Crystal structure of chlorido{[3-(η⁵-cyclopenta-dienyl)-2,2,3-trimethyl-1-phenylbutylidene] azanido-κN}[η²(N,O)-N,N-dimethylhydroxylaminato]titanium(IV), C₂₀H₂₇ClN₂OTi

Source of material

The title compound was obtained as a by-product during the preparation of [{η⁵-C₅H₄CMe₂CMe₂C(Ph) = N-κN}TiCl₂] from [{η⁵-C₅H₄CMe₂CMe₂C(Ph) = N-κN}TiCl(NMe₂)] as described in literature [5]. The by-product was formed in low yield (less than 10%) by a yet undisclosed mechanism (see the Discussion section) and crystallized from a concentrated toluene solution at 277 K as red prisms. NMR (benzene-d₈) ¹H: δ 0.86, 0.91, 0.94, 0.99(4x s, 3 H, CMe₂), 2.45, 2.86(2x s, 3 H, NMe₂) 5.68, 5.96, 6.04, 6.90(4x m, 1 H, C₅H₄), 6.98–7.17 (m, 5 H, Ph) ppm.

Experimental details

The structure was solved by direct methods (SHELXS [1]) and refined by full-matrix least-squares based on F² (SHELXL [1]). All hydrogen atoms were included at their calculated positions and treated as riding atoms with the U_iso values assigned to a multiple of U_eq of their bonding carbon atom. The structural drawing and all numerical parameters were obtained with a recent version of the PLATON program [4].

Discussion

The chemistry of group 4 metal complexes bearing bifunctional cyclopentadienyl-ketimide ligands was recently studied aiming at functional group transformations and potential utilization in catalysis [5], [6], [7]. The title compound resulted as an undesired side-product of the conversion of titanium half-sandwich complex [{η⁵-C₅H₄CMe₂CMe₂C(Ph)=N-κN}TiCl(NMe₂)] to the corresponding dichloride using Me₃SiCl in toluene under exclusion of air and moisture [5]. The η²-N,N-dimethylhydroxylaminato moiety (cf. the figure) formed from the original dimethylamido ligand by an unknown mechanism, however, adventitious oxidation by air might be suspected as a plausible pathway since a similar transformation of the dimethylamido ligand by dioxygen was observed for tantalum(V) amido silyl complexes [8]. Related cyclopentadienyl titanium complexes with the η²-hydroxylaminato moiety were previously obtained by different approaches including reactions of Me₂NOSiMe₃ with CpTiCl₃ [9] or N,N-dialkylhydroxylamines or nitroso compounds with methyl titanium precursors [10, 11] .

The title compound crystallizes in a centrosymmetric monoclinic space group P2₁/c as a racemate (the chiral center being situated at the unsymmetrically substituted titanium atom). The Cp-tethered ketimide moiety forms a formally six-membered azametallacycle with a non-planar conformation. The intramolecular tethering causes a significant distorsion of the ketimide from the ideal (close to linear) arrangement for its coordination to titanium. This distorsion can be characterized by the C12—N1—Ti1 angle of 145.62(9)° and it compares well with the values for related structurally characterized half sandwich Ti complexes [{η⁵-C₅H₄CMe₂CMe₂C(Ph)=N-κN}TiCl(NMe₂)] (145.71(8)°) and [{η⁵-C₅H₄CMe₂CMe₂C(Ph)=N-κN}TiCl₂] (146.52(12)°) or the metallocene compound [(η⁵-C₅Me₅){η⁵-C₅H₄CMe₂CMe₂C(Ph)=N-κN}TiCl] (150.16(11)°) [5]. The overall conformation of the Cp-ketimide ligand in the title compound is similar to the mentioned reference complexes. The distance Ti1—N1 1.8971(11) Å is only slightly longer than in [{η⁵-C₅H₄CMe₂CMe₂C(Ph)=N-κN}TiCl(NMe₂)] (1.8877(10) Å). The Ti1—N2 bond of the η²-hydroxylaminato moiety is obviously much longer than in the above dimethylamido complex (2.1473(12) Å vs. 1.8797(10) Å), the value, however, compares reasonably well with that of the complex [(η⁵-C₅H₅)TiCl₂(η²-ONMe₂)] (Ti-N distance of 2.128(2) Å) [9]. The Ti-O distance in the title compound, 1.8796(10) Å, is again only slightly longer than that reported for the discussed reference complex (1.866(1) Å) [9]. It shall be noted that no significant intermolecular interactions were detected in the title structure.