Exploring sulfur donor atom coordination chemistry with La(II), Nd(II), and Tm(II) using a terphenylthiolate ligand.

To expand the range of donor atoms known to stabilize 4fn5d1 Ln(II) rare-earth metal (Ln) ions beyond the C, N, and O first row main group donor atoms, the Ln(III) sulfur donor terphenylthiolate iodide complexes, LnIII(SAriPr6)2I (AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2, Ln = La, Nd) were reduced to form LnII(SAriPr6)2 complexes. These Ln(II) species were structurally characterized, analyzed by density functional theory (DFT) calculations, and compared to Tm(SAriPr6)2, which was synthesized from TmI2(DME)3.

Synthesis of Crystallographically Characterizable Bis(cyclopentadienyl) Sc(II) Complexes: (C5H2tBu3)2Sc and {[C5H3(SiMe3)2]2ScI}1.

The synthesis of previously unknown bis(cyclopentadienyl) complexes of the first transition metal, i.e., Sc(II) scandocene complexes, has been investigated using C5H2(tBu)3 (Cpttt), C5Me5 (Cp*), and C5H3(SiMe3)2 (Cp″) ligands. Cpttt2ScI, 1, formed from ScI3 and KCpttt, can be reduced with potassium graphite (KC8) in hexanes to generate dark-red crystals of the first crystallographically characterizable bis(cyclopentadienyl) scandium(II) complex, Cpttt2Sc, 2. Complex 2 has a 170.6° (ring centroid)-Sc-(ring centroid) angle and exhibits an eight-line EPR spectrum characteristic of Sc(II) with Aiso = 82.6 MHz (29.6 G). It sublimes at 200 °C at 10-4 Torr and has a melting point of 268-271 °C. Reductions of Cp*2ScI and Cp″2ScI under analogous conditions in hexanes did not provide new Sc(II) complexes, and reduction of Cp*2ScI in benzene formed the Sc(III) phenyl complex, Cp*2Sc(C6H5), 3, by C-H bond activation. However, in Et2O and toluene, reduction of Cp*2ScI at -78 °C gives a dark-red solution, 4, which displays an eight-line EPR pattern like that of 1, but it did not provide thermally stable crystals. Reduction of Cp″2ScI, in THF or Et2O at -35 °C in the presence of 2.2.2-cryptand, yields the green Sc(II) metallocene iodide complex, [K(crypt)][Cp″2ScI], 5, which was identified by X-ray crystallography and EPR spectroscopy and is thermally unstable. The analogous reaction of Cp*2ScI with KC8 and 18-crown-6 in Et2O gave the ligand redistribution product, [Cp*2Sc(18-crown-6-κ2O,O')][Cp*2ScI2], 6, as the only crystalline product. Density functional theory calculations on the electronic structure of these compounds are reported in addition to a steric analysis using the Guzei method.

Reductive C-O Cleavage of Ethereal Solvents and 18-Crown-6 in Ln(NR2)3/KC8 Reactions (R = SiMe3).

