Pyridylamido hafnium complexes with a silylene bridge: synthesis and olefin polymerization.

The synthesis and characterisation of six novel Cs-symmetric pyridylamido hafnium complexes with a silylene bridge of the type [ArPy(R2Si)NAr']HfAlk2 are reported. Four complexes have been structurally characterised using single crystal X-ray diffraction. Appreciable differences between the solid state structures of these complexes and the pyridylamido hafnium complexes with a CRR' bridge were noted. Reactions with B(C6F5)3, [Ph3C][B(C6F5)4] and [HMe2NPh][B(C6F5)4] yielded active catalysts for the homopolymerisations of propene and 1-hexene and ethene/1-octene copolymerization. In spite of the Cs-symmetry of the precatalysts, isotactically enriched polypropylene and poly(1-hexene) were obtained. The fact that the mechanism of the catalyst activation includes the insertion of alkene into the Hf-CAr bond was demonstrated. It was found that the structures of Ar and the R2Si bridge influence the activity, molecular weight capability and 1-octene affinity of the catalysts.

Reactivity of C1 -Symmetric Heteroarylamido Hafnium Complexes towards Unsaturated Electrophilic Molecules: Development of New Families of Olefin Polymerization Catalysts.

Heteroarylamido hafnium post-metallocenes with [C,N,N] ligands were functionalized by the insertion of small electrophilic unsaturated molecules into the CAr -Hf bond of the ligand, which gave rise to various 1,1- and 1,2-insertion complexes with the modified ligands of previously unknown [O,N,N], [N,N,N], [O,N,N,N], and [N,N,N,N] types. It was found that C1 symmetry of the starting complexes, in some cases, results in the formation of two diastereoisomers after 1,1- or 1,2-insertion. Most of the obtained novel complexes were shown to form active catalysts for olefin polymerization in the presence of MAO (methylaluminoxane). Thus, a new approach to the feasible and easy "late stage" modification of the precatalyst structure has been suggested; it can be performed immediately prior to the polymerization tests without isolation and even in a high-throughput manner.

Cationic zinc enolates as highly active catalysts for acrylate polymerization.

Unprecedented cationic zinc enolates have been generated by a novel activation route involving the amido to imino ligand transformation with B(C6F5)3, structurally characterized, and utilized as highly active catalysts for the production of high molecular weight polyacrylates at ambient temperature.

Rapid phosphorus triester hydrolysis catalyzed by bimetallic tetrabenzimidazole complexes.

Bimetallic complexes based on the binucleating ligand N,N,N',N'-tetrakis[(2-benzimidazolyl)methyl]-2-hydroxy-1,3-diaminopropane (1L) and its new toluoyl ester derivative (2L) catalyze the hydrolysis of phosphorus triesters at ambient temperature with activities rivalling the fastest known systems.

Dizinc alkoxides and amides supported by binucleating bis(amidoamine) ligands.

