Ferromagnetic coupling in d1–d3 linear oxido-bridged heterometallic complexes: ground-state models of metal-to-metal charge transfer excited states

Linear heterobimetallic oxido-bridged d¹–d³ compounds are described which are proposed as models for magnetic coupling of MMCT excited states.

Laser Photolysis Studies of the Reaction of Chromium(III) Octaethylporphyrin Complex with Triphenylphosphine and Triphenylphosphine Oxide

The photoreaction of the chromium(III) octaethylpoprhyrin complex, [Cr(OEP)(Cl)(L)] (L = H2O, Py, OPPh3), in dichloromethane was studied using laser flash photolysis technique. Laser irradiation causes the generation of a coordinately unsaturated intermediate [Cr(OEP)(Cl)], which reacts with ligands in solution to give the parent complex, [Cr(OEP)(Cl)(L)], or a transient species, [Cr(OEP)(Cl)(H2O)], when L = Py or OPPh3. Once produced [Cr(OEP)(Cl)(H2O)] eventually exchanges the axial H2O ligand with L to regenerate [Cr(OEP)(Cl)(L)]. The kinetics of the axial ligand substitution reaction was followed spectrophotometrically, and the ligand-concentration dependence of the ligand exchange rate revealed that the reaction occurs via a limited dissociative mechanism. The photoreaction of [Cr(OEP)(Cl)(OPPh3)] containing excess PPh3 in the bulk solution leads to the unfavorable coordination of the PPh3 molecule to the chromium ion to give a transient complex, [Cr(OEP)(Cl)(PPh3)]. The dynamic and thermodynamic properties of [Cr(OEP)(Cl)(PPh3)] in solution are discussed on the basis of the kinetic parameters of the dissociation and association reactions of the PPh3 ligand together with the steric aspect of the molecular structure of the related complexes.

Chromium(III) Octaethylporphyrinato Tetracarbonylcobaltate: A Highly Active, Selective, and Versatile Catalyst for Epoxide Carbonylation

The development of a highly active and selective porphyrin-based epoxide carbonylation catalyst, [(OEP)Cr(THF)2][Co(CO)4] (1; OEP = octaethylporphyrinato; THF = tetrahydrofuran), is detailed. Complex 1 is a separated ion pair composed of a tetracarbonylcobaltate anion and an octahedral chromium porphyrin complex axially ligated by two THF ligands. Regarding the carbonylation of epoxides to beta-lactones, catalyst 1 exhibits excellent turnover numbers (up to 10,000) and turnover frequencies (up to 1670 h(-1)), with regioselective carbonyl insertion occurring between the oxygen and the sterically less hindered carbon of the epoxide substrate. Complex 1 is highly tolerant of nonprotic functional groups, carbonylating an array of aliphatic and cycloaliphatic epoxides, as well as epoxides with pendant ethers, esters, and amides. With careful control of reaction conditions in the carbonylation of glycidyl esters, the exclusive production of either the beta- or gamma-lactone isomer was achieved. Through analysis of reaction stereochemistry, a mechanism for the formation of gamma-lactone products was proposed. Overall, a broad array of synthetically useful lactones has been synthesized in a rapid and selective fashion by catalytic carbonylation using [(OEP)Cr(THF)2][Co(CO)4].

Laser photolysis studies of the photochemical reactions of chromium(III) octaethylporphyrin and tetramesitylporphyrin complexes in toluene solution

A nanosecond laser photolysis study was carried out for the Cr(III) porphyrin complexes of 2,3,7,8,12,13,17,18-octaethylporphyrin, [Cr(OEP)(Cl)(L)], and of 5,10,15,20-tetramesitylporphyrin, [Cr(TMP)(Cl)(L)], in toluene containing water and an excess amount of L (L = axial ligand). The laser photolysis generates the triplet excited state of the parent complex as well as a five-coordinate complex, [Cr(porphyrin)(Cl)], produced by the photodissociation of the axial ligand L. The yields for the formation of the triplet state and the photodissociation of L are found to markedly depend on the nature of both L and porphyrin ligand. The five-coordinate [Cr(porphyrin)(Cl)] readily reacts with both H(2)O and L in the bulk solution to give [Cr(porphyrin)(Cl)(H(2)O)] and [Cr(porphyrin)(Cl)(L)], respectively. The axial H(2)O ligand in [Cr(porphyrin)(Cl)(H(2)O)] is then substituted by the ligand L to regenerate the original complex [Cr(porphyrin)(Cl)(L)]. In principle, the substitution reaction takes place by the dissociative mechanism: the first step is the dissociation of H(2)O from [Cr(porphyrin)(Cl)(H(2)O)], followed by the reaction of the five-coordinate [Cr(porphyrin)(Cl)] with the ligand L to regenerate [Cr(porphyrin)(Cl)(L)]. The rate constants for this ligand substitution reaction are found to exhibit bell-shaped ligand concentration dependence. The detailed kinetic analysis revealed that both ligands L and H(2)O in toluene make a hydrogen bond with the axial H(2)O ligand in [Cr(porphyrin)(Cl)(H(2)O)] to yield dead-end complexes for the substitution reaction. The reaction mechanisms are discussed on the basis of the substituent effects of the porphyrin peripheral groups and the kinetic parameters determined from the temperature dependence of the rate constants.