Erwann Le Coz

Joanna Hammoud

Thierry Roisnel

Marie Cordier

Vincent Dorcet

Samia Kahlal

Jean-Francois Carpentier

Jean-Yves Y Saillard
email: jean-yves.saillard@univ-rennes1.fr

Yann Sarazin
email: yann.sarazin@univ-rennes1.fr

Jean-François Carpentier

J F Orcid

Carpentier

Bonding in Barium Boryloxides, Siloxides, Phenoxides and Silazides. A comparison with the lighter alkaline earths

Keywords:

come

Introduction

The heavy alkaline earths calcium, strontium and barium (Ae = Ca, Sr, Ba) are gaining increasing recognition as suitable alternatives in molecular catalysis to more expensive and often more toxic transition metals. [1][2][3][START_REF]Early Main Group Metal Catalysis: Concepts and Reactions[END_REF] Versatile catalysts built on these three metals have shown for example excellent performances in a panel of reactions such as the polymerisations of styrene [START_REF] Harder | [END_REF] and cyclic esters, [6] or the hydrogenation, hydroamination and hydrophosphination of unsaturated bonds. [7][8][9][10][11][12][13][14][15][16][17][18] Recently, they have also proved competent in catalysed dehydrocouplings for the selective formation of amidoboranes, [19][20][21] silazanes, [22][23][24][25][26] borasiloxanes [27][28] and asymmetric disiloxanes. [29] Mechanistic and kinetic investigations have revealed that for a same ligand environment, and all parameters being otherwise equal, kinetics in these catalysed reactions usually increase according to Ca < Sr < Ba. [1][2][3][START_REF]Early Main Group Metal Catalysis: Concepts and Reactions[END_REF] A notable exception is the cyclohydroamination of aminoalkenes, where the opposite trend has been consistently observed. [7,30] On the whole, barium hence displays the highest turnover numbers. However, it also is with this metal that the high reactivity specific of the heavy alkaline earths is most difficult to tame. [31] Heavy alkaline-earth metals form d 0 complexes where bonding is ruled by electrostatic and steric factors, with little, or no, orbital contribution. In solution, heteroleptic complexes [{Lig}AeX], where {Lig} -is a monoanionic ancillary ligand and X -is often an amide, an alkyl or a hydride, often engage in ligand redistribution equilibria that generate ill-controlled mixtures unsuitable for catalysis. Stopping these phenomena, which are most troublesome for the largest and most electropositive element barium, still constitutes a challenge in modern alkaline-earth molecular chemistry. Yet, thanks to the use of carefully devised ligands, e.g. β-diketiminates, [12,32] iminoanilides, [12] carbazolates, [33][34] phenoxides, [6,35- 36] or tris(pyrazolyl)borates, [37] the number of stable, heteroleptic barium complexes has been growing steadily in recent years. Beyond homogenous catalysis, these ligands have led to breakthroughs such as the recent preparation of soluble barium-hydrides. [31,[37][38] Despite the overall importance of alkoxides as inorganic ligands, [START_REF] Bradley | Alkoxo and Aryloxo Derivatives of Metals[END_REF] well-defined Ba-alkoxide remain scarce due to the large size, propensity for high coordination numbers, and high oxophilicity and electropositivity of the Ba 2+ dication. Conventional alcohols, e.g. t BuOH, yield polymeric and insoluble Ba-alkoxides. [35] Rare examples of soluble compounds include [Ba{κ [START_REF]Early Main Group Metal Catalysis: Concepts and Reactions[END_REF] -N(CH2CH2O)(CH2CH2OH)2}2], [START_REF] Poncelet | [END_REF] [Ba2(μ2-OCPh3)3(OCPh3).(thf)3] [42] and [Ba{κ 3 -O(CH2CH2O)2}{κ 3 -O(CH2CH2OH)2}2]. [43] The addition of CF3 groups in α position to the alkoxide give fluoroalkoxides with enhanced stability and solubility, as in [Ba{OC(CF3)3}2.(dme)3]. [44] Oxygen-based derivatives of barium alkoxides exist. A number of Ba-phenoxides have been described since the report on [Ba(O-2,6-t Bu2-4-Me-C6H2)(thf)3], [45] e.g. the dimer [Ba(μ-I)(O-2,6-t Bu2-4-Me-C6H2).(thf)3]2 [36] or the dinuclear [Ba2(μ-O-2,6-Ph2-C6H3)3(O-2,6-Ph2-C6H3)]. [46] Siloxides have produced base-stabilised Ba species, such as Caulton's seminal [Ba2(μ2-OSi t Bu3)3(OSi t Bu3).thf], [42] and [Ba(OSiPh3)2 (15-crown-5).thf] [47] or [Ba3(OSiPh2OSiPh2O)3.(tetraglyme)2]. [48] We recently used the bulky silanol (Me3Si)3SiOH to generate the unsolvated Ba-siloxide [Ba2{μ2-10.1002/chem.202101687

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OSi(SiMe3)3}3{OSi(SiMe3)3}] (12, Figure 1), and its heteroleptic congener [Ba{μ2-OSi(SiMe3)3}{N(SiMe3)2}]2 (22); both compounds are dimeric in the solid state. [29] Besides, we have also reported the preparation and structural characterisation of the first barium boryloxides, notably the two-coordinate monomer [Ba(OB{CH(SiMe3)2}2)2] (3) and its amido, dimeric parent [Ba{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (42, Figure 1). [27] We had reasoned that the presence of boron or silicon atoms would lower the π-donating ability of the O atoms in these ligands, and that, combined with the steric bulk imparted by multiple SiMe3 groups, they would generate soluble, low-coordinate barium complexes.

Figure 1. Solvent-free, low-coordinate barium siloxides and boryloxides; [27,29] R = SiMe3.

Although we succeeded in preparing low-coordinate complexes that proved valuable dehydrocoupling precatalysts, [27][28][29] our curiosity was aroused by the variety of coordination patterns in complexes 12-42. The homoleptic boryloxide 3 is monomeric, while 12 is a dimer with three bridging siloxides and a single terminal one. In the heteroleptic siloxide/amide dimer 22, the siloxides are bridging the two metals and the amido moieties occupies terminal position, as expected with alkaline earths and other oxophilic elements. On the other hand, the opposite situation is (uncharacteristically) observed for the boryloxide/amide 42.

As part of our continuing efforts to devise low-coordinate and reactive barium complexes suited to molecular catalysis, we are reporting here on several new homoleptic barium boryloxides. In addition, a number of heteroleptic barium complexes that combine bulky boryloxo, siloxo, phenoxo or amido ligands are also presented. The structural motifs in these compounds are analysed on the basis of X-ray diffraction crystallography, NMR spectroscopy and DFT computations, with the aim of providing an appraisal of the criteria that enable the formation of potentially useful barium precatalysts. Besides, the structural comparison in a homologous series of heteroleptic Ae-boryloxide/amide complexes is also provided. It highlights the differences in the bonding patterns involving magnesium, calcium, strontium or barium. In this context, we note that no strontium boryloxide has been reported to date, whereas the dimer [Ca{μ-OB(CH(SiMe3)2)2}{N(C6F5)2}]2 is the sole known calcium boryloxide. [49] A handful of magnesium congeners have been prepared before, notably the dimeric [Mg{OB(mes)2}R.thf]2 (R = Me, Bu; mes = mesityl = 2,4,6-Me3-C6H2) [50] and the β-diketiminato [{BDI DiPP }MgOBPh2.(L)], where L is either a neutral organoboronic ester or N,N-dimethylaminopyridine. [51] 10.1002/chem.202101687

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Results and Discussion

Synthesis of homoleptic barium boryloxides

Prior to this work, existing barium boryloxides were limited to 3, 42 and to the solvated 3•toluene and 3•(thf)2. [27,29,52] We set out to prepare new complexes using the borinic acids {(Me3Si)2CH}2BOH, [53] (mes)2BOH, [54][55] (Tripp)2BOH, [54][55] {Fmes}2BOH [56] and (C6F5)2BOH [57][58][59] (Tripp = 2,4,6-i Pr3-C6H2, Fmes = 2,4,6-(CF3)3-C6H2, Scheme 1). These known compounds were prepared on multigram scales and high yields upon modification of published protocols. All are crystalline colourless solids with good solubility in organic solvents. Their identity was confirmed by NMR spectroscopy and X-ray diffraction crystallography (see the Supporting Information, Figures S49-S53). They are monomeric in the solid state, except for (C6F5)2BOH which forms a cyclic trimer with a non-planar B3O3 core; yet, it is monomeric in solution in toluene-d8 at room temperature. [60] Their respective chemical shifts in 11 Scheme 1. Optimised syntheses of borinic acids. [53][54][55][56][57][58][59] 10.1002/chem.202101687

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Following the synthesis of [Ba(OB{CH(SiMe3)2}2)2] (3), [27] a monomer stabilised by intra-and intermolecular Ba . Beyond the fact that this borinic acid is prone to condensation with loss of H2O and concomitant formation of (mes)2B-O-B(mes)2, the several reactions that were carried out only afforded the oxocluster [Ba4(O){OB(mes)2}6] as the sole isolable product, probably due to the release of water in the reaction medium. This cluster, a colourless crystalline material, was authenticated by XRD analysis (Figure S54). The reactivity of (mes)2BOH observed here is different from that with more robust magnesium precursors, although the reported synthesis of [Mg{OB(mes)2}Me.(thf)]2 was said to be erratic and to give low yields. [50] We were also unable to obtain Ba-boryloxides from the electronpoor (C6F5)2BOH. Regardless of the conditions, all attempted reactions generated intractable mixtures of compounds which contained various borates, as indicated by 11 The molecular solid-state structure of 52 is depicted in Figure 2. The complex is a centrosymmetric O-bridged dimer with a rhomboidal Ba2O2 core. Each barium is three-coordinate in a distorted trigonal pyramidal geometry (Σθ(Ba1) = 320.03(7)°). This pyramidalisation results from the presence of Ba The molecular structure of 62 displayed in Figure 3 shows this complex to also exist as an Obridged dimer with three-coordinate barium atoms. The complex is stabilised by a pattern of secondary

