²((Catch Phrase)) DOI: 10.1002/anie.200((will be filled in by the editorial staff)) A Silylene Stabilized by a -Donating Nickel(0) Fragment María Frutos, Nasrina Parvin, Antoine Baceiredo, David Madec, Nathalie Saffon-Merceron, Vicenç Branchadell, Tsuyoshi Kato* Abstract: A donor-stabilized silylene 4 featuring a Ni(0)-based date, the only known compounds of this type V are germylene-, donating ligand was synthesized. Complex 4 exhibits a pyramidalized stannylene- and plumbylene-based complexes (VII-X).6789 [6-9] DFT and nucleophilic Si(II) center and shows a peculiar behavior due to calculations predicted that, although germylenes and stannylenes are the cooperative reactivity of Si and Ni centers. Calculations indicate able to form complexes of type V, lighter analogues (silylenes and that the orientation of Ni-ligands with respect to the silylene moiety carbenes) tend to form classical R2E:→M complexes IV, [7] although is crucial in determining the role of Ni-fragment (Lewis acid or Lewis several methanediide-TM complex, featuring a pyramidalized carbon base) towards silylene. Indeed, a simple 90° rotatation of Si-Ni-bond, centre, have been described. 10 [10] To the best of our knowledge, reverses the role of Ni, and transforms a classical silylene→Ni(0) complexes V involving a silylene fragment M→SiR2 remain elusive, complex into an unprecedented Ni(0)→silylene complex. and no clear synthetic access has been proposed, despite their potential usefulness as an extended model of Lewis base-stabilized Transition metals basically act as Lewis acids and thus form silylenes VI. various complexes I interacting with Lewis base ligands. Despite less common, transition metals also act as Lewis bases1[1] to interact with p n Lewis acids (A) to form non-classical M→A complexes II. This non- L M A M E E M E E classical mode of interaction (known as Z-ligands2[2] or metal-only I II III IV V M VI L Lewis pairs3[3]) leads to new methods to tune/improve properties of transition metals. 4[4] Singlet divalent group-14 species III such as ML 3 PCy3 Pb carbenes and their heavier analogues (R2E, E = C, Si, Ge…), featuring L3ClM MClL 3 iBu Sn N N iBu E Pt a divalent E center with a lone pair orbital (n) and a vacant orbital X X Cl Cl M = Rh, Ir M = Pd, Pt PCy3 (p), present an ambiphilic character. Consequently, like in the case VII L = PMe3 VIII L = PMe3 IX E = Ge, Sn, P, X = Cl, Br of transition metals, there are potentially two coordination modes via -electron donation: (i) either from R2E to metal [R2E:→M] IV or, Ar N N Me3Si SiMe3 (ii) from metal to ER2 [M→ER2] V. Classical R2E:→M complexes Si Cl NHC PMe3 IV, presenting a planar E atom, categorized as Fischer- or Schrock- Ge Ni O Si Ni Si Ni Cl Ar N NHC types complexes, 5[5] are ubiquitous and play an important role in Cl Si PMe3 Ar N N Me3Si SiMe3 Me3Si Ar = 2,6-(Ph2CH)2 C6H3 synthetic chemistry. In contrast, and not surprisingly, M→ER2 X XI XII complexes V, characterized by a strongly pyramidalized E center as base-stabilized metallylene VI, are less common and only a few NHC Si Ni examples are known. How could the formation of such non-classical Cl Ar N NHC complexes V be promoted? On descending a group in the periodic table, the nucleophilicity of divalent atom (E) decreases (increasing Me3Si XII-bis s-character of lone pair) and the unoccupied pπ orbital becomes more Lewis acidic. Therefore, heavier divalent species (E = Ge, Sn, Pb) Figure 1. Classical and non-classical metallylene complexes present stronger tendency to form M→ER2 complexes V. Indeed, to Here, we would like to report the synthesis of an unusual 16e- [] Dr. M. Frutos, Dr. N. Parvin, Dr. A. Baceiredo, Dr. D. nickel(0)-silylene complex 4, presenting a strongly pyramidalized