Synthesis, Characterization, and Catalytic Exploration of Mononuclear Mo(VI) Dioxido Complexes of (Z)-1-R-2-(4',4'-Dimethyl-2'-oxazolin-2'-yl)-eth-1-en-1-ates.

The coordination chemistry of the title ligands with Mo metal centers was investigated. Thus, the synthesis and characterization (NMR, X-ray diffraction) of four mononuclear formally Mo(6+) complexes of (Z)-1-R-2-(4',4'-dimethyl-2'-oxazolin-2'-yl)-eth-1-en-1-ates (L: R = -Ph, -Ph-p-NO2, -Ph-p-OMe and -t-Bu), derived from the part enols (LH), is described. The resulting air-stable MoO2L2 complexes (1-4) exist, as shown by single-crystal X-ray diffraction experiments, in the cis-dioxido-trans(N)-κ2-N,O-L conformation in the solid state for all four examples. This situation was further probed using semi-empirical PM6(tm) calculations. Complexes 1-4 represent the first Mo complexes of this ligand class and, indeed, of Group 6 metals in general. Structural and spectroscopic comparisons were made between these and related Mo(6+) compounds. Complex 1 (R = -Ph) was studied for its ability to selectively catalyze the production of poly-norbornene from the monomer in the presence of MAO. This, unfortunately, only resulted in the synthesis of insoluble, presumably highly cross-linked, polymeric and/or oligomeric materials. However, complexes 1-4 were demonstrated to be highly effective for catalyzing benzoin to benzil conversion using DMSO as the O-transfer agent. This catalysis work is likewise put into perspective with respect to analogous Mo(6+) complexes.

Replacing the Z-phenyl Ring in Tamoxifen® with a para-Connected NCN Pincer-Pt-Cl Grouping by Post-Modification †.

Post-modification of a series of NCN-pincer platinum(II) complexes [PtX(NCN-R-4)] (NCN = [C6H2(CH2NMe2)2-2,6]-, R = C(O)H, C(O)Me and C(O)Et), X = Cl- or Br-) at the para-position using the McMurry reaction was studied. The synthetic route towards two new [PtCl(NCN-R-4)] (R = C(O)Me and C(O)Et) complexes used above is likewise described. The utility and limitations of the McMurry reaction involving these pincer complexes was systematically evaluated. The predicted "homo-coupling" reaction of [PtBr(NCN-C(O)H-4)] led to the unexpected formation of 3,3',5,5'-tetra[(dimethylamino)methyl]-4,4'-bis(platinum halide)-benzophenone (halide = Br or Cl), referred to hereafter as the bispincer-benzophenone complex 13. This material was further characterized using X-ray crystal structure determination. The applicability of the pincer complexes in the McMurry reaction is shown to open a route towards the synthesis of tamoxifen-type derivatives of which one phenyl ring of Tamoxifen® itself is replaced by an NCN arylplatinum pincer fragment. The newly synthesized derivatives can be used as potential candidates in anti-cancer drug screening protocols. Two NCN-arylpincer platinum tamoxifen type derivatives, 5 and 6, were successfully synthesized and of 5 the separation of the diastereomeric E-/Z-forms was achieved. Compound 6, which is the pivaloyl protected NCN pincer platinum hydroxy-Tamoxifen® derivative, was obtained as a mixture of E-/Z-isomers. The new derivatives were further analyzed and characterized with 1H-, 13C{1H}- and 195Pt{1H}-NMR, IR, exact mass MS and elemental analysis.

On the Electronic Nature of Bis((Z)-1-(benzo[d]oxazol-2-yl)-3.3.3-trifluoroprop-1-en-2-ate)palladium.

The electronic nature of the recently reported complex, bis((Z)-1-(benzo[d]oxazol-2-yl)-3.3.3-trifluoroprop-1-en-2-ate)palladium, is re-investigated by a combination of spectroscopy (NMR, IR, magnetic moment, etc.) and Density Functional Theory (DFT: B3LYP 6-31G*/LANL2DZ). In contrast to the recent report, the title complex displays all the properties of diamagnetism and hence retains the properties of a formally Pd(II) square planar complex with a bis-κ2-N,O-donor ligand set. A modified synthetic route is also presented which improves the yield of the compound.

