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Metal vapour synthesis of zero-valent nickel phosphine complexes and their characterization by 31P NMR spectroscopy
Polyhedron
Vol.8,
0277-5387/89 $3.&l+ .30 0 1988 Pergmon Press plc
No. 1, pp. 13-15, 1989
Printed in Great Britain
METAL
VAPOUR SYNTHESIS OF ZERO-VALENT NICKEL PHOSPHINE COMPLEXES AND THEIR CHARACTERIZATION BY 31P NMR SPECTROSCOPY KEITH J. FISHER
and ELMER
C. ALYEA”
Guelph-Waterloo Centre for Graduate Work in Chemistry, Guelph Campus, Department of Chemistry and Biochemistry, Guelph, Ontario, Canada NlG 2Wl (Received 3 May 1988 ; accepted 21 June 1988) Abstract-A series of Ni[P(p-X-&H,),], complexes has been synthesized by the cocondensation of Ni atoms with various solvents at 77 K, followed by warm-up to room temperature in the presence of the excess para-substituted triphenylphosphine. The complexes show a linear correlation between the coordination chemical shift, A6(31P), and both the pK, of the phosphine ligands and the Hammett sigma function of the para-substituent. The ditertiary phosphine complexes Ni(PhaP(CHz),PPh& (n = l-4), also prepared by metal vapour synthesis, display the expected 31P chemical shift “chelate ring” effect for the n = 2 case.
EXPERIMENTAL
Zero-valent nickel complexes of phosphines and phosphites have been extensively studied.‘,’ General syntheses utilize displacement of an organic ligand such as cyclopentadiene or cyclooctadiene from its nickel complex or reduction of a nickel(I1) complex with NaBH,, A1R3, Zn dust or excess ligand ; however, the metal vapour technique has been employed for a few Ni(PR3), complexes.3 The importance of steric effects in determining the exchange equilibria among phosphorus ligands in these complexes as well as the ease of substitution of CO from Ni(CO), led Tohnan to the cone angle concept for describing the bulk of phosphorus donor ligands.“6 Tolman observed that electronic effects strongly influenced the rate constants for ligand dissociation from Ni(PR3)+ ’ In initiating metal vapour synthesis (MVS) in our laboratory, we have explored the experimental conditions for the formation of zero-valent nickelphosphine complexes. Particular interest was focussed on para-substituted triarylphosphines to allow a correlation of 31P chemical shifts for the Nip(P-X-C6H4)3]4 complexes with parameters that reflect the electronic effects of the para-substituent.
A Torrovap MVS system was employed to cocondense nickel atoms with various solvents. Typically, ca 0.4 g of Ni were evaporated using the electron gun source and a pressure of less than lo-’ tori-, with about 100 cm3 of the solvent being codeposited on the inner wall of the 5 dm3 rotating flask, which was immersed in a liquid dinitrogen bath. Following the cocondensation, normally requiring about 2 h, the reaction flask was allowed to warm until the matrix melted. The solvated metal solution was syphoned into a receiving flask held at 77 K. The flask was filled with dinitrogen and removed from the MVS system. All or portions of these solutions of solvated nickel atoms were reacted with excess phosphine ligands at various temperatures (see text) to produce the nickel(O) phosphine complexes. All solvents were dried using standard procedures and degassed before use by freeze-pumpthaw cycles. Dinitrogen was dried and deoxygenated by passage over heated BTS catalyst. Samples for NMR spectral measurements were sealed in 10 mm tubes due to the dioxygen sensitivity of some of the complexes. 3’P NMR spectra were recorded at ambient temperature on a Bruker WH-400 spectrometer in the FT mode using 85% H,PO, as external standard.
