Mechanistic features of boron-iodine bond activation of B-iodocarboranes

Article abstract of DOI:10.1021/om0008575

Oxidative addition of the B-I bond of 9-iodo-m-carborane to [(Ph₃P)_nPd] (n = 3, 4) is reversible, the equilibrium being shifted to the Pd(0) and the iodocarborane. In the presence of [(Ph₃P)₄Pd] and [Bu₄N]Br in THF, 9-iodo-m-carborane undergoes halide exchange to produce 9-bromo-m-carborane. Coordinatively unsaturated Pd(0) and hydrido Pd(II) species generated upon thermal decomposition of [(Ph₃P)₂Pd(Ph)(O₂CH)] and [(Ph₃P)₂Pd₂(Ph)₂ (μ-O₂CH)₂] reduce 9-iodo-m-carborane to m-carborane with 100% selectivity. The thermal decomposition of [(Ph₃P)₂Pd₂(Ph)₂ (μ-O₂CH)₂] in the presence of excess 9-iodo-m-carborane and PhI (1:1) results in the formation of m-carborane (3%) and [(Ph₃P)₂Pd₂(Ph)₂ (μ-I)₂] (97%), whose structure was confirmed by single-crystal X-ray diffraction. X-ray analysis of 9-iodo-m-carborane and m-carboran-9-yl(phenyl)iodonium tetrafluoroborate shows that in the iodonium salt the B-I bond is longer by ca. 0.03 A? than in the iodocarborane. In contrast, the C-I bond distances in carboran-9-yl(phenyl)iodonium tetrafluoroborate (2.111(2) A?) and in iodobenzene (2.098(4) A?) are only marginally different. The elongation of the B-I bond, not the C-I bond, likely contributes to (i) the enhanced reactivity of B-carboranyl(phenyl)-iodonium cations toward nucleophiles and (ii) the remarkably high selectivity of these S_N reactions that occur exclusively at the boron atom. A new crystallographic form of 9,10-diiodo-m-carborane is reported.

Full text of DOI:10.1021/om0008575

Organometallics 2001, 20, 523-533
523
Mech a n istic F ea tu r es of Bor on -Iod in e Bon d Activa tion
of B-Iod oca r bor a n es
William J . Marshall, Robert J . Young J r., and Vladimir V. Grushin*
Central Research and Development, E. I. DuPont de Nemours & Co., Inc.,†
Experimental Station, Wilmington, Delaware 19880-0328
Received October 6, 2000
Oxidative addition of the B-I bond of 9-iodo-m-carborane to [(Ph3P)nPd] (n ) 3, 4) is
reversible, the equilibrium being shifted to the Pd(0) and the iodocarborane. In the presence
of [(Ph3P)4Pd] and [Bu4N]Br in THF, 9-iodo-m-carborane undergoes halide exchange to
produce 9-bromo-m-carborane. Coordinatively unsaturated Pd(0) and hydrido Pd(II) species
generated upon thermal decomposition of [(Ph3P)2Pd(Ph)(O2CH)] and [(Ph3P)2Pd2(Ph)2(µ-
O2CH)2] reduce 9-iodo-m-carborane to m-carborane with 100% selectivity. The thermal
decomposition of [(Ph3P)2Pd2(Ph)2(µ-O2CH)2] in the presence of excess 9-iodo-m-carborane
and PhI (1:1) results in the formation of m-carborane (3%) and [(Ph3P)2Pd2(Ph)2(µ-I)2] (97%),
whose structure was confirmed by single-crystal X-ray diffraction. X-ray analysis of 9-iodo-
m-carborane and m-carboran-9-yl(phenyl)iodonium tetrafluoroborate shows that in the
iodonium salt the B-I bond is longer by ca. 0.03 Å than in the iodocarborane. In contrast,
the C-I bond distances in carboran-9-yl(phenyl)iodonium tetrafluoroborate (2.111(2) Å) and
in iodobenzene (2.098(4) Å) are only marginally different. The elongation of the B-I bond,
not the C-I bond, likely contributes to (i) the enhanced reactivity of B-carboranyl(phenyl)-
iodonium cations toward nucleophiles and (ii) the remarkably high selectivity of these SN
reactions that occur exclusively at the boron atom. A new crystallographic form of 9,10-
diiodo-m-carborane is reported.
In tr od u ction
the catalytic methods widely employed in the chemistry
of haloarenes.5
Functional derivatives of icosahedral closo-carboranes
C2B10H12 have received much attention due to their use
in synthesis and potential applications in medicine.1
While in most instances more easily accessible C-
functionalized carboranes are reported, considerable
progress has also been made in the chemistry of B-
substituted derivatives of o-, m-, and p-carboranes.2
Readily available via direct electrophilic iodination,
B-carboranyl iodides have become key starting materi-
als for the preparation of various carborane derivatives
containing a boron-element bond. To solve the problem
of the notoriously poor reactivity of the B-I bond of
iodocarboranes toward nucleophiles,2,3 two distinct strat-
egies for B-I activation have been developed. One of
the two employs the Pd-catalyzed cross-coupling reac-
tions4 (e.g., the Kumada-type coupling; eq 1) similar to
Alternatively, the B-I bond is activated by converting
a B-carboranyl iodide to the corresponding carboranyl-
(4) (a) Zakharkin, L. I.; Kovredov, A. I.; Ol’shevskaya, V. A.;
Shaugumbekova, Zh. S. Izv. Akad. Nauk SSSR, Ser. Khim. 1980, 1691.
(b) Zakharkin, L. I.; Kovredov, A. I.; Savel’eva, I. S.; Safronova, E. V.
Zh. Obshch. Khim. 1981, 51, 2383. (c) Zakharkin, L. I.; Kovredov, A.
I.; Ol’shevskaya, V. A. Izv. Akad. Nauk SSSR, Ser. Khim. 1981, 2159.
(e) Zakharkin, L. I.; Kovredov, A. I.; Ol’shevskaya, V. A. Zh. Obshch.
Khim. 1981, 51, 2807. (f) Zakharkin, L. I.; Kovredov, A. I.; Ol’shevskaya,
V. A.; Shaugumbekova, Zh. S. J . Organomet. Chem. 1982, 226, 217.
(g) Zakharkin, L. I.; Kovredov, A. I.; Ol’shevskaya, V. A.; Antonovich,
V. A. J . Organomet. Chem. 1984, 267, 81. (h) Chistovalova, N. M.;
Akhrem, I. S.; Reshetova, E. V.; Vol’pin, M. E. Izv. Akad. Nauk SSSR,
Ser. Khim. 1984, 2342. (i) Zakharkin, L. I.; Kovredov, A. I.; Ol’shevskaya,
V. A. Izv. Akad. Nauk SSSR, Ser. Khim. 1985, 888. (j) Kovredov, A. I.;
Shaugumbekova, Zh. S.; Petrovskii, P. V.; Zakharkin, L. I. Zh. Obshch.
Khim. 1989, 59, 607. (k) Gal’chenko, G. L.; Tamm, N. B.; Pavlovich,
V. K.; Ol’shevskaya, V. A.; Zakharkin, L. I. Metalloorg. Khim. 1990,
3, 259. (l) Gal’chenko, G. L.; Tamm, N. B.; Pavlovich, V. K.; Ol’shevskaya,
V. A.; Zakharkin, L. I. Metalloorg. Khim. 1990, 3, 414. (m) Zakharkin,
L. I.; Ol’shevskaya, V. A. Synth. React. Inorg. Met.-Org. Chem. 1991,
21, 1041. (n) Li, J .; Logan, C. F.; J ones, M., J r. Inorg. Chem. 1991, 30,
4866. (o) J iang, W.; Knobler, C. B.; Curtis, C. E.; Mortimer, M. D.;
Hawthorne, M. F. Inorg. Chem. 1995, 34, 3491. (p) Zheng, Z.; J iang,
W.; Zinn, A. A.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem. 1995,
34, 2095. (q) J iang, W.; Harwell, D. E.; Mortimer, M. D.; Knobler, C.
B.; Hawthorne, M. F. Inorg. Chem. 1996, 35, 4355. (r) Zakharkin, L.
I.; Ol’shevskaya, V. A.; Guseva, V. V. Russ. Chem. Bull. 1998, 47, 524.
(s) Zakharkin, L. I.; Balagurova, E. V.; Lebedev, V. N. Russ. J . Gen.
