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Steric control of thiocyanate coordination in palladium(II) — diphosphine complexes
INORG.
NUCL,
CHEM.
LETTERS
Vol. 10, pp. 125-128, 1974.
Pergamon
Press. Printed in Great Britain.
STERIC CONTROL OF THIOCYANATE COORDINATION IN PALLADIUM(II) - DIPHOSPHINE COMPLEXES Gus J. Palenik,* W. L. Steffen and M. Mathew Center for Molecular Structure, Department of Chemistry University of Florida, Gainesville, Florida, 32611 and Marinda Li and Devon W. Meek* Department of Chemistry, The Ohio State University Columbus, Ohio 43210 (Received 27 September 1973)
The thiocyanate ion is an ambidentate ligand that can coordinate either through the sulfur or nitrogen atom (1). The o c c u r r e n c e of S-bonded thiocyanate complexes of palladium(II) when amine ligands a r e present but N-bonded thiocyanates when phosphines a r e present has been rationalized in t e r m s of a Ir - bonding hypothesis (2, 3). Although the change of thiocyanate from S- to N-bonded could be brought about by bulky amines (4), the Y - bonding arguments for phosphines persisted (5). The presence of N-bonded and S-bonded thiocyanates in Pd(dppn)(NCS)(SCN)(6), dppn = (C6IH5)2PCH2CH2CH2N(CH3) 2, appeared to support these ~r - b o n d i n g arguments.
However, when Pd(dpe)(SCN)(NCS),dpe = (C6I,I5)2PCI,I2CI,I2P(C6I,I5)2,
was shown to have both types of thiocyanate bonding (7), the question of steric control with phosphine ligands was raised.
We now wish to report our studies on the two complexes
Pd(dpm)(SCN)2, dpm = (C6H5)2PCH2P(C6H5)2, and Pd(dpp)(NCS)2 , dpp = (C6H5) 2 PCH2CH2CH2P(C6H5) 2. The series of complexes [~P2P(CH2)nP¢2]Pd(CNS)2 , CNS does not specify the mode of attachment, shows a change in the mode of coordination from S,S for n=l, to S,N for n=2, to N, N for n=3.
Therefore, we a r e able to show that at constant
~r - bonding capabilities, the coordination is controlled completely by s t e r i c effects.
Our
studies suggest that the ~r - bonding arguments which a r e usually given a r e i n c o r r e c t and that palladium-phosphine complexes contain N-bonded species for steric reasons alone. After the completion of our studies, the preparation of Pd(vpp)(SCN)2, vpp is c i s - 1 , 2 bisdiphenylphosphinoethylene (C6H5)2PCI-I=CHP(C6H5) 2 was reported (8) and also supports the concept of sterie control of thiocyanate coordination in palladium-phosphine complexes. The syntheses of Pd(dpm)(SCN)2 and Pd(dpp)(NCS)2 were c a r r i e d out by the addition of the ligand in CH2C12 to an ethanolic solution containing the Pd(SCN)4-2 ion. a r e in agreement with the formulations. b=29. 353,
c=9.884A, ~=119.86",
group P21/n.
The analyses
Crystal data for Pd(dpm)-(SCN)2: a=10. 426,
DM=I. 54 g / c m -3, for Z=4, DC=I. 536 g / c m -3
space
Crystal data for Pd(dpp)(NCS)2: a=14. 774, b=9.181, c=21. 182 A, ~--95.48",
* Address enquiries to these authors. 125
126
DIPHOSPHINE COMPLEXES
Vol. 10, No. 1
DM=I. 48 g / c m -3, for Z=4, DC=I. 475 g / c m -3, s p a c e group I 2 / a (non-standard setting of C2/c).
The data for both compounds w e r e m e a s u r e d using a Syntex P i d i f f r a c t o m e t e r with
g r a p h i t e - m o n o c h r o m a t i z e d MoK~ r a d i a t i o n . cases.
