Synthesis and Reactions of Group 4 Imido Complexes Supported by Cyclooctatetraene Ligands

Abstract

The reactions of the pseudo-two-coordinate titanium imido complexes [Ti(NtBu)(COT)] (1) (COT = η8-C8H8), [Ti(NtBu)(COT‘ ‘)] (2) (COT‘ ‘ = η8-1,4-C8H6(SiMe3)2), and [Ti(NAr)(COT)] (3) (Ar = 2,6-iPr2C6H3) with a variety of organic substrates are reported. Reaction of 1 with CO2, tBuNCO, or ArNCO and reaction of 3 with CO2 or tBuNCO afforded the organic products tBuNCO, tBuNCNtBu, tBuNCNAr, ArNCO, and ArNCNtBu, respectively, and a titanium oxo species. These reactions proceeded via an initial [2 + 2] cycloaddition to form an N,O-bound intermediate of the type [Ti{N(R)C(O)R‘}(COT)], which subsequently underwent a retrocycloaddition to give an organic product and the titanium oxo species. In contrast, reaction of 3 with ArNCO led to the formation of the N,N-bound [2 + 2] cycloaddition product [Ti{N(Ar)C(O)N(Ar)}(COT)] (7). In general, the reactions of 1 and 3 with CS2 and isothiocyanates also resulted in an initial [2 + 2] cycloaddition to form an N,S-bound intermediate of the type [Ti{N(R)C(S)R‘}(COT)], which also subsequently underwent a retrocycloaddition to give an organic product and a metal sulfide species. However, the N,S-bound compound [Ti{N(Ar)C(S)S}(COT)] (10) was stable to retrocycloaddition and was isolated. Proton transfer reactions occurred between pinacol and compounds 1−3 to form the bis(alkoxide) species [Ti{OC(Me)2C(Me)2O}(COT)] (11) (from 1 or 3) or [Ti{OC(Me)2C(Me)2O}(COT‘ ‘)] (12) (from 2) and the corresponding free amine. The reactions between 1−3 and 2 equiv of the thiols tBuSH and HS-2,4,6-iPr3C6H2 all resulted in the oxidation of the thiol to the disulfides tBuS−StBu and (2,4,6-iPr3C6H2)S−S(2,4,6-iPr3C6H2). Treatment of 1 with tBuNC in the presence of 1,3,5,7-cyclooctatetraene led to formal nitrene group transfer and the formation of the Ti(II) species [Ti(COT)(η4-C8H8)] (13) and tBuNCNtBu. The analogous reactions between 2 and 3 and tBuNC resulted in the formation of the adducts [Ti(NtBu)(COT‘ ‘)(CNtBu)] (15) and [Ti(NAr)(COT)(CNtBu)] (17), and similarly the reaction between 1 and pyridine led to the isolation of the adduct [Ti(NtBu)(COT)(py)] (19) (py = pyridine). Complex 19 was crystallographically characterized. DFT studies indicated that the interaction between pyridine and the Ti center in 19 and tBuNC and the Ti center in 17 was electrostatic in nature. Complexes of the type [Ti(NR)(COT)(AlMe3-xClx)] (R = tBu, x = 0 (20); R = Ar, x = 0 (21); R = tBu, x = 1 (22); R = Ar, x = 1 (23)) were formed through the reactions of 1 and 3 with AlMe3 and AlMe2Cl, and DFT studies indicated that they contained four-membered metallacyclic rings. Attempts to prepare monomeric zirconium imido cyclooctatetraene complexes through the reactions of [Zr2(μ-NR)2Cl4(THF)x] (R = tBu, x = 3; R = 2,6-Me2C6H3(Ar‘), x = 4) with K2COT, Li2COT‘ ‘·1.8(THF), or Li2[COT*] (COT* = 1,4-C8H6(SiMe2tBu)2) were unsuccessful. Only the crystallographically characterized dimeric species [Zr2(μ-NAr‘)2(COT‘ ‘)2] (24) was isolated.