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

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.