A v a r i a b l e speed 0 - 2 0 s c a n was used in both
The s t r u c t u r e s w e r e solved by the heavy a t o m method and refined by f u l l - m a t r i x
l e a s t - s q u a r e s techniques to an R of 0.049 for 2814 o b s e r v e d r e f l e c t i o n s in the c a s e of P d (dpm)~CN)2 and an R of 0. 037 for 1728 r e f l e c t i o n s for Pd(dpp)(NCS)2. The p e r t i n e n t s t r u c t u r a l data have been s u m m a r i z e d in F i g u r e 1, t o g e t h e r with the data of Pd(dpe)(NCS)(SCN) and Pd(dppn)(NCS)(SCN) for c o m p a r i s o n .
We s e e that f o r the t h r e e
ligands ~2P(CH2)nl~2 , n = l - 3 , lengthening the carbon chain between the phosphorus a t o m s leads to a l a r g e i n c r e a s e in the P - P d - P angle while m a i n t a i n i n g e s s e n t i a l l y constant e l e c t r o n i c character.
The i n c r e a s e in the P - P d - P angle r e s u l t s in an i n c r e a s i n g s t e r i c i n t e r a c t i o n
between the phenyl groups and the coordinated t h i o c y a n a t e s .
Consequently, s i n c e the s t e r i c
r e q u i r e m e n t s for S-bonded t h i o c y a n a t e s a r e m o r e s e v e r e (the P d - S - C angle is about 110" v e r s u s a P d - N - C angle of about 180°), the mode of coordination a l s o changes.
The
e x i s t e n c e of Pd(vpp)(SCN)2 as the S-bonded i s o m e r s u p p o r t s t h e s e s t e r i c a r g u m e n t s . The C=C d i s t a n c e of 1.33A is s h o r t e r than C-C of 1.54A which in e s s e n c e pulls the phenyl groups away f r o m the t h i o c y a n a t e groups allowing both ~hiocyanates to be S-bonded. The above s e r i e s offers a c l e a r delineation of ?r - bonding f r o m s t e r i c effects and c l e a r l y d e m o n s t r a t e s that the mode of thiocyanate coordination to Pd is s t e r i c a l l y c o n t r o l l e d .
The
s a m e t r e n d s exist in solution although a l l t h r e e compounds show the p r e s e n c e of both N and S-bonded t h i o c y a n a t e s in d i c h l o r o m e t h a n e (9).
T h e r e is a d r a m a t i c i n c r e a s e in the
amount of the N-bonded s p e c i e s with i n c r e a s i n g c h e l a t e chain length, a s expected on the b a s i s of s t e r i c effects. T h e s e r e s u l t s can be extended to the s e l e n o c y a n a t e ion a s w e l l as to the third row transition elements.
Since the s e l e n i u m a t o m is l a r g e r and the M-Se bond longer than M-S.
t h e r e a r e lower s t e r i c r e q u i r e m e n t s for SeCN- r e l a t i v e to SCN-.
The paucity of linkage
i s o m e r s of SeCN- is then a m a n i f e s t a t i o n of this lowered s t e r i c r e q u i r e m e n t .
Finally,
the l a c k of linkage i s o m e r s involving t h i r d row t r a n s i t i o n e l e m e n t s i s a l s o d i r e c t l y r e l a t e d to the l a r g e s i z e of t h e s e e l e m e n t s , reducing the s t e r i c i n t e r a c t i o n s between ligands.
We a r e g r a t e f u l for a D e p a r t m e n t of C h e m i s t r y P o s t d o c t o r a l F e l l o w s h i p ( M . M . ) , a G r a d u a t e School F e l l o w s h i p ( W . S . ) , a grant of c o m p u t e r t i m ~ f r o m the Computing C e n t e r (to G. J. P. ) and the National Science Foundation for financial s u p p o r t (M. L. and D. W. M. ).
Vol.10,No.1
DIPHOSPHINECOMPLEXES
127
c
¢R/c--c/•
Pd
S
(.)
