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Unusual Reactivities of N-Heterocyclic Carbenes Upon Coordination to the Platinum(II)–Dimethyl Moiety

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Unusual Reactivities of N-Heterocyclic Carbenes Upon Coordination to the Platinum(II)–Dimethyl Moiety Fortman, G C; Scott, N M; Linden, A; Stevens, E D; Dorta, R; Nolan, S P (2010). Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(II)-dimethyl moiety. Chemical Communications, 46(7):1050-1052. Postprint available at: http://www.zora.uzh.ch University of Zurich Posted at the Zurich Open Repository and Archive, University of Zurich. Zurich Open Repository and Archive http://www.zora.uzh.ch Originally published at: Chemical Communications 2010, 46(7):1050-1052. Winterthurerstr. 190 CH-8057 Zurich http://www.zora.uzh.ch Year: 2010 Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(II)-dimethyl moiety Fortman, G C; Scott, N M; Linden, A; Stevens, E D; Dorta, R; Nolan, S P Fortman, G C; Scott, N M; Linden, A; Stevens, E D; Dorta, R; Nolan, S P (2010). Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(II)-dimethyl moiety. Chemical Communications, 46(7):1050-1052. Postprint available at: http://www.zora.uzh.ch Posted at the Zurich Open Repository and Archive, University of Zurich. http://www.zora.uzh.ch Originally published at: Chemical Communications 2010, 46(7):1050-1052. Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(II)-dimethyl moiety Abstract Unsaturated NHCs of varying steric bulk undergo a series of unusual oxidative addition and reductive elimination processes upon binding to the Pt(Me)2 fragment. CREATED USING THE RSC COMMUNICATION TEMPLATE (VER. 3.1) - SEE WWW.RSC.ORG/ELECTRONICFILES FOR DETAILS ARTICLE TYPE www.rsc.org/xxxxxx | XXXXXXXX Unusual Reactivities of N-Heterocyclic Carbenes upon Coordination to the Platinum(II)-Dimethyl Moiety George C. Fortman,a Natalie M. Scott,a Anthony Linden,c Edwin D. Stevens,b Reto Dorta*c and Steven P. Nolan*a,b 5 Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X First published on the web Xth XXXXXXXXX 200X DOI: 10.1039/b000000x Unsaturated NHCs of varying steric bulk undergo a series of (C–H) and reductive elimination (C–H and C–C) processes unusual oxidative addition and reductive elimination processes upon binding to the platinum center. 10 upon binding to the (COD)Pt(Me)2 fragment. 55 We anticipated a rather straightforward reactivity between relatively small N-heterocyclic carbene ligands MeIMe Me i N-heterocyclic carbenes are now well-established and (%Vbur=25.5), I Pr (%Vbur=27.9), or ICy (%Vbur=26.1) and 8 versatile entities for late-transition metal compounds and have (COD)Pt(Me)2. Indeed, mixing (COD)Pt(Me)2 with 2 proven to be often superior ancillary ligands in homogeneous equivalents of the respective ligand in benzene at room 1 catalytic processes. Among such catalytic transformations, 60 temperature leads to fast substitution of the COD moiety and, 15 alkane (and in particular methane) C–H bond activation and after appropriate workup, to high yields of complexes cis- Me Me i functionalization certainly remains one of the most ( IMe)2Pt(Me)2 (1), cis-( I Pr)2Pt(Me)2 (2), and cis- 2 challenging tasks. While a variety of homogeneous metal (ICy)2Pt(Me)2 (3). Since these compounds represented a new compounds have shown potential for this kind of process, class of NHC-platinum complexes, complete characterization ‡ results with platinum complexes of the Shilov-type are 65 included X-ray diffraction studies. Ball-and-stick drawings of 20 particularly interesting for the functionalization of methane. cis-1, cis-2 and cis-3 are incorporated in Scheme 2. Due to the Intense research has gone into trying to understand individual steric crowding of the two cis-configured NHC ligands in steps of the Shilov chemistry with Pt(II)-dimethyl or Pt(IV)- these compounds, deviations from the expected square-planar methyl model compounds surrounded with neutral or anionic geometry are already apparent in these complexes. The bond 3 nitrogen-based chelates and phosphine ligands. These 70 angles between the NHCs and the platinum center increase 25 insightful studies have subsequently been implemented in an when going from cis-1 to cis-2 to cis-3, and lead to a 4 † improved catalytic process for the conversion of methane. contraction in the CH3–Pt–CH3 angles (Table S1). Me Despite the obvious similarity between NHCs and the ligands (COD)Pt normally used in this type of chemistry and despite the fact Me + 2 NHC - COD that interesting palladium-catalyzed methane reactivity has 75 30 been disclosed using a chelating N-heterocyclic carbene ligands,5 efforts to develop Shilov-type chemistry with platinum-NHC compounds have been minimal. For instance, we are aware of only two reports in the literature where NHC 6 ligands are part of alkyl- or aryl-containing platinum species, 80 35 and apparently simple model compounds such as (NHC)2Pt(Me)2 or (NHC)2Pt(Me)Cl are unknown at present. 