- Email: [email protected]
Mono- and dinuclear cationic rhodium(I) complexes with phosphine oxides and related ligands
Journal of Organometallic Chemistry, Elsevier
Sequoia
S-A_,
Lausanne
-
234 Printed
(1982) 205-217 in The Netherlands
MONO- AND DINUCLEAR CATIONIC RHODIUM(I) PHOSPHINE OXIDES AND RELATED LIGANDS
R. USON,
L-A.
Department (Received
ORO,
M-A.
CIRIANO,
of Inorganic Chemistry,
March
2nd,
F.J. LAHOZ
and M.C.
COMPLEXES WITH
BELL0
Uniuersiiy of Zaragoza, Zaragoza (Spain)
1982)
The syntheses of mononuclear cationic complexes of general formula [Rh(COD) (OER,),] C104 and [Rh(COD) (OER,)(PPh,)]ClO, (E = P, As) are described. Carbonylation of complexes of the types [ Rh(COD) (OER&] C104 and [ Rh( COD) ( 0PyR),]C104 (OPy R = substituted pyridine N-oxides), sometimes carried out in the presence of triphenylphosphine, leads to complexes of the types [Rh(C0)2(0ER3)2]C104, [Rh(CO) (OER&(PPh3)]C104, [Rh(CO)(OER3) (PPh&] C104, [Rh(CO)(OPyR)2(PPh,)]C10q and [Rh(CO)(OPyR)(PPh,),]ClO,. Dinuclear complexes of the general formula [Rh&-OER&(C0)2(PPh3)2] (ClO,), are prepared from the solvate [Rh(CO) (PPhB)(Me&O),]ClO~ and the appropriate ligand. Related complexes of the type [Rh2(~-OER3)2(COD)2](C104)2 are similarly obtained.
Introduction The coordination chemistry of phosphine oxides has been reviewed [1,2]. In particular, a large number of coordination metal complexes of the first series transition metals [3,4], lanthanides [ 51 and actinides [ 6,7] with phosphine oxides as ligands have been described; but complexes with metals of the second and third transition series have been less studied. On the other hand, phosphine oxides have been used in selective extraction processes of platinum metals [Slo] and as additives in some catalytic reactions [ 11-131. These ligands are also formed.during the preparation [ 141 or decomposition [ 151 of some phosphine rhodium(I) complexes. As far as we know, the only previously reported phosphine oxide rhodium(I) complex is RhCl(CO),(OPCy,) whose structure has been determined by X-ray diffraction [IS]. A few rhodium(II1) complexes of the type RhCl(ylid)z(OPRS) [17] and [Rl~(dimethylglyoximate)2(OPPh,)] *02 [ 181 have been reported. Following our earlier studies on rhodium(I) complexes with pyridine 0022-328X/82/000~00001$02.75
0
1982
Elsevier
Sequoia
S.A.
206
N-oxides as ligands [19], we describe below a family of rhodium(I) complexes incorporating phosphine oxides.
Results and discussion Complexes CIO, types
of the [Rh(COD)
(OER3)JC104
and [Rh(COD)
(OER,)(PPh,)]-
Treatment of acetone solutions of the solvated species [Rh(COD)(Me&O),]ClO, [ZO] with either triarylphosphine oxides or triphenylarsine oxide gives yellow solutions from which complexes I-V can be isolated as yellow crystalline solids (eq. 1). [Rln(COD)(Me,CO),]ClO,
+ 2 OER3 + [Rh(COD)(OER&]ClO,
(OER3 = OP@-ClC,H& (I), 0PPh3 (II), 0P@-MeC6&)3 OP@-MeOC6H& (IV), OAsPh, (V))
+x Me&O
(1)
(III),
Addition of stoichiometric amounts (l/l) of triphenylphosphine to dichloromethane solutions of the complexes I-V results in displacement of one mol of 0ER3 and formation of the mixed complexes [Rh(COD) (OER3) (PPh,)]ClO, (complexes VI-X). Alternatively, these complexes can be obtained by successive additions of stoichiometric amounts of OER3 and triphenylphosphine (l/l) to the solvated species [Rh(COD) (Me,CO),]ClO, (eq. 2). CRh(COD)(Me,CO),]ClO,
+ OER, + PPh, --f ]Rh(COD)(OER,)(PPh,)]CIOs +x Me&O
(2)
The complexes VI-X were isolated as yellow crystalline solids. Table 1 lists the analytical results, molar conductivities, yields and IR data for the complexes I-X. Their spectroscopic data are in accordance with square-planar structures around the rhodium atom. Thus, their IR spectra show v(P0) or ~(As0) of the coordinated phosphine or amine oxide [21] as an intense band in the 11701140 region or at ca. 870 cm-l, respectively, shifted towards lower wavenumbers than those of the free ligands. In addition, the bands at ca. 1100 and 620 cm-’ are characteristic of the uncoordinated anion C104- [22]. Consequently, the complexes are l/l electrolytes [23] in acetone. The 31P NMR spectrum of the complex II displays a singlet resonance at 39.7 ppm * from two phosphorus nuclei in equivalent environments. On the other hand, the 3 ‘P NMR of the complex VII shows the expected two resonances from triphenylphosphine oxide and triphenylphosphine at 39.5 ppm * (s) and 28.6 ppm (d, ‘J( Rh-P) 15 5 Hz), respectively. Complexes of the [Rh(CO)ZLJC104 type (i) L = Phosphine and arsine oxides. Bubbling of carbon monoxide at atmo-
spheric pressure through dichloromethane solutions of the complexes [Rh(COD) (OER,),]ClO, (I-V) results in displacement of 1,5icyclooctadiene and formation in solution (eq. 3) of cisdicarbonyl species, which were detected
* The possible coupling *J(Rh-P)
was not observed.
[Rh(COD)(OPPh~)~1ClO~
[Rh(COD){OP(PMcC~H~)~}21C1O4
[Rh(COD){OP(PMeOC~H4)3)
[ Rh(COD)(OAsPh 3) 21C104
[Rh(COD){OP(P-ClC~H4)~)(PPh~)1ClO~
[Rh(COD)(OPPh3)(PPh$lCl04
tRh(COD){OP(PMeC~l~~)~}(PPh~)lC10~
[Rh(COD){OP(P-MeOC~H~)~}(PPh3)3C104
[Rh(COD)(OAsPh~)(PPh~)JCl04
II
III
IV
V
VI
VII
VIII
IX
X
(5.2) 4.4 (4.4) 4.3 (4.1) B.0 (5.0) 5.8 (5.4) 5.4 (5.1) (if)
(67.3) 5G.O (55,3) 54.5 (55.4) 62.6 (62.1) 62.5 (63.2) 59.8 (60.0) 58.9
..-
._.
..-
.
.~
-
139
(5.7) 505
(6301) 57.7
(S9,O)
126
(4.9) 6.1
,(60,9) 62,5
._ -...
-
129
136
136
135
123
131
129
(3.4) 5.0
129
3.7
49.3
H
.
(ohm”
Anf
(49.2) 6008
C
Found (Calcd.) (%)
..-
cm2 mol-I)
_
.~~.-
77b
86O
870
86tJ
72O
88
aa
GC
13
88
(%I
Yield
__
YIELDS AND IR DATA FOR THE COMPLEXES [Rh(COD)(OER3)21ClO,q
c Yields according to cq. 2, b Yields according to method (ii) (experimcntol port).
