Pyridine complexes of platinum(II)

123 P Y R I D I N E C O M P L E X E S OF P L A T I N U M ( I I )

ct L--~~t .*--N ~ - Z Ct

(1)

M. O r c h l n a n d P. J. S c h m i d t

Department o[ Chemistry, University o] Cincinnati Cincinnati, Ohio 45221 (U.S.A.)

CONTENTS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . !1. Preparation and Properties of Pyridine Complexes . . . . . . . . . . A. Olefin Complexes (1, L=olefin) . . . . . . . . . . . . . . . B. Acetylene Complexes (1, L=acetylene) . . . . . . . . . . . . . C. Carbonyi Complexes (1, L=CO) . . . . . . . . . . . . . . . III. Dipole Moments of the Pyridine Complexes (1) . . . . . . . . . . . IV. The Infrared Spectra of the Olefin Complexes (1, L=olefin) . . . . . . A. Introduction B. The Olefin Stretching Frequency . . . . . . . . . . . . . . . C. The Metal-Nitrogen Stretching Frequency . . . . . . . . . . . . D. The Olefin-Platinum Stretching Frequency . . . . . . . . . . . . V. The Infrared Spectra of the Carbonyl Complexes (1, L=CO) . . . . . . VI. Chemical Shift Data in the NMR Spectra of 1 . . . . . . . . . . . A. The Pyridine Protons . . . . . . . . . . . . . . . . . . . B. The Oiefin Protons . . . . . . . . . . . . . . . . . . . 1. Rotational Phenomena . . . . . . . . . . . . . . . . . a. Introduction . . . . . . . . . . . . . . . . . . . b. Rotation in 2A,6-Trimethylpyridinc Complexes . . . . . . . . VII. Significance of the Coupling Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A° Introduction B. Coupling Constants of the Pyridine Protons. Effect of Z . . . . . . C. Coupling Constants" of the Olefin Protons . . . . . . . . . . . . D. The Evaluation of the trans Effect . . . . . . . . . . . . . . E. The Coordinating Ability of Solvents . . . . . . . . . . . . . . F. Correlation of Infrared and NMR Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G. Ligand Exchange H. NMR Evidence for ~--+~ Ligand Rearrangement . . . . . . . . . . VIII. References . . . . . . . . . . . . . . . . . . . . . . . .

I.

INTRODUCTION

The extensive investigations carried out in our laboratory on the preparation and properties of pyridine N-oxide complexes of Pt n, analogous to the pyridine complexes, 1, have been summarized recently. 1 The influence of ligands u p o n each other, the effect of the N-oxide moiety on the multiple b o n d character of the trans unsaturated ligand, L, as well as ligand lability and solvent interaction with the complex have been studied. The present review summarizes the w o r k we have been doing on the pyridine complexes, 1, where L is an unsaturated ligand with its o w n = a n d ~* systems. The pertinent literature data will also be summarized. Although the pyridine N-oxide ligands in complexes of Pt" do not appear to be involved in back-bonding with the metal, when pyridines are the ligands, the T-electron system of the ring can interact with the d-orbitals of the metal atom. In so doing, the pyridines compete for the d-T* bonding of the metal to other unsaturated ligands simultaneously coordinated.

Reviews 1968

123 123 123 124 t24 124 124 124 125 125 125 125 127 127 128 128 128 128 130 130 130 131 131 131 131 132 133 134

Donation of d-electrons from the platinum to the antibonding orbitals of a phenyl ring has been observed in phenylplatinum complexes. 2 By using pyridines in place of pyridine N-oxides it should be possible to exert greater influence on the properties of the ligand, L, because of the stronger interaction of the pyridine with the metal d-orbitals. Pyridine readily replaces the pyridine N-oxide from an olefin-pyridine N-oxide c o m p l e x ) reflecting the stronger coordinating ability of the pyridine.

II.

PREPARATION AND PROPERTIES OF PYRIDINE COMPLEXES

A.

OLEFIN COMPLEXES

Treatment of an aqueous solution of Zeise's Salt, [CI3Pt(CzH4)]K with one equivalent of the substituted pyridine precipitates the complex 1, (L----C,H~). Treatment with excess pyridine not only displaces the chloride but replaces the olefin as well to

124

M. ORCHINAND P. I. SCHMIDT

form either the bis-PyzPtClz 4.s or the tetrakis[Pt(Py)4]CI2. s The olefin complexes may be recrystallized from chloroform or methylene chloride or they may be reprecipitated from chlroform solution by dilution with pentane. The synthetic route wherein the dimer is reacted with pyridine is of interest:

IlL DIPOLE MOMENTS OF THE PYRIDINE COMPLEXES (1)

Dipole moment measurements of the pyridine Noxide complexes, 2, showed that the moments for the carbonyl complexes 2 ( L = C O ) are higher than those of the olefin complexes :~ ( L = C z H 4 ) ) In the pyridine series cI

!~, /CI /CI ~)Pt c l ) P t , ~ ~ + 2 Py---}2CI~PtPy(C,H,)

L'-"+t-,- O-N~-Z (t)

When 4-pentylpyridine is used in this reaction 6 either the cis or the trans isomer can be generated depending on reaction conditions. The preparation of complexes 1, where L is an olefin other than ethylene, is readily achieved by displacing ethylene with other olefins in accordance with Anderson's procedure. 7 The complexes we have prepared are shown in Table I.

ct

(2)

the same relationship holds and the carbonyls again have the larger dipole moments. The dipole data for these complexes are listed in Table II. As expected, the dipole increases as the electron-releasing power of Z increases.

Table II.

Dipole Moment Data for Pyridine Complexes CI

Table I.

Complexes 1 and Their Melting Points '~

[ LPtCh(4-Z-pyridine) ] Z/L C~H~ cis-C, Hs trans-C,Hs CO OCH~ 142 132 79 118 CH3 119 108 138 126 C2H5 128 C~H5 b 131 CH2OH l 12 101 102 H l l0 122 99 COCH3 142 125 80 CChCH3 142 76 113 126 NO2 131 151 CN 230 180 ~ 200 170 (CH3h c 154 135 a Many of these complexes decompose rather than melt. Decomposition temperatures depend on the rate of heating and are not sharp. The recorded temperatures are those at which appreciable decomposition commences, b Did not decolorize up to 270". c This is 2,4,6-trimethyipyridine.

B.

ACETYLENE COMPLEXES

Alkyne complexes of 1 may be prepared from the pyridine complexes by the replacement of ethylene with the appropriate acetylene. Another procedure 8'9 involves the displacement of ethylene from an acetone solution of K[Pt(C2H,)CI3] followed by reaction in aqueous solution with pyridine. Dimer splitting with pyridine is also practicable.

I .,- N ~ - Z L-,.~)t C!

