The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN] fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN] core with |M = ± 7/2⟩ ground Kramers doublet.
The synthesis,s tructural characterization, and reactivity of the first two-coordinate cobalt complex featuring am etal-element multiple bond [(IPr)Co(NDmp)] (4;I Pr = 1,3-bis(2',6'-diisopropylphenyl)imidazole-2-ylidene;D mp = 2,6-dimesitylphenyl) is reported. Complex 4 was prepared from the reaction of [(IPr)Co(h 2 -vtms) 2 ]( vtms = vinyltrimethylsilane) with DmpN 3 .A nX -ray diffraction study revealed its linear CÀCoÀNc ore and as hort CoÀNd istance (1.691 (6) ). Spectroscopic characterization and calculation studies indicated the high-spin nature of 4 and the multiplebond character of the Co À Nbond. Complex 4 effected grouptransfer reactions to CO and ethylene to form isocyanide and imine,r espectively.I ta lso facilitated E À H( E= C, Si) s-bond activation of terminal alkyne and hydrosilanes to produce the corresponding cobalt(II) alkynyl and cobalt(II) hydride complexes as 1,2-addition products.
The reactions of [Co(PMe)] with the bulky organic azides, DippN and DmpN [Dipp, 2,6-diisopropylphenyl; Dmp, 2,6-di(2',4',6'-trimethylphenyl)phenyl], afforded the cobalt(II) terminal imido complexes [(MeP)Co(NAr)] (Ar = Dipp, 1; Dmp, 2). The cobalt imido complexes in their solid states show trigonal pyramidal coordination geometry and long Co-N(imido) separations (ca. 1.71 Å). Spectroscopic characterization and theoretical studies indicated their low-spin cobalt(II) nature. Reactivity studies on 1 revealed its nitrene-transfer reactions with PMe and CO, the imido/oxo and imido/sulfido exchange reactions with PhCHO and CS, and the single-electron oxidation reaction by ferrocenium cation to form cobalt(III) imide.
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