Mixed-ligand di-mmm-chloro-bridged rhodium dimers as key intermediates in the synthesis of acyclic (ppp-allyl)-closo-rhodacarboranes

Article abstract of DOI:10.1016/j.mencom.2013.07.004

The mixed-ligand di-m-chloro-bridged rhodium dimers [(h²-C ₈H₁₄)(h⁴-2-methylbuta-1,3-diene)Rh(m-Cl)] ₂ and [(h²-C₈H₁₄)(h⁴-2,3- dimethylbuta-1,3-diene)Rh(m-C

Full text of DOI:10.1016/j.mencom.2013.07.004

Mendeleev
Communications
Mendeleev Commun., 2013, 23, 193–195
Mixed-ligand di-m-chloro-bridged rhodium dimers as key intermediates
in the synthesis of acyclic (p-allyl)-closo-rhodacarboranes
Konstantin I. Galkin, Fedor M. Dolgushin, Alexander F. Smol’yakov,
Ivan A. Godovikov, Elena A. Sergeeva and Igor T. Chizhevsky*
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5085; e-mail: chizbor@ineos.ac.ru
DOI: 10.1016/j.mencom.2013.07.004
The mixed-ligand di-m-chloro-bridged rhodium dimers [(h2-C8H14)(h4-2-methylbuta-1,3-diene)Rh(m-Cl)]2 and [(h2-C8H14)(h4-2,3-
dimethylbuta-1,3-diene)Rh(m-Cl)]2 were synthesized and characterized. These compounds are key precursors in the preparative syn-
thesis of the acyclic (p-allyl)-closo-rhodacarborane complexes [3-{p-(R''-allyl)}-1-R-2-R'-closo-3,1,2-RhC2B9H9], which were
obtained earlier via the one-pot metallation of the K+ salts of the [7-R-8-R'-nido-7,8-C2B9H10]– anions with [(h2-C8H14)4Rh2(m-Cl)2] in
the presence of substituted buta-1,3-diene ligands.
Previously, we reported a convenient synthesis of new acyclic
(p-allyl)rhodacarboranes of the general formula [3-{p-(R''-allyl)}-
1-R-2-R'-closo-3,1,2-RhC2B9H9] (where R'' is an aliphatic sub-
stituent).1 These (p-allyl)-closo-rhodacarboranes were prepared
by room-temperature one-pot reactions between the K+ salts
of [7-R-8-R'-nido-7,8-C2B9H10]– anions 1a–c and di-m-chloro-
bridged reagent [(h2-C8H14)4Rh2(m-Cl)2] 2 in the presence of an
excess of aliphatic 1,3-dienes, as exemplified by Scheme 1. The
molecular structures of these p-allyl complexes were determined
by X-ray diffraction analysis, which revealed the presence of a
rare Me···Rh agostic bond in these molecules,1 in accord with
their high stability in both a solid state and solution.
[(h4-diene)RhCl]2, can be isolated in the reaction with 2-methyl-
buta-1,3-diene (isoprene) 3 and 2,3-dimethylbuta-1,3-diene 4
regardless of the ratio between reactants.2 At the same time,
according to published data,3 if complex 2 was used as a starting
material in the displacement reaction with acyclic 1,3-dienes
3 or 4, the formation of electron-deficient mononuclear Rhi
species [(h4-1,3-diene)(h2-cyclooctene)RhCl] might be expected.
We surmised that the latter mixed-ligand complexes could exist
in a dimeric di-m-chloro-bridged form which is stable in both
a solid state and solution.
Indeed, the reaction of 2 with either 3 or 4 reported here
definitely evidences the dimeric mixed-ligand structure of the
products formed, [(h2-C8H14)(h4-2-methylbuta-1,3-diene)RhCl]2
5 and [(h2-C8H14)(h4-2,3-dimethylbuta-1,3-diene)RhCl]2 6, re-
spectively (Scheme 2).† Complex 6 was purified by dissolving in
n-hexane and cooling at –78°C; then, the resulting crystalline
material was recrystallized from CH2Cl2 at –20°C to give single
crystals suitable for a low-temperature X-ray diffraction study.
