Asymmetric bis-PNP pincer complexes of zirconium and hafnium-a measure of hemilability

Article abstract of DOI:10.1039/d0dt03544k

Asymmetrically-bound pyrrolide-based bis-PNP pincer complexes of zirconium and hafnium have been formed. The [κ2-PNPPh][κ3-PNPPh]MCl2 species are in direct contrast to previous zirconium PNP pincer complexes. The pincer ligands are fluxional in their binding and the energy barrier for exchange has been approximated using VT-NMR spectroscopy and the result validated by DFT calculations.

Full text of DOI:10.1039/d0dt03544k

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Asymmetric bis-PNP pincer complexes of zirconium
and hafnium – a measure of hemilability†
Cite this: Dalton Trans., 2020, 49,
16653
a
a
b
a
Celia Idelson, Leah Webster, Tobias Krämer * and F. Mark Chadwick
*
Received 13th October 2020,
Accepted 9th November 2020
DOI: 10.1039/d0dt03544k
rsc.li/dalton
Asymmetrically-bound pyrrolide-based bis-PNP pincer complexes thesis in 2012 the phenyl substituted pyrrolide-PNP pincer
2
Ph
3
of zirconium and hafnium have been formed. The [κ -PNP ][κ - ligand has been relatively underused, especially given both the
Ph
PNP ]MCl
PNP pincer complexes. The pincer ligands are fluxional in their reactivity.
binding and the energy barrier for exchange has been approxi- Our interest was also piqued by the recent resurgence of
mated using VT-NMR spectroscopy and the result validated by DFT early transition metal pincer chemistry. Initially pioneered by
2
species are in direct contrast to previous zirconium commercial availability of the precursors and its varied
6
–14
1
5
calculations.
Fryzuk and others from the 1970s onwards, it remains
remarkably underdeveloped when compared with the later
transition metals. However recent work has shown rich chem-
istry within early transition metal pincer complexes which are
capable of facilitating extremely challenging transformations,
Pincer ligands, tridentate ligands where a central ligating
moiety is flanked by two others, occupy a privileged position
in coordination chemistry. Their modular nature allows a
chemist to fine-tune both the steric and electronic properties
16
for example the activation of methane.
1
of the metal centre. Such design has allowed the development
R
With the pyrrolide-based [PNP ] ligand (where R represents
of active catalysts for a vast array of transformations. When
first used by Van Koten in 1989 the term pincer ligand strictly
applied to an ‘LXL’ ligand, where a central anionic carbon
atom was flanked by two donors (which would force a meridia-
nal geometry). This definition has been broadened to include
several different central heteroatom donors including O, Si
and N. For the last, an archetypical motif is that of PNP,
where a central pyridine is flanked by methylene phosphines
Fig. 1, top). The non-innocence of this ‘LLL’ ligand has been
exploited to accomplish difficult bond activations. A some-
the substituents on the phosphines), the formally anionic pyr-
rolide should bind strongly, however the phosphorus centres
will form comparatively weaker interactions with early tran-
sition metals versus later transition metals. This provides the
2
tantalising prospect of hemilability. Early transition metal
R
[
PNP ] complexes have exclusively had alkyl substituents in
1
R
R
t
the past. Sc[PNP ] and Y[PNP ] (R = Cy, Bu) have been shown
to facilitate C–H activation, hydrosilylation and hydrogenation
of olefins and support reactive methylidene complexes.
(
17–19
3
what less frequently deployed PNP pincer ligand is one where
4
,5
the pyridine ring is replaced by a pyrrole ring, giving a for-
R
mally anionic ‘LXL’ ligand ([PNP ], Fig. 1, bottom). What we
found particularly interesting is that, for both PNP congeners,
the phosphorus centres are usually further substituted with
alkyl substituents rather than aryl groups. Since its first syn-
a
Department of Chemistry, Molecular Sciences Research Hub, Imperial College
London, White City, Shepherds Bush, London, W12 0BZ, UK.
