Complexes of silver(I), thallium(I), lead(II) and barium(II) with bis[3-(2-pyridyl)pyrazol-1-yl]phosphinate: one-dimensional helical chains and discrete mononuclear complexes

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Abstract
Reaction of 3-(2-pyridyl)pyrazole with POBr 3 in toluene–NEt 3 afforded not the expected tris(pyrazolyl)phosphine oxide but the partially hydrolysed compound bis[3-(2-pyridyl)pyrazol-1-yl]phosphinate (as its triethylammonium salt). This compound has two potentially chelating N,N′-bidentate arms linked by an apical PO 2 - group. Reaction with AgNO 3 , Tl(O 2 CMe), Pb(NO 3 ) 2 or Ba(NO 3 ) 2 in dry MeCN followed by recrystallisation afforded crystals of the complexes [AgL]·2H 2 O, [TlL]·MeOH, [PbL 2 ]·H 2 O and [(BaL 2 ) 3 ]·6MeCN·2H 2 O respectively, all of which have been crystallographically characterised. The compound [AgL]·2H 2 O contains infinite helical chains (AgL) ∞ in which each ligand donates one N,N′-bidentate arm to each of two metals and each metal ion is four-co-ordinated by two arms from different ligands. The strands are held together in the crystal by a complex network of hydrogen bonds involving lattice water molecules and also by aromatic π-stacking interactions. The compound [TlL]·MeOH is likewise a one-dimensional helical polymer of TlL units, with each ligand bridging two metals and each Tl ion in a ‘2 + 3’ co-ordination geometry with two short bonds to ligands (2.87 Å): there is an obvious gap in the co-ordination sphere due to a stereochemically active lone pair. A combination of interstrand aromatic π-stacking interactions and hydrogen-bonding interactions involving the lattice MeOH molecule is present. The compound [PbL 2 ]·H 2 O is in contrast a discrete mononuclear complex, four-co-ordinated just by one bidentate arm from each of the two ligands with the other bidentate arms pendant: again there is a stereochemically active lone pair. The metal geometry is approximately trigonal bipyramidal with the lone pair in an equatorial position. The lattice water molecule is hydrogen bonded to three different complex units. The compound [(BaL 2 ) 3 ]·6MeCN·2H 2 O contains mononuclear BaL 2 and dinuclear Ba 2 L 4 units: the former is ten-co-ordinate, with each ligand acting as an N 4 O donor and a phosphinate oxygen atom participating in co-ordination, whereas in the latter each Ba II is nine-co-ordinated and two of the ligands are bridging, donating their four N atoms to one metal ion and a phosphinate oxygen atom to the other.

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