Department of Physical Chemistry, Faculty of Chemistry and Pharmacy, University of Sofia, 1 James Bourchier Avenue, 1164 Sofia, Bulgaria.
J Phys Chem A. 2013 Jan 24;117(3):670-8. doi: 10.1021/jp312258j. Epub 2013 Jan 14.
The magnetic properties of a series of manganese(II) diacetylacetonate and dihexafluoroacetylacetonate hybrid-spin complexes with neutral pyridine-based organic radicals were characterized theoretically by DFT calculations. Three stable radicals, in which a radical group is bound in either para or meta position with respect to the pyridine nitrogen atom, were considered. The correct stable structures and multiplets of the complexes were obtained by full geometry optimization starting from an ideal structure. A total of three important geometry descriptors of the complexes were monitored and related to their magnetic characteristics. These structural parameters are (i) the torsion angle governing the conjugation of the organic radical m-PyNO (anti versus gauche), (ii) the coordination geometry of the acetyl acetonate ligands around the metal ion (square versus rhombic), and (iii) the relative orientation of the organic radical with respect to the acetyl acetonate plane (parallel versus perpendicular). It was found that the magnetic properties are not sensitive to the orientation of the radicals with respect to the equatorial plane but do depend on the conformation of the organic radicals. Even a spin switch between the ferromagnetic (S = (7)/(2)) and antiferromagnetic (S = (3)/(2)) ground state was found to be feasible for one of the complexes upon variation of the organic radical geometry, namely, the dihedral angle between the organic radical moiety and the pyridine ring. The pattern of molecular orbital overlap was determined to be the key factor governing the exchange coupling in the modeled systems. Bonding π-type overlap provides antiferromagnetic coupling in all complexes of the para radicals. In the meta analogues, the spins are coupled through the σ orbitals. A low-spin ground state occurs whenever a continuous σ-overlap pathway is present in the complex. Ferromagnetic interaction requires σ-π orthogonality of the pyridine atomic orbitals and/or π-antibonding Mn-pyridine natural orbital overlap. Using an estimate of the donor-acceptor energy stabilization, the affinity of a given Mn(II) d-orbital to mix with the sp(2) orbital from pyridine can be predicted.
一系列中性基于吡啶的有机自由基的二乙酰丙酮合锰(II)和二六氟乙酰丙酮合锰(II)混合自旋配合物的磁性质通过 DFT 计算进行了理论表征。考虑了三种稳定的自由基，其中自由基基团在吡啶氮原子的对位或间位结合。从理想结构开始，通过全几何优化获得了配合物的正确稳定结构和多重态。共监测了配合物的三个重要结构参数，并将其与磁特性相关联。这些结构参数是：（i）控制有机自由基 m-PyNO（反式与 gauche 式）共轭的扭转角，（ii）金属离子周围乙酰丙酮配体的配位几何（正方形与菱形），以及（iii）有机自由基相对于乙酰丙酮平面的相对取向（平行与垂直）。研究发现，磁性对自由基相对于赤道面的取向不敏感，但与有机自由基的构象有关。甚至发现，对于其中一个配合物，通过改变有机自由基的几何形状，即有机自由基部分与吡啶环之间的二面角，自旋可以在铁磁（S = (7)/(2)）和反铁磁（S = (3)/(2)）基态之间切换。确定分子轨道重叠模式是控制模型体系中交换耦合的关键因素。在所有对位自由基配合物中，π 型键合重叠提供反铁磁耦合。在间位类似物中，自旋通过 σ 轨道耦合。只要在配合物中存在连续的 σ 重叠途径，就会出现低自旋基态。铁磁相互作用需要吡啶原子轨道的 σ-π 正交性和/或π-反键 Mn-吡啶自然轨道重叠。通过估计给体-受体能量稳定化，可以预测给定 Mn(II)d 轨道与吡啶的 sp(2)轨道混合的亲和力。