Madec, Dr. T. Kato and nucleophilic divalent silicon center, which can be regarded as a Laboratoire Hétérochimie Fondamentale et Appliquée silylene complex stabilized by coordination of -donating Ni(0) (UMR 5069), Université de Toulouse, CNRS, 118 route de ligand. DFT calculations indicate that the orientation of Ni-ligands Narbonne, F-31062 Toulouse (France) E-mail: tsuyoshi.kato@univ-tlse3.fr relative to silylene fragment is crucial in determining the coordination Homepage: https://lhfa.cnrs.fr/ (Equipe - ECOIH) mode of R2Si-Ni0L2 complexes, and a simple 90° rotation of Si-Ni bond reverses the role of Ni-fragment which turns from Lewis acid to Dr. N. Saffon-Merceron Lewis base (IV → V). Institut de Chimie de Toulouse (UAR 2599), Université de 16e-Ni(0) complexes featuring silylene ligands (XI, XII) usually Toulouse, CNRS, 118 route de Narbonne, F-31062 present a short Si=Ni double bond and two trigonal planar Si and Ni Toulouse (France) centers which are perpendicular to each other. 11 [11] We have Prof. V. Branchadell computationally studied Ni0-silylene complexes by hypothesizing Departament de Química that the Si=Ni -bond can be broken through a rotational distortion Universitat Autònoma de Barcelona around SiNi-bond axis and thus changing the coordination mode. 08193 Bellaterra (Spain) Calculations predict that such a 90° SiNi-bond rotation in complex Supporting information for this article is available on the model H2Si=Ni(PMe3)2 1, affording rotamer 2, is only moderately WWW under http://www.angewandte.org or from the exergonic (G1→2 = 6.6 kcal/mol) and leads to a dramatic geometrical author. modification (Figure 2). Indeed, rotamer 2 exhibits a strongly 1 pyramidalized Si center (°Si = 305.9°) and a considerably elongated decrease of HOMO-LUMO energy gaps (EHOMO-LUMO: 3.49 eV for Si-Ni bond (2.249 Å) compared to that of 1 (2.118 Å). This structural 1 vs 2.96 eV for 2 and 4.19 eV for XII vs 2.73 eV for XII-bis), and feature of 2 is in agreement with a Si atom holding a lone pair of therefore an increased reactivity of the non-classical complexes (type electrons and a reduced Si-Ni multiple bonding character. Indeed, it V) compared to classical ones (type IV). In contrast, in the case of the was found that the negative charge of H2Si fragment in 2 [-0.40 (H2Si) corresponding carbene complexes [R2C-Ni(PMe3)2, R = H, F], such a / +0.40 (Ni(PMe3)2] is increased from that in 1 [-0.17 (H2Si) / +0.17 geometrical change was not observed by the C-Ni bond rotation and (Ni(PMe3)2] and that the Wiberg and Mayer bond orders of the Si-Ni the classical carbene→Ni complex (with a planar carbon center and a bond of 2 (0.602 and 0.812) are smaller than those calculated for 1 short C=Ni double bond) was calculated to be the only stable isomer. (0.820 and 1.163). Furthermore, intrinsic bond orbital (IBO) In order to verify the theoretical results, we have considered the analysis[12]12 of 1 clearly indicates the presence of a Si=Ni double use of a rigid planar bridging system connecting silylene- and metal- bond constituted of R2Si:→Ni -donation (Si-Ni -bonding orbital fragments which imposes such a bond rotated geometry of silylene- with a main occupancy at Si, Figure 1a) and -back donation from Ni Ni complex of non-classical complexes of type V. For this purpose, to Si (-bonding orbital with a main occupancy at Ni, Figure-1b). In we employed the phosphine-stabilized (amino)(chloro)silylene 313[13] marked contrast, in the case of rotamer 2, a large part of Si-Ni - as a precursor, in which the amino substituent and the phosphine bonding electrons is localized at the Ni atom (0.38 at Si and 1.60 at ligand are linked by a planar olefin bridge. Similarly to the synthesis Ni, Figure-1c), in