Synthesis and reactivity of coordinatively unsaturated halocarbonyl molybdenum PNP pincer complexes

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.