Poly(pyrazolyl)aluminate complexes containing aluminum-hydrogen bonds

Article abstract of DOI:10.1021/ic201541c

The treatment of LiAlH₄ with 2, 3, or 4 equiv of the 3,5-disubstituted pyrazoles Ph₂pzH or iPr₂pzH afforded [Li(THF)₂][AlH₂(Ph₂pz)₂] (97%), [Li(THF)][AlH(Ph₂pz)₃] (96%), [Li(THF)₄] [Al(Ph₂pz)₄] (95%), and [Li(THF)][AlH(iPr ₂pz)₃] (89%). The treatment of ZnCl₂ with [Li(THF)][AlH(Ph₂pz)₃] afforded Zn(AlH(Ph ₂Pz)₃)H (70%). X-ray crystal structures of these complexes demonstrated κ² or κ³ coordination of the aluminum-based ligands to the Li or Zn ions. The treatment of [Li(THF)][AlH(Ph₂pz)₃] with MgBr₂ or CoCl ₂ in THF/Et₂O solutions, by contrast, afforded the pyrazolate transfer products Mg₂Br₂(Ph₂pz) ₂(THF)₃·2THF (25%) and Co₂Cl ₂(Ph₂pz)₂(THF)₃·THF (23%) as colorless and blue crystalline solids, respectively. An analogous treatment of [Li(THF)][AlH(Ph₂pz)₃] with MCl₂ (M = Mn, Fe, Ni, Cu) afforded metal powders and H₂, illustrating hydride transfer from Al to M as a competing reaction path.

Full text of DOI:10.1021/ic201541c

COMMUNICATION
pubs.acs.org/IC
Poly(pyrazolyl)aluminate Complexes Containing
AluminumꢀHydrogen Bonds
Christopher J. Snyder, Mary Jane Heeg, and Charles H. Winter*
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States
S Supporting Information
b
have only included methyl groups on the group 13 metal,
apparently to avoid the high reactivity of the AlꢀH or GaꢀH
bond. Supporting this contention, the synthesis of Na[GaH2-
(pz)2] was claimed upon the treatment of GaH3(NMe3) with
Napz and pzH, but it was noted that the treatment of Na[GaH2-
(pz)2] with nickel or copper salts led to the evolution of H2 and
no complexes could be isolated.3 We have recently reported
the atomic layer deposition of MB2O4 (M = Ca, Sr, Ba) films
from CaTp2, SrTp2, and Ba(TpEt2)2 [TpEt2 = hydrotris(3,5-
diethylpyrazolyl)borate] and water and obtained precise 2:1 B/M
ratios in the films.6 We have also recently reported potassium
complexes containing energetic dihydrobis(tetrazolyl)borate ligands.7
In view of the small number of poly(pyrazolyl)aluminate
complexes reported to date,2ꢀ4 the lack of derivatives containing
AlꢀH bonds, the potential use of complexes containing these
ligands as film growth precursors to metal aluminate films
with controlled M/Al ratios, and the relationship of poly-
(pyrazolyl)aluminates to so far unknown potentially highly
energetic poly(tetrazolyl)aluminates, we sought to explore the
preparation of ligands of the formula AlHn(R2pz)4ꢀnꢀ (R2pz =
3,5-disubstituted pyrazolyl). Herein, we report the synthesis,
structure, and properties of lithium complexes containing AlH2-
ABSTRACT: The treatment of LiAlH4 with 2, 3, or 4 equiv
of the 3,5-disubstituted pyrazoles Ph2pzH or iPr2pzH
afforded [Li(THF)2][AlH2(Ph2pz)2] (97%), [Li(THF)]-
[AlH(Ph2pz)3] (96%), [Li(THF)4][Al(Ph2pz)4] (95%),
and[Li(THF)][AlH(iPr2pz)3] (89%). The treatment of ZnCl2
with [Li(THF)][AlH(Ph2pz)3] afforded Zn(AlH(Ph2Pz)3)H
(70%). X-ray crystal structures of these complexes demon-
strated k2 or k3 coordination of the aluminum-based
ligands to the Li or Zn ions. The treatment of [Li(THF)]-
[AlH(Ph2pz)3] with MgBr2 or CoCl2 in THF/Et2O solutions,
by contrast, afforded the pyrazolate transfer products Mg2Br2-
(Ph2pz)2(THF)3 2THF (25%) and Co2Cl2(Ph2pz)2(THF)3
THF (23%) as colorless and blue crystalline solids, respec-
tively. An analogous treatment of [Li(THF)][AlH(Ph2pz)3]
with MCl2 (M = Mn, Fe, Ni, Cu) afforded metal powders
and H2, illustrating hydride transfer from Al to M as a
competing reaction path.
