Facile anaerobic oxidative dehydrogenation promoted by bases: The case of copper-2,4-dihydro-1H-benzo[d][1,3]oxazine complexes

Article abstract of DOI:10.1016/j.ica.2009.10.016

Two new 2,4-dihydro-1H-benzo[d][1,3]oxazines (L₁ and L₂) were prepared by condensation of 2-quinolinecarboxaldehyde and 2-amino-benzyl alcohols and tested as N,N'-bidentate ligands toward CuCl₂. Treatment of the resulting copper(II) derivatives with Et₃N promoted an oxidative dehydrogenation yielding the corresponding copper(I) [Cu(L-ox)Cl] complexes, 2, (L-ox = 4H-benzo[d][1,3]oxazine). The [Cu(L₂-ox)Cl] species, 2b, was characterized by single crystal X-ray diffraction, showing a trigonal geometry at the metal center and reacted with PPh₃ and CO, affording [Cu(L₂-ox)(PPh₃)Cl], 4b, and [Cu(L₂-ox)(CO)Cl], 6b, respectively. The latter species, stable in the solid state, was structurally characterized by diffraction methods and showed tetrahedral coordination of the Cu(I) ion.

Full text of DOI:10.1016/j.ica.2009.10.016

Inorganica Chimica Acta 363 (2010) 324–329
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Facile anaerobic oxidative dehydrogenation promoted by bases: The case of
copper-2,4-dihydro-1H-benzo[d][1,3]oxazine complexes
*
G. Attilio Ardizzoia , Stefano Brenna, Fulvio Castelli, Simona Galli, Norberto Masciocchi
Dipartimento di Scienze Chimiche e Ambientali, Università degli Studi dell’Insubria, Via Valleggio, 11, 22100 Como, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2009
Received in revised form 19 October 2009
Accepted 23 October 2009
Available online 29 October 2009
Two new 2,4-dihydro-1H-benzo[d][1,3]oxazines (L1 and L2) were prepared by condensation of 2-quino-
linecarboxaldehyde and 2-amino-benzyl alcohols and tested as N,N’-bidentate ligands toward CuCl2.
Treatment of the resulting copper(II) derivatives with Et3N promoted an oxidative dehydrogenation
yielding the corresponding copper(I) [Cu(L-ox)Cl] complexes, 2, (L-ox = 4H-benzo[d][1,3]oxazine). The
[Cu(L2-ox)Cl] species, 2b, was characterized by single crystal X-ray diffraction, showing a trigonal geom-
etry at the metal center and reacted with PPh3 and CO, affording [Cu(L2-ox)(PPh3)Cl], 4b, and [Cu(L2-
ox)(CO)Cl], 6b, respectively. The latter species, stable in the solid state, was structurally characterized
by diffraction methods and showed tetrahedral coordination of the Cu(I) ion.
Keywords:
Copper
Oxidative dehydrogenation
Carbon monoxide
Carbonyl complexes
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
the latter compounds undergo oxidative dehydrogenation yielding
the copper(I) [Cu(L1-ox)Cl] and [Cu(L2-ox)Cl] complexes (where L-
The interest on 1,3-oxazine molecules has recently increased,
mainly by reason of their well documented bioactivity [1,2] and
their versatility as synthetic intermediates [3]. Specifically, ben-
zo-1,3-oxazines are known to possess antianginal activity [4],
together with antihypertensive influence [5] and effectiveness as
antirheumatic agents [6]. In addition, they showed remarkable
properties when coordinated to transition metals and their cata-
lytic efficiency was proved to be usually higher when compared
to the extensively used 1,3-oxazolines. For example, remarkable
acceleration in addition to good enantiomeric excess is reported
for Pd-catalyzed asymmetric allylic alkylation [7], while Kandas-
amy et al. obtained relatively high conversions in styrene oxidation
promoted by oxo-vanadium complexes [8]. Strong donation from
ox are the corresponding 4H-benzo[d][1,3]oxazines). The molecu-
lar and crystal structure of these species is described, together with
the stoichiometry of the oxidative dehydrogenation reaction giving
rise to their formation. The reactivity of [Cu(L2-ox)Cl] towards
PPh3, molecular oxygen and carbon monoxide was investigated.
In particular, its exposure to CO allowed to prepare the monomeric
[Cu(L2-ox)(CO)Cl] carbonyl species, very stable in the solid state
and structurally characterized at room temperature.
