Silicon/Germanium Triple-Bonded Complexes ⇑ ⇑ Hisako Hashimoto , Hiromi Tobita

Coordination Chemistry Reviews 355 (2018) 362–379

Contents lists available at ScienceDirect

Coordination Chemistry Reviews

journal homepage: www.elsevier.com/locate/ccr

Review Recent advances in the chemistry of transition metal–silicon/germanium triple-bonded complexes ⇑ ⇑ Hisako Hashimoto , Hiromi Tobita

Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan article info abstract

Article history: The chemistry of complexes bearing triple bonds between transition metals and silicon or germanium Received 4 June 2017 has made considerable progress in the last two decades. For base-free complexes of this type, neutral Received in revised form 19 September and cationic complexes of Groups 5, 6, 7, 8, and 10 transition metals are known so far. The synthetic 2017 routes for these complexes can be categorized into two patterns: (1) reactions starting from stable diva- Accepted 22 September 2017 lent Group 14 element species (Category I), and (2) reactions starting from tetravalent Group 14 element Available online 20 October 2017 species (Category II). These synthetic reactions are described ﬁrst, and then characteristic structures and This review is dedicated to Prof. Pierre spectroscopic properties are discussed. Finally, several unique reactions originating from the peculiar Braunstein to celebrate his 70th birthday. reactivity of the metal-silicon/germanium triple bonds of silylyne and germylyne complexes are presented. Keywords: Ó 2017 Elsevier B.V. All rights reserved. Transition-metal triple-bonded complex Silicon Germanium Synthesis Structure Reactivity

Contents

1. Introduction ...... 363 2. Synthesis of germylyne and silylyne complexes ...... 363 2.1. Synthetic methods...... 363 2.2. Synthesis of germylyne complexes using germanium(II) halides...... 364 2.2.1. Salt metathesis followed by ligand elimination (Method A) ...... 364 2.2.2. Reactions between germanium(II) halides and labile neutral metal complexes (Method B)...... 365 2.3. Synthesis of germylyne complexes starting from NHC-stabilized germanium(II) halides (Method C) ...... 366 2.4. Synthesis of germylyne complexes starting from germanium(IV) hydrides (Method E) ...... 366 2.5. Ligand substitution reactions ...... 367 2.6. Synthesis of silylyne complexes using NHC-stabilized silicon(II) halides (Method C) ...... 367 2.7. Synthesis of silylyne complexes starting from silicon(IV) hydrides ...... 368 2.7.1. Abstraction of a halide/hydride group from halo/hydro-silylene complexes (Method D) ...... 368 2.7.2. Dehydrogenation from silylene complexes having a (H)M@Si(H) moiety (Method E)...... 368 3. Structures and properties of germylyne and silylyne complexes ...... 369 3.1. Structures and bonding...... 369

Abbreviations: Cp, g5-cyclopentadienyl; Cp*, g5-pentamethylcyclopentadienyl; Cy, cyclohexyl; DFT, density functional theory; DMAP, 4-(dimethylamino)pyridine; DME, 1,2-dimethoxyethane; dmpe, 1,2-bis(dimethylphosphino)ethane; dppe, 1,2-bis(diphenylphosphino)ethane; Eind, 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl; Idipp, 1,3-bis (2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene; IR, infrared; iPr, isopropyl; Me, methyl; Mes, mesityl; MeIMe, 1,3,4,5-tetramethyl-1,3-dihydro-2H-imidazol-2- ylidene; MeIiPr, 1,3-diisopropyl-4,5-dimethyl-1,3-dihydro-2H-imidazol-2-ylidene; NHC, N-heterocyclic carbene; NMR, nuclear magnetic resonance; Ph, phenyl; SIdipp, 1,3-bis t t (2,6-diisopropylphenyl)imidazolidin-2-ylidene; Bu, tert-butyl; Tbb, C6H2-2,6-[CH(SiMe3)2]2-4- Bu; THF, tetrahydrofuran; Trip, 2,4,6-triisopropylphenyl; VE, valence electron; WBI, Wiberg bond indices; Xyl, 2,6-dimethylphenyl. ⇑ Corresponding authors. Fax: +81 22 795 6543. E-mail addresses: [email protected] (H. Hashimoto), [email protected] (H. Tobita). https://doi.org/10.1016/j.ccr.2017.09.023 0010-8545/Ó 2017 Elsevier B.V. All rights reserved. H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 363

3.2. 29Si NMR spectroscopic properties ...... 372 4. Reactions of germylyne and silylyne complexes ...... 373 4.1. Reactions with nucleophiles ...... 376 4.2. Reactions with unsaturated organic molecules ...... 376 4.3. Some other reactions ...... 378 5. Outlook ...... 378 Acknowledgements ...... 378 References ...... 378

1. Introduction metal–silicon/germanium triple-bonded complexes is becoming increasingly clear. In this article, we will review the synthesis, Multiple bonds between transition metals and main-group ele- structures, properties, and reactions of all reported, isolable sily- ments hold a prominent position in coordination chemistry and lyne and germylyne complexes. organometallic chemistry, because of the uniqueness of their bonding modes and fascinating reactions that can only be achieved by these bonds. Needless to say, the most important examples of 2. Synthesis of germylyne and silylyne complexes compounds having these multiple bonds are carbene and carbyne complexes, and a number of excellent books and review articles 2.1. Synthetic methods have been published on their chemistry [1–8]. „ On the other hand, complexes having multiple bonds between Synthetic methods for preparing M E triple bonded complexes transition metals and heavier Group 14 elements such as silicon (M = transition metal, E = Si, Ge) are summarized in Scheme 1. and germanium have been much less extensively studied, mainly These methods are classified into two categories (I and II) depend- because the chemical properties of the compounds of silicon and ing on the starting Group 14 element species. Methods A–C in germanium are greatly different from those of carbon. For instance, the double- and triple-bonded species of silicon and germanium are stable only when they have bulky groups on silicon and germanium, and this makes the research on the reactions of M@E and M„E complexes (M = transition metal, E = Si, Ge) corresponding to alkene and alkyne metathesis reactions difficult. Nevertheless, a significant number of silylene complexes (M@Si) and germylene complexes (M@Ge) have been synthesized in these three decades, and based on the synthetic effort for these complexes, the chemistry of transition metal-silicon/germanium double bonded complexes has made steady progress. You can find several review articles on the synthesis, structures, and properties of silylene and germylene complexes [9–19]. As for the transition metal–silicon/germanium triple-bonded complexes, the first example, a germylyne complex (M„Ge), was reported by Simons and Power in 1996, which was prepared by the reaction of chlorogermylene and an anionic molybdenum complex [20]. They used a stable divalent germanium species bearing a halogen substituent and a very bulky group as a precursor. With respect to the silicon analogue, a neutral, genuine silylyne complex (M„Si) was first reported by Filippou et al. in 2010. They developed a new chlorosilylene having a very bulky group and stabilized by an NHC (N-heterocyclic carbene) as a precursor, and they achieved the synthesis by the reaction of this species with an anionic molybdenum complex [21]. In both cases, divalent Group 14 element species are used as key precursors. Importantly, these synthetic routes starting from divalent Group 14 element species are different from those of carbyne complexes from the viewpoint of the reaction mode, because many carbyne complexes are derived from the corresponding carbene complexes, i.e., prepared via conversion of M@C double bonds to M„C triple bonds. Therefore, if we can convert M@Si or M@Ge double bonds into M„Si or M„Ge triple bonds, the diversity of substituents and ligands in accessible silylyne and germylyne complexes will become much wider. Indeed, the authors have discovered by accident a new reaction that converts a germylene complex to a germylyne complex quantitatively, which was reported in 2012 [22]. We also subsequently reported a different type of reaction that converts a silylene complex to silylyne „ complex in 2016 [23,24]. During the last few years, although the Scheme 1. Synthetic methods for M E triple bonded complexes (M = transition metal, M0 = alkali metal, X = halide, Y = counter anion, L = ligand, LA = Lewis acid, number of examples is still small, the chemistry of transition LB = Lewis base, R = substituent, R0 = alkyl, Sub = substrate). 364 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

Category I are based on reactions using stable divalent Group 14 element halides (:ERX). Method A is the salt metathesis between anionic metal complexes and :ERX species followed by ligand elimination. Method B is the reaction between germanium(II) halides and labile neutral metal complexes involving halide migration to the metal. Method C (a) involves salt metathesis between anionic metal complexes and base-stabilized Group 14 element(II) halides followed by abstraction of the base, while Method C (b) involves abstraction of the halide in the last step. Methods D and E in Category II are based on reactions using tetravalent Group 14 element species such as hydrosilanes and hydrogermanes. Method D (a) is the reaction of metal complexes with hydrosilanes/hydrogermanes bearing a halo substituent (H2EXR) and subsequent abstrac- Scheme 4. Synthesis of aminogermylyne complex 7. tion of the halide from the resulting M@E(X)R complexes, while Method D (b) is the reaction of metal complexes with hydrosilanes/hydrogermanes (H3ER) and subsequent abstraction of the hydride from the resulting M@E(H)R complexes. Method E (a) is the reaction of metal complexes with hydrosilanes/hydrogermanes

(H3ER) and subsequent dehydrogenation of the resulting (H)M@E (H)R complexes with organic substrates, while in Method E (b) the dehydrogenation is accomplished by stepwise proton and hydride abstraction from the resulting (H)M@E(H)R complexes. About 50 terminal germylyne complexes have been isolated Scheme 5. Synthesis of germylyne complexes 9 and 10. and most of them have been prepared by one of Methods A, B, C, or E. In contrast, only 9 silylyne complexes are currently known and these have been prepared according to Methods C, D, or E. Besides these preparative routes, ligand substitution reactions can afford some germylyne complexes and hence are included in this section.

