Cp* as a removable protecting group: Low valent Zn(i) compounds by reductive elimination, protolytic and oxidative cleavage of Zn-Cp*

Article abstract of DOI:10.1039/c3dt51230d

Zn-Cp* bond cleavage reactions leading to novel monovalent cationic zinc species are presented (Cp* = pentamethylcyclopentadienyl). The treatment of [Zn₂Cp*₂] with two equiv. of [H(Et ₂O)₂][BAr₄^F] (BAr₄ ^F = B{C₆H₃(CF₃)₂} ₄) yields the triple-decker complex [Cp*₃Zn ₄(Et₂O)₂][BAr₄^F] (1) via protolytic removal of a Cp* ligand as Cp*H, whereas the reaction with an equimolar amount of [FeCp₂][BAr₄^F] (Cp = cyclopentadienyl) results in the formation of [Cp*Zn₂(Et ₂O)₃][BAr₄^F] (2) under oxidative cleavage of a Cp* ring giving decamethylfulvalene, (Cp*) ₂, and [FeCp₂] as by-products. The molecular structures of compounds 1 and 2 are established by single-crystal X-ray diffraction studies. A new synthetic pathway for the formation of [Zn₂Cp* ₂] based on the reductive elimination of Cp*H from in situ formed Cp*ZnH is presented.

Full text of DOI:10.1039/c3dt51230d

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Cp* as a removable protecting group: low valent Zn(I)
compounds by reductive elimination, protolytic and
oxidative cleavage of Zn–Cp*†
Cite this: Dalton Trans., 2013, 42, 10540
Kerstin Freitag,a Hung Banh,a Chelladurai Ganesamoorthy,a Christian Gemel,a
Rüdiger W. Seidelb and Roland A. Fischer*a
Zn–Cp* bond cleavage reactions leading to novel monovalent cationic zinc species are presented (Cp* =
F
pentamethylcyclopentadienyl). The treatment of [Zn2Cp*2] with two equiv. of [H(Et2O)2][BAr4F] (BAr4
=
B{C6H3(CF3)2}4) yields the triple-decker complex [Cp*3Zn4(Et2O)2][BAr4F] (1) via protolytic removal of a
Cp* ligand as Cp*H, whereas the reaction with an equimolar amount of [FeCp2][BAr4F] (Cp = cyclopenta-
dienyl) results in the formation of [Cp*Zn2(Et2O)3][BAr4F] (2) under oxidative cleavage of a Cp* ring
giving decamethylfulvalene, (Cp*)2, and [FeCp2] as by-products. The molecular structures of compounds
1 and 2 are established by single-crystal X-ray diffraction studies. A new synthetic pathway for the for-
mation of [Zn2Cp*2] based on the reductive elimination of Cp*H from in situ formed Cp*ZnH is
presented.
Received 10th May 2013,
Accepted 29th May 2013
DOI: 10.1039/c3dt51230d
www.rsc.org/dalton
compounds, the knowledge on related cationic species is still
rather limited. The existence of [Zn2]2+ was first evidenced in
1. Introduction
Although [Zn2Cp*2] (Cp* = pentamethylcyclopentadienyl) was 1967 in a zinc solution of fused ZnCl2,19 but up to now, only
discovered nearly a decade ago by Carmona et al.,1 a possible the dimethylaminopyridine stabilized [Zn2]2+ dication
reaction mechanism for the formation of this remarkable ZnI– [Zn2(dmap)6][Al{OC(CF3)3}4]2 (dmap = 4-dimethylaminopyri-
ZnI bonded species has not yet been fully elucidated. First pos- dine) has been isolated and characterized.15 These results
tulations have been made by Schnepf and Himmel in 2005 suggest that a selective removal of the stabilizing/protecting
where the authors discussed the involvement of active zinc, Cp* moieties from [Zn2Cp*2] is feasible in general and the syn-
formed in disproportionation steps during the reaction, which thetic success will depend on finding the right reaction con-
