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
Holzhacker, C.; Stöger, B.; Carvalho, M. D.; Ferreira, L. P; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Realista, S.; Gil, A.; Calhorda, M. J.; Müller, D.; Kirchner, K.* Dalton Trans. 2015, 44, 13071.
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 4.9μB 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,TADBH3)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.
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 4.9μB 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,TADBH3)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.