Reaction of B10H14 with para-(OHC)C6H4(COOH) in aqueous KOH gives the [nido-6-CB9H11-6-(C6H4-para-COOH)](-) anion I which upon cluster closure with iodine in alkali solution gives the [closo-2-CB9H9-2-(C6H4-para-COOH)](-) anion 2; an analogous procedure with B10H14 and glyoxalic acid OHCCOOH gives the [closo-2-CB9H9-2-(COOH)](-) anion 4 via the [arachno-6-CB9H13-6-(COOH)](-) anion 3

Franken, A.

Kennedy, J.D.

Kilner, C.A.

English

White Rose Research Online

promoting access to White Rose research papers      Universities of Leeds, Sheffield and York http://eprints.whiterose.ac.uk/    This is an author produced version of a paper, subsequently published in CHEMICAL COMMUNICATIONS. (This paper has been peer-reviewed but does not include final publisher proof-corrections or journal pagination.)    White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/3932  Published paper Franken A,  Kilner CA,  Kennedy JD (2004) Polyhedral monocarbaborane chemistry. Carboxylic acid derivatives of the [closo-2-CB9H10] (-) anion CHEMICAL COMMUNICATIONS 3 (328-329)    White Rose Research Online eprints@whiterose.ac.uk   Polyhedral monocarbaborane chemistry. Carboxylic acid derivatives of the [closo-2-CB9H10]− anion.  Andreas Franken, Colin A. Kilner and John D. Kennedy. Department of Chemistry, University of Leeds, Leeds, UK LS2 9JT. andreas.franken@vanderbilt.edu, colink@chem leeds.ac.uk, johnk@chem.leeds.ac.uk. ________________________________________________________________________________  Reaction of B10H14 with para-(OHC)C6H4(COOH) in aqueous KOH gives the [nido-6-CB9H11-6-(C6H4-para-COOH)]− anion 1 which upon cluster closure with iodine in alkali solution gives the [closo-2-CB9H9-2-(C6H4-para-COOH)] − anion 2; an analogous procedure with B10H14 and glyoxalic acid OHCCOOH gives the [closo-2-CB9H9-2-(COOH)]−  anion 4 via the [arachno-6-CB9H13-6-(COOH)]− anion 3.  ________________________________________________________________________________  There is interest in the functionalization of stable closo boranes and closo dicarbaboranes for use as  synthons or building-blocks for potential life-science 1,2 and  nanoarchitectural applications.3,4,5 Our laboratory has become interested in extending this functionalisation chemistry to the intermediate monocarbaboranes. Monocarbaboranes offer complementary charge and polarity possibilities compared to the boranes on one hand and to the dicarbaboranes on the other hand.  In this regard, monocarbaborane units with synthetically useful amine residues {NH2} are readily available. These {NH2} units may be directly bound to the monocarbaborane cluster, as in the [1-(H2N)-closo-1-CB11H11]− anion that is isolatable from the classical route to the closo twelve-vertex monocarbaborane clusters,6,7  or less directly bound, as in the isomers of  [para-(H2N)-C6H4-closo-CB9H9]− pioneered by Sivaev and co-workers.8,9 Carboxylate units are complementary to {NH2} groupings in synthetic reactions,9,10 and in this context we here report preliminary results on the hitherto elusive carboxylic acid {COOH} derivatives of the closo {CB9} residue. We have established examples that show that the acid unit can be bound either directly to the carbon atom of the constituents of the cluster, or indirectly, here by utilization of a {para-C6H4(COOH)} unit bound to the cluster. Both procedures make initial use of the Brellochs Reaction11 for the direct generation of functional ten-vertex {CB9} monocarbaboranes from the reaction of functional aldehydes with nido-B10H14, and further demonstrate the useful versatility of this reaction.    