Synthesis and Structures of Complexes Demonstrating the Coordinative Versatility of the 2,4-Diimino-3-phosphinopentene Anion (γ -Phosphino-β-diketiminate)

Article abstract of DOI:10.1021/ic0351444

The synthesis and characterization of a 2,4-diimino-3-phosphinopentene anion (γ-phosphino-β -diketiminate) is reported and enables diversification of the β-diketiminate ligand framework, which has been widely employed across the periodic table. Phosphines are observed to adopt the γ-position of the ligand rather than the N,N′ chelate. While aluminum and lithium adopt the familiar N,N′ chelate arrangement with the new 2,4-diimino-3-phosphinopentene anion ligand, reactions with AsCl ₃ or SbCl₃ result in substitution at the β-methyl position on the ligand backbone, realizing novel P→E (E = As or Sb) intramolecular coordination. The chemistry of the 2, 4-diimino-3-phosphinopentene anion can be monitored by the ³¹P NMR chemical shifts, which are distinctively diagnostic of the coordinative engagement of the ligand.

Full text of DOI:10.1021/ic0351444

Inorg. Chem. 2004, 43, 734−738
Synthesis and Structures of Complexes Demonstrating the Coordinative
Versatility of the 2,4-Diimino-3-phosphinopentene Anion
(γ-Phosphino-â-diketiminate)
,†
Neil Burford,* Mark D’eon,† Paul J. Ragogna,† Robert McDonald,‡ and Michael J. Ferguson‡
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3, and
X-Ray Crystallography Laboratory, Department of Chemistry, UniVersity of Alberta,
Edmonton, Alberta T6G 2G2, Canada
Received October 2, 2003
The synthesis and characterization of a 2,4-diimino-3-phosphinopentene anion (γ-phosphino-â-diketiminate) is reported
and enables diversification of the â-diketiminate ligand framework, which has been widely employed across the
periodic table. Phosphines are observed to adopt the γ-position of the ligand rather than the N,N′ chelate. While
aluminum and lithium adopt the familiar N,N′ chelate arrangement with the new 2,4-diimino-3-phosphinopentene
anion ligand, reactions with AsCl3 or SbCl3 result in substitution at the â-methyl position on the ligand backbone,
realizing novel PfE(E) As or Sb) intramolecular coordination. The chemistry of the 2,4-diimino-3-phosphinopentene
anion can be monitored by the 31P NMR chemical shifts, which are distinctively diagnostic of the coordinative
engagement of the ligand.
Introduction
â-Diketiminate ligands of the general type 1 (2,4-diimi-
nopentene anion) have been influential in the development
of catalytic processes and have facilitated the discovery of
unusual bonding arrangements for many elements.1 Chelate
complexes of the type 2 are common, but are not observed
for the electron-rich p-block elements. Our attempts to
develop phosphorus derivatives of 2 have realized the
unusual backbone substitution of the extensively utilized
reagent 3Li (Dipp ) 2,6-diisopropylphenyl) with chlo-
rodiphenylphosphine, which gives 2-amino-4-imino-3-phos-
phinopentene (γ-phosphino-â-diketimine) 4.2,3 The auxiliary
phosphine of 4 provides a 31P NMR probe for the â-diketim-
inate moiety so that distinct chemical shifts are observed for
the chelate complexes 5E (E ) Li or AlCl2), and the unusual
Results and Discussion
five-membered heterocyclic structures 6E are formed for E
) AsCl2 or SbCl2.
Diketiminate anions (1) have demonstrated a versatile
chelate coordination chemistry with a wide range of metals
and some p-block elements.4-14 Complexes of the electron-
rich elements of groups 15,15,16 16, and 17 are notably absent
* Author to whom correspondence should be addressed. E-mail:
Neil.Burford@dal.ca. Tel: (902) 494-3681. Fax: (905) 494-1310.
† Dalhousie University.
‡ University of Alberta.
