Review
Phosphinic acids as building units in materials chemistry

https://doi.org/10.1016/j.ccr.2020.213748Get rights and content

Highlights

  • The properties of phosphinic acids fall between phosphonic and carboxylic acids.

  • Phosphinate-based ligands lead to a variety of coordination polymer architectures.

  • Phosphinates are used to improve the efficiency of dye-sensitized solar cells.

  • Phosphinate capping agents lead to unique nanoparticle properties.

Abstract

Diaryl and dialkyl phosphinic acids are diverse groups of ligands capable of bonding to a wide variety of metal ions. This review provides a brief overview of their synthesis methods, and summarizes various functions and binding motifs phosphinic acids can have in the area of materials chemistry. The combination of these characteristics allows for many applications in coordination polymers, modification of surfaces, synthesis of nanoparticles, and sol–gel synthesis. The possibility of tuning electronic and steric properties by two aryl/alkyl substituents directly attached to phosphorus atom translates into unique properties of materials that cannot be replicated by the utilization of analogous phosphonates or carboxylates. Recent developments in the field and perspective directions of future research are also highlighted.

Introduction

Phosphinic acids of the general formula R1R2POOH, where R1 and R2 are hydrogen, alkyl, or aryl groups, are versatile ligands capable of forming a wide range of binding motifs (Fig. 1) [1]. The acids can be regarded as derivatives of phosphonic acids RPO(OH)2, where one –OH group is replaced by an Ri group. If R1 and R2 substituents are different, phosphinic acids are chiral, although the chirality can be lost upon deprotonation. Phosphinic acid esters, which retain their chirality, are an important intermediate in the preparation of p-stereogenic compounds for asymmetric syntheses and catalytic reactions [2]. This review, however, deals predominantly with coordinated phosphinates in their deprotonated, achiral form or after their racemization through deprotonation. The presence of two oxygen atoms capable of coordination results in similar coordination motives of phosphinic and carboxylic acids, with the obvious specificity resulting from sp3 hybridisation on phosphorus in phosphinates as opposed to sp2 hybridisation on carbon in carboxylates (for details see recent review [1]). The presence of two substituents on phosphorus atom imparts phosphinic acids with unique steric properties that have been utilized to improve the desired properties on the molecular level as well as in solid state chemistry. The acidity of phosphinic acids lies between the acidity of carboxylic and phosphonic acids (compare pKa = 4.76 for acetic acid, 2.38 for methylphosphonic acid, and 3.08 for dimethylphosphinic acid) [3]. The most acidic phosphinic acid reported to date is meta-carboranylphosphinic acid with pKa value of 1.32 [4]. The purely inorganic carborane substituent gives the P-OH group more acidic character than its closest organic analogue, phenyl group in phenylphosphinic acid (pKa 1.77). The phosphinate group also falls between carboxylates and phosphonates in terms of Pearson’s theory of hard and soft acids and bases [5]. This intermediate position, along with the synthetic tunability brought by the presence of two substituents, makes phosphinates unique molecules with programmed properties for utilization in the field of material chemistry.

From the practical point of view ligands coordinated via a P-containing group provide a 31P NMR spectroscopic handle that allows for characterization and identification of individual species even in complicated mixtures. For example, 31P NMR can be used for identification of molecular intermediates during the synthesis of ZnO nanoparticles [6] (details see Section 5). Solid state NMR experiments enable differentiating and quantification of coordination modes – non-bonded acid, acid coordinated through one oxygen, or coordinated through both oxygen atoms [7]. Another instrumental technique that can provide insight into phosphinate bonding is Fourier transform infrared spectroscopy (FTIR). Bonding of phosphinic acids to metal ions or surfaces is reflected in changes to O–H and O-P-O vibrations. Furthermore, in the case of phosphinates bearing hydrogen atom as one of the substituents, the P-H stretching mode appears in an area around 2400 cm−1 and does not overlap with vibrations of common groups, and thus provides an easily identifiable tool. On oriented samples, such as metal oxide surfaces, polarized FTIR can be used to elucidate the orientation of attached molecules [8]. In this case, three mutually nearly orthogonal vibrational modes, the P-H stretching, and symmetric and antisymmetric O-P-O stretching were analysed to determine the orientation of phosphinate molecules on the surface [9].

