Intramolecular Coupling of η2-Iminoacyl and η2-Acyl Fuctions af Group 4 and Group 5 Metal Centers: Structure and Spectroscopic Properties of the Resulting Enamidolate and Enediamide Complexes

Article abstract of DOI:10.1021/ja00254a027

A series of group 4 and group 5 metal aryloxide compouds containing enamidolate or enediamide ligation have been prepared by the intramolecular coupling of η²-acyl and η²-iminoacyl functional groups.For the group 4 metal reaction of the mono-iminoacyl's (ArO)2M(η²-R'NCR)(R) (M = Ti, Ar = 2,6-diisopropylphenyl, R = CH2Ph; M = Zr, Ar = 2,6-di-tert-butylphenyl, R = CH2Ph) with carbon monoxide (700-1000 psi) leads directly to the corresponding enamidolate (I) with no evidence for the presumed intermediate mixed acyl, iminoacyl compound.However, the isolable bis η²-iminoacyl compounds (ArO)2M(η²-R'NCR)2 (M = Ti, Zr, Hf; ArO = 2,6-diisopropyl-, 2,6-diphenyl-,and 2,6-di-tert-butylphenyl; R = CH3, CH2Ph) only undergo intramolecular coupling to the corresponding enediamide derivatives (II) on thermolysis in hydrocarbon solvents.In contrast, attempts to prepare a bis η²-iminoacyl derivative of Ti(OAr-2,6-Ph2)2(η²-PhNCCH2SiMe3)(CH2SiMe3) (OAr-2,6Ph2 = 2,6-diphenylphenoxide) by addition of PhNC led only to the isolation of a tris-inserted, coupled complex Ti(OAr-2,6Ph2)2 (III) (R = CH2SiMe3) containig an enediamide chelate with an imine substituent on the carbon backbone.The tantalum bis η²-iminoacyl Ta(OAr-2,6Me2)3(η²-xyNCCH2Ph)2 undergoes the clean formation of the enediamide Ta(OAr-2,6Me2)3 (IV) on thermolysis at 110 deg C.Single-crystal X-ray diffraction studies on six compounds of types I, II, III, and IV have been carried out.In the enamidolate compounds Ti(OAr-2,6Prⁱ2)2t> (Ia) and Zr(OAr-2,6-Bu^t2)2 (Ib) both molecules were found to contain a pseudotetrahedral geometry about the metal.The 5-membered metallacycle formed by the enamidolate chelate ring was found to be nonplanar with the metal atom lying out of the plane of the OC=CN backbone.Despite fold angles of 51.1 deg and 50.0 deg, respectively, solution NMR spectra indicated that inversion of the bent metallacycle was facile.For the related group 4 metal enediamide compounds Ti(OAr-2,6Prⁱ2)2t> (IIc), Zr(OAr-2,6Bu^t2)2 (IId) and Ti(OAr-2,6Ph2)2 (III) (R = CH2SiMe3) a similar, nonplanar geometry for the chelate ring is observed.However, in this case a significant barrier (12-17 kcal mol^-1) to inversion of the bent metallacycle ring is indicated by analysis of the temperature dependence of the ¹H NMR spectra of compounds of type II and III.In contrast the compound Ta(OAr-2,6Me2)3 (IV) was found to contain an essentially planar metallacycle, the two nitrogen atoms of the enediamide chelate occupying one axial and one equatorial position about the 5-coordinate, trigonal-bipyramidal, metal atom.Structural parameters for all of the coupled products obtained in this study are totally consistant with them being viewed as containing enamidolate and enediamide ligands bound to Ti^IV, Zr^IV, and Ta^V, d⁰-metal centers with little support for an alternate situation...