Metal−Olefin Interactions in M(CO)₅(cycloolefin) (M = Cr, Mo, W; Cycloolefin = Cyclopropene to Cyclooctene):  Strain Relief and Metal−Olefin Bond Strength

Abstract

Density functional theory calculations on the title compounds indicate that metal−olefin bond strengths follow the trend for cyclic olefin strain energies. It was found, however, that the proportionality between metal−olefin bond energy and strain energy is not evenly distributed throughout the olefin series. For instance, cyclopropene and cyclobutene are expected to bind to the metal much more weakly than would be anticipated on the basis of their strain energies. A bond energy decomposition analysis reveals that the metal−olefin interaction is responsible for strain relief in the cycloolefins by means of the rehybridization of the olefinic carbons. However, the geometrical changes accompanying this rehybridization, namely CC elongation and olefin pyramidalization, involve an energetic cost that is paid at the expense of the bonding interaction energy. Nonpyramidalized strained olefins such as cyclopropene and cyclobutene undergo large conformational changes, to the detriment of their large attractive interaction energies. It was found that a cyclic olefin that is already deformed, such as trans-cyclooctene, interacts strongly with a metal to relieve strain but does not suffer much energy-costly reorganizations. This, thus, constitutes an energy benefit to the metal−trans-cyclooctene bond strength.

The significance of the alkene size and the nature of the metal ion in metal–alkene complexes: a theoretical study

“…The strain induced on the alkene when it interacts with the metal ion is achieved by both elongation (stretching of the π-bond) of the π-bond and bending of the substituents (called pyramidalization) in the olefins. 30 The reorganization energy of methyl substituted cycloalkene complexes is, in general, higher than with their cycloalkene analogues, indicating that a higher strain is induced in the former case upon complexation with metal ions.…”

Transient anions of cis- and trans-cyclooctene studied by electron-impact spectroscopy

“…Our finding is also in line with the conclusion that the increased stability of the metal-olefin complex in TCO is a consequence of the CQC bond being pre-relaxed in the strained olefin for the d p (metal)-p* (olefin) bond, and not a consequence of a stabilized p* orbital. 1 The large values of the CQC stretch vibrational cross sections are indicative of a substantial CQC bond lengthening brought by electron attachment. The nearly symmetrical shape and large width of the attachment band provide a second indication of a substantial geometry change as a consequence of the electron attachment, that is, a large difference between the adiabatic and vertical attachment energies.…”

“…Geometrical distortion is an interesting means of activating chemical bonds, an important example being the deformation of the CQC bond in trans-cyclooctene (TCO) as compared to cis-cyclooctene (CCO). The distortion has dramatic consequences on bonding in metal-olefin complexes, 1 leads to an increasing number of applications of TCO as a ''voracious dienophile'' for bioorthogonal labeling, [2][3][4] assists ring-opening metathesis polymerization 5 and has biological implications -TCO counteracts ethylene at the receptor level in plants. 6 In this paper we investigate what are the changes in the frontier orbitals, in particular the normally unoccupied orbital, between CCO and TCO, and whether they could contribute to the enhanced reactivity of TCO.…”

Flow Photochemical Syntheses of trans‐Cyclooctenes and trans‐Cycloheptenes Driven by Metal Complexation

“…[79] Cyclic olefins such as transcyclooctene interact strongly with metals due to strain relief with relatively minimal energetic cost associated with the reorganization of the hydrocarbon framework. [80] Our strategy was also influenced by classic studies on photoprotonation of cyclic alkenes by Marshall, Kropp and Beauchemin. [81][82][83] A schematic of the apparatus for preparing trans-cyclooctenes is shown in Figure 3.…”

“…Our experiments were based on the well‐known observation that trans ‐cyclooctene, but not cis ‐cyclooctene, forms a complex with AgNO 3 . Cyclic olefins such as trans‐ cyclooctene interact strongly with metals due to strain relief with relatively minimal energetic cost associated with the reorganization of the hydrocarbon framework . Our strategy was also influenced by classic studies on photoprotonation of cyclic alkenes by Marshall, Kropp and Beauchemin .…”