Synthesis and π-tweezer properties of tripodcobalt-bisalkynyl compounds [CH3C(CH2PPh2)3Co(C≡CR)2] - Application to the oxidative coupling of alkynyl groups

Article abstract of DOI:10.1002/1099-0682(200009)2000:9<1953::aid-ejic1953>3.0.co;2-1

tripodCoCl₂ [CH₃C(CH₂PPh₂)₃CoCl₂] (1) reacts with alkynyl-lithium reagents RC≡CLi to produce tripodCo(C≡CR)₂ (2). The neutral paramagnetic Co¹¹ species 2 can be reduced to the anionic diamagnetic Co¹ compounds 2^-, which can also be obtained by treating tripodCoCl (3) with two equivalents of LiC≡CR. Compounds 2 contain an M(C≡CR)₂ group with the alkynyl substituents in cis-positions to each other. The expectation that they might therefore act as π-tweezer ligands is born out by the characterisation of tripodCo(C≡CR)₂-NiCO (4). The cis-position of the alkynyl groups in 2 should also facilitate their oxidative coupling. This type of reaction is, in fact, observed when compounds 2 are treated with one-electron oxidants. Reaction of 2 with Cp₂Fe⁺ leads to [tripodCo(η²-RC≡CC≡CR)]⁺ (5) in which one of the triple bonds of the diyne acts as a four-electron donor ligand while the other one remains uncoordinated. One-electron oxidation of 2 thus initiates a two-electron oxidative coupling of the alkynyl groups with concomitant one-electron reduction of Co¹¹ (2) to Co¹ (5). All compounds were characterised by the usual analytical and spectroscopic techniques including EPR spectroscopy. The structure of compounds 5 is exemplified by an X-ray analysis of [tripodCo(η²-tBuC≡CC≡CtBu)](PF₆) (5b).

Full text of DOI:10.1002/1099-0682(200009)2000:9<1953::aid-ejic1953>3.0.co;2-1

FULL PAPER
Synthesis and π-Tweezer Properties of tripodCobalt-Bisalkynyl Compounds
[CH3C(CH2PPh2)3Co(CϵCR)2] ؊ Application to the Oxidative Coupling of
Alkynyl Groups
Rolf Rupp,[a] Gottfried Huttner,*[a] Heinrich Lang,[b] Katja Heinze,[a] Michael Büchner,[a] and
Eric Robert Hovestreydt[c]
Dedicated to Prof. Peter Paetzold on the occasion of his 65th birthday
Keywords: Tripod ligands / Cobalt / π-Tweezers / Oxidative Coupling / Alkyne complexes
tripodCoCl2 [CH3C(CH2PPh2)3CoCl2] (1) reacts with alkynyl-
lithium reagents RCϵCLi to produce tripodCo(CϵCR)2 (2).
The neutral paramagnetic CoII species 2 can be reduced to
the anionic diamagnetic CoI compounds 2−, which can also
2 are treated with one-electron oxidants. Reaction of 2 with
Cp2Fe+ leads to [tripodCo(η2-RCϵCCϵCR)]+ (5) in which one
of the triple bonds of the diyne acts as a four-electron donor
ligand while the other one remains uncoordinated. One-elec-
be obtained by treating tripodCoCl (3) with two equivalents tron oxidation of 2 thus initiates a two-electron oxidative
of LiCϵCR. Compounds 2 contain an M(CϵCR)2 group with
the alkynyl substituents in cis-positions to each other. The
expectation that they might therefore act as π-tweezer li-
coupling of the alkynyl groups with concomitant one-elec-
tron reduction of CoII (2) to CoI (5). All compounds were char-
acterised by the usual analytical and spectroscopic tech-
gands is born out by the characterisation of niques including EPR spectroscopy. The structure of com-
tripodCo(CϵCR)2−NiCO (4). The cis-position of the alkynyl
groups in 2 should also facilitate their oxidative coupling.
This type of reaction is, in fact, observed when compounds
pounds 5 is exemplified by an X-ray analysis of [tripodCo(η2-
tBuCϵCCϵCtBu)](PF6) (5b).
