Ultrafast Ring Closure Energetics and Dynamics of Cyclopentadienyl Manganese Tricarbonyl Derivatives

Abstract

Ring closure following flash photolysis in alkane solvents has been detected for several complexes in the series (η5-C5H4R)Mn(CO)3 where R = COCH3 (1), COCH2SCH3 (2), CO(CH2)2SCH3 (3), COCH2OCH3 (4), (CH2)2CO2CH3 (5), and CH2CO2CH3 (6). In each case where ring closure occurs, a metal CO is ultimately substituted by the side chain functional group. Photoacoustic calorimetry studies reveal that ring closures occur with rate constants faster than 107 s-1 or between 106 and 107 s-1, or in some cases the ring closure is biphasic with rate constants in both ranges. The enthalpies of CO dissociation followed by ring closure for 2 and 3 are the same (12 kcal/mol) and more favorable than those for 4−6 (25−15 kcal/mol). Studies of 1−3 by transient picosecond to microsecond infrared spectroscopy confirm biphasic dynamics for 2 and 3: ring closure occurs immediately (k > 5 × 109 s-1) and at slower rates (k = 108−106 s-1). We propose that some ring closure occurs before solvent coordination and that the remaining ring closure, resulting in the displacement of solvent, is much slower. The relationships of the rates and energetics of ring closure to structure and quantum yields are discussed.

Photochemistry of Chromium Arene Tricarbonyl Complexes with Tethered Pyridinyl and Propenyl Groups: Investigations of the Effect of Ring Size on Chelate Formation, Structure, and Linkage Isomerization

“…These results are in contrast to those for (η 5 -C 5 H 4 R)Mn(CO) 3 , where the quantum yields are found to depend on the side chain structure (R) and in some cases unit quantum yields are observed. 8 Some combination of two mechanisms may account for the quantum yield independence of chain length. First, the quantum yield is determined by the partitioning of the excited states between CO dissociation (0.77) and relaxation back (0.23) to the ground state (no luminescence has been observed for chromium arene tricarbonyls).…”

“…For example, we reported that the quantum yields for photoinitiated chelation of manganese cyclopentadienyl tricarbonyl derivatives with different tethered functional groups varied from 0.65 to 1.00. 8 Unit quantum yields demand that every absorbed photon results in CO dissociation with no cage CO recombination. 9 For comparison, the quantum yield for the analogous bimolecular substitution of CpMn(CO) 3 was only 0.65 and was found to be independent of ligand structure.…”

Electron transfer triggered sulfoxide isomerization in ruthenium and osmium complexes

“…Both unimolecular and bimolecular pathways for isomerization were observed. This work was followed up by an ultrafast chelation study of [Cr( 6 -C 6 H 5 R)(CO) 3 ], where R is a pendant thioether, and the manganese compound [64][65][66][67]. Time resolved IR spectroscopy reveals both a solvent dependent and solvent free pathway for CO displacement by the thioether on the picosecond to nanosecond timescale.…”

Chelate ring closing and opening behavior in cyclopentadienyl cobalt(III) complexes with pendant nitrogen functional group

“…Since the rate constant k 1 describes the rupture of the chelated Co-N bond and the formation of a solvated reaction intermediate, the smallest k 1 value found for 3c implies that the metal-nitrogen bond in 3c is the most difficult one to rupture. The rate of chelate ring closing in coordinatively unsaturated metal complex is known as an ultrafast process and depends on the forming ring size [59][60][61][62]. The solvated ring opened reaction intermediate I S depicted in Scheme 2 are expected to be nearly identical in electronic property of the metal in spite of the different functional groups in their side chains.…”