Diamine Bis(phenolate) as Supporting Ligands in Organoactinide(IV) Chemistry. Synthesis, Structural Characterization, and Reactivity of Stable Dialkyl Derivatives

The homoleptic compounds [U­(salan-R2)2] (R = Me (1), tBu (2)) were prepared in high yield by salt-metathesis reactions between
UI4(L)2 (L = Et2O, PhCN) and 2 equiv
of [K2(salan-R2)] in THF. In contrast, the reaction
of the tetradentate ligands salan-R2 with UI3(THF)4 leads to disproportionation of the metal and to
mixtures of U­(IV) [U­(salan-R2)2] and [U­(salan-R2)­I2] complexes, depending on the ligand to M ratio.
The reaction of K2salan-Me2 ligand with U­(IV)
iodide and chloride salts always leads to mixtures of the homoleptic
bis-ligand complex [U­(salan-Me2)2] and heteroleptic
complexes [U­(salan-Me2)­X2] in different organic
solvents. The structure of the heteroleptic complex [U­(salan-Me2)­I2(CH3CN)] (4) was determined
by X-ray studies. Heteroleptic U­(IV) and Th­(IV) chloride complexes
were obtained in good yield using the bulky salan-tBu2 ligand. The new complexes [U­(salan-tBu2)­Cl2(bipy)] (5) and [Th­(salan-tBu2)­Cl2(bipy)] (8) were crystallographically
characterized. The salan-tBu2 halide complexes
of U­(IV) and Th­(IV) revealed good precursors for the synthesis of
stable dialkyl complexes. The six-coordinated alkyl complexes [Th­(salan-tBu2)­(CH2SiMe3)2] (9) and [U­(salan-tBu2)­(CH2SiMe3)2] (10) were prepared
by addition of LiCH2SiMe3 to the chloride precursor
in toluene, and their solution and solid-state structures (for 9) were determined by NMR and X-ray studies. These complexes
are stable for days at room temperature. Preliminary reactivity studies
show that CO2 inserts into the An–C bond to afford
a mixture of carboxylate products. In the presence of traces of LiCl,
crystals of the dimeric insertion product [Th2Cl­(salan-tBu2)2(μ-η1:η1-O2CCH2SiMe3)2(μ-η1:η2-O2CCH2SiMe3)] (11) were isolated. The structure
shows that CO2 insertion occurs in both alkyl groups and
that the resulting carboxylate is easily displaced by a chloride anion.

通过UI₄(L)₂（L = 二乙醚(Et₂O)、苯甲腈(PhCN)）与2当量的[K₂(salan-R₂)]在四氢呋喃(THF)中发生复分解反应(salt-metathesis)，以高产率制备得到同配体配合物(homoleptic complex)[U(salan-R₂)₂]（R = 甲基(Me)对应化合物1、叔丁基(tBu)对应化合物2）。与之相对，四齿配体(tetradentate)salan-R₂与UI₃(THF)₄的反应会发生金属歧化反应(disproportionation)，生成U(IV)价态的[U(salan-R₂)₂]与[U(salan-R₂)I₂]配合物的混合物，产物组成取决于配体与金属(M)的投料比。在不同有机溶剂中，K₂salan-Me₂配体与U(IV)价态的碘化物、氯化物盐反应时，总会得到同配体双配体(bis-ligand)配合物[U(salan-Me₂)₂]与杂配体(heteroleptic)配合物[U(salan-Me₂)X₂]的混合物。通过X射线衍射分析(X-ray studies)，确定了杂配体配合物[U(salan-Me₂)I₂(CH₃CN)]（化合物4）的晶体结构。使用位阻较大的(bulky)salan-tBu₂配体，可高产率制备得到杂配体U(IV)与Th(IV)的氯化物配合物。新配合物[U(salan-tBu₂)Cl₂(bipy)]（化合物5）与[Th(salan-tBu₂)Cl₂(bipy)]（化合物8）经晶体学表征(crystallographically characterized)得以确认。U(IV)与Th(IV)的salan-tBu₂卤化物配合物(halide complex)是合成稳定二烷基配合物的优良前驱体(precursor)。通过向甲苯中的氯化物前驱体中加入三甲基硅基甲基锂(LiCH₂SiMe₃)，制备得到六配位烷基配合物(six-coordinated alkyl complex)[Th(salan-tBu₂)(CH₂SiMe₃)₂]（化合物9）与[U(salan-tBu₂)(CH₂SiMe₃)₂]（化合物10）；其中化合物9的溶液与固态结构分别通过核磁共振(NMR)与X射线衍射分析得以确定。该类配合物在室温下可稳定存在数日。初步反应性研究表明，二氧化碳(CO₂)可插入锕系元素(An)-C键中，生成羧酸酯类产物的混合物。在痕量氯化锂(LiCl)存在的条件下，可分离得到二聚型插入产物(dimeric insertion product)[Th₂Cl(salan-tBu₂)₂(μ-η¹:η¹-O₂CCH₂SiMe₃)₂(μ-η¹:η²-O₂CCH₂SiMe₃)]（化合物11）的晶体。晶体结构表明，二氧化碳可在两个烷基位点均发生插入反应，且生成的羧酸酯基团可轻易被氯离子置换。