One pot conversion of benzophenone imine into the relevant 2-aza-allenium

Article abstract of DOI:10.1039/c6cc08702g

Interaction of common imines with WCl₆ in organic solvents revealed very unusual features in the context of imine chemistry. Ph₂=NH was converted in one pot into the 2-aza-allenium species [Ph₂C=N=CPh₂]⁺via N₂ release, with [Ph₂C=NH₂]⁺ being a co-product. PhCH=N^tBu underwent C-H bond activation resulting in the formation of [C≡N] containing derivatives, together with [PhCH=NH^tBu]⁺

Full text of DOI:10.1039/c6cc08702g

ChemComm
View Article Online
View Journal
COMMUNICATION
One pot conversion of benzophenone imine into
the relevant 2-aza-allenium†
Cite this:DOI: 10.1039/c6cc08702g
a
b
b
b
Marco Bortoluzzi,* Tiziana Funaioli, Fabio Marchetti,* Guido Pampaloni,
c
d
Received 29th October 2016,
Accepted 1st December 2016
Calogero Pinzino and Stefano Zacchini
DOI: 10.1039/c6cc08702g
www.rsc.org/chemcomm
Interaction of common imines with WCl
revealed very unusual features in the context of imine chemistry. compounds with tungsten hexachloride, herein we describe
Ph
CQNH was converted in one pot into the 2-aza-allenium the unprecedented, one pot conversion of benzophenone imine
species [Ph
6
in organic solvents In the context of our exploration of the reactivity of nitrogen
8
2
+
+
2
CQNQCPh
2
]
via N
2
release, with [Ph
2
CQNH
2
]
being into the relevant 2-aza-allenium. It should be noted that the
t
a co-product. PhCHQN Bu underwent C–H bond activation resulting information available in the literature on the reactivity of WCl
6
in the formation of [CRN] containing derivatives, together with with imine derivatives has been limited to the synthesis of
t
+
t
t
9
[
PhCHQNH Bu] .
W[NQC( Bu
) ]
2 2 4
by reductive addition of LiNQC( Bu
2 2 6
) to WCl .
The reaction between WCl
6
and benzophenone imine was
Imines are among the most important functional groups in conducted in dichloromethane under an argon atmosphere,
organic chemistry and have been employed as precursors for a and selectively proceeded to give a mixture of two products,
1
variety of nitrogen derivatives. In particular, imines may provide i.e. [Ph CQNQCPh ][WCl ], 1, and [Ph CQNH ][WCl ], 2, with
2
2
6
2
2
6
2
convenient synthetic routes to chiral amines, which are useful slight prevalence of the latter. When the reaction was conducted
intermediates in medicinal chemistry, and to access N-containing in 1,2-dichloroethane at reflux temperature, 1 and 2 were
3
4
natural products.
afforded in comparable amounts (see Scheme 1 and the ESI†
The imine group [CQNR] is isoelectronic to carbonyl for experimental details).
compounds, and in fact its chemistry is dominated by nucleo-
Compounds 1 and 2 were unambiguously characterized by
philic additions. Otherwise diverse activation reactions have IR and NMR spectroscopy, and magnetic analysis, this analysis
ꢀ
been reported involving N–H and both aliphatic and aromatic providing a result consistent with the presence of the [WCl ]
6
5
8
C–H bonds.
anion. Salient spectroscopic features of 1 are the IR absorptions
Although the range of imine transformations may regard the (1860, 1846, 1826 cm , solid state) and the C NMR resonance
ꢀ
1
10
13
10
ultimate cleavage of the [CQN] moiety, as is in the hydrolytic (157.0 ppm, CD
2
Cl
conversion to aldehydes/ketones, the elimination of the N atom moiety. X-ray quality crystals of 1ꢁCH
in its molecular form is typically not observed. representation is shown in Fig. 1, together with relevant, main
Benzophenone imine, Ph Cl consists of an ionic
CQNH, is an easily available substance bonding parameters. Compound 1ꢁCH
2
solution) ascribable to the 2-aza-allenium
6
2
Cl were isolated: an ORTEP
2
2
2
2
ꢀ
+
and one of the most commonly employed imines in synthetic packing of [WCl ] anions, [Ph CQNQCPh ] cations and
6
2
2
7
2b,5b
chemistry, also in view of its relative resistance to hydrolysis.
