Binding of specialty phosphines to metals: Synthesis, structure, and solution calorimetry of the phosphirane complex [PtMe2(iPrBABAR-Phos)2]

Article abstract of DOI:10.1021/om020642q

The complex [PtMe₂(^iPrBABAR-Phos)₂] (3) was prepared in a clean and quantitative ligand substitution reaction from [PtMe₂(cod)] (1; cod = η⁴-1,5-cyclooctadiene) and the phosphirane ^iPrBABAR-Phos (2). The structure of 3 was determined by X-ray diffraction. The Pt-P bonds (～2.26 A) lie in the shorter range of Pt^II-P bonds, although the ¹J(¹⁹⁵Pt³¹P) coupling (1840 Hz) is quite small. The enthalpy for this ligand substitution reaction was measured by solution calorimetry and found to be exothermic by 11.8 kcal/mol, a relatively low exothermicity for a reaction involving a tertiary phosphine in this Pt system. Calculations using density functional theory (DFT) on the B3LYP level were applied using the simplified model reaction [PtH₂(cod)] + 2(H₂N)PC₂H₄ → [PtH₂{(H₂N)PC₂H₄}₂)] + cod, and these also gave a rather low substitution enthalpy (-17 kcal/mol). A charge decomposition analysis (CDA) was performed for Pt(II) and Pt(0) complexes with the simple P-amino phosphirane (H₂N)PC₂H₄ and PH₃ as ligands. Contrary to expectations, it is found that the phosphirane acts as a relatively good electron donor, while its electron-acceptor properties are not significantly different from those of other phosphines. The particularly low reaction enthalpy may thus be due to a low directionality of the donor orbitals toward the metal center.

Full text of DOI:10.1021/om020642q

2202
Organometallics 2003, 22, 2202-2208
Articles
Bin d in g of Sp ecia lty P h osp h in es to Meta ls: Syn th esis,
Str u ctu r e, a n d Solu tion Ca lor im etr y of th e P h osp h ir a n e
Com p lex [P tMe2(iP r BABAR-P h os)2]
Ce´cile Laporte, Gilles Frison, and Hansjo¨rg Gru¨tzmacher*
Chemistry Department HCI H131, ETH-Ho¨nggerberg, CH-8093 Zu¨rich, Switzerland
Anna C. Hillier, William Sommer, and Steven P. Nolan
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
Received August 8, 2002
The complex [PtMe2(iPrBABAR-Phos)2] (3) was prepared in a clean and quantitative ligand
substitution reaction from [PtMe2(cod)] (1; cod ) η4-1,5-cyclooctadiene) and the phosphirane
iPrBABAR-Phos (2). The structure of 3 was determined by X-ray diffraction. The Pt-P bonds
1
(∼2.26 Å) lie in the shorter range of PtII-P bonds, although the J (195Pt31P) coupling (1840
Hz) is quite small. The enthalpy for this ligand substitution reaction was measured by
solution calorimetry and found to be exothermic by 11.8 kcal/mol, a relatively low
exothermicity for a reaction involving a tertiary phosphine in this Pt system. Calculations
using density functional theory (DFT) on the B3LYP level were applied using the simplified
model reaction [PtH2(cod)] + 2(H2N)PC2H4 f [PtH2{(H2N)PC2H4}2)] + cod, and these also
gave a rather low substitution enthalpy (-17 kcal/mol). A charge decomposition analysis
(CDA) was performed for Pt(II) and Pt(0) complexes with the simple P-amino phosphirane
(H2N)PC2H4 and PH3 as ligands. Contrary to expectations, it is found that the phosphirane
acts as a relatively good electron donor, while its electron-acceptor properties are not
significantly different from those of other phosphines. The particularly low reaction enthalpy
may thus be due to a low directionality of the donor orbitals toward the metal center.
In tr od u ction
more stable products with electron-rich metals via an
insertion of the metal center into one P-C bond.8-10
BABAR-Phos, on the other hand, allows the synthesis
of fairly stable Rh(I) and Pt(0) complexes which are
active hydrosilylation11 and recyclable hydroboration
catalysts.12 Also, the metallaphosphetane formation
with BABAR-Phos was found to be reversible and
controllable by the coligands.
Given the utility of BABAR-Phos, we became inter-
ested in evaluating the binding energy as one key
parameter for complex stability. A priori, it is not
expected that phosphiranes would be strong σ-donors,
because the donor orbital localized at the phosphorus
Phosphiranes contain a three-membered heterocycle
with at least one phosphorus atom.1-3 The sum of bond
angles at the phosphorus center is small (<260°), which
confers particular properties to these molecules; among
these, their resistance to oxidation is the most notewor-
thy for practical reasons. In the past few decades a
number of metal complexes were prepared with phos-
phiranes as ligands in various research groups.1-6 We
had developed the synthesis of BABAR-Phos, a phos-
phirane derivative in which the PC2 cycle is incorpo-
rated into a polycyclic framework and which is easily
prepared from simple starting materials.7 Monocyclic
phosphiranes tend to form metallaphosphetanes as the
(7) Liedtke, J .; Loss, S.; Alcaraz, G.; Gramlich, V.; Gru¨tzmacher,
H. Angew. Chem. 1999, 110, 1724; Angew. Chem. Int. Ed. 1999, 38,
1623.
(8) See: Carmichael, D.; Hitchcock, P. B.; Nixon, J . F.; Mathey, F.;
Ricard, L. J . Chem. Soc., Dalton Trans. 1993, 1811 and references cited
therein.
