Diisocyanoarene-linked pentacarbonylvanadate(I-) ions as building blocks in a supramolecular charge-transfer framework assembled through noncovalent π-π And contact ion interactions

Article abstract of DOI:10.1039/c2dt30488k

Three synthetic routes to the unusual supramolecular complex ([Cp ₂Co]₂[{(OC)₅V}₂(μ-1,4-CNC ₆Me₄NC)])_∞, which was crystallographically characterized, are presented. The dianion [{(OC) ₅V}₂(μ-1,4-CNC₆Me₄NC)] ^2- constitutes the first subvalent organometallics featuring a diisocyanoarene linker.

Full text of DOI:10.1039/c2dt30488k

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COMMUNICATION
Diisocyanoarene-linked pentacarbonylvanadate(I−) ions as building blocks in
a supramolecular charge-transfer framework assembled through noncovalent
π–π and contact ion interactions†
Tiffany R. Maher, John J. Meyers, Jr., Andrew D. Spaeth, Krista R. Lemley and Mikhail V. Barybin*
Received 29th February 2012, Accepted 2nd April 2012
DOI: 10.1039/c2dt30488k
red solution, from which air- and moisture sensitive, magenta-
coloured [Et4N]2[{(OC)5V}2(μ-CNC6Me4NC)] (1a) was isolated
in a ca. 60% yield (Scheme 1). The νCO/νCN region of the infra-
red spectrum of 1a in CH3CN (Fig. 1) is qualitatively similar to
those previously observed for several mononuclear complexes
[Et4N][M(CO)5(CNR)] M = (V,6,7 Ta8) and constitutes a
“classic” example of a pattern expected for a [M(CO)5(CNR)]z
motif with essentially perfect local C4v symmetry.9 The νCO
Three synthetic routes to the unusual supramolecular
complex ([Cp2Co]2[{(OC)5V}2(μ-1,4-CNC6Me4NC)])∞, which
was crystallographically characterized, are presented. The
dianion [{(OC)5V}2(μ-1,4-CNC6Me4NC)]2− constitutes the
first subvalent organometallics featuring a diisocyanoarene
linker.
Advances in organometallic crystal engineering critically rely on
the use of noncovalent phenomena, such as π-stacking, hydrogen
bonding, and charge transfer interactions, to assemble novel
supramolecular materials.1 Our recent interest in low-valent, di-
isocyanoarene-bridged organometallics2 has led us to explore the
possibility of incorporating metallocene and metal carbonyl
complexes, which are arguably the two most ubiquitous motifs
in organometallic chemistry, into new charge-transfer coordi-
nation platforms featuring conjugated linear diisocyanoarene
linkers.
cis
bands at 1951 (m) and 1831 (vs) cm−1 correspond to the A1
and E stretching modes, respectively, of the CO ligands posi-
tioned cis to the isocyanide groups in 1a. The remaining allowed
νCO band of A1 symmetry that arises primarily from stretching of
the carbonyl ligands trans to the isocyanides is obscured by the
very intense E band, as is the case for many other [V(CO)5L]−
anions not engaged in contact ion interactions with the accompa-
nying cations.9 The importance of V(dπ) → diisocyanodurene
(pπ*) interaction in 1a is nicely corroborated by a 57 cm−1
depression in νCN upon complexation of the free 1,4-
CNC6Me4NC linker (νCN = 2118 cm−1 in CH3CN).
The reactivity profile of V(CO)6, the only isolable odd-elec-
tron binary metal carbonyl, has been largely limited to one-elec-
tron reduction or disproportionation in the presence of donor
ligands/solvents to give [V(CO)6]−.3 A few interactions of
V(CO)6 with phosphines have led to mixed zero-valent substi-
tution products.4 In addition, Ellis et al. have shown that V(CO)6
reacts with excess CNXyl (Xyl = 2,6-dimethylphenyl or xylyl)
to afford neutral trans-V(CO)2(CNXyl)4.5 Herein, we report on
an unusual supramolecular ensemble containing a hitherto
unknown diisocyanoarene-bridged, subvalent bimetallic motif,
which is accessible either from V(CO)6 or [V(CO)6]−. While
several group 5 mononuclear species [M(CO)5(CNR)]− (R =
Scheme 1
t
i
Me, Ph, Cy, Bu, Pr, CH2CO2Et) have been prepared from the
labile complexes [M(CO)5L]− by the groups of Ellis (L = NH3)6
and Rehder (L = DMSO)7 via ligand exchange, no X-ray struc-
tural data on such complexes are available.
