Syntheses, X-ray structures and redox behaviour of the group 14 bis-boraamidinates

: The solid-state structures of the complexes M[PhB(μ-N- t-Bu) 2 ] 2 ( 1a, M= Ge; 1b , M = Sn) were determined to be spirocyclic with two orthogonal boraamidinate ( bam ) ligands N,N’ - chelated to the group 14 centre. Oxidation of 1b with SO 2 Cl 2 afforded the thermally unstable, blue radical cation {Sn[PhB(μ-N-t -Bu) 2 ] 2 } •+ , identified by EPR spectroscopy supported by DFT calculations, whereas the germanium analogue 1a was inert towards SO 2 Cl 2 . The reaction between Li 2 [PhB(μ-N-t -Bu) 2 ] 2 and SnCl 2 or PbI 2 in 2:1 molar ratio in diethyl ether produced the novel heterotrimetallic complexes Li 2 Sn[PhB(μ-N- t-Bu) 2 ] 2 ( 2b ) and (Et 2 O·Li)LiPb[PhB(μ-N- t-Bu) 2 ] 2 ( 2c ·OEt 2 ), respectively. By contrast, treatment of Li 2 [PhB(μ-N- t-Bu) 2 ] 2 with C 4 H 8 O 2 ·GeCl 2 yielded the germanium(IV) complex 1a via a redox process. The X-ray structures of 2b and 2c ·THF revealed polycyclic arrangements in which one bam ligand is N , N ′-chelated to the Sn(II) or Pb(II) atom and one of the Li + cations, while the second bam ligand exhibits a unique bonding mode, bridging all three metal centres. The fluctional behaviour of 2b was investigated by variable temperature, multinuclear NMR spectroscopy

The most intriguing consequence of the 2-charge is the facile tendency for redox transformations to occur in which the dianion B is oxidized to the corresponding monoanion radical [bam] •-(C).This paramagnetic ligand can be stabilized through chelation, e.g., to early main-group metals (4,5).In the case of group 13 metals it has been possible to isolate stable neutral radicals.
For example, the intensely coloured paramagnetic species {M[PhB(μ-N-t-Bu)2]2} • ( M = Al, dark red; M = Ga, Dark green) are produced by one-electron oxidation of the corresponding anions with iodine, and the X-ray structures of these spirocyclic complexes have been determined (Scheme 1) (4).The SOMO of these neutral radicals is located primarily and equally in p-orbitals on the four nitrogen atoms of the two bam ligands; there is very little electron density on the group 13 centres.

Scheme 1.
Synthesis of spirocyclic {M[PhB(μ-N-t-Bu)2]2} • radicals 4 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 (5).However, the green indium-containing radical is not sufficiently stable to be isolated in the solid state.The boron analogue, although it is thermally very stable, could not be obtained in a pure form owing to the formation of other products via a competing reaction pathway (5).Thallium(I) mono-bam complexes have also been structurally characterized; they form aggregated chains incorporating metallophilic Tl···Tl interactions in the solid state (6).
Although the metathesis reactions of two equivalents of Li2bam reagents with MCl4 (M = Ge, Sn) afford the corresponding M(bam)2 complexes (7,8), the synthesis of a silicon analogue (M = Si) by this method has not been reported.The known silicon complexes have only one bam ligand chelated to silicon in complexes of the type RR′Si(bam); they were prepared by indirect approaches rather than metathesis (9,10).The solid-state structures of M(bam)2 complexes (M = Ge, Sn) have not been determined.Interestingly, the reaction of Li2[PhB(-N-t-Bu)2] with PbCl4 produces the dimeric lead(II) complex {Pb[PhB(-N-t-Bu)2]}2, which is preferably prepared by using PbCl2 as a source of lead(II) (11).There are no reports of complexes in which two bam ligands are chelated to a group

Reagents and 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%), GeCl4 (Strem, 99.99%), C4H8O2 were prepared as described earlier (13).The solvents n-hexane, toluene, Et2O and THF were dried by distillation over Na/benzophenone under an argon atmosphere prior to use.Elemental analyses were performed by Analytical Services, Department of Chemistry, University of Calgary.

