Mustapha Hamdaoui

Fan Liu

Yann Cornaton

Xingyu Lu

Xiaohuo Shi

Jiyong Liu

Bernhard Spingler
email: spingler@chem.uzh.ch

Jean-Pierre Djukic
email: djukic@unistra.fr

Simon Duttwyler
email: duttwyler@zju.edu.cn

L=net

Bu

L = Net

Pr

= Pme

PEt Ph

An Iridium-Stabilized Borenium Intermediate

Exploration of new organometallic systems based on polyhedral boron clusters has the potential to solve challenging chemical problems such as the stabilization of highly reactive intermediates and transition state-like species postulated for E-H (E = H, B, C, Si) bond activation reactions. We p f c c B-H c f a hydroborane by a new iridium boron cluster complex. The product of this reaction is an unprecedented and fully characterized transition metal-stabilized boron cation or borenium.Moreover, this intermediate bears an unusual intramolecular B•••H interaction between the hydrogen originating from the activated hydroborane and the cyclometallated metal-bonded boron atom of the boron cluster. This B•••H interaction is proposed to be an arrested insertion of hydrogen into the B cage -metal bond and p f "c -w k " pp f c f c . T "cw k " process is supported by the hydrogen-deuterium exchange observed at the boron cluster, and a mechanism is proposed on the basis of theoretical methods with a special focus on the role of noncovalent interactions. All new compounds were isolated and fully characterized by NMR spectroscopy and elemental analysis. Key compounds were studied by single crystal X-ray diffraction. c) This work: Evidence for iridium-stabilized borenium and iridium "cage-walking"

INTRODUCTION

A fundamental principle of chemistry is the stabilization of reactive species by ligation to transition metals, an important strategy to study structure and reactivity and towards the development of novel methodologies. There is one distinct class of ligands/ligand constituents that deserves particular attention: Carboranes. 1 Many attractive features of carboranes, especially superior inertness, are manifested in one of its most famous members: The carba-closododecaborate anion [CB 11 H 12 ] -(CB11) and its derivatives are robust boron clusters with unique properties and have found numerous applications (Figure 1a). 1,2 However, derivatization of CB11 is faced with synthetic challenges, especially with respect to selectively breaking only one of its eleven B-H bonds for subsequent B-functionalization. [START_REF]For general reviews on B-H activation of CB11 and derivatives, see: a) Sivaev, I. B. Functional Group Directed B-H Activation of Polyhedral Boron Hydrides by Transition Metal Complexes (Review)[END_REF] While our group [START_REF] Bar F R = H | BAr f 4 = [B{3,5-C6H3(CF3)2}4])[END_REF] and others 5 have achieved the selective B-H activation of CB11 and derivatives (i.e., cage B-H activation) and characterization of the resulting metal-boryl cyclometallated complexes, little is known about the potential of these metal-boryl complexes for the E-H (E = H, B, C, Si) activation of external small molecules. How would metal-boryl complexes of CB11 behave towards, e.g., external R 2 B-H? Would a metal transfer between B cage -metal and R 2 B-H occur to give B cage -H and R 2 B-metal, and would it be possible to trap any species along the reaction pathway? These questions constitute the initial motivation behind our study.

Interaction of boranes R 2 BH with transition metal complexes bearing various ligands has been widely studied. The resulting metal-(R 2 BH) adducts have been described as " "  B-H type complexes 6,7 with often promising catalytic activity for subse-quent transformations. 8 A variety of metal-(R 2 BH) adducts have been found to ff c c f " "  B-H type complexes. 8 A continuum of bonding was proposed 9 depending on the extent of B-H elongation upon metal-coordination with f w : " "  B-H > elongated metal-(-B-H) > B-metal-H oxidative addition. 8f,9-11 The bonding in the oxidative addition situation results from complete cleavage of the B-H bond to give a metal complex with formally anionic BR 2 -and H -ligands. Ways of effecting B-H bond cleavage with retention of the oxidation state of the metal center while keeping the BR 2 and H fragments in the coordination sphere remain much less investigated. This would provide a means to form a metal-stabilized boron cation (borenium) BR 2 + , along with a metal-bound formal hydride ligand H -. B f f [ →B 2 ] + are positively charged tricoordinate boron species. [START_REF] Ryschkewitsch | A Trigonal Boron Cation[END_REF][START_REF] Tan | Recent advances in borenium catalysis[END_REF] Most of them exhibit higher electrophilicity (Lewis acidity) in comparison with their neutral counterparts BR 3 . Well-characterized organic c p x f p [ →B 2 ] + are known (Figure 1b), and many have been shown to play a role in (hydro)borylation catalysis of various substrates. [START_REF] Tan | Recent advances in borenium catalysis[END_REF][START_REF] Vrána | A. Acc c c p c c c surgery with N-heterocyclic carbenes[END_REF] Enhanced electrophilic reactivity in [ →B 2 ] + often originates from the high magnitude of positive charge at the boron atom, as shown computationally with large positive values of NBO (Natural Bond Orbital) charge.

As far as we know, there has been no reported example of a well-defined transition metal-stabilized borenium complex. The cationic manganese complex [Mn(CO) 4 (PR 3 )(η 1 -BH 3 •PMe 3 )] + Mn-A was shown to undergo formal heterolysis to the hydride Mn-B and the tetracoordinated phosphine-stabilized boron cation [H 2 B(PMe 3 ) 2 ] + B (Scheme 1). 15a Theoretical charge calculations showed that coordination of [H 3 B•PMe 3 ] to the cationic Mn fragment leads to a significant charge polarization of the coordinated B-H bond, which eventually facilitates B-H cleavage. However, no evidence for a direct manganese-boron bond could be seen either in Mn-A or the product mixture.

Scheme 1. Thermal decomposition of Mn-A into Mn-B and B.

Metal-boryl complexes are generally regarded as bearing an anionic boryl ligand BR 2 -. 16 It is worth mentioning that in some studies a certain electrophilic character of the boryl fragment was invoked. For instance, in the neutral CpFe(CO) 2 B(catechol) complex, Hartwig and coworkers observed Lewis acidic character based on the reactivity towards diethylamine. [START_REF] Hartwig | Transition Metal Boryl Complexes: Structure and Reactivity of CpFe(CO)2Bcat and CpFe(CO)2BPh2[END_REF] Some authors prepared metal complexes incorporating a ligand that stabilized a borenium-like moiety through a main group element, however, without direct metal-boron bond. For example, Oestreich and coworkers prepared ruthenium complexes with a sulfur-stabilized formal borenium ion, 14k while Vlaisavljevich and Daly and coworkers described azamacrocyle-coordinated boreniums as part of molybdenum complexes. 14d Brunker and coworkers proposed that the iron center of the [Cp*Fe(azaferrocene-BH 2 )] + cation contributes to the stabilization of the boron center. 14i Transition metals as Lewis bases have been extensively studied by Braunschweig and others. 18 As an early example involving iridium(III), Bergman and Andersen reported that the Ir(III) complex Cp*IrPR 3 H 2 reacts with AlPh 3 to form the corresponding Lewis adduct Cp*IrPR 3 H 2 →AlPh 3 ; other closely related species were described in the same paper. [START_REF] Golden | Adduct Formation and Single and Double Deprotonation of Cp*(PMe3)Ir(H)2 with Main Group Metal Alkyls and Aryls: Synthesis and Structure of Three Novel Ir-Al and Ir-Mg Heterobimetallics[END_REF] This study supports the idea that Cp*IrPR 3 H 2 can behave as a Lewis base towards Lewis acids. We have also recently shown that an iridium-silane metallacyclic adduct can be described as the donor-acceptor adduct [Cp*(ppy)(H)Ir(III)→SiR 3 ] + , wherein the SiR 3 moiety has substantial silylium character (ppy = 2-phenylenepyridine). 20 Ir(I) and other low-valent metal complexes behaving as Lewis bases are much more common and have been known since the work by Shriver [START_REF] Shriver | Basicity of a Transition Metal. A Boron Trifluoride Adduct of Biscyclopentadienyltungsten Dihydride[END_REF] and Parshall [START_REF] Parshall | Borane Complexes of Transition Metals[END_REF] in the 1960's. In his survey of the work by Johnson and Shriver, 23 Schmid "many metal complexes have lone pairs of electrons and therefore behave as Lewis bases. Addition of such bases to the Lewis acids BR 3 affords coordinate metal-boron bonds." [START_REF] Schmid | Metal-Boron Compounds -Problems and Perspectives[END_REF] Thus, it is reasonable to propose a similar idea for the interaction between electron-rich metal complexes and formal borenium fragments.

Two critical questions arise: 1) Is there a way to prepare isolable transition-metal-stabilized borenium complexes? 2) What is the nature of the metal-boron bonding in this case? Answering these questions could 1) provide a new design principle for taming the reactivity of unstable borenium species, and 2) bring new insights into the electronic structure of metal-boryl complexes and hence widen their potential applications. This study is an attempt to answer these questions by describing the reaction between a borane R 2 BH and a cyclometallated iridium-carboranyl complex and showing that the resulting iridium-borane adduct is best viewed as a metal-stabilized borenium ion (Figure 1c). Experimental and theoretical data support the presence of an unusual B cage •••H interaction, which can be viewed as an arrested B cage -H bond formation (Figure 1c). Furthermore, we also discovered "cage-walk " pp r surface of the boron cluster. Deuterium labeling experiments provide evidence for this process, and on the basis of theoretical calculations, a probable mechanism is identified.