Introduction
0) 2 are accessible, and how these bisalkynyl species may be
used
as
π-tweezers
in
derivatives
such
as
The coordination chemistry of the [tripodcobalt] template
[CH3C(CH2PPh2)3Co] is characterised by the persistent
pentacoordination in [tripodCoL2]nϩ compounds, regard-
less of the oxidation state (CoIII: d6, CoII: d7, CoI: d8).[1,2]
While only pentacoordination is known for d6- and d7-elec-
tron configurations, lower oxidation states allow for
pseudo-tetrahedral coordination.[2a,2b,3Ϫ8] With the geomet-
ric constraint imposed by the facial η3-coordination of the
tripod ligand, the co-ligands L in [tripodCoL2] are necessar-
ily cis to each other.
[tripodCo(CϵCR)2ϪNiCO] (4). Compounds 2 may also
serve as reagents for the oxidative coupling of the alkynyl li-
gands.
Results and Discussion
Cobalt(II) Derivatives
[tripodCoCl2] (1)[2b] reacts with two equivalents of the al-
kynyllithium compound LiCϵCR (RϭPh, tBu, TMS) to
give the corresponding bisalkynyl derivatives 2aϪc.[2a,2b]
With this in mind, the synthesis of cis-bisalkynyl derivat-
ives [tripodCo(CϵCR)2]nϩ is a rewarding goal in view of
the particular reactivity patterns documented for some spe-
cific cis-[LnM(CϵCR)2] cis-bisalkynyl compounds such as
[Cp2M(CϵCR)2] (M ϭ Ti, Zr, Hf).[9] It is shown in this
paper how the compounds [tripodCo(CϵCR)2]nϩ (n ϭ Ϫ1,
[a]
Anorganisch-Chemisches Institut der Universität Heidelberg,
Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
Fax: (internat.) ϩ49(0)6221/545-707
E-mail: g.huttner@indi.aci.uni-heidelberg.de
[b]
Technische Universität Chemnitz, Institut für Chemie,
Lehrstuhl Anorganische Chemie,
Straße der Nationen 62, D-09107 Chemnitz, Germany
Fax: (internat.) ϩ49(0)371/531-1833
E-mail: heinrich.lang@chemie.tu-chemnitz.de
[c]
Bruker-AXS GmbH, Product Support,
In order to produce 2 in high yields, the reaction has to
be performed at Ϫ80 °C in THF. Even at this low temper-
D-76181 Karlsruhe, Germany
Fax: (internat.) ϩ49(0)721/595-4506
Eur. J. Inorg. Chem. 2000, 1953Ϫ1959
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ1948/00/0909Ϫ1953 $ 17.50ϩ.50/0
1953

R. Rupp, G. Huttner, H. Lang, K. Heinze, M. Büchner, E. R. Hovestreydt
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ature, the reactions are spontaneous and an immediate col-
our change from blue (1 in THF[2b]) to brown (2) is ob-
served. After changing the solvent from THF to toluene,
with subsequent filtration, compounds 2 are obtained, by
evaporation of the solvent, as microcrystalline black-brown
powders. Derivatives of 1 that have only one alkynyl sub-
stituent and one remaining chloro substituent are not ac-
cessible by this procedure. If only one equivalent of
LiCϵCR is used, a mixture of 1 and 2 results. This is in
agreement with the observation that a 1:1 mixture of the
clean compounds
1
and
2
does not result in
alkynylϪchloro exchange.
While single crystals of 2 suitable for X-ray analysis could
not be obtained, analytical data unequivocally established
the structure as shown for 2. Osmometric determination of
the molecular mass of 2a shows that compounds 2 are
monomeric (see Experimental Section). Mass spectrometry
shows the molecular ion in each case (Table 1). The CϵC
bonds of 2 give rise to a characteristic ν(CC) absorption
band in each case (Table 1). Compounds 2 are paramag-
netic with an effective magnetic moment of 1.8 BM meas-
ured for 2a at 25 °C (see Experimental Section).
