A paramagnetic heterobimetallic polymer: Synthesis, reactivity, and ring-opening polymerization of tin-bridged homo- and heteroleptic vanadoarenophanes

Article abstract of DOI:10.1021/ja510884h

The synthesis of the first tin-bridged bis(benzene) vanadium and trovacene sandwich compounds and the investigation of their oxidative addition and insertion behavior are reported. The vanadoarenophanes and the corresponding platinum insertion products were fully characterized including electrochemical and electron paramagnetic resonance (EPR) measurements. Controllable ring-opening polymerization of the heteroleptic tin-bridged [1]trovacenophane using Karstedts catalyst yields a high molecular weight polymer (up to M_n = 89 200 g·mol^-1) composed of d⁵-vanadium metal centers in the main chain, making it a rare example of a spin-carrying macromolecule. Magnetic susceptibility measurements (SQUID) confirm the paramagnetic scaffold with repeating S = 1/2 centers in the main chain and suggest antiferromagnetic interactions between adjacent spin sites (Weiss constant θ = -2.9 K).

Full text of DOI:10.1021/ja510884h

Article
pubs.acs.org/JACS
A Paramagnetic Heterobimetallic Polymer: Synthesis, Reactivity, and
Ring-Opening Polymerization of Tin-Bridged Homo- and
Heteroleptic Vanadoarenophanes
Holger Braunschweig,*,† Alexander Damme,† Serhiy Demeshko,‡ Klaus Duck,† Thomas Kramer,†
̈
Ivo Krummenacher,† Franc Meyer,‡ Krzysztof Radacki,† Sascha Stellwag-Konertz,†
and George R. Whittell§
†Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
̈
‡Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstrasse 4, 37077 Gottingen, Germany
̈
̈
̈
§School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, United Kingdom
S
* Supporting Information
ABSTRACT: The synthesis of the first tin-bridged bis-
(benzene) vanadium and trovacene sandwich compounds
and the investigation of their oxidative addition and insertion
behavior are reported. The vanadoarenophanes and the
corresponding platinum insertion products were fully charac-
terized including electrochemical and electron paramagnetic
resonance (EPR) measurements. Controllable ring-opening
polymerization of the heteroleptic tin-bridged [1]-
trovacenophane using Karstedt’s catalyst yields a high
molecular weight polymer (up to Mn = 89 200 g·mol−1) composed of d5-vanadium metal centers in the main chain, making
it a rare example of a spin-carrying macromolecule. Magnetic susceptibility measurements (SQUID) confirm the paramagnetic
scaffold with repeating S = 1/2 centers in the main chain and suggest antiferromagnetic interactions between adjacent spin sites
(Weiss constant Θ = −2.9 K).
protocol,13 other sandwich compounds have also been the
INTRODUCTION
■
subject of investigations. Recently, several diamagnetic metal-
loarenophanes derived from [M(η6-C6H6)2] (M = Cr, Mo)14
and [M(η5-C5H5)(η7-C7H7)] (M = Ti, Cr)15 have been
prepared and, in some instances, successfully polymerized by
ROP.15d,16 Some systems, like the sandwich compounds of
titanium,17 chromium,18 or rhodium,19 show the potential to
access different oxidation states. Furthermore, cationic ansa-
derivatives are known, for example, of cobaltocene12e,20 or
[Ti(η5-C5H5)(η8-C8H8)].21
Despite numerous examples of paramagnetic, strained ansa
compounds, their electrochemical and polymerization behavior
has only been scarcely studied so far.22 In particular, bridged
complexes of bis(benzene) vanadium [V(η6-C6H6)2]14g,23 and
the corresponding isoelectronic mixed sandwich complex
[V(η5-C5H5)(η7-C7H7)] (also known as trovacene)24 have
long been a subject of great interest. Nevertheless, the first
polymer derived thereof was only obtained in 2008 by Pt-
catalyzed ROP of [V(η6-C6H5)2SiMeiPr] (1),25 yielding
polymers with silicon in the main chain (Scheme 1).24c
Because of the high density of unpaired spins, these polymers
are expected to exhibit remarkable physical properties with
Since the first report of high molecular weight polymers based
on ferrocene by Withers and Seyferth1 and Brandt and
Rauchfuss,2 the preparation of poly(metallocenes) has received
growing interest. There have been various synthetic approaches
to poly(ferrocenes), including the condensation of unstrained
dihaloboryl derivatives with triethylsilane to give borylene-
bridged macromolecules.3 In 1992, Manners’ group achieved a
breakthrough in this respect with the so-called ring-opening
polymerization (ROP) of strained ansa ferrocene precursors.4
During the following years several variants of this method were
developed including anionic, photoinduced, thermal, or
transition-metal-mediated polymerization.5 Most of these
ROP reactions rely on the use of single-atom bridged ansa
ferrocenes, in which the nature of the bridging moiety can vary
from group 13 (B, Al, In, Ga)6 to group 14 (Si, Ge, Sn),7 group
15 (P, As),1,7a,8 and group 16 elements (S, Se).9 Although
main-group elements represent the majority of bridging units, it
has also been possible to incorporate early and late transition
metals of group 4 (Ti, Zr, Hf)10 and group 10 (Ni, Pt)11 into
the ligand-sphere of metallocenes. Moreover, several para-
magnetic bridged and unbridged 17-electron ferrocenium
species are known.12
While ferrocene plays a prevalent role in this field because of
its ease of preparation and well-established dilithiation
Received: October 23, 2014
Published: January 5, 2015
© 2015 American Chemical Society
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Article
course of the reaction, the color changed from brown to red in
the case of 5 and from gray to purple in the case of compound
6. After workup and recrystallization from pentane at −70 °C,
the products were obtained as crystalline solids in moderate
yields (5, 29%; 6, 38%). The complexes were characterized by
elemental analysis and mass spectrometry. In both cases, the
molecular ion peaks at m/z = 439 clearly confirm the expected
composition of 5 and 6. Crystals suitable for solid-state
characterization by X-ray diffraction could be obtained by
recrystallization from pentane at −30 °C (Figure 1). While
compound 5 crystallizes in the monoclinic space group C2/c,
compound 6 crystallizes in the monoclinic space group P21/c.
