Structural and spectroscopic evidence for weak metal-metal interactions and metal-substrate exciplex formations in d10 metal complexes

Structural investigations of homometallic di- and tri-nuclear coinage metal complexes with bridging phosphine ligands, [M2(P∩P)2]X2 and [M3(P∩P∩P)2]X3 (M = Cu(I), Au(I), and Ag(I); X = anion), have revealed close intramolecular metal-metal and metal-anion contacts in their crystal lattices, thus providing solid-state evidence for metallophilicity and metal-substrate interactions in two-coordinate d10 metal ions. To verify the existence of weak closed-shell metal-metal interactions, UV-vis absorption and resonance Raman spectroscopic techniques have been employed. Comparisons of the absorption spectra of [M2(P∩P)2]2+ and [M3(P∩P∩P)2]3+ show that the spin- and dipole-allowed [ndσ* → (n + 1)pσ] transition red-shifts in energy from [M2(P∩P)2]2+ to [M3(P∩P∩P)2]3+. This spectral assignment was confirmed by resonance Raman spectroscopy, which reveals stronger metal-metal interaction in the 1[(n + 1)pσ, ndσ*] state compared to the ground state. Photoluminescence from the 3[(n + 1)pσ, ndσ*] excited state was also recorded for the [M2(P∩P)2]X2 and [M3(P∩P∩P)2]X3 solids, where X is a non-coordinating anion. The spectroscopic data for these metal-centered transitions are supported by theoretical calculations. Perturbations of the MM bonded excited states through interaction with neighboring solvent molecules or anions lead to exciplex formations with emission in the visible region. The sensitivity of photoluminescence of two-coordinate d10 metal complexes to metal-ligand coordination provides an entry to new classes of luminescent sensory materials for substrate-binding processes. The 3[(n + 1)pσ, ndσ*] excited states of [M2(P∩P)2]2+ (M = Cu and Au) systems are powerful reductants and light-induced multi-electron photocatalysis by these systems have been observed.