Rhenium Complexes with p-Fluorophenylisocyanide

p-Fluorophenylisocyanide (CNPh pF ) reacts with [Re(CO) 5 Br] under stepwise exchange of the carbonyl ligands depending on the conditions applied. The reaction stops with the formation of fac -[Re(CO) 3 Br(CNPh pF ) 2 ] in boiling THF. An ongoing carbonyl exchange is observed at higher temperatures, e.g. in refluxing toluene, with the final formation of the [Re(CNPh pF ) 6 ] + cation. The progress of the reactions has been studied by 19 F NMR spectroscopy and the structures of [Re(CO)Br(CNPh pF ) 4 ] and [Re(CNPh pF ) 6 ](BPh 4 ) have been elucidated by X-ray diffraction. The frequent use of an hexakis(isocyanide) complex of technetium ( 99m Tc-Sestamibi or Cardiolite) [1–3] in diagnostic nuclear medicine has also stimulated the related chemistry of rhenium. [4–9] Rhenium complexes are frequently used as (almost) non-radioactive mimics for the corresponding technetium compounds, but there are also two beta-emitting nuclei ( 186 Re: half-life 90 h, E ßmax = 1.09 MeV and 188 Re: half-life 16.9 h, E ßmax = 2.12 MeV ), [10] which are under discussion for applications in nuclear medical therapy. Particularly 188 Re is readily available for the clinical sites from an alumina-based 188 W/ 188 Re generator system, from which the isotope can be eluted when it is required. [11]

p-Fluorophenylisocyanide (CNPh pF ) reacts with [Re(CO) 5 Br] under stepwise exchange of the carbonyl ligands depending on the conditions applied.The reaction stops with the formation of fac-[Re(CO) 3 Br(CNPh pF ) 2 ] in boiling THF.An ongoing carbonyl exchange is observed at higher temperatures, e. g. in refluxing toluene, with the final formation of the [Re(CNPh pF ) 6 ] + cation.The progress of the reactions has been studied by 19 F NMR spectroscopy and the structures of [Re(CO)Br(CNPh pF ) 4 ] and [Re(CNPh pF ) 6 ](BPh 4 ) have been elucidated by X-ray diffraction.
[6][7][8][9] Rhenium complexes are frequently used as (almost) non-radioactive mimics for the corresponding technetium compounds, but there are also two beta-emitting nuclei ( 186 Re: half-life 90 h, E ßmax = 1.09MeV and 188 Re: half-life 16.9 h, E ßmax = 2.12 MeV ), [10] which are under discussion for applications in nuclear medical therapy.Particularly 188 Re is readily available for the clinical sites from an alumina-based 188 W/ 188 Re generator system, from which the isotope can be eluted when it is required. [11][14][15][16] For both fields, corresponding CO/CNR exchange reactions are of interest and for the most alkyl isocyanides reactions starting from [Re(CO) 5 Br] or [Re-(CO) 3 (X) 3 ] + ,2À complexes (X = solvent, halide) form products with the fac-tricarbonyl core. [17]Under mild conditions, frequently two isocyanides are introduced and products of the composition [Re(CO) 3 (X)(CNR) 2 ] + ,0 are formed, [5,13,[17][18][19] while the introduction of a third isocyanide ligands usually requires a halide scavenger. [12,18]A slightly different behavior has been observed for aryl isocyanides, which may allow an ongoing carbonyl exchange depending on their substitutions.First reports about this point can be found in an early paper of Treichel and Williams, [17] and during our experiments with technetium and rhenium complexes with sterically encumbered m-terphenyl isocyanides we found that particularly the pfluorinated isocyanide CNp-FAr DarF2 (see Scheme 1) does not stop with the replacement of three carbonyl ligands.Irrespective of its steric bulk it forms the tetrakis(isocyanide) complex [Re(CO)Br(CNp-FAr DarF2 ) 4 ] (3) (Scheme 1). [20]The tris(isocyanide) complex 2 could be isolated as well for rhenium, while a similar reaction with the common technetium starting material (NBu 4 )[Tc 2 (CO) 6 (Cl 3 )] gives quantitatively the tetrakis compound.Such a difference in the reactivity is not unexpected keeping in mind the slower kinetics in exchange reactions of rhenium compared with technetium. [21]ery recently, we found that the bulky CNp-FAr DarF2 shows a remarkable reactivity also with other synthons such as fac-[Tc(CO) 3 Cl(CN t Bu) 2 ] (CN t Bu = t-butylisocyanide), where it replaced selectively the three carbonyl ligands and gave the mixed-isocyanide complex [Tc(CNp-FAr DarF2 ) 3 Cl(CN t Bu) 2 ]. [22] On the basis of DFT calculations, [22] we address the observed reactivity to the fluorine substituent on the central phenyl ring, while the steric bulk of the ligands limits the extend of potential exchange reactions.For a further insight into the electronic and steric effects of such reactions, we performed reactions of (NBu 4 )[Tc 2 (CO) 6 Cl 3 ] and [Tc(CO) 3 Cl(CN t Bu) 2 ] with p-fluorophenylisocyanide (CNPh pF ), during which the entire coordination sphere of technetium was replaced and the [Tc(CNPh pF ) 6 ] + cation was formed. [22]Having in mind the slower ligand exchange kinetics for the 'third transition row' elements, we decided to extend these experiments also to related rhenium complexes.
Attempted reactions of [Re(CO) 3 Cl(CN t Bu) 2 ] with CNp-FAr DarF2 did not result in the replacement of carbonyl ligands as was observed for the technetium analog.Even after prolonged heating (3 days!) in toluene there was no evidence for the formation of defined mixed isocyanide complexes and an gradual decomposition of the ligand and intermediate products was observed by subsequently recorded 19 F NMR spectra.
