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.