Tetranuclear Pentaammineruthenium Complexes Bridged by .pi.-Conjugated Tetracyano Ligands Related to TCNE: Syntheses and Spectroscopy of Different Oxidation States

Abstract

The complexes { ~~~-TCNX)[RU(NH~)~]~}"+, TCNX = TCNE (tetracyanoethene), TCNQ (tetracyano-p-quinodimethane), TCNP (2,3,5,6-tetracyanopyrazine), and TCNB (1,2,4,5-tetracyanobenzene), were synthesized in the respective ( 8 f ) states with PF6-(TCNE, TCNQ, TCNP) or CF3S03-counterions (TCNB). IR, UV/vis/near-IR, and XPS spectroscopies were used to characterize the tetranuclear (8f) ions which exhibit four equivalent nitrile and pentaamminemetal groups and intense long-wavelength optical absorptions. The (8+) ions are easily oxidized to (IO+) species and reduced stepwise to (7+) and ( 6 f ) ions; EPR, Whishear-IR, and, in part, IR spectroelectrochemical techniques were applied to these oxidation states in the case of the TCNE, TCNQ, and TCNB systems. The complexes may be described as fully delocalized symmetrical tetrametalla n systems { (TCNX6-)[Ru"f"4(NH3)5]4}"+ with partially reduced ligands and fractionally oxidized metal centers. The extent "d" of metal-to-ligand electron transfer in the ground state is higher for the TCNQ and TCNE complexes than for the tetranuclear TCNP and TCNB compounds.The coordinatively unsaturated pentaammineruthenium fragment [Ru(NH3)5]"+ has played a prominent role in the development of our understanding of thermal and photoinduced electron transfer reactivity. Highlights include ion-or molecule-bridged heterodinuclear complexes L,M-@-X)-Ru(NH3)5 as examples for inner-sphere electron transfer processes,' the homodinuclear Creutz-Taube ion (l), which is now recognized as a delocalized 1 (class III) mixed-valent system? and various ruthenium-modified cytochromes as models for long-range electron transfer in proteins.The [Ru(NH3)5]"+ complex fragment is distinguished by unusually inert bonds formed from the metal to nitrogen donor ligands in both the n = 2 and n = 3 states, by generally reversible redox processes Ru(II/III) at convenient potentials and with small reorganization energies, and by the hydrophilic nature of the resulting complexe~.~ Furthermore, the 4d5 or 4d6 ruthenium species are generally assumed to have well-defined electronic structures with low-spin configurations and chargetransfer excited states lying lower than ligand-field excited configuration^.^ Although a vast number of symmetrical dinuclear complexes has been reported in relation to the Creutz-Taube system (1) ' Dedicated to Henry Taube on the occasion of his 80th birthday.

Tetracyanoethylene (TCNE): The Characteristic Geometries and Vibrational Absorptions of Its Numerous Structures

“…[2,4,58,61,[83][84][85][86] Owing to their valence ambiguous nature, further analysis is warranted, especially structural determinations. 4+ [50] have average ñ CN absorptions of 2215 and 2254 cm À1 , [49] .…”

“…The latter ruthenium-containing complex can either be oxidized or reduced, whereby the average ñ CN absorptions for these redox species are shifted to lower values. [86] This result indicates that the ruthenium ions are either oxidized or reduced, as it is unlikely that [TCNE]C 3À is present. These formulations assume a localized model for the electronic structure, which may not be valid.…”

“…8+ [86] have average ñ CN absorptions at 2140, 2135, and 2137 cm À1 , respectively, suggesting the presence of [TCNE] 2À , as [Os [TCNE] 2À )] 8+ , respectively. The latter ruthenium-containing complex can either be oxidized or reduced, whereby the average ñ CN absorptions for these redox species are shifted to lower values.…”

A Methyl Tricyanoethylenecarboxylate‐Based Room‐Temperature Magnet

“…MeTCEC underwent a reversible one-electron reduction at -0.05 V (vs. saturated calomel electrode (SCE) in MeCN), [12] that is ca. 0.3 V harder to reduce than TCNE, but easier to reduce than tetracyanopyrazine (-0.31 V), [13] and a reaction with V(CO) 6 was expected. Reaction of MeTCEC with V(CO) 6 in CH 2 Cl 2 rapidly occurred and the system turned dark blue with CO evolution, and a dark precipitate formed suggesting the formation of an organometallic polymer similar to V …”

Room‐Temperature Organic‐Based Magnet (Tc≈50 °C) Containing Tetracyanobenzene and Hexacarbonylvanadate(−I)

“…Hence, we sought additional examples of electron acceptors with a structure related to TCNP, and preferably one that could be chemically modified or attached to a polymer backbone. Herein, we report a new, the second, room-temperature organic-based magnet prepared from the reaction of [ V 0 (CO) 6 [19] ). Nonetheless, immediate reactions occurs forming a dark, amorphous product.…”