Synthesis and Characterization of a High-Symmetry Ferrous Polypyridyl Complex: Approaching the ⁵T₂/³T₁ Crossing Point for Fe^II

Electronic structure theory predicts that, depending on the strength of the ligand field, either the quintet (⁵T₂) or triplet (³T₁) term states can be stabilized as the lowest-energy ligand-field excited state of low-spin octahedral d⁶ transition-metal complexes. The ³T₁ state is anticipated for second- and third-row metal complexes and has been established for certain first-row compounds such as [Co­(CN)₆]^3–, but in the case of the widely studied Fe^II ion, only the ⁵T₂ state has ever been documented. Herein we report that 2,6-bis­(2-carboxypyridyl)­pyridine (dcpp), when bound to Fe^II, presents a sufficiently strong ligand field to Fe^II such that the ⁵T₂/³T₁ crossing point of the d⁶ configuration is approached if not exceeded. The electrochemical and photophysical properties of [Fe­(dcpp)₂]²⁺, in addition to being of fundamental interest, may also have important implications for solar energy conversion strategies that seek to utilize earth-abundant components

Probing Reaction Dynamics of Transition-Metal Complexes in Solution via Time-Resolved Soft X-ray Spectroscopy

We report the first time-resolved soft x-ray measurements of solvated transition-metal complexes. L-edge spectroscopy directly probes dynamic changes in ligand-field splitting of 3d orbitals associated with the spin transition, and mediated by changes in ligand-bonding

Synthesis and Characterization of a High-Symmetry Ferrous Polypyridyl Complex: Approaching the ⁵T₂/³T₁ Crossing Point for Fe^II

Electronic structure theory predicts that, depending on the strength of the ligand field, either the quintet (⁵T₂) or triplet (³T₁) term states can be stabilized as the lowest-energy ligand-field excited state of low-spin octahedral d⁶ transition-metal complexes. The ³T₁ state is anticipated for second- and third-row metal complexes and has been established for certain first-row compounds such as [Co­(CN)₆]^3–, but in the case of the widely studied Fe^II ion, only the ⁵T₂ state has ever been documented. Herein we report that 2,6-bis­(2-carboxypyridyl)­pyridine (dcpp), when bound to Fe^II, presents a sufficiently strong ligand field to Fe^II such that the ⁵T₂/³T₁ crossing point of the d⁶ configuration is approached if not exceeded. The electrochemical and photophysical properties of [Fe­(dcpp)₂]²⁺, in addition to being of fundamental interest, may also have important implications for solar energy conversion strategies that seek to utilize earth-abundant components

Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy

Solution-phase photoinduced low-spin to high-spin conversion in the Fe-II polypyridyl complex [Fe(tren(py)(3))](2+) (where tren(py)3 is tris(2-pyridylmethyliminoethyl)amine) has been studied via picosecond soft X-ray spectroscopy. Following (1)A(1) -> (MLCT)-M-1 (metal-to-ligand charge transfer) excitation at 560 nm, changes in the iron L-2- and L-3-edges were observed concomitant with formation of the transient high-spin T-5(2) state. Charge-transfer multiplet calculations coupled with data acquired on low-spin and high-spin model complexes revealed a reduction in ligand field splitting of similar to 1 eV in the high-spin state relative to the singlet ground state. A significant reduction in orbital overlap between the central Fe-3d and the ligand N-2p orbitals was directly observed, consistent with the expected ca. 0.2 angstrom increase in Fe-N bond length upon formation of the high-spin state. The overall occupancy of the Fe-3d orbitals remains constant upon spin crossover, suggesting that the reduction in a-donation is compensated by significant attenuation of pi-back-bonding in the metal ligand interactions. These results demonstrate the feasibility and unique potential of time-resolved soft X-ray absorption spectroscopy to study ultrafast reactions in the liquid phase by directly probing the valence orbitals of first-row metals as well as lighter elements during the course of photochemical transformations

Probing Reaction Dynamics of Transition-Metal Complexes in Solution via Time-Resolved X-ray Spectroscopy

We report measurements of the photo-induced Fe(II) spin crossover reaction dynamics in solution via time-resolved x-ray absorption spectroscopy. EXAFS measurements reveal that the iron?nitrogen bond lengthens by 0.21+-0.03 Angstrom in the high-spin transient excited state relative to the ground state. XANES measurements at the Fe L-edge show directly the influence of the structural change on the ligand-field splitting of the Fe(II) 3d orbitals associated with the spin transition

Femtosecond Soft X-ray Spectroscopy of Solvated Transition-Metal Complexes : Deciphering the Interplay of Electronic and Structural Dynamics

We present the first implementation of femtosecond soft X-ray spectroscopy as an ultrafast direct probe of the excited-state valence orbitals in solution phase molecules. This method is applied to photoinduced spin crossover of [Fe(tren(py)(3))](2+), where the ultrafast spin state conversion of the metal ion, initiated by metal-to-ligand charge transfer excitation, is directly measured using the intrinsic spin state selectivity of the soft X-ray L-edge transitions. Our results provide important experimental data concerning the mechanism of ultrafast spin state conversion and subsequent electronic and structural dynamics, highlighting the potential of this technique to study ultrafast phenomena in the solution phase

Femtosecond Soft X-ray Spectroscopy of Solvated Transition-Metal Complexes: Deciphering the Interplay of Electronic and Structural Dynamics

We present the first implementation of femtosecond soft X-ray spectroscopy as an ultrafast direct probe of the excited-state valence orbitals in solution-phase molecules. This method is applied to photoinduced spin crossover of [Fe(tren(py)₃)]²⁺, where the ultrafast spin-state conversion of the metal ion, initiated by metal-to-ligand charge-transfer excitation, is directly measured using the intrinsic spin-state selectivity of the soft X-ray L-edge transitions. Our results provide important experimental data concerning the mechanism of ultrafast spin-state conversion and subsequent electronic and structural dynamics, highlighting the potential of this technique to study ultrafast phenomena in the solution phase