Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8]4+ (M=MoIV, WIV) from Superacids

Article abstract of DOI:10.1002/anie.202002366

Octacyanometalates K₄[Mo(CN)₈] and K₄[W(CN)₈] are completely protonated in superacidic mixtures of anhydrous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH)

Full text of DOI:10.1002/anie.202002366

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Accepted Article
Title: Isolation and structural characterization of eight-fold protonated
octacyanometallates [M(CNH)8]4+ (M=MoIV,WIV) from
Superacids
Authors: Malte Sellin, Valérie Marvaud, and Moritz Malischewski
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202002366
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Isolation and structural characterization of eight-fold protonated
octacyanometallates [M(CNH)8]4+ (M=MoIV,WIV) from Superacids
Malte Sellin,[a] Valérie Marvaud[b] and Moritz Malischewski*[a]
Dedicated to the 75th anniversary of Professor Konrad Seppelt
[a]
[b]
M. Sellin, Dr. M. Malischewski
Freie Universität Berlin
Institut für Chemie und Biochemie, Anorganische Chemie
Fabeckstrasse 34-36, 14195 Berlin (Germany)
E-mail: moritz.malischewski@fu-berlin.de
Dr. V. Marvaud
Sorbonne Université, IPCM-CNRS-UMR8232, cc 229
4 place Jussieu,75252 Paris Cedex 05 (France)
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Octacyanometallates K4[Mo(CN)8] and K4[W(CN)8] are
completely protonated in superacidic mixtures of anhydrous
hydrogen fluoride and antimony pentafluoride. The resulting
hydrogen isocyanide complexes [Mo(CNH)8]4+ [SbF6]-4 and
[W(CNH)8]4+ [SbF6]-4 are the first examples of eight-coordinate
homoleptic metal complexes containing hydrogen isocyanide (CNH)
ligands. The complexes were crystallographically characterized,
revealing hydrogen-bonded networks with short N…H…F contacts.
Low-temperature NMR measurements in HF confirmed rapid proton
exchange even at -40 °C. Upon protonation, ṽ(C≡N) increases of
about 50 cm-1 which is in agreement with DFT-calculations.
networks as well as polynuclear complexes and cluster
[20-24]
compounds
have been reported. The ease of oxidation of
[M(CN)8]4- (M=Mo,W) to [M(CN)8]3- and the accessibility of an
excited triplet state for [M(CN)8]4- (M=Mo,W) by light irradiation
make octacyanometallates suitable building blocks for
photomagnetic materials,[25-28] while paramagnetic [M(CN)8]3-
(M=Mo,W) are promising building blocks for single-molecule
magnets.[29]
While treatment of octacyanometallates with hydrogen chloride
gives adducts of the neutral acids H4[M(CN)8]
• 6 H2O
(M=Mo,W),[30] H4[W(CN)8] • 4 HCl • 12 H2O[31] and H4[Mo(CN)8] •
2 O(C2H5)2 • CH3OH • 2 H2O,[32] complete (octa-) protonation is
The lability of metal cyanides towards acids is well-known and
often a subject of safety warnings since highly toxic hydrogen
cyanide might be released. In general, protonation of metal
bound cyano ligands (M-C≡N) at the terminal nitrogen atom
leads to the corresponding metal complex with hydrogen
isocyanide as a ligand (M-C≡N-H)+.[1-2] In contrast, only a small
number of metal complexes with hydrogen cyanide as a ligand
are known (M-N≡C-H).[3-9] Although hydrogen isocyanide CNH is
a good ligand for transition metals, it can be substituted by donor
solvents (e.g. water) or nucleophilic counteranions.[10]
Subsequently the liberated hydrogen isocyanide CNH can
isomerize to its thermodynamically more stable tautomer
hydrogen cyanide HCN.
achieved by reacting K4[M(CN)8]
• 2H2O (M=Mo,W) with
anhydrous hydrogen fluoride and a large excess of antimony
pentafluoride SbF5. Although the fully protonated species
[M(CNH)8] [SbF6]4 (M=Mo,W) are only slightly soluble in
anhydrous hydrogen fluoride at room temperature, their solubility
can be slightly increased by adding small amounts of sulfur
dioxide SO2 as co-solvent. Highly-moisture sensitive yellow
crystals form upon slow cooling to -75°C besides colourless
crystals of KSbF6.