E-mail: m.chadwick@imperial.ac.uk
b
Department of Chemistry, Maynooth University, Maynooth, Co. Kildare, Ireland.
E-mail: tobias.kraemer@mu.ie
†
Electronic supplementary information (ESI) available: Experimental details,
1
variable temperature H NMR spectra, and further crystallographic details.
CCDC 2023865–2023867. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/d0dt03544k
Fig. 1 The general structures of pyridine-based and pyrrolide-based
R
PNP ligands. Emboldened [PNP ] refers exclusively to the anionic pyrro-
lide-PNP, where R indicates the substituents on the phosphine.
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Ph
Recently Nishibayashi and co-workers have shown that
Anticipating a [PNP ]ZrCl
3
stoichiometry in the product,
tBu
tBu
tBu
Ph
Ti[PNP ], V[PNP ] and Zr[PNP ] complexes are able to initial reactions of [PNP ]Li·Et O with ZrCl ·2THF were
In order to explore the steric carried out in a 1 : 1 ratio. However these reactions would give
and electronic profile of pyrrolide-based PNP ligands we broad P{ H} NMR spectra and complex H NMR spectra. The
2
4
2
0,21
catalyse dinitrogen reduction.
3
1
1
1
Ph
wished to investigate the synthesis and properties of the phe- synthesis was repeated with a 2 : 1 excess of [PNP ]Li·Et O (to
2
Ph
nylated derivative [PNP ] (Scheme 1).
4
ZrCl ·2THF). Despite the presumed excess of ligand in the
The synthesis of [PNP ]H and its subsequent lithiation has reaction mixture, after one hour over 95% of the [PNP
Ph
Ph
]
been previously reported and is analogous to the alkylated Li·Et O had been consumed. Subsequent work-up yielded the
2
1
2,17,20–29
congeners.
4
A variety of lithium bases have been used product in good yield (74% with respect to ZrCl ·2THF). Single
in the literature but in our hands the conditions outlined in crystals could be grown by layering a saturated THF solution
Scheme 1 and the ESI† worked best. The lithium salt could be with pentane and showed the structure to be an asymmetrical
2
Ph
3
Ph
Ph
isolated from an Et
2
O solution in very good yield (86%), but bis-pincer complex: [κ -PNP ][κ -PNP ]ZrCl
2 2 2
, [PNP ] ZrCl ,
even after prolonged exposure to dynamic vacuum (16 h, <1 × Fig. 3. Asymmetric binding of two pincer ligands is extremely
−
2
30
1
0
mbar) one equivalent of Et O remained (this was not rare. An analogous synthesis furnished the hafnium deriva-
2
Ph
Ph
found to affect onward reactivity). Dimeric {[PNP ]Li·THF}
2
2 2
tive, [PNP ] HfCl , and crystals suitable for X-ray diffraction
was crystallised by layering a THF solution with pentane. The could be grown similarly (see ESI†). This complex is the first
structure is shown in Fig. 2.
A dimeric structure of {Li[PNP ]}
time a pyrrolide-PNP motif has been ligated onto hafnium. We
has been previously have seen no evidence for a mono-[PNP ] complex, even when
iPr
Ph
2
iPr
reported. In {Li[PNP ]}
2
the Li bridges through the pyrrole ZrCl
4
·2THF is in excess and the Li salt is added slowly and at
nitrogens, but in {[PNP ]Li·THF}2 it preferentially binds low temperature.
through The zirconium and hafnium complexes have very similar
2
5
Ph
a
π-interaction with the pyrrolide ring. This
3
π-interaction is significantly slipped towards an η coordi- structures. Both crystallise in P ˉ1 with near identical unit cells.
nation. This may be induced by the presence of the THF Such similarity is common. The metal centre adopts a near-
donor. It is unclear whether the dimer remains in solution perfect pentagonal bipyramidal geometry (Cl(1)–Zr(1)–Cl(2):
3
1
1
7
1
with both P{ H} and Li{ H} NMR showing broad singlets 170.50(4)°; ∑(equatorial angles) = 365.7°). One phosphorus
and no coupling resolved.
centre is clearly pendant. Of the remaining phosphorus
centres two, P(2) and P(3), are trans to the bisector of an N–Zr–
P angle and one, P(1), is trans to the P–Zr–P angle bisector.