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734 Inorganic Chemistry, Vol. 43, No. 2, 2004
10.1021/ic0351444 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/16/2003

The 2,4-Diimino-3-phosphinopentene Anion
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systems to 4 have been synthesized by alternative methods.41
As illustrated in Scheme 1, formation of 4 is envisaged to
involve nucleophilic displacement of chloride from Ph2PCl
by C-C π-electron density of the nacnac anion 3, rather
than by the sterically hindered nonbonding electron density
at the nitrogen sites. Subsequent migration of the proton at
carbon-3 of the intermediate 8 to one of the imine nitrogen
centers reestablishes conjugation for the five atom frame to
give 4.
The structure of 4 compares and contrasts the γ-addition
product of GeCl3 with 3, which has been definitively
characterized as 9a, exhibiting N-Câ distances (Table 2)
that illustrate a distinct diimine arrangement.42 The proposed
phosphine intermediate 8 is topologically analogous to the
conformer 9b, prompting questions about the stability of the
germanium derivative 9a. Most notable is the distinct
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Inorganic Chemistry, Vol. 43, No. 2, 2004 735

Burford et al.
Table 1. Crystal Data
4
5Li
6As
6Sb
5AlCl2
empirical formula
C41H51N2P
C41H50LiN2P‚
C7H8
700.87
triclinic
P1h
10.1138(4)
11.1175(5)
19.644(1)
77.193(1)
84.797(1)
78.208(1)
2106.0(2)
2
C41H50AsCl2N2P‚
1.5C6H6
864.78
triclinic
P1h
11.446(1)
12.462(1)
16.902(1)
94.920(2)
104.742(1)
90.251(2)
2322.0(3)
2
C41H50Cl2N2PSb‚
0.5C5H12
830.52
monoclinic
C2/c
34.699(3)
11.305(1)
20.995(2)
90
96.025(2)
90
8190.0(0)
8
1.347
0.0547
0.1525
C41H50Al1Cl2N2P
formula weight
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
602.81
monoclinic
P21/n
10.690(1)
36.444(4)
10.821(1)
90
118.846(1)
90
3692.7(4)
4
699.68
orthorhombic
Pbca
18.387(1)
20.247(1)
21.232(1)
90
90
90
7904.0(8)
8
1.176
Z
D
C (Mg m-3
)
1.084
1.105
0.0474
0.1312
1.021
1.237
0.0374
0.1010
1.001
R (I > 2σ(I))a
wR (all data)a
GOF Sa
0.0492
0.1389
1.057
0.0564
0.1092
1.007
1.040
2
2
2
a R(F [I > 2σ(I)]) ) ∑||Fo| - |Fc||/∑|Fo|; wR(F2 [all data]) ) [∑w(Fo2 - Fc )2/∑w(Fo )2]1/2; S (all data) ) [∑w(Fo2 - Fc )2/(n - p)]1/2 (n ) no. of data;
p ) no. of parameters varied, w ) 1/[σ2(Fo ) + (aP)2 + bP] where P ) (Fo2 + 2Fc )/3 and a and b are constants suggested by the refinement program [see
2
2
Supporting Information]).
Scheme 1. Proposed Mechanism for the Formation of 4
Table 2. Selected Bond Lengths and NMR Parameters for Derivatives
of 3, 4, 5, 6, and 9
P-Cγ
(Å)
N-E
(Å)
N-Câ
(Å)
δ 1Hd
δ
31P
compound
(ppm) (ppm)
ref
4
1.813(2) 0.97(2)a
1.338(2) 12.6
1.307(2)
1.318(3) 12.1
1.341(3)
-11 this work
3H
0.97(4)a
0.92(4)
43
5Li
1.808(2) 1.899(3)b 1.318(2)
d
-1 this work
intramolecular N-Ge interaction observed in 9a, which may
be responsible for mediating the proton affinity or nucleo-
philicity of the imine site, and is perhaps due to the greater
Lewis acidity of GeCl3 over that of PPh2.
1.908(3)
1.310(2)
Et2O-3Li
1.917(4)b 1.324(3)
4
1.912(4)
1.325(3)
THF-3Li
Hex-3Li
1.958(5)b 1.325(3)
4
4
1.894(7)b 1.323(4)
1.919(7)
1.325(4)
3Li
1.892(7)b 1.321(5)
4
1.903(7)
1.337(5)
1.300(3) 12.5
1.325(3)
1.338(5) 12.7
1.298(6)
6As
6Sb
5AlCl2
3AlCl2
3Al
1.764(2) 0.82(3)a
48 this work
1.782(4) 0.85(5)a
23 this work
1.830(3) 1.861(2)c 1.334(3)
d
-2 this work
1.867(2)
1.339(3)
1.8843(9)c 1.341(1)
1.8663(9) 1.348(1)
1.957(2)c 1.340(3)
4
Retention of the amine/imine bifunctionality upon incor-
poration of a phosphine substituent in 4 provides the
opportunity for subsequent diketiminate metathesis reactions.
