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Cyclopentadienyl and Benzene Complexes
Chapter VI Cyclopentadienyl and Benzene Complexes
A. DICYCLOPENTADIENYL COMPLEXES
A characteristic of the transition metals is the formation of cyclopentadienyl complexes of the type Cp M (Cp = C H ). These are also called metallocenes. The best known is, of course, ferrocene (dicyclopentadienyliron), but these complexes are also known for all the 3d elements (bar copper) and many of the 4d and 5d elements. The tendency for them to be formed appears to decrease down any given triad, if one may judge from reports of successful syntheses. A characteristic of the 3d metallocenes is that they are all, except ferrocene and the cobalticinium cation, (Cp Co ), paramagnetic. Nickelocene (Cp Ni) is a good example; it has a magnetic moment of 2.86 B.M. corresponding to the presence of two unpaired electrons. Unlike cobaltocene (Cp Co) which readily loses one electron to give the exceedingly stable C p C o ion, nickelocene does not lose two electrons to give the (hypothetical) diamagnetic C p N i ion. Instead, nickelocene has a high tendency to (effectively) lose one cyclopentadienyl ring. This can occur in a number of ways. 2
5
5
+
2
2
2
+
2
2+
2
254
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
863
In fact, with some ligands both cyclopentadienyl rings are removed ; for example, 57
Cp Ni + 4KCN -> 2K+Cp- + K Ni(CN) 2
2
Cp Ni + 6NH -> Ni(NH ) 2
3
Cp Ni + 4PPh 2
3
3
2+ 6
4
+ 2Cp-
(Ph P) Ni 3
4
The palladium and the platinum analogs of nickelocene remain unknown. Wilkinson prepared, but did not fully characterize, a red complex, [(C H )2Pd] , from sodium cyclopentadienide and palladium acetylacetonate. It was thermally very unstable. Fischer and Schuster-Woldan obtained (C H ) Pt , a more stable green complex, in low yield from sodium cyclo pentadienide and PtCl in hexane. The NMR spectrum showed a singlet, with satellites arising from the coupling of all the Cp protons to both P t nuclei; this indicated the presence of a Pt-Pt bond. From the infrared and ultraviolet spectra, however, these authors concluded that the molecule had σ- and 77bonded C H groups, and was presumably a fluxional molecule in solution. Bonding in ferrocene and its analogs has been extensively discussed and a good summary has been given by Rosenblum. In simple terms each cyclo pentadienyl ligand acts as a 6-electron donor [to M(II)] and can be thought of as occupying three coordination positions about the metal. Hence, the metallocenes with two rings parallel to each other and with the metal atom sand wiched between them can be regarded as pseudooctahedral complexes. For ferrocene, the effective atomic number formalism is obeyed. Nickelocene is usually regarded as isostructural with ferrocene, though suggestions have been 865
5
5
w
866
5
5
4
2
2
195
5
5
867
Β. MONOCYCLOPENTADIENYL COMPLEXES
255
made (to account for some of the reactions described above) that the two un paired electrons may be localized to some extent on two carbons of one ring. The apparent dislike which Pd(II) and Pt(II) have to forming dicyclopentadienyl complexes may be understood in terms of the much lower tendency for these metals to adopt octahedral (or pseudooctahedral) stereochemistry, in contrast to Ni(II) for which this is quite common. It should be pointed out, however, that although the dicyclopentadienyls are known for Rh and Ir, and have been studied, they are exceedingly unstable thermally and dimerize (via C-C bonds) even at very low temperatures, ' 868
869
(M = Rh, Ir)
Fritz and Schwartzhans have reported the cis and trans isomers of (Me S)2Pt(C H )2. The C H rings are probably σ-bonded here, and these complexes should be regarded as simple derivatives of square planar Pt(II). The NMR spectrum, however, showed the Cp proton resonances as a singlet and indicated this molecule to be fluxional. X-Ray investigations and low temperature NMR studies are necessary to completely characterize this complex and (C H ) Pt2. 870
2
5
5
5
5
5
5
4
