Low-temperature 1,2 →1,7 isomerization of sterically crowded icosahedral closo-((2,3,8-η3):(5,6-η2)- norbornadien-2-yl)rhodacarborane via the formation of a pseudocloso intermediate.

Article abstract of DOI:10.1021/om050125i

The metalation reaction of the K⁺ salt of the carborane anion [7,8-(4'-MeC₆H₄)₂-7,8-nido-C₂B ₉H₁₀]- (1) with [((2,3,5,η⁴)-C ₇H₇-2-CH₂OH)RhCl]₂ (2) in a solution of CHCl₃ or C₆H₆ at ambient temperature produced two diastereomeric closo complexes, [2,2-((2,3,8-η³):(5, 6-η²)C₇H₇CH₂)-1,8-(4'MeC ₆H₄)₂-2,1,8-closo-RhC₂B ₉H₉] (5 and 6), the products of the low-temperature 1,2 → 1,7 carbon atom isomerization of a (nonisolable) [3,3-((2,3,8-η³): (5,6-η²)-C₇H ₇CH₂)-1,2-(4'-MeC₆H₄) ₂-3,1,2-closo-RhC2B9H9]. The overall architecture and stereochemistry of diastereomers 5 and 6 have been determined crystallographically. It has been established that the isomerization reaction proceeds through the intermediate pseudocloso type complex [3,3-((2,3,8-η³):(5,6-η₂) -C₇H₇CH₂)-1,2-(4'-MeC₆H ₄)₂ 3,l,2-pseudoleloso-RhC₂B ₉H₉] (4), which was isolated and characterized by microanalysis, multinuclear NMR spectroscopy, and a single-crystal X-ray diffraction study.

Full text of DOI:10.1021/om050125i

2964
Organometallics 2005, 24, 2964-2970
Low-Temperature “1,2 f 1,7” Isomerization of Sterically
Crowded Icosahedral closo-((2,3,8-η3):(5,6-η2)-
Norbornadien-2-yl)rhodacarborane via the Formation of
a Pseudocloso Intermediate. Molecular Structures of
[3,3-((2,3,8-η3):(5,6-η2)-C7H7CH2)-1,2-(4′-MeC6H4)2-
3,1,2-pseudocloso-RhC2B9H9] and 1,2 f 1,7 Isomerized
Products
Alexander V. Safronov, Fedor M. Dolgushin, Pavel V. Petrovskii, and
Igor T. Chizhevsky*
A. N. Nesmeyanov Institute of Organoelement Compounds of the RAS, 28 Vavilov Street,
119991 Moscow, Russian Federation
Received February 22, 2005
The metalation reaction of the K+ salt of the carborane anion [7,8-(4′-MeC6H4)2-7,8-nido-
C2B9H10]- (1) with [((2,3,5,6-η4)-C7H7-2-CH2OH)RhCl]2 (2) in a solution of CHCl3 or C6H6 at
ambient temperature produced two diastereomeric closo complexes, [2,2-((2,3,8-η3):(5,6-η2)-
C7H7CH2)-1,8-(4′-MeC6H4)2-2,1,8-closo-RhC2B9H9] (5 and 6), the products of the low-
temperature “1,2 f 1,7” carbon atom isomerization of a (nonisolable) [3,3-((2,3,8-η3):
(5,6-η2)-C7H7CH2)-1,2-(4′-MeC6H4)2-3,1,2-closo-RhC2B9H9]. The overall architecture and ster-
eochemistry of diastereomers 5 and 6 have been determined crystallographically. It has been
established that the isomerization reaction proceeds through the intermediate pseudocloso
type complex [3,3-((2,3,8-η3):(5,6-η2)-C7H7CH2)-1,2-(4′-MeC6H4)2-3,1,2-pseudocloso-RhC2B9H9]
(4), which was isolated and characterized by microanalysis, multinuclear NMR spectroscopy,
and a single-crystal X-ray diffraction study.
Introduction
{1,8-R2-2,1,8-closo-MC2B9} or {1,2-R2-4,1,2-closo-MC2B9}
cluster structure, where M represents platinum-,4 nickel-
,5 rhodium-,6 and molybdenum-containing moieties.7 In
the reactions between [Mo(η3-C3H5)(CO)2]+ and [7,8-Ph2-
nido-7,8-C2B9H8R′]n- (R′ ) H, n ) 1; R′ ) 9-Me2S, 10-
Me2S, n ) 0),7 resulting in isomeric complexes of {2,1,8-
closo-MoC2B9R′} architecture, the preisomerized metalla-
carborane intermediates of the general formula
[Ph2(CO)2(η3-C3H5)MoC2B9H8R′]n- have been success-
fully isolated. Of particular interest is a nonicosahedral
Although it was known for nearly 30 years that steric
overcrowding in closo-metallacarboranes caused by
bulky cage substituents can dramatically reduce the
temperature of their polyhedral rearrangement,1,2 the
importance of such low-temperature processes for the
mechanistic study of isomerization of icosahedral het-
eroboranes has been widely recognized only recently.
Interest in this area has been revived by Welch,3 who
has obtained the first experimental results on the
mechanism of the low-temperature isomerization of the
overcrowded (nonisolable) transition-metal metallacar-
boranes [1,2-R2-3,1,2-closo-MC2B9H9] (R is mostly Ph
and, more rarely, other substituents) into complexes of
(3) (a) Welch, A. J. Steric Effects in Metallacarboranes. In Metal
Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.;
Wiley-VCH: Weinheim, Germany, 1999; p 26 (a review). (b) Barbera,
G.; Dunn, S.; Fox, M. A.; Garrioch, R. M.; Hodson, B. E.; Low, K. S.;
Rosair, G. M.; Teixidor, F.; Vin˜as, C.; Welch, A. J.; Weller, A. S. In
Contemporary Boron Chemistry; Davidson, M. G., Hughes, A. K.,
Marder, T. B., Wade, K., Eds.; Royal Society of Chemistry: Cambridge,
U.K., 2000; p 329.
(4) (a) Baghurst, D. R.; Copley, R. C. B.; Fleischer, H.; Mingos, D.
M.; Kyd, G. O.; Yellowlees, L. J.; Welch, A. J. J. Organomet. Chem.
