Electron Solvation and the Unique Liquid Structure of a Mixed‐Amine Expanded Metal: The Saturated Li–NH3–MeNH2 System

Abstract Metal–amine solutions provide a unique arena in which to study electrons in solution, and to tune the electron density from the extremes of electrolytic through to true metallic behavior. The existence and structure of a new class of concentrated metal‐amine liquid, Li–NH3–MeNH2, is presented in which the mixed solvent produces a novel type of electron solvation and delocalization that is fundamentally different from either of the constituent systems. NMR, ESR, and neutron diffraction allow the environment of the solvated electron and liquid structure to be precisely interrogated. Unexpectedly it was found that the solution is truly homogeneous and metallic. Equally surprising was the observation of strong longer‐range order in this mixed solvent system. This is despite the heterogeneity of the cation solvation, and it is concluded that the solvated electron itself acts as a structural template. This is a quite remarkable observation, given that the liquid is metallic.

Alkali metals demonstrate an exceptional solubility in NH 3 , yielding intensely colored conducting solutions that have fascinated chemists since the time of Sir Humphry Davy. [1,2] Va rying the concentration of metal in these liquids dramatically alters the electronic,magnetic,and structural properties of the solutions,a nd enables us to experimentally determine the manner in which liquid systems accommodate excess electron density.A talow concentration of metal/electrons, the solutions are electrolytic, whereby the metal valence electrons have been ionized into solution and exist as solvated electrons propagating between solvent cavities. [3] Increasing the concentration results in metallization in the liquid phase, which for the Li À NH 3 system occurs at amere 4mol %metal (MPM). [2] Interestingly at lower temperatures,b elow T C = 210 K, the point of the Mott-type metal-insulator transition (MIT) is obscured by ap ronounced liquid-liquid phase separation. [2] This illustrates that the localized and delocalized electron states do not readily co-exist, which is dramatically manifested by the fact that the more concentrated metallic solution floats above the dilute electrolytic phase for T < T c . [2,4] Above 8MPM the solutions do not exhibit phase separation, appearing golden up to the concentration limit of 20 MPM, [5] thee xpanded metal Li(NH 3 ) 4 . [6,7] Thec oncentration and temperature dependence of this liquid-liquid phase separation has been mapped through multi-element NMR spectroscopy. [8] Along with metal concentration, chemical tunability of the electronic properties of these systems can be achieved by varying the amine.L ithium will also dissolve in MeNH 2 to yield as ystem whereby solvated electrons transition to am etallic state. [3,[9][10][11][12] TheM IT in Li-MeNH 2 occurs at 15 MPM, an otably higher concentration than in Li-NH 3 . No liquid-liquid phase separation has been detected across the full concentration range of Li-MeNH 2 ,a nd the solution remains ad eep blue,a lbeit with am etallic luster. The conductivity of liquid Li(MeNH 2 ) 4 is 400 W À1 cm À1 (compared to 15 000 W À1 cm À1 in Li(NH 3 ) 4 ), which is close to Motts minimum conductivity limit, demonstrating that the Li-MeNH 2 system lies just on the metallic side of the MIT.
Thed iffering metallic properties of Li-NH 3 and Li-MeNH 2 are also reflected in their distinct liquid structures. [13,14] All metallic metal amines share the trait of being highly structured liquids, [13][14][15][16] with distinct M (solv) -M (solv) correlations.I nb oth Li-NH 3 and Li-MeNH 2 solutions,L ii s found to be four-coordinate,w hich has subsequently been found to be the case in the gas phase, [17][18][19] and through computational investigation. [1,[19][20][21] Thevolumetric expansion of these liquid metals across their insulator-metal transition is also accompanied by the appearance of void spaces,which is presumably linked to the conduction electron density.I nLi-NH 3 these voids take the form of channels between Li(NH 3 ) 4 units, [13] whereas in Li(MeNH 2 ) 4 the voids are spatially isolated. [14] This is concordant with magnetic measurements that suggest electronic conduction in Li(MeNH 2 ) 4 is via rapid migration of electrons between these polaronic voids,w hich begin to localize in the solid state. [22,23] Theq uestion then is how am ixture of Li(NH 3 ) 4 and Li(MeNH 2 ) 4 will behave.S urprisingly,p erhaps,t hese mixed amine solutions have only previously been studied by optical