Protonated Organic Diamines as Templates for Layered and Microporous Structures: Synthesis, Crystal Chemistry, and Structural Trends among the Compounds Formed in Aqueous Systems Transition Metal Halide or Nitrate–Diamine–Selenious Acid

Systematic studies of crystalline compounds formed in aqueous systems containing aliphatic diamines, divalent transition metal halides, and selenious acid resulted in the discovery of a large family of new complex species corresponding to several new structure types. With ethylenediamine (en), layered (enH2)[M(HSeO3)2X2] compounds are the most commonly formed species which constitute a significant contribution to the family of layered hydrogen selenites containing neutral [M(HSeO3)2] (M = Mg, Mn, Co, Ni, Cu, Zn, Cd) 2D building blocks. In contrast to some previous suggestions, piperazine (pip), as well as its homologue N-methylpiperazine, mostly give rise to quite different, sometimes more complex, structures of varied dimensionality while the (pipH2)[M(HSeO3)2X2] compounds are formed only with M = Cu and Cd. In addition, metal-, halide-, or selenium-free by-product species are observed. The SeIV can be present in a multitude of forms, including H2SeO3, HSeO3−, SeO32−, and Se2O52−, reflecting amazing adaptability to the shape of the templating cations.


Introduction
The use of "lone-pair" cations and halide or nitrate anions is well known to lead to numerous open-framework, porous, and non-centrosymmetric structures [1][2][3][4][5]; addition of magnetically active cations of dand f -elements frequently gives rise to unusual architectures and magnetic responses.Yet, most attention is dedicated to fully deprotonated acid residues, e.g., selenites, tellurites, or iodates, while the acid salts and other hydrogenbonded networks have as yet received less attention, although at least some of these species exhibit several attractive properties [6][7][8].
Among halides, ammonium compounds are complete structural analogs of the rubidiumand cesium-containing species [13,16], while among nitrates, they contribute to the compound with the most complex interlayer architecture, (NH 4 NO 3 ) 3 [Cu(HSeO 3 ) 2 ] [9], which was tentatively attributed to the ability of the NH 4 + cations to form strong directional hydrogen bonds to the nitrate anions.This suggests that some other species, particularly organic ammonium cations, may also be incorporated into the interlayer space.This was initially demonstrated by the preparation of cadmium-containing compound (enH 2 )[Cd(HSeO 3 ) 2 Cl 2 ] (enH 2 2+ = ethylenediammonium cation, H 3 NCH 2 CH 2 NH 3 2+ ) [21]; later, isostructural cobalt and copper-based compounds were reported [19].The layer topology in their structure is essentially the same as that in [M(H The existence of a related piperazinium compound was also noted in [21]; however, no data have been reported nor possible analogs mentioned.Yet, ethylenediammonium and piperazinium cations are characterized by nearly the same distances between the ammonium hydrogen bond donor centers, though the number of "active" N-H bonds is different (6 vs. 4).In the current study, we performed an extensive search for possible analogs among hydrogen selenite-halides of diprotonated ethylenediamine (enH 2 2+ ), piperazine (pipH 2 2+ ), N,N -dimethlethylenediamine (dmedaH 2 2+ ), and N-methylpiperazine (mpipH 2 2+ ) species (Figure 1) and transition metals reported earlier to form the [M II (HSeO 3 ) 2 ] slabs, namely Co, Cu, Zn, and Cd.In addition, test experiments were also performed with magnesium, manganese, and nickel halides.
l. Sci.2023, 24,14202 2 of 20 [17][18][19][20], and cadmium [21] containing species were reported.The crystal chemistry of this family is relatively diverse and the overwhelming majority of structure types and their representatives are the copper compounds.The structures of these compounds contain quasi-planar [M II (HSeO3)2] 0 layers formed by Cu 2+ cations and hydrogen-bonded dimers of HSeO3 − anions forming a distorted quasi-square grid.The interlayer space can be either empty, as in Cu(HSeO3)2 [22], filled with water molecules, as in Cu(HSeO3)2•2H2O [9], various metal-halide [11][12][13][15][16][17]20] and metal-nitrate [14] slabs, or more complex species such as [M II (H2O)4][Cu(HSeO3)2Cl2] series (M II = Mn-Zn, Fe excepted) [10,18] and [Co(H2O)4][Co(HSeO3)2Cl2] [20].It is noteworthy that the bromide analogs of the two latter compounds, as well as alkali-cobalt selenite chlorides [17], have not been reported, while within the (A I X)[Cu(HSeO3)2] series (A I = Na, K, Rb, Cs, NH4) [13], as well as (RbX)[Zn(HSeO3)2] [16], bromides are full structural analogs of the corresponding chlorides.Several nitrate-containing copper compounds are also known [9,16].Among halides, ammonium compounds are complete structural analogs of the rubidium-and cesium-containing species [13,16], while among nitrates, they contribute to the compound with the most complex interlayer architecture, (NH4NO3)3[Cu(HSeO3)2] [9], which was tentatively attributed to the ability of the NH4 + cations to form strong directional hydrogen bonds to the nitrate anions.This suggests that some other species, particularly organic ammonium cations, may also be incorporated into the interlayer space.This was initially demonstrated by the preparation of cadmium-containing compound (enH2)[Cd(HSeO3)2Cl2] (enH2 2+ = ethylenediammonium cation, H3NCH2CH2NH3 2+ ) [21]; later, isostructural cobalt and copper-based compounds were reported [19].The layer topology in their structure is essentially the same as that in [M(H2O The existence of a related piperazinium compound was also noted in [21]; however, no data have been reported nor possible analogs mentioned.Yet, ethylenediammonium and piperazinium cations are characterized by nearly the same distances between the ammonium hydrogen bond donor centers, though the number of "active" N-H bonds is different (6 vs. 4).In the current study, we performed an extensive search for possible analogs among hydrogen selenite-halides of diprotonated ethylenediamine (enH2 2+ ), piperazine (pipH2 2+ ), N,N-dimethlethylenediamine (dmedaH2 2+ ), and N-methylpiperazine (mpipH2 2+ ) species (Figure 1) and transition metals reported earlier to form the [M II (HSeO3)2] slabs, namely Co, Cu, Zn, and Cd.In addition, test experiments were also performed with magnesium, manganese, and nickel halides.The outcomes of the solution syntheses depended mostly on the nature of the organic species used.The majority of the new species belong to the targeted hydroselenite family; however, representatives of other, more compositionally simple families were also observed.These can be roughly collected into the following groups: (i) the "layered hydroselenites" containing all elements involved; (ii) framework selenite-diselenites with no incorporated halide or nitrate; (iii) ion-molecular crystals of ethylenediammonium salts and selenious acid which do not contain the divalent metal; and (iv) halometallates The outcomes of the solution syntheses depended mostly on the nature of the organic species used.The majority of the new species belong to the targeted hydroselenite family; however, representatives of other, more compositionally simple families were also observed.These can be roughly collected into the following groups: (i) the "layered hydroselenites" containing all elements involved; (ii) framework selenite-diselenites with no incorporated halide or nitrate; (iii) ion-molecular crystals of ethylenediammonium salts and selenious acid which do not contain the divalent metal; and (iv) halometallates of organic cations which do not incorporate selenium.