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Chemistry -A European Journal the formation of one-dimensional coordination polymers in the solid state, although there is no spectroscopic indication that this arrangement is maintained in solution. The structure of the dimeric solvate 62.(thf)2 shares some common features with that of 62, namely the presence of multiple intramolecular Ba•••F contacts (Figure S46). However, due to the coordination of thf molecules, each metal is now four-coordinate, and does not feature intermolecular Ba•••F interaction; as a result, in contrast with 62, 62.(thf)2 shares does not form a coordination polymer. As suggested by Ruhlandt-Senge over a decade ago, [61] Ae•••F-C secondary interactions such as in 62 and has been used effectively to obtain a variety of highly electrophilic Ae species in recent years. [6,49,62,63] 10.1002/chem.202101687

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Synthesis of heteroleptic barium complexes

The synthesis of barium complexes was extended to heteroleptic boryloxides and other related species, namely phenoxides and siloxides (Scheme 3), to complement the first examples of these compounds recently introduced: the amido-bridged dimer [Ba{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (42), [27] and the dinuclear [Ba{μ2-OSi(SiMe3)3}{N(SiMe3)2}]2 (22) bridged by a siloxide. [29] The ambition was to assess which of the bulky boryloxides, the siloxide (Me3Si)SiO -and the regular hexamethyldisilazide (Me3Si)2N -would provide suitable electronic density and/or steric shielding to generate solvent-free barium and perhaps, by extrapolation, other alkaline-earth complexes.

All reactions described here were carried out in toluene at room temperature.

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The NMR spectroscopic data recorded for crystalline materials in benzene-d6 (for 72 and 82) and thf-d8 (for 92 and 102) are unremarkable. Most 11 B and 29 Si resonances could be located, except in two cases, for 92 and 102, most probably due to high fluxionality of the systems in solution; low temperature NMR provided no assistance. A summary of representative NMR data for these complexes, as well as those for the homoleptic complexes described herein and for complexes 12-42, is collated in Table 1.

[Ba{μ2-O(2,6-Ph2-C6H3)}{OSi(SiMe3)3}]2 102 thf-d8 - - not detected
The molecular solid-state structure of [Ba{μ2-OB(Tripp)2}{N(SiMe3)2}]2 (72) shows the complex to be a centro-symmetric boryloxide-bridged dimer (Figure 4). Each barium is three-coordinate, with a [Ba{μ2-OSi(SiMe3)3}(OB{CH(SiMe3)2}2)]2 (82) forms a Cs-symmetric dimer bridged by the two siloxides (Figure 5), with a symmetry plane going through the barium and two terminal oxygen atoms.

The asymmetric unit contains three independent and similar molecules, only one of which is discussed here. Unlike in for instance 52 and 72, the Ba2O2 core is not planar, as Ba2 protrudes out of the best plane defined by Ba1, O21 and O51 presumably due to the steric constraints imposed by the very bulky ligands. Each barium is three-coordinate, but the geometry about Ba1 and Ba2 is very different due to [29] see Figure 1) and, for instance, the Ci-symmetric

52.

The Ba-O interatomic distances are unremarkable and compare well with those in the barium boryloxides and siloxides described previously here and elsewhere. [27][28][29] Complex 82 exhibits four short Ba-H interatomic distances in the range 2.493(6)-2.777(7) Å (not depicted) that may indicate the presence of Ba•••H-C anagostic interactions contributing to its stability; however, no spectroscopic evidence for the existence of such interactions could be detected. [Ba{μ2-OB(Tripp)2}{OSi(SiMe3)3}]2 (92) forms a centro-symmetric dimer bridged by Oboryloxide atoms, with a planar Ba2O2 rhomboidal core and terminal siloxides (Figure 6). The geometry around ]. [46] The values of the Ba1-O1 interatomic distance and Ba1-O1-

Summary of crystallographic data for barium complexes

In combination with our earlier contributions, [27][28][29] the results disclosed herein indicate that a range of soluble barium boryloxides, siloxides and phenoxides 12-102 are synthetically available. They feature a diversity of structural patterns in the molecular solid-state, with the main ones (nuclearity, structural type, coordination number and presence of non-covalent interactions) highlighted in Table 2. Most of these compounds exist as dimeric species in the solid state, where two different bridging modes for the Ba2O4 core have been identified: a classical and largely prevailing one with two bridging and two terminal ligands, and a minor one, with three bridging groups and a single terminal ligand as in 12 and its thf adduct 12.thf. [29] This latter, unusual arrangement is reminiscent of that found in the molecular structure of the alkoxide [Ba2(μ2-OCPh3)3(OCPh3)(thf)3]. [42] Other than this complex, there are very few examples of discrete homometallic barium alkoxides indexed in the CCDC database [66] for simple alkoxo groups, [67] even though the structures of several fluoroalkoxides, as in 10.1002/chem.202101687

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[Ba{OC(CF3)3}2(dme)3], [42] and those of heterobimetallic species, e.g. [Zr2Ba(μ2-O t Bu)6(O t Bu)4], [68] have been described. Taken collectively, these data also point to the importance of Ba [ Examination of the data for the six heteroleptic complexes obtained so far, namely 22, 42 and 72-102, indicate that the relative position (bridging vs terminal) of a given ligand in dimeric complexes changes with its identity. Hence, the propensity of a given anion to occupy the bridging position increases according to the empirical relationships displayed in Figure 8. No information can be provided about (Fmes)2BO -, as we failed to obtain structurally characterised heteroleptic complexes with this ligand. The boryloxide {(Me3Si)2CH}2BO -seems to be unable to generate dinuclear species, supposedly cation (rionic = 1.35 Å), and the resulting ability to accommodate bulky ligands and additional secondary interactions, must also be a contributing factor. Although it seems reasonable to hypothesise so, it is unclear at this stage to which extent the relationships displayed in Figure 8 can be extrapolated to the comparatively smaller calcium and strontium (rionic = 1.00 and 1.18 Å). Comparison between alkaline earths.

Ba2{μ2-OSi(SiMe3)3}3{OSi(SiMe3)3}] 12 C 3 & 4 Ba•••H-C [29] [Ba2{μ2-OSi(SiMe3)3}3{OSi(SiMe3)3}.thf] 12.thf C 4 & 4 - [29] [Ba{μ2-OSi(SiMe3)3}{N(SiMe3)2}]2 22 B 3 - [29] [Ba(OB{CH(SiMe3)2}2)2] 3 A 2 Ba•••H-C [27] [Ba(OB{CH(SiMe3)2}2)2.toluene] 3.tol A 2 Ba•••C(π) [28] [Ba(OB{CH(SiMe3)2}2)2.(thf)2] 3.(thf)2 A 4 - [27] [Ba{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 42 B 3 - b [Ba{μ2-OB(Tripp)2}{OB(Tripp)2}]2 52 B 3 Ba•••C(π) b [Ba{μ2-OB(Fmes)2}{OB(Fmes)2}]2 62 B c 3 Ba•••F b [Ba{μ2-OB(Fmes)2}{OB(Fmes)2}.thf]2 62.thf B 4 - b [Ba{μ2-OB(Tripp)2}{N(SiMe3)2}]2 72 B 3 Ba•••C(π) b [Ba{μ2-OSi(SiMe3)3}(OB{CH(SiMe3)2}2)]2 82 B d 3 Ba•••H-C b [Ba{μ2-OB(Tripp)2}{OSi(SiMe3)3}]2 92 B 3 Ba•••C(π) b [Ba{μ2-O(2,6-Ph2-C6H3)}{OSi(SiMe3)3}]2 102 B 3 Ba•••C(π)
The heteroleptic complexes [Ae{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (Ae = Mg, 112; Ca, 122; Sr, 132)

were prepared in 53-84% yields by reaction of [Ae{N(SiMe3)2}2]2 with {(Me3Si)2CH}2BOH. In conjunction with the barium complex [Ba{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (42), [27] they afford a complete series of homologous heteroleptic dimers. The molecular structures of the three new complexes were determined by XRD analysis performed on single crystals. Like 42, they all form dimer bridged by the amido groups, while the boryloxides are located in terminal positions. Although not identical (vide infra), the structural arrangement is similar across the four complexes, hence only the strontium complex 132 is detailed here; the ORTEP representations 112 and 122 can be found in the Supporting Information (Figures S47-S48) with a list of representative metric parameters.

Complex 132 forms a C1-symmetric dimer with a flat, rhomboidal Sr2N2 core (Figure 9). The

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Chemistry -A European Journal of such heteroleptic dimer is [Sr{μ-N(SiMe3)2}{N(C6F5)2}]2, [69] where the Sr-Nbr interatomic distances, in the range 2.598(2)-2.634(2) Å, match well those in 132 (2.601(2)-2.648(2) Å). The two boron atoms are also in a perfect trigonal planar environment (Σθ(B1) = 360.0(3)°; Σθ(B2) = 359.9(3)°). The main structural features are very similar across the three amide-bridged complexes 112, 122

and 132 (all C1-symmetrical, although the metric parameters is geometrical arrangements about the two metals atoms in both 112 and 122 are almost identical, resulting in a pseudo-C2 symmetry), and they resemble those in the known barium congener 42. The most representative data are compiled in Table 3.

