Supporting Information

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1 Supporting Information Wiley-VC Weinheim, Germany

2 From a Stable Silylene to a Mixed-Valent Disiloxane and an Isolable Silaformamide-Borane Complex with Considerable Silicon-Oxygen Double Bond Character Shenglai Yao, Markus Brym, Christoph van Wüllen, and Matthias Driess* Technische Universität Berlin, Institute of Chemistry: Metalorganics and Inorganic Materials, Sekr. C2, Strasse des 17. Juni 135, D Berlin (Germany) matthias.driess@tu-berlin.de Contents A. Syntheses S2 B. X-Ray Diffraction Studies S11 C. Computational Detail S16 D. References S25 1

3 A. Syntheses General Considerations. All experiments and manipulations were carried out under dry oxygen-free nitrogen using standard Schlenk techniques or in an MBraun inert atmosphere drybox containing an atmosphere of purified nitrogen. Solvents were dried by standard methods and freshly distilled prior to use. The starting material LSi (1) i ii and 2 O B(C 6 F 5 ) 3 were prepared according to literature procedure. 1, 13 C, 19 F, 11 B, 29 Si, NMR spectra were recorded with Bruker spectrometers AM 200 and AM 400, respectively. Chemical shifts of the deuterated solvents in 1 NMR data: benzene-d 6 : δ(c 6 D 5 ) = 7.15 ppm, CD 2 Cl 2 : δ(cdcl 2 ) = 5.32 ppm. Chemical shifts of the deuterated solvents in 13 C NMR data: benzene-d 6 : δ = ppm, CD 2 Cl 2 : δ(cd 2 Cl 2 ) = 53.1 ppm; 29 Si NMR data: SiMe 4 (external, δ = 0 ppm); 19 F NMR data: CFCl 3 (external, δ = 0 ppm); 11 B NMR data: BF 3 -diethylether adduct (external, δ = 0 ppm). 3 B(C 6 F 5 ) 3 : A solution of 2 O B(C 6 F 5 ) 3 ( 1.87 g, 3.53 mmol) in toluene (20 ml) was added dropwise to a solution of 1 (1.57 g, 3.53 mmol) in toluene (15 ml) at -60 C with stirring. After complete addition the reaction mixture was allowed to warm up to room temperature and stirred for 1 h. Volatiles were removed in vacuo and the residue extracted with C 2 Cl 2 (25 ml). Filtration and subsequent concentration (to about 8 ml) afforded, after 1 day of cooling at -20 C, colorless crystals of 3 B(C 6 F 5 ) 3 (2.29 g, 2.35 mmol, 67%). M.p. 206 C (decomp.). 1 NMR ( Mz, CD 2 Cl 2, 298K) (see Figure 1): δ = 1.05 (d, 3 J = 7 z, 6, CMe 2 ), 1.10 (d, 3 J = 7 z, 6, CMe 2 ), 1.14 (d, 3 J = 7 z, 6, CMe 2 ), 1.20 (d, 3 J = 7 z, 6, CMe 2 ), 1.94 (s, 6, Me), 2.78 (sept, 3 J = 7 z, 2, CMe 2 ), 3.09 (sept, 3 J = 7 z, 2, CMe 2 ), 5.64(br, 1, Si), 5.90 (s, 1, γ-c), (m, br, 6, arom. ). 13 C{ 1 } NMR ( Mz, CD 2 Cl 2, 298K): δ = 24.3 (NCMe); 22.7, 24.3, 24.8, 24.9 (CMe); 29.3, 29.5 (CMe); (γ-c); 125.1, 125.5, 130.2, 134.7, 144.7, (NC, 2,6-i-Pr 2 C 6 3 ); 124.1, 135.4, 137.9, 140.4, 146.7, (br. B(C 6 F 5 ) 3 ). 19 F{ 1 } NMR ( Mz, CD 2 Cl 2, 298K): δ = (d, br, J F-F = 21.5, o-f), (t, J F-F = 20.5z, p-f), (t, br, J F-F = 18.3 z m-f). 29 Si NMR (79.49 Mz, CD 2 Cl 2, 298K): δ = -61.5(d, 1 J Si = 307 z). 11 B{ 1 }NMR ( Mz, CD 2 Cl 2, 298K): δ = EI-MS: m/z (%): (1, [M + ]), (3, [M-Me] + ), (3, [M- i Pr] + ),