The molecular structure of 6 is shown in Figure 1.‡ In a crystal,
the molecule of 6 occupies a special position at an inversion
R''
H
H
H
[( 2-C8H14)4Rh2( -Cl)2] 2,
R'
1,3-diene
K+
Rh
R
H
C6H6, 20 °C
R'
R
1a R + R' = -(ortho-xylylene)
1b R = R' = Me
1c R + R' = -(CH2)3
R'' = Alk
Scheme 1
Rh
Apart from this particular synthetic motif, efforts have been
made toward establishing a plausible mechanism by which these
reactions may proceed so as to have efficient access to other
p-allyl rhodium carborane complexes which can be used as
unmodified catalyst precursors for the selective hydroformylation
of alkenes.1 As a result, we were encouraged to find possible
organometallic intermediate complexes which could be easily
formed from starting di-m-chloro-bridged reagent 2 and acyclic
1,3-dienes and, at the same time, would be active metallation
species in reactions with [nido-7,8-R',R''-7,8-C2B9H12]– mono-
anions giving rise to the desired acyclic (p-allyl)-closo-rhoda-
carboranes in good yields.
Evidently, this three-component one-pot method used for the
synthesis of the above (p-allyl)-closo-rhodacarboranes1 is con-
ceptually based on two-step reactions which apparently involve
the displacement of cyclooctene ligands in 2 by a given 1,3-diene
at the first step. It is well known that in the case of alkene–
rhodium dimers, such as [(C2H4)2Rh(m-Cl)]2, only one of two
possible diene–rhodium complexes, either [(h4-diene)2RhCl] or
Cl
2
R
Rh Cl
Cl Rh
2
+
R
hexane, 20 °C
R
3 R = H
4 R = Me
5 R = H
6 R = Me
Scheme 2
Synthesis of the dimeric complexes [(h2-C8H14) ( h4-diene)RhCl]2 5, 6
†
(general procedure). To a slurry of complex 2 in 6 ml of n-hexane (distilled
before use under argon) 1,3-diene 3 or 4 (a threefold molar excess) was
added dropwise with stirring. The resulting mixture was stirred for 1 h at
ambient temperature and then kept overnight at –78°C. Crystalline solids
that formed were collected by filtration and dried in vacuo affording 5 or
6, respectively. Complex 6 can be recrystallized from CH2Cl2 at –20°C to
give single crystals suitable for X-ray diffraction.
© 2013 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2013, 23, 193–195
C(11)
lengths in the 2,3-(dimethyl)butadiene ligand and the Rh–C(78M)
C(12)
C(10)
C(9)
bond length in the cyclooctene ligand [C(12M), C(34M) and
C(78M) are the midpoints of the double bonds in these ligands]
are significantly different being 1.984(1), 1.998(1) and 2.147(1)
Å, respectively. To the best of our knowledge, the only precedent
of crystallographically characterized dimeric di-m-chloro-bridged
five-coordinate Rhi complexes with a comparable molecular
architecture is [(h2,k1-C10H8NBoc)(CO)RhCl]2 (where C10H8NBoc
is N-Boc-azabenzonorbornadiene).4
C(13)
C(14)
C(8)
C(7)
Cl(1A)
Rh(1A)
Rh(1)
Cl(1)
C(1)
C(3)
C(4)
C(2)
Mixed-ligand complexes 5 and 6 are key intermediates in
the formation of agostic closo-(p-allyl)rhodacarboranes species
of these series. It was found that both 5 and 6 react with 1b in
C6H6 at room temperature to give acyclic (p-allyl)-closo-rhoda-
carboranes: [3-{(1-3-h3)-C5H9}-1,2-Me2-closo-3,1,2-RhC2B9H9]
7a,b (two isomers) and [3-{(1-3-h3)-C6H11}-1,2-Me2-closo-3,1,2-
RhC2B9H9] 8, respectively, in high yields. The structures of
complexes 7a,b and 8 were deduced by a comparison of their
1H NMR spectra with those of authentic samples which have been
prepared earlier via the one-pot method1 outlined in Scheme 1.
