The bis(ferrocenyl)phosphenium ion, [Fc2P]+, reported by Cowley et al. (J. Am. Chem. Soc. 1981, 103, 714–715), was the only claimed donor‐free divalent phosphenium ion. Our examination of the molecular and electronic structure reveals that [Fc2P]+ possesses significant intramolecular Fe⋅⋅⋅P contacts, which are predominantly electrostatic and moderate the Lewis acidity. Nonetheless, [Fc2P]+ undergoes complex formation with the Lewis bases PPh3 and IPr to give the donor–acceptor complexes [Fc2P(PPh3)]+ and [Fc2P(IPr)]+ (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene)

Beckmann, Jens

Malaspina, Lorraine A.

Mebs, Stefan

Mischin, Alexandra

Olaru, Marian

English

Institutional Repository of the Freie Universität Berlin

German Edition: DOI: 10.1002/ange.201913081Carbene AnaloguesInternational Edition: DOI: 10.1002/anie.201913081The Bis(ferrocenyl)phosphenium Ion RevisitedMarian Olaru, Alexandra Mischin, Lorraine A. Malaspina, Stefan Mebs,* and Jens Beckmann*Dedicated to Professor Klaus Roth on the occasion of his 75th birthdayAbstract: The bis(ferrocenyl)phosphenium ion, [Fc2P]+,reported by Cowley et al. (J. Am. Chem. Soc. 1981, 103,714–715), was the only claimed donor-free divalent phosphe-nium ion. Our examination of the molecular and electronicstructure reveals that [Fc2P]+ possesses significant intramolec-ular Fe···P contacts, which are predominantly electrostatic andmoderate the Lewis acidity. Nonetheless, [Fc2P]+ undergoescomplex formation with the Lewis bases PPh3 and IPr to givethe donor–acceptor complexes [Fc2P(PPh3)]+ and [Fc2P-(IPr)]+ (IPr= 1,3-bis(2,6-diisopropylphenyl)imidazole-2-yli-dene).Divalent phosphenium ions, [R2P]+, are highly reactive, six-valence-electron species that contain an electron lone pairand a vacant p orbital.[1] Due to the positive charge they aresignificantly more Lewis acidic than the isoelectronic sily-lenes, R2Si,[2] a property also shared with the related silylcations, [R3Si]+.[3] Unlike those, only two substituents areavailable to shield the phosphorus atom and to prevent thecounter anion from coordination. Consequently, the vastmajority of phosphenium ions reported in the literature areelectronically stabilized by substituents or ligands with donoratoms that compensate the electron deficiency, which dra-matically reduces the Lewis acidity and reactivity.[1] Althoughthese species are unarguably cationic and many even divalent,they formally possess more than six valence electrons.[1] Theseelectron-rich phosphenium ions also include the bis(supracy-clopentadienyl)phosphenium ion Cp*2P+.[4] The only notableexception seems to be the bis(ferrocenyl)phosphenium ion[Fc2P]+, reported by Cowley et al. in 1981, which allegedlylacks any donor atoms.[5] On the basis of 57Fe Mçssbauer and31P NMR spectroscopy it was concluded that the positivecharge was formally situated at phosphorus and that the ironatoms are in the oxidation state II. The claim was furthersupported by the ability of [Fc2P]+ to react as a Lewis acidtowards the Lewis base n-Bu3P, giving rise to the donor–acceptor complex [Fc2P(Pn-Bu3)]+.[6] Unfortunately, neither[Fc2P]+ nor [Fc2P(Pn-Bu3)]+ have been fully characterized.[5,6]Our interest in kinetically stabilized phosphenium ions andtheir heavier Group 15 analogues[7] prompted us to inves-tigate the molecular and electronic structure of [Fc2P]+ andtwo related donor–acceptor complexes.The reaction of FcLi[8]with i-Pr2NPCl2 and the subsequenttreatment with water-free HCl provided Fc2PCl (1) as yellowcrystals in 63% yield (Scheme 1).