Synthesis and Structures of Lead(II) Complexes with Substituted Derivatives of the Closo-Decaborate Anion with a Pendant N3 Group

In this work, we studied lead(II) and cobalt(II) complexation of derivatives [2-B10H9O(CH2)2O(CH2)2N3]2− and [2-B10H9O(CH2)5N3]2− of the closo-decaborate anion containing pendant azido groups in the presence of 1,10-phenanthroline and 2,2′-bipyridyl. Mononuclear [PbL2{An}] and binuclear [Pb2L4(NO3)2{An}] lead complexes (where {An} is the N3-substituted boron cluster) were isolated and studied by IR spectroscopy and elemental analysis. The mononuclear lead(II) complex [Pb(phen)2[B10H9O(CH2)2O(CH2)2N3] and the binuclear lead(II) complex [Pb2(phen)4(NO3)2[B10H9O(CH2)5)N3] were determined by single-crystal X-ray diffraction. In complex [Pb2(phen)4(NO3)2[B10H9O(CH2)5)N3], the boron cluster is coordinated by the metal atom only via the 3c2e MHB bonds. In complex [Pb(phen)2[B10H9O(CH2)2O(CH2)2N3], the coordination environment of the metal includes BH groups of the boron cluster and the oxygen atom of the exo-polyhedral substituent. When the reaction was performed in a CH3CN/water mixture, the binuclear lead(II) complex [(Pb(bipy)NO3)(Pb(bipy)2NO3)(B10H9O(CH2)2O(CH2)2N3)]·CH3CN·H2O was isolated, where the boron cluster acts as a bridging ligand between lead atoms coordinated by the boron cage via the O atoms of the substituent and/or the BH groups. In the course of cobalt(II) complexation, the starting compound (Ph4P)2[B10H9O(CH2)5N3] was isolated and its structure was also determined by X-ray diffraction. Although a number of lead(II) complexes with coordinated N3 are known from the literature, no complexes with the boron cluster coordinated by the pendant N3 group involved in the metal coordination have been isolated.

Derivatives of higher boron anions [B n H n ] 2− (n = 10, 12) with cyclic substituents of the oxonium type are easily modified, affording closo-borates with pendant functional groups [29][30][31][32][33][34][35][36][37].These reactions proceed under the action of a wide range of nucleophilic reagents and are accompanied by the opening of a cyclic substituent.Functional groups attached through the alkoxy spacer chain and isolated in this way can enter into further functionalization reactions, including fragments of biologically active molecules [32,38,39].Boron cluster anion derivatives containing a pendant azido group are of particular interest as objects for direct application in click chemistry [40][41][42][43].
Anions [B n H n ] 2− (n = 10, 12) and their derivatives are currently of considerable interest in the synthesis of complex compounds.To date, quite a lot of examples of their application both as counterions [44][45][46] and as polydentate and polytope ligands [47,48] have been described.Closo-borates with pendant functional groups have great potential in complexation due to the presence of several coordination centers: a Pearson soft boron cluster, an alkoxy spacer group, and an introduced pendant fragment.Varying the structure of the spacer (both by length and the presence of donor atoms of various types) and the pendant fragment opens up great prospects in the synthesis of metal coordination compounds with different Pearson hardness/softness levels, as well as complexes with the desired structures and properties.For example, the lead(II) complexation with hydroxycloso-decaborates with different types of binding of the OH group to the boron cluster leads to mono-, binuclear, and polymeric complexes [49].Research in this area is of interest both from the point of view of fundamental science and from the point of view of practice, for example, for the synthesis of functional boron-containing materials [50,51].
It is known that the azide ion is able to be coordinated by lead(II) in various ways; in particular, it can act as a tridentate (µ 1,1,1 -N 3 or µ 1,1,3 -N 3 ) and a bidentate bridging ligand (µ 1,1 -N 3 or µ 1,3 -N 3 ) [52].The obtained derivatives can be used in the synthesis of organometallic coordination polymers; therefore, studies in this field of lead(II) coordination chemistry are topical.
The aim of this work is to synthesize and study the closo-decaborate anion derivatives containing pendant azido groups in lead(II) and cobalt(II) complexation in the presence of azaheterocyclic ligands, namely 1,10-phenanthroline and 2,2 ′ -bipyridyl.

