Further Evidence for ‘Extended’ Cumulene Complexes: Derivatives from Reactions with Halide Anions and Water

Article abstract of DOI:10.1002/chem.201905399

Reactions of [Ru{C=C(H)-1,4-C₆H₄C≡CH}(PPh₃)₂Cp]BF₄ ([1 a]BF₄) with hydrohalic acids, HX, results in the formation of [Ru{C≡C-1,4-C₆H₄-C(X)=CH₂}(PPh₃)₂Cp] [X=Cl (2 a-Cl), Br (2 a-Br)], arising from facile Markovnikov addition of halide anions to the putative quinoidal cumulene cation [Ru(=C=C=C₆H₄=C=CH₂)(PPh₃)₂Cp]⁺. Similarly, [M{C=C(H)-1,4-C₆H₄-C≡CH}(LL)Cp]BF₄ [M(LL)Cp’=Ru(PPh₃)₂Cp ([1 a]BF₄); Ru(dppe)Cp* ([1 b]BF₄); Fe(dppe)Cp ([1 c]BF₄); Fe(dppe)Cp* ([1 d]BF₄)] react with H⁺/H₂O to give the acyl-functionalised phenylacetylide complexes [M{C≡C-1,4-C₆H₄-C(=O)CH₃}(LL)Cp’] (3 a–d) after workup. The Markovnikov addition of the nucleophile to the remote alkyne in the cations [1 a–d]⁺ is difficult to rationalise from the vinylidene form of the precursor and is much more satisfactorily explained from initial isomerisation to the quinoidal cumulene complexes [M(=C=C=C₆H₄=C=CH₂)(LL)Cp’]⁺ prior to attack at the more exposed, remote quaternary carbon. Thus, whilst representative acetylide complexes [Ru(C≡C-1,4-C₆H₄-C≡CH)(PPh₃)₂Cp] (4 a) and [Ru(C≡C-1,4-C₆H₄-C≡CH)(dppe)Cp*] (4 b) reacted with the relatively small electrophiles [CN]⁺ and [C₇H₇]⁺ at the β-carbon to give the expected vinylidene complexes, the bulky trityl ([CPh₃]⁺) electrophile reacted with [M(C≡C-1,4-C₆H₄-C≡CH)(LL)Cp’] [M(LL)Cp’=Ru(PPh₃)₂Cp (4 a); Ru(dppe)Cp* (4 b); Fe(dppe)Cp (4 c); Fe(dppe)Cp* (4 d)] at the more exposed remote end of the carbon-rich ligand to give the putative quinoidal cumulene complexes [M{C=C=C₆H₄=C=C(H)CPh₃}(LL)Cp’]⁺, which were isolated as the water adducts [M{C≡C-1,4-C₆H₄-C(=O)CH₂CPh₃}(LL)Cp’] (6 a–d). Evincing the scope of the formation of such extended cumulenes from ethynyl-substituted arylvinylene precursors, the rather reactive half-sandwich (5-ethynyl-2-thienyl)vinylidene complexes [M{C=C(H)-2,5-^cC₄H₂S-C≡CH}(LL)Cp’]BF₄ ([7 a–d]BF₄ add water readily to give [M{C≡C-2,5-^cC₄H₂S-C(=O)CH₃}(LL)Cp’] (8 a–d)].

Full text of DOI:10.1002/chem.201905399

A Journal of
Accepted Article
Title: Further evidence for ‘extended’ cumulene complexes: derivatives
from reactions with halide anions and water
Authors: Michael R Hall, Rachel R Steen, Marcus Korb, Alexandre N
Sobolev, Stephen A Moggach, Jason M Lynam, and Paul J.
Low
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10.1002/chem.201905399
Chemistry - A European Journal
Further evidence for ‘extended’ cumulene complexes:
derivatives from reactions with halide anions and water.
Michael R. Hall,a Rachel R. Steen,b Marcus Korb,a Alexandre N. Sobolev,a Stephen
A. Moggach,a,c Jason M. Lynam,b* Paul J. Lowa*
a School of Molecular Sciences, University of Western Australia, 35 Stirling
Highway, Crawley, WA, 6009, Australia
b Department of Chemistry, University of York, Heslington, York YO10 5DG, UK
c Centre for Microscopy, Characterisation and Analysis, University of Western
Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia
JML: jason.lynam@york.ac.uk
PJL: paul.low@uwa.edu.au
Supporting information and the ORCID identification numbers for the authors of this
article can be found under http://XXXXX. The Supporting Information contains
detailed synthetic procedures, crystallographic details and additional plots of
molecular structures, and plots of 1H, 13C{1H} NMR and mass spectra.
Abstract
Reactions of [Ru{C=C(H)-1,4-C6H4C≡CH}(PPh3)2Cp]BF4 ([1a]BF4) with hydrohalic
acids, HX, results in the formation of [Ru{C≡C-1,4-C6H4-C(X)=CH2}(PPh3)2Cp] [X
1
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= Cl (2a-Cl), Br (2a-Br)], arising from facile Markovnikov addition of halide anions
to the putative quinoidal cumulene cation [Ru(=C=C=C6H4=C=CH2)(PPh3)2Cp]+.