(a)
S
(~')
Pd
2.3/64/
~1.998
S
¢
~/
.c~c
q \p/
N
¢
Pd 2.051/ / N
~ N
2.0/63/ N
Pd
(~)
~ 2.295 s ('0
Figure 1. A c o m p a r i s o n of the geometry about the Pd atom in the four complexes:
(a) Pd(dpm)(SCN)2, (b) Pd(dpe)(SCN)(NCS), (c) Pd(dpp)(NCS)2 and (d) Pd(dpDn)(SCN)(NCS). The angles in parenthesis given in a, b and d are the tilt of the thiocyanate group from the plane defined by the Pd and the four coordinated atoms.
DIPHOSPHINE COMPLEXES
128
Vol. 10, No. 1
References 1.
A.H. Norbury and A . I . P . Sinha, Quart. Revs., 2..~4, 69(1970), have reviewed ambidentate ligands.
2.
A. Turco and C. Pecile, Nature (London), 191, 66(1961), first introduced the idea of ligand control of the mole of thiocyanate coordination. Their ~ -bonding arguments have been extended by some MO calculations (3) which are used to support the ~r-bonding arguments.
3.
A.I~. Norbury, J. Chem. Soc. (A), 1089(1971).
4.
F. Basolo, W.H. Baddley and J . L . Burmeister, Inorg. Chem., _3, 1202(1964).
5.
These arguments are found in many textbooks such as J . E . Huheey, '~inorganic Chemistry: Principles of Structure and Reactivity, ' ~ a r p e r and Row, New York 1972, p. 409 or F. Basolo and R.G. Pearson, "Mechanisms of Inorganic Reactions," 2nct ed., J. Wiley and Sons, New York, 1967, p. 296.
6.
G.R. Clarkand G.J. Palenik, Inorg. Chem., 9_, 2754(1970)
7.
G. Beran and G.J. Palenik, J . C . S . Chem. Comm., 1354(1970)
8.
K.K. Chow and C.A. McAuliffe, Inorg. Nucl. Chem. Letters , 8, 1031(1972).
9.
D.W. Meek, P . E . Nipcon and V.I. Meek, J. Amer. Chem. Soc., 9_22, 5351(1970).
NUCL,
CHEM.
LETTERS
Vol. 10, pp. 125-128, 1974.
Pergamon
Press. Printed in Great Britain.
STERIC CONTROL OF THIOCYANATE COORDINATION IN PALLADIUM(II) - DIPHOSPHINE COMPLEXES Gus J. Palenik,* W. L. Steffen and M. Mathew Center for Molecular Structure, Department of Chemistry University of Florida, Gainesville, Florida, 32611 and Marinda Li and Devon W. Meek* Department of Chemistry, The Ohio State University Columbus, Ohio 43210 (Received 27 September 1973)
The thiocyanate ion is an ambidentate ligand that can coordinate either through the sulfur or nitrogen atom (1). The o c c u r r e n c e of S-bonded thiocyanate complexes of palladium(II) when amine ligands a r e present but N-bonded thiocyanates when phosphines a r e present has been rationalized in t e r m s of a Ir - bonding hypothesis (2, 3). Although the change of thiocyanate from S- to N-bonded could be brought about by bulky amines (4), the Y - bonding arguments for phosphines persisted (5). The presence of N-bonded and S-bonded thiocyanates in Pd(dppn)(NCS)(SCN)(6), dppn = (C6IH5)2PCH2CH2CH2N(CH3) 2, appeared to support these ~r - b o n d i n g arguments.
However, when Pd(dpe)(SCN)(NCS),dpe = (C6I,I5)2PCI,I2CI,I2P(C6I,I5)2,
was shown to have both types of thiocyanate bonding (7), the question of steric control with phosphine ligands was raised.
We now wish to report our studies on the two complexes
Pd(dpm)(SCN)2, dpm = (C6H5)2PCH2P(C6H5)2, and Pd(dpp)(NCS)2 , dpp = (C6H5) 2 PCH2CH2CH2P(C6H5) 2. The series of complexes [~P2P(CH2)nP¢2]Pd(CNS)2 , CNS does not specify the mode of attachment, shows a change in the mode of coordination from S,S for n=l, to S,N for n=2, to N, N for n=3.
