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Conformational isomerism in π-C3H4XCo(CO)2PY3
INORG.
NUCL.
CHEM.
LETTERS
Vol. 9,
pp. 75-79,
1973.
Perlamon
Preae. Printed
in
Great
CONFORMATIONAL ISGMERISM IN ~-C3H4XCO(O3) 2PY3 H. L. Clarke and N. J.. Fitzpatrick Department of Chemistry, University College, Dublin, Ireland (Received 27 July 1972)
Conformational isomerism of two types has been reported in organometallic compounds: (i) isomerism due to restricted rotation about a M-L bond as in, for example, ~-CsHsFe(CO)2SiCIzCH 3 (i), ~-C5HsFe(O3)z (o-CsHs) (2) and ~-CsHsMO(O3)z(~-C3H4 R) (S), (ii) isomerism due to restricted rotation within a phosphorus-containing ligand as in ccmpounds
of the type ~-C6H6Cr(CO)2L (4) and ~-C7HTMo(CO)IL (5). Evidence for isomerism of the l a t t e r type in .~-C3H4XCO(CO)2PY3 (X=H, 1-CH3,2-(~t3, 2-C1 and Y--OC6Hs, 0(3-I3 and C6H5) is given herein, based on the m u l t i p l i c i t y of v (CO) bands ifl the infra-red spectra.
The
effects of the ~-allyl moiety, of the substituents on it in the I- and 2-positions, and of the Y groups are shown.
Experiment al The air-sensitive ~-C~4XCo(CO)3 compounds were prepared by a published method (6,7) and purified by fractional distillation. ~-C3HsCo(OD)zP(C6H5) 3 was prepared by the addition of P(C6Hs) 3 to an ethereal solution of ~-C3H5Co(O3)3 (8).
The orange crystalline material
was isolated by removal of solvent from the filtered reaction mixture.
75
l~'imin,
76
CONFORMATIONAL ISOMERISM
Vol. 9, No. 1
Analysis: required: C 66.03%, H 4.7~; found: C 66.24%, H 4.88%. Most other products were very unstable in air. Infra-red spectra were recorded using Perkin-Elmer 457 and 337 spectrometers in the 17OO-2200 an -I region, and were calibrated using -I water vapour and DCI. Accuracy is expected to be _+ 1 cm
Results and Discussion A M(CO) 2 group has C2v symmetry and is therefore expected to have carbonyl vibrations of species A 1 + BI, the former corresponding to a symmetric stretching of the carbonyls {S(AI) =~(r I + r2) where S -symmetry coordinate, and r corresponds to stretching of a CO group} and therefore having higher frequency.
For all the compounds consid1 ered, the antis)ametric B 1 mode {S(BI) --~-(r! - r2)} is more intense than the A 1 (the ratio is ~ 3:2)
Thus the angle OC-M-CO is - iOO °
(9).
The ge~netry of the ~-C3H4XCo(CO)2PY 3 is therefore similar to that of ~-CsHsFe(CO)2Sn(C6Hs) 3 (iO) and Co (CO) 2 (NO)PY 3
(ii).
The Table gives the carbonyl bands found in each compound, along with other examples where isomerism did not occur.
Four main observat-
ions are noted from the Table: (i) compounds containing P(OC6Hs) 3 show most isomerism; (ii) the most pronounced isomerism in ~-C3H4XCo(CO)2PY 3 occurs when X = 2-CI and 2-CH3; (iii) the P(O(~2)3CCH 3 compound has only two carbonyl bands; (iv) c~upounds containing P(C6Hs)3, P(OCH3) 3 show some degree of isomerism, while those containing P(n-C4Hg) 3 do not. Two possible explanations of conformational isomerism have been proposed (4) for the compounds ~-C6H6Cr(CO)2PY3:
Vol. 9, No. 1
CONFORMATIONAL ISOMERISM
77
TABLE
Infra-red spectra of ~-C3H4XCo(CO)2PY3 in the v(CO) region. Solvent is cyclohexane in each case.
Relative peak heights are
in parentheses (asym. refers to asymmetry of the main band, not to a separate band).
