Borate Ligand Research Articles

Scorpionates of the “Tetrahedral Enforcer” Variety as Ancillary Ligands for Dinitrogen Complexes of First Row Transition Metals (Cr–Co)

Dinitrogen complexes of the first row transition metals Cr–Co supported by 2nd generation hydrotris(pyrazolyl)borate (TpR,R′) ligands were characterized by structure and reactivity. Compounds of the type (TpR,R′M)n(N2) (n = 1,2) were prepared and evaluated as precursors for small molecule activation, with specific application to reactions of O2.

Isolable Ethylene Complexes of Copper(I), Silver(I), and Gold(I) Supported by Fluorinated Scorpionates [HB{3‐(CF3),5‐(CH3)Pz}3]– and [HB{3‐(CF3),5‐(Ph)Pz}3

The group 11 metal adducts [HB{3‐(CF3),5‐(CH3)Pz}3]M(C2H4) (M = Au, Ag, and Cu; Pz = pyrazolyl) have been synthesized via a metathesis process using [HB{3‐(CF3),5‐(CH3)Pz}3]Na and CF3SO3Cu, CF3SO3Ag, AuCl and ethylene. The related [HB{3‐(CF3),5‐(Ph)Pz}3]Ag(C2H4) has also been synthesized using [HB{3‐(CF3),5‐(Ph)Pz}3]Na(THF), CF3SO3Ag and ethylene. These group 11 metal ethylene complexes are white solids and form colorless crystals. They have been characterized by NMR spectroscopy and X‐ray crystallography. The gold‐ethylene adduct [HB{3‐(CF3),5‐(CH3)Pz}3]Au(C2H4) shows large upfield NMR shifts of the ethylene proton and carbon signals relative to the corresponding peaks of the free ethylene, indicating relatively high Au→ethylene backbonding. NMR chemical shift data suggest that the silver complexes of both the tris(pyrazolyl)borate ligands [HB{3‐(CF3),5‐(CH3)Pz}3]– and [HB{3‐(CF3),5‐(Ph)Pz}3]– exhibit the weakest interaction with ethylene as compared to the respective copper and gold complexes. X‐ray crystal structures reveal that the gold atom in [HB{3‐(CF3),5‐(CH3)Pz}3]Au(C2H4) binds to scorpionate in κ2‐fashion while the related silver adduct features a κ3‐bonded scorpionate. [HB{3‐(CF3),5‐(CH3)Pz}3]Cu(C2H4) has a scorpionate that binds to copper with two short Cu–N bonds and one long Cu–N distance.

Mononuclear Iron‐(hydro/semi)quinonate Complexes Featuring Neutral and Charged Scorpionates: Synthetic Models of Intermediates in the Hydroquinone Dioxygenase Mechanism

Neutral and anionic scorpionate ligands have been employed to generate active‐site models of hydroquinone dioxygenases (HQDOs). While the nonheme Fe center in nearly all HQDOs is coordinated to one Asp (or Glu) and two His residues, 1,2‐gentisate dioxygenase (GDO) is unique in featuring a three His triad instead. A synthetic GDO model was therefore prepared with the neutral tris(4,5‐diphenyl‐1‐methylimidazol‐2‐yl)phosphine (Ph2TIP) ligand. The gentisate substrate was mimicked with the bidentate ligand 2‐(1‐methylbenzimidazol‐2‐yl)hydroquinonate (BIHQ). X‐ray diffraction analysis of the resulting complex, [Fe(Ph2TIP)(BIHQ)]OTf (1a), revealed a distorted square‐pyramidal geometry. Structural and electrochemical data collected for 1a were compared to those previously reported for [Fe(Ph2Tp)(BIHQ)] (1b), which features an anionic hydridotris(3,5‐diphenylpyrazol‐1‐yl)borate (Ph2Tp) ligand. Oxidation of 1a and 1b provides the corresponding FeIII complexes (2a/2b) and the crystal structure of 2b is reported. Both complexes undergo reversible deprotonation to yield the brown chromophores, 3a and 3b. Detailed studies of 3a and 3b with spectroscopic (UV/Vis absorption, EPR, resonance Raman) and computational methods determined that each complex consists of a high‐spin FeII center ferromagnetically coupled to a p‐semiquinonate radical (BISQ). The (de)protonation‐induced valence tautomerization described here resembles key steps in the putative HQDO mechanism.

