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
Benjamin Rudin, Lukas Eberle, Joachim Ballmann
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 arsa -derivative 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.