Intramolecular dehydrogenation of alkyl groups at niobium aryloxide centers: Bonding and reactivity of the ensuing niobacyclopropane ring

The niobium dichloride [Nb(OC6H3Pr2i-2,6)(3)Cl-2] (OC6H3Pr2i-2,6 = 2,6-diisopropylphenoxide) undergoes reduction (2 Na per Nb)) in tetrahydrofuran (thf) solvent to produce the dark green complex [Nb(OC6H3Pri-eta(2)-CMe=CH2)(OC6H3Pr2i-2,6)(2)(thf)](1). The solid-state structure of 1 shows the alpha-methylvinyl group to be strongly eta(2)-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 d(2)-Nb(III) aryloxide followed by beta-hydrogen abstraction/elimination of H-2. The thf ligand in 1 can be substituted by pyridine (py) and Bu(t)NC to yield the adducts [Nb(OC6H3Pri-eta(2)-CMe=CH2)(OC6H3Pr2i-2,6)(2)(L)(2)] (L = py, 2; L = Bu(t)NC, 3). The solid-state structure of 3 shows both isocyanide ligands to be terminally bound with no interaction with the eta(2)-olefin fragment; The spectroscopic properties of these three adducts give insight into the degree of pi-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 Ph(2)C=O leads to a 2-oxaniobacyclopentane ring in 4 while PhC=CPh produces a niobacyclopent-3-ene derivative, [Nb(OC6H3Pri-CMeCH(2)CPh=CPh)(OC6H3Pr2i-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 HOC6H3Pr2i-2,6 and PhNH(2) the compounds [Nb(OC(6)H(3)Pr(i)CMe(2))(OC6H3Pr2i-2,6)(3)] (8) and [Nb(OC(6)H(3)Pr(i)CMe(2))(OC6H3-Pr-2(i)-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.