Tetraalkoxyaluminates of nickel(II), copper(II), and copper(I)

Article abstract of DOI:10.1021/ic701729y

The syntheses and structural details of tetraisopropoxyaluminates and tetra-tert-butoxyaluminates of nickel(II), copper-(I), and copper(II) are reported. Within the nickel series, either Ni[Al(OⁱPr) ₄]₂·2HOⁱPr, with nickel(II) in a distorted octahedral oxygen environment, or Ni[Al(OⁱPr) ₄]₂·py, with nickel(II) in a square-pyramidal O₄N coordination sphere, or Ni[(ⁱPrO)(¹BuO) ₃Al]₂, with Ni(II) in a quasi-tetrahedral oxygen coordination, has been obtained. Another isolated complex is Ni[ ⁱPrO)₃AlOAl(OⁱPr)₃]·3py (with nickel(II) being sixfold-coordinated), which may also be described as a NiO species trapped by two Al(OⁱPr)₃ Lewis acid-base systems stabilized at nickel by three pyridine donors. Copper(I) compounds have been isolated in three forms: [(ⁱPrO) ₄Al]Cu·2py, [(^tBuO)₄Al]Cu·2py, and Cu₂[^tBuO)₄Al]₂. In all of these compounds, the aluminate moiety behaves as a bidentate unit, creating a tetrahedrally distorted N₂O₂ copper environment in the pyridine adducts. In the base-free copper(I) tert-butoxyaluminate, a dicopper dumbbell [Cu-Cu 2.687(1) A] is present with two oxygen contacts on each of the copper atoms. Copper(II) alkoxyaluminates have been characterized either as Cu[^tBuO)₄Al]₂, {Cu(ⁱPrO)[( ⁱPrO)₄Al]}₂, and Cu[^tBuO) ₃(ⁱPrO)Al]₂ (copper being tetracoordinated by oxygen) or as [ⁱPrO)₄Al]₂Cu·py (pentacoordinated copper similar to the nickel derivative). Finally, a copper(II) hydroxyaluminate has been isolated, displaying pentacoordinate copper (O₄N coordination sphere) by dimerization, with the formula {[( ^tBuO)₄Al]Cu(OH)·py}₂. The formation of all of these isolated products is not always straightforward because some of these compounds in solution are subject to decomposition or are involved in equilibria. Besides NMR [copper(I) compounds], UV absorptions and magnetic moments are used to characterize the compounds.

Full text of DOI:10.1021/ic701729y

Inorg. Chem. 2008, 47, 1204−1217
Tetraalkoxyaluminates of Nickel(II), Copper(II), and Copper(I)
Michael Veith,*,†,‡ Kroum Valtchev,† and Volker Huch†
Institute of Inorganic Chemistry, Saarland UniVersity, Saarbruecken, Germany, and
Leibniz-Institut fuer Neue Materialien (INM), Saarbruecken, Germany
Received September 3, 2007
The syntheses and structural details of tetraisopropoxyaluminates and tetra-tert-butoxyaluminates of nickel(II), copper-
(I), and copper(II) are reported. Within the nickel series, either Ni[Al(OiPr)4]2 2HOiPr, with nickel(II) in a distorted
octahedral oxygen environment, or Ni[Al(OiPr)4]2
py, with nickel(II) in a square-pyramidal O4N coordination sphere,
or Ni[(iPrO)(tBuO)3Al]2, with Ni(II) in a quasi-tetrahedral oxygen coordination, has been obtained. Another isolated
complex is Ni[(iPrO)3AlOAl(OiPr)3]
3py (with nickel(II) being sixfold-coordinated), which may also be described as
a “NiO” species trapped by two Al(OiPr)3 Lewis acid
base systems stabilized at nickel by three pyridine donors.
Copper(I) compounds have been isolated in three forms: [(iPrO)4Al]Cu 2py, [(tBuO)4Al]Cu 2py, and Cu2[(tBuO)4Al]2.
‚
‚
‚
−
‚
‚
In all of these compounds, the aluminate moiety behaves as a bidentate unit, creating a tetrahedrally distorted
N2O2 copper environment in the pyridine adducts. In the base-free copper(I) tert-butoxyaluminate, a dicopper dumbbell
[Cu
have been characterized either as Cu[(tBuO)4Al]2,
tetracoordinated by oxygen) or as [(iPrO)4Al]2Cu
py (pentacoordinated copper similar to the nickel derivative). Finally,
a copper(II) hydroxyaluminate has been isolated, displaying pentacoordinate copper (O4N coordination sphere) by
dimerization, with the formula py 2. The formation of all of these isolated products is not always
[(tBuO)4Al]Cu(OH)
−
Cu 2.687(1) Å] is present with two oxygen contacts on each of the copper atoms. Copper(II) alkoxyaluminates
{
Cu(iPrO)[(iPrO)4Al] 2, and Cu[(tBuO)3(iPrO)Al]2 (copper being
}
‚
{
‚ }
straightforward because some of these compounds in solution are subject to decomposition or are involved in
equilibria. Besides NMR [copper(I) compounds], UV absorptions and magnetic moments are used to characterize
the compounds.
Introduction
Double alkoxides of bivalent late transition metals with
aluminum(III) represented by the general formula M[Al-
(OR)4]2 [where M ) MnII, FeII, CoII, NiII, and CuII and R )
Since the pioneering work carried out by Meerwein and
Bersin,1 the field of heterometallic alkoxides has been rapidly
growing.2 The increasing interest is due to their potential
application as precursors for oxide-based ceramic materials
using either sol-gel3 or chemical vapor deposition (CVD)
techniques.4
i
Me, Et, Pr, and OtBu] were synthesized by Mehrotra and
co-workers.5 Predictions of the structural features of the
heterometallic alkoxides were made on the basis of spec-
troscopic and magnetic measurements.6 The alkoxo anions
[Al(OR)4]- have been reported to behave as ambidentate
ligands depending on the nature of the central metal atom
and on the steric bulk of the alkoxy group involved. In
view of that mentioned above, it is of considerable interest
to explore the chemistry and bonding characteristics of
the tetraalkoxyaluminates with respect to the later 3d
metals.
* To whom correspondance should be addressed. E-mail: veith@mx.
uni-saarland.de.
† Saarland University.
‡ INM.
(1) Meerwein, H.; Bersin, T. Liebigs Ann. Chem. 1929, 476, 113-150.
(2) (a) Veith, M.; Mathur, S.; Mathur, Ch. Polyhedron 1998, 17, 1005-
1034. (b) Mehrotra, R. C.; Singh, A.; Sogani, S. Chem. ReV. 1994,
94, 1643-1660. (c) Mehrotra, R. C.; Singh, A.; Sogani, S. Chem.
Soc. ReV. 1994, 215-225. (d) Caulton, K. G.; Hubert-Pfalzgraf, L.
G. Chem. ReV. 1990, 90, 969-995.
(3) Mehrotra, R. C. Chemtracts: Anal., Phys., Inorg. Chem. 1990, 2, 389-
399.
(4) (a) Jones, A. C. Chem. Vap. Deposition 1998, 4, 169-179. (b)
Herrmann, W. A.; Huber, N. W.; Runte, O. Angew. Chem. 1995, 107,
2371-2390.
(5) (a) Singh, J. V.; Jain, N. C.; Mehrotra, R. C. Synth. React. Inorg.
Met.-Org. Chem. 1979, 9 (1), 79-88. (b) Garg, G.; Dubey, R. K.;
Singh, A.; Mehrotra, R. C. Polyhedron 1991, 10, 1733-1739.
(6) (a) Mehrotra, R. C.; Singh, J. Can. J. Chem. 1984, 62, 1003-1007.
(b) Mehrotra, R. C.; Singh, J. V. Z. Anorg. Allg. Chem. 1984, 512,
221-230. (c) Stampp, E.; Hillebrand, U. Z. Naturforsch. 1979, 34b,
262-265.
1204 Inorganic Chemistry, Vol. 47, No. 3, 2008
10.1021/ic701729y CCC: $40.75
© 2008 American Chemical Society
Published on Web 01/11/2008

Tetraalkoxyaluminates of Ni(II), Cu(II), and Cu(I)
Table 1. Crystal Data and Structure Solution Parameters of Compounds 4, 5, 7, and 9-11
compound
empirical
formula
4
5
7
9
10
11
C30H72Al2NiO10
C29H61Al2NNiO8
C33H57Al2N3NiO7
C30H70Al2Cu2O10
C29H61Al2CuNO8
C22H38AlCuN2O4
fw
705.55
orthorhombic
Pbca
664.46
monoclinic
P21/c
720.49
monoclinic
C2/c
771.90
monoclinic
P21/n
669.29
monoclinic
P21
485.06
orthorhombic
Pccn
cryst syst
space group
unit cell dimens:
a (Å)
10.678(2)
18.222(4)
44.127(9)
13.084(3)
17.054(3)
17.805(4)
12.034(2)
16.732(3)
17.281(3)
10.178(2)
16.774(3)
12.956(3)
9.250(2)
16.373(3)
13.473(3)
17.064(3)
9.400(2)
16.945(3)
b (Å)
c (Å)
R (deg)
â (deg)
97.81(3)
95.25(3)
93.36(3)
99.41(3)
γ (deg)
V (Å3)
8586(3)
8
3936(2)
4
3465(1)
4
2208.1(8)
2
2013.0(7)
2
2718.0(9)
4
Z
densitycalc
1.092
1.121
1.381
1.161
1.104
1.185
(Mg/m3)
abs coeff (mm-1
F(000)
)
0.535
3088
0.577
1440
0.661
1544
1.044
828
0.625
722
0.862
1032
cryst size (mm3)
θ range for data
collcn
0.5× 0.44 × 0.2
0.6 × 0.45 × 0.3
0.4 × 0.35 × 0.2
0.60 × 0.19 × 0.12
0.5 × 0.35 × 0.28
0.3 × 0.25 × 0.15
2-18.52
1.6-22.56
2-24.94
2.43-24.02
1.53-22.45
2.7-24.15
reflns collected
indep reflns
[I > 2σ(I)]
data/param
GOF in F 2
final R indices
(wR2)
18 234
3122
5154
3740
11 141
2821
13 309
3311
2693
2693
16 000
2040
3122/388
1.045
0.059 (0.145)
5154/370
1.055
0.061 (0.146)
2821/210
0.970
0.070 (0.18)
3311/218
0.997
0.047 (0.123)
2693/370
1.130
0.047 (0.138)
2040/137
1.025
0.064 (0.17)
[I > 2σ(I)]
R indices (all data)
largest diff peak
and hole (e/Å3)
0.081 (0.161)
0.336 and -0.222
0.091 (0.173)
0.386 and -0.283
0.093 (0.20)
0.590 and-0.346
0.074 (0.134)
0.390 and -0.326
0.055 (0.153)
0.318 and -0.237
0.099 (0.19)
0.352 and -0.350
(OtBu)3 were prepared by the reaction of aluminum metal with
isopropyl alcohol and tert-butanol, respectively, following a
published procedure.8 Anhydrous nickel(II), copper(II), and copper-
(I) chlorides were prepared by literature methods.9
Physical Methods. 1H and 13C NMR were obtained on a Bruker
AC-200 spectrometer, and chemical shifts were referenced to
solvent resonances. Magnetic moments in solution were measured
by the Evans method10 and were adjusted for diamagnetic contribu-
tions using Pascal’s constants. The LECO CHN 900 elemental
analyzer was used for elemental analyses. UV-vis spectra were
recorded on a Pye Unicam SP8-100 or a Perkin-Elmer Lambda
35. The metal contents of the new compounds were obtained by
titrimetric procedures.
Crystallography. Diffraction intensities were collected on a Stoe
IPDS or Siemens Stoe AED 2 diffractometer equipped with
graphite-monochromated Mo KR radiation. The structures were
solved by direct methods and refined with the full-matrix least
squares on F 2 using the SHELXS-97 and SHELXL-97 programs.11
Anisotropic thermal parameters were assigned to all non-H atoms.
The H atoms were generated geometrically. The crystallographic
data are summarized in Tables 1 and 2. Selected bond lengths and
angles for 4, 5, 7, 9-11, and 12-17 are listed in Tables 3 and 4.
Full data are available upon request.
The preparation of mixed-metal oxides with spinel struc-
tures using either a sol-gel or microemulsion-mediated sol-
gel process has been the subject of several papers.7 We were
exploring the possibility of using these compounds as single-
source precursors in a CVD process. In the course of our
investigations, we were faced with a series of problems
concerning the preparation and, in particular, the purification
of the compounds. Furthermore, the double isopropoxides
appeared to partly decompose upon distillation. As a result
of this, the metal content of the materials obtained by the
CVD process varied significantly in the expected values. In
order to gain insight into the behavior of the mixed-metal
alkoxides, which is necessary for control of the CVD process,
the present study was undertaken.
This paper is concerned with the synthesis and structural
characterization of the heterometalic alkoxides of nickel(II)
and copper(II) containing [Al(OR)4]- groups (R ) OiPr and
OtBu). Our interest was focused on their thermal stability
and, in particular, on the decomposition of the isopropoxides
upon vaporization. To the best of our knowledge, there is
no example of copper(I) aluminates reported in the literature
so far. Therefore, this work was later extended to the
synthesis and characterization of the corresponding copper-
(I) derivatives.
(7) (a) Otero Arean, C.; Penarroya Mentruit, M.; Lopez Lopez, A. J.; Parra,
J. B. Colloids Surf. A 2001, 180 (3), 253-258. (b) Escalona Platero,
E.; Otero Arean, C.; Parra, J. B. Res. Chem. Intermed. 1999, 25 (2),
187-194. (c) Meyer, F.; Hempelmann, R.; Mathur, S.; Veith, M. J.
Mater. Chem. 1999, 9, 1755-1763.
(8) Adkins, H. J. Am. Chem. Soc. 1922, 44, 2175-2180.
(9) Brauer, G. Handbuch der PraeparatiVen Anorganischen Chemie;
Ferdinand Enke Verlag: Stuttgart, Germany, 1978.
Experimental Section
General Comments. All operations were carried out under an
atmosphere of purified and dry nitrogen using a modified Stock
apparatus. All solvents were dried by distillation from sodium. The
hydrocarbon solvents were stored over sodium, isopropyl alcohol,
tert-butanol, and pyridine over molecular sieves. Al(OiPr)3 and Al-
(10) Evans, D. F. J. Chem. Soc. 1959, 2003-2005.
(11) (a) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure
Solution; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (b)
Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Deter-
mination; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 3, 2008 1205

