The chelate ligand dipyrido [3,2-a:2',3'-c] phenazine (dppz), its 11,12-dimethyl derivative dmdppz, and corresponding complexes with [Ru(bpy)2]2+ were studied in multiply reduced states by low-temperature cyclic voltammetry and UV/vis and EPR spectroscopy. The (dm)dppz ligands are reduced in two reversible steps, followed by a very moisture-sensitive third step. Highly resolved EPR and 'H-ENDOR spectra of the intermediate anion radicals were obtained and analyzed. The results are interpreted using a HMO/McLachlan perturbation approach of x spin populations and orbital energies. Three low-lying unoccupied x molecular orbitals can be identified as phenazinetype (bi, lowest) and as the ^(b,) and xfo) orbitals of the a-diimine moiety. Complexes with the N(4),N(3)-bound [Ru(bpy)2]2+ fragment show at least six reversible one-electron reduction steps in rigorously dried DMF at 200 K; the first four persistent reduced states were characterized by EPR and UV/vis spectroscopy. The EPR spectra of the first three reduced states of the complexes show a signal which proves the occupation of the phenazine-localized x* orbital of (dm)dppz by a single electron, the stepwise reduction of the bpy ligands resulting in temperaturedependent intensity loss of that EPR signal. The very basic quadruply reduced state exhibits EPR characteristics which are typical for Ru(II)-bound a-diimine anion radicals. All assignments are supported by UV/vis spectra and analyses of redox potential values. Because the very easily protonated higher reduced states are not sufficiently persistent for EPR and UV/vis characterization, further assignments could thus be based only on the analysis of redox potential values. The particular composite electronic structure of the complexes with differing redox and "optical" orbitals is related to their "light switch" behavior, i.e. to the absence of luminescence quenching in a nonaqueous environment.

The mononuclear title complexes can exist as stereoisomers due to the unsymmetrically chelating a-azimino functionality of the abpy ligand. Both fac and mer isomers of [Ru(abpy)3]2+ and two out of the three possible diastereoisomers of [Ru(abpy)2(bpy)]2+ were isolated via HPLC and identified by virtue of their NMR spectra. Electrochemical potentials for several one-electron reduction steps and UV/vis spectroelectrochemical features vary to a small but detectable degree for the respective pairs of isomers. The absence of a room-temperature EPR signal for one of the two isolated isomers of [RuII(abpy)(abpy)(bpy)]*+ demonstrates the structural requirements for intramolecular electron transfer. A 99'101Ru EPR hyperfine coupling of about 0.8 mT was established for the previously reported radical ion [Ru(abpy)(bpy)2],+. The free -azimino chelate sites in mononuclear complexes of abpy render these systems suitable for the construction of polynuclear compounds. The exceptional «--acceptor capability of abpy is illustrated by the reversible acceptance of at least eight electrons by the dinuclear complex {[Ru(bpy)2] 2(M,7?4-abpy)}".

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Reaction of the diphosphazane-bridged diruthenium derivatives Ru2(M-CO)(CO)4[u-(RO)2PN(Et)P(OR)2]2 (R = Me, 'Pr) with the electron acceptors TCNX (X = Q, E) affords products of the type {Ru2(CO)5[/it-(R0)2PN-(Et)P(OR)2]2(s1-TCNX)}(TCNX) containing a TCNX radical anion in both the inner and outer coordination sphere of the cation. The contribution of the coordinated TCNX*™ to the EPR/ENDOR and UV/vis/near-IR spectra of these salts is partly obscured by that of the external, completely dissociated ¿^/.-symmetric TCNX-, but the infrared vibrational spectra and, in particular, the cyclic voltammograms show clearly the presence of both coordinated and noncoordinated species. Four distinct and reversible waves appear in benzonitrile; two correspond to the TCNX0/™ and TCNX™/* 12™ couples of the external TCNX, while the other pair, shifted by ca. 0.3 V to more positive potentials because of rather weak Ru -* TCNX back-donation, are readily assigned to the corresponding redox couples of the coordinated TCNX. The external TCNQ-anion of the TCNQ salts is readily exchanged for the diamagnetic tetraphenylborate anion to give compounds of the type {Ru2(CO)5^-(RO)2PN(Et)P(OR)2]2(V-TCNQ)}(BPh4) (R = Me, 'Pr), thus enabling UV/vis/near-IR, IR, and EPR spectra of the coordinated TCNQto be recorded in CH2CI2 without interference from the external TCNQanion; the IR and EPR data confirm the low symmetry of the 7;'-bound TCNQ-. With the aid of HMO/McLachlan perturbation calculations and ENDOR measurements it proved possible, for the first time, to record and reproduce EPR spectra of a coordinated TCNQanion; the and 14N hyperfine coupling constants used to simulate the EPR spectra fit the pattern calculated for a TCNQ-bonded in the tj'-mode to one metal center. Replacement of the external TCNEin the TCNE salts by a diamagnetic anion could not be achieved, but since the external TCNEis more easily oxidized than bound TCNE-, selective oxidation of the former in situ to diamagnetic neutral TCNE could be accomplished, thus allowing EPR spectra of the coordinated TCNEto be recorded in CH2CI2 without interference from external TCNE*™. Successful reproductions of the highly resolved EPR spectra showed that, like TCNQ-, TCNEis bonded in the Tj'-mode to one metal center. In situ oxidation of {Ru2(CO)5[At-(RO)2PN(Et)P(OR)2]2(i?1-TCNX)}+ (R = Me, 'Pr) affords the completely oxidized dications containing a neutral coordinated TCNX ligand; these dications, as well as the reduced species {Ru2(C0)5[#i-('Pr0)2PN(Et)P(0'Pr)2]2(77'-TCNQ)}, were characterized by UV/vis/near-IR spectroscopy.

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