4 (28, [B(C 6 F 5 ) 3 ] + ). Elemental analysis (%) calcd for C N 2 SiOBF 15 : C, 57.91;, 4.34; N, Found: C, 57.63;, 4.32; N, Figure 1: 1 NMR spectrum of [3-B(C 6 F 5 ) 3 ], (L)Si()O B(C 6 F 5 ) 3. The sample was recrystallized in C 2 Cl 2 and contained co-crystallized solvent molecules C 2 Cl 2 ; the isopropyl groups are chemically inequivalent due to hindered rotation of the aryl groups at nitrogen. IR (KBr, cm -1 )(see Figure 2): ν = 428 (w), 449 (w), 459 (w), 481 (w), 493 (w), 540 (w), 569 (w), 576 (w), 595 (w), 604 (w), 619 (w), 655 (w), 671 (w), 681 (m), 704 (w), 728 (w), 736 (w), 747 (w), 764 (w), 773 (w), 795 (m), 856 (w), 934 (w), 978 (s), 1024 (m), 1058 (w), 1088 (s), 1165 (s, Si= 16 O str. and other vibrations), 1197 (w), 1227 (w), 1249 (w), 1278 (m), 1319 (m), 3

5 1363 (m), 1370 (s), 1384 (s), 1463 (s), 1516 (s), 1547 (s), 1590 (w), 1644 (w), 2223 (w, Si- 1 str.), 2876 (w), 2937 (w), 2969 (m), 3068 (w), 3434 (w). Figure 2: IR spectrum of [3-B(C 6 F 5 ) 3 ], (L)Si()O B(C 6 F 5 ) 3. Assignment of the Si- and Si=O vibrational modes in accord with isotope labelling experiments (see Figure 4 and Figure 5). (DL)Si(D)O B(C 6 F 5 ) 3, was prepared similarly to [3-B(C 6 F 5 ) 3 ] using D 2 O B(C 6 F 5 ) 3 instead of 2 O B(C 6 F 5 ) 3 : 1 NMR ( Mz, CD 2 Cl 2, 298K) (see Figure 3), 29 Si NMR (79.49 Mz, CDCl 3, 298K): δ = -61.5(t, 1 J DSi = 47 z). 4

6 Figure 3: 1 NMR spectrum of (DL)Si(D)O B(C 6 F 5 ) 3 (ca. 80%) and (L)Si()O B(C 6 F 5 ) 3 (ca. 20%). The sample was recrystallized in C 2 Cl 2 and contained co-crystallized solvent molecules C 2 Cl 2. IR (KBr, cm -1 ; see Figure 4): ν = 423 (w), 449 (w), 457 (w), 481 (w), 541 (w), 568 (w), 576 (w), 595 (w), 603 (w), 611 (w), 625 (w), 654 (w), 669 (m), 681 (m), 685 (w), 704 (w), 727 (w), 737 (w), 747 (w), 765 (m), 773 (m), 800 (m), 856 (w), 869 (w), 935 (w), 976 (s), 1024 (m), 1061 (w), 1086 (s), 1165 (s, Si= 16 O str. and other vibrations), 1197 (w), 1225 (w), 1249 (w), 1260 (w), 1278 (m), 1319 (m), 1370 (m), 1381 (s), 1462 (s), 1515 (s), 1546 (s), 1591 (w), 1616 (w, Si- D str.), 1644 (w), 2223 (w, Si- 1 str.), 2875 (w), 2936 (w), 2972 (m), 3067 (w), 3398 (w). 5

7 Figure 4: IR spectrum of (DL)Si(D)O B(C 6 F 5 ) 3 (ca. 80% Si-D). Assignment of the Si-D, and Si=O vibrational modes in accord with isotope labelling experiments (see Figure 2 and Figure 5). (L)Si() 18 O B(C 6 F 5 ) 3 was prepared similarly to [3 B(C 6 F 5 ) 3 ] using 18 2 O B(C 6 F 5 ) 3 instead of 2 O B(C 6 F 5 ) 3. IR (KBr, cm -1 ; see Figure 5): ν = 425 (w), 449 (w), 458 (w), 482 (w), 540 (w), 569 (w), 576 (w), 595 (w), 602 (w), 618 (w), 629 (w), 654 (w), 667 (w), 679 (m), 704 (w), 728 (w), 736 (w), 743 (w), 748 (w), 763 (w), 772 (w), 793 (m), 855 (w), 934 (w), 949 (w), 979 (s), 1023 (m), 1058 (w), 1086 (s), 1112 (m, Si= 18 O str.), 1163 (s), 1197 (w), 1225 (w), 1249 (w), 1278 (m), 1317 (m), 1363 (m), 1370 (m), 1381 (m), 1464 (s), 1515 (s), 1543 (m), 1589 (w), 1644 (w), 2223 (w, Si- 1 str.), 2875 (w), 2936 (w), 2970 (m), 3069 (w). 6