Cyanoscorpionate Ligands: Agostic Interactions in a Series of Metal Complexes Containing the Tris(4‐cyano‐3‐phenylpyrazolyl)borate and Bis(4‐cyano‐3‐phenylpyrazolyl)borate Ligands

Scorpionate chemistry has long featured sandwich complexes with two trispyrazolylborate (Tp) or two bispyrazolylborate (Bp) ligands coordinated to a transition metal. Less common are complexes in which one Tp and one Bp ligand are coordinated to the metal. We report herein five such complexes of NiII, MnII, CoII, or FeII involving Tp and Bp ligands comprising either 3‐phenyl‐4‐cyanopyrazole or 3‐phenylpyrazole, as well as a related CoII complex with two Tp ligands, one of which is coordinated in a κ2 fashion. All complexes feature an agostic interaction with a B–H hydrogen atom.

Activation of heteroallenes by coordinatively unsaturated nickel(ii) alkyl complexes supported by the hydrotris(3-phenyl-5-methyl)pyrazolyl borate (Tp(Ph,Me)) ligand.

Activation of sulfur containing heteroallenes by nickel(ii) alkyl complexes supported by the bulky hydrotris(3-phenyl-5-methylpyrazolyl)borate (Tp(Ph,Me)) ligand is described. Exposure of Tp(Ph,Me)NiCH2Ph (1a) and Tp(Ph,Me)NiCH2Si(CH3)3 (1b) to CS2 resulted in formation of the insertion products Tp(Ph,Me)Ni(η(2)-CS2)CH2Ph (2a) and Tp(Ph,Me)Ni(η(2)-CS2)CH2Si(CH3)3 (2b) in moderate yields. Reaction of 1a and MeNCS produced two species in a 1 : 1 ratio, identified as Tp(Ph,Me)Ni(η(2)-MeNC)CH2Ph (3) and Tp(Ph,Me)Ni(η(2)-MeNCS)SCH2Ph (4). Isolation of the unexpected insertion product (3) prompted an investigation into the activity of 1a-b in the presence of isocyanides (i.e.(t)BuNC), which resulted in isolation of Tp(Ph,Me)Ni(η(2-t)BuNC)CH2Ph (5a) and Tp(Ph,Me)Ni(η(2-t)BuNC)CH2Si(CH3)3 (5b). Similarly, reaction of 1a with OCS led to the isolation of a rare example of a Ni(i) carbonyl species Tp(Ph,Me)NiCO (6). Alternatively, complex 6 was also formed by exposure of 1a-b to an atmosphere of CO. Isolation of the intermediate species (Tp(Ph,Me)Ni(η(2)-CO)CH2TMS (7b) and Tp(Ph,Me)Ni(CO)(C(O)R, (8a-b) with R = Ph, TMS)) shed light on the formation of such species.

Scorpionate Hydrotris(indazolyl)borate Ligands as Tripodal Platforms for Surface‐Mounted Molecular Gears and Motors

This microreview describes the use of scorpionate hydrotris(indazolyl)borate ligands to build structurally rigid bifunctional surface‐mounted molecular gears or motors. The article starts with the synthesis of 6‐functionalized indazoles and their corresponding scorpionate tris(indazolyl)borate tripodal ligands. Thioether groups allow the tripodal anchoring of such ligands on metallic surfaces, whereas ester functions enable anchoring on insulating surfaces. Finally, we present their coordination to ruthenium penta(aryl)cyclopentadienyl fragments for the preparation of prototypes of molecular gears and motors. The synthesis of the tetraferrocenylruthenium complex, a molecular motor that has been shown to exhibit controlled clockwise or anticlockwise unidirectional rotation, is also detailed.

Unprecedented Borane, Diborane(3), Diborene, and Borylene Ligands via Pt-Mediated Borane Dehydrogenation.

Reactions of an aryldihydroborane with a Pt(0) complex lead to a range of novel products, including complexes with bridging diborene and diborane(3) ligands and a complex with both borylene and borane (M → B) ligands. The products imply varying degrees of dehydrogenation of the boron centers with concomitant formation of boron-boron bonds, which in one case is later broken. These reactions show that although the dehydrocoupling of dihydroboranes is not a straightforward process in this case, the reactions are capable of connecting boron atoms in unusual ways, leading to unprecedented bonding motifs.