Table 1. Analytical data for compounds 2
Figure 1. EPR spectrum of 2b in THF at 298 K (top), simulations
of the two individual components (middle) and composed simu-
lated spectrum (bottom)
885), while FAB-positive mass spectra demonstrate the
presence of CpCoϩ (m/z ϭ 189).
All compounds show an EPR pattern characteristic of
pentacoordinated [tripodCoIIL2] species (Table 1).[2c] Solu-
tion EPR spectra at 25 °C (Figure 1, top) revealed the pres-
ence of two species with individual g factors and hyperfine
splitting constants (Figure 1, middle spectra) which, by ana-
logy to literature data, were interpreted as being due to
idealised square pyramidal and idealised trigonal bipyram-
idal coordination modes.[2c] Powder EPR spectra (25 °C
and Ϫ173 °C) show only one broad resonance with no re-
Due to difficulties in the purification of the [Cp2Co][tri-
podCo(CϵCPh)2] salt, a different preparative strategy to
produce 2a؊ was used. The cobalt(I) compound [tri-
podCoCl] (3)[2a,2b] was reacted with two equivalents of
LiCϵCPh to produce the cobaltate(I) compound 2a؊,
ϩ
which was isolated as its PPh4 salt.
7
solved coupling to the 57Co (I ϭ /2) nucleus (Table 1).
Cobalt(I) Derivatives
Cyclovoltammetric studies of 2a in CH2Cl2 indicated that
compounds 2 should undergo one-electron reduction to
produce the corresponding anionic CoI species
[tripodCo(CϵCR)2]Ϫ (Table 1). Reversible reduction of 2a
is observed at Ϫ574 mV with reference to a SCE (Table 1).
Elemental analysis and mass spectra are in agreement
Treating 2a with an equimolar amount of Cp2Co in toluene with the assigned structure (see Experimental Section). The
led to a brown powder similar to 2a in colour. FAB-negative ν(CC) vibration of 2a؊ at 2049 cmϪ1 (see Experimental Sec-
mass spectra of this powder show a signal for 2a؊ (m/z ϭ tion) is shifted to a lower wave number than that of 2a
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Synthesis and π-Tweezer Properties of tripodCobalt-Bisalkynyl Compounds
FULL PAPER
(2068 cmϪ1) (Table 1), as would be expected if some charge Table 2. Analytical data for compounds 4
is delocalised into the π-system of the ligand. Owing to the
prolonged measuring time required, 13C NMR spectra of
the sensitive PPh4ϩ2a؊ could not be obtained. A sharp 31P
NMR signal at δ ϭ 32.1 and a well-resolved 1H NMR spec-
trum clearly show that 2a؊ is a diamagnetic CoI compound.
Anion 2a؊ conforms to the 18-electron rule, and this may
be one of the reasons for the observation that, regardless of
the stoichiometry of LiCϵCPh and 3 (2:1 or 1:1), 2a؊ is
inevitably obtained and there is no indication of the forma-
tion of [tripodCoCϵCPh].
Reactions of the Bisalkynyl Compounds 2
In the chemistry of π-tweezer ligands, CuI and AgI have
been found to be specifically prone to engage in bonding
with π-tweezers.[9c] When solutions of 2 are treated with
salts of these cations, however, the reaction is complicated
by the propensity of 2 towards oxidation. Cyclovoltamme-
try of 2a shows that the compound is irreversibly oxidised
(Table 1) and this is what presumably happens when com-
pounds 2 are treated with potentially oxidising metal ions.
Preparative oxidation of 2 with Cp2Feϩ results in a trans-
formation of 2 into 5.
The propensity of 2 to act as a π-tweezer compound is
seen by the reaction of 2 with Ni(CO)4 to give compounds
4. A NiCO moiety is retained in the π-tweezer entity of 2.
The structure of compounds 4 is in agreement with their
analytical and spectroscopic data. Mass spectra of 4 show
a prominent peak for [Mϩ Ϫ CO] in each case (Table 2).