Treatment of in situ generated [Et4N][V(CO)5(DMSO)]7 with
0.5 equiv of 1,4-diisocyanodurene (1,4-diisocyano-2,3,5,6-tetra-
methylbenzene) in THF at ambient temperature afforded a dark
Department of Chemistry, The University of Kansas, 1251 Wescoe Hall
Drive, Lawrence, KS 66045, USA. E-mail: mbarybin@ku.edu;
Fax: +1-785-864-5396; Tel: +1-785-864-4106
†Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic and analytical data, details of the X-ray and
DFT studies. CCDC 862922. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2dt30488k
Fig. 1 Infrared spectra of 1a (black) and 1b (red) in CH3CN.
Dalton Trans., 2012, 41, 7845–7848 | 7845
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The electronic spectrum of 1a in CH3CN in the visible region
(Fig. S3†) features an intense vanadium-to-diisocyanodurene
charge transfer absorption (λmax = 489 nm). This charge transfer
band exhibits positive solvatochromism10 and undergoes a
736 cm−1 hypsochromic shift upon changing the solvent from
CH3CN to significantly less polar CH2Cl2.
The 51V NMR (132 MHz) spectrum of 1a in CD3CN at 25 °C
shows a singlet resonance at −1903.9 ppm11 versus neat VOCl3.
This main peak is accompanied by two 13CO satellite doublets
1
of relative intensities 4 : 1 at −1904.2 ppm with J(13C–51V) =
116.8 Hz and −1903.9 ppm with 1J(13C–51V) = 167.5 Hz,
respectively, as well as by a weak singlet at −1904.0 ppm due to
the C18O-containing isotopomer (Fig. S7†). These observations
nicely parallel Rehder’s earlier findings12 that the 51V NMR
pattern for [V(CO)6]− features distinguishable resonances for the
minor isotopomers [V(CO)5(13CO)]− and [V(CO)5(C18O)]−.
Both of these are slightly shifted upfield from the main peak at
−1955 ppm because of the different deshielding effects of the
12C16O, 13C16O and 12C18O ligands on the 51V nucleus
(Fig. S8†).
Fig. 2 The supramolecular framework of 1b.
Cation metathesis of 1a with [Cp2Co][BF4] in CH2Cl2 was
driven by precipitation of [Cp2Co]2[{(OC)5V}2(μ-CNC6-
Me4NC)] (1b) from the reaction mixture (Scheme 1). It is impor-
tant to note that photolysis of [Cp2Co][V(CO)6]13 in the
presence of excess DMSO proved sluggish and afforded only a
30% yield of 1b upon treating the intermediate [Cp2Co]-
[V(CO)5(DMSO)] with 0.5 equiv of 1,4-CNC6Me4NC (eqn (1)).
Our preferred route to compound 1b involves addition of 1,4-di-
isocyanodurene to 2 equiv of V(CO)6 in heptane at −70 °C fol-
lowed by reduction of the presumed thermally unstable, zero-
valent intermediate [{V(CO)5}2(μ-1,4-CNC6Me4NC)] with 2
equiv of cobaltocene in CH2Cl2 to afford 1b in a good yield
(Scheme 1). Since 1b is only very sparingly soluble in CH2Cl2
or THF, its samples are easily freed from any [Cp2Co][V(CO)6]
impurity by washing the product with either of these solvents.
Employing the above protocol, we also accessed [Cp2Co]-
[V(CO)5(CNXyl)] (2), which may be viewed as a mononuclear
congener of 1b (eqn (2)).
Fig. 3 Molecular structure of the V2-dianion in 1b (50% thermal ellip-
soids). Selected bond distances (Å) and angles (°): V–C1 2.027(2), V–
C10 1.919(3), V–C11 1.967(3), V–C12 1.953(3), V–C13 1.960(3), V–
C14 1.959(3), C1–N1 1.173(3), N1–C2 1.388(3), C10–O10 1.174(3),
C11–O11 1.145(3), C12–O12 1.155(3), C13–O13 1.146(3), C14–O14
1.148(3), V–C1–N1 178.9(2), C1–N1–C2 177.7(3).
well-defined channels with a solvent accessible volume of
102 Å3 (Fig. 2).15 These channels appear to be populated by dis-
ordered CH3CN molecules of crystallization.