Preparation of zwitterion-type chromium(II) complexes for ethylene oligomerization

Lithium bis(phosphino)aluminate (4, [Et2Al(PPh2)2]Li·(THF); 5, [Et2Al(PCy2)2]Li·(THF)2), lithium bis(amino)aluminate (6, [Et2Al(NC4H8)2]Li·(THF); 7, [Et2Al(NiPr2)2]Li; 8, [iBu2Al(NiPr2)2]Li), and potassium bis(phosphino)borate ligands (11, [(C6F5)2B(PPh2)2]K; 12, [(C6F5)2B(PCy2)2]K) were prepared. Reaction of 4–6 with CrCl3(THF)3 did not provide any discrete chromium complexes, but the reaction of 5–6 with CrCl2(THF) yielded zwitterion-type Cr(II) complexes (13, [Et2Al(PCy2)2]2Cr(II); 14, [Et2Al(NC4H8)2]2Cr(II)). Reaction of THF-free 7–8 with CrCl3(THF)3 resulted in the formation of Cl-bridged Cr(II) dimer complexes {[Et(Cl)Al(NiPr2)2]Cr(II)-μ2-Cl}2 (15) and {[iBu(Cl)Al(NiPr2)2]Cr(II)-μ2-Cl}2 (16). A similar Cl-bridged Cr(II) dimer complex {[(C6F5)2B(PCy2)2]Cr(II)-μ2-Cl}2 (17) was also obtained by reacting the borate ligand 12 with CrCl2(THF). The structures of 13–17 were elucidated by X-ray crystallography. None of the prepared zwitterion-type Cr(II) complexes 13–17 exhibited any activity after activation with MAO, supporting catalytic cycle involving Cr(I/III) oxidation states in ethylene oligomerization.

Synthesis of Biomimetic Zinc Complexes for CO2 Activation and the Influence of Steric Changes in the Ttz Ligands [Ttz = Tris(triazolyl)borate

Carbon dioxide fixation with zinc complexes of the anionic tris(triazolyl)borate (Ttz) ligand offers biomimetic structures in a hydrophilic environment. However, the synthetic result depends greatly on the steric bulk of the Ttz ligand. Synthetic routes adapted from the synthesis of (Tp)ZnOH have been applied to bulky Ttz ligands, but side products often result because Ttz frequently forms complexes with +1 metals or cations used in the synthesis. Nonetheless, (TtztBu,Me)ZnOH (1) has been successfully synthesized and crystallographically characterized, but only via the hydrolysis of the zinc (hexamethylsilyl)azide complex (6). With less bulky ligands, namely TtzPh,Me, routes expected to form LZnOH lead instead to bis(ligand) products, namely [(TtzPh,Me)2Zn] (12). The use of ligands of intermediate steric bulk has led to the isolation of (TtziPr2)ZnOH (2a) which fixes atmospheric CO2 to produce [(TtziPr2)Zn]2(μ‐η1‐η2‐CO3) (2b). This activation of CO2 from air is irreversible. We report the crystal structures of 2b and related molecules used for synthesis and structural comparison: (TtziPr2)ZnCl (7), [(TtziPr2)Cu(μ‐OH)]2 (9), and [(TtziPr2)Cu(μ‐Cl)]2 (10).

ChemInform Abstract: Chemistry of the P‐Block Elements with Anionic Scorpionate Ligands

AbstractReview: 147 refs.

Synthesis and structural characterization of two half-sandwich nickel(II) complexes with the scorpionate ligands

The synthesis and characterization of two new halfsandwich mononuclear nickel(II) complexes with the scorpionate ligands, [k3-N, N',N''-Tpt-Bu, MeNiI] (1) and [k3-N,N',N''-Tpt-Bu, MeNiNO3] (2), are reported. These complexes have been fully characterized by elemental analyses and infrared spectra. Their molecular structures were determined by single crystal X-ray diffraction. The nickel(II) ion of complex 1 is in a four-coordinate environment, in which the donor atoms are provided by three nitrogen atoms of a hydrotris(pyrazolyl) borate ligand and one iodide atom, while that of complex 2 is in a five-coordinate environment with three nitrogen atoms from a hydrotris(pyrazolyl)borate ligand and two oxygen atoms from a nitrate ion.

