Light-Driven Ring Slippage in [Re(η7-C7H7)(η5-C7H9)]+ and the Inertness of Its Technetium Homologue.

Here, we present the light-driven reactions of [Re(η7-C7H7)(η5-C7H9)]+ (1+) with nitriles, phosphines, and isocyanides, which are added to 1+ via a ring slippage of the tropylium cation from η7 to η3, forming [Re(η3-C7H7)(η5-C7H9)(L)2]+ (L= acetonitrile 2+; 2-phenylacetonitrile 3+; 1,3,5-triaza-5-phosphoadamantane (PTA) 4+; tert-butyl isocyanide 6+; benzyl isocyanide 7+) and [Re(η3-C7H7)(η5-C7H9)(L)]+ with L = (ethane-1,2-diyl)bis(diphenylphosphane) (dppe) 5+. To compare the reactivities of rhenium and technetium, we also investigated the synthesis of [99Tc(η6-C10H8)2]+, its substitution of naphthalene with cyclohepta-1,3,5-triene to obtain [99Tc(η7-C7H7)(η5-C7H9)]+, and its reactivity (or lack thereof) with light.

Exploring Rhenium Arene Piano-Stool Chemistry with [Re(η6-C6H6)(NCCH3)3]+: A Powerful Semi-Solvated Precursor.

Thermal treatment of the ReIII hydride complex [ReH(η5-C6H7)(η6-C6H6)]+ in CH3CN results in the formation of [Re(η6-C6H6)(NCCH3)3]+. This semi-solvated complex is remarkably stable under an ambient atmosphere and exhibits a fast CH3CN self-exchange, which facilitates substitution reactions. The CH3CN ligands are replaced by σ-donating phosphines such as trimethyl phosphine (PMe3), triphenyl phosphine (PPh3), or the bidentate 1,2-bis(diphenylphosphino)ethane (dppe) to afford [Re(η6-C6H6)(NCCH3)3-x(PR3)x]+ (if R = Me, then x = 2; if R = Ph, then x = 1 or 2) or [Re(η6-C6H6)(dppe)(NCCH3)]+, respectively. [Re(η6-C6H6)(NCCH3)3]+ also reacts with π-acceptors such as 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), or CO (1 atm) to give [Re(η6-C6H6)(L)(NCCH3)]+ (L = bipy or phen) and [Re(η6-C6H6)(CO)(NCCH3)2]+, respectively. The latter does not show any signs of decomposition after being exposed to an ambient atmosphere for multiple days. Additionally, [Re(η6-C6H6)(NCCH3)3]+ reacts with π-donors such as the dienes 2,3-dimethyl-1,3-butadiene (DMBD), norbornadiene (NBD), or 1,5-cyclooctadiene (COD) to give [Re(η6-C6H6)(η4-diene)(NCCH3)]+ (diene = DMBD, NBD, and COD). All three complexes are extremely stable and do not decompose during purification by preparative high-performance liquid chromatography (aqueous acidic gradient). In the presence of 18-crown-6, [Re(η6-C6H6)(NCCH3)3]+ reacts with lithium cyclopentadienyl to give the sandwich complex [Re(η5-C5H5)(η6-C6H6)]. Loss of the coordinated benzene was observed when treating [Re(η6-C6H6)(NCCH3)3]+ with diphenylacetylene (PhC≡CPh), yielding the tetra-coordinated [Re(NCCH3)(η2-PhC≡CPh)3]+.

One Electron Changes Everything: Synthesis, Characterization, and Reactivity Studies of [Re(NCCH3)6]3.

Oxidation of [Re(NCCH3)6]2+ with the thianthrene radical cation results in the formation of [Re(NCCH3)6]3+, one of the very rare cases of a fully solvated +3 complex. It was fully characterized by spectroscopy and X-ray structure analysis. In contrast to its reduced analogue, [Re(NCCH3)6]3+ exhibits a much faster CH3CN exchange. Hence, substitution reactions proceed at 20 °C within minutes. Its potential as a versatile precursor for ReIII chemistry was examined with a series of substitution reactions. The more lipophilic analogue [Re(NCPh)6]3+ was synthesized by nitrile exchange in benzonitrile (NCPh). The Re(II) analogue of [Re(NCPh)6]3+, [Re(NCPh)6]2+, forms by AgI-mediated oxidation of in situ formed [Re(η6-C6H6)(NCPh3)3]+ in NCPh. The same synthetic strategy is feasible for the synthesis of [Re(NCCH3)6]2+ as well. [Re(NCCH3)6]3+ reacts with 1,4,7-trithiacyclononane (C6H12S3) to yield sevenfold-coordinated [Re(κ3-C6H12S3)2(NCCH3)]3+. The reaction of [Re(NCCH3)6]3+ with 1 equiv of (NBu4)X produces the ReIII monohalide complexes [ReX(NCCH3)5]2+ (X = Cl, Br, I). Mixed ReIII dihalides (trans-[ReXY(NCCH3)4]+) were obtained by treating [ReX(NCCH3)5]2+ with a second equivalent of (NBu4)Y (if X = Cl, Y = Br, I; if X = Br, Y = I). Because of this fast CH3CN exchange, [Re(NCCH3)6]3+ is a very suitable precursor for new ReIII complexes.

Watching Hydrogens Migrate: Step by Step from [ReI(η6-C6H6)2]+ to [ReIII(η3-C6H9)(η6-C6H6)(NCCH3)2]2.

