Synthesis and fluxional behavior of protonated 1,3-diene complexes of ruthenium(0), osmium(0), rhodium(I), and iridium(I) containing 2,3-dimethylene-5,6,7,8-dibenzobicyclo[2.2.2]octane (ddbo, C18H14). Single-crystal x-ray study of the agostic complex [Ru(C18H15)(.eta.-C6H3Me3)]PF6

The 1,3-diene 2,3-dimethylene-5,6,7,8-dibenzobicyclo[2.2.2] octane (ddbo, C18H14, 6) forms complexes Ir(eta-C5H5)(C18H14) (7), M(eta-C5Me5)(C18H14) [M = Rh (8), Ir (9)], and M(eta-1,3,5-C6H3Me3)(C18H14) [M = Ru (10), Os (11)], which react with aqueous HPF6 or triflic acid to give fluxional monoprotonated cations [Ir(eta-C5H5)(C18H15)]+ (14), [Rh(eta-C5Me5)(C18H15)]+ (12), [Ru(eta-C6H3Me3)(C18H15)]+ (13), and [Os(eta-C6H3Me3)(C18H15)]+ (15). Crystals of 13.PF6 are orthorhombic, space group P2(1)2(1)2(1), with a = 14.243 (2) angstrom, b = 13.554 (2) angstrom, c = 12.894 (2) angstrom, and Z = 4. The structure was solved by heavy-atom methods and refined by least-squares analysis to R = 0.034 and R(w) = 0.043 for 2020 unique observed reflections. The C18H15 moiety is bound to a (eta-6-mesitylene)ruthenium unit through a eta-3-enyl linkage [Ru-C(1) = 2.180 (8) angstrom, Ru-C(2) = 2.196 (7) angstrom, Ru-C(17) = 2.148 (7) angstrom] and a two-electron, three-center interaction Ru-H(18C)-C(18) to the terminal methyl group of the butenyl moiety [Ru-C(18) = 2.357 (9) angstrom, C(18)-H(18C) = 0.98 (8) angstrom, Ru-H(18C) = 1.81 (8) angstrom, angle C(17)-C(18)-H(18C) = 108 (5)-degrees, angle C(18)-H(18C)-Ru = 112 (6)-degrees]. The NMR (H-1, C-13) spectra of the rhodium complex 12 at-60-degrees-C are consistent with a similar ground-state agostic eta-3-methallyl structure, but unexpectedly, the H-1 NMR spectrum of 13 at -100-degrees-C resembles that expected for a hydrido (1,3-diene) structure. It is suggested that in 13 reversible C-H bond cleavage occurs more easily than M-H bond cleavage, whereas 12 shows the more usual reverse behavior. The NMR and IR spectra of 14 show this complex to have a hydrido 1,3-diene structure; the same is probably true for 15, although a eta-3-methallyl structure with a strong agostic interaction cannot be excluded. Treatment of 13 and 15 with tert-butyl isocyanide causes irreversible hydride transfer to ddbo, giving [M(eta-C6H3Me3)(C18H15)(CN-t-Bu)]PF6 [M = Ru (17), Os (16)].