Electronic Transition and Magnetic Coupling Regulation in Trimetallic Complexes Featuring a New Bridging Ligand Obtained by Oxidative Addition

A series of trimetallic complexes [Fe^III(μ-L)(py)]₂M^II(py)_n (n = 2, M^II = Mn^II, 1; Fe^II, 2; Co^II, 3; Zn^II, 4; n = 3, M^II = Cd^II, 5) with a new bridging ligand L^4- (deprotonated 1,2-N¹,N²-bis(2-mercaptoanil) oxalimidic acid) were synthesized and fully characterized by elemental analysis, single-crystal X-ray crystallography, IR, and Mössbauer spectra. Interestingly, the bridging ligand was obtained by oxidative addition of the (gma^•)^3- ligand from the mononuclear precursor Fe(gma)py (gma = glyoxal-bis(2-mercaptoanil)). In the obtained complexes, the bridging ligand L^4- coordinates to the terminal Fe^III ions (intermediate-spin with S_Fe = 3/2) by the N, S atoms, and coordinate to the central metal M^II ion by the four O atoms. The resonance structure of the bridging ligand can be described as the two 4π-electron delocalized systems connected by one single-bond (C₁-C₂), which is different from the electronic structure of the precursor Fe(gma)py. Remarkably, the magnetic coupling interaction can be regulated through the central metal. The ferromagnetic coupling constant J gradually decreases as M^II changes from Fe^II to Co^II and Mn^II, while the paramagnetic behaviors are presented when M^II = Zn^II and Cd^II, confirmed by the magnetic susceptibility measurements and further supported by using the PHI program. Furthermore, the bridging ligand to the terminal Fe^III charge transfer (LMCT) transitions emerged in all complexes but the central Fe^II to terminal Fe^III charge transfer (MMCT) only presented in complex 2, strongly supported by the UV/vis-NIR electronic spectra and TDDFT calculations.

Playing with Pearson's concept: orthogonally functionalized 1,4-diaza-1,3-butadienes leading to heterobinuclear complexes

By reacting 1,2-diketones and ortho- diphenylphosphinoyl aniline in the presence of zinc(ii) as a templating agent, cationic zinc(ii) complexes of novel phosphine oxide functionalized 1,4-diaza-1,3-butadiene ligands are acessible. Herein the zinc(ii) site is bound to all four donor atoms of the ligand. Depending on the flexibility of the 1,4-diaza-1,3-butadiene backbone, the bonds to zinc(ii) from the 1,4-diaza-1,3-butadiene donors can be broken. Reaction with oxalate cleaves the zinc(ii) coordination completely and makes accessible the free ligands possessing orthogonal (N,N: soft; O,O: hard) sets of donor sites. This allows for the specific coordination of soft and hard Lewis acids and thus for the generation of heterobimetallic complexes, here exemplarily shown for the combination of palladium(ii) (soft) and zinc(ii) (hard) centres.

Molecular and electronic structures of iron(II)/(III) complexes containing N,S-coordinated, closed-shell o-aminothiophenolato(1-) and o-iminothiophenolato(2-) ligands

The coordination chemistry of the ligands o-aminothiophenol, H(abt), 4,6-di-tert-butyl-2-aminothiophenol, H[L(AP)], and 1,2-ethanediamine-N,N'-bis(2-benzenethiol), H(4)('N(2)S(2')), with FeCl(2) under strictly anaerobic and increasingly aerobic conditions has been systematically investigated. Using strictly anaerobic conditions, the neutral, air-sensitive, yellow complexes (mu-S,S)[Fe(II)(abt)(2)](2) (1), (mu-S,S)[Fe(II)(L(AP))(2)](2).8CH(3)OH (2), and (mu-S,S)[Fe(II)('H(2)N(2)S(2'))](2).CH(3)CN (3) containing high spin ferrous ions have been isolated where (abt)(1-), (L(AP))(1-), and ('H(2)N(2)S(2'))(2-) represent the respective N,S-coordinated, aromatic o-aminothiophenolate derivative of these ligands. When the described reaction was carried out in the presence of trace amounts of O(2) and [PPh(4)]Br, light-green crystals of [PPh(4)][Fe(II)(abt)(2)(itbs)].[PPh(4)]Br (4) were isolated. The anion [Fe(II)(abt)(2)(itbs)](-) contains a high spin ferrous ion, two N,S-coordinated o-aminophenolate(1-) ligands, and an S-bound, monoanionic o-iminothionebenzosemiquinonate(1-) pi radical, (itbs)(-). Complex 4 possesses an S(t) = 3/2 ground state. In the absence of [PPh(4)]Br and presence of a base NEt(3) and a little O(2), the ferric dimer (mu-NH,NH)[Fe(III)(L(AP))(L(IP))](2) (5a) and its isomer (mu-S,S)[Fe(III)(L(AP))(L(IP))](2) (5b) formed. (L(IP))(2-) represents the aromatic o-iminothiophenolate(2-) dianion of H[L(AP)]. The structures of compounds 2, 4, and 5a have been determined by X-ray crystallography at 100(2) K. Zero-field Mössbauer spectroscopy of 1, 2, 3, and 4 unambiguously shows the presence of high spin ferrous ions: The isomer shift at 80 K is in the narrow range 0.85-0.92 mm s(-1), and a large quadrupole splitting, |DeltaE(Q)|, in the range 3.24-4.10 mm s(-1), is observed. In contrast, 5a and 5b comprise both intermediate spin ferric ions (S(Fe) = 3/2) which couple antiferromagnetically in the dinuclear molecules yielding an S(t) = 0 ground state.