Complex Free Pyridine b L Z Ix (D) IX (D) CO CH3 6.6 2.61 CO OCH~ 6.5 2.96 CO COCH3 4.4 2.41 CO CO~CH~ 5.0 CO CN 3.8 1.65 CO C~Hs 6.4 C~H4 OCH~ 5.5 2.96 CzH~ CH~ 4.5 (4.35) c 2.61 C2H, C2Hs 4.3 2.65 2.22 C2H, H 3.8 (3.75) c C2H, C,Hs 4.2 C2H, CO~CH~ 5.5 2.41 C2H, COCH~ 3.1 1.65 C2H~ CN 1.7 a The dipole moment measurements were made by the procedure described by I. Chatt and B. L. Shaw, ]. Chem. Soc., 705 (1959) using a W.T.W. Dipoimeter, Type DMOI and dry benzene at 25*. Av/to was assumed to be 0.6 for every complex. AP' is assumed to be 15 percent of .P and the latter values are calculated from the values given by A. Vogel, ]. Chem. Soc., 1842 (1948). • A. Katritsky, E. Randall, and L. Sutton, ]. Chem. Soc., 1709 (1957). rValues from I. Chatt, R. G. Guy, and L. A. Duncanson, ]. Chem. Soc., 827 (1961).

IV. THE INFRARED SPECTRA OF THE OLEFIN COMPLEXES OF 1 (L=olefln)

C.

CARBONYL COMPLEXES

The facile displacemenP ° of ethylene from its complexes 1 ( L = C z H 0 by carbon monoxide made available the complexes 1 ( L = C O ) listed in Table I. Carefully dried solvents must be employed to avoid decomposition of these complexes and the precipitation of elemental platinum.

A.

INTRODUCTION

Infrared spectra of the complexes 1 (L=olefin) are of interest in connection with three major features of the bonding: (a) the C = C bonding; (b) the metalnitrogen bonding; and (c) the metal-olefin bonding. lnorganica Chimica Acta

125

Pyridine Complexes o~ Platinum(ll)

B.

THE OLEFIN STRETCHING FREOUENCY

The lowering of the olefin stretching frequency, which occurs when the free olefin is complexed to a transition metal, is well documented. The values n for the cis-2-butene-pyridine complexes are listed in Table III. These frequencies are essentially independent of the electronic character of Z and they are approximately 20 cm -~ higher than the corresponding pyridine N-oxide complexes. The pyridines are much stronger bases than the corresponding N-oxides. It might be expected that this additional charge on the metal in the pyridine series would be relieved by enhanced back-bonding with the ~* orbital of the olefin. However, the results show the contrary; the olefin appears to have more double bond character in the pyridine series. It is possible that the greater charge brought into the vicinity of the metal atom by the pyridine is relieved by d-T* back-bonding with the pyridine.

"

Table III.

....

Infrared Spectra a o/

Z

pl~

(CHj)) b CHs H CH2OH COOCH~ COCH3 CI CN

L = 4---Z--PyO '~c,c

1.29 c 0.79 c

1506 1503

---0.41 a

1503

0.36 a --1.17 •

1506 1504

L = 4---Z---Py pK, "ac.c 6.03 t 5.27 ! 5.41 t 3.24 f 3.50 !

1523 1522 1522 1523 1521 1522

1.90 f

1521

a Halocarbon mull. b This is 2,4,6-trimethylpyridine. cA. R. Katritzky and F. I. Swinbourne, ]. Chem. Soc., 6707 (1965). a T. Kubota and H. Miyazaki, Bull. Chem. Soc. (lapan), 39, 2057 (1966). e I. H. Nelson, R. G. Garvey, and R. O. Ragsdale, ]. Heterocycl. Chem., 4, 591 (1967). t p. T. T. Wong and D. G. Brewer, Can. ]. Chem., 46, 131 (1968).

C.

THE METAL-NITROGEN STRETCHING FREQUENCY

Fritz and Sellman ~z have indentified a far infrared band at 242 cm -~ in a transopyridine ethylene platinum(Il) complexes as the metal-nitrogen vibration. For the complexes containing a substituted ethylene, the band is at approximately 252 cm -~. This band, as these investigators have shown, is significantly dependent upon the nature of Z, as can be seen from their data in Table IV.

D.

THE OLEFIN-PLATINUM STRETCHING FREQUENCY

Recently, ~3.t4as infrared and normal coordinate analyses of Zeise's Salt, K[CI3Pt(C2H4)]H20, and Zeise's Dimer, [CI2Pt(C2H4)]z, were reported. In the ethylene-pyridine complexes, '2 bands associated with the olefin-platinum bond are located at approximately 380 cm -~ and 480 cm-l; both frequencies remain Reviews 1968

-,, Table IV.

Infrared

Ct

ctr 2o,

Z

/ ct Platinum-Nitrogen Vibration Z NH2 CH3 CH2OH H Br Cl COCH3 COzH CN NOz

Pt-N (cm-') 287 294 290 242 222 227 191 207 205 206

approximately constant as Z varies. The corresponding bands for the cis-2-butene pyridine complexes occur at virtually the same values and they too are invariant. This lack of dependence has been ascribed t2 to a distorted geometry of the pyridine systems with respect to the plane of coordination and only weak contribution of or-bonding in the a-~ interaction between platinum and ethylene.

V. THE INFRARED SPECTRA OF THE CARBONYL COMPLEXES (1, L = C O )

The carbonyl stretching frequencies which appear in the infrared spectra of complexes 1 ( L = C O ) are particularly amenable to experimental study, since ordinarily only one intense, easily identifable band is present. The position of this b a n d in some carbonyl complexes is shown in Table V. The data show what appears to be a small but definite trend toward higher Vc=_o with increasing electron-withdrawing character of Z. In contrast, in the pyridine N-oxide series the Vc-o is largely independent of the nature of Z, and this is also true for the aniline complexes. 16'~7 As previously mentioned, there are better back-bonding possibilities with pyridine. Models also show that there is considerable hindrance to free rotation around the N - O bond in the pyridine N-oxide complexes. The nature of the bonding of the carbonyl group in these complexes has been discussed t,t7 previously. A study of the effect of solvent TM on the infrared spectra of the carbonyl complexes in the pyridine Noxide series 2 ( L = C O ) revealed some unusual behavior. The infrared spectra of acetone solutions of most of these pyridine N-oxide complexes showed two carbonyl bands, the relative intensities of which changed with time. These results are shown in Table V. A typical example of the change which occurs with the 4---CH3-PyO-PtCI2CO in acetone is shown in Figure 1. The band at 2108 cm -~ observed initially for the 4-methyl derivative diminishes on standing and a new band at 2088 cm -~ appears. The rate of the development of the new band was found to be a function of the quantity of water present in the acetone. The chemical shift of the water in the

126

M:

ORCHIN

AND P .