reduces ZnCp*2.2 This possibility has been ruled out by ditions and sufficiently stabilizing ligand settings for the
Carmona et al. as Rieke-zinc was not strong enough to achieve resulting [Zn2]2+ species. As for the smooth removal of Cp*
reduction and conversion into [Zn2Cp*2].3 Until now, the groups from related GaCp* species we were able to demon-
F
F
favoured perception of the reaction scheme is based on radical strate the suitability of [H(Et2O)2][BAr4
]
(BAr4
=
F
mechanisms in which [Zn2Cp*2] is formed by dimerization of B{C6H3(CF3)2}4) and [FeCp2][BAr4 ] (Cp = cyclopentadienyl). The
the monovalent, radical fragments [Cp*Zn˙].4 The first reports first reagent allows the abstraction by protolytic splitting of
on [Zn2Cp*2] were mostly dealing with the structure and Cp*H, the latter one yields selective oxidation of the Cp*
bonding situation of the Cp* stabilized Zn2 molecule itself, moiety forming decamethylfulvalene as a by-product. Thus,
F
F
recent studies also focus on the synthesis of new Zn2 contain- the reaction of GaCp* with [H(Et2O)2][BAr4
]
(BAr4
=
ing derivatives [Zn2R2] (R ≠ Cp*)4–17 as well as the reactivity of B{C6H3(CF3)2}4) results in the formation of the cation
[Zn2Cp*2] towards aryl or alkyl alcohols in the presence of [Ga2(C5Me5)]+.20 Accordingly, Braun et al. synthesized the
stabilizing nitrogen donors.18 In contrast to neutral organo ZnI triple-decker complex [Zn2Cp*3][BAr4F], emerging from the
reaction of [ZnCp*2] with [H(Et2O)2][BAr4F].21 In addition, we
succeeded in the oxidative cleavage of Ga–Cp* bonds with
[FeCp2][BAr4F] in [M(GaCp*)4] (M = Ni, Pd, Pt) yielding [GaNi-
(GaCp*4)]+ and [(µ2-Ga)nM3(GaCp*)6]n+ (n = 2 for M = Pd and
n = 1 for M = Pt).22 Herein, we report on selective Zn–Cp* clea-
vage reactions of [Zn2Cp*2] leading to cationic species with an
intact ZnI–ZnI bond: [Cp*3Zn4(Et2O)2][BAr4F] is accessible by
protonation of [Zn2Cp*2] with [H(Et2O)2][BAr4F], while
aInorganic Chemistry II, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum,
Universitätsstraße 150, 44780 Bochum, Germany. E-mail: roland.fischer@rub.de
bAnalytical Chemistry, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum,
Universitätsstraße 150, 44780 Bochum, Germany
†Electronic supplementary information (ESI) available. CCDC 928866 and
928867. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt51230d
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oxidation with [FeCp2][BAr4F] selectively leads to semi-depro- be assigned to [M − (Et2O)2]+ (see the ESI†). Two more signals
tected [Cp*Zn2(Et2O)3][BAr4F]. These studies of selective Cp* at m/z = 602 and 536 indicate the loss of one and two zinc
removal from [Zn2Cp*2] lead us to a new and reliable synthetic atoms, respectively (see the ESI†).
route for the preparation of [Zn2Cp*2] based on a Cp*H reduc-
tive elimination reaction from the precursors [ZnCp*2] and 2.3. Molecular structure of [Cp*3Zn4(Et2O)2][BAr4F] (1)
ZnH2(s) most likely involving in situ formed [Cp*ZnH].
Compound 1·2C6H5F crystallizes in the monoclinic space
group P21/c with four molecules in the unit cell. While Zn1
and Zn4 are η5 bound to terminal Cp* rings, two more Zn
atoms (Zn2 and Zn3) are η3 bound to one bridging Cp* ring
resulting in a “bent” triple-decker sandwich structure (Fig. 1).
