Halogenation of the Hexaphosphabenzene Complex [(Cp*Mo)2(μ,η6:η6‐P6)]: Snapshots on the Reaction Progress

Abstract The oxidation of [(Cp*Mo)2(μ,η6:η6‐P6)] (1) with halogens or halogen sources was investigated. The iodination afforded the ionic complexes [(Cp*Mo)2(μ,η3:η3‐P3)(μ,η1:η1:η1:η1‐P3I3)][X] (X=I3 −, I−) (2) and [(Cp*Mo)2(μ,η4:η4‐P4)(μ‐PI2)][I3] (3), while the reaction with PBr5 led to the complexes [(Cp*Mo)2(μ,η3:η3‐P3)(μ‐Br)2][Cp*MoBr4] (4) [(Cp*MoBr)2(μ,η3:η3‐P3)(μ,η1‐P2Br3)] (5) and [(Cp*Mo)2(μ‐PBr2)(μ‐PHBr)(μ‐Br)2] (6). The reaction of 1 with the far stronger oxidizing agent PCl5 was followed via time‐ and temperature‐dependent 31P{1H} NMR spectroscopy. One of the first intermediates detected at 193 K was [(Cp*Mo)2(μ,η3:η3‐P3)(μ‐PCl2)2][PCl6] (8) which rearranges upon warming to [(Cp*Mo)2(μ‐PCl2)2(μ‐Cl)2] (9), [(Cp*MoCl)2(μ,η3:η3‐P3)(μ‐PCl2)] (10) and [(Cp*Mo)2(μ,η4:η4‐P4)(μ‐PCl2)][Cp*MoCl4] (11), which could be isolated at room temperature. All complexes were characterized by single‐crystal X‐ray diffraction, NMR spectroscopy and their electronic structures were elucidated by DFT calculations.


Introduction
The halogenation of white phosphorus is the first step on an industrial scale to transform P 4 to organophosphorus derivatives. First publications on the halogenation of white phosphorus date back more than 120 years. [1,2] In 1940, Wyllie et al. proved that the reaction of P 4 with I 2 leads to P 2 I 4 or PI 3 , depending on the used stoichiometry, while the reaction of P 4 with Br 2 leads to PBr 3 . [3] Based on these results, in 1994, Tattershall et al. reported four series of compounds resulting from the reaction of P 4 with I 2 , Br 2 or ICl that were identified by NMR spectroscopy. [4] Later on, Stoppioni et al. presented the first example of a halogenation of coordinated white phosphorus in the coordination sphere of a transition metal by iodine, which resulted in the monocation [{CpRu(PPh 3 ) 2 } 2 (μ,η 1 :η 1 -P 4 H 2 I)] + ). [5] In 2019, Peruzzini et al. reported the ruthenium-mediated halogenation of white phosphorus, resulting in the complex [Cp*Ru(PCy 3 )(μ,η 2 :η 4 -P 4 Cl 2 )RuCp*] bearing the unprecedented P 4 Cl 2 moiety. [6] Recently, the stepwise degradation of white phosphorus coordinated to a Ru(II) complex upon the reaction with iodine was postulated by DFT computations. [7] The key role of the metal was demonstrated since the proposed mechanism differs significantly from the concerted one that is valid for uncoordinated white phosphorus. [8] As an alternative access, P 4 moieties already converted to polyphosphorus complexes might be a valuable approach to functionalized polyphosphorus complexes.

Results and Discussion
The reaction of 1 with an excess of iodine (6 equiv.) in CH 2 Cl 2 , followed by the layering of the reaction solution with npentane, leads to [(Cp*Mo) 2 (μ,η 3 :η 3 -P 3 )(μ,η 1 :η 1 :η 1 :η 1 -P 3 I 3 )][I 3 ] (2-I 3 ) as a red crystalline compound in 54 % isolated yield (Scheme 2). When a stoichiometric amount of I 2 was used (3 equiv.), a few crystals of the similar complex 2-I in which I 3 À is replaced by I À as counterion were isolated, together with [(Cp*Mo) 2 (μ,η 4 :η 4 -P 4 )(μ-PI 2 )][I 3 ] (3) as black blocks in a crystalline yield of 2 % (Scheme 2, cf. Supporting Information). The ESI-MS spectrum of the latter reaction solution shows the molecular ion peak of 2 and 3, together with the one of the paramagnetic complex [(Cp*Mo) 2 (μ-I) 4 ][I 3 ]. The latter compound was already described by Poli et al. [15] Possible other products as for instance PI 3 or P 2 I 4 could not be detected in the 31 P{ 1 H} NMR spectrum of the reaction solution which shows only the signals of 2 (see below). The formation of 3 could not be ascertained by 31 P NMR spectroscopy due it is paramagnetic nature. Since no signals of the starting material were detected, its full conversion can be assumed. The variable temperature (VT) 31 P{ 1 H} NMR spectrum of this reaction solution recorded from À 80°C to room temperature shows that the formation of 2 starts already at low temperatures, with 2 being the only P-containing diamagnetic product that could be detected. The spectra at higher temperatures are silent because 2 precipitates completely from the solution already at low temperatures (cf. SI for VT-31 P{ 1 H} NMR) and the remaining products are paramagnetic.
