Synthesis of Intramolecular P/Al-Based Frustrated Lewis Pairs via Aluminum-Tin-Exchange and their Reactivity toward CO2

Frustrated Lewis pairs (FLPs) composed of acidic alane and basic phosphane functions, separated by a xanthene linker, can be prepared through the corresponding Me₃Sn derivative and methyl aluminum compounds with elimination of Me₄Sn. This way MeClAl-, Cl₂Al- and (C₆F₅)₂Al- moieties could be introduced and the resulting FLPs are stabilized by a further equivalent of the alane precursors. In contact with the FLPs CO₂ is bound via the C atom at the phosphane functions and the two O atoms at the Al centers. The residues at the latter determine the binding strength. Hence, in case of MeClAl CO₂ capture occurs at higher pressure and under ambient conditions CO₂ is released again, while for Cl₂Al and (C₆F₅)₂Al CO₂ binding becomes irreversible. The results of DFT calculations rationalize these findings by the high thermodynamic stabilization in case of more electronegative residues, which concomitantly lead to higher barriers, and in case of (C₆F₅)₂Al further stabilization is achieved through a low reorganization energy.     

Simple, general, and efficient synthesis of meso-substituted borondipyrromethenes from a single platform

An unprecedented synthesis of 8-substituted-borondipyrromethenes is described starting from 8-thiomethylbodipy 1. Aryl, heteroaryl, alkenyl, and organometallic boronic acids smoothly reacted with 1 in the presence of a catalytic amount of Pd(0) and a stoichiometric amount of Cu(I)-2-thienylcarboxylate under neutral conditions to give the corresponding Bodipy analogues in good to quantitative yields (20 examples). A remarkable reactivity was observed in some cases, e.g., ferrocenylboronic acid gave the product in 98% isolated yield after only 10 min at 55 degrees C.     

Lower branch coherent states in shear flows: transition and control

Lower branch coherent states in plane Couette flow have an asymptotic structure that consists of O(1) streaks, O(R(-1)) streamwise rolls and a weak sinusoidal wave that develops a critical layer, for large Reynolds number R. Higher harmonics become negligible. These unstable lower branch states appear to have a single unstable eigenvalue at all Reynolds numbers. These results suggest that lower branch coherent states control transition to turbulence and that they may be promising targets for new turbulence prevention strategies.     

Synthesis and reactivity studies of iminoboryl complexes

A range of new iminoborylcomplexes of the type [L(n)M-B[triple bond]N-R], which are isoelectronic with sigma-alkynyl complexes [L(n)M-C[triple bond]C-R], was obtained by systematically varying the metal M, the coligands L, and the nitrogen bound substituent R. Selected examples include, for example, trans-[(Cy3P)2(Br)Pt(B[triple bond]N iBu)], which is characterized by a sterically less demanding N-R group or the unprecedented rhodium species cis,mer-[(Br)2(Me3P)3Rh(B[triple bond]NSiMe3)]. All compounds were fully characterized in solution by multinuclear NMR spectroscopy and, where appropriate, in the solid state by X-ray crystallography. Subsequent reactivity studies revealed that particularly the combination of smaller N-R groups with Pt-B linkages of increased stability opens up opportunities for novel reactivity patterns of this class of compounds. Within the scope of these study, we inter alia succeeded in synthesizing the unusual bridged boryl species 1,4-trans-[{(Cy3P)2(Br)Pt(B{NH iBu}NH)}2C6H4] and a complex bearing both an acetylide ligand and an iminoboryl ligand, respectively.     

Accurate thermochemistry from quantum chemical calculations?

The answer to the title question is definitely “yes” – at least for fairly small molecules. Computational procedures, namely the Weizmann (Wn) and Gaussian-3 (G3) family of methods, the complete basis set extrapolation scheme (CBS-x), the “high accuracy extrapolated ab initio thermochemistry” (HEAT) as well as the “correlation consistent composite approach” (ccCA), aimed at energies with chemical accuracy or even better (sub kJ?mol^?1) are described and several applications illustrating the level of accuracy that can be achieved are presented.     