8 Figure 5: IR spectrum of (L)Si() 18 O B(C 6 F 5 ) 3 (ca. 97% Si= 18 O). Assignment of the Si- and Si=O vibrational modes in accord with isotope labelling experiments (see Figure 4 and Figure 6). 7

9 Figure 6. Comparison of the IR spectra of [3 B(C 6 F 5 ) 3 ] ( black) and (L)Si()= 18 O B(C 6 F 5 ) 3 (gray) in the range of 750 cm -1 to 1350 cm -1. Assignment of the Si=O vibrational modes in accord with isotope labelling experiments (see Figure 2 and Figure 5). 8

10 Compound 4: A Schlenk tube containing a solution of LSi 1 (0.49 g, 1.1 mmol) in n-hexane (10 ml) was connected to another Schlenk tube containing water ( ml, 0.55 mmol) and hexane (20 ml) by a glass joint so that water vapor can slowly diffuse into the Schlenk tube with the solution of 1. The set-up was then placed in a refrigerator at 4 C for 4 days. The color of the reaction solution turned slowly from yellow to brown, affording dark brown crystals of 4 (yield: 0.26 g, 0.29 mmol, 52%). M.p. 67 C (decomp.). 1 NMR ( Mz, C 6 D 6, 298K) (see Figure 7): δ = (m, 96, CMe 2 ); 1.22, 1.33, 1.49, 1.52 (s, 18, NCMe); 3.23, 3.52, 3.87, 4.06 (s, 8, NC 2 ); (m, 16, CMe 2 ); 4.72, 4.78 (s, 2, Si, 29 Si satellites: 1 J Si = 273 z); 4.76, 4.83, 5.21, 5.37 (s, 4, γ-c); ( m, br, 24, arom. ). 13 C{ 1 } NMR ( Mz, C 6 D 6, 298K): δ = (NCMe, i-pr); 87.8, 88.3 (NCC 2 ); 100.7, (γ-c); (2,6-i-Pr 2 C 6 3, NC). 29 Si NMR (79.49 Mz, C 6 D 6, 298K): δ = (d, Si), -53.7(d, Si), -9.6(s, silylene-si), -7.9 (s, silylene-si). EI-MS: m/z (%): 907.5(30, [M + ]), 892.5(100, [M-Me] + ), 874.5(21, [M- i Pr] + ). Elemental analysis (%) calcd for C N 4 Si 2 O: C, 76.81;, 9.13; N, Found: C, 74.55;, 9.01; N, IR (KBr, cm -1 ): ν = 437 (w), 702 (w), 731 (w), 759 (m), 787 (m), 803 (m), 822 (w), 859 (w), 906 (w), 934 (m), 961 (m), 981 (m), 1057 (s), 1101 (s), 1176 (s), 1222 (w), 1254 (m), 1276 (m), 1322 (m), 1363 (s), 1382 (s), 1438 (s), 1464 (s), 1488 (m), 1552 (s), 1623 (s), 2233 (w), 2868 (m), 2928 (s), 2962 (s), 3060 (m), 3629 (w). 9

11 Figure 7: 1 NMR spectra of two samples of 4 as mixtures of two diastereomers (isomer a and isomer b); different ratios of the isomers a and b are observed due to the presence of a chiral Si center (Si * ) and hindered rotation around the Si-O bonds. In sample 1, the ratio of a:b is ca. 3:1; in sample 2, the ratio of a:b is ca. 1:6. Additionally, the assignment is based on 1-13 C COSY of sample 2 (see Figure 8). 10

