Properties of Metal Hydrides of the Iron Triad.

Metal hydride complexes are essential intermediates in hydrogenation reactions. The hydride-donor ability determines the scope of use of these complexes. We present a new, simple mass-spectrometry method to study the hydride-donor ability of metal hydrides using a series of 18 iron, cobalt, and nickel complexes with N- and P-based ligands (L). The mixing of [(L)MII(OTf)2] with NaBH4 forms [(L)MII(BH4)]+ (M = Fe, Co, Ni) that can be detected by electrospray ionization mass spectrometry. Energy-resolved collision-induced dissociations of [(L)MII(BH4)]+ provide threshold energies (ΔECID) for the formations of [(L)MII(H)]+ that correlate well with the hydride donor ability of the metal hydride complexes. We studied the vibrational and electronic spectra of the generated metal hydrides, assigned their structure and spin state, and demonstrated a good correlation between ΔECID and the M-H stretching vibration frequencies. The ΔECID also correlates with reaction rates for hydride transfer reactivity in the gas phase and known reactivity trends in the solution phase.

Unmasking the Iron-Oxo Bond of the [(Ligand)Fe-OIAr]2+/+ Complexes.

ArIO (ArI = 2-(tBuSO2)C6H4I) is an oxidant used to oxidize FeII species to their FeIV-oxo state, enabling hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions at low energy barriers. ArIO, as a ligand, generates masked Fen═O species of the type Fe(n-2)-OIAr. Herein, we used gas-phase ion-molecule reactions and DFT calculations to explore the properties of masked iron-oxo species and to understand their unmasking mechanisms. The theory shows that the I-O bond cleavage in [(TPA)FeIVO(ArIO)]2+ (12+, TPA = tris(2-pyridylmethyl)amine)) is highly endothermic; therefore, it can be achieved only in collision-induced dissociation of 12+ leading to the unmasked iron(VI) dioxo complex. The reduction of 12+ by HAT leads to [(TPA)FeIIIOH(ArIO)]2+ with a reduced energy demand for the I-O bond cleavage but is, however, still endothermic. The exothermic unmasking of the Fe═O bond is predicted after one-electron reduction of 12+ or after OAT reactivity. The latter leads to the 4e- oxidation of unsaturated hydrocarbons: The initial OAT from [(TPA)FeIVO(ArIO)]2+ leads to the epoxidation of an alkene and triggers the unmasking of the second Fe═O bond still within one collisional complex. The second oxidation step starts with HAT from a C-H bond and follows with the rebound of the C-radical and the OH group. The process starting with the one-electron reduction could be studied with [(TQA)FeIVO(ArIO)]2+ (22+, TQA = tris(2-quinolylmethyl)amine)) because it has a sufficient electron affinity for electron transfer with alkenes. Accordingly, the reaction of 22+ with 2-carene leads to [(TQA)FeIIIO(ArIO)]2+ that exothermically eliminates ArI and unmasks the reactive FeV-dioxo species.

Tuning the H-Atom Transfer Reactivity of Iron(IV)-Oxo Complexes as Probed by Infrared Photodissociation Spectroscopy.

Reactivities of non-heme iron(IV)-oxo complexes are mostly controlled by the ligands. Complexes with tetradentate ligands such as [(TPA)FeO]2+ (TPA=tris(2-pyridylmethyl)amine) belong to the most reactive ones. Here, we show a fine-tuning of the reactivity of [(TPA)FeO]2+ by an additional ligand X (X=CH3 CN, CF3 SO3- , ArI, and ArIO; ArI=2-(t BuSO2 )C6 H4 I) attached in solution and reveal a thus far unknown role of the ArIO oxidant. The HAT reactivity of [(TPA)FeO(X)]+/2+ decreases in the order of X: ArIO > MeCN > ArI ≈ TfO- . Hence, ArIO is not just a mere oxidant of the iron(II) complex, but it can also increase the reactivity of the iron(IV)-oxo complex as a labile ligand. The detected HAT reactivities of the [(TPA)FeO(X)]+/2+ complexes correlate with the Fe=O and FeO-H stretching vibrations of the reactants and the respective products as determined by infrared photodissociation spectroscopy. Hence, the most reactive [(TPA)FeO(ArIO)]2+ adduct in the series has the weakest Fe=O bond and forms the strongest FeO-H bond in the HAT reaction.