Reductive Ring Opening of a Cyclo-Tri(phosphonio)methanide

Article Cite This: Organometallics XXXX, XXX, XXX-XXX

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Reductive Ring Opening of a Cyclo-Tri(phosphonio)methanide Dication to a Phosphanylcarbodiphosphorane: In Situ UV-Vis Spectroelectrochemistry and Gold Coordination Sivathmeehan Yogendra,† Stephen Schulz,† Felix Hennersdorf,† Sarath Kumar,‡ Roland Fischer,§ and Jan J. Weigand*,† †

Department of Chemistry and Food Chemistry, TU Dresden, 01062 Dresden, Germany Department of Inorganic and Physical Chemistry, Indian Institute of Science, 560012 Bangalore, India § Department of Inorganic Chemistry, TU Graz, 8010 Graz, Austria ‡

S Supporting Information *

ABSTRACT: The formal two-electron reduction of the cyclotri(phosphonio)methanide dication 12+ results in a ring-opening reaction via C−P bond cleavage to yield the unique phosphanylfunctionalized carbodiphosphorane 2. In situ spectroelectrochemical investigations of the reduction of dication 12+ and the oxidation of 2 give insights into the mechanism of this unusual and reversible bond cleavage reaction. Compound 2 features in total three lone pairs of electrons, facilitating the preparation of mono-, di-, and trigold complexes.

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Scheme 1. (I) Stabilization of Ylides by the Introduction of α-Phosphonio Groups, (II) Preparation of the Carbodiphosphorane (Ph3P)2C, and (III) Reactivity of 12+ toward Fluoride Ions

hosphorus ylides are considered as reactive 1,2-dipolar compounds that are now indispensable in organic chemistry, especially in the Wittig olefination.1 The reactivity of phosphorus ylides strongly depends on the α substituents around the ylidic carbon atom. Neutral α-phosphonio ylides of type A (Scheme 1, I) feature a strongly nucleophilic carbon atom, rendering them inter alia as excellent ligands for metal complexation.2 The introduction of two phosphonio substituents as electron-withdrawing groups to give cations of type B stabilizes the ylidic carbanion by forming negative hyperconjugation from the lone pair of the C atom to the antibonding P−R orbitals of the phosphonio groups (lone pair → σ*(P−R)).2a,3 This facilitates the isolation of bisylidic carbodiphosphoranes bearing a formally dianionic C atom in the zero oxidation state (Scheme 1, II).4 The two lone pairs at the carbon atom of carbodiphosphoranes with σ and π symmetry enable the simultaneous coordination of two metal-based Lewis acids5 and allow the stabilization of unique electron-deficient main-groupelement Lewis acids.6 Successful isolation of functionalized bisylides was also accomplished by the introduction of imidazolio7 or sulfonio8 groups as electron-withdrawing substituents. The implementation of three phosphonio groups into a ylidic carbanion gives tri(phosphonio)methanide salts of type C, where the lone pair is strongly delocalized around the CP3 moiety.9 In this context, we have recently reported on the fluorophilic cyclo-tri(phosphonio)methanide dication 12+ (Scheme 1, III), which was prepared from a trifluoromethyl© XXXX American Chemical Society

Special Issue: Organometallic Chemistry in Europe Received: August 2, 2017

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DOI: 10.1021/acs.organomet.7b00597 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics sulfonyl-phosphonium dication10 via an intramolecular electrophilic aromatic substitution.11 Dication 12+ represents a waterresistant Lewis acid which is capable of binding fluoride ions selectively and reversibly from an organic/aqueous biphasic solution to give fluorophosphorane [1-F]+. Among other effects, the ring strength of the five-membered cycle was identified as a key factor for the high fluoride affinity of 12+. This exceptional electrophilicity prompted us to investigate the redox reactions of dication 12+. A preparative access to the formal two-electron-reduction reaction product of dication 12+ was found by using KC8 as a chemical reduction reagent. The reaction of 1[OTf]2 with 2 equivalents of KC8 results in a ring-opening reaction via a C−P bond cleavage to yield the unique phosphanyl-functionalized carbodiphosphorane 2 isolated as a highly air and moisture sensitive yellow solid in very good yield (81%, Scheme 2a).