The high reactivity accessible from the reduction of the tris(amide) complexes Ln(NR2)3 (R = SiMe3) with potassium graphite in the presence of a variety of ethers is demonstrated by crystal structures of six different types of products of C-O bond cleavage reactions with Ln = Y, Ho, Er, and Lu. Specifically, 1,2-dimethoxyethane (DME) can be cleaved in Ln(NR2)3/KC8 reactions as shown by three different types of crystals: [K (crypt)][(R2N)3Y(OCH2CH2OCH3)], 1-Y, [(R2N)2Y(µ-OCH2CH2OCH3-κO,κO')]2, 2-Y, and [K2(18-c-6)3]{[(R2N)3Lu]2[(µ-OCH2CH2O)]}, 3-Lu (18-c-6 = 18-crown-6; crypt = 2.2.2-cryptand). THF can be ring opened by the Y(NR2)3/KC8 reaction system, as shown by crystals of the butoxide, [K(crypt)][(R2N)3Y(OCH2CH2CH2CH3)], 4-Y. The cyclic ether, oxetane, OC3H6, ring opens in Ln(NR2)3/KC8 reactions to form crystals of the propoxide, [K(18-c-6)(OC3H6)][(R2N)3Ln(OCH2CH2CH3)], 5-Ln, for Ln = Ho and Er. In Et2O, the Y(NR2)3/KC8 reactions do not attack the solvent, but C-O cleavage of 18-c-6 is observed to form {[(R2N)2]Y[µ-η1:η1-O2(C10H20O4)K]}2, 6-Y. These Ln(NR2)3/KC8 C-O cleavage reactions are typically accompanied by C-H bond activation reactions, which form cyclometalates such as [K(crypt)]{(R2N)2Ln[N(SiMe3)(SiMe2CH2)-κC,κN]}, 7-Ln (Ln = Y, Ho, Er), and [K(18-c-6)]{(R2N)2Y[N(SiMe3)(SiMe2CH2)-κC,κN]}, 8-Y, which are common decomposition products of Ln(NR2)3 reactions. In addition, in this study, the hydride complex, [K(18-c-6)][(R2N)3YH], 9-Y, was isolated. NMR analysis indicates that the yttrium reactions form mixtures that consistently contain the yttrium cyclometalates 7-Y and 8-Y as major components. These results show the diversity of available reaction pathways for the Ln(NR2)3/KC8 system and highlight the inherent difficulties in isolating Ln(II) complexes containing the [Ln(NR2)3]1- anion.

London Dispersion Effects in a Distannene/Tristannane Equilibrium: Energies of their Interconversion and the Suppression of the Monomeric Stannylene Intermediate.

Reaction of {LiC6 H2 -2,4,6-Cyp3 ⋅Et2 O}2 (Cyp=cyclopentyl) (1) of the new dispersion energy donor (DED) ligand, 2,4,6-triscyclopentylphenyl with SnCl2 afforded a mixture of the distannene {Sn(C6 H2 -2,4,6-Cyp3 )2 }2 (2), and the cyclotristannane {Sn(C6 H2 -2,4,6-Cyp3 )2 }3 (3). 2 is favored in solution at higher temperature (345 K or above) whereas 3 is preferred near 298 K. Van't Hoff analysis revealed the 3 to 2 conversion has a ΔH=33.36 kcal mol-1 and ΔS=0.102 kcal mol-1 K-1 , which gives a ΔG300 K =+2.86 kcal mol-1 , showing that the conversion of 3 to 2 is an endergonic process. Computational studies show that DED stabilization in 3 is -28.5 kcal mol-1 per {Sn(C6 H2 -2,4,6-Cyp3 )2 unit, which exceeds the DED energy in 2 of -16.3 kcal mol-1 per unit. The data clearly show that dispersion interactions are the main arbiter of the 3 to 2 equilibrium. Both 2 and 3 possess large dispersion stabilization energies which suppress monomer dissociation (supported by EDA results).

Terpene dispersion energy donor ligands in borane complexes.

Structural characterization of the complex [B(ß-pinane)3] (1) reveals non-covalent H⋯H contacts that are consistent with the generation of London dispersion energies involving the ß-pinane ligand frameworks. The homolytic fragmentations of 1, and camphane and sabinane analogues ([B(camphane)3] (2) and [B(sabinane)3] (3)) were studied computationally. Isodesmic exchange results showed that London dispersion interactions are highly dependent on the terpene's stereochemistry, with the ß-pinane framework providing the greatest dispersion free energy (ΔG = -7.9 kcal mol-1) with Grimme's dispersion correction (D3BJ) employed. PMe3 was used to coordinate to [B(ß-pinane)3], giving the complex [Me3P-B(ß-pinane)3] (4), which displayed a dynamic coordination equilibrium in solution. The association process was found to be slightly endergonic at 302 K (ΔG = +0.29 kcal mol-1).

Inhibition of Alkali Metal Reduction of 1-Adamantanol by London Dispersion Effects.

