Anionic Amino Acid [closo-1-CB₉H₈-1-COO-10-NH₃]⁻ and Dinitrogen Acid [closo-1-CB₉H₈-1-COOH-10-N₂] as Key Precursors to Advanced Materials: Synthesis and Reactivity

Abstract

Amino acid [closo-1-CB(9)H(8)-1-COO-10-NH(3)](-) (4) was prepared by amination of iodo acid [closo-1-CB(9)H(8)-1-COOH-10-I](-) (1) with LiHMDS in a practical and reproducible manner. The apparent dissociation constants, pK(2) = 5.6 and pK(1) > 11, were measured for 4[NMe(4)] in 50% aq. EtOH. Diazotization of 4 with NO(+)PF(6)(-) under mildly basic conditions afforded stable dinitrogen acid [closo-1-CB(9)H(8)-1-COOH-10-N(2)] (5). Activation parameters (DeltaH(++) = 33.9 +/- 1.4 kcal mol(-1) and DeltaS(++) = 10 +/- 3.5 cal mol(-1) K(-1)) for thermolysis of its methyl ester [closo-1-CB(9)H(8)-1-COOMe-10-N(2)] (11) in PhCN were established, and the heterolysis of the B-N bond is believed to be the rate-determining step. Electrochemical analysis showed a partially reversible reduction process for 11 (E(1/2)(red) = -1.03 V) and 5(-) (E(1/2)(red) = -1.21 V), which are more cathodic than reduction of [closo-1-CB(9)H(9)-1-N(2)] (17). The dinitrogen acid 5 was reacted with pyridine and N,N-dimethylthioformamide, to form pyridine acid 6 and protected mercapto acid 7, respectively, through a boronium ylide intermediate 18. Compound 7 was converted to sulfonium acid 8. The molecular and crystal structures for 5 [C(2)H(9)B(9)N(2)O(2) monoclinic, P2(1)/n, a = 7.022(2) A, b = 11.389(4) A, c = 12.815(4) A, beta = 96.212(5) degrees ; V = 1018.8(6) A(3), Z = 4,], 6 [C(7)H(14)B(9)NO(2), monoclinic, P2(1)/n, a = 14.275(4) A, b = 12.184(3) A, c = 30.538(8) A, beta = 95.377(4) degrees ; V = 5288(3) A(3), Z = 16], and 8 [C(7)H(19)B(9)O(2)S, monoclinic, P2(1)/c, a = 15.988(5) A, b = 19.377(6) A, c = 9.655(3) A, beta = 98.348(5) degrees; V = 2959.4(16) A(3), Z = 8] were determined by X-ray crystallography and compared with results of density functional theory (DFT) and MP2 calculations. Electronic structures of 5, 6, and related species were elucidated with electronic spectroscopy and assessed computationally at the B3LYP/6-31G(d,p), MP2/6-31G(d,p), and ZINDO//MP2 levels of theory.

Synthesis and Characterization of 12-Pyridinium Derivatives of the [closo-1-CB11H12]− Anion

“…This structural assignment is supported by high-resolution mass spectrometry (HRMS) analysis of the hydrolyzed NMR sample. Similar adducts were observed in thermolysis of dinitrogen derivatives of [closo-1-CB 9 H 8 -1-COOMe-10-N 2 ], 12 [closo-1-CB 9 H 9 -1-N 2 ], 11 and [closo-B 10 H 8 -1,10-(N 2 ) 2 ] 19 in organic nitriles. If correct, derivative 14c would represent the first experimentally observed dinitrogen derivative of 12-vertex closo-borates.…”

“…Previous MP2/6-31+G(d,p)//MP2/6-31G(d,p) level calculations in 50% aqueous EtOH dielectric medium demonstrated 12,14 that the 12-ammonium group in the parent [closo-1-CB 11 H 11 -12-NH 3 ] (4b[H]) is more acidic than the 10-ammonium group in [closo-1-CB 9 H 9 -10-NH 3 ], whose carboxylic acid derivative [closo-1-CB 9 H 8 -1-COOH-10-NH 3 ] has been diazotized successfully in pyridine solutions giving an isolable stable dinitrogen acid [closo-1-CB 9 H 8 -1-COOH-10-N 2 ]. 12 This suggests that amine 4b[H] can also undergo partial deprotonation in pyridine solutions to the extent sufficient for effective diazotization and formation of transient dinitrogen derivative 14b.…”

“…14 The subsequent reaction of ylide 15b with 4-methoxypyridine is strongly exothermic (ΔH = −68.6 kcal mol −1 ), which is typical for boronium ylides of the 10-vertex analogues. 12 Further DFT calculations indicate that the stability of the dinitrogen species 14 depends on the nature of the antipodal (12) 3.186 (4) 3.243 (6) …”

“…We sought to improve the properties of materials derived from monocarbaborates A and B by focusing on structures IIA and IIB, in which the onium fragment is attached to the B(10) and B (12) positions of the anions, respectively. Pyridinium and sulfonium zwitterions IIA, prepared from relatively stable 10-dinitrogen derivatives of the [closo-1-CB 9 H 10 ] − anion (A), showed great promise.…”

“…The requisite amine 4b[NMe 4 ] was obtained in ∼50% yield by Pd-catalyzed amination 12,17 of iodo derivative [closo-1-CB 11 H 11 -12-I] − Cs + (6b[Cs]), which was prepared by iodination 18 of [closo-1-CB 11 H 12 ] − anion (B) with I 2 in AcOH (Scheme 2). The iodination reaction leads, however, to a mixture of two regioisomers, namely, the 12-and 7-iodo derivatives 6b and 7b, respectively, in ∼3:1 ratio.…”