Electrocatalytic proton reduction by phosphido-bridged diiron carbonyl compounds: Distant relations to the H-cluster?
Electrocatalytic proton reduction by phosphido-bridged diiron carbonyl compounds: Distant relations to the H-cluster?
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DOI：
10.1021/ic049746e
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发表时间：
2004-09-06
影响因子：
4.6
通讯作者：
Best, SP
中科院分区：
文献类型：
--
作者：
Cheah, MH;Borg, SJ;Best, SP
通过光谱电化学、连续流动和间断流动技术，已确定了在质子存在下Fe - 2(μ - PPh₂)₂(CO)₆（1）还原过程中形成的中间体。这些观察结果所提出的电催化质子还原机制对伏安法进行了数字模拟，与在一系列酸浓度下于四氢呋喃中进行的测量结果高度吻合。电催化质子还原机制涉及首先形成双阴离子1²⁻，它在进一步还原和释放氢气之前被两次质子化。1²⁻的单次和双次质子化形式的红外光谱表明其结构分别对应于[FeH(CO)₃(μ - PPh₂)₂Fe(CO)₃]⁻（1H⁻）和FeH(CO)₃(μ - PPh₂)₂FeH(CO)₃（1H₂）。1的巯基和二巯基类似物在双电子还原步骤中表现出电催化质子还原，这意味着相应的双电子还原双质子化物种相对于氢气释放是不稳定的。1H₂的稳定性很可能是由于扁平的{2Fe2P}核中铁中心之间的弱相互作用。1H₂在没有还原电位时是稳定的，而1H⁻会迅速重排为先前描述为[Fe - 2(μ - PPh₂)(μ - CO)(PHPh₂)(CO)₅]⁻（1H⁻w）的产物。1²⁻的另一种质子化产物，先前被表述为[Fe - 2(μ - PPh₂)₂(μ - CO)H(CO)₅]⁻，根据一系列光谱测量结果已重新表述为[Fe - 2(μ - PPh₂)(μ - CO)(CO)₆]⁻（2）。报道了1、1²⁻、1H(w)⁻和2的溶液EXAFS测量结果，这些结果得出与模型无关的Fe - Fe距离分别为2.61（1）、3.58（1²⁻）、2.58（1H(w)⁻）和2.59埃（2）。1H(w)⁻和2都存在Fe - Fe键是所提出结构的一个关键方面，这有力地支持了基于光谱证据的推断。溶液EXAFS对不同结构模型的拟合给出的统计数据与所提出的结构相符。
Intermediates formed during reduction of Fe-2(mu-PPh2)(2)(CO)(6) (1) in the presence of protons have been identified by spectroelectrochemical, continuous-flow, and interrupted-flow techniques. The mechanism for electrocatalytic proton reduction suggested by these observations yields digital simulation of the voltammetry in close agreement with measurements conducted in THF over a range of acid concentrations. The mechanism for electrocatalytic proton reduction involves initial formation of the dianion, 1(2-), which is doubly protonated prior to further reduction and dihydrogen elimination. The IR spectra of the singly and doubly protonated forms of 12- indicate structures corresponding to [FeH(CO)(3)(mu-PPh2)(2)Fe(CO)(3)](-) (1H(-)) and FeH(CO)(3)(mu-PPh2)(2)FeH(CO)(3) (1H(2)), The thiolato and dithiolato analogues of 1 exhibit electrocatalytic proton reduction associated with the two-electron reduction step, and this implies that the corresponding two-electron reduced doubly protonated species is unstable with respect to dihydrogen elimination. The stability of 1H(2) is most likely to be due to the weak interactions between the iron centers of the flattened {2Fe2P} core. Whereas 1H(2) is stable in the absence of a reducing potential, 1H(-) rearranges rapidly to a product previously described as [Fe-2(mu-PPh2)(mu-CO)(PHPh2)(CO)(5)](-) (1H(-)w). Another protonation product of 1(2-), previously formulated as [Fe-2(mu-PPh2)(2)(mu-CO)H(CO)(5)](-), has been reformulated as [Fe-2(mu-PPh2)(mu-CO)(CO)(6)](-) (2) on the basis of a range of spectroscopic measurements. Solution EXAFS measurements of 1, 1(2-), 1H(w)(-), and 2 are reported, and these yield model-independent Fe-Fe distances of 2.61 (1), 3.58 (1(2-)), 2.58 (1H(w)(-)), and 2.59 Angstrom (2). The presence of an Fe-Fe bond for both 1H(w)(-) and 2 is a key aspect of the proposed structures, and this strongly supports the deductions based on spectroscopic evidence. The fits of the solution EXAFS to different structural models give statistics in agreement with the proposed structures.