Desymmetrization of Dicationic Diboranes by Isomerization Catalyzed by a Nucleophile

Abstract Cationic monoboranes exhibit a rich chemistry. By constrast, only a few cationic diboranes are known, that all are symmetrically substituted. In this work, the first unsymmetrically substituted dicationic diboranes, featuring sp2–sp2‐hybridized boron atoms, are reported. The compounds are formed by intramolecular rearrangement from preceding isomeric symmetrically substituted dicationic diboranes, a process that is catalyzed by nucleophiles. From the temperature‐dependence of the isomerization rate, activation parameters for this unprecedented rearrangement are derived. The difference in fluoride ion affinity between the two boron atoms and the bonding situation in these unique unsymmetrical dicationic diboranes are evaluated.


Introduction
Diboranes with sp 2 hybridization of the two boron atoms are valuable reagents in modern synthetic chemistry,and they are used in numerous borylation and diboration reactions. [1] TheL ewis acidity of diboranes with weak p-donor substituents is exceptionally high, and allows,f or example,f or spontaneous dihydrogen activation with the symmetrically substituted tetra(o-tolyl)-diborane. [2] In the diboranes commonly applied for synthesis (for example B 2 cat 2 or B 2 pin 2 , where cat denotes the catecholate and pin the pinacolate group), Lewis acidity is attenuated by p-donor substituents. [1] Unsymmetrically substituted diboranes feature polarized BÀBb onds and reveal enhanced reactivity towards various substrates.T hus,t he external addition of one equivalent of aL ewis basic co-reagent to symmetrical diboranes with sp 2hybridized boron atoms is usually required to activate diboranes for further reactions. [3,4] Fore xample,t he addition of achiral base allows enantioselective borylation reactions. [5] Base addition turns these compounds into nucleophiles, [4] especially if anionic activators such as alkoxides are used. [6] Neutral diboranes that are apriori unsymmetrical have also been reported, for example,p inBBMes 2 [7] (where Mes denotes the mesityl group).
Charge is another way to vary the Lewis acidity of monoboranes,a ss hown in boronium cations with sp 3hybridized boron atoms,b orenium cations with sp 2 -hybridized boron atoms,o re ven borinium cations with sp-hybridized boron atoms. [8] Generally,t he Lewis acidity increases with decreasing number of substituents (boronium < borenium < borinium). Consequently,a lso the rewarding synthesis of cationic diboranes has been achieved in recent years (see Figure 1). In compounds 1, [9] 2, [10] and 3, [11] the two boron atoms are sp 3 -hybridized. Compound 4, [12] isolated in small amounts,and compound 5 [13] are the only known examples of dicationic diboranes with sp 2 -hydridization of the two boron atoms. [14] Importantly,all hitherto known dicationic diboranes are symmetrically substituted, and lack the advantageous effect of apolarized BÀBbond. Unsymmetrically substituted dicationic diboranes remain unknown to date.
Herein we report the comprehensive characterization of unsymmetrically substituted dicationic diboranes and acomputational evaluation of their bonding situation. They are obtained by isomerization from initially formed symmetrically substituted diboranes.T he isomerization process is elucidated by spectroscopy and computation and broadens our understanding of an emerging class of synthetically useful reagents. Figure 1. Collection of known, symmetrically substituted,d icationic diboranes with sp 3 and sp 2 hybridizedb oron atoms. Dur = 2,3,5,6tetramethylphenyl.

Results and Discussion
In our experiments,t he diborane B 2 Cl 2 (NMe 2 ) 2 was reacted with three neutral guanidine donors L1-L3 (Figure 2) in the presence of ac hloride abstraction reagent (AlCl 3 , GaCl 3 or SiMe 3 OTf). While L3 is commercially available and L1 known from previous reports, [15,16] the synthesis for L2 had to be developed (see the Supporting Information for further details). Thec ompounds were sorted with respect to their oxidation potentials (L1 [15,16] < L2 (this work) < L3 [17] ) measured by cyclic voltammetry in CH 2 Cl 2 solutions (as shown in Figure 2). Inoue et al. used SbCl 5 as chloride abstraction reagent for the synthesis of 5 starting with diborane B 2 Cl 2 (NMe 2 ) 2 . [13] However,this abstraction reagent could not be applied in our reactions,asfirst tests showed that it oxidizes the electron rich ligands (for example, L2)t ot he dication instead of abstracting chloride from the diborane reagent.