X-ray crystallography
Crystallographic data for 1a, 1b, 2b and 2c•THF are summarized in Table 1. 4 Crystals of were coated with Paratone 8277 oil and mounted on a glass fibre.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 was applied to the net intensities.The structures were solved by direct methods using SHELXS-97 (15) and refined using SHELXL-97 (16).After full-matrix least-squares refinement of the non-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.
In the structure of 2b, the Li2 and Sn1 atoms were disordered with the atoms statistically distributed over the two atomic sites.The two atoms were constrained to locate in the same position and the anisotropic thermal parameters were restricted to be equal.In the final refinement, the site occupation factors were ca.93 and 7%, respectively.The scattering factors for the neutral atoms 7 were those incorporated with the programmes.

Computational Details
All calculations were done with the Gaussian 03 program using density functional theory (17).Molecular structures were optimized using the hybrid PBE1PBE exchange-correlation functional (18) together with the Ahlrichs' TZVP basis sets (19); an effective core potential basis set of similar valence quality was used for the heavy tin nuclei (20).Hyperfine coupling constants were then calculated by single-point calculations employing the optimized geometries and the basis set combination used for optimizations.The orbital plot was obtained by the program gOpenMol (21).

Oxidation of Sn[PhB(μ-N-t-Bu)2]2 (1b) with SO2Cl2
A solution of SO2Cl2 (0.040 mL, 0.067 g, 0.50 mmol) was dissolved in 10.0 mL of diethyl ether; 1.0 mL (0.05 mmol) of this solution was added to a solution of 1b (0.058g, 0. The reaction mixture was allowed to reach room temperature in 30 min and it was then heated to 35 o C for 12 h.The precipitate of LiI was removed by filtration and the solvent was evaporated under vacuum giving a pale yellow, amorphous powder (0.294 g). 1 H NMR spectroscopy revealed a mixture of products among which the lead(II) complex, {Pb[PhB(-N-t-Bu)2]}2, was identified (

