Mario Bakardjiev

Phosphacarborane Chemistry: The Synthesis of the Parent Phosphadicarbaboranesnido-7,8,9-PC2B8H11 and [nido-7,8,9-PC2B8H10]−, and Their 10-Cl Derivatives − Analogs of the Cyclopentadienide Anion

The reaction of the carborane nido-5,6-C2B8H12 (1) with PCl3 in dichloromethane in the presence of a “proton sponge” [PS = 1,8-bis(dimethylamino)naphthalene], followed by hydrolysis of the reaction mixture, resulted in the isolation of the eleven-vertex nido-phosphadicarbaboranes 7,8,9-PC2B8H11 (2) and 10-Cl-7,8,9-PC2B8H10 (10-Cl-2), depending on the ratio of the reactants. Both of these compounds can be deprotonated by PS to give the nido anions [7,8,9-PC2B8H10]− (2−) and [10-Cl-7,8,9-PC2B8H9]− (10-Cl-2−). The molecular geometries of all compounds were optimized by ab initio methods at a correlated level of theory [RMP2(fc)] using the 6-31G* basis set and their correctness was assessed by a comparison of the experimental 11B NMR chemical shifts with those calculated by the GIAO-SCF/II//RMP2(fc)/6-31G* method. Moreover, the structure of 10-Cl-2− was determined by an X-ray diffraction analysis. The anionic compounds 2− and 10-Cl-2− are analogs of the Cp (Cp = η5-C5H5−) anion. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Reductive Degradation of nido‐1‐CB8H12 into Smaller‐Cage Carborane Systems via New Monocarbaboranes [arachno‐5‐CB8H13]− and closo‐2‐CB6H8

Treatment of the nido-1-CB8H12 (1) carborane with NaBH4 in THF at ambient temperature led to the isolation of the stable [arachno-5-CB8H13]- (2(-)), which was isolated as Na+[5-CB8H13]-.1.5 THF and PPh4 +[5-CB8H13]- in almost quantitative yield. Compound 2(-) underwent a boron-degradation reaction with concentrated hydrochloric acid to afford the arachno-4-CB7H13 (3) carborane in 70 % yield, whereas reaction between 2(-) and excess phenyl acetylene in refluxing THF gave the [closo-2-CB6H7]- (4-) in 66 % yield. Protonation of the Cs+4(-) salt with concentrated H2SO4 or CF3COOH in CH2Cl2 afforded a new, highly volatile 2-CB6H8 (4) carborane in 95 % yield, the deprotonation of which with Et3N in CH2Cl2 leads quantitatively to Et3NH+[2-CB6H7](-) (Et3NH+4(-)). Both compounds 4- and 4 can be deboronated through treatment with concentrated hydrochloric acid in CH2Cl2 to yield the carbahexaborane nido-2-CB5H9 (5) in 60 % yield. New compounds 2-, 3, and 4 were structurally characterised by the ab initio/GIAO/MP2/NMR method. The method gave superior results to those carried out using GIAO-HF when relating the calculated 11B NMR chemical shifts to experimental data.

Azatricarbaborane 7-t-Bu-arachno-7,1,5,12-NC3B8H12 and Parent Tricarbaboranes nido-[5,6,9-C3B7H10]- and -5,6,9-C3B7H11

The neutral azatricarbaborane 7-t-Bu-arachno-7,1,5,12-NC(3)B(8)H(12), isolated as a side product (yield 2%) from the new synthesis of 7-t-BuNH2-nido-7,8,9-C(3)B(8)H(10) (yield 70%), can be easily converted to the first parent representatives of the 10-vertex nido family of tricarbaboranes, [5,6,9-C(3)B(7)H(10)]- and 5,6,9-C(3)B(7)H(11).

Unusual interaction of alkynes with nine-vertex arachno-monocarbaboranes 4-CB8H14 and [4-CB8H13]-.