Furthermore, the backbone phosphine substitution offers a
valuable 31P NMR spectroscopic probe. The reaction mixture
of 4 with BuLi exhibits a single 31P NMR resonance (δ )
-1 ppm), and the isolated solid has been crystallographically
characterized as the toluene solvate of 5Li shown in Figure
2. 31P NMR spectra of equimolar mixtures of 5Li with AlCl3
in toluene indicate a quantitative reaction (δ ) -2 ppm),
and the solid state structure (Figure 3) of the isolated product
5AlCl2 confirms exchange of lithium by dichloroalane in
the “mouth” of the chelate. Equimolar mixtures of 5Li with
ECl3 (E ) As, Sb) in toluene also exhibit quantitative
metathesis (31P NMR: δ ) 48 ppm and 23 ppm, respec-
tively); however, the introduction of ECl2 contrasts that of
AlCl2, engaging the organic backbone of the ligand through
auxiliary interaction with the phosphine 6E (E ) As, Sb).
The solid state structures are shown in Figures 4 and 5,
confirming an amine/imine 6E formulation with ECl2 bridges
13
42
1.957(2)
2.419(2)
1.342(3)
1.273(3)
1.268(4)
9
a E ) H. b E ) Li. c E ) Al. d Not observed.
between a â-methylene carbon center and the γ-phosphine.
Table 2 lists P-Cγ, N-E, and N-Câ distances as well as
1H and 31P NMR chemical shifts for derivatives of 3, 4, 5,
6, and 9.
The As and Sb centers in 6E both exhibit distorted
disphenoidal (“seesaw”) geometry with Cl-E-Cl bond
angles of 173.85(2)° and 172.12(5)°, respectively, implying
stereochemical activity of the lone pair on As and Sb. One
E-Cl distance is elongated with respect to the other [As-
Cl, 2.3786(6) Å, 2.5243(6) Å; and Sb-Cl, 2.689(2) Å, 2.521-
(2) Å]. In contrast to the symmetric frameworks of 5Li and
5AlCl2, the N-Câ distances in 6As and 6Sb are inequivalent
(Table 2) due to the remote asymmetric incorporation of the
pnictogen fragment and are consistent with imine/amine
distinction of the nitrogen centers. The P-Cγ distances in
736 Inorganic Chemistry, Vol. 43, No. 2, 2004

The 2,4-Diimino-3-phosphinopentene Anion
Figure 4. Solid state structure of 6As. Thermal ellipsoids are drawn at
50% probability, all hydrogen atoms have been omitted for clarity (except
that bound to nitrogen), and the 1.5 molecules of benzene solvate are not
shown.
Figure 2. Solid state structure of 5Li‚toluene. Thermal ellipsoids are drawn
at 50% probability, and all hydrogen atoms have been omitted for clarity.
Figure 3. Solid state structure of 5AlCl2. Thermal ellipsoids are drawn at
50% probability, and all hydrogen atoms have been omitted for clarity.
Figure 5. Solid state structure of 6Sb. Thermal ellipsoids are drawn at
50% probability, all hydrogen atoms have been omitted for clarity (except
that bound to nitrogen), and the 0.5 molecule of pentane solvate is not
shown.
6E are shorter than in 4, while those in 5Li and 5AlCl2 are
consistent with 4.
1H NMR spectra of 6E derivatives display a characteristic
resonance at δ ≈ 12 ppm (Table 2) consistent with the
presence of an N-H proton, while this signal is not observed
for 5Li and 5AlCl2. The 31P NMR chemical shifts of these
complexes (Table 2) are diagnostic of the coordination mode
of the ligand, with P,C chelate complexes (6E) more
dramatically downfield shifted from the free ligand (4) than
N,N′ chelated complexes (5).
