Organoaluminium complexes incorporating an amido phosphine chelate with a pendant amine arm

Article abstract of DOI:10.1039/b502873f

The palladium-catalyzed aryl animation of l-bromo-2-fluorobenzene with N,N-dimethylethylenediamine quantitatively produces N-(dimethylaminoethyl)-2- fluoroaniline, which subsequently reacts with KPPh₂ in 1,4-dioxane to afford N-(dimethylaminoethyl)-2-diphenylphosphinoaniline (H[PNN]). The reactions of trialkylaluminium with H[PNN] in toluene generate the corresponding aluminium dialkyl complexes [PNN]A1R₂ (R = Me, Et, i-Bu). The solution NMR spectroscopic and X-ray crystallographic studies are indicative of a trigonal bipyramidal geometry for these aluminium complexes in which the amino nitrogen atom is trans to the phosphorus donor of the [PNN]^- ligand. This study presents rare examples of structurally characterized, five-coordinate aluminium hydrocarbyl complexes supported by phosphine-derived ligands. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502873f

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F U L L P A P E R
Organoaluminium complexes incorporating an amido phosphine
chelate with a pendant amine arm†
Wei-Ying Lee and Lan-Chang Liang*
Department of Chemistry and Center for Nanoscience & Nanotechnology, National Sun Yat-sen
University, Kaohsiung, 80424, Taiwan. E-mail: lcliang@mail.nsysu.edu.tw;
Fax: +886-7-5253908
Received 25th February 2005, Accepted 19th April 2005
First published as an Advance Article on the web 29th April 2005
The palladium-catalyzed aryl amination of 1-bromo-2-fluorobenzene with N,N-dimethylethylenediamine
quantitatively produces N-(dimethylaminoethyl)-2-fluoroaniline, which subsequently reacts with KPPh2 in
1,4-dioxane to afford N-(dimethylaminoethyl)-2-diphenylphosphinoaniline (H[PNN]). The reactions of
trialkylaluminium with H[PNN] in toluene generate the corresponding aluminium dialkyl complexes [PNN]AlR2
(R = Me, Et, i-Bu). The solution NMR spectroscopic and X-ray crystallographic studies are indicative of a trigonal
bipyramidal geometry for these aluminium complexes in which the amino nitrogen atom is trans to the phosphorus
donor of the [PNN]− ligand. This study presents rare examples of structurally characterized, five-coordinate
aluminium hydrocarbyl complexes supported by phosphine-derived ligands.
All solvents were reagent grade or better and purified by stan-
Introduction
dard methods. The trialkylaluminium complexes were obtained
There has been considerable interest in recent years in the
in solutions from Tokyo Chemical Industry (TCI) company.
search for appropriate ancillary ligands for the preparation
All other chemicals were obtained from commercial vendors
of organoaluminium complexes, due to their increasing role
(Aldrich, Strem) and used as received. The NMR spectra were
recorded on Varian instruments. Chemical shifts (d) are listed as
parts per million downfield from tetramethylsilane, and coupling
in polymerization chemistry.1 While the vast majority of the
preceding studies involved chelating ligands with exclusively
hard donor atoms,1–20 we chose to examine the aluminium
constants (J) and peak widths at half-height (Dv1/2) are in hertz.
complexes of hybrid ligands that contain both hard and soft
donors. In this regard, we have recently prepared a series
1H NMR spectra are referenced using the residual solvent peak
at d 7.16 for C6D6 and d 7.27 for CDCl3. 13C NMR spectra are
of mononuclear aluminium dialkyl complexes supported by
referenced using the residual solvent peak at d 128.39 for C6D6
bidentate diarylamido phosphine ligands (1, Fig. 1)21 and
and d 77.23 for CDCl3. The assignment of the carbon atoms for
explored the subsequent catalytic polymerization. In a parallel
all new compounds is based on DEPT 13C NMR spectroscopy.
