Theodore A. Budzichowski

Silasesquioxanes as ligands in inorganic and organometallic chemistry

Abstract Silasesquioxanes are an interesting class of ligands for both main group and transition-metal elements. A variety of coordination environments can be supported by silasesquioxane ligands, and it is now possible to prepare metallasilsesquioxanes containing practically any stable element from Groups 1015. This article surveys the synthesis, characterization, and reactivity of silasesquioxane ligands. Structure, bonding and structural dynamics of both silasesquioxanes and metallasilasesquioxanes are also discussed. The chemistry of silasesquioxanes and metallasilasesquioxanes can provide important insights into the chemistry of silica and silica-supported transition-metal catalysts. The uniqueness of silasesquioxanes as models for silica is discussed and several systems which provide molecular-level insights into the surface chemistry of silica and silica-supported catalysts are discussed.

Syntheses of highly-functionalized polyhedral oligosilsesquioxanes

Abstract The hydrolytic condensation of p-ClCH2C6H4SiCl3 in aqueous acetone affords [p-ClCH2C6H4SiO3/2]8 (1), a synthetically useful precursor for the syntheses of octafunctional polyhedral oligosilsesquioxanes [p-XCH2C6H4SiO1.5]8, including 2 (X = I), 3 (X = OH), 4 (X = ONO2), 5 (X = OAc), 6 (X = p-nitrobenzoyl), and 7 (X = methylterephthaloyl).

New polyhedral oligosilsesquioxanes via the catalytic hydrogenation of aryl-containing silsesquioxanes

The hydrolytic condensation of RSiCl3 (R = benzyl, m-tolyl, 3,5-dimethylphenyl) gives good yields of the corresponding octameric aryl silsesquioxanes (1). A single-crystal X-ray diffraction study of highly soluble 1 (R = benzyl) reveals that highly efficient crystal packing can be accomplished without the inclusion of solvent or the strong intermolecular π-stacking arrangements that normally lead to poor solubility properties. The catalytic hydrogenation of aryl polyhedral oligosilsesquioxanes (POSS) affords high yields of the corresponding aliphatic silsesquioxanes. These new silsesquioxanes display thermal and physical properties comparable to the corresponding aryl-containing POSS but generally have much greater solubilities in common organic solvents. The catalytic hydrogenation of [Ph12Si12O20] affords iso-[Cy12Si12O20], which possesses local C2v rather than D6h symmetry.

Is it possible to stabilise complexes with a tungsten–phosphorus triple bond?

[W(CO)2{η4-(But2C2P2)W(CO)5}{η-2-(ButCP)W(CO)5}]1 is prepared by treatment of ButCP with [W(CO)5THF]; the reaction of [W2(OBut)6] with ButCP in the presence of [M(CO)5THF](M = Cr, W) affords novel compounds of the type [(ButO)3WP→M(CO)5](M = Cr, W), the first examples of a formal WP containing complex.

Oxide-base adducts of aluminum: X-ray crystal structures of Me3Al(OPPh3), Me3Al(ONMe3) and [(CH3)3SiO]3Al(OPPh3)

Abstract The crystalline molecular structures of three Lewis adducts of aluminum are reported. The structures of the 1 : 1 adducts between trimethylaluminum and triphenylphosphine oxide, 1 : Me 3 Al(OPPh 3 ), trimethylaluminum and trimethylamine-N-oxide; 2 : Me 3 Al(ONMe 3 ); and 3 : [(CH 3 ) 3 SiO] 3 Al(OPPh 3 ) are discussed, and a comparison is made to the previously characterized aluminosilsesquinoxane complex ( c -C 6 H 11 ) 7 Si 7 O 12 Al(OPPh 3 ) ( 5 ).

Pyridine, isocyanide, carbodiimide and allene adducts of hexakis (trifluoromethyl t-botoxy) ditungsten. A comparison of ligand binding to W2(OtBu)6 and W2(OCMe2CF3)6

