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 [(η-CH)M(dppe)(N(H)CCHNO)][PF]/[(η-CH)M(dppe)(NCCHNO)] were investigated for their pH-switching second-order nonlinear optical (SONLO) properties. Compounds [(η-CH)M(dppe)(NCCHNO)] (M = Fe, Ru) and compound [(η-CH)Ru(dppe)(N(H)CCHNO)][PF] were fully characterized by (H-, C-, P-) NMR, cyclic voltammetry, and elemental analysis, and compounds [(η-CH)Fe(dppe)(NCCHNO)] and [(η-CH)Ru(dppe)(N(H)CCHNO)][PF] were further characterized by single-crystal X-ray diffraction; the synthesis of [(η-CH)Fe(dppe)(N(H)CCHNO)][PF] was unsuccessful. Time-dependent density functional theory calculations were performed using PBE0 and CAM-B3LYP functionals to evaluate the first hyperpolarizability (β) 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 [(η-CH)M(dppe)(NCCHNO)], with β[Ru] ≈ 1.2β[Fe], and (ii) a 3-fold reduction in β[Ru] upon protonation, in complex [(η-CH)Ru(dppe)(N(H)CCHNO)], forecasting [(η-CH)Ru(dppe)(NCCHNO)]/[(η-CH)Ru(dppe)(N(H)CCHNO)] complexes as on/off, pH-switchable SONLO forms.