Homoleptic tetrahydrometalate anions MH4-(M = Sc, Y, La). Matrix infrared spectra and DFT calculations

Article abstract of DOI:10.1021/ja020112l

Laser-ablated Sc, Y, and La atoms react with molecular hydrogen upon condensation in excess argon, neon, and deuterium to produce the metal dihydride molecules and dihydrogen complexes MH₂ and (H₂)MH₂. The homoleptic tetrahydrometalate anions ScH₄^-, YH₄^-, and LaH₄^- are formed by electron capture and identified by isotopic substitution (D₂, HD, and H₂ + D₂ mixtures). Doping with CCl₄ to serve as an electron trap virtually eliminates the anion bands, and further supports the anion identifications. The observed vibrational frequencies are in agreement with the results of density functional theory calculations, which predict electron affinities in the 2.8-2.4 eV range for the (H₂)ScH₂, (H₂)YH₂, and (H₂)LaH₂ complexes, and indicate high stability for the MH₄^- (M = Sc, La, Y) anions and suggest the promise of synthesis on a larger scale for use as reducing agents.

Full text of DOI:10.1021/ja020112l

Published on Web 05/31/2002
-
Homoleptic Tetrahydrometalate Anions MH4 (M ) Sc, Y, La).
Matrix Infrared Spectra and DFT Calculations
Xuefeng Wang and Lester Andrews*
Contribution from the Department of Chemistry, UniVersity of Virginia,
CharlottesVille, Virginia 22904-4319
Received January 22, 2002
Abstract: Laser-ablated Sc, Y, and La atoms react with molecular hydrogen upon condensation in excess
argon, neon, and deuterium to produce the metal dihydride molecules and dihydrogen complexes MH2
-
and (H2)MH2. The homoleptic tetrahydrometalate anions ScH4-, YH4-, and LaH4 are formed by electron
capture and identified by isotopic substitution (D2, HD, and H2 + D2 mixtures). Doping with CCl4 to serve
as an electron trap virtually eliminates the anion bands, and further supports the anion identifications. The
observed vibrational frequencies are in agreement with the results of density functional theory calculations,
which predict electron affinities in the 2.8-2.4 eV range for the (H2)ScH2, (H2)YH2, and (H2)LaH2 complexes,
and indicate high stability for the MH4- (M ) Sc, La, Y) anions and suggest the promise of synthesis on a
larger scale for use as reducing agents.
Introduction
Experimental and Theoretical Methods
The experimental methods for reacting laser-ablated transition metal
Homoleptic hydrometalate anions as anionic subunits are
found in ternary solid-state materials containing hydrogen and
transition, alkali or alkaline earth metals.1 These materials have
been intensively studied for many years because of efficient
storage of hydrogen for commercial vehicular applications and
hydrogenation in organic synthesis.2-4 In addition, yttrium and
lanthanum hydride films are known to undergo optical changes
with hydrogen loading and to have applications as optical
switches.5
atoms with H2 and identifying the reaction products with matrix infrared
spectra have been described previously,9-11 and the same methods were
used here for the reaction of Sc, Y, and La with H2. Laser-ablated metal
atoms were co-deposited with 3-5% H2 (or D2, HD) in excess neon
or argon and with pure deuterium onto a 3.5 K CsI window. Infrared
spectra were recorded after deposition, annealing, and UV-vis irradia-
tion. Several experiments were done with 0.1% CCl4 added to 3% H2
in neon or argon to serve as an electron trap,10,11 and the absorptions
due to anions were reduced markedly from the spectra of 3% H2
samples.
Recent laser-ablation matrix-isolation experiments with pal-
ladium gave Pd atoms, which interact with H2 to form the Pd-
(H2) complex; this complex captures an electron to give the
linear isolated PdH2- molecular anion.6 The analogous (HPdH)2-
dianions surrounded by alkali metal cations have been recently
characterized in solid materials.7 In addition late first-row
transition metal dihydride anions have been observed in the gas
phase.8 We wish to report matrix-isolation studies of Sc, Y,
and La + H2 systems, which produce dihydrogen complexes
(H2)xMH2 (x ) 1, 2) and stable MH4- anions. Characterization
of such species is relevant to understanding H2 storage, optical
switching devices, and hydrogenation in organic synthesis.