RESULTS AND DISCUSSION

Synthesis of the carboranylpyridine ligand 1

We began our study with the synthesis of carboranylpyridine 1 as a ligand to use in the cyclometallation reaction with iridium (vide infra). 1 was prepared in 73% yield by Negishi coupling of the Czincate of CB11 with 2-bromopyridine (Scheme 2). A reported method [START_REF] Kanazawa | Mediated C-C Cross-Coupling Reaction of Monocarba-closododecaborate Anion for the Synthesis of Functional Molecules[END_REF] for making 1 via the C-cuprate of CB11 was not fully

Scheme 2. Synthesis of 1 (L1 = (o-OMe-C 6 H 4 ) 3 P; L2 = XPhos, 2-dicyclohexylphosphino-2′ 4′ 6′- triisopropylbiphenyl).
reproducible in our hands (see the Supporting Information for details). Ligand 1 was fully characterized by NMR spectroscopy, high resolution mass spectrometry and elemental analysis. Carboranylpyridinium zwitterion 2, protonated 1, was prepared by reaction with aqueous HCl in Et 2 O (Scheme 3). Compound 2 was fully characterized by NMR spectroscopy, elemental analysis and X-ray diffraction. In the crystal structure, selected observed distances (Å)are C1-C2 1.4954(15), C1-B4 1.7212(18), C2-C3 1.3826(17) and C2-N1 1.3464(16). Scheme 3. Synthesis and X-ray crystal structure of 2 (with 25% displacement ellipsoids, acetone solvent molecule and H atoms except for N1-H omitted for clarity).

Synthesis of Cp*Ir(NCMe)(κ 2 -B,N-carboranylpyridine) (3)

Ligand 1 was then used for the cyclometallation reaction with one equivalent of the iridium(III) precursor [Cp*Ir(NCMe) 3 ][BF 4 ] 2 (Scheme 4). When the reaction was conducted in the absence of base (25 °C, 4 h), NMR monitoring of the reaction mixture revealed that only 50% of 1 was converted to the new iridium cyclometallated product 3. The other 50% of 1 was converted to 2, the product of N-protonation. Given the 1:1 stoichiometry of the products 3:2 and that no added base was present in the reaction mixture, it is likely that the proton in 2 originates from the upper belt B-H bond of 1 that is activated by cyclometallation/deprotonation (indicated in red in Scheme 4). 26 This observation also suggests that an external base might be needed to assist in the B-H activation step, as was shown by calculations for our previously reported systems. 4f Indeed, when the cyclometallation reaction was conducted in the presence of excess NaOAc as the base, 1 was fully converted to 3 (55 °C, 22 h); no formation of 2 was noticed. Purification of complex 3 by two successive filtrations over Celite provided samples of 3 in 58% yield. Traces of [Et 4 N][BF 4 ] (<2%), a by-product Scheme 4. Synthesis of 3 from 1 in the presence of NaOAc (58% isolated yield); when no external base was used, only 50% of 3 was generated (not isolated).

of the reaction, could not completely be removed even after several attempts of doing so by recrystallization, column chromatography (Al 2 O 3 or SiO 2 gel), or washing with deionized water. All these methods led to either very low yields or significant decomposition of the iridium complex 3 to unidentified species. Complex 3 was fully characterized using NMR spectroscopy, elemental analysis and X-ray diffraction. The X-ray crystal structure of 3 is shown in Figure 2. Complex 3 crystallizes in the triclinic crystal system with the centrosymmetric space group P and exhibits a piano-stool pseudo-tetrahedral geometry around the iridium center. It is a five-membered cyclometallate similar to related structures of iridacycles obtained with the anionic CB11 ligand framework reported earlier. The Ir-B1 bond length of 2.122(2) Å is in agreement with literature values for CB11-Ir(III) complexes supported by pyrrolidine amide and tosyl amidecarboranyl ligands (Ir-B cage 2.128(5) Å and 2.123(7) Å, respectively); 4f,g see Table 1 for details.

Synthesis of the CB11-based iridium hydride 6 by reaction of complex 3 with MeONa

Reaction of complex 3 with NaOMe in MeOH afforded clean conversion of 3 to the new iridium hydride 6 within 24 h (Scheme 5). This synthetic method is based on the work of Norton and Templeton on metal hydrides. 27,[START_REF] Hu | Synthesis, Electrochemistry, and Reactivity of New Iridium(III) and Rhodium(III) Hydrides[END_REF] Complex 6 was isolated in 59% yield and fully characterized by NMR spectroscopy, elemental analysis and single crystal X-ray diffraction. In the 1 H NMR spectrum in CD 2 Cl 2 , complex 6 features an upfield singlet resonance at -14.7 ppm, characteristic of the hydride ligand, along with a singlet resonance at 1.87 ppm assigned to the Cp* ligand. These values are in good agreement with those reported for the 2phenylpyridyl-supported Rh and Ir hydride complexes by Norton (ppm in THF-d 8 ): -12.64 for RhH, 1.85 for RhCp*; -15.21 for IrH, 1.89 for IrCp*). [START_REF] Hu | Synthesis, Electrochemistry, and Reactivity of New Iridium(III) and Rhodium(III) Hydrides[END_REF] In the 11 B{ 1 H} NMR spectrum, a broad singlet at - 17.7 ppm is assigned to the resonance of the cage boron atom bonded to iridium. When compared to the B cage -Ir signals observed for complexes 3 and 4, the boron resonance in 6 is shifted upfield by approximately 1.5 and 2.5 ppm, respectively.

Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a concentrated solution of 6 in CD 2 Cl 2 . The crystal structure of 6 is shown in Figure 2, and selected distances are listed in Table 1. The compound crystallizes in the monoclinic crystal system with the centrosymmetric space group P2 1 /n. Similar to complex 3, the geometry of 6 can be described as a pseudotetrahedral arrangement of the chelating carborane pyridine ligand, the hydride and the Cp* ring around the iridium center. The hydride ligand was located and refined. The Ir-H bond length of 1.51(8) Å is comparable to M-H bond lengths reported by Norton [START_REF] Hu | Synthesis, Electrochemistry, and Reactivity of New Iridium(III) and Rhodium(III) Hydrides[END_REF] for the 2-phenylpyridine Rh andIr hydrides (1.51(3) Å for Rh-H and1.53(4) Å for Ir-H). The Ir-B cage andIr-N1 bonds in 6 (2.094(4) and2.088(4) Å) are slightly contracted by approximately 0.03 Å compared to 3 (2.122(2) and2.112(4) Å). The origin of this contraction is unknown, but it can tentatively be assigned to the shift from formal cationic to neutral iridium.

Reaction of 3 with pinacolborane to give adduct 7

The reactivity of 3 with respect to the activation of an external B-H bond, as opposed to intramolecular cage B-H bond activation in the reaction 13, was probed using pinacolborane (HBPin). The reaction between complex 3 (0.036 mmol) and pinacolborane (0.42 mmol) was conducted in CD 2 Cl 2 at room temperature and monitored by 1 H and 11 B NMR spectroscopy. According to 1 H NMR Scheme 6. Synthesis of complex 7 starting from iridacycle 3.

analysis, 3 was fully converted to a monomeric Cp*Ir-based species alongside free MeCN (1:1 ratio based on signal integration), suggesting that full displacement of NCMe from 3 had occured concomitant with addition of HBPin to iridium. Upon repeating the latter experiment on a larger scale (0.162 mmol of 3, 0.65 mmol of HBPin), the new species was isolated in 90% yield and identified as the iridium-pinacolborane adduct (κ 2 -B,Ncarboranylpyridine)•••H•••IrCp*(BPin) 7 (Scheme 6). Complex 7 was fully characterized using multinuclear NMR spectroscopy, elemental analysis and single crystal X-ray diffraction. It is highly air-sensitive, but it is inert over days under an atmosphere of dry nitrogen in the solid state and in dry solvents such as dichloromethane and 1,2-dichlorobenzene.

Bonding analysis of complex 7 using NMR spectroscopy: Evidence for an unusual B cage •••H•••Ir interaction

A series of NMR spectroscopic analyses revealed that an unusual B cage •••H•••Ir interaction exists in 7, making it a unique boron cluster complex because to the best of our knowledge, no similar precedent exists in the literature.

In addition to the 11 B NMR cage B-H signals, complex 7 exhibits a 11 B resonance at 31.6 ppm, which is assigned to the iridiumbound boron atom B Pin . It appears as a broad singlet without any noticeable coupling pattern and is 3.5 ppm downfield from the resonance of free pinacolborane. Another broad singlet 11 B resonance prepared by the optimized procedure is observed at -16.7 ppm, which corresponds to the iridium-bound cage boron atom (designated B 1 ). These signals have width at half height of 220 and 81 Hz, respectively, and decoupling from 1 H does not lead to sharpening. In the 1 H NMR spectrum, the bridging H nucleus (designated H 1 ) appears as a broad doublet-like signal at -9.19 ppm with integration = 1, a line width at half height of 100 Hz and a peak-to-peak distance of 41 Hz (Figure 3a). It is clearly distinct from the other B cage -H shifts, which fall in the range of 2.1 to 1.6 ppm. This signal sharpens significantly upon broad-band decoupling from 11 B to become an apparent singlet, indicating no coupling to nearby H nuclei. This phenomenon of broad doublet-like peak appearance (a partially collapsed quartet with intensification of the two inner lines) and sharpening upon { 11 B} decoupling has been observed for σ-borane complexes and also organoboron compounds in which an increased H-B distance is associated with decreased 1 H-11 B scalar coupling. 29 The resonance of the bridging hydrogen nucleus required a more extensive analysis: Did its characterisics stem from an interaction with B Pin , B 1 , or both? And how could the bonding be interpreted in the context of 1 H NMR data of chemically related compounds?