12 ppm (t1 7.0 ppm (t2) Figure 8: 1-13 C COSY of 4 (sample 2). B. X-Ray Diffraction Studies Single-Crystal X-ray Structure Determinations: Crystals were each mounted on a glass capillary in perfluorinated oil and measured in a cold N 2 flow. The data of [3 B(C 6 F 5 ) 3 ] and 4 were collected on an Oxford Diffraction Xcalibur S Sapphire at 150 K (Mo-Kα radiation, λ = Å). The structures were solved by direct methods and were refined on F 2 with the SELX-97 iii software package. The positions of the atoms were calculated and considered isotropically according to a riding model (Exceptions are the hydrogen atoms on silicon which have been found in the electron density map). A disordered solvent molecule in the data of [3 B(C 6 F 5 ) 3 ] had to be removed using the squeeze command in the Platon Software package. iv CCDC contains the supplementary crystallographic data for this paper. These 11

13 data can be obtained free of charge via or by ing or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: Compound [3 B(C 6 F 5 ) 3 ]: Monoclinic, space group C2/c, a = (18), b = (18), c = (5) Å, β = (10), V = 9084(2) Å 3, Z = 8, ρ calc = Mg/m 3, μ(mokα = mm -1, collected reflections, 7942 crystallographically independent reflections [R int = ], 5917 reflections with I > 2σ(l), θ max = 25.00, R(F o ) = (I > 2σ(l)), wr(f 2 o ) = (all data), 618 refined parameters; GOOF = Figure 9. Molecular structure of [3 B(C 6 F 5 ) 3 ]. Thermal ellipsoids (C1-C5, N1, N2, Si1, 1, O1 and B1) are drawn at 50% probability level. ydrogen atoms (except for 1) are omitted for clarity. 12

14 Table 1: Selected interatomic distances and angles of [3 B(C 6 F 5 ) 3 ]. distances(pm) angles( ) Si(1)-O(1) (16) B(1)-O(1)-Si(1) (14) O(1)-B(1) 150.3(3) O(1)-Si(1)-N( (9) Si(1)-N(2) (19) O(1)-Si(1)-N( (9) Si(1)-N(1) (19) N(2)-Si(1)-N( (9) N(1)-C(2) 134.7(3) C(2)-N(1)-Si(1) (15) N(2)-C(4) 135.5(3) C(4)-N(2)-Si(1) (15) C(1)-C(2) 149.2(3) N(1)-C(2)-C(3) 121.4(2) C(2)-C(3) 139.8(3) N(1)-C(2)-C(1) 120.7(2) C(3)-C(4) 138.5(3) N(2)-C(4)-C(3) 121.0(2) C(4)-C(5) 150.5(3) N(2)-C(4)-C(5) 120.6(2) C(3)-C(2)-C(1) 117.8(2) C(4)-C(3)-C(2) 126.0(2) C(3)-C(4)-C(5) 118.5(2) Compound 4: Triclinic, space group P-1, a = (3), b = (14), c = (6) Å, α = (17), β = 74.78(2), γ = (15), V = (11) Å 3, Z = 2, ρ calc = Mg/m 3, μ(mokα = mm -1, collected reflections, crystallographically independent reflections [R int = ], 8420 reflections with I > 2σ(l), θ max = 27.50, R(F o ) = (I > 2σ(l)), wr(f 2 o ) = (all data), 637 refined parameters; GOOF =

15 Figure 10. Molecular structure of 4. Thermal ellipsoids (C1-C5, C30-C34, N1-N4, Si1, Si2, O1, 1, 1a and 1b) are drawn at 50% probability level. ydrogen atoms (except for 1, 1a and 1b) are omitted for clarity. 14

16 15 Table 2: Selected interatomic distances and angles of compound 4. distances(pm) angles( ) Si(1)-O(1) O(1)-Si(2) Si(1)-N(1) Si(1)-N(2) N(1)-C(2) N(2)-C(4) C(1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) Si(2)-N(3) Si(2)-N(4) N(3)-C(31) N(4)-C(33) C(30)-C(31) C(31)-C(32) C(32)-C(33) C(33)-C(34) (13) (13) (15) (14) 139.3(2) 137.9(2) 142.2(3) 140.4(2) 137.9(2) 148.6(2) (14) (15) 136.3(2) 137.6(2) 148.8(2) 138.7(2) 139.3(2) 145.4(2) O(1)-Si(1)-N(1) O(1)-Si(1)-N(2) N(1)-Si(1)-N(2) Si(1)-O(1)-Si(2) C(2)-N(1)-Si(1) O(1)-Si(2)-N(4) O(1)-Si(2)-N(3) N(4)-Si(2)-N(3) C(4)-N(2)-Si(1) N(1)-C(2)-C(3) N(1)-C(2)-C(1) N(2)-C(4)-C(5) C(3)-C(4)-N(2) C(3)-C(2)-C(1) C(4)-C(3)-C(2) C(3)-C(4)-C(5) C(31)-N(3)-Si(2) C(33)-N(4)-Si(2) N(3)-C(31)-C(32) N(3)-C(31)-C(30) N(4)-C(33)-C(32) N(4)-C(33)-C(34) C(32)-C(31)-C(30) C(31)-C(32)-C(33) C(32)-C(33)-C(34) (7) (7) 99.53(7) (8) (12) (7) (7) 96.40(6) (11) (16) (16) (15) (15) (16) (16) (15) (11) (11) (14) (15) (15) (15) (15) (15) (15)

