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L3C3P3: Tricarbontriphosphide Tricyclic Radicals and Cations

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L3C3P3: Tricarbontriphosphide Tricyclic Radicals and Cations Author Manuscript Title: L3C3P3: Tricarbontriphosphide Cage Radicals and Cations Stabilized by Cy- clic (alkyl)(amino)carbenes Authors: Hansjorg¨ Grutzmacher,¨ Prof.; Zhongshu Li; Yuanfeng Hou; Yaqi Li; Alexan- der Hinz; Jeffrey Harmer; Chen-Yong Su; Guy Bertrand This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofrea- ding process, which may lead to differences between this version and the Version of Record. To be cited as: 10.1002/ange.201710099 Link to VoR: https://doi.org/10.1002/ange.201710099 L3C3P3: Tricarbontriphosphide Cage Radicals and Cations Stabilized by Cyclic (alkyl)(amino)carbenes Zhongshu Li,[a] Yuanfeng Hou,[a] Yaqi Li,[a] Alexander Hinz,[b] Jeffrey R. Harmer,*[c] Cheng-Yong Su,[a] Guy Bertrand,[e] and Hansjörg Grützmacher*[a,d] [a] Dr. Z. Li, Y. Hou, Y. Li, Prof. Dr. C.-Y. Su, Lehn Institute of Functional Materials (LIFM) School of Chemistry Sun Yat-Sen University 510275 Guangzhou, China E-mail: [email protected] [b] Dr. A. Hinz University of Oxford, Chemistry Research Laboratory 12 Mansfield Road, OX1 3TA, Oxford, UK E-mail: [email protected] [c] Assoc. Prof. Dr. J. R. Harmer. Centre for Advanced Imaging, University of Queensland, Brisbane, QLD, 4072, Australia E-mail: [email protected] [d] Prof. Dr. H. Grützmacher Department of Chemistry and Applied Biosciences ETH Zurich 8093 Zurich, Switzerland E-mail: [email protected] [e] Prof. Dr. G. Bertrand UCSD/CNRS Joint Research Chemistry Laboratory Department of Chemistry University of California San Diego La Jolla, CA 92521–0403, USA E-mail: [email protected] 1 This article is protected by copyright. All rights reserved Dedicated to Prof. Dr. Dieter Fenske on the oaccasion of his 75 th birthday. Abstract: Alkynes usually oligomerize to give rings with a conjugated -electron system. In contrast, phosphaalkynes, R-CP, frequently give compounds with polycyclic structures which are thermodynamically more stable than the corresponding -conjugated isomers. Here we report the syntheses of the first C3P3 cages with either radical or cation ground states stabilized by cyclic (alkyl)(amino)carbenes (CAACs). These compounds may be considered as examples of tricarbontriphosphide coordinated by carbenes and are likely formed via trimerization of the corresponding mono-radicals CAAC-CP• . The mechanism for the formation of these tricarbontriphosphide cage radicals has been rationalized by a combination of experiments and DFT calculations. Organo-phosphorus compounds with CnPn cage skeletons that are exclusively composed of carbon and phosphorus atoms are still relatively rare and their properties remain to be explored. To the best of our knowledge, all reported CnPn cages are prepared by oligomerization of phosphaalkynes, mainly tBu-CP.[1] For instance, phosphaalkyne tetramers A and B (Scheme 1) could be prepared via thermally induced or metal mediated oligomerization.[2,3] Other cage types of tetramers, pentamers, and even hexamers are also obtained from the corresponding [1] phosphaalkynes. Remarkably, stable cage skeletons of the composition C3P3 seemingly have never been isolated while 1,3,5-triphosphabenzenes C and 1,3,5-triphospha-Dewar-benzenes D are well established and their conversion into cage compounds with additional reagents is documented.[1,3b] This agrees well with calculations on (H,C,P)3 isomers, which predict that the cyclic triphosphabenzenes are more than 30 kcal mol–1 lower in energy than prismane cages.