Pentadienyl chemistry of the heavy alkaline-earth metals revisited

Article abstract of DOI:10.1039/c4dt00545g

Open-metallocenes of the heavy alkaline-earth metals [(η⁵- Pdl′)₂M(thf)_n] (M = Ca (1), Sr (2), n = 1; M = Ba (3), n = 2; Pdl′ = 2,4-tBu₂C₅H₅) are readily prepared by salt-metathesis between MI₂ and KPdl′ and characterized by NMR spectroscopy and X-ray diffraction studies. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00545g

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Pentadienyl chemistry of the heavy alkaline-earth
metals revisited†
Cite this: Dalton Trans., 2014, 43,
6614
Matthias Reiners, Ann Christin Fecker, Matthias Freytag, Peter G. Jones and
Marc D. Walter*
Received 21st February 2014,
Accepted 25th February 2014
DOI: 10.1039/c4dt00545g
www.rsc.org/dalton
Open-metallocenes of the heavy alkaline-earth metals [(η5-Pdl’)2- been successfully prepared with tBu-substituted cyclopenta-
M(thf)n] (M = Ca (1), Sr (2), n = 1; M = Ba (3), n = 2; Pdl’ = 2,4- dienyl,5,35,36 pyrrolyl37 and P-heterocyclic38,39 ligands, and all
tBu2C5H5) are readily prepared by salt-metathesis between MI2 and exhibit good solubility in aliphatic and aromatic hydrocarbon
KPdl’ and characterized by NMR spectroscopy and X-ray diffraction solvents. Within our research program on pentadienyl
studies.
ligands,40 we therefore decided to revisit the heavy alkaline-
earth metals.
Over the last decade organometallic calcium, strontium and
barium complexes have attracted some interest,1–3 e.g. as pre-
cursors for metalorganic vapour deposition (MOCVD)4,5 or as
catalyst systems in polymerisation and hydrofunctionalization
reactions.6–9 Nevertheless, the isolation and characterization
of these highly reactive compounds required the development
of suitable ligands and synthetic techniques. In this context,
the η5-coordinate cyclopentadienyl ligand has played a promi-
nent role in stabilizing half-sandwich and metallocene
complexes.10–16 More recently, these investigations have been
extended to allyl systems, which can adopt both π-(η3) and
σ-(κC) bonding modes.17–25 Pentadienyls, with their ability to
adopt κC-, η3- and η5-coordination modes, occupy an inter-
mediate position between the cyclopentadienyl and allyl
systems, and their coordination chemistry with transition
metals has been well established.26–31 Surprisingly, reports on
group 2 complexes (Be,32 Mg,33 and Ca34) show only two of
them are structurally characterized, [(κC-2,4-Me2C5H5)2Mg-
(tmeda)]33 and [(η5-Pdl′)2Ca(thf)] (Pdl′ = 2,4-tBu2C5H5; 1).34
Overby and Hanusa report that the Pdl′ ligand provides only
limited solubility, thus precluding the synthesis of the open
metallocenes of the heavier homologues, Sr and Ba,34 but this
is in contrast to the general observation that sterically
demanding substituents such as tBu and SiMe3 enhance solu-
bility and stabilize the corresponding metal complexes. For
instance, several heavy alkaline-earth metal complexes have
The potassium salt KPdl′41 was prepared according to a
modified literature procedure† and can be crystallized as its
thf-adduct from saturated THF solutions at −30 °C.‡ A snap-
shot of the molecular structure of [(thf)K(μ–η5:η5-Pdl′)]∞ is
shown in Fig. 1 and reveals that the U-shaped η5-Pdl′ anion is
coordinated in the solid state by two potassium cations to
form a zigzag polymeric chain via a 21 screw axis. Selected
bond distances and angles are listed in Table 1. The C–C bond
distances within the pentadienyl moiety (C1–C5) follow the
typical short–long–long–short pattern. The conformation angle
χ is defined as the angle between the two planes, [centroid
(C1–C5)–C3–M] and [centroid(C1′–C5′)–C3′–M], and allows the
mutual orientation of the two pentadienyl moieties to be quan-
tified. In [(thf)K(μ-Pdl′)]∞ the pentadienyl ligands adopt a
nearly anti-ecliptic conformation with χ = 178.5°, thereby mini-
mizing repulsive interactions between the sterically demand-
ing tBu-groups.
Institut für Anorganische und Analytische Chemie, Technische Universität,
Braunschweig, Hagenring 30, 38106 Braunschweig, Germany.
E-mail: mwalter@tu-bs.de; Fax: +49 531 391 5387; Tel: +49 531 391 5312
†Electronic supplementary information (ESI) available: Experimental, NMR,
crystallographic and computational data. CCDC 982693–982696. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt00545g
Fig. 1 ORTEP diagram of [(thf)K(μ-Pdl’)]x (50% probability ellipsoids).
