Tris Pyrazolyl Borate Ligands Research Articles

Copper-Catalyzed Sulfimidation in Aqueous Media: a Fast, Chemoselective and Biomolecule-Compatible Reaction.

Performing transition metal-catalyzed reactions in cells and living systems has equipped scientists with a toolbox to study biological processes and release drugs on demand. Thus far, an impressive scope of reactions has been performed in these settings, but many are yet to be introduced. Nitrene transfer presents a rather unexplored new-to-nature reaction. The reaction products are frequently encountered motifs in pharmaceuticals, presenting opportunities for the controlled, intracellular synthesis of drugs. Hence, we explored the transition metal-catalyzed sulfimidation reaction in water for future in vivo application. Two Cu(I) complexes containing trispyrazolylborate ligands (Tpx ) were selected, and the catalytic system was evaluated with the aid of three fitness factors. The excellent nitrene transfer reactivity and high chemoselectivity of the catalysts, coupled with good biomolecule compatibility, successfully enabled the sulfimidation of thioethers in aqueous media. We envision that this copper-catalyzed sulfimidation reaction could be an interesting starting point to unlock the potential of nitrene transfer catalysis in vivo.

Tuning the optical and magnetic properties of lanthanide single-ion magnets using nitro-functionalized trispyrazolylborate ligands.

We report the synthesis, crystal structures, photophysical and magnetic properties of 11 novel lanthanide complexes with the asymmetrically functionalized trispyrazolylborate ligand 4-nitrotrispyrazolylborate, 4-NO2Tp-: [Ln(4-NO2Tp)3] (Ln = La-Dy, except Pm). In-depth photophysical characterization of the ligands via luminescence, reflectance and absorption spectroscopic techniques, decay lifetimes, quantum yields supported by time-dependent density functional theory (TD-DFT) and natural bond order (NBO) analysis reveal that n-NO2Tp- ligands are dominated by intra-ligand charge transfer (ILCT) transitions and that second-sphere interactions are critical to the stabilization of the T1 state of n-NO2Tp- ligands and hence their ability to sensitize Ln3+ emission. The luminescence properties of the complexes indicate that 4-NO2Tp- is a poor sensitizer of Ln3+ emission, unlike 3-NO2Tp-. Moreover, [Nd(4-NO2Tp)3] (crystallized as a hexane solvate) displays single-molecule magnet (SMM) properties, with longer relaxation times and larger barrier than the non-functionalized [NdTp3], attributed to the addition of the NO2-group and subsequent rigidification of the molecular structure.

Carbene-Controlled Regioselective Functionalization of Linear Alkanes under Silver Catalysis.

Control of the regioselectivity in the functionalization of C-H bonds of linear alkanes C2H2n+2 via carbene transfer from diazo compounds is restricted to the use of rhodium-based catalysts, which govern the reaction outcome employing donor-acceptor diazo reagents. At variance with that catalyst-controlled strategy, we present an alternative approach in which employing the appropriate silver complexes containing trispyrazolylborate ligands as catalysts with large differences in their steric and electronic properties, the regioselection is mainly governed by the diazo reagent, which leads to the functionalization of primary or secondary sites of linear alkanes (lacking any activating or directing groups). Donor-acceptor aryl diazoacetates exclusively provide the functionalization of the secondary sites of hexane or pentane, whereas acceptor ethyl diazoacetate leads to an unprecedented level of primary functionalization.

Introducing the Catalytic Amination of Silanes via Nitrene Insertion.

The direct functionalization of Si–H bonds by the nitrene insertion methodology is described. A copper(I) complex bearing a trispyrazolylborate ligand catalyzes the transfer of a nitrene group from PhI=NTs to the Si–H bond of silanes, disilanes, and siloxanes, leading to the exclusive formation of Si–NH moieties in the first example of this transformation. The process tolerates other functionalities in the substrate such as several C–H bonds and alkyne and alkene moieties directly bonded to the silicon center. Density functional theory (DFT) calculations provide a mechanistic interpretation consisting of a Si–H homolytic cleavage and subsequent rebound to the Si-centered radical.

