Three pyridyl modified Cu(II)/Cd(II)-diphosphonates: Syntheses, crystal structures and properties

Three examples of M(II)-diphosphonates, [Cu3(H3LH)4]·2(OH)·2H2O 1, [Cu3(H2L)2(H2O)2] 2, and [Cd(H2LH)]·H2O 3 based on 1-hydroxy-2-(3-pyridyl)ethylidene-1,1-diphosphonic acid (HxL/HxLH: x = 0–5, H5L = (H4C5N)CH2C(OH)(PO3H2)2, H5L-H = (H4C5NH)CH2C(OH)(PO3H2)2), have been hydrothermally obtained and characterized by powder X-ray diffraction, elemental analysis, TG-DSC and IR. The single-crystal X-ray diffractions reveal that compounds 1 and 3 are one-dimensional chain structures (linear for 1 and ladderlike for 3) constructed by binuclear units [M2O2]n, simultaneously organic pyridine-ring suspending both sides, and compound 2 shows a two-dimensional inorganic-organic alternate arrangement layer built from 1-D ladderlike inorganic chain with trinuclear units [Cu3(OPO)4]n via pyridine-ring linker. The results of electrochemical measurements indicate that both 2 and 3 are significant negatively shifted by 0.17 V and 0.13 V (0.33 V for 2 and 0.37 V for 3), respectively, while 1 was red-shifted by 0.87 V (1.37 V), compared with the ligand H5L (0.50 V). Moreover fluorescent measurements reveal that compounds 1–3 display fluorescent emission bands, 383 nm and 425 nm for 1, 382 nm and 425 nm for 2 and 311 nm, 378 nm and 422.5 nm for 3 (λex = 235 nm), caused by intraligand π*–π emission state of the ligand H5L (λex = 233 nm). Magnetic data indicate that compound 1 exhibits weak ferromagnetic interactions within 1-D linear chain, but compound 2 gives an antiferromagnetic behavior within 1-D ladderlike chain. The energy levels of the frontier molecular orbitals of 1∼3 are obtained from DFT calculations (EHOMO 1: −15.23 eV, 2: = −9.74 eV, 3: −11.5 eV), and the low HOMO–LUMO gaps of 1 (0.38 eV), 2 (0.20 eV) and 3 (0.38 eV) mean that high chemical reactivity for three compounds.