Download Fluxional behavior in platinum(II) complexes of the tridentate ligand

ELSEVIER
lnorganica ChimicaActa251 (1996) 89-93
Fluxional behavior in platinum(II) complexes of the tridentate ligand
1,1,1 -tris (diphenylphosphinomethyl) ethane. Evidence for the importance
of cis effects in an associative mechanism 1
A.
Rauf Khan ~, Pierre D. Harvey t,.., Steven M. Socol b , .
a Board of intermediate and Secondary Education, (NWFP). Bannu. Pakistan
b Department de Chimie. Universit~de Sherbrook,'. Sherbrooke, Que,. JIK 2Rl. Canada
Received23 February 1996;revised5 June 1996
Abstract
Fluxional behavior is observed in complexes of the type [PtX2(triphos)] (triphos= l,l,l-tris(diphenylphosphinomethyt)ethene) where
X--CN and NCS, but not CI, Br, I, CH3 or C6H~.The existence of intramolecular exchange in the former two complexes is attributed to
smaller stefic requirements of the anions. An associate mechanism which involves a five-coordinateintermediate is proposed for ti~eexchange.
Extended Hfickel Molecular Orbital Calculations (EHMO) .suggestthat the rates of exchange are not thermodynamically controlled. The
electronic structures of some of the intermediates are addressed by EHMO.
Keywords: Fluxionalbehavior:Platinumcomplexes;Tridentatephosphineligand complexes
1. Introduction
We recently reported a series of complexes of the type
[PtX2(tripnos)] ( X = B r , CI, CN, C6H5 and NCS; triphos
= l,l,l-tris(diphenylphosphinomethyl)ethanc) [1]. When
X = C N or NCS, the complexes show fluxional behavior at
room temperature which contrasts to the stereochemical
rigidity found previously in [Pt(CH3)2(triphos) ] [2]. The
recently prepared complexes c i s - [ P t ( t r i p h o s ) ( S R ) ( H ) ]
(R-- H and Ph) are also reported to show fluxional behavior
at room temperature [ 3 ]. We would like to report evidence
herein which indicates that the rate of intramolecular
exchange is dependent on the size of the ligand cis to the
phosphorus atom undergoing exchange, and that the
exchange proceeds through a five-coordinate intermediate.
121.5 MHz. The syntheses and 31p NMR chemical shifts
for the complexes [PtCl~(dpp)], [Pt(CN)2(dpp)],
[Pt(CNS)2(lriphos) ] and [Pt(CN)2(triphos) ] were previously described [ 1].
The MO calculations were of the extended H ~ k e l type
(EHMO) [4] using a modified version of the WolfsbergHelmholz formula [5]. The atomic parameters used for C
[ ~ ] , N [4c], P [6], H [4c], and Pt [6] were flora the
literature. The Pt-P, Pt-C, Pt-S, C-N, S-H, C - H and P-H
bonds were fixed at 2.27, 2.15, 2.20, 1.16, 1.40 ,~, (the sum
of the covalent radii), 1.00 and 1.43 ,~, respectively. A
detailed description of the graphic programs in this work has
been reported [7].
3, Results and discussion
2. Experimental
Recently, Vacca and co-workers prepared the complexes
( R = H or Ph), and observed
fluxional behavior [3]. The authors presented evidence that
the exchange was intramolecular with the initially uncoordinated phosphorus undergoing exchange with the phosphorus atom coordinated cis with respect to the hydride
ligand. It was postulated that the presence ofa hydrideligand,
because of its high cis effect [ 8], is necessary for fluxional
behavior in square-planar Pt(II)-triphos complexes. Subsecis-[Pt(triphos)(SR)(H)]
The 31p NMR spectra were recorded in CH2C!2 solutions on a Bruker WIvl-300 spectrophotometer operating at
* Correspondingauthors.
' A,R.K.and S.M.S.wouldlike to dedicatethis workto the memoryof
their teacher, colleague and friend, ProfessorDevon W. Meek (died 7
December 1988) in whose laboratories the compoundsdescribed in this
paper wereoriginallysynthesized.
0020-1693196/$15.00 © 1996ElsevierScienceS,A. All rights reserved
P!!S0020-1693(96)05256-5
90
A.R. Khan et al. / Inorganica Chimica Acta 251 (1996)89-93
quently, it was shown that [ PtX2 (triphos) ] complexes where
X = CN or NCS are also fluxional at room temperature, but
where X=CI, Br, CH, or C6H5 the complexes are stereochemically rigid on the 3]p NMR time scale [!,2]. The
exchange was proposed to be occurring by an associative
type mechanism in conjunction with concurrent rearrangement of the five-coordinate intermediates [ 3,9].
