Rutas Persuasivas de los spots.
8.1. Antecedentes e hipótesis de investigación.
The metal substrates studied in this chapter thus far are of electron rich nature, being based on low-valent metal centres, while those with reduced electron density are yet to be explored. This naturally leads to the exploration of the reactivity of 4.2 with substrates bearing less electron density at the metal centre. Based on previous work by Santos49 and Hill,50 suitable rhenium and molybdenum metal precursors were chosen based on the established stability in the analogous NaBm derivatives.
On a preparative scale [Mo(CO)3(6-C7H8)] was combined with a solution of 4.2 in THF in an attempt to generate the anion [Mo{H2Al(mt)2}(CO)3]‒ for subsequent capture by ClSnPh3 (Scheme 4.16). The resulting yellow supernatant from the combination of [Mo(CO)3(6-C7H8)] with 4.2 was instantly accompanied by insoluble black solid
deposition. Numerous CO bands were detected in the IR spectra of the supernatant fraction (CO = 1807, 1824, 1917 cm-1). Amongst these, none appeared to fall within similar values to those reported for the boron analogue Na[Mo(CO)3{3-H,S,S’- H2B(mt)2}]‒(CO = 1817, 1882, 2002 cm-1).51 However, bands at 1717 and 1782 cm-1 might be indicative of the presence of Al–H units.
Scheme 4.16: Attempted synthesis of molybdenum stannyl complex, [Mo(SnPh3)(CO)3{3-H,S,S’-H2Al(mtMe)2}].
The supernatant fraction containing the postulated Li[Mo(CO)3{3-H,S,S’-H2Al(mt)2}] salt was subsequently treated with SnPh3Cl. An instantaneous colour change from yellow to orange occurred and several bands were observed in the IR spectrum, which likely corresponded to the anticipated product [Mo(SnPh3)(CO)3{3-H,S,S’-H2Al(mt)2}] (CO = 1878, 1906, 2002, AlH = 1719 cm-1) and additional carbonyl containing by-products ( = 1817 and 1940 cm-1). Unfortunately, the precipitate obtained following purification had poor solubility in C6D6. Interpretation of the 1H NMR spectrum obtained suggested that the products contained methimazole-based functionality but was absent of metal hydride units, thus disfavouring a complex with an Al–H–Mo interaction. Attempts to identify the products formed through crystallographic means were unsuccessful.
Coordination of [H2Al(mt)2] to rhenium proved to be slightly more promising. The complex [ReBr(CO)3(THF)2] was prepared by heating [ReBr(CO)5] in THF under reflux for 20 hours52 and then was subsequently treated with a stoichiometric amount of 4.2 at –78°C (Scheme 4.17).
Scheme 4.17: Attempted synthesis of [Re(CO)3{3-H,S,S’-H2Al(mt)2}].
The reaction was monitored by IR spectroscopy and the spectrum obtained following the addition of 4.2 contained absorptions at CO= 1887 (broad) and 2008 cm-1. There was an absence of aluminium-hydride AlH and AlHRe absorptions, which would be expected to occur at frequencies below 1800 cm-1. The CO absorption bands are distinct from those of the precursor [ReBr(CO)3(THF)2] (2029, 1915, 1894 cm-1), and similar to Santos’ [Re(CO)3{3-H,S,S’-H2B(mt)2}] (1900 and 2000 cm-1).49 Poor solubility of the reaction mixture (in CD2Cl2 or C6D6) precluded the acquisition of good quality 1H NMR spectra, however, interpretation of the spectrum obtained suggests that the crude contained a mixture of products. Crystals of differing morphology (red blocks and white needles) suitable for crystallographic analysis were obtained from a concentrated solution of the crude sample in benzene and were identified as [Re2(-N,S- mt)2(CO)6(Hmt)2] 4.9x (Figure 4.6) and [Re2(-N,S-mt)(-S:2-N,S-mt)(CO)6(Hmt)2] 4.9y (Figure 4.7).
Figure 4.6: a) and b) Molecular structure of [Re2(-N,S-mt)2(CO)6(Hmt)2] 4.9x.0.5(C6H6) in two different orientations (CH hydrogens and solvent omitted, displacement ellipsoids shown at 50% probability). Selected bond lengths (Å) and angles (°): Re1–S1
2.545(1), Re1–S2 2.518(9), Re1–N2i 2.196(3), S2–C4 1.732(4), S1–Re1–S2 86.36(3), Re1–S2–C4 89.71(2), N2i–Re1–S2 88.99(9), Re1–N2–C4 130.4(3).
