Ivanović-Burmazović Ivana, Hamza Mohamed S A, van Eldik Rudi
Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Germany.
Inorg Chem. 2006 Feb 20;45(4):1575-84. doi: 10.1021/ic0514694.
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.
对七配位的[Fe(dapsox)(L)₂]ClO₄配合物（H₂dapsox = 2,6 - 二乙酰基吡啶 - 双（半草酰胺），L = 溶剂或其去质子化形式）与硫氰酸盐的取代行为进行了详细的动力学研究，研究了硫氰酸盐浓度、温度和压力对其的影响，研究在质子性（乙醇、酸化乙醇和甲醇）和非质子性（二甲基亚砜）有机溶剂中进行。配体的活性和取代机理强烈依赖于所选溶剂（即依赖于溶剂解和质子解过程）。在醇溶液中，可以观察到两个溶剂分子都被硫氰酸盐取代，而在二甲基亚砜中只发生一步取代。对于这两个取代步骤，[Fe(dapsox)(L)₂]ClO₄在甲醇和乙醇中表现出相似的机理行为，这最能通过活化体积的值体现出来（甲醇：ΔV(I) = +15.0 ± 0.3 cm³ mol⁻¹，ΔV(II) = +12.0 ± 0.2 cm³ mol⁻¹；乙醇：ΔV(I) = +15.8 ± 0.7 cm³ mol⁻¹，ΔV(II) = +11.1 ± 0.5 cm³ mol⁻¹）。根据所报道的活化参数，对于甲醇和乙醇中的反应，建议第一步取代反应采用离解（D）机理，第二步取代反应采用D或离解交换（I(d)）机理。这与醇溶液中主要存在的醇氧基[Fe(dapsox)(ROH)(OR)]物种一致。相比之下，在MES中pH为5.1时，硫氰酸盐取代水 - 羟基[Fe(dapsox)(H₂O)(OH)]配合物的活化参数测定为：ΔH = 72 ± 3 kJ mol⁻¹，ΔS = +38 ± 11 J K⁻¹ mol⁻¹，ΔV = -3.0 ± 0.1 cm³ mol⁻¹，并考虑到压力对所用缓冲液的影响，建议采用离解交换机理。向醇溶液中加入三氟甲磺酸（分别向甲醇和乙醇中加入[HOTf] = 10⁻³和10⁻² M）导致第一步取代反应的机理发生剧烈转变，基于正逆反应获得的活化参数和相应的体积曲线，建议采用缔合交换（Ia）机理。第二步取代反应仍通过I(d)或D机理进行（酸化甲醇：ΔV(II) = +9.2 ± 0.2 cm³ mol⁻¹；酸化乙醇：ΔV(II) = +10.2 ± 0.2 cm³ mol⁻¹）。发现二甲基亚砜中的第一步取代反应减慢了几个数量级，并遵循缔合交换机理（ΔS = -50 ± 9 J K⁻¹ mol⁻¹，ΔV(I) = -
