Imidazolin-2-iminato Ligand-Supported Titanium Complexes as Catalysts for the Synthesis of Urea Derivatives

The
reactions of tetrakis­(dimethylamido)­titanium­(IV) [Ti­(NMe2)4] with three different imidazolin-2-imines (ImRNH; R = tert-butyl (tBu), mesityl
(Mes), and 2,6-diisopropylphenyl (Dipp)) afforded the corresponding
titanium imidazolin-2-iminato complexes [(ImRN)­Ti­(NMe2)3] (R = tBu, 1a;
R = Mes, 1b; R = Dipp, 1c). Treatment of
complex 1a with two different carbodiimides [R′NCNR′;
R′ = cyclohexyl (Cy) and isopropyl (iPr)]
resulted in the formation of imidazolin-2-iminato titanium mono­(guanidinate)
complex of the type [(ImRN)­Ti­(R′NC­(NMe2)­NR′) (NMe2)2 (R′ = iPr; R = tBu (2a), R = Dipp
(2c); R′ = Cy, R = tBu (3a)], as yellow solid in 94% yield. However, a similar reaction
of 1b and 1c with 2 equiv of phenyl isocyanates
at ambient temperature resulted in the formation of corresponding
titanium bis­(ureate) complexes [(ImRN)­Ti­{κ2-OC­(NMe2)­NPh}2(NMe2)] (R = Mes, 4b and R = Dipp, 4c). Three equivalents of phenyl
isothiocyanate reacted with complex 1c to afford respective
titanium tris­(thioureate) complex [(ImDippN)­Ti­{κ2-SC­(NMe2)­NPh}2­{κ1-SC­(NMe2)­NPh}] (6c). The molecular
structures of 1a–c, 2a, 2c, 3a, 4c, and 6c were established by X-ray diffraction analyses, and, from the solid-state
structures of 1a–c, 2a, 2c, 3a, 4c, and 6c, it was confirmed that the imidazolin-2-iminato titanium bond in
each case is very short and possesses a multiple-bonding character.
The imidazolin-2-iminato titanium complex 1c was utilized
as a precatalyst for the addition of amine N–H bond to phenyl
isocyanate. High yields of the corresponding urea derivatives were
achieved under mild conditions. The mechanistic study of the aforementioned
catalytic reaction was performed, and the active catalyst complex 7b was isolated using 2 equiv of iminopyrrole [2-(2,6-iPr2­C6H3NCH)­C4H3NH] and the complex 4b. The molecular
structure of 7b was thereafter established.

四(二甲基氨基)钛(IV) [Ti(NMe₂)₄] 与三种不同的咪唑啉-2-亚胺（imidazolin-2-imine）（ImRNH；R = 叔丁基（tBu）、均三甲苯基（Mes）和2,6-二异丙基苯基（Dipp））发生反应，得到相应的钛咪唑啉-2-亚胺基配合物[(ImRN)Ti(NMe₂)₃]（R = tBu，1a；R = Mes，1b；R = Dipp，1c）。将配合物1a与两种不同的碳二亚胺（carbodiimide）[R′N=C=NR′；R′ = 环己基（Cy）和异丙基（iPr）]进行反应，得到通式为[(ImRN)Ti(R′NC(NMe₂)NR′)(NMe₂)₂]的咪唑啉-2-亚胺基钛单胍基配合物（guanidinate complex），具体包括R′=iPr、R=tBu（2a），R′=iPr、R=Dipp（2c）；R′=Cy、R=tBu（3a），产物为黄色固体，产率达94%。然而，配合物1b和1c在室温下与2当量异氰酸苯酯（phenyl isocyanate）反应，得到相应的钛双脲基配合物（ureate complex）[(ImRN)Ti{κ²-OC(NMe₂)NPh}₂(NMe₂)]（R = Mes，4b；R = Dipp，4c）。3当量异硫氰酸苯酯（phenyl isothiocyanate）与配合物1c反应，得到相应的钛三硫脲基配合物（thioureate complex）[(ImDippN)Ti{κ²-SC(NMe₂)NPh}₂{κ¹-SC(NMe₂)NPh}]（6c）。通过X射线衍射分析（X-ray diffraction）确定了1a~c、2a、2c、3a、4c及6c的分子结构；从这些配合物的固态结构可以证实，每种情况下的咪唑啉-2-亚胺基钛键均极短，且具有多重键特征。咪唑啉-2-亚胺基钛配合物1c被用作预催化剂（precatalyst），用于胺的N–H键与异氰酸苯酯的加成反应。在温和条件下可高产率得到相应的脲衍生物。对上述催化反应开展了机理研究（mechanistic study），并通过2当量亚胺基吡咯[2-(2,6-iPr₂C₆H₃N=CH)C₄H₃NH]与配合物4b反应，分离得到活性催化配合物7b，随后确定了其分子结构。