The results presented here support the idea that the rate of
fluxionality in Pt(II)-triphos complexes is a function of the
ligand cis to the phosphorus atom undergoing exchange. The
high cis effect previously reported for the coordinated hydride
iigand [3] is most likely due to its small size which would
help stabilize a five-coordinate intermediate as opposed to
any electronic properties. When the temperature of a solution
of [Pt(CN)2(triphos) ] is raised to - 4 0 °(2 in CH2CI2 (see
Fig. 1), all the 3~p NMR signals collapse into the baseline
while for [Pt(triphos)SH(H)], the coalescence occurs at
60 °(2 in C6DsCD3 [3]. This indicates that exchange is more
rapid in the former complex. The reason for this may be, in
part, statistical since in [Pt(CN)2(triphos) ] two phosphorus
atoms are cis to two small CN ligands, while in
[Pt(triphos)SH(H)] only one phosphorus atom is cis to a
small hydride. In addition, it was also previously postulated
that the rr-acceptor ability of the cyanide ion may serve to
enhance the electrophilic nature of the metal center [ 1,10].
The 3~p NMR resonances for [Pt(NCS)2(triphos)] are
only slightly broadened at room temperature which indicates
a slower exchange rate [ 1 ]. The most favored coordination
mode of thiocyanate in these complexes is the geminite configuration where one th~.ocyanate is N-bound while the other
is S-bound [ 1,11 ]. The more sterically demanding S-bound
thiocyanate may serve to inhibit the formation of a fivecoordinate intermediate.
F~'e Tripho~
37"C
30'C
--I
1
=20"¢
-40)C
.60. C
L.__
i
1
i
5
0
-5
"70"C
_
-10
i
i
i
-15
-20
-25
i
-30
-35
40
Chemica! shift (ppm}
Fig. !. Variable temi~mture 3~P{'H} NMR of [Pt(CN)z(tfiphos) j in
CH:CI:.
The structural properties of the tridentate ligand are also
of importance in determining the rate of intramolecular
exchange in square-planar complexes of tridentate ligands.
While [Pt(CH3)2(tripod)] is rigid, the 3~p NMR of the
analogous complex of the linear triphosphine .:~.p,
PhP(CH2CI-I:CH~PPh2)2 is slightly broadened at room temperature. The complex of the more sterically demanding
Cyttp, PhP(CHzCH2CH2P(C~HI~)2)2, however, is rigid in
toluene solution up to 100 °C [2].
Additional suppor~ for the presence of a five-coordinate
intermediate was obtained in experiments where diphenylmethylphosphine was added to dichloromethane solutions
of [PtClz(dpp)] and [Pt(CN)z(dpp)] ( d p p = l , 3 bis(diphenylphosphmo)propane) in molar ratios of 1:1. In the
case of [PtCl2(dpp)], the added ligand causes a chemical
reaction which results in the formation of cis. and trans[ PtCI:( C6H5) 2CHAP)2], unreacted [ PtCI2 (dpp) ] and some
unidentified products as shown by 3~p NMR. When this same
experiment wag done with [Pt(CN)2(dpp)] and added
P(C6Hs)2CH3, the 31p{'H} NMR resonance due to
[Pt(CN)2(dpp)] is slightly broadened whereas the resonance due to added ligand is much broadened from - 80 to
- 2 0 °C as shown in Fig. 2. This result suggests a weak
interaction between the added diphenylmethylphosphineand
the platinum metal center.
4. EHMO computations
The electronic structures of the Pt(PH3)3(X)(X') model
compounds (X = X' = CH3, CN, and X = SH (axial and
equatorial), X ' = H ) (equatorial and axial)) have been
obtained theoretically using EHMO. The structures were
ideal trigonal bipyramidal geometries, and the distances were
found from crystallography [ 12], and were kept constar~t.
Crystal field theory predicts that the energy levels for the d
orbitals are dz2> d : _ ? = d~..> dx~= d.~.=.The EHMO computations do indeed predict an ordering close to the crystal field
theory, but the formation of the MO's with the ligands slightly
modifies this scheme. For example, the CN derivative exhibits L U M O + x (x=O-3) as ~r* (CN) orbitals. Such a phenomenon is not uncommon for cyano complexes [ 10]. The
LUMO for the CH3 and SH compounds correlate with the
LUMO + 4 in the CN intermediate, and are composed of Pt
d : and ligand lone pair orbitals (n; except for H which does
not contribute to the MO's). The interactions between these
series of orbitals are anti-bonding (i.e. M-L tr* ). A MO
picture of these orbitals as well as the H O M O - x (x = 0-3)
is shown in Fig. 3 for Pt(PH3)3(SH)~q(H)~ as an example.
