Ammonia solutions of alkali metals

One of the most interesting processes in electrically initiated polymerization was an initiation with the solvated electron proposed by Laurin and Parravano (22), who studied electro-anionic polymerization of 4-vinylpyridine in liquid ammonia solution of alkali metal salts in the temperature range — 33 to — 78° C. Rapid and efficient polymerization occurred and conversions of monomer to polymers formed exclusively at the cathode in the form of an orange-red, porous, solid deposit, suggesting the formation of a pile of living polymers. 

Explosive salts such as NaNO can be prepared by the reaction of NO and liquid ammonia solutions of alkali metals. The unstable free arid, HNO, is thought to be an intermediate in many redox reactions of nitrogen compounds. 

Although alkali metal/liquid ammonia reductions (Birch reductions) of simple alkenes is difficult, presumably as a result of the very high energy of an ethene type LUMO, the corresponding reduction of non-terminal alkynes to trawi -alkenes is an efficient and useful synthetic tool for accessing trans-alkenes [116]. The mechanism for this reaction (Scheme 69), involves the homogeneous reduction of the alkyne to the corresponding anion radical by the solvated electrons present in liquid ammonia solutions of alkali metals. 

Ten or so years later, in the 1930s, Edward Zintl conducted a series of more systematic studies of these systems. He carried out potentiometric titrations of liquid ammonia solutions of alkali metals with various p metal salts, typically halides. Thus, the titration of a sodium solution with lead(II) iodide revealed that the green anionic species in solution are Pbg. Zintl and coworkers also discovered that... 

Identification of a species with a negative elementary charge in ammonia solutions of alkali metals, by conductivity measurements. C. Kraus"... 

Reduction of a variety of organic functional groups has long been carried out using ammonia solutions of alkali metals (Birch and SubbaRao, 1972). Given the strongly electropositive character of lanthanides such as ytterbium (which features a 4f 6s electron configuration), it follows that ytterbium/ammonia solutions should convert a,i8-unsatiu ated ketones to saturated ketones, alkynes to trans-alkenes and aromatics to 1,4-dihydroaromatics (White and Larson, 1978). 

Solutions of alkali metals in liquid ammonia are used in organic chemistry as reducing agents. The deep blue solutions effectively contain solvated electrons (p. 126), for example... 

The chemical resistance of PTFE is almost universal It resists attack by aqua regia, hot fummg nitnc acid, hot caustic, chlorine, chlorosulfonic acid, and all solvents. Despite this broad chemical resistance, PTFE is attacked by molten alkali metals, ammonia solutions of such metals, chlorine trifluoride, and gaseous fluonne at elevated temperature and pressure PTFE swells or dissolves m certam highly fluonnated oils near its melting point. Specific lists of chemicals compatible with PTFE are available [/.8]... 

Solutions of alkali metals in liquid ammonia have been developed as versatile reducing agents which effect reactions with organic compounds that are otherwise difficult or impossible/ Aromatic systems are reduced smoothly to cyclic mono- or di-olefins and alkynes are reduced stereospecifically to frani-alkenes (in contrast to Pd/H2 which gives cA-alkenes). 

These compounds are in many ways analogous to the solutions of alkali metals in liquid ammonia (p. 77). 

Solution of alkali metals in liquid ammonia, containing the so-called solvating electrons, may be used as an alternative homogeneous system to initiate polymerization by an electron transfer process. This system suffers, however, from complications resulting from proton transfer from ammonia leading to the formation of NH2- ions, which in turn initiate further polymerization.4... 

Solutions of alkali metals in liquid ammonia have been studied by many techniques. These include electrical conductivity, magnetic susceptibility, nuclear magnetic resonance (NMR), volume expansion, spectroscopy (visible and infrared), and other techniques. The data obtained indicate that the metals dissolve with ionization and that the metal ion and electron are solvated. Several simultaneous equilibria have been postulated to explain the unique properties of the solutions. These are generally represented as follows ... 

Reduction of a chemical species involves the gain of electrons by that species. Because the solutions of alkali metals in liquid ammonia contain free electrons, they are extremely strong reducing agents. This fact has been exploited in a large number of reactions. For example, oxygen can be converted to superoxide or peroxide ions. 

The dissolution of sulfur in ammonia has been known for more than 100 years [17]. The identification of the chemical species in these solutions was a matter of confusion until the identification of S4N and 83 , by Chivers and Lau [18] and Bernard et al. [19], using Raman spectroscopy. When considering the species formed in the dissolution process, it is quite remarkable that this dissolution is reversible sulfur is recovered after evaporation of ammonia. These solutions are strongly colored (blue), mainly due to the electronic absorption band of S4N at 580 nm. It must be mentioned that this dissolution is moderately fast at room temperature (but much slower than the dissolution of alkali metals) and that the rate is much slower when temperature decreases. It should also be mentioned that concentrated solutions of sulfur in liquid ammonia can be used as the solution at the positive electrode of a secondary battery. The solution at the negative electrode can be a solution of alkali metal in liquid ammonia [20], the electrodes being... 