pursuit, chemistry involving the tridentate variations of amido
19F, 31P, and 27Al NMR spectra are referenced externally using
phosphine ligands such as 2 and 3 is also investigated. Ligands of
CFCl3 in CHCl3 at d 0, 85% H3PO4 at d 0, and AlCl3 in D2O at
these types represent amido phosphine derivatives bearing sym-
d 0, respectively. Routine coupling constants are not listed. All
metric and unsymmetric donor arms, respectively, for binding
NMR spectra were recorded at room temperature in specified
to a metal. Though available, examples of aluminium complexes
1
solvents. The H–31P correlation spectrum of [PNN]Al(i-Bu)2
containing tridentate amido phosphine ligands are extremely
was acquired on a Varian Inova 500 MHz instrument using
the HMBC sequence. Elemental analysis was performed on a
Heraeus CHN-O Rapid analyzer. With multiple attempts, we
were not able to obtain satisfactory analysis for aluminium
rare.22 We present herein the preparation and characterization of
organoaluminium derivatives incorporating 3, a ligand that may
also be regarded as an o-phenylene derived amido phosphine
chelate that contains an aliphatic pendant arm.
complexes due to extreme air- and moisture-sensitivity of these
compounds.
X-Ray crystallography
Table 1 summarizes the crystallographic data for all structurally
characterized compounds. Data were collected on a Bruker-
Nonius Kappa CCD diffractometer with graphite monochro-
˚
mated Mo-Ka radiation (k = 0.7107 A). Structures were solved
by direct methods and refined by full matrix least squares
procedures against F2 using maXus or WinGX crystallographic
Fig. 1 o-Phenylene derived chelating amido phosphine ligands.
software package. All full-weight non-hydrogen atoms were re-
fined anisotropically. Hydrogen atoms were placed in calculated
positions. The crystals of [PNN]AlEt2 were of poor quality but
sufficient to establish the identity of this molecule.
CCDC reference numbers 264438–264441.
Experimental
General procedures
See http://www.rsc.org/suppdata/dt/b5/b502873f/ for cry-
stallographic data in CIF or other electronic format.
Unless otherwise specified, all experiments were performed
under nitrogen using standard Schlenk or glovebox techniques.
Synthesis of N-(dimethylaminoethyl)-2-fluoroaniline
† Electronic supplementary information (ESI) available: Molecular
structures of [PNN]AlEt2 and [PNN]Al(i-Bu)2. See http://www.rsc.org/
suppdata/dt/b5/b502873f/
A Schlenk flask was sequentially charged with tris(dibenzyl-
ideneacetone)dipalladium (Pd2(dba)3, 26 mg, 0.028 mmol,
1 9 5 2
D a l t o n T r a n s . , 2 0 0 5 , 1 9 5 2 – 1 9 5 6
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Table 1 Crystallographic data for H[PNN], [PNN]AlMe2, [PNN]AlEt2, and [PNN]Al(i-Bu)2
Compound
H[PNN]
[PNN]AlMe2
[PNN]AlEt2
[PNN]Al(i-Bu)2
Formula
C22H25N2P
348.41
1.165
Orthorhombic
P212121
7.2152(2)
11.5571(3)
23.8166(8)
90
90
90
1986.0(1)
4
200(2)
C24H30AlN2P
404.45
1.181
C26H34AlN2P
432.50
1.162
Monoclinic
P21/n
9.2620(3)
29.066(1)
9.8432(5)
90
111.149(3)
90
2471.4(2)
4
200(2)
C30H42AlN2P
488.61
1.146
M
D
calc/g cm−3
Crystal system
Triclinic
Triclinic
¯
¯
Space group
P1
P1
˚
a/A
9.9523(2)
11.1101(3)
11.7142(3)
80.2789(9)
74.9271(9)
65.7727(9)
1137.63(5)
2
11.1571(1)
13.1017(2)
20.6768(3)
73.4062(5)
89.8788(6)
78.4122(6)
2832.70(6)
4
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
T/K
293(2)