Density functional theory (DFT) frequency calculations were
performed to determine the structures and frequencies of new metal
hydrides using the Gaussian 98 program.12 The B3PW91 density
functional and 6-311++G(d,p) basis set and SDD pseudopotential13,14
(9) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697.
(10) (a) (Sc + O2): Chertihin, G. V.; Andrews, L.; Rosi, M.; Bauschlicher, C.
W., Jr. J. Phys. Chem. A 1997, 101, 9085. (b) (Sc + O2): Bauschlicher, C.
W., Jr.; Zhou, M. F.; Andrews, L.; Johnson, J. R. T.; Panas, I.; Snis, A.;
Roos, B. O. J. Phys. Chem. A 1999, 103, 5463. (c) (Y, La + O2): Andrews,
L.; Zhou, M. F.; Chertihin, G. V.; Bauschlicher, C. W., Jr. J. Phys. Chem.
A 1999, 103, 6525.
(11) (a) Zhou, M. F.; Andrews, L. J. Am. Chem. Soc. 1998, 120, 11499. (b)
Zhou, M. F.; Andrews, L. J. Am. Chem. Soc. 1999, 121, 9171.
(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6;
Gaussian, Inc.: Pittsburgh, PA, 1998.
* Corresponding author. E-mail: lsa@virginia.edu.
(1) King, R. B. Coord. Chem. ReV. 2000, 200-202, 813 and references therein.
(2) Bronger, W. Angew. Chem., Int. Ed. Engl. 1991, 30, 759.
(3) Firman, T. K.; Landis, C. R. J. Am. Chem. Soc. 1998, 120, 12650.
(4) Chen, J.; Kuriyama, N.; Xu, Q.; Takeshita, H. T.; Sakai, T. J. Phys. Chem.
B 2001, 105, 11214.
(5) Huiberts, J. N.; Griessen, R.; Rector, J. H.; Wijngaarden, R. J.; Dekker: J.
P.; de Groat, D. G.; Koeman, N. J. Nature 1996, 380, 231.
(6) (a) Andrews, L.; Wang, X.; Alikhani, M. E.; Manceron, L. J. Phys. Chem.
A 2001, 105, 3052. (b) Andrews, L.; Alikhani, M. E.; Manceron, L.; Wang,
X. J. Am. Chem. Soc. 2001, 122, 11011.
(13) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Perdew, J. P.; Wang,
Y. Phys. ReV. B 1992, 45, 13244.
(14) (a) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265. (b) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123.
(7) Olofsson-Martensson, M.; Haussermann, U.; Tomkinson, J.; Noreus, D. J.
Am. Chem. Soc. 2000, 122, 6960 and reference therein.
(8) Miller, A. E. S.; Feigerle, C. S.; Lineberger, W. C. J. Chem. Phys. 1986,
84, 4127.
9
7610
J. AM. CHEM. SOC. 2002, 124, 7610-7613
10.1021/ja020112l CCC: $22.00 © 2002 American Chemical Society

Homoleptic Tetrahydrometalate Anions MH - (M ) Sc, Y, La)
A R T I C L E S
4
-
Table 1. Infrared Absorptions (cm-1) for ScH4-, YH4-, and LaH4
in Excess Argon, Neon, and Deuterium at 3.5 K
argon
neon
deuterium
H
2
D
2
HD
H
2
D
2
HD
D
2
-
ScH4
1377.0
1302.7 1321.3
980.2
949.8
523.5
494.2
1291.4
1225.4 1233.3
915.7
1387.3
1318.8
992.3
961.5
526.6
497.1
1295.9
1232.4
890.8
920.5
1305.1
527.1
946.2
882.3
797.8
961.1
887.9
788.7
953.4
884.4
783.2
527.6
-
YH4
1227.3
482.7
884.5
492.9
454.1
482.4
-
LaH4
1183.1
1158.5
1096.3
825.0
1114.1
1112.4 1101.8
833.5
Figure 1. Infrared spectra in 1590-860 cm-1 region for laser-ablated
yttrium co-deposited with argon/hydrogen samples: (a) 5% H2 in argon
deposited at 3.5 K for 60 min, (b) after annealing to 16 K, (c) after λ >
240 nm photolysis for 15min, (d) 5% HD in argon deposited at 3.5 K for
70 min, (e) after annealing to 16 K, (f) 5% D2 in argon deposited at 3.5 K
for 60 min, and (g) after annealing to 16 K.