Results and Discussion
Derivatives of the closo-decaborate anion containing a pendant 2)) were obtained by multistage synthesis starting from the [B 10 H 10 ] 2− anion (Scheme 1), as reported in [53].
Anions [BnHn] 2− (n = 10, 12) and their derivatives are currently of considerable interest in the synthesis of complex compounds.To date, quite a lot of examples of their application both as counterions [44][45][46] and as polydentate and polytope ligands [47,48] have been described.Closo-borates with pendant functional groups have great potential in complexation due to the presence of several coordination centers: a Pearson soft boron cluster, an alkoxy spacer group, and an introduced pendant fragment.Varying the structure of the spacer (both by length and the presence of donor atoms of various types) and the pendant fragment opens up great prospects in the synthesis of metal coordination compounds with different Pearson hardness/softness levels, as well as complexes with the desired structures and properties.For example, the lead(II) complexation with hydroxy-closo-decaborates with different types of binding of the OH group to the boron cluster leads to mono-, binuclear, and polymeric complexes [49].Research in this area is of interest both from the point of view of fundamental science and from the point of view of practice, for example, for the synthesis of functional boron-containing materials [50,51].
It is known that the azide ion is able to be coordinated by lead(II) in various ways; in particular, it can act as a tridentate (µ1,1,1-N3 or µ1,1,3-N3) and a bidentate bridging ligand (µ1,1-N3 or µ1,3-N3) [52].The obtained derivatives can be used in the synthesis of organometallic coordination polymers; therefore, studies in this field of lead(II) coordination chemistry are topical.
The aim of this work is to synthesize and study the closo-decaborate anion derivatives containing pendant azido groups in lead(II) and cobalt(II) complexation in the presence of azaheterocyclic ligands, namely 1,10-phenanthroline and 2,2′-bipyridyl.
the formation of closo-decaborates containing pendant azido groups were monitored using 11 B and 11 B{ 1 H} NMR spectroscopy.For example, in the 11 B{ 1 H} NMR spectrum of the closo-decaborate anion, there were only two signals with an integral intensity ratio of 1:4 from two apical and eight equatorial boron atoms, respectively.In the 11 B{ 1 H} spectrum of the product of the reaction with 1,4-dioxane and salt K[B 10 H 9 O(CH 2 ) 4 O], there was one signal at 7.7 ppm from the ipso-boron atom, two signals at 0.8 and 6.4 ppm assigned to two chemically non-equivalent apical vertices, two signals at −21.5 and −23.4 ppm corresponding to two pairs of equatorial boron atoms adjacent to the ipso position, and one signal at −29.9 ppm assigned to three boron atoms opposite the substituted position.Only one signal at 7.7 ppm did not split into a doublet in the 11 B NMR spectrum.This spectral pattern makes it possible to unambiguously determine the type of compounds obtained.The 11 B{ 1 H} NMR spectrum of the product of the reaction of the 1,4-dioxane derivative with the [B 10 H 10 ] 2− anion with sodium azide retained the general form characteristic of closo-decaborates with a substituent in the equatorial belt; however, it exhibited a significant rearrangement of signals from boron atoms when compared with the spectrum of the initial derivative of the closo-decaborate anion.In particular, the signal from the ipso boron atom at 7.7 ppm shifted towards the strong field up to −1.9 ppm due to a change in the type of the substituent atom from oxonium to alkoxy and a decrease in the polarity of the B-O bond.The signals from two nonequivalent apical vertices of the polyhedron approached each other in the spectrum of the reaction product and changed their positions from −0.8 and −6.4 ppm to −3.8 and −4.8 ppm, respectively.These characteristic changes in the spectra of the products were associated with changes in the valence and charge of the oxonium oxygen atom.The structure of the exo-polyhedral substituent was determined by 1 H NMR and IR spectroscopies.
The obtained compounds 1 and 2 can be considered organic azides with a framework substituent in the form of a boron cluster.It should also be noted that the obtained compounds are very stable in light and air, and in solutions up to a temperature of 100 • C they do not undergo transformations in the azido group.In terms of thermal and kinetic stability, they can be compared with aryl and adamantylazide, which are known coordination compounds with transition metals.In these complexes, the organoazide group shows various coordination possibilities.The organoazide can be monodentate coordinated; this type is characteristic of copper, silver, cobalt, and palladium complexes [54][55][56][57][58]. µ 1,1 -N 3 bridging coordination is known for a heterometallic zirconium-iridium complex [59].η 2 -Coordination is observed in the case of some nickel complexes [60].Quite a large group is formed by complexes with µ 1,1,1 -N 3 coordination of the azido moiety; this type of binding is characteristic of tantalum, vanadium, tungsten, and titanium ions [61][62][63][64][65][66].Often, in complexes of this type, the azide moiety is spontaneously or by thermal effect converted to an imido species by extrusion of the N 2 molecule.
Substituted derivatives with a pendant N 3 group were subsequently used as ligands in lead(II) complexation reactions in the presence of organic ligands phen and bipy.Solid lead(II) nitrate was added to a solution of salt ( ). Organic ligands bipy and phen were added to the reaction solution (in a metal-to-ligand ratio of 1:2) to stabilize the lead(II) complexes formed.The complexation was carried out according to Scheme 2.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 16  6)).The IR spectra of the obtained compounds 3-6 contain a broad, intense band of stretching vibrations ν(BH) near 2400 cm −1 assigned to uncoordinated BH bonds and a band ν(BH) MHB , which is observed as a shoulder in the region of 2300-2200 cm −1 .The presence of bands in the region 2300-2200 cm −1 indicates the coordination of the boron cage via the MHB bond.A broad band of the stretching vibrations ν(NO) is observed in the IR spectra of complexes 5 and 6.The spectra of all compounds contain bands in the region 1700-600 cm −1 , which indicate the presence of coordinated bipy and phen molecules.