(enH 2 )[M(HSeO 3 ) 2 X 2 ] (X = Cl and Br)
Representatives of this structure type have been found among compounds of Co, Cu, Zn, and Cd, that is, for all M 2+ cations where the formation of [M(HSeO 3 ) 2 ] layers had been reported (vide supra); the new contributors to this series are Mn 2+ , Ni 2+ , and probably Mg 2+ (Table 1).Similar to (AX)[M(HSeO 3 ) 2 ] (M = Cu, Zn) and in contrast to [M (H 2 O) 4 ][M(HSeO 3 ) 2 ], both chlorides and bromides were found to exist.Bromides have not been observed yet for Mg 2+ and Ni 2+ ; most likely these compounds exist but are less stable and more sensitive to the preparation conditions.Numerous attempts to prepare acceptable quality crystals for the elusive (enH 2 )[Mg(HSeO 3 ) 2 Cl 2 ] have not been successful; the cell metrics and positions of non-hydrogen atoms correspond to the same arrangement.
of organic cations which do not incorporate selenium.

(enH2)[M(HSeO3)2X2] (X = Cl and Br)
Representatives of this structure type have been found among compounds of Co, Cu, Zn, and Cd, that is, for all M 2+ cations where the formation of [M(HSeO3)2] layers had been reported (vide supra); the new contributors to this series are Mn 2+ , Ni 2+ , and probably Mg 2+ (Table 1).Similar to (AX)[M(HSeO3)2] (M = Cu, Zn) and in contrast to [M(H2O)4][M(HSeO3)2], both chlorides and bromides were found to exist.Bromides have not been observed yet for Mg 2+ and Ni 2+ ; most likely these compounds exist but are less stable and more sensitive to the preparation conditions.Numerous attempts to prepare acceptable quality crystals for the elusive (enH2)[Mg(HSeO3)2Cl2] have not been successful; the cell metrics and positions of non-hydrogen atoms correspond to the same arrangement.
In the structures of (enH2)[M(HSeO3)2X2] (M = Cd, Co, Cu, Mn, Zn; X = Cl, Br), the metal cations center slightly distorted trans-MO4X2 octahedra (Figure 2).The bond valence sums on M 2+ cites agree well with the oxidation state of 2. The distortion indices for the octahedra (Table 2) were calculated using the Vesta suite [23].The largest anisotropy in the bond lengths is observed among the Cu and Zn compounds.The difference between the apical and equatorial bonds in [CuO 4 Br 2 ] is as much as 0.871Å (distortion index = 0.168).In the most symmetrical [CdO 4 Cl 2 ] octahedron, this difference drops to 0.293 Å (distortion index = 0.055).The strong distortion of the Cu and Zn polyhedral can be explained considering the low CFSE values and the Jahn-Teller effect (first order for the former and second order for the latter).Another reason for the distortion of the MO 4 X 2 octahedra is extrinsic and caused by hydrogen bonds, both to oxygen and halogen vertices, from the (enH 2 ) 2+ or (pipH 2 ) 2+ templates, where the size of halogen also matters.The volumes of the MO 4 X 2 octahedra vary from 14.27 Å 3 (CuO 4 Cl 2 ) to 19.06 Å 3 (CdO 4 Br 2 ).The M-X bond distances agree well with the reported values, e.g., Cd-Br distances of 2.7198(3) Å are only slightly shorter than those in the CdBr 6 octahedra in CdBr 2 (2.785(4) Å); the same applies to the Cd-Cl bond distances in (enH 2 )[Cd(HSeO 3 ) 2 Cl 2 ] (2.5910(7) Å) and CdCl 2 (2.637(4) Å).Selenium forms the expected SeO 3 ψ-tetrahedron with two shorter (1.67-1.69Å) and one longer (1.75-1.77Å) bond to the OH group (Figure 2) as expected for the HSeO 3 − anion.A typical crystal structure of the (enH 2 )[M(HSeO 3 ) 2 X 2 ] (M = Cd, Co, Cu, Mn, Zn, Ni; X = Cl, Br) is shown in Figure 3a,b.The M 2+ cations reside in centers of MO 4 X 2 octahedra formed by the non-protonated oxygen atoms of the four HSeO 3 − anions; these are very similar to those in the corresponding chloride compound (2.2881(17) and 2.3082(18) Å, respectively [21]).Each of the protonated nitrogen atoms of the enH 2 2+ cation forms hydrogen bonds: two to the bromide anion and one to the oxygen atoms of two HSeO 3 − species (Figure 3c).The halide anions' hydrogen bonds are weakest (3.196(5)-3.435(3)Å).Therefore, the enH 2 2+ cation forms the maximal number (6) of possible hydrogen bonds.Very similar environments are also observed in the compounds of manganese, cobalt, nickel, copper, and zinc.