(9) Ajulu, F. A.; Carmichael, D.; Hitchcock, P. B.; Mathey, F.;
Meidine, M. F.; Nixon, J . F.; Ricard, L.; Riley, M. L. J . Chem. Soc.,
Chem. Commun. 1992, 750.
(10) Al J uaid, S. S.; Carmichael, D.; Hitchcock, P. B.; Marinetti, A.;
Mathey, F.; Nixon, J . F. J . Chem. Soc., Dalton Trans. 1991, 905.
(11) Liedtke, J .; Loss, S.; Widauer, Ch.; Gru¨tzmacher, H. Tetrahe-
dron 2000, 56, 143 and references cited therein (Symposium in Print).
(12) Liedtke, J .; Ru¨egger, H.; Loss, S.; Gru¨tzmacher, H. Angew.
Chem. 2000, 112, 2596; Angew. Chem., Int. Ed. 2000, 39, 2479.
(1) Mathey, F.; Regitz, M. In Phosphorus Heterocyclic Chemistry:
The Rise of a New Domain; Mathey, F., Ed.; Pergamon: Amsterdam,
2001; pp 17-55.
(2) Dillon, K. B.; Mathey, F.; Nixon, J . F. Phosphorus: The Carbon
Copy; Wiley: New York, 1998; p 182.
(3) Mathey, F. Chem. Rev. 1990, 90, 997.
(4) Me´zailles, N.; Fanwick, P. E.; Kubiak, C. P. Organometallics
1997, 16, 1526.
(5) Marinetti, A.; Mathey, F.; Ricard, L. Organometallics 1993, 12,
1207.
(6) Ajulu, F. A.; Hitchcock, P. B.; Mathey, F.; Michelin, R. A.; Nixon,
J . F.; Pombeiro, A. J . L. J . Chem. Soc., Chem. Commun. 1993, 142
and references cited therein.
10.1021/om020642q CCC: $25.00 © 2003 American Chemical Society
Publication on Web 04/30/2003

Binding of Specialty Phosphines to Metals
Organometallics, Vol. 22, No. 11, 2003 2203
Sch em e 1. Syn th esis of [P tMe2(iP r BABAR-P h os)2]
(3)
has high s-orbital character and hence is strongly
contracted. On the other hand, phosphiranes may act
as better π-acceptors as compared to other phosphines,
because the strong pyramidalization of the P-coordina-
tion sphere leads to decreasing energies of unoccupied
π-type orbitals which are mainly P-C antibonding in
character.10,13
F igu r e 1. Molecular structure of 3. Thermal ellipsoids are
drawn at 50% probability; hydrogen atoms were omitted
for clarity. Selected bond lengths (Å) and angles (deg) (the
two phosphirane ligands show very similar structures, and
averaged values are given here): Pt(1)-P(1) ) 2.257(1),
Pt(1)-P(1a) ) 2.268(1), Pt(1)-C(19) ) 2.090(4), Pt(1)-
C(20) ) 2.082(5), P(1/1a)-N(1/1a) ) 1.676(4), P(1/1a)-C(4/
4a) ) 1.808(4), P(1/1a)-C(5/5a) ) 1.828(4), C(4/4a)-C(5/
5a) ) 1.571(5), N(1/1a)-C(1/1a) ) 1.485(5), N(1/1a)-C(16/
16a) ) 1.499(5); P(1)-Pt(1)-P(1a) ) 90.36(4), C(20)-
Pt(1)-C(19) ) 86.3(2), C(19)-Pt(1)-P(1) ) 91.51(15),
C(20)-Pt(1)-P(1a) ) 91.85(14), C(20)-Pt(1)-P(1) ) 177.79-
(14), C(19)-Pt(1)-P(1a) ) 172.82(14), C(4/4a)-P(1/1a)-
C(5/5a) ) 51.2(2), N(1/1a)-P(1/1a)-C(4/4a) ) 102.5(2), N(1/
1a)-P(1/1a)-C(5/5a) ) 103.7(2), N(1/1a)-P(1/1a)-Pt(1) )
123.0(1), C(4/4a)-Pt(1)-P(1/1a) ) 123.1(2), C(5/5a)-P(1/
1a)-Pt(1) ) 130.6(1), C(5/5a)-C(4/4a)-P(1/1a) ) 65.1(2),
C(4/4a)-C(5/5a)-P(1/1a) ) 63.8(2); Σ°(N) ) 350.3, Σ°(P) )
257.4.
Resu lts a n d Discu ssion
Syn th esis a n d Str u ctu r e of [P tMe2(iP r BABAR-
P h os)2]. As the system of choice, we investigated the
reaction of [PtMe2(cod)] (1) with iPrBABAR-Phos (2). The
binding energy of various monodentate phosphines,
PR3,14 and bis(chelates), R2P-X-PR2,15 with 1 has been
determined previously by solution calorimetry and
therefore allows a ranking of the binding qualities of
BABAR-Phos. Furthermore, these studies showed that
the PtMe2 fragment has neither predominant electron
acceptor nor electron donor properties, which may be a
useful quality for the evaluation of σ/π donor/acceptor
properties of additional ligands. The reaction of 1 with
2 equiv of 2 in methylene chloride at room temperature
proceeds smoothly and gives the Pt(II) iPrBABAR-Phos
complex as the only product (Scheme 1).
complexes with electron-poor phosphines such as pyr-
rolylphosphines, P(pyrl)3.14
Crystals suitable for X-ray analysis were grown from
a concentrated CH2Cl2 solution layered with n-hexane.
