The highly surprising behaviour of diphosphine ligands in iron-catalysed Negishi cross-coupling

Abstract

Iron-catalysed cross-coupling is undergoing explosive development, however, mechanistic understanding lags far behind synthetic methodology. Herein we find the activity of irondiphosphine complexes in the Negishi coupling of benzyl halides is strongly dependent on the diphosphine but the ligand does not appear to be coordinated to the iron during turn-over.This was determined using time-resolved in operando X-ray absorption fine structure spectroscopy, employing a custom-made flow-cell and confirmed by 31 P NMR spectroscopy.While the diphosphine ligands tested are all able to coordinate to iron(II), in the presence of excess zinc(II), as in the catalytic reaction, they coordinate predominantly to the zinc. Furthermore, combined synthetic and kinetic investigations implicate the formation of a putative mixed Fe-Zn(dpbz) species prior to the rate-limiting step of catalysis. These unexpected findings may not only impact upon the field of iron-catalysed Negishi cross-

Stereoselective Alkyne Hydrogenation by using a Simple Iron Catalyst

“…[10,11] The latter catalyst formed from Fe(acac) 2 and DIBAL-He nabled highly selective semihydrogenation to Z-alkenes. [16] For Fe(acac) 2 ,t he pronounced whiteline was only significantly reduced after addition of 3equivalents of DIBAL-H, which corresponded to ah igher concentration of the reductant than the catalytically active speciesf ormedf rom Fe(acac) 2 and 2equivalents of DIBAL-H. The XANESs pectra unambiguously show the reductiono fb oth Fe(acac) 2 and Fe(hmds) 2 upon addition of DIBAL-H ( Figure 2).…”

“…[16] We performed X-ray absorption near-edge structure( XANES) and extendedXray absorption fine structure (EXAFS) analysiso ft he catalyst combinations: Fe(hmds) 2 /DIBAL-H and Fe(acac) 2 /DIBAL-H. [17] The former catalyst contained ab ulky,r edox-stable amido ligand that was easily displaced by af ormal hydride delivered from DIBAL-H. [16] We performed X-ray absorption near-edge structure( XANES) and extendedXray absorption fine structure (EXAFS) analysiso ft he catalyst combinations: Fe(hmds) 2 /DIBAL-H and Fe(acac) 2 /DIBAL-H. [17] The former catalyst contained ab ulky,r edox-stable amido ligand that was easily displaced by af ormal hydride delivered from DIBAL-H.…”

“…As an element-specific ands hort-range sensitive method, X-ray absorption spectroscopy is ap owerful toolt oo btain information about the oxidation state and coordination environment of the absorbinga toms in solution and thus highly instructive toward the investigation of catalysts pecies. [16] We performed X-ray absorption near-edge structure( XANES) and extendedXray absorption fine structure (EXAFS) analysiso ft he catalyst combinations: Fe(hmds) 2 /DIBAL-H and Fe(acac) 2 /DIBAL-H. [17] The former catalyst contained ab ulky,r edox-stable amido ligand that was easily displaced by af ormal hydride delivered from DIBAL-H. The resultant catalyste xhibited very high activity in the complete hydrogenation of alkynes to the alkanes and the [2+ +2+ +2]-cyclotrimerization.…”

“…DIBAL-H) and the formation of agglomerates, in agreement with previous studies. [16] For Fe(acac) 2 ,t he pronounced whiteline was only significantly reduced after addition of 3equivalents of DIBAL-H, which corresponded to ah igher concentration of the reductant than the catalytically active speciesf ormedf rom Fe(acac) 2 and 2equivalents of DIBAL-H. In contrast, the Fe(hmds) 2 spectrum, which was significantly different to unreduced Fe(acac) 2 possibly owing to the presence of pronounced p-resonance in the FeÀNb onds, changed with 0.5 equivalents of DIBAL-H. [17] Interestingly,t he XANES spectra of [Fe(hmds) 2 + 2equiv.D IBAL-H] and [Fe(acac) 2 + 4equiv.D IBAL-H] showed very similar spectral shapes.…”

Expanding the family of tetrahalide iron complexes: Synthesis, structure and biological applications

“…The first, an undecanuclear mixed metal cluster containing iron, gallium and potassium bridged by bromides, by Linti et al [16] Here the central iron is coordinated to four bromides with the axial positions being occupied by bridging oxides. The second is a trinuclear iron cluster with bridging bromide ligands with β-diketiminate terminal ligands by Jones et al [17] The third example is a mixed trinuclear cluster with a central iron ion coordinated to four bridging bromides that coordinate a zinc ion capped by THF and p-tolyl groups by Bedford et al [18] In these examples all the bromide ligands coordinated to iron are bridging and therefore do not constitute a formally tetra anionic metal centre as seen in Fe(II)Cl4(HAmpy)2. The other three examples are polymeric in nature; the first example from Reiff et al is a catena-bridged ferrous bromide derivative with water molecules in the axial positions.…”