These differing phosphorus environments are reflected in the
3
1
1
low temperature P{ H} NMR studies. The individual bond
lengths are similar to the comparable ones in Nishibayashi’s
tBu
bis(tert-butyl)phosphinopyrrolide
complex
{[PNP ]ZrCl2
21
(
µ-Cl)}
2
.
Though unsymmetrical binding in a PNP pincer ligand
is unprecedented κ -[PNP] binding has been observed before
in the homoleptic [κ -PNP ]2Mn complex and in a DMAP
2
2
tBu
Ph
n
17,22
Scheme 1 The synthesis of [PNP
]
2
MCl
2
, M = Zr, Hf. (i) BuLi in substituted Sc complex.
No significant disorder is
Ph
hexane and toluene, −78 °C then RT for 1 hour, THF extraction. (ii)
ZrCl ·2THF/HfCl ·2THF.
observed in the crystal structure of either [PNP
2 2
] MCl
4
4
Ph
Fig. 3 The single crystal X-ray structure of [PNP
2 2
] ZrCl . Thermal
probability ellipsoids at 50%, hydrogen and non-ipso phenyl carbons
omitted for clarity. Selected bond distances (Å) and angles (°): Zr(1)–Cl(1)
2.4412(10), Zr(1)–Cl(2) 2.4193(11), Zr(1)–N(1) 2.260(3), Zr(1)–N(2) 2.264(3),
Zr(1)–P(1) 2.7935(10), Zr(1)–P(2) 2.8267(10), Zr(1)–P(3) 2.8390(11), Zr(1)–
P(4) 4.836(1), Cl(1)–Zr(1)–Cl(2) 170.50(4), P(2)–Zr(1)–N(1) 67.06(9), N(1)–
Zr(1)–P(1) 67.96 (9), P(1)–Zr(1)–N(2) 81.51(9), N(2)–Zr(1)–P(3) 67.97(9),
P(2)–Zr(1)–P(3) 81.20(3). ∑(equatorial angles) 365.7°.
Ph
2
Fig. 2 The single crystal X-ray structure of {[PNP ]Li·THF} . Thermal
probability ellipsoids at 50%, hydrogen and non-ipso phenyl carbons
omitted for clarity. Symmetry related labels marked with an apostrophe.
Selected bond distances (Å): Li(1)–N(1)’ 2.532(5), Li(1)–C(1)’ 2.573(6),
Li(1)–C(2)’ 2.544(6), Li(1)–C(3)’ 2.453(6), Li(1)–C(4)’ 2.441(5), Li(1)–N(1)
2.042(4), Li(1)–O(1) 1.957(5), Li(1)–P(1) 2.867(4).
16654 | Dalton Trans., 2020, 49, 16653–16656
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species, implying the structures to be static in crystallographic replaced by Me groups. Despite the differences in electronic
conditions. and steric properties of the resulting phosphines, we observe
We were curious to investigate whether the structure in the no qualitative effect on the proposed ligand exchange pathways
solid-state reflects the behaviour in solution. Pincer ligands that would alter our conclusions. A comparison of the geome-
Ph
often have the dissociation of a single arm of the ligand tries of the starting complex [PNP ] ZrCl and its truncated
2
2
Me
inferred in mechanistic pathways and our system could allow variant [PNP
us to investigate experimentally the energetics of this.
