Different Coordination Modes of the Ph₂PC_sp³PPh₂ Pincer Ligand in Rhodium Complexes as a Consequence of C_sp³–H Metal Interaction

Abstract

Starting from commercially available 4,4′-di-tert-butyldiphenylmethane the pincer ligand bis­(4-tert-butyl-2-(diphenylphosphino)­phenyl)­methane (PCP) was prepared in two steps in moderate yield. Treatment of a solution of RhCl3·3H2O in a mixture of isopropyl alcohol and toluene with equimolar amounts of PCP gave the dimeric rhodium complex 1. In an electrophilic metalation a facially coordinated pincer complex is formed. When PCP is treated with [CODRhCl]2 in a solution of pyridine, the square-pyramidal complex 2 is generated where the bis-phosphine PCP acts as bidentate ligand that coordinates in a cis fashion. SnCl2 inserts into the Rh–Cl bond of 2, which results in an oxidative addition of one of the methylene C–H bonds to form the Rh­(III) complex 3, where the PCP ligand coordinates in a meridional way. A 2 equiv portion of PCP reacts with 1 equiv of [CODRhCl]2 in the presence of the electron-donating ligands HPhPC6H4NMe2, PPh2Py, and PPh3, respectively, as well as with stanna- and germa-closo-dodecaborate to give the octahedral Rh­(III) complexes 4–8. Attempts to remove the HCl with KOtBu from complexes 4–6 produces the planar Rh­(I) compounds 9 and 10. No carbene formation has been observed.

Cationic mono and dicarbonyl pincer complexes of rhodium and iridium to assess the donor properties of PCcarbeneP ligands

“…A third class of PCP pincer ligands incorporate an sp 2 hybridized carbene donor in the anchoring position; [12][13][14] here the ligands can be formally neutral or dianionic depending on the character of the carbene donor and the metal to which the ligand is bound. For example, the PC carbene P ligand system IV 15 behaves in a "Fischer-like" manner when bound to iridium 16 or rhodium(I), 17,18 but in a "Schrock-like" way when supporting nickel(II) 19 complexes. Indeed, intermediate behaviour along this continuum is observed in the palladium chemistry associated with these ligands.…”

Facile generation of iridium PCcarbeneP pincer complexes via water elimination from an alcohol proligand

“…The flexibility of PC sp 3P ligands, related to 1, to adopt both fac and mer configurations is well-documented. 18 As the signal intensities from II diminish, signals for product 2 grow in intensity. Isomerisation of the tridentate PCP ligand from fac to mer generates III.…”