The result of the data refinement (see Table 3 in the
Experimental Section) is shown in Figure 1. Because
the individual bond lengths in the two coordinated
BABAR-Phos ligands in 3 do not vary much, selected
averaged bond lengths and angles are given in the
caption to Figure 1. In the structure of 3, the two methyl
groups and the two BABAR-Phos ligands occupy cis
positions in the expected square-planar coordination
sphere of the d8 valence electron configured Pt(II)
center. For steric reasons, the isopropyl groups at the
BABAR-Phos nitrogen center lie on the same side with
respect to the central molecular plane, which we also
observed for the cationic [Rh(iPrBABAR-Phos)4]+ com-
plex.12 The lengths of the Pt-P bonds in 3 (2.257(1),
2.268(1) Å) are slightly shorter than those in Pt(II)
alkylphosphine complexes and approach values ob-
served in complexes with electron-poorer phosphines.
After recrystallization, 3 is obtained as a white
microcrystalline powder in about 87% isolated yield. In
the 31P NMR spectrum, one sharp resonance at -69.6
ppm is detected, accompanied by characteristic satellites
separated by 1840 Hz due to coupling with the 195Pt
nucleus. The 31P chemical shift is not significantly
different from the ones observed in Pt(0) complexes with
BABAR-Phos ligands7,11 and is shifted by about 80 ppm
to higher frequencies in comparison to the free ligand
2 (-153.5 ppm). While in the Pt(0)-BABAR-Phos
1
complexes J (195Pt31P) is rather large (>4200 Hz) and
is in the upper range of known values, the 195Pt31
P
coupling in 3 is quite small and falls into the range
which is typically observed for Pt(II) complexes with
σ-donating alkyl-substituted phosphines. Pt(II) com-
plexes with electron-poor/π-accepting phosphines fre-
quently show larger couplings (>2000 Hz).14,16 On the
other hand, the platinum-bonded methyl groups are
shifted to rather high frequencies (1H NMR: 0.98 ppm,
2J (195Pt1H) ) 78.2 Hz) which is typical for Pt(II)
The platinum-methyl distances in
3 (2.082(5),
2.090(4) Å) are also in the lower range of previously
observed lengths. Note, however, that the variations of
the Pt-P and Pt-Me distances in [PtMe2(PR3)2] and
[PtMe2(PP)] complexes (PP ) chelating diphosphine) are
in general comparatively small (Pt-P ) 2.24-2.34 Å;
Pt-Me ) 2.07-2.26 Å) even for sterically and/or elec-
tronically rather different phosphines. To this end, we
conclude therefore that no particular steric or electronic
(13) Nguyen, M. T.; Van Praet, E.; Vanquickenborne, L. G. Inorg.
Chem. 1994, 33, 1153.
(14) Haar, C. M.; Nolan, S. P.; Marshall, W. J .; Moloy, K. G.; Prock,
A.; Giering, W. P. Organometallics 1999, 18, 474-479.
(15) Smith, D. C., J r.; Haar, C. M.; Stevens, E. D.; Nolan, S. P.
Organometallics 2000, 19, 1427-1433.
(16) Pregosin, P. S. Annu. Rep. NMR Spectrosc. 1986, 17, 285.

2204 Organometallics, Vol. 22, No. 11, 2003
Laporte et al.
Ta ble 1. Com p a r ison of P er tin en t Str u ctu r a l
P a r a m eter s of (i) BTMP BABAR-P h os,11a (ii)
[P t(iP r BABAR-P h os)2(P P h 3)2],7 (iii)
Ch a r t 1. Dep iction of Str u ctu r es Ca lcu la ted a t th e
B3LYP Den sity F u n ction a l Level of Th eor ya
[P t(iP r BABAR-P h os)(d vtm s)],7,11 (iv) 3, a n d (v) a
tBu -P h osp h ir a n iu m Ca tion 11 (CN ) Coor d in a tion
Nu m ber )
Bond Lengths (Å)
Za
a1
a2
b
c
1.850(3)
1.831(5)
1.812(6)
1.808(4)
1.774(3)
1.863(3)
1.856(5)
1.821(7)
1.828(4)
1.774(3)
1.532(4)
1.558(7)
1.576(9)
1.571(5)
1.601(6)
1.738(2)
1.699(4)
1.684(6)
1.676(4)
1.628(4)
Pt(0), CN 4
Pt(0), CN 3
Pt(II) (3)
tBu+
Bond Angles (deg)
R2 R3
48.8(1) 66.0(2) 65.2(2)
Pt(0), CN 4 49.9(2) 64.2(2) 65.8(2)
Pt(0), CN 3 51.4(3) 64.0(3) 64.6(3) 101.7(3) 103.1(3)
Za
R1
â1
â2
a
Selected bond lengths (Å) and angles are given for 4, 5,
6b, and 9.
98.8(1)
99.5(2) 101.0(2)
99.0(1)
which can be compared with substitution enthalpies for
a series of structurally related [PtMe2(PR3)2]14 and
[PtMe2(PP)]15 complexes that were obtained in a similar
manner. In the series with the monodentate phosphines,
the highest enthalpy, -34.3 ( 0.2 kcal/mol, was ob-
served for PMe3 as the sterically less demanding
electron-donor phosphine and the lowest value, -19.2
( 0.2 kcal/mol, was obtained for PCy3 (Cy ) cyclohexyl).
