Structural characteristics of redox-active pyridine-1,6-diimine complexes: Electronic structures and ligand oxidation levels

N-aryl and N-alkyl substituted pyridine-2,6-diimines (pdi) are useful tridentate ligands that are redoxactive and can coordinate to main group elements, transition metal ions, lanthanides and actinides. The neutral (pdi)(0) ligand can accept up to four electrons generating a monoanion (pdi(center dot))(1-) pi-radical, singlet or triplet dianion (pdi)(2-) or (pdi(center dot center dot))(2-), a trianionic pi-radical (pdi(center dot))(3-), and a singlet tetraanion (pdi)(4-). Upon this stepwise reduction the four C-N bond distances (C-py-N-py and C-imine-N-imine 1 increase and at the same time the two C-py-C-imine bond distances decrease. We show here that the single structural parameter Delta = [(d2 + d2')/2 - (d1 + d1' + d3 + d3')/4] varies in a linear fashion with increasing reduction of the (pdi(0))-ligand. Delta represents therefore a powerful structural parameter for the determination of the oxidation level of this ligand in a given complex provided the central metal ion does not exhibit significant pi-backdonation effects (M -> pdi(0)) as found in compounds with a neutral ligand and where M is a 2nd or 3rd row transition metal ion with d(n) configuration and n >= 6. (C) 2018 Elsevier B.V. All rights reserved

From Ylides to Doubly Yldiide-Bridged Iron(II) High Spin Dimers via Self-Protolysis

A synthetic strategy for the preparation of novel doubly yldiide bridged iron(II) high spin dimers ([(mu(2)-C)FeL](2), L = N(SiMe3)(2), Mesityl) has been developed. This includes the synthesis of ylide-iron(II) monomers [(Ylide)-FeL2] via adduct formation. Subsequent self-protolysis at elevated temperatures by in situ deprotonation of the ylide ligands results in a dimerization reaction forming the desired bridging mu(2)-C yldiide ligands in [(mu(2)-C)FeL](2). The comprehensive structural and electronic analysis of dimers [(mu(2)-C)FeL](2), including NMR, Mossbauer, and X-ray spectroscopy, as well as X-ray crystallography, SQUID, and DFT calculations, confirm their high-spin Fe-II configurations. Interestingly, the Fe2C2 cores display very acute Fe-C-Fe angles (averaged: 78.6(2)degrees) resulting in short Fe center dot center dot center dot Fe distances (averaged: 2.588(2) angstrom). A remarkably strong antiferromagnetic coupling between the Fe centers has been identified. Strongly polarized Fe-C bonds are observed where the negative charge is mostly centered at the mu(2)-C yldiide ligands

Electronic structures of "low-valent" neutral complexes [NiL2]0 (S = 0; L = bpy, phen, tpy) - An experimental and DFT computational study

The electronic structures of the neutral complexes [Ni(bpy)2]0, [Ni(tpy)2]0, and [Ni(phen)2]0 (bpy = 2,2-bipyridine; tpy = 2,2':6',2''-terpyridine; phen = 1,10-phenanthroline) have been investigated by a combination of UV/Vis spectroscopy, magnetochemistry, and X-ray crystallography {the structures of [Ni(bpy)2]0, [Ni(tpy)Cl]0, [Cu(bpy)2](PF6)2 (reinvestigation), and [(bpy)2Cu(μ2-NO3)Cu(bpy)2](PF6)3 are reported herein}. To further clarify the electronic structures of the title compounds and elucidate those of the electron transfer series produced by their reduction and oxidation, broken-symmetry (BS) density functional theory (DFT) calculations were performed for [Ni(bpy)2]m (m= 2+, 1+, 0, 1–,2–), [Ni(bpy)3]m (m= 3+, 2+, 1+, 0, 1–, 2–), [Ni(tpy)2]m (m=3+, 2+, 1+, 0) and, for calibration purposes, [Zn(phen)(NH3)xCl(2-x)]0 (x = 2, 1, 0). Through the aforementioned studies, it was shown that none of the so called "low-valent" Ni complexes contain zero-valent Ni. Instead, they each contain at least one (bpy·)1–, (tpy·)1–, or (phen·)1– π-radical monoanionic ligand. More specifically, they are all four-coordinate and possess the following electronic structures: [NiI(bpy·)(bpy0)] (delocalized), [NiI(tpy·)(tpy0)] (delocalized), and [NiII(Mephen·)2] (Mephen = 2,9-dimethyl-1,10-phenanthroline). In addition, all members of the series [Ni(bpy)2]m with m ≤ 1+ contain a NiI ion (d9, SNi = 1/2), and the redox processes that link them are ligand-centered. A similar finding was made for six-coordinate [Ni(bpy)3]m and [Ni(tpy)2]m, but in this case complexes with m ≤ 2+ contain Ni ions possessing an invariant +II oxidation state (d8, SNi = 1)