2. Experimental
2.1. General information
the metal center to the p
*-orbitals of oxazine ring has been invoked
to justify these uncommon performances [9]. Finally, benzoxazines
are important building blocks in the development of high-perfor-
mance polymeric materials: polybenzoxazines, usually obtained
by thermal ring-opening polymerization of the corresponding
benzoxazine monomers [10] provide advanced mechanical and
flame-retardant properties [11] or low-surface-free-energy materi-
als suitable for biotechnological applications [12,13].
In this context, we synthesized two novel 2,4-dihydrobenzox-
azine-type ligands (L1 and L2) (see Chart 1) and proved their reac-
tivity toward CuCl2 to form dimeric [Cu(L1)Cl2]2 and [Cu(L2)Cl2]2
copper(II) derivatives. Properly treated in anaerobic conditions,
All reactions were carried out under purified nitrogen using stan-
dard Schlenk techniques. The solvents were dried and distilled
according to standard procedures prior to use. Anhydrous CuCl2,
2-quinolinecarboxaldehyde, 2-amino-benzyl alcohol, 2-amino-5-
chloro-benzyl alcohol, triphenylphosphine, tributylamine and tri-
ethylamine (Aldrich) were used as purchased. Infrared spectra were
recorded on a Shimadzu Prestige 21 FTIR, NMR spectra were ac-
quired on a Bruker 400 Avance instrument, elemental analyses were
obtained with a Perkin–Elmer CHN Analyser 2400 Series II. Gas (H2)
detection was performed with a VG2 Quadrupolar Mass Spec (0–
200 amu) equipped with a SEM detector. Mass Spectra (FAB+) were
recorded on a Thermo-Finnigan LCQ Advantage (APCI) model.
Thermogravimetric analyses were recorded on a Netzsch STA
409 PC instrument, at a heating rate of 10 K minꢀ1 in the presence
* Corresponding author. Tel.: +39 0312386470; fax: +39 0312386449.
E-mail address: attilio.ardizzoia@uninsubria.it (G.A. Ardizzoia).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.10.016

G.A. Ardizzoia et al. / Inorganica Chimica Acta 363 (2010) 324–329
325
vacuum. 1a: Yield 85%. Anal. Calc. for C34H28Cl4Cu2N4O2: C 51.46,
H 3.56, N 7.06. Found: C 51.79, H 3.83, N 7.13%. 1b: Yield 91%. Anal.
Calc. for C34H26Cl6Cu2N4O2: C 47.35, H 3.04, N 6.50. Found: C 47.06,
H 3.21, N 6.31%.
2.5. Synthesis of [Cu(L1-ox)Cl], 2a
A suspension of [Cu(L1)Cl2]2 (200 mg, 0.252 mmol) in acetoni-
trile (10 ml) kept under nitrogen, was treated with Et3N (175 ll,
1.26 mmol) and stirred at room temperature for 2 h. Then it was
filtered and the solid was washed with diethylether. Yield 46%.
Anal. Calc. for C17H12ClCuN2O: C 56.83, H 3.37, N 7.80. Found: C
56.96, H 3.41, N 7.86%. IR (nujol)
m
/cmꢀ1 1594 (C@N). 1H NMR
Chart 1. Molecular structure of 2,4-dihydro-1H-benzo[d][1,3]oxazine (a), 4H-
benzo[d][1,3]oxazine (b) and ligands L1 and L2 (c).
(400 MHz, (CD3)2CO): d 6.03 (2H, s, CH2O), 7.08–8.53 (10H, CHaryl).
Pale brown single crystals suitable for X-ray investigation were ob-
tained by slowly cooling a hot solution of species 2a in acetonitrile.
of a N2 atmosphere. High-pressure experiments were conducted
using a 100 ml PARR stainless steel autoclave reactor equipped
with an Ashcroft Duralife (3000 psi) pressure gauge and a PARR
4842 temperature controller.
2.6. Synthesis of [Cu(L2-ox)Cl], 2b
A suspension of [Cu(L2)Cl2]2 (200 mg, 0.232 mmol) in acetoni-
trile (10 ml) kept under nitrogen, was treated with Et3N (162 ll,
2.2. Synthesis of ligand L1
1.16 mmol) and stirred at room temperature for 2 h. Then it was
filtered and the solid was washed with diethylether. Yield 49%.