1993, 447, C14. (b) Welch, A. J.; Weller, A. S. J. Chem. Soc., Dalton
Trans. 1997, 1205. (c) Robertson, S.; Ellis, D.; Rosair, G. M.; Welch,
A. J. J. Organomet. Chem. 2003, 680, 286.
(5) (a) Garrioch, R. M.; Kuballa, P.; Low, K. S.; Rosair, G. M.; Welch,
A. J. J. Organomet. Chem. 1999, 575, 57. (b) Robertson, S.; Ellis, D.;
Rosair, G. M.; Welch, A. J. Appl. Organomet. Chem. 2003, 17, 518.
(6) (a) Hodson, B. E.; Ellis, D.; McGrath, T. D.; Monaghan, J. J.;
Rosair, G. M.; Welch, A. J. Angew. Chem., Int. Ed. 2001, 40, 715. (b)
Tatusaus, O.; Vin˜as, C.; Kiveka¨s, R.; Sillanpa¨a¨, R.; Teixidor, F. Chem.
Commun. 2003, 2458.
(1) (a) Warren, L. F.; Hawthorne, M. F. J. Am. Chem. Soc. 1970,
92, 1157. (b) Kaloustian, M. K.; Wiersma, R. J.; Hawthorne, M. F. J.
Am. Chem. Soc. 1971, 93, 4912. (c) Kaloustian, M. K.; Wiersma, R. J.;
Hawthorne, M. F. J. Am. Chem. Soc. 1972, 94, 6679. (d) Hawthorne,
M. F.; Gallahan, K. P.; Wiersma, R. J. Tetrahedron 1974, 30, 1795. (e)
Dastin, D. F.; Evans, W. J.; Jones, C. J.; Wiersma, R. J.; Gong, H.;
Chan, S.; Hawthorne, M. F. J. Am. Chem. Soc. 1974, 96, 3085. (f)
Evans, W. J.; Jones, C. J.; Sˇtibr, B.; Grey, R. A.; Hawthorne, M. F. J.
Am. Chem. Soc. 1974, 96, 7405. (g) Doi, J. A.; Mizusawa, E. A.; Knobler,
C. B.; Hawthorne, M. F. Inorg. Chem. 1984, 23, 1482. (h) Baker, R. T.;
Delaney, M. S.; King, R. E., III; Knobler, C. B.; Long, J. A.; Marder, T.
B.; Paxson, T. E.; Teller, R. G.; Hawthorne, M. F. J. Am. Chem. Soc.
1984, 106, 2965. (i) Long, J. A.; Marder, T. B.; Behnken, P. E.;
Hawthorne, M. F. J. Am. Chem. Soc. 1984, 106, 2979.
(2) (a) Hanusa, T. P.; Todd, L. J. Polyhedron 1985, 4, 2063. (b) Carr,
N.; Mullica, D. F.; Sappenfield, E. L.; Stone, F. G. A. Inorg. Chem.
1994, 33, 1666. (c) Fallis, K. A.; Mullica, D. F.; Sappenfield, E. L.; Stone,
F. G. A. Inorg. Chem. 1994, 33, 4927. (d) Carr, N.; Mullica, D. F.;
Sappenfield, E. L.; Stone, F. G. A. Organometallics 1992, 11, 3697.
(7) (a) Dunn, S.; Rosair, G. M.; Thomas, R. L.; Weller, A. S.; Welch,
A. J. Angew. Chem., Int. Ed. 1997, 36, 645. (b) Dunn, S.; Rosair, G.
M.; Weller, A. S.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1998,
1065. (c) Dunn, S.; Garrioch, R. M.; Rosair, G. M.; Smith, L.; Welch,
A. J. Collect. Czech. Chem. Commun. 1999, 64, 1013.
10.1021/om050125i CCC: $30.25 © 2005 American Chemical Society
Publication on Web 05/05/2005

Sterically Crowded Icosahedral Rhodacarboranes
Organometallics, Vol. 24, No. 12, 2005 2965
Scheme 1
structure of these intermediate complexes, as deter-
mined in two cases by X-ray diffraction studies.7a,b
It should, however, be noted that products of the very
first step of metalation reactions, which might be
expected to retain a regular closo structure {1,2-R2-
3,1,2-closo-MC2B9} (where R is a bulky substituent),
have never been isolated. This is evidently due to the
spontaneous conversion of such overcrowded (presumed
transient) complexes into complexes that adopt a struc-
ture with broken8 (pseudocloso) and/or significantly
strained9 (semipseudocloso) C-C polyhedral connectiv-
ity or with heavily slipped distortions in a 3,1,2-closo-
MC2B9 framework.4c In the course of isomerization
reactions these species might, in principle, be further
transformed either to nonicosahedral preisomerized
complexes such as those discussed above7a,b or, in the
case of “1,2 f 1,7” isomerization processes, directly into
isomeric products of {1,8-R2-2,1,8-closo-MC2B9} cluster
structure via, for instance, the triangular face rotational
mechanism. However, despite the success in the syn-
thesis and structural characterization of a number of
pseudocloso8 and semipseudocloso9 type metallacarbo-
ranes, none of these species, until very recently,10a,b have
been reported to be reactive toward the low-temperature
polyhedral rearrangement.
In accord with our preliminary communication,10a we
wish to report the metalation reaction of the K+ salt of
a new [7,8-(4′-MeC6H4)2-nido-7,8-C2B9H10]- anion (1)
with the known metalation reagent [((2,3,5,6-η4)-C7H7-
2-CH2OH)RhCl]2 (2; C7H7 is 2,5-norbornadien-2-yl).11
The nido-carborane salt 1 was synthesized by base
(KOH) degradation of the neutral closo-carborane pre-
cursor [1,2-(4′-MeC6H4)2-1,2-closo-C2B10H10] (3), which
was prepared by direct reaction between (4-CH3C6H4)2C2
and B10H14 in 71% yield. The metalation reaction of 1
has been found to proceed according to the low-
temperature “1,2 f 1,7” carbon atom isomerization
scheme through the intermediate pseudocloso species
[3,3-((2,3,8-η3):(5,6-η2)-C7H7CH2)-1,2-(4′-MeC6H4)2-3,1,2-
pseudocloso-RhC2B9H9] (4), resulting finally in the two
diastereomeric complexes [2,2-((2,3,8-η3):(5,6-η2)-C7H7-
CH2)-1,8-(4′-MeC6H4)2-2,1,8-closo-RhC2B9H9] (5 and 6).
