Elisabeth Kaifer

Cooperative Transformations of Small Molecules at a Dinuclear Nickel(II) Site

The synergetic action of two nickel(II) ions embedded in a tunable dinucleating ligand matrix (illustrated) allows manifold cooperative reactions within the bimetallic pocket, such as the hydration of nitriles, the hydrolysis of esters, and the degradation of urea to cyanate. Related mononuclear complexes have been prepared in order to further elucidate mechanistic aspects of these transformations.

On the Electronic Structure of NiII Complexes That Feature Chelating Bisguanidine Ligands

In this work we report on the syntheses and properties of several new Ni complexes featuring the chelating bisguanidines bis(tetramethylguanidino)benzene (btmgb), bis(tetramethylguanidino)naphthalene (btmgn), and bis(tetramethylguanidino)biphenyl (btmgbp) as ligands. All complexes were structurally characterized by single-crystal X-ray diffraction and quantum chemical calculations. A detailed inspection of the magnetic susceptibility of [(btmgb)NiX(2)] and [(btmgbp)NiX(2)] (X=Cl, Br) revealed a linear temperature dependence of chi(-1)(T) above 50 K, which was in agreement with a Curie-Weiss-type behavior and a triplet ground state. Below approximately 25 K, however, magnetic susceptibility studies of the paramagnetic d(8) Ni complexes revealed the presence of a significant zero-field splitting (ZFS) that results from spin-orbit mixing of excited states into the triplet ground state. The electronic consequences that might arise from the mixing of states as well as from a possible non-innocent behavior of the ligand have been explored by an experimental charge density study of [(btmgb)NiCl(2)] at low temperatures (7 K). Here, the presence of ZFS was identified as one potential reason for the flat angle-spherical Cl-Ni-Cl deformation potential and the distinct differences between the angle-spherical X-Ni-X valence angles observed by experiment and predicted by DFT. An analysis of the topology of the experimentally and theoretically derived electron-density distributions of [(btmgb)NiCl(2)] confirmed the strong donor character of the bisguanidine ligand but clearly ruled out any significant non-innocent ligand (NIL) behavior. Hence, [(btmgb)NiCl(2)] provides an experimental reference system to study the mixing of certain excited states into the ground state unbiased from any competing NIL behavior.

Tunable TACN/pyrazolate hybrid ligands as dinucleating scaffolds for metallobiosite modeling—dinickel(II) complexes relevant to the urease active site

Abstract Two new dinucleating pyrazole ligands HL1 and HL2 bearing appended 1,4-diisopropyl-1,4,7-triazacyclononane (iPr2TACN) side arms in the 3- and 5-positions have been synthesized by multi-step synthetic procedures. HL1 and HL2 differ by the length of the spacer between the pyrazole and the iPr2TACN, which is a novel means of tuning the metal–metal-separation in pyrazolate-based bimetallic compounds. Complexes [L1Ni2(H3O2)](ClO4)2 (5) and [L2Ni2(OH)](BPh4)2 (6b) have been characterized structurally, where the bridging H3O2 moiety in 5 is reactive and allows cooperative substrate transformations within the bimetallic pocket, while the tightly bound OH bridge in 6b is inert. Using the dinickel scaffold 5, reactivity relevant to the urease active site has been probed. Under anhydrous conditions, 5 reacts with parent and N-substituted urea to yield complexes 8a–c featuring N,O-bridging ureate anions. At high temperatures, 8a–c lose ammonia to give the cyanate-bridged species 10. This is able to bind a second cyanate at one of the nickel ions forming 11. Complexes with bridging acetate (7), O,O-bridging carbamate (9) and N,O-bridging methyl carbamate (12) have been studied for comparison, and mutual interconversions of the dinickel complexes have been investigated. Complexes 5, 6b, 7, 8a–c, 9, 11 and 12 have been characterized by X-ray crystallography.

Synthesis of a stable B2H5(+) analogue by protonation of a double base-stabilized diborane(4).

