Sterically controlled double nucleophilic addition reactions of (η6-arene)(η6-[2.2]paracyclophane)ruthenium(II) complexes and reactions to form highly fluxional agostic cyclohexenyls

Abstract

Action of the hydride source Na[BH₄] on the (η⁶-arene)(η⁶-[2.2]paracyclophane)ruthenium(II) complexes [Ru(η⁶-C₁₆H₁₆)(η⁶-arene)][BF₄]₂(arene = benzene 1a, p-cymene 1b, 1,2,4,5-tetramethylbenzene 1c, pentamethylbenzene 1d or hexamethylbenzene 1e) results exclusively in the addition of two hydride nucleophiles to the non-cyclophane arene ring, giving the neutral (1,3-diene)ruthenium(0) complexes [Ru(η⁶-C₁₆H₁₆)(η⁴-diene)](diene = C₆Me₆H₂4, C₆Me₅H₃6, C₆ Me₄H₄7 or MeC₆H₆CHMe₂8). Complex 4 is the 1,3-diene isomer of the previously reported 1,4-diene compound [Ru(η⁶-C₁₆H₁₆)(η⁴-3,6-C₆Me₆H₂)]2(formed in the reduction of 1e by Red-Al {Na[AIH₂(OCH₂CH₂OMe)₂]}). In complexes 1a–1e, attack on the [2.2]paracyclophane ligand is not observed, implying that nucleophilic additions to these compounds are not charge controlled. This contrasts with previously reported related reactions which give bis(cyclohexadienyl) complexes. Attempts to prepare functionalised diene complexes derived from 1a and 1e were largely unsuccessful although the functionalised cyclohexadienyl compounds [Ru(η⁶-C₁₆H₁₆)-(η⁵-C₆R₆X)][BF₄]9–11(R = H or Me; X = Me or OMe) are prepared. Deprotonation of the hexamethylbenzene complex 1e results in the formation of the exo-methylene species [Ru(η⁶-C₁₆H₁₆){η⁵-C₆Me₅(CH₂)}][BF₄]12 and [Ru(η⁶-C₁₆H₁₆){η⁴-C₆Me₄(CH₂)₂}]13. Complexes 4, 6 and 7 react with HBF₄ to generate agostic cyclohexenyl compounds [Ru(η⁶-C₁₆H₁₆)(η³-C₆Me_nH_9–n)][BF₄](n= 6 16, 5 20 or 4 21). However, reaction of 2 with HBF₄ gives the exocyclic agostic complex [Ru(η⁶-C₁₆H₁₆){η³-(HCH₂)(CH₂)C₆Me₄H₄}][BF₄]18. In 16 the agostic cyclohexenyl ligand is bound via an endocyclic allylic functionality whereas in the isomer, 18, the metal is bound externally to the ring. Deprotonation of 18 with LiBuⁿ results in the abstraction of the agostic proton to generate [Ru(η⁶-C₁₆H₁₆){η⁴-(CH₂)₂C₆Me₄H₄}]19, which is isomeric with 2 and 4 and contains a saturated C₆Me₄H₄(CH₂)₂ ring bound to the metal centre by two exocyclic olefinic functionalities. The mechanisms for the formation of these compounds have been probed by deuteriation studies and their extensive dynamic behaviour investigated by variable-temperature ¹H NMR spectroscopy.