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The titanium tert-butyl imido compounds [Ti(NBu t )Cl 2 (NC 5 H 4 R-4) n ] (R = H, n = 2 or 3; R = Bu t , n = 2) have been found to be entry points to the half-sandwich η 5 -cyclopentadienyl derivatives [Ti(η 5 -C 5 RЈ 4 RЉ)(NBu t )Cl(NC 5 H 4 R-4)] (R = Bu t , RЈ = RЉ = H or Me; R = H, RЈ = H, RЉ = H, Me or Pr i ; R = H, RЈ = Me, RЉ = Me, Et or C 4 H 7 ), the η 5 -1,2,3-trimethylindenyl species [Ti(η 5 -C 9 H 4 Me 3 )(NBu t )Cl(NC 5 H 4 Bu t -4)] and the bis(η 5 -cyclopentadienyl) compound [Ti(η 5 -C 5 H 5 ) 2 (NBu t )(NC 5 H 5 )], the crystal structure of which has been determined. The complex [Ti(η 5 -C 5 H 5 )(NBu t )Cl(NC 5 H 5 )] readily loses pyridine under vacuum in the solid state to form the binuclear complex [Ti 2 (η 5 -C 5 H 5 ) 2 (µ-NBu t ) 2 Cl 2 ]. Treatment of [Ti(η 5 -C 5 Me 4 R)(NBu t )Cl(NC 5 H 5 )] (R = Me or Et) with Na[C 5 H 5 ] gives the corresponding mixed-ring sandwich derivatives [Ti(η 5 -C 5 H 5 )(η 5 -C 5 Me 4 R)(NBu t )Cl(NC 5 H 5 )]. Addition of Li[C 9 H 7 ] to [Ti(η 5 -C 5 H 5 )(NBu t )Cl(NC 5 H 5 )] gave the η 5 -cyclopentadienyl, η 3 -indenyl mixed-ring analogue [Ti(η 5 -C 5 H 5 )(η 3 -C 9 H 7 )(NBu t )(NC 5 H 5 )]. The complex [Ti(η 5 -C 5 H 5 ) 2 (NBu t )(NC 5 H 5 )] undergoes a roomtemperature cyclopentadienyl ligand-redistribution reaction with [Ti(NBu t )Cl 2 (NC 5 H 5 ) 2 ] forming [Ti(η 5 -C 5 H 5 )(NBu t )Cl(NC 5 H 5 )] in quantitative yield. Variable-temperature NMR spectra for the half-sandwich complexes show that the co-ordinated pyridine exchanges with free pyridine via an associative mechanism. The compound [Ti(η 5 -C 5 H 5 )(η 5 -C 5 Me 4 Et)(NBu t )(NC 5 H 5 )] is also fluxional and exhibits reversible pyridine dissociation at higher temperatures and restricted rotation about the Ti᎐N (pyridine) bond at lower temperatures.

Reaction of [Ti(NR)Cl2(py)3] (R=tBu or 2,6-iPr2C6H3) with K(2)[COT] (COT=C8H8) or Li2[COT''] (COT''=1,4-C8H6(SiMe3)2) gave the monomeric complexes [Ti(NR)(eta8-COT)] or [Ti(NR)(eta8-COT'')], respectively. The pseudo-two coordinate, "pogo stick" geometry for these complexes is unique in both early transition-metal and cyclooctatetraenyl ligand chemistry. In contrast, reaction of [Ti(N-2,6-Me2C6H3)Cl2(py)3] with K2[COT] gave the mu-imido-bridged dimer [Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2]. It appears that as the steric bulk of the imido and C8 ring substituents are decreased, dimerisation becomes more favourable. Aryl imido COT complexes were also prepared by imido ligand exchange reactions between anilines and [Ti(NtBu)(eta(8)-COT)] or [Ti(NtBu)(eta(8)-COT'')]. The complexes [Ti(NtBu)(eta(8)-COT)], [Ti(N-2,6-iPr2C6H3)2(eta8-COT)] and [Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2] have been crystallographically characterised. The electronic structures of both the monomeric and dimeric complexes have been investigated by using density functional theory (DFT) calculations and gas-phase photoelectron spectroscopy. The most striking aspect of the bonding is that binding to the imido nitrogen atom is primarily through sigma and pi interactions, whereas that to the COT or COT'' ring is almost exclusively through delta symmetry orbitals. A DFT-based comparison between the bonding in [Ti(NtBu)(eta8-COT)] and the bonding in the previously reported late transition-metal "pogo stick"complexes [Os(NtBu)(eta6-C6Me6)], [Ir(NtBu)(eta5-C5Me5)] and [Ni(NO)(eta5-C5H5)] has also been undertaken.