In Scheme 2 a mechanistic rationalization of the formal twoelectron-reduction process with likely radical cationic intermediate [1]+• after the first electron transfer followed by the ring-opening reaction and a final electron transfer to radical cation [2]+• are depicted (ECE mechanism, where E denotes electron transfer and C a homogeneous follow-up reaction step). This reaction motivated a more detailed investigation of the redox reaction of dication 12+ to 2 by the use of cyclic voltammetry (CV), square wave voltammetry (SWV), and in situ UV−vis spectroelectrochemistry (UV−vis SEC) during CV measurements. Dication 12+ shows a reduction reaction that appears to be irreversible in the CV measurement (Figure 2) at a peak potential of EP = −2.18 V (vs E1/2(Cp2Fe/Cp2Fe+)) with a formal potential of E1/2 = −2.02 V from the SWV measurement. This nonreversible reaction can possibly be assigned to the reduction of dication 12+ to 2. In the second cycle of the CV curve an irreversible emerging reoxidation process appears at Ep = −0.35 V. CV and SWV assign this process to a reoxidation of the reduction product 2 formed at the electrode after a feasible follow-up reaction coupled to the electron transfers. A minor side reaction process (marked by asterisks) occurs according to the high moisture sensitivity of the reduction product (likely a reversible reduction process of protonated 2: [2-H]+/[2-H]•; Chapter 2.5 in the Supporting Information). In situ UV−vis SEC under ultradry conditions12 is used to obtain further information about the product of the reduction reaction as well as the reoxidation reaction and their assignment to dication 12+ and 2. An in situ UV−vis measurement in a double-compartment cuvette cell over four cycles of CV (Figure 1, left) shows the formation of a species with an absorbance maximum at 333 nm during the reduction process. The potential product 2 of the reduction reaction is again consumed during the reoxidation reaction, forming back the starting material dication 12+. As deduced from this overall chemically reversible process, reduction reactions of dication 12+ coupled to a potential follow-up reaction to the stable final

Scheme 2. Reaction of 1[OTf]2 with 2 equivalents of KC8 to give 2 and a Postulated Electrochemical Pathwaya

Legend: a) +2 KC8, −2 KOTf, THF, −60 °C to room temperature, 81%; b) E < −2.02 V; c) E > −0.43 V vs E1/2(Cp2Fe/Cp2Fe+), THF, 0.1 M [nBu4N][OTf], Pt-mesh electrode, 0.5 mm cuvette cell.

a

Figure 1. In situ UV−vis SEC measurement (v = 15 mV/s) of dication 12+ (0.6 mM) in THF/0.1 M [nBu4N][OTf] at a platinum-mesh electrode in a double-compartment cuvette cell (d = 0.5 mm): (left) 2D plot of differential UV−vis spectra during the CV measurement (four cycles); (upper right) cyclic voltammogram; (lower right) absorbance of 12+ (red line) and 2 (blue line) during the measurement of the CV. B


