Enticing cobalt into planarity: can a pair of diolato ligands make it happen?
Enticing cobalt into planarity: can a pair of diolato ligands make it happen?
复制标题
吸引钴平面化：一对二醇配体能否实现这一目标？
作者：
X. Wurzenberger;C. Neumann;P. Klüfers
最近发现了分子四配位高自旋 (S = 2) 铁 (II) 中心，其具有或多或少的方形平面 FeO4 发色团。四个氧供体原子由两个二醇螯合物配体提供。对于高自旋 d Fe 中心，我们将四面体配位环境向方形平面配位环境的扁平化解释为由四面体的 Jahn-Teller (JT) 不稳定性驱动。具体来说，氧供体原子在 xy 平面中的空间分布和 z 轨道中高自旋 d 配置的唯一 b 自旋电子应该导致负电荷的有利分离。这种电荷分离有助于补偿方形平面基序内增加的配体间排斥力。因此值得注意的是，使用全氟频哪醇螯合剂（(FpinH 2) 2 ，Fpin = 全氟频哪醇；为了与我们小组之前的工作保持一致，我们使用这个缩写；在参考文献[1b]中，使用缩写 ddfp 代替 (FpinH 2) 2 )，这不仅仅是平面高自旋双二醇铁 (II) 的第二个例子最近还报道了一种高自旋 d 钴 (II) 类似物。由于四面体高自旋钴 (II) 配合物缺乏 JT 不稳定性，因此与铁 (II) 情况相比，四面体到方形平面的转变应该不太有利。然而，据报道却相反。一方面，根据我们的结果，通过 DFT 方法计算了铁 (II) 配合物从四面体到平面的过渡的平滑、平坦的曲线（图 1，曲线标签 Fe）。另一方面，对于四面体构象大约 40 kJmol 1 不太稳定的钴物种，发现了对平面结构的明显偏好（图 1，曲线标签 Coexc）。参考文献 1 的作者得出的结论是，平面配合物的形成仅由配体的特性驱动，而不是由中心金属的特定电子构型驱动。 [1b]引发了我们希望在此解决的争议。我们的出发点是 Ref. 最令人惊讶的结果。 [1b]，即DFT计算发现的四面体高自旋d钴(II)构象异构体的显着不稳定性。因此，我们从计算部分开始了研究。为此，参考文献的作者使用了相同的程序。 [1b]（BP/TZ2P，默认设置的 ADF 程序包）用于计算从平面到四面体形式的钴(II)络合物的松弛[势能]表面扫描。因此，参考文献的支持信息中描述的平面形式具有相同的轨道布居。得到[1b]。此外，向四面体的扫描（图 1）会产生与参考文献 图 4 所示相同的 40 kJ mol 1 上升。 [1b]。然而，在扫描结束时，四面体物种的 b 自旋构型是 (x y,z)(xy,xz,yz) 类型中不太稳定的构型之一。 x z 和 yz d 轨道被 b 自旋占据（自始至终都使用图 2 中定义的四面体物质的标准笛卡尔轴）。因此，获得了电子激发构型，特别是在扫描结束时。通过从四面体末端开始扫描，获得了整个扫描曲线中更稳定的物质（图 1，曲线标签“Cogs”）。就图 2 中的轴而言，最小值类似于两个 b 自旋的 (xz)(y) 配置，其中 y 轨道携带一些 s 混合物，以将自旋集中在 xz 平面上。继续走向平面构象，交叉点相交，电子构型切换到源自 b 自旋的 (x y)(z) 构型的状态。结果，通过考虑钴中心的电子基态构型，获得了平坦的扫描曲线，该曲线没有显示出 Coexc 曲线中发现的四面体构象的意外不稳定性。此外，预期有所不同。 图 1. 双(全氟频哪醇)金属盐 (II) 物质在从平面 (SP-4) 到四面体 (T4) 构象的转变路径上的相对能量。详情请见正文。横坐标：二面角M(Odiol 1)2/M(Odiol 2)2（d见参考文献[1a]，V见参考文献[1b]；08指矩形扭曲平面构象，908指扭曲四面体构象）；纵坐标：相对能量；对于最稳定的 Fe 或 Co 物质，DE = 0。
Molecular four-coordinate high-spin (S = 2) iron(II) centers with a more-or-less square-planar FeO4 chromophore have recently been discovered. The four oxygen donor atoms were provided by two diolato chelate ligands. For the highspin d Fe centers, we interpreted the flattening of a tetrahedral coordination environment towards a square-planar one as being driven by the tetrahedron s Jahn–Teller (JT) instability. Specifically, the spatial distribution of the oxygen donor atoms in, say, the xy plane and the only b-spin electron of the high-spin d configuration in the z orbital should result in a favorable separation of the negative charge. This charge separation helps to compensate the increased interligand repulsion within the square-planar motif. It was therefore remarkable that, with the perfluoropinacolato chelator ((FpinH 2) 2 , Fpin = perfluoropinacol; for the sake of consistency with a previous work by our group, we use this abbreviation; in Ref. [1b], the abbreviation ddfp is used instead of (FpinH 2) 2 ), not only a second example of a planar high-spin bisdiolato iron(II) but also a high-spin d cobalt(II) analogue was recently reported. Since a tetrahedral high-spin cobalt(II) complex is devoid of JT instability, the tetrahedral-to-square-planar transformation should be less favorable compared to the iron(II) case. The opposite, however, was