7186
J . Phys. Chem. 1991, 95,7186-7188
NMR Spectroscopy of Cobalt(II1) Porphyrin Complexes. 4. ''C Relaxatlon Times and Intramolecular Motion of Axlal N-Methyllmidazole L. Cassidei, Department of Chemistry, University of Bari, Bari. Italy
H.Bang, J. 0. Edwards,* and R. G. Lawler Department of Chemistry, Brown University, Providence, Rhode Island 02912 (Received: December 4, 1990; In Final Form: April 26, 1991)
Carbon- 13 spin-lattice relaxation times (Ti's) have been measured for the protonated carbons in [CO(TPP)(CHJ~)~+]BF,and [CO(TPP)(P~)~+]BF,in CDC13 and CDCll near room temperature. It is found that those carbons with C-H vectors parallel to principal rotation axes, and off-axis carbons of the Py ligands, have identical T,'sand yield the same values of the reorientation correlation time, 7, = (1.0-1.5) X 1O-Io s, for both complexes, the longer 7, being associated with higher viscosity. Off-axis carbons of the CH31m ligands have significantly longer Ti's, consistent with internal reorientation of the CH31m groups about the Co-N bonds. An estimate of 1 X 1Olo s-l is obtained for the diffusion coefficient, Di,associated with this internal motion. This motion is ca. 10 times faster than overall rotation of the complex. It is proposed that the anomalously narrow sgCOline observed for the CH31m complex arises from a decrease in the effective electric field gradient at the cobalt nucleus brought about by libration of the axial ligands. The results for Py ligands confirm an earlier report indicating the absence of internal motion in that complex.
Introduction In spite of the similarity in size and shape (see Figure l ) , the cobalt(II1) porphyrins with axial pyridine (Py) and N-methylimidazole (CH31m) exhibit large differences in 59C0N M R line widths.' The Py complexes have lines that range from 15 to 47 times as wide as those of CH31m, for a series of solvents and porphyrins. On the basis of model calculations using rotational correlation times from T, (or rotational diffusion coefficients, Do) from the Stokes-Einstein-Debye equation*J
0-Py m-Py P-PY c8
0-Ph m-Ph p-Ph TI mol
and electric field gradients from the cobalt ammine studies of Eaton? the estimated line widths for rigid complexes seemed more consistent with observation for the Py than for CH31m as axial ligand. Because of the similar sizes of these ligands, differences in T , for the two types of complexes should be minimal. The purpose of the I3Crelaxation time measurements reported here was to provide independent estimates of 7, that might be used to determine the quadrupole coupling constants, QCC, for the two complexes. Simultaneously it has been possible to demonstrate the presence of rotation of the cobalt-tdigand nitrogen single bond in the case of CH31m and confirm the absence of such rotation in the case of the Py ligand of [CO(TPP)(P~)~+]BF,as reported earlier by Huet and Gaudemeras To find out if the difference indeed is in the QCC, it is necessary to show that the rotational correlation times T, are comparable. Measurements of 13CN M R spin-lattice times, which have proven to be a sensitive probe of molecular reorientation in solvents: have , - [Cobeen carried out on [ C O ( T P P ) ( C H ~ I ~ ) ~ + ] B Fand (TPP)(Py)2+]BF4-. Considerable similarity has been found in the ( I ) Paper 3 of this series: Bang, H.; Cassidei, L.; Danford, H.; Edwards, J. 0.;Hagen, K. 1.; Krueger, C.; Lachowitz, J.; Schwab, C. M.; Sweigart, D. A.; Zhang. Z. Magn. Reson. Chem. 1989. 27. 117. (2) For discussions on relaxation and N M R line widths, see: (a) Bocl5, R. T.; K i d , R. 0.Annu. Rep. NMRSpectrmc. 1983.13,319-385. (b) Kidd, R. G. In N M R of Newly Accessible Nuclel; Laszlo, P., Ed.: Academic Press: New York, 1983; Vol. 1, pp 103-131. (c) Lyerla, J. R. Jr.; Levy, G. C. Topics in Carbon-13 NMR Spectroscopy,W h y : New York, 1974; Vol. I, Chapter I.