1 2 3 N N N N N N Scheme 2. Synthesis and ball-and-stick drawings of cis- Me Me i 85 ( IMe)2Pt(Me)2 (1), cis-( I Pr)2Pt(Me)2 (2), and cis- MeIMe MeIiPr ICy 40 (ICy)2Pt(Me)2 (3). %Vbur = 25.5 %Vbur = 27.9 %Vbur = 26.1 The increasing distortions in complexes 1-3 already hinted to N N N N N N possible problems when accommodating sterically more IMes IPr ItBu 90 demanding NHC ligands. For the next bulkier ligand in our 45 %Vbur = 34.2 %Vbur = 35.8 %Vbur = 37.0 series, namely IMes (%Vbur=34.2), reaction with Scheme 1. Unsaturated N-heterocyclic carbenes used with (COD)Pt(Me)2 in benzene and subsequent evaporation gave 7 corresponding %Vbur steric parameter values. two compounds in variable ratio depending on the reaction time used. In situ reactions in C6D6 revealed relatively fast Herein, we report our results on the coordination behavior of 95 displacement of COD by the two IMes ligands. Based on these 50 monodentate, unsaturated NHCs of varying steric bulk studies, preparative reactions were devised. Workup of the 7 (Scheme 1) with (COD)Pt(Me)2, and show how the bulkier reaction after 10 minutes gave the first complex in pure form ligand members undergo a series of unusual oxidative addition while a reaction time of 2 hours led to exclusive generation of This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 1 the second compound. This latter complex was fully Activation of the isopropyl groups of the aromatic side chains characterized and turned out to be the unusual, 60 in IPr is less straightforward than benzylic C–H activation in cyclometallated derivative cis-(IMes)(IMes’)Pt(Me) (5).‡ The IMes and has not yet been documented.10 We therefore turned X-ray structure of 5 (Scheme 3, below) shows that the two our attention to the interaction of IPr (%Vbur=35.8) with 5 NHC ligands remain in a cis orientation with the (COD)Pt(Me)2. Here, interaction of 2 equivalents of IPr with cyclometallated side chain trans to the normal IMes ligand. In (COD)Pt(Me)2 at room temperature was less straightforward 1 13 spite of its ready transformation into 5, H and C NMR 65 and led to a mixture of complexes. Gratifyingly, slight heating characterization of the first compound, that we presumed to be of the benzene or toluene solution (50°-80°C) gave a yellow- cis-(IMes)2Pt(Me)2 (4), was possible. In addition, when a orange fluorescent solution from which a single, bright yellow 10 benzene solution of 4 is layered with diethyl ether, X-Ray complex was isolated in high overall yield after workup. quality crystals are swiftly formed and permit the unequivocal Complete characterization showed the compound to be the ‡ ‡ confirmation of its structure (Scheme 3, above). The cis 70 homoleptic 14-electron platinum(0) complex (IPr)2Pt (6). The orientation of the ligands, together with the steric most likely reaction pathway leading to 6 is shown in Scheme requirements of the mesityl wingtips of IMes, leads to a 4 and involves initial coordination of two IPr ligands. The 15 highly distorted structure in the solid state. In order to resulting cis-configured (IPr)2Pt(Me)2 species would then accommodate both IMes ligands, the imidazole rings cannot undergo ready C–C reductive elimination of ethane because of acquire the usual co-linear arrangement with the carbene- 75 the steric pressure exerted by the two IPr ligands on the platinum bond and tilt away and towards the two Pt–methyl methyl groups. Such reactivity is highly unusual and we are groups (Bond angles between the plane of the imidazole ring not aware of reports on C–C reductive elimination of simple † 11 20 and platinum in 4: 159.2(1)° and 163.0(1)°, see Table S1). methyl/alkyl groups from platinum(II) compounds. In the Furthermore, one of the ortho-methyl groups of IMes solid state, complex 6 shows the expected linear two- approaches the metal center (3.569(5) Å). The overall 80 coordinate arrangement with somewhat shorter NHC–Pt bond structure certainly helps explain the propensity of 4 to lengths than in the Pt(II) complexes (Table S1).† undergo subsequent intramolecular C–H activation of one of 25 the benzylic positions of the ligand. A straightforward N reaction pathway accounting for the formation of 5 is outlined N Me in Scheme 3 and involves initial C–H activation of 4 to give a 85 Me + 2 IPr - Me–Me (COD)Pt Pt six-coordinate platinum(IV) complex which subsequently Me - COD Me undergoes reductive elimination of a molecule of methane to N N 30 give 5.
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