title: Synthesis and reactivity of TADDOL-based chiral Fe(II) PNP pincer complexes-solution equilibria between κ(2)P,N- and κ(3)P,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 κ(2)P,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(κ(3)P,N,P-PNP-R,TAD)X2] and [Fe(κ(2)P,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 κ(2)P,N-PNP-R,TAD ligand can be detected by (31)P{(1)H} 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(κ(2)P,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(κ(3)P,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(κ(3)P,N,P-PNP-iPr,TAD)(CO)2X](+). CO addition most likely takes place at the five coordinate complex [Fe(κ(3)P,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 κ(2)P,N- to a κ(3)P,N,P-coordination mode followed by CO coordination to [Fe(κ(3)P,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.
authors.affiliation: Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria. kkirch@mail.tuwien.ac.at.
authors.forename: Christian
authors.initials: C
authors.lastname: Holzhacker
authors.forename: Berthold
authors.initials: B
authors.lastname: Stöger
authors.forename: Maria Deus
authors.initials: MD
authors.lastname: Carvalho
authors.forename: Liliana P
authors.initials: LP
authors.lastname: Ferreira
authors.forename: Ernst
authors.initials: E
authors.lastname: Pittenauer
authors.forename: Günter
authors.initials: G
authors.lastname: Allmaier
authors.forename: Luis F
authors.initials: LF
authors.lastname: Veiros
authors.forename: Sara
authors.initials: S
authors.lastname: Realista
authors.forename: Adrià
authors.initials: A
authors.lastname: Gil
authors.forename: Maria José
authors.initials: MJ
authors.lastname: Calhorda
authors.forename: Danny
authors.initials: D
authors.lastname: Müller
authors.forename: Karl
authors.initials: K
authors.lastname: Kirchner
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry; D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004148:Dioxolanes / Q000138:chemical synthesis / Q000737:chemistry; D004583:Electrons; D005296:Ferrous Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D054879:Halogenation; D008024:Ligands; D000432:Methanol / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry; D008958:Models, Molecular; D012996:Solutions; D015204:Spectroscopy, Mossbauer
chemical_list: D056831:Coordination Complexes; D004148:Dioxolanes; D005296:Ferrous Compounds; D008024:Ligands; D012996:Solutions; C516101:alpha,alpha,alpha',alpha'-tetraaryl-1,3-dioxolane-4,5-dimethanol; D002248:Carbon Monoxide; D000432:Methanol