Magnetic effects of lanthanide bonding Lanthanide coordination compounds have attracted attention for their persistent magnetic properties near liquid nitrogen temperature, well above alternative molecular magnets. Gould et al . report that introducing metal-metal bonding can enhance coercivity. Reduction of iodide-bridged terbium or dysprosium dimers resulted in a single electron bond between the metals, which enforced alignment of the other valence electrons. The resultant coercive fields exceeded 14 tesla below 50 and 60 kelvin for the terbium and dysprosium compounds, respectively. —JSY

A small but growing number of molecular compounds have been isolated featuring divalent lanthanides with 4f n 5d z 2 1 electron configurations. While the majority of these possess trigonal coordination geometries, we previously reported the first examples of linear divalent metallocenes Ln(Cp iPr5 ) 2 (Ln = Tb, Dy; Cp iPr5 = pentaisopropylcyclopentadienyl). Here, we report the synthesis and characterization of the remainder of the Ln(Cp iPr5 ) 2 (1-Ln) series (including Y and excluding Pm). The compounds can be synthesized through salt metathesis of LnI 3 and NaCp iPr5 followed by potassium graphite reduction for Ln = Y, La, Ce, Pr, Nd, Gd, Ho, and Er, by in situ reduction during salt metathesis of LnI 3 and NaCp iPr5 for Ln = Tm and Lu, or through salt metathesis from LnI 2 and NaCp iPr5 for Ln = Sm, Eu, and Yb. Single crystal X-ray diffraction analyses of 1-Ln confirm a linear coordination geometry with pseudo-D 5d symmetry for the entire series. Structural and ultraviolet−visible spectroscopy data support a 4f n+1 electron configuration for Ln 2+ = Sm, Eu, Tm, and Yb and a 4f n 5d z 2 1 configuration for the other lanthanides ([Kr]4d z2 1 for Y 2+ ). Characterization of 1-Ln (Ln = Y, La) using electron paramagnetic resonance spectroscopy reveals significant s−d orbital mixing in the highest occupied molecular orbital and hyperfine coupling constants that are the largest reported to date for divalent compounds of yttrium and lanthanum. Evaluation of the room temperature magnetic susceptibilities of 1-Ln and comparison with values previously reported for trigonal Ln 2+ compounds suggests that the more pronounced 6s−5d mixing may be associated with weaker 4f−5d spin coupling.

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the Oxygen Evolving Complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S2 state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure−electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [MnIIIMn3IVO4] cuboidal complexes as spectroscopic models of the S2 state of the OEC. Resembling the oxidation state and EPR spectra of the S2 state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed.

The S 3 state is the last semi-stable state in the water splitting reaction that is catalyzed by the Mn 4 O 5 Ca cluster that makes up the oxygen-evolving complex (OEC) of photosystem II (PSII). Recent high-field/frequency (95 GHz) electron paramagnetic resonance (EPR) studies of PSII isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus have found broadened signals induced by chemical modification of the S 3 state. These signals are ascribed to an S 3 form that contains a fivecoordinate Mn IV center bridged to a cuboidal Mn IV 3 O 4 Ca unit. High-resolution X-ray free-electron laser studies of the S 3 state have observed the OEC with all-octahedrally coordinated Mn IV in what is described as an open cuboid-like cluster. No five-coordinate Mn IV centers have been reported in these S 3 state structures. Here, we report high-field/frequency (130 GHz) pulse EPR of the S 3 state in Synechocystis sp. PCC 6803 PSII as isolated in the presence of glycerol. The S 3 state of PSII from Synechocystis exhibits multiple broadened forms (≈69% of the total signal) similar to those seen in the chemically modified S 3 centers from T. elongatus. Field-dependent ELDOR-detected nuclear magnetic resonance resolves two classes of 55 Mn nuclear spin transitions: one class with small hyperfine couplings (|A| ≈ 1−7 MHz) and another with larger hyperfine couplings (|A| ≈ 100 MHz). These results are consistent with an all-Mn IV 4 open cubane structure of the S 3 state and suggest that the broadened S 3 signals arise from a perturbation of Mn4A and/or Mn3B, possibly induced by the presence of glycerol in the as-isolated Synechocystis PSII.

The MnCaO oxygen-evolving complex (OEC) of photosystem II catalyzes the light-driven oxidation of two substrate waters to molecular oxygen. ELDOR-detected NMR along with computational studies indicated that ammonia, a substrate analogue, binds as a terminal ligand to the Mn4A ion trans to the O5 μ oxido bridge. Results from electron spin echo envelope modulation (ESEEM) spectroscopy confirmed this and showed that ammonia hydrogen bonds to the carboxylate side chain of D1-Asp61. Here we further probe the environment of OEC with an emphasis on the proximity of exchangeable protons, comparing ammonia-bound and unbound forms. Our ESEEM and electron nuclear double resonance (ENDOR) results indicate that ammonia substitutes for the W1 terminal water ligand without significantly altering the electronic structure of the OEC.