Ancillary Ligand Effects upon the Photochemistry of Mn(bpy)(CO)3X Complexes (X = Br–, PhCC–)

Photochargeable Mn-Based Metal-Organic Framework and Decoupled Photocatalysis

Designing multifunctional materials that mimic the light-dark decoupling of natural photosynthesis is a key challenge in the field of energy conversion. Herein, we introduce MnBr-253, a precious metal-free metal-organic framework (MOF) built on Al nodes, bipyridine linkers and MnBr(CO)3(bipyridine) complexes. Upon irradiation, MnBr-253 colloids demonstrate an electron photocharging capacity of ~42 C ⋅ g-1 MOF, with state-of-the-art photocharging rate (1.28 C ⋅ s-1 ⋅ g-1 MOF) and incident photon-to-electron conversion efficiency of ~9.4 % at 450 nm. Spectroscopic and computational studies support effective electron accumulation at the Mn complex while high porosity and Mn loading account for the notable electron storage performance. The charged MnBr-253 powders were successfully applied for hydrogen evolution under dark conditions thus emulating the light-decoupled reactivity of photosynthesis.

Mechanistic insights into CO2 conversion to CO using cyano manganese complexes

Without the use of a photosensitizer, [Mn(bpy)(CO)₃(CN)] (MnCN) can photochemically form [Mn(bpy)(CO)₃]^-, the active species for CO₂ reduction. While cases of the axial X-ligand dissociating upon irradiation of fac-[M(N-N)(CO)₃X] complexes (M = Mn or Re; N-N = bipyridine (bpy) ligand; X = halogen or pseudohalogen) are well documented, the axial cyanide ligand is retained when either [Mn(bpy)(CO)₃(CN)] or [Mn(mesbpy)(CO)₃(CN)], MnCN(mesbpy), are irradiated anaerobically. Infrared and UV-vis spectroscopies indicate the formation of [Mn(bpy)(CO)₂(MeCN)(CN)] (s-MnCN) as the primary product during the irradiation of MnCN. An in-depth analysis of the photochemical mechanism for the formation of [Mn(bpy)(CO)₃]^- from MnCN is presented. MnCN(mesbpy) is too sterically hindered to undergo the same photochemical mechanism as MnCN. However, MnCN(mesbpy) is found to be electrocatalytically active for CO₂ reduction to CO. Thus providing an interesting distinction between photochemical and electrochemical charge transfer.

Elucidating the mechanism of photochemical CO2 reduction to CO using a cyanide-bridged di-manganese complex

The complex, [{[Mn(bpy)(CO)3]2}(μ-CN)]+ (Mn2CN+), has previously been shown to photochemically reduce CO2 to CO.

In Situ Solution-State Characterization of MOF-Immobilized Transition-Metal Complexes by Infrared Spectroscopy

Transition-metal catalysts immobilized on the surface of Metal-organic frameworks (MOFs) are being utilized for an ever-increasing number of reactions ranging from couplings to olefin oligomerization. While these reactions are usually performed in solution, unlike their homogeneous counterparts, the insolubility of the MOF systems makes it difficult to obtain detailed mechanistic information by in situ spectroscopic analysis in solution. In this report, we present a synthetic method to solubilize these systems by grafting oligomers on the surface of the MOF particles, making it possible to characterize these species by transmission infrared (IR) spectroscopy. The fundamental photochemistry of these catalysts was also studied and compared to that of their homogeneous counterparts. This work establishes a proof of principle for in-solution monitoring of heterogeneous catalysts.

Sterically hindered Re- and Mn-CO2 reduction catalysts for solar energy conversion

Novel molecular Re and Mn tricarbonyl complexes bearing a bipyridyl ligand functionalised with sterically hindering substituents in the 6,6'-position, [M(HPEAB)(CO)3(X)] (M/X = Re/Cl, Mn/Br; HPEAB = 6,6'-{N-(4-hexylphenyl)-N(ethyl)-amido}-2,2'-bipyridine) have been synthesised, fully characterised including by single crystal X-ray crystallography, and their propensity to act as catalysts for the electrochemical and photochemical reduction of CO2 has been established. Controlled potential electrolysis showed that the catalysts are effective for electrochemical CO2-reduction, yielding CO as the product (in MeCN for the Re-complex, in 95 : 5 (v/v) MeCN : H2O mixture for the Mn-complex). The recyclability of the catalysts was demonstrated through replenishment of CO2 within solution. The novel catalysts had similar reduction potentials to previously reported complexes of similar structure, and results of the foot-of-the-wave analysis showed comparable maximum turnover rates, too. The tentative mechanisms for activation of the pre-catalysts were proposed on the basis of IR-spectroelectrochemical data aided by DFT calculations. It is shown that the typical dimerisation of the Mn-catalyst was prevented by incorporation of sterically hindering groups, whilst the Re-catalyst undergoes the usual mechanism following chloride ion loss. No photochemical CO2 reduction was observed for the rhenium complex in the presence of a sacrificial donor (triethylamine), which was attributed to the short triplet excited state lifetime (3.6 ns), insufficient for diffusion-controlled electron transfer. Importantly, [Mn(HPEAB)(CO)3Br] can act as a CO2 reduction catalyst when photosensitised by a zinc porphyrin under red light irradiation (λ > 600 nm) in MeCN : H2O (95 : 5); there has been only one reported example of photoactivating Mn-catalysts with porphyrins in this manner. Thus, this work demonstrates the wide utility of sterically protected Re- and Mn-diimine carbonyl catalysts, where the rate and yield of CO-production can be adjusted based on the metal centre and catalytic conditions, with the advantage of suppressing unwanted side-reactions through steric protection of the vacant coordination site.