Therefore, we a r e able to show that at constant
~r - bonding capabilities, the coordination is controlled completely by s t e r i c effects.
Our
studies suggest that the ~r - bonding arguments which a r e usually given a r e i n c o r r e c t and that palladium-phosphine complexes contain N-bonded species for steric reasons alone. After the completion of our studies, the preparation of Pd(vpp)(SCN)2, vpp is c i s - 1 , 2 bisdiphenylphosphinoethylene (C6H5)2PCI-I=CHP(C6H5) 2 was reported (8) and also supports the concept of sterie control of thiocyanate coordination in palladium-phosphine complexes. The syntheses of Pd(dpm)(SCN)2 and Pd(dpp)(NCS)2 were c a r r i e d out by the addition of the ligand in CH2C12 to an ethanolic solution containing the Pd(SCN)4-2 ion. a r e in agreement with the formulations. b=29. 353,
c=9.884A, ~=119.86",
group P21/n.
The analyses
Crystal data for Pd(dpm)-(SCN)2: a=10. 426,
DM=I. 54 g / c m -3, for Z=4, DC=I. 536 g / c m -3
space
Crystal data for Pd(dpp)(NCS)2: a=14. 774, b=9.181, c=21. 182 A, ~--95.48",
* Address enquiries to these authors. 125
126
DIPHOSPHINE COMPLEXES
Vol. 10, No. 1
DM=I. 48 g / c m -3, for Z=4, DC=I. 475 g / c m -3, s p a c e group I 2 / a (non-standard setting of C2/c).
The data for both compounds w e r e m e a s u r e d using a Syntex P i d i f f r a c t o m e t e r with
g r a p h i t e - m o n o c h r o m a t i z e d MoK~ r a d i a t i o n . cases.
A v a r i a b l e speed 0 - 2 0 s c a n was used in both
The s t r u c t u r e s w e r e solved by the heavy a t o m method and refined by f u l l - m a t r i x
l e a s t - s q u a r e s techniques to an R of 0.049 for 2814 o b s e r v e d r e f l e c t i o n s in the c a s e of P d (dpm)~CN)2 and an R of 0. 037 for 1728 r e f l e c t i o n s for Pd(dpp)(NCS)2. The p e r t i n e n t s t r u c t u r a l data have been s u m m a r i z e d in F i g u r e 1, t o g e t h e r with the data of Pd(dpe)(NCS)(SCN) and Pd(dppn)(NCS)(SCN) for c o m p a r i s o n .
We s e e that f o r the t h r e e
ligands ~2P(CH2)nl~2 , n = l - 3 , lengthening the carbon chain between the phosphorus a t o m s leads to a l a r g e i n c r e a s e in the P - P d - P angle while m a i n t a i n i n g e s s e n t i a l l y constant e l e c t r o n i c character.
The i n c r e a s e in the P - P d - P angle r e s u l t s in an i n c r e a s i n g s t e r i c i n t e r a c t i o n
between the phenyl groups and the coordinated t h i o c y a n a t e s .
Consequently, s i n c e the s t e r i c
r e q u i r e m e n t s for S-bonded t h i o c y a n a t e s a r e m o r e s e v e r e (the P d - S - C angle is about 110" v e r s u s a P d - N - C angle of about 180°), the mode of coordination a l s o changes.
The
e x i s t e n c e of Pd(vpp)(SCN)2 as the S-bonded i s o m e r s u p p o r t s t h e s e s t e r i c a r g u m e n t s . The C=C d i s t a n c e of 1.33A is s h o r t e r than C-C of 1.54A which in e s s e n c e pulls the phenyl groups away f r o m the t h i o c y a n a t e groups allowing both ~hiocyanates to be S-bonded. The above s e r i e s offers a c l e a r delineation of ?r - bonding f r o m s t e r i c effects and c l e a r l y d e m o n s t r a t e s that the mode of thiocyanate coordination to Pd is s t e r i c a l l y c o n t r o l l e d .
