Synthesis and thermochemical study of ligand substitution reactions of aminobis(phosphines), Ph2P(R)NPPh2, with [Me 2Pt(COD)]

Article abstract of DOI:10.1016/j.ica.2005.03.021

The reactions of aminobis(phosphines), Ph₂PN(R)PPh₂ (R = H, Me, Et, ⁿPr, ⁿBu, Ph), with Me₂Pt(COD) led to the formation of four-membered metallacycles in good yield. The enthalpies of the ligand subst

Full text of DOI:10.1016/j.ica.2005.03.021

Inorganica Chimica Acta 358 (2005) 2817–2820
www.elsevier.com/locate/ica
Note
Synthesis and thermochemical study of ligand substitution
reactions of aminobis(phosphines), Ph2P(R)NPPh2,
with [Me2Pt(COD)]
a,
a
b
b,
*
Maravanji S. Balakrishna *, Srinivasan Priya , William Sommer , Steven P. Nolan
a
Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai 400 076, India
b
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148, USA
Received 22 September 2004; accepted 6 March 2005
Available online 18 April 2005
Abstract
The reactions of aminobis(phosphines), Ph2PN(R)PPh2 (R = H, Me, Et, nPr, nBu, Ph), with Me2Pt(COD) led to the formation of
four-membered metallacycles in good yield. The enthalpies of the ligand substitution reactions are measured by the anaerobic solu-
tion calorimetry in THF at 30 ꢀC.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Aminobis(phosphines); Thermochemistry; Enthalpy; Platinum complexes
1. Introduction
destabilizing strain energy caused by the formation of
chelate ring. The thermochemical measurements can as-
sess the metal–ligand interactions in organometallic sys-
tems effectively [6,7]. Nolan and coworkers [8–13] and
others [14,15] have extensively studied the steric and elec-
tronic influences in the transition metal complexes con-
taining tertiary phosphines and arsines using solution
calorimetry. As a part of our interest in organometallic
chemistry of phosphorus based ligands [16–21], we de-
scribe in this paper the synthesis and the solution calori-
metric studies of platinum (II) complexes of RN(PPh2)2
(R = H, Me, Et, nPr, nBu, Ph) (see Scheme 1).
The utility of chelating bis(phosphine) ligands in orga-
nometallic chemistry and in homogeneous catalysis [1] is
well documented. The platinum complexes of bis(phos-
phines) are of continuing interest owing to their potential
use in processes such as coupling reactions [2], olefin
reduction [3], and hydroformylation reactions [4,5]. In a
homogeneous catalytic process, the reactivity at the metal
center can be better understood by knowing the entropy
of the system along with the metal–ligand bond disrup-
tion energy. The bond disruption energy value is a net ef-
fect of the stabilizing metal–ligand bond and the
2. Experimental
Abbreviations: dpype, bis(dipyridylphosphino)ethane; dppf, bis
(diphenyl-phosphino)ferrocene; diop, (2,2-dimethyl-1,3-dioxolan-4,5-
diyl)bis(methylene) bis(diphenylphosphine); dmpe, bis(dimethyl-
phosphino)ethane; depe, bis(diethylphosphino)ethane; dcpe, bis
(dicyclohexylphosphino)ethane
2.1. General considerations
All experimental manipulations were performed un-
der an inert atmosphere of dry nitrogen or argon, using
standard Schlenk techniques. All the solvents were puri-
fied by conventional procedures and distilled prior to
*
Corresponding authors. Tel.: +91 22 25767181; fax: +91 22
25767152/2225723480.
E-mail addresses: krishna@iitb.ac.in (M.S. Balakrishna), SNolan@
uno.edu (S.P. Nolan).
0020-1693/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.03.021