On the basis of an analysis of the products formed in the course of reaction between B2H6 and the deuterated magic acid (FSO3D·SbF5), Olah et al. postulated the B2H5 + cation in 1988 as a short-lived intermediate formed by protonation of B2H6 and subsequent H2 elimination. [1] The cation had previously been observed in the gas phase after photoionization of B2H6. [2] However, a salt of this cationic boron hydride has not been synthesized on a preparative scale to date. Because of the scarceness of experimental information, the cation was the subject of several quantum-chemical calculations. These calculations found a global energy minimum structure with three bridging hydrogen atoms (Scheme 1). The distance between the two boron atoms is 149.5 pm according to HF/6-31G*, and 151.8 pm according to more recent QCISD(T)/6-311G** calculations. These values might argue for B B bonding, although it has been shown in many cases that a short distance does not automatically imply the presence of a significant chemical bond. To obtain more information, the cation was subjected to an NBO charge and Wiberg bond analysis. The NBO analysis suggested a charge of 0.20e on each boron atom, and the Wiberg bond analysis returned an index of 1.0 for the B B bond, which is clearly different to the situation in, for example, B2H6. The detailed description of the bonding situation in species such as this with multicenter bonds is still the subject of debate. Herein we report the synthesis of the first cationic binuclear borohydride [B2H3L2] , where L is the anionic guanidinate ligand 1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2a]pyrimidinate (hpp)). The geometry is similar to that of B2H5 , with two bridging hydrogen atoms being replaced by two hpp units. The synthesis commences with the complex hppH·BH3 (1), which can be dehydrogenated, via [BH2(hpp)]2 (2), to give the doubly base-stabilized diborane(4) [BH(hpp)]2 (3). Although we have previously reported the structure of 3, we have only now found a route to 3 in high yield and purity. The initial thermal dehydrogenation of 1 proved not to be the ideal route to 3 ; however, we recently showed that 1 can be catalytically dehydrogenated with [{Rh(1,5-cod)Cl}2] in toluene at 80 8C to give 2 (with a B···B separation of 306.5 pm). This species, dissolved in toluene, can be further dehydrogenated at 114 8C in the presence of catalytic amounts of [{Rh(1,5-cod)Cl}2] to yield 3 [Eq. (1)], with a B B bond

A boron–boron coupling reaction between two ethyl cation analogues

The design of larger architectures from smaller molecular building blocks by element–element coupling reactions is one of the key concerns of synthetic chemistry, so a number of strategies were developed for this bottom-up approach. A general scheme is the coupling of two elements with opposing polarity or that of two radicals. Here, we show that a B–B coupling reaction is possible between two boron analogues of the ethyl cation, resulting in the formation of an unprecedented dicationic tetraborane. The bonding properties in the rhomboid B4 core of the product can be described as two B–B units connected by three-centre, two-electron bonds, sharing the short diagonal. Our discovery might lead the way to the long sought-after boron chain polymers with a structure similar to the silicon chains in β-SiB3. Moreover, the reaction is a prime textbook example of the influence of multiple-centre bonding on reactivity. A stable tetranuclear boron dication with a rhomboid B4 skeleton has been formed by B–B coupling between two diborane cations. In the course of this unusual reaction — which is not feasible for the isolobal ethyl cation analogues — two electron-precise B–B bonds are converted into two B–B–B three-centre bonds.

What Makes a Strong Organic Electron Donor (or Acceptor)

Organic electron donors are of importance for a number of applications. However, the factors that are essential for a directed design of compounds with desired reduction power are not clear. Here, we analyze these factors in detail. The intrinsic reduction power, which neglects the environment, has to be separated from extrinsic (e.g., solvent) effects. This power could be quantified by the gas-phase ionization energy. The experimentally obtained redox potentials in solution and the calculated ionization energies in a solvent (modeled with the conductor-like screening model (COSMO)) include both intrinsic and extrinsic factors. An increase in the conjugated π-system of organic electron donors leads to an increase in the intrinsic reduction power, but also decreases the solvent stabilization. Hence, intrinsic and extrinsic effects compete against each other; generally the extrinsic effects dominate. We suggest a simple relationship between the redox potential in solution and the gas-phase ionization energy and the volume of an organic electron donor. We finally arrive at formulas that allow for an estimate of the (gas-phase) ionization energy of an electron donor or the (gas-phase) electron affinity of an electron acceptor from the measured redox potentials in solution. The formulas could be used for neutral organic molecules with no or only small static dipole moment and relatively uniform charge distribution after oxidation/reduction.

Combining NMR of dynamic and paramagnetic molecules: fluxional high-spin nickel(II) complexes bearing bisguanidine ligands.

A detailed nuclear magnetic resonance (NMR) study was carried out on a series of paramagnetic, tetrahedrally coordinated nickel(II) dihalide complexes featuring chelating guanidine ligands. A complete assignment of the NMR signals for all complexes was achieved by sophisticated NMR experiments, including correlation spectra. The effects of halide exchange, as well as the variation in the guanidine-metal bite angles on the paramagnetic shifts, were assessed. The paramagnetic shift was derived with the aid of the diamagnetic NMR spectra of the analogous Zn complexes, which were synthesized for this purpose. The experimentally derived paramagnetic shift was then compared with the values obtained from quantum chemical (DFT) calculations. Furthermore, variable-temperature NMR studies were recorded for all complexes. It is demonstrated that NMR spectroscopy can be applied to evaluate the rate constants of fast fluxional processes within paramagnetic and catalytically active metal complexes.