The bridging Cp* ligand and the terminal one bound to Zn1
are each disordered over two sites. The Zn–Zn distances are
almost identical with values of 2.310(1) Å for Zn1–Zn2 and
2.307(1) Å for Zn3–Zn4. These bond distances are virtually the
same as the one found in [Zn2Cp*2] (2.305(3) Å) or other Zn2R2
compounds, e.g. [Zn2TpMe22] (Tp = tris(3,5-dimethylpyrazolyl)
hydridoborate).1,14 The approximate Zn–Cp*centroid distances
for the terminal ligands (Zn1–Cp*centroid 1.93 Å, Zn4–Cp*centroid
1.89 Å) are slightly shortened as compared to the mother
compound [Zn2Cp*2] (2.04 Å) or other Zn2R2 compounds like
[{Zn2(η5-C5Me5)(µ-OC5Me5)-(pyr-py)}2] (2.004 Å) or [Cp*2ZnZn-
(dmap)2] (2.033 Å) (dmap = 4-dimethylaminopyridine).1,18,27
However, these ZnCp*centroid distances are all slightly
elongated in comparison to the recently published triple-
decker complex of ZnII, [Zn2Cp*3][BAr4F] (Zn–Cp*centroid 1.828
and 1.830 Å).21 The Zn2–CCp* and Zn3–CCp* distances in 1
2. Results and discussion
2.1. Protolytic cleavage of Zn–Cp* – synthesis of
[Cp*3Zn4(Et2O)2][BAr4F] (1)
The reaction of [H(Et2O)2][BAr4F] with two equivalents of
[Zn2Cp*2] at −25 °C in fluorobenzene leads to the novel double
sandwich complex [Cp*3Zn4(Et2O)2][BAr4F] (1) as a white
powder in yields around 70% (Scheme 1). Colourless crystals
of 1·2C6H5F suitable for single crystal X-ray diffraction were
obtained by slow diffusion of n-hexane into a saturated fluoro-
benzene solution of 1 at −30 °C. The pure solid of 1 is stable
under an inert gas atmosphere at −30 °C but decomposes
rapidly in the solid state at room temperature under formation
of a metallic precipitate.
2.2. Spectroscopic features of [Cp*3Zn4(Et2O)2][BAr4F] (1)
The 1H NMR spectrum of 1 in CD2Cl2 at room temperature
F −
shows the expected set of signals for one [BAr4
] anion (δ =
7.57 (s, 4H) and 7.73 (s, 8H) ppm) and two coordinated diethyl
ether solvent molecules (δ = 1.22 (m, 12H) and 3.63 (m, 8H
ppm)) as well as one signal for the Cp* groups at δ = 2.06 (s,
45H) ppm. Low temperature 1H NMR measurements at −60 °C
show no splitting of the signal which indicates a fast fluxional
process interchanging the terminal and bridging Cp* units.
strongly vary between 2.256(3)–2.451(3) (Zn2–CCp*
) and
2.316(4)–2.572(3) (Zn3–CCp*), due to the η3 binding mode of
the central Cp* ring. This can also be observed in the related
[Zn2Cp*3]+, where the respective distances vary between 2.060(3)
and 2.049(3) Å and 2.396(3) and 2.050(3) Å caused by an η1
binding mode.21 Both Zn–O bonds (Zn2–O2 2.204(4) Å, Zn3–
O1 2.234(4)) are elongated as compared to literature known
The 19F NMR spectrum exhibits one signal at δ = 62.9 ppm for
F −
the CF3 groups of the [BAr4
] anion, as expected. Owing to
the instability of 1 in solution, no 13C NMR spectrum could be
obtained. The FTIR spectrum of 1 shows absorption bands
typical of the Cp* units at 2955, 2887 and 2844 cm−1 and a
band at 1260 cm−1 ascribed to the C–F vibration. Both absorp-
tion bands are well comparable to literature known
examples.23–26
Liquid injection field desorption ionization mass spec-
trometry (LIFDI-MS) displays a signal at m/z = 668, which can
Fig. 1 Molecular structure of the cation [Zn4Cp*3(Et2O)2]+ in the crystal of
1·2C6H5F. Displacement ellipsoids are drawn at the 30% probability level. For
the sake of clarity, disorder of the Cp* ligands and hydrogen atoms are omitted.
