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

The rare earth (RE) aqua 4-nitrophenylacetate (4npa) complexes {[RE(4npa)₃(H₂O)₂]·2H₂O}_n (RE = La (<b>1La</b>), Nd (<b>2Nd</b>)), [Ce(4npa)₃(H₂O)₂]_n (<b>3Ce</b>), and {[RE₂(4npa)₆(H₂O)]·2H₂O}_n (RE = Gd (<b>4Gd</b>), Dy (<b>5Dy</b>), Y (<b>6Y</b>), Er (<b>7Er</b>), Yb (<b>8Yb</b>)) were synthesised by salt metathesis reactions of RE^III 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 = <b>4Gd</b>, which was determined to be isomorphous with the <b>5Dy</b> and <b>7Er</b> complexes by X-ray powder diffraction (XRPD). All the complexes crystallise as one-dimensional polymers linked by bridging carboxylates. Complexes (<b>1La</b>-<b>3Ce</b>) have mononuclear repeating units with two coordinated waters and ten coordinate RE ions, <b>1La</b> and <b>2Nd</b> also have two waters of crystallization, but <b>3Ce</b> has none. By contrast, complexes (<b>4Gd</b>-<b>8Yb</b>) have binuclear repeating units with a single coordinated water. Isomorphous <b>5Dy</b> and <b>7Er</b> have one nine coordinate and one eight coordinate metal ion, whilst isomorphous <b>6Y</b> and <b>8Yb</b> have two eight coordinate RE ions. In some cases, bulk powders have structures different from the corresponding single crystals. For example, bulk <b>1La</b> is isomorphous with <b>3Ce</b> owing to the loss of water of crystallization, and <b>8Yb</b> exhibits coordination isomerism between single crystals and microcrystalline powder. Weight loss corrosion tests revealed that {[Dy₂(4npa)₆(H₂O)]·2H₂O}_n (<b>5Dy</b>) has the greatest inhibition efficiency (89%) of the complexes (<b>1La</b>-<b>8Yb</b>). 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 <b>5Dy</b>, <b>6Y</b>, <b>7Er</b>, and <b>8Yb</b>, and the corrosion inhibition is optimised with a midrange size.