Suzanne J. Parrott

This review describes recent developments in the chemistry of both first and second generation 99m-technetium-based imaging agents. The material is presented according to the biological target for the agent, and where possible actual images are presented to indicate the type of information available to the clinician. Beta emitting isotopes of rhenium offer a possible method for the in situ treatment of cancerous tissue using analogous targeting strategies to those for technetium. Recent developments in the relevant coordination chemistry of rhenium and their extension to in vitro and in vivo studies are presented.

The reaction of [Re 2 O 3 (S 2 CNEt 2 ) 4 ] with catechol in acetone yielded the dark orange complex [ReO{O(OH)C 6 H 4 }(S 2 CNEt 2 ) 2 ] 1. The crystal structure shows a distorted-octahedral geometry with the oxo group trans to the monodentate catecholate ligand. The Re–O (catechol) bond length is typical of a Re–O single bond and implies little trans influence of the oxo ligand. Reaction of [Re 2 O 3 (S 2 CNEt 2 ) 4 ] with 1,4-dihydroxybenzene yielded the red-brown dimer [{ReO(S 2 CNEt 2 ) 2 } 2 (C 6 H 4 O 2 -1,4)] 2, in which the dianionic ligand bridges two rhenium centres. With 2-amino-4-methylphenol [ReO(OC 6 H 3 NH 2 -2-Me-4)(S 2 CNEt 2 ) 2 ] 3 was obtained containing the ligand co-ordinated in a monodentate mode. Reaction of [Re 2 O 3 (S 2 CNEt 2 ) 4 ] with dithiolate proligands such as ethane-1,2-dithiol yielded [NEt 2 H 2 ][ReOL 2 ], L = C 2 H 4 S 2 -1,2 4, C 6 H 4 S 2 -1,2 5 or MeC 6 H 3 S 2 -3,4 6, where degradation of the dithiocarbamate ligands to form the diethylammonium counter ion occurs. Reaction of 1 with bidentate phosphines yielded green complexes of the general formula [Re(S 2 CNEt 2 ) 2 L][BPh 4 ], where L = Me 2 PCH 2 CH 2 PMe 2 7 or Ph 2 PCH 2 CH 2 PPh 2 8. These reactions can be contrasted to the inactivity of these phosphine ligands towards [Re 2 O 3 (S 2 CNEt 2 ) 4 ]. The reaction of [Re 2 O 3 (S 2 CNEt 2 ) 4 ] with the bidentate phosphinothiolate proligand PPh 2 (C 6 H 4 SH-2) in acetone at room temperature yielded the red-orange rhenium(III) complex [Re{PPh 2 (C 6 H 4 S-2)} 2 (S 2 CNEt 2 )]·Me 2 CO 9. The crystal structure revealed a distorted-octahedral geometry. The sulfur donors of the phosphinothiolate ligands adopt a cis configuration. Reaction of [Re 2 O 3 (S 2 CNEt 2 ) 4 ] with the tridentate phosphinothiolate proligand PPh(C 6 H 4 SH-2) 2 proceeded at room temperature to yield the red rhenium(V) complex [ReO{PPh(C 6 H 4 S-2) 2 }(S 2 CNEt 2 )] 10. Its crystal structure shows a distorted-octahedral geometry. Reaction of [Re 2 O 3 (S 2 CNEt 2 ) 4 ] with SiMe 3 Cl yielded the new rhenium(V) precursor [ReCl 2 (S 2 CNEt 2 ) 2 ][BPh 4 ] 11 which permits investigation into rhenium dithiocarbamate chemistry without an oxo core.

The reaction of [ReCl3O(PPh3)2] in toluene with 2-diphenylphosphinoaniline yielded the brown rhenium(V) amido/imido-complex [ReCl2(NC6H4PPh2-2)(HNC6H4PPh2-2)]1. This represents the first example of a metal complex where a 2-aminophenylphosphine is fully deprotonated to give an N,P chelated imido-complex. The crystal structure of 1 shows a distorted-octahedral geometry with the imido-nitrogen trans to chloride whereas the amido-nitrogen is disposed trans to a phosphorus. The Re–N length of 1.988(4)Å for the amido nitrogen is consistent with protonation of the nitrogen in the chelated ligand. A Re–N bond length of 1.757(4)Å and a Re–N–C angle of 137.8(3)° for the deprotonated imide nitrogen imply substantial multiple bonding. The oxo-complex [Re(8-HNC9H6N)2O(PPh3)][BPh4]2 was the unexpected product in the reaction of [ReCl3O(PPh3)2] with 8-aminoquinoline in ethanol. It has a distorted-octahedral structure with two chelated amide ligands. The two quinoline-ring nitrogens as well as the two amido-nitrogens are arranged in a mutually cis configuration. The oxo and triphenylphosphine ligands occupy the remaining co-ordination sites. Reaction of [ReCl3(NPh)(PPh3)2] in ethanol with the sterically hindered 3-trimethylsilylpyridine-2-thiol (Htspyt) yielded the green rhenium(V) phenylimido-complex [Re(NPh)(PPh3)(tspyt)2][BPh4]3. Reaction of the unsubstituted pyridine-2-thiol (Hpyt) with the parent imido-complex yielded the analogous complex [Re(NPh)(PPh3)(pyt)2][BPh4]4. The crystal structure of 3 reveals a distortedoctahedral configuration of the ligands about the central rhenium atom. The imide and phosphine ligands are cis with the remaining sites being occupied by the S,N chelated pyridinethiolate ligands. A ReN bond length of 1.725(8)Å and a Re–N–C bond angle of 166.0(6)° are indicative of the imide ligand functioning as a four-electron donor.