Our study of the halogenation of [{CpMo(CO) 2 } 2 (μ,η 2 :η 2 -P 2 )] showed that the nature of the halogen (I 2 vs. Br 2 ) plays a decisive role regarding the type of the resulting compounds. [10] Hence, the reaction of [(Cp*Mo) 2 (μ,η 6 :η 6 -P 6 )] (1) with PBr 5 as a bromine source was carried out. Due to the higher reactivity of bromine (as compared to iodine), the reaction of 1 with an excess of PBr 5 (6 equiv.) in CH 2 Cl 2 was carried out at À 40°C and the reaction solution was slowly warmed up to À 20°C. Precipitation of the concentrated reaction solution with cold npentane led to a green precipitate which was dissolved in THF and layered with toluene affording a few crystals of [(Cp*Mo) 2 (μ,η 3 :η 3 -P 3 )(μ-Br) 2 ][Cp*MoBr 4 ] (4) (2 % yield). The supernatant mother liquor was decanted off and, while warming up to room temperature, crystals of the neutral species [(Cp*MoBr) 2 (μ,η 3 :η 3 -P 3 )(μ-P 2 Br 3 )] (5) were formed in 17 % yield. Performing the reaction under the same conditions but evaporating all volatiles at À 10°C, washing the residue with npentane and toluene and finally recrystallizing from a mixture of CH 2 Cl 2 /n-pentane, crystals of [(Cp*Mo) 2 (μ-PBr 2 )(μ-PHBr)(μ-Br) 2 ] (6) could be isolated (9 % yield; Scheme 2). Additionally, also a few crystals of the known side product [(Cp*MoBr 2 ) 2 (μ-Br) 2 ] [16] were formed. The 31 P{ 1 H} NMR spectrum of the reaction solution shows, among the signals corresponding to 4 [17] and 6, a broad singlet at 230 ppm corresponding to PBr 3 which overlaps with one of the two signals of 6. Signals corresponding to 5 could not be detected in the 31 P NMR spectrum of the freshly prepared reaction solution, but were only detected after storing the sample at room temperature for five days. This indicates that 5 is not an initial product of the reaction of 1 with PBr 5 . Preparing an NMR sample by mixing a precooled solution of 1 and PBr 5 and performing a VT 31 P{ 1 H} NMR experiment (starting at À 80°C) shows the formation of a very complex reaction mixture (cf. SI) from which no known complexes could be identified. By warming to room temperature, signals corresponding to 6 could be detected. Since the signal of PBr 3 is always detected when using PBr 5 as a reagent, the question arises which part of it results from the halogenation of 1. 31 P NMR spectroscopic investigations showed that only about 30 % of the PBr 3 originates from PBr 5 while the remaining 70 % follows from the bromination of 1 (see Supporting Information).
