Metal-binding motifs of alkyl and aryl phosphinates; versatile mono and polynucleating ligands

An analysis of 552 structures of metal complexes of alkyl and arylphosphinates in the Cambridge Crystallographic Database shows that the phosphinate ligating group is remarkably versatile and is able to adopt ten different binding motifs in both mono- and polynuclear complexes in which an individual phosphinate group can bind to up to five metal atoms. The majority of both homo- and heteroleptic complexes contain Msingle bondOsingle bondPR2double bond; length as m-dashOsingle bondM units in oligomeric and polymeric structures. In many heteroleptic complexes ligands containing hydrogen bond donors form strong bonding interactions with the phosphinate, generating pseudochelated structures. Similar pseudochelates, −Osingle bondPR2double bond; length as m-dashO⋯Hsingle bondOsingle bondPR2double bond; length as m-dashO, are formed when both a phosphinate and its parent phosphinic acid are coordinated to a single metal atom. Such structures feature also in the solution chemistry involved in metal extraction processes using phosphinate ligands. As might be expected, many of the binding motifs found in phosphinate complexes are similar to those in carboxylate complexes but there are fewer examples of phosphinates being used to form metal organic frameworks.