Untersuchungen zum strukturdynamischen Verhalten von 2‐Alkenylzink‐Verbindungen

Abstract

Die IR-und Raman-Spektren von LBsungen der Allylzinkverbindungen 1 -10 zeigen Schwingungsbanden zwischen 1655 und 1606 cm-', die fur isolierte C = C-Bindungen in 11'-Allylmetallsystemen charakteristisch sind. Die bei den Diorganozinkverbindungen RzZn [R: -CH,CH = CH, (l), -CH,CMe = CH, (2), -CH,CH = CHMe (3) und -CH,CH = CMe, (5)j untersuchte Temperaturabhangigkeit der 'H-und 13C-NMR-Spektren beweist das Vorliegen valenzdynamischer q'-Allylmetallsysteme, deren Dynamik abhangig von Struktur, Ltisungsmittel und Konzentration bei tiefen Temperaturen eingefroren werden kann. Aus den durch Linienformanalyse der 'H-NMR-Spektren von 2 gewonnenen kinetischen Daten muR fur die Allylumlagerung eine Reaktionsordnung von 2.2 ? 0.3 angenommen werden. Der Mechanismus der Umlagerung wird diskutiert. Im Gegensatz zu den flussigen Verbindungen 3-5 werden in den IR-und Raman-FestkorperSpektren der kristallisierten Diorganozinkverbindungen mit symmetrischer Allylgruppe 1 und 2 sowie der ebenfalls kristallisierten Allylzinkchloride 6 -9 langwellige Verschiebungen der Schwingungsbande auf 1550-1480 cm-' beobachtet. Dies 1aRt auf einen Aufbau des Kristallgitters mit delokalisierten Allylgruppen schlieRen. Nur das an C-3 der Allylgruppe zweifach substituierte 10 zeigt auch im kristallinen Zustand die fur q'-Allylsysteme typischen Bandenlagen. Investigation of the Dynamic Structural Behavior of 2-Alkenylzinc CompoundsThe solution IR and Raman spectra of the allyl zinc compounds 1 -10 have absorptions between 1655 and 1606 cm-' which are characteristic of isolated C = C bonds in q'-ally1 metal compounds. The temperature dependence of the 'H and I3C NMR spectra of the diorganozinc compounds R,Zn [R: -CH,CH = CH, (l), -CH,CMe = CH2 (2), -CH,CH = CHMe (3), and -CH2CH = C(Me), (5)] shows that these q'-ally1 metal systems are fluxional; the rates of exchange are dependent upon the structure, solvent, and concentration, and at low temperature are reduced sufficiently to reach the region of slow exchange. Kinetic data derived from a line shape analysis of the 'H NMR spectra of 2 indicate that the order of reaction for the ally1 rearrangement is 2.2 ? 0.3. The mechanism of this rearrangement is discussed.In contrast to the spectra of 3-5, which are liquids, the IR and Raman spectra of the crystalline diorganozinc compounds 1 and 2 (which have a symmetrical allyl group), and the allylzinc chlorides 6 -9, which are also crystalline, show C = C stretching modes which are shifted to lower wave number (1550-1480 cm-'). From this observation it may be concluded that in the crystal lattice the allyl groups are delocalized. Only the spectrum of crystalline 10, in which the allyl group is doubly substituted, has absorption bands that are typical of an q'-allyl system.

Structurally Defined Allyl Compounds of Main Group Metals: Coordination and Reactivity

“…IR and Raman spectroscopy as well as solid‐state CP MAS 13 C NMR measurements have been used to investigate the bonding mode of allyl ligands in the solid state 66. 68 These analytical techniques allow differentiation between bonding modes with localized or delocalized π‐electron systems. However, only limited information can be obtained about the nature of the metal–allyl interaction, as allyl ligands with delocalized π‐electron systems can show σ‐type MC allyl (M=metal) bonding (Scheme , type C ) and allyl ligands with localized π‐electron systems can be involved in π‐type MC allyl bonding (Scheme , types F – H ).…”

“…The former two methods allow discrimination between allyl ligands with localized and delocalized π electrons, as described in Section 3.1. The band ascribed to the asymmetric stretching of the allyl ligand is usually used as a diagnostic tool (localized type: $\tilde \nu $ =1590–1660 cm −1 ; delocalized type: $\tilde \nu $ =1450–1580 cm −1 ) 68. More detailed information can be obtained by NMR spectroscopy (see also Section 3.3).…”

“…Such processes have been reported for compounds with η 1 ‐ or η 3 ‐allyl ligands in the ground state 31. 57a, 68 If these exchange phenomena are fast on the NMR time scale, an equivalence of all four methylene protons is observed, thus resulting in an A 4 X coupling pattern (Figure 1 c). If they take place on a time scale that is approximately equal to the corresponding resolution of the NMR experiment, the resulting spectra reflect intermediates between the ground state (η 1 or η 3 bonding) and the fluxional situation (A 4 X pattern).…”

“…A second mechanism has been suggested, as there are allyl compounds of main group metals that seem not to favor π‐type metal–allyl interactions (Scheme ) 65. 68 Consequently, this bimolecular mechanism involves only σ‐type metal–allyl interactions, and a transition state featuring a μ 2 ‐η 1 :η 1 coordination mode has been suggested (see Scheme , C ). Determination of the molecularity of the exchange reaction is required to differentiate experimentally between these two mechanisms 68.…”

“…68 Consequently, this bimolecular mechanism involves only σ‐type metal–allyl interactions, and a transition state featuring a μ 2 ‐η 1 :η 1 coordination mode has been suggested (see Scheme , C ). Determination of the molecularity of the exchange reaction is required to differentiate experimentally between these two mechanisms 68. In addition, activation parameters of the allyl‐exchange reactions have been determined by means of variable‐temperature (VT) NMR spectroscopic measurements in combination with line‐shape analysis.…”