Veith et al.
Table 2. Crystal Data and Structure Solution Parameters of Compounds 12-17
compound
empirical
formula
12
13
14
15
16
17
C30H68Al2CuO8
C30H68Al2NiO8
C32H72Al2CuO8
C26H46AlCuN2O4
C32H72Al2Cu2O8
C42H84Al2Cu2N2O10
fw
674.34
monoclinic
P21/n
669.51
orthorhombic
P212121
702.40
monoclinic
P21/n
541.17
triclinic
P1h
765.94
triclinic
P1h
958.15
triclinic
P1h
cryst syst
space group
unit cell dimens:
a (Å)
9.976(2)
16.361(3)
25.451(5)
9.317(2)
17.258(3)
26.922(5)
10.068(2)
16.493(3)
25.809(5)
9.820(2)
11.993(2)
13.846(3)
83.03(3)
73.62(3)
81.39(3)
1541.5(5)
2
9.536(2)
10.209(2)
11.387(2)
96.91(3)
103.95(3)
96.40(3)
1056.6(4)
1
10.275(2)
10.668(2
13.277(3)
75.40(3)
74.85(3)
79.11(3)
1347.6(5)
1
b (Å)
c (Å)
R (deg)
â (deg)
92.29(3)
93.05(3)
γ (deg)
V (Å3)
Z
4150.7(14)
4
4328.9(14)
4
4280(1)
4
densitycalc
1.079
1.027
1.090
1.166
1.204
1.181
(Mg/m3)
abs coeff (mm-1
F(000)
)
0.606
1468
0.524
1464
0.590
1532
0.766
580
1.088
412
0.870
514
cryst size (mm3)
θ range for data
collcn
0.5 × 0.4 × 0.35
0.4 × 0.1 × 0.08
0.8 × 0.4 × 0.3
0.4 × 0.35 × 0.2
0.3 × 0.22 × 0.2
0.5 × 0.4 × 0.28
2.63-24.22
1.51-24.00
1.6-22.50
1.54-22.50
1.86-23.97
2.63-23.96
reflns collected
indep reflns
[I > 2σ(I)]
data/param
GOF in F 2
final R indices
(wR2)
22 740
6574
3841
3841
5584
4376
4031
3430
6600
3075
8237
3863
6574/370
0.953
0.079 (0.219)
3841/445
1.123
0.076 (0.164)
5584/388
1.071
0.067 (0.18)
4031/363
1.104
0.045 (0.117)
3075/199
1.082
0.031 (0.083)
3863/157
1.77
0.096 (0.261)
[I > 2σ(I)]
R indices (all data)
largest diff peak
and hole (e/Å3)
0.12 (0.25)
0.771 and -0.282
0.13 (0.21)
0.339 and -0.180
0.088 (0.20)
0.621 and -0.340
0.056 (0.128)
0.232 and -0.404
0.038 (0.098)
0.362 and -0.233
0.11 (0.27)
2.038 and -0.701
Syntheses and Reactions. {Ni[Al(OiPr)4]2} (1). A solution of
K[Al(OiPr)4], prepared by the reaction of potassium metal (2.46 g,
62.8 mmol) in isopropyl alcohol (100 mL) and Al(OiPr)3 (12.83 g,
62.82 mmol), was added to a suspension of NiCl2 (4.071 g, 31.41
mmol) in isopropyl alcohol (20 mL). The reaction mixture was
refluxed for 20 h for the completion of the reaction. Toluene (50
mL) was added to the mixture, and the precipitated KCl was
removed by filtration. The solvent was evaporated off under reduced
pressure, and the remaining violet liquid was filtered once again
to remove the last solid residues. Yield of crude product: 15.63 g
(85%). The vacuum distillation (90 °C, 10-3 mbar) yielded 11.95
g (65%) of 1 as a violet liquid, which contains a small amount of
Al(OiPr)3 (1-2%). Compound 1 could be obtained in a pure form
as a crystalline solid after removal of the solvent from {Ni[Al(Oi-
Pr)4]2‚2HOiPr} (4) in vacuum (50 °C, 10-3 mbar). Anal. Calcd for
C24H56Al2NiO8: C, 49.24; H, 9.64; Al, 9.22; Ni, 10.03. Found: C,
the solution changed from violet to yellow. Upon cooling at ambient
temperature, yellow crystals of 4 were formed. The first crop (2.156
g) was isolated by filtration, and the filtrate was concentrated to
yield a second crop (1.161 g). Total yield: 67%. Anal. Calcd for
C30H72Al2NiO10: C, 51.07; H, 10.29; Al, 7.65; Ni, 8.32. Found:
C, 51.21; H, 10.52; Al, 7.63; Ni, 8.34. UV-vis (isopropyl alcohol;
λ
max, nm (ꢀ, M-1 cm-1)): 786 (3.5), 733 (sh, 2.5), 423 (16).
Magnetic moment obtained by the Evans method (C6H6/HOiPr/
C6D6): 3.25 µB.
{Ni[Al(OiPr)4]2‚py} (5). Equimolar reaction between 1 and
pyridine in toluene yielded a dark-yellow oil, which crystallized
slowly at ambient temperature. Anal. Calcd for C29H61Al2NNiO8:
C, 52.42; H, 9.25; N, 2.11; Al, 8.12; Ni, 8.83. Found: C, 52.62;
H, 9.61; N, 1.96; Al, 7.71; Ni, 8.69. UV-vis [toluene; λmax, nm (ꢀ,
M-1 cm-1)]: 782, 739, 599, 504, 424 (54). Magnetic moment
obtained by the Evans method (C6H5CH3/C6D6): 3.18 µB.
{Ni[Al(OiPr)4]2‚2py} (6) was prepared by the reaction of 1 with
an excess of pyridine used as a solvent. Upon concentration, green
crystals from 6 formed. Anal. Calcd for C34H66Al2N2NiO8: C,
54.92; H, 8.95; N, 3.77; Al, 7.26; Ni, 7.89. Found: C, 55.86; H,
9.13; N, 3.55; Al, 6.16; Ni, 7.11. UV-vis [toluene; λmax, nm (ꢀ,
M-1 cm-1)]: 756 (3), 691 (sh), 406 (10).
48.50; H, 9.78; Al, 9.27; Ni, 9.86. UV-vis [cyclohexane; λmax
,
nm (ꢀ, M-1 cm-1)]: 424 (24), 525 (111), 578 (97), 781 (11), 819
(12). Magnetic moment obtained by the Evans method (C6H6/
C6D6): 3.19 µB.
{(iPrO)Ni[Al(OiPr)4]}2 (3). 1 (3.64 g, 6.22 mmol) was allowed
to reflux for approximately 70 h, yielding 0.90 g of 3 as blue solid
residue. The obtained sublimate, which consisted of 1 and Al(Oi-
Pr)3, was re-treated in the same way to yield a second crop of 0.48
g of 3 (total yield: 58%). Anal. Calcd for C30H70Al2Ni2O10: C,
47.24; H, 9.25; Al, 7.08; Ni, 15.39. Found: C, 47.59; H, 9.14; Al,
7.21; Ni, 15.70. UV-vis [cyclohexane; λmax, nm (ꢀ, M-1 cm-1)]:
440 (59), 531 (244), 588 (200), 787 (22), 820 (26). EI-MS (90 eV,
m/e): 745 ([M - CH3]+, 642 [M - OiPr]+, 627 [M - OiPr -
CH3]+, 583 [M - OiPr - OiPr]+, 569 [M - Ni(OiPr)2 - CH3]+.
Magnetic moment obtained by the Evans method (C6H12/C6D6):
3.41 µB.
[NiOAl2(OiPr)6‚3py] (7) was obtained as a side product by the
synthesis of 6 when water residues were present in the solvent.
For analysis, blue crystals were selected from the mixture of
crystals. Anal. Calcd for C33H57Al2N3NiO7: C, 55.01; H, 7.97; N,
5.83. Found: C, 54.83; H, 8.05; N, 5.62.
{Cu[Al(OiPr)4]2} (2) was prepared analogously to 1 by the
reaction of anhydrous CuCl2 (4.73 g, 35.1 mmol) and K[Al(Oi-
Pr)4] prepared by the reaction of potassium metal (2.75 g, 70.3
mmol) in isopropyl alcohol (100 mL) and Al(OiPr)3 (14.36 g, 70.3
mmol). Yield: 19.47 g (94%) of a viscous bluish-green liquid. The
distillation at 95 °C and 2 × 10-2 mbar is accompanied by
decomposition, yielding an impure product. For this reason, 2 was
{Ni[Al(OiPr)4]2‚2HOiPr} (4). 1 (4.10 g, 7.00 mmol) was
dissolved in isopropyl alcohol (10 mL), whereupon the color of
1206 Inorganic Chemistry, Vol. 47, No. 3, 2008

