Intramolecular Dehydrogenation of Alkyl Groups at Niobium Aryloxide Centers: Bonding and Reactivity of the Ensuing Niobacyclopropane Ring

Abstract

The niobium dichloride [Nb(OC6H3Pri2-2,6)3Cl2] (OC6H3Pri2-2,6 = 2,6-diisopropylphenoxide) undergoes reduction (2 Na per Nb)) in tetrahydrofuran (thf) solvent to produce the dark green complex [Nb(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)2(thf)] (1). The solid-state structure of 1 shows the α-methylvinyl group to be strongly η2-bound to the niobium metal center. The dehydrogenation of the ortho-isopropyl group of an aryloxide group to generate 1 is argued to proceed via CH bond activation within a d2-Nb(III) aryloxide followed by β-hydrogen abstraction/elimination of H2. The thf ligand in 1 can be substituted by pyridine (py) and ButNC to yield the adducts [Nb(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)2(L)2] (L = py, 2; L = ButNC, 3). The solid-state structure of 3 shows both isocyanide ligands to be terminally bound with no interaction with the η2-olefin fragment. The spectroscopic properties of these three adducts give insight into the degree of π-back-bonding between the metal and the chelated olefin fragment. The metallacyclopropane ring in 1 will undergo ring expansion (coupling of the olefin) with a variety of unsaturated substrates. The addition of Ph2CO leads to a 2-oxaniobacyclopentane ring in 4 while PhC⋮CPh produces a niobacyclopent-3-ene derivative, [Nb(OC6H3Pri-CMeCH2CPhCPh)(OC6H3Pri2-2,6)2] (5), which was structurally characterized. In the case of addition of PhC⋮CH to 1 the initial, kinetic product 6a, possibly containing a 2-phenylniobacyclopent-2-ene ring, is thermally isomerized to the 3-phenylniobacyclopent-2-ene compound 6b. With the protic reagents HOC6H3Pri2-2,6 and PhNH2 the compounds [Nb(OC6H3PriCMe2)(OC6H3Pri2-2,6)3] (8) and [Nb(OC6H3PriCMe2)(OC6H3Pri2-2,6)2(NHPh)] (9) are produced. The solid-state structure of 8 confirmed the presence of the cyclometalated aryloxide formed via protonation of the methylene group in 1.