]
2
ZrCl
level of theory, reveals that both give satisfactory agreement
Variable temperature NMR studies were therefore undertaken with the experimental counterpart (see ESI†). Ligand exchange
Fig. 4). occurs via a dissociative mechanisms, involving initial decoor-
At room temperature a single P{ H} environment is dination of one phosphine arm followed by coordination of
observed, indicating fluxionality in the structure. However, on P4. In the case of P1/P4 exchange, the rate-determining step is
2
optimised at the BP86/SDD/6-31G(d,p)
3
1
(
3
1
1
3
1
1
‡
−1
cooling to 203 K the P{ H} resolves into four clear environ- associated with P1 decoordination (ΔG = + 11.7 kcal mol ),
ments. For both Zr and Hf congeners a high-field sharp singlet while for the P2/P4 and P4/P3 exchange processes their
is observed at approximately δ −15.0, assigned as the free respective overall barriers of ∼16.5 kcal mol− represent rate
1
Ph
31
1
pendant phosphine ([PNP ]H has a single P{ H} NMR reso- determining steps that are associated with a small rearrange-
Ph
nance at δ −16.0). For [PNP
2 2
] ZrCl the remaining resonances ment of the pyrrolide ring (Fig. 5). This rearrangement shifts
resolve into a doublet at δ 16.3, and two remaining signals are the pyrrolide ring onto the vacant coordination site initially co-
Ph
overlapping at δ 15.0. In the case of [PNP
2 2
] HfCl the reso- ordinated by the leaving phosphine. We note that we here
nances resolve more clearly. Two of the low-field resonances report energies uncorrected for London dispersion effects.
appear as doublet of doublets [presumably corresponding to Inclusion of D3 correction across a large range of tested func-
P(1) and P(3)] and a well-defined triplet [P(2)]. Assignments are tionals adds up to 10 kcal mol− to barrier heights (see ESI†).
1
proposed since P(2) is effectively identically trans to P(1) and
P(3), therefore extremely similar coupling is expected.
Obtaining accurate ligand binding energies remains a chal-
lenge for quantum chemical methods. In particular, the use
3
2
Upon slowly warming the sample coalescence phenomena of polarisable continuum models often restricts the accuracy
for both samples were observed. Coalescence of all resonances of predictions of dissociative processes. This is due to them
occurs simultaneously; however it is unlikely that direct falling short of correctly describing intermolecular solute-
exchange of the pendant phosphorus with all three others is solvent interactions on one hand, and both intramolecular dis-
degenerate. A more likely alternative is that P4 exchange with persive interactions and intermolecular solvent–solvent inter-
P2 and P3 is of similar energy, which could undergo sequential actions on the other. It has been argued that the solvent can
exchange with either P1 or P4. Using this model the exchange significantly attenuate inter- and intramolecular dispersive
‡
33,34
can be modelled as single-site exchange, and therefore a ΔG = interactions.
Nonetheless, we believe that the suggested
11.3 kcal mol− can be estimated for [PNP] ZrCl from the mechanism for the ligand exchange is plausible and provides
variable temperature P{ H} NMR (coalescence temperature a rationale for the experimentally observed temperature-depen-
49 K, for [PNP] HfCl coalescence temperature 270 K, dence of the NMR resonances.
1
+
2
2
3
1
1
2
2
2
−1
‡
ΔG = + 12.1 kcal mol ). These values agree well with those
calculated using DFT (vide infra). The somewhat elevated value
for the hafnium congener could arise from the stronger Hf–P
bond strength and implies a dissociative pathway for exchange.
In order to elucidate plausible mechanisms for the fluxio-
nal ligand exchange process, we performed DFT calculations
using a truncated model complex in which Ph groups were
31
1
Ph
Me
Fig. 4 The variable temperature P{ H} NMR of [PNP
]
2
MCl
2
(M = Zr,
Fig. 5 Summary of ligand exchange pathways for [PNP
2 2
] ZrCl . Free
Hf) in d -toluene. Red spectrum is measured at 298 K, blue spectrum is
measured at 203 K. For spectra at 10 K increments see ESI.†
8
Energies (BP86/def2-TZVP corrected for toluene solvent) of the rate-
determining step in each pathway are given in kcal mol−
.
1
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Conflicts of interest
There are no conflicts to declare.
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