Pentacyano(pyridine)chromate(III):  Synthesis, Characterization, and Photochemistry

Abstract

The Cr(CN)(5)(py)(2)(-) anion (py = pyridine) has been prepared by acid-promoted methanolysis of Cr(CN)(6)(3)(-) followed by reaction with pyridine, isolated as the potassium salt, and characterized by absorption spectra (lambda(max): 403 and 256 nm in H(2)O; 411 nm in Me(2)SO) and phosphorescence, observed in Me(2)SO (lambda(max), 774 nm; tau = 56 &mgr;s at 20 degrees C) but not in H(2)O. In acid aqueous solution the complex decomposes stepwise to Cr(H(2)O)(5)(py)(3+); by contrast, the thermal reaction in Me(2)SO leads to Cr(CN)(5)(Me(2)SO)(2)(-) with first-order kinetics (k(25) = 9.8 x 10(-)(7) s(-)(1), DeltaH() = 138 +/- 8 kJ mol(-)(1)). Ligand-field (LF) band irradiation results in substitution of py and CN(-). The quantum yields, measured by ligand analysis, spectrophotometry, and HPLC, are as follows: Phi(py) = 0.08, Phi(CN) = 0.01 in H(2)O (pH 7.2, phosphate buffer) and Phi(py) = 0.04, Phi(CN) = 0.002 in Me(2)SO. The preference for py release obeys the prediction of the Vanquickenborne-Ceulemans, additive angular overlap model (AOM). A notable feature of this complex is that both types of ligands are pi acceptors, and the pi effect of py on bond labilization is evidenced by comparison with the photolysis of Cr(CN)(5)(NH(3))(2)(-). Irradiation of the intense UV absorption due to overlap of charge-transfer (CT) and pi --> pi, py localized transitions causes the increase of both quantum yields, suggesting the involvement of higher-energy states besides the LF ones. Co(sep)(3+) (sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane = sepulchrate) quenches the phosphorescence (k(q) = 1.6 x 10(9) M(-)(1) s(-)(1)) but has no effect on the photoreaction efficiencies: the photochemistry is thus inferred to originate entirely from the lowest quartet excited state(s) in competition with intersystem crossing. The marked solvent effects on the absorption spectrum, on the emission behavior, on the thermal reactivity, on the photolysis quantum yields, and, in particular, on the Phi(py)/Phi(CN) ratio, are discussed in terms of the proneness of the cyanide ligand to either protonation or hydrogen bonding and of solvent orientation toward anionic complexes.

Synthesis and studies of water-soluble Prussian Blue-type nanoparticles into chitosan beads

“…In our opinion the observed spin-glass behavior must be ascribed to intra-particle surface spin disordered and, most likely to the spin disorder into each nanoparticles taking into account the low particle size and weak spin coupling characterizing these nanomaterials. 40 were prepared according to literature procedures. The chitosan spheres were synthesized as previously described.…”

“…The next step of our approach consists in the intrapore formation of the CPNPs by the consecutive growth of the cyano-bridged network into the pores (Scheme 2). For this reason, the composite beads M 2+ /chitosan were treated with a methanolic solution of (N(C 4 H 9 ) 4 ) 3 [M 0 (CN) 6 ] (M 0 = Fe 39 or Cr 40 ) thoroughly washed with methanol and dried in vacuo. For each sample, the consecutive treatment with M 2+ /[M 0 (CN) 6 ] 3À was repeated twice in order to obtain a homogeneous incorporation of the coordination polymer network with a maximal amount.…”

“…The CN stretching frequency of a free CN À ion is 2080 cm À1 , whereas upon coordination to a metal ion, it shifts to higher frequencies, such as observed for (N(C 4 H 9 ) 4 ) 3 [M 0 (CN) 6 ]. 39,40 The IR spectra of 1a-6a show two characteristic absorption bands in the CN stretching region (Table 2): the high frequency bands can be attributed to the stretching of the CN ligand bridged between M 2+ and M 03+ in the M 2+ -CN-M 03+ mode and the low frequency bands can be assigned to the presence of the linkage isomer with the M 2+ -NC-M 03+ coordination mode, as it was reported for the bulk cyano-bridged coordination polymers. [41][42][43][44] It should be also noted that the IR spectra of the nanocomposites present the characteristic bands of the chitosan matrix.…”

Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices

“…Cr(en) 3 3+ and Cr(NH 3 ) 5 CN 2+ ) [42,[103][104][105]. There are a few cases for which little to no reaction quenching is detected under conditions of total phosphorescence quenching, namely, for Cr(CN) 5 X n− complexes [91,[106][107][108]. Note that such complexes have very high ligand field strengths, prohibiting BISC and leaving prompt quartet reactivity as the only (quartet) reaction channel available.…”