Noninnocence of the ligand glyoxal-bis(2-mercaptoanil). The electronic structures of [Fe(gma)]2, [Fe(gma)(py)] x py, [Fe(gma)(CN)]1-/0, [Fe(gma)I], and [Fe(gma)(PR3)(n)] (n = 1, 2). Experimental and theoretical evidence for "excited state" coordination

The electronic structure of the known iron complexes [Fe(gma)](2) (S(t) = 0) (1)(6) and [Fe(gma)(py)].py (S(t) = 1) (2)(7) where H(2)(gma) represents glyoxal-bis(2-mercaptoanil) has been shown by X-ray crystallography, Mössbauer spectroscopy, and density functional theory calculations to be best described as ferric (S(Fe) = 3/2) complexes containing a coordinated open-shell pi radical trianion (gma(*))(3)(-) and not as previously reported(6,7) as ferrous species with a coordinated closed-shell dianion (gma)(2)(-). Compound 1 (or 2) can be oxidized by I(2) yielding [Fe(III)(gma)I] (S(t) = 1/2) (3). With cyanide anions, complex 1 forms the adduct [(n-Bu)(4)N][Fe(III)(gma(*))(CN)] (S(t) = 1) (4), which can be one-electron oxidized with iodine yielding the neutral species [Fe(III)(gma)(CN)] (S(t) = 1/2) (5). With phosphines complex 1 also forms adducts(7) of which [Fe(III)(gma(*))(P(n-propyl)(3))] (S(t) = 1) (6) has been isolated and characterized by X-ray crystallography. [Fe(II)(gma)(P(n-propyl)(3))(2)] (S(t) = 0) (7) represents the only genuine ferrous species of the series. Density functional theory (DFT) calculations at the BP86 and B3LYP levels were applied to calculate the structural as well as the EPR and Mössbauer spectroscopic parameters of the title compounds as well as of the known complexes [Zn(gma)](0/)(-) and [Ni(gma)](0/)(-). Overall, the calculations give excellent agreement with the available spectroscopic information, thus lending support to the following electronic structure descriptions: The gma ligand features an unusually low lying LUMO, which readily accepts an electron to give (gma(*))(3)(-). The one-electron reduction of [Zn(gma)] and [Ni(gma)] is strictly ligand centered and differences in the physical properties of [Zn(gma(*))](-) and [Ni(gma(*))](-) are readily accounted for in terms of a model that features enhanced back-bonding from the metal to the gma LUMO in the case of [Ni(gma(*))](-). In the case of [Fe(gma)(PH(3))], [Fe(gma)(py)], and [Fe(gma)(CN)](-) an electron transfer from the iron to the gma LUMO takes place to give strong antiferromagnetic coupling between an intermediate spin Fe(III) (S(Fe) = 3/2) and (gma(*))(3)(-) (S(gma) = 1/2), yielding a total spin S(t) = 1. Broken symmetry DFT calculations take properly account of this experimentally calibrated electronic structure description. By contrast, the complexes [Fe(gma)(PH(3))(2)] and [Fe(PhBMA)] feature closed-shell ligands with a low-spin Fe(II) (S(Fe) = S(t) = 0) and an intermediate spin central Fe(II) (S(Fe) = S(t) = 1), respectively. The most interesting case is provided by the one-electron oxidized species [Fe(gma)(py)](+), [Fe(gma)I], and [Fe(gma)(CN)]. Here the combination of theory and experiment suggests the coupling of an intermediate spin Fe(III) (S(Fe) = 3/2) to the dianionic ligand (gma)(2)(-) formally in its first excited triplet state (S(gma) = 1) to give a resulting S(t) = 1/2. All physical properties are in accord with this interpretation. It is suggested that this unique "excited state" coordination is energetically driven by the strong antiferromagnetic exchange interaction between the metal and the ligand, which cannot occur for the closed-shell form of the ligand.