To Split or Not to Split: [AsCCAs]-Coordinated Mo, W, and Re Complexes and Their Reactivity toward Molecular Dinitrogen

Tolane-Based Phosphino- and Arsino-Ruthenium Complexes in Three Different Oxidation States: Ru(I), Ru(II) and Ru(III)

Targeting Ru(III) and Ru(I) η2-alkyne species, 2,2'-(iPr2E)2-substituted diphenylacetylenes (1-E, E=P, As) were employed for the preparation of [ECCE]-coordinated ruthenium complexes. The reactions between 1-E and cis-(MeCN)2(COD)RuCl2 led to the required Ru(II) starting materials cis-[ECCE]RuCl2(MeCN) (3-E). Upon oxidation of 3-E with PhICl2, the Ru(III) target complexes [ECCE]RuCl3 (7-E) were detectable for E=P and E=As, but only the arsa-derivative 7-As was obtained in a pure form, namely via oxidation of cis-[AsCCAs]RuCl2(THT) (THT=tetrahydrothiophene). Upon reduction of compounds 3-E, a hitherto unprecedented Ru(I) η2-alkyne complex, [PCCP]RuCl (9), was obtained for E=P. The former square planar Ru(I) complex (9) was characterized comprehensively and examined in detail by means of DFT and CASSCF calculation. Upon treatment of 9 with TlPF6, a diamagnetic μ-Tl-bridged compound (10) with a nearly linear Ru-Tl-Ru array was formed and isolated in high yields. Careful analysis of the bonding situation suggested that the Ru-Tl-Ru moiety in 10 is best interpreted in terms of a 3c-4e- bond.

Chemocatalytic Conversion of Dinitrogen to Ammonia Mediated by a Tungsten Complex

Whereas molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase, corresponding tungsten complexes were in most cases found to be catalytically inactive. Herein, we present a modified pentadentate tetrapodal (pentaPod) phosphine ligand in which two dimethylphosphine groups of the pentaPodMe (P5Me) ligand have been replaced with phospholanes (Pln). The derived molybdenum complex [Mo(N2)P5Pln] generates 22 and the analogous tungsten complex [W(N2)P5Pln] 7 equivalents of NH3 from N2 in the presence of 180 equivalents of SmI2(THF)2/H2O, rendering the latter the first tungsten complex chemocatalytically converting N2 to NH3. In contrast, the tungsten complex [W(N2)P5Me] generates ammonia from N2 only in a slightly overstoichiometric fashion. The reasons for these reactivity differences are investigated with the help of spectroscopic and electrochemical methods.

Synthesis of Collidine from Dinitrogen via a Tungsten Nitride

The synthesis of pyridines from dinitrogen in homogeneous solution is known to be challenging considering that an N2 cleavage step needs to be combined with two N-C coupling steps. Herein, a tungsten complex bearing a tailor-made 2,2'-(tBu2As)2-substituted tolane ligand scaffold was shown to split N2 to afford the corresponding tungsten nitride, which is not the case for the corresponding (iPr2As)2-substituted derivative. The former nitride was then reacted with 2,4,6-trimethylpyrylium triflate, which led to the formation of a tungsten oxo complex, along with collidine. Over the course of this reaction, the O atom of the pyrylium starting material was replaced with an N atom via a hitherto unprecedented skeletal editing process.