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. 2024 Jul 18;63(30):13820–13824. doi: 10.1021/acs.inorgchem.4c01934

Rhodium(I) Complexes with a η1-Fluorenyl-P-phosphanylphosphorane Ligand

Javier Eusamio †,‡, Nil Saumell †, Anton Vidal-Ferran †,‡,§, Arnald Grabulosa †,‡,*
PMCID: PMC11289756  PMID: 39023280

Abstract

graphic file with name ic4c01934_0005.jpg

The first example of a P-phosphanylphosphorane, Flu=PCy2–PCy2 (L2; Flu = 9-fluorenyl), has been easily prepared by P-phosphination of lithiated 9-dicyclohexylphosphinofluorene (FluPCy2, L0) with chlorodicyclohexylphosphane. L2 constitutes a new type of P(III)–P(V) organophosphorus compound, a σ3λ3–σ4λ5 species that is stable under an inert atmosphere in the solid state. The reaction of L2 with [Rh(diene)2]BR4 causes metalation of the benzylic carbon (C9) of fluorene, giving κ2-C,P complexes in which fluorene is coordinated in the η1 form. A complex with the weakly coordinating BArF anion has been isolated and fully characterized, including its crystal structure obtained by X-ray diffraction.

Short abstract

Treatment of 9-dicyclohexylphosphinofluorene (FluPCy2, L0; Flu = 9-fluorenyl) with n-BuLi/PClCy2 has surprisingly yielded the P(III)−P(V) phosphane−phosphorane Flu=PCy2−PCy2 (L2), formed by P-phosphination, instead of the expected diphosphane, L1. Compound L2 reacts with [Rh(diene)2]BR4, coordinates through the P(III) atom, and causes the metalation of C9 of fluorene, giving κ2-C,P complexes in which fluorene is coordinated in the η1 form.

Complexes with the fluorenyl ligand are significant because of their excellent performance in olefin polymerization with early transition metals.1 This is due to the versatility of the bonding of fluorenyl, encompassing η5, η3 and η1 coordination modes, easily interchanged during catalysis, a process known as ring slippage. Despite this, η1-coordination is relatively uncommon, even for late transition metals. A way to enforce it is by using of fluorenes with a coordinating side arm that modulates the steric and electronic properties of the ligand, producing cyclometalated complexes. For rhodium, only a handful of η1-fluorenyl complexes of this type have been structurally characterized (Figure 1, top).

Figure 1.

Figure 1

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Rhodium complexes with a η1-fluorenyl (top, A–D), ligand L2, and derived complexes C1–C4 presented in this work (middle) and examples of reported species related to L2 (bottom, E–H).

Maron and Bourissou2 reported complex A, using a fluorenyl-phosphazene ligand coordinated in a κ2-C,N mode. Much later, Lavigne and César3 described B, with a N-heterocyclic carbene ligand coordinated in κ2-C,C fashion while Iwai and Sawamura4 studied triarylmethane–monophosphanes and reported C, bearing a fluorenyl-phosphane coordinated in a κ2-C,P fashion. Finally, Sadow5 described Rh complexes with bis(oxazolinyl)fluorenylphosphane, coordinated in a κ2-C,N fashion, such as D. In this contribution we describe rhodium(I) complexes C1–C4, with an unprecedented cyclometalated phosphanylphosphorane ligand L2, a P(III)-P(V) species (Figure 1, middle).

Compounds such as L2 are unknown but other P(III)-P(V) species have been described.6−8 Some species related to L2 are given in Figure 1 (bottom). Appel9,10 and simultaneously Karsch11 reported several P-phosphanylphosphoranes (Figure 1, compounds E and F). Slightly later, Karsh12−14 described tetraphosphorus compound G. Much more recently, Ponikiewski15−18 has reported phosphanylphosphaalkenes, like the fluorenyl-substituted compound,19H. In this contribution, we detail the unexpected20 synthesis of L2, its characterization and its coordination to rhodium(I).

Inagaki20 took advantage of the relative acidity of the methylene protons of fluorene to prepare 9,9-bis(di-R-phosphino)fluorenes (R = Cy, Ph). He described the synthesis of monophosphane L0 (Scheme 1), without characterization details. We obtained it as a white solid in 90% yield, featuring a singlet at δP = +13.4 ppm in the 31P{1H} NMR spectrum. L0 is sensitive to oxidation so it was boronated for storage (δP = +35.3 ppm), although in low yield.

Scheme 1. Attempted Synthesis of L1 and Obtention of L2.

Scheme 1

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Following our interest in methylene-bridged diphosphanes,21,22 we reasoned that fluorene should be an excellent platform for single-atom-bridged, electron-rich diphosphanes,23,24 and we attempted the synthesis20 of L1 from L0 (Scheme 1).