Phosphinic acids are often used as flotation agents, flame retardants, ionic liquids (e.g., for separation of cations or as lubricating agents) [10], [11], [12], and for medicinal applications as peptide analogues that comprise the tetrahedral phosphorus moiety to replace an internal amide bond [13]. The area of organic–inorganic hybrids formed by organophosphorus acids was reviewed by Mutin et al. in 2003 [14]. Since then, the topic has undergone fast development with much of the research being focused on phosphonic acids and less on phosphinic acids [15]. Herein, we highlight the potential that phosphinic acids hold in the field of material chemistry, for the formation of coordination polymers, as capping agents for nanoparticles, as surface modifying agents, and in sol–gel chemistry. Various phosphinate ligands discussed in this review are shown in Fig. 2.

Section snippets

Synthesis of phosphinic acids

Only few phosphinic acids are commercially available, which implies the need for their synthesis before applications directly in laboratories. A review on synthetic procedures producing phosphinic acids has been published earlier [16]. Nevertheless, we would like to highlight common synthetic strategies, which are summarized in Scheme 1. Phosphinic acids or their esters can be conveniently prepared by hydrolysis or alcoholysis of the reactive PCl2 group (Scheme 1A) [17]. Hypophosphorous acid or

Coordination polymers

As reported by Carson et al. [1], phosphinic acids exhibit a broad range of coordination modes (Fig. 1), which makes them interesting building blocks for the construction of coordination polymers. As was already mentioned in Section 1, phosphinates, phosphonates, and carboxylates exhibit some similarities in their coordination behaviour. Despite that, phosphinates behave in their own unique way when forming coordination compounds. For example, in basic conditions, carboxylates form layered

Surface modifications by phosphinic acids

The surfaces of metal oxides and other materials can be covalently functionalized by organic molecules bearing various anchoring groups, including silane, phosphonate, carboxylate, or phosphinate. Silanes (e.g., methoxysilanes and halogensilanes) are the most commonly used. They rapidly form covalent bonds with surface atoms, although especially trifunctionalised organosilanes tend to undergo intra-monolayer crosslinking causing oligomerization [79]. Phosphonic acids are not as prone to

Phosphinic acids in nanoparticle syntheses

As was discussed in Section 4, phosphinic acids readily form strong coordination bonds with many transition metals and metal oxides. Therefore, phosphinic acids can be utilized as capping agents during the synthesis of nanoparticles to stabilize the nanoparticle size, prevent nanoparticle aggregation, or tune nanoparticle dispersibility. The advantage of phosphinic acids is that they can bear two alkyl/aryl substituents and therefore allow for tuning the sterical or hydrophobic/hydrophilic

Phosphinic acids in sol–gel syntheses

Simple phosphinic acids or ionic liquids containing phosphinate anions were used as porogens in sol–gel synthesis. Thus, the addition of 2 into Ti(O-i-Pr)4 sol resulted in the formation of xerosols/xerogels containing an oxo-alkoxide cluster [Ti(O)(O-i-Pr)(O2PPh2)]4 [123]. The authors documented that the resulting material displays a surface area up to 110 m2 g−1, but the origin of the porosity remains unclear.

Later, phosphinate 25 was incorporated in the form of ionic liquid by the sol–gel

Conclusions and outlook

This review illustrates various possibilities phosphinic acids offer for construction of coordination polymers or modification of functional surfaces. Although phosphinic acids have not gained as much attention as closely related phosphonic or carboxylic acids, the recent development documented in this review shows that phosphinic acids bring unique properties to the field of material chemistry. The properties of phosphinates fall in many ways between the properties of carboxylates and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Czech Science Foundation (No. 20-04408S). The authors are grateful to the working group Interactions of Inorganic Clusters, Cages, and Containers with Light within the AV21 Strategy of the Czech Academy of Science.

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