A convenient method for the generation of {Rh(PNP)}+ and {Rh(PONOP)}+ fragments: Reversible formation of vinylidene derivatives

Article abstract of DOI:10.1039/c8dt05049j

The substitution reactions of [Rh(COD)₂][BAr^F₄] with PNP and PONOP pincer ligands 2,6-bis(di-tert-butylphosphinomethyl)pyridine and 2,6-bis(di-tert-butylphosphinito)pyridine in the weakly coordinating solvent 1,2-F₂C₆H₄ are shown to be an operationally simple method for the generation of reactive formally 14 VE rhodium(i) adducts in solution. Application of this methodology enables synthesis of known adducts of CO, N₂, H₂, previously unknown water complexes, and novel vinylidene derivatives [Rh(pincer)(CCHR)][BAr^F₄] (R = tBu, 3,5-tBu₂C₆H₃), through reversible reactions with terminal alkynes.

Full text of DOI:10.1039/c8dt05049j

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A convenient method for the generation of
{Rh(PNP)}+ and {Rh(PONOP)}+ fragments:
Cite this: DOI: 10.1039/c8dt05049j
reversible formation of vinylidene derivatives†
Received 21st December 2018,
Accepted 28th January 2019
Matthew R. Gyton,
Thomas M. Hood
and Adrian B. Chaplin
*
DOI: 10.1039/c8dt05049j
rsc.li/dalton
The substitution reactions of [Rh(COD)2][BArF4] with PNP and
PONOP pincer ligands 2,6-bis(di-tert-butylphosphinomethyl)pyri-
dine and 2,6-bis(di-tert-butylphosphinito)pyridine in the weakly
coordinating solvent 1,2-F2C6H4 are shown to be an operationally
simple method for the generation of reactive formally 14 VE
rhodium(I) adducts in solution. Application of this methodology
enables synthesis of known adducts of CO, N2, H2, previously
unknown water complexes, and novel vinylidene derivatives
[Rh(pincer)(CCHR)][BArF4] (R = tBu, 3,5-tBu2C6H3), through revers-
ible reactions with terminal alkynes.
Scheme 1 Latent sources of the {Rh(PNP)}+ and {Rh(PONOP)}+ frag-
ments (X = O, CH2). COE = cyclooctene; COD = 1,5-cyclooctadiene.
Introduction
Phosphine-based pincers are a prominent ligand class in con-
temporary organometallic chemistry and catalysis, conferring
thermal stability whilst permitting a wide range of metal-based
reactivity.1,2 From a fundamental perspective, their capacity to
support the generation of reactive low-coordinate metal frag-
ments provides an attractive framework for gaining insights
into the mechanism of small molecule activation reactions.
The synthesis and characterisation of a rhodium(I) σ-methane
complex in solution is a particularly outstanding example,
with direct mechanistic relevance to C(sp3)–H bond activation
and alkane dehydrogenation processes mediated at iridium
homologues.2,3 Other group 9 highlights include the C(sp2)–H
bond oxidative addition of aryl halides4,5 and the selective
C(sp3)–F bond activation of fluorocarbons.6 In the context of
supporting such endeavours and building on preceding work
(Scheme 1),4,7–9 we herein report a convenient method for
accessing reactive formally 14 VE rhodium(I) adducts of the
widely studied PNP (2,6-bis(di-tert-butylphosphinomethyl)pyri-
dine) and PONOP (2,6-bis(di-tert-butylphosphinito)pyridine)
ligands, exploiting metastable coordination of 1,5-cycloocta-
diene (COD) in the weakly coordinating 1,2-difluorobenzene
solvent (DFB).10 We showcase this methodology for the prepa-
ration of previously unknown water complexes and the revers-
ible formation of vinylidene derivatives.