A series of alkali metal 1-adamantoxide (OAd1 ) complexes of formula [M(OAd1 )(HOAd1 )2 ], where M=Li, Na or K, were synthesised by reduction of 1-adamantanol with excess of the alkali metal. The syntheses indicated that only one out of every three HOAd1 molecules was reduced. An X-ray diffraction study of the sodium derivative shows that the complex features two unreduced HOAd1 donors as well as the reduced alkoxide (OAd1 ), with the Ad1 fragments clustered together on the same side of the NaO3 plane, contrary to steric considerations. This is the first example of an alkali metal reduction of an alcohol that is inhibited from completion due to the formation of the [M(OAd1 )(HOAd1 )2 ] complexes, stabilized by London dispersion effects. NMR spectroscopic studies revealed similar structures for the lithium and potassium derivatives. Computational analyses indicate that decisive London dispersion effects in the molecular structure are a consequence of the many C-H⋅⋅⋅H-C interactions between the OAd1 groups.

Designing a Solution-Stable Distannene: The Decisive Role of London Dispersion Effects in the Structure and Properties of {Sn(C6H2-2,4,6-Cy3)2}2 (Cy = Cyclohexyl).

The reaction of 1 equiv of the dimeric lithium salt of a new London dispersion effect donor ligand {Li(C6H2-2,4,6-Cy3)·OEt2}2 (Cy = cyclohexyl) with SnCl2 afforded the distannene {Sn(C6H2-2,4,6-Cy3)2}2 (1). The distannene remains dimeric in solution, as indicated by its room-temperature 119Sn NMR signal (δ = 361.3 ppm) and its electronic spectrum, which is invariant over the temperature range of -10 to 100 °C. The formation of the distannene, which has a short Sn-Sn distance of 2.7005(7) Å and greatly enhanced stability in solution compared to that of other distannenes, is due to increased interligand London dispersion (LD) attraction arising from multiple close approaches of ligand C-H moieties across the Sn-Sn bond. DFT-D4 calculations revealed a dispersion stabilization of dimer 1 of 38 kcal mol-1 and a dimerization free energy of ΔGdimer = -6 kcal mol-1. In contrast, the reaction of 2 equiv of the similarly shaped but less bulky, less hydrogen-rich Li(C6H2-2,4,6-Ph3)·(OEt2)2 with SnCl2 yielded the monomeric stannylene Sn(C6H2-2,4,6-Ph3)2 (2), which is unstable in solution at ambient temperature.

Hydrides, Halides, and Polymers: Some Unexpected Intermediates on the Routes to First-Row Transition Metal M{N(SiMe3)2}n (n = 2, 3) Complexes.

The reaction of 2 equiv of LiN(SiMe3)2·Et2O with TiCl3(NMe3)2 or VCl3(NMe3)2 afforded the dimeric, halide bridged complexes [Ti(µ-Cl){N(SiMe3)2}2]2 (1) or [V(µ-Cl){N(SiMe3)2}2]2 (2) in moderate yields. The reduction of titanium complex 1 with 3 equiv of 5% (wt) Na/NaCl gave the mixed metal titanium/sodium hydride cluster Ti2(µ-H)2{N(SiMe3)2}3{N(SiMe3)(SiMe2CH)}(Na) (3), which was formed by activation of two C-H bonds at a single methyl group of one of the bis(trimethylsilyl)amide ligands. Attempts to form the analogous vanadium complex by reduction of 2 gave only intractable products. Treatment of Co{N(SiMe3)2}2 with 1 equiv of BrN(SiMe3)2 (which was previously shown to give the unique three-coordinate cobalt(III) trisamide Co{N(SiMe3)2}3) afforded the polymeric [(µ-Br)Co{µ-N(SiMe3)(SiMe2CH2CH2Me2Si)(Me3Si)µ-N}Co(µ-Br)]∞ (4) as a second product, which was shown by a structural analysis to possess a carbon-carbon bond formed between the two ligands. Attempts to isolate manganese and iron complexes analogous to 4 were unsuccessful. The role of bromine in these reactions was further studied by examining the reaction of 0.5 equiv of elemental bromine with [Mn{N(SiMe3)2}2]2 or [Co{N(SiMe3)2}2]2, which for manganese was shown to give the previously reported manganese trisamide Mn{N(SiMe3)2}3 but for cobalt gives the dimeric amide-bridged [Co(Br){µ-N(SiMe3)2}]2 (5).

