Microwave-assisted synthesis of tricarbonyl rhenacarboranes: Steric and electronic effects on the 1,2 → 1,7 carborane cage isomerization

Article abstract of DOI:10.1021/ic0617543

A series of tricarbonyl rhenacarboranes {[M][Re(CO)₃(RR′ C₂B₉H₉)]} (M = Na, K) were synthesized in water using microwave radiation with reaction times of less than 15 min. The novel complexes were isolated in good yields (57-94%) as either 3,1,2-(R = H: R′ = CH₂Pyr 6; R′ = CH₂Cy, 20) or 2,1,8-(R = H: R′ = H, 4; R′ = CH₂PyrMe 12; R′ = CH ₂PyrH, 13; R′ = Pyr, 15; R′ = Ph, 17; R = R′ = Bn, 19) metallacarboranes and characterized by multinuclear (¹H, ¹¹B, ¹³C) and NOE NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray crystallography in the case of compounds 12 and 13. Carborane cage isomerization from the original 1,2 configuration to the 1,7 orientation occurred in cases where significant steric crowding was present at the metal center. Incorporation of a methylene spacer between the carborane cage and the six-membered ring as in 7 and 20 decreased steric strain such that the 3,1,2 configuration was maintained. Conversion of the 3,1,2 complex 6 to the 2,1,8 isomers 12 and 13 takes place at room temperature upon methylation or protonation of the pyridyl ring, indicating that electronic effects also play a significant role in the isomerization process.

Full text of DOI:10.1021/ic0617543

Inorg. Chem. 2007, 46, 2148−2158
Microwave-Assisted Synthesis of Tricarbonyl Rhenacarboranes: Steric
and Electronic Effects on the 1,2
f 1,7 Carborane Cage Isomerization
Andrea F. Armstrong† and John F. Valliant*,‡
Department of Chemistry, McMaster UniVersity, Hamilton, ON, Canada, and
Departments of Chemistry and Medical Physics & Applied Radiation Sciences,
McMaster UniVersity, Hamilton, ON, Canada, L8S 4M1
Received September 15, 2006
A series of tricarbonyl rhenacarboranes
microwave radiation with reaction times of less than 15 min. The novel complexes were isolated in good yields
(57 94%) as either 3,1,2-(R H: R′ ) CH2Pyr 6; R′ ) CH2Cy, 20) or 2,1,8-(R H: R′ ) H, 4; R′ ) CH2PyrMe
12; R′ ) CH2PyrH, 13; R′ ) Pyr, 15; R′ ) Ph, 17; R ′ ) Bn, 19) metallacarboranes and characterized by
{[M][Re(CO)3(RR′C2B9H9)]} (M ) Na, K) were synthesized in water using
−
)
)
)
R
multinuclear (1H, 11B, 13C) and NOE NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray
crystallography in the case of compounds 12 and 13. Carborane cage isomerization from the original 1,2 configuration
to the 1,7 orientation occurred in cases where significant steric crowding was present at the metal center. Incorporation
of a methylene spacer between the carborane cage and the six-membered ring as in 7 and 20 decreased steric
strain such that the 3,1,2 configuration was maintained. Conversion of the 3,1,2 complex 6 to the 2,1,8 isomers 12
and 13 takes place at room temperature upon methylation or protonation of the pyridyl ring, indicating that electronic
effects also play a significant role in the isomerization process.
Introduction
ways are the diamond-square-diamond6 and triangular face
rotation7 “mechanisms”, but these are best considered
schematic representations of the process rather than true
reaction mechanisms. Furthermore, the 1,2 f 1,7 isomer-
ization process appears to vary from system to system. For
example, in situ NMR studies have shown that the platina-
carborane [1-Ph-3,3-(PMe2Ph)2-3,1,2-closo-PtC2B9H10] is
converted directly from the 3,1,2 species to its 2,1,8 isomers
(Scheme 1).8 In this instance, no intermediate is observed,
and migration of either carborane C atom can occur. In
contrast, an analogous isomerization to yield the rhodium
complex [2,2-(2,3,8-η3):(5,6-η2)-C7H7CH2)-1,8-(4′-MeC6H4)2-
The synthesis of ortho-closo-dicarbadodecaborane C2B10H12
from acetylene and decaborane was first reported in 1963.1
The same year, the thermally induced conversion of this
molecule from the ortho isomer to meta carborane at ca. 500
°C was described;2 further heating (615 °C) yielded the para
isomer.3 Numerous transition metal complexes of substituted
ortho carboranes have since been prepared4 where it has been
observed that the introduction of sterically demanding
substituents on the carborane carbon atoms decreases the
barrier to isomerization so that the more thermally stable
2,1,8-metallacarboranes are obtained at much lower temper-
atures.5
(4) For a recent review, see Corsini, M.; de Biani, F. F.; Zanello, P. Coord.
Chem. ReV. 2006, 250, 1351-1372.
Despite substantial documentation of their occurrence, little
remains known regarding the mechanism of these carborane
cage isomerization reactions. Two widely disseminated path-
(5) See, for example, (a) Kaloustain, M. K.; Wiersema, R. J.; Hawthorne,
M. F. J. Am. Chem. Soc. 1971, 93, 4912-4913. (b) Welch, A. J.;
Weller, A. S. J. Chem. Soc. Dalton. Trans. 1997, 1205-1212. (c)
Garrioch, R. M.; Low, K. S.; Rosair, G. M.; Welch, A. J. J. Organomet.
Chem. 1999, 575, 57-62. (d) Robertson, S.; Ellis, D.; Rosair, G. M.;
Welch, A. J. Appl. Organomet. Chem. 2003, 17, 518-524. (e)
Robertson, S.; Ellis, D.; Rosair, G. M.; Welch, A. J. J. Organomet.
Chem. 2003, 680, 286-293.
* To whom correspondence should be addressed. Phone: +1-905-525-
9140 ext 22840. Fax: +1-905-522-2509. E-mail: valliant@mcmaster.ca.
† Department of Chemistry.
‡ Departments of Chemistry and Medical Physics & Applied Radiation
Sciences.
(6) Lipscomb, W. N. Science 1966, 153, 373-378.
(1) Heying, T. L.; Ager, J. W.; Clark, S. L.; Mangold, D. J.; Goldstein,
H. L.; Hillman, M.; Polak, R. J.; Szymanski, J. W. Inorg. Chem. 1963,
2, 1089-1092.
(2) Grafstein, D.; Dvorak, J. Inorg. Chem. 1963, 2, 1128-1133.
(3) Papetti, S.; Heying, T. L. J. Am. Chem. Soc. 1964, 86, 2295-2295.
(7) Kaesz, H. D.; Bau, R.; Beall, H. A.; Lipscomb, W. N. J. Am. Chem.
Soc. 1967, 89, 4218-4220.
(8) Baghurst, D. R.; Copley, R. C. B.; Fleischer, H.; Mingos, D. M. P.;
Kyd, G. O.; Yellowlees, L. J.; Welch, A. J.; Spalding, T. R.; O’Connell,
D. J. Organomet. Chem. 1993, 447, C14-C17.
2148 Inorganic Chemistry, Vol. 46, No. 6, 2007
10.1021/ic0617543 CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/16/2007

MicrowaWe-Assisted Synthesis of Tricarbonyl Rhenacarboranes
Scheme 1. Carborane Cage Isomerization of a Platinacarborane
energy to overcome the activation energy barrier to afford
isomerization. Notably, the incorporation of a methylene
spacer between the carborane cage and the phenyl ring
decreased steric strain at the bonding face of the carborane
ligand such that the mono-benzyl species {[K][3,3,3-(CO)3-
1-Bn-3,1,2-closo-ReC2B9H10]} (Bn ) CH2C6H5) displayed
the expected 3,1,2 conformation, with the two carborane C
atoms adjacent to one another on the C2B3 bonding face of
the cage. Prior to this, carborane cage isomerization had not
been reported for rhenacarborane complexes.