Scheme 1. Transition Metal-Catalyzed Polymerization of
Paramagnetic Vanadium Sandwich Compounds
regard to magnetism or conductivity. Elschenbroich et al.
showed that silicon-bridged dimers of bis(benzene) vanadium26
as well as di- and tetranuclear trovacenyl units with tin as spacer
show a pronounced intramolecular electronic and magnetic
communication.27
In this context, the synthesis of new, strained tin-bridged
paramagnetic ansa sandwich compounds became a major focus
of our studies. While several metallocenes and metalloarenes
with tin in the bridging unit were successfully synthesized,7c,d
the preparation of derivatives with a single-atom linker remains
challenging.15e,28 It is important to note that the bond between
the ipso-carbon and tin is rather weak, so that bulky substituents
are required to allow their isolation and avoid spontaneous
ring-opening reactions.15e In spite of having a lower tilt angle α
in comparison to their lighter analogues, the ease of bond
cleavage makes these molecules excellent precursors for ROP
(see Table 1 for a definition of the tilt angle α).7d
Figure 1. Molecular structures of [V(η6-C6H5)2SntBu2] (5, left) and
[V(η5-C5H4)(η7-C7H6)SntBu2] (6, right). Hydrogen atoms and
ellipsoids of the substituents on the tin atom are omitted for clarity.
Symmetry-related positions (5: −x + 1, y, −z + 3/2) are labeled with
_a. Thermal ellipsoids are displayed at the 80% (5) and 50% (6)
probability level. Selected bond lengths [pm] and angles [deg]: 5, V−
XBz 168.7, C1−Sn−C1_a 85.4(1), α = 15.6(1), β = 34.9, δ = 168.0; 6,
V−XCp 213.0, V−XCht 150.1, C1−Sn−C2 87.1(2), α = 12.8(2), βCp
29.5, βCht = 44.8, δ = 170.1 (XBz = centroid of the C6H5 ring, XCp
centroid of the C5H4 ring, XCht = centroid of the C7H6 ring).
=
=
Table 1. Structural Parameters of [1]- and [2]Bis(benzene)
Vanadoarenophanes (Bz = C6H5 Ring)
Herein, we report the synthesis and properties of strained,
tin-bridged [1]vanadoarenophanes, which have been fully
characterized by electron ionization mass spectrometry (EI-
MS), X-ray diffraction, UV−vis, and EPR spectroscopy.
Furthermore, we present reactivity studies toward [Pt(PEt3)3]
and the formation of a paramagnetic polymer derived from a
tin-bridged trovacene.
5
926b
11
α [°]
βBz [°]
δ [°]
15.6(1)
34.9
19.9
23
7.5(5)
Sn, 15.1; Pt, 9.7
173.6
RESULTS AND DISCUSSION
168.0
■
Synthesis and Characterization of [V(η6-C6H5)2SntBu2]
(5) and [V(η5-C5H4)(η7-C7H6)SntBu2] (6). The tin-bridged
vanadoarenophanes were prepared by salt elimination reactions
between the respective dilithiated sandwich compounds and di-
tert-butyltin dichloride at −78 °C (Scheme 2). During the
The insertion of a single-atom tin bridge distorts the
carbocyclic ring planes from a parallel arrangement (5, V−CBz
217.7(2)−224.3(2) pm; 6, V−CCp 222.8(3)−228.3(3) pm, V−
CCht 215.7(3)−220.1(3) pm), resulting in moderate deforma-
tion angles of δ = 168.0° (5) and δ = 170.1° (6) (see Tables 1
and 2). As expected, the tilt angles of 5 (α = 15.6(1)°) and 6 (α
= 12.8(2)°) differ slightly because of the larger interannular
Scheme 2. Synthesis of the [1]Stannavanadoarenophanes 5
and 6
Table 2. Structural Parameters of [1]- and
[2]Trovacenophanes (Cp = C5H4 Ring; Cht = C7H6 Ring)
2b24c
6
1024c
12
α [°]
18.3(2)
31.8
12.8(2)
29.5
17.7(2)
29.3
6.7(2)
7.5
βCp [°]
βCht [°]
δ [°]
46.4
44.8
45.2
1.5
166.1
170.1
166.1
175.1
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distance in trovacene in comparison with bis(benzene)
vanadoarenophane (11) as red-brown and platina-di-tert-
butylstanna[2]trovacenophane (12) as gray powder in
moderate yields (11, 47%; 12, 39%; Scheme 4).
vanadium. Furthermore, the Cipso−Sn bonds are considerably
twisted out of the ring planes (5, β = 34.9°; 6, βCp = 29.5, βCht
=
The insertion products were characterized by elemental
analysis and X-ray diffraction. Suitable crystals for X-ray
structural analysis were obtained by slow crystallization from
tetrahydrofuran (THF) solutions at −30 °C (see Figure 2). In
44.8), and the Cipso−Sn−Cipso angles deviate significantly from
an ideal tetrahedral angle (5, θ = 85.4(1)°; 6, θ = 87.1(2)°).
Because of the higher covalent radius of Sn compared to its
lighter congeners, the [1]stannavanadoarenophanes exhibit less
molecular strain than the corresponding silicon or germanium
compounds ([V(η6-C6H5)2Si(C3H6)] (9), α = 19.9°;26b [V(η5-
C5H4)(η7-C7H6)SiMeiPr] (2b), α = 18.3(2)°; [V(η5-C5H4)(η7-
C7H6)GeMe2] (10), α = 17.7(2)°).24c
In addition, we investigated the fate of compound 5 upon air
exposure (Scheme 3). Related ansa complexes like [V(η6-
C6H5)2BNiPr2] or [V(η6-C6H5)2Si2Me4] show decomposition
to a vanadium-containing precipitate and the free ligand
system.23c Likewise, controlled decomposition of 5 leads to
an insoluble solid of unknown composition and the free ligand,
1
as judged by H NMR spectroscopy (δ = 1.32 ppm (s, 18 H,
Figure 2. Molecular structures of [V(η6-C6H5)2Sn(tBu2)Pt(PEt3)2)]
(11, left) and [V(η5-C5H4)(η7-C7H6)Sn(tBu2)Pt(PEt3)2] (12, right).