17 C. Computational Details Quantum chemical calculations on model compounds in sections C.1 and C.2 were done based on density functional theory (DFT) with a B3LYP hybrid exchange-correlation functional v and polarized valence triple zeta (TZVP) vi basis sets. The calculations have been performed with the Gaussian03 vii program. The simulation of the isotope shift in the IR spectrum of compound [3 B(C 6 F 5 ) 3 ] (Section C.3, Fig. 13) were done on a somewhat smaller model (with 2,6-Me 2 C 6 3 instead of 2,6- i Pr 2 C 6 3 aryl groups at the nitrogen atoms of L). The simulated spectra are based on vibrational frequencies and IR intensities from a quantum chemical calculation at DFT level (B3LYP functional) with the 6-31G* basis set, using the Gaussian03 program. This data has been converted to plot data with the GaussView program viii. C.1. Calculated energy differences between model silaformyl compounds RSi=O and their hydroxo silylene tautomers RSi-O (B3LYP/TZVP level) For each compound selected bond lenghts, Si=O vibrational frequencies, and the total energy (in atomic units) as well as the energy of RSi-O relative to RSi=O (in kj/mol) are listed. a) data for unsupported silanones vs. hydroxo silylenes R Si O versus R Si O R=: r(si O) pm r(si O) pm ν(si=o) 1205 cm 1 ν(si O) 828 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= 14.6 kj/mol 16

18 R=C 3 : r(si O) pm r(si O) pm ν(si=o) 1213 cm 1 ν(si O) 820 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= 8.5 kj/mol R=Si 3 : r(si O) pm r(si O) pm ν(si=o) 1184 cm 1 ν(si O) 810 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= 5.7 kj/mol R=N 2 : r(si O) pm r(si O) pm r(si N 2 ) pm r(si N 2 ) pm ν(si=o) 1240 cm 1 ν(si O) 887 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= 38.2 kj/mol b) data for silanones vs. hydroxo silylenes stabilized by a Me 3 N donor at silicon NMe 3 NMe 3 R Si O versus R Si O R=: r(si O) pm r(si O) pm r(si NMe 3 ) pm r(si NMe 3 ) pm ν(si=o) 1144 cm 1 ν(si O) 725 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol 17

19 R=C 3 : r(si O) pm r(si O) pm r(si NMe 3 ) pm r(si NMe 3 ) pm ν(si=o) 1145 cm 1 ν(si O) 708 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol R=Si 3 : r(si O) pm r(si O) pm ν(si=o) 1128 cm 1 ν(si O) 718 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol R=N 2 : r(si O) pm r(si O) pm r(si N 2 ) pm r(si N 2 ) pm r(si NMe 3 ) pm r(si NMe 3 ) pm ν(si=o) 1163 cm 1 ν(si O) 713 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol c) data for silanones vs. hydroxo silylene stabilized by a Me 3 N donor at Si and a BF 3 acceptor at O NMe 3 NMe 3 R Si O BF 3 versus R Si O BF 3 R=: r(si O) pm r(si O) pm 18

20 r(si NMe 3 ) pm r(si NMe 3 ) pm r(o B) pm r(o B) pm ν(si=o) 1095 cm 1 ν(si O) 610 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol R=Si 3 : r(si O) pm r(si O) pm ν(si=o) 1067 cm 1 ν(si O) 603 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol R=C 3 : r(si O) pm r(si O) pm r(si NMe 3 ) pm r(si NMe 3 ) pm r(o B) pm r(o B) pm ν(si=o) 1099 cm 1 ν(si O) 593 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol R=N 2 : r(si O) pm r(si O) pm r(si N 2 ) pm r(si N 2 ) pm r(si NMe 3 ) pm r(si NMe 3 ) pm r(o B) pm r(o B) pm ν(si=o) 1105 cm 1 ν(si O) 582 cm 1 E(tot)= a.u. E(tot)= a.u. E(rel) = 0 E(rel)= kj/mol 19

