Structural analysis of the conformational flexibility of tris(pyrazolyl)borate ligands and their analogues

Database analysis and molecular mechanics were used to determine the conformational flexibility of tridentate scorpionate ligands. The tris(pyrazolyl)methane and tris(pyrazolyl)borate ligands act like molecular vises, opening their tripodal structure for larger metals and closing around smaller metal ions. Tris(3-tert-butylpyrazolyl)methane has significant preference for larger metal ions than its unsubstituted parent compound. Tris(pyrazolyl)methanes and tris(pyrazolyl)borates have similar conformational flexibilities. Placing sterically hindered groups on the central carbon or boron has only a minor effect on the geometry of the tris(pyrazolyl)methanes and tris(pyrazolyl)borates. However, it does influence the flexibility of the ligands, particularly when they have to open far from their ideal geometry, which commonly occurs.     

A chiral alkyltris(pyrazolyl)borate ligand: synthesis of [(Ipc)B(pz)3Mn(CO)3] and [(Ipc)B(pz)3Ru(p-cymene)]PF6 (Ipc = Isopinocampheyl)

The application of the reagent (Ipc)BCl(2) (Ipc = isopinocampheyl) in the synthesis of a new tris(pyrazolyl)borate ligand having an Ipc substituent on boron is described. The sodium salt is a convenient precursor for the preparation of the complexes [(Ipc)tris(pyrazolyl)borato]tricarbonylmanganese and [[(Ipc)tris(pyrazolyl)borato](p-cymene)ruthenium](+), whose X-ray crystal structures are reported. While little distortion of the B(pz)(3)M unit is observed in these complexes, steric interaction between the Ipc group and the 3-positions of the pyrazolyl rings is noted to lead to distortion of the angles around the B-C bond.     

A practical synthesis of tris(pyrazolyl)methylaryls

The preparation of three tris(pyrazolyl)toluidines from trifluoromethylaniline reagents is described that likely takes advantage of (quinoidal) resonance-stabilized activation of the C-F bonds. Subsequent transformations lead to two additional (for a total of five new) tris(pyrazolyl)methylaryls. This simple reaction is remarkable because only one other tris(pyrazolyl)methylaryl has been reported previously, because it is usually very difficult to activate fluoroalkane C-F bonds, and because of the potential scope of the reaction.     

A convenient preparative method for anionic tris(substituted pyrazolyl)methane ligands

The synthesis of tris[3-(6-carboxypyridin-2-yl)pyrazol-1-yl]methane is described in a linear multi-step protocol. The pyridyl-pyrazolyl arms are first constructed before being condensed with chloroform. Careful study of the condensation reaction shows the presence of an isomeric form of the tris(pyrazolyl)methane derivative in which one of the pyrazolyl substituents is linked through the nitrogen atom at the 2 position of the pyrazol. After acid-catalysed isomerisation to the desired isomer, the intermediate compound was subjected to a carboalkoxylation reaction and a subsequent hydrolysis. These are some rare examples of reactions directly occurring on the tris(pyrazolyl)methane platforms.     

A remarkable inversion of structure-activity dependence on imido N-substituents with varying co-ligand topology and the synthesis of a new borate-free zwitterionic polymerisation catalyst

Ethylene polymerisation productivities of tris(pyrazolyl)methane-supported catalysts [Ti(NR){HC(Me2pz)3}Cl2] show a dramatically different dependence on the imido R-group compared to those of their TACN analogues, attributed to differences in fac-N3 donor topology; when treated with AliBu3, the zwitterionic tris(pyrazolyl)methide compound [Ti(N-2-C6H4tBu){C(Me2pz)3}Cl(THF)] also acts as a highly active, single site catalyst (TACN = 1,4,7-trimethyltriazacyclononane).     

A simple,rapid and effective protocol for synthesis of bis(pyrazolyl)methanes using nickel–guanidine complex immobilized on MCM-41

Research on Chemical Intermediates - In this paper, a rapid, easy and very efficient method for the synthesis of bis(pyrazolyl)methanes has been reported in the presence of nickel–guanidine...     