While no aluminium coordination is evident in structures 4.9x and 4.9y, they are nevertheless useful illustrations of the versatile modes of coordination of the mt‒ ligand. The asymmetric unit of 4.9x consists of a ‘Re(mt)(CO)3(Hmt)’ fragment where a centre of inversion yields the symmetrical bimetallic rhenium complex 4.9x. The methimazole- based unit binds in a monodentate (-S) manner while the methimazolyl groups bridge in a -S/-N fashion, each to a different rhenium centre. Monodentate coordination of Hmt is exemplified through the thione (S1) two-electron donor, with a longer rhenium- sulfur bond than the formally anionic bridged form (Re1–S1 2.545(1) cf. Re1–S2 2.518(9) Å). Both coordination modes have comparably longer rhenium-sulfur bond lengths than in the compact tridentate ligand of [Re(CO)3{3-H,S,S’-H2B(mt)2}] (Re1–S1 2.486 Å).
In addition to the coordination modes described above, structure 4.9y displays a third mode of mt‒ bonding observed as chelation via nitrogen (N1) and sulfur (S3) donors to a common rhenium (Re1) centre while bridging through the sulfur (S3) atom to the second rhenium (Re2) centre. The versatile bonding exhibited by the sulfur atom results in elongation of the chelated Re1–S3 bond to 2.617(2) Å while the Re2–S3 bond length in 4.9y resembles that of the thione coordination in 4.9x (Re2–S3 2.545(2) 4.9y and Re1–S1 2.545(1) Å 4.9x).
Figure 4.7: Molecular structure of [Re2(-N,S-mt)(-S:2-N,S-mt)(CO)6(Hmt)] 4.9y.C6H6 and space filing representation (hydrogens and solvent omitted, groups simplified,
displacement ellipsoids shown at 50% probability). Selected bond lengths (Å) and angles (°): Re1–S3 2.617(2), Re1–N1 2.182(6), S3–C7 1.751(7), Re1–N3 2.165(6), N3–C11 1.333(9), S2–C11 1.737(6), Re2–S2 2.520(2), Re2–S1 2.524(2), Re2–S3 2.545(2), S3–Re1–N1 64.96(2), Re1–N1–C7 103.3(5), S3–C7–N1 114.3(6), Re1–S3–C7 77.2(2), S3–Re1–N3 87.82(2), N1–Re1–N3 81.9(2), Re1–N3–C11 129.0(4), S2–C11–N3
127.1(5), Re2–S2–C11 99.4(2), S2–Re2–S3 85.28(6), S1–Re2–S3 91.93(6), S1–Re2–S2 89.37(6).
4.4
Future Work
The reactivity of Li[H2Al(mt)2].THF has only been explored with second and third row transition metals featuring predominantly electron rich precursors and relying on ligand displacement (NCMe, PPh3, THF, C7H8) for coordination of Li[H2Al(mt)2]. A consideration lies in the judicious choice of metal precursors where scrambling of ligands is less likely. The chloro-bridged dimers [M2Cl2(Cp*)2] (M = Rh, Ir) and tetramer [Ru4Cl4(Cp*)4] are examples of substrates featuring a robust Cp* unit occupying one face of the metal and a vacant coordination site upon cleavage of the chloro-bridge. This reduces the number of ligands available for scrambling while providing an available vacant site for coordination.
Coordination of Li[H2Al(mt)2] to less electron rich metal substrates (group 4 and 5 transition metals) are presently unexplored and may offer a more compatible match should Hard Soft Acid Base considerations play a role. Similarly, little work has been directed to the first-row transition metals and main group metals, which provides many available avenues for investigation in the future.
4.5
Conclusion
The aluminium pro-ligand, Li[H2Al(mt)2].THF 4.2, was successfully prepared and confirmed through spectroscopy and X-ray diffraction analysis. While the ligand can be easily and reliably prepared in high yield, subsequent coordination to metal precursors in groups 6–8 has failed to deliver complexes with an intact HnAl(mt)2 (n = 1, 2) ligand. In addition to the greater reactivity and lower stability of 4.2 compared to the well-behaved Na[H2B(mt)2] analogue, several generalisations can be drawn concerning the behaviour of 4.2 towards metal substrates. In competition with initial sulfur coordination, the pro-ligand retains sufficiently reactive Al–H bonds that it may serve as a hydride donor. Coordination of the aluminium functionality generates a thermolabile kinetic product that converts to known complexes of greater stability over time, devoid of the aluminium centre of interest. This conversion occurs with degradation of 4.2 to liberate free mt‒, which readily coordinates to the metal centre through various binding modes. Although the work with 4.2 herein has yet to afford stable aluminium containing complexes, the potential for reactivity with other elements of the periodic table remains an uncharted territory providing avenues for future research.
4.6
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