ReviewThe transition-metal chemistry of amidinatosilylenes, -germylenes and -stannylenes
Graphical abstract
Introduction
“Tetrylene” is a chemical term that refers to a compound having a group 14 element in the +2 oxidation state. Heavier tetrylenes (HTs), also known as heavier carbene analogs, group 14 metalenes or group 14 metalylenes, are compounds of fundamental interest in main-group chemistry [1]. The simplest members of this family are species of general formula MX2 (M = Si, Ge, Sn, Pb; X = anionic group). Due to (i) their marked dual Lewis acid–base character (in their fundamental state they possess both a lone-pair of electrons and a vacant p orbital on the M atom; Fig. 1), (ii) the high polarity of their MX bonds, and (iii) their great versatility (four M atoms are available and X can virtually be any anionic group), they display a unique and very rich reactivity [2], [3]. For example, they are able to activate small molecules, insert into organic and inorganic σ-bonds, add to unsaturated substrates, promote cycloadditions, participate in redox processes, form donor–acceptor Lewis adducts and act as σ-donor–π-acceptor ligands in TM complexes (Fig. 1).
The coordination chemistry of HTs [3], [4], although covering a wide range of TMs, is still far from the maturity achieved by that of their carbene analogs and, particularly, that of N-heterocyclic carbenes (NHCs) [5], because systematic reactivity studies (stoichiometric or catalytic) involving HT–TM complexes are still comparatively scarce. This underdevelopment can be mainly ascribed to some synthetic problems and to the general lower stability of HTs and also of their TM complexes toward air, moisture, and/or substitution processes. In contrast to NHCs, coordinated HTs are generally weakly bonded to TMs (the MTM bond strength decreases on going down group 14 column of the Periodic Table [6]) and are prone to undergo oxidation and/or hydrolysis processes [7]. Additionally, the syntheses of most of the HT–TM complexes reported to date are generally tackled using free HT reagents, which are very air-sensitive. However, the use of pure NHCs is often unnecessary for the preparation of NHC–TM complexes (for example, imidazol-2-ylidenes can be generated in situ from readily available imidazolium salts). Silylene–TM complexes are specially difficult to prepare because silicon(II) precursors are not commercially available and a reduction of silicon(IV) reagents is required. Other methods that do not require the use of pure HT reagents to form HT–TM complexes have been developed mainly for the synthesis of silylene derivatives [4c]. For example, the double salt metathesis of MR2X2 (X = halide) species with appropriate metallic precursors (e.g., K2[Fe(CO)4]) [8], the abstraction of anionic X groups from XR2M–TM (X = Cl, OTf) complexes using electrophilic reagents (e.g., Na[BPh4]) [9], the 1,2-migration of a substituent (generally H) of an MHR2 ligand, mediated by the generation of a vacant coordination site on the TM [10], the capture of photochemically generated transient MR2 fragments by an appropriate metal center [11], the metathesis of dianionic [MR2]2− species with metallic dihalide precursors (e.g., [TiCp2Cl2]) [12], etc. However, despite the synthetic and stability issues mentioned above, the last 40 years have witnessed an incessant research activity devoted to prepare and study the chemistry of novel HT–TM complexes. These investigations have been associated with the emergence of new kinds of HT molecules in the main-group chemistry arena.
A very general classification of HT ligands is summarized in Fig. 2. They can be simple (S) or donor-stabilized (DS). Simple HTs can be acyclic (equipped with two terminal anionic groups; Sa) or cyclic (in which the M atom is chelated by a dianionic fragment, Sc). Donor-stabilized HTs formally result from an intramolecular (DSintra) or intermolecular (DSinter) interaction of a two-electron-donor group (D) with the M atom of simple HTs.