2.2. Synthesis of germylyne complexes using germanium(II) halides

2.2.1. Salt metathesis followed by ligand elimination (Method A) The synthesis of the ﬁrst example of M„E triple bonded complexes was achieved by Power and coworkers in 1996. They obtained the molybdenum germylyne complex Cp(CO)2Mo„Ge 5 (C6H3-2,6-Mes2)(1) (Cp = g -C5H5, Mes = 2,4,6-trimethylphenyl) by salt metathesis between GeCl(C6H3-2,6-Mes2) and Na[CpMo (CO) ](Scheme 2) [20]. The complex 1 was isolated in 34% yield 3 Scheme 6. Synthesis of germylyne complex 11.

Scheme 2. Synthesis of germylyne complex 1.

Scheme 7. Synthesis of germylyne complexes 12a–c.

Scheme 3. Synthesis of germylyne complexes 2–4. Chart 1. Structures of germylyne complexes 13–15. H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 365 as red crystals. This success was soon followed by the synthesis of germylyne complexes of all series of Group 6 metals, Cp(CO)2- M„Ge(C6H3-2,6-Trip2) (M = Cr (2), Mo (3), W (4), Trip = 2,4,6- triisopropylphenyl), which were isolated in 54–60% yields, by the same group (Scheme 3) [25]. In the chromium and tungsten systems, they also isolated metallogermylene complexes having a

MAGe single bond Cp(CO)3MAGe(C6H3-2,6-Trip2)(5: M = Cr, 6: M = W) as intermediates, which were indeed converted into germylyne complexes 2 and 4 with the formation of the M„Ge triple bond along with CO dissociation upon heating or irradiation. There is a previous attempt using a similar approach to prepare an Fe„Ge triple bond by CO dissociation from a metallogermylene t having a C6H2-2,4,6- Bu3 group on Ge, which resulted in intramolecular CAH activation [26]. These results indicate that the choice of the substituents on the Group 14 elements and the metal fragment are important in the isolation of these species. In a similar method, recently, Jones and coworkers synthesized an ⁄ ⁄ aminogermylyne complex Cp(OC)2Mo„N(Ar )(SiMe3)(7){Ar =C6 H2[C(H)Ph2]2-Me-2,6,4} in high yield, through a metallogermylene ⁄ Scheme 9. Synthesis of germylyne complexes 20 and 21a. intermediate Cp(OC)3MoAN(Ar )(SiMe3)(8)(Scheme 4) [27]. Filippou’s group reported that the reactions of a germylenoid Li

(THF)3GeCl2C(SiMe3)3, which is known to act as a halogermylene in solution [28], with Li[MoCp(CO)3] and K[WCp(CO)3] afforded germylyne complexes Cp(OC)2M„Ge[C(SiMe3)3](9: M = Mo, 73%; 10: M = W, 60%) (Scheme 5) [29]. This method can be regarded as a modiﬁed version of Method A. Furthermore, very recently, Filippou’s group reported the syn- 3 thesis of a niobium germylyne complex [(j -tmps)(CO)2Nb„Ge (C6H3-2,6-Mes2)] (11) [tmps = MeSi(CH2PMe2)3] as the ﬁrst example of Group 5 metal complex by the reaction of an anionic niobium complex and a halogermylene as shown in Scheme 6 [30a].

2.2.2. Reactions between germanium(II) halides and labile neutral metal complexes (Method B) In 2000, Filippou and coworkers demonstrated a new synthetic strategy using labile neutral metal complexes as starting materials. Thus, the reaction of halogermylenes [GeCp⁄X] (X = Cl, Br, I) with dinitrogen complex trans-[W(N2)2(dppe)2] (dppe = Ph2PCH2CH2- PPh2) in boiling toluene led to the formation of halido(germylyne)- Scheme 10. Synthesis of germylyne complex 22a. 1 ⁄ tungsten complexes trans-(X)(dppe)2W„Ge(g -Cp )(12a: X = Cl, 12b: X = Br, 12c: X = I) in 65–78% yields (Scheme 7) [31,32].In these reactions, dissociation of two molecules of dinitrogen opens formally two vacant sites, which allows addition of the GeAX bond W Cl NaA to form the M„Ge triple bond. In a similar method, the molybde- hv OC ⁄ Ge C H F, r .t. „ 1 Et O, r.t. 6 5 num analogues trans-(X)(dppe)2Mo Ge(g -Cp )(13a: X = Cl, 13b: 2 OC + + X = Br) [32] and some germylyne complexes bearing alkyl- − CO Idipp − NaCl 24 (56%) W A W CO Cl OC OC Ge OC Ge: OC Id ipp 25 hv (61%) 23 Idipp − NaCl NaA + C H F, r.t. − CO 6 5 CO OC W A OC Ge: dipp Idipp Idipp = NNdipp 26 (79%) dipp = 2,6-diisopropylphenyl A ={B[C6H3-3,5-(CF3)2]4}

Scheme 11. Synthesis of germylyne complex 25.

1 ⁄ substituted diphosphine ligands, trans-(X)(depe)2M„Ge(g -Cp ) (14a: M = Mo, X = Cl; 14b: M = Mo, X = Br; 15: M = W, X = Cl,

depe = Et2PCH2CH2PEt2) [33] were prepared (Chart 1). Analogous germylyne complexes having a substituent bulkier than the g1- ⁄ Scheme 8. Synthesis of germylyne complexes 16–19. Cp group, trans-(Cl)(Me3P)4M„Ge(C6H3-2,6-Mes2)(16: M = Mo, 366 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

17: M = W), were obtained using cis-[M(N2)2(PMe3)4] (M = Mo, W) as starting complexes (Scheme 8) [34]. Interestingly, one electron oxidation of 16 and 17 with [Ph3C][B(C6F5)4] afforded 17 valence electron (VE) germylyne complexes 18 and 19, respectively, which are the ﬁrst open-shell complexes having a M„E triple bond (Scheme 8). This dinitrogen elimination method has been successfully applied to the synthesis of germylyne complexes of Group 8 metal [30a,b] as well as analogous triple bonded complexes of tin [35,36] and lead [37–39]. Filippou’s group also showed that electron-rich phosphine com- 2 plexes [Mo(PMe3)6] and [W(g -CH2PMe2)(H)(PMe3)5] became good precursors for the synthesis of germylyne complexes. Thus, treatment of these complexes with GeCl(C6H3-2,6-Trip2) in toluene at room temperature gave chlorido(germylyne) complexes trans-

(Cl)(Me3P)4M„Ge(C6H3-2,6-Trip2)(20: M = Mo, 21a:M=W)in 64% and 67% yields, respectively (Scheme 9) [40]. Scheme 13. Synthesis of germylyne complex 28 via stepwise proton and hydride The ﬁrst germylyne complex of Group 7 metal was obtained abstraction. employing a similar phosphine complex. The reaction of [ReCl

(PMe3)5] with GeCl(C6H3-2,6-Trip2) in toluene at 60 °C afforded an equilibrium mixture containing the starting complex [ReCl

(PMe3)5], [(Cl)(Me3P)4Re@Ge(Cl)(C6H3-2,6-Trip2)], and mer- [(Cl)2(Me3P)3Re„Ge(C6H3-2,6-Trip2)] (22a), from which 22a was isolated in 80% yield by removing the PMe3 molecule produced during the reaction (Scheme 10) [41].

2.3. Synthesis of germylyne complexes starting from NHC-stabilized germanium(II) halides (Method C)

Very recently, Filippou’s group reported the synthesis of a cationic germylyne complex starting from an NHC-stabilized germanium(II) halide (Scheme 11) [42]. The NHC-stabilized metal- ⁄ lochlorogermylene Cp (OC)3WGeCl(Idipp) (23) (Idipp = 1,3-bis (2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) has ⁄ been prepared by us by the reaction of GeCl(Idipp) with Li[Cp W Scheme 14. Substitution reactions giving germylyne complexes 31 and 32a–d. (CO)3] [43]. CO elimination from 23 by photoirradiation and subsequent chloride abstraction from the resulting chlorogermylene 24 ⁄ afforded [Cp (OC)2W„Ge(Idipp)]{B[C6H3-3,5-(CF3)2]4}(25), which was isolated as a red-brown solid in 61% yield [44]. In this system, another route from 23 was also attempted but it was not successful: Chloride abstraction from 23 proceeded but CO elimination from the resulting cationic metallogermylene 26 did not occur.

2.4. Synthesis of germylyne complexes starting from germanium(IV) hydrides (Method E)

The above-mentioned methods using germanium(II) halides as a source of the germylyne ligand are most commonly used to

Scheme 15. Substitution reactions giving germylyne complexes 21b–d.

synthesize germylyne complexes but are limited to speciﬁc substituents, by which the germanium(II) halides are stabilized. On the other hands, germanium(IV) hydrides such as trihydrogermanes are available with a variety of substituents and can be used as convenient starting materials. Trihydrogermanes have previ- ously been used for synthesizing germylene complexes having MAH and GeAH bonds [45–47]. This type of germylene complexes Scheme 12. Synthesis of germylyne complex 28. can be converted into germylyne complexes if the two hydrogen H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 367

Scheme 20. Synthesis of silylyne complex 38. Scheme 16. Substitution reactions giving germylyne complexes 22b and 22c.

⁄ germylene complexes [Cp (OC)2W@Ge(H)C(SiMe3)3][HNHC] (30a: NHC = MeIMe, 30b: NHC = MeIiPr) were observed as intermediates and then these were gradually converted into 28 with

releasing H2NHC. This observation clearly indicates that stepwise proton and then hydride abstraction proceed in this system (Method E (b)).

