Sterically crowded 1,4-diiodobenzene as a precursor to difunctional hypervalent iodine compounds.

A bulky 1,4-di-iodobenzene having four adjacent para-tBu-C6H4 group (Ar') substituents (1) was used to prepare the di-hypervalent iodine compound 1,4-[I(OAc)2]2-2,3,5,6-Ar'4-C6 (2). Despite the steric encumbrance of the iodine center by the flanking aryl substituents, compound 2 undergoes ready cyclization under mild conditions (excess CF3COOH at 55 °C, 30 min) to afford a dicyclic di-iodonium di-triflate salt 3. The single crystal structures of compounds 2 and 3 were examined and compared to the formerly characterized precursor 1. The para-tert-butyl groups on these compounds also render the compounds more soluble than multifunctional hypervalent iodine (HVI) compounds. HVI compounds having multiple iodine(III) centers are increasingly of interest for applications as recyclable reagents, materials precursors, and as Lewis acids.

Luminescent 1H-1,3-benzazaphospholes.

2-R-1H-1,3-Benzazaphospholes (R-BAPs) are an interesting class of σ2P heterocycles containing P[double bond, length as m-dash]C bonds. While closely related 2-R-1,3-benzoxaphospholes (R-BOPs) have been shown to be highly photoluminescent materials depending on specific R substituents, photoluminescence of R-BAPs has been previously limited to an example having a fused carbazole ring system. Here we detail the synthesis and structural characterization of a new R-BAP (3c, R = 2,2'-dithiophene), and compare its photoluminescence against two previously reported R-BAPs (3a, R, R' = Me and 3b, R = 2-thiophene). The significant fluorescence displayed by the thiophene derivatives 3b (φ = 0.53) and 3c (φ = 0.12) stands in contrast to the weakly emissive methyl substituted analogue 3a (φ = 0.08). Comparative computational investigations of 3a-c offer insights into the interplay between structure-function relationships affecting excited state relaxation processes.

Remote Substituents as Potential Control Elements for the Solid-State Structures of Hypervalent Iodine(III) Compounds.

Hypervalent iodine (HVI) compounds are very important selective oxidants often employed in organic syntheses. Most HVI compounds are strongly associated in the solid state involving interactions between the electropositive iodine centers and nearby electron lone pairs of electronegative atoms. This study examines the impact of remote substituents on select families of HVI compounds as means to achieve predictable two-dimensional extended solid-state materials. Crystallographic analyses of 10 HVI compounds from several related classes of λ3 organoiodine(III) compounds, (diacetoxyiodo)benzenes, (dibenzoatoiodo)benzenes, [bis(trifluoroacetoxy)iodo]benzenes, and µ-oxo-[(carboxylateiodo)benzenes], provide insights into how remote substituents and the choice of carboxylate groups can impact intermolecular interactions in the solid state.

Organophosphorus decorated lithium borate and phosphate salts with extended π-conjugated backbone.

Several new bifunctional salts, Li[B(DPN)2], Li[B(DPN)(ox)] and Li[P(DPN)3], have been prepared from the phosphorus(v)-containing chelating ligand 2,3-dihydroxynaphthalene-1,4-(tetraethyl)bis(phosphonate) (H2-DPN). The new lithium salts were characterized by a variety of spectroscopic techniques. Both H2-DPN and Li[B(DPN)2] have been structurally characterized by X-ray crystallographic methods. These salts are related to materials being examined for use in electrolyte solutions for lithium-ion battery (LIB) applications.

Preferential N-H⋯:C[double bond splayed right] hydrogen bonding involving ditopic NH-containing systems and N-heterocyclic carbenes.

