Homoleptic hexa and penta gallylene coordinated complexes of molybdenum and rhodium

Article abstract of DOI:10.1021/ic200699f

The reactions of molybdenum(0) and rhodium(I) olefin containing starting materials with the carbenoid group 13 metal ligator ligand GaR (R = Cp*, DDP; Cp* = pentamethylcyclopentadienyl, DDP = HC(CMeNC₆H ₃-2,6-ⁱPr₂

Full text of DOI:10.1021/ic200699f

ARTICLE
pubs.acs.org/IC
Homoleptic Hexa and Penta Gallylene Coordinated Complexes of
Molybdenum and Rhodium†
Timo Bollermann, Thomas Cadenbach, Christian Gemel, Kerstin Freitag, Mariusz Molon, Vanessa Gwildies,
and Roland A. Fischer*
Anorganische Chemie II - Organometallics & Materials, Ruhr-Universit€at Bochum, D-44780 Bochum, Germany
S Supporting Information
b
ABSTRACT: The reactions of molybdenum(0) and rhodium-
(I) olefin containing starting materials with the carbenoid group
13 metal ligator ligand GaR (R = Cp*, DDP; Cp* = penta-
methylcyclopentadienyl, DDP = HC(CMeNC6H3-2,6-iPr2)2)
were investigated and compared. Treatment of [Mo(η4-
butadiene)3] with GaCp* under hydrogen atmosphere at
100 °C yields the homoleptic, hexa coordinated, and sterically
crowded complex [Mo(GaCp*)6] (1) in good yields g50%.
Compound 1 exhibits an unusual and high coordinated octahe-
dral [MoGa6] core. Similarly, [Rh(GaCp*)5][CF3SO3] (2) and
[Rh(GaCp*)5][BArF] (3) (BArF = B{C6H3(CF3)2}4) are pre-
pared by the reaction of GaCp* with the rhodium(I) compound
[Rh(coe)2(CF3SO3)]2 (coe = cyclooctene) and subsequent anion exchange in case of 3. Compound 2 features a trigonal
bipyramidal [RhGa5] unit. In contrast, reaction of excess Ga(DDP) with [Rh(coe)2(CF3SO3)]2 does not result in a high
coordinated homoleptic complex but instead yields [(coe)(toluene)Rh{Ga(DDP)}(CF3SO3)] (4). The common feature of 2 and
4 in the solid state structure is the presence of short CF3SO2O Ga contacts involving the GaCp* or rather the Ga(DDP) ligand.
3 3 3
Compounds 1, 2, and 4 have been fully characterized by single crystal X-ray diffraction, variable temperature 1H and 13C NMR
spectroscopy, IR spectroscopy, mass spectrometry, as well as elemental analysis.
’ INTRODUCTION
Important synthetic procedures for the preparation of transi-
tion metal complexes of ER (R = bulky organic group) include
ligand substitution reactions at [LnM] using ER compounds as
reagents. Many binary carbonyls (L = CO) give substitution
products on reaction with ER under liberation of CO, for
example, [Fe2(CO)9] or [Co2(CO)8].12 In particular, GaCp*
reacts with heteroleptic olefin containing carbonyl complexes,
for instance [(nbd)Mo(CO)4] or [(cht)Fe(CO)3] (nbd = 2,
5-norbornadiene, cht = cycloheptatriene), to give [(GaCp*)2-
Mo(CO)4] and [Fe2(GaCp*)3(CO)6], respectively.12 However,
the successive substitution of CO (or other strong acceptor
ligands) is limited by the coordination of GaCp*. The strong
σ-donor abilities of GaCp* increase the overall electron density
of the transition metal on coordination and thus lead to stronger
π-back bonding of the remaining CO ligands, which then cannot
be substituted anymore. Therefore, homoleptic GaCp* containing
complexes can only be obtained by substitution of labile and very
weak π-acceptor ligands such as ethene or acetonitrile. The same
situation is observed for other ER ligands. Thus, [Ni(GaCp*)4],13
the analogue to [Ni(CO)4], is readily available from [Ni(cod)2]
(cod = 1,5-cyclooctadiene) and likewise [M(GaCp*)4]14
Low valent group 13 metal ER species (E = Al, Ga, In; R =
bulky, mono anionic substituents; i.e., Cp* = pentamethylcyclo-
pentadienyl; alkyl, aryl, dialkylamide, β-diketiminate, guanidi-
nate, amidinate, or halide) have emerged as interesting carbenoid
ligands in coordination chemistry exhibiting metal-atom ligators.1ꢀ9
Comparison of the ligand properties of GaCp* and Ga(DDP) is
quite representative for the whole family of ER ligands. The steric
and electronic situations differ characteristically in both cases.
While in GaCp* the sp hybridized Ga(I) center is coordinated by
the rather flexible, comparably soft Cp* group, showing facile
haptotropic shifts, in Ga(DDP) on the contrary, the sp2 hybri-
dized Ga(I) is stabilized by a coordinatively rigid, and electro-
nically hard β-diketiminate. In general, ER ligands are more or
less isolobal to CO, PR3, and N-heterocyclic carbenes, that is,
they exhibit very strong covalent σ-donation as well as typically
weak π-accepting properties and substantial electrostatic (ionic)
contributions to the overall bonding interaction.10,11 While
in GaCp* the two vacant p-orbitals are quite populated by the
π-donating Cp* group, this effect is less pronounced in case of
Ga(DDP), creating a more Lewis acidic gallium center. In both
cases, however, the (residual) Lewis acidity at the gallium is
usually increased on coordination of the gallium to a transition
metal center.
Received: April 6, 2011
Published: May 17, 2011
r
2011 American Chemical Society
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(M = Pd, Pt) can be prepared from [Pt(cod)2] and
[Pd(tmeda)(CH3)2] (tmeda = tetramethylethylenediamine).
