Synthesis and properties of oxygen-centered tetradecaimido hexatantalum clusters

Article abstract of DOI:10.1002/anie.200603019

A New Class of Metal-Imido Complex: Octahedral hexatantalum clusters [(ArN)₁₄Ta₆O] (see picture; C gray, N blue, O red, Ta orange, Ar = Ph) were formed in good yields by treatment of [Bn ₃Ta=NtBu] (Bn = benzyl) or [Ta(NMe₂)₅] with an excess of the appropriate aniline and an equivalent of water. Crystallographic analysis revealed the presence of a central oxygen atom, which was further identified by electrospray mass spectrometry using ¹⁷O/ ¹⁸O-enriched material. (Figure Presented).

Full text of DOI:10.1002/anie.200603019

Angewandte
Chemie
DOI: 10.1002/anie.200603019
Cluster Compounds
Synthesis and Properties of Oxygen-Centered Tetradecaimido
Hexatantalum Clusters**
Jamin L. Krinsky, Laura L. Anderson, John Arnold,* and Robert G. Bergman*
Hexanuclear transition-metal clusters are well-known, but
examples displaying closely packed organic substituents are
scarce.[1] The terminal oxo ligands in hexamolybdate and
hexatungstate have been replaced to varying degrees by
imido[2] or hydrazido[3] groups, and cyclopentadienyl-func-
tionalized hetero-hexanuclear clusters have been prepared.[4]
Still, incorporation of organic groups into the bridging sites
has been more challenging.[5] Herein we report a high-yielding
synthesis of neutral, oxygen-centered hexatantalum clusters
with all other metal valencies occupied by terminal and
m3 arylimido ligands. Although chemically inert in their
neutral form,[6] these “nearly homoleptic” tantalum imido
species possess electronic structures which may facilitate
interesting redox-promoted transformations.
sensitive red crystalline material in low and erratic yield.
Subsequent characterization showed that this material was
the octahedral cluster [(PhN)14Ta6O] (2; Scheme 2).
Scheme 2.
The discovery of these new compounds occurred seren-
dipitously during our investigation of the hydroamination of
alkynes by anilines with the imido catalyst [Bn3Ta NtBu]
Single-crystal X-ray diffraction analysis[8] of 2 showed a
centrosymmetric hexatantalum cluster[9] with six terminal and
eight facially bridging phenylimido ligands (Figure 1).
Whereas only two nonequivalent sets of phenyl resonances
are observed in the 1H NMR spectrum of 2, in the solid state
the cluster core is slightly distorted from Oh symmetry and the
phenylimido ligands pack so that the potential D2d symmetry
is not realized. A central atom was located that was initially
assumed to be nitrogen[10] on the basis of precedent from
known nitride-centered polyimido clusters.[11] However, the
structure was refined equally well with oxygen in this position,
=
(1).[7] Reaction mixtures often developed a red color during
the course of the reaction, and catalyst deactivation was
always observed within 24 hours regardless of the amount of 1
added (Scheme 1). A control experiment revealed that treat-
ment of 1 with excess aniline at 1358C resulted in the
precipitation of a sparingly soluble, highly air- and moisture-
Scheme 1.
[*] J. L. Krinsky, Prof. J. Arnold, Prof. R. G. Bergman
Department of Chemistry
University of California, Berkeley
Berkeley, CA 94720-1460 (USA)
Fax: (+1)510-642-7714
E-mail: arnold@berkeley.edu
rbergman@berkeley.edu
Dr. L. L. Anderson
Department of Chemistry
University of California, Irvine
Irvine, CA 92697-2025 (USA)
[**] We are indebted to Dr. Fred Hollander and Dr. Allen Oliver at the UC
Berkeley CHEXRAY X-ray crystallographic facility and Dr. Kathleen
Durkin at the Molecular Graphics Facility (NSF grant CHE-0233882)
for crystallographic and computational consultation, respectively,
as well as Dr. John Greaves at UC, Irvine for mass spectrometry
data. We also thank Dr. Alexandr Shafir for providing a preliminary
structure of 2 obtained by using MoKa radiation. This work was
supported by NIH grant GM-25459 to R.G.B. and DOE funding to
J.A.
