Reversible cooperative dihydrogen binding and transfer with a bis-phosphenium complex of chromium

Abstract

The reversible reaction of H₂ with a bis-phosphenium complex of chromium provides a rare example of 3d transition metal/phosphenium cooperativity. Photolysis induces the activation of H₂ and yields a spectroscopically detectable phosphenium-stabilized (σ–H₂)-complex, readily showing exchange with gaseous H₂ and D₂. Further reaction of this complex affords a phosphine-functionalized metal hydride, representing a unique example of reversible H₂ cleavage across a 3d M=P bond. The same species is also accessible via stepwise H⁺/H⁻ transfer to the bis-phosphenium complex, and releases H₂ upon heating or irradiation. Dihydrogen transfer from the H₂-complex to styrene is exploited to demonstrate the first example of promoting hydrogenation with a phosphenium complex.

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Introduction

Cooperative metal ligand reactivity, implying that both the metal and a ligand of a transition metal complex participate in a bond activation process, has emerged as a new concept in homogeneous organometallic catalysis.¹ Classical examples for the utilization of such metal/ligand cooperativity (MLC) are Noyori's^2,3 approach to transfer-hydrogenation with amine/amide ruthenium complexes and Milstein's⁴ introduction of pyridine-based PNP-pincer ligands to the field. More recently, it was pointed out that the implementation of MLC through functional pincer ligands has in particular stimulated developing (de)hydrogenation reactions which use complexes of 3d metals as catalysts.⁵

Growing interest in the advance of MLC has inspired activities that are not only directed at rationally improving catalyst design, but also at searching for new combinations of cooperating ligands and 3d metals. While cooperative hydrogen activation using N–M bonds is well known,^1,5 H₂ addition across a P–M bond was long confined to rare cases involving second and third row transition metals.⁶ Only recently, the group of Thomas discovered the 1,2-addition of H₂ across the covalent Co–P bond of a PPP-pincer complex I (Scheme 1) as the first example for cooperative activation of H₂ on a P–M bond engrossing a 3d-metal.⁷ However, complex II proved inactive in reactions involving hydrogen transfer to other substrates, isotope exchange, or release of H₂. To the best of our knowledge, reversible cooperative hydrogen activation on a P–M bond including a first row transition metal remains still unknown.

Scheme 1 Reported examples of cooperative hydrogenation of metal–phosphorus bonds in complexes of 3d-transition metals (R = 2,6-iPr₂C₆H₃).

We had recently reported on the transformation of an N-heterocyclic phosphenium complex III into a phosphine-complex IV through stepwise addition of H⁺/H⁻ to a Mn=P bond (Scheme 1).⁸ Even if straight addition of H₂ across the double bond was not accomplished, the observation of the reverse reaction, viz. thermally or photochemically induced dehydrogenation of IV to afford III and H₂, implies that the forward transformation should in principle be feasible as well. Recollecting the potential redox non-innocence of the N-heterocyclic N₂P-donor units in I and III,⁹ which is expressed in invoking in both cases identical limiting descriptions as either anionic N-heterocyclic phosphido (NHP⁻) or cationic N-heterocyclic phosphenium (NHP⁺) moieties, the complexes are electronically quite similar. If one considers further that II is likewise accessible from I through H⁺/H⁻ transfer from ammonia borane,⁷ the formation of IV and the hydrogenolysis of I can then be viewed as being closely related.

In view of the fact that MLC in diaminophosphenium complexes had already been established for cycloaddition and E–H-bond activation reactions,¹⁰ further exploration of the chemistry of these species is deemed a promising strategy in search of a viable route to cooperative hydrogenation of a bond between a phosphorus atom and a base metal. Along these lines, we investigated the reactivity of a bis-phosphenium complex of chromium towards H₂ and report here on the first reversible addition of H₂ across a double bond between phosphorus and a 3d-metal. This reaction involves a photochemically assisted activation step representing an unprecedented mechanism in the chemistry of phosphorus-based ligands. Moreover, we demonstrate that transfer of the ingested H₂ molecule to a different substrate is feasible and enables hydrogenation of an olefin.