Hydrogen atoms and ellipsoids of the substituents on the tin and
phosphorus atoms are omitted for clarity. Thermal ellipsoids are
displayed at the 80% (11) and 50% (12) probability levels. Selected
bond lengths [pm] and angles [deg]: 11, V−XBz 163.9 and 166.6, Pt−
P1 229.6(2), Pt−P2 237.1(2), C1−Sn−Pt 108.9(2), C2−Pt−Sn
82.9(2), C2−Pt−P1 173.8(2)°, α = 7.5(5), βCp = 15.1, βCht = 9.7, δ =
173.6; 12, V−XCp 188.3, V−XCht 144.8, Pt−P1 229.3(1), Pt−P2
234.6(1), C1−Sn−Pt 108.8(1), C2−Pt−Sn 84.8(1), C2−Pt−P1
SntBu2), 7.14−7.64 ppm (several m, 10 H, phenyl groups)). On
the other hand, the trovacene compound 6 did not show any
signs of controlled decomposition upon air exposure.
Scheme 3. Decomposition Pathway of [V(η6-C6H5)2SntBu2]
(5) upon Air Exposure
179.1(2)°, α = 6.7(2), βCp = 7.5, βCht = 1.5, δ = 175.1 (XBz
centroid of the C6H5 ring, XCp = centroid of the C5H4 ring, XCht
centroid of the C7H6 ring).
=
=
comparison to the single-atom bridged complexes, the oxidative
addition of a Pt0 fragment decreases the tilt angle and increases
the deformation angle (11, α = 7.5(5)°, δ = 173.6°; 12, α =
6.7(2)°, δ = 175.1°), consistent with a reduction of ring strain.
The lighter analogues [V(η6-C6H5)2SiMeiPr]25 and [V(η5-
C5H4)(η7-C7H6)SiMe2] (α = 10.6°, δ = 171.9°)30 react in a
similar manner, whereas the products of the oxidative addition
of the silicon-bridged bis(benzene) vanadium compounds have
never been structurally characterized to the best of our
knowledge. Furthermore, the insertion products show the
expected square-planar environment at the Pt moiety (11, C2−
Pt−Sn 82.9°, C2−Pt−P1 173.8(2)°; 12, C2−Pt−Sn 84.3(2)°,
C2−Pt−P1 179.1(2)°) with Pt−P1 bonds that are slightly
shorter than those between Pt and P2 (11, Pt−P1 229.6(2) pm,
Pt−P2 237.1(2) pm; 12, Pt−P1 229.3(1) pm, Pt−P2 234.6(1)
pm), as a result of the trans influence of the organotin
fragment. Moreover, we note a regioselective bond cleavage of
the Cipso−Sn bond between the cycloheptatrienyl ring and the
bridging element in compound 12, as was previously observed
in other systems ([V(η5-C5H4)(η7-C7H6)SiMe2]30 and [Ti(η5-
C5H4)(η5-C7H6)SntBu2]).15e The observed regioselectivity is
consistent with the rationale that the Cipso−Sn bond associated
with the higher β angle is more susceptible to bond cleavage.
ROP Studies. With these results in hand and the knowledge
that Karstedt’s catalyst, Pt2[(CH2CHSiMe2)2O]3, successfully
initiates the polymerization of similar systems,24c,25 we
attempted to polymerize the [1]stannavanadoarenophanes 5
and 6. Thus, toluene solutions of each compound were
combined with Karstedt’s catalyst (1−3.0 mol%) and then
heated at 60 °C for ∼12 h. Subsequently, the resulting mixtures
were precipitated into hexane to purify the polymeric material
from short-chain oligomers. In the case of 5, all attempts to
Reactivity of 5 and 6 toward [Pt(PEt3)3]. As our initial
attempts to initiate an anionic polymerization29 of compounds
5 and 6 failed and, in addition, no hints for a thermal ROP
could be observed in differential scanning calorimetry (DSC)
measurements (see Figures S1 and S2, Supporting Informa-
tion),7d the compounds were studied with regard to the
activation of the Cipso−Sn bond by transition metal compounds.
Previous work has shown that [1]stannaferrocenophanes and
other tin-bridged sandwich compounds undergo carbon-
element bond activation by various Pt complexes.7d,15e,28
To this end, compound 5 and 6 were treated with an
equimolar amount of [Pt(PEt3)3] in toluene and heated to 60
°C over a period of 24 h (Scheme 4). The reaction resulted in
the isolation of platina-di-tert-butylstanna[2]bis(benzene)-
Scheme 4. Oxidative Addition of the Cipso−Sn Bond in 5 and
6 to [Pt(PEt3)3]
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produce polymeric structures by varying the mol% of the
catalyst resulted in either decomposition of the monomer or
isolation of the starting material. However, the addition of 2.5
mol% and 3.0 mol% Karstedt’s catalyst to a toluene solution of
6 resulted in an immediate color change from light purple to
dark green and gave a light purple solid in 46% yield (13a, 3.0
mol%) and a deep purple solid in 44% yield (13b, 2.5 mol%)
after precipitation into hexane (Scheme 5).