agreement with a Ni→Si -donation. Furthermore, of complex XII,[11b] the two successive additions of equimolar the IBO analysis also indicates the presence of a lone pair localized amounts of Ni(COD)2, and N-heterocyclic carbene (NHC), to on the Si atom in 2 with no contribution at Ni (1.90 at Si and 0.00 at chlorosilylene 3, in fluorobenzene at RT, afford silylene-Ni(0) Ni, Figure-1d), instead of -bonding orbital. These results complex 4 (Scheme 1). The reaction probably proceeds via the demonstrate that a 90° rotation of Si-Ni bond in 1 induces, not only insertion of Ni into the Si(II)-P-bond followed by the COD ligand the SiNi--bond breaking but also the reversal of the role of Ni(0)- substitution by NHC at the Ni center. Complex 4 was isolated as air- fragment: from Lewis acid (coordination center) to Lewis base sensitive crystals from a THF solution at RT (yield 70 %). (coordinating ligand). °Si = 360.0 °Ni = 351.1 °Si = 305.9 °Ni = 358.9 PMe3 G = 6.6 kcal/mol PMe3 H Si Ni 110.79 Si Ni 161.1° H 2.118 Å Si-Ni bond rotation by 90° H 2.249 Å PMe3 PMe3 H 1 2 3.94 eV EHOMO-LUMO 2.96 eV a) c) Scheme 1. Synthesis of Ni-stabilized silylene 4 Si-Ni of 1 [1.69 (Si) / 0.25 (Ni)] Si-Ni of 2 [0.38 (Si) / 1.60 (Ni)] b) d) Si-Ni of 1 [0.41 (Si) / 1.55 (Ni)] LPSi of 2 [1.92 (Si) / 0.00 (Ni)] Figure 3. Molecular structures of 4. Thermal ellipsoids represent 30 % probability. H and disordered atoms are omitted for clarity. Selected Figure 2. Calculated free energy G (kcal/mol) for the isomerization of bond lengths [Å] and angles [°]:Si1-Ni 2.178(1), N1-Si1 1.840(2), Si1- silylene-Ni(0) complex 1 to the corresponding rotamer 2 by a 90° Cl 2.196(1), Ni-C1 1.934(2), Ni-P 2.126(1), P-C2 1.762(2), C2-C3 rotation of Si-Ni bond as well as the geometry and selected intrinsic 1.359(3), C3-N1 1.360(2), Cl-Si1-N1 95.35(6), Cl-Si1-Ni 110.94(4), N1- bond orbitals (IBOs) of each rotamer (1 and 2). Values in parenthesis Si1-Ni 115.29(6), Si1-Ni-C1 101.53(6), C1-Ni-P 164.51(6), Si1-Ni-P are the contributions of Si-Ni - and -bonding electrons at Si and Ni 93.87(2), Ni-P-C2 117.05(7), P-C2-C3 129.90(16), C2-C3-N1 atoms. 129.62(18), C3-N1-Si1 117.17(13). °Si = 321.58°. Torsion angles: C1- Ni-Si1-Cl = 33.16(7)°, P-Ni-Si1-N1 = 38.21(7)° Driess' complex XII[11b] undergoes a similar isomerization with a relatively small energy (GXII→XII-bis = 12.1 kcal/mol) affording Even though silylene complex 4 slowly decomposes in solution rotamer XII-bis (Figure 1) with a strongly pyramidalized Si center (THF, fluorobenzene) (t1/2 = 2 days at RT, and t1/2 = 2 h, at 80 °C), it (°Si = 337.73°) and a single Si-Ni bond (2.244 Å). Of particular was characterized by NMR spectroscopy. In the 29Si-NMR spectrum, interest, such geometrical and electronic modifications of silylene- a doublet signal appears at 138.1 ppm (2JSiP = 71.8 Hz) in the region nickel complexes (1→2 and XII→XII-bis) induce a considerable of that of the related acyclic (chloro)aminosilylene-Ni(0) complex 2 XII ( = 123.2 ppm)[11b] but significantly downfield compared to the (LUMO, -1.389 eV) corresponds to the anti-bonding *-orbital of Si- starting phosphine-stabilized silylene 3 (-10.5 ppm).[13] The 31P-NMR Ni bond with a large coefficient on the Ni atom. Similar to the case spectrum displays a singlet signal at 91.4 ppm, also downfield shifted of XII-bis, the HOMO-LUMO energy gap of 4 (EHOMO-LUMO: 3.03 compared to 3 (75.0 ppm). A doublet signal observed at 186.7 ppm in eV) is calculated to be small, suggesting an enhanced reactivity. As 13C-NMR spectrum was attributed to the divalent carbon of NHC expected, IBO analysis of 4 show the same pattern of Ni→Si -bond ligand. The relatively large carbon-phosphorus coupling constant and Si-lone pair (nSi) orbitals as those calculated for 2 (Figure-1c,d) (2JCP = 50.4 Hz) is in good agreement with the T-shaped geometry (see the supporting information). around the Ni atom with the NHC ligand in a trans-position relative to the phosphine.