Real-time monitoring of the Suzuki–Miyaura reaction using mass spectrometry during sequential addition of the various reaction components suggests that a dynamic series of equilibria exist in these solutions. Depending on conditions, the boronic acid can be dehydrated, deprotonated, hydroxylated (or alkoxylated), or fluorinated. Palladium–phosphine species present include Pd(0) (to which the aryl iodide rapidly oxidatively adds), the Pd­(II) aryl iodide complex, a cationic Pd­(II) species formed by dissociation of the iodide ligand, and the Pd­(II) bisaryl complex that ultimately extrudes the product through reductive elimination. No fluorinated or hydr­(alk-)­oxylated palladium complexes were observed under catalytic conditions. Several transmetalation combinations were excluded as reactive partners, but several possibilities remain, and more than one mechanism is likely to be operative, even under similar conditions.

The donor properties of five different PCcarbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(i) and rhodium(i) carbonyl cations. The ligands feature dialkyl phosphine units (R = (i)Pr or (t)Bu) linked to the central benzylic carbon by either an ortho-phenylene bridge, or a 2,3-benzo[b]thiophene linker; in the former, substituent patterns on the phenyl linker are varied. The carbonyl complexes are synthesized from the (PCcarbeneP)M-Cl starting materials via abstraction of the chlorides in the presence of CO gas. In addition to the expected mono carbonyl cations, products with two carbonyl ligands are produced, and for the rhodium example, a novel product in which the second carbonyl ligand adds reversibly across the Rh[double bond, length as m-dash]C bond to give an η(2) ketene moiety was characterized. The IR data for the complexes shows the 2,3-benzo[b]thiophene linked system to be the poorest overall donor, while the phenyl bridged ligands incorporating electron donating dialkyl amino groups para to the anchoring carbene are very strongly donating pincer arrays.

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.

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