3
3
oly(pyrazolyl)borate ligands of the formula B(pz)nH4ꢀnꢀ (pz =
ꢀ
(Ph2pz)2ꢀ, AlH(Ph2pz)3ꢀ, AlH(iPr2pz)3ꢀ, and Al(Ph2pz)4
P
C3H3N2; Bp, n = 2; Tp, n = 3) are very common.1 In spite of the
widespread use of Bp and Tp ligands and C-substituted versions
thereof, there has been little development of the heavier group 13
analogues.2 An early report described the synthesis of Na-
[AlMe2pz2] and Na[Alpz4], but no characterization was given for
Na[AlMe2pz2]; the only data for Na[Alpz4] was an aluminum
microanalysis, and attempts to prepare transition-metal derivatives
containing these new aluminum-based ligands were unsuccessful.3
Since this initial report, crystallographically characterized alu-
minum derivatives have been limited to Na[AlMe2(Me2pz)2]
(Me2pz = 3,5-dimethylpyrazolyl),4a {[Na(THF)][AlMe2-
(tBu2pz)2]}2 (tBu2pz = 3,5-di-tert-butylpyrazolyl),4b and[Na(THF)]-
[AlMe(tBu2pz)3].4b Na[AlMe2(Me2pz)2] contains a complex
ligand coordination mode that does not resemble the common
k3-N,N,H mode typically observed for Bp complexes;4a the N
atoms in {[Na(THF)][AlMe2(tBu2-pz)2]}2 engage in a k3-N,N,
N coordination mode that involves a k1 interaction between the
Na ion and one pyrazolyl group and a k2 interaction with the
other pyrazolyl group,4b and [Na(THF)][AlMe(tBu2pz)3] con-
tains k4 bonding between the Na ion and the N atoms that entails
k1 interactions with two pyrazolyl groups and a k2 interaction
with the third pyrazolyl group.4b Synthetic routes to gallium
analogues are more numerous and include metal complexes
containing the GaMe2(pz)2ꢀ and GaMe(pz)3ꢀ ligands.2,3,5 No-
tably, the heavier analogues of Bp and Tp ligands reported to date
ligands. The AlH2(Ph2pz)2ꢀ and AlH(R2pz)3ꢀ ligands exhibit k2-
and k3-coordination modes to the Li ion, in analogy to common
bonding motifs of Bp and Tp ligands to metal ions.1 The complex
Zn(AlH(Ph2pz)3)H has been prepared by a salt metathesis route
from ZnCl2 and contains a k3-AlH(Ph2pz)3ꢀ ligand bonded to
the Zn ion. Attempted salt metathesis reactions with other metal
halides leads to pyrazolate or hydride transfer.
The treatment of LiAlH4 solutions in tetrahydrofuran (THF)
with 2, 3, or 4 equiv of Ph2pzH at ambient temperature led to
[Li(THF)2][AlH2(Ph2pz)2] (1, 97%), [Li(THF)][AlH(Ph2pz)3]
(2, 96%), and [Li(THF)4][Al(Ph2pz)4] (3, 95%) as colorless
crystalline solids (Scheme 1).8 A similar treatment of LiAlH4
with 3 equiv of iPr2pzH afforded [Li(THF)][AlH(iPr2pz)3] (4,
89%) as colorless crystals (Scheme 1).8 Complexes 1ꢀ4 were
characterized by spectral and analytical techniques and by X-ray
crystal structure determinations. The IR spectra showed AlꢀH
stretches for 1, 2, and 4 at 1826, 1934 and 1869 cmꢀ1, respectively.
The 1H NMR spectra of 1ꢀ4 showed resonances consistent with
Ph2pz or iPr2pz groups and THF ligands. In addition, 1, 2, and 4
exhibited broad resonances at δ 5.04, 4.66, and 5.55 for the Al-
bound H atoms. Analytically pure crystals of 1 showed a 50:50
mixture of 1 and 2 in the 1H and 13C{1H} NMR spectra at 23 °C
Received: July 19, 2011
Published: August 30, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
COMMUNICATION
Scheme 1. Synthesis of 1ꢀ4
Scheme 3. Synthesis of 6 and 7 from 2
pyrazolate transfer to the metal centers occurred. The 1H NMR of
6 contained resonances from the Ph2pz and THF ligands, but no
resonances from the AlꢀH or MgꢀH bond were observed. The
treatment of 2 with other MCl2 (M = Mn, Fe, Ni, Cu) at ambient
temperature in THF led to gas evolution over 0.5 h and the
formation of insoluble silver-black (M = Mn, Fe, Ni) or copper-
colored (M = Cu) powders, which are presumably the metals.
Perspective views of 2, 5, and 6 are shown in Figure 1.
Complexes 2 and 5 contain k3-N,N,N-AlH(Ph2pz)3 ligands, with
LiꢀN distances of 2.061(2), 2.072(2), and 2.137(3) Å and ZnꢀN
distances of 2.042(4), 2.061(4), and 2.112(4) Å, respectively.
