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15

N NMR shielding tensors in bent nitrosyl complexes of cobalt q Elaine A. Moore a,*, Joan Mason b a

Department of Chemistry, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK b University Chemical Laboratory, Lens®eld Road, Cambridge, UK Received 10 March 2001; accepted 30 March 2001

Abstract Ab initio calculations of nitrogen NMR chemical shielding tensors are presented for square±pyramidal cobalt model complexes with an apical nitrosyl ligand as a function of the geometry of the CoNO grouping. It is shown that the isotropic nitrogen shielding decreases with increasing N±O distance (in the range 101±116 pm) and with increasing CoN distance (in the range 173±193 pm). With increasing CoNO angle, in the models studied, the isotropic shielding initially decreases up to about 1408 and then increases. For complexes with the same CoNO geometry and S-, N- or O-ligating coligands, the shielding increases in the order S , N; O: The variation of the shielding with CoNO geometry is mainly due to variations in the shielding tensor component parallel to the NO bond which mixes n(N) and p p(NyO) orbitals. The calculations give no evidence for differing CoNO geometries producing similar isotropic shieldings but very different spans and skews of the shielding tensor, as given by the experiment in some series of compounds. It is likely that there is CoNO motional averaging (libration or spinning) in the solid state, as described for [Co(NO)(TPP)], in complexes for which anomalously small spans are observed. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Co complexes; NO; NMR shielding; DFT; Motional averaging

1. Introduction The last decade has uncovered the versatility of nitric oxide NO as a signalling molecule, in biological systems involved in the regulation of blood vessels, in communication within the brain, in immunology and in many pathological processes [1±6]. The linkage of a transition metal to NO is of particular interest since NO binds to heme proteins, acting as an allosteric trigger, as O2 does. Cobalt nitrosyl tetraporphinates with bent apical nitrosyl, such as the tetraphenylporq

Dedicated to Professor Graham A. Webb on the occasion of his 65th birthday. * Corresponding author. Tel.: 144-1908-655028; fax: 144-1908858327. E-mail address: [email protected] (E.A. Moore).

phinate [Co(NO)TPP], are useful models for oxygenated protoheme. NO has also recently been reported to bind to cobalt in cobalamin [7,8]. Transition metal nitrosyls are unusually versatile in that the M±NO bond can be bent, like the M±O bond in M±O2, or linear, like the M±C bond in M±CO, depending on the ligand ®eld. Complexes with strongly bent MNO groups are usefully regarded as having NO 2 attached to the metal so that the ligand is isoelectronic with O2 and the bonding in these square±pyramidal complexes is similar to that in oxyhemoglobin. The calculated bond orders for the NO bond are around 1.7, comparable to those found for aromatic nitrosyls [9], lending support to a picture of the ligand binding as NO 2. Nitrogen NMR is peculiarly suitable for the study of M±NO bonding and bending, since bending brings

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0022- 286 0( 01) 00775-X

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into play low-energy n(N) ! p p(NO) excitations which have dramatic effects on NMR shieldings and coupling constants. In bent M±NyO there is a net bonding interaction between the cobalt dz 2 orbital and the p p orbital of NO in the MNO plane, but the overlap of this NO orbital with dxz (where xz is the MNO plane) is smaller than in linear nitrosyls. The nitrogen lone pair n(N) occupies a non-bonding orbital formed from these three. Its stabilisation due to the interaction with dz 2 is responsible for the preference of {MNO} [8] systems such as low spin Co(III) nitrosyls for a square pyramidal arrangement with a CoNO angle of about 1208 [10,11]. The p p orbital is the antibonding orbital formed by the overlap of the p p NO orbital perpendicular to the CoNO plane with the dyz orbital on cobalt. A calculation of the energy levels of [IrCl4(NO)] 22 as a function of Ir±N±O angle shows the energy gap between the n and p p levels increasing with decreasing angle from 180 to 1208 [11]. Increasing the Co±N distance, and decreasing the overlap of cobalt and nitrogen orbitals, raises the n orbital energy and lowers the p p orbital, decreasing the n±p p energy gap. Increasing the N±O distance, weakening the NO bond and strengthening the Co±N interaction, increases the n±p p gap, with some compensation from the weakening of the p(NO) bond. If the n±p p excitation energy dominates the shielding, this should decrease with increase in the CoNO angle and the Co±N and NyO distances. Other factors come into play, including changes in the relevant orbitals, as the CoNO angle approaches 1808. If the n(N)±p p excitation energy is crucial, the shielding should decrease with changes in the coligating atoms as N . O . S; although the radial factor r 23 in the shielding, acting as a nephelauxetic in¯uence, increases with decreasing electronegativity of the coligands as O , N , S: A study of 15N and 59Co NMR chemical shielding for a series of square±pyramidal complexes with bent apical nitrosyl and unsaturated bis-chelating or porphin-related basal ligands [12] suggested that the 15 N shielding tends to increase with a decrease in the Co±N distance, increase in the NyO bond distance and an increase in the MNO bond angle; but change in a particular parameter is often accompanied by other changes in geometry, or in the nature of the ligating atoms in the basal plane.