I. SCIIMIIYr

Tsble V. Carbonyl Stretching Frequencies (cm -~) in the Infra red Spectra of Complexes, LPtCI2(CO) Z/L

CHCI3

OCH:C_~Hs OCH~ C(CH3)) CHj C~Hs H C~H5 CH2OH Cl CO~CH) COCH; CN NO2

4--Z--PyO Acetone

2118 2116

2104, 2085 2107, 2087

2117

2108, 2088

2122

2106, 2087

2127 2124

CHCh

4.-Z--Py Acetone

4--Z--Aniline CHCI) Acetone

2128 2129

2131 2132 2132 2133 2134 2134 2139 2135 2140 2147

2096 2087

2123, 2087 a

2131

2122,2085

2126, 2087 a

2132

2122,2085

2128, 2087 a

2132

2122, 2085

2128, 2087 2128, 2087 2128, 2087 2133, 2087 2134, 2087

2130

2123, 2086

* Small peaks.

? oc-~z

nmr spectrum moved downfield with time as the 2088 cm -t band was increasing. The rate at which the 2088 cm -t band appeared was also a function of the nature of Z; the weakest base (most electron-withdrawing Z) producing the lower band fastest. kS"

4,5

8.l

lit

J~7

210

312

~7

Cl

$0.2

t- 0¢.a~ t . ~ 210Qcm"l

Figure 1. The carbonyl bands of (CH~PyO)Pt(CO)CI2 in lmcetone solution. Concentration of complex, 0.13 mmol/ml. Concentration of H20, 0.16 mmol/ml. * Time elapsed from preparation of solution (in min).

This two-band effectts in acetone is observed in the spectra of the pyridine complexes as shown in Table V. The reason for the observed changes are of considerable interest and importance. Several explanations are possible: the complexes may be partially dissociated in solution; they may be highly solvated; they may be undergoing a trans--~is isomerization ,possibly via a dimeric species; or any or all combinations of the above possibilities may be occurring. Although a choice of reasons cannot be made with certainty at this time, the most attractive explanation is that the pyridine is being partially displaced by the solvent and that the new carbonyl band represents the carbonyl stretch of the solvent(s) coordinated species Pt(CO)C12S. In the case of the pyridine Noxide complexes, recent work 19 shows that when such complexes are dissolved in solvents of high coordinating power, partial displacement of the PyO occurs. The spectral results suggest the possibility that the solvated species is responsible for the low-frequency second band. The intensity of this band increases with decreasing basicity of the pyridine (Figure 2). The appearance and intensity of this lower band is also dependent upon the solvent; the better coordinating solvents give the low band. In the pyridine series

z'*

9 z,,¢..o~

t.~

z.mat

Figure 2. The carbonyl bands of (4--Z--Py)Pt(CO)CI2 in acetone solution.

the second band is not observed in chloroform (Table V). If the solvated species were responsible for this new band, then its position should be constant, and independent of the ligand as well as of the nature of Z; this is observed in both series. Although the high band has a range approximately 35 cm -~ for these complexes in acetone, the low band has a range of only 5 cm-k This would seem to indicate a common species for both series of complexes. Finally, it is instructive to note that in the pyridine N-oxide series in acetone, the lower band becomes more intense than the original, and with strong electronwithdrawing substituents may be the only band. With the pyridine complexes, however, the highfrequency band is always present, although the intensity does decrease with decreasing basicity of the pyridine. This behavior is consistent with the stronger coordinating ability of the substituted pyridines and the greater difficulty of its replacement to give the solvent complex with the low frequency band. As we shall see, the nmr data also suggest the possibility that a displacement of the pyridine by the solvent is occurring. Although the formation of a dimer intermediate and resulting trans--~cis isomerization is possible, there is little evidence to support such reactions. It is known that the strong trans-directing ethylene facilitates dimer formation '~ and such a dimer could be the precursor of the cis-isomer? ~ However, the corresponding dimer in our series; namely, the lnorganica Chimica Acta

Pyridine Complexes o~ Platinum(H) platinum-carbonyl dimer has been reported 2t" to have vc;o at 2152 cm -I (mull) and such a high frequency band was never observed in our acetone solutions. The partial or complete displacement of ligand by solvent as our infrared data seem to indicate and which nmr data also support is important in connection with the catalytic activity of complexes such as 1 and related complexes. The solvent may either retard" or aid z3 catalysis. Solvent interaction with bridged dimers of Pt n is well established. Thus in ethanol, ethylene platinous chloride dimer, [ P t C l r (C2H4)]2, exists p r e d o m i n a n t l f 4 as the monomer, trans-[PtCl2(CzH4)(CzHsOH)], while in refluxing acetone '~ it is approximately 70% reversibly dissociated to trans-[PtCIz(C2H4)(CH3COCH3)]. Even in the monomeric form, solvent interaction readily occurs; Zeise's Salt, K[PtCI3(CzH4)] in water can form ~°'~ trans-[PtCl2(C2H4)(HzO)] as the predominant species.

protons is probably due to the increased time the pyridine spends in the vicinity of the metal at the low temperature and the enhanced shielding effect of this nearby platinum atom. A similar but even more modest upfield shift has been reported 27 for the H, protons of pyridine N-oxide complexes of tin and lead. Shifts of the pyridine protons on complexation with a protonic acid '~s.3° have been reported and here the downfield shifts of the 8 and y protons are larger than the shifts of the ¢x protons. Table VI shows that there is an increasing downfield shift of both cc and [3 protons (except for Z = C N ) with decreasing basicity and this effect persists in the complexes. Probably the strong electronwithdrawing substituents in the 4-position make the ring nitrogen atom quite positive, thus deshielding the 0c proton. The electron-withdrawing substituent reduces electron.density at the ~-carbon and so both ¢z and [3 protons are shifted downfield. Since both of these effects arise out of the electron-withdrawing character of the substituent, it is not surprising that there should be a relationship between the chemical shift and the ¢r value of the substituent. The data to be reported below on the coupling constants indicate that with the ethylene complexes all pyridines are exchanging rapidly at room temperature. This means .that the chemical shift data for the pyridine protons taken at 30" and reported in Table VI are time-averaged values. This is also true of the values for the butenes when Z is electron-withdrawing because here again coupling is absent at 30". However, even for other Z substituents, it is likely that the chemical shift values of Table VI are averaged shifts for free and complexed pyridine, since lowering the temperature to - 4 5 ° produces the same upfield shift in Ha as does ethylene when it is the trans

Vl. CHEMICAL SHIFT DATA IN THE NMR SPECTRA OF 1 A.