Figure 1. ORTEP diagram of [(PhN)14Ta6O] (2) at 50% probability.
Hydrogen atoms have been removed for clarity. Selected bond lengths
(Š): Ta-O 2.2035(5), 2.2079(4), 2.2119(4); Ta-Nterminal 1.790(3),
1.791(4), 1.793(4). C gray, N blue, O red, Ta orange.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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369

Communications
and thus alternate methods of characterization were
employed to resolve this issue (see below).
Irreproducible yields obtained in the synthesis of 2 led us
to suspect the adventitious presence of a necessary reactant.
Whereas oxygen sources such as peroxides, pyridine N-oxide,
or phosphine oxides failed to yield 2, the addition of water
prior to heating of the reaction mixture resulted in consistent
yields of up to 57%. Surprisingly, tantalum oxide was also
moderately effective as an oxygen source. This observation
provides an explanation for the variable yield without added
water: tantalum oxide may have remained on the surface of
the glassware from one reaction to the next and may not have
been inert as was assumed.
Figure 2. UV/Vis absorption spectra of 2 (solid trace), 3 (dotted), and
4 (dashed) in THF. Inset: visible-wavelength absorption onsets.
The reaction is not limited to aniline itself. Substituted
anilines p-toluidine and p-anisidine provided [(p-
MeC6H4N)14Ta6O] (3) and [(p-MeOC6H4N)14Ta6O] (4) in
48% and 49% yield, respectively. Although p-chloro-, p-
fluoro-, and p-trifluoromethylaniline did not furnish isolable
products, m-chloroaniline gave [(m-ClC6H4N)14Ta6O] (5) in
modest yield (15%). High-quality crystals of 5 were obtained
detected no emission between 300 and 1000 nm. The large
number and density of vibrational modes accessible to these
clusters probably makes thermal relaxation the dominant
excited-state decay pathway.
Results of orbital calculations[13] (X3LYP/LACVP*)[14] on
Ci-optimized 2 agree with the expected bonding scheme
wherein all occupied frontier orbitals are ligand-centered.
Both the geometry and orbital structure of optimized 2 are
very similar to those calculated from the crystallographically
determined coordinates.
from
a
reaction mixture, and structural elucidation[12]
revealed a configuration analogous to that of 2.
A screening of alternate metal sources showed that
commercially available pentakis(dimethylamido)tantalum
provided yields comparable to those obtained by employing
1 and allowed for the preparation of 2 and 3 on a much larger
scale. Thus, it is likely that many reported reactions involving
addition of an arylamine to tantalum coordination complexes
have resulted in the formation, but not the detection, of these
clusters.
The calculated LUMO of 2 is delocalized over all six
tantalum centers and is primarily nonbonding (Figure 3). This
bonding scheme is consistent with the observed optical
properties in that ligand-to-metal charge-transfer processes
should be very facile. Furthermore, the computational results
suggest that the title compounds may exhibit interesting
redox behavior. Indeed, cursory reactivity studies have shown
that although 2 and 3 are inert towards unsaturated small
molecules, they are easily reduced to stable anions. Prelimi-
nary data suggest that further study of this novel family of
compounds will yield new insights into the nature of
polynuclear metal–imido bonding.
The presence of oxide in the cluster core was confirmed by
electrospray mass spectrometry studies on 3 and 4 using THF
as the matrix. Owing to extensive fragmentation and sub-
sequent reaction with the matrix, none of the observed
fragments corresponded to combinations of the cluster core
and arylimido fragments. However, many of the high-
molecular-weight fragments (up to ca. 2000 Da) possessed
masses that were 16 Da greater for 4 (MeO) than for 3 (Me).
Therefore, these fragments apparently each retained one
imido ligand. Samples of 3 and 4 that had been prepared using
17O- and 18O- enriched water yielded spectra in which each
high-mass fragment from the nonlabeled cluster was accom-
panied by fragments that were one and two Da heavier,
including those that were inferred to still possess an imido
ligand (see the Supporting Information). These data provide
strong evidence for assignment of these compounds as oxide-
centered clusters, with the oxide deriving from added water.