2.5. Ligand substitution reactions Scheme 17. Synthesis of the cationic germylyne complex 33. Filippou’s group demonstrated that some halido(germylyne) complexes underwent ligand substitution reactions to afford several new germylyne complexes. Thus, the reaction of 12c with

KBH4 in refluxing THF afforded the hydrido(germylyne) complex 1 ⁄ trans-(H)(dppe)2W„Ge(g -Cp )(31) in 95% yield (Scheme 14) [49]. Nucleophilic substitution of the iodide ligand in 12c also occurred by a variety of pseudohalides to give the corresponding 1 ⁄ germylyne complexes trans-(X)(dppe)2W„Ge(g -Cp ) [X = NCO Scheme 18. Synthesis of the cationic germylyne complex 34. (32a), N3 (32b), NCS (32c), CN (32d)]. By analogous substitution reactions, trans-(X)(Me3P)4W„Ge(C6H3-2,6-Trip2)[X=I(21b), H (21c), NCS (21d)] (Scheme 15) [40] and rhenium complexes mer- atoms can be removed. The authors demonstrated the effective- [(X)2(Me3P)3Re„Ge(C6H3-2,6-Trip2)] [X = I (22b), H (22c)] ness of such strategy by using a trihydrogermane having an alkyl (Scheme 16) [41] were synthesized. On the other hand, treatment group (Scheme 12) [22]. The precursor germylene complex ⁄ ⁄ 5 of 12a with Li[B(C6F5)4] in the presence of MeCN afforded the catio- Cp (OC)2(H)W@Ge(H)[C(SiMe3)3](27) (Cp = g -C5Me5) was pre- 1 ⁄ nic complex trans-[(MeCN)(dppe)2W„Ge(g -Cp )][B(C6F5)4](33) pared by the reaction of Cp⁄W(CO) Me with H GeC(SiMe ) [45]. 3 3 3 3 (Scheme 17) [49]. Similarly, treatment of rhenium complex 22a Treatment of 27 with mesitylisocyanate MesNCO in toluene at with Na[BPh4] in the presence of PMe3 afforded a cationic complex 80 °C led to the almost quantitative formation of germylyne trans-[(Cl(Me3P)4Re„Ge(C6H3-2,6-Trip2)][BPh4](34)(Scheme 18) complex Cp⁄(OC) W„Ge[C(SiMe ) ](28) via dehydrogenation 2 3 3 [41]. Interestingly, thermolysis of 14a and 15 in the solid state (Method E (a)). In this reaction, MesNCO was hydrogenated to form afforded novel dinuclear complexes trans-[Cl(depe)2W„Ge)]2 MesHNHCO in 90% NMR yield. A five-membered chelate complex [M@Mo (35), W (36)], in which the two W„Ge bonds are con- Cp⁄(CO) W{GeH(OCH@NMes)[C(SiMe ) ]} (29) was isolated as a 2 3 3 nected by a GeAGe single bond (Scheme 19) [33]. main product when the reaction was carried out at room temperature, which was proved to be converted into 28 completely upon 2.6. Synthesis of silylyne complexes using NHC-stabilized silicon(II) heating. The reaction mechanism including the five-membered halides (Method C) intermediate 29 was proposed based on DFT theoretical calculations [22]. Silylyne complexes have been the most challenging synthetic The authors recently found that a similar dehydrogenative con- targets among the family of heavier carbyne complexes. This is version of 27 into 28 also occurred by addition of NHC, i.e. MeIMe partly because there have been no stable silicon(II) halides that (MeIMe = 1,3, 4,5-tetramethyl-1,3-dihydro-2H-imidazol-2-ylidene) correspond to the germanium(II) halides used for preparing or MeIiPr (1,3-diisopropyl-4,5-dimethyl-1,3-dihydro-2H-imidazol- 2-ylidene) (Scheme 13) [48]. In this reaction, anionic

Scheme 19. Formation of dinuclear germylyne complexes 35 and 36. Scheme 21. Synthesis of silylyne complex 40a. 368 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

dibromosilylene (Scheme 21) [51]. Salt metathesis between Li[Cp⁄-

Cr(CO)3] and SIdipp–SiBr2 [SIdipp = 1,3-bis(2,6-diisopropylphenyl) imidazolidin-2-ylidene] produced the corresponding bromosilylene complex 39. Bromide abstraction from 39 afforded ⁄ [Cp (CO)2Cr„Si(SIdipp)]{B[C6H3-3,5-(CF3)2]4}(40a) as the ﬁrst sylylyne complex of chromium, which was isolated as dark red crystals in 89% yield [Method C (b)]. The silylyne complex 40a is reported to be thermally stable up to 158 °C. Analogous complexes of molybdenum (40b) and tungsten (40c)(Table 2) are also prepared [30,42]. Very recently, his group also synthesized a niobium silylyne 3 complex [(j -tmps)(CO)2Nb„Si(Tbb)] (41), a silicon analogue of germylyne complex 11, by a method similar to that described in Scheme 6 but using E-1,2-dibromodisilene (Tbb)(Br)Si@Si(Br) Scheme 22. Synthesis of silylyne complex 41. (Tbb) instead of GeClR (Scheme 22) [30].

2.7. Synthesis of silylyne complexes starting from silicon(IV) hydrides

2.7.1. Abstraction of a halide/hydride group from halo/hydro-silylene complexes (Method D) In 2003, Mork and Tilley reported the synthesis of a cationic hydrido(silylyne) complex starting from dihydrochlorosilane. They prepared a chlorosilylene complex Cp⁄(dmpe)(H)Mo@SiClMes (42) as the precursor of a silylyne complex by the reaction of Cp⁄(dmpe) 3 Mo(g -benzyl) (dmpe = Me2PCH2CH2PMe2) with H2SiClMes. Sub- sequent chloride abstraction from 42 with Li[B(C6F5)4] led to a ⁄ quantitative formation of [Cp (dmpe)(H)MoSiMes][B(C6F5)4](43) (Scheme 23) [52] [Method D (a)]. The X-ray crystal structure analysis revealed that 43 bears structure B with an SiH interaction as shown in Scheme 23 in the solid state, while the NMR study Scheme 23. Synthesis of silylyne complex 43. suggests that 43 adopts structure A (or C) in solution. In 2013, Tilley’s group synthesized a cationic hydrido(silylyne) complex of osmium having no SiH interaction by hydride abstraction (Scheme 24) [53]. The precursor silylene complex ⁄ i Cp ( Pr3P)(H)Os@Si(H)Trip (44) was synthesized by the reaction of Os ⁄ i 3 H3SiTrip a primary silane H SiTrip with Cp ( Pr P)Os( -CH Ph) [54]. Treat- i C H ,r.t. 3 3 g 2 Pr 3P 6 6 ⁄ i − toluene ment of 44 with a strong Lewis acid B(C6F5)3 gave [Cp ( Pr3P)(H) Os„SiTrip][HB(C F ) ](45) [Method D (b)]. Complex 45 was found i 6 5 3 Tri p = C 6H2-2,4,6-Pr 3 to be fairly thermally unstable in solution (t1/2 = 30 min in C6D5Br) but was isolated as red crystals in 67% yield.

B(C F ) i Tr ip 6 5 3 i Os Pr @ Os Pr P A 2.7.2. Dehydrogenation from silylene complexes having a (H)M Si(H) i Si 3 Si Pr3P pentane, r.t. H moiety (Method E) H i H iPr Pr The authors found that dehydrogenation from the silicon analogue of germylene complex 28,Cp⁄(OC) (H)W@Si(H)[C(SiMe ) ] 44 A =[HB(C6F5)3] 45 (67%) 2 3 3 (46), is possible by stepwise addition of Lewis base and Lewis acid t ( 1/2 =30mininC6D5Br) (Scheme 25) [23]. The precursor silylene complex 46 was synthe-

Scheme 24. Synthesis of silylyne complex 45. NN

i germylyne complexes by methods A and B. However, recent signif- H SiTsi (MeI Pr) W 3 W H icant progress in the use of N-heterocyclic carbenes (NHCs) for sta- OC Me OC Si toluene toluene bilization of low-valent silicon compounds made it possible to use OC L hv OC or r.t. H Tsi r.t. silicon(II) halides as an NHC-stabilized form [50]. Filippou and − Me H, − L coworkers successfully utilized the NHC-stabilized silicon(II) 46 halides to synthesize silylyne complexes. Thus, they treated Me IMe-SiClR (R = C6H3-2,6-Trip2) with Li[CpMo(CO)3] in toluene H Me i B(C6F5)3 W ° Me W [H I Pr] OC SiMe3 at 100 C, which gave the IMe-stabilized silylene complex 37 Si Si OC C H F, r.t. C (Scheme 20) [21]. Subsequent abstraction of the NHC by the strong OC 6 5 OC SiMe3 Me i Tsi − [HB(C6F5)3][H I Pr] SiMe Lewis acid B(p-tol)3 in boiling xylene afforded the molybdenum 3 silylyne complex Cp(CO)2Mo„Si(C6H3-2,6-Trip2)(38) [Method C 47 (84%) 48 (15%) (a)]. The complex 38 was isolated as a brick-red air sensitive solid H „ L=CO,MeCN i in 53% yield and structurally conﬁrmed to have a simple Mo Si [HMe I Pr] = NN bond with no other interaction, for the ﬁrst time. Tsi = C(SiMe3)3 In a similar approach, Filippou’s group also synthesized a cationic chromium silylyne complex by using NHC-stabilized Scheme 25. Synthesis of silylyne complex 48. H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 369