127

THE PYRIDINE PROTONS

In Table VI are listed the chemical shift data for the pyridine protons in a variety of free pyridines and in the 15 corresponding pyridine (L=olefin) complexes of structure 1. The spectra were taken at room temperature and at - 4 5 °. In the free pyridine, lowering the temperature causes a downfield shift of both ~z and ~3 protons, but in the complexes, except for Z=CO2CH3, the low temperature causes an upfield shift of the cx protons although the ~3 protons are still shifted downfield. As we shall show, the pyridine spends part of its time free and part complexed. The upfield shift of the

Table Vl. Chemical Shifts of the Pyridine Protons in 1,3-Dichloro-2-olefin-4-(4-Z-pyridine)platinum(lI) Complexes in DCCh CI L-*"

~t -'- N ~ - Z

L in Complex Free Pyridine 30" n - 45°

trans-2-Butene

Ethylene 30"

-45 °

30"

-45 8.63 6.98

°

cis-2-Butene 30" - 45°

Z = OCH3 H,

8.43 6.82

8.48 6.83

8.67 6.98

8.60 7.02

8.67 6.92

H,

8.47

8.55

8.78

8.72

8.68

Hj

7.13

7.15

7.32

7.42

7.25

Z=H H. HD

8.61 7.26

8.69 7.36

8.95 7.55

8.89 7.61

8.90 7.45

H.,

7.67

7.76

7.97

8.02

8.82

8.83

7.87

7.89

9.07 8.04

8.84

8.92

7.57

7.68

HI

8.71 6.93

8.63 6.99

8.66

8.68

8.66

7.34

7.27

7.34

8.83 7.51

8.90 7.45

8.83 7.53

7.87

7.95

7.88

7.96

9.10 8.14

9.04 8.00

9.06 8.09

9.03 7.99

9.00 8.07

9.25

9.12

9.16

9.12

9.14

9.12

7.82

7.83

7.72

7.84

7.74

7.83

Z=CH~

Z=CO=CH3 H. HI Z=CN H° Hj

a The 30" temperature is :t:3*. R~iews 1968

128

M. ORCHIN AND P. 1. SCHMIDT

ligand. There appears to be very little difference between cis- and trans-2-butene in ,their effect on the chemical shift values of the pyridine protons despite the difference in temperature at which these isomeric olefinic complexes exhibit coupling (vide infra) and despite the fact that the cis isomer is probably more stable) 1~2.~ Recently, chemical shift data for pyridine protons in corfiplexes of the type trans-[PtCl2(Py)L] have been determined 34 and found to be rather insensitive to the nature of L.

B.

THE OI,EFINIC PROTONS

The chemical shift values of the ethylene, cis-2butene, and trans-2-butene protons in complex 1 are shown in Table VII. The values were obtained at two temperatures in each case. Complexation of the olefin to platinum results in an upfield shift of the proton signal. The extent of this shift is influenced" by the ligand trans to the olefin. The data shoW" that when Z is CN, the chemical shift of the olefin protons is closest to the free olefin.

Table VII.

coalescence into one absorption signal, after eliminating other possibilities, was ascribed to free rotation around the coordination axls. On cooling the solution to -20", the absorptions at 2.77 and 1.12 ppm were each split into two pairs of doublets. When rotation occurs around the coordination axis, the it-bond may be considered as remaining intact, but the ~-bond is alternately partially broken and remade as two perpendicular d-orbitals alternately

C xc. . O~

c/CH, C",-..C..S.C0

c,/Pt/:y

o ~H

CHs

0

CI

/C

H

(a) (b) Figure 4. Olefin twist in acetylacetonato complex.

Chemical Shifts of Olefinic Protons in the 1,3-Dichloro-2-olefin-4-(4-Z-pyridine)platinum(ll) Complexes Ethylene ----44.5*C + 33.5"C

Z OCHs CHs H COOCH3 CN

4.90 4.89 4.94 4.97 4.99

4.85 4.88 4.93 4.93 4.99

Chemical Shifts* (8) cis-2-Butene ---44.5* C + 33.5"C 5.63 5.66 5.67 5.74 5.80

5.66 5.68 5.70 5.74 5.78

trans-2-Butene --44.5° C + 33.5"C 5.49 5.49 5.51 5.56 5.60

5.46 5.47 5.52 5.53 5.62

° All spectra were run at a 10% w/w concentration in CDCI~ using TMS as the internal reference.

1.

Rotational Phenomena in Platinum Complexes.

(a) Introduction. Cramer 35 has shown that the ethylene coordinated to rhodium(I) in the compound bis(ethylene)~-cyclopentadienylrhodium(I), Figure 3, rotates freely at room temperature. Each ethylene has two sets of protons, the inner, Hi, and outer, Ho, protons. In chloroform solution at room temperature there are two broad peaks at 2.77 and 1.12 ppm due to Ho and Hi respectively. However, at 57", the two peaks converge to a single band at 1.93 ppm. This

become involved. In the [Cl(acetylacetonato)(propene)Pt] complex, 36 two geometric isomers are possible, Figure 4 (a) and (b). (The olefin is viewed from above so that only the top carbon atom is shown.) The relative amounts of isomers may be deduced by freezing out the olefin rotation which occurs rapidly enough to coalesce the proton resonances at room temperature. Relative peak heights indicate isomer ratios below the coalescence temperature and the averaged chemical shift above coalescence also permits estimation of the ratio of isomers.

(b) Rotation in 2,4,6-Trimethylpyridine Complexes.

i'\

/÷"

/o (a)

(b) Figure 3. The bis(ethylcnc)complex of rhodium. (The ~cyclopentadienylring is omitted for clarity.) (a) Top view. (b) Side view.

In Zeise's anion, ethylene is known to be oriented at right angles to the square plane in the crystal state." In 3, rotation around the P t - N coordination bond as an axis is possible, but molecular models show that 2,6 methyl substitution would favor a geometry in which the pyridine ligand lies perpendicular to the square plane as shown. Similar steric hindrance by ortho substituents has been suggested in ortho.substituted aryl nickel complexes.~

lnorganica Chimica Acta

Pyridine Complexes o/ Platinum(II)

~ H~c..H

{{

R/C-.H

it was not as pronounced, and a difference of only 2 cps in the chemical shift was observed (Figure 6). Since the assumption is made that the pyridine is relatively fixed in these complexes, a single signal for the o-methyls in the styrene and t-butylethylene

H .--O43

Pt ct

"-~-'m' g

(3) ( R = H ,

CH3, P h , t--C,H,)

In the complex 3 ( R = H ) , the o-methyl groups are in magnetically equivalent environments with respect to ethylene. If, however, an unsymmetrical olefin were complexed in place of ethylene and the pyridine moiety were ~fixed~, the o-methyls would become nonequivalent and such nonequivalence should be discernible by nmr techniques. Accordingly, the complexes 3 (R=CH3, C(CH3)a, Ph) were prepared. A single nmr resonance for the o-methyls would be presumptive evidence for free rotation of the olefin; splitting of the singlet at low temperature would be consistent with slowing down such rotation.

TaMe

VIii.

NMR

129

Figure 5. N M R s p e c t r a of o r t h o a n d p a r a m e t h y l p r o t o n s of t r i m e t h y l p y r i d i n e .

