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(PNPMe-Ph)(CO)2X2] (X I, Br, Cl, F), featuring the new PNP pincer ligand N,N'-bis(diphenyl-phosphino)-N,N'-methyl-2,6-diaminopyridine (PNPMe-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(PNPMe-Ph)(CO)(2)I-2] and [Mo(PNPMe-Ph)(CO)(2)Br-2] were obtained by reacting [Mo(PNPMe-Ph)(CO)(3)] with stoichiometric amounts of I-2 and Br-2, respectively. Alternatively, these complexes were obtained upon treatment of [MoX2(CO)(3)(CH3CN)(2)] (X = I, Br) with 1 equiv. of PNPMe-Ph. On the other hand, in the case of X Cl, [Mo(PNPMe-Ph)(CO)(2)Cl-2] was afforded by the reaction of [Mo(CO)(4)(m-Cl)Cl](2) with 1 equiv. of PNPMe-Ph. The equivalent procedure also worked for X Br. Finally, addition of 1 equiv. of 1-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate to [Mo(PNPMe-Ph)(CO)(3)] yielded the analogous fluorine complex [Mo(PNPMe-Ph)(CO)(2)F-2]. 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(PNPMe-Ph)(CO)(2)X-2] were obtained, with the parent PNP-Ph ligand, i. e., featuring NH spacers, cationic seven-coordinate complexes of the type [Mo(PNP-Ph)(CO)(3)X]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. (C) 2014 Elsevier B. V. All rights reserved.