Against the background that the reaction of 1 towards a bromine source was predictably more complex than the one with iodine, the question arises as to what would be the difference in the reactivity towards an even stronger halogenating agent as for instance chlorine. Therefore, a time-dependent 31 P{ 1 H} NMR study of the reaction of 1 with an excess of PCl 5 (6 equiv.) was carried out at 193 K ( Figure 1a). The signal of 1 could not be detected, which proves that its conversion is complete already at 193 K after fifteen minutes. The first 31 P{ 1 H} NMR spectrum (t 1 = 15 minutes) shows four resonances centered at δ = 420.8, 329.3, 145.1 and À 361.7 ppm in a 1 : 2 : 2 : 1 integral ratio, corresponding to an AMM'OO'X spin system (Figure 1a,b) which can be assigned to 7, based on its similarity with 2 (see below). In addition, a singlet corresponding to PCl 3 (δ = 220.0 ppm) and a singlet at 6.5 ppm which could not be assigned were detected. 7 could not be isolated due to its high instability. Attempts to crystallize it led to the isolation of [(Cp*Mo) 2 (μ,η 3 :η 3 -P 3 )(μ-PCl 2 ) 2 ][PCl 6 ] (8) instead (74 % yield, scheme 2). Based on the 31 P NMR spectroscopic data, the first product of the reaction of 1 with PCl 5 is 7 which then converts to 8, where its signals start to appear in the timedependent NMR study after one hour (t 2 = 1 h; figure 1a). Solutions of 8 in CH 2 Cl 2 at room temperature are stable for less than one hour, afterwards the color of the solution starts to change from bright red to dark brown. Its decomposition was monitored by NMR spectroscopy (see Supporting Information). The 31 P{ 1 H} NMR spectrum of the crystals of 8 dissolved in
Moreover, the intensity of the signal corresponding to PCl 3 (220.2 ppm) increases with the temperature, while the one of [PCl 6 ] À decreases until it disappears completely at room temperature (cf. SI for VT 31 P{ 1 H} NMR). By performing the reaction of 1 with PCl 5 directly at room temperature, 9 and 10 can be isolated in 3 % and 4 % crystalline yield, respectively (Scheme 2). The 31 P{ 1 H} NMR spectrum of the reaction solution at 25°C shows the characteristic signals of 9 (δ = 317.2 ppm), 10 (see below) and PCl 3 (δ = 220.2 ppm) among other signals that could not be assigned (cf. SI). Roughly 78 % of the PCl 3 originates from 1, which reflects the low yields of 9 and 10. [18] Attempts to isolate other products of the chlorination led to the isolation of the 30 VE cationic triple-decker compound [(Cp*Mo) 2 (μ,η 4 :η 4 - 11 is well soluble in CD 2 Cl 2 but paramagnetic (cf. Supporting Information) and therefore could not be detected by NMR spectroscopy, but was identified by single crystal X-ray diffraction analysis. In the 1 H NMR spectrum of 11, the signal of the anion [Cp*MoCl 4 ] À in 11 could be detected at À 13.9 ppm as a broad singlet (ω 1/2 = 170 Hz), which is in line with the reported chemical shift. [19] When 1 was reacted with three equiv. of PCl 5 , the neutral analogue of 11, i. e. [(Cp*Mo) 2 (μ,η 4 :η 4 -P 4 )(μ-PCl 2 )] (12), could be isolated in 10 % yield. [20] The 31VE triple-decker complex 12 is paramagnetic, but no signals could  be detected by EPR spectroscopy, probably due to its triplet spin state. Crystals of 11 were alternatively obtained by layering a solution of 8 with n-pentane in CH 2 Cl 2 at room temperature after few days. Therefore, together with 9 and 10, 11 represents another conversion product of 8 (Scheme 2). Additional proof of this is provided by the 1 H NMR spectrum of crystals of 8 dissolved in CD 2 Cl 2 which also shows, after 15 days at room temperature, the broad singlet of the counterion of 11, [Cp*MoCl 4 ] À (cf. Supporting Information). [21] The 31 P NMR spectra of 2 and 7 are very similar, both of them showing an AMM'OO'X spin system (cf. Supporting Information). The central phosphorus atom of the allylic-like P 3 unit (P X ) resonates at high field (À 327 ppm for 2 and À 362 ppm for 7), while the peripheral P atoms (P M /P M' ) resonate at lower field (349 ppm for 2 and 382 ppm for 7) and show the largest 1 J PP coupling constant (~380 Hz). Similar chemical shifts and coupling constants were found for the allylic-like P 3 unit in the complexes 5, 10 and 8 (for details see Supporting Information), although 5 shows an ADHKX spin system (Figure 3, top) due to the chemically inequivalent peripheral P atoms of the allylic-like P 3 unit. The central P atom (P A ) of the P 3 X 3 subunit resonates at low field (380 ppm for 2 and 421 ppm for 7), while the peripheral P atoms (P O /P O' ) resonate at moderately higher field (5 ppm for 2 and 145 ppm for 7) and show a smaller 1 J PP coupling constant (~280 Hz; Figure 1b). Within the PBrPBr 2 unit of 5, the 1 J PP coupling of 420 Hz is rather large and an additional large coupling (J PAPH = 70 Hz) is observed within the P 3 unit. Similarly, PÀ P couplings can be detected between the PCl 2 and the P 3 units (up to 35 Hz) in 8 ( Figure 1c) and 10 (up to 41 Hz, Figure 3, bottom). Additionally, the resonance signal of the PCl 6 À anion in 8 is observed at À 296 ppm in the 31 P NMR spectrum. Compound 8 shows rather broad signals at room temperature, which sharpen by lowering the temperature to À 40°C (Figure 1c).