Thus, 4-carboxybenzaldehyde, para-(OHC)C6H4(COOH), reacts in strong alkaline solution with nido-B10H14 to form the ten-vertex [nido-6-CB9H11-6-(C6H4-para-COOH)]−  anion 1, isolatable in 62 % yield as its [NEt4]+ salt 1a.12(a) In strong alkaline solution anion 1 thence reacts with elemental iodine with cage closure to form the [closo-2-CB9H9-2-(C6H4-para-(COOH)]− anion 2, isolatable as its [NEt4]3[CB9H9(C6H4COOH)]2Br double salt 2a in 73 % yield.12(b)  In a similar manner, by the successive dissolution in strong alkaline solution of glyoxylic acid monohydrate, [OHCCO2H(OH2)], and nido-B10H14, the [arachno-6-CB9H13-6-(COOH)]− anion 3 can be obtained in 53 % yield as its [NEt4]+ salt 3a. 12(c) In strong alkaline solution anion 3 undergoes cage closure with elemental iodine to give the [closo-2-CB9H9-2-(COOH)]− anion 4, isolatable as its [NEt4]+ salt 4a in 92 % yield.12(d)  Both new closo anions 2 and 4 are characterised by single-crystal X-ray diffraction analysis,13 anion 4 in its [NEt4]+ salt 4a, and anion 2 in the [NEt4]3[closo-2-CB9H9-2-(para-C6H4COOH)]2Br double salt 2a (Figure 1). Salient interatomic dimensions are given in the caption to Figure 1.                     Figure 1. Crystallographically determined molecular structures13 of (left) the [closo-2-CB9H9-2-(C6H4-para-COOH)]− anion 2 in its [NEt4]3[CB9H9(C6H4COOH)]2Br  double salt 2a and (right) the [closo-2-CB9H9-2-(COOH)]− anion 4 in its [NEt4]+ salt 4a. In 2a the C(2)-C(21) distance is 1.497(4), the C(24)C(27) distance is 1.492(4) Å, the O(1)C(27) distance is 1.207(4) Å, the O(2)C(27) distance is 1.320(4) Å and the O(1)C(27)O(2)  angle is 123.5(3)°; within the cluster, C(2)B(1) is 1.637(4), C(2)B(3) is 1.753(5), C(2)B(5) is 1.769(4), C(2)B(6) is 1.752(4) and C(2)B(9) is 1.756(4) Å. There are two independent molecules, A and B, of 4 in the unit cell of 4a; the molecular structure of only one of these (anion A) is shown. Dimensions for the anions A and B are closely related (see text). In anion A, the C(2)C(3) distance is 1.465(3) Å, the O(1)C(3) distance is 1.221(2) Å, the O(2)C(3) distance is 1.293(2) Å and the O(1)C(3)O(2) angle is 123.14(18)°; within the cluster, C(2)B(1) is 1.603(3), C(2)B(3) is 1.703(3), C(2)B(5) is 1.761(3), C(2)B(6) is 1.754(3) and C(2)B(9) is 1.730(3) Å. In anion B, the C(2)C(3) distance is 1.473(3) Å, the O(1)C(3) distance is 1.188(2) Å, the O(2)C(3) distance is 1.291(2) Å and the O(1)C(3)O(2) angle is 121.57(19)°; within the cluster, C(2)B(1) is 1.606(3), C(2)B(3) is 1.765(3), C(2)B(5) is 1.698(3), C(2)B(6) is 1.732(3) and C(2)B(9) is 1.732(3) Å.   The crystallographically determined cluster structure of anion 2 is relatively symmetrical across the B(1)C(2)B(10) plane. By contrast, the crystallographic analysis of intramolecular C-B distances in anion 4 gives an apparent significant asymmetry about C(2). There are two independent molecules of 4 in the crystal of 4a.  