This small binding energy is certainly caused by the
high steric bulk. Note that electron-poor/π-accepting
phosphines with “normal” steric requirements such as
P(pyrl)3 (pyrl ) pyrrolyl) are also relatively weakly
bound; i.e., ∆Hr ) -20.4 ( 0.2 kcal/mol. As expected,
for chelating bisphosphines in the series [PtMe2(PP)],
slightly higher binding enthalpies are found than with
two monodentate phosphines. Also, alkyl-substituted
derivatives give larger values (>|-35| kcal/mol) than
aryl-substituted ones and with (pyrl)2P-(CH2)2-P(pyr)2
the lowest value (-21.7 kcal/mol) was measured. To this
end, BABAR-Phos gives by far the lowest substitution
enthalpy measured to date.
Qu a n t u m -Ch em ica l Ca lcu la t ion s. Calculations
were performed using B3LYP density functional theory
(DFT) in order to gain additional insight into the
electronic structure of platinum phosphirane complexes.
In particular, we were interested in evaluating the
binding quality of a phosphirane toward a Pt(II) center
using charge decomposition analysis (CDA) as intro-
duced by Frenking et al.17 As detailed in the Experi-
mental Section, this method corresponds to a “quanti-
fied” Dewar-Chatt-Duncanson model and gives in-
formation about the amount of LfM donation, d, LrM
back-donation, b, and LTM repulsive interactions, r.
Second, we wanted to compute the substitution enthalpy
for a model reaction close to the one we have investi-
gated in Scheme 1. The model compounds we used for
this investigation are depicted in Chart 1. As frequently
observed, the distances from the metal to its neighboring
Pt(II)
51.2(2) 65.1(2) 63.8(2) 102.5(2) 103.7(2)
53.6(2) 63.2(1) 63.2(1) 108.2(2) 108.2(2)
tBu+
a
R ) 3,5-bis(trifluoromethyl)phenyl (BTMP).
interactions are reflected in the metric parameters of
the platinum coordination sphere.
For comparison, the structural parameters of the PC2
cycle in (i) a noncomplexed BABAR-Phos (with 2,5-
(CF3)C6H2 instead of iPr at the nitrogen atom), (ii) in
the four-coordinate Pt(0) complex [Pt(iPrBABAR-Phos)2-
(PPh3)2],7 (iii) in the three-coordinate Pt(0) complex [Pt-
(
iPrBABAR-Phos)(dvtms)] (dvtms ) divinyltetrameth-
ylsiloxane),7,11 (iv) 3, and (v) the tert-butylphosphiranium
cation11 are listed in Table 1.
As is generally observed for phosphines, the bonds
from the phosphorus atom to its neighboring atoms,
here the P-C bonds a1 and a2 and the P-N bond c,
become shorter when the P lone pair is involved in
bonding. Also, the bond angles R1, â1, and â2 are
widened. These effects are provoked by increasing
electron donation from the phosphorus atom to its
bonding partner Z, whereby the positive atomic charge
at the P center increases. As the data in Table 1 show,
the PfZ donation increases in the order Z ) Pt(0) CN
4 < Pt(0) CN 3 < Pt(II) < tBu+ (CN ) coordination
number), placing the complexes between the noncoor-
dinated ligand and the phosphiranium salt in which the
PftBu+ interaction can be regarded as pure donation.
The structural data show that PfPt electron donation
makes a strong contribution to the electronic interaction
of BABAR-Phos with platinum centers. If PtfP back-
donation plays a dominant role, P-C and P-N bond
lengthening is expected (because the transfer of electron
density from the metal to the phosphine PX3 occurs in
P-X antibonding orbitals).
Solu tion Ca lor im etr y. The substitution reaction
shown in Scheme 1 is clean and quantitative at T ) 298
K at reaction times below 1 h. As is detailed in the
Experimental Section, an enthalpy ∆Hr of -11.8 ( 0.2
kcal/mol was determined from several experiments,
(17) Dapprich, S.; Frenking, G. J . Phys. Chem. 1995, 99, 9352.

Binding of Specialty Phosphines to Metals
Organometallics, Vol. 22, No. 11, 2003 2205
F igu r e 2. Comparison of PH3 and (H2N)PC2H4 rotation barriers relating the conformers 6b and 6d at the B3LYP level
of theory.
Ta ble 2. Ch a r ge Decom p osition An a lysisa a n d In ter a ction En er gy Ein t (in k ca l/m ol) for th e P h osp h in es P H3
(8) a n d (H2N)P C2H4 (9) in P la tin u m (II) (en tr ies 1 a n d 2) a n d P la tin u m (0) Com p lexes (en tr ies 3 a n d 4)
Ca lcu la ted a t th e B3LYP /II Level
entry no.
phosphane/complex
d
b
d/b
r
∆
Eint
1
2
3
4
5
6
PH3 (8)/6b
(H2N)PC2H4 (9)/6b
PH3 (8)/6c
(H2N)PC2H4 (9)/6c
0.461
0.598
0.454
0.604
0.382
0.494
0.171
0.203
0.180
0.219
0.229
0.275
2.70
2.95
2.52
2.76
1.67
1.80
-0.379
-0.274
-0.383
-0.272
-0.597
-0.342
0.004
-0.018
0.004
-0.019
0.020
32.0
34.1
32.3
34.4
28.3
31.1
PH3 (8)/[Pt(PH3)3] (11)19
(H2N)PC2H4 (9)/[Pt(PH3)2{(H2N)PC2H4}] (12)
-0.038
a
Donation d, back-donation b, repulsive polarization r, and residual term ∆.
