Synthesis and reactivity of TADDOL-based chiral Fe(ii) PNP pincer complexes-solution equilibria between κ2P,N- and κ3P,N,P-bound PNP pincer ligands

Abstract

Treatment of anhydrous FeX₂ (X = Cl, Br) with 1 equiv. of the asymmetric chiral PNP pincer ligands PNP-R,TAD (R = iPr, tBu) with an R,R-TADDOL (TAD) moiety afforded complexes of the general formula [Fe(PNP)X₂]. In the solid state these complexes adopt a tetrahedral geometry with the PNP ligand coordinated in κ²P,N-fashion, as shown by X-ray crystallography and Mössbauer spectroscopy. Magnetization studies led to a magnetic moment very close to 4.9μ_B reflecting the expected four unpaired d-electrons (quintet ground state). In solution there are equilibria between [Fe(κ³P,N,P-PNP-R,TAD)X₂] and [Fe(κ²P,N-PNP-R,TAD)X₂] complexes, i.e., the PNP-R,TAD ligand is hemilabile. At −50 °C these equilibria are slow and signals of the non-coordinated P-TAD arm of the κ²P,N-PNP-R,TAD ligand can be detected by ³¹P{¹H} NMR spectroscopy. Addition of BH₃ to a solution of [Fe(PNP-iPr,TAD)Cl₂] leads to selective boronation of the pendant P-TAD arm shifting the equilibrium towards the four-coordinate complex [Fe(κ²P,N-PNP-iPr,TAD^BH₃)Cl₂]. DFT calculations corroborate the existence of equilibria between four- and five-coordinated complexes. Addition of CO to [Fe(PNP-iPr,TAD)X₂] in solution yields the diamagnetic octahedral complexes trans-[Fe(κ³P,N,P-PNP-iPr,TAD)(CO)X₂], which react further with Ag⁺ salts in the presence of CO to give the cationic complexes trans-[Fe(κ³P,N,P-PNP-iPr,TAD)(CO)₂X]⁺. CO addition most likely takes place at the five coordinate complex [Fe(κ³P,N,P-PNP-iPr,TAD)X₂]. The mechanism for the CO addition was also investigated by DFT and the most favorable path obtained corresponds to the rearrangement of the pincer ligand first from a κ²P,N- to a κ³P,N,P-coordination mode followed by CO coordination to [Fe(κ³P,N,P-PNP-iPr,TAD)X₂]. Complexes bearing tBu substituents do not react with CO. Moreover, in the solid state none of the tetrahedral complexes are able to bind CO.