7, the smaller corresponding angle (MoAGeAN = 155.81(8)°) has been explained by the steric repulsion between the ligands [27]. (Eind)SiH3 H W W The small angle of 52 (WASiAC = 142.0(2)°) is due to the presence OC Me OC Si toluene of isocarbonyl coordination of CO ligand to the silylyne silicon atom OC N OC H Eind r.t. in this system (vide infra). The second structural feature is the sig- − MeH, − C H N 49 Me i 5 5 NN ( I Pr) niﬁcant shortening of the MAE bond lengths of these M„E triple- THF bonded complexes compared with the corresponding MAE double r.t. bond (if available) or single bond lengths. These two structural fea- Et Et tures are also characteristic of carbyne complexes with M„C triple Et Et [H MeIiPr] W Eind bonds [1–8]. OC Si Eind = For the Nb system, 11 is an only example of germylyne com- OC Et Et H plexes. The NbAGe bond length of 11 is 2.3579(4) Å that is ca. Et Et 50(78%) 0.3 Å shorter than the NbAGe single bonds of known germyl com- B(C F ) plexes [30]. The NbAGeAC angle is 164.0(1)°, indicating its slightly 6 5 3 toluene, r.t., i bent structure. − [HB(C F ) ][HMeI Pr] 6 5 3 For the Cr system, 2 is also currently an only example of germy- W lyne complexes. The CrAGe bond length of 2 is 2.1666(4) Å that is OC Eind Si A 0.5 OC W ca. 0.42 Å shorter than the Cr Ge single bond (2.590(2) Å) of met- CO OC A A Si Si allogermylene Cp(OC)3CrGe(C6H3-2,6-Trip) (5) [25]. The Cr Ge C CO OC Eind W Eind angle is 175.99(6)°, indicating an almost linear coordination of the 51 germylyne ligand. These structural data also show that 2 has a CrAGe triple bond and this fact is also supported by theoretical cal- 52(68%,crystals) culations. The bond order (WBI = Wiberg bond indices) for the A Scheme 26. Synthesis of silylyne complex 51 and its dimer 52. Cr Ge bond in a model compound of 2, Cp(OC)2CrGeMe, is calculated to be nearly three times larger than that for the CrAGe single

bond of a model compound of 5, Cp(OC)3CrGeMe (2: 1.41, 5: 0.42) sized in a manner similar to that of 28 using H3SiC(SiMe3)3 instead Me i [56]. Interestingly, the melting point is reported to be 240–242 °C, of H3GeC(SiMe3)3 [55] and was treated with I Pr, which gave the ⁄ Me i indicating the high thermal stability of 2 at least in the solid state. anionic silylene complex [Cp (OC)2W@Si(H)C(SiMe3)3][H I Pr] For the Mo and W systems, there are more than 20 examples of (47) in 84% yield. Treatment of 47 with B(C6F5)3 afforded the neutral tungsten silylyne complex Cp⁄(OC) W„Si[C(SiMe ) ] germylyne complexes and most of them are complexes of the type 2 3 3 „ „ A (48), which was isolated as orange crystals in 15% yield [Method of either Cp(OC)nM GeR or X(R3P)4M GeR. The Mo Ge bond E (b)]. By applying the same methodology to aryltrihydrosilane lengths of all molybdenum germylyne complexes are within the H SiEind (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), range of 2.27–2.31 Å and there is no large difference between these 3 A through preparation of neutral silylene complex 49 and anionic two types of complexes. The W Ge bond lengths are also within a A silylene complex 50 as intermediates, Cp⁄(OC) W„Si(Eind) (51) similar range (2.28–2.33 Å). These M Ge bond lengths are much 2 @ was synthesized (Scheme 26) [24]. The silylyne complex 51 is in shorter than reported M Ge double bond lengths. For example, A equilibrium with its dimer 52, which was isolated as yellow the W Ge bond length of 28 (2.2830(6) Å) is ca. 0.15 Å (6%) shorter ⁄ @ crystals in 66% yield and characterized by X-ray diffraction study. than that of germylene complex Cp (OC)2(D)W Ge(D)[C(SiMe3)3] (27-d2) (2.4289(8) Å) [45]. The WAGeAC linkage (173.39(16)°)is The ratio of 51:52 is approximately 1:4 in C6D5CD3 at room temperature, but the monomer 51 becomes predominant at 360 K. nearly linear. DFT calculations show the presence of a r-bond and two p-bonds in the WAGe bond [22]. Filippou’s group synthesized an open-shell 17 VE (valence electron) germylyne complex 3. Structures and properties of germylyne and silylyne 18 by one electron oxidation of 18 VE complex 16 (Scheme 8 in complexes the previous section), and determined the crystal structures of both 16 (Fig. 1) and 18 (Fig. 2) [34]. These complexes have nearly 3.1. Structures and bonding identical MoAGe bond lengths (16: 2.3041(3), 18: 2.3023(4) Å) and the MoAGeAC angles (16: 176.27(8), 175.3(1)°), implying that Much attention has been paid on the structure and bonding of the open shell complex 18 has a MoAGe triple bond similar to that the M„E triple bonded complexes since the ﬁrst isolation of this of the closed shell complex 16. However, this one-oxidation chan- type was achieved, and detailed analysis on their structures and ged the other MoAelement bond lengths: The MoAP bonds of 18 properties by theoretical calculations using energy decomposition (av. 2.536(1) Å) are longer than those of 16 (av. 2.475(9) Å), while analysis (EDA) [56–60] as well as spectroscopic and crystal struc- the MoACl bond (2.4878(9) Å) of 18 is shorter than that of 16 ture analyses of them have been reported in several excellent arti- (2.5380(7) Å). According to their DFT calculations, the HOMO of cles. Here we overview mainly the experimental aspects of 16 (mainly dxy character) is oriented perpendicular to the germylyne and silylyne complexes coupled with some theoretical ClAMoAGe axis and is non-bonding with respect to the germylyne aspects. Principal structural parameters of germylyne complexes ligand and chlorido ligand, but bonding with respect to the phos- (Nb, Cr, Mo, W, Re) and silylyne complexes (Nb, Cr, Mo, W) deter- phine ligands; the p-bonding involves back-donation from the ⁄ mined by X-ray crystal structure analysis together with their prop- ﬁlled metal dxy orbitals into the r (PAC) orbitals of the phosphine erties are listed in Tables 1 and 2, respectively. 29Si NMR data are ligands. Therefore, one-electron oxidation of 16 weakens the MAP also included in Table 2. p-back bonding leading to an elongation of the MAP bonds. In The most characteristic structural feature of these complexes is comparison, the MoACl distance decreases upon oxidation due to the linearity of the MAEAC (substituent) linkage (E = Ge, Si). All the the decreasing ionic radius of the metal, whereas the MoAGe dis- complexes except aminogermylyne complex 7 and dimeric silylyne tance is not affected. The WAGe bond length of the cationic com- complex 52 have almost linear MAEAC linkages as indicated by the plex 33 (Fig. 3) is also close to those of neutral complexes (12, 31, 1 ⁄ MAEAC (substituent) angles that are in the range of 164–179°, and 32) having the same fragment [(dppe)2WGe(g -Cp )]. In addi- regardless of the metals, elements, and the charge. In the case of tion, melting points of complexes bearing sterically demanding 370 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

Table 1 Principal structural parameters and properties of germylyne complexes.

Compound Structural parameters Properties Reference d(MAGe)/Å MAGeAC/° (CCDC: #)

Me 11 2.3579(4) 164.0(1) Deep magenta [30a] Si (decomp, 284 °C) (CCDC: 1553387)

Me2P Me2P PMe2 Nb OC OC Ge R R=C6H3-2,6-Mes2

iPr 2 2.1666(4) 175.99(6) Red crystals [25] (mp. 240–242 °C) Cr iPr iPr OC Ge OC iPr i i Pr Pr

1 2.271(1) 172.2(2) Red crystals [20]

Mo OC Ge OC

i Pr 3 2.272(8) 174.25(14) Orange-red crystals [25] (mp. 233–235 °C) Mo iPr iPr OC Ge OC iPr iPr i Pr 7 2.2811(4) 155.81(8) Orange crystals [27] Ph (mp. 230–234 °C) Mo Ph OC HC Ge OC N

Me3Si HC Ph Ph 9 2.2805(2) 169.60(3) Red-orange crystals [29] (mp. 85 °C) (CCDC: 855756) Mo OC Ge SiMe3 OC C SiMe3 SiMe3 P 13a (Cl) 2.3185(6) 172.0(1) Orange-brown microcrystals [32] P 13b (Br) 2.3103(6) 171.6(2) 13a (mp. 237 °C, decomp), 13b: (228 °C, decomp) (CCDC: X Mo Ge 180855 (13a) 180856 (13b) P P

PP dppe, X = Cl, Br P 14b 2.2798(5) 177.46(8) Brown-orange crystals [33] P (mp. 155 °C, decomp) (CCDC: 259718) Br Mo Ge

P P

PP Et2PCH2CH2PEt2 16 2.3041(3) 176.27(8) Wine-red crystals [34] (mp. 229 °C, decomp) (CCDC: 847001) PMe3 PMe3 Cl Mo Ge

Me3P PMe3

18 2.3023(4) 175.3(1) Persimmon-red crystals [34] (96 °C, decomp) (CCDC: 847003) PMe3 PMe3 Cl Mo Ge

Me3P PMe3 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 371

Table 1 (continued)

