Ring closure kinetics of bidentate hemilabile P,N and P,S ligands on a platinum(II) complex

Article abstract of DOI:10.1002/1099-0682(200203)2002:3<629::AID-EJIC629>3.0.CO;2-J

Complexes of the type cis-[PtPh₂(CO)(η¹-P-N)] and cis-[PtPh₂(CO)(η¹-P-S)], where bidentate phosphorus-nitrogen and phosphorus-sulfur ligands are bound to the metal centre in a monodentate fashion [P-N = Ph₂

Full text of DOI:10.1002/1099-0682(200203)2002:3<629::AID-EJIC629>3.0.CO;2-J

FULL PAPER
Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a
Platinum(II) Complex
Raffaello Romeo,*[a] Luigi Monsu’ Scolaro,[a,b] Maria Rosaria Plutino,[a] Andrea Romeo,[a]
Francesco Nicolo’,[a] and Alessandro Del Zotto[c]
Keywords: N,P ligands / Kinetics / Platinum / Hydrogen bonds / Chelate effect
Complexes of the type cis-[PtPh2(CO)(η1-P−N)] and cis-
[PtPh2(CO)(η1-P−S)], where bidentate phosphorus−nitrogen
and phosphorus−sulfur ligands are bound to the metal centre
in a monodentate fashion [P−N = Ph2PC5H4N (Ph2PPy),
Ph2P(CH2)2C5H4N (ppye), Ph2P(o-C6H4)NMe2 (PNMe2),
Ph2P(CH2)nNMe2 (n = 2, 3, i.e., peNMe2 and ppNMe2) and
P−S = Ph2P(CH2)2SEt (P−SEt), Ph2P(CH2)nSPh (n = 1, 2, i.e.,
P−CH2SPh and P−SPh)], were prepared in situ by reaction of
the hybrid ligands with cis-[PtPh2(CO)(SEt2)]. In each case,
from the reaction of the ligands with cis-[PtPh2(Me2SO)2]. A
single-crystal X-ray diffractometric study of cis-
[PtPh2(P−SEt)] (18) was performed. The crystal packing
showed linear chains originated by weak intermolecular
Pt···H−C hydrogen bonding interactions. The chelation
kinetics of P−X in the cis-[PtPh2(CO)(η1-P−X)] complexes
have been monitored in [D]chloroform by 1H and 31P{1H}
NMR. The rates of ring closure were found to be strongly
dependent on the nature (S or N) and steric hindrance of the
the first observed process was the fast substitution of diethyl chelating end of the monocoordinated bidentate P−X ligand,
sulfide by the phosphanyl group leading to the monosubsti- and on the size of the ring formed. In contrast, ring size plays
tuted ring-open cis-[PtPh2(CO)(η1-P−X)] (X = N or S) com- a negligible role, if any, in the dechelation reactions of cis-
plexes, which were characterised in solution by 1H and
[PtPh2(S−S)] [S−S = 1,2-bis(phenylthio)ethane, dpte and 1,3-
31P{1H} NMR spectroscopy. These initially formed species un- bis(phenylthio)propane, dptp] using diphosphanes (dppm
dergo a slow ring closure process with extrusion of carbon
monoxide and formation of the chelate [PtPh2(P−X)] prod-
and dppp) as reagents. These kinetic data, together with
those of previous work, give useful insight into the factors
ucts, except in the case of the short-bite Ph2PPy and controlling cyclisation reactions and the stability of the rings
P−CH2SPh ligands and of ppNMe2, where ring closure was
in square planar platinum(II) complexes.
not observed. The chelate complexes were isolated as solids
Introduction
the rate of ring closure that, usually, is much faster than the
rate of attack of the first end of the bidentate ligand on the
Factors affecting the ease of cyclisation of bifunctional metal centre. An old practice of overcoming this difficulty
organic chains, such as ring strain and the probability of is to render the uncoordinated end of the ligand inactive by
end-to-end joining, have been well understood and reco- steric hindrance or protonation. This latter approach has
gnised in physical organic chemistry, through detailed kin- been used with the formation of certain five- and six-mem-
etic studies. At this stage they are at the basis of synthetic bered chelated rings involving amino acids,[3] amino alco-
procedures in making rings of all sizes as well as macro- hols,[4] amino sulfides,[5] or diamines,[6Ϫ12] in platinum(II)
cycles.[1,2] By contrast, in coordination or organometallic complexes. The rate of ring closure can be modulated
chemistry, kinetic studies on the formation of chelated rings through an appropriate control of the acidity of the solu-
are relatively scarce because of the difficulty of monitoring tion but, in most cases, it is not possible to separate the rate
constant for ring closure from the acid dissociation con-
stant of the protonated end of the ligand.
[a]
Dipartimento di Chimica Inorganica, Chimica Analitica e
Chimica Fisica and ICTPN Ϫ CNR, Sezione di Messina,
`
Universita di Messina,
An alternative approach involves the generation in solu-
tion of coordinatively unsaturated species by photolysis, in
the presence of a bidentate ligand LϪL, as for M(CO)6
(M ϭ Cr, Mo, W) complexes.[13Ϫ18] The reaction involves
photodissociation of one CO ligand to yield an extremely
reactive M(CO)5 species which captures LϪL, leading to
an M(CO)5(η1-LϪL) intermediate with a monocoordinate
bidentate ligand. Thereby, ring closure follows with loss of
Salita Sperone, 31-Vill. S. Agata, 98166 Messina, Italy
Fax: (internat.) ϩ 39-090/393756
E-mail: Raf.Romeo@chem.unime.it
[b]
[c]
`
INFM, Unita di Messina
Salita Sperone, 31-Vill. S. Agata, 98166 Messina, Italy
`
Dipartimento di Scienze e Tecnologie Chimiche Ϫ Universita
di Udine
Via Cotonificio, 108, 33100 Udine, Italy
Supporting information for this article is available on the
WWW under http://www.eurjic.com or from the author.
