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1. Synthesis, structural studies, and redox chemistry of bimetallic [Mn(CO) 3 ] and [Re(CO) 3 ] complexes

   https://doi.org/10.1039/D0DT03666H  

   Henke, Wade C. ; Kerr, Tyler A. ; Sheridan, Thomas R. ; Henling, Lawrence M. ; Takase, Michael K. ; Day, Victor W. ; Gray, Harry B. ; Blakemore, James D. ( March 2021 , Dalton Transactions)

   null (Ed.)

   Manganese ([Mn(CO) 3 ]) and rhenium tricarbonyl ([Re(CO) 3 ]) complexes represent a workhorse family of compounds with applications in a variety of fields. Here, the coordination, structural, and electrochemical properties of a family of mono- and bimetallic [Mn(CO) 3 ] and [Re(CO) 3 ] complexes are explored. In particular, a novel heterobimetallic complex featuring both [Mn(CO) 3 ] and [Re(CO) 3 ] units supported by 2,2′-bipyrimidine (bpm) has been synthesized, structurally characterized, and compared to the analogous monomeric and homobimetallic complexes. To enable a comprehensive structural analysis for the series of complexes, we have carried out new single crystal X-ray diffraction studies of seven compounds: Re(CO) 3 Cl(bpm), anti -[{Re(CO 3 )Cl} 2 (bpm)], Mn(CO) 3 Br(bpz) (bpz = 2,2′-bipyrazine), Mn(CO) 3 Br(bpm), syn - and anti -[{Mn(CO 3 )Br} 2 (bpm)], and syn -[Mn(CO 3 )Br(bpm)Re(CO) 3 Br]. Electrochemical studies reveal that the bimetallic complexes are reduced at much more positive potentials (Δ E ≥ 380 mV) compared to their monometallic analogues. This redox behavior is consistent with introduction of the second tricarbonyl unit which inductively withdraws electron density from the bridging, redox-active bpm ligand, resulting in more positive reduction potentials. [Re(CO 3 )Cl] 2 (bpm) was reduced with cobaltocene; the electron paramagnetic resonance spectrum of the product exhibits an isotropic signal (near g = 2) characteristic of a ligand-centered bpm radical. Our findings highlight the facile synthesis as well as the structural characteristics and unique electrochemical behavior of this family of complexes. 

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2. Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low‐Valent [Cp*Rh] Complexes**

   https://doi.org/10.1002/chem.202103970  

   Henke, Wade C. ; Stiel, Jonah P. ; Day, Victor W. ; Blakemore, James D. ( January 2022 , Chemistry – A European Journal)

   Ligands based upon the 4,5‐diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9′‐dimethyl‐4,5‐diazafluorene (Me₂daf) can stabilize low‐valent complexes through charge delocalization into its conjugated π‐system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me₂daf is protective, blocking Brønsted acid‐base chemistry commonly encountered with other daf‐based ligands. Electronic absorption spectroscopy and single‐crystal X‐ray diffraction analysis of a family of eleven new compounds, including the unusual Cp*Rh(Me₂daf), reveal features consistent with charge delocalization driven by π‐backbonding into the LUMO of Me₂daf, reminiscent of behavior displayed by the workhorse 2,2′‐bipyridyl ligand. Taken together with spectrochemical data demonstrating clean conversion between oxidation states, our findings show that 9,9′‐dialkylated daf‐type ligands are promising building blocks for applications in reductive chemistry and catalysis.

    

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3. CO 2 Activation with Manganese Tricarbonyl Complexes by an H‐Atom Responsive Benzimidazole Ligand

   https://doi.org/10.1002/chem.202300796  

   Singh, Kundan K. ; Gerke, Carter S. ; Saund, Simran S. ; Zito, Alessandra M. ; Siegler, Maxime A. ; Thoi, V. Sara ( September 2023 , Chemistry – A European Journal)

   Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, Mn^I(HL)(CO)₃Br (1 a‐Br) and Mn^I(MeL)(CO)₃Br (1 b‐Br) (where HL=2‐(2’‐pyridyl)benzimidazole; MeL=1‐methyl‐2‐(2’‐pyridy)benzimidazole) and assayed their electrocatalytic properties for CO₂reduction. A redox‐active pyridine benzimidazole ancillary ligand in complex1 a‐Brdisplayed unique hydrogen atom transfer ability to facilitate electrocatalytic CO₂conversion at a markedly lower reduction potential than that observed for1 b‐Br. Notably, a one‐electron reduction of1 a‐Bryields a structurally characterized H‐bonded binuclear Mn(I) adduct (2 a’) rather than the typically observed Mn(0)‐Mn(0) dimer, suggesting a novel method for CO₂activation. Combining advanced electrochemical, spectroscopic, and single crystal X‐ray diffraction techniques, we demonstrate the use of an H‐atom responsive ligand may reveal an alternative, low‐energy pathway for CO₂activation by an earth‐abundant metal complex catalyst.