Similarly, [M{C=C(H)-1,4-C6H4-C≡CH}(LL)Cp ]BF4 [M(LL)Cp´ = Ru(PPh3)2Cp
([1a]BF4); Ru(dppe)Cp* ([1b]BF4); Fe(dppe)Cp ([1c]BF4); Fe(dppe)Cp* ([1d]BF4)]
react with H+/H2O to give the acyl-functionalised phenylacetylide complexes
[M{C≡C-1,4-C6H4-C(=O)CH3}(LL)Cp´] (3a-d) after workup. The Markovnikov
addition of the nucleophile to the remote alkyne in the cations [1a – d]+ is difficult to
rationalise from the vinylidene form of the precursor and is much more satisfactorily
explained from initial isomerisation to the quinoidal cumulene complexes
[M(=C=C=C6H4=C=CH2)(LL)Cp´]+ prior to attack at the more exposed, remote
quaternary carbon. Thus, whilst representative acetylide complexes [Ru(C≡C-1,4-
C6H4-C≡CH)(PPh3)2Cp] (4a) and [Ru(C≡C-1,4-C6H4-C≡CH)(dppe)Cp*] (4b) reacted
with the relatively small electrophiles [CN]+ and [C7H7]+ at the β-carbon to give the
expected vinylidene complexes, the bulky trityl ([CPh3]+) electrophile reacted with
[M(C≡C-1,4-C6H4-C≡CH)(LL)Cp´] [M(LL)Cp´ = Ru(PPh3)2Cp (4a); Ru(dppe)Cp*
(4b); Fe(dppe)Cp (4c); Fe(dppe)Cp* (4d)] at the more exposed remote end of the
carbon-rich ligand to give the putative quinoidal cumulene complexes
[M{C=C=C6H4=C=C(H)CPh3}(LL)Cp´]+, which were isolated as the water adducts
[M{C≡C-1,4-C6H4-C(=O)CH2CPh3}(LL)Cp´] (6a – d). Evincing the scope of the
formation of such extended cumulenes from ethynyl-substituted arylvinylene
precursors, the rather reactive half-sandwich (5-ethynyl-2-thienyl)vinylidene
complexes [M{C=C(H)-2,5-cC4H2S-C≡CH}(LL)Cp´]BF4 ([7a – d]BF4 add water
readily to give [M{C≡C-2,5-cC4H2S-C(=O)CH3}(LL)Cp´] (8a – d)].
2
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Graphical Abstract
M C C
L
C C H
L
E+
H
C C
E
M C C
L
L
Reactions of 4-ethynyl metal phenyl- or thienyl-acetylide complexes with
electrophiles produce reaction products consistent with the formation of quinoidal and
thienyl cumulenes.
Keywords
Cumulenes; alkynes; metal acetylide; vinylidene
Introduction
The coordination chemistry of vinylidenes (:C=CR2) and allenylidenes (:C=C=CR2),
which are the first members of a family of unsaturated carbene ligands, has been well
developed through many years of persistent investigation. In contrast, very few
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examples of complexes with butatrienylidene (:C=C=C=CR2) and longer cumulated
ligands have been isolated due the highly reactive nature of the extended unsaturated
carbon chain.[1] The existence of these highly conjugated species is more usually
inferred from the nature of reaction products,[2] with a pattern of alternate
electrophilic [C(α), C(γ), C(ε), etc] and nucleophilic [C(β), C(δ), C( ζ), etc] character
of the carbon atoms along the cumulated chain being identified.[3]
More recently, a range of intriguing electronic and electrical properties associated
with cumulated carbon chains have been identified, such as an increasing electronic
transmission with increasing length through even-carbon cumulenes[4] and
fascinating, helical orbital character in odd-carbon C2-symmetric cumulenes.[5] The
identification of similarly helical molecular orbitals in a range of bimetallic C4-
bridged radical cations that would require valence bond descriptions between the
butadiyndiyl (-C≡C-C≡C-) and cumulated butatrienylidiene (=C=C=C=C=) forms
provides further indications of the emerging areas of interest in the chemistry and
electronic structures of cumulated carbon chains.[6]
In light of these various interests in cumulated carbon chains, the isolation of
complexes trans-[Ru{C≡C-2,5-R2-C6H2-4-C(Cl)=CH2}Cl(dppm)2] (C, R = H, Me;
Scheme 1) from reactions of the (4-ethynylphenyl)vinylidene complexes trans-
[Ru{C=C(H)-2,5-R2C6H2-4-C≡CH}Cl(dppm)2]BF4 (A) with [NBu4]Cl points to a
further new line of investigation.[7] The formation of these unusual Markovnikov
addition products is consistent with the isomerisation of the (4-
ethynylphenyl)vinylidene ligand to a transient quinoidal cumulene B, a suggestion
4
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which was supported by DFT based calculations and explorations of possible
mechanistic pathways (Scheme 1).
R
R
PPh2
PPh2
PPh2
Ph2P
Ph2P
Ph2P
Ph2P
Ph2P
Ph2P
H
C
H
H
CH2
Cl
Cl Ru
C
R
C
Cl Ru
C
C
C
C
Cl Ru
C
C
C
PPh2
PPh2
PPh2
R
R
Cl
R
C
H
A
B
C
Scheme 1. The reaction of vinylidene complexes A to give Markovnokov addition
products C via putative quinoidal cumulenes B (R = H, Me).[7]
The literature provides some additional hints at other examples of such
rearrangements and a wider scope of quinoidal and related arylene extended or spaced
cumulene complexes. The Markovnikov product [Mn{C=C(H)-1,4-C6H4-
C(Br)=CH2}(CO)2Cp] was obtained more than 30 years ago from reaction of
[Mn{C=C(H)-1,4-C6H4-C≡CH}(CO)2Cp] with HBr,[8] although no discussion of a
potential mechanism of formation could be entered into at the time of the original
report. Some 20 years later, a cumulated thiophene derivative was specifically
proposed to account for a diamagnetic by-product formed upon oxidation of [Fe(C≡C-
2,5-cC4H2S-C≡CH)(dppe)Cp*] on the basis of mass and Mössbauer spectra,[9]
although the identity of this species remains to be fully confirmed.
