Chandan Nandi

Arindam Roy

Ketaki Kar

Marie Cordier

Sundargopal Ghosh

Cluster Growth Reactions: Structures and Bonding of Metal-Rich Metallaheteroboranes Containing Heavier Chalcogen Elements

Keywords: cobalt, decaborane, metallaheteroboranes, chalcogen, boron

) analogs, we have studied the reaction of 10-vertex nido-[(Cp*Co)2B6H6E2] (Cp* = η 5 -C5Me5, 1: E = Se and 2: E = Te) with [Co2(CO)8] under thermolytic conditions. All these reactions yielded face-fused clusters, [(Cp*Co)2B6H6E2{Co(CO)}(μ-CO){Co3(CO)6}] (3: E = Se and 4: E = Te). Further, when clusters 3 and 4 were treated with [Co2(CO)8], they underwent further cluster build-up reactions leading to the formation of 16-vertex doubly-face-fused clusters [(Cp*Co)2B6H6E2{Co2(CO)2}(μ-CO)2{Co4(CO)8}] (5: E = Se and 6: E = Te). Cobaltaheteroboranes 3 and 4 comprise one icosahedron {Co4B6E2} and one square pyramidal {Co 3 B 2 } moiety, whereas 5 and 6 are made with one icosahedron {Co 4 B 6 E 2 } and two square pyramidal {Co3B2} cores. In an attempt to generate heterometallic metal-rich clusters, we have explored the reactivity of decaborane(14) analog nido-[(Cp*Co)2B7TeH9] (7) with [Ru3(CO)12] at 80 °C, which afforded face-fused 13-vertex cluster [(Cp*Co)2B7H7Te{Ru3(CO)8}] (8). Cluster 8 is a rare example of a metal-rich metallaheteroborane in which one icosahedron {Co2Ru2B7Te} and a tetrahedron {Ru2B2} units are fused through a common {RuB2} triangular face. Further, the treatment of nido-[(Cp*Co)2B6S2H4(CH2S2)] (9) with [Fe2(CO)9] afforded 11-vertex nido-[(Cp*Co)2B6S2H4(CH2S2){Fe(CO)3}] (10). The core structure of 10 is similar to that of [C2B9H11] 2-with a 5-membered pentahapto coordinating face. All the synthesized metal-rich metallaheteroboranes have been characterized by multinuclear NMR spectroscopy, IR spectroscopy, ESI-mass, and structurally solved by single-crystal X-ray diffraction analysis.

Furthermore, theoretical investigations gave insight into the bonding of such higher-nuclearity clusters containing heavier chalcogen atoms.

INTRODUCTION

Due to the presence of three valence electrons, boron is electron deficient and forms a covalently bonded three-dimensional network producing unusual structures both with the main group and transition metal elements. 1,2 Polyhedral boranes and their derivatives containing heteroatoms, such as carboranes, metallacarboranes, metallaboranes, and metallaheterobranes, with different organic moieties have shown significant influence in many areas of organic and inorganic chemistry, [3][START_REF] Hosmane | Metallacarboranes of d-and f-Block metals[END_REF][5][START_REF]ESI + ): calculated for [C32H36B6Co8O12Te2+H[END_REF] for example, as catalysts, nanomaterials, polymers, ceramics and in BNCT (boron-neutron-capture-therapy). 4a,7 Apart from their applications, polyhedral boranes as well as their metal and heteroatom incorporated derivatives have shown a wide range of unusual synthetic and structural chemistry. 5a Over the last fifty years, many main group pioneers, such as Lipscomb, 1a Grimes, 3a Fehlner, 4c,5a Kennedy, 5b Shore, 5c Hawthorne, [START_REF] Hawthorne | The Role of Chemistry in the Development of Boron Neutron Capture Therapy of Cancer[END_REF] Welch, 8 Xie, [START_REF] Zheng | Synthesis and X-ray characterization of and 16-vertex closo-carboranes[END_REF] and us 10 have reported various boron, carbon, chalcogen and transition metal-enriched single-cage as well as condensed polyhedral clusters. However, syntheses of these large polyhedra from small building block units are typically uncontrolled and mechanistically undefined. [START_REF] Ghosh | Expansion of iridaborane clusters by addition of monoborane. Novel metallaboranes and mechanistic detail[END_REF] The progress in this area was further enhanced by the development of both electron counting rules [START_REF] Wade | Skeletal electron counting in cluster species. Some generalisations and predictions[END_REF] and isolobal principle [START_REF] Hoffmann | Building Bridges Between Inorganic and Organic Chemistry (Nobel Lecture)[END_REF] that provided the groundwork for understanding the structural relationships between different clusters and their bonding motifs.