[9] Chloride abstraction fromFc2PCl (1) was achieved using NaBArF4 (ArF= 3,5-(F3C)2C6H3), which produced [Fc2P][BArF4] (2) as darkbrown (almost black) crystals in 87% yield (Scheme 1).[10]Scheme 1. Synthesis and 31P NMR (CD2Cl2) chemical shifts of 1–4.[*] Dr. M. Olaru, A. Mischin, L. A. Malaspina, Prof. Dr. J. BeckmannInstitut ffr Anorganische Chemie und KristallographieUniversit-t BremenLeobener Straße 7, 28359 Bremen (Germany)E-mail: j.beckmann@uni-bremen.deDr. S. MebsInstitut ffr ExperimentalphysikFreie Universit-t BerlinArnimallee 14, 14195 Berlin (Germany)E-mail: stebs@chemie.fu-berlin.deSupporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201913081.T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co.KGaA. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properlycited.AngewandteChemieCommunications1581Angew. Chem. Int. Ed. 2020, 59, 1581 –1584 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimThe reaction of 2 with PPh3 and 1,3-bis(2,6-diisopropylphe-nyl)imidazole-2-ylidene (IPr) gave rise to the donor–acceptorcomplexes [Fc2P(PPh3)][BArF4] (3) and [Fc2P(IPr)][BArF4](4), respectively, which were isolated as orange crystals in86% and 83% yield.Unlike Fc2PCl (1), the borate salts [Fc2P][BArF4] (2),[Fc2P(PPh3)][BArF4] (3), and [Fc2P(IPr)][BArF4] (4) showclear ion separation in the crystal lattice (Figure 1).[11] Themost striking features of 2 are the intramolecular Fe···Pcontacts and the associated distortion of the molecularstructure of the [Fc2P]+ cation, which are absent in 1, 3, and4. The molecular structure of [Fc2P]+ is strongly asymmetricand displays two distinctively different P-C bonds.The P1-C10 bond length in 2 (1.801(5) c) is comparableto the related P-C bond lengths of 1 (1.799(2) and 1.805(2)c),3 (1.805(1) and 1.810(1) c), and 4 (1.818(2) and 1.802(2)c)while the P1-C31 bond (1.714(6)c) is significantly shorter, byalmost 0.1c. The latter value is closer to a P-C double bondthan a P-C single bond, which to some degree allows theinterpretation of 2 as being a phosphafulvenyl cation ratherthan a phosphenium cation. This view is further supported bythe rather large 1J(31P–13C) coupling of 57 Hz.[12] The differ-ence in the P-C bond lengths seems to be reversely correlatedwith the Fe–P distances; that is, Fe1-P1 (2.613(1)c) isconsiderably shorter than Fe2-P1 (3.062(1)c). The twodifferent dip angles, a1 (36.58) and a2 (21.88), defined as theangles made by the P-C bond vectors with the Cp plane,reflect the same trend (Figure 1). The dip angles in 2,although smaller than that observed for [Fc(t-Bu)MeSi]+(44.88) remain noticeably larger than those observed forsome Lewis acidic ferrocenylboranes such as FcBBr2 (18.98,17.78) and FcB(C6F5)2 (168), or the carbenium ion [FcCPh2]+(20.78).[13] The C10-P1-C31 angle in 2 (107.8(3)8) is wider thanthe related C-P-C angles in 1 (99.72(5)8), 3 (103.50(6)8), and 4(104.04(9)8). In 2, the tilt angles between the Cp rings in Fc1(containing Fe1, 13.38) and Fc2 (containing Fe2, 12.38) aresimilar to that observed for [Fc(t-Bu)MeSi]+ (11.68) andrepresent a significant deviation from the expected situationin 1, where the Cp rings of the two Fc units are essentiallyparallel. For 3 and 4 only one of the Fc units showsa significant tilt angle, albeit of lesser value (5.28 in 3; 4.78in 4). In the donor–acceptor complexes 3 and 4, the P1-P2(2.2335(5) c) and P1-C1 (1.865(2)c) bond distances com-pare well with those found in the related compounds [Ph2P-(PPh3)][GaCl4][14] (2.220(6)c) and [Ph2P(SIMes)][B(C6F5)4](1.861(4)c, SIMes= 1,3-dimesitylimidazolidin-2-ylidene).