Sterically encumbered pyrrolyl ligands and their incorporation into the cycloheptatrienyl zirconium coordination sphere

Article abstract of DOI:10.1039/c2dt32433d

An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPyr^tBu₂Me₂) and a subsequent reaction with KH yield K(Pyr^tBu₂Me₂) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl (Im) ligands were used in salt metathesis reactions with [(η⁷-C₇H ₇)ZrCl(tmeda)] (7) to afford a series of azatrozircenes: [(η⁷-C₇H₇)Zr(η⁵-Pyr ^tBu₂)] (1-Zr), [(η⁷-C₇H ₇)Zr(η⁵-Pyr^tBu₂Me₂)] (2-Zr), [(η⁷-C₇H₇)Zr(η⁵- Pyr^tBu₃)] (3-Zr) and [(η⁷-C ₇H₇)Zr(η⁵-Im^tBu₃)] (4-Zr), which were characterized by NMR spectroscopy and elemental analysis. In addition, the molecular structures of 2-H, 2-K·18-crown-6, 1-Zr, 2-Zr and 4-Zr were determined by X-ray diffraction analysis, revealing η⁵- rather than κ¹-N-coordination of the N-heterocyclic ligands. Cone angle measurements on the sandwich complexes 1-Zr-4-Zr showed that their nitrogen-containing ligands belong to the class of very sterically encumbered π-ligands, but DFT calculations suggest lower stabilities compared to their all-carbon analogues. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt32433d

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Sterically encumbered pyrrolyl ligands and their
incorporation into the cycloheptatrienyl zirconium
Cite this: DOI: 10.1039/c2dt32433d
coordination sphere†
Markus Kreye, Andreas Glöckner, Constantin G. Daniliuc, Matthias Freytag,
Peter G. Jones, Matthias Tamm and Marc D. Walter*
An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPyrtBu Me ) and a subsequent reac-
2
2
tion with KH yield K(PyrtBu Me ) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl
(Im) ligands were used in salt metathesis reactions with [(η7-C7H7)ZrCl(tmeda)] (7) to afford a series of
2
2
7
5
tBu2Me2
azatrozircenes: [(η7-C7H7)Zr(η5-PyrtBu )] (1-Zr), [(η -C7H7)Zr(η -Pyr
)] (2-Zr), [(η7-C7H7)Zr(η5-PyrtBu3)]
2
(3-Zr) and [(η7-C7H7)Zr(η5-ImtBu )] (4-Zr), which were characterized by NMR spectroscopy and elemental
analysis. In addition, the molecular structures of 2-H, 2-K·18-crown-6, 1-Zr, 2-Zr and 4-Zr were deter-
mined by X-ray diffraction analysis, revealing η5- rather than κ1-N-coordination of the N-heterocyclic
ligands. Cone angle measurements on the sandwich complexes 1-Zr–4-Zr showed that their nitrogen-
containing ligands belong to the class of very sterically encumbered π-ligands, but DFT calculations
suggest lower stabilities compared to their all-carbon analogues.
3
Received 12th October 2012,
Accepted 7th November 2012
DOI: 10.1039/c2dt32433d
www.rsc.org/dalton
discovery of ferrocene, [(η5-C5H5)2Fe].7 However, while azaferro-
cene [(η5-C5H5)Fe(η5-Pyr)] (Pyr = C4NH4) and moderately substi-
Introduction
We have recently employed bulky cyclopentadienyl (Cp) and tuted analogues such as [(η5-C5Me5)Fe(η5-Pyr)] or [(η5-C5H5)Fe-
Me2
indenyl (Ind) ligands for the stabilization of unusual mole- (η5-PyrMe )] (Pyr
= 2,5-dimethylpyrrolyl) have been exten-
2
cules of iron,1 manganese1e,2 and thorium.3 In this context sively studied,8 the corresponding diazaferrocenes [(η5-Pyr)2Fe]
cone angle measurements on a series of [(η7-C7H7)Zr(η5-Cp)] are unstable species even when the permethylated PyrMe
4
and [(η7-C7H7)Zr(η5-Ind)] complexes have allowed us to assess ligand is used.9 Only the bulky 2,5-di(tert-butyl)pyrrolyl ligand
in a realistic manner the size of such ligands bearing different (PyrtBu ) provides sufficient steric protection for the synthesis
2
substituents.4 In general, many substituents can impose steric and structural characterization of the elusive diazaferro-
bulk, and a number of them have been used for various pur- cenes.10 Subsequently, this ligand has also been employed for
poses.5,6 However, we prefer tert-butyl groups because of their other transition,11 rare-earth12 and main-group13 metal (sand-
simple NMR spectra, which is particularly useful when dealing wich) complexes, which predominantly exhibit the η5-coordi-
with paramagnetic compounds.1,2 To further explore the nation mode. The alternative σ-coordination via the formation
advantages of sterically encumbered π-ligands in the prepa- of a strong metal–nitrogen bond is disfavoured by the steric
ration of kinetically stabilized complexes, we decided to turn demand of the cylindrical tert-butyl groups in α-positions.
our attention to pyrrolyl systems. This class of ligands is for- Similar observations have been reported for a series of zirconium
mally derived from the replacement of a CH group in the cyclo- complexes incorporating the phenyl-substituted pyrrolyl ligand,
14
pentadienyl ring by an isolobal and isoelectronic nitrogen PyrPh
.