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Table 1 Selected bond distances and angles
[(thf)K(μ-Pdl′)]x
1
2a
3
M–C (pdl) (Å, range)
M–C1/5 (pdl) (Å, average)
M–C2/4 (pdl) (Å, average)
M–C3 (pdl) (Å, average)
M–C (pdl) (Å, average)
M–Pdlcentroidb (Å, average)
C1–C5 (Å, mean)
M–O (Å)
3.0166(19)–3.235(3)
3.159(65)
3.056(16)
2.982(50)
3.082 0.083
2.70
3.20
2.7577(18)
165.5
2.7908(10)–2.6681(10)
2.722(54)
2.768(31)
2.728(28)
2.738 0.040
2.28
3.18
2.4074(7)
145.6
2.786(3)–2.978(3)/2.800(3)–3.010(3)
2.902(59)/2.907(78)
2.888(20)/2.896(27)
2.805(27)/2.817(23)
2.887 0.053/2.884 0.060
2.45/2.46
3.19/3.18
2.5383(17)/2.5224(17)
143.5/143.6
2.9627(14)–3.1937(17)
3.147(33)
3.113(10)
2.981(26)
3.100 0.068
2.70
3.21
2.7488(11)/2.7943(12)
130.9
53.1
Pdlcentroid–M–Pdlcentroidb (°)
αc
29.9
28.8
30.3/33.4
χ
178.5
175.0
176.1/176.0
124.8
a Two independent molecules in the asymmetric unit. b Pdlcentroid is the centroid of the pentadienyl ligand. c α is defined as the angle formed by
the two dienyl planes.
In contrast to the previous report,34 the salt metatheses of
two equivalents of KPdl′ with MI2 (M = Ca, Sr, Ba) proceed
smoothly at ambient temperature and yield the corresponding
open-metallocenes [(η5-Pdl′)2M(thf)n] (M = Ca (1), Sr (2), n = 1;
M = Ba (3), n = 2) in crystalline form and moderate yields
(eqn (1)).†
THF
5
2 KPdl′þMI2 ꢀꢀꢀꢀꢀꢀ! ½ðη -Pdl′Þ2MðthfÞnꢁ þ 2 Kl
M¼Ca; Sr; Ba
M ¼ Cað1Þ; n ¼ 1
M ¼ Srð2Þ; n ¼ 1
M ¼ Bað3Þ; n ¼ 1
ð1Þ
Fig. 2 ORTEP diagrams of 2 and 3 (50% probability ellipsoids). For 2
only one of the two independent molecules in the asymmetric unit is
shown. For 3 the two thf molecules are disordered over two positions,
but only one conformation is shown.
Nevertheless, we also found that the anionic Pdl′ ligand
behaves as a strong base and readily undergoes radical coup-
ling to 2,4,7,9-tetra-tert-butyl-1,3,7,9-decatetraene.41,42 There-
fore, it is imperative for the synthesis of these complexes to
use metal iodides MI2 of high purity. Trace amounts of NH3 or
I2 as a result of the MI2 preparation must be removed, other- better comparison, to include our data in Table 1. In all cases,
wise the yield of the salt metathesis reaction is dramatically the Pdl′ ligand adopts a η5-U coordination mode and the struc-
reduced and insoluble material is obtained. However, pure tural features of these complexes resemble those of 1, except
open-metallocenes 1–3 are readily soluble in aliphatic that two THF ligands are coordinated in the case of Ba.
hydrocarbons (pentane, hexane), aromatic solvents (benzene, Whereas one THF ligand resides in the usual position at the
toluene) and ethers (tetrahydrofuran, diethyl ether), and can open edge of one Pdl′ ligand, the second THF molecule is posi-
readily be crystallized as their thf adducts from concentrated tioned at the backside of the second pentadienyl ligand. In
pentane solutions at −30 °C.‡ Since our synthetic protocol [(η5-2,4-Me2C5H5)2Zr(CO)2] the two CO ligands feature
a
yields very soluble products, we also recorded H and 13C{1H} similar orientation with respect to the dienyl ligands.43 The
NMR spectra in C6D6 of these complexes and as expected, the conformational angles χ range from 175° for 1 to 124.8° for 3
NMR spectra of these complexes are very similar. In addition, and the predominant driving force for these arrangements is
no dynamic behaviour of the Pdl′ ligands is observed at the minimization of repulsive interactions between the Pdl′
1
ambient temperature in solution.†
and thf ligands. Furthermore, the C–C distances within the
The molecular structures of 2 and 3 are shown in Fig. 2, pentadienyl system show a distinct short–long–long–short-
and relevant bond distances and angles are listed in Table 1. pattern. Previous studies on [(η5-2,4-Me2C5H5)3Nd], which also
In addition significantly improved X-ray diffraction data for shows ionic bonding between the dienyl ligand and the metal
the open-calcocene 1 were obtained, so we decided, for a cation, established that the M–C bonds to the formally
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K, space group P212121, Z = 4 (monomers), μ(CuKα) = 2.5 mm−1, 18 651 reflec-
tions measured, 3697 independent reflections (Rint = 0.108). The final R1 values
were 0.0498 (I > 2σ(I)) and 0.0572 (all data). The final wR(F2) values were 0.1245
(I > 2σ(I)) and 0.1304 (all data). The goodness of fit on F2 was 1.02.
charged C1-, C3-, and C5-positions are shorter than those to
the uncharged C2- and C4-positions.44 For the complexes 1–3
the M–C distances vary significantly and the shortest is found
to be the central carbon atom of the Pdl′ ligand (C3-position)
consistent with the observation on [(η5-2,4-Me2C5H5)3Nd].
