Amino acid-lanthanide interactions--2. The X-ray crystal structures of lanthanide and calcium complexes of 1-amino cyclohexane-1-carboxylic acid (Acc 6 ), [Nd 2 (Acc 6 ) 6 (H 2 O) 6 ](ClO 4 ) 6 (H 2 O) 6 (1), [Er 2 (Acc 6 ) 4 (H 2 O) 8 ](ClO 4 ) 6 (H 2 O) 1 1 (2) and [Ca 5 (Acc 6 ) 1 2 (H 2 O) 6 ](ClO 4 ) 1 0 (H 2 O) 4 (3)