Scheme 1. Preparation of [M(CNH)8]4+ [SbF6]-4 (M=Mo,W)
The superacidic mixtures HF/AsF5 or HF/SbF5 have recently
been used for the protonation of organic nitriles[11-12] and even
HCN[13] as well as for the preparation of highly electrophilic
organic cations.[14-15] Even though one could expect that the use
of superacids should immediately lead to the destruction of
polycyanometallates, these systems have the advantage that
even the formed AsF6 or SbF6 anions are very weak
nucleophiles and therefore much weaker ligands than the CNH
ligands which are formed upon protonation.
[Mo(CNH)8]4+ [SbF6]-4 • 2HF and [W(CNH)8]4+ [SbF6]-4 • 2HF
both crystallize in the monoclinic space group P21/n and are
isomorphous. The central metal is coordinated by
8
(crystallographically different) protonated cyanide / hydrogen
isocyanide ligands (M-CNH), resulting in a slightly distorted
square-antiprismatic coordination geometry. The question
whether the ligands are coordinated to the metal via carbon (M-
C≡NH) or nitrogen (M-N≡CH) could be clearly answered by
comparing the R factors and atomic displacement parameters of
both structure solutions (see supporting information). Due to the
high overall data quality the positions of all hydrogen atoms
could be located via difference-electron-density map.
-
-
Although the first reports on octacyanometallates [M(CN)8]4-
(M=MoIV,WIV) date to the beginning of the 20th century,[16-19] they
got a lot of attention from coordination and magnetochemists in
the past decades. Since the early 2000s,
octacyanometallate-based supramolecular
a
plethora of
coordination
1
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The M-C-N-H bonds are close to linear (varying from 172-179°),
while all terminal hydrogen atoms of the hydrogen isocyanide
ligands form strong hydrogen bonds to the fluorine atoms of the
SbF6- ions and co-crystallized HF molecules. In the molybdenum
compound the MCNH…F distances are in the range of 1.718(3)-
1.994(4) Å. Thus, the N-(H)-F distances are relatively short
(2.584(2)-2.709(2) Å) and in a similar range as in protonated
nitriles with hexafluorometallate counteranions (2.5-2.8 Å).[11-13]
Additionally, frequency calculations turned out to be even more
problematic. The comparison between the calculations for
[M(CNH)8]4+, [M(NCH)8]4+, [M(CNH)8]4+ • 8 HF and [M(NCH)8]4+
•
8 HF (M=Mo,W) with the reaction products was inconclusive.
Probably this is caused by the high ionic charges and strong
hydrogen bonding which are insufficiently modelled in the
calculations.
The IR spectra of [M(CNH)8]4+ [SbF6]-4 • 2HF (M=Mo,W) both
display a very broad band above 3000 cm-1 which can be
attributed to N-H stretching. Additionally, a weak band at 1615
cm-1 can be assigned to N-H bending, since both bands were
shifted during deuteration experiments with DF/SbF5. While an
isotopic ratio of 1.37 is observed for the δ(NH)/δ(ND)
deformation vibrations (close to the theoretical value of 1.41) the
corresponding value for the ν(NH)/ν(ND) stretching vibrations is
only ≈1.2. Similar effects have been observed before[35] and are
caused by strong hydrogen-bonding which has a larger influence
on stretching vibrations than on deformation vibrations.