Eur. J. Inorg. Chem. 2002, 629Ϫ638
WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1434Ϫ1948/02/0303Ϫ0629 $ 17.50ϩ.50/0
629

R. Romeo et al.
FULL PAPER
another CO ligand. If this latter process is too rapid, it is thioether, a dimethylamino or a pyridyl function, charac-
necessary to use flash photolysis techniques to initiate the terised by different steric and electronic properties, and sep-
kinetics.[19Ϫ24] In some cases the intimate nature of the ther- arated from the phosphorus atom by a backbone of variable
mal process was revealed by the determination of the vol- length. The reaction takes place very rapidly at room tem-
umes of activation.[20Ϫ24]
perature and produces the monosubstituted species in
In the study of the reactivity of the complex cis- quantitative yield. The most interesting feature of this reac-
[PtPh2(CO)(SEt2)] (1) with pyridines[25] and amines[26] we tion concerns the selectivity of the nucleophilic site of at-
became aware that this substrate contains a labile SEt2 and tack of the unsymmetrical ligand that is invariably the pho-
a relatively inert CO ligand in the trans positions to the sphanyl group. The phosphorus atom enters the coordina-
firmly bonded phenyl groups. The substrate showed a re- tion sphere of the metal centre leaving the remaining nitro-
markable selectivity between the two nucleophilic ends of gen or sulfur atoms free. This is in line with the sequence
an unsymmetrical bidentate ligand, with a clear preference of reactivity, phosphane[32] ϾϾ amine[26] Ͼ pyridine[25] of
for the most basic and the less hindered end. Thus, we have a monodentate ligand replacing SEt2 from 1, while the sub-
exploited this property to prepare stable ring-open species sequent substitution of the carbonyl group takes place very
such as cis-[PtPh2(CO)(η1-NH2(CH2)4NH2)] and cis- slowly. Thus, the pattern of behaviour of the PϪX ligands
[PtPh2(CO)(η1-NH2(CH2)2NHPh)] in solution, to intercept reflects the different nucleophilicity of the two donors (P
and characterise monocoordinate reaction intermediates of and S or N) of the ligand. This peculiar selectivity of the
the type cis-[PtPh2(CO)(η1-NϪN)] or cis-[PtPh2(CO)(η1- substrate between the two ends of an unsymmetrical ligand
PϪP)] in the reactions of this complex with diamines,[27] was already shown in the reactions with dinitrogen ligands
aminopyridines,[27] or diphosphanes,[28] and to identify the such as 2-(aminomethyl)pyridine and 2-(2-aminoethyl)-
factors controlling the rates of their final conversion into pyridine.[27]
the chelate products with extrusion of CO.
In this paper we have extended the study to so-called
‘‘hemilabile’’ ligands. New classes of unsymmetrical bident-
ate ligands (PϪX) were recently developed and used in or-
ganometallic chemistry, which associate the phosphorus
atom with donor atoms such as oxygen,[29] nitrogen[30] or
sulfur.[31] Indeed it was expected, and it has often been ob-
served, that the non-phosphorus donor X of the ligand is
weakly coordinated in solution to the metal centre, generat-
ing potential catalytically active systems, by providing, un-
der mild conditions, a vacant site for the coordination and
activation of organic substrates. To complete the outline of
reactivity of 1 we have chosen a series of hybrid ligands
PϪX of different chain length, bearing a diphenylphos-
phanyl group and a substituted sulfur or nitrogen atom of
variable electronic and steric properties. The high lability of
the SEt2 group in the starting substrate, combined with the
very high nucleophilic power of the phosphorus donor of
the bidentate ligands, leads ineluctably to the rapid forma-
tion of cis-[PtPh2(CO)(η1-PϪX)] and to a subsequent rela-
tively slow ring closure to the final product [PtPh2(PϪX)].
The kinetics of ring closure of PϪX ligands, together with
those of dinitrogen and diphosphane ligands studied previ-
ously,[27,28] give a useful insight into the factors controlling
these cyclization reactions and the stability of the rings in
square-planar complexes.
Scheme 1
The
Results and Discussion
complexes
[PtPh2(SEt2)]2
(2)
and
cis-
Synthesis and Structural Characterisation of the Complexes
[PtPh2(Me2SO)2] (3) proved to be very useful synthons for
Complexes cis-[Pt(Ph)2(CO)(η1-PϪX)] (4؊11), in which the synthesis of chelate complexes. The addition of an equi-
PϪX acts as a P-bound monodentate ligand, were prepared molar amount of a PϪX ligand to a dichloromethane solu-
in situ by treating equimolar amounts of ligand and com- tion of 3, according to Equation (1) leads to the rapid sub-
plex 1. Sketches of the heteronuclear ligands used are given stitution of both sulfoxide ligands, yielding chelate com-
in Scheme 1. The PϪX unsymmetrical species contain on pounds in a pure crystalline form and in almost quantitat-
one side a diphenylphosphanyl group and on the other a ive yield.
630
Eur. J. Inorg. Chem. 2002, 629Ϫ638

Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a PtII Complex
FULL PAPER
cis-[PtPh2(Me2SO)2] ϩ PϪX Ǟ cis-[PtPh2(PϪX)] ϩ 2 Me2SO
(1)
In a similar fashion, these compounds were formed start-
ing from the dinuclear complex 2, in the presence of a mod-
erate excess of SEt2.The cyclization reaction failed in the
case of the short-bite Ph2PPy and PϪCH2SPh ligands and
also ppNMe2, but was successful for the formation of the
chelate cis-[PtPh2(SϪS)] complexes, containing the dithi-
oether ligands dpte (12) and dptp (13). The isolation of the
chelate complexes as solid compounds allowed for their
1
complete characterisation through H and 31P{1H} NMR
spectroscopy. The complexes 14؊18 are characterised by
the presence, on the coordination plane, of two phenyl
groups in the mutual cis position and of a 5- or 6-membered
ring, arising from the coordination of the hybrid chelating
ligand.
Figure 1. Perspective view and atom numbering scheme of one of
the two equal complex units of compound 18; thermal ellipsoids
are drawn at 30% probability, while the H size is arbitrary
X-ray Structure of 18
The crystallographic asymmetric unit contains two inde- centre, but the solid state is a racemic mixture due to
pendent molecules which are related by a pseudo two-fold centrosymmetric space group.
screw axis 21 parallel to cell edge a. However, both are es-
The most interesting feature of the solid state is the pres-
sentially identical structurally (Table 1) and one of them is ence of a weak nonconventional intermolecular hydrogen-
illustrated in Figure 1. The main differences concern the ar- bonding interaction between each platinum atom and the
rangement of the phenyl groups. The η1-phenyl rings are terminal methyl group of the diphenylphosphanyl ligand of
almost equally oriented with respect to the Pt coordination the adjacent complexes (Figure 2). This interaction corre-
plane in the two complexes [53.1(2) vs. 52.8(2)° and 99.0(4) sponds to a 3-centre-4-electron (3c-4e) model, the three
vs. 95.2(4)° for C1Ph and C7Ph planes, respectively] forming centres being the C, H and Pt atoms and the four electrons
a reciprocal dihedral angle of 71.0(4) vs. 63.9(3)°. The same belonging two each to the CϪH bond and to the platinum
2
behaviour is exhibited by the phenyl groups of the di- dz orbital.[36] The two CϪH···Pt interactions are almost
˚
phenylphosphane fragment, with the C13Ph ring forming a equal [C(28)H···Pt ϭ 2.94 vs. 2.88A, C(28)···Pt ϭ 3.68(1)
˚
dihedral angle of 79.2(2) vs. 66.1(3)° with the other C19Ph vs. 3.680(8) A, C(28)ϪH···Pt ϭ 135 vs. 141°]. Despite the
plane. The square-planar coordination around the two fairly high electron density available on the metal ion,[37] it
platinum atoms is almost identical and the geometric values seems that the Pt···H interaction is weak owing to the low
are in agreement with those of other arylplatinum[26,33] electropositive character of the interacting hydrogen atom.
