In Search of the [PhB( μ -N t Bu) 2 ] 2 As • Radical: Experimental and Computational Investigations of the Redox Chemistry of Group 15 Bis-boraamidinates

DFT calculations for the group 15 radicals [PhB( μ -N t Bu) 2 ] 2 M • (M = P, As, Sb, Bi) predict a pnictogen-centered SOMO with smaller contributions to the unpaired spin density arising from the nitrogen and boron atoms. The reactions of Li 2 [PhB( μ -NR) 2 ] (R = t Bu, Dipp) with PCl 3 afforded the unsolvated complex LiP[PhB( μ -N t Bu) 2 ] 2 ( 1a ) in low yield and ClP[PhB( μ -NDipp) 2 ] ( 2 ) both of which were structurally characterized. Efforts to produce the arsenic-centered neutral radical, [PhB( μ -N t Bu) 2 ] 2 As • , via oxidation of LiAs[PhB( μ -N t Bu) 2 ] 2 with one-half equivalent of SO 2 Cl 2 yielded the Zwitterionic compound, [PhB( μ -N t Bu) 2 As( μ -N t Bu) 2 B(Cl)Ph] ( 3 ), containing one four-coordinate boron center with a B-Cl bond. The reaction of 3 with GaCl 3 produced the ion-separated salt, [PhB( μ -N t Bu) 2 ] 2 As + GaCl 4-( 4 ), which was characterized by X-ray crystallography. The reduction of 3 with sodium naphthalenide occurred by a two-electron process to give the corresponding anion [{PhB( μ -N t Bu) 2 } 2 As] - as the sodium salt. Voltammetric investigations of 4 and LiAs[PhB( μ -N t Bu) 2 ] 2 ( 1b ) revealed irreversible processes. Attempts to generate the neutral radical [PhB( μ -N t Bu) 2 ] 2 As • from these ionic complexes via in situ electrolysis did not produce an EPR-active species.


Introduction
The dianionic boraamidinate (bam) ligand, [RB(NR')2] 2-, is formally isoelectronic with the extensively studied monoanionic amidinate (am) ligand [RC(NR')2] -.Recent work on early main group and transition-metal complexes has established that the 2-charge on the bam ligand results in some interesting fundamental differences compared to the behavior of the monoanionic am ligand.
The 2-charge lowers the requirement for ancillary ligands around high oxidation-state metal centers.
2][3] Perhaps the most intriguing consequence of the 2-charge is the facile tendency for redox transformations to occur in which the the [bam] 2-dianion is oxidized to the corresponding monoanion radical [bam] -• .4][5] In the case of certain Group 13 metals it has been possible to isolate, stable neutral radicals, for example the highly colored spirocyclic systems {M[PhB(μ-N t Bu)2]2} • (M = Al, Ga) and determine their X-ray structures. 4The SOMO in these radicals is located primarily and equally in p-orbitals on the four nitrogen atoms of the two bam ligands; there is very little unpaired electron density on the Group 13 centers.The corresponding boron and indium-containing radicals {M[PhB(μ-N t Bu)2]2} • (M = B, In) were characterized in solution by EPR spectroscopy. 5However, the indium radical is not sufficiently stable to be isolated in the solid state, while the boron analogue, although it is thermally very stable, could not be obtained in a pure form.
In a recent investigation we reported the first examples of heavy Group 15 boraamidinates   in which the versatile coordinating ability of the bam ligand was highlighted. 6In addition to the mono-bams ClM[PhB(μ-N t Bu)2] (A; M = As, Sb, Bi) with a potentially useful M-Cl reactive site, the unusual 2:3 bam complexes, M2[PhB(μ-N t Bu)2]3 (B; M = Sb, Bi), displayed a unique bonding arrangement in which two metal centers are each N,N′-chelated by one bam ligand and the two 4 [M(bam)] + units are bridged by the third [bam] 2-ligand.For the 1:2 Group 15 bam systems, both solvated complexes (Et2O)LiM[PhB(μ-N t Bu)2]2 (C; M = As, Bi) and unsolvated ladder structures LiM[PhB(μ-N t Bu)2]2 (D; M = As, Sb) were observed; these complexes showed interesting fluxional behavior in solution (Berry pseudorotation) in the case of M = Sb, Bi. 6 Although monofunctional phosphorus-containing bam complexes of the type XP[PhB(μ-N t Bu)2] (X = Cl, Br; type A) are known, 7 a bis-bam complex LiP[PhB(μ-N t Bu)2]2 (type C or D) has not been reported.
