Johannes Bender

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.

Suboxides with Complex Anions : The Suboxoindate Cs9InO4

Salty metal: In the suboxometallate Cs(9)InO(4), metallic cesium columns (see picture; blue) lie next to ionic oxoindate(III) columns. The chemistry of the suboxides is thus expanded to structures containing complex anions.

On the energetics of P-P bond dissociation of sterically strained tetraamino-diphosphanes.

The homolytic P-P bond fission in a series of sterically congested tetraaminodiphosphanes (R2N)2P-P(NR2)2 ({4}2-{9}2, two of which were newly synthesized and fully characterized) into diaminophosphanyl radicals (R2N)2P˙ (4-9) was monitored by VT EPR spectroscopy. Determination of the radical concentration from the EPR spectra permitted to calculate free dissociation energies ΔGDiss(295) as well as dissociation enthalpies ΔHDiss and entropies ΔSDiss, respectively. Large positive values of ΔGDiss(295) indicate that the degree of dissociation is in most cases low, and the concentration of persistent radicals--even if they are spectroscopically observable at ambient temperature--remains small. Appreciable dissociation was established only for the sterically highly congested acyclic derivative {9}2. Analysis of the trends in experimental data in connection with DFT studies indicate that radical formation is favoured by large entropy contributions and the energetic effect of structural relaxation (geometrical distortions and conformational changes in acyclic derivatives) in the radicals, and disfavoured by attractive dispersion forces. Comparison of the energetics of formation for CC-saturated N-heterocyclic diphosphanes and the 7π-radical 3c indicates that the effect of energetic stabilization by π-electron delocalization in the latter is visible, but stands back behind those of steric and entropic contributions. Evaluation of spectroscopic and computational data indicates that diaminophosphanyl radicals exhibit, in contrast to aminophosphenium cations, no strong energetic preference for a planar arrangement of the (R2N)2P unit.

Crystal Structures of New Alkali Metal-rich Oxometallates: Rubidium Aluminate Tetrahydroxide, Rb9(AlO4)(OH)4, Rubidium Orthogallate, Rb5GaO4, Cesiumbis-Chromate(IV) Oxide, Cs10(CrO4)2O, and Cesium Diindate, Cs8In2O7

Several new alkali metal oxometallates with anions built up from tetrahedral [MO4] units were obtained in reactions aimed at the formation of alkali metal suboxometallates or by thermally decomposing the latter. Rubidium orthoaluminate tetrahydroxide Rb9(AlO4)(OH)4 crystallizes with a new structure type (space group P21/c, a = 13.116(1), b = 6.9266(5), c = 18.934(2) A , β = 92.05(1)°, V = 1719.0(3) Å3, Z = 4, R1 = 0.0352) and contains orthoaluminate anions [AlO4]5− and isolated hydroxide anions. Rubidium orthogallate Rb5GaO4 crystallizes with the Na5GaO4 structure type (space group Pbca, a = 6.9318(5), b = 21.309(2), c = 11.740(1) Å, V = 1734.2(3) Å3, Z = 8, R1 = 0.0423) with isolated orthogallate anions [GaO4]5−. Cesium chromate oxide Cs10(CrO4)2O adopts the Cs10(GeO4)2O structure type (space group P21/c, a = 12.903(1), b = 11.4523(8), c = 19.074(3) Å , β = 127.903(8)°,V = 2223.9(4) Å3, Z = 4, R1 = 0.0326) with orthochromate(IV) anions [CrO4]4− and isolated oxide anions. In all orthometallates the anions [MO4]n− deviate only slightly from ideal tetrahedral symmetry. Cesium diindate Cs8In2O7 crystallizes with the Cs8Fe2O7 structure type (space group P21/c, a = 7.4307(6), b = 18.6181(14), c = 7.2639(6) Å , β = 119.225(8)°, V = 877.0(1) Å3, Z = 2, R1 = 0.0349). A single-crystal structure investigation at r. t. has shown linear diindate units, but the temperature dependence of the libration angles from TLS studies for the bridging oxygen atom suggests a slightly bent and dynamically disordered diindate anion. Graphical Abstract Crystal Structures of New Alkali Metal-rich Oxometallates: Rubidium Aluminate Tetrahydroxide, Rb9(AlO4)(OH)4, Rubidium Orthogallate, Rb5GaO4, Cesiumbis-Chromate(IV) Oxide, Cs10(CrO4)2O, and Cesium Diindate, Cs8In2O7