Isolation and structural characterization of the first thermally robust and air stable Cr(4+) bent-metallocene complex

Article abstract of DOI:10.1039/b311352c

The first thermally robust and air stable bent-sandwich chromocene complex with chromium in the +4 oxidation state has been isolated and fully characterized.

Full text of DOI:10.1039/b311352c

Isolation and structural characterization of the first thermally robust
and air stable Cr(4+) bent-metallocene complex†
Piet-Jan Sinnema‡, Justin Nairn, Ralph Zehnder, Pamela J. Shapiro,* Brendan Twamley and Alex
Blumenfeld
Department of Chemistry, University of Idaho, Moscow, 83844-2343 ID, USA. E-mail: shapiro@uidaho.edu;
Fax: +01 208 885 6173; Tel: +01 208 885 5785
Received (in West Lafayette, IN, USA) 16th September 2003, Accepted 23rd October 2003
First published as an Advance Article on the web 24th November 2003
5
5
The first thermally robust and air stable bent-sandwich
chromocene complex with chromium in the +4 oxidation state
has been isolated and fully characterized.
complex,
(5).
[Me4C2(h -C5H4){h -C5H3B(C6F5)3}]Cr(CN)CNXyl
The combination of green, high-spin (meff = 3.85 mB) 3 with one
equivalent of xylyl isocyanide produces brown, low-spin (meff
=
The chemistry of chromocene has remained elusive due to the
metallocene’s reluctance to adopt a bent-sandwich geometry and its
susceptibility to ring loss. The strategy that we have employed to
override these tendencies and gain access to bent-sandwich
chromocene complexes is to introduce an interannular bridge
between the cyclopentadienyl rings of the chromocene that is too
short to span a parallel ring geometry.1,2 Brintzinger and coworkers
demonstrated the effectiveness of this approach with their synthesis
1.75 mB) 4 (Scheme 1). The X-ray crystal structure of 4 was
determined. An ORTEP drawing of the complex is shown in Fig.
2.§ A particularly interesting feature of the structure is a p-stacking
interaction between the isocyanide ligand and one of the C6F5 rings
of the boryl group, with a distance of 3.331 Å from the centroid of
the arene ring plane to the center of the CN bond. The C6F5 ring is
practically coplanar with the xylyl ring, with a dihedral angle of
2.4° between the two ring planes. Although there are now several
examples of early transition metal bent-metallocene complexes
bearing an anionic B(C6F5)3 substituent on one of the cyclopenta-
dienyl rings7 this is the first time an intramolecular p-stacking
interaction between the B(C6F5)3 group and a ligand on the metal
has been identified.
5
5
of Me4C2(h -C5H4)2CrCO,3 which is thermally stable, unlike (h -
C5H4)2CrCO, which undergoes CO loss to form the more stable
5
16e- (h -C5H5)2Cr.4 Recently we reported the first bent metal-
locene complexes of Cr(3+) and Cr(4+).5 Whereas the 3+ oxidation
state is quite prevalent in organochromium chemistry, the 4+
oxidation state is rare and, prior to our examples, unknown for
chromocene.6 The 18e-, diamagnetic Cr(4+) species 15a and 25b
(Fig. 1) that we reported previously are only meta-stable,
decomposing above 225 °C to form paramagnetic species, some of
which have been structurally characterized. One decomposition
A dramatic effect of the borate substituent on the redox
properties of the ansa-chromocene complex is revealed in cylic
voltammetry measurements on 4. The complex exhibits a reversible
Cr2+/3+ couple at 21523 mV, which, significantly, is 350 mV lower
5
than that of [Me2C4(h -C5H4)2CrCNtBu,2 a non-borylated analog.
5
product formed by 2 is the 15e-, zwitterionic complex [Me4C2(h -
Complex 4 also exhibits a reversible Cr3+/4+ couple at +24 mV.
5
5
C5H4){h -C5H3B(C6F5)3}]Cr (3).5b Complex 3 reacts with various
[Me4C4(h -C5H4)2CrCNtBu, by contrast, exhibits an irreversible
s-donating ligands like CO, isocyanides and phosphines to give the
anodic peak at +200 mV. This prompted us to investigate the
5
5
17e-complexes [Me4C2(h -C5H4){h -C5H3B(C6F5)3}]CrL. In this
communication we highlight one of these species, the xylyl-
5
5
isocyanide
(CNXyl)
complex
[Me4C2(h -C5H4){h -
C5H3B(C6F5)3}]CrCNXyl (4), which is oxidized by AgCN to form
the first thermally robust and air-stable Cr(4+) bent-metallocene
Fig. 1 Meta-stable Cr(4+) ansa-metallocene complexes.
† Electronic supplementary information (ESI) available: details of synthesis
and characterization for 4 and 5. See http://www.rsc.org/suppdata/cc/b3/
b311352c/
‡ Current address: Molecular Inorganic Chemistry, University of Gro-
ningen, Neijenborgh 4, 9747 AG, Groningen. The Netherlands.
Fig. 2 Molecular structure of 4. Selected bond lengths (Å) and angles (°).
Cr1–Cent(C1-5) 1.834(3), Cr1–Cent(C8-12) 1.830(3), Cr1–C17 1.944(3),
C17–N1 1.171(4), C2–B1 1.635(4), Cp < Cp 43.5, Cent-Cr1-Cent 142.7,
Cr–C17–N1 178.6(3).
Scheme 1 Preparation of complexes 4 and 5 from complex 3.
C h e m . C o m m u n . , 2 0 0 4 , 1 1 0 – 1 1 1 T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
110

