Volume 12 Number 5 7 February 2021 Chemical Pages 1571–1946 Science rsc.li/chemical-science ISSN 2041-6539 EDGE ARTICLE Agustí Lledós, Salvador Conejero et al. Reversible carbon–boron bond formation at platinum centers through σ-BH complexes Chemical Science EDGE ARTICLE Reversible carbon–boron bond formation at Cite this: Chem. Sci., 2021, 12, 1647 platinum centers through s-BH complexes† All publication charges for this article Pablo Rı́os, a Rocı́o Martı́n-de la Calle,a Pietro Vidossich,b have been paid for by the Royal Society Francisco José Fernández-de-Córdova,a Agustı́ Lledós*c and Salvador Conejero *a of Chemistry A reversible carbon–boron bond formation has been observed in the reaction of the coordinatively unsaturated, cyclometalated, Pt(II) complex [Pt(ItBuiPr0 )(ItBuiPr)][BArF], 1, with tricoordinated boranes HBR2. X-ray diffraction studies provided structural snapshots of the sequence of reactions involved in the process. At low temperature, we observed the initial formation of the unprecedented s-BH complexes [Pt(HBR2)(ItBuiPr0 )(ItBuiPr)][BArF], one of which has been isolated. From 15 to +10  C, the s-BH species undergo a carbon–boron coupling process leading to the platinum hydride derivative [Pt(H)(ItBuiPr– BR2)(ItBuiPr)][BArF], 4. Surprisingly, these compounds are thermally unstable undergoing carbon–boron bond cleavage at room temperature that results in the 14-electron Pt(II) boryl species [Pt(BR2)(ItBuiPr)2] [BArF], 2. This unusual reaction process has been corroborated by computational methods, which Received 6th October 2020 Accepted 9th November 2020 indicate that the carbon–boron coupling products 4 are formed under kinetic control whereas the platinum boryl species 2, arising from competitive C–H bond coupling, are thermodynamically more DOI: 10.1039/d0sc05522k stable. These findings provide valuable information about the factors governing productive carbon– rsc.li/chemical-science boron coupling reactions at transition metal centers. have been a few reports on the utilization of platinum 1. Introduction complexes for the formation of C–B bonds, either in stoichio- The formation of C–B bonds, mediated by transition metal metric reactions or in catalytic processes.5 Turculet and Stra- complexes, has become an important tool for the synthesis of diotto reported the dehydrogenative coupling of B–H/C–H organoboron compounds,1 an important family of organic bonds at a Pt(II) center,5a whereas Chatani, Tobisu et al. sug- molecules in synthetic organic chemistry and in materials gested the participation of s-BH complexes in the platinum- science.2 From the mechanistic point of view, there have been catalyzed borylation of arenes.5b,c However, the interaction of several reports that point to the participation of s-BH (s- tricoordinated hydroboranes with platinum centers leading to borane) complexes as key intermediates in the formation of the s-BH complexes has proven elusive.6 In addition, examples of carbon–boron bonds in metal-alkyl or -aryl compounds. For transition metal s-borane alkyl complexes [M(R)(HBR2)] are example, Hartwig et al. have shown, by means of DFT calcula- extremely rare, and those that have been isolated did not show tions, that s-BH complexes of rhodium and tungsten are tran- further reactivity involving the formation of carbon–boron sient species in the borylation of alkanes, where the driving bonds.7 Isolation of this type of compounds might offer an force is the formation of the C–B bonds (Scheme 1).3 invaluable opportunity for determining the factors that can lead Although most of this type of C–H borylation processes have been carried out using rhodium and iridium systems,4 there a Instituto de Investigaciones Quı́micas (IIQ), Departamento de Quı́mica Inorgánica, CSIC and Universidad de Sevilla, Centro de Innovación en Quı́mica Avanzada (ORFEO-CINQA), C/ Américo Vespucio 49, Sevilla, 41092, Spain. E-mail: sconejero@ iiq.csic.es b Molecular Modeling & Drug Discovery Laboratory, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genoa, Italy c