A tin-complexation strategy for use with diverse acylation methods in the preparation of 1,9-diacyldipyrromethanes

The acylation of dipyrromethanes to form 1,9-diacyldipyrromethanes is an essential step in the rational synthesis of porphyrins. Although several methods for acylation are available, purification is difficult because 1,9-diacyldipyrromethanes typically streak extensively upon chromatography and give amorphous powders upon attempted crystallization. A solution to this problem has been achieved by reacting the 1,9-diacyldipyrromethane with Bu(2)SnCl(2) to give the corresponding dibutyl(5,10-dihydrodipyrrinato)tin(IV) complex. The reaction is selective for dipyrromethanes that bear acyl groups at both the 1- and 9-positions but otherwise is quite tolerant of diverse substituents. The diacyldipyrromethane-tin complexes are stable to air and water, are highly soluble in common organic solvents, crystallize readily, and chromatograph without streaking. Four methods (Friedel-Crafts, Grignard, Vilsmeier, benzoxathiolium salt) were examined for the direct 1,9-diacylation of a dipyrromethane or the 9-acylation of a 1-acyldipyrromethane. In each case, treatment of the crude reaction mixture with Bu(2)SnCl(2) and TEA at room temperature enabled facile isolation of multigram quantities of the 1,9-diacyldipyrromethane-tin complex. The diacyldipyrromethane-tin complexes could be decomplexed with TFA in nearly quantitative yield. Alternatively, use of a diacyldipyrromethane-tin complex in a porphyrin-forming reaction (reduction with NaBH(4), acid-catalyzed condensation with a dipyrromethane, DDQ oxidation) afforded the desired free base porphyrin in yield comparable to that obtained from the uncomplexed diacyldipyrromethane. The acylation/tin-complexation strategy has been applied to a bis(dipyrromethane) and a porphyrin-dipyrromethane. In summary, the tin-complexation strategy has broad scope, is compatible with diverse acylation methods, and greatly facilitates access to 1,9-diacyldipyrromethanes.     

Direct synthesis of palladium porphyrins from acyldipyrromethanes

[reaction: see text] Palladium porphyrins are valuable photosensitizers and luminescent agents in biology and materials chemistry. New methodology is described wherein a 1-acyldipyrromethane is converted into the palladium chelate of a trans-A(2)B(2) porphyrin via a one-flask reaction. The reaction entails self-condensation of the 1-acyldipyrromethane in refluxing ethanol containing KOH (5-10 mol equiv) and Pd(CH(3)CN)(2)Cl(2) (0.6 mol equiv) exposed to air. This direct route to palladium porphyrins is more expedient than the four steps of the traditional synthesis: (1) reduction of the 1-acyldipyrromethane; (2) acid-catalyzed condensation; (3) oxidation of the porphyrinogen intermediate; and (4) metal insertion. The new synthesis requires neither acid nor DDQ and formally entails only a 2e(-) + 2H(+) oxidation overall versus the traditional multistep synthesis which requires a 2e(-) + 2H(+) reduction per each 1-acyldipyrromethane (4e(-) + 4H(+) overall) followed by a 6e(-) + 6H(+) oxidation. The analogous reaction of a 1,9-diacyldipyrromethane and a dipyrromethane also gives the palladium porphyrin. Seven palladium porphyrins have been prepared in yields of 25-57%. The direct route also can be used with Cu(OAc)(2).H(2)O to give the copper porphyrin albeit in low yield. In summary, this methodology readily affords palladium porphyrins directly from acyldipyrromethanes.     

9-Acylation of 1-acyldipyrromethanes containing a dialkylboron mask for the alpha-acylpyrrole motif

