Paramagnetic Semiconducting Se-Mn Clusters: A Mn3Se4-Stabilized Selenide Radical Intermediate and Its Aggregated Derivatives

The transition metal-stabilized heavy main group radicals are extremely scarce due to their highly reactive natures, making them difficult to be isolated and identified. We report here a rare class of the Se radical-containing manganese carbonyl anionic cluster, [(mu-Se)(mu 3-Se2)2Mn3(CO)9]center dot 2- (1), which was successfully obtained from the one-pot reaction of Se powder and Mn2(CO)10 in concentrated KOH/MeOH/MeCN solutions at 90 degrees C. Dianion 1 and its dimeric cluster, [(mu 4-Se2){(mu 3-Se2)2Mn3(CO)9}2]4- [(1)2], could undergo the reversible Se-Se bond breakage or reformation by the thermal cracking of (1)2 or self-dimerization of 1, showing the mu-Se center dot- radical character of 1. Complex 1 could react with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to form the Se radical-captured complex [(mu-Se(TEMPO)) (mu 3- Se2)2Mn3(CO)9]2- (1-TEMPO) or could react with alkylene bromides (CH2)nBr2 (n = 1, 2) to give the Mn4-based oxidative coupling products, [(mu 4-Se2)(mu-Se2LSe)2Mn4(CO)12]2- (L = CH2, 2-CH2; Se, 2-Se). In addition, dianion 1 and its aggregated derivatives (1)2, 1-TEMPO, 2-CH2, and 2-Se exhibited unusual paramagnetic properties with the spin-state switching from S = 1 (Mn) + 1/2 (Se) to S = 1 (Mn), in which their magnetic centers were proved to be mixed-valent Mn atoms and the mu-Se center dot- radical, as evidenced by Evans method, superconducting quantum interference device, X-ray photoelectron spectra, electron paramagnetic resonance, and density functional theory calculations. Importantly, these clusters showed semiconducting behaviors with low and tunable energy gaps (1.50-2.01 eV) and varied electrical conductivities (2.52 x 10-8-4.58 x 10-9 S/cm), where efficient electron transports mainly arose from C-H(phenyl)center dot center dot center dot O(carbonyl) interactions within the solid-state frameworks.

Automated summary

This study reports a rare class of manganese carbonyl cluster containing selenium radicals, which exhibit high reactivity and can react with other compounds. Additionally, these clusters show good paramagnetic properties and semiconducting behavior.