Results and Discussion
Synthesis and Characterization of the pseudo-
closo- and closo-Rhodacarborane Complexes 4-6.
We have previously described a simple synthetic pro-
cedure for a series of 12-vertex hydrocarbon-containing
complexes [(η-L)-closo-MC2B9H9R,R′] (M ) Rh, Ir; L )
cyclodienyl, cycloenyl ligands), which is based on the
room-temperature reaction of the K+ salts of the iso-
meric nido-carborane monoanions [7,n-R,R′-nido-7,n-
C2B9H10]- (n ) 8, 9; R, R′ ) H, Alk) with the dimeric
complexes [M(η4-diene)Cl]2.12 In a similar reaction of 1
with 2 in a solution of benzene or chloroform for about
1 h, a mixture of metallacarborane species 4-6 in 63
and 56% yields (total content), respectively, was ob-
tained (Scheme 1). As can be expected, the overcrowded
icosahedral closo-rhodacarborane [3,3-((2,3,8-η3):(5,6-η2)-
C7H7CH2)-1,2-(4′-MeC6H4)2-3,1,2-closo-RhC2B9H9] was
not detected among the reaction products.
It is important to note that the relative ratio of
complexes 4-6 is influenced markedly by both the
reaction time and the nature of solvent used in the
reaction. When the reaction of 1 and 2 in benzene is
terminated at an earlier stage, the ratio of 4 to mixture
of 5 and 6 was higher, whereas a longer period of
stirring of the reagents favored the formation of com-
plexes 5 and 6 as predominant products. The same is
true for the reaction of 1 with 2 in chloroform. These
experimental results, coupled with the fact that the
formation of complexes 5 and 6 was irreversible in these
reactions, suggest that 4 is an apparent intermediate
in this sequence. All products 4-6 have been individu-
ally isolated from the reaction mixture using prepara-
tive column chromatography on silica gel and fractional
crystallization using a mixture of CH2Cl2/n-hexane. The
multinuclear NMR spectra and analytical data are
entirely consistent with the formulations of all three
(vide infra).
(8) (a) Lewis, Z. G.; Welch, A. J. J. Organomet. Chem. 1992, 430,
C45. (b) Brain, P. T.; Bu¨hl, M.; Cowie, J.; Lewis, Z. G.; Welch, A. J. J.
Chem. Soc., Dalton Trans. 1996, 231. (c) Welch, A. J.; Weller, A. S.
Inorg. Chem. 1996, 35, 4548. (d) Gra¨dler, U.; Weller, A. S.; Welch, A.
J.; Reed, D. J. Chem. Soc., Dalton Trans. 1996, 335. (e) Kim, J.-H.;
Lambrani, M.; Hwang, J.-W.; Do, Y. J. Chem. Soc., Chem. Commun.
1997, 1761. (f) McWhannel, M. A.; Rosair, G. M.; Welch, A. J.; Teixidor,
F.; Vin˜as, C. J. Organomet. Chem. 1999, 573, 165.
(9) (a) Tomas, R. L.; Rosair, G. M.; Welch, A. J. J. Chem. Soc., Chem.
Commun. 1996, 1327. (b) Tomas, R. L.; Welch, A. J. J. Chem. Soc.,
Dalton Trans. 1997, 631. (c) Teixidor, F.; Vin˜as, C. Inorg. Chem. 1998,
37, 5394. (d) Garrioch, R. M.; Rosair, G. M.; Welch, A. J. J. Organomet.
Chem. 2000, 614-615, 153. (e) Llop, J.; Vin˜as, C.; Teixidor, F.; Victori,
L.; Kiveka¨s, R.; Sillanpa¨a¨, R. Organometallics 2001, 20, 4024.
(10) (a) Safronov, A. V.; Zinevich, T. V.; Dolgushin, F. M.; Petrovskii,
P. V.; Chizhevsky, I. T. Abstracts Mark Vol’pin Memorial International
Symposium, Moscow, Russian Federation, May 18-23, 2003; Abstract
no. P 9, p 83. (b) McIntosh, R.; Ellis, D.; Welch, A. J.; Rosair, G.
Abstracts Third European Meeting on Boron Chemistry (EUROBO-
RON 3), Pruhonice-by Prague, Czech Republic, September 12-16,
2004, Abstract no. O 17.
Complex 4 has been characterized by 1H, 13C{1H}, and
11B/11B{1H} NMR spectroscopy. The 1H NMR spectrum
(12) (a) Zinevich, T. V.; Safronov, A. V.; Vorontsov, E. V.; Petrovskii,
P. V.; Chizhevsky, I. T. Russ. Chem. Bull. (Engl. Transl.) 2001, 1702;
Izv. Akad. Nauk, Ser. Khim. 2001, 50, 1620. (b) Safronov, A. V.;
Zinevich, T. V.; Dolgushin, F. M.; Vorontsov, E. V.; Tok, O. L.;
Chizhevsky, I. T. Russ. Chem. Bull. (Engl. Transl.) 2001, 2254; Izv.
Akad. Nauk, Ser. Khim. 2001, 50, 2152. (c) Safronov, A. V.; Zinevich,
T. V.; Dolgushin, F. M.; Vorontsov, E. V.; Tok, O. L.; Chizhevsky, I. T.
J. Organomet. Chem. 2003, 680, 111. (d) Safronov, A. V.; Zinevich, T.
V.; Dolgushin, F. M.; Tok, O. L.; Vorontsov, E. V.; Chizhevsky, I. T.
Organometallics 2004, 23, 4970.
(11) (a) Zakharkin, L. I.; Chizhevsky, I. T.; Zhigareva, G. G.;
Petrovskii, P. V.; Polyakov, A. V.; Yanovsky, A. I.; Struchkov, Yu. T.
J. Organomet. Chem. 1988, 358, 449. (b) Felekidis, A.; Goblet-Stachow,
M.; Lie´geois, J. F.; Pirotte, B.; Delarge, J.; Demonceau, A.; Fontaine,
M.; Noels, A. F.; Chizhevsky, I. T.; Zinevich, T. V.; Bregadze, V. I.;
Dolgushin, F. M.; Yanovsky, A. I.; Struchkov, Yu. T. J. Organomet.