The presence of the coordinated carbonyl group is evident
from the corresponding ν(CO) IR bands for all compounds
4 (Table 2). This prominent band is observed at about 2000
cmϪ1, which is in good agreement with the observation of
the corresponding bands in π-tweezer compounds
[(RCp)2M(CϵCR)2ϪNiCO] (M ϭ Ti, Zr, Hf) at about the
The alkynyl groups of 2 undergo oxidative coupling to
same wavenumbers.[9] This indicates that direct result in 1,3-diyne ligands with one of the two triple bonds
metalϪnickel interactions are not really very important for being coordinated to the cobalt centre. The process is
the stability of this type of compound; if they were, the CO brought about by a single-electron transfer-step such that,
group would be sensitive to the presence of different types while the alkyne groups are oxidised in a formal two-elec-
of LnM entities to which the alkynyl groups are bound in tron oxidation, the cobalt centre is reduced from CoII in 2
[LnM(CϵCR)2].[9] In agreement with this interpretation is to CoI in 5. The coupling of σ-bonded alkynyl compounds
the observation that cyclovoltammetric reduction of 4b is not without precedent: with rhodium as the central atom
(Ϫ605 mV vs. SCE, see Experimental Section) occurs at in [RhIII(CϵCϪPh)2(SnPh3)(PiPr3)2], oxidative coupling by
about the same potential as that of 2a (Ϫ574 mV vs. SCE, I2 to give trans-[RhI(η2-PhCϵCCϵCPh)(PiPr3)2I] is form-
Table 1). Compounds 4 are paramagnetic as are the parent ally similar to the transformation of 2 into 5, since reduc-
compounds 2 from which they are derived. The EPR spec- tion of the central metal (in this case reduction of RhIII to
tra of compounds 4 (Table 2) closely resemble those of 2 RhI) necessarily accompanies the oxidative coupling of the
(Table 1) with only minor changes in g values and coupling alkynyl ligands.[10,11]
constants. With 4c, the hyperfine structure is not resolved
The salts 5·PF6 are obtained as analytically and spectro-
and one broad resonance is observed at g ഠ 2.05. The fact scopically pure green powders (Table 3) which show the
that two overlapping signals are observed for 4, with the peak for the molecular ion in their FAB mass spectra in
spectral pattern being similar to that observed for the par- each case. They are diamagnetic and show well-resolved
ent compounds 2, may be explained by the same type of NMR spectra. Their 31P NMR resonances are observed in
reasoning as discussed for 2. With 4, it is of course neces- the range δ ϭ 35 to 38 (Table 3). 1H and 13C NMR spectra
sary to assume that the bisalkynyl-Ni moiety changes place reveal the signals of each constitutional group in the ex-
as a single unit such that the idealised trigonal bipyramidal pected range (Table 3). The presence of one coordinated
and idealised square pyramidal coordination geometries and one uncoordinated triple bond is evident from the cor-
may interchange.
responding 13C NMR signals (Table 3). The presence of an
Eur. J. Inorg. Chem. 2000, 1953Ϫ1959
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R. Rupp, G. Huttner, H. Lang, K. Heinze, M. Büchner, E. R. Hovestreydt
FULL PAPER
uncoordinated triple bond is also indicated by a weak template, can be set free from the metal by extensive car-
ν(CC) IR band at 2100Ϫ2200 cmϪ1 (Table 3). Chemical bonylation.[13] The presence of the PF6 counterion is cor-
proof for the existence of an uncoordinated reactive triple roborated by a strong ν(PF) vibration and by the corres-
bond is found in the reaction of 5a with [Co2(CO)8].
ponding 31P NMR signal in each case (Table 3).
The structure of 5 is confirmed by an X-ray analysis of
the PF6 salt of 5b (Table 4 and 5). A general view of 5b is
given in Figure 2 (left), as well as a projection of the struc-
ture onto the plane of the three phosphorus atoms (right).