In the current study we were primarily interested in radicals of the type {M[PhB(μ-N t Bu)2]2} • (M = Group 15 element) in order to determine the effect of the additional pair of electrons on the molecular and electronic structures of the Group 15 systems in comparison with those of the well-characterized Group 13 analogs (M = Al, Ga). 4,5In this context we report (a) the results of DFT calculations of the molecular structures and EPR parameters of the pnictogen-centred radicals 7 [PhB(μ-NR)2]2M • (M = P, As, Sb, Bi; R = Me, t Bu) (b) the synthesis and X-ray structures of LiP[PhB(μ-N t Bu)2]2 (1a) and ClP[PhB(μ-NDipp)2]2 (2) (c) the synthesis and X-ray structures of the Zwitterionic complex [PhB(μ-N t Bu)2As(μ-N t Bu)2B(Cl)Ph] (3) and the ion-separated salt [PhB(μ-5 N t Bu)2]2As + GaCl4 -(4) (d) the chemical reduction of 3 and the cation [PhB(μ-N t Bu)2]2As + , and (e) voltammetric studies of the bis(boraamidinate)arsenic system.

Experimental Section
General Procedures.All reactions and the manipulations of products were performed under an argon atmosphere by using standard Schlenk techniques or an inert atmosphere glove box.The compounds PhBCl2 (Aldrich, 97%), GaCl3 (Aldrich, 99.99%), AsCl3 (Alfa, 99%), [ n Bu4N]Br3 (Aldrich, 98%) and NO[SbF6] (Aldrich, 99.9%) were used as received.PCl3 (Aldrich, 99%) was distilled prior to use.LiN(H) t Bu was prepared by the addition of n BuLi to a solution of anhydrous t BuNH2 in n-hexane at -10 o C and its purity was checked by 1 H NMR spectroscopy.The compounds 6 (1b) were prepared as described earlier.The solvents n-hexane, toluene, Et2O and THF were dried by distillation over Na/benzophenone, and the solvents dichloromethane and acetonitrile were distilled from CaH2 under an argon atmosphere prior to use.Electrochemical grade tetrabutylammonium hexafluorophosphate [ n Bu4N][PF6] (Fluka) was used as the supporting electrolyte and was kept in a dessicator prior to use.Ferrocene, prepared following the procedure described by Jolly, 9 was sublimed prior to use.Elemental analyses were performed by Analytical Services, Department of Chemistry, University of Calgary and Canadian Microanalytical Service Ltd., Vancouver, British Columbia.Spectroscopic Methods.The 1 H, 7 Li, 11 B, 13 C, 31 P and 71 Ga NMR spectra were obtained in CD2Cl2, d8-THF or d8-toluene at 23 ºC on a Bruker DRX 400 spectrometer operating at 399.59, 155.30, 128.20, 100.49, 161.77 and 121.87 MHz, respectively. 1H and 13 C spectra are referenced to the solvent signal and the chemical shifts are reported relative to (CH3)4Si. 7Li and 11  software.The cell design consisted of a conventional three electrode set-up with a 3.0 mm diameter glassy-carbon (GC), a 1.6 mm diameter platinum (Pt), or a 1.6 mm diameter gold (Au) working electrode, a Pt wire auxiliary and a silver wire quasi-reference electrode.The reference electrode was separated from the bulk solution by a fine-porosity frit.CVs were obtained over scan rates of 0.1 -20 V s -1 .All potentials are reported vs. the operative formal potential, / 0 Fc /0  E , for the Fc + /Fc redox couple (Fc = ferrocene), which was used as an internal standard.The electrodes were polished with 0.05 micron alumina on a clean polishing cloth (Buehler, USA), rinsed with distilled water, and dried with tissue paper prior to use.The solution was purged with dry nitrogen for 10 min directly before use, and a blanket of nitrogen gas covered the solution during all experiments.The cell design for all Simultaneous Electrochemical Electron Paramagnetic Resonance (SEEPR) experiments has been described previously. 10This cell was placed inside a Bruker EMX 113 spectrometer operating at X-band frequencies (9.8 GHz) at 18 ± 2 °C.