Chem. 1997, 536-537, 405. (c) Chizhevsky, I. T.; Yanovsky, A. I.;
Struchkov, Yu. T. J. Organomet. Chem. 1997, 536-537, 51 (a review).

2966 Organometallics, Vol. 24, No. 12, 2005
Safronov et al.
Figure 1. ORTEP representations of the molecular structures of the two enantiomeric species A (left) and B (right) for
complex 4, with thermal ellipsoids drawn at the 30% probability level. The hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles
(deg) for Complex 4
of 4 in C6D6 revealed a set of eight signals characteristic
of a η3:η2-norbornadienyl ligand and, in addition, the
resonances of the cage arene substituent protons; all of
these resonances are observed in the expected integral
ratios. Two of the dienyl ligand resonances at δ 4.32 and
3.89 ppm were slightly broadened singlets and, there-
fore, were attributed to the syn and anti protons,
respectively, at the exocyclic C(8) atom of the allylic
moiety; it is known that resonances of such geminal
protons appear in the 1H NMR spectra with an ex-
Rh(3)-C(1)
2.194(5)
2.182(5)
2.252(7)
2.239(6)
2.242(6)
2.197(10)
2.118(9)
2.223(10)
2.247(9)
2.327(4)
2.210(13)
Rh(3)-C(3′′)
Rh(3)-C(5′′)
Rh(3)-C(6′′)
C(1)‚‚‚C(2)
C(1)-B(4)
C(1)-B(6)
C(2)-B(6)
C(2)-B(7)
B(4)-B(8)
B(7)-B(8)
2.259(18)
2.294(14)
2.347(15)
2.539(7)
1.633(7)
1.747(7)
1.748(7)
1.629(7)
1.836(9)
1.807(9)
Rh(3)-C(2)
Rh(3)-B(4)
Rh(3)-B(7)
Rh(3)-B(8)
Rh(3)-C(2′)
Rh(3)-C(3′)
Rh(3)-C(5′)
Rh(3)-C(6′)
Rh(3)-C(8′)
Rh(3)-C(2′′)
2
tremely small J(H,H) coupling constant.13 The reso-
nances originating from the nonequivalent methylene
protons at the C(7) atom were observed as close AB
doublets. It is noteworthy that in the 13C{1H} NMR
spectrum of 4 the resonances arising from the carbocy-
clic ligand have different values of J(103Rh,C) coupling,
which are strictly dependent on the position of the
carbon atoms in the hydrocarbon framework. Specifi-
cally, those carbon resonances which are derived from
the double or allylic bonds of the coordinated hydrocar-
bon ligand are mostly doublets, while others are, in fact,
singlets. Taking into account these observations and
using a 2D [1H-13C]-HETCOR correlation technique, we
have assigned all resonances in the 1H and 13C{1H}
NMR spectra of 4. In the 11B/11B{1H} NMR spectra of 4
the resonances of the cage boron atoms are observed as
partly overlapping doublets of 1:2:1:1:2:1:1 relative
intensities between δ +25.5 and -19.5 ppm. Both the
observed range of these resonances and the weighted-
average 11B chemical shift of δ(11B) ) +5.45 ppm are
in good agreement with those values normally found in
the 11B/11B{1H} NMR spectra of typical pseudocloso
complexes.8 It was therefore of interest to confirm
unequivocally the molecular structure of 4 in the solid
state by a single-crystal diffraction study.
C(2)-Rh(3)-C(1)
B(4)-C(1)-B(6)
B(6)-C(1)-Rh(3)
B(7)-C(2)-B(6)
B(6)-C(2)-Rh(3)
70.92(18)
111.3(4)
97.2(3)
112.5(4)
97.6(3)
C(1)-B(4)-B(8)
C(1)-B(6)-C(2)
C(2)-B(7)-B(8)
B(7)-B(8)-B(4)
112.8(5)
93.2(4)
112.8(4)
115.3(4)
square C(1)Rh(3)C(2)B(6) open face with a contracted
Rh(3)‚‚‚B(6) interatomic distance of 2.971(6) Å. Both
C(1)‚‚‚C(2) and Rh(3)‚‚‚B(6) distances found in 4 can be
compared to those found in other known pseudocloso-
metallacarboranes: for instance, in [3-(η6-C6H6)-1,2-Ph2-
3,1,2-pseudocloso-RuC2B9H9 (2.485 and 2.946 Å)8b and
[3-(η5-C9Me7)-1,2-Ph2-3,1,2-pseudocloso-RhC2B9H9 (2.485
Å)8d (2.491 and 2.960 Å). All these structural peculiari-
ties established for 4 are thus typical for closo-to-
pseudocloso deformation studied in detail by Welch et
al.3
The rhodium atom in 4 is attached to a norbornadie-
nyl fragment via an η2-olefin C(5′)-C(6′) bond and an
η3-allylic bond involving C(2′), C(3′), and C(8′) carbon
atoms. Although 4 is a chiral molecule due to the
asymmetry of the η2:η3-norbornadienyl ligand, the cen-
trosymmetric crystal studied by X-ray diffraction is
naturally racemic. It should, however, be noted that the
carbocyclic ligand in the structure of 4 proved to be
disordered over two mirror-related positions, forming
two enantiomeric species A and B which share a
common exocyclic C(8′) atom. In the X-ray diffraction
experiment these components A and B have been
successfully modeled with a ratio of 40:60, respectively.
Figure 1 shows the perspective views of each of enan-
tiomeric species A and B; selected bond lengths and
angles for 4 are given in Table 1. The low accuracy of
the determination of the norbornadienyl ligand carbon
positions, due to the disorder, does not warrant any
further discussion of geometrical details of this ligand
in the structure of 4.
Diffraction-quality single crystals of 4 were grown
from a CH2Cl2/n-hexane mixture. According to the
results of the X-ray diffraction study (Figure 1), complex
4 adopts a full pseudocloso structure with a cage
C(1)‚‚‚C(2) separation of 2.539(7) Å. In the known
nondeformed closo analogues [3,3-((2,3,8-η3):(5,6-η2)-
C7H7CH2)-1,2-R2-3,1,2-closo-RhC2B9H9] (R ) H,11b Me11a
)
distances between the cage carbon atoms are 1.58 Å (an
average value of three independent molecules) and 1.637
Å, respectively. The lengthening of the cage C(1)‚‚‚C(2)
distance in 4 results in generation of an essentially
(13) Fedorov, L. A. Usp. Khim. 1970, 39, 1389.

