The Semiquinone−Ruthenium Combination as a Remarkably Invariant Feature in the Redox and Substitution Series [Ru(Q)_n(acac)_3−n]^m,n= 1−3;m= (−2), −1, 0, +1, (+2); Q = 4,6-Di-tert-butyl-N-phenyl-o-iminobenzoquinone

Abstract

Three new compounds, [Ru(Q(*-))(acac)(2)] = 1, [Ru(Q(*-))(2)(acac)] = 2, and [Ru(Q(*-))(3)] = 3, were obtained and characterized as Ru(III) complexes with 4,6-di-tert-butyl-N-phenyl-o-iminobenzosemiquinone (Q(*-)) ligands. All three systems show multiple electron transfer behavior, which was analyzed using electron paramagnetic resonance (EPR) and UV-vis-near-infrared (NIR) spectroelectrochemistry. (1)H NMR spectroscopy and a crystal structure analysis suggest antiferromagnetically spin-spin coupled Ru(III) and Q(*-) in 1, similar to that in the related compound 4 with unsubstituted o-iminobenzosemiquinone. However, in contrast to 4(n) (Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399), the system 1(m) exhibits unambiguously metal-centered electron transfer, producing ions [Ru(IV)(Q(*-))(acac)(2)](+) = 1(+) and [Ru(II)(Q(*-))(acac)(2)](-) = 1(-), both with EPR-evidenced ligand-based spin, as also supported by DFT calculations. Compared with the related redox system [Ru(Q)(bpy)(2)](k) (5(k)) (k = 0-3), the spectroelectrochemical similarity suggests corresponding electronic structures except for the 1(+)/5(3+) pair (Ru(IV)(Q(*-))(acac)(2)](+) (1(+)) versus [Ru(III)(Q(0))(bpy)(2)](3+) (5(3+))). Compound 2, a three-spin system [Ru(III)(Q(*-))(2)(acac)] obtained in the all-cis configuration, possesses a complicated magnetic behavior including strong intramolecular antiferromagnetic coupling (J(Ru-Q), on the order of -10(3) cm(-1) and J(Q-Q), -10(2) cm(-1)) and weak intermolecular antiferromagnetic and ferromagnetic interactions. Strong intramolecular coupling leads to one unpaired electron at low temperatures, as also supported by the radical-type EPR signal of the solid and of solutions, which diminishes at higher temperatures. The up-down-up spin arrangement for the ground state of {(Q(*-))-Ru(III)-(Q(*-))} (S = 1/2) is confirmed by DFT calculations for 2. Oxidation to 2(+) leaves the UV-vis-NIR spectrum almost unchanged, whereas reduction to 2(-) and 2(2-) produces low-energy absorptions. The ligand-centered spin for 2(2-) = [Ru(II)(Q(*-))(Q(2-))(acac)](2-) suggests the [Ru(II)(Q(*-))(2)(acac)](-) formulation for 2(-). Compound 3, obtained as a structurally characterized mer isomer, has a predominantly ligand-centered highest occupied molecular orbital (HOMO), as evident from the EPR signal of the intermediate 3(+) and as supported by DFT calculations. In contrast, electron addition proceeds to yield a metal/ligand mixed spin intermediate 3(-) according to EPR, in agreement with ca. 25% calculated metal character of the lowest unoccupied molecular orbital (LUMO). The near-infrared absorption of 3 at 1280 nm corresponds to the HOMO-LUMO transition (ligand-to-metal/ligand-to-ligand charge transfer). Oxidation to 3(+) produces a weak broad band at about 2500 nm, while the reduction to 3(-) gives rise to an intense absorption feature at 816 nm. The valence state alternatives are being discussed for all spectroelectrochemically accessible species, and the individual results are compared across this unique subst...