X
Y=OC6HS
Y=OO-I3
Y=C6H 5
Y:n-C4H 9
B1
A1
H 1-Ot 3
1974(10) 1962(10)
1984(6)asym 1971(7)asym
2019(6) 2012(6)
2-013 2-Ci
1962(i0) 1978(10)
1972(7) 1992(8.5)
2026(5) 2039(5)
H 1-C}I3 2-CH 3 2-CI
1953(10) 1946(10) 1949(10) 1965(10)
1961(5)asym 1956(5)asym 1959(5)asym 1975(5)asym
H 1-CIt3 2-(J-I3 2-CI
1951(10) 1939(10) 1939(10) 1952(2) 1958(I0) 1974(2)
H l-(]-I3 2-CH3 2-Cl
2-(J-13-~T-CzH4Co(CO)2 P(O(M2)3C(}I3
1933(10) 1923(10) 1923(10) 1947(i0)
1968(10)
2014(6) 2029(6)
2007(5) 2002(5) 2005(5) 2015(5) 2003(8) 1994(8)
1993(8)
2004(sh)
2008(8)
2020(sh)
1992(7) 1982(7) 1982(7) 2001(7) 2015(7)
78
CONFORMATIONAL ISOMERISM
Vol. 9, No:, 1
(a) restricted rotation about the M-P bond, involving eclipsed and staggered conformations; (b) restricted rotation within the ligand. Point (i) can be explained by (b) but not by (a), since if the isomerism were due to the eclipsed and staggered conformations of the groups attached to the cobalt and phosphorus, the smaller Y-OfH 3 would be expected to show more isomerism.
However if the isomerism is
due to restricted rotation within the ligand the larger Y=OC6H 5 group is expected to produce more isomerism. in the case of n-CsH5Fe(CO)IPY 3
It is interesting to note that
(4),the Y=O(~ 3 compound showed more
isomerism than the Y=OC6H S compound.
This is explained by (b), where
the CsH5 group fixes the phenyl group in P(OC6Hs) 3 and allows the smaller (H 3 group in P(OfM3) 3 restricted rotation, whereas the smaller ~-C3H 5 group allows restricted rotation of the phenyl group and relatively free rotation of the methyl group.
Thus the amount of isomerism is decided
by the relative sizes of the n-moiety and the phosphorus ligand.
If
the ligand has groups that are too large isomerism does not occur due to no rotation; on the other hand, small groups also fail to produce isomerism due to free rotation. Point (ii) adds further support to (b) as the explanation of isomerism in n-C3H4XCo(CO)2PY3, since an eclipsed conformation would not be more favoured in the X=2-CH 3 and 2-CI compounds than in the X=H compound, asst~ing similar orientations of the n-C3H4X ligadds.
However, according
to (b), the n-allyl compound is expected to allow relatively free rotation about the P-O-C bonds, because of its small size, while the CH 3 and C1 groups in the 2-position restrict the rotation more and, therefore, give more o f t h e s e c o n d i s o m e r .
Vol. 9, No. 1
CONFORMATIONAL ISOMERISM
79
(iii) This shows that isomerism is not due to different orientations of the ~-allyl group. (iv) This may also be explained by isomerism within the PY3 ligand. The decrease in the amount of isomerism when Y=C6H 5 and O(}13 relative to that when Y=OC6H 5 is explained on the basis of restricted rotation within the ligand.
The O(}I5 group is smaller than the OC6H 5 group, and hence
the former produces less isomerism due to freer rotation. having Y=
C6H 5
shows less isomerism than the Y=
OC6H 5
The compound compound due
to the phenyl groups in the former being less free to rotate.
No isomer-
ism occurs in the Y--n-C4H9 compound due to a fixed conformation. Variable temperature studies in n-octane showed that the ratio of the 1962 and 1972 cm -I bands in 2-(}~3-CsH4Co(CO)2P(OC6Hs) 3 varied little with temperature, changing from i0:7 to 10:8 over the range 15°-60° The spectra were also obtained using CS 2 as solvent.
However, cyclo-
hexane gave better resolution of bands than the more polar CS 2. References i. W. Jetz and W.A.G. Graham, J. Amer. Chem. Soc., 89, 2773 (1967). 2.
F.A. Cotton and T. J. Marks, ~bid., 91, 7525 (1969).
3. J. W. Faller and M. J. Incorvia, Inorg. Chem., 7, 840 (1968). 4.
D.A. Brown, H. J. Lyons and A. R. Manning, Inorg. Chim. Agta, 4, 428 (1970).
5.
T. W. Beall and L. W. Houk, Inorg. Chem., 11, 915 (1972).
6.
R. F. Heck and D. S. Breslow, J, Amer. Chem. Soc., 82, 750 (1960).
7.
W. R. McClellan, H. H. Hoehn, H. N. Cripps, E. L. Muetterties and B. W. Howk, ibid., 8.~.3,1601 (1961).
8.
R. F. Heck, ib4d., 85, 655 (1963).
9.
W. Beck, A. Melnikoff and R. Stahl, Chem. Bet., 99, 5721 (1966).
iO.
R. F. Bryan, J. Chem. Soc. (A), 192 (1967).
Ii.
A. Poletti, A. Foffani and R. Cataliotti, Spectrochim. Acta, 26A, 1063 (1970).
NUCL.