2818
M.S. Balakrishna et al. / Inorganica Chimica Acta 358 (2005) 2817–2820
Table 1
Spectral data for the complexes [Me2Pt{Ph2PN(R)PPh2-jP,jP}]
(7–12)
Ph2
PPh2
PPh2
Me
Me
P
Me
Me
R
N
Pt
+
N
R
Pt
P
1JPt–P
d CH3
(in ppm)
2JPt–H
Ph2
R
dP
(in ppm)
R = H, Me, Et, nPr, nBu, Ph
10 11 12
(in Hz)
(in Hz)
7
8
9
H (7)
Me (8)
Et (9)
nPr (10)
nBu (11)
Ph (12)
19.3
47.1
46.7
47.7
47.8
49.3
3138
1519
1594
1547
1547
4079
1.16
0.83
0.73
0.50
0.54
1.11
47.7
40.3
46.9
44.7
40.2
70.1
Scheme 1.
use. The aminobis(diphenylphosphines), RN(PPh2)2
n
n
(R = H, Me, Et, Pr, Bu, Ph), and Me2Pt(COD) were
prepared according to the literature procedures. The
1H and 31P NMR spectra were recorded using Varian
VXR 300, Bruker AMX 400, Gemini 300 and Oxford
400 spectrometers; Me4Si was used as internal standard
(ca. 2 h). Control reactions with Hg and phosphine show
no reaction. The enthalpy of ligand substitution
(ꢀ22.4 0.3 kcal/mol) listed in Table 1 represents the
average of at least three individual calorimetric determi-
nations with all species in solution. The enthalpy of
solution of (COD)PtMe2 (+6.25 0.23 kcal/mol) in
THF was determined using identical methodology.
1
for the H NMR and 85% H3PO4 as external standard
for the 31P NMR. Positive values indicate downfield
shifts. Only materials of high purity as indicated by
NMR spectroscopy were used in the calorimetric exper-
iments. Calorimetric measurements were performed
using a Calvet calorimeter (Setaram C-80), which was
periodically calibrated using the TRIS reaction [22] or
the enthalpy of solution of KCl in water [23]. The calo-
rimeter has been previously described [24,25]. The exper-
imental enthalpies for these two standard reactions were
compared very closely to literature values.
2.4. Syntheses
The complexes 7–12 were synthesized according to
the following typical procedure.
A solution of Ph2PN(R)PPh2 (0.089 mmol) (R = H,
Me, Et, nPr, nBu or Ph) in THF (7 mL) was added drop-
wise to a solution of Me2Pt(COD) (30 mg, 0.089 mmol)
in the same solvent at 25 ꢀC and the reaction mixture
was heated to ꢁ60 ꢀC for 6 h. The volatiles were re-
moved under reduced pressure and the residue was
washed with n-hexane (3 · 5 mL). The colorless off-
white solid was recrystallized in CH2Cl2-n-hexane (2:1)
mixture at 0 ꢀC. The spectral data of the complexes
7–12 are given in Table 1.
2.2. NMR titrations
Prior to every set of calorimetric experiments involv-
ing a new ligand, a precisely measured amount ( 0.1
mg) of (COD)PtMe2 was placed in an NMR tube along
1
with THF and >1.2 equivalents of ligand. Both H and
31P{1H}NMR spectra were measured within 1 h of mix-
ing; both indicated that the reactions were clean and
quantitative. These conditions are necessary for accurate
and meaningful calorimetric results and were satisfied
for all reactions investigated.
The NMR tube reactions of Ph2PN(R)PPh2 (0.059
mmol) (R = Et, nPr, nBu) with Me2Pt(COD) (20 mg,
0.059 mmol) in CH2Cl2 at 25 ꢀC resulted in the forma-
tion of a mixture of complexes, [Me2Pt{Ph2PN(R)PPh2-
n
jP,jP}] and [Me2Pt{Ph2PN(R)PPh2-jP}2] (R = Et, Pr,
2.3. Solution calorimetry
nBu) in ꢁ25% and 75% yields, respectively. The 31P
NMR data are given in Table 2.
In a representative experimental trial, the mixing ves-
sels of the Setaram C-80 were cleaned, dried in an oven
maintained at 145 ꢀC, and then taken into the glove box.
A sample of Me2Pt(COD) (20.3 mg, 0.0609 mmol) was
massed into the lower vessel, which was closed and
sealed with 1.5 mL of mercury. A solution of
PPh2N(nBu)PPh2 (56.4 mg, 0.0128 mmol) in THF
(4 mL) was added, and the remainder of the cell was
assembled, removed from the glove box, and inserted
into the calorimeter. The reference vessel was loaded
in an identical fashion with the exception that no plati-
num complex was added to the lower vessel. After the
calorimeter had reached thermal equilibrium at 30 ꢀC
(ca. 2 h), it was inverted, thereby allowing the reactants
to mix. The reaction was considered complete after the
calorimeter had once again reached thermal equilibrium
3. Results and discussion
The
reactions
of
aminobis(phosphines),
n
n
PPh2N(R)PPh2 (R = H, 1; Me, 2; Et, 3; Pr, 4; Bu, 5;
Ph, 6), with equimolar quantity of Me2Pt(COD) in
Table 2
31P NMR data for the complexes [Me2Pt{Ph2PN(R)PPh2-jP}2]
1
2
R
dP(Pt) (in ppm) dP(un) (in ppm) JPt–P (in Hz) JP–P (in Hz)
Et
56.2
35.4
32.4
35.9
1345
1344
1348
34.7
34.3
34.8
nPr 53.1
nBu 56.5