[n]Cycloparaphenylene-Pillar[5]arene Bismacrocycles: Their Circularly Polarized Luminescence and Multiple Guest Recognition Properties

Both pillar[n]arenes (P[n]As) and [n]cycloparaphenylenes ([n]CPPs) play an important role in supramolecular chemistry. Herein, we report the precise synthesis of two multifunctional bismacrocycles [n]CPP-P[5]A by integrating P[5]A into the [n]CPP backbone. The photoluminescence quantum yield (Φ_F) of the bismacrocycles was found to show a dramatic increase relative to the corresponding [n]CPPs. The chiral enantiomers (pR)/(pS)-[8]CPP-P[5]A were successfully isolated by chiral HPLC, and showed promising properties of circularly polarized luminescence (g_lum≈0.02). In addition, [n]CPP-P[5]A bismacrocycles are capable of binding pyridinium salts and fullerene derivatives with high affinity and specificity within the two distinct cavities. Transient absorption studies showed that photo-induced electron transfer occurs in [10]CPP-P[5]A⊃C₆₀ complex. Our results suggest that [n]CPP-P[5]A are potentially useful in CPL-active materials, multiple guest recognition and supramolecular polymer preparation.     

Synergizing Electron and Heat Flows in Photocatalyst for Direct Conversion of Captured CO2

We report a ternary hybrid photocatalyst architecture with tailored interfaces that boost the utilization of solar energy for photochemical CO₂ reduction by synergizing electron and heat flows in the photocatalyst. The photocatalyst comprises cobalt phthalocyanine (CoPc) molecules assembled on multiwalled carbon nanotubes (CNTs) that are decorated with nearly monodispersed cadmium sulfide quantum dots (CdS QDs). The CdS QDs absorb visible light and generate electron-hole pairs. The CNTs rapidly transfer the photogenerated electrons from CdS to CoPc. The CoPc molecules then selectively reduce CO₂ to CO. The interfacial dynamics and catalytic behavior are clearly revealed by time-resolved and in situ vibrational spectroscopies. In addition to serving as electron highways, the black body property of the CNT component can create local photothermal heating to activate amine-captured CO₂, namely carbamates, for direct photochemical conversion without additional energy input.     

Spectroscopic Signatures of Internal Hydrogen Bonds of Brønsted-Acid Sites in the Zeolite H-MFI

The location of Brønsted-acid sites (bridging OH groups, b-OH) at different crystallographic positions of zeolite catalysts influences their reactivity due to varying confinement. Selecting the most stable b-OH conformers at each of the 12 T-sites (T=Si/Al) of H-MFI, a representative set of 26 conformers is obtained which includes free b-OH groups pointing into the empty pore space and b-OH groups forming H-bonds across five- or six-membered rings of TO₄ tetrahedra. Chemically accurate coupled-cluster-quality calculations for periodic models show that the strength of internal H-bonds and, hence, the OH bond length vary substantially with the framework position. For 11 of the 19 H-bonded b-OH groups examined, our predictions fall into the full width at half maximum range of the experimental signals at 3250±175 cm⁻¹ and 7.0±1.4 ppm which supports previously debated assignments of these signals to H-bonded b-OH sites.     

Atomic force microscopy based thermal lithography of poly(tert-butyl acrylate) block copolymer films for bioconjugation

In this paper, we report on the local thermal activation of thin polymer films for area-selective surface chemical modification on micrometer and nanometer length scales. The thermally induced activation of tert-butyl ester moieties in polystyrene- block-poly(tert-butyl acrylate) (PS- b-PtBA) block copolymer films leads to the formation of pending carboxylic acid groups, which are among the versatile functionalities for subsequent bioconjugation. From Fourier transform infrared (FTIR) spectroscopic analyses, the apparent activation energy (Ea) for the tert-butyl ester deprotection in thin films was calculated to be 93 +/- 12 kJ/mol, which is in good agreement with values reported for the bulk. The availability of the deprotected carboxylic acid groups in subsequent wet chemical grafting reactions on neat thermolyzed films was confirmed by covalently immobilizing fluoresceinamine and amino end-functionalized poly(ethylene glycol) (PEG-NH2) using established 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) chemistry. Local thermal deprotection on micrometer and sub-micrometer length scales was achieved by scanning thermal microscopy using an atomic force microscope with heatable probe tips. Passivating PEG and fluoresceinamine layers were selectively covalently coupled to locally deprotected areas as small as 370 nm x 580 nm.