Results and discussion

Our entry point to this chemistry was the synthesis of complex 2 (Scheme 2), representing the formal hydrogenation product of known bis-phosphenium complex 4 (ref. 11) (see Scheme 3), from [Cr(CO)₃(naphthalene)] and two equivalents of secondary diazaphospholene 1 in THF. The product gives rise to two ³¹P NMR signals indicating the presence of NHP⁺ (δ³¹P 232 ppm) and secondary phosphine (δ³¹P 141 ppm, ¹J_PH = 341 Hz) moieties. In accord with this assignment, the ¹H NMR spectrum displays the expected signals attributable to phosphorus (δ¹H 8.86) and metal bound (δ¹H −6.34) hydrogens. We presume that the reaction is initiated by exchange of the coordinated arene in the metal precursor by THF to yield highly reactive [Cr(CO)₃(THF)₃],¹² which is then converted into a transient bis-phosphine-complex 3a. Formation of 2 is finally completed by 1,2-H-shift under dissociation of the last THF ligand.

Scheme 2 Synthesis of complex 2. Reagents and conditions: (i) [Cr(CO)₃(L)], THF, 16 h at r.t., – L; (ii) −Do; (iii) +Do (MeCN) (R = 2,6-iPr₂C₆H₃, L = naphthalene, Do = THF (3a), MeCN (3b)).

Scheme 3 Interconversion between 2 and 4 by H⁺/H⁻ transfer and cooperative dehydrogenation. Reagents and conditions: (i) 12 h 80 °C, C₆D₆, –H₂; (ii) ([H(OEt₂)₂)][BAr^f₄], C₆H₆, – 2 Et₂O; (iii) Li[BEt₃H], C₆H₆, 30 min, – Li[BAr^f₄], – BEt₃ (R = 2,6-iPr₂C₆H₃, Ar^f = 3,5-C₆H₃(CF₃)₂).

Experimental support for the proposed rearrangement was first obtained from ¹H-EXSY NMR spectra, revealing reversible chemical exchange between metal- and phosphorus-bound hydrogens on a sub-second time scale. Moreover, titrating a solution of 2 in C₆D₆ with CD₃CN afforded dynamic equilibrium mixtures in which both 2 and 3b could be directly observed by NMR spectroscopy. The assignment of 3b as bis-phosphine complex grounds on the observation of a single ³¹P NMR signal (δ³¹P 156.7) and the AA′XX′-type splitting of the resonance of the P-bound hydrogen atoms. Confirmation was found in an X-ray diffraction study of a single crystal, which had separated serendipitously from an equilibrated solution in C₆H₆/CH₃CN (1 : 3.5) and contained a 1 : 1 mixture of 2 and 3b. Complex 3b (Fig. 1, bottom) features two phosphine ligands with P–Cr distances (Cr2–P3 2.241(2), Cr2–P4 2.225(2) Å) ranging at the lower end of known bond lengths in comparable chromium phosphine complexes (Cr–P 2.313(62) Ň) and five-membered rings adopting flat envelope conformation. Hydride complex 2 (Fig. 1, top) contains only one phosphine ligand (tetrahedral coordination at phosphorus, envelope-shaped ring, Cr1–P1 2.224(2) Å), while the second phosphorus atom exhibits the planar coordination environment and shortened P–Cr distance (Cr1–P2 2.150(2) Å) that are deemed characteristic for carbene-analogous NHP⁺ complexes.¹³

Fig. 1 Molecular structure of 2 (top) and 3b (bottom) in the crystal. Carbon-bound hydrogen atoms were omitted for clarity and Dipp-substituents were displayed using a wire model. Thermal ellipsoids were drawn at the 50% probability level. For 2, only one of four possible orientations of the disordered part of the molecule is shown and the metal-bound hydrogen atom (the position of which could not be refined) is missing but the position of both hydrogen atoms is ascertained by spectroscopic data (see ESI for details†). Selected interatomic distances [Å]: 2: Cr1–P1 2.224(2), Cr1–P2 2.150(2); 3b: Cr2–P3 2.241(2), Cr2–P4 2.225(2). Sum of X–P–Y angles involving heavy atoms [°]: 2: P1 344.2(2), P2 358.3(4); 3b: P3 339.9(2), P4 345.1(2). “Envelope” fold angles along the N–N vectors of C₂N₂P-rings [°]: 2: P2 8.4(3), P1 20.1(3); 3b: P4 23.3(3), P3 17.3(4).

