Annellated 1,3,4,2-Triazaphospholenes-Simple Modular Synthesis and a First Exploration of Ligand Properties.

The successful use of 1,3,4,2-triazaphospholenes (TAPs) as organo-catalysts stresses the need for efficient synthetic routes to these molecules. In this study, we establish the [1 + 4]-cycloaddition of PBr3 to azo-pyridines as a new approach to preparing pyrido-annellated TAPs in a single step from easily available precursors. The modular assembly of the azo-component via condensation of primary amines and nitroso compounds along with the feasibility of post-functionalization at the P-Br bond under conservation of the heterocyclic structure allows, in principle, to address a wide range of target molecules, which is illustrated by prototypical examples. The successful synthesis of a transition metal complex confirms for the first time the ability of a TAP to act as a P-donor ligand. Crystallographic studies suggest that hyperconjugation effects and intermolecular interactions induce a qualitatively similar ionic polarization of the P-Br bonds in TAPs as in better known isoelectronic diazaphospholenes.

A PH-functionalized dicationic bis(imidazolio)diphosphine.

Reaction of the iodide salt of a secondary imidazolio-iodophosphine [(L)PHI]I (L+ = 1,3-diarylimidazolium-yl) with an imidazolio-phosphide (L)PH in the presence of GaI3 afforded the isolable salt of a dicationic, bis(imidazolio)-substituted dihydro-diphosphine [(L)2P2H2][GaI4]2. Non-preparative formation of the cationic diphosphines was also observed upon spontaneous "dehalo-coupling" of [(L)PHI]+, or in reactions of [(L)PHI]I and (L)PH in the absence of GaI3. Further reaction of [(L)2P2H2]2+ with (L)PH produced an iodide salt of a known (bis)imidazolio-diphosphide monocation [(L)2P2H]+. The identity of cationic diphosphines and diphosphides was established by single-crystal X-ray diffraction studies. NMR spectroscopy revealed that dications [(L)2P2H2]2+ exist as a mixture of meso- and rac-diastereomers in solution. Computational studies confirmed the stereochemical assignment of the isomers observed, and gave insight into the bonding situation of the diphosphine dications.

Synthesis and Ambiphilic Reactivity of Metalated Diorgano-Phosphonite Boranes.

Unprecedented metalated phosphonite boranes were prepared from PH-substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self-condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry.

Proton transfer vs. oligophosphine formation by P-C/P-H σ-bond metathesis: decoding the competing Brønsted and Lewis type reactivities of imidazolio-phosphines.

Studies of the protonation and alkylation of imidazolio-phosphides and deprotonation of imidazolio-phosphines reveal a complex behaviour that can be traced back to an interplay of Brønsted-type proton transfers and Lewis-type P-P bond formation reactions. As a consequence, the expected (de)protonation and (de)alkylation processes compete with reactions producing cyclic or linear oligophosphines. A careful adjustment of the conditions allows us to selectively address each reaction channel and devise specific synthesis methods for primary, secondary and tertiary imidazolio-phosphines, imidazolio-alkylphosphides, and cyclic oligophosphines, respectively. Mechanistic studies reveal that oligophosphines assemble in sequential P-P bond formation steps involving the condensation of cationic imidazolio-phosphines viaσ-bond metathesis and concomitant elimination of an imidazolium ion. Imidazolio-phosphides catalyse these transformations. Computational model studies suggest that the metathesis proceeds in two stages via an initial nucleophilic substitution under expulsion of a carbene, and a subsequent proton transfer step that generates an imidazolium cation and provides the driving force for the whole transformation. As energy barriers are predicted to be low or even absent, different elementary steps are presumed to form a network of mutually coupled equilibrium processes. Cyclic oligophosphines or their dismutation products are identified as the thermodynamically favoured final products in the reaction network.

Isolable Diaminophosphide Boranes.

Metalation of secondary diaminophosphine boranes by alkali metal amides provides a robust and selective access route to a range of metal diaminophosphide boranes M[(R2 N)2 P(BH3 )] (M=Li, Na, K; R=alkyl, aryl) with acyclic or heterocyclic molecular backbones, whereas reduction of a chlorodiaminophosphine borane gave less satisfactory results. The metalated species were characterized in situ by NMR spectroscopy and in two cases isolated as crystalline solids. Single-crystal XRD studies revealed the presence of salt-like structures with strongly interacting ions. Synthetic applications of K[(R2 N)2 P(BH3 )] were studied in reactions with a 1,2-dichlorodisilane and CS2 , which afforded either mono- or difunctional phosphine boranes with a rare combination of electronegative amino and electropositive functional disilanyl groups on phosphorus, or a phosphinodithioformate. Spectroscopic studies gave a first hint that removal of the borane fragment may be feasible.

Reversible cooperative dihydrogen binding and transfer with a bis-phosphenium complex of chromium.

The reversible reaction of H2 with a bis-phosphenium complex of chromium provides a rare example of 3d transition metal/phosphenium cooperativity. Photolysis induces the activation of H2 and yields a spectroscopically detectable phosphenium-stabilized (σ-H2)-complex, readily showing exchange with gaseous H2 and D2. Further reaction of this complex affords a phosphine-functionalized metal hydride, representing a unique example of reversible H2 cleavage across a 3d M[double bond, length as m-dash]P bond. The same species is also accessible via stepwise H+/H- transfer to the bis-phosphenium complex, and releases H2 upon heating or irradiation. Dihydrogen transfer from the H2-complex to styrene is exploited to demonstrate the first example of promoting hydrogenation with a phosphenium complex.