discrepancy most probably results from differences in
polydispersity and limitations with the Stokes−Einstein
model. The larger population by size would appear to arise
from aggregation of the polymer coils at the concentration
investigated (2 mg·mL−1), as there is no corresponding peak in
the GPC chromatogram. This would suggest that THF is a
thermodynamically marginal solvent for 13a and 13b. For
comparison, the polymerization of the silicon-bridged species
2a and 2b under comparable conditions produces polymers
with a significantly lower number-average degree of polymer-
ization of 38 (2a: Mn = 10 000 g·mol−1, PDI = 2.31, Rh = 3.4
nm) and 20 (2b: Mn = 5 600 g·mol−1, PDI = 1.64, Rh = 2.3
nm), respectively.24c
Scheme 5. Polymerization of 6 Using Karstedt’s Catalyst
EPR Spectroscopy. Fluid solution continuous-wave (CW)
EPR spectra of all compounds were recorded at X-band, the
results of which are summarized in Table 3. The EPR spectra of
Table 3. Summary of EPR Spectroscopic Parameters for the
Different [n]Vanadoarenophanes
aiso(51V) [G]
giso
To assess the polymeric nature of 13a and 13b, the
molecular weight was determined by gel permeation
chromatography (GPC) in THF, relative to a series of
monodisperse polystyrene standards. In case of 13a, the
number-average molecular weight (Mn) was determined to be
19 300 g·mol−1 with a polydispersity index (PDI = Mw/Mn) of
1.82 (see Figure S3 in Supporting Information). This value
corresponds to a number-average degree of polymerization
(DPn) of 44 (m = 438.09 g·mol−1). Using just 2.5 mol% of the
catalyst, a polymer with a higher Mn of 43 200 g·mol−1 (DPn =
99) and PDI of 2.40 could be isolated (Figure S4, Supporting
Information). The chromatograms for 13a and 13b, however,
exhibited significant tailing to lower molecular weight, which
suggested that interactions with the GPC columns were
complicating the size-exclusion process. To investigate these
phenomena further and their effect on the GPC-determined
molecular weights, compound 13b was reanalyzed using THF
that contained 0.1 w/w% [n-Bu4N]Br and toluene (1% v/v) as
the flow rate marker. Under these conditions, a Mn of 89 200 g·
mol−1 (DPn = 204) and PDI 1.82 were obtained, along with a
more Gaussian peak shape (see Figure S5, Supporting
Information). The determination of a larger molecular weight
in this instance is consistent with a reduction in polar column−
coil interactions, as a result of charge screening by the additive.
This finding suggests that the values obtained with pure THF
should be viewed as underestimates of the true values. While it
is stable under an inert atmosphere, exposure to air leads to
decomposition of the polymeric structure within 1h, as
indicated by a distinct color change from purple to brown.24c
It was therefore important to confirm that the material eluted
unchanged from the GPC column, as this technique cannot be
performed with the rigorous exclusion of air. Dynamic light
scattering (DLS) was conducted on 13b as a THF solution,
prepared with the exclusion of both air and moisture. This
technique revealed a bimodal distribution of hydrodynamic
radii (Rh) of 3.8 and 14.5 nm, which represented 62.8 and
37.2% of the scattering volume, respectively (Figure S6,
Supporting Information). Relative to monodisperse polystyrene
samples in solution, these sizes correspond to molecular
weights of 20 700 and 225 400 g·mol−1.31 The former is less
than that observed by GPC under comparable conditions (Mn
= 43 200 g·mol−1) but confirms that high polymer is present
prior to interactions with the GPC columns and air. This
5
57.8
68.5
62.3
71.1
1.987
1.984
1.993
1.988
1.974
a
6
11
12
13a
13b
a
a
No signal found.
the monomeric ansa complexes 5 and 6 as well as the platinum
inserted species 11 and 12 display eight hyperfine lines (I = 7/2
for 51V) that vary in their line widths and spacings as a result of
g and a anisotropies that are not completely averaged out in
solution (see Figure 3 and Figure S7, Supporting Information).
Taking the tumbling of the molecules into consideration
(rotational correlation times of 0.1−1 ns in pentane and THF
solutions), the EPR spectra can be accurately simulated (see
Figure 3. CW EPR spectra of 5 (a) and the corresponding platinum
C−Sn insertion complex 11 (b) in pentane (5) and THF solution
(11) at room temperature. The shoulders at the mI = 7/2 transitions
seen for compound 5 are likely due to further anisotropic
contributions from slow Brownian reorientation.
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also Figure S7, Supporting Information).32 Detailed studies by
Elschenbroich and co-workers have shown that the vanadium
hyperfine coupling constant correlates with the tilt angle α in
these [n]vanadoarenophanes.14g,24a By decreasing the tilt angle
(less molecular strain), higher hyperfine coupling constants are
observed. The same trend is found for the dinuclear transition-
metal complexes 11 and 12 in comparison to their
mononuclear counterparts 5 and 6, which is consistent with
extension of the ansa bridge through Pt insertion into the C−
Sn bond.
In contrast, polymer 13a displays a single broad line (line
width = 140 G) with no resolved metal hyperfine splittings (see
Figure S8, Supporting Information), thereby precluding the
possibility to determine the hydrodynamic radius via the
correlation time. The spectral profile, however, is in line with
the formation of a ring-opened microstructure including
multiple nonequivalent paramagnetic centers and is further
consistent with the data of a silicon-containing trovacenyl
polymer.24c The longer-chain polymer 13b exhibit no EPR
signal down to 20 K, suggesting that the spin-lattice relaxation
becomes extremely short due to interactions between the
unpaired electrons.
UV−Vis Spectroscopy. The electronic structure of the
compounds was probed by solution UV−vis spectroscopy in
THF in a range of 220−700 nm. The spectrum of [V(η6-
C6H5)2SntBu2] (5) (see Figure S9, Supporting Information)
shows two absorption bands at 325 and 436 nm that are only
slightly shifted in comparison to [V(η6-C6H6)2] (320 and 445
nm).33 This finding is in line with the known dependence of
the UV−vis transition with the tilt angle of the bis(benzene)
vanadium complexes, as already observed for [V(η6-
C6H5)2SiMeiPr] (428 nm) and [V(η6-C6H5)2BNiPr2] (416
nm), which show blue-shifted absorptions with increasing tilt
angles.23c According to the literature, the high-energy bands can
be ascribed to charge transfer as well as intraligand transitions
and the low-energy bands represent transitions from the singly
occupied molecular orbital (SOMO) into molecular orbitals
with ligand, metal, or mixed ligand-metal character. The
absorption bands in between can be assigned to metal-to-
ligand charge-transfer transitions.23c,33 Correspondingly, [V(η6-
C6H5)2Sn(tBu2)Pt(PEt3)2)] (11) (see Figure S9 in Supporting
Information), with a longer bridge and thus a smaller tilt angle,
displays red-shifted absorption bands (341 and 462 nm)
compared to 5. In contrast to ferrocene, which shows red-
shifted UV−vis bands with increasing tilt angles, ansa
complexes derived from [V(η6-C6H6)2] seem to follow an
opposite trend (see Table 4 and Figure S9, Supporting
Information).23c The reason for this different behavior has
not yet been established in detail.
