Air and water stable germacarbonyl compounds

Germacarbonyl compounds are the germanium analogs of carbonyl compounds requiring an inert atmosphere for stability. Making these compounds survive the ambient conditions was not feasible given the lability of the Ge 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 E bonds (E = O, S, Se, Te). However, the first examples of germacarbonyl compounds synthesized under ambient conditions by taking advantage of dipyrromethene ligand stabilization are detailed here; the isolated compounds are thiogermanone 3, selenogermanone 4, thiogermacarboxylic acid 6, selenogermacarboxylic acid 7, thiogermaester 9, selenogermaester 10, thiogermaamide 12, and selenogermaamide 13 with GeE bonds (E = S, Se). Compounds 12 and 13 can react under atmospheric conditions with copper(i) halides offering air and water stable monomeric 14–15 and dimeric 16–19 copper(i) complexes (halide = Cl, Br, I). Apart from just binding, selectivity was also observed; thiogermaamide 12 and selenogermaamide 13 bind CuCl and CuBr, respectively, when treated with a mixture of copper(i) halides.


Synthesis and spectra
With the knowledge that dipyrrinate stabilized monochlorogermylenes are air and water stable, 4a,b we studied the utility of DPMGeCl (1) to afford air and water stable thiogermaacid and selenogermaacid chlorides. The treatment of compound 1 with excess elemental sulphur/selenium in toluene (12 h, rt) gave no product. At a high temperature (60 C), the desired thiogermaacyl and selenogermaacyl chlorides were formed along with an inseparable unidentied side product. It is anticipated that the -I effect of chlorine may be the reason for this result; therefore, compounds with other functional groups were reacted with chalcogens. Phenyl germylene DPMGePh (2) was synthesized in 95% yield as an air and water stable solid through the reaction of germylene 1 with phenyl lithium at −20 C in toluene for 12 h (see ESI; Scheme S1 ‡). As the handling of phenyl lithium requires an inert atmosphere, phenyl germylene 2 was synthesized under a nitrogen atmosphere using a dried solvent. As anticipated, the reactions of compound 2 under ambient conditions with stoichiometric amounts of elemental sulphur and selenium occurred smoothly in toluene at room temperature for 1 h to afford thiogermanone DPMGe(S)Ph (3) and selenogermanone DPMGe(Se)Ph (4) in 95% and 93% yields (Scheme 1). THF and DCM as solvents instead of toluene did not offer germanones 3 and 4 cleanly. As phenyl germylene 2 did not react with elemental tellurium at room temperature or high (60 C) temperature, tellurogermanone was not isolable. Similarly, the reactions of compound 2 with nitrous oxide, N-(methyl)morpholine-N-oxide, and pyridine N-oxide also did not occur, prohibiting the synthesis of germanone with a Ge]O bond. A possible reason for this could be the bulkiness of the mesityl groups protecting the germylene center. Monoanionic N-heterocyclic ligand stabilized germylenes have offered germanones indirectly, which means that germylene reacted with N 2 O to form a m-oxo dimer and the reaction of this dimer with a suitable Lewis acid afforded germanone. 4c,d Considering this aspect, it is anticipated that the reaction of germylene 2 with an oxygen transfer agent does not occur due to the steric effect posed by the mesityl groups for the m-oxo dimer formation. Concerning the reaction with elemental tellurium, the large size of tellurium may prohibit its interaction with the germanium(II) center heavily guarded by bulky mesityl groups.
Compounds 3-4, 6-7, 9-10, and 12-13 are the rst examples of air and water stable heavy carbonyl compounds (Table 1); this stability reveals the ability of the bulky DPM ligand to protect the polar Ge]E bonds (E ¼ S, Se). The air and water stability of these germacarbonyl compounds was monitored by 1 H NMR spectroscopy (see ESI; Fig. S7, S8, S11, S12, S16, S19, S20, S24, S25, S28, S29, S38, S39, S43, and S44 ‡). The air stability was checked for up to 10 days and it was found that all the compounds were stable. Concerning water stability, the germacarbonyl compounds 3, 4, 9, 10, 12, and 13 are stable in water for 2, 4, 3, 5, 2, and 5 days, respectively (Table 1; the indicated stability refers to the duration for which the compounds show no detectable sign of decomposition). The thiogermacarboxylic and selenogermacarboxylic acids displayed poor water stability; selenogermacarboxylic acid 7 is stable for 6 h, while thiogermacarboxylic acid 6 is not stable and produces DPMH (2%) aer just 10 min of water addition. It is anticipated that two electronegative atoms, such as oxygen and S/Se attached to germanium, are responsible for this observation. These atoms make germanium more electrophilic; therefore, compounds 6 and 7 are more reactive toward water than the other compounds. Among all the germacarbonyl compounds, selenogermacarbonyl compounds are more stable than the corresponding thiogermacarbonyl compounds, perhaps due to the stronger Ge]Se bond in selenogermacarbonyl compounds than the Ge]S bond in thiogermacarbonyl compounds (Table  1) 3e,h-j Theoretical calculations on thiogermanone 3, selenogermanone 4, thiogermaamide 12, and selenogermaamide 13, offer evidence for this assumption; the Wiberg bond index (WBI) for the Ge]S bond in compounds 3 (1.457) and 12 (1.419) is marginally lower than that of compounds 4 (1.484) and 13 (1.439) with a Ge]Se bond.
Furthermore, to explain the observed stability of the germacarbonyl compounds, the NPA charges of the atoms in the Ge]E bond and the nature of the HOMO of dipyrrinate stabilized thiogermaamide 12 (E ¼ S) and selenogermaamide 13 (E ¼ Se) were analyzed and compared with those of aminotroponiminate and amidinate stabilized thio-and selenogermaamides (see computational details in the ESI ‡). As no signicant differences were seen, it was concluded that these electronic properties could not explain the observed air and water stability of dipyrrinate compounds with Ge]S/Se bonds. Therefore, it is anticipated that the steric protection offered by the mesityl groups of the dipyrrinate ligand may provide air and water stability. To test this, the isolation of Ph DPMGe] S(N(TMS) 2 ) with phenyl groups at the a and a ′ positions of the DPM ligand instead of the mesityl groups was tried. Surprisingly, it was not possible to synthesize the required germylene precursor ( Ph DPMGeCl) by reacting the in situ generated Ph DPMLi with GeCl 2 $(1,4-dioxane) until now. This result highlights the mesityl groups' role in offering stability.

