Multiple Bonds Between Metal Atoms / 07-Technetium Compounds

262Multiple Bonds Between Metal Atoms Chapter 7

A series of triply metal–metal-bonded Tc24+ tertiary phosphine complexes of the general formula Tc2Cl4(PR3)4 have been prepared and characterized.61 These compounds are intermediates for a number of other ditechnetium complexes in the same, higher (see Section 7.4), or lower oxidation states. The Tc2Cl4(PR3)4 complexes are prepared from mononuclear Tc4+ precursors of the type trans-TcCl4(PR3)2. The starting materials with the alkyl phosphines PEt3 and PPrn3 are prepared as blue solids via exchange with the known bis-triphenylphosphine compound trans-TcCl4(PPh3)2.62 Precursors with less basic phosphines, viz., trans-TcCl4(PMe2Ph)2 and trans-TcCl4(PMePh2)2, are prepared as green crystalline solids by treating a suspension of NH4TcO4 in THF with chlorotrimethylsilane and excess PR3 followed by column chromatography on silica gel.61 The mononuclear phosphine complexes are then combined with 1 equiv of zinc powder in dry, O2-free benzene or THF in a Schlenk flask. Sonication (in a commercially available ultrasonic water cleaning bath) of the suspensions for 6 h results in almost quantitative yield of the air-sensitive purple Tc2Cl4(PR3)4 compounds that can be recrystallized from benzene/(Me3Si)2O:

The complexes are readily soluble in aromatic solvents, THF, and methylene chloride. They are diamagnetic, exhibit sharp 1H and 31P{1H} NMR spectra, and show a series of weak absorptions in the VIS/NIR region between 488 and 770 nm. The spectroscopic features, as might be expected, are quite similar to those of the related Re24+ compounds, Re2Cl4(PR3)4. Therefore, the lowest energy bands near 770 nm may be assigned as forbidden β*Α/* transitions by analogy with that of the lowest energy transition in Re2Cl4(PPrn3)4.63 The structures of three of the complexes were elucidated. All consist of two trans-TcCl2(PR3)2 fragments that are rotated 90° with respect to each other, to give an eclipsed geometry with approximate D2d symmetry. The Tc–Tc distances are 2.133(3) Å, 2.127(1) Å, and 2.1384(5) Å for the PEt3, PMe2Ph and PMePh2 complexes, respectively. Two reversible oxidation couples were measured electrochemically. Depending on the phosphine ligand, the first oxidation wave is found between -0.48 (PEt3) and -0.26 V (PMePh2) and the second at +0.88 and +0.92 V vs. Fc+/Fc, respectively. The reversibility implies that syntheses of the monoand dicationic species [Tc2Cl4(PR3)4]2+/+ are possible. Indeed, mild chemical oxidation of Tc2Cl4(PMe2Ph)4 with FcPF6 in acetonitrile produced green [Tc2Cl4(PMe2Ph)4]+ in high yield (see Section 7.4). An attempt to oxidize Tc2Cl4(PMe2Ph)4 to [Tc2Cl4(PMe2Ph)4]2+ with 2 equiv of [p-BrC6H4)3N]SbCl6 in acetonitrile was unsuccessful and resulted in the isolation and structural characterization of the monomeric Tc(IV) adduct TcCl4(PMe2Ph)2·2SbCl3.64

Treatment of Tc2Cl4(PR3)4 (PR3 = PEt3 or PMe2Ph) with 2 equiv of bis(diphenylphosphino)ethane (dppe) in refluxing toluene results in displacement of the monodentate phosphine ligands and formation of the pale pink `-isomer of Tc2Cl4(dppe)2 (60% yield) whose structure was determined by X-ray crystallography,58 and is depicted in 7.1. The `-Tc2Cl4(dppe)2 isomer has a twist or torsion angle of 35(2) Å and a Tc–Tc separation of 2.117(1) Å. When Tc2Cl4(PMe2Ph)4 is refluxed with a ten-fold excess of dppe for 1 h, the mauve _-isomer of Tc2Cl4(dppe)2 is formed in 80% isolated yield. The _-isomer has an eclipsed conformation and an average Tc–Tc bond length of 2.15[1] Å. Davison and coworkers have examined the reaction of Tc2Cl4(PEt3)4 with 2 equiv of bis(dimethylphosphino)methane (dppm) in refluxing benzene

