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

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

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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. It should be noted that while intramolecular activation of the benzylic C–H bond of the aromatic side chain in IMes 6 9 is known for both Ru and Rh, the example here represents the 90 Scheme 4. Proposed reaction pathway for the formation of 6

first case of such reactivity with a group 10 metal. with ball-and-stick drawing of (IPr)2Pt (6).

t 35 The bulkiest NHC member in our series, namely I Bu

(%Vbur=37.0), chooses yet another reactivity pathway to alleviate 95 the severe steric crowding which would arise from a compound t of formula cis-(NHC)2Pt(Me)2. Reaction of 2 equivalents of I Bu with (COD)Pt(Me)2 in diethyl ether leads to slow precipitation of Me + 2 IMes 40 (COD)Pt 4 a white powder that is isolated in good yield after workup. The - COD Me 1H NMR spectrum showed a 1:1 stoichiometry between the t 100 methyl and I Bu ligands, but several resonances indicated that the t complex formed was not cis-(I Bu)2Pt(Me)2. To elucidate the structure of the compound, crystals were grown from slow

45 diffusion of diethyl ether into a concentrated CH2Cl2 solution and C–H act. were subjected to an X-ray diffraction study.‡ The crystal t t 105 structure of cis-(nI Bu)(aI Bu)Pt(Me)2 (7, Scheme 5) shows coordination of one of the two ItBu moieties via the C4 position N t N H of the central imidazole ring. By repositioning one of the Bu side - Me–H t 50 Mes Pt chains through abnormal binding of I Bu, the overall crowding is 5 Mes N Me considerably reduced and the X-Ray structural data permit the N Me Mes 110 quantification of the steric pressure around the Pt center. This can

be gauged by measuring the %Vbur values for both normal (37.0) and abnormal (29.4) ItBu ligands present in 7. As the numbers

55 Scheme 3. Probable reaction pathway for the formation of 4 clearly indicate, an abnormal binding mode relieves significantly

and 5 with ball-and-stick drawings of cis-(IMes)2Pt(Me)2 (4), the steric bulk around the Pt center, rendering the sterics of this and of cyclometallated (IMes)(IMes’)Pt(Me) (5). 115 coordination mode more similar to the overall %Vbur values of less sterically demanding, normally bound complexes 1-3.

A closer inspection of the X-ray structure of 7 also indicates 60 data can be obtained free of charge from The Cambridge Crystallographic that the normally bound ItBu ligand approaches the metal Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

considerably and both C(7)–Pt and C(11)–Pt distances are 1 (a) N-Heterocyclic Carbenes in Synthesis (Ed.: S. P. Nolan), Wiley- within the range of weak agostic interactions with distances of VCH, Weinheim, 2006; (b) N-Heterocyclic Carbenes in Transition 5 3.278(8)Å and 3.193(8)Å (closest H–Pt distances; 2.45(7)Å 65 Metal Catalysis; (Ed.: F. Glorius), Topics in Organometallic and 2.61(7)Å).12 Both these X-ray data as well as the fact that Chemistry, Vol. 21, Springer, Berlin, 2007; (c) S. Diez-Gonzalez, N. ItBu is sterically more demanding than IMes and IPr support Marion, S. P. Nolan, Chem Rev., 2009, 109, 3612. 2 (a) Activation and Catalytic Reactions of Saturated Hydrocarbons in the mechanistic proposal for its generation (Scheme 5). the Presence of Metal Complexes (Ed.: A. E. Shilov, G. B. Shul’pin), Accordingly, the pathway to 7 would proceed via 70 Springer, Berlin, 2000; (b) Activation and Functionalization of C–H 10 intermolecular C–H activation of the alkenyl C–H bond of the Bonds (Ed. K. I. Goldberg, A. S. Goldman), Oxford University Press, t Oxford, 2004. imidazole fragment by a transient (I Bu)Pt(Me)2 complex to 3 For a recent review, see; M. Lersch, M. Tilset, Chem. Rev., 2005, give an unsaturated Pt(IV) species. Subsequent formal C–H 105, 2471. reductive elimination via hydrogen migration from the 75 4 R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh, H. Fujii, platinum center to the C2 position of the imidazole moiety Science, 1998, 280, 560. 15 would generate the desired complex 7. We believe complex 7 5 M. Muehlhofer, T. Strassner, W. A. Herrmann, Angew. Chem. Int. to be the first example of abnormal NHC formation on a Ed., 2002, 41, 1745. 6 (a) E. M. Prokopchuk, R. J. Puddephatt, Organometallics, 2003, 22, monometallic complex from a free monodentate carbene.13 80 563; (b) R. Lindner, C. Wagner, D. Steinborn, J. Am. Chem. Soc., tBu 2009, 131, 8861. Me + ItBu N Me (COD)Pt Pt 7 For calculation of the steric bulk (%Vbur), the web-based Me - COD 20 N Me computational tool (SambVca) developed by Cavallo and co-workers t Bu was used: https://www.molnac.unisa.it/OMtools/sambvca.php. See H 85 supporting information for details. 8 For complexation of corresponding monophosphines and C–H act. + N N tBu tBu thermochemical data, see: C. M. Haar, S. P. Nolan, W. J. Marshall, K. G. Moloy, A. Prock, W. P. Giering, Organometallics, 1999, 18,