Hydrogen bonding plays a critical role in maintaining order and structure in complex biological and synthetic systems. N-heterocyclic carbenes (NHCs) represent one of the most versatile tools in the synthetic chemistry toolbox, yet their potential as neutral carbon hydrogen bond acceptors remains underexplored. This report investigates this capability in a strategic manner, wherein carbene-based hydrogen bonding can be assessed by use of ditopic NH-containing molecules. N-H bonds are unique as there are three established reaction modes with carbenes: non-traditional hydrogen bonding adducts (X-H⋯:C[double bond splayed right]), salts arising from proton transfer ([H-C[double bond splayed right]]+[X]-), or amines from insertion of the carbene into the N-H bond. Yet, there are no established rules to predict product distributions or the strength of these associations. Here we seek to correlate the hydrogen bond strength of symmetric and asymmetric ditopic secondary amines with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr, a representative NHC). In symmetric and asymmetric ditopic amine adducts both the solid-state (hydrogen bond lengths, NHC interior angles) and solution-state (1H Δδ of NH signals, 13C signals of carbenic carbon) can be related to the pK a of the parent amine.

Three Ways Isolable Carbenes Can Modulate Emission of NH-Containing Fluorophores.

Fluorescent molecules and materials that exhibit emission changes in response to analytes are of great interest across multiple disciplines. Herein, we investigate the response of NH-containing fluorophores carbazole and 2-phenylbenzimidazole (Ph-BIM) with two representative isolable singlet carbenes. Specifically, N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and cyclic (alkyl)(amino)carbene (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene (EtCAAC) were discovered to afford three different types of reaction products with carbazole and Ph-BIM. Depending on the reaction pair, hydrogen bonding (1), NH-insertion (2,3), or proton transfer (4) products can be isolated, each displaying variable photophysical responses. These products have been structurally authenticated by single crystal X-ray diffraction and NMR spectrometric methods. Studies of the solution state behavior of 1-4 reveals that these adducts are labile and can reversibly dissociate to free carbenes and fluorophores to varying extents. These equilibria produce concentration dependent solution state behavior as identified and quantified via UV-visible absorption, emission, 1H DOSY, and NMR spectroscopic measurements.

Controlling the Emissive Activity in Heterocyclic Systems Bearing C═P Bonds.

The photophysical properties of a series of heteroatom substituted indoles are explored to identify chemical means to control their emissive activity. In particular, we consider impacts of changes in the conjugated backbone, where the C═N bonds of benzoxazoles are replaced by C═P bonds (benzoxaphospholes). The effects of extending the π-conjugation, incorporating various secondary heteroatoms (X-C═P), and enforcing planar rigidity are also examined. Our computational analysis explains the higher fluorescence efficiency observed with extended π-conjugation and highlights the importance of maintaining molecular planarity at both ground- and emissive-state geometries.

An isolable magnesium diphosphaethynolate complex.

The reaction of magnesium chloride with two equivalents of sodium phosphaethynolate, Na[OCP]·(dioxane)2.5 (1), yields a magnesium diphosphaethynolate complex, [(THF)4Mg(OCP)2] (3). The formation of compound 3 goes through a monosubstituted chloromagnesium phosphaethynolate Mg(OCP)Cl (2). The structure of 3 was determined via a single crystal X-ray diffraction study. For comparison, we also report the structure of a monomeric sodium phosphaethynolate complex, [Na(OCP)(dibenzo-18-crown-6)] (4).

Synthesis of P2C2O2 and P2CO via NHC-mediated coupling of the phosphaethynolate anion.

The reaction of the chloroimidazolium chloride salt, [NHC-Cl][Cl], NHC = C{N(2,6-iPr2C6H3)CH}2 (1) with two equivalents of sodium phosphaethynolate, Na[OCP]·(dioxane)2.5, results in the formation of NHC-{cyclo-(CO)-P2-C(O)} (2) and NHC-P2-C(O)-NHC (3). Notably, in the presence of free NHC ligand, compound 2 converts to compound 3via extrusion of CO at elevated temperatures. The nature of the bonding in these complexes was probed computationally and spectroscopically.

Insertion of sodium phosphaethynolate, Na[OCP], into a zirconium-benzyne complex.

