Low-temperature photochemistry of (eta/sup 5/-C/sub 5/R/sub 5/)Fe(CO)/sub 2/Mn(CO)/sub 5/ (R = H, Me): substitution by P-donor ligands and kinetics of thermal Fe-Mn bond homolysis
Low-temperature irradiation of (eta/sup 5/-C/sub 5/R/sub 5/)Fe(CO)/sub 2/Mn(CO)/sub 5/ (R = H,Me) results in loss of CO as the only detectable photoprocess (Phi/sub app/ for CO = 10/sup -3/ at 313 nm, and Phi/sub 313//Phi/sub 366/ = 20 at 93 K) and yields a coordinatively unsaturated dinuclear photoproduct, (eta/sup 5/-C/sub 5/R/sub 5/)FeMn(CO)/sub 6/. It should be appreciated that CO loss and Mn-Fe bond cleavage are competitive processes with CO loss far more dominant at low temperature in a rigid glass. Warming of a glass containing (eta/sup 5/-C/sub 5/R/sub 5/)FeMn(CO)/sub 6/ in the presence of PR/sub 3/ (R = Ph, OPh) results in formation of (eta/sup 5/-C/sub 5/R/sub 5/)Fe(CO)/sub 2/Mn(COP)/sub 4/PR/sub 3/. Spectroscopic evidence, including IR, UV-vis, NMR, and MS, supports the conclusion that substitution of CO by PR/sub 3/ in (eta/sup 5/-C/sub 5/R/sub 5/)Fe(CO)/sub 2/Mn(CO)/sub 5/ occurs exclusively on the Mn atom. It was further determined that Phi/sub diss/ for (eta/sup 5/-C/sub 5/H/sub 5/)Fe(CO)/sub 2/Mn(CO)/sub 5/ at room temperature in the presence of P(OPh)/sub 3/ in CCl/sub 4/ is 0.89 +/- 0.08 and 0.89 +/- 0.1 at 313 and 366 nm, respectively, and Phi/sub app/ for (eta/sup 5/-C/sub 5/H/sub 5/)Fe(CO)/sub 2/Mn(CO)/sub 2/Mn(CO)/sub 4/P(OPh)/sub 3/ is 0.26 +/- 0.03 and 0.29 +/- 0.04 at 313 and 366 nm, respectively, in the same solution. The products (eta/sup 5/-C/sub 5/R/sub 5/)Fe(CO)/sub 2/Mn(CO)/sub 4/PPh/sub 3/ are labile with respect to Fe-Mn bond cleavage and a kinetic analysis yielded activation parameters for this thermal reaction: ..delta..H/sup double dagger/ = 106 +/- 6 and 84.3 +/- 4 kJ mol/sup -1/ for R = H, Me, respectively, and ..delta..S/sup double dagger/ = 41.5 +/- 40 and -21.4 +/- 23 J mol/sup -1/ K/sup -1/ for R = H and Me, respectively.
- Research Organization:
- Massachusetts Institute of Technology, Cambridge
- OSTI ID:
- 6084587
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 109:15
- Country of Publication:
- United States
- Language:
- English
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ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
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PHOTOCHEMISTRY
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ACTIVATION ENERGY
CHEMICAL BONDS
CHEMICAL REACTION KINETICS
EXPERIMENTAL DATA
INFRARED SPECTRA
LIGANDS
LOW TEMPERATURE
MASS SPECTROSCOPY
NMR SPECTRA
NUCLEAR MAGNETIC RESONANCE
ULTRAVIOLET SPECTRA
VISIBLE SPECTRA
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ENERGY
HYDROCARBONS
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NUMERICAL DATA
ORGANIC COMPOUNDS
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400500* - Photochemistry