Metal−Hydride Bond Activation and Metal−Metal Interaction in Dinuclear Iron Complexes with Linking Dinitriles:  A Synthetic, Electrochemical, and Theoretical Study

Abstract

The dinuclear iron(II)-hydride complexes [[FeH(dppe)(2)](2)(mu-LL)][BF(4)](2) (LL = NCCH=CHCN (1a), NCC(6)H(4)CN (1b), NCCH(2)CH(2)CN (1c); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and the corresponding mononuclear ones, trans-[FeH(LL)(dppe)(2)][BF(4)] (2a-c) were prepared by treatment of trans-[FeHCl(dppe)(2)], in tetrahydrofuran (thf) and in the presence of Tl[BF(4)], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal-metal interaction was detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent Fe(II)/Fe(III) 1a(+) complex. The latter either undergoes heterolytic Fe-H bond cleavage (loss of H(+)) or further oxidation, at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic processes, respectively, which were established by digital simulation. Anodically induced Fe-H bond rupture was also observed for the other complexes and the detailed electrochemical behavior, as well as the metal-metal interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic addition, by BF(4)(-), to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation potential of the complexes, the electrochemical P(L) and E(L) ligand parameters were also estimated for the dinitrile ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.

Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)-TEMPO Catalytic Systems

“…The solutions were saturated with N 2 by bubbling this gas before each run, the redox potentials of the complexes were measured by CV in the presence of ferrocene as the internal standard, and their values are quoted relative to the SCE by using the [Fe(g 5 -C 5 H 5 ) 2 ] 0/? redox couple (E = 0.42 V vs. SCE) [34][35][36].…”

“…For example, an almost quantitative yield (99.3 %) of acetophenone is achieved after 1 h at 150°C (10 W) for 3, in the presence of TEMPO (Table 2, entry 9), which is much higher than the 67 % yield Scheme 2 MW-assisted oxidation of 1-phenylethanol to acetophenone catalysed by 1-3 Fig. 3 Crystal structure of complex 3 with atomic labelling scheme [34][35][36] as internal standard at a scan rate of 200 mVs -1 a Anodic wave generated upon scan reversal following the reduction wave b Proton reduction (shifts to -1.18 V at a vitreous C electrode) c Oxidation of Cl - obtained for the same reaction time but at 70°C ( Table 2, entry 12). Furthermore, at this temperature, only 42.7 % (3) of acetophenone is obtained for the extended reaction time of 2 h ( Table 2, entry 14).…”

Electron-donor/acceptor properties of carbynes, carbenes, vinylidenes, allenylidenes and alkynyls as measured by electrochemical ligand parameters

“…Other indirect methods for the estimate of E s , b and/or P L have been used for certain types of 18-electron complexes, such as [M s LL 0 ] [31][32][33] and [M s L n ] [5,13] Values of the P L and E L parameters for selected ligands are shown in Table 2 [14,18,21,27,[29][30][31][32][33][34]36,[41][42][43][44][45][46][47][48][49][50][51][52][53][54], whereas E s , b and S M , I M parameters are listed in Tables 3 [5,14,27,30,[32][33][34][35]42,[44][45][46][47]49,55,56] and 4 [18,19,…”

“…Half-sandwich b {Mo(g 7 -C 7 H 7 )(dppe)} + 1.16 c 1.04 [55], TW c {Mo(g 7 -C 7 H 7 )( t Bu-dab)} + 1.14 0.68 [ dependent on their electronic and structural features [5,6,20,21] such as infra-red data [28,35,53,[59][60][61][62][63], the TolmanÕs electronic parameter (TEP) [64] for phosphines and a computed electronic parameter (CEP) [26] based (as TEP) on the infrared A 1 m(CO) frequency in [NiL(CO) 3 ] (also dependent on the electronic effect of L), the HammettÕs and related constants [23,24,26,42,60,65], the photoelectron binding energies or gas-phase vertical ionization potentials [66], the energy of a metal-to-ligand charge transfer [21] and chemical reactivity [14,43,62,63,67,68]. Ligand coordination criteria have also been proposed [14,15,42,48] on the basis of the electrochemical parameters.…”

C-Homoscorpionate Oxidation Catalysts—Electrochemical and Catalytic Activity

“…However, the irreversible character of the oxidation wave of the former (indicative of a chemical reaction following the electron-transfer step, with a resulting shift of the oxidation potential) preclude a reliable comparison between the measured potentials for these complexes. In the case of Mo(0 or II) complexes a second single-electron oxidation process is detected (not shown in Table 1 3 ] signals the instability of the resulting cationic Mo(III) complexes, which then rapidly decompose with probable CO loss [95] and, for the hydride compound, by deprotonation [96][97][98]. The first oxidation potentials of all these tricarbonyl complexes are much lower than that of the parent hexacarbonyl compound, on account of the replacement of three carbonyls in the latter by the more electron-donating C-scorpionate ligands [24,27,28].…”