Stable Spirocyclic Neutral Radicals: Aluminum and Gallium Boraamidinates

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Stable Spirocyclic Neutral Radicals: Aluminum and Gallium Boraamidinates
• are obtained by the oxidation of their corresponding anions with iodine, and EPR spectra supported by DFT calculations show that the spin density is equally delocalized over all four nitrogen atoms in these spiroconjugated systems.
The study of radicals of the heavier main-group elements is a fascinating undertaking that, through a combination of X-ray structural studies, EPR spectroscopy and theoretical calculations, provides informative insights into the bonding arrangements in odd-electron systems. 1 In Group 13 chemistry, many of the socalled stable radicals 2 are obtained as charged species, that is, in salts as either anion or cation radicals.The neutral paramagnetic complexes of the type [M(dbdab)2] • (M = Al, Ga; dbdab = 1,4-ditert-butyl-1,4-diazabutadiene) have been formulated as (dbdab −• )M(dbdab 2− )M(III) complexes, in which the spin is located on one of the dbdab ligands, on the basis of EPR 3a and UV-PES spectra.3b The boraamidinate ligand [RB(NR′)2] 2− (A) 4 is isoelectronic with the extensively studied amidinate anions [RC(NR′)2] − (B). 5 Early work on ligands of the type A was limited primarily to complexes of Group 4 or Group 14 metals. 4Interest in these dianionic ligands has been rekindled recently through reports of (a) complexes in which the ligand A bridges very short M≡M triple bonds (M = Mo, W), 6a (b) trisubstituted octahedral Group 4 dianions, 6b and (c) the spirocyclic Group 13 anions {[PhB(μ-N t Bu)2]2M} − (1b, M = Ga; 1c, M = In). 7The first structural characterizations of lithium derivatives of A, which form dimeric or trimeric clusters, have also been described. 8An intriguing feature of the latter reagents is the formation of coloured solutions upon oxidation, which are thought to contain anion radicals of the type [RB(NR′)2] −• . 9We report here the first examples of the stabilization of these paramagnetic chelating ligands, which is achieved via coordination to Group 13 metal centres in the neutral spirocyclic radicals {[PhB(μ-N t Bu)2]2M} • (2a, M = Al; 2b, M = Ga). 10in a boiling mixture of benzene and diethyl ether produces the aluminum boraamidinate complex {μ-Li(OEt2)[PhB(μ-N t Bu)2]2Al} (4a).† Subsequent reaction of either 4a or {μ-Li(OEt2)[PhB(μ-N t Bu)2]2Ga} 7 (4b) with iodine immediately generates dark red or dark green solutions, respectively (Scheme 1).† Dark red crystals of 2a and dark green crystals of 2b suitable for X-ray structural determinations ‡ were grown from concentrated diethyl ether solutions at 258 K.These radicals are stable in the solid state under an inert atmosphere at room temperature for weeks.

Treatment of one equivalent of the dilithiated boraamidinate
X-ray structural determinations revealed that complexes 2a and 2b are isostructural.The molecular structures and pertinent structural parameters for 2a and 2b are depicted in Figure 1.In each case, the molecule lies on a crystallographic two-fold axis which imposes crystallographic equivalence on the two Scheme 1 Oxidation of 4a and 4b with iodine.− (M = Al, Ga).This is confirmed by X-ray structural analyses of 2a and (vide supra) that reveal only slight deviations from idealized structures (Figure 1).The retention of the D2d symmetry in 2a and 2b is in sharp contrast to the observations for the related diazabutadiene radicals [M(dbdab)2] • (M = Al, Ga) (vide supra) 3 in which the two dbdab ligands exhibit distinctly different metrical parameters.This was explained recently by using DFT calculations, 12 which revealed that the HOMOs of the D2d symmetric diamagnetic anions [M(dbdab)2] − (M = Al, Ga) are doubly degenerate orbitals of esymmetry.Upon oxidation, an unstable partially filled degenerate set of orbitals is formed that incurs a Jahn-Teller distortion.Hence, the resulting radicals adopt C2v symmetry with mixedvalent dbdab ligands and localized spin density. 12In comparison, the diamagnetic anions in 4 are also D2d symmetric, however, the HOMO of these systems is not comprised of a degenerate set of orbitals.Thus, neither Jahn-Teller distortion nor spin localization are of consequence upon oxidation.
Figures 3a and 4a show the experimental EPR spectra of 2a and 2b, respectively.Both radicals give intensely coloured solutions (vide supra) in diethyl ether or benzene that persist for days (M = Al) or several hours (M = Ga) at room temperature.Excellent simulations of the spectra were obtained by using the hyperfine coupling (hfc) constants given in Table 1.In general, there is a very good agreement between the experimental and the calculated hfc values (Table 1). 14Hence, the spectral simulations and DFT calculations indicate uniform spin delocalization throughout both boraamidinate ligands in 2a and 2b and confirm the retention of spirocyclic structures in solution.
In summary, the neutral radicals 2a and 2b are the first examples of complexes in which a boraamidinate anion radical [PhB(N t Bu)2] −• is stabilized by coordination to a metal centre.They also provide an unique example of spiroconjugation in a purely inorganic framework. 16he isolation of these paramagnetic complexes suggests that redox behaviour may be a more important feature of boraamidinate complexes than heretofore recognized.CCDC-267896 and 267897.See http://www.rsc.org/suppdata/cc/??/???????? for crystallographic data in CIF or other electronic format.

Fig. 3
Fig. 3 Experimental (a) and simulated (b) X-band EPR spectra of a diethyl ether solution of 2a at 295 K.

Fig. 4
Fig. 4 Experimental (a) and simulated (b) X-band EPR spectra of a diethyl ether solution of 2b at 295 K.