2 1C104
[Rh(COD){OP(PClC~H~)3)2lClO4
RESULTS, MOLAR CONDUCTIVITIES,
I
Complex
(PPh3 )lClO,,
ANALYTICAL
TABLE 1
-
876~s
1145s
1149vs
llG2vs
1149s
870s
1142~11
1146vs
115ovs
1 lG&
IJ(E0) (cm-l )
-
AND [Rh(COD)(OERs)-
-
-
Complex h
Yield
90 74 72'
123 132 117 137 12G 134
3.4 (309) 3.4 (3.3) -
RQ
83
30G
1987
1987
2000
1964
1978
2100,2022
2102,203O
50 b 122
-
2090,200G
’
8G
132
-
2100,202G
-
3.8 (3.7) 3.4 (3.3) 2.6 (2.4) b.1 (4.7) 4.5 (4.3) 3.9 (4.0) 4.6 (4.1) r ,::;, 3.9 (8.7) GB.7 (86.0) GO.2 (G0.G) 44,2 (44.7) G9.5 (GE&) 62.3 (63.0) 67.9 (G8.1) 62.9 (G2.4) 606 (60.1) 67.4 (G&l)
v(C0)
117Gm
1200s
1207m
84Gm
1155s
llG'lm,sh
llGGs,sh
860s
114Gvs
Other
Charncteristic IR bends (cm-')
80”
136
N
(%)
H
(ohm" cm2 mol'l) C
Found(culcd,) (%)
RESULTS, MOLAR CONDUCTIVITIES,YIELDS AND II-t DATA FOR THE COMPLEXES [Rh(C0)2L2]C104,[Rh(CO)L2(PPb3)]CI04 AND [Rh(CO)L(PPh3)2]C104
ANALYTICAL
TABLE2
G3,8 (63.9) G4.G (64.8) G2,O (62.0) G0.G (Gl.3) 66.7 (66.4) 698 (69.8) GE.3 (G&7) bG.1 (G&2) 67-8 (G8.4) G&O (G&4) 63.6 (6300) 68.3 (68.3) 4.6 (402) 4,3 (4,4) 4,4 (45) 4,G (4,G) 4.3 (4,3) 4,3 (4.8)
(Z) 4.1 (4.6) 4.3 (4.2) 4.0 (3.8) (t::)
4.8 (4.4)
1980 1990
GE 66
137 139 138 137 141
-
Gl
1990
2010
1203m
1988
9oc
138
43
1210m
2000
93c
139
2010
1224s
1990
9oc
141
3.2 (381) 1,8 (1.6) 1.9 (1,6) -
GO
880m
-
1980
121
G7’
llG8m
19RG
137
1lGGs
1986
1lGOm
9oc
97“d
1994
138
134
770
-
a Yields according to eq. 3. b Yields according to method (ii) (cxpcrimenlaI pnrt). c Yields nccording to eq, G. rLYields according to eq, G,
xxx1
xxx
XXIX
XXVIII
XXVII
XXVI
xxv
XXIV
XXIII
XXII
XXI
xx
210 by IR spectroscopy. [Rh(COD)(OER,),]C104
+ 2 CO = [Rh(CO),(OER,)JCQ
+ COD
(3)
By addition of a large volume of ether with carbon monoxide bubbling through the solution, complexes XI and XII are precipitated as yellow crystalline solids (see Table 2). Solid mixtures of the dicarbonyl complexes and the starting material are obtained in other cases, suggesting that the reaction in eq. 3 is an equilibrium. Confirming this, addition of 1,Ejcyclooctadiene to complexes XI and XII leads to [Rh(COD) (OER,),]ClO, derivatives. The dicarbonyl complexes XI-XIV can also be obtained by addition of the appropriate phosphine or amine oxide to acetone solutions of [Rh(CO),(Me&O),]ClO, obtained by treating [RhCl(CO),], with silver perchlorate. The analytical results, molar conductivities, yields and IR data for the complexes XI-XIV are given in Table 2. Their IR spectra exhibit two intense ir(C0) bands typical of cis-dicarbonyl rhodium complexes [ 241, the v(P0) or v(As0) band for the coordinated oxigen-donor ligand and the characteristic absorptions of the uncoordinated perchlorate anion. In accord with the proposed skucture, the 31P NMR spectrum of complex XI shows a singlet signal at 47.1 ppm. (ii) L = Pyridine N-oxides_ Bubbling of carbon monoxide through dichloromethane solutions of the complex [Rh(COD) (OPyR)2]C10, [19] (OPyR = 4-NO,PyO, 4-MePyO, 4-MeOPyO) gives the cis-dicarbonyl complexes which display two intense v(C0) bands in the 2100-2000 cm-l region. However, attempts to isolate them were unsuccesful, and the starting materials were recovered. Complexes of the [Rh(CO)L2(PPh3)]C104 type (i) L = Phosphine and amineoxides. Treatment of dichloromethane solutions of the complexes of the type [Rh(COD) (OER3),]C104 (I-V) with carbon monoxide and subsequent addition of the stoichiomekic amount of kiphenylphosphine leads to the complexes of the type [Rh(CO) (OER3)JPPh3)]C104 (es- 4). [Rh(COD)(OER,),]ClO,~+
CO f PPh3 + [Rh(CO)(OER3)2(PPh3)]C104
+ COD
(4) The complexes XV and XVI are isolated from the solution as reasonably pure yellow solids, whilst the analogous derivatives with ligands of the type 0PCp-RC6H4)3 are contaminated with [Rh(CO) (OER3) (PPh3)2]C104 derivatives. The latter are the result of repeated recrystaIlizations of the mixtures. An alternative route for the preparation of the complexes XV and XVI involves the addition of a stoichiomekic amount of kiphenylphosphine (l/l) to dichloromethane solutions of complexes XI and XII. Their IR spectra show a single band Y(CO) along with Y(PO) or v(As0) of the oxygen-donor ligand and the characteristic bands of the perchlorate ion. They are l/l electrolytes (see Table 2). The 31P NMR spectrum of complex XV consists of two resonances: a doublet at 48.6 ppm (d, ‘J(Rh-P) 182 Hz) from the tiphenyl phosphine and a broad band centered at 40.5 ppm from both oxygen-donor ligands, which are apparently equivalent_
211
(ii) L = Pyridine N-oxides. Carbonylation of the complexes of the type [Rh(COD) (OPyR)JClO, followed by careful addition of triphenylphosphine similarly leads to the formation of the compounds [Rh(CO) (OPyR),(PPh,)]ClO+ The complexes XVII and XVIII are isolated from the solutions as yellow crystalline solids (see Table 2). However, a mixture of [Rh(CO) (4-N02PyO)2(PPh3)]C104 and [Rh(CO) (4-NO,PyO) (PPh&]C104 is obtained when the pyridine ring bears an electron withdrawing substituent such as a nitro group. Recrystallization of the mixture gives the latter complex. The IR spectra of the complexes XVII and XVIII shows a single v(C0) band between 2000-1980 cm-* and an intense absorption v(N0) around 1200 cm-l, characteristic of coordinated pyridine N-oxide [ 191, in accord with the proposed formula. Complexes of the [Rh(CO)L(PPh& C104 type Carbonylation of the complexes of the type [Rh(COD)(OER&]CIO, (I-V) and [Rh(COD) (OPyR)JC104 [19] (OPyR = 4-NO&O, 4-MePyO, 4-MeOPyO) in dichloromethane at atmospheric pressure followed by addition of triphenylphosphine in molar ratio l/2 gives directly (eq. 5) the complexes of the type [Rh(CO)L(PPhs)2]C104, which were isolated as yellow crystalline solids (complexes XIX-XXVI, Table 2). [Rh(COD)L,]ClO,
+ CO + 2 PPh3 -+ [Rh(CO)L(PPh,),]ClO,
+ COD + L
(5)
They can also be prepared by replacement of acetone in the complex [Rh(CO) (Me&O) (PPh&]Cl04 [ 251 by the oxygen-donor ligand L (eq. 6). [Rh(CO)(Me,CO)(PPh,),1C104
+ L + [ Rh(CO)L(PPh,),]ClO,
+ Me&O
(6) The complexes XIX-XXVI are l/l electrolytes in acetone, and their spectra support square-planar structures in which the two phosphine ligands are in positions trans. Thus, the 31P NMR spectrum of complex XX shows a resonance at 38.5 ppm due to the phosphine oxide and a doublet signal at 28.0 ppm (d, ‘J(Rh-P) 125 Hz) characteristic of trans-bisphosphine rhodium(I) complexes [26]. All the complexes display in their IR spectra a single Y(CO) band, v(PO), v(As0) or v(N0) bands of the coordinated oxygen-donor ligand, and bands of the perchlorate anion (see Table 2). The complexes [Rh(CO) (DMSO) (PPh3)JCI0, (XXVII) and [Rh(CO)(diox)(PPh,),]C104 (XXVIII) were prepared similarly by carbonylation (DMSO = dimethylsulfoxide and diox = dioxane). The related compounds with sulfurdonor ligands [Rh(CO) (THT) (PPh3)2]C1O4 (XXIX), [Rh(CO) (SPPh3)(PPh3)JC104 (XXX) and [Rh(CO) (SEh) (PPh3)2]C104 (XxX1) (THT = tetrahydrothiophene; SPPh3 = triphenylphosphine sulfide; SE& = diethylthioether) are similarly obtained. Dinuclear complexes of the [Rh2(r_L-OER3)2(CO)2(PPh3)J (CIO,), type Bubbling of carbon monoxide through dichloromethane solutions of the complexes of the type [Rh(COD) (OER3) (PPh,)]ClO, (VI-X) gives rise to complex reactions. Two v(C0) bands characteristic of cis-dicarbonyl rhodium(1) complexes, among others, are detected in solution. These solutions yield yellow solids which are a mixture of apparently monocarbonyl rhodium(I)
212
complexes. Recrystallization of the crude solids gives complexes of the general formula [Rh2(1_t-OER3)2(C0)2(PPh3)21(C104)2 (complexes XXXII-XXXVI, Table 3) which were the main products. These dinuclear complexes are obtained directly by reaction of the species [Rh(CO) (PPh,) (Me2CO),]C104 with the appropriate phosphine oxide or triphenylarsine oxide (eq. 7), and are isolated as orange-yellow solids. 2 [Rh(CO)(PPh,)(Me,CO),1CIO,
+ 2 0ER3 +
[Rh2@-OER,),(CO),(PPhS)z](C104
)* + 2 3cMe&O
(7)
Measurements of the equivalent conductivities in nitromethane or acetone solutions of the complexes XXXII-XXXVI at different concentrations give plots of the Onsager equation (A, - A, = m), for which B values are characteristic of 2/l electrolytes in these solvents [27,28] (see Table 3). Their IR spectra show only one zJ(C0) band in the region of terminal carbonyls, suggesting a structure for the complex ion in which the ligands 0ER3 act as bridges between the two rhodium atoms and each carbonyl and triphenylphosphine ligands is bonded to one rhodium atom. Structures with phosphine oxides bridging two metal atoms have been previously proposed for complexes of the type [MfOPPh,),] (ClO& M = Fe, Co, Ni [29 J . Similar structures with pyridine N-oxides of the type [ Cu( ~_t-OPy)Cl,], have been confirmed by X-ray diffraction studies [ 303. For copper complexes with pyridine N-oxides there is not a clearcut difference between the position of v(N0) for bridging and terminal ligands. No substantial differences in the position of v(P0) and v(As0) are found between complexes XXXII-XXXVI and complexes in which the ligands are terminal. The 3*P NMR spectrum of the complex XXXHI consists of a band centered at 41.1 ppm from the oxygen-donor ligands and a doublet resonance at 48.5 ppm (d, ‘J(Rh-P) 177 Hz) due to both triphenylphosphine ligands in equivalent environments_ The data suggest the following structure : -
P”3 I
2+
In addition, complex XXXIII undergoes symmetrical bridge cleavage on treatment with triphenylphosphine, giving [Rh(CO) (OPPh3) (PPh&]C104.