Anal. Calc. for C17H11Cl2CuN2O: C 51.86, H 2.82, N 7.11. Found: C
To a solution of 2-quinolinecarboxaldehyde (750 mg, 4.77 mmol)
in 25 ml of ethanol, 2-amino-benzyl alcohol (590 mg, 4.79 mmol)
was added. After addition of 2–3 drops of glacial acetic acid, the solu-
tion was refluxed for 6 h. Then it was cooled to room temperature
and the solvent was removed under reduced pressure. The crude
product was dissolved in CH2Cl2 and washed with a saturated solu-
tion of NaHCO3; the organic layer was dried over Na2SO4 and evap-
orated to dryness. The resulting white solid was used without
further purification. Yield 83%. Anal. Calc. for C17H14N2O: C 77.84,
51.40, H 3.05, N 7.01% . IR (nujol): m
/cmꢀ1 1599 (C@N). 1H NMR
(400 MHz, (CD3)2CO): d 5.63 (2H, s, CH2O), 7.34–8.58 (9H, CHaryl).
Red orange single crystals suitable for an X-ray analysis were
obtained by slow diffusion of diethylether into a solution of 2b in
chlorobenzene.
2.7. Synthesis of [Cu(L2)Cl], 3b
H 5.38, N 10.68. Found: C 77.51, H 5.32, N 10.93%. IR (nujol):
m/
To a solution of CuCl (100 mg, 1.01 mmol) in acetonitrile
(10 ml) L2 was added (300 mg, 1.01 mmol). The yellow suspension
was stirred at room temperature for 2 h. The resulting reddish
solution was reduced to small volume (3 ml) and diethylether
(10 ml) added. The solid was filtered, washed with diethylether
and dried under vacuum. Yields: 88%. Anal. Calc. for C17H13Cl2Cu-
N2O: C 51.59, H 3.31, N 7.08. Found: C 51.71, H 3.10, N 7.32%. IR
cmꢀ1 3353 (N–H). 1H NMR (400 MHz, CDCl3): d5.05 (1H, part of an
2
2
AB system, JAB 14.4 Hz), 5.29 (1H, part of an AB system, JAB
14.4 Hz), 5.84 (1H, s), 6.88 (1H, t, 3J 7.4 Hz), 6.90 (1H, d, 3J 7.9 Hz),
7.01 (1H, d, 3J 7.5 Hz), 7.17 (1H, t, 3J 7.7 Hz), 7.59 (1H, t, 3J 7.6 Hz),
7.76 (1H, d, 3J 8.4 Hz), 7.79 (1H, s), 7.86 (1H, d, 3J 8.1 Hz), 8.21 (1H,
d, 3J 8.5 Hz), 8.30 (1H, d, 3J 8.5 Hz); 13C NMR (100 MHz, CDCl3):
68.9 (CH2O), 84.9 (CH), 117.9, 119.9, 119.9, 122.3, 125.4, 127.4,
128.0, 128.1, 128.6, 129.9, 130.2, 137.6, 141.7, 147.5, 156.9.
(nujol):
m
/cmꢀ1 3283 (N–H). 1H NMR (400 MHz, C6D6,): d4.66
(1H, part of an AB system, 2JAB 14.2 Hz), 4.78 (1H, part of an AB sys-
2
tem, JAB 14.8 Hz), 5.65 (1H, s), 5.82 (1H, s, br, NH), 6.34 (1H, d, 3J
8.6 Hz), 6.73 (1H, s), 7.04 (1H d, 3J 10.4 Hz), 7.47 (3H, m), 7.62
(1H, d, 3J 8.6 Hz), 7.0 (1H, t, 3J 8.3 Hz), 8.31 (1H, d, 3J 7.5 Hz).
2.3. Synthesis of ligand L2
To a solution of 2-quinolinecarboxaldehyde (600 mg, 3.82 mmol)
in 25 ml of ethanol, 2-amino-5-chloro-benzyl alcohol (610 mg,
3.87 mmol) was added. After addition of glacial acetic acid (2–3
drops), a white solid formed and the suspension was stirred at room
temperature for 6 h. Then the solid was filtered off and washed with
pentane. Yield 85%. Anal. Calc. for C17H13ClN2O: C 68.81, H 4.42, N
2.8. Synthesis of [Cu(L2)(PPh3)Cl], 4b
A suspension of 3b (100 mg, 0.253 mmol) in CH2Cl2 (10 ml) was
treated with triphenylphosphine (66 mg, 0.252 mmol) and quickly
turned into a red solution. After 1 h stirring the solvent was re-
moved under reduced pressure and the residue treated with dieth-
ylether. After filtration, a red–orange solid was isolated. Yields:
84%. Anal. Calc. for C35H28Cl2CuN2OP: C 63.88, H 4.29, N 4.26.
