A molecular bis(isocyanide)silver(I) nitrate complex

Article abstract of DOI:10.1515/znb-2006-0102

Bis(p-tolylsulfonylmethylisocyanide)silver nitrate is obtained from the reaction of the isocyanide ligand with silver nitrate in chloroform regardless of the applied ratio of the reactands (1:1 or 2:1). The crystal structure of the product has been determined. In the complex molecule with C₂ symmetry, the nitrate anion is attached to the silver center as an η²-chelating ligand. Owing to this approach of the NO₃ ligand, the RNC-Ag-CNR axis (R = 4-Me-C₆H₄-SO ₂-CH₂) is bent [from 180 to 162.1(2)°], but the geometry of the nitrate is not significantly distorted, suggesting only weak coordinative bonding. The structure is thus intermediate between that of a molecular complex with a tetrahedral coordination and that of an ionic compound with linear coordination of the silver center.

Full text of DOI:10.1515/znb-2006-0102

A Molecular Bis(isocyanide)silver(I) Nitrate Complex
B. Djordjevic, O. Schuster, and H. Schmidbaur
Department Chemie, Technische Universita¨t Mu¨nchen,
Lichtenbergstraße 4, D-85747 Garching, Germany
Reprint requests to Prof. H. Schmidbaur. E-mail: H.Schmidbaur@lrz.tum.de
Z. Naturforsch. 61b, 6 – 10 (2006); received November 17, 2005
Bis(p-tolylsulfonylmethylisocyanide)silver nitrate is obtained from the reaction of the isocyanide
ligand with silver nitrate in chloroform regardless of the applied ratio of the reactands (1:1 or 2:1). The
crystal structure of the product has been determined. In the complex molecule with C2 symmetry, the
nitrate anion is attached to the silver center as an η2-chelating ligand. Owing to this approach of the
NO3 ligand, the RNC-Ag-CNR axis (R = 4-Me-C6H4-SO2-CH2) is bent [from 180 to 162.1(2)◦], but
the geometry of the nitrate is not significantly distorted, suggesting only weak coordinative bonding.
The structure is thus intermediate between that of a molecular complex with a tetrahedral coordination
and that of an ionic compound with linear coordination of the silver center.
Key words: Isocyanide Complex, Silver(I) Complex, Nitrate Complex
Introduction
the counterions X. Complexes with trigonal-planar
three- and tetrahedral four-coordination are in fact
most common, while linear two-coordination of
silver(I) by isocyanide ligands is more the excep-
tion [33– 42]. Recent examples include a series of
(isocyanide)silver(I) carboxylates where the RCOO−
groups are in O,O’-bridging positions between sil-
ver(I) centers [43]. Similar structural characteristics
were found for related complexes with tertiary
phosphine ligands [44 – 47].
In the course of preparative studies in the chemistry
of gold(I)/silver(I) isocyanide complexes we have re-
cently obtained and investigated a compound, in which
AgNO3 is the central unit of a 1:2 complex with the
ligand 4-Me-C6H4-SO2-CH2-NC. This isocyanide is
commercially available and has been employed as a
ligand in earlier studies [48], but structures of the com-
plexes have become known only recently [30]. The title
complex has an interesting structure which is reported
in this contribution.
Isocyanide complexes of gold(I) of the types
(RNC)AuX, [(RNC)AuL]+ X− and [(RNC)Au
(CNR)]+ X− are currently receiving considerable
interest owing to the linear “rigid-rod” structures
of their molecules and cations, respectively [1 – 4].
These structures with strictly two-coordinate Au+
centers are an excellent basis for the construction of
supramolecular frameworks and mesogenic phases,
not only assembled via standard coordinative bonding,
but also supported by Au—Au aurophilic interactions.
Many of these aggregates are strongly luminescent,
and their photophysical properties can be tuned by
selecting appropriate substituents R, auxiliary ligands
L and counterions X [5 – 32].