The 31 P{ 1 H} NMR spectrum of 6 shows two doublets with a coupling constant of 38 Hz, corresponding to the two nonequivalent phosphorus atoms. The signal at the highest field can be assigned to the PH ligand which splits in the 31 P NMR spectrum into a doublet due to the coupling with the proton ( 1 J PH = 444 Hz). For 9, only one singlet was detected (cf. Supporting Information). The 31 P NMR chemical shifts and coupling constants for all complexes were determined by iterative simulation of the experimental spectra (see Supporting Information). Apart from 7 which is extremely unstable even at low temperatures in solution and could not be isolated, the solidstate structures of all other products were determined by single crystal X-ray diffraction analysis (Figure 4), revealing that the halogenation of the 28VE cyclo-P 6 -containing triple-decker complex [(Cp*Mo) 2 (μ,η 6 :η 6 -P 6 )] (1) leads to the formation of dinuclear monocations and neutral species which, in most cases, retain their triple-decker geometry. However, in some cases, degradation to [(Cp*MoBr 2 ) 2 (μ-Br) 2 ], [(Cp*Mo)(I 4 )] À or [Cp*MoX 4 ], the latter being observed as an anion in 4 and 11, was detected. For all complexes, the MoÀ Mo bond lengths are below the sum of their covalent radii (3.08 Å) [22] and longer than the corresponding distance in the neutral cyclo-P 6 complex 1 (2.647(1) Å) [13] and in the related monocation [(Cp*Mo) 2 (μ,η 6 :η 6 -P 6 )][FAI] ([FAI] = [FAI{OC 6 F 10 (C 6 F 5 )} 3 ], 2.661(8) Å). [14] They vary from 2.691(2) Å in 9 to 2.920(6) Å in 10. The only exception is observed within 4 with a MoÀ Mo distance of 2.579(6) Å. The Cp* ligands in the compounds 2-I 3 , 3, 4, 6, 8, 9 and 11 are almost coplanar to each other, being only minimally tilted (tilt angle of 1°to 7°). In compounds 5 and 10, the coplanarity of the ligands is lost, and the Cp* ligands are tilted by 40°and 38°, respectively.
The molecular structure of 2-I 3 shows an allylic-P 3 and a P 3 I 3 ligand each of them bridging the two {Cp*Mo} fragments (Figure 4). In the P 3 I 3 unit, the central P atom does not coordinate to molybdenum. Compared to the PÀ P bond lengths in 1 [13] (average: 2.171(3) Å and in the range of PÀ P single bonds (2.22 Å) [23] ), the P3-P4 (2.139(5) Å) and P4-P5 (2.138(6) Å) distances are shortened. Similar bond lengths were observed for the cationic complex [(Cp*Mo) 2 (μ,η 6 :η 6 -P 6 )][FAI] [14] whose cyclo-P 6 undergoes a bis-allylic distortion upon one electron oxidation (average: 2.136(10) Å). The P1-P2 (2.216(5) Å) and the P1-P6 (2.220(5) Å) distances in the P 3 I 3 ligand are longer than the one in 1, and correspond to PÀ P single bonds, while the distance between P2 and P6 with 2.459(5) Å is too long for a usual single bond, but considerably below the sum of the van der Waals radii (3.80 Å). [22] DFT calculations for compound 2-I 3 reproduce this distance (2.434 Å) well, but no bonding interaction could be detected between these two P atoms (the Mayer bond order (BO) is lower than 0.1 and no orbital overlap occurs). The P···P distances between the two P 3 ligands are rather long (P2···P3 2.652(5) Å and P5···P6 2.665(5) Å). Nevertheless, DFT calculations indicate the presence of a P···P interaction as shown by BOs of 0.16 and 0.18 for P2···P3 and P5···P6, respectively (see also Supporting Information). A similar cyclo-P 3 I 3 moiety was recently postulated by DFT computations as one of the intermediates involved in the last steps of the iodine-induced stepwise degradation of the P 4 ligand in [Cp*Ru(dppe)(� 1 -P 4 )], [7] but without any experimental evidence. Therefore, 2-I 3 represents the first example of an isolated polyphosphorus complex bearing a P 3 I 3 ligand. The presence of an allylic P 3 ligand is recurrent among the products of the halogenation of 1, as in 4, 5, 8 and 10, which will be discussed together therefore. In the allylic P 3 ligand of 4, the P1À P2 (2.126(2) Å) and the P2-P3 (2.118(19) Å) bond lengths are shortened compared to the PÀ P
The solid-state structure of 6 contains only two P atoms, in the form of a PBr 2 and a PHBr bridging ligand, with a nonbonding distance between the two phosphorus atoms of 2.762(9) Å. A similar structure was observed for compound 9, which bears two equivalent bridging PCl 2 ligands, separated from each other by 2.894(6) Å.
