Highly reactive scandium phosphinoalkylidene complex: C-H and H-H bonds activation

Article abstract of DOI:10.1021/jacs.6b13081

The first scandium phosphinoalkylidene complex was synthesized and structurally characterized. The complex has the shortest Sc-C bond lengths reported to date (2.089(3) ?). DFT calculations reveal the presence of a three center π interaction in the complex. This scandium phosphinoalkylidene complex undergoes intermolecular C-H bond activation of pyridine, 4-dimethylamino pyridine and 1,3-dimethylpyrazole at room temperature. Furthermore, the complex rapidly activates H₂ under mild conditions. DFT calculations also demonstrate that the C-H activation of 1,3-dimethylpyrazole is selective for thermodynamic reasons and the relatively slow reaction is due to the need of fully breaking the chelating effect of the phosphino group to undergo the reaction whereas this is not the case for H₂.

Full text of DOI:10.1021/jacs.6b13081

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Communication
A Highly Reactive Scandium Phosphinoalkylidene
Complex: C–H and H–H Bonds Activation
Weiqing Mao, Li Xiang, Carlos Alvarez Lamsfus, Laurent Maron, Xuebing Leng, and Yaofeng Chen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b13081 • Publication Date (Web): 09 Jan 2017
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A Highly Reactive Scandium Phosphinoalkylidene Complex: C–H
and H–H Bonds Activation
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Weiqing Mao,† Li Xiang,† Carlos Alvarez Lamsfus,‡ Laurent Maron,*,‡ Xuebing Leng,† Yaofeng
Chen*,†
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, P. R. China
‡LPCNO, CNRS & INSA, Université Paul Sabatier, 135 Avenue de Rangueil, 31077 Toulouse, France
Supporting Information Placeholder
ABSTRACT: The first scandium phosphinoalkylidene complex
was synthesized and structurally characterized. The complex has
the shortest Sc–C bond lengths reported to date (2.089(3) Å). DFT
calculations reveal the presence of a three center π interaction in
the complex. This scandium phosphinoalkylidene complex underꢀ
goes intermolecular C–H bond activation of pyridine, 4ꢀ
dimethylamino pyridine and 1,3ꢀdimethylpyrazole at room temꢀ
perature. Furthermore, the complex rapidly activates H2 under
mild conditions. DFT calculations also demonstrate that the C–H
activation of 1,3ꢀdimethylpyrazole is selective for thermodynamic
reasons and the relatively slow reaction is due to the need of fully
breaking the chelating effect of the phosphino group to undergo
the reaction whereas this is not the case for H2.
Scandium
methyl
chloride
[LSc(Me)Cl]
(L
=
[MeC(NDIPP)CHC(NDIPP)Me]ꢀ) was prepared as reported by
Piers and coworkers.7 Lithium salt Li[CH(SiMe3)PPh2](THF) was
synthesized by using the method reported by Peterson,8 and the
complex was isolated and characterized by NMR spectroscopy
(1H, 13C{1H}, 31P{1H}) and elemental analysis. A salt metathesis
of
Li[CH(SiMe3)PPh2](THF) in toluene at room temperature providꢀ
ed scandium methyl phosphinoalkyl complex
scandium
methyl
chloride
[LSc(Me)Cl]
with
a
Mononuclear transition metal alkylidene (or carbene) complexꢀ
es have attracted intense attention, and a great number of monoꢀ
nuclear transition metal alkylidene complexes have been syntheꢀ
sized in past decades.1 One exception is those with rareꢀearth
metal ions. Due to HOMO/LUMO orbital energies mismatch
between the d0 rareꢀearth metal ions and the alkylidene groups, the
formation of mononuclear complexes is unfavorable.2 The carꢀ
bene groups have a strong tendency to bind more than one rareꢀ
earth metal ion or one rareꢀearth metal ion and two other metal
ions.3,4 For the purpose of stabilization of such mononuclear rareꢀ
earth metal complexes, methandiide dianions with P(V) substituꢀ
ents, such as [C(PPh2NSiMe3)2]2ꢀ [C(PPh2S)2]2ꢀ and
[C(SiMe3)PPh2S]2ꢀ, were employed.5 However, the reactivity of
such rareꢀearth metal complexes is quite sluggish. The observed
low activity is certainly due to the using of strong electronic
withdrawing P(V) substituents, pincerꢀtype structure and/or fourꢀ
member chelating. With this in mind, we carried out a study on
the synthesis of scandium phosphinoalkylidene complex. Comꢀ
pared to the P(V) substituted analogues, this ligand is less exꢀ
plored in the earlyꢀtranslation metal complexes,6 and no rareꢀearth
metal complexes of that type have been reported. Herein, we
report the synthesis and bonding analysis (DFT) of the first scanꢀ
dium phosphinoalkylidene complex. This complex is highly reacꢀ
tive and is able to activate pyridine, 4ꢀdimethylamino pyridine,
1,3ꢀdimethylpyrazole and H2 under mild conditions.
