Insight into Binding of Alkanes to Transition Metals from NMR Spectroscopy of Isomeric Pentane and Isotopically Labeled Alkane Complexes

Abstract

Alkane complexes of the type Cp'Re(CO)2(alkane) (Cp' = cyclopentadienyl or (isopropyl)cyclopentadienyl; alkane = isotopomers of n-pentane and cyclopentane) have been characterized using NMR spectroscopy following photolysis of Cp'Re(CO)3 in the appropriate alkane at 163-193 K. In the case of n-pentane, three different complexes are observed corresponding to binding of the three different types of carbon in this alkane. ROESY NMR experiments indicate that these isomeric complexes are slowly interconverting intramolecularly at 173 K. The order of the energetically preferred site of coordination is methylene (C2) approximately central methylene (C3) > methyl (C1) but with a spread of <0.2 kcal mol-1. Isotopic perturbation of resonance (IPR) experiments, conducted on several isotopomers of (i-PrCp)Re(CO)2(1-pentane), showed a large shielding of the 1H NMR chemical shift of the proton in a bound CHD2 moiety (delta -3.62) and CH2D (delta -2.64) compared with that of a bound CH3 moiety (delta -1.99). Likewise, the value of 1JCH for the coordinated methyl group of isotopomers of (i-PrCp)Re(CO)2(1-pentane) reduces in the order CH3 > CH2D > CHD2. This suggests that the alkane coordinates in an eta2-C,H fashion with a rapid exchange of bound hydrogen or deuterium within a methyl or methylene group, and that binding of a hydrogen atom is preferred over a deuterium by an amount of 0.23 +/- 0.03 kcal mol-1.

The role of alkane coordination in C–H bond cleavage at a Pt(II) center

“…This finding is only consistent with case 2: fast geminal but slow vicinal COH interchange. By way of comparison, in a metastable rhenium-pentane complex (observed at low temperature by NMR) geminal exchange is fast on the NMR time scale, whereas vicinal exchange takes place much more slowly (rates on the order of 1-10 s Ϫ1 at 173 K) (27).…”

“…Jones and coworkers have shown that in a rhodium system (TpЈLRh with L ϭ CNCH 2 CMe 3 ), coordination of secondary COH bonds is preferred over primary COH bonds by a factor of 1.5 (27); for the rhenium system cited above, coordination of pentane at the 2-and 3-positions is (very slightly) favored over statistical values (27 The failure to observe any evidence for terminal olefin adducts in the reactions of n-pentane, n-hexane, and n-heptane with 2-d 0.43 might also be taken to suggest a preference for internal reaction. However, it seems more likely that the terminal olefin complex is rapidly isomerized to the internal isomers via the olefin insertion/␤-H elimination sequence.…”

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.…”

Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment

“…The intermediacy of -alkane complexes has been found to play a critical role in these activation reactions (11)(12)(13)(14). Binding of alkanes has been studied experimentally by using time-resolved IR (15)(16)(17) and NMR spectroscopies (18).…”

“…Experimental investigation of this system has established the thermodynamic preference for a wide variety of hydrocarbon substrates, as indicated in Table 1 (19)(20)(21). Of particular note is the comparison of the activation of a methyl C-H bond in pentane vs. a methylene C-H bond in cyclohexane.…”