Allowing the direct interaction of: N -aryl α-diimines with a high valent metal chloride: One-pot WCl6-promoted formation of quinoxalinium salts

Article abstract of DOI:10.1039/c7dt03226a

The full potential of a high valent metal chloride as both a chlorinating and an oxidative agent was explored by allowing WCl₆ to react with N-(2,6-diisopropylphenyl) α-diimines, in CH₂Cl₂ at room temperature. These α-diimines underwent unprecedented conversion to quinoxalinium cations via intramolecular C-N coupling.

Full text of DOI:10.1039/c7dt03226a

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Allowing the direct interaction of N-aryl α-diimines
with a high valent metal chloride: one-pot WCl6-
promoted formation of quinoxalinium salts†
a
Cite this: DOI: 10.1039/c7dt03226a
Received 31st August 2017,
Accepted 14th September 2017
Niccolò Bartalucci,a Marco Bortoluzzi,b Tiziana Funaioli,a Fabio Marchetti,
*
DOI: 10.1039/c7dt03226a
Guido Pampalonia and Stefano Zacchini
c
rsc.li/dalton
The full potential of a high valent metal chloride as both a chlori- metal chlorides, the reactivity of NbCl5,9 TaCl5,10 MoCl5 and
nating and an oxidative agent was explored by allowing WCl6 to WCl6 12 with α-diimines has aroused a recent and notable con-
react with N-(2,6-diisopropylphenyl) α-diimines, in CH2Cl2 at room sideration. However, all the described reactions have been per-
temperature. These α-diimines underwent unprecedented conver- formed with prior reduction, by one or two electrons, of the
11
sion to quinoxalinium cations via intramolecular C–N coupling.
metal centre. This strategy quenches the activation ability of
the strongly acidic metal chlorides,13–15 so to straightforwardly
afford coordination compounds with intact, redox active
α-diimine ligands. A similar outcome has been achieved by
allowing TiX4 (X = Cl, Br)2d,16 and MCl5 (M = Nb, Ta)16a,17 to
react with dilithium(1,4-diaza-1,3-dienyl) salts.
α-Diimines (also called 1,4-diaza-1,3-dienes = DAD) constitute
a fascinating class of compounds whose steric and electronic
properties may be finely tuned by varying the substituents on
the NvCCvN skeleton.1 Due to their versatility and robust-
ness, α-diimines have been widely employed as ancillary che-
lating ligands in coordination chemistry,2 assisting a variety of
stoichiometric and catalytic transformations.3 α-Diimines are
prone to undergoing a stepwise reduction (up to two electron
reduction),1c,4 and for this reason they are considered as redox
non-innocent ligands when bound to a metal centre,2b,5 poss-
ibly determining some ambiguity in the assignment of the oxi-
dation states.6 On the other hand, the oxidation chemistry of
α-diimines still remains, surprisingly, an almost unknown
field. As a matter of fact, the electro-oxidation behaviour of
non coordinated α-diimines has not been clearly assessed
hitherto.7 Besides, the reactivity studies with high valent metal
compounds (oxidation state of the metal ≥4), that are poten-
tial, strong oxidizing agents, are exceedingly rare in the litera-
ture.4,8 With reference to homoleptic, high valent transition
Herein, we describe for the first time the reactions of two
α-diimines with a high valent metal chloride, in a weakly coordi-
nating solvent (dichloromethane) and in the absence of further
reactants (e.g., reductants). Thus the 1 : 1 molar reaction of
WCl6 with N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-buta-
diene (DADDip
)
afforded the quinoxalinium salt [{2,6-
C6H3(CHMe2)2}N(CH)2NCC(CHMe2)(CH)3C][WCl6], 1a, in 60%
yield (Scheme 1).18 Compound 1a was isolated and then charac-
terized by spectroscopic methods and single crystal X-ray diffr-
action (see Fig. 1, and Table SI1 in the ESI†). The analogous 1b
was obtained in 45% yield from WCl6 and 2,3-bis(2,6-diiso-
propylphenylimino) butane (Me-DADDip), see Scheme 1.