Departament de Quı́mica, Universitat Autònoma de Barcelona, Edici Cn, Cerdanyola del Vallès, 08193, Spain. E-mail: agusti@klingon.uab.es † Electronic supplementary information (ESI) available: Experimental details, spectroscopic, computational data (PDF). X-ray crystallographic data (CIF). Cartesian coordinates of the selected DFT optimized structures (XYZ). CCDC 2033931–2033934. For ESI and crystallographic data in CIF or other electronic Scheme 1 s-BH complexes proposed as intermediates in the bor- format see DOI: 10.1039/d0sc05522k ylation of C–H bonds of alkanes reported by Hartwig et al.3 © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2021, 12, 1647–1655 | 1647 Chemical Science Edge Article to competitive carbon–boron or carbon–hydrogen coupling products. In this article, we report the spectroscopic observation and isolation of platinum s-BH complexes that undergo either a carbon–boron coupling process or formation of platinum- boryl complexes as a function of temperature. The whole sequence of events underlying these processes has been fol- lowed by NMR and X-ray diffraction studies and investigated by DFT calculations. 2. Results and discussion 2.1 Synthesis of Pt(II) boryl complexes In previous studies we have reported that some low-coordinate Fig. 1 ORTEP representation of the cation of complex 2c. Hydrogen cationic platinum(II) complexes can interact with tetra- atoms (except those of the NH moieties) have been omitted for clarity. coordinated boranes, L$BH3 (L ¼ pyridine, amine), leading to Selected bond distances ( A) and angles ( ): Pt1–B1, 1.988(6); Pt1–C1, stable adducts or to catalytic boron–nitrogen coupling products 2.015(5); Pt1–C11, 2.016(5); B1–N6, 1.424(7); B1–N5, 1.433(7); C1–Pt1– C11, 173.5(2); B1–Pt1–C1, 92.9(2); B1–Pt1–C11, 93.6(2). (amine–borane dehydrocoupling).8 In this context we have examined the reactivity of these platinum systems with tri- coordinated boranes derived from dioxa- and diaza-boroles. We have chosen cationic platinum(II) complex [Pt(It- surrounded by only three ligands, with a formal vacant site trans BuiPr0 )(ItBuiPr)][BArF], 1, as starting material (where ItBuiPr is 1- to the boryl moiety. Very weak, at best, agostic interactions are tert-butyl-3-isopropylimidazol-2-ylidene, and ItBuiPr0 its cyclo- present in the molecule (the closest C to Pt is 3.1  A away).10 The  Pt–B distance of 1.988(6) A is similar to that reported by metalated form through the tert-butyl substituent), since this system has been successfully used for the isolation of related s- Braunschweig in related systems of the type trans-[(Cy3P)2Pt SiH derivatives.9 When complex 1 was treated with either {B(X)X0 }].11 pinacol- (HBpin, a) or catechol–borane (HBcat, b), or 1,3,2- benzodiazaborolane (HBdab, c) in CH2Cl2 at 25  C, boryl 2.2 Low-temperature reactivity studies: characterization and complexes 2a–c were formed quantitatively according to NMR isolation of s-BH Pt(II) complexes spectroscopy (Scheme 2). The reaction proceeds smoothly but reaction times vary considerably from dioxaboroles to the dia- In order to look for intermediates in the reaction process, and in zaborole, from 10 min to 15 h. The 1H NMR spectra of these particular to detect s-BH complexes, low temperature NMR species reveal a highly symmetric environment of the NHC experiments have been carried out. The reaction of 1 with ligands, indicating their chemical equivalence. a slight excess of pinacolborane in an NMR tube in CD2Cl2 at The 11B{1H} NMR spectra exhibit two sets of signals, one for 30  C led to a clean reaction with formation of a new species the Pt–B fragment and the other for the BArF anion. The former that, according to multinuclear NMR, is the s-BH complex 3a resonate at d 12.7, 14.7 and 8.8 for 2a–c, respectively, showing (Scheme 3). The 1H NMR is characterized by a broad signal in coupling to 195Pt in the range 1390–1700 Hz (see ESI†). Crystals the hydride region at 3.93 ppm, showing coupling to 195Pt of 2c suitable for X-ray diffraction studies were grown by slow (1JH,Pt ¼ 357 Hz). This signal