Fig. 3 collects the most representative examples of HTs that have been most used as ligands in TM complexes. As of the early stages of the HT–TM chemistry, the readily accessible dichlorides GeCl2·dioxane, SnCl2 and PbCl2 (I in Fig. 3) and their TM complexes have been used to prepare more sophisticated HTs and HT–TM derivatives via transmetallation procedures [2], [4]. The synthesis of some SiCl2 derivatives of TMs [3a,b] has followed the recent discovery (Roesky, 2009) that the elusive SiCl2 molecule can be stabilized with bulky NHCs (II) [13]. Since their discovery by Lappert 40 years ago, the diamido- or dialkyl-HTs M(HMDS)2 or M{CH(TMS)2}2 (M = Ge, Sn, Pb; III), or related HTs also equipped with bulky anionic groups, have been incorporated in a large number of TM complexes [4e,f,g]. The cyclic heavier tetrylenes reported by Veith et al., particularly M{(NtBu)2SiMe2} (M = Ge, Sn; IV) [2h] have been integrated in several metal complexes since the 80s [14]. Note that the silylene versions of both Lappert's and Veith's HTs, namely, Si(HMDS)2 [15] and Si{(NtBu)2SiMe2} [16], are known but they are stable only at low temperatures. A major breakthrough in the coordination chemistry of HTs was the synthesis by Denk et al. in 1994 of the first stable N-heterocyclic silylene, namely, Si{(NtBu)2C2H2} (V) [17], which is a silicon analog of Arduengo's type NHCs [5]. Its coordination chemistry (or that of related systems equipped with different NR groups, saturated backbones, benzoannulated rings, etc.) was then extensively investigated [3j,k], giving also rise to a few catalytic applications [18]. Related N-heterocyclic stannylenes [19] and germylenes [20] were known before silylene V was reported, but their coordination chemistry has not been studied to a comparable degree [3], [21], [22]. Of particular interest is the work developed by Hahn et al. with bidentate benzoannulated germylenes and stannylenes (VI) [22]. The group of Kira isolated the first dialkylsilylene, the cyclic Si[C(TMS)2C2H4] (VII), in 1999 [23], but its use as a ligand is hitherto restricted to a few metal complexes of groups 10 and 11 [3], [24]. Very recently, in 2006, attempting the synthesis of a β-diketiminate-stabilized silylene (see below), Driess and co-workers isolated the first heterofulvene-like silylenes (VIII) [25a], which have allowed, for example, the remarkable stabilization of elusive nickel(0) arene complexes [25b–e]. Finally, the bottom part of Fig. 3 displays the donor-stabilized HTs that have been most used as ligands in coordination chemistry. These HTs contain β-diketiminato (IX) [3], [26], β-diketonate (X) [4], [27], aminotropominato (XI) [3], [28], 2,6-bis(D)phenyl (D = CH2NMe2, P(O)(OiPr)2) (XII) [29], or amidinato (XIII) [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61] groups attached to the group 14 metal atom.
Among the family of intramolecularly stabilized HTs, those containing amidinato fragments are currently being increasingly utilized in TM coordination chemistry. In fact, since the first report, only seven years ago by Jones et al. [32], of the tungsten and iron amidinatogermylene derivatives [W(CO)5{Ge(Diip2tbam)Cl}] and [FeCp{Ge(Diip2tbam)}(CO)2], nearly one hundred amidinato-HT–TM complexes have been prepared, covering metals of almost all the TM groups of the Periodic Table (group 3 is the only exception). Noteworthy, contrasting with the few catalytic applications that have been found for other stable HT–TM complexes in the last 40 years [4], [18], [62], various amidinato-HT–TM complexes have already been identified as active catalysts for important transformations of organic substrates, such as ketone hydrosilylations [46], [48], [2 + 2 + 2] cycloadditions [52], arene C–H borylations [53] and Kumada [56], Negishi [56] and Sonogashira [58] cross-couplings.
Despite the prominent role that amidinato-HTs are currently playing as HT ligands in TM complexes, this chemistry has not been specifically reviewed. In order to fill this gap, this contribution comprehensively reviews the synthesis, structural features, theoretical investigations, reactivity and catalytic applications of amidinato-HT–TM complexes (also including group 12 metals), covering the literature published up to the end of 2014. Related guanidinato derivatives are also considered.
Section snippets
Amidinato-HT ligands
Fig. 4 shows the most common synthetic routes to amidinato-HTs. Amidinatogermylenes, -stannylenes and -plumbylenes are commonly prepared by simple transmetallation of lithiated amidinates with the corresponding group 14 metal dihalides, leading to heteroleptic or homoleptic amidinato-HTs depending on the stoichiometry used [63]. However, as silicon(II) precursors are not readily available, the synthesis of amidinatosilylenes requires a challenging reduction step of silicon(IV) precursors. The
Amidinato-HT metal complexes
For clarity, the TM complexes whose chemistry is surveyed in this review are classified according to the column (group) of the Periodic Table to which the TM atom belongs. Within each group, the reactions are roughly in chronological order, i.e., those that were reported earlier are discussed before those that were reported later. Relevant data for the known amidinato-HT–TM complexes of the metals of each TM group of the Periodic Table (including group 12), such as yield, color, 29Si or 119Sn
Conclusions
Amidinato-HTs have recently boosted the coordination chemistry of HTs. In fact, nearly one hundred amidinato-HT–TM complexes, with examples belonging to almost all the TM groups of the Periodic Table (group 3 is the only exception), have been prepared after first members of this family were reported in 2008 by Jones et al. (compounds 5 and 31) [32].
The current high interest in the use of amidinato-HTs as ligands in coordination chemistry can be attributed to a combination of the following
Acknowledgements
This work has been supported by a European Union Marie Curie action (FP7-2010-RG-268329), by Spanish MINECO-FEDER research projects (CTQ2010-14933, RYC2012-10491 and CTQ2013-40619-P), and by a research grant from the Government of Asturias (GRUPIN14-009).
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