Compound
Air stability a (days) Water stability b (day(s)) DPMGe(S)Ph (3) 1 0 2 DPMGe(Se)Ph (4) 1 0 4 DPMGe(S)OH (6) 10 Not stable DPMGe(Se)OH (7) 10 0.25 DPMGe(S)OEt (9) 1 0 3 DPMGe(Se)OEt (10 1 0 2 a Air stability was checked for up to 10 d only; therefore, they may be stable for a considerable period beyond this 10 d. For example, our experience with compounds 13 and 16 reveals that they did not start to decompose even aer one month of storage under ambient conditions. b Formation of 1-2% of DPMH was seen aer the specied period of water stability. Thiogermaamide 12 and selenogermaamide 13, apart from reacting independently with CuX (X ¼ Cl, Br, I), showed a novel aspect of selective binding towards a particular copper halide when a mixture of copper halides is present (Scheme 2). The reaction of thiogermaamide 12 with an equimolar mixture of CuX (X ¼ Cl, Br, I) in toluene for 15 min at room temperature exclusively gave compound 14 by reacting with CuCl only (Scheme 2). In contrast, selenogermaamide 13, under the same reaction conditions, reacted selectively with CuBr and gave compound 18 (Scheme 2). Even when thiogermaamide 12 was reacted with a mixture of CuX containing one equivalent of copper chloride and an excess of copper bromide and copper iodide (three equivalents each), it reacted only with copper chloride affording copper chloride complex 14 (Scheme 2). The result was the same for selenogermaamide 13; its reaction with a mixture of CuX salts containing copper chloride, copper bromide, and copper iodide in a ratio of 3 : 1 : 3 gave selectively copper bromide complex 18 (Scheme 2). Pearson's HSAB principle may better explain the observed selectivity. Among compounds 12 and 13, the Ge]S bond of thiogermaamide 12 is more polarized than that of selenogermaamide 13 (vide supra). The NPA charge on the sulphur (−0.826) of compound 12 is higher than that on the selenium (−0.685) of compound 13 (see computational details in the ESI ‡). These factors suggest that the soness of the sulphur in compound 12 is less than that of compound 13's selenium atom. For the copper(I) halides, copper(I) has the least soness when attached to chlorine (see computational details in the ESI ‡). Considering all these aspects, it is anticipated that the soness of sulphur in compound 12 closely matches the soness of copper(I) in CuCl rather than the copper(I) atom of CuBr/CuI. Extending the same argument to compound 13, the soness of its selenium matches the copper(I)'s soness in CuBr. Furthermore, compounds 12 and 13 did not react with AgX (X ¼ Cl, Br, I) and AuX (Cl, I).
Interestingly, compounds 14-19 are the rst examples of germacarbonyl compound stabilized transition metal complexes that are air and water stable. This feature was achievable due to the favorable steric protection and electronic stabilization offered by the bulky dipyrrinate ligand to the Ge]E/Cu moieties in these complexes. Akin to the methodology followed with germacarbonyl compounds, these copper(I) complexes' stability was studied using 1 H NMR spectroscopy (see ESI; Fig. S49, S50, S54, S55, S60, S61, S65, S66, S70, S71, S76, and S77 ‡). The complexes were stable in air up to the monitored period of 10 days. Regarding water stability, thiogermaamide stabilized copper(I) complexes 14, 16, and 17 were stable for 3 h, 1 day, and 3 days, respectively. It is explicit from the data that moving from chloride to iodide, the water stability increases. The same trend is seen for the selenogermaamide stabilized copper(I) complexes 15, 18, and 19; they were stable for 3 h, 12 h, and 2 days, respectively ( Table 1).

Conclusions
The rst examples of germacarbonyl compounds 3-4, 6-7, 9-10, and 12-13 that are stable under ambient conditions were synthesized and structurally characterized. Though thiogermanone 3, selenogermanone 4, thiogermacarboxylic acid 6, selenogermacarboxylic acid 7, thiogermaester 9, and selenogermaester 10 did not bind with copper(I) halides, thiogermaamide 12 and selenogermaamide 13 did react under ambient conditions providing copper(I) complexes (14-19) that are also stable outside inert atmospheres. The air and water stabilities of these germacarbonyl compounds and copper(I) complexes were studied using 1 H NMR spectroscopy; the stability of these compounds is due to the precise thermodynamic and kinetic stabilizations provided by a bulky dipyrromethene ligand. Uniquely, selective binding of thiogermaamide 12 and selenogermaamide 13 towards Cu(I)Cl and Cu(I)Br was noticed when they were reacted with a mixture of Cu(I)X, respectively (X ¼ Cl, Br, I).

Data availability
The experimental and computational data associated with this article are provided in the ESI. ‡

Conflicts of interest
There are no conicts to declare.