The HOMO for the CH3',and CN derivatives are mainly composed of Pt dx2_y~and x o (n = non bonding) orbitals with
anti-bonding interactions: Conversely, the HOMO for the SH
compounds exhibits the formation of MO's involving the d~y
and PH3 lone pairs and S px orbital, also in an anti-bonding
fashion. A correlation energy diagram (Fig. 4) predicts a
crossing between HOMO and HOMO-1 upon going from
A.R. Khan et al. , inorganica Chimica Acta 251 (1996) 89-93
li
9~
I
I
i
i
I
I
5
0
-5
-';0
-15
"'
I
'
-20
'I
-25
Fig. 2. Variable temperature 3,p{IH I NMR of [Pt(CN);(dpp) ]/diphenylmethylphosphine l:t in CH2C!2.
LUMO
HOMO
HOMO- I
'"'l'15I'dxY~
~" I
~../.(5)
-12,0
HOMO-2
HOMO-3
Fig. 3. Me representation of the LUMO and HOMO-x (x=O-3) for
Pt(PH3)3( SH)~( H),~ modelcompound.The fouratomgroupsare PH3,the
two atom group is SIt, and the single point atom is H.
CH3 to CN to the SH derivatives. The crossing is due to the
fact that the presence of S p orbitals interacting with the d~
metal orbital to form a ~ * system raises the M e energy.
Therefore, the description of the HOMO-1 is the same as
stated above, except reverse. Finally, the quasi-degenerated
HOMO-2 and HOMO-3 are practically pure d~ and dR Pt
orbitals for the CH3 and CN derivatives. In the SH compounds, the HOMO-2 involves some slight ¢'* interactions
between the Pt dyz and free S lone pair. This interaction
explains the larger energy difference between HOMO-2 and
HOMO-3.
Fig. 4. CorrelationEHMOenergydiagram for the four modelimennedia~
(i.e. the CH3,CN and SH derivatives). Onlythe LUMO (upper scale) and
the HOMO-x (x=0-3) (lower scale) a~ represemcd. Note that for the
CN derivative, the LUMO+x (x=0-3) are the ~r* (CN), and tl~ the
LUMO-I-4is also representedfor the con'elation.
The relative rates of substitution in square planar complexes are affected by steric hindrance, metal oxidation state
and hgand field stabilization of the intermediates. These intermediates most likely exhibit the classical trigonal bipyramid
geometry, similar to that responsible for the fluxional behavior. By using PH3 in the computations, the steric effects and
t,he complexity of the MO diagram are minimized. Since the
92
A.R. Khan et al. / lnorganica Chimica Acta 251 (1996) 89-93
Pt atom does not change its oxidation state, one can address
so!ely the ligand field stabilization of the 18-electron fivecoordinate intermediates. By following the M * energy correlation diagram for CH3, CN and SH~ for filled orbitals, one
can note an energy decrease for all the MO's going from CH 3
to CN and for the ~ , d~zand d,~_ ~jn (x) orbitals going from
CH3 to CN to SH~. This observation suggests that the CN
and SI-I~ intermediates are more stable than for that of CH3,
and that the presence of metal-ligand dTr-pTr interactions
may be responsible for the fluxionality. By comparing the
total EHMO energies of the square planar starting complexes
and the trig,hal bipyramidal intermediates, however, an identical difference in EHMO energy for both the CH3 and CN
compounds is noted. This phenomenon comes from tLe fact
that the square planar complexes undergo the same stabilization effects (metal-ligand dzr-p~- interactions). This
observation suggests that the fluxionality is kinetica!!y rather
than thermodynamically controlled.
We have also followed the EHMO total energies as a function of PH3 coordination onto the square planar Pt complex
for X = CN and CH3 in order to generate square pyramidal
intermediates (Scheme 1). The Pt-PH3 distance was varied
from 3.67 ~-, a distance above the sum of the van der Waals
radii (3.60 A), to 2.27 A, a distance normally encountered
for Pt-P bonds in such compounds according to X-ray data
[ 12]. "~'heenergy differences going from the cis-Pt(PH3) 2X2
to Pt(PH3) 3Xz (square pyramid) were 2.264 eV for X = CN
and 2.855 eV for X = CH3. This first result predicts that the
energy barrier is greater for alkyl derivatives. The EHMO
total energy calculations were then made upon moving one
of the equatorial X ligands downward to the second axial
position in increments of 10° while keeping the XPtX angle
90 °. The graph of the EHMO total energy versus the
P(axial)PtX angle 0 exhibits a minimum at 0 ~ 135°. This
result indicates that the 18-electron penta-coordinated intermediate does prefer a bipyramidal structure. These energy
PHj
ll=P I
L
H3F~P'!t~L
PH+
stabilizations are 1.325 eV for X = CN, and 0.748 eV for
X-~ CH3. At this point the PPtP angles are 90*. Finally, we
placed the P(axial)PtX angle at 1350 where the equatorial
plane is now described by two PH 3 ligands and one X group,
with both PPtX angles equal at 135°. The equatorial PPtP
angle was then varied from 90 to 180* while keeping the PPtX
angles equal. The graph of the EHMO total energy versus
this angle also exhibits a minimum in the neighborhood of
120°. For X = CN and CH3, these angles were approximately
135 and 106°, respectively. We cannot explain this difference
at this point. The extra stabilization gain by this motion is
0.630 eV for X = CN and 1.479 eV for X = CH3. At this point
the CN derivative is still ~ 0.24 eV more stable than the CH3
complex.