The particular bifunctional feature of the ER ligands in
contrast to their isolobal C or P ligator analogues mentioned
above is the combination of reduction (insertion) and coordina-
tion properties which is nicely shown by the synthesis of
[Pd(GaCp*)4] from [Pd(CH3)2(tmeda)] and 5 equiv of GaCp*
with [Cp*Ga(CH3)2] as the stoichometric byproduct. The iso-
electronic cationic congeners [M(GaCp*)4]þ (M = Cu, Ag)
could also be obtained by such type of reactions.15 Also, oligo-
nuclear complexes [Pd3(AlCp*)6],16 [PdM(GaCp*)5] (M = Pd,
Pt),14,16 [Pd3(InCp*)8],14,16 and group 11 metal dimers
Scheme 1. Synthesis of Compound 1
Complex 1 crystallizes in the triclinic space group P1. Most
interestingly, the molybdenum center of the metal core [MoGa6]
is surrounded by six GaCp* ligands in an almost perfect
octahedral fashion as determined by the continuous shape
measure SQ(P) = 0.195 (Figure 1).26,27 Only small deviations
from the perfect octahedral coordination environment can be
observed with respect to the bond angles. For a more detailed
structural description of 1 see the Supporting Information of our
previous communication.25 Achieving such a high coordination
number around the transition metal center is only possible
because of asymmetrically coordinated Cp* units to the gallium
centers and therefore haptotropic shifts resulting in coordination
modes from η1-(Ga4) to η5-(Ga1, Ga2, Ga3, Ga5, Ga6). Beside,
the shortened MoꢀGa bond distances in 1 compared to
[fac-(Cp*Ga)3Mo(CO)3]28 and [cis-Mo(Cp*Ga)2(CO)4],13 the
average η5-Cp*centroidꢀGa distance (2.069 Å) is somewhat shor-
tened compared to the free ligand (2.081 Å; monomer in the gas
phase).29 This shortening is an indirect measure for the polarity of
the MoꢀGa bond, an indication for the strong σ-donor property of
the Cp*Ga fragment and a measure of the increased electrophilicity
at the Ga center upon coordination to Mo.
[Cu2(GaCp*)(μ-GaCp*)3Ga(CF3SO3)3]17 and [M2(GaCp*)3
15,17
(μ-GaCp*)2][CF3SO3]2
(M = Cu, Ag) have been isolated
under optimized conditions and reaction stoichiometries with the
right choices of the transition metal precursors.
Despite the wealth of work done to coordinate ER to metal
centers across the periodic table, and particularly in contrast to
the d10 metal complexes [M(ER)4], corresponding to [M(CO)4],
the homoleptic, mononuclear complexes [M(ER)n] (n > 4),
which should be analogues to the classic penta- or hexacarbonyl
complexes, [M(GaCp*)5] (Fe, Ru, Os) and [M(GaCp*)6] (M =
Cr, Mo, W), have not been reported, so far.18ꢀ21 In all known
cases, olefins could not be fully substituted from [M(olefin)x] or
[M(olefin)x(PR3)y] starting materials and heteroleptic products
[LnM(GaCp*)m] (L = olefin, PR3) were isolated instead.22 In
fact, no homoleptic complexes of the general formula [Ma(ER)b]
have been reported for coordination numbers greater than four.
However, we like to note two exceptions at this point of the
introduction. First, the complexes [M(AlCp*)5] (M = Fe, Ru)
exhibit penta coordinated [MAl5] cores, but the actual structures
are CꢀH activated isomers.23 Second, the cation [Rh(GaCp*)4-
(GaCH3)]þ has been described and represents a formal analogue
to [Rh(CO)5]þ. However, this complex with a [RhGa5] core is
not fully homoleptic, and the synthesis is quite special.24 Never-
theless, these examples demonstrate, that homoleptic, closed shell
18 electron complexes [M(ER)n] with n > 4 are a valid target, at
least for R = Cp*.
We have already communicated the complex [Mo(GaCp*)6]
(1) as the crucial starting material for the synthesis of the
unprecedented zinc-rich compound [Mo(ZnCp*)3(ZnCH3)9].25
Below we like to highlight and discuss the synthesis, structural
and spectroscopic properties of [Mo(GaCp*)6] in detail on its
own right, as it represents the important and still only example of
a homoleptic, hexa gallylene coordinated transition metal com-
plex. In addition, we will report the related penta gallylene
coordinated [Rh(GaCp*)5]þ and its characterization as the second
fully homoleptic example of the targeted family of transition
metal complexes [M(ECp*)n] (n > 4). The compounds will be
put into the context of the different reaction behavior and ligand
properties of GaCp* and Ga(DDP).
The 1H NMR (one signal at 1.96 ppm) as well as the routine
13C{1H} NMR spectrum of pure 1 in C6D6 at room temperature
reveal the expected signals for the Cp* unit.25 Noteworthy, the
high fluctuation of the Cp* groups around the six gallium atoms
in the ligand sphere is still present even at ꢀ78 °C (low
1
temperature H NMR measurements in d8-toluene). The ab-
sorptions in the FTIR spectrum of 1 assigned to the Cp* group at
2943, 2877 as well as 2829 cmꢀ1 are in the typical range and are
quite comparable with Cp* absorptions in the first homoleptic
GaCp* containing compound [Ni(GaCp*)4] (2958, 2906, and
2855 cmꢀ1).13 The ν(MoꢀGa) vibrations are expected in the
near IR region (below 200 cmꢀ1) and were outside the range of
measurement. At this point we like to highlight an important
breakthrough in the mass spectrometric characterization of metal-
rich molecules. Quite a number of attempts to obtain meaningful
mass spectrometric data on compounds [Ma(ECp*)b] using stan-
dard techniques for sample transfer and ionization failed. How-
ever, by using liquid injection field desorption ionization mass
spectrometry (LIFDI-MS) and a special configuration of the
ionization chamber gave excellent results. In the ideal case, this
method allows the identification of the molecular ion peak [M].þ
of such compounds. Herein, the molecular ion peak [M].þ of
1 at m/z = 1326 as well as one fragment signal at m/z = 1190
for [M ꢀ Cp*]þ, could be nicely detected (see Supporting
Information).