Molecular dynamics of four-coordinate carbene-Cu(I) complexes employing tris(pyrazolyl)borate ligands

A series of three- and four-coordinate Cu(I) complexes was prepared employing a N-heterocyclic carbene and various scorpionate trispyrazolylborate (Tp) ligands. The NMR spectra of the four-coordinated complexes suggest C3 symmetry for the Tp ligands despite the asymmetric scorpionate η3-binding fashion observed by single crystal X-ray diffraction. Variable temperature (VT) 1H and 13C NMR spectroscopy was used to elucidate the dynamic process that equilibrates the different structural isomers. We report that the rapid exchange of the pyrazoles in the Tp ligand was hampered but not fully inhibited at −88 °C. Lastly, we show how the electrophilicity of the carbene and the hapticity of the Tp moiety both control the photophysics: more electrophilic carbenes red-shift the luminescence and bulkier Tp suppress modes of nonradiative decay (knr).

A Dicationic Bismuth(III) Lewis Acid: Catalytic Hydrosilylation of Olefins

Lewis acid promoted activation of inert chemical bonds is central to catalysis. The presence of a highly electrophilic central atom having minimum interactions with anions and solvent molecules is a requisite for a reactive Lewis acid. This requirement for a strong Lewis acid is met with by designing a reactive cation of a heavy element, bismuth, bearing the tridentate trispyrazolylborate ligand. The coordination sphere of the electrophilic bismuth dication is stabilized by very weak interactions with the halogen atoms of the weakly nucleophilic tetrakis(pentafluorophenyl)borate anion, chlorobenzene, and ortho‐dichlorobenzene in the solid state. The high electrophilicity at bismuth is demonstrated by the ability of the dication to efficiently catalyze the addition of Si–H to olefins.

Trispyrazolylborate Ligands Supported on Vinyl Addition Polynorbornenes and Their Copper Derivatives as Recyclable Catalysts.

Polynorbornenes prepared by vinyl addition polymerization and bearing pendant alkenyl groups serve as skeletons to support trispyrazolylborate ligands (Tpx ) built at those alkenyl sites. Reaction with CuI in acetonitrile led to VA-PNB-Tpx Cu(NCMe) (VA-PBN=vinyl addition polynorbornene) with a 0.8-1.4 mmol incorporation of Cu per gram of polymer. The presence of tetracoordinated copper(I) ions was been assessed by FTIR studies on the corresponding VA-PNB-Tpx Cu(CO) adducts, in agreement with those on discrete Tpx Cu(CO). The new materials were employed as heterogeneous catalysts in several carbene- and nitrene-transfer reactions, showing a behavior similar to that of the homogeneous counterparts but also being recycled several times maintaining a high degree of activity and selectivity. This is the first example of supported Tpx ligands onto polymeric supports with catalytic applications.

An unusual bis-heteroscorpionate complex with anomalous ligands: [tris(3,4-dibromo-5-phenylpyrazolyl)hydroborato][hydrotris(3-neopentylpyrazolyl)borato]nickel(II).

Tridentate trispyrazolylborate (Tp) ligands, introduced by Trofimenko in 1966, have been widely utilized in metal coordination chemistry because of the relatively facile synthetic modification of their electronic and steric factors. The title heteroscorpionate, [Ni(C27H16BBr6N6)(C24H40BN6)], features one ligand, namely hydrotris(3-neopentylpyrazolyl)borate, that has previously displayed variable steric effects, and a brominated ligand, namely tris(3,4-dibromo-5-phenylpyrazolyl)hydroborate, that, atypical in trispyrazolylborate chemistry, coordinates such that the less bulky pyrazole substituent is oriented facing toward the metal ion. The potential molecular threefold symmetry in scorpionates can allow axial chirality. Although crystallized in the centrosymmetric P-1 space group, a closer inspection of the structure of the title compound reveals axial diastereomers.

Copper(I) Complexes of Naphthyl‐Substituted Fluorinated Trispyrazolylborate Ligands with Ethene and Carbon Monoxide