L = 4-dimethylaminopyridine (dmap) (1.1 equiv) or pyridine (py) (1.1 equiv) CH 2 Cl 2 , 25 °C, 14 h -MeCN NaOMe (1.2 equiv) MeOH (H 2 O traces),
In order to address these questions, we carried out selective 1 H{ 11 B} hetero-nuclear decoupling experiments using an isolated sample of 7 in CD 2 Cl 2 (293 K; 500/160 MHz). First, when a 1 H NMR spectrum was recorded with selective decoupling from the B Pin resonance (δ( 11 B) = 31.6 ppm), the H 1 signal at -9.19 ppm remained unchanged in comparison to the regular 1 H NMR spectrum (Figure 3a, middle). In contrast, when a 1 H NMR spectrum was recorded with selective decoupling from the B 1 resonance (δ( 11 B) = -16.7 ppm), the signal at -9.19 ppm became a sharp singlet (Figure 3a, right). This result clearly shows the presence of an interaction between the H 1 and the cage boron atom B 1 .

Furthermore, complex 7 was also analyzed using 1 H{ 11 B}-11 B{ 1 H} HSQC NMR spectroscopy, a method commonly used for detection of direct one-bond heteronuclear scalar-coupling. A cross-peak correlation was observed between the H 1 and B 1 (Figure 3b). However, no such correlation could be detected between H 1 and B Pin .

These results strongly suggest that the appearance of the H 1 signal is predominantly caused by its interaction with B 1 , with no detectable contribution from B Pin of the pinacolborane ligand. In addition, it is unlikely that 2 J scalar coupling involving the iridium metal center (B 1 -Ir-H 1 ) plays a role, because then a through-bond coupling to B Pin (B Pin -Ir-H 1 ) should be present as well.

As one more NMR tool to investigate the solution phase behavior of complex 7 and in particular regarding the question of involvement of 2 J scalar coupling through iridium, variabletemperature (VT) measurements were performed. Hartwig and coworkers used this method to probe the origin of the coupling of the hydrogen nucleus in a series of σ-borane complexes. 29d The authors "indirect evidence for scalar coupling between the boron and the hydrogen atoms" by noticing a "virtual self decoupling" [START_REF] Pople | The effect of quadrupole relaxation on nuclear magnetic resonance multiplets[END_REF] in the B-H 1 H NMR resonances upon decreasing the temperature, a phenomenon also previously reported for compounds containing a proton directly attached to a quadrupolar nucleus such as in [BH 4 ] -complexes. [START_REF] Marks | Structure and Dynamics in Metal Tetrahydroborates. I. Nuclear Magnetic Resonance Studies of Zirconium and Hafnium Tetrakis(Tetrahydroborates)[END_REF] In the VT-NMR analysis of 7 indeed a temperature-dependent sharpening was observed compared to the shape and width of the signal of H 1 detected at -9.19 ppm at room temperature (temperature range +30 to -80 °C in CD 2 Cl 2 ; see the Supplementary Information for details).

Overall, these NMR experiments provide clear evidence that the coupling observed for the H 1 nucleus is predominantly due to its interaction with the cage boron atom B 1 . A substantial contribution by B 1 -Ir-H 1 two-bond scalar coupling and also 1 J or 2 J coupling to B Pin are not supported by the data. One must keep in mind, however, that this bonding interpretation only holds as long as we consider complex 7 in the solution state. Moreover, the absence of significant coupling between H 1 and B Pin in solution should not be interpreted as absence of any H 1 •••B Pin interaction. [START_REF] Câmpian | Comparisons of Photoinduced Oxidative Addition of B-H, B-B, and Si-H Bonds at Rhodium(η 5 -Cyclopentadienyl)Phosphine Centers[END_REF] Based on the solid-state structure and computational studies that are presented in the following sections, the bonding situation is more complex.

Finally, a comparison of 1 H NMR data underscores the unique position that complex 7 occupies in boron cluster chemistry (Figure 4). Free pinacolborane, as an example of a common organoboron compound, exhibits a H-B resonance that appears as a quartet with 1 J B,H = 174 Hz and ca. 560 Hz line width due to coupling to the quadrupolar 11 B nucleus (80% natural isotopic abundance, I = 3/2; coupling to the remaining 20% 10 B with I = 3 is disregarded in this discussion). For 1, the 1 H-B cage signals exhibit 1 J B,H = 140 Hz/ width = ca. 450 Hz and fall in the shift range of 2.04-1.63 ppm. The data for pinacolborane and 1 are in agreement with full covalent character of the B-H bonds. In the complex Rh agostic , which shows agostic-like B-H•••Rh interactions, a contraction of the resonance of the two coordinated hydrogen atoms is observed with a quartet featuring 1 J B,H = 90 Hz and line width = ca. 300 Hz. [START_REF] Zhang | III)-Catalyzed Room Temperature Alkenylation-Annulation of closo-Dodecaborate Anions via Double B-H Activation[END_REF] These values are consistent with a decreased covalent character, and the interaction with Rh(III) is additionally associated with a shielding that results in ( 1 H) = -1.71 ppm. A further trend of upfield shifts is seen in 7 and 6. As discussed above, the properties of the nucleus H 1 in 7 thus correspond to a substantially reduced bonding to B 1 concomitant with incipient Ir-hydride character. Finally, the values for the hydride ligand in 6 are consistent with complete cleavage of the H-B cage bond and formal :H -character.

X-ray diffraction study of complex 7

Single crystals of 7 suitable for X-ray diffraction were obtained by liquid diffusion of hexane into a CD 2 Cl 2 solution at -30 °C. The solid-state structure of 7 is shown in Figure 5 and can be viewed as a pinacolborane adduct of the [pyridylcarborane-IrCp*] fragment. Considering the carboranylpyridine backbone, the B Pin moiety and the H1 atom as formal individual ligands, complex 7 possesses a four-legged piano-stool geometry. The Ir1-N1 bond length of 2.1052(12) Å lies between the values found for 3 and 6 (Table 1). On the other hand, the Ir1-B1 distance of 7 is 2.1738(16) Å, which exceeds the corresponding Ir1-B1 distances of 3 and 6 (2.122(2) Å and 2.094(5) Å, respectively). Although the differences are small, they indicate a weakening of the Ir-B cage bond in 7. This interpretation is consistent with the calculated optimized geometry of 7 and the experimental and theoretical results on iridium "cage walking" on the cluster surface (vide infra). Determination of the position of a hydrogen atom close to a transition metal center is a common issue and must be carried out carefully. Atom H1 was located on the electron density map and refined freely without constraints. The Ir1-H1 distance is 1. 47(2) Å, close to the value of 1.51(8) Å found for 6. Distances of 1.80(3) Å and 1.88(2) Å are found for B Pin -H1 and B1-H1, respectively. Clearly, these values in 7 are beyond the sum of the covalent radii of boron and hydrogen (1.15 Å) [START_REF] Pyykkö | Molecular Single-Bond Covalent Radii for Elements 1-118[END_REF] and also the B-H distance of 1.194 Å obtained by computational studies on free pinacolborane [START_REF] Pandey | Structure and coordinate bonding nature of the rhenium-σ-borane complexes[END_REF] The distances of >1.8 Å suggest cleavage of the B Pin -H bond but at the same time incomplete formation of the B cage -H bond. In other words, the H1 ligand is held in a hovering position between the two boron centers. Interestingly, the refinement of the diffraction data places H1 closer to B Pin than to B1. This is in contrast to the solution-phase NMR data, which show coupling between H 1 and B 1 but not between H 1 and B Pin . This disparity points to the subtle effects of the solid state vs solution phase, i.e., the preferred geometry of pure 7 in the crystal lattice as opposed to the equilibrium geometry of the solvated complex. Furthermore, it is in line with a shallow energy surface with regard to the H1 position. Detailed aspects of the bonding situation are discussed in the computational section.

BPin is surrounded by Ir1, O1 and O2 in a perfect trigonalplanar coordination geometry. The sum of angles around BPin is 359.99(13)°, and the distance of BPin to the plane through Ir1, O1 and O2 is <0.01 Å. Such planarity at boron centers has been found in the X-ray analysis of metal-(-B-H)complexes, as exemplified by Cp 2 Ti(HBcat) 2 (cat = catecholborane), for which Hartwig and coworkers interpreted it as a sign of  back donation from Ti to the p orbital at boron (MO calculations). [START_REF] Hartwig | Catecholborane Bound to Titanocene. Unusual Coordination of Ligand σ-Bonds[END_REF] A comparison of structural parameters of 7 with those of 6 and complexes with agostic-like interactions is provided in Figure S11. Similar to the conclusions drawn from the discussion of NMR data in Figure 4, the comparison of X-ray data suggests that the bonding situation in 7 is distinct from those in agostic-like complexes and in 6 and that the H1 atom is held in a unprecedented position above the cluster surface.

X-ray photoelectron spectroscopy

Reactivity of 7

In order to investigate the hypothesis of the borenium character of the Ir-bound BPin moiety experimentally, the reactivity of 7 was probed towards several Lewis bases (LBs) of varying strength.

Reaction of 7 with three equivalents of 4-N,Ndimethylaminopyridine (dmap) rapidly gave a product mixture consisting of mainly the iridium monohydride complex 6 and the known 14e cationic adduct [(dmap) 2 BPin] + in a 1:1 ratio along with Ir-dmap complex 4 (Scheme 7). The identity of the three products was confirmed by comparison of the NMR spectra with those of independently prepared samples (see the Supplementary Information). Hence, the displacement of the [BPin] + moiety from 7 by the Lewis base dmap allows one to picture 7 as a borenium adduct of the formal representation [6→BPin] + .