becomes narrower upon 11B diffusion of CH2Cl2 solutions into pentane. Fig. 1 shows an decoupling (see Fig. S11 in the ESI†) and it has been assigned to ORTEP-type view of the cation. The platinum atom is the bridging Pt–H–B proton. The 11B{1H} spectrum shows, besides the signal for the BArF anion and some excess of HBpin, a very broad signal at 21.8 ppm.12 A similar behavior has been observed in the reaction of 1 with HBcat leading to 3b (1H NMR: 3.28 ppm, 1JH,Pt ¼ 327 Hz; 11B{1H} NMR: 21.3 ppm) and with HBdab producing 3c (1H NMR: 2.32 ppm, 1JH,Pt ¼ 339 Hz; 11B Scheme 2 Reaction of complex 1 with hydroboranes. Scheme 3 Formation of s-BH complexes 3a–c at low T. 1648 | Chem. Sci., 2021, 12, 1647–1655 © 2021 The Author(s). Published by the Royal Society of Chemistry Edge Article Chemical Science {1H} NMR: 18.6 ppm). All these sigma complexes are stable at temperatures ranging from 15 to +10  C, with complex 3c being the most stable. In line with these ndings, DFT calcu- lations estimate favorable DG of formation at 30  C of 2.3, 0.6 and 3.1 kcal mol1 for 3a, 3b and 3c, respectively (see Fig. S33 in the ESI†). The increased thermal stability of complex 3c has allowed us to obtain crystals suitable for X-ray diffraction analysis by slow diffusion of CH2Cl2 solution into pentane at 25  C (Fig. 2). The platinum center is found in a nearly planar environment, with two NHCs and one –CH2– unit occupying three coordination sites and the B–H moiety the fourth. The angle dened by atoms C13–Pt1–H1 and Pt1–H1–B1 are 168.5(1) and 105(2) , respec- tively, very close to those observed in the related s-SiH deriva- Fig. 3 Localized orbital analysis of 3c. The molecular structure is tive [Pt(HSiEt3)(ItBuiPr0 )(ItBuiPr)][BArF] (173.1(1) and 103(2) ).9 overlaid with the centroids of the localized orbitals (small green dots). Dotted lines highlight the Pt–B and Pt–H distances. The Pt1–H1 and H1–B1 bond distances are 1.69(4)  A and 1.19(3)  A, respectively. The latter is slightly elongated with respect to the B–H bond of boranes13 and comparable to related s-BH As in the previously characterized h1-SiH platinum primary aminoborane complexes.14 The Pt1/B1 distance of complexes,9 the coordination of the s-bonded ligand is very 2.31 A is considerably larger than the sum of covalent radii (2.2  exible. We computed the energetic cost of deforming the Pt– A), suggesting that, if any, the interaction between these two H–B angle in 3c (see Table S4 and Fig. S40†). Starting from the atoms is weak.15 In fact, topological analysis of the electron energy minimum at 101.6 , closing the angle to 78 or opening density and its laplacian (see ESI†) revealed the presence of it to 154 requires less than 6 kcal mol1. More acute angles bond critical points (BCPs) between Pt and H, and H and B, yet (68 ) lead to borane dissociation instead of B–H bond cleavage, no BCP between Pt and B was observed.16 This fact suggests that which contrasts with what was observed between 1 and hydro- the coordination mode is better described as h1-BH instead of silanes, for which closing the Pt–H–Si angle led ultimately to the the common h2-BH mode reported for other transition Si–H oxidative addition.9 Importantly, QTAIM analysis did not metals.6,13,17 Bonding interactions were further analyzed in reveal any BCP between Pt and B in structures with acute Pt–H– terms of localized orbitals.18 The centroid of the localized B angles (Fig. S38 and S39†). orbital corresponding to the B–H bond (XBH, Fig. 3) is close to the H center, in agreement with a marked hydride character of this H atom, and is displaced from the B–H axis towards Pt, 2.3 Thermal evolution of s-BH Pt(II) complexes: reversible indicative of a perturbed sigma bond. At the same time, one Pt C–B coupling en route to boryl complexes centroid (XPt) is slightly moved away (0.431  A) from the nucleus, As mentioned before complexes 3a–c are only marginally ther- compared to the other Pt centroids (0.345–0.398  A), and point- mally stable, and upon warming at temperatures above 15  C ing to B. We have