Unexpectedly, a different result was found when treating L0 with n-BuLi/PClCy2, following the reported procedure.20 The 31P{1H} NMR spectrum contained two doublets at δP = +23.50 and −2.36 ppm with a coupling constant of 308.9 Hz. It was thought that the bulkiness and rigidity of the molecule could render the two dicyclohexylphosphino moieties of L1 nonequivalent and coupled “through-space”25 but heating a sample up to 60 °C in benzene-d6 did not give any hints of coalescence either in 1H or 31P{1H} NMR spectra.

The very large JP–P suggested a compound with a direct P–P bond, ruling out diphosphane L1, although the exact mass was the expected one [m/z 559.3672, matching with M(L1) – H], indicating that the product was an isomer of L1. In addition, in the 13C{1H} NMR spectrum a quaternary non aromatic carbon appears as a doublet of doublets (δC = 55.4 ppm; JCP = 79.6, 3.5 Hz), which must correspond to the C9 carbon of fluorene. All these data are consistent with the formation of compound L2 instead of L1 (Scheme 1). L2 can be described as a P-phosphanylphosphorane, with a double P–C bond. In the coordination number (σ)–valency (λ) nomenclature, L2 is a σ3λ3–σ4λ5 species. Compound L2 was obtained in a 60% yield as a pearl white solid, which is stable in the solid state under an inert atmosphere. In contrast, when the solid was exposed to air for a few days, NMR spectroscopy showed that it decomposed giving L0 (δP = +13.4 ppm) and the known secondary phosphane oxide HP(O)Cy2 (δP = +49.5 ppm)26 together with other species. The decomposition is accelerated by light. The same reaction has been observed for related compounds and has been attributed to hydrolysis.9

The formation of L2 occurs by an unexpected P-phosphination instead of the reported C-phosphination of the carbanion derived from L0. The formation of an α-carbanion increases the electron density of the phosphorus atom. In the case of L0 this effect and the cyclohexyl groups makes the phosphorus so nucleophilic that C-phosphination is completely suppressed and L1 is not formed. This was observed by Appel9,10 (E and F in Figure 1) and very recently Rufanov27 reported the P-alkylation of 9-fluorenyldiphenylphosphane with alkyl halides, although the reaction with chlorodiphenylphosphane was in the carbon, as reported by Inagaki.20

The attention was then turned to the complexation of L2 to see whether the P–C double bond would remain upon coordination.28 Treatment of [Rh(nbd)2]BF4 with 1 equiv of L2 gave a red solid after workup (C1; Scheme 2).

Scheme 2. Complexation of Phosphanephosphorane L2 to Rhodium(I) Moieties.

Scheme 2

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The 31P{1H} NMR spectrum of C1 (Figure S1, top) presented two doublets of doublets, at δP = +88.7 (J = 163.6 and 21.5 Hz) and −19.7 ppm (J = 163.6 and 144.7 Hz). The strong shielding with respect to L2 is typical for the formation of four-membered phosphametallacycles and is due to the ring contribution (ΔR).29 We encountered it in complexes of methylene-bridge diphosphanes.21,22

Since the reactivity of [Rh(diene)2]X (X = weakly coordinating anion) can depend on the diene,30,31L2 was reacted with [Rh(cod)2]BF4. Compound C2 was obtained, but it was impurified with unidentified species. Finally, the weakly coordinating anion can be noninnocent,32 and given our experience with the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) anion,33−37 we reacted L2 with [Rh(cod)2]BArF. In this case, the reaction was cleaner, giving complex C3 in a pure form after recrystallization. This complex presents two doublets of doublets in the 31P{1H} NMR spectrum (Figure S1, bottom) at δP = +88.5 (J = 178.7 and 18.5 Hz) and −31.8 ppm (J = 178.6 and 136.9 Hz).

The 31P{1H} NMR spectra of C1–C3 clearly show that the P–P structure remains after coordination, with a 2JPP of 178.7 Hz for C3. The more shielded signal must correspond to the P(III) of the dicyclohexylphosphino moiety coordinated to rhodium, with a standard 1JPRh of 136.9 Hz for C3. However, the peak at lower fields has a much smaller JPRh of 18.5 Hz, which can not correspond to a species with a direct P–Rh bond. Another interesting feature of C3 are the chemical shifts of the cod unsaturated groups. In the 1H NMR spectrum, they appear as two broad singlets at δH = 4.64 and 2.47 ppm, integrating two protons each. The latter chemical shift is extremely low for alkene protons, even considering that cod is coordinated to rhodium. This suggests that one of the double bonds of cod is affected by the aromatic ring currents of the π electrons of the fluorenyl group, causing a strong shielding.38 In addition, the 2D 1H–13C{1H} NMR HSQC spectrum shows that while the sp2 carbon atoms of cod bound to the deshielded protons appear, in the 13C{1H} NMR spectrum, as the expected doublet (δC = 82.7 ppm; JCRh = 9.0 Hz), the carbons of the shielded protons resonate as a triplet (δC = 91.4 ppm; J = 10.4 Hz). This can only be explained by a coupling with rhodium and phosphorus with a similar constant, suggesting that one of the cod double bonds is trans to a phosphorus atom, but not the other.