9.44. Found: C 68.74, H 4.31, N 9.38%. IR (nujol): m
/cmꢀ1 3325 (N–
H). 1H NMR (400 MHz, CDCl3): d4.99 (1H, part of an AB system, 2JAB
14.6 Hz), 5.23 (1H, part of an AB system, 2JAB 14.6 Hz), 5.85 (1H, s),
6.83 (1H, d, 3J 8.5 Hz), 6.99 (1H, s), 7.11 (1H dd, 3J 6.6 Hz, 4J 1.9 Hz),
7.60 (1H, t, 3J 7.5 Hz), 7.73 (1H, d, 3J 8.5 Hz), 7.77 (1H, t, 3J 7.7 Hz),
7.86 (1H, d, 3J 8.1 Hz), 8.17 (1H, d, 3J 8.5 Hz), 8.26 (1H, d, 3J 8.5 Hz).
13C NMR (100 MHz, CDCl3): d 68.4 (CH2O), 84.6 (CH), 119.0, 119.8,
123.6, 124.7, 125.2, 127.5, 128.0, 128.1, 128.6, 129.8, 130.3, 137.7,
140.2, 147.4, 156.3.
Found: C 63.97, H 4.54, N 4.05%. IR (nujol):
m
/cmꢀ1 3283 (N–H).
1H NMR (400 MHz, CD2Cl2): d4.78 (1H, part of an AB system, JAB
2
2
14.8 Hz), 4.82 (1H, s, N–H), 5.12 (1H, part of an AB system, JAB
14.7 Hz), 5.80 (1H, s), 6.34–8.52 (24H, CHaryl). 31P NMR
(162 MHz, CD2Cl2): d – 2.72 (s).
2.9. Synthesis of [Cu(L2-ox)(PPh3)Cl], 5b
2.4. Synthesis of [Cu(L1)Cl2]2, 1a and [Cu(L2)Cl2]2, 1b
To a suspension of 2b (150 mg, 0.38 mmol) in toluene (10 ml),
triphenylphosphine (110 mg, 0.42 mmol) was added. The suspen-
sion was gently refluxed for 3 h, and then it was cooled to room
temperature. The red solid was filtered off and washed with dieth-
ylether. Yield 76%. Anal. Calc. for C35H26Cl2CuN2OP: C 64.08, H 3.99,
Anhydrous CuCl2 (250 mg, 1.86 mmol) was added to a solution
of the appropriate ligand (1.94 mmol) in methanol (10 ml). The
yellow suspension was stirred for 3 h at room temperature. Then
the solid was filtered, washed with diethylether and dried under

326
G.A. Ardizzoia et al. / Inorganica Chimica Acta 363 (2010) 324–329
N 4.27. Found: C 63.75, H 4.00, N 4.24%. IR (nujol): m
/cmꢀ1 1599
(C@N), 1084 (PPh3). 1H NMR (400 MHz, CD2Cl2): d 5.45 (2H, s),
7.07–8.39 (24H, br, CHaryl). 31P NMR (162 MHz, CD2Cl2): d – 3.02
(s).
2.10. Synthesis of [Cu(L2-ox)(CO)Cl], 6b
In a steel system, 200 mg (0.51 mmol) of 2b were suspended in
ethanol (20 ml) and kept under 50 atmosphere of CO, at 80 °C, for
6 h. Then, the system was slowly cooled to room temperature, and
opened to air. Yellow crystals of 6b were collected by removing the
solvent, washed with EtOH (2 ml) and dried in a stream of carbon
monoxide. Yield 68%. Anal. Calc. for C18H11Cl2CuN2O2: C 51.26, H
Scheme 1. Synthesis of ligands L1 (X = H) and L2 (X = Cl)
2.63, N 6.64. Found: C 51.34, H 2.65, N 6.57%. IR (nujol): m
/cmꢀ1
patterns of an AB system centered at 5.17 ppm (L1) and 5.11 ppm
(L2) were attributed to the CH2OH group, while the singlets at
5.84 ppm (L1) and 5.85 ppm (L2) were clearly assigned to the CH
in position 2 of the oxazine moiety. In the 13C NMR (CDCl3), the
presence of only two resonances in the aliphatic region, attributed
to the CH and CH2 carbon atoms of the 2,4-dihydrobenzoxazine
moiety, confirmed the proposed structure. Moreover, the infrared
spectrum, showed the sharp stretching at about 3300 cmꢀ1 due
to the N–H proton.