    

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4. Ruthenium terpyridine Phenol-Substituent supports PCET and semiquinone-like behavior

   https://doi.org/10.1016/j.poly.2023.116582  

   Moffa, Katherine L ; Teahan, Claire N ; Montgomery, Charlotte L ; Shepherd, Samantha L ; Dickenson, John C ; Benson, Kaitlyn R ; Olsen, Mark ; Boyko, Walter J ; Bezpalko, Mark ; Kassel, W Scott ; et al ( November 2023 , Polyhedron)

   Ligands play a central role in dictating the electronic properties of metal complexes to which they are coordinated. A fundamental understanding of changes in ligand properties can be used as design principles for more efficient catalysts. Designing ligands that have multiple protonation states that will change the properties of the coordination complex would be useful as potential ways of controlling catalysis, for example, as an on/off switch where one redox state exists below thermodynamic potential and another exists above. Thus, phenol moieties built into strongly coordinating ligands, like that of tpyPhOH (4′-(4-hydroxyphenyl)-2,2′:6′,2′’-terpyridine) may provide such a handle. Herein, we report the electrochemical and spectral characterization, and the crystallographic and computational analysis of two ruthenium analogs: [Ru(tpy)(tpyPhOH)](PF6)2 and [Ru(tpyPhOH)2] (PF6)2 (tpy =2,2′:6′,2′’-terpyridine). Cyclic voltammetry and differential pulse voltammetry indicate that two redox events occur, one of which is pH independent and we hypothesize that these follow an electrochemical- chemical-electrochemical (ECE) mechanism. XRD results of the ruthenium complexes’ protonated forms are generally consistent with expected bond lengths and angles and are in agreement with computational modeling. The properties are compared to a previously reported analog that contains the –OH group directly connected to terpyridine, [Ru(tpyOH)2](PF6)2, where tpyOH is 4′-hydroxy-2,2′:6′,2′’-terpyridine, with some intriguing differences. Overall, these data indicate that the phenyl-substituent decouples the phenol such that it behaves both as an electron withdrawing substituent and a location for a ligand centered oxidation event to occur. 

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5. Elucidating the mechanism of photochemical CO 2 reduction to CO using a cyanide-bridged di-manganese complex

   https://doi.org/10.1039/d2dt02506j  

   Cohen, Kailyn Y. ; Reinhold, Adam ; Evans, Rebecca ; Lee, Tia S. ; Kuo, Hsin-Ya ; Nedd, Delaan G. ; Scholes, Gregory D. ; Bocarsly, Andrew B. ( November 2022 , Dalton Transactions)

   The complex, [{[Mn(bpy)(CO) 3 ] 2 }(μ-CN)] + (Mn2CN+), has previously been shown to photochemically reduce CO 2 to CO. The detailed mechanism behind its reactivity was not elucidated. Herein, the photoevolution of this reaction is studied in acetonitrile (MeCN) using IR and UV-vis spectroscopy. Samples were excited into the Mn I → π* bpy metal-to-ligand charge transfer (MLCT) absorption band triggering CO loss, and rapid MeCN solvent ligation at the open coordination site. It is concluded that this process occurs selectively at the Mn axial ligation site that is trans to the C-end of the bridging cyanide. Upon further photolysis, the metal–metal bonded dimeric species, [(CO) 3 (bpy)Mn–Mn(bpy)(CO) 3 ] (Mn–Mn) is observed to form under anaerobic conditions. The presence of this dimeric species coincides with the observation of CO production. When oxygen is present, CO 2 photoreduction does not occur, which is attributed to the inability of Mn2CN+ to convert to the metal–metal bonded dimer. Photolysis experiments, where the Mn–Mn dimer is formed photochemically under argon first and then exposed to CO 2 , reveal that it is the radical species, [Mn(bpy)(CO) 3 ˙ ] ( Mn˙ ), that interacts with the CO 2 . Since the presence of Mn–Mn and light is required for CO production, [Mn(bpy)(CO) 3 ˙] is proposed to be a photochemical reagent for the transformation of CO 2 to CO. 

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