Borate Ligand Research Articles

Robust magnetic anisotropy of a monolayer of hexacoordinate Fe(ii) complexes assembled on Cu(111)

The tris pyrazolyl borate ligand imposes a rigid scaffold around Fe(ii) ensuring a robust magnetic anisotropy when the molecules assembled as monolayers suffer from the dissymmetric environment of the substrate/vacuum interface.

Platinum(II) Complexes with Bis(pyrazolyl)borate Ligands: Increased Molecular Rigidity for Bidentate Ligand Systems.

The structural motif of platinum(II) complexes bearing cyclometalating N-heterocyclic carbene ligands can be used to design deep-blue phosphors for application in organic light-emitting diodes. However, the photophysical properties of the resulting molecules are also highly dependent on the auxiliary ligand. These often allow molecular deformations in the excited state which contribute to non-radiative decay processes that diminish the attainable quantum yield. The use of bis(pyrazolyl)borate-based auxiliary ligands enforces a high molecular rigidity due to their unique geometry. The steric crowding in the coordination sphere inhibits deformation processes and results in highly efficient deep-blue platinum(II) emitters with CIE coordinates below (0.15; 0.15).

Green‐Blue Phosphorescent Iridium(III) Complexes with Near Unitary Quantum Yield

AbstractIridium(III) complexes with cyclometalating ligands can achieve excellent properties for use as phosphorescent emitters in organic light emitting diodes. A series of complexes with 2‐phenyl‐pyridine and bis(pyrazolyl)borate ligands was investigated with regard to this application. The sky‐blue to green emitters showed nearly unitary quantum yields and short excited state lifetimes.

Isolable Copper(I) η2-Cyclopropene Complexes.

Treatment of bis(pyrazolyl)borate ligand supported [(CF3)2Bp]Cu(NCMe) with 1,2,3-trisubstituted cyclopropenes produced thermally stable copper(I) η2-cyclopropene complexes amenable to detailed solution and solid-state analysis. The [(CF3)2Bp]Cu(NCMe) also catalyzed [2 + 1]-cycloaddition chemistry of terminal and internal alkynes with ethyl diazoacetate affording cyclopropenes, including those used as ligands in this work. The tris(pyrazolyl)borate [(CF3)2Tp]Cu(NCMe) is a competent catalyst for this process as well. The treatment of [(CF3)2Tp]Cu with ethyl 2,3-diethylcycloprop-2-enecarboxylate substrate gave an O-bonded rather than a η2-cyclopropene copper complex.

Charge-Separated and Lewis Paired Metal-Organic Framework for Anion Exchange and CO2 Chemical Fixation.

Charge-separated metal-organic frameworks (MOFs) are a unique class of MOFs that can possess added properties originating from the exposed ionic species. A new charge-separated MOF, namely, UNM-6 synthesized from a tetrahedral borate ligand and Co2+ cation is reported herein. UNM-6 crystalizes into the highly symmetric P43n space group with fourfold interpenetration, despite the stoichiometric imbalance between the B and Co atoms, which also leads to loosely bound NO3 - anions within the crystal structure. These NO3 - ions can be quantitatively exchanged with various other anions, leading to Lewis acid (Co2+ ) and Lewis base (anions) pairs within the pores and potentially cooperative catalytic activities. For example, UNM-6-Br, the MOF after anion exchange with Br- anions, displays high catalytic activity and stability in reactions of CO2 chemical fixation into cyclic carbonates.

Dihydrogen Adduct (Co-H2 ) Complexes Displaying H-Atom and Hydride Transfer.

The prototypical reactivity profiles of transition metal dihydrogen complexes (M-H2 ) are well-characterized with respect to oxidative addition (to afford dihydrides, M(H)2 ) and as acids, heterolytically delivering H+ to a base and H- to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H-atom or hydride transfer from the σ-H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S= and an anionic S=0 Co-H2 adduct, both supported by a trisphosphine borane ligand (P3 B ). The thermally stable metalloradical, (P3 B )Co(H2 ), serves as a competent precursor for hydrogen atom transfer to t Bu3 ArO⋅ . What is more, its anionic derivative, the dihydrogen complex [(P3 B )Co(H2 )]1- , is a competent precursor for hydride transfer to BEt3 , establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M-H2 complexes known.