We succeeded in isolating single crystals of complexes 4 and 6•3CH 3 CN, which were suitable for further X-ray diffraction studies.
The crystallographically independent part of the triclinic unit cell (P-1) of complex 4 includes the binuclear complex [Pb(phen act as bridging ligands; each anion is coordinated to one lead atom through the oxygen atom O1(O3) and the B6B9 edge (B16B19) and to the second metal atom along the B9B10 edge (B19B20), forming two pairs of threecenter two-electron Pb-HBBH bonds.Thus, the coordination environment of each lead atom includes four nitrogen atoms from two phenanthroline molecules, an oxygen atom of the exo-polyhedral substituent, and two edges, B6B9 and B9B10, from two anions (Figure 1).The Pb-O bond lengths are 2.443(7) and 2.562(7) Å, the Pb-B bond lengths fall in the range 3.358(12)-3.593(11)Å, and the Pb-H bond lengths are 3.044(2)-3.7752(14) Å (Table 1).The exo-polyhedral substituent of one of the anions is disordered with a population of 0.65:0.35,with large thermal vibrations of the azide group.6)).The IR spectra of the obtained compounds 3-6 contain a broad, intense band of stretching vibrations ν(BH) near 2400 cm −1 assigned to uncoordinated BH bonds and a band ν(BH)MHB, which is observed as a shoulder in the region of 2300-2200 cm −1 .The presence of bands in the region 2300-2200 cm −1 indicates the coordination of the boron cage via the MHB bond.A broad band of the stretching vibrations ν(NO) is observed in the IR spectra of complexes 5 and 6.The spectra of all compounds contain bands in the region 1700-600 cm −1 , which indicate the presence of coordinated bipy and phen molecules.
We succeeded in isolating single crystals of complexes 4 and 6·3CH3CN, which were suitable for further X-ray diffraction studies.
The crystallographically independent part of the triclinic unit cell (P-1) of complex 4 includes the binuclear complex [Pb(phen)2[µ-B10H9O(C2H4)O(C2H4)N3]]2 (Figure 1).Anions [B10H9O(C2H4)O(C2H4)N3] 2− act as bridging ligands; each anion is coordinated to one lead atom through the oxygen atom O1(O3) and the B6B9 edge (B16B19) and to the second metal atom along the B9B10 edge (B19B20), forming two pairs of three-center two-electron Pb-HBBH bonds.Thus, the coordination environment of each lead atom includes four nitrogen atoms from two phenanthroline molecules, an oxygen atom of the exo-polyhedral substituent, and two edges, B6B9 and B9B10, from two anions (Figure 1).The Pb-O bond lengths are 2.443(7) and 2.562(7) Å, the Pb-B bond lengths fall in the range 3.358(12)-3.593(11)Å, and the Pb-H bond lengths are 3.044(2)-3.7752(14) Å (Table 1).The exo-polyhedral substituent of one of the anions is disordered with a population of 0.65:0.35,with large thermal vibrations of the azide group.Almost all phenanthroline ligands are slightly distorted from a perfectly flat state (RSMD ranges from 0.038 to 0.066 Å).The angles between the PbNN and phen planes lie in the range of 153.9 • -167.3 • , which is associated with the formation of intra-and intermolecular π-π stacking interactions between the ligands.Molecules connected in this way form 1D polymer chains (Figure 2), which are interconnected through the CH. ..HB contacts.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 16 Almost all phenanthroline ligands are slightly distorted from a perfectly flat state (RSMD ranges from 0.038 to 0.066 Å).The angles between the PbNN and phen planes lie in the range of 153.9°-167.3°,which is associated with the formation of intra-and intermolecular π-π stacking interactions between the ligands.Molecules connected in this way form 1D polymer chains (Figure 2), which are interconnected through the CH…HB contacts.The crystallographically independent part of the triclinic unit cell (P-1) of complex 6•3CH3CN contains the binuclear complex [(Pb2(Phen)4(NO3))2[µ-B10H9O(CH2)5N3]], in which the [B10H9O(CH2)5N3] 2− anion acts as a bridging ligand, and there are three solvate molecules of acetonitrile (Figure 3).The coordination environment of each lead atom includes two phen molecules, one nitrate ion, and one B5B8B9 and B3B6B7 face forming the environment of the Pb1 and Pb2 atoms, respectively.The Pb-B bond lengths in complex The crystallographically independent part of the triclinic unit cell (P-1) of complex 6•3CH 3 CN contains the binuclear complex [(Pb 2 (Phen) 4 (NO 3 )) 2 [µ-B 10 H 9 O(CH 2 ) 5 N 3 ]], in which the [B 10 H 9 O(CH 2 ) 5 N 3 ] 2− anion acts as a bridging ligand, and there are three solvate molecules of acetonitrile (Figure 3).The coordination environment of each lead atom includes two phen molecules, one nitrate ion, and one B5B8B9 and B3B6B7 face forming the environment of the Pb1 and Pb2 atoms, respectively.The Pb-B bond lengths in complex 6•3CH 3 CN are significantly shorter than those for complex 4, on average by 0.23 Å (Table 2).The average value of Pb-H contacts in complex 6 is shorter by 0.5 Å.The exo-polyhedral substituent in this case does not participate in coordination and is bonded to neighboring complexes only through CH. ..O and CH. ..N contacts.The angles between the PbNN and phen planes lie in the range of 163.8 • -178.4 • , which is also associated with the formation of π-π stacking interactions between phenanthroline ligands.Solvent molecules are located in channels of square cross-sections parallel to axis a (Figure 4).