[4] 2 This article is protected by copyright. All rights reserved Scheme 1. Possible structures for tetra- or trimers of RCP (R = H, organyl groups). Singlet N-heterocyclic carbenes (NHCs)[5] and cyclic diamidocarbenes (DACs)[6] may stabilize otherwise reactive molecules and recently cyclic and acyclic C2P2 skeletons as possible forms of “dicarbondiphosphide” could be isolated with these as substituents.[7] Here we report the synthesis of neutral tricarbontriphosphide radicals + (L3C3P3) and their oxidation to the corresponding cations (L3C3P3) (L = CAAC). Phosphaketenes like 1 are easily prepared from chlorodiazaphospholenes and Na(OCP)[7] and are quantitatively rearranged to phosphallenes L=C=P[PO(NDipp)2(CH)2] upon reaction with diisopropylphenyl (Dipp) substituted [7a,8] NHC or DAC as carbene L. These phosphaallenes cleanly react with KC8 to give the anionic oxydiazaphospholene 3 and cyclic or acyclic dicarbondiphosphide [8] compounds L2C2P2 (L = NHC, DAC). Carbene-bound CP radicals L=C=P 4 may be formed as first intermediates. We reasoned that with sterically less demanding carbenes higher oligomers of these radicals may be obtained. Cyclic (alkyl)(amino)carbenes, CAACs, can be easily prepared on a multi-gram scale and their steric demand can be facilely tuned.[9] Phosphaketene 1[8] was reacted with CAACs L1 – L5 (Scheme 2). All reactions yield phosphaallenes 2 as bright pink powders in good to excellent yields which show two doublets in the 31P NMR spectra 1 at δ = 99.1 ppm (averaged) and δ = 17.5 ppm (averaged) with a coupling constant JP– 13 P = 398.3 Hz (averaged). In the C NMR spectra, doublet resonances in the range 1 from δ = 273.1 ppm (2a) to δ = 282.8 ppm (2e) with JP–C coupling constants of about 20 Hz are observed for the carbon nucleus bonded directly to CAACs which is in between that of structurally similar phosphaallenes stabilized with NHC (289.5 ppm)[7a] or DAC (249.8 ppm).[8] This ordering of the resonance frequencies from high 3 This article is protected by copyright. All rights reserved to low values reflects the π-electron acceptor strength of carbenes which increases in the order NHC < CAAC < DAC.[9] The structures of 2b and 2e were determined by single crystal X-ray diffraction analysis. As an example, the structure of 2b is shown in Figure 1a (see the Supporting Information for further details on all the reported structures herein). These molecules contain a slightly bent C2-C1-P1 unit (average angle at C1 is 172.0) with contracted C=C (C1–C2 1.326 Å, averaged) and C=P (C1– P1 1.627 Å, averaged) double bonds as compared to typical double bonds (C=C 1.34 Å; C=P 1.69 Å).[10] Scheme 2. Synthesis of 2, 3, 5, and 6. Compounds 2a – c react cleanly with one equivalent of KC8 in tetrahydrofuran (THF) 4 This article is protected by copyright. All rights reserved to give compound 3. Further products were not detected by 31P NMR spectroscopy but the reaction solutions showed very strong EPR signals (vide infra). After work-up the neutral tricarbontriphosphide radicals (L3C3P3) 5a – c were isolated in excellent yields (> 82%) as extremely air-sensitive green powders. The isolation of these cage radicals is a very strong indication for the formation of intermediate L=C=P• radicals 4. The molecular structures of 5a – c were determined unambiguously by single-crystal X-ray diffraction methods. As an example the structure of 5b is shown in Figure 1b (all substituents on the CAAC ring were omitted for clarity). a b c d Fig 1. Plots of the molecular structure of 2b, 5b, 6e, and 7b. Ellipsoids are set to 50% probability; H atoms, solvent molecules, anion, and substituents on CAAC for 5b and 7b are omitted for clarity. Selected distances [Å] and angles [°]: a) 2b: P1-P2 2.2360(6), P1-C1 1.623(2), C1-C2 1.324(3), P2-P1-C1 99.79(8), P1-C1-C2 173.18(16); b) 5b: P1-P2 2.6846(6), P1-P3 2.2082(6), P1-C1 1.8494(18), C1-P2 5 This article is protected by