Coordination chemistry of amine bis(phenolate) titanium complexes: tuning complex type and structure by ligand modification

Niobium Complexes Supported by Chalcogen-Bridged [OEO]-Type Bis(phenolate) Ligands (E = S, Se): Synthesis, Characterization, and Phenylacetylene Polymerization

Trichloro niobium(V) complexes 3 and 4 with the sulfur- or selenium-bridged [OEO]-type bis(phenolate) ligands (E = S, Se) were synthesized and fully characterized on the basis of their NMR spectroscopic data and X-ray crystallographic analysis. In the crystalline state of 4, the [OSeO]-core of the ligand was coordinated to the niobium center in a fac-fashion. The corresponding tribenzyl niobium(V) complexes 5 and 6 were also prepared by the reactions of 3 and 4 with 3 equivalents of PhCH2MgCl in toluene. The X-ray diffraction analysis of 6 revealed that the distorted six-coordinated niobium center incorporated in the [OSeO]-type ligand took a mer-fashion, and one benzyl ligand was coordinated to the niobium center by η2-fashion. Complexes 5 and 6 were tested for the phenylacetylene polymerization that produced poly(phenylacetylene)s (PPAs), oligomers, and triphenylbenzenes (TPBs) depending on the chalcogen atom in the [OEO]-type ligand.

Cytotoxic homoleptic Ti(iv) compounds of ONO-type ligands: synthesis, structures and anti-cancer activity

Reacting variously substituted dianionic tridentate ONO-type acylhydrazone ligands with titanium(iv) tetra(isopropoxide) gave a new class of eight homoleptic titanium(iv) compounds showing exceptional stability and promising cytotoxicity.

Aggregated initiators: defining their role in the ROP of rac-lactide

Reported examples of aggregated initiators for the ring-opening polymerisation (ROP) of lactide often lack detailed investigations as to the nature of the active species, making it difficult to reconcile ligand design with performance. Here, we offer additional stability to the polynuclear titanium complexes, TiL(OiPr) (L = 9-14), through a bridging carboxylate anchored to the supporting amine bis(phenolate) ligands. An in-depth study of solution-state behaviour determined the process of assembly was driven by interactions between the carboxylate and a vacant site on a neighbouring titanium centre. Furthermore, we establish that mononuclear units form dynamic mixtures of polynuclear aggregates, with a clear relationship between nuclearity of the aggregates and the steric bulk on the ligand. Smaller aggregates displayed increased activity towards the ROP of rac-lactide. Furthermore, addition of a chiral centre, on the ligand framework, was investigated as a route to influence the selectivity of the polymerisation via easily-accessible initiators.

Well-defined Ti4 pre-catalysts for the ring-opening polymerisation of lactide

The synthesis and full characterisation of four discrete tetrametallic titanium complexes is reported. These well-defined compounds are isostructural in the solid state and share the same general formula: Ti₄(μ-O)₂L₄ (L = 1, 2, 3 or 4). Using a combination of NMR techniques the complexes are found to be stable in solution, even at elevated temperatures. Further studies show that the carboxylate moieties of the supporting amine bis(phenolate) ligands can be displaced by a more strongly coordinating solvent. This reversible process causes the coordinatively saturated Ti₄(μ-O)₂L₄ complexes to separate into two Ti₂(μ-O)L₂ subunits which we envisaged would be catalytically active. Proof-of-concept experiments establish that all four of these complexes display catalytic activity in the ring-opening polymerisation of rac-lactide. These aggregates can therefore be viewed as air and moisture stable pre-catalysts for a range of reactions.

Aluminum complexes containing biphenolate phosphine ligands: synthesis and living ring-opening polymerization catalysis

This report describes the synthesis, structure, and reactivity of aluminum complexes containing tridentate biphenolate phosphine ligands of the type [RP(2-O-3,5-C₆H₂tBu₂)₂]^2- (R = tBu (2a), Ph (2b)). Alkane elimination of AlMe₃ with one equiv. of H₂[2a] or H₂[2b] in THF at 0 °C cleanly affords colorless crystalline [2a]AlMe(THF) (3a) and [2b]AlMe(THF) (3b), respectively. An X-ray diffraction study of 3a showed it to be a five-coordinate THF-bound species, whose coordination geometry is best described as trigonal bipyramidal, having the phosphorus donor and THF at axial positions. Treatment of either in situ prepared or isolated methyl complexes 3a,b with one equiv. of benzyl alcohol in toluene or THF generated their corresponding benzyloxides {[2a,b]Al(μ₂-OCH₂Ph)}₂ (4a,b). An X-ray diffraction study of 4a revealed a dimeric structure, in which the coordination geometry of aluminum is also distorted trigonal bipyramidal with the tridentate 2a ligand being facially bound. In the presence of one equiv. of benzyl alcohol, complex 3a is a competent catalyst precursor for the living ring-opening polymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA), producing poly(ε-caprolactone) and poly(rac-lactide), respectively, in a controlled manner. As such, well-defined block copolymers of ε-CL with rac-LA can also be prepared by catalytic 3a. Kinetic studies revealed that 3a catalyzes the polymerization of rac-lactide at a rate 2-fold faster than that of 3b, indicating the significance of the P-substituent effect on this catalysis. Interestingly, the polymerization rate of rac-lactide by 3a is 16.5 times faster than that of l-lactide under otherwise identical conditions.