1,9-Diacyldipyrromethanes are important precursors to porphyrins, yet synthetic access remains limited owing to (1) poor conversion in the 9-acylation of 1-acyldipyrromethanes and (2) handling difficulties because acyldipyrromethanes typically streak upon chromatography and give amorphous powders upon attempted crystallization. A reliable means for converting a dipyrromethane to a 1-acyldipyrromethane-dialkylboron complex was recently developed, where the dialkylboron (BR(2)) unit renders the complex hydrophobic and thereby facilitates isolation. Herein a refined preparation of 1,9-diacyldipyrromethanes is presented that employs the 1-acyldipyrromethane-BR(2) complex as a substrate for 9-acylation. The dialkylboron unit provides protection for the alpha-acylpyrrole unit. 9-Acylation requires formation of the pyrrolyl-MgBr reagent and the presence of 1 equiv of a nonnucleophilic base to quench the proton liberated upon alpha-acylation. Reaction of the 1-acyldipyrromethane-BR(2) complex (1 equiv) with mesitylmagnesium bromide (2 equiv) followed by the addition of an acylating agent (S-2-pyridyl thioate or acid chloride, 1.1 equiv) gives the corresponding 1,9-diacyldipyrromethane-BR(2) complex. The acylation method afforded 1,9-diacyldipyrromethane-BR(2) complexes with limited or no chromatography in yields of 64-92%. The 1,9-diacyldipyrromethane-BR(2) complexes are stable to routine handling, are readily soluble in common organic solvents, crystallize readily, and can now be prepared in multigram quantities through use of stoichiometric quantities of reagents.     

New route to ABCD-porphyrins via bilanes

A new strategy for preparing porphyrins that bear up to four different meso-substituents (ABCD-porphyrins) relies on two key reactions. One key reaction entails a directed synthesis of a 1-protected 19-acylbilane by acid-catalyzed condensation at high concentration (0.5 M) of a 1-acyldipyrromethane and a 9-protected dipyrromethane-1-carbinol (derived from a 9-protected 1-acyldipyrromethane). Three protecting groups (X) were examined, including thiocyanato, ethylthio, and bromo, of which bromo proved most effective. The bilanes were obtained in 72-80% yield, fully characterized, and examined by 15N NMR spectroscopy. The second key reaction entails a one-flask transformation of the 1-protected 19-acylbilane under basic, metal-templating conditions to give the corresponding metalloporphyrin. The reaction parameters investigated for cyclization of the bilane include solvent, metal salt, base, concentration, temperature, atmosphere, and time. The best conditions entailed the 1-bromo-19-acylbilane at 100 mM in toluene containing DBU (10 mol equiv) and MgBr2 (3 mol equiv) at 115 degrees C exposed to air for 2 h, which afforded the magnesium porphyrin in 65% yield. The magnesium porphyrin is readily demetalated to give the free base porphyrin. A stepwise procedure (which entailed treatment of the 1-(ethylthio)-19-acylbilane to oxidation, metal complexation, desulfurization, carbonyl reduction, and acid-catalyzed condensation) was developed but was much less efficient than the one-flask process. The new route to ABCD-porphyrins retains the desirable features of the existing "2 + 2" (dipyrromethane + dipyrromethane-1,9-dicarbinol) method, such as absence of scrambling, yet has significant advantages. The advantages include the absence of acid in the porphyrin-forming step, the use of a metal template for cyclization, the ability to carry out the reaction at high concentration, the lack of a quinone oxidant, avoidance of use of dichloromethane, and the increased yield of macrocycle formation to give the target ABCD-metalloporphyrin.     

Brominated boron dipyrrins: synthesis, structure, spectral and electrochemical properties

meso-Anisyl boron dipyrrins (BODIPYs) 1-6 containing one to six bromines at the pyrrole carbons have been synthesized by treating meso-anisyl dipyrromethane with 'n' equivalents of N-bromosuccinimide in THF at room temperature followed by oxidation with DDQ, neutralization with triethylamine and further complexation with BF(3)·OEt(2). The brominated compounds were characterized by HR-MS mass, detailed (1)H, (19)F and (11)B NMR and X-ray diffraction studies. The crystal structures solved for compounds 2-6 indicate that the boron dipyrrinato framework comprised two pyrrole rings and one six membered boron containing ring in one plane like other reported BODIPYs. However, the dihedral angle between the BODIPY core and the meso-anisyl group varied from 48° to 88° and the meso-anisyl ring has an almost perpendicular orientation in penta 5 and hexabrominated 6 BODIPYs. The absorption and emission studies showed a bathochromic shift and reached a maximum for tetrabrominated derivative 4, after which there was no change in the peak maxima for penta 5 and hexabrominated 6 derivatives. However, the quantum yields were reduced with the increasing number of bromines. The electrochemical studies revealed that brominated BODIPY compounds 1-6 are easier to reduce compared to unsubstituted meso-anisyl BODIPY 8 and the reduction potential is linearly related to the number of Br groups.     

