Spin Crossover and Long‐Lived Excited States in a Reduced Molecular Ruby

Abstract

The chromium(III) complex [CrIII(ddpd)2]3+ (molecular ruby; ddpd=N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) is reduced to the genuine chromium(II) complex [CrII(ddpd)2]2+ with d4 electron configuration. This reduced molecular ruby represents one of the very few chromium(II) complexes showing spin crossover (SCO). The reversible SCO is gradual with T1/2 around room temperature. The low‐spin and high‐spin chromium(II) isomers exhibit distinct spectroscopic and structural properties (UV/Vis/NIR, IR, EPR spectroscopies, single‐crystal XRD). Excitation of [CrII(ddpd)2]2+ with UV light at 20 and 290 K generates electronically excited states with microsecond lifetimes. This initial study on the unique reduced molecular ruby paves the way for thermally and photochemically switchable magnetic systems based on chromium complexes complementing the well‐established iron(II) SCO systems.

A Near‐Infrared‐II Emissive Chromium(III) Complex

“… [35a] On the other hand, [Cr(ddpd) 2 ] 3+ with its fused six‐membered electron‐rich ddpd chelates exhibits metal‐centered reduction at −0.48 V vs. NHE to give the spin‐crossover compound [Cr II (ddpd) 2 ] 2 + . [35b] The electron‐rich character of our anionic dpc ligand, combined with its six‐membered chelate rings suggest an analogous metal‐centered reduction to give [Cr II (dpc) 2 ] 0 , but cathodically shifted with respect to [Cr(ddpd) 2 ] 3+ due to the high electron density at the metal center. The latter characteristics furthermore facilitate the observation of a Cr IV/III redox couple at 0.46 V vs. Fc +/0 (−0.17 V vs. NHE), which does not seem to be detectable for the other complexes in Figure 1 .…”

“…The electron‐poor pyridine ligands of fused‐five‐membered tpy chelates in [Cr(tpy) 2 ] 3+ lead to a situation in which the first reduction process is ligand‐centered, occurring at +0.1 V vs. NHE to give [Cr III (tpy⋅ − )(tpy)] 2+ with no trace of Cr II [35a] . On the other hand, [Cr(ddpd) 2 ] 3+ with its fused six‐membered electron‐rich ddpd chelates exhibits metal‐centered reduction at −0.48 V vs. NHE to give the spin‐crossover compound [Cr II (ddpd) 2 ] 2 + [35b] . The electron‐rich character of our anionic dpc ligand, combined with its six‐membered chelate rings suggest an analogous metal‐centered reduction to give [Cr II (dpc) 2 ] 0 , but cathodically shifted with respect to [Cr(ddpd) 2 ] 3+ due to the high electron density at the metal center.…”

Investigation of Luminescent Triplet States in Tetranuclear CuI Complexes: Thermochromism and Structural Characterization

“…We already applied this technique successfully to as eries of transition metal complexes, including several binuclear Cu 2 I 2 complexes. [1,8,9,[32][33][34] On the basis of quantum chemical calculations with respect to relative energiesa nd molecular orbitals the high and low energy emissionb ands of such Cu 4 I 4 clustersa re assignedt ot ransitions from triplet metal/halide-to-ligand charge transfer ( 3 M/XLCT) states and cluster-centered ( 3 CC) states to the ground state in the vast majority of cases. [10, 15-19, 22, 23, 28] For a series of presented compounds within this work the relative population of these electronically excited states can be modulated by changingt he temperature.…”

“…The involved luminescent states are generally long‐lived triplet states so that time‐resolved step‐scan FTIR spectroscopy (nanosecond to microsecond time scale) is a very suitable tool to analyze the electronically excited states. We already applied this technique successfully to a series of transition metal complexes, including several binuclear Cu 2 I 2 complexes [1, 8, 9, 32–34] . On the basis of quantum chemical calculations with respect to relative energies and molecular orbitals the high and low energy emission bands of such Cu 4 I 4 clusters are assigned to transitions from triplet metal/halide‐to‐ligand charge transfer ( 3 M /XLCT) states and cluster‐centered ( 3 CC) states to the ground state in the vast majority of cases [10, 15–19, 22, 23, 28] .…”

The overlooked NIR luminescence of Cr(ppy)3

“…S3-S4). While polypyridine Cr III complexes are reduced either at the ligand to give radical anions or at the metal center to give Cr II , 17,20,22,30,31 fac-Cr(ppy) 3 is not reduced up to potentials of -2.2 V vs. ferrocene (ESI, Fig. S3), likely due to the electron-rich ppyligand.…”