Alkynes R(1)R(2)C(2) react with the neutral monocarbaborane arachno-4-CB(8)H(14) (1) at elevated temperatures (115-120 degrees C) under the formation of the derivatives of the ten-vertex dicarbaborane nido-5,6-C(2)B(8)H(12) (2) of general formula 9-Me-5,6-R1,R2-nido-5,6-C(2)B(8)H(9) (where R1,R2 = H,H 2a; Me,Me 2b; Et,Et 2c, H,Ph 2d, and Ph,Ph 2e) in moderate yields (26-52%). Side reaction with PhC(2)H also yields 1-Ph-6-Me-closo-1,2-C(2)B(8)H(8) (3d). In contrast, the reaction between [arachno-4-CB(8)H(13)](-) anion ((-)) and PhC(2)H produces a mixture of the closo anions [1-CB7H8]- (4-) and [1-CB6H7]- (5-) (yields 32 and 24%, respectively). Individual compounds were isolated and purified by liquid chromatography and characterized by NMR spectroscopy ((11)B, (1)H and (13)C) combined with two-dimensional [(11)B-(11)B]-COSY and (1)H-{(11)B(selective)}NMR techniques.

Diphosphacarborane analogues of ferrocene: The synthesis of two isomeric twelve-vertex closo-[(η5-C5H5)FeP2CB8H9] complexes

The reaction of the Tl+ salt of the [nido-7,8,9-P2CB8H9]- anion (1-) with [CpFe(CO)2I](Cp =eta(5)-C5H5) in refluxing mesitylene for 12 h gives mixed-sandwich [1-Cp-closo-1,2,3,4-FeP2CB8H9] (2) (yield 63%). Reaction of the PPh4+ salt of the isomeric [nido-7,8,10-P2CB8H9]- anion 3- with [CpFe(CO)2I] in refluxing mesitylene gives [1-Cp-closo-1,2,3,5-FeP2CB8H9]4 (yield 56%), isomeric with 2. Compound 4 also results (yield 92%) from the sublimation of 2 under argon at ca. 350 degrees C. The constitution of all compounds is established by mass spectrometry, IR spectroscopy and multinuclear NMR spectroscopy (1H, 11B, 31P, and 13C; two-dimensional [11B-11B]-COSY, and 1H- 11B(selective)), further confirmed in the case of 4 by a single-crystal X-ray diffraction analysis.

Skeletal Alkylcarbonation (SAC) Reactions as a Simple Design for Cluster–Carbon Insertion and Cross‐Coupling: High‐Yield Access to Substituted Tricarbollides from 6,9‐Dicarba‐arachno‐decaborane(14)

The works by Brellochs and others on degradative insertion of the aldehyde carbon into the structure of the [6-HOarachno-B10H13] 2 dianion and some metal–carbonyl C-insertion reactions suggested that C=O carbon incorporation procedures might be, in principle, applicable to other cluster systems. To show that this synthetic approach is indeed viable, we report, herein, our preliminary results on a simple and convenient synthesis of the carbon-substituted elevenvertex nido tricarbaboranes (tricarbollides). The reactions are characterized by net inclusion of the C R vertex into the cluster of arachno-6,9-C2B8H14 through the acyl chloride C=O group, which results in an effective cross-coupling between R and the tricarbollide cage. Scheme 1 (path A) shows that reactions involving the arachno-6,9-C2B8H14 (1) dicarbaborane, [3] two equivalents of Et3N (in situ generator of [arachno-6,9-C2B8H13] (1 ) and HCl scavenger), NaH (H2O scavenger), and acyl chlorides RCOCl (exemplified by R=Me, Ph, and Naph (1-Naphthyl)), followed by treatment with aqueous NaOH, and precipitation with R4NCl (R=Me or Et) led to the isolation of a series of the monoanionic [8-R-nido-7,8,9-C3B8H10] compounds (2 ) (2 a : R=Me; 2 b : R= Ph; 2 c : R=Naph), which were isolated in yields up to 85 % (unoptimized, Table 1). The reactions of path A are in accord with the simplified stoichiometry of Equation (1), comprising the deprotonation of 1 along with Cl and H2O elimination.

