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NMR study of (C6H5)3-nPXn (X = Cl, Br, I; n = 0-3) and (C6H6)3-n PXnCr(CO)5 compounds
Journal a f Molecular
Shucture, 65 (1980) 239-247 Elsevier Scientific Publishing Company, Amsterdam
NMR STUDY OF (C6H5)3-nP& Px, Cr(CO)5 COMPOUNDS
E_ VINCENT*,
L. VERDONCK
and Inorganic
(Received
1979)
6 September
Printed in The Netherlands
(X = Cl, Br, I; n = O-3)
and G. P. VAN
Laboratory for General B-9000 Gen f (Belgium)
-
AND (C,H,),-,
DER KELEN**
Chemistry-B,
UniuersiCy
of Gerzt, Kr&shn
271,
ABSTRACT The 3’P chemical shift of the (C, H, )a_nPX, ligands (X = Cl, Br, I; n = O-3) is dominated by the electronegativity of the substituents. TIbonding is only important for derivatives with three strongly electronegative substituents. The J’P chemical shift of the corresponding complexes (C, H, ),-, PX, Cr(CO), is governed by the simultaneous effects of the electronegativity, steric hindrance and ‘JTbonding. The resonance parameter, s’, indicates an increasing (pti, +dp )n and (dc, +dp)n electron delocalization with halogen substitution_ INTRODUCTION
In a recent study the "P [l] and 13C [Z] NMR parameters of the (C6H5)3-n PCl, CrtCO), complexes were reported. The irregular trends observed may be due to d-orbital effects. In the literature there is some controversy about the influence of the d-orbital participation on the 6 31P chemical shifts. Ionin [S] states that the 6~~ of tricoordinate phosphorus derivatives is mainly determined by the hybridizational state of the phosphorus lone pair, whereas for tetracoordinate phosphorus compounds the density of the d electrons on the phosphorus atom is the dominant factor. Filling of the 3cl orbitals leads to a high field shift of the resonance signal. According to Letcher and Van Wazer [4] , however, d-orbital participation would result in a deshielding. In this report the bromide and the iodide analogues are studied to further elucidate the problem. The stqdy of the 613c of the phenyl carbon and carbonyl carbon atoms could lend additional support to the proposed interpretations.
*Scholar of the Institute for Scientific Research in Industry and Agriculture (I.W.O.N.L.). **To whom correspondence shouldbe addressed. 0022~2860/80/0000-0000~$02.25
0 1980
Elsevier Scientific Publishing Company
EXPERIMENTAL
Synthesis Ligands: L = (CgHs)=, PX, (n = O-3; X = Cl, Br, I) Where possible, commercially available ligands were used after appropriate purification. ( C6H5 )2 PBr was prepared using a halogen exchange reaction [ 5-91 between (C6H5)2PCl and PBr, giving a yellow viscous liquid with b.p. 146°C (2 mm Hg). C,H, PI2 could be prepared by reaction of anhydrous LiI with phenyIphosphorusdichloride [IO] . In another method the (CsHS P)4 was treated with I, producing the C,H, PI, via the intermediate (C, HS )* P212 [ 111. In both methods a dark brown liquid with b-p. 138-140°C (1 mm Hg) was obtained. Cleaving the phosphorus-phosphorus bond in (C6H5)4P2 with I2 produced (C6H5)2PI 1121 a brown-red oil with b-p. 181°C (5 mm Hg). Complexes: LCr(CO)S The synthesis of the chromium carbonyl complexes was based on the method developed by Strohmeier and co-workers 113-l 51. Cr(C0)6
9
{ Cr(CO), *} T2F CO + THFCr(CO),
5 LCr(CO)s
+ THF
Instrumentation The basic instrument was a Bruker HFXs NMR spectrometer extended with a Fourier-Transform module with spectrum accumulation. The 13C spectra were recorded at 22.63 MHz on saturated solutions in CDC13. The chemical shift was calculated with respect to TMS using the convention 6 = “S -
Yef
(ppm)
- lo6
The 3’ P spectra were recorded at 36.43 MHz on saturated solutions in CDC13. The reference used here in the chemical shift notation was P(OCH3)3. RESULTS
AND
31P chemical
DISCUSSION
shift
In general the interpretation of the 3’ P chemical shifts is based on the paramagnetic term [16] of the magnetic screening of the 31P nuclei (p=--
2e2h2 3m2c2
.-.