Figure 1. Selected short H…F contacts < 2 Å (in orange) in the
crystal structure of [Mo(CNH)8]4+ [SbF6]-4 • 2HF. Ellipsoids
shown at 50% probability; C grey, N blue, H white, Mo turquoise,
F green, Sb lavender
The Mo-C bond lengths in [Mo(CNH)8]4+ [SbF6]-4 • 2HF
(2.140(2)-2.168(2) Å) are very similar to the Mo-C bond lengths
in K4[Mo(CN)8] • 2H2O (Mo-C 2.163(5) Å).[33] However, changes
in C-N-bond lengths are more significant. In the fully protonated
species, the C-N bond lengths are in the range of 1.128(3)-
1.136(3) Å and therefore shorter than in the potassium salt
(1.152(6) Å). While the former value resembles more the C-N
bond length in protonated nitriles,[11] the latter is more similar to
Figure 2. IR-Spectra showing the shifted CN stretching
frequency upon protonation
IR and Raman spectra of [M(CNH)8]4+ [SbF6]-4 • 2HF (M=Mo,W)
both reveal an increase of the CN stretching vibration of about
50 cm-1 compared to K4[M(CN)8] • 2H2O. A similar blue-shift has
already been observed in IR spectra of neutral
polycyanometallate acids.[36-40] This bond-strengthening effect
upon protonation is caused by the increased polarisation of the
carbon nitrogen bond. This observation goes in line with the
shortening of the carbon-nitrogen distance in the solid state
structure.
free HCN (1.157(1) Å).[34] Bond lengths in [W(CNH)8]4+ [SbF6]-4
•
2HF are very similar to the analogous Mo compound (see Table
1). This finding is also supported by DFT calculations
(M06L/Def2TZVP) on both [M(CNH)8]4+ and [M(CNH)8]4+ • 8 HF
(M=Mo,W). The latter was chosen as a model to simulate the
influence of hydrogen bonding in the crystal. Interestingly, the
comparison revealed that C-N bond lengths were totally
unaffected while M-C bond lengths decreased slightly in the
calculated HF solvates. However, it has to be stated that the
calculated M-C bond lengths were significantly longer than the
experimentally found values.
Table 2. Experimental IR data in cm-1
[Mo(CNH)8]4+
3082 (b)
[Mo(CND)8]4+
2529 (b)
[W(CNH)8]4+
3030 (b)
[W(CND)8]4+
2525 (b)
ν(NH)/ν(ND)
ν(CN)
Table 1. Experimental and calculated bond distances in Å
2166 (m)
1960 (b)
2145 (m)
1620 (m)
1970 (b)
Compound
M-C (exp).
C-N (exp.)
M-C (calc.)
2.203
C-N (calc.)
1.146
δ(NH)/δ(ND)
1615 (m)
1182 (m)
1180 (m)
[Mo(CNH)8]4+
2.140(2)-
2.168(2)
1.128(3)-
1.136(3)
[W(CNH)8]4+
2.142(2)-
2.169(2)
1.127(3)-
1.137(3)
2.211
1.147
Despite the relatively low solubility of [M(CNH)8]4+ [SbF6]-4
(M=Mo,W) in pure anhydrous HF even at room temperature, it
was possible to record NMR spectra of the products by using a
2
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solvent mixture of HF and SO2 at -40 °C. The 14N-NMR of a
solution of [Mo(CNH)8]4+ [SbF6]-4 shows a broad, unresolved
peak at (δ= -182 ppm) which is significantly shifted compared to
K4Mo(CN)8 in water (δ= -95 ppm). A similar shift was observed
for the protonation of acetonitrile (δ(CH3CN= -134 ppm;
δ(CH3CNH+)= -241 ppm).[11]
Only one signal at δ= 121 ppm is displayed in the 13C NMR
spectrum which indicates high-field-shift upon protonation
compared to aqueous K4(Mo(CN)8 (δ= 149 ppm).
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W4+ ions. The resulting square-antiprismatic complexes
[M(CNH)8]4+ (M=MoIV,WIV) are diamagnetic and fulfil the 18-
electron-rule.
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Acknowledgements
Gefördert durch die Deutsche Forschungsgemeinschaft (DFG) –
Projektnummer 387284271 – SFB 1349. Computing time was
made available by HPC Service of ZEDAT, FU Berlin.
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Keywords: cyanides • hydrogen bonds • isocyanide ligands •
protonation • superacidic systems
3
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Entry for the Table of Content
All eight cyano groups in octacyanometallates Mo(CN)84- and W(CN)84- are protonated under superacidic conditions without formation
of HCN. Instead homoleptic complexes of Mo4+ and W4+ with eight hydrogen isocyanide ligands are formed and fully characterized.
4
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