Systematic Method for the Incorporation of the {(η6-Arene)Fe} Fragment into Carborane Cages via [(η6-Arene)Fe]2+ Dications. A Series of [3-(η6-Arene)-closo-3,1,2-FeC2B9H11] Complexes. Reliable Synthesis of Polymethylated [(η6-Arene)2Fe]2+ Cations

Developed was a systematic method for the incorporation of the {(eta(6)-arene)Fe} fragment into a carborane cage. Reactions between [(eta(6)-arene)(2)Fe](PF(6))(2) salts and Tl(2)[nido-7,8-C(2)B(9)H(11)] in refluxing (CH(2))(2)Cl(2) generated a series of [3-(eta(6)-arene)-closo-3,1,2-FeC(2)B(9)H(11)] neutral complexes with variable types of polymethylated arene ligands in the structure. The structures of durene and hexamethylbenzene complexes were established by X-ray diffraction analyses. The work also shows that the original high-temperature method for the preparation of the [(eta(6)-arene)(2)Fe](2+) dications fails for mesitylene and durene compounds, which was overcome by a reliable room-temperature modification.

Additive character of electron donation by methyl substituents within a complete series of polymethylated [1-(η6-Me9n)C6H(6-n))-closo-1,2,3-FeC2B9H11] complexes. Linear correlations of the NMR parameters and Fe(II/III) redox potentials with the number of arene methyls.

A systematic method for the incorporation of the {(η(6)-Me(n)C(6)H(6-n))Fe} fragment into the dicarbollide cage was developed based on reactions between [(η(6)-Me(n)C(6)H(6-n))(2)Fe][PF(6)](2) salts (1) and Tl(2)[nido-7,8-C(2)B(9)H(11)]. These reactions proceed with elimination of one arene ligand to generate a complete series of the neutral [1-(η(6)-Me(n)C(6)H(6-n))-closo-1,2,3-FeC(2)B(9)H(11)] (2) complexes with n = 1-6 in yields ranging 15-70% depending on the arene. The structures of mesitylene and pentamethylbenzene complexes were established by X-ray diffraction analyses. All compounds were characterized by (11)B and (1)H NMR measurements, mass spectra, melting points and elemental analyses. Correlations between selected (1)H and (11)B NMR parameters and the Fe(II/III) redox potentials and the number of arene methyls for complexes 2 are linear. These facts establish direct evidence for a strictly additive character of electron donation by the methyl substituents to the arene ring and further to the Fe center and the second (dicarbollide) ligand.Correlations between the number of arene methyls (n) and selected (1)H and (11)B NMR parameters or the Fe(II/III) redox potentials for complexes [1-(η(6)-MenC(6)H(6-n))-closo-1,2,3-FeC(2)B(9)H(11)] are of strictly linear character.

Monocarbaborane chemistry. Preparation and characterisation of [4-CB8H9]−, the ‘missing’closo-carbaborane anion

Thermolysis in the solid state of Cs+[arachno-CB9H14]-, or of Cs+[nido-CB9H12]-, or the oxidation of nido-1-CB8H12 with I2 in THF at -78 degrees C in the presence of NEt3, gives the first nine-vertex closo monocarbaborane, the stable [closo-4-CB8H9]- anion, in yields of 56, 61 and 75%, respectively.

Unique Stereocontrol in Carborane Chemistry: Skeletal Alkylcarbonation (SAC) versus Exoskeletal Alkylmethylation (EAM) Reactions†

Reactions between the arachno-6,9-C2B8H14 (1) dicarbaborane and acyl chlorides, RCOCl (2), are subject to stereocontrol that completely changes the nature of the reaction products. While most chlorides produce the 8-R-nido-7,8,9-C3B8H11 (3) tricarbollides (by skeletal alkylcarbonation=SAC), bulky RCOCls (2; where R=1-adamantyl, 2 a; 1-mesityl, 2 b; 9-anthranyl, 2 c; 1-naphthyl, 2 d) in 1,2-dichloroethane (DCE) in the presence of triethylamine at 40-60 °C gave a series of entirely different 1-R-2-CH3-closo-1,6-C2B8H8 (4) dicarbaboranes upon acidification with conc. H2SO4 (by exosleletal alkylmehylation=EAM). Both types of reactions seem to proceed via a common [8-R-nido-7,8,9-C3B8H10](-) (3(-)) anion which in the EAM case is unstable because of steric crowd and undergoes rearrangement via the isomeric [R-nido-7,8,10-C3B8H10](-) tricarbollide structures which, on protonation, undergo reductive extraction of one CH vertex to generate the 2-CH3 substituent in structure 4.