D a t a a for t h e C o m P l e x e s

,-

-(1%

Cl

/

\

R' H Arab b

CH3 Amb

Ph -46 °

Amb

t---C,H, --60*

Arab

--46 °

7.00 10 Multiplet centered at 4.80 ... c 3.17 12 2.32 1.32

7.03 10 ... c

t

~i, t b,-n, 8H,,~, |,,-x.x~ ~io-c.3 l,,-.-c,3 6,-c,3 6~

7.05 9 4.80 62 3.17 12 2.37 None

6.99 9 Muhiplet centered at 4.65 ... n 2.90 13 2.20 1.57 doublet

7.02 9 ... c ... c 3.03 13 2.20 1.57 doublet

6.95 10 Muitiplets c e n t e r e d at 4.65, 5.44 . . . ,, 2.97 13 2.18 7.80, 7.48 multiplet

7.00 10 ... c ... c 2.93, 2.80 ~13 2.12 ...c

... c 3.03, 3.00 -12 2.20 1.15

a All c h e m i c a l shifts a r e in parts p e r million (8) a n d c o u p l i n g c o n s t a n t s in cycles p e r s e c o n d . T h e r e s o n a n c e p e a k of CHCI3 (8 7.27) w a s u s e d as an internal s t a n d a r d , b A m b i e n t m a c h i n e t e m p e r a t u r e , c Not e x a m i n e d in detail, a p. K a p l a n a n d M. O r c h i n , lnorg. Chem., 6, 1096 (1967), d i s c u s s platinum-olefin c o u p l i n g in vinyl c o m p l e x e s .

The nmr spectral data are given in Table VIII. Our discussion shall concern itself only with the signals for the o-methyl groups. When the propylene complex 3 (R=CH3) was examined only a single triplet (Figure 5) was observed for the o-methyls and there was no signifcant change at -46". Owing to instrumental limitations, the temperature could not be much further lowered. For 3 (R--Ph) the single triplet observed in the nmr at room temperature was split into two overlapping triplets at --60"; the chemical shifts of the two triplets were 8 2.93 (h,,-c-3 ~ 12 cps) and 6 2.80 (b,-c.3 " 12 cps), a chemical shift difference of 7.8 cps at 60 Me. The same effect was observed with compound 3 ( R = t-butyl) although Reviews 1968

complexes implies a. rotation of the vin¥1 compound around the coordination axis. Lowering the temperature • freezes~, out the rotation and permits observation of the more stable conformer in the nrar spectra. Alternately, it is possible that the olefin is fxed and the pyridine rotating at room temperature and frozen out at low temperature. Or, both moieties may be free to rotate at room temperature and both frozen out at low temperatures. Either of these is not as likely as the ~fixed~ pyridine and the rotating ethylene. Rapid intermolecular exchange of free and complexed olefin would also account for the observation of a single absorption for the o-methyls. However, the spectrum of 3 (R----Ph) shows platinum-

M. ORCHINAND P. I. SCHMIDT

130

olefin coupling at room temperature and thus this explanation is excluded.

R.c,~,

R.!-c, M,

~°c Figure 6. NMR spectra of ortho methyl protons of trimetylpyridine for R substituted complexes.

Failure to observe splitting with the propylene Oamplex $ (R--CH3) may mean barrier requiring_ temperatures lower than -60 ~ or if - / ~ t h ~ f f e c t of the propylene on the o-methyl groups is too small to be observed across the square plane.

VII.

SIGNIFICANCE OF THE COUPLING COMPLEXES

A.

The chemical shift of the H. protons in pyridine occurs in a region of the nmr spectrum in which there is relatively little interference from other protons. The ~Pt isotope with which these protons couple is present in 33.7 percent natural abundance; hence the coupling is readily recognized because of the relatively high intensity of the signals. The large coupling constants of about 34 cps permit easy resolution. Typical spectra are shown in Figure 7 (a) and (b).

Pt-

t Ha

%

/

"/3

~N

I

t H~

6

n.

COUPLINGCONSTANTS OF THE PYRIDINEPROTONS. EFFECT OF Z

INTRODUCTION

.

The observation of coupling indicates that the pyridine is complexed to platinum for a sufficiently long period of time on the nmr time scale to permit the platinum and pyridine protons to couple. When the coupling is lost, the pyridine has been labilized and must be rapidly exchanging with other ligands or with the solvent. If the complex were completely dissociated, the chemical shift of the protons would occur at exactly the same position as the free ligand and since this is generally not the case, exchange must be occurring. If the temperature of the sample is lowered and the exchange process slowed, coupling may again occur. The temperature below which coupling is observed and above which it disappears thus can be a sensitive and powerful tool for evaluating the pyridine lability. The effects of Z-substituents, the relative coordinating ability of solvents, the effect of other ligands simultaneously coordinated can all be evaluated by studying the coupling of the pyridine protons and its temperature dependence. The coupling of the olefin protons with "sPt also provides a powerful tool for evaluating the behavior of the coordinated olefin. Thus, exchange with other ligands or with solvent can be followed. The magnitude of the coupling constants has been used to determine geometry of the coordinated olefin. Some of the individual features that affect coupling will now be considered.

9J

(a) 30°

ik~

(b) -- 50~

[~,H

Figure 7. NMR spectra of complex at (a) 30° and (b) --51Y' in DCCIj solution.

In the pyridine N-oxide complexes, 2, the H~ protons are separated by four bonds from the platinum atom. No coupling between these protons and the ~Pt isotope was observed although four-bond coupling is known; 39'4° low temperature experiments have not been reported. In the pyridine complexes, which are considerably more soluble than their pyridine N-oxide counterparts, three bonds separate H, from the platinum. Here coupling may occur; its occurrence dcT~ends upon the temperature, the nature of Z, and the nature of the trans ligand, Lfl '4~'42 Table IX gives data on the coupling constants observed in the nmr spectra of some of the complexes 1. This table shows that when ethylene is present on the metal the temperature must be increasingly lowered as the electron-withdrawing power of Z increases in order to observe such coupling. Even with the most electron-releasing substituent, OCH3, 25" is about the maximum temperature at which coupling can be observed. The failure to observe coupling is taken to indicate that exchange of the pyridine with solvent is taking place rapidly on the nmr time scale. Table IX also shows that when the hutches are complexed to the metal and Z is strongly electron-withdrawing, coupling with H~ protons occurs only at low temperatures. lnorganica Chimica Acta

131

Pyridine Complexes ot Platinum(ID

Table IX. Coupling Constants a in Some 1,3-Dichloro-2-olefin-4-(4-Z-pyridine)platinum(I1) Complexes (l) (H,)2C= C(H.h trans-H.(CH))C = C(CH3)H. cis-H.(CHOC= C(CH3)H. Z/L

Jr,-ru

I,,- ~

OCH~ CH)

60.7 60.5 60.5 61.7 62.6

34.5 31 36 36 36.5

H COOCH) CN

( 23* ) ( 3* ) (--14.5") (--14.5") (--51")

l..-ra

I,.-..

