Alkali Cation Effects on Redox-Active Formazanate Ligands in Iron Chemistry

Abstract

Noncovalent interactions of organic moieties with Lewis acidic alkali cations can greatly affect structure and reactivity. Herein, we describe the effects of interactions with alkali-metal cations within a series of reduced iron complexes bearing a redox-active formazanate ligand, in terms of structures, magnetism, spectroscopy, and reaction rates. In the absence of a crown ether to sequester the alkali cation, dimeric complexes are isolated wherein the formazanate has rearranged to form a five-membered metallacycle. The dissociation of these dimers is dependent on the binding mode and size of the alkali cation. In the dimers, the formazanate ligands are radical dianions, as shown by X-ray absorption spectroscopy, Mössbauer spectroscopy, and analysis of metrical parameters. These experimental measures are complemented by density functional theory calculations that show the spin density on the bridging ligands.

Dinuclear Complexes of Flexidentate Pyridine‐Substituted Formazanate Ligands

“…[1][2][3] Formazanates have garnered considerable attention in coordination chemistry due to their ligand-based redox processes, which may facilitate multielectron redox transformations, [4] bond activations [5] and excited-state charge separation. [6] Av ariety of formazanate complexes of many main-group metals [7][8][9][10][11][12][13][14][15][16] andf irst-and second-row transition metals [17][18][19][20][21][22][23] have been described.T hese studies demonstrate the versatile coordination chemistry of formazanate ligandsa nd provides ignificant insight into the opticala nd redox properties of these compounds.S ome coppercomplexesc an also mediate oxygen activation, [24,25] certain cobalt and iron complexes exhibit unique magnetic characteristics, [17,26] and boronc omplexes in many cases feature not only the tunable redoxp roperties but also visible to nearinfrared photoluminescence, [9][10][11][12][13] finding applicationsa sc ellimaging agents [27,28] and electrochemiluminescence emitters. [10] Our group has expanded the coordination chemistry of formazanatest ot hird-row transition metals with as eries of hetero-leptic cyclometalated platinumc omplexes and bis-cyclometalated iridium complexes, [29][30][31] and accessed homoleptic azo-iminate platinum complexes and azo-1,2,3-triazolide iridium complexesv ia hydrogenative cleavage or [3+ +2]...…”

Selective Conversion of CO2 into Isocyanate by Low‐Coordinate Iron Complexes

“…This is in stark contrast with compound 1, which fully converted to 1·THF in the presence of just asingle equivalent of THF. [10] AvantHoff analysis ( Figure S4) of 2 in THF gave DH8 8 = À4.3 AE 0.2 kcal mol À1 and DS8 8 = À16.9 AE 0.6 cal mol À1 K À1 .T he negative reaction entropy is consistent with binding of two THF molecules to form two molecules of monomeric THF-containing complexes.T he magnetic moment in [D 8 ]THF at room temperature from the Evans method is 4.9 AE 0.2 m B per monomeric unit, which is consistent with amononuclear high-spin Fe 2+ (S = 2) complex.…”

“…We recently reported that Fe[N(SiMe 3 ) 2 ] 2 [9] gives access to low-coordinate Fe formazanate complex 1 (Scheme 2), and that THF contamination of solvent or starting materials results in the formation of complex 1·THF. [10] Herein, we show that these complexes both react with CO 2 to selectively give trimethylsilylisocyanate (TMSNCO) and the corresponding siloxide complex, and explain the unusually strong influence of THF through kinetic studies and through the isolation of akey intermediate.…”

“…One of the FeÀObonds is in the same plane as the formazanate ligand, and is significantly shorter than its counterpart (1.954(1) vs.2 .056 (2) ), which sticks out of the Fe formazanate plane,s imilar to the THF ligand in the solid state structure of 1·THF. [10] Theshort FeÀFe distance of 2.9916(5) in 2 is conducive to antiferromagnetic coupling of the two high-spin Fe 2+ centers,d escribed above.…”

Formazanate coordination compounds: synthesis, reactivity, and applications

“…Another study from these authors examined the effect of the countercation on the structures and reactivity of a series of derivatives of 21. 31 It was shown that when the countercation is one of the alkali metals (Na + , K + , Rb + or Cs + ), the compounds are dimeric in the solid state (22a-d, Scheme 7). In these dimers, the formazanate ligands coordinate in the 'open' form to give five-membered metallacycles, in which the 'pendant' terminal N-atom bridges to another Fe center.…”