Tetrazole-Functionalized Organoboranes Exhibiting Dynamic Intramolecular N→B-Coordination and Cyanide-Selective Anion Binding.

Starting from two different cyano-functionalized organoboranes, we demonstrate that 1,3-dipolar [3+2] azide-nitrile cycloaddition can serve to generate libraries of alkyl-tetrazole-functionalized compounds capable of intramolecular N→B-Lewis adduct formation. Due to the relatively low basicity of tetrazoles, structures can be generated that exhibit weak and labile N→B-coordination. The reaction furnishes 1- and 2-alkylated regio-isomers that exhibit different effective Lewis-acidities at the boron centers, and vary in their optical absorption and fluorescence properties. Indeed, we identified derivatives capable of selectively binding cyanide over fluoride, as confirmed by 11B NMR. This finding demonstrates the potentialities of this synthetic strategy to systematically fine-tune the properties of lead structures that are of interest as chemical sensors.

Molecular Engineering of Metalloporphyrins for High-Performance Energy Storage: Central Metal Matters.

Porphyrin derivatives represent an emerging class of redox-active materials for sustainable electrochemical energy storage. However, their structure-performance relationship is poorly understood, which confines their rational design and thus limits access to their full potential. To gain such understanding, we here focus on the role of the metal ion within porphyrin molecules. The A2 B2 -type porphyrin 5,15-bis(ethynyl)-10,20-diphenylporphyrin and its first-row transition metal complexes from Co to Zn are used as models to investigate the relationships between structure and electrochemical performance. It turned out that the choice of central metal atom has a profound influence on the practical voltage window and discharge capacity. The results of DFT calculations suggest that the choice of central metal atom triggers the degree of planarity of the porphyrin. Single crystal diffraction studies illustrate the consequences on the intramolecular rearrangement and packing of metalloporphyrins. Besides the direct effect of the metal choice on the undesired solubility, efficient packing and crystallinity are found to dictate the rate capability and the ion diffusion along with the porosity. Such findings open up a vast space of compositions and morphologies to accelerate the practical application of resource-friendly cathode materials to satisfy the rapidly increasing need for efficient electrical energy storage.

Preparation of Structurally and Electronically Diverse N → B-Ladder Boranes by [2 + 2 + 2] Cycloaddition.

We report the synthesis of a series of eight N → B-ladder boranes through cobalt-mediated cyclotrimerization of (2-cyanophenyl)-dimesitylborane with different dialkynes. The resulting tetracoordinate boranes show variable electrochemical and optical properties depending on the substitution pattern in the backbone of the coordinating pyridine-derivatives. While boranes containing alkyl-substituted pyridines show lower electron affinities than the known parent compound, boranes featuring π-extended pyridine derivatives show higher electron affinities in the range of acceptor substituted triarylboranes. All derivatives show larger Stokes shifts (8790-6920 cm-1) compared to the N → B-ladder borane coordinated by an unsubstituted pyridine.

Electronic Properties and Solid-State Packing of Isocyanofulvenes and Their Gold(I) Chloride Complexes.

Two methods for the synthesis of isocyanofulvenes are reported, and a series of ligands of the type R2═CH-NC (4a-g, R2 = a, 9-fluorenylidene; b, 9-(2,7-dioctyloxylfluorenylidene); c, 9-(3,6-dimethoxyfluorenylidene); d, 9-(3,6-dioctylfluorenylidene); e, 5-dibenzo[a,d]cycloheptenylidene; f, 9-thioxanthenylidene; and g, 2,5-dimethyl-3,4-diphenylcyclopentadienylidene) were prepared along with their gold(I) chloride complexes (R2═CH-NC-AuCl, 5a-f). A comprehensive study of the properties of the precursors, free ligands, and gold(I) complexes is reported and complemented by DFT calculations. Solid-state structure of two complexes (5a and 5c) show extensive aurophilic interactions and π-π stacking of the ligands. The metal centers are not involved in optical transitions. However, metal coordination leads to a consistent bathochromic shift in the absorption spectra, which signifies the effective conjugation between the isocyano group and the π-systems of the ligands. Furthermore, an additional DFT study of carbonyl complexes of the type R2═CH-NC-M(CO)5 (M = Cr, Mo, and W; R2═CH-NC = 4a) indicates very effective metal-to-ligand charge transfer when isocyanopentafulvenes are used as ligands.