Chem. 1998, 68, 922. (t) Zakharkin, L. I.; Ol’shevskaya, V. A.;
Zhigareva, G. G. Russ. J . Gen. Chem. 1998, 68, 925. (u) Harakas, G.;
Vu, T.; Knobler, C. B.; Hawthorne, M. F. J . Am. Chem. Soc. 1998, 120,
6405. (v) Craciun, L.; Custelcean, R. Inorg. Chem. 1999, 38, 4916. (w)
Ol’shevskaya, V. A.; Zakharkin, L. I. Spec. Publ., R. Soc. Chem. 2000,
253, 229.
† Contribution No. 8128.
(1) For selected recent reviews, see: (a) Hawthorne, M. F.; Maderna,
A. Chem. Rev. 1999, 99, 3421. (b) Lesnikowski, Z. J .; Shi, J .; Schinazi,
R. F. J . Organomet. Chem. 1999, 581, 156. (c) Hawthorne, M. F. Mol.
Med. Today 1998, 4, 174. (d) Soloway, A. H.; Tjarks, W.; Barnum, B.
A.; Rong, F.-G.; Barth, R. F.; Codogni, I. M.; Wilson, J . G. Chem. Rev.
1998, 98, 1515. (e) Hawthorne, M. F.; Zheng, Z. Acc. Chem. Res. 1997,
30, 267. (f) Godovikov, N. N.; Balema, V. P.; Rys, E. G. Usp. Khim.
1997, 66, 1125. (g) Hawthorne, M. F.; Yang, X.; Zheng, Z. Pure Appl.
Chem. 1994, 66, 245. (h) Vinogradova, S. V.; Valetskii, P. M.; Kabachii,
Yu. A. Usp. Khim. 1995, 64, 390. (i) J ones, M., J r. Adv. Carbene Chem.
1994, 1, 161. (j) Hawthorne, M. F. Angew. Chem., Int. Ed. Engl. 1993,
32, 950. (k) Sivaev, I. B.; Solntsev, K. A.; Kuznetsov, N. T. Koord. Khim.
1993, 19, 194. (l) Bregadze, V. I. Chem. Rev. 1992, 92, 209.
(2) Grushin, V. V.; Bregadze, V. I.; Kalinin, V. N. J . Organomet.
Chem. Libr. 1988, 20, 1.
(3) Zakharkin, l. I.; Kalinin, V. N. Izv. Akad. Nauk SSSR, Ser. Khim.
1971, 2310.
10.1021/om0008575 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 01/12/2001

524 Organometallics, Vol. 20, No. 3, 2001
Marshall et al.
Sch em e 1
(phenyl)iodonium salt, which undergoes highly selective
nucleophilic substitution at the boron atom under mild
conditions (eq 2).6
Given the importance of B-iodocaboranes in synthe-
sis, it is surprising that there have been no literature
reports describing mechanistic studies of B-I activation
with Pd(0) complexes (eq 1). Furthermore, structural
aspects of B-I activation via iodonium ion intermedi-
ates (eq 2) have received insufficient attention. In this
paper, we report unexpected results of our studies of
reactivity of 9-iodo-m-carborane toward Pd(0) complexes
and precise X-ray structures of 9-iodo-m-carborane and
m-carboran-9-yl(phenyl)iodonium tetrafluoroborate. A
new crystallographic form of 9,10-diiodo-m-carborane is
also reported.
of equal intensity emerge at 22.3 and -5.5 ppm from
[(Ph3P)2Pd(Ph)I] and Ph3P, respectively (eq 3). As the
reaction rapidly occurs, the originally yellow solution
decolorizes due to the quantitative formation of the two
colorless products.
Resu lts a n d Discu ssion
Derivatives of more stable m-carborane (1,7-dicarba-
closo-dodecaborane) were selected for our studies to
avoid the nucleophile-induced degradation reaction of
the carborane framework.7
In sharp contrast, when 9-iodo-m-carborane (10 molar
equiv) was added to [(Ph3P)4Pd] in toluene-d8 at 25 °C,
the yellow solution did not decolorize, and no visible sign
of any reaction was observed after the solution was kept
at room temperature for 12 h. The 31P NMR spectra of
the sample before and after the addition of the iodocar-
borane were indistinguishable, containing only one
broad (∆ν1/2 ) 90 Hz) resonance at ca. 16 ppm from
[(Ph3P)4Pd].9 Heating the solution for 3 h at 70 °C, a
temperature commonly used to run some Pd-catalyzed
cross-coupling reactions of iodocarboranes, led to a
marginally noticeable darkening. The signal from
[(Ph3P)4Pd] shifted upfield by ca. 1.5 ppm, while retain-
ing its line shape, and a very weak sharp singlet
resonance at 12.4 ppm appeared (ca. 2% of the intensity
of the main signal) due to formation of [(Ph3P)2PdI2].
Addition of PhI (2 molar equiv per Pd) resulted in
instant consumption of the Pd(0), as if there were no
iodocarborane present. The spectral pattern changed
immediately, with the broad resonance disappearing
and two singlets (1:1) appearing at 22.3 and -5.5 ppm
from [(Ph3P)2Pd(Ph)I] and Ph3P, respectively. The weak
resonance at 12.4 ppm from [(Ph3P)2PdI2] remained
unchanged. GC-MS analysis of the mixture revealed
the presence of the unreacted 9-iodo-m-carborane and
trace amounts of m-carborane. As no deuterium incor-
poration from the solvent (toluene-d8) into the m-
carborane product took place (MS), the source of hydro-
gen for the B-I bond reduction was probably the Ph3P
ligands. In a similarly designed experiment, no reaction
was observed between [(Ph3P)4Pd] and 9-iodo-m-carbo-
rane in THF at room temperature. The unreacted
Pd(0) complex was isolated in the form of [(Ph3P)3Pd]
in 73% yield (see the Experimental Section).
Reaction s of 9-Iodo-m-car bor an e with [(P h 3P )4M]
(M ) P d , P t). Oxidative addition of the B-I bond of
iodocarboranes to a zerovalent Pd species is believed to
be the first key step of the catalytic cycle that produces
the desired carborane derivative (Scheme 1).4f,n-p Since
B-iodocarboranes and iodobenzene are closely related
in terms of stability and reactivity,2 one would expect
oxidative addition reactions of iodocarboranes to Pd(0)
to exhibit mechanistic features similar to those involv-
ing iodobenzene. This, however, did not appear to be
the case.
Iodobenzene reacts readily and irreversibly with
[(Ph3P)4Pd] to give [(Ph3P)2Pd(Ph)I] in quantitative yield
within seconds at room temperature (eq 3).8 Yellow
solutions of [(Ph3P)4Pd] in benzene, toluene, or THF
exhibit a broad 31P NMR singlet at ca. 17 ppm due to
ligand exchange processes.9 When PhI is added to these
solutions, the broad resonance at 17 ppm from [(Ph3P)4-
Pd] disappears, while two new sharp singlet resonances
(5) See, for example: Tsuji, J . Palladium Reagents and Catalysis:
Innovations in Organic Synthesis; Wiley: Chichester, 1995.
(6) (a) Grushin, V. V. Acc. Chem. Res. 1992, 25, 529. (b) Grushin,
V. V.; Demkina, I. I.; Tolstaya, T. P. J . Chem. Soc., Perkin Trans. 2
1992, 505. (c) Grushin, V. V.; Demkina, I. I.; Tolstaya, T. P. Inorg.
Chem. 1991, 30, 4860. (d) Grushin, V. V.; Demkina, I. I.; Tolstaya, T.
P.; Galakhov, M. V.; Bakhmutov, V. I. Metalloorg. Khim. 1989, 2, 727.
(e) Demkina, I. I.; Grushin, V. V.; Vanchikov, A. N.; Tolstaya, T. P.;
Orlinkov, A. V. Zh. Obshch. Khim. 1987, 57, 1341. (f) Grushin, V. V.;
Shcherbina, T. M.; Tolstaya, T. P. J . Organomet. Chem. 1985, 292,
105. (g) Grushin, V. V.; Tolstaya, T. P.; Lisichkina, I. N. Izv. Akad.
Nauk SSSR, Ser. Khim. 1983, 2165. (h) Grushin, V. V.; Tolstaya, T.
