A unique nido exo-arachno equilibrium involving [(PPh3)2(CO)OsB5H9] and its base adducts: Crystal and molecular structure of [{(PPh3)2(CO)OsB4H7}(bh2·PPh2Me)]

Full text of DOI:10.1021/ja962192x

J. Am. Chem. Soc. 1997, 119, 631-632
631
Scheme 1
A Unique Nido Exo-Arachno Equilibrium Involving
[(PPh3)2(CO)OsB5H9] and Its Base Adducts:
Crystal and Molecular Structure of
[{(PPh3)2(CO)OsB4H7}(BH2‚PPh2Me)]†
Lawrence Barton,* Jonathan Bould, Hong Fang,
Kevin Hupp, Nigam P. Rath, and Charles Gloeckner1
Department of Chemistry, UniVersity of MissourisSt. Louis
St. Louis, Missouri 63121
ReceiVed June 28, 1996
Electron-counting rules for clusters allow predictions to be
made about structural changes that result from the addition of
electron pairs to cluster molecules.2 The anticipated effect is
that addition of electron pairs to a cluster results in the opening
up of the structure in the sequence closo, nido, arachno, hypho.3
There are some simple examples in the literature of systems in
which addition of an electron pair to a small nido-borane, in
the form of an equivalent of Lewis base, forms a conventional
arachno-cluster. This results when CN- is added to B5H9.4 In
most of the known cases, however, two electron pairs are added
to the nido-borane to afford a hypho-cluster5 or the added base
cleaves the borane.6 The structurally characterized adduct
species include B5H9‚2PMe3,5a B4H8‚(Me2NCH2)2,5b,c B5H9‚-
(Ph2P)2CH2,5c,d B5H9‚(Ph2PCH2)2,5c,d B5H9‚(Me2NCH2)2,5c,d and
B6H10‚2PMe3.7 In addition, B6H10‚PMe38 has also been identi-
fied from spectroscopic data. In this paper we describe the
formation of a unique exo-arachno cluster, from the addition
of phosphine bases to [(PPh3)2(CO)OsB5H9] (1),9a in a process
which is reversible.
If a mixture of [(PPh3)2(CO)OsB5H9] (1) (0.26 mmol) and
PPh3 (0.35 mmol) in CH2Cl2 is refluxed for 14 h under N2, a
pale-yellow, air stable crystalline solid of formulation [(PPh3)2-
(CO)OsB5H9‚(PPh3)] (2) is obtained in 57% yield. The same
species had previously been isolated as a minor product in the
preparation of 1.9b Mass spectral data10 provided the formula-
tion but identification of 2 eluded us for some time because
precedence suggested an arachno-type structure1,2 and satisfac-
tory elemental analysis data were precluded by the presence of
solvent in the lattice.11 The 11B NMR spectrum for 212 gave
four signals in 2:1:1:1 ratio, and the 1H{11B} spectra suggested
the presence of five terminal and four bridging H atoms, two
of them bridging to the metal atom. Selective decoupling
experiments indicated that only one terminal H resonance was
associated with the boron resonance of intensity 2, suggesting
that one boron atom must have a substituent. The low-
temperature 31P NMR spectrum showed three inequivalent P
atoms, one of which was broad suggesting coupling to boron.
The 11B NMR spectrum of 2 is quite similar to that of the
osmapentaborane [(PPh3)2(CO)OsB4H8] (3),9a except that one
the resonances has been shifted downfield by about 16 ppm
and there is an additional resonance B(6) whose chemical shift
of -34.4 ppm is close to that for BH3‚PR3 compounds (ca. -37
ppm).13 It is similarly comparable to the 11B spectrum of the
substituted derivative [(PPh3)2(CO)OsB4H7(C4H9)].14 These
observations suggested that we had an osmapentaborane with
a pendent BH2‚PPh3 group bonded to one of the basal boron
atoms. Selective decoupling experiments indicated that the
pendent boron was bonded to a boron adjacent to the Os atom,
therefore we presume that the reaction described in eq 1 and
illustrated in steps 1-3 of Scheme 1 had taken place. Although
† Suggested name: [2-Carbonyl-2,2-bis(triphenylphosphine)-3-{(diphe-
nylmethylphosphine)boryl}-nido-2-osmapentaborane].