The M-NM transition has been a topic of interest from the days of Sir Humphry Davy when sodium and potassium were discovered till then only high-density elements such as Au, Ag and Cu with lustre and other related properties were known to be metallic. A variety of materials exhibit a transition from the nonmetallic to the metallic state because of a change in crystal structure, composition, temperature or pressure. While the majority of elements in nature are metallic, some of the elements which are ordinarily nonmetals become metallic on application of pressure or on melting accordingly, silicon is metallic in the liquid state and nonmetallic in the solid state. Metals such as Cs and Hg become nonmetallic when expanded to low densities at high temperatures. Solutions of alkali metals in liquid ammonia become metallic when the concentration of the alkali metal is sufficiently high. Alkali metal tungsten bronzes... 

Further evidence is given in Section 5.5 for solutions of alkali metals in ammonia. 

Experimental evidence in support of this explanation is the fact that lithium added to a solution of lithium iodide in ethylenediamine dissolves without imparting a blue color to the solution—i.e., reacts immediately to give the amide. By contrast, lithium added to a solution of lithium chloride in ethylenediamine dissolves and imparts a deep blue color to the solution. The catalytic effect of iodide anion may be related to the effect of iodide anion on the electron spin resonance (ESR) absorption of solutions of alkali metals in liquid ammonia. Catterall and Symons (2) observed a drastic change in the presence of alkali iodides but very little change in the presence of alkali bromides or chlorides. They attributed this change to interaction of the solvated electron with the 6 p level of the iodide anion. 

The very dilute solutions of alkali metals in ammonia thus come close io presenting the chemist with the hypothetical ultimate base, the free electron (Chapter 9). As might be expected, such solutions are metaslable, and when catalyzed, the electron is leveled to the amide ion ... 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

A variety of strong reducing agents, including solutions of alkali metals in liquid ammonia, sodium solubilized by crown ethers or cryptands in tetrahydrofuran (THF), and alkali metal naphthalenides in THF, have been found to reduce M2(CO)10 and/or [M(CO)5] (M = Mn and Re) to the respective [M(CO)4]3- however, [Re(CO)5] has often been observed to be... 

Nature of Electronide and Spinide Solutions of Alkali Metals in Liquid Ammonia and Similar Solvents... 

Solvated electrons in ammonia are formed in equilibrium with metal ions dissolved in this medium (76). Analogous behavior was reported for ethylenediamine (42). On mixing ethylenediamine solutions of alkali metals with water, hydrated electrons were claimed to he formed as transients (43). 

Solutions of alkali metals in liquid ammonia at all concentrations, with the exception of cesium, are less dense than either of the constituents. This behavior for metal ammonia solutions is unique in that the expansion in volume is much larger than that shown on forming solutions of normal electrolytes or non-electrolytes. 

Although NH2 ion initiates polymerization of styrene it is known (17) that this monomer, as well as some other compounds containing C=C double bonds, are rapidly reduced by a solution of alkali metals in liquid ammonia. Undoubtedly, the first step of such a reduction is represented by the equation... 

Fig. 8. Classification of ESR spectra (—296 K) of solutions of alkali metals in a variety of nonaqueous solvents rM is the electron-cation encounter lifetime, and A is the metal hyperfine coupling constant, in hertz THF = tetrahydrofuran, EA = ethylamine, DG = diglyme, MA = methylamine (—220 K), 1,2PDA = 1,2-propanediamine, EDA = ethylenediamine, AM = ammonia (—240 K).

After Abe s work the problem again lay dormant for a number of years until it was taken up by Wilmarth and his co-workers. Claeys, Baes, and Wilmarth (29) in 1948 reported that a liquid ammonia solution of potassium metal rapidly catalyzed o-p H2 conversion, a half-time in solution of 37 sec. being obtained at —53°. In order to establish that this result was due to dissolved metal and not to amide ion impurity, Claeys, Dayton, and Wilmarth (30) studied the o-p H2 conversion in the presence of potassium amide in liquid ammonia. Rates were obtained comparable with those occurring with the metal solution. The mechanism of the conversion was different for the two cases, however, since the amide solution also catalyzed exchange between gaseous deuterium and liquid ammonia, while the metal solution did not. It was assumed that the metal acted by a paramagnetic mechanism and the amide ion by a chemical mechanism. In the same note Claeys, Dayton, and Wilmarth (30) reported confirmation of Wirtz and Bonhoeffer s results on the aqueous alkali system and questioned the validity of Abe s objections. 

Intercalation of alkali-metal atoms into zirconium disulphide has been reviewed.254 Only alternate layers of vacant sites are occupied by alkali-metal atoms when ZrS2 is treated with a dilute solution of alkali metal in liquid ammonia.255 At higher con-... 

In the S l mechanism of aromatic substitution the initiating step is the formation of a radical anion. In order to distinguish the process from the route described above (SR+N1) in which a radical cation plays a crucial role, the symbol S l has been used17. Creation of the radical anion can occur by several procedures. The reaction can be electrochemically initiated, a solvated electron in a solution of alkali metal in liquid ammonia may be involved or a radical anion may be used as the source of electrons. The most common source of electrons is, however, the nucleophile itself involved in the substitution reaction. In many cases the electron transfer from nucleophile to substrate is light-catalysed and the process is then sometimes referred to as S l Ar. Although the nucleofugic group in S l... 

To a considerable extent, solutions of alkali metals in solvents such as methylamine or ethylenediamine exhibit similar properties to the ammonia solutions. Some of the alkali metals also dissolve in ethers and tetrahydrofuran, but that chemistry will not be described here. 

---