The β‐Agostic Structure in (C₅Me₅)₂Sc(CH₂CH₃): Solid‐State NMR Studies of (C₅Me₅)₂Sc−R (R=Me, Ph, Et)

Abstract

Multinuclear solid-state NMR studies of Cp* 2 ScÀR (Cp* = pentamethylcyclopentadienyl;R = Me,P h, Et) and DFT calculations show that the ScÀEt complex contains a b-CH agostic interaction. The static central transition 45 Sc NMR spectra show that the quadrupolar coupling constants (C q ) follow the trend of Ph % Me > Et, indicating that the Sc À R bond is different in Cp* 2 ScÀEt compared to the methyl and phenyl complexes.Analysis of the chemical shift tensor (CST) shows that the deshielding experienced by Cb in ScÀCH 2 CH 3 is related to coupling between the filled s C-C orbital and the vacant p * ScÁÁÁHC orbital.Agostic M···H À Ci nteractions are important in many catalytic cycles.The a-CH agostic interaction is proposed to direct stereoselective propylene insertion reactions, [1] or to stabilize syn-alkylidene complexes that produce stereoselective alkene products in olefin metathesis reactions. [2] Complexes containing b-CH agostic interactions are more common, and are often encountered in transition-metal-catalyzed olefin polymerization reactions. [3] b-Agostic MÀEt complexes contain acute M-C-C angles and short M···HÀCc ontacts that distort the alkyl fragment from ideal geometries expected for sp 3 carbons.Inthe absence of asolid-state structure,the solution 1 HNMR spectra of agostic complexes contain signals shifted upfield compared to the free alkane,a nd the 1 J CH is usually significantly lower than about 125 Hz for sp 3 CÀHb onds. [3] In their studies of s-bond metathesis reactions involving Cp* 2 ScÀR, Bercaw and co-workers isolated Cp* 2 ScÀEt. [4] The solid-state structure of Cp* 2 ScÀEt was disordered, [5] and solution 1 Ha nd 13 CNMR data did not conclusively identify a b-agostic structure.T he b-agostic structure in Cp* 2 Sc À Et is supported by the presence of low energy C À Hbands in the IR spectrum, and the unexpected slow ethylene insertion kinetics compared to other Cp* 2 ScÀRcomplexes. [6] Though THF-free Cp* 2 LnÀEt complexes are rare,C p* 2 YÀEt was recently isolated and analysis of the solid-state structure obtained from X-ray diffraction studies unambiguously establishes the b-agostic structure in this complex. [7] Solid-state NMR spectroscopy could, in principle,d ifferentiate between the non-agostic and Sc···H À Ca gostic structures of Cp* 2 Sc À Et (Figure 1). Scandium has one NMR active nucleus ( 45 Sc, 100 %a bundant; I = 7/2) with ag yromagnetic ratio close to carbon. Static central transition (CT) solid-state NMR of quadrupolar nuclei is arich source of information for studies of complex structures. [8] Solid-state 45 Sc NMR has been used to study small molecules, [9] crystalline porous materials, [10] and glassy solids. [11] Theb road NMR powder patterns from these experiments are dominated by the interaction of the nuclear electric quadrupole moment, eQ, with the electric field gradient (EFG) tensor, which is reported as the quadrupolar coupling constant (C q )i nM Hz. Them agnitude of C q depends on the symmetry of the molecule,and the nature of the bonds in the first coord...