[8c] The molecular structure of 414[14] reveals a strongly pyramidalized Si1 center (°Si = 321.58°) similarly to the case of the phosphine- stabilized silylene 3 (°Si = 292.0)[13] and an elongated Si-Ni bond (2.178 Å) compared to other Ni(0)-silylene complexes (2.075 - 2.133 Å) (Figure 3).10] This value is within the range of Ni-Si single bonds.15[15] These structural data of 4 are in agreement with a non- classical complex V (Ni→silylene) with a lone pair on the Si atom and a reduced Si-Ni multiple bonding character. The Ni(0) site presents a planar and T-shaped geometry (°Ni = 359.91) with a large P-Ni-C1NHC angle (164.51°), in contrast to the case of silylene- Ni(NHC)2 complex XII with a trigonal planer Ni site (CNHC-Ni-CNHC = 111.4°).[11] A similar geometry was observed for other Scheme 2. Reactions of Ni-stabilized chlorosilylene complex 4 M(0)→metallylene complexes IX-X (M = Ni, Pt)[8,9] as well as for L2Ni→Lewis acid complexes.16 [16] In agreement with the MO analysis, contrary to other silylene-Ni a) b) complexes presenting an electrophilic character at the Si atom,[11a,17]17 silylene complex 4 presents a nucleophilic Si center, which has been demonstrated by the immediate reaction with MeOTf, leading to Si- methylated Ni(II) complex 5 (Scheme 2). In contrast, a Lewis base such as iso-propyl isocyanide coordinates on the metal center to form a tetra-coordinate Ni0 complex 6. A less pyramidalized Si(II) center (°Si = 349.69°) and a shorten Si-Ni bond [2.1108(5) Å] in 6 compared to those of 4 [°Si = 321.58°, Si-Ni: 2.1780(7) Å] suggest an enhanced Si→Ni -back donation in 6 probably due to the geometrical modification at Ni(0) center (T-shape → distorted tetrahedral). LUMO (*Si-Ni) : -1.389 eV G (kcal/mol) H Cl NHC c) d) Si Ni Ar N H PR2 (6.3) (13.4) 13.2 (25.9) H H NHC (19.5) 0.0 (15.6) Cl Si Ni NHC Ar N PR2 Cl Si -13.2 -13.7 -14.2 Ni + H2 Ar N PR2 Cl NHC Cl H NHC H Cl - 11 H Si Ni H H Si Ni H Si 25.6 Ni NHC H NHC 4 Ar N PR2 Ar N PR2 Ar N PR2 H Si Ni Cl Ar N PR2 HOMO (nSi) : -4.418 eV 7 7-cis 8-cis 8 Figure 4. Frontier molecular orbitals [LUMO (a, b) and HOMO (c, d)] of Figure 5. Calculated reaction pathways for the hydrogenation of 4 and 4 (isosurface level = ±0.05 e/(a.u.)3) and their energy levels (eV) calculated relative Gibbs energies G (kcal/mol) of each intermediate calculated at the M06/Def2TZVP level. and product 8 from that of 4+H2. In parenthesis are calculated Gibbs energy barriers Gǂ (kcal/mol). To gain more insight into the electronic structure of Ni0→silylene complex 4, DFT calculations have been performed at the Silylene complex 4 also readily reacts with H2 at RT to afford a M06/Def2TZVP//M06/6-31G(d) level of theory (Figure 4). The formal 1,2-dihydrogen adduct 7 which slowly isomerizes to the optimized structure of 4 agrees quite well with the experimentally corresponding isomer 8 by the substituent exchange of H and Cl on observed one (Si-Ni: 2.227 Å, Si-N: 1.831 Å, Si-Cl: 2.19 Å, C1-Ni- the Si and Ni atoms respectively (Scheme 2). Although intermediate P: 169.47°, °Si = 319.37°). The highest occupied molecular orbital 7 could not be isolated, the two 1H-NMR signals corresponding to the (HOMO, -4.418 eV) corresponds to the lone pair orbital mainly Si-H (4.92 ppm, JHH = 6.5 Hz, JPH = 5.6 Hz, JSiH = 176.5 Hz) and Ni- localized on the Si atom and the lowest unoccupied bond orbital H (-6.49 ppm, JHH = 6.5 Hz, JPH = 56.5 Hz, JSiH = 86.0 Hz) appearing 3 in the characteristic regions indicate its formation. Typically, the in two steps: i) reductive