Separation of Isomers and Mechanisms of Inversion of Stereochemistry of Group 9 d⁶ Tris-Chelate Complexes of Hinokitiol

Abstract

Tris-chelate complexes of Co­(III), Rh­(III), and Ir­(III) with 4-isopropyltropolone (hinokitiol or β-thujaplicin) form by the substitution of carbonate and chloride ligands from group 9 trivalent metal salts. The new complexes are neutral, are readily soluble in most organic solvents, and are brightly colored with strong charge transfer bands. The fac isomers of Co­(hino)3 and Rh­(hino)3 were isolated from the mixture by fractional recrystallization from ethanol. The remaining mixtures were respectively enriched by 5:3 and 4.4:3 for the mer isomer. The 1H NMR data show that the complexes exhibit remarkable stereochemical lability, which is unusual for diamagnetic d6 group 9 metals, with rotational barriers of 14.2 and 18.2 kcal/mol found for the inversion of stereochemistry of Co­(hino)3 and Rh­(hino)3. The low activation barriers, as well as the analysis of some key structural parameters, suggest that the inversion of stereochemistry occurs via a trigonal-twist (Bailar) mechanism. Facile substitution of a single hinokitiol ligand in the cobalt complex with ethylenediamine to form [Co­(en)­(hino)2]Cl also indicates that the tris-chelates are substitutionally and configurationally labile.

Three-Center M–H–B Bonds Are Strong Field Interactions. Synthesis and Characterization of M(CH2NMe2BH3)3 Complexes of Titanium, Chromium, and Cobalt

“…Octahedral cobalt­(III) compounds almost always have large activation free energies for the Δ/Λ racemization process; in fact, most are stereochemically rigid on the NMR time scale (and even on the chemical time scale). – To our knowledge, the only exceptions to this generalization are cobalt tris­(tropolonate) and tris­(dithiocarbamate) complexes, in which the ligands potentially are electronically non-innocent; these complexes have activation free energies Δ G ⧧ of between 14 and 15 kcal mol –1 at either 5 or 25 °C. ,, As shown from the variable temperature NMR studies above, BDAM complex 4 has by far the smallest Δ/Λ racemization barrier (Δ G ⧧ = 10.1 kcal mol –1 at 25 °C) seen for any octahedral cobalt­(III) complex.…”

Coordination Chemistry of the Parent Dithiocarbamate H2NCS2–: Organometallic Chemistry and Tris-Chelates of Group 9 Metals

“…The activation barrier for the inversion of stereochemistry of the complexes increases on going down the column: this is the expected trend that we have also observed in our study of tris-chelate complexes of substituted tropolones. 14 To exclude DMSO solvation reactions as the origin of these differences, the variable-temperature behavior of the diethyl derivative of rhodium was investigated in DMSO and benzene.…”

“…The resulting barriers are respectively 16.4 ± 0.3 and 17.1 ± 0.3 kcal/mol. The activation barrier for the inversion of stereochemistry of the complexes increases on going down the column: this is the expected trend that we have also observed in our study of tris-chelate complexes of substituted tropolones …”

“…In our study of tris-chelate complexes of hinokitiol, 14 we have measured low barriers for the inversion of stereochemistry even for the complex Ir(hino) 3 . This is important in the context of determining PVED, since this means the molecules are not inert toward racemization.…”

“…Recently we studied the tris-chelate complexes of a tropolone derivative, 4-isopropyltropolone, as potential candidates for PVED measurements . Using dynamic NMR studies as well as the determination of key structural parameters, we have concluded that the complexes have remarkable low-temperature fluxional behavior even with the heavier elements rhodium and iridium.…”