Probing Fe–V Bonding in a <i>C</i>₃‑Symmetric Heterobimetallic Complex

Direct metal–metal bonding of two distinct first-row transition metals remains relatively unexplored compared to their second- and third-row heterobimetallic counterparts. Herein, a recently reported Fe–V triply bonded species, [V­(^<i>i</i>PrNPPh₂)₃FeI] (<b>1</b>; Kuppuswamy, S.; Powers, T. M.; Krogman, J. P.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Vanadium–iron complexes featuring metal–metal multiple bonds. <i>Chem. Sci.</i> <b>2013</b>, <i>4</i>, 3557–3565), is investigated using high-frequency electron paramagnetic resonance, field- and temperature-dependent ⁵⁷Fe nuclear gamma resonance (Mössbauer) spectroscopy, and high-field electron-electron double resonance detected nuclear magnetic resonance. From the use of this suite of physical methods, we have assessed the electronic structure of <b>1</b>. These studies allow us to establish the effective <i><b>g̃</b></i> tensors as well as the Fe/V electro-nuclear hyperfine interaction tensors of the spin <i>S</i> = ¹/₂ ground state. We have rationalized these tensors in the context of ligand field theory supported by quantum chemical calculations. This theoretical analysis suggests that the <i>S</i> = ¹/₂ ground state originates from a single unpaired electron predominately localized on the Fe site

Spin Crossover in Fe(II) Complexes with N₄S₂ Coordination

Reactions of Fe­(II) precursors with the tetradentate ligand <i>S,S</i>′-bis­(2-pyridylmethyl)-1,2-thioethane (bpte) and monodentate NCE^– coligands afforded mononuclear complexes [Fe­(bpte)­(NCE)₂] (<b>1</b>, E = S; <b>2</b>, E = Se; <b>3</b>, E = BH₃) that exhibit temperature-induced spin crossover (SCO). As the ligand field strength increases from NCS^– to NCSe^– to NCBH₃^–, the SCO shifts to higher temperatures. Complex <b>1</b> exhibits only a partial (15%) conversion from the high-spin (HS) to the low-spin (LS) state, with an onset around 100 K. Complex <b>3</b> exhibits a complete SCO with <i>T</i>_1/2 = 243 K. While the γ-<b>2</b> polymorph also shows the complete SCO with <i>T</i>_1/2 = 192 K, the α-<b>2</b> polymorph exhibits a two-step SCO with the first step leading to a 50% HS → LS conversion with <i>T</i>_1/2 = 120 K and the second step proceeding incompletely in the 80–50 K range. The amount of residual HS fraction of α-<b>2</b> that remains below 60 K depends on the cooling rate. Fast flash-cooling allows trapping of as much as 45% of the HS fraction, while slow cooling leads to a 14% residual HS fraction. The slowly cooled sample of α-<b>2</b> was subjected to irradiation in the magnetometer cavity resulting in a light-induced excited spin state trapping (LIESST) effect. As demonstrated by Mössbauer spectroscopy, an HS fraction of up to 85% could be achieved by irradiation at 4.2 K