Treatment of a solution of anhydrous CuCl2 in methanol, kept
under inert atmosphere (N2 or Ar), with L1 or L2 (1:1 molar ratio),
afforded a yellow suspension, from which it was possible to isolate
the corresponding Cu(II) complexes, analyzing as [Cu(L1)Cl2] and
[Cu(L2)Cl2], respectively. Indeed, the sharp stretching at about
3300 cmꢀ1 detected in the IR spectrum of the free ligands, turned
into a broad band in the region 3100–3300 cmꢀ1, thus suggesting
that, in the solid, inter-molecular hydrogen-bonds are probably
present. A fab(+) mass spectra finally revealed the dimeric nature
of these derivatives (Figs. S1–S3, Supplementary material) hence
formulated as [Cu(L1)Cl2]2, 1a, and [Cu(L2)Cl2]2, 1b.
2059 (CO), 1609 (C@N). TGA: loss of CO molecule at 158 °C.
2.11. Determination of the stoichiometry of the oxidative
dehydrogenation
PPh3 (31 mg, 0.12 mmol) was added to
[Cu(L2)Cl2]2 (1b) (100 mg, 0.12 mmol) in acetonitrile (8 ml) kept
under nitrogen. The suspension was treated with Et3N (83.5 l,
a suspension of
l
0.60 mmol) and stirred at room temperature for 2 h. The solid
was then filtered and washed with diethylether. Yields of 2b:
49% (22.4 mg). The mother liquors were reduced to half the volume
and diethylether (10 ml) was added causing the precipitation of a
red–orange solid. This was filtered, washed with diethylether and
dried in vacuum. Yields (4b): 48.5% (37 mg).
In a different experiment, 100 mg of [Cu(L2)Cl2]2 (1b) in 10 ml
of CH3CN were treated with Et3N (83.5 ll, 0.60 mmol). After re-
moval of the solid 2b, the bulk was analyzed via infrared spectros-
copy, revealing the presence of Et3NHCl (vN–H at 2602 and
2496 cm-1, broad). Moreover, quantitative analysis of the residue
via 1H NMR (CDCl3), using hexamethylbenzene as internal stan-
dard, allowed to determine a 1:2 molar ratio between the starting
dimer and Et3NHCl.
3.2. Oxidative dehydrogenation of L1 and L2
2.12. X-ray crystallography
The addition of a base (Et3N or Bu3N) to yellow suspensions of
1a or 1b (CH3CN) resulted in the formation of a pale brown, air sta-
ble, solid. On the basis of analytical and spectroscopic data, the
resulting complexes were formulated as [Cu(L1-ox)Cl], 2a, and
[Cu(L2-ox)Cl], 2b. That is, the base promoted the oxidative
dehydrogenation of ligands L1 or L2 to the corresponding 4H-
benzo[d][1,3]oxazine (L1-ox or L2-ox). This reaction is well docu-
mented for amines or alcohols coordinated to a transition metal
[17]. In previously reported reactions of this type [18] the substrate
was converted into an imine or a ketone, via electron transfer
involving the metal which, at the end of the process, was recovered
in its initial oxidation state. In such a mechanism, the presence of
an exogenous oxidizing agent (generally O2) is essential [19]. In
contrast, in our specific case, the dehydrogenation reaction took
place in anaerobic conditions, with concomitant Cu(II) reduction.
Actually, when a suspension of 2 in acetonitrile (or methanol)
was refluxed in the presence of water and/or molecular oxygen,
no reaction took place, thus proving the decisive role of the base
in promoting the oxidative dehydrogenation of the ligands. More-
The single crystal X-ray data for 2b and 6b were collected on an
Enraf Nonius CAD-4 automated diffractometer using graphite-
monochromated Mo Ka radiation (k = 0.71073 Å). The unit cells
were determined on the basis of the setting angles of 25 randomly
distributed reflections in the 10 < h < 15° range. The data collec-
tions were performed in the 3.0 < h < 25.3° range by applying the
x
-scan mode [Dx = 1.1 + (0.35tanh)]. The data were corrected for
absorption and Lorenz-polarization effects. The structure was
solved by direct methods [14] and refined by full-matrix least-
squares on F2 [15]. All the non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were made riding their parent atoms
with an isotropic temperature factor 1.2 times that of their parent
atoms.