Investigation of compound 6 by UV−vis spectroscopy (see
Figures S10 and S11, Supporting Information) reveals an
absorption band at 213 nm with a shoulder at ca. 320 nm and a
weaker band at longer wavelength (611 nm). In contrast to the
spectroscopic data of 6, the platinum-inserted complex 12 (see
Figures S10 and S11, Supporting Information) shows two
shoulders at ca. 290 and 350 nm together with a strong
absorption band at 216 and a weak one at 571 nm. Similarly,
polymer 13a (see Figure S12, Supporting Information)
undergoes electronic transitions in the same energy range
with two peaks at 269 and 284 nm, two shoulders at ca. 310
and 456 nm, and two maximum absorption wavelengths (460
and 582 nm). Conversely, the longer-chain polymer 13b (see
Figure S13, Supporting Information) shows an absorption band
Table 4. Correlation between λmax and the Tilt Angle α
λmax [nm]
ε [L·mol−1·cm−1
]
α [°]
[V(η6-C6H5)2BNiPr2]23c
416
2753
2458
2925
3157
1123
64
29.4
20.9
15.6
7.5
[V(η6-C6H5)2SiMeiPr]23c
428
5
436
11
462
[V(η6-C6H6)2]33
445
6
611
12.8
6.7
12
571
104
13a
460, 582
467, 602
563
13b
[V(η5-C5H5)(η7-C7H7)]34
33
at 258 nm, two ill-defined shoulders at ca. 310 and 370 nm as
well as two maximum absorptions at 467 and 602 nm. For
comparison, the spectrum of the parent compound [V(η5-
C5H5)(η7-C7H7)] shows absorption bands at 243 nm and a
shoulder at 274 nm, which can be assigned to intraligand
charge-transfer processes.34 In addition, two ill-defined
shoulders at around 300 and 340 nm, resulting from ligand-
to-metal and metal-to-ligand charge-transfer transitions, are
observed.34a Finally, the broad absorption maximum at 571 nm
can be attributed to electronic transitions from the SOMO to
orbitals with metal- or ligand-character.34a It should be noted
that this data cannot provide a definite correlation between the
observed absorption bands and the existing molecular strain of
the compounds (Table 4).
Cyclic Voltammetry. The redox behaviors of the parent
sandwich compounds (5 and 6) and the polymers (13a/b)
were examined by cyclic voltammetry in THF solution (see
Figures S14, S15, S18, and S19 in the Supporting Information).
While 5 displays an irreversible oxidation wave at Epa = −0.8 V,
6 shows an anodically shifted quasi-reversible oxidation wave at
E1/2 = −0.2 V (potentials vs the ferrocene/ferrocenium redox
couple), in agreement with the different electronic structures of
the two bis(arene) complexes. In the case of 6, an additional
reduction event at Epc = −3.1 V is observed that can be ascribed
to a metal-centered one-electron reduction process giving an
18-electron complex.35 The electrochemical data for the
bimetallic complexes 11 and 12 suggest that the two-atom
bridge exerts an electron-donating effect on the vanadium
center relative to that of the single-atom bridge as the oxidation
potentials of 11 (E1/2 = −1.2 V) and 12 (E1/2 = −0.67 V) are
both shifted to more negative values than those of 5 and 6,
respectively (Figures S16 and S17, Supporting Information).
Interestingly, the oxidation wave of the bis(benzene) derivative
[V(η6-C6H5)2Sn(tBu2)Pt(PEt3)2)] (11) is chemically reversible,
presumably a consequence of the reduced molecular strain. All
these findings are consistent with previously established trends
related to the electronic influence of ring substituents and ansa
bridges for other vanadium bis(arene) systems.14g,35,36
In contrast, the cyclic voltammogram of polymer 13a exhibits
two distinct oxidation waves (E1/2(1) = −0.25 V, E1/2(2) =
−0.06 V) with small peak separations of 20 and 50 mV,
respectively (see Figure 4). The appearance of two successive
oxidation events (ΔE = 0.19 V) is strongly suggestive of some
degree of electronic communication between the vanadium
centers through the tin bridge. A similar redox behavior has
been found for ferrocenyl polymers, such as poly(ferrocenyl)-
silane or -germane.37 In addition, spawned by the reduction
processes starting around Epc = −3.0 V, a corresponding
oxidation wave at Epa = −1.8 V is observed (see Figure S18 in
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interactions through space, or of the same order of magnitude
at best.
CONCLUSION
■
The synthesis of paramagnetic tin-containing ansa compounds
derived from [V(η6-C6H6)2] and [V(η5-C5H5)(η7-C7H7)] has
been reported, and their constitution has been unambiguously
confirmed by elemental analysis, EI-MS, and X-ray diffraction.
Because of the molecular strain present in [V(η6-
C6H5)2SntBu2] (5) and [V(η5-C5H4)(η7-C7H6)SntBu2] (6),
the bridging C−Sn bond readily oxidatively adds to bis-
(triethylphosphine)platinum(0), thereby indicating the possi-
bility for metal-induced ring-opening polymerization reactions.
While the bis(benzene) vanadium complex (5) did not show
any sign of polymerization, [V(η5-C5H4)(η7-C7H6)SntBu2] (6)
yielded a high molecular weight polymer (Mn = 89 200 g·mol−1,
PDI = 1.82) using Karstedt’s catalyst. The magnetic behavior as
examined by SQUID indicates that the unpaired spins on the
vanadium centers behave essentially independently in terms of
contribution to the magnetic susceptibility of the polymer,
whereas some degree of electronic metal−metal communica-
tion can be deduced from cyclic voltammetry measurements.
Figure 4. Cyclic voltammogram of [V(η5-C5H4)(η7-C7H6)SntBu2]n
(13a) in THF/0.1 M [n-Bu4N][PF6] at room temperature. Scan rate =
250 mV·s−1.
the Supporting Information). The electrochemical behavior of
13b parallels that of 13a, except for the reduction events, which
are missing (see Figure S19 in the Supporting Information).
