Multiple Bistability in Quinonoid-Bridged Diiron(II) Complexes: Influence of Bridge Symmetry on Bistable Properties

Abstract

Quinonoid bridges are well-suited for generating dinuclear assemblies that might display various bistable properties. In this contribution we present two diiron(II) complexes where the iron(II) centers are either bridged by the doubly deprotonated form of a symmetrically substituted quinonoid bridge, 2,5-bis[4-(isopropyl)anilino]-1,4-benzoquinone (HL2') with a [O,N,O,N] donor set, or with the doubly deprotonated form of an unsymmetrically substituted quinonoid bridge, 2-[4-(isopropyl)anilino]-5-hydroxy-1,4-benzoquinone (HL5') with a [O,O,O,N] donor set. Both complexes display temperature-induced spin crossover (SCO). The nature of the SCO is strongly dependent on the bridging ligand, with only the complex with the [O,O,O,N] donor set displaying a prominent hysteresis loop of about 55 K. Importantly, only the latter complex also shows a pronounced light-induced spin state change. Furthermore, both complexes can be oxidized to the mixed-valent iron(II)-iron(III) form, and the nature of the bridge determines the Robin and Day classification of these forms. Both complexes have been probed by a battery of electrochemical, spectroscopic, and magnetic methods, and this combined approach is used to shed light on the electronic structures of the complexes and on bistability. The results presented here thus show the potential of using the relatively new class of unsymmetrically substituted bridging quinonoid ligands for generating intriguing bistable properties and for performing site-specific magnetic switching.

Homo‐ and Heterobinuclear Cu and Pd Complexes with a Bridging Redox‐Active Bisguanidino‐Substituted Dioxolene Ligand: Electronic Structure and Metal–Ligand Electron‐Transfer

“…In the heterobinuclear complex, the Pd II is coordinated by the softer bisguanidine side of L and the Cu II by the harder dioxolene side (in line with the HSAB concept). [19] In the last years, our group developed an ew class of redoxactive bis-bidentate ligands denoted GFAs (guanidino-functionalized aromatic compounds) [20] with ac harger egime (neutral to positive) opposite to that of the negatively charged tetraoxolene ligands. Oneelectron oxidation of both complexes was found to be quasi-reversible in CV experiments, and chemical one-electron oxidationw as achieved with NO + (SbF 6 À ).…”

“…et al, [14a] some compounds were tested as antitumorr eagents [15b] and others patented by Wella as hair dyes. [19] In the last years, our group developed an ew class of redoxactive bis-bidentate ligands denoted GFAs (guanidino-functionalized aromatic compounds) [20] with ac harger egime (neutral to positive) opposite to that of the negatively charged tetraoxolene ligands. [15] In their catechol or semi-quinone [17] form, homo-a nd heterobinuclear complexes with a1 ,2-dioxo-4,5-diimido framework are realized, but not in their quinone form.…”

“…Recently,ahomobinuclear Fe complex was characterized with as imilar [O,O;N,O]-quinone-type ligand. [19] In the last years, our group developed an ew class of redoxactive bis-bidentate ligands denoted GFAs (guanidino-functionalized aromatic compounds) [20] with ac harger egime (neutral to positive) opposite to that of the negatively charged tetraoxolene ligands. The Lewis representations of two examples, namely 1,2,4,5-tetrakis-(tetramethylguanidino)-benzene (2a) [21] and 2,3,5,6-tetrakis-(tetramethylguanidino)-pyridine (2b) [22b, c] are shown in Scheme 1.…”

Bis(pyrazolato) Bridged Diiron Complexes: Ferromagnetic Coupling in a Mixed‐Valent HS‐FeII/LS‐FeIII Dinuclear Complex

“…[20][21][22] The magnetic coupling as well as the extent of delocalization according to the Robin and Day classification [23] may vary widely and depend on, inter alia, the individual coordination environments and the nature of the bridging ligands. [24][25][26][27][28][29][30] Although the majority of metalloproteins and model complexes containing HS-Fe II and HS-Fe III ions display valence localized electronic structures and an S = 1/2 ground state, [31,32] some mixed-valent systems show a propensity for ferromagnetic coupling through a double-exchange mechanism. [33] For example, ferromagnetic coupling was observed in [Fe 2 (OH) 3 (tmtacn) 2 ] 2+ (tmtacn = 1,4,7-trimethyltriazacyclononane) arising from significant overlap of the magnetic orbitals due to the small Fe•••Fe distance of 2.51 Å, affording an overall S = 9/2 ground state.…”

Benzoquinonoid-bridged dinuclear actinide complexes

“…Bridging quinoid ligands have been used extensively in transition metal chemistry 1 to study properties such as electron transfer, 2 spin-spin coupling, 3 mixed valency, 4 valence tautomerism 5 and single-molecule magnetism (SMM). 6 Donor atoms are included in the extended π-conjugation of quinoid ligands, which makes this ligand class well suited to studying intermetallic communication.…”