1
[LSc{CH(SiMe3)PPh2}Me] (1) in 66% yield. The H NMR specꢀ
trum of 1 in C6D6 clearly shows two featured signals at δ = 0.64
and ꢀ0.82 ppm for Sc−CH3 and Sc−CH(SiMe3)PPh2, respectively.
The solid state structure of 1 was also obtained (the supporting
information, Figure S1), in which the scandium center adopts a
distorted square pyramidal geometry with two nitrogen atoms of L
and carbon and phosphorus atoms of [CH(SiMe3)PPh2]ꢀ forming
the basal plane and one methyl ligand occupying the apical posiꢀ
tion. Complex 1 is stable in benzene, toluene and THF at room
o
temperature, but eliminates methane in THF at 50 C to give a
mononuclear
scandium
phosphinoalkylidene
complex
[LSc{C(SiMe3)PPh2}THF] (2) as shown in Scheme 1. Complex 2
was isolated in 63% yield, and characterized by NMR spectroscoꢀ
py (1H, 13C{1H}, 31P{1H}) and elemental analysis. In the 13C{1H}
NMR, the alkylidene carbon (Sc−C(SiMe3)PPh2) signal appears at
1
δ = 151.7 (d, JPꢀC = 103 Hz), which is significantly downshifted
in comparison with that of the alkyl carbon (Sc−CH(SiMe3)PPh2)
in 1 (46.8 ppm), in agreement with a sp2 carbon. Interestingly, this
carbon signal is also downshifted in comparison with that of the
alkylidene
[{MeC(NDIPP)CHC(Me)(NCH2CH2
carbon
(Sc−C(SiMe3)PPh2S)
in
N(iPr)2)}Sc{C(SiMe3)PPh2S}] (117.0 ppm).5k Complex 2 was
further characterized by single crystal Xꢀray diffraction; its moꢀ
lecular structure is shown in Figure 1. Complex 2 has similar
coordination geometry as that of complex 1 but with the anionic
methyl group replaced by a neutral THF. The Sc–C bond length in
Scheme 1. Synthesis of Scandium Phosphinoalkylidene Com-
plex 2.
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2 (2.089(3) Å) is 0.20 Å shorter than that in 1 (2.292(4) Å), indiꢀ
N
N
cating a bondꢀorder increasing. To the best of our knowledge, the
Sc–C bond length in 2 is the shortest Sc–C bond length to date.9
On the other hand, the Sc–P and C–P bond lengths in 2 (2.597(1)
and 1.743(3) Å) are slightly shorter than those in 1 (2.638(1) and
1.790(4) Å).
3
4
5
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9
DIPP
DIPP
Sc
P
H
N
N
N
C
N
R
SiMe3
DIPP
Sc
P
H
C
Ph
SiMe3
Ph
R
THF
Ph
4
R = H ( )
88%yield;
Ph
toluene
r. t. 0.5 h
3
NMe2 (5) 78%yield
-THF
2
N
N
H2, 1-hexene
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N
N
N
N
DIPP
DIPP
toluene
r. t. 0.5 h
toluene
r. t. 12 h
DIPP
DIPP
Sc
P
Sc
H
C
H
N
C
SiMe3
SiMe3
N
P
Ph
Ph
Ph
7
Ph
, 70% yield
6
, 78% yield
Figure 1. Molecular structure of complex 2 (ball and stick repreꢀ
sentation). DIPP isopropyl groups and hydrogen atoms were
omitted for clarity.