previously characterized similar interactions (3a), 0  C (3b) or +10  C (3c) undergo a rearrangement that in cationic Pt(II) s-silane complexes, which adopt a rare h1-SiH involves the cleavage of the B–H bond and the formation of coordination mode.9 a new C–B bond leading to complexes 4a–c (Scheme 4). The reaction is particularly clean in the case of complex 3a, for Fig. 2 Molecular structure of the cationic component as determined by X-ray diffraction of complex 3c (ellipsoids at 30% probability). Selected bond distances ( A) and angles ( ): Pt1–H1, 1.69(4); H1–B1, 1.19(3), Pt1–C13, 2.054(2); Pt1–H1–B1, 105(2); C13–Pt1–H1, 168.5(1). Scheme 4 Rearrangement of complexes 3a–c. © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2021, 12, 1647–1655 | 1649 Chemical Science Edge Article which a progressive formation of 4a is observed before it value of 1906 Hz is observed in 4c. Since the 1JH,Pt values are undergoes further rearrangement (see below). When complex sensitive to the presence and nature of ligands trans to the 3a is allowed to stand at 5  C for ca. 70 min it undergoes B–H hydride, the interaction of the oxygen atom with the platinum bond cleavage of the borane leading to hydride species 4a, center in 4b is weaker than in 4a whereas, expectedly, the arising from a C–B coupling process with the Pt–CH2 unit. In nitrogen atom has a stronger interaction with the platinum the 1H NMR spectrum the signal of the BH proton of complex 3a atom in 4c.21 This has important implications in the subsequent progressively disappears, whereas a new narrow signal in the reactivity. All complexes 4 evolve at 25  C into boryl species 2, hydride region at 27.11 ppm simultaneously appears, showing but the time required is slightly different between 4a and 4b, a large coupling constant to 195Pt of 2167 Hz. This value is and markedly different for 4c. The fastest is 4b, requiring smaller than that observed for complex [Pt(H)(ItBuiPr)2][BArF] 10 min to be fully converted into 2b, while 4a rearrange in ca. (1JH,Pt ¼ 2496 Hz).19 In addition, the diastereotopic Pt–CH2 35 min. On the other hand, 4c undergoes the C–B bond cleavage protons in 3a merge into a singlet resonance at 2.44 ppm in 4a,20 aer 15 h. Thus, it appears that the stronger the interaction of that correlates in the 1H, 13C-HSQC spectrum with a very broad the X (X ¼ O, N) atom of the boryl fragment with the platinum resonance at 26.8 ppm. The 11B{1H} NMR spectrum shows a very center in complexes 4, the longer it takes to transform 4 into the broad signal at 32.9 ppm. All these data support the platinum- boryl species 2. Therefore, we reasoned that de-coordination of hydride structure shown in Scheme 4, but denite evidence the X atom to open a vacant site at the platinum center is came from X-ray diffraction studies of complex 4a obtained at required for the cleavage of the C–B bond leading to the boryl 20  C. Fig. 4 shows the cationic fragment of complex 4a. The species 2. In order to prove this, intermediate 4a was reacted at hydride ligand has been localized in the difference Fourier map. 5  C with tBuNC (Scheme 5), leading to the 16-electron Pt(II) The structure conrms the formation of a C–B bond, as antic- complex 5. This compound is very stable, and no cleavage of the ipated by NMR analysis. Moreover, one of the oxygen atoms of C–B bond is observed at 25  C in solution, therefore corrobo- the pinacol moiety is interacting with the platinum center (Pt–O rating the need for the vacant site to promote the rearrange- bond distance, 2.247(3)  A), trans to the hydride ligand, which ment of hydride complexes 4 into boryl species 2. explains the reduction of the 1JH,Pt coupling constant of the hydride compared to that in [Pt(H)(ItBuiPr)2][BArF].19,21 2.4 Mechanistic analysis of C–B and C–H bond formation With respect to complexes 3b and 3c, a similar rearrange- The observed reactivity of the 1 + HBR2 systems summarized in ment is observed, although in a different range of temperatures. Scheme 4 suggests a sequential mechanism in which rst C–B However, as