Deuterium Migration in {(1,4,7-Triazacyclononane)Rh(alkyl)D[P(OMe)₃]}⁺. Evidence for Mobility of Rhodium along Linear Alkane Chains in Inferred Rhodium−Alkane Complexes

Abstract

With increasing evidence for the existence of complexes of intact alkanes with transition metals ("σ complexes") as intermediates in many oxidative-addition/reductive-elimination reactions of alkanes (equation 1), there is considerable interest in learning about their structure, bonding, energetics, and dynamical properties. [1][2][3][4][5] Previously, we reported 3d indirect evidence for the methane complex [Cn*Rh(CH 4 )(PMe 3 )] + . 6 We now present indirect evidence that σ complexes of higher alkanes with rhodium in the triamine environment also form as transient intermediates, and that the metal moves along the chain with a barrier that is slightly below the barrier to alkane dissociation.Alkyl hydride complexes [CnRh(H)(R)(L)]X {Cn ) 1,4,7triazacyclononane; 6 R ) Me, Et, Bu, hexyl, and decyl; L ) P(OMe) 3 ; X -) -OTf (triflate, CF 3 SO 3 -) or -BAr 4 ( -B[3,5-(CF 3 ) 2 C 6 H 3 )] 4 )} have been prepared in 21-32% overall yields from RhCl 3 (H 2 O) n by methods completely analogous to those reported for [CnRh(H)(R)(PMe 3 )]X (R ) Me, Et) 7 and fully characterized. In benzene from 40 to 80 °C, the [CnRh(H)(R)-(L)](BAr 4 ) all react with clean first-order kinetics and quantitative formation of [CnRh(H)(C 6 H 5 )(L)](BAr 4 ), which has also been characterized. Arrhenius plots of the benzene reaction give ∆H q ranging from 29.7 to 33.1 kcal/mol and ∆S q from 9.6 to 18.4 eu. Reaction rates of the Et, Bu, hexyl, and decyl derivatives are very similar, while the methyl compound is about 10-fold slower. The size of ∆S q is consistent with a dissociative mechanism for replacement of alkane by benzene, and there is no dependence of the rate of loss of hexane from [CnRh(H)(hexyl)(L)](BAr 4 ) on benzene concentration in C 6 F 6 solution. 8

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

“…Early examples are summarized by Crabtree, Hall, and Perutz as well as by Shilov and Shulpin. [3,4,14] + (tacn = triazacyclononane), [78] [Tp*RhH-(CH 2 R) L] (Tp* = tris(3,5-dimethylpyrazolyl)borate), [12,79] [{H 2 Si(C 5 H 4 ) 2 }WH(Me)], [80] [Tp*PtH(Me) 2 ], [81] and related platinum complexes. [13,40] Two classes of Pt-catalyzed alkane-activation reactions have received particular attention.…”

Der σ‐CAM‐Mechanismus: σ‐Komplexe als Schlüssel der σ‐Bindungsmetathese bei späten Übergangsmetallen

“…Intermediär auftretende σ‐Methankomplexe sorgen für die Äquilibrierung zwischen den [Pt(D)(CH 3 ) 2 ] + ‐ und den [Pt(H)(CH 2 D)(CH 3 )] + ‐Derivaten 77. Ähnliche Alkanintermediate wurden für die Bildung und Austauschreaktionen etlicher Alkyl‐Hydrid‐Komplexe wie [RhH(CH 2 R)(tacn)L] + (tacn=Triazocyclononan),78 [Tp*RhH(CH 2 R)L] (Tp*=Tris(3,5‐dimethylpyrazolyl)borat),12, 79 [{H 2 Si(C 5 H 4 ) 2 }WH(Me)],80 [Tp*PtH(Me) 2 ]81 und damit verwandter Platinkomplexe postuliert 13. 40…”

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

“…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 .…”