Four CuI complexes of the two new hydridotrispyrazolylborate ligands hydridotris[3‐trifluoromethyl‐5‐(1‐naphthyl)pyrazol‐1‐yl]borate (= [TpCF3,1Nt]–) and hydridotris[3‐trifluoromethyl‐5‐(2‐naphthyl)pyrazol‐1‐yl]borate (= [TpCF3,2Nt]–) have been synthesized by reaction of the sodium salts of the ligands with CuI under either ethene or carbon monoxide atmosphere. Single crystal X‐ray diffraction analyses yielded the molecular structures of the three compounds [Na2(TpCF3,1Nt)2(µ‐acetone)2], [Cu(TpCF3,1Nt)(C2H4)] and [Cu(TpCF3,2Nt)(C2H4)]. The CuI ions are in approximate tetrahedral geometries comprising the three nitrogen donors of the tridentate ligand and the η2‐coordinated ethene ligands. The sodium ion in [Na2(TpCF3,1Nt)2(µ‐acetone)2] is found in a distorted octahedral coordination geometry formed by three N‐donors of the facially coordinating tridentate ligand, two O‐donors of bridging acetone ligands and a fluoride of a bridging –CF3 group with a particularly short Na···F distance. The chemical shifts of the ethene ligands in [Cu(TpCF3,1Nt)(C2H4)] and [Cu(TpCF3,2Nt)(C2H4)] are 5.00 ppm and 4.96 ppm respectively on 1H‐NMR and 85.9 ppm and 85.8 ppm on 13C‐NMR spectroscopy. The CO stretching frequencies in the IR spectra of [Cu(TpCF3,1Nt)(CO)] and [Cu(TpCF3,2Nt)(CO)] are found at 2103 and 2109 cm–1, respectively.

Utilizing the Trispyrazolyl Borate Ligand for the Mimicking of O2-Activating Mononuclear Nonheme Iron Enzymes.

Mononuclear, O2-activating nonheme iron enzymes are a fascinating class of metalloproteines, capable of realizing the most different reactions, ranging from C-H activation, via O atom transfer to C-C bond cleavage, in the course of O2 activation. They can lead us the way to achieve similar reactions with comparable efficiency and selectivity in chemical laboratories, which would be highly desirable aiming at accessing value-added products or to achieve degradation of unwanted compounds. Hence, these enyzmes motivate attempts to construct artificial low-molecular weight analogues, mimicking structural or functional characteristics. Such models can, for instance, provide insights about which of the features inherent to an active site are essential and guarantee the enzyme function, and from this kind of information the minimal requirements for a biomimetic or bioinspired complex that may be applied in catalysis can be derived. On the other hand, they can contribute to an understanding of the enzyme functioning. In order to create such replicates, it is important to faithfully mimic the surroundings of the iron centers in their active sites. Most of them feature two histidine residues and one carboxylate donor, while a few exhibit a deceptively simple (His)3Fe active site. For the simulation of these, the trispyrazolyl borate ligand (Tp) particularly offers itself, as the facial arrangement of three pyrazole donors is reminiscent of the three histidine-derived imidazole donors. The focus of this Account will be on bioinorganic/biomimetic research from our laboratory utilizing Tp ligands to develop molecular models for (i) two representatives of the (His)3Fe-enzyme family, namely, the cysteine dioxygenase (CDO) and acetyl acetone dioxygenase (Dke1), (ii) a related but less well-explored variant of the CDO-the 2-aminoethanethiol dioxygenase-as well as (iii) the 2-His-1-carboxylate representative 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO). The CDO catalyzes the dioxygenation of cysteine with O2 to give cysteine sulfinic acid, which could be mimicked at TpFe units in a realistic manner. Furthermore, the successful dioxygenation of 2-aminoethanethiol at the same complex metal fragments lends further support to the hypothesis that the active sites of CDO and the one of 2-aminoethanethiol dioxygenase, whose structure is unknown, are quite similar. Dke1 is capable of cleaving diketones and ketoesters to give the corresponding carboxylic acids and α-keto aldehydes, and Tp-based models have achieved comparable C-C bond cleavage reactions. The ACCO develops ethylene from ACC in the course of oxidation, and recently this has been achieved the first time for a TpFe model, too.

Highly tunable fluorinated trispyrazolylborates [HB(3-CF3-5-{4-RPh}pz)3](-) (R = NO2, CF3, Cl, F, H, OMe and NMe2) and their copper(I) complexes.