799.8
788.8
the 1227.3 and 482.7 cm-1 bands by 30%. With D2/Ar the upper
band shifts to 882.3 cm-1 and the lower band shifts below our
measurable region. The HD/Ar experiments gave three dou-
blets: at 1291.4 and 1225.4 cm-1, 915.7 and 884.5 cm-1 in the
upper region and at 492.9 and 454.1 cm-1 in the lower region.
The H2 + D2 experiments produced all of the above product
bands. In solid neon similar bands were observed at 1233.3,
482.4 cm-1 with H2, 887.9 cm-1 with D2, and doublets at 1295.9
and 1232.4 cm-1, 920.5 and 890.8 cm-1 with HD. A stronger
D2 counterpart band at 884.4 cm-1 was observed in pure solid
deuterium. The observed frequencies are collected in Table 1.
In addition, doping with CCl4 eliminated the 1227.3 and 482.7
cm-1 bands in argon and the 1233.3 and 482.4 cm-1 bands in
neon, which strongly suggests an anion identification.10,11 The
absorptions due to YH (1470.4 cm-1), YH2 (1578.1, 1542.5
+
cm-1), YH2 (1459.8, 1397.8 cm-1), YH3 (1385.1 cm-1), and
(H2)xYH2 complexes (1363, 1337, 1309 cm-1) will be discussed
in detail in a later paper.
Figure 2. Infrared spectra in the 1500-1060 and 580-430 cm-1 regions
for group 3 metal atom-hydrogen reaction products: (a) laser-ablated
scandium co-deposited with 5% H2 in argon at 3.5 K for 60 min, (b) after
annealing to 16 K, (c) after λ > 240 nm photolysis for 15 min, (d) after
annealing to 25 K, (e) laser-ablated yttrium co-deposited with 5% H2 in
argon at 3.5 K for 60 min, (f) after annealing to 16 K, (g) laser-ablated
lanthanum co-deposited with 5% H2 in argon at 3.5 K for 60 min, and (h)
after annealing to 16 K.
La + H2. Analogous experiments were done for laser-ablated
lanthanum atom reactions with hydrogen in solid argon, neon,
and pure deuterium. Infrared spectra for H2 are shown in Figure
2 and the anion product absorptions are listed in Table 1. A
new band appeared at 1114.1 cm-1 on deposition with H2 in
argon, increased 3-fold on broadband photolysis, and slightly
increased on further annealing to 25 K. Deuterium counterparts
appeared at 797.8 cm-1, and HD and H2 + D2 experiments gave
1183.1, 1112.4, 833.5, and 799.8 cm-1 bands. The neon matrix
spectra are very similar to argon matrix spectra; the 1101.8 cm-1
band with H2 and the 788.7 cm-1 band with D2 are the major
product bands, and with HD four bands at 1158.5, 1096.3, 825.0,
and 788.8 cm-1 were observed. A related band appeared at 783.2
cm-1 in pure deuterium. Unfortunately no bending mode is
observed in our measurable lower region. A similar CCl4
experiment for this system eliminated the 1114.1 cm-1 band.
Again other species, LaH3 (1263.6 cm-1), LaH2 (1320.9, 1283.0
cm-1), (H2)LaH2 (1287.1, 1235.3 cm-1), and higher hydrogen
complexes were also trapped in the solid argon matrix.
Sc + H2. Likewise, as shown in Figure 2, the strongest
absorption at 1305.1 cm-1 in the reaction of Sc with H2/Ar
exhibits a lower 527.1 cm-1 component. The upper band shifts
to 946.2 cm-1 with D2/Ar. Similar isotopic absorptions with
HD/Ar were observed at 1377.0 and 1302.7 cm-1, 980.2 and
949.8 cm-1, and 523.5 and 494.2 cm-1. In addition the reactions
in neon gave 1321.3, 527.6 cm-1 with H2, 961.1 cm-1 with D2,
were employed, and the calculated frequencies were used to assign
vibrational spectra. All geometrical parameters were fully optimized
and the harmonic vibrational frequencies were obtained analytically at
the optimized structures.