Proof of Concept Studies Directed Towards Designed Molecular Wires: Property-Driven Synthesis of Air and Moisture-Stable Polystannanes.

Polystannanes with azobenzene moieties designed to protect the Sn-Sn backbone from light- and moisture-induced degradation are described. The azo-stannyl precursor 3 (70 %) is converted in good yields (88-91 %) to the mono- (4), and dichlorostannanes (5), by sequential chlorination, followed by further reduction of 5 to the dihydride (6) using NaBH4 (78 %). All stannanes were characterised by NMR (1 H, 13 C, 119 Sn) spectroscopy and HRMS; in addition, 3, 4 and 5 were structurally elucidated using X-ray diffraction analysis. Metal-free dehydrocoupling of 6 at RT leads exclusively to homopolymer (7-i) displaying an initial solution 119 Sn NMR signal (δ=-196 ppm) that migrates to -235 ppm after 10 days (7-f). In contrast, metal-catalyzed dehydrocoupling of 6 in toluene at RT leads directly 7-f. Random co-polymers formed from 6 and (nBu)2 SnH2 at 4:1 (8 a) and 1:1 (8 b) ratios were compared to the alternating polystannane (9) prepared by the reaction of 6 with (nBu)2 Sn(NEt2 )2 . DFT calculations of 3-6 indicate that hypercoordination at Sn is influenced by substituents and by solvation. Homopolymer 7 was found to have unprecedented moisture and light stability in the solid state for >6 months.

Tautomerism and metal complexation of 2-acylmethyl-2-oxazolines: a combined synthetic, spectroscopic, crystallographic and theoretical treatment.

A synthetic, structural and theoretical investigation into the solid-state, solution and gas phase structure(s) of six 2-acylmethyl-4,4-dimethyl-2-oxazolines is reported. Four of these materials, viz.α-[(4,5-dihydro-4,4-dimethyl-2-oxazolyl)methylene]benzenemethanol (3a), α-[(4,5-dihydro-4,4-dimethyl-2-oxazolyl)methylene]-(4-nitrobenzene)methanol (3b), 1-(4,5-dihydro-4,4-dimethyl-2-oxazolyl)-3,3-dimethyl-1-buten-2-ol (3d) and (E)-1-phenyl-2-((3aR)-3,3a,8,8a-tetrahydro-2H-indeno[1,2-d]oxazol-2-ylidene)ethanone (3f) have been characterised in the solid-state by single crystal X-ray diffraction studies. These data represent the first solid-state structural studies of this class of compounds and details the first synthesis and full characterisation of chiral derivative 3f. All four of these materials are shown to exist in the solid phase in the enamine tautomeric form (e.g., 3a is best described as 2-[4,4-dimethyl-2-oxazolidinylidene]-1-phenylethanone) and it is suggested (NMR, IR) that this isomeric form is likely also retained in solution (e.g., CDCl3) as the more stable isomer. An investigation of the relative gas phase stabilities of the three possible (i.e., the (Z)-enol, keto and enamine) isomers of all five compounds by DFT at the B3LYP/6-311G(d) level of theory confirms the latter as the most stable form. The energy differences between the enamine and keto tautomers have been calculated to be the lowest for derivative 3d. These results are compared and contrasted with the previously reported NMR studies of such compounds which have identified the keto form as being a minor (albeit solution) tautomer. Equilibrium solution tautomer distributions for 3d are found to be solvent dependent. The protonated form of 3a, isolated as the HSO4(-) salt (i.e.4a), has been further characterised in the solid state by single crystal X-ray diffraction. These data represent the first example of a protonated oxazoline to be structurally elucidated and confirms that upon protonation, the keto (oxazoline) tautomer is the energetically favoured form in the solid-state. This observation is further supported by DFT studies for the gas phase protonated forms of such materials. Further DFT (B3LYP/6-311G(d)) calculations employing the SM8 or SMD solvation models were then applied to address the observed solution isomeric distribution for 3d; these results corroborate the gas phase theoretical treatment and also yield values that predict the higher solution stability of the enamine form as observed, although they fail to account for the existence of the keto form as a minor solution state tautomer. To access the availability of an enol-form, via hypothetical de-protonation to the enolate, compound 3a was treated with hydrated Cu(NO3)2 in EtOH solution. The resulting isolated green-coloured product (5), the first metal derivative of this entire class of ligands, is best described (IR, X-ray diffraction) as a coordinated enolate complex, i.e., Cu(3a-H)2. Complex 5 crystallizes in the P21/c space group with four molecules in the unit cell. The coordination geometry around the formal Cu(2+) metal centre is determined to be highly distorted square planar in nature (τ4 = 0.442). TD-DFT is used to give a reasonable explanation for the intensity of the absorbance band observed in the visible region for solutions of 5. These latter experiments strongly suggest that the title class of compounds may have considerable potential as ligands in coordination chemistry and/or metal-mediated catalysis.

