[closo-B10H8-10-PhI-1-COOH]− Anion: An Intermediate for Functional Anionic Carboxylate Ligands

A potentially general intermediate, [closo-B10H8-10-PhI-1-COOH]–, for a class of functional anionic carboxylic acids, [closo-B10H8-10-X-1-COOH]2–, was obtained in four steps and 26% overall yield from [closo-B10H10]2–. It was converted to the pyridinium derivative (X = C5H5N+) and subsequently to coordination complexes with (phen)2Cu2+ and (phen)2Zn2+ ions. Both the acid and Zn(II) complex exhibit a cage-to-pyridine charge-transfer band. The availability of such acids opens access to functional metal-ion complexes with compensated charges.

−19 For these reasons, access to new functional carboxylic acids and those with unusual properties remains of continued importance and interest.
Carboxylic acids derived from the [closo-B 10 H 10 ] 2− anion (A), 20−22 such as diacid 1a (Figure 1), 23,24 are most unusual: the high negative charge density on the {closo-B 10 } cluster makes the carboxyl group weakly acidic 24 and easy to protonate, 25 which presumably indicates a facile coordination of metal ions.The latter feature opens up the possibility of accessing metal complexes with functional carboxylic acids 1 possessing an additional cage-delocalized negative charge (a total of 2− or 3− for the carboxylate anion, depending on X).For instance, we have demonstrated intramolecular photoinduced charge-transfer (CT) behavior of pyridinium derivatives of A, 26 which suggests the possibility of photocontrolled basicity of the carboxyl group and metal binding properties if a properly functionalized carboxylic acid is used.Unfortunately, carboxylic acids of the general structure [closo-B 10 H 8 -10-X-1-COOH] 2− are limited to only three derivatives in addition to the dicarboxylic acid 1a: 10-cyano 1b, 24 10-(B 10 H 9 -1-CO) 1c, 27 and 10-carbonyl 1d. 25 Therefore, there is a need to develop access to a larger pool of carboxylic acids of structure 1 with a broader range of substituents X.Such acids are potentially attractive building blocks for functional materials and other derivatives through functional group transformations.
−30 Herein we describe the synthesis of acid The preparation of acid 1e[Et 4 N] relies on our recently discovered method for activation of the nitrile group toward hydrolysis by N-methylation. 25 It was determined that the nitrile 26 2[Et 4 N] readily reacts with CF 3 SO 3 Me in CH 2 Cl 2 , giving the nitrilium zwitterion 3 in 68% yield after recrystallization.Interestingly, an attempt to obtain the parent carboxylic acid (X = H) by methylation of mononitrile [closo-B 10 H 9 -1-CN] 2− failed and led to a complex mixture of products.The nitrilium 3 was relatively stable under acidic conditions but quickly hydrolyzed upon treatment with base, which eventually led to formation of the carboxyl group.
In a one-pot procedure, nitrile 2[Bu 4 N] was reacted with CF 3 SO 3 Me, and the resulting 3 was treated with aqueous  The structures of acid 1e[Et 4 N] and intermediate 3 were confirmed with single-crystal XRD analysis (Figure 2).The experimental B−I distances of 2.179(3) and 2.190(6) Å, respectively, are similar to those in the reported 29 [closo-B 10 H 8 -1,10-(PhI) 2 ] (avg.2.178 Å).The B−COOH bond length in 1e[Et 4 N] is 1.582(4) Å, which is comparable with 1.573(2) Å in 1d. 25 The C�NMe group in 3 is the most interesting: it is nearly linear [α CNMe = 177.9(6 The synthetic utility of acid 1e[Et 4 N] was demonstrated by replacing PhI with the pyridinium group and the formation of metal complexes.Thus, the reaction of 1e[Et 4 N] with pyridine at 85 °C gave the pyridinium derivative 1f[Et 4 N], which was isolated in 68−80% yield (Scheme 1).As in the case of acid 1e, dehydration of acid 1f under acidic conditions and formation of the corresponding carbonyl derivative were also observed.
The acid 1f[Et 4 N] was transformed to the double sodium salt 6[2Na] by passing it in MeCN through a Dowex ionexchange column and treatment of the eluate with NaOH (2 equiv).The subsequent addition of Cu(phen) 2 (NO 3 ) 2 •H 2 O 34 resulted in a precipitate, which was recrystallized from hot MeCN with a few drops of H 2 O, giving green copper(II) complex 6[Cu] in 84% yield (Scheme 1).Similarly, off-white zinc(II) complex 6[Zn] was obtained in 86% yield from 6[2Na] and Zn(phen) 2 (NO 3 ) 2 •2H 2 O. 35 Both complexes were characterized by solution NMR spectroscopy (6[Zn]), IR, mass spectrometry, and combustion analysis, demonstrating their expected composition and stoichiometry.IR spectroscopy revealed that the C�O peak of the acid 1f at v CO = 1633 cm −1 is absent in the salts 6[Cu] and 6[Zn] (see the SI), suggesting that both oxygen atoms of the COO group coordinate to the metal ion.Attempts at obtaining a single crystal suitable for XRD analysis were unsuccessful, and only morphologically homogeneous microcrystalline powders were obtained (see the SI for details).
Electronic absorption spectroscopy revealed a CT band at 330 nm tailing to 400 nm in the carboxylate dianion 6[2Na] in  an MeCN/H 2 O solution and a broad band tailing to 360 nm in solid Zn(phen) 2 (NO 3 ) 2 •2H 2 O (Figure 3).Analysis of the solid-state spectrum of complex 6[Zn] showed features present in the Zn precursor below 350 nm and a broad tailing absorption above 360 nm, presumably related to the cage-topyridine CT.
In summary, the carboxylic acid 1e[Et 4 N], a versatile intermediate to a potentially broad class of anionic carboxylic acids [closo-B 10 H 8 -10-X-1-COOH] 2− , was obtained in four steps and about 26% overall yield from the parent anion A. The potential of the PhI group as a convenient synthetic handle for the introduction of functional groups X was demonstrated by the preparation of photoactive pyridinium acid 1f[Et 4 N] and subsequently its complexes 6[M] with the (phen) 2 Cu 2+ and (phen) 2 Zn 2+ ions.The acid exhibits an intramolecular CT band also present in the solid-state 6[Zn] complex.The readily available intermediate 1e[Et 4 N], and the documented chemistry of the phenyliodonium zwitterions open up access to an unexplored class of functional and possibly polytopic zwitterions (e.g., X = N 3 , SCN, azines) anionic carboxylic acids with applications in metal-ion complexes with balanced charges.

