Tris(pyrazolyl)phosphine Oxides. Synthesis and Coordination Chemistry with Copper(I)

Abstract

A set of substituted tris(pyrazolyl)phosphine oxides (OP(pz x ) 3 ) has been prepared in high yield and applied as neutral scorpion-type ligands. The P apex provides a convenient spectroscopic handle. Substitution at the 3-position of the pyrazolyl ring influences the steric demands of the ligand, while substitution at the 5-position enhances the stability. Copper(I) acetonitrile complexes of the OP(pz x ) 3 ligands were prepared and tested in ligand exchange reactions with PPh 3 and CO. The ν(CO) values of the carbonyl complexes demonstrate the electron-withdrawing properties of the ligands. These observations show that OP(pz x ) 3 ligands are an interesting extension of the widely used scorpion-type ligands.

Tris(pyrazolyl)phosphine Oxide and Tris(triazolyl)phosphine Oxide Scorpion Ligands

“…Both ligand systems and some transition-metal complexes thereof are experimentally known, such as those shown in Figure 2. [9,20] The geometry of the lowest-energy form of the C Me2 ligand is nearly C 3 -symmetric. Evidently, introduction of the methyl groups causes a "ring flip" of two of the pyrazolyl rings of C. Introduction of the phenyl substituent on the triazole rings (E Ph ) has little effect on the geometry of E, which is hardly surprising as the phenyl substituents are positioned far from both the PO apex and the other rings.…”

“…Replacing the acetonitrile ligand for a CO group becomes slightly exothermic by Յ1.5 kcal mol -1 , but the calculated CO stretching frequencies are smaller by 25 cm -1 for the pyrazolyl ligand and 12 cm -1 for the triazolyl ligand (Table 1). Presumably, the substituents contribute to an increase in electron density of the metal centers, which results in a slightly increased πback-donation to CO. [9] …”

“…We next tried to introduce a CO ligand by exposing a solution of the presumed tris(triazolyl)phosphine oxide Cu I complex in CH 2 Cl 2 to a CO atmosphere in an analogous freeze-pump-thaw procedure that was used for the HCand OP-based pyrazolyl ligands B [31] and C. [9] Whereas at ambient temperature the reaction mixture was a suspension under N 2 , thawing under CO gave a clear solution from which a precipitate resulted upon addition of pentane. This material did not show a CO frequency by IR spectroscopy in contrast to products obtained from the same procedure used for ligands B and C. We presume that the CO ligation to the [E Ph Cu][PF 6 ] complex is relatively weak and that the labile carbonyl species formed loses its ligand once the CO pressure is released.…”

“…[9] For example, Cu I coordinates to each of the "free" nitrogen atoms of the three pyrazolyl groups with the fourth ligand being CO, triphenylphosphine, or acetonitrile, for example, [C Me2 Cu-(NCMe)][PF 6 ] (Figure 2, left). These ligands are readily synthesized in high yield from OPCl 3 and pyrazole in the presence of a base and form complexes with transition metals.…”

A new synthetic route to the electron-deficient ligand tris(3,4,5-tribromopyrazol-1-yl)phosphine oxide

“…The latter ligands are, however, difficult to prepare, requiring numerous purification steps for a number of substitution patterns (Bigmore et al, 2005). Our group presented recently two different alternatives for accessing neutral tris(pyrazolyl) ligands, namely the corresponding phosphanes and phosphine oxides (Tazelaar et al, 2012(Tazelaar et al, , 2014(Tazelaar et al, , 2016. We describe here an adaptation of the previously published procedures to the synthesis of an electron-poor ligand, i.e.…”