In this paper we describe efforts to more conclusively establish and generalise the
formation of metal complexes with extended cumulene ligands. Thus, isolation of a
series of Markovnikov addition products from reactions of halide anions and water
with (4-ethynylphenyl)vinylidene complexes [M{C=C(H)-1,4-C6H4-
C≡CH}(LL)Cp´]BF4 and (5-ethynyl-2-thienyl)vinylidene complexes [M{C=C(H)-
5
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2,5-cC4H2S-C≡CH}(LL)Cp´]BF4 evince the formation of quinoidal cumulenes on a
variety of ruthenium and iron half-sandwich platforms ([M(LL)Cp´ = Ru(PPh3)2Cp,
Ru(dppe)Cp*, Fe(dppe)Cp, Fe(dppe)Cp*]). Selected examples of halide addition and
substitution reactions of trans-[RuCl{C=C(H)-1,4-C6H4-C≡CH}(dppe)2]+ have also
been obtained. Furthermore, reactions of electrophilic trityl cations with similarly
composed (4-ethynylphenyl)acetylide complexes [M(C≡C-1,4-C6H4-C≡CH)(LL)Cp´]
further support the formation of reactive quinoidal cumulenes
[M{C=C=C6H4=C=C(H)CPh3}(LL)Cp´]+, which are isolated as the Markovnikov
water adducts [M{C≡C-1,4-C6H4-C(=O)CH2CPh3}(LL)Cp´].
Results and Discussion
The facile reaction of trans-[Ru{C=C(H)-2,5-R2-C6H2-4-C≡CH}Cl(dppm)2]BF4 (R =
H, Me) with [NBu4]Cl to give trans-[Ru{C≡C-2,5-R2-C6H2-4-
C(Cl)=CH2}Cl(dppm)2]BF4 (Scheme 1)[7] prompted consideration of initial,
exploratory NMR-scale reactions of the analogous dppe-complex trans-[Ru{C=C(H)-
1,4-C6H4-C≡CH}Cl(dppe)2]BF4 with a range of tetrabutylammonium halide salts
[NBu4]X (X– = Cl–, Br–, I–; Scheme 2). The formation of further examples of products
formally derived from Markovnikov addition of HX to the terminal alkyne moiety
was clearly evinced by the characteristic resonances from the methylene protons at
5.42 and 5.72 (X = Cl, JHH = 1.8 Hz), 5.67 and 6.07 (X = Br, JHH = 2.1 Hz), and 6.00
and 6.44 (X = I, JHH = 1.8 Hz) ppm, with further support garnered from mass
spectrometry. However, the product solutions derived from addition of bromide and
iodide were complicated by the observation of competing chloride addition products,
suggesting halide scrambling of the metal-ligated chloride ligand through exchange
with the added halide, giving rise to a mixture of the various metal-halide derivatives
6
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of the various vinyl halide products trans-[Ru{C≡C-1,4-C6H4-C(X)=CH2}Xʹ(dppe)2]
(X, Xʹ = Br/Cl, I/Cl) (Scheme 2). Clearly, the degenerate exchange of the coordinated
and free chloride nucleophile used in our initial study would not be evinced in the
isolated product.
R
R
PPh2
PPh2
PPh2
Ph2P
Ph2P
Ph2P
H
H
H
CH2
X
Cl Ru C C
PPh2
R
C
Cl Ru C C
PPh2
C C
Cl Ru C C
PPh2
Ph2P
C
Ph2P
Ph2P
R
R
D
X
R
- Cl-
+ Cl-
C
H
R
R
PPh2
PPh2
Ph2P
Ph2P
Ph2P
CH2
X
CH2
X
X
X Ru C C
PPh2
C
Ru C C
PPh2
C
Ph2P
R
R
Scheme 2 Markovnikov addition of X– (X– = Cl–, Br–, I–) to the transient cumulene
species trans-[RuCl(C=C=C6H4R2=C=CH2)(dppe)2]+ (D) and Ru–X scrambling
To simplify the reaction profile by eliminating this side reaction, attention was
subsequently turned to the half-sandwich derivative
[Ru{C=C(H)C6H4C≡CH}(PPh3)2Cp]BF4 ([1a]BF4),[10] prepared here from
[RuCl(PPh3)2Cp] and an excess of 1,4-diethynylbenzene in the presence of NH4BF4
(Scheme 3) . The vinylidene cation, [1a]+, was crystallographically characterised as
the PF6 salt (Figure S1). Reaction of [1a]BF4 with [NEt4]Cl proceeded slowly (48
hours) and gave a mixture of the chlorovinyl complex
[Ru{C≡CC6H4C(Cl)=CH2}(PPh3)2Cp] (2a-Cl), together with the parent acetylide
[Ru(C≡CC6H4C≡CH}(PPh3)2Cp]. Separation of the two acetylide products proved
difficult, and so the reaction was simply performed whilst introducing a slow flow of
dry HCl for the duration of the reaction, resulting in the quantitative formation of 2a-
Cl. The bromide analogue 2a-Br was prepared in a similar manner from [NBu4]Br /
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HBr, albeit in lower isolated yield (40%), as might be expected given the decreased
nucleophilicity of bromide vs chloride in aprotic solvents (Scheme 3).