Rational syntheses of porphyrins bearing up to four different meso substituents

Porphyrins bearing specific patterns of substituents are crucial building blocks in biomimetic and materials chemistry. We have developed methodology that avoids statistical reactions, employs minimal chromatography, and affords up to gram quantities of regioisomerically pure porphyrins bearing predesignated patterns of up to four different meso substituents. The methodology is based upon the availability of multigram quantities of dipyrromethanes. A procedure for the diacylation of dipyrromethanes using EtMgBr and an acid chloride has been refined. A new procedure for the preparation of unsymmetrical diacyl dipyrromethanes has been developed that involves (1) monoacylation with EtMgBr and a pyridyl benzothioate followed by (2) introduction of the second acyl unit upon reaction with EtMgBr and an acid chloride. The scope of these acylation methods has been examined by preparing multigram quantities of diacyl dipyrromethanes bearing a variety of substituents. Reduction of the diacyl dipyrromethane to the corresponding dipyrromethane-dicarbinol is achieved with NaBH(4) in methanolic THF. Porphyrin formation involves the acid-catalyzed condensation of a dipyrromethane-dicarbinol and a dipyrromethane followed by oxidation with DDQ. Optimal conditions for the condensation were identified after examining various reaction parameters (solvent, temperature, acid, concentration, time). The conditions identified (2.5 mM reactants in acetonitrile containing 30 mM TFA at room temperature for <7 min) provided reaction without detectable scrambling (LD-MS) for aryl-substituted dipyrromethanes, and trace scrambling for alkyl-substituted dipyrromethanes. The desired porphyrins were obtained in 14-40% yield. The synthesis is compatible with diverse functionalities: amide, aldehyde, carboxylic acid, ester, nitrile, ether, bromo, iodo, ethyne, TMS-ethyne, TIPS-ethyne, perfluoroarene. In total 30 porphyrins of the types A(3)B, trans-A(2)B(2), trans-AB(2)C, cis-A(2)B(2), cis-A(2)BC, and ABCD were prepared, including >1-g quantities of three porphyrins.     

Porphyrin complexes containing coordinated BOB groups: synthesis, chemical reactivity and the structure of [BOB(tpClpp)]2+

The reactions of boron halides with free base porphyrins under conditions where partial hydrolysis of the boron halides can occur give diboron porphyrin complexes containing BOB moieties in which each boron is bonded to two porphyrin nitrogen atoms. BF(3).OEt(2) with H(2)(por) gives B(2)OF(2)(por) (por = tpp, ttp, tpClpp, oep) which has an asymmetric structure in which one boron lies in the porphyrin plane (B(ip)) while the other lies above it (B(oop)). BCl(3).MeCN with H(2)(por) gives B(2)O(2)(BCl(3))(2)(por) which contains a four-membered B(2)O(2) ring and is stable only in the presence of excess BCl(3). BBr(3) with Li(2)(tpClpp) gives the dicationic complex [B(2)O(tpClpp)](2+) as its [BBr(4)](-) salt, and is the first example of a boron porphyrin containing three-coordinate boron to be structurally characterised. B(2)O(2)(BCl(3))(2)(por) can be chromatographed on basic alumina to give the hydroxyboron complex B(2)O(OH)(2)(por), which is deduced from its NMR spectra and DFT calculations to have a structure analogous to B(2)OF(2)(por). The OH protons are shifted upfield to near delta -4 (B(oop)-OH) and -10 (B(ip)-OH) by the diamagnetic porphyrin ring current. The reaction of either B(2)O(2)(BCl(3))(2)(por) or B(2)O(OH)(2)(por) (por = ttp, tpClpp) with alcohols (ROH, R = Et, 4-C(6)H(4)CH(3)) gives B(2)O(OR)(2)(por), which can in turn be converted to B(2)O(OR)(OH)(por) by repeated chromatography. The reaction of PhBCl(2) with H(2)(por) (por = ttp, tpClpp) gives B(2)O(Ph)(OH)(por) which has been characterised by spectroscopy in concert with DFT calculations. It is a further example of the B(2)OF(2)(por) structural type, in which the phenyl group is coordinated to the out-of-plane boron and the OH group to the in-plane boron, as are its derivatives B(2)O(Ph)(X)(tpClpp) (X = F, OEt). Steric drivers for the facile hydrolysis of haloboron porphyrins relative to their dipyrromethene and expanded porphyrin counterparts are discussed.     

