Spin crossover in phosphorus- and arsenic-bridged cyclopentadienyl-manganese(ii) dimers

Abstract

Manganocene reacts with LiE(SiMe 3 ) 2 (E = P or As) to give Mn{l-E(SiMe 3 ) 2 }] 2 , where E = P (1) or As (2). The temperature dependence of the magnetic susceptibility in 1 and 2 is due to antiferromagnetic exchange and to spin-crossover (SCO). Compound 2 shows two-step SCO with hysteresis, involving high-spin (S = 5/2) and intermediate-spin S = (3/2) Mn(II).The high-spin/low-spin bistability of spin-crossover (SCO) transition metal complexes is a fascinating property that attracts considerable interest because of its potential applications in molecular switches.1 The largest class of SCO compounds comprises monometallic, octahedral iron(II) complexes with six N-donor ligands, whose bistability involves interconversions of the high-spin (t 2g )4 (e g ) 2 and the low-spin (t 2g ) 6 (e g ) 0 states. 2 SCO has also been observed in five-coordinate iron(II) complexes, 3 a tetrahedral iron(II) complex, 4 and in monometallic complexes of chromium(II), manganese(III) and cobalt(II).5 Polymetallic, exchange-coupled SCO compounds with bridging N-donor ligands are less common, however they are attractive synthetic targets because interactions between SCO centres could lead to significant enhancements in cooperativity and bistability properties.6 Although considerable progress has been made with N-donor ligands, the different electronic properties of ligands based on heavier pnictogens such as phosphorus and arsenic could provide an alternative method of influencing the interplay between magnetic exchange and SCO. Thus, we now report the structures and magnetic properties of the phosphorus-and arsenic-bridged cyclopentadienyl-manganese(II) dimers [CpMn{m-E(SiMe 3 ) 2 }] 2 , with E = P (1) or As (2). In 1 and 2, antiferromagnetic exchange occurs concurrently with thermally induced two-step SCO involving the high-spin S = 5/2 and the rare intermediate-spin S = 3/2 states of manganese(II).Compounds 1 and 2 were synthesized by reacting Cp 2 Mn with LiE(SiMe 3 ) 2 (E = P, As) (Scheme 1). The structures of 1 and 2 were determined by X-ray diffraction,w and are very similar, both consisting of pnictogen-bridged dimers of general formula [(Z 5 -Cp)Mn{m-E(SiMe 3 ) 2 }] 2 . The dimers have approximate molecular D 2h symmetry, and the {CpME 2 } coordination environments have approximate C 2v symmetry. Assuming that an Z 5 -Cp ligand formally occupies three coordination sites, each metal atom in 1 and 2 is five-coordinate.The formally five-coordinate, 15-valence-electron phosphidebridged dimanganese compound [CpMn{m-P(SiMe 3 ) 2 }] 2 (1) crystallizes with two independent molecules in the unit cell, 1a and 1b, which are structurally similar and lie about independent inversion centres ( Fig. 1, S1). In 1a, the two Mn(II) centres are bridged by two m-[(Me 3 Si) 2 P] À ligands. The Mn(1)-P(1) and Mn(1)-P(1A) bond distances in 1a are 2.5075(5) and 2.5123(5) Å and the P-Mn-P and Mn-P-Mn angles are 93.83(2) and 86.17(2)1, respectively. The arsenide-bridged dimanganese compound 2 has only one independent molecule in the unit cell, which li...

Synthesis, and characterization of low- and high-spin manganese(II) complexes of polyfunctional adipoyldihydrazone: Effect of coordination of N-donor ligands on stereo-redox chemistry

“…Based on the analytical data, these complexes are suggested to have the compositions [Mn II (npahH 2 )], [Mn II (npahH 2 )(A) 2 ] and [Mn II (npahH 2 )(NN)] (where A = py, (2); 2-pic, (3); 3-pic, (4); 4-pic, (5); NN = bpy, (6); phen, (7). The complexes are air-stable and insoluble in water and common organic solvents such as ethanol and acetone, but they dissolve freely in coordinating solvents like DMSO and DMF.…”

“…Because of its higher spin-pairing energy amongst bivalent 3d ions, the ligands with very strong ligand fields only can induce low-spin character on manganese ion. Low-spin Mn(II) complexes have been known with cyano ligands [6,17], phosphine ligands [7], oxime ligands [8,19], and dithiochelate ligands [9]. Recently, a few low-spin manganese(II) complexes have been reported by various groups [10], and also by us in our earlier reported work with ligand bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone [11].…”

“…for complex (1) suggests that it is a low-spin complex. Very few low-spin complexes of di-and tri-valent manganese are currently known [6][7][8][9][10][11][17][18][19]. Although the complex (1) should show l B value corresponding to one unpaired electron, considering orbital contribution, the l B value exhibited by the complex (1) should fall around ca.…”

“…The cyclic voltammograms of 2 mmol solutions of the ligand and complexes (1), (2) and (4) to (7) in DMSO under nitrogen with 0.1 M TBAP as a supporting electrolyte were run at several scan rates. The data have been set out in Tables 4 and 5 at scan rates of 5 and 100 mV/s.…”

A Spin Crossover Transition in a Mn(II) Chain Compound

“…25%-ca. 60% between 300 and 400 K. Again, this behavior is very rare, since there are very few Mn(II) complexes showing SCO [42][43][44], and as far as we know, although partial, this is the first time it has been observed in a Mn(II) chain.…”