100(2)
Diffractometer
Kappa CCD
Mo Ka, 0.71073
50.04
Kappa CCD
Mo Ka, 0.71073
50.04
Kappa CCD
Mo Ka, 0.71073
50.00
Kappa CCD
Mo Ka, 0.71073
50.22
˚
Radiation, k/A
◦
2hmax
/
Total reflections
9678
3473
0.0745
0.145
11107
3992
0.0497
0.171
11346
3909
0.0867
0.162
33913
9981
0.0771
0.148
Independent reflections
Rint
l/mm−1
Data/restraints/parameters
Goodness of fit
Final R indices [I > 2r(I)]: R1, wR2
R indices (all data): R1, wR2
3473/0/226
1.100
0.0537, 0.1314
0.0844, 0.1669
3992/0/254
1.054
0.0501, 0.1309
0.0848, 0.1589
3909/0/272
1.036
0.0695, 0.1691
0.1146, 0.1921
9981/0/642
1.054
0.0558, 0.1443
0.0834, 0.1776
0.49%
equiv),
racemic-2,2ꢀ-bis(diphenylphosphino)-1,1ꢀ-
filtered. All volatiles were removed in vacuo to yield a reddish
brown oil. Diethyl ether (20 mL) was added resulting in a pale
yellow solution containing a black precipitate. The diethyl ether
solution was passed through a pad of Celite to remove the
insoluble solid, which was further washed with diethyl ether
(20 mL × 2) until the washings were colorless. Pentane (10 mL)
was added to the combined diethyl ether solutions (total volume
60 mL) and the solution was cooled to −35 ◦C for 1 d to afford
the product as colorless crystals, which were isolated and dried in
vacuo; yield 1.237 g (79.8%). 1H NMR (C6D6, 500 MHz) d 7.47
(m, 4, Ar), 7.27 (td, 1, Ar), 7.14 (td, 1, Ar), 7.06 (m, 6, Ar), 6.68
(t, 1, Ar), 6.62 (dd, 1, Ar), 5.58 (d, 1, JHP = 6 Hz, NH), 2.86 (td,
binaphthyl (rac-BINAP, 27 mg, 0.043 mmol, 0.75% equiv),
sodium tert-butoxide (0.770 g, 8.00 mmol, 1.4 equiv),
N,N-dimethylethylenediamine (0.503 g, 5.71 mmol), 1-bromo-
2-fluorobenzene (1.000 g, 5.71 mmol), and toluene (25 mL)
under nitrogen. The reaction mixture was heated to reflux with
stirring for 2 d. Toluene was removed in vacuo and the reaction
was quenched with deionized water (25 mL). The product was
extracted with CH2Cl2 (25 mL) and the organic portion was
separated from the aqueous layer, which was further extracted
with CH2Cl2 (10 mL × 3). The combined organic solution
was dried over MgSO4 and filtered. All volatiles were removed
in vacuo to yield a viscous, tan oil that was directly used for
the subsequent nucleophilic phosphanylation without further
purification, yield 0.948 g (91.1%). 1H NMR (CDCl3, 500 MHz)
d 7.03 (t, 1, Ar), 6.99 (d, 1, Ar), 6.72 (t, 1, Ar), 6.65 (dq, 1, Ar),
4.57 (br s, 1, NH), 3.19 (t, 2, NCH2CH2), 2.59 (t, 2, NCH2CH2),
2.29 (s, 6, NMe2). 19F NMR (CDCl3, 470.5 MHz) d −137.64.
1
2, NCH2), 2.14 (t, 2, NCH2), 1.86 (s, 6, NMe2). 31P{ H} NMR
1
(C6D6, 202.3 MHz) d −19.3. 31P{ H} NMR (THF, 80.953 MHz)
1
1
d −19.0. 31P{ H} NMR (pentane, 80.953 MHz) d −17.4. 31P{ H}
NMR (Et2O, 80.953 MHz) d −18.3. 13C{ H} NMR (C6D6,
1
125.5 MHz) d 152.1 (d, JCP = 17.2 Hz, C), 137.0 (d, JCP
=
9.9 Hz, C), 135.3 (d, JCP = 3.6 Hz, CH), 134.7 (s, CH), 134.5
(s, CH), 131.4 (s, CH), 129.1 (d, JCP = 3.5 Hz, CH), 120.1 (d,
1
1
13C{ H} NMR (CDCl3, 125.5 MHz) d 151.4 (d, CF, JCF
=
2
238.3 Hz), 136.8 (d, FCCN, JCF = 11.8 Hz), 124.3 (d, CH,
JCP = 9.2 Hz, C), 117.7 (d, JCP = 1.8 Hz, CH), 110.9 (d, JCP =
JCF = 3.1 Hz), 116.1 (d, CH, JCF = 6.8 Hz), 114.0 (d, CH, JCF
=
2.6 Hz, CH), 57.9 (s, NCH2), 45.1 (s, NMe2), 41.7 (s, NCH2).
Anal. Calcd. for C22H25N2P: C, 75.82; H, 7.24; N, 8.04. Found:
C, 75.47; H, 7.24; N, 7.93%.
18.2 Hz), 111.8 (d, CH, JCF = 3.6 Hz), 57.6 (s, NCH2), 44.9 (s,
NMe2), 40.6 (s, NCH2).