and
1-(ethylthio)-2-(diphenylphosphanyl)ethane
com- This hydrogen interaction alternatively connects the two in-
plexes.[31d,34] Both the two PtϪP and PtϪS bonds [mean dependent complexes of adjacent asymmetric units along
˚
length 2.280(3) and 2.352(2) A, respectively] show a signi- the crystallographic axis a. The crystal packing is consti-
ficant elongation caused by the strong trans influence of the tuted by infinite columns parallel to a and interconnected
η1-phenyl acting as a powerful σ-donor, as already reported by van der Waals interactions as a monodimensional poly-
for the compound cis-[PtPh2(SMe2)2].[35] The chelating li- mer. The weakness of the hydrogen bond in the solid state
gand forms a five-membered puckered ring with a bite precludes its existence in solution, as evidenced by the ab-
1
angle of 85.97(7) vs. 86.30(8)°. The sulfur atoms display a sence in the H NMR spectrum of 18, in CDCl3, of upfield
tetrahedral arrangement acting as a stereogenic akyrotopic or downfield chemical shifts or of scalar 1H-195Pt couplings.
˚
Table 1. Selected bond lengths [A], bond and torsional angles [°] for the two independent molecules in the crystal unit
PtϪC(1)
PtϪC(7)
PtϪP
2.069(8)
2.033(7)
2.282(2)
2.353(2)
2.039(8)
2.016(8)
2.277(2)
2.352(2)
PϪC(13)
PϪC(19)
PϪC(25)
1.807(8)
1.817(8)
1.841(7)
1.813(9)
1.800(9)
1.835(9)
PtϪS
C(7)ϪPtϪC(1)
PϪPtϪS
88.5(3)
85.97(7)
90.6(3)
86.30(8)
C(7)ϪPtϪP
C(1)ϪPtϪS
94.2(2)
91.7(2)
91.8(2)
91.9(2)
SϪPtϪC(1)ϪC(2)
SϪPtϪPϪC(13)
PϪC(25)ϪC(26)ϪS
Ϫ51.6(6)
Ϫ101.6(3)
53.5(7)
Ϫ50.3(6)
Ϫ109.7(3)
37(1)
PϪPtϪC(7)ϪC(8)
SϪPtϪPϪC(19)
PtϪSϪC(27)ϪC(28)
96.2(6)
132.4(3)
Ϫ40.8(8)
95.9(6)
125.5(3)
Ϫ43(1)
Eur. J. Inorg. Chem. 2002, 629Ϫ638
631

R. Romeo et al.
FULL PAPER
tion in the case of P-containing potentially chelating li-
gands.[38] Furthermore, the chemical shift values reported
here for the PϪS species are very close to that found for
other PtII derivatives with the same ligands.[31e] Finally, che-
lation in compound 14 (where the ligand ppye forms a six-
membered ring) is proved on the basis of the existence of a
195Pt-1H scalar coupling between the metal and a proton of
the ‘‘far-from-P’’ side of the ligand. In fact, the H6 nucleus
of the pyridine ring at δ ϭ 8.55 exhibits a 3JPtH coupling of
23 Hz, while conversely the same proton in the correspond-
ing ring-open complex 5 exhibits no coupling.
Ring Closure Reactions
Figure 2. View of crystallographic asymmetric unit showing the
intermolecular H bonds between the two independent complexes
and the adjacent units translated along the crystallographic a axes
(x Ϯ 1, y, z); shaded unit is the complex reported in Figure 1;
thermal ellipsoids at the 10% probability level while the hydrogen
size is arbitrary
As described above, the formation of the chelate com-
plexes occurs in two steps, according to Equation (2): (i) a
fast formation of the ring-open species, on mixing equiva-
lent amounts of complex and ligand, (ii) a subsequent slow
ring closure. The course of the reaction can be effectively
monitored in [D]chloroform by H or 31P{1H} NMR spec-
1
troscopy, through the changes in intensity of appropriate
signals.
NMR Characterisation
Selected 1H and 31P{1H} NMR resonances for ring-open
and chelate compounds are given in Table 2. Within the
chelate compounds 14؊18 the average value of 1JPtP for the
31P{1H} signal is found at 1885 Hz, typical for a phos-
phorus atom trans to a σ-PtϪC bond.[28,32] A remarkable
decrease of the 1JPtP coupling constant is found for the ring-
(2)
Typical time-resolved NMR spectra for a kinetic run with
open derivatives 4؊11 (1JPtP average value 1603 Hz). For ppye are shown in Figure 3. The mol fraction F ϭ [A]/([A]
those ligands (peNMe2, PNMe2, PϪSPh and PϪSEt) which ϩ [B]) of the unchanged complex A was obtained by integ-
form a 5-membered ring upon chelation, the coordination ration of the signals and the first-order rate constant kc
of the N or S donor is confirmed by the marked downfield [sϪ1] for ring closure was obtained from a nonlinear least-
shift of the δ31P values. In fact, an increase of ca. 20 ppm squares fit of the experimental data to F ϭ c1 ϩ c2 exp(Ϫkct),
is observed for the PϪN ligands (in complexes 15 and 16), with c1, c2, and kc as the parameters to be optimised.
while a higher ∆δ (ca. 30 ppm) is found for the PϪS ligands A similar analysis can be performed using the mol fraction
(in complexes 17 and 18). These features are in accordance of the chelate product B. In order to account for the
with the criterion proposed by Garrou for assessing chela- possible re-entry of carbon monoxide into the coordina-
Table 2. Selected 1H and 31P NMR spectroscopic data for ring-open cis-[Pt(Ph)2(CO){η1-(PϪX)}] (numbers in square brackets) and
chelate [PtPh2(PϪX)] complexes[a]
No.