1 AE
241
1
where AE represents the mean excitation energy of the electrons and r~ , 0 1 are mean values, with r equal to the distance between the nucleuspand 0 7‘3d respectively a 3p, 3d electron. P and D represent the electron unbalance in the p and d orbit& respectively. Thus on substitution, the 3’P chemical shift will be affected by three major factors: (a) Electronegative substituents cause a contraction of the orbitals, increasing and resulting in deshielding. (b) Substitution in a symmetric compound causes a rehybridization. The valence angles become different and the electron imbalance increases. This effect is also deshielding. The angles and indirectly the imbalance are also influenced by steric factors. Hence angle distortion by introduction of heavy atoms or voluminous groups leads to deshielding (steric factor). (c) Formation of n bonds has a marked influence on the d-orbital population of phosphorus and will affect r and D. As mentioned in the Introduction the effect of 7~bonding on 631~is still a point of discussion. 31P chemical shift in (C6H5)3-nPXn (n = O-3; X = Cl, Br, I) Substitution of a phenyl group by an X atom increases &lp (deshielding) (Table 1). The nearly constant increment of S+ as n increases indicates a dominating influence of the electronegativity effect. The atomic charge on P calculated by an electronegativity equalization procedure [ 1’7, 181 correlates well with 6+
(Fig. 1, Table 1) and offers additional
proof.
(A model
with
bond orders of unity and formal charges of zero is used throughout.) However, the 6~1~sequence expected for the PX3 derivatives is not found. The sequence 631pBr > Cl > I can be rationalized by accepting a substantial (Pc, + c&)x contribution in the P-Cl bonds. Since only strongly electronegative atoms can contract the 3d(P) orbitals sufficiently to make them suitable for ?r bonding, (px +c&).rr contributions are only important for X=F and Cl. The data for n = 0, 1, 2, where the sequence is as expected on electronegativity grounds (Cl > Br > I), show that in trivalent phosphorus compounds 7rbonding can be neglected except for derivatives with three strong electronegative substituents. 31P chemical shift in (C,H,)3-,PX,Cr(C0)5 (n = O-3) derivatives (Fig. 2) (C,H,),-,PCL,Cr(CO), derivatives. Substitution of a phenyl group in (C6H, )3PCr(CO), by a chlorine atom causes a downfield shift of 95 ppm (Table 1). The major contribution to this shift arises from the effect of the 1 electronegative chlorine atom increasing the factor 7 - Further, the break0 down of the local CgV symmetry around P and the inequality of the valence
242 TABLE
1
3’P NMR chemical plexes, LCr( CO),
shifts of the free ligands,
L = (C,H,),_,PX,
Charge
Free ligand PCl, C, H, PCI, (C,H, ),PCl (C, H, ), P PBr, C, H, PBr 2 (C,H, )?PBr
on P calculated
on Pa
Free ligand
Complex:LCr(CO),
+ 78.8 + 20.1 -59.3 -146.1
+ 45.6 + 62.7 + 10.6 -85.6
0.15745 0.10194 0.04647 -0.00895
+ 86.1 + 10.4 -68.9
-45.2 + 22.9 -4.7
0.11077 0.07125 0.03135
+ 31.9 -43.7 -101.8
PI, C, H, PI, (C, H, ), PI
A
and the com-
631~(p.p.m.)
L = (C,H,),-,PX,
=Charge
(n = O-3),
0.05891 0.03655 0.01393
-258.7 -84.3 -49.5
by an electronegativity
equalization
procedure
(refs.
17, 18).
631p
*lOC l-
0l-
-100
l-
(I
-200
I-
-J
, (I
I
2
Fig. 1. Plot of 531~vs. n for the ligands
3
(C, H, ),-,
”
PX, .
angles O$O # Ok1 should increase the 3p imbalance “P”. However, since the Van der Waals radii {phenyl = 1.7 A (perpendicular to the ring) and Cl = 1.8 A ) are comparable, this contribution cannot be important.
243
0
1
2
Fig. 2. Plot of 631~ vs. n for the complexes
3
-il
(C, H, )J-nPX, Cr(CO), .
According to the “resonance” parameter 6’ (see also 13C) bpo + dr)n bonding increases on substitution and according to the AS’ values we can also deduce an increase in (do, +c&)n bonding. The energies of the P3a! and C13p orbitals are comparable; so (PC1+c&)n bonding also seems a reasonable possibility. However, with only one Cl atom on P, the effects of the contributions of the latter on the chemical shift are probably slight. Disubstitution causes a downfield shift of 52.1 p.p.m. By comparison with the monochloro derivative the contribution to the chemical shift due to the 3p imbalance P can be considered almost constant. However, the “resonance” parameter 6’ and 06’ (see also ’ 3C) indicate an increase of the (p. + dp )n and (d,+ dp)n contributions. The same statement holds for (PC1+ dp)n bonding according to ESCA measurements [19]. As a result the paramagnetic influence of the electronegativity effect is partially compensated for by the TT bonding. On trisubstitution the chemical shift trend is reversed: an upfield shift of 17.1 p.p.m. is observed. The decrease of the imbalance P because of the restored CjV symmetry, together with a net increase of the n bonding now overcompensate the deshielding contributions.
