Pogo-Stick Iron and Cobalt Complexes: Synthesis, Structures, and Magnetic Properties

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(μ-I)]₂ (1-M, M = Fe, Co; Cp' = η⁵-1,2,4-tri-tert-butylcyclopentadienyl) with [Im^DippNLi]₂ (Im^DippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal half-sandwich compounds [Cp'M(NIm^Dipp)] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid-base reaction between [Cp'Co{N(SiMe₃)₂}] (3-Co) and Im^DippNH. The electronic and magnetic properties of 2-M and 3-Co were probed by ⁵⁷Fe Mössbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

Cobalt, nickel and copper complexes with glycinamide: structural insights and magnetic properties

Ten new compounds of Co, Ni and Cu with glycinamide (HL = glycinamide): [Co(H₂O)₂(HL)₂]Cl₂ (1a), [Co(H₂O)₂(HL)₂]Br_1.06Cl_0.94 (1b), [Co(H₂O)₂(HL)₂]I₂ (1c), [Ni(H₂O)₂(HL)₂]Cl₂ (2a), [Ni(H₂O)₂(HL)₂]Br_0.94Cl_1.06 (2b), [Ni(H₂O)₂(HL)₂]I₂ (low and room temperature polymorph, 2c_LT and 2c_RT), [CuCl₂(HL)₂] (3a), [CuBr_1.3Cl_0.7(HL)₂] (3b) and {[Cu(HL)₂]₂[Cu₂I₆]}_n (3c), as well as glycinamide hydroiodide (H₂LI) and a new polymorph of glycinamide hydrochloride (β-H₂LCl) were prepared and characterized by single-crystal X-ray diffraction, infrared spectroscopy, thermal analysis (TG/DTA) and ESR spectroscopy. 1a, 1b, 2a and 2b are isostructural, as well as 1c and 2c_RT, while the Cu compounds (3a–c) have entirely different molecular structures. All investigated compounds are mononuclear with exception of the 1D coordination polymer 3c. Compound 3c contains copper ions in the mixed oxidation state Cu(i) and Cu(ii) with interesting magnetic properties. Paramagnetic behaviour was found in 1a, 1b, 3a and 3b. Temperature induced polymorphic transformation was observed in 2c. Compounds 1a and 3a showed moderate antiproliferative activity and selectivity toward the human breast tumor cell line MCF-7.

We report the crystal structures and magnetic properties of nine mononuclear complexes and one 1D coordination polymer containing Cu^II/Cu^I ions.

Detection of a charge-separated catalyst precursor state in a linked photosensitizer-catalyst assembly

We have designed two new supramolecular assemblies based on Co(ii)-templated coordination of Ru(bpy)3(2+) (bpy = 2,2'-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle, which are of interest as proton reduction catalysts. The self-assembled photocatalyst precursors were structurally characterized by Co K-edge X-ray absorption spectroscopy and solution-phase X-ray scattering. Visible light excitation of one of the assemblies has yielded instantaneous electron transfer and charge separation to form a transient Co(i) state which persists for 26 ps. The development of a linked photosensitizer-cobaloxime architecture supporting efficient Co(i) charge transfer is significant since it is mechanistically critical as the first photo-induced electron transfer step for hydrogen production, and has not been detected in previous photosensitizer-cobaloxime linked dyad assemblies. X-band EPR spectroscopy has revealed that the Co(ii) centres of both assemblies are high spin, in contrast to most previously described cobaloximes, and likely plays an important role in facilitating photoinduced charge separation. Based on the results obtained from ultrafast and nanosecond transient absorption optical spectroscopies, we propose that charge recombination occurs through multiple ligand states present within the photosensitizer modules. The studies presented here will enhance our understanding of supramolecular photocatalyst assembly and direct new designs for artificial photosynthesis.