Further support to this view was obtained from the following reaction. When complex 7 was treated with excess pyridine (py), the product distribution was different: the iridium-py complex 5 was the major Ir complex, accompanied by formation of the iridium monohydride complex 6 and the product of 1,4-hydroboration of pyridine, H-py-Bpin 38 in a 1:0.4:0.2 ratio. The expected [(py) 2 BPin] + cation could not be detected at this stage. However, addition of excess HBPin (ca. 80 equiv/7) to this reaction mixture resulted in the detection of [(py) 2 BPin] + alongside the previously observed products 6, H-py-Bpin and 5 with an approximate relative distribution of 0.4:1:7.3:0.2 (10% of py was converted).

Scheme 7. Reaction of 7 with 4-N,N-dimethylaminopyridine.

Interestingly, further NMR monitoring of this reaction mixture (1 day at 25 °C) showed full consumption of pyridine, 5, and [(py) 2 BPin] + . In fact, the new mixture mainly consisted of the product of hydroboration of pyridine, H-py-Bpin, together with 7, 6 and free HBPin (in a 50:1:0.1:1.2 ratio). Thus, these observations show that 1) complete and selective 1,4-hydroboration of pyridine can be achieved catalytically, and 2) complex 7 is a key intermediate for this process as it provides the source of the additional two key intermediates: the iridium monohydride complex 6 and [(py) 2 BPin] + . The iridium-py complex 5 observed during the course of the reaction should be considered as the resting form of the transient catalytic species [carboranylpyridine-Ir-Cp*(□)] (□ = vacant coordination site).

Next, reaction of 7 with a slight excess of [n-Bu 4 N]F (TBAF) (moisture-free 1 M THF solution) gave a mixture consisting of 6, [F 2 BPin] -and an iridium complex assigned to be the iridium-THF complex, formed in a 1:1:2 relative ratio (see the Supporting Information). Free HBPin that was formally released from 7 (and substituted by THF) could not be detected, suggesting that it was transformed into one or several products through an unknown process. Again, these observations support the electrophilic character of the BPin group, with formation of the resulting product [F 2 BPin] -being most likely due to the remaining Lewis acidity of transient FBPin. [START_REF] Cybulski | C-F Bond Activation of P(C6F5)3 by Ruthenium D p x : c f " " Ru(PPh3)3H(Halide) Complex, Ru(PPh3)3HF[END_REF] On the other hand, the reaction of 7 with excess Et 3 N in CD 2 Cl 2 was incomplete after stirring the mixture for 15 h at room temperature (see the Supporting Information). Nonetheless, NMR analysis showed characteristic peaks of 6 alongside those attributed to [(NEt 3 )BPin] + and unchanged 7 in a 0.3:0.3:1 ratio. The steric encumbrance of NEt 3 and the resulting unfavorable access to the electrophilic BPin ligand is most likely responsible for the observed low conversion.

Reactivity of 7 against either bulkier or less electron-rich LBs than dmap or py was also investigated (see the Supporting Information). For example, no reaction was observed between 7 and 2,6-dimethylpyridine, penta-fluoropyridine, triethylsilane or para-Me-benzonitrile. On the other hand, 7 reacted smoothly with some of phosphines, such as PPh 3 , (p-Me-C 6 H 4 ) 3 P, or (p-CF 3 -C 6 H 4 ) 3 P, by formal substitution of the HBPin with the phosphine ligand at the iridium center. However, there was no reaction between 7 and XPhos (a known electron-rich but bulky phosphine). This contrasting reactivity of phosphines leads us to suggest that the initial center of attack of the phosphine occurs at the electrophilic BPin ligand followed by attack of the hydride at the phosphinestabilized [BPin] + group. Also, when 7 is dissolved in neat MeCN-d 3 or THF-d 8 , NMR analysis of the resulting solutions indicated 95% and 70% conversion of 7 into the deutero analogue of 3 and the iridium-(THF-d 8 ) complex, respectively. Free HBPin (50%) could only be detected in the former reaction (7 + MeCNd 3 ). Furthermore, 7 was inert (within the time of analysis) in solutions of fluorobenzene/CD 2 Cl 2 , C 6 D 6 /CD 2 Cl 2 and 1,2difluorobenzene.

Next, we wondered: How would 7 react with an electrophile or acid? For this, we chose diphenyl acetic acid (dpaa) as an electrophile a convenient source of proton, the simplest of all electrophiles. We selected it because it is a solid and can be dried reliably, making it suitable for small-scale reactions under controlled conditions. According to 1 H and 11 B NMR-spectroscopic analysis, addition of dpaa to a solution of 7 in CD 2 Cl 2 led to no noticeable change of the spectra when compared to those of 7. We interpret the lack of reactivity of 7 against dpaa as another piece of corroboration for the Lewis acidic character of the BPin ligand.

An extended series of reactivity experiments of 7 towards nucleophilic and electrophilic reagents, such as K[MeBF 3 ], EtMgBr, HNTf 2 , MeOTf, [Me 3 O][BF 4 ] Et 3 SiOTf, and Nfluorobenzenesulfonimideis outlined in the Supporting Information (p. Sxx). They are all consistent with borenium character of the BPin fragment.

Deuterium-labeling experiment: Evidence for CB11 deuteration via iridium cage-walking

When deuterated pinacolborane DBPin (0.43 M in C 6 H 6 /C 6 D 6 ; for its preparation, see the Supporting Information) was used instead of HBPin for the formation of 8, the deuterated analogue of 7, an intriguing observation was made. Monitoring of the reaction conducted in CD 2 Cl 2 at 25 °C by 1 H{ 11 B} NMR spectroscopy indicated that the initially formed iridium-DBPin adduct 8 slowly converted into the iridium-HBPin adduct 9a and its regioisomers 9b-d (Scheme 8). Exactly one boron atom of the cage in 8 became covalently deuterated via formal insertion of the Ir-bound D into the Ir-B cage bond. The experimental data are best explained by "c -w k " 40 of iridium over the five boron atoms of the upper cage belt; thus ultimately yielding the observed mixture containing 20% of 8 and 80% of 9a-d. On the one hand, the kinetic curve shown in Figure 7a reveals that already 20 minutes after mixing of the starting materials, a considerable amount of initially formed 8 is converted to 9a-d, and a saturation plateau at 80% 9a-d is reached after approximately 3 hours at 25 °C. The ultimate 20%/80% ratio of 8/9a-d was not exceeded during the time of NMR monitoring even though a slight excess of DBPin was used as indicated in Scheme 8. On the other hand, support for the single site boron deuteration in 9a-d came from highresolution mass spectrometric analysis (ESI-HRMS, negative mode) of the quenched NMR reaction mixture described above. The major species detected were the mono-deuterated carboranylpyridine ligand 1-[D] and the mono-deuterated pyridylfree [CB 11 H 11 D] -anion CB11-[D] (Figures 7b/c); we suspect that loss of the pyridine ring, affording CB11-[D], results from fragmentation of 1-[D] in the mass spectrometer.

In a control experiment, reaction of the carboranylpyridine ligand 1 with excess DBPin in CD 2 Cl 2 but in the absence of iridium at room temperature for several hours did not lead to deuteration of Scheme 8. Formation of deuterated pinacolborane adduct 8 using D-BP "c -w k " f upper-belt boron vertices. The final equilibrium mixture consisting of 8 (20%) and 9a-d (80%) is reached within hours at 25 °C; see also Figure 7a. the cage boron atoms, as verified by NMR spectroscopy and ESI-HRMS analysis. Furthermore, 1 H and 11 B NMR spectroscopic analysis of a mixture containing DBPin (C 6 H 6 /C 6 D 6 solution as prepared for this study) and CD 2 Cl 2 that was stirred at room temperature over several hours did not show any H/D exchange to give HBPin. Therefore, the combined experimental data strongly suggest that the selective mono-deuteration requires the involvement of iridium and rules out competetive reactions with the solvents used.

To gain further insight into the cage-walking hypothesis, a variable temperature NMR study was conducted on a sample of 7 in 1,2-dichlorobenzene-d 4 . Upon heating the sample from 25 °C to 140 °C, the resulting 1 H{ 11 B} NMR spectra indicated gradual broadening of most of the resonances. When the sample was heated to 140 °C, the resonance corresponding to the iridiumbound hydride has significantly broadened with peak intensity almost fully reduced to the baseline (line width at half height >200 Hz) in comparison to the same resonance recorded at 25 °C (line width at half height = 8 Hz with selective { 11 B} decoupling from B 1 ). However, no true coalescence occurred even at 140 °C. Therefore, based on the Gutowsky-Holm approximation and interpreting the lineshape change as incipient coalescence, the energy barrier to the overall process of cage-walking can roughly be estimated to be 22 kcal/mol < ΔG ‡ < 30 kcal/mol, consistent with the value of 28 kcal/mol found by computations (vide infra).

Computational Study

Why does such unusual B cage •••H 1 interaction exist in complex 7? One hypothesis is that the B cage •••H 1 interaction is an arrested B cage -H 1 bond formation; this would explain the elongated Ir-B cage distance observed in the X-ray of 7 as compared to that of 3 or 6 (Table 1). Previous investigators have studied the bonding in metal-c p x f "remanent" •••H interactions, which were suggested to arise from the incomplete oxidative addition of the Si-H bond to the metal center. 41 A comparable B•••H interaction was postulated in a pincer-boryl/Rh-hydride complex 41d and was calculated for a transition state in metalmediated C-H borylation. 41e We decided to explore possible answers to this intriguing phenomenon by addressing the formation and structure of 7 as well as the interactions among B cage , H 1 , Ir and B Pin using a series of computational techniques. Conversion of 3 into 7. The formation of 7 from 3 involves the decoordination of NCMe and the coordination of HBPin, which could proceed simultaneously or successively (Figure 9). The simultaneous decoordination of NCMe/coordination of HBPin proceeds through a high activation barrier (ΔG ‡ = +38 kcal/mol) and should thus be ruled out. The decoordination of NCMe from 3, yielding a 16-electron complex X, is weakly endergonic with ΔG = +8 kcal/mol. The subsequent coordination of HBpin to X to yield 7

x c w ΔG = -16 kcal/mol and seems to proceed in one single step where the B Pin -H 1 bond is elongated as both B Pin and H 1 coordinate to Ir. Any attempt to locate a stable reaction intermediate where the B Pin -H 1 bond still persists while one or both atoms are getting coordinated to Ir before 7 proved vain.