Rational routes to formyl-substituted chlorins

Two distinct approaches have been developed for the synthesis of chlorins bearing formyl groups: (1) reaction of an acetal-substituted 1-acyldipyrromethane with 2,3,5,6-tetrahydro-1,3,3-trimethyldipyrrin to give upon hydrolysis a 5-formylchlorin and (2) Pd-mediated coupling of a bromochlorin with a one-carbon synthon (hydroxymethyl tributyltin or CO) to give a 13-, 15-, or 3,13-formylchlorin. The zinc chlorins exhibit long-wavelength peak absorption maxima ranging from 626 to 667 nm, indicating the wavelength tunability afforded by formyl substitution.     

DFT survey of monoboron and diboron corroles: regio- and stereochemical preferences for a constrained, low-symmetry macrocycle

The structures of a number of mono- and diboron corrole complexes have been optimized using DFT methods in order to establish regio- and stereochemical preferences for bonding of one or two boron atoms to the corrole macrocycle. The formulations of the complexes were suggested either from preliminary experimental results (to be reported elsewhere) or by analogy with related diboron porphyrin compounds. The computational results suggest for the monoboron corroles BF(2)(H(2)corrole) and BPhH(H(2)corrole) that the regioisomer in which the boron is bound to a dipyrromethene site adjacent to the bipyrrole is preferred over the other possible regioisomers in which boron coordinates either in the bipyrrole or in the dipyrromethene site opposite the bipyrrole. In the N-substituted corrole complexes there are only two possiblities and, for each complex, the regioisomer with boron in the dipyrromethene site adjacent to the bipyrrole is lower in energy. For all four monoboron complexes the stereoisomers in which boron and both its substituents are displaced out of the mean N(4) plane are more stable than the boron in-plane stereoisomers. These regio- and stereochemical preferences are rationalised by an analysis of the deformations to the corrole macrocycle and the geometry at the boron atoms. The lowest energy structures in all cases correspond to the least strained configurations. In addition, all four complexes show significant BFHN hydrogen bonding and BHHN dihydrogen bonding interactions, which are maximised in the lowest energy configurations for each structure, indicating that these are important additional stabilising interactions. Three different regioisomers, each with cisoid or transoid stereochemistry were optimised for the diboron complex PhBOB(corrole) which contains a bridging BOB group. The dipyrromethene/dipyrromethene isomer is more stable than either of the dipyrromethene/bipyrrole isomers and cisoid stereochemistry is preferred over transoid. This contrasts with porphyrin complexes containing BOB groups for which both stereochemical possibilities are observed, and reflects the contracted size of the corrole macrocycle. Three further diboron corroles were investigated, the diboranyl cation [B(2)(corrole)](+) and its one- and two-electron reduced derivatives B(2)(corrole) and [B(2)(corrole)](-). These calculations were undertaken to determine whether the site of reduction of [B(2)(corrole)](+) is likely to be the diboron moiety or the macrocycle. The B-B bond lengths do not shorten upon reduction and an analysis of the molecular orbitals of each species indicates that reduction will be most likely to occur at the macrocycle, offering a potential route to an example of the two-electron reduced corrole ligand, an analogue of the 20-electron isophlorin ligand observed in the corresponding reduced porphyrin complex B(2)(porphine).     

