Synthesis, characterization and crystal structure of diamagnetic tetracoordinated chromium (II) complexes supported by α-diimine ligands

Article abstract of DOI:10.1016/j.inoche.2006.03.011

The α-diimine ligands Ar{single bond}N{double bond, long}C(R){single bond}C(R){double bond, long}N{single bond}Ar (ai, bi, c) react with CrCl₃(thf)₃ 1 in the presence of zinc to give new diamagnetic tetrahedral chromium (II) complexes supported by α-diimine ligands 2ai (i = 1-2), 2bi (i = 1-2) and 2c in good yields. All compounds were characterized by NMR and IR spectroscopy. The solid-state structure of 2b2, which crystallized in the Orthorhombic space group P2₁2₁2₁ with unit cell parameters a = 11.825(5) A?, b = 12.034(4) A? and c = 16.393(8) A?, was established by X-ray crystallography.

Full text of DOI:10.1016/j.inoche.2006.03.011

Inorganic Chemistry Communications 9 (2006) 1023–1025
www.elsevier.com/locate/inoche
Synthesis, characterization and crystal structure of diamagnetic
tetracoordinated chromium (II) complexes supported by
a-diimine ligands
a
b
a,
*
Thouraya Turki , Taha Guerfel , Faouzi Bouachir
a
Laboratoire de Chimie de Coordination, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia
b
Laboratoire de Chimie de Solide, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia
Received 15 December 2005; accepted 8 March 2006
Available online 24 March 2006
Abstract
The a-diimine ligands ArAN@C(R)AC(R)@NAAr (ai, bi, c) react with CrCl3(thf)3 1 in the presence of zinc to give new diamagnetic
tetrahedral chromium (II) complexes supported by a-diimine ligands 2ai (i = 1–2), 2bi (i = 1–2) and 2c in good yields. All compounds
were characterized by NMR and IR spectroscopy. The solid-state structure of 2b2, which crystallized in the Orthorhombic space group
˚
˚
˚
P212121 with unit cell parameters a = 11.825(5) A, b = 12.034(4) A and c = 16.393(8) A, was established by X-ray crystallography.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Diazadiene ligands; Chemical reduction; Chromium (II) complexes
The coordination chemistry of 1,4-diazabutadiene (dab)
ligands, bis (arylimino)acenaphtene (Ar-BIAN) ligands, bis
(phenylimino)camphene (Ph-BIC) and their complexes [1–
8] has attracted much interest due to their unusual electron
donor and acceptor properties [9,10]. Their complexes have
been reported essentially for late transition metals. Orga-
nometallic compounds with diazadiene (dad) ligands have
been extensively studied in recent years. Group 10 com-
pounds with specially designed ligands containing a bulky
side group have found practical uses as olefin polymerisa-
tion catalysts [11–14]. Poli et al. have recently reported
about the synthesis of MoCl2(iPr2-dad)2 [15].
cyclooctene (eco) afforded high yields of the unprecedented
olefin-substituted derivatives fac-M(CO)3(iProp-dab)(g2-
eco) [18]. Recently, Poli et al. have reported the controlled
radical polymerisation of styrene with new CpMoCl2(dad)
complexes (dad:RN@CHACH@NR with R = Ph, p-Tol,
2,6-iPr2C6H3 and iPr) [19] and have presented results dem-
onstrating that CpMoCl2(iPr2-diazadiene) is an efficient
catalyst for the controlled free radical polymerisation of
methylacrylate, butylacrylate and styrene in the presence
or absence of a catalytic promoter [20].
Diazadiene complexes of the type (dad)CrCl2 have not
been known, only few analogues have been recently pre-
pared in our laboratory [(dad)MCl2, M = Mo, W]
[21,22]. Similar complexes [CrX2(LÙL) with phosphorous
ligands [LÙL = dippe:1,2-bis(di-i-propylphosphino)ethane]
have been reported [23].
Ziegler et al. have recently shown a variety of
(dad)M(CO)4 [16,17] with M = Cr, Mo,W. Photolysis of
M(CO)4(iProp-dab) [M = Mo, W; iProp-dab = 1,4-diiso-
propyl-1,4-diazadbuta-1,3-diene] in the presence of (E)-
In the present paper, we describe the preparation of
divalent chromium (II) compounds (2ai, 2bi, 2c) [24] with
bulky aryl-substituted a-diimine ligands and the structure
of the Cr(II) species as determined in the solid state by
X-ray crystallography. Despite their formal 12-electron
*
Corresponding author. Address: INSAT, Chemistry, Avenue de la
Terre, Zone Urbaine Nord BP676 Cedex, Tunis 1080, Tunisia. Tel.: +216
98 503 997; fax: +216 71 704 329.
E-mail address: bouachirf@yahoo.de (F. Bouachir).
1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.03.011

