Synthesis and Hydrogen Uptake Property of Rare-Earth/Transition-Metal
Hydride Clusters
Takanori Shima and Zhaomin Hou(Organometallic Chemistry Laboratory, RIKEN)
Hydrogen is thought to be an ideal fuel for vehicles and portable electronics because it isnon-polluting, and forms an environmentally benign by product –water– on combustion. The
recent increasing interest in hydrogen as a fuel warrants careful studies into structural chemistryof this element. Thus, the structure of soluble molecular multinuclear hydride clusters, whosecores resemble little fragments of metals with hydrogen atoms attached, have been actively
investigated to understand the interactions of hydrogen with metal-containing materials forhydrogen storage and also for the development of new catalysts for hydrogen transformations.
We recently found that rare earth metal polyhydride cluster 1 can show unique structure,
which possesses 4-coordinate tetrahedral hydrogen in the center of the metal core, and its uniquereactivities toward a variety of unsaturated substrates.1-7 During these studies, we became
interested in the use of the rare earth metal polyhydride clusters as building blocks for thesynthesis of rare-earth and d-block heteronuclear polyhydride clusters, which are expected tohave more unique structures and properties of their own. Here we show the rare-earth andd-block heteronuclear clusters, 2a and 2b, which can be formed by the reaction of 1 withCp'
Cp*M(PMe3)H5 (M = Mo, W;
HCp*
Cp* = η5-C5Me5). The UV
irradiation of 2a and 2b yielded“phosphine free” clusters, 3aand 3b. (Fig.1). While cluster
3b can oxidatively add one
molecule of hydrogen in aprocess that can be reversed byUV irradiation, cluster 3a canrelease one molecule ofhydrogen to afford more
electronicallyunsaturated
cluster 5, which can bereversed by the addition ofhydrogen (Fig.1).

OY
H
H
Cp' = C5Me4SiMe3
YHCp'H
H Y Cp'H
YH
Cp'1
Cp*M(PMe3)H5
H2
M = Mo, W-H2

Cp* PMe3
M
H
H H
Y Cp'
Cp' Y
H
HH
HHY Y
H
H
Cp' Cp'
M = Mo (2a)W (2b)
Cp* = C5Me5H

H
H
H W
H
Cp' H HH Y
HY
Cp' YCp'
H H H
HH
YCp'4M=W

Cp*PMe3hνH
Cp' YHHM
H

H2HHhν
Y Cp'

H
HHY Y
H
H
Cp' Cp'
M = Mo (3a)W (3b)

M = Mo
H2Cp*
H

H2Cp' Y

MoH

H

H

HY Y

H

H
H
Cp' Cp'5YCp'

H

Fig.1. Synthesis of Y4-M heteropentametallic polyhydrides 2 to 5.

Compound 5 reacts with hydrogen in the solid state. Of particular note is that in thecrystalline state 5 reacts with hydrogen while retaining its crystallinity. The crystal of 5
underwent a change in unit cell dimensions without degradation of crystallinity when it wasexposed to N2/H2 mixed gas. Fig. 2A shows the changes in the unit cell parameters, a, b, c, β,and V, and the average of metal–metal distances with reaction time. The a, V values andY1–Mo1 distance are almost constant, while b, β values and Y3–Y4 distance decrease whereas
the other parameters (c axis, Y2–Mo1, Y3–Mo1, Y4–Mo1, Y1–Y2, Y1–Y3, Y2–Y3, Y2–Y4)
increase. The elongation of metal–metal distances may provide the space for hydrogen atom,which is formed at the center of the metal framework cavity, while the shortening of the Y3–Y4
distance may indicate that the hydrogen atom is newly formed and bridging between Y3 and Y4.
Fig. 2B-E shows the real time in situ observation of hydrogen addition reaction followed by useof composite difference electron density maps at each reaction step. A gradual increase of theelectron density of the(deg)
(Å)
92.25
3.80
Ahydrogens (blue color),
Y2-Mo1
3.70
92.20
which are in interstitial
3.60
βY3-Mo1
3.50
92.15
position and bridging
3.40(Å)3.30
position between Y3Y1-Mo1
30.8
3.20
and Y4, is observed as
Y4-Mo130.7
3.10(Å)the reaction proceeds.
Y2-Y33.65
b30.6
3.60
Thisobservation
30.5
Y1-Y2
Y3-Y4
Y1-Y3
3.55
indicates that the crystal
30.4

3.50

18.1of 5 is gradually
becoming the crystal ofY2-Y4

3.45

c18.0(Å3)

V

17.97300

13.3

a13.20102030407250
50

B060 h

102030405060 h

CH 2 /N 2 (7/93)
rtDE

H 2 /N 2 (7/93)

H 2 /N 2 (7/93)

rt

rt

Fig. 2. Real time in situ observation of the hydrogen uptake process by a single crystal of 5:(A) The change of the unit cell parameters and the average of metal–metal distances withreaction time. (B) Composite difference electron density maps created with theVisualization for Electronic and Structural Analysis (VESTA). Cp’, Cp* and hydride ligands
are omitted for clarity. A crystal of 5 reacted with H2/N2 mixed gas (7/93) at room
temperature for 11 h (C), 19 h (D), and 57 h (E), respectively, using inner one-third data-1-3(sinθ/λ cutoff = 0.33 Å ). The contour surface (blue) corresponds to the value of 0.3 eÅ .

3a. Although there aresome examples of theintraandinter-molecular reaction
retaining
their
crystallinity, to our best
knowledge, this is thefirst example of anintermolecularhydrogenuptakereaction in which thesingle crystal integrity
is fully preserved.

References(1) Z. Hou, M. Nishiura, T. Shima, Eur. J. Inorg. Chem. 2007, 2535–2545. (2) X. Li, J. Baldamus, M.
Nishiura, O. Tardif, Z. Hou, Angew. Chem. Int. Ed. 2006, 45, 8184–8188. (3) T. Shima, Z. Hou, J. Am.
Chem. Soc. 2006, 128, 8124–8125. (4) D. Cui, M. Nishiura, Z. Hou, Angew. Chem. Int. Ed. 2005, 44,959–962. (5) O. Tardif, D. Hashizume, Z. Hou, J. Am. Chem. Soc. 2004, 126, 8080–8081. (6) D. Cui,O. Tardif, Z. Hou, J. Am. Chem. Soc. 2004, 126, 1312–1313. (7) O. Tardif, M. Nishiura, Z. Hou,Organometallics 2003, 22, 1171–1173.