The formation of 6E (E ) As, Sb) is envisaged to occur
via Scheme 2. Inter- or intramolecular tautomerism gives
access to a methylene site at the â-carbon center. In this
context, deprotonation of anion 3 is rare but a complex of
the dianionic conjugate base of 3 has recently been reported,44
and a methylene isomer has been observed for a germanium
complex.45 Nucleophilic attack at ECl3 (E ) As, Sb) will
promote chloride elimination, supported by coordination by
the neighboring phosphine center to give a 5-membered ring
system, not uncommon for phosphorus.23,26,46
Conclusions
A series of complexes of the 2,4-diimino-3-phosphino-
pentene anion demonstrate unusual coordinative versatility
that goes beyond conventional wisdom for the extensively
studied and exploited diketiminate framework. N,N′ chelate
complexes are observed for lithium and aluminum while the
electron-rich elements of group 15 (P, As, Sb) access remote
sites by proton substitution of the organic backbone. Con-
sequential introduction of the Ph2P substituent at the γ
(45) Ding, Y.; Ma, Q.; Roesky, H. W.; Herbst-Irmer, R.; Uson, I.;
Noltemeyer, M.; Schmidt, H. G. Organometallics 2002, 21, 5216-
(43) Feldman, J.; McLain, S. J.; Parthasarathy, W.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514-1516.
(44) Harder, S. Angew. Chem., Int. Ed. 2003, 42, 3430-3434.
5220.
(46) Lochschmidt, S.; Schmidpeter, A. Z. Naturforsch. 1985, 40b, 765-
773.
Inorganic Chemistry, Vol. 43, No. 2, 2004 737

Burford et al.
5Li. nBuLi in hexanes (1.6 M, 0.11 mL) was added dropwise to
a solution of 4 (0.11 g, 0.18 mmol) in toluene (3 mL) at RT. The
mixture was stirred for 2 h, concentrated in vacuo (≈0.5 mL), and
crystallized at -30 °C. Subsequent washing with cold n-pen-
tane gave clear, colorless crystals, 0.05 g, 42%. mp 270-274 °C.
FT-IR (cm-1): solid material is extremely air sensitive and as such,
a reliable IR spectrum could not be obtained. Anal. Calcd for
C41H50Li1N2P (found): C: 80.9 (80.6), H: 8.3 (8.0), N: 4.6 (4.2).
Scheme 2. Proposed Mechanism for the Formation of 6E
7
31P{1H} NMR: -1 ppm. Li NMR: -0.4 ppm.
6As. 1.6 M nBuLi in hexanes (0.10 mL) was added dropwise to
a solution of 4 (0.10 g, 0.17 mmol) in toluene (3 mL) at 0 °C. The
mixture was stirred for 2 h at room temperature and then added to
a solution of AsCl3 (0.003 g, 0.17 mmol) in toluene (3 mL) -30
°C. The mixture was stirred for 12 h at RT. LiCl was filtered and
the solvent was removed and the solid was recrystallized from
benzene to give brown crystals, 0.06 g, 44%. mp (dec): 177 °C.
Anal. Calcd for C41H50AsCl2N2P (found): 65.8 (66.1); H, 6.7 (6.8);
N, 3.8 (3.8). FT-IR (cm-1): 1537(1), 1190(9), 1101(5), 801(10),
778(3), 746(8), 688(6), 523(4), 477(2), 436(7). 31P{1H} NMR: 48
ppm.
6Sb, 1.6 M nBuLi in hexanes (0.10 mL) was added dropwise to
a solution of 4 (0.10 g, 0.17 mmol) in toluene (3 mL) at RT; the
mixture was stirred for 2 h and then added to a solution of SbCl3
(0.04 g, 0.17 mmol) in toluene (3 mL) at -78 °C. The mixture
was stirred for 12 h and allowed to warm to RT. LiCl was filtered,
and the solvent was removed; the solid was recrystallized using
vapor diffusion (toluene/pentane) to give colorless crystals, 0.03
g, 22%. mp (dec): 124 °C. Anal. Calcd for C41H50SbCl2N2P
(found): C 62.0 (60.8); H 6.3 (6.7); N 3.5 (2.7). FT-IR (cm-1):
693(8), 745(3), 778(5), 796(4), 1258(7), 1295(9), 1321(10), 1406-
(6), 1436(2), 1564(1). 31P{1H} NMR: 23 ppm.