Complex8
Scheme 2. Microwave-Assisted Formation of {[Na][2,2,2-(CO)3-8-
PhOH-1-R-2,1,8-closo-ReC2B9H9]} (R ) H, PhOH)13
This microwave-assisted methodology has proven amen-
able to working with 99mTc at the tracer level;13 however, in
the synthesis of clinically relevant species, it is imperative
to be able to accurately predict which isomer of a given
carborane complex will form. The vast majority of the
metallacarboranes known to undergo 1,2 f 1,7 carborane
cage isomerization contain either phenyl or p-tolyl substit-
uents on the carborane C atoms:5 no investigation of the
amount of steric bulk required to induce isomerization has
yet been conducted. An additional factor which may affect
the isomerization barrier is the electronic character of the
substituent(s) on the carborane cage. In order to further our
understanding of the isomerization process of these com-
plexes, we report here the synthesis of a series of tricar-
bonyl rhenium carboranes, wherein the carborane cage has
been mono- or disubstituted with a variety of moieties. For
simplicity, the structures discussed here are compiled in
Chart 1.
2,1,8-closo-RhC2B9H9] proceeds via a stable pseudo-closo
intermediate; X-ray crystallography revealed that the two
carborane C atoms of this species remain adjacent but not
bonded to one another (dC‚‚‚C ) 2.539(7) Å).9
Our research interests involve the synthesis of [Re(CO)3]-
carborane complexes and their radioactive 99mTc counterparts
as synthons for preparing molecular imaging and therapy
agents. Preparative scale syntheses are done with rhenium
in order to develop a synthetic methodology for reactions
with technetium, which does not possess a non-radioactive
isotope. The rhenium congeners also serve as characterization
(HPLC) standards for tracer level work with 99mTc. We have
recently reported10,11 the aqueous synthesis of rhenacarbo-
ranes and their 99mTc congeners which is a significant
advance over the traditional anhydrous reaction conditions
that were formerly thought to be required for the synthesis
of these complexes. However, the long reaction times of 18
h to 7 days are not suitable for 99mTc syntheses due to the
relatively short half-life (t1/2 ) 6.02 h) of that isotope.
As microwave heating is capable of decreasing reaction
times significantly,12 we investigated its use in the preparation
of metallacarborane complexes and demonstrated that it
results in decreased reaction times of ca. 15 min, as well as
substantial improvements in product yields.13 Over the course
of this work, it was observed that the reaction of [Re(CO)3-
(H2O)3]Br with mono- or bis-phenol-carborane resulted in
the exclusive formation of the 2,1,8 isomers {[Na][2,2,2-
(CO)3-8-PhOH-1-R-2,1,8-closo-ReC2B9H9]} (R ) H, PhOH,
Scheme 2), as the high pressure (20 atm) and temperature
(200 °C) attained in the microwave reactor provide sufficient
Experimental
Reagents and General Procedures. Decaborane and closo-
ortho-carborane 1 were purchased from Katchem Ltd.; nBuLi (2.5
M, 1.6 M solutions in hexanes), 3-cyclohexyl-1-propyne, KF,
MeSO3CF3, and HSO3CF3 were purchased from Sigma-Aldrich and
used without further purification. Benzyl chloride (Baker) and NaF
(EM Science) were also purchased. Compounds prepared according
to literature procedures were [Re(CO)3(H2O)3]Br,14 [1-CH2C5H4N-
closo-C2B10H11] (8),15 [1-C5H4N-closo-C2B10H11] (14),16 and [1-Ph-
closo-C2B10H11].1 Solvents were purchased from Caledon, dried over
calcium hydride (CH2Cl2, CH3CN, and toluene) or Na/benzophe-
none (Et2O), and freshly distilled prior to use.
Instrumentation. Multi-NMR spectra were recorded on a Bruker
DRX-500 spectrometer with chemical shifts reported in ppm relative
to the residual proton signal of the deuterated solvent (1H NMR)
or the carbon signal of the solvent (13C NMR). Boron-11 NMR
spectra were referenced externally to BF3‚Et2O. Proton NOE
experiments were conducted on a Bruker AV-600 NMR spectrom-
eter. Infrared spectra were acquired using a BioRad FTS-40 FT-IR
spectrometer. All spectra were recorded at 22 °C. A Biotage Initiator
Sixty Microwave Reactor was employed for reactions requiring
microwave radiation. Thin layer chromatograms (Merck F254 silica
gel on aluminum plates) were visualized using 0.1% PdCl2 in 3 M
HCl(aq) and/or UV light. Purification of all products was effected
(9) Safronov, A. V.; Dolgushin, F. M.; Petrovskii, P. V.; Chizhevsky, I.
T. Organometallics 2005, 24, 2964-2970.
(10) Sogbein, O. O.; Merdy, P.; Morel, P.; Valliant, J. F. Inorg. Chem.
2004, 43, 3032-3034.
(11) (a) Sogbein, O. O.; Green, A. E. C.; Schaffer, P.; Chankalal, R.; Lee,
E.; Healy, B. D.; Morel, P.; Valliant, J. F. Inorg. Chem. 2005, 44,
9574-9584. (b) Sogbein, O. O.; Green, A. E. C.; Valliant, J. F. Inorg.
Chem. 2005, 44, 9585-9591.
(12) Tierney, J. P.; Lidstro¨m, P. MicrowaVe Assisted Organic Synthesis;
CRC Press: Boca Raton, FL, 2005.
(13) Green, A. E.C.; Causey, P. W.; Louie, A. S.; Armstrong, A. F.;
Harrington, L. E.; Valliant, J. F. Inorg. Chem. 2006, 45, 5727-
5729.
(14) Lazarova, N.; James, S.; Babich, J.; Zubieta, J. Inorg. Chem. Commun.
2004, 7, 1023-1027.
(15) Wang, X.; Jin, G.-X. Organometallics 2004, 23, 6319-6322.
(16) Coult, R.; Fox, M. A.; Gill, W. R.; Herbertson, P. L.; MacBride, J.
A.H.;Wade, K. J. Organomet. Chem. 1993, 462, 19-29.
Inorganic Chemistry, Vol. 46, No. 6, 2007 2149

Armstrong and Valliant
Chart 1. Structures of Compounds Discussed (Shaded Circle Represents BH)
either by flash chromatography using Ultrapure Silica Gel from
Silicycle (70-230 mesh), or by using a Biotage SP1 autopurification
system. Low-resolution mass spectra were obtained on a Waters/
micromass GCT-ToF spectrometer using electron impact ionization
or a Waters/Micromass Quattro Ultima spectrometer using elec-
trospray ionization. High-resolution mass spectra were obtained on
a Waters/Micromass Q-ToF Ultima Global spectrometer.
Preparation of {[K][2,2,2-(CO)3-2,1,8-closo-ReC2B9H11]} (4).
The potassium salt of [7,8-nido-H2C2B9H10] (2) (0.0636 g, 0.369
mmol) and [Re(CO)3(H2O)3]Br (0.1765 g, 0.437 mmol) were
weighed into a microwave vial (5 mL) and dissolved in aqueous
ethanol (10%, 4 mL). The vial was crimp-sealed, and the opaque
reaction mixture was heated in a microwave reactor at 200 °C for
20 min. An additional portion of [Re(CO)3(H2O)3]Br (0.0605 g,
0.150 mmol) was added, and the reaction mixture was heated at
200 °C for 40 min, resulting in a clear yellow solution with a small
amount of black precipitate. The solvent was removed by rotary
evaporation; the product was isolated as a tan semisolid (0.1429 g,
0.323 mmol, 87%) by silica gel chromatography using an automated
purification system and a solvent gradient of 3-30% MeOH in
1
CH2Cl2. H NMR (d4-methanol, δ): 2.12 (br, 1 H, CH), 1.58 (br,
1 H, CH). 11B{1H} NMR (d4-methanol, δ): -7.4, -8.1, -10.7,
-11.2, -11.6, -14.7, -18.8, -20.2, -21.6 (all resonances equal
in intensity). 13C{1H} NMR (d4-methanol, δ): 200.58 (CdO), 37.13
(br, CH), 29.12 (br, CH). IR (neat): 2555 (br, νB-H) 2001 (νCdO),
1894 cm-1 (br, νCdO). TLC (20% MeOH/CH2Cl2): Rf ) 0.37.
2150 Inorganic Chemistry, Vol. 46, No. 6, 2007

MicrowaWe-Assisted Synthesis of Tricarbonyl Rhenacarboranes
HRMS (ES-) m/z for C5H11O3B9Re1: calcd 402.1163, observed
402.1146 [M-].
5.18 (br, 1 H, CHcarborane), 4.67 (s, 3 H, CH3), 4.57 (s, 2 H, CH2).
