Dual nucleophilic substitution at a W(II) η2-coordinated dIIodo acetylene leading to an amidinium carbyne complex

Article abstract of DOI:10.1039/c5cc10397e

The synthesis and reactivity of a W(ii) C₂I₂ complex towards various nucleophiles are described. Soft, aprotic nucleophiles like 4-dimethylaminopyridine (DMAP) lead to substitution of one CO at tungsten, whereas reaction with an excess of benzylamine results in a dual nucleophilic substitution at the alkyne moiety involving the rearrangement to a novel cationic amidinium carbyne complex.

Full text of DOI:10.1039/c5cc10397e

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Dual nucleophilic substitution at a W(II)
2
g -coordinated diiodo acetylene leading
to an amidinium carbyne complex†
Cite this: Chem. Commun., 2016,
2, 2616
5
Received 18th December 2015,
Accepted 30th December 2015
Kai Helmdach, Julia Ruger, Alexander Villinger and Wolfram W. Seidel*
¨
DOI: 10.1039/c5cc10397e
www.rsc.org/chemcomm
2 2
The synthesis and reactivity of a W(II) C I complex towards various
nucleophiles are described. Soft, aprotic nucleophiles like 4-dimethyl-
aminopyridine (DMAP) lead to substitution of one CO at tungsten,
whereas reaction with an excess of benzylamine results in a dual
nucleophilic substitution at the alkyne moiety involving the rear-
rangement to a novel cationic amidinium carbyne complex.
2 2
With the exception of C F , dihalogen acetylenes are known for
Scheme 1 Synthesis of complexes 1-PF
ii) 2-PF : CH CN; 3-PF : DMAP, CH Cl ; (iii) PhCH
6
to 4-PF
6
; (i) [Fc][PF
6
], CH
2
Cl .
2
Cl
2
;
more than a century. Despite their apparent preparative potential
the coordination chemistry of these alkynes is only sporadically
explored, which might be attributed in parts to the explosive
(
6
3
6
2
2
2
NH (xs.), CH
2
2
2ꢀ
2ꢀ
nature of free dihalogen acetylenes. Structurally characterized assembly of polynuclear complexes with bridging C
2
S
2
(acdt
)
mononuclear examples are restricted to W(IV) C
2
X
2
complexes moieties, we recently expanded this chemistry to mixed S,N donor
complex
)] presented by S u¨ nkel. Both which is prone to nucleophilic substitution by N-donors opening
systems are not suitable for nucleophilic substitution in the the access to a novel amidinium carbyne complex (Scheme 1).
ꢀ
with [WY
Dehnicke and [(PPh ) Pt(C Cl
3 2 2 2
5
]
2 2
moieties (X = Cl, Br, I; Y = Cl, Br) published by alkyne complexes. Herein, we present a side-on W(II) C I
1
2
7
2
complex due to competing reactions. In particular, late transition
Adapting a general method by Templeton the Z -C I complex
2 2
0
metals tend to react with dihalogen acetylenes by undergoing 1-PF is prepared by oxidation of the W(I) compound Tp W(CO)
6
3
2
0
oxidative addition. However, substitution of one bromide was {Tp = hydridotris(3,5-dimethylpyrazolyl)borate} with ferrocenium
3
successfully realized by Filippou and coworkers at a mononuclear salt in the presence of C
2 2
I . The reaction is monitored by in situ
4
W(II) bromo alkyne complex and by Okazaki and coworkers at a IR spectroscopy showing a distinctive pair of absorption bands
ꢀ
1
tetranuclear Fe(I) alkyne cluster. In order to open a general access (CH
2 2
Cl 2074, 2006 cm ) in the CO region. In solution this
to complexes with a variety of heteroatom substituted alkynes, we compound decomposes markedly within several hours even
4
started investigations with C
2 2
I and d -centres like W(II) and Re(III) when treated under strict anaerobic conditions. However, further
in sterically encumbered coordination environments. Most evidence is provided by ESI mass spectrometry in CH CN showing
3
+
+
recently our group succeeded in isolating a bis(alkyne) rhenium the complex cation 1 (814.95 m/z) and the acetonitrile derivative 2
