Synthesis and Reactivity of Oxametallacyclic Niobium Compounds by Using α,β‐Unsaturated Carbonyl Ligands

Reduction of mono(cyclopentadienyl)niobium complexes [NbCp R Cl 4 ] [Cp R = C 5 Me 4 H ( 1 ), C 5 H 4 SiMe 2 Cl ( 2 ), C 5 H 4 SiMe 3 ( 3 )] with Na/Hg in the presence of methyl methacrylate [MMA, CH 2 =C(Me)C(O)OMe ( a )], methyl acrylate [MA, CH 2 =CHC(O)OMe ( b )] and mesityl oxide [MO, CMe 2 =CHC(O)Me ( c )] afforded the corresponding derivatives [NbCp R Cl 2 (LL)] [Cp R = C 5 Me 4 H, LL = MMA ( 1a ); Cp R = C 5 H 4 SiMe 2 Cl, LL = MMA ( 2a ), MA ( 2b ), MO ( 2c ); Cp R = C 5 H 4 SiMe 3 , LL = MMA ( 3a ), MA ( 3b )] in variable yields depending on both the cyclopentadienyl and the α,β‐unsaturated carbonyl compounds. The reactivity of these complexes was studied toward protic and Lewis acids. Addition of triflic acid TfOH (Tf = CF 3 SO 2 ) to 3b gave the triflate complex [NbCp R Cl 2 {(CH 2 ) 2 C(O)OMe}(OTf)] [Cp R = C 5 H 4 SiMe 3 ( 4 )]. The Lewis acids E(C 6 F 5 ) 3 (E = B, Al) reacted with complexes 2b and 3b to give the three‐membered metallacyclic (or η 2 ‐enone) compounds [NbCp R Cl 2 {η 2 ‐CH 2 =CHC(OMe){O · E(C 6 F 5 ) 3 }}] [Cp R = C 5 H 4 SiMe 2 Cl, E = B ( 5 ), Al ( 6 ); Cp R = C 5 H 4 SiMe 3 , E = B ( 7 ), Al ( 8 )], which decomposed to the corresponding adducts MA · E(C 6 F 5 ) 3 . The same reaction with the 2a and 3a derivatives only allowed the observation of the corresponding adducts MMA · E(C 6 F 5 ) 3 . Complexes 2a , b and 3a , b reacted with CO with elimination of the acrylate compounds, MA and MMA, respectively, to give the carbonylniobium(III) compounds [NbCp R Cl 2 (CO) 2 ] 2 [Cp R = C 5 H 4 SiMe 2 Cl ( 9 ), C 5 H 4 SiMe 3 ( 10 )]. Analogous reactions with CNAr showed the elimination of the free MA and MMA compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)