Franc Meyer

Tuning the Activity of Catechol Oxidase Model Complexes by Geometric Changes of the Dicopper Core

Dicopper(II) complexes of a series of different pyrazolate-based dinucleating ligands [L1](-)-[L4](-) have been synthesized and characterized structurally and spectroscopically. A major difference between the four complexes is the individual metal-metal separation that is enforced by the chelating side arms of the pyrazolate ligand scaffold: it varies from 3.45 A in 2 x (BF4)4 to 4.53 A in 4 x (ClO4)2. All complexes have been evaluated as model systems for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechole (DTBC) as the test substrate. They were shown to exhibit very different catecholase activities ranging from very efficient to poor catalysts (k(obs) between 2430+/-202 and 22.8+/-1.2 h(-1)), with an order of decreasing activity 2 x (ClO4)4 > 1 x (ClO4)2 > 3 x (ClO4)2 >> 4 x (ClO4)2. A correlation of the catecholase activities with the variation in Cu...Cu distances, as well as other effects resulting from the distinct redox potentials, neighboring groups, and the individual coordination spheres are discussed. Saturation behavior for the rate dependence on substrate concentration was observed in only two cases, that is, for the most active 2 x (ClO4)4 and for the least active 4 x (ClO4)2, whereas a catalytic rate that is almost independent of substrate concentration (within the range studied) was observed for 1 x (ClO4)2 and 3 x (ClO4)2. H2O2 was detected as the product of O2 reduction in the catecholase reaction of the three most active systems. The structures of the adducts of L3Cu2 and L4Cu2 with a substrate analogue (tetrachlorocatecholate, TCC) suggest a bidentate substrate coordination to only one of the copper ions for those catalysts that feature short ligand side arms and correspondingly exhibit larger metal-metal separations; this possibly contributes to the lower activity of these systems. TCC binding is supported by several H-bonding interactions to water molecules at the adjacent copper or to ligand-side-arm N-donors; this emphasizes the importance of functional groups in proximity to the bimetallic active site.

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.

Difunctional Pyrazole Derivatives − Key Compounds en Route to Multidentate Pyrazolate Ligands

Straightforward syntheses for some basic functional pyrazole derivatives are reported. These compounds serve as starting materials for the preparation of a series of new 3,5-donor-substituted multifunctional pyrazole compounds, which may act as compartmental dinucleating ligand scaffolds for the controlled assembly of bimetallic complexes. The thp-protected 3,5-bis(chloromethyl)pyrazole (1), and in particular the more reactive 3,5-bis(bromomethyl) analogue 3, permit efficient attachment of various N-donor side arms through nucleophilic substitution reactions. The preparation of the enantiomerically pure (methoxymethyl)pyrrolidine derivative 2e by this route makes use of the intrinsic C2 symmetry of the pyrazole-based framework, and a dizinc complex 2e·(ZnCl2)2 has been characterized crystallographically. The thp-protected pyrazoledicarbaldehyde 6 affords access to pyrazole derivatives bearing aldimine donor side arms, which may be viewed as coupled dinucleating versions of α-diimine ligands. DIBAH reduction of thp-protected dimethyl 1H-pyrazole-3,5-dicarboxylate 4 yields the unsymmetrically functionalized methyl 3-hydroxymethyl-1H-pyrazole-5-carboxylate 8, which has been analyzed by X-ray crystallography. Compound 8 is shown to provide a route to unsymmetrical pyrazole ligands with different chelating side arms in the heterocycle 3- and 5-positions, such as the thioether/amine system 11.

Controlled assembly of a nickel(II) azido alternating chain from dinuclear building units

Abstract An alternating linear chain of nickel(II) ions and azide has been prepared in a controlled fashion from dinuclear building blocks, in which an end-to-end binding mode of the azido ligand is predetermined by the compartmental dinucleating pyrazolate scaffold. The structure and unexpected magnetic properties of the coordination polymer are reported.