complex with C I . In fact, we seek for suitable starting materials (827.98 m/z). Accordingly, facile stoichiometric substitution with
5
2
2
for substitution reactions in the metal complex, because our CH
3
CN or 4-dimethylaminopyridine (DMAP) led to 2-PF
6
and
work is focused on polynuclear complexes bearing heterodonor 3-PF
6
, respectively, both of which are more stable compared
6
8
6
substituted alkynes as bridging ligands. Based on the successful with 1-PF . Both compounds were characterized by IR, MS, EA,
1
13
H and C NMR spectroscopy. The C
1
symmetry of these
complexes is reflected in typical NMR signal patterns for the
Tp ligand as found for related compounds. The C resonances
of the alkyne carbon atoms, which are detected at 165 to 175 ppm
Institut f u¨ r Chemie, Universit a¨ t Rostock, Albert-Einstein-Straße 3a, 18059 Rostock,
Germany. E-mail: wolfram.seidel@uni-rostock.de
0
9,10
13
†
Electronic supplementary information (ESI) available: Synthetic procedures and
characterization data of all new compounds. CCDC 1428746 and 1428747. DFT
calculation details including enthalpies, bond dissociation energies, Cartesian
coordinates of computationally studied complexes and transition state represen-
tation. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5cc10397e
(
2 2
cf. free C I : 0.53 ppm), are distinctive features of these complexes
11
pointing to the 4e donor character of the alkyne. The molecular
structure of the complex cation 3 determined by single crystal
X-ray diffraction is depicted in Fig. 1; essential metric parameters
+
2
616 | Chem. Commun., 2016, 52, 2616--2619
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6
Fig. 2 Molecular structure of one of the two independent cations of 4a-PF .
Thermal ellipsoids are drawn at 50% probability. Selected bond lengths [Å] and
angles [1]: W1–C1 1.802(9), W1–C17 1.987(12), W1–C18 2.000(11),
W1–N3 2.183(8), W1–N5 2.207(8), W1–N7 2.276(7), C1–C2 1.453(12),
C2–N1 1.349(12), C2–N2 1.318(11), W1–C1–C2 168.6(8), N1–C2–N2
6
Fig. 1 Molecular structure of the cation of 3-PF . Thermal ellipsoids
are drawn at 50% probability. Selected bond lengths [Å] and angles [1]:
W1–C1 2.015(10), W1–C2 2.041(10), W1–C10 1.962(12), W1–N1 2.181(8),
W1–N2 2.234(9), W1–N4 2.194(8), W1–N6 2.159(8), C1–C2 1.276(14),
C1–I1 2.056(11), C2–I2 2.083(10), C1–C2–I2 138.9(8), C2–C1–I1 138.8(8).
1
20.3(9).
13
bond character. Accordingly, the C NMR spectrum exhibits one
are given in the figure caption. The alkyne is bound in a standard set of signals for the benzyl groups (CH 47.4 ppm, C 135.4 ppm,
2
ipso
1
3
side-on fashion thereby aligning with the W–CO axis. This typical Cm,o,p 126.9–129.3 ppm). The essential C NMR resonances of
0
arrangement of the [Tp W(CO)(L)(C
2
R
2
)] moiety is attributed to p the C
2
unit are detected in the low field beyond 150 ppm. The
bonding optimization between filled metal d and divergent ligand resonance at 159.9 ppm is assigned to the central amidinium
10
p* orbitals. The alkyne C1–C2 bond is extended to 1.276(14) Å (cf. carbon C2, which was further verified by a cross peak in the
3
C
1
2
I
2
: 1.19 Å) and the iodine atoms are being bent back to about gHMBC NMR spectrum indicating a JC,H coupling between the
401. Resulting from the respective hybridization change, the C–I benzyl methylene protons and C2. The chemical shift is in
bond lengths C1–I1 2.056(11) Å and C2–I2 2.083(10) Å also differ agreement with those reported for amidines. The resonance
12
significantly from the free alkyne (cf. 1.99 Å) being rather furthest downfield at 248.1 ppm is caused by the metal bound
1
3
13
comparable to the bond lengths found in C I (cf. 2.11 Å).
