Transcript My half-century with alkali metals From alkali metal anions to anionic electrons James L.

My half-century with alkali metals
From alkali metal anions to anionic electrons
James L. Dye, Department of Chemistry
Michigan State University
East Lansing, Michigan, USA
Alkali Metals, Period 1 of the Periodic Table
Alkali metals isolated by Sir Humphry Davy, 1807
100 year dogma: oxidation states only 0 and +1
Predictable? Boring? Nothing new?
A Half-Century with Alkali Metals
• Alkali metals dissolved in liquid ammonia,(1954- 63)
• Alkali metal anions in solutions in amines, (1961-74)
• Crystalline salts with alkali metal anions (Alkalides)
(1974-2000)
• Crystalline salts with electrons as anions (Electrides)
(1983-2002)
• Inorganic electrides in silica zeolites (2002-04)
• Alkali metals in silica gel (2003-Present)
Metal-Ammonia Solutions
• Species are solvated cations and solvated electrons.
• Properties are essentially independent of metal.
Normalized spectra of Na (open) and K (solid) in liquid ammonia
Douthit and Dye ,1960
In Amines the Spectra Depend on the Metal
After 7 years of confusion, the peaks were
assigned to alkali metal anions
Oxidation States, Na+,Na0, Na-?
• For a century, chemical dogma had insisted that
alkali metal compounds could only contain the +1
oxidation state.
• With the assignment by us and others in 1969-70 of
the metal-dependent optical peaks in amines and
ethers to alkali metal anions, the -1 oxidation state
was introduced.
• Low solubility limited the usefulness of solutions
that contained M+ and M- until the advent of cation
complexants.
Some Early Complexants for Alkali Cations
O
O
O
O
O
O
O
N
N
O
O
O
O
O
Cryptand(2.2.2)
18 crown 6
O
N
O
O
O
O
15 crown 5
N
N
N
N
N
Hexamethyl Hexacyclen
(HMHCY)
Recipe for Crystals with Alkali Metal Anions
+
M’ = Li-Cs, M = Na-Cs: Crystallize from concentrated solution
More than 40 such alkalides have now been made by us
and structurally characterized
A Sodide with Separated Anions
K+
Per methyl Aza
cryptand[2.2.2]
With K+
ALKALIDES
M+(HMHCY)Na-
Alkalides are not always just complexed cations and
isolated alkali metal anions
Contact ion-pair
Rb- ion-pairs and chains of anions
Sodium Hydride is Normally Na+H-
+
-
Here we have an Inverse Sodium Hydride!
Could We Crystallize a Salt with
Electrons as the Anions?
• We had crystalized M- salts from
M+(complexant) and M- in solution.
• Could we crystallize e- salts from
M+(complexant) and e- in solution?
• It took 12 years, but we finally succeeded!
• Would M+· e- be metallic?
Ion-Packing in Cs+(15C5)2e-
Just like the sodide with
Na- removed!
Schematic of Cavity-Channel Structure of
Cs+(15C5)2e-
Shape of the Void Spaces in Cs+(15C5)2e-
View Down the Channel
Side View
Modeling developed with T. F. Nagy,
D. Tomanek & S. D. Mahanti
Theoretical Electron Density Contour Surfaces
for Cs+(15C5)2e-
Singh, Krakauer, Haas & Pickett, Nature, 365, 39 (1993)
Channels and Cavities in Other Electrides
Cs+(18C6)2e-
K+(15C5)2e-
Li+(C211)e-
K+(C222)e-
Magnetic Susceptibility of Three Electrides
Solid lines are fits to 1D Heisenberg Chain Model
Conductivities of Electrides
(Defect dominated)
We worked 16 years to find a complexant that wouldn’t get “eaten”!
The First Electride that is Stable at Room Temperature
(2002)
e-
Na-
Na+(Tri-ring aza222)e-
Optical Spectrum of Film
Mikhail Redko
Ned Jackson
Could We Make Inorganic Electrides
Inspired by alkali metals in alumino-silicate zeolites (Kasai, 1965)
Many studies of alkali metals in zeolites over the years
Although called an electride,
here the electron is shared
By 4 sodium cations
A Pure SiO2 Zeolite (ITQ-4)
~7 Å Diam.
channels
Alkali metal addition and ionization
would provide one electron per cation
(an inorganic electride?)