*Author to whom correspondence should be addressed. 13
14
K. J. FISHER
and E. C. ALYEA
RESULTS AND DISCUSSION
As briefly mentioned in early MVS reports, Ni(PR& complexes are readily prepared by cocondensing nickel atoms with various solvents at 77 K and adding excess phosphine ligands. Allowing for the loss of approximately 40% of the evaporated Ni atom that deposits on the electron focussing shield, conversion of solvated Ni atoms to Ni(PR& is essentially quantitative if the phosphine ligands are added at or only slightly above 77 K. Suitable solvents used in our experiments included toluene, toluene/lO% dimethoxyethane, acetone, methylcyclohexane and methylcyclohexane/20% dimethoxyethane. Of these solutions, only that of toluene/lO% dimethoxyethane still showed the production of a small amount of Ni(PR& when warmed to 253 K before addition of the phosphine ligand. Klabunde and coworkers7 have shown earlier that toluene solutions of Ni atoms decompose to Ni metal if allowed to warm above 193 K. We observe that if Ni atoms are cocondensed with toluene containing norbornadiene the resultant brown, clear solution appears stable at room temperature, and reacts with dppe to give Ni(dppe)z. The red solution obtained by codepositing Ni atoms with methylcyclohexane containing indene at 77 K is also stable at room temperature and does not react with PPh3 or dppe. The nature of the solution species, probably Ni(indene)*, is under further investigation. Similarly, we note that the dark brown solution obtained by evaporating Ni atoms into an acetone solution containing acetylacetone is stable at room temperature. Reaction of this solution occurs with both PPh3 and dppe to give paramagnetic species of yet unknown structure ; in the presence of dioxygen rather than phosphine, Ni(acac), is isolated in 86% yield. 31P NMR spectral data for the six Ni[P(p-XC6H5)3]4complexes prepared in this study by MVS are compiled in Table 1. Only Ni[P(C,H,),], was studied previously, with Tohnan4 reporting that no detectable 31P resonance was observed for a satu-
rated solution of the complex in toluene, even with excess P(C6H5)3 being added. This type of behaviour, with fast exchange and extensive dissociation according to the equation NiL, I
NiL3 + L
was later contrasted with that exhibited by other nickel(O) phosphine and phosphite complexes. 5Our data, obtained in toluene solutions in the presence of excess phosphine, indicates that Ni[P(C,H,),], shows an extreme in this type of behaviour. All of the other substituted phenylphosphine complexes give readily detected 3’P resonances even in the presence of only a slight excess of the phosphine. A plot of the coordination chemical shift A6(6NiL, - 6L) with the basicity of the phosphine, given by pK,, is shown in Fig. 1 for the six com38
36 -.
34.
A(PP~) 32.
_:>__; & 287 -0.600
-0.600
-0.400
-0.200
PK,
(C&J&’
2.73 1.03 1.97 3.84 4.57 8.65
bCGH4)3P
281
:
I
:
0123456789
(p-F&H&P (p-Me&H&P (p-MeOC,Hd3P (p-Me2NC6H,)3P a Solvent, toluene.
0.200
0.
:
:
:
:
:
:
PK, Fig. 1. Correlations of 3’P data with pK, and op for substituted triphenylphosphine nickel(O) complexes.
63’p” 0.0 0.23 0.06 -0.14 - 0.27 -0.60
0.000
u
Table 1. 3’P NMR data of Ni(PR,), compounds (NiL3 L
(1)
-4.2 -7.3 -8.3 -7.0 -8.7 - 10.6
6 3
‘P(NiL4)” 25.5 22.5 22.6 25.2 24.3 25.4
A6 29.7 29.8 30.9 32.2 33.0 36.0
15
Synthesis of zero-valent nickel phosphine complexes Table 2. 3’P NMR data of NiL2 compounds (L = diphosphine)
L (Ph 2P)2CH 2, dppm (Ph2P)&H,, dppe (Ph,P)&Hs, dppp (Ph,P),C.,H,, dppb Ni(PMePh,),b
63’P (Free ligand) -21.4 -12.3 - 17.2 - 15.8 -26.2
d3’P (NiL,)”
A6
5.6 44.7 12.7 17.7 4.7
27.0 67.0 29.9 33.5 30.9
a Solvent, toluene. bLiterature values are - 27.9, + 2.9 and + 30.8, respectively. 5