In molecule A, C(2)-B(3) is much shorter than C(2)-B(5),  and, conversely, C(2)-B(6) is much longer than C(2)-B(9). At first sight, it is tempting to ascribe this to differential trans effects arising from C=O versus COH in the {COOH} group, but molecule B shows the converse asymmetry, with C(2)-B(3) being much longer than C(2)-B(5),  and C(2)-B(6) is much shorter than C(2)-B(9), suggesting a crystallographic artefact. A more definitive difference between anions 2 and 4 is that the C(2)-B(1) distance in the directly carboxylated cluster species 4 is shorter than C(2)-B(1) in 2, and C(2)-C(3) in 4 is shorter than both C(2)C(21) and C(24)-C(27) in 2, which suggest some multiple-bonding character in the cluster to carboxylate linkage C(2)-C(3) in 4. There is also an interesting difference in intermolecular interaction behaviour when the anions in 2a and 4a are compared.  Whereas hydrogen bonding between the carboxylic acid residues in 2a results in an association of pairs of anions 2 to give conventional carboxylic acid dimeric units, for anion 4 the hydrogen bonding in 4a gives the less common planar assembly of four {COOH} units, rather than the dimeric configuration. We hope to address these and related phenomena in more detail when this and related work is complete and presented in full description.   It is anticipated that these carboxylic acid species, and analogous derivatives of related cluster species, will constitute very useful intermediates for molecular architectural constructions and for the synthesis of molecules to be investigated for potential life-science applications. They offer opportunity for derivatisation for further useful functionalization, e.g. for acid chloride formation which could be useful for the generation of amide linkages, and for reduction to alcohols and subsequent reoxidation to aldehydes, both of which offer further synthetic potential. Also, in principle, they can be isomerised to their closo-1-monocarbadecaborane isomers,8 permitting more subtle permutations in molecular configuration that could be useful in the fine tuning of any emerging effect chemistry. We are currently devising experimentation to examine some of these possibilities.    Acknowledgements  We thank the UK EPSRC and the UK DTI for support  References  1. A. H. Soloway, W. Tjarks, B. A. Barnum, F.-G. Rong, R. F. Barth, I. M. Codogni and J. G. Wilson,  Chem. Rev., 1998, 98, 1515. 2. (a) M. F. Hawthorne, Angew. Chem. Int. Ed. Eng., 1993, 32, 950; (b) M. F. Hawthorne and A. Maderna, Chem. Rev., 1999, 99, 3421. 3. (a) X. Yang, W. Jiang, C. B. Knobler and M. F. Hawthorne,  J. Am. Chem. Soc. 1992, 114, 9719; (b) M. F. Hawthorne and Z. Zheng, Acc. Chem. Res., 1997, 30, 267; (c) M. D. Mortimer, W. Jiang, Z. Zheng, R. R. Kane, I. T.Chizhevsky, C. B. Knobler and M. F. Hawthorne, Rods, Rings, Balls and Strings, in Modular Chemistry, ed. J. Michl, 1997, pp 551-564, Kluwer Academic, Dordrecht; (d) J. Thomas and M. F. Hawthorne, Chem. Commun., 2001, 1884. 4. See, for example (a) J. Müller, K. Baše, T. F. Magnera and J. Michl, J. Am. Chem. Soc. 1992, 114, 9721; (b) U. Schöberl, J. Müller, K. Baše, M. Ibrahim P. N. Ibrahim, T. F. Magnera, D. E. David, J. Michl, Polymer Preprints, 1993, 34, 81; (c) P. Schwab, M. D. Levin and J. Michl, Chem. Rev., 1999, 99, 1863; and (d) H. Yao and R. N. Grimes, J. Organomet. Chem., 2003, in press (from J. Organomet. Chem.  website). 5. (a) H. M. Colquhoun, P. L. Herbertson, K. Wade, I. Baxter and D. J. Williams, Macromolecules, 1998, 31, 1694; (b) E. S. Alexseyava, M. A. Fox, J. A. K. Howard, J. A. H. Macbride and K. Wade, Appl. Organomet. Chem., 2003, 17, 499. 6. B. Štíbr, Chem. Rev., 1992, 92, 225.  7. J. Plešek, T. Jelínek, E. Drdáková, S. Heřmánek and B. Štíbr, Collect. Czech. Chem. Comm., 1984, 49, 1559.  