244
(C,H, )s-nPX, Cr(COJs (X = Br, I). Comparing the chemical shift of the corresponding bromide and iodide derivatives with the chlorides, we observe for monosubstitution a chemical shift sequence 6,i> a,,> 6r in accordance with the electronegativity. However, for X = I the low field shift on phenyl substitution cannot be ascribed only to the difference in electronegativity between the phenyl group and the I atom. Steric hindrance causing angle distortion and affecting the imbalance P is related to the Van der Waals radii. So the imbalance P will be the dominant factor for the iodide derivatives and will also be significant for the bromides. On_disubstitution the sequence remains 6,, >6,,>6, but, compared to the monosubstitution product, 6, already shows a high field shift. None of the discussed effects can be responsible for this behaviour. However, ligands with extensive electron clouds surrounding the atom under study increase the shielding; this is called the “heavy atom” effect [20]. Obviously the sequence for this effect is I > Br > Cl. On trisubstitution the CgV symmetry is restored and achieves shielding through the decrease of the imbalance P. For X = Br, I, (px + cZr)nis insignificant due to the large energy gap between the 3& and respectively 4p,, and 5p1 orbitals. Thus, the strong high field shifts must be ascribed to the imbalance P and heavy atom effects. 13C NMR In general, the contributions of the pammagnetic term are supposed the dominating factor in determining the 13C chemical shifts. (JP
=-
e21z2
2 m2c2
l
1 -. AE
to be
1 .p 0 r3p
(For symbolism see 31P chemical shift). 13C NMR of the carbonyl carbon atoms From the experimental chemical shift data collected in Table 2 the following trends can be derived: increases on substitution of a carbonyl group by a phosphine ligand. (1) +o of the group or atoms on (2) 613co decreases with increasing electronegativity phosphorus. in axial position > 613~~ in equatorial position. (3) +o Substitution of a CO group by a phosphine ligand increases the population of the antibonding 7~* orbitals of the carbonyl groups and causes a decrease of the mean excitation energy, AE, as stated earlier [Zl, 221 for comparable compounds. So the mechanism has a deshielding effect on the 61Jo. Stepwise substitution of a phenyl group in the phosphine ligand by halogen atoms has a shielding effect, because under the influence of the electronegative group on P the P3d orbitals contract, which results in an increase of the r-acceptor capacity. The subsequent redistribution of the n-electron
245 TABLE
2
I3C NMR chemical shifts and J( 3*P J3C) in LCr(CO), L
coupling constants of the carbonyl
carbon atoms
J3I p_l3
%xw2q.) b.p.m.)
PCI, C, H, PCI, (C,H, ),PCl (C,H, ),P
213.5 214.9 216.7 218.4
17.1 15.4 15.3 13.2
219.0 220.4 221.9 223.1
4.3 2.7 5.5 7.1
PSr, C,H,PBr, (C,H, ),PBr
214.3 215.4 216.9
14.7 13.9 14.0
219.2 220.8 221.1
6.1 4.2 4.9
PI,a C,H,PI, (GH, )zPT
217.0 217.8
12.8 14.2
221.6 222.9
3.1 4.9
Cr(CO), aSolubility
aax.)
(Hz)
:613~~
=
211.9 p.p.m.
in organic solvents too low.
density with a decrease of the (Cr + CO)n bonding, causes an increase of AE. Additional proof of this hypothesis can be found in the observation that on substitution the trend of the 613, changes is decreasing according Cl > Br > I. This sequence reflects the change of the effective nuclear charge on P and the subsequent orbital contraction, with the electronegativity of the substituents. The ‘ITorbit& of an axial car-bony1 group can come into interaction with two d orbitals (dzy and d, ) of the central Cr atom whereas the equatorial carbonyl group interacts with only one d orbital (d,, or d,). Redistribution of -rr-electrondensity will affect the axial carbonyl group more than the equatorial one resulting in 613c(axs > 83cces.j.
