Reactivity of the CpMn(CO)₂−XR Bond [X = Cl, Br]: A Kinetic Study Using Rapid-Scan FTIR Spectroscopy

Abstract

The mechanism and energetics of the displacement of dihaloalkanes from the photolytically generated CpMn(CO)2−XR (X = Cl, Br) complexes by 2,6-lutidine have been studied using rapid-scan FTIR. The substitution reactions proceed through an Id mechanism, and theoretical calculations indicate that the Mn−XR bond is mostly broken in the transition state. Activation enthalpies of 16−17 kcal/mol for the Mn−ClR and 18−19 kcal/mol for the Mn−BrR complexes are only slightly lower than previous thermodynamic measurements of the Mn−haloalkane bond dissociation enthalpies. The rate of displacement of bromoalkanes was found to be ∼10 times slower than for the analogous chloroalkanes. Ab initio calculations suggest that this difference in reactivity is primarily due to a more stable interaction between the Mn center and bromoalkanes.

Oxidative Addition of Haloalkanes to Metal Centers: A Mechanistic Investigation

“…At 213 K this species is stable and demonstrates no further reactivity. Since CpMn(CO) 2 (ClCH 2 Cl) and other CpMn(CO) 2 (XR) (RX = Cl(CH 2 ) 2 Cl, Cl(CH 2 ) 3 Cl, Cl(CH 2 ) 6 Cl) complexes have CO band positions similar to those observed here, this dicarbonyl species is identified as the chloro-bound complex CpRe(CO) 2 (ClCH 2 Cl) ( 1 ). DFT calculations are consistent with this assignment and predict a 24.3 kcal/mol Re–ClCH 2 Cl bond dissociation enthalpy (BDE).…”

A unified set of experimental organometallic data used to evaluate modern theoretical methods

“…Schultz et al have evaluated the performance of 42 functionals for 21 transition metal compound's bond dissociation enthalpies (MLBE21/05 dataset). 33 Table 1 List of organometallic complexes used in the benchmark study 46 (13) η 2 -3-hexyne 37 (3) η 2 -cycloheptene 47 (14) η 2 -benzene 52 (4) η 2 -cyclooctene 47 (15) TES 53 (5) η 2 -furan 36 (16) THF 53 (6) S(n-But) 2 48 (7) tetrahydrothiophene a (8) THF 49 (9) thiophene a (10) toulene 50 (11) triethyl silane (TES) 51…”

Manganese Alkane Complexes: An IR and NMR Spectroscopic Investigation

“…There are numerous mechanistic studies that implicate alkane complexes as reaction intermediates in processes such as C−H oxidative addition and reductive elimination, ,,− metathesis processes especially σ-CAM, − protonation of metal alkyls, , simple ligand substitution processes, ,,, and alkane adsorption on metal surfaces . Recent examples include borylation of alkanes (a σ-CAM mechanism), , equilibrium isotope effects, isotope exchange and chain walking at rhodium alkyl hydrides, and C−H activation at platinum. , It follows that alkane complexes may be formed in a variety of ways including reaction with an alkane, reductive coupling of a metal alkyl hydride, and protonation of a metal alkyl.…”

“…9 The σalkane complexes represent the intermolecular analogues of agostic complexes in which the saturated C-H bond that interacts with the metal is bound as a chelate with the support of more conventional metal-ligand interactions. 10 There are numerous mechanistic studies that implicate alkane complexes as reaction intermediates in processes such as C-H oxidative addition and reductive elimination, 3,4,[11][12][13][14] metathesis processes especially σ-CAM, [15][16][17] protonation of metal alkyls, 18,19 simple ligand substitution processes, 3,4,20,21 and alkane adsorption on metal surfaces. 22 Recent examples include borylation of alkanes (a σ-CAM mechanism), 15,16 equilibrium isotope effects, 23 isotope exchange and chain walking at rhodium alkyl hydrides, 24 and C-H activation at platinum.…”