Electronic Structure of Square Planar Bis(benzene-1,2-dithiolato)metal Complexes [M(L)₂]^z (z = 2−, 1−, 0; M = Ni, Pd, Pt, Cu, Au): An Experimental, Density Functional, and Correlated ab Initio Study

The three diamagnetic square planar complexes of nickel(II), palladium(II), and platinum(II) containing two S,S-coordinated 3,5-di-tert-butylbenzene-1,2-dithiolate ligands, (LBu)2-, namely [MII(LBu)2]2-, have been synthesized. The corresponding paramagnetic monoanions [MII(LBu)(LBu•)]- (S = 1/2) and the neutral diamagnetic species [MII(LBu•)2] (M = Ni, Pd, Pt) have also been generated in solution or in the solid state as [N(n-Bu)4][MII(LBu)(LBu•)] salts. The corresponding complex [CuIII(LBu)2]- has also been investigated. The complexes have been studied by UV−vis, IR, and EPR spectroscopy and by X-ray crystallography; their electro- and magnetochemistry is reported. The electron-transfer series [M(LBu)2]2-,-,0 is shown to be ligand based involving formally one (LBu•)- π radical in the monoanion or two in the neutral species [MII(LBu•)2] (M = Ni, Pd, Pt). Geometry optimizations using all-electron density functional theory with scalar relativistic corrections at the second-order Douglas−Kroll−Hess (DKH2) and zeroth-order regular approximation (ZORA) levels result in excellent agreement with the experimentally determined structures and electronic spectra. For the three neutral species a detailed analysis of the orbital structures reveals that the species may best be described as containing two strongly antiferromagnetically interacting ligand radicals. Furthermore, multiconfigurational ab initio calculations using the spectroscopy oriented configuration interaction (SORCI) approach including the ZORA correction were carried out. The calculations predict the position of the intervalence charge-transfer band well. Chemical trends in the diradical characters deduced from the multiconfigurational singlet ground-state wave function along a series of metals and ligands were discussed

Coordination Modes, Oxidation, and Protonation Levels of 2,6-Pyridinediimine and 2,2 ':6 ',2 '-Terpyridine Ligands in New Complexes of Cobalt, Zirconium, and Ruthenium. An Experimental and Density Functional Theory Computational Study

The syntheses and molecular and electronic structures of the following complexes have been established by single crystal X-ray crystallography and UV-vis-NIR spectroscopy, and verified by density functional theory calculations (DFT B3LYP): [(eta(5)-Cp)(2)Zr-IV(tpy(2-))](0) (S = 0) 1, [(eta(5)-Cp)(2)Zr-IV-((OMe)pdi(2-))](0) (S = 0) 2, [Co-II((OMe)pdi(center dot))(eta(2)-BH4)](0) (S = 0) 4, [Ru-II((OMe)pdi-H)Cl(PPh3)(2)](0) (S = 0) 5, cis-[Ru-II((OMe)pdi(0))Cl-2(PPh3)](0) (S = 0) 6, and [Run(II)(eta(2)-(OMe)pdi(0))(eta(2)-(OMe)pdi-H)(2)](0) (S = 0) 7, with (tpy(0)) being neutral 2,2':6',2'-terpyridine, (tpy(center dot))(1-) its pi radical anion, (tpy(2-))(2-) its dianion; ((OMe)pdi(0)) neutral 2,6-bis(4-methoxyphenylmethylimine)pyridine, ((OMe)pdi(center dot))(1-) its radical anion and ((OMe)pdi(2-))(2-) its dianion; ((OMe)pdi-H)(1-) represents the deprotonated form of the ((OMe)pdi(0)) ligand where deprotonation takes place at the meta-position of the pyridine ring. Density functional theory calculations using the B3LYP functional were performed, establishing geometry optimized molecular and electronic structures. The structural parameter Delta = [(average distance C-py-C-imine) - (av. distance C-py - N-py + av. distance C-imine-N-imime)] is introduced for the characterization of the oxidation level of pdi (and analogously of tpy) ligands of M(pdi) (or M(tpy)) motifs for first row transition metals. The M(L-0) unit in second and third row low-valent transition metal ion complexes may exhibit significant pi-backdonation M -> L-0 structural effects