Selective synthesis of meso-naphthylporphyrins

A series of novel meso-(8-substituted naphth-1-yl)porphyrins has been synthesized creating derivatives with a tight recognition environment above the porphyrin plane. The selective synthesis of single atropisomers is discussed. Condensation of bisnaphthaldehyde 12 with phenyldipyrromethane unexpectedly led to selective synthesis of the alpha,alpha-5,10-bridged isomer 14. A mechanism is proposed for this unusual scrambling, and alternative syntheses of alpha,alpha-5,15-bisnaphthylporphyrins are described. Synthesis of 5,15-analogues can be achieved by employing (pentafluorophenyl)dipyrromethane or via presynthesis of a bis(dipyrromethane) derivative 22 (from bisnaphthaldehyde 12) and subsequent condensation with benzaldehyde.     

Matrix isolation infrared spectroscopic and theoretical study of the hydrolysis of boron dioxide in solid argon

The reaction of boron dioxide with water molecule has been studied using matrix isolation infrared spectroscopy. The boron dioxide molecules produced by codeposition of laser-evaporated boron atoms with dioxygen react spontaneously with water molecules to form OB(OH)2, which is characterized to have a doublet ground-state with two OH groups in the cis-trans form. Isotopic substitution results indicate that the hydrolysis process proceeds via a concerted two hydrogen atom transfer mechanism. The cis-trans-OB(OH)2 molecule is photosensitive; it decomposes to the OH x OB(OH) complex upon broadband UV-visible irradiation. The OH x OB(OH) complex is determined to have a (2)A' ground-state with a bent C(s) symmetry, in which the terminal oxygen atom of the OB(OH) fragment is hydrogen bonded with the hydroxyl radical. The OH x OB(OH) complex recombines to the cis-trans-OB(OH)2 molecule upon sample annealing.     

Synthesis and crystal structure of a meso-trialkynyl[28]hexaphyrin

A 5,15,25-trialkynyl [28]hexaphyrin(1.1.1.1.1.1) is the main product of the boron trifluoride-catalysed condensation of meso-anthryl dipyrromethane and triisopropylsilylpropynal; it absorbs strongly in the near-infrared, out to 1093 nm.     

Dual fluorescence from an isonido ReIII rhenacarborane phosphine complex, [7,10-mu-H-7-CO-7,7-(PPh3)2-isonido-7,8,9-ReC2B7H9

The complex [7,10-mu-H-7-CO-7,7-(PPh3)2-isonido-7,8,9-ReC2B7H9] has been synthesized by treatment of the complex salt [NHMe3][3,3-Cl2-3,3-(CO)2-closo-3,1,2-ReC2B9H11] with PPh3 in refluxing THF (tetrahydrofuran) and isolated as intensely colored orange-red microcrystals. Spectroscopic NMR and IR data have suggested that the product has a highly asymmetric structure with two inequivalent PPh3 ligands and a single CO ligand. Measurement of 11B NMR spectra in particular have indicated seven distinct boron vertexes, although the resulting cage degradation by removal of two BH vertexes was confirmed only following X-ray crystallographic analysis, which revealed the pentadecahedral isonido-7,8,9-ReC2B7 architecture. The 11B NMR resonances span an enormous chemical shift range (Deltadelta = 113), and this appears to be a direct consequence of the deshielding of the boron vertex directly opposite the quadrilateral |ReCCB| aperture. The new complex has been shown by electrochemical measurements to undergo a reversible one-electron oxidation. Digitally simulated cyclic voltammograms support a proposed square scheme (E(1/2) = 0.58, 0.69 V vs ferrocene) involving a reversible isonido-closo transition of the metallacarborane cage. Most unusually for a metallacarborane complex, ambient temperature solutions in CH2Cl2 and DMF have been shown to be intensely turquoise-blue fluorescent (lambda(em) = 442 nm, Phi = 0.012). Fluorescence spectroscopy measurements in MeTHF (2-methyltetrahydrofuran) glass at 77 K have indicated that the likely cause of such a broad emission is dual fluorescence (lambda(em) = 404, 505 nm), with both emissions displaying vibronic structure. Following excited-state lifetime decay analysis, the emissive behavior has been accredited to metal-perturbed 1IL states, with the lower energy emission arising from a slight geometric distortion of the initially excited complex.     