Dinuclear complexes of the [Rh,(Ir-OER3)2(C00)~(C104)2 type In analogy with the reactions represented in eq. 7, reaction of the solvate [Rh(COD) (Me&O),]ClO, with a stoichiometric amount of ligands OER, gives the complexes of the type [Rh&-OER,),(COD)J (ClO.&, which are iso-
RESULTS, YIELDS, CONDUCTIVITY
[Rhz{~CP(pMeOC6H4)3)2(C0)2(PPh3)2l(CIC4)2
(Rh2(~OAsPh3)2(CO)2(PI’h3)21(C104)2
[RII~{~-CP(~CIC~R~)~}~(COD)~I(CIC~)~
[Rh201-OPPh3)2(COD)2lClOq)2
[Rh,{~.OP@-Mc,IJ,),},(CCW21(CIO&
~R~~{~OP~~MeOC~H~~~}~~COD~~l~ClO~~~
tRh2(Cc.OAsPh3)2(COD)2l(C104)2
xxxv
XXXVI
XXXVII
XXXVIII
XXXIX
XL
XL1
52
89
999'
19Gb
90 (4.9) 4,6 (4-3)
246 f~
266 b
280 b
GO
32
(3.6) 4.9
73oe
487 b
583 b
637 b
699 b
nc
27Gc
__ _ __.
_ .~__ _ ------
_. .~. .
--.
852s
1146s
1164s
1165s,s11
844s
114Gs
1148vs&
1158,113Gs
1142vs,sh
v (EO)
16Gb
1963
1980
1986
1980
1988
v (CO)
llG2s
..-_-
cm2 mol’l)
IR dnta (cm-l )
AND
146 b
164b
1G2b
2ofic
156b
16Ab
170b
lG0 b
AM (ohm”
Conductivity data
(4.6) 5.8 (5.3) G.1
70
(Z) 3.9
72
58
(4.5) 4.9 (4.2)
81
(3.Q) 6,l
63
(3J) 4.G
3.3
H
(%)
Yield
COMPLEXES [I~l~,(~O~R,),(CO),(PI’h~)~](C104)~
[271. b Meosured in nitromcthnnc solution. c Measured in occtone solution.
[R~~2{~OP(~MeC~H4)~}2(C0)2(PPh~)2l(CIC~)2
XXXIV
c Slope found for Onsager’s equation Ae = A, - DJC
[Rh2~OPph3)2(CO),(PPh3)2l(ClO4)2
(GO.8) 67.7 (87.6) 68.8 (GDJ) 6608 (GG.8) G&2 (64.G) 48-3 (48.1) G2,G (83-O) 5508 (66.2) Gl.,b (61,3) 49.6 (49.3)
49.8 [Rh2{~OP(~ClC~~I4)~)2(CO)2(PPh3)2l(C104)2
C
Found (calcd.) (%)
AND IR DATA FOR TIIE DINUCLEAR
xxx111
xxx11
Complex
[Rh,(~CER,),(CCD),I(CIO4)2
ANALYTICAL
TABLE 3
214
lated as hygroscopic yellow solids (complexes XXXVII-XLI). Their analyses, molar conductivities, yields and IR data are collected in Table 3. Measurements of equivalent conductivities of nitromethane solutions of the complexes XXXVI-XL at various concentrations lead to values of coefficient B in the Onsager equation between those corresponding to l/l and l/2 electrolytes. Evaporation of these solutions gives yellow solids different from the starting materials, which contain nitromethane, and consequently the low B values are not completely unexpected. The 31P NMR spectra of the complexes XXXVIII and XL display a resonance at 40.2 and 41.2 ppm, respectively, similar in position to the resonance of the related complex [Rh2(p-OPPh&(CO),(PPh,)J (C104)2. On the other hand, conductivity data for acetone solutions of [ Rhz(l-(-0_AsPh3)2(COD)2] (ClO,), (XLI) are compatible (B = 999) [28] with the proposed dinuclear structure. Complexes XXXVI-XL1 undergo symmetrical bridge cleavage with triphenylphosphine, giving the mononuclear species [ Rh(COD) (OER,) (PPh,)] CIO,. Experimental C, H and N analyses were carried out with a Perkin-Elmer 240 B microanalyzer. IR spectra were recorded on a Perkin-Elmer 577 spectrophotometer (over the range 4000-200 cm-l) using Nujol mulls between polyethylene sheets or dichloromethane solutions in cells with NaCl windows. Conductivities were measured with a Philips PW 9501/01 conductimeter in ca. 5 X 10e4 M nitromethane or acetone solutions and, when stated, at various concentrations in the range 5 X 10-3-10-4 M. The 31P NMR spectra were recorded on a Varian
FTSOA spectrometer using CDC13 solutions. Chemical shifts are given in ppm relative to external 85% H3P04, with a positive sign indicating a shift to lower field. CRhCl(CGD)l. 13x1, CRh’%CO)W’h,)l. [321, CRhCUCO).l. C331, RhCl(C0) (PPh& [ 341 and [ Rh(COD) (OPyR)2]C104 [ 191 were prepared as described elsewhere. The triarylphosphine oxides were prepared by oxidizing the phosphines in acetone with a slight excess of dilute hydrogen peroxide [ 351, purified by vacuum-melting and recrystallized from acetone_ Triphenylphosphine oxide and triphenylarsine oxide were obtained from Fluka A.G. and recrystallized from acetone. Pyridine Woxides were prepared by standard methods [36,37] and vacuum-distilled or recrystallized before use. The solvents were distilled before use. AlI the reactions were carried out at room temperature and atmospheric pressure with 0.1-0.2 mmol of the reagents_ Preparation of complexes of the type [Rh(COD)(OER,)JClO,
(I-V) The phosphine or arsine oxide (0.40 mmol) was added to a solution of [Rh(COD) (Me2CO),]CI0, (obtained by treating [RhCI(COD)], (0.049 g, 0.10
mmol) with AgC104 (0.041 g, 0.20 mmol) in 10 ml of acetone and removing the precipitated AgCl by filtration). Vacuum evaporation to dryness left yellow solids which were suspended in hexane, filtered off, washed with hexane, and air-dried.
215
Preparation of complexes of the type [Rh(COD)(OER3)(PPh3)]C104
(VI-X)
(i) To an acetone solution of equimolecular amounts of PPh, and the respective phosphine or amine oxide was added a stoichiometric amount of [Rh(COD)(Me,CO),]ClO,. The solutions were vacuum-concentrated to ca. 1 ml and slow addition of ether (VII and X) or hexane (VI, VIII and IX) gave yellow crystalline solids, which were filtered off, washed thoroughly with ether or hexane, and air-dried. (ii) PPh3 (l/l) was added to the above-described dichloromethane solutions of complexes I-V, work-up was as in (i). This method gave higher yields for complexes VII and X (see Table 1).
Preparation of complexes of the type [Rh(CO),(OER,)JCiO, (XI-XIV) (i) Bubbling of carbon monoxide for 15 min through a very concentrated dichloromethane/ether solution of [ Rh(COD) (OPPh,),] C104 led to formation of a pale-yellow crystalline solid. Precipitation was completed by addition of ether and bubbling of carbon monoxide for another 15 min. The resulting crystals were filtered off, washed with ether and air-dried_ Complex [Rh(C0)2(0AsPh3)2]C104 (XII) was synthesized in an analogous manner. (ii) Addition of OP@-CIC,H,), (0.079 g, 0.21 mmol) to an acetone solution of [Rh(CO),(Me,CO),]ClO. (obtained by treating [RhCI(C0)J2 (0.020 g, 0.05 mmol) with AgClO, (0.021 g, 0.10 mmol) in the absence of light and under argon) led to a yellow solution. The precipitated AgCl was filtered off and the filtrate was concentrated to dryness to give an off-yellow oil. This was extracted with ca_ 50 ml of ether, and the extract was evaporated to dryness and the resulting solid washed with hexane and vacuum-dried. Complex XIV was ebtained in a similar way. Complexes XI and XII can be obtained by this method, but in lower yields.
Preparation of complexes of the type [Rh(CO)L,(PPh,)]C104
(XV-XVIiI)
A stoichiometric amount (l/l) of PPh3 in dichloromethane was added to a solution of [Rh(C0)2L2]C10, (obtained by 15 min bubbling of carbon monoxide through a dichloromethane solution of complexes of the type
[Rh(COD)L,]C104, where L = 0ER3 (complexes I-V) or L = OPyR [19]. After 30 min stirring the solution was concentrated under reduced pressure to ca. 1 ml. The complexes were crystallized by slow addition of ether. The yellow solids were filtered off, washed with ether, and air-dried. Complexes in which L was a substituted pyridine N-oxide were crystallized by prolonged stirring with ether_
gave oils which
Preparation of complexes of the type [Rh(CO)L(PPh,)JC104 Complexes XIX-XXVI_ (i) Bubbling of carbon monoxide through a dichloromethane solution of [Rh(COD) (OPPh3)JC104 (0.052 g, 0.06 mmol) and subsequent addition of PPh3 (0.031 g, 0.12 mmol) gave a intense yellow solution. After 30 min stirriug, the solution was evaporated to dryness to give a yellow solid, which was suspended in hexane, filtered off, washed with hexane and vacuum-dried. The other complexes of this type were synthesized analogously using the corresponding [Rh(COD)L,]’ complexes as starting materials, but ether was used as precipitating agent for complexes with substituted pyridine N-oxides.
