A systematic approach to the generation of long-lived metal alkane complexes: combined IR and NMR study of (Tp)Re(CO)2(cyclopentane)

Abstract

Short wavelength photolysis of (Tp)Re(CO)(3) (Tp = tris(pyrazol-1-yl)borate) at low-temperature in cyclopentane yielded (Tp)Re(CO)(2)(cyclopentane), an alkane complex with three nitrogen ligands that was characterised by NMR spectroscopy.

Characterisation of Alkane σ‐Complexes

“…Such studies have proven successful in supporting the results of experimental observations;30, 54, 74, 84, 110 however, the ability of theoretical calculations to predict experimentally observable alkane complexes has been less reliable. For example, theoretical results prompted Brookhart to unsuccessfully attempt protonation of the cationic palladium complex, [Pd(PONOP)Me][BAr F 4 ], to form the dicationic methane complex, [Pd(PONOP)(CH 4 )][BAr F 4 ] 2 ,112 and conversely, a study comparing methane adducts of “TpRe” and “CpRe” fragments, was prognostic of the thermodynamic stabilising effects of the Tp ligand (later verified experimentally by Duckett and George83—see above) 108. Disconcertingly, the results of independent studies of similar or even identical systems often differ greatly 112.…”

“…Numerous experiments demonstrating large IPR effects in partially deuterated alkanes are in themselves evidence for a η 2 ‐ C,H binding mode given that this effect hinges upon differing zero‐point energies of terminal and bridging CH/D bonds (compare modes 2 and 4 that would be devoid of terminal CH/D bonds for methylene and methyl bound groups, respectively) 29. 30, 83…”

“…More frequently, for recently reported alkane complexes, NMR and IR methods to experimentally confirm alkane complex assignment have been used in conjunction 52. 74, 79, 83, 84, 111…”

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

“…The first area concerns characterization and stabilization of metal‐alkane complexes generated in situ . In most cases, studies have been carried out in low temperature matrices or solutions using half‐sandwich complexes or complexes with multidentate ligands such as Tp (tris‐pyrazolylborate) . Ball and Geftakis characterized the cyclopentane complex CpRe(CO) 2 (C 5 H 10 )] (Cp = η 5 ‐C 5 H 5 ) using 1 H NMR spectroscopy, at ∼180 K. This is the most stable σ‐alkane complex characterized in solution with a lifetime of ∼1 h. A η 2 ‐(CH) σ‐coordination to the metal center has been proposed based on the decrease in the CH coupling constant .…”

Potassium–Alkane Interactions within a Rigid Hydrophobic Pocket

“…Examples detected by NMR spectroscopy include [(C 5 R 5 )Re(CO) 2 -(alkane)], [3,4] [(C 5 R 5 )M(CO)(PF 3 )(alkane)] (M = Re or Mn), [4] [TpRe(CO) 2 (alkane)], [5] [(PONOP)Rh(CH 4 …”