We start the discussion with the results obtained with the strongest electron donor, L1.F irst, we tested reactions with the non-oxidizing chloride abstraction reagents GaCl 3 and AlCl 3 .H owever, reaction of L1 with two equivalents of B 2 Cl 2 (NMe 2 ) 2 and four equivalents of GaCl 3 yielded the salt [(L1)(GaCl 2 ) 2 ](GaCl 4 ) 2 ( Figure 3), which crystallized from the reaction mixture and was isolated with ay ield of 89 %. Hence,i nstead of chloride abstraction from the diborane, GaCl 3 underwent self-ionization to give GaCl 2 + stabilized by L1.T he use of an excess of GaCl 3 also led not to the desired product and as imilar pathway was observed with AlCl 3 .
Next, we tried Me 3 SiOTf as chloride abstraction reagent. To our delight, this reaction gave the tetracationic bis-(diborane) P1 in ayield of 80 %. Thestructure is in line with sp 2 -hybridization of all boron atoms in P1.T he two B À B bonds are 1.688(8) and 1.700(6) long,w hich are in the typical range for B À Bs ingle bonds. [18] Noticeable is the distortion of the aromatic backbone between the C2/C3/C4 and C5/C6/C1 plane of 12.78 8 ( Figure 4). We suggest the reason for the distortion to be the steric demand of the NMe 2 groups in combination with the presence of one distorted sixmembered ring on both sides of the aromatic backbone. Theoxidation of ligand L1 can be excluded owing to the bond lengths in the central C 6 ring and the absence of the characteristic deep green color of L1 2+ .N oteworthily,b isdiborane P1 turned out as very robust. Heating asolution of P1(OTf) 4 in CH 3 CN to at emperature of 80 8 8Cf or 24 ha lso did not lead to traceable changes.
These changes indicate that in both cases the positive charge is delocalized into the guanidino groups.
Notably,all of the B À Nbonds are significantly shorter in P2 isomer with two sp 2 -hybridized boron atoms that could establish p-interactions compared to P3 with two sp 3 -hybridized boron atoms.
Interestingly, 1 HNMR spectroscopic analysis of the reaction product indicated am ixture of the two isomeric dicationic diboranes P4 and P4 isomer (Figure 7) in ar atio of 43:57.
Thei somerization from P4 to P4 isomer proceeded at room temperature in solution. Thus,u nder the given conditions,i t was not possible to obtain P4 in pure form. However, both isomers crystallized from aCH 3 CN/Et 2 Osolution at À40 8 8Cin the form of cube-shaped crystals.C areful crystal picking enabled SCXRD analysis of both isomers (Figure 7). Some structural data of P4 to P4 isomer are compared in Table 1. The B À Bb ond is slightly longer in P4 isomer ,b ut both B À Bb ond lengths are in ar egion typical for B À Bs ingle bonds. [18] Both compounds display long N1ÀC7 and N4ÀC12 bond distances (compare with free L3;these bond distances measure 1.291 (3) and 1.301(3) [20] ), indicating charge delocalization into the  . Illustration of the structures of the dicationic diboranes P2 isomer and P3 (ellipsoids set at 50 %probability).A ll hydrogen atoms and the GaCl 4 À counterions are omitted. [26] Figure 7. Top: Reaction leading to the isomers P4 and P4 isomer .F rom the experiments we propose P4 to be the kinetic and P4 isomer the thermodynamic product. Bottom:S tructures of the two isomers P4 and P4 isomer in the solid state (ellipsoids set at 50 %probability).A ll hydrogen atoms and the GaCl 4 À counterions are omitted. [26] guanidino groups.P ure P4 was finally obtained as TfO À salt by reacting B 2 Cl 2 (NMe 2 ) 2 with L3 at À30 8 8Cinthe presence of MeSiOTf in o-difluorobenzene solution. Thes uspension was warmed to room temperature and stirred for additional 2h.
