(Pyridine)pentaamminechromium(III). Synthesis, Characterization, and Photochemistry

Abstract

The Cr(NH(3))(5)(py)(3+) ion has been obtained by metathesis of Cr(NH(3))(5)(Me(2)SO)(3+) in pyridine, isolated as the perchlorate salt, and characterized by absorption (lambda(max) at 467, 352, and 260 nm) and emission spectra (lambda(max) at 668 nm, tau = 2.0 &mgr;s at 20 degrees C in water) and by the py aquation rate (k = 5 x 10(-)(4) s(-)(1) at 80 degrees C). Ligand-field (LF) band irradiation in acid aqueous solution (10(-)(2) M HClO(4)) induces photoaquation of py (Phi = 0.26) and NH(3) (Phi = 0.16). HPLC indicates that the latter reaction gives rise to both cis- and trans-Cr(NH(3))(4)(py)(H(2)O)(3+), with the predominance of the cis isomer. This is the first Cr(NH(3))(5)X(z+)() species where Phi(x) > Phi(NH)3: the result is compared with the predictions of various photolysis models and is taken as chemical evidence for pi-acceptance by the py ligand. The photostereochemistry is also discussed. The phosphorescence is totally quenched by Cr(C(2)O(4))(3)(3)(-) (k(q) = 2.7 x 10(9) M(-)(1) s(-)(1)), while the photoreactions are only in part. On 470-nm excitation, the Phi(py)/Phi(NH)()3 ratio is approximately 1 and approximately 2 for the unquenchable and the quenchable contributions, respectively. Such a difference, suggesting at least two reactive precursors, can be interpreted in terms of the photochemistry proceeding from either the lowest doublet and quartet excited states or, alternatively, from the (4)E and (4)B(2) states. Irradiation of the very distinct absorption of coordinated pyridine results in both doublet-state emission and loss of py and NH(3). Comparison of this photobehavior with the LF results gives an efficiency of 0.7 for conversion of the py-localized pipi states into the Cr-localized LF states, confirmed by the wavelength dependence of the relative emission yields. Some py release (Phi = 0.03) is concluded to originate in the pipi states.

Photochemistry and Photophysics of Chromium(III) Complexes

“…Consistent with this expectation, in the complex Cr(NH 3 ) 5 (py) 3+ , the observed preference for photoaquation of the weaker-field py over ammonia can be attributed to the weakening of both the Cr-py σand π-bonding in the excited state. 102 This is the first pentammine to show preferential loss of the weaker-field ligand, and the result is in accord with the predictions of the AOM 4 and some early models. [103][104][105] The observation also confirmed the π-acceptor nature of the pyridine ligand, which had been disputed.…”

“…Another interesting ligand is pyridine; it is a π acceptor like cyanide but as its bonding strength to Cr is smaller, it may be better suited to reveal information about the role of π-bonding in the excited-state chemistry. Consistent with this expectation, in the complex Cr(NH 3 ) 5 (py) 3+ , the observed preference for photoaquation of the weaker-field py over ammonia can be attributed to the weakening of both the Cr−py σ- and π-bonding in the excited state . This is the first pentammine to show preferential loss of the weaker-field ligand, and the result is in accord with the predictions of the AOM 4 and some early models. − The observation also confirmed the π-acceptor nature of the pyridine ligand, which had been disputed.…”

“…For reaction by loss of axial ammonia ligand from the lowest 4 E state, the stereochemical prediction (Figure 3) is exclusive formation of cis isomer, whereas experiment showed an estimated 20-30% of trans isomer was present. This trans isomer could of course arise from a stereoretentive photoaquation of axial ammonia, but the authors 102 preferred to consider it as a concerted substitution with stereochemical change and attributed the reaction to loss of equatorial ammonia from the 4 E state. This generates, in its excited state, the tbp intermediate with an equatorial pyridine ligand for which the allowed entry of water trans to pyridine leads to the observed trans product.…”

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

“…As part of a program aimed at gaining knowledge of the ground-state and excited-state chemistry of chromium(III) cyano complexes, we have devised the synthesis of the pyridine (py) derivative Cr(CN) 5 (py) 2- , explored its thermal reactivity, and studied its photophysical and photochemical behavior in water and Me 2 SO. This work complements our earlier reports on the related species Cr(CN) 5 (NH 3 ) 2- 11 and Cr(NH 3 ) 5 (py) 3+ …”

“…Regarding this feature, Cr(CN) 5 (py) 2- is the first chromium(III) system where both types of ligands are π acceptors. The investigation was also prompted by the search of indications on whether there is any particular role of py in a doublet-state reaction, a possibility that was suggested, but was not straightforwardly testable, in our previous investigation of Cr(NH 3 ) 5 (py) 3+ …”