In several studies of gold(I) isonitrile complexes
(RNC)AuX it has been shown that the introduction of
the nitrate ligand (X = NO3) leads to unusual organi-
zational patterns, very different e. g. from those of the
correspondinghalides (X = Cl, Br, I). In the aggregates,
the linearity of the N ≡ C-Au-O axis is always retained
and the flat nitrate groups appear to be readily accom-
modated in the oligomers or polymers [16, 17, 20, 22].
Results
The preparative experiments aimed at the synthe-
sis of both 1:1 and 1:2 complexes of AgNO3 with
p-tosylmethylisocyanide. However, the reactions with
both stoichiometries in chloroform as a solvent were
found to give only the 1:2 complex. With the 1:1 sto-
ichiometry, one half of the ligand was recovered and
The structures of the silver(I) complexes
(RNC)AgX, [(RNC)AgL]+ X− and [(RNC)2Ag]+
X− are more flexible, with the Ag+ centers becoming
three- or four-coordinate through significant interac-
tions with additional donor sites of the ligands L or the yield of the 1:2 complex accordingly was close to
c
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B. Djordjevic et al. · A Molecular Bis(isocyanide)silver(I) Nitrate Complex
7
50% (equ. (1)). While AgNO3 is only sparingly sol-
uble in chloroform, it dissolves rapidly upon addition
of the isocyanide, and colourless crystals are formed
after completion of the reaction. The product can be
redissolved in acetone, and from these solutions single
crystals of the complex can be grown upon careful la◦y-
ering with pentane at room temperature (m. p. 162 C
with decomposition).
AgNO3+2 (4−Me−C6H4−SO2−CH2−NC)
(1)
→ (4−Me−C6H4−SO2−CH2−NC)2AgNO3.
Solutions of the complex in chloroform-d1 show
1
Fig. 1. Molecular structure of bis(p-tosylmethylisocyan-
ide)silver(I) nitrate with point group C2 symmetry (ORTEP,
50% probability ellipsoids, hydrogen atoms omitted).
the expected sets of signals in the 1H and 13C{ H}
NMR spectra. The isocyanide carbon atoms gives
rise to a broad resonance at δC 165 ppm, with
the multiplicity arising from the couplings J(14N-
13C) and J(109/107Ag-13C) which remain unresolved
owing to ligand exchange processes in solution. In
the FAB mass spectrum (p-nitrobenzyl alcohol ma-
trix) the cation [(p-TosCH2NC)2Ag]+ is detected at
m/z 498 (83%), with [(p-TosCH2NC)Ag]+ (100%)
as the parent ion. The solid (in KBr) shows an IR
absorption ν(NC) at 2197.5 cm−1, very similar to
the reference value of 2195 cm−1 reported for [(p-
Tol)NC]2AgNO3 [35].
Crystals of the complex (from acetone/pentane) are
monoclinic (space group C2/c, Z = 4). The unit cell
contains some sub-stoichiometric, residual solvent,
which was found to be disordered. Cation and anion of
the complex are not separated. The nitrate group is at-
tached symmetrically to the silver atom as an O,O’-η 2
chelating ligand. The ion-pair has crystallographically
imposed C2 symmetry with the twofold axis passing
through the silver atom and the atoms N11 and O12 of
the nitrate group (Fig. 1).
Selected bond lengths [A] and angles [◦]: Ag1-C1
˚
2.106(4), Ag1-O11 2.571(2), S1-O1 1.435(3), S1-O2
1.440(2), S1-C2 1.795(4), S1-C11 1.744(4), N1-C1 1.133(4),
N1-C2 1.415(5), N11-O11 1.252(3), N11-O12 1.244(4);
C1-Ag1-C1’ 162.08(19), Ag1-C1-N1 169.7(3), C1-N1-C2
175.5(3), N1-C2-S1 110.8(2), O11-N11-O11’ 118.9(3),
O11-N11-O12 120.53(17).