Results
Investigations into the reactions of laser-ablated Y, La, and
Sc atoms with H2 will be presented.
Y + H2. Infrared spectra for the yttrium-hydrogen and
yttrium-deuterium stretching regions are illustrated in Figure
1 for laser-ablated yttrium atoms co-deposited with H2/Ar, HD/
Ar, and D2/Ar. The strong upper absorption at 1227.3 cm-1
tracks with a lower absorption at 482.7 cm-1, which is compared
in Figure 2. Annealing to 16 K sharpens and increases these
spectral features and changes the relative intensities of several
absorptions; broadband photolysis increases 1578.1, 1542.5,
1459.8, 1397.8, and 1385.1 cm-1 absorptions and slightly
increases the integrated absorbances of the 1227.3 and 482.7
cm-1 bands, and further annealing to 20 K (not shown) increases
9
J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002 7611

A R T I C L E S
Wang and Andrews
Table 2. Calculated Vibrational Frequencies (cm-1) and
Intensities (km/mol) for Tetrahedral ScH4-, YH4-, and LaH4
Anions with Td Symmetrya
-
stretching fundamentals ν3 (t2) of the tetrahedral anions YH4
-
-
and YD4 on the basis of the following evidence. In the HD
experiments the above bands split into 1291.4 and 1225.4 cm-1
and 915.7 and 884.5 cm-1 doublets, respectively, on symmetry
lowering, which are due to symmetric and antisymmetric YH2
and YD2 stretching fundamentals in YH2D2-. The antisymmetric
ScH -
ScD -
ScH D -
2 2
4
4
1456.2(a1, 0)
1030.1(a1, 0)
1389.4(462), 977.2(4730),
1304.1(1167), 942.8(708)
521.1(217), 488.4(349),
422.4(270), 374.2(93)
478.0(0)
1305.5(t2, 1219 × 3)
939.9(t2, 638 × 3)
525.9(t2, 434 × 3)
380.2(t2, 203 × 3)
-
YH2 and YD2 modes in YH2D2 are very close to the ν3 (t2)
520.1(e, 0)
367.9(e, 0)
modes observed in YH4- and YD4-, respectively, and the new
symmetric modes lie higher by about 64.1 cm-1 for the YH2
mode and 33.4 cm-1 for the YD2 mode in YH2D2- anion. This
pattern of four Y-H(D) stretching modes for YH2D2- verifies
a tetrahydride species as described for ZrH4 and HfH4.15
Furthermore, the strong absorption at 482.7 cm-1 in argon,
YH -
YD -
YH D -
4
4
2 2
1364.2 (a1, 0)
965.0 (a1, 0)
1309.0 (492), 919.7 (464)
1241.3 (t2, 1255 × 3) 886.0 (t2, 644 × 3) 1240.0 (1202), 888.3 (711)
502.0 (e, 0)
476.4 (t2, 516 × 3)
355.1 (e, 0)
341.1 (t2, 250 × 3)
434.4 (0)
489.2 (225), 444.4 (428),
379.9 (325), 347.7 (152)
which splits to 492.9 and 454.1 cm-1 with HD, is due to the
LaH -
LaD -
LaH D -
+
antisymmetric ν4 (t2) bending mode of YH4-. Finally, YH2
,
4
4
2 2
YHD+ and YD2+ (Figure 1) and the ArxH+ and ArxD+ species16
are observed in these experiments, which provides for charge
balance in the matrix.
