Striking Differences between the Solution and Solid-State Reactivity of Iron PNP Pincer Complexes with Carbon Monoxide

Abstract

Several new iron(II) complexes of the types [Fe(PNP)X 2 ] (X = Cl, Br) containing tridentate PNP pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine have been prepared. They all exhibit intermolecular Fe-X 3 3 3 H-N hydrogen bonds, forming supramolecular networks in the solid state. In the case of X = Cl these compounds react readily with gaseous CO both in the solid state and in solution to give selectively the octahedral complexes cis-and trans-[Fe(PNP)(CO)-(Cl) 2 ], respectively, whereas with X = Br mixtures of cis and trans isomers are obtained. These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in all cases, and heating solid samples of either cis-or trans-[Fe(PNP)(CO)(X) 2 ] leads to complete regeneration of analytically pure [Fe(PNP)(X) 2 ]. M€ ossbauer spectroscopy confirmed the high-spin nature of the parent five-coordinate Fe(II) complex (δ = 0.80(1) mm s -1 ) and the shift to two different low-spin octahedral species after reaction with CO in the solid (δ = 0.13(1) mm s -1 ) or in solution (δ = 0.15(1) mm s -1 ). Magnetization studies led to a magnetic moment close to 4.9 μ B , reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl 2 ], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (e3.4 kcal mol -1 ). The cis-trans isomerization reaction was found to take place with a low energy barrier (10.8 kcal mol -1 ), after initial loss of chloride, and involves also spin-state changes.

Synthesis and characterization of iron complexes based on bis-phosphinite PONOP and bis-phosphite PONOP pincer ligands

“…The observed proton chemical shifts for iPr PONOPFeCl 2 fall in ranges similar to those observed for the related paramagnetic complex, iPr PNNNPFeCl 2 reported by Benito-Garagorri and coworkers. 27 …”

“…26 There also have been reports of R PNNNP iron complexes [ R PNNNP = 2,6-(R 2 PNH) 2 (C 5 H 3 N); R = i Pr], which exhibit intermolecular hydrogen bonding and the formation of supramolecular solid state structures. 27 Also notable are the bis-phosphinite R POCOP [ R POCOP = 2,6-(R 2 PO) 2 (C 6 H 3 ); R = i Pr, Ph] iron systems. These ligands present an additional challenge as metallation requires activation of the C-H bond.…”

A new class of PN3-pincer ligands for metal–ligand cooperative catalysis

“…They could both be synthesized using a similar procedure to those for the symmetrical PN 3 P-pincer ligands. Upon treatment of 2 equivalents of iPr 2 PCl with the respective 2,6-diaminopyrimidinein the presence of NEt 3 in THF, these air-stable ligands were isolated in good yields (68-85%) [76] (Scheme 10). 3 in tetrahydrofuran (THF) [63].…”

“…Coordinatively unsaturated paramagnetic (S=2) Fe(II) complexes (Fe12 and Fe13) were achieved by the reaction of PN 3 P tBu (L4) and PN 3 P iPr (L5) ligands with FeCl 2 , respectively [76,79,80] (Scheme 13). The pentacoordinate geometry of Fe13·THF was confirmed by the X-ray single crystal structure which also suggests that "basal" (considering a distorted square pyramid) chloride leads to a hydrogen-bond interaction with one of the N-H moieties of the neighboring complex.…”

Unusual Anion Effects in the Iron‐Catalyzed Formation of 3‐Hydroxyacrylates from Aromatic Aldehydes and Ethyl Diazoacetate

“…As part of our ongoing research on the synthesis and reactivity of iron(II) pincer complexes [10] we have recently prepared cationic complexes of the types trans-[Fe(PNP)(CO) 2 Cl] + . [11] These complexes are unusual as they feature two CO ligands in a mutual trans position making one of them comparatively labile, which may thus be replaced by other potential ligands.…”