The syntheses and single crystal X-ray structural analysis of five novel hetero-and homometallic µ 3 -oxo trinuclear cluster with the formula [Fe 2) and Co (4.7; 55.37; +11.2; +10.9). A limiting D mechanism is assigned to these exchange reactions. This assignment is based on a first-order rate law, the detection of intermediates, the positive and large entropies and volumes of activation. The order of reactivity k Co > k Ni is expected for a D mechanism at these metal centers: their low exchange rates are due to their strong binding with the 4-Rpy donor. Surrounded by oxygen donors the d 5 iron(III) usually reacts associatively; however, here due to low affinity of this ion for nitrogen the mechanism is D and the rate of exchange is very fast, even faster than on the divalent ions. There is no significant effect of the divalent ion in cluster 2, 3, and 5 on the exchange rates of 4-Phpy at the iron center, which seems to indicate that the specific electronic interactions between the three ions making the clusters do not influence the Fe III -N bond strength.

The isomerization dynamics of five labile octahedral Co(II) compounds have been investigated by variable temperature and pressure (1)H and (19)F NMR spectroscopy in dichloromethane solution. The X-ray crystal structure of the two tris-chelates, [Co(HFA)(2)bpic] (1) and [Co(TTFA)(2)bpy] (2), show a distorted octahedral arrangement of the 4 oxygen and 2 nitrogen donor atoms, with bidentate ligand bite angles smaller than 90 degrees. On the other hand, in the three bis-chelates, trans(N)-[Co(HFA)(2)(CH(3)py)(2)](3), cis(N)-cis(CF(3))-trans(S)-[Co(TTFA)(2)(CH(3)py)(2)](4), and trans(N)-trans(CF(3))-[Co(TTFA)(2)(CF(3)py)(2)](5), the replacement of the bidentate nitrogen donor ligands by two monodentate Rpy ligands leads to relaxed structures with almost regular octahedral arrangements of the donor atoms (HFA = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato anion; TTFA = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato anion; bpy = 2,2'-bipyridine; bpic = 4,4'-dimethyl-2,2'-bipyridine). In solution the five complexes are stereolabile and all possible isomers are formed: from one for 1 up to five for 4 and 5. All cis-N isomers form pairs of enantiomers, whereas the trans-N isomers are achiral. A solid state structure/isomerization mechanism/rate correlation has been established for the isomerization dynamics of these Co(II) tris- and bis-chelates. The two tris-chelate complexes 1 and 2, with a distorted octahedral solid state structure, show one and three isomers in solution and isomerize/tautomerize very rapidly according to Bailar twist mechanisms. The three bis-chelate complexes 3, 4, and 5, with a close to octahedral symmetry in the solid state, show two, five, and five isomers, respectively. They isomerize/tautomerize 3 orders of magnitude slower as the tris-chelates, by an intramolecular dissociative mechanism involving a ring-opening of an arm of a bidentate ligand to form a TBP intermediate with a dangling bidentate ligand. The results of this first systematic investigation of the isomerization mechanisms of highly labile Co(II) complexes are supported by the NMR observed exchange paths (up to five for complexes for 4 and 5), the variable temperature (185 to 312 K) and pressure (up to 200 MPa) activation parameters, and a detailed analysis of the solid state structures.

The cobalt(II) complexes [CoL2(R2-Py)2] (1-4) where HLA = 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione, R2-Py = 4-methylpyridine (1), HLB = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione, R2-Py = 4-methylpyridine (2), 4-phenylpyridine (3) and S-(-)-1-(4-pyridyl)ethanol (4) were prepared by two-step reactions. X-ray structure analysis of [CoLA2(CH3-Py)2] revealed the {trans(N)-trans(CF3)-trans} configuration for the complex obtained by crystallization from ethanol. A dynamic equilibrium between the five possible stereoisomers was observed for each complex 1-4 in solution by 19F NMR spectroscopy. The criteria used for full NMR assignment (180-265 K) include comparison of integral ratios, cis(N) and trans(N) differentiation in presence of the chiral amine [S-(-)-1-(4-pyridyl)ethanol], effect of solvent polarity on the relative stabilities of the five isomers and observation of trans influences in a mixture of complexes. Thermodynamic parameters for the equilibria between the isomers of 2 in CD2Cl2 (DeltaHi,j, DeltaSi,j and Ki,j) were obtained from signal integrals. The two trans(N) isomers are slightly more stable than the three cis(N) isomers at low temperature [DeltaGdegreesi,j (max) = 2.8 kJ mol(-1) at 179.8 K], but this stability difference almost vanishes with increasing temperature [DeltaGdegreesi,j (max) = 1.0 kJ mol(-1) at 265.0 K]. The values found for DeltaHdegreesi,j are relatively small and largely entropy compensated.

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The labile cobalt (II) complex [Co(TUF)2(Pic)2] with 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (TUF) bidentate and 4-methylpyridine (Pic) monodentate ligands was studied by variable temperature (VT) 19F NMR from 184 to 312 K. In contrast to the solid state, all the isomers are present in a CD2Cl2 solution. Thermodynamic parameters (?H° and ?S°) of each isomer with reference to the trans(N)-trans(CF3)-trans isomer were obtained from the fitting of experimental mole fraction values. A kinetics investigation of the isomerization processes was performed by lineshape analysis of the spectra using the Kubo-Sack formalism. The suggested bidentate bond-rupture mechanism involves the breaking of the weakest Co–O bond (closest to the CF3 withdrawing group) with the TUF ligand.