New Linear Tricobalt Complex of Di(2-pyridyl)amide (dpa), [Co₃(dpa)₄(CH₃CN)₂][PF₆]₂

Abstract

Reaction of the linear tricobalt compound Co3(dpa)4Cl2 (1) (dpa = di(2-pyridyl)amide) with silver hexafluorophosphate in acetonitrile yields [Co3(dpa)4(CH3CN)2][PF6]2 (2). Two crystalline forms are obtained from the same solution, namely, a monoclinic (P2(1)) form 2xCH3CNx2Et2O and a triclinic (P1) form, 2x3CH3CN. The tricobalt units in both crystals are essentially symmetrical, though this is not required by crystal symmetry, with Co-Co distances in the range 2298-2304 A. Each of the two terminal Co atoms is coordinated to an acetonitrile molecule with Co-N distances in the range 2068-2111 A at 213 K. The spiral arrangement of ligands gives an overall idealized D4 point group symmetry for the cation [Co3(dpa)4(CH3CN)2]2+ . Chiral crystals of both delta and lambda configurations in the P2(1) form have been isolated. The absolute configurations were determined by X-ray crystallography and their mirror-image circular dichroism spectra measured. The D4 symmetry of the cation appears to be preserved in solution as judged by the presence of only five proton resonance signals in the 1H NMR spectrum. Magnetic susceptibility measurements in the solid state indicates that 2 has a doublet ground state and exhibits an increase of the effective moment at high temperature (approximately 160 K) due to a spin crossover process.

Excited‐State Dynamics of the Metal String Complex Co3(dpa)4(NCS)2 from Femtosecond Transient Absorption Spectra

“…The low-energy bands with small absorption coefficient centered at 450 and 560 nm are assigned to spin-allowed, metal-centered, ligand-field (LF) d-d transitions. Clerac et al [21] assigned the visible bands of [Co 3 (dpa) 4 (CH 3 CN) 2 ][PF 6 ] 2 based on measurements in electrochemical experiments; their absorption spectrum is similar to ours, thus indicating that the axial ligand has no effect on the absorption in the visible region. The shoulder band at 400 nm is assigned to the LMCT transition according to their assignment.…”

Enantiopure Chiral Coordination Polymers Based on Polynuclear Paddlewheel Helices and Arsenyl Tartrate

“…30% in all cases. The maximum expected yield is 50%, given that the clusters are not expected to interconvert in solution [25,26,29,30]. The use of 0.5 or 1 equivalent of (NBu 4 ) 2 2 species, but in lower yields than in the previously-described direct synthesis.…”

“…Recently, an approach based on anion exchange has been employed by our group to resolve a racemic mixture of [Co3(dpa)4(MeCN)2] 2+ clusters [29]. For this purpose, inexpensive chiral dianions based on commercially available (2R, [30]. Furthermore, significant X-ray natural circular dichroism (XNCD) signals were also detected at the Co K-edge in circularly polarized X-ray absorption experiments made on oriented single crystals [29].…”

“…ECD measurements in DMF solutions of the two pairs of enantiomers demonstrate the efficiency of the chiral resolution strategy Recently, an approach based on anion exchange has been employed by our group to resolve a racemic mixture of [Co 3 (dpa) 4 (MeCN) 2 ] 2+ clusters [29]. For this purpose, inexpensive chiral dianions based on commercially available (2R,3R)-(+)-or (2S,3S)-(−)-tartaric acid were employed, affording the molecular species [∆-Co 3 (dpa) 4 [30]. Furthermore, significant X-ray natural circular dichroism (XNCD) signals were also detected at the Co K-edge in circularly polarized X-ray absorption experiments made on oriented single crystals [29].…”

Conductivity Measurements of Paddlewheel Dimetal Complexes with Metal-Metal Multiple Bonds

“…The initial compounds synthesized are a Co3 complex [3] that contains a chain of three metal centers with single bonds between the metals, and a Re2 complex [4] that contains a quadruple bond. These complexes were chosen for initial studies because they have a large difference in the metalmetal bond order and they are both robust and air stable.…”