In a single case, a crystal with quite different metrics was picked, which was found to belong to a new compound (pipH 2 )[Cd(HSeO 3 ) 2 Cl 2 ]•2H 2 O (Table 3) with a related composition but a totally different structure (Figure 5).In this case, two symmetry-independent Cd 2+ cations reside in trans-CdO 2 Cl 4 octahedra (Figure 5a) which share common Cl•••Cl edges to form chains (Figure 5b).The HSeO 3 − anions do not form pairs; they act as hydrogen bond acceptors from the pipH 2 2+ cations (Figure 5c).The [CdCl 2 (HSeO 3 ) 2 ] 2− chains are also involved in hydrogen bonding with water molecules.
The newcomer to this family is Mn 2+ .Overall, combinations of (protonated) SeO 3 2− and Se 2 O 5 2− are rather common in inorganic frameworks based on various s-, d-, and f -metals [25][26][27][28][29][30].Their templating by various organic species is very likely to result in a variety of complex and elegant architectures.In the meantime, such species were not observed with the branched mpipH 2 2+ cation.A likely reason for this is the same as in the previous case: either the branched structure or the shape of the organic moiety.
The newcomer to this family is Mn 2+ .Overall, combinations of (protonated) SeO3 2− and Se2O5 2− are rather common in inorganic frameworks based on various s-, d-, and fmetals [25][26][27][28][29][30].Their templating by various organic species is very likely to result in a variety of complex and elegant architectures.In the meantime, such species were not observed with the branched mpipH2 2+ cation.A likely reason for this is the same as in the previous case: either the branched structure or the shape of the organic moiety.
In the (pipH2)[M(HSeO3)(Se2O5)]2 (M = Co, Mn), the divalent cations center nearly regular MO6 octahedra (Figure 6a), with a mean distance of 2.1039(18) and 2.181(4) Å, respectively.Three symmetry-independent Se atoms contribute to the Se1Se2O5 2− and HSe3O3 − anions (Figure 6b).The latter associate into chains aligned along a via.The Se1O3 and Se2O3 polyhedra share vertices with the MO6 octahedra to form layers aligned in parallel (Figure 6c).Overall, a porous framework is formed (Figure 6d) with cavities filled by the piperazinium cations.The latter are for hydrogen bonds to the oxygen atoms of the framework.The channel size, estimated as the distance between the opposite oxygen atoms, is 4.13 × 5.58 Å.In the (pipH 2 )[M(HSeO 3 )(Se 2 O 5 )] 2 (M = Co, Mn), the divalent cations center nearly regular MO 6 octahedra (Figure 6a), with a mean distance of 2.1039(18) and 2.181(4) Å, respectively.Three symmetry-independent Se atoms contribute to the Se1Se2O 5 2− and HSe3O 3 − anions (Figure 6b).The latter associate into chains aligned along a via.The Se1O 3 and Se2O 3 polyhedra share vertices with the MO 6 octahedra to form layers aligned in parallel (Figure 6c).Overall, a porous framework is formed (Figure 6d) with cavities filled by the piperazinium cations.The latter are for hydrogen bonds to the oxygen atoms of the framework.The channel size, estimated as the distance between the opposite oxygen atoms, is 4.13 × 5.58 Å.

