Electronic Structure and Reactivity of a Well-Defined Mononuclear Complex of Ti(II)

A facile and high-yielding protocol
to the known Ti­(II) complex trans-[(py)4­TiCl2] (py = pyridine) has been developed. Its electronic
structure has been probed experimentally using magnetic susceptibility,
magnetic circular dichroism, and high-frequency and high-field electron
paramagnetic resonance spectroscopies in conjunction with ligand-field
theory and computational methods (density functional theory and ab initio methods). These studies demonstrated that trans-[(py)4­TiCl2] has a 3Eg ground state (dxy1dxz,yz1 orbital occupancy), which, as a result of spin–orbit
coupling, yields a ground-state spinor doublet that is EPR active,
a first excited-state doublet at ∼60 cm–1, and two next excited states at ∼120 cm–1. Reactivity studies with various unsaturated substrates are also
presented in this study, which show that the Ti­(II) center allows
oxidative addition likely via formation of [Ti­(η2-R2E2)­Cl2(py)n] E = C, N intermediates. A new Ti­(IV) compound, mer-[(py)3(η2-Ph2C2)­TiCl2], was prepared by reaction with Ph2C2, along with the previously reported complex trans-(py)3TiNPh­(Cl)2, from reaction with
Ph2N2. Reaction with Ph2CN2 also yielded a new dinuclear Ti­(IV) complex, [(py)2(Cl)2­Ti­(μ2:η2-N2CPh2)2­Ti­(Cl)2], in
which the two Ti­(IV) ions are inequivalently coordinated. Reaction
with cyclooctatetraene (COT) yielded a new Ti­(III) complex, [(py)2­Ti­(η8-COT)­Cl], which is a rare
example of a mononuclear “piano-stool” titanium complex.
The complex trans-[(py)4­TiCl2] has thus been shown to be synthetically accessible, have
an interesting electronic structure, and be reactive toward oxidation
chemistry.

本研究开发了一种简便且高产率的合成方法，用于制备已知的二价钛配合物反式-[(py)₄TiCl₂]（py = 吡啶，pyridine）。采用磁化率测试、磁圆二色谱（magnetic circular dichroism）、高频高场电子顺磁共振光谱技术，并结合配体场理论（ligand-field theory）与计算方法（密度泛函理论（density functional theory）和从头算方法（ab initio methods）），对该配合物的电子结构开展了实验表征。
上述研究表明，反式-[(py)₄TiCl₂]的基态为³Eg轨道构型（电子占据dxy¹dxz,yz¹）；受自旋轨道耦合（spin–orbit coupling）效应影响，其基态自旋双重态具有电子顺磁共振活性，在约60 cm⁻¹处存在第一激发态双重态，在约120 cm⁻¹处存在两个次激发态双重态。
本研究同时报道了该配合物与多种不饱和底物的反应性研究，结果显示二价钛中心可通过形成[Ti(η²-R₂E₂)Cl₂(py)ₙ]（E = C, N）中间体发生氧化加成（oxidative addition）反应。
通过与二苯乙炔（Ph₂C₂）反应，制备得到了新型四价钛配合物mer-[(py)₃(η²-Ph₂C₂)TiCl₂]（面式-[(py)₃(η²-Ph₂C₂)TiCl₂]）；与二苯基二氮烯（Ph₂N₂）反应则得到了此前已报道的配合物反式-[(py)₃Ti=NPhCl₂]。
与二苯基重氮烷（Ph₂CN₂）反应还得到了新型双核四价钛配合物[(py)₂(Cl)₂Ti(μ₂:η²-N₂CPh₂)₂Ti(Cl)₂]，该配合物中两个四价钛离子的配位环境并不等价。
与环辛四烯（cyclooctatetraene, COT）反应则得到了新型三价钛配合物[(py)₂Ti(η⁸-COT)Cl]，该配合物属于罕见的单核“钢琴凳”型钛配合物。
综上，反式-[(py)₄TiCl₂]不仅可通过简便方法实现高产率合成，其电子结构颇具研究价值，同时在氧化化学领域表现出良好的反应活性。