Justin F. Binder

Assessing the Ligand Properties of NHC-Stabilized Phosphorus(I) Cations

The isolation and full characterisation of a series of cationic metal-carbonyl complexes bearing an N-heterocyclic carbene stabilised phosphorus(I) ligand are reported. Specifically, the mononuclear coordination complexes [LM(CO)5 ][BPh4 ] (M=Cr, Mo, W), [LFe(CO)4 ][BPh4 ] and the dinuclear complexes [LMn2 (CO)8 ][BPh4 ] and [LCo2 (CO)6 ][BPh4 ], in which L=[bis(1,3,4,5-tetramethylimidazol-2-ylidene)phosphanide]+ , have all been isolated in the solid state and structurally confirmed by single-crystal X-ray diffraction. The dicationic platinum complex trans-[L2 PtCl2 ][BPh4 ]2 is also reported and fully characterised. The donor ability of [L]+ has been assessed by IR spectroscopy of its metal-carbonyl complexes and by using DFT calculations. The results suggest that [L]+ is a weak π-acceptor with moderate donor strength and thus it bridges the gap that exists between phosphines and amines in terms of ligand properties. Collectively, these molecules represent the first crystallographically characterised cationic metal-carbonyl derivatives of a PI -centred ligand and are a rare collection of cationic metal-carbonyl complexes.

Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)

We present herein the optimized synthesis of a triphosphenium bromide salt. Apart from being a versatile metathesis reagent, this unusually stable low-valent-phosphorus-containing compound acts as a useful P+ transfer agent. Unlike traditional methods employed to access low-coordinate phosphorus species which usually require pyrophoric phosphorus-containing precursors (white phosphorus, Tris(trimethylsilyl)phosphine, etc.), or harsh reducing agents (alkali metals, potassium graphite, etc.), the current approach does not involve pyrophoric or explosive reagents and can be done on large scales (>20 g) in excellent yields by undergraduates with basic air-free synthetic training. The bromide counter ion is readily exchanged with other anions such as tetraphenyl borate (described herein) using typical salt metathesis reagents to obtain materials with desired properties and reactivities. The versatility of this P+ transfer approach is exemplified by the reactions of these triphosphenium precursors with an N-heterocyclic carbene and an anionic bisphosphine, each of which readily displace the neutral bisphosphine to give an NHC-stabilized phosphorus(I) cation and a phosphorus(I) containing zwitterion, respectively.

1,3,5-Triazine(trithiophenylcarboxylate) esters form metastable monotropic nematic discotic liquid crystal phases

ABSTRACT Conformationally flexible cores that generate large free volume when stacked and have an insufficient number of side-chains for filling the lateral space around each core have been proposed as design criteria for discotic liquid crystals that display nematic rather than columnar mesophases. Presented here are 1,3,5-triazines substituted with three thiophene carboxylate ester groups that fulfil these design criteria. Nine derivatives with linear, branched, and chiral ester chains were synthesised and their mesomorphism was studied by polarised optical microscopy, differential scanning calorimetry, and powder x-ray diffraction. Triazines with linear propyl to octyl chains display monotropic nematic discotic mesophases that crystallise within hours or days or when mechanically agitated. Packing structures for the nematic phases are proposed based on the single crystal structure of the ethyl ester. Their unusually slow nucleation and crystal growth can be attributed to the large number of possible low energy conformations that arise from the conformationally flexible core. Incorporation of racemic and chiral branched chains expectedly lowers the melting points of these derivatives but, unexpectedly, enhances nucleation and crystal growth well above the temperature ranges of their hypothetical nematic phases. Chiral nematic discotic mesophases were obtained for binary mixtures of derivatives with chiral branched and linear chains. GRAPHICAL ABSTRACT

Synthesis of Heteroleptic Phosphorus(I) Cations by P+ Transfer

Reported are general synthetic approaches for the syntheses of asymmetrically substituted phosphorus(I) cations by P+ transfer from [dppeP]+ (dppe = 1,2-bis(diphenylphosphino)ethane). The first method grants access to acyclic derivatives and is accomplished by the sequential substitution of dppe using first a sterically encumbered ligand which cannot form a stable homoleptic complex, followed by a second equivalent of a less sterically demanding ligand. The second method grants access to cyclic derivatives and utilizes asymmetric hybrid phosphine/N-heterocyclic carbene ligands. Interplay between the different ligand types and their stoichiometries relative to those of [dppeP]+ also allows for the isolation of symmetrical derivatives with pendant phosphines.

Synthesis of Heavy Dicyanamide Homologues from Air-Stable Precursors

A convenient synthesis of dicyanophosphide and dicyanoarsenide anions is reported. These heavy homologues of the long-known and fundamentally important dicyanamide anion were formed through the nucleophilic displacement of bis(diphenylphosphino)ethane (dppe) from the pnictogen+ transfer agents [dppePn][BPh4 ] (Pn=P, As) by exposure to cyanide salts. The protocol requires three synthetic steps from commercially available materials and the [dppePn][BPh4 ] salts are remarkably temperature, air, and moisture stable. All products have been fully characterized by spectroscopic methods and by single-crystal X-ray diffraction, and the electronic structures of the DCPn anions have been assessed computationally.