Structural analysis of 7. The X-ray structure of 7 was reoptimized at the ZORA 42 -PBE [START_REF] Perdew | Generalized Gradient made Simple[END_REF] -D4(EEQ) [START_REF] Caldeweyher | A Generally Applicable Atomic Charge Dependent London Dispersion Correction[END_REF] /all electron-TZP 45 /COSMO 46 (CH 2 Cl 2 ) level of theory in order to elucidate the nature of the interactions involved in its stability. From this point on, the inter-actions from the theoretically optimized structure of 7 are discussed. In this computed geometry, the calculated H 1 -B Pin distance (1.804 Å) is longer by around 0.6 Å in 7 as compared to the free HBPin (1.196 Å) and longer by around 0.04 Å as compared to the experimental value (1.88(2) Å, Table 1). However, H 1 remains closer to B Pin than to B 1 by 0.25 Å, suggesting that the interaction of H 1 with B Pin is still stronger than that with B 1 . B Pin and B 1 are almost equally close to Ir (Δd = 0.054 Å), suggesting a similar interaction of both boron atoms with Ir. Extended Transition State-Natural Orbital for Chemical Valence (ETS-NOCV) [START_REF] Mitoraj | A Combined Charge and Energy Decomposition Scheme for Bond Analysis[END_REF] analyses were undertaken with complex 7 using two arbitrary fragmentation schemes to evaluate the orbital interactions occurring between the BPin fragment and the hydrido ligand H with the rest of the molecule (Figure 8; see also the Supporting Information). In both fragmentation modes, the ETS-NOCV analysis suggests that some weak degree of covalent interaction persists between the Ir-bound hydrido ligand and the BPin ligand. When the interaction of a cationic BPin fragment with the anionic molecular residue was considered, the ETS-NOCV analysis clearly indicates that the main donation of electron density is from metal centered orbitals and the hydrido ligand towards a doughnutshaped area located in the B Pin -Ir-H 1 interspatial triangle. The NCI plots 48 isosurfaces depicted in Figure S13 tend to indicate the B Pin •••H 1 bond to be covalent assisted by noncovalent interactions, while the B 1 •••H 1 interaction seems purely noncovalent; the interactions of Ir with the three aforementioned atoms have the covalent character of a coordination bond (see the Supporting Information for a more detailed analysis).

A quantitative insight into the nature of the interactions around the Ir-B Pin -H 1 -B 1 was sought using the Independent Gradient Model (IGM) 49 -based δg inter descriptor and Intrinsic Bond Strength Index (IBSI), [START_REF] Klein | New Way for Probing Bond Strength[END_REF] Quantum Theory of Atoms In Molecules ion (QTAIM)-based Bader charges, [START_REF] Bader | Atoms in Molecules: A Quantum Theory[END_REF] and Interacting Quantum Atoms (IQA) 52 analysis (Tables S15-17). [START_REF] Karmazin | The IGM-δg inter descriptor is a measure of the electron sharing between two user-defined fragments, assessing the mutual density Ruhlmann[END_REF] Figure 8. Specifiy a) and b) ETS-NOCV analysis of the bonding interaction between the BPin + fragment and the anionic moiety of 7. The deformation density isosurface plot Δρ1 (0.005 a.u.) is associated with an orbital interaction energy of -145 kcal/mol. Electron density transfer operates from the red to the blue volumes upon interaction between fragments.

The IBSI and IQA analysis indicate that the H 1 •••B Pin interaction retains a very weak covalent character in 7, yet being several-fold weaker than in free HBPin. On the other hand, the interaction between H 1 and B 1 appears primarily noncovalent and very weak (more than two-fold weaker than the H 1 •••B Pin interaction) with virtually no ionic contribution. The δg inter analysis however gives a stronger interaction of H 1 with the carboranylpyridine (CBPy) ligand than with the BPin ligand. This might be an indication of the propensity of CBPy to attract H 1 and modify the nature of the B Pin •••H 1 •••B 1 interaction (see the Supporting Information for an extended discussion). The IBSI and IQA analysis of the interactions between Ir and H 1 , B Pin or B 1 categorizes these three interactions as coordination bonds. The Ir•••B Pin interaction is the weakest among all these interactions, with a preponderant attractive covalent part that counterbalances a repulsive ionic part. QTAIMbased Bader charges 50 (Table S16) confirm the repulsive ionic interaction between Ir and both B Pin and B 1 as these three atoms present positive charges. Moreover, the Bader charge at B Pin is more than five-fold that of B 1 , justifying the large ionic repulsion in the Ir-B Pin bond. The IGM-δg inter analysis also gives the interaction of the CBPy and BPin ligands with Ir stronger than their respective interaction with H 1 , the interaction of Ir with CBPy being stronger than the one with BPin. All these results tend to support the borenium-like formulation for complex 7 and suggest that the H 1 -B Pin bond still holds a dominant covalent character in 7, being however largely weakened by the coordination to Ir and the noncovalent attractive interaction of H 1 with B 1 .

Furthermore, it appears that H 1 could move rather freely towards B 1 , as illustrated by the formation of the so-c " c complex 7a (the structural features of which are more in accord with those observed for 7 in solution) preparing the bond breaking/forming events to occur 54 p "c w k " process. The formation of such a reactant complex modifies the nature of the B Pin •••H 1 •••B 1 interaction, reinforcing the borenium character of the complex. "Cage-walking" process. As previously proposed by Peryshkov and coworkers for Ru-carborane complexes 40d "cw k " p c occurs following a rotation of the carborane relative to the metal center. In the present case, two mechanisms have been investigated entailing that: a) the rotation is accompanied by a decoordination-recoordination of the BPin + , meaning "c -w k " c cc via the generation of 6; b) the rotation occurs while the BPin + ligand is still coordinated to Ir, i.e.

"c -w k " c cc in 7. Figure 9 sums up all the different considered mechanisms , i.e., proposed and also improbable pathways are included).

The decoordination of a BPin + cation from 7 to yield 6 is highly unfavourable (ΔG = +89 kcal/mol). Even considering that another HBPin molecule could decoordinate the BPin + cation, yielding 6 and the elusive [H(BPin) 2 ] + cation, is associated with a barrier of ΔG = +67 kcal/mol. Moreover, the activation barrier for the rotation of the carborane in 6 is high as well (ΔG ‡ = +41 kcal/mol), ruling out the p f "c -w k " via 6 as the preferred path.

The rotation of the carborane in 7 seems more energetically accessible (ΔG ‡ = +28 kcal/mol). The transition state TS1 corresponding to the cage rotation can apparently be reached by a rotation c (" w " " w " Scheme S12). Neither pathway can a priori be excluded. Further investigation f c "c -w k " seem to occur directly from 7 but rather from closely related complex designated 7a (resembling the proposed experimental equilibrium structure in CD 2 Cl 2 solution). In a preliminary step, H 1 is transferred from B Pin to B 1 generating this so-called reactant complex 7a (ΔG(7→7a) = +2 kcal/mol, with virtually no discernable transition state). Then the cage rotation occurs through TS1 to give 7a' in an inwards manner, with an activation barrier for the cage rotation of ΔG ‡ = +26 kcal/mol. Any attempt of localization of such a reactant complex corresponding to the outwards rotation of the cage failed. Even though an outwards mechanism cannot be completely ruled out, the inwards mechanism is favored.

A mechanism involving a direct intramolecular H/H exchange between the Ir-bound hydrogen and one of the nearby cage boronbound hydrogens was also considered (step 7'→7'' in Figure 8). The calculated energy barrier of ΔG ‡ = +47 kcal/mol is considerably higher than that for 7→7' and therefore not a preferred pathway. Quite informative is the IGM-based analysis [START_REF] Karmazin | The IGM-δg inter descriptor is a measure of the electron sharing between two user-defined fragments, assessing the mutual density Ruhlmann[END_REF] of the interactions existing behind the relative position of H 1 , the orientation of the BPin ligand and, to a lesser extent, the orientation of the cage in the starting complex 7 and the reactant complex 7a that reveal the predisposition (or so-c " -c " 54c ) for the inwards cage rotation (see the Supporting Information for more details).