PϪX
1H
31P
[4]
[5]
14
[6]
15
[7]
[8]
16
[9]
[10]
17
PPh2Py
ppye
ppye
8.83 (H6-py)
20.1 (1636)
12.9 (1592)
16.4 (1886)
9.8 (1585)
31.0 (1915)
13.8 (1598)
11.1 (1679)
31.1 (1990)
12.9 (1593)
11.4 (1570)
41.6 (1803)
11.5 (1576)
40.2 (1832)
2.74 (CH2ϪP), 2.55 (CH2ϪN), 8.39 (H6-py)
3.59 (CH2ϪP), 2.36 (CH2ϪN), 8.55 (23) (H6-py)
2.01 (CH3ϪN)
peNMe2
peNMe2
ppNMe2
PϪNMe2
PϪNMe2
PϪCH2SPh
PϪSPh
PϪSPh
PϪSEt
2.72, 2.57 (18) (CH3ϪN), 2.34
2.02 (CH3ϪN), 1.94, 1.83, 1.43
2.71 (CH3ϪN)
3.10 (17) (CH3ϪN)
3.37 (13.0) (CH2ϪP)
2.66, 2.26
2.99 (CH2ϪP), 2.38 (CH2ϪS)
2.32, 2.24 (CH2, CH3ϪCH2ϪS), 1.07 (CH3ϪCH2ϪS)
2.80 (CH2ϪP), 2.66 (CH3ϪCH2ϪS), 2.49 (CH2ϪS), 1.20 (CH3ϪCH2ϪS)
[11]
18
PϪSEt
[a]
1
In CDCl3 at 298.2 K. Chemical shifts (δ) are reported in ppm downfield from TMS. Coupling constants with 195Pt (3JPtH and JPtP
)
in Hz are given in parentheses. Peaks reported in bold were used for monitoring kinetics.
632
Eur. J. Inorg. Chem. 2002, 629Ϫ638

Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a PtII Complex
FULL PAPER
tion sphere of the metal ion, all the rate data were recalcu-
lated by using SCIENTIST[39] and the equation Ϫd[A]/dt ϭ
kc [A] Ϫ k2 [B] [CO], where [A], [B] and [CO] are the con-
centrations of the ring-open complex, the chelate complex
and carbon monoxide, respectively; kc and k2 are the rate
Table 3. Summary of kinetic data for ring closure of dinitrogen,
diphosphane and hybrid PϪX (X ϭ N, S) ligands[a]
constants for ring closure [sϪ1] and ring opening [Ϫ1 Ϫ1].
s
For the short-bite ligands Ph2PPy (4), PϪCH2SPh (9) as
well as for ppNMe2 (7) the only reaction observed was the
fast formation of the ring-open species stable in solution for
a long time before decomposition occurs. A summary of
the relevant kinetic data for ring closure reactions are given
in Table 3 together with data relative to diphosphanes and
dinitrogen symmetrical and unsymmetrical ligands ob-
tained in previous work.[27,28]
[a]
[c]
[b]
In CDCl3 at 298 K, unless otherwise mentioned. From ref.[28]
This work. From ref.[27], in CD2Cl2 as solvent. Abbreviations
[d]
for ligands not encountered in the text: dppe ϭ 1,2-bis(diphenyl-
phosphanyl)ethane; en ϭ 1,2-ethylenediamine; 2-ampy ϭ 2-(amin-
omethyl)pyridine; N-(Ph)en ϭ N-phenylethylendiamine; dap ϭ 1,3-
diaminopropane; 2-aepy ϭ 2-(2-aminoethyl)pyridine; dpy ϭ dipy-
rid-2-ylamine; dps ϭ dipyrid-2-yl sulfide; dppb ϭ 1,4-bis(diphenyl-
phosphanyl)butane; dab ϭ 1,4-diaminobutane
(PϪP ϭ dppm, dppe, dppp) showed a single-stage conver-
sion from the starting complex to the final chelate com-
pound, according to Equation (3).
Figure 3. 1H NMR spectral changes associated with ring closure
on cis-[PtPh2(CO)(η1-ppye)] (5) in CDCl3 at 298 K; the signals
monitored are those of the H6-py proton at δ ϭ 8.39 for the ring-
open and at δ ϭ 8.55 for the chelate [PtPh2(ppye)] (14) complex, re-
spectively
(3)
1
For example, the H NMR spectrum of [PtPh2(dpte)] in
CDCl3 at 298 K showed a broad singlet for the methylene
3
protons (at δ ϭ 3.03; JPtH ϭ 16.6 Hz) and a doublet (δ ϭ
7.38; JPtH ϭ 76 Hz) in the aromatic region, relative to the
Dechelation Reactions
3
Complexes of the type [PtPh2(SϪS)] [SϪS ϭ dpte (12), ortho-protons of the coordinated phenyl rings. By addition
dptp (13)] were easily prepared in high yield by reaction of of dppm, in an 1:1 ratio, these peaks begin to decrease in
2 with the appropriate dithioether ligand SϪS in dichloro- intensity and new signals appear which belong to
methane at room temperature. Any attempt to prepare plat- [PtPh2(dppm)] (methylene proton signals at δ ϭ 4.36), to-
inum(II) derivatives with shorter or longer SϪS ligands [i.e. gether with a sharp singlet at δ ϭ 3.10 of free dpte. The
bis(phenylthio)methane and 1,4-bis(phenylthio)butane] 31P{1H} NMR spectrum of the reaction product shows a
failed, due to the instability of four- and seven-membered singlet at δ ϭ Ϫ 37.6 with platinum satellites of 1392 Hz,
chelate rings. The analysis of the 1H NMR spectral changes typical of the four-membered diphosphane chelate ring.
1
during the course of the reaction with diphosphane ligands The H and 31P{1H} NMR spectra of the compound agree
Eur. J. Inorg. Chem. 2002, 629Ϫ638
633

R. Romeo et al.
FULL PAPER
with its formulation as a diaryl(phosphanyl)platinum che- mechanism obeys Equation (5) which reduces to kobs ϭ k1
late complex.[28] No intermediate species were detected in ϩ k2 [PϪP], when kϪ1 ϽϽ k3 [PϪP].
solution during the reaction and addition of excess dppm
does not significantly affect the reaction. The same behavi-
our was observed using dppe or dppp as reagents, with
formation of the phosphane-chelated derivatives
[PtPh2(dppe)] and [PtPh2(dppp)], respectively.
