Rare-earth polyhydride complexes bearing Bis(phosphinophenyl)amido pincer ligands

Article abstract of DOI:10.1002/anie.201006812

PNP downsizes hydride: Hydrogenolysis of dialkyl precursors results in trinuclear hexahydride complexes A (Me-PNP^iPr= {4-Me-2-(iPr ₂P)-C₆H₃}₂N). Treatment with [NEt₃H][BPh₄] gives the first cationic trinuclear polyhydrides B. Cationic binuclear trihydride complexes C (only the central structural units are shown in the picture; Cblack, Hblue, Ored, Ygreen) have also been obtained for the first time.

Full text of DOI:10.1002/anie.201006812

DOI: 10.1002/anie.201006812
Rare-Earth Polyhydrides
Rare-Earth Polyhydride Complexes Bearing
Bis(phosphinophenyl)amido Pincer Ligands**
Jianhua Cheng, Takanori Shima, and Zhaomin Hou*
Rare-earth (Group 3 and lanthanide) metal polyhydride
complexes consisting of the dihydride species “LLnH2” with
one ancillary ligand per metal center are receiving intense
current interest because of their fascinating structure and
reactivity, which are exquisitely different from their mono-
hydride relatives “L2LnH” with two supporting ligands.[1] The
nuclearity of the rare-earth polyhydride complexes is gen-
erally dependent on the steric bulk of the ancillary ligands. An
increase of ligand hindrance usually leads to a decrease of the
nuclearity and increase of the reactivity of the resulting
hydride clusters. To date, several hexanuclear,[1a,2a] tetranu-
clear,[2a,3] and trinuclear[2b,4,5] polyhydride complexes have
been reported. However, despite recent progress in this area,
the number of well-defined rare-earth polyhydride complexes
is still very limited. Most of the ancillary ligands employed to
date for rare-earth polyhydride complexes are either steri-
formation of the trinuclear polyhydride complex
1
[{(PNPiPr)YH2}3] was observed, as indicated by the H NMR
spectrum (6.04 ppm, quartet, 1JY-H = 16.8 Hz, in C6D6).
Unfortunately, however, a single crystal suitable for X-ray
structure determination was not obtained because of the high
solubility of this hydride compound in all common organic
solvents. To make a hydride compound with better crystal-
linity, the dialkyl complex with a methyl-substituted analogue
of the PNPiPr ligand [(Me-PNPiPr)Y(CH2SiMe3)2] (1-Y, Me-
PNPiPr = {4-Me-2-(iPr2P)-C6H3}2N)[8] was then used. To our
delight, the hydrogenolysis of 1-Y in toluene under 10 atm H2
successfully afforded the corresponding trinuclear hexahy-
dride complex [{(Me-PNPiPr)YH2}3] (2-Y) as pale yellow
crystals in 92% yield of isolated product (Scheme 1). The
cally demanding cyclopentadienyl derivatives such as
[3]
C5Me4SiMe3
or scorpionate tris(pyrazolyl)hydroborate
units,[2] although the use of tetraazacycloamido[4] or pyridy-
lamido[5] ligands was also described. Well-defined cationic
rare-earth polyhydrides have remained scarce,[6] and a
binuclear polyhydride complex consisting of “LLnH2” has
not been reported to date. Herein, we report the synthesis and
structural characterization of a new family of rare-earth
polyhydride complexes supported by the rigid bis(phosphi-
nophenyl)amido (PNP) ligands, including the first examples
of cationic trinuclear and binuclear rare-earth polyhydrides.
As a possible precursor to a rare-earth polyhydride
complex bearing a PNP ligand, we first chose the yttrium
dialkly complex [(PNPPh)Y(CH2SiMe3)2(thf)] (PNPPh = {2-
(Ph2P)-C6H4}2N), which has shown novel polymerization
activity.[7] However, the hydrogenolysis reaction of
[(PNPPh)Y(CH2SiMe3)2(thf)] with H2 seemed very compli-
cated, and no characterizable product was obtained. In
contrast, when an analogous dialkyl complex with a bis(dii-
Scheme 1. Synthesis of PNP-ligated trinuclear polyhydride complexes.
analogous Lu hydride cluster 2-Lu was also obtained in a
similar way.
