Isolation of Neutral, Mono‐, and Dicationic B2P2 Rings by Diphosphorus Addition to a Boron−Boron Triple Bond

Abstract The NHC‐stabilised diboryne (B2(SIDep)2; SIDep=1,3‐bis(2,6‐diethylphenyl)imidazolin‐2‐ylidene) undergoes a high‐yielding P−P bond activation with tetraethyldiphosphine at room temperature to form a B2P2 heterocycle via a diphosphoryldiborene by 1,2‐diphosphination. The heterocycle can be oxidised to a radical cation and a dication, respectively, depending on the oxidant used and its counterion. Starting from the planar, neutral 1,3‐bis(alkylidene)‐1,3‐diborata‐2,4‐diphosphoniocyclobutane, each oxidation step leads to decreased B−B distances and loss of planarity by cationisation. X‐ray analyses in conjunction with DFT and CASSCF/NEVPT2 calculations reveal closed‐shell singlet, butterfly‐shaped structures for the NHC‐stabilised dicationic B2P2 rings, with their diradicaloid, planar‐ring isomers lying close in energy.

Cyclic compounds of boron and phosphorus have attracted the attention of main-group chemists for decades. [1] The early discoveries in this field focussed on oligomers of the phosphinoboranes, R 2 PBR 2 . [2] In comparison to their lighter congeners, the aminoboranes, R 2 NBR 2 , the reluctance of phosphorus to adopt a planar geometry and engage in pbonding with boron leads to a higher propensity for intermolecular B À P association and formation of four-membered (A, Scheme 1) and six-membered heterocycles, and indeed polymers. [3] A number of four-membered 1,3-diphospha-2,4diboretanes (B) have also been reported, [4] largely from unsuccessful attempts to prepare monomeric RP = BR species. [5] These compounds contain pyramidalised phosphorus atoms that can act as ligands for transition metals. [4e, 6] The majority of developments in P-B ring systems in the last two decades relate to the synthesis of a 1,3-diphospha-2,4diboretanediyl diradicaloid (C, Scheme 1) by Bertrand and co-workers. [7] The first example of such a compound, cyclo-(BtBu) 2 (PiPr 2 ) 2 , was prepared by reacting the diborane(4) 1,2-B 2 Cl 2 tBu 2 with LiPiPr 2 . The presumed 1,2-diphosphinodiborane intermediate undergoes a rearrangement to cleave the BÀB s bond and generate the four-membered ring. This compound possesses p single bond character between the two boron atoms despite the large distance between these atoms (2.57 ), with a low-lying B À B antibonding LUMO. Theoretical studies [8] on diradicaloids C revealed large singlettriplet gaps of + 23.4 to + 33.7 kcal mol À1 , while the population of the LUMO from distinct two-electron-in-two-orbital approaches was found to be 0.169 to 0.19 electrons, indicating weak but distinct diradical character. This was supported by examples of diradical-like reactivity, [9] including reactions with Me 3 SnH and BrCCl 3 . Other derivatives of C were later prepared, with the conclusion that such compounds have two structural isomers [10] -the aforementioned planar diradicaloids and butterfly-type structures containing a conventional boronÀboron s bond (D, Scheme 1)-which can interconvert with the appropriate selection of substituents. [10a] For several years, our group has been interested in the chemistry of low-valent boron species stabilised by strong Lewis bases. [11] Of particular interest are compounds with boronÀboron multiple bonds, [12] diborenes and diborynes, which are able to reductively cleave a wide range of polar [13] and non-polar [14] bonds. We recently used this methodology to develop a new route to covalent boron À phosphorus bonds, namely the hydrophosphination of diborenes and diborynes. [15] This work included the catalyst-free reaction of the diboryne B 2 (SIDep) 2 (E, SIDep = 1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene) with HPPh 2 to yield a hydro(phosphino)diborene (F, Scheme 1). We recognised herein a potential opportunity to prepare B 2 P 2 compounds, related to C and D by addition of two further valence electrons, via the activation of a phosphorus À phosphorus bond.
As a starting point, we again chose diboryne E due to the relatively low steric demand of the flanking carbenes compared to other derivatives. Treatment with the diphosphane P 2 Et 4 produced a gradual colour change from red to dark green. After 6 h, a new signal was observed at 33 ppm in the 11 B NMR spectrum in an approximate 1:1 ratio with unreacted starting material. This is indicative of diborene formation (the equivalent P-substituted boron atom in F displays a resonance at 38 ppm) and suggests that the expected 1,2-diphosphination across the BÀB bond had occurred to yield compound 1 (Figure 1).