Heptacoordinate Heteroleptic Salan (ONNO) and Thiosalan (OSSO) Titanium(IV) Complexes: Investigation of Stability and Cytotoxicity

Seven heptacoordinate titanium(IV) complexes were synthesized based on the concept of hetero-bis-chelate stabilization of salan (ONNO) and thiosalan (OSSO) titanium(IV)alkoxides with 2,6-pyridinedicarboxylic acid (dipic) and derivatives thereof. The resulting compounds were investigated in a solid by X-ray diffraction and in solution by NMR spectroscopy. A thiosalan (OSSO) titanium(IV) complex could be isolated and its conformational stabilization by dipic was shown by (1)H NMR spectroscopy to lead to nonfluxional behavior even at room temperature. The stability of selected complexes was assessed at pH 1.9, 6.8, and 12.1 by an UV-vis monitored hydrolysis study with >5 Mio. equivalents of water. Even at pH 12.1 [L(1)Ti(dipic)(1)] showed t1/2 of more than 2 days. The cytotoxicity of all compounds was investigated in two human carcinoma cell lines. IC50-values in the range of cisplatin were achieved by all tested compounds except for [L(4)Ti(dipic)(1)], which was proven to be nontoxic. The functionalization of dipic was thus well tolerated and did neither interfere with the stability nor the cytotoxicity of the heteroleptic complexes.

Bis(phenolate)amine-supported lanthanide borohydride complexes for styrene and trans-1,4-isoprene (co-)polymerisations

New bis(phenolate)amine-supported neodymium borohydride complexes and their previously reported samarium analogues were tested as catalysts for the polymerisation of styrene and isoprene.

Zirconium complexes stabilized by amine-bridged bis(phenolato) ligands as precatalysts for intermolecular hydroamination reactions

Pentacoordinate zirconium complexes 5 and 7 stabilized by amine-bridged bis(phenolato) ligands are more active than hexacoordinate complexes 1–4 in catalyzing intermolecular hydroamination reactions.

Toward the understanding of radical reactions: experimental and computational studies of titanium(III) diamine bis(phenolate) complexes

Radical reactions of titanium(III) [Ti((tBu2)O2NN')Cl(S)] (S = THF, 1; S = py, 2; (tBu2)O2NN' = Me2N(CH2)2N(CH2-2-O-3,5-(t)Bu2C6H2)2) are described. Reactions with neutral electron acceptors led to metal oxidation to Ti(IV), [Ti((tBu2)O2NN')Cl(TEMPO)] (4) being formed with the TEMPO radical and [Ti((tBu2)O2NN')Cl2] (9) with PhN═NPh. [Ti((tBu2)O2NN')Cl2] was also formed when [Ti((tBu2)O2NN')Cl(S)] was oxidized by [Cp2Fe][BPh4], but the [Cp2Fe][PF6] analogue yielded [Ti((tBu2)O2NN')ClF] (8). The reactions of [Ti((tBu2)O2NN')Cl(S)] with O2 gave [Ti((tBu2)O2NN')Cl]2(μ-O) (3). The DFT calculated Gibbs energy for the above reaction showed it to be exergonic (ΔG298 = -123.6 kcal·mol(-1)). [Ti((tBu2)O2NN')(CH2Ph)(S)] (S = THF, 5; py, 6) are not stable in solution for long periods and in diethyl ether gave 1:1 cocrystals of [Ti((tBu2)O2NN')(CH2Ph)2] (7) and [Ti((tBu2)O2NN')Cl]2(μ-O) (3), most probably resulting from a disproportionation process of titanium(III) followed by oxygen abstraction by the resulting Ti(II) species. The oxidation of [Ti((tBu2)O2NN')(κ(2)-{CH2-2-(NMe2)-C6H4})] (10), which is a Ti(III) benzyl stabilized by the intramolecular coordination of the NMe2 moiety, led to a complex mixture. Recrystallization of this mixture under air led to a 1:1 cocrystal of two coordination isomers of the titanium oxo dimer (3). In one of these isomers, one metal is pentacoordinate and the dimethylamine moiety of the diamine bis(phenolate) ligand is not bonded to the metal, displaying a coordination mode of the ligand never observed before. The other titanium center is distorted octahedral with two cis-phenolate moieties. In the second unit, the coordination of the two ancillary ligands to the titanium centers reveals mutually cis-phenolate groups in one-half of the molecule and trans-coordinated in the other titanium center, keeping a distorted octahedral environment around each titanium.