Several new dizinc complexes that are supported by dianionic bis(amidoamine) ligands are reported. Reaction of N,N'-bis(2-dimethylaminoethyl)dibenzofuran-4,6-diamine ((Me)LH(2)) with 2 equiv of EtZn(O(i)Pr) forms the dizinc bis(alkoxide) (Me)LZn2(O(i)Pr)2 (1), which was isolated in 76% yield. Similarly, (Me)LH2 reacts cleanly with EtZn(OPh) and EtZn(OCHPh2) to form (Me)LZn2(OPh)2 (2) and (Me)LZn2(OCHPh2)2 (3), respectively. The solid-state structures of 1 and 2 feature puckered [Zn2(mu-OR)2]2+ cores, with short intermetal separations (2.81-2.88 Angstroms). Overall, the molecules have approximate (noncrystallographic) C2v symmetry. The use of the more-hindered (i)Pr-substituted ligand N,N'-bis(2-diisopropylaminoethyl)dibenzofuran-4,6-diamine (i(Pr)LH2) to prepare zinc alkoxides gave similar results. Thus, reaction of i(Pr)LH2 with 2 equiv of EtZn(OPh), EtZn(OMe), EtZn(OCHPh2), and EtZn(OCH2Ph) forms i(Pr)LZn2(OPh)2 (4), i(Pr)LZn2(OMe)2 (5), i(Pr)LZn2(OCHPh2)2 (6), and i(Pr)LZn2(OCH2Ph)2 (7), respectively (isolated yields 48-63%). At 70 degrees C, C6D6 solutions of 6 undergo beta-hydride transfer with 2 equiv of benzaldehyde to form 7 and benzophenone in quantitative yield (according to 1H NMR spectroscopy). Benzene solutions of 1 react with 1 equiv of trimethylsilyl trifluoromethanesulfonate (Me3SiOTf) to form (Me)LZn2(O(i)Pr)(OTf) (8) in 70% isolated yield. In the solid state, 8 features a bridging alkoxide donor as well as a 1,3-bridging triflate group. The previously reported dinuclear organozinc species (Me)LZn2Ph2 (9) reacts with 1 equiv of tert-butylamine to form the protonolysis product (Me)LZn2(Ph)(NH(t)Bu) (10) in 66% isolated yield. The solid-state structure of 10 (two independent molecules) reveals a somewhat asymmetric [Zn2(mu-Ph)(mu-NH(t)Bu)]2+ core with short Zn-Zn separations [2.6761(5) and 2.6518(5) Angstroms]. In CD2Cl2 solution, the Ph bridge of 10 undergoes rapid reversible cleavage. Cleavage of this bridging interaction followed by rotation about the Zn-Ph bond and re-formation of the bridging interaction results in exchange of the inequivalent ortho (and meta) protons of the phenyl ligand. Variable-temperature 1H NMR spectroscopic data indicate that this exchange occurs with DeltaG = 12.7(1) kcal.mol(-1) (-27 degrees C). At 75 degrees C, toluene solutions of (Me)LH2 react with 2 equiv of EtZnNH(t)Bu to form the dizinc bis(amido) product (Me)LZn2(NH(t)Bu)2 (11) in 46% isolated yield. The solid-state structure of 11 (two independent molecules) features a puckered and fairly symmetric [Zn2(mu-NH(t)Bu)2]2+ core with short intermetal separations [2.775(1), 2.760(1) Angstroms].

Doxazolidine, a proposed active metabolite of doxorubicin that cross-links DNA.

A crystal structure establishes doxoform as a dimeric formaldehyde conjugate of the oxazolidine of doxorubicin. Doxoform is a prodrug of doxazolidine, a monomeric doxorubicin formaldehyde-oxazolidine. Both doxoform and doxazolidine inhibit the growth of cancer cells at 1-4 orders of magnitude lower concentration than doxorubicin. They also inhibit the growth of cancer cells better than doxsaliform, a prodrug for an acyclic doxorubicin-formaldehyde conjugate. Doxoform rapidly hydrolyzes to doxazolidine, which then hydrolyzes to doxorubicin with a half-life of 3 min in human serum at 37 degrees C. Both doxoform and doxazolidine are taken up by multidrug-resistant MCF-7/Adr cells 3- to 4-fold better than doxorubicin. A molecular model suggests that doxazolidine can cross-link DNA by direct reaction with a G-base in a tautomeric form with synchronous ring opening of the oxazolidine. These results point to doxoform being a prodrug for doxazolidine that is the reactive species that directly cross-links DNA.

The direct alpha-zincation of amides, phosphonates and phosphine oxides by H-Zn exchange.

Stoichiometric or catalytic quantities of simple 2 degrees amines greatly increase the rate of H-Zn exchange between ZnPh2 and a range of relatively non-acidic substrates, allowing for the convenient and direct preparation of alpha-functionalized organozincs.

Dititanium complexes of preorganized binucleating bis(amidinates).

