Bond Forming Reactions Involving Isocyanides at Diiron Complexes

inorganics Review Bond Forming Reactions Involving Isocyanides at Diiron Complexes Rita Mazzoni 1 , Fabio Marchetti 2,*, Andrea Cingolani 1 and Valerio Zanotti 1,* 1 Dipartimento di Chimica Industriale “Toso Montanari”, University of Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy; [email protected] (R.M.); [email protected] (A.C.) 2 Dipartimento di Chimica e Chimica Industriale, University of Pisa, Via G. Moruzzi 13, I-56124 Pisa, Italy * Correspondence: [email protected] (F.M.); [email protected] (V.Z.); Tel.: +39-051-209-3694 (V.Z.) Received: 28 January 2019; Accepted: 14 February 2019; Published: 26 February 2019 Abstract: The versatility of isocyanides (CNR) in organic chemistry has been tremendously enhanced by continuous advancement in transition metal catalysis. On the other hand, the urgent need for new and more sustainable synthetic strategies based on abundant and environmental-friendly metals are shifting the focus towards iron-assisted or iron-catalyzed reactions. Diiron complexes, taking advantage of peculiar activation modes and reaction profiles associated with multisite coordination, have the potential to compensate the lower activity of Fe compared to other transition metals, in order to activate CNR ligands. A number of reactions reported in the literature shows that diiron organometallic complexes can effectively assist and promote most of the “classic” isocyanide transformations, including CNR conversion into carbyne and carbene ligands, CNR insertion, and coupling reactions with other active molecular fragments in a cascade sequence. The aim is to evidence the potential offered by diiron coordination of isocyanides for the development of new and more sustainable synthetic strategies for the construction of complex molecular architectures. Keywords: isocyanide; diiron complexes; aminocarbyne; aminocarbene; imidoyl; multicomponent reactions 1. Introduction Since the discovery of isocyanide-based multicomponent reactions (MCRs) (e.g., the Passerini and Ugi reactions), isocyanides (CNR) have been recognized as extremely valuable synthetic tools for the construction of complex molecules, particularly heterocycles [1–4]. Development of MCRs takes advantage of the complex nature of isocyanides, which can act both as nucleophiles and electrophiles. On the other hand, isocyanides are long-known ligands, and their reactivity can be considerably modified upon transition-metal coordination [5–7]. Indeed, both the nucleophilicity and the electrophilicity of isocyanides can be controlled and enhanced upon metal coordination. As an example, isocyanide ligands have been successfully used for obtaining both carbene [8,9] and carbyne complexes [10]. In general, interest in isocyanide chemistry involving metal species is still growing [11,12]. One reason for this is a resurgent interest in isocyanides as C1 synthons driven by the development of transition-metal-catalyzed isocyanide insertion reactions [13–16]. Similarly to CO, isocyanides can be incorporated into catalytic mechanisms based on oxidative addition, isocyanide insertion, and reductive elimination. This emerging strategy, based on CNR insertion into M-element bonds, offers tremendous opportunities for the synthesis of fine chemicals containing a nitrogen functionality, and is largely dominated by Pd-catalyzed or Pd-promoted reactions [17–22]. Considerable efforts have been made in order to expand the research field to other transition- metals, given the strong interest in replacing noble and rare metals with more abundant and sustainable Inorganics 2019, 7, 25; doi:10.3390/inorganics7030025 www.mdpi.com/journal/inorganics Inorganics 2018, 6, x FOR PEER REVIEW 2 of 18 Inorganics 7 given the2019 strong, , 25 interest in replacing noble and rare metals with more abundant and sustainable2 of 17 transition metals, such as iron. Indeed, the number of Fe-catalyzed reactions is rapidly expanding, buttransition those involving metals, such isocyanides as iron. Indeed, are still the limited number [23–25]. of Fe-catalyzed reactions is rapidly expanding, but thoseThis involving mini-review isocyanides focuses are on still reactions limited involving [23–25]. isocyanides and diiron complexes, aimed at the formationThismini-review of C–C or focuses C–heteroatom on reactions bonds involving for the isocyanides construction and diironof more complexes, complex aimed molecular at the architectures.formation of C–C The orinterest C–heteroatom in diiron bonds complexes for the (ins constructiontead of complexes of more containing complex molecular one Fe atom) architectures. is based onThe two interest major in features: diiron complexes First, two (instead adjacent of Fe complexes atoms might containing provide one cooperative Fe atom) is