The
s a m e t r e n d s exist in solution although a l l t h r e e compounds show the p r e s e n c e of both N and S-bonded t h i o c y a n a t e s in d i c h l o r o m e t h a n e (9).
T h e r e is a d r a m a t i c i n c r e a s e in the
amount of the N-bonded s p e c i e s with i n c r e a s i n g c h e l a t e chain length, a s expected on the b a s i s of s t e r i c effects. T h e s e r e s u l t s can be extended to the s e l e n o c y a n a t e ion a s w e l l as to the third row transition elements.
Since the s e l e n i u m a t o m is l a r g e r and the M-Se bond longer than M-S.
t h e r e a r e lower s t e r i c r e q u i r e m e n t s for SeCN- r e l a t i v e to SCN-.
The paucity of linkage
i s o m e r s of SeCN- is then a m a n i f e s t a t i o n of this lowered s t e r i c r e q u i r e m e n t .
Finally,
the l a c k of linkage i s o m e r s involving t h i r d row t r a n s i t i o n e l e m e n t s i s a l s o d i r e c t l y r e l a t e d to the l a r g e s i z e of t h e s e e l e m e n t s , reducing the s t e r i c i n t e r a c t i o n s between ligands.
We a r e g r a t e f u l for a D e p a r t m e n t of C h e m i s t r y P o s t d o c t o r a l F e l l o w s h i p ( M . M . ) , a G r a d u a t e School F e l l o w s h i p ( W . S . ) , a grant of c o m p u t e r t i m ~ f r o m the Computing C e n t e r (to G. J. P. ) and the National Science Foundation for financial s u p p o r t (M. L. and D. W. M. ).
Vol.10,No.1
DIPHOSPHINECOMPLEXES
127
c
¢R/c--c/•
Pd
S
(.)
(a)
S
(~')
Pd
2.3/64/
~1.998
S
¢
~/
.c~c
q \p/
N
¢
Pd 2.051/ / N
~ N
2.0/63/ N
Pd
(~)
~ 2.295 s ('0
Figure 1. A c o m p a r i s o n of the geometry about the Pd atom in the four complexes:
(a) Pd(dpm)(SCN)2, (b) Pd(dpe)(SCN)(NCS), (c) Pd(dpp)(NCS)2 and (d) Pd(dpDn)(SCN)(NCS). The angles in parenthesis given in a, b and d are the tilt of the thiocyanate group from the plane defined by the Pd and the four coordinated atoms.
DIPHOSPHINE COMPLEXES
128
Vol. 10, No. 1
References 1.
A.H. Norbury and A . I . P . Sinha, Quart. Revs., 2..~4, 69(1970), have reviewed ambidentate ligands.
2.
A. Turco and C. Pecile, Nature (London), 191, 66(1961), first introduced the idea of ligand control of the mole of thiocyanate coordination. Their ~ -bonding arguments have been extended by some MO calculations (3) which are used to support the ~r-bonding arguments.
3.
A.I~. Norbury, J. Chem. Soc. (A), 1089(1971).
4.
F. Basolo, W.H. Baddley and J . L . Burmeister, Inorg. Chem., _3, 1202(1964).
5.
These arguments are found in many textbooks such as J . E . Huheey, '~inorganic Chemistry: Principles of Structure and Reactivity, ' ~ a r p e r and Row, New York 1972, p. 409 or F. Basolo and R.G. Pearson, "Mechanisms of Inorganic Reactions," 2nct ed., J. Wiley and Sons, New York, 1967, p. 296.
6.
G.R. Clarkand G.J. Palenik, Inorg. Chem., 9_, 2754(1970)
7.
G. Beran and G.J. Palenik, J . C . S . Chem. Comm., 1354(1970)
8.
K.K. Chow and C.A. McAuliffe, Inorg. Nucl. Chem. Letters , 8, 1031(1972).
9.
D.W. Meek, P . E . Nipcon and V.I. Meek, J. Amer. Chem. Soc., 9_22, 5351(1970).