In addition, the reaction of 6 with 1d (R = Me, R' = Ph) was found
to afford the new p-allyl derivative [3-{(1-3-h3)-C6H11}-1-Me-
2-Ph-closo-3,1,2-RhC2B9H9] isolated in 56% yield (total content)
as an inseparable mixture of two diastereomers 9a,b (Scheme 3).
These isomeric complexes were characterized by elemental analysis
and multinuclear NMR spectroscopy, although the precise stereo-
chemistry of 9a,b cannot be assigned based on these data only.§
C(5)
C(6)
Figure 1 ORTEP representation of the molecular structure of complex 6
with thermal ellipsoids drawn at a 50% probability level. Hydrogen atoms are
omitted for clarity. Selected interatomic distances (Å) and bond angles (°):
Rh(1)–Cl(1) 2.5001(3), Rh(1)–Cl(1A) 2.5098(3), Rh(1)–C(7) 2.250(1),
Rh(1)–C(8) 2.260(1), Rh(1)–C(1) 2.063(1), Rh(1)–C(2) 2.159(1), Rh(1)–C(3)
2.164(1), Rh(1)–C(4) 2.085(1), C(1)–C(2) 1.447(2), C(2)–C(3) 1.417(2),
C(3)–C(4) 1.445(2), C(7)–C(8) 1.381(2), C(8)–C(9) 1.510(2), C(7)–C(14)
1.504(2), Rh(1)···Rh(1A) 3.6392(2); Cl(1)–Rh(1)–Cl(1A) 86.83(1), Rh(1)–
Cl(1)–Rh(1A) 93.17(1).
centre, thus having a centrosymmetric structure. Two crystallo-
graphically equivalent [(h2-C8H14)(h4-C6H10)RhCl] fragments
in 6 are linked together by two symmetrical m-chloro bridges
[Rh–Cl, 2.5001(3) and 2.5098(3) Å], while the butadiene and
cyclooctene ligands are in trans position to each other. Each
rhodium atom in 6 is bound to one 2,3-dimethylbuta-1,3-diene
ligand, which acts as an h4-coordinating ligand, and one weakly
coordinated cyclooctene ligand. Of particular interest is the
coordination geometry of the rhodium atoms, as determined in
structure 6. Assuming that each m-chloro ligand occupies only
one coordination site and the Rh–Rh bond in 6 is excluded
[Rh···Rh, 3.6392(2) Å], the rhodium atoms in this complex have
a coordination number of five. The Rh–(m-Cl2)–Rh four-mem-
bered ring is planar with the Rh(m-Cl)2 angle of 86.83(1)°.
Although the cyclooctene ligand in 6 is located rather far from
the rhodium centre, it still remains coordinated to each of the
rhodium atoms. Note that the Rh–C(12M) and Rh–C(34M) bond
Hsyn
Hanti
Hsyn
Hanti
CH2
Me
Me
CH2
H
H
Ph
Rh
Rh
H
6
Me
Me
K+
+
Me
Ph
Me
Me
Ph
C6H6,
20 °C
1d
9a
9b
Scheme 3
§
Diastereomeric complexes [3-{(1-3-h3)-C6H11}-1-Me-2-Ph-closo-3,1,2-
RhC2B9H9] 9a,b. To a solution of 6 (65 mg, 0.1 mmol) in 4 ml of degassed
benzene, K salt 1d7 (66 mg, 0.25 mmol) was added in one portion as a
solid. After 2 h of vigorous stirring, the resulting red solution was treated
by column chromatography on silica gel, eluting a colored fraction with
a mixture of CH2Cl2–hexane (1:2). After the removal of the solvent in
vacuo and recrystallization of the residue from a mixture of CH2Cl2–
hexane, 43 mg (53%) of isomeric complexes 9a,b were obtained as a light
red amorphous solid.