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Organometallics

process at Ep = −0.29 V (E1/2 = −0.43 V) that is comparable to the reoxidation of 2 in Figure 2 and a nonreversible rereduction process at Ep = −2.07 V in the second cycle in accordance with the start of the measurement from the redox state of reduction product 2. No further evidence for a persistent intermediate in the formal two-electron-reduction process has been found. In situ UV−vis SEC data during a CV measurement of carbodiphosphorane 2 under ultradry conditions are depicted in Figure 3. The UV−vis data show an overall chemical reversible switching between the starting material 2 and the oxidation product 12+. Dication 12+ (Figure 1) and neutral 2 (Figure 3) show exactly the same redox behavior, but in an inverted manner. The UV absorbance of dication 12+ (Figure 3, lower right, blue line) and neutral 2 (red line) show the expected correlation to the nonreversible peaks in the in situ CV (Figure 3, upper right). Since the overall two-electron reduction coupled to a homogeneous follow-up reaction is quite uncommon, the method of chronoamperometry with multiple potential steps under in situ UV−vis monitoring was used. Switching between the open circuit potential, the oxidation potential Eox = 0.30 V and the rereduction potential Erered = −2.56 V results in a current vs time profile and an absorbance vs time profile (Figure 4, left) from the in situ UV−vis (Figure 4, right) data. On the basis of the assumption of full conversion of the thinlayer compartment of the cuvette cell volume during the reduction reaction a theoretical charge for a two-electron process can be calculated12 (Figure 4, dotted line) which is consistent with the experimental data. From this measurement a repeated switching between 2 and 12+ consumes ≈ 2e− per molecule 12+ within the tolerances of the method. On the basis of the molar extinction coefficient of the chemically prepared carbodiphosphorane 2 at 333 nm absorbance and of dication 12+ at 243 nm a correlation to the number of electrons for both the reduction (Figure S2.4 in the Supporting Information: in situ UV−vis MPCA of 12+ vs UV−vis of 2) and reoxidation (Figure S2.5 in the Supporting Information: in situ UV−vis MPCA of 2 vs UV−vis of 12+) are in accordance with an overall

Figure 2. Cyclic voltammogram (v = 0.1 V/s) and square wave voltammogram of triflate salt 1[OTf]2 (1.67 mM) in THF/0.1 M [nBu4N][OTf] at a 3 mm platinum-disk electrode.

reduction product 2 causes the nonreversible peak shape of this process in the cyclic voltammogram (vide inf ra). The CV curve based on the UV−vis absorbance changes for the starting material 12+ (Figure 1, right, red line) as well as for the potential reduction product 2 (Figure 1, right, blue line) are in accordance with the in situ CV (Figure 1, upper graph). A cyclic switching between dication 12+ starting material and its stable reduction product 2 at the nonreversible CV peak positions is observed over four cycles. Only slight changes in the relative absorbance are observed after the measurement cycles because of the UV sensitivity of 12+.12 In situ UV−vis SEC under full conversion conditions (Figure S2.2 in the Supporting Information) and UV−vis multi pulse chronoamperometry (UV−vis MPCA, Figure S2.3 in the Supporting Information) further support this observation.12 The observed chemical reversibility of the reduction process of dication 12+ (Scheme 2b,c) to neutral 2 in the in situ SEC prompted us to take a more detailed look into the oxidation process of 2. A CV and SWV measurement of 2 in THF (Figure S2.1 in the Supporting Information) shows a nonreversible oxidation

Figure 3. In situ UV−vis SEC measurement (v = 15 mV/s) of carbodiphosphorane 2 (1.18 mM) in THF/0.1 M [nBu4N][OTf] at a platinum-mesh electrode in a double-compartment cuvette cell (d = 0.5 mm): (left) 2D plot of differential UV−vis spectra during the CV measurement (three cycles); (upper right) cyclic voltammogram; (lower right) absorbance of 2 (red) and 12+ (blue) during the CV measurement. C


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Organometallics

Figure 4. In situ UV−vis multi pulse chronoamperometry of carbodiphosphorane 2 (1.18 mM) in THF/0.1 M [nBu4N][OTf] at a platinum-mesh electrode in a double-compartment cuvette cell (d = 0.5 mm); (left, top to bottom) potential profile, chonoamperogram (I has been corrected by I0 to compensate for the additional diffusion of 2 form the bulk part of the cuvette cell to the electrode), chronocoulogram, chronoabsorptiometry; (right) 2D plot of in situ differential UV−vis spectra during the MPCA measurement.