B. MONOCYCLOPENTADIENYL COMPLEXES Quite a number of these are known for all these elements in the (II) oxida tion state and also for Pt(lV). Complexes of the type CpNiXL (L = R P, CO, etc; X = alkyl, aryl, halide, etc.) are well k n o w n . " Recently Cross and W a r d l e have reported the synthesis of the analogous palladium and platinum complexes, e.g., 3
3 6 7
3 7 0 , 3 7 0 a
370b
PhMgBr
(Ph P) Pd Br + CpTl 3
2
2
> CpPdBr(PPh )
4
3
> CpPdPh(PPh ) 3
These complexes are all relatively stable. Fischer and his c o - w o r k e r s have described the preparation of the cyclopentadienylpalladium and cyclopentadienylplatinum nitrosyls. 871,872
M(NO)Cl + CpNa
p e n t a n
%
(Cj\
Μ—NO
(M = Pd, Pt)
256
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
The nickel analog has been known for a long time and has been shown to be a symmetric top molecule, with a linear Ni-N-O group, in the gas phase. Fischer and Schuster-Woldan concluded from the infrared spectra of the three complexes that they were isostructural and that the M-NO bonding increased from Ni to P t . Both the nickel and platinum cyclopentadienyl carbonyl dimers are known, but their structures are quite different. The nickel complex has bridging carbonyls (VI-1), whereas the platinum one has not (VI-2). Similar trends are observed in many di- and polynuclear carbonyl complexes, for example, [CpM(CO) ] , for Μ = Fe, Ru, and O s . 873
874
872
863,875
876
2
2
(VI-l)
(VI-2)
The analogous palladium complex has not yet been prepared. Smidt and Jira in 1959 described a brown complex, from cyclopentadiene and aqueous palladium chloride, which they proposed to be C H P d C l . However, the nature of this compound is not clear, and it may well be a nonstoichiometric polymer. Robinson and Shaw reported the reaction between cyclopentadiene and methanolic sodium chloropalladate to give a yellow precipitate which darkened rapidly. No pure product could be isolated. In view of the preparation of pentamethylcyclopentad/ewepalladium chloride (VI-3a) (which is stable) by Balakrishnan and Maitlis, the yellow solid prepared by Robinson and Shaw may be cyclopentad/ewepalladium chloride (VI-3b) which then reacts further to give unknown products. 877
5
5
634
616
R
PdCl
2
(VI-3a) (R = Me) (VI-3b) (R = H)
By far the most common cyclopentadienyl complexes of palladium are those with 7r-allylic, π-cyclobutadiene, 77-1,5-cyclooctadiene, or 77-enyl ligands. Some examples include
Β. MONOCYCLOPENTADIENYL COMPLEXES
257
J 2
(Μ = Pd, Pt)
Apart from sodium cyclopentadienide (in ethereal solvents) cyclopentadienylthallium and eyclopentadienyliron dicarbonyl bromide have been used as sources of the Cp group. The latter reagents offer some advantages in that they can be used under milder conditions than N a C H ~ . For example, tetramethylcyclobutadienenickel chloride undergoes attack at carbon as well as at nickel with N a C H - ~ (this volume, Chapter IV, Section F,2). The structure of allylcyclopentadienylpalladium has been determined by Minasyants and Struchkov. They showed that both groups were 7r-bonded to the metal, and that all the Cp carbons and all the allylic carbons were at the same distances from the metal (2.25 and 2.05 A, respectively), but that the two ligands were not parallel. NMR studies on a variety of cyclopentadienylpalladium complexes of this type showed that the Cp group was symmetrically bonded. There was no evi dence for fluxional behavior. The ultraviolet spectrum and the polarographic behavior of C H PdCp have been r e p o r t e d . The cyclopentadienyl group is also easily cleaved from palladium. For example, Maitlis et al. showed that whereas the alkoxycyclobutenylnickel complex (VI-4a) reacted with HBr to give the (7r-cyclobutadiene)(7r-cyclopentadienyl)nickel cation (VI-5), the palladium complex (VI-4b), even under the mildest conditions, lost the cyclopentadienyl ring to give (VI-5). This work was extended by Gubin et al., who showed that the cyclopenta dienyl group in C H PdCp was easily displaced by a variety of reagents to give allylpalladium complexes. The authors say that this cleavage occurs with both electrophilic and nucleophilic reagents under acidic, neutral, or basic condi tions. +
5
+
6 7 3
5
6 7 5
5
214
773,776
3
5
612
773
3
5
5
258
