# Synthesis, Structures and Corrosion Inhibition Properties of 4-Nitrophenylacetato-Rare-Earth(III) 1D Coordination Polymers

## TL;DR

This paper reports the synthesis and corrosion inhibition properties of rare-earth coordination polymers with 4-nitrophenylacetate ligands.

## Contribution

The study introduces new rare-earth coordination polymers and identifies one with high corrosion inhibition efficiency.

## Key findings

- Complex 5Dy showed the highest corrosion inhibition efficiency (89%) among the tested complexes.
- The coordination structures of the complexes vary with the rare-earth element and hydration state.
- Coordination isomerism was observed in some complexes between single crystals and powders.

## Abstract

The rare earth (RE) aqua 4-nitrophenylacetate (4npa) complexes {[RE(4npa)3(H2O)2]·2H2O}n (RE = La (1La), Nd (2Nd)), [Ce(4npa)3(H2O)2]n (3Ce), and {[RE2(4npa)6(H2O)]·2H2O}n (RE = Gd (4Gd), Dy (5Dy), Y (6Y), Er (7Er), Yb (8Yb)) were synthesised by salt metathesis reactions of REIII chlorides or nitrates with sodium 4-nitrophenylacetate Na(4npa) in aqueous ethanol. The structures of all the complexes were determined by single-crystal X-ray diffraction (SCXRD) except for RE = 4Gd, which was determined to be isomorphous with the 5Dy and 7Er complexes by X-ray powder diffraction (XRPD). All the complexes crystallise as one-dimensional polymers linked by bridging carboxylates. Complexes (1La–3Ce) have mononuclear repeating units with two coordinated waters and ten coordinate RE ions, 1La and 2Nd also have two waters of crystallization, but 3Ce has none. By contrast, complexes (4Gd–8Yb) have binuclear repeating units with a single coordinated water. Isomorphous 5Dy and 7Er have one nine coordinate and one eight coordinate metal ion, whilst isomorphous 6Y and 8Yb have two eight coordinate RE ions. In some cases, bulk powders have structures different from the corresponding single crystals. For example, bulk 1La is isomorphous with 3Ce owing to the loss of water of crystallization, and 8Yb exhibits coordination isomerism between single crystals and microcrystalline powder. Weight loss corrosion tests revealed that {[Dy2(4npa)6(H2O)]·2H2O}n (5Dy) has the greatest inhibition efficiency (89%) of the complexes (1La–8Yb). The activities are comparable to those of the corresponding 4-hydroxyphenylacetates (4hpa) and far superior to those of 2-hydroxyphenylacetates (2hpa) and the unsubstituted phenylacetates. Whilst the coordination numbers generally decline with the lanthanoid contraction, there are deviations around 5Dy, 6Y, 7Er, and 8Yb, and the corrosion inhibition is optimised with a midrange size.