the temperature increases, mixtures of these and Pt–H bonds are formed (compounds 4a–c) and then compounds together with intermediates 4b, c and the nal cleaved22 leading to C–H and Pt–B bonds (compounds 2a–c). To boryl complexes 2b, c coexist, hampering full characterization unravel atomic and electronic rearrangements taking place in of intermediates 4b, c (see Fig. S25–S28 in the ESI†). Despite these systems, we carried out DFT calculations (M06 functional) this, the signals observed in the hydride region of the 1H NMR in dichloromethane solvent (SMD continuum model) using spectra are consistent with formation of these transient species species 1 and HBpin (a). Details of the calculations are given in (4b: 24.7 ppm, 1JH,Pt ¼ 2459 Hz at 5  C; 4c: 21.06; 1JH,Pt ¼ the ESI.† Contrary to expectations, however, we found that it is 1905 Hz at 20  C), as those in the 11B{1H} NMR (4b: 33.4 ppm; not possible to directly interconvert 4a into 2a. All attempts 4c: 29.6 ppm). It's worth noting the variations in the magnitude made to nd such a pathway resulted in high energy species (see of the 1JH,Pt coupling constants of these Pt–H hydrides. In 4b the value of 2459 Hz is higher than that of complex 4a, and close to the hydride derivative [Pt(H)(ItBuiPr)2] [BArF], whereas a smaller Fig. 4 Molecular structure of the cationic component as determined Scheme 5 Synthesis of complex 5 (top) and molecular structure of its by X-ray diffraction of complex 4a (ellipsoids at 30% probability). cationic component (bottom). 1650 | Chem. Sci., 2021, 12, 1647–1655 © 2021 The Author(s). Published by the Royal Society of Chemistry Edge Article Chemical Science High Energy Transition States section in ESI†). The solution to this issue came by recognizing the analogy with the borylation of alkanes (Scheme 1).3 Considering the s-BH complex (here 3a) the system can evolve towards the formation of a C–B bond (complex 4a) or with the formation of a C–H bond (complex 2a). Moreover, when raising the temperature, 4a transforms into 2a (Scheme 6). We started the exploration from s-BH complex 3a. However, with the trans-NHC arrangement in 3a, all transition states for transferring a H atom to the cyclometalated methylene frag- ment and forging a Pt–B bond, yielding complex 2a (C–H bond formation), or transferring the Bpin and forming a Pt–H bond, yielding 4a (C–B bond formation), are high in energy (34.0 and 28.5 kcal mol1 above the reagents, respectively; see Fig. S48 Fig. 5 Optimized geometries of s-BH complexes with a cis- and S49 in the ESI†) and inconsistent with the experimental arrangement of NHC ligands. observations. We thus searched for alternative routes. Pt(NHC)2 fragments, even with bulky NHC ligands, are known to be rather exible around the C–Pt–C angle.23 In The computed Gibbs energy prole assuming a cis arrange- previous works we have shown that trans–cis isomerizations of ment of the NHC ligands is depicted in Fig. 6. 3a-cis is the Pt(NHC)2 fragments can occur in their reactions with H2 24 and starting point of the C–B bond coupling pathway (Fig. 6, green silanes.19 Thus, we investigated mechanisms containing species trace). From this point, the B–H bond is cleaved through TS1 with a cis geometry of the NHC ligands. Starting from complex (15.6 kcal mol1) in a process where the boryl group is trans- 1, trans–cis isomerization requires overcoming a barrier of ferred to the methylene fragment (C–B bond formation) at the 12.8 kcal mol1 and yields species 1-cis, 11.1 kcal mol1 above same time that a Pt–H bond is created. The resulting hydride the origin (see Fig. S50 in the ESI†) presumably due to the species Int1 (7.5 kcal mol1) features an agostic interaction increased steric bulk between the isopropyl chain of the cyclo- through one of the C–H bonds from the CH2 group (H–Pt ¼ 1.89 metalated NHC ligand and the other NHC fragment. Coordi-  A, C–H–Pt ¼ 107.5 )10 that stabilizes the vacant position of the nation of HBpin to 1-cis gives a s-BH complex, for which we complex. Further stabilization is observed when one of the