plexes;also shownisa plota~d~tith the Hanunett sigma function of thepuru-substituent on the phenyl groug. S&e the Qi~rysphosphines are &k&y similar (0 = 145”),6 the two correlations were expected to demonstrate the differing ekctronic properties of the ligands. As expected, a good correlation was found between increased 46, i.e. more deshielding of P, and the increased basicity of the triarylphosphine. The departure of the data for Ni[p(C,H,),], from the correlation is attributed to the greater rate of exchange and degree of dissociation, which would lead to a smaller AS due to ian avera&n+ 0%‘9 &n&s. T%e apparenk k~er Cbissotia~~Dn‘jD, the J_D-X--q&,),P j% =cj, F> ccom_rhexesis no’rewofiby anb may be’mkcalive DS c&Wrii~ I+?& W-J? A &fr, lfi +br_ &&W9Wg%fz;~U%E&.z&f~~~?r~S. %..%&k“ -z &ii possibility, what is nevertheless clear from our 31P IXMR data is that the chemical shifts o6served for this series of ?-&mp-x< ,$f 5)3j4c~piexes dir&ly reflect the g-bonding ability of the substituted trirU-&?h0s&k&. %g&W&. 31P NMR spectral data for the four Wi~fi,P(CH,),PPfi& (n = 14) complexes prepared in this MVS study are given in Table 2. The AS values are near 30 ppm for three of the complexes (n = 1, 3 and 4), i.e. similar to those found for the Ni[P(p-X-C,H,),], complexes as well as the closer analogue, Ni(PMePh,),. The Ni(dppe), complex, however, shows the expected anomalous
behatiour welt docuueatkd hbr ditetiq
p&oaphiae
complexes having five-membered chelate rings. ’ The riog 0zmtrikmon in this c.rkse,By Wm~arison to the chemical shift value for Ni(PMePh&, the closest monodentate analogue, is 36 ppm. Acknowle4qementsK.J.F. thanks the University of Khartoum, Sudan for a research leave. E.C.A. thanks NSERC of Canada for an operating grant and support of the Southwestern Ontario NMR Centre. Purchase of the Torrovap system was made possible by a BILD grant from the Pruvince of Ontario and an award from the P&&E&? s~&FuIz~ d~U2kn&~ &G&&L
‘I. C. M%mdra an6 5. Ctiti, Zero-v&en1 ‘zompounbs of Metals. Academic Press, New York (1974). 2. C. A. McAuliiEe and W. Levasoq, Phoqhtie,. Arsine Isffd S~&ne C0m~kw i$&E Tr&m Ekmtx~. Elsevier, New York (1979). 3. I *R. Blackharaw and D . YcwxLg,,Metal vapour sp thesis in Organometallic Chemistry. Springer, Berlin (1979), 4. C. A. Tolman, J. Am. Chem. Sot. 1970,92,2956. 5. C. A. Tolman, W. C. Seidle and L. W. Gosser, J. Am. Chem. Sot. 1974, %, 53. 6. C. A. Tolman, Chem. Rev. 1977,77,3 13. 7. K. J. Klabunde, H. F. Efner, T. 0. Murdock and R. Roppel, J. Am. Chem. Sot. 1976,98,1021. 8. P. Garrou, Chem. Rev. 1981,81,229.
Vol.8,
0277-5387/89 $3.&l+ .30 0 1988 Pergmon Press plc
No. 1, pp. 13-15, 1989
Printed in Great Britain
METAL
VAPOUR SYNTHESIS OF ZERO-VALENT NICKEL PHOSPHINE COMPLEXES AND THEIR CHARACTERIZATION BY 31P NMR SPECTROSCOPY KEITH J. FISHER
and ELMER
C. ALYEA”
Guelph-Waterloo Centre for Graduate Work in Chemistry, Guelph Campus, Department of Chemistry and Biochemistry, Guelph, Ontario, Canada NlG 2Wl (Received 3 May 1988 ; accepted 21 June 1988) Abstract-A series of Ni[P(p-X-&H,),], complexes has been synthesized by the cocondensation of Ni atoms with various solvents at 77 K, followed by warm-up to room temperature in the presence of the excess para-substituted triphenylphosphine. The complexes show a linear correlation between the coordination chemical shift, A6(31P), and both the pK, of the phosphine ligands and the Hammett sigma function of the para-substituent. The ditertiary phosphine complexes Ni(PhaP(CHz),PPh& (n = l-4), also prepared by metal vapour synthesis, display the expected 31P chemical shift “chelate ring” effect for the n = 2 case.
EXPERIMENTAL
Zero-valent nickel complexes of phosphines and phosphites have been extensively studied.‘,’ General syntheses utilize displacement of an organic ligand such as cyclopentadiene or cyclooctadiene from its nickel complex or reduction of a nickel(I1) complex with NaBH,, A1R3, Zn dust or excess ligand ; however, the metal vapour technique has been employed for a few Ni(PR3), complexes.3 The importance of steric effects in determining the exchange equilibria among phosphorus ligands in these complexes as well as the ease of substitution of CO from Ni(CO), led Tohnan to the cone angle concept for describing the bulk of phosphorus donor ligands.“6 Tolman observed that electronic effects strongly influenced the rate constants for ligand dissociation from Ni(PR3)+ ’ In initiating metal vapour synthesis (MVS) in our laboratory, we have explored the experimental conditions for the formation of zero-valent nickelphosphine complexes. Particular interest was focussed on para-substituted triarylphosphines to allow a correlation of 31P chemical shifts for the Nip(P-X-C6H4)3]4 complexes with parameters that reflect the electronic effects of the para-substituent.