Compound Structural parameters Properties Reference d(MAGe)/Å MAGeAC/° (CCDC: #)

i Pr 4 2.2767(14) 170.9(3) Red crystals [25] (mp. 158–160 °C) W iPr iPr OC Ge OC iPr i i Pr Pr 10 2.2842(6) 170.5(1) Red-brown crystals [29] (mp. 86 °C) (CCDC: 855757) W OC Ge SiMe3 OC C SiMe3 SiMe3 P 12a (Cl) 2.302(1) 174.04(3) 12a, 12b: [31,32] P 12b (Br) 2.293(1) 172.4(2) Orange-brown crystals, (CCDC: X WGe 12c (I) 2.3060(9) 172.6(2) 12c: 142852 (12a) Red-brown crystals [32] P P (CCDC: PP dppe, X = Cl, Br, I 180857 (12b) 180858) (12c) 17 2.3106(4) 176.3(1) Dark brown-red crystals [34] (mp. 230 °C, decomp) (CCDC: 847002) PMe3 PMe3 Cl WGe

Me3P PMe3

19 2.321(2) 172.9(4) Persimmon-red crystals [34] (92 °C, decomp) (CCDC: 847004) PMe3 PMe3 Cl WGe

Me 3P PMe3

iPr 21a (Cl) 2.338(1) 177.9(3) 21a: Brown crystals [40] 21b (I) 2.3206(4) 175.59(5) (mp. 184 °C, decomp) (CCDC: i PMe3 Pr ° ° PMe iPr 21c (H) 2.324(1) 178.9(2) 21b: Red brown crystals (mp. 194 C, decomp 198 C) 608272 (21a) 3 ° X WGe 21c: Red-brown crystals (mp. 189 C, decomp) 608273 (21b) iPr 608274 (21c) Me3P i PMe3 Pr iPr X=Cl,I,H + 25 2.2813(4) 168.7(1) Red-brown crystals [42] (mp. 147 °C) (CCDC: 1524137) W OC Ge OC Idipp

Idipp = dipp NNdipp dipp = 2,6-diisopropylphenyl 28 2.2830(6) 173.39(16) Orange crystals [22] (CCDC: 788617) W OC Ge SiMe3 OC C SiMe3 SiMe3 P 31 (H) 2.310(1) 176.8(1) 31: Red-orange crystals [49] P 32a (NCO) 2.2991(9) 172.0(1) 32a: Orange crystals (CCDC: X W Ge 32d (CN) 2.3184(6) 172.2(1) 32d: Green crystals 239416 (31) 239417 (32a) P P 239418 (32d)

PP dppe,X=H,NCO,CN P 33 2.3030(8) 174.31(7) Amber-colored crystals [49] P (CCDC:239419) MeCN WGe

P P

PP dppe

(continued on next page) 372 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

Table 1 (continued)

Compound Structural parameters Properties Reference d(MAGe)/Å MAGeAC/° (CCDC: #) ° P P 36 2.3087(5) 175.13(3) Green-brown crystals (mp. 240 C, decomp) [33] P P (W–Ge–Ge) (CCDC: 259719) X WGeGe W X

P P P P

PP Et2PCH2CH2PEt2 i Pr 22a (Cl) 2.2597(6) 164.6(1) 22a: Apple-green crystals (mp. 194 °C, decomp) [41] 22b (I) 2.2609(3) 167.99(7) 22b: Brown crystals (mp. 183 °C, decomp) (CCDC:901048 (22a) i PMe3 Pr 22c (H) 2.2678(8) 171.3(2) 22c: Orange-red crystals 901049 (22b) i Pr PMe3 901050 (22c) X Re Ge i X Pr i PMe3 Pr iPr X=H,Cl,I i ° Pr 34 2.2772(2) 171.84(7) Red-brown crystals (decomp at 130 C) [41] (CCDC: 901053) i PMe3 Pr iPr PMe3 Cl Re Ge iPr Me3P i PMe3 Pr

iPr

⁄ aryl substituents such as 3, 7, 16, and 21 are reported to be fairly and 2.3872(7) Å for Cp (OC)2(Me3Si)Mo@SiMes2 [63]]. Although high (190–240 °C). 43 has a hydrido ligand bridging over the MoASi bond, the short For the Re system, there are four examples including one catio- MoASi bond length and almost linear MoASiAC linkage (170.9 nic complex. The ReAGe bond lengths of 22a–c and 34 are within (2)°) suggests that 40a has a considerable silylyne character. The the range of 2.26–2.28 Å and are the shortest ever reported for tungsten complex 48 also has a very short WASi bond (2.2297 ReAGe bonds [41]. These values are ca. 0.4 Å (14%) shorter than (9) Å), which is comparable with those of 43 and 38. This value is those reported for ReAGe single bonds (2.633(3)–2.6661(5) Å) ca. 0.1 Å (5%) shorter than that of the shortest known WASi double ⁄ [61] and ca. 0.21 Å (7%) shorter than the ReAGe bond in [Re(GePh2- bond length of the anionic silylene complex [Cp (OC)2W@Si(H)C Me i OTf)Cp(NO)(PPh3)] (2.4738(6) Å), in which the ReAGe bond is sug- (SiMe3)3][H I Pr] (47) (2.3367(17) Å) [64]. These parameters con- gested to have a double bond character because of the ionic firm the presence of a WASi triple bond in 48. In the case of 51 hav- GeAOTf bond [62]. The ReAGeAC linkage of these rhenium com- ing a plate-shaped aryl group (Eind), because there is an plexes are almost linear. These features confirm the presence of equilibrium between the monomer 51 and its dimer 52 in solution, ReAGe triple bonds in these complexes. The cationic complex 34 less soluble 52 crystallized out. The X-ray crystal structure deter- (mp. 130 °C, decomp) is less stable than neutral complexes 22a–c mination of this crystal revealed that 52 is a dimeric silylyne com- (mp. 184–194 °C). plex, in which an oxygen atom of one of two CO ligands in a In addition to the above-mentioned germylyne complexes, monomer unit coordinates to the silylyne silicon atom of the other those of Ni and Pt are reported as examples of Group 10 metal monomer unit. By this isocarbonyl coordination, the WASiAC complexes [30a,c]. (Eind) linkage is fairly bent (av. 143.1°) and the WASi bond length For sylylyne complexes, the structures of 8 examples (1 for Nb, (av. 2.341 Å) becomes close to the shortest W@Si double bond 1 for Cr, 3 for Mo, 3 for W) have been determined by X-ray crystal- length of 47. lography. The crystal structures of 38, 40a, and 48 are illustrated in Figs. 4–6, respectively. The NbASi bond length of 41 is 2.327(2) Å, 3.2. 29Si NMR spectroscopic properties which is 0.21–0.35 Å shorter than the NbASi single bonds of silyl complexes (2.541–2.679 Å) [30a]. The CrASi bond length of the 29Si NMR chemical shifts and the MASi coupling constants (if cationic silylyne complex 40a is 2.1220(9) Å, which is 0.05 Å (2%) available) are powerful tools to know the situation of the Si atom shorter than that of the precursor neutral bromosilylene complex bonded to metals (Table 2). The cationic molybdenum complex 39 (2.1716(7) Å) and is 0.28 Å (12%) shorter than those of CrASi 43 exhibits a 29Si NMR signal at 289 ppm, which is greatly shifted single bonds (average value: 2.399 Å) [51]. Filippou and coworkers downfield from that (182 ppm) of the precursor, i.e., silylene com- performed theoretical calculations on this system 40a. The bond plex 42, indicating the presence of a strong triple-bond character cleavage energy (BCE) of the CrASi bond is calculated to be [52]. The neutral molybdenum silylyne complex 38 similarly 504.4 kJ mol1, which is considerably higher than that of the CrASi shows a 29Si NMR signal at very low field (320.1 ppm) [21]. The ⁄ + 29 single bond of the cationic metallosilylene [Cp (OC)3CrASiSIdipp] Si NMR signal of neutral tungsten silylyne complex 48 also (166.2 kJ mol1). The WBI value of 40a (1.42) is nearly three times appears at very low field (339.1 ppm) [23], which is lower than of that of the cationic metallosilylene (0.50). These data completely that of the precursor, i.e., neutral silylene complex 46 support the presence of a CrASi triple bond in 40a. The MoASi (275.3 ppm) [55], and anionic silylene complex 47 (286.1 ppm) bond lengths of the cationic hydrido(silylyne) complex 43 (2.219 [64]. This signal of 48 is accompanied by satellite signals due to (2) Å) and neutral silylyne complex 38 (2.2241(7) Å) are almost the coupling with 183W nuclei (I = 1/2). The 1J(WASi) coupling con- the same and these are shorter than the known MoASi double stant (316.2 Hz) is much larger than those for base-free tungsten bond lengths [e.g. 2.288(2) Å for Cp⁄(dmpe)(H)Mo@Si(Cl)Mes [52] silylene complexes (91–155 Hz) [55,65,66], indicating that the W H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 373

Table 2 Principal structural parameters, properties, and 29Si NMR data of silylyne complexes.