Numerous complementary approaches to the expansion of cluster networks are available especially to build higher vertices metallaborane and metallaheteroborane clusters. For example, (i) condensation reactions by using monoborane reagents, such as [BH3•THF] or [LiBH4•THF], 10a,b (ii) insertion or fragmentation by using metal carbonyl fragments or borane, [START_REF]ESI + ): calculated for [C32H36B6Co8O12Te2+H[END_REF] and (iii) the intercluster fusion of smaller to higher clusters such as tetrahedral, octahedral, and trigonal-prismatic, square antiprismatic, or icosahedral fragments with one, two or more atoms ACCEPTED MANUSCRIPT / CLEAN COPY boron, metal, and other main group elements are rare. [START_REF] Ghosh | Expansion of iridaborane clusters by addition of monoborane. Novel metallaboranes and mechanistic detail[END_REF] As a result, finding viable synthetic methods for such higher nuclearity clusters became of interest.

Chart 1. Polyhedral and macropolyhedral metallaboranes and metallaheteroboranes.

In our continuing efforts to synthesize metallaboranes and metallaheteroboranes, we have recently synthesized and characterized a 19-vertex face-fused cluster closo- [(Cp*Co) 3 B 15 H 10 {Cp*CoH}] (I). [START_REF] Zafar | Cluster Fusion: Face-fused Macropolyhedral Tetracobaltaboranes[END_REF] As shown in Chart 1, I is composed of one icosahedron {Co3B9}, one tetrahedral {B4} and one bicapped square-antiprismatic {CoB9} core, which are fused by one {B3} triangular and one {B4} butterfly face. [START_REF] Zafar | Cluster Fusion: Face-fused Macropolyhedral Tetracobaltaboranes[END_REF] Kennedy and co-workers have reported a 19vertex nickeladithiaborane [(PPh3)NiS2B16H12(PPh3)] (II) comprising a 9-vertex nido-{NiB8} and one 12-vertex closo-{NiSB10}, which are fused by a triangular {NiB2} face. [START_REF] Kaur | Macropolyhedral boroncontaining cluster chemistry: nineteen-vertex [(PPh3)NiS2B16H12(PPh3)] and eighteen-vertex S2B16H14(PPh3)[END_REF] Subsequently, ACCEPTED MANUSCRIPT / CLEAN COPY the same group has synthesized an 18-vertex macropolyhedral diiridaborane(III), composed of one 8-vertex nido-{IrB7} and one 12-vertex closo-{IrB11}, fused by two common boron atoms. 16 Although several macropolyhedral metallaboranes and metallathiaboranes are known in the literature, single-cage, and condensed clusters containing heavier chalcogen atoms, especially selenium and tellurium are limited. Recently, we have isolated a fused cluster, 13-vertex closo-[(Cp*Co)2B6H6Se2{(CO)8Ru3}] (IV), composed of one {Co2Ru2B6Se2} icosahedron and a triangle {Ru3}, fused through {Ru2} edge. 6b Spalding and co-workers have isolated a 12-vetex Bfluorinated closo-[(η 7 -C7H7)F2MoTeB10H8] (V), which is the first example of a paramagnetic transition element complex having Te atom in the cluster core. [START_REF] Di Biani | structure, spectroscopic and electrochemical study of the paramagnetic ACCEPTED MANUSCRIPT / CLEAN COPY compound [2-(η 7 -C7H7)-7,11-F2-2,1-closo-MoTeB10H8[END_REF] Further, in the quest for polyhedral clusters comprising heavier chalcogen atoms, we have recently isolated a 9-vertex nido-[(Cp*Co) 2 B 5 H 5 Te 2 ] (VI). [START_REF] Joseph | Chemistry of Triple-Decker Sandwich Complexes Containing Four-Membered Open B2E2 Rings (E = S or Se)[END_REF] As a result, isolation of higher vertices clusters comprising heavier chalcogen atoms following a cohesive synthetic route became appealing. In this paper, we have re-examined the chemistry of decaborane (14) analogs with different metal carbonyls, such as [Co2(CO)8], [Ru3(CO)12], and [Fe2(CO)9] that afforded higher nuclearity metallaheteroborane clusters with unusual geometries. Herein, we discussed the syntheses, structure, and bonding of unusual 14-and 16-vertex face-fused clusters comprising group 16 heavier elements.