PEG-SO3H as a new, highly efficient and homogeneous polymeric catalyst for the synthesis of bis(indolyl)methanes and 4,4��-(arylmethylene)-bis(3-methyl-1-phenyl-1Hpyrazol-5-ol)s in water

In this work, a green, simple and highly efficient procedure for the synthesis of bis(indolyl)methanes and 4,4??- (arylmethylene)-bis(3-methyl-1-phenyl-1H-pyrazol-5-ol)s [as an important class of bis(pyrazolyl)methanes] is described. The condensation of indoles or 1-phenyl-3-methylpyrazol-5-one with carbonyl compounds catalyzed by poly(ethylene glycol)-bound sulfonic acid (PEG-SO₃H) in water affords the title compounds in high yields and relatively short reaction times.     

Bis(pyrazolyl)(thioimidazolyl)borate ligands: the missing member in the N3...S3 scorpionate series

The anionic bis(pyrazolyl)(thioimidazolyl)borate ligands BpMt(R) with R = tert-butyl and isopropyl were obtained as their potassium salts by reacting potassium tris(pyrazolyl)borate with the corresponding thioimidazoles in the melt at 150 degrees C. They were applied to form some tetrahedral zinc complexes and identified by the crystal structures of (BpMt(t-Bu))ZnCl and (BpMt(i-Pr))Zn-SC(6)H(4)-p-Cl.     

ScorpoPhos: a novel phosphine-nitrogen ligand containing a tris(pyrazolyl)borate ligand core

A new tris(pyrazolyl)borate ligand bearing phosphine donor groups appended to the 3-position of the pyrazolyl rings is reported, and the hemilabile behaviour of this tris-N,P ligand in coordination with K+, Tl+ and Cu+ ions is investigated.     

Synthesis of a macrobicycle incorporating the tris(pyrazolyl)methane ligand

Steric crowding of the 3-position of tris(pyrazolyl)borate and -methane ligands has produced tetrahedral metal complexes with controlled reactivity. As an alternative, we propose to incorporate the tris(pyrazolyl)methane chelate in a macrobicyclic structure in order to create a cavity with well-defined dimensions and shape. Acid-catalyzed equilibration of excess of the new pyrazole 3-(1H-pyrazol-3-yl)benzenemethanethiol acetate with HC(3,5-Me(2)pz)(3) followed by hydrolysis affords a functionalized tris(pyrazolyl)methane, which reacts with 1,3,5-tris(bromomethyl)benzene in K(2)CO(3)/DMF to give the title compound. [structure: see text]     

Coordination chemistry of a terpyridine-tris(pyrazolyl) ditopic ligand

A new ditopic ligand, 4'-(4-(2,2,2-tris(1H-pyrazol-1-ido)ethoxymethyl)phenyl)-2,2':6',2'-terpyridine (pzt), has been prepared and its coordination chemistry studied. Metal ions with a preference for octahedral geometry form ML(2) complexes that are readily isolated and characterised, with the metal ion being bound to the terpyridine sites of both ligands. Other metal ions bind to the terpyridine site of just one ligand. In the case of silver(i), a dinuclear M(2)L(2) complex has been isolated in which each silver ion is coordinated to the terpyridine site of one ligand and to a single pyrazolyl donor group from the second ligand. Evidence for binding of metal ions to the tris(pyrazolyl) binding site was obtained by electrospray mass spectrometry and NMR techniques. The free ligand and three metal complexes, including the disilver complex, have been characterised by X-ray crystallographic techniques.     

Bis‐ and Tris(pyrazolyl)borate/Methane‐Stabilized PIII‐Centered Cations

We report the synthesis of [H₂B(pz)₂PR]⁺, [H₂C(pz)₂PR]⁺², [HB(pz)₃P]⁺², and [HC(pz)₃P]⁺³ (H₂B(pz)₂=bis(pyrazolyl)borate; H₂C(pz)₂=bis(pyrazolyl)methane; HB(pz)₃=tris(pyrazolyl)borate; HC(pz)₃=tris(pyrazolyl) methane; R=Ph, Cy or Et₂N) by reaction of the corresponding neutral or anionic ligands with chlorophosphines in the presence of TMSOTf. The structures of these compounds were determined by X‐ray crystallographic analysis and the nature of their bonding was examined using density functional theory.     

