Conventional and unconventional alkyne activations by Ru and Os for unprecedented dimetalated quinolizinium complexes

Article abstract of DOI:10.1039/d0cc03480k

Two types of unexpected quinolizinium complexes were obtained from the reactions between pyridine-functionalized propargylic alcohol HCCC(OH)(Ph)(CH2(2-py)) (L1) and cis-[M(L^L)2Cl2] (M = Ru, Os; L^L = 1,1-bis(diphenylphosphino)methane (dppm), 2,2′-bipyri

Full text of DOI:10.1039/d0cc03480k

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Conventional and unconventional alkyne
activations by Ru and Os for unprecedented
Cite this:DOI: 10.1039/d0cc03480k
dimetalated quinolizinium complexes†
Received 15th May 2020,
Accepted 1st July 2020
ab
a
Chi-Fung Yeung,
Hau-Lam Shek,
‡
Lai-Hon Chung, ‡a Sheung-Ying Tse,
a
a
a
ab
Man-Kit Tse,
Shek-Man Yiu
and Chun-Yuen Wong
*
DOI: 10.1039/d0cc03480k
rsc.li/chemcomm
Two types of unexpected quinolizinium complexes were obtained monometalated heterocyclic complexes were obtained, signifying
from the reactions between pyridine-functionalized propargylic the potential of ‘‘metal-induced cyclization of heteroatom-
R
alcohol HC CC(OH)(Ph)(CH2(2-py)) (L1) and cis-[M(L^L)2Cl2] functionalized alkynes’’ for the preparation of new metalated
(M = Ru, Os; L^L = 1,1-bis(diphenylphosphino)methane (dppm), heterocyclic complexes.5c, f,k,l,6 We herein report our serendipitous
2,20-bipyridine (bpy)). Their molecular structures revealed that L1 findings on the isolation of 2 types of dimetalated quinolizinium
can be activated by Ru and Os via the conventional ‘‘vinylidene- complexes from the reactions between pyridine-functionalized
involving’’ or unconventional ‘‘non-vinylidene-involving’’ pathways. propargylic alcohol HCRCC(OH)(Ph)(CH2(2-py)) (L1) and
dichloride complexes cis-[M(L^L)2Cl2] (M = Ru, Os; L^L =
Activation of alkynes by transition-metal complexes has long 1,1-bis(diphenylphosphino)methane (dppm), 2,20-bipyridine (bpy)).
been one of the most vital research interests in organometallic It is worth mentioning that while a variety of traditional and
chemistry.1 Regarding the interactions between d6 Ru(II) and metal-catalyzed synthetic strategies have been developed for
Os(II) centers and terminal alkynes, the formation of metal– the preparation of organic quinolizinium compounds due to
vinylidene species via alkyne–vinylidene rearrangement has their interesting biological and material applications,7 this
been well established and supported by many isolated and work represents the first report on the preparation of their
structurally characterized Ru– and Os–vinylidene complexes.2,3 stable metalated counterparts and exploration of their coordi-
However, a number of recent studies on the Ru/Os-induced nation chemistry.8 Moreover, the formation mechanisms for
cyclizations of heteroatom-functionalized alkynes suggested that the complexes isolated in this work provides insight into the
alkynes can be activated via unconventional ‘‘non-vinylidene- pathways of Ru/Os-induced alkyne activations/transformations.
involving’’ pathways.4,5 More specifically, the Ru/Os centers can
Ru- and Os-quinolizinium complexes 1q(OTf) and 2q(OTf)
act as electrophiles and induce cyclization of heteroatom- respectively were initially prepared from the reactions between
functionalized alkynes without vinylidene intermediacy. Evidently, cis-[Ru/Os(dppm)2Cl2] and L1 in CH2Cl2 with NaOTf as a mild
the ‘‘vinylidene-involving’’ pathways should not be taken for chloride abstracting agent at room temperature (Scheme 1).
granted regarding the Ru/Os-mediated alkyne transformations.
This synthetic method is not desirable as metallafuran complexes
With the goal to gain control on the Ru(II)- and Os(II)- 1f(OTf)2 and 2f(OTf)2 were also isolated from the reaction mix-
induced alkyne activations, we have initiated research activi- tures and as the major products. Reaction temperature was found
ties on isolating and analyzing the intermediates or products to be an important parameter in controlling the product selectiv-
from the reactions between heteroatom-functionalized alkynes ity, in which the q to f ratio increases with elevating temperature.
and low-valent transition metal precursors. Gratifyingly, This ratio can be further improved by performing the reactions in
several series of unprecedented and structurally well-defined refluxing MeOH.