RESULTS AND DISCUSSION

Reactivity of

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In order to synthesize metal-rich metallaheteroboranes, we were curious to explore a similar kind of reactivity with a different type of decaborane (14) analogs, and hence, we treated [(Cp*Co)2B7TeH9], nido-7 [START_REF] Nandi | Directed Syntheses of CS2-and CS3-Bridged Decaborane-14 Analogues[END_REF] with [Co2(CO)8]. Unfortunately, the reactivity of nido-7 with [Co2(CO)8] did not generate mono face-fused clusters. This may be due to the presence of a single chalcogen atom in the decaborane( 14) core that could not offer the necessary coordination environment as observed in the case of nido-1 and 2. 4), B1-Co4 2.126(4), B1-Te1 2.439(4), B4-Te2 2.414(4), B4-Co1 2.150(4), Co5-Co6 2.4319 (7), Co4-Te1 2.5058(5), Co3-Co4 2.7470 (7); B6-B1-Co2 118.9(2), Te2-Co3-Te1 107.553 (19).

To provide some insight into the electronic structures and bonding relationship of such high nuclearity metallaheteroborane clusters, the DFT calculations using the Gaussian 16 program with a B3LYP/def2-TZVP level of theory for 3 and 4 (Cp analogs of 3 and 4) have been carried out (see computational details in the supporting information (SI)). The calculated bond distances and the [START_REF] Ghosh | Expansion of iridaborane clusters by addition of monoborane. Novel metallaboranes and mechanistic detail[END_REF] B NMR resonances for 3 and 4 are in good accordance with the structural and S1 andS2). The frontier MO analyses clearly show a higher HOMO-LUMO gap in 3 (~2.577 eV) as compared to 4 (~2.500 eV). This may be due to the presence of different chalcogen atoms in their cluster framework (Table S6). As shown in Figure 3, the HOMOs of 3 and 4 are predominantly concentrated on Co (d-orbitals), whereas their LUMOs display mainly antibonding interaction along the Co-Co bond in the icosahedron core.

On the other hand, the HOMO-1 and HOMO-2 of 3 and 4, respectively, show notable bonding interaction with three Co-centers at one side of the icosahedron. In addition, the antibonding interaction along the Co-E (E = Se, Te) bond on other side of the icosahedron resulted in bulging out of the metal d-orbital outwards. As a result, one might assume further interaction with metal carbonyl fragments leading to additional cluster growth reactions. The donating nature can also be observed from the natural charge and natural valence population calculations, achieved from NBO analyses of 3 and 4 (Table S4). show minor differences (Table S1). The ESI-MS spectra display isotopic patterns at m/z 1308.5886 [M+H] + and 1405.5727 [M+H] + for 5 and 6, respectively. Although all the spectroscopic assignments suggest similar structures for 5 and 6, the X-ray analyses provided their core structures.

The X-ray quality crystals suitable for 5 and 6 were obtained at -5 °C by slow evaporation of a CH2Cl2-hexane solution. As shown in Figure 4, the solid-state X-ray structures of ACCEPTED MANUSCRIPT / CLEAN COPY hydrogens. They are comparable to B-H-B hydrogens. [START_REF] Zafar | Cluster Fusion: Face-fused Macropolyhedral Tetracobaltaboranes[END_REF] Theoretical insight into the bonding situation of 3-6 showed that the Wiberg bond indices (WBIs) of the B-Hb interaction are significantly higher than those of Co-Hb bond and in the range of terminal B-H bond (Table S3).