11B{1H} NMR (d6-acetone, δ): -2.2 (1 B), -4.5 (1 B), -8.7 (2
B), -11.4 (3 B), -12.1 (3 B). 13C{1H} NMR (d6-acetone, δ):
152.38, 149.19, 147.15, 133.36, 128.91 (CAr), 72.40 (Ccarborane),
64.79 (CHcarborane), 47.92 (CH2), 39.15 (CH3). IR(neat): 3047 (νC-H),
2594 (br, νB-H), 1262 cm-1 (νS-O). HRMS (ES+) m/z for
C9H20NB10: calcd 250.2604, observed 250.2625 [M+].
Preparation of {[Na][3,3,3-(CO)3-1-CH2C5H4N-3,1,2-closo-
ReC2B9H10]} (6). Method A. The closo-carborane 7 (19.1 mg,
0.0812 mmol) was combined with KF (44.9 mg, 0.773 mmol) in a
microwave vial (5 mL) and mixed with aqueous ethanol (10%, 4
mL); the vial was crimp-sealed. The resulting suspension was heated
in a microwave reactor at 195 °C for 12 min to form the nido-
carborane 5 in situ. [Re(CO)3(H2O)3]Br (42.9 mg, 0.106 mmol)
was added, and the reaction mixture heated for 15 min at 190 °C.
The solvent was removed by rotary evaporation, and the product
isolated (36.5 mg, 0.0685 mmol, 84%) via silica gel chromatography
using an automated purification system with a gradient of 3-30%
MeOH in CH2Cl2.
Method B. The potassium salt of 5 (23.8 mg, 0.101 mmol) was
combined with KF (40.6 mg, mmol) and [Re(CO)3(H2O)3]Br (51.7
mg, 0.128 mmol) in a microwave vial (5 mL). Aqueous ethanol
(10%, 4 mL) was added, the vial crimp-sealed, and the reaction
mixture was heated at 190 °C for 15 min in a microwave reactor.
The solvent was removed by rotary evaporation, and the product
isolated (37.5 mg, 0.0704 mmol, 87%) via silica gel chromatography
using an automated purification system with a gradient of 3-30%
MeOH in CH2Cl2. Spectroscopic data were consistent with those
reported previously.11a
Preparation of {2,2,2-(CO)3-8-[(N-Me)CH2C5H4N]-2,1,8-closo-
ReC2B9H10} (12). Method A. The closo-carborane 11 (47.0 mg,
0.118 mmol) was combined with NaF (32.3 mg, 0.767 mmol) and
aqueous ethanol (10%, 3 mL) in a crimp-sealed microwave vial (5
mL). The resulting suspension was heated for 8 min at 195 °C in
a microwave reactor, resulting in a clear solution with a dense white
precipitate. A solution of [Re(CO)3(H2O)3]Br (54.6 mg, 0.135
mmol) in aqueous ethanol (10%, 1 mL) was added, and the reac-
tion mixture was heated for 15 min at 190 °C; a second aliquot of
[Re(CO)3(H2O)3]Br (27.3 mg, 0.067 mmol) in EtOH(aq) (10%, 0.5
mL) was added, and the reaction mixture was heated for an
additional 5 min at 190 °C. The solvent was removed via rotary
evaporation, and the product 12 isolated as a yellow oil (56.2 mg,
0.110 mmol, 93%) by silica gel chromatography using an automated
purification system and a gradient of 2-20% MeOH in CH2Cl2.
Method B. The potassium salt of 6 (37.5 mg, 0.0704 mmol)
was dissolved in CH2Cl2 (10 mL) under an inert (nitrogen)
atmosphere. A freshly prepared solution of MeSO3CF3 in CH2Cl2
(5% v/v, 0.16 mL, 0.0707 mmol) was added to the reaction mixture,
which was stirred for 18 h. The reaction mixture was washed with
water (3 × 8 mL), then dried over Na2SO4 prior to removal of the
solvent by rotary evaporation, leaving the product 12 as a pale
yellow solid (28.0 mg, 0.0548 mmol, 78%). X-ray quality crystals
were grown from CH2Cl2/hexanes (ca. 1:2). 1H NMR (d6-acetone,
δ): 9.04 (d, 1 H, Pyr), 8.64 (t, 1 H, Pyr), 8.11 (t, 1 H, Pyr), 8.04
(d, 1 H, Pyr), 4.55 (s, 3 H, CH3), 3.82 [AA′, 2 H, CH2, ∆δ AA′ )
Preparation of [1-Bn-closo-C2B10H11] (8) and [1,2-Bn2-closo-
C2B10H10] (9). Closo-ortho-carborane 1 (1.0865 g, 7.544 mmol)
was dissolved in Et2O (40 mL) and cooled to -78 °C under an
inert (nitrogen) atmosphere. nBuLi (9.4 mL, 15.0 mmol) was added,
and the resulting white slurry was stirred for 50 min prior to addition
of benzyl chloride (1.3 mL, 1.43 g, 11.3 mmol). After 3 h, the
reaction mixture was warmed slowly to 22 °C; stirring was
continued for an additional 18 h. The yellow slurry was washed
with water (3 × 20 mL), then dried over Na2SO4 prior to removal
of the solvent by rotary evaporation. The two products were
separated by silica gel chromatography using an automated
purification system with a gradient of 2-20% CH2Cl2 in hexanes.
[1-Bn-closo-C2B10H11] (8). Colorless crystals (0.2484 g, 1.06
mmol). 1H NMR (d6-acetone, δ): 7.37 (m, 3 H, CHAr), 7.28 (m, 2
H, CHAr), 4.49 (br, 1 H, CHcarborane), 3.69 (s, 2 H, CH2). 11B{1H}
NMR (d6-acetone, δ): -1.6 (1 B), -4.8 (1 B), -8.4 (2 B), -10.1
(2 B), -10.9 (2 B), -11.8 (2 B). 13C{1H} NMR (d6-acetone, δ):
136.47, 131.02, 129.54, 128.93 (CAr), 77.15 (br, Ccarborane), 62.76
(br, CHcarborane), 43.92 (CH2). IR (neat): 3066 (νC-H), 2594 cm-1
(br, νB-H). TLC (10% CH2Cl2/hexanes): Rf ) 0.27. HRMS (EI+)
m/z for C9H18B10: calcd 234.2416, observed 234.2412 [M+].
[1,2-Bn2-closo-C2B10H10] (9). White powder (0.2153 g, 0.660
2
46.0 Hz, J (1H-1H) ) 15 Hz], 1.84 (br, s, 1 H, CHcarborane). 11B-
{1H} NMR (d6-acetone, δ): -5.4 (1 B), -8.5 (2 B), -9.0 (1 B),
-11.0 (1 B), -11.7 (1 B), -18.0 (1 B), -19.7 (1 B), -20.0 (1 B).
13C{1H} NMR (d6-acetone, δ): 199.58 (CdO), 156.10, 147.50,
145.84, 133.10, 127.59 (CAr), 49.36 (Ccarborane), 47.51 (CH2), 42.85
(CH3). IR (neat): 2549 (br, νB-H), 2000 (νCdO), 1890 cm-1 (br,
νCdO). TLC (10% MeOH in CH2Cl2): Rf ) 0.41.
Preparation of {2,2,2-(CO)3-8-[(N-H)CH2C5H4N]-2,1,8-closo-
ReC2B9H10} (13). The sodium salt of compound 6 (57.7 mg, 0.112
mmol) was combined with CH2Cl2 (15 mL) under an inert (nitrogen)
atmosphere, resulting in a cloudy light yellow liquid. A freshly
prepared solution of HSO3CF3 in CH2Cl2 (5% v/v, 0.20 mL, 0.113
mmol) was added to the reaction mixture. After 15 min, a white
solid had formed while the supernatant liquid was now clear. After
13 h, the reaction mixture was washed with water (3 × 10 mL) to
remove the dense white solid. The organic phase was dried over
Na2SO4 prior to removal of the solvent by rotary evaporation,
leaving the product 13 as a pale yellow solid (44.8 mg, 0.0909
mmol, 81%). X-ray quality crystals were grown from MeOH/
1
mmol). H NMR (CDCl3, δ): 7.37 (m, 3 H, CHAr), 7.23 (m, 2 H,
CHAr), 3.64 (s, 2 H, CH2). 11B{1H} NMR (CDCl3, δ): -3.9 (2 B),
-9.5 (4 B), -10.2 (4 B). 13C{1H} NMR (CDCl3, δ): 135.18,
130.59, 128.80, 128.29 (CAr), 79.57 (br, Ccarborane), 41.59 (CH2).
IR (neat): 3070 (νC-H), 3030 (νC-H), 2583 cm-1 (br, νB-H).