Wrapping an organic reducing reagent in a cationic boron complex and its use in the synthesis of polyhalide monoanionic networks.

The reaction between BF(3)⋅OEt(2) and one of two guanidines, 1,8-bis(tetramethylguanidinyl)naphthalene (btmgn) and 1,2,4,5-tetrakis(tetramethylguanidinyl)naphthalene (ttmgn), yields the salts [(btmgn)(BF(2))]BF(4) and [(ttmgn)(BF(2))(2)](BF(4))(2). NMR spectroscopic data show that the boron atoms in the cation and anion exchange in the case of [(ttmgn)(BF(2))(2)](BF(4))(2), but not in the case of [(btmgn)(BF(2))]BF(4). The rate constant for this exchange was estimated to be 4 s(-1) at 80 °C for solutions in CH(3)CN. These salts were subsequently used for the reduction of dihalides Br(2) or I(2) to give polyhalide salts. We report the synthesis and first complete characterization (including structural analysis) of salts that contain pentabromide monoanions. In these salts, the Br(5)(-) anions interact to give dimeric units or polymeric chains. Our results are compared to previous quantum chemical calculations on the gas-phase structure of the Br(5)(-) anion. The possible pathways that lead to the polyhalides are evaluated. In the case of [(ttmgn)(BF(2))(2)](BF(4))(2), reduction is accompanied by ttmgn oxidation, whereas in the case of [(btmgn)(BF(2))]BF(4) reduction is initiated by aromatic substitution.

Cooperative Binding of Nitrile Moieties Within a Bimetallic Pocket: Enforcing Side-On π-Interaction With a High-Spin Nickel(II) Site

Different cooperative binding modes of nitriles within the bimetallic pocket of a pyrazolate-based compartmental dinickel(II) site have been studied. The H3O2-bridged dinuclear complex 1 reacts with cyanamide to yield 4, in which a secondary hydrogencyanamido(1–) bridge spans the two metal centers at an unusually short metal–metal distance imposed by the primary ligand matrix. In 5, a single 2-cyanoguanidine (cnge) molecule is N-bound to one nickel(II) ion through its nitrile part and is coordinated to the adjacent metal site through an amido nitrogen. The characteristics of the coordination spheres of the metal centers suggest an additional side-on π-bonding interaction of the nitrile moiety with the second high-spin nickel(II) ion. This unusual interaction is corroborated by comparing the IR bands for the ν(C≡N) stretching vibration of 5 with those of complex 6, which has two end-on bound cnge molecules, and those of the related mononuclear complex 7, which lacks a second nickel(II) ion. The nature of the π-bonding interaction in 5 is further analyzed by DFT calculations on relevant model systems. Even though the π-bonding is found to be very weak, it does include some backbonding from occupied 3d MOs at the second high-spin nickel(II) ion to the π* MOs of the nitrile. Such an unconventional π-interaction is suggested to be enforced by the constrained fixation of the nitrile unit within the highly organized coordination pocket of the bimetallic framework. In contrast, the bifunctional 2-hydroxybenzonitrile is accommodated by the distinct binding of the nitrile and phenolate functions to the different metal centers in 8, which confirms that the simultaneous binding of both an OR-function and an end-on bound nitrile is indeed feasible within the active site pocket. Such a situation is reminiscent of the bimetallic effect that has been assumed to enable the cooperative hydration of nitriles at the dinickel(II) site of 1. Complexes 4·(ClO4)2, 5·(ClO4)2, 6·(ClO4)3, 7·(ClO4)(BPh4), and 8·(ClO4)2 have been characterized structurally by X-ray crystallography.

First dinuclear B(II) monocations with bridging guanidinate ligands: synthesis and properties.

Reaction between the diborane (4) B2Cl2(NMe2)2 and Li(hpp) (hpp-=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) leads to [(Me2N)B(mu-hpp)] 2. This species can be protonated by HCl.OEt2 to give [(Me2HN)B(mu-hpp)]2Cl2 featuring two B(II) cations with direct B-B bonding. The unsymmetrical monocation [(Me2N)B2(mu-hpp)2(NHMe2)]+ is also obtained. [(Me2HN)B(mu-hpp)]2Cl2 eliminates NHMe2 in a slow reaction leading to [ClB2(mu-hpp)2(NHMe2)]Cl and ultimately, presumably, to [ClB(mu-hpp)]2. We report the crystal structures of the two monocations [ClB2(mu-hpp)2(NHMe2)]Cl and [(Me2N)B2(mu-hpp)2(NHMe2)]Cl. The experimental results are accompanied by some quantum chemical density-functional theory calculations.