Synthesis and isolation of crystalline alkali metal arene radical anions

Abstract

Certain embodiments are directed to a composition comprising a complex of the general formula [M(crown ether)(solvent).sub.n][arene.sup.], wherein M is an alkali metal and method of making the same.

Claims

1. A composition comprising a crystalline solid having a formula:
[M(crown ether)(solvent).sub.n][arene.sup.] wherein M is an alkali metal selected from lithium, sodium, or potassium; crown ether is selected from 18-crown-6 polyether, 15-crown-5 polyether, or 12-crown-4 polyether; the solvent is a polar, coordinating organic solvent; n is 1, 2, 3, 4, 5, or 6; and the arene radical is a biphenyl, naphthalene, anthracene, or perylene radical.

2. The composition of claim 1, wherein the crown ether is 18-crown-6 polyether.

3. The composition of claim 1, wherein the alkali metal is lithium.

4. The composition of claim 1, wherein the arene is naphthalene.

Description

DESCRIPTION OF THE DRAWINGS

(1) The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

(2) FIG. 1. Representative solid-state structures of (a) 3, (b) 4, (c) 8, and (d) 12.

DESCRIPTION

(3) Aromatic hydrocarbons, by virtue of their relatively low-lying * LUMO, can be readily reduced by alkali metals to give intensely colored, open-shell monoanions. The first well-defined arene radical, viz sodium anthracenide, was reported in 1914 by Wilhelm Schlenk during the formative years of organometallic chemistry..sup.1 Over a century later, the chemistry of arene radicals continues to command attention. From an electronic perspective, these species display unique aromatic character and have been shown to exhibit long-range magnetic ordering in the solid-state..sup.2 Chemically, arene radicals find utility in a wide range of applications including use as initiators in anionic polymerization,.sup.3 as potent organic reducing agents and bases,.sup.4 models for graphitic battery materials,.sup.5 and are key intermediates in Birch reductions..sup.6

(4) Given the radical nature of arene monoanions, it is well-known that these compounds are highly sensitive and prone to adventitious oxidation, thus requiring preparation and handling under rigorously anaerobic and anhydrous conditions. These reagents are kinetically unstable, having short storage times,.sup.4b and are often freshly prepared in-situ (necessitating multi-step titration to accurately determine concentration) with product equilibriums that are highly solvent dependent..sup.1c,7 Moreover, attempts to isolate them from solution often results in disproportionation..sup.7b It is not then surprising that only a handful of radical arene monoanions, utilizing a variety of metals and arenes, have been isolated and characterized in the solid-state..sup.2b-f,6,8

(5) These complicating factors negatively affect the utility of these reagents. In order to circumvent many of the aforementioned issues, a general procedure was developed for the solid-state isolation of arene radicals. Described herein is a synthetic method for solid-state structures using twelve arene radical monoanions as examples, as well the characterization of the same.

(6) Initially, the storage of concentrated THF solutions of M[arene.sup.] (M=Li, Na, K; arene=biphenyl, naphthalene, anthracene) at 25 C. produced large, crystalline blocks of the respective anion within hours. However, all attempts to isolate these crystals failed as the solids were observed to rapidly desolvate within seconds upon removal from solution, producing intractable gummy, oils.

(7) The inventors contemplated that desolvation effects could be mitigated by addition of a chelating base to sequester the alkali metal cations and protect their coordination sphere from solvent loss. Accordingly, treatment of M[arene.sup.] in THF with 1 equiv of 18-crown-6 and subsequent storage at 25 C. affords crystalline solids of [M(18-c-6)(THF).sub.n][arene.sup.] (eq 1) in all cases. Gratifyingly, upon removal from solution and drying under vacuum, the products retain their shape and form. While 18-crown-6 has been previously employed for the successful solid-state isolation of the potassium complexes [K(18-c-6)(THF).sub.2][C.sub.10H.sub.8] and [K(18-c-6)(THF).sub.2][C.sub.14H.sub.10],.sup.2b the inventors have found this common and relatively inexpensive reagent suitable for use with both lithium and sodium metals, thus avoiding the need for specialized and size-specific crown ethers in these reactions.

