Irradiation Temperature Dependence of the Photomagnetic Mechanisms in a Cyanido-Bridged Cu^II₂Mo^IV Trinuclear Molecule

Abstract

We report a new bimetallic cyanido-bridged trinuclear complex [Cu(enpnen)][Mo(CN)]·6.75HO (1) (enpnen = N,N'-bis(2-aminoethyl)-1,3-propanediamine) that shows reversible photomagnetic effect. The photo-induced increase of magnetization is characterized by the irradiation temperature-dependent shapes of the χ T( T) plots and different magnetization values at low temperature in high magnetic field, suggesting multiple photoexcited states. The photomagnetic effect in 1 is explained through two possible processes simultaneously: the light-induced metal-to-metal charge transfer (MMCT) in the Cu-NC-Mo pair and the light-induced excited spin-state trapping (LIESST) effect in Mo center. A numerical model assuming the simultaneous existence of three possible states after irradiation: the MMCT Cu-NC-Mo-CN-Cu state, the LIESST Cu-NC-Mo-CN-Cu state, and the ground-state Cu-NC-Mo-CN-Cu was applied to the data and resulted in Cu-Mo exchange coupling constants J = 11 cm and J = 109 cm for the MMCT and LIESST mechanisms induced states, respectively. Fractions of respective states after irradiations at different temperatures were also calculated, shedding light on the influence of irradiation temperature on the photomagnetic mechanism. The proposed model can provide the interpretative framework for testing and refinement of the mechanism of photomagnetic effect in other coordination networks with cyanido-bridged Cu-[Mo(CN)] linkages.

Experimental and theoretical insights into the photomagnetic effects in trinuclear and ionic Cu(ii)–Mo(iv) systems

“…33 Furthermore, the spectrum of 1 in this range is similar to the spectra of 3 and K 4 Mo, which results from their structural similarities: 1 and K 4 Mo contain isolated [Mo(CN) 8 ] 4− anions, while 3 has weakly linked Cu(II) centres in the V-shaped trinuclear molecule with an average Cu-N distance of 2.353 Å and an obtuse Cu⋯Mo⋯Cu angle equals 131.52°. 29 For 2, the spectrum is comparable to 4, and is significantly diverse from other compounds, with intense peaks around 2160 cm −1 originating from the vibrations of short bridging cyanides in the V-shaped trinuclear molecules with average Cu-N distances of 1.966 and 1.975 Å for 2 and 4, respectively, and acute Cu⋯Mo⋯Cu angles equal 74.57°and 73.44°for 2 and 4, respectively. In the "fingerprint" region, the spectra of 1, 2 and 4 are similar, while the spectrum of 3 differs significantly from other compounds, which is the result of different copper(II) coordination environments.…”

“…These maxima are typically attributed to the Cu(II) d-d transitions in the various coordination environments. 29,45 The remaining bands are routinely assigned to ligand-field (LF) bands (in the 250-500 nm region) and metal-to-ligand charge transfer (MLCT) bands of [Mo(CN) ations of the absorption spectra of 1-4 are essential for further analysis of the mechanism of photomagnetic phenomena.…”

“…Up to now, photomagnetic processes in Cu(II)-Mo(IV) systems have been considered in terms of two possible mechanisms: metal-to-metal charge transfer (MMCT): paramagnetic Cu II (S = 1/2)⋯Mo IV-LS (S = 0)⋯Cu II (S = 1/2) → ferromagnetic Cu I (S = 0)⋯[Mo V-LS -Cu II ](S total = 1), 27,[29][30][31]33,36,[38][39][40][41]43,44,[46][47][48][49][50][51][52][53][54][55] and a Light-Induced Excited Spin-State Trapping (LIESST) effect on the Mo(IV) centre: paramagnetic Cu II (S = 1/2) ⋯Mo IV-LS (S = 0)⋯Cu II (S = 1/2) → ferromagnetic [Cu II -Mo IV-HS -Cu II ](S total = 2). [27][28][29][31][32][33]35,37,42,45 The latter case may result from the photoinduced formation of an intermediate geometry between the ideal geometries of TDD-8 and SAPR-8 (Fig. 5) 32,35,60 or the photodissociation of the single cyanide leading to a reduction of the coordination number to 7.…”

“…Furthermore, the excited states induced in the LIESST process are more stable than the MMCT one, due to the nearly two orders of magnitude higher energy barrier and lifetime, showing T relax around 250 K and slightly above 100 K, respectively. 29,76 Bearing in mind the aforementioned characteristics of the mechanism for photomagnetic effects, structural features and the results of experimental and theoretical spectroscopic studies of 1-4 it is possible to predict the most preferred mechanism of the photomagnetic phenomenon. The ionic compound 1 with isolated [Mo(CN) 8 ] 4− anions most likely exhibits the LIESST-induced effect due to the absence of Cu(II) ⋯NC-Mo(IV) bridges (Fig.…”

“…[13][14][15][16][17][18][19][20][21][22][23][24] The greatest successes in developing photomagnetic materials were achieved for octacyanidometallate based systems, 25,26 in particular for copper(II)-molybdenum(IV) ones. 27 Furthermore, it was found that Cu II -Mo IV photomagnetic assemblies can be based on polynuclear molecules, [28][29][30][31][32][33][34][35][36][37][38][39][40][41] chains, 33,[42][43][44] layers 33,[45][46][47] and three-dimensional networks 33,[48][49][50][51][52][53][54][55] with at least one bridging cyanide per single copper(II) centre. Surprisingly, ionic systems with an isolated [Mo(CN) 8 ] 4− anion were not considered, assuming that the photomagnetic phenomenon in Cu II -Mo IV compounds originates from the light-induced MMCT mechanism.…”