Cobalt Half-Sandwich, Sandwich, and Mixed Sandwich Complexes with Soft Tripodal Ligands

Abstract

Reaction of sodium hydrotris(methimazolyl)borate (NaTm(Me)) with cobalt halides leads to the formation of paramagnetic pseudotetrahedral [Co(Tm(Me))X] (X = Cl, Br, I), of which the bromide has been crystallographically characterized. Mass spectrometry reveals the presence of higher molecular weight fragments [Co(Tm(Me))(2)](+) and [Co(2)(Tm(Me))(2)X](+) in solution. Aerial oxidation in donor solvents (e.g. MeCN) leads to formation of the [Co(Tm(Me))(2)](+) cation, which has been crystallographically characterized as the BF(4)(-), ClO(4)(-), Br(-), and I(-), salts. Attempts to prepare the mixed sandwich complex, [Co(Cp)(Tm(Me))](+), resulted in ligand decomposition to yield [Co(mtH)(3)I]I (mtH = 1-methylimidazole-2-thione), but with the more electron donating methylcyclopentadienyl (Cp(Me)) ligand, [Co(Cp(Me))(Tm(Me))]I was isolated and characterized. Electrochemical measurements reveal that the cobalt(III) Tm(Me) complexes are consistently more difficult to reduce than their Tp and Cp congeners.

Syntheses, Structures and Electronic Properties of Zwitterionic Iron(II) and Cobalt(II) Complexes Featuring Ambidentate Tris(pyrazolyl)methanide Ligands

“…The redox potential can be shifted substantially to more positive values by employing more electron-withdrawing Cp ligands [62] Hence, in light of the negative potential of 3 (E 0 1/2 =À0.32 V), the hexamethyl-substituted Me Tpmd ligand [63] can be classified in terms of electron-donating properties as being similar to Trofimenkos hexamethyl-substituted Tp* ligand (see Table 4). [64] Furthermore, these electrochemical results confirm our earlier assumption that the metal cations experience more or less the same ligand environment in Tp R and R Tpmd complexes. Owing to the intrinsic single negative charge, both differ from the neutral R Tpm analogues, which means that sandwich-type complexes of divalent metal cations with R Tpm ligands are dicationic, whereas those with Tp R and R Tpmd ligands are neutral.…”

Structural and Theoretical Insights into Metal–Scorpionate Ligand Complexes

“…The σ donor, π donor bonding of Tm is consistent with the lowest CO and NO stretching frequencies in these complexes, but also with the weak field behaviour observed in other systems. [11,12] Likewise, the simple σ donor Tp to metal interactions and the weak π acceptor properties of Cp are also consistent with the ordering of their properties. It is clear that the term "soft" in relation to the Tm R ligands is useful.…”

“…2 ], there is a transition from low spin (Cp) to spin-crossover (Tp) to high spin (Tm Me ) magnetic behaviour, suggesting that the spectrochemical series for these ligands lies in the order Cp Ͼ Tp Ͼ Tm Me . [11] This is further supported by the cobalt chemistry of these ligands, [12] in which a tetrahedral (weak field) complex [ we observed that the carbonyl stretching frequencies of the iodo and η 3 -allyl species also lie in the order Cp Ͼ Tp Ͼ Tm Me , suggesting that Tm is in fact the strongest electron donor of the three tripodal ligands and Cp the weakest. It is notable that Tp, as the moiety which relies on σ interactions, is found in the middle of this short series and not at either of its termini.…”

“…For instance, the reduction potentials of the complexes [Co III (L) 2 ] + confirm that the Tm Me complex is the most difficult to reduce, consistent with Tm Me being the strongest electron donor. [12] Furthermore, we found that we were unable to stabilise tin(II) complexes with the Tm Me ligand, obtaining only the tin(IV) adducts. In this situation oxidation occurred even under anaerobic conditions, [10m] Scheme 2.…”

First Row Transition Metal Mediated Redox Reactions of Hydrazines and Isonitriles

“…Other N-substituted variants besides the parent Tm Me which have been isolated with the first-row transition-metals titanium, iron, cobalt, and nickel. 71,[86][87][88] Additionally, metallaboratranes have been synthesized for all of the group VIII, IV, and V metals through an activation of the hydride to form a metal-boron bond, ultimately achieving κ 4 -S,S,S,B coordination. [89][90][91][92][93][94][95][96] Overall, examples of metal complexes bearing Tm or one of its derivatives exist with nearly every transition metal.…”