reported. On the one hand, in line with our result, a smooth, flat curve was calculated for the iron(II) complex s tetrahedral-to-planar transition by a DFTapproach (Figure 1, curve label Fe). On the other hand, a clear preference for the planar structure was found for the cobalt species for which the tetrahedral conformation is approximately 40 kJmol 1 less stable (Figure 1, curve label Coexc). With their conclusion that the formation of a planar complex would be driven by the ligand s characteristics only, and not the specific electron configuration of the central metal, the authors of Ref. [1b] initiated a controversy that we wish to address herein. Our starting point was the most surprising result of Ref. [1b], namely the marked instability of the tetrahedral high-spin d cobalt(II) conformer found by the DFT calculations. We thus started our investigation with the computational part. For this, the same procedure as used by the authors of Ref. [1b] (BP/TZ2P, ADF program package in its default settings) was applied to calculate a relaxed [potential energy] surface scan from the planar to the tetrahedral form of the cobalt(II) complex. As a result, the same orbital population for the planar form depicted in the Supporting Information of Ref. [1b] was obtained. Moreover, the scan towards the tetrahedron (Figure 1) results in the same 40 kJ mol 1 ascent depicted in Figure 4 of Ref. [1b]. At the end of the scan, however, the b-spin configuration of the tetrahedral species was one of the less stable configurations of the (x y,z)(xy,xz,yz) type. The x z and yz d orbitals were occupied by the b spins (standard Cartesian axes for tetrahedral species as defined in Figure 2 are used throughout). Hence, an electronically excited configuration was obtained, particularly at the scan s end. More-stable species throughout the entire scan curve were obtained by starting the scan at the tetrahedral end (Figure 1, curve label “Cogs”). The minimum resembled, in terms of the axes in Figure 2, an (xz)(y) configuration of the two b spins, with the y orbital carrying some s admixture to concentrate the spin in the xz plane. Proceeding towards the planar conformation, an intersection was crossed where the electronic configuration switched to a state that was derived from the (x y)(z) configuration for the b spins. As a result, by allowing for the electronic ground-state configuration at the cobalt center, a flat scan curve was obtained that does not show the unexpected instability of the tetrahedral conformation found in the Coexc curve. Moreover, the expected differFigure 1. Relative energies of bis(perfluoropinacolato)metallate(II) species on the transition path from the planar (SP-4) to the tetrahedral (T4) conformation. For details see the text. Abscissa: the dihedral angle M(Odiol 1)2/M(Odiol 2)2 (d as in Ref. [1a], V in Ref. [1b]; 08 refers to a rectangularly distorted planar conformation, 908 refers to a distorted tetrahedral conformation); ordinate: relative energy; DE = 0 for the most stable Fe or Co species.