3. Results and discussion
3.1. Synthesis of ligands L1 and L2 and of complexes [Cu(L)Cl2]2
over, the presence of Et3NHCl in the bulk as a side product (vN–H
:
As reported by Holly and Cope [16] the synthesis of benzox-
azine-type ligands from the condensation of 2-quinolinecarboxal-
dehyde with 2-amino-benzyl alcohols is known to proceed via
the formation of the intermediate imine species, which undergoes
nucleophilic attack on the sp2 carbon from the adjacent OH group
(see Scheme 1). After hydrogen shift, 2,4-dihydro-benzoxazine
moieties (L) are produced, as established by analytical and spectro-
scopic data. In their 1H NMR spectra (CDCl3), the typical roof
2602 and 2496 cmꢀ1, broad), and the absence of any N–H vibration
in the IR spectrum of 2, concurred to prove the deprotonation of L.
Finally, in the 1H NMR (acetone-d6) of 2, a single resonance in the
aliphatic region at 6.03 ppm (2a) and 5.63 ppm (2b) was detected
and easily related to the CH2O moiety. Actually, in the resulting
complexes the ligand shows a conformation close to planarity
around the metal center, thus making the former AB system col-
lapsing into a singlet.

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327
3.3. Crystal structure of complex [Cu(L2-ox)Cl]
mechanism illustrated in Scheme 2. In this hypothesis, one ligand
of the dimer is oxidized to the 4H-benzo[d][1,3]oxazine form,
while the residuary (reduced) ligand is found (in the mother li-
quors) weakly coordinated to the second copper(I) center (3b).
Several attempts to isolate this latter species in a pure form
failed, unidentified species (probably CuCl/L2 mixtures) being al-
ways obtained. Nevertheless, a derivative analyzing as [Cu(L2)Cl]
was obtained by reacting CuCl with one equivalent of ligand L2
in acetonitrile (see Section 2, compound 3b). However, as a proof
of the nominal presence of CuCl in solution, gaseous NH3 was bub-
bled in the reaction medium after removing 2b by filtration: the
instantaneous precipitation of white [Cu(NH3)2]Cl occurred, thus
confirming the presence of copper(I) species in solution (bubbling
gaseous NH3 through suspensions of 3b in methanol does not
cause any apparent reaction). Moreover, in another experiment,
the addition of an excess of PPh3 to the reaction medium allowed
to quantitatively collect [Cu(PPh3)3Cl] (as established by elemental
analysis and 31P NMR data) [26].
On the other hand, a stoichiometric amount of PPh3 added to the
reaction medium allowed the isolation of [Cu(L2)(PPh3)Cl], charac-
terized by 1H, 31P NMR and elemental analysis. In fact, after having
removed 2b by filtration and reduced the solvent to half the vol-
ume, an orange solid can be isolated. In the 1H NMR (CD2Cl2, RT)
the compound showed the characteristic roof pattern centered at
4.95 ppm (JAB = 14.8 Hz) related to the CH2 moiety of ligand L2, to-
gether with the singlet at 5.80 ppm (CH between the two heteroat-
oms). In the 31P NMR, a resonance at ꢀ2.71 ppm was easily
attributed to the bound PPh3. These values were in total agreement
with those obtained for the species [Cu(L2)(PPh3)Cl] prepared in a
pure form directly from 3b. Indeed, treatment of a suspension of
[Cu(L2)Cl] (3b) with one equivalent of PPh3, in CH2Cl2 at room tem-
perature, resulted in the formation of a deep red solution, from
which it was possible to isolate a red-to-orange solid analyzing
as [Cu(L2)(PPh3)Cl] (4b) and showing identical spectral features
(IR, 1H, 31P NMR) as the aforementioned species obtained by indi-
rect synthesis.
X-ray structural characterization of species 2a and 2b prove
their formulation. The room temperature single crystal X-ray anal-
ysis on 2a disclosed its metrics [orthorhombic, Pcab, a = 9.047(2),
b = 14.598(5), c = 22.070(11) Å, V = 2915(2) Å3] and its main struc-
tural features, thus allowing to describe it as a monomeric complex
of [Cu(L1-ox)Cl] formula, displaying a tri-coordinated Cu(I) metal
center. However, its crystal structure is on the whole of insufficient
quality, due to the disorder affecting the L1-ox ligand. Indeed, the
disorder makes it impossible to distinguish the aromatic from
the aliphatic nitrogen containing rings. The presence of a chlorine
substituent on one of the ‘external’ rings of L2-ox eliminates this
problem. For this reason and seeing that L1 and L2 show the same
chemical behavior, we have focalize the present study on copper
derivatives of L2.
