Preface
Directly after the incipient finding of the PCP-pincer platform, as a monoanionic,
tridentate ligand in organometallic chemistry, in the late 1970s, the novel pincermetal compounds found extensive use as catalysts in homogeneous catalysis, opening new pathways in, for example, organic synthesis. Initially, the pincer motif
comprised a well-defined Donor-C(anion)-Donor (DCD) ligand architecture
consisting of a central, covalent metal-carbon, bond, often from a sp
2 -C(aryl)
donor, and two additional interactions with the neutral donor heteroatom (D or D
0 )
of two flanking substituents. In parallel, the full potential of the planar DCD,
potentially 6-electron donating pincer motif was explored. The metal, the nature of
the donor sites D, and type of central two-electron donor atom, from C to N, Si, etc.
was varied. Moreover, it appeared that a modular approach for the synthesis of
ligands with architectures, supporting a planar DCD coordination motif, could be
followed as well as its subsequent (cyclo-)metallation. Much of this development
was spurred by the, often, surprisingly high thermal and kinetic stability of the
resulting DCD-pincer metal complexes. This earlier work is partly covered by the
“Topics in Organometallic Chemistry Series,” Volume 40 (Organometallic Pincer
Chemistry) and Volume 54 (The Privileged Pincer-Metal Platform: Coordination
Chemistry and Applications). The greater part of this earlier research involves
chemistry in which the pincer ligand acts as innocent ligand: i.e., it is merely the
mutual speciation and electronic properties of the three 2e-donor sites of the pincer
ligand that is playing its role in affecting the metal properties. Nevertheless, occasionally, it was already observed that incidentally electronic communication along
the central metal-(anionic) donor site is playing a distinct role in modulating
processes at the metal center.
The research presented in this volume showcases research in which the tridentate
pincer motif, rather than behaving as an “innocent ligand,” is playing an active role,
i.e., metal–ligand cooperation (MLC) occurs involving profound, and often reversible, changes in the binding and electronic situation between the pincer platform and
the metal site. A first demonstration of this “non-innocent” behavior of the pincer
ligand in the pincer-metal platform was the discovery by Milstein and coworkers of a
v
Directly after the incipient finding of the PCP-pincer platform, as a monoanionic,
tridentate ligand in organometallic chemistry, in the late 1970s, the novel pincermetal compounds found extensive use as catalysts in homogeneous catalysis, opening new pathways in, for example, organic synthesis. Initially, the pincer motif
comprised a well-defined Donor-C(anion)-Donor (DCD) ligand architecture
consisting of a central, covalent metal-carbon, bond, often from a sp
2 -C(aryl)
donor, and two additional interactions with the neutral donor heteroatom (D or D
0 )
of two flanking substituents. In parallel, the full potential of the planar DCD,
potentially 6-electron donating pincer motif was explored. The metal, the nature of
the donor sites D, and type of central two-electron donor atom, from C to N, Si, etc.
was varied. Moreover, it appeared that a modular approach for the synthesis of
ligands with architectures, supporting a planar DCD coordination motif, could be
followed as well as its subsequent (cyclo-)metallation. Much of this development
was spurred by the, often, surprisingly high thermal and kinetic stability of the
resulting DCD-pincer metal complexes. This earlier work is partly covered by the
“Topics in Organometallic Chemistry Series,” Volume 40 (Organometallic Pincer
Chemistry) and Volume 54 (The Privileged Pincer-Metal Platform: Coordination
Chemistry and Applications). The greater part of this earlier research involves
chemistry in which the pincer ligand acts as innocent ligand: i.e., it is merely the
mutual speciation and electronic properties of the three 2e-donor sites of the pincer
ligand that is playing its role in affecting the metal properties. Nevertheless, occasionally, it was already observed that incidentally electronic communication along
the central metal-(anionic) donor site is playing a distinct role in modulating
processes at the metal center.
The research presented in this volume showcases research in which the tridentate
pincer motif, rather than behaving as an “innocent ligand,” is playing an active role,
i.e., metal–ligand cooperation (MLC) occurs involving profound, and often reversible, changes in the binding and electronic situation between the pincer platform and
the metal site. A first demonstration of this “non-innocent” behavior of the pincer
ligand in the pincer-metal platform was the discovery by Milstein and coworkers of a
v