Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds

Abstract

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d 6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L CNC ), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L CNC ) 2 ](PF 6 ), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(L CNC ) 2 ](PF 6 ) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L CNC ) 2 ](PF 6 ), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

“…The ILCT states of our Zn II complexes complement the different types of photoactive excited states reported recently for first-row and other Earth-abundant transition-metal complexes, which includes the classical MLCT states for d 6 complexes (Cr 0 , Mn I , Fe II , Co III ), ,,,− square-planar d 8 compounds (Ni II ) , and four-coordinate d 10 complexes (Cu I ), , ligand-to-metal charge-transfer (LMCT) states for Ti IV , Zr IV , , Mn IV , Fe III and Co III , − − metal-centered (MC) states for V III , Cr III and Co III , − as well as ligand-to-ligand charge-transfer (LLCT) excited states for two-coordinate Cu I complexes. − Given these findings, it seems reasonable to conclude that Zn II complexes with charge-transfer and triplet excited states would perhaps deserve greater attention in future studies aiming to discover new photophysics and photochemistry in first-row transition-metal complexes.…”

“…Tetrahedral copper­(I) complexes with luminescent and redox-active metal-to-ligand charge-transfer (MLCT) excited states have long been known , and nowadays represent a well-developed compound class with applications in light-emitting devices, − dye-sensitized solar cells, and photocatalysis. − In the 3d 10 valence electron configuration of Cu I , there are no metal-centered (MC) excited states that can depopulate the photoactive 3 MLCT states, , unlike for analogous 3d 6 (Co III , Fe II , Mn I , Cr 0 ) − or 3d 8 (Ni II ) compounds, , in which nonradiative MLCT deactivation by MC states can be undesirably fast . Consequently, complexes with the 3d 10 configuration are predisposed for obtaining long-lived and strongly emissive excited states, as illustrated for example by recently reported linear two-coordinate Cu I compounds with exceptionally strongly luminescent ligand-to-ligand charge-transfer (LLCT) excited states. − …”

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes

“…However, for the 3d 6 and 3d 8 valence electron configurations, the presence of low-lying distorted MC states represents a major challenge . In second- and third-row transition metal complexes with the d 6 and d 8 configurations, the respective MC states are often at higher energies than emissive MLCT or other charge-transfer (CT) states, , making nonradiative relaxation less prevalent and leading to favorable photophysical and photochemical behavior of complexes made from Ru II , Ir III , − Pt II , − or Au III . , Among 3d metals, Fe II has received the most attention concerning the relaxation of MLCT into MC states, − complemented recently by studies of isoelectronic Co III , Mn I , and Cr 0 compounds. − Thus, there is now a substantial body of literature on the photophysics of 3d 6 compounds, whereas 3d 8 complexes have remained underexplored in comparison …”

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

“… 56 − 59 Advances in ligand design, photophysical techniques, and theoretical understanding 60 have made an unexpectedly broad range of transition metals in different oxidation states useable for photophysical or photochemical applications. In particular, luminescent first-row transition metal complexes with V, 61 − 64 Cr, 65 − 73 Mn, 53 , 74 − 76 Fe, 28 , 34 , 42 , 77 − 79 Co, 71 , 80 − 800 Ni, 83 − 86 and Cu 87 − 96 with different types of electronically excited states featuring promising photoreactivity and photoluminescence behavior have been discovered recently. 56 , 57 , 59 , 97 , 98 …”

“…Recently, there has been increased interest in first-row transition metal elements, partly because they are cheaper and more abundant than the traditionally used metals from the platinum group, and because they seem to offer ample opportunities for groundbreaking discoveries. − Advances in ligand design, photophysical techniques, and theoretical understanding have made an unexpectedly broad range of transition metals in different oxidation states useable for photophysical or photochemical applications. In particular, luminescent first-row transition metal complexes with V, − Cr, − Mn, ,− Fe, ,,,− Co, ,− Ni, − and Cu − with different types of electronically excited states featuring promising photoreactivity and photoluminescence behavior have been discovered recently. ,,,, …”