Tetraalkoxyaluminates of Ni(II), Cu(II), and Cu(I)
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 4, 5, 7, and
9-11
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 12-17
{Cu[Al(OiPr)(OtBu)3]2} (12)
{Ni[Al(OiPr)4]2‚2HOiPr} (4)
Cu1-O2 1.918(4) O2-Cu1-O1 119.4(2)
Cu1-O1 1.926(4) O2-Cu1-O4 139.0(2)
Cu1-O4 1.927(4) O1-Cu1-O4 78.0(2)
Cu1-O3 1.929(4) O2-Cu1-O3 77.9(2)
Al1-O6 1.631(6) O1-Cu1-O3 138.2(2)
Al1-O5 1.637(6) O4-Cu1-O3 115.1(2)
Al1-O4 1.768(4) O6-Al1-O5 119.5(4)
Al1-O1 1.789(4) O6-Al1-O4 110.1(4)
O5-Al1-O1 112.1(3)
O4-Al1-O1 86.0(2)
O8-Al2-O7 119.3(5)
O8-Al2-O3 112.8(4)
O7-Al2-O3 111.4(3)
O8-Al2-O2 110.8(4)
O7-Al2-O2 112.0(3)
O3-Al2-O2 85.7(2)
Ni1-O1 2.039(5) O1-Ni1-O4
Ni1-O4 2.055(5) O1-Ni1-O2
Ni1-O2 2.059(5) O4-Ni1-O2
Ni1-O3 2.061(5) O1-Ni1-O3
Ni1-O10 2.093(5) O4-Ni1-O3
94.6(2) O8-Al2-O7
73.4(2) O8-Al2-O4
99.1(2) O7-Al2-O4
98.6(2) O8-Al2-O3
73.2(2) O7-Al2-O3
112.3(3)
120.7(3)
106.7(3)
118.8(3)
107.1(3)
88.5(2)
113.5(3)
120.6(3)
106.5(3)
118.0(3)
106.9(3)
88.2(2)
Ni1-O9 2.124(5) O2-Ni1-O3 168.7(2) O4-Al2-O3
Al2-O8 1.652(7) O1-Ni1-O10 86.9(2) O6-Al1-O5
Al2-O7 1.733(6) O4-Ni1-O10 173.9(2) O6-Al1-O1
Al2-O4 1.750(5) O2-Ni1-O10 87.0(2) O5-Al1-O1
Al2-O3 1.765(5) O3-Ni1-O10 100.7(2) O6-Al1-O2
Al1-O6 1.659(7) O1-Ni1-O9 176.0(2) O5-Al1-O2
Al2-O8 1.624(7) O5-Al1-O4 114.0(3) Al1‚‚‚Cu1‚‚‚Al2 174.93(6)
Al2-O7 1.637(5) O6-Al1-O1 110.2(3)
Al2-O3 1.771(4)
Al2-O2 1.784(4)
Cu1‚‚‚Al1 2.795(2)
Al1-O5 1.711(6) O4-Ni1-O9
87.5(2) O1-Al1-O2
Cu1‚‚‚Al2 2.800(2)
Al1-O1 1.750(5) O2-Ni1-O9 103.0(2) Al1‚‚‚Ni1‚‚‚Al2 138.14(8)
Al1-O2 1.766(5) O3-Ni1-O9
Ni1‚‚‚Al1 2.851(2)
85.2(2)
{Ni[Al(OiPr)(OtBu)3]2} (13)
Ni-O1 1.939(7)
Ni-O2 1.938(7)
Ni-O4 1.951(7)
Ni-O3 1.954(7)
Al1-O5 1.65(1)
Al1-O6 1.67(1)
O1-Ni-O2
77.1(3)
O2-Al1-O1
84.9(3)
Ni1‚‚‚Al2 2.857(3)
O1-Ni-O4 129.7(3)
O2-Ni-O4 129.4(3)
O1-Ni-O3 126.0(3)
O2-Ni-O3 125.1(3)
O7-Al2-O8 117.3(7)
O7-Al2-O3 112.5(5)
O8-Al2-O3 112.2(6)
O7-Al2-O4 114.0(5)
O8-Al2-O4 110.8(6)
{Ni[Al(OiPr)4]2‚py} (5)
Ni1-O4 1.991(3) Ni1‚‚‚Al2
Ni1-O1 1.996(3) Ni1‚‚‚Al1
2.884(2) O6-Al1-O1
2.893(2) O5-Al1-O1
113.0(2)
109.5(2)
111.0(2)
113.2(2)
87.5(2)
115.1(3)
114.6(3)
111.6(2)
115.1(3)
109.7(2)
87.6(2)
O4-Ni-O3
77.5(3)
Ni1-N1 2.001(5) O4-Ni1-O1 141.5(2)
Ni1-O3 2.050(3) O4-Ni1-N1 111.9(2)
Ni1-O2 2.054(3) O1-Ni1-N1 106.6(2)
Al1-O6 1.666(5) O4-Ni1-O3 74.6(1)
Al1-O5 1.676(4) O1-Ni1-O3 102.3(1)
Al1-O1 1.772(4) N1-Ni1-O3 94.1(2)
Al1-O2 1.774(4) O4-Ni1-O2 101.5(1)
Al2-O8 1.642(5) O1-Ni1-O2 74.5(1)
Al2-O7 1.687(5) N1-Ni1-O2 96.4(2)
Al2-O3 1.762(4) O3-Ni1-O2 169.5(1)
Al2-O4 1.775(4) O6-Al1-O5 118.6(2)
O6-Al1-O2
O5-Al1-O2
O1-Al1-O2
O8-Al2-O7
O8-Al2-O3
O7-Al2-O3
O8-Al2-O4
O7-Al2-O4
O3-Al2-O4
Al1-O2 1.772(7) O5-Al1-O6 119.4(7)
O3-Al2-O4
85.8(3)
Al1-O1 1.805(8) O5-Al1-O2 110.3(5) Al1‚‚‚Ni‚‚‚Al2 173.3(1)
Al2-O7 1.63(1)
Al2-O8 1.64(1)
O6-Al1-O2 113.1(5)
O5-Al1-O1 111.7(5)
Al2-O3 1.789(7) O6-Al1-O1 112.3(5)
Al2-O4 1.802(8)
Ni‚‚‚Al1 2.831(3)
Ni‚‚‚Al2 2.838(3)
Al2‚‚‚Ni1‚‚‚Al1 149.86(6)
{Cu[Al(OtBu)4]2} (14)
Cu1-O4 1.930(3) O4-Cu1-O2 134.1(2)
Cu1-O2 1.935(3) O4-Cu1-O3 118.8(2)
Cu1-O3 1.941(3) O2-Cu1-O3 77.6(2)
Cu1-O1 1.943(3) O4-Cu1-O1 77.8(1)
Al1-O5 1.644(5) O2-Cu1-O1 124.0(1)
Al1-O6 1.648(5) O3-Cu1-O1 133.4(1)
Al1-O4 1.787(4) O5-Al1-O6 119.1(3)
Al1-O1 1.794(3) O5-Al1-O4 113.7(3)
O6-Al1-O1 111.3(3)
O4-Al1-O1 85.6(2)
O8-Al2-O7 118.5(4)
O8-Al2-O3 113.4(3)
O7-Al2-O3 111.7(3)
O8-Al2-O2 110.7(3)
O7-Al2-O2 112.3(3)
O3-Al2-O2 85.8(2)
[NiAl2O(OiPr)6‚3py] (7)
Ni1-O4 1.990(4) O4-Ni1-N2 180.000(1) N2-Ni1-O1′ 104.09(9)
Ni1-N2 2.014(5) O4-Ni1-N1
Ni1-N1 2.021(4) N2-Ni1-N1
92.3(1)
87.8(1)
O1-Ni1-O1′ 151.8(2)
O2-Al1-O3 106.6(2)
O2-Al1-O4 117.3(2)
Ni1-O1 2.025(3) N1′-Ni1-N1 175.5(2)
Al1-O2 1.624(3) O4-Ni1-O1
Al1-O3 1.642(3) N2-Ni1-O1 104.09(9) O2-Al1-O1 116.3(2)
Al1-O4 1.676(1) N1′-Ni1-O1 87.4(1) O3-Al1-O1 105.3(2)
Al1-O1 1.726(3) N1-Ni1-O1 93.7(1) O4-Al1-O1 93.1(2)
75.91(9) O3-Al1-O4 117.5(2)
Al2-O8 1.637(5) O6-Al1-O4 110.0(3) Al2‚‚‚Cu1‚‚‚Al1 173.71(5)
Al2-O7 1.643(4) O5-Al1-O1 112.3(2)
Al2-O3 1.778(4)
Ni1‚‚‚Al1 2.752(2) O4-Ni1-O1′ 75.91(9)
Al2-O2 1.788(4)
Cu1‚‚‚Al2 2.815(2)
{(iPrO)Cu[Al(OiPr)4]}2 (9)
Cu1‚‚‚Al1 2.816(2)
Cu-O1
Cu-O1′
Cu-O3
Cu-O2
Al-O5
Al-O4
Al-O2
Al-O3
Cu‚‚‚Al
1.918(3) O1-Cu-O1′
1.921(3) O1-Cu-O3
82.0(1) O5-Al-O2 116.2(2)
104.3(1) O4-Al-O2 115.0(2)
{Cu[Al(OtBu)4]‚2py} (15)
1.947(3) O1′-Cu-O3 160.1(1) O5-Al-O3 116.4(2)
Cu1-N1 1.949(3) N1-Cu1-N2 137.4(1)
Cu1-N2 1.968(3) N1-Cu1-O1 112.0(1)
Cu1-O1 2.232(2) N2-Cu1-O1 102.9(1)
Cu1-O2 2.246(2) N1-Cu1-O2 111.8(1)
Al1-O4 1.708(2) N2-Cu1-O2 102.9(1)
O4-Al1-O3 109.8(1)
O4-Al1-O1 117.2(1)
O3-Al1-O1 109.5(1)
O4-Al1-O2 116.9(1)
O3-Al1-O2 109.6(1)
69.17(8) O1-Al1-O2 92.6(1)
1.951(3) O1-Cu-O2
1.675(4) O1′-Cu-O2 104.1(1) O2-Al-O3
1.685(4) O3-Cu-O2
1.793(3) O5-A1-O4
1.794(3)
158.7(1) O4-Al-O3 116.0(2)
85.1(1)
77.0(1)
107.3(3)
Al1-O3 1.715(2) O1-Cu1-O2
Al1-O1 1.756(2)
2.846(1)
Cu‚‚‚Cu′ 2.898(1)
Al1-O2 1.759(2)
Cu1‚‚‚Al1 2.991(1)
{Cu[Al(OiPr)4]2‚py} (10)
[CuAl(OtBu)4]2 (16)
Cu1-O2 1.884(2) Al1-O3 1.691(2) O2-Cu1-O1 176.44(7)
Cu1-O1 1.964(5) Cu1‚‚‚Al1
Cu1-O3 1.973(5) Cu1‚‚‚Al2
2.854(2) O7-Al1-O4
2.956(3) O5-Al1-O1
113.6(3)
117.2(3)
110.7(3)
85.9(2)
114.3(5)
110.0(4)
115.6(5)
109.9(4)
113.1(4)
91.7(3)
Cu1-O4 1.976(5) O1-Cu1-O3 170.3(2)
Cu1-N1 2.020(7) O1-Cu1-O4 76.7(2)
Cu1-O2 2.316(6) O3-Cu1-O4 96.6(2)
Al1-O5 1.695(6) O1-Cu1-N1 92.0(3)
Al1-O7 1.706(7) O3-Cu1-N1 92.1(2)
Al1-O4 1.780(6) O4-Cu1-N1 158.7(3)
Al1-O1 1.809(6) O1-Cu1-O2 115.2(2)
Al2-O6 1.687(9) O3-Cu1-O2 72.7(2)
Al2-O8 1.692(8) O4-Cu1-O2 100.8(3)
Al2-O2 1.754(6) N1-Cu1-O2 100.3(3)
Al2-O3 1.807(6) O5-Al1-O7 111.6(3)
O5-Al1-O4 115.6(3)
O7-Al1-O1
O4-Al1-O1
O6-Al2-O8
O6-Al2-O2
O8-Al2-O2
O6-Al2-O3
O8-Al2-O3
O2-Al2-O3
Cu1-O1 1.889(2) Al1-O4 1.715(2)
Cu1-Cu1′ 2.687(1) Al1-O2 1.800(2)
Cu1‚‚‚Al1 2.953(1) Al1-O1 1.802(2)
O3-Al1-O4 120.4(1)
O3-Al1-O2 111.5(1)
O4-Al1-O2 106.0(1)
O3-Al1-O1 113.4(1)
O4-Al1-O1 102.4(1)
O2-Al1-O1 101.1(1)
{(HO)Cu[Al(OtBu)4]·2py}2 (17)
Cu1-O1′ 1.925(7) O1′-Cu1-O1
Cu1-O1 1.926(8) O1′-Cu1-N1
78.2(4) N1-Cu1-O3
94.6(3) O2-Cu1-O3
95.2(2)
69.8(2)
Al1‚‚‚Cu1‚‚‚Al2 133.63(8)
Cu1-N1 2.016(7) O1-Cu1-N1 166.8(3) O5-Al1-O4 111.3(3)
Cu1-O2 2.033(5) O1′-Cu1-O2 171.7(3) O5-Al1-O3 117.6(3)
Cu1-O3 2.393(5) O1-Cu1-O2
Al1-O5 1.707(6) N1-Cu1-O2
93.6(3) O4-Al1-O3 111.1(3)
93.2(3) O5-Al1-O2 115.1(3)
{Cu[Al(OiPr)4]‚2py} (11)
Cu1-N1 1.945(4) N1-Cu1-N1′ 145.9(3) O2-Al1-O2′ 103.0(3)
Cu1-O1 2.312(3) N1-Cu1-O1 108.4(2) O2-Al1-O1′ 116.3(2)
Al1-O2 1.720(4) N1-Cu1-O1′
Al1-O1 1.736(3) O1-Cu1-O1′
Al1-O4 1.725(6) O1′-Cu1-O3 111.9(3) O4-Al1-O2 109.7(3)
Al1-O3 1.762(6) O1-Cu1-O3
Al1-O2 1.828(5)
Al1-O2 1.828(5)
97.7(3) O3-Al1-O2
90.5(3)
00.0(2) O2-Al1-O1 113.7(2)
67.0(2) O1′-Al1-O1 94.6(2)
Inorganic Chemistry, Vol. 47, No. 3, 2008 1207

