A new heterometallic multiligand 3D coordination polymer: synthesis
and structure of [Pb(OH)]
n[Ag(SCN)(CN)]
n.
Eliano Diana*
a,Giuliana Gervasio
a, Emanuele Priola
aand Elisabetta Bonometti
aReceived (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x
A new coordination 3D polymer of silver and lead ions with ambidentate mixed ligands has been prepared and characterized.
The goal to build coordination polymers can be achieved by a strategic control of both metal and ligand employed. Cyanide and thiocyanate ligands are very versatile because of their ambidentate nature and the presence of coordinating element of different hardness, that can permit the cross-link among the metals. Recently, the ability of thiocyanate to form Ag2S21
and Ag3S3 rings2 has been exploited to build 3D coordination
polymers, and cyanide group has been usefully employed as a linker of [Ag5(SCN)4(bipy)2]nn+ columns in the 3D
coordination polymer [Ag5(SCN)4(CN)(bipy)2]n 3. In the
present work this possibility has been explored with two d10
metals, Ag(I) and Pb(II), and both CN- and SCN- ligands.
This choice is due to the interesting luminescence properties usually found with d10 polynuclear metal complexes4 and to
the ability of Pb(II) ion to form polynuclear hydroxo species in aqueous solution5.
The title compound has been prepared by mixing in a Dewar vessel a boiling water solution of lead(II) thiocyanide with a water solution of potassium dicyanoargentate (experimental details are in S.I.). After slow cooling (12h), transparent acicular crystals suitable for x-ray analysis have been obtained.
a)
b)
Figure 1. ORTEP plot(thermal ellipsoids at 50%) of a):
[Pb(OH)]nn+ ribbons and b): [Ag(CN)(µ3-SCN)]nn- layers.
The Ag(SCN)(CN)Pb(OH) compound (Table S1 and S2) is formed by two types of polymers linked through weak N···Pb interactions (2.639(7)Å). One moiety is formed by [Pb(OH)]nn+ribbons (parallel to b axis), where OH- acts as a
µ3-bridge over three Pb ions (see Figure 1a).The other moiety
is a corrugated layer (nearly parallel to (100) plane) where sulphur atom of SCN- ion bridges three Ag(CN) units (see
figure 1b); the CN- ligands lay over and below the [Ag(CN)
(µ3-SCN)]nn- layers and link the [Pb(µ3-OH)]nn+ribbons.
(Figure 2).
The [Pb(µ3-OH)]nn+ polymer has been formed probably
during the dissolution of lead thiocyanate in boiling water and then acting as aggregation centre of the crystalline packing. The coordination sphere of lead(II) is formed by three OH oxygen atoms (Pb···O 2.406(4) Å av.) and three nitrogen atoms: two N atoms belong to the cyanide ion and the contact is quite short ( 2.639(7) Å ) with respect to the third N atom at the tip of the thiocyanate ion (2.972(7) Å). Pb ions shows an emispheric coordination similar to that of other compounds (cf. Pb(SCN)2)6.
Silver atoms of [Ag(CN)(µ3-SCN)]nn- show the typical
tetrahedral coordination and, as metal ions softer than Pb2+,
bond the soft sulphur atom of thiocyanate ion. In fact the borderline Pb(II) interacts better with nitrogen and oxygen atoms.
Figure 2 Packing plot of the [Pb(OH)]n[Ag(SCN)(CN)]n
compound, showing the link between the [Ag(CN)(µ3
-SCN)]nn- layers (blue) and the [Pb(µ3-OH)]nn+ ribbons (red).