Sterically Crowded Icosahedral Rhodacarboranes
Organometallics, Vol. 24, No. 12, 2005 2967
Figure 3. ORTEP representation of the molecular struc-
ture of complex 6 with thermal ellipsoids drawn at the 50%
probability level (top) and projection on the CB4 pentagonal
face of the carborane ligand (bottom). The hydrogen atoms
are omitted for clarity.
Figure 2. ORTEP representation of the molecular struc-
ture of complex 5 with thermal ellipsoids drawn at the 50%
probability level (top) and projection on the CB4 pentagonal
face of the carborane ligand (bottom). The hydrogen atoms
are omitted for clarity.
5 and 6 are indeed diastereomers which, however, differ
not only in the stereochemistry of the η3:η2-norborna-
dienyl ligands but also in their conformational disposi-
tion relative to the carborane cage. Thus, in the isomer
5 the carbocyclic ligand adopts a conformation where
the allylic unit is projected onto the B(7)-B(3) bond and
partly the arene substituent at the C(8) atom of the
lower pentagonal belt (Figure 2). In the isomer 6 the
allylic moiety is projected onto the B(6)-B(11) bond of
the CB4 open face; in this case, the exocyclic carbon atom
C(8′) of the allylic group is positioned on the opposite
side with regard to the lower belt arene substituent
(Figure 3).
On the basis of the X-ray structural data the SS and
RS configurations should be assigned to the chiral C(1)
and C(1′) centers of those enantiomers of 5 and 6, which
are depicted in Figures 2 and 3, respectively. The
relative configurations of racemic crystals of 5 and 6
can thus be designated as SS/RR and RS/SR, respec-
tively.14
The 1H NMR spectra of complexes 5 and 6 show many
similarities with that of 4, especially in the patterns
associated with the η3:η2-norbornadienyl ligand. When
this was taken into account, the assignment of proton
resonances observed in the spectra of 5 and 6 was made
by analogy with the spectrum of 4. In the 11B{1H} NMR
spectra of 5 and 6 the resonances were mostly separated
peaks. These appeared in the range usually observed
for the closo-metallacarborane complexes: i.e., at higher
field than the resonances in the spectrum of 4. Accord-
ingly, the calculation for complexes 5 and 6 revealed
δ(11B) values of -6.60 and -7.13 ppm, respectively,
and these are also typical for complexes with regular
closo structures. Considering all these spectroscopic data
and taking into account the fact that both the carbocy-
clic and the carborane ligands in 5 and 6 do not possess
molecular symmetry, it is reasonable to assume that
these species are diastereomers.
To determine the overall architecture and stereo-
chemistry of 5 and 6, their X-ray diffraction studies have
been undertaken. Well-formed single crystals of com-
plexes 5 and 6 suitable for X-ray diffraction study were
obtained by slow recrystallization from a mixture of
CH2Cl2 and n-hexane. The molecular structures of 5 and
6 in two projections are shown in Figures 2 and 3,
respectively, and selected bond lengths and angles are
given in Table 2. Single-crystal X-ray diffraction studies
of these complexes clearly established that their archi-
tecture is based on {2,1,8-closo-RhC2B9} cluster units,
thus confirming the fact of polyhedral rearrangement
of the initial nido-carborane 1 during the metalation
reaction. The crystallographic data also confirmed that
In contrast to 4, neither 5 nor 6 show any significant
distortions of closo icosahedral geometry. Within the
carborane ligands, which are η5 coordinated by the
rhodium atom in both 5 and 6, the bond lengths and
angles between carbon and boron atoms are as expected,
(14) As the cage C(1) atom in 5 (or C(1′) atom in 6) is formally six-
coordinated, the Cahn-Ingold-Prelog notation seems to be not ap-
plicable for the assignment of absolute configuration of these centers.
Nevertheless, while naming the enantiomers of 5 and 6 (Figures 2 and
3), we used the extension of Cahn-Ingold-Prelog rules, which takes
advantage of the fact that all of the B(4), B(5), or B(6) atoms are on
the same side of the Ru(1), C(13), B(3) triangle, and any of the former
boron atoms, if treated as the fourth position on the tetrahedron, should
be considered as the lowest priority substituent at the chiral C(1)
center.

2968 Organometallics, Vol. 24, No. 12, 2005
Safronov et al.
Table 2. Selected Bond Lengths (Å) and Angles
(deg) for Complexes 5 and 6
5
6
Rh(2)-C(1)
2.285(2)
2.166(3)
2.161(3)
2.184(3)
2.203(3)
2.194(2)
2.130(3)
2.279(3)
2.289(3)
2.319(3)
1.526(4)
1.531(4)
1.546(4)
1.453(4)
1.372(4)
1.530(4)
1.533(4)
1.540(4)
1.386(4)
1.709(4)
1.750(4)
1.852(4)
1.848(4)
1.797(4)
98.6(2)
101.1(2)
101.5(2)
123.3(2)
124.4(3)
106.0(2)
104.8(2)
98.5(2)
102.3(2)
100.8(2)
106.9(2)
106.5(2)
94.5(2)
2.284(5)
2.124(5)
2.200(6)
2.187(5)
2.202(6)
2.231(7)
2.134(5)
2.293(5)
2.310(7)
2.292(5)
1.543(9)
1.545(8)
1.571(9)
1.449(8)
1.318(9)
1.501(9)
1.567(9)
1.557(8)
1.366(8)
1.726(7)
1.737(7)
1.837(8)
1.859(8)
1.793(8)
100.3(5)
100.9(5)
100.3(5)
123.5(6)
122.7(6)
104.8(5)
105.9(5)
100.1(5)
103.6(5)
101.5(5)
104.3(5)
109.0(5)
92.4(5)
Rh(2)-B(3)
Rh(2)-B(6)
Rh(2)-B(7)
Rh(2)-B(11)
Rh(2)-C(2′)
Rh(2)-C(3′)
Rh(2)-C(5′)
Rh(2)-C(6′)
Rh(2)-C(8′)
C(1′)-C(2′)
C(1′)-C(6′)
C(1′)-C(7′)
C(2′)-C(3′)
C(2′)-C(8′)
C(3′)-C(4′)
C(4′)-C(5′)
Figure 4. Monitoring of the 400.13 MHz (C6D6) 1H NMR
spectrum of 4 in the 8.0-0.7 ppm region.