Synthesis of Pharmacologically Active Bis(indolyl) and Tris(indolyl) Derivatives Using Chlorotrimethylsilane

Chlorotrimethylsilane is found to be a comparatively fast and efficient catalyst for carrying out electrophilic substitution reactions of indoles with various aldehydes/ketones/triethylorthoformate, yielding excellent amount of bis(indolyl)methanes/tris(indolyl)methanes. The merits of this protocol are avoidance of any external energy source, minimal reaction time, simple and easy procedure and high yield under solvent free room temperature condition. The versatility of this method has been tested with various aldehydes/ketones and received satisfactory results.     

Silica-bonded DABCO hydrogen sulfate ((SB-DABCO)HSO<Subscript>4</Subscript>): a new dual-interphase catalyst for the diversity-oriented pseudo-five-component synthesis of bis(pyrazolyl)methanes and novel 4-[bis(pyrazolyl)methane]phenylmethylene-bis(indole)s

Silica bonded n-propyl-4-aza-1-azoniabicyclo[2.2.2]octane hydrogen sulfate ((SB-DABCO)HSO₄) was prepared as a new interphase dual-catalyst and was successfully applied for the diversity-oriented synthesis of bis(pyrazolyl)methanes via a one-pot pseudo-five-component condensation reaction. This approach provides a versatile range of available substrates and high diversity of desired products. Moreover, ((SB-DABCO)HSO₄) was successfully applied for the synthesis of some novel [bis(pyrazolyl)methane]-bis(indole)s as more complex molecules.     

Silver-catalyzed [2,3]-rearrangement of halonium ylides derived from allyl and propargyl halides and alkyl diazoacetates

A silver(I) complex derived from a polyfluorinated tris(pyrazolyl)borate effectively catalyzes carbene transfer to allylic and propargylic halides, leading to the formation of alpha-haloacetate derivatives.     

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands

Although a multitude of studies have explored the coordination chemistry of classical tripodal ligands containing a range of main‐group bridgehead atoms or groups, it is not clear how periodic trends affect ligand character and reactivity within a single ligand family. A case in point is the extensive family of neutral tris‐2‐pyridyl ligands E(2‐py)₃ (E=C?R, N, P), which are closely related to archetypal tris‐pyrazolyl borates. With the 6‐methyl substituted ligands E(6‐Me‐2‐py)₃ (E=As, Sb, Bi) in hand, the effects of bridgehead modification alone on descending a single group in the periodic table were assessed. The primary influence on coordination behaviour is the increasing Lewis acidity (electropositivity) of the bridgehead atom as Group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.     

Benzene-o-dithiolate ligands as versatile building blocks in supramolecular chemistry

Polydentate ligands with benzene-o-dithiolato donor groups are useful building blocks in supramolecular coordination chemistry. The coordination chemistry of bis- and tris(benzene-o-dithiolato) ligands and mixed benzene-o-dithiolato/catecholato ligands is reviewed. These ligands exhibit a versatile coordination chemistry both in solution and in the solid state.     

The elusive tripodal tris(2-pyridyl)borate ligand: a strongly coordinating tetraarylborate

Tris(2-pyridyl)borates are introduced as a new robust and tunable "scorpionate"-type ligand family. A facile synthesis of this hitherto unknown ligand and its complexation to Fe(II) are described; the optical and electrochemical properties of the resulting iron complex are compared to complexes derived from tris(pyrazolyl)borate, tris(2-pyridyl)aluminate, and corresponding charge-neutral ligands.     

Syntheses of aluminum and zinc alkyl complexes of a highly fluorinated tris(pyrazolyl)borate using [HB(3,5-(CF3)2Pz)3]Ag(THF) as the ligand transfer agent

Dimethylaluminum or ethylzinc complexes of highly fluorinated tris(pyrazolyl)borate ligand [HB(3,5-(CF(3))(2)Pz)(3)](-) can be obtained in excellent yield from the reaction between the silver adduct [HB(3,5-(CF(3))(2)Pz)(3)]Ag(THF) and the metal alkyl reagent Me(3)Al or Et(2)Zn. The X-ray crystal structure of [HB(3,5-(CF(3))(2)Pz)(3)]AlMe(2) shows that the tris(pyrazolyl)borate ligand coordinates to the aluminum center in kappa(2)-fashion. [HB(3,5-(CF(3))(2)Pz)(3)]ZnEt features the typical kappa(3)-bonded ligand.