6•3CH3CN
are significantly shorter than those for complex 4, on average by 0.23 Å (Table 2).The average value of Pb-H contacts in complex 6 is shorter by 0.5 Å.The exo-polyhedral substituent in this case does not participate in coordination and is bonded to neighboring complexes only through CH…O and CH…N contacts.The angles between the PbNN and phen planes lie in the range of 163.8°-178.4°,which is also associated with the formation of π-π stacking interactions between phenanthroline ligands.Solvent molecules are located in channels of square cross-sections parallel to axis a (Figure 4).6•3CH3CN are significantly shorter than those for complex 4, on average by 0.23 Å (Table 2).The average value of Pb-H contacts in complex 6 is shorter by 0.5 Å.The exo-polyhedral substituent in this case does not participate in coordination and is bonded to neighboring complexes only through CH…O and CH…N contacts.The angles between the PbNN and phen planes lie in the range of 163.8°-178.4°,which is also associated with the formation of π-π stacking interactions between phenanthroline ligands.Solvent molecules are located in channels of square cross-sections parallel to axis a (Figure 4).It is interesting that when we used the water/acetonitrile mixture to perform the lead complexation reaction (instead of CH3CN, used to prepare complexes 3-6), we succeeded in isolating complex 7 for starting compound 1 (see Scheme 3).
The IR spectrum of complex 7 is similar to related complex 4; no significant changes could be found.However, the elemental analysis data indicated a 3:2 ratio of M:L instead of the 4:2 ratio found for complexes 3-6.
The IR spectrum of complex 7 is similar to related complex 4; no significant changes could be found.However, the elemental analysis data indicated a 3:2 ratio of M:L instead of the 4:2 ratio found for complexes 3-6.
Complex 7, similar to compound 4, forms 1D polymer units due to π-π stacking interactions between bipy molecules of neighboring complexes.The units are interconnected through CH. ..HB contacts (Figure 6), forming channels parallel to axis c, which are filled with disordered solvent molecules.Complex 7, similar to compound 4, forms 1D polymer units due to π-π stacking interactions between bipy molecules of neighboring complexes.The units are interconnected through CH…HB contacts (Figure 6), forming channels parallel to axis c, which are filled with disordered solvent molecules.Complex 7, similar to compound 4, forms 1D polymer units due to π-π stacking interactions between bipy molecules of neighboring complexes.The units are interconnected through CH…HB contacts (Figure 6), forming channels parallel to axis c, which are filled with disordered solvent molecules.In spite of the ability of lead(II) to form compounds with the N 3 group coordinated by different methods [52], it seems that the softer boron cluster is preferable to form the coordination environment of the metal atom as compared to the azido group.We concluded that the N 3 group is too hard to be coordinated by lead(II) in the presence of BH groups.
Therefore, we tried to study cobalt(II) complexation because this metal is harder compared to lead(II).A number of cobalt(II) complexes with organoazides coordinated are known [57]; therefore, the formation of cobalt(II) complexes with boron clusters coordinated via the azide functional group could be expected.The experiments performed for compound 2 in the case of using Co(II) as a complexing agent revealed that the closodecaborates with a pendant N 3 functional group act as extremely weakly coordinating ligands in the absence of chelating ligands.In an attempt to obtain a Co(II) complex, when the closo-decaborate anion containing an azide pendent group was allowed to react with CoCl 2 