A slightly broadened new signal at À44 ppm in the 31 P NMR spectrum alongside unreacted P 2 Et 4 (d = À33 ppm) was also consistent with this assignment. The mixture was left to stand at room temperature for a further 12 h. Rather than the expected complete conversion to 1, a further colour change to orange had occurred, accompanied by selective generation of a new set of NMR signals. While a broad signal in the 31 P NMR at À9.6 ppm suggested a significantly different phosphorus environment, a triplet resonance at À18.9 ppm ( 1 J BP = 125 Hz) in the 11 B NMR was more revealing, suggesting bonding between boron and two equivalent phosphorus atoms. Slow evaporation of a hexane solution of the compound provided red crystals suitable for X-ray diffraction, allowing confirmation of its identity as 1,3bis(alkylidene)-1,3-diborata-2,4-diphosphonio-cyclobutane 2 (Figure 1 b). The B 2 P 2 ring is an almost perfect square, and lies on a crystallographic inversion centre. The BÀP bonds are of equal length within experimental error (1.913 (2), 1.917 (2) ) and the internal angles at boron (P-B-P = 89.8(1)8) and phosphorus (B-P-B = 90.2(1)8) are right angles. The Nheterocyclic carbene units are also planar with respect to the central ring, and the short B À C bond of 1.468 (3) is in the range of a double bond rather than a dative interaction. Attempts to crystallise compound 1 eventually produced a single crystal of sufficient quality to confirm its connectivity by X-ray diffraction (see SI), but unfortunately the data are insufficient for discussion of structural parameters.
The B 2 P 2 ring in 2 has two electrons more than that in D, on account of the donor capability of the NHC units, which leads to a completely closed-shell system. Nevertheless, their structural similarities are striking. The distance between the two boron atoms is larger in 2, at 2.713(3) , indicating even less bonding interaction between these atoms than in D. In order to better understand the bonding in these B 2 P 2 rings, DFT calculations at the B3LYP-D3(BJ)/def2-SVP level of theory [16] were performed ( Figure 2). As expected from the influence of the NHC substituents, the HOMO (LUMO) of D correlates with the HOMOÀ1 (HOMO) of 2. The HOMOÀ1 of 2 describes a formally p-bonding orbital derived predominantly from the p orbitals of the boron atoms, with a small degree of overlap. A small amount of delocalisation to the carbon atoms of the NHCs distinguishes this orbital from the HOMO of D. The HOMO of 2 represents the mutually outof-phase B = C p bonds and is weakly p antibonding with respect to the two boron atoms. The population of both bonding and antibonding p(BB) orbitals results in a Mayer bond order (MBO) [17] for the BÀB interaction of < 0.05, in contrast to D, which shows a substantial degree of BÀB p bonding (MBO = 0.274). The MBOs of 1.600 and 1.592 for the B À C bonds of 2 clearly indicate significant double bond character, while the P À B bonds (MBO = 0.974-0.988) are single bonds.
In light of these insights into the electronic structure of 2, we anticipated that the compound should undergo facile oxidation. Indeed, cyclic voltammetry experiments revealed two distinct, seemingly reversible oxidation events at À0.76 and À1.45 V vs. Fc/Fc + (Figure 1 c). The first oxidation is attributed to a single-electron process leading to a radical cation, while the second oxidation would be expected to lead to a singlet diradicaloid of the form D or its corresponding closed-shell singlet butterfly-shaped counterpart. The apparent reversibility of these two redox processes is plausible as the B À B bond that is formed/broken is very weak and is accompanied by relatively minor structural changes (i.e. bending of the B 2 P 2 ring).
