Andreas Grohmann

Spin-State Patterns in Surface-Grafted Beads of Iron(II) Complexes

Novel strategies for the design of functional materials are in increasing demand, as the down-scaling of lithographic processes (the top-down approach) will soon encounter the fundamental physical limits of miniaturization. One of the fascinating perspectives of molecular electronics is information storage at the single-molecule level, on the basis of arrays of molecular switches. Spin-crossover (SCO) compounds hold considerable potential in this context. SCO can occur in octahedral transition-metal complexes in which the metal ion has a d to d electron configuration. The transition may be stimulated externally, by a change in temperature or pressure, or by irradiation. SCO is entropydriven and, in the solid state, is influenced strongly by intermolecular interactions, such as hydrogen bonding or p–p stacking. Such interactions give rise to cooperativity between SCO complexes within the ensemble. High cooperativity can cause the change in spin state to be accompanied by hysteresis, which confers bistability on the system and thus a memory effect. A viable reading/writing procedure, that is, a means of reproducible actuation on the single-molecule level, is a formidable challenge that has yet to be met, but in this way SCO compounds could serve in devices of unsurpassable storage density. In principle, reliable information storage could be achieved even in the absence of hysteresis, provided the energy difference between low-spin state and high-spin state of the complexes within the SCO ensemble is sufficiently large (on the order of several kT). A large number of spin-crossover systems are known, with complexes of iron(II) the most numerous, both in solution and in the solid state. Usually, ferrous iron is in a quasi-octahedral N6 coordination environment, and switching occurs between a low-spin (LS, A1g/t2g , S= 0) and a high-spin state (HS, T2g/ t2g eg , S= 2). SCO systems have been characterized by physical techniques including M ssbauer and UV/Vis spectroscopy, magnetic susceptibility measurements, and diffraction methods, applied to the bulk solids. Many attempts have been made to obtain SCO materials in the form of thin films, multilayers, or nanocrystals. Recent strategies include the sequential assembly of coordination polymers on metal or biopolymer supports (such as gold or chitosan) and the preparation in polymeric matrices, in surface-grown multilayer thin films incorporating iron(II) coordination polymers, and in nanoparticulate iron(II) complexes. Our approach is to use spin-switchable iron(II) complexes of bis(pyrazolyl)pyridine ligands, with a variety of substituents that can serve as surface anchors depending on the kind of substrate. We studied the spin state of adsorbates at the single-molecule level with scanning tunneling microscopy (STM) techniques at room temperature (298 K). For the present study, we chose [Fe(L)2](BF4)2 (1; L= ligand), whose synthesis, solid-state structure, and spin behavior have been reported in detail. The solid-state structure of the dication in 1 is shown in Figure 1a. The magnetic susceptibility of 1

Gas-Phase C−H and N−H Bond Activation by a High Valent Nitrido-Iron Dication and 〈NH〉-Transfer to Activated Olefins

A tetrapodal pentadentate nitrogen ligand (2,6-bis(1,1-di(aminomethyl)ethyl)pyridine, 1) is used for the synthesis of the azido-iron(III) complex [(1)Fe(N3)]X2 where X is either Br or PF6. By means of electrospray ionization mass spectrometry, the dication [(1)Fe(N3)]2+ can be transferred into the gas phase as an intact entity. Upon collisional activation, [(1)Fe(N3)]2+ undergoes an expulsion of molecular nitrogen to afford the dicationic nitrido-iron species [(1)FeN]2+ as an intermediate, which upon further activation can intramolecularly activate C-H- and N-H bonds of the chelating ligand 1 or can transfer an NH unit in bimolecular reactions with activated olefins. The precursor dication [(1)Fe(N3)]2+, the resulting nitrido species [(1)FeN]2+, and its possible isomers are investigated by mass spectrometric experiments, isotopic labeling, and complementary computational studies using density functional theory.

Spin Crossover in a Vacuum-Deposited Submonolayer of a Molecular Iron(II) Complex

Spin-state switching of transition-metal complexes (spin crossover) is sensitive to a variety of tiny perturbations. It is often found to be suppressed for molecules directly adsorbed on solid surfaces. We present X-ray absorption spectroscopy measurements of a submonolayer of [Fe(II)(NCS)2L] (L: 1-{6-[1,1-di(pyridin-2-yl)ethyl]-pyridin-2-yl}-N,N-dimethylmethanamine) deposited on a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. These molecules undergo a thermally induced, fully reversible, gradual spin crossover with a transition temperature of T1/2 = 235(6) K and a transition width of ΔT80 = 115(8) K. Our results show that by using a carbon-based substrate the spin-crossover behavior can be preserved even for molecules that are in direct contact with a solid surface.

