Mixed-Valent Dicobalt and Iron–Cobalt Complexes with High-Spin Configurations and Short Metal–Metal Bonds

Cobalt–cobalt
and iron–cobalt bonds are investigated in coordination complexes
with formally mixed-valent [M2]3+ cores. The
trigonal dicobalt tris­(diphenylformamidinate) compound, Co2(DPhF)3, which was previously reported by Cotton, Murillo,
and co-workers (Inorg. Chim. Acta 1996, 249, 9), is shown to have an energetically isolated,
high-spin sextet ground-state by magnetic susceptibility and electron
paramagnetic resonance (EPR) spectroscopy. A new tris­(amidinato)­amine
ligand platform is introduced. By tethering three amidinate donors
to an apical amine, this platform offers two distinct metal-binding
sites. Using the phenyl-substituted variant (abbreviated as LPh), the isolation of a dicobalt homobimetallic and an iron–cobalt
heterobimetallic are demonstrated. The new [Co2]3+ and [FeCo]3+ cores have high-spin sextet and septet ground
states, respectively. Their solid-state structures reveal short metal–metal
bond distances of 2.29 Å for Co–Co and 2.18 Å for
Fe–Co; the latter is the shortest distance for an iron–cobalt
bond to date. To assign the positions of iron and cobalt atoms as
well as to determine if Fe/Co mixing is occurring, X-ray anomalous
scattering experiments were performed, spanning the Fe and Co absorption
energies. These studies show only a minor amount of metal-site mixing
in this complex, and that FeCoLPh is more precisely described
as (Fe0.94(1)Co0.06(1))­(Co0.95(1)Fe0.05(1))­LPh. The iron–cobalt heterobimetallic
has been further characterized by Mössbauer spectroscopy. Its
isomer shift of 0.65 mm/s and quadrupole splitting of 0.64 mm/s are
comparable to the related diiron complex, Fe2(DPhF)3. On the basis of spectroscopic data and theoretical calculations,
it is proposed that the formal [M2]3+ cores
are fully delocalized.

本研究针对形式上具有混合价态[M₂]³+核的配位配合物中的钴-钴键与铁-钴键展开探究。此前由Cotton、Murillo及其合作者报道的三(二苯甲脒合)二钴三角配合物Co₂(DPhF)₃（Inorg. Chim. Acta 1996, 249, 9），经磁化率与电子顺磁共振（electron paramagnetic resonance, EPR）波谱表征，被证实存在能量孤立的高自旋六重态基态。本研究引入了一种全新的三(脒基)胺配体平台：通过将三个脒基供体连接至顶端胺基，该配体平台可提供两个独立的金属结合位点。利用苯基取代的衍生物（缩写为LPh），我们成功分离得到一种二钴均双金属配合物与一种铁-钴异双金属配合物。新型的[Co₂]³+与[FeCo]³+核分别具有高自旋六重态与七重态基态。其固态结构表征显示，金属-金属键长极短：Co-Co键长为2.29 Å，Fe-Co键长为2.18 Å；其中后者为目前已报道的最短铁-钴键长。为确定铁与钴原子的配位位点，并判断是否存在Fe/Co位点混杂现象，我们开展了覆盖铁与钴吸收能的X射线反常散射（X-ray anomalous scattering）实验。结果表明该配合物仅存在微量的金属位点混杂，FeCoLPh的精确结构应描述为(Fe₀.₉₄(1)Co₀.₀₆(1))(Co₀.₉₅(1)Fe₀.₀₅(1))LPh。该铁-钴异双金属配合物进一步通过穆斯堡尔谱学（Mössbauer spectroscopy）进行了表征：其同质异能移位为0.65 mm/s，四极裂分为0.64 mm/s，与相关的二铁配合物Fe₂(DPhF)₃较为相近。基于光谱数据与理论计算，我们提出形式上的[M₂]³+核为完全离域的。