H
H
H
X
CH2
X
Ru C C
Ru C C
PPh3
C C
Ru C C
PPh3
C
Ph3P
Ph3P
Ph3P
PPh3
X = Cl (2a-Cl); Br (2b-Br)
C
C
[1a]+
H
Scheme 3. The preparation of [2a-X] (X = Cl, Br) from [1a]+ via a proposed
quinoidal cumulene intermediate.
The structures of 2a-Cl and 2a-Br were determined by single crystal X-ray diffraction
(Figure 1, Table 1). Structural parameters of the terminal C(X)=CH2 moieties are
consistent with those of similar vinyl-halide analogues, such as trans-[Ru{C≡C-2,5-
R2-C6H2-4-C(Cl)=CH2}Cl(dppm)2] (R = H, Me)[7] and [Mn{C=C(H)-1,4-C6H4-
C(Br)=CH2}(CO)2Cp].[8] The pattern of alternating long / short / long bond lengths
along the Ru-C1-C2-C3 chain is typical of ruthenium alkynyl complexes,[11] but
rather less pronounced in the case of 2a-Cl, which may be indicative of a small
contribution from a more cumulated resonance structure stabilised by the
electronegative Cl atom.
8
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Table 1. Selected crystallographically determined bond lengths (Å) and angles (°) for
2a-Cl and 2a-Br.
2a-Cl
2a-Br
Ru1-P1
Ru1-P2
Ru1-C1
C1-C2
C2-C3
C6-C9
C9-C10
C9-X
2.2916(5)
2.2881(5)
2.005(2)
1.216(3)
1.433(3)
1.480(3)
1.336(3)
1.751(2)
2.2898(15)
2.2852(15)
2.030(7)
1.173(9)
1.446(9)
1.480(10)
1.328(9)
1.847(7)
P1-Ru1-P2
Ru1-C1-C2
C1-C2-C3
C6-C9-C10
C10-C9-X1
99.430(17)
176.81(16)
171.5(2)
99.26(5)
175.0(5)
169.5(6)
127.6(6)
114.8(6)
126.86(19)
117.25(17)
9
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Figure 1. ORTEP (50% probability) representations of the molecules: (a) [Ru{C≡C-
1,4-C6H4-C(Cl)=CH2}(PPh3)2Cp] (2a-Cl); (b) [Ru{C≡C-1,4-C6H4-
C(Br)=CH2}(PPh3)2Cp] (2a-Br) showing the atom labeling schemes. For clarity, only
the H-atoms on C(10) are shown.
The isolation of 2a-Cl and 2a-Br, together with the spectroscopic observation of
vinylhalide products formed from the formal Markovnikov addition of HX to the
pendent ethynyl moiety in trans-[Ru{C=C(H)-1,4-C6H4-C≡CH}Cl(dppe)2], gave
10
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10.1002/chem.201905399
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confidence that the rearrangement of the ethynyl-substituted phenylvinylidene
complexes to a quinoidal cumulene ligand reported earlier[7] could be extended to a
broader range of metal-ligand fragments and nucleophiles. To advance these studies,
further members of the family of half-sandwich ethynyl vinylidene complexes
[M{C=C(H)-1,4-C6H4-C≡CH}(LL)Cp]BF4 [M(LL)Cp´ = Ru(dppe)Cp* ([1b]BF4);
Fe(dppe)Cp ([1c]BF4); Fe(dppe)Cp* ([1d]BF4)] were prepared from the respective
half-sandwich chloride complexes [MCl(LL)Cpʹ] and isolated as the [BF4]- salts in ca.
60% yield in each case. The vinylidene salt [1b]BF4 was crystallographically
characterised by way of further example (Figure S2).
R5
R5
R5
H
H2O
-H+
H
H
O
M
C C
M
C
C
C C
M
C
C
C
CH3
L
L
L
L
L
L
C
C
H
M(LL)Cp'
M(LL)Cp'
[1a]+
[1b]+
[1c]+
[1d]+
Ru(PPh3)2Cp
Ru(dppe)Cp*
Fe(dppe)Cp
Fe(dppe)Cp*
3a
3b
3c
3d
Ru(PPh3)2Cp
Ru(dppe)Cp*
Fe(dppe)Cp
Fe(dppe)Cp*
Scheme 4. The preparation of acyl complexes 3a – d from water addition to [1a – d]+
via putative quinoidal cumulene complexes.
Treatment of each of [1a – d]BF4 with a moist solution of HBF4•Et2O in CH2Cl2
resulted in the formation of the acetophenone derivatives [M{C≡C-1,4-
C6H4C(=O)Me}(LL)Cp´] (3a – d) over the course of ca. 48 hours, which were
isolated following chromatographic purification and crystallisation (Scheme 4). The
complexes were readily identified from the characteristic IR ν(C≡C) and ν(C=O)
11
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10.1002/chem.201905399
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bands between ca. 2040 – 2060 and 1664 – 1669 cm-1, respectively. The acyl moiety
was further evinced by a low field 13C{1H} NMR resonance in the range 190 – 200
ppm each case and a singlet in the 1H NMR spectrum near 2.5 ppm. The structures of
3b – 3d were also determined by single-crystal X-day diffraction analysis (Figure 2,
Figure S3, Figure S4), and metric data are summarised in Table 2, although the
relatively low precision of the structure determination of 3c precludes extensive
comparison with the other structures.
