Systematic syntheses and structural characterizations of two isomeric phosphadicarbaboranes: 6-R-arachno-6,8,9-PC2B7H11 and 6-R-arachno-6,5,7-PC2B7H11. The first 10-vertex phosphadicarbabor

Article abstract of DOI:10.1021/ic000728g

The first 10-vertex phosphadicarbaboranes, 6-R-arachno-6,8,9-PC₂B₇H₁₁ 1 (R = Ph 1a or Me 1b) and 6-Me-arachno-6,5,7-PC₂B₇H₁₁ 2 have been synthesized in good yields and structurally characte

Full text of DOI:10.1021/ic000728g

4
996
Inorg. Chem. 2000, 39, 4996-4997
Systematic Syntheses and Structural Characterizations of Two Isomeric Phosphadicarbaboranes:
-R-arachno-6,8,9-PC 11 and 6-R-arachno-6,5,7-PC 11. The First 10-Vertex Phosphadicarbaboranes
6
2
B
7
H
2 7
B H
Daewon Hong, Scott E. Rathmill, Daniel E. Kadlecek, and Larry G. Sneddon*
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323
ReceiVed June 30, 2000
The development of high-yield methods for heteroatom inser-
tion has been the critical step needed to enable the systematic
syntheses of heteroatom-polyborane clusters. Recently, we
reported a new, efficient method for the syntheses of thia- and
phosphapolyborane clusters and applied this method to the
synthesis of a range of new 11-vertex thia- and phosphaboranes,
thia- and phosphadicarbaboranes, and the first thiaphosphaborane.1
In this communication, we report the syntheses and structural
characterizations of the two isomeric clusters, 6-R-arachno-6,8,9-
Scheme 1. Reaction Sequences Leading to the Formation of
1 and 2
PC
2
B
7
H
11 1 (R ) Ph 1a or Me 1b) and 6-Me-arachno-6,5,7-
PC
2 7
B H11 2, which are the first examples of 10-vertex phosphadi-
carbaboranes.
As outlined in Scheme 1, 1a and 1b are synthesized from
2
adjacent-carbon arachno-4,5-C
2
B H
7
13, while 2 is derived from
3
2 7
the nonadjacent-carbon arachno-4,6-C B H13. Both syntheses
involve initial deprotonation of the starting dicarbaboranes with
Proton Sponge (1,8-bis(dimethylamino)naphthalene), followed by
the reaction of the anions with RPCl to produce metathesis
2
products. These intermediates were not isolated, but structures
in which the RClP groups are substituted at either terminal or
bridging sites on the original dicarbaborane frameworks can be
reasonably proposed. Further reaction with an additional 1 equiv
of Proton Sponge induces dehydrohalogenation and cage insertion
of the RP group to yield the final phoshadicarbaborane products
which were obtained in 68% (1a), 49% (1b), and 50% (2) isolated
yields as air-sensitive oils. Their compositions are established by
both elemental analyses and mass spectrometry.
2
the CH resonances of 1a and 1b resolve, even at room
temperature, into doublets with J13 -31 ) 49 (1a) and 43 Hz (1b).
C
P
The 11B NMR spectra of 1a and 2 are presented in Figure 1,
5
According to cluster electron counting methods, 1 and 2 have
26 skeletal electrons and should adopt 10-vertex arachno
geometries based on an icosahedron missing two vertices, but a
number of isomeric structures are possible based on this frame-
work. Since the compounds are oils, crystallographic determina-
tions have not been possible. However, the structures of both
isomers have been firmly established using DFT/GIAO/NMR
calculations (Gaussian 94) in conjunction with two-dimensional
and the observed spectral patterns are in agreement with C
1
and
C
s
cage symmetries, respectively, for the two isomers. The
resonances at -12.0 and -19.7 ppm in the spectrum of 1a both
show fine structure characteristic of bridge-hydrogen coupling.
1
The H NMR spectrum of 1a (and 1b) shows, in addition to the
phenyl (methyl 1b) and seven terminal BH resonances, an
intensity-one bridge-hydrogen resonance and three cage-CH
resonances with one of the CH resonances at a high-field shift
COSY 11B- B NMR studies. Density functional theory (DFT)
11
6
(
-0.72 ppm 1a, -0.90 ppm 1b) characteristic of an endo-
was first used to obtain optimized molecular geometries and
1
2
hydrogen of a cage-CH group. The H NMR spectrum of 2
exhibits a methyl resonance and the terminal BH resonances in
their expected 1:1:1:2:2 ratios, along with intensity-two bridge-
(3) Garrett, P. M.; George, T. A.; Hawthorne, M. F. Inorg. Chem. 1969, 8,
2
008-2009.
(
4) (a) Wrackmeyer, B. In Progress in NMR Spectroscopy; Emsley, J. W.,
Feeney, J., Sutcliffe, L. H., Eds.; Pergamon: New York, 1979; Vol. 12,
pp 227-259. (b) Gragg, B. R.; Layton, W. J.; Niedenzu, K. J.
Organomet. Chem. 1977, 132, 29-36.
5) (a) Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 1-66. (b)
Williams, R. E. Chem. ReV. 1992, 92, 177-207 and references therein.
13
hydrogen and cage-CH resonances. The room temperature C-
1
{
H} NMR spectrum of 2 contains a single resonance with a
multiplet structure arising from both boron and phosphorus
coupling, but when recorded at temperatures (-83 °C) low enough
to thermally decouple 11B coupling, the resonance resolves into
a sharp doublet, J13 -31 ) 52 Hz, consistent with the carbon and
(
4
(6) For some examples of the use of computational methods for the structural
characterization of polyboranes, see ref 1 and the following: (a) B u¨ hl,
M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1992, 114, 477-491. (b) B u¨ hl,
M.; Gauss, J.; Hofmann, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1993,
C
P
phosphorus being in adjacent positions in the cage framework.
The proton-coupled 13C NMR spectrum of 1a (and 1b) shows
the expected two cage-carbon resonances, one of which appears
1
15, 12385-12390. (c) Onak, T.; Tran, D.; Tseng, J.; Diaz, M.; Arias,
J.; Herrera, S. J. Am. Chem. Soc. 1993, 115, 9210-9215. (d) Diaz, M.;
Jaballas, J.; Arias, J.; Lee, H.; Onak, T. J. Am. Chem. Soc. 1996, 118,
as an apparent triplet and the other as a doublet indicating CH
2
4405-4410. (e) Bausch, J. W.; Matoka, D. J.; Carroll, P. J.; Sneddon,
and CH cage units, respectively. In their 13C{ H} NMR spectra,
1
L. G. J. Am. Chem. Soc. 1996, 118, 11423-11433. (f) Jaballas, J.; Onak,
T. J. Organomet. Chem. 1998, 550, 101-109. (g) Cranson, S. J.; Fox,
M. A.; Greatrex, R.; Greenwood, N. N. J. Organomet. Chem. 1998, 550,
207-212. (h) Hofmann, M.; Fox, M. A.; Greatrex, R.; Williams, R. E.;
Schleyer, P. v. R. J. Organomet. Chem. 1998, 550, 331-340. (i) Onak,
T.; Jaballas, J.; Barfield, M. J. Am. Chem. Soc. 1999, 121, 2850-2856.
(
(
1) Shedlow, A. M.; Sneddon, L. G. Inorg. Chem. 1998, 37, 5269-5277.
2) (a) He rˇ m a´ nek, S.; Jel ´ı nek, T.; Ple sˇ ek, J.; Sˇ t ´ı br, B.; Fusek, J. J. Chem.
Soc., Chem. Commun. 1987, 927-928. (b) Sˇ t ´ı br, B.; He rˇ m a´ nek, S.;
Ple sˇ ek, J. Inorg. Synth. 1983, 22, 237-239.
1
0.1021/ic000728g CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/03/2000