elimination at the Ni center and formation large trans coupling-constants (2JSi-H = 86.0 Hz, 2JP-C(NHC) = 82.8 Hz) of a Si-C bond to generate a SiIV-Ni0 complex 15 with the Si-H moiety suggest a square planar geometry around the NiII atom in 7 with the interacting with the Ni(0) center, and then ii) oxidative addition of the Ni–H function at the trans-position relative to the Si atom. The Si-H moiety to the Ni center to give the final nickel(II)-hydride structure of dihydrosilane-Ni(II) complex 8 was confirmed by X-ray complex 16. Although clean NMR spectra of 14 could not be obtained diffraction analysis.[14] due to the similar kinetics of the two isomerization steps (14 → 15 Silylene-Ni 4 complex also readily reacts with 2,3-dimethyl-1,3- and 15 → 16), careful NMR analysis of the reaction mixture allowed butadiene at RT to give a mixture of two SiVI-NiII complexes (9 and to detect characteristic signals for the Si-H-Ni moiety of 15 in the 1H- 10) with a 1 : 1 ratio, which are formally formed by either a C-H NMR spectrum ( = -3.48 ppm, 2JPH = 4.8 Hz, 1JSiH = 104.1 Hz)19 [19] insertion or a [4+1] cycloaddition at the Si center followed by a 1,2- and in the 29Si-NMR spectrum ( = -15.8 ppm, 2JSiP = 28.6 Hz),20[20] migration of chlorine atom to the Ni center, respectively (Scheme 2). strongly supporting the formation of 15 as an intermediate. Of particular interest, complex 9 isomerizes at 100 °C to give cycloadduct 10 in 2 h. Calculations predict that both reactions start with a formal 1,2-addition of butadiene to the Si-Ni moiety, either via NHC N C N iPr an ene-type reaction of the allylic C-H group, or by a [4+2]- (II) (0) (IV) (II) Ph Si Ni Ph Si Ni H cycloaddition reaction, followed by the isomerization of the resulting PhLi 4 Ar N PR2 Ar N PR2 intermediates via 1,2-migrations of the ligands onto the Si and Ni -78 °C -78°C → RT atoms to give the experimentally obtained 9 and 10 (see Supporting 6 hours 13 16 Information). In both cases, the first step is the most costly step (1,2- > -30°C t1/2 (RT) = 15 min. RT addition or [4+2]-cycloaddition), with very similar energy barriers (Gǂ = 21.3 and 20.7 kcal/mol, respectively), which is consistent with the formation of 9 and 10 in the same proportions. Calculations also N N N iPr C N C indicate that the isomerization of 9 to 10 proceeds via a H (IV) iPr (IV) Ph Si Ni (II) Ph Si Ni (0) tautomerization of 9 to generate a zwitterionic intermediate 12 that H Ar N PR2 RT cyclizes to give 10 (Figure 6). The experimental thermal activation Ar N PR2 required for this isomerization (100 °C) is in agreement with a 14 15 considerably high energy barrier (Gǂ9→12 = 37.9 kcal/mol) and the endergonic nature of the tautomerization step (G9 → 12 = 30.3 Scheme 3. Synthesis of phenyl-substituted silylene 13 and its kcal/mol). isomerization. NHC In conclusion, a donor-stabilized silylene complex 4 with a Ni(0)- 9 H Si Ni Cl 10 based donating ligand (Ni0→silylene complex) was successfully G‡ = 37.9 kcal/mol G‡ = 0.7 kcal/mol G = 30.3 kcal/mol Ar N PR2 G = -50.8 kcal/mol synthesized. Complex 4, exhibiting a pyramidalized and nucleophilic Si(II) center, shows a peculiar chemical behavior due to the 12 cooperative reactivity of the Si and Ni centers. Of particular interest, calculations demonstrate that the orientation of Ni-ligands with Figure 6. Calculated reaction pathway for the isomerization of 9 to 10. respect to the silylene moiety is a crucial factor in determining the role of Ni-fragment (Lewis acid or Lewis base) towards silylene, and a simple 90° rotatation of the Si-Ni bond reverses the role of Ni, In order to check the substituent effect on the reactivity/stability transforming a classical silylene→Ni(0) complex into an original of the Ni→Si