Compound 29Si{1H} NMR (d) Structural parameters Properties Reference (M„Si) d (MASi)/Å MASiAC/° (CCDC: #)

Me 41 267.8 (br) 2.327(2) 159.2(2) Red-brown crystals [30a] ° Si (C4D8O) (decomp 258 C) (CCDC: 1553388)

Me2P Me2P PMe2 Nb OC Si OC Tbb

40a 40a: 127.8 40a: 2.1220(9) 40a: 169.76(9) 40a: Dark-red crystals thermally stable 40a [51]

(M = Cr) (C6D5Cl) 40b: 2.2212(9) (mp. 158 °C, decomp) (CCDC: 965778) M OC 40b 40c: 2.224(3) 40b,c [42] Si (M = Mo) OC SIdipp 40c M=Cr,Mo,W (M = W)

iPr 38 320.1 2.2241(7) 173.49(8) Brick-red crystals [21]

(C6D6) (CCDC: 760808) i i Mo Pr Pr OC Si i OC Pr

i i Pr Pr 43 289 2.219(2) 170.9(2) Amber crystals [52]

(JSiH = 15 Hz) [MoAH 1.85(5), (CCDC: 194947)

Me2 Mo (C6H5F/C6D6 = 10:1) SiAH 1.39(5)] P Si P H Me2 48 339.1 2.2297(9) 173.71(11) Orange crystals [23]

(JWSi = 316. 2 Hz) (CCDC:1444469) W (C D ) OC 6 6 Si SiMe3 C OC SiMe3 SiMe3 51 302 – – There is an equilibrium between this [24] Et Et Et (C6D5CD3, 360 K) monomer 51 and its dimer 52 in W solution. The monomer 51 becomes OC Si Et OC dominant at 360 K. Et (Eind) Et Et Et monomeric form 52 237.8 2.3389(14), 142.00(16), Yellow crystals [24]

(JWSi = 303.9 Hz) 2.3433(14) 144.16(17) (CCDC:1472739)

W (C6D6, 298 K) [SiO 1.722(4), OC Si Eind OC 1.730(4)] CO Eind Si CO W

dimeric form

45 321 ––(t1/2 = 30 min in C6D5Br, r.t.) [53]

i (C6D5Br) Os Pr i Pr3P Si H iPr iPr

and Si atoms in 48 use the orbitals having high s characters to form comparison with the corresponding signal of the precursor, i.e., the WASi bond. These data of 48 is consistent with its WASi triple- bromosilylene complex 39 (74.8 ppm). bonded structure in solution. The monomeric tungsten complex 51 exhibits a 29Si NMR signal at 302 ppm at 360 K [24]. This signal is 4. Reactions of germylyne and silylyne complexes shifted downﬁeld from that of the dimer 52 (237.9 ppm measured at 298 K) and the chemical shift is close to that of 48 (339.1 ppm), As documented in several review articles [1–8], carbyne com- supporting that 51 is a monomeric silylyne complex. The cationic plexes react with various organic substrates at the M„C triple osmium complex 45 also shows a 29Si NMR signal at 321 ppm bonds and are used for transformation of organic molecules.

(C6D5Br), which is comparable with those of the above- Germylyne and silylyne complexes are also expected to show mentioned silylyne complexes [53]. Although there is no crystal interesting reactivity arising from the M„E triple bonds (E = Ge, structure of 45, this chemical shift is a strong evidence for the pres- Si). Some comparisons of structural features (geometry, bond ence of an OsASi triple bond. In contrast, the 29Si NMR signal of the polarity, and strength of the triple bonds) and reactivity between cationic chromium complex 40a appears at 127.0 ppm, which is carbyne complexes and silylyne/germylyne complexes are summa- shifted far upﬁeld compared with those of the other silylyne rized in Chart 2. Although the number of reports on the reactivity complexes [51]. This value, however, is also shifted downﬁeld in of germylyne and silylyne complexes is still small, discoveries of 374 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

Fig. 3. Crystal structure of the cation part of 33 (CCDC: 239419). Hydrogen atoms are omitted for clarity.

Fig. 1. Crystal structure of 16 (CCDC: 847001). Hydrogen atoms are omitted for clarity.

Fig. 2. Crystal structure of the cation part of 18 (CCDC: 847003). Hydrogen atoms Fig. 4. Crystal structure of 38 (CCDC: 760808). Hydrogen atoms are omitted for are omitted for clarity. clarity. H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 375

N N

M M C(SiMe3)3 OC OC Ge Ge o OC C(SiMe ) tolu ene, 2 0 C OC 3 3 N N

9:M=Mo 53:M=Mo(69%) 10:M=W 54:M=W(79%)

Scheme 27. Reactions of germylyne complexes 9 and 10 with NHC.

OR W +ROH W OC OC Ge Ge C6D6,r.t. OC OC H C(S iMe3)3 C(SiMe3)3 28 55a:R=Me(83%) 55b:R=Et(67%) i 55c:R= Pr(53%)

Scheme 28. Reactions of germylyne complexes 28 with ROH.

Fig. 5. Crystal structure of the cation part of 40a (CCDC: 965778). Hydrogen atoms are omitted for clarity. 2 + + + [NEt4 ] [Li ][Li]2 Mo R Mo R OC Mo R OC Si Si OC Si OC OC Me OC Me N3 Me 56b (quant.) 56c (75%) 57 (88%)

[NEt4]N 3 LiMe 2LiMe Et O, 2 DME/hexane DME, r.t. −60 oC −70 oC

Mo OC Si OC [NMe4]Cl R 2KC8 38

DME DME/pentane 2 r.t. r.t. i i Pr Mo Pr [NEt +][K+] OC Si 4 Mo R 2 i OC OC Pr Si iPr OC i Pr Cl i Pr R=C6H3-2,6-Trip2 56a (72%) 58 (90%)

Scheme 29. Reactions of silylyne complex 38 with anionic nucleophiles.

Fig. 6. Crystal structure of 48 (CCDC: 1444469). Hydrogen atoms are omitted for clarity.

Scheme 30. Reactions of germylyne complex 28 with arylaldehydes.

novel reactions are gradually increasing. In this section, all the reactions reported recently will be mentioned. Exceptions are ligand substitution reactions at the metal centers and oxidation reactions that have already been included in the previous Sec- tion 2.5, because these reactions lead to the formation of new Chart 2. Comparison between carbyne and silylyne/germylyne complexes. M„E triple bonded complexes. 376 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

4.1. Reactions with nucleophiles

It is well known that there are two classes of carbyne complexes, i.e., Fischer-type ones with electrophilic carbon centers and Schrock-type ones with nucleophilic carbon centers. In the case of M„E triple-bonded complexes with E = Ge, Si, only Fischer-type complexes have been reported so far. Filippou and coworkers showed that their germylyne complexes 9 and 10 have electrophilic germanium centers and the reactions with NHC cleanly produced NHC adducts 53 and 54, respectively (Scheme 27) [29]. The authors examined the reactions of germylyne complex 28 with alcohols, which afforded hydrido(alkoxygermylene) complexes 55a–c quantitatively (by 1H NMR) (Scheme 28) [67]. The formation of 55a–c indicates that the W„Ge bond of 28 is polar- ized in W(d)AGe(d+) fashion and 28 is therefore regarded as a Fischer-type germylyne complex. Filippou’s group also demonstrated the high electrophilicity at the silicon center in the silylyne complex 38 by the reactions with Scheme 32. Reactions of germylyne complex 28 with a,b-unsaturated ketones. several anionic nucleophiles (Scheme 29) [68]. Treatment of 38 with one equiv. of (NMe4)Cl, (NEt4)N3, or LiMe gave unprecedented anionic silylene complexes 56a, 56b, and 56c, respectively. Treat- ment with two equivalents of LiMe resulted in double addition of Me anions to 38 to afford very rare dianionic silyl complex 57.

Reduction of 38 with two equivalents of KC8 gave dianionic silaﬂu- orenyl complex 58 as a result of the insertion of silicon into a CAC bond of the substituent. These examples demonstrate the high potential utility of silylyne complexes to synthesize new complexes having metal–silicon single and double bonds.

4.2. Reactions with unsaturated organic molecules

The authors have found that the tungsten germylyne complex 28 reacted with several carbonyl compounds through incorporation of two molecules of the substrates. Thus, the reactions of 28 with arylaldehydes at room temperature resulted in the exclusive formation of four-membered metallacycles 59a–c as a mixture of two diastereomers (Scheme 30) [67]. The metallacycles 59a–c con- tain an OCH2R group on the germanium atom and an acyl ligand (A(R)C@O) that bridges W and Ge. This means that the formyl proton of one aldehyde is transferred to another molecule of aldehyde on the W„Ge triple bond. A proposed reaction mechanism is

O δ- δ+ H R [W] G e Tsi [W] Ge Tsi coordination H of the O atom O δ δ [2+2] 28 R + - a cycloaddition H H R

O Tsi Scheme 33. Proposed mechanisms for the reactions of germylyne complex 28 with [W] Ge [W] Ge [W] = Cp*(CO)2W a,b-unsaturated ketones. Tsi H O O R R 59a-c b illustrated in Scheme 31, which involves the three key steps, (1) [2 hydrogermylation C−Hbond +2] cycloaddition between RCH@O and the W„Ge triple bond of of the C=O bond activation A H 28 (a to b), (2) C H activation followed by the formation of the R base-stabilized metallogermylene intermediate (b to c), and (3) O H O H coordination of the second RCHO molecule to the metal- Tsi H R Tsi [W] Ge [W] Ge logermylene followed by hydride migration from W to the coordi- nated aldehyde (c to d). This mechanism has been theoretically O O coordination supported by DFT calculations of Xie and coworkers [69]. R of the O atom R d c When a,b-unsaturated ketones (enones) were used as substrates, another mode of incorporation of the two substrates was Scheme 31. A proposed mechanism for the reaction of 28 with arylaldehydes. observed. Treatment of 28 with 2 equiv of a-unsubstituted enones H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 377

O Eind W OC Si W R R OC C 0.5 OC O dimer Si OC Eind C6D6 52 51 r. t. R R 62a:R=H i 62b Pr toluene :R=Me N C N O i r.t. Pr toluene R R r.t.