Compounds 3 and 4 are soluble in benzene to about
10 mm and were recrystallized without decomposition. Both
are significantly more air-sensitive than 2, decomposing in
solution over about two days if stored in polypropylene-
capped glass vials inside an inert-atmosphere glove box.
Nonetheless, a sample of 3 in C6D6 showed no decomposition
after it was heated at 1358C for two weeks under rigorously
air-free conditions. Compound hues vary from red/orange (5)
to red/black for the more electron-rich 4 (Figure 2, insert).
Compounds 2, 3, and 4 exhibit intense UV absorptions quite
similar in energy to those of the anilines from which they are
derived (Figure 2). Fluorescence measurements, however,
Figure 3. LUMO of Ci-optimized 2 (X3LYP/LACVP*) viewed along the
fourfold axis of the cluster core (nonbonding, localized on six tantalum
centers). Ph groups have been removed for clarity.
Experimental Section
[(ArN)14Ta6O] (method 1): Compound
1 (100 mg, 0.190 mmol,
1 equiv) was dissolved in benzene (1 mL). The aniline (0.950 mmol,
5 equiv) was treated with degassed water (0.6 mL, 0.03 mmol,
0.16 equiv) by microsyringe and then dissolved in benzene (0.5 mL).
370
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Angewandte
Chemie
The two solutions were then combined. Hexane (4 mL) was added
and the mixture was heated at 1358C for 24 h in a sealed vessel, during
which time the product precipitated as microcrystalline material.
After cooling to room temperature, the product was washed with
Et2O (2 ” 2 mL), then pentane (2 ” 2 mL). Isotopically enriched
samples were prepared in an identical fashion using 17O/18O-enriched
water.
2: Material collected from the reaction mixture was analytically
pure. Yield: 43.3 mg, 0.018 mmol, 57%; 1H NMR (500 MHz,
[D8]THF, 228C, TMS): d = 7.68 (dd, J(H,H) = 8.5 Hz, 1.0 Hz, 16H),
7.13 (m, 16H), 6.98 (m, 12H), 6.92 (m, 8H), 6.71 (m, 6H), 6.07 ppm
(dd, J(H,H) = 8.5 Hz, 1.0 Hz, 12H); 13C{1H} NMR (125.8 MHz,
[D8]THF, 228C, TMS): d = 157.84, 156.83, 129.80, 129.20, 128.35,
126.87, 125.27, 124.86 ppm; UV/Vis (THF): lmax(onset) = 280 nm
(580 nm). Elemental analysis (%) calcd for C84H70N14OTa6: C 42.09,
H 2.94, N 8.18; found: C 42.23, H 2.89, N 8.86.
Keywords: cluster compounds · computer chemistry ·
imide ligands · structure elucidation · tantalum
.
[1] a) M. T. Pope, A. Mueller, Angew. Chem. 1991, 103, 56 – 70;
Angew. Chem. Int. Ed. Engl. 1991, 30, 34 – 48; b) P. Gouzerh, A.
Proust, Chem. Rev. 1998, 98, 77 – 111.
[2] a) J. B. Strong, G. P. A. Yap, R. Ostrander, L. M. Liable-Sands,
A. L. Rheingold, R. Thouvenot, P. Gouzerh, E. A. Maatta, J.
Am. Chem. Soc. 2000, 122, 639 – 649; b) W. Clegg, R. J.
Errington, K. A. Fraser, S. A. Holmes, A. Schaefer, J. Chem.
Soc. Chem. Commun. 1995, 455 – 456.
[3] H. Kang, J. Zubieta, J. Chem. Soc. Chem. Commun. 1988, 1192 –
1193.
[4] a) T. M. Che, V. W. Day, L. C. Francesconi, M. F. Fredrich, W. G.
Klemperer, W. Shum, Inorg. Chem. 1985, 24, 4055 – 4062; b) J. R.
Harper, A. L. Rheingold, J. Am. Chem. Soc. 1990, 112, 4037 –
4038.
[5] V. G. Kessler, G. A. Seisenbaeva, Inorg. Chem. Commun. 2000,
3, 203 – 204.
[6] D. E. Wigley in Progress in Inorganic Chemistry, Vol. 42, Wiley,
New York, 1994, pp. 239 – 482.