216 (ii) To an acetone solution of [Rh(CO)(Me,CO)(PPh3)JC104 (0.100 g, 0.12 mmol) was added a stoichiometric amount of the relevant ligand (0.12 mmol). The solution was concentrated to ca. 1 ml, and slow addition of hexane (complexes XIX-XXIII) or ether (complexes XXIV-XXVI) yielded yellow crystalline solids, which were filtered off, washed with hexane or ether, and vacuum-dried_ Complexes XXVII-XXXI_ A stoichiometric amount of the corresponding Oor S-donor ligand (0.10 mmoi) was added to an acetone solution of [Rh(COD)(Me,CO),]C104 (0.10 mmol). Bubbling of carbon monoxide for 10 min through this solution and addition of PPh3 (0.052 g, 0.20 mmol) gave a yellow solution which was stirred for 30 min and vacuum-evaporated to ca. 1 ml. These complexes were precipitated by slow addition of ether as yellow solids, which were filtered off, washed with ether and vacuum-dried_ Preparafion of complexes (,XXXII-XXXVI)
of the type
[Rh,(p-OER,),(CO),(PPh,)J
(CIOJ,
Complexes of this type were each made by two routes, which are illustrated below_ (i) A solution of AgC104 (0.025 g, 0.12 mmol) in 5 ml of acetone was treated with an acetone solution of [RhCl(CO)(PPh,)], (0.052 g, 0.06 mmol) in the absence of light and the mixture was stirred for 15 min. The precipitated AgCl was filtered off and a solution of triphenylphosphine oxide (0.033 g, 0.12 mmol) in 10 ml of acetone was slowly added to the filtrate_ The solution was vacuum-evaporated to dryness and the resulting orange-yellow solid was washed with hexane and vacuum-dried. Complexes XxX11, XXXIV-XXXVI were prepared analogously_ The preparations of these complexes, which are hygroscopic, were carried out under argon. (ii) Bubbling of carbon monoxide for 15 min through dichloromethane solutions of [ Rh(COD) (OERs) (PPh3)]C104 (VI-X), followed by concentration under reduced pressure to ca. 1 ml and addition of hexane gave orange-yellow solids, which were filtered off, washed with hexane, and vacuum-dried_ They were recrystallized twice from dichloromethane solutions by slow addition of hexane. The recrystallized complexes were washed with hexane and vacuumdried_ Preparation of complexes (XXXVrl--XLr)
of the type [Rh2(p-OER3)2(COD)J(CI04),
To an acetone solution of ]Rh(COD) (Me2CO),]C104 under an argon atmosphere was slowly added the stoichiometric amount of an acetone solution of the 0ER3 ligand (Rh/OERS, l/l)_ Evaporation to dryness gave dark-yellow oils which were kept under vacuum for 1 h. Addition of dichloromethane gave yellow solutions, which were vacuum-concentrated to dryness. The resulting yellow solids were washed several times with hexane and vacuum-dried. These complexes are hygroscopic, except for [Rh&-OAsPh,),(COD).1(ClO,),_ References 1 N-M. Karayannis, C.M.Miku.lskiand.L.L. Pytlewski.Inorg.Chim.ActaRev..5(1971) 2 Z_A_ShekaandE.I_Smyavskaya.Usp.Khim.Koord.Soedin.,<1975)113;Chem.Ahstr..85 115909a.
69. (1976)
217 3 CM. hIIkuIski. L-L. PytIewski and N.M. Karayannis. Synth. React. Inorg. Met-org. Chem.. 9 (1979) 401_ 4 M.W.G. De Bolster. C. Boutkan. T.A. Van der Knaap. L. Van Zweeden. I.E. Kortram and W.L. Groeneveld. 2. Anorg. Aug. Chem.. 443 (1978) 269. Chem. Abstr.. 90 (1979) 145122j. 5 DC. Bradley. J-S. Ghotra. F-A. Hart. M-B. Hu&.house and P.R. Raithby. 3. Chem. Sot.. Ddton (1977) 1166. 6 J.G.H. du Preez. B.J. GeIIatIy and M-L. Gibson. J. Chem. Sot.. Dalton (1977) 1062 and references therein. 7 (a) J-H. Burns. Inorg. Chem.. 20 (1981) 3868. (b) N-W. AIcock. S. Espenis. K.W. BagnaB and W. Hdan-Yun. J. Chem. Sot.. Dalton, (1978) 638. 8 L-N. Lomakina. T-1. Ignat’eva. S.A. Pisareva. T. Ya Medved and M-1. Kabachnik. Zh. Anal. Khim.. 35 (1980) 86. Chem. Abstr., 92 (1980) 208352~. 9 F.E. Alekseevna and KS. Vasil’evich. Mater. Vses. Nauchn. Stud. Konf.: Khim.. 13th (1975) 14. Chem. Ahstr.. 86 (1977) 61157d. 10 V.N. DaniIova, M.G. SuIeimano~ra. G.V. ShiIina. G.A. Anistratenko. N.G. Feshchenko. A.P. hlarchenko. G.K. Fedorova and L-1. Samarai. Ukr_ Khim. Zh.. 41 (1075) 1200. Chem. Abstr.. 84 (1976) 65775k. 11 K. Kuemmerle and H. Heise. Ger. Offen. 2.837.022. Chem. Abstr.. 92 (1980) 214359x. 89 (1978) 146443f; ibid.. 78 63.307. Chem. 12 I. Kuriyama. Japan Kokai. 78 63.308. Chem. Abstr.. Abstr.. 89 (1978) 129066t. A.J. FaneIIi and F.C. Rauch. U.S. 3.884.975. Chem. Abstr.. 83 (1975) 115331q:A.J. FaneIIi, G-M_ Blank and F.C. Rauch. U.S. 3.801.639. Chem. Abstr.. 80 (1974) 145478~. 14 J-A. Osbom. F.H. Jardine. J.F. Young and G. Wilkinson. J. Chem. Sot. (1966) 1711. 15 Yu.N. Kukushkin. N.P. Fedyanin and A-1. Mokhov. Zh. Neorg. Khim.. 25 (1980) 784. Chem. Abstr.. 92 (1980) 190561d: Yu.N. Kukushkin. L-1. Danilina. A-1. Osokin and V.P. Botel’nikov. Zh. Neorg. Khim.. 24 (1979) 2253. Chem. Abstr.. 91 (1979) 203593x. 16 G. Bandoh. D.A. Clemente, G. DeganeIIo. G. Carturan. P. Uguaghati and II. BeIIuco. J. Organometal.
13
17 18 19 20 21 22 23
24 25 26 27 28 29 30 31 32 33 34 35 36 37
Chem.. 71 (1974) 125. S.A. Dias. A.W. Downs and W-R. McWhinnie. J. Chem. Sot.. Dalton. (1975) 162. M-V. KIyuev. B-G. Rogachev. Yu.hI. ShuI’ga and h1.L. Khidekel. Izv. Akad. Nauk. SSSR. Ser. Khim.. 8 (1979) 1869. Chem. Abstr.. 91 (1979) 221624n. R. Uson. LA. Oro. hI.A. Ciriano and F.J. Lahoz. J. Organometal. Chem.. 217 (1981) 251. R.R. Schrock and J.A. Oshorn. J. Amer. Chem. Sot.. 93 (1971) 3089. D.hI.L. Goodgame. M. Goodgame and P.J. Hayward. J. Chem. SOL Dalton, (1970) 1352; G.A. RodIey. D.M.L. Goodgame and F.A. Cotton. J. Chem. SIX.. (1965) 1499. J. Peone and L. Vaska. Angew. Chem. Int. Ed.. 10 (1971) 511. W.J. Geary. Coord. Chem. Rev.. 7 (1971) 81. R. Uson. L.A. Oro. hI.A. GarraIda and C. Claver. J. Organometal. Chem.. 105 (1976) 365. A.J. Deeming and I. Rothwell, J. Chem. Sot., Dalton. (1980) 1259. P.S. Pregosin end R-W. Kunz. NMR. Basic Principles and Progress. Springer Verlag. Berlin, vol. 16. 1979. p_ 110 and ss. R-D. Feltham and R.G. Hayter. J. Chem. Sot.. (1964) 4587. R. Us&i. J. Gimeno. J. Fomies and F. Martinez. Inorg. Chfm. Acta. 50 (1981) 173. CM. hIikuIski. X5-J. Strocko. L-L. PytIewski and hI.hl. Labes. J. Inorg. NucI. Chem.. N.M. Karayannis, 33 (1971) 2691; ibid.. 33 (1971) 3185. W.H. Watson. Inorg. Chem.. 8 (1969) 1879 and references therein. G. Giordano and R.H. Crabtree. Inorg. Synth.. 19 (1979) 218. D.F. Steele and T.A. Stephenson. J. Chem. SIX.. Dalton. (1972) 2161. J.A. h¶cCIeverty and G. Wilkinson. Inorg. Synth.. 8 (1966) 211. J.A. McCIeverty and G. Wilkinson. Inorg. Synth., 8 (1966) 214. D-B_ Copley. F_ Fairbrother. J.R. Miller and A. Thompson, Proc. Chem. Sot.. (1964) 300. E. Ochiai. J. Org. Chem.. 18 (1953) 534. E. Ochfai. Aromatic Amine Oxides, Elsevier. Amsterdam, 1967.
