Intramolecular aromatic C -H bond activation in aryloxide ligands by niobium(V) and tantalum(V)

Robert Wayne Chesnut, Purdue University

Abstract

Compounds of formula M(OAr)$\sb{\rm n}$R$\sb{\rm {5-n}}$ (M = Nb, Ta; OAr = 2,6-diphenylphenoxide; R = alkyl; n = 2, 3) were synthesized and characterized by microanalysis, $\sp1$H NMR and $\sp{13}$C NMR. These complexes were studied for cyclometalation reactions. M(OAr)$\sb3$(CH$\sb3)\sb2$ cyclometalated to yield (ArO)$\sb2$M(O-C)(CH$\sb3$) and CH$\sb4$ (O-C = OC$\sb6$H$\sb3$PhC$\sb6$H$\sb4$) at temperatures ranging from 112$\sp\circ$ to 150$\sp\circ$. Similarly M(OAr)$\sb2$(CH$\sb3)\sb3$ cyclometalated to yield M(OAr)$\sb2$(O-C) (CH$\sb3$) at 200$\sp\circ$. The benzyl compounds Ta(OAr)$\sb2$(CH$\sb2$-C$\sb6$H$\sb4$-CH$\sb3)\sb3$ and Ta(OAr)$\sb3$(CH$\sb2$C$\sb6$H$\sb4$-CH$\sb3)\sb2$ cyclometalated at lower temperatures (100$\sp\circ$-175$\sp\circ$) than did the corresponding methyl compounds. Attempts to prepare Ta(OAr)$\sb3$(n-C$\sb4$H$\sb9)\sb2$ led to formation of (ArO)$\sb2$Ta(O-C) (n-C$\sb4$H$\sb9$) at room temperature. The mechanism of cyclometalation was examined by using Ta(OAr)$\sb3$-(CH$\sb3)\sb2$ cyclometalation as a model. An isotopic labelling study demonstrated that there is no methylidene intermediate in the reaction. Activation parameters for the reaction are $\Delta$H$\sp{\ddagger}$ = 25.3 $\pm$ 0.8 kcal mol$\sp{-1}$ and $\Delta$S$\sp{\ddagger}$ = $-$15 $\pm$ 5 cal mol$\sp{-1}$ K$\sp{-1}$. The negative $\Delta$S$\sp{\ddagger}$ is consistent with an electrophilic aromatic substitution mechanism or a 4-center, 4-electron mechanism. A series of asymmetrically substituted triarylphenols was prepared for use in studying substituent effects on the cyclometalation of Ta(Oar$\sp\prime$-X)$\sb2$(CH$\sb2$-C$\sb6$H$\sb4$CH$\sb3)\sb3$ (OAr$\sp\prime$-X = 2,4-Ph$\sb2$-C$\sb6$H$\sb2$-C$\sb6$H$\sb4$X; X = H, CH$\sb3$, OCH$\sb3$, Cl). Substituents cause a ring metalation preference in the order Cl $>$ OCH$\sb3$ $>$ CH$\sb3$ $>$ H. Since this order is inconsistent with an electrophilic substitution mechanism, a 4-center, 4-electron process is the probable mechanism. The observed substituent order is a reflection of the ease with which the substituted ring conjugates with the central aryloxide ring to produce a coplanar ring conformation.

Degree

Ph.D.

Advisors

Rothwell, Purdue University.

Subject Area

Chemistry

COinS