X-ray structures of M[PhB(μ-N-t-Bu)2]2 (1a, M = Ge; 1b, M = Sn)
The germanium(IV) and tin(IV) complexes 1a and 1b were obtained in excellent yields (ca. 85 %) by a modification of the literature synthesis in which the dilithium reagent Li2[PhB(-N-t-Bu)2] was isolated and purified prior to reaction with MCl4 (M = Ge, Sn) in a 2:1 molar ratio in diethyl ether.The NMR spectroscopic data (see Experimental Section) showed minor variations compared with the literature values (7).Attempts to make the silicon analogue Si[PhB(μ-N-t-Bu)2]2 by a similar procedure were unsuccessful.
The X-ray structural determinations of 1a and 1b confirmed the expected spirocyclic structures with two orthogonal bam ligands N,N′-chelated to the distorted tetrahedral metal centres (Fig. 1).In both complexes the molecule lies on a crystallographic two-fold axis that imposes equivalence on the two bam ligands.The diamagnetic complexes 1a and 1b are isostructural with the paramagnetic group 13 analogues {M[PhB(μ-N-t-Bu)2]2} • (M = Al, Ga) (4).The centrosymmetric germanium(IV) complex 1a represents the first structurally characterized germanium bam complex (1).
Selected bond lengths and bond angles for 1a and 1b are summarized in Table 2.The The EPR spectrum of the blue solution in diethyl ether displays an eight-line pattern and a highly broadened lineshape (Fig. 2); the g-value of this radical is 2.0055.Several attempts to resolve additional fine structure in the EPR spectrum by variations in the solvent, concentration of the solution and temperature, as well as changes in the measurement parameters, were unsuccessful.A reasonable simulation of the observed spectrum is obtained by using a line width of 2.85 G (95% couplings that result from the spin density of the unpaired electron while overestimating those which arise from spin-polarization effects (4,5).The undulating baseline of the EPR spectrum and the low intensity humps visible at both ends of the spectrum most likely arise from the presence of small amounts of isotopomers which contain the spin-active tin nuclei ( 117 Sn, I = ½, 7.6 %; 119 Sn, I = ½, 8.6 %).The magnitudes of the two Sn couplings cannot, however, be obtained with the current computational method due to the indirect treatment of relativistic effects via effective core potential basis sets.The observed broad lineshape can, at least in part, be attributed to the numerous smaller couplings to the hydrogen atoms of the t-Bu groups. 6
The reaction between Li2[PhB(μ-N-t-Bu)2] and SnCl2 in a 2:1 molar ratio in diethyl ether proceeded cleanly to produce Li2Sn[PhB(μ-N-t-Bu)2]2 (2b) in 75 % yield (eq. [1]).The unsolvated heterotrinuclear complex 2b was characterized in solution by multinuclear NMR spectroscopy and in the solid state by single crystal X-ray crystallography. [ The reaction of Li2[PhB(μ-N-t-Bu)2] with PbI2 in 2:1 molar ratio was conducted under various conditions in an effort to produce the analogous lead(II) complex Li2Pb[PhB(μ-N-t-Bu)2]2 (2c).The 1 H NMR spectrum of the reaction mixture, however, revealed several products, one of which was identified as the previously reported mononuclear lead(II) complex, {Pb[PhB(μ3-N-t-Bu)2]}2 (11).In an alternative synthetic approach, this dimeric Pb(II) complex was prepared and isolated prior to treatment with two equivalents of Li2[PhB(μ-N-t-Bu)2].However, this two-step method also afforded a mixture of products from which only a few crystals of 2c were isolated as the tetrahydrofuran solvate, (THF The reaction of Li2[PhB(μ-N-t-Bu)2] with C4H8O2•GeCl2 in 2:1 molar ratio in diethyl ether produced several products ( 1 H NMR). Consequently, the reaction was repeated in 1:1 molar ratio in an attempt to form the unknown germanium(II) complex, Ge[PhB(μ-N-t-Bu)2], for subsequent reaction with Li2[PhB(μ-N-t-Bu)2]; this two-step approach was previously found to be necessary for the successful synthesis of the aluminum complex (Et2O•Li)Al[PhB(μ-N-t-Bu)2]2 (4).The 1 H NMR spectrum of the product(s) showed three equally intense resonances for N-t-Bu -groups at 1.41, 1.39 and 1.30 ppm (d8-toluene, 23 o C).The chemical shift of 1.30 ppm is identical with the value observed for the t-Bu groups in the germanium(IV) complex Ge[PhB(-N-t-Bu)2]2 (1a) (8), suggesting that a redox process has occurred.The resonances at 1.41 and 1.39 are tentatively assigned to the t-Bu groups attached to the three-and four-coordinate nitrogen atoms of the dimeric complex germanium(II) complex {Ge[PhB(μ-N-t-Bu)2]}2; however, attempts to isolate this second product were unsuccessful.