P.; Lisichkina, I. N. Izv. Akad. Nauk SSSR, Ser. Khim. 1982, 2633. (i)
Grushin, V. V.; Tolstaya, T. P.; Lisichkina, I. N.; Vanchikov, A. N. Dokl.
Akad. Nauk SSSR 1982, 264, 868.
In the hope to obtain an isolable B-carboranyl
(7) Hawthorne, M. F.; Young, D. C.; Garrett, P. M.; Owen, D. A.;
Schwerin, S. G.; Tebbe, F. N.; Wegner, P. A. J . Am. Chem. Soc. 1968,
90, 862.
platinum complex,10 [(Ph3P)4Pt] and a 7-fold excess of
(8) Fitton, P.; Rick, E. A. J . Organomet. Chem. 1971, 28, 287.
(9) Mann, B. E.; Musco, A. J . Chem. Soc., Dalton Trans. 1975, 1673.

B-I Bond Activation of B-Iodocarboranes
Organometallics, Vol. 20, No. 3, 2001 525
9-iodo-m-carborane were reacted in toluene-d8. How-
ever, no sign of any reaction was observed (31P NMR)
at room temperature, nor after this mixture was kept
at 100 °C for 2.5 h.
The lack of NMR-observable reactivity of 9-iodo-m-
carborane toward [(Ph3P)4M] (M ) Pd, Pt) was surpris-
ing because the conditions used for these experiments
were comparable to those commonly employed for the
cross-coupling reactions of B-iodocarboranes, which do
occur in the presence of Pd catalysts but not in their
absence.4
Rea ction s of 9-Iod o-m -ca r bor a n e w ith [(P h 3P )4-
P d ] in th e P r esen ce of [Bu 4N]X (X ) I, Br ). Con-
vincing evidence for B-I activation with Pd(0) was
obtained by reacting 9-iodo-m-carborane with [(Ph3P)4-
Pd] in the presence of [Bu4N]Br.
Halide anions are known11,12 to promote oxidative
addition reactions of Pd(0) complexes due to the forma-
tion of electron-rich anionic phosphine palladium ha-
lides. Keeping that in mind, we reacted 9-iodo-m-
carborane, [(Ph3P)4Pd], and [Bu4N]I in a 10:1:1 molar
ratio in THF. After 3 days at ambient temperature and
then 3 h at 60 °C no 31P NMR spectral evidence for any
reaction was observed, except for the formation of trace
amounts of [(Ph3P)2PdI2] and m-carborane (GC-MS).
Both these products were also formed in comparable
quantities when the reaction was conducted in the
absence of the tetrabutylammonium salt in THF or
toluene (see above).
A similar 31P NMR spectral pattern was observed
when an excess of [Bu4N]Br was used instead of
[Bu4N]I. However, GC-MS analysis of the reaction
solution revealed the formation of 9-bromo-m-carborane.
After 12 h at 30 °C and then 2 h at 55 °C, the ratio of
9-bromo-m-carborane to 9-iodo-m-carborane was mea-
sured at 1:7 and 1:5, respectively. Therefore, [(Ph3P)4-
Pd] does cleave the B-I bond of iodocarboranes and is
capable of promoting the halogen exchange reaction
under mild conditions (eq 4). This result suggests that
B-iodocarboranes may undergo Pd-catalyzed nucleo-
philic displacement reactions with anionic nucleophiles
(e.g., Br-, Cl-, F-, N3-, CN-, SCN-, etc.).
In an attempt to shift equilibrium 5 to the right,
toward a carboranyl Pd(II) complex, and thus increase
the probability of its isolation, we decided to react
9-iodo-m-carborane with low-ligated and more reactive
[(Ph3P)nPd] (n ) 1 or 2). It has been established that
the actual reactive triphenylphosphine Pd(0) species
which oxidatively adds the C-I bond of iodoarenes is
dicoordinate 14-e [(Ph3P)2Pd], which is present in solu-
tions of [(Ph3P)4Pd] or [(Ph3P)3Pd] in very low concen-
trations.12,13
We chose to generate [(Ph3P)2Pd] and [(Ph3P)Pd] via
the thermal decomposition of phenyl palladium formato
complexes, [(Ph3P)2Pd(Ph)(O2CH)] and [(Ph3P)2Pd2-
(Ph)2(µ-O2CH)2], respectively.14,15 Both reactions were
carried out in standard 5 mm NMR tubes, and the
products were analyzed by NMR spectroscopy and then
by GC-MS (see the Experimental Section for details).
When [(Ph3P)2Pd(Ph)(O2CH)] was decomposed in the
presence of a 3-fold excess of 9-iodo-m-carborane in
benzene-d6, B-I activation did rapidly occur under very
mild conditions (1.5 h at 25 °C). However, instead of an
observable/isolable σ-B-carboranyl Pd complex, this
smooth and highly selective reaction resulted in the
formation of m-carborane (GC-MS), along with [(Ph3P)2-
Pd(Ph)(I)] and [(Ph3P)2Pd] in a 1:1:1 ratio (eq 6). The
former of the two complexes was reliably identified in
1
situ by its 31P, 13C, and H NMR spectral characteris-
tics.8,16-18 The highly unstable [(Ph3P)2Pd] exhibited a
broadened singlet resonance at 21.7 ppm (31P NMR),19
which rapidly disappeared upon addition of PhI to the
sample, due to the formation of [(Ph3P)2Pd(Ph)(I)]. At
that point, the 31P NMR spectrum contained only one
sharp signal at 22.3 ppm from [(Ph3P)2Pd(Ph)(I)].20
The thermal decomposition of [(Ph3P)2Pd2(Ph)2(µ-O2-
CH)2] to generate “[(Ph3P)Pd]” is known14 to require a
higher temperature (50-65 °C). When this formato Pd
dimer was decomposed in the presence of 9-iodo-m-
carborane (3.3 equiv per Pd) in benzene-d6 (40 min at
55 °C), again no formation of an NMR observable
Rea ction s of 9-Iod o-m -ca r bor a n e w ith [(P h 3P )2-
P d (P h )(O2CH)] a n d [(P h 3P )2P d 2(P h )2(µ-O2CH)2].
From the above data it is clear that the reaction between
[(Ph3P)4Pd] and 9-iodo-m-carborane is reversible, the
equilibrium (eq 5) being shifted to the left.
(11) Amatore, C.; Azzabi, M.; J utand, A. J . Am. Chem. Soc. 1991,
113, 8375.
(12) For a recent review, see: Amatore, C.; J utand, A. Organomet.
Chem. 1999, 576, 254.
(13) Fauvarque, J . F.; Pflu¨ger, F.; Troupel, M. J . Organomet. Chem.
1981, 208, 419.
(14) Grushin, V. V.; Bensimon, C.; Alper, H. Organometallics 1995,
14, 3259.
(15) (a) An alternative way to generate “[(Ph3P)Pd]” is the reaction
between [(Ph3P)2PdCl2] and alkali.15b As the latter may cause the
degradation reaction of the carborane skeleton,7 we did not use this
method. (b) Grushin, V. V.; Alper, H. Organometallics 1993, 12, 1890.
(16) Garrou, P.; Heck, R. F. J . Am. Chem. Soc. 1976, 98, 4115.
(17) Pilon, M. C.; Grushin, V. V. Organometallics 1998, 17, 1774.
(18) Flemming, J . P.; Pilon M. C.; Borbulevitch O. Ya.; Antipin M.
Yu.; Grushin, V. V. Inorg. Chim. Acta 1998, 280, 87.
(19) Urata, H.; Suzuki, H.; Morooka, Y.; Ikawa, T. J . Organomet.
Chem. 1989, 364, 235.
(10) (a) It has been claimed10b that m-carboran-9-ylmercury chloride
reacts with [(Ph3P)3Pt] in benzene at 20 °C to produce stable [(Ph3P)2-
Pt(C2B10H11)Cl], which has been characterized by elemental analysis
(C, H, Cl, and P) and a melting point. Neither yield nor spectroscopic/
structural data have been reported for the isolated material. (b)
Zakharkin, L. I.; Pisareva, I. V. Izv. Akad. Nauk SSSR, Ser. Khim.
1978, 252.

526 Organometallics, Vol. 20, No. 3, 2001
Marshall et al.
Sch em e 2
No incorporation of deuterium in the m-carborane
molecule was observed (MS) for reactions 6 and 7.
