Chromium Complexes with Benzanellated N-heterocyclic Phosphenium Ligands - Synthesis, Reactivity and Application in Catalytic CO2 Reduction.

A chromium complex carrying two benzanellated N-heterocyclic phosphenium (bzNHP) ligands was prepared by a salt metathesis approach. Spectroscopic studies suggest that the anellation enhances the π-acceptor ability of the NHP-units, which is confirmed by the facile electrochemical reduction of the complex to a spectroscopically characterized radical anion. Co-photolysis with H2 allowed extensive conversion into a σ-H2-complex, which shows a diverse reactivity towards donors and isomerizes under H-H bond fission and shift of a hydride to a P-ligand. The product carrying phosphenium, phosphine and hydride ligands was also synthesized independently and reacts reversibly with CO and MeCN to yield bis-phosphine complexes under concomitant Cr-to-P-shift of a hydride. In contrast, CO2 was not only bound but reduced to give an isolable formato complex, which reacted with ammonia borane under partial recovery of the metal hydride and production of formate. Further elaboration of the reactions of the chromium complexes with CO2 and NH3BH3 allowed to demonstrate the feasibility of a Cr-catalyzed transfer hydrogenation of CO2 to methanol. The various complexes described were characterized spectroscopically and in several cases by XRD studies. Further insights in reactivity patterns were provided through (spectro)electrochemical studies and DFT calculations.

The Noble Addendum of a Phosphenium Ligand to a Base Metal: Coordination, Activation, and Hydrogenation of Alkenes and Alkynes on a Chromium Complex.

The synthesis of a new bis-NHP complex (NHP=N-heterocyclic phosphenium) of chromium via salt metathesis and studies of its reactivity are reported. Photochemical reactions with H2 and selected olefins give rise to non-isolable H2- and π-alkene complexes identified spectroscopically, while internal alkynes react via activation of the triple bond to yield isolable metalla-phospha-cyclobutenes characterized by spectroscopic and XRD data. DFT studies give a preliminary account of the bonding in H2- and alkene-complexes and explain the different reactivity towards alkenes and alkynes as the consequence of kinetic effects. Photolysis of the bis-NHP-complex in the presence of H2 and olefins or alkenes enables the catalytic hydrogenation of the organic substrates, while the π-ethene complex mediates the catalytic hydrogenation of ethene in a dark reaction. The similarities and differences between both catalytic processes are shortly discussed.

Reversible Binding of Hydrogen and Styrene Coordination on a Manganese Phosphenium Complex.

The reactions of two complexes [(R NHP)Mn(CO)4 ] (R NHP=N-arylated N-heterocyclic phosphenium) with H2 at elevated pressure (≈4 bar) were studied by NMR spectroscopy. Irradiation with UV light initialized in one case (5 a, R=Dipp) the unselective formation of (R NHP-H)MnH(CO)4 ] (6 a) via cooperative addition of H2 across the Mn=P double bond. In the other case (5 b, R=Mes), addition of H2 was unobservable and the reaction proceeded via decarbonylation to a dimeric species [(R NHP)2 Mn2 (CO)7 ] (7 b) that was isolated and identified spectroscopically. Taking into account the outcome of further reaction studies under various conditions in the absence and presence of H2 , both transformations can be explained in the context of a common mechanism involving decarbonylation to 7 a,b as the first step, and the different outcome is attributable to the fact that 7 b is unreactive towards both H2 and CO while 7 a is not. DFT studies relate this divergence to deviations in the molecular constitution and stability arising from a different level of steric congestion. Preliminary studies suggest further that 5 a/H2 as well as 6 a enable the photo-induced hydrogenation of styrene to ethyl benzene, even if the mechanism and possibly catalytic nature of this process remain yet unknown.

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 neutral analogue of a phosphamethine cyanine.

Reaction of an imidazolio-phosphide with an N-heterocyclic bromo-borane and NaH afforded a neutral analogue of a phosphamethine cyanine cation. DFT studies were used to analyse the dative bonding across P-C/B bonds and the conformational preferences and imply that the observed conformation is imposed by sterics.

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.

Formation and properties of inorganic Si-contaminant compounds.