CHEM.
LETTERS
Vol. 9,
pp. 75-79,
1973.
Perlamon
Preae. Printed
in
Great
CONFORMATIONAL ISGMERISM IN ~-C3H4XCO(O3) 2PY3 H. L. Clarke and N. J.. Fitzpatrick Department of Chemistry, University College, Dublin, Ireland (Received 27 July 1972)
Conformational isomerism of two types has been reported in organometallic compounds: (i) isomerism due to restricted rotation about a M-L bond as in, for example, ~-CsHsFe(CO)2SiCIzCH 3 (i), ~-C5HsFe(O3)z (o-CsHs) (2) and ~-CsHsMO(O3)z(~-C3H4 R) (S), (ii) isomerism due to restricted rotation within a phosphorus-containing ligand as in ccmpounds
of the type ~-C6H6Cr(CO)2L (4) and ~-C7HTMo(CO)IL (5). Evidence for isomerism of the l a t t e r type in .~-C3H4XCO(CO)2PY3 (X=H, 1-CH3,2-(~t3, 2-C1 and Y--OC6Hs, 0(3-I3 and C6H5) is given herein, based on the m u l t i p l i c i t y of v (CO) bands ifl the infra-red spectra.
The
effects of the ~-allyl moiety, of the substituents on it in the I- and 2-positions, and of the Y groups are shown.
Experiment al The air-sensitive ~-C~4XCo(CO)3 compounds were prepared by a published method (6,7) and purified by fractional distillation. ~-C3HsCo(OD)zP(C6H5) 3 was prepared by the addition of P(C6Hs) 3 to an ethereal solution of ~-C3H5Co(O3)3 (8).
The orange crystalline material
was isolated by removal of solvent from the filtered reaction mixture.
75
l~'imin,
76
CONFORMATIONAL ISOMERISM
Vol. 9, No. 1
Analysis: required: C 66.03%, H 4.7~; found: C 66.24%, H 4.88%. Most other products were very unstable in air. Infra-red spectra were recorded using Perkin-Elmer 457 and 337 spectrometers in the 17OO-2200 an -I region, and were calibrated using -I water vapour and DCI. Accuracy is expected to be _+ 1 cm
Results and Discussion A M(CO) 2 group has C2v symmetry and is therefore expected to have carbonyl vibrations of species A 1 + BI, the former corresponding to a symmetric stretching of the carbonyls {S(AI) =~(r I + r2) where S -symmetry coordinate, and r corresponds to stretching of a CO group} and therefore having higher frequency.
For all the compounds consid1 ered, the antis)ametric B 1 mode {S(BI) --~-(r! - r2)} is more intense than the A 1 (the ratio is ~ 3:2)
Thus the angle OC-M-CO is - iOO °
(9).
The ge~netry of the ~-C3H4XCo(CO)2PY 3 is therefore similar to that of ~-CsHsFe(CO)2Sn(C6Hs) 3 (iO) and Co (CO) 2 (NO)PY 3
(ii).
The Table gives the carbonyl bands found in each compound, along with other examples where isomerism did not occur.
Four main observat-
ions are noted from the Table: (i) compounds containing P(OC6Hs) 3 show most isomerism; (ii) the most pronounced isomerism in ~-C3H4XCo(CO)2PY 3 occurs when X = 2-CI and 2-CH3; (iii) the P(O(~2)3CCH 3 compound has only two carbonyl bands; (iv) c~upounds containing P(C6Hs)3, P(OCH3) 3 show some degree of isomerism, while those containing P(n-C4Hg) 3 do not. Two possible explanations of conformational isomerism have been proposed (4) for the compounds ~-C6H6Cr(CO)2PY3:
Vol. 9, No. 1
CONFORMATIONAL ISOMERISM
77
TABLE
Infra-red spectra of ~-C3H4XCo(CO)2PY3 in the v(CO) region. Solvent is cyclohexane in each case.
Relative peak heights are
in parentheses (asym. refers to asymmetry of the main band, not to a separate band).