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

“…The residue from toluene solutions of 4 contained signals consistent with the formation of Cp* 2 Sc(CH 2 C 6 H 5 ) as well signals that most likely correspond to a mixture of the previously reported ortho- , meta- , and para -tolyl Cp* 2 Sc(C 6 H 4 CH 3 ) complexes . The crude material obtained from the reduction in Et 2 O displayed a signal corresponding to unreacted Cp* 2 ScI along with signals, which suggested the formation of Cp* 2 Sc(OCH 2 CH 3 ) and the previously reported Cp* 2 Sc(CH 2 CH 3 ) . These three signals appeared in an approximate 1:1:0.8 ratio.…”

“…The mixture immediately became dark red in color and was allowed to stand at this temperature for 18 h. After the mixture was allowed to warm to ambient temperature and stirred for 1 h, the solution was decanted from the black solids, presumably graphite, at the bottom of the vial, and the solvent was removed from this solution under reduced pressure to give a yellow residue. This crude residue was then dissolved in C 6 D 6 for study by 1 H NMR spectroscopy, which showed resonances consistent with the presence of Cp* 2 ScI, Cp* 2 ScCH 2 CH 3 , and a third complex that we have tentatively assigned as Cp* 2 ScOCH 2 CH 3 . 1 H NMR (500 MHz, C 6 D 6 ): δ = 4.49 (q, 2H, OC H 2 CH 3 (Cp* 2 ScOEt), J HH = 6.9 Hz), 1.93 (s, 30H, CH 3 ( Cp* 2 ScOCH 2 CH 3 )), 1.91 (s, 30H, CH 3 ( Cp* 2 ScI)), 1.86 (s, 30H, CH 3 ( Cp* 2 ScCH 2 CH 3 ), 1.33 (t, 3H, CH 3 (Cp* 2 ScOCH 2 C H 3 ), J HH = 6.9 Hz), 0.99 (q, 2H, CH 2 (Cp* 2 ScC H 2 CH 3 ), J HH = 8.2 Hz), 0.17 (t, 3H, CH 3 (Cp* 2 ScCH 2 C H 3 ), J HH = 8.2 Hz).…”

“…41 The crude material obtained from the reduction in Et 2 O displayed a signal corresponding to unreacted Cp* 2 ScI along with signals, which suggested the formation of Cp* 2 Sc(OCH 2 CH 3 ) and the previously reported Cp* 2 Sc(CH 2 CH 3 ). 50 These three signals appeared in an approximate 1:1:0.8 ratio. While some other minor signals were apparent in this spectrum, none could be attributed to known compounds.…”

“…solution under reduced pressure to give a yellow residue. This crude residue was then dissolved in C 6 D 6 for study by1 H NMR spectroscopy, which showed resonances consistent with the presence of Cp* 2 ScI,41 Cp* 2 ScCH 2 CH 3 ,50 and a third complex that we have tentatively assigned as Cp* 2 ScOCH 2 CH 3 . 1 H NMR (500 MHz, C 6 D 6 ): δ = 4.49 (q, 2H, OCH 2 CH 3 (Cp* 2 ScOEt), J HH = 6.9 Hz), 1.93 (s, 30H, CH 3 (Cp* 2 ScOCH 2 CH 3 )), 1.91 (s, 30H, CH 3 (Cp* 2 ScI)), 1.86 (s, 30H, CH 3 (Cp* 2 ScCH 2 CH 3 ), 1.33 (t, 3H, CH 3 (Cp* 2 ScOCH 2 CH 3 ), J HH = 6.9 Hz), 0.99 (q, 2H, CH 2 (Cp* 2 ScCH 2 CH 3 ), J H H = 8.2 Hz), 0.17 (t, 3H, CH 3 (Cp* 2 ScCH 2 CH 3 ), J HH = 8.2 Hz).…”

Structure and bonding in rhodium coordination compounds: a 103Rh solid-state NMR and relativistic DFT study

“…Rh(CO) 2 Cp*, the only compound herein with an η 5 -coordinated cyclopentadienyl ring, has a δ iso = −1150 ppm, the lowest in this set, which is indicative of an extremely shielded 103 Rh nucleus as is typical for δ iso values of metal nuclides in metallocenes. 132–135 The relationships between the 103 Rh chemical shift tensor parameters and structure and bonding in this set of Rh complexes is further explored with DFT calculations (see Sections 3.2 and 3.3).…”