McMurry reactions of (g5-acetylcyclopentadienyl) cobalt-(g4-tetraphenylcyclobutadiene) with benzophenone: ketone couplings and a pinacol/pinacolone rearrangement

The reaction of (g4-C4Ph4) Co[g5-C5H4–C(@O)Me], 5, with benzophenone under McMurry conditions (TiCl4/Zn/THF) gives the hetero-coupled product (g4-C4Ph4)Co[g5-C5H4–C(Me)@CPh2], 7, together with the dicobalt species: trans-(g4-C4Ph4)Co[(g5-C5H4– C(Me)@C(Me)-g5-C5H4] Co(g4-C4Ph4), 9, and the pinacolone Me[(g4-C4Ph4)Co(g5-C5H4)]2C–C(@O)Me, 10. The latter is apparently formed from the pinacol by migration of an (g4-C4Ph4)Co[(g5-C5H4)] group. Preferential migration of the cobalt sandwich moiety rather than a methyl group is rationalized in terms of a favored transition state involving a metal-stabilized cation. The products 7, 9 and 10, and also the ketone (g4-C4Ph4)Co[g5-C5H4–C(@O)Et], 6, were all characterized by X-ray crystallography

Comparison of two field-induced Er-III single ion magnets

10.1039/c9dt02434dDALTON TRANSACTIONS484115679-1568

Relevance of chemical vs electrochemical oxidation of tunable carbene iridium complexes for catalytic water oxidation

Based on previous work that identified iridium(III) Cp* complexes containing a C,N-bidentate chelating triazolylidene-pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C 5Me 5 –) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron-donating substituents into the pyridyl unit of the chelating ligand (H, a; 5-OMe, b; 4-OMe, c; 4-tBu, d; 4-NMe 2, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure-activity relationship with electron-donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOF max = 2500 h – 1 for complex 10 with a MeO group on pyr and a OEt-substituted triazolylidene, compared to 700 h – 1 for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN-mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation

Relevance of chemical vs electrochemical oxidation of tunable carbene iridium complexes for catalytic water oxidation

\u3cp\u3eBased on previous work that identified iridium(III) Cp* complexes containing a C,N-bidentate chelating triazolylidene-pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C \u3csub\u3e5\u3c/sub\u3eMe \u3csub\u3e5\u3c/sub\u3e \u3csup\u3e–\u3c/sup\u3e) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron-donating substituents into the pyridyl unit of the chelating ligand (H, a; 5-OMe, b; 4-OMe, c; 4-tBu, d; 4-NMe \u3csub\u3e2\u3c/sub\u3e, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure-activity relationship with electron-donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOF \u3csub\u3emax\u3c/sub\u3e = 2500 h \u3csup\u3e–\u3c/sup\u3e \u3csup\u3e1\u3c/sup\u3e for complex 10 with a MeO group on pyr and a OEt-substituted triazolylidene, compared to 700 h \u3csup\u3e–\u3c/sup\u3e \u3csup\u3e1\u3c/sup\u3e for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN-mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation. \u3c/p\u3

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling

International audiencePinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn-III(3,5-diCl-sal(2)(323))]BPh4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc -> Pc -> P1 -> P1((1/2)). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn-Teller-active Mn3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc -> P1 and P1 -> P1((1/2)) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1 -> P1((1/2)) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled