Terminal vs bridging imido ligation in complexes of zirconium(IV) and hafnium(IV): structural characterization of a .mu.-[.eta.1(N):.eta.2(N,C)] imido ligand

Abstract

The reaction of 4 equiv of LiNHAr (Ar = 2,6-06 3' 2) with a solution of ZrCl4(THF)2 in THF/pyridine yields Zr(=NAr)(NHAr)2(py)2 (1) in high yield. Zr(=NAr)(NHAr)2(py)2 reacts with 1 and 2 equiv of Me3SiCl in THF/pyridine to provide Zr(=NAr)(NHAr)Cl(py)2 (2) and Zr(=NAr)Cl2(py)3 (3), respectively. ZrCl4(THF)2 reacts with 4 equiv of LiNHAr, followed by 2 equiv of Me3SiCl (in THF), to afford Zr(=NAr)Cl2(THF)2 (4) in a high-yield, one-pot synthesis. Hf(=NAr)Cl2(THF)2 ( 5) is prepared similarly. Zr(=NAr)(NHAr)Cl(py)2 reacts with 1 equiv of K[N(SiMe3)2] in THF to provide Zr(=NAr)(NHAr) [N(SiMe3)2] (py)2 (6), which could not be converted into a bis(imido) species. Both Hf(=NAr)Cl2(THF)2 and Zr(=NAr)Cl2(THF)2 react with K2(C8H8) to provide [(?/8-C8H8)Zr(=NAr)]2 (7) and [(?;8-C8H8)Hf(=NAr)]2 (8) in high yield. Zr(=NAr)Cl2(THF)2 reacts with 1 equiv of Li(C3H4Me) in THF to provide [(r/5-C5H4Me)Zr(=NAr)Cl]2 (9) in moderate yield. An X-ray structural determination of this compound reveals a dimeric structure with bridging imido ligands. [(tj5-C5H4-

Titanium Imido Complexes of Cyclooctatetraenyl Ligands

“…[32,33] Although there has previously been one report of imido complexes bearing cyclooctatetraenyl co-ligands, these complexes were neither monomeric nor structurally characterised. [27] The Ti=NÀtBu and Ti···COT (ring centroid) distances of 1.699(6) and 1.369 in 1 are within the expected ranges for terminal titanium tert-butylimido and titanium(iv) h 8 -cyclooctatetraenyl complexes, respectively. [32,33] The COT ring is planar and the complex is approximately C s symmetric.…”

“…The dimethylphenyl group of the imide is bent slightly towards the Ti atom in the same asymmetric unit, so the Ti···Ti vector makes an angle of 84.78 with the best plane of the phenyl ring. Neither of the ipsocarbon atoms of the bridging m-N-2,6-Me 2 C 6 H 3 ligands displays any interaction with the Ti centre in contrast to the significant interactions seen in similar zirconium, [27] samarium [38] and uranium [39][40][41][42] complexes. This lack of interaction could be for steric reasons or because titanium is smaller than these other metals.…”

“…Unsymmetrically bridging imides have previously been observed for several different transition-metal complexes and this effect increases p donation to the metal centre. [27,35,38] The ligands of the complex are slightly distorted from their regular geometries. The dimethylphenyl group of the imide is bent slightly towards the Ti atom in the same asymmetric unit, so the Ti···Ti vector makes an angle of 84.78 with the best plane of the phenyl ring.…”

“…[26] We noticed that although cyclopentadienyl and to a lesser extent arene co-ligands have been widely used in transitionmetal imido chemistry, there has only been one report of imido complexes bearing cyclooctatetraenyl co-ligands, namely the dinuclear m-arylimido derivatives [M 2 (m-N-2,6-iPr 2 C 6 H 3 ) 2 (h 8 -COT) 2 ] (M= Zr, Hf). [27] Therefore, we were interested to see if using the smaller titanium congener would lead to a mononuclear derivative. Only two families of one-legged piano stool ('pogo stick') imido complexes 2 ].…”

Mechanistic Investigation of Cycloreversion/Cycloaddition Reactions between Zirconocene Metallacycle Complexes and Unsaturated Organic Substrates

“…The Zr-N1 and Zr-N2 bond distances are 2.117(3) and 2.1222(2) Å, respectively, and are close to those seen in several other structurally similar zirconocene amido complexes. 25,27,28 The bond lengths for N1-Cl and N2-C1 are virtually the same, 1.393(4) and 1.408(4) Å, respectively (Table 1; see Table 2 for crystallographic data). The shorter N3-Cl bond length of 1.278(4) Å indicates greater electron density in the exocyclic C-N bond of the four-membered zirconacycle.…”