The molecular structures determined for 1q(ClO4)ꢀCH2Cl2
a Department of Chemistry, City University of Hong Kong, Tat Chee Avenue,
and 2q(ClO4)ꢀCH2Cl2 revealed that the 2-phenylquinolizinium
Kowloon, Hong Kong SAR. E-mail: acywong@cityu.edu.hk
b State Key Laboratory of Terahertz and Millimeter Waves City University of Hong
moieties derived from L1 behave as k2(C4,C6)-bidentate ligands
where the coordinating C atoms are the C4 and C6 of the
quinolizinium (labelled as C9 and C1 respectively in Fig. 1), and
the ten non-hydrogen atoms on each of the quinolizinium
skeleton are essentially coplanar. All the M–C bond distances
(2.075(4)–2.100(5) Å) are indicative of Ru–C or Os–C single bond
Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR
† Electronic supplementary information (ESI) available: Full experimental
details. CCDC 2001062–2001068. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d0cc03480k
‡ These authors contributed equally to this work.
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Scheme 1 Synthesis of metalated quinolizinium and metallafuran com-
plexes under different reaction conditions.
Fig. 2 Perspective views of dications 1f and 2f as represented by 50%
probability ellipsoids (hydrogen atoms are omitted and phenyl rings on
dppm are represented by gray sticks for clarity).
the isomerization involves cleavage of the picolinyl group from
the propargylic carbon on L1, and bond formation between the
picolinyl nitrogen and Cb on the five-membered metallacycle.
The metallacycles are essentially planar (sum of the interior
angles are 539.6–540.01), and the C1–C2 and C2–C3 distances
(1.351(14)–1.429(14) Å) are between typical C–C single and
double bonds, consistent with the mesomeric representation
depicted in Scheme 1.
Structural analysis on the k2(C4,C6)- and k2(C,O)-bidentate
ligands in q and f respectively suggests that they are products
from two competing reaction mechanisms (Scheme 2). The
formation of q is proposed to go through a combination of
vinylidene rearrangement and dehydration, then 6-exo-dig cycliza-
tion to give monometalated quinolizinium species, and finally
metalation at the C6 of quinolizinium to give dimetalated quino-
Fig. 1 Perspective views of monocations 1q and 2q as represented by
50% probability ellipsoids (hydrogen atoms are omitted and phenyl rings
on dppm are represented by gray sticks for clarity).
character. The highly distorted sp2 hybridization of the meta- lizinium complexes (Scheme 2(a)). The vinylidene intermediacy
lated carbons suggests that the M–C bonding interaction in 1q was consolidated by the isolation of a monocationic Os–vinylidene
and 2q cannot be described as Fischer carbene as in RuII/OsII/ complex 2v from the reaction between cis-[Os(dppm)2Cl2] and
IrIII–pyridylidene complexes.9 The bite angles of the k2(C4,C6)- HCRCC(OH)(Ph)(CH2(6-Br-2-py)) (L2; Scheme S1, ESI†), in which
2-phenylquinolizinium moieties span from 63.7(2) to 64.6(2)1, the Br substituent was intentionally introduced to prevent post-
which are even smaller than those of dppm in 1q and 2q vinylidene transformations. On the other hand, the formation of
(70.4(1)–71.0(1)1). The four-membered chelate rings formed f can be rationalized as a result of a ‘‘non-vinylidene-involving’’
by the M, C1, N1, and C9 atoms are highly planar, with the pathway, which involves metal-induced 5-exo-dig cyclization and
sum of the interior angles close to 3601 (1q: 360.01, 2q: 360.11). a series of post-transformations including rearrangement of a
Comparison with the structure determined for the non- picolinyl group as depicted in Scheme 2(b).
metalated 2-phenylquinolizinium ion Q (Fig. 3) shows that
The temperature-dependent q to f ratio in the reactions between
the bond lengths of the 2-phenylquinolizinium moieties cis-[Ru/Os(dppm)2Cl2] and L1 deserves further discussion. It was
remain nearly the same but the +C1–N–C9 are strained upon recently reported that the reaction between cis-[Os(dppm)2Cl2] and
metalation with Ru or Os (from 119.6(2)1 to 105.4(4)–105.9(3)1). pyridyl ynone HCRC(CQO)(2-py) at room temperature gave
The 2-methylpyridinium-functionalized metallafuran struc- Os-indolizinone complex via a ‘‘non-vinylidene-involving’’ pathway,
tures determined for dications 1f and 2f revealed that the whereas performing the reaction at elevated temperature gave the
propargylic alcohols isomerize as k2(C,O)-bidentate ligands vinylidene-derived phosphonium-ring-fused osmafuran complex
and coordinate to the metal centers (Fig. 2). More specifically, (Scheme S2, ESI†).5l,6d,e Apparently the activation barrier for the
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Fig. 3 Perspective views of 3 and Q as represented by 50% probability
ellipsoids (hydrogen atoms on 3 are omitted for clarity).