= 16 Hz) in the 13C{1H} NMR spectrum and is shifted significantly downfield in comparison to the unsubstituted carbodiphosphorane (Ph3P)2C (δ = 12.5 ppm, 1JCP = 127 Hz).12 This effect is explained by a donor−acceptor interaction between one lone pair of electrons of the C atom and the σ*(P1−C13) orbital (vide inf ra). The molecular structure of compound 2 is depicted in Figure 5 and displays the expected bent geometry for the C1 atom

two-electron process. The UV−vis spectra are also in very good agreement with the in situ data, further confirming the assignment to carbodiphosphorane 2 and dication 12+ in the in situ measurements. During the fast potential change no indication of intermediates can be found in the UV absorbance. Combining the results from the preparative synthesis of 2 and the in situ UV−vis SEC data of 12+ and 2, the nonreversible character of the reduction and oxidation is most likely explained by the ring opening reaction (vide supra) as a necessary followup reaction step in the reduction of 12+ to 2. From in situ UV− vis CV and MPCA measurements no further hints of transient intermediates on the time scale of the experiments were found. Since the P−C bond cleavage and formation take place during the reduction of 12+ and oxidation of 2, a chemical step is necessarily involved in the mechanism. A description of the electrode mechanism as a likely ECE mechanism (Scheme 2, bottom) is favorable, when taking a very fast chemical follow-up reaction into account. Peak parameters from CV measurements (Figure S2.10 − S2.16 in the Supporting Information) at scan rates up to 750 V s−1 are in accordance with this hypothesis. An alternative EEC electrode reaction mechanism is to the best of our knowledge less favorable, because the P−C bond cleavage as well as the P−C bond formation only take place after an electron transfer. Further mechanistic investigations are necessary to prove the hypothetical ECE electrode reaction mechanism.12 However, the nonreversible appearance of the two-electron reduction and oxidation reaction as well as the over the whole electrode reaction reversible reaction between dication 12+ and neutral 2 coupled to a homogeneous reaction step are a rare example in the chemistry of reactive main-groupelement species. The 31P{1H} NMR spectrum of 2 displays an AMX spin system. Part A appears as a doublet at δ(PA) = −17.7 ppm (3JPP = 21 Hz), which is assigned to the phosphanyl moiety. The M part appears as a dd resonance at δ(PM) = −6.9 ppm for the −Ph2PV− moiety (2JPP = 93 Hz and 3JPP = 21 Hz) and the X part reveals a doublet at δ(PX) = −3.4 ppm for the Ph3P substituent. The distinct ddd resonance for the central C atom is observed at δ = 18.1 ppm (1JCP = 136 Hz, 1JCP = 134 Hz, 4JCP

Figure 5. Molecular structure of 2 (hydrogen atoms are omitted for clarity, and thermal ellipsoids are displayed at 50% probability).

with a slightly larger P2−C19−P3 angle of 140.74(8)° in comparison to the respective P−C−P angle in (Ph3P)2C (131.7(3)°).5b,13 The short P−C bonds in 2 (C19−P2 1.636(1) Å, C19−P3 1.642(2) Å) are in accordance with those observed in other carbodiphosphoranes and refer to a strong degree of negative hyperconjugation. The short P1···C19 contact (3.035(2) Å, ∑vdW(C, P) 3.5 Å)14 and the linear arrangement of the C19···P1−C13 fragment are noticeable (178.40(4)°), confirming a certain degree of donor−acceptor interaction. The angles P1−C1−C6 (120.80(9)°) and P2− C6−C1 (120.09(9)°) are widened in comparison to those in precursor 12+ (P2−C3−P2 114.9(2)°, P1−C2−C3 113.9(2)°), which leads to reduced angle tension and thus indicates that the ring opening might be a driving force for the formation of 2. D