Synthesis and biological activities of 4-N-(anilinyl-n-[oxazolyl])-7-chloroquinolines (n=3' or 4') against Plasmodium falciparum in in vitro models.

The synthesis (Pd-mediated coupling strategy) and characterization (NMR, IR, elemental analysis, etc.) of a short series of quinoline-oxazole hybrid compounds has been carried out. These materials are found to be moderately active against Plasmodium falciparum in vitro, with activities in the sub-micromolar range, and to display acceptable cytotoxicity to mononuclear leukocytes. Chemical modification strategies, with the intention to increase the biological potency of this new class of anti-malarial agents, are discussed.

Unusual cationic trinuclear nickel clusters incorporating oxazolines or N,N,N',N'-tetramethylethylene-1,2-diamine: applications in olefin polymerization.

The synthesis and characterization of two rare examples of the nickel(II)-containing trinuclear clusters of the general formula µ(3)-halido-µ(3)-hydroxotris(µ-halido)tris(L) trinickel(II) halide [halide = Cl (2), Br (3); L = 4,4-dimethyl-2-(o-anilinyl)-2-oxazoline] are described. These materials are compared and contrasted to the "parent" chloride salt (1) of this series (L = N,N,N',N'-tetramethylethylene-1,2-diamine and halide = Cl) and its congeners; 2 and 3 represent the first oxazoline-containing clusters of this structural class. Both 1 and 2 are shown to be active catalysts for the polymerization of olefins (styrene, methyl methacrylate, etc.) using a stoichiometric equivalent of methylaluminoxane as the copromoter, a situation that gives good yields of syndiotactic rich polymers. Density functional theory (B3LYP/6-31G*/LANL2DZ) is employed to hypothesize the likely origin of the activity demonstrated by these compounds.

Synthesis and characterisation of κ2-N,O-oxazoline-enolate complexes of nickel(ii): explorations in coordination chemistry and metal-mediated polymerisation.

The synthesis and characterisation (UV-Vis, IR, X-ray diffraction, etc.) of a series of Ni(ii) complexes derived from both known and novel 2-acylmethyl-2-oxazolines (2a-g: i.e., (Z)-1-R-2-(4,4'-dimethyl-2'-oxazolin-2'-yl)eth-1-en-1-ol; R = -Ph, -2-furanyl, -p-NO2-Ph, -t-Bu, -2-thiofuranyl, p-NC-Ph, -CF3) is reported. These Ni materials (3a-g) represent the first group 10 metal complexes of this ligand class. All derivatives reported are paramagnetic (S = 1) compounds of formulae Ni(κ2-N,O-L)2 where L represents an enolate of structure (Z)-1-R-2-(4',4'-dimethyl-2'-oxazolin-2'-yl)eth-1-en-1-ate formed via proton loss from 2. The air- and moisture-stable metal complexes feature a less typical distorted seesaw-shaped disposition of binding atoms around the metal centre for six structurally characterised (X-ray) examples. Preliminary investigations indicate that 3a (R = -Ph) is a useful catalysts for olefin polymerisation in the presence of alkylaluminum reagents.