Iron carbonyl, nitrosyl, and nitro complexes of a tetrapodal pentadentate amine ligand: Synthesis, electronic structure, and nitrite reductase-like reactivity

The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms a series of octahedral iron(II) complexes of general formula [Fe(L)(1)]Xn with a variety of small-molecule ligands L at the sixth coordination site (L = X = Br, n = 1 (2); L = CO, X = Br, n = 2 (3); L = NO, X = Br, n = 2 (4); L = NO+, X = Br, n = 3 (5); L = NO2-, X = Br, n = 1 (6)). The bromo complex, which is remarkably stable towards hydrolysis and oxidation, serves as the precursor for all other complexes, which may be obtained by ligand exchange, employing CO, NO, NOBF4, and NaNO2, respectively. All complexes have been fully characterised, including solid-state structures in most cases. Attempts to obtain single crystals of 6 produced the dinuclear complex [Fe2[mu 2-(eta 1-N: eta 1-O)-NO2](1)2]Br2PF6 (7), whose bridging NO2- unit, which is unsupported by bracketing ligands, is without precedent in the coordination chemistry of iron. Compound 2 has a high-spin electronic configuration with four unpaired electrons (S = 2), while the carbonyl complex 3 is low-spin (S = 0), as are complexes 5, 6 and 7 (S = 0 in all cases); the 19 valence electron nitrosyl complex 4 has S = 1/2. Complex 4 and its oxidation product, 5 ([Fe(NO)]7 and [Fe(NO)]6 in the Feltham-Enemark notation) may be interconverted by a one-electron redox process. Both complexes are also accessible from the mononuclear nitro complex 6: Treatment with acid produces the 18 valence electron NO+ complex 5, whereas hydrolysis in the absence of added protons (in methanolic solution) gives the 19 valence electron NO. complex 4, with formal reduction of the NO2- ligand. This reactivity mimicks the function of certain heme-dependent nitrite reductases. Density functional calculations for complexes 3, 4 and 5 provide a description of the electronic structures and are compatible with the formulation of iron(II) in all cases; this is derived from the careful analysis of the combined IR, ESR and Mössbauer spectroscopic data, as well as structural parameters.

Self-assembled monolayers of a bis(pyrazol-1-yl)pyridine-substituted thiol on Au(111).

Self-assembled monolayers (SAMs) of a bis(pyrazol-1-yl)pyridine-substituted thiol (bpp-SH) on Au (111)/mica were studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Using substrates precoated with perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA), preparation at elevated temperatures yields highly ordered layers whose structure is described by a rectangular (5 x radical3) unit cell containing one molecule. The bis(pyrazol-1-yl)pyridine (bpp) units exhibit pi-stacking along the 112 direction, and they are tilted significantly. We conclude the three imine nitrogen atoms in the bpp headgroup adopt a trans,trans arrangement.

A Structurally Characterised Pair of Dicobalt(III) Peroxo/Superoxo Complexes with C2-Symmetrical Tetrapodal Pentadentate Amine Ligands, and Some Reactivity en route

The tetrapodal pentaamine ligand 2,6-bis(1′,3′-diamino-2′-methylprop-2′yl)pyridine (pyN4, 1) provides square-pyramidal coordinated cobalt(II) building blocks, which have been used in the synthesis of the singly-bridged dicobalt(III) µ2-η1:η1-peroxo and superoxo complexes [(1)Co−O2−Co(1)]4+/5+, isolated as the chloride, bromide, and mixed chloride/dithionate and bromide/dithionate salts. The peroxo complex is accessible by the classical route involving oxygenation of [(pyN4)CoII], but has also been obtained from a cobalt(III) precursor in air, which implies that the pentaamine 1 acts as a multi-electron reductant. Oxidation of the peroxo complex with chlorine generated in situ from an HCl/H2O2 mixture (H2O2 derived from partial peroxo complex hydrolysis) generates the superoxo complex. Both cations have highly symmetrical solid state structures, locked in transoid Co−O−O−Co conformations by two pairs of intramolecular hydrogen bonds. Each involves two protons in the equatorial Co(NH2R)4 plane of one half of the molecule and the bridge oxygen atom in the other half. A significant difference between the two structures is the orientation of the O2 bridge, which is coplanar with the pyridine rings of the coordination caps in the peroxo complex and at right angles in the superoxo complex. The reactivities of the complexes in acidic and basic media have been explored, and the mononuclear bromo complex [(1)CoBr]2+ was isolated in one of the products. Dithionite, intended as an external reducing agent in the reaction of Na3[CoIII(CO3)3] with 1, instead yields the S-sulfito complex cation [(1)Co(SO3)]+, by disproportionation of S2O42− to give SO32− and S2−. All complexes have been characterised by 1H, 13C NMR, IR, Raman, UV/Vis, and EPR spectroscopy (as applicable), elemental analysis and X-ray structure determination, and the cyclic voltammetry parameters of the peroxo/superoxo pair of complexes have been determined.

High intrinsic barriers against spin-state relaxation in iron(II)-complex solutions.

Slow relaxation: Exergonic high-spin→low-spin relaxation after photoexcitation has been found to be exceedingly slow in a class of iron(II) complexes with hexadentate imine ligands. The thermal activation barriers that arise between the quintet- and singlet-spin manifolds are the highest ever recorded for spin crossover of isolated molecules in free solution (see figure).