X-ray Crystallography. Crystals of LiP
[PhB(μ-N t Bu)2As(μ-N t Bu)2B(Cl)Ph] (3) and [PhB(μ-N t Bu)2]2As(GaCl4) (4) were coated with Paratone 8277 oil and mounted on a glass fiber.Diffraction data were collected on a Nonius KappaCCD diffractometer using monochromated MoK radiation ( = 0.71073 Å) at -100 °C.The data sets were corrected for Lorentz and polarization effects, and empirical absorption correction hydrogen atoms with anisotropic thermal parameters, the hydrogen atoms were placed in calculated positions [C-H = 0.98 Å for C(CH3)3 and 0.95 Å for phenyl hydrogens].The isotropic thermal parameters of the hydrogen atoms were fixed at 1.2 times to that of the corresponding carbon for phenyl hydrogens, and 1.5 times for C(CH3)3.In the final refinement the hydrogen atoms were riding on their respective carbon atoms.
Similarly to the structures of LiM[PhB(-N t Bu)2]2 (M = As, Sb), 6 the lithium and phosphorus atom positions in the structure of 1a were disordered with the atoms distributed over the two atomic sites.The two atoms were not constrained to locate in the same position, but the anisotropic thermal parameters were restricted to be equal.The site occupation factors were fixed to be 50% for these atoms.The crystal structure of 4 exhibits two discrete ion pairs in which the second GaCl4 -anion shows disorder in the chlorine atom positions.In the final refinement, the site occupation factors for these atoms were approximately 80/20 %.The scattering factors for the neutral atoms were those incorporated with the programs.Crystallographic data are summarized in Table 1.
Computational Details.The geometries of all studied compounds were optimized by using DFT.The calculations utilized both the hybrid PBE0 exchange-correlation functional for Me substituted systems as well as its non-hybrid GGA variant PBEPBE for t Bu substituted systems. 13lrichs' triple-zeta valence basis sets augmented by one set of polarization functions (TZVP) were employed in all calculations. 14For heavy atoms antimony and bismuth, the corresponding ECP basis sets were used. 14,15 ppropriate molecular point groups and the resolution-of-the-identity approximation were used to improve the efficiency of the calculations.Geometry optimizations were done with Turbomole 5.9.1 16 (R = t Bu) and Gaussian 03 17 (R = Me) program packages.
Hyperfine coupling constants were calculated for all paramagnetic systems in their geometry-optimized structures by using both a non-relativistic and a scalar relativistic (ZORA) approach 18 within the unrestricted Kohn-Sham formalism.The non-relativistic calculations utilized the same basis sets as the geometry optimizations, but only the hybrid version of the PBEPBE functional was employed.However, for the heavier nuclei antimony and bismuth, the use of ECP basis sets prevents the direct determination of hyperfine couplings using the same method.In addition, relativistic calculations are essential in order to obtain more than a qualitative accuracy for systems containing heavy nuclei.Thus, relativistic calculations for systems with Sb and Bi atoms were carried out.The calculations utilized the PBEPBE GGA functional 13 together with the large TZ2P STO-type basis sets. 19The hyperfine coupling constant calculations were done with the Gaussian 03 17 (non-relativistic) and ADF 2007.01 20 program packages (relativistic).The values reported in Table 2 are non-relativistic for the lighter nuclei and scalar-relativistic for the heavier atoms antimony and bismuth.

Synthesis of ClP[PhB(μ-NDipp)2]2 (2).
A solution of Li2[PhB(μ-NDipp)2] (0.726 g, 1.60 mmol) in Et2O (30 mL) was added to a solution of PCl3 (0.14 mL, 1.60 mmol) in Et2O (10 mL) at -80C.The reaction mixture was stirred for ½ h at -80 o C and 18 h at 23 o C. The volatiles were removed in vacuo and the product was then extracted with n-hexane.The precipitate of LiCl was removed by filtration and the solvent was evaporated under vacuum yielding an oily product that solidified to give a pale yellow powder of 2 after 24 h at 23 o C (0.648 g, 1.30 mmol, 80%).Anal.