C(4′)-C(7′)
C(5′)-C(6′)
C(1)-B(3)
C(1)-B(6)
species 5 and 6 in a ratio of 6:1. This is in good
agreement with the comparative chemical experiments.
Thus, stirring of 1 and 2 in solution of benzene for 1
week at room temperature followed by column chroma-
tography on silica gel resulted in the formation of a
mixture of compounds 5 and 6 in 30% yield and in a
ratio of 5:1, close to that observed in the 1H NMR
monitoring experiment.
B(3)-B(7)
B(6)-B(11)
B(7)-B(11)
C(2′)-C(1′)-C(6′)
C(2′)-C(1′)-C(7′)
C(6′)-C(1′)-C(7′)
C(8′)-C(2′)-C(3′)
C(8′)-C(2′)-C(1′)
C(3′)-C(2′)-C(1′)
C(2′)-C(3′)-C(4′)
C(3′)-C(4′)-C(5′)
C(3′)-C(4′)-C(7′)
C(5′)-C(4′)-C(7′)
C(6′)-C(5′)-C(4′)
C(5′)-C(6′)-C(1′)
C(4′)-C(7′)-C(1′)
B(3)-C(1)-B(6)
C(1)-B(3)-B(7)
C(1)-B(6)-B(11)
B(11)-B(7)-B(3)
B(7)-B(11)-B(6)
Concluding Remarks
We have synthesized the new nido-carborane anion
[7,8-(4′-MeC6H4)2-nido-7,8-C2B9H10]- (1) and examined
its room-temperature metalation reaction with the
dimeric rhodium complex [((2,3,5,6-η4)-C7H7CH2OH)-
RhCl]2 (2; C7H7 is 2,5-norbornadien-2-yl). The reaction
has been found to proceed according to the low-temper-
ature “1,2 f 1,7” carbon atom isomerization scheme,
producing the two η3:η2-allylolefinic type diastereomeric
complexes 5 and 6 of {2,1,8-closo-RhC2B9} cluster
structure as final products. During metalation of nido-
carborane 1 the pseudocloso complex [3,3-((2,3,8-η3):
(5,6-η2)-C7H7CH2)-1,2-(4′-MeC6H4)2-3,1,2-pseudocloso-
RhC2B9H9] (4) is formed, which was individually iso-
lated and structurally characterized. Since this species
was shown to isomerize at room temperature in solution
to give complexes 5 and 6, it should be regarded as an
apparent isomerization intermediate.
108.1(2)
109.9(2)
109.9(2)
106.5(2)
105.0(2)
108.2(4)
110.7(4)
108.8(4)
105.2(4)
106.6(4)
with C-B ) 1.69-1.75 Å and B-B ) 1.73-1.86 Å. The
coordination bonding mode of the rhodium atom with
respect to the η3:η2-norbornadienyl ligands in 5 and 6
is quite similar to that found in a number of structurally
related (η3:η2-norbornadien-2-yl)-closo-rhodacarboranes11
as well as complex 4 reported in this work.
Although the pseudocloso complex 4 is quite stable
in the solid state, in solution it undergoes irreversible
transformation to form the diastereomeric complexes 5
and 6 (Scheme 2), which has been proven by in situ
Experimental Section
Scheme 2
General Considerations. All reactions and manipulations
except for column chromatography were carried out under an
atmosphere of dry argon using standard Schlenk techniques.
All solvents, including those used for column chromatography,
as eluents were dried under appropriate drying agents and
distilled under argon prior to use. Chromatographic columns
(ca. 25 cm in length and 2 cm in diameter) packed with silica
gel (Merck, 230-400 mesh) were used for purification of the
complexes. The starting bis(p-tolyl)acetylene used for the
preparation of carborane 3 was synthesized by a four-step
procedure fully analogous to that described for Ph2C2.15 The
dimeric rhodium complex 2 was prepared according to the
NMR studies. In particular, the 1H NMR monitoring of
the isomerization reaction of 4 in solution of C6D6
(Figure 4) revealed the formation of the diastereomeric
(15) Tietze, L.-F.; Eicher, T. Reactions and Syntheses in the Organic
Chemistry Laboratory; University Science Books: Mill Valley, CA,
1989.

Sterically Crowded Icosahedral Rhodacarboranes
Organometallics, Vol. 24, No. 12, 2005 2969
Table 3. Crystal Data, Data Collection, and Structure Refinement Parameters for Complexes 4-6
4
5
6
formula
mol wt
C
24H32B9Rh
C24H32B9Rh
520.70
C24H32B9Rh
520.70
520.70
cryst color, habit
temp, K
orange plate
293(2)
orthorhombic
Pbca
16.191(3)
15.763(3)
19.781(4)
yellow plate
115(1)
monoclinic
P21/c
yellow prism
110(1)
monoclinic
P21/n
cryst syst
space group
a, Å
11.495(2)
15.988(3)
13.396(3)
100.015(4)
2424.4(8)
4
12.764(1)
12.327(1)
15.863(2)
100.063(2)
2457.6(4)
4
b, Å
c, Å
â, deg
V, Å3
5049(2)
Z
8
d(calcd), g cm-3
diffractometer
scan mode
θmax, deg
1.370
1.427
1.407
Enraf-Nonius CAD4
SMART 1000 CCD
ω and æ
θ-5/3θ
25.0
6.89
30.0
7.17
28.1
7.07
µ(MoKR), cm-1
(λ ) 0.710 73 Å)
no. of unique rflns (Rint
)
4447 (0.0000)
2164
0.0419
0.1090
7048 (0.0333)
5886
0.0459
0.1107
5967 (0.0891)
3301
0.0587
0.1455
no. of obsd rflns (I > 2σ(I))
R1 (on F for obsd rflns)a
wR2 (on F2 for all rflns)b
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )2]/∑w(Fo2)2}1/2
.
published method.11a The 1H, 11B/11B{1H}, and 13C/13C{1H} as
well as 2D correlation NMR spectra were recorded on Bruker
AMX-400 (1H at 400.13 MHz, 13C at 100.61 MHz, and 11B at
128.33 MHz) and Avance-300 instruments (1H at 300.13 MHz).
IR spectra were obtained on a Carl-Zeiss M-82 spectrometer.