ꢀ
Indeed, 2b crystallizes in the triclinic P1 space group and con-
sists of [Cu(L2-ox)Cl] mononuclear complexes in which the Cu(I)
metal center, possessing an almost planar trigonal stereochemis-
try, is tris-coordinated by one chlorine atom and the nitrogen
atoms of a chelating L2-ox ligand (see Fig. 1). Mononuclear three-
coordinate complexes of transition metals are rather uncommon
[20] and generally very bulky ligands are required to stabilize these
electronically unsaturated species [21]. On the other hand, some
benzo[d][1,3]oxazine-complexes have already been reported for
palladium(II) and platinum(II), with [22] or without [23] a struc-
tural characterization. A few rhodium(II) species are also known
[24,25]. Thus, to the best of our knowledge, complex 2b represent
the first example of structurally characterized copper(I)
benzo[1,3]oxazine complexes.
3.4. Mechanistic investigation on the oxidative dehydrogenation
The mechanism of generation of species 2a and 2b considerably
differs from that commonly accepted for oxidative dehydrogena-
tion. The loss of two hydrogen atoms is usually associated either
to consecutive one-electron oxidations via the formation of free-
radical intermediates, or to a two-electron transfer from the ligand
to the metal usually involving a metal-hydride [17]. Yet, in our
case, the reaction of 1b with Et3N, in the presence of a radical scav-
enger as 2,2,6,6-tetramethylpiperidinyl-1-oxy (tempo), again
afforded complex 2b. Thus, any pathway implying the generation
of free-radicals was plausibly excluded. On the other hand, neither
the presence of transient hydride species leading to the formation
of H2 was admissible, as no gas evolution was registered in the bulk
at a 10ꢀ9 ppm scale detection level via mass spectrometry.
The aforementioned evidences, joined to a mindful analysis of
the yields of products in the reaction of [Cu(L2)Cl2]2 with PPh3 in
the presence of a base (see Experimental), suggest the strictness
of the postulated stoichiometry for the oxidative dehydrogenation
of ligand L2 bound to Cu(II) in 1b.
3.5. Reactivity of complex [Cu(L2-ox)Cl]
[Cu(L2-ox)Cl], even though a Cu(I) species, proved to be surpris-
ingly stable towards oxidation. Actually, treatment of a suspension
of 2b in ethanol or dichloromethane under oxygen (1 atm), did not
provide any oxidation product. Even when higher temperatures
and pressures in ethanol or toluene (namely, at 80 °C and
50 atm) were applied, no reaction with oxygen occurred.
We therefore postulated a concomitant reduction of the two
copper(II) centers of the dinuclear species 1b, most likely favored
by the bridging chloride ligands, and proposed the stoichiometric
On the contrary, when 2b was refluxed in toluene with a stoi-
chiometric amount of PPh3, a red suspension formed, from which
it was possible to recover a brick-red compound, later formulated
as [Cu(L2-ox)(PPh3)Cl], 5b. The infrared spectrum of the solid
showed the PPh3 stretching at 1084 cmꢀ1, together with the ligand
fingerprint, while the single resonance at ꢀ3.02 ppm in the 31P
NMR spectrum (CD2Cl2, RT) was clearly attributed to the triphenyl-
phosphine coordinated to the copper(I) center.
The propensity shown by [Cu(L2-ox)Cl] to bind PPh3 suggested
its affinity to p-acceptor ligands, and prompted us to react it with
carbon monoxide. Because of the limited affinity of Group 11 met-
als to carbon monoxide, much less examples of structurally charac-
terized copper-carbonyl species have been reported in the
literature than for other late transition metals [27]. In this context,
we treated a suspension of 2b in ethanol with CO (50 atm), at
80 °C, for 6 h. This afforded the novel compound [Cu(L2-ox)(CO)Cl],
Fig. 1. Ortep drawing, at 30% probability level, of the monomeric [Cu(L2-ox)Cl]
complex, 2b. Significant bond distances and angles at the metal center: Cu–Cl
2.099(2) Å; Cu–N 2.053(3), 2.055(4) Å; Cl–Cu–N 138.3(1), 140.6(1)°; N–Cu–N
81.0(1)°.

328
G.A. Ardizzoia et al. / Inorganica Chimica Acta 363 (2010) 324–329
Scheme 2. Stoichiometry for the reaction of [Cu(L2)Cl2]2, 1b, in the presence of a base (the reciprocal disposition of ligands L2 in 1b is arbitrary).