Therefore, the intermediate formation of free carboranyl
radicals via homolytic cleavage of the Pd-B bond can
be ruled out. A mechanism proposed for reactions 6 and
7 likely involves the intermediate formation of B-car-
boranyl palladium hydrides, which are expected22 to
undergo facile and irreversible reductive elimination of
carborane (e.g., Scheme 3). The B-carboranyl hydrido
palladium complexes likely form due to I/H and/or
σ-phenyl/σ-carboranyl exchange, similar to the previ-
ously studied Me/Ph exchange.23 A mechanism involving
oxidative addition of the B-I bond to a palladium(II)
hydrido intermediate to produce a Pd(IV) carboranyl
hydrido species is unlikely. Had this happened, [(Ph3P)2-
Pd2(Ph)2(µ-I)2] would have been produced as a result of
the decomposition of the dinuclear formato complex. It
is to be emphasized that [(Ph3P)2Pd2(Ph)2(µ-I)2] forms
quantitatively when [(Ph3P)2Pd2(Ph)2(µ-O2CH)2] is de-
composed in the presence of iodobenzene.14
The experiments with the formato complexes dem-
onstrate that the B-I bond of 9-iodo-m-carborane is
easily cleaved by Pd(0) species bearing only one or two
Ph3P ligands. However, like the B-I activation with
[(Ph3P)4Pd] (eq 5), these reactions are also reversible,
with the equilibria being shifted toward the Pd(0)
species. It is particularly remarkable that even when
[(Ph3P)2Pd] and 9-iodo-m-carborane were both present
in solution in high concentrations, no formation of a
carboranyl Pd complex was detected by 31P NMR.
Oxid a t ive Ad d it ion t o P d (0): Iod ob en zen e vs
9-Iod o-m -ca r bor a n e. As seen from the above, while
often exhibiting very similar reactivity,2 PhI and the
iodocarborane behave differently when reacted with the
zerovalent palladium species, [(Ph3P)nPd]. Two major
distinctions merit emphasis.
carboranyl palladium complex took place, but [(Ph3P)2-
Pd(Ph)I], m-carborane, and Pd metal formed instead.
The stoichiometry of this markedly selective reaction
is shown in eq 7. Like in the previous experiment, the
σ-phenyl complex [(Ph3P)2Pd(Ph)I] that formed in >98%
1
selectivity21 was unambiguously identified by its H, 13C,
and 31P NMR parameters, whereas m-carborane was
found by GC-MS analysis of the reaction mixture after
the NMR runs.
1. While oxidative addition of iodoarenes to Ph3P
complexes of zerovalent Pd is virtually irreversible (eq
3), 9-iodo-m-carborane appears to react with [(Ph3P)n-
Pd] reversibly. The equilibrium between the product, a
B-carboranyl Pd(II) complex, and the Pd(0) is shifted
to the latter (eq 5).
2. Complexes of the type [(Ph3P)2Pd(Ar)I] undergo
C-P rather than C-I reductive elimination. As a result,
the thermal decomposition reactions of σ-aryl phosphine
complexes of palladium lead to the formation of arylphos-
phonium salts. Both stoichiometric24,25 and catalytic25,26
reactions of this type have been reported. In contrast,
B-carboranyl iodo Pd complexes readily undergo reduc-
tive elimination of the iodocarborane but not [9-m-
C2B10H11PPh3]+. The latter is known to be a stable
To compare reactivities of 9-iodo-m-carborane and
iodobenzene toward Pd(0), the formato dimer [(Ph3P)2-
Pd2(Ph)2(µ-O2CH)2] was decomposed in a benzene-d6
solution containing excess quantities of 9-iodo-m-car-
borane and iodobenzene in a 1:1 molar ratio. A highly
selective reaction was observed, leading almost exclu-
sively to [(Ph3P)2Pd2(Ph)2(µ-I)2] and small amounts
(<3%) of m-carborane (Scheme 2).
(22) Grushin, V. V. Chem. Rev. 1996, 96, 2011.
(20) Small amounts (<10%) of Ph3P (31P NMR: δ ) -5.5 ppm) and
Pd metal were also present due to expected14 disproportionation of the
unstable [(Ph3P)2Pd] to [(Ph3P)3Pd] and Pd during and after the
formation of the dicoordinate complex before the addition of PhI. Being
in fast ligand exchange, [(Ph3P)2Pd] and small amounts of [(Ph3P)3-
Pd] exhibited one broadened singlet at 21.7 ppm (see above). It is
remarkable that in the presence of 9-iodo-m-carborane the dispropor-
tionation of [(Ph3P)2Pd] appeared to be noticeably slower than usual,14
perhaps due to stabilization via Pd‚‚‚I-B10C2H11 coordination. For a
review of complexes of the type R-X-M (X ) halogen), see: Kulawiec,
R. J .; Crabtree, R. H. Coord. Chem. Rev. 1990, 99, 89. See also: Butts,
M. D.; Scott, B. L.; Kubas, G. J . J . Am. Chem. Soc. 1996, 118, 11831,
and references therein.
(23) Ozawa, F.; Fujimori, M.; Yamamoto, T.; Yamamoto, A. Orga-
nometallics 1986, 5, 2144. Ozawa, F.; Hidaka, T.; Yamamoto, T.;
Yamamoto, A. J . Organomet. Chem. 1987, 330, 253. Ozawa, F.;
Kurihara, K.; Fujimori, M.; Hidaka, T.; Toyoshima, T.; Yamamoto, A.
Organometallics 1989, 8, 180.
(24) (a) Sakamoto, M.; Shimizu, I.; Yamamoto, A. Chem. Lett. 1995,
1101. (b) Vicente, J .; Arcas, A.; Bautista, D.; Tiripicchio, A.; Tiripicchio-
Camellini, M. New J . Chem. 1996, 20, 345. (c) Vicente, J .; Arcas, A.;
Bautista, D.; J ones, P. G. Organometallics 1997, 16, 2127. (d) Goodson,
F. E.; Wallow, T. I.; Novak, B. M. J . Am. Chem. Soc. 1997, 119, 12441.
(e) Vicente, J .; Abad, J .-A.; Frankland, A. D.; Ram´ırez de Arellano, M.
C. Chem. Eur. J . 1999, 5, 3066. (f) Grushin, V. V. Organometallics
2000, 19, 1888.
(21) In addition, ca. 1% of [(Ph3P)2PdI2] (31P NMR: δ ) 12.6 ppm)
also formed. There was one more weak (ca. 0.5%) 31P NMR signal
observed (δ ) 21.6 ppm), which was not identified.
(25) Ziegler, C. B.; Heck, R. F. J . Org. Chem. 1978, 43, 2941.
(26) Migita, T.; Nagai, T.; Kiuchi, K.; Kosugi, M. Bull. Chem. Soc.
J pn. 1983, 56, 2869.

B-I Bond Activation of B-Iodocarboranes
Organometallics, Vol. 20, No. 3, 2001 527
Sch em e 3
Sch em e 4
species forming isolable salts.27 Moreover, our attempts
to extend the Pd-catalyzed arylation of Ph3P with ArI26
to the synthesis of [9-m-C2B10H11PPh3]+I- from 9-iodo-
m-carborane and triphenylphosphine failed. Not even
traces of carboranyl phosphonium salts were detected.28
Scheme 4 illustrates the difference in trends for reduc-
tive elimination from σ-phenyl and σ-B-carboranyl Pd
iodo complexes.
The exact structure of B-carboranyl palladium iodo
complexes, the products of the B-I oxidative addition
to [(Ph3P)nPd], remains unknown. It is conceivable that
due to the large effective steric bulk of the σ-carboranyl
ligand, this complex may be cis-[(Ph3P)2Pd(I)(C2B10H11)]29
or a dimer containing one phosphine per metal, e.g.,
[(Ph3P)2Pd2(µ-I)2(C2B10H11)2], which may exist in equi-
librium with mononuclear [(Ph3P)Pd(I)(C2B10H11)].30
The B-I reductive elimination is likely to be governed
by the three-center synchronous mechanism,31 rather
than the two-step mechanism32 that would involve
metal-iodine bond ionization, followed by nucleophilic
attack of the I- on the ipso-boron atom. The empty σ-
(B-Pd)* antibonding orbital of the carboranyl Pd com-
plex (LUMO) is “hidden” inside the polyhedron and
therefore inaccessible for interaction with the HOMO
of a nucleophile.