M.S. Balakrishna et al. / Inorganica Chimica Acta 358 (2005) 2817–2820
2819
THF at 60 ꢀC gave mononuclear complexes 7–12,
respectively. In contrast, when the reactions were carried
out in dichloromethane at room temperature a mixture
of complexes with ligands exhibiting both chelating (7–
12) and monodentate modes of coordination
(Me2Pt{Ph2PN(R)PPh2-jP}2 (R = Et, nPr, nBu)) was
obtained; evidence from the 31P NMR spectroscopic
data [27]. The chemical shifts due to the coordinated
phosphorus centers appear as doublets in the range of
53.1–56.5 ppm with Pt satellites, and the uncoordinated
phosphorus centers also appear as doublets in the range
of 32.4–35.9 ppm (see Table 2). The 31P NMR spectra of
the complexes 7–12 show single resonances (Table 1)
data. Therefore, any variation in the structures and en-
thalpy of reaction should be mainly due to the steric/bite
angle and electronic effects of the phosphine ligands em-
ployed in the study. As in all the complexes, the phos-
phorus environments are the same; the changes in the
enthalpy value are attributed to the substituents on the
nitrogen centers. As shown in Table 3, the enthalpy of
substitution ranges from ꢀ20.7(3) kcal/mol for the com-
plex 7 to ꢀ23.7(3) kcal/mol for the complex 8. The over-
Ph < nBu < Me. In other words, the reaction with the li-
gand having methyl substituent is more exothermic and
the one with hydrogen substituent is least exothermic.
The present enthalpy data allow for comparison to
prior thermochemical studies for the reaction of chelat-
ing aminobis(phosphines) in ruthenium complexes of
the type, Cp0Ru(P ꢁ P)Cl (Cp0 = g5-C5H5, g5-C5H5
[21]. The ligand 6 forms more stable complex with
Cp0Ru(P ꢁ P)Cl when compared to the ligand 2, but
the trend is exactly reverse with Me2Pt(COD). The rea-
son may be the steric and electronic differences attrib-
uted to the other ancillary ligands. However, the
results in the present study are comparable with the
same in the analogous complexes containing bis(phos-
phines) such as dpype, dppf and diop with P–C–P
framework [13]. It is observed that the better r-donors
such as dmpe, depe and dcpe form thermodynamically
n
all order of stability is as follows: H < Et ꢁ Pr ꢁ
1
with JPt–P couplings in the range expected for cisplati-
1
num (II) chelating complexes. The JPt–P values for the
N-alkyl derivatives are in the range of 1519–1594 Hz,
whereas the NH and N-phenyl derivatives show larger
1
values. Similar differences in the JPt–P values were ob-
served between N-phenyl- and N-methylaminobis(diph-
enylphosphites) in chelate complexes, [Cl2Pt{P(OPh)2}2
1
NR] (R = Ph, JPt–P = 3628 Hz; R = Me, JPt–P = 5041
1
1
Hz), and obviously no clear explanation [26]. The H
NMR spectra of the complexes also support the forma-
tion of the complexes 7–12.
3.1. Solution calorimetry
more
aminobis(phosphines).
stable
complexes
compared
to
the
1
As determined by both H and 31P NMR spectros-
copy, the reactions of aminobis(phosphines) (1–6) with
[Me2Pt(COD)] appear quantitative for all ligands inves-
tigated by solution calorimetry at 30 ꢀC in THF. The
4. Conclusions
i
reaction of isopropyl amine derivative, PrN(PPh2)2,
with Me2Pt(COD) was not clean under similar reaction
conditions. The enthalpies for the reaction of Me2Pt
(COD) with aminobis(phosphines) to give the complexes
7–12 are presented in Table 3. These values include the
enthalpy of solution of Me2Pt(COD) (+6.25 0.23
kcal/mol) in THF. All the complexes of this series pos-
sess identical coordination environment around the plat-
inum center as evidenced by the 31P NMR spectroscopic
The reaction enthalpies of aminobis(phosphines) are
similar to the analogous bis(phosphines) with PCP
framework [13]. The relative stability of the chelate com-
plexes is influenced by the substituents present on the
nitrogen center with better r-donors inducing more
thermodynamic stability to the complexes. The coordi-
nation behavior of the aminobis(phosphines) depends
on the reaction conditions, the stoichiometry and the
type of substituents present on both phosphorus and
the nitrogen centers. The complexes of the type
[Me2Pt{Ph2PN(R)PPh2-jP}2] can be used as synthons
to design binuclear complexes and high nuclearity clus-
ters. The research in this direction is in progress.
Table 3
Enthalpies of substitution in the reaction
Me2Pt(COD)
P~P(Soln)
+
(Soln)
THF
COD
+
Me2Pt(P~P)(Soln)
(Soln)
30 oC
Acknowledgments
Complex
Ligands (P ꢁ P)
DH (kcal/mol)
Department of Science and Technology (DST) and
Council of Scientific and Industrial Research (CSIR),
New Delhi, are acknowledged for the financial support
of this work. S.P. is thankful to the CSIR, New Delhi,
for JRF and SRF fellowship. We also thank the Regio-
nal Sophisticated Instrument Center (RSIC), IIT,
7
Ph2PN(H)PPh2
Ph2PN(Me)PPh2
Ph2PN(Et)PPh2
Ph2PN(nPr)PPh2
Ph2PN(nBu)PPh2
Ph2PN(Ph)PPh2
ꢀ20.7(3)
ꢀ23.7(3)
ꢀ21.4(3)
ꢀ21.0(2)
ꢀ22.4(3)
ꢀ21.4(3)
8
9
10
11
12

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[14] G. Kiss, K. Zhang, S.L. Mukerjee, C.D. Hoff, G.C. Roper, J.
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