Figure 3. Molecular structure of complex 5 (ball and stick repreꢀ
sentation). DIPP isopropyl groups and hydrogen atoms (except the
H30 atom) were omitted for clarity.
In contrast to the remarkable stability of titanium phosphinoalꢀ
kylidene complex [(PNP)Ti(CHPPh2)Ph] (PNP = [N{2ꢀP(iPr)2ꢀ4ꢀ
methylphenyl}2]ꢀ) and scandium alkylidene complexes with P(V)
Figure 2. HOMO (3 centres πꢀtype orbital) of complex 2.
substituent
[LʹSc{C(SiR3)PPh2S}]
(Lʹ
=
In order to get more insights into the nature of the bonding in
complex 2 and especially on the Sc−C one, DFT calculations were
carried out. For comparison purposes, a similar analysis was done
on complex 1. In complex 2, the HOMO is clearly a 3 center(3c)
πꢀtype orbital mainly involving the Sc−C and P−C bonds(Figure
2), whereas the HOMOꢀ1 and HOMOꢀ2 are the Sc−C and P−C σ
bonds, respectively (see the supporting information). The associꢀ
ated Lewis structure is therefore of allylicꢀtype with π electron
delocalization between the Sc, C and P centers, different from that
in (C5Me5)Ta(PMe3)(2ηꢀCHPMe2) where the electron is localꢀ
ized.6g Natural Bonding Orbital (NBO) analysis was then perꢀ
formed. In 2, the two σ bonds were found in the bonding interacꢀ
tion and a 3c bond was also found between Sc, C and P atoms, in
line with the Lewis structures drawn from the MOs. In the same
way, the associated Wiberg Bond Indexes (WBI) are 0.75 for the
Sc−C bond, 0.34 for the ScꢀP and 1.21 for the P−C bond, indicatꢀ
ing delocalized electron densities. For the latter, the comparison
with the bonding in complex 1 is informative. Indeed, the WBI
are strongly reduced with respect to complex 2 (0.39 for the Sc−C
bond 0.36 for ScꢀP and 0.99 for the P−C bond), in line with the
lack of π interaction in the alkyl complex 1. Therefore, DFT
results indicate the alkylidene character of complex 2 with a π
density delocalized in between the Sc, C and P centers.
[MeC(NDIPP)CHC(Me)(NCH2CH2NMe2)]ꢀ, [MeC(NDIPP)CH
C(Me)(NCH2CH2N(iPr)2)]ꢀ; Rʹ = Me, Ph),5k, 6k complex 2 rapidly
undergoes an intramolecular C−H bond activation to give comꢀ
plex 3 accompanied by release of THF in C6D6 at room temperaꢀ
1
ture (Scheme 2). The H NMR spectral monitoring also revealed
an equilibrium between the complexes 2 and 3 (see Figure S29 of
the supporting information), and the equilibrium constant of the
reaction at room temperature is 1.64 according to the equation Ke
= ([3][THF])/([2]). Complex 3 was prepared in 69% isolated yield
by a scaledꢀup reaction in toluene, and THF was removed under
vacuum during the reaction for promoting the conversion of 2 into
3. The molecular structure of 3 was determined by single crystal
Xꢀray diffraction (the supporting information, Figure S2). While
in the presence of pyridine, complex 2 rapidly converts into a
scandium
pyridyl
phosphinoalkyl
complex
[LSc{CH(SiMe3)PPh2}(C5H4N)] (4). The mechanism by which
complex 4 is formed can occur by two plausible pathways: 1) Path
A, a 1,2ꢀaddtion of pyridine C−H bond to the Sc−C(alkylidene)
bond of 2; 2) Path B, an intramolecular C−H bond activation first
to generate complex 3, followed by a σꢀbond metathesis of 3 with
pyridine. The isotopic labeling experiment by using 2 with pyriꢀ
dineꢀd5 was carried out, which cleanly produced the isotopomer
[LSc{CD(SiMe3)PPh2}(C5D4N)], and no deuterium incorporation
was observed into the aryl group of L. Therefore, the scandium
pyridyl phosphinoalkyl complex 4 is formed via Path A. Complex
2 also readily reacts with 4ꢀdimethylamino pyridine (DMAP) at
room temperature to afford a C−H bond activation product 5.
