Twisting and Tilting 1,1′-Bis(dialkylphosphino)ferrocene Bound to Low Valent Tricarbonylmaganese(I to −I)

Deciphering the direct heterometallic interaction in κ3-bis(donor)ferrocenyl-transition-metal complexes

Ligands featuring a 1,1'-bis(donor)ferrocene motif can adopt various binding modes. Among them, the κ3 binding mode, which involves interaction between the iron center of the ferrocene unit and the transition metal is the most unique. Although various examples highlight the interaction itself, the exact quantification of its strength remains uncertain. In our computational study, we systematically investigate the nature of this unique heterometallic bond, demonstrating that the electron density at the transition metal primarily governs the heterobimetallic interaction. On the other hand, the contribution of the ipso-carbon atoms of the cyclopentadiene ring is not negligible. We demonstrated that isodesmic reactions provide the most quantifiable data regarding the interaction. If the transition metal center is complexed with good electron-donor ligands or its positive charge is compensated by the negative charge of the ligands, the interaction with the electron-rich iron center recedes into the background. Finally, we highlighted the importance of the accurate computational description of these systems.

Gradual Coordination and Reversible P-P Bond Activation of a P3 -Unit with Transition Metal Carbonyls

Coordination of a stereochemically defined P3 -chain to a series of transition metal carbonyls [M(CO)x ]z- (M = Mn (x = 5, z = 1), Fe (x = 4, z = 2) or Co (x = 4, z = 1)) is explored using a [3]ferrocenophane scaffold. A gradual transition from η1 -, η2 - to η3 -coordination is observed where in the η2 -mode the terminal positions of the phosphorus chain are bridged. With an excess of cobalt carbonyl successive P-P bond activation and gradual separation of the central phosphorus atoms from the phosphorus chain has been observed. This process is reversible and with suitable reagents such as methyl lithium, the P3 -unit is regenerated in stereospecific manner. The bonding situation and steps of the gradual P-P bond activation are investigated by DFT calculations as well as experimental methods (e.g., NMR spectroscopy, X-ray crystallography).

Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation

Understanding the thermodynamics of paramagnetic transition metal hydride complexes, especially of the abundant 3d metals, is important in the design of electrocatalysts and organometallic catalysts. The pK_a^MeCN([MHL_n]⁺/[ML_n) of paramagnetic hydrides in MeCN are estimated for the first time using the ligand acidity constant (LAC) equation where contributions to the pK_a^MeCN from each ligand are simply added together, with the sum corrected for effects of charge and 5d metals. The pK_a^LAC-MeCN([MHL_n]⁺/ML_n) of over 200 hydride complexes MHL_n are used, along with their electrochemical potentials from the literature, in an uncommonly applied thermochemical cycle in order to reveal systematic trends in the redox couples M^III/II and M^V/IV (M = Cr, Mo, W), Mn^II/I, Re^VI/V and Re^IV/III, M^III/II and M^IV/III (M = Fe, Ru, Os), and M^III/II and M^II/I (M = Co, Rh, and Ir) and allow the estimation of the bond dissociation free energies BDFE(MH) of the unoxidized hydrides MHL_n and the prediction of the electrochemical potential for their oxidation. Density functional theory (DFT) calculations are used to validate the pK_a^LAC-MeCN values of hydrides of W^III, Mn^II, Fe^III, Ru^III, Co^II, and Ni^III. When a pK_a^LAC-MeCN is less than zero for a given complex [MHL_n]⁺, the oxidation of MHL_n is irreversible due to proton loss from the oxidized complex to the solvent. When pK_a^LAC-MeCN ≫ 0, the oxidation is reversible when there is no gross change in the coordination geometry upon a change in the redox state. Twenty paramagnetic hydrides prepared in bulk all have pK_a^LAC-MeCN > 8.

(Spectro)electrochemical and Electrocatalytic Investigation of 1,1'-Dithiolatoferrocene-Hexacarbonyldiiron

Hexacarbonyldiiron bridged by a 1,1'-dithiolatoferrocene, [Fe(C₅H₄S)₂{Fe(CO)₃}₂] (1), was synthesized, and the electrochemistry showed reversible oxidation at the Fe(C₅H₄S)₂ site and quasi-reversible reduction at the hexacarbonyldiiron site. Spectroelectrochemical techniques showed reduction-induced ligand isomerization, where the thiolate ligand went from bridging to terminal and one carbon monoxide ligand moved to a quasi-bridging position; this mechanism was further supported by cyclic voltammetry simulation and density functional theory calculations. Complex 1 showed electrocatalytic activity toward hydrogen-evolving reaction.

Beyond Common Metal-Metal Bonds, κ3 -Bis(donor)ferrocenyl→Transition-Metal Interactions

Ligands with 1,1'-bis(donor)ferrocene motif are capable of a wide range of binding modes, including the trans chelation mode in which there is a Fe-M interaction (κ³ -D,Fe,D), in the form of a dative Fe→TM bond (TM=transition metal). This Minireview will explore the nature of this Fe-TM interaction thorough select examples as well as how to characterize a Fe→TM dative bond using physical, computational, and spectroscopic techniques.