Synthetic, structural, and solution thermochemical studies in the dimethylbis(phosphine)platinum(II) system. Dichotomy between structural and thermodynamic trends

Article abstract of DOI:10.1021/om9807001

Reaction enthalpies of the complex (COD)PtMe₂ (COD = η⁴-1,5-cyclooctadiene) with an extensive series of unidentate phosphines have been measured by solution calorimetry. The molecular structures of cis-P₂PtMe₂ f

Full text of DOI:10.1021/om9807001

474
Organometallics 1999, 18, 474-479
Syn th etic, Str u ctu r a l, a n d Solu tion Th er m och em ica l
Stu d ies in th e Dim eth ylbis(p h osp h in e)p la tin u m (II)
System . Dich otom y betw een Str u ctu r a l a n d
Th er m od yn a m ic Tr en d s
Christopher M. Haar and Steven P. Nolan*
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
William J . Marshall and Kenneth G. Moloy
Central Research and Development, E. I. du Pont de Nemours & Co., Inc.,
Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328
Alfred Prock and Warren P. Giering
Department of Chemistry, Metcalf Science and Engineering Center, Boston University,
Boston, Massachusetts 02215
Received August 17, 1998
Reaction enthalpies of the complex (COD)PtMe2 (COD ) η4-1,5-cyclooctadiene) with an
extensive series of unidentate phosphines have been measured by solution calorimetry. The
molecular structures of cis-P2PtMe2 for P ) PEt3, PMe2Ph, P(pyrrolyl)3, and PCy3 have been
determined by single-crystal X-ray diffraction. The relative stabilities of the resulting P2-
PtMe2 complexes are strongly influenced by the size (cone angle) of the incoming phosphine,
with larger cone angles resulting in less thermodynamically stable complexes. Crystal-
lographic and 31P NMR data, however, do not reflect the enthalpic stability scale and are
more closely correlated to the electronic (ø) character of the phosphine ligands. The strength
of the Pt-P interaction, as determined from these structural data, is greatest for phosphines
with electron-withdrawing substituents, regardless of phosphine size or reaction enthalpy.
In tr od u ction
The intelligent design of any chemical synthesis
involves an understanding of the factors influencing the
stability of the target compound. It has long been
recognized that the nature of the coordinated phosphine-
(s) can exert a powerful influence on the structural and
catalytic chemistry of transition-metal complexes.1-3
Owing to platinum’s well-developed reaction chemistry,
synthetic routes to σ-bonded organoplatinum(II) com-
plexes are numerous.4 In particular, complexes of the
type (COD)PtRX (COD ) η4-1,5-cyclooctadiene; R, X )
alkyl, aryl, halide) are known for many combinations
of R and X (e.g., 1). The lability of the COD-Pt
interaction renders these compounds convenient sources
of organoplatinum fragments for coordination to Lewis
bases such as phosphines, according to eq 1. In an effort
to better understand the factors which influence the
stability of bis(phosphine)platinum(II) species, a ther-
mochemical study of the reaction in eq 1 was under-
taken for a series of unidentate tertiary phosphines (2)
of varying steric and electronic character. The enthalpic
data are discussed in terms of both steric and electronic
properties of the incoming phosphines and provide an
interesting contrast to NMR spectroscopic and X-ray
structural data for this series of complexes.
(1) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(2) Parshall, G. W.; Ittel, S. Homogeneous Catalysis; Wiley: New
York, 1992.
Resu lts a n d Discu ssion
(3) Pignolet, L. H. In Homogeneous Catalysis with Metal Phosphine
Complexes; Pignolet, L. H., Ed.; Plenum: New York, 1983.
(4) See for example: Anderson, G. K. In Comprehensive Organo-
metallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: Oxford, U.K., 1995; Vol. 9, pp 431-531.
1
As determined by H and 31P NMR spectroscopy, the
reaction illustrated by eq 1 is notably fast, clean, and
quantitative for a wide variety of tertiary phosphines.