CONCLUSION

This work is a new contribution to the organometallic chemistry of carboranes, an fascinating class of compounds pioneered more than 60 years ago by Hawthorne 545 , and also to the chemistry of metal-hydroborane complexes. [START_REF] Peryshkov | Coordination Chemistry of Carborane Clusters: Metal-Boron Bonds in Carborane, Carboranyl, and Carboryne Complexes[END_REF] The main findings of this study are: 1. The iridium boryl complex of the carba-closo-dodecaborate anion (3), obtained by intramolecular B cage -H activation, mediates the B-H activation of external pinacolborane, leading to the new iridium-hydroborane adduct 7. Calculations suggest that formation of 7 from 3 proceeds by the decoordination of NCMe, the coordination of HBPin and an essentially barrierless B-H activation. 2. Despite being highly air-sensitive, 7 was isolated in good yield and fully characterized, including single crystal X-ray diffraction. 3. According to experimental and theoretical data, 7 is best described as an iridium-stabilized borenium complex, representing the first case of a well-characterized transition metal-borenium adduct. 4. Solution NMR data of 7 are consistent with cleavage of the BPin B-H bond to such an extent that only the B cage -H 1 but not the H 1 -B Pin interaction can be detected. 6. Calculations suggest that the CBPy boron cluster to some extent also contributes to the stabilization of the formal BPin + moiety through noncovalent interactions. This assumption gains support from the IGM-based Δg inter (attr) fragment pairs interaction data (Table S17): Very similar Δg inter (attr) values were obtained for both the Ir-BPin (0.861 a.u.) and CBPy-BPin (0.797 a.u.), indicating a comparable extent of attractive stabilization of the formal BPin + fragment by both the Ir and CBPy fragments. This feature is also apparent upon closer analysis of the experimental X-ray structure of 7: the planar geometry around the coordinated BPin ligand and its orientation predispose the empty p orbital to face both one of the methyl groups of Cp* and two hydrogen atoms of the boron cage cluster . 7. Our findings on "c -w k " w 7 are the first description of this process for the carba-closo-dodecaborate anion CB11. "Cage-walking" has already been described for C 2 B 10 H 10 -based boron clusters 40a-e , and was first described by Hawthorne in 1990 for the aluminum-boryl cages exo-nido-9,10-(-AlR 2 )(-H) 2 -7,8-C 2 B 9 H 10 (R = CH 3 , C 2 H 5 ), 40f although a similar phenomenon had already observed in a series of polyhedral boranes. [START_REF] Park | Novel exo-nido-Cupracarborane with an Unusual Solution Fluxionality[END_REF] 8. This work includes a detailed DFT study of "cage-walking" process that takes the role of noncovalent interactions (NCIs) into consideration. The calculations suggest the relative rotation of the cage vs Ir in 7 occurs through transition state TS1, with an activation barrier of ΔG ‡ = +26 kcal/mol, thus a process accessible at room temperature and in line with experimental observations. In addition, closer analysis of NCIs found by computations on 7 suggests that an intimate balance between attractive and repulsive NCIs, involving the CBPy, BPin, and the hydride fragments, and also to a smaller extent nearby cage B-H bonds, contribute to the "c -w k " c with a preference for inwards rotation. 9. The results set the stage for designing "c -walker" s with tunable properties, such as rotational speed, based on the choice of the metal/carboranyl/H-E (E = main group element) triad. These are in turn anticipated to enable a better understanding of the organometallic chemistry of carboranes and allow for selective and multiple B cage -H activation and functionalization.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Experimental details, analytical data and details of calculations (PDF). CIF files for the X-ray crystal structures of 2, 3, 6 and 7. These structures have been deposited with the Cambridge Crystallographic Data Centre (deposition numbers CCDC 2170710-2170713).

penetration by the deviation of the true electron density gradient from a non-interacting reference. 48 The maximum value of the δg inter between two fragments can give an indication on the nature of the interaction between them. The δg inter descriptor can also be integrated over the entire space (Δg inter score) to measure the total (attractive, non-bonding and repulsive) interaction between the two fragments, or over the space where the sign of the electron density Hessian second derivative (λ2) is positive (Δg inter (attr) score) to focus on the attractive part of the interactions whilst leaving non-bonding and repulsive parts aside. IBSI is another efficient x w p f " " f p c f G . B δg inter score and IBSI have previously been successfully applied in mechanistic studies for the rationalization of the discrimination between competing reaction pathways and proven particularly useful in assessing reaction predispositions in soc " c c p x ". See: Wu, F.; Deraedt, C.; Cornaton, Y.;

Figure 1 .

 1 Figure 1. a) The carba-closo-dodecaborate anion [CB11H12] - (CB11); b) selected examples of known organo-borenium adducts [L→BR2] + ; c) Main findings of this work.

Figure 2 .

 2 Figure 2. X-ray crystal structures of 3 and 6 (with 25% displacement ellipsoids; countercation of 6 and H atoms except for H1 omitted for clarity).

Figure 3 .

 3 Figure 3. NMR-spectroscopic analysis of the Bcage•••H moiety in 7: (a) 1 H resonance, 1 H resonance with selective { 11 B}decoupling from BPin and 1 H resonance with selective { 11 B}decoupling from cage boron atom B 1 (CD2Cl2, 293 K, 500 MHz); (b) 1 H{ 11 B}-11 B{ 1 H} HSQC NMR spectrum showing the cross-correlation between H 1 and B 1 (CD2Cl2; 293 K; 500/160 MHz).

Figure 4 .

 4 Figure 4. 1 H NMR data for the highlighted hydrogen atoms in free pinacolborane, carboranylpyridine 1, a dodecaborate-based Rh(III) complex with agostic-like B-H•••Rh interactions, complex 7 and iridium hydride 6; width = signal width at half height. Check data of 6 prepared by the optimized procedure

  -8.8 -9.0 -9.2 -9.4 ppm -8.8 -9.0 -9.2 -9.4 ppm -8.8 -9.0 -9.2 -9width = ca. 560 Hz width = ca. 450 Hz width = ca. 300 Hz width = 100 Hz width = 9 Hz

Figure 5 .

 5 Figure 5. X-ray crystal structure of 7 (with 25% displacement ellipsoids; disordered Me groups of BPin and H atoms except for H1 omitted for clarity).

Figure 6 .

 6 Figure 6. Ir 4f region of the X-ray photoelectron spectra of 3, 6 and 7, [Cp*IrCl2]2 and IrCl3•H2O.

Figure 7 .

 7 Figure 7. Experimental evidence for iridium "cage-walking" as displayed in Scheme 8: (a) NMR monitoring of the evolution of the equilibrium of the reaction 3 → 8 (20%) / 9a-d (80%). The percentage of 9a-d was determined by integration of the 1 H NMR signal at -9.2 ppm; (b/c) HRMS analysis of the quenched reaction mixture after equilibration showing the isotopic distribution of 1-[D] (*) and CB11-[D] (HRMS = high resolution mass spectrometry, recorded in (-)-ESI mode; # = unidentified species).

Figure 9 .

 9 Figure 9. Gibbs free energy diagram at the ZORA 42 -PBE 43 -D4(EEQ) 44 /all electron-TZP 45 /COSMO 46 (CH2Cl2) level of the different considered mechanisms involved in the ligand exchange and the "cage-walking" process. For details, see the manuscript text.

  5. The B cage •••H 1 bonding interaction can be regarded as an arrested hydrogen insertion into the Ir-B bond en route to B-H bond formation. This finding is significant as the B•••H bonding interaction in 7 resembles that of a similar B•••H interaction in a [CpM(CO) n CH 3 •••H•••B(OCH 2 ) 2 ] transition state proposed by Hall,

  41e

Table 1 .

 1 Selected distances (Å) in the X-ray crystal structures of

3, 6 and 7 and calculated values for 7.

	B1-H1 -20 to 25 °C, 3 h --1) nBuLi, THF 25 °C, 1 h Method-A 2) CuI, THF BPin-H1 --			[Cu] C	----	1.88(2) Pd(OAc) 2 /L 1 N Br 1.80(3)	2.059 1.804					NaOAc (13 equiv) (1.2 equiv) [Cp*Ir(NCMe) 3 ][BF 4 ]	2	C	N	Ir
	C H				not isolated	(<50% conversion) [Et 4 N]Br for cation exchange (cat.)	C	N			N	-HOAc 4 N][BF -[Et 4 ] -Na[BF MeCN, 55 °C, 22 h 4 ]	B	NCMe
														3 58%
												C
	[Me 3 NH] +				C [Zn]		Br	[Et 4 N] +				B	H
	Method-B	1) nBuLi, Et 2 O 25 °C, 3 h 2) ZnCl 2 , THF					N Pd(OAc) 2 /L 2 (cat.)	1 73% with respect to [Me 3 NH][CB 1 1 H 12 ]	[Et	4 N] +		[Cp*Ir(NCMe)	3 ][BF	4 ]	2	C	N	H
			-20 to 25 °C, 16 h			[Et 4 N]Br for				1		(1.1 equiv)	3 (50%)	+
						isolated	cation exchange						MeCN, 25 °C, 4 h
						as a solid
														(ratio based on
														NMR spectra)
														2 (50%)
			N					N
								H
			C	aq. HCl		C
			Et	2 O, 25 °C
			-Et	4 NCl
	[Et	4 N] +
			1					2 64%			Bond Ir1-N1 Ir1-N2 Ir1-B1 Ir1-H1 Ir1-BPin	3 2.1216(18) 2.0154(19) 2.122(2) ----	6 2.088(4) --2.094(5) 1.51(7) --	7 2.1052(12) --2.1738(16) 1.47(2) 2.0974(18)	7 (calc.) 2.114 --2.155 1.592 2.101

Substitution of NCMe in 3 by other L-type ligands

	Reaction of 3 with either N,N-dimethylaminopyridine (dmap) or
	pyridine (py) allowed the synthesis of the corresponding com-
	plexes Ir-dmap 4 and Ir-py 5 (Scheme 5). Both products were
	characterized by multinuclear NMR spectroscopy and elemental
	analysis. Complex 4 was isolated in 74% yield. The exact yield
	for complex 5 could not be determined because of its poor solubil-
	ity in most of the tested solvents (see the Supporting Information
	for details). For instance, 1 H and 11 B NMR spectra of 5 recorded
	in CD 2 Cl 2 showed significant amounts of impurities attributed to
	residual CB11 and [Et 4 N][BF 4 ], which are much more soluble than 5. 1 H NMR spectra of 4 and 5 both show peaks characteriz-
	ing the Ir-dmap and Ir-py heterocycle moieties in addition to the
	pyridyl ring attached to C cage . In the 11 B NMR spectrum of 4, the
	resonance for B cage -Ir was observed at -15.2 ppm. The B cage -Ir
	signal for the Ir-py complex 5 could not be unambiguously as-
	signed due to overlap with the other boron resonances.
	Scheme 5. Synthesis of complexes 4, 5 and 6 starting from
	iridacycle 3.