Selected interatomic distances (Å) and angles (°): Zn1–Zn2 2.310(1), Zn3–Zn4
2.307(1), Zn2–O2 2.204(4), Zn3–O1 2.234(4), Zn2–C31 2.442(3), Zn2–C32
2.256(4), Zn2–C33 2.402(3), Zn3–C33 2.572(3), Zn3–C43 2.316(4), Zn3–C35
2.412(4), Zn1–Cp*centroid 1.935, Zn4–Cp*centroid 1.899; Cp*centroid–Zn1–Zn2 165.86,
Cp*centroid–Zn3–Zn4 170.92.
Scheme 1 Synthesis of [Zn2Cp*2] by reductive elimination of Cp*H and for-
mation of the cations [Cp*3Zn4(Et2O)2]+ and [Cp*Zn2(Et2O)3]+ from [Zn2Cp*2] by
selective protolysis or oxidative cleavage.
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Zn–OEt2 bond distances, which range from 1.997 Å in
[Ph2SiO]8[AlO(OH)]2[AlO2]2[Zn(OH)]2·2(OEt2) to 2.163 Å in [(R,R)-
(N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethylene-
diamino)(diethyl ether)zinc(II)], indicating a rather weak Zn–
OEt2 interaction in 1.28,29 The angles Zn–Zn–Cp*centroid exhibit
values of 165.86(4)° (Zn2–Zn1–Cp*centroid) and 170.92(4)° (Zn3–
Zn4–Cp*centroid), respectively, which results in a significant
deviation from linearity, possibly due to steric reasons. This
deviation can also be observed in similar compounds with
additional coordinated donor ligands to the ZnZnCp* unit, e.g.
[Zn2Cp*(OArMes)(pyr-py)2] (175.6°) or [Zn2Cp*(µ-OC5Me5)(pyr-py)]2
(167.2°).18
2.4. Oxidative cleavage of Zn–Cp* – synthesis of
semi-protected [Cp*Zn2(Et2O)3][BAr4F] (2)
Treatment of [Zn2Cp*2] with an equimolar amount of [FeCp2]-
[BAr4F] at room temperature in diethyl ether leads to an oxi-
Fig. 2 Hydrogen atoms and the disorder of the complex are omitted for clarity.
Selected interatomic distances (Å) and angles (°): Zn1–Zn2 = 2.324(2), ca. Zn2–
Cp*centroid = 1.92, Zn1–O1 = 2.117(4), Zn1–O2 = 2.096(4), Zn1–O3 = 2.076(4);
dative cleavage of
a
Cp* moiety and formation of
[Cp*Zn2(Et2O)3][BAr4F] (2) as a grey powder with a yield of 71%
(Scheme 1). Ferrocene and decamethylfulvalene (Cp*)2 can
both be observed as stoichiometric by-products by in situ 1H
NMR spectroscopy. Although the compound is stable as a
microcrystalline powder at −30 °C under an inert gas atmos-
phere, it gradually decomposes at room temperature in the
solid state and more rapidly in solutions of fluorobenzene,
diethyl ether or dichloromethane under precipitation of
metallic particles.
Cp*centroid–Zn2–Zn1
= 174.47, Zn2–Zn1–O1 = 121.99(15), Zn2–Zn1–O2 =
120.54(12), Zn2–Zn1–O3 = 121.71(14).