CH Cl , displaying normal van der Waals contacts. The struc-
2 2
ture of the [WCl6] anion has been previously reported as a
ꢀ
a
University Ca’ Foscari Venezia, Dipartimento di Scienze Molecolari e Nanosistemi,
Via Torino 155, I-30170 Mestre (VE), Italy. E-mail: markos@unive.it;
Web: http://www.unive.it/data/people/5591286; Tel: +39 041 234 8561
University of Pisa, Dipartimento di Chimica e Chimica Industriale,
Via Moruzzi 13, I-56124 Pisa, Italy. E-mail: fabio.marchetti1974@unipi.it;
Web: http://www.dcci.unipi.it/fabio-marchetti.html; Tel: +39 050 2219245
ICCOM-CNR, Area della Ricerca, Via G. Moruzzi 1, I-56124 Pisa, Italy
University of Bologna, Dipartimento di Chimica Industriale ‘‘Toso Montanari’’,
Viale Risorgimento 4, I-40136 Bologna, Italy
b
c
d
†
Electronic supplementary information (ESI) available: Experimental details,
X-ray crystallography, characterization of the products, and DFT structures. CCDC
509118. For ESI and crystallographic data in CIF or other electronic format see
1
DOI: 10.1039/c6cc08702g
6
Scheme 1 WCl -promoted activation of benzophenone imine.
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.

View Article Online
Communication
ChemComm
We performed several experiments aimed to give insight into
the mechanism of the reaction. In general, nitrogen compounds
react with WCl6 via initial single electron transfer from the
8
former to the latter. Conversely, this route does not appear to
be the prevailing one with reference to the Ph CQNH/WCl /
2
6
+
2 2 2
CH Cl reaction system. Indeed the radical cation [Ph CNH]
was EPR observed in traces at ca. 193 K, disappearing at higher
temperatures.
1
7
The reaction of Ph
2
CQNH with NOBF
4
as an oxidant
proceeded via the intermediacy of [Ph
identified by EPR (Fig. S1, ESI†), and the salt [Ph CQNH ][BF ]
2
4
CQNH][BF ], 3, clearly
1
8
2
2
4
was finally isolated in 70% yield. Accordingly, electrochemical
and IR and UV-Vis spectroelectrochemical analyses (see Fig. S2
Fig. 1 View of the structure of 1 with key atoms labelled. Displacement
ellipsoids are at the 50% probability level. Selected bond lengths (Å) and
angles (1): W–Cl 2.419(8)–2.460(11), N(1)–C(1) 1.46(4), N(1)–C(2) 1.42(4), and S3, ESI†) revealed an irreversible oxidation of benzophenone
C(1)–N(1)–C(2) 160(3); sum at C(1) 360(3), sum at C(2) 360(3).
2 2 2
imine in CH Cl occurring at +1.12 V (vs. FeCp ), followed by
fast, presumable quenching of the resulting radical cation
with hydrogen atom captured from the medium, resulting in
8
+
miscellaneous salt. However, the crystallographic determination the formation of [Ph
2 2
CQNH ] .
+
of [Ph CQNQCPh ] is new, even though related 2-aza-allenium
In agreement with all the experimental outcomes, the synthesis
2
2
11
cations were characterized in the past.
of 1 should not be attributed to the well known oxidative power
8
,19
According to gas chromatographic analysis, the self-condensation of WCl6.
DFT calculations were consistent with this point
process leading to 1 takes place with the release of molecular and allowed tracing a plausible reaction mechanism (Scheme 2;
nitrogen. This is a new feature in the landscape of the hugely the relevant DFT-optimized geometries are provided as ESI†).
investigated chemistry of imines, including the interaction with
The reaction may start with the migration of a chloride ligand
to the [CQN] moiety, preliminarily affording Ph C(Cl)NH–WCl
2 5
1
2
20
high valent transition metal chlorides different from WCl
6
.
To the best of our knowledge, 2-aza-allenium salts have been (Scheme 2, step a). Although the related DG is approximately 0,
obtained to date from a variety of multicomponent systems, i.e. such a hypothesized reaction matches well the previously reported
1
3
11b
i
bis(iminomethane)/triflic anhydride, alkene/N-alkylnitrilium,
formation of ( Pr)CH(Cl)NMe–C (Q O)Cl, as a stable product,
11c
i
21
chlorocarbonyl isocyanate/ketone/SbCl
5
,
alkylation or acylation from ( Pr)CHQNMe and phosgene. The key to the establishment
10b,14
of N-substituted imine,
abstractor,
nitrilium/imine/diazomethane.