’ RESULTS AND DISCUSSION
Reaction of [Mo(η4-butadiene)3] with excess GaCp* in
toluene under 3 bar hydrogen atmosphere at 100 °C yields
[Mo(GaCp*)6] (1) in reproducible yields around 50% as an
orange powder with n-butane as the byproduct (Scheme 1).25
Single crystals of 1 being suitable for X-ray diffraction studies
can be obtained by heating up a saturated mesitylene solution
and slowly cooling it to ꢀ30 °C over several days. The result of
the structure solution and refinement is shown in Figure 1.
Our related investigations on the coordination chemistry of
GaCp*and Ga(DDP) toward d6 metal centers, however, gave no
equally satisfying preparative or spectroscopic results similar to 1.
For instance, use of bulky Ga(DDP) instead of GaCp* according
to Scheme 1 does not lead to any specific product formation like
partial substitution of butadiene and coordination of Ga(DDP)
toward the Mo center. The analogous reaction of [W(η4-
butadiene)3] failed too, and no pure products could be isolated.
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Figure 1. Molecular structure of [Mo(GaCp*)6] (1) in the solid state as determined by single crystal X-ray diffraction (left) and superimposition of the
ideal (black) and experimental (gray) polyhedra (right). The displacement ellipsoids are shown at 50% probability level, hydrogen atoms are omitted for
clarity. Selected interatomic distances (Å) and angles (deg): Mo1ꢀGa4 2.384(1), Mo1ꢀGa1 2.458(1), Mo1ꢀGa5 2.474(1), Mo1ꢀGa3 2.476(1),
Mo1ꢀGa6 2.485(1), Mo1ꢀGa2 2.493(1), Ga4ꢀC31 2.088(4), Ga1ꢀCp*centroid 2.127, Ga2ꢀCp*centroid 2.091, Ga3ꢀCp*centroid 2.040, Ga5ꢀCp*centroid
2.057, Ga6ꢀCp*centroid 2.024, Ga4ꢀMo1ꢀGa1 87.92(2), Ga4ꢀMo1ꢀGa5 88.77(2), Ga4ꢀMo1ꢀGa3 85.12(2), Ga1ꢀMo1ꢀGa5 175.22(3),
Ga4ꢀMo1ꢀGa6 174.35(3), Ga3ꢀMo1ꢀGa2 176.39(3), Ga1ꢀMo1ꢀGa3 90.39(2), Ga5ꢀMo1ꢀGa3 92.77(2), Ga1ꢀMo1ꢀGa6 91.15(2),
Ga5ꢀMo1ꢀGa6 92.49(2), C31ꢀGa4ꢀMo1 168.01(12), Ga3ꢀMo1ꢀGa6 89.31(2), Ga4ꢀMo1ꢀGa2 91.32(2), MoꢀGa1ꢀCp*centroid 175.21,
MoꢀGa2ꢀCp*centroid 167.42, MoꢀGa3ꢀCp*centroid 170.30, MoꢀGa5ꢀCp*centroid 167.36, MoꢀGa6ꢀCp*centroid 166.04.
Scheme 2. Synthesis of Compounds 2 and 4
This difference is explained by the much stronger bonded
butadiene in the tungsten case which has been first described
by Gausing and Wilke who carried out substitution reactions of
CO and cot (cot = cyclooctatetraene) using [M(η4-butadiene)3]
(M = Mo, W).30 Herein, variation of the reaction conditions
using higher hydrogen or high temperatures give any substitution
of the olefin from the tungsten metal. Furthermore, the chro-
mium homologue of 1 was targeted by the reaction of [Cr-
(C10H8)2]31 with GaCp*. However, no reasonable product
formation was seen in several NMR spectroscopic experiments
under a wide variety of reaction conditions. Consequently, a
different synthetic approach has to be developed to yield the Cr
and W homologues of 1, at least by using much more substitu-
tion labile starting Cr and W compounds.
rhodium(I) dimer [Rh(coe)2(CF3SO3)]2 with stoichiometric
amounts of 10 equiv of GaCp* in fluorobenzene at 60 °C results
in the formation of [Rh(GaCp*)5][CF3SO3] (2) in very good
yields g80%.
Compound 2 dissolves well in polar solvents like fluoroben-
zene, dichloromethane, or tetrahydrofuran (thf) and is stable for
several weeks when stored under an inert gas atmosphere at
1
ꢀ30 °C. As expected, the H NMR spectrum of 2 in CD2Cl2
shows one signal at 1.91 ppm for the equivalent ring protons of
the GaCp* ligand, again indicating fluxional processes. First, a fast
exchange of the CF3SO3 group between the Ga centers has to be
taken in to account and second, the exchange of equatorial and
axial positions in the [RhGa5] core (as it is quite typical for penta
coordinated species). Likewise for 1, the fluctuation of the Cp*
groups can be observed up to ꢀ78 °C determined by low
Accordingly and on the basis of the existence of the high
coordinated complexes 1 and [Rh(GaCp*)4(GaCH3)][BArF],
we then targeted the isolation of the corresponding, fully
homoleptic cation [Rh(GaCp*)5]þ stabilized in the form of
an appropriate salt. As shown in Scheme 2, treatment of the
1
temperature H NMR measurements. The routine 13C{1H}
NMR spectrum illustrates no unusual features and gives rise to
two signals at 11.09 (GaC5Me5) and 117.21 (GaC5Me5) ppm.
