Mössbauer spectroscopy. The computational analysis also pointed to the central
pyridine ring in the tpy framework bearing most of the ligand-based spin density,
which also supported the crystallographically determined bond length perturbations
within the tpy skeleton. Strong overall covalency between the metal and ligand
radical orbitals lends support for strong pi-backbonding. Furthermore, application of
these systems in the catalytic hydrosilylation of olefins with tertiary silanes was also
reported. Alongside the results obtained by Chirik, Nakazawa and co-workers
developed complexes with substituents on both the flanking pyridine rings of tpy
for the same catalytic purpose [88].
Strikingly different reactivity was reported for the reaction between the high-spin
Co
II (CH 2 SiMe 3 ) 2 (Py) 2 precursor and tpy. Unlike for the Fe case, where both alkyl
ligands were preserved, Chirik reported that pyridine ligand displacement at cobalt
occurs concomitantly with ejection of an alkyl radical from Co to generate the square
planar complex Co
II (tpy
À• )(R), very similar as was observed in case of the
Ni-dimethyl precursor before by Vicic (vide supra) [89]. Antiferromagnetic coupling between the ligand-centred and the Co-centred unpaired electron resulted in an
overall low-spin configuration for this system. The fate of the alkyl radical nor the
possible intermediacy of a Co
I
(tpy)(CH 2 SiMe 3 ) species that undergoes metal-toligand single-electron transfer was not further commented on. This complex was also
applied in the context of catalytic alkene hydroaddition, showing remarkable
Markovnikov selectivity for the hydroboration of styrene, but although the pincer
character of the tpy ligand was discussed in the context of the suppressed competitive deactivation pathways or substrate inhibition for diene or arene containing
substrates, the potential role of the ligand redox activity was not elaborated. For this
particular type of catalytic species, the ligand acting as an ‘electron reservoir’ may
tune the spin state/oxidation state at the respective metal centre (iron or cobalt) and
thus tune Lewis acidity and reactivity of the metal ion and modify back-electron
transfer to acceptor ligands such as alkenes and dienes.
Follow-up research by Chirik revealed that with cobalt dichloride or cobalt bis
(acetate), the tpy ligand behaves as a redox-innocent pincer platform, giving rise to,
e.g. high-spin five-coordinated Co
II (OAc) 2 (
Ar tpy) [90]. This species was shown to
undergo significant rearrangement by ligand scrambling upon reaction with
pinacolborane, resulting in inter alia six-coordinate Co(
Ar tpy) 2 . This species,
which was also independently synthesized, showed temperature-dependent EPR
signatures corresponding to S ¼ ½ (10 K) or S ¼
3 / 2 (r.t.), which could relate to
ligand redox-chemistry. Not surprisingly, this coordinatively saturated species
proved inactive for hydroboration catalysis but likely relates to a deactivation
pathway promoted by pinacolborane under catalytic conditions.
In the context of dinitrogen activation chemistry with earth-abundant base metals,
Chirik reported on the (oxidative and) reductive (electro)chemistry of a dinuclear
[{Mo(
Ph tpy)} 2 (μ 2 -N 2 )] dication featuring a linear bridging N 2 fragment [91]. This
chemistry was intended to circumvent chemical reactivity observed for related
Mo 2 (μ 2 -N 2 ) species featuring a bis(imino)pyridine pincer platform (vide infra),
where the ligand undergoes irreversible bond cleavage chemistry upon reaction
with ammonia (or hydrazine) [69]. Precursor to this dinitrogen-bridged complex
Redox-Active Pincer Ligands
147
pyridine ring in the tpy framework bearing most of the ligand-based spin density,
which also supported the crystallographically determined bond length perturbations
within the tpy skeleton. Strong overall covalency between the metal and ligand
radical orbitals lends support for strong pi-backbonding. Furthermore, application of
these systems in the catalytic hydrosilylation of olefins with tertiary silanes was also
reported. Alongside the results obtained by Chirik, Nakazawa and co-workers
developed complexes with substituents on both the flanking pyridine rings of tpy
for the same catalytic purpose [88].
Strikingly different reactivity was reported for the reaction between the high-spin
Co
II (CH 2 SiMe 3 ) 2 (Py) 2 precursor and tpy. Unlike for the Fe case, where both alkyl
ligands were preserved, Chirik reported that pyridine ligand displacement at cobalt
occurs concomitantly with ejection of an alkyl radical from Co to generate the square
planar complex Co
II (tpy
À• )(R), very similar as was observed in case of the
Ni-dimethyl precursor before by Vicic (vide supra) [89]. Antiferromagnetic coupling between the ligand-centred and the Co-centred unpaired electron resulted in an
overall low-spin configuration for this system. The fate of the alkyl radical nor the
possible intermediacy of a Co
I
(tpy)(CH 2 SiMe 3 ) species that undergoes metal-toligand single-electron transfer was not further commented on. This complex was also
applied in the context of catalytic alkene hydroaddition, showing remarkable
Markovnikov selectivity for the hydroboration of styrene, but although the pincer
character of the tpy ligand was discussed in the context of the suppressed competitive deactivation pathways or substrate inhibition for diene or arene containing
substrates, the potential role of the ligand redox activity was not elaborated. For this
particular type of catalytic species, the ligand acting as an ‘electron reservoir’ may
tune the spin state/oxidation state at the respective metal centre (iron or cobalt) and
thus tune Lewis acidity and reactivity of the metal ion and modify back-electron
transfer to acceptor ligands such as alkenes and dienes.
Follow-up research by Chirik revealed that with cobalt dichloride or cobalt bis
(acetate), the tpy ligand behaves as a redox-innocent pincer platform, giving rise to,
e.g. high-spin five-coordinated Co
II (OAc) 2 (
Ar tpy) [90]. This species was shown to
undergo significant rearrangement by ligand scrambling upon reaction with
pinacolborane, resulting in inter alia six-coordinate Co(
Ar tpy) 2 . This species,
which was also independently synthesized, showed temperature-dependent EPR
signatures corresponding to S ¼ ½ (10 K) or S ¼
3 / 2 (r.t.), which could relate to
ligand redox-chemistry. Not surprisingly, this coordinatively saturated species
proved inactive for hydroboration catalysis but likely relates to a deactivation
pathway promoted by pinacolborane under catalytic conditions.
In the context of dinitrogen activation chemistry with earth-abundant base metals,
Chirik reported on the (oxidative and) reductive (electro)chemistry of a dinuclear
[{Mo(
Ph tpy)} 2 (μ 2 -N 2 )] dication featuring a linear bridging N 2 fragment [91]. This
chemistry was intended to circumvent chemical reactivity observed for related
Mo 2 (μ 2 -N 2 ) species featuring a bis(imino)pyridine pincer platform (vide infra),
where the ligand undergoes irreversible bond cleavage chemistry upon reaction
with ammonia (or hydrazine) [69]. Precursor to this dinitrogen-bridged complex
Redox-Active Pincer Ligands
147