X-ray crystallographic studies revealed that the com-
plexes 2-Yand 2-Lu are isostructural. The ORTEP drawing of
2-Y is shown in Figure 1; selected interatomic distances are
listed in Table 1. Each Y atom in 2-Y is bonded to a PNP
sopropylphosphinophenyl)amido
ligand
[(PNPiPr)Y-
(CH2SiMe3)2] (PNPiPr = {2-(iPr2P)-C6H4}2N) was used, clean
Table 1: Selected interatomic distances [ꢀ] for 2-Y and 3-Y.
[*] Dr. J. Cheng, Dr. T. Shima, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory and Advanced Catalyst
Research Team, RIKEN Advanced Science Institute
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Fax: (+81)48-462-4665
2-Y
3-Y
Y1···Y2
Y1···Y3
Y2···Y3
Y(1,2,3) H1
Y(1,2,3) H2
3.4829(6)
3.4678(6)
3.1648(7)
2.31(4), 2.22(4), 2.32(4)
2.06(4), 2.37(4), 2.34(4)
2.12(4), 2,27(4)
2.12(4), 2.21(4)
2.07(4), 2.11(4)
2.07(4), 2.24(4)
2.367(3)
3.4789(6)
3.4411(6)
3.4812(6)
2.37(4), 2.30(4), 2.43(4)
2.37(4), 2.30(4), 2.33(4)
2.11(3), 2.07(3)
2.08(3), 2.11(3)
2.03(4), 2.05(4)
À
E-mail: houz@riken.jp
À
[**] This work was supported by Grants-in-Aid for Scientific Research (S)
(No. 21225004) and for Young Scientists (B) (No. 21750068) from
JSPS. J.C. is grateful to RIKEN for a FPR fellowship. We thank Dr. H.
Koshino for technical assistance in 11B NMR spectroscopy meas-
urements.
À
Y(1,3) H3
À
Y(1,2) H4
À
Y(2,3) H5
À
Y(2,3) H6
À
Y N (av.)
2.309(3)
2.900(1)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006812.
À
Y P (av.)
2.948(1)
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Communications
Scheme 2. Synthesis of cationic trinuclear pentahydride complexes.
diffraction studies were obtained from saturated toluene
solutions. Similar to the neutral precursors 2-Ln, the cationic
hydride clusters 3-Ln are also isostructural. The {Ln3H5} core
structure of 3-Y is shown in Figure 2 (right) together with its
Figure 1. ORTEP plot of 2-Y with thermal ellipsoids at the 30%
probability level. The methyl groups and hydrogen atoms in the PNP
ligands have been omitted for clarity.
ligand in a typical k3-(P,N,P) mode. Two of the six hydride
ligands adopt a m3 bonding mode, each of which caps one side
of the Y3 plane. The remaining four hydride ligands bridge the
three edges of the Y3 triangle in a m2 fashion, and therefore,
one the of the three Y···Y edges is bridged by two m2-H
ligands, while the other two Y···Y edges are each bridged by
only one m2-H ligand, in addition to the m3-H capping ligands.
As a consequence, the Y2···Y3 edge (3.1648(7) ꢀ) bridged by
two m2-H ligands is significantly shorter than the other two
Y···Yedges (3.4829(6) and 3.4678(6) ꢀ, Table 1). This Y2···Y3
edge represents the shortest Y···Y distance ever reported, as
far as we are aware. The Y3H6 core structure in 2-Y is thus in
contrast to those of other reported trinuclear hexahydrides,
such as [{(TpiPr2)YH2}3] (TpiPr2 = tris(3,5-diisopropyl-1-pyra-
zolyl)hydroborate), which possesses one m3-H and five m2-H
ligands,[2b] and [{(Me3TACD)YH2}3] (Me3TACD-H = 1,4,7-
trimethyl-1,4,7,10-tetraazacyclododecane),[4] in which all six
hydride ligands adopted the same m2-H bonding mode. The
Y···Y distances in these previous examples range from
3.3423(3) to 3.6841(2) ꢀ.
