Anions derived from squaric acid form interionic pi-stack and layered, hydrogen-bonded superstructures with organometallic sandwich cations: The magnetic behaviour of crystalline [(eta(6)-C6H6)(2)Cr](+)[HC4O4](-)

Depending on the stoichiometric ratio, squaric acid (3,4-dihydroxy -3-cyclo butene-1,2-dione, H(2)SQA) reacts with [(eta(6)-C6H6)(2)Cr] in THF to form the crystalline material [(eta(6)-C6H6)(2)Cr]-[HSQA] (1) and in water to yi eld [{(eta(6)-C6H6)(2)Cr}(2)][SQA] . 6 H2O (3); it also reacts with [(eta(5 )-C5H5)(2)Co][OH] in water to form [{(eta(5)-C5H5)(2)Co}(2)][SQA] . 6 H2O ( 4). Compound 1 is almost isostructural with the previously reported salt [( eta(5)-C5H5)(2)Co][HSQA] (2); its structure is based on pi-pi stacks betwee n the benzene ligands and the hydrogen squarate anionic chains (pi-pi dista nce 3.375 Angstrom). Compounds 3 and 4 are isomorphous and have a structure in which layers organometallic cations intercalate with layers of water mo lecules hydrogen bonded to squarate dianions. All crystals contain charge-a ssisted C-H delta(+)... O delta(-) hydrogen bonds between the organometalli c and the organic components, while negative O-H(-)... O(-) and O-H ... O(2 -1) interactions are present in the pairs 1/3 and 2/4, respectively. In con strast to most organic salts of [(eta(5)-C6H6)(2)Cr](+) and [(eta(5)-C5H5)( 2)Co](+) which are yellow, crystals of compounds 1-4 are orange. Reflectanc e spectra measured on the crystalline material I show the presence of an in tense tail that can be assigned to a charge-transfer transition through the [(eta(6)-C6H6)(2)Cr](+)/ [HSQA] pi-stacking interactions, while the pi sta cking in 2 causes only a broadening of the band. The magnetic behaviour of 1 and 3 has been investigated by SQUID magnetometry. Both compounds are cha racterised by a weak antiferromagnetic interaction between the S=1/2 Cr cen tres of the [(eta(6)-C6H6)(2)Cr](+) cations, which is significantly stronge r in 1 due to the pi-stacking with the HSQA(-) anions.