1 Introduction
Phosphorus-containing ligands have been widely utilized in various organometallic
reactions and transition metal catalyses. The profound impact of various tertiary
phosphines as a neutral donor has been well-recognized, due to their excellent
character, versatile enough to play important roles in transition metal catalytic
processes, such as hydrogenation, hydroformylation, and polymerization. To control
the electronic and steric properties of phosphorus-containing ligands, various substituents have been synthetically incorporated. An electronic factor can be easily
managed by employing alkyl and aryl groups, as well as heteroatoms to give a P-E
bond (E ¼ O, N, or Si). A steric property nicely defined by “cone angle” suggested
by Tolman in 1970 [1] can be controlled by utilizing substituents of different sizes.
With their relative ease of synthetic preparation, various alkyl and aryl groups have
been employed to control the steric and electronic properties of transition metal
complexes, which allows to achieve high stereoselectivity. Their advantageous
effects have been well perceived from a wide range of organometallic studies. One
of the advantages of working with phosphorus-containing organometallic complexes
is the convenience of using
31 P nuclear magnetic resonance (NMR) spectroscopy.
Having the phosphorus nuclear spin of ½ with high sensitivity, its NMR spectroscopic data can be conveniently collected and utilized not only to characterize
various diamagnetic species but also to study kinetics in order to obtain a mechanistic understanding of organometallic reactions [2].
Various mono- and multidentate ligands having phosphorus atom(s) as a donor
moiety have been synthesized and widely employed. In particular, tridentate
pincer systems are an important class of ligand to produce four-coordinate square
planar metal complexes, in which various catalytic reactions effectively occur at
the site trans to a central moiety. By incorporating heteroatom(s) in central and/or
side arm(s) along with P donor(s), the local geometry about a metal center can be
finely tuned, and thus it is effective in controlling the electronic structure of a
metal complex and regulating the energy of the metal-based frontier molecular
orbitals (FMOs). Thus, various phosphorus-containing pincer ligands have been
designed and prepared for the last several decades. Most of phosphorus-containing
ligands are conventionally categorized as spectator ligands, which do not directly
interact with substrates. Although they are tightly connected to the metal center,
there is no formal redox change or bond formation/cleavage occurring at a P site
during chemical reactions. In contrast, actor ligands cooperatively assist the
adjacent metal by providing electrons, protons, and/or functional groups, and
thus they are actively involved in chemical transformations. Within phosphoruscontaining pincer systems, several examples reveal metal-ligand cooperativity
(MLC), which has been recently recognized as a promising way to expand the
role of transition metals in organometallic catalysis. To be actively engaged in
chemical reactions, several redox noninnocent ligands have been prepared by
employing π-conjugated systems such as porphyrin [3], pyridine [4–6], catecholate
[7, 8], aminophenolate [9, 10], and salen [11] as cooperative sites. For example,
the Milstein group reported various reactions employing phosphorus-containing
72
S. Kim et al.
Phosphorus-containing ligands have been widely utilized in various organometallic
reactions and transition metal catalyses. The profound impact of various tertiary
phosphines as a neutral donor has been well-recognized, due to their excellent
character, versatile enough to play important roles in transition metal catalytic
processes, such as hydrogenation, hydroformylation, and polymerization. To control
the electronic and steric properties of phosphorus-containing ligands, various substituents have been synthetically incorporated. An electronic factor can be easily
managed by employing alkyl and aryl groups, as well as heteroatoms to give a P-E
bond (E ¼ O, N, or Si). A steric property nicely defined by “cone angle” suggested
by Tolman in 1970 [1] can be controlled by utilizing substituents of different sizes.
With their relative ease of synthetic preparation, various alkyl and aryl groups have
been employed to control the steric and electronic properties of transition metal
complexes, which allows to achieve high stereoselectivity. Their advantageous
effects have been well perceived from a wide range of organometallic studies. One
of the advantages of working with phosphorus-containing organometallic complexes
is the convenience of using
31 P nuclear magnetic resonance (NMR) spectroscopy.
Having the phosphorus nuclear spin of ½ with high sensitivity, its NMR spectroscopic data can be conveniently collected and utilized not only to characterize
various diamagnetic species but also to study kinetics in order to obtain a mechanistic understanding of organometallic reactions [2].
Various mono- and multidentate ligands having phosphorus atom(s) as a donor
moiety have been synthesized and widely employed. In particular, tridentate
pincer systems are an important class of ligand to produce four-coordinate square
planar metal complexes, in which various catalytic reactions effectively occur at
the site trans to a central moiety. By incorporating heteroatom(s) in central and/or
side arm(s) along with P donor(s), the local geometry about a metal center can be
finely tuned, and thus it is effective in controlling the electronic structure of a
metal complex and regulating the energy of the metal-based frontier molecular
orbitals (FMOs). Thus, various phosphorus-containing pincer ligands have been
designed and prepared for the last several decades. Most of phosphorus-containing
ligands are conventionally categorized as spectator ligands, which do not directly
interact with substrates. Although they are tightly connected to the metal center,
there is no formal redox change or bond formation/cleavage occurring at a P site
during chemical reactions. In contrast, actor ligands cooperatively assist the
adjacent metal by providing electrons, protons, and/or functional groups, and
thus they are actively involved in chemical transformations. Within phosphoruscontaining pincer systems, several examples reveal metal-ligand cooperativity
(MLC), which has been recently recognized as a promising way to expand the
role of transition metals in organometallic catalysis. To be actively engaged in
chemical reactions, several redox noninnocent ligands have been prepared by
employing π-conjugated systems such as porphyrin [3], pyridine [4–6], catecholate
[7, 8], aminophenolate [9, 10], and salen [11] as cooperative sites. For example,
the Milstein group reported various reactions employing phosphorus-containing
72
S. Kim et al.