Article

Organometallics Compound 2 represents a rare example of a carbodiphosphorane that features in total three donor functionalities. Consequently, 2 features a multidentate donor for metal coordination. The reaction of 2 with [AuCl(tht)] in a 1:1 ratio results in the formation of the corresponding gold complex 3, where the AuCl is coordinated to the carbodiphosphorane moiety (yield 96%, Scheme 3a). In sharp contrast to the air-

Scheme 4. Reaction of 3 with NH4BF4 To Give the Complex 4[BF4]a

Scheme 3. Reaction of 2 with 1−3 equivalents of [AuCl(tht)] (tht = Tetrahydrothiophene) to give mono-, di-, and trigold Complexes 3, 5, and 6a

a

Legend: a) +NH4BF4, −NH3, THF, 12 h, room temperature, 55%.

donor for the second equivalent of AuCl rather than the carbon atom (yield 93%, Scheme 3b). The 31P{1H} NMR spectrum of 5 measured at 22 °C reveals three broad resonances at δ = 25.8 ppm (ν1/2 = 50 Hz), 21.5 ppm (ν1/2 = 130 Hz), and 10.8 ppm (ν1/2 = 95 Hz), indicating the presence of a dynamic process (Figure 6, top). a Legend: a) +[AuCl(tht)], −tht, THF, room temperature, 96%; b) + 2[AuCl(tht)], −2 tht, THF, room temperature, 93%; c) +3[AuCl(tht)], −3tht, THF, room temperature, 98%.

stable congener [AuCl((Ph3P)2C)],15 complex 3 is obtained as a highly reactive and air sensitive solid, which readily decomposes in C−H acidic solvents such as CH2Cl2, CH3CN, DMSO, and DMF and is insoluble in THF, oC6H4F2, and C6H5F. However, it is poorly soluble in o-C6H4Cl2 (∼4 mg/mL) but decomposes slowly, as indicated by 31P NMR spectroscopic investigations showing decomposition of up to 7% after 12 h (Figure S3.1 in the Supporting Information).12 The 31P{1H} NMR spectrum of 3 was recorded in o-C6D4Cl2 at −10 °C in order to decrease the decomposition rate and reveals an AMX spin system. The A part resonates as a doublet at δ(PA) = −18.1 ppm consistent with the phosphanyl moiety. The M and the X parts are assigned to the Ph3PV− and −Ph2PV− substituents, respectively, and are observed at lower field due to the metal coordination with resonances at δ(PM) = 8.6 ppm and δ(PX) = 18.7 ppm (2JPP = 52 Hz and 3JPP = 23 Hz), respectively. Complex 3 can be protonated with NH4BF4 to give quantitatively complex 4[BF4] (isolated yield 55%, Scheme 4). This reaction likely proceeds via protonation of the carbon atom to give intermediate I. A related derivative represents the known salt [(Ph3P)2CH(AuCl)][OTf].5c However, hypothetical intermediate I is not stable and subsequently rearranges to the air-stable complex 4[BF4]. This possible AuCl rearrangement of complex 3 also explains the unusual high reactivity toward C−H acidic solvents (vide supra). The 31P{1H} NMR spectrum of 4[BF4] displays the expected AMX spin system where the A part is assigned to the Ph2PIII(AuCl) moiety with a resonance observed at lower field at δ(PA) = 19.6 ppm in comparison to 3. The M and X parts are assigned to the −Ph2PV− and Ph3PV− moieties, respectively, and also display downfield-shifted resonances at δ(PM) = 24.8 ppm and δ(PX) = 28.3 ppm (2JPP = 9.1 Hz, 3JPP = 20.5 Hz). The reaction of 2 with [AuCl(tht)] in a 1:2 ratio forms the digold complex 5, in which the phosphanyl moiety acts as the

Figure 6. (top) Variable-temperature 31P{1H} NMR spectra of 5 measured in CD2Cl2 at 22, −28, and −68 °C. A1M1X1 and A2M2X2 spin systems are assigned to two different rotamers of 5. (bottom) 31 1 P{ H} 31P{1H} EXSY NMR spectrum of 5 (−48 °C, mixing time 0.2 s, CD2Cl2; unidentified side product is marked with an asterisk).