TLC (10% CH2Cl2/hexanes): Rf ) 0.18. HRMS (EI+) m/z for
C16H24B10: calcd 326.2809, observed 326.2810 [M+].
1
CH2Cl2/hexanes (ca. 1:5:10). H NMR (d6-acetone, δ): 11.6 (br,
Preparation of {[1-(N-Me)CH2C5H4N-closo-C2B10H11][O3SCF3]}
(11). The closo-carborane 7 (96.2 mg, 0.409 mmol) was dis-
solved in CH2Cl2 (15 mL) under an inert (nitrogen) atmosphere,
yielding a clear colorless solution. A freshly prepared solution of
MeSO3CF3 in CH2Cl2 (5% v/v, 0.92 mL, 0.406 mmol) was added
to the reaction mixture. After 18 h, the solvent was removed from
the clear pale yellow solution leaving 11 as a yellow oil which
solidified to a fine yellow powder (0.1301 g, 0.326 mmol, 80%)
NH), 9.03 (d, 1 H, Pyr), 8.77 (t of d, 1 H, Pyr), 8.20 (t, 1 H, Pyr),
8.10 (d, 1 H, Pyr), 3.71 [AA′, 2 H, CH2, ∆δ AA’ ) 59.3 Hz, J
2
(1H-1H) ) 14.8 Hz], 1.80 (br, s, 1 H, CHcarborane). 11B{1H} NMR
(d6-acetone, δ): -5.72 (1 B), -8.38 (2 B), -9.35 (1 B), -11.2 (1
B), -11.86 (1 B), -18.06 (1 B), -19.61 (1 B), -19.93 (1 B).
13C{1H} NMR (d6-acetone, δ): 199.82 (CdO), 154.83, 146.84,
142.88, 130.11, 126.53 (CAr), 50.38 (Ccarborane), 44.77 (CH2), 29.21
(CHcarborane). IR (neat): 2545 (br, νB-H), 2001 (νCdO), 1902 cm-1
(br, νCdO). TLC (20% MeOH in CH2Cl2): Rf ) 0.25. HRMS (ES-)
m/z for C11H16B9O3NRe: calcd 494.1581, observed 494.1591 [M-].
1
upon standing for 12 h. H NMR (d6-acetone, δ): 9.21 (d, 1 H,
CHAr), 8.77 (t, 1 H, CHAr), 8.34 (d, 1 H, CHAr), 8.24 (t, 1 H, CHAr),
Inorganic Chemistry, Vol. 46, No. 6, 2007 2151

Armstrong and Valliant
Table 1. Crystallographic Data
Preparation
of
{[K][2,2,2-(CO)3-8-C5H4N-2,1,8-closo-
ReC2B9H10]} (15). [1-C5H4N-closo-C2B10H11] 14 (18.0 mg, 0.0813
mmol) was combined with KF (51.2 mg, 0.881 mmol) and
[Re(CO)3(H2O)3]Br (38.0 mg, 0.0941 mmol) in a microwave vial
(5 mL). Aqueous ethanol (10%, 4 mL) was added to the crimp-
sealed vial, and the resulting suspension was heated for 15 min at
200 °C in a microwave reactor. A second portion of [Re(CO)3-
(H2O)3]Br (31.0 mg, 0.0767 mmol) was added, and the reaction
mixture was heated for an additional 10 min at 190 °C. The solvent
was removed via rotary evaporation, and the product isolated (39.7
mg, 0.0764 mmol, 94%) by silica gel chromatography using an
automated purification system and a gradient of 3-30% MeOH in
parameter
11
12
13
formula
fw
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
C10H20NO3SF3B9 ReC12H19NO3B9 ReC12H21NB9O4
388.63
P2(1)
508.79
P2(1)/c
7.4000(2)
15.4930(4)
15.3055(4)
90
92.328(2)
90
1753.30(8)
4
526.79
P2(1)/n
9.4919(2)
12.8949(3)
15.9481(4)
90
96.7330(10)
90
1938.54(8)
4
6.8158(6)
10.9599(9)
12.8039(10)
90
95.531(5)
90
952.00(14)
2
173(2)
1.54178
T, K
λ, Å
173(2)
0.71073
1.928
173(2)
0.71073
1.805
1
CH2Cl2. H NMR (d4-methanol, δ): 8.28 (d, 1 H, Pyr), 7.64 (t of
d
calcd, g cm-3 1.356
d, 1 H, Pyr), 7.57 (d, 1 H, Pyr), 7.19 (t, 1 H, Pyr), 1.81 (br, s, 1 H,
CHcarborane). 11B{1H} NMR (d4-methanol, δ): -4.2 (1 B), -6.4 (1
B), -7.0 (2 B), -10.8 (1 B), -11.6 (1 B), -17.4 (1 B), -18.7 (2
B). 13C{1H} NMR (d4-methanol, δ): 200.14 (CdO), 161.41,
148.03, 138.11, 124.72, 123.09 (CAr), 57.73 (Ccarborane), 28.82
(CHcarborane). IR (neat): 2555 (br, νB-H), 2011 (νCdO), 1904 cm-1
(br, νCdO). TLC (15% MeOH in CH2Cl2): Rf ) 0.09. HRMS (ES-)
m/z for C12H23B9O3NRe: calcd 480.1425, observed 480.1413 [M-].
µ, mm-1
F(000)
Ra
1.685
408
0.0417
0.1121
6.942
968
0.0265
0.0477
6.285
1008
0.0463
0.0586
b
Rw
a R ) [∑||Fo| - |Fc|]/[∑|Fo|] for reflections with I > 2.00σ(I). b Rw
)
2
2
2
{[∑w(Fo - Fc )2]/[∑w(Fo )2]}1/2 for all reflections.
Preparation of {[K][3,3,3-(CO)3-1-CH2Cy)-closo-ReC2B9H10]}
(21). [1-CH2Cy-closo-C2B10H11] 20 (20.1 mg, 0.0832 mmol) was
combined with KF (47.6 mg, 0.819 mmol) and [Re(CO)3(H2O)3]-
Br (40.9 mg, 0.101 mmol) in aqueous ethanol (10%, 4 mL) in a
microwave vial (5 mL). The reaction mixture was crimp-sealed
and heated at 200 °C for 15 min in a microwave reactor. A second
portion of [Re(CO)3(H2O)3]Br (34.8 mg, 0.0861 mmol) was added,
and the reaction mixture was heated at 200 °C for 10 min. The
resulting colorless solution was evaporated to dryness under a
stream of N2(g). The product 21 was isolated as a light brown oil
(31.4 mg, 0.0583 mmol, 70%) by silica gel chromatography using
an automated purification system and a gradient of 3-30% MeOH
Preparation of {[K][2,2,2-(CO)3-1,8-Bn2-2,1,8-closo-ReC2B9H9]}
(19). [1,2-Bn2-closo-C2B10H10] 9 (24.9 mg, 0.0767 mmol) was
combined with KF (51.1 mg, 0.879 mmol) and [Re(CO)3(H2O)3]-
Br (35.7 mg, 0.0883 mmol) in aqueous ethanol (10%, 4 mL) in a
microwave vial (5 mL). The reaction mixture was sealed and heated
at 200 °C for 15 min in a microwave reactor. A second portion of
[Re(CO)3(H2O)3]Br (15.0 mg, 0.0371 mmol) was added, and the
reaction mixture was heated at 200 °C for 10 min; a third (10 min)
and fourth (15 min) heating with additional [Re(CO)3(H2O)3]Br
(15.0 mg, 0.0371 mmol; 34.4 mg, 0.0851 mmol) was required for
complete consumption (by 11B NMR) of the carborane ligand. The
solvent was removed via rotary evaporator, and the product 19 was
isolated (27.2 mg, 0.0437 mmol, 57%) using an automated
purification system and a gradient of 3-30% MeOH in CH2Cl2.