N-substituted imine/hydride or chloride of the [NCN] skeleton of 1 seems to be the nucleophilic attack of a
nitrile oxide/diazoketone/isocyanate, and second benzophenone imine to the new C–Cl bond (step b). The
1
0a,15
16
1
0b
5 2 2
resulting compound, i.e. [WCl (NHCPh NHCPh )]Cl, was observed
Scheme 2 Relative Gibbs energies of selected DFT-optimized intermediates (see Fig. S6–S11, ESI†) along the proposed pathway for the WCl
conversion of benzophenone imine to 2-aza-allenium. Black lines: C-PCM/oB97X calculations. Red lines: EDF2 calculations.
6
-mediated
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2016

View Article Online
ChemComm
Communication
as a stationary point by gas-phase DFT calculations (EDF2).
Further optimization at the C-PCM/oB97X level (dichloromethane
as a continuous medium) showed the highly favourable release
of HCl to give WCl (NCPh NHCPh ) (step c), the isomer
5
2
2
ꢀ1
WCl (NHCPh NCPh ) being less stable by about 37 kcal mol
.
5
2
2
Then 2-aza-allenium is likely to be generated together with
ꢀ
the W(VI) imido anion [WCl
5
NH] (step d). HCl elimination
ꢀ
from [WCl
5
NH] is thermodynamically favourable by about
ꢀ
1
7
.1 kcal mol (step e). The following, possible reaction of the
ꢀ
ꢀ
4
resulting nitride species, [WCl N] , with Cl (produced in the
reaction ambient, see Scheme 2, steps c and e) should finally
ꢀ
6
Scheme 3 WCl -promoted imine C–H bond activation.
lead to [WCl ] and N , in agreement with the experimental
6
2
22
evidence. We propose that the N–N bond formation is promoted
by the stepwise oxidation of [WCl N] by still unreacted WCl ,
5 6
2ꢀ
and a plausible pathway is detailed in Scheme S6 (ESI†). In
particular, the key step might be the intermolecular coupling of
2
3
two N-chloroimido mononuclear complexes.
2
The reaction of WCl with the commercially available ( Bu) CQNH
t
6
yielded an insoluble solid material, whose unambiguous
identification was not possible. However, the reaction proceeded
with release of some N , thus suggesting that the WCl -induced
2
6
denitrogenative behaviour of imines might hold some general
characteristics. We moved to extend our study to the chemistry
of Schiff bases, i.e. imines bearing an alkyl/aryl substituent
on the nitrogen. The 1 : 1 molar reaction in dichloromethane Scheme 4 Relative Gibbs energies (C-PCM/oB97X calculations) of
t
selected DFT-optimized intermediates (see Fig. S17 and S18, ESI†) along
the proposed pathway for the WCl -mediated conversion of benzylidene-
6
of WCl
6
with N-benzylidene-tert-butylamine, PhCHQN Bu, led
t
to the isolation of [PhCHQNH Bu][WCl ], 4, in 48% yield. The
H NMR spectrum of 4 (CD CN solution) shows two doublets
6
tert-butylamine to the relevant nitrilium.
1
3
accounting for intercoupling CH and NH protons (at 8.77 and
3
t
1
0.85 ppm, respectively; JHH = 17.6 Hz). We collected spectro- adduct WCl
6
(PhCHQN Bu) was not observed as a stationary
t
2
t
5
scopic evidence indicating that the imine PhCHQN Bu acts as a point. WCl (k -C,N-PhCQN Bu) is prone to undergoing
probable source of self-protonation via C–H bond activation. double electron transfer from the organic ligand to the metal
ꢀ
First, a solid material was isolated from the reaction mixture centre, thus generating [WCl
5
]
and the nitrilium cation (step b,
ꢀ
1
ꢀ
whose IR spectrum (solid state) exhibited an intense band at DG = ꢀ13.6 kcal mol ). The W(V) anion, [WCl
6
] , as detected in 4,
ꢀ1
IV
ꢀ
2
267 cm , accounting for a nitrilium group. The latter is prone to may be afforded by the interaction of [W Cl ] with unreacted
5
VI
31,32
losing the tert-butyl cation, possibly converting into 2-methylpropene W Cl .