Dihydrogen Adduct (Co–H2) Complexes Displaying H‐Atom and Hydride Transfer

AbstractThe prototypical reactivity profiles of transition metal dihydrogen complexes (M‐H2) are well‐characterized with respect to oxidative addition (to afford dihydrides, M(H)2) and as acids, heterolytically delivering H+ to a base and H− to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H‐atom or hydride transfer from the σ‐H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S= and an anionic S=0 Co‐H2 adduct, both supported by a trisphosphine borane ligand (P3B). The thermally stable metalloradical, (P3B)Co(H2), serves as a competent precursor for hydrogen atom transfer to tBu3ArO⋅. What is more, its anionic derivative, the dihydrogen complex [(P3B)Co(H2)]1−, is a competent precursor for hydride transfer to BEt3, establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M‐H2 complexes known.

Enhancing Tris(pyrazolyl)borate-Based Models of Cysteine/Cysteamine Dioxygenases through Steric Effects: Increased Reactivities, Full Product Characterization and Hints to Initial Superoxide Formation.

The design of biomimetic model complexes for the cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) is reported, where the 3‐His coordination of the iron ion is simulated by three pyrazole donors of a trispyrazolyl borate ligand (Tp) and protected cysteine and cysteamine represent substrate ligands. It is found that the replacement of phenyl groups—attached at the 3‐positions of the pyrazole units in a previous model—by mesityl residues has massive consequences, as the latter arrange to a more spacious reaction pocket. Thus, the reaction with O2 proceeds much faster and afterwards the first structural characterization of an iron(II) η 2‐O,O‐sulfinate product became possible. If one of the three Tp‐mesityl groups is placed in the 5‐position, an even larger reaction pocket results, which leads to yet faster rates and accumulation of a reaction intermediate at low temperatures, as shown by UV/Vis and Mössbauer spectroscopy. After comparison with the results of investigations on the cobalt analogues this intermediate is tentatively assigned to an iron(III) superoxide species.

Structural, Spectral, and Optical Characterization of Potassium bis(2-methyllactato)borate Hemihydrate Crystal

Potassium bis(2-methyllactato)borate hemihydrate (KMB) was grown by a solvent evaporation technique. Crystal structural analysis revealed that the potassium cation was pseudo-octahedrally coordinated by five O atoms from four bis(2-methyllactato)borate (MB) ligands and half-occupied water. The sharp peaks in the powder x-ray diffraction (PXRD) pattern confirmed the perfect crystalline nature of the KMB. Transparency was observed in the range from 215 nm to 1100 nm, and the bandgap of KMB was determined to be 2.15 eV. The molecular structure of KMB was established by interpreting the functional group vibrations through vibrational spectroscopy. The molecular structure was further confirmed by the nuclear magnetic resonance (NMR) spectral technique. The peak followed by valley nature observed is due to the self-focusing behavior of KMB. The minimum transmission near the focus in the open aperture curve is attributed to the reverse saturation absorption nature of KMB. The third-order susceptibility (χ(3)) of KMB is estimated to be 4.17694 × 10−5 esu. These results suggest that KMB can be used in optical sensors as well as other photonic and optoelectronic applications.

Crystal structure of bis(5-bromo-1,10-phenanthroline-κ2 N,N')bis-[di-hydro-bis-(pyrazol-1-yl)borato-κ2 N 2,N 2']iron(II) toluene disolvate.

The structure determination of the title compound was undertaken as part of a project on the modification and synthesis of new spin-crossover (SCO) compounds based on octa-hedral FeII bis-(pyrazol-yl)borate complexes. In the course of these investigations, the compound [Fe(C6H8BN4)2(C12H7BrN2)] was synthesized, for which magnetic measurements revealed an incomplete spin-crossover behaviour. Crystallization of this compound from toluene led to the formation of crystals of the toluene disolvate, [Fe(C6H8N4B)2(C12H7N2Br)]·2C7H8. Its asymmetric unit comprises two discrete metal complex mol-ecules and two toluene solvent mol-ecules. One of the latter is severely disordered and its contribution to the diffracted intensities was removed using the SQUEEZE routine [Spek (2015 ▸). Acta Cryst. C71, 9-18]. In each complex mol-ecule, the FeII cation is coordinated by the two N atoms of a 5-bromo-1,10-phenanthroline ligand and by two pairs of N atoms of chelating di-hydro-bis(pyrazol-1-yl)borate ligands in the form of a slightly distorted octa-hedron. The discrete complexes are arranged in columns along the a-axis direction with the toluene solvate mol-ecules located between the columns. The 5-bromo-1,10-phenanthroline ligands of neighbouring columns are approximately parallel and are slightly shifted relative to each other, indicating π-π inter-actions.