Bis-[3]Ferrocenophanes with Central >E-E'< Bonds (E, E'=P, SiH): Preparation, Properties, and Thermal Activation.

Invited for this month's cover picture are the groups of Professors Rudolf Pietschnig at the University of Kassel, Professor Dietrich Gudat at the University of Stuttgart and Professor László Nyulászi at the Budapest University of Technology and Economics. The cover picture shows the thermally induced homolytic cleavage of the central P-P bond in a phosphorus-rich bis-ferrocenophane furnishing P-centered radicals (as evidenced by the computed spin-density highlighted in blue). The central P6 unit in the title compound is a structural analog of the connecting unit in Hittorf's violet phosphorus, which links the orthogonally arranged tubular entities. A portrait of the German physicist Johann Wilhelm Hittorf is included. Read the full text of their Full Paper at 10.1002/open.201900182.

Bis-[3]Ferrocenophanes with Central >E-E'< Bonds (E, E'=P, SiH): Preparation, Properties, and Thermal Activation.

A series of bis-[3]ferrocenophanes of the general type Fe(C5H4E')2E-E(E'C5H4)2Fe (E=P, SiH and E'=PtBu, NneoPentyl, NSi(CH3)3) with an isolobal molecular framework have been prepared and characterized by heteronuclear NMR spectroscopy and X-ray crystallography. The thermal dissociation behavior with respect to homolytic fission of the central bond generating phosphorus centered radicals was investigated using EPR spectroscopy and quantum chemical calculations.

Controllable access to P-functional [3]ferrocenophane and [4]ferrocenophane frameworks.

Condensation of secondary 1,1'-diaminoferrocenes with phosphorus trihalides (PCl3 or PBr3) yielded either P-halo-1,3-diaza-2-phospha[3]ferrocenophanes or 1,1'-diaminoferrocenediyl-bis(dichlorophosphines), respectively. The latter provide controlled access to both [3]- and [4]ferrocene frameworks. Thus, reductive coupling with magnesium gave diaza-diphospha[4]ferrocenophane-annelated tetraphosphetanes, while a reaction with LiNMe2 produced a P-chloro-diazaphospha[3]ferrocenophane. Condensation of diaminoferrocenes and aminodichlorophosphines were mostly unselective, but afforded in one case a 3-amino[3]ferrocenophane. All reaction products were characterised by spectroscopic and single-crystal XRD studies. DFT studies indicate that the product selectivity in the reactions studied depends on a combination of kinetic and thermodynamic effects, which correlate subtly with the steric bulk of the N-substituents. Cyclic voltammetry measurements revealed that the ferrocenophanes can undergo multiple oxidation events, the first of which may according to DFT studies be located in both the ferrocene and aminophosphine units. The quantum chemical studies provided also insight into stereochemical aspects like ring strain in the ferrocenophane units.

Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes.

N-Heterocyclic phosphenium (NHP) and nitrosonium (NO+) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox "noninnocent" behavior. We report here on mixed complexes [(NHP)M(CO) n(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)3(NO)] or PPN[Cr(CO)4(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV-vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and π-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO-) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/NO complexes is closely akin to that of the Hieber anion, [Fe(CO)3(NO)]-. The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)2(NO)] (4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character, and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange or trapped with electrophiles (H+, SnPh3+) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M═P double bond.

N-Heterocyclic Phosphenium Complex of Manganese: Synthesis and Catalytic Activity in Ammonia Borane Dehydrogenation.

A neutral N-heterocyclic phosphenium complex of manganese was synthesised by a metathesis approach and characterised by IR, NMR, and XRD studies. The short P-Mn distance suggests a substantial metal-ligand double bond character. Reaction with a hydride produced an anionic phosphine complex, which was also characterised by IR and NMR spectroscopy and, after anion exchange, a single-crystal XRD study. Protonation of the anion occurs at the metal to yield a neutral phosphine metal carbonyl hydride, which releases dihydrogen upon irradiation with UV light. These reactions confirm the electrophilic nature of the phosphenium ligand and suggest that the title complex might undergo reactions displaying metal-ligand cooperativity. Surprisingly, reaction with ammonia borane (AB) did not proceed under transfer hydrogenation of the Mn=P double bond but through the catalytic dehydrogenation of AB. The phosphenium complex behaves here as a class II catalyst, which dehydrogenates AB to NH2 BH2 that was trapped with cyclohexene. Computational model studies led to the identification of two possible catalytic cycles, which differ in the regioselectivity of the initial AB activation step. In one case, the activation proceeds as cooperative transfer hydrogenation of the Mn=P bond, whereas in the other case a H+ /H- pair is transferred to the phosphorus atom and a nitrogen atom of the phosphenium unit, resulting in a ligand-centred reaction in which the metal fragment acts merely as stabilising substituent. Unexpectedly, this pathway, which constitutes a new reaction mode for phosphenium complexes, seems to be better in accord with experimental findings on the course of the catalysis.