In contrast, group 4 metal complexes involving smaller
2
atom. Therefore, it is not surprising that the first pyrrolyl com- derivatives, such as the commercially available Pyr or PyrMe
plexes of iron and manganese were reported shortly after the ligands,15 frequently favour κ1-N-coordination, which can be
ascribed to the weaker donor properties of the pyrrolyl compared
2
to the cyclopentadienyl π-system.16 In this context, it should be
noted that theoretical studies suggest a fluxional process in
group 4 pyrrolyl complexes, interconverting the two possible
coordination modes,17 and experimental support for this has
been provided by dipyrrolylmethane complexes of titanium.18
Whereas most of the investigations on bulky pyrrolyl
ligands and their complexes were reported by Kuhn and
Institut für Anorganische und Analytische Chemie, Technische Universität
Braunschweig, Hagenring 30, 38106 Braunschweig, Germany.
E-mail: mwalter@tu-bs.de; Fax: +49 531-391-5309
†Electronic supplementary information (ESI) available: Additional molecular
structures, determination of cone angles and computational details. CCDC
902358–902364. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt32433d
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Fig. 1 Bulky heterocyclic π-ligands discussed in this paper.
Scheme 1 Synthesis of K(PyrtBu Me ) (2-K).
2
2
co-workers in the early 1990s,10,11,13 sterically demanding
pyrrolyl ligands have remained largely neglected in the last two
decades. Therefore, we decided to start our investigations
with a familiar and well-defined complex fragment, namely
(η7-C7H7)Zr, before moving our efforts to unexplored areas.
The cycloheptatrienyl zirconium scaffold is accessible from the
half-sandwich precursor [(η7-C7H7)ZrCl(tmeda)], which is an
excellent starting material for the incorporation of monoanio-
nic ligands into the (η7-C7H7)Zr coordination sphere.4,19,20 In
this context various η5-ligands have been coordinated to this
fragment to form the corresponding trozircene complexes. The
trivial name trozircene is derived from tropylium, zirconium
and cyclopentadienyl. A prefix such as “half-open”, “phospha”
or “aza” is used to differentiate the η5-ligand from the conven-
tional cyclopentadienyl ligand. Most relevant for this manu-
script is a series of phosphatrozircenes of the type [(η7-C7H7)Zr-
(η5-C4PR4)], because they contain phospholyl ligands, the
heavier analogues of pyrrolyl systems.19d
Fig. 2 ORTEP diagram of 2-H with thermal displacement parameters drawn at
50% probability. Selected bond lengths (Å): N–C1 1.3784(16), C1–C2 1.3763(18),
C2–C3 1.4372(16), C3–C4 1.3827(17), C4–N 1.3764(16), N–H1 0.89(2).
from 6 dissolved in an NH3-saturated methanol solution
(Scheme 1, see the Experimental section for details). The col-
ourless product can be handled in air, but should be stored
under an inert atmosphere to avoid decomposition. One of the
degradation products is a hydroperoxide, which we structurally
characterised (see ESI† for details). This observation can pre-
sumably be ascribed to a reaction with oxygen, which is con-
sistent with recent reports on the controlled oxidation of
pyrroles.28 The molecular structure of 2-H resembles that of
In addition to the established 2,5-di(tert-butyl)pyrrolyl
10–13
ligand (PyrtBu , 1),
we wish to address in this contribution
2
HPyrtBu (Fig. 2).13c In particular, the electron delocalization
the following, scarcely used, nitrogen-containing heterocycles
2
(Fig. 1): 2,5-di(tert-butyl)-3,4-dimethylpyrrolyl (PyrtBu Me , 2),
2
2
within the pyrrole ring is disturbed, as shown by a distinct
short–long–short pattern in the C–C bond lengths of the
C1–C2–C3–C4 moiety. For instance, the C1–C2 bond distance
is at 1.3763(18) Å markedly shorter than the C2–C3 bond dis-
tance of 1.4372(16) Å.
21
2,3,5-tri(tert-butyl)pyrrolyl (PyrtBu , 3) and 2,4,5-tri(tert-butyl)-
3
22
imidazolyl (ImtBu , 4).
3
The potassium salt, K(PyrtBu Me ) (2-K), can readily be pre-
pared in high yield from 2-H and KH in refluxing THF. 2-K has
a low solubility in non-coordinating solvents, but was crystal-
lized from toluene as its 18-crown-6 adduct, 2-K·18-crown-6.
The molecular structure is shown in Fig. 3 and exhibits an
η5-coordination mode with the potassium atom clearly posi-
tioned underneath the five-membered ring; the K–N distance
is 3.0223(10) Å, with K–C distances of 3.0223(10)–3.1828(11) Å.
Despite significant differences, the C–C bond distances within
the five-membered ring are much more similar than in 2-H.