Edward, J. T. J . Chem. Educ. 1970, 47, 261. (4) Au-Yeung, S. C. F.; Eaton, D. R. J . Mogn. Reson. 1983.52.351, 366. ( 5 ) Huet, J.; Gaudemer, A. Org. Magn. Reson. 1981, 15, 347. (3)
0.33 (1.9) 0.33 (1.9) 0.30 (1.7) 0.30 (1.8) 0.43 (1.8) 0.43 (1.8) 0.33 (2.0) 0.31
0.39 (1.9) 0.39 (1.9) 0.41 (2.0) 0.36 (1.9) 0.56 (1.9) 0.56 (1.9) 0.39 (1.9) 0.39
0.26 (1.8) 0.26 (1.8) 0.25 (1.8) 0.24 (1.8) 0.35 (1.7) 0.35 (1.8) 0.26 (1.9) 0.25
"The 13C chemical shifts relative to TMS in sample A were 146.7 (o-Py), 142.2 (CJ, 139.3 (C ipso), 137.4 @-Py), 135.6 (C& 133.8 (o-Ph), 128.4 (p-Ph), 127.3 (m-Ph), 122.0 (m-Py), and 119.5 (C meso). bSample conditions: A, CDCI,, 0.065 M,30 OC; B, CDzClz, 0.13 M,25 OC (7 = 0.416 cP); C, CDCI,, 0.13 M,30 "C (7 = 0.514 CP). TABLE II: TI (seconds) and NOE Values for Protonated Carbons in [Co(TPP)(CHJ~I)~+]BF,TI (NOE)
carbon" c-2' c-4'
c-5' C8
o-Ph m-Ph p-Ph TI,
, Ab 0.92 (2.1) 1.10 (1.9) 0.58 (1.8) 0.28 (1.8) 0.41 (1.7) 0.41 (1.7) 0.32 (1.8) 0.30
B 1.35 (2.0) 1.46 (2.1) 0.84 (2.1) 0.40 (1.9) 0.66 (1.9) 0.66 (1 -9) 0.45 (1.9) 0.43
"The I3C chemical shifts relative to TMS in sample A were 143.1 (C, = Cl), 140.5 (C ipso), 134.6 (C& 134.1 (o-Ph), 132.3 (C-2'), 128.2 (pPh), 127.0 (m-Ph), 123.2 (C-4'), 119.8 (C meso), 118.7 (C5'), and 33.8 (N-CHI). bSample conditions: A, CDCI,, 0.065 M,30 OC; 9, CDICII, 0.065 M, 25 OC. T , and in the TI, values, but axial ligand motion is not the same because an extra libration of the axial CH31m ligand has been found.
Results The 13C spin-lattice relaxation times (TI) and the nuclear Overhauser effect (NOE) enhancement factors of each mono-
0022-3654/91/2095-7 186$02.50/0 @ 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7187
N M R of Cobalt(II1) Porphyrin Complexes TABLE 111: htinrtes of DI for [Co(TPP)b+]BF,-
TI,"
PC
carbon
19: deg
Ab 10 (2)
c-5'
72 71 37
Phlm-Py
60
1.2'
c-2' c-4'
13 (3 9 (3
d
10
(3
12 (fb 9 (3
2.4 (0.92) 2.6 (1.10) 1.4 (0.58)
IO-'ODi, s-'
(Ti,*)
B A (Co(TPP)(CH,Im)2']BFi
A
B 3.6 (1.35) 3.9 (1.46) 2.0 (0.84)
B
0.9 (!I) 1.4 (3 0.9
(A:;)
[c0(Tpp)(pY)2+1BF420 (0.33)
1.4 (;::I 1.8 (;:!I 1.3 (I:!)
0.02'
'Valuts taken from Scheidt9for Co(TPD)(CH,lm). bSample conditions: A, CDCl,, 0.065 M, 30 OC; 9,CD2C12,0.065 M, 25 OC. Ti, for these samples are 0.30 and 0.43 s. respectively, giving rise to Do of 1.1 X IO9 and 1.6 X IO9 PI. CHighand low error estimates (1 standard deviation) are m . cSample given as super- and subscripts, respectively, based on an assumed standard deviation of 5% for TI values. dTl,* from eq 2 as p conditions: CDCI,, 0.065 M, 30 OC; TI, = 0.31; Do = 1.2 X IO9 s-l. fNot significantly different from zero.
-
TABLE 1% 9Co Chemical Shifts and Line Widths for [CO(TPP)bt]BF, a'=, WI2, 1 0 1 O r c QCC, ~ temp, concn, ligand mm Hz s MHz solvt OC M MeIm 8491 1706 1.5 9.4 CDCI, 30 0.065 Melm 8451 1090 1.0 9.2 CD,C12 25 0.065 Py 8120 17000 1.1 35 CD2CIZ 25 0.13
'The chemicals shifts are referred to external 0.1 M K,CO(CN)~in DzO. bEstimated from values of Do obtained from TI,. where the constant in the wavy brackets has the value 2.27 X 1Olo s - ~when Tc-H = 1.OSA for both complexes. The overall rotational diffusion coefficient Do for the Py and CH31m complexes is 1.1 X lo9 s-I, corresponding to 7c = 1.5 X s for samples A. For the carbons of ligands (third group) with potential for rotational motion, we define the relaxation time, TI,*, by the
B
1 Figure 1. Structure of the cobalt(l1l) tetraphenylporphyrincomplex with two axial N-methylimidazole ligands. In the formula [Co(TPP)(CH,lm),+]BF,-, TPP stands for the dianion of tetraphenylporphyrin. The carbon atoms on the porphyrin periphery are numbered 1-20; numbers 5, IO, 15, and 20 are the meso positions; numbers 2, 3, 7, 8, 12, 13, 17, and 18 are C, positions. Carbons on imidazole are numbered 2', 4', and 5'. The ipso carbons of the phenyl groups are attached to the meso carbons of the porphyrin ring.