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES Ph
Br-
(VI-4a) Μ = Ni (VI-4b) M = Pd PdBr
2
(VI-6) (—PdCl
+ CH
(—PdCl
+ C H HgCl
5
6
Pd—)
FeCl
(
3
PdCl
5
5
+ Cp Fe 2
Some transformations which do not involve cleavage of the Cp group are possible under basic or neutral conditions. 672
Ph,
,Ph FeBr
_
K Fe(CN) 4
Ph,
,Ph
6
Br-
4
Ph
Ph Co (CO) 2
8
RO Ph Ph'
Maitlis et al. were also able to transfer the cyclopentadienyl and the cyclo butadiene group onto cobalt by reaction of [Ph C PdCp]Br with Co (CO) . 612
4
4
2
8
C. BENZENE COMPLEXES
259
Robinson and Shaw in 1963 reported the preparation of (7r-cyclopentadienyl)trimethylplatinum(IV) from iodotrimethylplatinum tetramer. 878
Pt^Me Me
The complex was shown to be monomeric, and on the basis of its NMR spectrum (two singlets, with satellites due to coupling to Pt), Robinson and Shaw suggested the Cp ring was 7r-bonded. This was disputed by Fritz and Schwartzhans on the basis of the chemical shifts of the resonances, an unreliable criterion at best. Semion et al* have briefly reported an X-ray crystal structure determination of CpPtMe and have shown that the Cp ring was indeed 7r-bonded and parallel to the plane of the three methyls. Approximate distances were Pt-C (cyclopentadienyl) 2.2 A, Pt-C (methyl) 2.05 A. No palladium analogs of this complex are known. 195
870
19
3
C. BENZENE COMPLEXES While mono- and bisarene complexes are well established among elements in the early and middle groups of the transition metals, they become progressively more rare in group VIII. Here, in fact, only the very electron-rich hexamethylbenzene (HMB) will act as a ligand to give isolable complexes. Bis-HMB complexes of Fe(0), Fe(I), Co(0), Co(I), Co(II), Rh(I), and Rh(II) have been described by Fisher and Lindner. These authors have also recently described the synthesis of the first such nickel complex, (HMB) Ni , and have obtained evidence for the formation of a very unstable (and not isolated) [(HMB Ni] ion. . These complexes are paramagnetic [/x for the Ni(II) complex is 3.0 B.M.], since they have more electrons than are required by the effective atomic number formalism, and are very sensitive to hydrolysis. No details of the structures of these complexes are known. The nickel(II) complex was prepared by heating HMB with NiBr in the presence of AlBr and was characterized as the hexachloroplatinate(IV), [(HMB) Ni] (PtCl ) -. It is perhaps, therefore, a little surprising that benzene complexes of palladium, formally in the (I) oxidation state, are known. In fact, these complexes probably play a very important role in the coupling and oxidation reactions of arenes with Pd(II) salts. Allegra et al. have reported the formation of two complexes of this type by the classical Fischer synthesis of arene-metal complexes. ' When the melt 880
2+
2
+
2
881
eff
2
3
2+
2
2
881
6
157
158
260
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
obtained by heating PdCl , aluminum, aluminum trichloride, and benzene at 80° was allowed to cool, large brown crystals of diamagnetic complexes with formula (C H PdAlCl ) , or (C H PdAl Cl ) , were obtained. The latter complex was isolated when a large excess of A1C1 was used. The complexes were only stable in the solid; they were insoluble in hydrocarbons and disproportionated in tetrahydrofuran to give equal amounts of palladium and PdCl . 2
6
6
4
2
6
6
2
7
2
3
2
THF
(C H PdAlCl ) 6
6
4
2C H + Pd + PdCl + 2AICI
2
6
6
2
3
X-Ray structure determinations on both complexes have been carried out (Figs. VI-1 and V I - 2 ) . ' The basic features are similar. In each case two Pd atoms, 2.57(1) A apart, are sandwiched between two benzene rings. The metal-metal distances observed are much shorter than in palladium metal, 2.75 A. 157
158
FIG. VI-1. The structures of (C H PdAlCl ) (X = CI) and (QH PdAl Cl ) (X = A1C1 ). After Allegra et al. * ' * 6
1
6
4
2
6
2
7
2
4
7 15
(a) (b) FIG. VI-2. (a) The arrangement of the metal atoms with respect to the benzene rings in (C H PdAl Cl ) . (b) A projection perpendicular to the average ring plane of the complex (C H PdAlCl ) . * 6
6
6
6
2
7
2
15
4
2