In (Se/Te) p-orbitals with antibonding interaction along the metal-metal bond same as observed in 3 and 4 (see computational details in SI). Further, the HOMO-8 of 5 shows trimetallic-bonding interaction between the cobalt centers in which one of the Co atoms directly involved in the fusion of cluster and the HOMO-29 of 6 represents an extended orbital overlap of cobalt dorbitals and (Se/Te) p-orbitals (Figure 5). Topology analyses using the QTAIM approach and Wiberg bond indices also support the overall bonding scenario (Tables S2 andS5). Further, the 1 H-11 B{ 1 H} HSQC NMR experiment of 8 was performed that established a correlation between the bridging hydrogens and the boron resonance observed at δ = 14.9 ppm (Figure S41). The 11 B{ 1 H} NMR spectrum of 8 shows peaks at δ = 50.0, 39. 4, 35.8, 14.9, and 5.6 ppm with an intensity ratio of ~1:1:1:3:1. In addition, the infrared spectrum (IR) of Co1-Te1 2.4692( 9), Co2-B2 2.026( 7), Ru1-Te1 2.6571( 7), Ru1-B7 2.451( 7), Ru1-Ru3 2.8175( 7), Ru2-Ru3 2.7242( 7), Ru3-Te1 2.6674( 6), Te1-B3 2.397( 7); B3-Co1-B7 94.2(3), B7-Co1-Te1 64.24 (19). The HOMO-4 of 8 (Cp analog of 8) shows a significant overlap of the dz 2 orbitals of Ru3 and Ru2, indicating notable bonding interaction between two Ru centers (Figure S55). Moreover, the Wiberg Bond Index (WBI) of 0.4084 between two Ru centers validates the Ru-Ru bonding interaction. Further, as shown in Figure 7, the HOMO-27 shows a three-center overlap of orbitals involving two Ru atoms and one Te atom extended along the icosahedron core. Laplacian electron density plot of 8 also depict the above bonding picture (Figure 7). However, all our attempts to get 12-vertex icosahedral core failed. Recently, we have isolated a {CS2} supported decaborane (14) analog, [(Cp*Co)2B6S2H4(CH2S2)], nido-9 having an open face. [START_REF] Nandi | Directed Syntheses of CS2-and CS3-Bridged Decaborane-14 Analogues[END_REF] Thus, with a similar objective, we have carried out the reaction of nido-9 with the same set of reactants that afforded purple solid 10 (28% yield, Rf = 0.282) (Scheme 3). The 11 B{ 1 H} NMR spectrum displays resonances in ~2:2:1:1 intensity ratio at δ = 34. 8, 30.7, 26.2, and 6.9 ppm for 10. The 1 H NMR displays a peak at δ = 1.65 ppm, which confirms the presence of a single Cp* environment. Further, the 1 H and 13 C{ 1 H} NMR spectrum of 10 confirms the presence of the methylene group. In addition, the IR spectrum displays distinctive peaks for BH t and CO ligands for 10. The ESI-MS spectrum of 10 shows a peak at 739.0144 [M] + that corresponds to the molecular formula of C24H36B6Co2FeO3S4. Finally, the molecular structure of 10 was confirmed by X-ray diffraction analysis. As shown in Figure 8

Reactivity of nido

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The HOMO of 10 (Cp analog of 10) shows significant bonding interactions between B and Co atoms along with the antibonding interactions between Co and S atoms of methanedithiolate (CH2S2) moiety (Figure S56). In addition, the HOMO-4 shows directed p-orbitals on S-atoms of the decaborane core that is inclined towards the pentagonal face (Figure 8(b)). Thus, one may assume that this molecule might show further cluster growth reaction if treated with two-electron metal or main group fragments. As a result, we have studied the reactions of nido-10 with [Co2(CO)8] both under room temperature and thermolytic conditions. However, the objective of getting closo-icosahedral from nido-10 was not achieved. This may be due to the fact that {Fe(CO3)} and {CS2} bridged units in nido-10 are oriented in such a fashion that prevents bonding interaction between the S-atom and additional metal carbonyl fragment. X-ray Structure Analysis: Single-crystals of 3-6, 8, and 10 were grown by slow diffusion of a solution of CH2Cl2-hexane at 5 ˚C. The X-ray data were collected and integrated for 3 and 4 using a Bruker APEX-II CCD, for 5 using Bruker Axs Kappa Apex2 Diffractometer, for 6 using D8 VENTURE Bruker AXS, for 8 using Bruker D8 VENTURE diffractometer with PHOTON II detector and APEXII Bruker-AXS with graphite monochromated Mo-Kα (λ = 0.71073 Å)

MS (ESI

radiation at 296(2) K (for 3, 4, 5, and 8), 150(2) K (for 6 and 10). The structures were solved using SHELXS-97, SHELXT-2014 20 and refined using SHELXL-2014, SHELXL-2017, SHELXL-2018. 21 Olex2 software was used to draw the X-ray structures for all synthesized clusters. [START_REF] Dolomanov | OLEX2: a complete structure solution, refinement and analysis program[END_REF] Crystallographic data have been deposited with the Cambridge Crystallographic Data

Center as supplementary publications CCDC-2160195 (3), 2151737 (4), 2151736 (5), 2144494 (6), 2174391 (8), and 2151735 (10) contain crystallographic data. These data can be obtained

ASSOCIATED CONTENT

Solid-state X-ray structures; ESI-MS spectra; Cluster expansion reactions of 10-vertex decaborane( 14) analogs with various metal carbonyls.

