Intramolecular Electron Transfer from Coordinated Ligand-Radicals Morton Z. Hoffman and Kevin D. Whitburn Boston University, Boston, MA 02215 Intermolecular electron transfer involving transition metal coordination complexes is usually described as being of two types: outer-sphere and inner-sphere ( I ) . In the outer-sphere case, electron transfer occurs via a weak coupling mechanism in which there is an interaction between the potential energy surfaces of the electron donor and acceptor; the primary coordination spheres of the reactant complexes remain intact during the transfer process. Despite the fact that these surfaces are only part of a multidimensional representation of the wave functions of the species, their interaction can be described in classical term; and rate constants of the processes can he correlated with thermodynamic and kinetic parameters according to a number of quantitative relationships (e.g., Marcus equations) (2). Inner-sphere electron transfer, on the other hand, is a strong coupling phenomenon in which a chemical bond is formed between the donor and the acceptor; the reactants Denetrate the coordination soheres of each other before the k ~ e c t r o n is transferred.'The reaction of Co(NH&CI2+ with Craq2+to form Coaq2+and Cr(H20)&I2+ is one of the classic examples of the process ( 3 ) .It is clear that a t some point along the reaction coordinate, the species (NH3)5CoClCr(H20)5" is formed and that an electron is transferred intramolecularly from the Cr(I1) center to the Co(II1). The inner-sphere mechanism can he formulated according to reactions (1)-(3) in which the reactants diffuse together to form, in the first instance, a precursor complex which undergoes intramolecular transfer of an electron from the donor center to the acceptor center. MnC-L + D- Mnt-L-D(1) Mn+-L-DMI"-I)+-L-D (2) M(~-II+-L--D
M("-I>+-L
+D
(3)
Diffusion apart of the resulting successor complex completes the act. The overall reaction (4) has the stoichiometry of electron transfer from the donor D- to the metal center
Mn+ M"+-L + D-
+D (4) The question has been raised as to the involvement of the ligand in the electron transfer process. Focusing on the intramolecular step (2) for a moment, it is clear that for the electron to get from D- to M"+, it must go through or around the ligand L. The "chemical" or "radical-ion" mechanism of electron transfer considers that a t some moment in time, a reduced ligand species exists as an intermediate. As depicted in reaction (51, the coordinated ligand-radical species has the reducing electron localized on the ligand; its decay (h.d represents the transmission of the electron into the metal-centered orbitals or, to be consistent with the principle of microscopic reversibility, hack into orbitals localized on the oxidized donor. M"+-L-D-
F?
M(~-II+-L
M"+--L:-D
a M("-ll+-L--D
(5)
If the coordinated ligand-radical species is an important intermediate, one can immediately recognize certain molecular parameters of the ligand that could he important in determining the ease of transfer (4). The reducibility of L will limit
the extent to which a L- species can be formed; the orbital nature of L will dictate the symmetry characteristics of the wave function of the added electron (5). The coordination between L and the metal center can have a number of lead-in erouns: carboxvlate and aromatic nitroeen base are the two kost'cbmmon..~he linknge of the donoyto the ligand can he far from the lead-in group ("remote" attack) or a t the lead-in gronp ("adjacent" attack) representing a spatial separation of the locus of electron density in the donor from the ultimate acceptor position (6).If L is an a m iatic moiety, the isomeric position of D- relative to the lead411gronp may be important (7). The Radialion Chemical Approach From the point of view of a radiation chemical approach to the investigation of intramolecular electron transfer, we should recognize the interactions that are possible between radicals and metal complexes. Reducing radicals (R' ) could reduce the metal center directly (reaction (fi)) Mrt-L + R: -.1\.; 2-I)+-L + R (6) or reduce the ligand (reaction (7)) forming a coordinated ligand-radical. Mn+-L + R + M"+-L- + R (7) Hydroxyl radicals and hydrogen atoms could engage in hydrogen-abstraction reactions a t the ligand (reaction (8)) M"+-LH
+ R.
-
M"+-L.
+ RH
(8)
or add to aromatic rings (reaction (9)) M"+--L + R. M"+-L-R. (9) in some instances .OH could act as an outer-sphere oxidizing agent and H. could reduce metal centers by an inner-sphere path. The coordinated ligand-radicals formed in reactions (7)-(9) would be capable of undergoing intramolecular electron transfer so that a strategy emerges to study the phenomenon. Because of the special properties of the Co(III)/Co(II) couple, Co(II1) complexes have been used a great deal to examine electron transfer reactions. Co(II1) is a d%svstem with six electrons in the tz, non-bonding orbital (t2C~j. Addition of one electron into the metal core generates initially the low-spin Co(I1) center, so called because a single unpaired electron resides in the e, anti-bonding orbital (tz,fiegl). However, the gronnd-state form of Co(I1) that exists in aqueous solution is "high-spin" with a tz85eC2configuration of three unpaired electrons. Although a certain amount of speculation about the role o; low-spin Co(I1) as an intermediate has been raised (81, the evidence a t the moment is that spin equilibration in Co(I1) is complete within s (9).In addition to this rapid process which renders back electron transfer from Co(I1) improbable, the lability of high-apin Co(I1) in aqueous solution results in the formation of Coaq2+ as the final product. Thus, when the electron is captured by Co(III), the system rapidly relaxes in an irreversible manner. The kinetics of labilization of Co(I1) are of importance to note when considering the time frame of intramolecular Volume 58
Number 2
February 1981
119
electron transfer. When CO(NH&~+is reduced, the resulting high-spin Co(NH&?+ (after spin equilibration) exchanges the NH3 ligands for Hz0 according to the stoichiometry of reaction (10).
Ammonia exchange studies have shown that the first NH3 is labilized in
