Novel and stable metal-metal-bonded diruthenium(I) complexes containing TCNX0/.bul.- in both the inner and the outer coordination sphere (TCNX = TCNE, TCNQ). A combined EPR/ENDOR-, UV/visible/near-IR-, and IR-spectroscopic and electrochemical investigation

Abstract

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.

Studies on TMPD:TCNB; a Donor-Acceptor with Room Temperature Paramagnetism and n-π Interaction†

“…The peak to peak linewidth, ∆H pp , is 0.06 mT for g ⊥ and the width at half height, ∆H 1/2 , is 0.09 mT for g || . This result compares well to the literature g-values for the radical anions of ~2.002 for similar acceptors like TCNQ and TCNE [25]. In contrast, TMPD:TCNQ forms a π-π complex with a diamagnetic ground state.…”

Ultrafast exciton decay in PbS quantum dots through simultaneous electron and hole recombination with a surface-localized ion pair

“…Figure 1A also shows absorption spectra of reduced TCNQ produced by bulk electrolysis in the presence of TBAPF 6 ("TBA + /TCNQ 1-", red), as described in the Experimental Methods section, and of a dispersion of PbS QDs with 2.5 added equivalents of TCNQ (black) after subtraction of the spectrum of QDs alone. The shape of the spectrum of TCNQ in the presence of QDs is very similar to the spectrum of electrochemically reduced TCNQ, which contains characteristic TCNQ 1-absorption features at 686, 749 and 850 nm; [1,12,13] these spectra indicate that spontaneous electron transfer from PbS QDs to TCNQ forms a stable QD 1+ /TCNQ 1-complex. From the intensity of the signal corresponding to neutral TCNQ (centered at 400 nm, not shown), we calculate that the mixture of QDs with TCNQ contains 2.4 µM of neutral TCNQ in solution, and therefore (4.2 -2.4 =) 1.8 µM of reduced TCNQ in the sample (there is no signal from TCNQ 2-).…”

“…Using the observed absorbance of reduced TCNQ at 842 nm (0.026 a.u), we calculate the extinction coefficient of the surface-adsorbed, reduced TCNQ to be 14,000 cm -1 M -1 , which is a factor of 3 smaller than the published value of 43,300 cm -1 M -1 in CH 2 Cl 2 . [12] For comparison, we have scaled the spectrum of pre-electrolyzed TCNQ 1-such that the two spectra correspond to the same concentrations of reduced TCNQ; we see, again, that the signal from free TCNQ 1-in solution is a factor of 3 larger than that of the TCNQ reduced in situ.…”

Electron transfer reactions of (C5R5)2(CO)2Ti (R=H or Me) with TCNE or TCNQ

“…Changes in the IR spectrum of 1 on stepwise reduction to the monoanion (top) and from the monoanion to the dianion (bottom) from spectroelectrochemistry in CH 2 Cl 2 /0.1 M Bu 4 NPF 6 . contrast, the oxidized 2 ' showed an unresolved signal at 2.0030, compatible with a complex of TCNQ +( [20].…”