A similar bonding picture was established for the 18-crown-6
2
2
Results and discussions
Ligand synthesis
The Paal–Knorr reaction between the appropriate diketone and
ammonium acetate (NH4OAc) is a well-established procedure
for the synthesis of HPyrtBu (1-H) which is also the starting
2
material for the preparation of HPyrtBu (3-H) and 2,5-di(tert-
3
butyl)-3-trimethylsilylpyrrole.21,23,24 In contrast, the synthesis
of HPyrtBu Me (2-H) requires some modifications, because the
reaction of 2,2,4,5,7,7-hexamethyloctan-3,6-dione (6), obtained
from oxidative coupling of 2,2-dimethylpent-3-one (5),25 with
NH4OAc affords the corresponding furan instead of the
desired pyrrole.26 Kuhn and co-workers eventually succeeded
in isolating this pyrrole in 60% yield from 6 and NH3 in an
autoclave at 8 bar and 50 °C.27 We have essentially followed
their protocol, but have accomplished the cyclization by using
milder conditions that do not require special equipment.
2
2
tBu2
adduct of K(CptBu ) (Cp
= 1,3-di(tert-butyl)cyclopentadi-
2
enyl), which was prepared by an acid–base reaction from [K(18-
crown-6)OtBu] and the corresponding cyclopentadiene.29
In contrast, κ1-N-coordination was established for (thf)2Li-
13c
(PyrtBu ),
which may be explained either by the smaller
2
metal and/or by the higher flexibility of the two THF ligands
compared to 18-crown-6, thereby reducing steric interactions
Thus, HPyrtBu Me (2-H) is now synthesized in a Schlenk tube
2
2
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Fig. 4 ORTEP diagram of 1-Zr with thermal displacement parameters drawn at
50% probability. Selected bond lengths (Å): Zr–CCht 2.318(3)–2.356(3), Zr–C8
2.470(3), Zr–C9 2.551(3), Zr–C10 2.546(3), Zr–C11 2.466(3), Zr–N 2.400(2).
Fig. 3 ORTEP diagram of 2-K·18-crown-6 with thermal displacement para-
meters drawn at 50% probability. The hydrogen atoms of the crown-ether have
been omitted for clarity. Selected bond lengths (Å): K–N 3.0223(10), K–C1
3.0687(11), K–C2 3.1828(11), K–C3 3.1816(11), K–C4 3.0759(11), K–O1
2.8085(8), K–O2 2.9331(8), K–O3 2.9559(9), K–O4 2.8206(8), K–O5 2.9420(9),
K–O6 2.8825(8), N–C1 1.3717(14), C1–C2 1.4054(15), C2–C3 1.4277(16), C3–C4
1.4076(16), C4–N 1.3727(14).
with the tert-butyl groups. Remarkably, the molecular structure
1
of (tmeda)Li(PyrPh ) also exhibits the κ -N-coordination mode,
4
with a Li–N σ-bond of 1.928(4) Å (see ESI† for details).
Synthesis and characterization of azatrozircenes
Azatrozircenes are readily accessible by salt metathesis reactions
tBu2Me2
of [(η7-C7H7)ZrCl(tmeda)] (7) with Na(PyrtBu ), K(Pyr
) and
2
K(PyrtBu ), respectively, affording the 16-electron sandwich
3
complexes [(η7-C7H7)Zr(η5-PyrtBu )] (1-Zr), [(η7-C7H7)Zr(η5-
2
Fig. 5 ORTEP diagram of 2-Zr with thermal displacement parameters drawn at
50% probability. Selected bond lengths (Å): Zr–CCht 2.3306(19)–2.3537(17),
Zr–C8 2.4506(15), Zr–C9 2.5825(15), Zr–C10 2.5869(15), Zr–C11 2.4592(11),
Zr–N 2.3560(13).
PyrtBu Me )] (2-Zr) and [(η -C7H7)Zr(η -Pyr )] (3-Zr) in mode-
7
5
tBu3
2
2
rate to good yields as purple compounds. Likewise, the imid-
azolyl derivative, [(η7-C7H7)Zr(η5-ImtBu )] (4-Zr), can also be
3
1
prepared (Scheme 2). The H NMR spectra of 1-Zr–4-Zr exhibit
the typical singlet for the C7H7 ligand at ca. 5.2 ppm, while the
13C NMR spectra show a distinct resonance at ca. 82 ppm.
The remaining resonances can unambiguously be assigned to
the η5-ligands; for instance, the resonances assigned to the
α-carbon atoms in 1-Zr appear at δ 150.8 in the 13C NMR spec-
trum, whereas the β-carbon atoms are shifted to significantly
higher fields (δ 101.4).
Despite their high solubility in hydrocarbon solvents, crys-
tals suitable for X-ray diffraction analysis were grown by
cooling saturated pentane solutions of 1-Zr and 2-Zr, and their
molecular structures are shown in Fig. 4 and 5 along with per-
tinent bond distances. To date, our efforts to establish the
molecular structure of the PyrtBu complex 3-Zr failed, because
3
a clear distinction between the position of the nitrogen atom
and the only CH group within the five-membered ring was not
possible (presumably because of disorder). Therefore, density
functional theory (DFT) calculations at the M06 level of theory
were undertaken and the calculated structure for 3-Zr is
included in the ESI;† it resembles those obtained experi-
mentally for 1-Zr and 2-Zr.