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and obtained a 60% yield of the fuchsia colored Tc2Cl4(µ-dppm)2. Crystallographic studies of the complex confirm the `-isomer with a twist angle of c. 51° and a Tc–Tc bond length of 2.1126(7) Å.65

7.1

Acidification of acetonitrile/methylene chloride solution (1:5) of Tc2Cl4(PEt3)4 with HBF4(Et2O) (>8 equiv) followed by heating to c. 50 °C provides the bright blue solvated Tc24+ complex [Tc2(NCCH3)10](BF4)4 in 80% isolated yield:66

Blue crystals are obtained by recrystallization from acetonitrile/ether. The procedure was adopted from a similar one that Dunbar and coworkers used to prepare [Re2(NCCH3)10]- (BF4)4.67,68 The dinuclear tetracation can also be prepared, albeit in lower yields, from higher valent starting materials such as (Bu4N)2TcCl6 and (Bu4N)2Tc2Cl8.66 Attempts to obtain a satisfactory structure from X-ray data collected on [Tc2(NCCH3)10](BF4)4 were unsuccessful. Treatment of [Tc2(NCCH3)10](BF4)4 with 8 equiv of thallium triflate, Tl(O3SCF3), gave the triflate substituted complex [Tc2(NCCH3)8(O3SCF3)2](BF4)2 as a blue solid. It consists of two Tc(NCCH3)4 fragments which are linked by a short Tc–Tc triple bond of 2.122(1) Å. The pseudo-planar [Tc(NCCH3)4] units are staggered with respect to each other, resulting in a torsion angle of 43.5° (Fig. 7.7).

If the axial triflate ligands are ignored, the remainder of the cation has approximate D4d symmetry. A ground-state configuration of μ2/4β2β*2 is expected. The absence of β-bonding between the technetium atoms implies a low energy rotation barrier between the two fragments, and the molecule adopts the sterically favored staggered geometry. The 1H NMR spectrum of [Tc2(NCCH3)10](BF4)4 in CD3NO2 contained two separate signals for coordinated acetonitrile at β 3.0 and β 2.0 ppm in a ratio of 4:1. These are assigned as the equatorial and axial nitrile ligands, respectively. The 1H NMR spectrum of the same complex in CD3CN initially shows resonances for the equatorial nitriles at β 2.95 and free CH3CN at β 1.95 ppm, i.e., the axial nitriles rapidly exchange with the deuterated solvent. The electrochemistry of [Tc2(NCCH3)10](BF4)4 in acetonitrile/0.1 M (Bu4N)PF6 shows a reversible one electron reduction at -0.82 V vs Fc+/Fc which prompted a search for the [Tc2(NCCH3)10]3+ cation.

264Multiple Bonds Between Metal Atoms Chapter 7

Fig. 7.7. View of the [Tc2(NCCH3)8] unit of [Tc2(NCCH3)8(O3SCF3)2](BF4)2 looking down the Tc–Tc bond.

Bright blue acetonitrile solutions of [Tc2(NCCH3)10](BF4)4 gradually lose their color when exposed to fluorescent light. While initially this color change was believed to be a consequence of deterioration of the glove box atmosphere where the samples were stored, this is not the case. Rather, a rare example of the photochemical scission of a metal–metal multiple bond was discovered.69

In this case [Tc2(NCCH3)10]4+ is converted to the solvated mononuclear Tc2+ complex [Tc(NCCH3)6]2+. Under preparative conditions, using a 1000 W Hg vapor lamp, concentrated solutions of [Tc2(NCCH3)10](BF4)4 are photolyzed for c. 90 min and pale yellow [Tc(NCCH3)6](BF4)2 (95% yield) is precipitated by careful addition of diethyl ether. The structure of [Tc(NCCH3)6](BF4)2 was determined and revealed a Tc center coordinated to six acetonitrile ligands, with almost ideal octahedral symmetry. The magnetic moment of the low spin d5 complex is 2.1 B.M. as determined by the Evans method. Monitoring of the photochemical reaction indicated the process is not a simple one-step conversion. At least two intermediates are involved in the formation of the final product. A possible mechanism could be photoexcitation to a mixed-valent, charge-separated Tc1+–Tc3+ species that undergoes bond cleavage and subsequent comproportionation to the observed Tc2+ species. Of note, [Tc2(NCCH3)10]4+ is stable in refluxing acetonitrile and the Re–Re triple bond of [Re2(NCCH3)10]4+ cannot be broken under similar photolytic conditions.69