Ion-Molecular Crystals and Halometallates
Our previous studies have demonstrated that careful inspection of various crystals formed in selenite-containing systems may lead to discovery of unusual architectures, including first representatives of new intriguing families [31][32][33][34].Therefore, all good quality crystals, including colorless (i.e., those evidently not containing Mn 2+ , Co 2+ , Ni 2+ , or Cu 2+ ), were studied (Table 3).Formation of diammonium polyhalometallates was detected in many halide-containing runs, as illustrated by (pipH 2 )ZnCl which contain both tetrahedrally and octahedrally coordinated cadmium [37].The presence of excess acid (selenious of trifluoroacetic) liberates some free hydrohalic acid, which probably contributes to formation of these side products.In the structure of (pipH 2 )[ZnCl 4 ](H 2 O) (Figure 7a), Zn 2+ centers a [ZnCl 4 ] 2− tetrahedron (<Zn-Cl> = 2.2763 Å) connected to pip by hydrogen bonds (Figure 7b).The Cl atoms accept two hydrogen bonds (2.856(4) and 2.375(2) Å) from the pipH 2 2+ cation, and one from the water molecule (2.566(3) Å).A close chemical composition was also found for the tetrachlorozincate of mpip (Figure 7c,d); however, due to the different size and shape of the cation, as well as the lower number of hydrogen bonds (three for mpipH 2 2+ vs. four for pipH 2 2+ ), the structure is also different (Figure 7e,f).The mpipH 2 2+ cation has three "active" nitrogen-bound hydrogens, which form two single hydrogen bonds and one bifurcated hydrogen bond accepted by Cl-from two different [ZnCl 4 ] 2− anions and one water molecule.The hydrogen atoms of the latter also form bonds to the chlorozincate anions.
The metal-free structure of (enH2)X2•2H2SeO3 (X = Cl and Br) (Figure 8) is the first "organic analog" of the AX•nH2SeO3 compounds (A is an alkali cation) [33,34], wherein the Se atom is also coordinated to three oxygen atoms, two of which are protonated to The metal-free structure of (enH 2 )X 2 •2H 2 SeO 3 (X = Cl and Br) (Figure 8) is the first "organic analog" of the AX•nH 2 SeO 3 compounds (A is an alkali cation) [33,34], wherein the Se atom is also coordinated to three oxygen atoms, two of which are protonated to form the H 2 SeO 3 molecule (Figure 8a).In the structure of halides (take X = Br for example), these species and the enH 2 2+ cations, as well as Br − , link via hydrogen bonding to form a framework wherein the organic and inorganic parts form sublayers distantly reminiscent of the AX•nH 2 SeO 3 [33,34] (Figure 8a-c).The H 2 SeO 3 molecules also form hydrogen bonds between each other (Figure 8d).
The nitrate derivatives, (enH 2 )(NO 3 ) 2 •2H 2 SeO 3 and (pipH 2 )(NO 3 ) 2 •2H 2 SeO 3 , have close chemical compositions but also different crystal structures (Figure 9).In the former structure (Figure 9a,c), the H 2 SeO 3 species form chains aligned along b.They are decorated by the nitrate groups attached via hydrogen bonds to form ribbons.The enH 2 2+ cations are situated between these.As in the previous case, both architectures can be considered to be pseudo-layered, yet with a different topology; the layers are formed by changed and neutral moieties.Both organic species form hydrogen bonds to oxygens of both NO 3 − and H 2 SeO 3 moieties (Figure 9b,d).
form the H2SeO3 molecule (Figure 8a).In the structure of halides (take X = Br for example), these species and the enH2 2+ cations, as well as Br − , link via hydrogen bonding to form a framework wherein the organic and inorganic parts form sublayers distantly reminiscent of the AX•nH2SeO3 [33,34] (Figure 8a-c).The H2SeO3 molecules also form hydrogen bonds between each other (Figure 8d).The nitrate derivatives, (enH2)(NO3)2•2H2SeO3 and (pipH2)(NO3)2•2H2SeO3, have close chemical compositions but also different crystal structures (Figure 9).In the former structure (Figure 9a,c), the H2SeO3 species form chains aligned along b.They are decorated by the nitrate groups attached via hydrogen bonds to form ribbons.The enH2 2+ cations are situated between these.As in the previous case, both architectures can be considered to be pseudo-layered, yet with a different topology; the layers are formed by changed and neutral moieties.Both organic species form hydrogen bonds to oxygens of both NO3 − and H2SeO3 moieties (Figure 9b,d).