The results of our studies may account for the poor
yields of the Suzuki-type coupling of B-iodocarboranes
with phenylboronic acid.4s Because oxidative addition
of the B-I bond to Pd(0) is reversible and thermody-
namically disfavored (see above), solutions of [(Ph3P)n-
Pd] and a iodocarborane contain σ-B-carboranylpalla-
dium species in a low concentration. Both RLi and
RMgX are reactive enough to efficiently trap (carbora-
nyl)Pd(I) intermediates present in low concentrations,
converting them to (carboranyl)Pd(R), which then re-
ductively eliminate the product, carboranyl-R. Arylbo-
(27) Grushin, V. V.; Tolstaya, T. P.; Lisichkina, I. N.; Grishin, Yu.
K.; Shcherbina, T. M.; Kampel, V. Ts.; Bregadze, V. I.; Godovikov, N.
N. Izv. Akad. Nauk SSSR, Ser. Khim. 1982, 472.
(28) (a) No carboranylphosphonium salt formation was observed
when 9-iodo-m-carborane and Ph3P in the presence of Pd(OAc)2 were
heated in xylenes at 130 °C for 30 h, i.e., under reaction conditions
certainly suitable for the formation of tetraarylphosphonium salts from
Ph3P and ArI.26 It has been shown4p that in certain cases CuI can
promote the Pd-catalyzed cross-coupling reactions of B-iodocarboranes
and RMgX. However, when the reaction between 9-iodo-m-carborane
and Ph3P was conducted in THF in the presence of both Pd(OAc)2 (7%)
and CuI (35%), no carboranylphosphonium salt formed after 15 h at
65 °C. (b) The formation of a B-P bond has recently been reported in
the reaction of a C-monophosphino-nido-carborane with [(Ph3P)2-
PdCl2]: Vinas, C.; Nunez, R.; Texidor, F.; Sillanpaa, R.; Kivekas, R.
Organometallics 1999, 18, 4712.
(30) Palladium complexes containing a σ-B-carboranyl ligand have
been reported. See, for example: Kalinin, V. N.; Usatov, A. V.;
Zakharkin, L. I. Metalloorg. Khim. 1989, 2, 54. Ryabov, A. D.; Eliseev,
A. V.; Sergeenko, E. S.; Usatov, A. V.; Zakharkin, L. I.; Kalinin, V. N.
Polyhedron 1989, 8, 1485. See also: Corcoran, E. W., J r.; Sneddon, L.
G. Mol. Struct. Energ. 1988, 5, 71.
(31) Hartwig, J . F. Acc. Chem. Res. 1998, 31, 852.
(32) Goldberg, K. I.; Yan, J . Y.; Breitung, E. M. J . Am. Chem. Soc.
1995, 117, 6889.
(29) Oxidative addition of ArI to Pd(0) triphenylphosphine complexes
produces cis-[(Ph3P)2Pd(I)Ar], which isomerize to the trans-isomer
rapidly and quantitatively. See: Casado, A. L.; Espinet, P. Organo-
metallics 1998, 17, 954.

528 Organometallics, Vol. 20, No. 3, 2001
Marshall et al.
Ta ble 1. Selected Cr ysta llogr a p h ic Da ta for [(P h 3P )2P d 2(P h )2(µ-I)2], [9-m -C2B10H11IP h ]+BF 4-, 9-m -C2B10H11I,
a n d 9,10-m -C2B10H10I2
-
[(Ph3P)2Pd2(Ph)2(µ-I)2]
[9-m-C2B10H11IPh]+BF4
9-m-C2B10H11
I
9,10-m-C2B10H10I2
empirical formula
fw
C48H40I2P2Pd2
1155.40
C8H16B11F4I
434.03
C2H11B10
270.11
I
C2H10B10I2
396.00
cryst size, mm
cryst syst
space group
temp, K
0.40 × 0.43 × 0.44
monoclinic
P21/c (No. 14)
173
0.24 × 0.30 × 0.40
monoclinic
P21/ c (No. 14)
173
0.09 × 0.14 × 0.19
monoclinic
P21/c (No. 14)
173
0.14 × 0.18 × 0.23
orthorhombic
Pnma (No. 62)
173
a, Å
b, Å
c, Å
14.388(1)
9.189(1)
17.362(1)
109.11(1)
2169.0
7.072(1)
20.564(1)
11.952(1)
99.71(1)
1713.3
11.430(1)
7.013(1)
13.288(1)
105.34(1)
1027.2
13.843(1)
12.827(1)
7.000(1)
â, deg
volume, Å3
Z
1242.9
4
2.116
4
4
4
calcd density, g·cm-1
µ(Mo), cm-1
diffractometer
1.753
23.31
1.682
18.72
1.746
30.16
49.55
Bruker SMART
1K CCD
3.0-66.3
SADABS
60971
Bruker SMART
1K CCD
4.0-56.6
SADABS
19836
Bruker SMART
1K CCD
3.7-56.6
SADABS
6249
Bruker SMART
1 K CCD
5.9-56.6
SADABS
13450
2θ range, deg
abs corr
no. of reflns collected
no. of unique reflns used
in refinement (I > 3σ(I))
no. of params refined
data-to-param ratio
R1, %
6662
3282
1946
1270
244
27.27
1.8
281
11.66
1.7
162
11.99
1.8
70
18.01
1.5
Rw, %
2.1
1.9
2.0
1.7
error of fit
0.85
0.82
0.83
0.70
ronic acids (Suzuki) and organotin compounds (Stille)
are much less nucleophilic. In many instances it is
transmetalaition that is the rate-determining step of the
Suzuki and Stille coupling reactions.33 As a result,
ligand exchange to produce (carboranyl)Pd(Ar) from
(carboranyl)Pd(I) and ArB(OH)2 is too slow to compete
with side processes. The latter may involve, for example,
ortho-palladation of coordinated PPh3 to give a carbo-
ranyl Pd hydride which produces carborane upon B-H
reductive elimination.
X-r a y Str u ctu r e of [(P h 3P )2P d 2(P h )2(µ-I)2]. The
thermal decomposition of [(Ph3P)2Pd2(Ph)2(µ-O2CH)2] in
the presence of iodobenzene and 9-iodo-m-carborane
furnished [(Ph3P)2Pd2(Ph)2(µ-I)2] as the main product
(see Scheme 2 and text above). As the reaction occurred,
the iodo dimer product precipitated slowly to form X-ray
quality crystals. We took this opportunity to determine
an X-ray structure of [(Ph3P)2Pd2(Ph)2(µ-I)2], whose poor
solubility and limited thermal stability24a obstruct
crystal growth by conventional methods.
Selected crystallographic data for [(Ph3P)2Pd2(Ph)2-
(µ-I)2] are presented in Table 1. The molecule sits on a
crystallographic center of symmetry with the asym-
metric unit containing one-half of the dimer in anti
configuration (Figure 1). Coordination bond distances
and angles for [(Ph3P)2Pd2(Ph)2(µ-I)2] are presented in
Table 2, along with similar parameters previously
determined for [(Ph3P)2Pd(Ph)I]18 and [(Ph3P)2Pd2-
(PhCO)2(µ-I)2].15b As seen from Table 2, [(Ph3P)2Pd2-
(Ph)2(µ-I)2] displays almost ideal square-planar geom-
etry around Pd, as expected. The Pd-I bonds distances
are different, the one trans to the carbon (2.7185(3) Å)
being noticeably longer than the Pd-I bond trans to the
phosphine ligand (2.6645(4) Å). Therefore, the σ-Ph
ligand exerts stronger trans-influence than PPh3. A
similar trend has been observed previously for the X-ray
structure of [(Ph3P)2Pd2(PhCO)2(µ-I)2],15b except that the
even longer Pd-I bond trans to the benzoyl ligand
(2.766(1) Å) points to PhCO being a stronger trans-
influencing ligand than Ph. All coordination bond angles
found in [(Ph3P)2Pd2(Ph)2(µ-I)2] and [(Ph3P)2Pd2(PhCO)2-
(µ-I)2] are similar.