protonated carbon in [CO(TPP)(P~)~+]BF~have been measured under three sets of conditions, and the results are reported in Table I. Results for similar measurements with [Co(TPP)(CH31m)2']BF[ are reported in Table 11. It is convenient to divide the carbon-proton dipoles into three subsets. In the first subset are those dipoles that presumably maintain their orientation with respect to the molecular axes and may therefore be used to obtain the rotational correlation time, T ~ from , the complex as a whole; these include the 0 carbons (C, = C2, C3, etc.) on the porphyrin ring, the para carbons (p-Ph) on the meso-phenyl groups, and the para carbons (p-Py) on the axial pyridine ligands. The second subset is the ortho and meta C-H groups (0-Ph and m-Ph) on the phenyl rings; these are expected to change their orientation due to a carbon-carbon libration involving the dihedral angle between the phenyl groups and the porphyrin planeas The third subset is the C-H groups on axial ligands (0-Py and m-Py on pyridine; C-2', C-4', and C-5' on N-methylimidazole) whose motion will be modified by rotation about the cobalt-nitrogen bond. The values of TI which is the average of the T I values for the first subset, are aTh given in Tables I and 11. It is immediately apparent that T I , for the two complexes are the same (0.31 and 0.30 s) within an estimated experimental error of f0.02 s (determined by replicate measurements on samples of the A type). Using the relationship6
where K = 2.27 X 1 O l o s2, A = 1/4(3 cos2 0 - 1)2, B = 3/4(sin2 28), and C = 3/4(sin4e), 6 being the angle formed by the pertinent C-H vector and the axis of rotation. The 8 values were estimated from the structure reported by S ~ h e i d t . From ~ p we can obtain the intramolecular diffusion coefficient, Di, via the relationship p
=1
+ Di/Do
(4)
Note that in the limit where Di goes to zero or when the C-H vector coincides with the rotation axis (e = 0), eqs 2 and 3 are the same. Estimates for six values of Di (three carbons each on CH,Im for two sets of experimental data) are given in Table 111. The average value of Difrom series A is 1.1 X 10'O s-I, while from series B it is 1.5 X loio s-I. Similar calculations for the pyridine complex gave a value of Di not significantly different from zero, in agreement with the results of Huet and G a ~ d e m e r . ~ In Table IV, a summary of data obtained from sgCON M R of these complexes is presented. Noteworthy is the difference between the values of quadrupole coupling constant (QCC) for CH31m and Py complexes. ~
~~
(6)Doddrell, D.; Glushko, V.; Allerhand, A. J . Chem. Phys. 1972, 56, 3683. (7) Equation 3 assumes the extreme narrowing limit in which the product
w 2 r 2 < 1 , where the highest frequency amiated with the C-H spin system is wH + wc = 1.58 X lo9 rad s-' (XL-ZOO spectrometer). The maximum estimated value of r, = 1/6D6(Table 111) is 1.5 X 1O-Io s. Thus the product w2r2 never ex& IO%, the approximate limit of error in measuring TI. The
extreme narrowing approximation is even better for the C-H subset in CH&m because of the approximate 10-fold shortening of the correlation time 8880ciatd with the internal rotation. (8) (a) Dais, P. Magn. Reson. Chem. 1981,25, 141. The definition of D, as given in this reference is incorrect. For the correct definition, using the same notation, see ref 8b. (b) Wen, J. Q.;Grutzner, J. B.J. Org. Chem. 1986, 51, 4220. (c) Woessnert, D. E. J . Chrm. Phys. 1962, 37, 647. (9) (a) Scheidt, W. R. J . Am. Chem. Soc. 1974, 96, 90. See also: (b) Abraham, R. J.; Medforth, C. J. Magn. Reson. Chem. 1987, 25, 432.