13C NMR of the carbon atoms in the phenyl moiety 13C spectra of monosubstituted benzene derivatives can give information about the resonance effect between the ring system and the substituent. The corrected chemical shift 123, 241 of the carbon atom in para position 6’ = is a good measure of the resonance effect since it is admitted = &z(4) - h(3.5) that the contributions to the 13C chemical shift from the inductive effect are identical in the meta and para positions. All 6’ data (Table 3) for the free ligands as well as for the complexes are positive, indicating a (ptiqg+dP)n delocalization from the ring to phosphorus. Substitution of a phenyl group by a more electronegative X atom induces a positive charge on the P atom. For the free ligands this can be compensated by two mechanisms @ring+dp)7r or (px + dr)n . However, it is generaIly
TABLE
3
"C NMR chemical shiftsandJ( 31P--"C)couplingconstantsofthe phenylcarbonatomsin theligandsL =(C,H,),-,PX, (n = 0;1,2)and inthecomplexesLCr(CO), Compound
(G H, ),P
(C,H, ),PCl C, H,PCl,
(C,H,),PCr(CO), (C,H,),PCICr(CC), C,H,PCi,Cr(CO), (C,H,),PBr C,H,PBr,
(C, H, ): PBrCr(CO), C,H,PBrzCr(CO),
C(2,6) J
6
J
6
J
6
J
139.2 140.3 142.1 136.9 140.3 144.6
12.0 33.0 52.5 36.1 27.6 22.7
135.5 133.6 131.7 134.4 132.3 129.5
19.8 23.9 31.0 11.5 14.2 15.9
130.2 130.4 130.6 130.2 130.3 130.5
7.3 7.1 8.1 9.5 10.3 11.0
130.4 131.9 134.3 131.8 133.0 133.9
0 0.2 0 1.5 0 3.7 2.4 1.6 1.4 1.7 2.7 1.2 1.7 3.4 -0.3
139.5 139.5 143.6
60.3 24.4 13.7
134.6 132.9 132.8 130.1
23.7 31.0 13.9 15.6
130.7 130.7 130.2 130.2
7.1 7.6 10.0 10.3
132.5 134.6 132.9 133.8
0 0 2.0 2.2
1.8 3.9 2.7 0.9 3.6 -0.3
135.4 134.4 133.6 131.1
23.4 28.8 13.2 13.9
130.3 130.4 130.1 129.9
6.1 7.1 9.8 9.8
131.9 133.8 132.8 133.3
0 0 2.2 2.2
1.6 3.4 2.7 3.4
(C,H, ),pI C,H,P12 (C,H,)zPIWCO), C,H,PI:Cr(CO), “6’
= 6C(4)
-
k(85).
C(3,5)
C(1) 6
138.5
bA6’
=
20.5
h'(complex)-6'
(free
C(4) Wa
ASnb
1.1 0.0
ligand).
accepted that (px +&)n contributions are only important for X = F, Cl and can be neglected for X = Br, I (see also 3’ P NMR). Furthermore, the 6’ data indicate an increasing &,,p +dp)n contribution with progressive halogen substitution in accordance with an increasing positive charge on P. On coordination with the transition metal Cr the positive charge on P becomes more pronounced. For the compensation of the induced charge a supplementary mechanism (de + dp)n is now operating. On substitution the 6’ data of the complexes parallel those of the i?ee ligands. However, the difference between successive substitution steps decreases indicating a partial compensation of the positive charge on P by (da+- dp)n contributions. data decrease with halogen substitution and The A 6’ = 6 ‘complex - Glligand even become negative. This is probably due to the increasing importance of the n-bonding contribution from the transition metal. Indeed we can presume that a d-d orbital overlap will be more effective than a p-d overlap. So on halogen substitution the (dti +dp)n contribution increases while the (pring+dp)~ contribution is pulled back. ACKNOWLEDGEMENTS
The authors thank F. Persyn for cooperation in recording the NMR spectra. E. Vincent is extremely grateful to the I.W.O.N.L. for a research scholarship.