Significant amounts of isopropyl chloride were clearly
recognized by NMR/GC-MS analyses on the liquids distilled
aUniversity of Pisa, Dipartimento di Chimica e Chimica Industriale, Via Moruzzi 13,
I-56124 Pisa, Italy. E-mail: fabio.marchetti1974@ unipi.it;
http://www.dcci.unipi.it/fabio-marchetti.html; Tel: +39 050 2219245
bUniversity Ca’ Foscari Venezia, Dipartimento di Scienze Molecolari e Nanosistemi,
Via Torino 155, I-30170 Mestre, VE, Italy
cUniversity of Bologna, Dipartimento di Chimica Industriale “Toso Montanari”,
Viale Risorgimento 4, I-40136 Bologna, Italy
†Electronic supplementary information (ESI) available: Details of experimental
procedures (including Scheme SI1), X-ray crystallography, electrochemical
studies (including Table SI3 and Fig. SI1), computational studies (including
Scheme SI2 and Fig. SI2–SI6), NMR spectra (Fig. SI7–SI12). CCDC 1551416
(1a·CH2Cl2) and 1551419 (2b). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7dt03226a
Scheme 1 α-Diimine to quinoxalinium conversion promoted by WCl6.
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Fig. 2 Cyclic voltammograms of DADDip recorded at a Pt electrode in
CH2Cl2 solution containing [nBu4N][PF6] 0.2 M. Scan rate 0.1 V s−1
.
(a) Cyclic voltammetry obtained starting the scan toward negative poten-
tials from +0.8 V; (b) 2 cycles voltammetry between −0.8 and 1.8 V.
First, the voltammetric investigation of
a series of
Fig. 1 View of the structure of 1a with key atoms labeled. Displacement
ellipsoids are at the 50% probability level. Only one of the two indepen-
dent cation–ion pair present within the unit cell is represented.
α-diimines (see Chart 1 in the ESI† and Fig. 2 for DADDip) at a
Pt disk electrode was performed in CH2Cl2. The examined
compounds underwent, in addition to the well established
one-electron reduction, an oxidation process at potentials
from the WCl6/DADDip/CD2Cl2 and WCl6/Me-DADDip/CD2Cl2 ranging between 1.43 and 1.70 V.33 These values would allow,
reaction systems, respectively. By using the same method, we in principle, a chemical oxidation by WCl6 in CH2Cl2 (E°′ ca.
detected limited amounts of alkyl halides as produced from 1.6 V vs. Ag/AgCl/KCl).34
the combination of WCl6 with some other N-aryl α-diimines
Notwithstanding, variable temperature EPR analysis of a
(see Experimental for details), suggesting that the activation WCl6/DADDip mixture in CH2Cl2 did not allow to detect any
process observed for DADDip and Me-DADDip might hold some trace of [DADDip]+. Otherwise, the electrochemical oxidation of
general character.
DADDip to [DADDip]+ did not lead to final formation of the qui-
The crystal structure of 1a consists in an ionic packing of noxalinium cation contained in 1a (see comparison of UV
[WCl6]− anions19 and [{2,6-C6H3(CHMe2)2}N(CH)2NCC(CHMe2) spectra in Fig. SI1†). Accordingly, the reaction in CH2Cl2 of
(CH)3C]+ cations. The bonding parameters of the quinoxali- DADDip with NOBF4, i.e. a typical oxidizing agent whose power
nium core do not significantly differ from those previously is comparable to that of WCl6,34 did not give the quinoxali-
reported for N-alkyl-quinoxalium cations.20 Quinoxalinium nium derivative among a complicated mixture of products.
salts have aroused interest as possible chemosensors,21 in These facts suggest that the synthesis of 1a, albeit containing a
synthetic organic chemistry22 and for their pharmaceutical23 W(V) anion, cannot be the consequence of a simple electron
and photophysical properties.24 They are usually generated transfer from DADDip to W(VI). In agreement with these experi-
via the condensation of α-diketones with 1,2-phenyl- mental observations, key intermediates along a plausible reac-
enediamines.24a,25a,b,26
quinoxalinium salts have been synthesized to date,25b,27 and Fig. SI2–SI6†).
