Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex.

In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O2-activating iron enzymes. This work details an electronic-structure investigation of [FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm-1 in the E(dxy → dxz,yz) excited state. Both values are similar to those measured for [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm-1. We can unequivocally assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation energy for dxz,yz → dx2-y2 is significantly higher (∼12 000 cm-1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2-y2 orbital to eventually lift it above dz2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.

A trans-1,2 End-On Disulfide-Bridged Iron-Tetracarbene Dimer and Its Electronic Structure.

A disulfide-bridged diiron complex with [Fe-S-S-Fe] core, which represents an isomer of the common biological [2Fe-2S] ferredoxin-type clusters, was synthesized using strongly σ-donating macrocyclic tetracarbene capping ligands. Though the complex is quite labile in solution, single crystals were obtained, and the structure was elucidated by X-ray diffraction. The electron-rich iron-sulfur core is found to show rather unusual magnetic and electronic properties. Experimental data and density functional theory studies indicate extremely strong antiferromagnetic coupling (-J > 800 cm(-1)) between two low-spin iron(III) ions via the S2(2-) bridge, and the intense near-IR absorption characteristic for the [Fe-S-S-Fe] core was assigned to a S → Fe ligand-to-metal charge transfer transition.

Decay of iron(V) nitride complexes by a N-N bond-coupling reaction in solution: a combined spectroscopic and theoretical analysis.

Cryogenically trapped Fe(V) nitride complexes with cyclam-based ligands were found to decay by bimolecular reactions, forming exclusively Fe(II) compounds. Characterization of educts and products by Mössbauer spectroscopy, mass spectrometry, and spectroscopy-oriented DFT calculations showed that the reaction mechanism is reductive nitride coupling and release of dinitrogen (2 Fe(V)≡N→Fe(II)-N=N-Fe(II)→2 Fe(II)+N2). The reaction pathways, representing an "inverse" of the Haber-Bosch reaction, were computationally explored in detail, also to judge the feasibility of yielding catalytically competent Fe(V)(N). Implications for the photolytic cleavage of Fe(III) azides used to generate high-valent Fe nitrides are discussed.

Iron Azides with Cyclam-Derived Ligands: Are They Precursors for High-Valent Iron Nitrides in the Gas Phase?

The FeIII azide complexes [FeIII (N3 )cyclam-ac]PF6 (1⋅PF6 ), [FeIII (N3 )Me3 cyclam-ac]PF6 (2⋅PF6 ), and trans-[FeIII (N3 )2 cyclam]ClO4 (3⋅ClO4 ) (cyclam=1,4,8,11-tetraazacyclotetradecane; cyclam-ac=1,4,8,11-tetraazacyclotetradecane-1-acetate; Me3 cyclam-ac=4,8,11-trimethyl-1,4,8,11-tetraazacyclotetra-decane-1-acetate) are studied in the gas phase with special emphasis on the formation of high-valent iron nitrides by collision-induced dissociation. Whereas the azide complex with unsubstituted cyclam-acetate 1 as major fragmentation expels N2 to form a high-valent FeV nitride complex, a similar process is not observed for its methyl-substituted congener. In contrast, loss of an azide radical results in iron reduction to FeII . Thus, the gas-phase behavior is parallel to the results obtained in spectroscopic studies of photolyzed frozen solution. The diazide complex 3 mainly fragments via consecutive losses of HN3 without change in the iron oxidation state. However, small amounts of dinitrogen loss and thus FeV nitride formation are also observed. While it is assumed that the FeV nitride complex detected by Mössbauer spectroscopy in frozen solution is still coordinated by an azide in the trans position to the nitride, both the complex [FeV (N)(N3 )(cyclam)]+ still bearing an intact second azide and the coordinatively unsaturated [FeV (N)(cyclam-H)]+ are observed in the gas phase.

Ultrafast primary processes of an iron-(III) azido complex in solution induced with 266 nm light.

The ultrafast photo-induced primary processes of the iron-(III) azido complex, [Fe(III)N(3)(cyclam-acetato)] PF(6) (1), in acetonitrile solution at room temperature were studied using femtosecond spectroscopy with ultraviolet (UV) excitation and mid-infrared (MIR) detection. Following the absorption of a 266 nm photon, the complex undergoes an internal conversion back to the electronic doublet ground state at a time scale below 2 ps. Subsequently, the electronic ground state vibrationally cools with a characteristic time constant of 13 ps. A homolytic bond cleavage was also observed by the appearance of ground state azide radicals, which were identified by their asymmetric stretching vibration at 1659 cm(-1). The azide radical recombines in a geminate fashion with the iron containing fragment within 20 ps. The cage escape leading to well separated fragments after homolytic Fe-N bond breakage was found to occur with a quantum yield of 35%. Finally, non-geminate recombination at nanosecond time scales was seen to further reduce the photolytic quantum yield to below 20% at a wavelength of 266 nm.

The photochemistry of [Fe(III)N3(cyclam-ac)]PF6 at 266 nm.

The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [Fe(III)N(3)(cyclam-ac)]PF(6) ([1]PF(6)), where cyclam-ac represents the 1,4,8,11-tetraazacyclotetradecane-1-acetate ligand, has been shown to be wavelength-dependent, leading either to the rare high-valent iron(V) nitrido complex [Fe(V)N(cyclam-ac)]PF(6) ([3]PF(6)) after cleavage of the azide N(α)-N(ß) bond, or to a photoreduced Fe(II) species after Fe-N(azide) bond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1 in acetonitrile solution at room temperature has been investigated using step-scan and rapid-scan time-resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron-containing photochemical product, it is shown that 266 nm excitation of 1 results exclusively in the cleavage of the Fe-N(azide) bond, as was suspected from earlier steady-state irradiation studies. In argon-purged solutions of [1]PF(6), the solvent-stabilized complex cation [Fe(II)(CH(3)CN)(cyclam-ac)](+) (2red) together with the azide radical (N(3)(.)) is formed with a relative yield of 80%, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step-scan experiments with a higher time resolution reveal the formation of azide anions (N(3)(-)) during the first 500 ns after photolysis, with a yield of 20%. These azide ions can subsequently react thermally with 2red to form [Fe(II)N(3)(cyclam-ac)] (1red) as a secondary product of the photochemical decomposition of 1. Molecular oxygen was further used to quench 1red and 2red to form what seems to be the elusive complex [Fe(O(2))(cyclam-ac)](+) (6).

The quest for ring opening of oxaphosphirane complexes: a coupled-cluster and density functional study of CH(3)PO isomers and their Cr(CO)(5) complexes.

Opening gambit: A high-level theoretical study on the relative stabilities of oxaphosphirane isomers and their Cr(CO)(5) complexes is reported (see picture). Furthermore, thermodynamics and kinetics of possible ring-opening reactions of these complexes in the presence of a {Cp(2)Ti(III)Cl} fragment are theoretically investigated. The C--O bond cleavage is predicted to be the most efficient pathway, thus leading to reactive intermediates that are attractive for synthetic applications.A high-level theoretical study on the relative stabilities of oxaphosphirane isomers and their Cr(CO)(5) complexes is reported. Furthermore, thermodynamics and kinetics of possible ring-opening reactions of these complexes in the presence of a {Cp(2)Ti(III)Cl} fragment are theoretically investigated. The C--O bond cleavage is predicted to be the most efficient pathway thus leading to reactive intermediates that are attractive for synthetic applications. The ring-opening reaction is predicted to not lead to the most favorable product (a coordinated phosphinidene oxide species). Rather, the ring-opening product is separated by a substantial barrier of about 24 kcal mol(-1) from the thermodynamically most favorable species.