8. I. Sivaev, S. Sjoberg and V. Bregadze, Abstracts Eleventh International Meeting on Boron Chemistry (IMEBORON XI), Moscow, Russia, July 28 - August 2 , 2002, Abstract no. CA-15, p. 69. 9. N. J. Bullen, A. Franken, C. A. Kilner and J. D. Kennedy, Chem. Commun., 2003, 1684. 10.  A. Franken, C. A. Kilner and J. D. Kennedy, Inorg. Chem. Commun., 2003, 6, 1104. 11.  B. Brellochs, in Contemporary Boron Chemistry, Eds. M. Davidson, A. K. Hughes, T. B. Marder and K. Wade, Royal Society of Chemistry, Cambridge, England (2000), pp. 212-214. 12. All reactions were carried out under dry N2, followed by work-up in air. (a) Anion 1.  para-(OHC)C6H4(COOH) (15 g, 100 mmol) was added to a stirred solution of KOH (11.2 g, 200 mmol) in H2O (100 ml) at 0 °C, and after 5 min, B10H14 (2.0 g, 16.4 mmol) was added. The orange solution was stirred for 3 h at ca. 20 °C, cooled to 0 °C, aqueous HCl (5 %, 100 ml) added dropwise, the solution filtered, and [NEt4]+Br− (4.20 g, 20 mmol) added to the filtrate. The [NEt4]+ salt 1a of the [nido-6-CB9H11-6-(C6H4-para-COOH)]−anion 1 precipitated as a white solid, which was filtered off and dried in vacuo (2.46 g, 10.2 mmol, 62 %). NMR, (CD3)2CO, 294−299 K, ordered as assignment δ(11B)/ppm [δ(1H)/ppm]: BH(5,7) +2.2 [+3.36], BH(9) –1.5 [+2.98], BH(1,3) –4.4 [+2.49], BH(8,10) –12.2 [+2.02], BH(2) –26.2 [+0.63], BH(4) –37.5 [+0.47] with δ(1H) for µH(8,9)/(9,10) at −3.30 ppm; additionally δ(1H)(COOH) +9.92 (broad), δ(1H)(Ph) +7.85 (2H) and +7.23 (2H) (both apparent doublets), and δ(1H)(Et) at +3.49 (8H, quartet) and +1.40 ppm (12H, triplet), also δ(13C)(COOH) +173.0, δ(13C)(Ph) +153.2 (1C), +135.7 (1C), +128.9 (2C) and +127.0 (2C), with δ(13C)(cluster) +62.7 and δ(13C)(Et) +7.2 and +52.4 ppm. (b) Anion 2. A solution of 1a (2.46 g, 10.2 mmol) in aqueous HCl (10 %, 100 ml) was extracted with Et2O (3 × 50 ml), the organic layers separated, combined, aqueous  KOH (ca.1 M; 200 ml) added, and the Et2O evaporated in vacuo. Elemental I2 (2.0 g, 8.0 mmol) was added to the resulting alkaline aqueous solution, which was then stirred for 3 h at ca. 20 °C, then cooled to 0 °C, whereupon  Na2SO3 (1.26 g, 10 mmol) and aqueous HCl (5 %, 100 ml) were added, the solution filtered, and [NEt4]+Br− (2.1 g, 10.0 mmol) added to the filtrate. After brief stirring, the [NEt4]+ / [NEt4]Br double salt 2a of the [closo-2-CB9H9-2-(C6H4-para-COOH)]− anion 2 precipitated as a white microcrystalline solid, which was filtered off and dried in vacuo (4.32 g, 7.45 mmol, 73 %). NMR, (CD3)2CO, 294-299 K, ordered as assignment δ(11B)/ppm [δ(1H)/ppm]: BH(10) +2.2 [+3.89], BH(1) –2.7 [+3.38], BH(4) –20.8 [+1.29], BH(6,7) -25.5 [+0.92], BH(3,5) -28.4 [+1.73], BH(7,8) –28.4 [+0.64]; additionally δ(1H)(COOH) +10.79 (broad), δ(1H)(Ph) at +7.72 (2H) and +6,99 (2H) (both apparent doublets) and δ(1H)(Et) at +3.48 (16H, quartet) and +1.39 ppm (24H, triplet); also δ(13C)(COOH) at +167.1, δ(13C)(Ph) at +150.9 (1C), +141.9 (1C), +128.5 (2C) and +126.7 (2C), δ(13C)(cluster) at +47.12 and δ(13C)(Et) at +7.1 and +52.5 ppm. (c) Anion 3.   [OHCCO2H(OH2)] (9.2 g, 100 mmol) was added to a stirred solution of KOH (11.2 g, 200 mmol) in H2O (100 ml) at 0 °C, and, after 5 minutes, B10H14 (2.0 g, 16.4 mmol) was added. The yellow solution was stirred for 2 h at ca. 20 °C. [NEt4]+Br− (4.2 g, 20 mmol) added, the solution cooled to 0 °C and aqueous HCl (5 %, 150 ml) added dropwise. The [NEt4]+ salt 3a of the [arachno-6-CB9H13-6-(COOH)]− anion 3 precipitated as a white solid that was filtered off and dried in vacuo (2.68 g, 8.73 mmol, 53 %). NMR, (CD3)2CO, 294−299 K, ordered as assignment δ(11B)/ppm [δ(1H)/ppm]: BH(4) –0.6 [+2.35], BH(2) –9.4 [+2.02], BH(5,7) –9.4 [+2.43], BH(9) -20.0 [+1.14], BH(8,10) –27.9 [+0.89] and BH(1,3) –39.1 [+0.46], with δ(1H) for µH(7,8)/(5,10) at −3.61 and for CH(6) at –0.75 ppm; additionally δ(1H)(COOH) +10.14 (broad), δ(1H)(Et) at +3.45 (8H, quartet) and +1.38 (12H, triplet) ppm, with δ(13C)(COOH) at +172.4, δ(13C)(cluster) at + 17.0 and δ(13C)(Et) at +7.0 and +51.9 ppm. (d) Anion 4. A solution of  3a (2.68 g, 8.73 mmol) in aqueous HCl (10 %, 100 ml) was extracted with Et2O (3 × 50 ml), the organic layers separated and combined, then aqueous  KOH (ca. 1.5 M; 300 ml) was added to these, and the Et2O evaporated in vacuo. Elemental I2 (6.0 g, 24 mmol) was added to the resulting alkaline aqueous solution, which was then stirred for 3 h at ca. 20 °C, cooled to 0 °C, and then Na2SO3 (2.92 g, 23 mmol), and [NEt4]+Br− (2.10 g, 10 mmol) were added, followed by aqueous HCl (5 %, 200 ml) dropwise. After brief stirring, the white microcrystalline precipitate of the [NEt4]+ salt 4a of the [closo-2-CB9H9-2-(COOH)]− anion 4 was filtered off, washed with H2O (50 ml) and dried in vacuo  (1.31 g, 8.03 mmol, 92 %). NMR,  (CD3)2CO, 294-299 K, ordered as assignment δ(11B)/ppm [δ(1H)/ppm]: BH(10) +0.81 [+3.76], BH(1) –5.2 [+3.53], BH(4) –21.7 [+1.20], BH(6,7) −26.7 [+0.92], BH(3,5) −29.2 [+1.49], BH(7,8) –29.7 [+0.51]; additionally δ(1H)(COOH) +10.52 (broad), δ(1H)(Et) at +3.52 (8H, quartet) and +1.40 ppm (12H, triplet), with δ(13C)(COOH) at +173.6, (13C)(cluster) at +45.8 and (13C)(Et) at +7.1 and +52.4 ppm. 13. Crystallographic data. The [NEt4]3[CB9H9(C6H4COOH)]2Br double salt 2a of anion 2: C40H88B18BrN3O4: M = 949.62, monoclinic (colourless prism from Et2O-acetone), Space Group C2/c, a = 23.2400(4), b = 20.6240(4), c = 11.8850(2) Å, β = 103.3860(10)°, U = 5541.74(17) Å3, Dcalc = 1.138 Mg m-3, Z = 4, Mo-Kα, λ = 0.71073 Å, µ = 0.781 mm−1, T = 150(2) K, R1 = 0.065 for 4834 reflections with I > 2σ(I), and wR2 = 0.1722 for all 6354 unique reflections; CCDC 211907. The [NEt4]+ salt 4a of anion 4: C20H60B18N2O4: M = 587.28, monoclinic (colourless prism from Et2O-acetone), Space Group C2/c, a = 30.0062(3), b = 15.1936(2), c = 16.7407(2) Å, β = 112.0290(10)°, U = 7074.93(14) Å3, Dcalc = 1.106 Mg m-3, Z = 8, Mo-Kα, λ = 0.71073 Å, µ = 0.063 mm−1, T = 150(2) K, R1 = 0.0726 for 5668 reflections with I > 2σ(I), and wR2 = 0.2306 for all 6957 unique reflections; CCDC 211908. Methods and programs were standard (Z. Otwinowski, W. Minor, Methods Enzymol.  1997, 276, 307; COLLECT, Data Collection Strategy Program, Nonius,1999.  Graphical Abstract  B10H14 with para-(OHC)C6H4(COOH) or OHCCOOH in aqueous KOH gives the [nido-6-CB9H11-6-(C6H4-para-COOH)]− anion or the [arachno-6-CB9H13-6-(COOH)]− anion,  respectively; oxidation with I2 thence gives the [closo-2-CB9H9-2-(C6H4-para-COOH)] − anion or the [closo-2-CB9H9-2-(COOH)]−  anion, respectively.                     

Polyhedral monocarbaborane chemistry. Carboxylic acid derivatives of the [closo-2-CB9H10](-) anion

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Polyhedral monocarbaborane chemistry. Carboxylic acid derivatives of the [closo-2-CB9H10](-) anion

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