atoms are all slightly too long in the B3LYP calculations
in comparison with X-ray crystallographic structures.18
Gross structural features are, however, correctly rep-
resented, as the Pt-Pphosphirane bonds in 6b or 7 are
found to be slightly shorter than the Pt-PH3 bonds in
5 or 6b and the overall agreement of the calculated
structures with experiment is satisfying. In particular,
the structural changes within the phosphirane unit
upon complexation are reproduced: i.e., the P-C and
P-N bonds becoming shorter and the C-C bond of the
PC2 cycle becoming longer when 9 is coordinated. Also,
the C-P-C angle is widened, while the inner-ring
P-C-C angles are narrowed when the phosphirane 9
is coordinated.
platinum atom. This observation was made also for
Pt(0) complexes with monosubstituted phosphines PH2R
with R * H.19 However, the barrier for rotation around
the Pt-P(phosphirane) bond via the transition state
(TS) 6c is very low (0.62 kcal/mol) and is not signifi-
cantly higher than the rotation barrier for the PH3
ligand in 6b (via TS 6a at 0.12 kcal/mol) or 6d (via TS
6e at 0.38 kcal/mol). This finding can be interpreted in
terms of the absence of any particular π-interaction
between the phosphirane 9 and the (PH3)PtH2 fragment
(neither π-donation or π-acception) in one specific plane
of the complex.
We discuss here the results of the CDA for the most
stable complex, 6b, although the structure 6c is closer
to the experimental structure in the sense that the
nitrogen atom does not lie in the central plane of the
complex. However, the CDA gives very similar results,
as indicated in Table 2. The calculated values for
donation, d, back-donation, b, and repulsion, r, for the
PH3 ligand 8 (entries 1 and 3) and phosphirane ligand
9 are listed in Table 2. In all cases, the residual term
∆ is small, which is a prerequisite to allow a meaningful
discussion. For comparison we have also included the
data for the tris(phosphine) Pt(0) complex [Pt(PH3)3]
(11)19 and the bis(phosphine) phosphirane complex
[Pt(PH3)2{(H2N)PC2H4}] (12).
For the CDA, the mixed [PtH2(PH3)(H2NPC2H4)]
complex 6 seemed especially interesting because an
internal comparison of the model phosphirane (H2N)-
PC2H4 (9) versus the phosphine PH3 (8) is possible. Both
phosphines are exposed to the strong trans influence of
a hydride ligand, while their mutual influence should
be small, being bonded in cis positions. Searching on
the potential energy surface (PES) for the most stable
structure of 6, we found two stable rotational conform-
ers, denoted as 6b and 6d in Figure 2, which differ by
only 0.04 kcal/mol. In both conformers, the PC2 cycle
adopts a position such that the nitrogen atom of the NH2
group lies almost in the central molecular plane includ-
ing the phosphorus atoms, hydrogen atoms, and the
In contrast to our expectations, we find that LfM
donation, d, increases more than LrM back-donation,
b, on going from PH3 (8) to (H2N)PC2H4 (9). As a net
effect the d/b ratio for 9 is larger than for 8, and this is
(18) For Pt(0) phosphine complexes the Pt-P bonds are about 0.03-
0.05 Å longer than the lengths determined experimentally; see: Udin,
J .; Dapprich, S.; Frenking, G.; Yates, B. F. Organometallics 1999, 18,
458.
(19) Frison, G.; Gru¨tzmacher, H. J . Organomet. Chem. 2002, 643-
644, 285.

2206 Organometallics, Vol. 22, No. 11, 2003
Laporte et al.
or PPh3 to a CH2Cl2 solution of [PtMe2(iPrBABAR-
Phos)2] (3) rapidly (<10 min) and quantitatively leads
to a displacement of the two BABAR-Phos ligands and
formation of [PtMe2(PR3)2] (R ) OMe,21 δ(31P) 137.9,
Ch a r t 2. Su bstitu tion Rea ction s Tr a n sfor m in g
[P tH2(cod )] (4) Su ccessively in to [P tH2(P H3)2] (5) f
[P tH2(P H3){(H2N)P C2H4}] (6b) f
[P tH2{(H2N)P C2H4}2] (7)a
1
1J PtP ) 3120 Hz; R ) OPh, δ(31P) 119.7, J PtP ) 3058
1
Hz; R ) Ph,22 δ(31P) 26.7, J PtP ) 1899 Hz). The
intermediate formation of the mixed complex [PtMe2-
1
(BABAR-Phos){PPh)3}] (δ(31P) -61.1, J PtP ) 2002 Hz,
1
2J PP ) 13 Hz (BABAR-Phos); δ(31P) 23.1, J PtP ) 1843
Hz, 2J PP ) 13 Hz (PPh3)) was only observed with PPh3,
where the reaction is slightly slower.