The molecular structure of the V2-dianion within 1b features
slightly distorted octahedral coordination around the V(I−)
centres (Fig. 3). For 1b, the C1–N1 distance is 0.010 Å longer
while the N1–C2 bond is 0.017 Å shorter compared to corre-
sponding parameters documented for free 1,4-CNC6Me4NC.16
Albeit small, both of the above differences are statistically sig-
nificant according to the 3σ criterion and reflect significant V
(dπ) → CNC6Me4NC(pπ*) back-bonding interaction. Due to the
trans-influence of the isocyanide ligand, which has a greater
σ-donor/π-acceptor ratio than CO, the V–C10 bond is notably
shorter while the C10–O10 bond is longer than the V–C and
C–O distances, respectively, documented for all other V–C–O
units in 1b. The [Cp2Co]+ cations in 1b exhibit the Co–C bond
lengths of 2.022(3)–2.037(3) Å and the Co–Cp(centroid) dis-
tances of 1.63 Å (Fig. S1†), which are typical of other cobalto-
cenium salts.17
ð1Þ
ð2Þ
The air- and light sensitive complex 1b is modestly soluble in
acetonitrile and can be recrystallized from CH3CN–Et2O to
afford dark violet, nearly black microcrystalline solid. The single
crystal X-ray analysis of 1b confirmed its nature as a charge
transfer salt between [{V(CO)5}2(μ-1,4-CNC6Me4NC)] and two
cobaltocene units.‡ Only a half of the V2-dianion and one
[Cp2Co]+ cation are crystallographically independent. As illus-
trated in Fig. 2, the aromatic moieties of the 1,4-diisocyanodur-
ene linker undergo π-stacking aggregation14 with intercentroid
distances between the six-membered rings being 3.58 Å. Such
noncovalent π–π interactions form arrays of perfectly staggered
[{V(CO)5}2(μ-1,4-CNC6Me4NC)]2− dianions that are insulated
from one another by layers of the [Cp2Co]+ cations. The arrange-
ment of the cobaltocenium ions within the structure of 1b creates
While a satisfactory crystallographic analysis of the charge-
transfer salt [Cp2Co][V(CO)6] proved impossible in the past due
to “massive disorder of [V(CO)6]− in the crystal”,18 the well-
ordered framework of 1b now allows detailed geometric appreci-
ation of the interionic interactions between the cobaltocenium
7846 | Dalton Trans., 2012, 41, 7845–7848
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Table 1 Half-wave potentials (in volts) for 1a, 1b, 2, and [V(CO)6]−a
E1/2 (VI−/0
)
E1/2 (V0/I
)
E1/2 (CoII/III
)
Solvent
1ab
−0.69
−0.69
−0.75
−0.35
−0.38
−0.39
−0.38
CH3CNb
CH3CNb
CH3CNb
CH3CNb
Acetonec
1bb
−1.35
−1.35
−1.32
—
2b
[Cp2Co][V(CO)6]
[Na(diglyme)2][V(CO)6]c −0.36
0.06
a Potentials vs. Cp2Fe/Cp2Fe+. b This work. c Ref. 21.
The shoulders at 1847 and 1824 cm−1, that accompany a
Fig. 4 A fragment of the structure of 1b showing interionic contacts
(in Å). All hydrogen atoms are omitted for clarity.
more intense peak at 1839 cm−1, probably correspond to the B1
trans
and A1
νCO modes, respectively, the former being IR-forbid-
den under perfect C4v symmetry. This may suggest perturbation
of the dianion’s C4v geometry9 in 1b by contact ion pairing
(CIP), e.g., akin to those shown in Fig. 4, even in this polar
solvent.
and carbonylvanadate(I−) ions in the solid state. The X-ray
structure of 1b shows multiple cation–anion contacts at or under
4 Å. These contacts, depicted in Fig. 4, are somewhat shorter
than the CO⋯Co (4.29 Å) and CO⋯Cp(centroid) (3.91 Å) non-
bonded interactions documented by Kochi and Bockman for the
contact ion pair [Cp2Co]+[Co(CO)4]−.19 Each of the pentacarbo-
nylvanadate(I−) moieties in 1b displays close interactions with
four cobaltocenium units at distances dCo(A)–V = 5.38 Å, dCo(B)–V
= 5.53 Å, dCo(C)–V = 5.73 Å, and dCo(D)–V = 6.48 Å (Fig. 4).
