Substitution behaviour of an unusual seven-coordinate iron(III) complex in different solvents

Molecular Dynamics Studies of the Ho(III) Aqua-tris(dibenzoylmethane) Complex: Role of Water Dynamics

The seven-coordinate Ho(III) aqua-tris(dibenzoylmethane)(DBM) complex, referred to as Ho-(DBM)₃·H₂O, was first reported in the late 1960s. It has a threefold symmetric structure, with Ho at the center of three dibenzoylmethane ligands and hydrogen-bonded water to ligands. It is considered that the hydrogen bonds between the water molecule and the ligands surrounding Ho play an important role in the formation of its symmetrical structure. In this work, we developed new force-field parameters for classical molecular dynamics (CMD) simulations to theoretically elucidate the structure and dynamics of Ho-(DBM)₃·H₂O. To develop the force field, structural optimization and molecular dynamics were performed on the basis of ab initio calculations using the plane-wave pseudopotential method. The force-field parameters for CMD were then optimized to reproduce the data obtained from ab initio calculations. Validation of the developed force field showed good agreement with the experimental crystalline structure and ab initio data. The vibrational properties of water in Ho-(DBM)₃·H₂O were investigated by comparison with bulk liquid water. The vibrational motion of water was found to have a characteristic mode originating from stationary rotational motion along the c-axis of Ho(III) aqua-tris(dibenzoylmethane). Contrary to expectations, the hydrogen-bond dynamics of water in Ho-(DBM)₃·H₂O were found to be almost equivalent to those of bulk liquid water except for librational motion. This development route for force-field parameters for CMD and the establishment of water dynamics can advance the understanding of water-coordinated metal complexes with high coordination numbers such as Ho-(DBM)₃·H₂O.

Solvent Tuning of the Substitution Behavior of a Seven-Coordinate Iron(III) Complex

A detailed kinetic study of the substitution behavior of the seven-coordinate [Fe(dapsox)(L)2]ClO4 complex (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide), L = solvent or its deprotonated form) with thiocyanate as a function of the thiocyanate concentration, temperature, and pressure was undertaken in protic (EtOH and acidified EtOH and MeOH) and aprotic (DMSO) organic solvents. The lability and substitution mechanism depend strongly on the selected solvent (i.e., on solvolytic and protolytic processes). In the case of alcoholic solutions, substitution of both solvent molecules by thiocyanate could be observed, whereas in DMSO only one substitution step occurred. For both substitution steps, [Fe(dapsox)(L)2]ClO4 shows similar mechanistic behavior in methanol and ethanol, which is best reflected by the values of the activation volumes (MeOH DeltaV(I) = +15.0 +/- 0.3 cm(3) mol(-1), DeltaV(II) = +12.0 +/- 0.2 cm(3) mol(-1); EtOH DeltaV(I) = +15.8 +/- 0.7 cm(3) mol(-1), DeltaV(II) = +11.1 +/- 0.5 cm(3) mol(-1)). On the basis of the reported activation parameters, a dissociative (D) mechanism for the first substitution step and a D or dissociative interchange (I(d)) mechanism for the second substitution step are suggested for the reaction in MeOH and EtOH. This is consistent with the predominant existence of alcoxo [Fe(dapsox)(ROH)(OR)] species in alcoholic solutions. In comparison, the activation parameters for the substitution of the aqua-hydroxo [Fe(dapsox)(H2O)(OH)] complex by thiocyanate at pH 5.1 in MES were determined to be DeltaH = 72 +/- 3 kJ mol(-1), DeltaS = +38 +/- 11 J K(-1) mol(-1), and DeltaV = -3.0 +/- 0.1 cm(3) mol(-1), and the operation of a dissociative interchange mechanism was suggested, taking the effect of pressure on the employed buffer into account. The addition of triflic acid to the alcoholic solutions ([HOTf] = 10(-3) and 10(-2) M to MeOH and EtOH, respectively) resulted in a drastic changeover in mechanism for the first substitution step, for which an associative interchange (Ia) mechanism is suggested, on the basis of the activation parameters obtained for both the forward and reverse reactions and the corresponding volume profile. The second substitution step remained to proceed through an I(d) or D mechanism (acidified MeOH DeltaV(II) = +9.2 +/- 0.2 cm(3) mol(-1); acidified EtOH DeltaV(II) = +10.2 +/- 0.2 cm(3) mol(-1)). The first substitution reaction in DMSO was found to be slowed by several orders of magnitude and to follow an associative interchange mechanism (DeltaS = -50 +/- 9 J K(-1) mol(-1), DeltaV(I) = -1.0 +/- 0.5 cm(3) mol(-1)), making DMSO a suitable solvent for monitoring substitution processes that are extremely fast in aqueous solution.