The readily available complex [Ti(NB~t)Cl~(Butpy)~] (Butpy = 4-tert-butylpyridine) is a useful precursor to a range of sandwich-and half-sandwich titanium imido derivatives including [Ti(q-C9H4Me3) (NBut)CI(Butpy)] (C9H4Me3 = trimethylindenyl), [Ti(q-C5H5)2(NBuf)(Butpy)], [Ti(dm bpz)(NBuf)CI(Butpy)] [dmbpz = tris(3,5-dimethylpyrazolyl) borate] and [Ti(r14-Me,taa)(NBut)l { n = 4 or 8; Met4 or 8)taa = (tetra-or octa-)methyl-dibenzotetraaza[l4]annulene respectively}. Much of the current interest in transition metal-ligand multiple bonds has focused on the early transition metals.1-5 Reactive group 4 imido transients such as Bergman's [Zr(q5-CSH5)2(NBut)] ,2 and Wolczanski's [zr(B~t~SiNH)~(NSiBut~)l and [Ti(B~t~SiO)~(NSiBut~)13 have demonstrated a rich chemistry including [2 + 21 cycloadditions and alkane and arene C-H bond activations. Group 4 metal-ligand multiply-bonded complexes which have reactive M=E functionalities yet are sufficiently kinetically stable to be isolated are rare. The few examples include the monomeric oxo-and sulfido-derivatives [Ti(q4-Me4taa)(E)] (Me4taa = tetramethyldibenzotetraaza[l4]annulene; E = 0, S), first reported by Goedken in 1982.4 The first fully characterised monomeric titanium imido complexes were first reported as late as 1990 by Roesky and Rothwell.5 We describe here the synthesis of monomeric h R = H 3 2bR=Me \. / 1 4

The reactions of the pseudo-two-coordinate titanium imido complexes [Ti(N t Bu)(COT)] (1) (COT ) η 8 -C 8 H 8 ), [Ti(N t Bu)(COT′′)] (2) (COT′′ ) η 8 -1,4-C 8 H 6 (SiMe 3 ) 2 ), and [Ti(NAr)(COT)] (3) (Ar ) 2,6i Pr 2 C 6 H 3 ) with a variety of organic substrates are reported. Reaction of 1 with CO 2 , t BuNCO, or ArNCO and reaction of 3 with CO 2 or t BuNCO afforded the organic products t BuNCO, t BuNCN t Bu, t BuNCNAr, ArNCO, and ArNCN t Bu, 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 CS 2 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) 2 C(Me) 2 O}(COT)] (11) (from 1 or 3) or [Ti{OC(Me) 2 C(Me) 2 O}(COT′′)] (12) (from 2) and the corresponding free amine. The reactions between 1-3 and 2 equiv of the thiols t BuSH and HS-2,4,6-i Pr 3 C 6 H 2 all resulted in the oxidation of the thiol to the disulfides t BuS-S t Bu and (2,4,6-i Pr 3 C 6 H 2 )S-S(2,4,6-i Pr 3 C 6 H 2 ). Treatment of 1 with t BuNC 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)(η 4 -C 8 H 8 )] ( 13) and t BuNCN t Bu. The analogous reactions between 2 and 3 and t BuNC resulted in the formation of the adducts [Ti(N t Bu)(COT′′)(CN t Bu)] ( 15) and [Ti(NAr)(COT)(CN t Bu)] (17), and similarly the reaction between 1 and pyridine led to the isolation of the adduct [Ti(N t Bu)-(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 t BuNC and the Ti center in 17 was electrostatic in nature. Complexes of the type [Ti(NR)(COT)(AlMe 3-x Cl x )] (R ) t Bu, x ) 0 (20); R ) Ar, x ) 0 (21); R ) t Bu, x ) 1 (22); R ) Ar, x ) 1 ( 23)) were formed through the reactions of 1 and 3 with AlMe 3 and AlMe 2 Cl, and DFT studies indicated that they contained four-membered metallacyclic rings. Attempts to prepare monomeric zirconium imido cyclooctatetraene complexes through the reactions of [Zr 2 (µ-NR) 2 Cl 4 (THF) x ] (R ) t Bu, x ) 3; R ) 2,6-Me 2 C 6 H 3 (Ar′), x ) 4) with K 2 COT, Li 2 COT′′‚1.8-(THF), or Li 2 [COT*] (COT* ) 1,4-C 8 H 6 (SiMe 2 t Bu) 2 ) were unsuccessful. Only the crystallographically characterized dimeric species [Zr 2 (µ-NAr′...