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

Abstract

The anionic tris(pyrazolyl)borates, or scorpionates, have proven to be extremely useful ligands. Neutral tris(pyrazolyl)methane ligands, however, are difficult to prepare and require numerous purification steps for a number of substitution patterns. We have previously outlined two different routes for accessing neutral tris(pyrazolyl) ligands. We describe here an adaptation of the previously published procedures for the synthesis of the electron-poor ligand tris(3,4,5-tribromopyrazol-1-yl)phosphine oxide, CBrNOP. Similar electron-deficient ligands have been proven to unlock unique chemistry for the anionic scorpionates. The title perbrominated tris(pyrazolyl)phosphine oxide displays a network of halogen bonds in the solid state. All the bonds in the pyrazole ring are rather similar to the reported borate analogues, which makes this molecule promising as a ligand for applications where very electron-poor metal complexes are required.

Phosphine Oxides as Spectroscopic Halogen Bond Descriptors: IR and NMR Correlations with Interatomic Distances and Complexation Energy

“…For example, in [52,53], the crystal adducts of various phosphine oxides (Ph 2 (Me)P=O, Ph 3 PO) with strong halogen donors such as pentafluoroiodoebenzene C 6 F 5 I and 1,4-diaryl-5-iodotriazole are described. Finally, for a series of crystals containing molecules with P=O groups, the presence of short X···O contacts (X = Cl, Br, I), which can be interpreted as halogen bonds, has been established with the help of X-ray analysis and quantum-chemical calculations [54][55][56][57][58][59][60][61]. Many other examples of XBs with phosphine oxides and related compounds can be found in CCDC data, which allowed us to perform an extensive database search and analyze the distributions of geometric parameters, as described in Section 4.…”

Crystal structures of 5-bromo-1-arylpyrazoles and their halogen bonding features

“…A quick search in the CSD revealed 17 hits [41][42][43][44][45][46][47][48][49][50][51][52][53] for 5-bromopyrazole structures, 14 of them presenting short halogen contacts involving 5-Br-pyrazole. These 14 structures are discussed in 10 articles and to date, the remaining structures 49,52,53 appear in the CSD as communications only, without associated articles.…”

“…However, of the 10 published papers only three make clear reference to the presence of halogen bonding involving the 5-Br atom without any further study. 41,47,48 As we had previously investigated the halogen bond propensity in 5-iodinated pyrazoles, we hypothesized that 5-bromopyrazoles should possess similar features, despite the relatively smaller magnitude of the σ-hole of the Br atom compared with that of the I atom. Having this in mind we have synthesized and obtained single crystals of five 5-bromopyrazoles which displayed good diffraction quality.…”

“…However, of the 10 published papers only three make clear reference to the presence of halogen bonding involving the 5-Br atom without any further study. 41,47,48…”

“…[33][34][35][36][37] We also previously reported the halogen bonding properties in 5-iodopyrazoles which we found to be favorable substrates for investigating such interactions. [38][39][40] Proceeding further in attempting to understand halogen bonding, our attention was drawn to 5-bromopyrazoles, as we observed that crystal structures containing a pyrazole brominated at the 5 position are rather scarce (with only 17 hits in the Cambridge Structural Database (CSD)) [41][42][43][44][45][46][47][48][49][50][51][52][53] and usually represented by transition metal complexes. Another reason for pursuing these compounds is that from our previous studies we observed that even when the Br atom is not attached at the 5 position of the pyrazole, it is prone to halogen bonding.…”