Thea ddition of catalytic amounts of KF in the presence of [18]-crown-6 accelerates the P 4 ! P 4isomer isomerization drastically.I n our experiments (see the Supporting Information for details), the process was completed in less than 15 min at 22 8 8C, whereas it took 2h (t 1/2 = 44 min, 25 8 8C) in the absence of KF (for the dications with TfO À counterions). Hence the isomerization is catalyzed by nucleophiles.T o obtain more information about the effect of nucleophiles,w er epeated the isomerization experiments with P4(GaCl 4 ) 2 in CD 3 CN at different temperatures (T = 293.2, 313.6, and 324.4 K). To our surprise,t he best fit of the experimental data was obtained now by assuming az eroorder rate law (see the Supporting Information), in contrast to the first-order rate constants derived for the isomerization of P4(TfO) 2 .However,the rate decrease upon increase of the concentration argues for amore complex mechanism (see the Supporting Information). An activation enthalpy of DH°= 67.09 AE 4.36 kJ mol À1 and an activation entropy of DS°= À101.3 AE 10.9 Jm ol À1 K À1 were derived from an Eyring plot of the zero order rate constants (see the Supporting Information for details). Hence the activation enthalpy was higher, but the activation entropy lower than for isomerization of P4(TfO) 2 ,r esulting in ar elatively small change of the activation free energy from DG°2 98K = 93.48 AE 0.17 kJ mol À1 for P4(TfO) 2 to 97.86 AE  (5) ---- 0.20 kJ mol À1 for P4(GaCl 4 ) 2 .T he change from first-order reaction for P4(TfO) 2 to zero-order for P4(GaCl 4 ) 2 as well as the significant negative entropy of activation in both cases cannot be understood by athermal (nucleophile-free) isomerization process.F urthermore,t he calculated transition state energy for the purely thermal intramolecular isomerization amounts to am uch higher value of DG°2 98K= 181 kJ mol À1 (Supporting Information, Figure S24). One possible explanation might be the catalysis by Cl À ions generated in small quantities in ar ate-determining pre-equilibrium reaction from GaCl 4 À .Arelated phenomenon was reported for the iodination of acetone,where the enol form as reactive species is generated in small amounts in the rate-controlling step from the unreactive keto form. [23] Herein, the reaction follows ap seudo zero-order kinetic on the iodine.S ince the isomerization process even took place when as uspension of P4(GaCl 4 ) 2 in CH 2 Cl 2 was stirred for 4d(see the Supporting Information), we could exclude as ignificant role of the solvent CD 3 CN as nucleophile.
Next, the experimental data was backed-up by further quantum chemical calculations.Structure optimizations at the B3LYP + D3/def2-TZVP level of theory reproduced the structure of both isomers very well (see the Supporting Information). Theu nsymmetrical isomer was preferred over the symmetric one (DG 298K (P1 isomer ÀP1) = À105 kJ mol À1 (rearrangement of both diborane units), DG 298K (P2 isomer ÀP2) = À43 kJ mol À1 , DG 298K (P4 isomer ÀP4) = À49 kJ mol À1 ;F igure 9). Calculations with inclusion of solvation (COSMO with e r = 37.5) found adifference in the Gibbs free energy of DG 298K = À60 kJ mol À1 in favor of P4 isomer .Hence the symmetric isomers are the kinetic and the unsymmetrical isomers the thermodynamic reaction products.Bycontrast, in the case of compound 5 synthesized by Inoue et al. (see Figure 1), the symmetric isomer is preferred by À11 kJ mol À1 (see the Supporting Information).
Theb arrier for isomerization appears to depend on the electron-donor capacity of the guanidine substituent. The computed transition state for the isomerization of P4! P4 isomer revealed, that the migration of ag uanidino group causes the barrier, but not the subsequent migration of the À NMe 2 unit. This situation should persist likewise for the case of an ucleophile catalyzed isomerization. With the relatively weak electron donor L 3 ,t he barrier is below 100 kJ mol À1 . With L2,t he barrier should be smaller, since the unsymmetrical product P2 isomer is formed immediately,and we were not able to detect its symmetric isomer P2.Although L1 is the strongest electron donor (the compound with the lowest oxidation potential), only the symmetric isomer P1 is formed, and not the unsymmetrical P1 isomer .A tf irst glance this observation contradicts the prediction that the barrier lowers with increasing electron donor character of the guanidine substituents.H owever,t he presence of the second dicationic diborane unit strongly reduces the electron-donor character of L1 (the electron donor capacity of [(B 2 (NMe 2 ) 2 )L1] 2+ is certainly lower than that of L2 or L3).
Thee xperimental 11 BNMR chemical spectra of P4 isomer (d = 37.7 and 30.7 ppm) featured the presence of two substantially different boron atoms,a nd thus of ap olarized B À Bb ond. To understand the electronic structure and bonding situation of P4 isomer in more depth, further quantum chemical calculations and bond analysis tools were carried out. Theb ond polarization in P4 isomer was inspected first by NBO charge analysis.Indeed, the boron atom coordinated to the guanidines is substantially more positive (+ 0.84) than the boron atom in the B(NMe 2 ) 2 unit (+ 0.58). In agreement to that, the fluoride ion affinity of the guanidino substituted boron center is by 130 kJ mol À1 higher as that of the B(NMe 2 ) 2 unit. This unbalanced charge distribution can be rationalized by the stronger p-donor properties of the NMe 2 units in comparison to the positively charged guanidino functionalities (see below). Thei nfluence of the additional amino groups in P2 isomer can also be unraveled by NBO charge analysis.T he boron atom coordinated to the guanidines becomes less positive (+ 0.72) than in P4 isomer (+ 0.84) owing to the better p-donor capability of the ligand. Theb oron in the B(NMe 2 ) 2 unit remains almost unchanged (+ 0.60). Indeed, the weaker polarization of the B À Bb ond within P2 isomer is in line with the decreasing 11 BNMR shift separation (D(d 11 B) = 3.0 ppm) in comparison to P4 isomer (D(d 11 B) = 7.0 ppm) and exemplifies the unique handle to control bond ionicity by alteration of substituents at the bisguanidinium donor entity.
