Synthesis and Straightforward Quantification Methods of Imino Nitroxide-Based Hexaradical Architecture on a Cyclotriphosphazene Scaffold

Abstract

The synthesis of a homogeneous neutral hexaradical architecture consisting of six imino nitroxide radical moieties covalently bonded on a cyclotriphosphazene scaffold was reported. The synthesis of hexaradical imino nitroxide compounds follows the Ullman procedure involving the condensation of 2,3-bis(hydroxylamino)-2,3-dimethylbutane with hexa-(4-formylphenoxy)cyclotriphosphazene (3) followed by oxidation of the condensation product hexa-[4-(1-hydroxy-4,4,5,5-tetramethyl-2-imidazoline-2-yl)phenoxy]cyclotriphosphazene (2) by NaIO. Characterization of hexaradical was performed by X-ray and SQUID in solid state and by EPR, absorption spectroscopy, and electrochemistry in solution. CV of 1 shows an oxidation peak at 1.184 V (vs SCE) and a reduction peak at -0.883 V, both characteristics of the presence of phenyl imino nitroxide (7) moieties, suggesting that the contribution of the cyclotriphosphazene core is negligible. Attention was particularly focused on developing methods, UV-vis spectroscopy and square-wave voltammetry, to quantify the number of radicals in a way to confirm easily and rapidly the polyradicals' structure.

Cyclotriphosphazene, a scaffold for 19 F MRI contrast agents

“…The P-N (from 1.574(4) Å to 1.585(4) Å) and P-O (from 1.569(3) Å) to 1.580(3) Å) bond lengths are in good agreement with the previous structure of this type of cyclotriphosphazene, as well as the P−N−P (122.7(3)°) and N−P−N (117.2(2)° and (118.2(3°) angles. [10][11] All lengths for C-O bonds (from 1.450(5) Å to 1.460(6) Å) involving the first C atom of the phenyl ring and the O atom attached to the P atom of the cyclotriphosphazene ring are in the normal ranges. The 3-D network of 1 is built from C-F … F-C interactions between neighboring molecules and lead to planes perpendicular to the [100] direction of the unit-cell (Fig.…”

Revisiting the Ullman's Radical Chemistry for Phthalocyanine Derivatives

“…4 and 10 show the typical split Qb and characteristico ff ree-based phthalocyanines. In the visible range, the specific weak transitions of imino nitroxide radical at about 450 nm [7] are simply overshadowed by those of phthalocyanine. Bb ands around 350 nm are almosti dentical to each other.N oteworthy is that the extension of the conjugation pathway introduced by the imino nitroxider adical moiety of 10 (l max = 441 nm) leads to a1 1nmr ed shift of the charge transfer band in comparison to that of 4 (l max = 430 nm).…”

“…Bb ands around 350 nm are almosti dentical to each other.N oteworthy is that the extension of the conjugation pathway introduced by the imino nitroxider adical moiety of 10 (l max = 441 nm) leads to a1 1nmr ed shift of the charge transfer band in comparison to that of 4 (l max = 430 nm). In the visible range, the specific weak transitions of imino nitroxide radical at about 450 nm [7] are simply overshadowed by those of phthalocyanine. Both molecules exhibit one main fluorescencee mission band at 699 nm, but as anticipated, the introduction of the radical moiety result in an 8-fold decrease of the fluorescence intensity.F luorescence quantum yield in THF were found to be F F(4) = 0.25 and F F(10) = 0.03 for 4 and 10,r espectively,t hus confirming the quenching of the fluorescence.…”

“…[1,2] Ar enewed interest in their chemistry has comei nt he spotlight because of their involvementi no rganic electrode-active materials [3] or fluorophore-nitroxide hybrid probe. [7] Condensationp roducts suffer from low solubility.C onsequently,t heir purificationa nd characterization are always ad ifficult task and the Ullman's synthesis is general-ly continued withoutf urtherp urification. However,i ns ome cases the reactiong oes further and the 1,3-dihydroxyimidazolidine derivative undergoes dehydration to 1-hydroxy-2-imidazoline.…”

“…[6] In very exceptional cases,d ehydration takes place even at room temperature. [7] Condensationp roducts suffer from low solubility.C onsequently,t heir purificationa nd characterization are always ad ifficult task and the Ullman's synthesis is general-ly continued withoutf urtherp urification. Oxidation of 1,3-dihydroxyimidazolidine or 1-hydroxy-2-imidazoline condensation product can be carried out by employing variouso xidants such as NaIO 4 ,P bO 2 ,M nO 2 ,o re ven in air to afford the corresponding nitronyl nitroxides (Way I) and imino nitroxide (Way II).…”