(1) Analytical Sciences Center, Monsanto Company, 800 N. Lindbergh
Boulevard, St. Louis, MO 63167.
(2) (a) Wade, K. J. Chem. Soc., Chem. Commun. 1971, 792. (b) Rudolph,
R. W.; Pretzer, W. P. Inorg. Chem. 1971, 11, 1974. (c) Mingos, D. M. P.
Nature (Phys. Sci.) 1972, 236, 99.
(3) (a) Williams, R. E. Inorg. Chem. 1971, 10, 210. (b) Williams, R. E.
AdV. Inorg. Chem. Radiochem. 1976, 18, 67.
(4) Taylor, J. G.; Wallbridge, M. G. H. Polyhedron 1985, 4, 321.
(5) (a) Fratini, A. V.; Sullivan, G. W.; Denniston, M. L.; Hertz, R. K.;
Shore, S. G. J. Am. Chem. Soc. 1974, 96, 3013. (b) Colquhoun, H. G. J.
Chem. Res. 1978, 451. (c) Alcock, N. N.; Colquhoun, H. G.; Sawyer, J. F.;
Wallbridge, M. G. H. J. Chem. Soc., Dalton Trans. 1982, 2243. (d) Alcock,
N. N.; Colquhoun, H. G.; Haran, G.; Sawyer, J. F.; Wallbridge, M. G. H.
J. Chem. Soc., Chem. Commun. 1977, 368.
[(PPh3)2(CO)OsB5H9] + PPh3 h
[(PPh3)2(CO)OsB5H9‚(PPh3)] (1)
the NMR spectra for [(PPh3)2(CO)OsB5H9] leave some doubt
about its structure, we have recently determined the structure
by X-ray analysis and confirmed that the bridging H atoms are
symmetrically disposed, as seen in Scheme 1.15 The reversibility
of the reaction was established by heating a purified sample of
2 in CDCl3 at 40 °C for 17 h, which resulted in recovery of
53% of 1. Heating 2 for extended periods at 50 °C results in
the formation of [(PPh3)2(CO)OsB4H8] and BH3‚PPh3. This
suggests that the process shown in step 4 of Scheme 1 also
occurs.
(6) For example, see: (a) Parry, R. W.; Edwards, L. J. J. Am. Chem.
Soc. 1959, 81, 3554. (b) Shore, S. G. in Boron Hydride Chemistry;
Muetterties, E. L., Ed.; Academic Press: New York, 1975; Chapter 3. (c)
Kodama. G. In AdVances in Boron and the Boranes; Liebman, J. F.,
Greenberg, A., Williams, R. E., Eds.; VCH Publishers, Inc.: Weinheim,
FRG, 1988; Chapter 6.
(11) Anal. Found for C55H52P3B5OsO: C, 59.03; H, 5.06. Calcd: C,
61.84; H, 5.10.
(12) Boron (160.5 MHz), proton (500.1 MHz), and phosphorus (202.5
MHz) NMR data in ppm (CDCl3 solution, 298 K) for 2 (assignment [δ
(11B) (1H in parentheses)]): δ(11B) B(4) [+9.3 (+5.3)], B(3) [+9.3 (no
H)], B(5) [-16.4 (+1.3)], B(1) [-28.9 (+0.26)], B(6) [-34.4 (+1.95,
(7) Mangion, M.; Hertz, R. K.; Denniston, M. L.; Long, J. R.; Clayton,
W. R.; Shore, S. G. J. Am. Chem. Soc. 1976, 98, 449.
(8) Kameda, M.; Kodama, G. Inorg. Chem. 1981, 20, 1072. The unstable
species B6H10‚PMe3 is considered to be the precursor to the belt-shaped
B6H10‚2PMe3.
2
+2.05)]; δ(1H) µH3,4 [-1.4], µH4,5 [-2.3], µH2,5 [-9.2, J(31P-1H)trans
) 42 Hz)], µH2,3 [-10.4]; δ(31P) P(1) [+14.9, t, J ) 9.3 Hz], P(2) [+9.9,
d, J ) 9.3 Hz], P(3) [+24.5, br s].