Table 3. Analytical data for compounds 5
Table 4. Bond lengths [pm], bond angles [°] and torsion angles [°]
for 5b
Table 5. Crystal data for compound 5b
This reaction, as performed under standard conditions,
gives [tripodCo(η2-PhCϵCCϵCPhϪCo2(CO)6)]ϩ as the
main product.[12] This is apparent from the FAB mass spec-
tra of its PF6 salt which show the molecular ion (m/z ϭ
1171, see Experimental Section) together with the peaks
corresponding to the consecutive loss of up to five CO
groups. The last CO group is lost together with the Co2
entity, which was part of the coordinated Co2(CO)6 unit,
apparently in one step leading to the mass peak of [tri-
podCo(η2-PhCϵCCϵCPh)]ϩ 5a. The IR data of the
Co2(CO)6 adduct of 5a are also in agreement with its for-
mulation as 5a·Co2(CO)6 (see Experimental Section).[12]
The 31P NMR resonance of this compound is found at δ ϭ
36.9 (see Experimental Section). Under carbonylating con-
ditions, the carbonyl derivative [tripodCo(CO)2]ϩ is formed
as a minor side-product, as indicated by its 31P NMR res-
onance at δ ϭ 24.1 (see Experimental Section) and by the
FAB-MS signal of the molecular ion (m/z ϭ 739, see Ex-
From this second view in Figure 2, it becomes clear that
perimental Section).[2f] This observation also shows that the there are at least two sets of chemically different phos-
diyne, once it has been formed as a ligand at the [tripodCo] phorus nuclei in 5b. The fact that only one 31P NMR signal
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Eur. J. Inorg. Chem. 2000, 1953Ϫ1959

Synthesis and π-Tweezer Properties of tripodCobalt-Bisalkynyl Compounds
FULL PAPER
Figure 2. General view and projection of the structure of the cation of 5b
is observed means that the coordinated alkyne group is free
Based on the formalism of electron counting, the alkynes
to adopt different rotational positions relative to the [tri- in 5 should act as four-electron donors since the com-
podCo] scaffolding, with the vector radiating from the co- pounds are diamagnetic and behave as 18-electron com-
balt centre to the midpoint of the coordinated alkyne bond pounds,[14] while a 16-electron species [tripodCoL]ϩ with L
(C6ϪC7) interpreted as the axis of this rotation. With the acting as a two-electron donor should be paramagnetic.[4Ϫ7]
coordinated alkyne bond seen as just one donor ligand, the This follows from simple crystal-field-type arguments as
coordination geometry around the cobalt may be described well as from experience.[4Ϫ7,14] Assuming that the alkyne
as idealised tetrahedral.
ligands in 5 act as four-electron donor moieties is in good
The tBu substituent of the alkyne in 5b makes the coord- agreement with the established rules correlating the 13C
inated triple bond a rather bulky one. The rotational posi- NMR resonances of coordinated alkyne groups with the
tion of the alkyne ligand is such that the tBu substituent number of electrons which these groups donate to the
(C14, Figure 2) of the coordinated alkyne ligand (C6ϪC7, metal.[15] The signals of the carbon atoms of the coordin-
Figure 2) is in an eclipsed position with respect to the ated alkyne group in 5a are observed at approximately δ ϭ
CoϪP3 bond. It follows that the tBuϪCϵC substituent 160Ϫ180 and those of 5b occur at δ ϭ 164Ϫ200 (Table 3).