■ ASSOCIATED CONTENT
[closo-B 10 H 8 -10-IPh-1-COOH] − (1e[Et 4 N]) containing the PhI leaving group in the B(10) position as a versatile intermediate for the preparation of B(10)-functionalized carboxylic acids 1.We demonstrate the utility of 1e[Et 4 N] by transformation to 10pyridinium acid 1f and the preparation of coordination complexes with Cu(II) and Zn(II).Synthetic work is supported with single-crystal X-ray diffraction (XRD) analysis and augmented with spectroscopic studies.

Figure 1 .
Figure 1.Structures of the parent anion A and carboxylic acids 1.Each unsubstituted green vertex corresponds to a B−H group.
NaOH and then HCl to complete the hydrolysis process, giving the desired acid 1e contaminated with the carbonyl derivative 4 (Scheme 1).The latter was formed by the dehydration of 1e under acidic conditions.To convert 4 to the acid 1e, the mixture was treated with NaHCO 3 to neutralize HCl, and products were extracted and passed through a Dowex ion-exchange column to remove the remaining [Bu 4 N] + counterion.Treatment of the eluate with controlled amounts of [Et 4 N] + OH − gave the acid 1e[Et 4 N] isolated in 54−59% overall yield.It is remarkable that the labile PhI + group was unaffected during the sequence of transformations of 2[Bu 4 N] to 1e[Et 4 N], which includes basic, acidic, and hydrolytic conditions.In contrast, the COOH group in 1e can be easily esterified.Thus, passing a MeOH solution of 1e[Et 4 N] through a Dowex column, followed by neutralization with controlled amounts of [Et 4 N] + OH − , gave ester 5[Et 4 N] in 88% yield (Scheme 1).Passing salt 1e[Et 4 N] through Dowex exchange resin, followed by evaporation to dryness, gave essentially pure carbonyl derivative 4 in 88% yield (Scheme 1).

Scheme 1 .
Scheme 1. Synthesis of Carboxylic Acid 1e and Its Derivatives a

Figure 2 .
Figure 2. Displacement ellipsoid diagrams for 1e[Bu 4 N] (left) and 3 (right) with pertinent geometrical dimensions.For other geometrical parameters, see the text and SI.Thermal ellipsoids are at the 50% probability level.The molecule of solvent and cation are omitted for clarity.Color codes: C, gray; B, green; O, red; I, purple.