All these data shows that activation of the P–C double bond of L2 has occurred so C1–C3 are cyclometalated rhodium(I) compounds, having a Rh–C bond with the bis(benzylic) carbon C9 of fluorene (Scheme 2).

It is interesting to note that in complexes C1–C3 ligand L2 acts as bidentate ligand in a κ2-C,P fashion. In other words, L2 can be formally viewed as a zwitterionic P-phosphanylphosphonium fluorenide ligand that uses the C–P double bond electron pair for coordination, as the right resonance (mesomeric) structure shows (Figure 1, middle). This octet rule compliant structure should be the predominant one.

To produce a complex with a more labile ligand, C3 was dissolved in fluorobenzene and pressurized with 4 bar of hydrogen, to substitute the cod ligand by fluorobenzene39 and give complex C4 (Scheme 2). After 1 h, 31P{1H} NMR spectroscopy indicated only 10% conversion.40 Several attempts were carried out at longer reaction times, but C3/C4 mixtures with many other peaks in the 31P{1H} NMR spectrum were invariably obtained and for long reaction times, rhodium black appeared.

Complexes C1–C4 turned out to be very stable and a concentrated fluorobenzene solution of the C3/C4 90:10 mixture spontaneously yielded a crop of beautiful, dark red cubic crystals that were analyzed by X-ray diffraction.41

The asymmetric unit contains one molecule of the complex and one BArF anion. The rhodium atom is coordinated to a mixture cyclooctadiene (85%) and fluorobenzene (15%). Figure 2 displays the two different metal cations that can be extracted from the crystal structure, and Table S1 gives the more informative geometric parameters.

Figure 2.

Figure 2

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Ellipsoid plot of the cations of C3 (top) and C4 (bottom) with ellipsoids drawn at the 50% probability level. Hydrogen atoms and BArF anions have been omitted for clarity.

The Rh1B–P1B(Cy2)–P2B(Cy2) fragment is crystallographically identical in the two cations, but they differ in the coordinated fluorenyl ligand and obviously in the ancillary ligand (cod for C3 or fluorobenzene for C4).

Both structures show that the rhodium(I) center is coordinated to the P1B phosphorus of the phosphane group of L2 and to fluorene, coordinated in a σ,η1 fashion, by carbon C33B or C33′, which corresponds to C9 in the standard numbering of fluorene. In the cod cation, the two double bonds of cod are η2,η2-coordinated to Rh, as expected, completing the expected square-planar geometry of the metal. In the fluorobenzene cation, the arene is coordinated in a η6 fashion, giving, formally, a pentacoordinate geometry around the rhodium center.

L2 acts as a bidentate, κ2-C,P-coordinated ligand, giving an essentially planar Rh1B–P1B–P2B–C33B/C33′ four-membered ring for both cations of Figure 2. There are, however, differences because in C3 (Figure 2, top) the distance Rh1B–C33B [2.212(4) Å] is considerably longer than the distance Rh1B–C33′ [2.012(15) Å] of C4 (Figure 2, bottom) and the opposite happens with the C33B/C33–P2B distances. As a consequence, ligand L2 has a smaller bite angle in C3 [79.84(11)°] than in C4 [85.6(4)°]. This can be due to the bulkier (tridimensional) nature of cod compared to 2D fluorobenzene, causing a larger value of the Rh–fluorene distance in C3 compared to C4. In addition, the fluorenyl substituent is much more planar in the complex with cod than in the complex with fluorobenzene. All of these differences are not unexpected because formally C3 is a tetracoordinated, square-planar, 16e– complex, while C4 is a pentacoordinated, 18e– complex.

In conclusion, P-phosphination of L0 has produced the P-phosphanylphosphorane L2, whose coordination to [Rh(diene)2]BR4 has given cyclorhodated complexes, containing a η1-coordinated fluorenyl ligand. 9-Fluorenyl is an electron-releasing, sterically demanding substituent42 that has produced phosphanes for catalysis.43−45 The metalation of fluorene presented here should further increase electron donation and steric crowding to the metal. Additionally, it is likely that similar compounds with other substituents should be accessible. We are currently studying this in our laboratories.

Acknowledgments

We thank AGAUR (2021-SGR-01107) and MICINN (PID2020-115658GB-I00) for financial support. We thank Dr. Jordi Benet-Buchholz for X-ray analysis of C3/C4. This paper is dedicated to Dr. Mercè Font-Bardia on the occasion of her retirement.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c01934.

  • Figure S1, Table S1, full experimental details (no uncommon hazards are noted), and NMR, IR and MS spectra of the new compounds (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic4c01934_si_001.pdf (25.1MB, pdf)

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