[15]No other major structural features were observed around the[Fc2P]+ fragments in these complexes, which indicates that theligands effectively compensate most of the positive charge onP1. This was also evident by NMR spectroscopy. Theformation of 2 was confirmed by the observation at a highchemical shift of its 31P NMR resonance signal at 184.3 ppm[10](CD2Cl2), shifted by more than 100 ppm compared to1 (CD2Cl2, 82.4 ppm). In solution, the ferrocenyl groups of 2are magnetically equivalent. Upon coordination of the Lewisbase, the 31P{1H} resonance signal assigned to the [Fc2P]+fragment shifted to @0.5 ppm in the case of 3 (d, 1J(31P–31P)=369 Hz) and @21.9 ppm in the case of 4.In the gas phase the optimized molecular structure of 2 isnearly symmetric, with the two P-C bonds of 1.75c and twointramolecular P-Fe distances of 2.78c. For a more detailedanalysis of the bonding situation a comprehensive analysiswas conducted including the real-space bonding indicators(RSBI) based upon the atoms in molecules (AIM),[16] non-covalent interaction (NCI) index,[17] and the electron local-izability indicator (ELI-D) methods[18] (see Figure 2 andTables S3–S5). The AIM bond topological analysis of theelectron density (ED) shows bond paths (and thus bondcritical points, bcp) for all primary P-C, C-C, and C-Hinteractions as well as for one secondary H···H contactbetween the two ferrocenyl groups (Figure 2a). However, itdoes not show all 20 conceivable Fe-C bond paths, which isa common feature in AIM and related to the flat ED gradientin the conical Fe–C5H5 area.[19] This may also explain whythere is no bcp formed between the P atom and the two Featoms. To get a more detailed view into that, the EDdistribution was mapped on the surface of the AIM atomicbasin of the P atom, which discloses strong ED accumulationsalong the two P-C axes but no apparent accumulation in theP···Fe area, suggesting this interaction to be noncovalent(Figure 2b). The NCI complements AIM in that it uncoversregions in space where (weak) noncovalent interactions occureven if no AIM bond paths were observed.[20] Accordingly,ring-shaped and red-colored NCI basins are obtained forevery Fe–C5H5 contact, suggesting dominant covalent metalto cyclopentadienyl interactions, as well as small green areascorresponding to the weak H···H contacts (Figure 2c).Figure 1. Molecular structures of Fc2PCl (1) and the cations [Fc2P]+ (2),[Fc2P(PPh3)]+ (3), and [Fc2P(IPr)]+ (4) showing 30% probability ellip-soids. The hydrogen atoms and counter ions are omitted for clarity.Selected bond lengths [b] and angles [8]: for 1: P1-C10 1.799(2), P1-C30 1.805(2), P1-Cl1 2.0984(7), Cl1-P1-C10 99.685, Cl1-P1-C3099.72(5), C10-P1-C30 99.92(7); for 2 : Fe1-P1 2.613(1), Fe2-P1 3.062(1),P1-C10 1.801(5), P1-C31 1.714(6), C10-P1-C31 107.8(3), a1 36.5, a221.8; for 3 : P1-P2 2.2335(5), P1-C10 1.805(1), P1-C30 1.810(1), P2-P1-C10 106.09(4), P2-P1-C30 95.86(4), C10-P1-C30 103.50(6); for 4 : P1-C11.865(2), P1-C10 1.818(2), P1-C30 1.802(2), C1-P1-C10 96.08(9), C1-P1-C30 103.50(9), C10-P1-C30 104.04(9).AngewandteChemieCommunications1582 www.angewandte.org T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1581 –1584Notably, localized and blue-colored NCI areas are obtainedalong the P-Fe axis, indicating an (almost) purely noncovalentatom–atom contact. This is supported by the ELI-D, whichdoes not show any P-Fe bonding basins (Figure 2d). Toinvestigate whether regions of increased electron localizabil-ity are formed between the P and the Fe atoms, the ELI-Ddistribution was mapped on the lone-pair basins of the P atom(Figure 2e) as well as