A visual inspection reveals an η5-coordination of the pyrrolyl
ligands in 1-Zr and 2-Zr, which can be attributed to the steri-
cally demanding substituent in the α-positions to the nitrogen
atom. The formation of a Zr–N σ-bond is prevented, because
the tert-butyl groups would then point directly towards the
(η7-C7H7)Zr moiety leading to unfavourable interactions as
depicted in Fig. 6.
Scheme 2 Synthesis of complexes 1-Zr–4-Zr.
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Fig. 6 Destabilizing interactions in the κ1-N-coordination mode (left), which
are circumvented in the η5-coordination mode (right), while the free electron
pair on the nitrogen atom remains inaccessible.
Table 1 Structural parameters and cone angles for 1-Zr–4-Zr (distances in Å,
angles in °)a
1-Zr
2-Zr
3-Zrb
4-Zr
Fig. 7 ORTEP diagram of 4-Zr with thermal displacement parameters drawn at
50% probability. Selected bond lengths (Å): Zr–CCht 2.3341(11)–2.3471(11), Zr–
C8 2.4465(10), Zr–C9 2.5318(10), Zr–C10 2.5246(10), Zr–N1 2.4743(9), Zr–N2
2.4631(9).
Zr–Chtcent
Zr–Pyrcent
Cht–Zr–Pyr
α
1.669
2.185
167.8
9.03
1.674
2.182
167.3
7.12
1.681
2.208
169.1
6.80
1.667
2.191
179.3
3.08
Θ
Ω
111.9
102.7
124.4
101.9/53.4
127.8
104.3
117.9
101.9
a Cht = cycloheptatrienyl; Cht–Zr–Pyr represents the angle between the
centroid of the cycloheptatrienyl ring, the metal and the centroid of
the five-membered ring; α = angle between the two ligand planes; for
the definition of Θ and Ω refer to ref. 4. b Data based on a calculated
structure.
tri(tert-butyl)imidazolyl ligand, the N-analogue to 1,2,4-tri(tert-
butyl)cyclopentadienyl. Because of its symmetry, the structural
elucidation is simplified and no difficulties as for 3-Zr were
encountered (vide supra). Probably for the same reason as dis-
cussed above, the ligand in 4-Zr also adopts an η5-coordination
mode (Fig. 7). In the present case, however, the Zr–N bond dis-
tances are noticeably longer, 2.4743(9) and 2.4631(9) Å, com-
pared to the pyrrolyl complexes 1-Zr and 2-Zr. It should be
noted that 4-Zr together with 1-Zr and 2-Zr are rare examples
of structurally characterized zirconium complexes with nitro-
gen-containing π-ligands. To the best of our knowledge, the
only other examples with non-bridged ligands involve the
Both cyclic ligands deviate from a perfectly parallel orien-
tation; the angle between the two least-squares planes is 9.0°
and 7.1° for 1-Zr and 2-Zr, respectively (Table 1); if one prefers
to make this judgment based on centroid–Zr–centroid′ angles,
these values are 167.8° and 167.3°. In any case, the observed
bending results from the asymmetric coordination of the
pyrrolyl ligand, which is a common feature.11–13 In both cases,
the Zr–N bond is short, 2.400(2) and 2.3560(13) Å for 1-Zr and
2-Zr respectively, whereas the metal carbon bonds are not only
significantly longer, but they also increase on going from the
α- to the β-C atoms (2,5- vs. 3,4-position). For instance, the
PyrPh system;14,30 furthermore, no metal complexes incorpor-
2
ating the PyrtBu Me ligand have been reported in the literature.
Recently, we have used the (η7-C7H7)Zr fragment as a refer-
ence system for cone angle measurements on a series of cyclo-
pentadienyl and indenyl ligands. In this context, two angles
were defined: The angle Θ describes the size in the xy-dimen-
sion, while the angle Ω is related to the size in the z-direction
(towards the metal) and depends only on the nature of the sub-
stituent.4 Since the Zr–centroid distances to the pyrrolyl ligand
fall in the same range as observed for their cyclopentadienyl
counterparts, a direct comparison of their cone angles is pos-
sible. Naturally, the values for Ω resemble those previously
determined for the tert-butyl substituent. Of greater interest is
the angle Θ, where the formal substitution of a CH group by
N comes into play. In this context, the first related pair is the
2
2
averaged values for the PyrtBu Me complex are 2.455 vs.
2.585 Å. Therefore, an η3:η2-mode might be an alternative
description for the pyrrolyl coordination mode. This, however,
is not consistent with the internal C–C bond distances, which
are fairly uniform, 1.397(4)–1.415(4) Å for 1-Zr and 1.413(2)–
1.431(2) Å for 2-Zr. Remarkably, the bulky substituents also
block the free electron pair on the nitrogen atom (Fig. 6),
which otherwise could have led to the formation of dimers in
the solid state, as observed for the related phosphatrozircenes.19d
A closer inspection indicates that the wedge is slightly smaller
for 2-Zr because of the additional methyl substituents in
the 3,4-positions, which push the adjacent tert-butyl groups
towards the nitrogen atom as shown by C(tBu)–C–N angles of
118.3° and 120.6° for 2-Zr and 1-Zr, respectively. As expected,20
the metal bonds to the formally trianionic cycloheptatrienyl
ligand are significantly shorter than to the η5-ligand; conse-
quently, because of the larger diameter of the seven-membered
ring, the Zr–centroid distances to the seven-membered ring are
dramatically shorter (Table 1).
