Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity

Abstract

Developments in palladium chemistry have been spurred predominantly by the outstanding application potential of this metal in catalysis. The quest for new ligands in order to modulate the catalytic activity and selectivity of the palladium center has been greatly stimulated by the discovery of N-heterocyclic carbenes as formally neutral, strongly donating, and covalently binding ligands. Abnormal variations of N-heterocyclic carbenes, even though known (yet not recognized) for 30 years, have received very little attention until recently. In parts this may have been due to the fact that the free abnormal carbene ligand is much less stable than the normal carbene analogues. In the last decade, significant progress has been made in abnormal carbene palladium chemistry and reliable synthetic routes as well as promising catalytic applications have been developed. As a consequence, these type of complexes have gradually transformed from laboratory curiosities to unique formally neutral ligands with exceptional donor ability. Here, the advances in abnormal carbene palladium chemistry are summarized. In an attempt to stimulate the entry of newcomers in this fascinating field of research, elementary aspects of synthesis are discussed as well as progress in characterization of the complexes. Most recent (catalytic) applications may highlight the potential of this rapidly growing area of palladium chemistry.

Normal and Abnormal N-Heterocyclic Carbene Ligands

“…Such a separation has indeed been predicted by a theoretical analysis of the bimetallic complex 10 ( Fig. Likewise, most X-ray structure analyses of C4-metalated imidazolylidenes reveal a longer C]C bond (1.36-1.38 Å ), 46 which suggests a more delocalized double bond than in imidazolylidenes that bind the metal via C2. 45 The bonding in this complex features a high-lying occupied molecular orbital centered on the C]CdPd fragment as donor moiety that interacts with the LUMO of the [Ag(NCMe) 3 ] + fragment, thus providing the most relevant contribution for stabilizing the PddAg bond in complex 10 ( Fig.…”

Platinum(ii) and platinum(iv) complexes stabilized by abnormal/mesoionic C4-bound dicarbenes

“…Moreover, short contacts between the chloride and the methyl groups of DMSO were identified. The distances C17- 25 H···Cl and C16-H···Cl are 2.801 and 2.756 Å, respectively, and may point to weak intramolecular hydrogen bonding interactions in the solid state. and BF4 -anion omitted for clarity).…”

“…2 Hence, 25 stabilization of high oxidation states will provide useful insights for understanding the reactivity of such intermediates. A suitable approach consists of using strong Lewis base ligands such as Nheterocyclic carbenes (NHCs) 3 for accessing high-valent metal centers.…”

Rational Design of Ligand Precursors to Prepare Abnormal (Mesoionic) and Normal Carbene Complexes and Zwitterionic CX‐Type Palladacycles (X = C, N)

“…The Pd–C bond length in 7c is comparable to those of related trans pyridine/abnormal carbene complexes 11. It is slightly shorter than the average bond length of 1.99(3) Å (seen in 41 crystallographically determined Pd complexes with abnormal carbene ligands) 4d. An important structural difference between the two structures is the orientation of the heterocyclic ring with respect to the coordination plane (which contains PdCl 2 CN atoms).…”