Synthesis and Reactivity of Tantalum Complexes Supported by Bidentate X₂ and Tridentate LX₂ Ligands with Two Phenolates Linked to Pyridine, Thiophene, Furan, and Benzene Connectors: Mechanistic Studies of the Formation of a Tantalum Benzylidene and Insertion Chemistry for Tant

Using either alkane elimination or salt metathesis methods, tantalum complexes have been prepared with new ligand systems with tridentate bis(phenolate)donor (donor = pyridine, furan, and thiophene) or bidentate bis(phenolate)benzene arrangements. The ligand framework has two X-type phenolates connected to the flat heterocyclic L-type donor at the 2,6- or 2,5- positions or to the 2,6- positions of benzene via direct ring−ring (sp2−sp2) linkages. Solid-state structures of these complexes show that in all cases the ligands bind in a mer fashion, but with different geometries of the LX2 frameworks. The pyridine-linked system binds in a Cs-fashion, the furan-linked system in a C2v-fashion, and the thiophene-linked system in a C1-fashion. A bis(phenolate)pyridine tantalum tribenzyl species (7), upon heating in the presence of dimethylphenylphosphine, generates a stable benzylidene complex by α-hydrogen abstraction with loss of toluene and PMe2Ph trapping. This process was found to be independent of PMe2Ph concentration with ΔH⧧ = 31.3 ± 0.6 kcal·mol−1 and ΔS⧧ = 3 ± 2 cal·mol−1·K−1, and the kinetic isotope effect kH/kD = 4.9 ± 0.4, consistent with a mechanism involving rate determining α-hydrogen abstraction with loss of toluene, followed by fast phosphine coordination to the resulting benzylidene species. An X-ray structure determination reveals that the benzylidene π-bond is oriented perpendicular to the oxygen−oxygen vector, in accord with the prediction of DFT calculations. Tantalum alkyl complexes with the benzene-linked bis(phenolate) ligand (Ta(CH3)2[(OC6H2-tBu2)2C6H3] (16), Ta(CH2Ph)2[(OC6H2-tBu2)2C6H3] (17), and TaCl2CH3[(OC6H2-tBu2)2C6H4] (18)) are obtained with (to afford pincer complexes) or without cyclometalation at the ipso-position. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzene-diyl ring reveals an unexpected mechanism for the metalation of bis(phenol)benzene with TaCl2(CH3)3 to generate 18. This process involves protonolysis of a methyl group, followed by C-H/Ta-CH3 σ bond metathesis leading to cyclometalation of the linking ring, and finally protonation of the cyclometallated group by the pendant phenol. TaCl2CH3[(OC6H2-tBu2)2C6H4] was found to undergo σ bond metathesis at temperatures over 90 °C to give the pincer complex