Investigation of Structural Changes of Cu(I) and Ag(I) Complexes Utilizing a Flexible, Yet Sterically Demanding Multidentate Phosphine Oxide Ligand

Abstract

The syntheses of a sterically demanding, multidentate bis(quinaldinyl)phenylphosphine oxide ligand and some Cu(I) and Ag(I) complexes thereof are described. By introducing a methylene group between the quinoline unit and phosphorus, the phosphine oxide ligand gains additional flexibility. This specific ligand design induces not only a versatile coordination chemistry but also a rarely observed and investigated behavior in solution. The flexibility of the birdlike ligand offers the unexpected opportunity of open-wing and closed-wing coordination to the metal. In fact, the determined crystal structures of these complexes show both orientations. Investigations of the ligand in solution show a strong dependency of the chemical shift of the CH 2 protons on the solvent used. Variable-temperature, multinuclear NMR spectroscopy was carried out, and an interesting dynamic behavior of the complexes is observed. Due to the introduced flexibility, the quinaldinyl substituents change their arrangements from open-wing to closed-wing upon cooling, while still staying coordinated to the metal. This change in conformation is completely reversible when warming up the sample. Based on 2D NMR spectra measured at −80 °C, an assignment of the signals corresponding to the different arrangements was possible. Additionally, the copper(I) complex shows reversible redox activity in solution. The combination of structural flexibility of a multidentate ligand and the positive redox properties of the resulting complexes comprises key factors for a possible application of such compounds in transition-metal catalysis. Via a reorganization of the ligand, occurring transition states could be stabilized, and selectivity might be enhanced.

Coinage Metal Complexes of Bis(quinoline‐2‐ylmethyl)phenylphosphine‐Simple Reactions Can Lead to Unprecedented Results

“…The synthesis of bis(quinolin‐2‐ylmethyl)phenylphosphine ( bqmpp ) has been previously reported by our group [29] . The crystal structure of bqmpp is now provided in the Supporting Information (Figure S1).…”

“…The synthesis of bis(quinolin‐2‐ylmethyl)phenylphosphine ( bqmpp ) has been previously reported by our group. [29] The crystal structure of bqmpp is now provided in the Supporting Information (Figure S1). For the synthesis of the copper(I) complex, bqmpp is dissolved in dry, degassed acetonitrile (MeCN) and the resulting mixture transferred to the equivalent amount of [Cu(MeCN) 4 ]BF 4 dissolved in MeCN with a cannula under inert gas.…”

“…In the cationic complex 1 , two phosphine ligands bridge two copper(I) ions, similar to the silver(I) hexafluoroantimonate compound of the respective phosphine oxide previously reported by our group, where oxygen takes over the coordination task of phosphorus. [29] Similar to the previously reported phosphine oxide ligand, the structure of the ligand in 1 can be compared to a bird, where one ligand spreads both quinaldinyl substituents in an open‐wing arrangement, whereas the other ligand displays a closed‐wing arrangement with a N3⋅⋅⋅N4 distance of 3.082(5) Å. The Cu1 atom exhibits a distorted square pyramidal geometry with angles ranging from 83.3(1)–134.4(1)° and a τ 5 value of 0.17.…”

“…Unless otherwise specified, all reagents and solvents were purchased from commercial sources and used as received. Complex [AuCl(tht)] [41] and ligand C 26 H 21 N 2 P [29] were prepared according to literature procedures. The solvents used were dried, freshly distilled and degassed prior to use.…”

“…[40] All complexes show a broad phosphorus resonance in the 31 P NMR spectrum at room temperature, indicating a dynamic behavior in solution similar to the complexes of the respective phosphine oxide published previously by our group. [29] The reason for the dynamics can be explained similarly and is most likely due to a rapid movement of the quinaldinyl rings in solution at room temperature. The corresponding variable temperature NMR spectra of compounds 1, 3 and 5 are shown in the Supporting Information.…”