2
2
1,3-di(tert-butyl)cyclopentadienyl (CptBu
)
and the PyrtBu
2
2
ligand. Here, the Θ angle of 116.2° for the all-carbon ligand
exceeds the corresponding value for the pyrrolyl ligand by 4.3°
(Table 1). Similarly, starting from 1,2,4-tri(tert-butyl)cyclopenta-
tBu3
dienyl (CptBu ), the first CH/N replacement affords Pyr
3
and the second exchange gives ImtBu . In this series, the angle
3
Θ decreases from 132.0° to 127.8° to 117.9°. It can certainly be
questioned whether the cone angle concept is useful to predict
(kinetic) stabilities of pyrrolyl complexes, because it does not
take a potential rearrangement to a κ1-N-coordination mode
into account, which leads to an electron-deficient and
The formal substitution of the remaining CH group in
PyrtBu by an isoelectronic nitrogen atom is realized in the
3
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presumably more reactive metal centre. In addition, pyrrolyl Avance III 400 or a Bruker Avance II 300 spectrometer at
ligands are less efficient π-donors than their cyclopentadienyl ambient temperature unless stated otherwise. The residual
counterparts,16 and the stability of the η5-coordination mode solvent signal was used as a chemical shift reference (δH = 7.16
is also expected to be different. Although it was recently con- for benzene, 7.26 for chloroform, 3.58 for α-H of THF) for the
cluded, based on DFT calculations, that both types of ligands 1H spectra and the solvent signal (δC = 128.06 ppm for
have comparable bond dissociation enthalpies in group benzene, 77.17 for chloroform, 67.21 for α-C of THF) for the
4 metal complexes,17b,c the formal ligand exchange reaction in 13C spectra. The assignment follows the numbering:
this system provides a different picture.
ꢁ
tBu2
½ðη7-C7H7ÞZrðη5-CptBu Þꢀ þ ðPyr
Þ
!
2
ꢁ
tBu2
½ðη7-C7H7ÞZrðη5-PyrtBu Þꢀ þ ðCp
Þ
2
Here, the free enthalpy of ΔG0 = +12.5 kcal mol−1 (ΔH0
=
Elemental analyses were performed by combustion and gas
chromatographic analysis with an Elementar varioMICRO
instrument. 2,2-Dimethylpent-3-one,31 [(η7-C7H7)ZrCl(tmeda)]
+12.1 kcal mol−1) indicates a pronounced thermodynamic
difference between the related CptBu and PyrtBu ligands.
2
2
12a
(7)19a and Na(PyrtBu
)
were prepared according to the litera-
2
Similarly, the replacement of the cyclopentadienyl ligand in
tBu3
−
[(η7-C7H7)Zr(η5-CptBu )] by (Pyr
)
is also endergonic and
3
ture. KH, 18-crown-6 and FeCl3 were obtained commercially
and used without further purification. K(PyrtBu ) and K(Im
)
tBu3
3
endothermic (ΔG0 = 11.3 kcal mol−1, ΔH0 = 9.8 kcal mol−1).
were synthesized from HPyrtBu and HIm
,
24a,22a respectively,
tBu3
3
Significantly larger values for ΔG0 = 23.9 kcal mol−1 and ΔH0 =
and KH as described below for K(PyrtBu Me ).
2
2
21.8 kcal mol−1 are obtained when (CptBu
)
is exchanged for
−
3
−
(ImtBu ) , which suggests an even weaker Zr–imidazolyl bond.
3
SYNTHESIS OF 2,2,4,5,7,7-HEXAMETHYLOCTAN-3,6-DIONE (6). The
preparation followed a published procedure.25 A 2 L three-neck
round-bottom flask equipped with a gas-outlet, dropping
funnel and a mechanical stirrer was charged with diisopropyl-
amine (45 mL, 0.318 mol) and THF (300 mL). At 0 °C, n-butyl-
lithium in hexane (1.6 M, 200 mL, 0.318 mol) was added
dropwise. After stirring for 30 min, the reaction mixture was
cooled to −78 °C and 2,2-dimethylpent-3-one (32.95 g,
0.289 mol) was added dropwise, resulting in a colourless sus-
pension, which was stirred for an additional 2 h at −78 °C and
then treated dropwise with anhydrous FeCl3 (51.58 g,
0.318 mol) in DMF (250 mL) (note: when exposed to air, anhy-
drous FeCl3 slowly degrades leading to decreasing yields.
Therefore, it should be used freshly or stored under N2).25 The
mixture was allowed to warm up to RT and stirred overnight.