kobs ϭ k1 k3 [PϪP]/ (kϪ1 ϩ k3 [PϪP]) ϩ k2 [P-P]
(5)
Reaction according to Equation (3) (PϪP ϭ dppm,
dppp) was carried out under pseudo-first-order conditions
in dichloromethane at 298.2 K, using spectrophotometric
techniques. The kinetics were monitored by repetitive scan-
ning of the spectrum at suitable times in the wavelength
range 315Ϫ350 nm, where the absorbance of the free dipho-
sphane ligand is negligible. The reactions went to comple-
tion, the final spectra being identical to those of authentic
samples of [PtPh2(PϪP)] complexes.[28] Abstract factor ana-
lysis[40] of the spectral changes confirmed the presence of
only two absorbing species in solution, i.e. the starting com-
plex [PtPh2(SϪS)] and the final diphosphane product
[PtPh2(PϪP)]. The pseudo-first-order rate constants kobs
calculated from the kinetic traces, are listed in the Sup-
porting Information (Table SI1, see footnote on the first
page of this article). On plotting kobs as a function of the
entering diphosphane concentration straight lines were ob-
tained with a y axis intercept different from zero. Linear
regression analysis applied to the rate data and fitted with
the simple two-term Equation (4) gave the following values
of the rate constants for the two systems: (i) for
[PtPh2(dpte)], k1 ϭ (0.004 Ϯ 0.0008) sϪ1, k2 ϭ (1.04 Ϯ 0.03)
Scheme 2
Thus, the values derived for k1 and k2 represent the rate
constants for ring opening upon dissociation of the PtϪS
bond (k1 [sϪ1]) and upon bimolecular attack of the nucle-
ophile (k2 [Ϫ1sϪ1]). The values of k1 and k2 derived for the
two complexes do not show appreciable differences, indicat-
ing that both the pathways for dechelation (dissociative and
associative) are hardly affected by the length of the ring
(five- or six-membered).
The Chelate Effect

Ϫ1sϪ1 in the reaction with dppm, and k1 ϭ (0.007 Ϯ
0.0001) sϪ1, k2 ϭ (1.86 Ϯ 0.05) Ϫ1sϪ1 with dppp; (ii) for
The formation constant of a metalϪchelate complex is
[PtPh2(dptp)] k1 ϭ (0.004 Ϯ 0.0008) sϪ1, k2 ϭ (1.00 Ϯ 0.03) related to the rate constants of the separate bond-making

Ϫ1sϪ1 in the reaction with dppm.
and bond-breaking steps in Scheme 3 by the expression
Kf ϭ k1k3/k2k4. For octahedral systems, such as CoIII and
NiII chelates, there is increasing evidence indicating that the
kobs ϭ k1 ϩ k2 [P-P]
(4)
We know from previous studies[37] that complexes con- processes of metalϪligand bond formation (k1 and k3) and
taining ligands with an array of donor atoms of the type loss of a monodentate ligand (k2) hardly affect the stability
cis-[Pt(C,C)(S,S)] (C ϭ alkyl or aryl; S ϭ thioether or sul- of the chelate. An unexpected slowness of the ring-opening
foxide) show a great propensity to undergo substitution of reaction (k4) accounts for the high formation constant of
one sulfur ligand through a dissociative mechanism. The the chelates,[42] while the effect has traditionally been asso-
reasons for the observed behaviour are a combination of (i) ciated with a high chelate ring-closing rate constant (k3).
ground-state destabilisation (lengthening of the PtϪS Two different explanations have been given to account for
bond), (ii) increase of electron density at the metal centre the slowness of this dissociatively activated pathway (k4).
preventing the approach of weak nucleophiles, and (iii) sta- For ethylendiamine complexes of relatively small ions, Fun-
bilisation of the coordinatively unsaturated three-coordin- ahashi et al.[43] have proposed that the elongation of the
ated intermediate. The most likely mechanism that can be MϪN separation, along the coordinate reaction, forces the
envisaged for substitution reactions of the chelated metalϪnitrogenϪcarbon (MϪNϪC) angle to decrease,
[PtPh2(SϪS)] complexes, on the basis of the behaviour of thus increasing the energy involved in the formation of the
strictly similar substrates containing monodentate thioe- transition state. In the case of a monodentate amine such
thers,[41] is depicted in Scheme 2, and involves: (i) dissociat- angular distortion around the leaving nitrogen donor is ab-
ive opening of the chelate SϪS ring (k1 path) to yield a sent. Very recently, Blackman, Clark et al.[44] have given an
ring-open three-coordinated 14-electron intermediate, (ii) alternative explanation for the much slower rate of chelate
competition for it between ring closure (kϪ1 path) and the ring-opening in [Co(cyclen)(diamine)]3ϩ compared to the
attack of one end of the diphosphane ligand (via k3) to loss of NH3 from [Co(cyclen)(NH3)2]3ϩ, based on the re-
form a doubly ring-open species and, (iii) final fast ring duced ability of the former system to allow the bond angle
closure of PϪP to yield the observed products. The pathway expansion required to produce the SN1CB trigonal-bipyr-
via k2 represents the usual direct bimolecular attack of the amidal intermediate. Formation of the latter from the con-
nucleophile on the substrate. The rate law derived for this jugated base [Co(cyclen-H)(diamine)]2ϩ requires expansion
634
Eur. J. Inorg. Chem. 2002, 629Ϫ638

Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a PtII Complex
FULL PAPER
of the bond angles in the equatorial plane from ca. 90° to much greater nucleophilicity of PPh2 compared to pyridine.
120°, and the in-plane bidentate ligand will hinder this ex- In the case of ppNMe2, steric congestion brought about
pansion, while the loss of NH3 from [Co(cyclen- by alkyl substitution on the donor nitrogen atom impedes
H)(NH3)2]2ϩ is not restricted in this way. Clearly, the energy ring closure.
barrier for the rate-determining CoϪN bond cleavage and
The opposite role of k3 and k4 in octahedral and square-
the formation of the dissociative five-coordinate interme- planar complexes stems from the different mode of activa-
diate is increased by the structural restriction imposed by tion (dissociative vs. associative). In light of the above con-
the retention of the diamine chelate character.
siderations, elongation of the ring combined with the use of
a weak donor atom at one end of the chelating ligand,
would be a way of increasing the ‘‘hemilability’’ of hybrid
ligands in square-planar complexes and their possible effici-
ency as catalysts.
Scheme 3
The mechanistic picture for ring opening and closure in Experimental Section
a square-planar complex appears to be much simpler than
General Procedures and Chemicals: All solvents were dried accord-
that of an octhaedral complex, since both processes (k3 and
k4) are characterised by an associative mode of activation.