The first oxazoline adduct of Zn(acac)(2): bis-(acetyl-acetonato-κO,O')(2-phenyl-2-oxazoline-κN)zinc(II).

The title material, [Zn(C(5)H(7)O(2))(2)(C(9)H(9)NO)], was synthesized by the treatment of bis-(acetyl-acetonato)zinc(II) monohydrate with 2-phenyl-2-oxazoline. The Zn atom is coordinated by two chelating acetyl-acetonate groups and one oxazoline ligand in the apical position of a slightly distorted square-pyramidal metal-ligand geometry.

Enhanced product selectivity promoted by remote metal coordination in acceptor-free alcohol dehydrogenation catalysis.

A bimetallic [Ir(3+)]2 complex was synthesized based on a bridging 1,2,3-triazole ligand that coordinates to one Cp*Ir unit as N,N-bidentate chelate, and to the other as a C,C-bidentate ligand. When compared to monometallic homologues, the bimetallic complex shows greatly enhanced product selectivity for the acceptorless dehydrogenation of alcohols; spectroscopic and electrochemical analysis suggest significant alteration of the metal properties in the bimetallic system compared to the monometallic species, which offers a rationale for the observed high selectivity.

Transfer hydrogenation with abnormal dicarbene rhodium(iii) complexes containing ancillary and modular poly-pyridine ligands.

Treatment of an abnormal dicarbene ligated rhodium(iii) dimer with 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen) or 2,2':6',2''-terpyridine (terpy) results in coordination of the N-donor ligands and concomitant cleavage of the dimeric structure. Depending on the denticity of the pyridyl ligand, this situation retains one (L = terpy) or two (L = bipy, phen) flexible sites for substrate coordination. In the case of the bipy complexes, modification of the electron density at Rh, without directly affecting the steric environment about the metal centre, was achieved by the incorporation of electron-donating or electron-withdrawing substituents on the bipy backbone. The dicarbene pyridyl complexes were active in transfer hydrogenation catalysis of benzophenone at 0.15 mol% catalyst loading in a iPrOH/KOH mixture. The catalysts displayed a strong characteristic colour change (yellow to purple) after activation which allowed for visual monitoring of the status of the reaction. The colour probe and the robustness of the active catalysts proved useful for catalyst recycling. The catalytic activity sustained over five consecutive substrate batch additions and gave a maximum overall turnover number of 3100.

Synthesis and catalytic applications of 1,2,3-triazolylidene gold(i) complexes in silver-free oxazoline syntheses and C-H bond activation.

A series of novel 1,2,3-triazolylidene gold(i) chloride complexes have been synthesised and fully characterised. Silver-free methodologies for chloride ion abstraction of these complexes were evaluated for their potential as Au-based catalyst precursors. Using simple potassium salts or MeOTf as chloride scavengers produced metal complexes that catalyse both the regioselective synthesis of oxazolines and the C-H activation of benzene or styrene for carbene transfer from ethyl diazoacetate. These results indicate that Ag-free activation of 1,2,3-triazolylidene gold(i) chloride complexes is feasible for the generation of catalytically active Au triazolylidene species. However, silver-mediated activation imparts substantially higher catalytic activity in oxazoline synthesis.

Hetero-aggregate compounds of aryl and alkyl lithium reagents: a structurally intriguing aspect of organolithium chemistry.