Structural Trends in Organic Hydrogen Selenites
Table S1 contains 24 entries for hydrogen selenites containing organic cations.To determine the topology of mutual positioning of the organic and inorganic parts, we used the approach developed in [38].For each structure, a diagram is constructed where green triangles designate the partially or fully protonated SeO 3 groups, with the OH vertices highlighted in brown, the organic species are shown as blue rectangles, and the water molecules are shown as red ovals.These structures can be divided into three groups: those containing only HSeO 3 − (Figure 10), those also containing H 2 SeO 3 molecules (Figure 11), and those containing crystal water molecules (Figure 12).
In the structures of organic hydrogen selenites (BH) + •HSeO 3 − (where B designates protonated organic matter), one can clearly distinguish the segregation of organic and inorganic species into layer-like areas, irrespective of the size and shape of the former.In the latter, the HSeO 3 − species aggregate into dimers via hydrogen bonding.This trend is visible even in such complex structures as that of the phenylalanine and guanylurea derivatives,    The trend in segregation of the organic and inorganic matter can also be followed in the few structures containing neutral molecules of selenious acid (Figure 11).When both constituents are neutral (e.g., in co-crystals of amino acids), we can consider the result to be a hybrid organo-inorganic composite.A more complex pattern is observed for (enH 2 )(NO 3 ) 2 •2H 2 SeO 3 and (pipH 2 )(NO 3 ) 2 •2H 2 SeO 3 , where there are several hydrogen bond donors and acceptors.
Addition of water molecules to these organo-inorganic architectures results in "delamination" into three types of layers; those formed by water molecules are either sandwiched between the organic matter layers or between those and the selenite groups (Figure 12).Note that the number of these entries is very small; no structure is known containing neutral molecules of both selenious acid and water.
Overall, the structures discussed here can be considered as complex organo-inorganic architectures.The segregation of the organic and inorganic parts can be understood considering that the former species aggregate due to the dominating Van der Waals (hydrophobic) interactions; they interact with the latter only via hydrogen bonding.The donors of "linking" hydrogen bonds are the protonated nitrogen atoms of the organic matter, while the recipients are the inorganic anions, including HSeO 3 − and H 2 SeO 3 ; the latter also interact via hydrogen bonding, but only between each other (Figure 3e), which is likely the cause of their aggregation.The size and shape of the organic molecules do not affect the type of hydrogen bonding.
the approach developed in [38].For each structure, a diagram is constructed where green triangles designate the partially or fully protonated SeO3 groups, with the OH vertices highlighted in brown, the organic species are shown as blue rectangles, and the water molecules are shown as red ovals.These structures can be divided into three groups: those containing only HSeO3 − (Figure 10), those also containing H2SeO3 molecules (Figure 11), and those containing crystal water molecules (Figure 12).S1.S1.S1.S1.
In the structures of organic hydrogen selenites (BH) + •HSeO3 − (where B designates protonated organic matter), one can clearly distinguish the segregation of organic and inorganic species into layer-like areas, irrespective of the size and shape of the former.In the latter, the HSeO3 − species aggregate into dimers via hydrogen bonding.This trend is visible even in such complex structures as that of the phenylalanine and guanylurea deriva- )X 2 (H 2 SeO 3 ) 2 (e), (pipH 2 )(NO 3 ) 2 (H 2 SeO 3 ) 2 (f).The organic species are identified in Table S1.S1.S1.
In the structures of organic hydrogen selenites (BH) + •HSeO3 − (where B designates protonated organic matter), one can clearly distinguish the segregation of organic and inorganic species into layer-like areas, irrespective of the size and shape of the former.In the latter, the HSeO3 − species aggregate into dimers via hydrogen bonding.This trend is visible even in such complex structures as that of the phenylalanine and guanylurea deriva-  S1.

