic acid in forming metallo-organic framework: Synthesis, characterization and first crystal structure of a dimeric Ti(IV) complex

Two novel dinuclear Ti(IV) complexes of the ligand, 4,4′-methylene-bis (3-hydroxy-2-naphthalene carboxylic acid) (H 4L) or pamoic acid having compositions, [(HL) 2Ti 2(μ-O)(DMF) 2]·(DMF) 6 ( 1) and [(L) 2Ti 2(μ-O)(DMF) 2]·(DMF) 4(4,4′-Bipy-2H)(H 2O), ( 2) have been synthesized and characterized by analytical and spectral methods and the structure has been established by single crystal XRD. Unlike the reported polymeric structures observed in case of H 4L or pamoic acid, the anti-conformation of H 4L changes to syn - orientation to avoid poly-metallic complex formation, as noticed in 1 and 2. The dimeric Ti(IV) units stack in the lattice to form helical columns and the space between the adjacent columns is being filled by the solvent molecules in 1 and solvent plus the protonated 4,4′-bipy in the lattice of 2 and thus the neighbor columns are connected through weak interactions.

Structural characterization of dinuclear Ti(IV) complexes of rigid tetradentate dianionic diamine bis(phenolato) ligands; effect of steric bulk on coordination features

A small difference in diamine bis(phenolato) ligands, namely an additional single methylene unit, directs formation of dinuclear Ti(IV) complexes rather than mononuclear ones as characterized by X-ray crystallography. Varying steric bulk of the ligand affects the coordination number in the dinuclear complexes and the ligand to metal ratio. A ligand with reduced steric bulk leads to a L 2Ti 2(O iPr) 4 type complex featuring two octahedral metal centers bridged only by the two phenolato ligands, whereas a bulky ligand leads to a Ti 2(μ-L 1)(μ-O iPr) 2(O iPr) 4 type complex with a single chelating ligand, two bridging isopropoxo ligands, and two terminal isopropoxo groups on each of the two metal centers, which are of trigonal bi-pyramidal geometry.

Synthesis and Conformational Analysis of Constrained Ethylene-Bridged Bis(hydroxylamino-1,3,5-triazine) Compounds as Tetradentate Ligands; Structure of Rigid Dinuclear Ti(IV) Complex

Ethylene-bridged bis(hydroxylamino-1,3,5-triazine) compounds, that may serve as tetradentate ligands, were synthesized in three steps from 2,4,6-trichloro-1,3,5-triazine. These compounds demonstrate different rotation restrictions around the C(Ar)-N bonds due to their distinctive electronic structure as apparent from their resonative contributors. A dinuclear complex Ti 2(mu-L)2(OiPr)4 (L = bis(triazine)) was synthesized where each octahedral Ti(IV) center is also bound to two isopropoxo groups. The complex rigidity is manifested in a significant deviation from planarity of the aromatic systems, and relatively long Ti-N bonds compared to mononuclear analogous complexes. Increased ligand lability in this complex brings about diminished cytotoxicity toward colon and ovarian cells.

Ring-Opening Reactions of Tetrahydrofuran versus Alkyne Complexation by Group 4 Metallocene Complexes Leading to General Consequences for Synthesis and Reactions of Metallocene Complexes

The reduction of certain group 4 metallocene dichlorides by magnesium or lithium in the presence or absence of Me3SiC2SiMe3 in THF or toluene was investigated, giving in the case of titanium the dinuclear Ti(III) complex [rac-(ebthi)Ti(μ-Cl)]2 (1). For zirconium the 1-oxa-2-zirconacyclohexane 2 was formed by ring-opening reaction of rac-(ebthi)Zr(η2-Me3SiC2SiMe3) with THF. As a byproduct from the synthesis of Cp*2Zr(η2-Me3SiC2SiMe3) starting from Cp*2ZrCl2 another 1-oxa-2-zirconacyclohexane (3) was obtained by ring-opening reaction of THF via the dinuclear complex Cp*2Zr(Cl)-(CH2)4O−Zr(Cl)Cp*2 (4). In the case of hafnium the analogous dinuclear complex Cp*2Hf(Cl)−(CH2)4O−Hf(Cl)Cp*2 (5) and 1-oxa-2-hafnacyclohexane (6) were the main products of the reaction, inhibiting the synthesis of Cp*2Hf(η2-Me3SiC2SiMe3) (7). The tendency for ring opening of THF initiated by metallocenes increases in the series Ti, Zr, Hf, thus leading to consequences for the synthesis of metallocene complexes.