(
6
1
24
a H NMR resonance could be detected at 4.50 ppm ). When the
reaction mixture was treated with a weakly basic water solution, a the WCl
significant amount of benzonitrile was generated and then clearly consequence of possible imine oxidation by WCl
recognized by NMR and GC-MS (see the ESI† for details), Scheme 3. irreversible oxidation process was found for PhCHQN Bu in
Also the WCl /PhCHQNPh reaction system afforded some CH Cl solution (at +1.12 V vs. FeCp ), followed by fast degra-
benzonitrile (detected by GC-MS) after work-up. dation of the in situ generated imine radical cation to give
Electrochemical analyses support the idea (Scheme 4) that
t
6
-promoted C–H activation within PhCHQN Bu is not a
. Indeed one
6
t
6
2
2
2
3
3
t
+
t
+
The formation of the nitrilium [PhCRN Bu] from the parent [PhCHQNH Bu] (see Fig. S4 and S5, ESI†).
t
imine PhCHQN Bu, albeit far from holding possible synthetic
In summary, in view of the promising potential of high
implications, is an unusual feature in the context of imine valent transition metal chlorides to promote unusual activation
2
5
34
chemistry. Indeed the conversion of nitriles into imines is a reactions of organic species, herein we have reported a study
2
6
viable process, but not vice versa. Otherwise nitrilium cations, on the interaction of WCl
6
with a selection of simple, commercial
0
+
0
[
RCRNR ] (R = alkyl group or H), have been detected as elusive imines. The reactions proceed according to unusual pathways,
2
7
intermediates in organic reactions and have been commonly and especially the unprecedented, denitrogenative one pot con-
prepared by alkylation of nitriles with alkyl halides or strong version of benzophenone imine into the relevant 2-aza-allenium
protonating agents.
2
8
2
9
might open the door to a new perspective in the scenery
According to DFT outcomes (Scheme 4), imine C–H bond of the chemistry of imines. Two points deserve to be remarked:
t
cleavage takes place at the first stage of the WCl /PhCHQN Bu (1) 2-aza-allenium species have been accessible to date only by
6
ꢀ
1 30
interaction (Scheme 4, step a; calculated DG = ꢀ20.6 kcal mol ),
2
elaborated synthetic protocols and (2) N elimination from imines
the released HCl being presumably responsible for the side has never been reported hitherto. According to experimental and
t
+
formation of [PhCHQNH Bu] . The hypothetical preliminary DFT outcomes, the benzophenone imine pseudo-dimerization
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.

View Article Online
Communication
ChemComm
3
5
seems to be triggered by the chlorinating behaviour of WCl
rather than the oxidative one.
6
12 F. Marchetti, G. Pampaloni and S. Zacchini, Polyhedron, 2016, 115,
9
9–104.
3 G. B o¨ ttger, A. Geisler, R. Fr ¨o hlich and E.-U. W u¨ rthwein, J. Org. Chem.,
997, 62, 6407–6411.
14 (a) R. Kupfer, S. Meier and E.-U. W u¨ rthwein, Chem. Ber., 1992, 125,
487–2492; (b) M. Krestel, R. Kupfer, R. Allmann and E.-U. Wiirthwein,
Chem. Ber., 1987, 120, 1271–1279.
T. B. Rauchfuss, F. Arrigoni and G. Zampella, Organometallics, 2016, 15 B. Samuel and K. Wade, Chem. Commun., 1968, 1081–1082.
5, 2782–2792; (b) S. Pramanik, R. R. Reddy and P. Ghorai, J. Org. 16 A. Hamed, E. Wiilleir, J. C. Jochims, A. L. Zsolnai and G. Buttner,
Chem., 2015, 80, 3656–3663; (c) J. Zhang, A. Ugrinov and P. Zhao, Tetrahedron, 1989, 45, 5825–5836.