Overall Remarks
The new compounds observed in our studies form several new families containing both known and new structure types, and their diversity and elegance is quite amazing.It is also evident that at least one half of these can be considered to be the first representatives of significantly more numerous groups.Due to almost unlimited diversity of organic hydrogen bond donors with both rigid and flexible backbones and, correspondingly, the topology of hydrogen bond directions, the structural chemistry of their derivatives also seems to be hardly restricted.However, some relationships can be traced and some new families predicted.
The most common structure, previously reported for (enH 2 )[M(HSeO 3 ) 2 Cl 2 ] (M = Cd [21], Cu, and Co [19]) is observed for the majority of metal di-cations studied, including bromides and compounds of zinc.Three newcomers to this family are Mn 2+ , Mg 2+ , and Ni 2+ , although the latter two only with some reluctance.The electronic structure of the metal di-cation is therefore not sufficient; the size of Mg 2+ is relatively close to Zn 2+ or Mn 2+ [55].We may tentatively assume, however, that because its electronic structure is different, the lack of d-orbitals hinders formation of Mg-X bonds even in the presence of a large excess of competing water molecules.Examples are quite abundant when, even in halide-rich media, products of ambient-temperature crystal hydrates mostly contain the initial [Mg(H The non-rigid and relatively small ethylenediammonium cation, which forms as many as six hydrogen bonds, easily fits into the space between the hydroselenite-halide [M(HSeO 3 ) 2 X 2 ] 2− layers.With more voluminous and more rigid piperazinium, the layered structure is observed only for Cu 2+ (which is the commonest contributor to the layered hydroselenite family, vide supra) and the largest Cd 2+ .Even then, the layered hydroselenite structure seems to lie on the stability edge, as exemplified by the formation of the structurally unrelated dihydrate.With smaller Mn 2+ , Co 2+ , or Zn 2+ , only the "microporous" hydroselenite-diselenites are formed; Mg 2+ also does not contribute to this family.This 3D framework is also relatively flexible and can host both linear-chain (CH 2 ) 4 (NH 3 ) 2 2+ and piperazinium cations.Finally, the branched N-methylpiperazinium cation probably has no chance to contribute to any of these, and simpler and unconstrained structures of tetrahalometallates are formed instead; most starting solutions containing this cation produced no crystals at all.In targeted attempts to produce crystals of (BH 2 )(NO 3 ) 2 •2H 2 SeO 3 with B = pip and mpip, the formed solution produced the target crystals within several weeks, while the latter remained viscous.It should be noted, however, that nearly all solutions containing selenious acid become syrupy, and nearly glassy at the last stages of evaporation, which probably hinders crystallization.
The structures of the organic species containing both selenium and organics can thus be divided into three groups: (i) the "layered hydroselenites", containing all constituents, (ii) the halide-free "microporous structures", and (iii) the metal-free "ion-molecular crystals".For the first family, we predict the existence of new members with relatively flexible nonbranched or short-branched backbones containing ammonium groups at the opposite ends which would fit the size of the [M(HSeO 3 ) 2 ] grids, e.g., α,ω-polymethylenediammonium cations, + H 3 N(CH 2 ) n NH 3 + .Note, however, that at least some of these can also contribute to the halide-free structures [26].Formation of double Langmuir-like layers of alkylammonium cations, similar to Cs 2 [Co(HSeO 3 ) 2 Cl 2 ] [17], is also possible.As the Cu-and Co-based compounds exhibit magnetic ordering [19], it may be of interest to see how the intralayer interactions would be affected by varying the interlayer distances by changing the spacer size.Some of these investigations are now in progress.
The second family of structures contains the [M 2 (HSeO 3 ) 2 (Se 2 O 5 ) 2 ] 2− frameworks with a limited inner space, so more voluminous species such as mpipH 2 2+ are probably too large to fit therein; the alternative found in our experiments is the formation of simpler structures of halometallates.Therefore, long chains of larger-cycle diamines (or polyamines) will most likely not be "admitted", including for compounds with branched backbones.Other topologies are known, however, e.g., based on vanadyl cations, where such species act as templates for chiral porous frameworks [60].
The third group, represented as yet by a pair of isostructural compounds, enH 2 X 2 •2H 2 SeO 3 (X = Cl and Br) and the non-analogous nitrate enH 2 (NO 3 ) 2 •2H 2 SeO 3 , also promises to be diverse as the hydrogen-bonded nets of H 2 SeO 3 molecules and are expected to be the very flexible and adaptable.Another example is (pipH 2 )(NO 3 ) 2 •2H 2 SeO 3 .Due to the unlimited variability of the organic species, which need not necessarily be hydrogen bond donors or acceptors (similar to alkali cations), one can expect formation of various structures, including non-centrosymmetric structures, by careful choice of the corresponding templates.Formation of non-centrosymmetric organically templated inorganic selenites has been documented [8,60]; further perspectives in this direction are quite promising.Note that while nitrates and halides can contribute to similar structures when these anions form purely ionic bonds to the cationic species (alkali cations), the analogies disappear when hydrogen bonding is enforced.The very symmetrical halide anions can form hydrogen bonds in all possible directions, while the rigid planar nitrate group has a fixed geometry of hydrogen bond acceptors.
The few structures of selenium-free halometallates observed in our work also fit very well into the structural trends observed in this relatively well-addressed family.The noncentrosymmetric structure of (pipH 2 )[ZnBr 4 ] is a full analog of (pipH  [63]; however, the latter was not structurally characterized.Thus, the family hosts at least four non-centrosymmetric and one complex layer structure.Its future also seems intriguing.

