Solid Phase Nitrosylation of Enantiomeric Cobalt(II) Complexes

Abstract

Accompanied by a change in color from red to black, the enantiomorphic phases of the cobalt complexes of a chiral salen ligand (L2−, Co(L)·CS2, and Co(L) (L = LS,S or LR,R)) chemisorb NO (g) at atmospheric pressure and rt over hours for the CS2 solvated phase, and within seconds for the desolvated phase. NO is installed as an axial nitrosyl ligand. Aligned but unconnected voids in the CS2 desorbed Co(LR,R)·CS2 structure indicate conduits for the directional desorption of CS2 and reversible sorption of NO, which occur without loss of crystallinity. Vibrational circular dichroism (VCD) spectra have been recorded for both hands of LH2, Zn(L), Co(L)·CS2, Co(L), Co(NO)(L), and Co(NO)(L)·CS2, revealing significant differences between the solution-state and solid-state spectra. Chiral induction enables the detection of the νNO band in both condensed states, and surprisingly also the achiral lattice solvent (CS2 (νCS at 1514 cm−1)) in the solid-state VCD. Solution-state spectra of the paramagnetic Co(II) complex shows a nearly 10-fold enhancement and more extensive inversion of polarity of the vibrations of dominant VCD bands compared to the spectra of the diamagnetic compounds. This enhancement is less pronounced when there are fewer polarity inversions in the solid state VCD spectra.

Solid-state vibrational circular dichroism for pharmaceutical purposes

“…[16,17] Although VCD has been routinely utilized in the determination of absolute configuration in solutions, the use of VCD for solid-state analysis (ssVCD) has, thus far, been reported only recently. [3,[18][19][20][21][22][23] This is mainly because liquids provide an easy-to-measure ideal isotropic environment. On the other hand, natural anisotropy of the solid-phase results in various artifacts in ssVCD spectra, [24,25] which must be adequately resolved for a reliable analysis.…”

Structure Activity Relationships for Reversible O2 Chemisorption by the Solid Phases of Co(salen) and Co(3F-salen)

“…Since this seminal work, the Co­(salen) scaffold has been studied extensively in numerous modifications incorporating, e.g. , electron withdrawing and donating groups and bulky substituents, in a continuing search for practical materials containing this unit. − A prevailing observation was that a ligand trans to the O 2 binding site tunes this function, reminiscent of the O 2 binding heme enzymes. Indeed, when dissolved in non-coordinating solvents, Co II (salen) does not bind O 2 strongly without the additional stabilization of the Co III –O 2 bond by the increase in electron density at the cobalt center induced by a trans coordinating ligand. , …”

NO Reactions Inside Crystals

“…[22] It is not, however, necessary to deploy a MOF, the crystalline solid-state of molecular Co II (salen) can reversibly bind and store NO for long periods as Co III (NO)(salen) with desorption trigged by heating (100 °C). [23] Recently observations of NO conversion inside the solid state of metal-organic materials were described: The Fe 2 + -substituted Zn 4 O-(terephthalate) 3 MOF mediates the disproportionation of NO with formation of N 2 O and a N-coordinated Fe nitrite. [24] Similarly surface-bound nickel porphyrins have been shown to disproportionate NO also to nitrite and N 2 O.…”

“…Another MOF, [M 2 (2,5‐dihydroxyterephthalate)(H 2 O) 2 ] ⋅ 8H 2 O (M=Co, Ni), can chemisorb and store NO for several months with desorption triggered by moisture [22] . It is not, however, necessary to deploy a MOF, the crystalline solid‐state of molecular Co II (salen) can reversibly bind and store NO for long periods as Co III (NO)(salen) with desorption trigged by heating (100 °C) [23] . Recently observations of NO conversion inside the solid state of metal‐organic materials were described: The Fe 2+ ‐substituted Zn 4 O(terephthalate) 3 MOF mediates the disproportionation of NO with formation of N 2 O and a N ‐coordinated Fe nitrite [24] .…”