(C8ϪC9ϪC10, Figure 2) at the other carbon centre of the The uncoordinated triple bonds give rise to signals between
coordinated alkyne (C7, Figure 2) is in a staggered position δ ϭ 77 and δ ϭ 101 in these compounds (Table 3). These
with respect to the CoϪP1 and CoϪP2 bonds (Figure 2). latter shift values compare favourably with the 13C NMR
The steric strain imposed by the orientation of the alkyne shifts reported for uncoordinated 1,3-diynes. In cobalt(I)
ligand is mirrored by the geometric parameters of the compounds in which alkyne ligands can only act as two-
CoϪP3 part of the compound. The distance CoϪP3 (224 electron donors (due to the electron count), 13C NMR res-
pm) is distinctly larger than the CoϪP1 (220 pm) and onances of δ ഠ 100 are generally observed.[16] The CϪC
CoϪP2 (216 pm) distances. The P3ϪCoϪP2 (92.5°) and bond lengths in these compounds are found to be about
P3ϪCoϪP1 (96.3°) angles are definitely larger than the 127 pm, which is definitely shorter than the C6ϪC7 dis-
angle between the CoϪP1 and CoϪP2 bonds (84.6°). The tance (131 pm, Table 5) in 5b where the alkyne acts as a
rotational positions of the ‘‘phenyl-petals’’ also respond to four-electron donor.[16b]
this steric strain; the phenyl groups at P3 show distinctly
The bonding of acetylenes at [tripodCo]ϩ fragments has
larger rotation angles than those at P1 and P2 (Figure 2, previously been reported, although no NMR spectroscopic
Table 5).
The coordinated triple bond (C6ϪC7: 131.3 pm, Table 4
data were given.[17]
and 5) is approximately 11 pm longer than the uncoordin-
ated one (C8ϪC9: 120.0 pm, Table 4 and 5). As expected,
the coordination around the carbon atoms of the free triple
bond is linear (C7ϪC8ϪC9: 176.6°; C8ϪC9ϪC10: 177.8°,
Conclusion
tripodCo-bisalkyne compounds are easily prepared by the
Table 4 and 5), but the bond angles at the carbon atoms of reaction of tripodCo-halides with LiCϵCR. Starting from
the coordinated triple bond (C14ϪC6ϪC7: 134.5°; [tripodCoCl2] (1), neutral CoII derivatives [tri-
C6ϪC7ϪC8: 140.1°, Table 4 and 5) are distinctly different podCo(CϵCR)2] (2) are obtained. Anionic CoI derivatives
from 180°. The degree of this bending and the lengthening [tripodCo(CϵCR)2]Ϫ (2؊) are found when [tripodCoCl] (3)
of the coordinated bond are indicative of a strong interac- is used as the starting material. The bisalkynyl compounds
tion between the alkyne and the metal.[15]
2 are found to act as π-tweezer ligands with Ni(CO). It is
Eur. J. Inorg. Chem. 2000, 1953Ϫ1959
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R. Rupp, G. Huttner, H. Lang, K. Heinze, M. Büchner, E. R. Hovestreydt
FULL PAPER
LiCϵCPh (2 mmol) in Et2O (5 mL) was added at Ϫ80 °C. The re-
action mixture was stirred for 10 min at this temperature and then
allowed to warm to 25 °C resulting in a colour change from beige
to red-brown. The solvent was evaporated and the residue dissolved
in CH2Cl2. Ph4PCl (1 mmol, 375 mg) was added and the reaction
mixture stirred for 20 min. After removal of the solvent under re-
duced pressure, the residue was redissolved in toluene and filtered
to remove LiCl. Evaporation of the solvent produced PPh4ϩ2a؊ as
a red-brown, air-sensitive powder. Ϫ 1H NMR (CD2Cl2): δ ϭ 1.28
(br. s, 3 H, CH3), 2.46 (br. s, 6 H, CH2), 7.07Ϫ7.48 (m, 60 H, Har).
observed that they undergo one-electron oxidation which
results in oxidative coupling of the alkynyl ligands to pro-
duce 1,3-diynes and concomitant reduction of CoII to CoI
to produce [tripodCo(η2-RCϵCCϵCR)]ϩ (5).
Experimental Section
General Remarks: Unless otherwise noted, all manipulations were
carried out under argon by means of standard Schlenk techniques
All solvents were dried by standard methods and distilled under
argon.[18] The CD2Cl2 used for NMR spectroscopic measurements
was degassed by three successive ‘‘freeze-pump-thaw’’ cycles and
Ϫ
31P NMR (CD2Cl2): δ ϭ 26.8 (s, PPh4ϩ), 32.1 (s, CH2ϪP). Ϫ
IR (KBr): ν(CC) ϭ 2049 (w) cmϪ1. Ϫ MS (FAB-negative): m/z ϭ
885 [MϪ]. Ϫ C81H69CoP4 (1225.27): calcd. C 79.40, H 5.68; found
C 78.21, H 5.82. Ϫ M.p. 145 °C (dec.).
