A Manganese(III) Complex That Exhibits Spin Crossover Triggered by Geometric Tuning

Abstract

The design and synthesis of molecules with potential for information storage and processing is a focus in current materials research, an area in which bulk metal oxides still predominate. The potential of discrete transition-metal spincrossover (SCO) complexes in this area is now starting to be realized as photoinduced switching emerges as an increasingly efficient trigger for spin conversion.[1] As the field continues to grow, it is important to identify new candidates with improved thermal or optical responses and also to fine-tune the molecular design for successful incorporation into devices.Although the phenomenon is theoretically possible for octahedral d [4][5][6][7] [3,4] it is considered exceptional. The best-known example is [Mn(trp)] (trp 3À = tris{1-(2-azolyl)-2-azabuten-4-yl}amine), [3] and investigations of its physical properties continue. [5, 6] The only other well-characterized example [CrI 2 (depe) 2 ][4] (depe = 1,2-bis-(diethylphosphino)ethane) also continues to attract attention.[7] An early report of some Mn III complexes, which were not structurally characterized, attributes the temperature dependence of the magnetic moment to exchange interactions, although SCO is a more likely interpretation. [8] An important driving force for SCO is the change in metal-ligand bond length arising from the increased vibrational entropy in the HS state. The concomitant decrease in ligand-field splitting energy reduces the ligand-field stabilization energy (LFSE) associated with the maximum population of the t 2g orbitals. The reluctance of octahedral d 4 ions to undergo thermal SCO may be due to the very minor gain in LFSE on crossover from 5 E to 3 T as only one electron is transferred. In most crystal fields, the small LFSE gain for d 4 ions is outweighed by the energetic penalty of pairing two electrons, which results in both an increase in the destabilizing Coulomb repulsion energy p c and a decrease in the stabilizing exchange energy p e . [9] Schiff base ligands with N 4 (O À ) 2 , N 2 (O À ) 2 , and N 3 (O À ) 3 donor sets that are known to generate ligand fields close to the crossover value for Fe III [10] tend to stabilize manganese in its + III oxidation state, in which it is normally HS. Recently, however, we have observed that modulation of the orientation of the N 4 (O À ) 2 donors in a series of hexadentate ligands has a dramatic effect on the spin state of Mn III complexes and that gradual thermal SCO may be attained when the phenolate oxygen donors are trans to each other. This geometry is achieved by varying the length of the alkyl chains in the starting tetraamine. The use of a triethylenetetraamine link (in L1) orients the oxygen donors cis to each other and produces a HS Mn III complex, whereas lengthening the link by two methylene groups (L2) orients the oxygen donors in trans geometry and thermal SCO results.To illustrate this phenomenon, we report the structures and temperature dependence of the magnetic susceptibility of [MnL1]NO 3 ·EtOH (1) and [MnL2]NO 3 (2), which exhibit differ...

A manganese(III) complex that exhibits spin crossover behavior

“…3. At 77 K, the magnetic moment (l eff ) converges close to the expected LS value of 2.8 lB (S = 1), whereas, at 300 K, the HS (S = 2) limit is entropically inaccessible, as this result shows a one-electron spin crossover [14]. The HS/LS states ratio is 76% at 300 K. Temperature hysteresis of this complex was measured, the shape of the curve only indicates a gradual spin transitions (similar to Fig.…”

“…Recently, however, Morgan and his coworkers [14] have observed that modulation of the orientation of the N 4 (O À ) 2 donors in a Schiff base ligand has a dramatic LS M HS transition effect on the spin state of Mn III complexes and that gradual thermal SCO may be attained when the phenolate oxygen donors are trans-to each other [14].…”

Synthesis and self-assembly of spin-labile and redox-active manganese(iii) complexes

“…28 This criterion might be less relevant for the manganese series, since the equally spin-stable methoxy-50 substituted analog of 3a has α = 87.8. 12 Fig. 4 ORTEP representations of the Mn III complex cations 3a and 4a (a and b, respectively, 30% probability ellipsoids, only one of the two independent residues shown), and 5a (c, 50% probability ellipsoids).…”

“…While most of the bond lengths in 5a at 173 K are similar to those in the methoxy-functionalized complex I at 5 RT, which is known to be in high spin configuration, partial spin crossover may be supported by the short M-N amine bonds in 5a. In addition, the angular distortion parameter (Σ = 54.5) is between the parameters calculated for the high and the low spin states of complex I (70.7 and 45.0, respectively), 12 and hydrogens to two oxygen atoms of the nitrate counterion. Complexes of 5a are linked together by a similar hydrogen bonding motif as 3a, involving a NO 3 -counterion and a cocrystallized H 2 O molecule.…”

“…The magnetic properties of selected complexes have been investigated over a temperature range of 300-50 K. Akin to related complexes, 12 crystalline samples of 3a × 1.75 H 2 O and 3c displayed a temperature-independent magnetic moment (µ eff = 4.85 and 4.56 µ B , respectively). 21 This value is in 45 agreement with the expected spin-only magnetic moment calculated for a high spin complex with four unpaired electrons (S = 2, µ eff = 4.90 µ B ).…”