Fataftah Majed S, Mercado Brandon Q, Holland Patrick L
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, CT-06511, USA.
Chemistry. 2023 Nov 13;29(63):e202301962. doi: 10.1002/chem.202301962. Epub 2023 Oct 4.
The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe (μ-CCO) ] complex with two carbon bridges (4) is S= , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant. In contrast, one-electron reduction of the [Fe (μ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S= ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.
固氮酶中铁钼辅因子（FeMoco）中的碳化物配体连接着不同氧化态的铁原子，然而，由于该辅因子结构复杂，很难识别其介导磁交换相互作用的能力。在此，我们描述了两种具有碳基烯酮亚基桥连配体的混合价态二铁配合物，并将碳桥与更为常见的硫桥进行比较。具有两个碳桥的[Fe(μ-CCO)]配合物（4）的基态为S = ，在穆斯堡尔时间尺度上其价态是离域的，电子跳跃的热垒较小，这源于较低的Fe-C力常数。相比之下，具有一个碳桥的[Fe(μ-CCO)]配合物（2）单电子还原后得到一个具有高自旋基态（S = ）的混合价态物种，且Fe-Fe距离收缩了1 Å。对后者的光谱、磁性和计算研究表明，Fe-Fe键合相互作用导致了完全的价态离域。对5中近红外价间电荷转移跃迁的分析表明，双交换常数（B）非常大，在780 - 965 cm范围内。这些结果表明，碳桥在稳定混合价态铁二聚体的价态离域基态方面极为有效。
