Pentacyanoamminechromate(III). Synthesis, characterization, and photochemistry

Abstract

The Cr(CN)3(NH3)2' ion was synthesized by irradiation of Cr(CN)63' in methanol and reaction with NH3, isolated as the potassium salt, and characterized by analysis, chromatography, aquation, absorption spectroscopy (Xmax at 397 and 320 nm, as predicted theoretically), and phosphorescence (Xmax at 777 nm, = 32 ms at 20 °C in Me2SO). Ligand field excitation causes release of both NH3 and CN". The respective quantum yields are 0.08 and 0.06 in H20, 0.05 and 0.007 in Me2SO, and 0.11 and 0.003 in dimcthylformamide. The prevalence of NH3 loss is in agreement with the angular overlap model but not with previous photolysis theories. The medium markedly affects the absorption spectrum, the emission behavior, the photoreaction efficiencies, and especially the ,/ratio; the changes are interpreted in terms of solvent orientation and hydrogen bonding. The phosphorescence is observed in aprotic solvents but not in H20 and is quenched by Co(sep)3+. In Me2SO, at least 90% of each photoreaction mode remains unquenchcd upon complete doublet-state quenching. Both photoprocesses are concluded to proceed from the lowest quartet excited state prior to intersystem crossing. Results for Cr(CN)63" photolyzed in Me2SO in the absence and in the presence of Co(scp)3* are also reported, and the two systems are compared.

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.…”

Photochemistry and Photophysics of Chromium(III) Complexes

“…88 Some parallels with this behavior can be found in mixed cyanoam(m)ine complexes. Cyano complexes have been extensively studied [90][91][92][93][94][95][96][97][98][99][100] by the Perugia group of Riccieri and Zinato and co-workers. Zinato has written an excellent review 24 of their synthesis, properties, spectroscopy, thermal kinetics, photochemistry, and photophysics.…”

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

“…The key step in the preparation of pentacyano(pyridine)chromate(III) is the obtainment of substitutionally labile Cr(CN) 5 (MeOH) 2- . A previously devised route to this intermediate was the photolysis of Cr(CN) 6 3- in methanol, with the drawback of overly long irradiation periods and small absolute product yields . The alternative procedure developed in this study, is H + -promoted methanolysis of CN - (eq 1), accomplished by interaction of equimolar amounts of hexacyanochromate(III) and sulfamic acid.…”

“…Assignments can be made by analogy with the spectra of Cr(CN) 5 (NH 3 ) 2- and Cr(NH 3 ) 5 (py) 3+ . In the pentacyanoammine species, the maximum at 251 nm and the shoulders at 263 and 220 nm have been attributed to charge-transfer (CT) transitions involving cyanide, on the grounds of their close similarity to the features of Cr(CN) 6 3- and of their disappearance upon aquation of all CN - groups however, this interpretation was later questioned, and the ligand-to-metal character, namely, t 1u (σ,CN) → t 2g (π,Cr), was proposed. , In the pentaammine(pyridine) species, the resemblance of the 260-nm peak with that of free pyridine identifies py-localized, fully allowed π → π* transitions .…”

“…The pentacyanochromates(III) with X = H 2 O, , OH - , , Me 2 SO, DMF (dimethylformamide), and MeCN have been identified as the products of thermolysis 1,3,4 or photolysis 2,5-7 of Cr(CN) 6 3- in the corresponding solvents, and that with X = NCS - has been separated in solution from mixtures of Cr(CN) 6 - n (NCS) n 3- anions formed by interaction of CN - with Cr(NCS) 6 3- . , However, obstacles to the study of these compounds have been the difficult isolation of adequate quantities and, sometimes, the instability toward CN - loss even in moderately acidic media . The only pentacyano complexes thus far obtained as solid salts are K 3 [Cr(CN) 5 (OH)]·H 2 O and K 2 [Cr(CN) 5 (NH 3 )], the latter prepared and photochemically characterized in this laboratory.…”