Each metal atom is also bonded to one CI atom [at 2.45(1) A ; the array Cl-Pd-Pd-Cl is very nearly linear] which is also attached to an A1C1 or A1 C1 group. The bonding of the metal atoms to the benzene rings is most unusual. In (C H PdAl Cl ) , the benzene rings have at least two different orientations with respect to the Pd-Pd axis, one of which is shown in Fig. VI-2a. The other is the mirror image of this. The metal atoms are not symmetrically arranged with respect to the benzenes, but each metal atom appears to be within a reasonable bonding distance (2.2-2.5 A) of two carbon atoms and out of the bonding 3
6
6
2
7
2
2
7
C. BENZENE COMPLEXES
261
range (2.6-3.5 A) of the remainder in each ring. However, the e.s.d.'s of the Pd-C distances are large, 0.03A. In (C H PdAlCl ) , the structure of which is better resolved, the ( C H ) P d core is of D symmetry. Again (Fig. VI-2b) the metal atoms are not arranged symmetrically with respect to each ring. In this complex each metal is within reasonable bonding range of two carbon atoms of each ring [2.26(2), 2.37(2) A] and out of range of the remainder (2.77, 3.03,3.38, 3.50 A). The e.s.d.'s on the C-C bond lengths are too large to allow any conclusions to be drawn about the benzene ring, but it does appear to be distorted slightly away from planar, two of the carbons being bent 7° out of the plane of the other four toward the metal atoms. These complexes have no direct parallel elsewhere. The bonding of each metal atom to the benzene is more reminiscent of the edge-bonded copper and silver complexes C H CuAlCl and C H AgAlCl (VI-7) than of the sym metrically 7r-bonded complexes such as dibenzenechromium (VI-8) or benzenechromium tricarbonyl. * The metal-carbon bond lengths in the silver 6
6
4
2
6
6
2
2
2 h
882
6
6
4
883
6
6
4
884
(VI-7)
(VI-8)
complex (VI-7) are much longer [2.47(6), 2.57(6) A] than in either of the benzenepalladium complexes, and the orientation of the metal with respect to the ring is also different. There appears to be a wide variety of behavior possible for benzene rings complexed to a m e t a l . ' Davidson and Triggs have studied the oxidative coupling of benzene to biphenyl, "catalyzed" by palladium acetate. When the reaction was carried out in acetic acid in the presence of 0.5 Μ perchloric acid below 60°, no acetoxylation or precipitation of metal occurred, but the color of the solution turned magenta. From this solution the authors isolated, on addition of acetic anhydride, the very explosive Pd(I) complex, (C H Pd · H 0 · 0 0 ) „ , together with a 50 % yield of biphenyl. The complex decomposed above 80° in acetic acid, but benzene was not oxidized, nor were acetoxy radicals formed. On addition of chloride ion, the complex disproportionated with the formation of metal and PdCl ~. Oxidizing agents such as oxygen, bromine, or permanganate oxidized the metal to Pd(II), but Cr(VI) or Pb(IV) did not, except in the presence of halide ion. The 885
886
159
6
6
2
4
2
4
262
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
complex appeared to be paramagnetic since no benzene resonances were seen until after halide was added and disproportionation to Pd(0) and Pd(II) had occurred. This suggested the absence of metal-metal bonding between Pd(I) atoms in the complex. The ultraviolet-visible spectrum was also in agreement with the presence of a d ion. The complex could also be obtained from palladium acetate and 1-hexene or PhB(OH) (phenylboronic acid) provided benzene and perchloric acid were present. This particular complex does not therefore appear to be an inter mediate in the reaction 2 C H -> C H C H , but is formed simultaneously, and probably plays a vital role in the overall process, 9
2
6
6
C H + Pd(II) 6
6
2PhPd(II) 2Pd(I)
6
5
6
•
5
PhPd(II) + H+ Ph + 2Pd(I) 2
Pd° + PdCl 2
4
The decomposition of two PhPd(II) species to biphenyl appears to be best regarded as a concerted reaction since no phenyl radicals are produced. The benzene appears to be necessary in order to stabilize Pd(I). However, benzene π complexes are probably intermediates in the electro philic palladation reactions. These and the above reactions are more fully discussed in Chapters II and III, Volume II, Section D.