Sequoia
S-A_,
Lausanne
-
234 Printed
(1982) 205-217 in The Netherlands
MONO- AND DINUCLEAR CATIONIC RHODIUM(I) PHOSPHINE OXIDES AND RELATED LIGANDS
R. USON,
L-A.
Department (Received
ORO,
M-A.
CIRIANO,
of Inorganic Chemistry,
March
2nd,
F.J. LAHOZ
and M.C.
COMPLEXES WITH
BELL0
Uniuersiiy of Zaragoza, Zaragoza (Spain)
1982)
The syntheses of mononuclear cationic complexes of general formula [Rh(COD) (OER,),] C104 and [Rh(COD) (OER,)(PPh,)]ClO, (E = P, As) are described. Carbonylation of complexes of the types [ Rh(COD) (OER&] C104 and [ Rh( COD) ( 0PyR),]C104 (OPy R = substituted pyridine N-oxides), sometimes carried out in the presence of triphenylphosphine, leads to complexes of the types [Rh(C0)2(0ER3)2]C104, [Rh(CO) (OER&(PPh3)]C104, [Rh(CO)(OER3) (PPh&] C104, [Rh(CO)(OPyR)2(PPh,)]C10q and [Rh(CO)(OPyR)(PPh,),]ClO,. Dinuclear complexes of the general formula [Rh&-OER&(C0)2(PPh3)2] (ClO,), are prepared from the solvate [Rh(CO) (PPhB)(Me&O),]ClO~ and the appropriate ligand. Related complexes of the type [Rh2(~-OER3)2(COD)2](C104)2 are similarly obtained.
Introduction The coordination chemistry of phosphine oxides has been reviewed [1,2]. In particular, a large number of coordination metal complexes of the first series transition metals [3,4], lanthanides [ 51 and actinides [ 6,7] with phosphine oxides as ligands have been described; but complexes with metals of the second and third transition series have been less studied. On the other hand, phosphine oxides have been used in selective extraction processes of platinum metals [Slo] and as additives in some catalytic reactions [ 11-131. These ligands are also formed.during the preparation [ 141 or decomposition [ 151 of some phosphine rhodium(I) complexes. As far as we know, the only previously reported phosphine oxide rhodium(I) complex is RhCl(CO),(OPCy,) whose structure has been determined by X-ray diffraction [IS]. A few rhodium(II1) complexes of the type RhCl(ylid)z(OPRS) [17] and [Rl~(dimethylglyoximate)2(OPPh,)] *02 [ 181 have been reported. Following our earlier studies on rhodium(I) complexes with pyridine 0022-328X/82/000~00001$02.75
0
1982
Elsevier
Sequoia
S.A.
206
N-oxides as ligands [19], we describe below a family of rhodium(I) complexes incorporating phosphine oxides.
Results and discussion Complexes CIO, types
of the [Rh(COD)
(OER3)JC104
and [Rh(COD)
(OER,)(PPh,)]-
Treatment of acetone solutions of the solvated species [Rh(COD)(Me&O),]ClO, [ZO] with either triarylphosphine oxides or triphenylarsine oxide gives yellow solutions from which complexes I-V can be isolated as yellow crystalline solids (eq. 1). [Rln(COD)(Me,CO),]ClO,
+ 2 OER3 + [Rh(COD)(OER&]ClO,
(OER3 = OP@-ClC,H& (I), 0PPh3 (II), 0P@-MeC6&)3 OP@-MeOC6H& (IV), OAsPh, (V))
+x Me&O
(1)
(III),
Addition of stoichiometric amounts (l/l) of triphenylphosphine to dichloromethane solutions of the complexes I-V results in displacement of one mol of 0ER3 and formation of the mixed complexes [Rh(COD) (OER3) (PPh,)]ClO, (complexes VI-X). Alternatively, these complexes can be obtained by successive additions of stoichiometric amounts of OER3 and triphenylphosphine (l/l) to the solvated species [Rh(COD) (Me,CO),]ClO, (eq. 2). CRh(COD)(Me,CO),]ClO,
+ OER, + PPh, --f ]Rh(COD)(OER,)(PPh,)]CIOs +x Me&O
(2)
The complexes VI-X were isolated as yellow crystalline solids. Table 1 lists the analytical results, molar conductivities, yields and IR data for the complexes I-X. Their spectroscopic data are in accordance with square-planar structures around the rhodium atom. Thus, their IR spectra show v(P0) or ~(As0) of the coordinated phosphine or amine oxide [21] as an intense band in the 11701140 region or at ca. 870 cm-l, respectively, shifted towards lower wavenumbers than those of the free ligands. In addition, the bands at ca. 1100 and 620 cm-’ are characteristic of the uncoordinated anion C104- [22]. Consequently, the complexes are l/l electrolytes [23] in acetone. The 31P NMR spectrum of the complex II displays a singlet resonance at 39.7 ppm * from two phosphorus nuclei in equivalent environments. On the other hand, the 3 ‘P NMR of the complex VII shows the expected two resonances from triphenylphosphine oxide and triphenylphosphine at 39.5 ppm * (s) and 28.6 ppm (d, ‘J( Rh-P) 15 5 Hz), respectively. Complexes of the [Rh(CO)ZLJC104 type (i) L = Phosphine and arsine oxides. Bubbling of carbon monoxide at atmo-
spheric pressure through dichloromethane solutions of the complexes [Rh(COD) (OER,),]ClO, (I-V) results in displacement of 1,5icyclooctadiene and formation in solution (eq. 3) of cisdicarbonyl species, which were detected
* The possible coupling *J(Rh-P)
was not observed.
[Rh(COD)(OPPh~)~1ClO~
[Rh(COD){OP(PMcC~H~)~}21C1O4
[Rh(COD){OP(PMeOC~H4)3)
[ Rh(COD)(OAsPh 3) 21C104
[Rh(COD){OP(P-ClC~H4)~)(PPh~)1ClO~
[Rh(COD)(OPPh3)(PPh$lCl04
tRh(COD){OP(PMeC~l~~)~}(PPh~)lC10~
[Rh(COD){OP(P-MeOC~H~)~}(PPh3)3C104
[Rh(COD)(OAsPh~)(PPh~)JCl04
II
III
IV
V
VI
VII
VIII
IX
X
(5.2) 4.4 (4.4) 4.3 (4.1) B.0 (5.0) 5.8 (5.4) 5.4 (5.1) (if)
(67.3) 5G.O (55,3) 54.5 (55.4) 62.6 (62.1) 62.5 (63.2) 59.8 (60.0) 58.9
..-
._.
..-
.
.~
-
139
(5.7) 505
(6301) 57.7
(S9,O)
126
(4.9) 6.1
,(60,9) 62,5
._ -...
-
129
136
136
135
123
131
129
(3.4) 5.0
129
3.7
49.3
H
.
(ohm”
Anf
(49.2) 6008
C
Found (Calcd.) (%)
..-
cm2 mol-I)
_
.~~.-
77b
86O
870
86tJ
72O
88
aa
GC
13
88
(%I
Yield
__
YIELDS AND IR DATA FOR THE COMPLEXES [Rh(COD)(OER3)21ClO,q
c Yields according to cq. 2, b Yields according to method (ii) (experimcntol port).