(8) ##STR00001##

(9) The number of ancillary solvent molecules in [M(18-c-6)(THF),][arene.sup.] cannot be readily quantified by NMR spectroscopy due to significant signal broadening, a consequence of the compounds' inherent paramagnetism. On the other hand, the high crystallinity of these compounds makes them very amenable to X-ray diffractometry, thus allowing for unambiguous composition determination (Table 1).

(10) TABLE-US-00001 TABLE 1 Isolated Arene Radical Monoanions and Electronic Properties. Compound % Yield E.sub.1/2 (V).sup.a (nm) .sub.eff (.sub.B).sup.b [Li(18-c-6)][C.sub.10H.sub.8] (1) 37 3.09 294, 318, 327, 376, 435, 468, 799 1.53 [Na(18-c-6)(DME)][C.sub.10H.sub.8] (2) 79 3.09 294, 327, 373, 442, 469, 798, 875 1.67 {[K(18-c-6)][:.sup.2-C.sub.10H.sub.8]}.sub. (3) 80 3.13 294, 326, 374, 445, 469, 771, 856 2.11 [Li(.sup.3-18-c-6)(THF).sub.2][C.sub.12H.sub.10] (4) 42 3.18 411, 643, 829 2.08 [Na(18-c-6)(THF).sub.2][C.sub.12H.sub.10] (5) 53 3.15 410, 451, 653, 837 2.24 [K(18-c-6)(THF).sub.2][C.sub.12H.sub.10] (6) 37 3.17 403, 649, 831 2.20 [Li(18-c-6)][C.sub.14H.sub.10] (7) 30 2.53 328, 348, 358, 369, 407, 550, 598, 1.67 641, 659, 696, 735, 759, 813, 921 [Na(18-c-6)(DME)][C.sub.14H.sub.10] (8) 87 2.48 329, 342, 353, 359, 368, 379, 407, 1.90 546, 595, 638, 659, 698, 728, 752, 813, 922 [K(18-c-6)(THF).sub.2][C.sub.14H.sub.10] (9).sup.c 31 2.49 328, 348, 358, 367, 407, 550, 598, 1.97 640, 661, 698, 733, 759, 813, 925 [Li(.sup.3-18-c-6)(DME)][C.sub.20H.sub.12]0.5C.sub.20H.sub.12 (10) 55 2.20, 2.87 322, 388, 411, 437, 579, 688, 740, 2.20 761, 782, 813, 847, 903, 1007 [Na(18-c-6)(DME)][C.sub.20H.sub.12] (11) 84 2.19, 2.77 323, 393, 414, 438, 465, 580, 692, 2.29 737, 782, 813, 849, 906, 1010 [K(18-c-6)(THF).sub.2][C.sub.20H.sub.12] (12) 38 2.28, 2.80 323, 393, 413, 438, 580, 689, 741, 2.10 757, 778, 812, 847, 903, 1008 .sup.aReferenced vs Fc.sup.0/+. .sup.bGuoy balance measurement. .sup.cKnown structure, see reference 2b.

(11) Crystals of [M(18-c-6)(THF).sub.n][arene.sup.] harvested from THF solutions are typically of satisfactory size and shape for X-ray crystallographic analyses. In a few instances, most often with the lithium and sodium salts of naphthalene and perylene, fine needles too small for crystallographic characterization are produced. However, recrystallization of these compounds from DME solutions does yield X-ray quality crystals..sup.9