the adjacent P-C bonding basin(Figure 2 f). Although the lone-pair basin of the P atomshows a small excrescence in direction of the Fe atom, noindications for increased electron localizability are present forboth basin types, supporting the weak noncovalent nature ofthe P···Fe contact. A quantitative measure is given by theRaub–Jansen Index (RJI),[21] which overlaps ELI-D basinswith AIM basins and proves that only 0.05e of the P atomsELI-D lone-pair basin are located within each AIM atomicbasin of the two Fe atoms.Complementary to the ED based methods, molecularorbital (MO) and natural bond orbital (NBO) calculations of2 were carried out. The HOMOs contain Fe(dxy, dx2@y2),P(s,px,py), and C(s,p) contributions, which are responsible forthe Fe-C and P-C interactions. The LUMO is given by theempty pz orbital of the P atom, whereas higher LUMOs aregiven by antibonding Fe-C contributions (see the SupportingInformation). The Wiberg bond index (WBI) of the P-Cinteractions is 1.04, thus excluding hyperconjugation. The P-CNBOs are populated with 1.96e, 67% of which stems from theC atomsQ atomic orbitals. p-Type contributions are 66% forthe sp2-C atoms and 81% for the P atoms confirming WBI.The WBIs of the P···Fe contacts are as small as 0.152,emphasizing once again the low covalent bond character.[22]The elusive phosphenium ions [Me2P]+ and [Ph2P]+ werepredicted to be Lewis superacids in the gas phase.[23] Wefinally calculated the fluoride ion affinity (FIA) of 2(670 kJmol@1), which is substantially smaller than that of[Me2P]+ (935 kJmol@1), and only slightly smaller than that of[Ph2P]+ (789 kJmol@1). Yet the value is still larger than that ofSbF5 (480 kJmol@1), which classifies [Fc2P]+ (2) also asa Lewis super acid.[24]In summary, we investigated the molecular and electronicstructure of the phosphenium ion [Fc2P]+ (2) first reported byCowley et al. in 1981[5, 6] Despite their previous assumption, itpossesses significant intramolecular Fe···P contacts, whichdistort the ideal geometry and increase the coordinationnumber at the P atom. These Fe···P contacts are predom-inantly electrostatic and moderate the Lewis acidity. None-theless, [Fc2P]+ (2) is still a Lewis superacid and undergoescomplexation with typical Lewis bases, such as PPh3 and IPr,to give the donor–acceptor complexes [Fc2P(PPh3)]+ (3) and[Fc2P(IPr)]+ (4). We are currently studying the utility of 2 forthe activation of small molecules.AcknowledgementsThe Deutsche Forschungsgemeinschaft (DFG) is gratefullyacknowledged for financial support.Conflict of interestThe authors declare no conflict of interest.Keywords: bond analysis · carbene analogue ·divalent phosphorus · ferrocene · phosphenium ionsHow to cite: Angew. Chem. Int. Ed. 2020, 59, 1581–1584Angew. Chem. 2020, 132, 1597–1600Figure 2. Real-space bond indicator (RSBI) analysis of the cations [Fc2P]+ (2). a) AIM bond topological analysis, b) ED distribution mapped on thesurface of the AIM atomic basin of the P atom, c) NCI basins indicating noncovalent interactions, d) ELI-D distribution indicating regions ofelectron localizability, e) ELI-D distribution mapped on the lone-pair basin of the P atom, and f) the adjacent P-C bonding basin.AngewandteChemieCommunications1583Angew. Chem. Int. Ed. 2020, 59, 1581 –1584 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. 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The Bis(ferrocenyl)phosphenium Ion Revisited

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The Bis(ferrocenyl)phosphenium Ion Revisited

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