Borylation in the Second Coordination Sphere of Fe^II Cyanido Complexes and Its Impact on Their Electronic Structures and Excited-State Dynamics

Abstract

Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B­(C6F5)3 resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two FeII cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the FeII-based t2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B­(C6F5)3 attachment at the cyanido N lone pair.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

“…32 Borylation of the peripheral nitrogen atoms of the cyanido ligands presumably further strengthens the ligand field, 33 yet the detectable excited-state lifetime of 28 ps in [Fe(bpy)(BCF) 4 ] 2− (BCF = tris(pentafluorophenyl)) (Figure 3c) was attributed to an MC rather than an MLCT state. 34 Further MLCT excited- state tuning in iron(II) cyanido complexes is possible through variation of the α-diimine ligands. 35 N-Heterocyclic carbene (NHC) ligands emerged as better alternatives to polypyridines as far as long 3 MLCT lifetimes are concerned, largely owing due their stronger σ-donor properties.…”

Nanosecond Metal-to-Ligand Charge Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials

“…In solvents of high acceptor number (and therefore higher E MLCT ), the metalcentered states may become relatively lower in energy, leading to more rapid deactivation. 45 As E MLCT decreases (with lower AN), deactivation via MC states becomes less significant, but direct deactivation to the ground state increases in accordance with the energy gap law.…”

“…[40][41][42] (This may be viewed as an intermolecular extension of covalent Lewis acid-cyano adducts which have been recently investigated. [43][44][45] ) The energies of the filled dπ and the empty d z 2 should therefore drop in acidic environments. The energy of the tetraimine π system is expected to stay relatively constant in contrast, 41 so E MLCT concomitantly increases (Chart 1), as observed experimentally.…”

Boronated Cyanometallates

“…45,46 Most recently, Wenger et al provided a comprehensive study of the electronic structural properties of Fe(II) boronated cyanometallates as a function of the number of cyanide ligands. 47 Studies of cyanometallates have been driven by the promise of applications in magnetic materials involving single-molecule/ Prussian-Blue analogues and in electrode materials for solidstate batteries. 18,48−50 Of direct relevance to our research is the performance of reversible cyanometallates as electrolytes in redox flow batteries.…”

“…Notably, boronation also has similar effects on the spectroscopic and photophysical properties of heteroleptic iridium­(III) and copper­(I) diimine complexes. , Of interest is that BCF/BPh 3 adducts of heteroleptic rhenium­(I) diiminecarbonyls have shown promise as selective sensors for cyanide anions . Also, Wenger et al boronated heteroleptic ruthenium­(II) and iridium­(III) diimine complexes and demonstrated their high activity for energy transfer and photoredox catalysis. , Most recently, Wenger et al provided a comprehensive study of the electronic structural properties of Fe­(II) boronated cyanometallates as a function of the number of cyanide ligands …”