7188 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 Discussion
It is convenient to list the conclusions that can be derived from the experimental results. ( 1 ) The rotational correlation times 7, in CD2C12are the same for both complexes. Also the values of TI for the 9,and CH31m complexes are 0.31 s and 0.30 s in CD?13 at 30 OC at a concentration of 0.065 M. (2) The T Ivalues for pPy and p-Ph are the same, indicating that overall rotation for the Py complex is isotropic.*c Because of the similarity of the values of we assume that rotation of the CH31m complex is also isotropic. (3) For ortho and meta positions on the phenyl groups, the values of T I are longer than T I , by a factor of about 1.4, irrespective of axial ligand nature. (4) For ortho and meta positions on axial Py,the values of T I are the same as (or just slightly longer than) values of T,,. (5) The TIvalues for axial CH31m are significantly longer than the T I , values. The individual values of Difor C-2', C-4', and C-5' may not be identical, but all are about a factor of 10 larger than Do. (6) The extra motion indicated by the Divalue for CH31m is appreciably slower than 'free rotation". This can be seen by comparing the observed values of T Ifor these carbons with those estimated for infinite Di.The motion of the CH31m ligand might therefore be considered a libration (incompletely free rotation) about the cobalt-to-nitrogen-3' bond, analogous to similar motions suggested for the meso-phenyl groups.s (7) The large line width difference (previously reported1) between CH31m and Py complexes can be ascribed to the lower effective value of QCC shown when the axial ligand is CH31m. (8) TI,, Tlm(0- and m-Ph), and Di(CH31m) all seem to depend on solvent viscosity. The Tl's increase as expected if Do becomes smaller at low viscosity. It is surprising, however, that Di seems to increase slightly with increasing viscosity and/or decreasing temperature. The decreased Ti's observed for the Py complex at increased concentration also remains unexplained. It should be noted also that both complexes fit the cobalt(II1) d6 low spin category and that ligand replacement in the coordination sphere is a very slow process. Thus none of the relaxations can be ascribed to an exchange type of process. The most important question to be raised on the basis of the results is: What prevents the cobalt to nitrogen bond of pyridine from undergoing a libration similar to that of the analogous CH31m bond? There appear to be two factors that might bear on this but that cannot be distinguished as yet. One is that the A bonding may not be the same for Py and for CH31m. The
Cassidei et al. arguments for a Ir-bonding barrier for the Py complex have been summarized by Huet and G a ~ d e m e r .The ~ second factor is that there is steric hindrance to rotation by pyridine, which is of decreased importance for CH31m. There are two arguments for this second factor. It is expected from bond angles that the ortho hydrogens of Py project closer to the porphyrin plane than do the C-2' and C-4' hydrogens of CH3Im.I0 The other argument is based on sgCOchemical shifts for the two complexes. The shift, 6, for [ C O ( T P P ) ( P ~ ) ~ + ] B is F353 ~ ppm lower than for [Co(TPP)(CH,Im),+]BFi in CH2CI,. In the unhindered pentammine complexes, C O ( N H ~ ) ~ Ahowever, ~ ~ + , 6 for Am = Py is 57 ppm higher than for Am = CH31m. The correspondence between low 6 and steric hindrance is reinforced by observations that low shifts are obtained when TPP is replaced by the bulky TMP (tetramesityl) and TDCPP (tetrakis(2,6-dichlorophenyl)) porphyrins." Experimental Section Compounds. The complexes were prepared and identified as
described ear1ier.l
NMR Spectroscopy. The "C spectra were obtained from samples in IO-" tubes by using broadband proton decoupling on a Varian XL-200 spectrometer at 50.31 MHz. Typical conditions were a 4000-Hz spectral width, 100-3000 transients, and 32 K data points. Temperature was held constant with a Varian temperature controller. Spin-lattice relaxation times were measured by using the standard inversion-recovery sequence and calculated from peak intensities by exponential regression using three parameters. Sixteen T values were used for each experiment. The repetition rate was maintained at greater than 3 times the longest TI of interest. The typical 90' pulse width was 18 fis. NOE measurements were made in standard fashion by gated decoupling. Data were collected by using a low-power 'Hdecoupling method (Waltz-16).I2 Delays of 10Tlwere used between the 90° pulses. The estimated errors were less than 10% for relaxation times and 15% for NOE measurements. Acknowledgment. We are grateful to C.N.R. (Italy) for a Summer Research Fellowship to L.C., to National Institutes of Health for financial support, and to Professor D. A. Sweigart for helpful discussions. (10) Scheidt, W. R.; Lec, Y.J. In Srrucr. Bonding 1987, 64, 1 . (1 1) Bang, H.; Edwards, J. 0.;Lawler, R. G.; Reynolds, K.; Sweigart, D. A,, to be submitted. (12) Shaka, A. J.; Keeler. J.; Freeman. R. J. Map.Reson. 1983,53,313.