Careful addition of conc. HCl (37%, 63.12 g, 0.641 mol) in
water (300 mL) at 0 °C, addition of diethyl ether (400 mL) and
extraction with diethyl ether (3 × 100 mL) gave an organic
phase, which was washed with water (3 × 100 mL) and brine
(2 × 100 mL) and finally dried over MgSO4. After solvent
removal, the crude product was distilled at 40–44 °C and
0.08–0.02 mbar, yielding a mixture of two diastereomers (4 : 1)
as pale yellow, viscous liquid (19.57 g, 60%). 1H NMR
(200 MHz, CDCl3), major diastereomer: δ 3.32–3.23 (m, 2 H,
CHMe), 1.12 (s, 18 H, tert-butyl), 1.07–1.04 (m, 6 H, Me);
minor diastereomer: δ 3.40–3.36 (m, 2 H, CHMe), 1.16 (s, 18 H,
tert-butyl), 0.89–0.86 (m, 6 H, Me). 13C NMR (50 MHz, CDCl3):
δ 220.5 (Cq, CvO), 219.3 (Cq, CvO), 45.0 (Cq, tert-butyl),
44.4 (Cq, tert-butyl), 43.7 (CH, CHMe), 43.5 (CH, CHMe), 27.2
(CH3, tert-butyl), 26.6 (CH3, tert-butyl), 18.7 (CH3, Me), 16.1
Conclusions
The most frequently employed bulky pyrrolyl system is the 2,5-
di(tert-butyl)pyrrolyl ligand and landmark contributions were
mainly made by Kuhn, Schumann, and their respective co-
workers.10–13 As part of our interest in sterically encumbered
π-ligands,1–4 we have now gained insight into the synthesis
and coordination chemistry of several nitrogen-containing
systems by using the cycloheptatrienyl zirconium scaffold as a
reference. According to the DFT calculations, the coordination
to the (η7-C7H7)Zr fragment decreases in stability in the order
cyclopentadienyl > pyrrolyl > imidazolyl. While the cone angles
measurements confirmed that the pyrrolyl ligands used in the
present study can impose severe steric demand, the thermo-
dynamic differences of conventional cyclopentadienyl and pyr-
rolyl ligands will certainly have an impact on the stability and
reactivity of complexes involving other metals, e.g. late tran-
sition and f-block metals, and are still not fully understood.
Therefore, further investigations are currently ongoing to shed
light on the special and sometimes surprising characteristic
features of bulky pyrrolyl ligands, and the results will be pub-
lished in due course.
Experimental section
General
All synthetic and spectroscopic manipulations were carried out (CH3, Me).
under an atmosphere of purified nitrogen, either in a Schlenk
U2Me2
SYNTHESIS OF HPyrtB
(2-H). The synthesis was based on a
apparatus or in a glovebox. Solvents were dried and deoxyge- modified literature procedure.27 A Schlenk tube with a Teflon
nated either by distillation under a nitrogen atmosphere from screw cap was charged with the diketone 6 (19.57 g, 0.087 mol)
sodium benzophenone ketyl (THF) or by an MBraun GmbH and methanol (250 mL) saturated with NH3 was added. The
solvent purification system (all other solvents). NMR data were closed vessel was heated to 60 °C overnight followed by four
recorded on a Bruker DPX 200, a Bruker DRX 400, a Bruker days at 110 °C (Caution! For safety reasons, the reaction
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Dalton Transactions
should be performed in a closed hood). The mixture was 7 H, C7H7), 1.79 (s, 6 H, Me), 1.29 (s, 18 H, tert-butyl).13C{1H}
neutralized (ca. pH 6) with aqueous HCl (3 M) and the organic NMR (75 MHz, C6D6): δ 145.1 (Cq, C-2/-5), 119.8 (Cq, C-3/-4),
phase was extracted with diethyl ether (4 × 150 mL). The 82.7 (CH, C7H7), 34.8 (Cq, tert-butyl), 31.4 (CH3, tert-butyl),
combined organic extracts were washed with water (2 × 11.8 (CH3, Me). Anal. calcd for C21H31NZr (388.70): C, 64.89;
150 mL) and brine (200 mL) and dried over MgSO4. The crude H, 8.04; N, 3.60. Found: C, 64.27; H, 8.23; N, 3.52.
7
5
U
3
product was purified by column chromatography (silica,
SYNTHESIS OF [(η -C7H7)Zr(η -PyrtB )] (3-Zr). A solution of
hexane) and isolated as a colourless solid (10.85 g, 60%), K(PyrtBu ) (0.255 g, 0.931 mmol) in THF (20 mL) was slowly
3
which should be stored under an inert atmosphere. Single added to a stirred solution of [(η7-C7H7)ZrCl(tmeda)] (0.311 g,
crystals were obtained from
a concentrated hexamethyl- 0.931 mmol) in THF (20 mL). After 12 h the resulting purple
disiloxane solution at −24 °C. 1H NMR (400 MHz, CDCl3): δ solution was taken to dryness and the product was extracted
7.47 (br. s, 1 H, NH), 2.07 (s, 6 H, Me), 1.33 (s, 18 H, tert-butyl). with pentane. After solvent removal, sublimation under high
13C{1H} NMR (100 MHz, CDCl3): δ 130.8 (Cq, C-2/-5), 112.9 vacuum (0.1 mbar) at 150 °C afforded a purple solid (0.231 g,
1
(Cq, C-3/-4), 32.2 (Cq, tert-butyl), 30.3 (CH3, tert-butyl), 10.9 60%). H NMR (300 MHz, C6D6): δ 5.89 (s, 1 H, CH); 5.29 (s,
(CH3, Me).
