Redox-Induced Hydrogen Bond Reorientation Mimicking Electronic Coupling in Mixed-Valent Diruthenium and Macrocyclic Tetraruthenium Complexes

Abstract

We present the coordination-driven self-assembly of three tetranuclear metallacycles containing intracyclic NH2, OH, or OMe functionalities through the combination of various isophthalic acid building blocks with a divinylphenylene diruthenium complex. All new complexes of this study were characterized by means of nuclear magnetic resonance spectroscopy, ultrahigh-resolution ESI mass spectrometry, cyclic and square wave voltammetry and, in two cases, X-ray diffraction. The hydroxy functionalized macrocycle 4-BOH and the corresponding half-cycle 2-OH stand out, as their intracyclic OH···O hydrogen bonds stabilize their mixed-valent one- (2-OH, 4-BOH) and three-electron-oxidized states (4-BOH). Despite sizable redox splittings between all one-electron waves, the mixed-valent monocations and trications do not exhibit any intervalence charge-transfer band, assignable to through-bond electronic coupling, but nevertheless display distinct IR band shifts of their charge-sensitive Ru­(CO) tags. We ascribe these seemingly contradicting observations to a redox-induced shuffling of the OH···O hydrogen bond(s) to the remaining, more electron-rich, reduced redox site.

A “Pretender” Croconate-Bridged Macrocyclic Tetraruthenium Complex: Sizable Redox Potential Splittings despite Electronically Insulated Divinylphenylene Diruthenium Entities

“…The increased ring strain and the ensuing unusually large torsion at the vinyl phenylene linkages in the present macrocycle seem to have a detrimental effect on the absorptivity, as the molar extinction coefficient is smaller by ca. 25% as compared to those obtained for related isophthalate-bridged systems [27,29,32,86]. This band is replicated well by the quantum chemical calculations (λ calc = 574 nm).…”

“…Through an iterative process of varying reaction time, temperature, and sol were able to identify conditions that afforded pure 2 Ru2Ph-Croc, namely: (i) usin rated (27 mM) solution of Ru2Ph-Cl in benzene as the tetranuclear macrocycle is uble in this solvent than the octanuclear congener and hence precipitates first; ducting the reaction at a slightly elevated reaction temperature of 35 °C; and (iii) the reaction time to 19 h, as the smaller macrocycle constitutes the kinetic product reaction times led to incomplete conversion of difficult-to-separate Ru2Ph-C longer reaction times lead to increasing amounts of the larger macrocycle. 2 Ru2 could be ultimately obtained in a yield of 21%, which is only modest when com what can be achieved for similar dicarboxylate-linked macrocycles [25,[28][29][30][31][32]. Un reasons are that a significant portion of the tetranuclear macrocycle remains in and cannot be separated from slightly better soluble 4 Ru2Ph-Croc and the difficu The high purity of isolated 2 Ru 2 Ph-Croc is evident from the 1 H-, 13 C-, and 31 P-NMR spectra (see Figure 4 and S3-S5 of the Supplementary Materials) as well as by mass spectrometry (see Figure S6 of the Supplementary Materials).…”

“…The spectrum in fluid solution was hence recorded at −20 °C. Under these conditions, a slightly anisotropically broadened signal is observed with a g ave -value of 2.026 (see Figure 12 , bottom left), which is again typical of such species [ 27 , 29 , 32 , 86 ]. The slightly increased g value points to higher spin densities at the ruthenium ions.…”

“…Shorter reaction times led to incomplete conversion of difficult-to-separate Ru 2 Ph-Cl, while longer reaction times lead to increasing amounts of the larger macrocycle. 2 Ru 2 Ph-Croc could be ultimately obtained in a yield of 21%, which is only modest when compared to what can be achieved for similar dicarboxylate-linked macrocycles [25,[28][29][30][31][32]. Underlying reasons are that a significant portion of the tetranuclear macrocycle remains in solution and cannot be separated from slightly better soluble 4 Ru 2 Ph-Croc and the difficulties encountered when separating the precipitated solid from the biphasic supernatant.…”

“…The occupied frontier orbitals of such repeat units spread over the entire Ru-CH=CH-Aryl entity [ 22 , 23 , 24 ], while the carboxylate linker represents a local node, thereby attenuating electronic coupling in the mixed-valent states [ 25 , 26 ]. In contrast, structurally similar macrocyclic ruthenium complexes built from {Ru}-CH=CH-Aryl-CH=CH-{Ru} entities, capable of forming fully delocalized mixed-valent radical cations, and isophthalate, terephthalate, isonicotinate or pyrrole-, furan- or thiophene-2,5-dicarboxylate linkers do not exhibit any signs of electronic coupling between the π-conjugated diruthenium entities, as the dicarboxylate linkers do not contribute to the relevant redox orbitals [ 25 , 27 , 28 , 29 , 30 , 31 , 32 ].…”