TETRAPODAL PENTADENTATE NITROGEN LIGANDS: ASPECTS OF COMPLEX STRUCTURE AND REACTIVITY

Publisher Summary This chapter describes tetrapodal pentadentate nitrogen ligands. Tetrapodal pentadentate ligands have the ability to stabilize transition metal centers possessing a single labile coordination site, thereby, providing attractive platforms for reactivity studies in complexes of overall octahedral geometry. The chapter introduces a highly symmetrical aliphatic NN4 ligand, which consists of a “central” pyridine unit and four equivalent primary amino groups (“pyN4”, 1). This polyamine has predominantly σ donor character, is expected to render the coordinated metal ion electron-rich, imposes virtually no steric constraints, and may thus be construed as a “chelating analogue” of the pentaammine donor set (NH 3 ) 5 . The ligand may be construed as a chelating analogue of the pentaammine donor set has predominant σ-donor character, thereby, providing an electron-rich coordination environment for the metal center. This gives rise to unusual reactivity, such as the observed reduction of coordinated nitrite to NO.

Copper(II)- and Proton-Assisted Condensation Reactions of a Tetrapodal Pentaamine with Acetone: Formation of “Podand-cum-Macrocycle” Copper Complexes and a Protonated Bis(aminal)

The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms mononuclear complexes with CuII, as shown for a series of compounds of the type [(1)Cu]X2 (X = Br, SCN, PF6, ClO4). The coordination environment of the copper ion is square-pyramidal, with the pyridine nitrogen atom of 1 in the apical position. The axial bond Cu−Npy is significantly longer than the four Cu−N bonds at the base of the pyramid, average values being 2.16 A and 2.03 A, respectively. When [(1)Cu]X2 (X = PF6, ClO4) is refluxed with an excess of acetone in methanol in the absence of base, the corresponding bis(isopropylideneimine) complexes [{(nac)2pyN4}Cu]X2 are obtained, in which two diametrically opposite primary amino groups of 1 have condensed with the ketone. The complex cations, which are now chiral, again have square-pyramidal coordinated CuII. In the presence of sodium methoxide, the condensation of [(1)Cu]X2 with acetone proceeds further, leading to the formation of one diacetone-amine-imine linkage in the product. A 12-membered 1,5,9-triazamacrocycle, incorporating the 2,6-disubstituted pyridine unit, is thus formed. Two podand nitrogen donors of the NN4 set (which retains its square pyramidal topology) remain: a primary amine and an isopropylidene imine group. The product forms as a mixture of both conceivable isomers (azomethine groups at the base of the pyramid cis or trans), which were separated and structurally characterised as the tribromocuprate(I) and hexafluorophosphate salts, respectively. The condensation of metal-free 1 with acetone in the presence of two equivalents of HBr yields the bis(aminal) salt (2,6-C5H3N{CMe[cyclo-CH2NH2C(CH3)2NHCH2-]}2)Br2, in which one of the secondary amino groups in each diazacyclohexane ring is protonated, as ascertained by X-ray crystallography. The aminal is a metal-free tautomer of the bis(isopropylideneimine) ligand (nac)2pyN4, to which it converts upon reaction with CuII under suitable conditions, providing an alternative synthetic route to complexes of the type [{(nac)2pyN4}Cu]X2.

Conformational Flexibility of the Square-Pyramidal Coordination Cap in a Series of Octahedral Nickel(II) Pentaamine Complexes – Magnetochemical Characterization of the Singly μ-Cl-Bridged Nickel(II) Dimer [(pyN4)Ni-Cl-Ni(pyN4)](PF6)3 [pyN4 = 2,6-Bis(1′,3′-diamino-2′-methylprop-2′-yl)pyridine]

The architecture of the tetrapodal pentaamine ligand 2,6-bis(1′,3′-diamino-2′-methylprop-2′-yl)pyridine (pyN4, 1) allows it to coordinate to nickel(II) as a square pyramidal coordination cap. The pyridine nitrogen atom occupies an apical position of the coordination octahedron, while four equivalent pendent primary amino groups occupy the equatorial positions, with a sixth coordination site remaining for a monodentate ligand. Exchange of this ligand is facile, and a series of complexes [(1)NiX]n+ (X = OH2, OClO3, NCS, N3, {Cl-Ni(pyN4)}) has been prepared and characterised by elemental analysis, IR and UV/Vis spectroscopies (as applicable), and X-ray structure determination. While the solid state structures show varying degrees of distortion of the ligand cap 1 from C2v symmetry, a polynucleating coordination mode has not been observed. The ligand enables the synthesis of dinuclear nickel(II) complexes containing a single bridging ligand, as exemplified by the singly -chloro bridged complex [(1)Ni–Cl–Ni(1)](PF6)3. This complex has an antiferromagnetically coupled ground state of total spin ST = 0, as determined from variable-temperature magnetic susceptibility measurements.