Reduced and Superreduced Diplatinum Complexes

A d⁸–d⁸ complex [Pt₂(μ-P₂O₅(BF₂)₄]^4– (abbreviated Pt­(pop-BF₂)^4–) undergoes two 1e^– reductions at <i>E</i>_1/2 = −1.68 and <i>E</i>_p = −2.46 V (vs Fc⁺/Fc) producing reduced Pt­(pop-BF₂)^5– and superreduced Pt­(pop-BF₂)^6– species, respectively. The EPR spectrum of Pt­(pop-BF₂)^5– and UV–vis spectra of both the reduced and the superreduced complexes, together with TD-DFT calculations, reveal successive filling of the 6pσ orbital accompanied by gradual strengthening of Pt–Pt bonding interactions and, because of 6pσ delocalization, of Pt–P bonds in the course of the two reductions. Mayer–Millikan Pt–Pt bond orders of 0.173, 0.268, and 0.340 were calculated for the parent, reduced, and superreduced complexes, respectively. The second (5–/6−) reduction is accompanied by a structural distortion that is experimentally manifested by electrochemical irreversibility. Both reduction steps proceed without changing either d⁸ Pt electronic configuration, making the superreduced Pt­(pop-BF₂)^6– a very rare 6p² σ-bonded binuclear complex. However, the Pt–Pt σ bonding interaction is limited by the relatively long bridging-ligand-imposed Pt–Pt distance accompanied by repulsive electronic congestion. Pt­(pop-BF₂)^4– is predicted to be a very strong photooxidant (potentials of +1.57 and +0.86 V are estimated for the singlet and triplet dσ*pσ excited states, respectively)

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