Synthesis of [PhB(μ-N t Bu)2As(μ-N t Bu)2B(Cl)Ph] (3).
A solution of 1b (0.271 g, 0.50 mmol) in diethyl ether (25 mL) was cooled to -80 o C and a solution of SO2Cl2 (0.040 mL, 0.067 g, 0.50 mmol) in diethyl ether (1.0 mL) was added via syringe.The reaction mixture was stirred for ½ h at -80 o C and ca. 4 h at room temperature.LiCl was removed by filtration and the solvent was evaporated under vacuum to give 3 as a white solid (0.243 g, 85%). 1  Synthesis of [PhB(μ-N t Bu)2]2As + GaCl4 -(4).A solution of 3 (0.285 g, 0.50 mmol) in diethyl ether (20 mL) was cooled to -80 o C and a solution of GaCl3 (0.088 g, 0.50 mmol) in diethyl ether (10 mL) was added via cannula.The reaction mixture was stirred for ½ h at -80 o C and ca. 2 h at room temperature.The precipitate was allowed to settle and the solvent was decanted via cannula.
The product was then washed with Et2O (2 x 20 mL) and dried under vacuum giving 4 as a white, spectroscopically pure powder (0.306 g, 82%). 1  primarily on the Group 15 center (Figure 1a).For the bismuth-containing bis-bam radical, the calculations predict a highly distorted C1 symmetric structure with one long (2.51 Å) and one short (2.16 Å) Bi-N bond as well as a SOMO which is mainly located on the nitrogen atoms of one of the two chelating bam-ligands (Figure 1b).As a consequence, large (several hundred Gauss) hyperfine coupling constants for the Group 15 center are expected in the EPR spectra of the pnictogencentered radicals [PhB(μ-NMe)2]2M • (M = P, As and Sb). 7In contrast, the EPR spectra of the analogous bismuth species should display at least ten orders of magnitude smaller Bi coupling.
Interestingly, calculations done for the more realistic N t Bu substituted derivatives gave a C2symmetric trigonal bipyramidal geometry also for the bismuth species.Hence, the N t Bu-substituted bismuth species is also predicted to be a pnictogen-centered radical.This change in geometry most likely originates from a combination of steric reasons (i.e.due to increased steric bulk of the N t Bu substituents) and electronic factors (i.e. a slight change in MO characteristics due to the difference in N-substituent).The optimized geometries of all of the lighter Group 15 congeners are in good agreement with the structures obtained for the NMe derivatives and show only small changes in their calculated hyperfine coupling constants (see Table 2   , the reaction between PCl3 and Li2[PhB(μ-N t Bu)2] was carried out in 1:2 molar ratio. 6The new phosphorus-containing complex LiP[PhB(μ-N t Bu)2]2 (1a) was isolated in 25% yield (Scheme 1) and characterized in solution by multinuclear NMR spectroscopy and in the solid state by a single crystal X-ray structure determination.

Scheme 1.
The 1 H NMR spectrum of 1a at room temperature in toluene shows a doublet and two singlets in a 1:2:1 intensity ratio for the t Bu groups and a multiplet for phenyl hydrogens in the expected intensity ratios.The 7 Li, 11 B and 31 P NMR spectra exhibit singlets at 0.94, 34.9 and 87.1 ppm, respectively.The NMR spectroscopic data for 1a resemble those of the corresponding arsenic complex LiAs[PhB(μ-N t Bu)2]2 (1b), 6 suggesting a similar ladder structure for 1a.
The single crystal X-ray crystallographic analysis confirmed the isostructural nature of 1a and the heavier Group 15 analogs LiM[PhB(μ-N t Bu)2]2 (M = As, Sb). 6 As illustrated in Figure 2a, the molecular structure of 1a is comprised of two four-membered rings, BN2Li and BN2P, connected by Li-N and P-N bonds to form a tricyclic compound with both three and four-coordinate nitrogens.
