C–H and C–C agostic interactions in cycloalkyl tris(pyrazolyl)boratoniobium complexes

Abstract

H 11 (4),) have been prepared from Tp Me2 NbCl 2 (MeC᎐ ᎐ ᎐ CMe). The cyclopropyl complex 2a shows no sign of C-H agostic interactions either in the solid state (X-ray) or in solution. In contrast, the NMR spectra of 3 and 4 are temperature dependent as a consequence of an equilibrium between a major α-agostic species and a minor non-agostic one. Hybrid QM/MM calculations are used to rationalise the behaviour of these cycloalkyl species, and illustrate the subtle interplay of steric and electronic effects in these systems.

A Rhodium Complex with One Rh⋅⋅⋅CC and One Rh⋅⋅⋅HC Agostic Bond

“…In contrast, compared with free Binor-S (as determined from the structure of the tris-hydroxylated derivative [14] ), the CÀC bond that is in close proximity with the metal is lengthened by % 0.1 , whereas the two other CÀC bonds in the cyclopropane unit remain unchanged (Scheme 2). A similar, but slightly smaller, increase in CÀC bond length (0.049 ) has been noted in D. [6] The DFT calculated structure of 1 (see below; see Supporting Information for full details) is in close agreement with the experimentally determined structure, showing a lengthening of the C21ÀC25 bond (1.620 ), while the calculated CÀH bond lengths (1.104/1.105 ) are not significantly lengthened compared with the other calculated CÀH distances (cf. C24ÀH24 1.104 ).…”

“…The metal-mediated activation of carbon-carbon single bonds is the subject of significant contemporary interest, [1] with the putative intermediates in such processes being complexes in which the metal is bound through a CÀC s interaction. [2] Complexes showing such M···CC interactions are extremely rare, and are limited to intramolecular (agostic) interactions that have been reported for complexes A-D; [3][4][5][6] although in one case (A) the bonding mode has been queried. [7] The lack of well-characterized examples of such species is in contrast with M···HC agostic complexes that are well established; [2,[8][9][10] and further examples of genuine, fully-characterized examples of M···C À C s interactions are of clear importance regarding the fundamental study of CÀC activation processes.…”

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

“…[112][113][114] In 1992, Tilley et al had already suggested that "a transient intermediate which may form prior to a four-center transition state is a complex with coordinated SiÀH, HÀH, or Si À Si s bonds." [115] There are now a few examples of C À C agostic complexes, [65,[116][117][118][119][120] but intermolecular s-C À C complexes are not known as far as we are aware. We look forward to the discovery of evidence for s-EÀE' complexes and their reactivity.…”

C–C σ complexes of rhodium

“…Milstein and colleagues have characterized a number of PCP-and PCN-type rhodium pincer complexes (e.g., Scheme 2, complexes III) in solution that are proposed to have agostic M⅐⅐⅐C-C interactions on the basis of NMR data and a solid-state structure that shows a closer than van der Waals separation between Rh and C(2, [33][34][35]. Etienne and colleagues have described the niobium cyclopropyl complex IV that shows a close Nb⅐⅐⅐C-C contact and lengthening of the C-C distance in the cyclopropyl group consistent with an agostic C-C⅐⅐⅐Nb interaction (36,37). An early report of an agostic C-C complex recently has been reassessed by Maseras and Crabtree (38) as one that has a close nonbonding contact with a C-C bond and not an agostic species.…”