Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes

The
transition between spin states in d-block metal complexes has
important ramifications for their structure and reactivity, with applications
ranging from information storage materials to understanding catalytic
activity of metalloenzymes. Tuning the ligand field (ΔO) by steric and/or electronic effects has provided spin-crossover
compounds for several transition metals in the periodic table, but
this has mostly been limited to coordinatively saturated metal centers
in octahedral ligand environments. Spin-crossover complexes with low
coordination numbers are much rarer. Here we report a series of four-coordinate,
(pseudo)­tetrahedral Fe­(II) complexes with formazanate ligands and
demonstrate how electronic substituent effects can be used to modulate
the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo
spin-crossover in solution with T1/2 above
room temperature (300–368 K). While structural analysis by
X-ray crystallography shows that the majority of these compounds are
low-spin in the solid state (and remain unchanged upon heating), we
find that packing effects can override this preference and give rise
to either rigorously high-spin (6) or gradual spin-crossover
behavior (5) also in the solid state. Density functional
theory calculations are used to delineate the empirical trends in
solution spin-crossover thermodynamics. In all cases, the stabilization
of the low-spin state is due to the π-acceptor properties of
the formazanate ligand, resulting in an “inverted” ligand
field, with an approximate “two-over-three” splitting
of the d-orbitals and a high degree of metal–ligand covalency
due to metal → ligand π-backdonation. The computational
data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is
present at the C–Ar or N–Ar rings, which is ascribed
to the opposing effect on metal–ligand σ- and π-bonding.

d区金属配合物的自旋态转变对其结构与反应性具有重要影响，其应用场景涵盖信息存储材料开发，乃至金属酶催化活性的机理研究。通过空间效应与/或电子效应调控配体场（ligand field），已为周期表中的多种过渡金属制备出自旋交叉（spin-crossover）化合物，但此类研究大多局限于八面体配体环境中的配位饱和金属中心。低配位数的自旋交叉配合物则极为罕见。本研究报道了一系列由甲臜配体（formazanate ligand）配位的四配位（准）四面体铁(II)配合物，并阐明了取代基电子效应可用于调控溶液中S=0与S=2自旋态间的热诱导转变。该六种配合物在溶液中均发生自旋交叉，其转变半温度（T1/2）高于室温（300~368 K）。尽管X射线晶体学结构分析显示，多数此类配合物在固态下为低自旋态（且加热时无变化），但本研究发现堆积效应可突破这一偏好，使固态配合物呈现严格高自旋（化合物6）或逐步自旋交叉行为（化合物5）。本研究通过密度泛函理论（density functional theory）计算，阐明了溶液中自旋交叉热力学的经验性变化趋势。在所有体系中，低自旋态的稳定化源于甲臜配体的π受体特性，由此形成“反转”配体场：d轨道分裂近似为“二比三”，且因金属→配体π反馈键作用，金属-配体间具有高度共价性。计算数据表明，对位取代基的电子特性对体系的影响取决于其连接在C-芳环还是N-芳环上，这一现象可归因于其对金属-配体σ键与π键作用的相反影响。