1-(2,3,4-Trihydroxybenzylideneamino)-8-hydroxynaphthalene-3,6-disulfonic acid as reagent for spectrophotometric determination of boron in plants

A highly sensitive and selective method has been developed for spectrophotometric determination of boron in plants, the method based on the color reaction of new reagent 1-(2,3,4-trihydroxybenzylideneamino)-8-hydroxynaphthalene-3,6-disulfonic acid (THBA) with boron (III). In an ammonium acetate solution of pH 8.0, boron(III) reacts with THBA to form a 1:2 yellow complex which has a maximum absorption peak at 430 nm. The reaction can complete within 90 min and the absorbance of the complex remains maximum and almost constant at least for 24 h under a temperature range from 0 to 35 °C. The apparent molar absorptivity and Sandell's sensitivity are 2.95 × 10⁴ l mol⁻¹ cm⁻¹ and 0.00036 ng cm⁻², respectively. The limit of quantification, limit of detection and relative standard deviations were found to be 5.1, 1.5 ng ml⁻¹ and 1.12%, respectively. Under the optimum conditions, the absorbency of the complex (λ_max = 430 nm) increases linearly with concentration up to 0.8 μg ml⁻¹ of boron(III). The influences of foreign ions on the determination of boron were investigated in detail. Most of foreign ions can be tolerated in considerable amounts. Experiments have indicated that THBA as chromogenic reagent for spectrophotometric determination of boron has excellent analytical characteristics. Its sensitivity is more than 4.2-fold that of azomethine-H, and stability is advantage over other derivatives of azomehine-H remarkably. Moreover, the synthesis of THBA and its physicochemical properties of THBA were also investigated in detail. Proposed method has been applied to the determination of boron in plants with satisfactory results.     

Theoretical Investigations on the Structures and Properties of the Side-on Complexes B_2(N_2)_2 and Monocyclic B_n(N_2)_n~m (n=3～6,m=-1～+2)

The structures and energies of the side-on complexes B2(N2)2 and monocyclic Bn(N2)nm (n = 3～6,m = -1～+2) between N2 (1∑+g) and B (2P) have been investigated by the DFT-B3LYP and MP2 methods at the 6-311+G(2d) and aug-cc-pVTZ levels. The analyses of NICS (Nucleus Independent Chemical Shifts),NBO (nature bond orbital),AIM (atoms in molecules) and frontal orbitals have been used to reveal the origin of coordination bond between the π-electron donor N2 group and B atom,accompanied by the comparison with the end-on complexes. The results have indicated that the side-on coordination complexes can be formed due to the relative strong fluidity of the π-electrons,and the nature of the coordination bond has been exposed to be that the N2 group offers 1πu electron to the 2p orbital of boron. The coordinate energies of the side-on complexes are less than those of the end-on complexes. Furthermore,the aromaticity of side-on coordination complex is weaker than that of the corresponding end-on coordination complex.     

Effect of boron on the surface tension of molten silicon and its temperature coefficient

The influence of boron concentration (C(B)/mass%) on the surface tension of molten silicon has been investigated with the sessile drop method under oxygen partial pressure P(O(2))=1.62x10(-25)-2.63x10(-22) MPa, and the results can be summarized as follows. The surface tension increases with C(B) in the range below 2.09 mass%, and the maximum increase rate of the surface tension is about 30 mN m(-1)(mass% C(B))(-1). The temperature coefficient of the surface tension, ( partial differential sigma/ partial differential T)C(B), was found to increase with the boron concentration in molten silicon. At the interface between molten silicon and the BN substrate, a discontinuous Si(3)N(4) layer was reckoned to form and the layer might prevent BN from dissolving into the molten silicon. Since dissolved boron from the BN substrate into the molten silicon is below 0.054 mass% and the associated increase in surface tension is below 1.5 mN m(-1), the contamination from the BN substrate on the surface tension can be ignored. The relation between the surface tension and C(B) indicates negative adsorption of boron and can be well described by combining the Gibbs adsorption isotherm with the Langmuir isotherm.     