A. DICYCLOPENTADIENYL COMPLEXES
A characteristic of the transition metals is the formation of cyclopentadienyl complexes of the type Cp M (Cp = C H ). These are also called metallocenes. The best known is, of course, ferrocene (dicyclopentadienyliron), but these complexes are also known for all the 3d elements (bar copper) and many of the 4d and 5d elements. The tendency for them to be formed appears to decrease down any given triad, if one may judge from reports of successful syntheses. A characteristic of the 3d metallocenes is that they are all, except ferrocene and the cobalticinium cation, (Cp Co ), paramagnetic. Nickelocene (Cp Ni) is a good example; it has a magnetic moment of 2.86 B.M. corresponding to the presence of two unpaired electrons. Unlike cobaltocene (Cp Co) which readily loses one electron to give the exceedingly stable C p C o ion, nickelocene does not lose two electrons to give the (hypothetical) diamagnetic C p N i ion. Instead, nickelocene has a high tendency to (effectively) lose one cyclopentadienyl ring. This can occur in a number of ways. 2
5
5
+
2
2
2
+
2
2+
2
254
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
863
In fact, with some ligands both cyclopentadienyl rings are removed ; for example, 57
Cp Ni + 4KCN -> 2K+Cp- + K Ni(CN) 2
2
Cp Ni + 6NH -> Ni(NH ) 2
3
Cp Ni + 4PPh 2
3
3
2+ 6
4
+ 2Cp-
(Ph P) Ni 3
4
The palladium and the platinum analogs of nickelocene remain unknown. Wilkinson prepared, but did not fully characterize, a red complex, [(C H )2Pd] , from sodium cyclopentadienide and palladium acetylacetonate. It was thermally very unstable. Fischer and Schuster-Woldan obtained (C H ) Pt , a more stable green complex, in low yield from sodium cyclo pentadienide and PtCl in hexane. The NMR spectrum showed a singlet, with satellites arising from the coupling of all the Cp protons to both P t nuclei; this indicated the presence of a Pt-Pt bond. From the infrared and ultraviolet spectra, however, these authors concluded that the molecule had σ- and 77bonded C H groups, and was presumably a fluxional molecule in solution. Bonding in ferrocene and its analogs has been extensively discussed and a good summary has been given by Rosenblum. In simple terms each cyclo pentadienyl ligand acts as a 6-electron donor [to M(II)] and can be thought of as occupying three coordination positions about the metal. Hence, the metallocenes with two rings parallel to each other and with the metal atom sand wiched between them can be regarded as pseudooctahedral complexes. For ferrocene, the effective atomic number formalism is obeyed. Nickelocene is usually regarded as isostructural with ferrocene, though suggestions have been 865
5
5
w
866
5
5
4
2
2
195
5
5
867
Β. MONOCYCLOPENTADIENYL COMPLEXES
255
made (to account for some of the reactions described above) that the two un paired electrons may be localized to some extent on two carbons of one ring. The apparent dislike which Pd(II) and Pt(II) have to forming dicyclopentadienyl complexes may be understood in terms of the much lower tendency for these metals to adopt octahedral (or pseudooctahedral) stereochemistry, in contrast to Ni(II) for which this is quite common. It should be pointed out, however, that although the dicyclopentadienyls are known for Rh and Ir, and have been studied, they are exceedingly unstable thermally and dimerize (via C-C bonds) even at very low temperatures, ' 868
869
(M = Rh, Ir)
Fritz and Schwartzhans have reported the cis and trans isomers of (Me S)2Pt(C H )2. The C H rings are probably σ-bonded here, and these complexes should be regarded as simple derivatives of square planar Pt(II). The NMR spectrum, however, showed the Cp proton resonances as a singlet and indicated this molecule to be fluxional. X-Ray investigations and low temperature NMR studies are necessary to completely characterize this complex and (C H ) Pt2. 870
2
5
5
5
5
5
5
4
B. MONOCYCLOPENTADIENYL COMPLEXES Quite a number of these are known for all these elements in the (II) oxida tion state and also for Pt(lV). Complexes of the type CpNiXL (L = R P, CO, etc; X = alkyl, aryl, halide, etc.) are well k n o w n . " Recently Cross and W a r d l e have reported the synthesis of the analogous palladium and platinum complexes, e.g., 3