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chemical oxidation of 4. Refluxing a solution of 4 in THF with
excess AgCN for 24 h resulted in the deposition of metallic silver
and the formation of complex 5, which was isolated as a dark
orange-red solid in 73% yield (Scheme 1). Alternatively, 3 reacts
readily with AgCN at room temperature to form a paramagnetic,
reddish brown species, which reacts slowly with CNXyl to form 5
in 54% yield. Efforts to identify the paramagnetic intermediate
formed in the reaction between 3 and AgCN are in progress. We
presume it to be a dimer or higher aggregate of the 16e-ansa-
chromocene cyanide derivative.
In summary, we have prepared and fully characterized the first
thermally robust, air stable bent metallocene complex of Cr(4+).
The electron releasing properties of the anionic –B(C6F5)3
substituent was key to achieving this result by lowering the redox
potential of the chromium and stabilizing the unusual Cr(4+)
oxidation state. A similar effect of the anionic boryl group on the
redox properties of metallocene complexes of other early transition
metals may be expected.
The authors are grateful to the National Science Foundation
(grant no. CHE-9816730) for its generous financial support.
An 18e-complex, 5 is diamagnetic, thermally robust (stable when
heated for 16 h at 70 °C in THF) and stable in air for at least 1 h.
We have characterized it thoroughly by 1H, 13C, 19F, and 11B NMR
spectroscopy and determined its molecular structure by X-ray
crystallography. An ORTEP drawing of the molecule is shown in
Fig. 3.§ Significantly, the boryl group is positioned over the much
bulkier xylyl isocyanide ligand, with which it retains its p-stacking
interaction. The same arrangement of ligands results when the CN2
and CNXyl ligands are introduced in reverse order onto chromium
(i.e. by reacting 3 with AgCN prior to introducing CNXyl). A
comparison of the Cp–Cp dihedral angles (5: 44.6° 4: 43.5°; 2:
42.5°; 3: 36.9°) and Cp–Cr–Cp angles (5: 133.6° 4: 142.7°; 2:
140.0°; 3: 149.3°) shows that introducing more ligands on
chromium results in greater tilting of the cyclopentadienyl rings
away from the equatorial wedge.
1H and 13C NMR spectra of 5 reflect the lack of symmetry in the
compound; all of the cyclopentadienyl ring carbons and hydrogens
are inequivalent, as are all four methyl groups along the ethanediyl
bridge. A variable temperature 19F NMR study revealed restricted
rotation of the –B(C6F5)3 substituent in the complex (ESI†). At 25
°C the three C6F5 rings appear equivalent, showing only three
resonances for the o, m and p-fluorines. At 234 °C, the resonance
for the p-fluorines at 2163 ppm decoalesces into three resonances.
Line shape analysis gave DH‡ = 7.6 ± 0.8 kcal mol21 and DS‡ =
11 ± 3 eu for the three site exchange. The decoalescence patterns of
the o- and m-fluorines are more complex since they are affected by
two processes, the Cp-Boryl rotation and the rotation of the
individual C6F5 rings. Ultimately, at 294 °C all 15 inequivalent
fluorine atoms are distinguishable.
Notes and references
§ Crystal data: 4 + C6H6: C49H34BCrF15N, M = 984.58, triclinic, space
¯
group P1 (#2), a = 11.864(3), b = 12.943(3), c = 15.545(2) Å, a = 72.61,
b = 73.68(1), g = 71.90(2)°, V = 2118.1(8) Å3, Z = 2, Dc = 1.544 g cm23
m(Mo–Ka) = 37.4 cm21, F(000) = 998. An orange parallelpiped of
dimensions 0.19 0.11 1.5C6H6:
0.07 mm3 was used.
,
3
3
5 +
C53H37BCrF15N2, M = 1049.66, monoclinic, space group P2(1)/n (#14), a
= 13.975(3), b = 19.331(4), c = 16.878(3) Å, b = 90.30(3), V =
4559.6(16) Å3, Z = 4, Dc = 1.529 g cm23, m(Mo–Ka) = 35.4 cm21
,
F(000) = 2132. A red block of dimensions 0.44 3 0.33 3 0.20 mm3 was
used. 27874 (60130) reflections were collected at 203(2) K (83(2) K) on a
Bruker SMART APEX diffractometer with Mo–Ka radiation using w scans
for 4 (5), respectively. The structures were solved by direct methods. All
atoms were refined anisotropically using full materix least squares based on
F2 to give R1 = 0.0645 (0.0445), wR2 = 0.1205 (0.1030) for 9713 (13043)
independent reflections [¡Fo¡ > 2s(¡Fo¡), 2q 5 55(60)°] and 610 (655)
parameters for 4 (5), respectively. CCDC reference numbers 219696 and
219697. See http://www.rsc.org/suppdata/cc/b3/b311352c/ for crystallo-
graphic files in .cif format.
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Fig. 3 Molecular structure of 5. Selected bond lengths (Å) and angles (°).
Cr1–Cent(C1-5) 1.853(2), Cr1–Cent(C8-12) 1.864(2), Cr1–C44
2.0136(16), C35–N1 1.1556(19), C10–B1 1.649(2), Cp < Cp 44.6, Cent-
Cr1-Cent 133.6, Cr–C35–N1 171.66(13).
C h e m . C o m m u n . , 2 0 0 4 , 1 1 0 – 1 1 1
111