Magnetic Susceptibility Measurements. The magnetic
properties of the monomeric species 6 and the corresponding
polymer 13b were studied using a SQUID magnetometer at an
applied magnetic field of 0.5 T in the temperature range from
295 to 2 K (Figure 5). The observed χMT value for 6 at room
EXPERIMENTAL SECTION
■
General Considerations. All operations were performed under an
atmosphere of argon by using either a glovebox or standard Schlenk
line techniques. Solvents were dried by standard procedures. [V(η6-
C6H5Li)2]·tmeda (7) (tmeda = N,N,N′,N′-tetramethylethylenedi-
amine),25 [V(η5-C5H4Li)(η7-C7H6Li)]·pmdta (8) (pmdta =
N,N,N′,N′,N′-pentamethyldiethylenetriamine),24c Cl2SntBu2,39 and
[Pt(PEt3)3]40 were prepared according to published procedures. The
Karstedt catalyst (2.1−2.3% w/w solution of Pt2[(CH2CHSiMe2)2O]3
in xylene) was obtained commercially (ABCR). Mass spectra were
recorded on a Varian 320-MS SQ mass spectrometer (EI, 70 eV), and
the MALDI/TOF spectra were recorded on a Bruker Autoflex II LRF
50 in a DCTB matrix (C17H18N2). Elemental analysis (C, H, N) was
performed by combustion and gas chromatographic analysis with an
Elementar Vario MICRO elemental analyzer. UV−vis spectra were
recorded on a JASCO V-660 UV−vis spectrometer in an inert argon
atmosphere glovebox.
Gel Permeation Chromatography. The gel permeation
chromatography (GPC) experiments for polymers 13a and 13b
were performed on an argon-flushed Agilent SECcurity GPC System
(1260 Infinity) with light scattering, refractive index, and viscometer
detector. The eluent was degassed and dried THF with a flow rate of
1.0 mL·min−1. All molecular weights are reported relative to a
monodisperse polystyrene standard. For polymer 13b gel permeation
chromatography was additionally performed on a Viscotek RImax
chromatograph, equipped with an automatic sampler, a pump, an
injector, and an inline degasser. The columns were contained within an
oven (35 °C) and consisted of styrene/divinylbenzene gels with pore
sizes ranging from 500 to 100 000 Å. THF containing 0.1% w/w
[nBu4N]Br was used as the eluent at a flow rate of 1.0 mL·min−1.
Samples were dissolved in the eluent (2 mg·mL−1, unless otherwise
stated), and toluene (1% v/v) was added to serve as a flow rate
marker. These were then stirred for 1 h at room temperature and
filtered with a Ministart SRP 15 filter (polytetrafluoroethylene
membrane of 0.45 μm pore size) before analysis.
Dynamic Light Scattering. Dynamic light scattering (DLS) was
performed on a Malvern Instruments Zetasizer Nano S using a 5 mW
He-Ne laser (633 nm) at 25 °C. The correlation function was acquired
in real time and analyzed with a function capable of modeling multiple
exponentials. The diffusion coefficients for the component particles
were then extracted. These were subsequently expressed as effective
hydrodynamic radii, by volume, using the Stokes-Einstein relationship.
Samples were prepared at 2 mg·mL−1 and filtered through a
Figure 5. χMT vs T plot for 6 (top) and 13b (bottom) at 5000 Oe.
The solid lines represent the best fits (see text).
temperature is 0.378 cm3·mol−1·K (or 1.74 μB), as expected for
one S = 1/2 spin carrier (0.375 cm3·mol−1·K or 1.73 μB for the
spin-only case with g = 2.0), and stays almost constant down to
50 K. Below 50 K, χMT decreases to 0.100 cm3·mol−1·K, which
is likely due to intermolecular antiferromagnetic spin coupling.
Simulation of the experimental data (see Experimental Section)
leads to best fit parameters g = 2.02 and a Weiss temperature Θ
= −4.9 K, reflecting the intermolecular magnetic interactions.
The Θ parameter lies in the range of −0.7 to −8.8 K found for
a series of trovacenyl-based complexes.38 Magnetic properties
of 13b are similar to those of 6, with best fit parameters g =
1.90 and Θ = −2.9 K. Interestingly, the Θ value for
mononuclear 6 is slightly larger than the Θ value for polymeric
13b, indicating that exchange coupling through covalent bonds
in 13b should be even smaller than the intermolecular
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membrane filter (0.2 μm pores) into an optical glass cuvette (10.0 mm
path length).
mL and stored at −70 °C for 3 days. The resulting purple solid was
washed with cold pentane (3 × 10 mL) at −78 °C, solved in pentane,
and filtered off. All volatiles were removed in vacuum to afford [V(η5-
C5H4)(η7-C7H6)SntBu2] (6) as a purple solid (256 mg, 0.58 mmol,
38%).
EPR Spectroscopy. CW EPR measurements at X-band (9.86
GHz) were carried out at room temperature using a Bruker ELEXSYS
E580 CW EPR spectrometer. The spectral simulations were
performed using MATLAB 8.0 and the EasySpin 4.5.1 toolbox.41
Cyclic Voltammetry. Cyclic voltammetry experiments were
performed using a Gamry Instruments reference 600 potentiostat. A
standard three-electrode cell configuration was employed using a
platinum disk working electrode, a platinum wire counter electrode,
and a silver wire, separated by a Vycor tip, serving as the reference
electrode. Formal redox potentials are referenced to the ferrocene/
ferrocenium redox couple ([Cp2Fe]+/0) either by using ferrocene or
decamethylferrocene (E1/2 = −0.427 V) as an internal standard.42
Tetra-n-butylammonium hexafluorophosphate ([n-Bu4N][PF6]) was
employed as the supporting electrolyte. Compensation for resistive
losses (iR drop) was employed for all measurements.