Synthesis of N-(dimethylaminoethyl)-
Synthesis of [PNN]AlMe2
2-diphenylphosphinoaniline, H[PNN]
Solid H[PNN] (100 mg, 0.28 mmol) was dissolved in toluene
(6 mL) and cooled to −35 ◦C. To this cold solution was added
AlMe3 (0.14 mL, 2.0 M in toluene, TCI, 0.28 mmol) dropwise.
After being stirred at room temperature for 2 d, the reaction
solution was filtered through a pad of Celite and concentrated
under reduced pressure to ca. 1 mL. Pentane (5 drops) was
layered on top and the solution was cooled to −35 ◦C to afford
the product as colorless crystals suitable for X-ray diffraction
analysis; yield 113.4 mg (97.7%). 1H NMR (C6D6, 500 MHz) d
7.54 (t, 4, Ar), 7.37 (t, 1, Ar), 7.20 (t, 1, Ar), 7.03 (m, 6, Ar), 6.66
(t, 1, Ar), 6.61 (t, 1, Ar), 2.84 (t, 2, NCH2), 2.16 (t, 2, NCH2),
A 250-mL Schlenk flask equipped with a condenser was flushed
with nitrogen thoroughly. To this flask was added KPPh2
(89 mL, 0.5 M in THF solution, Aldrich, 44.5 mmol). THF was
removed in vacuo and a solution of N-(dimethylaminoethyl)-2-
fluoroaniline (8.119 g, 44.5 mmol) in 1,4-dioxane (60 mL) was
added with a syringe. The transparent, ruby reaction solution
was heated to reflux for 7 d, during which time the reaction
1
condition was monitored by 31P{ H} NMR spectroscopy. All
volatiles were removed from the resulting orange solution
under reduced pressure and degassed deionized water (30 mL)
was added. The product was extracted with deoxygenated
dichloromethane (30 mL). The dichloromethane solution was
separated from the aqueous layer, from which the product
was further extracted with dichloromethane (15 mL × 2).
The combined organic solution was dried over MgSO4 and
3
1
1.75 (s, 6, NMe2), −0.31 (d, 6, JHP = 6.5, AlMe2). 31P{ H}
1
NMR (C6D6, 202.3 MHz) d −28.4 (Dv1/2 = 3.6). 31P{ H} NMR
(toluene, 80.95 MHz) d −27.7. 13C NMR (C6D6, 125.5 MHz) d
161.5 (JCP = 24.6), 136.6 (CH), 135.4 (JCP = 9.2), 134.0 (JCP
=
14.6, CH), 133.0 (CH), 129.1 (CH), 128.9 (JCP = 7.3, CH), 116.7
D a l t o n T r a n s . , 2 0 0 5 , 1 9 5 2 – 1 9 5 6
1 9 5 3

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(JCP = 18.2), 115.6 (JCP = 3.6, CH), 112.0 (JCP = 6.4, CH), 57.3
(NCH2), 45.2 (NCH2), 44.8 (NMe2), −6.1 (2JCP = 34.6, AlMe2).
27Al NMR (C6D6, 130.22 MHz) d 123.6 (Dv1/2 = 9113). Anal.
Calcd. for C24H30AlN2P: C, 71.27; H, 7.48; N, 6.93. Found: C,
69.29; H, 8.47; N, 6.52%.
(1)
Synthesis of [PNN]AlEt2
the fluorine compound can be isolated as a viscous oil in quan-
titative yield from the cross-coupling reaction26,27 of 1-bromo-
2-fluorobenzene with N,N-dimethylethylenediamine catalyzed
by Pd2(dba)3 in the presence of rac-BINAP as the cocatalyst
and sodium tert-butoxide as the base in refluxing toluene.
Subsequent nucleophilic phosphanylation28,29 with KPPh2 in
refluxing 1,4-dioxane produces H[PNN] as colorless crystals in
80% yield. Both H[PNN] and its fluorine precursor were fully
characterized by multinuclear NMR spectroscopy. Analogous
to those found for the bidentate analogues of the type 1,21,25
the NH proton in H[PNN] is observed as a doublet resonance
due to its internuclear coupling with the phosphorus atom
(JHP = 6 Hz). Colorless crystals of H[PNN] suitable for X-ray
diffraction analysis were grown from a concentrated pentane
solution at −35 ◦C. Crystallographic details are summarized in
Table 1. As depicted in Fig. 2, compound H[PNN] represents
itself as an excellent precursor for a tridentate meridional ligand,
at least from a geometric point of view. As anticipated, the two
phenyl rings at the phosphorus donors are orientated such that
they are nearly orthogonal to each other (dihedral angle 85.4◦).
