Photochemistry of CpMn(CO)3 and Related Derivatives: Spectroscopic Observation of Singlet and Triplet CpMn(CO)2
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Abstract
We report novel photochemistry derived from (η5-C5H5)Mn(CO)3 (1a), (η5-C5H4Me)Mn(CO)3 (1b),(η5-C5Me5)Mn(CO)3 (1c), and (η5-indenyl)Mn(CO)3 (1d). Photolysis (>261 nm, 1 h) of the parent tricarbonyl (1a–d), matrix isolated in argon at 10 K, yields two species: the expected singlet dicarbonyl 1(η5-L)Mn(CO)2 (12a–d) and an additional compound assigned as the triplet dicarbonyl 3(η5-L)Mn(CO)2 (32a–d). Density functional theory calculations (B3LYP/LANL2DZ) support the structural assignments for 12 and 32. Natural bond orbital population analyses of 12a and 32a explain the source of the large coupling (ΔνCO 153 cm–1) between the carbonyl stretching vibrations in 32a. The triplet isomer (32) is metastable, even at temperatures as low as 10 K. We determined the rate constants for the thermal isomerization 32 → 12 using dispersive kinetic analysis. As revealed by these rate constants, the triplet complexes display the following order of stability in this system: Ind ≫ Cp ≈ Cp′ > Cp*. The spectroscopy and kinetics observed in various matrices (Ar, CH4, and Xe) do not differ appreciably. Experimental and computational results suggest that the singlet–triplet energy gap (ΔEST) of CpMn(CO)2 (2a) must be smaller than previous estimates.
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