Reactions of cyclopentadienyl-amidinate titanium imido compounds with CO2: cycloaddition-extrusion vs. cycloaddition-insertion

Abstract

A combined experimental and DFT study of the reactions of cyclopentadienyl-amidinate titanium imido complexes with CO(2) is reported. Cycloaddition reactions of the aryl imido compounds Ti(eta-C(5)R(4)Me)(NAr){R(2)C(NR(1))(2)} (R = H or Me; R(1), R(2) = SiMe(3), Ph or (i)Pr, Me) with CO(2) gave the corresponding N,O-bound carbamate complexes Ti(eta-C(5)R(4)Me){N(Ar)C(O)O}{R(2)C(NR(1))(2)}. These reacted further with CO(2) by insertion into the Ti-N(Ar) bond to afford the new dicarboxylates Ti(eta-C(5)R(4)Me){OC(O)N(Ar)C(O)O}{R(2)C(NR(1))(2)} in which the original Ti=NAr bond has been completely cleaved. The X-ray structures of two of these have been determined. The CO(2) insertion reactions of the para-substituted phenyl carbamate complexes Ti(eta-C(5)Me(5)){N(-4-C(6)H(4)X)C(O)O}{MeC(N(i)Pr)(2)} (X = Me, CF(3) or NMe(2)) were first order with respect to both carbamate complex and CO(2) and the pseudo first order rate constants were effectively independent of the para substituent. The corresponding tert-butyl imido compounds Ti(eta-C(5)R(4)Me)(N(t)Bu){R(2)C(NR(1))(2)} also reacted with CO(2) to form N,O-bound carbamate complexes, Ti(eta-C(5)R(4)Me){N((t)Bu)C(O)O}{R(2)C(NR(1))(2)}. However, these did not insert a further molecule of CO(2) and instead extruded (t)BuNCO to form the crystallographically characterized oxo-bridged dimers [Ti(eta-C(5)R(4)Me)(mu-O){R(2)C(NR(1))(2)}](2). These reactions proceeded via transient terminal oxo intermediates, one of which was trapped by the addition of TolNCO (Tol = p-tolyl). DFT (B3PW91) calculations on Ti(eta-C(5)H(5))(NR){MeC(NMe)(2)} (R = Me, Ph, 4-C(6)H(4)Me, 4-C(6)H(4)NMe(2), 4-C(6)H(4)CF(3)) reacting with CO(2) showed that the second CO(2) insertion is thermodynamically favoured over isocyanate extrusion, and that the rates of the two processes are similar. Calculations on Ti(eta-C(5)R(5))(N(t)Bu){MeC(N(i)Pr)(2)} (R = H or Me) showed that increasing the steric bulk increases the thermodynamic favourability of the isocyanate extrusion process and significantly raises the activation barrier for the second CO(2) insertion, making the latter process impossible.

Reversible Dihydrogen Activation and Hydride Transfer by a Uranium Nitride Complex

“…Complex 4 is only the second example of ac rystallographically characterized formate complex obtained from the insertion of CO 2 into aU ÀHb ond and the first one containing ab ridging formate. Thec oncomitant cycloaddition of CO 2 to the parent imido linkage affords ab ridging k 3 -carbamate group.T his reactivity is ar are example of cycloaddition of CO 2 to ametal imido complex [22] and differs from that reported for U IV -bound terminal bulky aryl imido groups (U = N À Ar) that results in N = Cb ond methathesis and extrusion of the cyanate via ap roposed carbamate intermediate. The proton of the formate was located by X-ray diffraction and was identified in the 1 HNMR spectrum.…”

“…The proton of the formate was located by X-ray diffraction and was identified in the 1 HNMR spectrum. Thec oncomitant cycloaddition of CO 2 to the parent imido linkage affords ab ridging k 3 -carbamate group.T his reactivity is ar are example of cycloaddition of CO 2 to ametal imido complex [22] and differs from that reported for U IV -bound terminal bulky aryl imido groups (U = N À Ar) that results in N = Cb ond methathesis and extrusion of the cyanate via ap roposed carbamate intermediate. [23] Thelow steric pressure of the NH group and its bridging mode result in ahigh stability of the k 3 -O,O,N-carbamate group.Afew examples of U IV carbamate have been reported but they all present monoanionic k 2 -O,Ocarbamate groups.…”

Recent Advances in the Chemistry of Metal Carbamates

“…In this case, the geometry of the ligand forces the nitrogen atom to remain coordinated after CO 2 addition, thus stabilizing a rare example of monoanionic N,O-chelating carbamato ligand (D/1-n). The insertion of CO2 into M−N bonds has also been reported for Sc(III) [217], Ti(IV) [218][219][220], Ni(II) [221], Pd(II) [222] and Ir(III) [223,224] imido complexes. The dianionic carbamyldiide ligand thus generated usually remains coordinated to the metal via N,O (Scheme 21a).…”

Mild and High‐Yielding Molybdenum(VI) Dichloride Dioxide‐Catalyzed Formation of Mono‐, Di‐, Tri‐, and Tetracarbamates from Alcohols and Aromatic or Aliphatic Isocyanates

“…[12] Moreover, carbamates serve as lithiation-directing groups, [13] chiral auxiliaries [14] , constituents of transition metal complexes [15] and linkers. [16] The standard synthesis of a carbamate is by the addition of an alcohol to an isocyanate.…”