1H NMR (d4-methanol, δ): 7.24-6.98 overlapping (m, 10 H, CHAr),
1
in CH2Cl2. H NMR (d4-methanol, δ): 1.78 (br, t, 2 H, Cy), 1.67
(t, 2 H, Ccarborane-CH2, 3J ) 5.5 Hz], 1.63 (br, s, 1 H, CHcarborane),
1.61 (m, 2 H, Cy), 1.38 (m, 2 H, Cy), 1.26 (m, 2 H, Cy), 1.12 (m,
2 H, Cy), 0.77 (br, q, 2 H, Cy). 11B{1H} NMR (d4-methanol, δ):
-5.2 (1 B), -7.7 (2 B), -10.4 (1 B), -11.9 (2 B), -14.8 (1 B),
-20.0 (2 B). 13C{1H} NMR (d4-methanol, δ): 200.89 (CO), 55.11
(br, Ccarborane), 39.12 (Ccarborane-CH2), 35.64 (Cy), 29.42 (br,
CHcarborane), 27.51 (Cy), 27.35 (Cy), 27.32 (Cy). IR (neat): 2531
(br, νB-H), 1996 (νCdO), 1884 cm-1 (br, νCdO). TLC (15% MeOH
in CH2Cl2): Rf ) 0.13. HRMS (ES-) m/z for C12H23B9O3Re: calcd
499.2099, observed 499.2091 [M-].
2
3.05 [AA′, 2 H, CH2, ∆δ AA′ ) 26.0 Hz, J (1H-1H) ) 14 Hz],
2
2.93 [AA′, 2 H, CH2, J (1H-1H) ) 15 Hz]. 11B{1H} NMR (d4-
methanol, δ): -5.5 (2 B), -8.1 (2 B), -9.1 (1 B), -11.8 (1 B),
-15.7 (1 B), -17.5 (2 B). 13C{1H} NMR (d4-methanol, δ): 201.07
(CO), 142.41, 140.92, 131.28, 131.26, 128.46, 128.20, 127.18,
126.90 (CAr), 60.15 (br, Ccarborane), 57.01 (br, Ccarborane), 51.11 (CH2),
48.08 (CH2). IR (neat): 2534 (υB-H), 1999 (υCdO), 1904 (υCdO),
1870 cm-1 (υCdO). HRMS (ES-) m/z for C19H23B9O3Re: calcd
583.2101, observed 583.2098 [M-].
X-ray Analyses. Colorless crystals of 11 and 12 and a pale
yellow crystal of 13 were mounted on glass fibers. Measurements
of 11 were made on a Bruker D8 diffractometer with a SMART6000
CCD detector using Cu KR radiation rendered monochromatic via
cross-coupled parallel focusing mirrors. Data for 12 and 13 were
collected on a Bruker P4 diffractometer with a SMART1K CCD
detector using graphite-monochromated Mo KR radiation. The
program SMART17 was used for data collection; data processing
was carried out by use of the program SAINT;18 the data were
scaled using SADABS.19 Data were corrected for absorption using
the multiscan method. Crystallographic data are summarized in
Table 1. The structures were solved by direct methods and refined
by full matrix least-squares using SHELXL-97.20 For 11 and 12,
Preparation of [1-CH2Cy-closo-C2B10H11] (20). Decaborane
(0.6545 g, 5.355 mmol) was dissolved in a mixture of acetonitrile
(5.5 mL) and toluene (25 mL) and brought to reflux under an inert
(nitrogen) atmosphere. After 1 h, 3-cyclohexyl-1-propyne (0.85 mL,
0.72 g, 5.9 mmol) was added, and the reaction mixture was heated
at reflux for an additional 15 h. The solvent was removed by rotary
evaporation; the product 19 was isolated by silica gel chromatog-
raphy (10% CH2Cl2 in hexanes) as a colorless semisolid (0.7512
1
g, 3.125 mmol, 58%). H NMR (CDCl3, δ): 3.54 (br, CH), 2.09
[d, 2 H, Ccarborane-CH2,3J(1H-1H) ) 6 Hz], 1.78 (br, d, 2 H, Cy),
1.64 (m, 2 H, Cy), 1.48 (m, 1 H, Cy), 1.24 (m, 2 H, Cy), 1.15 (m,
2 H, Cy), 0.92 (m, 2 H, Cy). 11B{1H} NMR (CDCl3, δ): -1.82 (1
B), -5.22 (1 B), -8.91 (2 B), -10.78 (2 B), -11.35 (2 B), -12.68
(2 B). 13C{1H} NMR (CDCl3, δ): 75.33 (Ccarborane-CH2), 62.00
(CH2-CH), 45.93 (CH2), 37.93 (Cy), 34.06 (Cy), 30.98 (CHcarborane),
26.00 (Cy). IR (neat): 2930 (νC-H), 2857 (νC-H), 2590 cm-1 (br,
(17) Sheldrick, G. M. SMART, Release 6.45; Siemens Energy and Automa-
tion Inc.: Madison, WI, 2003.
(18) Sheldrick, G. M. SAINT, Release 6.45; Siemens Energy and Automa-
tion Inc.: Madison, WI, 2003.
(19) Sheldrick, G. M. SADABS (Siemens Area Detector Absorption
Corrections); Siemens Energy and Automation Inc.: Madison, WI,
2003.
ν
B-H). TLC (10% CH2Cl2/hexanes): Rf ) 0.41. HRMS (EI+) m/z
for C9H24B10: calcd 242.2809, observed 242.2804 [M+].
2152 Inorganic Chemistry, Vol. 46, No. 6, 2007

MicrowaWe-Assisted Synthesis of Tricarbonyl Rhenacarboranes
Scheme 3. Microwave-Assisted Formation of 3,1,2- and 2,1,8-[closo-
Re(CO)3(C2B9H11)]-
some hydrogen atoms were located from a difference map, while
the remainder were included at geometrically idealized positions;
for 13, all hydrogen atoms were included at geometrically idealized
positions. Non-hydrogen atoms were refined anisotropically. Com-
pound 13 crystallized with a molecule of MeOH in the lattice.
Thermal ellipsoid plots were created using Mercury 1.4.1.21
Discussion
Generation of 3 and 4. Closo-ortho-carboranes are known
to undergo fluoride-mediated deboronation reactions in water
heated to reflux;22 similarly, heating aqueous closo-ortho-
carborane C2B10H12 (1) in a microwave reactor at g190 °C
in the presence of excess MF (M ) Na, K) results in quanti-
tative conversion of 1 to the corresponding alkali metal salt
[M][7,8-nido-C2B9H12] (2) in less than 10 min. The nido-car-
borane 2 is readily differentiated from the starting material
by 11B NMR spectroscopy, as it produces a total of five
resonances, two of which are at significantly higher field
(-33.3, -38.1 ppm) than those of 1 (δ range: -2.0 to -13.2
ppm).
In order to develop a viable set of experimental conditions
for the microwave-assisted synthesis of rhenacarboranes, a
series of reactions was conducted between the unsubstituted
nido-carborane 2 and [Re(CO)3(H2O)3]Br in aqueous ethanol.
The reaction time was fixed at 10 min, while temperatures
of 100, 120, 140, 160, 180, and 200 °C were employed. The
crude reaction mixtures were then dissolved in d4-methanol
and analyzed by 1H and 11B NMR spectroscopy. The
reactions conducted at 100 and 120 °C were found to contain
only unreacted 2, which is consistent with our observation
that only ca. 5% conversion of 2 to the 3,1,2 rhenacarborane
3 (Scheme 3) can be accomplished by refluxing in water for
18 h. When the reaction temperature was increased to 140
°C, however, an additional albeit minor 1H NMR resonance
emerged downfield (2.92 ppm) of that of the starting material
2 (1.74 ppm). At 180 °C, this second species constituted
nearly 40% of the material detected by NMR spectroscopy;
an additional set of resonances was now readily distinguished
in the 11B NMR spectrum (Figure 1), though without the
two high-field peaks (ca. -33 and -38 ppm) that are
characteristic of nido-ortho-carboranes. These additional
resonances were attributed to the formation of the 3,1,2-
Figure 1. 11B{1H} NMR spectra of the microwave-assisted reaction of
2 with [Re(CO)3(H2O)3]Br at (A) 120, (B) 180, (C) 200 °C, and (D)
purified 4.
spectrum of 3 was recorded when it was originally synthe-
sized,23 the chemical shifts were not reported; however, the
chemical shift range observed was consistent with those of
other closo-(tricarbonyl)-rhenacarboranes.11a
The reaction conducted at 200 °C was expected to contain
a mixture of 2 and 3, with a greater proportion of 3. In
1
actuality, the H NMR spectrum indicated a 1:1:2 mixture
of 2 and 3 and a third compound, which gave rise to two
broad 1H NMR signals (2.21, 1.58 ppm) of equal intensities,
and several additional 11B NMR resonances. The reaction
mixture was also analyzed by electrospray mass spectrom-
etry, which revealed that the only anionic species present
were 2 and a peak corresponding to the tricarbonyl rhenium
complex 3, both of which exhibit diagnostic isotope patterns
due to the presence of boron (10B, 19.9%; 11B, 80.1%) and
rhenium (185Re, 37.4%; 187Re, 62.6%). Taken in concert,
these data suggested that the majority of the 3,1,2-rhena-
carborane 3 had been converted to the more thermodynami-
cally stable 2,1,8 isomer 4, which cannot be differentiated
from 3 by mass spectrometry, and displays C1 symmetry such
1
rhenacarborane 3 as the H NMR signal was in agreement
with the literature value.23 Though the 11B{1H} NMR
(20) Sheldrick, G. M. SHELXTL, Release 6.14; Siemens Crystallographic
Research Systems: Madison, WI, 2000.