We have shown above that the rates of dechelation of 5-
and 6-membered rings in the complexes [PtPh2(SϪS)]
(SϪS ϭ dpte, dptp) have very similar values. This is in line
with the results of previous studies showing that the rate
constants for ring opening of complexes of the type
[Pt(Me2SO)(NϪN)Cl]ϩ (NϪN ϭ en, tn, bn) do not differ
significantly from one complex to another.[10] The same
complexes, in their ring-open [Pt(Me2SO)(NϪNH)Cl2]ϩ
protonated forms, showed a marked dependence of the rate
of ring closure on the ring size. The same strong depend-
ence on ring size characterises the ring-closing kinetics of
trans-[PtCl2(NH3)(NϪNH)]ϩ.[12] Therefore, as for the rela-
tionships between the formation constant of square-planar
complexes and the size of the chelate ring, it is possible to
conclude that the rates of ring closure (k3) have a dominant
effect while ring opening (k4) makes a relatively small con-
tribution. The formation of 4-membered rings is hindered
by ring strain and the formation of large rings (7- or 8-
membered rings) must compete with other reaction path-
ways, in particular with the possibility of obtaining dinuc-
lear or polynuclear species. In this context, it is interesting
to mention the results of a recent work on chelation kinetics
of bidentate phosphane ligands on five-coordinate
[Ru(CO)4(η1-PϪP)] complexes, where the rates of ring clos-
ing (104 kobs/sϪ1 at 60 °C, in n-heptane) are as follows:
dppm (11.2), dppe (4.44), dppp (6.73), dppb (5.75). This
stands in striking contrast with the strong dependence of
the rates on ring size observed for the square-planar
[PtPh2(CO)(η1-PϪP)] complexes (Table 3 and ref.[28]).
ing to standard procedures by distillation under oxygen-free nitro-
gen from appropriate drying agents (i.e. dichloromethane from bar-
ium oxide and diethyl ether from sodium benzophenone ketyl).
Spectrophotometric grade dichloromethane was dried by distilla-
tion, degassed by several freeze-pump-thaw cycles and then stored
˚
over activated 4-A molecular sieves. CDCl3 was purchased from
Cambridge Isotopes Laboratories (D 99.8%). Solid phosphane li-
gands (Aldrich Chemical Co) triphenylphosphane (PPh3), bis(di-
phenylphosphanyl)methane (dppm), and 1,3-bis(diphenylphos-
phanyl)propane (dppp) were recrystallized from ethanol. 1,2-Bis-
(phenylthio)ethane (dpte), and 1,3-bis(phenylthio)propane (dptp)
were recrystallized from toluene before use. 2-(Diphenylphosphan-
yl)pyridine (PPh2Py) was purchased from Aldrich. The other bi-
dentate hybrid ligands PϪX {1-(diphenylphosphanyl)-2-(pyrid-2-
yl)ethane (ppye),[46] 1-(dimethylamino)-2-(diphenylphosphanyl)-
ethane (peNMe2),[47] 1-(dimethylamino)-3-(diphenylphosphanyl)-
propane
(ppNMe2),[47]
[o-(dimethylamino)phenyl]diphenyl-
phosphane (PNMe2),[48] 1-(diphenylphosphanyl)-2-(thioethyl)-
ethane
(PϪSEt),[49]
1-(diphenylphosphanyl)-2-(thiophenyl)-
ethane (PϪSPh),[49] (diphenylphosphanyl)(thiophenyl)methane
(PϪCH2SPh)[50]} were prepared according to published methods.
Instrumentation: 1H and 31P{1H} NMR spectra were recorded at
298 K with a Bruker AMX R-300 spectrometer operating with a
broad-band probe at 300.13 and 121.50 MHz for 1H and 31P nuclei,
respectively. Chemical shifts (δ) refer to SiMe4 for 1H and to ex-
ternal H3PO4 (85%) for 31P{1H} NMR. The temperature within
the NMR tube was calibrated by use of the ethylene glycole
method.[51] UV/Vis data were collected with a rapid-scanning
HewlettϪPackard Model 8452 A spectrophotometer. Infrared
spectra were recorded in dichloromethane solution using
a
PerkinϪElmer FT-IR model 1730 spectrometer. Microanalysis was
performed by Redox Analytical Laboratories, Milan, Italy.
The nucleophilic power of the uncoordinated end of the X-ray Data Collection and Structure Refinement of 18: Crystal data:
C28H29PPtS (623.63), monoclinic, P21/c (no. 14), a ϭ 12.230(3),
chelating ligand is also a factor of major importance in con-
trolling the rates of ring closure (k3), as suggested by the
data collected in Table 3. The differences in reactivity are
the same as those found in the n0Pt nucleophilicity scale
which controls the rates of bimolecular substitution reac-
tions on platinum(II).[45] An attractive example comes from
the comparison of the rates of ring closure by dppp, ppye,
and ppNMe2 with a 6-membered ring. As a matter of fact,
the difference of velocity between dppp and ppye encom-
˚
b ϭ 24.911(3), c ϭ 16.493(2) A, β ϭ 90.789(15)°, V ϭ 5025(1)
3
A , Z ϭ 8, Dcalcd. ϭ 1.649 Mg/m3, F(000) ϭ 2448, µ (Mo-Kα) ϭ
˚
5.745 mmϪ1, R1 ϭ 0.044/0.070 and wR2 ϭ 0.098/0.112 for 7123/
9763 observed [I Ͼ 2σ(I)]/all independent reflections, GOF ϭ
1.024. Air-stable, pale yellow crystals of 18 were obtained by slow
diffusion of n-hexane into a concentrated dichloromethane solution
of the complex. For the data collection, a prismatic single crystal
(0.33 ϫ 0.25 ϫ 0.10 mm) was mounted on a Siemens P4 automated
four-circle single-crystal diffractometer and measured with graph-
˚
passes at least five orders of magnitude, in line with the ite-monochromated Mo-Kα radiation (λ ϭ 0.71073 A). 12164
Eur. J. Inorg. Chem. 2002, 629Ϫ638
635

R. Romeo et al.
FULL PAPER
1
reflections were collected using the ω/2θ scan technique up to 2θ ϭ
52°. No crystal decay was detected from the check reflections mon-
cis-[PtPh2(η1-P؊SPh)(CO)] (10): H NMR (CDCl3):δ ϭ 2.66 (m,
2 H), 2.26 (m, 2 H). 31P{1H} NMR (CDCl3): δ ϭ 11.4 (1JPtP
ϭ
itored every 197 measurements. Intensities were evaluated by pro- 1570 Hz). IR: νCO ϭ 2068 cmϪ1
.