Organolithium compounds are often depicted as mononuclear species. However, such compounds are in fact aggregated species and can form hetero-aggregates containing different organic groups, including heteroatom groups. In reactions involving organolithium reagents, the "pure" homo-aggregate organolithium compound can change into a hetero-aggregate, which has a different structure and reactivity to the homo-aggregate. This fact is often overlooked. When there are chiral centers in the organolithium reagent or the substrate, diastereoselective self-assembly (the preferential formation of a particular diastereoisomeric aggregate) plays a role. The importance of these contributions in understanding the structure and reactivity patterns of organolithium reagents is the focus of this Minireview.

Synthesis and characterisation of the first transition metal complex of zoxazolamine (2-amino-5-chlorobenzoxazole): the X-ray crystal structure determination of [ZnCl2(eta 1-Nbenzoxazole-2-amino-5-chlorobenzoxazole)2].

The synthesis and characterisation (NMR, X-ray, elemental analysis) of the first transition metal complex of Zoxazolamine (1: 2-amino-5-chlorobenzoxazole), viz. [ZnCl(2)(1)(2)] (2) is described; complex 2 is obtained in 77% yield from the treatment of 1 with ZnCl(2) in acetone solution. The Zn compound is a mononuclear species (X-ray) with a distorted tetrahedral array of ligands around the metal centre with the title ligand bound to Zn via the benzoxazole ring N-atom. The structural properties of 2 are discussed in relation to other mononuclear Zn halide complexes.

(Phosphinoalkyl)silanes. 4.(1) Hydrozirconation as a Non-Photochemical Route to (Phosphinopropyl)silanes: Facile Assembly of the Bis(3-(diphenylphosphino)propyl)silyl ("biPSi") Ligand Framework. Access to the Related Poly(3-(dimethylsilyl)propyl)phosphines R(n)()P(CH(2)CH(2)CH(2)SiMe(2)H)(3)(-)(n)() (n = 1, R = Ph; n = 0).

Treatment of SiEt(3)(CH=CH(2)) with ZrCp(2)HCl (Schwartz's reagent) followed by reaction with PPh(2)Cl provides a high-yield (75%) route to Ph(2)PCH(2)CH(2)SiEt(3), and accordingly hydrozirconation of CH(2)=CHCH(2)SiHMe(2) affords the intermediate ZrCp(2)(CH(2)CH(2)CH(2)SiHMe(2))Cl (2). The latter, which is very sensitive to hydrolysis and reacts with HCl forming SiHMe(2)Pr(n)() and with NBS or I(2) affording SiHMe(2)CH(2)CH(2)CH(2)X (X = Br (3), I (4)), behaves similarly with PPh(2)Cl, PPhCl(2), or PBr(3) undergoing cleavage to the known Ph(2)PCH(2)CH(2)CH(2)SiMe(2)H (i.e. chelH, A) and the novel bis- and tris(silylpropyl)phosphines PhP(CH(2)CH(2)CH(2)SiMe(2)H)(2) (5) and P(CH(2)CH(2)CH(2)SiMe(2)H)(3) (6), respectively, with concomitant formation of ZrCp(2)Cl(2). Corresponding hydroboration of allylsilanes is facile, but subsequent phosphine halide cleavage yields (phosphinoalkyl)silanes only as constituents of intractable mixtures. Hydrozirconation followed by phosphination with PPh(2)Cl also converts SiHMe(CH(2)CH=CH(2))(2) to SiHMe(CH(2)CH(2)CH(2)PPh(2))(2) (i.e. biPSiH, B) together with a propyl analogue Ph(2)PCH(2)CH(2)CH(2)SiMe(Pr(n)())H (7) of A (ca. 2:1 ratio), as well as SiH(CH(2)CH=CH(2))(3) to a mixture (ca. 5:2:1 ratio) of SiH(CH(2)CH(2)CH(2)PPh(2))(3) (i.e. triPSiH, C), a new analogue SiH(Pr(n)())(CH(2)CH(2)CH(2)PPh(2))(2) (8) of B, and a further analogue Ph(2)PCH(2)CH(2)CH(2)SiHPr(n)()(2) (9) of A. A further analogue SiH(2)(CH(2)CH(2)CH(2)PPh(2))(2) (10) of biPSiH (B) is obtained similarly starting from SiH(2)(CH(2)CH=CH(2))(2). Steric control of silylalkyl cleavage from 2 is indicated by the fact that, like PPh(2)Cl (which forms B), two further biPSiH analogues SiH(Me)[CH(2)CH(2)CH(2)P(n-hex)(2)](2) (11) and SiH(Me)(CH(2)CH(2)CH(2)PPhBz)(2) (12) were obtained using P(n-hex)(2)Cl (i.e. n-hex = CH(3)(CH(2))(4)CH(2)-) or PPhBzCl (i.e. Bz = -CH(2)C(6)H(5)), respectively, whereas neither PPr(i)(2)Cl nor PBu(t)(2)Cl led to (phosphinoalkyl)silane formation. The surface-substrate linking reagent Ph(2)PCH(2)CH(2)CH(2)Si(OEt)(3) (D) is formed efficiently by similar means from Si(OEt)(3)(CH(2)CH=CH(2)). NMR data ((1)H, (13)C, (29)Si, (31)P) for 2-12 have been measured and are discussed.