[7] L. L. Anderson, J. Arnold, R. G. Bergman, Org. Lett. 2004, 6,
2519 – 2522.
3: Material collected from the reaction mixture was analytically
pure. Yield: 39.2 mg, 0.015 mmol, 48%; 1H NMR (500 MHz, C6D6,
228C, TMS): d = 8.03 (d, J(H,H) = 8.5 Hz, 16H), 6.87 (d, J(H,H) =
8.0 Hz, 12H), 6.81 (d, J(H,H) = 8.5 Hz, 16H), 6.47 (d, J(H,H) =
8.0 Hz, 12H), 2.08 (s, 18H), 1.96 ppm (s, 24H); 13C{1H} NMR
(125.8 MHz, C6D6, 228C, TMS): d = 155.49, 155.13, 134.25, 133.68,
130.18, 128.91, 126.78, 125.12, 21.17, 20.75 ppm; UV/Vis (THF):
lmax(onset) = 285 nm (600 nm). Elemental analysis (%) calcd for
C98H98N14OTa6: C 45.74, H 3.84, N 7.62; found: C 46.10, H 4.08,
N 7.87.
[8] Crystal
data
and
structure
refinements
for
2:
C84H70N14OTa6·4C6H6, Mr = 2689.67 gmolꢀ1, crystal dimensions
1
4: Yield: 43.4 mg, 0.016 mmol, 49%; H NMR (500 MHz, C6D6,
3
¯
0.05 ” 0.05 ” 0.04 mm , triclinic, space group P1, a = 14.137(3),
228C, TMS): d = 8.19 (d, J(H,H) = 9.0 Hz, 16H), 6.74 (d, J(H,H) =
9.0 Hz, 16H), 6.63 (s, 24H), 3.12 (s, 18H), 3.09 (s, 24H); 13C{1H} NMR
(125.8 MHz, C6D6, 228C, TMS): d = 157.53, 157.45, 151.70, 151.63,
127.96, 126.22, 114.90, 113.75, 55.34, 55.17 ppm; UV/Vis (THF):
lmax(onset) = 300 nm (630 nm). The product was recrystallized by
vapor diffusion of pentane into a benzene solution of 4 prior to
submission for elemental analysis. Elemental analysis (%) calcd for
C98H98N14O15Ta6: C 42.07, H 3.53, N 7.01; found: C 42.62, H 3.74,
N 6.90.
5: After heating, the solvent was removed under vacuum. The
orange oil was dissolved in toluene (1 mL), into which pentane was
then allowed to diffuse. The resulting crystalline material was
insoluble in nonreactive solvents. Yield: 13.4 mg, 0.0047 mmol,
15%. Elemental analysis (%) calcd for C84H56Cl14N14OTa6: C 35.28,
H 1.97, N 6.86; found: C 34.94, H 2.17, N 6.81.
b = 14.262(3), c = 14.343(3) Š, a = 76.192(4), b = 72.097(4), g =
60.491(4)8, V= 2381(9) Š3, Z = 1, 1calcd = 1.875 gcmꢀ3
, m =
8.59 mmꢀ1, F(000) = 1286, 32552 reflections measured, 13506
unique [R(int) = 0.0600], 2q = 3.60–67.348, 583 refined parame-
ters, R1 = 0.0501 (12494 observations where I > 2s(I)), wR2 =
0.1359, GoF = 1.026. Data were collected at 193(2) K on a
Bruker Platinum 200 CCD diffractometer (APEX2 v.2.02, Area-
Detector Software Package, Bruker AXS, Madison, WI, 1995–
99) with Si-h111i channel-cut crystal-monochromated synchro-
tron radiation (l = 0.77490 Š, Advanced Light Source beamline
11.3.1, Lawrence Berkeley National Laboratory, operated under
DOE contract DE-AC03-76SF00098) using w scans (0.38 per
1.5-s frame). All non-hydrogen atoms were refined anisotropi-
cally; hydrogen atoms were included in calculated positions but
not refined. The maximum and minimum peaks on the final
difference Fourier map corresponded to 2.820 and
ꢀ2.150 eꢀ Šꢀ3, respectively, and were located near the tantalum
atoms. General methods for integration, solution, and refine-
ment can be found in the Supporting Information. CCDC-
615636 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
2
(method 2): Pentakis(dimethylamido)tantalum (1.00 g,
2.49 mmol, 1 equiv) was dissolved in toluene (5 mL). A solution of
aniline (1.41 g, 15.14 mmol, 6.08 equiv) in toluene (10 mL) was added,
followed by water (7.5 mL, 0.42 mmol, 0.16 equiv). The mixture was
heated at reflux for 4 days, after which time the flask was cooled to
room temperature and the supernatant was removed. The solid was
washed with Et2O (2 ” 10 mL), then extracted with THF (2 ” 20 mL).