Figure 2. The metal hydride core structures of 2-Y and 3-Y.
neutral counterpart 2-Y (left) for comparison. Some selected
interatomic distances are summarized in Table 1. All three Y
atoms in the cationic complex 3-Y are almost equally
connected by the hydride ligands. Two of the five hydride
ligands adopt a m3 bonding mode, and the remaining three
hydride ligands are bonded in a m2 fashion, each bridging one
Y···Y edge. Therefore, the three Y···Y edges in 3-Y are almost
equal, in contrast with its neutral precursor 2-Y, in which one
of the Y···Y edges is much shorter than the other two because
of the additional bridging hydride ligand (Figure 2 and
Table 1). The hexagon formed by the three Y atoms and the
three m2-H atoms in 3-Y is almost planar, with a maximum
deviation of 0.129 ꢀ. The two m3-H atoms are located 1.213
À
and 1.248 ꢀ above and below the hexagonal plane. The Y
(m3-H) bonds in 3-Y (av. 2.35(4) ꢀ) are longer than the Y (m2-
H) bonds (av. 2.08(4) ꢀ), similar to the situation in 2-Y. The
1
À
The H NMR spectrum of 2-Y in C6D6 showed a quartet
(1JY-H = 17.0 Hz) at d = 6.08 ppm corresponding to the hydride
ligands, thus suggesting that the structure is fluxional and that
all six hydrides are equivalent on the time scale of NMR
spectroscopy. The hydrides in 2-Lu showed a broad singlet at
d = 9.70 ppm. These results are comparable with those
observed in other trinuclear polyhydride complexes.[2b,4]
On treatment with an equivalent of [NEt3H][BPh4] in
toluene (or THF) at room temperature, both 2-Y and 2-Lu
gave the corresponding cationic pentahydrides [(Me-
PNPiPr)3Y3H5][BPh4] (3-Ln; Ln = Y, Lu) in high yields, with
concomitant release of H2 and NEt3 (Scheme 2).
À
À
Y N and Y P bonds in the cationic complex 3-Y are
significantly shorter than those in the neutral precursor 2-Y
(Table 1).
It is also noteworthy that no direct bonding interaction
was observed between the cation [((Me-PNPiPr)3Ln3H5] and
the anion [BPh4] in 3-Y despite steric and electronic
unsaturation (compared to 2-Y) at the metal center. More-
over, the incorporation of a thf ligand into 3-Y was not
observed even when the complex was mixed with THF. These
results are in sharp contrast with what was observed in the
The cationic complexes 3-Y and 3-Lu are soluble in THF
and C6H5Cl and partly soluble in toluene and benzene but
almost insoluble in hexane. Single crystals suitable for X-ray
tetranuclear
cationic
polyhydride
complex
[(C5Me4SiMe3)4Y4H7][B(C6F5)4], which adopted a contact-
ion-pair structure in the THF-free form and gave the solvent-
1858
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Angew. Chem. Int. Ed. 2011, 50, 1857 –1860

separated ion pair [(C5Me4SiMe3)4Y4H7(thf)2][B(C6F5)4],
upon reaction with THF.[6] Complex 3-Y (as well as 3-Lu)
represents the first example of a structurally characterized
cationic trinuclear rare-earth polyhydride complex and also
the first example of a cationic rare-earth polyhydride complex
bearing a non-cyclopentadienyl ligand.