Unusual 13-and 14-vertex face-fused and 16-vertex doubly-face-fused polyhedral clusters (see pictures).

Scheme 1 .

 1 Scheme 1. Syntheses of face-fused and doubly-face-fused metallaheteroboranes 3-6.

Figure 1 .

 1 Figure 1. 1 H{ 11 B}-11 B{ 1 H} HSQC NMR spectra of 4 (left) and 5 (right).

Figure 2 .

 2 Figure 2. Solid-state structures of 3(a) and 4(b). Thermal ellipsoids are set at the 40% probability level. Note that, for clarity Cp* (attached to Co1 and Co2) and the carbonyl groups (attached to Co3, Co4, Co5, and Co6) are omitted. Selected bond lengths (Å) and angles (°). 3: B5-B6 1.784(12), B6-Co6 2.216(9), B1-Co2 2.128(8), B1-Co5 2.135(9), B1-Se2 2.249(8), B2-Se1 2.282(8), B5-Se1 2.160(8); B2-B1-Co2 117.4(5), Co2-B1-Co5 122.4(4), Co3-Co5-Co4 58.63(4); 4: B1-B6 1.741(5), B1-Co2 2.139(4), B1-Co4 2.126(4), B1-Te1 2.439(4), B4-Te2

Figure 3 . 6 (

 36 Figure 3. Selected molecular orbitals of 3 and 4. Isosurfaces are plotted at an isovalue of ±0.04

5 and 6 Figure 4 .

 64 Figure 4. Molecular structures of 5(a) and 6(b). (, , ,) are used to indicate symmetrygenerated atoms. Note that, Cp* ligands are attached to Co1, Co1 (for 5 and 6); and carbonyl ligands attached to Co2, Co2, Co2, Co2, Co3, Co3 (for 5) and Co2, Co2, Co3, Co3, Co3, Co3 (for 6) are omitted for clarity. Thermal ellipsoids are set at the 40% probability level. Selected bond lengths (Å) and angles (°). 5: Co1-B2 2.118(9), Co1-Se1 2.3177(18), Co2-Co3 2.4896(17), Co3-B2 2.117(10), Co3-Co3 2.651(3), Se1-B2 2.242(9), B2-B2 1.73(2); B2-Co1-B2 94.2(5), B2-Co1-Se1 60.5(3), B2-Co2-Co2 81.0(3), B2-Co2-Co3 52.9(2), Co2-Co3-Co2 58.33(6); 6: B1-Te1 2.444(5), Co2-Te1 2.4646(6), Te1-Co1 2.4661(9), Co1-B1 2.141(5), Co2-B1 2.137(5), Co2-Co3 2.4901(9), Co2-Co2 2.7938(15); B1-Te1-B1 81.7(2), B1-Te1-Co2 51.61(11), Co2-Te1-Co2 69.05(3), B1-Te1-Co1 51.69(11), Co2-Te1-Co1 99.28(3), B1-Co1-Te1 63.63(12).

  addition, the bond occupancy of the bridged B-Hb (eg. 1.720 for 4) is further nearer to the terminal B-H bond (eg. 1.943 for 4) that illustrates stronger B-Hb interaction as compared to Co-Hb interaction in Co-H-B.

Figure 5 .

 5 Figure 5. Selected molecular orbitals of 5 and 6. Isosurfaces are plotted at an isovalue of ±0.04

  -7 with [Ru3(CO)12]. Although nido-7 does not participate in cluster growth reaction with [Co2(CO)8], we were interested in checking the same reaction with [Ru3(CO)12]. As a result, we have studied the chemistry of nido-7 with [Ru3(CO)12]. The reactivity of nido-7 with [Ru3(CO)12] at 80 °C yielded 8 as violet solid (11% yield, Rf = 0.342) (Scheme 2). The 1 H NMR spectrum exhibit two different Cp* environments at δ = 1.76 and 1.63 ppm in ~1:1 ratio, which was also established by the 13 C{ 1 H} NMR spectrum. Cluster 8 shows one upfield resonance in the 1 H NMR spectrum at δ = -8.92 ppm, which is due to the presence of Ru-H-B proton(s).

8 8 .Figure 6 .