Arene substitution reactions in [M(η6-arene)2]0/2+ are well documented for Groups 6 and 8 but are essentially unknown for the manganese triad. Aiming to replace benzene in [ReI(η6-C6H6)2]+, we altered the hapticity of one coordinated benzene, which we found to be tunable stepwise from an η6 to an η3-allyl coordination mode. Reduction of [ReI(η6-C6H6)2]+ with hydrides gives [ReI(η5-C6H7)(η6-C6H6)]. Subsequent addition of acid yields [ReIIIH(η5-C6H7)(η6-C6H6)]+, which converts to [ReI(η4-C6H8)(η6-C6H6)NCCH3]+ in acetonitrile. Further protonation gives the title complex [ReIII(η3-C6H9)(η6-C6H6)(NCCH3)2]2+ by a rhenium-mediated, intramolecular hydride shift. Herein, we present a full mechanistic elucidation of these transformations based on NMR studies, isolation of reaction intermediates, and their full characterizations. The structural feature {ReIII(η6-C6H6)} is unprecedented. Direct arene exchange from [ReI(η6-C6H6)2]+ to [ReI(η6-arene)(η6-C6H6)]+ was found only under strongly acidic conditions in neat arene. The analogous chemistry of the lighter homologue technetium (99Tc) is distinctly different. Treatment of [TcI(η5-C6H7)(η6-C6H6)] with acid in acetonitrile yields only mixtures of [TcI(η6-C6H6)2]+ and [TcII(NCCH3)6]2+.

Fully Solvated, Monomeric ReII Complexes: Insights into the Chemistry of [Re(NCCH3)6]2.

The oxidation of [Re(η6-C10H8)2]+ with AgI in acetonitrile yields [Re(NCCH3)6]2+. This fully solvated ReII compound was characterized by spectroscopic methods and X-ray structure analyses. We show that [Re(NCCH3)6]2+ acts as a precursor complex for a variety of substitution reactions. Treatment with monodentate triphenylphosphine (PPh3) and bidentate 1,2-bis(diphenylphosphino)ethane (dppe) yields the complexes [trans-Re(PPh3)2(NCCH3)4]2+ and [trans-Re(dppe)2(NCCH3)2]+, respectively. [trans-Re(dppe)2(NCCH3)2]+ is oxidized under mild conditions by AgI to its ReII analogue [trans-Re(dppe)2(NCCH3)2]2+. Reactions of [Re(NCCH3)6]2+ with a halide mixture consisting of NaX and AgX (X = Cl, I) result in the formation of the corresponding ReIII complexes [trans-ReX2(NCCH3)4]+. [trans-ReBr2(NCCH3)4]+ can be obtained directly from [Re(η6-C10H8)2]+ by oxidation with FeBr3 in acetonitrile. The title compound is thus a convenient starting material for ReII and ReIII complexes by simple solvent exchange, which are otherwise difficult to access.

To Sandwich Technetium: Highly Functionalized Bis-Arene Complexes [99m Tc(η6 -arene)2 ]+ Directly from Water and [99m TcO4 ].

The labeling of (bio)molecules with metallic radionuclides such as 99m Tc demands conjugated, multidentate chelators. However, this is not always necessary since phenyl rings can directly serve as integrated, organometallic ligands. Bis-arene sandwich complexes are generally prepared by the Fischer-Hafner reaction. In extension of this, we show that [99m Tc(η6 -C6 R6 )2 ]+ -type complexes are directly accessible from water and [99m TcO4 ]- , even using arenes incompatible with Fischer-Hafner conditions. To unambiguously confirm the nature of these unprecedented 99m Tc complexes, their rhenium homologous have been prepared by substituting naphthalene ligands in [Re(η6 -C10 H8 )2 ]+ with the corresponding phenyl groups. The ease with which highly stable [99m Tc(η6 -C6 R6 )2 ]+ complexes are formed under standard labeling conditions enables a multitude of new potential imaging agents based on commercial pharmaceuticals or lead structures.

Chemistry at High Dilution: Dinuclear 99 m Tc Complexes.

The formation of di- or polynuclear complexes at nanomolar concentrations is generally too slow to be observed with 99m Tc. It is reported in this communication that an appropriate choice of potentially bridging ligands, herein thiols HS-R, accelerates the dimerization reaction to an extent that dinuclear complexes are formed at very high dilution. The dinuclear nature of [99m Tc2 (µ2 -SR)3 (CO)6 ]- is shown by chromatographic comparison, not only with its rhenium homologue as commonly done, but also with the true 99 Tc analogue.

Self-Assembled Multinuclear Complexes Incorporating 99m Tc.

Multinuclear complexes or clusters are rarely investigated in medicinal inorganic chemistry although they represent structural intermediates between molecules and nanomaterials. We present in this report two strategies towards 99m Tc-containing clusters. In a pre-assembly approach, the preformed but incomplete cluster fragment [Re3 (µ2 -OH)3 (µ3 -OH)(CO)9 ]- reacts with [99m Tc(CO)3 ]+ to the highly stable [99m TcRe3 (µ3 -OH)4 (CO)12 ] cube. The same structure self-assembles when reacting the mononuclear Re and 99m Tc precursors in one pot. Integrating the coordinating OH groups from Schiff bases in this concept leads straight to dinuclear, mixed-metal complexes of the type [99m TcRe(µ2 -O^N-R1 )2 (CO)6 ] in quantitative yields. Both strategies are unprecedented and open a future path towards clusters, incorporating a 99m Tc radiolabel while being decorated with targeting or cytotoxic moieties.