In order to investigate the bonding situation in complexes 2-12, DFT calculations at the D4-TPSSh(CPCM)/def2-TZVP level were conducted. [31] The geometric parameters of the complexes are well reproduced, including the distances between the different P n units in the molecules. In all complexes 2-12, a MoÀ Mo bond was detected, being in line with the relatively short MoÀ Mo distances. The Mayer bond order (BO) varies from 0.87 in 4 to 0.59 in 5 (MoÀ Mo distances 2.574 and 2.892 Å in the optimized geometries, respectively). The intrinsic bonding orbitals [32] representing the MoÀ Mo bond in 2 and the bonding within the Mo 2 P 3 unit in 4 are depicted in Figure 5.
The NBO [33] analysis is in agreement with the IBO analysis, although only in 4 and 6 an MoÀ Mo bond (MoÀ Mo 2.579 Å (4) and 2.709 (6)) is predicted by the NBO analysis, while in the other complexes a nonbonding NBO on each Mo with an occupancy of approx. 1e is partitioned instead of a MoÀ Mo bonding orbital. Although the P2À P6 distance in 2 is rather short, no bonding interaction could be detected by DFT calculations. In contrast, bonding interactions were detected between P2À P3 (2.649 Å) and P5-P6 (2.628 Å) as shown by BOs of 0. 16 [34] clearly shows a bonding interaction between the P 3 and the PCl 2 units in 8, among the expected bonds. A plot of IRI in the plane defined by the phosphorus atoms is depicted in Figure 5 showing the regions with notable chemical bond interaction (orange) and areas where weak interactions occur (green). The Electron Localization Function (ELF) [35] and Localized Orbital Locator (LOL) [36] also support these interactions (see Supporting Information). Complexes 3 and 11 are paramagnetic in solutions at room temperature, however, DFT calculations show that the singlet spin state of the cation in 3 and 11 is with 90 and 102 kJ·mol À 1 , respectively, more stable than the triplet spin state. The overall paramagnetic behavior of 11 might be due to the paramagnetic nature of the counter anions [Cp*MoCl 4 ] À , having a triplet spin state as determined experimentally. [37]

Conclusions
In summary, we showed that the halogenation of [(Cp*Mo) 2 (μ,η 6 :η 6 -P 6 )] (1) proceeds via a very complex pathway leading to a plethora of complexes containing different P n units such as P 4 , P 3 , P 2 and P 1 . An excess of halogens/halogen sources leads to phosphorus-free complexes of the type [Cp*MoX n ] (X = I, Br, Cl) as well as PX 3 (X = Br, Cl) as final reaction products. Additionally, we showed that besides iodination, which is known to be a powerful tool for the synthesis of new P n ligand complexes, bromination and chlorination can also be used for this purpose. Among the products of the iodination of the hexaphosphabenzene complex 1, the novel compound 2 bearing an unprecedented P 3 I 3 ligand could be isolated. With a bromine source, complex 5 could be isolated, representing the first example of a compound bearing a P 2 Br 3 unit as a bridging ligand between two Mo centers.
Whereas the products of the one-electron oxidation of 1 showed only a distortion of the hexagonal geometry of the P 6 middle deck, the use of halogens or halogen sources as oxidizing agents afforded a variety of new polyphosphorus compounds bearing synthetically useful novel P n X m units. As expected, the chemoselectivity of the reaction decreased with the enhancement of the oxidizing power of the halogen. However, with a strict control of the temperature it is possible to exert a good control of the reaction even with a stronger oxidant such as the chlorine source PCl 5 . First snapshots from the halogenation of the P 6 ligand complex were found, and even though the complete pathway could not be clarified, they gave useful information concerning the progress of the reaction. Thus, first an allylic fragmentation of the cyclo-P 6 unit occurs followed by the monohalogenation of the P atoms of one of the allylic P 3 units. Subsequently, a dihalogenation forming PX 2 units occurs, which is followed by the removal of the P atoms as PX 3 moieties. Only afterwards, the second allylic-P 3 unit appears to be halogenated.
This study adds triple-decker complexes to the class of polyphosphorus compounds that can be successfully halogenated and thus functionalized. Therefore, future investigations will focus on the halogenation of heterobimetallic triple-decker compounds as well as of E n ligand derivatives that combine the features of triple-decker complexes and separate E n units. Moreover, since the halogenation reactions in general have shown to be a powerful tool for the generation of halogenfunctionalized polyphosphorus ligands, future studies will also focus on the use of these products for further derivatizations.