We subsequently attempted to oxidise compound 2 on a preparative scale. Treatment with an equimolar amount of the weak oxidising agent [Fe(h 5 -C 5 H 5 ) 2 ][PF 6 ] in C 6 D 6 solution led to a rapid colour change from orange to pink and the disappearance of all signals in the NMR spectra, suggesting the formation of a paramagnetic species ([3][PF 6 ], Figure 1). The EPR spectrum of [3][PF 6 ] (see Supporting Information) exhibits a broad resonance at g = 2.0025 which is dominated by a 1:2:1 triplet created by a large hyperfine coupling to the two phosphorus nuclei (a( 31 P) = 51 MHz). After several hours, red crystals formed in the solution, allowing the isolation of the radical cation [3][PF 6 ] and the confirmation of its structure by X-ray diffraction (Figure 1 b). The structure of the B 2 P 2 ring is slightly perturbed from that of the neutral precursor 2, with a shortened BÀB distance of 2.632(5) and slightly wider internal angles at boron (ca. 92.58). The most significant difference to the neutral compound can be observed in the bonding of the SIDep moieties. The B À C bonds (B1 À C1 = 1.528(4), B2 À C2 = 1.534(4) ) are 0.06-0.07 longer than those in 2, while the NHCs now display a slightly twisted orientation towards the central ring (torsion angles between N-C-N and P-B-P planes = 19.58 and 16.58). Both of these parameters indicate a reduction in BÀC p bonding consistent with the removal of an electron from the HOMO of 2. The shortening of the B À B distance can also be rationalised by the removal of electron density from this orbital, which is weakly B À B antibonding in character. We conducted DFT calculations of the cation [3] + C to further investigate this system. Its frontier orbitals are very similar to those in 2, with the singly-occupied SOMO representing the B À C p bond and the weakly repulsive through-space B À B interaction, while the HOMO represents the in-phase C-B-B-C p interaction. The MBO values are consistent with removal of an electron from the HOMO of 2, with a reduction in the BÀC bond order to 1.344/1.357 and a marginal increase of the BÀB bond order to 0.054. It should be noted that the combination of 2 with [CuCl(SMe 2 )] provided the analogous CuCl 2 salt of the radical cation,  (Figure 3 b) and revealed a butterfly-type structure. Compared to the monocation of [3][PF 6 ], the B À B distance had decreased to 2.115(3) , with an angle between the two BBP planes of 648. This B À B distance remains larger than those found for the butterfly-type derivatives of D (Scheme 1), in which BÀB distances as short as 1.8 have been observed. [10a] In addition, while all PÀB bond lengths of the central motif UV/Vis spectroscopy was performed on one example of each of the different charge states of the B 2 P 2 systems prepared herein, namely the neutral 2 (l max = 506 nm), the cationic [3][PF 6 ] (l max = 538 nm) and the dicationic [4][BAr F 4 ] 2 (l max = 639 nm). A modest redshift was observed between the neutral and monocation species (32 nm), with a more substantial redshift between the mono-and dications (101 nm). Assuming that these absorptions correspond to [BAr F 4 ] 2 ) or SOMO-LUMO transitions (for [3][PF 6 ]), these results are qualitatively in line with the initial removal of one electron from a filled orbital followed by removal of an unpaired electron, thus leaving a vacant orbital.
Natural bond orbital (NBO) [18] analyses on the naked butterfly-type dication [4] 2+ (Figure 3 c) and the corresponding system of Bertrand, D (R = tBu, R' = iPr, see Figure S12), showed that in both cases a bent [19] BÀB s bond (MBO = 0.472) is formed, which leads to closed-shell singlet systems. Steric effects related to the bulky substituents of [4] 2+ prevent the boron atoms from binding at a shorter distance, affecting the systems overall stability. Consequently, the fully optimised, planar diradicaloid isomer of [4] 2+ , herein labelled planar [4] 2+ , is merely 3 kcal mol À1 higher in energy than [4] 2+ at the DFT level. Moreover, in contrast to its butterfly-type isomer, multireference CASSCF [20] /NEVPT2 [21] calculations on planar [4] 2+ predicted a LUMO occupation of 0.41 electrons and a singlet-triplet gap of 13.3 kcal mol À1 , which explains the two-fold diradical character of planar [4] 2+ (y 0 = 0.12) in comparison to that of C (R = tBu, R' = iPr, y 0 = 0.06; Scheme 1). As the HOMO of planar [4] 2+ features the trans-annular p-bonding characteristic of planar derivatives of C, [10a] barrierless thermal ring closure/opening processes that interconvert [4] 2+ and planar [4] 2+ are expected, which might allow both isomers to coexist in solution. Future work in our group will focus on the investigation of this bond-stretch [10a] isomerism between butterfly-type and planar diradicaloid structures in novel Lewis base-stabilised B 2 P 2 rings.
In summary, we have developed a synthetic strategy to prepare the first NHC-stabilised cyclo-B 2 P 2 compounds by the addition of a phosphorus À phosphorus bond to a boron À boron triple bond of a diboryne. In contrast to demonstrated reactions of other apolar EÀE bonds to NHC-stabilised diborynes, [14] the 1,2-addition product of a diphosphine, namely diphosphinodiborene 1, rearranges to form a product with a B 2 P 2 heterocyclic core, 2. Compound 2 is prone to oxidation to either a radical cation or a dication, depending on the nature of the reactants and counterions. The L 2 B 2 P 2 core of compound 2 is highly planar, with oxidation processes leading to planarity loss and decreased boronÀboron distances.