complexes, we have also tested the substitution of Ni(0)→silylene complex. In addition, calculations also predict that chloride on the Si atom in 4 by PhLi. The reaction of 4 with PhLi (1 the energy difference between both complex isomers is relatively eq.) proceeds smoothly at -80 °C to generate the corresponding small and, therefore, such a transformation can occur in solution for phenyl-substituted silylene complex 13 (Scheme 3). The formation of models without structural restrictions to prevent the Si-Ni bond 13 was indicated by the characteristic 29Si-NMR chemical shift for rotation and alter their reactivity. More detailed studies on their the Si(II) atom ( = 154.2 ppm, JSiP = 85.6 Hz). However, properties and reactivity are under investigation. phenylsilylene complex 13 is less stable than its precursor 4 and starts evolving above -30 °C via a 1,2-addition of the C-H moiety of NHC ligand across the Si-Ni fragment to give the silyl hydride SiIV-NiII Acknowledgements complex 14 (isomerization completed within 30 min at RT). The formation of 14 is supported by the characteristic signals observed for We are grateful to the ANR (MMdash and Si-POP), the DGA the Si-H group in the 1H-NMR spectrum ( = 5.36 ppm, 3JHP = 5.2 (RAPID-LIMS, Spherea/CNRS) and Marelli corporation (Marelli Hz, 1JSiH = 153.1 Hz) and for the CH2-Ni moiety in the 13C-NMR next-generation scholarship) and the Spanish AEI (grant PID2020- spectrum ( = 45.0 ppm, 2JCP = 12.8 Hz).18[18] The decreased HOMO- 116861GB-I00) for financial support of this work. LUMO energy gap (2.72 eV) calculated for 13 compared to 4 (3.03 eV) is in good agreement with its lower stability. Furthermore, at RT, Received: ((will be filled in by Received: ((will be filled in by the complex 14 isomerizes further by exchange of ligands (H and CH2) editorial staff)) on the Si and Ni centers to give a stable pincer-type nickel(II) hydride Published online on ((will be filled in by the editorial staff)) complex 16.[14] Contrary to the related isomerization of 7 to 8, proceeding via two simultaneous 1,2-migrations of ligands on the Si Keywords: Silylenes ·Density Functional calculations ·Donor ligands and Ni atoms (Figure 5), in the case of 14, the isomerization proceeds ·Metallacycles ·Nickel complexes 4 [1] D. F. Shriver, Acc. Chem. Res. 1970, 3, 231. [11] a) C. Watanabe, Y. Inagawa, T. Iwamoto, M. Kira, Dalton Trans. [2] A. Amgoune, D. Bourissou, Chem. Commun. 2011, 47, 859. 2010, 39, 9414; b) T. J. Hadlington, T. Szilvási, M. Driess, Angew. [3] J. Bauer, H. Braunschweig, R. D. Dewhurst, Chem. Rev. 2012, 112, Chem. Int. Ed. 2017, 56, 7470. 4329. [12] G. Knizia, J. Chem. 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Kato __________ Page – Page A Silylene stabilized by -Donating Nickel(0) Fragment A silylene complex stabilized by a Ni(0)-based donating ligand was successfully synthesized. This complex exhibits a pyramidalized and nucleophilic Si(II) atom and shows a peculiar chemical behavior due to the cooperative reactivity of the Si and Ni centers. DFT calculations demonstrated that a simple rotation of 90° on the Si-Ni bond reverses the role of Ni, transforming a classical silylene→Ni(0) complex into the corresponding Ni(0)→silylene complex. 1 D. F. Shriver, Acc. Chem. Res. 1970, 3, 231. 2 A. Amgoune, D. Bourissou, Chem. Commun. 2011, 47, 859. 3 J. Bauer, H. Braunschweig, R. D. Dewhurst, Chem. Rev. 2012, 112, 4329. 4 a) J. Becica, G. E. Dobereiner, Org. Biomol. Chem. 2019, 17, 2055; b) Ryan C. Cammarota, L. J. Clouston, C. C. Lu, Coord. Chem. Rev. 2017, 334, 100. 5 a) G. Frenking, M. Sola and S. F. Vyboishchikov, J. Organomet. Chem. 2005, 690, 6178; b) I. Dalmazio, H. A. Duarte, J. Chem. 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