Cyclodiphosphazanes with functionalities: Synthesis, reactivity and transition metal chemistry

Single Molecular Persistent Room-Temperature Phosphorescence and Circularly Polarized Luminescence from Binaphthol-Decorated Optically Innocent Cyclotriphosphazenes

Circularly polarized luminescence (CPL) features of BINOL-decorated cyclotriphosphazenes (CPs) are reported for the first time. The luminescence dissymmetry factor (glum ) of these compounds in chloroform solutions and polymethyl methacrylate (PMMA) thin films with wt 1 % doping concentrations are found to be 1.0×10-3 , and 2.9×10-3 , respectively. However, no CPL signal is observed for the pristine solids. The enantiomers (CP-(R)/CP-(S)) show ultraviolet photoluminescence (~350-360 nm) in solution and the solid state. These compounds show ~10 times larger absolute photoluminescence quantum yield (PLQY) than the simple BINOLs in the solutions state. In the solid state, CP-(R) shows larger PLQY than binaphthol-(R); in contrast, the S enantiomer shows lower PLQY than binaphthol-(S); this indicates that the isomer-dependent solid-state packing of these compounds plays a crucial role in controlling the PL. Thin films with more than 1 % doping concentration and pristine solids of these compounds do not show persistent room-temperature phosphorescence (pRTP) due to concentration-caused quenching. However, thin films with wt 1 % of these chiral emitters exhibit pRTP characteristics with a ~159-343 ms lifetime under vacuum. Theoretical calculations reveal that the cyclophosphazene acts as an optically innocent dendritic core, and the optical features of these compounds are dictated by the pendent BINOL chromophore.

Single- and double-bridged PNP ligands in chromium-catalysed ethylene oligomerisation

Chromium catalysts with diazaphospholane ligands have shown good activities and selectivities for ethylene tri- and tetramerisation. Oligomerisations with a doubly N-bridged cyclodiphosphazane result in a Schulz–Flory distribution of α-olefins.

N-Bridged Acyclic Trimeric Poly-Cyclodiphosphazanes: Highly Tuneable Cyclodiphosphazane Building Blocks

We have synthesized a completely new family of acyclic trimeric cyclodiphosphazane compounds comprising NH, Nⁱ Pr, N^t Bu and NPh bridging groups. In addition, the first NH-bridged acyclic dimeric cyclophosphazane has been produced. The trimeric species display highly tuneable characteristics so that the distance between the terminal N(H)R moieties can be readily modulated by the steric bulk present in the bridging groups (ranging from ≈6 to ≈10 Å). Moreover, these species exhibit pronounced topological changes when a weak non-bonding NH⋅⋅⋅π aryl interaction is introduced. Finally, the NH-bridged chloride binding affinities have been calculated and benchmarked along with the existing experimental data available for monomeric cyclodiphosphazanes. Our results underscore these species as promising hydrogen bond donors for supramolecular host-guest applications.

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.

Spirocyclic, macrocyclic and ladder complexes of coinage metals and mercury with dichalcogeno P2N2-supported anions

Reactions of the dianions [^tBuN(E)P(μ-N^tBu)]₂²⁻ (E = Se,S) with M(i) reagents (M = Ag, Au) or HgCl₂ produce complexes of [^tBu(H)N(E)P(μ-N^tBu)₂P(E)N^tBu]⁻ with spirocyclic, macrocyclic or ladder structures.

Steric C–N bond activation on the dimeric macrocycle [{P(μ-NR)}2(μ-NR)]2

The dimeric macrocyclophosphazane [{P(μ-N^tBu)}₂(μ-N^tBu)]₂ ( 1) was reacted with elemental selenium. An unexpected C–N cleavage reaction occurred producing P₄(μ-N^tBu)₃(μ-NH)₃Se₄ ( 2). The C–N bond cleavage is driven by the high steric ring strain present within the ring.

Chiral ditopic cyclophosphazane (CycloP) ligands: synthesis, coordination chemistry, and application in asymmetric catalysis

A series of dichlorocyclophosphazanes [{ClP(μ-NR)}2 ] containing chiral and achiral R groups was obtained from simple commercially available amines and PCl3 . Their condensation reactions with axially chiral biaryl diols yielded ansa-bridged chiral cyclophosphazane (CycloP) ligands. This highly modular methodology allows extensive elaboration of the ligand set, in which the chirality can be introduced at the diol bridge and/or the amido R group. This provides the possibility to observe match and mismatch effects in catalysis. A series of twenty CycloP ligands was synthesized and characterized by multinuclear NMR spectroscopy, HRMS, elemental analysis, and in selected cases, single-crystal X-ray diffraction. These studies show that all of the ditopic CycloP ligands are C2 symmetric, rendering their metal coordination sites symmetry equivalent. Two well-established enantioselective reactions were explored by using late-transition metal CycloP complexes as catalysts; the gold-catalyzed hydroamination of γ-allenyl sulfonamides and the asymmetric nickel-catalyzed three-component coupling of a diene and an aldehyde. The steric demands of the CycloP ligands have a subtle influence on the reactivity and selectivity observed in both reactions. Good enantiomeric ratios (e.r.) as high as 89:11 in the gold-catalyzed reaction and 92:8 in the nickel-catalyzed bis-homoallylation reaction were observed.

The Phospha-Michael addition product {(t-BuNH)P(μ-N-t-Bu)(2)P(=N-t-Bu)-C(=CH(2))CH(p-CH(3)O-C(6)H(4))-P(O)[(OCH(2)C(CH(3))(2)CH(2)O)]}

The title compound, 2-{2-[1,3-di-tert-butyl-4-(tert-butyl-amino)-2-(tert-butyl-imino)-1,3,2λ(5),4-diaza-diphosphetidin-2-yl]-1-(4-meth-oxy-phen-yl)prop-2-en-1-yl}-5,5-dimethyl-1,3,2λ(5)-dioxaphosphinan-2-one, C(31)H(57)N(4)O(4)P(3), was synthesized from the Phospha-Michael addition reaction of cyclo-diphos-pha-zane [(t-BuNH)P(μ-Nt-Bu)](2) and allenyl-phospho-nate [(OCH(2)C(CH(3))(2)CH(2)O)P(O)C(p-CH(3)O-C(6)H(4))=C=CH(2)]. In the crystal, N-H⋯O and C-H⋯O hydrogen bonds link the mol-ecules. The structure exhibits pseudosymmetry but attempts to solve it in a higher (monoclinic) space group were unsuccessful.