Correlating Bridging Ligand with Properties of Ligand-Templated [Mn^II₃X₃]³⁺Clusters (X = Br^–, Cl^–, H^–, MeO^–)

Abstract

Polynuclear manganese compounds have garnered interest as mimics and models of the water oxidizing complex (WOC) in photosystem II and as single molecule magnets. Molecular systems in which composition can be correlated to physical phenomena, such as magnetic exchange interactions, remain few primarily because of synthetic limitations. Here, we report the synthesis of a family of trimanganese(II) complexes of the type MnXL (X = Cl, H, and MeO) where L is a tris(β-diketiminate) cyclophane. The tri(chloride) complex (2) is structurally similar to the reported tri(bromide) complex (1) with the MnX core having a ladder-like arrangement of alternating M-X rungs, whereas the tri(μ-hydride) (3) and tri(μ-methoxide) (4) complexes contain planar hexagonal cores. The hydride and methoxide complexes are synthesized in good yield (48% and 56%) starting with the bromide complex employing a metathesis-like strategy. Compounds 2-4 were characterized by combustion analysis, X-ray crystallography, X-band EPR spectroscopy, SQUID magnetometry, and infrared and UV-visible spectroscopy. Magnetic susceptibility measurements indicate that the Mn clusters in 2-4 are antiferromagnetically coupled, and the spin ground state of the compounds (S = 3/2 (1, 2) or S = 1/2 (3, 4)) is correlated to the identity of the bridging ligand and structural arrangement of the MnX core (X = Br, Cl, H, OCH). Electrochemical experiments on isobutyronitrile solutions of 3 and 4 display broad irreversible oxidations centered at 0.30 V.

Cyclophanes as Platforms for Reactive Multimetallic Complexes

“…Facile access to an M 3 ( μ -H) 3 L Et/Me series (M = Fe, Co, Zn) is possible by reaction of an alkali trialkylborohydride with the corresponding M 3 X 3 L Et/Me complex (X = Cl − , Br − ). In contrast to work from monometallic analogs, 68 thermal or photochemical reductive elimination is not observed for the triFe and triCo tri(hydride) compounds. Given that the order of N 2 binding and H 2 reductive elimination from the E 4 state of FeMoco is unclear, we surveyed the reaction of the [Fe 3 H 3 ] 3+ compound with various substrates and discovered a very substrate specific insertion reactivity towards carbon dioxide.…”

Isolation of chloride- and hydride-bridged tri-iron and -zinc clusters in a tris(β-oxo-δ-diimine) cyclophane ligand

“…Based on the angle dependence on coupling proposed by Goodenough, Kanamori, and Anderson, we anticipate that the iron centres in Fe 3 Cl 3 (H 3 L) are antiferromagnetically coupled as are our previous trimetallic complex. 14,[40][41][42][43] There are differences for coordinating halides between 1 and our previous β-diketiminate complex with [Fe 3 Br 3 ] 3+ cluster. 14 For our previously reported trinucleating ligand, the ancillary ligands (e.g., chloride) lie in the M 3 plane, whereas the similarly μ-halides in 1 and 2 are out of the M 3 plane.…”

Terminal Hydride Complex of High-Spin Mn

“…Figure ). We postulated that such ligands would be well-suited to support terminal, 3d metal hydrides due to a (i) large, readily modulated steric profile able to hinder dimerization − ,− ,,, and (ii) high σ-donicity, which should act to suppress reductive elimination of H 2 or R L H. We report herein the synthesis and characterization of a terminal hydride complex of high-spin ( S = 5/2) Mn 2+ , ( tBu L )­MnH. Q-band EPR measurements show that this complex exists in a novel zero-field splitting regime, while 1,2 H ENDOR reveals that spin density on the hydride is substantially greater than that for any other previously reported synthetic complex.…”

“…In this process, structural and electronic assignments have relied heavily on comparisons to synthetic analogues, − the majority of which feature strong-field supporting ligands and are, as a result, low-spin. ,,, As useful as these model complexes have been, FeMoco is a relatively weak-field cluster, and, as such, transition-metal hydrides that adopt locally high-spin configurations are more faithful spectroscopic models. Metal hydrides that model the low-coordinate (≤5), weak-field Fe sites within FeMoco are not uncommon. ,− Within this group, however, only a handful bear the half-integer spin required for meaningful ENDOR analysis, let alone the d 5 or d 6 configurations expected for Fe 2+ and Fe 3+ , respectively …”

“…Figure 1). We postulated that such ligands would be well-suited to support terminal, 3d metal hydrides due to a (i) large, readily modulated steric profile able to hinder dimerization [26][27][28][29][32][33][34][35][36]40,43,44 and (ii) high σ-donicity, which should act to suppress reductive elimination of H 2 or R LH. We report herein the synthesis and characterization of a terminal hydride complex of high-spin (S = 5/2) Mn 2+ , ( tBu L)MnH.…”