Reductions of M{N(SiMe3)2}3 (M = V, Cr, Fe): Terminal and Bridging Low-Valent First-Row Transition Metal Hydrido Complexes and "Metallo-Transamination".

The reaction of the vanadium(III) tris(silylamide) V{N(SiMe3)2}3 with LiAlH4 in diethyl ether gives the highly unstable mixed-metal polyhydride [V(µ2-H)6[Al{N(SiMe3)2}2]3][Li(OEt2)3] (1), which was structurally characterized. Alternatively, performing the same reaction in the presence of 12-crown-4 affords a rare example of a structurally verified vanadium terminal hydride complex, [VH{N(SiMe3)2}3][Li(12-crown-4)2] (2). The corresponding deuteride 2D was also prepared using LiAlD4. In contrast, no hydride complexes were isolated by reaction of M{N(SiMe3)2}3 (M = Cr, Fe) with LiAlH4 and 12-crown-4. Instead, these reactions afforded the anionic metal(II) complexes [M{N(SiMe3)2}3][Li(12-crown-4)2] (3, M = Cr; 4, M = Fe). The reaction of the iron(III) tris(silylamide) Fe{N(SiMe3)2}3 with lithium aluminum hydride without a crown ether gives the "hydrido inverse crown" complex [Fe(µ2-H){N(SiMe3)2}2(µ2-Li)]2 (5), while treatment of the same trisamide with alane trimethylamine complex gives the iron(II) polyhydride complex Fe(µ2-H)6[Al{N(SiMe3)2}2]2[Al{N(SiMe3)2}(NMe3)] (6). Complexes 2-6 were characterized by X-ray crystallography, as well as by infrared, electronic, and 1H and 13C (complex 6) NMR spectroscopies. Complexes 1 and 6 are apparently formed by an unusual "metallo-transamination" process.

Low-Coordinate Iron Chalcogenolates and Their Complexes with Diethyl Ether and Ammonia.

Treatment of Fe{N(SiMe3)2}2 with 2 equiv of the appropriate phenol or thiol affords the dimers {Fe(OC6H2-2,6-But2-4-Me)2}2 (1) and {Fe(OC6H3-2,6-But2)2}2 (2) or the monomeric Fe{SC6H3-2,6-(C6H3-2,6-Pri2)2}2 (3) in moderate to excellent yields. Recrystallization of 1 and 2 from diethyl ether gives the corresponding three-coordinate ether complexes Fe(OC6H3-2,6-But2-4-Me)2(OEt2) (4) and Fe(OC6H3-2,6-But2)2(OEt2) (5). In contrast, no diethyl ether complex is formed by the dithiolate 3. The 1H NMR spectra of 4 and 5 show equilibria between the ether complexes and the base-free dimers. A comparison of these spectra with those of the dimeric 1 and 2 allows an unambiguous assignment of the paramagnetically shifted signals. Treatment of 1 with excess ammonia gives the tetrahedral diammine Fe(OC6H2-2,6-But2-4-Me)2(NH3)2 (6). Ammonia is strongly coordinated in 6, with no apparent loss of ammine ligand either in solution or upon heating under low pressure. In contrast, significantly weaker ammonia coordination is observed when dithiolate 3 is treated with excess ammonia, which gives the diammine Fe{SC6H3-2,6-(2,6-Pri2-C6H3)2}2(NH3)2 (7). Complex 7 readily loses ammonia either in solution or under reduced pressure to give the monoammine complex Fe{SC6H3-2,6-(2,6-Pri2-C6H3)2}2(NH3) (8). The weak binding of ammonia by iron thiolate 7 reflects the likely behavior of the proposed iron-sulfur active site in nitrogenases, where release of ammonia is required to close the catalytic cycle.