For all three complexes the Pd-C bond distances are
only marginally different, obviously due to the same
trans-influence of bridging iodine trans to the organic
ligand. The Pd-P bond distance in [(Ph3P)2Pd2(Ph)2(µ-
I)2] (2.265(1) Å) is slightly shorter than in [(Ph3P)2Pd2-
(PhCO)2(µ-I)2] (2.285(2) Å) and considerably shorter
than in [(Ph3P)2Pd(Ph)I] (2.338(1) and 2.342(1) Å), as
expected from the well-established order of trans-
influence R3P > halogen.
X-r a y Str u ctu r es of m -Ca r bor a n -9-yl(p h en yl)-
iod on iu m Tetr a flu or obor a te, 9-Iod o-m -ca r bor a n e,
a n d 9,10-Diiod o-m -ca r bor a n e. As mentioned above,
unlike B-iodocarboranes, B-carboranyl(phenyl)iodo-
nium cations exhibit remarkably high reactivity toward
nucleophiles, undergoing nucleophilic displacement re-
actions selectively at the boron atom (eq 2).6 The
mechanism of these reactions and similar reactions of
diaryliodonium salts involves nucleophilic attack on the
iodonium center, followed by reductive elimination of
the product from the resulting 10-I-3 intermediate
(Scheme 5).6a,b,34 The reductive elimination is symmetry-
forbidden from the ground-state T-shape tricoordinate
iodine(III) complex but allowed from its Y-shape con-
former which mediates the cis-trans isomerization (or
pseudorotation if the two lone electron pairs on the
iodine are viewed as substituents, so-called “phantom-
ligands”). Of the two substituents, the bulkier one
(carboranyl) reductively eliminates from the tricoordi-
nate complex probably because (i) the required obtuse
Nu-I-B angle is more easily attained than Nu-I-C
and (ii) elimination of the cumbersome carboranyl
ligand results in a larger decrease in steric strain.35
(33) Farina, V.; Krishnan, B. J . Am. Chem. Soc. 1991, 113, 9585.
Farina, V.; Kapadia, S.; Krishnan, B.; Chenjie, W.; Liebeskind, L. S.
J . Org. Chem. 1994, 59, 5905. Farina, V. Pure Appl. Chem. 1996, 68,
73. Louie, J .; Hartwig, J . F. J . Am. Chem. Soc. 1995, 117, 11598.

B-I Bond Activation of B-Iodocarboranes
Organometallics, Vol. 20, No. 3, 2001 529
F igu r e 1. Molecular structure of anti-[(Ph3P)2Pd2(Ph)2(µ-I)2] with thermal ellipsoids drawn at the 50% probability level.
All hydrogen atoms are omitted for clarity.
Ta ble 2. Selected Geom etr y P a r a m eter s for Com p lexes tr a n s-[(P h 3P )2P d (P h )2(µ-I)2],
tr a n s-[(P h 3P )2P d (P h )(I)],18 a n d [(P h 3P )2P d 2(P h CO)2(µ-I)2]15b
[(Ph3P)2Pd2(Ph)2(µ-I)2]
[(Ph3P)2Pd(Ph)(I)]18
[(Ph3P)2Pd2(PhCO)2(µ-I)2]15b
Interatomic Distances, Å
Pd-I (trans to P)
Pd-I (trans to C)
Pd-C
2.6645(4)
2.7185(3)
1.993(1)
2.265(1)
2.6862(12)
2.7658(11)
1.986(7)
2.7011(8)
2.029(4)
2.338(1); 2.342(1)
Pd-P
2.285(2)
Bond Angles, deg
Pd-I-Pd
I-Pd-I
I-Pd-P
I-Pd-P
I-Pd-C
I-Pd-C
P-Pd-C
92.68(1)
87.32(1)
97.41(1)
173.55(3)
172.47(5)
87.62(4)
88.08(4)
92.25(3)
87.75(3)
97.06(6)
174.62(5)
172.9(2)
87.1(2)
90.87(3); 95.24(3)
171.4(1)
88.0(1); 86.1(1)
88.2(2)
To gain further insight into the exceptional selectivity
of SN reactions of B-carboranyl(phenyl)iodonium salts,
we obtained and compared precise X-ray structures of
m-carboran-9-yl(phenyl)iodonium tetrafluoroborate (Fig-
ure 2) and 9-iodo-m-carborane (Figure 3).36 Crystal-
lographic data obtained from these studies are collected
in Table 1.
In the structure of the iodonium salt (Figure 2), the
cation and anion are separated. As seen from the
shortest intermolecular I-F (BF4-) distances found
(2.854(1) and 3.112(1) Å), no strong bonding contacts
between the anion and cation take place in the solid
state. The B-I-C bond angle of 96.72(7)° slightly
deviates from the expected35b ideal value of 90°, possibly
due to the steric bulk of the carboranyl ligand. The
strong electron-withdrawing effect of the iodonium
atom40 (for 9-m-C2H2B10H9I+, σI ) +1.39; σR ) 0 in
(36) Limited information has been accrued on molecular and crystal
structures of monosubstituted B-iodocarboranes and B-carboranyl
iodonium salts. In addition to the very early crystallographic studies
of 9-iodo-o-carborane37 and 9-iodo-m-carborane,38 molecular and crystal
structures of only few such compounds, namely, 2-iodo-p-carborane4o
and o-carboran-9-yl(phenyl)iodonium iodide,39 have been published.
Hawthorne and co-workers4o,p have also reported X-ray structures of
a series of di- and polyiodinated carboranes.
(34) Grushin, V. V. Chem. Soc. Rev. 2000, 29, 315.
(35) (a) Likewise, diaryliodonium cations [Ar-I-Ar′]+ containing
two aromatic substituents of different steric bulk arylate nucleophiles
selectively with the bulkier of the two aryls.6a,b,35b This phenomenon
is commonly observed when one of the aryls bears ortho substituent-
(s), whereas the other one does not. The selective binding of a
nucleophile to the ortho-substituted aryl is called “ortho-effect”. (b) For
reviews, see: Koser, G. F. In The Chemistry of Functional Groups,
Suppl. D; Patai, S., Rappoport, Z., Eds.; Wiley Interscience: Chichester,
1983; p 1173. Varvoglis, A. The Organic Chemistry of Polycoordinated
Iodine; VCH: New York, 1992.
(37) Andrianov, V. G.; Stanko, V. I.; Struchkov, Yu. T.; Klimova, A.
I. Zh. Strukt. Khim. 1967, 8, 707.
(38) Andrianov, V. G.; Stanko, V. I.; Struchkov, Yu. T. Zh. Strukt.
Khim. 1967, 8, 558.
(39) Ionov, V. M.; Subbotin, M. Yu.; Grushin, V. V.; Tolstaya, T. P.;
Lisichkina, I. N.; Aslanov, L. A. Zh. Strukt. Khim. 1983, 24, 139.
(40) (a) Grushin, V. V.; Demkina, I. I.; Tolstaya, T. P.; Galakhov,
M. V.; Bakhmutov, V. I. Metalloorg. Khim. 1989, 2, 727. (b) Mironova,
A. A.; Maletina, I. I.; Iksanova, S. V.; Orda, V. V.; Yagupol’skii, L. M.
Zh. Org. Khim. 1989, 25, 306.

530 Organometallics, Vol. 20, No. 3, 2001
Marshall et al.
F igu r e 2. Molecular structure of m-carboran-9-yl(phenyl)iodonium tetrafluoroborate with thermal ellipsoids drawn at
the 50% probability level. All hydrogen atoms are omitted for clarity.
Sch em e 5
chloroform)40a is manifested41 by the uncommonly large
C-C-C ipso angle of 123.2(2)°. In substituted benzenes,
the C-C-C ipso angle is an accurate structural mea-
sure of electronic effects of the substituent.41 The
C-C-C ipso angle is larger than 120° for electron-
withdrawing groups and smaller than 120° for electron-
donating substituents. For comparison, in the X-ray
structure of nitrobenzene42 the C-C-C ipso angle
(122.7(1)°) is less obtuse than in the carboranyl(phenyl)-
iodonium salt (Figure 2), indicating that the carbora-
nyliodonio substituent is a stronger electron acceptor
than a nitro group. The strong electron-accepting effect
of the phenyliodonio group (σI ) +1.35; σR ) 0 in
(41) Domenicano, A. In Accurate Molecular Structures: Their De-
termination and Importance; Domenicano, A., Hargittai, I., Eds.;
Oxford University Press: New York, 1992; p 437, and references
therein. Domenicano, A.; Vaciago, A.; Coulsen, C. A. Acta Crystallogr.