Synthesis
The starting compounds were the organic diamines, selenious acid, and corresponding metal halides or nitrates.Unimolar working solutions of all compounds were prepared first.Next, MX 2 , H 2 SeO 3 , and diamine solutions were mixed together to provide a 1:1:3 molar ratio.Generally, 1 mL of MX 2 solution was mixed with 3 mL of 1M H 2 SeO 3 .Next, 1 mL of the diamine solutions was added dropwise upon stirring with a small glass bar.The 50% excess of the selenious acid was shown in [10,13,21] to be necessary to suppress formation of insoluble metal selenites and competitive formation of halometallates, which easily form if extra hydrohalic acid is added [13].Commonly, precipitation or turbidity due to formation of MSeO 3 •aq was observed after diamine was introduced, which mostly disappeared after stirring and heating to 50-60 • C for 5 to 10 min on a hotplate.Until clearance, several drops of water were added to keep the solution volume constant.With M = Cu or Cd, which form less soluble selenites, the flaky green and white (respectively) precipitates were dissolved after heating and adding 0.1 to 0.7 mL of trifluoroacetic acid in the case of halides and 0.1-0.5 mL of 50% nitric acid for nitrates, according to the protocols developed earlier [13,15].(In one or two cases, traces of CdSeO 3 •aq persisted but dissolved over time when crystals of the targeted compounds started to form.)The halide solutions (colorless in case of M II = Mg, Zn and Cd, initially pink for Mn, violet for Co, green in the case of Ni, and blue in the case of Cu) were left to evaporate at ambient conditions.Crystallization started in a few weeks, after the color of halide-bearing solutions containing Co and Cu turned lilac (Co), green (Cu-Cl), or brownish (Cu-Br); the color of the Cu-and Co-containing crystals was bluish-green and dark violet, respectively.The intermediate change in solution colors likely indicates transient formation of [MX 4 2− ] due to free hydrohalic acid present in solution.No color change was observed for the solutions containing Mn and Ni (as well as in all experiments with nitrates).The Mn-and Ni-bearing crystals were grayish-pink and brownish yellow.The crystals grown in Mg-containing solutions were of relatively low quality.Sometimes, small amounts of red selenium were precipitated due to reduction of selenious acid, which did not affect the formation and growth of the target crystals.The exception was dmeda, which was rapidly oxidized by selenious acid; this compound was therefore excluded from further studies.The crystals were collected and kept in closed vials under a drop of the mother liquor.Further evaporation of the mother liquors led to repeated deposition of crystals with similar color and appearance.The majority of these runs provided the target hydroselenite halides for en, just a few for pip, and none for mpip.In most cases, crystals of some side products were also observed, which were formed either due to the off-stoichiometric reagents ratio of the initial charge, or instead of the targeted compound.Based on the combination of diamine, metal cation, and anion, these side products may contain either all constituents, or only some of these; those exhibiting new compositions and/or structures are discussed in detail.
A related pattern was observed for the nitrate solutions, except that the color of the crystals corresponded to those of the hydrated M 2+ cations.The only exception was observed in the case of Mn 2+ , which produced black needles of low quality; such a color change is indicative of oxidation of Mn 2+ into Mn 3+ or Mn 4+ .This compound will be addressed elsewhere.In these cases, no target compound was observed; crystals of a new compound (enH 2 )(NO 3 ) 2 •2H 2 SeO 3 were produced with all runs with en.With pip and mpip, no crystalline produces were observed for the nitrate runs.
As in the previous studies, in the last stages the liquors commonly became dark and viscous, produced no more crystals, and were discarded.It is worth noting that crystals were the first to form with en, next with pip, and with greater difficulty in the case of mpip; some solutions containing mpip produced no crystals but only viscous residues.
According to the classification suggested above, the produced species were: (i) (enH 2 )[M(HSeO 3 ) 2 X 2 ] (M = Cd, Co, Cu, Mn, Zn, Ni; X = Cl, Br) and (pipH 2 )[M(HSeO 3 ) 2 ].The side products most likely come from the off-stoichiometric ratio of the initial solutions; these were observed in most deposited samples.The three new compounds enH 2 X 2 •2H 2 SeO 3 (X = Cl, Br, and NO 3 ) were prepared in targeted runs by mixing the 1M working solutions of en, HX, and H 2 SeO 3 taken in a 1:2.1:2 volume ratio; large (up to several mm) colorless crystals were produced after several weeks of evaporation at ambient conditions.

