Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
1038
Can. J. Chem. Vol. 83, 2005
Fig. 1. Amide proligand set [R ′(NO)R]H.
Fig. 2. ORTEP depiction of the solid-state molecular structure of
[
DMP(NO)Ph]2Hf(CH2Ph)2(THF) (2a) (the THF ring at O3 has
been omitted for clarity).
Results and discussion
Easily assembled modular ligand systems are important in
expediting the discovery of catalyst systems for various or-
ganic transformations (5). Organic amides offer an easily
modified scaffold for the study of ligand-based effects in
their resultant transition metal complexes. A general syn-
thetic scheme for the preparation of organic amides is shown
in eq. [1]. Elimination of HCl upon reaction of an acid chlo-
ride and a primary amine results in the formation of the
desired amide product in high yields (75%–95% yield). Puri-
fication of these proligands for use in metal chemistry can
be accomplished by vacuum sublimation or recrystallization
from an appropriate solvent. Proligand 1 (N-2,6-dimethyl-
phenyl(phenyl)amide) was synthesized in excellent yield.
Purification via recrystallization from CHCl3 at –20 °C and
protonolysis of the Hf—C bonds proceeds very rapidly and
exothermically because of the low pKa of the organic amide
proligands.
Purification of this complex in moderate yield can be
achieved by recrystallization from a concentrated solution of
2a in hexanes, giving orange single crystals. X-ray crystallo-
graphic analysis of this complex revealed that the
bis(amidate)–dibenzyl species is monosolvated by THF in
the solid state, as shown in Fig. 2. Selected bond lengths and
angles are found in Table 1 and crystallographic details are
located in Table 2.
[1]
subsequent drying while heating under vacuum overnight
gave pure proligand 1 in 64% yield.
The solid-state structure of 2a exhibits pseudo-C2v sym-
metry in a slightly distorted pentagonal bipyramidal geome-
try. This coordination geometry has been previously
observed for seven-coordinate salen complexes of Hf (8),
acetylacetonato complexes of Zr (9), and structurally related
monothiocarbamate and dithiocarbamate complexes of Ti
(10, 11). Both amidate ligands in 2a are found in the equato-
rial plane, with the O atoms nearly trans to each other at
156°. A bound THF molecule occupies the final equatorial
coordination site. The plane containing Hf and the N and O
donor atoms is nearly perfectly planar, with the sum of the
bond angles about the Hf center being 360.2°. In the axial
positions, the two benzyl groups are located 169.8° apart. As
expected, the Hf1—O3 bond length of the coordinated THF
(2.236 Å) is significantly longer than the Hf1—O1 and
Hf1—O2 bond lengths (2.161 and 2.174 Å, respectively).
Bond lengths between the amidate N atoms and the Hf cen-
ter are somewhat longer than the corresponding Hf –
amidate O bond lengths (Hf1—N1 = 2.265 Å, Hf1—N2 =
2.258 Å), indicating that the amidate ligands are bound in an
alkoxy-imine fashion (2). Unexpected elongation of the C—
N bond in the ligand backbone is also observed; however,
the steric bulk at the N of the amidate ligands precludes the
formation of the anticipated shorter C—N bond. This ligand
binding motif has been seen previously by our group for
bis(amidate)-bis(amido) complexes of Ti and Zr (2, 4). There
are some variations from expected bond lengths, which may
be due to the unique π-stacking interactions present between
the coplanar N-aryl rings of the amidate ligands, whose cen-
troids are separated by approximately 3.6 Å. Intramolecular
Protonolysis reactions have been utilized successfully in
the synthesis of amidinate (6) and guanidinate (7) complexes
of the Group 4 metals. Given the structural similarity of the
amidate ligand motif, protonolysis was anticipated to be an
efficient route into analogous amidate complexes. The pro-
tonolysis methodology used to form this bis(amidate)–
bis(alkyl) Group 4 metal complex (2a) was facile and high
yielding (eq. [2]). Freshly prepared and recrystallized
Hf(CH2Ph)4 was combined with 2 equiv. of proligand 1
([DMP(NO)Ph]H) in a darkened Schlenk flask, which was
cooled to –78 °C before introduction of THF via cannula
transfer. Warming to room temperature resulted in the for-
[2]
mation of a bright red-orange solution, which was concen-
trated in vacuo to give a bright orange powder in 98% crude
yield. Low temperatures utilized during this process were
necessary to control the rate of reaction, as reaction at room
temperature results in the formation of the desired product
contaminated with a homoleptic side-product. In addition,
the starting material (Hf(CH2Ph)4) is well-known to exhibit
a high sensitivity towards ambient visible light, and the
© 2005 NRC Canada