An official website of the United States government
Herein we report an experimental and computational study of a family of four coordinated 14-electron complexes of Rh( iii ) devoid of agostic interactions. The complexes [X–Rh(κ 3 ( P,Si,Si )PhP( o -C 6 H 4 CH 2 Si i Pr 2 ) 2 ], where X = Cl (Rh-1), Br (Rh-2), I (Rh-3), OTf (Rh-4), Cl·GaCl 3 (Rh-5); derive from a bis(silyl)- o -tolylphosphine with isopropyl substituents on the Si atoms. All five complexes display a sawhorse geometry around Rh and exhibit similar spectroscopic and structural properties. The catalytic activity of these complexes and [Cl–Ir(κ 3 ( P,Si,Si )PhP( o -C 6 H 4 CH 2 Si i Pr 2 ) 2 ], Ir-1, in styrene and aliphatic alkene functionalizations with hydrosilanes is disclosed. We show that Rh-1 catalyzes effectively the dehydrogenative silylation of styrene with Et 3 SiH in toluene while it leads to hydrosilylation products in acetonitrile. Rh-1 is an excellent catalyst in the sequential isomerization/hydrosilylation of terminal and remote aliphatic alkenes with Et 3 SiH including hexene isomers, leading efficiently and selectively to the terminal anti-Markonikov hydrosilylation product in all cases. With aliphatic alkenes, no hydrogenation products are observed. Conversely, catalysis of the same hexene isomers by Ir-1 renders allyl silanes, the tandem isomerization/dehydrogenative silylation products. A mechanistic proposal is made to explain the catalysis with these M( iii ) complexes.
more »
« less
Origin of the 29 Si NMR chemical shift in R 3 Si–X and relationship to the formation of silylium (R 3 Si + ) ions
The origin in deshielding of 29 Si NMR chemical shifts in R 3 Si–X, where X = H, OMe, Cl, OTf, [CH 6 B 11 X 6 ], toluene, and O X (O X = surface oxygen), as well as i Pr 3 Si + and Mes 3 Si + were studied using DFT methods. At the M06-L/6-31G(d,p) level of theory the geometry optimized structures agree well with those obtained experimentally. The trends in 29 Si NMR chemical shift also reproduce experimental trends; i Pr 3 Si–H has the most shielded 29 Si NMR chemical shift and free i Pr 3 Si + or isolable Mes 3 Si + have the most deshielded 29 Si NMR chemical shift. Natural localized molecular orbital (NLMO) analysis of the contributions to paramagnetic shielding ( σ p ) in these compounds shows that Si–R (R = alkyl, H) bonding orbitals are the major contributors to deshielding in this series. The Si–R bonding orbitals are coupled to the empty p-orbital in i Pr 3 Si + or Mes 3 Si + , or to the orbital in R 3 Si–X. This trend also applies to surface bound R 3 Si–O X . This model also explains chemical shift trends in recently isolated t Bu 2 SiH 2 + , t BuSiH 2 + , and SiH 3 + that show more shielded 29 Si NMR signals than R 3 Si + species. There is no correlation between isotropic 29 Si NMR chemical shift and charge at silicon.