247 REFERENCES 1 M. F. Guns, E. G. Claeys and G. P. Van der Kelen, J. Mol. Struct., 54 (1979) 101. 2 M. F. Guns, E. G. Claeys and G. P. Van der Kelen, J. Mol. Struct., 53 (1979) 45. 3 B. I. Ionin, J. Gen. Chem. U.S.S.R., 38 (1968) 1618. 4 J. H. Letcher and J. R. Van Wazer, J. Chem. Phys., 44 (1966) 815. 5 W. Kuchen and W. Gruenewald, Chem. Ber., 98 (1965) 480. 6 W. Kuchen and W. Gruenewald, Angew. Chem., 75 (1963) 576. 7 H. H. Anderson, J. Am. Chem. Sot., 75 (1953) 1576. 8 J. Meisenheimer, Justus Liebigs Ann. Chem., 449 (1926) 213. 9 D. Seyferth and K. A. Braendle, J. Am. Chem. Sot., 83 (1961) 2055. 10 N. G. Feshchenko, T. V. Kovaleva and A. V. Kirsanov, J. Gen. Chem. U.S.S.R., 39 (1969) 2133. 11 H. Hoffmann and R. Gruenewald, Chem. Ber., 94 (1961) 186. 12 H. Hoffmann and R. Gruenewald, Chem. Ber., 93 (1960) 861. 13 W. Strohmeier and K. Gerlach, Chem. Ber., 94 (1961) 398. 14 W. Strohmeier. Angew. Chem., 21 (1964) 873. 15 W. Strohmeier and F. J. Mueller, Chem. Ber., 102 (1969) 3608. 16 C. T. Jameson and H. S. Gutowsky, J. Chem. Phys., 40 (1964) 1714. 17 W. L. Jolly and W. B. Perry, J. Am. Chem. Sot., 95 (1973) 5442. 18 W. L. Jolly and W. B. Perry, Inorg. Chem., 13 (1974) 2686. 19 E. Vincent, unpublished results. 20 G. C. Levy and G. L. Nelson, “C NMR for Organic Chemists, J. Wiley, 1972. 21 G. M. Bodner and L. J. Todd, Inorg. Chem., 13 (1974) 1335. 22 P. S. Braterman, D. W. Milne, E. W. Randall and E. Rosenberg, J. Chem. Sot. Dalton Trans., 10 (1973) 1027. 23 C. P. Nash and G. E. Maciel, J. Phys. Chem., 68 (1974) 832. 24 G. E. Maciel and J. J. Natterstad, J. Chem. Phys., 42 (1965) 2427.
Shucture, 65 (1980) 239-247 Elsevier Scientific Publishing Company, Amsterdam
NMR STUDY OF (C6H5)3-nP& Px, Cr(CO)5 COMPOUNDS
E_ VINCENT*,
L. VERDONCK
and Inorganic
(Received
1979)
6 September
Printed in The Netherlands
(X = Cl, Br, I; n = O-3)
and G. P. VAN
Laboratory for General B-9000 Gen f (Belgium)
-
AND (C,H,),-,
DER KELEN**
Chemistry-B,
UniuersiCy
of Gerzt, Kr&shn
271,
ABSTRACT The 3’P chemical shift of the (C, H, )a_nPX, ligands (X = Cl, Br, I; n = O-3) is dominated by the electronegativity of the substituents. TIbonding is only important for derivatives with three strongly electronegative substituents. The J’P chemical shift of the corresponding complexes (C, H, ),-, PX, Cr(CO), is governed by the simultaneous effects of the electronegativity, steric hindrance and ‘JTbonding. The resonance parameter, s’, indicates an increasing (pti, +dp )n and (dc, +dp)n electron delocalization with halogen substitution_ INTRODUCTION
In a recent study the "P [l] and 13C [Z] NMR parameters of the (C6H5)3-n PCl, CrtCO), complexes were reported. The irregular trends observed may be due to d-orbital effects. In the literature there is some controversy about the influence of the d-orbital participation on the 6 31P chemical shifts. Ionin [S] states that the 6~~ of tricoordinate phosphorus derivatives is mainly determined by the hybridizational state of the phosphorus lone pair, whereas for tetracoordinate phosphorus compounds the density of the d electrons on the phosphorus atom is the dominant factor. Filling of the 3cl orbitals leads to a high field shift of the resonance signal. According to Letcher and Van Wazer [4] , however, d-orbital participation would result in a deshielding. In this report the bromide and the iodide analogues are studied to further elucidate the problem. The stqdy of the 613c of the phenyl carbon and carbonyl carbon atoms could lend additional support to the proposed interpretations.
*Scholar of the Institute for Scientific Research in Industry and Agriculture (I.W.O.N.L.). **To whom correspondence shouldbe addressed. 0022~2860/80/0000-0000~$02.25
0 1980
Elsevier Scientific Publishing Company
EXPERIMENTAL
Synthesis Ligands: L = (CgHs)=, PX, (n = O-3; X = Cl, Br, I) Where possible, commercially available ligands were used after appropriate purification. ( C6H5 )2 PBr was prepared using a halogen exchange reaction [ 5-91 between (C6H5)2PCl and PBr, giving a yellow viscous liquid with b.p. 146°C (2 mm Hg). C,H, PI2 could be prepared by reaction of anhydrous LiI with phenyIphosphorusdichloride [IO] . In another method the (CsHS P)4 was treated with I, producing the C,H, PI, via the intermediate (C, HS )* P212 [ 111. In both methods a dark brown liquid with b-p. 138-140°C (1 mm Hg) was obtained. Cleaving the phosphorus-phosphorus bond in (C6H5)4P2 with I2 produced (C6H5)2PI 1121 a brown-red oil with b-p. 181°C (5 mm Hg). Complexes: LCr(CO)S The synthesis of the chromium carbonyl complexes was based on the method developed by Strohmeier and co-workers 113-l 51. Cr(C0)6
9
{ Cr(CO), *} T2F CO + THFCr(CO),
5 LCr(CO)s
+ THF
Instrumentation The basic instrument was a Bruker HFXs NMR spectrometer extended with a Fourier-Transform module with spectrum accumulation. The 13C spectra were recorded at 22.63 MHz on saturated solutions in CDC13. The chemical shift was calculated with respect to TMS using the convention 6 = “S -
Yef
(ppm)
- lo6
The 3’ P spectra were recorded at 36.43 MHz on saturated solutions in CDC13. The reference used here in the chemical shift notation was P(OCH3)3. RESULTS
AND
31P chemical
DISCUSSION
shift
In general the interpretation of the 3’ P chemical shifts is based on the paramagnetic term [16] of the magnetic screening of the 31P nuclei (p=--
2e2h2 3m2c2
.-.