C1 atom. Since the C NMR shifts of the carbyne Ca atom
2
4
Nucleophilic substitution of the iodide is achieved by choosing a depend rigorously on the substituent R in MRC–R moieties
protic nucleophilic agent like benzylamine. Within minutes the ranging from 220 ppm (R = OAr) to 556 ppm (R = Li), the
moss green solution changes to a brownish suspension. Completion electronic influence of the amidinium group is related to amine
0
of the reaction can be monitored by in situ IR spectroscopy in functionalized carbynes (cf. 248.6 ppm for Tp W(CO)
2
RCNEt
2
)
ꢀ1
16
CH
CN) and a single absorption at 3358 cm (nNH), which is [Tp W(CO)
indicative of secondary amine groups. After work-up, a green introduced by Templeton. The latter examples do not only share
2
Cl
2
, most notably by the appearance of a band at 1605 cm
presented by Filippou and most notably to phosphonium carbynes
ꢀ1
0
+
0
(n
2 3 2 3 6
RCPR ] (cf. 242.1 ppm for [Tp W(CO) RCPPh ][PF ])
17
1
3
powder of 4a-PF is obtained. Recrystallization from CH Cl /pentane similar C shifts compared to 4a-PF , but also similar CO vibration
6
2
2
a
6
ꢀ
1
0
led to single crystals, which were suitable for X-ray diffraction. bands (nCO = 2026, 1940 cm for [Tp W(CO)
2
RCPPh
3 6
][PF ]; nCO =
ꢀ1
Complex 4a-PF crystallizes in P2 /n with two crystallographically 2020, 1936 cm for 4a-PF ) indicating consequently a similar
6
1
6
independent complex cations, which show very similar bonding electronic situation at W. In fact, it is illustrative to describe both
parameters (Fig. 2). Surprisingly, the structure does not show complex types as carbonium complexes, which are donor stabilized
the expected side-on coordinated alkyne but a unique amidinium by either an N-heteroacyclic carbene or a phosphine (Scheme 2). The
+
carbyne complex instead. The W–C1 bond length is very short strong impact of positive charge on W in 4a is evident from the high
1
4
0
.802(9) Å and comparable to other WC triple bond lengths. The CO valence frequencies as compared with other [Tp W(CO) CR]
2
1
ꢀ
1
18
C1–C2 bond is rather long 1.453(12) Å showing a restricted p bond carbene complexes (cf. nCO = 1992, 1903 cm for R = H). An
character. In addition, the nearly linear bond angle of 168.6(8)1 for alternative vinylidene resonance form is rather incompatible with
W1–C1–C2 is in accordance with the description of a carbyne the NMR data and bond lengths of the WC
motif. The angle sum of 359.891 about C2 and the short C–N bond The value of a dative bond description for 4a (in the light of
a b
C moiety (vide supra).
+
1
5
19
lengths (average 1.330 Å) reflect the amidinium character
a recent disputation on the arrow symbolism) is highlighted
0
+
with strong p interactions and delocalization of the positive by comparison of (slightly truncated) [Tp (CO)
charge. H NMR data suggest a dynamic behaviour of the whole
amidinium group in solution. Despite the differing environment calculations. Herein, [Tp (CO) WCCO] , 7 , with CO as a ligand in
2
WCC(NHMe)
2
] ,
1
Me
+
4a , with two potential prototype complexes using DFT
0
+
+
2
0
+
of both amino benzyl groups just one set of broad signals is the donor/acceptor picture and, rather inversely, [Tp (CO) WCCH ] ,
2
2
+
detected for the benzyl groups (CH
and the acidic NH protons give rise to a broad resonance above serve as benchmarks. The geometry optimized structure of 8
.00 ppm. The rotation about the C1–C2 bond in the NMR time shows a rather long WC bond and a normal CC double bond
scale serves as an additional indication for a restricted double reflecting a standard vinylidene character of the C moiety (Fig. 3).