Synthesis and crystal structure of the rhodium(I) cyclooctadiene complex with bis(3-tert-butylimidazol-2-ylidene)borate ligand

The rhodium(I) cyclooctadiene complex with the bis(3-tert-butylimidazol-2-ylidene)borate ligand [H2B(Im t Bu)2]Rh(COD) C22H36BN4Rh, has been prepared, and its crystal structure is determined by X-ray diffraction. Complex exhibits slightly distorted square planar configurations around the metal center, which is coordinated by the bidentate H2B(Im t Bu)2 and one cyclooctadiene group. The Rh–Ccarbene bond lengths are 2.043(4) and 2.074(4) A, and the bond angle C–Rh1–C is 82.59°. The dihedral angle between two imidazol-2-ylidene rings is 67.30°.

Bright Cyan Phosphorescence of a (Phosphane)copper(I) Complex of the Tri­hydrido­pyrazolylborate Ligand H3B(3,5‐Ph2Pz)–

AbstractThe ligand trihydrido(3,5‐diphenylpyrazol‐1‐yl)borate([MpPh2]–) has been synthesized from 3,5‐diphenylpyrazole and sodium borohydride in N,N‐dimethylformamide. By using this ligand, three phosphane‐stabilized copper(I) complexes [Cu(MpPh2)(L2)] [L2 = (PPh3)2, 1,2‐bis(diphenylphosphanyl)ethane (dppe), or 1,2‐bis(diphenylphosphanyl)benzene (dppbz)] were synthesized. The complexes were characterized by multinuclear NMR spectroscopy, IR spectroscopy, and X‐ray crystallography. The crystal structures of these complexes show that the copper ions are in trigonal‐pyramidal geometries with the apical position formed by agostic Cu–H interactions between the CuI center and one of the hydride atoms of the borate ligand. Complexes [Cu(MpPh2)(PPh3)2] and [Cu(MpPh2)(dppbz)] are mononuclear, whereas complex [Cu2(MpPh2)2(μ‐dppe)2] is dinuclear with bridging dppe ligands. In the solid state, fluorescent emissions are observed in [Cu(MpPh2)(PPh3)2] and [Cu2(MpPh2)2(μ‐dppe)2] but not in [Cu(MpPh2)(dppbz)], which exhibits bright cyan phosphorescence at room temperature that shifts to green when the sample is cooled to 77 K. The phosphorescence of [Cu(MpPh2)(dppbz)] is attributed to mixed interligand and metal‐to‐ligand charge transfer 3(MLCT + π–π*) excited states.

A Single Nickel Catalyst for the Acceptorless Dehydrogenation of Alcohols and Hydrogenation of Carbonyl Compounds

A single homogeneous nickel(II) complex, supported by the tris(3,5-dimethylpyrazolyl)borate ligand and 2-hydroxyquinoline ancillary ligand, is shown to catalyze both acceptorless dehydrogenation of alcohols and hydrogenation of carbonyl compounds under mild conditions. Products from the catalytic reactions were isolated with good yields. A mechanistic investigation highlights the critical role of the 2-hydroxyquinoline ligand in the catalysis and argues against a stepwise dehydrogenation pathway.

Concurrent B-to-Pt Methyl Migration and B-Center Retention in Aerobic Oxidation of Methylborato Platinum(II) Complexes

A new anionic methoxy methylbis(2-pyridyl)borate ligand enables facile oxidation of the derived dimethylplatinum(II) complex, 10, with O2 in methanol to produce a mixture of two platinum(IV) complexes in combined quantitative yield, one supported by a modified ligand, dimethoxybis(2-pyridyl)borate, and the second supported by the original methoxy methylbis(2-pyridyl)borate. The former product results from the aerobic PtII-to-PtIV oxidation and B-to-PtIV methyl migration with subsequent bridging of the boron and PtIV centers by a methoxy group derived from the solvent. A similar reactivity was also demonstrated for 11, the methyl methoxo platinum(II) analogue of 10. Factors responsible for the product distribution are discussed.

Trimerization of Alkynes in the Presence of a Hydrotris(pyrazolyl)borate Iridium Catalyst and the Effect of Substituent Groups on the Reaction Mechanism: A Computational Study

The mechanism of the trimerization of alkynes in the presence of an Ir complex bearing a hydrotris(pyrazolyl)borate (Tp) ligand has been studied using density functional theory calculations at the B3LYP and M06 levels. In this reaction, the initial oxidative coupling of two alkyne molecules yields an iridacyclopentadiene intermediate, which reacts with a third alkyne molecule to give a benzene TpIr complex. There are two possible mechanisms for the formation of the benzene complex in this reaction, including the intramolecular [4+2] cycloaddition and Schore mechanisms. The formation of unsubstituted benzene was initially investigated using acetylene molecules, and then the oxidative coupling reaction of 1,4-dimethyl-2-butyne-1,4-dioate (CH3OCOC≡CCOOCH3) followed by the formation of the substituted benzene complex with 2-butyne (H3CC≡CCH3) was studied. It has been possible to clarify the favorable reaction pathway and the effects of different substituents on the reaction mechanism. In the unsubstituted rea...

