Radical Monocationic Guanidino-Functionalized Aromatic Compounds (GFAs) as Bridging Ligands in Dinuclear Metal Acetate Complexes: Synthesis, Electronic Structure, and Magnetic Coupling

Abstract

In this work, the oxidation of several new dinuclear metal (M) acetate complexes of the redox-active guanidino-functionalized aromatic compound (GFA) 1,2,4,5-tetrakis(tetramethylguanidino)benzene (1) was studied. The complexes [1{M(OAc)2}2] (M = Ni or Pd) were oxidized to the radical monocationic complexes [1{M(OAc)2}2](+ •). From CV (cyclic voltammetry) measurements, the Gibbs free enthalpy for disproportionation of [1{M(OAc)2}2](+ •) into [1{M(OAc)2}2] and [1{M(OAc)2}2](2+) could be estimated to be roughly +20 kJ mol(-1) in CH2Cl2 solution. A characteristic feature of the [1{M(OAc)2}2](+ •) complexes is the presence of intense metal-ligand charge-transfer bands in the electronic absorption spectra. The complex [1{Ni(OAc)2}2](+ •) combines three paramagnetic centers with four metal-centered unpaired electrons and a ligand centered π-radical and exhibits a sextet electronic ground state. Spin distribution of the Ni complexes was evaluated by paramagnetic (1)H and (13)C NMR and was correlated with calculations. The strong ferromagnetic metal-ligand magnetic coupling was studied in the solid state by magnetometric (SQUID) measurements and by quantum chemical (DFT) calculations. The temperature dependence of the paramagnetic NMR shift was used for the evaluation of the magnetic coupling between the Ni centers and the π-radical in solution.

Copper Complexes of New Redox‐Active 4,5‐Bisguanidino‐Substituted Benzodioxole Ligands: Control of the Electronic Structure by Counter‐Ligands, Solvent, and Temperature

“…This meanst hat the monocationic ligand is stabilized by complex formation.Asimilaro bservation was made for dinuclearc omplexes of ligand 1. [26] The complexes could also be reduced( Cu II !Cu I at E red = À1.04 Vf or [3(CuCl 2 )] and À0.86 Vf or [4(CuBr 2 )]), but this copper-centeredr edox process is irreversible ( Figure SI41 in the SupportingI nformation). Ac orresponding oxidation wave waso bserved at À0.27 Vf or the chlorido and at À0.22 Vf or the bromido complex.…”

Evaluation of the Synthetic Scope and the Reaction Pathways of Proton‐Coupled Electron Transfer with Redox‐Active Guanidines in C−H Activation Processes

“…The free radical monocation 1 • + is unstablet owards disproportionation into 1 and 1 2 + .H ence, it is not formed in mixtures of 1 and 1 2 + .O n the other hand, the radical monocationic form is stable as a bridging ligand in severald inuclearl ate-transition-metal complexes. [21,22] An umber of salts of 1 2 + were fully characterized, including 1(PF 6 ) 2 [23] and 1(BF 4 ) 2 [24] used in this work, and 1(I 3 ) 2 . [19] Also, the nitrogen-rich 1[N(CN) 2 ] 2 was prepared, [25] that melts above 200 8Ca nd decomposes smoothly at 220 8C, demonstrating its excellent thermal stability.I nt he dication 1 2 + , the bond length betweent he carbonsi n1 -a nd 2-positions and that between the carbonsi n4 -a nd 5-positions (all directly bound to guanidino groups) are considerably elongated, in line with the Lewis structure in Figure 1.…”

“…Hence, it is not formed in mixtures of 1 and 1 2+ . On the other hand, the radical monocationic form is stable as a bridging ligand in several dinuclear late‐transition‐metal complexes [21, 22] . A number of salts of 1 2+ were fully characterized, including 1 (PF 6 ) 2 [23] and 1 (BF 4 ) 2 [24] used in this work, and 1 (I 3 ) 2 [19] .…”

1,2,4,5‐Tetrakis(tetramethylguanidino)‐3,6‐diethynyl‐benzenes: Fluorescent Probes, Redox‐Active Ligands and Strong Organic Electron Donors

“…Most likely, oxidation initiates decomposition of the complex. This behaviour is in stark contrast to that of complexes [ 1 a {M(OAc) 2 } 2 ] (M=Cu, Ni or Pd), that could be reversibly oxidized in two one‐electron steps, allowing the isolation of salts of the monocation [ 1 a {M(OAc) 2 } 2 ] + (with radical monocationic 1 a ⋅ + units) and the dication [ 1 a {M(OAc) 2 } 2 ] 2+ (with dicationic 1 a 2+ units) …”

“…They could also be used in materials, for example, semiconducting devices or „low‐dimensional perovskites“ . Finally, GFAs like 1 a are versatile redox‐active ligands in late‐transition metal complexes, establishing stable ligand‐metal bonding in three redox states of the ligand (neutral, radical monocationic and dicationic) . In this context it should be noted that complexes with guanidine or guanidinate ligands as well as other N ‐heterocyclic imino (NHI) ligands/substituents are intensively studied.…”