Synthesis and Characterization of Heterobimetallic Tantalum-Rhodium and Tantalum-Iridium Complexes Connected by a Tantalacyclopentadiene Fragment

We report the synthesis of heterobimetallic Ta–Rh and Ta–Ir complexes bridged by a 2,5‐di‐tert‐butyltantalacyclopentadiene fragment. A mononuclear 2,5‐di‐tert‐butyltantalacyclopentadiene complex 2 was prepared by the reaction of (η2‐Me3SiC≡CSiMe3)TaCl3(dme) (1) with excess amounts of 3,3‐dimethylbut‐1‐yne in the presence of AlCl3. The tantalacyclopentadiene moiety of complex 2 served as a η4‐diene unit for coordinating the Rh and Ir centers; treatment of 2 with [M(μ‐Cl)(cod)]2 (M = Rh and Ir; cod = cycloocta‐1,5‐diene) in toluene gave TaRh(μ‐C4H2tBu2)Cl4(cod) (3) and [TaIr(μ‐C4H2tBu2)Cl4]2 (5), respectively. The X‐Ray diffraction study of 3 revealed a dative bond from an electron‐rich Rh toward an electron‐deficient Ta. Upon dissolving 3 in THF, [(thf)TaRh(μ‐C4H2tBu2)Cl3]2(μ‐Cl)2 (4) was isolated together with free cycloocta‐1,5‐diene. When complex 5 was treated with 1,2‐bis‐(diphenylphosphino)ethane (dppe), a monomeric Ta–Ir complex, TaIr(μ‐C4H2tBu2)Cl4(dppe) (6), was isolated. Ta–Rh and Ta–Ir heterobimetallic complexes 3 and 6 were reduced by a two‐electron process upon reaction with 2,3,5,6‐tetramethyl‐1,4‐bis(trimethylsilyl)‐1,4‐dihydropyrazine (7a: Si‐Me4‐DHP) or 2,5‐dimethyl‐1,4‐bis(trimethylsilyl)‐1,4‐dihydropyrazine (7b: Si‐Me2‐DHP) to afford the corresponding complexes TaM(μ‐C4H2tBu2)Cl2(L) (8: M = Rh, L = cod; 9: M = Ir, L = dppe), where the metallacycle moiety was assigned to have a tantalacyclopentadiene fragment with a large contribution of a tantalacyclopentatriene canonical form.