While the eight B, O, Met and N atoms are coplanar or very nearly so in 112-132 (see for instance the extremely short distances between the oxygen or boron atoms and the best average plane defined by the Ae2N2 planar core in these complexes), the two O-atoms and two B-atoms protrude out of the Ba2N2 planar core in the Ci-symmetric barium complex 42. This last compound is also the only one where the geometry about the metal atoms shows noticeable pyramidalisation (Σθ(Ba) = 348.16(9)°). The Ae-to-O and Ae-to-N interatomic distances globally increase expected on account of the differences between the ionic radii of the Mg 2 (0.74 Å), Ca 2+ (1.00 Å), Sr 2+ (1.18 Å) and Ba 2+ (1.35 Å) cations. The Ae-N-Ae' angle also widens as a result of increasing metal size. A particularly striking contrast between the a Distance from the oxygen atoms to the best average plane defined by the rhomboidal Ae2N2 core. b Distance from the boron atoms to the best average plane defined by the rhomboidal Ae2N2 core.

Computational analysis

We first examined the bonding within an homogeneous series of simple dinuclear homoleptic complexes with structural motif B (see Table 2), namely, [Ae{μ2-N(SiMe3)2}(N(SiMe3)2)]2, for which X-ray structural data are available for Ae = Mg, Ca, Sr and Ba. [70][71][72][73] For this purpose, DFT calculations were performed at the PBE0/Def2-TZVP-D3 level of theory (see Supporting Information for computational details). Relevant averaged computed data are provided in Table 4, together with some experimental metrical values. The four optimised geometries (left side of Figure 10), are in a very good agreement with their X-ray counterparts. Note that, in the four structures, the terminal nitrogen atoms (Nt) retain their trigonal planar bonding mode (Σθ(Nt) = 360°; Table 4). This is in line with the planarity of N(SiR3)3, [74] where hyperconjugation exists between the nitrogen lone pair and the three silyl groups.

The Nt planarity also allows for π donation to the Ae metal. However, the structures of the four 10.1002/chem.202101687

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Chemistry -A European Journal complexes differ substantially with respect to the rotational conformations of their terminal disilazides.

It is particularly noteworthy that in the crystal, the Ba and Mg complexes adopt different exact point symmetry groups: in [Ba{μ2-N(SiMe3)2}(N(SiMe3)2)]2 (Ci symmetry), the two planes defined by the two sets of (NtSi2) atoms remain roughly parallel to the BaNbrBaNbr rhombus (Nbr = bridging N-atom; Nt = terminal N-atom), while for the magnesium derivative (C2 symmetry), the two (NtSi2) planes form angles of 75.8° and 57.4° with the (BaNbr)2 core, respectively, and an angle of 46.8° between each other. [70] The DFT-optimised structures reproduce nicely these structural differences (Figure 10). The crystallographically determined molecular structures of the Ca and Sr species have no exact symmetry element in the crystal. That of Ca exhibits a rotational conformation of the terminal amido groups rather similar to that of Mg, but with the latter two terminal Nt(SiMe3) groups bent away from the Ae-Ae' axis.

This unsymmetrical structure was also found as the most stable by DFT (Figure 10). With strontium, the DFT-optimised geometry of [Sr{μ2-N(SiMe3)2}(N(SiMe3)2)]2 was found to be of C2 symmetry, but this time with the C2 axis lying along the Nbr-Nbr vector; hence, the four Ae and Nt atoms are not perfectly aligned. Although of a different symmetry group, this structure is quite similar to that of the Ba congener.

As a matter of fact, optimising [Sr{μ2-N(SiMe3)2}(N(SiMe3)2)]2 assuming Ci symmetry destabilises it by only 1.4 kcal/mol.

Contrarily to Nt atoms, the Ae centre, which adopts a perfect trigonal planar coordination in the case of Mg, shows increasing pyramidalisation when descending group 2 and drops to Σθ(Ba) = 350.8° in the case of Ba (Table 4). As a result, the two Nt atoms progressively move out of the AeNbrAeNbr plane; this out-of-plane shift is particularly noticeable in the case of Sr and Ba (~ 0.6 and 0.9 Å, respectively). Gradual pyramidalisation at Ae atoms in the [Ae{μ2-N(SiMe3)2}(N(SiMe3)2)]2 series is reminiscent of the bending observed in the heavier AeX2 (X = H or halide) species; [75] it is attributed to metal core polarisation and/or participation of the valence d-orbitals of Ae elements. [75][76][77][78][79] The latter has been described as resulting from a pseudo-Jahn-Teller effect from post Hartree-Fock calculations. [80] The atomic charges obtained from a natural atomic orbital (NAO) population analysis (Table 4)

indicate strong ionicity, with Ae-charges varying within the narrow range 1.82-1.87. The corresponding valence NAO occupations indicate that the valence s orbital very largely dominates the (weak) covalent component of the bonding in the case of Mg. On the other hand, with the heavier Ae elements, the valence (n-1)d orbitals are also significantly involved. Note also that with Ca, Sr and Ba, some core orbitals are also slightly depopulated to the benefit of their valence counterpart. From the atomic charges, it emerges that the weakly covalent interaction increases in the order Mg < Ba ≈ Sr < Ca. This is overall consistent with the analysis of the Ae-N Wiberg bond index (WBI) values, also listed in Table 4: Mg < Sr < Ba < Ca. This trend is dominated by the involvement of the valence d(Ae) orbitals. Partial occupation of the d(Ae) AOs is not the single factor inducing Ae pyramidalisation, [75][76][77][78][79] since it is maximal for Ae = Ca which pyramidalises only moderately. Part of the (n-1)d(Ae) population might also come from π-donation of the Nt lone pair, this effect working against pyramidalisation at Ae. At any rate, all these orbital effects are small and bonding in these complexes remains chiefly ionic. Finally, 10.1002/chem.202101687

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Chemistry -A European Journal Table 4. Selected averaged computed data across [Ae{μ2-N(SiMe3)2}(N(SiMe3)2)]2 (Ae = Mg, Ca, Sr, Ba). Wiberg bond indices are given in brackets. Experimental averaged metrical values [70][71][72][73] are reported in italics for comparison. Nt and Nb refer to the terminal and bridging ligand, respectively.

We now shift our attention to the analysis of the set of heteroleptic complexes [Ae{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (Ae = Mg, Ca, Sr, Ba, i.e. 112-132 and 42); relevant computed data are provided in Table 5. The four optimised geometries (Figure 10, right) were found to be rather floppy, a recognised feature of Ae-centred molecules, [79] with their lowest energy structure being of (or very close to) Ci symmetry. Their optimised metrical data are in good agreement with their X-ray counterparts (Tables 3 and5). As in the homoleptic series, the geometry about Ae pyramidalises when going down

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Chemistry -A European Journal oxygen atoms for sp hybridisation, and hence (slightly) larger π-donation to Ae. Taken collectively, the data of Table 5 suggests increasing covalency in the order Mg < Sr < Ba < Ca. However, as for the homoleptic series discussed above, the covalent component of the bonds remains weak compared to the ionic contribution. Several Ae•••H-C anagostic interactions are also present in all complexes of the heteroleptic set. For example, for the barium complex 42, eight Ba•••H-C contacts shorter than 3.20 Ǻ are seen for each Ba atom; this is fully consistent with the crystallographically established molecular structure for this complex. [27] Several additional Ba•••H contacts slightly larger than 3.20 Å are also detected. In order to provide a rationalisation of the empirical bridging ability proposed in Figure 8 Å. [81] The case of [Ba{μ2-OSi(SiMe3)3}{N(SiMe3)2}]2 (22), which does not possess any π system, is

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Chemistry -A European Journal through Grimme's empirical corrections [82] for 22 is 9.0 kcal/mol lower than in 22*. This difference is indicative of larger van der Waals and steric stabilisations in 22. Another part of these secondary nonbonding interaction energies is likely to be contained into the DFT electronic energy. On the other hand, in complexes 42 and 82, the dispersion energy actually favours the isomers 42* and 82* not seen experimentally. One may then infer that the preference for the actual structures of 42 and 82 is dictated by the direct ligand-to-Ba bonding, which is dominated by ionicity.

In summary, the ability of N-and O-based ligands for bridging two Ba 2+ cations is a composite of several contributions. With sufficiently non-bulky ligands, since the Ba-N or Ba-O bond is mainly ionic, the more negatively polarised (on N or O) ligand prefers occupying the bridging position, thus maximizing electrostatic interactions. In the case of homoleptic dinuclear complexes of such "small" ligands, since the bridging position is preferred, the number of bridging ligands will be maximised to three, as actually seen in 12 [29] and, for instance, in Ba2Cl4. [75,79] In these two examples, we found the triply bridged structural motif C (Table 2) to be more stable that the "regular" doubly bridged motif B by 13.6 and 9.2 kcal/mol respectively. Interestingly, the quadruply bridged structure [Ba(µ2-Cl)2]2 of D4h symmetry is also a local energy minimum for Ba2Cl4. [75] At our level of calculations, it is 15.0 kcal/mol less stable than the C3v Ba2(µ2-Cl)3Cl minimum. With four bridging chlorides, the Ba-Cl electrostatic interactions are maximised, but the repulsions between the ligands disfavour this configuration. With bulkier ligands, secondary metal-ligand bonding interactions are expected to come into the play. For example, Ba•••H-C anagostic interactions (~ 3 Å) are present in all our optimised structures and their (relative) weakness can be compensated by their large number. More specific but stronger interactions can also occur, such as the above-mentioned Ba•••C(π) more covalent interactions with ligands that contain a π-system (e.g. a C6 aromatic ring), but also electrostatic Ba•••F interactions in the case of fluorinated ligands. We have shown on model calculations that the magnitude of a single Ca•••F bonding energy could reach 9-14 kcal/mol. [49] Similar values can be expected for barium. It should be noted that the possibility for such secondary interactions could occur in both bridging and terminal positions of the ligand, making the induction of the bridging vs. terminal choice also dependant of other factors, such as the shape and flexibility of the co-ligand. Indeed, ligand•••ligand interactions play also a crucial role in the structural determinism of these complexes. They result from a compromise between steric repulsions and van der Waals bonding (dispersion forces), in particular through H•••H interactions.

