Observation of a Tungsten Alkane σ-Complex Showing Selective Binding of Methyl Groups Using FTIR and NMR Spectroscopies

Abstract

The alkane σ-complex (HEB)W(CO)(2)(pentane) (HEB = η(6)-hexaethylbenzene) is produced from the UV photolysis of (HEB)W(CO)(3) in alkane solvents at low temperature. IR and (1)H and (13)C NMR spectroscopic data are reported, representing the first NMR data for a group 6 alkane complex. Only binding of the methyl functionality of the pentane ligand was observed in (HEB)W(CO)(2)(pentane). This contrasts with the previously reported binding of pentane to rhenium fragments, wherein both methylene and methyl groups were observed to bind, with a slight preference for binding of the former. The reason for the preference for binding through the methyl group is investigated, and the steric requirement for the pentane to adopt an unfavorable gauche conformation when bound via a methylene is identified as a contributing factor.

Contrasting electronic requirements for CH binding and CH activation in d6 half‐sandwich complexes of rhenium and tungsten

“…The tungsten‐carbonyl fragment [(η 6 ‐C 6 H 6 )W(CO) 2 ] prefers the methylene CH bond over the methyl CH by ∼1.6 kcal/mol. This result supports the suggestion made by Ball and coworkers that the experimentally observed methyl CH binding selectivity in [(η 6 ‐C 6 Et 6 )W(CO) 2 (C 5 H 12 )] might be due to the steric demand of the bulky hexaethylbenzene (HEB) ligand.…”

“…A second aspect that has received significant attention is the intramolecular dynamic process, termed alkane chain walking in many metal‐alkane complexes . The use of 1 H NMR spectroscopy revealed a slow migration of the metal from one CH bond to another in the alkane by Ball and coworkers .…”

“…Binding preferences of two fragments [CpRe(CO) 2 ] and [CpRe(CO)PF 3 ] , which are formed on photoirradiation of [CpRe(CO) 2 PF 3 ] (Scheme ), for different CH bonds (1° and 2°/axial over equatorial) of pentane and cyclohexane have been elucidated . The importance of steric factors was brought out by Young et al who showed that the half‐sandwich W(0) complex [(η 6 ‐C 6 Et 6 )W(CO) 3 ] , selectively binds primary CH bonds of pentane on photo‐induced elimination of one of the CO ligands . However, steric aspects cannot be overplayed as the metal‐alkane binding energies are reduced in sterically encumbered systems.…”

“…In the cyclohexane complex [CpRe (CO) 2 (C 6 H 12 )] , the activation energies calculated by Ball and coworkers, for the 1,3‐(axial‐to‐axial) migration was ∼3.8 kcal/mol lower than the activation energy of the 1,2‐(axial‐to‐equatorial) migration . A direct transfer of the metal fragment from one end to the other end of pentane, a 1,5 migration, is observed in [(η 6 ‐C 6 Et 6 )W(CO) 2 (C 5 H 12 )] , which is presumably due to the preference for methyl coordination to methylene coordination . A similar situation exists in Tp complexes, studied by Jones and coworkers, where there is a competition between these three processes resulting in the selectivities observed in CH activation …”

Potassium–Alkane Interactions within a Rigid Hydrophobic Pocket

“…+ {PONOP = 2,6-(tBu 2 PO) 2 C 5 H 3 N}, [6] and [(C 6 Et 6 )W(CO) 2 -(n-pentane)], [7] but none of these complexes have proven sufficiently robust to allow isolation or crystallization. At the other end of the spectrum are the crystallographically characterized metal-alkane complexes [8] which have not been observed in solution.…”