Angew. Chem., Int. Ed., 2013, 52, 6681–6684; (d) M. E. Belowich and J. F. 17 Imine radical cations were claimed to be generated by FeCl
1
1
Notes and references
1
2
Selected recent references: (a) W.-Y. Chu, C. P. Richers, E. R. Kahle,
3
3
oxida-
tion in non aqueous environment: R. Leardini, D. Nanni, A. Tundo,
G. Zanardi and F. Ruggieri, J. Org. Chem., 1992, 57, 1842–1848.
Stoddart, Chem. Soc. Rev., 2012, 41, 2003–2024; (e) M. J. Campbell and
F. D. Toste, Chem. Sci., 2011, 2, 1369–1378.
2
(a) J. Liu, C.-G. Cao, H.-B. Sun, X. Zhang and D. Niu, J. Am. Chem. 18 C. J. Rhodes and H. Agirbas, J. Chem. Soc., Faraday Trans., 1990, 86,
Soc., 2016, 138, 13103–13106; (b) H.-J. Pan, Y. Zhang, C. Shan, Z. Yu, 3303–3308.
Y. Lan and Y. Zhao, Angew. Chem., Int. Ed., 2016, 55, 9615–9619; 19 N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877–910.
c) K. Xu, Y.-H. Wang, V. Khakyzadeh and B. Breit, Chem. Sci., 2016, 7, 20 (a) J.-C. M. Monbaliu, L. K. Beagle, J. Kovacs, M. Zeller, C. V. Stevens
(
3
313–3316; (d) E. Skucas, M.-Y. Ngai, V. Komanduri and M. J. Krische,
and A. R. Katritzky, RSC Adv., 2012, 2, 8941–8945; (b) A. K. Gupta,
J. Acharya, D. Pardasani and D. K. Dubey, Tetrahedron Lett., 2007, 48,
767–770.
Acc. Chem. Res., 2007, 40, 1394–1401.
(a) N. Kato, T. Shirai and Y. Yamamoto, Chem. – Eur. J., 2016, 22,
3
4
5
7
739–7742; (b) J. P. Mahajan, Y. R. Suryawanshi and S. B. Mhaske, 21 J. P. Chupp, J. Heterocycl. Chem., 1971, 677–679.
ꢀ
ꢀ
ꢀ
6
4 6
Org. Lett., 2012, 14, 5804–5807; (c) G. K. Friestad and A. K. Mathies, 22 Calculated DG for the reaction [WCl N] + 2Cl + 2WCl - 3[WCl ] +
Tetrahedron, 2007, 63, 2541–2569.
(a) M. Yus, J. C. Gonz ´a lez-G ´o mez and F. Foubelo, Chem. Rev., 2013, 23 (a) J. D. Lichtenhan, J. W. Ziller and N. M. Doherty, Inorg. Chem.,
ꢀ1
1/2N
2
is ꢀ133.9 kcal mol
.
1
13, 5595–5698; (b) R. Alam, A. Das, G. Huang, L. Eriksson, F. Himo
1992, 31, 4210–4212; (b) K. Dehnicke, Z. Anorg. Allg. Chem., 2003,
629, 729–743.
24 A. J. Kell, Langmuir, 2005, 21, 9741–9746. The related resonance due
to the methyl groups could not be unambiguously assigned.
and K. J. Szab ´o , Chem. Sci., 2014, 5, 2732–2738; (c) L. H. Choudhury
and T. Parvin, Tetrahedron, 2011, 67, 8213–8228.
(a) A. K. Pagire, S. Paria and O. Reiser, Org. Lett., 2016, 18, 2106–2109;
(
b) S. Camadanli, R. Beck, U. Fl o¨ rke and H.-F. Klein, Organometallics, 2009, 25 H. Bock and R. Dammel, J. Am. Chem. Soc., 1988, 110, 5261–5269.
8, 2300–2310; (c) Y. Yu. Scaffidi-Domianello, A. A. Nazarov, M. Haukka, 26 See for instance: (a) M. Chatterjee, H. Kawanami, M. Sato, T. Ishizaka,
2
M. Galanski, B. K. Keppler, J. Schneider, P. Du, R. Eisenberg and V. Y.
Kukushkin, Inorg. Chem., 2007, 46, 4469–4482; (d) J. Albert, J. Magali
Cadena, A. Gonz ´a lez, J. Granell, X. Solans and M. Font-Bardia, Chem. –
Eur. J., 2006, 12, 887–894.
T. Yokoyama and T. Suzuki, Green Chem., 2010, 12, 87–93; (b) T. Li,
I. Bergner, F. N. Haque, M. Zimmer-De Iuliis, D. Song and R. H.
Morris, Organometallics, 2007, 26, 5940–5949.