Having both compounds 2 and 4 (ref. 11) in hand, we were curious on studying their mutual interconversion by cooperative (de)hydrogenation (Scheme 3).

Starting from 2, we found that thermolysis at 80 °C resulted in evolution of H₂ and formation of bis-phosphenium complex 4 as the only phosphorus-containing product detectable by ³¹P NMR spectroscopy. The reverse reaction was readily achieved in two steps through initial protonation of 4 with [H(OEt₂)₂][BAr^f₄] (Ar^f = 3,5-C₆H₃(CF₃)₂) to afford cationic bis-phosphenium complex 5⁺, and subsequent treatment with super hydride.§ Even if the order of both steps differs from that employed for conversion of III into IV,⁸ these reactions confirm that 4 exhibits, like mono-phosphenium complex III,⁸ nucleophilic character at the metal and electrophilic character on the NHP⁺ ligands. Phosphenium complex 5[BAr^f₄] was isolated in crystalline form and unmistakably identified by its deshielded ³¹P NMR signal (δ³¹P 261, Δδ³¹P +21 ppm vs.4), the typical ¹H NMR signal of the metal bound hydride (δ¹H −7.9), and a single-crystal X-ray diffraction study. Cation 5⁺ (Fig. 2) features two NHP moieties adopting a carbene-like coordination mode^9b,13 distinguished by a planar coordination environment at the phosphorus atoms and short P–Cr distances (P1–Cr 2.135(1) Å, P2–Cr 2.138(1) Å).

Fig. 2 Molecular structure of the cation of 5[BAr^f₄] in the crystal. Carbon-bound hydrogens were omitted for clarity. Dipp-substituents were displayed using a wire model. Thermal ellipsoids were drawn at the 50% probability level. Selected interatomic distances [Å]: Cr1–P2 2.135(1), Cr1–P3 2.138(1), Cr1–H1 1.632(23); sum of X–P–Y angles [°]: P2 359.7(1), P3 359.9(1).

The observation of a formal addition of H₂ across the P=Cr bond of complex 4 by stepwise transfer of a H⁺/H⁻ pair brought up the question of whether the same product was accessible by direct activation of dihydrogen. We therefore studied the reactivity of 4 with H₂ by monitoring the NMR spectra of solutions of the complex in THF-[D₈] under H₂-atmosphere (1 to 8 bar). While no reaction was observed at ambient temperature, the spectrum of a solution that had been heated for 300 h at 60 °C under 8 bar of H₂ revealed indeed the formation of trace amounts of 2. This finding confirms that direct hydrogenolyis of the bis-phosphenium complex is indeed in principle feasible, but rather ineffective under the conditions chosen.

More promising results were obtained by performing the reaction under irradiation with a medium-pressure Hg-lamp. Inspection of the ¹H NMR spectrum of a solution of 4 recorded after 7.5 h of photolysis under H₂ (8 bar) revealed two signals with negative chemical shifts indicative of the formation of two new metal hydrides (Fig. 3).

Fig. 3 Expansion of the hydride region of the ¹H NMR spectrum of a 14 mM solution of 4 in THF-[D₈] after 7.5 h photolysis under 8 bar of H₂ with signal assignment.

Further analysis of 1D and 2D NMR spectra allowed us to assign one of these resonances to hydrogenation product 2 and to identify the second one as belonging to an isomeric complex featuring two equivalent NHP⁺ ligands (δ³¹P 234 ppm) and two metal-bound hydrogen atoms. Integration of suitable ¹H NMR signals indicated that 20% of bis-phosphenium complex 4 had been converted to 2 and 16% to the yet unknown second hydrogenation product. Extended irradiation led to growing in of additional resonances, which we attribute to decomposition products, but did not affect further significant changes in the distribution of the main products, suggesting that a photo-stationary state had been reached.