10.1021/om9807001 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 01/16/1999

The Dimethylbis(phosphine)platinum(II) System
Organometallics, Vol. 18, No. 4, 1999 475
Ta ble 1. En th a lp ies of Su bstitu tion (k ca l/m ol) a n d NMR Da ta for th e Rea ction
CH2Cl2
30 °C
(COD)PtMe2(soln) + 2PX3(soln)
8 (X3P)2PtMe2(soln) + COD(soln)
entry no.
complex
PX3
-∆Hrxn (kcal/mol)a
δ(31P) (ppm)b
J PtP (Hz)b
δ(1H(CH3)) (ppm)b
J PtCH (Hz)b
3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
3a
3a
3b
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
PMe3
PEt3
PMe2Ph
34.3(0.2)
32.9(0.3)
32.0(0.3)
29.0(0.2)
25.4(0.3)
24.3(0.2)
23.4(0.2)
23.2(0.1)
23.0(0.2)
22.6(0.3)
22.6(0.2)
21.5(0.3)
20.7(0.2)
20.4(0.2)
20.1(0.2)
19.2(0.3)
-23.1
9.3
1761
1843
1826
1854
1852
1917
1933
1876
1888
2048
1900
1850
1859
2506
2245
1828
0.30
0.28
0.43
0.31
0.19
0.28
0.29
0.37
0.36
0.54
0.34
0.46
0.32
0.69
0.66
0.29
66
66
68
68
68
69
69
70
69
66
69
70
67
70
70
67
-10.4
PMePh2
7.6
3.2
PiBu3
P(p-MeC6H4)3
P(p-MeOC6H4)3
P(p-ClC6H4)3
P(p-FC6H4)3
PPh2(pyrl)
PPh3
26.3
24.9
27.1
26.7
69.4
27.7
29.0
32.7
96.6
86.5
20.1
P(p-CF3C6H4)3
PiPr3
P(pyrl)3
PPh(pyrl)2
PCy3
a
b
Enthalpy values are reported with 95% confidence limits. NMR data were recorded in CD2Cl2 at 25 °C.
Of 17 phosphines examined, only PtBu3 failed to react,
presumably due to the steric bulk of this ligand; after 2
days at 30 °C, only a small amount of substitution was
apparent by NMR spectroscopy. In all cases, exclusively
cis-P2PtMe2 was formed. Reaction enthalpies, as well
as structural data determined by 31P NMR and X-ray
crystallography, can be interpreted in terms of the
classical steric (θ) and electronic (ø) parameters intro-
duced by Tolman5 and revised by Giering and others.6-8
This simple two-parameter model, however, fails to
provide a global description of all the observed phenom-
ena in this system. Instead, the most consistent cor-
relations occur primarily among subsets or families of
ligands, while correlations between thermochemical and
F igu r e 1. Enthalpy of reaction vs phosphine electronic
parameter for (COD)PtMe2 + 2PX3: (circles) P(alkyl)3
(slope 2.34, R ) 0.932); (squares) PMe3-xPhx; (diamonds)
structural measurements illustrate that these proper-
ties are not always coupled in a straightforward man-
ner.
P(p-XC6H4)3 (slope -0.189, R ) 0.769); (crosses) PPh3-x
-
Th er m och em ica l Mea su r em en ts. Measured en-
thalpies of reaction span a range of 15 kcal/mol (Table
1) and clearly establish that small-θ phosphines give
the most stable complexes in the P2PtMe2 system. The
large and positive slope of the PR3 line in Figure 1 (for
trialkylphosphines ø -θ9) indicates that the reaction
enthalpies display a strong, inverse dependence on the
cone angle of the incoming ligand. Such an observation
is intuitively satisfying, given that phosphine substitu-
tion occurs at mutually cis positions on platinum. Less
obvious, perhaps, is the observation that an isosteric5,10
series of aryl- and pyrrolylphosphines yield reaction
enthalpies which span a mere 4 kcal/mol range for 14.8
< ø < 41.4.7
(pyrl)x.
negative, which indicates that -∆H becomes larger as
the phosphines become better electron donors. Previous
thermochemical measurements have shown that, in the
absence of steric effects, relative enthalpic stability is
enhanced for electron-rich ligands if the incoming
phosphine interacts through the metal center with an
electron-accepting ligand, e.g., CO in Rh(CO)(Cl)-
(PR3)211,12 and (PR3)2Fe(CO)3.13-17 Conversely, in electron-
rich metal-centered systems such as the R[PNP]Rh(L)
system (R[PNP] ) (R2PCH2SiMe2)2N-; R ) i-Pr, Ph),18
in which the rhodium center is quite electron rich due
to both phosphine and amide donor properties, stability
is enhanced by electron-poor/π-accepting ligands such
as CO or pyrrolyl-substituted phosphines. The clearly
A small but significant ø effect is evidenced by the
nonzero slope of the line for P(p-XC6H4)3 and PPh3-x
-
(pyrl)x in Figure 1. Not surprisingly, the slope is
(11) Huang, A.; Marcone, J . E.; Mason, K. L.; Marshall, W. J .; Moloy,
K. G.; Serron, S.; Nolan, S. P. Organometallics 1997, 16, 3377-3380.
(12) Serron, S. A.; Nolan, S. P.; Moloy, K. G. Organometallics 1996,
15, 4301-4306.