Table 1
6b. Indeed, when the reaction between 2b and CO was performed
at room temperature (in EtOH or CH2Cl2), either by bubbling CO in
the bulk or under higher pressure (50 atm), no carbonylation prod-
ucts were detected. The intense, sharp absorption at 2059 cmꢀ1 no-
ticed in the IR spectrum of solid 6b was in agreement with those
observed in previously reported tetrahedral copper(I)–carbonyl
complexes [28]. The stability of species 6b in the solid was con-
firmed by TGA (see Fig. S4, Supporting information), where the loss
of CO occurred only at 158 °C (complex 2b being quantitatively re-
stored, as established by spectroscopic and analytical data).
Crystallographic and Structural Refinement Data for [Cu(L2-ox)Cl], (2b) and [Cu(L2-
ox)(CO)Cl] (6b).
[Cu(L2-ox)Cl], (2b)
[Cu(L2-ox)(CO)Cl] (6b)
Molecular formula
FW
Crystal system
Space group
a (Å)
C17H11Cl2CuN2O
393.74
C18H11Cl2CuN2O2
421.75
monoclinic
P21/n
13.274(4)
8.169(1)
15.609(3)
90.00
92.26(2)
90.00
1691.3(6)
4
1.656
1.621
298(2)
triclinic
ꢀ
P1
7.346(5)
9.636(3)
11.234(4)
84.75(3)
84.37(6)
78.37(6)
773.0(7)
2
1.691
1.762
298(2)
396
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
3.6. Crystal structure of complex [Cu(L2-ox)(CO)Cl]
V (Å3)
Z
Density (g cmꢀ3
)
A single crystal X-ray analysis revealed that 6b crystallizes in
the monoclinic P21/n space group and contains monomeric com-
plexes of [Cu(L2-ox)(CO)Cl] formula. The metal center, displaying
a tetrahedral stereochemistry, is coordinated by one chlorine atom,
the carbon atom of a carbonyl moiety and the nitrogen atoms of
one L2-ox ligand, showing N,N0-endobidentate coordination mode
(see Fig. 2). Notably, 6b easily loses CO at room temperature in
dichloromethane, thus suggesting that the stability of solid 6b
l
(Mo K
a
) (mmꢀ1
)
T (K)
F(0 0 0)
848
1.060
0.031, 0.075
0.041, 0.080
Goodness-of-fit (GOF) on (F2)
1.124
0.050, 0.109
0.076, 0.120
R1, wR2 [I > 2
R1, wR2 (all data)
r
(I)]
must be related to the (not easily interpretable) crystal lattice sta-
bilization (shortest CHꢁꢁꢁO distance 3.38 Å) (Table 1).
4. Conclusions
We have described the synthesis of two novel 2,4-dihydro-
1H-benzo[d][1,3]oxazine type ligands (L1, L2) and of their cop-
per(II) complexes. Treatment of the latter with a base promoted
the oxidative dehydrogenation of the ligands, yielding the corre-
sponding copper(I) [Cu(L-ox)Cl] species (L-ox = 4H-benzo[d]
[1,3]oxazine), with a concomitant reduction of CuCl2 to CuCl.
[Cu(L2-ox)Cl] proved to be unreactive towards oxidation, even
when treated with high-pressure molecular oxygen. On the con-
trary, it readily reacted with
p-acceptor molecules like PPh3 and
Fig. 2. Ortep drawing, at 30% probability level, of the monomeric [Cu(L2-ox)(CO)Cl]
complex, 6b. Significant bond distances and angles at the metal center: Cu–C
1.808(3) Å; Cu–Cl 2.2734(9) Å; Cu–N 2.084(2), 2.089(2) Å; C–Cu–Cl 114.08(9)°; C–
Cu–N 122.0(1), 124.5(1)°; Cl–Cu–N 104.49(7), 107.82(6)°; N–Cu–N 79.27(8)°.
CO, providing the corresponding copper(I) compounds, [Cu(L2-
ox)(PPh3)Cl] and the remarkably stable (in the solid) [Cu(L2-
ox)(CO)Cl].

G.A. Ardizzoia et al. / Inorganica Chimica Acta 363 (2010) 324–329
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Acknowledgment
[12] C.-F. Wang, Y.-C. Su, S.-W. Kuo, C.-F. Huang, Y.-C. Sheen, F.-C. Chang, Angew.
Chem. 118 (2006) 2306.
This work was supported by MIUR (Ministero Istruzione, Uni-
versità e Ricerca).
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Appendix A. Supplementary material
CCDC 695144 and 695145 contain the supplementary crystallo-
graphic data for 2b and 6b. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2009.10.016.
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