A Novel Facet of Carbonyliron-Diene Photochemistry:  The η⁴-s-transIsomer of the Classical Fe(CO)₃(η⁴-s-cis-1,3-butadiene) Discovered by Time-Resolved IR Spectroscopy and Theoretically Examined by Density Functional Methods

Abstract

The photolysis of Fe(CO)3(η4-s-cis-1,3-butadiene) (1) and Fe(CO)4(η2-1,3-butadiene) (2), formerly studied in low-temperature matrixes, is reexamined in cyclohexane solution at ambient temperature using time-resolved IR spectroscopy in the ν(CO) region. Flash photolysis of 2 (λexc = 308 nm) generates Fe(CO)3(η4-s-trans-1,3-butadiene) (5) as a transient product, which then rearranges to form the classical η4-s-cis-1,3-butadiene complex 1. Species 5, previously addressed as the coordinately unsaturated Fe(CO)3(η2-1,3-butadiene) (3), is also photogenerated from 1, in this case along with the very short-lived CO loss fragment Fe(CO)2(η4-1,3-butadiene) (τ < 4 μs under CO atmosphere). It decays by temperature-dependent first-order kinetics (τ = 13 ms at 25 °C; ΔH ⧧ = 17.3 kcal·mol-1) with nearly complete recovery of 1. According to density functional calculations at the BP86 level of theory, 5 resides in a distinct energy minimum, 20.3 kcal·mol-1 above 1 and separated from it by a barrier of 15.0 kcal·mol-1. Its computed structure involves a diene dihedral angle of 129°. Species 3 (with a diene dihedral angle of −150.1°), by contrast, is predicted to exist in a rather flat minimum, which makes it too short-lived for detection with our instrumentation. Flash photolysis of Fe(CO)5 generates the very short-lived (<1 μs) doubly unsaturated Fe(CO)3(solv) species in addition to the familiar Fe(CO)4(solv) fragment (τ = 10−15 μs), Fe2(CO)9 being the ultimate product in the absence of potential trapping agents other than CO. Deliberate contamination of the system with water gives rise to the formation of Fe(CO)4(H2O) as a longer lived transient (ca. 1 ms). In the presence of 1,3-butadiene, both 2 and 5 appear almost instantaneously. The latter decays, again in the millisecond time range, with formation of 1, thus providing clear evidence of a single-photon route from Fe(CO)5 to 1 in addition to the established two-photon sequence via the monosubstituted complex 2.

μ-η2:2-s-trans-Butadieneoctacarbonyldiiron(0) – Structural and spectroscopic properties

“…The major product in the first step is the mononuclear iron(0) complex [Fe(CO) 4 (h 2 -s-trans-C 4 H 6 )] ( Table 3). The signals of this intermediate complex slowly decreased and two signals at 85.8 and 40.9 ppm were observed that are assigned to the C atom of the CH and the CH 2 group of [Fe(CO) 3 (h 4 -s-cis-1,3-butadiene)], respectively, in close agreement to data reported in the literature 85.6 and 40.8 ppm [6]. …”

Tricarbonyl[η4-2-phenyl-4-(3-phenylallylidene)-4H-oxazol-5-one]iron(0)

“…An s-trans configuration of dienes coordinated to transition metals is much less common and has been reported for Cp 2 M (M = Zr and Hf) and CpMo(NO) moieties (Erker et al, 1985;Yasuda et al, 1985;Hunter et al, 1985Hunter et al, , 1986. Nevertheless, the corresponding (s-trans-butadiene)-Fe(CO) 3 has recently been observed by time-resolved IR spectroscopy and has been calculated by high-level DFT methods (Bachler et al, 2003).…”

The mechanistic investigations of photochemical decarbonylations and oxidative addition reactions for M(CO)5 (M = Fe, Ru, Os) complexes

“…This theoretical observation is in good agreement with many experimental results. 2,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][33][34][35][36][37] For the Ru(CO) 5 and Os(CO) 5 model compounds, both ruthenium and osmium pentacarbonyls have similar photochemical and thermal potential energy proles. The computations also show that both Ru(CO) 4 and Os(CO) 4 fragments feature a singlet electronic ground state.…”

“…[29][30][31][32] As a consequence, the study of the photochemistry of Fe(CO) 5 has been the subject of a great many experimental and theoretical studies that seek to determine the fundamental aspects of its reaction chemistry. [33][34][35][36][37] Along the entire group 8 metal triad (Fe, Ru, and Os), the photochemistry of Ru(CO) 5 and Os(CO) 5 has not been the subject of extensive study. [38][39][40][41][42][43][44][45][46][47] The reason for this could be that iron is cheap and environmentally benign but ruthenium and osmium are expensive and toxic.…”

“…[11][12][13][14][38][39][40][41][42][43][44][45][46][47] The activation of the Si-H bond in alkylsilane by this a d 8 organometallic photoproduct Fe(CO) 4 has been studied using picosecond time-resolved infrared (TRIR) spectroscopy. [62][63][64][65][66] Therefore, if the reactants and products have different spin states, the way in which the species in a particular spin state transform into the nal products of the same spin state by way of several intermediates of different spin state and the spin crossovers is of interest.…”