ligand from the parent compound [Zn2Cp*2] is replaced by
three Et2O molecules, while the second Zn atom is still co-
ordinated by an η5-Cp* moiety. In the crystal, the entire
complex is disordered over two sites. Compared to [Zn2Cp*2]
(2.305(3) Å) the Zn–Zn bond of 2 is slightly elongated (2.324(2)
Å).1 However this value is in the range of other reported ZnI–
ZnI bond lengths, such as [Zn2(dpp-bian)2] (dpp-bian = 1,2-bis-
[(2,6-diisopropylphenyl)imino]acenaphthene) (2.332(2) Å).7
The Zn2–Cp*centroid distance of approximately 1.92 Å is com-
parable with the Zn1–Cp*centroid and Zn4–Cp*centroid distances
of compound 1 and accordingly distinctly shorter than in
[Zn2Cp*2] (2.04 Å).1 Compound 2 shows a deviation of the
angle Zn1–Zn2–Cp*centroid (170.31°) from linearity, similar to
other ZnI–ZnI compounds with coordinated donor ligands
([Zn2(η5-C5Me5)(OArMes)(pyr-py)2] (175.6°), [Zn2(η5-C5Me5)-
(µ-OC5Me5)(pyr-py)]2 (167.2°) or [Zn2Cp*2(dmpa)2] (159.6°)).18,27
The Zn1–O1, Zn1–O2 and Zn1–O3 distances are 2.117(4), 2.096(4)
and 2.076(4) Å, respectively, which are very close to those of
other Zn–Et2O complexes such as [EtZn(OEt2)3][B(C6F5)4]
(2.110(2), 2.072(2) and 2.031(2) Å).30 There is a major differ-
ence between the Zn–Zn–O angles of compound 1 with
109.66(11) and 110.2(1)° and compound 2 whose values range
from 120.54(12) to 121.99(15)°.
2.5. Spectroscopic features of [Cp*Zn2(Et2O)3][BAr4F] (2)
The 1H NMR spectrum of 2 in CD2Cl2 at room temperature
exhibits two signals for the [BAr4 ]
F − anion (δ = 7.56 (s, 4H) and
7.71 (s, 8H)), two resonances corresponding to the coordinated
Et2O molecules (δ = 1.25 (t, 18H) and 3.69 (q, 12H) ppm) and
one peak for the Cp* moiety (δ = 2.08 (s, 15H) ppm). At room
temperature additional signals are observed due to decompo-
sition of the compound: at 8.24 and 8.02 ppm (BArF) in a ratio
of 1 : 2.
The coordinated Et2O molecules in 2 can be exchanged by
1
THF molecules at room temperature: the H NMR spectrum of
1 in THF-d8 at room temperature shows signals for non-
coordinated Et2O at δ = 3.39 (q) and 1.12 (t) ppm as well as two
F −
resonances for the [BAr4
] anion (δ = 7.58 (s, 4H) and 7.79 (s,
8H)) and one peak for the Cp* unit at δ = 2.07 (s, 15H) ppm.
Solutions of 1 in THF are apparently less temperature sensitive
than solutions in other solvents like fluorobenzene, CH2Cl2 or
Et2O. The FTIR spectrum of 2 reveals typical bands for the Cp*
ring at 2963, 2889 and 2845 cm−1 as well as a signal due to the
2.7. Mechanism of the [Zn2Cp*2] formation via elimination
of Cp*H
C–F vibration at 1265 cm−1
2.6. Molecular structure of [Cp*Zn2(Et2O)3][BAr4F] (2)
.
The convenient and high-yield elimination of Cp*H from
[Zn2Cp*2] by addition of an external proton source (intermole-
Colourless crystals appropriate for single crystal X-ray diffrac- cular attack on Zn–Cp*) prompted us to study the Cp*H elimi-
tion were obtained by slow diffusion of n-hexane into a fluoro- nation reaction also in systems with an internal hydrogen
benzene solution of 2 at −30 °C. Compound 2 crystallizes in source, i.e. zinc-hydride moiety. Notably, such a very selective,
the orthorhombic space group Pbca. Note that the cation is intramolecular reductive elimination reaction in main group
fully disordered. The coordination geometry of the Cp*Zn2O3 organometallic chemistry has recently been reported for the
unit resembles an “elongated” piano stool (Fig. 2). One Cp* aluminium complexes [Cp*AlH2] and [Cp*2AlH]: heating a
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(CF3)3}4]2,15 contain weakly coordinating anions, indicating
the need of such components for a successful isolation of the
di-zinc species. The selective Cp*H elimination turned out as
the key step in the formation of [Zn2Cp*2] from ZnH2 and
ZnCp*2 whereby [ZnCp*H] is suggested to be the crucial inter-
mediate and a new, reliable synthesis of [Zn2Cp*2] was develo-
ped. Apparently, efficient nucleophilic reaction mechanisms,
i.e. intramolecular hydride migration, are possible at the Zn
centre and allow selective Zn–Cp* bond splitting to yield the
desired ZnI compound. From these data it can be expected
that the lability of the Zn–Cp* bond towards both, electrophilic
(protonation) and nucleophilic (hydride) attack and as well
single electron oxidation allow new approaches to other cat-
ionic ZnI products. Moreover it is reasonable to assume that
the reported chemistry may offer valuable precursors and reac-
tion concepts for the formation of intermetallic Zn-rich
clusters.32
Scheme 2 Formation of [Zn2Cp*2] from [ZnCp*2] and ZnH2.