X
Y=OC6HS
Y=OO-I3
Y=C6H 5
Y:n-C4H 9
B1
A1
H 1-Ot 3
1974(10) 1962(10)
1984(6)asym 1971(7)asym
2019(6) 2012(6)
2-013 2-Ci
1962(i0) 1978(10)
1972(7) 1992(8.5)
2026(5) 2039(5)
H 1-C}I3 2-CH 3 2-CI
1953(10) 1946(10) 1949(10) 1965(10)
1961(5)asym 1956(5)asym 1959(5)asym 1975(5)asym
H 1-CIt3 2-(J-I3 2-CI
1951(10) 1939(10) 1939(10) 1952(2) 1958(I0) 1974(2)
H l-(]-I3 2-CH3 2-Cl
2-(J-13-~T-CzH4Co(CO)2 P(O(M2)3C(}I3
1933(10) 1923(10) 1923(10) 1947(i0)
1968(10)
2014(6) 2029(6)
2007(5) 2002(5) 2005(5) 2015(5) 2003(8) 1994(8)
1993(8)
2004(sh)
2008(8)
2020(sh)
1992(7) 1982(7) 1982(7) 2001(7) 2015(7)
78
CONFORMATIONAL ISOMERISM
Vol. 9, No:, 1
(a) restricted rotation about the M-P bond, involving eclipsed and staggered conformations; (b) restricted rotation within the ligand. Point (i) can be explained by (b) but not by (a), since if the isomerism were due to the eclipsed and staggered conformations of the groups attached to the cobalt and phosphorus, the smaller Y-OfH 3 would be expected to show more isomerism.
However if the isomerism is
due to restricted rotation within the ligand the larger Y=OC6H 5 group is expected to produce more isomerism. in the case of n-CsH5Fe(CO)IPY 3
It is interesting to note that
(4),the Y=O(~ 3 compound showed more
isomerism than the Y=OC6H S compound.
This is explained by (b), where
the CsH5 group fixes the phenyl group in P(OC6Hs) 3 and allows the smaller (H 3 group in P(OfM3) 3 restricted rotation, whereas the smaller ~-C3H 5 group allows restricted rotation of the phenyl group and relatively free rotation of the methyl group.
Thus the amount of isomerism is decided
by the relative sizes of the n-moiety and the phosphorus ligand.
If
the ligand has groups that are too large isomerism does not occur due to no rotation; on the other hand, small groups also fail to produce isomerism due to free rotation. Point (ii) adds further support to (b) as the explanation of isomerism in n-C3H4XCo(CO)2PY3, since an eclipsed conformation would not be more favoured in the X=2-CH 3 and 2-CI compounds than in the X=H compound, asst~ing similar orientations of the n-C3H4X ligadds.
However, according
to (b), the n-allyl compound is expected to allow relatively free rotation about the P-O-C bonds, because of its small size, while the CH 3 and C1 groups in the 2-position restrict the rotation more and, therefore, give more o f t h e s e c o n d i s o m e r .
Vol. 9, No. 1
CONFORMATIONAL ISOMERISM
79
(iii) This shows that isomerism is not due to different orientations of the ~-allyl group. (iv) This may also be explained by isomerism within the PY3 ligand. The decrease in the amount of isomerism when Y=C6H 5 and O(}13 relative to that when Y=OC6H 5 is explained on the basis of restricted rotation within the ligand.
The O(}I5 group is smaller than the OC6H 5 group, and hence
the former produces less isomerism due to freer rotation. having Y=
C6H 5
shows less isomerism than the Y=
OC6H 5
The compound compound due
to the phenyl groups in the former being less free to rotate.
No isomer-
ism occurs in the Y--n-C4H9 compound due to a fixed conformation. Variable temperature studies in n-octane showed that the ratio of the 1962 and 1972 cm -I bands in 2-(}~3-CsH4Co(CO)2P(OC6Hs) 3 varied little with temperature, changing from i0:7 to 10:8 over the range 15°-60° The spectra were also obtained using CS 2 as solvent.
However, cyclo-
hexane gave better resolution of bands than the more polar CS 2. References i. W. Jetz and W.A.G. Graham, J. Amer. Chem. Soc., 89, 2773 (1967). 2.
F.A. Cotton and T. J. Marks, ~bid., 91, 7525 (1969).
3. J. W. Faller and M. J. Incorvia, Inorg. Chem., 7, 840 (1968). 4.
D.A. Brown, H. J. Lyons and A. R. Manning, Inorg. Chim. Agta, 4, 428 (1970).
5.
T. W. Beall and L. W. Houk, Inorg. Chem., 11, 915 (1972).
6.
R. F. Heck and D. S. Breslow, J, Amer. Chem. Soc., 82, 750 (1960).
7.
W. R. McClellan, H. H. Hoehn, H. N. Cripps, E. L. Muetterties and B. W. Howk, ibid., 8.~.3,1601 (1961).
8.
R. F. Heck, ib4d., 85, 655 (1963).
9.
W. Beck, A. Melnikoff and R. Stahl, Chem. Bet., 99, 5721 (1966).
iO.
R. F. Bryan, J. Chem. Soc. (A), 192 (1967).
Ii.
A. Poletti, A. Foffani and R. Cataliotti, Spectrochim. Acta, 26A, 1063 (1970).