Scheme 2. Reactivity of Scandium Phosphinoalkylidene Com-
plex 2.
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Solid state structures of 4 and 5 were also obtained; the molecular
structure of 5 is shown in Figure 3, while that of 4 given in the
supporting information (Figure S3). In 4 and 5, the pyridyl and 4ꢀ
dimethylamino pyridyl ligands are both η2 bound with Sc–N bond
lengths of 2.154(2) and 2.150(3) Å and Sc–C bond lengths of
2.227(2) and 2.219(4) Å, respectively.
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Figure 4. Molecular structure of complex 5 (ball and stick repreꢀ
sentation). DIPP isopropyl groups and hydrogen atoms (except the
H30 atom) were omitted for clarity.
Figure 5. DFT computed enthalpy reaction profile for the reaction
of 2 with 1,3ꢀdimethylpyrazole at room temperature
DFT calculations were then carried out on the selective C−H
activation of 1,3ꢀdimethylpyrazole (Figure 5). The reaction with
1,3ꢀdimethylpyrazole begins by the coordination of the pyrazole
that induces the loss of the chelating effects of the phosphino
group in 2. This is overall disfavored by more than 20 kcal/mol.
Then, the C−H bond activation transition state (TS) can be
reached and the barrier is around 34 kcal/mol for the lowest in
line with a 12h reaction. The selectivity of the C−H bond activaꢀ
tion is both kinetic (lower barrier by 3.6 kcal/mol) and finally by
the formation of the product that is more stable when the methyl
on the nitrogen atom is activated. For comparison purposes, the
H2 activation was also computed (Figure 6). The coordination of
H2 does not imply the full disruption of the chelating effect of the
phosphino group in 2. The barrier is 15 kcal/mol, significantly
lower than the C−H activation of the 1,3ꢀdimethylpyrazole, in line
with a much faster reaction.
In the reaction of 2 with 1,3ꢀdimethylpyrazole, a selective C−H
bond activation occurs at the methyl group on nitrogen atom to
give complex 6 as shown in Scheme 2, and no other product was
observed. 1,3ꢀdimethylpyrazolyl ligand coordinates to the scandiꢀ
um center in a K2ꢀC,N fashion with Sc–N and Sc–C bond lengths
of 2.276(3) and 2.287(2) Å, respectively (Figure 4). It’s worthy to
note that the phosphorus atom of the phosphinoalkyl ligand is not
coordinated to the scandium ion. Furthermore, complex 2 rapidly
reacts with H2 under one atmosphere of H2 in benzene or toluene
at room temperature. The product quickly decomposes, but it can
be trapped by a reaction with 1ꢀhexene.10 The reaction gave a
scandium hexyl complex 7 in high yield, and the complex was
characterized by Xꢀray crystallography (see Figure S4 of the
supporting information). The formation of 7 implied that complex
2 activates H2 to produce a scandium hydride, which subsequently
undergoes an addition reaction with 1ꢀhexene. The isotopic labelꢀ
ing experiment of 2, D2 and 1ꢀhexene gave the isotopomer
[LSc{CD(SiMe3)PPh2}(CH2CH(D)C4H9)], and no deuterium
incorporation was observed into the aryl group of L. It was also
found that addition of THF to 7 causes hexane elimination to
o
regenerate the phosphinoalkylidene complex 2 at 50 C. Furtherꢀ
more, our initial study showed complex 2 can catalyze the hydroꢀ
genation of 1ꢀhexene.
Figure 6. DFT computed enthalpy reaction profile for the reaction
of 2 with H2 at room temperature.