(5) Tolman, C. A. Chem. Rev. 1977, 77, 313-348.
(6) Fernandez, A.; Reyes, C.; Wilson, M. R.; Woska, D. C.; Prock,
A.; Giering, W. P. Organometallics 1997, 16, 342-348 and references
therein.
(7) Fernandez, A. L.; Lee, T. Y.; Reyes, C.; Prock, A.; Giering, W. P.
Submitted for publication.
(8) Fernandez, A.; Reyes, C.; Lee, T.-Y.; Prock, A.; Giering, W. P.
Submitted for publication.
(9) Bartholomew, J .; Fernandez, A. L.; Lorsbach, B. A.; Wilson, M.
R.; Prock, A.; Giering, W. P. Organometallics 1996, 15, 295-301.
(10) Moloy, K. G.; Petersen, J . L. J . Am. Chem. Soc. 1995, 117,
7696-7710.
(13) Serron, S. A.; Nolan, S. P. Inorg. Chim. Acta 1996, 252, 107-
113.
(14) Li, C.; Stevens, E. D.; Nolan, S. P. Organometallics 1995, 14,
3791-3797.
(15) Li, C.; Nolan, S. P. Organometallics 1995, 14, 1327-1332.
(16) Luo, L.; Nolan, S. P. Inorg. Chem. 1993, 32, 2410-2415.
(17) Luo, L.; Nolan, S. P. Organometallics 1992, 11, 3483-3486.
(18) Huang, J .; Haar, C. M.; Nolan, S. P.; Marshall, W. J .; Moloy,
K. G. J . Am. Chem. Soc. 1998, 120, 7806-7815 and references therein.

476 Organometallics, Vol. 18, No. 4, 1999
Haar et al.
inverse, albeit weak, correlation of the reaction enthalpy
to ø in the present case is similar to the results observed
in the (acac)Rh(CO)(PR3) system (acac ) 2,4-pentanedi-
onato).19 This suggests that both (acac)Rh(CO) and
PtMe2 fragments lie in some intermediate domain,
perhaps one in which the synergistic bonding patterns
observed in the above-mentioned systems do not obtain.
Regression analysis of the thermochemical data does
not require the introduction of the aryl (Ear, which
accounts for the disparate trends observed in alkyl- and
arylphosphine physicochemical properties20) or pyrrolyl
(πpyrl, similar to Ear, but related to π-acceptor character
in N-pyrrolylphosphines8) electronic parameters in order
to achieve a satisfactory fit. The point of intersection of
the P(p-XC6H4)3 and PR3 lines in Figure 1 is close to ø
) 5, which indicates that Ear is not important.9 This is
confirmed by the regression analyses: when the Ear
parameter is included in the analysis of P(p-XC6H4)3 and
PR3, no improvement is seen in the correlation, while
the Ear coefficient has a very large standard error. The
π effect for the pyrrolyl ligands is equally problematic.
The inclusion of the πpyrl parameter also does not
improve the correlation, and the standard error of the
πpyrl coefficient, too, is large. Statistically and graphi-
cally, therefore, it appears that both aryl and pyrrolyl
effects are either small or entirely absent, and the
thermodynamic data for the 16 ligands can be analyzed
in terms of the two-parameter model shown in eq 2,
F igu r e 2. 31P chemical shift vs phosphine electronic
parameter (ø) in (PX3)2PtMe2: (circles) P(alkyl)3 (slope
-6.45, R ) 0.846); (squares) PMe3-xPhx; (diamonds) P(p-
XC6H4)3 (slope 0.312, R ) 0.843); (crosses) PPh3-x(pyrl)x.
∆Hrxn (kcal/mol) ) (0.35 ( 0.03)θ +
(0.19 ( 0.03)ø - (77.8 ( 3.6)r2 ) 0.946 (2)
which corresponds to steric and electronic contributions
of 71% and 29%, respectively. Thus, both large θ (larger)
and high ø (less basic) ligands destabilize (PX3)2PtMe2,
although steric effects seem to play a more significant
role.
