Carbon Nuclear Magnetic Resonance Study

Carbo n Nuclear Magnetic Resonance Study o f Rho dium-Olefin Complexes Summary The magnitude of the 103Rh-13C coupling constant and the substantial up:field shift of the ole:finic carbon resonance upon complexation suggest that there is consideraЫe s-character in ole:fin-rhodium bonds. То investigate the mode of bonding of ole:fins to transition metals we have examined the 13С n.m.r. spectra of а series of rhodium-ole:fin complexes. 13С N.m.r. spectra at natural abundance were obtained using а spectrometer described previously.1 ТаЫе 1 shows the spectral data for the ole:fin carЬon atoms of the rhodium complexes studied. 103Rh-13C Coupling of са. 14--16 Hz, and substantial up:field shift of the ole:finic resonance upon complexation are shown. The coupling constant appears small. \Vhen the small gyro­ magnetic ratio of 103Rh is accounted for, the reduced t coupling constant, K(103Rh-13C') is са. 3 х К(13С-1Н). Although electron-nuclear dipolar interactions can con­ tribute to scalar coupling,2 а Feпni contact interaction is still likely to Ье dominant since the reduced coupling co11stants are quite large. А study of (C 5H 5)Rh(o--allyl)­ (1r-allyl), :first prepared Ьу Powell and Shaw, 3 shows а 26 Hz coпpling for the rhodium-carbon o--bond. Assuming sp3hybridization for the o--allyl methylene carЬon one can determine that the s-character, р = 0·96 J. t From this the s-character of tl1e rhodium-ole:fin-carbon bond is estimated to Ье са. 15%. One extreme view of ole:fin­ metal Ьonding involves o--bond foпnation of an sp3-rehybrid­ ized olefin carbon and а dsp2 metal orbltal. Our estimate of 15% s-character implies а contribution from this bonding scheme of about 60%.t We have assumed that the scaling of J(163 Rh-13C) is dominated Ьу the s-character dependence of the Fermi contact term. J(13C­ H) is dependent on а second portion of the Fermi contact term as well, namely the effective nuclear charge (Z*) .' At present we must assume this term is constant since its variance in these rhodium complexes is not known. Cook and Wan estimate 28% s-character in (Ph 3 P) 2Pt(C 2 HJ and thus а 100% contribution from the u-bonded scheme.• t The previously noted substantial upfield shift for the olefinic carbon resonance upon complexation ranges from са. 50 to as large as 115 p.p.m. in the 2,3,5,6 positions of may Ье due to the fortuitous cancellation of factors in the coupling constant expression of equal magnitude but opposite sign, or it may Ье due to а difference in bonding of TABLE 1 codd с.н. Compound nbd< I (cod)Rh(C 5H 5) (cod)Rh(acac)d (cod)Rh(acac-I:< '8) (cod)Rh(acac-F8 ) (C 2H.) 2Rh(C 5H 5) (C 2H4) 2Rh(acac) (nbd)Rh(C 5H 5) 13С N.m.r. data for free and co-ordinated olefin carbon atoms & (p.p.m.)"' дсь (p.p.m.) J(13C-H) (Hz)c 150 65·0 71·29 174 48·9 152 130·4 +65·4 116·8 156 +51·8 116·5 154 +51-5 II4·6 152 +49·6 132·8 160 +61·2 156·5 158 +85·3 164·1 176 +115·2 л1оаRh_1зс) (Hz) 14 14 14 16 10 14 10 а Chemical shifts relative to CS2 = О p.p.m. ь де represents the shift upfield of the olefin resonance upon complexation. с± 5 Hz. d cod = cyclo-octa-1,5-diene, nbd = norЬornadiene, асас = acetylacetone. • In СС1 4 solution (W. W. Conover, Indiana University). norbornadiene.§ This resonance falls in the region where one would expect the resonances of saturated carЬon atoms. The 18С screening tensor has been shown to Ье dominated Ьу the paramagnetic shielding term, 7 which is dependent on changes in the electron density, in the ?Т-bond order, or in the average excitation energy, 8 (caused Ьу the mixing of excited states into the ground state due to the perturbation of the magnetic field). The u-bond model indicated above for olefin-metal bonding results in а substantial decrease in the ?Т-bond order of the olefin upon complexation and leads to an upfield shift. An increase in s-character is known empirically to lead to an upfield shift of the resonance. The magnitude and effect of а change in average excitation energy from free to co-ordinated olefin is not yet known. ТаЫе 2 gives the 13С n.m.r. data for the cyclopentadienyl carbon atoms of the rhodium complexes studied. The chemical shift values are nearly constant and no 103Rh-13C coupling is observed. The absence of 103RhJЗC coupling cyclopentadienyl and olefin ligands to а transition-metal atom. Thus it is possiЫe that olefin-metal bonds have substantial u-character while cyclopentadienyl-metal bonds have predominantly ?Т-Character. TABLE 2 13 С N.m.r. data for cyclopentadienyl carbon atoms Compound Бс (p.p.m.)"' J(13C-H) (Hz) 167 106·3 (cod)Rh(C 5H 5) 174 (C 2H4) 2Rh(C 5H 5) 105·9 172 108·0 (nbd)Rh(C 5H 5) 176 105·2 (CO).Rh(C 5H 5) а Chemical shifts relative to CS3 = О p.p.m. This work was supported Ьу the National Science Foundation. § А complexation shift of 4 p.p.m. for AgN0 8(olefin) complexes has been reported. 8 1 А. О. Clouse, D. Doddrell, S. В. Kahl, and L. J. Todd, Chem. Сотт., 1969, 729. 2 N. F. Ramsey, Phys. Rev., 1953, 91, 303. 8 J. Powell and В. L. Shaw, J. Сhет. Soc. (А), 1968, 583. 4 D. М. Grant and W. Litchman, J. Amer. Chem. Soc., 1965, 87, 3994. 5 С. D. Cook and К. У. Wan, J. Amer. Chem. Soc., 1970, 92, 2595. 8 R. G. Parker and J. D. Roberts, ]. Amer. Chem. Soc., 1970, 92, 743. 7 М. Karplus and J. А. Pople, ]. Chem. Phys., 1963, 38, 2803. 8 L. Salem, "The Molecular Orbltal Theory of Conjugated Systems," Benjamin, Ne,v York, 1966, р. 221.