1 AE
241
1
where AE represents the mean excitation energy of the electrons and r~ , 0 1 are mean values, with r equal to the distance between the nucleuspand 0 7‘3d respectively a 3p, 3d electron. P and D represent the electron unbalance in the p and d orbit& respectively. Thus on substitution, the 3’P chemical shift will be affected by three major factors: (a) Electronegative substituents cause a contraction of the orbitals, increasing and resulting in deshielding. (b) Substitution in a symmetric compound causes a rehybridization. The valence angles become different and the electron imbalance increases. This effect is also deshielding. The angles and indirectly the imbalance are also influenced by steric factors. Hence angle distortion by introduction of heavy atoms or voluminous groups leads to deshielding (steric factor). (c) Formation of n bonds has a marked influence on the d-orbital population of phosphorus and will affect r and D. As mentioned in the Introduction the effect of 7~bonding on 631~is still a point of discussion. 31P chemical shift in (C6H5)3-nPXn (n = O-3; X = Cl, Br, I) Substitution of a phenyl group by an X atom increases &lp (deshielding) (Table 1). The nearly constant increment of S+ as n increases indicates a dominating influence of the electronegativity effect. The atomic charge on P calculated by an electronegativity equalization procedure [ 1’7, 181 correlates well with 6+
(Fig. 1, Table 1) and offers additional
proof.
(A model
with
bond orders of unity and formal charges of zero is used throughout.) However, the 6~1~sequence expected for the PX3 derivatives is not found. The sequence 631pBr > Cl > I can be rationalized by accepting a substantial (Pc, + c&)x contribution in the P-Cl bonds. Since only strongly electronegative atoms can contract the 3d(P) orbitals sufficiently to make them suitable for ?r bonding, (px +c&).rr contributions are only important for X=F and Cl. The data for n = 0, 1, 2, where the sequence is as expected on electronegativity grounds (Cl > Br > I), show that in trivalent phosphorus compounds 7rbonding can be neglected except for derivatives with three strong electronegative substituents. 31P chemical shift in (C,H,)3-,PX,Cr(C0)5 (n = O-3) derivatives (Fig. 2) (C,H,),-,PCL,Cr(CO), derivatives. Substitution of a phenyl group in (C6H, )3PCr(CO), by a chlorine atom causes a downfield shift of 95 ppm (Table 1). The major contribution to this shift arises from the effect of the 1 electronegative chlorine atom increasing the factor 7 - Further, the break0 down of the local CgV symmetry around P and the inequality of the valence
242 TABLE
1
3’P NMR chemical plexes, LCr( CO),
shifts of the free ligands,
L = (C,H,),_,PX,
Charge
Free ligand PCl, C, H, PCI, (C,H, ),PCl (C, H, ), P PBr, C, H, PBr 2 (C,H, )?PBr
on P calculated
on Pa
Free ligand
Complex:LCr(CO),
+ 78.8 + 20.1 -59.3 -146.1
+ 45.6 + 62.7 + 10.6 -85.6
0.15745 0.10194 0.04647 -0.00895
+ 86.1 + 10.4 -68.9
-45.2 + 22.9 -4.7
0.11077 0.07125 0.03135
+ 31.9 -43.7 -101.8
PI, C, H, PI, (C, H, ), PI
A
and the com-
631~(p.p.m.)
L = (C,H,),-,PX,
=Charge
(n = O-3),
0.05891 0.03655 0.01393
-258.7 -84.3 -49.5
by an electronegativity
equalization
procedure
(refs.
17, 18).
631p
*lOC l-
0l-
-100
l-
(I
-200
I-
-J
, (I
I
2
Fig. 1. Plot of 531~vs. n for the ligands
3
(C, H, ),-,
”
PX, .
angles O$O # Ok1 should increase the 3p imbalance “P”. However, since the Van der Waals radii {phenyl = 1.7 A (perpendicular to the ring) and Cl = 1.8 A ) are comparable, this contribution cannot be important.