Reduction of [Tc2(NCCH3)10](BF4)4 in acetonitrile with 1 equiv of cobaltocene leads to a red-brown mixed-valence Tc1+–Tc2+ complex [Tc2(NCCH3)11](BF4)3 as shown in the following equation.70

Technetium Compounds 265

The reaction is performed at ambient temperature, and the product is isolated in 70% yield. The X-ray crystal structure reveals an unusual µ,δ1,δ2 coordination mode of one of the acetonitrile ligands which is bridging via its nitrogen atom between Tc centers and through the nitrile carbon to one of the Tc centers. The Tc–Tc separation is 4.04(2) Å indicating the total loss of metal–metal bonding. The electrochemical reduction of [Tc2(NCCH3)10]4+ at -0.82 V in acetonitrile is reversible using scan rates ranging from 20 mV/s to 250 mV/s. On the chemical time scale, the cation [Tc2(NCCH3)10]4+ is reduced and then undergoes reaction and rearrangement to the final (isolated) product. Electrochemical studies indicate the mixed-valence complex can be further reduced at -1.12 V, probably to a bridged Tc1+–Tc1+ complex. The observed oxidation at +0.25 V would correspond to a bridged Tc2+–Tc2+ complex. Although these compounds have not been isolated, the observed redox chemistry is unparalleled among homoleptic acetonitrile complexes.

7.6Hexanuclear and Octanuclear Technetium Clusters

For a particular metal oxidation state, an increase in cluster size should be paralleled by a decrease in the average M–M bond order. An example of this trend is provided by the pair of Mo2+ cluster anions [Mo2Cl8]4- and [Mo6Cl14]2- where the Mo–Mo bond order decreases from four to one as the number of pairwise Mo–Mo interactions for each Mo atom increases from one to four. For octahedral Tc3+ and Re3+clusters based upon the 24-electron M618+ cores, the average M–M bond order should be one, like that for the isoelectronic Mo612+ core. This expectation has not been realized for Tc and Re as halo species of the types [M6X12]6+ or M6X18 have not yet been prepared. A related question is how the properties and structures of Tc (or Re) hexanuclear clusters might change as the electron count increases, i.e., as the average metal oxidation state decreases. This question, to some degree at least, has been answered with the synthesis of molecules containing Tc612+, Tc611+, and Tc610+ cores.

The reduction of (Me4N)2TcCl6 or (Me4N)TcO4 in concentrated hydrochloric acid by molecular hydrogen (30-50 atm) in an autoclave at 140-180 °C yields a mixture of dark brown almost black crystals of different geometric shapes.25,71 These crystals are a mixture of two hexanuclear species, brown (Me4N)3{[Tc6Cl6(µ-Cl)6]Cl2} and black (Me4N)2[Tc6Cl6(µ-Cl)6]. The former is formed in high yield at 140-150 °C under an initial H2 pressure of 30-50 atm. Optimal conditions for the synthesis of (Me4N)2[Tc6Cl6(µ-Cl)6] are more forcing and require temperatures of 160-180 °C. The {[Tc6Cl6(µ-Cl)6]Cl2}3- and [Tc6Cl6(µ-Cl)6]2- clusters are derivatives of Tc611+ and Tc610+ cores with 31and 32-electron counts, respectively. Both compounds have been described as being paramagnetic. While a magnetic moment of ~1.7 B.M. for (Me4N)3{[Tc6(µ-Cl)6Cl6]Cl2} is consistent with the presence of one unpaired electron, a value of ~1.1 B.M. reported for (Me4N)2[(Tc6Cl6)Cl6], as well as an EPR signal, may be due to the presence of a paramagnetic impurity. The trigonal prismatic structure of the chloro anions is as represented in Fig. 7.8.72,73 The unsupported rectangular edge Tc–Tc bonds are very short, 2.16(1) Å for the [Tc6Cl12]- anion and 2.22(1) Å for [Tc6Cl12]2-, whereas

