Germanium Complexes with ONO Tridentate Ligands: O-H Bond Activation Control According to DFT Calculations

Polydentate ligands are used for thermodynamic stabilization of tetrylenes—low-valent derivatives of Group 14 elements (E = Si, Ge, Sn, Pb). This work shows by DFT calculations how the structure (the presence or absence of substituents) and type (alcoholic, Alk, or phenolic, Ar) of tridentate ligands 2,6-pyridinobis(1,2-ethanols) [AlkONOR]H2 and 2,6-pyridinobis(1,2-phenols) [ArONOR]H2 (R = H, Me) may affect the reactivity or stabilization of tetrylene, indicating the unprecedented behavior of Main Group elements. This enables the unique control of the type of the occurring reaction. We found that unhindered [ONOH]H2 ligands predominantly led to hypercoordinated bis-liganded {[ONOH]}2Ge complexes, where an E(+2) intermediate was inserted into the ArO-H bond with subsequent H2 evolution. In contrast, substituted [ONOMe]H2 ligands gave [ONOMe]Ge: germylenes, which may be regarded as kinetic stabilized products; their transformation into E(+4) species is also thermodynamically favorable. The latter reaction is more probable for phenolic [ArONO]H2 ligands than for alcoholic [AlkONO]H2. The thermodynamics and possible intermediates of the reactions were also investigated.


Introduction
In modern chemistry, selective synthesis of organometallic compounds in a determined oxidation state is an important problem. For transition metal complexes, this problem has been solved and is controlled by the oxidation state of the initial metal reagent [1,2]; for Main Group elements, the influence of organic ligand has a special role.
Different types of alkanolamines are used as such ligands. Interestingly, germylenes based on 2,6-bis(hydroxyalkyl)pyridines are monomeric, whereas stannylenes are dimeric [43] (Scheme 1; type A). The aggregation degree (monomer or dimer) for the related tetrylenes based on dialkanolamines is determined by the type of substituent at C and N atoms. In this case (Scheme 1; type C), monomeric germylenes for the voluminous ligands containing substituents at both C atoms in an α-position to O are formed; otherwise, the species formed are dimeric [44]. Stannylenes are dimeric despite the presence of substituents. Tetrylenes based on the related aminobis(methylenophenols) (type B) are monomeric [45]. All these Scheme 1. Structures of tetrylenes based on various aminoalcohols: tetrylenes based on 2,6-bis(hydroxyalkyl)pyridines (A); tetrylenes based on aminobis(methylenophenols) (B); tetrylenes based on dialkanolamines (C).
In continuation of our research (see above, Schemes 1 and 3) into germylenes [33][34][35][42][43][44], this work presents our theoretical results on their synthesis to understand how the oxidation state of the central atom is regulated by the steric properties of organic ligands In continuation of our research (see above, Schemes 1 and 3) into germylenes [33][34][35][42][43][44], this work presents our theoretical results on their synthesis to understand how the oxidation state of the central atom is regulated by the steric properties of organic ligands (1-H, 1-Me vs. 2-H, 2-Me), the oxidation state of which remains unchanged.

Results
The mechanism of activation of O-H bonds by tetrylenes has been studied in detail; it proceeds through the initial formation of adduct (R2E←O(H)R) followed by the migration of H to the E(+2) atom from a coordinated molecule [64] or another ROH molecule [61,65]. In this work, to study the details of the interaction of ONO-coordinating polydentate ligands with Ge(+2) species, we performed DFT calculations of the thermodynamic parameters of model reactions in toluene (Schemes 4 and 5), including the interaction between Lappert's germylene and unsubstituted (R = H) or substituted (R = Me) ligands. It should be noted that the parameters used for the calculations (B3LYP/DGDZVP for structure optimization) are correct; the main calculated structural parameters correlate well with the XRD data.

Results
The mechanism of activation of O-H bonds by tetrylenes has been studied in detail; it proceeds through the initial formation of adduct (R 2 E←O(H)R) followed by the migration of H to the E(+2) atom from a coordinated molecule [64] or another ROH molecule [61,65]. In this work, to study the details of the interaction of ONO-coordinating polydentate ligands with Ge(+2) species, we performed DFT calculations of the thermodynamic parameters of model reactions in toluene (Schemes 4 and 5), including the interaction between Lappert's germylene and unsubstituted (R = H) or substituted (R = Me) ligands. It should be noted that the parameters used for the calculations (B3LYP/DGDZVP for structure optimization) are correct; the main calculated structural parameters correlate well with the XRD data.  In continuation of our research (see above, Schemes 1 and 3) into germylenes [33][34][35][42][43][44], this work presents our theoretical results on their synthesis to understand how the oxidation state of the central atom is regulated by the steric properties of organic ligands (1-H, 1-Me vs. 2-H, 2-Me), the oxidation state of which remains unchanged.