Con clu sion . The thermochemical determination of
the substitution enthalpy, ∆Hr, for the reaction between
[PtMe2(cod)] (1) and iPrBABAR-Phos (2), yielding [PtMe2-
(
iPrBABAR-Phos)2] (3), resulted in the lowest value
(-11.8 kcal/mol) ever observed for a phosphine. Accord-
ingly, the BABAR-Phos ligands in 3 are readily dis-
placed by other phosphines. Since the Pt-P bonds in 3
are rather short and lie in the lower region of Pt-P
bonds, the above reaction gives another example for the
“short bond-low substitution enthalpy” paradox. This
does not necessarily mean that the M-P bonds are
intrinsically weak, since thermochemical measurements
give only the difference between states.14,15,23 However,
since no strong particular steric interactions are recog-
nized in the ligand sphere of complex 3 and the
structural changes in the free and complexed form of
the ligand are small,24 the low enthalpy is most likely
connected with a low Pt-P binding energy. A CDA
analysis shows that phosphiranes are not especially
poor electron donors or significantly strong electron
acceptors. This is indicated by the absence of a sizable
rotation barrier for the phosphirane (H2N)PC2H4 (9) in
the complex [Pt(PH3){(H2N)PC2H4}] (6b). Although
more substantiated conclusions must await a more
detailed theoretical analysis using a larger set of various
types of phosphines, we ascribe the particularly low
binding enthalpy for phosphiranes such as BABAR-Phos
to the fact that the donor orbitals (mainly the s and p
orbitals at phosphorus corresponding approximately to
the lone pair and the P-C bonds of the PC2 ring plane
perpendicular to the P-M axis) are only poorly directed
toward the metal center. This results in a poor overlap
and low binding energy, although the M-P bonds may
be rather short. However, further investigations are
needed to allow a generalization of the observations
reported here.
a
Relative energies, enthalpies (in boldface), and free ener-
gies (in italics) are given in kcal/mol. Enthalpies and free
energies are given at T ) 298 K.
true for Pt(II) and Pt(0). Given that PH3 is an especially
bad acceptor, d/b will be even smaller for other phos-
phines, making the difference between them and 9 even
larger.19,20 We find that the d/b ratio is smaller for
Pt(0); i.e., back-donation becomes relatively more im-
portant with the lower oxidized metal center in accord
with qualitative bonding models. Note that also the
intrinsic reaction energy, Eint, is higher for the phos-
phirane 9 than for PH3, regardless of the platinum
oxidation state. (Eint corresponds to the interaction
energy of the metal fragment MLn with the ligand L
when both have already adopted the structure within
the complex; i.e., they are electronically and structurally
prepared for bonding.)
To get a better picture of the thermodynamic substi-
tution energies, we calculated the reactions outlined in
Chart 2. The first step in our model reaction sequence
is the substitution of the cyclooctadiene (cod) ligand in
[PtH2(cod)] (4) by two PH3 ligands, which is modestly
exothermic (∆Hr ) -11.18 kcal/mol). The stepwise
substitution of each PH3 (8) by (H2N)PC2H4 (9) is
slightly exothermic for each step by -2.50 and -3.51
kcal/mol, respectively, leading to an overall reaction
enthalpy of about -17 kcal/mol, which is in reasonable
accord with the experimentally derived value.
The calculations show that phosphirane 9 is a stron-
ger ligand than PH3, which, however, is an especially
poorly bonded phosphine. Any other phosphine will be
a significantly stronger ligand. Also, the binding energy
will increase in the series PH2R < PHR2 < PR3 and
therefore especially tertiary phosphines will be more
strongly bound to the PtH2 fragment than a phos-
phirane.20
In accordance with the results from the solution
calorimetry and the DFT calculations are the following
simple substitution reactions, which we performed in
an NMR tube. Addition of 2 equiv of P(OMe)3, P(OPh)3,
(21) Yang, D.-S.; Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt,
R. J .; Tse, J . S. Inorg. Chem. 1990, 29, 2487-2495.
(22) Rashidi, M.; Fakhroeian, Z.; Puddephatt, R. J . J . Organomet.
Chem. 1991, 406, 261-268.
(23) Huang, J .; Haar, C. M.; Nolan, S. P.; Marshall, W. J .; Moloy,
K. G. J . Am. Chem. Soc. 1998, 120, 7806-7815 and references cited
therein.
(24) For the ensemble of reactions [PtMe2(cod)] + 2PR3 f [PtMe2-
(PR3)2] + cod, the contributions to the reaction enthalpy which are
caused by structural differences of the PtMe2 fragments on the left
and right sides of the reaction equation and the free and bonded forms
of cod should be almost equal. The contributions which are evoked by
different conformations of the substituents R in the free and complexed
forms of the phosphines are difficult to evaluate. However, as we
discussed above, we have no indication of any unfavorable steric
interactions of the BABAR-Phos ligands in the coordination sphere of
3. Furthermore, BABAR-Phos molecules have a very rigid structure,
which does not change much upon complexation. This can be shown
by the comparison between a number of related BABAR-Phos ligands
and their complexes, whose structures we determined by X-ray
analyses (Liedtke, J . ETH-Dissertation No. 13688, 2000). Therefore,
it can be assumed that the small enthalpy reported here is mainly
due to a low Pt-P binding energy.
(20) Frison, G.; Gru¨tzmacher, H.; Frenking, G. Unpublished results.

