2,2′-Diphosphino- and 2,2′-Diarsenotolanes and Their Fe-, Co-, and Ni-Complexes: Pnictogen-Dependent Cyclization Tendencies and Metal-Dependent Stability and Reactivity Patterns

Abstract

Common iron(II), cobalt(II), and nickel(II) bromides were reacted with 2,2′-bis(diisopropylphosphino)tolane 1-P (R = i Pr), which led to the isolation of P-heterocyclic chelate complexes (2-M, M = Fe, Co, and Ni) containing a new C−P and a new C−M bond. Attempts to reduce complexes 2-M (M = Fe, Co, and Ni), however, were unsuccessful, that is, no C−C coupling was observed, while diphosphadibenzo[a,e]pentalene formation was noticed in the majority of our experiments. To suppress the latter reactivity pattern, the reduced stability of As-ylides (in comparison to P-ylides) was exploited by preparing the arsaderivative of the ligand (1-As, R = i Pr), which was found to resist diarsadibenzo[a,e]pentalene formation. The lower tendency of 1-As to cyclize is also reflected in its reactions with MBr 2 (M = Fe, Co, and Ni). For M = Co, a κ 2 -As,As-coordinated complex (3-Co) with an uncoordinated alkyne backbone was obtained, while pincer formation was found to occur for M = Fe (3-Fe). Only in the case of nickel, an As-heterocyclic species (3-Ni) akin to 2-Ni was isolated. Upon reduction of 3-Ni, a trimetallic metallacyclopentadiene (4-Ni) was produced via alkyne C−C coupling, inferring that 3-Ni is prone to ring-opening under reducing conditions. Reduction of 3-Co was only successful in the presence of CO (2.5 atm), which led to the formation of a new As−C bond and the isolation of an As-heterocyclic cobalt tricarbonyl complex (5-Co). For 3-Fe, neither intermolecular C−C nor intramolecular As−C coupling was observed, but an unusual mixed-valent dimer with an [Fe 2 (alkyne) 2 ] + core (4-Fe) was generated and isolated upon reduction with Zn powder. Compounds 4-M (M = Ni and Fe) are discussed in the context of alkyne cycloadditions, given that related motifs have been proposed as key intermediates in these transformations.

[AsCCAs]-Coordinated Rhenium Hydrides and Their Reactivities Toward Unsaturated Hydrocarbons, Heterocumulenes, and CO2

“…In view of the high price of rhenium and severe issues upon scale-up to amounts of >200 mg, a strategy to circumvent ligand cyclization was considered mandatory and the recently reported arsa -ligand derivative ( 1-As ) was taken into consideration, which requires a more detailed explanation and a closer look at the former cyclization products: These undesirable species are formed via twofold nucleophilic attack at the alkyne unit, thus generating a diylide, which is known to readily decompose in CH 2 Cl 2 to give rise to the observed cationic oxidation products . Given that the formation of As-ylides is considerably less favorable (in comparison to P-ylides), similar ligand cyclization pathways are hampered substantially for 1-As and thus not even observed at elevated temperatures . Using this thermally robust and less air-sensitive ligand derivative, the corresponding pincer complex 2-As was prepared in excellent yields of 95% and isolated in gram quantities (see Scheme ).…”

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

“…Disappointingly, these efforts met with failure (W) and limited success (Re) 18 given that heating is required for the complexation of these kinetically inert 5d-metals, which inevitably leads to the formation of 4 (see Scheme 1). The arsa-derivative of the ligand (1-As), however, was shown to resist thermal cyclization reactions, 19 which prompted us evaluate whether tungsten and rhenium complexes of 1-As may be prepared and exploited for the activation of N 2 .…”

“…Disappointingly, these efforts met with failure (W) and limited success (Re) given that heating is required for the complexation of these kinetically inert 5d-metals, which inevitably leads to the formation of 4 (see Scheme ). The arsa -derivative of the ligand ( 1-As ), however, was shown to resist thermal cyclization reactions, which prompted us evaluate whether tungsten and rhenium complexes of 1-As may be prepared and exploited for the activation of N 2 . Considering the crucial role of molybdenum in the context of N 2 activation ( vide supra ), it seemed reasonable to include molybdenum in our studies and, thus, elucidate whether arsa -analogs of 2-P and 3-P may be prepared.…”