copyright. All rights reserved 1.8577(18), P2-C3 1.8578(18), C3-P1 1.8412(18), P2-C5 1.8828(18), P3-C5 1.8040(18), C1-C2 1.357(2), C3-C4 1.352(2), C5-C6 1.386(2), C5-P3-P1 94.40(6), P3-P1-C3 94.15(6), P3-P1-C1 91.61(6); c) 6e: P1-P1' 2.2508(8), P1-C1 1.6336(17), C1-C2 1.324(2), P1'-P1-C1 99.30(7), P1-C1-C2 175.67(14); d) 7b: P1-P2 2.755, P1-P3 2.2252(7), P1-C1 1.939(2), C1-P2 1.941(2), P2-C3 1.837(2), C3-P1 1.910(2), P2-C5 1.856(2), P3-C1 1.808(2), P3-C5 1.821(2), C1-C2 1.443(3), C3-C4 1.365(3), C5-C6 1.371(3), C5-P3-P1 84.05(9), C5-P3-C1 84.05(9), P3-P1-C3 99.22(7), P3-P1-C1 54.09(6). The structural parameters of all three compounds 5a – c vary only marginally (see the Supp. Info for details). The center of the molecules comprises a P3C3 cage which is bonded to three CAAC substituents. Within the cage, all C–P bonds are in the range of single C–P bonds (P–C 1.86 Å) with the exception of the C5-P3 bond which is slightly shorter (1.83 Å, averaged).[10] Likewise, all C=C bonds between the CAAC [10] groups and the C3P2 cage are in the range of double bonds (C=C 1.34 Å) with the exception of the C5=C6 bonds which are slightly longer (1.37 Å, averaged). These data indicate significant -electron conjugation across the P3–C5–C6 unit (vide infra). The P3–P1 bonds (2.22 Å, averaged) correspond to standard P-P single bonds bond lengths. The structures of 5a – c can be viewed as [4+2] cycloadducts between a C2P2 • ring in L2C2P2 and a L=C=P radical where the addition took place across the P1, P2 vector. Note that also in reactions between L2C2P2 and H2, a cis-specific addition to give L2C2(PH)2 took place at the phosphorus centers which are therefore considered to be the reactive centers of carbene stabilized dicarbondiphosphides.[8] These additions cause the basal C2P2 heterocycles to deviate from planarity as seen in L2C2P2 and dihedral angles of 38.9° (averaged) are observed in 5a – 5c.
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    Synthetic and Spectroscopic Investigations Enabled by Modular Synthesis of Molecular Phosphaalkyne Precursors Wesley J. Transue,y Junyu Yang,y Matthew Nava,y Ivan V. Sergeyev,z Timothy J. Barnum,y Michael C. McCarthy,{ and Christopher C. Cummins∗,y yDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA zBruker BioSpin Corporation, Billerica, MA 01821, USA {Harvard{Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA Received October 27, 2018; E-mail: [email protected] PPh3 Cl Abstract: A series of dibenzo-7-phosphanorbornadiene com- P 2 Ph3P=CHR R P 20–80 °C RC P pounds, Ph3PC(R)PA (1-R; A = C14H10, anthracene; R = Me, – [Ph3PCH2R]Cl – PPh , A Et, iPr, sBu), are reported to be capable of thermal fragmen- 3 tation to generate alkyl-substituted phosphaalkynes (RC≡P) ClPA 1-R concomitant with triphenylphosphine and anthracene. Facile preparation of these molecular precursors proceeds by treatment Scheme 1. Synthesis and fragmentation of alkyl-substituted phos- phaalkyne molecular precursors, Ph3P=C(R)PA (1-R; R = H, Me, of ClPA with the appropriate ylide Ph P=CHR (2 equiv). For i s 3 Et, Pr, Bu; A = C14H10). methyl, ethyl, and isopropyl substituents, the phosphaalkyne conversions are measured to be 56{73% in solution by quan- 31 titative P NMR spectroscopy. In the case of compound 1- H, 1-R compounds have been found to spontaneously release Me, the kinetic profile of its spontaneous unimolecular frag- the RC≡P payload into solution at or below room temper- mentation is investigated by an Eyring analysis. The resulting ature. This series of phosphaalkyne precursors has allowed 1-phosphapropyne is directly detected by solution NMR spec- us to pursue further exploratory reactivity and spectroscopic troscopy and gas phase rotational microwave spectroscopy.
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