Communications
Inorganic Chemistry, Vol. 39, No. 22, 2000 4997
Figure 1. 11B NMR spectra (160.5 MHz) of (a) 1a and (b) 2.
Figure 3. Comparisons of the experimental NMR shifts (ppm) and
11
assignments for compounds 1 and 2 (160.5 MHz B NMR; 125.8 MHz
13
31
C NMR, 25 °C; and 145.8 MHz P NMR) with the DFT/GIAO
calculated values (B3LYP/6-311G*//B3LYP/6-311G*) for structures I
and IV.
2 7 2 7
PC B H11 (I) and 6-Me-arachno-6,5,7-PC B H11 (IV) structures
are in excellent agreement with the experimentally determined
1
1
13
31
11
B, C, and P chemical shifts and the B assignments (from
11
11
2
D
B- B COSY experiments) for isomers 1 and 2. One
unanticipated result of the spectroscopic and structural analyses
is the fact that the CH carbon (C9) of compound 1 exhibits strong
2
coupling to the phosphorus, even though these two atoms are not
in adjacent cage positions.7
A comparison of the relative energies of the isomers in Figure
2
reveals that I is thermodynamically favored, with IV at
considerably higher energy. This difference in energy is in
agreement with the well-known tendency for electron-rich ele-
ments to favor lower coordinate vertices on the open faces of
8
polyhedral clusters. Thus, in I (and 1) the phosphorus and one
carbon are situated in the low-coordinate 6,9 cage positions,
whereas in IV (and 2) one of these sites is occupied by a boron.
The fact that a higher energy isomer such as 2 can be synthesized
using the Proton Sponge initiated in situ dehydrohalogenation
method again illustrates the advantages of this mild reaction
method for achieving kinetic products.
We will report in future publications our explorations of the
chemical properties of these phosphadicarbaboranes, including
their use for the syntheses of new families of both smaller-cage
phosphadicarbaboranes and metallaphosphadicarbaborane com-
plexes.
Figure 2. DFT calculated optimized geometries and relative energies
(
B3LYP/6-311G*//B3LYP/6-311G*) for isomeric arachno-RPC
2 7 11
B H
structures.
relative energies for the possible isomers shown in Figure 2 that
could result from phosphorus insertion at the different positions
on the open face of the two dicarbaboranes. Gauge independent
atomic orbital (GIAO) NMR shift calculations were then used to
predict the chemical shifts and assignments for each structure.
As shown in Figure 3, the calculated values at the B3LYP/6-
Acknowledgment. We thank the National Science Foundation
for support of this research.
3
11G*//B3LYP/6-311G* level for the 6-Me-arachno-6,8,9-
Supporting Information Available: Characterization data and
descriptions of the syntheses of compounds 1a, 1b, and 2. Tables listing
Cartesian coordinates and bond distances for the optimized geometries
at the B3LYP/6-311G* level for I-V. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
7) We also previously reported a long-range coupling between phosphorus
and carbon cage atoms in 6-R′-3,4-R -6,3,4-nido-PC ; see: Keller,
W.; Barnum, B. A.; Bausch, J. W.; Sneddon, L. G. Inorg. Chem. 1993,
2, 5058-5066.
8) (a) Williams, R. E. AdV. Inorg. Chem. Radiochem. 1976, 18, 67-142.
b) Bausch, J. W.; Rizzo, R. C.; Sneddon, L. G.; Wille, A. E.; Williams,
R. E. Inorg. Chem. 1996, 35, 131-135.
2
2 4 4
B H
3
(
(
IC000728G