F igu r e 3. Platinum-phosphorus coupling constants vs
phosphine electronic parameter (ø) in (PX3)2PtMe2: (circles)
P(alkyl)3; (squares) PMe3-xPhx; (diamonds) P(p-XC6H4)3;
(crosses) PPh3-x(pyrl)x.
properties of the phosphine ligands. Graphical analysis
of 31P{1H} chemical shifts clearly divides the set of 16
ligands into 4 distinct families (Figure 2). The slope of
the P(p-XC6H4)3 line indicates a small ø effect, with
downfield shifts for the more electron-poor phosphines.
The slope of the PR3 line differs greatly from that of
the P(p-XC6H4)3 line, thereby indicating a severe steric
effect, in agreement with the analysis of -∆H. It is
unclear whether the PCy3 point is anomalously low
(correlation of the PR3 data is greatly improved if the
PCy3 datum is neglected) or indicative of a steric
threshold (θst, the cone angle above which steric effects
change9). There appears to be no significant aryl effect,
since the lines for P(p-XC6H4)3 and PR3 intersect near
ø ) 5, and for the PMe3-xPhx series the chemical shift
trend is essentially that expected on the basis of sterics
alone. The large and systematic deviation of the data
for PPh3-x(pyrl)x indicates the significant influence of
the pyrrolyl substituent, consistent with either a π-effect
or simple inductive effects arising from the electrone-
gativity of the pyrrolyl group. (Of course, these phe-
nomena may be interrelated.)
NMR Sp ectr oscop y. Included in Table 1 are the
experimentally determined 31P and 1H NMR data for
all compounds investigated in the present study.21 These
data were typical of square-planar platinum with a cis
arrangement of phosphine and methyl ligands. The Pt-
CH3 resonances appear in 1H spectra as phosphorus-
coupled multiplets with 195Pt satellites (2J PtH ) 66-70
Hz), while 31P resonances are singlets with 195Pt satel-
lites (1J PtP, ) 1761-2506 Hz, within the typical range
for cis-bis(phosphine) species). It is interesting to note
1
that while 31P chemical shifts and J PtP were highly
dependent on the identity of the phosphine, virtually
none of this information is transferred across platinum
to the methyl groups: the Pt-CH3 signals display only
a weak downfield shift on going to higher ø phosphines,
2
while J PtH was basically insensitive to variation of the
phosphine ligand.
In contrast to thermochemical data, the 31P NMR
parameters are much more sensitive to the electronic
(19) Serron, S.; Huang, J .; Nolan, S. P. Organometallics 1998, 17,
534-539.
(20) Wilson, M. R.; Woska, D. C.; Prock, A.; Giering, W. P. Orga-
nometallics 1993, 12, 1742-1752.
(21) Spectroscopic data for many of these complexes have previously
been reported, but to ensure the greatest consistency during data
analysis, all NMR data in Table 1 were acquired from NMR titrations
performed prerequisite to the calorimetric measurements (see Experi-
mental Section). The data from the present study compare favorably
to the literature values.
Platinum-phosphorus coupling constants display the
same general trends as phosphorus chemical shifts, and
again, the ligands can be divided into four distinct
families (Figure 3). The ø effect is even more evident
across the P(p-XC6H4)3 series, with the better electron
donors displaying an increased coupling constant. In the

The Dimethylbis(phosphine)platinum(II) System
Organometallics, Vol. 18, No. 4, 1999 477
Ta ble 2. Cr ysta llogr a p h ic Da ta for th e Com p lexes P 2P tMe2
3b
3c
3n
3p ‚CH2Cl2
formula
fw
color
space group
a, Å
b, Å
C14H36P2Pt
461.48
light amber
P21/n (No. 14)
8.148(2)
17.543(5)
12.780(5)
93.67(3)
4
79.34
0.029
0.025
154
C18H28P2Pt
501.46
faint amber
C2/c (No. 15)
17.098(4)
13.949(3)
17.040(3)
108.73(2)
8
75.24
0.040
0.030
190
C26H30N6P2Pt
683.61
light amber
P21/c (No. 14)
9.252(1)
15.607(4)
18.638(3)
95.04(1)
4
54.31
0.036
0.033
316
C39H74Cl2P2Pt
870.97
colorless
Pbca (No. 61)
17.702(1)
18.102(1)
25.137(1)
c, Å
â, deg
Z
8
µ(Mo), cm-1
Ra
37.56
0.039
0.041
451
83 404
5589
2.5
a
Rw
no. of refined params
no. of data collected
no. of unique data, I > 3σ(I)
Rmerge (%)
8276
2963
2.6
9194
2276
3.2
6563
3940
1.5
a
R ) ∑(||Fo| - |Fc||)/∑|Fo|; Rw ) ∑w(|Fo| - |Fc|)2/∑w|Fo|.