In summary, scandium methyl phosphinoalkyl complex
[LSc{CH(SiMe3)PPh2}Me] (1) eliminates methane in THF to
produce
scandium
phosphinoalkylidene
complex
[LSc{C(SiMe3)PPh2}THF] (2). Xꢀray diffraction analysis reveals
the very short Sc−C bond length in 2, and DFT analysis indicates
electronic delocalization between the Sc, C and P centers. Comꢀ
plex 2 exhibits a much higher reactivity than the one reported to
date for scandium alkylidene complexes containing P(V) substituꢀ
ent.5k DFT investigations of the reaction mechanisms explained
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(4) For some selected references of the rareꢀearth metal complexes
the rate and the selectivity of the C−H activation of 1,3ꢀ
dimethylpyrazole because of the loss of Pꢀchelation to allow the
activation and the stability of the formed product. In the same
way, the easiness of the H2 activation appears to be due to the
possibility to maintain chelation by the phosphino group unlike
the C−H activation of 1,3ꢀdimethylpyrazole. This is in line with
the electronic delocalization found in the phosphinoalkylidene
ligand by DFT analysis. This complex 2 is therefore a unique
platform, and we are currently exploring its ability to perform
catalytic reactions and enlarging the family of rareꢀearth metal
phosphinoalkylidene complexes.
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ASSOCIATED CONTENT
Supporting Information
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P. L. Chem. Commun. 2008, 78, 1747. (d) Buchard, A.; Auffrant, A.;
Ricard, L.; Le Goff, X. F.; Platel, R. H.; Williams, C. K.; Le Floch, P.
Dalton Trans. 2009, 10219. (e) FustierꢀBoutignon M.; Le Goff, X.F.; Le
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T.; Mills, D. P.; Wooles, A. J. Chem. Soc. Rev. 2011, 40, 2164. (g) Mills,
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Experimental and computational details and a zip file containing
CIFs for 1−7. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
laurent.maron@irsamc.upsꢀtlse.fr (Laurent Maron);
yaofchen@mail.sioc.ac.cn (Yaofeng Chen)
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 21132002, 21325210 and
21421091), the State Key Basic Research & Development
Program (Grant No. 2012CB821600), the Strategic Priority Reꢀ
search Program of the Chinese Academy of Sciences (Grant No.
XDB20000000), and the CAS/SAFEA International Partnership
Program for Creative Research Teams. Laurent Maron is member
of the Institut Universitaire de France. The Humboldt foundation,
the Chinese Scholarship Council and the Chinese Academy of
Science are also acknowledged as well as CalMip for computing
time.
(7) Knight, L. K.; Piers, W. E.; FleuratꢀLessard, P.; Parvez, M.;
McDonald, R. Organometallics 2004, 23, 2087.
(8) Peterson, D. J. J. Org. Chem. 1968, 33, 780.
REFERENCES
(1) For some representative reviews of the transition metal alkylidene
(or carbene) complexes, see: (a) Astruc, D. New J. Chem. 2005, 29, 42. (b)
Elschenbroich, C. Organometallics, 3rd ed.; WileyꢀVCH, Weinheim,
Germany, 2006. (c) Schrock, R. R. Angew. Chem., Int. Ed. 2006, 45, 3748.
(d) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760.
(9) The previous shortest Sc–C bond length is 2.1134(18) Å observed in
[{MeC(NDIPP)CHC(Me)(NCH2CH2N(iPr)2)}Sc{C(SiMe3)PPh2S)],
the reference 5k.
see
(10) The complexes 2 and 3 both do not react with 1ꢀhexene.
(2) Giesbrecht, G. R.; Gordon, J. C. Dalton Trans. 2004, 2387.
(3) For reviews of the rareꢀearth metal complexes containing bridging
alkylidene groups, see: (a) Liu, Z. X.; Chen, Y. F. Sci. Sin. Chim. 2011, 41,
304. (b) Summerscales, O. T.; Gordon, J. C. RSC Adv. 2013, 3, 6682. (c)
Kratsch, J.; Roesky, P. W. Angew. Chem., Int. Ed. 2014, 53, 376.
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