2 1C104
[Rh(COD){OP(PClC~H~)3)2lClO4
RESULTS, MOLAR CONDUCTIVITIES,
I
Complex
(PPh3 )lClO,,
ANALYTICAL
TABLE 1
-
876~s
1145s
1149vs
llG2vs
1149s
870s
1142~11
1146vs
115ovs
1 lG&
IJ(E0) (cm-l )
-
AND [Rh(COD)(OERs)-
-
-
Complex h
Yield
90 74 72'
123 132 117 137 12G 134
3.4 (309) 3.4 (3.3) -
RQ
83
30G
1987
1987
2000
1964
1978
2100,2022
2102,203O
50 b 122
-
2090,200G
’
8G
132
-
2100,202G
-
3.8 (3.7) 3.4 (3.3) 2.6 (2.4) b.1 (4.7) 4.5 (4.3) 3.9 (4.0) 4.6 (4.1) r ,::;, 3.9 (8.7) GB.7 (86.0) GO.2 (G0.G) 44,2 (44.7) G9.5 (GE&) 62.3 (63.0) 67.9 (G8.1) 62.9 (G2.4) 606 (60.1) 67.4 (G&l)
v(C0)
117Gm
1200s
1207m
84Gm
1155s
llG'lm,sh
llGGs,sh
860s
114Gvs
Other
Charncteristic IR bends (cm-')
80”
136
N
(%)
H
(ohm" cm2 mol'l) C
Found(culcd,) (%)
RESULTS, MOLAR CONDUCTIVITIES,YIELDS AND II-t DATA FOR THE COMPLEXES [Rh(C0)2L2]C104,[Rh(CO)L2(PPb3)]CI04 AND [Rh(CO)L(PPh3)2]C104
ANALYTICAL
TABLE2
G3,8 (63.9) G4.G (64.8) G2,O (62.0) G0.G (Gl.3) 66.7 (66.4) 698 (69.8) GE.3 (G&7) bG.1 (G&2) 67-8 (G8.4) G&O (G&4) 63.6 (6300) 68.3 (68.3) 4.6 (402) 4,3 (4,4) 4,4 (45) 4,G (4,G) 4.3 (4,3) 4,3 (4.8)
(Z) 4.1 (4.6) 4.3 (4.2) 4.0 (3.8) (t::)
4.8 (4.4)
1980 1990
GE 66
137 139 138 137 141
-
Gl
1990
2010
1203m
1988
9oc
138
43
1210m
2000
93c
139
2010
1224s
1990
9oc
141
3.2 (381) 1,8 (1.6) 1.9 (1,6) -
GO
880m
-
1980
121
G7’
llG8m
19RG
137
1lGGs
1986
1lGOm
9oc
97“d
1994
138
134
770
-
a Yields according to eq. 3. b Yields according to method (ii) (cxpcrimenlaI pnrt). c Yields nccording to eq, G. rLYields according to eq, G,
xxx1
xxx
XXIX
XXVIII
XXVII
XXVI
xxv
XXIV
XXIII
XXII
XXI
xx
210 by IR spectroscopy. [Rh(COD)(OER,),]C104
+ 2 CO = [Rh(CO),(OER,)JCQ
+ COD
(3)
By addition of a large volume of ether with carbon monoxide bubbling through the solution, complexes XI and XII are precipitated as yellow crystalline solids (see Table 2). Solid mixtures of the dicarbonyl complexes and the starting material are obtained in other cases, suggesting that the reaction in eq. 3 is an equilibrium. Confirming this, addition of 1,Ejcyclooctadiene to complexes XI and XII leads to [Rh(COD) (OER,),]ClO, derivatives. The dicarbonyl complexes XI-XIV can also be obtained by addition of the appropriate phosphine or amine oxide to acetone solutions of [Rh(CO),(Me&O),]ClO, obtained by treating [RhCl(CO),], with silver perchlorate. The analytical results, molar conductivities, yields and IR data for the complexes XI-XIV are given in Table 2. Their IR spectra exhibit two intense ir(C0) bands typical of cis-dicarbonyl rhodium complexes [ 241, the v(P0) or v(As0) band for the coordinated oxigen-donor ligand and the characteristic absorptions of the uncoordinated perchlorate anion. In accord with the proposed skucture, the 31P NMR spectrum of complex XI shows a singlet signal at 47.1 ppm. (ii) L = Pyridine N-oxides_ Bubbling of carbon monoxide through dichloromethane solutions of the complex [Rh(COD) (OPyR)2]C10, [19] (OPyR = 4-NO,PyO, 4-MePyO, 4-MeOPyO) gives the cis-dicarbonyl complexes which display two intense v(C0) bands in the 2100-2000 cm-l region. However, attempts to isolate them were unsuccesful, and the starting materials were recovered. Complexes of the [Rh(CO)L2(PPh3)]C104 type (i) L = Phosphine and amineoxides. Treatment of dichloromethane solutions of the complexes of the type [Rh(COD) (OER3),]C104 (I-V) with carbon monoxide and subsequent addition of the stoichiomekic amount of kiphenylphosphine leads to the complexes of the type [Rh(CO) (OER3)JPPh3)]C104 (es- 4). [Rh(COD)(OER,),]ClO,~+
CO f PPh3 + [Rh(CO)(OER3)2(PPh3)]C104
+ COD
(4) The complexes XV and XVI are isolated from the solution as reasonably pure yellow solids, whilst the analogous derivatives with ligands of the type 0PCp-RC6H4)3 are contaminated with [Rh(CO) (OER3) (PPh3)2]C104 derivatives. The latter are the result of repeated recrystaIlizations of the mixtures. An alternative route for the preparation of the complexes XV and XVI involves the addition of a stoichiomekic amount of kiphenylphosphine (l/l) to dichloromethane solutions of complexes XI and XII. Their IR spectra show a single band Y(CO) along with Y(PO) or v(As0) of the oxygen-donor ligand and the characteristic bands of the perchlorate ion. They are l/l electrolytes (see Table 2). The 31P NMR spectrum of complex XV consists of two resonances: a doublet at 48.6 ppm (d, ‘J(Rh-P) 182 Hz) from the tiphenyl phosphine and a broad band centered at 40.5 ppm from both oxygen-donor ligands, which are apparently equivalent_
211
(ii) L = Pyridine N-oxides. Carbonylation of the complexes of the type [Rh(COD) (OPyR)JClO, followed by careful addition of triphenylphosphine similarly leads to the formation of the compounds [Rh(CO) (OPyR),(PPh,)]ClO+ The complexes XVII and XVIII are isolated from the solutions as yellow crystalline solids (see Table 2). However, a mixture of [Rh(CO) (4-N02PyO)2(PPh3)]C104 and [Rh(CO) (4-NO,PyO) (PPh&]C104 is obtained when the pyridine ring bears an electron withdrawing substituent such as a nitro group. Recrystallization of the mixture gives the latter complex. The IR spectra of the complexes XVII and XVIII shows a single v(C0) band between 2000-1980 cm-* and an intense absorption v(N0) around 1200 cm-l, characteristic of coordinated pyridine N-oxide [ 191, in accord with the proposed formula. Complexes of the [Rh(CO)L(PPh& C104 type Carbonylation of the complexes of the type [Rh(COD)(OER&]CIO, (I-V) and [Rh(COD) (OPyR)JC104 [19] (OPyR = 4-NO&O, 4-MePyO, 4-MeOPyO) in dichloromethane at atmospheric pressure followed by addition of triphenylphosphine in molar ratio l/2 gives directly (eq. 5) the complexes of the type [Rh(CO)L(PPhs)2]C104, which were isolated as yellow crystalline solids (complexes XIX-XXVI, Table 2). [Rh(COD)L,]ClO,
+ CO + 2 PPh3 -+ [Rh(CO)L(PPh,),]ClO,
+ COD + L
(5)
They can also be prepared by replacement of acetone in the complex [Rh(CO) (Me&O) (PPh&]Cl04 [ 251 by the oxygen-donor ligand L (eq. 6). [Rh(CO)(Me,CO)(PPh,),1C104
+ L + [ Rh(CO)L(PPh,),]ClO,
+ Me&O
(6) The complexes XIX-XXVI are l/l electrolytes in acetone, and their spectra support square-planar structures in which the two phosphine ligands are in positions trans. Thus, the 31P NMR spectrum of complex XX shows a resonance at 38.5 ppm due to the phosphine oxide and a doublet signal at 28.0 ppm (d, ‘J(Rh-P) 125 Hz) characteristic of trans-bisphosphine rhodium(I) complexes [26]. All the complexes display in their IR spectra a single Y(CO) band, v(PO), v(As0) or v(N0) bands of the coordinated oxygen-donor ligand, and bands of the perchlorate anion (see Table 2). The complexes [Rh(CO) (DMSO) (PPh3)JCI0, (XXVII) and [Rh(CO)(diox)(PPh,),]C104 (XXVIII) were prepared similarly by carbonylation (DMSO = dimethylsulfoxide and diox = dioxane). The related compounds with sulfurdonor ligands [Rh(CO) (THT) (PPh3)2]C1O4 (XXIX), [Rh(CO) (SPPh3)(PPh3)JC104 (XXX) and [Rh(CO) (SEh) (PPh3)2]C104 (XxX1) (THT = tetrahydrothiophene; SPPh3 = triphenylphosphine sulfide; SE& = diethylthioether) are similarly obtained. Dinuclear complexes of the [Rh2(r_L-OER3)2(CO)2(PPh3)J (CIO,), type Bubbling of carbon monoxide through dichloromethane solutions of the complexes of the type [Rh(COD) (OER3) (PPh,)]ClO, (VI-X) gives rise to complex reactions. Two v(C0) bands characteristic of cis-dicarbonyl rhodium(1) complexes, among others, are detected in solution. These solutions yield yellow solids which are a mixture of apparently monocarbonyl rhodium(I)
212
complexes. Recrystallization of the crude solids gives complexes of the general formula [Rh2(1_t-OER3)2(C0)2(PPh3)21(C104)2 (complexes XXXII-XXXVI, Table 3) which were the main products. These dinuclear complexes are obtained directly by reaction of the species [Rh(CO) (PPh,) (Me2CO),]C104 with the appropriate phosphine oxide or triphenylarsine oxide (eq. 7), and are isolated as orange-yellow solids. 2 [Rh(CO)(PPh,)(Me,CO),1CIO,
+ 2 0ER3 +
[Rh2@-OER,),(CO),(PPhS)z](C104
)* + 2 3cMe&O
(7)
Measurements of the equivalent conductivities in nitromethane or acetone solutions of the complexes XXXII-XXXVI at different concentrations give plots of the Onsager equation (A, - A, = m), for which B values are characteristic of 2/l electrolytes in these solvents [27,28] (see Table 3). Their IR spectra show only one zJ(C0) band in the region of terminal carbonyls, suggesting a structure for the complex ion in which the ligands 0ER3 act as bridges between the two rhodium atoms and each carbonyl and triphenylphosphine ligands is bonded to one rhodium atom. Structures with phosphine oxides bridging two metal atoms have been previously proposed for complexes of the type [MfOPPh,),] (ClO& M = Fe, Co, Ni [29 J . Similar structures with pyridine N-oxides of the type [ Cu( ~_t-OPy)Cl,], have been confirmed by X-ray diffraction studies [ 303. For copper complexes with pyridine N-oxides there is not a clearcut difference between the position of v(N0) for bridging and terminal ligands. No substantial differences in the position of v(P0) and v(As0) are found between complexes XXXII-XXXVI and complexes in which the ligands are terminal. The 3*P NMR spectrum of the complex XXXHI consists of a band centered at 41.1 ppm from the oxygen-donor ligands and a doublet resonance at 48.5 ppm (d, ‘J(Rh-P) 177 Hz) due to both triphenylphosphine ligands in equivalent environments_ The data suggest the following structure : -
P”3 I
2+
In addition, complex XXXIII undergoes symmetrical bridge cleavage on treatment with triphenylphosphine, giving [Rh(CO) (OPPh3) (PPh&]C104.