Figure 2. ORTEP (50% probability) of a molecule of [Ru{C≡C-1,4-C6H4-
C(=O)Me}(dppe)Cp*] (3b) showing the atom labeling scheme. H-atoms have been
omitted for clarity.
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Table 2. Selected crystallographically determined bond lengths (Å) and angles (°) for
3b, 3c and 3d
3b (M = Ru)
2.2501(7)
2.2648(7)
2.005(3)
1.233(4)
1.425(4)
1.486(4)
1.504(5)
1.223(4)
3c (M = Fe)
2.177(2)
3d (M = Fe)
M1-P1
M1-P2
M1-C1
C1-C2
C2-C3
C6-C9
C9-C10
C9-O1
2.1667(16)
2.1758(15)
1.903(5)
1.210(7)
1.438(7)
1.484(7)
1.498(8)
1.227(6)
2.163(2)
1.884(8)
1.229(11)
1.424(10)
1.474(10)
1.490(11)
1.228(8)
P1-M1-P2
M1-C1-C2
C1-C2-C3
C6-C9-C10
C10-C9-O1
82.55(3)
174.7(2)
171.9(3)
119.2(3)
120.7(3)
86.71(9)
175.4(7)
170.4(8)
119.2(6)
120.1(7)
85.28(6)
175.1(4)
171.0(5)
118.7(5)
120.5(5)
The M-C1 bond lengths of 2.005(3), 1.884(8) and 1.903(5) Å for 3b, 3c and 3d are
consistent with reported bond lengths for analogous Group 8 ‘piano-stool’ acetylide
complexes [e.g. Ru(PPh3)2Cp;[12] Ru(dppe)Cp*[11] Fe(dppe)Cp;[13] Fe(dppe)Cp*[11, 14]
with the shorter M-C and M-P bond lengths exhibited by the iron complexes 3c and
3d a result of the smaller atomic radius of iron. As with other metal acetylide
complexes, the M-C1-C2 fragment is essentially linear [174.7(2)° (3b); 175.4(7)°
]
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(3c); 175.1(4)° (3d)] and the C1-C2 bond lengths fall in a narrow range [C1-C2:
1.233(4) Å (3b); 1.229(11) Å (3c); 1.210(7) Å (3d)] due to the limited role played by
metal-acetylide back-bonding in the electronic structures of such complexes.[15] In
each case, the acyl fragment is characterised by typical C=O double bond [C9-O1:
1.223(4) Å (3b); 1.228(8) Å (3c); 1.227(6) Å (3d)] and C(9)-C(10) single bond [C9-
C10: 1.504(5) Å (3b); 1.490(11) Å (3c); 1.498(8) Å (3d)] lengths, similar to those
found in acetophenone.[16]
At present, the exact mechanistic role of acid in these reactions is unclear. Re and
colleagues have examined mechanisms of proton migration from ethynylvinylidene to
butatrientylidene supported by a {Ru(PMe3)2Cp}+ fragment, and proposed that the
isomerization proceeds via a deprotonation / re-protonation mechanism.[17] In our
case, it seems that the additional acid is important in promoting protonation at the
terminal carbon to give the quinoidal cumulene, albeit as a transient intermediate that
we have not yet succeeded in detecting spectroscopically in solution (Scheme 5). The
formation of the putative metal-stabilised cumulene is apparently key to the facile
addition of water. Reaction of PhC≡CH with HBF4.OEt2 under similar conditions did
not result in the formation of any PhC(O)Me, presumably reflecting that the
carbocation formed which would be formed under these conditions is far less stable
than the readily accessible ruthenium-containing quinoidal cumulene.
14
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H+
R5
R5
R5
H
H
M
C
C
C
CH
H+
M
C
C
C C
M
L
C C
H
L
L
L
L
L
C
C
H
Scheme 5. Protonation equilibria that would allow exchange of ethynyl-substituted
phenylvinylidene and quinoidal cumulene ligands on a half-sandwich metal complex
coordination sphere.
To gain some further insight and evidence for this concept of a deprotonation / remote
re-protonation conceptually illustrated in Scheme 5, we turned attention to the model
reactions of the metal acetylide complexes [M(C≡C-1,4-C6H4-C≡CH)(LL)Cpʹ] (4a –
d) with electrophiles. The prototypical ethynyl-substituted phenyl acetylide complex
[Ru(C≡C-1,4-C6H4-C≡CH)(PPh3)2Cp] (4a)[10] was allowed to react with electrophiles
of increasing steric demand, specifically 1-cyano-4-dimethylaminopyridinium
tetrafluoroborate ([CAP]BF4) as a convenient source of cyanogen, [CN]+,[18]
tropylium tetrafluoroborate ([C7H7]BF4) [19] and trityl tetrafluoroborate
([CPh3]BF4).[20] Whilst both [CAP]BF4 and [C7H7]BF4 afforded the anticipated
vinylidene complexes [Ru{C=C(CN)-1,4-C6H4- C≡CH}(PPh3)2Cp]BF4 ([5a-CN]BF4)
and [Ru{C=C(C7H7)-1,4-C6H4-C≡CH}(PPh3)2Cp]BF4 ([5a-C7H7]BF4), the reaction
of 4a with the very bulky trityl electrophile gave Ru{C≡C-1,4-C6H4-
C(=O)CH2CPh3}(PPh3)2Cp (6a) after workup (Scheme 6).