(9) (a) Bould, J.; Greenwood, N. N.; Kennedy, J. D. J. Organomet. Chem.
1983, 249, 11. (b) Bould, J.; Gloeckner, C.; Rath, N. P.; Barton, L.
Unpublished results.
(13) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectros-
copy of Boron Compounds; Springer-Verlag: Heidelberg, 1978; pp 340-
344.
(10) HRMS (Finnegan MAT 95/FAB/positive ion electrospray using
electric sector scanning and standardized with PEG1000 in MeOH/CHCl3
(14) Bould, J.; Pasieka, M.; Braddock-Wilking, J.; Rath, N. P.; Barton,
L.; Gloeckner, C. Organometallics 1995, 14, 5138.
(15) Bould, J.; Rath, N. P.; Barton, L. Unpublished results.
(1:1) with 10 mmol NH4OH) gave m/q for 2 (M - H2) of 12C55 H5231P311B5192
-
1
Os116O1 1066.3370 (obsd) and 1066.3346 (calcd).
S0002-7863(96)02192-0 CCC: $14.00 © 1997 American Chemical Society

632 J. Am. Chem. Soc., Vol. 119, No. 3, 1997
Communications to the Editor
the earlier study used much higher temperatures (100 °C) to
obtain comparable yields of 3.9a We propose that the formation
of the phosphine adducts 2 (or 4), and the ultimate degradation
to the osmapentaborane 3, proceeds as illustrated in Scheme 1.
Coordination of PPh3 results in the opening of the cage to afford
an unstable intermediate arachno system that rearranges, as
indicated in Scheme 1, to afford (PPh3)2(CO)OsB4H7(BH2PPh3).
Loss of phosphine‚borane affords the osmapentaborane (PPh3)2-
(CO)OsB4H8 (3). Obviously this latter step must involve the
incorporation of two additional H atoms from solution species
since (PPh3)2(CO)OsB4H7(BH2PPh3) (2) contains two fewer H
atoms than the combination of 3 and BH3‚PPh3. The observed
structure of 4 may be compared to the proposed structure for
B6H10‚PMe3, which, on the basis of NMR evidence, is postulated
to be a pentaborane cage with the BH2L group bridging adjacent
basal boron atoms rather than σ-bonded to a single basal B atom
as we observed for 4.8 The degradation of metallahexaboranes
to clusters containing pendent boron atoms is a process which
has only been suggested from ab initio calculations,18 but our
results suggest that it may be an important new reaction mode
for clusters.
Figure 1. Molecular structure of nido-[(PPh3)2(CO)OsB4H7](BH2‚PPh2-
Me) (4) with 30% probability thermal ellipsoids and the R groups on
P and the cage H atoms omitted. Selected interatomic distances (Å)
are from Os (B(5) 2.36(2), B(3) 2.40(2), B(1) 2.23(2)), from B(3) (B(4)
1.85(4), B(1) 1.75(3), B(6) 1.69(3)), and B(4)-B(5) 1.79(3) and B(6)-
P(3) 1.92(2). Angles (deg) include B(3)-B(6)-P(3) 110.8(13), B(4)-
B(3)-B(6) 134(2), and Os-B(3)-B(6) 130.7(12).
Attempts to obtain crystals of 2 suitable for X-ray analysis
were unsuccessful. However, we were able to demonstrate
similar chemistry with the bases PMe3 and PPh2Me, and the
adduct (PPh3)2(CO)OsB5H9‚(PPh2Me) (4) provided crystals
suitable for a good quality X-ray analysis.16 The structure of
4, represented in Figure 1, confirms that the species is an
osmapentaborane skeleton with a pendent BH2‚PR3 group. As
expected, the structure differs little from that of B5H9 except
for features influenced by the presence of the Os atom. Thus,
the angle B(5)-Os-B(3) is 69.3(6)° as opposed to 90° in B5H9,
and the apex-base and the base-base interboron distances are
slightly longer in 4 than in the latter.17 The angle B(1)-B(3)-
B(6) of 139(2)°, is similar to the B(1)-B(2)-H(2) angle in B5H9
of 130(2)°, indicating that the BH2‚PR3 moiety occupies a very
similar location to the terminal H atom it supplants in B5H9.
