An Objective Alternative to IUPAC's Approach To Assign Oxidation States

The IUPAC has recently clarified the term oxidation state (OS), and provided algorithms for its determination based on the ionic approximation (IA) of the bonds supported by atomic electronegativities (EN). Unfortunately, there are a number of exceptions and ambiguities in IUPAC's algorithms when it comes to practical applications. Our comprehensive study reveals the critical role of the chemical environment on establishing the OS, which cannot always be properly predicted using fix atomic EN values. By identifying what we define here as subsystems of enhanced stability within the molecular system, the OS can be safely assigned in many cases without invoking exceptions. New insights about the effect of local aromaticity upon OS are revealed. Moreover, we prove that there are intrinsic limitations of the IA that cannot be overcome. In this context, the effective oxidation state (EOS) analysis arises as a robust and general scheme to derive an OS without any external guidance.

Coordination isomerism in N-heterocyclic phosphenium thiocyanates

Two N-heterocyclic phosphines with exocyclic SCN substituents were synthesised via metathesis of chlorophosphine precursors with KSCN and fully characterised. The crystallographic studies reveal that the products exhibit pronounced structural differences. The thiocyanato unit binds in one case via the nitrogen atom to yield a molecular structure with a slightly elongated P–N single bond and, in the other case, via the sulfur atom to form a structure that is best described as an ion pair and forms a one-dimensional coordination polymer in the crystal. DFT calculations suggest that the P–N and P…S interactions can be described as covalent and dative bonds, respectively, and that the structural differences correlate with the different cation stabilities of the individual phosphenium cation fragments.

A very peculiar family of N-heterocyclic phosphines: unusual structures and the unique reactivity of 1,3,2-diazaphospholenes

This Perspective gives an account of the peculiar electronic and molecular structures of N-heterocyclic phosphines featuring either a single 1,3,2-diazaphospholene (DAP) ring with an exocyclic P-substituent X or two DAP rings linked by a P-P bond (bis-diazaphospholenyls), respectively, and their impact on the chemical properties of these molecules. The bonding situation in simple DAPs is epitomized by strong hyperconjugation between endocyclic π-type electrons and the exocyclic P-X bond. This interaction may induce a perceptible ionic polarization of the P-X bond which can persist even in the limit of a vanishing electronegativity gradient between P and X, and becomes visible in unusual geometric distortions of molecular structures and a unique chemical behaviour. Structural distortions are particularly evident in bond lengthening effects in P-halogen and P-phosphino derivatives R2P-DAP (with R2P ≠ DAP) which span the whole range from covalent molecules to contact ion pairs with a close relation to frustrated Lewis-pairs. The most significant impact on the chemical properties is found for P-phosphino- and P-hydrogen derivatives where reactions at substantially accelerated rates or totally new reaction modes can be observed, and new stoichiometric and first catalytic processes exploiting these features are currently emerging. The recently discovered bis-diazaphospholenyls differ from the simple derivatives as their central bond remains unpolarised as a consequence of the symmetric molecular structure. The occurrence of low-energy P-P bond homolysis that was nonetheless observed in one case is according to the results of thermochemical studies of P-P bond fission reactions attributable to the effects of steric congestion and induces chemical reactivity that can be considered complementary to that of the simple R2P-DAPs. Some concluding remarks will pay attention to a facet of DAP reactivity that has so far been widely neglected but is currently receiving increasing attention, namely well-defined ring-opening processes.

Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group's recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange.

Diazaphospholenes: N-heterocyclic phosphines between molecules and Lewis pairs

The interest in geometrically distorted or electronically polarized molecules is often motivated by the realization that unusual structures can engender unprecedented physical or chemical properties. 1,3,2-Diazaphospholenes are N-heterocyclic phosphines (NHPs) that have ring structures similar to N-heterocyclic carbenes (NHCs). Although NHPs were initially mainly of interest as precursors for carbene-analogous phosphenium cations, it was noted that they exhibit quite peculiar structural features and remarkable chemical behavior on their own. In this Account, we discuss both structure and chemistry in connection with the special bonding situation that is characterized by significant n(N)-σ*(P-X) hyperconjugation and induces a strong ionic polarization of the P-X bonds. This induced polarization is surprisingly maintained even when P and X have similar or like electronegativities (for example, X = H, P) and offers the possibility to design compounds with polarized homonuclear bonds. An exemption from the general pattern was only noted for some P-amino-NHPs in which reverse hyperconjugation weakens endocyclic P-N bonds and was predicted to facilitate ring fragmentation and formation of phosphinidenes. An important corollary of the P-X bond polarization in NHPs is a unique bond lengthening, which not only can be fine-tuned by adjusting intramolecular steric and electronic interactions but also responds to intermolecular influences and solvent effects. Insight from crystallographic, spectroscopic, and computational studies allows the development of concepts for controlled manipulation of the bonding, up to a point where P-X bonds are dominated by electrostatic interactions and species behave as Lewis pairs rather than covalent molecules. An appealing aspect lies in the fact that this P-X bond polarization induces reactivities that have hardly any precedence in phosphine chemistry. Examples include (i) reactions of P-hydrogen-substituted NHPs as hydride transfer reagents, (ii) metatheses and additions to multiple bonds (diphosphination) of phosphino-NHPs, which can be used to catalyze P-C cross-coupling reactions and to synthesize 1,2-bisphosphine ligands, (iii) cyclopentadienyl (Cp) transfer reactions of P-Cp-NHPs, and (iv) metal insertion into the P-X bonds of P-halogeno-NHPs. In many aspects, these reactions have potentially useful mechanistic or synthetic implications, and their future exploitation might help to convert NHPs from academically interesting species into useful reagents.