Dinuclear complexes of the [Rh,(Ir-OER3)2(C00)~(C104)2 type In analogy with the reactions represented in eq. 7, reaction of the solvate [Rh(COD) (Me&O),]ClO, with a stoichiometric amount of ligands OER, gives the complexes of the type [Rh&-OER,),(COD)J (ClO.&, which are iso-
RESULTS, YIELDS, CONDUCTIVITY
[Rhz{~CP(pMeOC6H4)3)2(C0)2(PPh3)2l(CIC4)2
(Rh2(~OAsPh3)2(CO)2(PI’h3)21(C104)2
[RII~{~-CP(~CIC~R~)~}~(COD)~I(CIC~)~
[Rh201-OPPh3)2(COD)2lClOq)2
[Rh,{~.OP@-Mc,IJ,),},(CCW21(CIO&
~R~~{~OP~~MeOC~H~~~}~~COD~~l~ClO~~~
tRh2(Cc.OAsPh3)2(COD)2l(C104)2
xxxv
XXXVI
XXXVII
XXXVIII
XXXIX
XL
XL1
52
89
999'
19Gb
90 (4.9) 4,6 (4-3)
246 f~
266 b
280 b
GO
32
(3.6) 4.9
73oe
487 b
583 b
637 b
699 b
nc
27Gc
__ _ __.
_ .~__ _ ------
_. .~. .
--.
852s
1146s
1164s
1165s,s11
844s
114Gs
1148vs&
1158,113Gs
1142vs,sh
v (EO)
16Gb
1963
1980
1986
1980
1988
v (CO)
llG2s
..-_-
cm2 mol’l)
IR dnta (cm-l )
AND
146 b
164b
1G2b
2ofic
156b
16Ab
170b
lG0 b
AM (ohm”
Conductivity data
(4.6) 5.8 (5.3) G.1
70
(Z) 3.9
72
58
(4.5) 4.9 (4.2)
81
(3.Q) 6,l
63
(3J) 4.G
3.3
H
(%)
Yield
COMPLEXES [I~l~,(~O~R,),(CO),(PI’h~)~](C104)~
[271. b Meosured in nitromcthnnc solution. c Measured in occtone solution.
[R~~2{~OP(~MeC~H4)~}2(C0)2(PPh~)2l(CIC~)2
XXXIV
c Slope found for Onsager’s equation Ae = A, - DJC
[Rh2~OPph3)2(CO),(PPh3)2l(ClO4)2
(GO.8) 67.7 (87.6) 68.8 (GDJ) 6608 (GG.8) G&2 (64.G) 48-3 (48.1) G2,G (83-O) 5508 (66.2) Gl.,b (61,3) 49.6 (49.3)
49.8 [Rh2{~OP(~ClC~~I4)~)2(CO)2(PPh3)2l(C104)2
C
Found (calcd.) (%)
AND IR DATA FOR TIIE DINUCLEAR
xxx111
xxx11
Complex
[Rh,(~CER,),(CCD),I(CIO4)2
ANALYTICAL
TABLE 3
214
lated as hygroscopic yellow solids (complexes XXXVII-XLI). Their analyses, molar conductivities, yields and IR data are collected in Table 3. Measurements of equivalent conductivities of nitromethane solutions of the complexes XXXVI-XL at various concentrations lead to values of coefficient B in the Onsager equation between those corresponding to l/l and l/2 electrolytes. Evaporation of these solutions gives yellow solids different from the starting materials, which contain nitromethane, and consequently the low B values are not completely unexpected. The 31P NMR spectra of the complexes XXXVIII and XL display a resonance at 40.2 and 41.2 ppm, respectively, similar in position to the resonance of the related complex [Rh2(p-OPPh&(CO),(PPh,)J (C104)2. On the other hand, conductivity data for acetone solutions of [ Rhz(l-(-0_AsPh3)2(COD)2] (ClO,), (XLI) are compatible (B = 999) [28] with the proposed dinuclear structure. Complexes XXXVI-XL1 undergo symmetrical bridge cleavage with triphenylphosphine, giving the mononuclear species [ Rh(COD) (OER,) (PPh,)] CIO,. Experimental C, H and N analyses were carried out with a Perkin-Elmer 240 B microanalyzer. IR spectra were recorded on a Perkin-Elmer 577 spectrophotometer (over the range 4000-200 cm-l) using Nujol mulls between polyethylene sheets or dichloromethane solutions in cells with NaCl windows. Conductivities were measured with a Philips PW 9501/01 conductimeter in ca. 5 X 10e4 M nitromethane or acetone solutions and, when stated, at various concentrations in the range 5 X 10-3-10-4 M. The 31P NMR spectra were recorded on a Varian
FTSOA spectrometer using CDC13 solutions. Chemical shifts are given in ppm relative to external 85% H3P04, with a positive sign indicating a shift to lower field. CRhCl(CGD)l. 13x1, CRh’%CO)W’h,)l. [321, CRhCUCO).l. C331, RhCl(C0) (PPh& [ 341 and [ Rh(COD) (OPyR)2]C104 [ 191 were prepared as described elsewhere. The triarylphosphine oxides were prepared by oxidizing the phosphines in acetone with a slight excess of dilute hydrogen peroxide [ 351, purified by vacuum-melting and recrystallized from acetone_ Triphenylphosphine oxide and triphenylarsine oxide were obtained from Fluka A.G. and recrystallized from acetone. Pyridine Woxides were prepared by standard methods [36,37] and vacuum-distilled or recrystallized before use. The solvents were distilled before use. AlI the reactions were carried out at room temperature and atmospheric pressure with 0.1-0.2 mmol of the reagents_ Preparation of complexes of the type [Rh(COD)(OER,)JClO,
(I-V) The phosphine or arsine oxide (0.40 mmol) was added to a solution of [Rh(COD) (Me2CO),]CI0, (obtained by treating [RhCI(COD)], (0.049 g, 0.10
mmol) with AgC104 (0.041 g, 0.20 mmol) in 10 ml of acetone and removing the precipitated AgCl by filtration). Vacuum evaporation to dryness left yellow solids which were suspended in hexane, filtered off, washed with hexane, and air-dried.
215
Preparation of complexes of the type [Rh(COD)(OER3)(PPh3)]C104
(VI-X)
(i) To an acetone solution of equimolecular amounts of PPh, and the respective phosphine or amine oxide was added a stoichiometric amount of [Rh(COD)(Me,CO),]ClO,. The solutions were vacuum-concentrated to ca. 1 ml and slow addition of ether (VII and X) or hexane (VI, VIII and IX) gave yellow crystalline solids, which were filtered off, washed thoroughly with ether or hexane, and air-dried. (ii) PPh3 (l/l) was added to the above-described dichloromethane solutions of complexes I-V, work-up was as in (i). This method gave higher yields for complexes VII and X (see Table 1).
Preparation of complexes of the type [Rh(CO),(OER,)JCiO, (XI-XIV) (i) Bubbling of carbon monoxide for 15 min through a very concentrated dichloromethane/ether solution of [ Rh(COD) (OPPh,),] C104 led to formation of a pale-yellow crystalline solid. Precipitation was completed by addition of ether and bubbling of carbon monoxide for another 15 min. The resulting crystals were filtered off, washed with ether and air-dried_ Complex [Rh(C0)2(0AsPh3)2]C104 (XII) was synthesized in an analogous manner. (ii) Addition of OP@-CIC,H,), (0.079 g, 0.21 mmol) to an acetone solution of [Rh(CO),(Me,CO),]ClO. (obtained by treating [RhCI(C0)J2 (0.020 g, 0.05 mmol) with AgClO, (0.021 g, 0.10 mmol) in the absence of light and under argon) led to a yellow solution. The precipitated AgCl was filtered off and the filtrate was concentrated to dryness to give an off-yellow oil. This was extracted with ca_ 50 ml of ether, and the extract was evaporated to dryness and the resulting solid washed with hexane and vacuum-dried. Complex XIV was ebtained in a similar way. Complexes XI and XII can be obtained by this method, but in lower yields.
Preparation of complexes of the type [Rh(CO)L,(PPh,)]C104
(XV-XVIiI)
A stoichiometric amount (l/l) of PPh3 in dichloromethane was added to a solution of [Rh(C0)2L2]C10, (obtained by 15 min bubbling of carbon monoxide through a dichloromethane solution of complexes of the type
[Rh(COD)L,]C104, where L = 0ER3 (complexes I-V) or L = OPyR [19]. After 30 min stirring the solution was concentrated under reduced pressure to ca. 1 ml. The complexes were crystallized by slow addition of ether. The yellow solids were filtered off, washed with ether, and air-dried. Complexes in which L was a substituted pyridine N-oxide were crystallized by prolonged stirring with ether_
gave oils which
Preparation of complexes of the type [Rh(CO)L(PPh,)JC104 Complexes XIX-XXVI_ (i) Bubbling of carbon monoxide through a dichloromethane solution of [Rh(COD) (OPPh3)JC104 (0.052 g, 0.06 mmol) and subsequent addition of PPh3 (0.031 g, 0.12 mmol) gave a intense yellow solution. After 30 min stirriug, the solution was evaporated to dryness to give a yellow solid, which was suspended in hexane, filtered off, washed with hexane and vacuum-dried. The other complexes of this type were synthesized analogously using the corresponding [Rh(COD)L,]’ complexes as starting materials, but ether was used as precipitating agent for complexes with substituted pyridine N-oxides.