The latter tend to favour pseudo-spherical shapes (as in 22) to the detriment of more open structures. The main learning to take away is that the bridging ability of a specific ligand depends of course on its various intrinsic features but also, in a significant part, on the way these characteristics can be expressed in relation with those of its other co-ligand. Therefore, it seems wise to consider the trend given in Figure 8 as relative to the considered ligand interplay. Introducing other ligand combinations may partly contradict this relative classification.

10.1002/chem.202101687

Accepted Manuscript

Chemistry -A European Journal

Concluding comments

Building on our earlier syntheses of low-coordinate barium-boryloxides and barium-siloxides, [27,29] we have prepared here several homoleptic and heteroleptic barium complexes supported by various monoanionic O-based ligands. The complexes form dinuclear species that have been structurally characterised by X-ray diffraction crystallography. The siloxide, boryloxide and even phenoxide ligands are shown to occupy in turn bridging or terminal positions, depending on the specific case and ligand combination. Extended to the structural data already available for complexes bearing these O-based ligands, including for heteroleptic compounds that also contain the [N(SiMe3)2] -amide, Table 2 highlights a diversity of coordination patterns, some of them being unexpected. on several occasions, [31,49] but also their structures in the solid state. In these complexes bearing "simple", monodentate ligand, the coordination of Lewis bases, such as thf, suppresses partly or entirely these secondary interactions. This needs not be always the case, as we have shown earlier that prevented the coordination of thf onto the metal. [83] The comparison of crystallographic data obtained for the homologous set of heteroleptic complexes [Ae{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (Ae = Mg, Ca, Sr and Ba) shows that structural features are generally well reproduced across the whole series, although some subtle variations are observed, notably with the smaller magnesium. It therefore feels legitimate to assume that the main conclusions drawn from the detailed examination of the structures of barium complexes can be extrapolated to, at least, calcium and strontium; this is also confirmed by DFT computations.

Our DFT calculations on series of homo-and heteroleptic dinuclear complexes of barium and related alkaline earths, including magnesium, stress on the largely prevailing ionic character of ligand-Ae bonding. The minor covalent component of the bonding is mainly controlled by the valence s atomic orbital of the alkaline earth, together with the valence d atomic orbital in the case of the heavier Ca, Sr and Ba elements. The participation of d atomic orbitals favours non-planarity of the coordination mode for the heavier Ae metals, associated with an increased floppiness of the complexes. The pre-eminence of the ionic character of the Ae-ligand bond favours bridging coordination, the number of bridging ligands (most often two) being regulated by steric factors. In the case of heteroleptic species, the relative ligand preference for bonding vs. terminal positions is not determined by the ligand abilities to make stronger/weaker direct terminal or bridging bonds with the metal, but rather by the secondary (but nonnegligible) interactions that can occur between the Ae metal and the ligand. This is well exemplified by Considering the importance of well understood and controlled structure-reactivity relationships for the design of efficient alkaline-earth homogeneous catalysts, and in particular for those constructed with oxygen-based ligands, the data and analysis presented herein should constitute a useful guideline to assist synthetic organometallic main group chemists in their future endeavours. Comforted with this knowledge and with the further confirmation of the key role of secondary interactions in alkaline-earth chemistry, we now intend to prepare the simplest and, arguably, most versatile of O-ligated complexes: soluble, well-defined barium alkoxides and their lighter congeners.

Figure 2 .

 2 Figure 2. ORTEP representation of the molecular structure of [Ba{μ2-OB(Tripp)2}{OB(Tripp)2}]2 (52). Noninteracting toluene molecule, H atoms and Me groups not represented for the sake of clarity. Ellipsoids at the 50% probability level. Ba•••C(π) interactions depicted with dashed lines. Selected interatomic distances (Å) and angles (°): Ba1-O1 = 2.5694(11), Ba1-O1 i = 2.8100(11), Ba1-O2 = 2.4167(11), Ba1-C2 i = 3.0854(15), Ba1-C3 i = 3.1994(15), Ba1-C7 i = 3.3157(16); O1-Ba1-O1 i = 81.19(4), O2-Ba1-O1 = 123.86(4), O2-Ba1-O1 i = 114.98(4), Ba1-O1-B1 = 143.11(10), Ba1-O1 i -B1 i = 105.40(9), Ba1-O1-Ba1 i = 98.82(4), Ba1-O2-B51 = 148.28(11), O1 i -B1 i -C2 i = 112.54(13), O1 i -B1 i -C17 i = 122.23(13). Symmetry transformations used to generate equivalent atoms: i, -x, -y, -z, T = [1, 0, 0].

62.(thf) 2

 2 constitute an efficient tool (along other types of secondary interactions, e.g. anagostic Ae•••H or, as above in 52, Ae•••C(π)) towards the stabilisation of low-coordinate alkaline-earth complexes. It

Figure 3 .

 3 Figure 3. ORTEP representation of the molecular structure of [Ba{μ2-OB(Fmes)2}{OB(Fmes)2}]2 (62). H atoms omitted for clarity. Ellipsoids at the 30% probability level. Selected interatomic distances (Å) and angles (°): Ba1-O1 = 2.435(3), Ba1-O51 = 2.623(3), Ba1-O51 i = 2.657(3), Ba1-F19 = 3.218(4), Ba1-F40 iii = 2.957(3), Ba1-F47 = 3.104(3) , Ba1-F68 = 3.284(3), Ba1-F76 i = 3.164(3), Ba1-F88 i = 2.933(2), Ba1-F90 = 2.939(3); Ba1-O1-B2 = 171.9(3), Ba1-O51-Ba1 i = 104.45(9). Symmetry transformations used to generate equivalent atoms: i, -x, -y, -z, T = [1, 0, 0]; ii, -x, -y, -z, T = [0, 0, 0].

  The boryloxidebridged [Ba{μ2-OB(Tripp)2}{N(SiMe3)2}]2 (72) was isolated in 67% yield following the equimolar reaction of [Ba{N(SiMe3)2}2.(thf)2] with (Tripp)2BOH. Treatment of 22 with two equivalents of {(Me3Si)2CH}2BOH afforded the quantitative and selective formation of the siloxo-bridged [Ba{μ2-OSi(SiMe3)3}(OB{CH(SiMe3)2}2)]2 (82) upon release of HN(SiMe3)2. The analogous reaction between 22 and the aromatic borinic acid (Tripp)2BOH yielded the dimer [Ba{μ2-OB(Tripp)2}{OSi(SiMe3)3}]2 (92), where the two metals are bridged by the boryloxides. Finally, the mixed siloxide/phenoxide [Ba{μ2-O(2,6-Ph2-C6H3)}{OSi(SiMe3)3}]2 (102) was obtained in 70% yield upon protonolysis of 22 with two

  distorted trigonal pyramidal environment (Σθ(Ba1) = 325.97(8)°) that reflects an η 1 -interaction with the ipso C-atom (Ba1-C41 = 3.1380(18) Å; the other Ba1-C(π) interatomic distances are 3.497(2) Å or greater) in a neighbouring Tripp fragment. The Ba-O distances in 72 (2.5932(13)-2.6789(13) Å) are in the range of those in other related complexes having boryloxides in bridging positions, e.g. 52 and 62.The O21-B22-C41 angle involving the carbon atom (C41) that interacts with Ba1 is somewhat narrower that the O21-B22-C23 with the non-interacting C atom, 119.07(17)° and 125.04(18)°, respectively. It must be noted that by contrast with 42 where the amides bridge the two metal atoms, in 72 they occupy terminal positions. Hence, the nature of the boron-bound alkyl/aryl groups clearly affects the π-donating character of the oxygen atom in different boryloxides.

Figure 4 .

 4 Figure 4. ORTEP view of the molecular structure of [Ba{μ2-OB(Tripp)2}{N(SiMe3)2}]2 (72) . H atoms omitted for clarity. Ellipsoids at the 30% probability level. Selected interatomic distances (Å) and angles (°): Ba1-N1 = 2.5512(17), Ba1-O21 = 2.6789(13), Ba1-O21 i = 2.5932(13), Ba1-C41 = 3.1380(18), O21-B22 = 1.334(2), N1-Si1 = 1.6825(18), N1-Si2 = 1.6803(19); N1-Ba1-O21 = 140.52(5), N1-Ba1-O21 i = 109.53(5), O21-Ba1-O21 i = 75.92(4). B22-O21-Ba1 = 105.69(11), B22-O21-Ba1 i = 149.24(12), Ba1-O21-Ba1 i = 104.08(4), O21-B22-C23 = 125.04(18), O21-B22-C41 = 119.07(17). Symmetry transformations: i, -x, -y, -z, T = [2, 0, 0].

Figure 5 .