(21) Mercury 1.4.1; CCDC: Cambridge, UK, 2001-2005.
(22) Fox, M. A.; MacBride, J. A. H.; Wade. K. Polyhedron 1997, 16, 2499-
2507.
(23) Hawthorne, M. F.; Andrews, T. D. J. Am. Chem. Soc. 1965, 87, 2496-
2496.
Inorganic Chemistry, Vol. 46, No. 6, 2007 2153

Armstrong and Valliant
Scheme 4. Microwave-Assisted Synthesis of 3,1,2-Rhenacarboranes 6 and 10
1
that two carborane CH resonances are expected in its H
NMR spectrum.
area by revisiting the synthesis of the 3,1,2-rhenacarborane
6. The potassium salt of this anion has been synthesized
previously by combining the corresponding nido-carborane
5 with [Re(CO)3]+ in boiling water over 24 h.11a The
spectroscopic data obtained for the product of the analogous
microwave-assisted reaction (Scheme 4), including the 11B
NMR spectrum, were consistent with those reported for 6,
indicating that the same isomer formed in both cases.11a
In order to confirm these assignments, efforts were made
to isolate each of the two isomers 3 and 4 by adjusting the
microwave reaction conditions. While it proved possible to
obtain 3 as the major species (ca. 80%), it was not formed
exclusively under any conditions, nor was it possible to
separate 2, 3, and 4 chromatographically. Experiments
directed at preparing the purported 2,1,8 isomer 4 were far
more successful. Starting either with the crude 3,1,2 isomer
3 prepared as outlined above, or from pure 2 and [Re(CO)3-
(H2O)3]Br, heating in a microwave reactor at 200 °C for 1
h was found to yield exclusively the 2,1,8 complex 4, which
was isolated by silica gel chromatography and characterized
spectroscopically. The 13C NMR spectrum of 4 contained
two resonances (37.13, 29.12 ppm) corresponding to the two
In order to definitively establish the configuration of the
carborane cage of 6, a NOE 1H NMR experiment was done
wherein the carborane CH resonance frequency was irradi-
ated in order to enhance the signals due to neighboring
protons. A significant enhancement of the signal due to the
diastereotopic methylene protons (3.38 ppm) was observed
(see Supporting Information), indicating close proximity to
the carborane CH. An identical experiment was performed
using the closo-carborane precursor 7, and a similar enhance-
ment in the methylene proton signal was observed. This
indicated that the distance between the carborane CH and
CH2 unit of 7 is comparable to that in the rhenacarborane 6,
and confirmed that the 3,1,2 isomer had indeed been isolated.
1
inequivalent CH units observed in the H NMR spectrum,
as well as a signal at 200.58 ppm due to the three CO ligands.
The presence of these CO units was also confirmed by
infrared spectroscopy, with two broad absorptions centered
at 2001 and 1894 cm-1 in addition to the B-H stretching
vibrations (2555 cm-1). A total of nine signals are readily
distinguished in the 11B NMR spectrum due to the decrease
in symmetry of the C2B9 cage upon isomerization. High-
resolution electrospray mass spectrometry was used to
confirm the identity of the product, with a characteristic
isotope distribution centered at 402.1146 m/z corresponding
to the molecular anion.
In order to examine what impact, if any, altering the
electron-withdrawing properties of the carborane substituent
on the carborane cage would have on the isomerization pro-
cess, it was decided to prepare the benzyl (Bn, CH2-C6H5)
analogue of 6. This was done by treating closo-ortho-
n
carborane 1 with 1.5 equiv of BuLi at -78 °C, followed
by addition of an excess of benzyl chloride or bromide: this
reaction produces a mixture of closo-benzyl carborane 8 and
closo-dibenzyl carborane 9, which can be separated by silica
gel chromatography. Further reaction of 8 with [Re(CO)3-
(H2O)3]Br in the presence of sodium fluoride yielded the
3,1,2 tricarbonyl rhenium complex 10 (Scheme 4), as
described in a preliminary communication.13 The 11B{1H}
NMR spectrum of 10 is essentially identical to that of 6,
displaying six signals in the 1:2:1:2:1:2 pattern that we
have found to be characteristic of monosubstituted 3,1,2
rhenacarboranes including the previously reported spe-
cies {[Na][3,3,3-(CO)3-1-R-3,1,2-closo-ReC2B9H10]} (R )
CH2CH2COOH, glucose).11a This pattern arises due to
incidental equivalences in the 11B NMR signals of three cage
boron atoms, since the molecular point group of these 3,1,2-
rhenacarboranes is C1.
Deriving from these and other experiments a set of
conditions that would be generally applicable to the synthesis
of tricarbonyl rhenacarboranes was challenging, as the
temperature at which complexation of a nido-carborane to
the [Re(CO)3]+ core occurs is highly dependent upon the
identity of the substituent(s) attached to the carborane cage.
For example, complexation of the pyridyl-derivatized nido-
carborane 5 with [Re(CO)3(H2O)3]Br is 90% complete after
20 min at 140 °C; conversely, only 8% conversion of
nido-phenyl carborane to a rhenacarborane is observed
under identical conditions. Ultimately, reaction conditions
of 15 min at 200 °C were selected as a starting point for
the synthesis of compounds of the type [M][Re(CO)3-
(RR′C2B9H9)], as they appear to be more than sufficient for
effecting the majority of the desired complexation reactions.
Pyridyl-Bearing Rhenacarboranes: Electronic Con-
tributions to Isomerization. We are not aware of any studies
done to date that have examined the impact of altering the
electron-donating properties of the carborane substituent on
the barrier to cage isomerization and decided to explore this
Both the 3,1,2 complexes 6 and 10 were subjected to
prolonged heating in the microwave reactor: 1H and 11B
NMR spectra were recorded afterward, but were found not
to undergo carborane cage isomerization despite the harsh
conditions (200 °C, 20 bar). This finding was slightly
2154 Inorganic Chemistry, Vol. 46, No. 6, 2007

MicrowaWe-Assisted Synthesis of Tricarbonyl Rhenacarboranes
Scheme 5. Microwave-Assisted Synthesis of the
2,1,8-Rhenacarborane 12
and 11 is the average C-N bond distance within the pyridyl
ring, which increased from 1.331(4) to 1.428(4) Å upon
methylation. The carborane cage C-C bond length of 11
[1.649(4) Å] is also slightly longer than that of 7 [1.622(4)
Å] due to the inductive electron-withdrawing effect of the
methylated pyridyl ring. The B-B distances in 11 range from
1.755(5) to 1.783(5) Å.
Figure 2. Thermal ellipsoid plot of 11 (30% probability ellipsoids); H
atoms omitted.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 11
C(1)-C(2)
1.649(4)
1.723(4)
1.711(4)
1.374(4)
1.783(5)
C(1)-C(1′)
1.527(4)
1.720(4)
1.736(4)
1.482(4)
1.785(5)
The closo-carborane 11 was converted to its nido analogue
in situ by combining it with sodium fluoride and heating
under microwave irradiation at 195 °C (Scheme 5) prior to
addition of the rhenium reagent [Re(CO)3(H2O)3]Br; the
product 12 was isolated by silica gel chromatography. The
1H NMR spectrum of 12 in d6-acetone displayed the expected
four resonances in the aromatic region, as well as a sharp
singlet at 4.55 ppm due to the methyl group, an AA′ pattern
at 3.82 ppm due to the diastereotopic protons of the
methylene unit, and a broad resonance attributed to the
carborane CH vertex (1.84 ppm). The corresponding signals
were observed in the 13C NMR spectrum, as well as a
resonance at 199.58 ppm, assigned to the three CO ligands.
Curiously, the 11B NMR spectrum of the product did not
display the 1:2:1:2:1:2 pattern that we have found to be
characteristic of 3,1,2-rhenacarboranes: a total of eight
distinct signals could be resolved, indicating that the carbo-
rane cage of this product had lower symmetry than those of
6 and 10.