Elemental analyses were performed by the Analytical Labora-
tory of the Institute of Organoelement Compounds of the RAS.
Preparation of [3,3-((2,3,8-η3):(5,6-η2)-C7H7CH2)-1,2-(4′-
MeC6H4)2-3,1,2-pseudocloso-RhC2B9H9] (4) and Diaster-
eomers [3,3-((2,3,8-η3):(5,6-η2)-C7H7CH2)-1,8-(4′-MeC6H4)2-
2,1,8-closo-RhC2B9H9] (5 and 6). To a mixture of 1 (0.136 g,
0.384 mmol) and 2 (0.1 g, 0.192 mmol) was added via syringe
10 mL of degassed chloroform. The resulting mixture was
stirred at room temperature for 0.5 h, while the suspension
became orange. The mixture of products formed was then
treated by column chromatography on silica gel. The first
yellow band was eluted with a 1:2 CHCl3/n-hexane mixture
to afford, after evaporation, 0.031 g (16%) of a yellow oil,
consisting of a crude mixture of 5 and 6 in the ratio of 1:2, as
Preparation of K[7,8-(4′-CH3C6H4)2-nido-7,8-C2B9H10
]
(1). To a solution of KOH (0.52 g, 9.202 mmol) in 50 mL of
absolute ethanol was added solid closo-carborane 3 (1 g, 3.067
mmol), and the resulting mixture was refluxed with stirring
for 72 h. After the reaction mixture was cooled to room
temperature, CO2 was bubbled through it until precipitation
of K2CO3 was completed. The solution was filtered and
evaporated to dryness, affording oily material. To this oil was
added 30 mL of dry benzene, and the mixture was refluxed
with a Dean-Stark trap until the liberation of water was
completed. The resulting mixture was evaporated to dryness,
affording 1 (1.06 g, 98%) as a white powder. IR (hexachlorob-
utadiene, cm-1): 2528 (νB-H). 1H NMR (acetone-d6, 400.13
MHz, J(H,H), Hz): δ 7.01 (d, 4H, J ) 8.0, C6H4), 6.65 (d, 4H,
J ) 8.0, C6H4), 2.05 (s, 6H, CH3), -1.74 (br m, 1H, B-H-B).
11B NMR (acetone-d6, 128.33 MHz, J(B,H), Hz): δ -7.45 (d,
2B, J ) 135), -13.75 (d, 1B, J ) 155), -15.9 (d, 2B, J ) 135),
1
estimated by H NMR spectroscopy. The mixture of 5 and 6
was repeatedly recrystallized from CH2Cl2/n-hexane solution
at 0 °C, finally affording 0.012 g (6.1%) of pure compound 6.
Anal. Calcd for C24H32B9Rh: C, 55.37; H, 6.15; B, 18.68.
Found: C, 55.44; H, 6.35; B, 18.59. IR (KBr, cm-1): 2570 (νB-H).
1H NMR (CDCl3, 400.13 MHz, J(H,H), Hz): δ 7.50 (d, 2H, J )
8.0, C6H4), 7.08 (d, 4H, J ) 7.7, C6H4), 6.90 (d, 2H, J ) 8.0,
C6H4), 5.59 (m, 1H, H5), 4.84 (br s, 1H, H8-syn), 4.22 (br s, 1H,
H8-anti), 3.77 (m, 1H, H3), 3.57 (m, 1H, H4), 3.33 (m, 1H, H1),
3.07 (m, 1H, H6), 2.33 (s, 3H, CH3), 2.26 (s, 3H, CH3), 1.78 (d,
1H, JAB ) 9.5, H7R (H7â)), 1.74 (d, 1H, JAB ) 9.5, H7â (H7R)).
13C{1H} NMR (CDCl3, 100.61 MHz, J(Rh,C), Hz): δ 141.26,
137.93, 136.88, 135.18, 128.50, 128.01, 125.01 (s, C6H4), 100.87
(d, J ) 1.9, C2), 84.67 (d, J ) 1.9, C8), 76.85 (d, J ) 2.9, C5),
73.55 (d, J ) 2.8, C6), 55.42 (s, C7), 45.90 (s, C4), 43.68 (s, C1),
36.65 (d, J ) 9.5, C3), 20.84, 20.74 (s, CH3). 11B NMR (CDCl3,
128.33 MHz, J(B,H), Hz): δ 0.95 (d, 2B, J ) 165), -1.0 (d,
1B, J ) 155), -4.6 (d, 2B, J ) 140), -6.5 (d, 1B, J ) 180),
-14.6 (d, 2B, J ) 160), -17.2 (d, 1B, J ) 145). The orange
band which was eluted next using the same mixture of solvents
gave, after evaporation, 0.08 g (40%) of complex 4, isolated as
an orange microcrystalline solid. Anal. Calcd for C24H32B9Rh:
C, 55.37; H, 6.15; B, 18.68. Found: C, 55.18; H, 6.19; B, 18.79.
IR (KBr, cm-1): 2536 (νB-H). 1H NMR (C6D6, 300.13 MHz;
J(H,H), Hz): δ 7.93 (d, 2H, J ) 7.8, C6H4), 7.66 (d, 2H, J )
8.0, C6H4), 7.16 (d, 2H, J ) 8.0, C6H4), 7.04 (d, 2H, J ) 7.8,
C6H4), 5.22 (m, 1H, H5(6)), 4.32 (br s, 1H, H8-syn), 3.96 (m, 1H,
-18.3 (d, 2B, J ) 150), -32.6 (dd, 1B, JB-H
) 46), -34.9 (d, 1B, J ) 140).
) 170, JB-H-B
term
Preparation of [1,2-(4′-CH3C6H4)2-1,2-closo-C2B10H10
]
(3). To a solution of freshly sublimed decaborane (1 g, 8.060
mmol) in 100 mL of degassed toluene was added 1.03 mL
(8.060 mmol) of N,N-dimethylaniline via syringe. After 5 min
of stirring, solid bis(p-tolyl)acetylene (1.66 g, 8.060 mmol) was
added to the solution, and the mixture was stirred at room
temperature additionally for 1 h. The solution was then kept
at 112-114 °C for 7.5 h and after cooling was decanted from
the solid residue and evaporated to dryness. To the yellow oil
thus obtained was added 100 mL of ice-cold ethanol, and white
crystals were collected by filtration, washed with ice-cold
ethanol (3 × 5 mL), and dried under vacuum to give 1.86 g
(71%) of analytically pure compound 3 as colorless crystals.
