Prominent Ligand Types in Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 25

Figure 2.4 Definition of %V bur .
Reproduced from Ref. 15.
from DFT-optimized structures can be larger than those calculated from
the CPK models by Tolman. 12 For instance, the cone angle of P(t-Bu) 3 using
the method of Tolman is 1821, 9 whereas an angle of 1941 is determined
by the computational method 12b (Table 2.1, Entry 20). Moreover, as noted by
Nolan and co-workers, the Tolman calculation is not valid for N-heterocyclic
carbene ligands as the steric bulk of these ligands is directed towards the
metal center, not away as in the case of tertiary phosphines. 14 Nolan and
co-workers therefore introduced the ''percentage buried volume'' (%V bur )to
quantify the steric demand of NHC ligands and also others that are mis-
represented by conventional Tolman cone angle calculations. 15 The %V bur is
defined as the percentage of the total volume occupied by a ligand of a
sphere with a defined radius (2.00 and 2.28 Å) and a metal at the core
(Figure 2.4). Using crystallographic data, the %V bur can be calculated using
the web-based SambVca software (Salerno molecular buried volume calcu-
lation), which is available free of charge. 16
A good correlation was found between %V bur and y for tertiary phos-
phines, 17 and it was also demonstrated that cone angles could be estimated
from the %V bur by linear regression based on this correlation. This method
provided a means not only to evaluate the steric properties of NHC ligands,
but also to allow for direct comparison with phosphines. Table 2.2 presents a
list of %V bur values.
2.2.2 Electronic Considerations
As illustrated in Figure 2.5, the phosphorus-palladium bond is a dative
interaction which arises primarily from s-donation from the phosphorus
electron lone pair to the unfilled d-orbitals of Pd (Lewis acid-base inter-
action). Tertiary phosphines also act as p-acceptors (p-acids), wherein the
filled metal d-orbitals overlap primarily with P-R s*-orbitals (Figure 2.5). 18
The electron-donating ability of the phosphine is significant in terms of
increasing the electron density on the Pd, thereby helping a more facile
oxidative addition. Various methods have been used to characterize the
electronic contributions from ancillary ligands; however, the separation of
the s-donor versus p-backbonding effects has proven particularly chal-
lenging. One of the most straightforward methods to quantify s-donation is