Wavelength-, Medium-, and Temperature-Dependent Competition between Photosubstitution and Photofragmentation in Ru₃(CO)₁₂and Fe₃(CO)₁₂: Detection and Characterization of Coordinatively Unsaturated M₃(CO)₁₁Complexes

Irradiation of 0.1 mM Ru₃(CO)₁₂(X = 313 nm) or 0.02 mM Fe₃(CO)₁₂(λ = 366 nm) in a methylcyclohexane or 2-methyltetrahydrofuran (2-MeTHF) glass at 90 K yields loss of one CO as the only IR detectable photoreaction to yield products formulated as M₃(CO)11 or M₃(CO)₁₁(2-MeTHF), respectively. An initially observed axially vacant form of Ru₃(CO)₁₁(II) having no bridging CO's rearranges at 90 K to an axially vacant form (III), having at least one bridging CO, also adopted by Fe₃(CO)₁₁in an alkane glass. An initially observed, equatorially substituted form of Ru₃(CO)₁₁(2-MeTHF) (I') rearranges at 90 K to III or a 2-MeTHF adduct of III. I' is extremely photosensitive with respect to further substitution by 2-MeTHF for up to three CO ligands. Ru₃(CO)₁₁(III) reacts with N₂or^I3CO to yield Ru₃(CO)₁₁(N₂) or axial-¹³CO-Ru₃(CO)₁₁(¹³CO) complexes, respectively. Ru₃(CO)₁₁and Fe₃(CO)₁₁react with C₂H₄to yield M₃(CO)₁₁(C₂H₄) complexes. M₃(CO)₁₁(III) reacts with PPh₃to yield Ru₃(CO)₁₁(PPh₃) at 298 K and Fe₃(CO)₁₁(PPh₃) at 195 K. Long wavelength excitation of Ru₃(CO)₁₂(X = 366 nm) or Fe₃(CO)₁₂(X = 436 nm) yields negligible photochemistry in alkane or 2-MeTHF glasses but yields associative photosubstitution of C₂H₄, C₅H_l0, and¹³CO but not N₂or 2-MeTHF for CO at 90 K. Long wavelength (X > 540 nm) excitation of Fe₃(CO)12 yields no photochemistry at 90 K but gives asymmetric fragmentation in C₂H₄-containing alkane solutions at 298 K to yield 1 equiv each of Fe(CO)₅, Fe(CO)₄(C₂H₄), and Fe(CO)₃(C₂H₄)₂; competitive photosubstitution occurs in the presence of PPh₃to yield Fe₃(CO)₁₁(PPh₃). At 195 K, the Fe₃(CO)₁₁L/Fe(CO)₅__n(L)n (L = C₂H₄, PPh₃; n = 0–2) product ratios increase with decreasing irradiation wavelength. Long wavelength (X > 420 nm) irradiation of 0.2 mM Ru₃(CO)₁₂in 195 K alkane solutions containing excess L = CO or C₂H₄initially yields 1 equiv each of Ru(CO)₄L and Ru₂(CO)₈L; Ru₂(CO)₈(C₂H₄) fragments at 195 K to yield 2 more equiv of Ru(CO)₄(C₂H₄). Long wavelength irradiation of Ru₃(CO)i₂in PPh₃-containing solutions at 195 K yields conversion to a CO-bridged product which reacts thermally at 195 K to form Ru₃(CO)_n(PPh₃), in competition with Ru₃(CO)₁₂regeneration; Ru(CO)₄(PPh₃) and Ru(CO)₃(PPh₃)₂are only observed as secondary photoproducts at 195 K. The low-temperature photochemistry of Ru₃(CO)₁₂is discussed in terms of a wavelength-dependent competition between dissociative loss of equatorial CO from higher energy excitation and generation of a nonradical, reactive isomer of Ru₃(CO)₁₂from long wavelength excitation. Implications of the new results for the photocatalyzed isomerization of 1-pentene to cis- and trans-2-pentene by M₃(CO)₁₂(M = Ru, Fe) precursors are discussed.