Note that the carbon atom of the CF3SO3 group was not detected
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in the 13C NMR spectrum under the standard measurement
conditions. The 19F NMR spectrum shows one signal in the
expected range at ꢀ78.7 ppm. In contrast to compound 1, the
molecular peak of 2 could not be detected by LIFDI mass
spectrometry, rather facile loss of {(Cp*)2Ga(CF3SO3)} occurs
and results in the detection of the fragment [M ꢀ {(Cp*)2-
Ga(CF3SO3)}]þ (m/z = 788). This observation of the species
[Rh(GaCp*)3(Ga)]þ under mass spectrometric conditions,
however, agrees nicely with the fluctuating behavior of 1 and 2
and the known ligand property of naked Gaþ together with the
leaving group properties of the neutral gallium(III) molecule
(Cp*)2Ga(CF3SO3).32 Furthermore, the rhodocenium cation
[RhCp*2]þ (m/z = 373) was detected in the mass spectrum as a
characteristic fragmentation and rearrangement product of 2.
FTIR spectroscopic results of 2 display the expected absorptions
for the Cp* groups at 2938, 2885, and 2834 cmꢀ1 comparable to
[Mo(GaCp*)6] as well as [Ni(GaCp*)4]. IR spectroscopy is a
popular but often ambiguous method to distinguish between
(contact) ion pair and covalent bonding modes of the triflate
anion acting as a weak nucleophile. Herein, the characteristic
absorption of ionic existing CF3SO3ꢀ is near 1280 cmꢀ1 and is
shifted to higher wave numbers around 1380 cmꢀ1 for more
covalent character.33 The IR spectrum of 2 shows a strong signal
at 1219 cmꢀ1 and weak absorptions at 1295 and 1373 cmꢀ1
which could confirm a more ionic than covalent situation in
compound 2. Deep red needles of 2 suitable for X-ray measure-
ments were obtained by slow diffusion of n-hexane into a satu-
rated fluorobenzene solution at room temperature over several
days. Compound 2 crystallizes in the triclinic space group P1.
The coordination geometry can be best described as a distorted
trigonal bipyramid (Figure 2). Caused by steric overcrowding,
the trigonal bipyramidal coordination geometry of the [RhGa5]
core is strongly distorted. A similar, yet distinctly weaker distor-
tion has also been found in [Rh(GaCp*)4(GaCH3Cp*)] and
[Rh(GaCp*)4(GaCH3)][BArF] (BArF = B{C6H3(CF3)2}4).24
The deviation from the ideal trigonal bipyramid is reflected in the
Ga5ꢀRh1ꢀGa2 angle (154.297(18)°) with a pronounced de-
viation from linearity. The equatorial angles also strongly deviate
from the ideal 120° angles and have values of 106.604(16)°
(Ga4ꢀRh1ꢀGa3), 143.643(17)° (Ga1ꢀRh1ꢀGa3) and
109.655(16)° (Ga1ꢀRh1ꢀGa4). The angles between equatorial
and axial ligands (ideally 90°) show values between 79.725(15)°
(Ga1ꢀRh1ꢀGa2) and 100.423(16)° (Ga5ꢀRh1ꢀGa4). The
RhꢀGa distances (average 2.3572 Å) are significantly elongated
with respect to the average RhꢀGa distance in [Rh(GaCp*)4-
(GaCH3)][BArF]24 (Ø 2.2957 Å) but comparable to other Rh-
GaCp* compounds, which show RhꢀGa distances between
2.3292(15) Å in [Cp*Rh(GaCp*)(CH3)2]34 and an average
value of 2.408 Å for [{Rh(η2,η2-nbd)(PCy3)(GaCp*)2}{BArF}]22
(nbd = 2,5-norbornadiene). With exception of the Cp* group
coordinated to Ga1, all Cp* ligands are η5 coordinated to the
gallium atoms with an average bond distance of 1.991 Å, com-
parable with free GaCp* (2.081(5) Å, monomer in the gas
phase).29 This value for GaꢀCp*centroid of 2 clearly reflects
the enhanced electrophilic character of the coordinated Ga
centers as compared with 1.25,29 The η1 coordination mode of
the Cp* moiety of Ga1 (Ga1ꢀC5 bond distance 2.055(3) Å)
is obviously a result of steric overcrowding at Rh and the
presence of a weak donorꢀacceptor interaction between Ga1
and the CF3SO3 anion. The Ga1ꢀO1 distance of 2.079(2) Å
is well comparable to the GaꢀO distances in the RGa-OSO2-
CF3 fragments (R = organic substituent) of, for example,
Figure 2. Molecular structure of 2 in the solid state as determined by
single crystal X-ray diffraction; displacement ellipsoids are shown at 50%
probability level, hydrogen atoms are omitted for clarity. Selected
interatomic distances (Å) and angles (deg): Rh1ꢀGa1 2.3326(4),
Rh1ꢀGa2 2.3393(4), Rh1ꢀGa3 2.3799(4), Rh1ꢀGa4 2.3800(4),
Rh1ꢀGa5 2.3542(4), Ga2ꢀCp*centroid 1.963, Ga3ꢀCp*centroid 1.996,
Ga4ꢀCp*centroid 2.039, Ga5ꢀCp*centroid 1.964, Ga1ꢀC5 2.055(3),
Ga1ꢀO1 2.079(2), Rh1ꢀGa2ꢀCp*centroid 171.87, Rh1ꢀGa3ꢀCp*centroid
166.79, Rh1ꢀGa4ꢀCp*centroid 161.73, Rh1ꢀGa5ꢀCp*centroid 156.29,
O1ꢀGa1ꢀRh1 115.94(7), O1ꢀGa1ꢀC5 92.74(12), C5ꢀGa1ꢀRh1
148.82(10), Ga5ꢀRh1ꢀGa2 154.297(18), Ga4ꢀRh1ꢀGa3 106.604(16),
Ga2ꢀRh1ꢀGa3 89.454(16), Ga5ꢀRh1ꢀGa4 100.423(16), Ga1ꢀRh1ꢀ
Ga3 143.643(17), Ga1ꢀRh1ꢀGa4 109.655(16), Ga1ꢀRh1ꢀGa5
83.584(15), Ga1ꢀRh1ꢀGa2 79.725(15).
