Nickel(II) and Palladium(II) Complexes with η5:κ1(N)-Coordinated Dicarbollide Ligands Containing Pendant Pyridine Group

A series of C- and B-substituted nido-carborane derivatives with a pendant pyridyl group was prepared. The synthesized compounds were used as ligands in the complexation reactions with bis(triphenylphosphine)nickel(II) and palladium(II) chlorides to give six new metallacomplexes with unusual η5:κ1(N)-coordination of the metal center. The single crystal structures of 1-(NC5H4-2′-S)-1,2-C2B10H11, 1-(NC5H4-2′-CH2S)-1,2-C2B10H11, Cs [7-(NC5H4-2′-CH2S)-7,8-C2B9H11] closo- and nido-carboranes and 3-Ph3P-3-(4(7)-NC5H4-2′-S)-closo-3,1,2-NiC2B9H10 and 3-Ph3P-3-(4(7)-NC5H4-2′-CH2S)-closo-3,1,2-NiC2B9H10 metallacarboranes were determined using single crystal X-ray diffraction.


Introduction
nido-Carborane [nido-7,8-C 2 B 9 H 12 ] − is one of the most important members of the family of polyhedral boron hydrides, located at the junction of inorganic and organic chemistry.nido-Carborane (from Latin nidus, meaning "nest") is formed upon the removal of the boron atom adjacent to the carbon atoms from the icosahedral ortho-carborane 1,2-C 2 B 10 H 12 , which has the closo-structure (a corruption of clovo, from Latin clovis, meaning "cage"), under the action of strong nucleophiles and has an open pentagonal face [1,2].After the removal of endo-hydrogen from nido-carborane, the resulting dicarbollide anion [nido-1,2-C 2 B 9 H 11 ] 2-can act as a ligand similar to a cyclopentadienyl one [3][4][5][6][7][8].It is known that transition metal complexes with cyclopentadienyl ligands containing pendant donor groups are good catalysts for various organic reactions [9][10][11][12][13][14][15][16], and some of them are also promising luminescent materials [17,18].One such pendant substituent is the pyridyl group [19,20].In addition to the purely scientific interest associated with various types of coordination of such ligands [21][22][23][24], it was shown that titanium complexes with the cyclopentadienyl ligand containing the 2-picolyl substituent exhibit high catalytic activity during the ethylene polymerization reaction [25].Therefore, the synthesis of nido-carboranes with a pendant pyridyl group and metallacarboranes based on them is of considerable interest.
In this contribution we describe the synthesis of new nido-carborane derivatives with a pendant pyridine group and nickel(II) and palladium(II) complexes thereof (Figure 1).
In this contribution we describe the synthesis of new nido-carborane derivatives with a pendant pyridine group and nickel(II) and palladium(II) complexes thereof (Figure 1).

Design of Dicarbollide Ligands with Pendant Pyridine Group: General Principles
There are two main points to consider when designing dicarbollide ligands with a pendant pyridyl group.The first of these is the presence and length of a spacer between the carborane cage and the pyridyl ring.This determines the size and stability of the metallocycle formed during coordination.Clearly, in the absence of such a spacer [49], the formation of only a strained four-membered metallocycle is possible, which is unfavorable for most d-metals.Indeed, in structurally characterized iridium complexes with pyridyl substituted nido-carborane [7-(NC5H4-2′)-7,8-C2B9H11] − , the latter is coordinated to the metal atom by the κ 2 -type through the nitrogen atom of the pyridyl group and the BH group of nido-carborane [58], rather than by the η 5 :κ 1 -type.At the same time, in the case of ligands with a monoatomic spacer between the carborane cage and the pyridyl ring, for example, [7-(NC5H4-2′-CH2)-7,8-C2B9H11] − , metal coordination occurs with the formation of a stable five-membered metallocycle according to the η 5 :κ 1 -type [50].As for complexes based on nido-carborane with a diatomic spacer between the carborane cage and the pyridine heterocycle, to the best of our knowledge, there is only one such complex [3,3- which was accidentally obtained by leaving the ortho-carborane complex trans-[Co(κ 2 (N,O)-1-NC5H4-2′-C(O)H-1,2-C2B10H11)2] in an acetone solution for several days or weeks under air [40].
The second point is the position of the substitution in the nido-carborane basket.Unlike the cyclopentadienide ligand, in which all the carbon atoms in the five-membered ring are identical, the pentagonal face of the dicarbollide ligand is formed by two carbon atoms and three boron atoms.In this case, by introducing a substituent at the carbon atom or boron atom, it is possible to vary both the properties of the ligand itself and the properties of the metal complexes based on it.Firstly, this is due to the different electronic

