Cationic ligands between σ-donation and hydrogen-bridge-bond-stabilisation of ancillary ligands in coinage metal complexes with protonated carbodiphosphoranes.

A series of copper(I) and silver(I) complexes is reported containing the PCP-type tridentate ligand [(dppm)2CH]+ with a protonated carbodiphosphorane (CDPH) as cationic central donor group (dppm = 1,1-bis(diphenylphosphino)methane). In comparison to the previously reported gold complex [({dppm}2CH)AuCl]+ [Reitsamer et al., J. Organomet. Chem., 2017, 830, 150], the corresponding silver and copper complexes exhibit a κC-coordinated CDPH-group, which is absent in case of gold. For the series of silver complexes, we demonstrate that the κC-coordination of the CDPH-group is dependant on the ancillary ligand and that hydrogen bonding of the CDPH-group to the ancillary ligand can be competitive. The ability for such hydrogen bonding is a unique characteristic of protonated CDPs in comparison to other cationic ligands, which might offer benefits for applications in homogeneous catalysis by hemilability and substrate activation with this group.

From carbones to carbenes and ylides in the coordination sphere of iridium.

The carbodiphosphorane-based iridium pincer complex (2) is demonstrated to rearrange in chlorinated organic solvents under cleavage of a P-C-bond to give a chelating phosphine ylide ligand. A detailed mechanistic investigation reveals that these types of donor groups are prone for P-C-bond cleavage in the coordination sphere of transition metal hydrido complexes. Finally, complex 2 is demonstrated to be an efficient hydrogen-borrowing catalyst.

Quantifying the Donor Strength of Ligand-Stabilized Main Group Fragments.

Unusual binding properties, enabling the stabilization of elusive species, and beneficial properties for homogeneous catalysts have been predicted and demonstrated for ligand-stabilized main group fragments, such as carbodiphosphoranes and -carbenes. However, the quantification and comparison of their binding properties by experimental means still represent major challenges. In this article, we describe a series of iridium(III) pincer complexes of the type [(PEP)IrCl(CO)(H)] q enabling the quantification of the donor strength of the central donor group E ( q = 0, +1, +2). Our investigations show that phosphine-stabilized boron(I) and carbon(0) compounds are exceptionally strong neutral donor groups in comparison to common spectator ligands in homogeneous catalysis such as carbenes and phosphines. Our experimental and computational results for the first time allow and justify the comparison of the donor strength of cationic, neutral, and anionic ligands. On the basis of quantum chemical investigations, we further demonstrate that the heavier homologues of phosphine-stabilized borylenes and carbon(0) compounds exhibit slightly diminished donor properties.

Carbodiphosphorane-based nickel pincer complexes and their (de)protonated analogues: dimerisation, ligand tautomers and proton affinities.

The reactivity patterns of carbodiphosphoranes (CDPs) as ligands are much less explored than those of isoelectronic analogues. In the current manuscript, we investigate the reactivity of the carbodiphosphorane-based PCP nickel(ii) pincer complex [({dppm}2C)NiCl]Cl (1) towards acids and bases, calculate proton affinities, analyse the bonding situation and tautomeric forms with the aim to evaluate whether CDPs can potentially act as cooperative ligands in catalysis (dppm = 1,1-bis(diphenylphosphino)methane). Our investigations show that different tautomeric forms are stable for the coordinated and the uncoordinated ligand. The protonated CDP-based complex 2 represents a rare example of a cationic donor group binding to a cationic metal centre. The continuous arm-deprotonation of 1 leads to the formation of remarkably stable dimers with Ni-C-P-C-metallacycles. In comparison to corresponding boron and amine-based ligands, the coordinated CDP-group exhibits the lowest proton affinity according to DFT calculations, indicating that coordinated CDP ligands can potentially serve as proton relay in cooperative catalysis.

Donor ligands based on tricoordinate boron formed by B-H-activation of bis(phosphine)boronium salts.

We report a novel method for the preparation of PBP-pincer complexes from bis(phosphine)boronium salts. The central (R3P)2HB-moiety in a palladium complex is demonstrated to be a L-type ligand, therewith completing a series of pincer-type complexes with Z-, X- and L-type boron-based ligands, respectively.