Selected structural parameters are given in Figure 1.
The results suggest a possible mechanism for the formation
of 1-[Fe(CO)3]B4H819 and 4-[Fe(CO)3]B5H920 from Fe(CO)5 and
B5H9. The former is prepared from the thermolysis of B5H9/
Fe(CO)5 mixtures. If the reaction is carried out with frequent
removal of excess CO, which is formed in the reaction, then
the principal product is [Fe(CO)3]B5H9,20 presumably because
CO serves the same role in the latter reaction as does the addition
of excess phosphine in the reactions of (PPh3)2(CO)OsB5H9
described herein. Our results also emphasize the importance
of not disregarding trace products in reactions. The initial
observation of 2 was as one of several trace byproducts which
also included (PPh3)2(CO)(H)OsB3H8, (PPh3)2(CO)OsB4H8, Li-
[(PPh3)2(H)OsB9H13], etc., formed in the preparation of (PPh3)2-
9b
(CO)OsB5H9.
The position of the equilibrium and the tendency to degrade
to the osmapentaborane 3 is dependent on the donor strength
of the phosphine base. Thus, PMe3 reacts with 1 to give an
analogue of the adduct 2 which on heating does not regenerate
1 but degrades to 3 with the elimination of BH3‚PMe3. The
PPh2Me adduct 4 is more stable than the PPh adduct 2, and an
equilibrium between 4 and 1 exists in which the former
predominates. Thermolysis of 1 under the same conditions (18
h at 50 °C) affords 3. This was observed previously, although
Acknowledgment. This work was supported by the NSF (grant
no. CHE-9311557), the Missouri Research Board, and a UM-St. Louis
a Research Incentive Award. We also acknowledge instrumentation
grants from the NSF (no. CHE-9309690 and CHE-9318696), the DOE
(grant no. DE-FG02-92CH10499), and the UM-St. Louis Center for
Molecular Electronics. Finally we acknowledge the cooperation of Dr.
Fred Hileman, Analytical Sciences Center, Monsanto, St. Louis.
Supporting Information Available: Crystallographic data, posi-
tional parameters, anisotropic displacement parameters, and bond
lengths and angles for 4 (8 pages). See any current masthead page for
ordering and Internet access instructions.
(16) Crystallographic data for 4: C51H53B5Cl3OOsP3, monoclinic, P21,
a ) 10.8064(1), b ) 12.0477(2), and c ) 19.9238(4) Å, â ) 98.258(1)°,
V ) 2567.03(7) Å3, Z ) 2, Dc ) 1.456 Mg m-3. Of the 12398 reflections
collected (Siemens SMART CCD diffractometer, Mo KR 293 K), 9312
were independent and 9306 were observed (4σ(Fo). All the cage hydrogens
except µH2,3 were located from difference Fourier maps but could not be
refined; thus, no cage H atoms were included in the final refinement. The
other H atoms were treated using appropriate riding models: S ) 1.014,
R1 ) 0.074, wR2 ) 0.187. Complete NMR data are available for 4, and
they will be included in the full report of this work.
JA962192X
(18) Mebel, A. M.; Morokuma, K.; Musaev, D. G. J. Am. Chem. Soc.
1994, 116, 3932.
(19) Greenwood, N. N.; Savory, G. C.; Sneddon, L. G.; Grimes, R. N.;
Davison, A.; Wreford, S. S. J. Chem. Soc., Chem. Commun. 1974, 718.
(20) Fehlner, T. P.; Ragaini, J. D.; Mangion, M.; Shore, S. G. J. Am.
Chem. Soc. 1976, 98, 7085.
(17) Onak, T. P.; Shore, S. G.; Yamauchi, M. Gmelin Handbook of
Inorganic Chemistry; Boron Compounds 20; Springer-Verlag: Berlin, 1979;
New Supplement Series Vol. 54, pp 14-15.