ACKNOWLEDGMENT Y.C. acknowledges the GENCI-IDRIS for providing access to the national HPC resources (grant A0100812469) and to the High Performance Computing Center of the University of Strasbourg for providing access to computing resources (grant g2021a248c). S.D. acknowledges the financial support by the National Natural Science Foundation of China (No. 21871231) and the Special Funds for Basic Scientific Research of Zhejiang University (No. 2019QNA3010 and K20210335).

AUTHOR INFORMATION

Notes

The authors declare no competing financial interests.

Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manuscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equation, concept, or theorem, etc., that is discussed in the manuscript.


𝑎 John A Tomsick
email: jtomsick@berkeley.edu

𝑏,𝑎 Steven E Boggs

𝑎 Andreas Zoglauer

𝑐 Dieter Hartmann

𝑐 Marco Ajello

𝑑 Eric Burns

𝑒 Chris Fryer

𝑓 ,𝑤 Chris Karwin

𝑓 Carolyn Kierans

𝑎 Alexander Lowell

𝑔 Julien Malzac

𝑏 Jarred Roberts

𝑎 Pascal Saint-Hilaire

𝑓 Albert Shih

Thomas Siegert

𝑖 Clio Sleator

𝑗 Tadayuki Takahashi

𝑘 Fabrizio Tavecchio

𝑖 Eric Wulf

𝑎 Jacqueline Beechert

𝑎 Hannah Gulick

𝑎 Alyson Joens

𝑎 Hadar Lazar

𝑓 ,𝑙 Eliza Neights

Juan Carlos

𝑎 Martinez Oliveros

𝑗 Shigeki Matsumoto

𝑗 Tom Melia

Hiroki Yoneda

𝑚 Mark Amman

𝑐 Dhruv Bal

𝑔 Peter Von Ballmoos

𝑐 Hugh Bates

𝑛 Markus Böttcher

𝑜 Andrea Bulgarelli

𝑝 Elisabetta Cavazzuti

𝑞 Hsiang-Kuang Chang

𝑎 Claire Chen

𝑞 Che-Yen Chu

Alex Ciabattoni

𝑝 𝑜 Luigi Costamante

𝑛 Lente Dreyer

𝑜 Valentina Fioretti

𝑝 Francesco Fenu

𝑟 Savitri Gallego

𝑘 Giancarlo Ghirlanda

𝑖 Eric Grove

𝑞 Chien-You Huang

𝑔 Pierre Jean

𝑐 Nikita Khatiya

𝑔 Jürgen Knödlseder

𝑠 Martin Krause

𝑐 Mark Leising

𝑓 ,𝑤 Tiffany R Lewis

𝑟 Jan Peter Lommler

𝑡 Lea Marcotulli

𝑓 ,𝑢 Israel Martinez-Castellanos

Saurabh Mittal

Michela Negro

𝑑 Samer

𝑎 Al Nussirat

𝑣 Kazuhiro Nakazawa

𝑟 Uwe Oberlack

𝑑 David Palmore

𝑜 Gabriele Panebianco

𝑜 Nicolo Parmiggiani

𝑓 Tyler Parsotan

Sean N Pike

𝑏 Field

𝑛 Hester Schutte

𝑐 Yong Sheng

𝑓 Alan P Smale

𝑖 Jacob Smith

𝑑 Aaron Trigg

𝑓 Tonia Venters

𝑗 Yu Watanabe

𝑤 Zhang 𝑓

The Compton Spectrometer and Imager

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Overview

The low-energy part of the gamma-ray regime -the "MeV bandpass" -has not been wellexplored due to observational challenges, such as high background levels. However, this is a part of the electromagnetic spectrum where we know that there are signals of great scientific interest, including nuclear lines from radioactive elements and the electron-positron annihilation line at 0.511 MeV. In addition, accreting black holes produce emission at these energies, and this is also a key regime for multimessenger astrophysics (MMA).

Future observations made by the COSI satellite mission will address all of these topics, and its key scientific goals are related to positrons, nuclear lines, polarimetry of accreting black holes, and MMA [START_REF] Tomsick | 37th International Cosmic Ray Conference[END_REF]. COSI is a Compton telescope, and the capabilities that make it unique are its combination of excellent energy resolution, which it gets from cryogenically-cooled germanium detectors, and its large field of view. Thus, COSI is optimized to make all-Galaxy and all-sky emission line images in the MeV bandpass, which will advance our understanding of creation and destruction of matter in our Galaxy.

Here, we provide an update on the COSI mission, focusing on developments since our previous ICRC presentation in July 2021 [START_REF] Tomsick | 37th International Cosmic Ray Conference[END_REF]. COSI was selected by NASA to move to Phase B in October 2021. This was followed by maturing of the requirements and the payload design, and COSI completed a combined System Requirements Review and Mission Definition Review during the first calendar quarter of 2023.

In this proceedings paper, we describe the COSI mission concept and design in Section 2. A summary of how the COSI design enables the key science goals is provided in Section 3. Examples of the extended portfolio of science that will be enabled by COSI are given in Section 4.

COSI mission concept and design

COSI operates as a Compton telescope when multiple photon interactions are detected in the instrument. This occurs when a gamma-ray enters the instrument and undergoes one or more Compton scattering events before the interactions end in a final photoabsorption event. When the full energy of a gamma-ray is deposited in the germanium detectors, there are two quantities that are obtained: the energy of the gamma-ray is determined by adding up the individual interaction energies; and the Compton angle measured from an axis defined by the first and second interactions defines an event circle on the sky ( [21] and references therein). Thus, it is essential that the detectors provide precise energy measurements and 3-dimensional interaction positions.

Figure 1a highlights the main subsystems of the COSI payload: 1. the germanium detectors enclosed in a vacuum cryostat; 2. the front-end electronics; 3. the active shields; and 4. the heat removal subsystem.

Semiconducting germanium detectors are used where gamma-ray interactions liberate electrons and holes which, under high-voltage, drift to opposite sides of the detectors. COSI uses double-sided strip detectors with 64 strips per side and 1.162 mm strip pitch to measure the 𝑥 and 𝑦 interaction positions. The depth (𝑧) position is determined by measuring the time difference between the rise of the electron and hole signals. In addition, the strips are surrounded by a guard ring electrode, which acts as a veto signal. The detectors are cooled to between 80 and 90 K using a mechanical cryocooler. Inside the cryostat, there are 16 detectors (4 stacks of 4), and each detector is 8 × 8 × 1.5 cm 3 . Figure 1b shows the 12 detectors (4 stacks of 3) that flew on a balloon platform. Special processing of the detectors is required [1].

The heart of the front-end electronics is an Application Specific Integrated Circuit (ASIC). The COSI ASIC has 32 channels with separate circuits for timing, which is required to measure the interaction depth, and spectroscopy [20]. Each detector side is read out using three ASICs: two for the 64 signal strips and one for the guard ring. Thus, each detector requires six ASICs, and COSI's 16 detectors use 96 ASICs.

The detectors are surrounded by active shields on four sides and underneath. The scintillator material used is bismuth germanium oxide (BGO), and they are read out by silicon photomultipliers (SiPMs). The shields have several functions, including passive background attenuation, active anticoincidence for background radiation coming from the side and bottom, anticoincidence of gamma-rays that escape from the germanium detectors, and finally as a gamma-ray transient monitor.

The largest consumer of power in the satellite is the cryocooler that cools the germanium detectors. The cryocooler is attached underneath the instrument. The cryostat heat removal system (CHRS) includes the cryocooler, heat pipes for conducting the heat from the cryocooler, and the radiators.

Measurement requirements for key science goals

The key science goals for COSI are: A. Uncover the origin of Galactic positrons; B. Reveal Galactic Element Formation; C. Gain insight into extreme environments with polarization; and D. Probe the physics of multimessenger events. Descriptions of how these goals connect to As-tro2020 decadal survey white papers can be found in [START_REF] Tomsick | 37th International Cosmic Ray Conference[END_REF] and [START_REF] Tomsick | [END_REF], and a condensed summary follows.

The primary measurements for science goals A and B involve making full Galaxy images at 0.511 MeV and at four nuclear line energies. The specific spectral resolution, angular resolution, and line sensitivity requirements for making these measurements are provided in Table 1. These measurements will allow COSI to determine if the 0.511 MeV emission from the Galactic bulge has substructure and will measure the latitudinal extension of the Galactic disk. They will also provide information about the electron-positron annihilation conditions across the Galaxy, including the width of the narrow line and the positronium fraction (by comparing the strength of the narrow line to the orthopositronium continuum). Measurements at 1.157 MeV probe 44 Ti from young (<1000 years old) supernova remnants. Measurements at 1.173 and 1.333 MeV will provide the first Galactic image of 60 Fe, which traces core-collapse supernovae that have occurred over the past millions of years. Finally, measurements at 1.809 MeV from 26 Al produced by massive stars will provide advances over past COMPTEL and INTEGRAL maps in terms of sensitivity and angular resolution. The COSI required line sensitivities are compared to INTEGRAL/SPI and COMPTEL in Figure 2a.

