Bjorn Askevold

Ammonia formation by metal–ligand cooperative hydrogenolysis of a nitrido ligand

Bioinspired hydrogenation of N(2) to ammonia at ambient conditions by stepwise nitrogen protonation/reduction with metal complexes in solution has experienced remarkable progress. In contrast, the highly desirable direct hydrogenation with H(2) remains difficult. In analogy to the heterogeneously catalysed Haber-Bosch process, such a reaction is conceivable via metal-centred N(2) splitting and unprecedented hydrogenolysis of the nitrido ligands to ammonia. We report the synthesis of a ruthenium(IV) nitrido complex. The high nucleophilicity of the nitrido ligand is demonstrated by unusual N-C coupling with π-acidic CO. Furthermore, the terminal nitrido ligand undergoes facile hydrogenolysis with H(2) at ambient conditions to produce ammonia in high yield. Kinetic and quantum chemical examinations of this reaction suggest cooperative behaviour of a phosphorus-nitrogen-phosphorus pincer ligand in rate-determining heterolytic hydrogen splitting.

Closed-shell and open-shell square-planar iridium nitrido complexes

Coupling reactions of nitrogen atoms represent elementary steps to many important heterogeneously catalysed reactions, such as the Haber-Bosch process or the selective catalytic reduction of NO(x) to give N(2). For molecular nitrido (and related oxo) complexes, it is well established that the intrinsic reactivity, for example nucleophilicity or electrophilicity of the nitrido (or oxo) ligand, can be attributed to M-N (M-O) ground-state bonding. In recent years, nitrogen (oxygen)-centred radical reactivity was ascribed to the possible redox non-innocence of nitrido (oxo) ligands. However, unequivocal spectroscopic characterization of such transient nitridyl {M=N(•)} (or oxyl {M-O(•)}) complexes remained elusive. Here we describe the synthesis and characterization of the novel, closed-shell and open-shell square-planar iridium nitrido complexes [IrN(L(t-Bu))](+) and [IrN(L(t-Bu))] (L(t-Bu)=N(CHCHP-t-Bu(2))(2)). Spectroscopic characterization and quantum chemical calculations for [IrN(L(t-Bu))] indicate a considerable nitridyl, {Ir=N(•)}, radical character. The clean formation of Ir(I)-N(2) complexes via binuclear coupling is rationalized in terms of nitrido redox non-innocence in [IrN(L(t-Bu))].

Learning from the Neighbors: Improving Homogeneous Catalysts with Functional Ligands Motivated by Heterogeneous and Biocatalysis

The future shortage of fossil-based resources requires enhanced efficiency for industrial processes. Catalysis will be a key field for the establishment of novel sustainable feedstock cycles for chemical synthesis. In addition, reactions associated with conversion and storage of regenerative energy, such as photochemical water splitting or chemical hydrogen storage, demand new reliable and abundant catalysts. 5] Approximately 80 % of all industrially synthesized chemicals, worth about 10 US-

A Square-Planar Ruthenium(II) Complex with a Low-Spin Configuration

(as of 2007), are presently produced by the application of catalysis for one or more reaction steps. Heterogeneous catalysis still represents the lion’s share (about 80 % in 2007). The advantages of heterogeneous catalysis, such as high thermal catalyst stability and easy product isolation, are clear. However, the multiphasic nature of heterogeneous catalysis and the structural and electronic complexity of catalyst surfaces generally render heterogeneous catalysis difficult to study. Therefore, despite tremendous success in creating a higher level of mechanistic understanding, catalyst development and improvement remains highly empirical. This minireview focuses on selected examples for efficient homogeneous catalysis based on functional ligands, which were inspired by heterogeneous and metalloenzyme catalysis. The strategies discussed are already established and cooperative catalysis by the action of either multiple metal centers or metal centers and cooperating ligands has been discussed by many authors as a useful concept to improve catalyst performance. By covering parts of our own recent results and related work, here we want to emphasize some basic principles associated with catalyst design from a more inorganic perspective, including topics, such as polymerization of organic and inorganic substrates, and applications relevant to energy conversion and storage.

Square-Planar Ruthenium(II) Complexes: Control of Spin State by Pincer Ligand Functionalization

The coordination chemistry of d ions of Group 8 is dominated by octahedral complexes. Four coordination is mainly observed in case of tetrahedral iron complexes, which exhibit an electronic high-spin configuration (S = 2, HS). With macrocyclic, chelating, and few monodentate ligands, square-planar, intermediate-spin (S = 1, IS) iron(II) complexes are known (Scheme 1, left). On the contrary, four-

A Ruthenium Hydrido Dinitrogen Core Conserved across Multielectron/Multiproton Changes to the Pincer Ligand Backbone

Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH2 CH2 PtBu2 )2 }], [RuCl{N(CHCHPtBu2 )(CH2 CH2 PtBu2 )}], and [RuCl{N(CHCHPtBu2 )2 }], in which the ruthenium(II) ions are in the extremely rare square-planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S=0) ground state and energetically low-lying triplet (S=1) state, the vinyl alkyl amido and the divinyl amido complexes exhibit unusual triplet (S=1) ground states as confirmed by experimental and computational examination. However, essentially non-magnetic ground states arise for the two intermediate-spin complexes owing to unusually large zero-field splitting (D>+200 cm(-1) ). The change in ground state electronic configuration is attributed to tailored pincer ligand-to-metal π-donation within the PNP ligand series.

Cooperative Aliphatic PNP Amido Pincer Ligands – Versatile Building Blocks for Coordination Chemistry and Catalysis

A series of ruthenium(II) hydrido dinitrogen complexes supported by pincer ligands in different formal oxidation states have been prepared and characterized. Treating a ruthenium dichloride complex supported by the pincer ligand bis(di-tert-butylphosphinoethyl)amine (H-PNP) with reductant or base generates new five-coordinate cis-hydridodinitrogen ruthenium complexes each containing different forms of the pincer ligand. Further ligand transformations provide access to the first isostructural set of complexes featuring all six different forms of the pincer ligand. The conserved cis-hydridodinitrogen structure facilitates characterization of the π-donor, π-acceptor, and/or σ-donor properties of the ligands and assessment of the impact of ligand-centered multielectron/multiproton changes on N2 activation. Crystallographic studies, infrared spectroscopy, and 15N NMR spectroscopy indicate that N2 remains weakly activated in all cases, providing insight into the donor properties of the different pincer ligand states. Ramifications on applications of (pincer)Ru species in catalysis are considered.