Abstract W2(OR)6 compounds where RtBu and CMe2CF3, both reversibly bind pyridine in hydrocarbon solvents to form adducts W2(OR)6L2. Pyridine binds more strongly to the fluoroalkoxide but the structural parameters of the compounds W2(OCMe2CF3)6(C6H5N)2 and W2(OtBu)6(4-CH3C6H4N)2 reveal an essentially identical W2O6N2 core with W  W = 2.39(1) A ,, W  O = 1.92–1.95 A and W  N = 2.26(1) A . Both compounds were crystallographically characterized in the space group C2/c and each molecule has rigorous C2 symmetry. Allene and 1,3-di-p-tolycarbodiimide form 1:1 adducts with W2(OCMe2CF3)6 in which the substrate is bound parallel to the M-M axis, i.e. μ-η2,η2-C3H4 and μ-η2,η2-ArNCNAr-W2(OCMe2CF3)6. Also W2(OCMe2CF3)6 and W2(OSitBuMe2)6 bind two equivalents of xylylisocyanide to afford W2(OR)6(η1-CNAr)2. For W2(OCMe2CF3)6(η1-CNAr)2, the molecular structure has been determined by X-ray crystallography and shows a nearly eclipsed central W2O6C2 skeleton with W  W = 2.44(1) A , W  O = 1.94(1) A (av.) and W  C = 2.14(1) A , whereas the WWO angles span the range of 105–114°, the WWC angles are 83(1)°. Similarly, W2(OCMe(CF3)2)4(NMe2)2 forms a bis adduct upon reaction with the isocyanide. However, the molecular structure of W2(OCMe(CF3)2)4(NMe2)2(η1-CNAr)2 shows a staggered arrangement of the two ligands about the ditungsten center where W  W = 2.382(1) A , W  O = 2.00(1) A (av.), W  N = 1.93(1) A (av.) and W  C = 2.14(1) A (av.) with a CWWC dihedral angle of 41.9°. These reactions and their products are compared for W2(OR)6 compounds where R  t Bu , t BuMe 2 Si and Cme 2 CF 3 .

Coordination chemistry of complexes with metalmetal multiple bonds: Reversible coordination of cyanide ion by dimolybdenum and ditungsten hexaalkoxides☆

Abstract The dimetal-hexaalkoxides of molybdenum and tungsten (MM) [1a : W2(OtBu)6, 1b : W2(OCH2tBu)6, 1c : Mo2(OtBu)6, 1d : Mo2(OtPr)6, 1e : Mo2(OCH2tBu)6] react reversibly with one equivalent of cyanide ion as its nBu4N salt in non-polar media to form mono-adducts of formula [nBu4N]+[M2(OR)6(CN)]− (2a–e). The spectroscopic data are consistent with the presence of a μ-CN moiety which interacts predominantly as a σ-donor, but do not discount alternative structures which exhibit rapid fluxionality. Addition of a second equivalent (or more) of nBu4NCN leads to the reversible formation of 1,2-diadducts of formula [nBu4N]2+[M2(OR)6(CN)2]2− (3a, 3b, 3d, 3e). Coordination of a second equivalent of cyanide is significantly less favorable than the first so that the consecutive equilibria may be measured. Determination of Keq for the equilibrium M2(OR)6+CN− ⇋ [M2(OR)6(CN)]− allows the first quantitative comparison of the Lewis acidity of the reactive dimetal hexaalkoxides. The identity of the metal and steric factors have a significant effect as shown by the following data: for the formation of 2a, ΔH° = − 11.2(1) kcal mol−1, ΔS° = − 18.6(7) eu; 2c, ΔH° = − 8.8(1) kcal mol−1, ΔS° = − 18.8(9) eu; 2d, ΔH° = − 10.5(1) kcal mol−1, ΔS° = − 18.0(8) eu. Complexes 2b and 2e showed no signs of dissociation under similar conditions and thermodynamic parameters associated with their formation may only be estimated (ΔH° = − 16.9 kcal mol−1, ΔS° = − 18.5 eu and ΔH° = − 13.5 kcal mol−1, ΔS° = − 18.5 eu respectively). At high temperatures the equilibrium becomes fast relative to the NMR time scale for molybdenum, and this allows an estimation of the rate of dissociation by dynamic lineshape analysis. For [Mo2(OtBu)6 (CN)]− (2c) ΔH‡ = 22.0(5) kcal mol−1, ΔS‡ = 13.5(8) eu while for [Mo2(OCH2tBu)6 (CN)]− (2e) ΔH‡ = 22.2(5) kcal mol−1, ΔS‡ = 13.9(8) eu. Line broadening was not observed for the tungsten alkoxide complexes 2a–b consistent with the greater thermodynamic strength of the W2(CN) bond (ΔΔH° = − 2.4 kcal mol−1). The second equilibrium [M2(OR)6(μ-CN)]− + CN− ⇋ [M2(OR)6(CN)2]2− shows a similar dependence on the nature of the metal and alkoxide: for formation of 3a, ΔH° = − 9.3(5) kcal mol−1, ΔS° = − 29(2) eu; for 3e, ΔH° = − 11.6(5) kcal mol−1, ΔS° = − 29(2) eu. In contrast, 3c was not detected under any conditions (ΔH° = − 6.9 kcal mol−1, ΔS° = − 29.2 eu, estimated) while 3b was undissociated under similar conditions (ΔH° = − 15.0 kcal mol−1, ΔS° = − 29.2 eu, estimated).