The C1-N1 distance compares well with the value
reported for p-Tos-CH2NCAuCl, where C1-N1 =
˚
1.131(6) A [30]. By contrast, the Ag1-C1 distance of
the title compound is very significantly longer than the
˚
Au1-C1 reference distance of 1.933(5) A [30], con-
firming the recently revised order of the standard radii
of the coinage metals [49]. All other structural parame-
ters of the isocyanide ligand show no anomalies.
The cation and the complete ion-pair have C2 sym-
metry and are thus chiral. The unit cell contains both
enantiomers related by a center of inversion.
Discussion
The configuration of the complex cation shows a
The structure of the title complex appears to rep-
bending of the C1-Ag1-C1’ axis away from the 180◦ resent an intermediate state between a fully ionic ex-
angle to a value of 162.1(2)◦ which is clearly due treme with linearly two-coordinate silver in a cation
to the approach of the nitrate anion. The Ag-O1 dis- [RNCAgCNR]+ associated with, but not coordinated
˚
tances are 2.571(2) A long, which is in good agree- to an independent nitrate anion of D3h symmetry on
ment with standard values for bonding Ag-O con- one hand, and an electroneutral complex of tetrahe-
tacts. The N-O bond lengths of the nitrate anion are drally four-coordinate silver ions with the nitrate anion
different with N11-O11(coordinated) at 1.252(3) and as a chelating ligand of C2v symmetry on the other.
˚
N11-O12(non-coordinated) at 1.244(4) A as expected This situation reflects the affinity of two-coordinate
for η2-coordinated nitrate, but the deviations of the silver(I) for additional donors including “hard” lig-
O-N-O angles from 120◦ are very small: 120.5(2), ands like NO3−. This affinity clearly is aiming not
118.9(3)◦.
only at trigonal-planar three-coordination, for which
The coordinated isocyanide has distances Ag1-C1 = the monodentate coordination mode of the nitrate
˚
2.106(4) and C1-N1 = 1.133(4) A, and angles would be sufficient, but rather at four-coordination
Ag1-C1-N1 = 169.7(3)◦ and C1-N1-C2 = 175.5(3)◦. with chelating nitrate.
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8
B. Djordjevic et al. · A Molecular Bis(isocyanide)silver(I) Nitrate Complex
Table 1. Crystal and structure solution data for bis(p-tosyl-
methylisocyanide)silver(I) nitrate.
And yet the approach of the nitrate anion is not
reaching an extreme equilibrium position with tetra-
hedral angles C-Au-C and C-Au-O: Partly owing to
the strain of the four-membered chelate ring, but partly
also owing to the reduction of the bond energies of the
Au-C bonds associated with the bending of the C-Au-C
angle away from linearity, the latter is only compressed
to 162.1(2)◦. From the results of an early study of the
vibrational spectra of di(p-tolylisocyanide)silver(I) ni-
trate it was concluded that the nitrate resides outside
the coordination sphere of the silver atom [36]. How-
ever, since the ν(CN) data are almost the same for the
two compounds (2198 vs. 2195 cm−1), an analogous
solid state structure can be assumed.
[(pTolSO2CH2NC)2Ag]NO3
Empirical formula
M
C18H18AgN3O7S2
560.34
monoclinic
C2/c
17.0363(6)
13.8518(4)
11.4364(3)
92.2009(10)
2696.81(14)
1.380
Crystal system
Space group
˚
a / A
˚
b / A
˚
c / A
β / ◦
3
˚
V / A
ρcalc / g cm−3
Z
4
1128
9.39
143
F(000)
µ(Mo-Kα ) / cm−1
T / K
The crystal structure of bis[2,4,6-tri(tbutyl)phenyl-
isonitrile]silver(I) hexafluorophosphatehas been deter-
mined and appears to be closely related: the octahedral
PF6 anion is attached to the silver cation via one of
its edge to give a molecular structure of C2 symme-
try [39]. The Ag-C and C≡N distances are 2.075(14)
Refls. Measured
Refls. Unique
Refined param. /restraints
R1[I ≥ 2σ(I)]
wR2a
8219
2344 [Rint = 0.073]
144 / 0
0.0381
0.1051
a = 0.0589, b = 3.3507
0.409 / −0.576
Weighting scheme −3
˚
σ
fin(max/min) / eA
˚
a
2
wR2 = {Σ[w(Fo − Fc2)2]/Σ[w(Fo2)2]}1/2; w = 1/[σ2(Fo2) +
and 1.148(17) A, respectively, and the C-Ag-C’ angle
is 156.1(6)◦. This structure confirms the high affinity
of two-coordinate silver cations towards anionic lig-
ands including even the poorest donors like CF3SO3
or BF4 [13]. None of these or other anions [25, 26] be-
comes attached to the gold cations in its 2:1 complexes
with isocyanides [1 – 4].