Pd-catalysed reactions of alkynes with model distannanes and poly[di-(n-butyl)]stannane.

A reinvestigation of Pd-catalysed alkyne (R'-C≡CH; R' = H, Ph) insertion chemistry involving R(3)SnSnR(3) (3a: R = Me; 3b: R = n-Bu) was undertaken. Model distannyl ethylenes 4a-b (Me(3)SnCH=CR'SnMe(3)) and 5a-b ((n-Bu)(3)SnCH=CR'Sn(n-Bu)(3)) were reproduced and further characterized by NMR ((119)Sn, (13)C, (1)H) and UV-Vis spectroscopy. In the presence of an excess of phenylacetylene, dimerization-carbostannylation of compound 4b yielded the new conjugated butadiene, (Z,Z)-1,4-bis(trimethylstannyl)-1,4-diphenyl-buta-1,3-diene (9). An X-ray structure determination of 9 reveals a symmetrical double-bond Z confirmation. Compound 9 was further characterized by NMR, UV-Vis spectroscopy, and MS. A DFT analysis of model compounds (4a-b, 5a-b, 9) and the experimental and theoretical λ(max) values from the UV-Vis spectra were also compared. Acetylene and phenylacetylene Pd-catalysed insertion into the backbone of poly[di-(n-butyl)]stannane 12 resulted in new, modest molecular weight, partially inserted alkene tin polymers (13a-b) that were also characterized by GPC, NMR, UV-Vis spectroscopy and elemental analysis.

Pincer oxazolines: emerging tools in coordination chemistry and catalysis--where to next?

A brief overview of the coordination chemistry aspects of pincer ligands and their complexes containing at least one oxazoline (i.e., 4,5-dihydro-2-oxazole) unit is presented. This historical perspective is placed into a context of the possible future direction(s) in this particular arena of pincer chemistry. These ideas are compared and contrasted to the overall direction of pincer chemistry since the first such complexes were reported in the 1970s.

Oxazoles revisited: On the nature of binding of benzoxazole and 2-methylbenzoxazole with the zinc and palladium halides.

A synthetic and structural (X-ray) investigation into the bonding modes of benzoxazole (box) and 2-methylbenzoxazole (Mebox) ligands with halide precursors of Zn and Pd has been undertaken to clarify earlier discrepancies concerning the nature of the bonding mode(s) of the two azoles. In four structurally characterised examples, all contain the title ligands in a κ(1)N bonding motif. Calculations at the density functional level (DFT) of theory (B3LYP) confirm the ground state stability of this class of coordination for several hypothetical Pd and Zn (gas phase) compounds. The attempt to obtain suitable crystalline material of PdCl(2)(box)(2) (i.e., 5) leads to substantial complex degradation. One minor product of this process has been identified (X-ray) as the diarylformamidinato complex C(26)H(22)N(4)O(4)Pd, presumably formed via a complex combination of the decomposition products of both free box and 5.