A complete series of halocarbonyl molybdenum PNP pincer complexes - Unexpected differences between NH and NMe spacers

In the present study a complete series of seven-coordinate neutral halocarbonyl Mo(II) complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] (X = I, Br, Cl, F), featuring the new PNP pincer ligand N,N'-bis(diphenylphosphino)-N,N'-methyl-2,6-diaminopyridine (PNP^Me-Ph), 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-Ph)(CO)₂I₂] and [Mo(PNP^Me-Ph)(CO)₂Br₂] were obtained by reacting [Mo(PNP^Me-Ph)(CO)₃] with stoichiometric amounts of I₂ and Br₂, respectively. Alternatively, these complexes were obtained upon treatment of [MoX₂(CO)₃(CH₃CN)₂] (X = I, Br) with 1 equiv. of PNP^Me-Ph. On the other hand, in the case of X = Cl, [Mo(PNP^Me-Ph)(CO)₂Cl₂] was afforded by the reaction of [Mo(CO)₄(μ-Cl)Cl]₂ with 1 equiv. of PNP^Me-Ph. The equivalent procedure also worked for X = Br. Finally, addition of 1 equiv. of 1-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate to [Mo(PNP^Me-Ph)(CO)₃] yielded the analogous fluorine complex [Mo(PNP^Me-Ph)(CO)₂F₂]. The modification of the ligand scaffold by introducing a Me group instead of H changed the properties of the PNP-Ph ligand significantly. While in the present case exclusively neutral seven-coordinate complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] were obtained, with the parent PNP-Ph ligand, i.e., featuring NH spacers, cationic seven-coordinate complexes of the type [Mo(PNP-Ph)(CO)₃X]X were afforded. DFT calculations indicated that the reactions are under thermodynamic control. The structures of representative complexes were determined by X-ray single crystal analyses.