Because the anions are not tightly bound in com-
pounds of this type, they can be readily mobilized in
polar solvents by solvation of the ionic components of
the complexes.
(ap)2 +bp]; p = (F 2 +2F 2)/3.
o
c
(560.34): calcd. C 38.12, H 2.93; found C 38.58, H 3.24.
NMR (CDCl3, 20 ◦C), 1H: δ = 7.85 (m, 2H, C6H4-2/4), 7.43
(m, 2H, C6H4-3/5), 2.39 (s, 3H, Me); 13C{ H}: δ = 165
1
(m, NC), 147.4 (s, C6H4-1), 133.1 (s, C6H4-4), 131.4 (s,
C6H4-2/6), 129.8 (s, C6H4-3/5), 62.4 (s, CH2), 22.6 (s, Me).
IR(KBr): 2197.5, ν(NC). MS(FAB): m/z 498 (83.7, L2Ag);
303 (100, LAg).
Crystal structure determination: A specimen of suit-
able quality and size was mounted on the end of a quartz
fiber in F06206R oil and used for intensity data collection
on a Nonius DIP2020 diffractometer, employing graphite-
monochromated Mo-Kα radiation. Intensity data were cor-
rected for absorption effects (DELABS from PLATON). The
structure was solved by a combination of direct methods
(SHELXS-97) and difference-Fourier syntheses and refined
by full matrix least-squares calculations on F2 (SHELXL-
97) [50]. The thermal motion of all non-hydrogen atoms was
treated anisotropically. All hydrogen atoms were calculated
Experimental Section
General: The experiments were carried out in dry sol-
vents under nitrogen; Jeol JNM-GX 270 and 400 NMR
spectrometers; Finnigan MAT 90 mass spectrometer; Jaseo
460 Plus infrared spectrometer; Kleinfeld Electrothermal
IA 9200 melting point apparatus. The reagents were com-
mercially available.
Preparation: p-Tolylsulfonylmethylisocyanide (200 mg, and allowed to ride on their parent atoms with fixed isotropic
1.02 mmol) was dissolved in 10 ml of chloroform and added contributions. During the structure refinement, the atoms of
to a suspension of silver nitrate (174 mg, 1.02 mmol) in a solvent molecule were observed but could not be mod-
10 ml of the same solvent with stirring at room temper- elled satisfactorily. The SQUEEZE routine in PLATON was
ature. The reaction flask was protected against light. Af- used to modify the HKL file. Further information on crys-
ter 3 h the product was recovered by filtration, washed tal data, data collection and structure refinement are summa-
with diethylether and dried in a vacuum; 286 mg (49% rized in Table 1. Anisotropic displacement parameters and ta-
yield). In an experiment with the reagents in the molar ratio bles of interatomic distances and angles have been deposited
2:1 the yield was almost quantitative. Single crystals were with the Cambridge Crystallographic Data Centre, 12 Union
grown from an ◦acetone solution upon layering with pen- Road, Cambridge CB2 1EZ, UK. The data are available on
tane, m. p. 162 C with decomposition. C18H18AgN3O7S2 request on quoting CCDC-291990.
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B. Djordjevic et al. · A Molecular Bis(isocyanide)silver(I) Nitrate Complex
9
Acknowledgements
This work was supported by Deutsche Forschungsge-
meinschaft and Fonds der Chemischen Industrie.
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