The ethene and carbon monoxide adducts of copper(I) with seven trispyrazolylborate ligands ([HB(3-CF3-5-{4-RPh}pz)3](-); R = NO2 (4a), CF3 (4b), Cl (4c), F (4d), H (4e), OMe (4f) and NMe2 (4g)) were synthesized and characterized. The ligands were synthesized from their corresponding pyrazoles and sodium tetrahydridoborate and were obtained as solvent adducts of their sodium salts after workup. When the pyrazole with the most electron-withdrawing substituent (R = NO2) is used the asymmetric ligand [HB(3-CF3-5-{4-NO2Ph}pz)2(3-{4-NO2Ph}-5-CF3pz)](-) (4a') is formed as the major product. Copper(I) complexes with ethene or CO as a co-ligand were prepared in good yields and were structurally characterized using (1)H NMR, (13)C NMR and infrared spectroscopy. Single crystal X-ray crystallography analyses revealed the structures of Na4a', Na4b, four copper ethene complexes and four copper carbonyl complexes. The structures of the copper(I) complexes show Cu(I) ions in pseudo-tetrahedral coordination environments consisting of three nitrogen atoms of the trispyrazolylborate ligand and the carbonyl or η(2)-coordinated ethene ligands, with nearly identical coordination environments around the Cu(I) ion. The compound [Na(4a')(H2O)]n crystallizes as one-dimensional chains with intermolecular NaO2N interactions. The sodium ions were found in severely distorted octahedral geometries with three nitrogen atoms from the trispyrazolylborate ligand, one aqua ligand and two oxygen atoms from the nitro group of an adjacent molecule. The compound [Na2(4b)2(μ-H2O)2] crystallizes as a centrosymmetric water-bridged dimer: two five-coordinate square-pyramidal sodium ions each are coordinated facially by three nitrogen atoms from a trispyrazolylborate ligand and two bridging water ligands. Below the base of the pyramidal structure one intermolecular and two intramolecular NaF short contacts are present. The (1)H and (13)C NMR spectra of the copper-ethene complexes show signals for the ethene ligands in the range of 4.84-4.96 ppm and 84.9-86.8 ppm respectively. The infrared spectra of the carbonyl complexes show CO stretching frequencies in the range of 2096-2120 cm(-1). Both the NMR signals for the ethene ligands and infrared signals for the carbonyl ligands were found to show good correlations with the Hammett σp parameters of the substituents on the phenyl rings of the ligands.

ChemInform Abstract: Chemo‐, Regio‐, and Stereoselective Silver‐Catalyzed Aziridination of Dienes: Scope, Mechanistic Studies, and Ring‐Opening Reactions.

Silver complexes bearing trispyrazolylborate ligands (Tpx) catalyze the aziridination of 2,4-diene-1-ols in a chemo-, regio-, and stereoselective manner to give vinylaziridines in high yields by means of the metal-mediated transfer of NTs (Ts = p-toluensulfonyl) units from PhI═NTs. The preferential aziridination occurs at the double bond neighboring to the hydroxyl end in ca. 9:1 ratios that assessed a very high degree of regioselectivity. The reaction with the silver-based catalysts proceeds in a stereospecific manner, i.e., the initial configuration of the C═C bond is maintained in the aziridine product (cis or trans). The degree of regioselectivity was explained with the aid of DFT studies, where the directing effect of the OH group of 2,4-diene-1-ols plays a key role. Effective strategies for ring-opening of the new aziridines, deprotection of the Ts group, and subsequent formation of β-amino alcohols have also been developed.

Chemo-, Regio-, and Stereoselective Silver-Catalyzed Aziridination of Dienes: Scope, Mechanistic Studies, and Ring-Opening Reactions

Silver complexes bearing trispyrazolylborate ligands (Tp(x)) catalyze the aziridination of 2,4-diene-1-ols in a chemo-, regio-, and stereoselective manner to give vinylaziridines in high yields by means of the metal-mediated transfer of NTs (Ts = p-toluensulfonyl) units from PhI═NTs. The preferential aziridination occurs at the double bond neighboring to the hydroxyl end in ca. 9:1 ratios that assessed a very high degree of regioselectivity. The reaction with the silver-based catalysts proceeds in a stereospecific manner, i.e., the initial configuration of the C═C bond is maintained in the aziridine product (cis or trans). The degree of regioselectivity was explained with the aid of DFT studies, where the directing effect of the OH group of 2,4-diene-1-ols plays a key role. Effective strategies for ring-opening of the new aziridines, deprotection of the Ts group, and subsequent formation of β-amino alcohols have also been developed.

Introducing Copper as Catalyst for Oxidative Alkane Dehydrogenation

The dehydrogenation of n-hexane and cycloalkanes giving n-hexene and cycloalkenes has been observed in the reaction of such hydrocarbons with hydrogen peroxide, in the presence of copper complexes bearing trispyrazolylborate ligands. This catalytic transformation provides the typical oxidation products (alcohol and ketones) with small amounts of the alkenes, a novel feature in this kind of oxidative processes. Experimental data exclude the participation of hydroxyl radicals derived from Fenton-like reaction mechanisms. DFT studies support a copper-oxo active species, which initiates the reaction by H abstraction. Spin crossover from the triplet to the singlet state, which is required to recover the catalyst, yields the major hydroxylation and minor dehydrogenation products. Further calculations suggested that the superoxo and hydroperoxo species are less reactive than the oxo. A complete mechanistic proposal in agreement with all experimental and computational data is proposed.