Studies aiming at a more comprehensive description of the unknown hydrogenation product revealed that the hydride signal displays a very short T₁ relaxation time (T_1min = 18 ms at 253 K), which is characteristic of dihydrogen complexes¹⁴ and led us to assign this product tentatively as (σ–H₂)-complex 6 (Scheme 4). Photochemical generation of H₂-complexes of chromium has precedence,¹⁵ and we could indeed confirm our initial assignment by isotope labelling studies. While the formation of specifically deuterated 2-[D₂] and 6-[D₂] during photolysis of 4 under D₂-atmosphere proved in the first place merely the uptake of a hydrogen molecule from the gas phase, crucial structural information was gained from an irradiation experiment that was conducted with a H₂/HD/D₂-mixture and afforded a mix of all three possible isotopomers of 6.

Scheme 4 Cooperative hydrogenation of 4. Reagents and conditions: (i) 8 bar H₂, hν; (ii) 8 bar H₂, 40 °C; (iii) vacuum 40 °C, several days.

Analysis of the hydride ¹H NMR signal of 6-[D₁] allowed us to determine a value of ¹J_HD = 31.8 Hz and, using the well-established relation between ¹J_HD and H–H distance (d_HH = 1.42 − 0.0167 × J_HD¹⁶), to calculate d_HH = 0.89 Å. Both the values of ¹J_HD and d_HH for 6 come close to reported data for H₂-complexes of chromium.¹⁵

NMR spectroscopic monitoring of the photolysis over time revealed that formation of 6 precedes that of 2 and that the steady-state molar fractions of both species grow with increasing H₂-pressure (see ESI†). Tempering reaction mixtures at 40 °C under 8 bar of H₂ for several days without irradiation led to an eventual increase of the signals of 2 at the expense of those of 6. As a corollary of these findings, we consider 6 as an intermediate in the formation of 2. It should be noted that initial bonding and activation of a H₂ molecule on the metal centre had also been postulated for the hydrogenolysis of I.⁷

Once formed, complex 6 was surprisingly thermally stable, showing no release of H₂ within 18 h at 20 °C in an atmosphere of argon (1 bar), and decaying only slowly upon tempering the reaction mixture in a previously evacuated NMR tube at 40 °C for several days. The observed recovery of bis-phosphenium complex 4 and H₂ under these conditions implies that the interchange between 4 and 6 under uptake or release of H₂ is not coupled to (de)carbonylation, as in other cases.¹⁵ Contrasting its inert behaviour in the absence of external H₂, complex 6 reacted via rapid incorporation of D₂ and release of H₂ upon exposure to a D₂-atmosphere. The failure to detect any HD or 6-[D₁] suggests that the reaction proceeds, as in other cases,¹⁵ by exchange of intact H₂/D₂ molecules. We presume that the isomerization 6 → 2 under H₂-pressure, which contrasts the slow dehydrogenation observed in the absence of a significant partial pressure of H₂, is most likely a bimolecular process.

In order to gain mechanistic understanding of the hydrogen activation on 4, we performed preliminary computational studies (at the ωB97xD/def2-tzvp-level of theory that had also been used to model similar reactions)⁸ on the hydrogenolysis of NMe-substituted complex 4^Me (see ESI for details†). The energy optimized molecular structures of 4^Me and 2^Me feature, in accord with the experimental findings on 4 and 2, NHP⁺ units with planar coordinated phosphorus atoms and short P–Cr bonds (P–Cr 2.076–2.077 Å) with partial multiple bond character. The formal addition of H₂ across a Cr=P bond of 4^Me is predicted to be faintly endergonic (ΔG⁰ = 0.6 kcal mol⁻¹) but impeded by a high kinetic barrier (ΔG^#TS2) = 54.8 kcal mol⁻¹, (see Fig. 4). The computed molecular structure of dihydrogen complex 6^Me has two structurally different NHP units, one of which adopts the same “carbene-like” binding mode as in 2^Me and 4^Me, while the other one exhibits a pyramidal coordination at phosphorus and an elongated Cr–P distance (P–Cr 2.635 Å). Conceptually, this bonding mode can be associated with a description of the NHP unit either as an 1e-ligand (assuming covalent P–Cr-bonding¹³) or an anionic phosphide moiety^9,13c with a P-centred lone-pair of electrons that does not interact with the metal. This view is supported by an NBO analysis,¹⁷ which supposes the presence of a lone-pair and a reduced partial charge (q(NHP) = +0.38 vs. 1.28 for the planar NHP ligand) on the pyramidal NHP unit. Similar shifts in coordination modes have been previously associated with redox-non-innocent behaviour of phosphenium ligands and were considered to reflect a close relation to the chemistry of nitrosyls.⁹ The manifestation of two unlike NHP ligands contradicts at first glance the experimental data observed for 6, but we located a low-lying (ΔG_rel = +4.4 kcal mol⁻¹) excited singlet state with two indistinguishable NHP ligands that may provide for facile dynamic equilibration of both binding modes. The open shell nature and low electronic excitation energy of the excited singlet (ΔE = 7.5 kcal mol⁻¹) and the likewise symmetrical triplet state (ΔE = 8.5 kcal mol⁻¹) suggest that the electronic structure of 6^Me may be more complex than anticipated from our preliminary computations. Hydrogenation of 4^Me to afford 6^Me is kinetically less disfavoured (ΔG^#(TS1) = 33.2 kcal mol⁻¹) than addition of H₂ across a P=Cr bond but strongly endergonic (ΔG⁰ = 22.8 kcal mol⁻¹), explaining our failure to access 6 in a thermal reaction.¶

