Aromaticity of 5- and 7-Membered Metallacyclocumulenes of Group 4

Abstract

Using the recent aromatic criteria, the conjugation and aromatic properties of group 4 metallacyclocumulenes Cp 2 M-η 4 −(R)CCCC−R (1) and Cp 2 M-η 3 (R−C CCC(R)−R(C) CR) (2) (M = Ti, Zr, Hf) were studied. To these nontrivial bonded metallacycles, the most recent universal criteria of aromaticity were applied that do not require any parametrisation such as isomerisation stabilization energy, electron density of delocalized bonds (EDDB), nuclear independent chemical shift, and gauge-included magnetically induced currents (GIMIC). All methods used indicated the absence of the common π-type aromaticity. The most efficient EDDB and GIMIC methods showed in-plane aromatic pathway in 5-membered cycle 1 against 7-membered 2. The distribution of the induced currents (ICs) for such unique aromaticity type was demonstrated. It shows that the diatropic ICs of 1 are located both inside and outside the cycle. The population analysis of molecular orbitals and natural orbitals for delocalized bonds revealed the abidance to the 4n + 2 electron rule. According to EDDB and GIMIC data, the degree of aromaticity for 1 tends to decrease from Ti to Zr and Hf. Article pubs.acs.org/Organometallics

Synthesis and Characterization of Dinuclear Allenediide Bridged Hafnocene(IV) Complexes

“…In this case, the selective formation of 2-Hf took place without decomposition (Figure S16) and the compound was stable in solution at 100 °C for 2 weeks (Figure S35). 13 C NMR analysis shows the central allene carbon atom at 182.8 ppm, which is in the same range as the value found for the zirconocene analogue 2-Zr (cf. δ 174.7 ppm).…”

“…These values are identical to those identified as secondary or follow-up products during formation of 2-Hf (vide supra). A 13 C resonance is found at δ 177.8 ppm (δ 171.4 ppm for 3-Zr 22 ), characteristic for the central allene carbon atom (Figure S5). The presence of an antisymmetric CCC vibration is illustrated by intense bands in the IR spectrum at ṽ1793 and 1775 cm −1 and weak band at 1774 and 1756 cm −1 in the Raman spectrum (Figures S55 and S56).…”

“…17,22 NMR spectra were recorded on Bruker AV300 and AV400 spectrometers. 1 H and 13 C chemical shifts were referenced to the solvent signal: benzene-d 6 (δ H = 7.16 ppm, δ C = 128.06 ppm), toluene-d 8 (δ H = 2.08 ppm, δ C = 20.43 ppm), and THF-d 8 (δ H = 3.58 ppm, δ C = 25.3 ppm). 54 Raman spectra were recorded on a LabRAM HR 800 Raman Horiba spectrometer equipped with an Olympus BX41 microscope with variable lenses.…”

“…1 H NMR (25 °C, benzene-d 6 , 300.2 MHz): δ 6.08 (s, 10H, Cp), 6.03 (s, 10H, Cp), 0.40 ppm (s, 2 J H,Si = 6.3 Hz, 18H, (SiCH 3 ) 3 ). 13 Synthesis of 3-Hf. White solids of Cp 2 HfCl 2 (151 mg, 0.40 mmol) and 1 (78 mg, 0.40 mmol) were placed in a Schlenk flask in the glovebox and dissolved in toluene (10 mL) at −78 °C.…”

“…1 H NMR (25 °C, benzened 6 , 300.2 MHz): δ 6.00 (s, 20H, Cp), 0.41 ppm (s, 2 J H,Si = 6.2 Hz, 18H, (SiCH 3 ) 3 ). 13 C NMR (25 °C, benzene-d 6 , 75.5 MHz): δ 177.8 (CCC), 109.7 (Cp), 106.1 (CCC), 3.9 ppm (SiCH 3 ). 29 Synthesis of Cp 2 HfI 2 .…”