5AlCl2. 5Li (0.10 g, 0.14 mmol) in toluene was added to a
solution of AlCl3 in toluene (0.02 g, 0.14 mmol) at RT. The mixture
was stirred for 12 h at RT, LiCl was filtered, and the solvent was
removed. The solid was recrystallized using vapor diffusion
(toluene/pentane) to give clear crystals, 0.07 g, 58%. mp: 202-
206 °C. Anal. Calcd for C41H50AlCl2N2P (found): C 70.4 (70.2);
H 7.2 (7.2); N 4.0 (4.2). FT-IR (cm-1): 470(14), 511(10), 535(6),
569(9), 596(7), 802(11), 901(18), 925(19), 1017(8), 1051(20), 1100-
(17), 1174(13), 1256(12), 1314(2), 1336(1), 1435(4), 1496(3), 1585-
(15), 3060(16). 31P{1H} NMR: -2 ppm.
position provides a useful and diagnostic 31P NMR probe
for the analysis of reaction mixtures in situ.
Experimental Section
General. All manipulations were carried out in a N2 filled
Innovative Technologies Drybox. All solvents were distilled prior
to use and dispensed in the drybox. CH2Cl2 was dried at reflux
over CaH2, P2O5, and again over CaH2. Hexane was dried at reflux
over K, Et2O was dried at reflux over Na/benzophenone, and toluene
was dried over Na. 3Li(Et2O) was prepared from literature methods,4
n
and Ph2PCl, BuLi (1.6 M in hexane), AsCl3, SbCl3, and AlCl3
were purchased from Aldrich Chemical Co. and used as received.
Solid samples were evacuated for 12 h and submitted to Desert
Analytics, Tucson, AZ, for determination of elemental composition.
IR spectra were collected on a Bruker VECTOR 22 FT-IR using
Nujol mulls and are reported with ranked intensities in parentheses.
Solution NMR data were collected on a Bruker AC-250 NMR
spectrometer at room temperature unless otherwise indicated.
Chemical shifts are reported in ppm relative to a reference standard
[100% SiMe4 (1H, 13C) and 85% H3PO4 (31P); 1.0 M LiCl (7Li)],
and both 1H and 13C NMR data were calibrated to an internal
reference signal (1H, CHDCl2, 5.32 ppm; 13C, CD2Cl2, 54.00 ppm).
Crystals were mounted under oil (Paratone or perfluoropolyether)
and placed in a cold stream of N2. X-ray diffraction data were
collected on a Bruker PLATFORM diffractometer with a sealed
tube generator and a SMART 1000 CCD detector at 193(2) K using
graphite-monochromated Mo KR (λ ) 0.71073) radiation (Table
1).
Preparative Procedures and Characterization Data. 4. A
solution of 3Li(Et2O) (3.24 g, 6.50 mmol) in toluene (50 mL) was
added to a cooled (-78 °C) solution of ClPPh2 (1.43 g, 6.50 mmol)
in toluene (15 mL). The reaction mixture was stirred for 12 h,
warmed to room temperature, and filtered. Slow removal of solvent
gave yellow crystals 3.22 g, 82.0%; mp 144-147 °C. Anal. Calcd
for C41H51N2P (found): C 81.7 (82.4), H 8.5 (8.2), N 4.7 (4.2).
FT-IR (cm-1): 1526(1), 1322(8), 1191(10), 1028(4), 795(9), 769-
(5), 742(3), 696(2), 555(6), 427(7). 31P{1H} NMR: -11 ppm.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada, the Killam Foun-
dation, the Canada Research Chairs Program, the Canada
Foundation for Innovation, the Nova Scotia Research and
Innovation Trust Fund, and the Walter C. Sumner Foundation
for funding, the Atlantic Region Magnetic Resonance Centre
for use of instrumentation, and the referees for their
comments.
Supporting Information Available: Figures depicting solid
state structures, tables of crystallographic data, and crystallographic
information files. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC0351444
738 Inorganic Chemistry, Vol. 43, No. 2, 2004