Two-Coordinate, Nonlinear Vanadium(II) and Chromium(II) Complexes of the Silylamide Ligand-N(SiMePh2)2: Characterization and Confirmation of Orbitally Quenched Magnetic Moments in Complexes with Sub-d5 Electron Configurations.

The two-coordinate metal amide complexes V{N(SiMePh2)2}2 (1) and Cr{N(SiMe2Ph)2}2 (2) were synthesized by reaction of two equivalents of LiN(SiMePh2)2 with VI2(THF)4 or CrCl2(THF)2 in n-hexane. Their crystal structures showed that they have bent coordination, N-V-N = 137.0(4)°, N-Cr-N = 139.19(5)°, at the metals. The vanadium complex (1) displayed no tendency to isomerize as previously observed for some V(II) amido complexes. Curie fits of SQUID magnetic measurements afforded magnetic moments of 3.36 (1) and 4.68 (2) µB, consistent with high-spin configurations. These values are lower than the spin-only values of 3.88 and 4.90 µB expected for d3 and d4 complexes, suggesting a significant unquenched orbital angular momentum contribution to the overall moment, which is lower as a result of the positive spin-orbit coupling constants.

Molecular Complexes Featuring Unsupported Dispersion-Enhanced Aluminum-Copper and Gallium-Copper Bonds.

The reaction of the copper(I) ß-diketiminate copper complex {(Cu(BDIMes))2(µ-C6H6)} (BDIMes = N,N'-bis(2,4,6-trimethylphenyl)pentane-2,4-diiminate) with the low-valent group 13 metal ß-diketiminates M(BDIDip) (M = Al or Ga; BDIDip = N,N'-bis(2,6-diisopropylphenyl)pentane-2,4-diiminate) in toluene afforded the complexes {(BDIMes)CuAl(BDIDip)} and {(BDIMes)CuGa(BDIDip)}. These feature unsupported copper-aluminum or copper-gallium bonds with short metal-metal distances, Cu-Al = 2.3010(6) Å and Cu-Ga = 2.2916(5) Å. Density functional theory (DFT) calculations showed that approximately half of the calculated association enthalpies can be attributed to London dispersion forces.

Characterization of the "Absent" Vanadium Oxo V(═O){N(SiMe3)2}3, Imido V(═NSiMe3){N(SiMe3)2}3, and Imido-Siloxy V(═NSiMe3)(OSiMe3){N(SiMe3)2}2 Complexes Derived from V{N(SiMe3)2}3 and Kinetic Study of the Spontaneous Conversion of the Oxo Complex into Its Imido-Siloxy Isomer.