Ta ble 3. P er tin en t Metr ica l P a r a m eter s in
P 2P tMe2 (P ) P Et3, P P h Me2, P P h 2Me, P (p yr l)3,
P Cy3)
P(2) distances are significantly different in 3p (2.344-
(1) and 2.330(1) Å, respectively), whereas in the remain-
ing complexes the two distances are essentially
indistinguishable. The significantly longer bonds in 3p
can be explained by the extreme bulk of the PCy3
ligands; the remaining Pt-P distances, however, cannot
be rationalized entirely on steric grounds. Bond lengths
do increase with θ, but 3n displays an anomalously
short Pt-P contact. Conversely, Pt-P distances de-
crease with increasing ø, although in this case the
exception is 3p , in which this contact is anomalously
long. The P(1)-Pt-P(2) angles display similar trends:
increasing from 95.0(1)° in 3c to 108.60(5)° in 3p , the
angle generally opens up with θ, but the angle in 3n is
somewhat smaller than predicted on the basis of the
other complexes. As might be expected, no clear rela-
tionship between P(1)-Pt-P(2) and ø appears to exist,
and either 3n or 3p could be considered deviant.
Average Pt-Cmethyl distances span a range from 2.086
Å (3c) to 2.129 Å (3p ) and appear somewhat less
sensitive to the identity of the phosphine ligand than
the Pt-P distances. In general, these distances increase
with θ and decrease with ø, although no obvious trends
are apparent. In contrast to the Pt-P distances, the Pt-
C(1) and Pt-C(2) distances are significantly different
in 3n (2.086(7) and 2.123(6) Å, respectively) and indis-
tinguishable in the other complexes (including 3p ).
Complementary to the observations for the phosphorus
ligands, C(1)-Pt-C(2) decreases as θ, and thus P-
(1)-Pt-P(2), increases, and the angle in 3n is somewhat
larger than expected. Again, no clear correlation be-
tween ø and C(1)-Pt-C(2) is evident.
Cor r ela tion s betw een Str u ctu r a l a n d Th er m o-
d yn a m ic Da ta . The most remarkable relationship
between structural and thermodynamic data is that
reaction enthalpies are smallest in the complexes with
the shortest Pt-P bond lengths, as is apparent in Figure
4. The exception is the PCy3 adduct, in which steric
contributions surely destabilize 3p relative to the other
complexes. Similarly, although bond length/strength
conclusions based on J MP can be misleading,23 there is
a general tendency toward lower reaction enthalpies in
complexes with higher J PtP values. The classic rule of
“shorter bond equals stronger bond” may appear to have
been violated, but it cannot be overemphasized that
thermochemical measurements such as described here
can only determine the total energy difference between
a
PEt3
PMe2Ph PMePh2 P(pyrl)3
PCy3
Bond Distances (Å)
Pt-P(1)
Pt-P(2)
Pt-C(1)
Pt-C(2)
2.290(1) 2.284(3) 2.285(2) 2.249(1) 2.344(1)
2.295(2) 2.285(3) 2.284(2) 2.252(2) 2.330(1)
2.102(5) 2.096(11) 2.122(6) 2.086(7) 2.132(6)
2.099(5) 2.075(11) 2.119(5) 2.123(6) 2.126(5)
Bond Angles (deg)
P(1)-Pt-P(2) 100.03(5) 95.0(1)
97.75(6) 97.36(7) 108.60(5)
93.0(2) 89.8(2) 85.8(2)
P(1)-Pt-C(1) 86.1(2) 90.2(3)
P(1)-Pt-C(2) 169.1(2) 175.1(3) 173.7(2) 173.5(2) 164.7(2)
P(2)-Pt-C(1) 173.7(2) 174.4(3) 168.9(2) 172.7(2) 165.1(2)
P(2)-Pt-C(2) 90.7(2)
C(1)-Pt-C(2) 83.1(2)
89.7(3)
85.1(4)
87.2(2)
81.9(2)
89.1(2) 86.6(2)
83.7(3) 79.1(2)
a
From ref 24.