Veith et al.
used without further purification. Anal. Calcd for C24H56Al2CuO8:
C, 48.84; H, 9.56; Al, 9.14; Cu, 10.77. Found: C, 49.17; H, 9.25;
Al, 9.14; Cu, 10.77. UV-vis [benzene; λmax, nm]: 714. Magnetic
moment obtained by the Evans method (C6H6/C6D6): 1.75 µB.
{(iPrO)CuII[Al(OiPr)4]}2 (9). Under prolonged storage, 2 gradu-
ally decomposed to 8, 9, and aluminum isopropoxide. After a period
of 6 months, the compound mixture was filtered, whereupon 9 partly
crystallized out from the filtered green oil. For analytical charac-
terization, blue crystals were selected. Anal. Calcd for C30H70Al2-
Cu2O10: C, 46.68; H, 9.14. Found: C, 46.43; H, 9.20.
{CuII[Al(OiPr)8]2‚py} (10). The reaction of 2 with a slight excess
of pyridine resulted in a bluish-green viscous solution. Under
storage, blue crystals from 10 were formed. However, small
amounts of 11 and aluminum isopropoxide were also present
because of the decomposition of 2, which is even favored by
pyridine. For analytical characterization, blue crystals were selected.
Anal. Calcd for C29H61Al2CuNO8: C, 52.04; H, 9.19; N, 2.09.
Found: C, 51.79; H, 9.60; N, 1.99.
sublimation at 120 °C and 10-3 bar yielded 1.076 g of 13 (94%).
Anal. Calcd for C30H68Al2NiO8: C, 53.82; H, 10.24; Al, 8.06; Ni,
8.77. Found: C, 53.05; H, 9.98; Al, 8.18; Ni, 8.59. UV-vis
[cyclohexane; λmax, nm]: 432, 535, 577, 806.
{CuII[Al(OtBu)4]2} (14). A solution of K[Al(OtBu)4], prepared
by the reaction of KOtBu (0.794 g, 7.08 mmol) in tert-butanol (25
mL) with Al(OtBu)3 (1.744 g, 7.08 mmol) dissolved in toluene (5
mL), was added to a stirred suspension of CuCl2 (0.476 g, 3.54
mmol) in tert-butanol (15 mL). The resulting mixture was refluxed
for 10 days to ensure the completion of the reaction. The
precipitated KCl was removed by filtration, and the solvent was
evaporated off under reduced pressure. Vacuum sublimation at 120
°C gave a mixture of Al(OtBu)3 and 14. The two compounds were
separated by fractional sublimation. After removal of [Al(OtBu)3]2
by heating of the solid mixture at 80 °C and 10-3 mbar, the product
was sublimed in vacuum at 120 °C, yielding 1.551 g of 14 (64%).
Anal. Calcd for C32H72Al2CuO8: C, 54.72; H, 10.33; Al, 7.68; Cu,
9.05. Found: C, 54.78; H, 10.53; Al, 7.14; Cu, 8.93. UV-vis
[cyclohexane; λmax, nm (ꢀ, M-1 cm-1)]: 931 (89). 1H NMR (200.1
MHz, C6D6, 294 K): δ 1.13 (br, 36H). 13C{1H} NMR (50.3 MHz,
C6D6, 294 K): δ 35.68, 65.3. Magnetic moment obtained by the
Evans method (C6H6/C6D6): 1.94 µB.
{CuI[Al(OiPr)4]‚2py} (11). CuCl (0.836 g, 8.445 mmol) and
K[Al(OiPr)4] (2.554 g, 8.445 mmol) were dissolved in pyridine (25
mL). The reaction mixture was allowed to reflux for 15 h. After
removal of the precipitated KCl by filtration, the solution was
concentrated under reduced pressure, whereupon pale-yellow to
colorless crystals of 11 were formed (yield: 72%). Anal. Calcd
for C22H38AlCuN2O4: C, 54.47; H, 7.90; N, 5.78; Al, 5.56; Cu,
13.10. Found: C, 54.87; H, 8.19; N, 5.85; Al, 5.58; Cu, 13.14. 1H
NMR (200.1 MHz, C6D6, 294 K): δ 1.31 (d, 24H), 4.44 (sept,
4H). 13C{1H} NMR (50.3 MHz, C6D6, 294 K): δ 28.58, 63.27.
CuI[Al(OiPr)4] (8) was obtained in a quantitative yield by the
decomposition of 11 at 80 °C in dynamic vacuum. Alternatively, 8
was obtained as a pale-green residue by the thermal decomposition
of 2 upon prolonged refluxing. Anal. Calcd for C12H28AlCuO4: C,
44.09; H, 8.63; Al, 8.25; Cu, 19.44. Found: C, 44.07; H, 8.39; Al,
{CuI[Al(OtBu)4]‚2py} (15). A solution of Al(OtBu)3 (2.357 g,
9.57 mmol) in toluene (25 mL) and a solution of KOtBu (2.357 g,
9.57 mmol) in tert-butanol (25 mL) and pyridine (5 mL) were
subsequently added to CuCl (0.947 g, 9.57 mmol). The resulting
mixture was refluxed for 7 days to ensure the completion of the
reaction. Filtration of KCl and removal of the solvent under reduced
pressure yielded 4.822 g of a crude product (93%). After recrys-
tallization from hot toluene/pyridine (5 mL/1 mL), pale-yellow
crystals of 15 were obtained in 77% yield. Alternatively, 15 could
be obtained by reacting CuOtBu with Al(OtBu)3 in a tert-butanol/
toluene/pyridine solution. Anal. Calcd for C26H46AlCuN2O4: C,
57.70; H, 8.57; N, 5.18; Al, 4.98; Cu, 11.74. Found: C, 57.68; H,
1
8.33; Cu, 18.58. H NMR (200.1 MHz, C6D6, 294 K): δ 1.44 (d,
1
24H), 4.57 (sept, 4H). 13C{1H} NMR (50.3 MHz, C6D6, 294 K):
δ 28.59, 63.92. 27Al SPE MAS NMR: δ 59.8. 13C SPE MAS
NMR: δ 28.56, 64.89. EI-MS (90 eV, m/e): 652 [M]+, 594 [M -
(CH3)2CO]+, 593 [M - OiPr ]+, 579 [M - (CH3)2CO - CH3]+.
{CuII[Al(OiPr)(OtBu)3]2} (12). tert-Butanol (25 mL) was added
to 2 (5.746 g, 9.736 mmol) dissolved in toluene (25 mL). The
solution was stirred overnight at ambient temperature, and then the
solvent was removed under reduced pressure. This procedure was
repeated several times until the resonances of isopropyl alcohol
disappeared from the NMR spectra of the solvent. The final solid
was sublimed in vacuum at 120 °C. The sublimate was dissolved
in toluene (2 mL), and the solution was filtered in order to remove
the insoluble copper(I) aluminum alkoxide species. After evapora-
tion of the volatiles, the residual solid mixture was fractionally
sublimed in vacuum at 80, 100, and 120 °C. The sublimate obtained
at 120 °C was sublimed again in vacuum at 100 and 120 °C. The
second crop (120 °C) was analyzed to be the desired product
(yield: 34%). Anal. Calcd for C30H68Al2CuO8: C, 53.43; H, 10.16;
Al, 8.00; Cu, 9.29. Found: C, 53.31; H, 10.28; Al, 8.45; Cu, 9.88.
UV-vis [cyclohexane; λmax, nm (ꢀ, M-1 cm-1)]: 850 (80). 1H NMR
(200.1 MHz, C6D6, 294 K): δ 1.1 (br, 36H). 13C{1H} NMR (50.3
MHz, C6D6, 294 K): δ 35.56, 35.68, 65.2 (br). Magnetic moment
obtained by the Evans method (C6H6/C6D6): 1.83 µB.
8.32; N, 5.13; Al, 4.95; Cu, 11.67. H NMR (200.1 MHz, C6D6,
294 K): δ 1.57 (s, 36H). 13C{1H} NMR (50.3 MHz, C6D6, 294
K): δ 33.96, 68.83.
{CuI[Al(OtBu)4]}2 (16). Upon prolonged heating at 100 °C and
10-2 mbar, 15 released pyridine and 16 was formed. The vacuum
sublimation of the crude product at 130 °C is accompanied by
decomposition, leaving appreciable residue (yield: 64%). The
analytically pure colorless product was obtained by recrystallization
of the sublimate from hot toluene. Anal. Calcd for C32H72Al2-
Cu2O8: C, 50.18; H, 9.47; Al, 7.05; Cu, 16.59. Found: C, 50.21;
H, 9.59; Al, 7.19; Cu, 16.82. 1H NMR (200.1 MHz, C6D6, 294 K):
δ 1.57 (s, 72 H). 13C{1H} NMR (50.3 MHz, C6D6, 294 K): δ 33.89,
72.21.
{(HO)Cu[Al(OtBu)4]‚py}2 (17). Crystals of 17 were obtained
from a toluene solution of 15 in the presence of air and humidity.
Anal. Calcd for C42H84Al2Cu2N2O10: C, 52.65; H, 8.84; N, 2.92.
Found: C, 52.18; H, 8.32; N, 2.60.
Results and Discussion
Bimetallic isopropoxides containing either nickel(II) or
copper(II) and aluminum(III) having the general formula {M-
[Al(OiPr)4]2} [where M ) NiII (1) and CuII (2)] have been
prepared according to eq 1 following established routes.5a
The mixed-metal alkoxides 1 and 2 were obtained as colored
liquids that are highly moisture-sensitive, miscible with
nonpolar organic solvents, and volatile. Upon heating under
reduced pressure, the compounds tend to undergo partial
decomposition, which makes it difficult (in the case of 1)
{Ni[Al(OiPr)(OtBu)3]2} (13). tert-Butanol (15 mL) was added
to 4 (1.149 g, 1.629 mmol) dissolved in toluene (15 mL). The
solution was stirred overnight at ambient temperature, and then the
solvent was removed under reduced pressure. This procedure was
repeated three times until the resonances of isopropyl alcohol
disappeared from the NMR spectra of the solvent. Vacuum
1208 Inorganic Chemistry, Vol. 47, No. 3, 2008