NiO/Poly(4-alkylthiazole) Hybrid Interface for Promoting Spatial Charge Separation in Photoelectrochemical Water Reduction.

Conjugated polymers are emerging as alternatives to inorganic semiconductors for the photoelectrochemical water splitting. Herein, semi-transparent poly(4-alkylthiazole) layers with different trialkylsilyloxymethyl (R3SiOCH2-) side chains (PTzTNB, R = n-butyl; PTzTHX, R = n-hexyl) are applied to functionalize NiO thin films to build hybrid photocathodes. The hybrid interface allows for the effective spatial separation of the photoexcited carriers. Specifically, the PTzTHX-deposited composite photocathode increases the photocurrent density 6- and 2-fold at 0 V versus the reversible hydrogen electrode in comparison to the pristine NiO and PTzTHX photocathodes, respectively. This is also reflected in the substantial anodic shift of onset potential under simulated Air Mass 1.5 Global illumination, owing to the prolonged lifetime, augmented density, and alleviated recombination of photogenerated electrons. Additionally, coupling the inorganic and organic components also enhances the photoabsorption and amends the stability of the photocathode-driven system. This work demonstrates the feasibility of poly(4-alkylthiazole)s as an effective alternative to known inorganic semiconductor materials. We highlight the interface alignment for polymer-based photoelectrodes.

Expansion of the scope of alkylboryl-bridged N → B-ladder boranes: new substituents and alternative substrates.

A series of new boranes capable of forming intramolecular N → B-heterocycles has been prepared and their properties have been studied by electrochemical methods and UV-vis-spectroscopy complemented by DFT calculations. A dimethylborane (BMe2), haloborane derivatives (BBr2, BF2, BI2) and mixed cyano/isocyano-borane (B(CN)(NC)) have been prepared by different techniques. Furthermore, 2'-alkynyl-substituted 2-phenylpyridines bearing terminal tert-butyl- and trimethylsilyl-groups are introduced as a new class of substrates for hydroboration. Successful hydroboration with either 9H-borabicyclo[3.3.1]-nonane (9H-BBN), dimesitylborane (Mes2B-H), or Piers' borane ((C6F5)2B-H, BPF-H) furnished new π-extended boranes capable of forming intramolecular six- or seven-membered N → B-heterocycles (tBuBBN, SiBPF), and, in the case of Mes2BH, formation of a sterically crowded styrylborane (SiBMes2) incapable of intramolecular N → B-coordination was observed. All the boranes listed above except BMe2 have been structurally characterized, and a study of their electrochemical properties showed that the systematic variation of the substituents on boron allows for the incremental variation of the electron affinity of the phenylpyridine-model system over a total range of >0.7 eV between alkylboranes (BMe2, BBN) and B(CN)(NC). B(CN)(NC) shows the strongest N → B-bond (≈175 kJ mol-1), and highest electron-affinity observed so far, and is the first example of a borane bearing an isocyano-substituent on boron.

Hydroboration as an Efficient Tool for the Preparation of Electronically and Structurally Diverse N→B-Heterocycles.

Ladder-type organoboranes featuring intramolecular N→B coordination have been prepared through hydroboration of a 2-(ortho-styryl)pyridine (PhPy) with a series of hydroboranes, including 9H-9-borabicyclo[3.3.1]nonyl (9H-BBN), BH3 ⋅THF, HBCl2 ⋅SMe2 , HB(C6 F5 )2 , and a 9H-9-borafluorene derivative. The hydroboration reaction results in highly regioselective borylation under mild conditions and gives the products in good to excellent yields. The molecular structure and electronic properties of the obtained boranes have been experimentally investigated in detail, and complemented with DFT calculations to further elucidate the origin of differences in optical and electronic properties. The electron affinity of the conjugated system can be controlled through variation of the borane, while the optical properties are likewise directly linked to the type and molecular structure of the substituents on boron. The broad substrate range shows that this preparative approach is widely applicable to introduce chemically diverse boryl groups into conjugated systems.

Generation of an N→B Ladder-type Structure by Regioselective Hydroboration of an Alkenyl-Functionalized Quaterpyridine.