Tungsten-containing polyhedral oligosilasesquioxanes: synthesis, structure and reactivity of (c-C6H11)7Si7O9(O3W[NMe2]3)

Abstract The reaction of the trisilanol (c-C6H11)7Si7O9(OH)3 (1) with an equimolar amount of W(NMe2)6 (2) produces the mixed amide/siloxide complex (c-C6H11)7Si7O9(O3W[NMe2]3) (5). The solid state structure of 5, as determined by a single-crystal X-ray diffraction study, contains a slightly distorted fac-octahedral WN3O3 unit with relatively short WN bonds [1.958(5) A]. The silasesquioxane framework easily accommodates the size and steric demands of the W(NMe2)3 substrate with SiOW = 162.4(3)° and WO = 1.947(4) A. Despite the obtuse SiOW linkages there does not appear to be exceptionally strong π-interactions between the oxygen atoms and the tungsten centre in this complex. Consistent with the steric bulk and poor electron donating capabilities of the siloxide ligand, complex 5 is relatively inert towards further protonolysis of the three remaining amide groups which are engaged in π-bonding to the metal centre.

Metathetic reactions involving ditungsten hexapivalate. Preparation and structures of W2(O2CtBu)2(OtBu)2(NiPr2)2, W2(O2CtBu)4(NiPr2)2, W2(O2CtBu)2(OtBu)4 and Na2W4O4Cp2(O2CtBu)6 solvate where CpC5H5

The reactions between W2(O2CtBu)6(M3−M) and each of Na/Hg, NaBH4, LiHBEt3 and LiMe in hydrocarbon/THF solutions yield W2(O2CtBu)4(M4−M). W2(O2CtBu)6 and KOtBu (2 equiv.) or W2(OtBu)6 react to give W2(O2CtBu)2(OtBu)4 (1) by ligand redistribution. Crystal data for 1 at − 172°C: a = 11.200(3), b = 16.331(5), c = 10.490(3) A, α = 95.71(2), β = 111.19(1), γ = 71.82(1)°, Z = 2, Dcalc = 1.685 g cm−3, space group P-1. A similar reaction involving W2(O2CtBu)6 and LiNPr2 i (2 equiv.) yields and isomeric mixture of W2(O2CtBu)4(NiPr2)2, for which one isomer, 2, has been crystallographically characterized. Crystal data for 2 at −164°C: a = 41.858(9), b = 10.009(2), c = 41.481(9) A, Z = 16, Dcalc = 1.608 g cm−3, space group C2/c. Compound 2 was shown to react with KOtBu (2 equiv.) to give a 6:1:1 mixture of products from which the major product W2(O2CtBu)2(NiPr2)2(OtBu)2 (3) was characterized. Crystal data for 3 at −174°C: a = 10.675(4), b = 9.836(4), c = 35.795(14) A, β = 96.76(2)°, Z = 4, Dcalc = 1.631 g cm−3, space group P21/c. The reaction between W2(O2CtBu)6 and NaCp (2 equiv.) in THF gave an orange hydrocarbon soluble crystalline compound Na2W4O4Cp2(O2CtBu)6·4tBuCOOH·toluene, (4· solvate). Crystal data for 4·solvate at −70°C: a = 13.065(2), b = 14.941(2), c = 12.996(2) A, β = 114.90(1), γ = 71.69(1)°, Z = 1, Dcalc = 1.67(2) g cm−3, space group P-1. In compounds 1, 2 and 3 there are Wz.3usco;;W bonds and each metal atom is coordinated to four ligand atoms. The Mz.3usco;M bond is spanned by a pair of cis-ppivalate ligands. The structure of the W4O4Cp2(O2Ct Bu)62− anion is a centrosymmetric bis-oxo bridged dimer [η5-C5H5)(η1-O2CtBu)2W (μ-η1, η1-O2CtBu)W (=O)(μ-O)]2 with a WW bonding distance of 2.537(1) A.

Two new structural types for d3-d3 dimers of molybdenum and tungsten: [K(18-crown-6)]+[Mo2(OCH2But)7]− and [W2{P(c-hexyl)2}3(OCH2But)3{HP(c-hexyl)2}]

Abstract The syntheses and structures (determined by single-crystal X-ray diffraction techniques) are reported for [K(18-crown-6)]+[Mo2(OCH2But)7]− and [W2{P(c-hexyl)3}3(OCH2But3{HP(c-hexyl)2}]. The former contains a single bridging (μ-OCH2But) ligand, six terminal alkoxide ligands and an MM triple bond. The latter contains three bridging (μ-PR2) ligands and an MM bond distance that is most consistent with the presence of a single bond. These two compounds are the first examples of M2L7 complexes with bridging ligands. They highlight the delicate balance of metal-metal vs metal-ligand bonding in the coordination chemistry of d3-d3 dimers of molybdenum and tungsten.