Mixed-Isocyanide Complexes of Technetium under Steric and Electronic Control

Abstract

Reactions of the alkyl isocyanide fac-[Tc­(CO)3(CNR)2Cl] complexes (2) (CNR = CN n Bu or CN t Bu) with the sterically encumbered isocyanide CNp-FArDarF2 [DArF = 3,5-(CF3)2C6H3] allow a selective exchange of the carbonyl ligands of 2 and the isolation of the mixed-isocyanide complexes mer,trans-[Tc­(CNp-FArDarF2)3(CNR)2Cl] (3). Depending on the steric requirements of the residues R, the remaining chlorido ligand can be replaced by another isocyanide ligand. Cationic complexes such as mer-[Tc­(CNp-FArDarF2)3(CN n Bu)3]+ (4a) or mer,trans-[Tc­(CNp-FArDarF2)3(CN n Bu)2(CN t Bu)]+ (6) have been prepared in this way and isolated as their PF6 – salts. mer,trans-[Tc­(CNp-FArDarF2)3(CN n Bu)2(CN t Bu)]­(PF6) represents to the best of our knowledge the first transition-metal complex with three different isocyanides in its coordination sphere. Since the degree of the ligand exchange seems to be controlled both by the electronic and steric measures of the incoming isocyanides, we undertook similar reactions with the sterically less demanding p-fluorophenyl isocyanide, CNPhpF, which indeed readily led to the hexakis­(isocyanide)­technetium­(I) cation through an exchange of all ligands in the staring materials [Tc2(CO)6(μ-Cl)3]− or fac-[Tc­(CO)3(CNR)2Cl]. The influence of the substituents at the isocyanide ligands in such reactions has been reasoned with the density functional theory-derived electrostatic potential at the accessible surface of the corresponding isocyanide carbon atoms.

The Chemistry of Phenylimidotechnetium(V) Complexes with Isocyanides: Steric and Electronic Factors

“…A more detailed assessment of the influence of electronic factors on the coordination behavior of highly substituted aryl isocyanides is available for a number of chromium(0) compounds, where a special role for aryl isocyanides with electron-withdrawing substituents was found [ 10 ]. Similar results were derived very recently for carbonyltechnetium(I) compounds, which react with differently substituted isocyanides in very different manners ( Scheme 1 ) [ 11 ]. It became evident that simple alkyl isocyanides were not able to replace carbonyl ligands, while this was readily possible with aryl isocyanides which have a fluorine substituent in 4-position.…”

“…The results obtained during reactions of tricarbonyltechnetium(I) complexes and other metal centers with isocyanides strongly indicate that this class of ligands should not be regarded in the same undifferentiated way as carbonyl surrogates, although a few correlations between the properties of some isocyanides and their highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) have been discussed in the past [ 23 , 24 , 25 , 26 ]. It has been found that electronic as well as steric effects strongly influence their coordination capabilities [ 10 , 11 , 12 , 13 , 14 , 15 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. The electronic effects can best be described by a consideration of the electrostatic potentials located at the donor carbon atoms.…”

“…Details of the calculations are outlined in Ref. [ 11 ]. The final self-consistent field densities were used for the construction of mappings of the electrostatic potentials onto the density isosurfaces (MO = 0.02; ρ = 0.004).…”

“…They revealed stunning substitution-dependent differences when normalized to the potential boundaries [e/ų] of the intermediate donor CNMe. The corresponding electrostatic potential maps for the isocyanides discussed in this paper are shown in Figure 1 , while corresponding maps for a large number of other isocyanides are published elsewhere [ 11 ].…”

“…The influence of the substituents at the isocyanide ligands in such reactions has been reasoned with the DFT-derived electrostatic potential at the accessible surface of the corresponding isocyanide carbon atoms. The corresponding S urface- A veraged D onor A tom P otential ( SADAP ) parameter allowed predictions concerning the reactivity of the individual isocyanides with the regarded d 6 systems in a synthetic and operationally convenient way [ 11 ]. It would now be interesting to see if the derived SADAP parameter is also suitable for electron-poor metal centers such as technetium(V) complexes, where no or almost no back-bonding to ligand orbitals should be expected.…”