Synthesis and reactivity of coordinatively unsaturated halocarbonyl molybdenum PNP pincer complexes

Abstract

In the present study a series of six-coordinate neutral 16e halocarbonyl Mo(II) complexes of the type [Mo(PNP^Me-iPr)(CO)X₂] (X = I, Br, Cl), featuring the PNP pincer ligand N,N′-bis(diisopropylphosphino)-N,N′-dimethyl-2,6-diaminopyridine (PNP^Me-iPr), were prepared and fully characterized. The synthesis of these complexes was accomplished by different methodologies depending on the halide ligands. For X = I and Br, [Mo(PNP^Me-iPr)(CO)I₂] and [Mo(PNP^Me-iPr)(CO)Br₂] were obtained by reacting [Mo(PNP^Me-iPr)(CO)₃] with stoichiometric amounts of I₂ and Br₂, respectively. In the case of X = Cl, [Mo(PNP^Me-iPr)(CO)Cl₂] was afforded by the reaction of [Mo(CO)₄(μ-Cl)Cl]₂ with 1 equiv. of PNP^Me-iPr. The equivalent procedure also worked for X = Br. The modification of the 2,6-diaminopyridine scaffold by introducing NMe instead of NH spacers between the aromatic pyridine ring and the phosphine moieties changed the steric properties of the PNP-iPr ligand significantly. While in the present case exclusively neutral six-coordinate complexes of the type [Mo(PNP^Me-iPr)(CO)X₂] were obtained, with the parent PNP-iPr ligand, i.e. featuring NH spacers, cationic seven-coordinate complexes of the type [Mo(PNP-iPr)(CO)₃X]X were afforded. Upon treatment of [Mo(PNP^Me-iPr)(CO)X₂] (X = Br, Cl) with Ag⁺ in CH₃CN, the cationic complexes [Mo(PNP^Me-iPr)(CO)(CH₃CN)X]⁺ were formed. Halide abstraction from [Mo(PNP^Me-iPr)(CO)Cl₂] in THF–CH₂Cl₂ afforded [Mo(PNP^Me-iPr)(CO)(THF)Cl]⁺. In keeping with the facile synthesis of monocationic complexes preliminary ESI-MS and DFT/B3LYP studies revealed that one halide ligand in complexes [Mo(PNP^Me-iPr)(CO)X₂] is labile forming cationic fragments [Mo(PNP^Me-iPr)(CO)X]⁺ which react with molecular oxygen in parallel pathways to yield mono and dioxo Mo(IV) and Mo(VI) species. Structures of representative complexes were determined by X-ray single crystal analyses.