Synthesis, Structure, and Properties of Low-Spin Manganese(III)−Poly(pyrazolyl)borate Complexes

Abstract

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (microeff = 2.9-3.8 microB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn-N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn-Teller distortion of a high-spin d4 center. N-Mn-N bite angles of slightly less than 90 degrees within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)degrees in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal-bis(pzb) complexes reveals systematic changes within the metal-(pyrazolyl)borate framework as the ligand is changed from pzTp to Tp to Tp*. These deformations consist of significant increases in M-N-N, N-B-N, and N-N-B angles and a minimal increase in Mn-N distance as a consequence of the steric demands of the 3-methyl groups. Less effective overlap of pyrazole lone pairs with metal atom orbitals resulting from the M-N-N angular displacement is suggested to contribute to the lower ligand field strength of Tp* complexes. Mn(pzb)2+ complexes undergo electrochemical reduction and oxidation in CH3CN. The electrochemical rate constant (ks,h) for reduction of t2g4 Mn(pzb)2+ to t2g3eg2 Mn(pzb)2 (a coupled electron-transfer and spin-crossover reaction) is 1-2 orders of magnitude smaller than that for oxidation of t2g4 Mn(pzb)2+ to t2g3 Mn(pzb)22+. ks,h values decrease as Tp* > pzTp > Tp for the Mn(pzb)2+/0 electrode reactions, which contrasts with the behavior of the comparable Fe(pzb)2+/0 and Co(pzb)2+/0 couples.

Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

“…2 All N–Mn–N adjacent angles for 1 are within 2.6° of the idealized 90° expected for O h geometry, and the Mn–N distances fall within the narrow range of 1.979 – 1.988 Å. Complex 2 has slightly longer Mn–N bond lengths (1.992 – 1.999 Å) and N–Mn–N adjacent angles even closer to 90° (89.75 – 90.49°).…”

“…Complex 2 has slightly longer Mn–N bond lengths (1.992 – 1.999 Å) and N–Mn–N adjacent angles even closer to 90° (89.75 – 90.49°). 2 The solid-state structure of [{Ph(MeIm) 3 B} 2 Mn III ] + ( 3 ) is similar to those of 1 and 2 , with Mn–C distances ranging from 2.014 – 2.037 Å, but with slightly smaller C–Mn–C adjacent angles of 85.82 – 87.32 Å. 10 …”

“…In contrast, the homoleptic Mn III complexes supported by the Tp − and Tp* − ligands (Figure 1; Tp* − = hydridotris(3,5-dimethylpyrazolyl)borate) both possess an S = 1 spin state, which is extremely unusual for Mn III centers. 2 Other than the hexacyanido complex, [Mn III (CN) 6 ] 3− , 3 the “classic” example of coordination favoring low-spin ground states, there are very few examples of Mn III coordination complexes with S = 1 ground states. These include an interesting class of complexes with a cis -N4O2 donor set (oximato-N, azo-N, and carboxylato-O), such as bis[((2-carboxylatophenyl)azo)benzaldoximato]manganate(III).…”

“…All that had been hitherto reported are the diffuse reflectance absorption spectra of 1 and 2 reveal major bands at 390 – 395 and 400 nm, respectively, with shoulders at 460 – 465 and 475 nm. 2,10 The higher-energy features have been attributed to metal-to-ligand charge transfer (MLCT) transitions, while the shoulders are presumed to derive from the 3 T 1g → 3 E g transition, using symmetry labels from the parent O h group. 2 Given the rarity of S = 1 Mn III centers, further exploration of the optical properties of these complexes is warranted.…”

“…2,10 The higher-energy features have been attributed to metal-to-ligand charge transfer (MLCT) transitions, while the shoulders are presumed to derive from the 3 T 1g → 3 E g transition, using symmetry labels from the parent O h group. 2 Given the rarity of S = 1 Mn III centers, further exploration of the optical properties of these complexes is warranted. In addition, better insight into the molecular basis for the significantly enhanced zfs of 3 is also of interest.…”