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]nvia 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 (EHOMO1: −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.