Anal. Calcd for C16H24B10: C, 59.26; H, 7.41; B, 33.33. Found:
C, 59.43; H, 7.74; B, 33.42. IR (hexachlorobutadiene, cm-1):
H
H
6(5)), 3.89 (br s, 1H, H8-anti), 3.39 (m, 1H, H3), 3.02 (m, 1H,
1(4)), 2.36 (br s, 4H, CH3, H4(1)), 2.21 (s, 3H, CH3), 1.16 (d,
1
2573 (νB-H). H NMR (acetone-d6, 400.13 MHz, J(H,H), Hz):
1H, JAB ) 9.7, H7R (H7â)), 0.97 (d, 1H, JAB ) 9.7, H7â (H7R)).
13C{1H} NMR (C6D6, 100.61 MHz, J(Rh,C), Hz): δ 149.62,
144.87, 136.71, 134.95 (C6H4), 105.26 (s br, C2), 104.47 (s.br,
C8), 68.05 (d, J ) 4.2, C5), 66.03 (d, J ) 4.8, C6), 55.19 (s, C7),
45.01 (s, C4), 43.19 (s, C1), 36.53 (d, J ) 7.6, C3), 20.76 (s, CH3).
δ 7.46 (d, 4H, J ) 8.4, C6H4), 7.05 (d, 4H, J ) 8.4, C6H4), 2.21
(s, 6H, CH3). 11B NMR (acetone-d6, 128.33 MHz, J(B,H), Hz):
δ -2.25 (d, 2B, J ) 150), -8.35 (d, 4B, J ) 230), -10.1 (d, 3B,
J ) 220), -10.7 (d, 1B, J ) 175).

2970 Organometallics, Vol. 24, No. 12, 2005
Safronov et al.
11B NMR (CDCl3, 128.33 MHz, J(B,H), Hz): δ 25.5 (d, 1B, J
) 135), 14.8 (d, 2B, J ) 140), 10.7 (d, 1B, J ) 155), 5.2 (d, 1B,
J ) 140), 3.2 (d, 1B, J ) 140), 2.5 (d, 1B, J ) 115), -1.75 (d,
1B, J ) 140), -19.5 (d, 1B, J ) 155).
collection, and structure refinement parameters are given in
Table 3. The structures were solved by direct methods and
refined by the full-matrix least-squares technique against F2
with the anisotropic temperature factors for all non-hydrogen
atoms except for carbon atoms of the disordered organic ligand
in 4, which were refined isotropically. Hydrogen atoms at the
carborane ligand in 4 and all hydrogen atoms in 5 and 6 were
located from the Fourier synthesis; hydrogen atoms of the
disordered organic ligand in 4 were placed geometrically. All
hydrogen atoms in 4-6 were included in the structure factor
calculations in the riding motion approximation. SHELXTL-
9716 was used throughout the calculations.
A similar reaction of 1 (0.136 g, 0.384 mmol) with 2 (0.1 g,
0.192 mmol) was carried out in 10 mL of degassed benzene
for 1 h. This, after exactly the same workup procedure as
described above, resulted in complexes 4 (0.080 g, 40%) and a
mixture of 5 and 6 (0.046 g, 23%) in a ratio of 2:1. Repeated
redissolution of a crude mixture of isomers 5 and 6 in CH2Cl2
with subsequent addition of a few drops of n-hexane afforded
0.009 g (4.5%) of pure isomer 5. Anal. Calcd for C24H32B9Rh:
C, 55.44; H, 6.35; B, 18.59. Found: C, 55.37; H, 6.15; B, 18.68.
IR (KBr, cm-1): 2570 (νB-H). 1H NMR (CDCl3, 400.13 MHz,
J(H,H), Hz): δ 7.43 (d, 2H, J ) 8.1, C6H4), 7.08 (d, 2H, J )
7.9, C6H4), 7.01 (d, 2H, J ) 8.1, C6H4), 6.88 (d, 2H, J ) 7.9,
C6H4), 5.31 (m, 1H, H5), 4.92 (br s, 1H, H8-syn), 4.46 (br s, 1H,
Acknowledgment. This research was supported by
the Russian Foundation for Basic Research (Grant No.
03-03-32651). We also gratefully acknowledge the fi-
nancial support of the Program of Basic Research of the
Chemistry and Materials Science Division of the RAS
(Project No. 05-07) and Leading Schools of the President
of the Russian Federation (LS-1060.2003.03).
H8-anti), 3.73 (m, 1H, H3), 3.53 (m, 1H, H4), 3.30 (m, 1H, H1),
3.05 (m, 1H, H6), 2.28 (s, 3H, CH3), 2.24 (s, 3H, CH3), 1.76
(dd, 2H, JAB ) 10.0, H7R (H7â)), 1.72 (dd, 2H, JAB ) 10.0, H7â
(H7R)). 13C{1H} NMR (CDCl3, 100.61 MHz, J(Rh,C), Hz): δ
141.77, 137.94, 136.97, 135.12, 130.48, 128.92, 128.50, 125.11
(s, C6H4), 101.21 (d, J ) 2.9, C2), 85.35 (s.br, C8), 77.23 (d, J )
4.2, C5), 72.49 (d, J ) 2.9, C6), 59.78 (s, C7), 45.71 (s, C4), 43.59
(s, C1), 36.49 (d, J ) 9.5, C3), 20.84, 20.74 (s, CH3). 11B{1H}
NMR (CDCl3, 128.33 MHz; J(B,H), Hz): δ 2.0 (1B), 0.9 (1B),
-2.0 (1B), -2.9 (1B), -6.35 (1B), -8.35 (1B), -14.25 (1B),
-15.1 (1B), -17.5 (1B).
Supporting Information Available: CIF files giving
X-ray crystallographic data for 4-6. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM050125I
X-ray Data Collection and Structure Refinement Pa-
rameters for Complexes 4-6. Details of crystal data, data
(16) Sheldrick, G. M. SHELXTL-97, V5.10, Program for Crystal
Structure Refinement; Bruker AXS Inc., Madison, WI 53719, 1997.