X-ray structures of Li2Sn[PhB(μ-N-t-Bu)2]2 (2b) and (THF•Li)LiPb[PhB(μ-N-t-Bu)2]2 (2c•THF)
Crystals of 2b and 2c•THF were obtained by recrystallization from boiling n-hexane and THF, respectively. 7The crystal structures of 2b and 2c•THF with the atomic numbering scheme are depicted in Fig. 4 and the pertinent bond parameters are summarized in Table 3 The molecular structure of the antimony complex 3 is comprised of two four-membered rings, BN2Li and BN2Sb, connected by Li-N and Sb-N bonds to form a tricyclic compound with both three and four-coordinate nitrogens (12).The introduction of a second Li + ion in 2b has a surprisingly small effect on the overall structure; the molecule retains the ladder-like backbone and cisoid arrangement of the PhBN-t-Bu -units.Most significantly, however, the Li2•••N4 contact in 2b (Fig. The fluctional behaviour observed for 2b is reminiscent of the exchange processes exhibited by the related group 15 complexes, 3 and 4•OEt2, which both showed only a single t-Bu resonance at room temperature (12).These types of fluctional processes in lithium derivatives of polyimido anions of p-block elements are known to have low activation energies (27).In the case of 3 and 4•OEt2, the fluctionality was determined to originate from a Berry pseudorotation (12).In a similar manner, the single resonance observed at room temperature in the 1 H NMR spectra of 2b can be attributed to a rapid exchange in which each of the four nitrogens, in turn, occupy the site that is not coordinated to the tin centre.12).Despite the introduction of a second Li + ion, the tin complex maintains the ladder-like backbone and cisoid arrangement of the bam ligands found in the antimony analogue.The intrusion of a second Li + ion in the lead complex results in rupture of the bond between the solvated Li + ion and one of the bam ligands, as well as cleavage of the fourth M-N bond (M = Bi, Pb) that exists in the structure of (Et2O•Li)Bi[PhB(μ-N-t-Bu)2]2.In view of these structural similarities it is, perhaps, not surprising that the variable temperature NMR spectra of Li2Sn[PhB(μ-N-t-Bu)2]2 show trends comparable to those previously observed for the group 15 complex LiSb[PhB(μ-N-t-Bu)2]2 (12), implying that the fluctional processes are also similar.a M = Ge, b M = Sn.Symmetry operation for the atoms marked with a single quote('); c -x, y, 0.5-z.;

Conclusions
d 1-x, y, 1.5-z.Hydrogen atoms have been omitted for clarity.The tin complex 1b is isostructural with 1a.

14 14Scheme 3 .
Scheme 3. Possible structures of the product of one-electron oxidation of 1b with SO2Cl2.

. The structures 18 reveal
novel polycyclic arrangements, which bear a close similarity with each other despite the introduction of a solvent molecule in 2c•THF.In both frameworks one of the bam ligands is N,N′chelated to both the M(II) atom (M = Sn, Pb) and one of the Li + ions while the second bam ligand exhibits a unique bonding motif by bridging all three metal centers (1).While one of the Li + ions (Li1) in both structures is coordinated to three nitrogens, the second lithium in 2c•THF is coordinated to two nitrogen atoms and the solvent molecule.The absence of the solvent molecule in 2b affords a third, albeit relatively weak, Li•••N close contact [2.557(6) Å] for the second lithium cation that results in a formal coordination number of five for the N4 atom (Fig. 4a).Lithium salts of the group 15 monoanions M[PhB(μ-N-t-Bu)2]2 -(3, M = Sb; 4, M = Bi) (12), which are isoelectronic with the group 14 dianions M[PhB(μ-N-t-Bu)2]2 2-(M = Sn, Pb), provide an interesting benchmark for comparison with the structures of 2b and 2c•THF (Scheme 4) Curiously, the lighter complexes in both group 14 (M = Sn) and group 15 (M = Sb) seem to favour unsolvated structures, whereas solvation of the Li + cation is observed for the heavier congeners (M = Pb, Bi).
B,13C and 119 Sn NMR spectra were obtained in d8-toluene at 23 ºC on a Similarly, the 119 Sn NMR spectra are referenced externally and the chemical shifts are Bruker DRX 400 spectrometer operating at 399.59, 155.30, 128.20, 100.49and 149.00 MHz, respectively. 1d13C spectra are referenced to the solvent signal and the chemical shifts are reported relative to (CH3)4Si.7Liand 11 B NMR spectra are referenced externally and the chemical 6 shifts are reported relative to a 1.0 M solution of LiCl in D2O and to a solution of BF3•Et2O in C6D6, respectively.