solution of [Cp*AlH2] leads quantitatively to metallic alu-
minium, Cp*H (and presumably H2). Accordingly, reductive
elimination of Cp*H from [Cp*2AlH] represents a new high
yield access to AlCp*.31 Indeed, the reaction of [ZnCp*2] and
ZnH2 in a 3 : 1 molar ratio in THF affords [Zn2Cp*2] and Cp*H
1
in yields around 65%. As shown by H NMR spectroscopy, the
only observed species are the reaction products [Zn2Cp*2] and
Cp*H as well as the starting compound [ZnCp*2]. The yield of
the products depends only on the molar ratio of starting com-
pounds. The reaction proceeds quantitatively, however, the
product [Zn2Cp*2] itself is reactive towards ZnH2 leading to
metallic Zn and Cp*H. By suitable adjustment of reactant con-
centration and molar ratios this undesired decomposition can
be slowed down very considerably and isolated yields of
[Zn2Cp*2] up to 65% (based on Zn) can be obtained. The com-
parison with [Cp*2AlH] suggests that the reaction mechanism
is likely to involve a soluble, unstable intermediate, presum-
ably [Cp*ZnH], which is formed via a heterogeneous reaction
of pure, insoluble ZnH2(s) and soluble [ZnCp*2]. This inter-
mediate [Cp*ZnH] spontaneously reacts with another equi-
valent of [ZnCp*2] to give [Zn2Cp*2] via reductive elimination
of Cp*H (Scheme 2). Preliminary kinetic studies are in agree-
ment with that reasoning (Fig. S8†), however, a more detailed
analysis of this reaction is needed and currently ongoing. The
results will be published elsewhere together with theoretical
calculations, similar to the related [Cp*2AlH] case.30
It should be noted that the possible pathway for [Zn2Cp*2]
formation suggested herein agrees well with the literature: an
appropriate mixture of [ZnCp*2], ZnCl2 and KH most likely gives
a similar distribution of intermediates and products as the com-
bination of [ZnCp*2] with pure, preformed ZnH2. However, the
reaction of [ZnCp*2] with ZnR2 (R = Et, Ph) at low temperature,
which is the second (and low yield) route to [Zn2Cp*2] known in
the literature, cannot be explained by a similar (intra/inter-
molecular) reductive Cp*H elimination mechanism.3
Acknowledgements
This work was funded by the German Research Foundation
(Fi-502/23-2). K.F. is grateful for a Ph.D. scholarship provided
by the German Chemical Industry Fund (https://www.vci.de/
fonds) and for the support of the Ruhr University Research
School (http://www.research-school.rub.de). We thank the
Linden CMS GmbH for support in mass spectrometry.
Notes and references
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3. Conclusions
In this contribution we presented two routes to unusual cat-
ionic low valent zinc species based on the selective cleavage of
a Cp* ligand from [Zn2Cp*2]. Protonation with [H(Et2O)2]-
[BAr4F] yields the triple-decker complex [Cp*3Zn4(Et2O)2]+ and
oxidation with [FeCp2][BAr4F] leads to the formation of semi-
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]
F
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