15
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E+
M(LL)Cp'
R5
[CN]+ (5a-CN)
[C7H7]+ (5a-C7H7)
E
Ru(PPh3)2Cp
M C C
L
L
[CN]+ (5b-CN)
[C7H7]+ (5b-C7H7)
Ru(dppe)Cp*
C
C
H
M(LL)Cp'
Ru(PPh3)2Cp
E+
R5
4a
4b
4c
4d
M C C
L
C CH
[CPh3]+
Ru(dppe)Cp*
Fe(dppe)Cp
Fe(dppe)Cp*
L
R5
H
M C C
L
C C
CPh3
L
H2O / -H+
M(LL)Cp'
R5
Ru(PPh3)2Cp
Ru(dppe)Cp*
Fe(dppe)Cp
Fe(dppe)Cp*
6a
6b
6c
6d
O
C
H2C CPh3
M C C
L
L
Scheme 6. The reactions of 4a – d with electrophiles (E+ = [CN]+, [C7H7]+, [CPh3]+).
The vinylidene complexes [5a-CN]BF4 and [5a-C7H7]BF4 were characterised by the
usual suite of spectroscopic methods, details of which are given in the Experimental
section. The complex 6a was characterised by the metal acetylide ν(C≡C) band at
16
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10.1002/chem.201905399
Chemistry - A European Journal
2058 cm-1 whilst the carbonyl fragment, presumably derived from water addition to
the putative cumulene (Scheme 6) exhibited a ν(C=O) band at 1673 cm-1 and a
distinctive low field resonance in the 13C{1H} NMR spectrum (δC 196.0 ppm). The
methylene moiety was evinced by 1H and 13C{1H} NMR resonances at δH 2.54 ppm
and δC 28.5 ppm, respectively, whilst the addition of the CPh3 fragment was clear
from the observation of the [M+H]+ ion in the ESI(+)-MS (1077.3193 amu; calculated
1076.2850 amu).
Similar reactions of [CAP]BF4 and [C7H7]BF4 with [Ru(C≡C-1,4-C6H4-
C≡CH)(dppe)Cp*] (4b) gave the analogous vinylidene complexes [Ru{C=C(CN)-1,4-
C6H4-C≡CH}(dppe)Cp*]BF4 ([5b-CN]BF4) and [Ru{C=C(C7H7)-1,4-C6H4-
C≡CH}(dppe)Cp*]BF4 ([5b-C7H7]BF4), the additional steric bulk at the
{Ru(dppe)Cp*} fragment apparently not being sufficient to prevent addition of the
electrophile to C(β). Again, the very bulky trityl cation was found to add at the
terminal carbon, presumably giving a cumulated intermediate, which reacted with
adventitious water resulting in formation and isolation of [Ru{C≡C-1,4-C6H4-
C(=O)CH2CPh3}(dppe)Cp*] (6b). Although 6a and 6b proved resistant to
crystallisation in a manner conducive to single-crystal X-ray analysis, the iron
compounds [Fe{C≡C-1,4-C6H4-C(=O)CH2CPh3}(dppe)Cp´] [Cp´ = Cp (6c), Cp*
(6d)], formed from entirely analogous reactions of [Fe(C≡C-1,4-C6H4-
C≡CH)(dppe)Cp´] [Cp´ = Cp (4c), Cp* (4d)][21] with [CPh3]BF4 (Scheme 6), both
proved somewhat more amenable to crystallisation. Although the crystal samples
were of poor quality, resulting in a high degree of thermal and static disorder in the
resulting structures, the crystallographic work allowed the unambiguous
17
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determination of the product structures (Figure 3, Table 3). The C1-C2 bond lengths
of 1.14(2) Å (6c) / 1.227(7) Å (6d) Å are consistent with the formal triple bond
character of these fragments, the C10-C11 bond lengths of 1.58(2) Å (6c) / 1.514(8) Å
(6d) Å correspond to sp3-sp3 C-C single bonds, whilst the C9-O1 bond lengths of
1.27(2) Å (6c) / 1.266(7) Å (6d) are consistent with that of a carbonyl group.
Together, these data support the formation of new C-C bonds to the trityl fragment
and the new C-O bonds of the carbonyl groups.
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Figure 3. ORTEP plots of molecules of (a) [Fe{C≡C-1,4-C6H4-
C(=O)CH2CPh3}(dppe)Cp] (6c) (20% probability; only one of three molecules in the
asymmetric unit shown) and (b) [Fe{C≡C-1,4-C6H4-C(=O)CH2CPh3}(dppe)Cp*] (6d)
(20% probability) and showing the atom labeling schemes. H-atoms and solvent of
crystallisation omitted for clarity.
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Table 3. Selected bind lengths (Å) and angles (°) from the crystallographically
determined structures of 6c and 6d.
6c
6d
Fe1-P1
Fe1-P2
Fe1-C1
C1-C2
C2-C3
C6-C9
C9-C10
C9-O1
C10-C11
2.165(6)
2.1941(15)
2.1824(15)
1.888(6)
1.227(7)
1.414(7)
1.461(10)
1.42(9)
2.164(7)
1.88(2)
1.14(2)
1.54(3)
1.47(2)
1.48(2)
1.27(2)
1.58(2)
1.266(7)
1.514(8)
P1-Fe1-P2
Fe1-C1-C2
C1-C2-C3
C6-C9-C10
C10-C9-O1
C9-C10-C11
86.1(2)
85.93(6)
176.9(4)
172.9(5)
123.2(8)
114.5(8)
122.4(9)
171(2)
157(3)
122.8(18)
118.5(19)
118.6(16)
The weight of evidence for quinoidal cumulene complexes derived from half-
sandwich iron and ruthenium complexes of 1,4-diethynyl benzene prompted further
consideration of the potential generality of such rearrangements from other
diethynylaryl ligand precursors. Given the lower aromaticity of thiophene relative to
20
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benzene, attention was briefly turned towards the search for complexes evincing the
formation of a putative thiophenylene-spaced cumulene ligand.