a mechanism distinctly different from those depicted in
Scheme 2. Based on the structure, the formation of 3 can be
rationalized as a combination of 6-endo-dig cyclization and
dehydration of L1 to give monometalated quinolizinium spe-
cies, and then metalation at the C2 of the phenyl ring to give 3
(Scheme 3(b)).
Scheme 2 Plausible mechanisms for the formation of q and f. [M] =
[Ru/Os(dppm)2]2+
.
The isolation of 1q, 2q and 3 from the reactions aforementioned
prompted us to explore the possibility to prepare these metalated
quinolizinium complexes by reacting 2-phenylquinolizinium ion
Q with cis-[Ru/Os(dppm)2Cl2] and cis-[Ru(bpy)2Cl2], but attempts to
react these metal precursors with Q in the absence/presence
of various bases did not afford metalated quinolizinium com-
plexes of any kind. Interestingly, L1 was found to cyclize in
acidified MeOH under refluxing condition to give Q, represent-
ing a new method to prepare quinolizinium compounds.10,11
The structure of Q was also determined by X-ray crystallography
(Fig. 3). It is worth mentioning that (i) all complexes isolated in
this work are stable in solid form under ambient conditions,
(ii) 1q and 2q remain intact in a variety of organic solvents for
weeks, with only partial decomposition (ca. 10%) observed at
high temperature (e.g. refluxing chlorobenzene) in the presence
of acid to give Q, (iii) 3 undergoes decomposition upon dis-
solution in acetone to give Q at room temperature, even in
darkness.
To conclude, the isolation of unprecedented quinolizinium
and metallafuran complexes in this this work demonstrated the
diverse alkyne activation pathways by Ru(II) and Os(II), where
1q/2q, 1f/2f and 3 were formed from distinct reaction mechan-
isms. The formation of 1q/2q and 3 from propargylic alcohol L1
highlighted the potential of ‘‘metal-induced cyclization of
heteroatom-functionalized alkynes’’ as a strategy to prepare
metalated heterocyclic complexes, and that of 1f/2f and 3
revealed the existence of unconventional ‘‘non-vinylidene-
involving’’ pathways regarding the Ru/Os–alkyne interactions.
The work described in this paper was supported by the
Research Grants Council of Hong Kong SAR (CityU 11207117
and T42-103/16-N) and City University of Hong Kong (7005096,
7005257 and 9680206).
alkyne–vinylidene rearrangement is relatively higher than that for
electrophilic cyclization of the pyridine-tethered terminal alkynes.
Although the generality of this statement is to be validated, the
current experimental finding reinforces the argument that alkynes
can be activated by Ru(II) and Os(II) via unconventional ‘‘non-
vinylidene-involving’’ pathways.
To expand the scope of this study, the reactions between cis-
[Ru/Os(bpy)2Cl2] and L1 were also investigated. Although the
reactions were messy and not all the products could be identified,
a Ru complex supported by two bpy and a 2-phenylquinolizinium
moiety was isolated and characterized (3(OTf), Scheme 3(a) and
Fig. 3); isolation and characterization of its Os analogue were not
successful. Interestingly, the 2-phenylquinolizinium moiety in 3
does not behave as a k2(C4,C6)- but k2(C3,CPh)-bidentate ligand,
i.e. the metalating C atoms are the C3 of the quinolizinium and
the C2 of the phenyl group. The Ru–C bond distances in 3
(2.026(3)–2.046(3) Å) are slightly shorter than those in 1q and 2q
(2.075(4)–2.100(5) Å), but still within the Ru–C single bond regime.
The bite angle of the k2(C3,CPh)-2-phenylquinolizinium is
80.61(12)1. Evidently, cis-[Ru(bpy)2Cl2] can activate L1 through
Conflicts of interest
Scheme 3 (a) Synthesis and (b) plausible mechanism for the formation of
3 [Ru] = [Ru(bpy)2]2+
.
There are no conflicts to declare.
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