Four different dianionic bis(amidinate) ligands ((iPr)L(DBF)(2)(-), (tBu,Et)L(DBF)(2)(-), (iPr)L(Xan)(2)(-), (tBu,Et)L(Xan)(2)(-)) featuring rigid dibenzofuran (DBF) and 9,9-dimethylxanthene (Xan) backbones have been used to prepare several new dititanium complexes. Reaction of the free-base bis(amidines) (LH(2)) with 2 equiv of Ti(NMe(2))(4) forms the hexaamido derivatives (iPr)L(DBF)Ti(2)(NMe(2))(6) (1), (tBu,Et)L(DBF)Ti(2)(NMe(2))(6) (2), (iPr)L(Xan)Ti(2)(NMe(2))(6) (3), and (tBu,Et)L(Xan)Ti(2)(NMe(2))(6) (4) in good yields. Compound 4, which features an unsymmetrically substituted bis(amidinate) ligand, was isolated as an 8:1 mixture of rotational diastereomers with C(2) and C(s)() symmetry, respectively. The two diastereomers interconvert upon heating, and at equilibrium the C(2) isomer is preferred thermodynamically by 0.2 kcal/mol. Compound 3 reacts with excess Me(3)SiCl in toluene to form the mixed amido-chloride derivative (iPr)L(Xan)Ti(2)(NMe(2))(2)Cl(4) (5) in low-moderate yield. Alternatively, 5 is also prepared by reaction of (iPr)L(Xan)H(2) with 2 equiv of Ti(NMe(2))(2)Cl(2) in good yield. Compound 3 reacts with CO(2) to form the red carbamate derivative (iPr)L(Xan)Ti(2)(NMe(2))(4)(O(2)CNMe(2))(2) (6) in moderate yield. Infrared data for 6 indicates bidentate coordination of the carbamate ligands. Metathesis reaction of (iPr)L(Xan)Li(2) with 2 equiv of CpTiCl(3) affords (iPr)L(Xan)Ti(2)Cp(2)Cl(4) (7) in moderate yield. Reduction of 7 with 1% Na amalgam in toluene solution affords the paramagnetic dititanium(III) complex (iPr)L(Xan)Ti(2)Cp(2)Cl(2) (8) in good yield. Structural studies reveal that 8 features two bridging chloride ligands. Reaction of the free-base bis(amidines) with 2 equiv of CpTiMe(3) forms the red sigma-alkyl derivatives (iPr)L(DBF)Ti(2)Cp(2)Me(4) (9), (tBu,Et)L(DBF)Ti(2)Cp(2)Me(4) (10), and (iPr)L(Xan)Ti(2)Cp(2)Me(4) (11) in good yields. Structural data are presented for compounds 4, 5, 8, and 9.

Synthesis and characterization of binucleating bis(amidinate) ligands and their dialuminum complexes.

Two general routes to binucleating bis(amidinate) ligands based on dibenzofuran and 9,9-dimethylxanthene backbones are reported. The free-base form of one of the ligands, (Ph,Mes)L(DBF)H(2), forms a 1:1 adduct with acetone. Single-crystal X-ray diffraction of this adduct reveals bidentate H-bonding of the bis(amidine) to the ketone oxygen. Bond lengths suggest that the individual H-bonds are relatively weak, yet IR spectroscopy shows a significant -26 cm(-1) shift for the carbonyl stretch relative to free acetone. Additionally, the new dialuminum complexes (i)(Pr)L(DBF)Al(2)Me(4) (3), (i)(Pr)L(Xan)Al(2)Me(4) (4), (t)(Bu,Et)L(DBF)Al(2)Me(4) (5), and (t)(Bu,Et)L(Xan)Al(2)Me(4) (6) are prepared by reaction of Al(2)Me(6) with the bis(amidines) in toluene solution. (1)H NMR spectroscopic studies indicate that 3 and 4 interact weakly with certain Lewis bases (DMSO, DMF, pyridine) to effect the exchange of the Al-bound Me groups. Other bases, such as THF and TMEDA, fail to interact. Solid-state structures for 3 and 4 are reported.

New dibenzofuran-bridged bis(amidoamine) and bis(ethylenediamine) ligands and their dinuclear zinc and aluminium complexes.

A new class of dibenzofuran-bridged bis(amidoamine) and bis(ethylenediamine) ligands are used to prepare structurally-characterized dinuclear zinc and aluminium complexes.

Tethered Bis-Amidinates as Supporting Ligands: A Concerted Elimination/σ-π Rearrangement Reaction Forming an Unusual Titanium Arene Complex.

A 1,4-cyclohexadiene dianion resonance structure makes a significant contribution to the structure of the (arene)Ti complex [LTi(η6 -PhCH3 )] (see picture on the right). Labeling experiments have demonstrated that the structure forms through an unusual elimination/σ-π rearrangement reaction in which [LTi(CH2 Ph)2 ] reacts readily with hydrogen; the use of the cyclohexane-linked bis-amidinate ligand L with constrained geometry makes the transformation possible.