The synthesis and characterization of V(═O){N(SiMe3)2}3 (1), V(═NSiMe3){N(SiMe3)2}3 (2), and V(═NSiMe3)(OSiMe3){N(SiMe3)2}2 (3) are described. Prior attempts to synthesize the vanadium(V) oxo complex 1 via salt metathesis of VOCl3 with the lithium or sodium silylamide salt had yielded either the putative rearranged species V(═NSiMe3)(OSiMe3){N(SiMe3)2}2 (3) or the oxo-bridged, dimetallic {(µ-O)2V2[N(SiMe3)2]4}. We now show that complex 1 is available by treatment of the vanadium(III) tris(silylamide) V{N(SiMe3)2}3 with iodosylbenzene. The imido complex 2 was obtained by treatment of V{N(SiMe3)2}3 with trimethylsilyl azide. Sublimation of 1 formed complex 3, which was determined to be V(═NSiMe3)(OSiMe3){N(SiMe3)2}2, on the basis of infrared, electronic, and 1H and 51V NMR spectroscopies. Crystallographic disorder precluded a complete structural characterization of 3, although a four-coordinate V atom, as well as severely disordered ligands, were apparent. Comparison of the vibrational spectra of 1 and 2 allowed an unambiguous assignment of the V-O (995 cm-1) and V-Nimide (1060 cm-1) stretching bands. The vibrational spectrum of complex 3 displayed strong absorbances at 1090 and 945 cm-1, indicative of its metal imide and metal siloxide moieties. The 1H NMR spectrum of 1 in deuterated benzene showed overlapping signals for the ligand protons proximal and distal to the oxo moiety at 0.52 and 0.38 ppm. The 1H NMR spectrum of 2 in deuterated methylene chloride displayed distinct signals for the imido (0.41 ppm) and amido (0.35 ppm) protons, whereas 1H NMR spectroscopy of 3 showed three signals in an intensity ratio consistent with the formula V(═NSiMe3)(OSiMe3){N(SiMe3)2}2. 51V NMR spectra of 1-3 revealed singlet resonances at -119 ppm (1), -24 ppm (2), and -279 ppm (3). The electronic spectra of 1-3 displayed single absorbances in the charge transfer region, consistent with their d0 electron configurations. Kinetic studies of the spontaneous conversion of complex 1 to 3 were used to determine the rate constants (ca. 0.0002 s-1 (63 °C), 0.0006 s-1 (73 °C), 0.002 s-1 (83 °C)) and activation energy (ca. 20 kcal/mol) of this first-order process.

Unexpected Coordination Complexes of the Metal Tris-silylamides M{N(SiMe3)2}3 (M = Ti, V).

The synthesis, molecular structures, and spectroscopic details of a series of isocyanide and nitrile complexes of the early first-row transition-metal tris(silyl)amides M{N(SiMe3)2}3 (M = Ti, V) are reported. Previously, first-row transition-metal tris(silyl)amides were generally thought to be incapable of forming complexes with Lewis bases due to their excessive steric crowding. However, it is now shown that simple treatment of the base-free trisamides with 2 equiv of an isocyanide or nitrile base at room temperature results in the formation of the trigonal bipyramidal complexes Ti{N(SiMe3)2}3(1-AdNC)2 (1), Ti{N(SiMe3)2}3(CyNC)2 (2), Ti{N(SiMe3)2}3(ButNC)2 (3), Ti{N(SiMe3)2}3(PhCN)2 (4), V{N(SiMe3)2}3(1-AdNC)2 (5), V{N(SiMe3)2}3(CyNC)2 (6), V{N(SiMe3)2}3(ButNC)2 (7), and V{N(SiMe3)2}3(PhCN)2 (8), which incorporate two donor ligands (1-AdNC = 1-adamantyl isocyanide, CyNC = cyclohexyl isocyanide, ButNC = tert-butyl isocyanide, PhCN = benzonitrile). All complexes display a characteristic increase in the frequency of the multiple bonded C-N stretching mode which is observed to be in the range of 2170-2190 cm-1 for the isocyanide complexes 1-3 and 5-7 and at 2250 cm-1 for the nitrile complex 8. This effect was not observed for the titanium nitrile complex 4, suggesting weak binding of the donor to titanium. Paramagnetic 1H NMR studies showed these complexes to have detectable, though extremely broadened, signals attributable to the trimethylsilyl groups of the amide ligands (δ = ca. 2.8 ppm for titanium isocyanide complexes, ca. 4.5-4.7 ppm for vanadium isocyanide complexes). A variable-temperature 1H NMR study showed that in solution these complexes exist as mixtures of the five-coordinate species and a putative four-coordinate species coordinating a single Lewis basic ligand. Electronic spectroscopy indicated that the vanadium complexes 5-8 bind the Lewis bases more strongly than the corresponding titanium complexes, where the spectra of complexes 1-4 are essentially identical to the base-free Ti{N(SiMe3)2}3 at the temperatures and concentrations studied. In contrast to these results, no corresponding complexes were detected for the metal silylamides M{N(SiMe3)2}3 (M = Cr, Mn, Fe, or Co) when treated with the isocyanide or nitrile bases.