61.0 60.3 62.4 62.3 63.2

33.2 32.8 33.6 33.4 (--24*) 34 (--25*)

68.4 68.1 69.0 69.0 70.4

33.5 33.5 b 34.5 34.1 (--9*) 34 ( 3*)

aWhen the temperature was lower than the ambient temperature of the probe (29-32") this is indicated in parentheses. 'l'he constants are in cycles per second. All spectra were taken in CDCI3 as 10-20% " / , solutions. The larger coupling, l,,-a,, for Z-'electron-withdrawing is most probably a reflection of the increased resolution at lower temperatures, bAt --45*, I'r,-a.=34.4 cps. Other complexes exhibit a similar increase at low temperatures, most probably because of better resolution.

C.

COUPLING CONSTANTS OF THE OLEFIN PROTONS

The data of Table IX imply that the olefinic moiety is more tightly bonded than the pyridine moiety in complexes 1 (L=olefin) since olefinic proton coupling with I~Pt is observed with all the complexes at even the highest temperature employed for the nmr studies. with all three oleffhs examined, the coupling constant increases as the basicity of the pyridine decreases. As was indicated earlier, the chemical shift values, 8, in each olefin series increase with decreasing basicity of the pyridine, Table VII; the increase is in the direction of the value of the free olefin. The increasing coupling constant accompanied by increasing 8 for the chemical shift has been observed previously43 and commented on by other workers. 44'45 At temperatures above 30* the olefin begins to undergo exchange, as indicated by line-broadening and, in acetone solution, exchange is rapid as indicated by loss of coupling. (s The larger coupling constants of the cis-2-butene complexes compared to the trans-2butene complexes are taken as further evidence of the twisting of the hydrogens to ward and the methyls away from the platinum atom. ~.(7 The exact mechanism of olefin exchange is not yet clear. The olefins with the largest coupling constants are most labile in solution; these are the complexes in which the pyridine moiety is the weakest base and which lose the H= proton coupling most readily. Conceivably an exchange between free pyridine and olefin might be occurring. It is known that excess pyridine will displace the olefin from an olefinpyridine complex:

D.

THE EVALUATION OF THE trans EFFECT

Since Chernyaev ~ introduced the concept of the trans effect in 1926 to describe the labilizing (ease

of replacement) effect that ligands have on the groups trans to them, a variety of theories have been advanced to account for this effect. Because unsaturated ligands with low-lying empty 7:* orbitals were found to exhibit high trans effects," it was assumed that the interaction of filled d-orbitals of the metal with the ~* ligand orbitals made the position trans to the unsaturated ligand particularly prone to nucleophilic attack either by solvent or by an incoming ligand. However, the rather recently discovered very high trans effect of hydride and methide required Reviews 1968

alternate explanations and these somewhat resemble the very early electrostatic postulates of Grinberg. 4~ In any case the trans effect is usually evaluated from kinetic data and these are frequently difficult to obtain and often difficult to interpret. In ordering the magnitude of the trans effect of ligands, the unsaturated ligands such as carbon monoxide, acetylenes, and the various olefins are frequently lumped together as having a very high trans effect. Relatively little has been published concerning the relative order of the trans effect of these unsaturated ligands. The coupling constant data give valuable information about relative labilizing effect on pyridine of these high trans labilizing groups. If we examine the nmr spectra of a series of complexes 1 holding Z and the solvent constant, the temperature at which coupling is observed may be used to evaluate the trans labilizing effect of L; the lower the temperature required to observe the coupling the greater the trans effect. The order for ligand L in the complexes 1 (in chloroform) when Z-----H was found to be C2H4>>ePCH=CH2~cis and transCH3CH = CHCH~ ~ CH3CH2CH:,CH = CH2 > CO) 7's°

E.

THE COORDINATING ABILITY OF SOLVENTS

As might be expected, solvents have a profound effect on ligand lability. The results of some studies with deacetone are given in Table X. In acetone, the coupling, which is present under identical conditions in chloroform solution, is lost indicating rapid exchange on the nmr time scale between the pyddine and solvent and the much stronger coordinating ability of acetone as compared to chloroform. The marked downfield shift of H~ protons in acetone solutions, which has also been observed by other workers:' probably reflects some solvent anisotropy rather than a coordination effect. It will be recalled that acetone facilitates the formation of the low frequency C - O stretching band in the carbonyl complexes 1 (L=CO). This behavior and the nmr behavior are thus consistent.

F.

CORRELATION OF INRARED AND NMR SPECTRA

The effect of the Z substituent upon the I-I, coupling in the nmr spectra of complexes 1 is profound. Such an effect is also present in the infrared spectra of the

132

M. OncmN AND P. I. ScnMl~r

"rabl¢ g.

Solvent Effects on the nmr Spectra of Ct

Ha

Z

Solvent a

OCH3

.

)

CD)COCD3

•

•

CH,

CDCh ~, CD3COCD~ •

• y,

/ Ha

\

HI~

6.

6s

6.

33 -45 33 -48

5.65 5.63 5.55 5.53

6.93 6.99 7.21 7.28

8.71 8.63 8.68 8.66

33.5 33.5 none 33. l

33 -45 30 --46

5.68 5.66 5.56 5.53

7.27 7.34 7.45 7.55

8.68 8.66 8.62 8.67

33.5 33.5 none none

Temperature (*C)

CDCI3

•

~CH 3 CL

I,,-a. (cps)

a All spectra were run at 60Me, using TMS as the internal reference. The solutions were prepared in a 10% " / . The temperature of the probe was calibrated employing methanol in the standard manner.

Table XI.

NMR Spectral Values a of the Reaction of (la) with (lb)

Ha

LL

Ha

HI~

"

Ha /

la

~ 40 34 -40

/

Ha

\

Hp

Ha

H~

ld

CH3

Chemical Shifts (6) Aromatic H, Ho H,

1.80 ---

5.79 4.88

7.72 7.32

9.13 8.78

. .. c 60.5

none none

36.0 ---

1.80 ---

5.69 4.90

7.73 7.25

9.16 8.72

... c 61.5

none none

36.2 ---

7.82 • 7.82 ¢ 7.37 • 7.37 •

9.10 e 9.10 • 8.67 e 8.67 •

. . . 63.2 60.5 ---

Oleflnic (la) (lb) (la)+(lb) (la) (lb) (la)+(lb) (la) (1c) (lb) (ld)

CI

HI~

lc

Temp. (*(2)

--H a

lb

Ha

Complex

ratio.

I (cps) CH~

Pt--H,

Pt--H,

Pt--CHs b

-43 ca 5.73 n 5.00 4.89 ca 5.73 d

2.50 2.50

.

.