Upon cooling to −68 °C the 31P{1H} NMR spectrum displays two comparable AMX spin systems, designated as A1M1X1 and A2M2X2. The A1M1X1 spin system resonates at δ(A1) = 9.5 ppm for Ph2PIII(AuCl)−, δ(M1) = 20.2 ppm for −Ph2PV−, and δ(X1) = 25.6 ppm for Ph3PV moiety (2JPP = 47 Hz and 3JPP = 8 Hz). E


Article

Organometallics The A2M2X2 spin system reveals the respective resonances at δ(A2) = 12.5 ppm, δ(M2) = 24.1 ppm, and δ(X2) = 25.2 ppm (2JPP = 48 Hz and 3JPP = 8 Hz). The 31P{1H}31P{1H} EXSY NMR spectrum proves a dynamic exchange process indicative of different rotamers of 5 (Figure 6, bottom).12 From the variable-temperature NMR spectra recorded between −30 and −80 °C, an increasing intensity is observed for the A1M1X1 spin system when reaching lower temperatures, as indicated by the different A1:A2 ratio of 1.5:1 at −30 °C compared to that of 2.5:1 at −80 °C. The reaction of 2 with [AuCl(tht)] in a 1:3 ratio results in the formation of the corresponding trigold complex 6, in which two AuCl moieties are coordinated to the carbon atom and one AuCl fragment to the phosphanyl group (yield 98%, Scheme 3c).

Table 1. Selected Bond Lengths (in Å) and Angles (in deg) of Crystallographically Characterized Complexes 2, 3, 4+, 5, and 6 2 3 4+ 5 6

P2−C19

P3−C19

P2−C19−P3

1.636(1) 1.696(2) 1.710(7) 1.689(3) 1.763(5)

1.642(2) 1.701(4) 1.706(6) 1.690(3) 1.774(5)

140.74(8) 126.0(2) 129.8(4) 131.4(2) 123.6(3)

indicating sp2 hybridization. As expected, the P2/P3−C19 bond lengths increase upon coordination or protonation at the C19 atom, as observed in complexes 3, 4+, and 5. This is explained by the reduced negative hyperconjugation of the π-type lone pair of electrons at the C19 atom into the σ*(P−R) orbitals in comparison to the carbodiphosphorane 2. Interestingly, the second gold coordination in 5 leads to a slight shortening of the P2/3−C19 bonds in comparison to those observed in complex 3. Complex 6 displays a distorted-tetrahedral bonding environment around the C19 atom and reveals the longest P2/3−C19 bonds due to the double auration of the carbon atom, disabling the aforementioned negative hyperconjugation. The Au−C19 bond lengths in 6 (Au3−C19 2.089 Å, Au2−C19 2.064 Å) are slightly longer than those observed in complexes 3 (Au1−C19 2.043 Å) and 5 (Au2−C19 2.037(3) Å) but are in the expected range of doubly gold coordinated carbodiphosphoranes.5b,c,16 In conclusion, the formal two-electron reduction of the highly electrophilic cyclo-tri(phosphonio)methanide dication 12+ results in the formation of the phosphanyl-functionalized carbodiphosphorane 2. Detailed investigations of the redox reaction by in situ UV−vis spectroelectrochemistry reveal the overall two-electron nature of this process with an reversible P−C bond cleavage as a follow-up reaction. The phosphanyl carbodiphosphorane 2 represents a rare example of such compounds with three donor functions. These donor functions were utilized to synthesize gold(I) complexes with one (3), two (5), and three (6) coordinated AuCl fragments.

Figure 7. Molecular structure of 4+ (selected hydrogen atoms are omitted for clarity, and thermal ellipsoids are displayed at 50% probability).

