Benzoquinone diimine bridged ruthenium ammines, a novel type of mixed-valence complexes

First binuclear complex of an N,N′,N″,N‴-tetraalkyl 2,5-diamino-1,4-benzoquinonediimine

“…The delocalization pathway also differs from that in complexes where the metal centres are linked by substituted p-benzoquinonediimines. 6,7 This important feature may be of wider occurrence in such systems than previously believed. 16,17 The molecular structure of 7 indicates that p-backbonding from the Pt orbitals into a p* orbital of the quinonediimine is occurring: the N(1)NC(1), C(1)-C(3A), C(3A)NC(2A) and C(2A)-N(2A) bonds of the bridge show alternating lengthening and shortening relative to those of the free ligand (Table 1).…”

“…Such compounds are expected to increase the scope of bonding patterns and electronic delocalization compared to other bridging nitrogen ligands such as terpyridine systems, 4 Creutz-Taube species 5 and p-benzoquinonediimine-bridged ligands. 6,7 The direct use of 2b has been shown to be unsuitable since coordination of metal fragments renders the benzoquinonediimine bridge more susceptible to nucleophilic attack. 8 In contrast, N-substitution by electron-donating groups should lead to more stable complexes toward dioxygen and/or water.…”

2,5‐Dioxido‐1,4‐benzoquinonediimine (H2L2−), A Hydrogen‐Bonding Noninnocent Bridging Ligand Related to Aminated Topaquinone: Different Oxidation State Distributions in Complexes [{(bpy)2Ru}2(μ‐H2L)]n (n=0,+,2+,3+,4+) and [{(acac)2Ru}2(μ‐H2L)]m (m=2−,−,0,+,2+)

“…A neutral 1,2,4,5‐tetraimino‐3,6‐diketocyclohexane‐bridged bis[Ru(bpy) 2 ] complex also showed a rather low K c value at 10 5 10. On the other hand, the diruthenium complex [{(NH 3 ) 5 Ru} 2 (μ‐H 2 L′)] 5+ involving the neutral bis‐monodentate p ‐benzoquinonediimine bridging ligand (H 2 L′)20 exhibits a K c value of 10 10 . The effects on K c of strongly σ‐donating NH 3 (and of acac − ) as ancillary ligands in comparison to π‐acidic bpy were demonstrated earlier in a series of tetrazine‐bridged diruthenium systems, [(bpy) 2 Ru(μ‐bptz)Ru(bpy) 2 ] 5+ ( K c =10 8.5 ),21 [(acac) 2 Ru(μ‐bptz)Ru(acac) 2 ] + ( K c =10 13.6 ),14b and [(NH 3 ) 4 Ru(μ‐bptz)Ru(NH 3 ) 4 ] 5+ ( K c =10 15.0 ),22 in which bptz=3,6‐bis(2‐pyridyl)‐1,2,4,5‐tetrazine.…”

An Experimental and Density Functional Theory Approach Towards the Establishment of Preferential Metal‐ or Ligand‐Based Electron‐Transfer Processes in Large Quinonoid‐Bridged Diruthenium Complexes [{(aap)2Ru}2(μ‐BL2–)]n+ (aap = 2‐Arylazopyridine)

“…[3] A large number of polyruthenium derivatives have been studied and a wide variation of coupling aspects, including moderately coupled localized class II, strongly coupled delocalized class III, and hybrid or borderline class II-III situations [4,5] have emerged that primarily depend on the electronic states of the specific set of bridging and ancillary ligands. Although different kinds of molecular frameworks have been utilized as bridging ligands for assembling metal fragments in place, only a limited number of diruthenium complexes with the following redox-active polynucleating quinonoid bridging (BL) units along with selective ancillary ligands (AL) are known so far: 1,10-phenanthroline-5, 6-dione (AL = acac - [6] /bpy [7] ), 1,10-phenanthroline-5,6-diimine (acac -), [6] 2,5-dioxido-1,4-benzoquininediimines (bpy/acac -), [8] 2,5-dioxido-1,4-benzoquinones (bpy), [9] 5,8-dioxido-1,4-naphthoquinone (bpy), [10] 1,4-dioxidoanthraquinone (bpy), [11] 1,5-dioxidoanthraquinone (bpy), [11] 1,2,4,5-tetraimino-3,6-diketocyclohexane (bpy), [12] 3,3Ј,4,4Ј-tetraoxybiphenyl (bpy), [13] 3,3Ј,4,4Ј-tetraiminobiphenyl (acac - [14] /bpy [15] ), and p-benzoquinonediimine(NH 3 ) [16] (bpy = 2,2Ј-bipyridine, acac -= acetylacetonate). The remarkable mixing of ruthenium and quinonoid frontier orbitals [17] is expected to facilitate the bridging-ligand-mediated intermetallic coupling processes in such systems; however, it also creates additional challenges in assigning the precise valence state combinations of the metal ion and the bridging ligand in the native state as well as towards the accessible electron-transfer processes.…”