ReducedN-Alkyl Substituted Bis(imino)pyridine Cobalt Complexes: Molecular and Electronic Structures for Compounds Varying by Three Oxidation States

Abstract

The stepwise 1-3 electron reduction of the N-alkyl substituted bis(imino)pyridine cobalt dichloride complexes, ((R)APDI)CoCl(2), was studied where (R)APDI = 2,6-(RN=CMe)(2)C(5)H(3)N, R = C(6)H(11) (Cy), CHMe(2) ((i)Pr). One electron reduction with either zinc metal or NaBEt(3)H furnished the bis(imino)pyridine cobalt monochloride compounds, ((R)APDI)CoCl. X-ray diffraction on the ((iPr)APDI)CoCl derivative established a distortion from square planar geometry where the chloride ligand is lifted out of the idealized cobalt-chelate plane. Superconducting Quantum Interference Device (SQUID) magnetometry on both compounds established spin crossover behavior with an S = 1 state being predominant at room temperature. Computational studies, in combination with experimental results, establish that the triplet spin isomer arises from a high spin Co(II) center (S(Co) = 3/2) antiferromagnetically coupled to a bis(imino)pyridine chelate radical anion, [PDI](-) (S(PDI) = 1/2). At lower temperatures, the Co(II) ion undergoes a spin transition to the low spin form (S(Co) = 1/2) and antiferromagnetic coupling gives rise to the observed diamagnetic ground state. Replacing the chloride ligand with a methyl group, namely ((R)APDI)CoCH(3), also yielded distorted compounds, albeit less pronounced, that are diamagnetic at room temperature. Two electron reduction of the ((R)APDI)CoCl(2) derivatives with excess 0.5% sodium amalgam or 2 equiv of NaBEt(3)H furnished the bis(chelate)cobalt complexes, ((R)APDI)(2)Co, while three electron reduction with 3 equiv of sodium naphthalenide yielded the cobalt dinitrogen anions, [Na(solv)(3)][((R)APDI)CoN(2)] (solv = THF, Et(2)O). Both bis(chelate) compounds were crystallographically characterized and determined to have S = 3/2 ground states by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy. Computational studies, in combination with metrical parameters determined from X-ray diffraction, establish a high spin (S(Co) = 3/2) cobalt(II) center with two bis(imino)pyridine chelate radical anions. Antiferromagnetic coupling between the two chelate centered radicals is mediated by a doubly occupied t(2g) cobalt orbital and gives rise to the observed overall quartet ground state.

Redox properties of cobalt(II) complexes with isoindoline-based ligands

“…Two of the six Co-N bond distances are shorter (*2.07 Å ) and four are longer (*2.14 Å ); thus, the geometry can be described as tetragonally elongated octahedral. This bond distance pattern is similar to the recently published bis-(chelate) cobalt(II) complexes of bis-(imino)pyridine ligands [31]. Comparison of X-ray structures of analogous M II (L 3 ) 2 complexes (M = Mn, Cu) to the structure of 3 reveals that the corresponding M-N ind bond distances (2.21, 2.07 and 1.99 Å for Mn, Co, and Cu, respectively; N ind is the negatively charged nitrogen donor of the central isoindoline moiety) decrease with the atomic number, in accordance with the order in the covalent radius for these metals [32].…”

Oxidative Addition of Carbon–Carbon Bonds with a Redox-Active Bis(imino)pyridine Iron Complex

“…Similar SCO behavior has been observed with bis(imino)pyridine iron imido ( S = 0 to S = 1) 51 and bis(chelate) bis(imino)pyridine cobalt ( S = 1/2 to S = 3/2) compounds. 52 The variable temperature magnetic data observed for ( iPr PDI)Fe(biphenyl) displays the same general trends as for ( Me PDI)Fe(biphenyl) (see Supporting Information) but the instability of the compound ( vide supra ) made collection of reliable magnetic data more challenging. In an attempt to circumvent these issues and to evaluate the influence of impurities, all magnetic samples were analyzed by Mössbauer spectroscopy prior to SQUID, MSB, or Evans measurements.…”

Electronic Effects in 4-Substituted Bis(imino)pyridines and the Corresponding Reduced Iron Compounds

“…This phenomenon was first identified in bis(imino)pyridine chemistry by Budzelaar and coworkers for the ( R PDI)CoR class of compounds 24,41 and later verified by our laboratory with the spin crossover behavior of alkyl-substituted bis(imino)pyridine cobalt halide complexes. 42 The temperature dependence of the chemical shifts of the ( iPr RPDI)FeN 2 compounds are more dramatic than the cobalt alkyls suggesting a more energetically accessible triplet state. The chemical shift dispersion as a function of temperature is likely a result of the singlet-triplet gap.…”