1024
T. Turki et al. / Inorganic Chemistry Communications 9 (2006) 1023–1025
counts, neither of the diazadiene complexes (2ai, 2bi, 2c)
shows any sign of agostic interactions in either the solid
state or solution.
show normal NMR spectra of diamagnetic compounds,
which has not been expected for d4 complexes shown in
1
the literature [26]. The H and 13C NMR chemical shifts
Reaction of CrCl3(thf)3 [25] with a-diimine ligand (ai, bi,
c) in the presence of zinc at ambient temperature resulted in
the formation of the Cr(II) coordination compounds (2ai,
2bi, 2c) (Scheme 1).
Compounds 2ai, 2bi and 2c crystallize as green or brown
crystals for 2ai, orange or orange brown for 2bi and yellow
for 2c from CH2Cl2/n-hexane. All these complexes are sol-
uble in methylene chloride, dimethylsulfoxide, acetone and
acetonitrile; they are air-sensitive and non-soluble in unpo-
lar solvents. These compounds have been characterized by
1H, 13C NMR and IR spectroscopy. The new complexes
(dad)CrCl2 exhibit spectroscopic data in agreement with
the proposed structures and they are low-spin, since they
of the ArAN@CRACR@NAAr subunit in the complexes
2ai, bi, c are slightly shielded in comparison to those of
the free ligands dab, Ar-BIAN and Ph-BIC similar to other
complexes [17]. The iminic carbon appears between 163
and 176 ppm. The IR spectra of complexes 2ai, 2bi and
2c are also instructive since, the mC@N stretching frequencies
are different from those of the free diazadiene ligands. It is
observed at 1590–1650 cmÀ1. The CrACl stretching vibra-
tion in the IR spectroscopy is observed at 540 cmÀ1 as a
weak bond which is consistent with known literature value
for other chromium complexes [27].
Single crystal of (o-MeAC6H4-BIAN)CrCl2 (2b2)[28]
suitable for X-ray structural study could be grown from
N
CH2Cl2/25˚C
CrCl3(thf)3
+
N
N
CrCl2
zinc
N
1
ai, bi, c
2ai, 2bi, 2c
5
10
6
7
4
3
7
9
NPh
NPh
4
3
2
N
N
R
R
2
5
=
1
1
NAr
ArN
6
NAr
Ar-BIAN[bi; i=1-2]
ArN
8
dab [ai; i=1-2]
Ph-BIC [c]
a1: Ar = 2,4,6-(CH3)3-C6H2; R = CH3
a2: Ar = 2,6-iPr2-C6H3; R = CH3
b1 : Ar = 2,6-iPr2-C6H3
b2 : Ar = o-Me-C6H4
Scheme 1.
Fig. 1. ORTEP representation of the molecular structure of complex 2b2 with 30% probability ellipsoids (Hydrogen atoms are omitteds for clarity).
˚
Selected bond lengths (A) and angles (°): CrAN1 = 2.070(8), CrAN2 = 2.074(8), CrACl1 = 2.190(3), CrACl2 = 2.197(4),N1AC12 = 1,283(12),
N2AC1 = 1.287(11), C1AC12 = 1.502(12), N1AC19 = 1.449(13), N2AC13 = 1.482(13), N1ACrAN2 = 81.2(3), Cl1ACrACl2 = 120.65(18),
N2ACrACl2 = 118.4(3), N1ACrACl1 = 111.9(2), CrAN1AC12 = 110.9(6), CrAN2AC1 = 112.4(6).