Reaction of the zirconium-benzyne complex [Cp2Zr(PMe3)(C6H4)] with sodium phosphaethynolate, Na[OCP], affords a zircono-phosphaalkene complex. Notably, unlike reactions of other transition metal complexes with Na[OCP] that yield the products of simple salt metathesis, this transformation represents novel Na[OCP] insertion chemistry and formation of an unusual solid state coordination polymer. The polymer is disrupted upon addition of Me3SiCl to afford a silyl-capped dimer that retains the zirconophosphaalkene functionality. Protonation of either form of zirconophosphaalkenes results in the formation of benzoylphosphine, PhC([double bond, length as m-dash]O)PH2.

Organoiodine(III) Reagents as Active Participants and Ligands in Transition Metal-Catalyzed Reactions: Iodosylarenes and (Imino)iodoarenes.

This chapter overviews the roles of transition metal complexes having the organoiodine(III) reagents iodosylarenes (ArIO) and (imino)iodoarenes (ArINR) as ligands in catalysis. Mechanistic implications are discussed.

Luminescent materials containing multiple benzoxaphosphole units.

A series of bisbenzoxaphospholes and a trisbenzoxaphosphole have been prepared and characterized. These extended π-conjugated heterocyclic phosphaalkenes display significant fluorescence in solution having quantum yields between 0.12 and 0.85. The solid state structure of a bisbenzoxaphosphole reveals evidence for extended conjugation, as do DFT calculations.

Phosphoryl-rich flame-retardant ions (FRIONs): towards safer lithium-ion batteries.

The functionalized catecholate, tetraethyl (2,3-dihydroxy-1,4-phenylene)bis(phosphonate) (H2 -DPC), has been used to prepare a series of lithium salts Li[B(DPC)(oxalato)], Li[B(DPC)2], Li[B(DPC)F2], and Li[P(DPC)3]. The phosphoryl-rich character of these anions was designed to impart flame-retardant properties for their use as potential flame-retardant ions (FRIONs), additives, or replacements for other lithium salts for safer lithium-ion batteries. The new materials were fully characterized, and the single-crystal structures of Li[B(DPC)(oxalato)] and Li[P(DPC)3] have been determined. Thermogravimetric analysis of the four lithium salts show that they are thermally stable up to around 200 °C. Pyrolysis combustion flow calorimetry reveals that these salts produce high char yields upon combustion.

An unusually unstable ortho-phosphinophenol and its use to prepare benzoxaphospholes having enhanced air-stability.

The primary phosphine 3,5-di-tert-butyl-2-phosphinophenol has been prepared and characterized. Oddly, the presence of a sterically demanding tert-butyl group adjacent to the PH2 centre renders the molecule very sensitive to loss of PH3 and formation of 3,5-di-tert-butyl-phenol in chloroform solutions in the presence of air. The process was catalyzed by HCl and dependent on the purity of CDCl3. Despite the instability of 3,5-di-tert-butyl-2-phosphinophenol, this material could be employed to produce a series of luminescent 2-R-4,6-di-tert-butyl-1,3-benzoxaphospholes having greater air stability than corresponding less bulky 2-R-1,3-benzoxaphospholes.

Comparison of 1,4-distyrylfluorene and 1,4-distyrylbenzene analogues: synthesis, structure, electrochemistry and photophysics.

A series of nine 1,4-distyrylfluorene derivatives (2) functionalized with substituents of variable electrondonating or -accepting capabilities was synthesised. The photophysical properties of the molecules were investigated, including UV/vis absorption, photoluminescence emission, and fluorescence quantum yields. Photophysical properties of chromophores 2 were found to exhibit significant solvatochromic effects, especially in the Stokes shift and photoluminescence maxima. The electrochemical properties of series 2 were also assessed by cyclic voltammetry and differential pulse voltammetry. Results of photophysical and electrochemical analyses were further supported by DFT calculations (B3LYP/6-31G*) and single crystal X-ray diffraction on select molecules. The contributions of intermolecular π-stacking and hydrogen bonding to crystal packing are discussed. A series of nine 1,4-distyrylphenylene derivatives (3) were also synthesised and similarly characterized for comparison to photophysical and solvatochromic effects observed in series 2. Properties of similarly-substituted molecules in series 2 and 3 were compared to one another in order to assess the influence of the 1,4-fluorenylene unit.