 886 Scheme 2. Synthesis of metallaheteroboranes 8.

  for 8 is computed by the addition of valance electrons of an icosahedron, [14×2 (Co) + 14×2 (Ru) + 4×7 (B) + 4×1 (Te) + 2 = 90] and a tetrahedron [15×2 (Ru) + 5×2 (B) = 40] followed by subtraction of common triangle [16×1 (Ru) 6×2 (B) = 28], which provided a total of 102 cluster valance electrons. This is being satisfied from the available cluster valance electrons for 8 [2×9 (Co) + 3×8 (Ru) + 2×5 (Cp*) + 7×3 (B) + 7×1 (H) + 8×2 (CO) + 1×6 (Te) = 102].

Figure 7 .

 7 Figure 7. (a) Frontier molecular orbital (HOMO-27) of 8. (b) Laplacian electron density plots of 8 of the plane Ru1-Te1-Ru3, where the charge depletion (∇ 2 ρ(r) > 0) areas shown by dashed black lines, while solid red lines indicate areas of charge concentration (∇ 2 ρ(r) < 0). Orange dots indicate BCPs (bond critical points).

  Scheme 3. Synthesis of metallaheteroborane 10.

Figure 8 .

 8 Figure 8. (a) Molecular structure of 10. Thermal ellipsoids are set at the 40% probability level. Cp* ligands attached to cobalt, carbonyl ligands attached to iron, and one CH2Cl2 molecule are omitted for clarity. Selected bond distances (Å) and angles (°): Co1-S1 2.2081(17), Co1-B32.072(7), Co1-B2 2.095(5), Fe1-S1 2.3033(18), S1-B3 1.867(7), B3-B3 1.857(13), S2-B3 1.856(7), S2-C13 1.817(6), B3-B4 1.799(10); B4-Co1-B1 87.7(3), B3-Co1-S1 51.6(2), B4-Co1-S1 92.3(3), S1-Fe1-S1 92.78(9), S2-B3-S1 118.4(4), S2-C13-S2 111.6(5); (b) Frontier molecular orbital (HOMO-4) of 10.

  the cluster expansion reactions of nido-[(Cp*Co)2B6H6E2] (1: E = Se and 2: E = Te) by employing [Co2(CO)8] yielded 14-vertex face-fused clusters 3 and 4. Further reaction of 14-vertex 3 and 4 with [Co2(CO)8] produced 16-vertex doubly-face-fused clusters 5 and 6. In addition, a 13-vertex face-fused heterometallic metallaheteroborane 8 has been isolated and structurally characterized. The cluster fusion that generated 14 and 16-vertex fused clusters is rare. Also, the core geometry of 8 in which an icosahedron and a tetrahedron unit are fused through a common triangular face is unique in metallaborane cluster chemistry. Furthermore, the chemistry of {CS2}-supported decaborane(14) analog nido-9 with [Fe2(CO)9] was explored, which yielded 11-vertex nido-10. The 5-membered open face of nido-10 is very similar to [C2B9H11] 2-. Thus, the explorations to evaluate the scope for further cluster growth reactions of nido-10 with small molecules and transition metal fragments are underway and we anticipate additional progress in the future.Experimental SectionGeneral Procedures and Instrumentation. All the syntheses were performed in an argon atmosphere using Schlenk line techniques. Under an argon atmosphere, dichloromethane, toluene, THF, and hexane were distilled using Na/benzophenone before use. All chemicals, such as [Co2(CO)8], [Ru3(CO)12], [Fe2(CO)9], were bought from Sigma Aldrich and used without purification. The starting materials nido-[(Cp*Co)2B6H6E2] (1: E = Se; 2: E = Te), 6b nido-[(Cp*Co)2B7TeH9], 7[START_REF] Nandi | Directed Syntheses of CS2-and CS3-Bridged Decaborane-14 Analogues[END_REF] and nido-[(Cp*Co)2B6S2H4(CH2S2)], 9[START_REF] Nandi | Directed Syntheses of CS2-and CS3-Bridged Decaborane-14 Analogues[END_REF] were synthesized according to the literature procedures. All the clusters described here are reproducible. These reactions were carried out multiple times, which provided the same product distribution. Dialuminium-ACCEPTED MANUSCRIPT / CLEAN COPY supported TLC plates (MERCK TLC Plates) were used to separate the reaction mixtures. The 400 and 500 MHz Bruker FT-NMR spectrometers were used to record NMR spectra. Residual solvent carbon (CDCl3, δ = 77.1 ppm) and protons (CDCl3, δ = 7.26 ppm) were used as a reference for the 13 C{ 1 H} and 1 H NMR spectra, respectively. The inverse gated decoupling (zgig) and power gated (zgpr) pulse sequences, respectively, were used to get the 11 B{ 1 H} and the 1 H{ 11 B} spectra. A commercial Bruker spectrometer has access to all pulse sequences. The ESI-MS of the synthesized clusters were recorded using the 6545 Qtof LC/MS and Qtof Micro YA263 HRMS instruments. The IR spectra were obtained in CH2Cl2 solvent using JASCO FT/IR-1400 spectrometer.Syntheses of metallaheteroboranes 3 and 4. a flame-dried Schlenk tube, under an argon atmosphere, the yellow solution of nido-1 (0.030 g, 0.048 mmol) in dry toluene (10 mL) was charged dropwise with a solution of [Co2(CO)8] (0.096 mmol, 0.033 g) in 5 mL dry toluene at room temperature. The reaction mixture was then left to stir for a further 16 h at 80 °C. The reaction mixture color was turned from yellow to deep brown. The solvent was removed under vacuum, and the residue was extracted through Celite using a CH2Cl2/hexane (30:70 v/v) mixture. After evaporation of the solvent, TLC plates were used for chromatographic work-up of the residue. Elution using a mixture of CH2Cl2/hexane (30:70 v/v) produced brown solid 3 (0.009 g, 18%; Rf = 0.356). By following similar synthetic strategies, the dark brown solution of nido-2 (0.041 mmol, 0.03 g) in 10 ml dry toluene was charged with [Co2(CO)8] (0.082 mmol, 0.028 g) that yielded brown solid 4 (0.012 g, 24%; Rf = 0.363).

