Manganese−Chalcocarbonyl Bond Strengths from Threshold Photoelectron Photoion Coincidence Spectroscopy

Abstract

The dissociation kinetics of CpMn(CO) 2 CX (X ) S, Se) complexes have been investigated by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Energy-selected CpMn(CO) 2 CX + ions sequentially lose the two carbonyl and chalcocarbonyl ligands upon photoionization. The resulting asymmetric time-of-flight peak shapes and breakdown diagrams were simulated, and the following 0 K appearance energies were determined for the fragment ions: CpMn(CO)CS + , 8.79 ( 0.05 eV; CpMnCS + , 9.42 ( 0.05 eV; CpMn + , 11.53 ( 0.05 eV; CpMn(CO)CSe + , 8.70 ( 0.05 eV; CpMnCSe + , 9.39 ( 0.05 eV; and CpMn + , 12.05 ( 0.05 eV. These values, combined with the adiabatic ionization energies, yield Mn-CO bond energies in CpMn(CO) 2 CX + (X ) S, Se) and CpMn(CO)CX + (1.20 ( 0.06, 0.63 ( 0.07 eV and 1.08 ( 0.06, 0.72 ( 0.07 eV, respectively) and Mn-CX bond energies in CpMnCX + (X ) S, Se, 2.12 ( 0.07 and 2.66 ( 0.07 eV, respectively).The 0 K heats of formation of the neutral and ionic species were determined using the heats of formation of the dissociation products, CpMn + , CO, and CX. Thus, it became possible to obtain Mn-CX bond energies in both parent ions and both first daughter ions. Room-temperature gaseous heats of formation are also given using ab initio thermal enthalpies. On average, in the ions, the Mn-CX bond energies are larger than Mn-CO bond energies by 1.04 eV (X ) S) and 1.61 eV (X ) Se). Using the appearance energy of CpMn + in the earlier TPEPICO studies of CpMn(CO) 3 , the carbonyl f calcocarbonyl substitution heats were determined for the neutral complexes.

Unimolecular Reaction Mechanism of an Imidazolin‐2‐ylidene: An iPEPICO Study on the Complex Dissociation of an Arduengo‐Type Carbene

“…Both propene-and methyl-loss fragment ions, [3] and [4], respectively, appear at nearly the same threshold ( Figure 3) and are produced with an almost constant branching ratio over a large energy range. As already mentioned, this indicates that the same intermediate imidazolium parent ion, [2], participates in the methyl loss, too.…”

“…As discussed above, a hydrogen-atom migration step must be involved prior to propene loss, [1]! [3]. If one of the terminal hydrogen atoms of the iPr group is moved to the one of the five positions of the imidazole ring, the resulting isomers span the 6.45 to 9.46 eV energy range (B3LYP) with respect to the neutral NHC.…”

“…[43,71] The [2]! [3], propene-loss reaction leads to a product energy of 8.21 eV (CBS-QB3) along a purely attractive reaction energy curve. Three high-energy alternative product ion structures, at 8.80-10.46 eV (CBS-QB3) above the neutral sample cannot compete efficiently with the lowest energy channel leading to [3].…”

“…[3], propene-loss reaction leads to a product energy of 8.21 eV (CBS-QB3) along a purely attractive reaction energy curve. Three high-energy alternative product ion structures, at 8.80-10.46 eV (CBS-QB3) above the neutral sample cannot compete efficiently with the lowest energy channel leading to [3]. These structures are not shown in Figure 4, but can be understood as the propene-loss products from the hydrogen-transfer isomer ions to the bivalent (8.80 eV), and the trivalent nitrogen atom (10.46 eV) or to the ethylene group (9.79 eV) of the imidazole ring.…”

“…[1,2] They are excellent s donors thanks to the lone pair of the carbon and can have good p-acceptor properties depending on their functionalization. Carbenes can thus be further stabilized in organometallic complexes, similarly to other marginally unstable s-donating ligands with a small HOMO-LUMO (highest occupied and lowest unoccupied molecular orbital) gap, such as CS, CSe, [3] or borylenes. [4][5][6][7] In the last 50 years, Nheterocyclic carbenes (NHCs; imidazolin-2-ylidenes, Im) have evolved from intermediates whose existence could only be demonstrated by kinetic and trapping studies in the 1960s [8][9][10] to stabilized forms currently being studied in numerous research groups.…”