A restricted number of N-aryl tion pathway to 1a were DFT calculated (see Scheme SI2 and
the crystallographic characterization of 1a provides the first
On theoretical grounds, it appears that the initial inter-
X-ray structure of a N-aryl-quinoxalium cation ever reported. It action WCl6–DADDip results in a favourable Cl-attack to one of
should be remarked here that the known activation reactions the two CvN moieties (Fig. SI2 and SI3†). Experimentally, the
of α-diimines are rare,5c,28 being usually limited to C–C and 1 : 1 reaction in CD2Cl2 of DADDip with PCl5, i.e. a typical chlor-
C–Cl coupling additions to the imine carbons.9,10,28–30 A tri- inating substance employed in organic synthesis,35 selectively
cyclic dipyrrolopyrrole was obtained by FeCl2-promoted oligo- proceeded with H/Cl substitution at one imine group to give
merization of a N-aryl α-diimine, accompanied by the loss of [{2,6-C6H3(CHMe2)2}NvCHC(Cl)vN{2,6-C6H3(CHMe2)2}]
as
half of one DAD fragment.31 The cyclization reaction presented the stable product (Scheme SI1†). Following chlorination of
herein has more affinity with the chemistry of 2,3-bis(diphenyl- the organic substrate, one-electron transfer from the newly
phosphino)-1,4-diazadienes, undergoing
closure upon coordination of the phosphine groups to group is concomitant with the migration of one chloride from the
10 metal halides.32
metal to isopropyl-substituted carbon atom. The cyclization
a peripheral ring formed ligand to the metal centre is likely to occur. This step
Several experiments, including electrochemical analyses step should then take place favoured by the presence of both
and spectroscopic studies, and DFT calculations allowed to an activated aromatic ring and a formally anionic nitrogen
elucidate some mechanistic aspects of the WCl6 promoted atom. The release of isopropyl chloride (see above) is likely to
cyclization reaction of DADDip
.
represent the driving force for the entire process. The radical
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Table 1 Selected bond distances (Å) and angles (°) for 2b
W(1)–Cl(1)
W(1)–Cl(3)
W(1)–N(1)
N(1)–C(1)
C(1)–C(2)
2.351(3)
2.296(3)
2.032(10)
1.344(15)
1.381(17)
W(1)–Cl(2)
W(1)–Cl(4)
W(1)–N(2)
N(2)–C(2)
2.349(3)
2.250(3)
2.054(9)
1.351(15)
Cl(1)–W(1)–Cl(2)
Cl(4)–W(1)–N(2)
N(1)–W(1)–N(2)
N(1)–C(1)–C(2)
C(2)–N(2)–W(1)
169.60(11)
160.9(3)
73.7(4)
113.4(11)
118.9(8)
Cl(3)–W(1)–N(1)
Cl(3)–W(1)–Cl(4)
W(1)–N(1)–C(1)
C(1)–C(2)–N(2)
161.7(3)
110.88(12)
120.1(8)
113.3(10)
Scheme 2 Synthesis of W(IV) coordination complexes from WCl6 and
α-diimines in toluene.
molecular C–N coupling. Based on experimental and compu-
tational outcomes, the most probable reaction pathway
exploits the rather unique synergic properties offered by
WCl6,13 acting as both a powerful chlorinating species13,36 and
a strong oxidant.19,34,37 We are currently working to develop
this concept in the direction to achieve unconventional trans-
formations of suitable organic substrates.
derived from isopropyl elimination (Fig. SI6†) could be oxi-
dized to the final quinoxalinium cation either by WCl5 (ΔG =
−7.5 kcal mol−1) or more easily by still unreacted WCl6 (ΔG =
−40.8 kcal mol−1).
The W(IV) coordination complex WCl4(DADDip), 2a,12a was
identified as a side-product in the synthesis of 1a. The solvent
polarity revealed to be crucial in regulating the 1a/2a ratio (see
ESI, page S13†). In fact, when toluene was employed as the
reaction medium in the place of dichloromethane, 2a became
the major product (60% yield, Scheme 2). It is noticeable that
this method may provide an alternative route to WIVCl4(DAD)
coordination complexes, in the absence of a Cl-abstracting
agent.12a We found that the reaction of WCl6 with DADDet in
toluene afforded the W(IV) complex 2b in 40% yield (Scheme 2).
The new compound 2b was characterized by analytical and
spectroscopic techniques, and by X-ray diffraction (Fig. 3,
Table 1). The structure of 2b resembles that previously
reported for 2a.12a
Conflicts of interest
There are no conflicts to declare.
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