For 5: yellow microcrystals, 51% yield. 1H NMR (400.13 MHz, CD2Cl2)
d: 5.50 (m, 4H, 1,2-CH, COE), 4.97 [br.t, 2H, H(3), Jt 7.3 Hz], 2.39
[br.s, 2H, H(1)syn], 2.35 [d, 2H, H(4)syn, Jvic 5.4 Hz], 2.16 (br.s, 8H,
3,8-CH2, COE), 2.06 (s, 6H, Me), 1.54 (br.s, 16H, 4,5,6,7-CH2, COE),
0.48 [br.s, 2H, H(1)anti], 0.43 [d, 2H, H(4)anti, Jvic 9.0 Hz]. 13C{1H} NMR
(100.61 MHz, CD2Cl2) d: 122.0 (1,2-CH, COE), 101.4 [C(2)], 85.6 [C(3)],
39.5 [C(1)], 36.9 [C(4)], 30.5, 27.1 (4,5,6,7-CH2, COE), 27.0 (3,8-CH2,
COE), 22.0 (Me). Found (%): C, 48.91; H, 6.88; Cl, 11.33. Calc. for
C26H44Cl2Rh2 (%): C, 49.31; H, 7.00; Cl, 11.20.
For 9a,b: IR (KBr, n/cm–1): 2548 (nBH). 1H NMR (600.22 MHz,
CD2Cl2; 9a:9b = 1:2.3*) d: 7.84* (dd, 2H, Ph, J1 7.7 Hz, J2 1.9 Hz), 7.51
For 6: yellow microcrystals, 65% yield. 1H NMR (400.13 MHz, CD2Cl2)
d: 5.52 (m, 4H, 1,2-CH, COE), 2.21 [br.s, 4H, H(1,4)syn], 2.15 (br.s, 8H,
3,8-CH2, COE), 1.97 [s, 12H, 2,3-Me], 1.54 (br.s, 16H, 4,5,6,7-CH2, COE),
0.24 [br.s, 4H, H(1,4)anti]. 13C{1H} NMR (150.93 MHz, CD2Cl2) d: 121.2
(1,2-CH, COE), 96.1 [C(2,3)], 37.5 [C(1,4)], 30.5, 27.1 (4,5,6,7-CH2,
COE), 27.0 (3,8-CH2, COE), 18.8 (2,3-Me). Found (%): C, 49.35; H, 6.85;
Cl, 11.56. Calc. for C28H48Cl2Rh2 (%): C, 50.83; H, 7.26; Cl, 10.74.
(d, 2H, Ph, J 7.4 Hz), 7.36–7.26 (m, 6H, Ph, Ph*), 4.25* [d, 1H, H(3')syn,
Jgem 1.8 Hz], 3.98 [d, 1H, H(3')syn, Jgem 2.2 Hz], 2.86 (s, 3H, Mecarb), 2.63
[br.s, 1H, H(3')anti], 2.55* [br.s, 1H, H(3')anti], 2.39* (s, 3H, Mecarb),
2.16* (s, 3H, Mesyn), 2.00 (s, 3H, Mesyn), 1.70* (d, 3H, 2'-Me, J 2.2 Hz),
1.67 (d, 3H, 2'-Me, J 2.0 Hz), 0.00* [s, 3H, Meanti (agost.)], –0.33 [s, 3H,
Meanti (agost.)]. 13C{1H} NMR (150.93 MHz, CD2Cl2, J
Rh) d: 147.0,
13C,103
144.1, 129.8, 129.5, 129.2, 128.9, 128.4, 127.4 (CAr, C* ), 117.9 [d,
Ar
‡
Crystal data for 6: C28H48Cl2Rh2, M = 661.38, orthorhombic, space
C(2'), J 5.4 Hz], 117.9* [d, C(2'), J 5.5 Hz], 116.1, 111.4 (Ccarb), 109.9*,
104.3* (Ccarb), 97.9* [d, C(1'), J 5.8 Hz], 96.9 [d, C(1'), J 5.9 Hz], 61.2
[d, C(3'), J 8.9 Hz], 60.3* [d, C(3'), J 9.3 Hz], 36.7 (Mecarb), 32.6*
group Pbca, a = 10.5900(4), b = 14.2438(5) and c = 18.1388(6) Å, V =
= 2736.1(2)Å3, dcalc = 1.606 g cm–3, Z = 4, MoKa radiation (l = 0.71073Å),
m = 1.416 mm–1, T = 100(2) K, 2qmax = 60°, R1 = 0.0169 for 3702 reflec-
tions with I > 2s(I), and wR2 = 0.0395 for all 3965 unique reflections
(Rint = 0.0296). The SHELXTL program package5 was used for the calcu-
lations.
