Anion-Induced Structural Diversity and Optical Chromism in a Series of Cyano-Bridged Heterometallic 3d-4f Coordination Polymers

Abstract

The self-assembly reaction of the neutral dicyano-bis(1,10-phenanthroline) iron(II) complex with lanthanide ions (Ln = Eu(III), Gd(III), Er(III)) provided two different classes of heterometallic cyano-bridged 3d-4f architectures depending on the nature of the counteranion, irrespective of the size of the 4f metal ion. Tetranuclear oligomers with a squared Fe2Ln2 core were isolated when using nitrate salts, whereas unusual 1D polymeric chains were obtained when resorting to triflate salts under the same synthetic conditions. It is shown that the different structural motifs have a remarkable impact on the thermal stability and the optical properties of the compounds, which display a notable optical ipsochromism of the parent Fe(II) complex upon coordination with the Ln ion. This effect is significantly more pronounced in the polymeric chain than in the Fe2Ln2 oligomer both in solution and in the solid state. Structural evidence suggests that this behavior is likely related to the geometry of the CN-Ln bridge. On the other hand, more extended π-stacking interactions in the oligomer give rise to a broad charge-transfer absorption (600–1500 nm), making this compound promising as NIR absorber. Density Functional Theory calculations and electrochemical studies demonstrate that the observed negative chromism originates from the stabilization of a mixed metal/cyanide character HOMO with respect to a phenanthroline-centered LUMO.

Spectral tuning and emission enhancement through lanthanide coordination in a dual Vis-NIR emissive cyanide-bridged heterometallic Ru(II)-Er(III) complex

“…38 . This band shows a notable hypsochromism in the cyanide-bridged heterometallic RuEr compound with respect to the parent Ru complex (λmax(RuEr) = 450 nm vs λmax(Ru) = 494 nm), which originates from the coordination of the CN groups to the Lewis acid Er(III) ion, as also observed in analogous cyanide-bridged Fe(II)-Ln(III) assemblies 37 . Such hypsochromic shift is also evident in the diffuse reflectance spectra of solid-state samples (Figure 3b) and results in a remarkable change in color of the parent Ru complex upon coordination to Er(III) both in solution and in the crystalline state (Figure 3c).…”

“…In this work, since the [Ru(bipy)2(CN)2] complex was intended to be further reacted to produce the heterometallic complex, purification steps were reduced to avoid time-consuming procedures. The obtained product (Ru) was then directly reacted with the erbium nitrate salt to obtain the heterometallic complex following a similar procedure previously used to afford heterometallic Fe-Ln architectures based on the Fe(II) analog [Fe(phen)2CN2] neutral complex 37 , where the nitrate counteranion remains coordinated to the Ln(III) metal center, serving as blocking ligand and dictating the geometry of the resulting compound. The reaction afforded crystals of the heterometallic tetranuclear [Ru2(bipy)4(CN)4Er2(NO3)4(H2O)4(ox)]⋅5H2O (RuEr) complex.…”

Spectral Tuning and Emission Enhancement through Lanthanide Coordination in a Dual Visible–Near-Infrared Emissive Cyanide-Bridged Heterometallic Ru(II)–Er(III) Complex

“…39 This band shows a notable hypsochromism in the cyanide-bridged heterometallic RuEr compound with respect to the parent Ru complex (λ max(RuEr) = 450 nm vs λ max(Ru) = 494 nm), which originates from the coordination of the CN groups to the Lewis acid Er(III) ion, as also observed in analogous cyanide-bridged Fe(II)−Ln(III) assemblies. 38 Such a hypsochromic shift is also evident in the diffuse reflectance spectra of solid-state samples (Figure 3b) and results in a remarkable change in color of the parent Ru complex upon coordination to Er(III) both in solution and in the crystalline state (Figure 3c). It is interesting to note that in the solid state, the spectral shape is significantly broadened (>100 nm) with respect to the solutions, as highlighted by the color differences for the two samples in the diverse phases illustrated in Figure 3c.…”

“…The spectra are characterized by a series of intense bands in the UV region and a prominent band in the visible, attributable to the [Ru(bipy) 2 (CN) 2 ] moiety in both compounds and typically assigned to 1 MLCT transitions as in most Ru(II) complexes . This band shows a notable hypsochromism in the cyanide-bridged heterometallic RuEr compound with respect to the parent Ru complex (λ max( RuEr ) = 450 nm vs λ max( Ru ) = 494 nm), which originates from the coordination of the CN groups to the Lewis acid Er(III) ion, as also observed in analogous cyanide-bridged Fe(II)–Ln(III) assemblies . Such a hypsochromic shift is also evident in the diffuse reflectance spectra of solid-state samples (Figure b) and results in a remarkable change in color of the parent Ru complex upon coordination to Er(III) both in solution and in the crystalline state (Figure c).…”

“…In this work, since the [Ru-(bipy) 2 (CN) 2 ] complex was intended to be further reacted to produce the heterometallic complex, purification steps were reduced to avoid time-consuming procedures. The obtained product (Ru) was then directly reacted with the erbium nitrate salt to obtain the heterometallic complex following a similar procedure previously used to afford heterometallic Fe-Ln architectures based on the Fe(II) analogue [Fe(phen) 2 CN 2 ] neutral complex, 38 where the nitrate counteranion remains coordinated to the Ln(III) metal center, serving as a blocking ligand and dictating the geometry of the resulting compound. The reaction afforded crystals of the heterometallic tetranuclear complex [Ru 2 A summary of the crystallographic parameters for RuEr is reported in Table 1.…”