243
0
1
2
Fig. 2. Plot of 631~ vs. n for the complexes
3
-il
(C, H, )J-nPX, Cr(CO), .
According to the “resonance” parameter 6’ (see also 13C) bpo + dr)n bonding increases on substitution and according to the AS’ values we can also deduce an increase in (do, +c&)n bonding. The energies of the P3a! and C13p orbitals are comparable; so (PC1+c&)n bonding also seems a reasonable possibility. However, with only one Cl atom on P, the effects of the contributions of the latter on the chemical shift are probably slight. Disubstitution causes a downfield shift of 52.1 p.p.m. By comparison with the monochloro derivative the contribution to the chemical shift due to the 3p imbalance P can be considered almost constant. However, the “resonance” parameter 6’ and 06’ (see also ’ 3C) indicate an increase of the (p. + dp )n and (d,+ dp)n contributions. The same statement holds for (PC1+ dp)n bonding according to ESCA measurements [19]. As a result the paramagnetic influence of the electronegativity effect is partially compensated for by the TT bonding. On trisubstitution the chemical shift trend is reversed: an upfield shift of 17.1 p.p.m. is observed. The decrease of the imbalance P because of the restored CjV symmetry, together with a net increase of the n bonding now overcompensate the deshielding contributions.
244
(C,H, )s-nPX, Cr(COJs (X = Br, I). Comparing the chemical shift of the corresponding bromide and iodide derivatives with the chlorides, we observe for monosubstitution a chemical shift sequence 6,i> a,,> 6r in accordance with the electronegativity. However, for X = I the low field shift on phenyl substitution cannot be ascribed only to the difference in electronegativity between the phenyl group and the I atom. Steric hindrance causing angle distortion and affecting the imbalance P is related to the Van der Waals radii. So the imbalance P will be the dominant factor for the iodide derivatives and will also be significant for the bromides. On_disubstitution the sequence remains 6,, >6,,>6, but, compared to the monosubstitution product, 6, already shows a high field shift. None of the discussed effects can be responsible for this behaviour. However, ligands with extensive electron clouds surrounding the atom under study increase the shielding; this is called the “heavy atom” effect [20]. Obviously the sequence for this effect is I > Br > Cl. On trisubstitution the CgV symmetry is restored and achieves shielding through the decrease of the imbalance P. For X = Br, I, (px + cZr)nis insignificant due to the large energy gap between the 3& and respectively 4p,, and 5p1 orbitals. Thus, the strong high field shifts must be ascribed to the imbalance P and heavy atom effects. 13C NMR In general, the contributions of the pammagnetic term are supposed the dominating factor in determining the 13C chemical shifts. (JP
=-
e21z2
2 m2c2
l
1 -. AE
to be
1 .p 0 r3p
(For symbolism see 31P chemical shift). 13C NMR of the carbonyl carbon atoms From the experimental chemical shift data collected in Table 2 the following trends can be derived: increases on substitution of a carbonyl group by a phosphine ligand. (1) +o of the group or atoms on (2) 613co decreases with increasing electronegativity phosphorus. in axial position > 613~~ in equatorial position. (3) +o Substitution of a CO group by a phosphine ligand increases the population of the antibonding 7~* orbitals of the carbonyl groups and causes a decrease of the mean excitation energy, AE, as stated earlier [Zl, 221 for comparable compounds. So the mechanism has a deshielding effect on the 61Jo. Stepwise substitution of a phenyl group in the phosphine ligand by halogen atoms has a shielding effect, because under the influence of the electronegative group on P the P3d orbitals contract, which results in an increase of the r-acceptor capacity. The subsequent redistribution of the n-electron
245 TABLE
2
I3C NMR chemical shifts and J( 3*P J3C) in LCr(CO), L
coupling constants of the carbonyl
carbon atoms
J3I p_l3
%xw2q.) b.p.m.)
PCI, C, H, PCI, (C,H, ),PCl (C,H, ),P
213.5 214.9 216.7 218.4
17.1 15.4 15.3 13.2
219.0 220.4 221.9 223.1
4.3 2.7 5.5 7.1
PSr, C,H,PBr, (C,H, ),PBr
214.3 215.4 216.9
14.7 13.9 14.0
219.2 220.8 221.1
6.1 4.2 4.9
PI,a C,H,PI, (GH, )zPT
217.0 217.8
12.8 14.2
221.6 222.9
3.1 4.9
Cr(CO), aSolubility
aax.)
