Synthesis, Structure, and Reactivity of (η⁴(5e)-Butadienyl)ruthenium Complexes: Crystal Structures of CpRu=C(Ph)-η³-(C(Ph)C(Ph)CH(Ph)), CpRuC(Ph)=C(Ph)-η²-(C(Ph)=CH(Ph))P(OMe)₃, and CpRu₂(µ-(Z)-C(Ph)=CH(Ph))(CO)₂(η⁴-C₄Ph₄)

The chemistry of the first example of a η⁴(5e)-butadienyl complex is described. Reaction of [CpRu-(NCMe)(η⁴-C₄Ph₄)[BF₄] with K[BHBu^s₃] affords CpRu═C(Ph)-η³-(C(Ph)C(Ph)CH(Ph)) (5), which was structurally identified by single-crystal X-ray crystallography. Compound 5 is triclinic, space group P1 (No. 2); a = 10.156 (5), b = 10.996 (5), c = 13.145 (6) Å, α = 112.12 (4), β = 101.20 (4), γ = 103.76 (4)°. The molecule contains an open-chain η⁴(5e)-butadienyl system, in which the CpRu fragment is double bonded to C(1) (Ru–C(1) = 1.896 (5) Å), the remaining three carbon ruthenium distances Ru–C(2) (2.204 (5) Å), Ru–C(3) (2.152 (4) Å), and Ru–C(4) (2.154 (6) Å) being appropriate for an η³-allyl system. A mechanism is proposed for the formation of 5 involving conrotatory ring-opening of an η³-cyclobutenyl intermediate carrying an endo hydrogen, which becomes a pseudo-syn hydrogen on the terminus of the C₄ chain. The complex 5 together with the partially characterized (Ru(PPh₃)(η⁴-C₄Ph₄))_x is also obtained by reacting [CpRu(PPh₃)(η⁴-C₄Ph₄)][BF₄] with K[BHBu^s₃]. In contrast, the same reaction in the presence of CO or P(OMe)₃ affords RuL₂(PPh₃)(η⁴-C₄Ph₄) (L = CO or P(OMe)₃). This is rationalized in terms of the intermediacy of Ru(PPh₃)(η⁴-C₄Ph₄)(η⁴-C₅H₆). In support of this the complex Ru(P(OMe)₃)(η⁴-C₄Ph₄)(η⁴-C₅H₆) (9) is formed as a stable crystalline material by reaction of [CpRu(P(OMe)₃)(η⁴-C₄Ph₄)][BF₄] with “H^-”. Thermolysis of 9 affords initially the isomeric complex CpRuC(Ph)═C(Ph)-η²-(C(Ph)═CH(Ph))P(OMe)₃ (12), which is also obtained by direct reaction of 5 with P(OMe)₃. The solid-state structure of 12 was established by X-ray crystallography. Compound 12 is monoclinic, space group Cc (No. 9); a = 18.226 (6), b = 12.190 (4), c = 27.447 (15) Å, α = 90, β = 94.31 (4), γ = 90°. The molecule is an η³(3e)-butadienyl complex, and significantly, along with the η⁴(5e) to η³(3e) transformation of the bonding mode of the butadienyl ligand, a stereomutation at C(4) has occurred on reaction of 5 with P(OMe)₃. Thermolysis of 9 finally leads to the formation of a mixture of η²-bonded hexatrienyl complexes 10 and 11 formed via an apparent orthometalation reaction. Additionally the reaction of hydride anion sources with [CpRu-(CO)(η⁴-C₄Ph₄)][BF₄] (1) was examined, leading to the formation of CpRu═C(Ph)-η³-(C(Ph)C(Ph)C-(CHO)Ph) (15) together with the X-ray crystallographically identified dinuclear complex CpRu₂(µ-CO)(µ-(Z)-C(Ph)═CH(Ph))(CO)₂(η⁴-C₄Ph₄) (16). Compound 16 is monoclinic, space group P2₁/c (No. 14); a = 15.763 (11), b = 15.928 (9), c = 18.914 (9) Å, α = 90, β = 112.58 (5), γ = 90°. The formation of 16 is rationalized in terms of the fragmentation of a cyclobutadiene complex.