found two possible orientations of the HBpin ligand relative to oxygen groups from Bpin coordinates to platinum as in 4a-cis the Pt(NHC)2 fragment (3a-cis and 3a-cis0 , Fig. 5). Both display (2.0 kcal mol1), due to its higher electron donation ability. This a h1-BH coordination mode, with geometrical parameters of the exchange of ligands takes place through TS2 (15.4 kcal mol1). Pt–H–B fragment resembling those of 3a. In 3a-cis Once 4a-cis is formed, cis to trans isomerization through TS3 (7.8 kcal mol1 above 1 + HBpin), the B atom is closer to the (13.6 kcal mol1) yields experimentally observed complex 4a, methylene unit than the H atom (C/B(H)pin ¼ 2.84  A vs. C/ which exhibits both NHC ligands in a trans geometry. During H(Bpin) ¼ 3.05  A) with a Pt–H–B angle of 100.5 . On the isomerization, coordination of the Bpin fragment through contrary, in 3a-cis0 (10.0 kcal mol1) HBpin places its H atom oxygen is lost, giving species Int2, which consists of a cationic closer to the methylene unit than the Bpin fragment (C/ Pt(II) hydride complex analogous to [Pt(H)(ItBu2)2]+, previously H(Bpin) ¼ 2.57  A vs. C/B(H)pin ¼ 3.54 A) with a Pt–H–B angle calculated by our group (the main difference being the presence of 109.2 . of Bpin in one of the carbene ligands and isopropyl instead of tert-butyl substituents).25 Finally, coordination of one oxygen of the Bpin fragment to the metal yields the C–B coupling product 4a, at 6.8 kcal mol1. 3a-cis0 has the right orientation of the HBpin ligand to start C–H bond coupling pathway (Fig. 6, red trace). From here, the H atom is transferred to the C atom at the same time than the Pt–B bond is formed. This step takes place through TS4 (22.1 kcal mol1), aer which the corresponding boryl complex Int3 is formed. Right aer, isomerization via TS5 (10.1 kcal mol1) gives the experimentally observed boryl derivative 2a, which is the thermodynamically most stable compound of the whole energy prole. From the Gibbs energy prole in Fig. 6 it can be inferred that, at low temperature, the product entailing crossing the lowest barrier will be formed (kinetic control). This is the hydride species 4a, arising from a C–B coupling process with the Pt–CH2 Scheme 6 Different reactivity pathways considered in DFT unit, which is formed from 3a-cis aer crossing a barrier of only calculations. © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2021, 12, 1647–1655 | 1651 Chemical Science Edge Article Fig. 6 Gibbs energy profile in dichloromethane for the C–B (green trace) and C–H (red trace) bond formation pathways (NHCs in cis) upon reaction between 1 and HBpin. Gibbs energies at 298 K in kcal mol1. The Gibbs energy of 1 + HBpin has been taken as zero-energy. 7.8 kcal mol1 (TS1). However, going back from 4a to 3a-cis and C–H coupling products and their interconversion. Calcu- requires an activation of only 22.4 kcal mol1, which can be lations have shown that rupture of B–H and H2C–Pt bonds and provided raising enough the temperature. At 25  C transition formation of either C–B and Pt–H or C–H and Pt–B bonds entail state TS4 (22.1 kcal mol1) can be overcome and the reaction is similar energy requirements. To reveal these bonding rear- driven by the formation of the most stable product, 2a (ther- rangements we applied a localized orbital approach, which modynamic control). The boryl complex 2a is formed via a C–H turned highly useful to analyze other organometallic reac- coupling process. The rationalization for the experimental tions.18 In this approach Kohn–Sham molecular orbitals are observations combines the C–B (Fig. 6, green trace) and C–H transformed into maximally localized orbitals (LMOs) and the (Fig. 6, red trace) bond formation pathways: at low tempera- centroids of these LMOs are computed for selected structures tures, C–B bond formation takes place through the aforemen- along the reaction path. The movement of the LMOs centroids tioned described mechanism, yielding 4a. Then, increasing the along the reaction describes the electronic rearrangements temperature to 25  C allows the