27 R. C. Cioc, H. D. Preschel, G. van der Heijden, E. Ruijter and R. V. A.
Orru, Chem. – Eur. J., 2016, 22, 7837–7842.
6
(a) J. H. Babler and B. J. Invergo, J. Org. Chem., 1981, 46, 1937–1938;
(
(
b) J. M. Sayer and P. Conlon, J. Am. Chem. Soc., 1980, 102, 3592–3600; 28 R. Abu-El-Halawa, Z. Naturforsch., 2009, 64b, 297–306.
c) R. W. Layer, Chem. Rev., 1963, 63, 489–510. 29 G. A. Olah and T. E. Kiovsky, J. Am. Chem. Soc., 1968, 4666–4672.
2
t
7
8
M. J. O’Donnell, Benzophenone imine, Encyclopedia of Reagents for 30 In the resulting W(VI) complex WCl
5
(k -C,N-PhCQN Bu), the ligand
Organic Synthesis, Wiley, 2001.
(a) M. Bortoluzzi, F. Marchetti, G. Pampaloni and S. Zacchini, Inorg.
Chem., 2016, 55, 887–893; (b) M. Bortoluzzi, F. Marchetti, G. Pampaloni 31 Calculated DG for the reaction [WCl ] + Cl + WCl - 2[WCl ]
and S. Zacchini, Chem. Commun., 2015, 51, 1323–1325; (c) M. Bortoluzzi,
has Z configuration, and all geometry optimizations starting from
2
t
5
E-WCl (k -C,N-PhCQN Bu) led to E to Z isomerisation.
ꢀ
ꢀ
ꢀ
6
5 6
ꢀ1
is ꢀ93.0 kcal mol
.
F. Marchetti, G. Pampaloni and S. Zacchini, Inorg. Chem., 2014, 53, 32 Attempts to obtain a stationary point composed by WCl
5
and the
radical PhCN Bu afforded in all the cases [PhCRN Bu][WCl ].
R. A. D. Soriaga, J. M. Nguyen, T. A. Albright and D. M. Hoffman, 33 (a) J. Larsen, K. A. Jørgensen and D. Christensen, J. Chem. Soc.,
t
t
3832–3838.
5
9
J. Am. Chem. Soc., 2010, 132, 18014–18016.
0 (a) H. Frey, A. Mehlhorn and K. Ruhlkann, Tetrahedron, 1987, 43,
Perkin Trans. 1, 1991, 1187–1190; (b) C. J. Rhodes, J. Chem. Soc.,
Faraday Trans. 1, 1988, 84, 3215–3222.
1
2
3
945–2954; (b) M. Al-Talib and J. C. Jochims, Chem. Ber., 1984, 117, 34 (a) M. Schubert and S. R. Waldvogel, Eur. J. Org. Chem., 2016,
222–3230.
1921–1936; (b) Y. Satoh and Y. Obora, Eur. J. Org. Chem., 2015,
5041–5054; (c) M. Schubert, J. Leppin, K. Wehming, D. Schollmeyer,
K. Heinze and S. R. Waldvogel, Angew. Chem., Int. Ed., 2014, 53,
2494–2497; (d) S. R. Waldvogel and S. Trosien, Chem. Commun.,
2012, 48, 9109–9119; (e) F. Marchetti and G. Pampaloni, Chem.
Commun., 2012, 48, 635–653.
1
1 (a) A. Kozma, G. Gopakumar, C. Fares, W. Thiel and M. Alcarazo,
Chem. – Eur. J., 2013, 19, 3542–3546; (b) A. H. Moustafa, W. Wirschun,
C. C. Freyardt, J. C. Jochims Konstanz and R. Abu-El-Halawa, J. Prakt.
Chem., 1997, 339, 615–622; (c) E.-U. Wiirthwein, R. Kupfer, P. H. M.
Budzelaar, C. Strobel and H. P. Beck, Angew. Chem., Int. Ed., 1985,
2
4, 340–341; (d) M. AL-Talib, I. Jibril, E.-Y. W u¨ rthwein, J. C. Jochims 35 M. Bortoluzzi, F. Marchetti, M. G. Murrali, G. Pampaloni and S. Zacchini,
and G. Hutter, Chem. Ber., 1984, 117, 3365–3373. Dalton Trans., 2015, 44, 8729–8738.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2016