OC W Eind Si W OC C OC Scheme 36. Reactions of germylyne complex 22a with CO and MeNC. i O Pr N N OC Eind i Si Pr R 64(80%) H O Et Et R unsaturated tungsten center gives the product. These mechanisms Et Et R have also been supported theoretically by Li and coworkers [71]. Eind = The dimeric tungsten silylyne complex 52 is highly reactive, R Et Et because it easily dissociates to the monomeric silylyne complex Et Et 63a : R = H (38%, 63%NMR) 63b : R = Me (39%,66%NMR) 51 in solution. Thus, treatment of 52 with diarylketones in toluene led to the instantaneous formation of [2+2] cycloaddition products Scheme 34. Reactions of silylyne complex 51 with diarylketones and carbodiimide. 62a–b, which were then gradually converted into siloxy complexes 63a–b having a silicon containing six-membered ring in the siloxy ligand (Scheme 34) [24]. The formation of the ring system means @ 3 RC(O)CH CH2 (R = Me, Et) at room temperature produced g -allyl the occurrence of the coupling of two diarylketones through o- complexes 60a and 60b bearing an oxagermacyclopentene frame- CH activation, CAC bond formation, and C@O bond cleavage in 3 work (Scheme 32) [70]. Both of the g -allyl complexes 60a and the course of this reaction. The X-ray crystal structure determina- 60b adopt the exo-anti geometry among four possible geometries tion of 62a, one of the intermediates, confirmed the initial occur- 3 at the g -allyl moiety, according to the X-ray diffraction study rence of the [2+2] cycloaddition between 51 and the ketone. A [2 and NMR analysis. On the other hand, when a-Me-substituted +2] cycloaddition also occurred in the reaction of 51 with diiso- @ enone MeC(O)C(Me) CH2 was employed, only one substrate propylcarbodiimide to afford a four-membered cyclic silylene 3 reacted with 28 to give an equilibrium mixture of 28 and g - complex 64, which was also structurally fully characterized. All allyl complex 61 even when a large excess of the enone was used. these reactions by germylyne complex 28 and silylyne complex Probably the a-Me group sterically prevents the approach of the 51 are unprecedented and imply high potential of the M„E second substrate molecule. This reaction displays an important triple-bonded complexes toward unique transformations of unsat- „ feature of the W Ge triple bond that can be completely cleaved urated organic substrates. and formed reversibly by the action of this substrate at room tem- Tilley and coworkers demonstrated that the cationic osmium perature. The authors proposed possible reaction mechanisms for silylyne complex 45 was activated by the positive charge, and formation of 60a, 60b, and 61 as shown in Scheme 33, in which underwent [2+2] cycloaddition with alkynes and phosphaalkyne, „ double [2+4] cycloaddition of the substrates to the W Ge bond i.e. non-polar or less polar unsaturated substrates (Scheme 35) is involved for the formation of 60a and 60b. On the other hand, [53]. Treatment of 45 with diphenylacetylene afforded metallasila- for the formation of 61, 1,3-H migration occurs on intermediate e cyclobutadiene 65 in high yield. A similar cycloaddition occurred instead of [2+4] cycloaddition to give carbene complex intermedi- with phosphaalkyne to give a corresponding four-membered ring ate h, and subsequent 1,2-migration of the germyl group to the product 66. These products were fully characterized @ carbene carbon followed by coordination of the C C bond to the spectroscopically. The regioselectivity of the cycloaddition to form 66 may be con- trolled either by the electronegativity (Pauling) of carbon (2.55) that is higher than that of phosphorus (2.19) or by the coordinating A ability of phosphorus to osmium that is stronger than that of car- A =[HB(C F ) ] i Os 6 5 3 bon. On the other hand, the reactions with terminal alkynes Pr3P Si H Trip resulted in activation of the C(sp)AH bond to give 67, according 45 H R Ph Ph to the NMR studies. On the basis of a deuterium-labeling experi- t C6D5Br, r.t. ment, the authors proposed a mechanism involving a direct CAH Bu C P C6H5Br C6H5Br „ r.t. −78 oC addition across the Os Si bond. The formation of the metallasilacyclobutadiene 65 is an impor- A A A tant cycloaddition reaction between the M„Si triple bond and the i „ Pr P Os Si Trip i i Os C C triple bond, which appeared in the scientific journal as the 3 Pr P Os Si Trip Pr P Trip H 3 3 Si H H first example. Around the same period, related [2+2] cycloaddition P: H Ph Ph t reactions between silylyne complexes with several alkynes have Bu R 65 t been found also by Filippou’s group [30d]. Furthermore it can be (88%) 66 (77%) 67a: R= Bu i anticipated that any catalytic reactions will be interesting and 67b:R=SiPr 3 exciting because they will probably follow the same types of cat- (byNMR) alytic processes that have been observed for the related transition

Scheme 35. Reactions of silylyne complex 45 with alkynes and phosphaalkyne. metal carbyne complexes, e.g., alkyne metathesis. 378 H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379

PMe 3 PMe3 5. Outlook PMe 3 N2 Cl Re Ge R + Me3P Mo N2 The chemistry of transition metal-silicon/germanium triple- Cl Me P toluene 3 o bonded complexes, i.e., silylyne and germylyne complexes, has PMe3 PMe3 60 C developed into a rich and exciting research ﬁeld over the last R=C H -2,6-Trip − 2N2 22a 6 3 2 two decades. The [2+n] cycloaddition reactions between the metal–silicon/germanium triple bond and the unsaturated bonds PMe of organic substrates have already been proved to occur, and sev- 3 PMe3 PMe 3 PMe3 eral subsequent reactions with intriguing reaction patterns have + Cl Mo Ge R Cl Re PMe3 also been discovered as described in this review. Although the

Me3P Me3P reactions catalyzed by silylyne or germylyne complexes have not PMe 3 PMe3 been discovered yet, the chance of developing such catalytic reac- 69 tions in the future appears to be high. Furthermore, the expected (by 1Hand31PNMR) catalytic reactions will be deﬁnitely interesting and exciting

Scheme 37. Reaction of germylyne complex 22a with Mo(N2)2(PMe3)4. because the reaction patterns of those catalytic reactions are con- sidered to be closely related to those of the reactions such as alkyne metathesis reaction etc. catalyzed by carbyne complexes. A challenging object of this ﬁeld for the future will be the discovery of new reactions catalyzed by silylyne or germylyne complexes to prepare the silicon or germanium compounds that are elusive or difﬁcult to prepare. An additional intriguing feature of silylyne and germylyne complexes that carbyne complexes hardly have is their potential frustrated Lewis pair reactivity and amphiphilic reactivity. These complexes having a structure with adjacent Lewis acid (electrophile: Si, Ge) and Lewis base (nucleophile: transition metal itself or the ligands bound to the metal) could become a new type of catalyst that selectively activate CAH, CAC, and other bonds under mild conditions. Therefore, another challenging object Scheme 38. Reaction of silylyne complex 40a with CO and the oxygenation of the will be the development of new catalysts featuring metal-silicon/ resulting product. germanium triple-bonded structures. We hope this review article will help achieving these objects by stimulating the curiosity and research of a wide variety of chemists. 4.3. Some other reactions

Besides the above-mentioned reactions, Filippou’s group Acknowledgements reported that the rhenium germylyne complex 22a reacted with CO and MeNC at the metal center accompanied by 1,2-chloride This work was supported partly by JSPS KAKENHI (the Ministry migration to afford chlorido(chlorogermylene) complexes 68a of Education, Culture, Sports, Science and Technology, Japan (Grant and 68b, respectively (Scheme 36) [41]. The same complex 22a Numbers. JP15H03782, JPK05444) and MEXT KAKENHI (a Grant- underwent a novel germylyne transfer reaction with dinitrogen in-Aid for Scientiﬁc Research on Innovative Areas, ‘‘Stimuli- responsive Chemical Species for the Creation of Functional molybdenum complex cis-[Mo(N2)2(PMe3)4] to produce molybdenum germylyne complex 69 as a main product (Scheme 37) [41]. Molecules” [#2408] (Grant Number No. JP24109011). A possible driving force for this germylyne transfer reaction is the stability of the molybdenum complex 69 that is higher than References that of the rhenium complex 22a. Indeed, the p-back donation to the germylyne ligand from Mo(II) in 69 is expected to be stronger [1] W.A. Nugent, J.M. Mayer, Metal-Ligand Multiple Bonds, Wiley, New York, 1988. than that from Re(III) in 22a due to the higher dp orbitals of the for- [2] E.O. Fischer, Adv. Organomet. Chem. 14 (1976) 1. mer, which makes 69 more stable than 22a. [3] R.R. Schrock, Acc. Chem. Res. 12 (1979) 98. [4] U. Schubert, Coord. Chem. Rev. 55 (1984) 261. Furthermore, Filippou’s group demonstrated that the reaction [5] R.R. Schrock, Chem. Rev. 102 (2002) 145. of the cationic chromium silylyne complex 40a with CO led to [6] K.H. Dötz, Angew. Chem. Int. Ed. Engl. 23 (1984) 587. ⁄ + [7] R.H. Grubbs, Handbook of Metathesis, Wiley-VCH, Weinheim, 2003. the formation of a cationic chromiosilylene [Cp Cr(CO)3Si(SIdipp)] ⁄ @ [8] H. Fischer, P. Hofmann, F.R. Kreissl, R.R. Schrock, U. Schubert, K. Weiss, Carbyne (70), which allowed access to a metallasilanone [Cp Cr(CO)3Si( O) Complexes, VCH, Weinheim, 1988. + (SIdipp)] (71) via oxygenation with N2O [51] (Scheme 38). In addi- [9] W. Petz, Chem. Rev. 86 (1986) 1019. ⁄ + [10] M.F. Lappert, R.S. Rowe, Coord. Chem. Rev. 100 (1990) 267. tion, a cationic metallogermylene [Cp W(CO)3Ge(Idipp)] relevant to 70 has recently been reported by one of the authors and cowork- [11] C. Zybill, H. Handwerker, H. Friedrich, Adv. Organomet. Chem. 36 (1994) 229. [12] H.K. Sharma, K. Pannell, Chem. Rev. 95 (1995) 1351. ers, which was prepared by a different method: chloride abstrac- [13] M. Haaf, T.A. Schmedake, R. West, Acc. Chem. Res. 33 (2000) 704. ⁄ tion from [Cp W(CO)3GeCl(Idipp)] [72]. [14] H. Ogino, Chem. Rec. 2 (2002) 291. In general, silylyne and germylyne complexes have been proved [15] H. Okazaki, H. Tobita, H. Ogino, Dalton Trans. (2003) 493. [16] R. Eaterman, P.G. Hayes, T.D. Tilley, Acc. Chem. Res. 40 (2007) 712. to undergo speciﬁc activations of CAH, C@O, C„C, and other bonds [17] B. Blom, M. Stoelzel, M. Driess, Chem. Eur. J. 19 (2013) 40. illustrated in Schemes 30, 32, 34, and 35. These unique bond acti- [18] L. Álvarez-Rodríguez, J.A. Cabeza, P. García- Álvarez, D. Polo, Coord. Chem. Rev. vations are attributable not only to the highly unsaturated and 300 (2015) 1. [19] M.C. Lipke, A.L. Liberman-Martin, T.D. Tilley, Angew. Chem. Int. Ed. 56 (2017) exposed metal-silicon/germanium triple bond that can readily 2260. interact with various unsaturated substrates but also to the high [20] R.S. Simons, P.P. Power, J. Am. Chem. Soc. 118 (1996) 11966. Lewis acidity of the silylyne and germylyne ligands that allows [21] A.C. Filippou, O. Chernov, K.W. Stumpf, G. Schnakenburg, Angew. Chem. Int. Ed. 49 (2010) 3296. these ligands to capture various Lewis basic substrates to facilitate [22] H. Hashimoto, T. Fukuda, H. Tobita, M. Ray, S. Sakaki, Angew. Chem. Int. Ed. 51 intramolecular CAH and other bond activations. (2012) 2930. H. Hashimoto, H. Tobita / Coordination Chemistry Reviews 355 (2018) 362–379 379