Combining organometallic and Werner-type coordination sites in highly preorganized heterobimetallic systems

A first example of a novel class of preorganized bimetallic complexes is reported, in which both an organometallic CpMn(COh fragment and a classical Werner-type coordination site are arranged in close proximity by means of a bridging pyrazolate. The synthetic route starts from N-protected bis(chloromethyl )pyrazole and involves sequential attachment of the manganese part and the chelating N-donor compartment,

Novel μ3‐Coordination of Urea at a Nickel(II) Site: Structure, Reactivity and Ferromagnetic Superexchange

A tetranuclear mixed-spin nickel(II) complex featuring two urea molecules in an unprecedented μ3-ĸN:ĸN′:ĸO coordination mode has been structurally characterized. The two central high-spin nickel(II) ions exhibit ferromagnetic coupling (J = +3.4 cm−1) mediated by the NH2–C–O linkages of the bridging urea. In solution the temperature dependence of the UV/Vis optical absorption and of the magnetic moment indicate that, upon cooling, additional solvent molecules bind to the terminal low-spin metal ions and a concomitant spin change occurs. In the solid state the coordinated urea ligands can be thermally degraded to cyanate within the grip of the multimetallic nickel(II) site.

Dinucleating Hybrid Ligands Providing a “Soft” P∩N and an Adjacent N-Rich Coordination Pocket − Controlled Synthesis of Unsymmetric Homodinuclear and Heterodinuclear Complexes

New unsymmetric dinucleating pyrazolate ligands with different chelating side arms in the 3- and 5-positions of the heterocycle have been prepared. These ligands provide a “soft” P∩N site and either an adjacent “N3” (HLa) or “N2S2” (HLb) coordination pocket, and they have been employed to build homo- and heterobimetallic complexes featuring various types of asymmetry. Both La and Lb form the mixed-spin dinickel(II) chloride systems 4 and 5, respectively, which in the former case dimerizes via Cl bridges to give a tetranuclear compound. In the heterobimetallic complex 6 the PdII is specifically housed within the P∩N compartment. All new complexes have been characterized by X-ray crystallography, and conformational and electronic coupling between the two metal ions of the dinuclear scaffolds have been analyzed.

Complexes of a bis(tetradentate) compartmental pyrazolate ligand: solution studies and solid state structures

The stability of Ni2+, Cu2+, and Zn2+ complexes with a bis(tetradentate) compartmental pyrazolate ligand (L) was determined in aqueous solution at 25xa0°C and Ixa0=xa00.5 M (KNO3). Thereby a series of mononuclear [MLHn] (nxa0=xa02, 1, 0, −1, and −2) and dinuclear species [M2LHm] (mxa0=xa0 0, −1, −2) was observed. The stability of [ML] and [M2L] increases in the order Ni2+xa0<xa0Zn2+xa0<xa0Cu2+, whereby in the case of Ni2+ no dinuclear species could be detected. Structural proposals for several species were corroborated by X-ray diffraction studies. In all cases the pyrazolate group has a pronounced tendency to bridge two metal ions.

An unusual hexanickel cage complex with μ- and μ3-chloro bridges and an interstitial μ6-chloride

A pyrazolate-based dinucleating ligand of the nbis(α-diimine) type forms an unusual hexanuclear nnickel(II) cage complex incorporating an interstitial nμ6-Cl atom.

1,3,5-Triazine-based tricopper(II) complexes: structure and magnetic properties of threefold-symmetric building blocks

Two new trinuclear complexes 1 and 2 that are composed of the three-directional ligand 2,4,6-tris(di-2-pyridylamino)-1,3,5-triazine (dipyatriz) and copper(II) chloride as well as a 1D polymeric zigzag system 3 assembled from trimetallic type 2 building blocks have been prepared and structurally characterized. While the triazine-N are not involved in metal coordination, each ligand bidentately binds to three copper ions via its three pairs of pyridine-N donors, and five-coordination of the copper is completed by chloride or dmso solvent molecules. Variable-temperature magnetic studies reveal weak antiferromagnetic coupling. Magnetic properties of the trinuclear entities and of the bis(micro-chloro) bridged pseudo-dimeric copper(II) subunits in 3 can be rationalized on the basis of the structural findings.