. . . . . . . . . . . . . . . . ca 34 ---

a The solutions were 10% " / , solute to solvent, b Double irradiation experiments have established a change in sign between 1P,-xn, and l,,-,-; B. F. G. lohnson, C. Holloway, O. Hulley, and I. Lewis, Chem. Commun., 1143 (1967). cToo weak to measure accurately, a The signal is too broad and weak to distinguish between the two butene complexes present. • Average values of the two complexes of each pyridin¢.

c o m p l e x e s 1, L = C O . As the pyridine basicity d e c r e a s e s t h e i n t e n s i t y of t h e l o w e r c a r b o n y l b a n d increases. S t u d i e s o n the s t r e n g t h o f the b o n d bet w e e n 4 - Z - p y f i d i n e s a n d c o p p e r r e v e a l large c h a n g e s as a f u n c t i o n of the Z s u b s t i t u e n t ; t h e w e a k e r the basi¢ity o f t h e p y r i d i n e , t h e w e a k e r t h e b o n d . ~ G.

LIGAND EXCRANGE

A l t h o u g h loss of t h e p y r i d i n e ~

proton coupling

w i t h ~gsPt is a l m o s t c e r t a i n l y d u e to l i g a n d e x c h a n g e w i t h s o l v e n t , it m a y c o n c e i v a b l y be d u e o n l y to a b o n d w e a k e n i n g p r o c e s s t h a t a c c o m p a n i e s the d e v e l o p m e n t of a h i g h l y s o l v a t e d species s3 w i t h o u t l i g a n d a c t u a l l y d e p a r t i n g f r o m t h e m e t a l site. In o r d e r to d e t e r m i n e w h e t h e r an a c t u a l e x c h a n g e o f l i g a n d s bet w e e n c o m p l e x e s is p o s s i b l e , the f o l l o w i n g e x p e r i m e n t w a s r e p o r t e d : ~ a 1 0 % *'/w s o l u t i o n o f 1,3-dichloro2-(cis-2-butene).4-(4-cyanopyridine)platinum( I I ) in d e u t e r o c h l o r o f o r m w a s p r e p a r e d a n d the n m r spec-

lnorganica Chimica Acta

Pyridine Complexes o/ Platinum(ll) trum recorded; an equal weight of 1,3-dichloro-2ethylene-4-(4-methylpyridine)platinum(II) was then added to the solution and the spectrum again recorded with the probe temperature at about 30*. The solution was then cooled to ---45* in the probe. The ethylene region of the spectrum at ---45°, Figure 8, shows that all four possible complexes are present. The data are recorded in Table XI. (bl

H.

153

NMR EVIDENCE FOR ~---),o" LIGAND REARRANGEMENT

The catalytic conversion of olefins to functionalized derivatives, e.g., ethylene to acetaldehyde, ~ as well as other olefin reactions carded out in the presence of transition metal complexes, are assumed to all involve oletin coordination via a =-complex followed by rearrangement to a ~-alkyl complex. The reaction of this type which has received most investigation as to mechanism is the insertion of olefins into metalhydrogen bonds: ~ RCH= CH2+ M--H~

Jm.a,-eo.s

R---CHr-CHr-CH2M

(2)

Although the indicated direction of addition is probably the preferred one, and when R and other ligands are large may be the exclusive one, ~ Markovnikov addition followed by elimination of MH is a commonly-accepted explanation for transition metal catalysis of olefin isomerization. In the many exampies of such isomerization where the hydride cannot be shown to be present as such, it is postulated to be the intermediate. In one important, example, ~ a hydride is added externally to the olefin complex to secure the ~ conversion:

I CH, l + I CO [ ~--CsH~---Fe+--II + NaBH. l I CHCH,

Figure 8. Ethylene region of the mixture of l(a) and l(b) at -44* (see Table XI1).

1_

(3)

co

~---CsHr-Fe(CO)r--CH(CH3)2 The appearance of the four complexes may be explained by either pyridine exchange alone, olefin exchange alone, or by complete scrambling. Since the olefin-platinum coupling was observed at 30* while the pyridine-platinum was not, it appears that the pyridine moieties are exchanging. The possibility of a slow olefin exchange cannot be entirely eliminated. A somewhat analogous olefin exchange between a rhodium complex and platinum complex has been reported .54 In order to evaluate further the possibility of olefin exchange, it is of interest to consider the time scale involved. Estimates 55 of the minimum metalligand lifetime, ":, can be made from the value of the spin coupling constants, since spin coupling can be observed when " r > + .

In our experiments, olefin

coupling is observed even at about 40 ° and since I ~ 6 4 cps the olefin must be on the metal for at least 0.016 sec. In this system 4-methylpyridine coupling is observed only at --45" and because 1 - 3 4 cps, the pyridine must be on the metal for at least 0.029 sec. However, since at 40" the olefin was already on for at least 0.016 sec., it is likely that at -45" it is on for periods considerably longer than one second and indeed may not be exchanging at all. Reviews 1968

Although the sequence of steps in the ~ conversion remains unsettled because intermediates have not been isolated, it is likely that nucleophilic (or hydride) attack occurs on the 7r-complex just prior to the ~ rearrangement. If the pyridine is exchanging in the above complexes, ds-pyridine added to the solution should become incorporated in the original complex. This does occur but, in addition, nmr evidence has been secured for the formation of an unstable ~-complex at -50* resulting from ~ rearrangement. The unstable compound is formed when deuterated pyridine is added to a chloroform solution of 1,3dichloro-2-ethylene-4-pyridine-platinum(II): Cl CH, I / ~ CH2[ I'-"Pt~"N ~-~-~l CI

Cl I / ~ + CsDsN= C,DsN---}CH2CHz--Pt+-N~_f) Cl

(4)

The evidence for the rearrangement is based exclusively on the nmr spectrum. The chemical shifts as well as the coupling constants for both species are listed in Table XII. The alkyl CH2 attached to platinum is characterized by occurring much farther upfield than the CH2 group of the olefin, and by having a larger P t - H coupling constant. ~'~'61 In addition,

134

M . ORCHIN AND P. I. SCnMWT

the presence of an additional CH~ group would be expected to split the original resonance peak into a triplet and this is observed (Figure 9). The second triplet (not shown) of the CHz group attached to pyridine, is partially buried in the resonance peak due to platinum-hydrogen coupling in ethylene, and can be observed by carefully varying the temperature to

Table Xll.

Chemical Shifts o and Coupling Constants b (_40 o)

Ha /

-.,

~t

~Ha

Q

/

\

la 0

D

o

o

O

O

Ct

2.648.9 Hz

~.. ~.~

~#~"•' ~ O ' c

~ 6.. l'~',-". l"sv,-.. 8c.2-,, l'~r,-x, ~¢.~-N

~-Complexc 4.87 7.525 7.96 8.855 60•6 35.6

I.,-.2 Figure 9.