Similar to the case for the digold complex 5, the 31P{1H} NMR spectrum of 6 reveals broad resonances at 27 °C which show at −50 °C one distinct AMX spin system (Figure S3.2 in the Supporting Information).12 The A part resonates as a doublet at δ(PA) = 14.1 ppm and is assigned to the Ph2P(AuCl) moiety. The M and X parts are assigned to the Ph3PV− and −Ph2PV− moieties, respectively, and display resonances at δ(PM) = 25.4 ppm and δ(PX) = 26.9 ppm, which are shifted slightly downfield due to the second gold coordination at the C atom in comparison to the complexes 3 and 5. The molecular structures of complexes 3, 4+, 5, and 6 are depicted in Figures 7 and 8, and selected geometrical parameters are given in Table 1. Complexes 3, 4+, and 5 reveal a trigonal-planar bonding environment around the C19 atom

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00597. Experimental procedures, characterization data, spectroscopic data, crystallographic data, and additional CV and in situ UV−vis spectroelectrochemical data (PDF)

Figure 8. Molecular structures of 3, 5, and 6 (hydrogen atoms are omitted for clarity, and thermal ellipsoids are displayed at 50% probability). F


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Organometallics Accession Codes

(7) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 3206−3209. (8) Dellus, N.; Kato, T.; Bagán, X.; Saffon-Merceron, N.; Branchadell, V.; Baceiredo, A. Angew. Chem., Int. Ed. 2010, 49, 6798−6801. (9) (a) Schmidbaur, H.; Strunk, S.; Zybill, C. E. Chem. Ber. 1983, 116, 3559−3566. (b) Karsch, H. H.; Zimmer-Gasser, B.; Neugebauer, D.; Schubert, U. Angew. Chem., Int. Ed. Engl. 1979, 18, 484−485. (10) Yogendra, S.; Hennersdorf, F.; Bauza, A.; Frontera, A.; Fischer, R.; Weigand, J. J. Chem. Commun. 2017, 53, 2954−2957. (11) Yogendra, S.; Hennersdorf, F.; Bauzá, A.; Frontera, A.; Fischer, R.; Weigand, J. J. Angew. Chem., Int. Ed. 2017, 56, 7907−7911. (12) For further information, see the Supporting Information (13) Hardy, G. E.; Zink, J. I.; Kaska, W. C.; Baldwin, J. C. J. Am. Chem. Soc. 1978, 100, 8001−8002. (14) Bondi, A. J. Phys. Chem. 1964, 68, 441−451. (15) Schmidbaur, H.; Zybill, C. E.; Müller, G.; Krüger, C. Angew. Chem., Int. Ed. Engl. 1983, 22, 729−730. (16) (a) Alcarazo, M.; Lehmann, C. W.; Anoop, A.; Thiel, W.; Fürstner, A. Nat. Chem. 2009, 1, 295−301. (b) Morosaki, T.; Wang, W.-W.; Nagase, S.; Fujii, T. Chem. - Eur. J. 2015, 21, 15405−15411.

CCDC 1560752−1560756 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.J.W.: [email protected]. ORCID

Felix Hennersdorf: 0000-0002-3729-030X Jan J. Weigand: 0000-0001-7323-7816 Author Contributions

S.Y. performed the synthesis and characterization, and S.S. performed the electrochemical and spectroelectrochemical measurements and the data analyses. Both authors contributed equally and share first authorship. F.H. and R.F. performed single crystal X-ray diffraction and data analysis, and S.K. assisted in the synthesis and characterization. J.J.W. supervised this project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the European Research Council (ERC, SynPhos 307616) and Fonds der Chemischen Industrie (FCI, Scholarship for F.H.). J.J.W. thanks the DFG for funding a diffractometer (INST 269/618-1)

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Reductive Ring Opening of a Cyclo-Tri(phosphonio)methanide

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