The extracts were combined and the solvent removed under vacuum.
Yield: 380 mg, 0.158 mmol, 38%.
[9] Possesses the minimal cluster core of hexatantalate; see: F.
Pickhard, H. Hartl, Z. Anorg. Allg. Chem. 1997, 623, 1311 – 1316.
[10] Possibly deriving from the tBuN moiety. An undetected proton
could provide charge balance.
[11] M. M. Banaszak Holl, P. T. Wolczanski, J. Am. Chem. Soc. 1992,
114, 3854 – 3858.
3 (method 2): p-Toluidine (1.36 g, 12.7 mmol, 5.10 equiv) and
water (7.5 mL, 0.42 mmol, 0.16 equiv) were combined and dissolved in
toluene
(5 mL).
Pentakis(dimethylamido)tantalum
(1.00 g,
2.49 mmol, 1 equiv) was then added, followed by n-octane (20 mL).
The mixture was heated at reflux for 2 days, after which time the flask
was cooled to room temperature and the supernatant was removed.
The solid was washed with Et2O (2 ” 10 mL), then extracted with hot
toluene (2 ” 20 mL). The extracts were combined and the solvent
removed under vacuum. Yield: 320 mg, 0.124 mmol, 30%.
[12] Crystal
data
and
structure
refinements
for
5:
C84H56Cl14N14OTa6·2C4H11N·2C6H6, Mr = 3161.92 gmolꢀ1, crystal
3
¯
dimensions 0.22 ” 0.14 ” 0.06 mm , triclinic, space group P1, a =
13.148(1), b = 13.796(1), c = 16.005(1) Š, a = 108.687(1), b =
General experimental, crystallographic, and computational pro-
96.277(1), g = 94.847(1)8, V= 2711.5(3) Š3, Z = 1, 1calcd
=
cedures, as well as coordinates for Ci-optimized
2 and mass
1.936 gcmꢀ3, m = 6.43 mmꢀ1, F(000) = 1510, 15509 reflections
measured, 10598 unique [R(int) = 0.0196], 2q = 4.62–52.788, 617
refined parameters, R1 = 0.0303 (8490 observations where I >
2s(I)), wR2 = 0.0713, GoF = 0.997. Data were collected at
168(2) K on a Bruker APEX diffractometer (SMART v.5.631,
Bruker AXS, Madison, WI, 1995–99) with graphite-monochro-
spectrometry data, can be found in the Supporting Information.
Received: July 26, 2006
Published online: December 8, 2006
Angew. Chem. Int. Ed. 2007, 46, 369 –372
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
371

Communications
mated radiation (l = 0.71073 Š) using w scans (0.38 per 10-s
atoms. CCDC-615637 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
frame). All non-hydrogen atoms were refined anisotropically
except those of tBuNH2, while hydrogen atoms were included in
calculated positions but not refined (no hydrogen atoms were
included for tBuNH2). The occupancies of disordered Cl2 were
also refined. The maximum and minimum peaks on the final
difference Fourier map corresponded to 1.309 and
ꢀ1.065 eꢀ Šꢀ3, respectively, and were located near the tantalum
[13] Jaguar, v.6.5, Schrodinger, LLC, New York, NY, 2005.
[14] Extension of B3LYP by Xu and Goddard to include the Perdew–
Wang 1991 gradient exchange correction. X. Xu, W. A. God-
dard III, Proc. Natl. Acad. Sci. USA 2004, 101, 2673 – 2677.
372
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Angew. Chem. Int. Ed. 2007, 46, 369 –372