21 C.2 Calculated interatomic distances, force constants, Si=O stretching frequencies, and bond orders of Si-O bonds (B3LYP/TZVP level) Bond orders are derived from the computed Si-O stretching force constants by the relation B.O. = k 2 (k in N/m). The bond order is thus linear in the force constant, and prototypical 436 molecules are used to fix the bond order to 1.0 for Me 3 Si OMe and to 2.0 for 2 Si=O. The calculated frequencies for these prototypes are in good agreement with the experimentally observed ones, which are 1202 cm -1 for 2 Si=O ix and 720 cm -1 for Me 3 SiOMe, x respectively. a) Prototypical Si-O double vs. single bonds Si O r =153.7 pm k =874 N/m ν =1205 cm -1 B.O. = 2.00 Me OMe Si Me Me r = pm k = 438 N/m ν = 714 cm -1 B.O. = 1.00 Note: These two bond orders are fixed to 1.0 and 2.0 by construction 20

22 b) Si=O bonds supported by nitrogen donors at the silicon atoms NMe 3 Si O r = pm k = 782 N/m ν = 1144 cm -1 B.O. = 1.79 N N Si O r = pm k = 807 N/m ν = 1154 cm -1 B.O. = 1.85 c) Si=O bonds stabilized by N-donors at Si and B-acceptors at O Me 3 N r = pm Si O BF 3 k = 701N/m ν = 1095 cm -1 B.O. = 1.60 N N Si O BF 3 r = pm k = 682 N/m ν = 1076 cm -1 B.O. =

23 d) Figure 11. Plot of the calculated force constants k (in N/m) vs. the square of the calculated vibrational frequencies (in 10 6 cm 2 ). Computed data from above (B3LYP/TZVP level). 22

24 C.3 Calculated IR Spectra (B3LYP/6-31G* level) Figure 12. Calculated IR spectra for a model of compound [3 B(C 6 F 5 ) 3 ] (with 2,6-Me 2 C 6 3 instead of 2,6- i Pr 2 C 6 3 aryl groups at the nitrogen atoms of L) and its 18 O Isotopmer (B3LYP/6-31G* quantum chemical calculations). Note that upon isotop substitution, the shift of the Si=O stretching vibration by 40 cm -1 is the only visible change in the spectrum. 23

25 C.4 Population analysis in terms of natural atomic orbitals (B3LYP/TZVP level) At the optimized geometry, a single-point calculation has been performed, triggering the NBO calculation through the NBOREAD keyword. The NBO directives BNDIDX and 3CBOND have been used, the latter allows for three-centre bonds in the description which suppresses the weight of non-lewis structures in the aromatic rings. The charges are quite extreme: +2.0 for Si, 0.72/ 0.71 for N, and 1.08 for O, much larger than the Mulliken charges (+0.88 for Si, 0.15 for N, 0.44 for O). The Wiberg bond indices (in the natural atomic orbital basis) are quite small for the polar bonds (0.54/0.53 for Si-N, 0.88 for Si-, and 0.74 for Si-O) as the bond orbitals are mainly localized on the more electronegative atom. It is clear that a description in terms of Lewis structure alone is not possible, since this would imply that the same orbitals could be obtained with a minimal basis set. Therefore, an NBO analysis also finds small populations in non-lewis structures (e. g. Rydberg and antibonding orbitals) which are usually quite small (well below 0.1 electrons). Because the silicon atom is tetracoordiniated, there is no classical Lewis structure featuring a double bond. owever, the two Si-N bonds are strongly polarized towards the nitrogen atoms because of the electronegativity difference, which implies that the corresponding antibonding orbitals are mainly located on the Si atom. The minus linear combination of these two has a constructive overlap with a p-type lone pair at the oxygen atom, and this orbital interaction establishes the double bond character. In our NBO analysis of [3 B(C 6 F 5 ) 3 ], we find significant deviations from the ideal values (2.0 for occupied orbitals associated to Lewis structures, 0.0 for non-lewis structures), namely 1.91/1.86 for two lone pairs at O, and 0.10/0.08 for the two Si-N antibonds (which are mainly located on the Si atom). This supports the picture of a donor-acceptor type π-bonding interaction between these orbitals, to which the partial double bond character of the Si-O bond is ascribed. 24

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