The molecular dynamics of a series of organometallic comple
xes covalently bound to amorphous silica surfaces is determined experimentally using solid-
state nuclear magnetic resonance (NMR) spectroscopy and density functional theory calculations (DFT). The determination is carried out for a series of alkylidene-based cataly
sts having the general formula [(
![](http://pubs.acs.org/images/entities/tbd1.gif)
SiO)M(ER)(
![](http://pubs.acs.org/images/entities/dbd_2.gif)
CH
tBu)(R′)] (M = Re, Ta, Mo or W; ER = C
tBu, NAr or CH
2tBu; R′ = CH
2tBu, NPh
2, NC
4H
4). Proton−carbon dipolar coupling con
stants and carbon chemical shift anisotropies (CSA) are determined experimentally by solid-
state NMR. Room-temperature molecular dynamics is quantified through order parameters determined from the experimental data. For the chemical shift anisotropy data, we validate and use a method that integrates
static values for the CSA obtained computationally by DFT, obviating the need for low-temperature measurements. Comparison of the room-temperature data with the calculations shows that the widths of the calculated
static limit dipolar couplings and CSAs are always greater than the experimentally determined values, providing a clear indication of motional averaging on the NMR time scale. Moreover, the dynamics are found to be significantly different within the series of molecular comple
xes, with order parameters ranging from <
Sz> = 0.5 for [(
![](http://pubs.acs.org/images/entities/tbd1.gif)
SiO)Ta(
![](http://pubs.acs.org/images/entities/dbd_2.gif)
CH
tBu)(CH
2tBu)
2] and [(
![](http://pubs.acs.org/images/entities/tbd1.gif)
SiO)Re(
![](http://pubs.acs.org/images/entities/tbd1.gif)
C
tBu)(
![](http://pubs.acs.org/images/entities/dbd_2.gif)
CH
tBu)(CH
2tBu)] to <
Sz> = 0.9 for [(
![](http://pubs.acs.org/images/entities/tbd1.gif)
SiO)Mo(
![](http://pubs.acs.org/images/entities/tbd1.gif)
NAr)(
![](http://pubs.acs.org/images/entities/dbd_2.gif)
CH
tBu)(R′) with R′ = CH
2tBu, NPh
2, NC
4H
4. The data also show that the motion is not isotropic and could be either a jump between two sites or more likely re
stricted librational motion. The dynamics are discussed in terms of the molecular
structure of the surface organometallic comple
xes, and the orientation of the CSAs tensor at the alkylidene carbon is shown to be directly related to the magnitude of the α-alkylidene CH ago
stic interation.