Direct bromination and iodination of the trispyrazolyl borate ligand in TpRu(nbd)Cl and the effect of Tp4−X ligands on redox potentials and catalysis

Traditional syntheses of Tp (trispyrazolylborate, HB(pz)3) ligands often require extreme conditions that are incompatible with some pyrazole derivatives. In contrast, an alternative direct functionalization of the Tp ligand could offer more compatible conditions. To prove this principle, direct bromination or iodination of TpRu(nbd)Cl is reported herein, and Sonogashira cross-coupling of the iodinated ligand with an alkyne could be performed at ambient temperature. This direct halogenation strategy allows rapid access to several halogenated Tp4−XRu(nbd)Cl derivatives from TpRu(nbd)Cl in a single step and to diversified ruthenium complexes having functionalized Tp ligands in minimal additional steps. The influence of the halogen substituents on the physical and chemical properties of the complexes was also examined. Ligand exchange in the presence of modified ligands was facile. Halogenation caused the standard reduction or oxidation potential to shift 200 mV in the positive direction and reduced efficacy in ruthenium catalyzed epoxyalkyne rearrangement during examination of a literature hypothesis relating catalysis to reduction and oxidation potentials.

Alkaline Earth Metal-Mediated Highly Iso-selective Ring-Opening Polymerization of rac-Lactide.

Alkaline earth (Ae) metal complexes of the aminophosphine borane ligand are highly active and iso-selective catalysts for the ring-opening polymerization (ROP) of rac-lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac-LA catalyzed by all Ae metal complexes had a first-order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex-mediated ROP of rac-LA is discussed, based on controlled kinetic experiments and molecular chain mobility.

Charge-Separated Metal–Organic Frameworks Derived from Boron-Centered Tetrapods

Metal–organic frameworks (MOFs) are an emerging class of microporous materials that have potential applications in a wide range of areas. As a subclass of MOFs, ionic MOFs, especially charge-separated MOFs, have been relatively less studied but possess unique features including strong host–guest interactions from the exposed charged centers. We report the synthesis and single-crystal structural characterization of five new charge-separated MOFs (UNM 1–5) based on two tetrapodal borate ligands: tetrakis(4-(4-pyridineethynyl)-2,3,5,6-tetrafluorophenyl)borate (T1) and tetrakis(4-(4-pyridyl)-2,3,5,6-tetrafluorophenyl)borate (T2) having rigid arms of different lengths and pyridine groups at the end of each arm. Coordination of these tetrapods with Cu(I), Cu(II), and Ag(I) ions under specific conditions led to a series of new charge-separated MOFs in single crystalline forms. UNM-1 and UNM-2/UNM-3, which crystallize respectively in tetragonal I4̅ space group and monoclinic C2/c space group, are derived from Cu(CH3CN)4BF4 and Cu(NO3)2 upon coordination with T1. On the other hand, coordination of T2 with Cu(CH3CN)4BF4 and AgBF4 respectively yielded UNM-4 and UNM-5 in the monoclinic I2/a space group. All these MOFs possess several degrees of interpenetration that are correlated with the arm lengths of ligands. Noticeably, UNM-1 is 4-fold interpenetrated, leading to the highest stability among all five MOFs, while still displaying an impressive Brunauer–Emmett–Teller (BET) surface area (SABET) of ca. 621 m2/g. Our findings highlight the versatility of tetrapodal borate ligands in engineering charge-separated MOFs with diverse structures and controlled functionality.