Tetraalkoxyaluminates of Ni(II), Cu(II), and Cu(I)
+ 2HOiPr a
(3)
i
i
i
and even impossible (in the case of 2) to isolate the pure
double isopropoxides by distillation.
{Ni[A1(O Pr)4]2}
{Ni[A1(O Pr)4]2‚2HO Pr}
1
4
MCl2 + 2K[Al(OiPr)4] f {M[Al(OiPr)4]2} + 2KClV (1)
By application of a dynamic vacuum (10-3 mbar),
compound 4 readily released 2 equiv of isopropyl alcohol
and was quantitatively converted to 1 (eq 3). In this way,
compound 1 was isolated as a crystalline solid in a pure form.
Unfortunately, no suitable crystal could be selected for a
single-crystal X-ray diffraction study. It is noteworthy to
mention that the crystalline state was not stable, and under
storage, the compound started slowly to “melt”. The gradual
liquefaction of compound 1, which proceeded each time at
ordinary room temperature, can be regarded as further
evidence for the decomposition of the pure substance toward
the equilibrium state according to eq 2a. A similar behavior
was also observed for the later runnings obtained by
fractional distillation, which is to be expected from the above-
mentioned equilibrium. On fractionation, the forerunnings
contained a significant surplus of aluminum isopropoxide.
Because of the gradual enrichment of the distilland with 3,
equilibrium (2a) was continuously shifted to the left and the
further runnings gave a pure product, which crystallized
spontaneously and remained solid for several hours before
liquefaction.
The adduct 5 was obtained by the equimolar reaction of 1
and pyridine as a viscous yellow mass, which slowly
crystallized under storage (eq 4). The pyridine donor
molecule is strongly bonded to the Ni atom and cannot be
removed in vacuum at room temperature. Upon heating under
reduced pressure, 5 partly distilled and partly decomposed
to pyridine and 1.
The dissolution of 1 in an excess of pyridine (eq 5)
produced a green solution, which on concentration gave the
bipyridine adduct 6 as large green crystals, which lost their
crystallinity upon removal of the mother liquor. In dynamic
vacuum, 6 eliminated a molecule of pyridine and was
converted to 5.
A prolonged refluxing of 1 under reduced pressure induced
decomposition, indicated by the release of a white sublimate
on the reflux condenser with analytical data corresponding
to [Al(OiPr)3]4. The blue residue in the receiver flask
corresponded in analysis to the composition NiAl(OiPr)5 (3).
Therefore, the following reactions (eqs 2a,b) can be sug-
gested. Both equilibria are continuously shifted to the right
because of the gradual crystallization of tetrameric aluminum
isopropoxide on the reflux condenser, which results finally
in the complete conversion of 1 into 3. The reversibility of
eq 2a was checked by reacting 3 with a stoichiometric
amount of aluminum isopropoxide, which indeed led to the
formation of 1. The interconversion reactions of various
aluminum isopropoxide oligomers have been extensively
studied so far.12
i
2
a
{Ni[A1(O Pr)4]2}
1
+ [A1(OiPr)3]2 (2a)
{(iPrO)Ni[A1(OiPr)4]}2
3
2[A1(OiPr)3]2 a [A1(OiPr)3]4
(2b)
In order to investigate possibilities to influence equilibrium
(eq 2a), we have studied the interactions of the constituents
using strong donor solvents. On the basis of the visible
spectra of 1, Mehrotra and co-workers suggested an equi-
librium between tetrahedral and octahedral nickel species in
a benzene solution, increasing proportions of octahedral
species in an isopropyl alcohol solution, and complete
conversion to the octahedral form in pyridine.6a The authors
further concluded that the tetraisopropoxyaluminate moieties
function both as bidentate and tridentate ligands. By contrast,
the investigations carried out in our laboratories do not
support such an interpretation. In nonpolar solvents, the
nickel coordination environment in 1 appears to be entirely
tetrahedral (see further on).
i
i
+ py f
+ py a
(4)
(5)
{Ni[A1(O Pr)4]2}
{Ni[A1(O Pr)4]2‚py}
1
5
i
i
{Ni[A1(O Pr)4]2‚py}
{Ni[A1(O Pr)4]2‚(py)2}
In the case of an isopropyl acohol solution at room
temperature, we found in the UV-vis spectrum between
11 000 and 30 000 cm-1 two bands at 12 630 cm-1 [ν2, 3A2g
5
6
The hydrolysis of compound 6 resulted in the formation
of the oxoalkoxide 7, which crystallized as a light-blue solid
(eq 6). Surprisingly, the addition of a stoichiometric amount
of water to a solution of 6 in pyridine did not lead to
complete conversion to 7. For X-ray and elemental analysis,
suitable crystals were separated from the product mixture.
3
3
3
f T1g(F)] and 23 680 cm-1 [ν3, A2g f T1g(P)], charac-
teristic of hexacoordinated nickel(II), indicating nearly a
complete conversion to the octahedral form, with the intensity
of the transition ν3 [3T1 f T1(P)] at 17 400/19 330 cm-1
3
for tetrahedral nickel(II) being negligibly small. Upon
concentration of the solution, a yellow solid crystallized and
was found to be an isopropyl alcohol adduct of compound
1. The analytical data of the isolated product are consistent
with the composition of 4, which was corroborated by a
single-crystal X-ray diffraction study.
i
+ H O + py f
{Ni[A1(O Pr)4]2‚2py}
2
6
+ 2HOiPr (6)
i
[NiA12O(O Pr)6‚3py]
7
(12) (a) Kleinschmidt, D. C.; Shiner, V. J., Jr.; Whittaker, D. J. Org. Chem.
1973, 38, 3334-3337. (b) Turova, N. Ya.; Kozunov, V. A.; Yanovskii,
A. I.; Bokii, N. G.; Struchkov, Yu. T.; Tarnopolskii, B. L. J. Inorg.
Nucl. Chem. 1979, 41, 5-11.
The dimeric nature of 3 in the vapor phase was established
by mass spectrometry. Compound 3 is soluble in nonpolar
solvents, but the partial removal of the solvent from such
Inorganic Chemistry, Vol. 47, No. 3, 2008 1209

Veith et al.
solutions did not lead to the formation of crystals. Cryo-
scopical measurements showed that the molecular association
of compound 3 in cyclohexane lies at 2.2, which indicates
the formation of further oligomeric species. In benzene, the
degree of oligomerization increases. Upon cooling a solution
of 3 in benzene to the freezing point of the solvent, a blue
solid precipitated, whereby the remaining soluble substance
in the solution exhibited a molecular association degree of
2.8 with respect to the monomeric unit of 3, which corre-
sponds to the empirical formula Ni2Al3(OiPr)13. This obser-
vation could be regarded as evidence for the existence of
further species containing Ni(OiPr)2 and Al(OiPr)3 units in
various stoichiometric ratios. Equation 7 gives a possible
explanation of the behavior of compound 3 in freezing
benzene. The hypothetical species [Ni2Al3(OiPr)13] was not
isolated, but the corresponding magnesium compound [Mg2-
Al3(OiPr)13] is known and has been structurally character-
ized.13
Obviously, the solidification of tetrameric aluminum iso-
propoxide is the driving force for this transformation.
i
a
2{Cu[A1(O Pr)4]2}
2
+ [A1(OiPr)3]2 (9a)
{(iPrO)CuII[A1(OiPr)4]}2
9
2[A1(OiPr)3]2 a [A1(OiPr)3]4
(9b)
The decomposition of 2 upon distillation is more complex
compared to the nickel derivative 1 (eq 10a-c). The analysis
of the volatiles by mass spectrometry coupled with the
distillation apparatus revealed as the main products isopropyl
alcohol and acetone, indicating a partial reduction of copper-
(II) to copper(I). This finding was also confirmed by NMR
analysis of the products isolated in a cooling trap. The
isopropyl alcohol to acetone ratio being 2.4 is significantly
higher than that expected from eq 10a. The high yield of
alcohol suggests a radical-based mechanism for this reaction.
The first step requires homolytic scission of a Cu-O bond
followed by generation of isopropyl alcohol from the initially
formed isopropoxy radical by capturing a H atom from
another isopropoxy group. An analogous mechanism has
been proposed for the thermal decomposition of some
copper(I) alkoxides by Whitesides and co-workers.14
3
2
{(iPrO)Ni[A1(OiPr)4]}
2 f
3
1
n
[Ni(OiPr)2]n + [Ni2A13(OiPr)13] (7)
1 forms crystalline adducts with donor solvents, whereas
3 is not stable and decomposes in the presence of the latter.
The addition of isopropyl alcohol or pyridine to a solution
of 3 in cyclohexane resulted in the precipitation of a blue
solid with analytical data corresponding to the formula Ni-
(OiPr)2. The quantitative analysis of the precipitate is in
agreement with eq 8.
II
i
I
i
f
+
2{Cu [A1(O Pr)4]2} 2Cu [A1(O Pr)4]
2
8
[A1(OiPr)3]2 + (CH3)2CO + HOiPr (10a)
II
i
a
2{Cu [A1(O Pr)4]2}
+ 2HOiPr f
{(iPrO)Ni[A1(OiPr)4]}2
2
+ [A1(OiPr)3]2 (10b)
{(iPrO)CuII[A1(OiPr)4]}2
3
9
1
n
[Ni(OiPr) ] +
(8)
i
i
{Ni[A1(O Pr)4]2‚2HO Pr}
2 n
{(iPrO)CuII[A1(OiPr)4]}
4
2 a
9
The copper compound 2 is a metastable viscous liquid.
The stoichiometric composition CuAl2(OiPr)8 of a freshly
prepared sample was confirmed by elemental analysis.
Moreover, the magnetic moment µeff ) 1.75 µB is comparable
with the spin-only value (1.73 µB), indicating divalent copper.
However, NMR spectroscopy revealed that the oil contained
approximately 15% Al(OiPr)3. Upon aging at ambient
temperature, the color of the oil changed slowly from bluish
green to green. This process was accompanied by the
precipitation of a white solid, which was analyzed as 8, and
the formation of colorless [Al(OiPr)3]4 crystals. After separa-
tion of the solids, the filtered oil was stored for several
months before further filtration. The storage and filtration
steps were repeated several times over a period of 3 years.
Finally, a blue solid slowly crystallized out from the filtered
green liquid. Surprisingly, the single-crystal diffraction
analysis revealed another stoichiometry for the blue com-
pound, namely, 9. These observations give rise to equilibria
(eq 9a,b) similar to the corresponding nickel compound 1.
2
n
[CuII(OiPr)2]n + [A1(OiPr)3]2 (10c)
The blue distillate was not stable, and after a while, a white
solid precipitated, which was found to be 8. During storage
for several weeks, the color of the distillate changed slowly
from blue to green and colorless crystals of tetrameric
aluminum isopropoxide were formed. Treatment of the
compound mixture with pyridine formed three sorts of
crystals, which were analyzed by X-ray diffraction: blue
crystals of 10, pale-yellow ones of 11, and colorless crystals
of [Al(OiPr)3]4. Treatment of a nondistilled sample of 2 with
pyridine gave mainly 10 (eq 11), which was accompanied
only by small amounts of 11 and aluminum isopropoxide.
On the contrary, after heating of 2 dissolved in pyridine, 10
failed to crystallize. As confirmed by NMR spectroscopy,
the heating induced nearly quantitative reduction of copper-
(II) to copper(I). The isopropyl alcohol/acetone ratio being
1.9 supports the assumption of a radical-based mechanism.
(13) Meese-Marktscheffel, J. A.; Fukuchi, R.; Kido, M.; Tachibana, G.;
(14) Whitesides, G. M.; Sadowski, J. S.; Lilburn, J. J. Am. Chem. Soc.
1974, 96, 2829-2835.
Jensen, C. M.; Gilje, J. W. Chem. Mater. 1993, 5, 755-757.
1210 Inorganic Chemistry, Vol. 47, No. 3, 2008