PR3 series, a steric threshold may exist at ø ) 5,
corresponding to θst ) 145°, consistent with the analysis
of Romeo and Alibrandi.22 Considered without a steric
threshold, however, the line for PR3 is essentially
parallel (although with much larger standard error for
the slope) to that for P(p-XC6H4)3, which would imply
no steric component and a significant aryl effect.9 The
effect of pyrrolyl substituents is clear from the large and
systematic variations of PPh3-x(pyrl)x data from the P(p-
XC6H4)3 line, though again it is unclear if these results
arise from increased Pt-P π interactions or from the
increased s-character of the PPh3-x(pyrl)x σ-orbital.23
X-r a y Str u ctu r a l Stu d ies. In an effort to clarify the
relationship between phosphine ligand steric and elec-
tronic properties and the thermochemical and spectro-
scopic data, single-crystal X-ray diffraction analyses of
the PEt3 (3b), PMe2Ph (3c), P(pyrl)3 (3n ), and PCy3 (3p )
adducts were performed (Table 2). Selected metrical
parameters for these complexes and the PMePh2 adduct
3d 24 are given in Table 3. In their gross structural
features, all five complexes are typical for square-planar
Pt(II). Although deviations from this idealized structure
are evident in each complex, no unusual interatomic
distances were observed.25
Average Pt-P bond lengths range from 2.250 Å in 3n
to 2.337 Å in 3p . Interestingly, the Pt-P(1) and Pt-
(22) Romeo, R.; Alibrandi, G. Inorg. Chem. 1997, 36, 4822-4830.
(23) Alyea, E. C.; Song, S. Inorg. Chem. 1995, 34, 3864-3873.
(24) Wisner, J . M.; Bartczak, T. J .; Ibers, J . A. Organometallics 1986,
5, 2044-2050.
(25) See, for example, ref 23 and references therein.

478 Organometallics, Vol. 18, No. 4, 1999
Haar et al.
Solu tion Ca lor im etr y. In a representative experimental
trial, the mixing vessels of the Setaram C-80 were cleaned,
dried in an oven maintained at 120 °C, and then taken into
the glovebox. A sample of (COD)PtMe2 (19.4 mg, 58.2 µmol)
was massed into the lower vessel, which was closed and sealed
with 1.5 mL of mercury. A solution of PPh3 (37.8 mg, 144 µmol)
in CH2Cl2 (4 mL) was added, and the remainder of the cell
was assembled, removed from the glovebox, and inserted into
the calorimeter. The reference vessel was loaded in an identical
fashion, with the exception that no platinum complex was
added to the lower vessel. After the calorimeter had reached
thermal equilibrium at 30.0 °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 (ca. 2 h). Control reactions with
Hg and phosphine show no reaction. The enthalpy of ligand
substitution (-22.6 ( 0.2 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 (+5.8 ( 0.1 kcal/mol) in neat CH2Cl2 was
determined using identical methodology.
F igu r e 4. Reaction enthalpy as a function of Pt-P bond
length in (PX3)2PtMe2 (slope 2.95, R ) 0.975).
states. Energy terms due to distortions in the Pt
coordination environment and internal ligand structure,
i.e., reorganization energies,18,26 may induce a net
destabilizing effect regardless of the intrinsic Pt-P bond
strength.
Syn th esis of Dim eth ylbis(ph osph in e)platin u m (II) Com -
p lexes. Unless otherwise noted, all new complexes were
prepared according to the following typical procedure. In the
glovebox, a Schlenk tube equipped with a magnetic stir bar
was charged with (COD)PtMe2 (79.6 mg, 0.239 mmol), P(p-
FC6H4)3 (151.1 mg, 0.478 mmol), and either THF or CH2Cl2
(3-5 mL). The vessel was sealed, removed from the glovebox,
and interfaced to a Schlenk line, where the mixture was stirred
for ∼2 h. The volatile components were removed in vacuo, and
the residue was triturated several times with CH2Cl2 (3-5 mL)
to ensure complete removal of cyclooctadiene. The colorless to
off-white residue was then taken up in a minimum of CH2Cl2.
Layering with pentane afforded colorless crystals of 3i after 1
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations were per-
formed under inert atmospheres of argon or nitrogen using
standard high-vacuum or Schlenk-line techniques, or in a
glovebox containing less than 1 ppm of oxygen and water.
Solvents, including deuterated solvents for NMR analysis,
were dried by standard methods27 and distilled under nitrogen
or vacuum-transferred before use. NMR spectra were recorded
using Varian Gemini 300 MHz or Varian Unity 400 MHz
spectrometers. Elemental analyses were performed by Desert
Analytics. Only materials of high purity as indicated by NMR
spectroscopy were used in the calorimetric experiments.
Calorimetric measurements were performed using a Calvet
calorimeter (Setaram C-80) which was periodically calibrated
using the TRIS reaction28 or the enthalpy of solution of KCl
in water.29 This calorimeter has been previously described,30,31
and typical procedures are described below. Experimental
enthalpy data are reported with 95% confidence limits.