 5 Figure 5. ORTEP view of the molecular structure of [Ba{μ2-OSi(SiMe3)3}(OB{CH(SiMe3)2}2)]2 (82). Only one of the three independent but very similar molecules in the asymmetric unit depicted. H atoms omitted for clarity. Ellipsoids at the 30% probability level. Selected interatomic distances (Å) and angles (°): Ba1-O1 = 2.431(3), Ba1-O21 = 2.549(3), Ba1-O51 = 2.635(3), Ba2-O5 = 2.551(3), Ba2-O31 = 2.386(3), Ba2-O21 = 2.584(3), O1-B2 = 1.331(6), O31-B32 = 1.326(6); O1-Ba1-O21 = 124.71(11), O1-Ba1-O51 = 156.57(11), O21-Ba1-O51 = 76.26(11), O21-Ba2-O31 = 120.37(11), O21-Ba2-O51 = 77.14(10), O31-Ba2-O51 = 117.11(12), B2-O1-Ba1 = 164.8(3), B32-O31-Ba2 = 170.20(4), O1-B2-C3 = 120.5(5), O1-B2-C10 = 122.6(5), C3-B2-C10 = 116.8(4), O31-B32-C33 = 120.3(4), O31-B32-C40 = 120.3(4), C33-B32-C40 = 119.3(4).

  10.1002/chem.202101687 Accepted Manuscript Chemistry -A European Journal each barium atom is trigonal pyramidal (Σθ(Ba1) = 314.9(6)°), with deviation from the expected trigonal planar arrangement due to the presence of Ba•••C(π) η 3 -interactions with flanking aromatic groups. The corresponding Ba-C interatomic distances are in the range 3.074(10)-3.265(10) Å. Note that the Ba1-O1-Si1 angle of 172.0(6)° is particularly wide, which suggests substantial sp-hybridisation at O1 due to the participation of the lone pairs of electrons to the formation of the O1-Ba1 and O1-Si1 bonds. This angle and the Ba1-O1 bond length (2.377(10) Å) match the pertaining ones in other dimeric complexes having the bulky siloxide OSi(SiMe3)3 in terminal positions, e.g. as in 12 (2.414(7) Å and 173.50(5)°).

Figure 6 .

 6 Figure 6. ORTEP view of the molecular structure of [Ba{μ2-OB(Tripp)2}{OSi(SiMe3)3}]2 (92). H atoms omitted for clarity. Ellipsoids at the 30% probability level. Selected interatomic distances (Å) and angles (°): Ba1-O1 = 2.377(10), Ba1-O11 = 2.778(10), Ba1-O11 i = 2.560(10), Ba1-C12 = 3.074(10), Ba1-C13 = 3.198(10), Ba1-C17 = 3.265(10); O1-Ba1-O11 = 130.0(3), O1-Ba1-O11 i = 109.9(3), O11-Ba1-O11 i = 75.0(4), Si1-O1-Ba1 = 172.0(6). Symmetry transformations used to generate equivalent atoms: i, -x, -y, -z, T = [ 1, 0, 0].

Figure 7 .

 7 Figure 7. ORTEP view of the molecular structure of [Ba{μ2-O(2,6-Ph2-C6H3)}{OSi(SiMe3)3}]2 (102). H atoms omitted for clarity. Ellipsoids at the 30% probability level. Selected interatomic distances (Å) and angles (°): Ba1-O1 = 2.3564(14), Ba1-O23 = 2.6728(14), Ba1-O23 i = 2.7152(14), Ba-C14 = 3.321(2), Ba1-C15 = 3.199(2), Ba1-C16 = 3.2031(19), Ba1-C24 i = 3.120(2), Ba1-C25 i = 3.252(2), Ba1-C29 i = 3.273(2); O1-Ba1-O23 = 111.75(5), O1-Ba1-O23 i = 107.11(5), O23-Ba1-O23 i = 78.87(4), Ba1-O1-Si1 = 166.68(9). Symmetry transformations used to generate equivalent atoms: i, -x, -y, -z, T = [ 1, 0, 0].

ba

  Arrangement of the Ba(X1)2, Ba2(μ-X1)2(X2)2 or Ba2(μ-X1)3(X2) core, without taking coordinated solvent molecule(s) into account. b This work. c Coordination polymer in the solid-state. d Cs-symmetric dimer.

Figure 8 .

 8 Figure 8. Relative bridging ability of O-based monoanionic ligands established from the structural data for the dinuclear heteroleptic barium complexes 22, 42 and 72-102. Br = bridging and term = terminal positions.

  geometry about the two Sr atoms differs slightly. The two B1-O1-Sr1 and B2-O2-Sr2 angles close to perfectly linear (178.90(16)° and 173.51(19)°) suggest the participation of the lone pairs at O to the formation of sp-hybridised orbitals, and all six B1-O1-Sr1-B2-O2-Sr2 atoms are nearly aligned. The geometry about each strontium is trigonal planar (Σθ(Sr1) = 359.99(11)°; Σθ(Sr2) = 359.79(11)°). The bridging position of the two N(SiMe3)2 -ligands is unusual. To our knowledge, the sole other example 10.1002/chem.202101687

Figure 9 .

 9 Figure 9. ORTEP view of the molecular structure of [Sr{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (132). H atoms omitted for clarity. Ellipsoids at the 50% probability level. Selected interatomic distances (Å) and angles (°): Sr1-N1 = 2.648(2), Sr1-N2 = 2.634(2), Sr1-O1 = 2.2751(18), Sr2-N1 = 2.601(2), Sr2-N2 = 2.635(2), Sr2-O2 = 2.2727(18), O1-B1 = 1.351(3), O2-B2 = 1.346(3); Sr1-N1-Sr2 = 95.19(7), Sr1-N2-Sr2 = 94.71(7), N1-Sr1-N2 = 84.59(7), N1-Sr1-O1 = 139.43(6), N2-Sr1-O1 = 135.97(7), N1-Sr2-N2 = 85.49(7), N1-Sr2-O2 = 132.46(7), N2-Sr2-O2 = 141.84(6), B1-O1-Sr1 = 178.90(16), B2-O2-Sr2 = 173.51(19), O1-B1-C1 = 121.8(2), O1-B1-C8 = 121.6(2), C1-B1-C8 = 116.6(2), O2-B2-C15 = 121.8(2), O2-B2-C22 = 121.1(2), C15-B2-C22 = 117.0(2).

group 2 ,

 2 to reach Σθ(Ba) = 343.7° in the case of the barium complex 42. The experimental trend along this series for the O and B out-of-plane shifting mentioned above is thus nicely reproduced by DFT. As seen for the homoleptic series of amides, the covalent component of the Ae-ligand bonding involves essentially the valence s(Ae) orbital in the case of Mg, and both s(Ae) and d(Ae) orbitals for the heavier elements. The experimental tendency for larger B-O-Ae angles in the cases of Ca and Sr is also reproduced by DFT. It is associated with larger B-O Wiberg indices, suggesting a larger tendency of the

  , the whole Ba heteroleptic series [Ba(μ2-X1)(X2)]2, namely 22, 42 and 72-102, was also calculated along with their hypothetical topological isomers [Ba(μ2-X2)(X1)]2 (denoted 22*, 42* and 72*-102*), where X1 and X2 are chosen among N(SiMe3)2, OB{CH(SiMe3)2}2, OSi(SiMe3)

42 and 72 -

 72 102 are in good agreement with their experimental X-ray analogues. In particular, the Ba•••H-C and Ba•••C(π) secondary interactions are well reproduced in the optimised structures. Consistently with the fact they were not observed experimentally, the topological isomers of 22*, 42*, 72*, 82*, 92*, and 102* were found systematically less stable by 17.5, 13.1, 24.2, 5.5, 10.8 and 7.1 kcal/mol, respectively. Therefore, the DFT simulations reproduce the empirical trend displayed in Figure8. As shown above, the nature of the Ba-O or Ba-N bonds in these complexes is largely ionic in character. It follows that the O and N atomic charge on the "free ligand" is likely to be an important parameter in the prediction of the bonding strength and mode: the more negative the charge, the stronger the bond, but also the more favoured the bridging position over the terminal one. A negatively charged atom is indeed expected to interact in a stronger way with two metal cations, as compared with a single one, provided the metal-ligand distances do not differ importantly. The absolute values of the computed N and O natural atomic charges, and the potential bridging preference with them, are increasing in the order:[O(2,6-Ph2-C6H3)] -(-0.72) < [OB(Tripp)2] -(-0.97) < [OB{CH(SiMe3)2}] -(-1.07) < [OSi(SiMe3)3] -(-1.29) < [N(SiMe3)2] -(-1.75).Obviously, this trend does not follow that given in Figure8; it is only consistent with the configurations of [Ba{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2(42) and [Ba{μ2-OSi(SiMe3)3}(OB{CH(SiMe3)2}2)]2(82). Although it is only marginal, one may then wonder about the potential ability of covalent bonding in these complexes. The ligand donating abilities are related to the energies and localisations of their lone-pair orbitals. Whereas a terminal ligand needs a single donating orbital (or a combination thereof) of -type, a bridging ligand needs to use two donating orbitals, one of -type and one of π-type. The more available the π-type orbital, the more privileged the bridging position over the terminal one. It turns out that all the investigated ligands have their HOMO containing the π-type lone pair, as illustrated in their Kohn-Sham MO diagrams (FigureS57). All of them have their -type lone pair lying much lower, except for [N(SiMe3)2] -in which the two donating orbitals are close in energy. From their π-type HOMO energies, the bridging preference increases as: [OB{CH(SiMe3)2}] -(-1.77 eV)  [OB(Tripp)2}] -(-1.70 eV) < [OSi(SiMe3)3] -(-1.55 eV) < [N(SiMe3)2] -(-1.37 eV) < [O(2,6-Ph2-C6H3)] -(-1.27 eV). Again, and as expected owing to the weak covalent character in these complexes, no clear agreement with the empirical trend in Figure 8 is seen. From these considerations, it follows that the secondary interactions play an important role in the isomer preference. Unsurprisingly, the Ba•••C(π) bonding in 72, 92 and 102 plays a key role in favouring the bridging position of the [OB(Tripp)2}] -and [O(2,6-Ph2-C6H3)] -ligands. The corresponding optimised Ba•••C distances of 3.0-3.3 Å are only moderately larger than that in BaCp2 (3.0-3.1 Ǻ)

  particularly telling: whereas the siloxide [OSi(SiMe3)3] -is less negatively polarised than [N(SiMe3)2] -, it still prefers the bridging position between the two Ba atoms. The optimised geometries of 22 and its topological isomer [Ba{μ2-N(SiMe3)2}{ OSi(SiMe3)3}]2 (22*, FigureS55) differ in their compactness, for 22 is rather spherical, while 22* is much more oblong. Consistently, the dispersion energy computed 10.1002/chem.202101687