N M R s p e c t r u m of ~-complex at --4tY'C.

change its chemical shift. The values given here, ~cnz-vt=2.44 ppm, ]Pt-ca2=83.4 cps, and I ~ - , = 7 . 1 , compare favorably with those for [Pt(CH3CH2)3C1]4 ~cnz-vt = 2.23ppm, lPt-cH2= 86.0 cps, Ja- H= 7.5 cps; s9 and for those of Pt(C3Hs), at -20"C, where 8cn2-P~= 2.26 ppm and IPt-cn2=83 cps?° A small degree of assymmetry within both triplets is observed over a range of 15"C and is considered to indicate an AA'XX' rather than an A2X2 spectrumP This is consistent with the observation that the apparent hydrogenhydrogen coupling varies over about 0.5 cycle. A certain hindrance to rotation about the carbon-carbon bond might be expected in this complex at -50"C. The "~-complex was converted to the it-derivative in at least a 30% yield when the pyridine was added to a 10% w/, solution in deuterochloroform in the temperature range -35" to -50"C. The relative concentration of the o-complex was highly dependent on the amount of base used as well as the temperature. Thus, a 1:1 ratio of base to complex produces very little o-complex, although some of the undeuterated pyridine is replaced from the ~-complex, while a 4 : 1 ratio of base to complex yielded 30-40% new species. In our experiments the optimum temperature for o-bond formation appears to be around -50"C. At -20"C to -300C the new species rapidly disappears, and at -10°C the ethylene peak of the ~-complex simultaneously present in solution broadens, presumably because of exchange with the excess pyridine present. Excess pyridine is known to displace the olefin from a pyridine olefin complexs but whether such a ~-->~ rearrangement is an essential preliminary step remains a provocative but unanswered question. The chemical shifts of the aromatic protons in the undeuterated pyridine molecule in the solution to

\

s~

Ib

NMR spectrum of (la)

• ~,~'¢V"'- ~ ' ~ "

/

~

Complex Plus Base c 4.81 7.89 7.775 8.69 60.5 none 2.44 83.4 4.55 7•1 a

Free Base c 7.36 7.76 8.69

a All c h e m i c a l shifts are g i v e n in p p m f r o n tetramethylsil a n e ( T M S ) as a n e x t e r n a l reference; c o u p l i n g c o n s t a n t s are given in cps. T h e internal reference w a s CHClj. b T h e c o u p l i n g c o n s t a n t s are given in cps. C T h e s o l u t i o n s w e r e p r e p a r e d in a 10%w/,, solute to solvent ratio. T h e Ds pyrid i n e w a s a d d e d in a p p r o x i m a t e l y a 5 / I ratio o f base to c o m p l e x , d T h e c o u p l i n g c o n s t a n t b e t w e e n vicinal h y d r o g e n s is a n a v e r a g e value, since t h e r e is s o m e v a r i a t i o n in magnitude with temperature.

which CsDsN was added indicate that the major portion of originally complexed CsHsN has been replaced by the deuterated species and is present, free, in solution• There are at least two additional pyridine species which, as yet, have not been categorized, due to the somewhat complex absorption pattern observed in the aromatic region.

Acknowledgment. The authors wish to thank Drs. F. and P. Kaplan for their many helpful discussion; Mr. T. Weft for skillful assistance; Engelhard Industries, Inc., for a generous supply of platinum; the National Science Foundation for a Traineeship to P.].S. and the donors to the Petroleum Research Fund of the American Chemical Society for a Grant to M.O. VIII.

REFERENCES

(1) M. Orchin and P. I. $chmldt, Coordln. Chtm. Rev., 3. 345 0968). (2) I. Char and B. L. Shaw, !. Chem. 5oc., 4020 (1959). (3) A, R. Brause, P h . D . The~sls. Unlvcrslt? of Cincinnati, Cincinnati, Ohio (1967). (4) P. D. Kaplan, P. Schmldt, and M. Orchln, I. Amer. Chem. See., 90, 4175 (1968).

(5) 1. S. Anderson, I. Chem See., 971 (1954). (6) I. Chatt and L. M. Venanzi, 1. Chem, See., 2787 0955). (7) 1. S. Anderson, I. Chem. See., 1042 (1936). (S) 1. Chatt, R. G. Guy, and L. A. Duncanson. l. Chem. See., 827 (19@1). (9) ]. Chatt, R. G. Guy, L. A. Duncanson, and D. T. Thompson, I. Chem. So¢., 5170 (1963).

(10 W. H. Clement and M. Orchln, I. Ori~momctnl. Chem., 3, 95 (1965). (11) P. Schrnidt and M. Orchin, lnorB. Chem., 6, 1260 (1967i. lnorganica Chimica Acta

Pyridine Complexes o/ Platinum(il) (12) H. P. Fritz and D. S¢llman, I. Organomctal. Chem., 6, 558 (1c~6). (13) M. ). Grogan and K. Nakamoto, I. Amer. Chem. Soc., 88, 5454 0966). (14) M. J, Grogan and K. Nakamoto, I. Amer. Chem. Soc., 90, 918 (1968). (15) |. Pradilla-Sorzano and |. P. Fackler, Jr., J. Mol. Spectrosc., 22, 80 (1967). (16) M. Rycheck, P h . D . Thesis, University of Cincinnati, Cincinnati, Ohio (1966). (17) A. R. Brause, M. Rycheck, and M. Orehln, l. Amer. Chem. Sot., 89, 6500 (1967). (18) T. A. Weft and M. Orchin. submitted for publication. 0 9 ) D. W. I-lerlocker, R. S. Drago, and V. 1. Meek, Inorg. Chem., 5, 2OO9 (1966). (20) S. 1. Lokken and D. S. Martln, |r., Inorg. Chem., 2, 562 (l~J). (21) ]. Chatt, N. Johnson, and B. L. Shaw, I. Chem. Soc., 1662 (l~) 2In) R. |. Irving and E. A. Magnu~son, I. Chem. Sot., 1860 (1956). (22) H. A. Taytm and I. C. Bailar, I. Amer. Chem. Soc., 89, 4337 0967). (23) F. H. ]ardln, J. A. Osborn, and G. Wilkinson, 1. Chem. Soc., A, 1574 (1~7). (24) D. G. McMane and D. S. Martin, Jr., lnorg. Chem., 7, 1169 (1968}. (25)). Chatt and A. A. Williams, I. Chem. Soc., 3061 (1951). (26) I. Laden and I. Chatt, J. Chem. Sot., 2936 (1955). (27) V. G. K u m a r Das and W . Kitchlng, Organometal. Reviews, In press. (28) 1. C. Smith and W. G. Schneider, Can. ]. Chem., 39, 1158 (IC~1). (20) V. M. S. Gil and J. N. Murrell, Trans, Faraday Soc., 60, 248 (1964). (30) R. A. Abramovltch and J. B. Davis, /. Chain. Soc., B, 1137 (t~). (31) H. B. lonassen and W. B. Klrsch, 1. Amer. Chem. Soc., 79, 1279 (1957). (32) ]. R. Joy and M. Orehin, I. Amer. Chem. Soc., 81, 310 0959). (33) G. C. Bond and P. B. Wells, Advan. in Catal., 15, 216 (1964). (34) I. Chatt and A. D. Westland, 1. Chem. 3oc., A, 88 (1968). (35) R. Cramer, I. Amer. Chem. Sot., 86, 217 (1964); 89, 5377 0967).

Reviews

1968

135

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