Carbon Dioxide Activation Assisted by a Bis(chlorodimethylsilyl)cyclopentadienyl Titanium Compound

Abstract

A great deal of interest has focused on the role of metal ions as the active centers in the fixation of CO 2 and its transformation.[1] Activation of CO 2 by hydroxo and oxo metal complexes to afford metal hydrogencarbonato and carbonato species, respectively, is related to the function of the carbonic anhydrase metalloenzyme, [2] which catalyzes the physiologically important hydration of CO 2 to hydrogencarbonate [Eq. (1)].This type of reaction is common for the late-transition metals [3] and is known for main-group organometallic species.[4] Nevertheless, the carbonato derivatives reported for complexes of Group 4-6 metals are synthesized by alternative methods [5] based on reactions of metal precursor compounds with carbonate salts X 2 CO 3 (X = K, Bu 4 N) and NH 4 HCO 3 or by methods that involve the disproportionation of CO 2 .Herein, we describe the use of the bis(chlorodimethylsilyl)cyclopentadienyl titanium(iv) compound 1, [6] which was reported previously for the in situ activation of CO 2 . The carbonato titanium(iii) derivative 3 was serendipitously obtained when a dilute solution of 1 in wet toluene was exposed to air for several days (Scheme 1). This reaction proceeded in better yield (43 % after purification) when a solution of 1 in toluene was treated with a saturated aqueous solution of CO 2 to give 3, which was isolated as an analytically pure and highly air-stable diamagnetic orange crystalline solid. However, hydrolysis of 1 carried out in the presence of NEt 3 resulted in no reaction with CO 2 and the m-oxo titanium(iv) derivative 2 and NEt 3 ·HCl being obtained (Scheme 1). The reaction of 1 with K 2 CO 3 in THF or toluene afforded a mixture of unidentified compounds that did not contain 3.Hydrolysis of the Group 4 metal/chloro complexes usually proceeds with initial transformation of the metal-chlorine bonds so that intermediate complexes are formed which contain rather uncommon, discrete terminal Group 4 metalhydroxo bonds.[7] These species subsequently condense to give polynuclear compounds stabilized by m-oxo bridges.[8] We propose that the carbonato complex 3 results from the in situ formation of intermediate SiÀOH/TiÀOH terminal bonds (see A and B in Scheme 1) and a further insertion reaction of CO 2 with simultaneous reduction to the highly stable titanium(iii) compound 3. This mechanism of formation is consistent with the high stability of 3, which remains unaltered when left for weeks in air; with the formation of the m-oxo complex 2 in the presence of a deprotonating agent; and also with the observed stability of 2, as it did not react with CO 2 to give 3 after several days at temperatures higher than 120 8C.The 1 H NMR spectrum (CDCl 3 , 258C) of complex 3 shows behavior expected for a C 2h -symmetric molecule with an A 2 B spin system for the cyclopentadiene (Cp) protons and with two resonances of the two nonequivalent methyl groups of the four equivalent {SiMe 2 } fragments (see Experimental Section). The resonances of the carbon atoms of the two equivalent bridging carbonato ...