Spectroscopic Comparison of Dinuclear Ti+ and Ti2+ μ‐η1:η1 Dinitrogen Complexes with Cp*/Pentafulvene and Amine/Amide Ligation: Moderate versus Strong Activation of N2 (Eur. J. Inorg. Chem. 2/2006)

AbstractThe cover picture shows that Raman spectroscopy can reveal drastically different N−N stretching frequencies in dinuclear, dinitrogen‐bridged titanium complexes. In the article by F. Tuczek et al. on p. 291 ff., the recently synthesized complex [{(η5‐C5Me5)Ti[η6‐C5H4C(p‐tolyl)2]}2(μ‐η1:η1‐N2)] (1; R. Beckhaus et al., Eur. J. Inorg. Chem. 2005, 1003) is compared to the dinuclear (dinitrogen)titanium complex [{(Me3Si)2NTiCl(TMEDA)}2(μ‐η1:η1‐N2)] (3; R. Duchateau et al., J. Am. Chem. Soc. 1991, 113, 8986). With respect to 3 [ν(NN) = 1284 cm−1], 1 exhibits an unusually high N−N stretching frequency [ν(NN) = 1749 cm−1]. These spectroscopic differences reflect different degrees of dinitrogen activation, which are explained by the fact that only one of the N2 π*‐orbitals is occupied in 1, whereas both π*‐orbitals are occupied in 3.

A new dinuclear Ti(IV)-peroxo-citrate complex from aqueous solutions. Synthetic, structural, and spectroscopic studies in relevance to aqueous titanium(IV)-peroxo-citrate speciation.

The wide use of titanium in applied materials has prompted pertinent studies targeting the requisite chemistry of that metal's biological interactions. In order to understand such interactions as well as the requisite titanium aqueous speciation, we launched investigations on the synthesis and spectroscopic and structural characterization of Ti(IV) species with the physiological citric acid. Aqueous reactions of TiCl(4) with citric acid in the presence of H(2)O(2) and neutralizing ammonia afforded expediently the red crystalline material (NH(4))(4)[Ti(2)(O(2))(2)(C(6)H(4)O(7))(2)].2H(2)O (1). Complex 1 was further characterized by UV-vis, FT-IR, FT- and laser-Raman, NMR, and finally by X-ray crystallography. Compound 1 crystallizes in the monoclinic space group P2(1)/n, with a = 10.360(4) A, b = 10.226(4) A, c = 11.478(6) A, beta = 107.99(2) degrees, V = 1156.6(9) A(3), and Z = 2. The X-ray structure of 1 reveals a dinuclear anionic complex containing a Ti(IV)(2)O(2) core. In that central unit, two fully deprotonated citrate ligands are coordinated to the metal ions through their carboxylate moieties in a monodentate fashion. The central alkoxides serve as bridges to the two titanium ions. Also attached to the Ti(IV)(2)O(2) core are two peroxo ligands each bound in a side-on fashion to the respective metal ions. NH(4)(+) ions neutralize the 4- charge of the anion in 1, further contributing to the stability of the derived lattice through H-bond formation. The structural similarities and differences with congener vanadium(V)-peroxo-citrate complexes may point out potential implications in the chemistry of titanium with physiological ligands, when the former is present in a biologically relevant medium.

Insight into the mechanism of the asymmetric addition of alkyl groups to aldehydes catalyzed by titanium-BINOLate species.

The asymmetric addition of alkyl groups to aldehydes catalyzed by BINOLate-titanium complexes has become the testing grounds to evaluate the potential of new BINOL-based ligands. We have investigated the mechanism of this reaction and report our findings here. Model systems for the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), based on mono-oxygen-alkylated BINOL ligands have been examined. Comparison of the reactivity and enantioselectivity of the mono-alkyl BINOL derivatives with those of BINOL indicate that the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), is not active in the asymmetric addition reaction. Several BINOLate-titanium complexes have been synthesized and characterized by X-ray crystallography. These include the dinuclear (BINOLate)Ti(O-i-Pr)(2).Ti(O-i-Pr)(4), which contains a bridging naphtholate and isopropoxy group, trinuclear (BINOLate)Ti(O-i-Pr)(2).[Ti(O-i-Pr)(4)](2), and trimeric [(BINOL)Ti(O-i-Pr)(2)](3). The solid-state and solution structures reported here indicate that (BINOLate)Ti(O-i-Pr)(2) prefers to bind to titanium tetraisopropoxide rather than to itself, explaining why no nonlinear effects are observed in the catalytic reaction. Additionally, experimental evidence suggests that the BINOLate-titanium species responsible for the catalytic and stoichiometric asymmetric addition reactions are different, indicating that the proposed intermediate, (BINOLate)Ti(R)(aldehyde)(O-i-Pr), is not involved in either of these processes. Reactions were examined using different sources of the alkyl group [ZnMe(2) or MeTi(O-i-Pr)(3)]. Under similar conditions, it was found that the product ee's were the same, independent of whether ZnMe(2) or Me-Ti(O-i-Pr)(3) was used as the source of the alkyl groups. This indicates that the role of the dialkylzinc is not to add the alkyl group to the carbonyl but rather to transfer the alkyl group to titanium. On the basis of these results, we hypothesize that the intermediate in the asymmetric addition involves (BINOLate)Ti(O-i-Pr)(2)(aldehyde).MeTi(O-i-Pr)(3).