266Multiple Bonds Between Metal Atoms Chapter 7

the triangular edge distances are indicative of much weaker Tc–Tc bonding. The average Tc–Tc distances for the latter bonds are 2.69(1) Å and 2.57(1) Å, respectively, in the two structures. The related 31e- trigonal prismatic bromide cluster (Me4N)3{[Tc6(µ-Br)6Br6]Br2}has been synthesized by the reaction of the analogous chloride complex with concentrated hydrobromic acid at 180 °C under a pressure of H2 in an autoclave.74 The structure is similar to (Me4N)3{[Tc6(µ-Cl)6Cl6]Cl2} with Tc–Tc distances of 2.154(5) Å and 2.702(2) Å.74 When (Et4N)2TcCl6 is reduced by H2 in concentrated hydrobromic acid under similar conditions, a different cluster is obtained. The resulting dark brown salt is (Et4N)2{[Tc6(µ-Br)6Br6]Br2}, a derivative of the diamagnetic Tc612+ core with a 30-electron count.74 The structure of this anion is similar to those of the 31and 32-electron species with Tc–Tc distances in (Et4N)2{[Tc6(µ-Br)6Br6]Br2} of 2.188(5) Å and 2.66(2) Å.74

Fig. 7.8. Structure of the trigonal prismatic cluster anion, [Tc6Cl6(µ-Cl)6]- in (Me4N)3{[Tc6(µ-Cl)6Cl6]Cl2. Capping chloride ions have been omitted. The structure of [Tc6Cl6(µ-Cl)6]2- is similar.

A detailed treatment of the bonding in 30to 32-electron chloro clusters by Wheeler and Hoffmann has shown that 30 electrons are involved in the metal–metal bonding and the additional one or two electrons occupy a weakly antibonding Tc–Tc orbital that is /* with respect to the dinuclear species and weakly bonding in the triangles.75,76 The 30 electrons are partitioned between three electron-rich Tc>Tc bonds and six Tc–Tc single bonds. The model developed for the hexanuclear chloro compounds does not fit the 30and 31-electron bromide clusters. Here the Tc–Tc bond length decreases on going from Tc612+ to Tc611+ which is inconsistent with population of an a2'' orbital that is /-antibonding within the dimers. It would seem that additional theoretical work will be needed to fully understand the bonding in these remarkable compounds.

Additional technetium cluster compounds of even higher nuclearity are synthesized from concentrated HBr and HI solutions. The series of octanuclear Tc bromide cluster compounds [Tc8(µ-Br)8Br4]Br·2H2O,77,78 (H5O2)[Tc8(µ-Br)8Br4]Br,78,79 and (H5O2)2[Tc8(µ-Br)8Br4]Br278 has been described and all are based upon the cluster unit shown in Fig. 7.9. The [Tc8(µ-Br)8Br4]+ cluster has properties consistent with the presence of one unpaired electron.80 The four types of Tc–Tc bonds in these clusters have quite different distances, as illustrated in the case of the [Tc8(µ-Br)8Br4]+ cluster by values for the distances Tc(1)–Tc(2), Tc(1)–Tc(4), Tc(3)–Tc(4), and Tc(3)–Tc(4A) of 2.145(2) Å, 2.689(2) Å, 2.521(2) Å and 2.147(2) Å, respectively. The clusters clearly possess four Tc–Tc bonds of high multiplicity. A consideration of the bonding in this cluster type has led to the conclusion75 that these four Tc–Tc bonds are electron-rich triple bonds and similar to the triple bonds present in trigonal prismatic [Tc6Cl12]n- clusters (see above). These four Tc2 units are then bound together by overlap of five β and β* type orbitals, four of which are associated mainly with bonds around the rhomboidal top and bottom faces,

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while the remaining pair of β- and β*-orbitals are concentrated on atoms located in the prisms’ shared face.75 The mixed bromide-iodide cluster (Bu4N)2[Tc8(µ-Br)4(µ-I)4Br2I2]I2 has been reported as the product of the reaction of (H5O2)2[Tc8(µ-Br)8Br4]Br2 with concentrated hydriodic acid and NBun4OH in acetone. The identity of the cluster was confirmed by a single crystal X-ray structure determination.81 In addition, an unusual ferrocinium salt of the Tc iodide cluster anion {[Tc6(µ-I)6I6]I2}3- has been isolated and structurally characterized.52

Fig. 7.9. Structure of [Tc8(µ-Br)8Br4]+ cation in [Tc8(µ-Br)8Br4]Br·2H2O.

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