The AlꢀN distances in 2 and 5 fall between 1.88 and 1.91 Å.
The NꢀAlꢀN and NꢀLiꢀN angles in 2 lie between 100.49(5)
and 110.58(5)° [avg = 105(5)°] and between 95.03(10) and
101.68(10)° [avg = 99(3)°] and are slightly smaller than the
109.5° value expected for tetrahedral geometry, probably because
of the tight chelate. The related values of 5 [NꢀAlꢀN, 101.7-
(3)ꢀ109.9(4)°, avg = 106(4)°; NꢀZnꢀN, 90.07(13)ꢀ97.85-
(14)°, avg = 94(4)°] are similar to those of 2. Complex 6 exists as
a dimer that is held together by bridging Ph2pz and THF
ligands. Each Mg ion is also bonded to one Br and one THF
ligand, and the molecule crystallizes with two THF solvates in
the lattice. The MgꢀN distances range from 2.098(2) to
2.108(2) Å, the MgꢀBr bond lengths are 2.478(1) and
2.482(1) Å, and the MgꢀO distances are 2.268(2) and
2.285(2) Å and 2.050(2) and 2.052(2) Å for the bridging and
terminal THF ligands, respectively. The X-ray crystal structures
of 1, 3, 4, and 7 are not described in detail herein but are
contained in the Supporting Information.
The aluminum analogues 1ꢀ4 of the well-known Bp, Tp, and
Bpz4ꢀ ligands are easily prepared upon the treatment of LiAlH4
with the appropriate stoichiometry of a pyrazole. The coordina-
tion modes of the AlH(R2pz)4ꢀnꢀ ligands in 1ꢀ4 are analogous
to the common bonding motifs of the Bp and Tp ligands.1
Complex 5 is obtained upon the treatment ꢀof 2 with ZnCl2,
which demonstrates that the AlH(Ph2pz)3 ligand can be
transferred from Li to Zn by a standard salt metathesis reaction.
However, this reaction is more complex because the second
chloride ion is also replaced by a hydride to afford 5. Hence,
hydride transfer from aluminum to metal with 1, 2, and 4 in
salt metathesis reactions is a potential complication that is
generally absent with the Bp and Tp ligands.1 The zinc hydrides
ZnHTptBu [TptBu = hydrotris(3-tert-butylpyrazolyl)borate]10a
and ZnHTpp-Tol,Me [Tpp-Tol,Me = hydrotris[3-(4-tolyl)-5-methyl-
pyrazolyl]borate]10b were prepared upon the treatment of ZnH2
with TlTptBu and ZnFTpp-Tol,Me, respectively, and, like 5, have
distorted tetrahedral geometries about the Zn ion. The treatment
of 2 with MgCl2 and CoCl2 afforded the pyrazolate complexes 6
and 7. This reaction path may be driven by the formation of
crystalline 6 and 7 and is consistent with previous reports
of pyrazolate transfer from boron to metals upon the treatment
of KBp and KTp with metal halides.11 Many metal(II) chlorides
Scheme 2. Synthesis of 5 from 2
in benzene-d6, and variable-temperature 1H NMR spectra taken
at ꢀ80 and +80 °C in toluene-d8 also showed ∼50:50 mixtures of
1 and 2. Complex 1 may exist in equilibrium with 2 and another
pyrazolyl species in solution. A pyrazolyl 4-CH resonance for a
1
possible third species was observed at δ 6.86 in the H NMR
spectrum in benzene-d6, at 23 °C, but the other resonances for
this species were of too low intensity to provide further insight.
We have recently reported that CaBp2(THF)2 decomposes to
CaTp2 and CaTp(BH4) upon thermolysis at 190 °C.9 The
weaker bond energies in 1, relative to CaBp2(THF)2, apparently
allow a similar process to occur at 23 °C. Additionally, the NMR
spectra of 1, 2, and 4 recorded at 23 °C in benzene-d6 showed
only one type of pyrazolyl carbon substituent, demonstrating
that there is rapid exchange between the 3- and 5-substituent
sites on the NMR time scale. A similar pyrazolate site exchange
process was observed in [Na(THF)][AlMe(tBu2pz)3].4b 1H
NMR spectra of 2 and 4 in toluene-d8 at ꢀ80 °C showed two
types of pyrazolyl carbon group resonances, confirming that the
exchange processes are slow at these temperatures.
