Electronic Properties of 4,4‘,5,5‘-Tetramethyl-2,2‘-biphosphinine (tmbp) in the Redox Series fac-[Mn(Br)(CO)₃(tmbp)], [Mn(CO)₃(tmbp)]₂, and [Mn(CO)₃(tmbp)]^-:  Crystallographic, Spectroelectrochemical, and DFT Computational Study

Abstract

Stepwise electrochemical reduction of the complex fac-[Mn(Br)(CO)(3)(tmbp)] (tmbp = 4,4',5,5'-tetramethyl-2,2'-biphosphinine) produces the dimer [Mn(CO)(3)(tmbp)](2) and the five-coordinate anion [Mn(CO)(3)(tmbp)](-). All three members of the redox series have been characterized by single-crystal X-ray diffraction. The crystallographic data provide valuable insight into the localization of the added electrons on the (carbonyl)manganese and tmbp centers. In particular, the formulation of the two-electron-reduced anion as [Mn(0)(CO)(3)(tmbp(-))](-) also agrees with the analysis of its IR nu(CO) wavenumbers and with the results of density functional theoretical (DFT) MO calculations on this compound. The strongly delocalized pi-bonding in the anion stabilizes its five-coordinate geometry and results in the appearance of several mixed Mn-to-tmbp charge-transfer/IL(tmbp) transitions in the near-UV-vis spectral region. A thorough voltammetric and UV-vis/IR spectroelectrochemical study of the reduction path provided evidence for a direct formation of [Mn(CO)(3)(tmbp)](-) via a two-electron ECE mechanism involving the [Mn(CO)(3)(tmbp)](*) radical transient. At ambient temperature [Mn(CO)(3)(tmbp)](-) reacts rapidly with nonreduced fac-[Mn(Br)(CO)(3)(tmbp)] to produce [Mn(CO)(3)(tmbp)](2). Comparison with the analogous 2,2'-bipyridine complexes has revealed striking similarity in the bonding properties and reactivity, despite the stronger pi-acceptor character of the tmbp ligand.

Electrocatalytic and Solar‐Driven CO2 Reduction to CO with a Molecular Manganese Catalyst Immobilized on Mesoporous TiO2

“…The product was isolated as an orange solid in 63 % yield and characterized by CHNP microanalysis, 31 P‐NMR spectroscopy, high‐resolution mass spectrometry, and infrared (IR) spectroscopy ($\bar{\nu }$ CO =2030, 1946, and 1930 cm −1 , Figure 1 a), which confirmed a fac‐ Mn tricarbonyl species 19. Full synthetic and characterization details can be found in the Supporting Information.…”

“…The electrochemical investigations establish the heterogenized MnP as the best‐performing Mn electrocatalyst to date, which was enabled by a dynamic TiO 2 | MnP interface and dimerization of the immobilized Mn catalyst. Finally, we present the first example of CO 2 reduction by a Mn catalyst driven by full UV/Vis solar‐spectrum irradiation, circumventing the typical photo‐instability13b, 19 of these compounds by combining the TiO 2 | MnP hybrid cathode in the dark with a CdS‐sensitized photoanode.…”

“…Manganese carbonyl compounds, such as MnP , show instability under illumination,19 and tend to undergo photolysis and release CO ligands 25. Consequently, the few reports of Mn‐based CO 2 reduction photocatalysis use monochromatic or narrowly filtered light to prevent decomposition of the catalyst 14, 25a, 26.…”

The CO/PC analogy in coordination chemistry and catalysis

“…Definitive evidences were given by the synthesis of a series of negatively charged biphosphinine complexes of different metals: dianionic homoleptic complexes of group 4 metals 7 were synthesized, [10] monoanionic complexes of group 7 (M = Mn, Re) 8, [11] dianionic complexes of group 8 (M = Fe, Ru) 9, [12] monoanionic complexes of group 9 metals (M = Co, Rh) 10, [13] and a 19 electron nickel(-1) complex 11 was characterized by EPR spectroscopy (Scheme 4). [14] These complexes were obtained using one of the two following methods : a first conventional method proceeds via reduction of preformed complexes with the appropriate number of electrons; a second approach relies on the prior synthesis of a mono radical anion of the biphosphinine or its dianion followed by complexation.…”

2,2′‐Biphosphinines and 2,2′‐Bipyridines in Homoleptic Dianionic Group 4 Complexes and Neutral 2,2′‐Biphosphinine Group 6 d6 Metal Complexes: Octahedral versus Trigonal‐Prismatic Geometries

“…[30] Thus, not surprisingly, biphosphinines proved to be very efficient ligands for the stabilization of electron-rich and even electron-excessive metal centers. Several anionic biphosphinine metal complexes were successfully synthesized and structurally characterized with Group 4, [31] 6, [32] 7, [33,34] 8, [35][36][37][38][39][40][41] 9 [41,42] and 10 [41,43,44] metals. During our studies on the synthesis of homoleptic Group 4 dianionic complexes of general formula [M(bp) 3 ] 2À (M = Ti, Zr, Hf), we were interested in a comparison between the respective p-accepting capacities of biphosphinines and their bipyridine analogues.…”