T. Turki et al. / Inorganic Chemistry Communications 9 (2006) 1023–1025
1025
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a mixture of methylene chloride and hexane solution at
ambient temperature. The molecular structure of 2b2 and
the associated atom-numbering scheme are depicted in
Fig. 1. The geometry around the chromium center in 2b2
is that of a distorted tetrahedral, where the relatively small
N1ACrAN2 bond angle of 81.2(3)° is a result of chelating
ligand steric constraints. The bond lengths CrAN1 and
˚
CrAN2 are in the order of 2.07 A, which are very close
˚
to other Cr-imino-nitrogen bonds (2.058 A) [29] and Aryl-
˚
chromium complexes (2.137 A) [30]. The N1AC12, N2AC1
and C1AC12 bond distances are equal to 1.28(12), 1.28(11)
and 1.50(12) in complex 2b2, confirming the conjugate di-
imine nature of this ligand [18].
The metal-chelate ring (CrAN2AC1AC12AN1) in com-
plex 2b2 is almost flat as indicated by the torsion angles of
À2.02 (1.20)°, 1.01 (0.95)° and 1.85 (1.02)° for N2AC1A
C12AN1, CrAN1AC12AC1 and CrAN2AC1 AC12,
respectively. This result is very similar to those of other
reported palladium acenaphtene complexes [31]. The o-
methylphenyl groups make an angle of approximately 90°
to the plane of the C@N bonds, due to the presence of
the o-methyl substituents as appears from the torsion
angles of 99.66 (1.09)°, À86.90 (1.22)°, 91.58 (1.10)° and
À94.64 (1.05)° for C1AN2AC13AC14, C1AN2AC13A
C18, C12AN1AC19AC24 and C12AN1A C19AC20,
respectively. These torsion angles are comparable to those
measured for nickel complexes [8].
[18] F.W. Grevels, K. Kerpen, W.E. Klotzbucher, K. Schaffner, Organo-
metallics 20 (2001) 4775–4792.
[19] F. Stoffelbach, R. Poli, P. Richard, J. Organomet. Chem. 663 (2002)
269–276.
[20] F. Stoffelbach, D.M. Haddleton, R. Poli, Eur. Poly. J. 39 (2003)
2099–2105.
[21] T. Turki, T. Guerfel, F. Bouachir, Polyhedron, in press.
[22] T. Turki, T. Guerfel, F. Bouachir, J. Organomet. Chem. in press.
[23] A.R. Hermes, R.J. Morris, G.S. Girolami, Organometallics 7 (1988)
2372.
[24] The diazadiene ligands: dab [32], Ar-BIAN, Ph-BIC [31], and
CrCl3(thf)3 [25] were obtained according to the literature proce-
dures. General method: In an inert atmosphere, CrCl3(thf)3 (1 equiv.)
was added to diazadiene ligand (3 equiv.) in 20 ml of anhydrous
CH2Cl2. Zinc powder was added (5 equiv.) as a reducing agent. The
product was precipitated by addition of hexane. Pure compound
was isolated as a solid by crystallization from methylene chloride/
hexane solution at ambient temperature. Yields ranged from 73 to
83%. Preparation of 2a2: Complex 2a2 was obtained in 74% yield by
the same procedure as a green solid. Decomposition = 345 °C; IR
In conclusion, a straightforward synthetic procedure for
chromium (II) complexes with a-diimine ligands was
described. The application of these complexes in catalytic
reactions such as polymerisation and copolymerisation of
functional alkenes (methyl acrylate, styrene) is in progress.
(mC@N = 1643; 1599 cmÀ1
; m
CrACl = 537 cmÀ1). Anal. Calc. for
C28H40N2CrCl2 (527): C, 63.75; H, 7.59; N, 5.31. Found: C, 64.05;
H, 7.62; N, 5.08. 1H NMR (300 MHz, CD2Cl2, 25 °C, d[ppm]) :
d = 1.40 À 1.05 (2d, 24H, JHH = 6.9 Hz, CHMe2) ; 2.25 (s, 6H, Me
on ligand); 2.82 (sept., 4H, CHACH3); 7.15–7.28 (m, 6H, Har on
ligand). 13C NMR( 75.5 MHz, CD2Cl2, 25 °C, d[ppm]): d = 19.45
(Me on ligand); 22.17 (Me on iPr group); 22.38 (Me on iPr group);
27.98 (CH on iPr group); 123.93 (Cmeta); 127.36 (Cpara);138.34
(Cipso); 169.25 (C@N).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.inoche.2006.03.011.
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[27] M.E. Jacox, D.E. Milligian, J. Chem. Phys. 51 (1969) 4143.
[28] Crystallographic data for 2b2: C26H20Cl2N2Cr; f.w. = 483.34;
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