1234.0 (a1, 0)
872.9 (a1, 0)
1182.7 (578), 828.7 (547)
1118.9 (t2, 578 × 3) 796.1 (t2, 753 × 3) 1117.8 (1427), 798.2 (823)
474.8 (e, 0)
420.0 (t2, 476 × 3)
335.9 (e, 0)
411.2(0)
299.5 (t2, 233 × 3) 450.8 (173), 391.9 (395),
-
334.3 (301), 314.0 (175)
In neon the Y-H and Y-D stretching modes in YH4 and
YD4 appeared at 1233.3 and 887.9 cm-1, respectively, and a
-
a Calculated at B3PW91/6-311++G(d,p)/SDD. Sc-H: 1.897 Å. Y-H:
2.063 Å. La-H: 2.232 Å.
similar four-band isotopic splitting pattern was observed for
YH2D2-. This is a typical blue shift for argon to neon matrices.17
A strong 884.4 cm-1 band in pure deuterium, which is reduced
and three pairs of bands at 1387.3 and 1318.8 cm-1, 992.3 and
961.5 cm-1, and 526.6 and 497.1 cm-1 with HD. Doping a 3%
H2/argon sample with 0.1% CCl4 resulted in reduction of the
1305.1, 527.1 cm-1 bands to <20% of their absorbance in Figure
2a, and in contrast with Figure 2b-d, little growth was observed
on annealing and photolysis. Finally, other scandium hydride
species ScH (1530.4 cm-1), and (H2)xScH2 complexes were also
trapped in the argon matrix.
DFT Calculations. DFT calculations were performed for
ScH4-, YH4-, and LaH4- to determine the structures, frequen-
cies, infrared intensities, and deuterium isotopic frequencies,
which are listed in Table 2. All three anions have Td symmetry
with bond lengths of 1.897 (Sc-H), 2.063 (Y-H), and 2.232
Å (La-H), respectively.
on broadband photolysis and recovered partially on further
-
annealing, is due to YD4
.
-
The strong degenerate ν3 (t2) mode computed for YH4 at
1241.3 cm-1 and YD4- at 886.0 cm-1 and the strong degenerate
-
ν4 (t2) mode for YH4 at 476 cm-1 are in excellent agreement
-
with observed values (Tables 1 and 2). For YH2D2 the
calculated Y-H stretching splitting of 69.0 cm-1, Y-D
stretching splitting of 31.4 cm-1, and Y-H2 bending splitting
of 44.8 cm-1 match very well with observed values of 66.0,
31.2, and 38.8 cm-1, respectively.
The anion identification is further confirmed with CCl4
additive, which serves as an electron trap agent. The elimination
of YH4 with CCl4 added to the sample shows that electron
trapping is very effective in this system.10,11 Hence, YH4 is
-
-
Similar DFT calculations were done for the (H2)MH2
complexes, which converged with C2V structures and gave real
frequencies. The strongest MH2 stretching modes are in excellent
agreement with the bands observed and assigned here. The MH2
stretching modes are 30-50 cm-1 lower in (H2)MH2 than in
identified here from the observation of four Y-H(D) stretching
absorptions on HD substitution, agreement with DFT frequency
calculations, and CCl4 doping to verify the anion charge.
LaH4-. The analogous tetrahedral lanthanum hydride anion
LaH4- was identified in the reaction of laser-ablated La atoms
with molecular hydrogen in argon, neon, and pure deuterium
matrix experiments, and the infrared absorptions are listed in
Table 1. The La-H and La-D stretching modes observed at
1114.1 and 797.8 cm-1, respectively, in argon exhibit tetrahy-
dride HD substitution; both La-H and La-D stretching modes
split into doublets at 1183.1, 1112.4 cm-1 and 833.5, 799.8
cm-1. The neon matrix absorptions at 1101.8 and 788.7 cm-1
show the same isotopic distributions and confirm the tetrahy-
dride assignment. However, in contrast to the blue matrix shift
-
the isolated MH2 molecules. The energy differences (MH4
minus (H2)MH2) provide electron affinities for (H2)MH2 and a
-
prediction of stability for the MH4 anions. These energy
differences are 2.79, 2.80, and 2.37 eV respectively for the Sc,
-
Y, and La species. Similar calculations for AlH4 and (H2)-
-
AlH2 yielded 2.97 eV, and the AlH4 anion is known to be
stable.
Discussion
The yttrium, lanthanum, and scandium tetrahydrometalate
anions will be identified based on isotopic substitution, com-
parison of argon, neon, and deuterium matrix spectra, and DFT
frequency calculations.