Figure 3 main interligand arrangement in crystalline Ag(SCN) (left) and 1 (right)
Raman spectra of 1 are coherent with the crystal structure. The spectra (figure S2 and table S3) are mainly characterized by the vibrational modes of the cyanide and thiocyanate ligands. In order to do a vibrational assignment it is useful to compare the SCN- interactions in crystalline AgSCN and in 1
(figure 3). Free SCN- ion has ν(CN) frequency at 2066 cm-1 7,
that becomes 2140 cm-1 in the AgSCN compound8 because of
the bridging coordination of the ligand. In 1 SCN- ligand is
linked with the sulfur atom to three silver, as in AgSCN, but the N∙∙∙Pb distance is 2.97 Å, longer than 2.16 Å of d(N∙∙∙Ag) in AgSCN9, that make the SCN- ligand in 1 more terminal
like. This is reflected on the ν(CN) frequency, that decreases to 2100 cm-1. Similarly behaves the ν(CS) vibrational mode,
that moves from 744 cm-1 in AgSCN10 to 721 cm-1 in 1. The
cyanide ligand behaves similarly:in crystalline AgCN the ligand has a linear bridge coordination and a ν(CN) frequency at 2164 cm-1 11. Compound 1 has a similar arrangement of
CN-, with a Pb∙∙∙N distance of 2.64Å, greater than Ag∙∙∙N
distance in AgCN compound(1.86Å), and a non–linear Pb-N-C angle of 158°. The elongation of the bridge disposition induces a lowering of the ν(CN) frequency, that we assigne to the 2120 cm-1 mode.
The low frequency region contains bands attributable to stretching and deformation modes involving the metal atoms. Vibrational modes involving the μ3-OH groups bonded to Pb
atoms are expected similar to the analogous [PbOH]+
polymers12: the two bands at 352 and 337 cm-1 are attributed
to O-Pb stretching. The modes of cyanide ligand show a very weak feature at 476 cm-1, attributable to ν(Ag-C), and a broad
band at 277 cm-1, assignable to the silver cyanide bending
δ(Ag-CN)11. These modes are very similar to those found in
crystalline AgCN, and this is reasonable by considering the equal (within the high e.s.d.’s) bond distance (d(Ag-C)=2.12(1)Å in 1, 2.15(6)Å in AgCN). Two main bands are assignable to modes involving the SCN ligand, the medium band at 451 cm-1, attributable to SCN bending, and the band
at 239 cm-1, assigned to Ag-S stretching. Also in this case no
significant differences are detected in the thiocyanate ligand spectra of crystalline AgSCN.
Both structural and vibrational data indicate the ability of both thiocyanate and cyanide ligand to interlink bands of hydroxo-lead polymers with the almost 2D network of Ag-S atoms (figure 2b). The solid state electronic absorption and emission spectrum of 1 are reported in fig. 4, and shows an absorption band at 301 nm and an emission at 360 nm. The absorption wavenumber is very similar to that of AgSCN (around 290 nm)13 that shows two phosphorescence
emissions at near 400 and 550 nm. These phosphorescences have been assigned to a T1S
0 transition of the SCN- ion.A
TD-DFT computed absorption spectrum obtained from a tetra-coordinated [Ag(SCN)3(CN)]3- silver fragment
(computational details are in SI, figure S4) reports two intense close transitions at 305 and 311 nm, assignable to HOMO-1LUMO and HOMOLUMO, two LM excitations, and this suggest that the absorption and emission of 1 can be attributed mainly to the silver centres.
250 300 350 400 450 500 In te ns it y (a rb it ra ry u ni ts ) wavelenght (nm) 301 360 416
Figure 4. Excitation and emission spectra of 1
This work has shown an intriguing ability of polydentate ligands like thiocyanate and cyanide to build and interlink 2D organometallic networks and underlines the crystal
engineering potential of
[Pb(µ
3-OH)]
nn+ribbons.
References
a Dipartimento di Chimica and Centro Interdipartimentale di
Cristallografia Diffrattometrica (CrisDi), Università di Torino, Via Pietro Giuria 7, Turin 10125, Italy. Fax: +3901167078550 Tel: +390116707572; E-mail: eliano.diana@unito.it
†Electronic Supplementary Information (ESI) available:
[Experimental and computational details, crystal data, Raman data, CSD:427447 ]. See DOI: 10.1039/b000000x/
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