Stable Molybdenum(0) Carbonyl Complex for Upconversion and Photoredox Catalysis

Abstract

Photoactive complexes with earth-abundant metals have attracted increasing interest in the recent years fueled by the promise of sustainable photochemistry. However, sophisticated ligands with complicated syntheses are oftentimes required to enable photoactivity with nonprecious metals. Here, we combine a cheap metal with simple ligands to easily access a photoactive complex. Specifically, we synthesize the molybdenum(0) carbonyl complex Mo(CO)3(tpe) featuring the tripodal ligand 1,1,1-tris(pyrid-2-yl)ethane (tpe) in two steps with a high overall yield. The complex shows intense deep-red phosphorescence with excited state lifetimes of several hundred nanoseconds. Time-resolved infrared spectroscopy and laser flash photolysis reveal a triplet metal-to-ligand charge-transfer (3MLCT) state as the lowest excited state. Temperature-dependent luminescence complemented by density functional theory (DFT) calculations suggest thermal deactivation of the 3MLCT state via higher lying metal-centered states in analogy to the well-known photophysics of [Ru(bpy)3]2+. Importantly, we found that the title compound is very photostable due to the lack of labilized Mo–CO bonds (as caused by trans-coordinated CO) in the facial configuration of the ligands. Finally, we show the versatility of the molybdenum(0) complex in two applications: (1) green-to-blue photon upconversion via a triplet–triplet annihilation mechanism and (2) photoredox catalysis for a green-light-driven dehalogenation reaction. Overall, our results establish tripodal carbonyl complexes as a promising design strategy to access stable photoactive complexes of nonprecious metals avoiding tedious multistep syntheses.

Vapor-phase synthesis of low-valent metal–organic frameworks from metal carbonyl synthons

“…Kitzmann et al have recently assigned a reversible couple at E 1/2 = À0.20 V (vs. Fc + /Fc) to the one-electron oxidation of the structurally related fac-Mo(CO) 3 (tpe) (tpe = 1,1,1-tris(pyrid-2-yl)ethane) complex in DMF. 22 Also shown in Fig. 5 and Fig.…”

“…[7][8][9][10][11][12][13][14][15][16] There is already considerable interest in developing methods to incorporate these functional units into existing MOF scaffolds, [17][18][19][20][21] while very recently they have also proven to be active photoredox catalysts. 22 The applications of MOFs, for example as electrocatalysts 23 or sensors, 24 often require their deposition on various substrates. 25 MOF synthesis and deposition protocols largely rely on solutionphase processes to control homogeneity and crystallization.…”

Spectroscopic Techniques to Unravel Mechanistic Details in Light‐Induced Transformations and Photoredox Catalysis

“…As such, the Cu(I) photosensitizer was excited with red light which led to the excited‐state reduction of dicyanoanthracene, whose corresponding radical anion also absorbed red light to perform challenging reactions. Photon upconversion via triplet‐triplet annihilation also represents a promising avenue that will take advantage of red‐light absorbing chromophore to activate blue or UV emitter for substrate photo‐activation [13,17n,86] Mechanistic investigations .…”

Coulomb interactions for mediator-enhanced sensitized triplet–triplet annihilation upconversion in solution

“…5,26,61–64 Also in systems where the comparably short lifetimes of the sensitizers call for high concentrations of the annihilators, filter effects can significantly limit the output of high-energy photons. 21,65–70 These performance improvements are important for applications in which the upconversion system is used as light source to drive secondary reactions. 1,71–76 With our new approach using a commercially available charge-adapted mediator in solution, a modular exchange of the annihilator to other anthracene-based structures is possible with similar enhancements of the upconversion quantum yields induced by the mediator.…”