Scheme 3. Synthesis of Compound 3
[Ga(thf)4H(CF3SO3)2][Ga(thf)2(CF3SO3)4]35 (thf = tetra-
hydofuran, GaꢀO = 1.947(9)ꢀ1.988(9) Å), [{2,6-(Me2NCH2)2-
C6H3}Ga(CF3SO3)2H][CF3SO3]36 (GaꢀO = 1.939(8)ꢀ1.962-
(9) Å), [{(CF3SO3)Bi(GaDDP)}2]37 (GaꢀO = 2.027(1) Å)
or [(TPP)Ga(CF3SO3).C7H8]38 (TPP = tetraphenylporphyrin,
GaꢀO = 1.963 Å). It should be mentioned that coordination of
the anion at Lewis acidic gallium centers is not observed in the
compound [Rh(GaCp*)4(GaCH3)][BArF] because of the much
weaker nucleophilic character of the BArF anion. We were thus led
to treat compound 2 with a slight excess NaBArF in CH2Cl2 at room
temperature which results in the formation of [Rh(GaCp*)5]-
[BArF] (3) as a micorcrystalline orange powder which is stable
under inert atmosphere at ꢀ30 °C for several weeks (Scheme 3).
Surprisingly, once compound 3 is isolated in pure solid form, it
is very labile when dissolved again in common polar solvents like
CH2Cl2 and thf. The first signs of decomposition are detected
during NMR measurements within 2 h resulting in elimination of
Cp*H and a series of further not assigned broad peaks in the Cp*
area. The initial deep red solution slowly turns yellow, and some
metallic precipitate is observed. Even at low temperatures around
ꢀ30 °C, crystallization of the pure compound 3 was not successful
and always gives rise to a yellow oil and metal deposition in the
Schlenk flask. However, quick preparation of fresh solutions in
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CD2Cl2 and immediate NMR measurements showed very
1
dominant signals in the H NMR spectrum at 1.91 ppm (s,
75H) for the Cp* groups as well as broad signals at 7.65 (br, 4H)
and 7.72 (br, 8H) ppm for the BArF anion. Low temperature 1H
NMR measurements gave results very similar to compounds 1
and 2. Thus, splitting of the initial one Cp* signal is not observed
at ꢀ78 °C, as it is the case for 1 and 2. The 19F NMR spectrum
shows one sharp signal at ꢀ62.9 ppm for the CF3 groups. 11B
NMR measurement displays one peak at ꢀ6.6 ppm in the
expected range. As discussed above, decomposition of 3 takes
place in solution after a few minutes. The 13C NMR measure-
ment has been done only with a minimum number of scans to
obtain signals for the pure product. Herein, peaks at 11.12
(GaC5Me5) and 117.45 (GaC5Me5) ppm for the GaCp* ligand
as well as signals for the BArF anion at 117.88 (s, para CH),
125.03 (q, ꢀCF3, J = 272.3 Hz,) 128.99 (s, ipso to the CF3 units)
and 135.23 ppm (s, ortho CH) were detected. The signal for the
carbon atom ipso to boron could not be observed because of low
scan number measurement as suggested above and also reported
in the literature for different BArF containing structures in the
past.39 Unfortunately, the LIFDI mass spectrum did neither
display the molecular ion peak [M].þ nor the characteristic
fragment [Rh(GaCp*)3(Ga)]þ as this was seen for 2, but rather
the rhodocenium cation [RhCp*2]þ (m/z = 373) as the domi-
nant peak. FTIR absorptions of 3 were detected at 2956, 2895,
and 2836 cmꢀ1 characteristic for the Cp* units as well as one
strong peak at 1264 cmꢀ1 for the CꢀF vibration. This distinctly
different behavior of 2 and 3 with respect to stability of the
[Rh(GaCp*)5]þ unit in solution and the Cp* transfer to Rh in
case of 2 and 3 in the course of redox reactions, that is,
Rh(I)fRh(III) and Ga(I)fGa(0), is quite puzzling and may
need further investigation based on density functional theory.
Also note, that the closely related cation [Rh(GaCp*)4(Ga-
CH3)]þ is perfectly stable as BArF salt in solution.
Figure 3. Molecular structure of 4 in the solid state as determined by
single crystal X-ray diffraction; displacement ellipsoids are shown at 50%
probability level, hydrogen atoms are omitted for clarity. Selected inter-
atomic distances (Å) and angles (deg): Rh1ꢀGa1 2.3696(6), Ga1ꢀO1
2.113(3), Ga1ꢀN1 1.957(4), Ga1ꢀN2 1.999(3), Rh1ꢀC9 2.125(4),
Rh1ꢀC16 2.136(4), C9ꢀC16 1.409(6), N1ꢀGa1ꢀN2 94.87(14),
N1ꢀGa1ꢀRh1 133.39(11), N2ꢀGa1ꢀRh1 124.55(10), O1ꢀGa1ꢀ
Rh1 107.00(8), N1ꢀGa1ꢀO1 95.28(14), N2ꢀGa1ꢀO1 90.37(13).
recrystallization from a saturated toluene solution at ꢀ30 °C.
