Synthesis and Characterization of Divalent Manganese, Iron, and Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene Ligand Environments

Two novel tripodal ligands, (BIMPN^Mes,Ad,Me)⁻ and (MIMPN^Mes,Ad,Me)^2–, combining two types of donor atoms, namely, NHC and phenolate donors, were synthesized to complete the series of N-anchored ligands, ranging from chelating species with tris­(carbene) to tris­(phenolate) chelating arms. The complete ligand series offers a convenient way of tuning the electronic and steric environment around the metal center, thus, allowing for control of the complex’s reactivity. This series of divalent complexes of Mn, Fe, and Co was synthesized and characterized by ¹H NMR, IR, and UV/vis spectroscopy as well as by single-crystal X-ray diffraction studies. Variable-temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed <i>high-spin</i> ground states for all divalent complexes and revealed a trend of increasing zero-field splitting |<i>D</i>| from Mn­(II), to Fe­(II), to Co­(II) complexes. Zero-field ⁵⁷Fe Mössbauer spectroscopy of the Fe­(II) complexes <b>3</b>, <b>4</b>, <b>8</b>, and <b>11</b> shows isomer shifts δ that increase gradually as carbenes are substituted for phenolates in the series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is evident. Particularly, the molecular structure of <b>1</b>in which a pyridine molecule is situated next to the Mn–Cl bondand those of azide complexes <b>2</b>, <b>4</b>, and <b>6</b> demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the corresponding symmetrical TIMEN^R ligands, allow for side access of, e.g., organic substrates, to the reactive metal center

A Copper(II) Thiolate from Reductive Cleavage of an <i>S</i>‑Nitrosothiol

<i>S</i>-Nitrosothiols RSNO represent circulating reservoirs of nitric oxide activity in the plasma and play intricate roles in protein function control in health and disease. While nitric oxide has been shown to reductively nitrosylate copper­(II) centers to form copper­(I) complexes and ENO species (E = R₂N, RO), well-characterized examples of the reverse reaction are rare. Employing the copper­(I) β-diketiminate [Me₂NN]­Cu, we illustrate a clear example in which an RS–NO bond is cleaved to release NO_gas with formation of a discrete copper­(II) thiolate. The addition of Ph₃CSNO to [Me₂NN]Cu generates the three-coordinate copper­(II) thiolate [Me₂NN]­CuSCPh₃, which is unstable toward free NO

A Copper(II) Thiolate from Reductive Cleavage of an <i>S</i>‑Nitrosothiol

<i>S</i>-Nitrosothiols RSNO represent circulating reservoirs of nitric oxide activity in the plasma and play intricate roles in protein function control in health and disease. While nitric oxide has been shown to reductively nitrosylate copper­(II) centers to form copper­(I) complexes and ENO species (E = R₂N, RO), well-characterized examples of the reverse reaction are rare. Employing the copper­(I) β-diketiminate [Me₂NN]­Cu, we illustrate a clear example in which an RS–NO bond is cleaved to release NO_gas with formation of a discrete copper­(II) thiolate. The addition of Ph₃CSNO to [Me₂NN]Cu generates the three-coordinate copper­(II) thiolate [Me₂NN]­CuSCPh₃, which is unstable toward free NO

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)­FeCl₂] (<b>1</b>) (PNP = <i>N</i>[2-P­(CHMe₂)₂-4-methylphenyl]₂^–), prepared from one-electron oxidation of [(PNP)­FeCl] with ClCPh₃, displays an unexpected <i>S</i> = 3/2 to <i>S</i> = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex <b>1</b> to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of <i>U</i>_eff = 32–36 cm^–1 (47–52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and Δ<i>E</i>_Q values for <b>1</b>, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of <b>1</b> reveals the <i>S</i> = 3/2 to be likely the ground state with the <i>S</i> = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of <b>1</b> to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of <b>1</b> collected between 30 and 300 K also reveals elongation of the Fe–P bond lengths and increment in the Cl–Fe–Cl angle as the <i>S</i> = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d­(<i>z</i>²) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet (⁴B) and the sextet (⁶A) states are close in energy

Fluoride Bridges as Structure-Directing Motifs in 3d-4f Cluster Chemistry

The use of kinetically robust chromium­(III) fluorido complexes as synthons for mixed 3d-4f clusters is reported. The tendency toward linear {Cr^III–F–Ln^III} units dictates the cluster topology. Specifically, we show that reaction of <i>cis</i>-[Cr^IIIF₂(NN)₂]­NO₃ (NN = 1,10-phenanthroline (“phen”) or 2,2′-bipyridine (“bpy”)) with Ln­(NO₃)₃·<i>x</i>H₂O produces isostructural series of molecular {Ln₂Cr₂} squares (<b>1</b>–<b>9</b>) with linear fluoride bridges. In a parallel fashion, <i>fac</i>-[Cr^IIIF₃L], where L = <i>N</i>,<i>N′</i>,<i>N</i>″-trimethyl-1,4,7-triazacyclononane (“Me₃tacn”), reacts with Nd­(NO₃)₃·6H₂O to form a fluoride-centered penta-nuclear complex and <i>fac</i>-[Cr^IIIF₃L′], with L′ = 1,1,1-tris-((methylamino)­methylethane) (“Me₃tame”), reacts with [Ln­(hfac)₃(H₂O)₂] (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone) to yield an isostructural series of {Ln₃Cr₂} (<b>10</b>–<b>14</b>) trigonal bipyramids with no central ligand. The formation of the latter is accompanied by a partial solvolysis of the Cr­(III) precursor but without formation of insoluble LnF₃. The magnetic properties of the gadolinium containing clusters allow quantification of fluoride-mediated, antiferromagnetic Gd–Cr exchange interactions of magnitude between 0.14 cm^–1 and 0.71 cm^–1 (<i>Ĥ</i> = <i>J</i>₁₂<b>Ŝ</b>₁·<b>Ŝ</b>₂ formalism) and vanishingly small <i>J</i>_Gd–Gd of 0.06(0) cm^–1. The large spin and small anisotropy together with weak exchange interactions in the {Gd₃Cr₂} (<b>11</b>) cluster give rise to a very large magneto-caloric effect of −Δ<i>S</i>_m = 28.7 J kg^–1 K^–1 (μ₀<i>H</i> = 90 to 0 kOe)