Binding of Specialty Phosphines to Metals
Organometallics, Vol. 22, No. 11, 2003 2207
Ta ble 3. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
Deta ils for th e Com p lex
Exp er im en ta l Section
Th eor etica l Meth od s. Full geometry optimizations for
systems 4-10 were carried out with the use of the B3LYP25,26
density functional level of theory and with the following basis
set. The Hay and Wadt small-core relativistic effective-core
potential with a valence shell of double-ú quality (441/2111/
21) was used on platinum.27 The 6-31G(d) basis set28,29 was
employed for all the other atoms, except for the hydrogen
atoms located on the platinum center, for which a set of 2p
polarization functions (6-31G(d,p) basis set) were added. Sets
of six Cartesian d functions were used in the basis sets
throughout these calculations. This corresponds to the stan-
dard “Basis Set II” defined by Frenking and collaborators,30
and the level of theory used in this study will hereafter thus
be denoted as B3LYP/II. The optimized structures were
characterized by harmonic frequency analysis as minima (all
frequencies real; 4, 5, 6b,d , 7-10) or transition states (only
one imaginary frequency; 6a ,c,e). These molecular orbital
calculations were performed with the Gaussian 98 programs.31
Total energies have been computed at the level of optimiza-
tion (B3LYP/II). The zero-point, thermal, and entropic correc-
tions from the B3LYP analyses were used, without scaling to
convert, in some cases, the B3LYP electronic energies to
enthalpies and free energies at 298 K.32
[P tII(iP r BABAR-P h os)2(CH3)2] (3)
identification code
0_75
empirical formula
C
38H42N2P2Pt
temp
wavelength
cryst syst
space group
293(2) K
0.710 73 Å
monoclinic
P21/n
unit cell dimens
a
13.0620(10)
b
12.5170(10)
c
20.4639(14)
R
90
â
92.990(3)
γ
90
V
3341.2(4) Å3
Z
4
calcd density
abs coeff
F(000)
1.558 Mg/m3
4.324 mm-1
1568
cryst size
data collection
0.5 × 0.5 × 0.2 mm
Siemens SMART PLATFORM
with CCD detector,
graphite monochromator
30 mm
detector dist
method; exposure time/frame ω scans; t ) 10 s
refinement method
full-matrix least squares on F2,
Metal-ligand donor-acceptor interactions were examined
in terms of charge donation, back-donation, and repulsive
polarization using the program CDA 2.1.33 This charge de-
composition analysis (CDA)17 is achieved by inspecting the
orbital contributions to the charge distributions in the complex
by (i) the mixing of the filled orbitals of the ligand, L, with
SHELXL97
θ range for data collection
1.81-28.28°
index ranges
-17 e h e 13, -16 e k e 16,
-22 e l e 27
no. of rflns collected
no. of indep rflns
abs cor
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
largest diff peak and hole
28 328
8286 (R(int) ) 0.0726)
empirical (SADABS)
8286/0/388
the unfilled orbitals at the metal-containing fragment, [ML1
]
n
(donation, d), (ii) the mixing of the unfilled orbitals of L with
the filled orbitals at [ML1n] (back-donation, b), (iii) the mixing
0.999
R1 ) 0.0360, wR2 ) 0.0687
R1 ) 0.0594, wR2 ) 0.0715
2.169 and -1.823 e Å-3
of the filled orbitals of L with the filled orbitals at [ML1
]
n
(repulsive polarization, r), and (iv) the mixing of the unfilled
orbitals of L with the unfilled orbitals at [ML1n] (residual
term, ∆).
Total energies and complete sets of Cartesian coordinates
for the optimized geometries are contained in the Supporting
Information.
Exp er im en ta l Meth od s. (a ) Gen er a l Con sid er a tion s.
All manipulations involving organoplatinum complexes were
performed under an inert atmosphere of argon, using standard
high-vacuum or Schlenk techniques, or in a MBraun glovebox
containing less than 1 ppm of oxygen and water. Only
materials of high purity as indicated by NMR spectroscopy
were used in the calorimetric experiments. The NMR spectra
were recorded on a Bruker DPX 250 MHz or DPX 300 MHz
spectrometer. 1H and 13C chemical shifts are calibrated against
1
the solvent signal (CDCl3, H NMR 7.27 ppm, 13C NMR 77.23
ppm; CD2Cl2, 1H NMR 5.32 ppm, 13C NMR 54.00 ppm). 31P
chemical shifts are calibrated against 85% H3PO4 as an
external standard. Calorimetric measurements were per-
formed using a Calvet calorimeter (Setaram C-80), which was
periodically calibrated using the Tris reaction34 or the enthalpy
of solution of KCl in water.35 The calorimeter has been
previously described.36,37 The experimental enthalpies for these
two standard reactions compared very closely to literature
values. The phosphine ligand used in this study was prepared
by the literature method.7
(25) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. 1988, B37, 785.
(26) Becke, A. D. J . Chem. Phys. 1993, 98, 5648.
(27) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 299.
(28) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28, 213.
(29) Francl, M. M.; Petro, W. J .; Hehre, W. J .; Binkley, J . S.; Gordon,
M. S.; DeFrees, D. J .; Pople, J . A. J . Chem. Phys. 1982, 77, 3654.
(30) Frenking, G.; Antes, I.; Bo¨hme, M.; Dapprich, S.; Ehlers, A. W.;
J onas, V.; Neuhaus, A.; Otto, M.; Stegmann, R.; Veldkamp, A.;.
Vyboishchikov, S. F. In Reviews in Computational Chemistry; Lipkow-
itz, K. B., Boyd, D. B., Eds.; VCH: New York, 1996; Vol. 8, pp 63-
144.
(31) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA,
1998.
(b) NMR Titr a tion s. Prior to every set of calorimetric
experiments involving a ligand, a precisely measured amount
((0.1 mg) of [PtMe2(cod)] (1) was placed in an NMR tube along
1
with CD2Cl2 and 2.1 equiv of ligand. Both H and 31P{1H} NMR
spectra were measured within 1 h of mixing; both indicated
the reactions were clean and quantitative. These conditions
are necessary for accurate and meaningful calorimetric results
and were satisfied for all reactions investigated.
