A Bis‐Manganese(II)–DOTA Complex for Pulsed Dipolar Spectroscopy

Abstract

High-spin gadolinium(III) and manganese(II) complexes have emerged as alternatives to standard nitroxide radical spin labels for measuring nanometric distances by using pulsed electron-electron double resonance (PELDOR or DEER) at high fields/frequencies. For certain complexes, particularly those with relatively small zero-field splitting (ZFS) and short distances between the two metal centers, the pseudosecular term of the dipolar coupling Hamiltonian is non-negligible. However, in general, the contribution from this term during conventional data analysis is masked by the flexibility of the molecule of interest and/or the long tethers connecting them to the spin labels. The efficient synthesis of a model system consisting of two [Mn(dota)](2-) (MnDOTA; DOTA(4-) =1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) directly connected to the ends of a central rodlike oligo(phenylene-ethynylene) (OPE) spacer is reported. The rigidity of the OPE is confirmed by Q-band PELDOR measurements on a bis-nitroxide analogue. The Mn(II) -Mn(II) distance distribution profile determined by W-band PELDOR is in reasonable agreement with one simulated by using a simple rotamer analysis. The small degree of flexibility arising from the linking MnDOTA arm appears to outweigh the contribution from the pseudosecular term at this interspin distance. This study illustrates the potential of MnDOTA-based spin labels for measuring fairly short nanometer distances, and also presents an interesting candidate for in-depth studies of pulsed dipolar spectroscopy methods on Mn(II) -Mn(II) systems.

Synthesis of Homoditopic Ligands with an Incrementable Rodlike Backbone

“…The use of tBuLi as a stronger base than LDA (for definitions of reagents, see the Experimental Section) was necessary to convert the remaining bromoalkyne into compound 2. [18] Similarly, the iodinated coupling partners 8 and 9 were obtained by a mono-Sonogashira reaction be- tween compound 3 and 0.75 equiv. Compound 4 was deprotected with TBAF to give the OPE linker 5 equipped with ethynyl end groups in gram amounts (Scheme 1).…”

“…[18] Compounds 7, 11 and 13 were oxidized to the corresponding carboxylic acids 16, 17 and 18 in nearly quantitative yields by using Oxone in DMF, as described previously for the corresponding shorter OPE spacer (Scheme 2). The reactions of compounds 10 and 12 with bromoacetyl bromide in the presence of NEt 3 gave products 14 and 15, respectively, equipped with a CH 2 Br moiety, which can react further with various modules through nucleophilic substitution reactions.…”

“…Finally, to illustrate the diversity of the functionalization methods available with these OPE linkers, compounds 14 and 15 were treated with the previously reported tri-Pp-DO3A [18] to give platforms 31 and 32. Compounds 16-18 were treated with commercially available tri-tBu-DO3A in the presence of HOBt and DCC or HATU as coupling agents to afford the protected platforms 27-29.…”

“…Reagents and chemicals were used without further purification. [18] The Triethylamine (TEA) was dried with CaH 2 , distilled under argon and stored over 4 Å molecular sieves under argon.…”

“…[8] Dinuclear lanthanide or Mn II complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and its counterpart missing an acetate arm (DO3A) linked through an aromatic spacer have also proved useful as contrast agents in magnetic acetylene repeat units were quickly obtained through a bidirectional approach. [16][17][18][19] The generation of ditopic platforms incorporating two ligands connected to a central rigid molecular wire of incrementing length is needed for diverse applications. The resulting homoditopic platforms are of interest in numerous fields ranging from supramolecular chemistry to materials science.…”