Acid hydrolysis and linkage isomerism of the cis-dicyanobis(ethylenediamine)chromium(III) ion

Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles. 3. Insights into the Temperature-Dependent Radiationless Deactivation of the 2Eg (Oh) Excited State of trans-[Cr(N4)(CN)2]+ Complexes

Macrocyclic complexes of the type trans-[Cr(N4)(CN)2]+, where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam demonstrate significant variation in their room-temperature excited-state behavior; namely, the lifetimes of the 2Eg (Oh) excited states are 335, 23, and 0.24 micros, respectively. The lifetimes of these complexes have been measured in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C. Arrhenius activation parameters were calculated from these data. There was very little variation in the values of the Arrhenius preexponential factor between these three complexes, whereas the value of Ea is 40.6 kJ/mol for the cyclam complex, 35.5 kJ/mol for the 1,11-C3-cyclam complex, and 22.3 kJ/mol for the 1,4-C2-cyclam complex. Thus, differences in the room-temperature excited-state lifetimes can be rationalized based on the competition between thermally independent nonradiative relaxation and a thermally activated channel. To test whether a photodissociation mechanism involving Cr-macrocyclic N bond cleavage is a plausible explanation for the thermally activated relaxation pathway, samples of the cyclam complex were photolyzed in acidified D(2)O. A marked increase in the lifetime after photolysis demonstrated the occurrence of photodeuteration and thus a likely photodissociation of a macrocyclic N.

Ground-State, Mode-Dependent Vibronic Coupling in Some Simple, Cyanide-Bridged Transition-Metal Donor-Acceptor Complexes

Patterns of the shifts in bridging cyanide-stretching frequencies have been examined in several fully saturated, &mgr;-cyano, bi- or trimetallic transition-metal donor-acceptor (D/A) complexes. An earlier (Watzky, M. A.; et al.Inorg. Chem. 1996, 35, 3463) inference that the bridging ligand nuclear and the D/A electronic coordinates are entangled is unequivocally demonstrated by the 32 cm(-)(l) lower frequency of nu(CN) for (NH(3))(5)Cr(CNRu(NH(3))(5))(4+) than for the cyanopentaamminechromium(III) parent. This contrasts to the 41 cm(-)(1) increase in nu(CN) upon ruthenation of (NH(3))(5)RhCN(2+). More complex behavior has been found for cis and trans trimetallic, donor-acceptor complexes. The symmetric combination of CN(-) stretching frequencies in trans-Cr(III)(MCL)(CNRu(II)(NH(3))(5))(2)(5+) complexes (MCL = a tetraazamacrocyclic ligand) shifts 100-140 cm(-)(1) to lower frequency, and the antisymmetric combination shifts less than about 30 cm(-)(1). This contrast in the shifts of the symmetric and the antisymmetric combinations of the CN stretches persists even in a trans complex with no center of symmetry. Two CN stretches have also been resolved in an analogous cis complex, and both shift to lower frequency by about 60 cm(-)(1). The net shift, summed over all the CN-stretching frequencies, is about the same for the bis-ruthenates of related dicyano complexes. A simple, symmetry-adapted perturbation theory treatment of the coupled vibrations is employed to deal with the opposing effects of the "kinematic" shifts (delta) of nu(CN) to higher frequency, expected in the absence of D/A coupling, and shifts ( f ) of nu(CN) to lower frequency that occur when D/A coupling is large. The Rh(III)- and Cr(III)-centered complexes correspond to different limits of this model: delta > f and delta < f, respectively. When referenced by means of this model to complexes with Rh(III) acceptors, the shifts in trimetallic complexes, summed over the symmetric and antisymmetric combinations of CN stretches, are about twice those of bimetallic complexes. Similarly referenced and summed over all bridging CN frequencies, the shifts of nu(CN) to lower energies are proportional to the oscillator strength of the electronic, donor-acceptor charge-transfer transition. The simplest interpretation of this correlation is that the donor-acceptor coupling in these systems is a function of the nuclear coordinates of the bridging ligand. This behavior of these complexes is semiquantitatively consistent with expectation for CN(-)-mediated vibronic (pseudo-Jahn-Teller) coupling of neighboring donors and acceptors, and the observed Ru(II)/CN(-) CT absorption parameters can be used in a simple, semiclassical vibronic model to predict shifts in nu(CN) that are in reasonable agreement with those observed.

Molecular Heme-Cyanide-Copper Bridged Assemblies: Linkage Isomerism, Trends in nu(CN) Values, and Relation to the Heme-a(3)/Cu(B) Site in Cyanide-Inhibited Heme-Copper Oxidases

Bovine heart cytochrome c oxidase and related heme copper oxidases are inhibited by cyanide, which binds at the binuclear heme-a(3)/Cu(B) site where dioxygen is reduced to water. To determine the mode of cyanide binding, heme-based binuclear complexes containing iron-cyanide-copper bridges in different oxidation states have been prepared by the reaction of [(py)(OEP)Fe(CN)] with Cu(II,I) precursors and structurally characterized by X-ray methods. Structures of two precursor complexes and two binuclear Cu(I)-CN-Cu(I) species are reported. The assembly [(py)(OEP)Fe-CN-Cu(Npy(3))](2+) has a nearly linear Fe(III)-CN-Cu(II) bridge containing low-spin Fe(III). The assemblies [(OEP)Fe-NC-Cu(MeNpy(2))](+) and [(OEP-CH(2)CN)Fe-NC-Cu(Npy(3))](+) exhibit the high-spin bridges Fe(III)-NC-Cu(I) and Fe(II)-NC-Cu(I), respectively. These are the first title bridges in these oxidation states. Bridge atom sequences are obtained from structural refinements of both linkage isomers; those for the reduced bridges are consistent with the soft-acid nature of Cu(I). Cyanide stretching frequencies respond to metal oxidation state and bridge geometry and, using data for solution and solid states, fall into the following ranges: Fe(III)-CN-Cu(II), 2120-2184 cm(-)(1) (11 examples); Fe(III)-NC-Cu(I), 2072-2100 cm(-)(1) (2 examples); Fe(II)-NC-Cu(I), 2099-2107 cm(-)(1) (1 example). These data are compared with nu(CN) values for the enzymes in different oxidation states. A nonlinear Fe(III)-CN-Cu(II) bridge (Cu-N-C = 150-160 degrees ) is consistent with the 2146-2152 cm(-)(1) range found for the fully oxidized enzymes. Bands that can be assigned with some certainty as Fe-CN vibrations in partially and fully reduced enzymes do not appear to correspond to Fe(III)-NC-Cu(I) and Fe(II)-NC-Cu(I) bridges but rather to Fe(II)-CN modes. The current work complements and extends our previous investigation (Scott and Holm, J. Am. Chem. Soc. 1994, 116, 11357) of linear and nonlinear Fe(III)-CN-Cu(II) bridges and is part of an investigation directed at providing a molecular basis of cyanide toxicity. (MeNpy(2) = bis(2-(2-pyridylethyl))methylamine; Npy(3) = tris(2-pyridylmethyl)amine; OEP = octaethylporphyrinate(2-), OEP-CH(2)CN = N-(cyanomethyl)octaethylporphyrinate(1-).)