3 6 7
3 7 0 , 3 7 0 a
370b
PhMgBr
(Ph P) Pd Br + CpTl 3
2
2
> CpPdBr(PPh )
4
3
> CpPdPh(PPh ) 3
These complexes are all relatively stable. Fischer and his c o - w o r k e r s have described the preparation of the cyclopentadienylpalladium and cyclopentadienylplatinum nitrosyls. 871,872
M(NO)Cl + CpNa
p e n t a n
%
(Cj\
Μ—NO
(M = Pd, Pt)
256
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
The nickel analog has been known for a long time and has been shown to be a symmetric top molecule, with a linear Ni-N-O group, in the gas phase. Fischer and Schuster-Woldan concluded from the infrared spectra of the three complexes that they were isostructural and that the M-NO bonding increased from Ni to P t . Both the nickel and platinum cyclopentadienyl carbonyl dimers are known, but their structures are quite different. The nickel complex has bridging carbonyls (VI-1), whereas the platinum one has not (VI-2). Similar trends are observed in many di- and polynuclear carbonyl complexes, for example, [CpM(CO) ] , for Μ = Fe, Ru, and O s . 873
874
872
863,875
876
2
2
(VI-l)
(VI-2)
The analogous palladium complex has not yet been prepared. Smidt and Jira in 1959 described a brown complex, from cyclopentadiene and aqueous palladium chloride, which they proposed to be C H P d C l . However, the nature of this compound is not clear, and it may well be a nonstoichiometric polymer. Robinson and Shaw reported the reaction between cyclopentadiene and methanolic sodium chloropalladate to give a yellow precipitate which darkened rapidly. No pure product could be isolated. In view of the preparation of pentamethylcyclopentad/ewepalladium chloride (VI-3a) (which is stable) by Balakrishnan and Maitlis, the yellow solid prepared by Robinson and Shaw may be cyclopentad/ewepalladium chloride (VI-3b) which then reacts further to give unknown products. 877
5
5
634
616
R
PdCl
2
(VI-3a) (R = Me) (VI-3b) (R = H)
By far the most common cyclopentadienyl complexes of palladium are those with 7r-allylic, π-cyclobutadiene, 77-1,5-cyclooctadiene, or 77-enyl ligands. Some examples include
Β. MONOCYCLOPENTADIENYL COMPLEXES
257
J 2
(Μ = Pd, Pt)
Apart from sodium cyclopentadienide (in ethereal solvents) cyclopentadienylthallium and eyclopentadienyliron dicarbonyl bromide have been used as sources of the Cp group. The latter reagents offer some advantages in that they can be used under milder conditions than N a C H ~ . For example, tetramethylcyclobutadienenickel chloride undergoes attack at carbon as well as at nickel with N a C H - ~ (this volume, Chapter IV, Section F,2). The structure of allylcyclopentadienylpalladium has been determined by Minasyants and Struchkov. They showed that both groups were 7r-bonded to the metal, and that all the Cp carbons and all the allylic carbons were at the same distances from the metal (2.25 and 2.05 A, respectively), but that the two ligands were not parallel. NMR studies on a variety of cyclopentadienylpalladium complexes of this type showed that the Cp group was symmetrically bonded. There was no evi dence for fluxional behavior. The ultraviolet spectrum and the polarographic behavior of C H PdCp have been r e p o r t e d . The cyclopentadienyl group is also easily cleaved from palladium. For example, Maitlis et al. showed that whereas the alkoxycyclobutenylnickel complex (VI-4a) reacted with HBr to give the (7r-cyclobutadiene)(7r-cyclopentadienyl)nickel cation (VI-5), the palladium complex (VI-4b), even under the mildest conditions, lost the cyclopentadienyl ring to give (VI-5). This work was extended by Gubin et al., who showed that the cyclopenta dienyl group in C H PdCp was easily displaced by a variety of reagents to give allylpalladium complexes. The authors say that this cleavage occurs with both electrophilic and nucleophilic reagents under acidic, neutral, or basic condi tions. +
5
+
6 7 3
5
6 7 5
5
214
773,776
3
5
612
773
3
5
5
258
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES Ph
Br-