system to undergo the reverse taking place in the bond-breaking and bond-forming processes. pathway, reaching 3a-cis and, aer a conformational change, In this way an arrow-pushing description of the C–B and C–H 3a-cis0 , which initiates the C–H coupling pathway leading to the coupling processes can be obtained from DFT calculations. most stable product 2a. The different energetic barriers Recently, we applied this approach to analyze C–Si bond observed from 3a-cis and 3a-cis0 leading to C–B or C–H coupling, coupling reactions.19 respectively, might have (at least in part) a steric origin. The Fig. 7 shows the relevant LMOs together with their orientation of the pinacol substituents in 3a-cis0 is likely centroids in three representative structures of the key chem- inducing some steric repulsions with the spectator NHC ligand ical step of each process. The corresponding arrow-pushing in cis to it, and this might account for the different Pt/B bond schemes are also depicted in Fig. 7. The same LMOs are distances (0.13  A) observed in both 3a-cis and 3a-cis0 . Reducing involved in both processes, one describing the H2C–Pt bond the steric effects on the borane might reduce the energy and another corresponding to the B–H bond. Similarly to 3c, differences. In fact, as stated before, in the reaction carried out in the initial structures 3a-cis and 3a-cis0 the centroid of the with HBcat, complexes 4b and 2b coexist in a certain range of B–H bond is displaced and very close to the H center, pointing temperatures, suggesting that, in this case, formation of C–B to the hydride character of this atom. In the formation of the and C–H bonds have very similar energetic barriers. C–H bond the centroid of the C–Pt bond moves from the C–Pt axis to the C–H axis, while the centroid of the B–H bond moves from the B–H axis to the B–Pt axis. This corresponds to 2.5 Electronic analysis of C–B and C–H coupling processes a sigma-CAM type reaction,26 in which, formally, a proton is Exploration of the potential energy surface of 1 + HBpin system being transferred to the methylene fragment and a boryl has allowed disclosing the mechanism for the formation of C–B 1652 | Chem. Sci., 2021, 12, 1647–1655 © 2021 The Author(s). Published by the Royal Society of Chemistry Edge Article Chemical Science Fig. 7 C–H (top) and C–B (bottom) bond forming events from the analysis of the movements of the centroids of the localized orbitals, rep- resented by small purple dots. Isosurfaces of the localized orbitals involved in the bond-breaking/bond-forming processes are also shown. For each process the deduced arrow-pushing scheme is also shown. moiety is being formed. Conversely, in the formation of the 3. Conclusion C–B bond the centroid of the C–Pt bond moves from the C–Pt axis to the B–C axis, while the centroid of the B–H bond moves The low-electron count Pt(II) complex [Pt(ItBuiPr0 )(ItBuiPr)] from the B–H axis to the H–Pt axis (again, a sigma-CAM [BArF], 1, reacts with tri-coordinated boranes in a set of reac- rearrangement). Formally, a borinium27 is being transferred tions that start with formation of s-BH complexes, 3, in which to the methylene fragment and a metal-hydride is being the interaction of the B–H fragment with the cationic platinum formed. Therefore, the possibility to generate both boryl and center adopts a rare h1-BH coordination mode. These species borinium species from a platinum s-BH complex is the evolve, as a function of temperature, by either formation of underlying reason that allows both C–B and C–H bond hydride complexes [Pt(H)(ItBuiPr0 -BR2)(ItBuiPr)][BArF], 4, or coupling processes to take place in this system. platinum boryl derivatives [Pt(BR2)(ItBuiPr)2][BArF], 2. We were able to detect species 3, 4 and 2 by means of low temperature © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2021, 12, 1647–1655 | 1653 Chemical Science Edge Article NMR spectroscopy and, in some cases, by X-ray diffraction, 931; (e) T. Ishiyama and N. Miyaura, J. Organomet. 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