B 1975, 31, 1630. Schultz, G.; Hargittai, I.; Seip, R. Z. Naturforsch. A
1981, 36, 669, and references therein.
(42) Boese, R.; Blaser, D.; Nussbaumer, M.; Krygowski, T. M. Struct.
Chem. 1992, 3, 363.

B-I Bond Activation of B-Iodocarboranes
Organometallics, Vol. 20, No. 3, 2001 531
F igu r e 3. Molecular structure of 9-iodo-m-carborane with
thermal ellipsoids drawn at the 50% probability level. All
hydrogen atoms are omitted for clarity.
F igu r e 4. Molecular structure of 9,10-diiodo-m-carborane
with thermal ellipsoids drawn at the 50% probability level.
chloroform)40a is manifested by the sum of B-B-B bond
angles at the substituted boron in the m-carboran-9-yl-
(phenyl)iodonium salt (304.6°), which is noticeably
larger than that measured for the structure of 9-iodo-
m-carborane (301.0°).
diiodo-m-carborane. Despite the different packing, mo-
lecular structures, i.e., bond distances and angles mea-
sured for the two crystallographic forms, appear very
similar (see ref 4p and the Supporting Information).
Of special interest was to compare the B-I and C-I
bond distances in the m-carboran-9-yl(phenyl)iodonium
salt with the corresponding parameters of 9-iodo-m-
carborane (Figure 3) and iodobenzene. While the dif-
ference between the C-I bond distance in the iodonium
cation (2.111(2) Å) and iodobenzene (2.098(4) Å)43 is
marginal, the B-I bond distance in the iodonium salt
(2.214(2) Å) is noticeably elongated, as compared to the
value determined for 9-iodo-m-carborane (2.180(2) Å).
The same trend was previously observed for the struc-
ture of o-carboran-9-yl(phenyl)iodonium iodide, for two
independent molecules of which the B-I and C-I bond
distances were measured at 2.261(9); 2.234(9) Å and
2.111(8); 2.104(8) Å, respectively.39 Importantly, the B-I
bond elongation is observed for both the ionic structure
of m-carboran-9-yl(phenyl)iodonium tetrafuloroborate
and covalent o-carboran-9-yl(phenyl)iodonium iodide, in
which the iodide is coordinated to the iodonium center.
Therefore, the B-I bond appears to be weakened before
and after nucleophilic attack on the central iodine,
whereas the C-I bond remains insignificantly activated,
if at all, in both the ionic and covalent structures. This
factor likely contributes to the selectivity of SN reactions
of the B-carboranyl(phenyl)iodonium salts, which all
occur exclusively with B-I rather than C-I bond
cleavage.
Exp er im en ta l Section
9-Iodo-m-carborane and 9,10-diiodo-m-carborane were
prepared by the iodination of m-carborane (Aldrich) with
I2/HIO3 in AcOH in the presence of H2SO4.44 m-Carbo-
ran-9-yl(phenyl)iodonium tetrafluoroborate was synthe-
sized by the oxidative condensation of 9-iodo-m-carbo-
rane with benzene in a K2S2O8/Ac2O/H2SO4 mixture,
followed by anion metathesis with aqueous HBF4.45 The
phenylpalladium formato complexes were prepared as
described in the literature.14 GC-MS and NMR spec-
troscopy were used for purity control of commercially
unavailable reagents prepared and used in this work.
All other chemicals were purchased from Aldrich,
Strem, and Lancaster chemical companies. Reactions
involving zerovalent complexes of Pd and Pt were
carried out in a glovebox. Deuterated solvents were
dried using standard procedures and stored over freshly
calcined molecular sieves (4 Å) in the glovebox. NMR
spectra were obtained with a Bruker Avance DPX 300
spectrometer. GC-MS analysis was carried out with a
Hewlett-Packard 6890 Series instrument.
1. R ea ct ion s of 9-Iod o-m -ca r b or a n e w it h
[(P h 3P )4M] (M ) P d , P t). (a) To a yellow solution of
[(Ph3P)4Pd] (6.0 mg; 0.005 mmol) in toluene-d8 (0.65 mL)
was added 9-iodo-m-carborane (15.0 mg; 0.055 mmol).
The iodocarborane easily dissolved without any notice-
able change in color. The solution was placed in a
standard 5 mm NMR tube, which was sealed with a
rubber septum. The 31P NMR spectra of this sample (A)
recorded immediately and 12 h after the preparation
were identical, exhibiting only one broad (∆ν1/2 ) ca.
90 Hz) resonance at ca. 16 ppm. Indistinguishable 31P
NMR spectral patterns were observed for a control
We also wish to mention an X-ray structure of 9,10-
diiodo-m-carborane (Table 1, Figure 4), which is differ-
ent from the structure of the same compound recently
reported by Hawthorne and co-workers.4p We grew
crystals of 9,10-diiodo-m-carborane from dichloromethane/
heptane, whereas Hawthorne et al.4p obtained crystals
of this diiodo derivative from pure dichloromethane. The
space group determined in this work (Pnma) and
previously (P21c)4p indicated polymorphism for 9,10-
(43) The gas-phase structure of iodobenzene was determined by
conjoint analysis of electron diffraction intensities and microwave
rotational constants: Brunvoll, J .; Samdal, S.; Thomassen, H.; Vilkov,
L. V.; Volden, H. V. Acta Chem. Scand. 1990, 44, 23.
(44) Stanko, V. I.; Brattsev, V. A.; Vostrikova, T. N.; Danilova, G.
N. Zh. Obshch. Khim. 1968, 38, 1348.
(45) Grushin, V. V.; Tolstaya, T. P.; Lisichkina, I. N. Dokl. Akad.
Nauk SSSR 1981, 261, 99; Dokl. Chem. (Engl. Transl.) 1981, 261, 456.

532 Organometallics, Vol. 20, No. 3, 2001
Marshall et al.
sample (B) prepared similarly, but without 9-iodo-m-
carborane. Heating sample A for 3 h at 70 °C (oil bath)
resulted in a slight darkening and an upfield shift of
the broad 31P NMR signal by ca. 1.5 ppm. Also, a very
weak sharp singlet resonance at 12.4 ppm appeared (ca.
2% of the intensity of the main signal) which was
assigned to [(Ph3P)2PdI2] (authentic sample). Addition
of PhI (1 µL; ca. 2 equiv per Pd) to sample A resulted in
instant decoloration and concomitant change in the
spectral pattern. The broad resonance disappeared and
two singlets (1:1) appeared at 22.3 ppm ([(Ph3P)2Pd-
(Ph)I]) and -5.5 ppm (Ph3P). The weak resonance at
12.4 ppm from [(Ph3P)2PdI2] remained unchanged. GC-
MS analysis indicated the presence of unreacted 9-iodo-
m-carborane and trace amounts of m-carborane. No
deuterium incorporation into the m-carborane product
was observed (MS).
3. Rea ction of 9-Iod o-m -ca r bor a n e w ith [(P h 3P )2-
P d 2(P h )2(µ-OOCH)2]. In a glovebox, a solution of
9-iodo-m-carborane (34 mg; 0.13 mmol) and freshly
prepared [(Ph3P)2Pd2(Ph)2(µ-OOCH)2] (18 mg; 0.02 mmol)
in benzene-d6 (0.8 mL) was placed in a 5 mm NMR tube
and sealed with a rubber septum. The 31P NMR spec-
trum of this sample exhibited one sharp singlet at 30.0
ppm from the formato complex.14 In the 1H NMR
spectrum, aromatic and HCOO (8.3 ppm) signals from
the formato complex14 and broad resonances (1.0-4.0
ppm) from the iodocarborane were observed. Heating
the sample at 55 °C (oil bath) for 40 min resulted in
full conversion of the formato complex and precipitation
of Pd metal. The 31P NMR spectrum of the thermolyzed
sample contained one sharp singlet at 22.5 ppm, which
was assigned to [(Ph3P)2Pd(Ph)I]. The assignment was
confirmed by obtaining 1H and 13C NMR spectra for the
sample, which were identical with those described in
the literature for [(Ph3P)2Pd(Ph)I].18,24f GC-MS analysis
after evaporation, extraction with heptane, and filtra-
tion through silica indicated the formation of m-carbo-
rane. No incorporation of deuterium was detected by
comparison of the mass spectrum of the product with
that from an authentic sample of m-carborane.