Low- and Mid-Valent Uranium Species Supported by Phenyltris(oxazolinyl)borate Ligands

The synthesis and characterization of a family of uranium derivatives featuring the phenyltris(4,4-dimethyl-2-oxazolinyl)borate (ToM) ligand framework was achieved. Uranium(IV) halide compounds, specifically ToMUCl3(THF) and ToMUI3, were generated by adding 1 equiv of Tl[ToM] to either UCl4 or UI3(THF)4, respectively. The tetravalent derivatives ToMUCl2(NPh2) and ToMUI(═N-Ad)(THF) were also synthesized, showing that ToM supports uranium amido and imido species. Attempts to make bis(phenyltris(oxazolinyl)borate) compounds were unsuccessful. However, when the smaller hydrotris(pyrazolyl)borate (Tp) ligand was used, a mixed bis(ligand) derivative, TpToMUI, was isolated. All of the compounds were characterized by 1H NMR, infrared, and electronic absorption spectroscopies. Where possible, X-ray crystallography was used to assess structural parameters.

NHC-Au(i) complexes bearing trispyrazolyl borate (Tp) ligands: efficient platforms for bimetallic species.

Treatment of NHC-AuCl (NHC = IPr and IMes) complexes with equimolar amounts of KTpR2 (R = Me, H) salts in tetrahydrofuran produces in high yields the heteroleptic complexes 3-6 with the general formula NHC-Au-TpR2. As the TpR ligand in complexes 3-6 features a k1-N type coordination toward the gold(i) center, the subsequent addition of group 10 and 11 metal precursors (NiII, PtII, CuII) results in the isolation of heterobimetallic Au/M complexes supported by TpR platforms. All new metal complexes have been fully characterized by elemental analysis and NMR spectroscopy, and in the case of 3, 4 and 6 by X-ray crystallography.

Nickel(ii)-methyl complexes adopting unusual seesaw geometries.

We report four-coordinate nickel(ii)-methyl complexes of tris-carbene borate ligands which adopt rare seesaw geometries. Experimental and computational results suggest the structural distortion from threefold symmetry results from a combination of electronic stabilization of the singlet state, strong field donors, and constrained angles from the chelating ligand.

Synthesis and structures of anionic rhenium polyhydride complexes of boron-hydride ligands and their application in catalysis.

The rhenium complex, [K(DME)(18-c-6)][ReH4(Bpin)(η2-HBpin)(κ2-H2Bpin)] 1, comprising hydride and boron ligands only, has been synthesized by exhaustive deoxygenation of the commercially available perrhenate anion (ReO4−) with pinacol borane (HBpin). The structure of 1 was analysed by X-ray crystallography, NMR spectroscopy, and DFT calculations. While no hydrides were located in the X-ray crystal structure, it revealed a trigonal arrangement of pinacol boron ligands. Variable-temperature NMR spectroscopy supported the presence of seven hydride ligands but further insight was hindered by the fluxionality of both hydride and boron ligands at low temperature. Further evaluation of the structure by Ab Initio Random Structure Searching (AIRSS) identified the presence of hydride, boryl, σ-borane, and dihydroborate ligands. This complex, either isolated or prepared in situ, is a catalyst for the 1,4-hydroboration of N-heteroaromatic substrates under simple operating procedures. It also acts as a reagent for the stoichiometric C–H borylation of toluene, displaying high meta regioselectivity in the borylated products. Reaction of 1 with 9-BBN resulted in HBpin substitution to form the new anionic tetra(dihydroborate) complex [K(DME)(18-c-6)][Re(κ2-H-9-BBN)4] 4 for which the hydride positions were clearly identified by X-ray crystallography. The method used to generate these isolable yet reactive boron–hydride complexes is direct and straightforward and has potential utility for the exploitation of other metal oxo compounds in operationally simple catalytic reactions.

Five-Membered Ruthenacycles: Ligand-Assisted Alkyne Insertion into 1,3-N,S-Chelated Ruthenium Borate Species.