The structures of the SH intermediates were also investigated. The molecular structures indicate two pathways for
the intermediate formation (Scheme 2). Path (a) leads to the
equatorial SH substituted derivative, and path (b) is given
for the axially substituted one. The EHMO total energy computation for the model compounds Pt(PH3)3(SH)eq(H)a~
(path (a)) and Pt(PH3)3(SH)~,(H)~q (path (b)) predict
that intermediate path (a) <path (b) by 0.62 eV. We have
also generated the square pyramid Pt (PH3) 3(SH) (H) intermediate with one of the PH3 at the axial position, and performed similar calculations as discussed above. First, EHMO
total energies were computed while the H group was moved
toward the axial position in increments of 10° while keeping
the HPt(SH) angle at 90 °. These calculations were repeated
where the SH group was moved to the axial position in the
same manner. Again the plots of the EHMO total energy
versus 0 exhibit minima around 135". The second step of
these computations was to consider the (PHa)2PtH and
(PH3) 2Pt (SH) equatorial planes for each intermediate where
the PPtH and PPtS angles are 135°, and change the equatorial
PPtP angles going from 90 to 180°. Again minima are
observed in the ,,, 1200 range. The sum of the stabilization
energies are 2.323 and 1.671 eV for Pt(PH3)a(SH)¢q(H)~
and Pt(PH3)3(SH)~(H)~q, respectively. The EHMO total
energy is also dependent upon the Pt-SH rotation angle. The
largest difference between the maxima and minima for a
rotation going from 0 to 3600 is 0.014 eV. The energy differences between the two geometric isomers range from approximately 0.62 to 0.65 eV. These results indicate that the
intermediate is indeed the equatorial SH substituted trig,hal
bipyramidal derivative. They also indicate that both steric
'~o
PRa
'~" R
3.671.227.i.
Hn~
PR~
~12o.
H
L
H 3 P ~
L
Scheme1.
R2 SH
Scheme2.
A.R. Khan et al. / lnorganica Chimica Acta 251 (1996) 89-93
hindrance (kinetic) and thermodynamic parameters converge towards the same product. Thus, the axially substituted
derivative was neglected in the above discussion.
Acknowledgements
A.R,K. acknowledges the United States Education Foundation and the Council for International Exchange of Scholars
for a Fullbright Fellowship, and also the government o f
Pakistan for granting a research leave. P.D.H. acknowledges
N S E R C and F C A R for financial support.
References
[I ] A.R. Khan, S.M. Socol, D.W. Meek and R. Yasmeen, lnorg. Chim.
Acta, 234 (1995) 109.
93
[2] K.D. Tau, R. Uriane, TJ. Mazanec and D.W. Meek, 1. Am. Chem.
Soc., I01 (1979) 6614.
[3] F. Cecconi, P. innoccnli, S. Midol|ini, S. Monefi, A. Vacca and £A.
Ramircz, J. Chem. Soc., Dalton Trans., ( 1991 ) 1129.
[4] (a) R. Hoffmann and W.M. Lipscomb, J. Chem. Phyv. 36 (1962)
2179; (b) J. Chem. Phys., 37 (1962) 2872; ( c ) 1t. Hoffmann,Z Chem.
Phys., 39 (1963) 1397.
[5] J.H. Ammeter, H.B. Burgi, J.C. Thibeault and R. Hoffmann, J. Am.
Chem. 5oc., 100 (1978) 3680.
[6] R.H. Sumn~rville and R. Hoffmann, J. Am. Chem. Sot-., 98 (1976)
7240.
[7] C. Mealli and D.M. Proserpio, J. Chem. Educ., 67 { 1990) 399.
[ 8 ] F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley,
blew York, 5th edn.. 1988, p.1330.
[9] P. Meakin, E.L. Muetterties and J.P. Jesson, J. Am. Chem. Soc.. 94
(1972) 5271.
[ 10] T. Zielger, J.K. Nagle, J.G. Snijders and EJ. Baerends, Z Ant Che~
Sac., I11 (1989) 5631.
[ 111 AJ. Paviglianiti, D.J. Minn, W.C. Fultz and J.L. Bum~ster, lnorg.
Chim. Acta, 159 (1989) 65.
[12] R.M. Kirchner, R.G. Little, K.D. Tan and D.W. Meek,,/. Organon~t.
Chem, 149 (1978) CI5.