  The experimentallyestablished propensity to occupy bridging positions, as opposed to terminal ones, in heteroleptic dimers,increases with [OB{CH(SiMe3)2}2)] -< [N(SiMe3)2] -< [OSi(SiMe3)3] -< [O(2,6-Ph2-C6H3)] -< [OB(2,4,6-i Pr3-C6H2)2] -.In the solid state, most of these complexes display secondary Ba•••H-C, Ba•••F and Ba•••C(π) secondary interactions which not only enhances profoundly their stability as demonstrated

  combinations of multiple Ca•••H-C, Ca•••F and Ca•••C(π)secondary interactions within sterically congested calcium complexes supported by functionalised aminoether-fluoroalkoxide effectively

  the compounds discussed herein, which exhibit a range of anagostic (and numerous) Ae•••H, covalent Ae•••C(π) and/or ionic Ae•••F interactions.

  ••H-C interactions, we were interested in obtaining other low-coordinate Baboryloxides to assess the role of secondary interactions towards the stabilisation of electrophilic barium complexes. Our initial attempts started off with (mes)2BOH, but it failed to react with [Ba{N(SiMe3)2}2]2

	or [Ba{N(SiMe3)2}2.(thf)2]

Table 1 .

 1 Multinuclear NMR data for complexes 12-102. Data for 12-42 are taken from references[27-29].

	Complex		Solvent	19 F NMR	11 B NMR	29 Si NMR
				(ppm)	(ppm)	(ppm)
	[Ba2{μ2-OSi(SiMe3)3}3{OSi(SiMe3)3}]	12	benzene-d6	-	-	-19.37
						-27.02
	[Ba{μ2-OSi(SiMe3)3}{N(SiMe3)2}]2	22	benzene-d6	-	-	-18.60
						-19.22
						-19.27
						-24.69
						-27.74
	[Ba(OB{CH(SiMe3)2}2)2]	3	benzene-d6	-	45.8	-3.44
	[Ba{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2	42	benzene-d6	-	43.7	-3.47
	[Ba{μ2-OB(Tripp)2}{OB(Tripp)2}]2	52	thf-d8	-	47.1	-
	[Ba{μ2-OB(Fmes)2}{OB(Fmes)2}]2	62	thf-d8	-56.58	34.1	-
				-63.98
	[Ba{μ2-OB(Tripp)2}{N(SiMe3)2}]2	72	benzene-d6	-	47.5	-18.57
	[Ba{μ2-OSi(SiMe3)3}(OB{CH(SiMe3)2}2)]2	82	benzene-d6	-	43.7	-3.38
						-19.86
						-27.84
	[Ba{μ2-OB(Tripp)2}{OSi(SiMe3)3}]2	92	thf-d8	-	not detected	-20.67
						-36.67

  fact that one of the terminal boryloxides (at Ba2) is tilted towards the core of the dimer, while the other (at Ba1) is pointing away from it. Hence, Ba1 rests in a trigonal planar geometry (Σθ(Ba1) = 357.54(19)°), while the geometry around Ba2 is pyramidalised (Σθ(Ba2) = 314.62(19)°). Overall, the geometrical arrangement in 82 appears to be intermediate between those in 22 (a Cs-symmetric dimer with 3 bridging siloxides and a single terminal one,

	Chemistry -A European Journal	10.1002/chem.202101687
		Accepted Manuscript

the

thf, 3.(thf)2 and 62.thf.

  •••F, Ba•••C(π) and even Ba•••H-C secondary interactions towards the stability of these complexes; these non-covalent interactions are broken upon coordination of Lewis bases, as in the solvated 12.

Table 2 .

 2 Summary of solid-state structural data for barium siloxides, boryloxides and phenoxides 12-102.

	Complex

  four complexes is the largely different B-O-Ae angles: while they are nearly perfectly linear for the calcium and strontium complexes 122 and 132, they are much less obtuse for the magnesium and barium derivatives 112 (157.40(3)° and 158.30(3)°) and 42 (160.00(15°).

	Chemistry -A European Journal						10.1002/chem.202101687
	Table 3. Summary of structural data across [Ae{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (Ae = Mg, Ca, Sr, Ba).
	Ae	Mg (112)		Ca (122)		Sr (132)		Ba (42) [27]
	Symmetry	C1 (pseudo-C2)	C1 (pseudo-C2)	C1		Ci
		Mg1	Mg2	Ca1	Ca2	Sr1	Sr2	Ba1 and Ba1 i
	Ae-N (Å) Ae-O (Å) B-O (Å) N-Ae-N' (°) N-Ae-O (°) Σθ(Ae) B-O-Ae (°) dO-plane (Å) a dB-plane (Å) b Ae-N-Ae' (°)	2.107(4) 2.117(4) 1.857(3) 1.341(5) 91.87(15) 131.46(16) 130.92(16) 136.22(4) 2.122(4) 2.4399(11) 2.4471(11) 2.634(2) 2.132(4) 2.4467(11) 2.4499(11) 2.648(2) 1.863(3) 2.1209(9) 2.1228(10) 2.2727(18) 2.2751(18) 2.3999(14) 2.601(2) 2.7210(16) 2.635(2) 2.8912(17) 1.351(5) 1.3353(17) 1.3376(18) 1.351(3) 1.346(3) 1.332(3) 91.05(14) 87.44(4) 87.21(4) 84.59(7) 85.49(7) 82.34(5) 135.47(4) 135.97(7) 132.46(7) 121.64(5) 136.67(15) 137.88(15) 136.30(4) 137.32(4) 139.43(6) 141.84(6) 144.18(5) 360.00(27) 359.84(26) 359.96(11) 360.00(7) 359.99(11) 359.79(11) 348.16(9) 157.4(3) 158.3(3) 178.65(9) 178.92(10) 178.90(16) 173.51(19) 160.00(14) 0.0493(30) 0.1389(30) 0.0342(9) 0.0368(9) 0.0529(18) 0.1647(18) 0.9776(14) 0.0384(49) 0.1263(49) 0.0268(15) 0.0481(15) 0.0655(29) 0.1487(29) 1.0758(22) 88.50(14)-88.51(14) 92.55(4)-92.79(4) 94.71(7)-95.19(7) 97.66(5)	Accepted Manuscript

  C2 axis parallel to the Ae-Ae' vector. b C2 axis parallel to the Nb-Nb' vector. c Distance of Nt to the (AeNb)2 mean plane.

						10.1002/chem.202101687
	Ae		Mg	Ca	Sr	Ba
	Symmetry	C2 a	C1	C2 b	Ci
	Ae-Nb (Å)	2.128 [0.025] 2.152	2.454 [0.045] 2.4786	2.609 [0.040] 2.635	2.803 [0.042] 2.822
	Ae-Nt (Å)	1.967 [0.038] 1.976	2.274 [0.097] 2.274	2.416 [0.080] 2.433	2.552 [0.088] 2.577
	Nb-Ae-Nb' (°)	95.7 95.9	88.6 89.2	84.3 83.9	82.7 81.9
	Nb-Ae-Nt (°)	132.1 132.2	134.6 134.8	135.6 136.8	134.1 134.8
	Σθ(Ae) (°)	360.0 360.0	357.8 358.8	355.6 357.5	350.8 351.5
	Σθ(Nt) (°)		360.0 360.0	359.7 359.7	359.8 359.9	360.0 360.0
	DNt-plane (Å) c	0.000 0.000	0.556 0.444	0.634 0.478	0.949 0.915
	Ae-Nb-Ae' (°)	84.3 84.1	91.3 90.7	95.7 96.1	97.3 98.1
	Atomic	Ae	1.88 3s 0.10 3p 0.01 3d 0.01	1.82 4s 0.08 4p 0.01 3d 0.09	1.84 5s 0.07 5p 0.01 4d 0.07	1.87 6s 0.07 6p 0.01 5d 0.07
	charges	Nb	-2.03	-1.93	-1.94	-1.92
		Nt	-1.97	-1.88	-1.88	-1.88

a

Table 5 .

 5 Selected computed data for the series [Ae{μ2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2 (Ae = Mg, Ca, Sr, Ba, 112-132 and 42). Wiberg bond indices are given into brackets.

  Distance of O to the (AeN)2 mean plane. b Distance of B to the (AeN)2 mean plane.