Building upon previous work, the chemistry of [(η6 -p-cymene)Ru{P(OMe)2 OR}Cl2 ], (R=H or Me) has been extended with [H2 B(mbz)2 ]- (mbz=2-mercaptobenzothiazolyl) using different Ru precursors and borate ligands. As a result, a series of 1,3-N,S-chelated ruthenium borate complexes, for example, [(κ2 -N,S-L)PR3 Ru{κ3 -H,S,S'-H2 B(L)2 }], (2 a-d and 2 a'-d'; R=Ph, Cy, OMe or OPh and L=C5 H4 NS or C7 H4 NS2 ) and [Ru{κ3 -H,S,S'-H2 B(L)2 }2 ], (3: L=C5 H4 NS, 3': L=C7 H4 NS2 ) were isolated upon treatment of [(η6 -p-cymene)RuCl2 PR3 ], 1 a-d (R=Ph, Cy, OMe or OPh) with [H2 B(mp)2 ]- or [H2 B(mbz)2 ]- ligands (mp=2-mercaptopyridyl). All the Ru borate complexes, 2 a-d and 2 a'-d' are stabilized by phosphine/phosphite and hemilabile N,S-chelating ligands. Treatment of these Ru borate species, 2 a'-c' with various terminal alkynes yielded two different types of five-membered ruthenacycle species, namely [PR3 {C7 H4 S2 -(E)-N-C=CH(R')}Ru{κ3 -H,S,S'-H2 B(L)2 }], (4-4'; R=Ph and R'=CO2 Me or C6 H4 NO2 ; L=C7 H4 NS2 ) and [PR3 {C7 H4 NS-(E)-S-C=CH(R')}Ru{κ3 -H,S,S'-H2 B(L)2 }], (5-5', 6 and 7; R=Ph, Cy or OMe and R'=CO2 Me or C6 H4 NO2 ; L=C7 H4 NS2 ). All these five-membered ruthenacycle species contain an exocyclic C=C moiety, presumably formed by the insertion of a terminal alkyne into the Ru-N and Ru-S bonds. The new species have been characterized spectroscopically and the structures were further confirmed by single-crystal X-ray diffraction analysis. Theoretical studies and chemical-bonding analyses established that charge transfer occurs from phosphorus to ruthenium center following the trend PCy3 <PPh3 <P(OPh)3 <P(OMe)3 .

Clicking Azides and Alkynes with Poly(pyrazolyl)borate-Copper(I) Catalysts: An Experimental and Computational Study

The synthesis of 1,4-disubstituted-1,2,3-triazoles under a copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) regime was accomplished in high yields and a regioselective manner by using two homoscorpionate poly(pyrazolyl)borate anions: tris(pyrazolyl)hydroborate (HB(pz)3−) and bis(pyrazolyl)hydroborate (H2B(pz)2−), which stabilized in situ the catalytically active copper (I) center. The [3+2] cycloaddition (32CA) reactions took place under strict click conditions, including room temperature and a mixture of environmentally benign solvents such as water/ethanol in a 1:1 (v/v) ratio. These click chemistry conditions were applied to form complex 1,2,3-triazoles-containing sugar moieties, which are potentially relevant from a biological point of view. Computational modeling carried out by DFT methodologies at the B3LYP/6-31G(d) level showed that the coordination of poly(pyrazolyl)borate-copper(I) to alkyne groups produced relevant changes in terms of generating a high polar copper(I)-acetylide intermediates. The analysis of the global and local reactivity indices explains correctly the role of poly(pyrazolyl)borate ligands in the stabilization and activation of the copper(I) catalyst in the studied 32CA reactions.

Tris(pyrazolyl)borate Copper Hydroxide Complexes Featuring Tunable Intramolecular H-Bonding.

A modular synthesis provides access to a series of new tris(pyrazolyl)borate ligands XpyMeTpK that possess a single functionalized pendant pyridyl (py) or pyrimidyl (pyd) arm designed to engage in tunable intramolecular H-bonding to metal-bound functionalities. To illustrate such H-bonding interactions, a series of [XpyMeTpCu]2(μ-OH)2 (6a-6e) complexes were synthesized from the corresponding XpyMeTpCu-OAc (5a-5e) complexes. Single crystal X-ray structures of three new dinuclear [XpyMeTpCu]2(μ-OH)2 complexes reveal H-bonding between the pendant heterocycle and bridging hydroxide ligands while the donor arm engages the copper center in an unusual monomeric DMAPMeTpCu-OH complex. Vibrational studies (IR) of each bridging hydroxide complex reveal reduced νOH frequencies that tracks with the H-bond accepting ability of the pendant arm. Reversible protonation studies that interconvert [XpyMeTpCu]2(μ-OH)2 and [XpyMeTpCu(OH2)]OTf species indicate that the acidity of the corresponding aquo ligand decreases with increasing H-bond accepting ability of the pendant arm.