Science goal C requires measurements of polarization, which COSI is sensitive to because the azimuthal component of the Compton scattering angle depends on the polarization of the incident gamma-ray. The COSI requirement focuses on the measurement of polarization for accreting black holes, and the flux limit given in Table 1 corresponds to being able to measure polarization in the 0.2-0.5 MeV band for the brightest three known Active Galactic Nuclei (AGN): Cen A, 3C 273, and NGC 4151. While COSI's gamma-ray measurements have connections to all of the other messengers (gravitational waves, neutrinos, and cosmic rays), it is the detection and reporting of short gammaray bursts (GRBs) from merging neutron stars that drives COSI requirements related to Goal D. COSI's large field of view and sensitivity are important for detecting short GRBs. In addition, the requirement that COSI provides public reports of short GRB localizations within an hour is met by automatically downlinking germanium detector data when gamma-ray transients are detected in the BGO shields.

Extended portfolio of science enabled

Beyond the key science goals, COSI's capabilities will enable a large number of additional scientific studies. We describe a few illustrative examples here, which are taken from an array of investigations planned by the COSI science team. COSI's design is optimized for emission line studies, but it still provides a significant improvement for continuum emission as shown in Figure 2b. Examples using COSI's emission line and continuum sensitivities are highlighted below.

Blazars, jet-dominated AGN with their jets pointing close to our line of sight, appear to emerge as one of the sources of astrophysical high-energy neutrinos. If these jet environments are efficient neutrino production sites, they are likely to be highly opaque to 𝛾𝛾 absorption of high-energy and very-high-energy gamma-rays, leading to suppression of the high-energy gamma-ray flux and the initiation of electromagnetic pair cascades. The emission from these cascades is expected to emerge primarily at MeV -sub-MeV X-ray / soft gamma-ray energies [e.g., 7,14]. This makes COSI the ideal instrument to test for possible correlations between very-high-energy neutrinos detected, e.g., by IceCube or KM3NeT, and MeV-flaring blazars, thus providing further evidence for the neutrino -blazar connection.

Observing gamma-rays at the MeV bandpass also benefits the study of cosmic dark matter (DM). Attractive DM candidates are predicted in the broad mass range of 10 -22 eV to 10 35 g, and COSI has the potential to search for many of them. In the ultralight mass region, where the DM mass 𝑀 DM << 1 eV, the axion-like particle (ALP) is an attractive candidate. COSI will be sensitive to the effects of ALPs on the flux and polarization of MeV gamma-rays emitted from, e.g., blazars [6]. In the light mass region (1 eV ≲ 𝑀 DM ≲ 100 GeV), many influential DM candidates have been proposed, including the sterile neutrino, dark photon, and light weakly interacting massive particle (WIMP). Those are predicted around the MeV mass scale, and COSI will observe MeV gamma-ray signals emitted by their decay and annihilation [2,3]. Moreover, the candidates also emit lowenergy positrons, which are captured by ambient electrons, form positronium, and contribute to 0.511 MeV emissions. In the heavy mass region where DM is heavier than the EW scale, the heavy WIMP is known to be a well-motivated candidate. Though it primarily emits GeV/TeV gamma-rays by its annihilation, MeV gamma-rays are also produced via inverse Compton scattering caused by energetic electrons from the annihilation. In the ultraheavy mass region where DM is heavier than the Planck scale, primordial black holes (pBH) with mass around 10 17 g could still compose all of the DM in the present universe. Such pBH would emit both MeV gamma-rays and positrons via Hawking radiation [3].

The prompt emission spectrum of GRBs is often described by the Band function. But the underlying radiation mechanisms are still debated. Synchrotron emission in either ordered or random magnetic fields can contribute to the prompt emission, while photospheric emission may also explain the observed spectrum. COSI can distinguish these mechanisms by measuring the MeV polarization, since they produce distinct polarization signatures [5,8,13,17]: synchrotron polarization degree depends on the orderness of the magnetic field, while the photospheric emission may produce polarization degree from 0 to 100% depending on the geometry of the Compton scattering, which is probably anisotropic in the prompt emission region. By observing the polarization degree of >30 GRBs, COSI will be able to tell synchrotron in an ordered magnetic field apart from synchrotron in a random magnetic field and photospheric models, since the latter two are characterized by a different number distribution of GRB prompt polarization. Additionally, COSI may observe the time-dependent polarization signatures for a few bright GRBs during its mission span, which can further probe the underlying physical conditions in those bright GRBs.

Thermonuclear supernovae (SN Ia) are important tools of modern cosmology and significant contributors to galactic nucleosynthesis and energetics, yet their progenitor systems and explosion mechanisms are poorly understood. As shown in Figure 2a, the expected peak flux of the prominent 0.847 MeV line of 56 Co decay is below the 10 6 s COSI sensitivity for a SN Ia at distance 20 Mpc. However, with COSI's complete coverage of the ∼10 7 s light curve peak and sensitivity to other lines, e.g., 1.238 MeV, COSI should detect 56 Co from SN Ia at this distance, within which there is roughly one per year. For nearer SN Ia, like the previous SN 2011fe and SN 2014J (see, e.g., [4]), COSI could monitor the line light curves and resolve their velocity profiles, yielding independent determinations of the synthesized 56 Ni mass, its distribution within the ejecta, and total ejecta mass and kinetic energy [START_REF] Leising | [END_REF]. These would provide important constraints on the thermonuclear explosions and white dwarf progenitor systems.

Classical nova explosions are caused by thermonuclear runaways of material accreted by white dwarfs in a close binary systems. The material ejected at a velocity of several thousands of km/s is enriched with freshly synthesized nuclei. Among these, the unstable 13 N and 18 F decay with a lifetimes of 10 min and 1.8 hr, respectively, by emitting positrons that rapidly annihilate in the dense ejecta in expansion. The resulting annihilation photons and their Comptonization produce a prompt emission (∼2 hr) of a 0.511 MeV line and a gamma-ray continuum <0.511 MeV [9, 10], which has not been detected yet [15,16]. The long-lived radioactive nuclei synthesized in the explosion are 7 Be in CO white dwarfs and 22 Na in ONe white dwarfs. Their lifetimes (53 days and 2.6 yr, respectively) are long enough for them to decay in a mostly transparent ejecta by emitting gamma-ray lines at 0.478 MeV and 1.275 MeV, respectively. The detection of the prompt emission with COSI would provide information on the outburst conditions and on the properties of the ejecta, while the detection of 0.478 MeV and 1.275 MeV lines would provide the type (CO or ONe) of the underlying white dwarf and the mass of released radioactive isotopes, which give unique constraints to explosive nucleosynthesis models.

COSI's continuum sensitivity will also provide new constraints on MeV-band emission from the Fermi Bubbles [12] and the Galactic Diffuse Continuum Emission (GDCE). The GDCE in the COSI energy band is dominated by inverse Compton radiation, produced by cosmic ray (CR) electrons up-scattering low-energy photons of the interstellar radiation field. There is also expected to be subdominant contributions from Bremsstrahlung radiation, unresolved point sources, and plausibly other sources as well. COSI's continuum sensitivity and wide field of view will enable measurements of a highly precise spectrum of the GDCE over the entire sky. Such measurements will play a major role in disentangling the different components of the emission, including searches for new physics, and they will likely lead to new insights regarding the sources of CR electrons in the Galaxy. More details regarding COSI's prospects for probing the GDCE can be found in the ICRC2023 proceedings paper by Karwin et al.

Figure 1 :

 1 Figure 1: (a) The COSI payload design. (b) The 12 germanium detectors (4 stacks of 3) in the COSI-balloon instrument.

1 . 2 ×

 12 10 -5 photons cm -2 s -1 at 0.511 MeV 3.0 × 10 -6 photons cm -2 s -1 at 1.157 MeV ( 44 Ti) 3.0 × 10 -6 photons cm -2 s -1 at 1.173 MeV ( 60 Fe) 3.0 × 10 -6 photons cm -2 s -1 at 1.333 MeV ( 60 Fe) 3.0 × 10 -6 photons cm -2 s -1 at 1.809 MeV ( 26 Al) Flux limit for polarizationb 1.4 × 10 -10 erg cm -2 s -1 (0.2-0.5 MeV) Reporting short GRB detections <1 hour reporting time ∼1 • localizations (accuracy depends on GRB fluence) 100 ms absolute time accuracy a 3𝜎 narrow line point source sensitivity in 2-years of survey observations. b For 50% minimum detectable polarization in 2-years of survey observations.

Figure 2 :

 2 Figure 2: Narrow-line (a) and continuum (b) sensitivities based on COSI's requirements compared to current and previous instruments. The sensitivity curves are for point sources at the 3-𝜎 level during 2 years of COSI survey time. Due to the all-sky coverage that COSI obtains, these sensitivities will be reached for every source in the sky.

Table 1 :

 1 COSI Requirements

	Parameter

• at 0.511 MeV and 2.1 • at 1.809 MeV Line Sensitivity a

satellite mission in development with a planned launch in 2027. COSI is a wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV. It provides imaging, spectroscopy, and polarimetry of astrophysical sources, and its germanium detectors provide excellent energy resolution for emission line measurements. Science goals for COSI include studies of 0.511 MeV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization measurements, and detecting and localizing multimessenger sources. The instantaneous field of view for the germanium detectors is >25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts and other gamma-ray flares over most of the sky. In the following, we provide an overview of the COSI mission, including the science, the technical design, and the project status.38th International Cosmic Ray Conference (ICRC2023) 26 July -3 August, 2023 Nagoya, Japan

Acknowledgements: COSI is a Small Explorer mission and is supported under NASA contract 80GSFC21C0059. The research of Dr. Lewis, Dr. Zhang, and Dr. Karwin was supported by an appointment to the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Oak Ridge Associated Universities under contract with NASA.

The Compton Spectrometer and Imager (COSI) is a NASA