(Hz)
:613~~
=
211.9 p.p.m.
in organic solvents too low.
density with a decrease of the (Cr + CO)n bonding, causes an increase of AE. Additional proof of this hypothesis can be found in the observation that on substitution the trend of the 613, changes is decreasing according Cl > Br > I. This sequence reflects the change of the effective nuclear charge on P and the subsequent orbital contraction, with the electronegativity of the substituents. The ‘ITorbit& of an axial car-bony1 group can come into interaction with two d orbitals (dzy and d, ) of the central Cr atom whereas the equatorial carbonyl group interacts with only one d orbital (d,, or d,). Redistribution of -rr-electrondensity will affect the axial carbonyl group more than the equatorial one resulting in 613c(axs > 83cces.j.
13C NMR of the carbon atoms in the phenyl moiety 13C spectra of monosubstituted benzene derivatives can give information about the resonance effect between the ring system and the substituent. The corrected chemical shift 123, 241 of the carbon atom in para position 6’ = is a good measure of the resonance effect since it is admitted = &z(4) - h(3.5) that the contributions to the 13C chemical shift from the inductive effect are identical in the meta and para positions. All 6’ data (Table 3) for the free ligands as well as for the complexes are positive, indicating a (ptiqg+dP)n delocalization from the ring to phosphorus. Substitution of a phenyl group by a more electronegative X atom induces a positive charge on the P atom. For the free ligands this can be compensated by two mechanisms @ring+dp)7r or (px + dr)n . However, it is generaIly
TABLE
3
"C NMR chemical shiftsandJ( 31P--"C)couplingconstantsofthe phenylcarbonatomsin theligandsL =(C,H,),-,PX, (n = 0;1,2)and inthecomplexesLCr(CO), Compound
(G H, ),P
(C,H, ),PCl C, H,PCl,
(C,H,),PCr(CO), (C,H,),PCICr(CC), C,H,PCi,Cr(CO), (C,H,),PBr C,H,PBr,
(C, H, ): PBrCr(CO), C,H,PBrzCr(CO),
C(2,6) J
6
J
6
J
6
J
139.2 140.3 142.1 136.9 140.3 144.6
12.0 33.0 52.5 36.1 27.6 22.7
135.5 133.6 131.7 134.4 132.3 129.5
19.8 23.9 31.0 11.5 14.2 15.9
130.2 130.4 130.6 130.2 130.3 130.5
7.3 7.1 8.1 9.5 10.3 11.0
130.4 131.9 134.3 131.8 133.0 133.9
0 0.2 0 1.5 0 3.7 2.4 1.6 1.4 1.7 2.7 1.2 1.7 3.4 -0.3
139.5 139.5 143.6
60.3 24.4 13.7
134.6 132.9 132.8 130.1
23.7 31.0 13.9 15.6
130.7 130.7 130.2 130.2
7.1 7.6 10.0 10.3
132.5 134.6 132.9 133.8
0 0 2.0 2.2
1.8 3.9 2.7 0.9 3.6 -0.3
135.4 134.4 133.6 131.1
23.4 28.8 13.2 13.9
130.3 130.4 130.1 129.9
6.1 7.1 9.8 9.8
131.9 133.8 132.8 133.3
0 0 2.2 2.2
1.6 3.4 2.7 3.4
(C,H, ),pI C,H,P12 (C,H,)zPIWCO), C,H,PI:Cr(CO), “6’
= 6C(4)
-
k(85).
C(3,5)
C(1) 6
138.5
bA6’
=
20.5
h'(complex)-6'
(free
C(4) Wa
ASnb
1.1 0.0
ligand).
accepted that (px +&)n contributions are only important for X = F, Cl and can be neglected for X = Br, I (see also 3’ P NMR). Furthermore, the 6’ data indicate an increasing &,,p +dp)n contribution with progressive halogen substitution in accordance with an increasing positive charge on P. On coordination with the transition metal Cr the positive charge on P becomes more pronounced. For the compensation of the induced charge a supplementary mechanism (de + dp)n is now operating. On substitution the 6’ data of the complexes parallel those of the i?ee ligands. However, the difference between successive substitution steps decreases indicating a partial compensation of the positive charge on P by (da+- dp)n contributions. data decrease with halogen substitution and The A 6’ = 6 ‘complex - Glligand even become negative. This is probably due to the increasing importance of the n-bonding contribution from the transition metal. Indeed we can presume that a d-d orbital overlap will be more effective than a p-d overlap. So on halogen substitution the (dti +dp)n contribution increases while the (pring+dp)~ contribution is pulled back. ACKNOWLEDGEMENTS
The authors thank F. Persyn for cooperation in recording the NMR spectra. E. Vincent is extremely grateful to the I.W.O.N.L. for a research scholarship.
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