Synthesis of meso-substituted porphyrins carrying carboranes and oligo(ethylene glycol) units for potential applications in boron neutron capture therapy

Selective delivery of 10B to tumours is one of the major remaining problems in boron neutron capture therapy (BNCT) of cancer. Porphyrins are selectively accumulated in tumours. Thus two series of carborane-carrying porphyrins were constructed, with additional functionality for attachment of uncharged potentially water-solubilising polyethers. 3-(1,2-Dicarbaclosododecaboran(12)-1-ylmethoxy)benzaldehyde was prepared by protection of the aldehyde of 3-(prop-2-ynyloxy)benzaldehyde as a dithioacetal, treatment with decaborane(14) and deprotection. Condensation with a 3-nitrophenyldipyrromethane gave a separable mixture of meso-(3-nitrophenyl)-meso-(3-carboranylmethoxyphenyl)porphyrins, resulting from extensive scrambling at the porphyrinogen stage. Similarly, condensation of 3-(1,2-dicarbaclosododecaboran(12)-1-yl)benzaldehyde with this dipyrromethane gave an analogous mixture of meso-(3-nitrophenyl)-meso-(3-carboranylphenyl)porphyrins. In this second series, the two regioisomeric bis(nitrophenyl)bis(carboranylphenyl)porphyrins could only be distinguished by X-ray crystallography, their NMR spectra being identical. The nitro groups of the mono(nitrophenyl)porphyrins and the bis(nitrophenyl)-porphyrins were reduced to the corresponding amines with tin(II) chloride and the monoamines were coupled with a omega-methoxy poly(ethylene glycol) chloroformate of mean MW 600 to give the MeOPEGylated tricarboranyl porphyrins.     

Relevance of the electronegativity of boron in eta5-coordinating ligands: regioselective monoalkylation and monoarylation in cobaltabisdicarbollide [3,3'-Co(1,2-C2B9H11)2]- clusters

Regioselective monoalkylation and monoarylation in cobaltabisdicarbollide clusters has been achieved starting from Cs[8-I-3,3'-Co(1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))] by cross-coupling reactions between a B-I fragment and an appropriate Grignard reagent in the presence of a Pd catalyst and CuI. A considerable number of monoalkylated and monoarylated derivatives have been synthesized, which allowed study of the influence of boron in metallocene-type ligands and the effect of alkyl and aryl substituents on boron in boron anionic clusters. Experimental data from UV/Vis spectroscopy, E(1/2) measurements, and X-ray diffraction analysis, and supported by EHMO and ab initio analyses, indicate that the participation of metal d orbitals in the HOMO is less than that in typical metallocene complexes. This can be explained in terms of the lower electronegativity of boron compared to carbon. Related to this is the -I character of alkyl groups when bonded to boron in boron anionic clusters, contrary to the common belief that alkyl groups are generally electron-releasing moieties.     

Electrochemical Determination of Boric Acid Using the Boric Acid–Tiron Complexation System

For monitoring the boron concentration in reverse osmosis (RO) water produced by desalination plants, a voltammetric determination method for the amount of electrochemically inactive boron has been developed by using the complexation with 4,5‐dihydroxybenzene‐1,3‐disulfonic acid (Tiron). Cyclic voltammetric measurements showed the oxidation peak of boric acid–Tiron 1 : 1 complex at 920 mV. This peak was successfully applied to the determination of boron in RO water with the LOD of 0.1 mg B dm^?3 by using differential pulse voltammetry. The redox and acid–dissociation behaviors of Tiron and its boron complex are found to depend on the properties of pH buffer reagent.     

11B solid-state NMR investigation of the rhamnogalacturonan II-borate complex in plant cell walls.

The boron in plant cell walls, which is water-insoluble and in the solid state, is solubilized by pectinase digestion to give a dimeric rhamnogalacturonan II-borate (dRG-II-B) complex. To clarify the nondestructive structure of boron present in plant cell walls (as represented by sugar beet fiber), we performed 192- and 96-MHz 11B solid state NMR measurements. The use of a high field magnet frequency of 192-MHz enabled us to observe 11B isotropic chemical shifts at -9.7 and -9.6 ppm for dRG-II-B and sugar beet fiber in the solid state, respectively, demonstrating that the boron in isolated dRG-II-B and in plant cell walls is present as a borate-diol ester (1:2). The observation of the magnetic field dependence of the chemical shift and lineshape for the borate-diol ester (1:2) by quadrupolar interaction suggested that the borate complex had a distorted tetrahedral boron structure.