In contrast, reactions with CNPh pF are more defined and the ligand exchange starting from [Re(CO) 5 Br] proceeds under a stepwise replacement of carbonyl ligands (Scheme 2).The rate of the exchange is generally slow, but can be controlled by the reaction temperature.In boiling THF, up to two CO ligands are replaced.The intermediate formation of the monosubstitution product [Re(CO) 4 Br(CNPh pF )] ( 4) is indicated by a 19 F NMR signal at 109.3 ppm, which could be recorded in the reaction mixture during the early stages of the reaction.Subsequent recordings of 19 F NMR spectra of the reaction mixture are shown in Figure 1, in which only the signals of two ligand exchange products could be detected.While no pure samples of the transient compound 4 could be isolated from such reactions, the final product fac-[Re(CO) 3 Br(CNPh pF ) 2 ] (5) was obtained as a colorless solid in good yields.
Prolonged heating of the above mentioned reaction mixture in THF did not give higher carbonyl/isocyanide exchange rates, even when additional Ag(PF 6 ) was added as Br À scavenger. 19F NMR spectra of such reaction mixtures indicate a gradual decomposition of CNPh pF in such solutions by the appearances of numerous 19 F signals (see Supporting Information).All our  attempts to isolate the expectes tris complex [Re-(CO) 3 (CNPh pF ) 3 ] + from such solutions failed.
An ongoing ligand exchange, however, is observed at higher temperatures, e. g. in boiling toluene, and species such as [Re(CO)Br(CNPh pF ) 4 ] and [Re(CNPh pF ) 6 ] + could be assigned unambiguously in the 19  Yellow crystals of compound 6 can be obtained by concentration of the mother liquor, filtration from the precipitated salt(s) and crystallization from CH 2 Cl 2 /n-pentane in a yield of approximately 30 %.The compound is readily soluble in polar solvents.The ν (CO) band appears at a low value of 1874 cm À 1 , but unlike in compound 5 also the ν (CN) frequency (2065 cm À 1) is bathochromically shifted against the value in the uncoordinated isocyanide indicating that CNPh pF participates in the withdrawal of electron density from the d 6 ion by π-backbonding in the tetrakis complex.
The ESI + mass spectrum of [Re(CO)Br(CNPh pF ) 4 ] ( 6) in acetonitrile shows an intense signal at m/z = 740.107,which can be assigned to the {MÀ Br + CH 3 CN} + ion, but also the molecular ion {M + Na} + is visible with lower intensity at m/z = 800.986.Similar to the spectrum of compound 5, also that of [Re(CO)Br-(CNPh pF ) 4 ] (6) gives evidence for the formation of dimeric species in the mass spectrometer (for details see Experimental and Supporting Information).
Single crystals of 6 were obtained by slow diffusion of npentane into a solution of the complex in CH 2 Cl 2 .Figure 2 shows an ellipsoid representation of the molecular structure of the complex with an almost perfectly planar equatorial coordination sphere formed by the four CNPh pF ligands.Minor deviations are only due to the C10-ReÀ C1/C4 angles being slightly larger than 90°due to the bulk of the CO ligand.Selected bond lengths and angles can found in the Supporting Information.
The colorless solid removed from the reaction mixture in toluene dissolves in MeOH.Addition of Na(BPh 4 ) to such a solution gives a colorless precipitate, which is readily soluble in dichloromethane.Single crystals of [Re(CNPh pF ) 6 ](BPh 4 ) were grown from CH 2 Cl 2 /n-pentane.Since there are hitherto only two examples of homoleptic hexakis(isocyanide)rhenium(I) cations are studied crystallographically, we undertook an X-ray diffraction study on compound 7. [9,29]  The ESI + mass spectrum of 7 is dominated by the molecular ion at m/z = 913.151and the ν (CN) stretch appears at 2074 cm À 1 in its IR spectrum.The latter value confirms the expected back-donation to the isocyanide ligands in the homoleptic rhenium(I) complex.
Summarizing, it can be stated that the introduction of a fluorine atom in 4-position of the phenyl ring increases the reactivity of arylisocyanides with d 6 metal centers such as Tc(I) or Re(I) carbonyl complexes and that depending on the reaction conditions applied a complete exchange of the coordination sphere of the metal ions and the formation of [M(CNPh pF ) 6 ] + cations (M = Tc, Re) can be achieved.The exchange rate is faster with technetium, where only the final products could be obtained, while for rhenium intermediates such as [Re(CO) 3 Br-(CNPh pF ) 2 ] or Re(COBr(CNPh pF ) 4 ] could be isolated.The results of the present communication can be regarded as another experimental proof of the old paradigm of Dieter Lentz that "fluorinated isocyanides are more than ligands with unusual properties". [30]
F NMR spectra of such solutions.Two more signals with an intensity ratio of 4 : 1 can be assigned to [ReBr(CNPh pF ) 5 ] or [Re(CO)(CNPh pF ) 5 ] + .The spectra of such a reaction sequence are shown as Supporting Information.In the course of the reaction, the initially clear mixture became turbid and gradually a colorless solid precipitated.This solid contains charged species (mainly [Re(CNPh pF ) 6 ]Br) which are only sparingly soluble in toluene, while the neutral species (mainly [Re(CO)Br(CNPh pF ) 4 ] (6)) remain in solution.
Figure 3 shows the structure of the [Re(CNPh pF ) 6 ] + cation.The coordination environment of the rhenium atom is an almost perfect octahedron with cis-CÀ ReÀ C angels between 87.2(2) and 92.7(2)°.Details about individual bond lengths and angles are contained in the Supporting Information.