pH-Switchability and Second-Order Nonlinear Optical Properties of Monocyclopentadienylruthenium(II)/iron(II) Tetrazoles/Tetrazolates: Synthesis, Characterization, and Time-Dependent Density Functional Theory Calculations

Tetrazole/tetrazolate monocyclopentadienyliron­(II) and ruthenium­(II) compounds of general formulas [(η⁵-C₅H₅)­M­(dppe)­(N₄(H)­CC₆H₄NO₂)]­[PF₆]/[(η⁵-C₅H₅)­M­(dppe)­(N₄CC₆H₄NO₂)] were investigated for their pH-switching second-order nonlinear optical (SONLO) properties. Compounds [(η⁵-C₅H₅)­M­(dppe)­(N₄CC₆H₄NO₂)] (M = Fe, Ru) and compound [(η⁵-C₅H₅)­Ru­(dppe)­(N₄(H)­CC₆H₄NO₂)]­[PF₆] were fully characterized by (¹H-, ¹³C-, ³¹P-) NMR, cyclic voltammetry, and elemental analysis, and compounds [(η⁵-C₅H₅)­Fe­(dppe)­(N₄CC₆H₄NO₂)] and [(η⁵-C₅H₅)­Ru­(dppe)­(N₄(H)­CC₆H₄NO₂)]­[PF₆] were further characterized by single-crystal X-ray diffraction; the synthesis of [(η⁵-C₅H₅)­Fe­(dppe)­(N₄(H)­CC₆H₄NO₂)]­[PF₆] was unsuccessful. Time-dependent density functional theory calculations were performed using PBE0 and CAM-B3LYP functionals to evaluate the first hyperpolarizability (β_tot) of the tetrazole/tetrazolate complexes and for a detailed analysis of the experimental data. Both functionals predict (i) high first hyperpolarizabilities for the tetrazolate complexes [(η⁵-C₅H₅)­M­(dppe)­(N₄CC₆H₄NO₂)], with β_tot[Ru] ≈ 1.2β_tot[Fe], and (ii) a 3-fold reduction in β_tot[Ru] upon protonation, in complex [(η⁵-C₅H₅)­Ru­(dppe)­(N₄(H)­CC₆H₄NO₂)]⁺, forecasting [(η⁵-C₅H₅)­Ru­(dppe)­(N₄CC₆H₄NO₂)]/[(η⁵-C₅H₅)­Ru­(dppe)­(N₄(H)­CC₆H₄NO₂)]⁺ complexes as on/off, pH-switchable SONLO forms.