Low-Valent Iron Carbonyl Complexes with a Tripodal Carbene Ligand

A bulky tris(carbene)borate ligand allows several low-valent iron carbonyl complexes to be isolated. One-electron reduction of the cationic iron(II) complex [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1) ([PhB(MesIm3)]− = phenyltris(1-mesitylimidazol-2-ylidene)borate) yields the low-spin (S = 1/2) iron(I) complex PhB(MesIm)3Fe(CO)2 (2), as determined by structural and spectroscopic methods. This complex can in turn be reduced to provide the anionic dicarbonyl complex [K][PhB(MesIm)3Fe(CO)2] (3), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of 3 yields the weakly acidic iron hydride PhB(MesIm)3Fe(CO)2H (4), which can also be isolated by treating the κ3-coordinated alkylborohydrido complex PhB(MesIm)3Fe(κ3-BH(CH2CH3)3) (5) with CO. The strong donor ability of the tris(carbene)borate ligand results in significant reduction of the CO bonds, as measured by IR spectroscopy.

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.

Homometallic Cubane Clusters: Participation of Three-Coordinated Hydrogen in 60-Valence Electron Cubane Core.

This work describes the synthesis, structural characterizations, and electronic structures of a series of novel homometallic cubane clusters [(Cp*Ru)2{Ru(CO)2}2BH(μ3-E)(μ-H)B(μ-H)3M], (2, M = Cp*Ru, E = CO; 3, M = Ru(Cp*Ru)2(μ-CO)3(μ-H)BH), E = BH), [(Cp*Ru)3(μ3-CO)(BH)3(μ3-H)3], 4, and [(Cp*Ru)2(μ3-CO){Ru(CO)3}2(BH)2(μ-H)B], 5 (Cp* = η(5)-C5Me5). These cubane clusters have been isolated from a thermally driven reaction of diruthenium analogue of pentaborane(9) [(Cp*RuH)2B3H7], 1, and [Ru3(CO)12]. Structural and spectroscopic studies revealed the existence of triply bridged hydrogen (μ3-H) atoms that participate as a vertex in the cubane core formation for compounds 2, 3, and 4. In addition, the crystal structure of these clusters clearly confirms the presence of an electron precise borane ligand (borylene fragment) which is triply bridged to the trimetallic units. Bonding of these novel complexes has been studied computationally by DFT methods, and the studies demonstrate that the cubane clusters 2 and 3 possess 60 cluster valence electrons (cves) with six metal-metal bonds. All the new compounds have been characterized in solution by mass spectrometry; IR; and (1)H, (11)B, and (13)C NMR studies, and the structural types were unequivocally established by crystallographic analysis of compounds 2-5.

Zirconium-Catalyzed Amine Borane Dehydrocoupling and Transfer Hydrogenation

κ5-(Me3SiNCH2CH2)2N(CH2CH2NSiMe2CH2)Zr (1) has been found to dehydrocouple amine borane substrates, RR′NHBH3 (R = R′ = Me; R = tBu, R′ = H; R = R′ = H), at low to moderate catalyst loadings (0.5–5 mol %) and good to excellent conversions, forming mainly borazine and borazane products. Other zirconium catalysts, (N3N)ZrX [(N3N) = N(CH2CH2NSiMe2CH2)3, X = NMe2 (2), Cl (3), and OtBu (4)], were found to exhibit comparable activities to that of 1. Compound 1 reacts with Me2NHBH3 to give (N3N)Zr(NMe2BH3) (5), which was structurally characterized and features an η2 B–H σ-bond amido borane ligand. Because 5 is unstable with respect to borane loss to form 2, rather than β-hydrogen elimination, and 2–4 do not exhibit X ligand loss during catalysis, dehydrogenation is hypothesized to proceed via an outer-sphere-type mechanism. This proposal is supported by the catalytic hydrogenation of alkenes by 2 using amine boranes as the sacrificial source of hydrogen.