The N(1)–C bond distance is slightly longer for the aliphatic
substituent than for the aromatic counterpart.
A Teflon-sealed reaction vessel containing a toluene solution
(6 mL) of H[PNN] (100 mg, 0.28 mmol) and AlEt3 (218.4 mg,
15% in hexane, TCI, 0.28 mmol) was heated in an oil bath to
100 ◦C for 24 h. After being filtered through a pad of Celite, the
reaction solution was concentrated under reduced pressure to
ca. 1 mL. The concentrated solution was layered with pentane
(5 drops) and cooled to −35 ◦C to afford the product as colorless
1
crystals; yield 86.1 mg (69.4%). H NMR (C6D6, 500 MHz) d
7.59 (td, 4, Ar), 7.37 (td, 1, Ar), 7.22 (td, 1, Ar), 7.03 (m, 6,
Ar), 6.67 (t, 1, Ar), 6.62 (t, 1, Ar), 2.83 (t, 2, NCH2), 2.15 (t, 2,
NCH2), 1.79 (s, 6, NMe2), 1.28 (t, 6, AlCH2CH3), 0.28 (dq, 4,
3JHP = 7, AlCH2CH3). Selective irradiation on the b-hydrogen
atoms at 1.28 ppm led to a doublet resonance for the a-hydrogen
1
atoms with 3JHP of 7 Hz. 31P{ H} NMR (C6D6, 202.3 MHz) d –
1
26.9 (Dv1/2 = 3.5). 31P{ H} NMR (toluene, 80.95 MHz) d −26.4.
13C NMR (C6D6, 125.5 MHz) d 161.7 (JCP = 24.5), 136.5 (CH),
135.5 (JCP = 8.2), 133.8 (JCP = 14.5, CH), 132.9 (CH), 129.1
(CH), 129.0 (JCP = 7.3, CH), 116.8 (JCP = 17.2), 115.8 (JCP
=
2.8, CH), 112.1 (JCP = 5.4, CH), 57.7 (NCH2), 45.2 (NCH2),
44.8 (NMe2), 11.5 (AlCH2CH3), 2.4 (2JCP = 28.2, AlCH2). 27Al
NMR (C6D6, 130.22 MHz) d 117.0 (Dv1/2 = 14599). Anal. Calcd.
for C26H34AlN2P: C, 72.20; H, 7.92; N, 6.48. Found: C, 69.12;
H, 7.68; N, 6.11%.
Synthesis of [PNN]Al(i-Bu)2
A Teflon-sealed reaction vessel containing a toluene solution
(6 mL) of H[PNN] (100 mg, 0.28 mmol) and Al(i-Bu)3 (379 mg,
15% in hexane, TCI, 0.28 mmol) was heated in an oil bath to
100 ◦C for 24 h. After being filtered through a pad of Celite, the
reaction solution was concentrated under reduced pressure to
ca. 1 mL. The concentrated solution was layered with pentane
(5 drops) and cooled to −35 ◦C to afford the product as colorless
crystals suitable for X-ray diffraction analysis; yield 93.0 mg
(66.3%). 1H NMR (C6D6, 500 MHz) d 7.59 (t, 4, Ar), 7.38
(t, 1, Ar), 7.28 (t, 1, Ar), 7.07 (m, 2, Ar), 7.03 (m, 4, Ar),
6.70 (t, 1, Ar), 6.63 (t, 1, Ar), 2.88 (t, 2, NCH2), 2.22 (t, 2,
NCH2), 1.97 (m, 2, AlCH2CHMe2), 1.84 (s, 6, NMe2), 1.00 (d,
6, AlCH2CHMeAMeB), 0.97 (d, 6, AlCH2CHMeAMeB), 0.33 (m,
2
3
2
2, JHH = 15, JHP = 7, AlCHAHB), 0.16 (m, 2, JHH = 14,
3JHP = 9, AlCHAHB). Selective decoupling of b-hydrogen atoms
at 1.97 ppm revealed two well-resolved doublet of doublets
Fig.
