Ligand Attachment Chemistry in the Preparation of PC_sp³P and PC_sp²P Complexes of Rhodium

Abstract

The attachment of electron-rich PCP pincer ligands bis(2-(dialkylphosphino)phenyl)methane (alkyl = isopropyl, tert-butyl) to rhodium via reactions with [(COE) 2 Rh(μ-X)] 2 (X = Cl, OSO 2 CF 3 ) through C−H bond activations is reported. The first C−H activation to produce PC sp 3 P derivatives is facile and favors products wherein the remaining benzylic C−H and the Rh−H hydrogens are transdisposed across the new Rh−C bond. For the less bulky isopropyl-substituted ligand, chlorido-or triflato-bridged dinuclear products are favored, while for the tert-butyl-adorned ligand, monomers are formed. The favoring of both trans C− H/Rh−H and dinuclear systems hampers the second C−H activation, necessary to form the (more desirable) PC carbene P derivatives. Through spectroscopic and structural investigations, the factors that influence the ligand attachment chemistry through successive C−H activations in these ligands are discussed.

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

“…† In addition to the terminal CO ligand, the structure confirms the addition of a CO to the RhvC double bond; the Rh1-C1 distance of 2.105(3) Å is significantly elongated from the 1.900(3) Å Rh-C1 distance found in 1-Cl. 18 The C1-C30 bond length of 1.442(5) Å is consistent with a C sp 3-C sp 2 single bond, while the C30-O1 length of 1.171(4) is similar to the C-O bond length of 1.162 Å in CO 2 . 51 Exposure of compounds 6-Cl or 6-CO to high vacuum resulted in loss of CO by reversal of the CO addition to the RhvC moiety; in the former case, this regenerated carbene chloride 1-Cl, while in the latter this allowed for isolation of the targeted cationic carbonyl complex of the PC carbene P ligand as a purple solid.…”

“…Details concerning the syntheses of the ligands and their rhodium(I) (1-Cl 18 ) or iridium(I) chloro carbene complexes (iPr-2-Cl, 16 tBu-2-Cl, 23 3-Cl, 35 and 4-Cl 25 ) have been previously published. The xanthene-based ligand variation seen in 5-Cl is, however, a new ligand/complex.…”

“…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.…”

Synthesis and reactivity of a PCcarbeneP cobalt(i) complex: the missing link in the cobalt PXP pincer series (X = B, C, N)

“… 13 Ozerov, and later Piers, introduced β-hydride elimination resistant systems. 14 All of these systems are reliant upon double C–H activation, and are thus restricted to metals capable of facile C–H activation ( i.e. noble metals).…”

An unusual endo-selective C-H hydroarylationof norbornene by the Rh(I)-catalyzed reactionof benzamides

“…The coordination of the N(sp 2 ) atom in the quinoline ring and the NH in amide 1 to a rhodium center gives the Rh(I)X species A . The electrophilic addition of norbornene to the rhodium complex A gives complex B , which undergoes a hydride shift to give the rhodium carbene complex C 41 . The oxidative addition of the ortho -C–H bond to the rhodium center followed by a hydride migration from the rhodium center to the carbene carbon gives E 42 , which undergoes reductive elimination to give the hydroarylation product 2 with the regeneration of the Rh(I) species.…”