Results
The mechanism of activation of O-H bonds by tetrylenes has been studied in detail; it proceeds through the initial formation of adduct (R2E←O(H)R) followed by the migration of H to the E(+2) atom from a coordinated molecule [64] or another ROH molecule [61,65]. In this work, to study the details of the interaction of ONO-coordinating polydentate ligands with Ge(+2) species, we performed DFT calculations of the thermodynamic parameters of model reactions in toluene (Schemes 4 and 5), including the interaction between Lappert's germylene and unsubstituted (R = H) or substituted (R = Me) ligands. It should be noted that the parameters used for the calculations (B3LYP/DGDZVP for structure optimization) are correct; the main calculated structural parameters correlate well with the XRD data. Thus, for   According to these DFT data using PCM method in toluene, it is evident that the substitution of N(SiMe3)2 ligands at a Ge atom is thermodynamically favorable, whereas the interaction with phenolic derivatives is more preferable in comparison with alkanolic derivatives. Indeed, the formation of 5-H/5-Me derivatives is one and a half times better than that of 3-H/3-Me (−60−(−63) vs. −44−(−46) kcal/mol). The reaction with the second equivalent of ONO ligand is twice less favorable. Interestingly, in the case of alkanolamines 3-H, 3-Me, introduction of substituents to the OCR2 atom decreases the possibility of this reaction, which fully corresponds to Jurkschat's data and our results (see Introduction). In contrast, for phenolic derivatives, this tendency is not so evident and, apparently, it is necessary in this case to take into account the steric effects of the substituents. However, it should be emphasized again that the reaction with phenolic derivatives is significantly more energetically favorable in comparison with alcoholic derivatives; indeed, formation of 4-H/4-Me is two times less preferable than that of 5-H/5-Me (−23 and −11 vs. −35 and −36 kcal/mol). Furthermore, for alcoholic ligands, the steric demands of the substituents (H vs. Me) may have a critical impact on the low favorability of interaction of the Ge(II) center with the second equivalent of the ligand.
It should be noted that the increase of the steric volume of the substituents R in ligand 1 (Me vs. H) diminishes the probability of the formation of compound 4. Apparently, this fact may be explained by the fac-disposition of ligand frameworks in the coordination sphere of Ge. In contrast, for phenolic ligands 2, the change of the steric size of substituents in α-positions of phenol rings has almost no effect on the thermodynamics of the reaction; in this case (see Schemes 3 and 6), the ligands' cores are in a mer-disposition in the coordination sphere of the central atom, i.e., are maximally separated in space.
Apparently, in the case of the model ligand 2 ([ Ar ONO R ]H2), the corresponding germylene [{ Ar ONO R }Ge:] obtained in situ at the first stage, due to the increased acidity of Scheme 6. Data of the XRD analysis for the known Ge(+4) bis-liganded compound (its adduct with a ligand molecule).
According to these DFT data using PCM method in toluene, it is evident that the substitution of N(SiMe 3 ) 2 ligands at a Ge atom is thermodynamically favorable, whereas the interaction with phenolic derivatives is more preferable in comparison with alkanolic derivatives. Indeed, the formation of 5-H/5-Me derivatives is one and a half times better than that of 3-H/3-Me (−60−(−63) vs. −44−(−46) kcal/mol). The reaction with the second equivalent of ONO ligand is twice less favorable. Interestingly, in the case of alkanolamines 3-H, 3-Me, introduction of substituents to the OCR 2 atom decreases the possibility of this reaction, which fully corresponds to Jurkschat's data and our results (see Introduction). In contrast, for phenolic derivatives, this tendency is not so evident and, apparently, it is necessary in this case to take into account the steric effects of the substituents. However, it should be emphasized again that the reaction with phenolic derivatives is significantly more energetically favorable in comparison with alcoholic derivatives; indeed, formation of 4-H/4-Me is two times less preferable than that of 5-H/5-Me (−23 and −11 vs. −35 and −36 kcal/mol). Furthermore, for alcoholic ligands, the steric demands of the substituents (H vs. Me) may have a critical impact on the low favorability of interaction of the Ge(II) center with the second equivalent of the ligand.