2
4.65 ppm, Ph 6.85–7.40 ppm); 8a , representing the complete absence of N-donor stabilization,
+
8
2
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+
Fig. 3). However, the short C1–C2 bond length in 4a (exp. 1.453 Å,
Me
+
calc. 1.423 Å in 4a ) combined with high calculated BDE values
support a regular covalent bond reflected in resonance structure A
(Scheme 2). Though, the relevance of resonance structure B is
corroborated by the reactivity of the related system with PMe .
3
+
+
17
An equilibrium 5b /6b was reported by Templeton and coworkers
including the crystallographic characterization of the double
adduct [Tp (CO) WC(PMe ) ] , 6b . Thus, preliminary attempts
2 3 2
0
+
+
+
to deprotonate 4a indicate an intricate behaviour which points
Scheme 2 Comparison of resonance formulas A and B for the amidinium
carbyne complex 4a-PF
and 6b-PF
6
and related phosphonium congeners 5a/5b-PF
6
to the competitive nucleophilic attack of the base at the W-bound
C atom.
21
6
.
Further DFT calculations and related experiments were
performed in order to get insights into the cause and mechanism
of the side-on to end-on rearrangement of the C moiety.
2
Remarkably, comparative calculations with the carbyne and
2
the corresponding Z -alkyne binding isomers reveal that the
side-on isomers are enthalpy favoured. This trend is strong for
0
2
+ 22
+
the known acetylene complex [Tp (CO)
2
W(Z -C
2 2
H )] , 8b
ꢀ1
(
DH = 13.4–20.1 kcal mol ), due to the 4e donor effect leading
to a 18e count (in contrast to isomer 8a ). Surprisingly, even for
the pair 4a / 4b the side-on isomer is slightly more stable
DH = 3.7–9.0 kcal mol depending on the functional used, see
+
Me + Me
+
ꢀ
1
(
ESI†). Apparently, the observed rearrangement must rather
be attributed to kinetic reasons. Experimental evidence for a
mechanistic hypothesis was sought by variation of the reaction
conditions. Usage of benzyl amide, KNHBn, instead of the
neutral amine changed the outcome dramatically leading to
intractable mixtures. In addition, a reduction of the amine
equivalents to two or even less instead of four did not lead to
Fig. 3 Geometry optimized structures and selected bond lengths (in Å) of
Me
+
+
4
a
(right) and 8a (left); calculations performed using DFT, pbe0/def2-
TZVP and ZORA.
The CQC bond is best described as a covalent bond formed from the isolation or even in situ detection (by IR or mass spectrometry)
0
+
2 2
the two triplet fragments [Tp W(CO) C] and CH . In contrast, the of any mixed substituted I/NHBn products. Consequently, the
Me
+
geometry optimized complex 4a displays a WC bond as short as second substitution step is considered to be much faster compared
found in 4a by X-ray crystallography and a distinctively longer CC to the first one. On this rationale a mechanistic concept is
bond compared with 8a . This bond is reasonably described as a developed which is centred on a rearrangement concomitant
+
+
donor acceptor bond of either the acyclic C(NHR) singlet carbene with the first substitution step rendering the terminal C-atom
2
0
+
or CO with the singlet species [Tp W(CO) C] . Remarkably, the particularly electrophilic and easily accessible for the second
2
difference in DH between the singlet and the triplet state for the nucleophilic attack (see ESI† for details).
0
+
metal based fragment [Tp W(CO)
2
C] is negligible (see ESI†).
2
0
This perception is substantiated by the Haaland criterion.
The homolytic fission of the CC single bond in 4a (considering
complex type 4a as an amidinium carbyne complex) into two
Me
+
+
doublet fragments resulted in a calculated BDE of 136.4 kcal
ꢀ
1
mol . In comparison, the BDE of the heterolytic fission leading
ꢀ
1
to two singlet species amounts to 119.1 kcal mol rendering
the latter more favourable (Scheme 3). This bias is much clearer
+
for the hypothetical CO adduct 7 (reflecting the high ionisation
energy of 14 eV for CO), while the WC bond length in the
geometry optimized structure is intermediate (1.869 Å, compare
Fig. 4 Calculated IR spectra (gas phase, upper part) of both isomers (end-
on 4a : black, side-on 4b : blue); experimental IR spectra (lower part)
ꢀ
1
Me
+
Me
+
Scheme 3 Calculated bond dissociation energies (BDE, values in kcal mol
)
Me
+
+
of 4a and 7 assuming homolysis into doublet species (left) and heterolysis
right), calculations performed using DFT and pbe0/def2-TZVP.
of a crude product sample of 4a-PF
the signals attributed to 4b .
