³⁵Cl and ⁷⁹Br NQR Spectra in Cp₂M Hal₂

Abstract

The 35Cl and 79,81Br NQR spectra of (C5H5)2M Hal2 (M = Ti, Mo, W, Zr, Hf; Hal = Cl, Br) are similar. They are quadruplets with splittings ranging from 2 to 7%. The averaged frequencies of chlorine and bromine are related by a linear dependence. The temperature dependence of 81Br in Cp2HfBr2 (in the range of 77 ≑ 224 K.) is of conventional character with frequencies slowly changing with temperature; phase transitions have not been observed.

Solid-State Chlorine NMR of Group IV Transition Metal Organometallic Complexes

“…[64][65][66] The M-Cl distances and Cl-M-X angles are similar for all of the chlorine sites, resulting in powder patterns that essentially correspond to that of a single site for each complex. Quantum-chemical calculations (see below) and NQR spectra acquired by Bryukhova and coworkers 45 confirm this for the Cp 2 MCl 2 series.…”

“…The difference between values of C Q in these two species must therefore arise from differences in Ti-Cl covalent bonding. In this light, Bryukhova and co-workers 44,45 have previously ascribed the smaller C Q in CpTiCl 3 to increased Ti-Cl π bonding in comparison with that in Cp 2 TiCl 2. This hypothesis is further discussed in Quantum-Chemical Calculations.…”

“…Bryukhova and co-workers 44,45 have previously hypothesized that increased Ti-Cl π bonding leads to a decrease in the observed 35/37 Cl EFGs. This hypothesis was used to explain the large decrease in C Q observed when the 35 Cl NQR data from CpTiCl 3 and Cp 2 TiCl 2 were compared.…”

“…complex and others via NQR spectroscopy. 44,45 The value of ν Q is related to the C Q value for an I ) 3 / 2 nucleus by the expression…”

“…To date, the largest value of C Q measured by 35 Cl SSNMR on a powdered sample is that of aluminum trichloride (C Q ) 9.4 MHz), 43 which has a 35 Cl powder pattern with a breadth of over 250 kHz at 14.1 T. 35 Cl quadrupolar parameters for halogenated transition-metal metallocenes fall into this intermediate regime. For instance, 35 Cl NQR spectra acquired by Bryukhova and co-workers 44,45 revealed values of C Q ranging from ∼16 MHz in CpTiCl 3 to 24 MHz in Cp 2 TiCl 2 , corresponding to 35 Cl SSNMR powder patterns 1-3 MHz in breadth at moderate magnetic field strengths (e.g., 9.4 T). The extreme breadths of such 35 Cl powder patterns necessitates the use of frequency-stepped NMR techniques, [46][47][48][49][50][51][52][53] where separate spin echoes are acquired at evenly spaced transmitter frequencies.…”