	Chemistry -A European Journal			10.1002/chem.202101687
	Ae Symmetry Ae-N (Å) Ae-O (Å) B-O (Å) N-Ae-N' (°) N-Ae-O (°) Σθ(Ae) (°) B-O-Ae (°) dO-plane (Å) a dB-plane (Å) b Ae-N-Ae' (°) Atomic charges Ae N O B	Mg (112) Pseudo-Ci 2.103 [0.029] 2.110 [0.030] 1.858 [0.042] 1.343 [1.022] 91.8 130.2 138.0 360.0 154.5 0.029 0.176 88.2 1.88 3s 0.11 3p 0.01 3d 0.01 -2.03 -1.31 1.18	Ca (122) Ci 2.403 [0.048] 2.516 [0.034] 2.117 [0.082] 1.336 [1.059] 84.9 126.9 148.0 359.8 174.1 0.126 0.145 95.1 1.85 4s 0.08 4p 0.01 3d 0.09 -1.94 -1.27 1.16	Sr (132) Ci 2.557 [0.040] 2.670 [0.027] 2.260 [0.065] 1.332 [1.065] 84.3 125.4 145.1 354.8 168.7 0.626 0.750 95.7 1.87 5s 0.08 5p 0.01 4d 0.07 -1.94 -1.28 1.16	Ba (42) Ci 2.717 [0.044] 2.851 [0.031] 2.391 [0.070] 1.330 [1.054] 83.2 121.9 138.7 343.7 158.2 1.173 1.351 96.8 1.88 6s 0.07 6p 0.01 5d 0.07 -1.93 -1.28 1.18	Accepted Manuscript
				3, O-2,6-Ph2-C6H3 and OB(Tripp)2 and
	X1 ≠ X2. The corresponding optimised geometries are given in Figures S55-56; those for complexes 22,

a

Acknowledgements

The authors thank the ANR for funding (ANR-17-CE07-0003-01) and for the provision of a PhD studentship to J. H. The GENCI (Grand Equipment National de Calcul Intensif) is acknowledged for HPC resources (Project A0050807367).

Entry for the Table of Contents

Barium and related alkaline-earth complexes bearing combinations of boryloxides, siloxides, phenoxides and amides have been structurally authenticated. Their structural patterns and bonding properties have been thoroughly investigated by DFT calculations. 10.1002/chem.202101687

Accepted Manuscript

Chemistry -A European Journal

Conflicts of interest

There are no conflicts of interest to declare.

Author Contributions

Notes and references

Accepted Manuscript

Chemistry -A European Journal


A L Londero

J Minella

F Schneider

D Deuschle

O Evrard

M Boeni

G Merten

JPG ; Minella

FJA ; Schneider

; Deuschle

; Menezes

D ; Evrard

O ; Boeni

; Merten

Quantifying the impact of no-till on

Keywords: hydrological monitoring, soil management, soil conservation, curve number, 42 infiltration, terraces, water losses. 43 44 45 1. Introduction Hydrological Processes

published or not. The documents may come   L'archive ouverte pluridisciplinaire

90 founded justifications at the end of the 1990s [START_REF] Caviglione | 674 Espaçamento entre terraços em plantio direto[END_REF]. The abandonment of 91 terracing occurred due to changes in the technological standards of grain production in Brazil 92 (larger agricultural properties, genetic engineering, and developments in the agricultural 93 machinery and equipment sector), erroneously implying this technique would be unnecessary 94 and reduce the efficiency of agricultural operations. Allied to this, the excessive use of 95 agricultural machinery in conditions that were often inadequate significantly increased the 96 problems of soil compaction, including reduced soil water infiltration and storage (Alaoui, 97 Rogger, Peth, & Blöschl, 2018, NRCS, 2004). 98 Estimating runoff is pivotal in soil and environment conservation practices, such as 99 using the Curve Number method (CN) [START_REF] Hawkins | Curve number hydrology: State 723 of the practice[END_REF]100 Mishra, Tyagi, Singh, & Singh, 2006). Runoff volume is estimated from rainfall volume and 101 CN values, which must be obtained from a set of monitored events in which rainfall and runoff 102 are considered. Despite the importance and versatility of the CN method, few studies have been 103 conducted in Brazil (Oliveira et al., 2016;[START_REF] Pruski | Comparação entre dois métodos para a 775 determinação do volume de escoamento superficial[END_REF]Tedela et al., 2013;104 Do Valle Junior, Rodrigues, & de Oliveira, 2019) to estimate CN values under Brazilian 105 conditions, and the correct parameterization of the CN is essential for validating the method for 106 application under different conditions. The surface and subsurface conditions of the NT carried 107 out in Brazil are very particular that must be adapted in hydrological and erosive modeling 108 [START_REF] Huang | Runoff and sediment responses to conservation 726 practices: Loess plateau of china[END_REF][START_REF] Polyakov | Long-term 772 runoff and sediment yields from small semiarid watersheds in southern Arizona[END_REF]. 109

The analysis of hydrological responses as a function of soil use and management can be 110 performed by comparing hydrographs in paired catchments (Rinaldo, Vogel, Rigon, and 111 Rodriguez-Iturbe, 1995;[START_REF] Nadal-Romero | Rainfall, run-off, and sediment transport 757 dynamics in a humid mountain badland area: Long-term results from a small catchment[END_REF][START_REF] Williams | Soil and water conservation in the Pacific Northwest 820 through no -tillage and intensified crop rotations[END_REF]112 Solyom and Tucker;[START_REF] Rose | An introduction to the environmental physics of soil, water, and watersheds[END_REF]. In NT, geomorphology control is crucial since erosive processes 113 are concentrated in the thalweg, and there is a great demand for information on the effects of 114 different conservationist management practices on controlling runoff at catchment scale. This 115 data is essential for choosing the best set of practices, considering farmers' interests and need 116 to protect water resources. In this context, this study aimed to evaluate the hydrological 117 variables obtained through the hydrological monitoring of catchments and paired slopes under 118 NT to evidence the effects of the farming practices on runoff. The objective was to generate 119 information from hyetographs and hydrographs capable of quantitatively demonstrating the 120 efficiency of the amount of biomass, chiseling, and terracing in reducing runoff. 121 122 123 2. Materials and Methods 124 125

The study is based on the monitoring of rainfall and liquid discharge representing the 126 volume of runoff in two zero-order catchments and four paired macroplots. These monitoring 127 units represent different surface conditions in NT. The information extracted from the events 128 was used to characterize the runoff in each of the studied conditions. Monitoring was performed 129 between 2014 and 2018 in southern Brazil (29°13'39"S, 53°40'38"W) in a region belonging to 130 the Southern Plateau, which was formed from successive volcanic spills with the formation of 131 extrusive volcanic rocks (Figure 1) and characterized by a smooth wavy relief and 5-13% 132 slopes. The region has a Cfa climate (humid subtropical) with hot and humid summers and dry 133 and cold winters, according to the Köppen classification (Alvares, Stape, Sentelhas, De Moraes 134 Gonçalves, & Sparovek, 2013). The soil has >30% of clay content and is classified as Rhodic 135 Nitisol (Dystric) [START_REF] Wrb | World Reference Base for Soil Resources 2014, update 2015. International soil 823 classification system for naming soils and creating legends for soil maps[END_REF]. The average annual rainfall is 1678 mm, evenly distributed 136 throughout the year. 137 138 Figure 1. Location of the experimental area and six monitoring units. 139 140

The six monitoring units were placed side by side on a slope with similar soil and relief 141 characteristics and representative of the physiographic conditions of the region. A previous 142 physical and chemical soil characterization of the slope demonstrated the homogeneity of the 143 area [START_REF] Deuschle | Erosion and 684 hydrological response in no-tillage subjected to crop rotation intensification in southern 685 Brazil[END_REF]Londero et al., 2016). A detailed topographic survey and analysis 144 of topographic attributes of the slope, slope shape, accumulated runoff, and moisture content of 145 the terrain were then performed using the digital elevation model (DEM). These indices were 146 used to define the monitoring units. Different sizes, positions, and directions of the units were 147 tested to seek the maximum similarity. Homogeneity was considered within the units and 148 between them. 149

Two different scales were used considering the area of contribution and form. One of 150 them was the hydrographic catchment with the largest contribution area and converging 151 curvature in the plane and a length slope of ~200m to evaluate the runoff control management 152 (broad-based terraces). The other scale explored was the slope scale with the smallest 153 contribution area with rectilinear curvature in the plane and length slope of ~80m to assess the 154 influence of soil and crop management. 155 156 157 2.1 Zero-order catchments and macroplots 158 159

The two zero-order catchments are paired and have similar soil and relief characteristics. 160 The catchments were named non-terraced catchment (NTC) and terraced catchment (TC) with 161 2.43 and 2.35 ha, respectively, with approximately 200 m in length, converging curvature in 162 the plane, and convex in the profile favoring the concentration and spread of runoff. Small 163 ridges (40 cm high) were built to delimitate the experimental units and prevent runoff from 164 coming in from outside of the catchment. Five medium-level infiltration terraces were built at 165 the TC. The terraces were distributed and dimensioned according to the estimated runoff 166 volume for a design rainfall of 110 mm based on a ten-year return period (RP) for 6 h of 167 duration. The basic infiltration velocity is measured in concentric rings in the order of 20 mm 168 h -1 . The terraces are 40-70 cm high, have ~2 m 2 cross-sectional areas, and were built with their 169 ends closed to prevent water from leaving the system and enable total runoff quantification 170 (Figure 2). 171 172 Figure 2. The terraces controlling the runoff after a rainfall event. 173

