Hydrogen evolution catalysis by terminal molybdenum-oxo complexes.

Stable complexes with terminal triply bound metal-oxygen bonds are usually not considered as valuable catalysts for the hydrogen evolution reaction (HER). We now report the preparation of three conceptually different (oxo)molybdenum(V) corroles for testing if proton-assisted 2-electron reduction will lead to hyper-reactive molybdenum(III) capable of converting protons to hydrogen gas. The upto 670 mV differences in the [(oxo)Mo(IV)]-/[(oxo)Mo(III)]-2 redox potentials of the dissolved complexes came into effect by the catalytic onset potential for proton reduction thereby, significantly earlier than their reduction process in the absence of acids, but the two more promising complexes were not stable at practical conditions. Under heterogeneous conditions, the smallest and most electron-withdrawing catalyst did excel by all relevant criteria, including a 97% Faradaic efficiency for catalyzing HER from acidic water. This suggests complexes based on molybdenum, the only sustainable heavy transition metal, as catalysts for other yet unexplored green-energy-relevant processes.

The Dual-Stereocontrol Mechanism: Heteroselective Polymerization of rac-Lactide and Syndioselective Polymerization of meso-Lactide by Chiral Aluminum Salan Catalysts.

We describe alkoxo-aluminum catalysts of chiral bipyrrolidine-based salan ligands that follow the dual-stereocontrol mechanism wherein a given combination of stereogeneities at the metal site and the proximal center of the last inserted lactidyl ("match") is active towards lactide having a proximal stereogenic center of the opposite configuration, while the diastereomeric combination of stereogeneities ("mismatch") is inactive towards any lactide. Polymerization of rac-LA by the enantiomerically pure catalysts was sluggish and gave stereoirregular poly(lactic acid) (PLA) because selective insertion to a match diastereomer gives a mismatch diastereomer. The racemic catalysts showed higher activity and led to highly heterotactic PLA following polymeryl exchange between two mismatched catalyst enantiomers. A succession of match diastereomers in selective meso-LA insertions led to syndiotactic PLAs reaching a syndiotacticity degree of α=0.96. This polymer featured a Tm of 153 °C matching the highest reported value, and the highest crystallinity (ΔHm =56 J g-1 ) ever reported for syndiotactic PLA.

Influence of Anionic Ligand Exchange in Latent Sulfur-Chelated Ruthenium Precatalysts.

Four new cis-dianionic S-chelated ruthenium benzylidenes were synthesized by chloride ligand exchange. The special cis-dianionic conformation of these complexes contributed to a particularly facile anion exchange process, producing room-temperature-latent precatalysts. Their catalytic activity was strongly influenced by the solvent used. The latent iodide complex very efficiently promoted ring-closing metathesis by heating in toluene. Conversely, carboxylate ligands produced quite poor catalysts, but could abstract chlorides from chlorinated solvents to transform into active precatalysts. In tetrahydrofuran (THF), the S-chelated dichloro complex was shown to promote cycloisomerization instead of metathesis; however, the metathesis activity in THF could be recovered in the presence of phenylacetylene as a cocatalyst. Under the same conditions, all the other complexes required addition of LiCl to mimic this dichotomous behavior.

Block-Stereoblock Copolymers of Poly(ϵ-Caprolactone) and Poly(Lactic Acid).

A magnesium complex of the type {ONNN}Mg-HMDS wherein {ONNN} is a sequential tetradentate monoanionic ligand is introduced. In the presence of an alcohol initiator this complex catalyzes the living and immortal homopolymerization of the lactide enantiomers and ϵ-caprolactone at room-temperature with exceptionally high activities, as well as the precise block copolymerization of these monomers in a one-pot synthesis by sequential monomer addition. Copolymers of unprecedented microstructures such as the PCL-b-PLLA-b-PDLA and PDLA-b-PLLA-b-PCL-b-PLLA-b-PDLA block-stereoblock microstructures that feature unique thermal properties are readily accessed.