Tetraalkoxyaluminates of Ni(II), Cu(II), and Cu(I)
The tendency of the copper(II) alkoxides to decompose to
copper(I) alkoxides appears to depend on the temperature
and the polarity of the medium.
case of tert-butanol only six out of eight isopropoxy groups
could be replaced by tert-butoxy groups. In the course of
our study, we repeated these reactions in order to elucidate
the structural features of the derivatives of the type
[MAl2(OiPr)8-R(OtBu)R], where M ) NiII and CuII.
II
i
II
i
+ py a
(11)
{Cu [A1(O Pr)4]2}
{Cu [A1(O Pr)4]2‚py}
2
10
2 was allowed to react at room temperature with an excess
of tert-butanol in toluene. After removal of the solvent under
reduced pressure, the procedure was repeated several times.
The progress of the reaction was followed by the determi-
nation of the isopropyl alcohol liberated in each step. The
first step resulted only in the partial replacement of ap-
proximately two isopropoxy groups. The lower substituted
derivatives [CuIIAl2(OiPr)8-R(OtBu)R] (R < 4) form bluish-
green oils. Further replacement of the isopropoxy groups
by tert-butoxy groups led to solidification and to a change
in the color to green. Because the starting material 2 is
always a mixture of compounds, the final product of the
alcoholysis reaction was also a complicated mixture of
various types of species. After sublimation, the mixture was
dissolved in toluene, and the insoluble copper(I) alkoxides
CuIAl(OiPr)4-γ(OtBu)γ (γ ) 1-4) were removed by filtration.
After evaporation of the solvent, the other compounds were
separated by fractional sublimation. A white sublimate
containing aluminum alkoxide species of the type [Al2(OiPr)6-δ
(OtBu)δ] (δ ) 5-6) was obtained at 80 °C. A mixture of
various derivatives [CuIIAl2(OiPr)8-R(OtBu)R] (R e 6) was
collected at 100 °C. Finally at 120 °C, the last crop contained
12, as confirmed by elemental and X-ray diffraction analysis.
Compound 12 is thermally stable and sublimes without
leaving any solid residues.
The green residue obtained after evaporation was partly
soluble in toluene, and the addition of hot pyridine caused
precipitation of a green solid, which was analyzed to be the
homometallic copper(II) isopropoxide. Evaporation of the
solvent from the filtered pyridine solution gave only crystals
of complex 11.
8 was partly present in the distillate because of its poor
volatility, but its main amount was found in the distillation
residue among 9 and [Cu(OiPr)2]n, both species of the type
[Cu(OiPr)2]m[Al(OiPr)3]2 (where m > 2). The species [Cu-
(OiPr)2]m[Al(OiPr)3]2 could be either molecular species or
colloidal Cu(OiPr)2 particles stabilized with aluminum iso-
propoxide on the surface, which makes them soluble in
nonpolar solvents. The treatment of the distillation residue
with pyridine transformed 8 into 11 according to eq 12a.
Heating of the solution accelerated the decomposition of 9,
and the species of the type [Cu(OiPr)2]m[Al(OiPr)3]2 to 11
as proposed in eq 12b,c was formed, which was accompanied
by the precipitation of CuII(OiPr)2. After filtration and
evaporation of the solvent from the solution, compound 11
was the only substance that was isolated.
I
i
I
i
+ 2py f
(12a)
Cu [A1(O Pr)4]
{Cu [A1(O Pr)4]‚2py}
8
11
The reaction of 1 and 4 with an excess of tert-butanol
was also examined briefly. In contrast to 2, the replacement
of the isopropoxy groups occurred quite readily at ambient
temperature and 13 was obtained in a quantitative yield. The
product was sublimed unchanged under reduced pressure.
The expected composition was confirmed by elemental and
X-ray analysis. In contrast to 1, derivative 13 is stereochemi-
cally rigid and shows a tendency neither to form adducts
with alcohol nor to undergo thermal decomposition.
{(iPrO)CuII[A1(OiPr)4]}2
+ 4py f
9
I
+ (CH ) CO + HOiPr (12b)
i
2{Cu [A1(O Pr)4]‚2py}
3 2
11
[Cu(OiPr)2]m[A1(OiPr)3]2 + 2py f
m - 2
n
+
[CuII(OiPr)2]n +
I
i
2{Cu [A1(O Pr)4]‚2py}
11
The bimetallic tert-butoxide 14 was obtained in analogy
to eq 1, reacting anhydrous CuCl2 with K[Al(OtBu)4] in a
toluene/tert-butanol solution following a procedure already
described in the literature.5b After removal of the precipitated
KCl and evaporation of the solvent, a green solid formed.
Apart from 14, the crude product of the reaction contained
[Al(OtBu)3]2 and further copper(II) species, which were
not volatile. [Al(OtBu)3]2 (80 °C and 10-2 mbar) and 14
(120 °C and 10-2 mbar) were separated from the green
residue by fractional sublimation. In contrast to the observa-
tions made by Mehrotra and co-workers, who describe
the compound as a green, sticky solid, upon distillation at
162 °C and 0.05 Torr, we got 14 as a yellow crystalline solid.
Elemental analysis and X-ray structure determination con-
(CH3)2CO + HOiPr (12c)
Attempted preparation of 9 by reaction between either
CuCl2, Li[Al(OiPr)4], and LiOiPr or CuCl2, K[Al(OiPr)4], and
KOiPr in equimolar proportions failed (eq 13). In the first
case, nearly quantitative reduction of copper(II) to copper-
(I) occurred, whereas the second route resulted in the
formation of 2 and [Cu(OiPr)2]n. The failure to prepare 9
via the second route could be due to its instability in
isopropyl alcohol.
+ MOiPr -//f
i
CuCl2 + M[A1(O Pr)4]
9
1
2
{(iPrO)CuII[A1(OiPr)4]}2 +
(13)
MC1V
M ) Li, K
1
firmed the composition as CuAl2(OtBu)8. The H NMR
spectrum of 14 displays at room temperature a single
resonance for the methyl protons of the OtBu groups.
However, the intensity of the signal (in a ratio to the solvent
resonances) corresponds to four OtBu groups and probably
Reactions of {M[Al(OiPr)4]2} [M ) NiII (1) and CuII (2)]
with various alcohols have been reported so far in the
literature.6a,b Mehrotra and co-workers described that in the
Inorganic Chemistry, Vol. 47, No. 3, 2008 1211

Veith et al.
originates from the terminal alkoxy groups located far away
from the paramagnetic center.
11 was initially obtained by the treatment of 2 with
pyridine. Interestingly, the efforts made to synthesize 14 by
the reaction of CuCl2 and K[Al(OtBu)4] in the presence of
pyridine also resulted in the formation of the corresponding
tert-butoxide derivative 15. Consequently, it appears that
pyridine favors the reduction of copper(II) to copper(I).
The preparation of 8 by a simple metathesis reaction was
not successful. Because of its poor solubility in common
organic solvents, 8 could not be separated from the precipi-
tated KCl. The addition of pyridine led to the dissolution of
the double isopropoxide as 11, which was obtained after the
workup of the reaction mixture (eq 14). An analogous
procedure was employed for the preparation of 15. Both
compounds were characterized by elemental analysis, NMR
spectroscopy, and single-crystal structure determination.
Compound 15 in a toluene solution is slowly reacting with
air (oxygen and water) to form the decomposition product
17, which has been characterized by X-ray diffraction on
single crystals (see below).
Figure 1. Graphical representation of 4, ellipsoids showing 50% prob-
ability. As in the following figures, the C atoms are represented as simple
balls; H atoms are omitted for clarity.
as found from NMR experiments. Anhydrous copper(I)
chloride did not react with K[Al(OtBu)4] even under reflux.
The addition of pyridine and tert-butanol facilitated both
reactions, and 14 was obtained in a quantitative yield. On
heating under reduced pressure, 14 released 2 equiv of
pyridine and 16 was formed (eq 15). The sublimation of 16
in vacuum (120 °C and 10-2 mbar) was accompanied by
decomposition, leaving some residue. Analytically pure
product was, nevertheless, obtained upon recrystallization
of the sublimate from hot toluene. The composition [CuAl-
(OtBu)4]2 was confirmed by crystal structure determination.
CuCl + K[A1(OR)4] + 2py f
{CuI[A1(OR)4]‚2py}
+ KClV (14)
R ) iPr (11) and tBu (15)
CuI[A1(OR)4] + 2py
{CuI[A1(OR)4]‚2py} f
R ) iPr (8) and tBu (16)
(15)
1
2
t
t
[CuA1(OtBu) ]
[CuO Bu]4 + [A1(O Bu)3]2 -//f
(17)
4 2
8
Upon heating under reduced pressure (80 °C and 10-2
mbar), 11 released pyridine and 8 was formed (eq 15).
Attempted sublimation initially delivered 8 in a rather low
yield (13%) as a white sublimate. However, prolonged
heating of 8 in vacuum (120 °C and 10-2 mbar) resulted in
decomposition, forming mostly aluminum isopropoxide and
metallic copper (eq 16). Consequently, 8 cannot be purified
by sublimation. The dimeric nature of 8 in the vapor phase
was established by mass spectrometry. Its molecular weight
could not be determined cryoscopically because of its
insufficient solubility in benzene and cyclohexane. In the
solid state, it probably has a polymeric structure.
1
2
CuCl + K[A1(OtBu)4] -//f
+ KC1 (18)
[CuA1(OtBu)4]2
8
Molecular and Solid-State Structures
A collection of the experimental setup and the most
important data concerning the X-ray diffraction analyses are
compiled in Tables 1 and 2. Selected bond lengths and angles
are summarized in Tables 3 and 4.
Compound 4. Figure 1 shows a graphic representation
of the molecular structure of 4 derived from a single-crystal
X-ray diffraction analysis. The pertinent bond lengths and
angles are compiled in Table 3. The structure of 4 (almost
point symmetry C2) is analogous to the corresponding
magnesium compound {Mg[Al(OiPr)4]2‚2iPrOH}15 and con-
sists of two aluminum-centered oxygen tetrahedra sharing
common edges with a nickel-centered octahedron. Apart from
the O atoms of the AlO4 tetrahedra, the fifth and sixth
coordination sites of the Ni atom are occupied by the O atoms
of the solvating isopropyl alcohol molecules, located in cis
position with respect to each other. The central Ni atom is
substantially distorted from the regular octahedral geometry
with trans O-Ni-O angles ranging from 168.6° to 175.9°
and cis angles in the range from 73.0° to 103.0°. The Ni-O
distances from the neutral alcohol ligands [ave 2.102(7) Å]
are larger than the bridging Ni-O distances [ave 2.045(6)
I
i
f
2Cu [A1(O Pr)4]
8
1
Cu + [A1(OiPr)3]4 + (CH3)2CO + HOiPr (16)
2
Alternatively, 2 can be used as a starting source for the
preparation of 8 according eq 10a. Prolonged refluxing of 2
in vacuum yielded 8 as a nearly colorless residue in the
receiver flask. The pale-green color of the product was due
to some [Cu(OiPr)2]n impurities.
Efforts made to prepare copper(I) aluminum tert-butoxide
by either Lewis acid-base adduct formation (eq 17) or
metathesis reaction (eq 18) were not successful. [Al(OtBu)3]2
and [CuOtBu]4 did not react in boiling toluene. The addition
of polar solvents such as tert-butanol, monoglyme, or dioxane
and prolonged refluxing of the reaction mixture for several
days resulted only in the formation of small quantities of 16
(15) Sassmannshausen, J.; Riedel, R.; Pflanz, K. B.; Chmiel, H. Z.
Naturforsch. 1993, 48b, 7-10.
1212 Inorganic Chemistry, Vol. 47, No. 3, 2008

