A Pyrrole-Based Pincer Ligand Permits Access to Three Oxidation States of Iron in Organometallic Complexes

Abstract

Treatment of FeCl2(thf)1.5 with the sodium salt of bis­(dicyclohexylphosphinomethyl)­pyrrole (NaCyPNP) in the presence of pyridine (py) affords the high-spin, five-coordinate iron­(II) complex [FeCl­(py)­(CyPNP)]. The chloride complex serves as a starting point for a variety of organometallic iron­(II) compounds including S = 1 square planar alkyls, [FeR­(CyPNP)] (R = Me, Et, Bn, Ph). Treatment of [FeCl­(py)­(CyPNP)] with NaBEt3H does not generate an analogous square-planar hydride complex, but rather a bimetallic high-spin iron­(II) species containing bridging hydride ligands. Analogous transmetalation reactions in the presence of carbon monoxide produced six-coordinate low-spin species [FeR­(CO)2(CyPNP)] (R = Et, Ph, and H). Reduction of [FeCl­(py)­(CyPNP)] under CO afforded the low-spin iron­(I) carbonyl complex [Fe­(CO)2(CyPNP)]. This species could be further reduced with KC8 to afford the anionic iron(0) complex K­[Fe­(CO)2(CyPNP)]·thf. The breadth of coordination numbers, spin states, and valencies supported by this pyrrole-based PNP ligand demonstrates the flexibility afforded by this pincer framework.

A PNNH Pincer Ligand Allows Access to Monovalent Iron

“…The IR spectra of the product in KBr shows the presence of two bands of equal intensities at 1970 and 1899 cm −1 ; in both THF and Et 2 O solution, the higher band overlaps a strong solvent absorption band at 1965 cm −1 , but the lower ν CO band appears at 1912 cm −1 . Only a few Fe I pincer dicarbonyl complexes have been reported with monoanionic ligands and PNN − is the least donating of these (see Figure and the Conclusions for more details). For the analogous but diamagnetic Co I complex [(PNN)Co(CO) 2 ], two bands are observed at 1990 and 1937 cm −1 .…”

“…Fe I complexes with low coordination number (two‐ or three‐coordinate) are HS, [22; 23] as are some four‐coordinate species . More typically, Fe I is found in square‐planar four‐ or five‐coordinate environments with strong‐field ligands such as N‐heterocyclic carbenes (NHCs), N donors such as porphyrins, β‐diketiminates (NacNac), multiple phosphine ligands, tripodal phosphines, or sulfur donors as well as pincer ligands (NNN and PNP), and is thus LS. Lastly, an interesting case of HS‐to‐LS conversion is the reversible reaction of HS [Cp′Fe(I i Pr 2 Me 2 )] (Cp′=1,2,4‐(Me 3 C) 3 C 5 H 2 , I i Pr 2 Me 2 =1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐yildene) with N 2 to give a LS complex .…”

“…We summarize in Figure the CO frequencies of mono‐ and zero‐valent iron complexes to facilitate comparison of the electronic features of the pincer ligands. Group A shows that pyridine is less donating than CH 2 P t Bu 2 , and comparison with group B shows that PNNB(pin) is more donating than pyridine, whereas pyrazolate is more donating than CH 2 P t Bu 2 .…”

Catalytic C−H Borylation Using Iron Complexes Bearing 4,5,6,7‐Tetrahydroisoindol‐2‐ide‐Based PNP‐Type Pincer Ligand

“…The reaction of 3‐Cl with 1 equiv of KHBEt 3 gave the corresponding hydride‐bridged diiron complex ( [3‐H] 2 ) in 75 % yield (Scheme b). A similar hydride‐bridged diiron complex bearing PNP pincer ligands was reported by Tonzetich and co‐workers . Detailed molecular structures of 3‐Ph , 3‐Me , and [3‐H] 2 were confirmed by X‐ray analysis.…”

An Iron‐Hydrogen Bond Resistant to Protonation and Oxidation

“…As far as iron hydrides are concerned, “( Cy PNP)FeH” has been shown to adopt a dimeric structure with the Cy PNP ligand (μ‐κ 2 ‐P,N : κ 1 ‐P) and the hydride bridging two iron centers [9] . The dimerization can be suppressed in the presence of another coordinating ligand such as CO, [9a] 2,2′‐bipyridine, [10] or PhPMe 2 , [11] or using a sterically more hindered PNP‐pincer ligand like tBu PNP [12] . We have been particularly interested in cis ‐( Cy PNP)Fe(CO) 2 H ( 1 ) reported by Tonzetich and co‐workers [9a] .…”