6
(measured in thf). The asterisks mark
+
(
2
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In addition, IR spectra of non-crystalline crude samples of
a-PF reveal the presence of a second minor complex species.
6
M. T ¨a nzler, M. Folk and A. Villinger, Chem. – Eur. J., 2013, 19,
14702–14711.
4
7
S. G. Feng, A. S. Gamble, C. C. Philipp, P. S. White and J. L. Templeton,
Organometallics, 1991, 10, 3504–3512.
According to frequency calculations side-on coordination can
be inferred from the comparison of the calculated and experimental
IR data (Fig. 4). The spectral pattern of the additional species
8 The side-on binding mode of C
2
I
2
in 1-PF
6
is not only demonstrated
by its derivatives but also by its H NMR spectrum, which indicates
local C symmetry at the W center.
1
ꢀ
1
ꢀ1
s
(
CO approximately 2045 cm and 1965 cm , coupled CC/CN
9
(a) J. L. Caldarelli, P. S. White and J. L. Templeton, J. Am. Chem. Soc.,
1992, 114, 10097–10103; (b) L. W. Francisco, P. S. White and
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P. S. White and J. L. Templeton, Organometallics, 1999, 18, 1897–1903;
ꢀ1
approximately 1680 cm , all marked with stars) is at least in
accord with the presence of a side-on isomer 4b in solution.
+
However, we have not been able to isolate this species in
substance so far.
(
d) W. W. Seidel, B. L. S ´a nchez, M. J. Meel, A. Hepp and T. Pape, Eur.
J. Inorg. Chem., 2007, 936–943; (e) W. W. Seidel, M. Schaffrath, M. J. Meel,
T. Hamilton, S. Castelar Ariza and T. Pape, Z. Naturforsch., 2007, 791–798.
0 D. S. Frohnapfel, S. Reinartz, P. S. White and J. L. Templeton,
In conclusion, we accomplished the nucleophilic substitution
1
2
of both iodo substituents in the cationic Z -diiodo acetylene
Organometallics, 1998, 17, 3759–3769.
complex 1-PF . The alternative substitution of one CO ligand in 11 The strong influence of spin–orbit interaction of iodine on the
6
2
chemical shift of a-carbon atoms ca. ꢀ70 pp(m) must be considered.
1
-PF
6
under retention of the Z -C
2
I
2
coordination is observed with
CN and DMAP. Dual
1
2 J. D. Dunitz, H. Gehrer and D. Britton, Acta Crystallogr., 1972, B28,
aprotic and softer nucleophiles like CH
3
1989–1994.
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6
0
by benzylamine led preferably to a rearrange- 13 (a) T. L. Khotsyanova, A. I. Kitaigorodsky and Y. T. Struckov, Zh. Fiz.
2
Khim., 1953, 27, 1330–1343; (b) B. C. Haywood and R. Shirley, Acta
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1
2
3
vinylidene rearrangement investigated by Werner and others.
1
While the latter is restricted to 2e donor alkyne forming vinylidene
complexes, the conversion of a 4e donor alkyne into a carbenium
carbyne complex (preserving the 18e count of the metal centre) is
rather unusual due to the instability of the carbenium group.
However, the specific substitution with amine substituents as
potential donors, which stabilize the carbenium centre by
resonance, allows the isolation of an unprecedented amidinium
carbyne complex. The presented synthesis seems to be without an
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(
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24,25
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2
0
complexes through k -N,N coordination and subsequent inves-
1
1
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6
ꢀ1
+
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1
ꢀ
ꢀ1
4
1987 cm and 1897 cm .
1
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6
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2
RC-Li, M = Mo, W
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6
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