It should be noted that the increase of the steric volume of the substituents R in ligand 1 (Me vs. H) diminishes the probability of the formation of compound 4. Apparently, this fact may be explained by the fac-disposition of ligand frameworks in the coordination sphere of Ge. In contrast, for phenolic ligands 2, the change of the steric size of substituents in αpositions of phenol rings has almost no effect on the thermodynamics of the reaction; in this case (see Schemes 3 and 6), the ligands' cores are in a mer-disposition in the coordination sphere of the central atom, i.e., are maximally separated in space.
Apparently, in the case of the model ligand 2 ([ Ar ONO R ]H 2 ), the corresponding germylene [{ Ar ONO R }Ge:] obtained in situ at the first stage, due to the increased acidity of phenolic groups in 2, participates in the further reaction with the ligand faster than Lappert [69]; a related result was observed only in interaction with thiophenols, giving (ArS) 3 GeH [70]. Unlike the cases stated, in our case, the driving force of this reaction, caused by the absence of a steric hindrance in the ligand, is the release of gaseous hydrogen and the formation of a bis-liganded complex, where the Ge atom is hexacoordinated. Such hypercoordinated derivatives are thermodynamically stable. The method of their synthesis presented in our work is promising for hypercoordinated compounds of Group 14 elements, which are widely used nowadays [71][72][73].
Interestingly, among several possible intermediates in this reaction, the most stable thermodynamically are species with pyridinyl coordinating bonding (II, where the coordination number (C.N.) of the Ge atom is 6; Scheme 7); other possible intermediates, including the O(H) bonding one, are less probable (I, with the C.N. of 6, and III, with the C.N. of 7, Scheme 7). We should emphasize that the related structure has been studied by XRD [74] (Scheme 7; E). [67], giving [R3SiO]3GeH [68] or [RsiO3GeH]4 [69]; a related result was observed only in interaction with thiophenols, giving (ArS)3GeH [70]. Unlike the cases stated, in our case, the driving force of this reaction, caused by the absence of a steric hindrance in the ligand, is the release of gaseous hydrogen and the formation of a bis-liganded complex, where the Ge atom is hexacoordinated. Such hypercoordinated derivatives are thermodynamically stable. The method of their synthesis presented in our work is promising for hypercoordinated compounds of Group 14 elements, which are widely used nowadays [71][72][73].
Interestingly, among several possible intermediates in this reaction, the most stable thermodynamically are species with pyridinyl coordinating bonding (II, where the coordination number (C.N.) of the Ge atom is 6; Scheme 7); other possible intermediates, including the O(H) bonding one, are less probable (I, with the C.N. of 6, and III, with the C.N. of 7, Scheme 7). We should emphasize that the related structure has been studied by XRD [74] (Scheme 7; E) We also studied the electronic structure of model germylenes 4-H ( Figure 1) and 6-H ( Figures S1 and S2, Supplementary Materials) and their UV absorption properties, which were obtained by the GaussView program; the spectra are presented in the form of Table  1 with the assignment of transitions between molecular orbitals (Table 1; for details see [75]). Interestingly, for both compounds, the HOMO -> LUMO transition is the most probable; this can be confirmed by the visual distribution of frontier orbitals (see Figure  1). As is evident, for the phenolic conjugated derivative 6-H, the UV bands are shifted into the red field. For 6-H, the UV bands correspond to the π-π transition. Interestingly, in intermediate II, the HOMO-LUMO transition ( Figures S3 and S4, Supplementary Materials) corresponds to a n-π transition due to the O atom. We also studied the electronic structure of model germylenes 4-H ( Figure 1) and 6-H ( Figures S1 and S2, Supplementary Materials) and their UV absorption properties, which were obtained by the GaussView program; the spectra are presented in the form of Table 1 with the assignment of transitions between molecular orbitals (Table 1; for details see [75]). Interestingly, for both compounds, the HOMO -> LUMO transition is the most probable; this can be confirmed by the visual distribution of frontier orbitals (see Figure 1). As is evident, for the phenolic conjugated derivative 6-H, the UV bands are shifted into the red field. For 6-H, the UV bands correspond to the π-π transition. Interestingly, in intermediate II, the HOMO-LUMO transition ( Figures S3 and S4, Supplementary Materials) corresponds to a n-π transition due to the O atom.