A Torrovap MVS system was employed to cocondense nickel atoms with various solvents. Typically, ca 0.4 g of Ni were evaporated using the electron gun source and a pressure of less than lo-’ tori-, with about 100 cm3 of the solvent being codeposited on the inner wall of the 5 dm3 rotating flask, which was immersed in a liquid dinitrogen bath. Following the cocondensation, normally requiring about 2 h, the reaction flask was allowed to warm until the matrix melted. The solvated metal solution was syphoned into a receiving flask held at 77 K. The flask was filled with dinitrogen and removed from the MVS system. All or portions of these solutions of solvated nickel atoms were reacted with excess phosphine ligands at various temperatures (see text) to produce the nickel(O) phosphine complexes. All solvents were dried using standard procedures and degassed before use by freeze-pumpthaw cycles. Dinitrogen was dried and deoxygenated by passage over heated BTS catalyst. Samples for NMR spectral measurements were sealed in 10 mm tubes due to the dioxygen sensitivity of some of the complexes. 3’P NMR spectra were recorded at ambient temperature on a Bruker WH-400 spectrometer in the FT mode using 85% H,PO, as external standard.
*Author to whom correspondence should be addressed. 13
14
K. J. FISHER
and E. C. ALYEA
RESULTS AND DISCUSSION
As briefly mentioned in early MVS reports, Ni(PR& complexes are readily prepared by cocondensing nickel atoms with various solvents at 77 K and adding excess phosphine ligands. Allowing for the loss of approximately 40% of the evaporated Ni atom that deposits on the electron focussing shield, conversion of solvated Ni atoms to Ni(PR& is essentially quantitative if the phosphine ligands are added at or only slightly above 77 K. Suitable solvents used in our experiments included toluene, toluene/lO% dimethoxyethane, acetone, methylcyclohexane and methylcyclohexane/20% dimethoxyethane. Of these solutions, only that of toluene/lO% dimethoxyethane still showed the production of a small amount of Ni(PR& when warmed to 253 K before addition of the phosphine ligand. Klabunde and coworkers7 have shown earlier that toluene solutions of Ni atoms decompose to Ni metal if allowed to warm above 193 K. We observe that if Ni atoms are cocondensed with toluene containing norbornadiene the resultant brown, clear solution appears stable at room temperature, and reacts with dppe to give Ni(dppe)z. The red solution obtained by codepositing Ni atoms with methylcyclohexane containing indene at 77 K is also stable at room temperature and does not react with PPh3 or dppe. The nature of the solution species, probably Ni(indene)*, is under further investigation. Similarly, we note that the dark brown solution obtained by evaporating Ni atoms into an acetone solution containing acetylacetone is stable at room temperature. Reaction of this solution occurs with both PPh3 and dppe to give paramagnetic species of yet unknown structure ; in the presence of dioxygen rather than phosphine, Ni(acac), is isolated in 86% yield. 31P NMR spectral data for the six Ni[P(p-XC6H5)3]4complexes prepared in this study by MVS are compiled in Table 1. Only Ni[P(C,H,),], was studied previously, with Tohnan4 reporting that no detectable 31P resonance was observed for a satu-
rated solution of the complex in toluene, even with excess P(C6H5)3 being added. This type of behaviour, with fast exchange and extensive dissociation according to the equation NiL, I
NiL3 + L
was later contrasted with that exhibited by other nickel(O) phosphine and phosphite complexes. 5Our data, obtained in toluene solutions in the presence of excess phosphine, indicates that Ni[P(C,H,),], shows an extreme in this type of behaviour. All of the other substituted phenylphosphine complexes give readily detected 3’P resonances even in the presence of only a slight excess of the phosphine. A plot of the coordination chemical shift A6(6NiL, - 6L) with the basicity of the phosphine, given by pK,, is shown in Fig. 1 for the six com38
36 -.
34.
A(PP~) 32.
_:>__; & 287 -0.600
-0.600
-0.400
-0.200
PK,
(C&J&’
2.73 1.03 1.97 3.84 4.57 8.65
bCGH4)3P
281
:
I
:
0123456789
(p-F&H&P (p-Me&H&P (p-MeOC,Hd3P (p-Me2NC6H,)3P a Solvent, toluene.
0.200
0.