absorption towards the limit of infinite dilution. [24] Herein we report the first studies of the liquid structure of Li-NH 3 -MeNH 2 in the concentrated regime at 20 MPM, an empirical stoichiometry of Li(NH 3 ) 2 (MeNH 2 ) 2 .W en ote that after equilibration these solutions appear to the eye as homogenous red/bronze liquids (Figure 1), which upon dilution demonstrate ap ronounced liquid-liquid phase separation whereby the lustrous phase floats above adeep-blue solution. We have observed this phase separation across av ery large concentration range of Li, from 2-19 MPM. As confirmation of the homogeneity at 20 MPM, Figure 1s hows that only as ingle feature is witnessed in the corresponding 7 Li NMR spectrum. This has proven to be an extremely sensitive indication of homogeneity in previous metal-amine studies. [8] Interestingly,t he 7 Li peak appears at higher ppm than both Li(NH 3 ) 4 and Li(MeNH 2 ) 4 , [8,12,25] suggesting ahigher conduction electron spin-density at the Li nucleus (or indeed larger molar spin susceptibility) in the mixed amine than in either of the parent amines.T he temperature dependence of the 7 Li Knight shift [8] and the Dysonian lineshape of the ESR are also indicative of ah omogeneous metallic system. [22,26,27] Neutron diffraction is au niquely powerful method for determining the liquid structure of metal amines,allowing for an umber of isotopically distinct samples to be measured (defined in the Experimental Section below). Figure 2 presents an example fit to an experimental total structure factor, F(Q), from samples Aand HinT able 1. Also shown is the difference spectrum [F6 Li (Q)ÀFnat Li (Q)] and its Fourier transform, the Li-centered total pair correlation function, DG Li (r). Thes ingle principal peak at 1.75 À1 ,a nd importantly the existence of asharp pre-peak at 0.97 À1 ,attest to the homogeneity of the 20 MPM solution. Thep re-peak is astructural feature witnessed in solvated-electron systems as they transition to the metallic state,and arises owing to strong intermediate range ordering in the liquid. [13][14][15] Integration of the principal peak in the DG Li (r)d istribution corresponds to the coordination number of Li, and is consistent with approximately 4s olvent molecules,a sr eported previously for other Li-RNH 2 systems. [13,14]    To determine the spatial arrangement of lithium and amine species in the system, the empirical potential structure refinement (EPSR) technique was invoked to produce as tructural model that was refined simultaneously to all the isotopically unique experimental F(Q)functions (Table 1). [28] Figure 3s hows the EPSR fits to the experimental data, with the EPSR model fitting well the experimentally obtained partial structure factors.T he corresponding real-space distribution of solvent molecules around the lithium ions is given in Figure 4. TheL i À Nd istance for both NH 3 and MeNH 2 is about 2.0 ,i nc lose agreement with that observed for both Li(NH 3 ) 4 and Li(MeNH 2 ) 4 .I ntegration of this first solvation shell gives an average co-ordination number of 2.01 and 1.78 for Li-MeNH 2 and Li-NH 3 ,r espectively.T he orientational distribution of the coordinated solvent about Li can be extracted from the EPSR model (Figure 4a-d). Only asmall distortion from tetrahedral geometry for agiven Li (solv) unit is found, with the MeNH 2 ligands acting to compress the tetrahedra slightly along the tetrahedral C 2 axis.T he dipole moment of NH 3 is orientated directly towards the cation, whereas the Li-MeNH 2 angle is about 1098 8.T he relative orientation of neighboring Li (solv) units is not isotropic,w ith the vertex (that is,t he NH 3 /MeNH 2 molecules) of one tetrahedral unit approaching the face and edges of the adjacent tetrahedral unit. Importantly,t he EPSR model also reveals the statistical distribution of ammonia and methylamine molecules in the [Li(NH 3 ) n (MeNH 2 ) m ] + tetrahedral complexes (Supporting Information, Figure S2). This distribution shows that for around 40 %o fc ations (n,m)i s( 2,2), for around 50 %i ti s (1,3) or (3,1), and for the remaining 10 %i ti s( 0,4) or (4,0). Thecation solvation is therefore significantly heterogeneous, and it is all the more surprising that we observe as harp diffraction pre-peak in our data, indicative of longer-range order in the liquid. In the absence of as ingle cationic motif, we must conclude that the solvated electron itself acts as as tructural template that drives homogenization. This hypothesis is consistent our observation of as ingle 7 Li Knight shift (Figure 1), and is truly remarkable given that the system is metallic.