Although the terminal dialkyne 2,5-diethynyl thiophene is relatively unstable as a neat
compound, in situ deprotection of a dilute solution of 2,5-
(trimethylsilylethynyl)thiophene provides a convenient entry route to metal
complexes derived from this species.[22] The reaction of excess 2,5-
diethynylthiophene with [MCl(LL)Cpʹ] [M(LL)Cp´ = Ru(PPh3)2Cp, Ru(dppe)Cp*,
Fe(dppe)Cp, Fe(dppe)Cp*] in methanol solution containing NH4BF4 at ambient
temperature allowed isolation of the vinylidene complexes [M{C=C(H)-2,5-cC4H2S-
C≡CH}(LL)Cp´]BF4 ([7a – d]BF4) as impure powders in low to moderate yield by
precipitation into Et2O (Scheme 7). The presence of the vinylidene in the crude
reaction product was evinced by observation of an ion corresponding to the cation
[M{C=C(H)-2,5-cC4H2S-C≡CH}(LL)Cp´]+ in the ESI(+)-MS, vinylidene ν(M=C=C)
bands in the IR spectra ([7a]+, 1624; [7b]+ 1621; [7c]+, 1625; [7d]+, 1609 cm-1) and
characteristic resonances in the 1H NMR spectra arising from the vinylidene (δH:
[7a]+, 5.44; [7b]+, 4.64; [7c]+, 5.17; [7d]+, 5.35 ppm) and ethynyl (δH: [7a]+, 3.45;
[7b]+, 3.27; [7c]+, 3.29; [7d]+, 3.43 ppm) protons. Whilst samples of [7b – d]BF4
were estimated to be 90 – 95% pure from analysis of the 31P{1H} NMR spectra,
[7a]BF4 proved to be rather reactive, with evidence of the carbonyl cation
[Ru(CO)(PPh3)2Cp]+ (ν(CO) 1980 cm-1) amongst other unidentified products within
the sample. Efforts to purify these samples were rather fraught, and so efforts were
made to establish the ligand framework and garner evidence for a similar cumulene
rearrangement process through derivative reactions of the crude samples of [7a –
d]BF4.
21
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R5
M Cl
L
+
C
C
C
S
C
H
H
L
M(LL)Cp'
R5
Ru(PPh3)2Cp
Ru(dppe)Cp*
Fe(dppe)Cp
Fe(dppe)Cp*
[7a]+
[7b]+
[7c]+
[7d]+
H
M C C
L
S
L
C
C
H
R5
M C C
S
C
L
CH2
L
H2O
M(LL)Cp'
R5
Ru(PPh3)2Cp
Ru(dppe)Cp*
Fe(dppe)Cp
Fe(dppe)Cp*
8a
8b
8c
8d
M C C
L
O
S
C
CH3
L
Scheme 7. The formation thiophenylethynyl vinylidene complexes [7a – d]+ and
reactions with water to give 8a – d.
The samples containing [7a – d]BF4 were allowed to react with moist HBF4.Et2O in
CH2Cl2 causing an immediate change in colour of the solution to an intense purple in
each case, which further evolved to green over the course of ca. 1hr. The difficulties
22
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encountered in obtaining pure samples of the thiophene-derived compounds [7a – d]+
and the relatively rapid hydration reactions to give 8a – d are consistent with the
proposal that the decreased aromaticity of thiophene permits facile rearrangement of
the ethynyl vinylidene to the extended cumulenes. Chromatographic workup on basic
alumina afforded the acyl acetylide complexes 8a – d, consistent with the facile
formation and reaction of the thienylcumulene intermediates (Scheme 7). These
compounds were characterised through the usual spectroscopic and spectrometric
methods, which included both ν(C≡C) and ν(CO) bands in the IR spectra, low field
resonances in the 13C{1H} NMR spectra associated with the carbonyl carbons, and
observation of the molecular ion, [M]+, or [M+H]+ in the ESI(+)-MS. The structures
of 8b and 8c were determined by single crystal X-ray diffraction analysis, confirming
the identity of the products (Figure 4, Table 4).
23
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Figure 4. ORTEP representations (30% probability) of molecules of (a) [Ru{C≡C-
1,4-cC4H2S-C(=O)CH3}(dppe)Cp*] (8b) and (b) [Fe{C≡C-1,4-cC4H2S-
C(=O)CH3}(dppe)Cp] (8c) showing the atom labelling scheme. H-atoms have been
omitted for clarity.
24
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Table 4. Selected bond lengths (Å) and angles (°) from the crystallographically
determined structures of 8b and 8c.