The reactivity of 2 toward metal(II) halides was investigated
to establish the relationship of hydrotris(pyrazolyl)aluminate
ligands to analogous Tp systems. The treatment of ZnCl2 with
2 in diethyl ether afforded Zn(AlH(Ph2Pz)3)H (5, 70%) as
colorless crystals upon workup (Scheme 2).8 Complex 5 was
identified from spectral and analytical data and by X-ray crystal-
lography. The IR spectrum of 5 showed an AlꢀH stretch at
1963 cmꢀ1 and a ZnꢀH stretch at 1842 cmꢀ1. The H NMR
1
spectrum of 5 contained a sharp ZnꢀH resonance at δ 4.72 and a
very broad AlꢀH signal at δ 4.40. While the solubility of 5 was
too low in benzene-d6 and other unreactive solvents to permit
collection of a 13C{1H} NMR spectrum, the 1H NMR spectrum
in benzene-d6 suggested two different types of phenyl groups.
In contrast to the formation of 5, the treatment of 2 with
MgBr2 or CoCl2 in THF/Et2O solutions afforded Mg2Br2-
(Ph2pz)2(THF)3 2THF (6, 25%) and Co2Cl2(Ph2pz)2-
3
(THF)3 THF (7, 23%) as colorless and blue crystalline solids,
3
respectively (Scheme 3).8 Complexes 6 and 7 were identified
from spectral and analytical data and by X-ray crystallography.
The IR spectra of 6 and 7 lacked AlꢀH stretches, suggesting that
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Inorganic Chemistry
COMMUNICATION
tetrazolyl groups would have high nitrogen contents, should be
highly endothermic, and would thus be relevant to ongoing efforts
to construct energetic metal salts for a variety of applications.15
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures and ana-
b
lytical and spectroscopic data for 1ꢀ7 and X-ray crystallographic
data for 1ꢀ7 in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: chw@chem.wayne.edu.
’ ACKNOWLEDGMENT
We are grateful to the Office of Naval Research (Grant
N00014-07-1-0105) for generous support of this work.
’ REFERENCES
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Figure 1. Perspective views of 2, 5, and 6 with selected bond lengths
(Å) and angles (deg): (top) 2, AlꢀN1 1.884(1), AlꢀN3 1.899(1),
AlꢀN5 1.893(1), LiꢀN2 2.072(2), LiꢀN4 2.061(2), LiꢀN6 2.137(3),
LiꢀO1 1.967(2), AlꢀH 1.46(1); N1ꢀAlꢀN3 110.58(5), N1ꢀAlꢀN5
100.49(5), N3ꢀAlꢀN5 104.53(5), N2ꢀLiꢀN4 98.8(1), N2ꢀLiꢀN6
101.7(1), N4ꢀLiꢀN6 95.0(1); (middle) 5, AlꢀN10 1.897(8), AlꢀN20
1.892(9), AlꢀN3 1.907(9), ZnꢀN1 2.112(4), ZnꢀN2 2.042(4),
ZnꢀN30 2.061(4); N10ꢀAlꢀN20 101.7(3), N10ꢀAlꢀN3 106.6(4),
N20ꢀAlꢀN3 109.0(4), N1ꢀZnꢀN2 90.1(1), N1ꢀZnꢀN30 93.9(1),
N2ꢀZnꢀN30 97.8(1); (bottom) 6, Mg1ꢀBr1 2.478(1), Mg2ꢀBr2
2.482(1), Mg1ꢀN1 2.098(2), Mg1ꢀN3 2.108(2), Mg2ꢀN2 2.104(2),
Mg2ꢀN4 2.105(2), Mg1ꢀO1 2.268(2), Mg1ꢀO2 2.050(2), Mg2ꢀO1
2.285(2), Mg2ꢀO3 2.052(2).
exhibited vigorous gas evolution upon the treatment with 2 and
concomitant formation of metallic precipitates. This behavior is
consistent with hydrogen transfer from aluminum to metal,
followed by H2 reductive elimination and the formation of metal
powders, and is similar to Storr’s observations upon the at-
tempted syꢀnthesis of nickel and copper complexes containing
GaH2(pz)2 ligands.3 Pathways that lead to hydride and pyr-
azolate transfer with 2 presumably also afford the neutral species
Al(Ph2pz)3 and [AlH(Ph2pz)2]n, but examples of these com-
plexes have not been crystallized in pure form so far. However,
Al(tBu2pz)312 and Al hydride pyrazolates13 have been reported.
The present work also implies that analogues of 1ꢀ4 containing
1,2,4-triazolyl and tetrazolyl groups should be easily prepared by
protonolysis routes between LiAlH4 and the hydrogen-substi-
tuted heterocycles because some of the corresponding B ligands
are known.1,7,14 Al-based ligands containing 1,2,4-triazolyl and
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C. H. Eur. J. Inorg. Chem. 2005, 330ꢀ337 and references cited therein.
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Hemling, H.; Hinrichs, W.; Lentz, D. Chem.—Eur. J. 1995, 1, 637–644 and
references contained therein. (b) Groshens, T. J. J. Coord. Chem. 2010,
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