Ichimura, Dye, Camblor and Villaescusa,
J. Am. Chem. Soc. 2002, 124, 1170-1171.
Proposed Cs+e- distribution in ITQ- 4, based
on the pair-distribution function
*Petkov, Billinge, Vogt,
Ichimura & Dye,
Phys Rev. Lett. (2002)
89 075502.
e- density
(schematic)
Cs+
Density functional calculation of Li et al*
(Cs has about 30% atomic character)
Cs+
Max. e- density
* Li, Z.; Yang, J.; Hou, J. G.; Zhu, Q.,
"Inorganic Electride: Theoretical Study
on Structural and Electronic Properties"
J. Am. Chem. Soc. 2003, 125, 6050-6051.
A New Inorganic Electride from Japan
[S.Matsuishi et al, Science 301,626 (2003)]
Thermally stable
Air stable
Low Work function
From semiconductor
to superconductor
Mayenite Electride Cage
[Ca24Al28O64](e-)4
1/3 of 2cages contain e4
Silica Gel as an Alkali Metal “Container”
(Developed with Michael Lefenfeld for SiGNa Chemistry, Inc.)
• Enormous capacity – 40 wt% metal is routine
• Liquid Na-K alloys absorbed at room temp.
• Heat treatment (Stage0 → Stage I) can often avoid
pyrophoric character
• Reducing power of parent metals retained
• Main questions: When Na0 is converted to Na+, where do the
electrons go? Why is this so temperature dependent?
SEM and TEM Images
Silica Gel
1.0 millimeter
Nanoscale structure has amorphous, connected
particles of 15 nm average size and similar-sized pores.
Bulk Synthesis of Na-K-SG
Pair Distribution Function for Stage I Na-SG
Fit of Na(s) structure
Simon Billinge
Mouath Shatnawi
Sodium metal nanocrystals are present in Stage I Na-SG
XRD Line of Na0 in Two Loadings of Na-SG-I
4
40%
25%
Average particle size (~10 nm) is independent of loading!
Na metal must migrate to fill some pores and empty others.
23Na
MAS-NMR of Stage I Na2K-SG
Effect of Loading (Na0 → Na+)
23Na
MAS-NMR of Stage I Na2K-SG
Effect of Silica Gel Pore Size
23Na
Static NMR of Na2K-SG-I
(Reversible Temperature Dependence)
Temp (C)
Na0/Na+
25
2.1
90
1.6
120
1.0
150
0.87
At 150 C we have 1.0
Na+ for every 2-4 Si!
How can surface sites
be so numerous?
Possible ‘Electride’ Model of Stage I Na-SG
Possible Addition of Electrons to Si
Expansion of coordination number of Si from 4 to 5
Potential Uses of M-SG Reagents
(For details see www.signachem.com)
1. M-SG(I)
Birch Reductions:
2. H2O
M-SG(I)
Wurtz Coupling:
Cl
M-SG(I)
Desulfurization:
S
CFCl3 + Na-SG(I) --> “C” + 3NaCl + NaF
H2 Production &
Solvent Drying:
Amine Detosylation:
2H2O + 2M-SG(I) --> 2MOH(SG) + H2
N
O S O
1. M-SG(I)
2. PhCOCl
N
C O
Possible Reactions as Temperature is Increased
Ring opening
1
Si Anion Formation
2
3
Si-Si Bond Formation
The ultimate products would be Na2SiO3 and Si
(With excess Na, NaSi would form).
Sodium Silicide – A Convenient High-Yield Hydrogen Source
4 Na + 4 Si
2 NaSi + 5 H2O
450 C
Na4Si4 (“NaSi”) (Stable in dry air)
Na2Si2O5 + 5 H2 (98 g H2 per kg NaSi)
Shows promise for portable fuel cells. This application is being
developed by SiGNa Chemistry, Inc.
Na4Si4 Structure and Unit Cell
Who actually did the work?
•
Early Work – Ahmed Ellaboudy, Margaret Faber, Judith Eglin, Mary Tinkham, Lauren Hill
•
Crystal Structures – Rui Huang, Donald Ward, Fred Tehan, Steven Dawes
•
Synthesis and Properties – Mike Wagner, Songzhan Huang, Kevin Moeggenborg, Deborah
Gilbert, Kerry Reidy, Erik Hendrickson, Andrew Ichimura, Qingshan Xie, Dick Phillips, Jineun
Kim, Andrezej Misiolek, Dae Ho Shin, Mikhail Redko, Partha Nandi, Daryl Wernette,
Stephanie Urbin, Kevin Cram, Andrea Alexander, Philip Bentley, Peter Lambert, Michael
Beach, Bryan Dunyak
•
Faculty Colleagues – Ned Jackson, Bill Pratt, Jim Harrison, Simon Billinge, Marc DeBacker,
•
NSF-Division of Materials Research, MSU Center for Fundamental Materials Research,
Dreyfus Foundation, SiGNa Chemistry, Inc.