Dinuclear Ti IV and Ti III complexes supported by calix[4]arene ligands. Binding alkali-metal cations inside and outside the cavity of calix[4]arenes

New dinuclear Ti IV and Ti III complexes with the calix[4]arene ligand C 28H 20O 4H 4 (H 4L) have been isolated from the reaction of Ti(NMe 2) 4, H 4L, and Na (or KC 8) in THF. X-ray analyses revealed a similar core structure for the two complexes Na 4(THF) 8[Ti IV 2(μ-O) 2L 2] ( 1) and K 4(THF) 8[Ti III 2(μ-NMe 2) 2L 2] ( 2). Two titanium atoms are bridged by two oxygen atoms in 1 and by two dimethylamido groups in 2 and are also supported by two deprotonated calix[4]arene ligands in a cone conformation. This resulted in a similar Ti⋯Ti separation of about 3.29 Å in 1 and 3.28 Å in 2 and in a distorted octahedral environment for each Ti center in both complexes. In contrast, in a novel complex 3, Na 2(THF) 6[Ti III 2L 2], two Ti III atoms are supported only by two deprotonated ligands. This results in a five-coordinate environment for both titanium(III) centers with the separation between them being 3.133(1) Å.

Bis(tetramethylcyclopentadienyl)titanium Chemistry. Molecular Structures of [(C5HMe4)(μ-η1:η5-C5Me4)Ti]2and [(C5HMe4)2Ti]2N2

Thermolysis of bis(tetramethylcyclopentadienyl)-stabilized titanium(III) compounds (C(5)HMe(4))(2)TiR (R = Me (2), Ph (3)) yields, in marked contrast with the bis(pentamethylcyclopentadienyl) analog, the dimeric product [(C(5)HMe(4))(mu-eta(1):eta(5)-C(5)Me(4))Ti](2) (4), With a bridging metalated tetramethylcyclopentadienyl ligand. The hydride (C(5)HMe(4))(2)TiH (5), synthesized by hydrogenolysis of 2 or 3, reacts with N-2 to form the dinuclear Ti(II) dinitrogen compound [(C(5)HMe(4))(2)Ti]N-2(2) (8) Under a dynamic vacuum, the dinitrogen complex 8 loses the N-2 ligand to give the titanocene (C(5)HMe(4))(2)Ti (10). The molecular structures of both 4 and 8 were determined by X-ray diffraction methods.

ChemInform Abstract: Dimeric versus Monomeric Titanium(III) Siloxide Complexes. Syntheses and Characterization of ((c‐C6H11)7(Si7O12)Ti)2 and ((c‐C6H11)7(Si7O12)Ti(C5H5N))2.

AbstractThe extremely air‐sensitive dinuclear Ti(III) complex (III) and its pleochroic bis(pyridine) adduct (V) are synthesized starting with the trisilanol (I) as demonstrated in the scheme.

Zur Reaktion von [Cp2TiCl2] und [CpTiCl3] mit E(SiMe3)2 (E = S, Se) Die Kristallstrukturen von [Cp3Ti2S2Cl], [Cp4Ti2S2Cl2], [Cp3Ti2Se2Cl] und [Cp4Ti4Se7O] / Reaction of [Cp2TiCl2] and [CpTiCl3 ] with E(SiMe 3)2 (E = S. Se) The Crystal Structures of [Cp3Ti2S2Cl], [Cp3Ti2S2Cl2], [Cp3Ti2Se2Cl] and [Cp4Ti4Se7O

Abstract The reaction of [Cp2TiCl2 ] with E(SiMe3)2 leads to dinuclear Ti complexes. In [Cp3Ti2S2Cl] (1) and [Cp3Ti2Se2Cl] (3) two μ2 -S(Se) ligands bridge the Cp2Ti and CpTiCl units, respectively in contrast to these, [Cp4Ti2S2Cl2] (2) contains a μ2η1-S2 bridge connecting two Cp2TiCl fragments. A similar reaction of [CpTiCl3] with Se(SiMe3)2 leads to the tetranuclear cluster [Cp4Ti4Se7O] (4). 4 consists of a Ti4 -tetrahedron which encloses an oxygen atom.