Reactions of co-ordinated ligands. Part 42. The synthesis and reactions of cationic η4-tetraphenylcyclobutadieneruthenium complexes; X-ray crystal structures of [Ru(NCMe)(η4-C4Ph4)(η-C5H5)][BF4] and [Ru{η5-CH(Ph)C(Ph)C(Ph)C(Ph)O}(η-C5H5)]

Abstract

Addition of Ag[BF₄] to a solution of [Ru₂(CO)₄(η-C₅H₅)₂] and PhC₂Ph in CH₂Cl₂ results in the formation of the η⁴-cyclobutadiene complex [Ru(CO)(η⁴-C₄Ph₄)(η-C₅H₅)][BF₄](1). U.v. irradiation of a solution of (1) in MeCN gives [Ru(NCMe)(η⁴-C₄Ph₄)(η-C₅H₅)][BF₄](2), structurally identified by X-ray crystallography. The η⁴-tetraphenylcyclobutadiene ligand is not co-ordinated symmetrically to the ruthenium as is indicated by Ru–C distances of 2.192(2), 2.208(3), 2.177(2), and 2.175(2)Å, a distortion which is related to the frontier orbitals presented by the d⁶ Ru(η-C₅H₅)L⁺ fragment (L = MeCN). The mechanism of formation of (1)is discussed. Tetramethylammonium bromide and chloride also react with (1) on u.v. irradiation in CH₂Cl₂ to form [RuX(η⁴-C₄Ph₄)(η-C₅H₅)][X = Br (4) or Cl (5)]. These complexes are also formed when (2) is reacted thermally with the corresponding halide anions. A related reaction occurs between (2) and the thiolate anion of Na[SC₆H₄Me-4] forming [Ru(SC₆H₄Me-4)(η⁴-C₄Ph₄)(η-C₅H₅)], however, the presence of water in this latter reaction leads to an unusual ring-opening reaction and the formation of [Ru{η⁵-CH(Ph)C(Ph)C(Ph)C(Ph)O}(η-C₅H₅)](7) whose structure was elucidated by X-ray crystallography. A η⁵(5 e)-bonded butadienolato ligand has been generated by nucleophilic attack of hydroxide ion on the η⁴-C₄Ph₄ ligand leading to ring-opening of an η³-cyclobutenyl complex. Protonation of (7) affords [Ru{η⁶-C₆H₅(CH₂CPhCPhCOPh)}(η-C₅H₅)][BF₄](8). Mixed C₅–C₆ sandwich cations are also formed on u.v. irradiation of (1) in the presence of alkynes RC₂R, and in this way [Ru(η⁶-C₆Ph₆)(η-C₅H₅)][BF₄](9), [Ru(η⁶-C₆Ph₅Me)(η-C₅H₅)][BF₄](10), [Ru(η⁶-C₆Ph₄Me₂)(η-C₅H₅)][BF₄](11), and [Ru{η⁶-C₆Ph₄(CO₂Me)₂}(η-C₅H₅)][BF₄](12) are obtained in good to moderate yield. Complex (9) is also formed on reaction of (5) with PhC₂Ph and Ag[BF₄], providing insight into this novel ring-expansion reaction. The related cationic complex [Ru(η⁶-C₆Et₆)(η-C₅H₅)][BF₄](13) is formed in low yield on reaction of EtC₂Et with [Ru₂(CO)₄(η-C₅ H₅)₂] and Ag[BF₄]. The low-temperature 100.6-MHz ¹³C-{¹H} n.m.r. spectrum of (13) is discussed in terms of rotational movement of the ethyl groups. Treatment of (9) and (11) with K[BHBu^s₃] leads to selective attack on the η-cyclopentadienyl ring and the formation of [Ru(η⁴-C₅H₆)(η⁶-C₆Ph₆)](14) and [Ru(η⁴-C₅H₆)(η⁶-C₆Ph₄Me₂)](15). The reduction of (9) with Na–Hg in thf gives (14) and [Ru(η⁵-exo-C₆Ph₆H)(η-C₅H₅)].