  .cam.ac.uk/data_request/cif. Crystal data of 3. C28H36B6Co6O8Se2; formula weight, Mr.= 1076.93; monoclinic; P21/n; a = 11.9660(7) Å, b = 18.4274(9) Å, c = 16.9432(6) Å, α = 90°, β = 95.015(2)°, γ = 90°; Z = 4; V = ACCEPTED MANUSCRIPT / CLEAN COPY Crystal data of 10. C24H36B6Co2FeO3S4•CH2Cl2; formula weight, Mr.= 824.26; orthorhombic; Pnma; unit cell, a = 17.4175(14) Å, b = 16.4575(12) Å, c = 11.7592(7) Å, α = 90°, β = 90°, γ = 90°; Z = 4; V = 3370.8(4) Å 3 ; μ = 1.833 mm -1 ; F(000) = 1680; ρcalcd = 1.624 g/cm 3 ; wR2 = 0.1744; R1 = 0.0833; 3518 independent reflections [2θ ≤ 52.736°], and parameters 149, Goodness-of-fit on F 2 = 1.171.

  Cp*, 30H) ppm; 11 B{ 1 H} NMR (CDCl3, 160 MHz, 22 °C, ppm): δ = 34.8, 30.7, 26.2, 6.9; 13 C{ 1 H} NMR (CDCl3, 22 °C, 100 MHz): δ = 98.8 (s, C5Me5), 41.5 (s, CH2S2), 9.1 (s, C 5 Me 5 ) ppm; IR (CH 2 Cl 2 ): ῡ = 2499 (BH t ), 2055, 2020 (CO) cm -1 .