Fig. 4 Computed free energy profile for the reaction of 4^Me with H₂ and wire-model representations of the molecular structures of calculated stationary states. Relative free energies ΔG⁰ (in kcal mol⁻¹) of electronic ground states (in black) and excited states (in blue) were obtained from DFT or TD-DFT calculations at the ωB97xD/def2-tzvp-level. 4^Me*^,vert and 4^Me* denote the energies of the first excited state of 4^Me after vertical electron excitation and subsequent structural relaxation (calculated at the ωB97xD/def2-tzvp//ωB97xD/def2-svp-level), respectively. 6^Me,rot refers to a conformer obtained by rotation of the pyramidal NHP unit in 6^Me.

A TD-DFT calculation on 4^Me allowed us to assign the energetically lowest electron excitation as a transition with MLCT-character that transfers electron density from the Cr-centred Kohn–Sham (KS) HOMO into the NHP-centred KS-LUMO (see ESI†). Energy optimization of the vertically excited state furnished a relaxed molecular geometry distinguished by one planar and one pyramidal NHP unit with similar characteristics as in 6^Me (Fig. 4). Adopting the notion of pyramidal and planar NHPs as 1e- and 3e-ligands,¹³ electronically excited 4^Me* can be pictured as complex with a 16 VE count that should be capable of binding a further Lewis donor. In accord with this hypothesis, a relaxed potential energy scan revealed that reaction of 4^Me* with H₂ allows accessing 6^Me in an exergonic process without having to pass a further energy barrier. Additional computations implied that binding of a solvent (THF) is likewise feasible, but less exergonic compared to formation of 6^Me (see ESI†). The energetic ordering of TS1 and TS2, representing competing pathways for the unimolecular decay of 6^Me, is qualitatively in accord with the observed formation of H₂ and 4 during thermolysis of 6.

The observed exchange with free dihydrogen brought up the question of whether complex 6 was also capable of transferring a H₂-molecule to another substrate. Studying the photolysis of solutions of 4 in THF-[D₈] under 8 bar of H₂ in the presence of an excess of styrene, we noticed indeed an over-stoichiometric formation of ethylbenzene with up to 99% conversion (Scheme 5 and Table 1). Control experiments revealed that no hydrogenation took place in the absence of 4, or when only phosphine 1 was present,|| whereas [Cr(CO)₃(naphthalene)] gave a single turnover, which is in accord with a stoichiometric rather than catalytic reaction. The incomplete consumption of styrene with low initial complex loadings (Table 1, entry 2) is presumably due to eventual conversion of 4 and 6 into inactive follow-up products. In accord with this conjecture, NMR studies indicated formation of an alkyl phosphine arising from formal hydrophosphination of styrene, along with minor amounts of 1 and further unknown phosphorus-containing species as by-products of the hydrogenation. Analysing the composition of a reaction mixture that had been irradiated for 20 min and was then stored in the dark for 18 h at 20 °C revealed further that the signal of 6 generated during photolysis had disappeared, while 2 was still present, and an approximate 10-fold excess of ethylbenzene (in relation to the molar amount of 6 consumed) had newly formed.