2
Molecular structure of H[PNN] with thermal ellipsoids
˚
drawn at the 35% probability level. Selected bond distances (A)
and angles (◦): P(1)–C(7) 1.829(4), P(1)–C(13) 1.836(4), P(1)–C(1)
1.841(4), N(1)–C(18) 1.372(5), N(1)–C(19) 1.441(5), N(2)–C(21)
1.437(7), N(2)–C(22) 1.448(6), N(2)–C(20) 1.459(5); C(7)–P(1)–C(13)
102.90(18), C(7)–P(1)–C(1) 102.81(17), C(13)–P(1)–C(1) 101.46(16),
C(18)–N(1)–C(19) 122.0(3), C(21)–N(2)–C(22) 110.8(4), C(21)–N(2)–
C(20) 111.6(4), C(22)–N(2)–C(20) 112.4(4).
resonances for the diastereotopic a-hydrogen atoms. 31P{ H}
1
NMR (C6D6, 202.3 MHz) d −25.1 (Dv1/2 = 4). 31P{ H} NMR
1
(toluene, 80.95 MHz) d −25.0. 13C NMR (C6D6, 125.5 MHz)
d 161.7 (JCP = 24.5), 137.0 (CH), 136.3 (JCP = 6.4), 135.3
(CH), 134.1 (JCP = 14.6, CH), 133.4 (JCP = 10.9, CH), 132.9
(CH), 116.5 (JCP = 17.2), 115.8 (CH), 112.5 (JCP = 5.4, CH),
57.7 (NCH2), 45.4 (NMe2), 45.1 (NCH2), 29.9 (AlCH2CHMe2),
27.9 (AlCH2CHMe2), 27.8 (AlCH2CHMe2), 24.1 (2JCP = 30.8,
AlCH2CHMe2). 27Al NMR (C6D6, 130.22 MHz) d 115 (Dv1/2
too broad to be determined). Anal. Calcd. for C30H42AlN2P: C,
73.74; H, 8.66; N, 5.73. Found: C, 72.85; H, 8.51; N, 5.76%.
Alkane elimination reactions of H[PNN] with trialkylalu-
minium in toluene produced the corresponding [PNN]AlR2
(R = Me, Et, i-Bu) in high yield (eqn. (2)).
(2)
Results and discussion
To assess the synthetic accessibility to ligands of type 3, we
first attempted the preparation of N-(dimethylaminoethyl)-2-
diphenylphosphinoaniline (H[PNN]) as it is readily available
following the previously established protocol.23–25 As shown in
eqn. (1),
These organoaluminium complexes are all colorless crystalline
solids that are readily available by layering pentane on top of the
concentrated reaction solutions at −35 ◦C. The solution NMR
data for these molecules are all consistent with five-coordinate
aluminium complexes having a mirror-plane symmetry passing
1 9 5 4
D a l t o n T r a n s . , 2 0 0 5 , 1 9 5 2 – 1 9 5 6

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Table 2 Selected NMR spectroscopic dataa
type 1.21 Consistently, two doublet resonances are observed for
the isobutylmethyl groups in [PNN]Al(i-Bu)2 in the H NMR
1
Compound
dHa(3JPH
)
dCa(2JPC
)
dP (Dv1/2
−19.3
)
dAl (Dv1/2)
spectrum. The aluminium atom in these dialkyl complexes
appears as a broad signal at ca. 120 ppm in the 27Al NMR
spectrum, a result that is notably different from that found for
the four-coordinate aluminium dialkyl complexes supported by
1,21 but consistent with what one anticipates for five-coordinate
aluminium dialkyl complexes.34–36 Similar 27Al chemical shift
and peak width at half-height have also been reported for the
five-coordinate organoaluminium phosphine complexes such as
tris[o-((diphenylphosphino)methyl)phenyl]aluminium.31
H[PNN]
[PNN]AlMe2
[PNN]AlEt2
[PNN]Al(i-Bu)2
−0.31 (7)
0.28 (7)b
0.33 (7)b
0.16 (9)b,c
−6.1 (35) −28.4 (4) 124 (9113)
2.4 (28) −26.9 (4) 117 (14599)
24.1 (31) −25.1 (4) 115 (NA)d
a All spectra were recorded in C6D6 at room temperature, chemical shifts
in ppm, coupling constants in Hz, peak widths at half-height in Hz.
b The coupling constants were determined by 1H NMR spectroscopy
with selective decoupling of b-hydrogen atoms. c The larger 3JPH found
for the signal at 0.16 ppm than that at 0.33 ppm was further confirmed
by 1H–31P correlation spectroscopy. d Not available; too broad to be
determined.
The solid-state structures of [PNN]AlMe2, [PNN]AlEt2, and
[PNN]Al(i-Bu)2 have been determined by X-ray crystallography.