Divergent [{ONNN}Mg-Cl] complexes in highly active and living lactide polymerization.

Chloro-magnesium complexes of divergent tetradentate-monoanionic {ONNN}-type ligands which formed as either mononuclear or dinuclear complexes are reported. These complexes catalyze the ring-opening polymerization of lactide with high activity and in a living manner, and enable the synthesis of well-defined stereo-diblock copolymers with Mn > 200 kDa, as well as stereo-n-block copolymers (n = tri, tetra) with high molecular weights by sequential monomer addition. Thermal and crystallographic characterizations revealed that even the very high molecular weight PLA copolymers crystallized in a stereocomplex phase, and that their degradation temperature was as high as 354 °C.

Group†4 Metal Complexes of Phenylene-Salalen Ligands in rac-Lactide Polymerization Giving High Molecular Weight Stereoblock Poly(lactic acid).

Phenylene-salalens-sequential tetradentate dianionic {ONNO}-type ligands that include an ortho-phenylene group bridging between an imine and amine internal donors bound to phenol arms with a broad variety of substitution patterns are described. Zirconium and hafnium complexes of the type [{ONNO}M(O-tBu)2 ] were formed as single diastereomers, which, according to crystallographic structures, featured the fac(around the amine)-mer(around the imine) wrapping mode. The reactivity and stereoselectivity in rac-lactide (rac-LA) polymerizations were found to depend on the substitution pattern: complexes featuring small groups on the imine-side phenol and bulky groups on the amine-side phenol exhibited the favorable combination of high activity and high isoselectivity (Pm ≤0.91). Isotactic stereoblock copolymers of high molecular weights were prepared. The polymers crystallized in the stereocomplex phase according to thermal (differential scanning calorimetry) and crystallographic analysis.

Selective Synthesis of Cyclic Carbonate by Salalen-Aluminum Complexes and Mechanistic Studies.

Salalen-aluminum complexes were synthesized and used as catalysts in the reactions of CO2 with different epoxides. The reaction of cyclohexene oxide and CO2 was thoroughly investigated. In particular, the effect of the reaction conditions (nature and equivalents of the co-catalyst, CO2 pressure, and temperature) and of the ligands (substituents on the ancillary ligand, nature of the labile ligand, and nature of the nitrogen-donor atoms) on the results of this reaction was studied. The cycloaddition reaction of CO2 with terminal epoxides bearing different functional groups was realized. Moreover, NMR mechanistic studies provided information on the catalytic cycle. Interestingly, the characterization of an intermediate species in the mechanism of the reaction of cyclohexene oxide with CO2 , catalyzed by one of the salalen-aluminum complexes, was accomplished.

Tailor-Made Stereoblock Copolymers of Poly(lactic acid) by a Truly Living Polymerization Catalyst.

Poly(lactic acid) (PLA) is a biodegradable polymer prepared by the catalyzed ring opening polymerization of lactide. An ideal catalyst should enable a sequential polymerization of the lactide enantiomers to afford stereoblock copolymers with predetermined number and lengths of blocks. We describe a magnesium based catalyst that combines very high activity with a true-living nature, which gives access to PLA materials of unprecedented microstructures. Full consumption of thousands of equivalents of L-LA within minutes gave PLLA of expected molecular weights and narrow molecular weight distributions. Precise PLLA-b-PDLA diblock copolymers having block lengths of up to 500 repeat units were readily prepared within 30 min, and their thermal characterization revealed a stereocomplex phase only with very high melting transitions and melting enthalpies. The one pot sequential polymerization was extended up to precise hexablocks having "dialed-in" block lengths.

Zinc Complexes of Sequential Tetradentate Monoanionic Ligands in the Isoselective Polymerization of rac-Lactide.

Zinc complexes of {ONNN}-type sequential tetradentate monoanionic ligands reacted with diethylzinc to give the mononuclear ethylzinc complexes. The benzyloxy complexes were formed readily and were found to be highly active as well as living/immortal catalysts for ring-opening polymerization of rac-lactic acid with a clear isospecific inclination. Chiral gas chromatography analysis revealed a mild preference for a given lactide enantiomer by the chiral catalysts.