Results and discussion
As an operationally simple entry point into the organometallic
chemistry of cationic rhodium(I) pincer complexes, we targeted
substitution reactions of the readily accessible (and commer-
cially available) rhodium precursor [Rh(COD)2][BArF4] (ArF
=
3,5-(CF3)2C6H3); postulating that alongside chelation of the
pincer, steric buttressing from the phosphine substituents
would promote efficient liberation of the comparatively bulky
COD ligand and access to reactive {Rh(pincer)}+ fragments in
solution.11 To this end, stoichiometric reactions between PNP/
PONOP and [Rh(COD)2][BArF4] in DFB under an argon atmo-
sphere were first studied in situ, resulting in quantitative
coordination of the pincer and establishment of dynamic
Department of Chemistry, University of Warwick, Gibbet Hill Road,
Coventry CV4 7AL, UK. E-mail: a.b.chaplin@warwick.ac.uk
†Electronic supplementary information (ESI) available: Full experimental details
and NMR spectra of new compounds and selected reactions (PDF). Primary
NMR data (MNOVA). CCDC 1886657–1886664. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c8dt05049j
equilibrium mixtures containing monomeric 1a (PNP, δ
31P
2
1
63.4, 52.6, JPP ∼320 Hz; PONOP, δ 202.5, JRhP = 134 Hz) as
31P
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ceeded to completion with the exception of 2c-O (ca. 95%),
which instead required use of isolated 1b-O to presumably
counter recoordination of COD (t ∼ 2 days).
Application of our methodology allows access to novel
rhodium(I) pincer adducts of water 2d. PNP variant 2d-CH2
1
(δ
31P
63.1, JRhP = 144 Hz) was formed quantitatively on
addition of excess degassed water (40 equiv.) to an equilibrium
mixture of 1a/b-CH2 generated from stoichiometric reaction
between [Rh(COD)2][BArF4] and PNP in DFB (t ∼ 3 h).
Consistent with the aforementioned reactivity trends PONOP
1
variant 2d-O (δ 199.1, JRhP = 150 Hz) was, however, only
31P
formed with 88% conversion after 3 days under these con-
ditions. Analytically pure samples of the 2d-CH2 (71% yield)
and 2d-O (49% yield) were obtained using 1a/b-CH2 or 1b-O,
respectively, and excess water (500 equiv./Rh). The structural
formulation of both new aqua complexes was confirmed using
a combination of NMR spectroscopy, X-ray diffraction (see
−
Scheme 2 Synthesis and reactivity of 1 (X = O, CH2). [BArF
anions omitted for clarity.
] counter
4
the major components and dimeric 1b as the minor com- Fig. 1 for 2d-O) and satisfactory microanalysis. The solid-state
ponents, alongside liberated COD (Scheme 2). The former structures of 2d are notable for and R –OH2 bond lengths of
were formed exclusively when the reactions were repeated in ca. 2.15 Å and the presence of an additional hydrogen bonded
the presence of 10 equivalents of COD, while the latter dica- water molecule in the lattice (RhOH2⋯O
H2 = 2.7–2.8 Å).13
tions were isolated in high yield by crystallisation from DFB/ As a means of showcasing our new methodology and as
̲
̲
̲
̲
̲
hexane (PNP, 90%; PONOP, 96%), structurally characterised in part of our work exploring C(sp)–H activation reactions
the solid state using X-ray diffraction and the bulk purity estab- mediated by cationic rhodium(I) pincers,14 we set about study-
lished by microanalysis (see Fig. 1 for 1b-O).† The synthetic ing the reactivity of the associated {Rh(pincer)}+ fragments
utility of this methodology was then assessed by reacting mix- with two bulky terminal alkynes (Scheme 3). Using isolated 1b-
tures of 1a/b, generated as described above in DFB, with CO, CH2 as the precursor, reaction with both HCCtBu and HCCAr′
1
H2, and N2 (1 atm) and analysis in situ using H and 31P NMR (Ar′ = 3,5-tBu2C6H3) in DFB under an argon atmosphere
spectroscopy. Introduction of strongly coordinating CO resulted in rapid (t < 5 min) and quantitative formation of
resulted in rapid (t < 5 min) and quantitative formation of the vinylidene derivatives 3a/b-CH2, the formation of which was
known carbonyl complexes 2a, with only free COD as the by- readily apparent due to their striking deep green colours. Both
product.7,8 Reactions of H2 and N2 with 1a/b likewise afforded complexes were subsequently isolated from solution and fully
the corresponding (known) adducts 2b (with concomitant characterised in solution and the solid state.† The NMR
hydrogenation of COD) and 2c,7,8,12 respectively, but under dis- spectra of 3a/b-CH2 are notable for the presence of vinylidene
parately longer timeframes for the PONOP (t ∼ 2 days) com- 13C resonances at δ 317.5/326.9 (located by HMBC experi-
pared to PNP complexes (t < 3 h). These reactions all pro- ments), time-averaged C2v symmetry – reconcilled by free
Fig. 1 Solid-state structures of 1b-O and 2d-O. Thermal ellipsoids drawn at 50% probability; anions and most hydrogen atoms omitted for clarity.