Interestingly, ultrafast infrared transient absorption spectroscopy
experiments by Spears et al. revealed two different electron
transfer rates for the Cp2Co|V(CO)6 contact ion pair.20 These
authors’ density functional theory (DFT) calculations suggested
two distinct stable geometries for [Cp2Co]+|[V(CO)6]−: one with
two oxygen atoms lying in the cleft between the two Cp rings at
In contrast to 1a, no significant solvatochromism of the
vanadium-to-diisocyanodurene charge transfer absorption (λmax
= 484 nm) was documented for 1b (Fig. S4†), thereby
suggesting limited solvent accessibility of the V2-dianion in 1b
compared to that in 1a. Interestingly, a relatively weak shoulder
at λmax ≈ 738 nm, which is obscured by the low energy tail of
the very intense metal-to-diisocyanodurene charge transfer band,
was observed for 1b (Fig. S4†). We tentatively assign this
shoulder as a CIP charge transfer band involving the vanadate(I−)
anions and the cobaltocenium cations in 1b. For the [Cp2Co]+[V
(CO)6]− ion pair, such interionic charge transfer occurs at ca.
620 nm in CH2Cl2 (Fig. S6†).18 The lower energy of the inter-
ionic charge transfer for 1b versus [Cp2Co]+[V(CO)6]− corre-
lates18 nicely with ≈300 mV smaller difference between the E1/2
redox potentials of the V(I−) donor and the Co(III) acceptor ions
documented for the former (Table 1).
In summary, we described the supramolecular ensemble
([Cp2Co]2[{(OC)5V}2(μ-1,4-CNC6Me4NC)])∞ held together via
synergistic π-stacking and CoIII/VI− contact ion interactions. We
hope that the availability of this unusual platform will offer new
opportunities in organometallic crystal engineering.1 Further
studies aimed at detailed understanding of interionic association
and photochemistry of 1a,b in solution are in progress.
d
Co–V = 5.4 Å and the other featuring two oxygen atoms contact-
ing the top of a cyclopentadienyl moiety at dCo–V = 6.1 Å.20
Very similar interactions are clearly recognizable in Fig. 4 for 1b
(dCo(A)–V = 5.38 Å and dCo(D)–V = 6.48 Å, respectively) and
involve the cis-CO ligands. In addition, two other cation–anion
contacts at dCo(B)–V = 5.73 Å and dCo(C)–V = 5.53 Å place the
corresponding [Cp2Co]+ cations in close proximity of the trans-
CO ligands (Fig. 4), which experience the greatest extent of
back-bonding from the V(I−) centres.
In addition to a reversible CoII/III wave, the cyclic voltammo-
grams of 1b and 2 in CH3CN feature two vanadium-centered
processes VI−/0 and V0/I, both of which are only partially revers-
ible presumably due to instability of the oxidized vanadium
species in CH3CN (cf. fast disproportionation of V(CO)6 in
donor solvents3). The V(I−) → V(0) oxidation for 1a,b and 2
occurs at substantially lower potentials with respect to the corre-
sponding process for the [V(CO)6]− ion (Table 1).21,22 Also, the
V2-dianion in 1a,b is 60 mV harder to oxidize than the [V-
(CO)5(CNXyl)]− anion in 2. This is consistent with a lower σ-
donor/π-acceptor ratio for the 1,4-diisocyanodurene ligand com-
pared to CNXyl. Indeed, while the LUMOs of 1,4-CNC6Me4NC
and CNXyl lack contributions from the methyl substituents, the
involvement of an additional isocyanide unit (an electron-with-
drawing group) in the LUMO of 1,4-CNC6Me4NC compared to
that of CNXyl, should facilitate the V(dπ) → isocyanoarene
(pπ*) interaction (Fig. S2†).
Acknowledgements
This work was funded by the NSF CAREER award
(CHE-0548212) and DuPont Young Professor Award to M.V.B.
The authors thank Drs Douglas R. Powell and Victor W. Day for
their expert advice in the crystallographic characterization of 1b.
Notes and references
‡Crystal data for 1b: C42H32Co2N2O10V2, Mw = 944.44, tetragonal,
space group P42/n, black needle, 0.46 × 0.16 × 0.10 mm3, T = 100(2) K,
a = b = 23.719(1) Å, c = 7.1617(8) Å, V = 4029.2(6) Å3, Z = 4, Dcalc
=
1.557 Mg m−3, μ = 1.319 mm−1, F(000) = 1912, R1(I > 2σ(I)) = 0.050,
wR2 (all data) = 0.129, GOF = 1.069 on F2. CCDC number 862922.
The IR pattern for 1b in acetonitrile shows broader and more
structured bands in the νCO/νCN region compared to 1a (Fig. 1).
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7848 | Dalton Trans., 2012, 41, 7845–7848
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