[23] T. Fukuda, T. Yoshimoto, H. Hashimoto, H. Tobita, Organometallics 35 (2016) [42] Y.N. Lebedev, U. Das, G. Schnakenburg, A.C. Filippou, Organometallics 36 921. (2017) 1530. [24] T. Yoshimoto, H. Hashimoto, N. Hayakawa, T. Matsuo, H. Tobita, [43] K. Inomata, T. Watanabe, H. Tobita, J. Am. Chem. Soc. 136 (2014) 14341. Organometallics 35 (2016) 3444. [44] We have also independently synthesized complex 25 by exactly the same [25] L. Pu, B. Twamley, S.T. Haubrich, M.M. Olmstead, B.V. Mork, R.S. Simons, P.P. method described in Scheme 11. H. Tobita, K. Inomata, T. Watanabe, 5th Asian Power, J. Am. Chem. Soc. 122 (2000) 650. Conference on Coordination Chemistry, 12–16 July 2015, Hong Kong. [26] P. Jutzi, C. Leue, Organometallics 13 (1994) 2898. [45] H. Hashimoto, T. Tsubota, T. Fukuda, H. Tobita, Chem. Lett. 38 (2009) 1197. [27] J. Hicks, T.J. Hadlington, C. Schenk, J. Li, C. Jones, Organometallics 32 (2013) [46] P.H. Hayes, R. Waterman, P.B. Glasser, T.D. Tilley, Organometallics 28 (2009) 323. 5082. [28] T. Ohtaki, W. Ando, Organometallics 15 (1996) 3103. [47] T.P. Dhungana, H. Hashimoto, H. Tobita, Dalton Trans. 46 (2017) 8701. [29] A.C. Filippou, K.W. Stumpf, O. Chernov, G. Schnakenburg, Organometallics 31 [48] T. Fukuda, H. Hashimoto, H. Tobita, J. Organomet. Chem. 848 (2017) 89. (2012) 748. [49] A.C. Filippou, A.I. Philippopoulos, P. Portius, G. Schnakenburg, Organometallics [30] (a) A.C. Filippou, D. Hoffmann, G. Schnakenburg, Chem. Sci. 8 (2017) 6290, The 23 (2004) 4503. preparation of germylyne and silylyne complexes of Group 8 and 10 [50] A.C. Filippou, O. Chernov, B. Blom, K.W. Stumpf, G. Schnakenburg, Chem. Eur. J. introduced in this literature are reported in the following doctoral 16 (2010) 2866. dissertations; [51] A.C. Filippou, B. Baars, O. Chernov, Y.N. Lebedev, G. Schnakenburg, Angew. (b) B. Blom, ‘‘Reactivity of Ylenes at Late Transition Metal Centers,” Chem. Int. Ed. 53 (2014) 565. Dissertation University of Bonn, 2011: The publisher Cuviller (ISBN 978-3- [52] B.V. Mork, T.D. Tilley, Angew. Chem. Int. Ed. 42 (2003) 357. 86955-907-0); (c) I. Papazoglou, ‘‘Unprecedented Tetrylidyne Complexes of [53] P.G. Hayes, Z. Xu, C. Beddie, J.M. Keith, M.B. Hall, T.D. Tilley, J. Am. Chem. Soc. Group 6 and Group 10 Metals,” Dissertation, University of Bonn, 2017): the 135 (2013) 11780. publisher Dr. Hut (ISBN 978-3-8439-3155-7); (d) O. Chernov, ‘‘Novel Silicon [54] P.G. Hayes, C. Beddie, M.B. Hall, R. Waterman, T.D. Tilley, J. Am. Chem. Soc. 128 Precursors for the Synthesis of the First Compounds with Metal-Silicon Triple (2006) 11780. Bonds, Dissertation, University of Bonn, 2012” (http://hss.ulb.uni-bonn.de/ [55] T. Watanabe, H. Hashimoto, H. Tobita, Angew. Chem. Int. Ed. 43 (2004) 218. 2012/2994/2994.htm). [56] K.K. Pandey, M. Lein, G. Frenking, J. Am. Chem. Soc. 125 (2003) 1660. [31] A.C. Filippou, A.I. Philippopoulos, P. Portius, D.U. Neumann, Angew. Chem. Int. [57] M. Lein, A. Szabo, A. Kovacs, G. Frenking, Faraday Disc. 124 (2003) 365. Ed. 39 (2000) 2778. [58] N. Takagi, K. Yamazaki, S. Nagase, Bull. Korean Chem. Soc. 24 (2003) 832. [32] A.C. Filippou, P. Portius, A.I. Philippopoulos, Organometallics 21 (2002) 653. [59] K.K. Pandey, A. Lledó, Inorg. Chem. 48 (2009) 2748. [33] A.C. Filippou, G. Schnakenburg, A.I. Philippopoulos, N. Weidemann, Angew. [60] K.K. Pandey, P. Patidar, P.P. Power, Inorg. Chem. 50 (2011) 7080. Chem. Int. Ed. 44 (2005) 5979. [61] Based on a survey of the Cambridge Structural Data Base, CSD version 5.37 [34] A.C. Filippou, A. Barandov, G. Schnakenburg, B. Lewall, M.V. Gastel, A. (November, 2015). Marchanka, Angew. Chem. Int. Ed. 51 (2012) 789. [62] K.E. Lee, A.M. Arif, J.A. Gladysz, Organometallics 10 (1991) 751. [35] A.C. Filippou, P. Portius, A.I. Philippopoulos, H. Rohde, Angew. Chem. Int. Ed. 42 [63] M. Hirotsu, T. Nunokawa, K. Ueno, Organometallics 25 (2006) 1554. (2003) 445. [64] T. Fukuda, H. Hashimoto, S. Sakaki, H. Tobita, Angew. Chem. Int. Ed. 55 (2016) [36] A.C. Filippou, A.I. Philippopoulos, G. Schnakenburg, Organometallics 22 (2003) 188. 3339. [65] K. Ueno, S. Asami, N. Watanabe, H. Ogino, Organometallics 21 (2002) 1326. [37] A.C. Filippou, H. Rohde, G. Schnakenburg, Angew. Chem. Int. Ed. 43 (2004) [66] T. Watanabe, H. Hashimoto, H. Tobita, Chem. Asian J. 7 (2012) 1408. 2243. [67] T. Fukuda, H. Hashimoto, H. Tobita, Chem. Commun. 49 (2013) 4232. [38] A.C. Filippou, N. Weidemann, G. Schnakenburg, H. Rohde, A.I. Philippopoulos, [68] A.C. Filippou, O. Chernov, G. Schnakenburg, Angew. Chem. Int. Ed. 50 (2011) Angew. Chem. Int. Ed. 43 (2004) 6512. 1122. [39] A.C. Filippou, N. Weidenmann, G. Schnakenburg, Angew. Chem. Int. Ed. 47 [69] X. Ye, L. Yang, Y. Li, J. Huang, L. Zhou, Q. Lei, W. Fang, H. Xie, Eur. J. Inorg. Chem. (2008) 5799. (2014) 1502. [40] A.C. Filippou, N. Weidemann, A.I. Philippopoulos, G. Schnakenburg, Angew. [70] T. Fukuda, H. Hashimoto, H. Tobita, J. Am. Chem. Soc. 136 (2014) 80. Chem. Int. Ed. 45 (2006) 5987. [71] M. Li, Q. Yang, R. Fang, J. Chem. Sci. 127 (2015) 1477. [41] A.C. Filippou, U. Chakraborty, G. Schnakenburg, Chem. Eur. J. 19 (2013) 5676. [72] K. Inomata, Y. Miyazaki, T. Watanabe, H. Tobita, J. Am. Chem. Soc. 137 (2015) 1193.