Tetraalkoxyaluminates of Ni(II), Cu(II), and Cu(I)
Figure 2. Graphical representation of 5, ellipsoids showing 50% prob-
ability (see also Figure 1).
Figure 3. Graphical representation of 6, ellipsoids showing 50% prob-
ability from a preliminary structure determination (see also Figure 1).
Å] because of the reduced donor capacity of the alcohol in
comparison to the charged alkoxo ligands.
The hydroxylic H atoms on O9 and O10 participate in
intramolecular hydrogen bonding with the terminal alkoxy
ligands (O7 and O5) evident in the short O9‚‚‚O7 ) 2.696
Å and O10‚‚‚O5 ) 2.695 Å separations (sum of O van der
Waals radii ) 3.00 Å).16 These hydrogen bridges result in
the formation of six-membered chelate cycles [Ni-µ2-O-
Al-O‚‚‚H-O] fused with the four-membered NiO2Al rings,
causing further stabilization of the molecule.
The aluminum centers are in a distorted tetrahedral
environment. The Al-O bond lengths are comparable with
those observed for {Mg[Al(OiPr)4]2‚2HOiPr} and follow the
trend Al-Oterm [ave 1.636(9) Å] < Al-Oterm [involved in
intramolecular hydrogen bonding; ave 1.708(8) Å] < Al-
Obridge [ave 1.757(7) Å]. The bridging O atoms are almost
planar [average sum of angles ) 357.9(5)°], accounting for
sp2 hybridization.
Figure 4. Graphical representation of 7, ellipsoids showing 50% prob-
ability (see also Figure 1).
Compound 6. An attempted X-ray structure determination
of 6 did not lead to a satisfactory solution because of the
poor quality of the crystals. An appreciable residual electron
density prevented the completion of the refinement. The
preliminary solid structure model exhibits a primitive triclinic
cell with two independent molecules in the asymmetric unit.
The most important feature of the structure (Figure 3) is its
similarity to the structure of 4. Apart from the two bidentate
[Al(OiPr)4] ligands, two solvating pyridine molecules situated
in cis position toward each other fill in the octahedral
coordination environment around the Ni atom.
Compound 7. The central core of 7 (compare Figure 4)
is built up of two planar NiO2Al four-membered rings fused
along the common Ni-µ3-O edge. The rings are coplanar
because of the requirements of the twofold symmetry axis
in the crystal passing through O4, Ni, N2, and C17. The
three N atoms of the pyridine molecules and the two bridging
alkoxo and oxo O atoms form a distorted octahedron around
the nickel center. Bond lengths and angles at the Ni atom
are comparable with those of compounds 4 and 5 (see also
Table 4). All bridging O atoms have trigonal-planar sur-
roundings [sum of angles 360.0(3)°]. The coordination figure
around the O4 atom bridging the nickel and the two Al atoms
[O4-Al ) 1.676(1) Å; O4-Ni ) 1.990(4) Å] is trigonal-
planar with a remarkable tendency of the Al1-O4-Al1a
angle to linearity [166.1(3)°], which could result from steric
reasons. The different functions of the O atoms in the
structure (oxo, bridging, and terminal alkoxy) are reflected
Compound 5. The molecular structure of 5, which has
almost point group symmetry C2 (with the quasi C2 axis
intersecting the pyridine ligand), consists of two planar AlO2-
Ni four-membered rings [sum of angles ) 360.0(2)°] joined
at the Ni atom to which a pyridine molecule is bonded
through the N atom. The nickel coordination figure NiO4N
can be described as a distorted trigonal bipyramid with a
bending of the axial O2-Ni-O3 angle from the ideal 180°
to 169.5(2)°. In Figure 2, the central NiAlON backbone of
the structure is shown, omitting all H atoms for clarity; some
important bond lengths and angles are assembled in Table
3. The two doubly bridging OiPr groups of each [Al(OiPr)4]
unit are bonded to the nickel center in an asymmetrical
fashion with short equatorial Ni-O1 and Ni-O4 distances
[ave 1.993(3) Å] and larger axial Ni-O2 and Ni-O3
distances [ave 2.055(3) Å]. The mean Ni-O bond length in
5 is 2.023(3) Å for the pentacoordinate Ni atom compared
to 2.064(7) Å for the hexacoordinate Ni atom in 4. The
bonding angles within the NiO2Al rings of the two com-
pounds are similar, being in 4 more acute at the nickel and
aluminum centers because of the higher coordination number
of nickel [73.0(3)° (4) in comparison with 74.5(1)° (5)]. As
expected, the terminal Al-O bond lengths of ave 1.668(5)
Å are shorter than the bridging ones [ave 1.771(4) Å].
(16) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
Inorganic Chemistry, Vol. 47, No. 3, 2008 1213

Veith et al.
Figure 5. Graphical representation of 9, ellipsoids showing 50% prob-
ability (see also Figure 1).
by a considerable difference in the Al-O values (respectively
1.676(2), 1.633(3), and 1.726(3) Å).
Figure 6. Graphical representation of 10, ellipsoids showing 50%
probability (see also Figure 1).
Compound 9. The crystal structure of 9 (Figure 5)
contains centrosymmetric dimeric molecules of the general
formula {(iPrO)Cu[Al(OiPr)4]}2. The backbone of the struc-
ture is made up of a planar Cu2O2 rhombus connected with
two planar CuO2Al four-membered rings [sum of angles )
359.99(13)°] through the Cu atoms. The distances within the
central Cu2O2 four-membered ring [Cu-O ) ave 1.920(3)
Å, Cu‚‚‚Cu ) 2.898(1) Å, and O‚‚‚O ) 2.519(3) Å)] are
quite similar to the distances found in compound 17 (see
below) containing a CuII (OH)2 unit [Cu-O(1) ) 1.925(7)
2
Å, Cu‚‚‚Cu ) 2.989(2) Å, and O‚‚‚O ) 2.465(2) Å)]. The
bonding angles O1-Cu-O1′ within the Cu2O2 four-
membered rings of the two compounds are similar, being in
9 somewhat larger at the copper centers because of the lower
coordination number of copper [81.97(12)° (9) in comparison
with 78.2(4)° in 17]. The coordination polyhedron around
the Cu atoms in 9 can be described as an extremely flattened
tetrahedron not far away from square-planar geometry. The
degree of bisphenoidic distortion could be expressed through
the angle between the Cu2O2 and CuO2Al planes [32.23-
(14)°]. This value lies far away from 90°, which is expected
for an ideal tetrahedron. The extent of tetrahedral compres-
sion can be seen as a compromise between the mutual
repulsion of the ligands and the crystal-field stabilization of
the square-planar geometry. The subsequent replacement of
the space-filling tert-butoxy by the smaller isopropoxy groups
in a Cu(OR)4 tetrahedron (R ) Bu and Pr) decreases the
repulsion of the ligands, and the geometry is shifted toward
the square-planar one. For the plane angles between the
CuO2Al planes in 14 and 12 or respectively between the
Cu2O2 and CuO2Al planes in 9, the following dispersion was
observed: 77.77° for Cu(OtBu)4 in 14, 68.54° for Cu(Oi-
Pr)2(OtBu)2 in 12, and 32.23° for Cu(OiPr)4 in 9. The
distances between O atoms and fourfold-coordinated Cu
atoms in 9 [ave 1.941(9) Å] are comparable with the average
Cu-O bond lengths of 1.922(5) and 1.933(5) Å in 12 and
14, respectively.
Figure 7. Graphical representation of 11, ellipsoids showing 50%
probability (see also Figure 1).
Compound 10. It is interesting to compare the solid-state
molecular structure of 10 (Figure 6) with the corresponding
one of the nickel compound 5 (Figure 2). Whereas the two
bidentate [Al(OiPr)4]- ligands in 5 are bonded to the nickel
center nearly symmetrically with respect to the quasi twofold
axis C2 passing through the Ni-N bond, in 10 the two
crystallographic inequivalent [Al(OiPr)4]- units are coordi-
nated to copper in an asymmetrical fashion. One of the
Cu-O bonds is about 0.36 Å longer than the others [Cu-
O2 ) 2.327(8) Å in comparison with Cu-O3 ) 1.962(6)
Å, Cu-O1 ) 1.963(6) Å, and Cu-O4 ) 1.985(6) Å]. The
NO4 coordination polyhedron around copper can be described
as a distorted tetragonal pyramid consisting of one N atom
and three strongly bonded O atoms in its base and a bridging
O atom at the apex. As may be seen by inspection of Table
3, this description is in accordance with the observed bond
lengths and angles.
Compounds 11 and 15. The solid-state structure of 11
(see also Figure 7 and Table 3 for distances and angles) is
built up of monomeric molecules in which Cu, Al, and two
bridging O atoms form a strictly planar four-membered ring
with a crystallographic twofold symmetry axis C2 passing
through Cu and Al. The coordination figure around Cu can
be described as an extremely distorted N2O2 tetrahedron with
two short Cu-N bonds [1.945(4) Å in comparison to the
longer Cu-N bond of 2.046 Å in [CuI(py)4]ClO4]17 forming
a large N1-Cu-N1a angle of 145.9(3)°. The small O1-
Cu-O1a angle of 67.0(2)° at the tetrahedral copper in 11 is
t
i
The Al atoms exhibit a distorted tetrahedral environment,
with O-Al-O angles ranging from 85.1(1)° to 116.4(2)°.
The expected dispersion of Al-O bond lengths in 9 [Al-
Oterm ) ave 1.680(4) Å < Al-Obridg ) ave 1.794(4) Å] is
quite similar to that found in 10 [Al-Oterm ) ave 1.691 (10)
Å and Al-Obridg ) ave 1.798(8)].
1214 Inorganic Chemistry, Vol. 47, No. 3, 2008