:
:
:
:
:
:
PK, Fig. 1. Correlations of 3’P data with pK, and op for substituted triphenylphosphine nickel(O) complexes.
63’p” 0.0 0.23 0.06 -0.14 - 0.27 -0.60
0.000
u
Table 1. 3’P NMR data of Ni(PR,), compounds (NiL3 L
(1)
-4.2 -7.3 -8.3 -7.0 -8.7 - 10.6
6 3
‘P(NiL4)” 25.5 22.5 22.6 25.2 24.3 25.4
A6 29.7 29.8 30.9 32.2 33.0 36.0
15
Synthesis of zero-valent nickel phosphine complexes Table 2. 3’P NMR data of NiL2 compounds (L = diphosphine)
L (Ph 2P)2CH 2, dppm (Ph2P)&H,, dppe (Ph,P)&Hs, dppp (Ph,P),C.,H,, dppb Ni(PMePh,),b
63’P (Free ligand) -21.4 -12.3 - 17.2 - 15.8 -26.2
d3’P (NiL,)”
A6
5.6 44.7 12.7 17.7 4.7
27.0 67.0 29.9 33.5 30.9
a Solvent, toluene. bLiterature values are - 27.9, + 2.9 and + 30.8, respectively. 5
plexes;also shownisa plota~d~tith the Hanunett sigma function of thepuru-substituent on the phenyl groug. S&e the Qi~rysphosphines are &k&y similar (0 = 145”),6 the two correlations were expected to demonstrate the differing ekctronic properties of the ligands. As expected, a good correlation was found between increased 46, i.e. more deshielding of P, and the increased basicity of the triarylphosphine. The departure of the data for Ni[p(C,H,),], from the correlation is attributed to the greater rate of exchange and degree of dissociation, which would lead to a smaller AS due to ian avera&n+ 0%‘9 &n&s. T%e apparenk k~er Cbissotia~~Dn‘jD, the J_D-X--q&,),P j% =cj, F> ccom_rhexesis no’rewofiby anb may be’mkcalive DS c&Wrii~ I+?& W-J? A &fr, lfi +br_ &&W9Wg%fz;~U%E&.z&f~~~?r~S. %..%&k“ -z &ii possibility, what is nevertheless clear from our 31P IXMR data is that the chemical shifts o6served for this series of ?-&mp-x< ,$f 5)3j4c~piexes dir&ly reflect the g-bonding ability of the substituted trirU-&?h0s&k&. %g&W&. 31P NMR spectral data for the four Wi~fi,P(CH,),PPfi& (n = 14) complexes prepared in this MVS study are given in Table 2. The AS values are near 30 ppm for three of the complexes (n = 1, 3 and 4), i.e. similar to those found for the Ni[P(p-X-C,H,),], complexes as well as the closer analogue, Ni(PMePh,),. The Ni(dppe), complex, however, shows the expected anomalous
behatiour welt docuueatkd hbr ditetiq
p&oaphiae
complexes having five-membered chelate rings. ’ The riog 0zmtrikmon in this c.rkse,By Wm~arison to the chemical shift value for Ni(PMePh&, the closest monodentate analogue, is 36 ppm. Acknowle4qementsK.J.F. thanks the University of Khartoum, Sudan for a research leave. E.C.A. thanks NSERC of Canada for an operating grant and support of the Southwestern Ontario NMR Centre. Purchase of the Torrovap system was made possible by a BILD grant from the Pruvince of Ontario and an award from the P&&E&? s~&FuIz~ d~U2kn&~ &G&&L
‘I. C. M%mdra an6 5. Ctiti, Zero-v&en1 ‘zompounbs of Metals. Academic Press, New York (1974). 2. C. A. McAuliiEe and W. Levasoq, Phoqhtie,. Arsine Isffd S~&ne C0m~kw i$&E Tr&m Ekmtx~. Elsevier, New York (1979). 3. I *R. Blackharaw and D . YcwxLg,,Metal vapour sp thesis in Organometallic Chemistry. Springer, Berlin (1979), 4. C. A. Tolman, J. Am. Chem. Sot. 1970,92,2956. 5. C. A. Tolman, W. C. Seidle and L. W. Gosser, J. Am. Chem. Sot. 1974, %, 53. 6. C. A. Tolman, Chem. Rev. 1977,77,3 13. 7. K. J. Klabunde, H. F. Efner, T. 0. Murdock and R. Roppel, J. Am. Chem. Sot. 1976,98,1021. 8. P. Garrou, Chem. Rev. 1981,81,229.