1
day. Yield: 143 mg (70%). H NMR (CD2Cl2): δ 0.36 (m, 6 H,
Pt(CH3)2, J PtCH ) 69 Hz), 6.8-7.4 (m, 24 H, C6H4F). 31P{1H}
3
NMR (CD2Cl2): δ 26.7 (s, J PtP ) 1888 Hz). Anal. Calcd for
C
38H30F6P2Pt: C, 53.22; H, 3.53. Found: C, 53.18; H, 3.70.
(P h 2(p yr r olyl)P )2P tMe2 (3j) was prepared from (COD)-
PtMe2 (72.8 mg, 0.218 mmol) and PPh2(pyrrolyl) (109.8 mg,
1
0.4370 mmol). Yield: 103 mg (65%), as off-white crystals. H
NMR (CD2Cl2): δ 0.52 (m, 6 H, Pt(CH3)2, J PtCH3 ) 71 Hz), 6.25
(s, 4 H, pyrrolyl), 7.0-7.5 (m, 24 H, pyrrolyl and C6H5). 31P-
{1H} NMR (CD2Cl2): δ 69.4 (s, J PtP ) 2048 Hz). MS (EI) for
C
34H34N2P2Pt: calcd m/e 713.161 (-CH3), 697.138 (-2CH3),
Complexes 3a -h ,k ,m ,p have been reported previously.22
found m/e 713.162, 697.132.
10
The ligands PPh2(pyrrolyl), PPh(pyrrolyl)2, and P(pyrrolyl)3
((p-CF 3C6H4)3P )2P tMe2 (3l) was prepared from (COD)-
PtMe2 (62.8 mg, 0.188 mmol) and P(p-CF3C6H4)3 (175.1 mg,
0.376 mmol). Yield: 183 mg (84%), as irregular, transparent
blocks which became opaque when dried in vacuo. 1H NMR
were synthesized according to literature procedures, as was
(COD)PtMe2.32 Triphenylphosphine and PiBu3 (Aldrich) and
PMePh2, PMe2Ph, PMe3, PEt3, PiPr3, PCy3, and P(p-XC6H4)3
(X ) F, Cl, Me, OMe, CF3) (Strem) were used as received.
NMR Titr a tion s. Prior to every set of calorimetric experi-
ments involving a new ligand, a precisely measured amount
((0.1 mg) of (COD)PtMe2 was placed in an NMR tube along
with CD2Cl2 and >1.2 equiv of ligand. Both 1H and 31P NMR
spectra were measured within 1 h of mixing; both indicated
that the reactions were clean and quantitative. These condi-
tions are necessary for accurate and meaningful calorimetric
results and were satisfied for all reactions investigated.
(CD2Cl2): δ 0.46 (m, 6 H, Pt(CH3)2, J PtCH ) 70 Hz), 7.4-7.7
3
(m, 24 H, C6H4CF3). 31P{1H} NMR (CD2Cl2): δ 29.0 (s, J PtP
)
1850 Hz). Anal. Calcd for C44H30F18P2Pt: C, 45.65; H, 2.61.
Found: C, 45.73; H, 2.76.
((p yr r olyl)3P )2P tMe2 (3n ) was prepared from (COD)PtMe2
(50.6 mg, 0.152 mmol) and P(pyrrolyl)3 (69.6 mg, 0.304 mmol).
Yield: 63 mg (61%), as white microcrystals. 1H NMR (CD2-
Cl2): δ 0.69 (m, 6 H, Pt(CH3)2, J PtCH ) 70 Hz), 6.27 (s, 6 H,
3
pyrrolyl), 6.68 (s, 6 H, pyrrolyl). 31P{1H} NMR (CD2Cl2): δ 96.6
(s, J PtP ) 2506 Hz). Anal. Calcd for C26H30N6P2Pt: C, 45.68;
H, 4.42; N, 12.29. Found: C, 45.61; H, 4.44; N, 12.15.
(26) Martinho Simo˜es, J . A.; Beauchamp, J . L. Chem. Rev. 1990,
90, 629-688.
(27) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: Oxford, U.K., 1988.
(28) Ojelund, G.; Wadso¨, I. Acta Chem. Scand. 1968, 22, 1691-1699.
(29) Kilday, M. V. J . Res. Natl. Bur. Stand. (U.S.) 1980, 85, 467-
481.
