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Issue 30, 2009
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Reactions of cyclopentadienyl-amidinate titanium imido compounds with CO2: cycloaddition-extrusionvs.cycloaddition-insertion

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Abstract

A combined experimental and DFT study of the reactions of cyclopentadienyl-amidinate titanium imido complexes with CO2 is reported. Cycloaddition reactions of the aryl imido compounds Ti(η-C5R4Me)(NAr){R2C(NR1)2} (R = H or Me; R1, R2 = SiMe3, Ph or iPr, Me) with CO2 gave the corresponding N,O-bound carbamate complexes Ti(η-C5R4Me){N(Ar)C(O)O}{R2C(NR1)2}. These reacted further with CO2 by insertion into the Ti–N(Ar) bond to afford the new dicarboxylates Ti(η-C5R4Me){OC(O)N(Ar)C(O)O}{R2C(NR1)2} in which the original Ti[double bond, length as m-dash]NAr bond has been completely cleaved. The X-ray structures of two of these have been determined. The CO2insertion reactions of the para-substituted phenyl carbamate complexes Ti(η-C5Me5){N(-4-C6H4X)C(O)O}{MeC(NiPr)2} (X = Me, CF3 or NMe2) were first order with respect to both carbamate complex and CO2 and the pseudo first order rate constants were effectively independent of the para substituent. The corresponding tert-butyl imido compounds Ti(η-C5R4Me)(NtBu){R2C(NR1)2} also reacted with CO2 to form N,O-bound carbamate complexes, Ti(η-C5R4Me){N(tBu)C(O)O}{R2C(NR1)2}. However, these did not insert a further molecule of CO2 and instead extruded tBuNCO to form the crystallographically characterized oxo-bridged dimers [Ti(η-C5R4Me)(μ-O){R2C(NR1)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(η-C5H5)(NR){MeC(NMe)2} (R = Me, Ph, 4-C6H4Me, 4-C6H4NMe2, 4-C6H4CF3) reacting with CO2 showed that the second CO2insertion is thermodynamically favoured over isocyanate extrusion, and that the rates of the two processes are similar. Calculations on Ti(η-C5R5)(NtBu){MeC(NiPr)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 CO2insertion, making the latter process impossible.

Graphical abstract: Reactions of cyclopentadienyl-amidinate titanium imido compounds with CO2: cycloaddition-extrusionvs.cycloaddition-insertion

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Publication details

The article was received on 27 Jan 2009, accepted on 24 Mar 2009 and first published on 27 Apr 2009


Article type: Paper
DOI: 10.1039/B901774G
Citation: Dalton Trans., 2009,0, 5960-5979

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

    A. E. Guiducci, C. L. Boyd, E. Clot and P. Mountford, Dalton Trans., 2009, 0, 5960
    DOI: 10.1039/B901774G

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