Synthesis and Reactivity of Low-Coordinate Iron(II) Fluoride Complexes and Their Use in the Catalytic Hydrodefluorination of Fluorocarbons

Abstract

Transition metal fluoride complexes are of interest because they are potentially useful in a multitude of catalytic applications, including C-F bond activation and fluorocarbon functionalization. We report the first crystallographically characterized examples of molecular iron(II) fluorides: [L(Me)Fe(mu-F)]2 (1(2)) and L(tBu)FeF (2) (L = bulky beta-diketiminate). These complexes react with donor molecules (L'), yielding trigonal-pyramidal complexes L(R)FeF(L'). The fluoride ligand is activated by the Lewis acid Et2O.BF3, forming L(tBu)Fe(OEt2)(eta1-BF4) (3), and is also silaphilic, reacting with silyl compounds such as Me3SiSSiMe3, Me3SiCCSiMe3, and Et3SiH to give new thiolate L(tBu)FeSSiMe3 (4), acetylide L(tBu)FeCCSiMe3 (5), and hydride [L(Me)Fe(mu-H)]2 (6(2)) complexes. The hydrodefluorination (HDF) of perfluorinated aromatic compounds (hexafluorobenzene, pentafluoropyridine, and octafluorotoluene) with a silane R3SiH (R3 = (EtO)3, Et3, Ph3, (3,5-(CF3)2C6H3)Me2) is catalyzed by addition of an iron(II) fluoride complex, giving mainly the singly hydrodefluorinated products (pentafluorobenzene, 2,3,5,6-tetrafluoropyridine, and alpha,alpha,alpha,2,3,5,6-heptafluorotoluene, respectively) in up to five turnovers. These catalytic perfluoroarene HDF reactions proceed with activation of the C-F bond para to the most electron-withdrawing group and are dependent on the degree of fluorination and solvent polarity. Kinetic studies suggest that hydride generation is the rate-limiting step in the HDF of octafluorotoluene, but the active intermediate is unknown. Mechanistic considerations argue against oxidative addition and outer-sphere electron transfer pathways for perfluoroarene HDF. Fluorinated olefins are also hydrodefluorinated (up to 10 turnovers for hexafluoropropene), most likely through a hydride insertion/beta-fluoride elimination mechanism. Complexes 1(2) and 2 thus provide a rare example of a homogeneous system that activates C-F bonds without competitive C-H activation and use an inexpensive 3d transition metal.

Influence of the Diphosphine Coordinated to Molybdenum and Tungsten Triangular M3S4 Cluster Hydrides in the Catalytic Hydrodefluorination of Pentafluoropyridine

“…Holland and coworkers have shown that β-dikeminato iron (II) fluoride complexes catalyse HDF of perfluoroaromatics and perflluoroalkenes in the presence of silanes [16]. In particular, HDF of pentafluoropyridine selectively affords the para-substituted product as the only HDF product, although high catalyst loadings are needed.…”

The Reactivity Patterns of Low-Coordinate Iron−Hydride Complexes

“…Using these ligands, we have isolated the only examples of iron-hydride complexes with a coordination number less than fiVe ( Figure 2). 20,21 Our studies on low-coordinate iron are complemented by those of Peters and co-workers, who use tridentate, strong-field tris(phosphino)borate (BP 3 ) supporting ligands that contain "soft" phosphine donors, and more often give low-spin electronic configurations. 22 In the BP 3 systems, it has not yet been possible to isolate a low-coordinate iron hydride, but the presence of a terminal hydride in four-coordinate complexes is strongly Figure 1.…”

“…Both complexes have roughly tetrahedral geometry at each iron atom. 20,21 implied by the isolation of products that result from activation of solvent or the supporting ligand. 23 Bridging hydrides have recently been observed in a few electronically unsaturated dinuclear complexes, 24,25 and borohydrides have been studied on iron-sulfur clusters.…”

Hoch oder niedrig? Zur Erlaubtheit von Spinzuständen in der Übergangsmetallchemie

“…Der Zustand S = 3/2 ist dagegen verboten, weil die Paarung des vierten und fünften Elektrons in den t 2g -Orbitalen entweder für beide Elektronen gleichermaßen gün-stig (S = 1/2) oder ungünstig ist (S = 5/ 2), aber nicht für das eine Elektron günstig sein kann und für das andere nicht (wie es ein Grundzustand mit S = 3/2 erfordern würde). Beim Tetraeder (T-ML 4 ) scheiden die beiden Spinzustände S = 3/2 und S = 1/2 aus, weil die Energiedifferenz zwischen den e-und t 2 [28,29] (1-3, Abbildung 8), z. B. ein Tripyrazolylborat (E = N), [30] Triphosphanborat (E = P), [31,32] Trithioborat (E = S) [33] oder Tris-(carben)borat.…”