Scheme 5 Photocatalytic hydrogenation of styrene.

Table 1 Results of the photocatalytic hydrogenation of styrene^a

Entry	Catalyst	Mol%	Irradn. time [h]	Conv.^b [%]	TON^c
a Conditions: 26 μmol styrene in THF-[D8] (0.5 mL), 8 bar H2, irradiation with a medium-pressure Hg lamp. b Conversion determined by integration of suitable ^1H NMR signals. c TON = n(C6H5Et)/n(complex). d L = naphthalene.
1	4	16	11.5	99	6
2	4	4	10.5	76	18.5
3	1	16	17	0	0
4	[Cr(CO)3(L)]^d	16	17	18	1.1
5	None	—	17	0	0

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We interpret these findings as confirmation that H₂-complex 6 is the active species in promoting olefin hydrogenation, while 2 acts merely as spectator. The discovery that complex 6 is obviously capable of accomplishing multiple turnovers without further photochemical activation is in accord with a catalytic mechanism. It is worthwhile mentioning that the observed hydrogenation of styrene, even if its performance cannot yet compete with established schemes for catalytic hydrogenation and requires further optimization, provides nonetheless first a proof of concept for the feasibility of using a phosphenium base–metal complex as hydrogenation catalyst.

Conclusions

In summary, we have accomplished the first reversible cooperative addition of molecular H₂ to a double bond between phosphorus and a first-row transition metal, and demonstrated further the transfer of the ingested H₂-molecule in the hydrogenation of an olefin. Key to this reactivity is the stimulation of the initially present bis-N-heterocyclic phosphenium complex by photolysis, which paves the way to formation of a Kubas-type “non-classical” (σ–H₂)-complex¹⁹ as crucial intermediate in both Cr=P hydrogenolysis and H₂-transfer. The finding that activation of H₂ occurs on the metal centre confirms earlier conjectures,⁷ and is also supported by computational studies. The observed reaction pathway is an unprecedented approach to using the specific reactivity of a P-donor ligand for generation of a vacant coordination site on an electronically saturated (18 VE) transition metal centre, even if it does not provide an example of metal–ligand cooperativity in a strict sense.⁵ Nonetheless, its feasibility relies crucially on the redox non-innocence of the phosphenium ligand and is inconceivable without cooperative interplay between metal and ligand. We are currently striving to improve the performance of NHP-complex-mediated hydrogenation and extend its application to further types of multiple bonds, and to explore the photolytic stimulation of NHP complexes as a more generally applicable tool for promoting metal binding and activation of other ligands than H₂.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Dr W. Frey and J. Trinkner (both Institute for Organic chemistry) for the collection of X-ray data sets and the acquisition of HRMS data, respectively, B. Förtsch for elemental analyses, B. Rau for coordination of NMR measurements, and R. Gayde for construction of the hydrogenation reactor. N. B. thanks T. J. Walter for extensive proofreading of the manuscript and several discussions. The computational studies were supported by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/467-1 FUGG (JUSTUS cluster).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures. Representations of NMR, MS, UV-vis spectra, results of crystallographic and computational studies. CCDC 2005812 and 2005814. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc03773g

‡ Median and standard deviation returned by a query in the CSD database for chromium complexes containing ligands of type PRY₂ (R = any non-metal substituent, Y = O- or N-based substituent).

§ The order of addition is important; adding the reagents in reverse order gave in this case no conclusive results.

¶ Preliminary DFT calculations suggest that formal replacement of NMe by NPh substituents exerts a marked energetic stabilization of the H₂-complex (the energy gap between 6^Ph and 4^Ph + H₂ computed at the ωB97xD/def2-tzvp//ωB97xD/def2-svp level of theory is by 9.1 kcal mol⁻¹ lower than between the NMe-derivatives). The change is still way too small to render the hydrogenation exergonic, but highlights the qualitative nature of our computational model. The effect on 2^Ph (stabilized by 0.9 kcal mol⁻¹ at the same level) is only minute.

|| This result allows excluding catalysis by phosphinyl radicals formed by photolysis of 1; see ref. 18.

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