The starred atoms are generated using the symmetry operation −x, 1 − y, 1 − z. Selected bond lengths (Å): 1b-O: Rh1–Cnt(C4,C5), 2.083(2); cf. 1b-
CH2, Rh1–Cnt(C4,C5), 2.058(2); 2d-O: Rh1–O4, 2.154(2); cf. 2d-CH2, 2.152(7).
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have made when studying C(sp)–H bond activation reactions
of cationic rhodium complexes of NHC-based pincer ligands.14
Conclusions
−
Scheme 3 Preparation of vinylidene complexes 3 (X = O, CH2). [BArF
]
4
counter anions omitted for clarity; a 3b-O formed as mixture with the
corresponding π-complex 2e-O.
We have demonstrated that reaction between the readily acces-
sible (and commercially available) rhodium precursor
[Rh(COD)2][BArF4] and two phosphine-based pincer ligands
(2,6-bis(di-tert-butylphosphinomethyl)pyridine and 2,6-bis(di-
tert-butylphosphinito)pyridine) in 1,2-F2C6H4 provides a opera-
tionally simple method for generating the corresponding
{Rh(pincer)}+ fragments, and subsequent synthesis of adducts
of CO, H2, N2 and H2O (2). This methodology is underpinned
by reversible binding of COD, that leads to dynamic equili-
brium mixtures of monomeric [Rh(pincer)(η2-COD)][BArF4] 1a
and dimeric [{Rh(pincer)}2(μ–η2:η2-COD)][BArF4]2 1b, the latter
of which were readily isolated by recrystallisation. This
methodology was used to study the reaction between
{Rh(pincer)}+ and bulky terminal alkynes, which resulted in
the (reversible) formation of novel vinylidene derivatives
[Rh(pincer)(CCHR)][BArF4] (R = tBu, 3a; 3,5-tBu2C6H3, 3b).
rotation about the RhvCvC vector – and single phosphorus
1
resonances with large coupling to 103Rh (δ 65.6/65.9, JRhP
=
31P
139/137 Hz). The solid-state structures affirm the structural
assignments and, in addition to the RhvC bond lengths of
1.81–1.84 Å that are fully inline with related rhodium pre-
cedents,15 notable for the adoption of both C2 (3a-CH2, Z′ = 2)
and Cs (3a-CH2, Z′ = 2; 3b-CH2) symmetric pincer ligand con-
formations; demonstrating the flexibility of lutidene-based
pincer backbones.16 In a similar manner, deep blue vinylidene
complex 3a-O was obtained from HCCtBu and 1b-O, but over a
more protracted time frame than the PNP analogue (ca. 18 h;
1
δ
206.5, JRhP = 144 Hz, δ13C(RhC) 330.0, C2v solution sym-
31P
metry; Fig. 2). Consistent with the aformetioned trends, the
reaction between {Rh(PONOP)}+ and HCCAr′ only resulted in
the formation of the green vinylidene derivative 3b-O (δ
31P
Conflicts of interest
208.4, 1JRhP = 143 Hz, δ13C(RhC) 339.5) as the minor component
There are no conflicts to declare.
(30%) of a mixture with the balance made up by the corres-
ponding π-complex [Rh(PONOP)(HCCAr′)][BArF4] (2e-O; δ
31P
194.6. JRhP = 129 Hz).17 This ratio was found even in use of
1
Acknowledgements
excess alkyne and the co-crystallisation of these two com-
pounds (with the ratio reflected in the corresponding crystallo-
graphic occupancies) precluded separation in this manner.† 18
Curiously, we note reaction of isolated 3 (in the case of 3b-
O, as a mixture with 2e-O) with CO (1 atm) resulted in com-
plete conversion to 2a in all cases with concomitant liberation
of the corresponding terminal alkynes (t < 90 min). Such
reversible vinylidene formation is well documented in ruthe-
nium complexes,19 and in line with related observations we
We thank the European Research Council (ERC, grant agree-
ment 637313) and Royal Society (UF100592, UF150675,
A. B. C.) for financial support. Crystallographic data for 3a-O
were collected using an instrument purchased through
support from Advantage West Midlands and the European
Regional Development Fund. All other crystallographic data
were collected using an instrument that received funding from
the ERC under the European Union’s Horizon 2020 research
and innovation programme (grant agreement no. 637313).
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