inorganics Article Transmetalation from Magnesium–NHCs—Convenient Synthesis of Chelating π-Acidic NHC Complexes Julian Messelberger 1, Annette Grünwald 1, Philipp Stegner 2, Laura Senft 3, Frank W. Heinemann 1 and Dominik Munz 1,* 1 Lehrstuhl für Anorganische und Allgemeine Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany; [email protected] (J.M.); [email protected] (A.G.); [email protected] (F.W.H.) 2 Lehrstuhl für Anorganische und Metallorganische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany; [email protected] 3 Lehrstuhl für Bioanorganische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-9131-85-27464 Received: 6 May 2019; Accepted: 19 May 2019; Published: 22 May 2019 Abstract: The synthesis of chelating N-heterocyclic carbene (NHC) complexes with considerable π-acceptor properties can be a challenging task. This is due to the dimerization of free carbene ligands, the moisture sensitivity of reaction intermediates or reagents, and challenges associated with the workup procedure. Herein, we report a general route using transmetalation from magnesium–NHCs. Notably, this route gives access to transition-metal complexes in quantitative conversion without the formation of byproducts. It therefore produces transition-metal complexes outperforming the conventional routes based on free or lithium-coordinated carbene, silver complexes, or in situ metalation in dimethyl sulfoxide (DMSO). We therefore propose transmetalation from magnesium–NHCs as a convenient and general route to obtain NHC complexes. Keywords: NHC; transmetalation; magnesium; palladium; carbene 1. Introduction N-heterocyclic carbene (NHC) ligands have become the powerhouse of modern transition metal chemistry [1–13]. This is largely due to the ease of tuning their electronic and steric properties targeting a specific application. After Arduengo’s report on the first crystalline carbene [14], it became common belief that NHCs should be considered strong σ-donor ligands with negligible π-acceptor properties. However, it was later realized that the π-acceptor properties of ancillary carbene ligands are equally important [15]. This led to the design of π-electron-withdrawing carbenes such as Bertrand’s cyclic (alkyl)(amino) carbenes (CAACs) [16–21], diamido carbenes (DACs) [22–25], or ferrocenium-decorated NHCs [26–29]. Also, NHCs with saturated backbones (saNHCs, imidazolidin-2-ylidenes) as well as benzannulated congeners (benzNHCs, benzimidazolin-2-ylidenes) show considerable backbonding capabilities. This is either due to the pyramidalization of the amino groups, which reduces the overlap with the π-system within the N-heterocycle, or to the π-acidic character of the benzannulated π-system. An excellent example of the importance of these π-effects is represented by conjugated organic singlet biradicaloids derived from carbene scaffolds, where the stability and electronic properties are largely dependent on the nature of the carbene [30–34]. Another outstanding example from transition metal chemistry is the capability of CAACs and saNHCs to stabilize low-valent metal complexes [35]. Inorganics 2019, 7, 65; doi:10.3390/inorganics7050065 www.mdpi.com/journal/inorganics Inorganics 2019, 7, 65 2 of 14 Inorganics 2019, 7, x FOR PEER REVIEW 2 of 14 34]We. Another showed outstanding recently that example a strong from ligand transition field as thatmetal exerted chemistry by carbene is the ligandscapability with of strongCAACsσ -donorand andsaNHCs strong toπ-acceptor stabilize low properties-valent metal stabilizes complexes multiple-bonded [35]. late-transition-metal complexes [36–38]. We thereforeWe showed became recently interested that ina strong the synthesis ligand field of complexes as that exerted with πby-acidic carbene 2,6-pyridine ligands with diNHC strong (CNC) σ- pincer-typedonor and ligands strong derivedπ-acceptor from properties the saNHC stabilizes or benzNHC multiple sca-ffbondedold bridged late-transition by a pyridine-metal moiety complexes [39–41 ]. In[36 contrast–38]. We to thetherefore ubiquitous became conventional interested in imidazoline-derived the synthesis of complexes NHCs, with examples π-acidic for 2,6 the-pyridine saturated imidazolidinediNHC (CNC derivatives) pincer-type are ligands still comparably derived from scarce the in saNHC the literature or benzNHC [42–44]. scaffold This applies bridged even by more a topyridine CNC pincer-type moiety [39 ligands–41]. In with contrast saNHC to orthe benzNHC ubiquitous congeners conventional [45– 50imidazolin]. In fact,e- onlyderived one NHCs, example hasexamples been reported for thefor saturated the saNHC imidazolidin derivatives,e derivatives which is