(b) To a yellow solution of [(Ph3P)4Pd] (115 mg; 0.10
mmol) in THF (8 mL) was added 9-iodo-m-carborane (52
mg; 0.19 mmol), which quickly dissolved upon gentle
shacking. No visible color changes occurred upon mix-
ing, nor after 2 h at room temperature. After hexane
(10 mL) was added and the mixture was kept overnight
at -20 °C, the precipitated solid (light yellow plates)
was collected, washed with hexanes, and dried under
vacuum to yield 65 mg (73%) of [(Ph3P)3Pd]. The
stoichiometry of the product was established by reacting
it with PhI. Before the reaction, the 31P NMR spectrum
of the yellow solid in benzene-d6 displayed only one
broad resonance at ca. 17 ppm. The solution rapidly
decolorized upon treatment with PhI, and the 31P NMR
spectrum of the resulting solution exhibited two reso-
nances, at 22.3 ppm from [(Ph3P)2Pd(Ph)I] and -5.5
ppm from Ph3P. Integration of the signals (2:1) allowed
for unambiguous determination of the stoichiometry of
the complex, i.e., [(Ph3P)3Pd] rather than [(Ph3P)4Pd].
4. Rea ction of 9-Iod o-m -ca r bor a n e w ith [(P h 3P )2-
P d (P h )(OOCH)]. In a glovebox, a solution of 9-iodo-
m-carborane (35 mg; 0.13 mmol) and [(Ph3P)2Pd(Ph)-
(OOCH)] (0.04 mmol) generated in situ14 from [(Ph3P)2-
Pd2(Ph)2(µ-OOCH)2] (19 mg; 0.02 mmol) and Ph3P (10.5
mg; 0.04 mmol) in benzene-d6 (0.8 mL) was prepared
in a 5 mm NMR tube and sealed with a rubber septum.
In accord with the literature data,14 the decomposition
of the formato complex started immediately and was
complete after 1 h at room temperature. The 31P NMR
spectrum of the decomposed sample exhibited a sharp
singlet at 22.5 ppm from [(Ph3P)2Pd(Ph)I] and a broad
resonance at 21.7 ppm from [(Ph3P)2Pd].19 The latter
resonance disappeared, while the former grew in inten-
sity upon addition of PhI. The formation of [(Ph3P)2Pd-
(c) A solution of [(Ph3P)4Pt] (13 mg; 0.01 mmol) and
9-iodo-m-carborane (20 mg; 0.07 mmol) in toluene-d8
was analyzed by 31P NMR immediately after prepara-
tion and then after 2.5 h at 100 °C (oil bath). No sign of
reaction was observed.
1
(Ph)I] was confirmed by H and 13C NMR spectral data
(see above). GC-MS analysis was carried out as de-
scribed in the previous experiment, to reveal the pres-
ence of nondeuterated m-carborane.
2. Reaction s of 9-Iodo-m-car bor an e with [(P h 3P )4-
P d ] in th e P r esen ce of Bu 4NX (X ) I, Br ). (a)
Experiment 1a was repeated with a sample prepared
with [(Ph3P)4Pd] (6 mg; 0.005 mmol), 9-iodo-m-carbo-
rane (15 mg; 0.055 mmol), and tetrabutylammonium
iodide (20 mg; 0.054 mmol) in THF (0.8 mL). The 31P
NMR spectral patterns observed were identical with
those described in experiment 1a.
5. Decom p osition of [(P h 3P )2P d 2(P h )2(µ-OOCH)2]
in th e P r esen ce of 9-Iod o-m -ca r bor a n e a n d Iod o-
ben zen e. A solution of [(Ph3P)2Pd2(Ph)2(µ-OOCH)2] (14
mg; 0.014 mmol), 9-iodo-m-carborane (27 mg; 0.1 mmol),
and iodobenzene (20.5 mg; 0.1 mmol) in benzene-d6 in
a 5 mm NMR tube was kept at 65 °C (oil bath) for 3 h.
The 31P NMR spectrum indicated full conversion of
the formato complex to [(Ph3P)2Pd2(Ph)2(µ-I)2] (27.7
ppm).15b,24f Due to its poor solubility, the iodo Pd dimer
precipitated almost completely from the reaction mix-
ture in the form of well-shaped dark red-orange crystals.
These were separated, washed with hexane (ca. 1 mL),
and dried under vacuum. The yield was 15.8 mg (97%).
A crystal from this crop was selected for X-ray analysis.
The combined mother liquor and hexane washings were
filtered through a short silica plug and analyzed by GC-
MS. The formation of m-carborane (ca. 3%) was de-
tected.
(b) A mixture of [(Ph3P)4Pd] (10 mg; 0.009 mmol),
9-iodo-m-carborane (35 mg; 0.13 mmol), and tetrabutyl-
ammonium bromide (250 mg; 0.78 mmol) in THF (4 mL)
was stirred at 30 °C overnight. The 31P NMR spectrum
of the solution exhibited a broad singlet at 16 ppm from
[(Ph3P)4Pd] and no other resonances. One-half of the
solution was evaporated and extracted with 2 mL of
heptane. GC-MS analysis of the heptane extract after
filtration through a short silica plug revealed the
presence of only 9-iodo-m-carborane (m/z ) 270) and
9-bromo-m-carborane (m/z ) 223) in a 7:1 ratio. After
the remaining half of the reaction mixture was kept at
55 °C (oil bath) for 2 h and then analyzed as described
above the ratio of 9-iodo-m-carborane to 9-bromo-m-
carborane was determined at 5:1.
6. Attem p ted P r ep a r a tion of [P h 3P (C2B10H11)]I
fr om 9-Iod o-m -ca r bor a n e a n d P h 3P in th e P r es-
en ce of a P d Ca ta lyst. A previously reported26 method

B-I Bond Activation of B-Iodocarboranes
Organometallics, Vol. 20, No. 3, 2001 533
for the Pd-catalyzed arylation of triarylphosphines with
aryl iodides was used. A solution of 9-iodo-m-carborane
(110 mg; 0.41 mmol), Ph3P (135 mg; 0.52 mmol), and
Pd(OAc)2 (5 mg; 0.02 mmol) in xylenes (1.5 mL) was
heated under nitrogen at 130 °C (oil bath) for 2 days.
The dark reaction mixture was evaporated, extracted
with CD2Cl2, and analyzed by 31P NMR. No formation
of the phosphonium cation, [Ph3PC2B10H11]+,27 was
detected. Similarly, when the reaction was repeated in
the presence of CuI (35 mol %) in THF at 65 °C for 3
days, no formation of [Ph3PC2B10H11]I was observed.
7. Sin gle-Cr ysta l X-r a y Diffr a ction Stu d ies. X-ray
quality crystals of [(Ph3P)2Pd2(Ph)2(µ-I)2] were obtained
directly from the reaction mixture, as described in
experiment 5. Crystals of m-carboran-9-yl(phenyl)iodo-
nium tetrafluoroborate, 9-iodo-m-carborane, and 9,10-
diiodo-m-carborane were grown from dichloromethane/
ether, hexane, and dichloromethane/heptane, correspond-
ingly. A Bruker SMART 1K CCD diffractometer (Mo KR
radiation) was used for X-ray analysis at -100 °C of all
four compounds. In all cases, SADABS correction was
applied. The structures were solved by direct methods
and refined by full-matrix least-squares on F. All non-
hydrogen atoms were refined anisotropically. For m-car-
boran-9-yl(phenyl)iodonium tetrafluoroborate and 9-iodo-
m-carborane all hydrogens refined reasonably and were
left in the final refinement. Because for [(Ph3P)2Pd2-
(Ph)2(µ-I)2] and 9,10-diiodo-m-carborane the refinement
of a few of the hydrogen atoms gave unrealistic bond
distances (0.79 and 1.07 Å, respectively), all of the
hydrogen atoms were idealized close to their previously
refined positions. The refinement converged to the R
values listed in Table 1. Full details of the crystal-
lographic studies are presented in the Supporting
Information.
Su p p or tin g In for m a tion Ava ila ble: Full details of the
X-ray crystallographic studies of [(Ph3P)2Pd2(Ph)2(µ-I)2], [9-m-
C2B10H11IPh]+BF4-, 9-m-C2B10H11I, and 9,10-m-C2B10H10I2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0008575