Soil contamination with inorganic contaminants such as lead (Pb), copper (Cu) and cadmium (Cd) is a major environmental issue. Silicon (Si) may reduce the mobility of the contaminants in the environment so that we studied the extent and the mechanisms of the interactions between Pb2+, Cd2+ and Cu2+ and silicic acid during its polymerization. We used tetraethyl orthosilicate as Si source and separately Pb(NO3)2, Cd(NO3)2 or Cu(NO3)2. Selectivity of Si towards the metals was tested in an equimolar solution of all three salts and the polymerizing Si source. Time-dependency of particle growth was examined using dynamic light scattering. Transmission electron microscopy was used for visualizing the particles. We characterized the solid phases by Fourier transform infrared (FTIR) and 29Si nuclear magnetic resonance (NMR) spectroscopy. Polymerized silica bound relative to the initial concentrations (10 mmol L-1) up to 2.1‰ Cd2+, 2‰ Cu2+ and 1.4‰ Pb2+. The FTIR spectra indicated an incorporation of the metals in the polymeric network. 29Si-NMR relaxation experiments showed an accelerating effect of Cu2+ on the 29Si longitudinal relaxation time. It appears that the proportion of the rapidly relaxing components decreases with increasing distance to the surface. This points to a predominant location of Cu centers close to the surface of the Si matrix. Thus, polymerizing silica may contribute to reduced metal mobility in the environment.

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.

Approaching Dissolved Species in Ammonoacidic GaN Crystal Growth: A Combined Solution NMR and Computational Study.

Solutions of gallium trihalides GaX3 (X=F, Cl, Br, I) and their ammoniates in liquid ammonia were studied at ambient temperature under autogenous pressure by multinuclear (71 Ga, 35 Cl, 81 Br) NMR spectroscopy. To unravel the role of pH, the analyses were done both in absence and in presence of ammonium halides, which are employed as mineralizers during ammonoacidic gallium nitride crystal growth. While gallium trifluoride and its ammoniate were found to be too sparingly soluble to give rise to a NMR signal, the spectra of solutions of the heavier halides reveal the presence of a single gallium-containing species in all cases. DFT calculations and molecular dynamics simulations suggest the identification of this species as consisting of a [Ga(NH3 )6 ]3+ cation and up to six surrounding halide anions, resulting in an overall trend towards negative complex charge. Quantitative 71 Ga NMR studies on saturated solutions of GaCl3 containing various amounts of additional NH4 Cl revealed a near linear increase of GaCl3 solubility with mineralizer concentration of about 0.023 mol GaCl3 per mol NH4 Cl at room temperature. These findings reflect the importance of Coulombic shielding for the inhibition of oligomerization and precipitation processes and help to rationalize both the low solubility of gallium halides in neutral ammonia solution and, in turn, the proliferating effect of the mineralizer during ammonoacidic gallium nitride formation.

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.

Variable Reactivity of a N-Heterocyclic Phosphenium Complex: P-C Bond Activation or "Abnormal" Deprotonation.

The reaction of an N-heterocyclic phosphenium complex of manganese with MeLi/Et3 NHCl under formal addition of CH4 to the Mn=P double bond can be reversed upon UV photolysis, providing a rare example for selective P-C(alkyl) bond activation. Action of LDA on the phosphenium complex does not proceed via attack at phosphorus but rather via C4-deprotonation to yield a unique P-analogue of an "abnormal" carbene. A transmetalation product of the original complex was fully characterized. The C-metalation is also applicable to bis-phosphenium complexes of other metals.

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.

Visible light assisted hydrogen generation from complete decomposition of hydrous hydrazine using rhodium modified TiO2 photocatalysts.

Hydrogen is considered to be an ideal energy carrier, which produces only water when combined with oxygen and thus has no detrimental effect on the environment. While the catalytic decomposition of hydrous hydrazine for the production of hydrogen is well explored, little is known about its photocatalytic decomposition. The present paper describes a highly efficient photochemical methodology for the production of hydrogen through the decomposition of aqueous hydrazine using titanium dioxide nanoparticles modified with a Rh(i) coordinated catechol phosphane ligand (TiO2-Rh) as a photocatalyst under visible light irradiation. After 12 h of visible light irradiation, the hydrogen yield was 413 µmol g-1 cat with a hydrogen evolution rate of 34.4 µmol g-1 cat h-1. Unmodified TiO2 nanoparticles offered a hydrogen yield of 83 µmol g-1 cat and a hydrogen evolution rate of only 6.9 µmol g-1 cat h-1. The developed photocatalyst was robust under the experimental conditions and could be efficiently reused for five subsequent runs without any significant change in its activity. The higher stability of the photocatalyst is attributed to the covalent attachment of the Rh complex, whereas the higher activity is believed to be due to the synergistic mechanism that resulted in better electron transfer from the Rh complex to the conduction band of TiO2.