The five hydride ligands of the cationic complex 3-Y
1
showed two sets of H NMR signals in C6D5Cl, with a broad
singlet at d = 5.53 ppm for the two m3-H ligands and a triplet at
d = 6.52 ppm (1JY-H = 31.7 Hz) for the three m2-H ligands, in
contrast with its neutral precursor 2-Y, which exhibited only a
quartet (d = 5.97 ppm, 1JY-H = 16.9 Hz) for all six hydrides
under similar conditions. Similarly, the hydrides in 3-Lu
showed two broad signals at d = 8.43 and 11.85 ppm. These
results suggest that the Ln3H5 core structure in the cationic
complexes 3-Ln is more rigid than the Ln3H6 unit in the
neutral complexes 2-Ln. Similar phenomena were also
observed in the C5Me4SiMe3-ligated tetranuclear polyhydride
analogues.[6]
Surprisingly to us, when the hydrogenolysis reaction of the
dialkyl complexes 1-Ln was carried out in the presence of
0.5 equivalent [NEt3H][BPh4] in THF, the binuclear cationic
trihydride complexes [(Me-PNPiPr)2Y2H3(thf)2][BPh4] (4-Ln;
Ln = Y, Lu) were obtained in 60–67% yields of isolated
product (Scheme 3), in sharp contrast with the reaction in the
Figure 3. ORTEP plot of the cationic part of 4-Y with thermal ellipsoids
at the 30% probability level. The methyl groups and hydrogen atoms
in the PNP ligands have been omitted for clarity. Selected interatomic
distances [ꢀ] and angles [8]: Y1–H1 2.08(5), Y1–H2 2.14(4), Y1–H3
2.17(4), Y1–N1 2.314(3), Y1–P1 2.9492(12), Y1–P2 2.9293(11), Y1–O1
2.372(2), Y2–H1 2.21(4), Y2–H2 2.09(4), Y2–H3 2.12(4), Y2–N2
3.321(3), Y2–P3 2.9349(12), Y2–P4 2.9266(12), Y2–O2 2.351(3),
Y1···Y2 3.3297(5); Y1-H1-Y2 102.0(12), Y1-H2-Y2 104.0(12), Y1-H3-Y2
102.1(12).
PNPiPr)LnH2(thf)x], formed by the hydrogenolysis of the
dialkyl complex 1-Ln, with a cationic monohydride species
[(Me-PNPiPr) LnH(thf)x][BPh4] resulting from the hydro-
genolysis of the cationic monoalkyl species [(Me-PNPiPr)Ln-
(CH2SiMe3)(thf)x][BPh4] that could be formed by reaction of
1-Ln with [NEt3H][BPh4] in THF.
In summary, we have demonstrated that the rigid
bis(phosphinophenyl)amido moiety {4-Me-2-(iPr2P)-C6H3}2N
can serve as an excellent supporting ligand for rare-earth
polyhydride complexes, leading to formation of a new family
of structurally characterizable trinuclear and binuclear poly-
hydride complexes, including the first examples of cationic
trinuclear and binuclear rare-earth polyhydrides. Studies on
the reactivity of these hydride clusters and on the analogous
rare-earth polyhydride complexes with other non-cyclopen-
tadienyl ancillary ligands are in progress.[10]
Scheme 3. One-pot synthesis of cationic binuclear trihydride com-
plexes.
absence of [NEt3H][BPh4], which yielded selectively the
trinuclear polyhydrides 2-Ln as described above.[9] The three
hydride ligands in 4-Y showed a triplet at d = 5.72 ppm with
1
1JY-H = 23.1 Hz in the H NMR spectrum in [D8]THF. Com-
Received: October 29, 2010
Published online: January 14, 2011
plex 4-Lu gave a singlet at d = 8.45 ppm.
Single crystals of 4-Y and 4-Lu were grown from THF/
hexane. These two complexes are isostructural. Figure 3
depicts the ORTEP structure of the cation part of 4-Y. The
two Y atoms are bridged by three hydride ligands, and each Y
atom is also bonded to a k3-PNP ligand and a thf ligand. The
Y2H3 core adopts a slightly distorted triangular bipyramid
geometry, with the three hydride ligands occupying the
equatorial positions and the two Y atoms in the apical sites.
Keywords: cationic complexes · hydrides · N,P ligands ·
rare earths
.
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The Y H bond lengths vary from 2.08(5) to 2.21(5) ꢀ, but fall
in the range of typical Y-(m2-H) distances. The Y···Y distance
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characterized cationic binuclear rare-earth polyhydride com-
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The present formation of 4-Ln could formally be viewed
as a combination of a neutral dihydride species such as [(Me-
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Communications
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[10] Structurally characterizable cationic binuclear trihydrides bear-
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by the hydrogenolysis of [{PhC(NAr*)2}Y(CH2SiMe3)2(thf)]
with 0.5 equivalents of [NEt3H][BPh4] in a THF solution.
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