8b (M = Ru)
8c (M = Fe)
2.1618(8)
2.1765(8)
1.890(3)
1.217(4)
1.413(4)
1.465(4)
1.501(5)
1.225(4)
M1-P1
M1-P2
M1-C1
C1-C2
C2-C3
C6-C7
C7-C8
C7-O1
2.2685(13)
2.2793(12)
1.973(5)
1.233(6)
1.407(7)
1.464(7)
1.496(7)
1.216(6)
P1-M1-P2
M1-C1-C2
C1-C2-C3
C6-C7-C8
C8-C7-O1
83.04(5)
171.2(4)
173.4(5)
117.6(5)
121.8(5)
86.72(3)
176.2(3)
171.7(3)
117.6(3)
121.2(3)
In addition to establishing the connectivity, the structural parameters of the terminal
methyl ketone group are consistent with those reported for the analogous
acetophenone derivatives 2b – d, as well as closely related organic compounds, such
as 2-bromo-5-acetylthiophene.[23]
25
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From a mechanistic perspective the addition of water to the putative quinoidal
cumulene provides further insight into the ruthenium-mediated hydration of
alkynes.[24] In the case of simple alkynes, there are two principle outcomes from such
a reaction, either Markovnikov addition to give a methyl ketone (thought to proceed
through attack of water to an η2-alkyne intermediate) or anti-Markovnikov addition
via a vinylidene complex which affords an aldehyde.[25] The regiochemical outcome
of the reaction is, therefore, dictated by the balance between the alkyne and
vinylidene tautomers of the coordinated alkyne, which is in turn controlled by the
choice of ligands within the coordination sphere of the metal.[26] For extended
cumulenes, the situation has an additional degree of complexity as the carbon atoms
of the ligand shown alternating electrophilic-nucleophilic behavior and in complexes
[M]=(C)n=CR2, nucleophilic addition to all the odd-numbered carbon atoms is
possible.[27] For example, in allenylidene ligands (n = 2) nucleophilic attack may
occur at Cα or Cγ, but only addition to the latter is productive. Therefore, the site(s) of
hydration (or more generally nucleophilic addition) depends on the relative
electrophilic character of the individual carbon atoms within the cumulene ligand and
the steric influence of the metal.
The selectivity of the nucleophilic addition reactions reported in this work is driven
by two factors. Firstly, our previous calculations on the addition of chloride to the
quinoindal cumulene ligand indicated that addition to the metal-bound carbon atom
was kinetically accessible but the resulting product was far less thermodynamically
stable than that arising from attack at the remote site, presumably due to the increased
steric hinderance on changing from a linear to bent ligand at the metal.[7] Secondly,
the presence of the spacer groups inhibits addition to the central unit of the extended
26
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carbon chain as regaining the aromaticity of the phenyl- or thienyl-substituents
following addition of halide anion or water is a key driving force for the reaction.
This work demonstrates that the formal hydration of alkynes may take place at centers
which are remote from the metal and, through appropriate choice of the metal and
ligand, may occur in a highly regioselective manner.
Conclusions
The reactions of [M{C=C(H)-1,4-C6H4-C≡CH}(LL)Cp ]BF4 [M(LL)Cp´ =
Ru(PPh3)2Cp ([1a]BF4); Ru(dppe)Cp* ([1b]BF4); Fe(dppe)Cp ([1c]BF4);
Fe(dppe)Cp* ([1d]BF4)] with halides and water afford the Markovnikov addition
products [Ru{C≡C-1,4-C6H4-C(X)=CH2}(PPh3)2Cp] [X = Cl (2a-Cl), Br (2a-Br)] and
[M{C≡C-1,4-C6H4-C(=O)Me}(LL)Cp´] (3a – d). These products are consistent with
reactions of the nucleophilic reagents with the putative quinoidal cumulene
complexes [M{C=C=C6H4C=CH2}(LL)Cp´]+. Water addition to [M{C=C(H)-2,5-
cC4H2S-C≡CH}(LL)Cp´]BF4 ([7a – d]BF4) proved even more facile, consistent with
the lower aromatic stabilisation energy of the thiophene ring. Further evidence for the
extended cumulene was found in reactions of [M(C≡C-1,4-C6H4-C≡CH)(LL)Cpʹ] (4a
– d) with [CPh3]+, which afford [M{C≡C-1,4-C6H4-C(=O)CH2CPh3}(LL)Cpʹ] (6a –
d), presumably via water addition to the cumulated complexes
[M{C=C=C6H4C=C(H)CPh3}(LL)Cpʹ]+. These observations hint at a wealth of
extended cumulene chemistry to be explored from re-arrangements of ethynyl
vinylidene complexes, and new routes for the activation of unsaturated organic
molecules within metal coordination spheres. Work to further explore these areas is
currently underway in our laboratories.
27
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Experimental
Full descriptions of synthetic procedures and characterization data are given in the
Supporting Information accompanying this article. Crystallographic data for the
structures reported in this paper can also be found in the Supporting Information of
this paper and have been deposited at the Cambridge Crystallographic Data Centre.
Copies of the [1a]PF6, [1b]BF4, 2a-Cl, 2a-Br, 3b, 3c, 3d, 6c, 6d, 8b, 8c
crystallographic data with CCDC numbers 1965102-1965112 can be obtained free of
charge via https://www.ccdc.cam.ac.uk/structures/, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.KCB21EZ,
UK (fax +441223336033; email deposit@ccdc.cam.ac.uk).
Acknowledgements We gratefully acknowledge support from the Forrest Research
Foundation and award of a Forrest Fellowship to M.K. M.R.H. holds a Dean’s
Excellence in Science PhD Scholarship from the Faculty of Science, University of
Western Australia. The authors gratefully acknowledge the facilities, and the
scientific and technical assistance of Microscopy Australian at the Centre for
Microscopy, Characterisation & Analysis, The University of Western Australia, a
facility funded by the University, State and Commonwealth Governments. This work
was made possible through a Research Collaboration Award from the University of
Western Australia.
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