In contrast, for the unpolarized TDADB,t he homolytic bond cleavage is clearly the favored process.According to the IUPAC definition, this would imply at ype of dative bond interaction between the two boron atoms in P4 isomer . Figure 9. Gibbs free energy change (from calculations, B3LYP + D3/ def2-TZVP;C OSMO e r = 37.5) and activation Gibbs free energy (from NMR experimentsa tvariable temperature for the dications with TfO À counterions in CH 3 CN solution) for the TfO À catalyzed P 4 !P 4isomer isomerization.
To unravel the question of dative vs.c ovalent BÀB bonding further, the electron density was inspected by QTAIM. Bond descriptors for the B À Bb onds revealed similar and predominantly covalent bond characteristics for both compounds,w ith the BÀBb ond in P4 isomer being more polarized/ionic in comparison to the symmetrical TDADB (Supporting Information, Table S5). Finally,e nergy decomposition analysis between the fragments of homolytic and heterolytic B À Bbond cleavage was performed. Several EDA studies suggested that the fragmentation that corresponds to the best description of the bonding situation (dative/heterolytic or covalent/homolytic) is the one with the smallest orbital interaction energy term DE orb ,a si tr equires the smallest change in electronic charge distribution to conform to the electronic structure of the molecule. [24] Then umerical results of the EDA( Supporting Information, Table S6) revealed that for both P4 isomer and TDADB,t he electronsharing bond is the more realistic representation. In turn, from the coulombic attraction energy it can be concluded, that the coulombic repulsion between the two cationic fragments after heterolytic bond cleavage rationalizes the strongly favored heterolytic dissociation in P4 isomer .Moreover, EDAr eveals as ignificant amount of dispersion interaction that further stabilizes the BÀBb ond in P4 isomer Importantly, none of the given results support adative BÀBbonding within P4 isomer that is pretended by the trend in homolytic vs. heterolytic bond dissociation enthalpies.
To conclude,from the various possible Lewis representations assembled in Figure 11, the most realistic ones are those in the box. Owing to the larger electron-donor capability of L2,t he structure in which all positive charge is accumulated on the bisguanidine unit is more important for P2 isomer than for P4 isomer .
Hence the degree of B À Bb ond polarization is efficiently tunable by the electron-donor character of the bisguanidine substituent at hand in our group. [25] It will be interesting to see whether this picture is also reflected in the reactivity of those compounds.I nafirst reactivity test, we were able to convert P4 isomer with KF in the presence of [18]-crown-6 to the suggested (for ad iscussion, see the Supporting Information) monocationic diborane P4 isomer F1 with am ixed sp 3 -sp 2 hybridization ( Figure 12). The 11 BNMR spectra displays two remarkable different signals at 36.06 and 7.42 ppm.

Conclusion
We have reported the first unsymmetrically substituted dicationic diboranes.Reactions between electron-rich bisguanidines and B 2 Cl 2 (NMe 2 ) 2 in the presence of chloride abstraction reagents first led to symmetrically substituted dicationic diboranes,w hich undergo nucleophilic catalyzed isomerization to the energetically preferred unsymmetrical diborane isomers.Acomparison between unsymmetrical diboranes with different bisguanidine substitutents indicates that the BÀBb ond polarization could be tuned by the electron-donor character of the bisguanidine.E xperiments and theoretical analysis of the fluoride ion affinity discloses significant differences between the two boron atoms.T o unleash the full potential of this new dicationic unsymmetrically substituted diboranes,adetailed analysis of the reactivity is under current investigations. Figure 10. Comparison betweeen heterolytic and homolytic BÀBb ond cleavage of P4 isomer .T he differenceint he fragmentation energy of the relaxed fragments is DE = À295 kJ mol À1 for the heterolytic bond cleavage. Figure 11. Possible resonancestructures for P4 isomer .A ccordingt oour quantum-chemical bond analysis, the structures in the box should be of higher relevance. Figure 12. Reaction of P4 isomer with KF in the presence of 18-crown-6 as suggested by NMR spectroscopy,H RESI mass spetrometry,and quantum chemical calculations (see the Supporting Information).