CCDC 921336 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
For details, see ‘Notice to Authors’, Mendeleev Commun., Issue 1, 2013.
(Mecarb), 21.5* (Mesyn), 20.4* (2'-Me, 2'-Me*), 20.2 [Me* (agost.), Mesyn],
anti
20.1 [Meanti (agost.)]. 11B NMR (193 MHz, CD2Cl2, J11B,1H) d: 20.1 (d,
J 147 Hz), 14.5 (d, J 125 Hz), 13.7 (d, J 147 Hz), 10.0 (d, J 144 Hz), 4.1 (d,
J 140 Hz), 3.3 (d, J 166 Hz), 2.4 (d, J 171 Hz), –1.3 (d, J 166 Hz), –2.2
(d, J 175 Hz), –3.1 (d, J 160 Hz), –3.9 (d, J 152 Hz), –4.7 (d, J 160 Hz), –5.6
(d, J 196 Hz), –6.5 (d, J 140 Hz), –18.7 (d, J 152 Hz), –20.0 (d, J 153 Hz).
Found (%): C, 43.71; H, 7.20; B, 22.83. Calc. for C15H28B9Rh (%): C, 44.09;
H, 6.91; B, 23.81.
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Mendeleev Commun., 2013, 23, 193–195
Nido-dicarbaundecaborate salts are known to display the acidic
the endo-deuteration of the (1-3-h3)-cyclooctenyl ligand at the
C(4)–C(8) carbon atoms had occurred during its prepara-
tion from [Cs][10-endo-D-7,8-(4'-MeC6H4)2-nido-7,8-C2B9H9].6
Further experiments to establish the mechanism of formation of
the acyclic (p-allyl)-closo-rhodacarboranes are currently under-
way in our laboratory.
character of the endo-hydrogen atom which is located over the
pentagonal C2B3-open face. Taking this into account, we propose
two alternative reaction sequences leading to the formation of
the (p-allyl)-closo-rhodacarborane complexes. The first involves
the initial attack of the endo-hydrogen either on the metal centre
of above intermediate complexes 5 or 6 followed by the addition of
metal hydride to one of the double bonds of the acyclic h4-diene
ligand or directly to a double bond of the diene ligand giving
rise to an unsymmetrical h3-allyllic unit. According to this, the
dianionic {nido-7-R-8-R'-C2B9}2– species generated in situ after
the elimination of the endo-hydrogen atom and potassium chloride
can undergo relatively fast metallation through an open face of
the cage ligand with (p-allyl)rhodium species ultimately forming
the agostic (p-allyl)-closo-rhodacarborane products.
This work was supported in part by the Russian Foundation
for Basic Research (grant no. 12-03-00102).
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Alternatively, these reactions can proceed through the initial
formation of exo-nido species, namely, either [exo-{Rh(h4-diene)-
(h2-C8H14)}-nido-7-R-8-R'-C2B9H10] or [exo-{Rh(h4-diene)}-
nido-7-R-8-R'-C2B9H10], which then undergoes intramolecular
attack by acidic nido-carborane endo-hydrogen on the metal
centre and subsequent exo-nido-to-closo rearrangement of the
diene–hydride intermediate. Hydride transfer from the metal
atom to the acyclic h4-diene ligand, similar to the above pathway,
would finally result in the desired (p-allyl)rhodium-carborane
complexes.
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Both of the pathways are consistent with our earlier observation
that, in [3-{(1-3-h3)-C8H8D5}-1,2-(4'-MeC6H4)2-pseudocloso-
3,1,2-RhC2B9H9], the species closely related to 7–9, mostly
Received: 6th March 2013; Com. 13/4080
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