˚
dried with 4 A molecular sieves. Ϫ EPR: Bruker ESP 300 E, X-
General Procedure for the Synthesis of Compounds 4: Compound 2
(1 mmol) was dissolved in toluene and treated with a threefold ex-
cess of Ni(CO)4. The reaction mixture was stirred for 2 h. No col-
our change took place, but a slow gas evolution was observed. The
solvent was removed under reduced pressure and the residue recrys-
tallised from CH2Cl2. After evaporation of the solvent, compounds
4 were obtained as black-brown microcrystalline powders.
band, standard cavity ER 4102, temperature control unit Euro-
therm B-VT 2000, external standard dipenylpicrylhydrazyl
(DPPH); Simulation: XSophe Computer Simulation Software
Suite, Version 1.0.4b, 1993Ϫ1999; XEPRView, Version 1.0, 1999;
Bruker Analytik, Rheinstetten, Germany. Ϫ NMR: Bruker Avance
DPX 200 at 200.12 MHz (1H), 50.323 MHz (13C{1H}),
81.015 MHz (31P{1H}), T ϭ 303 K; chemical shifts (δ) are given in
ppm with respect to CD2Cl2 (1H: δ ϭ 5.32; 13C: δ ϭ 53.8) as in-
ternal standard. 31P chemical shifts (δ) are given in ppm with re-
spect to 85% H3PO4 (31P: δ ϭ 0) as external standard. Ϫ IR:
Bruker IFS-66, KBr disks. Ϫ MS: Finnigan MAT 8230; Fast-atom
bombardment (FAB) xenon, matrix: 4-nitrobenzyl alcohol. Ϫ HR-
MS (FAB) JEOL JMS 700, matrix: 4-nitrobenzyl alcohol. Ϫ Mag-
netic measurements: Faraday balance with a Bruker electromagnet
B-E 15 C8, Bruker Field Controller B-H 15, Sartorius vacuum-
microbalance M25 D-S, Oxford Temperature Control Unit ITC-4;
calibration with K3[Fe(CN)6]. Ϫ Molecular weight determinations:
Knauer vapor-pressure osmometer No. 7311100000; calibration
with dibenzil. Ϫ Elemental analyses: Microanalytical Laboratory
of the Organisch-Chemisches Institut, Universität Heidelberg. Ϫ
Melting points: Gallenkamp MFB-595 010, melting points are not
corrected. Ϫ Cyclic voltammetry: Metrohm ‘‘Universal Meß- und
Titriergefäß’’, Metrohm GC electrode RDE 628, platinum elec-
trode, SCE electrode, Princeton Applied Research potentiostate
Model 273, 10Ϫ3  in 0.1  nBu4NPF6/CH2Cl2.
CV (4b, CH2Cl2, vs. SCE): E1/2 ϭ 605 mV (rev.), ∆E ϭ 90 mV.
General Procedure for the Synthesis of Compounds 5: Compound 2
(1 mmol) was dissolved in acetone (25 mL) and treated with
(Cp2Fe)PF6 (1 mmol, 332 mg). The colour immediately changed
from black-brown to green. The solvent was removed under re-
duced pressure and the resulting green residue was purified by chro-
˚
matography on SiO2 (ICN 32Ϫ63, 60 A) with PE (boiling range
40Ϫ60 °C)/CH2Cl2 (5:1). After evaporation of the solvent, the PF6
salts 5aϪc were obtained as fine black-green powders. Single crys-
tals of compound 5b suitable for X-ray diffraction were obtained
after three months by diffusion of PE (boiling range 40Ϫ60 °C)
into a solution of 5b in THF.