7 H, C7H7), 1.40 (s, 9 H, tert-butyl), 1.24 (s, 9 H, tert-butyl),
(2-K). HPyrtBu Me (5.00 g, 1.19 (s, 9 H, tert-butyl). 13C{1H} NMR (75 MHz, C6D6): δ 148.3
U2Me2
SYNTHESIS OF K(PyrtB
)
2
2
24.12 mmol) was dissolved in THF (50 mL) and added to a sus- (Cq, C-5); 145.0 (Cq, C-2), 136.3 (Cq, C-3), 104.7 (CH, C-4), 82.0
pension of KH (1.26 g, 31.36 mmol) in THF (150 mL). The (CH, C7H7), 35.8 (Cq, C-2-tert-butyl), 33.9 (CH3, C-3-tert-butyl),
mixture was refluxed for 4 h and stirred for 12 h at RT. 33.6 (Cq, C-5-tert-butyl), 33.4 (CH3, C-2-tert-butyl), 32.1 (Cq,
Filtration through a pad of Celite and solvent removal afforded C-3-tert-butyl), 30.9 (CH3, C-5-tert-butyl). Anal. calcd for
a light yellow powder, which was suspended in pentane C23H35NZr (416.75): C, 66.29; H, 8.46; N, 3.36. Found: C, 66.34;
(40 mL). The solid was collected on a frit, washed with H, 8.37; N, 3.39.
7
5
tBU
3
pentane (2 × 20 mL) and dried to give a colourless powder.
SYNTHESIS OF [(η -C7H7)Zr(η -IM )] (4-Zr). [(η7-C7H7)ZrCl-
Yield: 5.40 g (93%). 1H NMR (300 MHz, d8-THF): δ 2.02 (s, 6 H, (tmeda)] (0.122 g, 0.364 mmol) was dissolved in THF (15 mL)
Me), 1.21 (s, 18 H, tert-butyl). 13C{1H} NMR (75 MHz, d8-THF): followed by the slow addition of K(ImtBu
) (0.100 g,
3
δ = 139.3 (Cq, C-2/-5), 109.1 (Cq, C-3/-4), 34.3 (Cq, tert-butyl), 0.364 mmol) in THF (15 mL). After stirring was continued for
32.2 (CH3, tert-butyl), 12.8 (CH3, Me). Anal. calcd for C14H24KN 12 h, the solvent was removed under dynamic vacuum and the
(245.45): C, 68.51; H, 9.86; N, 5.71. Found: C, 68.51; H, 9.70; residue was extracted with pentane. Crystallization from
N, 5.35. Single crystals of the 18-crown-6 adduct were obtained pentane at −24 °C yielded purple crystals (0.090 g, 59%).
by refluxing equimolar amounts of the potassium salt and 1H NMR (300 MHz, C6D6): δ 5.33 (s, 7 H, C7H7), 1.36 (s, 9 H,
18-crown-6 in toluene until the mixture became clear, followed tert-butyl), 1.33 (s, 18 H, tert-butyl). 13C{1H} NMR (75 MHz,
by cooling to −24 °C. 1H NMR (300 MHz, d8-THF): δ 3.53 (s, 24 C6D6): δ 161.2 (Cq, C-2), 148.8 (Cq, C-4/-5), 83.4 (CH, C7H7),
H, CH2), 2.02 (s, 6 H, Me), 1.25 (s, 18 H, tert-butyl). Anal. calcd 34.7 (Cq, tert-butyl), 34.2 (Cq, tert-butyl), 33.0 (CH3, tert-butyl),
for C26H48KNO6 (509.76): C, 61.26; H, 9.49; N, 2.75. Found: 30.4 (CH3, tert-butyl). Anal. calcd for C22H34N2Zr (417.74):
C, 60.49; H, 9.35; N, 2.66.
C, 63.25; H, 8.20. Found: C, 62.08; H, 8.24.
7
5
U
2
SYNTHESIS OF [(η -C7H7)Zr(η -PyrtB )] (1-Zr). [(η7-C7H7)ZrCl-
X-ray diffraction studies
(tmeda)] (0.400 g, 1.198 mmol) was dissolved in THF (20 mL)
and Na(PrytBu ) (0.242 g, 1.198 mmol) in THF (5 mL) was Data were recorded at 100 K on Oxford Diffraction diffracto-
slowly added at RT, resulting in an immediate colour change meters using monochromated Mo Kα or mirror-focussed
from blue to dark purple. After stirring was continued for 2 h, Cu Kα radiation (Table 2). The structures were refined anisotro-
the solvent was removed under dynamic vacuum and the pically using the SHELXL-97 program.32 Hydrogen atoms were
product extracted with pentane. After filtration through a pad either (i) located and refined isotropically (for the NH hydro-
of Celite and solvent removal, the product was crystallized as gen of 2-H and for all hydrogen atoms directly attached to a
purple needles from a concentrated pentane solution at 5- or 7-membered ring); (ii) included as idealized methyl
−30 °C (0.334 g, 77%). 1H NMR (300 MHz, C6D6): δ 5.57 (s, groups and allowed to rotate but not tip or (iii) placed geo-
2 H, H-3/-4), 5.23 (s, 7 H, C7H7), 1.21 (s, 18 H, tert-butyl). metrically and allowed to ride on their attached carbon atoms.