As depicted in Figs. 3, S1, and S2 (see ESI†), respectively, the
X-ray structures of [PNN]AlMe2, [PNN]AlEt2, and [PNN]Al(i-
Bu)2 are rather similar. Table 3 summarizes the selected bond
distances and angles. The coordination geometry for these
molecules is best described as trigonal bipyramidal with the
amido nitrogen and the two a-carbon atoms occupying the
equatorial positions. The aluminium center lies virtually on
the equatorial plane, as indicated by the sum of the three
equatorial bond angles of 358.8◦ for [PNN]AlMe2, 358.6◦ for
[PNN]AlEt2, and 358.3◦ for [PNN]Al(i-Bu)2. The small dihedral
angle found between the N(amine)–Al–N(amide) and P–Al–
N(amide) planes in [PNN]AlMe2 (1.4◦), [PNN]AlEt2 (5.0◦), and
[PNN]Al(i-Bu)2 (4.5◦) is indicative of a meridional coordination
mode for the tridentate [PNN]− ligand. The Al–amide, Al–
amine, Al–C, and Al–P distances are all within the expected
values.8,31,37–42 For instance, the Al–P bond lengths of 2.756(1)
through the aluminium center and the three donor atoms of
the meridional [PNN]− ligand. Table 2 summarizes the selected
NMR data. In all cases, the two aluminium-bound alkyl ligands
are chemically equivalent, as are the methyl groups in the
pendant amine arm, as indicated by the 1H and 13C NMR
spectra. The phosphorus donor appears as a singlet resonance
at ca. −27 ppm for these molecules, a value that is shifted
relatively upfield from that of H[PNN] at −19 ppm. A similar
upfield change in the 31P chemical shift is also observed for
other phosphine coordinated aluminium complexes21,22,30,31 as
compared to the corresponding free phosphine ligands. The
coordination of the soft phosphorus donor in [PNN]− to the
hard aluminium center is further supported by the H and 13C
1
˚
˚
˚
A, 2.748(2) A, and 2.783(1) A for [PNN]AlMe2, [PNN]AlEt2,
NMR spectra. For instance, the methyl ligands in [PNN]AlMe2
are observed as a doublet resonance at −0.31 ppm with 3JHP of
and [PNN]Al(i-Bu)2, respectively, are comparable to those of the
1
6.5 Hz in the H NMR spectrum and a doublet at −6.1 ppm
2
1
with JCP of 34.6 Hz in the 13C{ H} NMR spectrum. Selective
decoupling of b-hydrogen atoms in [PNN]AlEt2 and [PNN]Al(i-
Bu)2 unambiguously elucidates the internuclear coupling of the
a-hydrogen atoms with the phosphorus donor. These results
are in sharp contrast to that of the aluminium dimethyl
complex supported by a salicylaldiminate ligand bearing a
pendant triarylphosphine donor, in which the phosphorus atom
is not associated with the aluminium center.32 Interestingly,
among the compounds investigated, [PNN]Al(i-Bu)2 exhibits
two well-resolved multiplet resonances for the a-hydrogen atoms,
consistent with the anticipated diastereotopic characteristic. The
chemical non-equivalence of the a-hydrogen atoms in the Al–
CH2R (R = H, Me, i-Pr) fragments is ascribable to the lack of
symmetry of these molecules with respect to internal rotation
involving the Al–C bonds.33 With smaller aluminium-bound
alkyls, rapid rotation about the Al–C bonds happens readily and
the diastereotopic a-hydrogen atoms are indistinguishable on the
NMR timescale, as has been described in aluminium chemistry
involving the bidentate diarylamido phosphine ligands of the
Fig. 3 Molecular structure of [PNN]AlMe2 with thermal ellipsoids
drawn at the 35% probability level.