in the CH 3 CN/H 2 O system, the initial derivative of the closo-decaborate anion (compound 2) was formed as a solid phase, the structure of which was determined by X-ray diffraction (Figure 7).According to the X-ray diffraction data, the structure of compound 2 consists of two Ph 4 P + cations and the [2-B 10 H 9 O(CH 2 ) 5 N 3 ] 2− anion (Figure 7).The IR and 11 B NMR data correlate with those reported [53].by different methods [52], it seems that the softer boron cluster is preferable to form the coordination environment of the metal atom as compared to the azido group.We concluded that the N3 group is too hard to be coordinated by lead(II) in the presence of BH groups.
Therefore, we tried to study cobalt(II) complexation because this metal is harder compared to lead(II).A number of cobalt(II) complexes with organoazides coordinated are known [57]; therefore, the formation of cobalt(II) complexes with boron clusters coordinated via the azide functional group could be expected.The experiments performed for compound 2 in the case of using Co(II) as a complexing agent revealed that the closo-decaborates with a pendant N3 functional group act as extremely weakly coordinating ligands in the absence of chelating ligands.In an attempt to obtain a Co(II) complex, when the closo-decaborate anion containing an azide pendent group was allowed to react with CoCl2 in the CH3CN/H2O system, the initial derivative of the closo-decaborate anion (compound 2) was formed as a solid phase, the structure of which was determined by X-ray diffraction (Figure 7).According to the X-ray diffraction data, the structure of compound 2 consists of two Ph4P + cations and the [2-B10H9O(CH2)5N3] 2− anion (Figure 7).The IR and 11 B NMR data correlate with those reported [53].At the same time, when organic ligands L (bipy or phen) were used as additional ligands in the same reaction and the reaction was performed in CH3CN, heteroleptic cobalt(II) complexes [Co(bipy)2Cl2] or [Co(phen)2Cl2] were isolated, according to the singlecrystal diffraction data [57,68].In both cases, the starting compound (compound 2) remained unreacted.It could be concluded that cobalt(II) seems to be too hard to form compounds with coordinated boron clusters and their derivatives.The reactions proceeded according to Scheme 4. At the same time, when organic ligands L (bipy or phen) were used as additional ligands in the same reaction and the reaction was performed in CH 3 CN, heteroleptic cobalt(II) complexes [Co(bipy) 2 Cl 2 ] or [Co(phen) 2 Cl 2 ] were isolated, according to the single-crystal diffraction data [57,68].In both cases, the starting compound (compound 2) remained unreacted.It could be concluded that cobalt(II) seems to be too hard to form compounds with coordinated boron clusters and their derivatives.The reactions proceeded according to Scheme 4.
(  7) Pb(NO 3 ) 2 (0.2 mmol) was dissolved in water (5 mL) and the obtained solution was added to a solution of (Ph 4 P) 2 [An] (2) (0.2 mmol) in acetonitrile (10 mL).A solution of ligand (phen) (0.4 mmol) in acetonitrile (10 mL) was added to the resulting solution.The reaction mixtures of pale-yellow color were kept in air.The formation of yellow crystals was observed within 24 h.Yield, 57%.Single crystal 7•CH 3 CN•H 2 O suitable for X-ray diffraction study was taken directly from the reaction solution.

Figure 1 .
Figure 1.Structure of complex 4 (left) and coordination environment of lead atoms in the complex (right).

Figure 1 .
Figure 1.Structure of complex 4 (left) and coordination environment of lead atoms in the complex (right).

Table 1 .
Selected bond lengths in the structure of 1.

Table 1 .
Selected bond lengths in the structure of 1.

Table 2 .
Selected bond lengths in the structure of complex 6•3CH 3 CN.