are Chirik’s still comparably iron complex scarce [51, 52in ].the This literature iron complex [42– 44]. This applies even more to CNC pincer-type ligands with saNHC or benzNHC congeners [45–50]. was synthesized by in situ deprotonation/metalation using iron hexamethyldisilazane (Fe[N(SiMe3)2]2) In fact, only one example has been reported for the saNHC derivatives, which is Chirik’s iron complex as a precursor. Note that this precursor is inconvenient to handle. Besides, because of its high air [51,52]. This iron complex was synthesized by in situ deprotonation/metalation using iron sensitivity, it has to be distilled prior to use [53,54]. Accordingly, we decided to investigate more hexamethyldisilazane (Fe[N(SiMe3)2]2) as a precursor. Note that this precursor is inconvenient to expedient and general routes to access metal complexes. handle. Besides, because of its high air sensitivity, it has to be distilled prior to use [53,54]. Accordingly,NHC complexes we decided of the to s-block investigate metals more are expedient still rarely and explored general [55 routes]. This to is access also true metal for complexes. magnesium, althoughNHC the complexes first example of the of ans-block NHC–magnesium metals are still complex rarely explored dates back [55] to. Th 1993is is [56 also]. Subsequent true for investigationsmagnesium, withalthough s-block the first metals example focused of [an57 –NHC68] largely–magnesium on anionic complex ligands dates becauseback to of1993 the [56] weak. magnesium–NHCSubsequent investigations bond [69 with–74]. s For-block magnesium–NHC metals focused [57 complexes,–68] largelyexamples on anionic of liga saturatednds because [75] of and benzannulatedthe weak magnesium [76,77] NHCs–NHC remain bond [69 extraordinarily–74]. For magnesium scarce. We–NHC hypothesized complexes, that examples transmetalation of saturated from magnesium–NHCs[75] and benzannulated should be[76,77 an exceptional] NHCs remain mild method extraordinar to synthesizeily scarc NHCe. We complexes. hypothesized In particular, that wetransmetalation were hoping for from a suppression magnesium– ofNHCs carbene-dimerization should be an exceptional processes mild as method well as ato beneficialsynthesize template NHC effcomplexes.ect by the magnesiumIn particular, metal.we were Accordingly, hoping for a suppression we were intrigued of carbene by-dimerization the low cost processes as well as as easewell of handlingas a beneficial of the template related magnesium effect by the compounds.magnesium metal. Inspiration Accordingly, came especiallywe were intrigued from two by reportsthe low in thecost literature, as well as in ease which of thehandling transmetalation of the related of anmagnesium anionic NHCcompounds. ligand toInspiration iron [73] came and ofespecially an acyclic diaminocarbenefrom two reports ligand in theto literature, copper [in78 which] was the reported. transmetalation In light of an the anionic recent NHC report ligand of heavy to iron alkaline [73] earth–NHCand of an complexesacyclic diaminocarbene embedded in ligand tridentate to copper coordination [78] was environmentsreported. In light [79 ],of we the decided recent report to explore of theheavy transmetalation alkaline earth from–NHC magnesium–NHCs complexes embedded in more in tridentate detail. Herein, coordination we report environment a convenients [79] method, we fordecided the synthesis to explore of the late-transition-metal transmetalation from complexes magnesium with–NHCs tridentate in more ligands detail. Herein, with π -acidicwe report NHCs a convenient method for the synthesis of late-transition-metal complexes with tridentate ligands with based on transmetalation from magnesium complexes. Notably, other routes commonly applied [80] π-acidic NHCs based on transmetalation from magnesium complexes. Notably, other routes failed entirely in our hands or gave low yields and/or impure products. Our results hence suggest commonly applied [80] failed entirely in our hands or gave low yields and/or impure products. Our transmetalation from magnesium–NHCs as a general and convenient method to access π-acidic and results hence suggest transmetalation from magnesium–NHCs as a general and convenient method chelating NHC complexes with high yields. to access π-acidic and chelating NHC complexes with high yields. 2. Results and Discussion 2. Results and Discussion Following our computational predictions [36,37], we decided to synthesize 2,6-pyridine diNHC Following our computational predictions [36,37], we decided to synthesize 2,6-pyridine diNHC (CNC)(CNC) complexes complexes withwith bulky 2,6 2,6-diisopropylphenyl-diisopropylphenyl