The B-N bond lengths in 1a show a slight inequality of ca.0.04 Å (Table 3).Expectedly, the P-N bond to three-coordinate nitrogen is ca.0.12 Å shorter than those involving the four-coordinate nitrogens.The central four-membered PN2Li ring in 1a is non-planar with mean torsion angles involving the four atoms of ca.6.5º.The three-coordinate boron and nitrogen atoms in 1a show a cis arrangement with respect to the central four-membered ring similar to that observed in LiM[PhB(μ-N t Bu)2]2 (M = As, Sb). 6 The oxidation of 1a with SO2Cl2 in both 2:1 and 1:1 molar ratios in diethyl ether was investigated.The attempted one-electron oxidation did not generate an EPR-active species, and both reactions resulted in multiple products.The 11 B NMR spectra of the reaction mixtures showed the presence of both three and four-coordinate boron centers, cf.formation of 3 described below.The 31 P NMR spectra revealed two main products with singlets at 6.0 and -69.9 ppm, comprising ca.70% of the phosphorus-containing compounds; however, no tractable products were isolated from either reaction.
With a view to generating the sterically protected phosphorus-centered radical [PhB(μ-NDipp)2]2P • , the reaction between PCl3 and Li2[PhB(μ-NDipp)2] was conducted in a 1:2 molar ratio.13 However, the 31 P NMR spectrum revealed a complex mixture of products.Consequently, we decided to attempt installation of the two bam ligands on the phosphorus center in a stepwise fashion, a process that had previously been found to be necessary for the successful synthesis of the bis-bam aluminum complex (Et2O•Li)Al[PhB(μ-N t Bu)2]2. 4The reaction between PCl3 and Li2[PhB(μ-NDipp)2] in a 1:1 molar ratio produced an 80 % yield of ClP[PhB(μ-NDipp)2] (2) (Scheme 1), which was characterized in solution by multinuclear NMR spectroscopy and in the solid state by a single crystal X-ray structure determination.Subsequent reaction between 2 and a second equivalent of Li2[PhB(μ-NDipp)2], however, also failed to generate the bis-bam complex, The 1 H NMR spectrum of 2 shows the expected resonances for the phenyl and Dipp groups.
The 11 B NMR spectrum exhibits a singlet at 32.8 ppm and a single resonance is observed at 189.8 ppm in the 31 P NMR spectrum, cf.δ ( 31 P) 180 for ClP[PhB(μ-N t Bu)2].8a The crystal structure of 2 (Figure 2b) confirms the isostructural relationship with the other structurally characterized Group 15 mono-bams, XM[PhB(μ-N t Bu)2] (X = Br, M = P;X = Cl, M = As, Sb, Bi). 6, 8a In common with the lighter Group 15 mono-bams, XM[PhB(μ-N t Bu)2] (X = Br, M = P; X = Cl, M = As), 2 does not exhibit the significant intermolecular M•••Cl close contacts that were observed for the heavier Group 15 congeners (X = Cl, M = Sb, Bi). 6 The P-N bonds in 2 (Table 3) are somewhat elongated (by ca.0.03 Å) compared to those in the tert-butyl derivative BrP[PhB(μ-N t Bu)2], 8a whereas the B-N bond lengths in 2 are in the typical range for the bam ligand. 1 The calculated P-Cl bond order in 2 (0.97) 22 is somewhat higher than the value of ca.0.83 observed for P-Br, As-Cl and Sb-Cl bonds in XM[PhB(μ-N t Bu)2] (X = Br, M = P; X = Cl, M = As, Sb). 6  suggesting the absence of a radical species.A multinuclear NMR spectroscopic analysis ( 1 H, 7 Li, 11 B, 13 C) of the reaction mixture showed the presence of some unreacted 1b, as well as a product that exhibits three N t Bu resonances with relative intensities 2:1:1 ( 1 H, 13 C), two different environments for boron centers ( 11 B) and no lithium atoms ( 7 Li).Specifically, singlets were observed at 37.3 and 7.6 ppm in the 11 B NMR spectrum, suggesting the presence of both threecoordinate and four-coordinate boron atoms, respectively.In the light of these NMR data, the reaction of SO2Cl2 with 1b was carried out in a 1:1 molar ratio and the product [PhB(μ-N t Bu)2As(μ-N t Bu)2B(Cl)Ph] (3) was isolated in 85% yield (eq. 1), and identified by an X-ray crystallographic analysis.