(VI-4a) Μ = Ni (VI-4b) M = Pd PdBr
2
(VI-6) (—PdCl
+ CH
(—PdCl
+ C H HgCl
5
6
Pd—)
FeCl
(
3
PdCl
5
5
+ Cp Fe 2
Some transformations which do not involve cleavage of the Cp group are possible under basic or neutral conditions. 672
Ph,
,Ph FeBr
_
K Fe(CN) 4
Ph,
,Ph
6
Br-
4
Ph
Ph Co (CO) 2
8
RO Ph Ph'
Maitlis et al. were also able to transfer the cyclopentadienyl and the cyclo butadiene group onto cobalt by reaction of [Ph C PdCp]Br with Co (CO) . 612
4
4
2
8
C. BENZENE COMPLEXES
259
Robinson and Shaw in 1963 reported the preparation of (7r-cyclopentadienyl)trimethylplatinum(IV) from iodotrimethylplatinum tetramer. 878
Pt^Me Me
The complex was shown to be monomeric, and on the basis of its NMR spectrum (two singlets, with satellites due to coupling to Pt), Robinson and Shaw suggested the Cp ring was 7r-bonded. This was disputed by Fritz and Schwartzhans on the basis of the chemical shifts of the resonances, an unreliable criterion at best. Semion et al* have briefly reported an X-ray crystal structure determination of CpPtMe and have shown that the Cp ring was indeed 7r-bonded and parallel to the plane of the three methyls. Approximate distances were Pt-C (cyclopentadienyl) 2.2 A, Pt-C (methyl) 2.05 A. No palladium analogs of this complex are known. 195
870
19
3
C. BENZENE COMPLEXES While mono- and bisarene complexes are well established among elements in the early and middle groups of the transition metals, they become progressively more rare in group VIII. Here, in fact, only the very electron-rich hexamethylbenzene (HMB) will act as a ligand to give isolable complexes. Bis-HMB complexes of Fe(0), Fe(I), Co(0), Co(I), Co(II), Rh(I), and Rh(II) have been described by Fisher and Lindner. These authors have also recently described the synthesis of the first such nickel complex, (HMB) Ni , and have obtained evidence for the formation of a very unstable (and not isolated) [(HMB Ni] ion. . These complexes are paramagnetic [/x for the Ni(II) complex is 3.0 B.M.], since they have more electrons than are required by the effective atomic number formalism, and are very sensitive to hydrolysis. No details of the structures of these complexes are known. The nickel(II) complex was prepared by heating HMB with NiBr in the presence of AlBr and was characterized as the hexachloroplatinate(IV), [(HMB) Ni] (PtCl ) -. It is perhaps, therefore, a little surprising that benzene complexes of palladium, formally in the (I) oxidation state, are known. In fact, these complexes probably play a very important role in the coupling and oxidation reactions of arenes with Pd(II) salts. Allegra et al. have reported the formation of two complexes of this type by the classical Fischer synthesis of arene-metal complexes. ' When the melt 880
2+
2
+
2
881
eff
2
3
2+
2
2
881
6
157
158
260
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
obtained by heating PdCl , aluminum, aluminum trichloride, and benzene at 80° was allowed to cool, large brown crystals of diamagnetic complexes with formula (C H PdAlCl ) , or (C H PdAl Cl ) , were obtained. The latter complex was isolated when a large excess of A1C1 was used. The complexes were only stable in the solid; they were insoluble in hydrocarbons and disproportionated in tetrahydrofuran to give equal amounts of palladium and PdCl . 2
6
6
4
2
6
6
2
7
2
3
2
THF
(C H PdAlCl ) 6
6
4
2C H + Pd + PdCl + 2AICI
2
6
6
2
3
X-Ray structure determinations on both complexes have been carried out (Figs. VI-1 and V I - 2 ) . ' The basic features are similar. In each case two Pd atoms, 2.57(1) A apart, are sandwiched between two benzene rings. The metal-metal distances observed are much shorter than in palladium metal, 2.75 A. 157
158
FIG. VI-1. The structures of (C H PdAlCl ) (X = CI) and (QH PdAl Cl ) (X = A1C1 ). After Allegra et al. * ' * 6
1
6
4
2
6
2
7
2
4
7 15
(a) (b) FIG. VI-2. (a) The arrangement of the metal atoms with respect to the benzene rings in (C H PdAl Cl ) . (b) A projection perpendicular to the average ring plane of the complex (C H PdAlCl ) . * 6
6
6
6
2
7
2
15
4
2