The stereoselectivity of bipyrrolidine-based sequential polydentate ligands around Ru(ii).

The Ru(ii) coordination chemistry of the sequential hexadentate, tetradentate and the novel hybrid pentadentate ligands assembled around the chiral bipyrrolidine core and including bipyridyl and pyridyl periphery units is described. The bipyridine group exhibited priority in binding over the bipyrrolidine group, which led to a diastereomer mixture in the case of the hexadentate ligand 1. Employing only monopyridyl or a combination of monopyridyl and bipyridyl peripheral groups restored the chiral induction ability to the bipyrrolidine core resulting in predetermined chiral-at-metal complexes for the ligands 3 and 4.

Intriguing Physical and Chemical Properties of Phosphorus Corroles.

The fluorescence intensity of phosphorus corroles increases upon meso-aryl C-F/C-H and P-OH/P-F substitutions, the latter affects corrole-centered redox processes more than C-H/C-F substitution on the corrole's skeleton, and the presence of F atoms allows for the first experimental insight into the electronic structures of oxidized corroles. Experimental and theoretical methodologies reveal that mono- but not bis-chlorosulfonation of the corrole skeleton is under kinetic control. Selective introduction of heavy atoms leads to complexes that are phosphorescent at room temperature.

Assembling Quasi-enantiomeric Octahedral Complexes of Different Metals via Quasi-racemate Crystallization.

Aiming at a general methodology for binary co-assembly of complexes of different metals through quasiracemate crystallization, the hexadentate ligand 1 comprised of the chiral bipyrrolidine core and two bipyridine peripheral arms is introduced. Ligand 1 was found to bind in a fully diastereoselective and uniform mode around Zn(II), Fe(II) and Cd(II) giving coordinatively inert octahedral "chiral-at-metal" complexes with the Δ4Λ2/Λ4 Δ2 wrapping mode. Equimolar mixtures of quasienantiomeric pairs of these complexes exhibited a clear tendency to pack as quasiracemates as was revealed from the crystallographic structures of [(R,R)-1-Zn](PF6)2/[(S,S)-1-Fe](PF6)2 and [(R,R)-1-Zn](PF6)2/[(S,S)-1-Cd](PF6)2, in an isomorphous fashion to that of the racemic compound [rac-1-Zn](PF6)2 in space group C2/c.

A Light-Activated Olefin Metathesis Catalyst Equipped with a Chromatic Orthogonal Self-Destruct Function.

A sulfur-chelated photolatent ruthenium olefin metathesis catalyst has been equipped with supersilyl protecting groups on the N-heterocyclic carbene ligand. The silyl groups function as an irreversible chromatic kill switch, thus decomposing the catalyst when it is irradiated with 254 nm UV light. Therefore, different types of olefin metathesis reactions may be started by irradiation with 350 nm UV light and prevented by irradiation with shorter wavelengths. The possibility to induce and impede catalysis just by using light of different frequencies opens the pathway for stereolithographic applications and novel light-guided chemical sequences.

Straightforward synthesis and catalytic applications of rigid N,O-type calixarene ligands.

Here, we report a simple one-step access to new rigid N,O-calixarene ligands which is based on copper-catalyzed amination at the lower rim. We also present the coordination properties of these ligands with some main group and transition metals leading to new complexes with superior catalytic activity, in several organic transformations, compared with calixarene metal complexes reported in the literature.

Diamondoid framework solid with Sn(OCH3)2-tetrapyridylporphyrin linkers, CuI nodes and [CuICl2]⁻ counter-ions.