more »
« less
- Award ID(s):
- 1800561
- PAR ID:
- 10227736
- Date Published:
- Journal Name:
- Dalton Transactions
- Volume:
- 49
- Issue:
- 45
- ISSN:
- 1477-9226
- Page Range / eLocation ID:
- 16453 to 16463
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Reaction of [CuH(PPh3)]6with 1 equiv. of Tl(OTf) results in formation of [Cu6TlH6(PPh3)6][OTf] ([1]OTf]), which can be isolated in good yields. Variable‐temperature1H NMR spectroscopy, in combination with density functional theory (DFT) calculations, confirms the presence of a rare Tl−H orbital interaction. According to DFT, the1H chemical shift of the Tl‐adjacent hydride ligands of[1]+includes 7.7 ppm of deshielding due to spin‐orbit effects from the heavy Tl atom. This study provides valuable new insights into a rare class of metal hydrides, given that[1][OTf]is only the third isolable species reported to contain a Tl−H interaction.more » « less
-
The ratio of silicon-29 nuclear magnetic resonance (NMR) coherence lifetimes for Q 4 and Q 3 sites under magic-angle spinning and a train of π pulses in a series of binary alkali silicate glasses is used to detect phase separation, even at small scales where the glass appears optically homogenous. This approach exploits the dependence of echo train coherence lifetimes on the residual heteronuclear dipolar coupling between Si-29 and the NMR active nuclei of neighboring network modifier cations. The shifted-echo phase incremented echo train acquisition NMR method is used to eliminate modulation of the echo train amplitudes due to J couplings across Si–O–Si linkages. A 2D Fourier and inverse Laplace transform of this dataset provides a correlation of the isotropic 29 Si chemical shift to echo train coherence lifetimes, giving a sensitive probe of phase separation as well as chemical composition and local structure of the different phases. The 29 Si Q 4 :Q 3 mean coherence lifetime ratios are 28.8, 23.8, and 5.8 in the phase-separated glasses, 0.05Li 2 O·0.95SiO 2 , 0.1Li 2 O·0.9SiO 2 and 0.05Na 2 O·0.95SiO 2 , respectively, while the ratio is reduced to 2.1, 1.6, and 1.6 in glasses not exhibiting signs of phase separation, 0.05K 2 O·0.95SiO 2 , 0.05Cs 2 O·0.95SiO 2 and 0.10Cs 2 O·0.90SiO 2 , respectively. Phase separation inhibition, through addition of alumina, is also verified in 0.07Li 2 O·0.02Al 2 O 3 ·0.91SiO 2 .more » « less
-
The concept of metalla-aromaticity proposed by Thorn–Hoffmann ( Nouv. J. Chim . 1979, 3, 39) has been expanded to organometallic molecules of transition metals that have more than one independent electron-delocalized system. Lanthanides, with highly contracted 4f atomic orbitals, are rarely found in multiply aromatic systems. Here we report the discovery of a doubly aromatic triatomic lanthanide-boron molecule PrB 2 − based on a joint photoelectron spectroscopy and quantum chemical investigation. Global minimum structural searches reveal that PrB 2 − has a C 2v triangular structure with a paramagnetic triplet 3 B 2 electronic ground state, which can be viewed as featuring a trivalent Pr(III,f 2 ) and B 2 4− . Chemical bonding analyses show that this cyclo-PrB 2 − species is the smallest 4f-metalla-aromatic system exhibiting σ and π double aromaticity and multiple Pr–B bonding characters. It also sheds light on the formation of the rare B 2 4− tetraanion by the high-lying 5d orbitals of the 4f-elements, completing the isoelectronic B 2 4− , C 2 2− , N 2 , and O 2 2+ series.more » « less
-
In this work, the structural and electrical properties of metalorganic chemical vapor deposited Si-doped β-(Al x Ga 1−x ) 2 O 3 thin films grown on (010) β-Ga 2 O 3 substrates are investigated as a function of Al composition. The room temperature Hall mobility of 101 cm 2 /V s and low temperature peak mobility (T = 65 K) of 1157 cm 2 /V s at carrier concentrations of 6.56 × 10 17 and 2.30 × 10 17 cm −3 are measured from 6% Al composition samples, respectively. The quantitative secondary ion mass spectroscopy (SIMS) characterization reveals a strong dependence of Si and other unintentional impurities, such as C, H, and Cl concentrations in β-(Al x Ga 1−x ) 2 O 3 thin films, with different Al compositions. Higher Al compositions in β-(Al x Ga 1−x ) 2 O 3 result in lower net carrier concentrations due to the reduction of Si incorporation efficiency and the increase of C and H impurity levels that act as compensating acceptors in β-(Al x Ga 1−x ) 2 O 3 films. Lowering the growth chamber pressure reduces Si concentrations in β-(Al x Ga 1−x ) 2 O 3 films due to the increase of Al compositions as evidenced by comprehensive SIMS and Hall characterizations. Due to the increase of lattice mismatch between the epifilm and substrate, higher Al compositions lead to cracking in β-(Al x Ga 1−x ) 2 O 3 films grown on β-Ga 2 O 3 substrates. The (100) cleavage plane is identified as a major cracking plane limiting the growth of high-quality Si-doped (010) β-(Al x Ga 1−x ) 2 O 3 films beyond the critical thicknesses, which leads to highly anisotropic and inhomogeneous behaviors in terms of conductivity.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/D0DT02099K