Magnetic Measurements (SQUID). Temperature-dependent
magnetic susceptibility measurements were carried out with a
Quantum-Design MPMS-XL-5 SQUID magnetometer equipped with
a 5 T magnet in the range from 295 to 2.0 K at a magnetic field of 0.5
T. The powdered sample was contained in a gelatin capsule and fixed
in a nonmagnetic sample holder. Each raw data file for the measured
magnetic moment was corrected for the diamagnetic contribution of
the gelatin capsule according to Mdia(capsule) = χg·m·H, with an
experimentally obtained gram susceptibility of the gelatin capsule. The
molar susceptibility data were corrected for the diamagnetic
contribution using the Pascal constants and the increment method
according to Haberditzl.43 Experimental data were modeled by using a
fitting procedure to the spin Hamiltonian for Zeeman splitting (eq
1):44
EPR (THF, 295 K): giso = 1.984, aiso(51V) = 68.3 G; MS(EI) m/z
(%) = 439 (2) [M+], 325 (8) [M+−(C4H9)2], 247 (5) [M+−(C4H9)
−(C7H6) −(CH3)3], 207 (13) [V(η5-C5H4)(η7-C7H6)2+], 178 (5)
[Sn(C4H10)+], 116 (7) [V(η5-C5H4)]+, 57 (100) [(C4H9)+]; MALDI/
TOF m/z: 689 [M+·C17H18N2]; UV−vis (THF): λmax (ε) = 213 nm
(47 075 L·mol−1·cm−1), 320 nm (sh), 611 nm (64 L·mol−1·cm−1);
elemental analysis (%) calcd. for C20H28SnV (439.07 g·mol−1): C
54.66, H 6.43; found: C 54.85, H 6.47.
[V(η6-C6H5)2SntBu2Pt(PEt3)2] (11). A mixture of [V(η6-
C6H5)2SntBu2] (5) (50 mg, 0.11 mmol) and [Pt(PEt3)3] (63 mg,
0.11 mmol) in benzene (5 mL) was heated at 60 °C over a period of
24 h. The red-brown solution was then filtrated over a short pad of
Celite, and all volatiles were removed under vacuum. The resulting
residue was washed with cold pentane (3 × 5 mL) at −78 °C and
vacuum-dried to yield [V(η6-C6H5)2SntBu2Pt(PEt3)2] (11) as a red-
brown solid (47 mg, 0.05 mmol, 47%).
EPR (THF, 295 K): giso = 1.993, aiso(51V) = 62.3 G; UV−vis
(THF): λmax (ε) = 341 nm (17 692 L·mol−1·cm−1), 462 nm (3 157 L·
mol−1·cm−1); elemental analysis (%) calcd. for C32H58P2PtSnV
(870.21 g·mol−1): C 44.12, H 6.72; found: C 44.02, H 6.72.
[V(η5-C5H4)(η7-C7H6)SntBu2Pt(PEt3)2] (12). A solution of [V(η5-
C5H4)(η7-C7H6)SntBu2] (6) (50 mg, 0.11 mmol) and [Pt(PEt3)3] (63
mg, 0.11 mmol) in benzene (5 mL) was heated at 60 °C over a period
of 72 h. All volatiles were removed from the greenish solution, and the
gray solid was washed with hexane (3 × 5 mL). The residue was solved
in toluene (10 mL) and filtrated, and the solvent was removed under
high vacuum to afford [V(η5-C5H4)(η7-C7H6)SntBu2Pt(PEt3)2] (12)
as a gray solid (39 mg, 0.05 mmol, 39%).
̂
⃗
H = gμBB·S
(1)
EPR (THF, 295 K): giso = 1.988, aiso(51V) = 71.1 G; UV−vis
(THF): λmax (ε) = 216 nm (39 118 L·mol−1·cm−1), 290 nm (sh), 350
nm(sh), 571 nm (104 L·mol−1·cm−1); elemental analysis (%) calcd. for
C32H58P2PtSnV (870.21 g·mol−1): C 44.12, H 6.72; found: C 44.54, H
6.73.
Temperature-independent paramagnetism (TIP) was included
according to χcalc = χ + TIP (TIP = 410·10−6 cm3·mol−1 for 6).
Intermolecular interactions were considered in a mean field approach
by using a Weiss temperature Θ.45 The Weiss temperature Θ (defined
as Θ = zJS(S + 1)/3k) relates to intermolecular interactions zJ of
−13.6 cm−1 for 6 and −8.1 cm−1 for 13b, where J is the interaction
parameter between two nearest-neighbor magnetic centers, k is the
Boltzmann constant (0.695 cm−1·K−1), and z is the number of nearest
neighbors.
Polymerization of [V(η5-C5H4)(η7-C7H6)SntBu2] (6). A greaseless
Schlenk flask was equipped with a solution of [V(η5-C5H4)(η7-
C7H6)SntBu2] (6) (13a, 100 mg, 0.23 mmol; 13b, 50 mg, 0.12 mmol)
in toluene (2 mL) and the Karstedt catalyst (13a, 0.35 mL, 3.0 mol %
Pt; 13b, 0.14 mL, 2.5 mol % Pt), and the color changed immediately
from purple to dark green. The mixture was heated to 60 °C for 24 h
and then precipitated into rapidly stirred hexane. The resulting solid
was dissolved in toluene, and the precipitation was repeated twice to
afford [V(η5-C5H4)(η7-C7H6)SntBu2]n (13a and 13b) as a light (13a)
or dark (13b) purple solid, which was dried in vacuum (13a, 46 mg,
46%; 13b, 22 mg, 44%).
[V(η6-C6H5)2SntBu2] (5). A solution of Cl2SntBu2 (453 mg, 1.49
mmol) in pentane (10 mL) was added dropwise to a slurry of [V(η6-
C6H5Li)2]·tmeda (7) (500 mg, 1.49 mmol) in pentane (10 mL) at
−78 °C over a period of 1 h. After complete addition, the reaction
mixture was allowed to warm to ambient temperature over 2 h and was
stirred for 1.5 h. The precipitated solid was then filtered off, and the
red-brown filtrate was reduced to 5 mL. After storing the solution for 3
days at −70 °C, a reddish-brown solid had formed that was washed
with cold pentane (3 × 5 mL) at −78 °C, and solved in pentane again,
and then the unsoluble compounds were filtered off. All volatiles were
removed in vacuum, and [V(η6-C6H5)2SntBu2] (5) can be isolated as a
yellowish-brown solid (188 mg, 0.43 mmol, 29%).