YH4-. In solid argon the 1227.3 cm-1 band increased on
annealing at the expense of (H2)YH2 complex absorption at
1363.4 cm-1. Broadband photolysis increased YH2 and (H2)-
YH2 and slightly increased the 1227.3 cm-1 absorption. The
deuterium counterpart band at 822.3 cm-1 showed the same
annealing and photolysis behavior. The 1227.3 and 882.3 cm-1
bands can be assigned to antisymmetric Y-H and Y-D
-
-
of YH4 for argon to neon, a red shift is observed for LaH4
.
This is probably due to repulsive interaction and the tight fit of
-
LaH4 into the more rigid argon matrix environment.
ScH4-. Assignment of 1305.1 and 527.1 cm-1 bands to the
ν3 and ν4 modes of ScH4 follows from its similar chemical
-
(15) (a) Chertihin, G. V.; Andrews, L. J. Am. Chem. Soc. 1995, 117, 6402. (b)
Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995, 99, 15004.
(16) Milligan, D. E.; Jacox, M. E. J. Mol. Spectrosc. 1973, 46, 460. Andrews,
L.; Ault, B. S.; Grzybowski, J. M.; Allen, R. O. J. Chem. Phys. 1975, 62,
2461.
(17) Jacox, M. E. Chem. Phys. 1994, 189, 149.
9
7612 J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002

Homoleptic Tetrahydrometalate Anions MH - (M ) Sc, Y, La)
A R T I C L E S
4
-
and spectroscopic behavior to YH4 and LaH4-. Scandium
molecules, and the (H2)MH2 complexes are formed spontane-
ously instead of tetrahydrides on the association of another H2
reagent because of the trivalent state of this metal group.
However, electrons are abundant in the ablation plume and (H2)-
MH2 can capture an electron to provide four valence electrons
and form the tetravalent anion. The spectra in Figures 1 and 2
summarize this process. The YH2 molecule complexes one or
two dihydrogen molecules on annealing to allow diffusion of
reagents (Figure 1, absorptions in the 1370-1300 cm-1 region).
atoms exhibit more thermal and photochemical reactions in low-
temperature matrices, giving ScH4 as the dominant product
after annealing and photolysis (Figure 2).
The ν3 (t2) mode for ScH4- is calculated slightly lower than
the observed neon matrix value whereas these modes for YH4
and LaH4 are calculated slightly higher as is expected.
Scandium is a more difficult calculation owing to competing
-
-
-
low-lying atomic configurations.18
-
The MH4 anion absorptions increased on annealing and
Comparisons. Note that in argon the Sc-H and La-H
stretching modes lie 77.8 cm-1 higher and 113.2 cm-1 lower,
respectively, than the Y-H stretching mode. The bending mode
broadband photolysis, which indicates the strong attraction for
-
electrons in this stable species. Furthermore, the same MH2D2
for ScH4- appeared at 527.1 cm-1, and for YH4- at 482.7 cm-1
,
,
anion is prepared from HD and H2 + D2 mixtures, which shows
that the anion is homoleptic; however, the MHD product is
spectroscopically distinct from MH2 and MD2.
and we can infer that this mode for LaH4- is below 400 cm-1
which is out of our measurable region. The antisymmetric M-H
stretching mode ν3 (t2) decreases from 1305.1 cm-1 for ScH4
-
M + H2 f MH2
(1)
(2)
-
-
to 1227.3 cm-1 for YH4 and to 1114.1 cm-1 for LaH4
,
suggesting a straightforward metal-hydrogen bond length
increase from ScH4- to YH4- and to LaH4-, respectively, which
is in line with our DFT computations.
MH2 + H2 f (H2)MH2
-
(H2)MH2 + e f MH4
(3)
The isoelectronic TiH4, ZrH4, and HfH4 molecules exhibit
ν3 (t2) modes at 1663.8, 1623.6, and 1678.4 cm-1, respectively,
in solid argon.15,19,20 The increase in frequency from ZrH4 to
HfH4 has been attributed to the relativistic bond-length contrac-
tion predicted for HfH4 (1.907 Å) relative to ZrH4 (1.912 Å).21,22
However, La and Hf are separated by the 14 elements of the
lanthanide series, and no such contraction occurs for La relative
to Y as is shown here.
