Tuning the Electronic Structure of Fe(II) Polypyridines via Donor Atom and Ligand Scaffold Modifications: A Computational Study

Abstract

Fe(II) polypyridines are an important class of pseudo-octahedral metal complexes known for their potential applications in molecular electronic switches, data storage and display devices, sensors, and dye-sensitized solar cells. Fe(II) polypyridines have a d(6) electronic configuration and pseudo-octahedral geometry and can therefore possess either a high-spin (quintet) or a low-spin (singlet) ground state. In this study, we investigate a series of complexes based on [Fe(tpy)2](2+) (tpy = 2,2';6',2″-terpyridine) and [Fe(dcpp)2](2+) (dcpp = 2,6-bis(2-carboxypyridyl)pyridine). The ligand field strength in these complexes is systematically tuned by replacing the central pyridine with five-membered (N-heterocyclic carbene, pyrrole, furan) or six-membered (aryl, thiazine-1,1-dioxide, 4-pyrone) moieties. To determine the impact of ligand substitutions on the relative energies of metal-centered states, the singlet, triplet, and quintet states of the Fe(II) complexes were optimized in water (PCM) using density functional theory at the B3LYP+D2 level with 6-311G* (nonmetals) and SDD (Fe) basis sets. It was found that the dcpp ligand scaffold allows for a more ideal octahedral coordination environment in comparison to the tpy ligand scaffold. The presence of six-membered central rings also allows for a more ideally octahedral coordination environment relative to five-membered central rings, regardless of the ligand scaffold. We find that the ligand field strength in the Fe(II) polypyridines can be tuned by altering the donor atom identity, with C donor atoms providing the strongest ligand field.

Electronic Modulation of the SOMO–HOMO Energy Gap in Iron(III) Complexes towards Unimolecular Current Rectification

“…Moreover, replacement of the electron‐donating alkoxo, −O(CH 2 ) 2 (OCH 3 ), substituent with the electron‐withdrawing nitro, −NO 2 , function in the ligand framework may further decrease the SOMO energy permitting lower applied potentials for molecular rectification. Similar electronic tune‐ups have been proposed recently for other iron species …”

“…Moreover,r eplacement of the electron-donating alkoxo, ÀO(CH 2 ) 2 (OCH 3 ), substituent with the electron-withdrawing nitro, ÀNO 2 ,f unctioni nt he ligand framework may furtherd ecrease the SOMO energy permitting lower appliedp otentials for molecular rectification.S imilar electronic tune-upsh ave been proposed recently for other iron species. [17][18][19] Complexes 1 and 3 were reported elsewhere. [8,9] The ligand H 2 L 2 (Scheme 2) was obtained by treating two equivalents of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with one equivalent of 4,5-dinitrobenzene-1,2-diamine [20] in methanol.…”

Ligand-Field-Dependent Behavior of Meta-GGA Exchange in Transition-Metal Complex Spin-State Ordering

“…Approximate density functional theory (DFT) remains the method of choice for computational discovery [4][5][6][7][8][9][10][11] , owing to its favorable balance of accuracy and efficiency. 12 Nevertheless, delocalization errors [13][14] and other biases in semi-local exchange approximations [15][16] (e.g., the generalized gradient approximation or GGA) produce systematic biases toward low-spin states [17][18] that prevent prediction of either qualitative (i.e., ground state identity) or quantitative (i.e., energetic splitting between states) spin-state properties.…”

Boosting Vis/NIR Charge‐Transfer Absorptions of Iron(II) Complexes by N‐Alkylation and N‐Deprotonation in the Ligand Backbone

“…Neither approach has yet succeeded in giving luminescent iron(II) complexes with sufficiently long 3 MLCT lifetimes. However, theoretical studies suggest that a suitably strong ligand field combined with low‐energy charge‐transfer states could possibly yield luminescent iron(II) complexes …”