13C{1H} NMR (75 MHz, C6D6): δ 150.8 (Cq, C-2/-5), 101.4 (CH, Special features: for 2-H, the anomalous dispersion was not
C-3/-4), 81.6 (CH, C7H7), 33.5 (Cq, tert-butyl), 31.0 (CH3, tert- significant, so the Friedel opposite reflections were averaged
butyl). Anal. calcd for C19H27NZr (360.65): C, 63.28; H, 7.55; and the Flack parameter is thus meaningless. For 3-Zr, the
2
N, 3.88. Found: C, 62.79; H, 7.58; N, 3.73.
Flack parameter refined to −0.01(2).
7
5
U2Me2
SYNTHESIS OF [(η -C7H7)Zr(η -PyrtB
)] (2-Zr). To a stirred
Computational details
solution of [(η7-C7H7)ZrCl(tmeda)] (0.272 g, 0.815 mmol) in
THF (20 mL) was added a THF solution (20 mL) of K(PyrtBu Me
)
All computations were performed using the density functional
2
2
(0.200 g, 0.815 mmol). After 12 h, the solvent was removed method M06 as implemented in the Gaussian09 program.33
from the purple solution. Extraction with pentane, filtration For all main-group elements (C, H and N) the all-electron
and crystallization from pentane at −24 °C afforded purple triple-ζ basis set (6-311G**) was used,34 whereas for zirconium
crystals (0.238 g, 75%). 1H NMR (300 MHz, C6D6): δ 5.14 (s, a small-core relativistic ECP together with the corresponding
Dalton Trans.
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Paper
Table 2 Crystallographic data
2-H
2-K·18-crown-6
1-Zr
2-Zr
4-Zr
Formula
C
14H25
N
C26H48NO6K
509.75
1.184
C19H27NZr
360.64
1.371
C21H31NZr
388.69
1.346
C22H34N2Zr
417.73
1.307
MW (g mol−1
)
207.35
1.019
dcalcd (g cm−3
)
Crystal size (mm)
T (K)
0.25 × 0.20 × 0.20
100(2)
0.25 × 0.20 × 0.10
100(2)
0.08 × 0.06 × 0.02
100(2)
0.20 × 0.15 × 0.05
100(2)
0.40 × 0.20 × 0.20
100(2)
F(000)
Crystal System
Space group
464
1112
Monoclinic
P21/n
752
Monoclinic
P21/n
816
880
Monoclinic
P21/c
Orthorhombic
P212121
Orthorhombic
P212121
a (Å)
b (Å)
c (Å)
α (°)
7.5358(2)
10.7021(2)
16.7625(3)
90
9.8304(2)
17.5251(4)
16.5980(3)
90
6.4001(3)
29.9375(12)
9.3920(4)
90
9.5908(3)
13.9470(4)
14.3385(3)
90
9.3606(2)
25.4410(4)
9.9949(3)
90
β (°)
90
90
90.083(2)
90
2859.47(10)
4
103.842(4)
90
1747.28(12)
4
90
90
116.929(3)
90
2122.12(9)
4
γ (°)
V (Å3)
1351.88(5)
4
1917.96(9)
4
Z
θ range (°)
4.90–75.92
1.00 to 2θ 150°
46 839
1646 [Rint = 0.0239]
0.428
0.031; 0.082
0.032; 0.083
1.087
2.32–28.28
1.00 to 2θ 56°
155 683
7090 [Rint = 0.0631]
0.223
0.034; 0.075
0.045; 0.079
1.015
5.07–76.09
1.00 to 2θ 145°
23 478
3589 [Rint = 0.0705]
5.082
0.034; 0.067
0.047; 0.072
1.044
2.55–30.89
1.00 to 2θ 61°
98 371
5864 [Rint = 0.0552]
0.574
0.025; 0.045
0.031; 0.046
1.058
2.42–30.93
0.98 to 2θ 61°
124 284
6426 [Rint = 0.0377]
0.525
0.023; 0.049
0.027; 0.051
1.086
Completeness
Collected reflections
Unique reflections; Rint
μ/mm−1
R1; wR(F2) [I > 2σ(I)]
R1; wR(F2) (all data)
GoF (F2)
Residual electron density
0.163, −0.183
0.336, −0.176
0.343/−0.636
0.304, −0.293
0.381/−0.306
double-ζ valence basis set was employed (Stuttgart RSC 1997
ECP).35 No symmetry restrictions were imposed (C1) and the
nature of the extrema (minima) was established with analytical
frequency calculations. The zero point vibration energy (ZPE)
and entropic contributions were estimated within the harmo-
nic potential approximation. The Gibbs free energy, ΔG, was
calculated for T = 298.15 K and 1 atm. Geometrical parameters
were reported within an accuracy of 10−3 Å and 10−1 degrees.
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Emmy-Noether program (WA 2513/2-1). MK is grateful to the
German Academic Exchange Service (DAAD) for a travel
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