Complex 4 crystallizes in the monoclinic space group P21/n. The
coordination environment of the Rhodium atom in 4 as deter-
mined by single crystal X-ray diffraction can be best described as a
piano stool configuration (Figure 3). The N1ꢀN2ꢀGa1ꢀRh1
plane is nearly trigonal planar with an angular sum of 352.81°.
The Rh1ꢀGa1 bond distance (2.3696(6) Å) is comparable
with RhꢀGa distances in other RhꢀGa complexes, that is,
[Rh(PPh3)2μ-Cl(Ga(DDP))]41 (2.3870(6)Å), [(coe)(benzene)
;Rh(GaDDP)Cl]41 (2.4037(8) Å), [Rh(η2,η2-cod)(GaCp*)3]-
[BArF]22 (cod = 1,5-cyclooctadiene) (Ø 2.379 Å) or [Rh(cod)
(IMes){Ga{[N(Ar)C(H)]2}}]42 (IMes = :C{N(C6H2Me3-2,4,6)-
CH}2, Ar = C6H3Pri2-2,6) (2.4259(6) Å). The Ga1ꢀO1 distance
(2.113(3) Å) is slightly longer than in compound 2 but lies well in
the range of galliumꢀoxygen distances found in RGa-OSO2CF3
fragments (R = organic substituent) of the structures discussed above
(vide supra).
The differences in the reaction behavior of low valent gallium
compounds GaR can be nicely illustrated on the basis of the
rhodium(I) precursor used in the preparation of compound 2. In
contrast to the formation of compound 2 by using GaCp* as
ligand, the reaction of Ga(DDP) with [Rh(coe)2(CF3SO3)]2 in
fluorobenzene at 50 °C and recrystallization from a saturated
toluene solution leads to the formation of the mono gallylene
complex [(coe)(toluene)Rh(GaDDP)(CF3SO3)] (4) (Scheme 2).
At this point, we like to highlight that the different choices of the
stoichiometric ratios of the reactants are based on different
reaction pathways in comparison to the less bulky GaCp* known
from several publications over the past years.17,40 The reaction
with Ga(DDP) involves three steps, that is, replacement of one
coe ligand accompanied by an insertion reaction of gallium into
the RhꢀO bond as well as coordination of one toluene molecule
during the crystallization process. Notably, a quite similar reac-
tion of Ga(DDP) with [Rh(coe)2Cl]2 was observed leading to
’ CONCLUSIONS
The reactions of olefin containing d6 and d9 metal starting
materials with GaR ligands (R = Cp* and DDP) lead to a full
substitution of the olefin ligands in the case of GaCp* to yield the
steric crowded transition metal complexes [Mo(GaCp*)6] (1),
[Rh(GaCp*)5][CF3SO3] (2), and [Rh(GaCp*)5][BArF] (3).
While [Mo(η4-butadiene)3] is sufficiently reactive only under
H2 atmosphere and higher temperatures, the reaction of [Rh-
(coe)2(CF3SO3)]2 with GaCp* takes place under mild condi-
tions without hydrogen. All three compounds 1ꢀ3 represent
proof of principle to obtain homoleptic, highly gallylene coordi-
nated congeners of the classic transition metal carbonyl com-
plexes [M(CO)n] for coordination numbers n > 4, despite of the
steric bulk of Cp* as the substituent at the gallium center. The
fluxional behavior and the facile haptotropic shift of the Cp*
ligand reduces the strain of the otherwise steric overcrowded
1
[(coe)(benzene)Rh(GaDDP)(Cl)].41 The H NMR spectrum
of 4 in C6D6 shows signals for one coordinated coe at 2.79
(m, 2H) ppm and 0.92ꢀ1.48 ppm (m, 12H) at which the latter
signals overlap with signals of the DDP moiety (24H, CH-
(CH3)2). The DDP unit gives rise to four further signals at 1.69
(s, 6H, CH3), 3.48 (m, 4H, CH(CH3)2), 5.20 (s, 1H, γ-CH) as
well as 6.99ꢀ7.15 (m, 6H, aryl). In addition the signals for a
coordinated toluene are found at 2.10 (s, 3H) and within the
multiplet at 6.99ꢀ7.15 ppm overlapping with the DDP unit. The
13C NMR as well as the 19F NMR spectrum do not show any
unusual features. Pure yellow crystals of 4 can be obtained by
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Inorganic Chemistry
ARTICLE
situation in case of rigid substituents. However, redox-reactions
and Cp* transfer may limit the stability of the complexes
[M(GaCp*)n] as it is the case of compound 3. In sharp contrast
to the versatile, soft coordinating properties of GaCp*, the
reaction of Ga(DDP) with [Rh(coe)2(CF3SO3)]2 does not lead
to a homoleptic complex with an all-Ga coordination sphere
around the Rh center, rather the heteroleptic mono gallium
complex [(coe)(toluene)Rh(GaDDP)(CF3SO3)] (4) was iso-
lated. This is certainly due to the rigidity of the DDP ligand and
the much increased steric bulk and concave ligand shape.3 In
addition, the overall positive charge at the species [Rh(GaCp*)5]þ
and [(coe)(toluene)Rh(GaDDP)]þ contributes to the more
electrophilic nature of the coordinated Ga centers which undergo
Lewis acidꢀbase adduct formation with the counterion triflate as
weak nucleophile. Similar observations have been made in
reactions of GaCp* and Ga(DDP) toward [Cu(CF3SO3)2].
