Nitrile Substituents at the Conjugated Dipyridophenazine Moiety as Infrared Redox Markers in Electrochemically Reduced Heteroleptic Ru(II) Polypyridyl Complexes

Ruthenium(II) complexes [Ru(tap)2(NN)]2+ (tap = 1,4,5,8-tetraazaphenanthrene, NN = 11-cyano-dipyrido[3,2-a:2′,3′-c]phenazine (11-CN-dppz) and 11,12-dicyano-dipyrido[3,2-a:2′,3′-c]phenazine (11,12-CN-dppz)) feature the CN groups as infrared (IR)-active redox markers. They were studied by cyclic voltammetry, UV–vis, and IR spectroelectrochemistry (SEC), and density functional theory calculations to assign the four 1e– reduction waves R1–R4 observed in dichloromethane. Generally, the NN ligands are reduced first (R1). For [Ru(tap)2(11,12-CN-dppz)]2+, R1 is sufficiently separated from R2 and delocalized over both tap ligands. Accordingly, IR SEC conducted at R1 shows a large red shift of the νs,as(CN) modes by −18/–28 cm–1, accompanied by a 4-fold enhancement of the νs(CN) intensity, comparably with reference data for free 11,12-CN-dppz. The first tap-based reduction of spin-doublet [Ru(tap)2(11,12-CN-dppz)]+ to spin-triplet [Ru(tap)2(11,12-CN-dppz)] at R2 decreased ν(CN) by merely −2 cm–1, while the intensity enhancement reached an overall factor of 8. Comparably, a red shift of ν(CN) by −27 cm–1 resulted from the 1e– reduction of [Ru(tap)2(11-CN-dppz)]2+ at R1 (poorly resolved from R2), and the intensity enhancement was roughly 3-fold. Concomitant 1e– reductions of the tap ligands (R2 and R3) caused only minor ν(CN) shifts of −3 cm–1 and increased the absorbance by overall factors of 6.5 and 8, respectively

Extreme Basicity of Biguanide Drugs in Aqueous Solutions: Ion Transfer Voltammetry and DFT Calculations

Ion transfer voltammetry is used to estimate the acid dissociation constants <i>K</i>_a1 and <i>K</i>_a2 of the mono- and diprotonated forms of the biguanide drugs metformin (MF), phenformin (PF), and 1-phenylbiguanide (PB) in an aqueous solution. Measurements gave the p<i>K</i>_a1 values for MFH⁺, PFH⁺, and PBH⁺ characterizing the basicity of MF, PF, and PB, which are significantly higher than those reported in the literature. As a result, the monoprotonated forms of these biguanides should prevail in a considerably broader range of pH 1–15 (MFH⁺, PFH⁺) and 2–13 (PBH⁺). DFT calculations with solvent correction were performed for possible tautomeric forms of neutral, monoprotonated, and diprotonated species. Extreme basicity of all drugs is confirmed by DFT calculations of p<i>K</i>_a1 for the most stable tautomers of the neutral and protonated forms with explicit water molecules in the first solvation sphere included

Ultrafast Wiggling and Jiggling: Ir₂(1,8-diisocyanomenthane)₄²⁺

Binuclear complexes of d⁸ metals (Pt^II, Ir^I, Rh^I,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M–M bonding is strengthened upon dσ* → pσ excitation. We report the bridging ligand dynamics of Ir₂(1,8-diisocyanomenthane)₄²⁺ (Ir­(dimen)), investigated by fs–ns time-resolved IR spectroscopy (TRIR) in the region of CN stretching vibrations, ν­(CN), 2000–2300 cm^–1. The ν­(CN) IR band of the singlet and triplet dσ*pσ excited states is shifted by −22 and −16 cm^–1 relative to the ground state due to delocalization of the pσ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ¹dσ*pσ state depend on the initially excited Franck–Condon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale Ir–Ir contraction and 5 ps Ir–Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ∼5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ³dσ*pσ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν­(CN) IR bandwidth oscillates with the frequency of the ν­(Ir–Ir) wave packet, ca. 80 cm^–1, indicating that the dephasing time of the high-frequency (16 fs)⁻¹ CN stretch responds to much slower (∼400 fs)⁻¹ Ir–Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal–metal unit and that the coherent Ir–Ir motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation