Class III Delocalization in a Cyanide‐Bridged Trimetallic Mixed‐Valence Complex

Abstract

The NIR and IR spectroscopic properties of the cyanide-bridged complex, trans-[Ru(dmap) 4 {(m-CN)Ru-(py) 4 Cl} 2 ] 3+ (py = pyridine, dmap = 4-dimethylaminopyridine) provide strong evidence that this trimetallic ion behaves as a Class III mixed-valence species, the first example reported of a cyanide-bridged system. This has been accomplished by tuning the energy of the fragments in the trimetallic complex to compensate for the intrinsic asymmetry of the cyanide bridge. Moreover, (TD)DFT calculations accurately predict the spectra of the trans-[Ru(dmap) 4 {(m-CN)Ru(py) 4 Cl} 2 ] 3+ ion and confirms its delocalized nature.Mixed-valence chemistry has been the focus of attention of inorganic chemists for more than 40 years, since the first example of a deliberate synthesis of a discrete binuclear system was reported by Creutz and Taube.[1] An early survey of the reported mixed-valence systems gave to rise to the Robin-Day classification, which consisted of three distinctive categories:[2] Class I, not interacting; Class II, interacting, but localized; and Class III, delocalized. A strong effort in the characterization of the inter-valence charge transfer (IVCT) of Class II systems was fueled by the two-state model proposed by Hush [3,4] and Sutin, [5,6] which provided an easy route to obtain valuable information about the parameters governing the electron transfer process. Later, the focus shifted to the exploration of systems close to delocalization, somewhere in the transition between Class II and Class III. Hence, a new Class, II/III, was proposed, [7] which comprises systems still localized, but solvent-averaged. The potential energy surface describing these systems possesses two minima of similar energy separated by a small barrier, so that both mixed-valence isomers coexist and interconvert into each other. This behavior has been demonstrated experimentally, [8,9] and the rate constant for electron transfer between the isomers has been measured. [10] In many of these nearly delocalized mixed-valence systems, the bridge plays an important role in the spectroscopy, [11][12][13] which requires a three-state model for an accurate interpretation. [14,15] Another focus has been the characterization of higher nuclearity systems, [16] which also present challenging spectroscopy. [17][18][19] We have recently reported [18] the spectroscopy of two trimetallic cyanide-bridged mixed-valence complexes of formula trans-[Ru(L) 4 {(m-CN)Ru(py) 4 Cl} 2 ] 3+ (1 3+ , where L = pyridine and 2 3+ , where L = 4-methoxypyridine; Table 1). These systems exhibit two transitions in the near infrared (NIR) that correspond to a charge transfer from the central ruthenium(II) unit to the neighboring terminal {-Ru III (py) 4 Cl} fragment (MM'CT), and a charge transfer between the distant {-Ru(py) 4 Cl} fragments (MMCT). The latter is the most intense, which suggests a strong interaction between the three metallic ions. These species demonstrate that diminishing the energy of the state located at the central Ru II (the "bridge") r...

Temperature dependence of spherical electron transfer in a nanosized [Fe14] complex

“…2b) Compared with the enriched sample, the natural [Fe 14 ] sample exhibits one additional quadrupole doublet in the spectrum at both high and low temperatures. Considering its isomer shift (IS) of 0.07–0.15 mm s −1 and quadrupole splitting (QS) of ~0.45 mm s −1 , the additional doublet can be assigned to Fe II -ls in the [Fe(Tp)(CN) 3 ] unit 21 . The result indicates that the reactant, [Fe III (Tp)(CN) 3 ] − , is completely reduced to [Fe II (Tp)(CN) 3 ] 2− during the formation of [Fe 14 ].…”

“…Prussian blue analogs have received considerable attention as they can demonstrate the coexistence of electron transfer and exchange interaction through a cyanide bridge 21,22 . The realization of partially reducing or oxidizing the interacting metal centers in a nanoarchitecture is proposed as a route to achieve the extensive multielectron transfer with novel magnetic and electronic properties 23,24 .…”

MMCT energy change in cyanidometal-bridged trinuclear complexes by changing the ligand electron donating ability

“…S10 (ESI †), the complexes 1 2+ -5 2+ exhibit similar absorption bands (from 18 000 to 28 000 cm À1 ) in the UV-vis area. These bands can be assigned as the metal-to-ligand charge transfer (MLCT) transition for the dp(Ru II )p*(i-PrPy) and dp(Fe II )p*(dppe) transitions according to the previous reports, 28,40,41,43,44 and do not exhibit any absorption band in the NIR region (Fig. S10, ESI †).…”

“…[24][25][26] All of these studies focussed on how the energy level of the bridge group (B) or the length of the bridge group (B) affects the electron communication of D and A ligands. 19,[24][25][26][27][28][29] Recently, our group has synthesized two groups of trimetallic cyanidometal-bridged Fe-NC-Ru-CN-Fe complexes for investigating the influence of the cyanidometal bridge on MMCT by changing the ligand on the central Ru from 2,2 0 -bipyridine to 4,4 0dimethyl -2,2 0 -bipyridyl. 31 However, the effect of the electrondonating or-withdrawing ability of D and A ligands on the delocalization properties of mixed-valence complexes has been less explored.…”

“…To date, many cyanidometal-bridged MV compounds have been synthesized for the study of MMCT and other properties. 7,28,[33][34][35][36][37] To better understand the influence of ligand substituents on MMCT in trinuclear cyanidometal-bridged MV compounds, we synthesized and characterized five trinuclear cyanidometal-bridged MV compounds that differ only in methyl substituents of cyclopentadiene (Fig. 1), trans-[CpMe x (dppe)Fe-NC-Ru(i-PrPy) 4 -CN-Fe(dppe)Cp- The experimental results show that the change trend of the MMCT energy from the remote terminal Fe II to Fe III in complexes N 3+ is not obvious, which may be due to the increase in the electron-donating ability of the Cp ligands on both the terminal electron-donating Fe II and electron-accepting Fe III ions, while the MMCT energy from neighboring Ru II to terminal Fe III in complexes N 4+ decreases with the increase of methyl substituents.…”