Modulation of an Anagostic Interaction in SiPSi-Type Pincer Platinum Complexes

Abstract

The reactivities of tris­(benzyldimethylsilyl)­phosphine [P­(o-C6H4-CH2SiMe2H)3] (1) and tris­(benzyldiphenylsilyl)­phosphine [P­(o-C6H4-CH2SiPh2H)3] (6) toward the same platinum precursor [Pt­(PPh3)3] are strikingly different. The reaction with 1 renders the trans disilyl platinum­(II) complex [Pt­{P­(o-C6H4-CH2SiMe2)2(o-C6H4-CHSiMe2)}­PPh3] (2) in which the ligand coordinates in a tridentate fashion while a new Si–C bond is formed from the third Si moiety. The most prominent feature is an anagostic interaction that is established at the apical position. In contrast, the reaction of [Pt­(PPh3)3] with 6 yields the hexacoordinated hydrido trisilyl platinum­(IV) complex [PtH­{P­(o-C6H4-CH2SiPh2)3}­PPh3] (7). We have studied the effect of the variation of the monodentate ligand in 2 by simple substitution reactions. We found a systematic variation of the chemical shift of the anagostic hydrogen in the 1H nuclear magnetic resonance spectrum of the corresponding PMe3, P­(OPh)3, and CO complexes that can in principle be ascribed to a varying degree of the π acceptor character of the ancillary ligand. However, theoretical calculations at the density functional theory level show only slight changes in the frontier orbitals in line with predominantly closed-shell electrostatic interactions.

Ortho-aryl substituted DPEphos ligands: rhodium complexes featuring C–H anagostic interactions and B–H agostic bonds

“…What was unanticipated is that in the parent DPEphos–H complex such interactions are also present – albeit only observed at very low temperature in solution. Similar properties (C–H⋯M, 2.23–3.01 Å, low-field chemical shifts and apical approaches of C–H groups to d 8 metal centres), have been discussed by others, including: Bergman, 44 Dyker, 42 Fairlamb, 45 and Sabo-Etienne, 38 Fig. 7 .…”

“…27,83 Common to all the Rh(I) DPEPhos-R complexes structurally described herein (with their associated NBD, CO or vinylborane co-ligands) is the observation of downfield-shifted signals in their 1 H NMR spectra that signal an anagostic M•••H-C interaction, 28 for which the steric bulk of the ligand determines the temperature at which they are observed. As discussed previously, 30,38,40,45 while such anagostic interactions are associated with weak Rh*CH and minimal  CH  Rh orbital donations, the driver for the downfield chemical shift of the C-H protons observed in the 1 H NMR spectrum does not come from these. Instead, the positioning of the anagostic hydrogen with reference to different regions of valence shell for the d 8 metal centre is important, as Scherer 30 has previously elegantly described for Rh(CAAC)(CO)Cl systems (CAAC = cyclic alkyl-aminocarbene).…”

“…an occupied dz 2 orbital. 30,[38][39][40] These are distinct from agostic, 28 3c-2e, bonds that are characterised by donation from a C-H bond into an unoccupied metal orbital and upfield chemical shifts in the 1 H NMR spectrum. The fluxional processes operating at room temperature mean these characteristic signals are only resolved on cooling, apart from for 2-i Pr in which the static structure makes them persistent.…”

B–N Cleavage in (9‐BBN)bis(pyrazolyl)borate NiII Complexes

“…As a class of bis(pyrazolyl)borate ligands, the chemistry of (9‐BBN)bis(pyrazolyl)borate [(BBN)Bp R ] – is far less developed[4e], [5a], compared with its dihydrobis(pyrazolyl)borate counterparts . In addition, the rigid feature of BBN bicyclic system in [(BBN)Bp R ] – ligands usually causes the terminal hydrogen (α‐C–H) to be forced into close proximity to the metal center, forming agostic, or anagostic interactions. This relatively new ligand combined with C–H ··· M interaction provides new potential to modulate the reactivity and catalytic applications of complexes of the type [(BBN)Bp R ]ML n .…”