13a: EPR (THF, 295 K): giso = 1.974; UV−vis (THF): λmax = 269
nm (maxima), 284 nm (maxima), 310 nm (sh), 456 nm (sh), 582 nm
(maxima); GPC (in THF versus polystyrene): Mw = 35 155 g·mol−1,
Mn = 19 294 g·mol−1, Mw/Mn = 1.82.
13b: UV−vis (THF): λmax = 258 nm (maxima), 310 nm (sh), 370
nm (sh), 467 nm (maxima), 602 nm (maxima); GPC (in THF versus
polystyrene): Mw = 103 666 g·mol−1, Mn = 43 222 g·mol−1, Mw/Mn =
2.40; GPC (in THF containing 0.1% w/w [nBu4N]Br): Mw = 162 400
g·mol−1, Mn = 89 200 g·mol−1, Mw/Mn = 1.82; DLS: Rh = 3.8 nm
(62.8%), 14.5 nm (37.2%).
Single-Crystal X-ray Structure Determination. The crystal data
of 5 and 11 were collected on a Bruker X8APEX diffractometer with a
charge-coupled device (CCD) area detector and multilayer mirror
monochromated MoKα radiation. The crystal data of 6 and 12 were
collected on a Bruker APEX diffractometer with a CCD area detector
and graphite monochromated MoKα radiation. The structures were
solved using direct methods, refined with the Shelx software package,46
and expanded using Fourier techniques. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included in structure
factor calculations. All hydrogen atoms were assigned to idealized
geometric positions.
EPR (THF, 295 K): giso = 1.987, aiso(51V) = 57.8 G; MS(EI): m/z
(%) = 439 (45) [M+], 382 (12) [M+−(C4H9)], 325 (100) [M+−
(C4H9)2], 275 (36) [Sn(C6H5)2+], 247 (15) [M+−(C4H9)2 −(C6H5)],
207 (13) [V(η6-C6H5)2]2+, 197 (29) [Sn(C6H5)+], 127 (12)
+
[V(C6H5)+], 57 (100) [C4H9 ]; UV−vis (THF): λmax (ε) = 325 nm
(18 752 L·mol−1·cm−1), 436 nm (2 925 L·mol−1·cm−1); elemental
analysis (%) calcd. for C20H28SnV (439.07 g·mol−1): C 54.66, H 6.43;
found: C 54.12, H 6.56.
[V(η5-C5H4)(η7-C7H6)SntBu2] (6). A slurry of [V(η5-C5H4Li)(η7-
C7H6Li)]·pmdta (8) (600 mg, 1.53 mmol) in pentane (20 mL) was
treated dropwise with a solution of Cl2SntBu2 (488 mg, 1.61 mmol) in
pentane (20 mL) at −78 °C over a period of 1 h. The purple reaction
mixture was allowed to warm to ambient temperature during 3.5 h and
stirred for 2.5 h. All volatiles were removed under vacuum, and the
residue was extracted in pentane. The solution was concentrated to 5
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Journal of the American Chemical Society
Article
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no.
CCDC-1029707 (5), CCDC-1029708 (6), CCDC-1029709 (11)
and CCDC-1029710 (12). These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Crystal data for 5: C20H28SnV, Mr = 438.05, orange block, 0.24 ×
0.12 × 0.04 mm3, monoclinic space group C2/c, a = 12.5094(12) Å, b
= 9.8617(9) Å, c = 15.3826(15) Å, β = 107.849(4)°, V = 1806.3(3) Å3,
Z = 4, ρcalcd = 1.611 g·cm−3, μ = 1.895 mm−1, F(000) = 884, T =
108(2) K, R1 = 0.0124, wR2 = 0.0313, 2216 independent reflections
[2θ ≤ 56.64°] and 104 parameters.
Crystal data for 6: C20H28SnV, Mr = 438.05, violet block, 0.18 ×
0.14 × 0.05 mm3, monoclinic space group P21/c, a = 12.8987(16) Å, b
= 9.3160(12) Å, c = 16.749(2) Å, β = 110.477(2)°, V = 1885.4(4) Å3,
Z = 4, ρcalcd = 1.543 g·cm−3, μ = 1.816 mm−1, F(000) = 884, T =
173(2) K, R1 = 0.0365, wR2 = 0.0782, 3682 independent reflections
[2θ ≤ 52.16°] and 205 parameters.
Crystal data for 11: C32H58P2PtSnV, Mr = 869.44, orange block,
0.178 × 0.099 × 0.047 mm3, monoclinic space group P21/c, a =
21.9178(14) Å, b = 18.6844(11) Å, c = 17.9740(11) Å, β =
109.179(3)°, V = 6952.2(7) Å3, Z = 8, ρcalcd = 1.661 g·cm−3, μ = 5.102
mm−1, F(000) = 3448, T = 100(2) K, R1 = 0.0709, wR2 = 0.0965,
13 538 independent reflections [2θ ≤ 52.08°] and 691 parameters.
Crystal data for 12: C32H58P2PtSnV, Mr = 869.44, colorless plate,
0.24 × 0.19 × 0.03 mm3, monoclinic space group P21/n, a = 11.812(2)
Å, b = 19.413(4) Å, c = 16.299(3) Å, β = 102.23(3)°, V = 3652.7(13)
Å3, Z = 5, ρcalcd = 1.976 g·cm−3, μ = 6.069 mm−1, F(000) = 2155, T =
173(2) K, R1 = 0.0660, wR2 = 0.1271, 7153 independent reflections
[2θ ≤ 52.04°] and 346 parameters.
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ASSOCIATED CONTENT
* Supporting Information
■
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EPR, GPC, DLS, cyclic voltammetry, and UV−vis data of all
species. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* h.braunschweig@uni-wuerzburg.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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This work was gratefully supported by the Deutsche
Forschungsgemeinschaft (DFG). We also thank Prof. Ian
Manners for assistance with polymer characterization.
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