C(1)-B(3)
C(1)-B(4)
C(1)-B(5)
C(2′)-N(1′)
B(7)-B(12)
C(2)-C(1)-B(3)
B(4)-C(1)-B(6)
C(2)-B(3)-B(4)
B(4)-B(5)-B(6)
B(7)-B(12)-B(9)
C(1)-B(6)
C(7′)-N(1′)
B(10)-B(12)
115.7(2)
B(3)-C(1)-B(5)
B(3)-C(1)-B(6)
B(3)-B(4)-B(5)
B(5)-B(6)-C(2)
B(8)-B(12)-B(10)
113.9(2)
113.4(2)
104.7(2)
108.4(2)
107.6(2)
113.6(2)
108.0(2)
103.8(2)
108.4(2)
surprising in light of the results described for the unsubsti-
tuted complex 3/4, where isomerization was observed even
at 170 °C. Though in all three cases (3, 6, and 10) the 2,1,8
isomer is more thermodynamically stable, the activation
energy barrier for a process involving the movement of a
benzyl or CH2-pyridyl substituent is rationalized to be much
larger than the corresponding process involving a cage CH
unit: thus, compound 3 isomerizes to 4, but compounds 6
and 10 remain unchanged.
To further alter the electronic nature of the carborane
substituent, methylation of the pyridyl ring of 7 was effected
using methyl triflate, yielding the closo-carborane 11, which
was characterized both spectroscopically and by X-ray
crystallography. The 1H NMR spectrum of 11 in d6-acetone
contained four resonances in the aromatic region as well as
sharp singlets at 4.67 and 4.57 ppm due to the CH3 and CH2
moieties, respectively; the carborane CH appeared as a broad
peak at 5.18 ppm, which is typical for a monosubstituted
closo-carborane. The 13C NMR spectrum was consistent with
this, though the anticipated quartet due to the [CF3SO3]-
carbon atom remained unresolved; the 11B NMR spectrum
of 11 was virtually unchanged from that of 7.
In order to assess whether compound 12 was in the 3,1,2
1
or 2,1,8 configuration, H NMR NOE experiments were
conducted on both the closo-carborane 11 and the rhena-
carborane 12 (see Supporting Information). Irradiation of the
carborane CH vertex of 11 resulted in a distinct enhancement
in the methylene signal, and to a lesser extent, of one
aromatic proton and the methyl resonance. However, when
the CH resonance frequency of the rhenacarborane 12 was
irradiated, no enhancement was observed in methylene signal,
presumably due to an increase in the distance between those
two moieties, suggesting that the 2,1,8 isomer had been
obtained.
The product was definitively identified by high-resolution
electrospray mass spectrometry ([M+] 251.2587 m/z) and
X-ray crystallography. A thermal ellipsoid plot of 11, which
crystallizes in the P21 space group, is shown in Figure 2;
crystallographic data and pertinent metrical parameters are
summarized in Tables 1 and 2, respectively. The solid-state
structure of 7 has been determined previously;24 as expected,
the most significant difference between the structures of 7
Following recrystallization of the product, confirmation
of this hypothesis was obtained by single-crystal X-ray
diffraction. As shown in Figure 3, complex 12 consists of a
1,7 carborane ligand that is coordinated to the [Re(CO)3]+
core through a CB4 bonding face. The carborane cage C atom
that is directly bonded to the CH2-pyridyl substituent has
migrated out of the bonding face of the ligand. The average
Re-B bond length (see Table 3) in 12 is 2.317(4) Å, while
the Re-C distance is slightly shorter at 2.301(3) Å, consistent
with the smaller size of carbon as compared to boron. A
greater range of B-B distances [1.744-1.814(5) Å] is seen
(24) Alekseyeva, E. S.; Batsanov, A. S.; Boyd, L. A.; Fox, M. A.; Hibbert,
T. G.; Howard, J. A. K.; MacBride, J. A. H.; Mackinnon, A.; Wade,
K. Dalton Trans. 2003, 475-482.
Inorganic Chemistry, Vol. 46, No. 6, 2007 2155

Armstrong and Valliant
Scheme 6. Room-Temperature Isomerization of 6 to 12 (R ) Me)
and 13 (R ) H)
1
A H NMR NOE experiment was then conducted in which
the carborane CH resonance of 13 was irradiated. Contrary
to what was seen for 6, no enhancement of the methylene
signal of 13 occurred, indicating that the distance between
the CH2 unit and the carborane CH vertex was greater in 13
than in 6. Final confirmation that isomerization of the
carborane cage had occurred upon protonation of 6 to yield
13 (Scheme 6) was obtained in the form of an X-ray crystal
structure (not shown) which revealed that 13 is isostruc-
tural with 6, with the carborane cage bonded to the rhen-
ium center through a CB4 bonding face, and the substi-
tuted carborane C vertex removed from the bonding face.
Corresponding metrical parameters for 12 and 13 were
in close agreement with one another (see Supporting Infor-
mation).
Figure 3. Thermal ellipsoid plot of 12 (50% probability ellipsoids); H
atoms omitted.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 12
B(3)-Re(2)
2.268(3)
2.353(4)
1.924(4)
1.721(5)
1.791(5)
1.814(5)
1.787(5)
75.29(13)
45.80(13)
107.6(2)
107.3(3)
B(6)-Re(2)
B(11)-Re(2)
C(1)-Re(2)
C(1)-B(6)
B(6)-B(10)
C(1)-B(5)
2.312(4)
2.333(4)
2.301(3)
1.755(5)
1.790(6)
1.714(5)
B(10)-Re(2)
Re(2)-C(8′)
C(1)-B(3)
B(3)-B(11)
B(10)-B(11)
B(5)-B(6)
C(1)-Re(2)-B(10)
B(3)-Re(2)-B(11)
C(1)-B(3)-B(11)
B(6)-B(10)-B(11)
C(1)-Re(2)-B(3)
B(3)-Re(2)-B(6)
B(3)-C(1)-B(6)
C(1)-B(6)-B(10)
44.25(12)
77.56(13)
111.3(3)
Steric Contributions to Isomerization. As the formation
of the 2,1,8 isomers 12 and 13 was evidently an electronic
effect, we explored the possibility of inducing a similar
process by increasing the steric crowding on the bonding
face of the carborane ligand: this can be done by eliminating
the methylene unit between the pyridyl ring and the carborane
cage. Closo-pyridyl carborane 14 was prepared according
to the literature procedure,16 then reacted with [Re(CO)3-
(H2O)3]Br in a microwave reactor at 200 °C. Following
purification by silica gel chromatography, the rhenacarborane
15 was characterized spectroscopically.
106.6(3)
for 12 than for 11 due to the significantly different electronic
environments of the boron atoms on the bonding face of the
carborane versus those in the rest of the cluster.
We were curious regarding the temperature required to
induce this isomerization process compared to that of 3/4,
as the energetic cost associated with movement of the
methylated pyridyl unit was expected to be much larger than
the CH unit of 3/4. This could potentially be determined by
preparing the 3,1,2 isomer of 12, and monitoring its behavior
as a function of temperature. To that end, the synthesis of
the 3,1,2 isomer of 12 was undertaken via the reaction of 6
with an equivalent of methyl triflate. The reaction mixture
was stirred at 22 °C in dichloromethane, and the product
characterized by multinuclear NMR spectroscopy. Surpris-
ingly, the spectra obtained for this supposed 3,1,2 isomer
were identical to those obtained of 12, indicating that the
3,1,2 rhenacarborane undergoes isomerization at room tem-
perature upon methylation (Scheme 6). This result clearly
demonstrates that the electronic nature of the substituent on
the carborane cage is as influential as steric consideration
in determining the activation energy barrier to carborane cage
isomerization.
1
The H NMR spectrum of 15 in d4-methanol contained
four resonances of equal intensities in the aromatic region,
as well as a broad singlet at 1.81 ppm. Incidentally, the
carborane CH of the closely related phenyl-bearing 3,1,2-
rhenacarborane 16 resonates at 3.95 ppm, while isomerization
to its 2,1,8 analogue 17 is accompanied by a decrease in
chemical shift to 1.75 ppm. This observation, combined with
the 11B NMR spectrum of 15, which did not display the
pattern typical of a 3,1,2 rhenacarborane, suggested that the
2,1,8 isomer had in fact been prepared. This was confirmed
by a 1H NMR NOE experiment, in which irradiation of the
carborane CH proton failed to result in any enhancement of
the signals of the pyridyl ring.