During the deposition process, electrons from laser ablation
-
of the target surface are captured in reaction 3 to form MH4
anions; hence, the addition of CCl4 minimizes the MH4- yield
by preferential capture of these ablated electrons.11 Photolysis
-
of the prepared samples increases the MH4- anion yield (ScH4
and LaH4- more than YH4-); this suggests that electrons from
photodetachment of a less stable anion in the matrix allow the
formation of the more stable MH4 anions. In the La case the
-
Here electron affinity plays an important role in the formation
of the metal tetrahydrometalate anions. The electron affinities
predicted from DFT, 2.79 eV for (H2)ScH2, 2.80 eV for (H2)-
YH2, and 2.37 eV for (H2)LaH2, are substantial and only slightly
lower than the 2.97 eV value calculated for (H2)AlH2 at the
ionization energy is sufficiently low (295 nm) for La atoms to
serve as a photoelectron source. Finally, the pronounced growth
-
of MH4 absorptions on annealing to 16-25 K is unusual as
ions are reactive, and higher annealing to 30-40 K markedly
decreases the MH4- absorptions. How then do the MH4- anions
increase on early annealing? The most likely additional trapping
site for ablated electrons in these experiments is as the hydride
anion, H- (the electron affinity of atomic hydrogen is 0.76 eV).25
The small H- ion should diffuse in the matrix and electron
transfer to species with higher electron affinities such as the
(H2)MH2 complexes.
-
same level of theory. The stable isolated AlH4 anion in an
argon matrix is attested by its formation through UV irradiation
(240 nm) of aluminum samples containing the (H2)AlH2
complex.23 Similar laser-ablation experiments give isolated
-
AlH4 at 1609.0 cm-1 in solid argon, which is in excellent
agreement with the matrix photolysis experiments.
A final comparison between electron detachment energies
-
-
predicted here for ScH4 and YH4 (2.79 and 2.80 eV,
respectively) and those for ScO2- and YO2- can be made. The
latter have been measured as 2.32 and 2.00 eV, respectively,24
and computed as 2.4 and 1.9 eV, respectively.10 In similar
investigations of the reactions of laser-ablated Sc, Y, and La
Conclusions
Matrix-isolation experiments and DFT calculations show that
Sc, Y, and La atoms energized by laser ablation react with
molecular hydrogen to form dihydrides and higher dihydrogen
complexes. Electron capture leads to stable Sc, Y, and La
-
atoms with O2, the stable MO2 anions were observed as the
-
tetrahydrometalate anions, which are isoelectronic with AlH4
.
-
major products; although the MO2 absorptions increased
Doping with CCl4 to serve as an electron trap almost eliminates
the anion absorptions and attests to the anion identification. DFT
calculations predict frequencies in agreement with the matrix
observations and electron affinities in the 2.4-2.8 eV range
for the (H2)MH2 complexes that form the stable MH4- anions.
The group 3 tetrahydride anions should be sufficiently stable
for synthesis on a larger scale and use as reducing agents.
slightly on annealing to 20-25 K, they were destroyed on full-
-
arc photolysis.10 In contrast, the MH4 ions increased on full-
arc photolysis. Hence, the tetrahydrometalate anions of Sc, Y,
and La are more stable than their dioxide anions.
Reaction Mechanisms. Laser-ablated metal atoms react with
hydrogen molecules to give the MH2 (M ) Sc, Y, La) dihydride
(18) Bauschlicher, C. W., Jr.; Langhoff, S. R. J. Chem. Phys. 1986, 85, 5936.
(19) Xiao, Z. L.; Hague, R. H.; Margrave, J. L. J. Phys. Chem. 1991, 95, 2696.
(20) Chertihin, G. V.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 8322.
(21) Pyykko, P.; Snijders, J. G.; Baerends, E. J. Chem. Phys. Lett. 1981, 83,
432.
Acknowledgment. We gratefully acknowledge support for
this work from N.S.F Grant CHE 00-78836
JA020112L
(22) Pyykko, P. Chem. ReV. 1988, 88, 563.
(23) Pullumbi, P.; Bouteiller, Y.; Manceron, L. J. Chem. Phys. 1994, 101, 3610.
(24) Wu, H.; Wang, L.-S. J. Phys. Chem. A 1998, 102, 9129.
(25) Berry, R. S. Chem. ReV. 1969, 69, 533.
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J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002 7613