Manganese Alkane Complexes: An IR and NMR Spectroscopic Investigation

Abstract

Manganese propane and manganese butane complexes derived from CpMn(CO)(3) were generated photochemically at 130-136 K with the alkane as solvent and characterized by FTIR spectroscopy and by (1)H NMR spectroscopy with in situ laser photolysis. Time-resolved IR spectroscopic measurements were performed at room temperature with the same laser wavelength. The ν(CO) bands in the IR spectra of the photoproducts in propane are shifted to low frequency with respect to CpMn(CO)(3), consistent with formation of CpMn(CO)(2)(propane). The (1)H NMR spectra conform to the criteria for alkane complexes: a high-field resonance for the η(2)-CH protons that shifts substantially on partial deuteration of the alkane and exhibits a coupling constant J(C-H) on (13)C-labeling of ca. 120 Hz. The NMR spectrum of each system exhibits two diagnostic product resonances in the high-field region for the η(2)-CH protons, corresponding to CpMn(CO)(2)(η(2)-C1-H-alkane) and CpMn(CO)(2)(η(2)-C2-H-alkane) isomers. Partial deuteration of the alkane at C1 results in characteristic strong isotopic perturbation of equilibrium of the η(2)-CH resonance of CpMn(CO)(2)(η(2)-C1-H-alkane). With propane-(13)C(1), the η(2)-CH resonance of CpMn(CO)(2)(η(2)-C1-H-alkane) isomer exhibits (13)C satellites with J(C-H) = 119 Hz. The corresponding resonance of CpMn(CO)(2)(η(2)-C2-H-alkane) is identified by use of propane-2,2-d(2). The lifetimes of the (η(2)-C1-H-alkane) isomers of the manganese complexes were determined by NMR spectroscopy as 22 ± 2 min at 134 K (propane) and 5.5 min at 136 K (butane). The corresponding spectra and lifetimes of the CpRe(CO)(2)(alkane) complexes were measured for reference (CpRe(CO)(2)(propane) lifetime ca. 60 min at 161 K; CpRe(CO)(2)(butane) 13 min at 171 K). The lifetimes determined by IR spectroscopy were similar to those determined by NMR spectroscopy, thereby supporting the assignments. These measurements extend the range of alkane complexes characterized by NMR spectroscopy from rhenium and rhodium derivatives to include less stable manganese derivatives.

Characterisation of Alkane σ‐Complexes

“…NMR data for manganese alkane complexes have also been reported by George and co‐workers, utilising systems analogous to the original rhenium system reported by Ball 111. 94 Generation of the alkane complexes [(η 5 ‐C 5 H 5 )Mn(CO) 2 (alkane)] (alkane= n‐ propane, n ‐butane, cyclopentane) (Scheme ) proceeded in an identical manner to that used to generate previously reported analogous rhenium alkane complexes (Scheme ).…”

“…These complexes represent the first NMR spectroscopic characterisation of a first row (3D) alkane complex. NMR characterisation performed at 130–136 K 111…”

A Rhodium–Pentane Sigma‐Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques

“…This proposed fluxional process is closely related to the chain‐walking events that have directly8b, 16c,16e and indirectly23 been shown to occur in transient, or only stable at low temperature, transition‐metal alkane σ‐complexes in solution (Scheme 4). With the pentane complex [ 3 ][BAr F 4 ], similar processes could well be occurring in the solid state, connecting terminal (M⋅⋅⋅H 3 C) and internal (M⋅⋅⋅H 2 C) σ‐binding motifs, and the estimated barriers to this (15–25 kJ mol −1 ) are similar to experimentally measured values derived from solution‐measurements at very low temperature 8b.…”

Photochemistry of framework-supported M(diimine)(CO) 3 X complexes in three-dimensional lithium carboxylate metal–organic frameworks: monitoring the effect of framework cations

“…However, this is due to the difference in the two experiments as the X-ray measurements were obtained following continuous-wave irradiation which can build up the desired photoproduct while the time-resolved experiments are performed following pulsed laser excitation and probes the events following the absorption of a single photon. The second reason why the rapid decay of the Mn(diimine)(CO) 2 moiety was surprising is that similar dicarbonyl nominally 16-electron intermediates such as CpMn(CO) 2 (alkane) live for much longer time scales (more than 100 µs) in solution at room temperature [22]. Further insight into the short lifetime comes from examining the TRIR trace at the parent ν(CO) band.…”