Single Crystal X-ray Study
The collected crystals were taken out of the mother liquors, attached to glass fibers, and transferred to a Rigaku XtaLAB Synergy-S diffractometer (Tokyo, Japan) equipped with a PhotonJet-S detector (Tokyo, Japan) operating with MoKα radiation at 50 kV and 1 mA.A single crystal was chosen and more than a hemisphere of data was collected with a frame width of 0.5 • in ω, and 5-15 s was spent counting for each frame.The data were integrated and corrected for absorption applying a multiscan type model using the Rigaku Oxford Diffraction programs CrysAlis Pro (Rigaku OD, 2015) (Tokyo, Japan).The experiments were performed at 150 K.The unit cell parameters were calculated by the least-squares method.The structures were solved by direct methods using WinGX version 2020.1 (Glasgow, UK) [64] and Olex2 version 1.3.0(Regensburg, Germany) [65] software.The final solutions include the coordinates and anisotropic thermal parameters of atoms except for hydrogens.Hydrogen atoms were located using the mathematical part of the Olex2 program.
Experimental data for the (enH 2 )[M(HSeO 3 ) 2 X 2 ] series are collected in Table 1 (except M = Mg, where the crystals were of low quality).All these compounds are isostructural; some trends are discussed below.The corresponding data for the piperazinium compounds are listed in Table 3.In contrast to the alkali-bearing family, in no case did we observe formation of organic-containing layered hydroselenite nitrates; the organic and inorganic matter crystallized separately yielded MSeO 3 •aq (or M(HSeO 3 ) 2 ); and with en and pip, new compounds (BH 2 )(NO 3 ) 2 •2H 2 SeO 3 were observed, which are considered below.The simplest side results are also discussed here, while some others with 3D structures will be reported in separate contributions.

Figure 1 .
Figure 1.Organic templates used in the current study.

Figure 1 .
Figure 1.Organic templates used in the current study.

Figure 8 .
Figure 8. Hydrogen bonding in the structures of enH2X2•2H2SeO3 (X = Cl and Br) (a-c).Projection of the structure onto the ac plane (d).

Figure 8 .
Figure 8. Hydrogen bonding in the structures of enH 2 X 2 •2H 2 SeO 3 (X = Cl and Br) (a-c).Projection of the structure onto the ac plane (d).

Figure 8 .
Figure 8. Hydrogen bonding in the structures of enH2X2•2H2SeO3 (X = Cl and Br) (a-c).Projection of the structure onto the ac plane (d).

Author Contributions:
Conceptualization, methodology, investigation, writing-original draft preparation; writing-review and editing, visualization: D.O.C., E.V.N., D.N.D., V.Y.G., T.A.O., D.V.S. and O.I.S.All authors have read and agreed to the published version of the manuscript.Funding: This research received no external funding.