Design of Dicarbollide Ligands with Pendant Pyridine Group: General Principles
There are two main points to consider when designing dicarbollide ligands with a pendant pyridyl group.The first of these is the presence and length of a spacer between the carborane cage and the pyridyl ring.This determines the size and stability of the metallocycle formed during coordination.Clearly, in the absence of such a spacer [49], the formation of only a strained four-membered metallocycle is possible, which is unfavorable for most d-metals.Indeed, in structurally characterized iridium complexes with pyridyl substituted nido-carborane [7-(NC 5 H 4 -2 )-7,8-C 2 B 9 H 11 ] − , the latter is coordinated to the metal atom by the κ 2 -type through the nitrogen atom of the pyridyl group and the BH group of nido-carborane [58], rather than by the η 5 :κ 1 -type.At the same time, in the case of ligands with a monoatomic spacer between the carborane cage and the pyridyl ring, for example, [  ] in an acetone solution for several days or weeks under air [40].
The second point is the position of the substitution in the nido-carborane basket.Unlike the cyclopentadienide ligand, in which all the carbon atoms in the five-membered ring are identical, the pentagonal face of the dicarbollide ligand is formed by two carbon atoms and three boron atoms.In this case, by introducing a substituent at the carbon atom or boron atom, it is possible to vary both the properties of the ligand itself and the properties of the metal complexes based on it.Firstly, this is due to the different electronic effects of the nido-carborane cage substituted at the boron and carbon atoms, which in the case of a short spacer can have a significant effect on the pendant donor group.Another less obvious point that can affect the stability and reactivity of metal complexes is the mutual orientation of the ligands.Due to the non-equivalence of atoms in the pentagonal face of the dicarbollide ligand, they interact differently with the metal atom, which leads to energetic inequality of different conformations due to the rotation of the dicarbollide ligand around the M-B(10) axis.In particular, for complexes of the d 8 metal ions, such as Ni(II), Pd(II), and Pt(II), the preferred orientation is one in which the angle θ between the L-M-L plane and the B(8}-M-Center(C(1)-C(2)) planes is 90 • [59][60][61][62].Another feature is the displacement of the metal atom from the center of the pentagonal face of the ligand towards the boron atoms, the so-called "slippage" of the dicarbollide ligand, which is especially characteristic of nickel(II) bis(dicarbollide) complexes [63,64].The introduction of substituents at the carbon atoms into the dicarbollide ligand can lead to a significant deviation of the angle θ from the ideal value due to steric repulsion between the ligands [65,66], and in extreme cases, this even results in the isomerization of the dicarbollide ligand [67,68].It is clear that the position of attachment of the pendant donor group to the dicarbollide ligand will have a significant effect on the structure, and consequently, the properties of the resulting metal complexes [52,69,70].
effects of the nido-carborane cage substituted at the boron and carbon atoms, which in the case of a short spacer can have a significant effect on the pendant donor group.Another less obvious point that can affect the stability and reactivity of metal complexes is the mutual orientation of the ligands.Due to the non-equivalence of atoms in the pentagonal face of the dicarbollide ligand, they interact differently with the metal atom, which leads to energetic inequality of different conformations due to the rotation of the dicarbollide ligand around the M-B(10) axis.In particular, for complexes of the d 8 metal ions, such as Ni(II), Pd(II), and Pt(II), the preferred orientation is one in which the angle θ between the L-M-L plane and the B(8}-M-Center(C(1)-C(2)) planes is 90° [59][60][61][62].Another feature is the displacement of the metal atom from the center of the pentagonal face of the ligand towards the boron atoms, the so-called "slippage" of the dicarbollide ligand, which is especially characteristic of nickel(II) bis(dicarbollide) complexes [63,64].The introduction of substituents at the carbon atoms into the dicarbollide ligand can lead to a significant deviation of the angle θ from the ideal value due to steric repulsion between the ligands [65,66], and in extreme cases, this even results in the isomerization of the dicarbollide ligand [67,68].It is clear that the position of attachment of the pendant donor group to the dicarbollide ligand will have a significant effect on the structure, and consequently, the properties of the resulting metal complexes [52,69,70].