Preparation of 5a·Co2(CO)6: Compound 5a (0.1 mmol, 103 mg)
was dissolved in acetone (10 mL). [Co2(CO)8] (0.15 mmol, 52 mg)
was added and the reaction mixture was stirred for 1.5 h. A
darkening of the solution was evident after a few minutes. The solv-
ent was removed under reduced pressure and the residue was
washed three times with Et2O. After evaporation of the solvent, the
Co2(CO)6 adduct of 5a was obtained as a black microcrystalline
powder.
General Procedure for the Synthesis of Compounds 2: Alkyne
(2 mmol) was dissolved in Et2O (5 mL) and deprotonated with
nBuLi (0.8 mL, 2.5  in n-hexane) at Ϫ80 °C. After 5 min, the reac-
tion mixture was allowed to warm to room temperature. [tri-
podCoCl2] (1 mmol, 753 mg) was dissolved in THF (25 mL) and
cooled to Ϫ80 °C. The acetylide was added at Ϫ80 °C with a syr-
inge, and a brown solution was immediately formed. After 10 min
at Ϫ80 °C, the reaction mixture was allowed to warm to room
temperature and the solvent was removed under reduced pressure.
The resulting black-brown residue was dissolved in toluene and
filtered to remove LiCl. After evaporation of the solvent, com-
pounds 2 were obtained as microcrystalline black-brown powders.
Attempts to obtain crystals suitable for X-ray crystallography were
unsuccessful. Compounds 2 are highly soluble in organic solvents
such as hexane, toluene, Et2O, and methanol such that crystallis-
ation by diffusion is unfeasible; slow evaporation of the solvent
leaves them as microcrystalline powders only. Cooling the solutions
down to Ϫ80 °C also did not result in single crystals.
31P NMR (CD2Cl2): δ ϭ 36.9 (s, CH2ϪP). Ϫ IR (KBr): ν(CO) ϭ
2100 (s), 2083 (s), 2054 (s), 2034 (s) cmϪ1. Ϫ MS (FAB): m/z ϭ
1171 [Mϩ], 1143 [Mϩ Ϫ CO], 1115 [Mϩ Ϫ 2CO], 1087 [Mϩ
3CO], 1059 [Mϩ Ϫ 4CO], 1031 [Mϩ Ϫ 5CO], 885 [Mϩ
Co2(CO)6].
Ϫ
Ϫ
X-ray Crystallographic Study: The measurement was carried out on
a Siemens AXS Smart CCD diffractometer using graphite-mono-
chromated Mo-Kα radiation. All calculations were performed us-
ing the SHELXT PLUS software package. The structure was solved
by direct methods with the SHELXS-97 program and refined with
the SHELXL-97 program.[19] Graphical handling of the structural
data during solution and refinement was performed with
XPMA.[20] Atomic coordinates and anisotropic thermal parameters
of the non-hydrogen atoms were refined by full-matrix least-squares
calculations. Table 5 lists the data for the structure determination.
µeff (2a, 25 °C) ϭ 1.8 BM; osmometric determination of the mo-
lecular mass of compound 2a: calcd./found: 885/887.
Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Preparation of PPh4[tripodCoI-(CϵCPh)2], PPh4ϩ2a؊: [tri-
podCoCl] (3) (1 mmol, 718 mg) was suspended in THF (20 mL). Crystallographic Data Centre as supplementary publication
1958 Eur. J. Inorg. Chem. 2000, 1953Ϫ1959

Synthesis and π-Tweezer Properties of tripodCobalt-Bisalkynyl Compounds
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Acknowledgments
We are indebted to the Deutsche Forschungsgemeinschaft (SFB
247) and the Fonds der Chemischen Industrie for financial support.
One of us (R. R.) is indebted to the Graduiertenkolleg ‘‘Selektivität
in der organischen und metallorganischen Synthese und Katalyse’’
for a scholarship. The unselfish cooperation of Dr. J. Groß and Mr.
T. Jannack (MS); Mr. D. Günauer (CV) and Mrs. A. Dittes; Mrs.
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Received February 15, 2000
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1959