TaCl2[(OC6H2-tBu2)2C6H3] (19) and methane (ΔH⧧ = 27.1 ± 0.9 kcal·mol−1; ΔS⧧ = −2 ± 2 cal·mol−1·K−1; kH/kD = 1.6 ± 0.2 at 125 °C). Ta(CH3)2[(OC6H2-tBu2)2C6H3] (16) was found to react with tBuNC to insert into the Ta-CH3 bonds and generate an imino-acyl species (23). Reaction of 16 with Ph2CO or PhCN leads to insertion into the Ta-Ph bond to give 21 and 22. Complexes 6, 7, 10, 11-P, 12, 13, 17, 18, 19-OEt2, 21, 22, and 23 have been structurally characterized by single crystal X-ray diffraction, and all show a mer binding mode of the diphenolate ligands, but the ligand geometry varies leading to C2v-, pseudo-Cs-, pseudo-C2-, and C1-symmetric structures.本研究采用烷烃消除法（alkane elimination）或盐复分解法（salt metathesis），合成了一系列基于新型配体体系的钽配合物（tantalum complexes）：该类配体体系包含三齿双酚盐给体配体（tridentate bis(phenolate) donor，给体分别为吡啶（pyridine）、呋喃（furan）与噻吩（thiophene）），或是双齿双酚盐苯（bidentate bis(phenolate)benzene）配体结构。该配体骨架包含两个X型酚盐配体（X-type phenolates），通过环-环（sp²-sp²）连接键，分别连接于平面杂环L型给体（flat heterocyclic L-type donor）的2,6-或2,5-位，或是苯环的2,6-位。 上述配合物的固态结构表明，所有配体均以面式（mer，meridional）配位模式结合，但LX2骨架（LX2 framework）的几何构型存在差异：吡啶连接的体系以Cs对称（Cs symmetry）模式配位，呋喃连接的体系以C2v对称（C2v symmetry）模式配位，而噻吩连接的体系则以C1对称（C1 symmetry）模式配位。 双酚盐吡啶三苄基钽物种（bis(phenolate)pyridine tantalum tribenzyl species，7）在二甲基苯基膦（dimethylphenylphosphine，PMe2Ph）存在下加热时，通过α-氢夺取（α-hydrogen abstraction）并脱甲苯（loss of toluene），同时被PMe2Ph捕获，生成稳定的亚苄基配合物。该反应的速率与PMe2Ph浓度无关，其活化焓ΔH⧧=31.3±0.6 kcal·mol⁻¹，活化熵ΔS⧧=3±2 cal·mol⁻¹·K⁻¹，动力学同位素效应kH/kD=4.9±0.4，这与决速步为α-氢夺取并脱甲苯、随后膦快速配位至生成的亚苄基物种的反应机理一致。X射线单晶衍射结构解析显示，该亚苄基的π键（benzylidene π-bond）取向与氧-氧矢量（oxygen−oxygen vector）垂直，这与密度泛函理论（DFT, Density Functional Theory）计算的预测结果相符。 针对苯环连接的双酚盐配体，我们得到了如下钽烷基配合物：Ta(CH3)2[(OC6H2-tBu2)2C6H3]（16）、Ta(CH2Ph)2[(OC6H2-tBu2)2C6H3]（17）以及TaCl2CH3[(OC6H2-tBu2)2C6H4]（18），这些配合物可在本位（ipso-position）发生或不发生环金属化（cyclometalation），从而得到钳形配合物（pincer complex）。 通过对酚氢与连接的1,3-亚苯基环（1,3-benzene-diyl ring）进行氘代标记（deuterium labeling），我们揭示了TaCl2(CH3)3与双酚苯发生金属化反应生成18的意外机理：该过程首先发生甲基的质子解（protonolysis），随后经历C-H/Ta-CH3 σ键复分解（σ bond metathesis）实现连接环的环金属化，最终由悬挂酚基（pendant phenol）对环金属化位点进行质子化。 TaCl2CH3[(OC6H2-tBu2)2C6H4]在90℃以上可发生σ键复分解反应，生成钳形配合物TaCl2[(OC6H2-tBu2)2C6H3]（19）与甲烷，其活化焓ΔH⧧=27.1±0.9 kcal·mol⁻¹，活化熵ΔS⧧=-2±2 cal·mol⁻¹·K⁻¹，125℃下的动力学同位素效应kH/kD=1.6±0.2。 Ta(CH3)2[(OC6H2-tBu2)2C6H3]（16）可与叔丁基异氰（tBuNC, tert-butyl isocyanide）发生反应，插入至Ta-CH3键中生成亚氨基酰基物种（imino-acyl species，23）。而16与二苯甲酮（Ph2CO, benzophenone）或苯甲腈（PhCN, benzonitrile）的反应则会插入至Ta-Ph键中，分别得到21与22。 配合物6、7、10、11-P、12、13、17、18、19-OEt2、21、22及23均通过单晶X射线衍射（single crystal X-ray diffraction）完成了结构表征，所有配合物均表现出双酚盐配体的面式配位模式，但配体几何构型的差异导致产物呈现C2v对称、准Cs对称、准C2对称及C1对称的结构。