(12) Examination of the solid-state structures of 1-12 (see FIG. 1) reveals that nearly all crystallize as non-interacting ion pairs with one notable exception. In 3, the [K(18-c-6)].sup.+ moiety is axially flanked by two bridging [C.sub.10H.sub.8].sup. anions forming a close contact network that gives rise to a 1D coordination polymer. Interestingly, each of the two bridging naphthalenes exhibits a distinct coordination mode. The first naphthalene ligates the potassium cations through .sup.2-binding where the two KC.sub.arene bond distances (avg. 3.13 ) and KC.sub.arene dihedral angle (120.30) are indicative of a typical -cation interaction. The second naphthalene engages each potassium through two longer KC.sub.arene bonds (avg. 3.45 ) with a notably more obtuse KC.sub.arene dihedral angle (150.3), parameters that are consistent with agostic interactions between potassium and the CH bonds of the naphthalene..sup.10It should be noted that the ion-separated analog [K(18-c-6)(THF).sub.2][C.sub.10H.sub.8] has been reported..sup.2b While the exact cause of this structural variation is not known, the difference is attributed to differing crystallization methods and conditions..sup.2b,9

(13) Yields of 1-12 fall within a wide range, from moderate to excellent (Table 1), with diminished yields most often a result of solution equilibrium effects or high solubility in THF..sup.1c,7b In contrast to standing solutions, 1-12 can be stored as solids under nitrogen for extended periods of time. When kept under strictly anhydrous and anaerobic conditions, it was have found that solid samples of 1-12 were unchanged after almost a year.

(14) The solution redox properties of each complex were examined by cyclic voltammetry (CV). In all cases, the compounds exhibit chemically reversible (i.sub.pc/i.sub.pa1) redox waves with E.sub.1/2 values in full agreement with known reduction potentials..sup.1c,4b,9 While it has been suggested that the identity of the alkali cation should have a detectable effect on the potential values,.sup.4b the inventors observe no systematic effects under their experimental conditions..sup.9 As anticipated, the reducing power of the arene radical monoanions follows the trend C.sub.20H.sub.12.sup.<C.sub.14H.sub.10.sup.<C.sub.10H.sub.8.sup.<C.sub.12H.sub.10.sup. (Table 1). It should be noted that while complexes 1-12 each have chemically accessible dianionic forms, we find only the perylene derivatives 10-12 display a second redox wave in their CV in THF at room temperature.

(15) The signature electronic absorption features of each arene.sup. type are seen in the UV-vis/NIR solution spectra of 1-12 (Table 1)..sup.1c,11 Between the complexes within a given arene.sup. class (e.g. 1 vs 2 and 3) the spectra are qualitatively similar; notably, though, the peak definitions and absorbance parameters are found to be cation dependent (without systematic trend). While these observations stand in contrast to the results found in the respective CV data, the electrochemical experiments are conducted in the presence of a vast excess of supporting electrolyte which may impede close M-arene.sup. pairing.

(16) The solid-state, room temperature magnetic susceptibilities of the open-shell compounds were measured (Gouy balance). The effective magnetic moments of 1-12 are unexceptional and found to range from 1.53 to 2.28.sub.B. These values are comparable to that found for [K.sub.2(THF)][C.sub.10H.sub.8] (1.69.sub.B per anion) and fall in line with the 1.7.sub.B calculated for an isolated S= system..sup.2c

(17) Following the protocol recently developed by Buchwald and co-workers for the sealing of air-sensitive palladium catalysts in paraffin as an oxygen and water exclusion barrier,.sup.12 it was found that the described arene radical monoanions can be sealed in paraffins, e.g. eicosane, and stored in air for at least several days without detectable degradation. By this method of encasing these arenides in paraffins, storage under aerobic and hydrous atmospheric conditions without specialized equipment becomes possible. Moreover, these paraffin mixtures can be used as easily handled delivery agents for chemical reactions and processes.

(18) The inventors have described a general and straightforward procedure for the solid-state isolation of arene radical monoanions using 18-crown-6 as a co-reagent. As proof of principle, the inventors have demonstrated through twelve examples that the methodology can be applied to a wide range of aromatic systems with varying counter cations to give highly crystalline, well-defined materials. These solids, as compared to their parent solutions, are remarkably stable, easily stored, and readily handledfurther enhancing the utility of these novel and important radical species.

REFERENCES

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