P=P bond photophysics in an Ar-P=P-Ar diphosphene.

The details of the photophysics of a diphosphene DmpP=PDmp (Dmp: 2,6-Mes(2)C(6)H(3)) have been examined experimentally and computationally. Femtosecond transient absorption spectroscopy has been used to probe the dynamics of the S(1) and S(2) excited states of DmpP=PDmp, through excitation at 480 and 400 nm, respectively. The molecule returns to S(0) on sub-nanosecond timescales; no irreversible photochemistry is observed. The S(2) state is observed in the transient spectra with an absorption feature at the red end of the visible spectrum. Its lifetime was measured to be 275 fs. The S(1) state does not absorb appreciably in the probe wavelength range. Excitation into either of these states leads to transient absorption signals in the 400-600 nm region that exhibit a rise time longer than the measured instrument response function, indicating that they do not arise from the initially excited state. These bands decay biexponentially, with lifetimes of ~20 ps and of a few hundred ps. Calculations at the CASSCF(8,6)/6-31G** and CASPT2(8,6)/6-31G**//CASSCF(8,6)/6-31G** levels support these assignments, and underpin an initial working model that involves participation of phenyl torsional twisting motions and the possibility of rapid intersystem crossing to the low-lying triplet manifold.

Naphthoxaphospholes as examples of fluorescent phospha-acenes.

Seven new fluorescent 2-R-naphtho[2,3-d]oxaphospholes (R-NOPs) (4a­g; R = tBu (a), Ad (b), C(6)H(5) (c), 4-MeC(6)H(4) (d), 4-ClC(6)H(4) (e), 4-BrC(6)H(4) (f ), 4-MeOC(6)H(4) (g)), have been synthesized by cyclocondensation reactions of benzimidoyl chlorides with 3-phosphino-2-naphthol (3). The compounds were characterized by multinuclear NMR, UV-vis, and fluorescence spectroscopy. Compounds 4a­d and 4g were characterized by cyclic voltammetry experiments. The solid state structures of compounds 4b and 4d were also determined by single-crystal X-ray diffraction experiments.

Long, directional interactions in cofacial silicon phthalocyanine oligomers.

Single crystal structures have been determined for the three cofacial, oxygen-bridged, silicon phthalocyanine oligomers, [((CH(3))(3)SiO)(2)(CH(3))SiO](SiPcO)(2-4)[Si(CH(3))(OSi(CH(3))(3))(2)], and for the corresponding monomer. The data for the oligomers give structural parameters for a matching set of three cofacial, oxygen-bridged silicon phthalocyanine oligomers for the first time. The staggering angles between the six adjacent cofacial ring pairs in the three oligomers are not in a random distribution nor in a cluster at the intuitively expected angle of 45° but rather are in two clusters, one at an angle of 15° and the other at an angle of 41°. These two clusters lead to the conclusion that long, directional interactions (LDI) exist between the adjacent ring pairs. An understanding of these interactions is provided by atoms-in-molecules (AIM) and reduced-density-gradient (RDG) studies. A survey of the staggering angles in other single-atom-bridged, cofacial phthalocyanine oligomers provides further evidence for the existence of LDI between cofacial phthalocyanine ring pairs in single-atom-bridged phthalocyanine oligomers.

A hybrid lithium oxalate-phosphinate salt.

The novel organophosphorus-containing lithium salt Li(THF)[(C(2)O(4))B(O(2)PPh(2))(2)] (1; THF = tetrahydrofuran) was synthesized and characterized using a variety of spectroscopic techniques. An X-ray structural analysis on crystals of 1 grown from THF reveals a dimeric structure [Li(THF)(C(2)O(4))B(O(2)PPh(2))(2)](2)·THF, whereby the two units of 1 are bridged via P-O···Li interactions. Compound 1 displays high air and water stability and is also thermally robust, properties needed of electrolytes for their possible use as electrolytes and/or additives in lithium-ion battery applications.