Mechanism for Iron-Catalyzed Alkene Isomerization in Solution

Abstract

ABSTRACT.Here we report nano-through microsecond time-resolved IR experiments of iron-catalyzed alkene isomerization in room-temperature solution. We have monitored the photochemistry of a model system, Fe(CO) 4 (η 2 -1-hexene), in neat 1-hexene solution. UV-photolysis of the starting material leads to the dissociation of a single CO to form Fe(CO) 3 (η 2 -1-hexene), in a singlet spin state. This CO loss complex shows a dramatic selectivity to form an allyl hydride, HFe(CO) 3 (η 3 -C 6 H 11 ), via an internal C-H bondcleavage reaction in 5-25 ns. We find no evidence for the coordination of an alkene molecule from the bath to the CO loss complex, but do observe coordination to the allyl hydride, indicating that it is the key intermediate in the isomerization mechanism. Coordination of the alkene ligand to the allyl hydride leads to the formation of the bis-alkene isomers, Fe(CO) 3 (η 2 -1-hexene)(η 2 -2-hexene) and Fe(CO) 3 (η 2 -1-hexene) 2 . Because of the thermodynamic stability of Fe(CO) 3 (η 2 -1-hexene)(η 2 -2-hexene) over Fe(CO) 3 (η 2 -1-hexene) 2 (ca. 12 kcal/mol), nearly 100% of the alkene population will be 2-alkene. The results presented herein provide the first direct evidence for this mechanism in solution and suggest modifications to the currently accepted mechanism.

Investigation of organometallic reaction mechanisms with one and two dimensional vibrational spectroscopy

“…where E(ω) is the incident electric field of the 800 nm fundamental, χ (2) is the second-order nonlinear susceptibility of the BBO crystal, and P (2) is the induced polarization [108,109]. The nonlinear process typically results in ~20% conversion of the 800 nm pulses (~200 µJ) into 400 nm pulses (~40 µJ).…”

“…Pump-probe spectroscopy has been extensively used to understand these types of dynamics [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], and this work applies the same pump-probe methodologies as well as a relatively new technique, two dimensional infrared spectroscopy, to the photophysics and reactions of several prototypical organometallic complexes [20][21][22][23][24][25][26].…”

“…17-electron (17e) metal-centered radicals are the most common transition-metal radicals and can be photochemically generated from organometallic [144,146,147]. These same dimers were also observed to participate in ligand substitution reactions to produce the products [CpM(CO) 2 …”

“…2 The 800 nm beam is rotated with a half-wave plate to a vertical polarization before being overlapped with the 400 nm beam into the BBO crystal. This process generates horizontally polarized 266 nm laser pulses with an energy of ~5 µJ.…”

“…The vertically polarized 400 nm beam (n e = 1.57; n o = 1.69) is polarized ~30˚ from the crystal axis to produce an effective refractive index of 1.66, satisfying the phase-matching condition n 400nm = n 800nm . 2 The vertically polarized 800 nm (n o = 1.66) and 400 nm (n o = 1.69) beams are polarized along the ordinary direction. The horizontally polarized 266 nm beam (n e = 1.61; n o = 1.76) is polarized ~45˚ from the crystal axis to produce an effective refractive index of 1.68, satisfying the phase-matching condition n 266nm = 2 ⁄ 3 n 400nm + 1 ⁄ 3 n 800nm .…”