Themanner in which excess electrons are accommodated can be determined by examining the liquid structures for void regions:s pherical volumes with circa 2.5 radii containing no atoms.P revious studies have demonstrated the existence of these regions in each of the metallic metal amines,w ith stark differences in their size and orientation, which correlate with their differing metallic properties.I nt he highly conducting Li-NH 3 liquid these voids are centered above the protons of the NH 3 ,f orming channels between solvated Li(NH 3 ) 4 units (hence the moniker expanded metal). In the poor metal, Li-MeNH 2 ,t he cavities are spatially isolated from one another,r emoved from the primary solvation environment of Li and bounded by the methyl groups of coordinated MeNH 2 .T he void-void distribution function in our mixed amine solution is presented in the Supporting Information, Figure S3, and shows weak inter-void correlations beyond around 4 . Figure 5s hows the orientation of void regions relative to an NH 3 and MeNH 2 molecule in the 20 MPM Li-NH 2 -MeNH 2 system. It is seen that the voids (and conduction electron density) lie around the circumference of NH 3 and above the amine group of MeNH 2 .T his structure is distinctly different to either the Li-NH 3 or Li-MeNH 2 system, and indicates that the electron density is drawn closer to Li, around the circumference of the solvating NH 3 /MeNH 2 molecules.
In conclusion, we have discovered that the mixed amine system Li(NH 3 ) 2 (MeNH 2 ) 2 forms ah ighly structured homogenous liquid metal. Each cation is tetrahedrally coordinated by approximately four solvent molecules,b ut there is significant heterogeneity in the NH 3 :MeNH 2 ratio around   individual Li + ions.T his lack of asingle cationic motif makes our observation of strong intermediate-range order in the solutions particularly surprising.E ven though the system is metallic,w et herefore turn to the solvated electron as ap otential structural template.I ndeed, we find that the system appears unique amongst current expanded metalamine solutions for the manner in which the excess electron density is accommodated. This is drawn closer to the lithium cations than in either the constituent lithium-ammonia or lithium-methylamine solutions,and we observe only asingle Knight shift on the lithium nuclei. Mixed-solvent metalamine solutions therefore present anew set of challenges for our understanding of electron solvation, and the nature of the concomitant phase separation of the localized and delocalized electronic states.

Experimental Section
H/D and 7 Li/ nat Li isotopic substitution experiments (where nat denotes natural abundance)w ere performed using the SANDALS diffractometer at the ISIS Spallation Neutrona nd Muon Source, UK. [29] Ac lean sample of Li metal was loaded under argon into as ealed flat-plate null-coherent scattering Ti/Zr container,w ith sample and wall thicknesses of 1mm. Thecontainer and sample were attached to as tainlesss teel gas-rig and evacuated to 10 À5 mbar.T o prepare asample of 20 MPM Li-NH 3 -MeNH 2 ,anequimolar ratio of NH 3 :MeNH 2 were premixed to the exact volume required, and cryopumped onto the Li sample at 60 K. This avoided any problem in the different boiling temperatures of the amines.T he sample was then isolated and warmed to 240 K. Thesamples were monitored for Bragg reflections in the warming process to ensure that the solutions were fully homogenized. Data were collected for ap eriod of 8h,a nd spectra were corrected for background, multiple scattering and absorption following standard protocols for SANDALS data.
Themeasured total structure factor for agiven system comprising n chemical species can be expressed as in Eq. (1): where c i denotes the atomic fraction of species i and b i is the bound coherent neutron scattering length for that species. S ab (Q)i st he Faber-Zimanpartial structure factor,and Q the scattering vector.The detailed method for extraction the partial structure factor from isotopically varying F(Q)v alues is detailed elsewhere. F(Q)i st he Fourier transform of the total pair correlationf unction G(r). The specific isotopic substitutions for prepared samples are given in Table 1. Theempirical potential structure refinement (EPSR) method was used to model the measured neutron total scattering data. [28] A structural model comprising 2000 species (Li and molecular NH 3 , MeNH 2 )i ssimultaneously refined to each of the isotopically unique experimental F(Q)s pectra (samples A-H in Table 1), enabling arigorous constraint to the resultant structural model, and extraction of atom specific pair-correlationf unctions, G ab (r). Thei nter-atom potentials in EPSR are comprised of aL ennard-Jones potential and Coulombict erm, and is iterated throughoutt he simulation until asuitable convergence to experimental F(Q)spectra is reached.T he seed potentials used in this work were the same as used previously for metal-amine liquid structure studies. [12][13][14][15] NMR spectra were recorded on aB ruker AV IIIHD 400 MHz spectrometer, referenced to LiCl in D 2 O. ESR was performeda tXband frequencies on aBruker EMXmicro spectrometer. Samples for each were prepared in the manner outlined above,w ithin spectrosil quartz tubes that were subsequently sealed. All handling and transfer of samples was conducted under cryogenic conditions.