Discussion
As a rule, E(+4) species form only in trace amounts in reactions of E(+2) with polydentate OH ligands like dialkanolamines [76]. A related process has been observed previously by Tzschach, Jurkschat et al. [77] in the reaction of RN(CH2CH2SH)2 with: Sn(OBu-t)2 at a controlled temperature to give E(+2) derivatives at ambient temperature or E(+4) species on heating.
Thus, we may conclude that the structure of the ligand governs the stability of tetrylene and determines the type of the products formed, which is unprecedented in the chemistry of tetrylenes. Another

Discussion
As a rule, E(+4) species form only in trace amounts in reactions of E(+2) with polydentate OH ligands like dialkanolamines [76]. A related process has been observed previously by Tzschach, Jurkschat et al. [77] in the reaction of RN(CH 2 CH 2 SH) 2 with: Sn(OBu-t) 2 at a controlled temperature to give E(+2) derivatives at ambient temperature or E(+4) species on heating.
Thus, we may conclude that the structure of the ligand governs the stability of tetrylene and determines the type of the products formed, which is unprecedented in the chemistry of tetrylenes.
Another  [56], where the interaction of R 2 E with HOR' yields R 2 E(OR') 2 in trace amounts. To the best of our knowledge, there is the single related example, where MesSeH reacts with [(Me 3 Si) 2 N]Ge: giving (MesS) 4 Ge and hydrogen [78]. In our case, the high reactivity of the R' 3 E-H bond towards the interaction with O-H is caused by hypercoordination in II [79].
Unlike the present case of ONO bis(phenolic) ligands, application of dialkanolamines as ligands for tetrylene synthesis resulted in a successful process; the absence of voluminous substituents at the C atom in an α-position to OH groups led only to dimeric, [GeO] 2 , species. Furthermore, using 2,6-bis(hydroxyalkyl)pyridines in reactions with Lappert's tetrylenes resulted in stabilized substituted tetrylenes due to alkoxydeamination reactions.
When comparing two structural features of tridentate ONO ligands, i.e., the presence of voluminous substituents near the donation atoms and the type of O atoms (alcoholic vs. phenolic), we found that the structure of 2,6-pyridinobis(1,2-phenols), [ Ar ONO R ]H 2 (R = H, Me), unprecedentedly determined the character of the product formed in the interaction with Lappert's germylene. For the successful synthesis of substituted tetrylenes, it is necessary to apply sterically voluminous substituents at positions close to the coordinating atoms (i.e., in an ortho-position to the OH group), otherwise, the intermediate tetrylenes formed in situ further react with a ligand faster, giving E(+4) species. At the same time, we showed that the careful choice of a ligand structure also enabled determining the oligomeric degree of the tetrylene. Thus, in comparison with alkyl amines, the presence of the Py donor group is critical for the formation of a monomeric structure, whereas the steric size of substituents determines the direction of the reaction between Lappert's tetrylene and a ligand. It should be noted that wide application of Ge(+2) and even hypercoordinated Ge(+4) complexes in chemistry and catalysis opens new prospects for these derivatives.
All model reactions studied (Schemes 4 and 5) are thermodynamically favorable, but the effect of steric factors, i.e., the presence of bulky substituents, may slow down the reactions, as observed in chemical experiments (see Scheme 3, right side). In this case, the activation energy of the synthesis of the hexacoordinated complex increased significantly.
Furthermore, the non-covalent interaction between the benzene rings can be important (dispersion interaction, D3 correction), but this effect is apparently weak and requires additional more specialized investigation. Interestingly, our calculation shows that the distances between hydrogen atoms of different ligands are about 3 Å. In principle, at such a distance, dispersion interactions can be neglected. Moreover, the dispersion interactions between phenyl rings can be significant at a distance between them up to 3.5 Å in the π-π interaction. In our systems, the dihedral angle between the phenyl rings is approximately 126 • . At the same time, the distance between the nitrogen atoms of the pyridyl groups is greater than 4 Å. Of course, dispersion interactions between hydroxyl and/or hydride groups can have a certain effect on the thermodynamic parameters in our chemical reactions. However, the difference between the calculated Gibbs energies in the reactions of systems with methyl and phenyl substituents is quite large (about 30%) and, in our opinion, does not affect the conclusions of the article.

Materials and Methods
DFT calculations details. The hybrid exchange-correlation functional (B3LYP) was used throughout the study because previous calculations had shown the B3LYP approach [80,81] to be a cost-effective method for studying metal-containing systems [82]; results obtained using B3LYP functionality are compared well with a large number of functionals incorporated in G09 and G16. Even at calculations of the thermodynamic parameters, the B3LYP results compare well with the highly exact G2MP2 method, as well as with the experimental values [83]. We used the DGDZVP basis set for all atoms at the B3LYP level. The DGDZVP is an all-electron, double-ζ valence polarized basis set optimized specifically for DFT methods [84,85]. We used the time-dependent M0-62X [86] density functional with the 6-311+G(d,p) basis set implemented by Gaussian 09 (2010) to explore the excited manifold and to compute the possible electronic transitions.
The calculations were performed with full geometry optimization; the Gaussian 09 program package was used [87]. The absence of imaginary vibration frequencies confirmed the stationary character of all compounds studied. The molecular orbitals and UV/visible spectra were visualized using the GaussView program. The UV spectra were calculated in PCM approximation [88] in toluene.

Conclusions
Our DFT calculations indicate that the structural features of organic tridentate ligands determine to a great extent the type of reactions observed at the interaction of tridentate