Each metal atom is also bonded to one CI atom [at 2.45(1) A ; the array Cl-Pd-Pd-Cl is very nearly linear] which is also attached to an A1C1 or A1 C1 group. The bonding of the metal atoms to the benzene rings is most unusual. In (C H PdAl Cl ) , the benzene rings have at least two different orientations with respect to the Pd-Pd axis, one of which is shown in Fig. VI-2a. The other is the mirror image of this. The metal atoms are not symmetrically arranged with respect to the benzenes, but each metal atom appears to be within a reasonable bonding distance (2.2-2.5 A) of two carbon atoms and out of the bonding 3
6
6
2
7
2
2
7
C. BENZENE COMPLEXES
261
range (2.6-3.5 A) of the remainder in each ring. However, the e.s.d.'s of the Pd-C distances are large, 0.03A. In (C H PdAlCl ) , the structure of which is better resolved, the ( C H ) P d core is of D symmetry. Again (Fig. VI-2b) the metal atoms are not arranged symmetrically with respect to each ring. In this complex each metal is within reasonable bonding range of two carbon atoms of each ring [2.26(2), 2.37(2) A] and out of range of the remainder (2.77, 3.03,3.38, 3.50 A). The e.s.d.'s on the C-C bond lengths are too large to allow any conclusions to be drawn about the benzene ring, but it does appear to be distorted slightly away from planar, two of the carbons being bent 7° out of the plane of the other four toward the metal atoms. These complexes have no direct parallel elsewhere. The bonding of each metal atom to the benzene is more reminiscent of the edge-bonded copper and silver complexes C H CuAlCl and C H AgAlCl (VI-7) than of the sym metrically 7r-bonded complexes such as dibenzenechromium (VI-8) or benzenechromium tricarbonyl. * The metal-carbon bond lengths in the silver 6
6
4
2
6
6
2
2
2 h
882
6
6
4
883
6
6
4
884
(VI-7)
(VI-8)
complex (VI-7) are much longer [2.47(6), 2.57(6) A] than in either of the benzenepalladium complexes, and the orientation of the metal with respect to the ring is also different. There appears to be a wide variety of behavior possible for benzene rings complexed to a m e t a l . ' Davidson and Triggs have studied the oxidative coupling of benzene to biphenyl, "catalyzed" by palladium acetate. When the reaction was carried out in acetic acid in the presence of 0.5 Μ perchloric acid below 60°, no acetoxylation or precipitation of metal occurred, but the color of the solution turned magenta. From this solution the authors isolated, on addition of acetic anhydride, the very explosive Pd(I) complex, (C H Pd · H 0 · 0 0 ) „ , together with a 50 % yield of biphenyl. The complex decomposed above 80° in acetic acid, but benzene was not oxidized, nor were acetoxy radicals formed. On addition of chloride ion, the complex disproportionated with the formation of metal and PdCl ~. Oxidizing agents such as oxygen, bromine, or permanganate oxidized the metal to Pd(II), but Cr(VI) or Pb(IV) did not, except in the presence of halide ion. The 885
886
159
6
6
2
4
2
4
262
VI. CYCLOPENTADIENYL AND BENZENE COMPLEXES
complex appeared to be paramagnetic since no benzene resonances were seen until after halide was added and disproportionation to Pd(0) and Pd(II) had occurred. This suggested the absence of metal-metal bonding between Pd(I) atoms in the complex. The ultraviolet-visible spectrum was also in agreement with the presence of a d ion. The complex could also be obtained from palladium acetate and 1-hexene or PhB(OH) (phenylboronic acid) provided benzene and perchloric acid were present. This particular complex does not therefore appear to be an inter mediate in the reaction 2 C H -> C H C H , but is formed simultaneously, and probably plays a vital role in the overall process, 9
2
6
6
C H + Pd(II) 6
6
2PhPd(II) 2Pd(I)
6
5
6
•
5
PhPd(II) + H+ Ph + 2Pd(I) 2
Pd° + PdCl 2
4
The decomposition of two PhPd(II) species to biphenyl appears to be best regarded as a concerted reaction since no phenyl radicals are produced. The benzene appears to be necessary in order to stabilize Pd(I). However, benzene π complexes are probably intermediates in the electro philic palladation reactions. These and the above reactions are more fully discussed in Chapters II and III, Volume II, Section D.