Scheme 1. Synthesis of Cs
The obtained carboranes were characterized using methods of 1 H, 13 C, and 11 B NMR and IR spectroscopy and mass spectrometry (See Supplementary Materials, Figure S1-S70 and Table S1).The 11 B NMR spectrum of Cs [1] in acetone-d 6 contains a singlet at −3.1 ppm and seven doublets at −6.8, −15.6, −18.4,−22.5, −24.3, −30.6, and −37.8 ppm with an integral intensity ratio of 1:1:1:2:1:1:1:1, which is significantly different from the spectrum of the N-protonated form H[1] [51], indicating a rather strong interaction between the carborane cage and the pyridine ring.The 1 H NMR spectrum of Cs [1], in addition to the signals of the CH and BH groups of the nido-carborane cage, contains a set of signals of the pyridyl group, which appears in the form of two doublets at 8.22 and 7.67 ppm and two triplets at 7.49 and 6.87 ppm.
To obtain a related nido-carborane with a pendant pyridyl group attached to the carbon atom, as a development of the known approach to the arylation and heteroarylation of 1-mercapto-ortho-carborane [71][72][73][74], we used the reaction of the trimethylammonium salt of 1-mercapto-ortho-carborane with 2-bromopyridine.The reaction in refluxing ethanol gave a To obtain a related nido-carborane with a pendant pyridyl group attached to the carbon atom, as a development of the known approach to the arylation and heteroarylation of 1-mercapto-ortho-carborane [71][72][73][74], we used the reaction of the trimethylammonium salt of 1-mercapto-ortho-carborane with 2-bromopyridine.The reaction in refluxing ethanol gave a mixture of the expected pyridyl derivative of ortho-carborane 1-(NC5H4-2′-S)-1,2-C2B10H11 (2) and its deboronation product as the N-protonated intramolecular salt [7-(HNC5H4-2′-S)-7,8-C2B9H11] (H [3]), which were separated using column chromatography on silica followed by the conversion of the latter to the cesium salt Cs[7-(NC5H4-2′-S)-7,8-C2B9H11] (Cs [3]) (Scheme 2).The cesium salt Cs [3] was also obtained via the deboronation of the corresponding ortho-carborane 2 with CsF in refluxing ethanol (Scheme 2).In the 1 H NMR spectrum of Cs[3], the signals of the pyridyl group appear as a doublet at 8.30 ppm, a triplet at 7.70 ppm, a doublet at 7.24 ppm, and a triplet at 7.02 ppm, demonstrating a significant difference in the electronic effects of the nido-carborane cage substituted at the boron and carbon atoms.
The solid-state structure of 1-(NC5H4-2′-S)-1,2-C2B10H11·HBr (2·HBr) (see Supplementary Materials) was determined using single crystal X-ray diffraction (Figure 2).In the 1 H NMR spectrum of Cs [3], the signals of the pyridyl group appear as a doublet at 8.30 ppm, a triplet at 7.70 ppm, a doublet at 7.24 ppm, and a triplet at 7.02 ppm, demonstrating a significant difference in the electronic effects of the nido-carborane cage substituted at the boron and carbon atoms.
The solid-state structure of 1- HBr) (see Supplementary Materials) was determined using single crystal X-ray diffraction (Figure 2).
The reaction of the trimethylammonium salt of 1-mercapto-ortho-carborane with 2bromomethyl pyridine followed by the deboronation of the resulting pyridine-containing ortho-carborane 1-(NC 5 H 4 -2 -CH 2 S)-1,2-C 2 B 10 H 11 (4) was used to prepare the nido-carborane derivative with a longer spacer between the carborane cage and the pendant pyridyl group Cs[7-(NC 5 H 4 -2 -CH 2 S)-7,8-C 2 B 9 H 11 ] (Cs [5]) (Scheme 3).Previously, this approach was used for the synthesis of various alkylsulfenyl derivatives of orthoand nido-carboranes including those containing various functional groups [75][76][77][78][79].The obtained carboranes were characterized using methods of 1 H, 13 C, and 11 B NMR and IR spectroscopy and mass spectrometry (See Supplementary Materials).In the 1 H NMR spectrum of 4 in acetone-d6, the signal of the methylene group appears as a singlet at 4.42 ppm, whereas in the spectrum of Cs [5], the signals of the methylene group appear as two doublets at 4.13 and 3.89 ppm ( 2 JHH = 12.8 Hz) due to chirality of the C-monosubstituted nido-carborane cage that causes protons to become diastereotopiс and magnetically inequivalent.
Int. J. Mol.Sci.2023, 24, x FOR PEER REVIEW 7 of 17 determined using single crystal X-ray diffraction (see Supplementary Materials).A general view of the nickelacarborane molecule is given in Figure 4.The orientation of the σ-donor ligands (the pendant pyridine and triphenylphosphine) with respect to the dicarbollide ligand significantly deviates from the ideal orientation with the θ angle between the N(1)-Ni(1)-P(1) plane and the B(8)-Ni(1)-Center-(C(1)-C(2)) plane being ~ 60°, and the pyridyl group is rotated around the B(9)-S(1) bond toward the carbon atoms of the dicarbollide ligand (Figure 3).No noticeable "slippage" of the dicarbollide ligand was found.
The orientation of the σ-donor ligands (the pendant pyridine and triphenylphosphine) with respect to the dicarbollide ligand strongly deviates from the ideal orientation with the θ angle between the N(1)-Ni(1)-P(1) plane and the B(8)-Ni(1)-Center-(C(1)-C( 2)) plane being ~16 • .The six-membered ring Ni(1)-C(1)-S(1)-C(3)-C(4)-N(1) adopts a highly distorted boat conformation with a nickel atom and a methylene group located at the bow and stern (Figure 4).No noticeable "slippage" of the dicarbollide ligand was found.1) adopts a highly distorted boat conformation with a nickel atom and a methylene group located at the bow and stern (Figure 4).No noticeable "slippage" of the dicarbollide ligand was found.
The cesium salt Cs [2] was also obtained by refluxing 2 (0.14 g, 0.55 mmol) in ethanol (10 mL) with cesium fluoride (0.17 g, 1.11 mmol) for 12 h.The precipitate formed was filtered off and the filtrate was evaporated under reduced pressure.The residue was dissolved in acetone (10 mL) and unreacted CsF was filtered off.The filtrate was evaporated in vacuo to give a white solid of Cs[3] (0.20 g, 97% yield).