Metal promoted conversion of aromatic amines to ortho-phenylenediimine derivatives by a radical coupling path

A radical path for the conversion of o-substituted arylamines to o-phenylenediimine derivatives is reported. In the presence of [Ru-II(PPh3)(3)Cl-2] (Ru-P), 2-(phenylthio)aniline ((LH2)-H-SN) acts as an o-amination agent. Reaction of (LH2)-H-SN with Ru-P in toluene promotes (4e + 4H(+)) oxidative dimerization affording an o-phenylenediimine complex of ruthenium(ii). Similarly, intermolecular coupling between (LH2)-H-SN and other arylamines has been achieved

Molecular and Electronic Structures of Tetrahedral Complexes of Nickel and Cobalt Containing N,N‘-Disubstituted, Bulky o-Diiminobenzosemiquinonate(1−) π-Radical Ligands

The reaction of 2 equiv of the bulky ligand N,N‘-bis(3,5-di-tert-butylphenyl)-1,2-phenylenediamine, H2[3LPDI], excess triethylamine, and 1 equiv of M(CH3CO2)2·4H2O (M = Ni, Co) in the presence of air in CH3CN/CH2Cl2 solution yields violet-black crystals of [NiII(3LISQ)2] CH3CN (1) or violet crystals of [Co(3L)2] (3). By using Pd(CH3CO2)2 as starting material, green-blue crystals of [PdII(3LISQ)2]·CH3CN (2) were obtained. Single-crystal X-ray crystallography revealed that 1 and 3 contain (pseudo)tetrahedral neutral molecules [M(3L)2] (M = Ni, Co) whereas in 2 nearly square planar, neutral molecules [PdII(3LISQ)2] are present. Temperature-dependent susceptibility measurements established that 1 and 2 are diamagnetic (S = 0) whereas 3 is paramagnetic with an S = 3/2 ground state. It is shown that 1 contains two π radical benzosemiquinonate(1−)-type monoanions, ((3LISQ)1-•, Srad = 1/2), and a central Ni(II) ion (d8; S = 1) which are antiferromagnetically coupled yielding the observed St = 0 ground state. This result has been confirmed by broken symmetry DFT calculations of 1. In contrast, the St = 3/2 ground state of 3 is more difficult to understand:  the two resonance structures [CoIII(3LISQ)(3LPDI)] ↔ [CoII(3LPDI)(3LIBQ)] might be invoked (for tetrahedral [CoII(3LISQ)2] containing an SCo = 3/2 with two antiferromagnetically coupled π-radical ligands an St = 1/2 is anticipated). Complex 2 is diamagnetic (S = 0) containing a PdII ion (d8, SPd = 0 in an almost square planar ligand field) and two antiferromagnetically coupled ligand radicals (Srad = 1/2). The electrochemistry and spectroelectrochemistry of 1,2, and 3 have been studied, and electron-transfer series comprising the species [M(L)2]z (z = 2+, 1+, 0, 1−, 2−) have been established. All oxidations and reductions are ligand centered

Molecular and Electronic Structure of [Mn^VN(cyclam−acetato)]PF₆. A Combined Experimental and DFT Study

From the reaction of Li(cyclam−acetate), MnCl2·4H2O, and KPF6 in methanol brown microcrystals of [MnIIICl(cyclam−acetato)]PF6 (1) were obtained in the presence of air (cyclam−acetic acid = 1,4,8,11-tetraazacyclotetradecane-1-acetic acid). The reaction of 1 in aqueous NH3 solution with NaOCl produced blue crystals of [MnVN(cyclam−acetato)]PF6 (2). Complexes 1 and 2 were characterized by single-crystal X-ray crystallography, IR and Raman, electronic absorption, and 1H, 13C, and 15N NMR spectroscopies. Their magnetochemistry as well as their electrochemistry have been investigated. The complexes [MnN(cyclam−acetato)]+/2+ were studied by theoretical calculations at the DFT and semiempirical levels in order to obtain more insight into the ground and excited states of the MnV⋮N unit. Structural and spectroscopic parameters were successfully calculated and compared to experiment. A pictorial description of the bonding has been developed