Tetraalkoxyaluminates of Ni(II), Cu(II), and Cu(I)
Figure 9. Graphical representation of 16, ellipsoids showing 50%
probability (see also Figure 1).
Figure 8. Graphical representation of 15, ellipsoids showing 50%
probability (see also Figure 1).
A similar structural pattern was observed in [CuN(SiMe2-
NEt2)2]2 containing a (CuNAlN)2 eight-membered ring with
linearly coordinated CuI atoms.19
associated with extremely large Cu-O distances [2.312(3)
Å in 11 in comparison with the mean Cu-O distance of
1.886(2) Å in 15]. This situation suggests increased ionic
character of the bonds between the Cu and O atoms.
Therefore, compound 11 can be better described as an ion
pair of the type [Cu(py)2]+[Al(OiPr)4]- with quite strong
Cu+‚‚‚OiPr interactions and a tendency of the N-Cu-N
angle to become linear. On the same line, all isopropoxy
groups at the Al atom are essentially terminal, which is
evident in the small difference between the terminal and
bridging OiPr groups [Al-Oterm ) 1.718(4) Å in comparison
with Al-Obridg ) 1.736(3) Å]. The ambient-temperature 1H
and 13C NMR spectra show only one type of isopropoxide
environment owing to the fluxional behavior of this ligand
in solution.
As shown in Figure 8, the molecular structure of 15 is
similar to the structure of 11, but the two substances are not
isostructural. The central four-membered CuO2Al ring in 15
is not planar, and the tetrahedral coordination environment
around copper is not as extremely distorted as that in 11.
The Cu-O distance of 2.239(3) Å is still significantly larger
than the sum of the effective ionic radii of Cu+ and O2-
(1.96 Å)18 but considerably shorter than the Cu-O bond
length in 11 [2.312(3) Å] because of the increase in
covalency of the Cu-O bond. This is also associated with
subsequent elongation of the Cu-N bonds [ave 1.958(4) Å]
and a decrease in the value of the N-Cu-N bond angle
[137.3(2)°]. The closer attachment of [Cu(py)2]+ to two of
the O atoms of the [Al(OtBu)4]- anion lengthens the Al-
The short CuI‚‚‚CuI separation of 2.687(1) Å, which is
smaller than the sum of the van der Waals radii (2.80 Å),
not only results from the small bite distance of the
[Al(OtBu)4]- ligands. Moreover, the bending of the two
O-Cu-O bridges toward each other is indicative of a very
weak metal-metal bonding caused both by weak dispersion
attraction between the filled 3d10 electron shells of the Cu+
ions and by a decrease of Pauli repulsion of the outer electron
shells by hybridization between 3d, 4s, and 4p orbitals.20
The average Cu-O bond length of 1.886(2) Å, which is
comparable with the Cu-O distances in other copper
alkoxides [1.852 Å in [Cu(OtBu)]421 and 1.840 Å in [LiCu-
(OtBu)2]422], indicates a covalent bonding between the Cu
and O atoms. Consequently, the geometry at the O atoms of
the bridging OtBu groups deviates from planarity, as shown
by the sum of their interbond angles [352.3(1)° for O1 and
357.6(2)° for O2].
Compounds 12-14. As is evident from the unit cell
parameters and atomic coordinates, the copper aluminates
14 and 12 are isomorphous and crystallize in the same space
group. Furthermore, 14 and 12 are isostructural with the
corresponding cobalt, nickel, and magnesium compounds.7c,23
The molecular structures of 14 and 12 display a spirocyclic
core made up of two almost planar CuO2Al four-membered
rings joined at the copper center (Figures 10 and 11). Both
copper and aluminum atoms are quasi-tetrahedrally coordi-
nated by the alkoxy ligands, with the distortion being more
significant at the copper center. The deviation from the ideal
tetrahedral geometry can be expressed in terms of distortions
with respect to the S4 symmetry axes of an ideal CuO4
tetrahedron. The elongation along the molecular Al-Cu-
Al axis is caused by the small O-Al-O bite of the
tetralkoxyaluminate ligands. On the other hand, the CuO4
tetrahedra are somewhat flattened as a result of a bisphe-
Obridg bonds by 0.046 Å compared to the terminal ones. The
difference in the structures of 11 and 15 may be attributed
to the higher inductive effect of tert-butyl compared to
isopropyl, with the oxygen getting more electron density in
the tert-butyl case.
Compound 16. The X-ray structural study of 16 reveals
the crystal to be built up of dimeric centrosymmetric
molecules, which have two [Al(OtBu)4] units linked by two
nearly linear O-Cu-O bridges [176.44(7)°] to form an
eight-membered ring in a chairlike conformation (Figure 9).
(19) Koban, A. Ph.D. Thesis, Saarland University, Saarbruecken, Germany,
1999.
(20) Wang, S.-G.; Schwarz, W. H. E. Angew. Chem., Int. Ed. 2000, 39,
1757-1761.
(21) Greiser, T.; Weiss, E. Chem. Ber. 1976, 109, 3142-3146.
(22) Purdy, A. P.; George, C. F. Polyhedron 1995, 14, 761-769.
(23) (a) Wolfanger, H. Ph.D. Thesis, Saarland University, Saarbruecken,
Germany, 1991. (b) Fritscher, E. Ph.D. Thesis, Saarland University,
Saarbruecken, Germany, 1997.
(17) Nilsson, K.; Oskarsson, A. Acta Chem. Scand., Ser. A 1982, 36, 605-
610.
(18) Huheey, J. E.; Keiter, E. A.; Keiter R. L. Inorganic Chemistry:
Principles of Structure and ReactiVity, 4th ed.; HarperCollins: New
York, 1993; references cited therein.
Inorganic Chemistry, Vol. 47, No. 3, 2008 1215

Veith et al.
Figure 10. Graphical representation of 12, ellipsoids showing 50%
probability (see also Figure 1).
Figure 12. Graphical representation of 13, ellipsoids showing 50%
probability; one of the enantiomeric forms is shown (see also Figure 1).
Figure 11. Graphical representation of 14, ellipsoids showing 50%
probability (see also Figure 1).
noidic distortion, which may be due to the Jahn-Teller
effect. Whereas the MO2Al four-membered rings in {M[Al-
(OtBu)4]2} (M ) Co, Ni, and Mg) are nearly perpendicular
to each other (88.7° for M ) Ni and 88.3° for M ) Mg),
the planes of the CuO2Al rings are twisted toward each other
to form smaller angles (77.77° in 14 and 68.54° in 12).
The degree of the tetrahedral compression can be seen as
a compromise between the mutual repulsion of the ligands
and the crystal-field stabilization of the square-planar
geometry. Therefore, the replacement of two out of the four
tert-butoxy groups with the less space-filling OiPr ligands
results in a more flattened CuO4 tetrahedron in 12. As a result
of this, the broad absorption band of the CuO4 chromophore
in 12 at 12 531 cm-1 is shifted toward larger wavenumbers
in comparison to 14 (10 741 cm-1). The distances between
O atoms and fourfold-coordinated Cu atoms of ave 1.922-
(5) and 1.933(5) Å respectively in 12 and 14 are smaller
than the average Cu-O bond length of ave 2.059 Å in 10
(square-pyramidal geometry). The expected dispersion of
Al-O bond lengths [Al-Oterm < Al-Obridg] is found in AlO4
tetrahedra with Al-Oterm distances of 1.632(8) Å in 12 and
1.643(7) Å in 14 and Al-Obridg distances of 1.778(6) Å in
12 and 1.787(6) Å in 14. The same structural pattern was
also observed for 13 (Figure 12). Whereas 12, 14, {Ni[Al-
(OtBu)4]2}, and {Mg[Al(OtBu)4]2} crystallize in the mono-
clinic space group P21/n, 13 crystallizes in the orthorhombic
space group P212121. The two bidentate [(µ2-OiPr)(µ2-OtBu)-
Al(OtBu)2] moieties tetrahedrally coordinated to the Ni atom
form two almost planar AlO2Ni four-membered rings [sum
of angles ) 359.6°] with a plane angle of 89.7(3)°, which is
almost 90° as expected for an ideal tetrahedron. The two
possible positions of the two bridging isopropoxy groups with
respect to each other along the molecular Al-Ni-Al axis
Figure 13. Graphical representation of 17, ellipsoids showing 50%
probability (see also Figure 1).
result in the formation of two geometric isomers sharing the
same crystallographic positions in this acentric space group.
As a result of this, only the isopropoxy group connected to
O2 could be clearly resolved. The other one sharing the same
positions with a tert-butoxy group was located with ap-
proximately 50% occupancy at O3 and O4.
The mean Ni-O bond length in 13 (coordination number
4) being 1.946(7) Å is smaller than the Ni-O distances in
5 [ave 2.023(3) Å] (coordination number 5) and in 4 [ave
2.064(7) Å] (coordination number 6). The Al-O bond
lengths in 13 exhibit the typical dispersion: Al-Oterm ) ave
1.649(10) Å < Al-Obridg ) ave 1.790(10) Å.
Compound 17. The crystal structure of 17 (Figure 13)
contains centrosymmetric dimers of the general formula
{(HO)Cu[Al(OtBu)4]‚py}2. The backbone of the structure
is made up of two CuO2Al four-membered rings spyrocy-
clically connected through Cu atoms with a central Cu2O2
rhombus. Each Cu atom has a further pyridine molecule
coordinated, increasing the coordination number at Cu to 5.
Distances and angles within the central four-membered ring
are quite similar to the Cu-OH distances found in other
compounds containing a CuII (OH)2 unit (Cu-O1 ) 1.925
2
Å, Cu‚‚‚Cu ) 2.989 Å, and O‚‚‚O ) 2.465 Å).24 The
distances between the hydroxylic oxygen O1 and the other
O atoms, e.g., O1‚‚‚O4 ) 3.128 Å, are larger than the sum
of O van der Waals radii (3.00 Å), so there is no structural
evidence for the existence of intramolecular hydrogen
bonding.
Like in 10, the O atoms of the bridging OtBu groups of
each [Al(OtBu)4] unit are asymmetrically bonded to the
1216 Inorganic Chemistry, Vol. 47, No. 3, 2008

Tetraalkoxyaluminates of Ni(II), Cu(II), and Cu(I)
copper centers, resulting in extremely different Cu-O bond
lengths: Cu-O3 ) 2.393(5) Å compared with Cu-O2 )
2.033(5) Å. The distance from the Cu atom to the N atom
of the solvating pyridine molecule being 2.016(7) Å is quite
similar to the Cu-N bond length of 2.029(8) Å in 10. The
coordination polyhedron around the Cu atom can be de-
scribed as a square pyramid with a N atom, two hydroxy
atoms, and a bridging O atom in its base and with the O3
atom at the apex.
The Al-O bond lengths do not fall into two different
categories (terminal-bridging) but follow a more elaborate
trend: Al-O5term ) 1.707(6) Å < Al-O4term (oriented
toward the hydroxylic O1) ) 1.725(6) Å < Al-µ2-O3apex
) 1.762(6) Å < Al-µ2-O2base ) 1.828(5) Å.
Acknowledgment. The authors thank the Fonds der
Chemischen Industrie and the BASF AG for their financial
support and the “Sonderforschungsbereich 277” as well as
the “Europa¨isches Graduiertenkolleg” GRK 532, both fi-
nanced by the Deutsche Forschungsgemeinschaft (DFG),
Bonn, Germany, for the provision of a postdoctoral grant to
K.V.
(24) (a) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto,
S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am.
Chem. Soc. 1992, 114, 1277-1291. (b) Lewis, D. L.; Hatfield, W.
E.; Hodgson, D. J. Inorg. Chem. 1972, 11, 2216-2221. (c) Lewis, D.
L.; Hatfield, W. E.; Hodgson, D. J. Inorg. Chem. 1974, 13, 147-152.
(d) Hammond, R. P.; Cavaluzzi, M.; Haushalter, R. C.; Zubieta, J. A.
Inorg. Chem. 1999, 38, 1288-1292. (e) Komaei, S. A.; van Albada,
G. A.; Haasnoot, J. G.; Kooijman, H.; Spek, A. L.; Reedijk, J. Inorg.
Chim. Acta 1999, 286, 24-29.
Supporting Information Available: Crystallographic data of
the structures in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC701729Y
Inorganic Chemistry, Vol. 47, No. 3, 2008 1217