effects, based on particularly two major effectivefeatures: in First, redox two reactions adjacent that Fe involv atomse mightcoordinated provide ligands. cooperative As an example, effects, particularly μ-dithiolate effective diiron has in beenredox very reactions actively that investigated involve coordinated as mimics ligands. of [FeFe]-H As an2ases example, active siteµ-dithiolate and as potential diiron has catalysts been very for hydrogen evolution [26–29]. The second peculiar feature of diiron complexes is the possibility of actively investigated as mimics of [FeFe]-H2ases active site and as potential catalysts for hydrogen observingevolution [bridging26–29]. Thecoordination second peculiar of a variety feature of of carbon-based diiron complexes ligands is the(including possibility isocyanides of observing and hydrocarbylbridging coordination ligands derived of a variety from of CNR carbon-based ligands), ligandswhich potentially (including isocyanidesprovide access and to hydrocarbyl activation modesligands and derived reaction from profiles CNR ligands), not observed which potentiallywhen the same provide ligands access are to bound activation to a modes single andmetal reaction atom [30–32].profiles not observed when the same ligands are bound to a single metal atom [30–32]. The review review covers covers most most of of the the research research work work done done by bysome some of us of in us this in thisfield, field, in the in last the two last decades.two decades. 2. Diiron Complexes with Isocyanide Ligands Diiron complexescomplexes containing containing isocyanide isocyanide ligands ligands have have been wellbeen known well known since the since 1970s. the Examples 1970s. Examplesinclude the include complexes the complexes [Fe2(CNEt) 9[Fe][332(CNEt),34] and9] [33,34] [Fe2(CNPh) and [Fe9][235(CNPh)], formally9] [35], derived formally and derived structurally and structurallysimilar to [Fe similar2(CO) 9to]. [Fe Beside2(CO) the9]. Beside homoleptic the homoleptic complexes complexes mentioned mentioned above, a above, prominent a prominent role has rolebeen has assumed been assumed by isocyanide by isocyanide complexes complexes derived derived from the from [Fe 2the(Cp) [Fe2(CO)2(Cp)4]2(CO) frame4] frame (Cp = (Cpη-C 5=H η5).- CDirect5H5). Direct replacement replacement of CO ofwith CO with CNR CNR in [Fe in 2[Fe(Cp)2(Cp)2(CO)2(CO)4] generally4] generally results results in in the the formationformation of mixtures of isocyanide complexes complexes [Fe [Fe22(Cp)(Cp)2(CO)2(CO)4−4n−(CNR)n(CNR)n] (nn]( =n 1–3),= 1–3), with with mono-isocyanide mono-isocyanide and and di- isocyanidedi-isocyanide complexes complexes being being prevalent prevalent (Scheme (Scheme 1,1 ,compounds compounds 1 1[36–38][36–38 ]and and 22 [39,40],[39,40], respectively). respectively). Diiron complexes of the typetype [Fe[Fe22(Cp)2(CNR)(CNR)44],], containing containing up up to to four four is isocyanideocyanide ligands, are also known [41]. [41]. Scheme 1. SelectedSelected examples of diiron complexes with isocyanide ligands. Alternative synthetic routes routes have have been been design designeded in in order order to to improve improve the the selectivity selectivity [42]; [42]; nevertheless, the the preparation preparation remains remains difficult, difficult, and and separation separation steps steps are generally are generally required. required. The situationThe situation is made is made more morecomplicated complicated by the by presence the presence of isomers, of isomers, in that inisocyanides that isocyanides can act both can actas terminalboth as terminaland bridging and bridgingligands. Indeed, ligands. CNR Indeed, shows CNR a higher shows tendency a higher toward tendency bridging toward coordination bridging t comparedcoordination to CO, compared and only to CO, with and the only bulky with CNBu thet bulkyligand CNBu does theligand terminal does coordination the terminal coordinationprevail [43]. prevailIsocyanide [43]. ligands, in diiron complexes, might also result from the transformation of other Isocyanide ligands, in diiron complexes, might also result from the transformation of other ligands. ligands. For example, alkylation (with MeSO3CF3) of the cyanide in the complex For example, alkylation (with MeSO CF ) of the cyanide in the complex [Fe (Cp) (CN)(CNMe )(CO) ] [Fe2(Cp)2(CN)(CNMe2)(CO)3] leads3 3 to the formation of the2 isocyanide2 complex2 3 leads to the formation of the isocyanide complex [Fe (Cp) (CNMe )(CO) (CNMe)]SO CF [44]. [Fe2(Cp)2(CNMe2)(CO)3(CNMe)]SO3CF3 [44]. A less predictable2 2 route2 to isocyanide3 complexes3 3 is Aprovided less predictable by the peculiar route to reaction isocyanide of complexesthe cyanate is (NCO provided−) with by the peculiarbridging reaction thiocarbyne

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