	δ = 35.6, 30.4, 24.5, 2.6; 13 C{ 1 H} NMR (22 °C, CDCl3, 125 MHz): δ = 99.3 (s, C5Me5), 9.6 (s, residue. Elution with a hexane/toluene (70:30 v/v) mixture afforded purple solid nido-10 (0.010
	C5Me5) ppm; IR (cm -1 , CH2Cl2): ῡ = 2484 (BHt), 2083 (CO). g, 28%; Rf = 0.282).
	4. MS (ESI + ): calcd. for [C28H36B6Co6O8Te2+Na] + : m/z 1196.7006, found: 1196.6965; 1 H{ 11 B} 10. MS (ESI + ): calcd. for [C24H36B6Co2FeO3S4] + : m/z 739.0168, found: 739.0144. 1 H{ 11 B} NMR
	NMR (CDCl3, 500 MHz, 22 °C): δ = 6.02 (br, 1H, BHt), 4.87 (br, 2H, BHt), 3.12 (s, 1H, BHt),
	(CDCl3, 500 MHz, 22 °C): δ = 4.95 (br, BHt, 1H), 3.88 (s, 2H, BHt), 3.47 (br, 1H, BHt), 4.10 (d,
	1.86 (s, 30H, C5Me5), -4.86 (Co-H-B, 2H) ppm; 11 B{ 1 H} NMR (22 °C, 160 MHz, CDCl3): δ =
	2H, CH2S2), 1.65 (s,
	38.9, 27.3, 26.6, 8.1 ppm; 13 C{ 1 H} NMR (22 °C, CDCl3, 125 MHz, ppm): δ = 98.8 (s, C5Me5),
	10.2 (s, C5Me5); IR (cm -1 , CH2Cl2): ῡ = 2485 (BHt), 2000 (CO).
	Syntheses of metallaheteroboranes 5 and 6. In a flame-dried Schlenk tube, under an argon
	atmosphere, a brown solution of 3 (0.020 g, 0.019 mmol) in dry toluene (10 mL) was charged
	with [Co 2 (CO) CH2Cl2/hexane (30:70 v/v) produced brown solid 5 (0.004 g, 16%; Rf = 0.214). Under similar
	reaction conditions, the dark brown solution of 4 (0.017 mmol, 0.020 g) in 10 ml dry toluene was
	charged with [Co2(CO)8] (0.034 mmol, 0.012 g) that yielded brown solid 6 (0.005 g, 21%; Rf =
	0.227).
	5. MS (ESI + ): calculated for [C32H36B6Co8O12Se2+H] + : m/z 1308.5895, found: 1308.5886;
	1 H{ 11 B} NMR (CDCl3, 500 MHz, 22 °C): δ = 4.87 (br, 2H, BHt), 1.82 (s, 30H, C5Me5), -4.74
	(Co-H-B, 4H) ppm; 11 B{ 1 H} NMR (160 MHz, 22 °C, CDCl3): δ = 24.7, 23.5 ppm; 13 C{ 1 H}
	NMR (CDCl3, 125 MHz, 22 °C, ppm): δ = 99.3 (s, C5Me5), 9.7 (s, C5Me5); IR (CH2Cl2, cm -1 ): ῡ
	= 2510 (BHt), 2018 (CO).
	ACCEPTED MANUSCRIPT / CLEAN COPY

+ ): calcd. for [C28H36B6Co6O8Se2+Na] + : m/z 1100.7246, found: 1100.7235; 1 H{ 11 B} NMR (500 MHz, CDCl3, 22 °C): δ = 4.53 (br, 1H, BHt), 4.15 (br, 1H, BHt), 3.65 (s, 2H, BHt), 1.77 (s, 30H, C5Me5), -4.42 (2H, Co-H-B) ppm; 11 B{ 1 H} NMR (160 MHz, CDCl3, 22 °C, ppm):

ACCEPTED MANUSCRIPT / CLEAN COPY 8 ] (0.013 g, 0.038 mmol) at room temperature. The reaction mixture was stirred for further 5 h at RT. The solvent was eliminated under vacuum, and the residue was extracted through Celite using a mixture of CH2Cl2/hexane (30:70 v/v). After the solvent evaporation, TLC plates were used for chromatographic work-up of the residue. Elution using a mixture of

- 6, 8, and 10. For Table of Contents Only Cluster Growth Reactions: Structures and Bonding of Metal-Rich Metallaheteroboranes Containing Heavier Chalcogen Elements Chandan

  1 H, 1 H{ 11 B},[START_REF] Hoffmann | Building Bridges Between Inorganic and Organic Chemistry (Nobel Lecture)[END_REF] C{ 1 H}, 11 B{ 1 H}, HSQC NMR spectra; IR spectra; additional DFT results; and optimized coordinates of synthesized clusters 3Nandi, a Arindam Roy, a Ketaki Kar, a Marie Cordier b and Sundargopal Ghosh* ,a

scheme of IIT Madras. C.N. and K.K. are grateful to DST-INSPIRE, A.R. is grateful to IIT Madras, India for research fellowships. Authors are grateful to Dr. P. K. Sudhadevi Antharjanam and V. Ramkumar for X-ray data analyses. The computational facility of IIT Madras is gratefully acknowledged.

ACKNOWLEDGMENT

This work was supported by SERB, New Delhi, India (grant no. CRG/2019/001280) and the Centre of Excellence on Molecular Materials and Functions under the Institution of Eminence ACCEPTED MANUSCRIPT / CLEAN COPY

Supporting Information

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AUTHOR INFORMATION

Corresponding Author

*E-mail: sghosh@iitm.ac.in (S.G.)

Notes

The authors declare no competing financial interest.