PhD Elisabeth Felbermair

Cross-Linked Cyanometallate Networks of Ni(II) and Fe(III) in Silica

Abstract

Reactions of NiII complexes of the type [Ni(polyamine)n]2+ (n = 1, 2, 3) and [Fe(CN)6]3 – commonly lead to cyanometallate networks by formation of cyanide bridges FeIII—C≡N–NiII. The resulting [Ni(polyamine)n]x[Fe(CN)6]y structures often exhibit ferromagnetism caused by magnetic coupling of unpaired electrons of the NiII and FeIII ions through the cyanide bridges.The polyamine ligands block coordination sites of NiII which limits the possibilities to form cyanide bridges. This favors a low-dimensional (1D, 2D) arrangement of the cyanometal- lates. Diamines or tetramines are typically used for this purpose as they have high affinity to NiII.

In this work such low-dimensional cyanometallates are embedded in SiO2 by means of sol- gel processing. A chemical link is established between the cyanometallate and SiO2 matrix to make the cohesion more stable. This is achieved by introducing linker groups into the organic blocking ligands. The linker consists of an alkyl chain with a trialkoxysilyl group that reacts with the SiO2 precursor tetraethoxysilane (TEOS) during sol-gel processing, thus forming covalent bonds. The influence of this process on the structure of the cyano- metallate network and its magnetic properties is studied by FTIR spectroscopy, SEM, EDX, SWAXS and SQUID.

Five NiII–FeIII cyanometallates with various blocking ligands are discussed in this work and the influence of the ligands on the structure and magnetic properties assessed. [Ni(AEAPTS)2]3[Fe(CN)6]2 (AEAPTS = N-(2-aminoethyl)-3-(trimethoxysilyl)propyl-amine) forms a 1D double chain structure with crystalline order. The 3D order is lost during sol- gel processing, but FTIR spectroscopy confirms the survival of the cyanometallate at least in small domains or isolated double chains. Embedded in SiO2 it shows magnetic order- ing below 22 K indicating ferromagnetism. The TC is 10.6 K, and µef f is 4.46 flB at room temperature and 8.60 flB at the maximum at ~15 K. [Ni(silyl-2,3,2-tetramine)2]3[Fe(CN)6]2, [Ni(bissilyl-2,3,2-tetramine)2]3[Fe(CN)6]2 and K[Ni(silylcyclam)][Fe(CN)6] show a similar structure as [Ni(AEAPTS)2]3[Fe(CN)6]2, and also lose the crystallinity upon sol-gel processing. These three cyanometallates embedded in SiO2 show magnetic behavior similar to [Ni(AEAPTS)2]3[Fe(CN)6]2 at high temperature (>50 K), but no magnetic ordering at low temperature. They are paramagnetic. [Ni(silyl-2,3,2-tetramine)2]3[Fe(CN)6]2 shows a µef f of 5.40 flB, [Ni(bissilyl-2,3,2-tetramine)2]3- [Fe(CN)6]2 a µef f of 6.81 flB and K[Ni(silylcyclam)][Fe(CN)6] a µef f of 3.34 flB. [Ni(silyl-6-aminocyclam)]3[Fe(CN)6]2 does not show an as well ordered crystalline phase before sol-gel processing, but FTIR spectroscopy confirms the formation of a cyanomet- allate network. A 1D arrangement can be concluded from SWAXS data. The compound embedded in SiO2 is also paramagnetic with a µef f of 4.23 flB.

A CuII–FeIII cyanometallate with AEAPTS as blocking ligand is synthesized analogous to [Ni(AEAPTS)2]3[Fe(CN)6]2 to study the influence of the metal ion on the structure and magnetic behavior. [Cu(AEAPTS)2]3[Fe(CN)6]2 shows crystallinity before sol-gel pro- cessing, but is likely a mixture of several structures. The material obtained after sol- gel processing lost the 3D order, but small domains or isolated chains of the cyanomet- allate survive, as in the analogous NiII structure. Paramagnetic behavior is found for [Cu(AEAPTS)2]3[Fe(CN)6]2 in SiO2 with a µef f of 3.52 flB.