25 474. tBu 90 9 (a) J. Huang, E. D. Stevens, S. P. Nolan, Organometallics, 2000, 19, N tBu N H H transfer Me 1194; (b) M. J. Chilvers, R. F. R Jazzar, M. F. Mahon, M. K. 7 tBu Pt Whittlesey, Adv. Synth. Catal., 2003, 345, 1111; (c) D. Giunta, M. N Me Hölscher, C. W. Lehmann, R. Mynott, C. Wirtz, W. Leitner, Adv. N Synth. Catal., 2003, 345, 1139; (d) K. Abdur-Rashid, T. Fedorkiw, A. tBu 95 J. Lough, R. H. Morris, Organometallics, 2004, 23, 86. 30 10 Activation of the methyl group of the ortho-isopropylphenyl fragment on a nacnac-type ligand is known for platinum, see; U. Scheme 5. Possible reaction pathway leading to the formation Fekl, K. I. Goldberg, J. Am. Chem. Soc., 2002, 124, 6804. t t of 7 with ball-and-stick drawing of cis-(nI Bu)(aI Bu)Pt(Me)2 11 For thermolytic C–C reductive elimination from strained (7). C(7)–Pt and C(11)–Pt distances; 3.278(8) and 3.193(8) Å. 100 platinacyclobutanes; (a) R. Di Cosimo, G. M. Whitesides, J. Am. Chem. Soc., 1982, 104, 3601. For thermolytic reductive C–C 35 elimination from diarylplatinum(II) compounds; (b) S. E. Himmel, G. In conclusion, the present results indicate that N-heterocyclic B. Young, Organometallics, 1988, 7, 2440; (c) R. K. Merwin, R. C. carbene ligands can be used to generate a remarkable range of Schnabel, J. D. Koola, D. M. Roddick, Organometallics, 1992, 11, high-yielding, unusual products when reacted with platinum- 105 2972. dimethyl precursors. All of the NHCs studied bind readily to 12 (a) A. Albinati, C. G. Anklin, F. Ganazzoli, H. Ruegg, P. S. Pregosin, 14 Inorg. Chem., 1987, 26, 503; (b) A. Albinati, C. Arz, P. S. Pregosin, 40 (COD)Pt(Me) . With the bulkier members of the NHCs 2 Inorg. Chem., 1987, 26, 508. tested, a series of unprecedented transformations have been 13 Examples of abnormal N-heterocyclic carbenes generated directly uncovered. Further surprises may be anticipated upon testing 110 from free NHCs are rare and either involve multidentate NHC the reactivity of complexes such as the ones described here chelates or metal clusters, see; (a) X. Hu, I. Castro-Rodriguez, K. and studies aimed at discovering and exploiting these Meyer, Organometallics, 2003, 22, 3016; (b) A. A. Danopoulos, N. Tsoureas, J. A. Wright, M. E. Light, Organometallics, 2004, 23, 166; 45 transformations are ongoing in our laboratories. (c) C. E. Ellul, M. F. Mahon, O. Saker, M. K. Whittlesey, Angew. 115 Chem. Int. Ed., 2007, 46, 6343; (d) M. R. Crittall, C. E. Ellul, M. F. Notes and references Mahon, O. Saker, M. K. Whittlesey, Dalton Trans., 2008, 4209. For a

a review on abnormal/remote NHCs, see; (e) O. Schuster, L. Yang, H. University of St Andrews, School of Chemistry, St Andrews, KY16 9ST, G. Raubenheimer, M. Albrecht, Chem. Rev., 2009, 109, 3445. UK, E-mail: [email protected], Homepage: http://chemistry.st- 14 Qualitatively, this fact can be appreciated through the simple and.ac.uk/staff/spn/group t b 120 observation that IMes, IPr and I Bu interact readily with the platinum 50 University of New Orleans, Department of Chemistry, 2000 Lakeshore t precursor, while similarly bulky P( Bu)3 does not bind to Dr, New Orleans, LA 70148, USA. (COD)Pt(Me)2 (see ref. 8a). Preliminary quantitative data via reaction c University of Zurich, Institute of Organic Chemistry, calorimetry support the view that NHCs bind better than PR3 to the Winterthurerstrasse 190, Zurich, 8057, Switzerland. Platinum-dimethyl moiety. † Electronic Supplementary Information (ESI) available: [Table S1 with 55 selected bond lengths and angles of complexes 1-7, experimental procedures]. See DOI: 10.1039/b000000x/ ‡ CCDC-741809 (1), CCDC-741808 (2), CCDC-741810 (3), CCDC- 741811 (4), CCDC-741812 (5), CCDC-741813 (6), and CCDC-741814 (7) contain the supplementary crystallographic data for this paper. These