◦
˚
Table 3 Selected bond distances (A) and angles ( ) for [PNN]AlMe2, [PNN]AlEt2, and [PNN]Al(i-Bu)2
a
Compound
[PNN]AlMe2
[PNN]AlEt2
[PNN]Al(i-Bu)2
Al–P
2.756(1)
1.901(2)
2.134(2)
1.967(3), 1.978(3)
2.748(2)
1.928(3)
2.115(4)
1.974(4), 1.993(4)
2.783(1)
1.914(2)
2.098(2)
Al–N(amide)
Al–N(amine)
Al–C
1.990(3), 1.998(3)
C–Al–C
120.1(2)
113.7(2)
117.6(1)
C–Al–N(amide)
C–Al–N(amine)
N–Al–N
120.8(1), 117.9(1)
98.3(1), 99.5(1)
83.11(9)
119.2(2), 125.7(2)
102.0(2), 98.3(2)
82.7(1)
121.2(1), 119.5(1)
98.8(1), 101.6(1)
82.84(9)
N(amide)–Al–P
N(amine)–Al–P
C–Al–P
75.77(7)
158.84(8)
91.4(1), 91.7(1)
75.7(1)
157.9(1)
93.0(1), 90.4(1)
75.67(7)
158.05(7)
88.69(8), 92.95(9)
a The asymmetric unit cell contains two independent molecules whose structural parameters are similar to one to the other. The data summarized
correspond to one of the molecules.
D a l t o n T r a n s . , 2 0 0 5 , 1 9 5 2 – 1 9 5 6
1 9 5 5

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five-coordinate phosphine complexes of aluminium hydrocarbyl
8 S. Dagorne, L. Lavanant, R. Welter, C. Chassenieux, P. Haquette and
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such as tris[o-((diphenylphosphino)methyl)phenyl]aluminium
31
˚
(2.7295 A average). It should be noted that examples of
structurally characterized, five-coordinate phosphine complexes
containing Al–C bond(s) are extremely rare.43 In sharp contrast
to the results described herein, the X-ray structure of the closely
related Al[N(SiMe2CH2PiPr2)2](CH2Ph)2 reveals that only two
rather than three donor atoms of the potentially tridentate
amido phosphine ligand are bound to the aluminium center,
thereby resulting in a four-coordinate species.22 Interestingly, the
Al–P distance is slightly longer for [PNN]AlR2 than for the four-
coordinate [1]AlR2, e.g., [o-(2,6-Me2C6H3N)C6H4PPh2]AlEt2
21
˚
(2.456(4) A), due perhaps to the accommodation of the two
14 A. V. Korolev, I. A. Guzei and R. F. Jordan, J. Am. Chem. Soc., 1999,
121, 11605–11606.
five-membered rings in the meridional tridentate ligand for the
former. It is also anticipated from a purely steric viewpoint that
the Al–P distance in a five-coordinate species is longer than that
in a four-coordinate compound. Nevertheless, the dissociation
of the phosphorus atom from aluminium seems not to occur
readily on the NMR timescale. As a result, the P–Al–N(amide)
angles for [PNN]AlR2 (ca. 75.7◦) are notably smaller than those
found for the aluminium complexes supported by 1, e.g., [o-(2,6-
iPr2C6H3N)C6H4PPh2]AlMe2 (82.1(2)◦),21 although both ligand
systems incorporate an o-phenylene for the connection of the
amido nitrogen and the phosphorus donors.
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In summary, a new monoanionic, tridentate amido phos-
phine ligand possessing a pendant amino functionality has
been prepared and employed for the coordination chemistry
of aluminium. Both rigid o-phenylene and flexible ethylene
moieties are incorporated in the ligand backbone of [PNN]−
for the connection of the three donor atoms. The chelating
feature of [PNN]− allows for the facile isolation of a series of five-
coordinate aluminium dialkyl complexes, which represent rare
examples of structurally characterized, five-coordinate phos-
phine complexes containing the Al–C bond(s). The phosphorus
donor is trans to the dimethylamino nitrogen atom in these trig-
onal bipyramidal molecules. Diastereotopic a-hydrogen atoms
are recognized for higher aluminium-bound alkyls in which the
rotation about the Al–C bonds is comparatively hindered on
the NMR timescale. The coordination of the soft phosphorus
donor to the hard aluminium center is confirmed by both
solution NMR spectroscopy and X-ray crystallography. Studies
involving the reactivity chemistry of these new molecules will be
the subjects of further reports.
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Acknowledgements
We thank the National Science Council of Taiwan for financial
support (NSC 93-2113-M-110-016) of this work and Prof.
Michael Y. Chiang (NSYSU) and Mr Ting-Shen Kuo (National
Taiwan Normal University) for crystallographic assistance.
35 We note that the multinuclear probe head gives a background signal
at ca. 60 ppm for 27Al NMR spectra. For relevant studies, see
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2217.
43 The only example available from the Cambridge Structure Database
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1 9 5 6
D a l t o n T r a n s . , 2 0 0 5 , 1 9 5 2 – 1 9 5 6