file fitting of a 96-steps peak scan among 2θ shells procedure[52]
1
cis-[PtPh2(η1- P؊SEt)(CO)] (11): H NMR (CDCl3): δ ϭ 1.07 (t,
3 H, CH3ϪCH2ϪS), 2.24 (m, 4 H, CH2 ϩ CH3ϪCH2ϪS), 2.32
(m, 2 H). 31P{1H} NMR (CDCl3): δ ϭ 11.5 (1JPtP ϭ 1576 Hz). IR:
and then corrected for Lorentz polarisation effects. Absorption
correction was applied by fitting a pseudo-ellipsoid to the azi-
muthal scan data of 20 suitable reflections with high χ angles.[53]
Data collection and reduction were performed by XSCANS[54] and
SHELXTL[55] packages. The accurate evaluation of the data set
excluded the orthorhombic crystal system. The structure was
solved by a combination of standard Direct Methods[56] and Four-
ier synthesis, and refined by minimising the function Σw(F2o Ϫ Fc2)2
with the full-matrix least-squares technique based on all 9763 inde-
pendent F2 [R(int) ϭ 0.0284], by using SHELXL97.[57] All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
included in the model refinement among the ‘‘riding model’’
method with the XϪH bond geometry and isotropic displacement
parameter depending on the parent atom X. An empirical extinc-
tion parameter was included in the last refinement cycles. The last
difference map showed the largest electron density residuals within
νCO ϭ 2066 cmϪ1
.
Chelate complexes with both dithioethers and hybrid PϪX ligands
were prepared by adding a stoichiometric amount of the ligand to
compound 2 or 3 in dichloromethane solution. The compounds
were precipitated in an almost quantitative yield by adding diethyl
ether or n-hexane (1:1, v/v) and cooling to Ϫ30 °C. The off-white
solids were washed with cold n-hexane and air-dried.
[PtPh2(dpte)] (12): 1H NMR (CDCl3): δ ϭ 3.03 (br. s, JPtH
ϭ
3
16.6 Hz, 4 H, CH2ϪS), 6.81 (m, 2 H, H4), 6.90 (dd, Jav ϭ 6.6,
4JPtH ϭ 12.0 Hz, 4 H, H3,5), 7.38 (d, 3JH,H ϭ 6.6, 3JPtH ϭ 76.0 Hz,
4 H, H2,6), 7.41 (m, 6 H, Hm,mЈ ϩ Hp PhϪS), 8.00 (m, 3JH,H ϭ 6.6,
4JH,H ϭ 2.2 Hz, 4 H, Ho,oЈ PhϪS). Yield: 95%. C26H24PtS2 (595.68):
calcd. C 52.42, H 4.06; found C 52.8, H 4.15.
3
1 A from the platinum atoms (max/min values 1.657/Ϫ1.437 eAϪ3).
The final geometrical calculations and drawings were carried out
with the PARST program[58] and the XPW utility of the Siemens
package, respectively.[59]
˚
˚
[PtPh2(dptp)] (13): 1H NMR (CDCl3): δ ϭ 2.26 (m, 2 H,
3
CH2ϪCH2ϪS), 3.28 (m, JPtH ϭ 13.3 Hz, 4 H, CH2ϪCH2ϪS),
6.69 (m, 2 H, H4), 6.78 (dd, 3Jav ϭ 7.1, 4JPtH ϭ 11.5 Hz, 4 H, H3,5),
7.23 (d, JH,H ϭ 7.1, JPtH ϭ 75.2 Hz, 4 H, H2,6), 7.28 (m, 6 H,
3
3
Synthesis of Complexes: The complexes cis-[PtPh2(CO)(SEt2)]
(1),[60] [PtPh2(SEt2)]2 (2),[60] and cis-[PtPh2(dmso)2] (3),[61] were syn-
thesised according to literature methods. Complexes in which the
heteronuclear PϪX ligands are bound to the metal ion in a monod-
entate fashion were formed upon mixing equimolar amounts of
complex 1 and of the appropriate ligand in CDCl3 solution, taking
advantage of the slow rate of ring closure. These reaction interme-
diates have been characterised in situ by 1H and 31P{1H} NMR
spectroscopy (T ϭ 298 K).
Hm,mЈ ϩ Hp PhϪS), 7.75 (m, JH,H ϭ 7.1, JH,H ϭ 2.7 Hz, 4 H,
Ho,oЈ PhϪS). Yield: 93%. C27H26PtS2 (609.71): calcd. C 53.19; H
4.30; found C 53.6, H 4.27.
3
4
[PtPh2(ppye)] (14): 1H NMR (CDCl3): δ ϭ 2.36 (m, 2 H, CH2),
3.59 (m, JPH ϭ 25 Hz, 2 H, CH2ϪP), 6.54 (m, 3 H), 6.85 (m, 1
2
H), 6.95 (m, 4 H), 7.08 (m, 2 H), 7.23 (m, 8 H), 7.40 (m, 4 H), 7.68
(m, 1 H), 8.55 (m, JPtH ϭ 23 Hz, 1 H, H6-py). 31P{1H} NMR
3
(CDCl3): δ ϭ 16.4 (1JPtP ϭ 1886 Hz). Yield: 92%. C31H28NPPt
(640.61): calcd. C 58.12, H 4.41, N 2.19; found C 58.3, H 4.32,
N 2.30.
cis-[PtPh2(η1-Ph2PPy)(CO)] (4): 1H NMR(CDCl3): δ ϭ 6.57 (m, 1
H), 6.65 (m, 2 H), 6.83 (m, 1 H, H3-py), 6.90 (m, 1 H), 7.04 (m, 2
H), 7.13 (m, 1 H, H5-py), 7.27 (m, 1 H, H4-py), 7.53Ϫ7.44 (m, 14
H), 8.83 (m, 1 H, H6-py).31P{1H} NMR (CDCl3): δ ϭ 20.1 (1JPtP ϭ
1
[PtPh2(peNMe2)] (15): H NMR (CDCl3): δ ϭ 2.34 (m, 2 H), 2.57
(s, 3JPtH ϭ 18 Hz, 6 H, CH3ϪN), 2.72 (m, 2 H), 6.60 (m, 1 H), 6.65
(m, 2 H), 6.82 (m, 1 H), 7.03 (m, 2 H), 7.20 (m, 2 H), 7.48Ϫ7.38 (m,
12 H). 31P{1H} NMR (CDCl3): δ ϭ 31.0 (1JPtP ϭ 1915 Hz). Yield:
89%. C28H30NPPt (606.60): calcd. C 55.44, H 4.98, N 2.31; found
C 55.6, H 5.07, N 2.42.