(P h (p yr r olyl)2P )2P tMe2 (3o) was prepared from (COD)-
PtMe2 (49.7 mg, 0.149 mmol) and PPh(pyrrolyl)2 (71.6 mg,
0.298 mmol). Yield: 82 mg (78%), as off-white microcrystals.
1H NMR (CD2Cl2): δ 0.66 (m, 6 H, Pt(CH3)2, J PtCH ) 73 Hz),
3
6.25 (s, 8 H, pyrrolyl), 6.64 (m, 4H, phenyl), 6.94 (s, 8 H,
pyrrolyl), 7.10 (m, 4 H, phenyl), 7.27 (m, 2 H, phenyl). 31P-
{1H} NMR (CD2Cl2): δ 86.5 (s, J PtP ) 2245 Hz). Anal. Calcd
for C30H32N4P2Pt: C, 51.06; H, 4.57; N, 7.94. Found: C, 50.69;
H, 4.44; N, 7.67.
(30) Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D. Inorg. Chem.
1986, 25, 4446-4448.
(31) Nolan, S. P.; Hoff, C. D.; Landrum, J . T. J . Organomet. Chem.
1985, 282, 357-362.
(32) Costa, E.; Pringle, P. G.; Ravetz, M. Inorg. Synth. 1997, 31,
284-286.

The Dimethylbis(phosphine)platinum(II) System
Organometallics, Vol. 18, No. 4, 1999 479
X-r a y Str u ctu r a l An a lyses of 3b,c,n . These structures
were determined from data collected on an Enraf-Nonius
CAD4 diffractometer equipped with Mo KR radiation and a
low-temperature apparatus (-75 °C). Data sets were corrected
for Lorentz and polarization effects and for absorption (azi-
muthal). The structures were solved by direct methods (MUL-
TAN) and refined by full-matrix least-squares techniques.
Atomic scattering factors, including anomalous terms for P and
Pt, were taken from ref 33. In the least-squares refinement,
the function minimized was ∑w(|Fo| - |Fc|)2 with the weights,
w, assigned as [σ2(I) + 0.0009I2]-1/2. Crystallographic highlights
are given in Table 2. Selected distances and angles are given
in Table 3. Because in each complex the refinement produced
some unrealistic C-H bond lengths, the hydrogen atoms were
placed in idealized positions close to their previously refined
positions. All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. Complete structural details, in-
cluding data collection and refinement, atomic coordinates,
anisotropic thermal parameters, and hydrogen atom positions,
are available as Supporting Information.
Pt, were taken from ref 33. In the least-squares refinement,
the function minimized was ∑w(|Fo| - |Fc|)2 with the weights,
w, assigned as [σ2(I) + 0.0009I2]-1/2. Crystallographic highlights
are given in Table 2. Selected distances and angles are given
in Table 3. Because the refinement produced hydrogen thermal
parameters larger than desired, the hydrogen atoms were
placed in idealized positions close to their previously refined
positions. All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. Two of the cyclohexyl groups (one
on each phosphine ligand) are badly disordered, and no
hydrogens are affixed to them. Complete structural details,
including data collection and refinement, atomic coordinates,
anisotropic thermal parameters, and hydrogen atom positions,
are available as Supporting Information.
Ack n ow led gm en t. The National Science Founda-
tion (Grant No. CHE-9631611) and Du Pont (Educa-
tional Aid Grant) are gratefully acknowledged for their
support of this research. We also thank Prof. Matt Tarr
for expert technical assistance.
X-r a y Str u ctu r a l An a lysis of 3p . The structure was
determined from data collected on a Rigaku RU300 R-AXIS
image plate area detector equipped with Mo KR radiation and
Su p p or t in g In for m a t ion Ava ila b le: Full crystallo-
graphic details, including listings of atomic coordinates, B
values, selected distances and angles, anisotropic thermal
parameters, and ORTEP drawings, for complexes 3b,c,n ,p .
This material is available free of charge via the Internet at
http://pubs.acs.org.
a
low-temperature apparatus (-100 °C). The data were
corrected for Lorentz and polarization effects, but not for
absorption. The structure was solved by direct methods using
teXsan (SIR-92) and refined by full-matrix least-squares
techniques. The refinement and analysis of the structure was
carried out using a package of local programs.34 Atomic
scattering factors, including anomalous terms for Cl, P, and
OM9807001
(34) Calabrese, J . C. Central Research and Development, E. I. Du
Pont de Nemours and Co., Inc., P.O. Box 80228, Wilmington, DE 19880-
0228, 1991.
(33) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, U.K., 1974; Vol. IV.