Four‐Center Oxidation State Combinations and Near‐Infrared Absorption in [Ru(pap)(Q)2]n (Q=3,5‐Di‐tert‐butyl‐N‐aryl‐1,2‐benzoquinonemonoimine, pap=2‐Phenylazopyridine)

“…The paramagnetic complexes ( μ eff =1.8–1.9 BM at ambient temperature) exhibit free radical‐type EPR spectra at 77 K in CH 2 Cl 2 with g values of approximately 1.995 without any detectable hyperfine splitting (Figure 1a) 8c,g. 13b One‐electron‐reduced species 2 was isolated during the chromatographic purification of the corresponding parent complex [ 2 ]ClO 4 and its EPR spectrum is shown in Figure 1b (see below).…”

“…The isomeric identity of 1 + – 4 + in isolated [ 1 ]ClO 4 –[ 4 ]ClO 4 as C ( ctt isomer; Scheme ) has been authenticated in one case through the determination of the single‐crystal X‐ray structure of representative compound [ 1 ]ClO 4 (Figure 2 and Tables 1 and 2, and Figure S3a in the Supporting Information). The possibility of variable redox‐states for all redox‐active components, Ru (Ru II , Ru III , Ru IV ),13b pap ((NN) 0 , (NN) .− , (NN) 2− )8c, 17, 18 and Q (Q 0 , Q .− , Q 2− )8 in [Ru(pap)(Q) 2 ] n leads to several valence structure alternatives. A comparison of the sensitive bond parameters of Q (C1O1/C21O2 1.303(3)/1.296(3) Å; C6N1/C26N2 1.348(4)/1.344(4) Å; CC( meta ) (average of C2C3, C4C5, C22C23, C24C25) 1.373(4) Å; Table 2 and Figure 2) and the charge‐state‐sensitive NN bond length of the pap ligand (1.292(3) Å) in the crystal of [ 1 ]ClO 4 with the well‐established distinctive bond lengths in different redox states of coordinated Q (CO/CN:1.22/1.30 Å for Q 0 ; 1.30/1.35 Å for Q .− ; 1.34/1.38 Å for Q 2− )8, 15 and pap (NN: ≈1.30 Å for pap 0 ; >1.35 Å for pap .− ; >1.40 Å for pap 2− )18 clearly suggests the presence of two semiquinoneimine ligands and unreduced pap, which leads to a three‐spin configuration (↑↓↓) for [Ru III (pap 0 )(Q .− )(Q .− )] + .…”

“…Strong antiferromagnetic coupling between the neighboring unpaired spins on Ru III and one of the iminosemiquinone moieties leaves the remaining spin on the second iminosemiquinone, which gives rise to narrow free‐radical‐type EPR signals with g values at about 1.995 for 1 + to 4 + in CH 2 Cl 2 at 77 K (Figure 1a) 8c. 13b The EPR spectrum of the related complex [Os III (bpy)(Q .− ) 2 ] + (bpy=2,2′‐bipyridine; Q: X=O, Y=NPh, Schemes and ) with the heavier 5d metal ion exhibits g factor anisotropy ( g 1 =1.981, g 2 =1.955, g 3 =1.803) due to the high spin‐orbit coupling constant of the Os III ion 19…”

“…Earlier studies were mostly confined to compounds with a single coordinated o ‐quinone {Ru(AL)(Q)} (AL=ancillary ligand)7, 8 or to homoleptic tris ‐complexes, {Ru(Q) 3 } 13. However, a few bis‐quinone systems have also been scrutinized: {Ru(AL)(Q) 2 } (Q: X=Y=O, Scheme ; AL=bpy (2,2′‐bipyridine),14a t BuPy,14b PPh 3 ,14c,d CO14e), {Ru(bpy)(Q) 2 } (Q: X=Y=NH, Scheme ),14f {Ru(acac)(Q) 2 } (X=O, Y=NPh, Schemes and ; acac=acetylacetonate)13b and {Ru(bpy)(Q) 2 } (X=O,Y=NAr, Schemes and ) 14g…”

“…The specific choice of iminoquinonoid ligand, Q with X=O, Y=NAr (Schemes and ) for this study has originated from our recent observations that ruthenium coordinated Q with X=O, Y=NAr exhibits remarkable variability in Ru–Q‐based intramolecular electron transfers 8c,g. 13b, 15 Moreover, unlike the well‐developed chemistry of first‐row transition‐metal ions of Q (X=O, Y=NAr),16 the corresponding ruthenium chemistry is restricted to rather few reports 8c,e,j. 13b, 14g Substitution of the neutral AL(s) by pap introduces an additional competitive component because the low‐lying π*(NN) orbitals of pap are susceptible to participating in facile electron uptake at relatively low potentials approaching that of Q 8c.…”