However, the average Ca–C1/5 distances (2.722(54) Å) in 1 are
essentially identical to the average Ca–C3 distances (2.728(28)
Å), a different trend is observed for the heavier homologues 2
and 3, for which the average M–C2/4 and M–C1/5 bonds
become progressively longer (Table 1). The increase in the
average M–C distances of 2.738(40) Å in 1 to 3.100(68) Å in 3
nicely mirrors the increase in ionic radii for heptacoordinate
Ca2+ (1.06 Å) and Sr2+ (1.21 Å) and octacoordinate Ba2+
(1.40 Å).45 These observations can be compared to those in
metal–allyl complexes, for which average M–C distances in
[(η3-1,3-(Me3Si)2C3H3)2Ca(thf)2] (2.654(4) Å),17 [(η3-C3H5)2(tri-
gylme-κ4)] (2.727(72) Å),46 and [(η3-1,3-(Me3Si)2C3H3)2Sr(thf)2]
(2.801 Å)21 are found. All attempts to prepare a momomeric
allyl Ba analogue resulted in the formation of a heterometallic
barium/potassium complex, [K(thf)Ba2(1,3-(Me3Si)2C3H3)5].21
This further supports the important features of the penta-
dienyl fragment in the stabilization of the heavy-alkaline earth
metals by adopting an intermediate position between the allyl
and cyclopentadienyl fragments. Furthermore, the average
M–C (M = Ca, Sr, Ba) bond distances in 1–3 are longer than
those in the related metallocenes, e.g. [(η5-1,3-(Me3Si)2C5H3)2-
M(thf)] (M = Ca (2.678(14) Å),47 Sr (2.82 Å)48) and [(η5-1,2,4-
(Me3C)3C5H2)2Sr(thf)] (2.87 Å).35 This presumably indicates a
more flexible and therefore weaker metal–ligand bonding in
the pentadienyl systems, and therefore a hapticity switch
should be relatively facile in the open group 2 metallocenes.
Crystal data for 1: C30H54OCa, M = 470.81, monoclinic, a = 9.9014(2) Å, b =
21.7075(4) Å, c = 14.4003(3) Å, β = 105.501(2)°, V = 2982.56(10) Å3, T = 100(2) K,
space group P21/n, Z = 4, μ(MoKα) = 1.9 mm−1, 78 853 reflections measured,
6171 independent reflections (Rint = 0.048). The final R1 values were 0.0296 (I >
2σ(I)) and 0.0306 (all data). The final wR(F2) values were 0.0785 (I > 2σ(I)) and
0.0794 (all data). The goodness of fit on F2 was 1.05.
Crystal data for 2: C30H54OSr, M = 518.35, triclinic, a = 9.7948(5) Å, b =
14.1834(5) Å, c = 23.3962(10) Å, α = 88.549(3)°, β = 79.821(2)°, γ = 71.131(2)°, V =
3
3025.4(2) Å , T = 100(2) K, space group P(1), Z = 4, μ(MoKα) = 1.8 mm−1, 132 306
ˉ
reflections measured, 15 422 independent reflections (Rint = 0.111). The final R1
values were 0.0544 (I > 2σ(I)) and 0.1022 (all data). The final wR(F2) values were
0.0724 (I > 2σ(I)) and 0.0825 (all data). The goodness of fit on F2 was 1.04.
Crystal data for 3: C34H62BaO2, M = 640.18, triclinic, a = 9.5411(3) Å, b =
12.0829(3) Å, c = 15.6171(5) Å, α = 96.320(2)°, β = 97.530(3)°, γ = 98.661(3)°, V =
3
1748.97(9) Å , T = 100(2) K, space group P(1), Z = 2, μ(MoKα) = 1.2 mm−1, 92 321
ˉ
reflections measured, 10 431 independent reflections (Rint = 0.031). The final R1
values were 0.0222 (I > 2σ(I)) and 0.0262 (all data). The final wR(F2) values were
0.0493 (I > 2σ(I)) and 0.0514 (all data). The goodness of fit on F2 was 1.05.
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We have extended the series of group 2 open-metallocenes to Sr
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the preparation of synthetically useful quantities. In all cases
the Pdl′ ligand adopts a η5-U coordination mode in the solid
state and in solution. It can be expected that the other group 2
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Acknowledgements
MDW acknowledges the financial support from the TU
Braunschweig through the “Zukunftsfonds” and from the
Deutsche Forschungsgemeinschaft (DFG) through the Emmy
Noether program (WA 2513/2).
Notes and references
‡Crystal data for [(thf)K(μ–η5:η5-Pdl′)]∞: C17H31KO, M = 290.52, orthorhombic,
a = 10.1190(2) Å, b = 10.7148(6) Å, c = 16.4367(9) Å, V = 1782.11(14) Å3, T = 130(2)
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