The crystal structure of 3 with the atomic numbering scheme is depicted in Figure 3a, and the pertinent bond parameters are summarized in Table 4.The structure consists of a spirocyclic arsenic complex in which the arsenic center is N,N′-chelated by a bam ligand and a chlorinated bam ligand with B-Cl bond.The solid-state structure is consistent with the observation of both three-and four-coordinate boron centers and three t Bu environments (bonded to nitrogens N1, N2 and N3/N4, respectively) in the solution NMR spectra.The B-Cl bond formation in the three atom NBN backbone is a novel feature in the chemistry of boraamidinates. 1 A similar arrangement for fourcoordinate boron can be found, however, in amidinate complexes of the Ph(Cl)B unit. 25,26  B-N bonds in the chlorinated bam ligand in 3 are ca.0.12 Å longer than those in the non-chlorinated ligand owing to the influence of the four-coordinate boron center in the former.
Concomitantly, the As-N bonds involving the chlorinated bam ligand are ca.0.07 Å shorter than those in the bam ligand.The As-N bond lengths to the bam ligand are comparable to those observed in the mono-bam, [PhB(μ-N t Bu)2]AsCl (1.839(5) and 1.846(5) Å), 6 while the corresponding bonds to the chlorinated ligand are the shortest reported for the arsenic bam complexes thus far; the As-N bond involving the three-coordinate nitrogen in 1b has a bond length of 1.789(3) Å. 6 These bond lengths are indicative of a Zwitterionic compound with the positive charge located mainly on arsenic and the negative charge on the four-coordinate boron center.The B-Cl bond length of 1.953(4) Å in 3 is significantly longer than those observed in amidinate complexes of the Ph(Cl)B unit (1.847(4)-1.882(6)Å), while the elongated B-N bond distances in 3 are comparable to the corresponding B-N bonds in the amidinate complexes. 25,26 he lengthened B-N bonds in 3 are, however, among the longest observed in the three atom NBN backbone of the bam ligand; similar bond lengths have been reported only when four-coordinate nitrogen atoms are involved. 1ongation of the B-N bonds and contraction of the As-N bonds results in a wider N-As-N angle (ca.5.5º ) within the chlorinated ligand and a narrower N-B-N angle (ca.8.1º) at the fourcoordinate boron.Although the N-B2-Cl and C-B2-Cl bond angles show ideal tetrahedral values, the remaining bond angles at B2 atom are somewhat distorted due to the involvement of this atom in the four-membered AsNBN ring with a bite angle at boron of 93.8(8) o .The three-coordinate boron atom shows a trigonal planar geometry (sum of bond angles is 359.9º), as is typical for bam ligands. 1 The four-membered AsNBN rings are slightly distorted from planarity with torsion angles of ca.ppm with a 1:2:1 intensity ratio), 6 suggests that a two-electron reduction with formation of the analogous sodium salt NaAs[PhB(μ-N t Bu)2]2, has occurred (Scheme 2).
Voltammetric studies performed on 1b in both dichloromethane and tetrahydrofuran on Pt, GC, and Au electrodes showed at least one oxidation process, which is also chemically and electrochemically irreversible, and takes place near the edge of the solvent window.This initial oxidation process gives rise to a return wave in THF which is offset by 1.5 V on Au (Figure 5), and by over 1.8 V on Pt (these waves are observed only after scanning through the +1.24V process).
The presence of this offset return wave may indicate that, upon oxidation of 1b, the lithium ion complexes with THF to give [PhB(μ-N t Bu)2]2As • and Li(THF)4 + .The intermediacy of such lithium complexes have been implicated previously in related systems. 33The return wave would then correspond to reduction of the electrochemically generated radical at +1.24 V. Two irreversible oxidation processes occurred in CH2Cl2 on GC, Pt, and Au, at +0.9 V and +1.• is likely to be thermodynamically unfavorable.Insofar as they do reflect the redox transformations of [Asbam2] y (y = -1, 0, +1), it appears that the strong complexation of lithium in 1b has a powerful stabilizing effect.Thus, the onset of oxidation of this species is found at least several volts more anodic than observed for the y = 0/+1 couple in 4.
Attempts to generate the radical [PhB(μ-N t Bu)2]2As • by SEEPR, an in situ method optimized for the detection of short-lived, electrochemically generated radicals, 10 by reductive electrolysis of 4 at -1.3 V in CH3CN or oxidative electrolysis of 1b at +0.9 V in CH2Cl2 or at 1.3 V in THF did not produce a detectable EPR-active species.