General Procedure for Synthesis of Metallacarboranes 6-11
To solution of nido-carborane derivative 2, 4 or 5 in anhydrous THF under argon atmosphere, the 3-fold excess of potassium tert-butoxide was added.The reaction mixture was stirred at ambient temperature for about 10 min and the 1.1-fold excess of phosphine complexes [(Ph 3 P) 2 MCl 2 ] (M=Ni, Pd) was added to one portion.Immediately, a dark brown solution was observed.The resulting mixture was stirred at ambient temperature for about 30 min and the solvent was evaporated to dryness under vacuum conditions.The target complex was isolated using column chromatography on silica using CH 2 Cl 2 as eluent.If necessary, an additional column chromatography on silica using hexane as eluent was performed to purify the complex from uncoordinated triphenylphosphine.

Single Crystal X-ray Diffraction Study
Single crystal X-ray diffraction experiments for 2•HBr, 4, Cs [5]•0.5Me 2 CO, 8, and 10 were carried out using SMART APEX2 CCD diffractometer (λ(Mo-Kα) = 0.71073 Å, graphite monochromator, ω-scans) at 140 K (See Supplementary Materials).Collected data were processed using the SAINT and SADABS programs incorporated into the APEX2 program package [83].The structure was solved using the direct methods and refined using the full-matrix least-squares procedure against F 2 in anisotropic approximation.The refinement was carried out with the SHELXTL program [84].The CCDC numbers (2294563 for 2•HBr, 2294561 for 4, 2294562 for Cs [5]•0.5Me 2 CO, 2294564 for 8, and 2294565 for 10) contain the Supplementary Materials for this paper.These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 9 October 2023).

Figure 1 .
Figure 1.General view of metal complexes based on nido-carboranes with pendant pyridine group.

Figure 1 .
Figure 1.General view of metal complexes based on nido-carboranes with pendant pyridine group.