216 (ii) To an acetone solution of [Rh(CO)(Me,CO)(PPh3)JC104 (0.100 g, 0.12 mmol) was added a stoichiometric amount of the relevant ligand (0.12 mmol). The solution was concentrated to ca. 1 ml, and slow addition of hexane (complexes XIX-XXIII) or ether (complexes XXIV-XXVI) yielded yellow crystalline solids, which were filtered off, washed with hexane or ether, and vacuum-dried_ Complexes XXVII-XXXI_ A stoichiometric amount of the corresponding Oor S-donor ligand (0.10 mmoi) was added to an acetone solution of [Rh(COD)(Me,CO),]C104 (0.10 mmol). Bubbling of carbon monoxide for 10 min through this solution and addition of PPh3 (0.052 g, 0.20 mmol) gave a yellow solution which was stirred for 30 min and vacuum-evaporated to ca. 1 ml. These complexes were precipitated by slow addition of ether as yellow solids, which were filtered off, washed with ether and vacuum-dried_ Preparafion of complexes (,XXXII-XXXVI)
of the type
[Rh,(p-OER,),(CO),(PPh,)J
(CIOJ,
Complexes of this type were each made by two routes, which are illustrated below_ (i) A solution of AgC104 (0.025 g, 0.12 mmol) in 5 ml of acetone was treated with an acetone solution of [RhCl(CO)(PPh,)], (0.052 g, 0.06 mmol) in the absence of light and the mixture was stirred for 15 min. The precipitated AgCl was filtered off and a solution of triphenylphosphine oxide (0.033 g, 0.12 mmol) in 10 ml of acetone was slowly added to the filtrate_ The solution was vacuum-evaporated to dryness and the resulting orange-yellow solid was washed with hexane and vacuum-dried. Complexes XxX11, XXXIV-XXXVI were prepared analogously_ The preparations of these complexes, which are hygroscopic, were carried out under argon. (ii) Bubbling of carbon monoxide for 15 min through dichloromethane solutions of [ Rh(COD) (OERs) (PPh3)]C104 (VI-X), followed by concentration under reduced pressure to ca. 1 ml and addition of hexane gave orange-yellow solids, which were filtered off, washed with hexane, and vacuum-dried_ They were recrystallized twice from dichloromethane solutions by slow addition of hexane. The recrystallized complexes were washed with hexane and vacuumdried_ Preparation of complexes (XXXVrl--XLr)
of the type [Rh2(p-OER3)2(COD)J(CI04),
To an acetone solution of ]Rh(COD) (Me2CO),]C104 under an argon atmosphere was slowly added the stoichiometric amount of an acetone solution of the 0ER3 ligand (Rh/OERS, l/l)_ Evaporation to dryness gave dark-yellow oils which were kept under vacuum for 1 h. Addition of dichloromethane gave yellow solutions, which were vacuum-concentrated to dryness. The resulting yellow solids were washed several times with hexane and vacuum-dried. These complexes are hygroscopic, except for [Rh&-OAsPh,),(COD).1(ClO,),_ References 1 N-M. Karayannis, C.M.Miku.lskiand.L.L. Pytlewski.Inorg.Chim.ActaRev..5(1971) 2 Z_A_ShekaandE.I_Smyavskaya.Usp.Khim.Koord.Soedin.,<1975)113;Chem.Ahstr..85 115909a.
69. (1976)
217 3 CM. hIIkuIski. L-L. PytIewski and N.M. Karayannis. Synth. React. Inorg. Met-org. Chem.. 9 (1979) 401_ 4 M.W.G. De Bolster. C. Boutkan. T.A. Van der Knaap. L. Van Zweeden. I.E. Kortram and W.L. Groeneveld. 2. Anorg. Aug. Chem.. 443 (1978) 269. Chem. Abstr.. 90 (1979) 145122j. 5 DC. Bradley. J-S. Ghotra. F-A. Hart. M-B. Hu&.house and P.R. Raithby. 3. Chem. Sot.. Ddton (1977) 1166. 6 J.G.H. du Preez. B.J. GeIIatIy and M-L. Gibson. J. Chem. Sot.. Dalton (1977) 1062 and references therein. 7 (a) J-H. Burns. Inorg. Chem.. 20 (1981) 3868. (b) N-W. AIcock. S. Espenis. K.W. BagnaB and W. Hdan-Yun. J. Chem. Sot.. Dalton, (1978) 638. 8 L-N. Lomakina. T-1. Ignat’eva. S.A. Pisareva. T. Ya Medved and M-1. Kabachnik. Zh. Anal. Khim.. 35 (1980) 86. Chem. Abstr., 92 (1980) 208352~. 9 F.E. Alekseevna and KS. Vasil’evich. Mater. Vses. Nauchn. Stud. Konf.: Khim.. 13th (1975) 14. Chem. Ahstr.. 86 (1977) 61157d. 10 V.N. DaniIova, M.G. SuIeimano~ra. G.V. ShiIina. G.A. Anistratenko. N.G. Feshchenko. A.P. hlarchenko. G.K. Fedorova and L-1. Samarai. Ukr_ Khim. Zh.. 41 (1075) 1200. Chem. Abstr.. 84 (1976) 65775k. 11 K. Kuemmerle and H. Heise. Ger. Offen. 2.837.022. Chem. Abstr.. 92 (1980) 214359x. 89 (1978) 146443f; ibid.. 78 63.307. Chem. 12 I. Kuriyama. Japan Kokai. 78 63.308. Chem. Abstr.. Abstr.. 89 (1978) 129066t. A.J. FaneIIi and F.C. Rauch. U.S. 3.884.975. Chem. Abstr.. 83 (1975) 115331q:A.J. FaneIIi, G-M_ Blank and F.C. Rauch. U.S. 3.801.639. Chem. Abstr.. 80 (1974) 145478~. 14 J-A. Osbom. F.H. Jardine. J.F. Young and G. Wilkinson. J. Chem. Sot. (1966) 1711. 15 Yu.N. Kukushkin. N.P. Fedyanin and A-1. Mokhov. Zh. Neorg. Khim.. 25 (1980) 784. Chem. Abstr.. 92 (1980) 190561d: Yu.N. Kukushkin. L-1. Danilina. A-1. Osokin and V.P. Botel’nikov. Zh. Neorg. Khim.. 24 (1979) 2253. Chem. Abstr.. 91 (1979) 203593x. 16 G. Bandoh. D.A. Clemente, G. DeganeIIo. G. Carturan. P. Uguaghati and II. BeIIuco. J. Organometal.
13
17 18 19 20 21 22 23
24 25 26 27 28 29 30 31 32 33 34 35 36 37
Chem.. 71 (1974) 125. S.A. Dias. A.W. Downs and W-R. McWhinnie. J. Chem. Sot.. Dalton. (1975) 162. M-V. KIyuev. B-G. Rogachev. Yu.hI. ShuI’ga and h1.L. Khidekel. Izv. Akad. Nauk. SSSR. Ser. Khim.. 8 (1979) 1869. Chem. Abstr.. 91 (1979) 221624n. R. Uson. LA. Oro. hI.A. Ciriano and F.J. Lahoz. J. Organometal. Chem.. 217 (1981) 251. R.R. Schrock and J.A. Oshorn. J. Amer. Chem. Sot.. 93 (1971) 3089. D.hI.L. Goodgame. M. Goodgame and P.J. Hayward. J. Chem. SOL Dalton, (1970) 1352; G.A. RodIey. D.M.L. Goodgame and F.A. Cotton. J. Chem. SIX.. (1965) 1499. J. Peone and L. Vaska. Angew. Chem. Int. Ed.. 10 (1971) 511. W.J. Geary. Coord. Chem. Rev.. 7 (1971) 81. R. Uson. L.A. Oro. hI.A. GarraIda and C. Claver. J. Organometal. Chem.. 105 (1976) 365. A.J. Deeming and I. Rothwell, J. Chem. Sot., Dalton. (1980) 1259. P.S. Pregosin end R-W. Kunz. NMR. Basic Principles and Progress. Springer Verlag. Berlin, vol. 16. 1979. p_ 110 and ss. R-D. Feltham and R.G. Hayter. J. Chem. Sot.. (1964) 4587. R. Us&i. J. Gimeno. J. Fomies and F. Martinez. Inorg. Chfm. Acta. 50 (1981) 173. CM. hIikuIski. X5-J. Strocko. L-L. PytIewski and hI.hl. Labes. J. Inorg. NucI. Chem.. N.M. Karayannis, 33 (1971) 2691; ibid.. 33 (1971) 3185. W.H. Watson. Inorg. Chem.. 8 (1969) 1879 and references therein. G. Giordano and R.H. Crabtree. Inorg. Synth.. 19 (1979) 218. D.F. Steele and T.A. Stephenson. J. Chem. SIX.. Dalton. (1972) 2161. J.A. h¶cCIeverty and G. Wilkinson. Inorg. Synth.. 8 (1966) 211. J.A. McCIeverty and G. Wilkinson. Inorg. Synth., 8 (1966) 214. D-B_ Copley. F_ Fairbrother. J.R. Miller and A. Thompson, Proc. Chem. Sot.. (1964) 300. E. Ochiai. J. Org. Chem.. 18 (1953) 534. E. Ochfai. Aromatic Amine Oxides, Elsevier. Amsterdam, 1967.