These results nicely show the importance of electronic and steric
properties of the two different Ga(I) ligands which effect their
reactivity toward substitution labile transition metal complexes,
not only leading to different products but also allowing for diffe-
rent reaction pathways.
single crystals. Yield: 0.785 g (51%). Anal. Calcd. for C60H90Ga6Mo1: C,
54.36; H, 6.84. Found: C, 53.86; H, 6.24. 1HNMRδH(C6D6), 1.96 (s, 90H,
1
1
C5Me5). H NMR δH(d8-toluene), 1.95 (s, 90H, C5Me5). H NMR
δH(d8-toluene, ꢀ78 °C), 1.99 (s, 90H, C5Me5). 13C{1H} NMR δC{H}
-
(C6D6), 117.11 (C5Me5), 11.91 (C5Me5). IR (ATR, cmꢀ1): 2943 (w),
2877 (m), 2829 (m), 2700 (w), 1478 (w), 1413 (m), 1363 (m), 1285 (w),
1011 (w), 934 (w), 789 (w), 723 (w), 639 (w), 587 (w), 414 (s). MS
(LIFDI, toluene): m/z 1326 [M].þ, 1190 [M ꢀ Cp*]þ.
[Rh(GaCp*)5][CF3SO3] (2). To a solution of [Rh(coe)2(CF3SO3)]2
(0.200 g, 0.212 mmol) in fluorobenzene (5 mL) GaCp* (0.443 g, 2.162
mmol) was added. The reaction mixture was heated at 60 °C for 1 h, then
the solvent was reduced in vacuo, the residue washed with n-hexane and
dried in vacuo to give a red-purple solid. Recrystallization of the crude
product by slow diffusion of n-hexane into a solution of 2 in fluor-
obenzene gave deep red single crystals. Yield: 0.435 g (81%) Anal. Calcd.
for C51H75F3S1O3Rh1Ga5: C, 48.11; H, 5.94; S, 2.51. Found: C, 47.73;
H, 5.83; S, 2.43. 1H NMR δH(CD2Cl2), 1.91 (s, 75H, C5Me5). 1H NMR
δH(d8-thf), 1.90 (s, 75H, C5Me5). 1H NMR δH(d8-thf, ꢀ78 °C), 1.89
(s, 75H, C5Me5). 13C{1H} NMR δC{H}(CD2Cl2), 117.21 (C5Me5),
11.09 (C5Me5). 19F{1H} NMR δ(CD2Cl2), ꢀ78.7 (s, SO3SF3). IR
(ATR, cmꢀ1): 2938 (w), 2885 (w), 2834 (w), 1580 (w), 1482 (w), 1409
(w), 1373 (w), 1285 (w), 1219 (s), 1179 (m), 1112 (m), 973 (s), 792
(w), 748 (w), 705 (w), 679 (w), 524 (w), 582 (w), 513 (w), 461 (w), 413
(w). MS (LIFDI, CH2Cl2): m/z 788 [M ꢀ {(Cp*)2Ga(CF3SO3)}]þ,
373 [RhCp*2]þ.
’ EXPERIMENTAL SECTION
[Rh(GaCp*)5][BArF] (3). To a solution of 2 (0.120 g, 0.094 mmol) in
CH2Cl2 (5 mL) NaBArF (0.105 g, 0.109 mmol) was added, and a further
5 mL of CH2Cl2 was added. The reaction mixture was stirred at room
temperature for 45 min and filtered to remove the generated colorless
NaOTf. The solution was concentrated approximately to 4 mL, and
n-hexane (12 mL) added which results in the precipitation of an orange
powder. The powder was isolated, the residue washed with n-hexane and
dried in vacuo to give a slightly orange solid. Yield: 0.184 g (94%) Anal.
Calcd. for BC88H96F24Rh1Ga5: C, 49.47; H, 4.40. Found: C, 47.40; H,
4.36 (for deviation of C value see explanation in the main text). 1H NMR
δH(CD2Cl2), 1.91 (s, 75H, C5Me5), 7.56 (br, 4H, BArF), 7.72 (br, 8H,
BArF). 1H NMR δH(CD2Cl2, ꢀ78 °C), 1.83 (s, 75H, C5Me5), 7.53 (br,
4H, BArF), 7.72 (br, 8H, BArF). 13C{1H} NMR δC{H}(CD2Cl2), 135.23
ppm (s, ortho CH, BArF), 128.99 (s, ipso to the CF3 units, BArF), 125.03
(q, ꢀCF3, J = 272.3 Hz, BArF), 117.88 (s, para CH, BArF), 117.21
(C5Me5), 11.09 (C5Me5). 19F{1H} NMR δ(CD2Cl2), ꢀ62.9 (s, CF3).
11B{1H} NMR δ(CD2Cl2), ꢀ6.6 (s, BArF). IR (ATR, cmꢀ1): 2956 (w),
2895 (w), 2836 (w), 1597 (w), 1411 (w), 1375 (w), 1341 (m), 1264 (s),
1152 (w), 1114 (s), 939 (w), 878 (w), 831 (w), 792 (w), 738 (w), 707
(w), 676 (w), 663 (w), 585 (w), 444 (w). MS (LIFDI, THF): m/z 373
[RhCp*2]þ.
[(coe)(toluene)Rh(GaDDP)(CF3SO3)] (4). [Rh(coe)2(CF3SO3)]2
(0.150 g, 0.159 mmol) and Ga(DDP) (0.162 g, 0.334 mmol) in
fluorobenzene (5 mL) were stirred at 50 °C for 2 h. The solvent was
reduced in vacuo, the residue washed with n-hexane and dried in vacuo
to give a yellow solid. Recrystallization of the crude product by cooling a
saturated toluene solution of 3 to ꢀ30 °C gave yellow single crystals.
