Base metal complexes featuring a new pyrazole-derived PCP pincer ligand

Abstract

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)₆] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ²PN-PCP-iPr)(CO)₄] were obtained featuring a κ²P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ³PCP-PCP-iPr)(CO)₃] was observed featuring a weak Mo–C bond. DFT calculations reveal that there is no agostic η²-C–H interaction. Treatment of [Mn₂(CO)₁₀], [Fe₂(CO)₉], [Co₂(CO)₈] and [Ni(COD)₂] afforded complexes [Mn(κ³PCP-PCP-iPr)(CO)₃], [Fe(κ³PCP-PCP-iPr)(H)(CO)₂], [Co(κ³PCP-PCP-iPr)(CO)₂] and [Ni(κ³PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ³-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me₃O]BF₄. Treatment of [Mn(κ³PCP-PCP-iPr)(CO)₃] and [Fe(κ³PCP-PCP-iPr)(H)(CO)₂] with [Me₃O]BF₄ resulted in the formation of the cationic complexes [Mn(κ³PCP-PCP^Me-iPr)(CO)₃]⁺ and [Fe(κ³PCP-PCP^Me-iPr)(Cl)(CO)₂]⁺. In the case of the latter, the chloride ligand seems to originate from the solvent CH₂Cl₂ undergoing a hydride chloride exchange. All complexes were characterized by means of ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.