258

Rainfall characterization is important to define the range of magnitude and patterns 259 that the set of monitored events provided. The information obtained from the runoff data 260 and its relationship with rainfall is essential to define the proportion of the rainfall that is 261 converted into runoff, which has a high potential for degradation. The information related 262 to lag times reflects the effects of the roughness generated by the different managements 263 applied in runoff propagation time. The effects are due to terracing in the catchments and 264 land surface roughness imposed by the amount of phytomass and/or chiseling in the 265 macroplots. The results of each variable were analyzed using the paired t-test to compare 266 the means of each monitoring unit. The mean of the variables of each unit is compared by 267 considering that each event can be paired with the observations in the other unit. Statistical 268 significance was expressed as a p-value, followed by a Student's t-distribution under the 269 null hypothesis that there is no difference between treatments [START_REF] Casella | Statistical inference[END_REF]. 270 271 b) Total apparent infiltration 272 273

The concept of apparent infiltration rate (I ap , mm h -1 ) [START_REF] Rose | An introduction to the environmental physics of soil, water, and watersheds[END_REF] was used to 274 demonstrate the infiltration pattern as a function of rainfall intensity (mm h -1 ). Analysis of all 275 instantaneous values led to Eq. 1, where I ap is calculated by the difference between the 276 instantaneous values of rainfall intensity (R, mm h -1 ) and runoff (Q, mm h -1 ). 277

ap i i I R Q   (1) 278 279
The analysis was performed with the paired values of R and I ap for all events to 280 characterize each monitoring unit The CN model (NRCS, 2004) is a widely used method in hydrological models to 288 estimate runoff generated in a rainfall event as a function of the rainfall (P), initial abstractions 289 (I a ), and the estimate of the maximum water storage capacity of the soil (S). The value of S is 290 estimated according to an empirical parameter called CN. This dimensionless parameter varies 291 between 0 and 100 and represents the soil characteristics regarding permeability, use, and soil 292 management. The CN parameter was empirically derived from a network of experimental 293 catchments in the USA to design conservation practices [START_REF] Hawkins | Curve number hydrology: State 723 of the practice[END_REF]. In this study, S 294 and CN values were estimated for each event and the different monitoring units, considering 295 the monitored values of rainfall (mm) and runoff (mm) using Eqs. 2 and 3: 296

297 (2)   2 5 2 4 5 S P Q Q PQ           298 299 (3)   25400 254 CN S   300 301 302 3. Results 303 304
The El Niño climate phenomenon occurred from 2014 to 2015 and generated above-305 average rainfall, increasing high-magnitude rainfall events. In 2016, the La Niña climate 306 phenomenon reduced the number of monitored events (NOAA, 2018). This time series favored 307 the analysis of the hydrological behavior in the monitoring units in contrasting conditions (e.g., 308 high soil moisture and low soil coverage). Moreover, runoff occurred in events of all 309 magnitudes, from small rainfall events (14 mm) to the largest one (160 mm). The boxplot of 310 Figure 4 shows the median (middle line), first and quartile (box), maximum and minimum 311 values (whiskers), and extreme values (points) of the R monitored at three different durations 312 (10, 30, and 60 min). The 30-min rainfall intensity (R 30 ) reached the average magnitude of 25 313 mm h -1 and several events with values >50 mm h -1 . 314 315 Figure 4. Rainfall intensity for 10, 30, and 60-min intervals. 316 317

The main characteristics of the rainfall to represent its magnitude are shown in Table 318 3. Six events (9%) had a RP of rainfall between 1 and 10 years, five events (8%) between 11 319 and 70 years, and one event (1.6%) exceeded the 100-year RP. 320 321 Table 3. Major rainfall events classified by decreasing return periods. 322 323 324 3.1 Zero-order catchments 325 326 a) Hydrological characteristics 327

The runoff in the study period, considering the 63 monitored events, is described in 328 Figure 5. The terraces (TC) significantly reduced runoff (p-value = 0.038). The terraces 329 overflowed in some events, especially in 2015, although there was a significant reduction in 330 runoff volume during these events. On average, the terraces reduced runoff by 56%, despite the 331 reduction tending to be greater in smaller events. It should be noted that 2015 was the year with 332 the highest number of high magnitude events (1413 mm of total monitored rainfall), where the 333 total runoff was 19% in the NTC and 10% in the TC. Furthermore, there was 781 mm of rainfall 334 monitored in 2017, which provided 13% of runoff in the NTC and 6% in the TC. 335 336 Figure 5. Rainfall and total runoff volume in the NTC and TC during rainfall events. 337 380 15% (p-value = 0.0391). The mean TLPC values were similar between NTC and TC, with no 381 significant differences between them. 382 383 b) Apparent infiltration 384 385

The apparent total infiltration (I ap ) was estimated using data pairs (I ap and R) and 386 considering 30-min intervals (Figure 9). The distribution generates different regions in the 387 graphs: a) R 30 < 25 mm h -1 , b) 25 < R 30 < 65 mm h -1 , and R 30 > 65 mm h -1 . In the first section, 388 I ap ≈ R and practically all the rainfall infiltrates. In the intermediate section, there is high data 389 dispersion due to the influence of soil moisture in the infiltration process (i.e., infiltration may 390 be higher or lower depending on the initial soil moisture in similar R values). In the final section, 391 there is a tendency to form an asymptote that expresses the maximum possible infiltration. 392 393 Figure 9. Total apparent infiltration model for 30-min rainfall intensities at the NTC and TC. 394 395

Equations 4 and 5 describe the apparent total infiltration model of each catchment. The 396 values of 95 and 154 mm.h -1 , in the NTC and TC, respectively, mathematically express the 397 asymptote of the curve and physically express the maximum infiltration limit when the 398 maximum area is saturated. 399

𝐼 𝑎𝑝 = 95 * [ 1 -𝑒𝑥𝑝 ( -𝑅 95 )] (4) 𝐼 𝑎𝑝 = 154 * [ 1 -𝑒𝑥𝑝 ( -𝑅

)]

(5) 400 401 c) Determination of the CN parameter 402 403

The CN values determined in both catchments for all events are shown in Figure 10. The 404 average CN values for the NTC and TC were 69 and 59, respectively, clearly showing the 405 effects of terracing. The figure also shows the analysis of CN behavior as a function of rainfall. 406 Moreover, there is a clear difference in the distribution of points between both catchments. 407 408 Figure 10. CN values as a function of the rainfall that occurred during the monitoring period. 409 410

The points are close to the curve that defines the CN 0 value at the TC, while the points 411 have greater dispersion and are farther from the CN 0 curve at the NTC, showing the inefficiency 412 of the system in controlling runoff. The CN 0 curve represents the minimum runoff value for a 413 given rainfall event (Soil Conservation Service Engineering Division, 1972)

  . The model obtained (Iap = f (R, I m ) shows the infiltration 281 rate behavior as a function of the rainfall intensity and an estimate of the maximum infiltration 282 rate of the unit (I m ). The model obtained in each unit and I m values were compared to determine 283 the influence of the controlling factors on infiltration in each unit. 284 285 c) Curve number method (CN-SCS) 286 287

Figure 1 .

 1 Figure 1. Location of the experimental area and six monitoring units.

Figure 2 .

 2 Figure 2. The terraces controlling the runoff after a rainfall event.

  Figure 3. H-flume for monitoring the flow rates installed in the lower part of the catchments and macroplots.

Figure 4 .

 4 Figure 4. Rainfall intensity for 10, 30, and 60-min intervals.

Figure 5 .

 5 Figure 5. Rainfall and total runoff volume in the NTC and TC during rainfall events.

Figure 6 .Figure 7 .

 67 Figure 6. Values of runoff coefficient (RC) in the NTC and TC.

Figure 8 .Figure 9 .Figure 10 .Figure 11 .

 891011 Figure 8. The behavior of three Lag Time variables (TLC, TLP, TLPC, and TLR) for the set of events monitored at NTC and TC.

Figure 12 .Figure 13 .Figure 14 .Figure 15 .Figure 16 .Figure 17 .

 121314151617 Figure 12. The difference of Qpeak on the events monitored in the four macroplots.

Table 1 .

 1 Soil, plant and water managements (treatments) used in each catchment and macroplot.

	.

Table 2 .

 2 Characteristic variables obtained in the analysis of hyetograms and hydrograms.

	Abbreviation	Description	Unit
	P	Rainfall	mm
	R 10 , R 30 e R 60	Maximum rainfall intensity at 10, 30 and 60 minute intervals	mm h -1
	RP	Return period	years
	RC	Runoff coefficient	%
	Q	Runoff	mm
	Q peak	Peak flow	L s -1
	TLP	Time between the beginning of rainfall and peak flow	min
	TLPC	Time between the hyetogram centroid and the peak flow	min
	TLC	Time between the hyetogram centroid and the hydrogram centroid	min
	TLR	Time between the beginning of rainfall and start of runoff	min
	T C	Time of concentration	min

Table 3 .

 3 Major monitored rainfall events classified by decreasing return periods.

	Date	P (mm)	TDP (min)	RP (years)
	05/13/2017	105	102	115.38
	02/10/2018	93	86	67.43
	10/08/2015	160	774	43.66
	11/19/2015	73	66	19.82
	10/18/2016	111	306	16.07
	03/15/2018	82	121	11.22
	01/27/2018	61	69	4.34
	06/07/2017	116	1012	2.1
	12/23/2015	79	230	2.05
	11/03/2014	51	56	1.86
	08/13/2017	80	294	1.35
	10/30/2014	69	192	1.04
	Where: P is the total accumulated rainfall; TDP is the rainfall total duration; and
	RP is the return period.

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