(c) Solu tion Ca lor im etr y. In a representative experiment,
the mixing vessels of the Setaram C-80 instrument were
(32) It has to be remembered that the accuracy of computed entropic
contributions to the free energies are suspect, due to the fact that (a)
gas-phase entropies are not the same as the solution-phase entropies,
which are chemically relevant, and (b) Gaussian 98 uses harmonic
oscillator partition functions for the low-frequency, hindered rotations
in the complexes.
(34) Ojelund, G.; Wadso¨, I. Acta Chem. Scand. 1968, 22, 1691-1699.
(35) Kilday, M. V. J . Res. Natl. Bur. Stand. (U.S.) 1980, 85, 467-
481.
(36) Nolan, S. P.; Hoff, C. D.; Landrum, J . T. J . Organomet. Chem.
1985, 282, 357-362.
(37) Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D. Inorg. Chem.
1986, 25, 4446-4448.
(33) Dapprich, S.; Frenking, G. CDA 2.1; Marburg, Germany, 1994.

2208 Organometallics, Vol. 22, No. 11, 2003
Laporte et al.
cleaned, dried in an oven maintained at 145 °C, and then taken
into the glovebox. A sample of [PtMe2(cod)] (1; 16.4 mg, 0.05
mmol) was weighed into the lower vessel, which was closed
and sealed with 1.5 mL of mercury. A solution of iPrBABAR-
Phos (2; 30.2 mg, 0.108 mmol) 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 calorim-
eter had once again reached thermal equilibrium (ca. 2 h).
Control reactions with Hg and phosphine show no reaction.
The enthalpy of ligand substitution (-6.0 ( 0.2 kcal/mol)
represents the average of at least three individual calorimetric
determinations. The enthalpy of solution of [PtMe2(cod)] (+5.8
( 0.1 kcal/mol) in CH2Cl2 was determined using identical
methodology. This corresponds to a total enthalpy of reaction
with all species in solution of -11.8 ( 0.2 kcal/mol. This value
can be compared to enthalpy values previously measured for
[PtMe2(PR3)2] complexes.14,15
solution was stirred for 1 h at room temperature. The meth-
ylene chloride phase was concentrated to 10% of its volume,
and hexane was added to precipitate pure complex (187 mg,
1
87%). 31P NMR (CD2Cl2): δ -69.63 (d, J PtP ) 1840 Hz). 1H
3
NMR (CD2Cl2): δ 0.90 (d, J HH ) 6.7, CH3 iPr, 6H), 0.98 (m,
2J PtH ) 78.2 Hz, 3J PH ) 1.6 Hz, CH3, 6H), 2.90 (m, 2J PH + 4J PH
3
3
) 5.1 Hz, J PtH ) 9.0 Hz, PCHCH, 2H), 4.05 (m, J HH ) 6.7
Hz, CH Pr, 1H), 4.98 (m, J PH + 6J PH ) 14.3 Hz, J PtH ) 11.1
Hz, CH benzyl, 1H), 6.94-7.20 (m, CH aryl, 8H). 13C NMR
(CD2Cl2): δ 0.89 (m, CH3, 1C), 3.01 (m, CH3, 1C), 21.60 (m,
i
3
4
PCHCH, 2C), 22.32 (m, CH3 iPr, 2C), 50.81 (m, CH Pr, 1C),
i
59.55 (m, CH benzyl, 2C), 124.26, 126.06, 127.67, 129.72 (s,
CH aryl, 8C), 132.24 (m, quat C, 2C), 136.32 (m, quat C, 2C).
Anal. Calcd for C38H42N2P2Pt: C, 58.23; H, 5.40. Found: C,
58.34; H, 5.48.
Ack n ow led gm en t is made to the ETH Zu¨rich and
the Swiss National Science Foundation for support of
this work. We are indebted to the Swiss Center for
Scientific Computing (CSCS) and the Competence Cen-
tre for Computational Chemistry (C4) of the ETH Zu¨rich
for providing computer time. The National Science
Foundation is gratefully acknowledged for support of
the work carried out at UNO. We thank the reviewers
for valuable comments.
(d ) X-r a y Cr ysta llogr a p h y. Single crystals of the complex
were obtained by slow diffusion of n-hexane into a concentrated
toluene solution. Crystallographic data for the complex were
collected on a Siemens SMART PLATFORM instrument with
CCD detector and graphite monochromator and are listed in
Table 3. The structure of the complex was solved using direct
methods and was refined against full matrix (versus F2) with
SHELXTL (version 5.0).38 Non-hydrogen atoms were treated
anisotropically; hydrogen atoms were refined on calculated
positions using the riding model.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates and equivalent isotropic displacement parameters,
bond distances and angles, anisotropic displacement param-
eters, and hydrogen coordinates and isotropic displacement
parameters for 3 and total energies and complete sets of
Cartesian coordinates for the optimized geometries of 4, 5, 6a -
e, and 7-10. This material is available free of charge via the
Internet at http://pubs.acs.org.
(e) Syn th esis of [P tIIMe2(iP r BABAR-P h os)2]. A 100 mg
(0.3 mmol) portion of [(1,2,5,6-η)-1,5-cyclooctadiene]dimethyl-
platinum(II)39 and 167 mg (0.6 mmol) of iPrBABAR-Phos7 were
dissolved in 20 mL of methylene chloride. The resulting yellow
OM020642Q
(38) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, A46, 467.
Sheldrick, G. M. SHELX 97; Universita¨t Go¨ttingen, Go¨ttingen, Ger-
many, 1997.
(39) Costa, E.; Pringle, P. G.; Ravetz, M. Inorg. Synth. 31, 284-
286; 38, 1623.