1636 Hz). IR: νCO ϭ 2070 cmϪ1
.
cis-[PtPh2(η1-ppye)(CO)] (5): 1H NMR (CDCl3): δ ϭ 2.55 (m, 2 H,
CH2), 2.74 (m, 2 H, CH2ϪP), 6.77 (m, 2 H), 6.89 (m, 3 H), 7.04
(m, 4 H), 7.50 (m, 8 H), 7.63 (m, 6 H), 8.39 (m, 1 H, H6-py).
31P{1H} NMR (CDCl3): δ ϭ 12.9 (1JPtP ϭ 1592 Hz). IR: νCO
ϭ
[PtPh2(PNMe2)] (16): 1H NMR (CDCl3): δ ϭ 3.10 (s, JPtH
ϭ
3
2066 cmϪ1
.
17 Hz, 6 H, CH3ϪN), 6.59 (m, 3 H), 6.85 (m, 1 H), 7.04 (m, 3 H),
7.14 (m, 2 H), 7.38 (m, 11 H), 7.53 (m, 2 H), 7.60 (m, 2 H). 31P{1H}
1
cis-[PtPh2(η1-peNMe2)(CO)] (6): H NMR (CDCl3): δ ϭ 2.01 (s, 6
NMR (CDCl3):
δ ϭ ϭ 1990 Hz). Yield: 90%.
31.1 (1JPtP
H, CH3ϪN). 31P{1H} NMR (CDCl3): δ ϭ 9.8 (1JPtP ϭ 1585 Hz).
C32H30NPPt (654.64): calcd. C 58.71, H 4.62, N 2.14; found C 58.8,
H 4.70, N 2.21.
IR: νCO ϭ 2062 cmϪ1
.
1
cis-[PtPh2(η1-ppNMe2)(CO)] (7): H NMR (CDCl3): δ ϭ 1.43 (m, [PtPh2(P؊SPh)] (17): 1H NMR (CDCl3): δ ϭ 2.38 (m, 2 H,
2 H), 1.83 (m, 2 H), 1.94 (m, 2 H), 2.02 (s, 6 H, CH3ϪN), 6.80 (m, CH2ϪS), 2.99 (m, 2JPH ϭ 24 Hz, 2 H, CH2ϪP), 6.76 (m, 4 H), 6.92
1 H), 7.05Ϫ6.90 (m, 7 H), 7.55Ϫ7.43 (m, 12 H). 31P{1H} NMR (m, 2 H), 7.19 (m, JPtH ϭ 76 Hz, 2 H, H2,6 trans to S), 7.40 (m,
3
(CDCl3): δ ϭ 13.8 (1JPtP ϭ 1598 Hz). IR: νCO ϭ 2071 cmϪ1
.
15 H), 7.99 (m, 2 H). 31P{1H} NMR (CDCl3): δ ϭ 41.6 (1JPtP
ϭ
1803 Hz). Yield: 87%. C32H29PPtS (671.69): calcd. C 57.22, H 4.35;
found C 57.6, H 4.45.
1
cis-[PtPh2(η1-PNMe2)(CO)] (8): H NMR (CDCl3): δ ϭ 2.71 (s, 6
H, CH3ϪN). 31P{1H} NMR (CDCl3): δ ϭ 11.1 (1JPtP ϭ 1679 Hz).
3
[PtPh2(P؊SEt)] (18): 1H NMR (CDCl3): δ ϭ 1.20 (t, JH,H
ϭ
IR: νCO ϭ 2064 cmϪ1
.
7.5 Hz, 3 H, CH3ϪCH2ϪS), 2.49 (m, 2 H, CH2ϪS), 2.66 (q,
cis-[PtPh2(η1-PCH2SPh)(CO)] (9): 1H NMR (CDCl3): δ ϭ 3.37 3JH,H ϭ 7.5 Hz, 2 H, CH3ϪCH2ϪS), 2.80 (m, 2 H, CH2ϪP), 6.72
2
3
3
(m, JPH ϭ 6, JPtH ϭ 13 Hz, 2 H, CH2ϪP), 6.77 (m, 1 H), 6.89 (m, 3 H), 6.86 (m, 1 H), 7.02 (m, 2 H), 7.19 (m, JPtH ϭ 74 Hz, 2
(m, 4 H), 7.00 (m, 5 H), 7.14 (m, 3 H), 7.47 (m, 8 H), 7.60 (m, 4 H, H2,6 trans to S), 7.49Ϫ7.34 (m, 12 H). 31P{1H} NMR (CDCl3):
H). 31P{1H} NMR (CDCl3): δ ϭ 12.9 (1JPtP ϭ 1593 Hz). IR: νCO ϭ δ ϭ 40.2 (1JPtP ϭ 1832 Hz). Yield: 90%. C28H29PPtS (623.65):
2068 cmϪ1
.
calcd. C 53.92, H 4.69; found C 54.1, H 4.61.
636
Eur. J. Inorg. Chem. 2002, 629Ϫ638

Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a PtII Complex
FULL PAPER
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Kinetics
[10]
[11]
[12]
NMR Measurements: The ring-closure reactions were initiated as
follows: 6·10Ϫ3 mmol of the hemilabile ligand was dissolved in
CDCl3 (0.3 mL) in a 5-mm NMR tube which was immersed in a
liquid nitrogen bath. After a few minutes, a [D]chloroform solution
of cis-[PtPh2(CO)(SEt2)] (6·10Ϫ3 mmol, 0.2 mL) was added over the
frozen solution of the ligand and the NMR tube was cooled again
to allow the complex solution to freeze over that of the ligand.
Then, the NMR tube was rapidly inserted into the probe of the
M. L. Tobe, A. Schwab, R. Romeo, Inorg. Chem. 1982, 21,
1185Ϫ1190.
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spectrometer thermostated at 298 K. H or 31P{1H} NMR spectra
1
were recorded, immediately after the sample temperature reached
an equilibrium value (i.e. stable lock signal), and at subsequent
appropriate time intervals. The first recorded spectrum only
showed signals relative to free sulfide at δ ϭ 2.57 and 1.27 and to
the ring-open cis-[Pt(Ph)2(CO)(η1-PϪX)] complex, indicating that
the displacement of SEt2 from 1 by the phosphorus end of all the
bidentate ligands is too fast at 298 K to be monitored by 1H NMR
spectroscopy. The ring-closure process was followed by monitoring
the decrease in intensity of the 1H or 31P{1H} signals associated
with the amount of unchanged cis-[PtPh2(CO)(η1-PϪX)] and the
matching increase of the signals due to the final chelate
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We are grateful to the Ministero dell’Universita e della Ricerca Sci-
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