We report on the synthesis of a new metal-organic framework (MOF) composed of Sn(OCH3)2-tetrakis(pyridin-4-yl)porphyrin linkers, Cu(+) connecting nodes and [CuCl2](-) counter-ions, namely poly[[bis(methanolato-κO)[µ5-5,10,15,20-tetrakis(pyridin-4-yl)porphyrin-κ(8)1κN(5):1'κN(10):1''κN(15):1'''κN(20):2κ(4)N(21),N(22),N(23),N(24)]copper(I)tin(II)] dichloridocuprate(I)], [CuSn(C40H24N8)(CH3O)2][CuCl2]. Its crystal structure consists of a single-framework coordination polymer of the organic ligand and the Cu(I) ions. The latter are characterized by a tetrahedral coordination geometry [with CN (coordination number) = 4], linking to the pyridyl N-atom sites of four different ligands and imparting to the positively charged polymeric assembly a diamondoid PtS-type topology. Correspondingly, every porphyrin unit is coordinated to four different Cu(I) connectors. The [CuCl2](-) anions occupy the intra-lattice voids, along with disordered molecules of the water crystallization solvent. The asymmetric unit of this structure consists of two halves of the porphyrin scaffold, located on centres of crystallographic inversion, and the Cu(+) and [CuCl2](-) ions. This report provides unique structural evidence for the formation of tetrapyridylporphyrin-based three-dimensional MOFs with a diamondoid architecture that have been observed earlier only on rare occasions.

Selective aryl-fluoride reductive elimination from a platinum(IV) complex.

A difluoro(mesityl)platinum(IV) complex underwent highly selective reductive elimination of 2-fluoromesitylene upon heating in toluene. Kinetic analysis and DFT calculations suggest that the CF coupling involves a five-coordinate Pt(IV) transient intermediate resulting from the rate-limiting dissociation of the pyridine ligand.

Porphyrins and Corroles with 2,6-Pyrimidyl Substituents.

Corroles and porphyrins with 2,6-pyrimidyl substituents are reported for the first time, together with the spectroscopic data and the crystal structures of the free-base porphyrin and of the phosphorus and cobalt complexes of the corrole.

Mechanistic Insight into the Stereochemical Control of Lactide Polymerization by Salan-Aluminum Catalysts.

Alkyl aluminum complexes of chiral salan ligands assembled around the 2,2'-bipyrrolidine core form as single diastereomers that have identical configurations of the N donors. Active catalysts for the polymerization of lactide were formed upon the addition of benzyl alcohol. Polymeryl exchange between enantiomorphous aluminum species had a dramatic effect on the tacticity of the poly(lactic acid) (PLA) in the polymerization of racemic lactide (rac-LA): The enantiomerically pure catalyst of the nonsubstituted salan ligand led to isotactic PLA, and the racemic catalyst exhibited lower stereocontrol. The enantiomerically pure catalyst of the chloro-substituted salan ligand led to PLA with a slight tendency toward heterotacticity, whereas the racemic catalyst led to PLA of almost perfect heterotacticity following an insertion/auto-inhibition/exchange mechanism.

Amino acids as chiral anionic ligands for ruthenium based asymmetric olefin metathesis.

Several amino acid ligands were introduced into the Hoveyda-Grubbs 2nd generation complex by a facile anionic ligand exchange. The chiral pre-catalysts obtained displayed enantioselectivity in asymmetric ring-closing and ring-opening cross-metathesis reactions. Reduction of the lability of the carboxylate ligands was found to be cardinal for improving the observed enantiomeric product enrichment.

Tuning the photophysical and redox properties of metallocorroles by iodination.

Facile procedures were developed for selective iodination of aluminum and gallium corroles; crystallographic characterization shows that the main structural aspects are not changed (the macrocycle remains planar). Absorption maxima are red-shifted by 3-5 nm/iodine, singlet lifetimes are reduced to <80 ps, and emissions from long-lived excited states come into effect. The iodinated corroles display prompt fluorescence, phosphorescence, and delayed thermal fluorescence, all at room temperature. The effect on redox potentials appears to be additive for each additional iodine and, surprisingly, is practically identical to that of the other three halides. The conclusions of this work are of large importance for the design of metallocorroles that are best suited for the various applications where metallocorroles are used as catalysts and photosensitizers.