To confirm that formation of the 2,1,8 isomer was indeed
caused by the introduction of a positively charged unit onto
the carborane substituent, rather than a result of the increased
size of the substituent upon methylation, we sought to prepare
the protyl analogue of 12 by reacting 6 with an equivalent
of triflic acid rather than methyl triflate. Notably, the 11B
NMR spectrum of this reaction product 13 was not consistent
with that of 6 as expected for two isostructural compounds.
For comparison with the variable-temperature experiment
conducted for the complexation of 2 with the rhenium
tricarbonyl core, a similar reaction series was done with the
nido-pyridyl carborane, generated by fluoride-mediated de-
boronation of 14. All reaction times were 10 min, while
temperatures of 100, 120, 140, 160, 180, and 200 °C were
1
employed. Analysis of the crude reaction mixtures by H
and 11B NMR spectroscopy revealed complete consumption
2156 Inorganic Chemistry, Vol. 46, No. 6, 2007

MicrowaWe-Assisted Synthesis of Tricarbonyl Rhenacarboranes
of the carborane ligand to form the 2,1,8 complex 15 at
temperatures of 160 °C and above. This was somewhat
surprising, given that the corresponding reaction of 2 is not
complete even after 15 min at 200 °C, and the pyridyl ring
at the ligand bonding face would be expected to provide
enough steric hindrance to slow this reaction considerably
relative to that of 2. The reaction conducted at 100 °C was
subjected to high-resolution electrospray mass spectrometry,
which confirmed the presence of the nido-carborane and
suggested the second species to be the rhenacarborane 15.
removing the bridging hydrogen atom of the nido-carborane
cage, thus assisting in the formation of the η5-complex.
To further probe the amount of steric crowding required
to induce carborane cage isomerization, reactions between
dibenzyl carborane 9 and [Re(CO)3(H2O)3]Br were under-
taken. We have demonstrated that the presence of one benzyl
unit was not sufficient to effect isomerization: the presence
of a second benzyl substituent will increase steric crowding
at the ligand bonding face considerably, though not nearly
as much as for bis-phenol carborane (Scheme 2) due to the
flexibility provided by the methylene spacers. The reagents
were combined with potassium fluoride and heated in the
microwave at 200 °C; the product 19 was isolated by silica
gel chromatography and identified by mass spectrometry
(583.2098 m/z [M-]) and multinuclear NMR spectroscopy.
1
However, the H NMR spectrum of the reaction indicated
that it contained a 1:1 mixture of unreacted nido-pyridyl
carborane and a second species which was inconsistent with
1
the H NMR spectrum obtained previously for 15. Most
notably, a broad singlet attributable to a carborane CH vertex
was observed at 4.21 ppm, rather than the expected resonance
seen at 1.81 ppm in compound 15. The 11B NMR spectrum,
though complicated by the presence of two species with
several overlapping signals, did not exhibit the characteristic
peaks of 15. Bearing in mind that isomerization of the closely
related phenyl-bearing 3,1,2-rhenacarborane 16 to its 2,1,8
1
Both the H and 13C NMR spectra clearly indicated the
presence of two inequivalent benzyl substituents: two distinct
signals (60.15, 57.01 ppm) were also detected in the carbon-
13 spectrum due to inequivalent carborane cage C atoms.
Thus, the dibenzyl rhenacarborane 19 was determined to be
in the 2,1,8 configuration that was observed for the sterically
crowded monosubstituted carboranes 15 and 17.
1
isomer 17 is accompanied by a decrease in the H NMR
A final compound of interest for the purposes of this study
was one with an intermediate amount of steric bulk between
the 3,1,2 complexes 6, 10, 12, and 13, and the 2,1,8 species
15 and 17. A logical target molecule was one that retains
the methylene spacer of the 3,1,2 complexes between the
carborane cage and the six-membered ring but possesses a
bulkier, e.g., cyclohexyl, ring after that unit. To that end,
the novel closo-carborane 20 was synthesized via an alkyne
insertion into the boron cage of decaborane and isolated in
modest yield (58%) via silica gel chromatography. Com-
pound 20 was then reacted with [Re(CO)3(H2O)3]Br at 200
°C; following purification by silica gel chromatography, the
rhenacarborane 21 was recovered in good yield (70%) and
characterized spectroscopically. The 1H and 13C NMR
spectrum of 21 in d4-methanol contained the expected
resonances due to the CH2-Cy (Cy ) C6H11, cyclohexyl)
unit and carborane CH, but gave little indication of the
configuration of the carborane cage. A 11B NMR spectrum
was subsequently obtained and found to display six reso-
nances in a 1:2:1:2:1:2 pattern, indicating that the original
ortho geometry of the carborane cage had been retained.
Unfortunately, attempts to confirm this utilizing a 1H NMR
NOE experiment were not successful due to significant
overlap of the BH, CH, and cyclohexyl resonances, rendering
it impossible to discern whether enhancement of the cyclo-
hexyl signals occurred upon irradiation of the carborane CH
resonance frequency. The identity of the novel rhenacarbo-
rane 21 was confirmed by high-resolution electrospray mass
spectrometry (499.2091 m/z [M-]).
chemical shift of the carborane CH from 4.1 to 1.76 ppm,
the data as a whole suggest that the second species present
in this reaction is the 3,1,2 analogue of 15, i.e., {[Na][3,3,3-
(CO)3-1-C5H4N-3,1,2-closo-ReC2B9H10]} (18).
The reaction conducted at 120 °C contained a 1:3:5
mixture (by 1H NMR integration) of 15, nido-pyridyl
carborane, and the species identified as 18, indicating that
this slight increase in reaction temperature was sufficient to
overcome the activation energy barrier to isomerization such
that a small amount of 18 had been converted to the more
thermodynamically stable product 15. As expected, when the
reaction temperature was increased to 140 °C, the same three
1
compounds were present but in a 5:1:3 ratio, based on H
NMR integrations: that is, the majority of the material
present had isomerized to yield 15.
An analogous variable-temperature study of the reaction
of nido-phenyl carborane with [Re(CO)3(H2O)3]Br yielded
no evidence for the initial formation of the 3,1,2 isomer:
the only species present in the reaction mixtures were nido-
phenyl carborane and 17, indicating that complexation of
the carborane ligand to the [Re(CO)3]+ core and isomeriza-
tion of the carborane cage occur simultaneously. Also, in
contrast to the experiments done with 14, no complex
formation could be detected at temperatures less than 160
°C, while only 50% conversion of the nido-carborane to 17
was observed at 200 °C. By analogy with the pyridyl-
carborane complexes 15 and 18, this temperature is well in
excess of that which is expected to be required for isomer-
ization of the 3,1,2 metallacarborane 16 to occur and explains
why only the 2,1,8 isomer 17 is observed from microwave-
assisted reactions. What remains unclear is the reason for
the drastic difference in the minimum temperature required
for formation of the two isosteric rhenacarboranes 16 and
18 at a constant reaction time of 10 min. A possible
explanation is that the basic pyridyl functionality assists in
Conclusion
The use of a microwave reactor has facilitated an
investigation of 1,2 f 1,7 carborane cage isomerization in
rhenacarborane complexes. For the unsubstituted carborane
2, cage isomerization occurs upon heating at ∼170 °C;
introduction of a CH2-R (R ) Ph, Pyr, Cy) unit increases
Inorganic Chemistry, Vol. 46, No. 6, 2007 2157

Armstrong and Valliant
the isomerization energy barrier such that the 3,1,2 config-
uration is retained. Removal of the methylene spacer
increases steric crowding at the metal center: relief of this
steric strain provides sufficient energy to overcome this
activation energy barrier, and the 2,1,8 complexes are
obtained. Though the mechanism of this process remains
unknown, our observation that the presence of a positively
charged substituent lowers the isomerization energy barrier
significantly may provide a starting point for further empiri-
cally or computationally based investigations into this
phenomenon.
Acknowledgment. X-ray crystallography data collection
and structure solution was carried out by Drs. C. Robertson,
L. E. Harrington, and J. Britten. Financial support of this
work was provided by NSERC (Canada). We thank Ms.
Anika Louie for helpful discussions regarding this research.
1
Supporting Information Available: 1H NMR and H NMR
NOE spectra for 11 and 12; crystallographic cif files for 11-13.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0617543
2158 Inorganic Chemistry, Vol. 46, No. 6, 2007