An unusual reactivity of 2-(1-alkenyl)-pyridines towards hydroboration with 9H-borabicyclo[3.3.1]nonane (9H-BBN) has been employed to selectively introduce two borane groups into a conjugated quaterpyridine. Quantitative conversion of the substrate was observed with exclusive regioselectivity. A molecular structure that allows intramolecular N→B coordination was generated. The effect of the ladder formation on the molecular structure and the electronic properties of the conjugated system have been investigated. The synthetic strategy demonstrated herein offers a facile access to N→B ladder-type structures from readily available substrates, and allows to simultaneously introduce several boron centers under mild conditions.

Head-to-Tail Regioregular Polythiazole Prepared via Kumada-Coupling Polycondensation.

Head-to-tail regioregular poly(4-alkylthiazoles) (C9-pTz, C13-pTz, alkyl = n-C9H19, n-C13H27) have been synthesized and spectroscopically and electrochemically characterized. The PTzs were obtained by transmetalation of 2-chloro-5-bromo-4-alkylthiazoles with iPrMgCl, followed by Kumada-coupling polycondensation. The polymers are largely insoluble in organic solvents but dissolve readily in the presence of, e.g., trifluoro acetic acid (TFA). Analyses of soluble trace fractions of the polymers gave number average molecular weights (Mn) of 1.9-2.4 kDa (C9-pTz, PDI ≈ 1.1-1.3) and 2.9-3.0 kDa (C13-pTz, PDI ≈ 1.1-1.2), as determined by gel permeation chromatography (GPC) relative to polystyrene standards, while the molecular weight of the bulk material is presumed to be considerably higher. Comparison of the 1H NMR spectra of the PTzs with a quaterthiazole model compound (4Tz) and head-to-head-tail-to-tail regioregular polybithiazole (PBTz) confirmed the head-tail structure and the high degree of regioregularity. The optical and electrochemical band gaps of C9-PTz were found to be similar to those of poly(3-hexylthiophene) (P3HT), while the frontier orbital levels are stabilized by 0.3-0.5 eV relative to those of the polythiophene. The synthesis of PTz via selective transmetalation of the precursor at the sterically hindered 5-position renders this synthesis a rare polycondensation of a reversed monomer. The implications for the polymerization mechanism are also discussed.

Metallopolymers featuring boratabenzene iron complexes.

Boratabenzene-derived metallocene analogues are introduced as versatile new building blocks for metallopolymers. Bis(borinato)iron(II) complexes of the type Fe(C(5)H(5)B-R)(2) (R: C≡CH, C≡CSiMe(3), 4-(1-benzyl-1,2,3-triazolyl) were synthesized and spectroscopically, structurally, and electrochemically characterized. Polymerization via Sonogashira-Hagihara coupling of the alkynyl-substituted derivative Fe(C(5)H(5)B-C≡CH)(2) with 2,5-bis(dodecyloxy)-1,4-diiodobenzene yields a film-forming polymer in 50-69% yield. Polymer batches were obtained with number-average molecular weights (M(n)) of 11.7 and 20.5 kDa and polydispersity indices (PDI) of 1.55 and 2.65, respectively, as determined by gel permeation chromatography relative to polystyrene standards. Click-type polymerization of Fe(C(5)H(5)B-C≡CH)(2) through azide-alkyne cycloaddition with 1,4-bis(4-azidobutoxy)benzene gives the corresponding metallopolymer with an M(n) of 5.7 kDa and a PDI of 2.29 in 87% yield.

Exploring the concept of aromaticity on complexes of a fourfold benzannulated cyclopentadienyl ligand.

The ligand dibenzo[c,g]fluorenide (Dbf(-)), combines a cyclopentadienide ligand and 1,1'-binaphthyl fragment in one molecule. Preliminary investigations confirmed the special electronic situation in this 6pi-electron donor by way of a series of novel transition-metal complexes. Herein, the electron delocalization was investigated in detail by means of DFT calculations in combination with calculations on the nucleus-independent chemical shifts (NICS). These results clearly prove that the Dbf(-) ion gains the largest aromatic stabilization among all benzannulated fluorenides, and there are two almost olefinic C=C bonds in this structure. These bonds undergo cyclopropanation when treated with ZnEt(2) and ClCH(2)I in a modified Simmons-Smith reaction.