(PhBP3)Cu(PPh3)] as a Surrogate of TpxCuL in Homogeneous Catalysis (PhBP3 = PhB(CH2PPh2)3; Tpx = Hydrotrispyrazolylborate)

The novel complexes [(PhBP3)Cu(L)] (PhBP3 = PhB(CH2PPh2)3, L = PPh3, 1; L = tetrahydrofuran, 2) have been synthesized and spectroscopically and, in the case of 1, structurally characterized. Both complexes undergo dissociation of the PPh3 or THF ligand in solution to deliver the (PhBP3)Cu unit, similar in geometry to and isoelectronic with TpxCu (Tpx = trispyrazolylborate ligand). A complete study of the catalytic properties of 1 has shown that it can be considered as a surrogate of TpxCu. The study includes (a) olefin cyclopropanation, cyclopropenation, aziridination, halocarbon addition, and radical polymerization, (b) X–H (X = O, N) functionalization by carbene insertion, and (c) furan conversion into dihydropyridines. Since the observed activity is quite comparable to that already described for TpxCu complexes, this new catalytic system could become an alternative to the latter.

A multi-scale approach to spin crossover in Fe(ii) compounds

We report here for the first time a multi-scale study on the concept of spin-crossover compounds, which integrates improved density functionals, a polarizable force field and hybrid QM/MM calculations. This multi-scale setup is applied to the temperature dependence of spin states of a Fe(II) compound with trispyrazolylborate ligands that exhibits spin-crossover. Our study shows a transition temperature of around 290 K, which is in perfect agreement with experimental results. Moreover, based on our data we provide the origin of why spin transition occurs in this iron-compound: it results directly from spin-state changes in the iron-compound that lead to more favourable electrostatic interactions for the high-spin state.

The Analysis of Hyperfine Shifts of Mono-Ligand High-Spin Cobalt(II) Pyrazolylborate Complexes

In this paper, we present the detailed study on the correlation of the nuclear magnetic resonance (NMR) parameters with the results of density functional theory calculations performed for paramagnetic high-spin cobalt(II) complexes with trispyrazolylborate ligands. This work focuses on estimation of dipolar and contact shifts in mono-ligand high-spin cobalt(II) pyrazolylborate systems along with discussion on 1H NMR properties of the mentioned tetra-, penta- and hexacoordinate complexes. The calculation results show frontier molecular orbitals that may be responsible for contact shift. The calculated contact shift values are compared with the dipolar shift and hyperfine ones.

Analysis of NMR shifts of high-spin cobalt(II) pyrazolylborate complexes

We present a detailed study on the correlation of the nuclear magnetic resonance (NMR) parameters with the results of density functional theory (DFT) calculations performed for paramagnetic high-spin cobalt(II) complexes with trispyrazolylborate ligands. This work is a first attempt to calculate dipolar and contact shifts in high-spin cobalt(II) pyrazolylborate systems. The calculation results show frontier orbitals that may be responsible for the contact shift. The contact shift values are compared with the dipolar shift ones. The latter shift values were both obtained from experimental data and calculated using DFT methods. We attempt to explain the dipolar effect, which may have a great share in NMR chemical shift of paramagnetic compounds.

The Effect of Catalyst Loading in Copper‐Catalyzed Cyclohexane Functionalization by Carbene Insertion

AbstractA study of the variables that affect the insertion of the :CHCO2Et group (formed from ethyl diazocetate, EDA) into the C–H bonds of cyclohexane in the presence of a TpxCu complex as the catalyst (Tpx = trispyrazolylborate ligand) has demonstrated an anomalous effect of the catalyst loading. The use of low concentrations of catalyst produces an increase in the yield of the C–H activation product ethyl cyclohexaneacetate. This effect has also been found in the case of other less elaborated catalysts such as [BpBr3Cu] or [(bipy)2Cu][I]. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Copper−Homoscorpionate Complexes as Active Catalysts for Atom Transfer Radical Addition to Olefins

Cu(I) complexes containing trispyrazolylborate ligands efficiently catalyze the atom transfer radical addition (ATRA) of polyhalogenated alkanes to various olefins under mild conditions. The catalytic activity is enhanced when bulky and electron donating Tpx ligands are employed. Kinetic data have allowed the proposal of a mechanistic interpretation that includes a Cu(II) pentacoordinated species that regulates the catalytic cycle.