Yield: 0.145 g (98%) Anal. Calcd. for C45H63F3S1O3N2Rh1Ga: C, 57.40;
H, 6.74; N, 2.97; S, 3.41. Found: C, 58.12; H, 6.32; N, 3.22; S, 3.32. 1H
NMR δH(C6D6), 6.99ꢀ7.15 (m, 11H, arylic DDP and C6H5CH3), 5.20
(s, 1H, γ-CH), 3.48 (m, 4H, CH(Me)2, 2.79 (m, 2H, COE), 2.10 (s, 3H,
C6H5CH3), 1.69 (s, 6H, CH3 groups, DDP), 1.48ꢀ0.92 (m, 36H,
CH(Me)2 and COE). 13C{1H} NMR δC{H}(C6D6), 168.60 (CN),
143.35 (ar), 137.90 (C1, toluene), 129.30 (CH 2,6; toluene) 127.39
(CH 3,5; toluene), 126.97 (ar), 125.66 (CH 4; toluene), 124.43 (ar),
100.62 (γ-C), 57.16 (d, CdC, JRhꢀC = 14.1 Hz), 34.90 (coe), 32.40
(coe), 26.46, 25.27 (coe), 24.40, 21.48 (CH3, toluene) (Note: not all
carbon signals of the DDP unit could be observed because of low scan
number measurement). 19F{1H} NMR δ(C6D6), ꢀ77.1 (s, SO3CF3).
General Remarks. All manipulations were carried out in an atmo-
sphere of purified argon using standard Schlenk and glovebox techni-
ques. Hexane and toluene were dried using an mBraun Solvent Purifi-
cation System. Fluorobenzene and mesitylene were dried by alumina
column under dry atmosphere. The final H2O content in all solvents
used was checked by Karl Fischer-Titration and did not exceed 5 ppm.
[Rh(coe)2(CF3SO3)]2,43 [Mo(η4-butadiene)3],30 GaCp*,12,44 Ga-
(DDP),45 and NaBArF 46 were prepared according to literature methods.
Elemental analyses were performed by the Microanalytical Laboratory of
the University of Bochum. NMR spectra were recorded on a Bruker
Avance DPX-250 spectrometer (1H, 250.1 MHz; 13C, 62.9 MHz; 19F,
235.3 MHz; 11B, 80.3 MHz) in C6D6 and CD2Cl2 at 298 K unless
otherwise stated. Chemical shifts are given relative to TMS and were
referenced to the solvent resonances as internal standards. The X-ray
diffraction intensities were collected on an Oxford Xcalibur2 diffract-
ometer with a Sapphire2 CCD. The crystal structures were solved by
direct methods using SHELXS-97 and refined with SHELXL-97.47 The
crystals were coated with a perfluoropolyether, picked up with a glass
fiber, and immediately mounted in the cooled nitrogen stream of the
diffractometer. CCDC 679006 (1), 819987 (2), and 819988 (4) contain
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. FT-IR spectra were
measured in an ATR setup with a Bruker Alpha FTIR spectrometer
under inert atmosphere in a glovebox. Mass spectrometry was measured
with a Jeol AccuTOF GCv; Ionization method: liquid injection field
desorption ionization (LIFDI; special ionization cell obtained from
Linden CMS GmbH, Leeste, Germany; http://www.linden-cms.de),
solvents: toluene (1 and 4), CH2Cl2 (2), and thf (3).
Syntheses. [Mo(GaCp*)6] (1). A sample of freshly prepared
[Mo(η4-C4H6)3] (0.300 g, 1.162 mmol) was introduced into a
FischerꢀPorter bottle and then dissolved in toluene (12 mL). After
addition of GaCp* (1.666 g, 8.129 mmol) the reaction mixture was
pressurized to 3 bar dihydrogen. The orange solution was warmed to
100 °C, whereupon a red microcrystalline precipitate was formed. After
stirring for further 16 h at 80 °C the reaction mixture was transferred into
a Schlenk tube. The red crystals were isolated by means of a cannula,
washed with a small amount of n-hexane, and dried in vacuo.
Recrystallization from mesitylene gave well formed dark red needle-shaped
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Inorganic Chemistry
ARTICLE
IR (ATR, cmꢀ1): 2934 (w), 2902 (w), 2841 (w), 2820 (w), 2020 (w),
1540 (w), 1502 (w), 1449 (w), 1424 (w), 1377 (m), 1349 (w), 1333 (w),
1296 (w), 1276 (w), 1245 (w), 1222 (m), 1191 (m), 1156 (m), 1089
(w), 1047 (w), 1003 (m), 930 (w), 896 (w), 873 (w), 852 (w), 827 (w),
789 (w), 773 (w), 753 (w), 718 (w), 701 (w), 686 (w), 621 (m), 576 (w),
562 (w), 538 (w), 514 (w), 502 (w), 458 (w), 434 (w). MS (LIFDI,
toluene): m/z 940 [M].þ, 830 [M ꢀ coe]þ, 791 [M ꢀ CF3SO3]þ.
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’ AUTHOR INFORMATION
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Corresponding Author
*E-mail: roland.fischer@ruhr-uni-bochum.de. Fax: (þ49)234
321 4174.
Notes
†Organo group 13 complexes of transition metals LVIII; for
communication LVII see ref 32.
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’ ACKNOWLEDGMENT
This work has been funded by the German Research Founda-
tion (Fi-502/23-1) T.B. and M.M. are grateful for Ph.D. scholar-
ships by the Fonds der Chemischen Industrie, Germany and for
support by the Ruhr University Research School (http://www.
research-school.rub.de/). The authors would like to thank the
Linden CMS GmbH, Germany and S. Bendix (Ruhr University
Bochum) for support in mass spectrometry.
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