15

N shielding tensors in bent apical nitrosyl ligands and 59Co shifts were measured for complexes with Nsubstituted salicylidene iminate (salim) ligands in the basal plane [13]. In these closely related complexes the isotropic 15N shifts cover a range of over 60 ppm, but d 11 displays an even greater range of 835 ppm. For all complexes for which the full tensor was obtained, the lowest-shielding component is s 11, with the 1 axis almost parallel to the N±O bond. This component is expected to arise mainly from the mixing of n(N) and p p(NO) orbitals and is very large and negative because of the low energy of the n(N) ! p p(NO) excitation. For the two complexes of fully determined structure, d 11 increases with the increasing angle. In these complexes as in other delocalised systems, the highest shielding component is d 33 where the 3 axis lies perpendicular to the CoNO plane. This component is expected to arise from the mixing of n/s and s p orbitals, which corresponds to a high excitation energy. Changes in the three components are not necessarily parallel. For example in going from [Co(NO)(N-Mesalim)2] to [Co(NO)(N-Phsalim)2], d 11 increases but d 22 and d 33 decrease whereas from [Co(NO)(N-Mesalim)2] to [Co(NO)(ketox)2], d 11 and d 33 decrease whilst d 22 increases. The observed order of shielding with a change in the coligating atoms on cobalt is S . N . O; but the CoNO geometry is sensitive to changes in the coligands on cobalt: the Co±N distance is shorter and the CoNO angle greater for S-ligating complexes than for N- and O-ligating complexes. The experimental data not always providing very clear trends, from one complex to another, may well be due to changes in more than one factor at the same time. Calculated shielding tensors allow us to determine the response of the nitrogen shielding tensors to changes in each factor in isolation. We have used ab initio calculations of the 15N shielding tensors, to throw light on the trends in the nitrogen shielding with changes in the chemical environment of the metal. 2. Calculations Two methods of calculation were employed, rpac 11.0 [14] to calculate shielding tensors from wavefunctions generated by gamess [15,16] and the giao

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349

Fig. 1. Model complexes used in the calculations: (a) N4-coligating, (b) N2O2-coligating, (c) S4-coligating.

[17] method in gaussian94/98 [18,19]. gamess and rpac 11.0 were run on a dec alpha 600 cluster at the Open University. gaussian94 calculations were performed on the vector computer facility at the University of London Computer Centre and on the Columbus super scalar facility at the Rutherford Appleton Laboratory (RAL). Gaussian98 calculations were performed on the Columbus super scalar facility at RAL. In the Gaussian98 calculations we used the DFT method with the hybrid functional B3LYP, which we have found to be effective in calculating N shielding in N2O3 [20]. The basis sets used were Ahlrichs VTZ basis set [21] 1 with polarization functions on all atoms. The calculated isotropic shieldings were close to those observed experimentally. The gamess calculations were run at the Hartree± Fock level with a 6-311G [22] basis set plus one d polarisation function on the oxygen and two d polarisation functions on the nitrogen of the nitrosyl group. On cobalt we used the minimal basis set of Huzinaga (MINI) [23]. Although small, this basis set has optimised exponents and contraction coef®cients for each element and does not constrain s and p functions to have the same coef®cients. We have shown that this set will give good results for Ti shielding [24]. In order to ®t the calculations on the available computer space, MINI basis sets were also used on the atoms forming the plane of ligands around the cobalt. 1 Basis sets were obtained from the Extensible Computational Chemistry Environment Basis Set Database, Version 9/11/00, as developed and distributed by the Molecular Science Computing Facility, Environmental and Molecular Sciences Laboratory which is part of the Paci®c Northwest Laboratory, P.O. Box 999, Richland, Washington 99352, USA, and funded by the U.S. Department of Energy. The Paci®c Northwest Laboratory is a multiprogram laboratory operated by Battelle Memorial Institue for the U.S. Department of Energy under contract DE-AC06 76RLO 1830. Contact David Feller or Karen Schuchardt for further information.

Further con®dence in this basis set comes from the structure obtained by geometry optimisation. The Ê optimised structures had Co±NO distances of 1.72 A Ê for the N4-ligating model (described below), 1.71 A Ê for the S4for the N2O2-ligating model and 1.728 A Ê (N4), ligating model and CoNyO distances of 1.151 A Ê (N2O2) and 1.107 A Ê (S4). These are all close 1.145 A to the values found experimentally. The CoNO angle was 1178 for all three models, slightly smaller than the observed values. Furthermore the optimised structures reproduce the asymmetry noted in a recent crystallographic determination of the structure of cobalt octaethylporphyrin [25]. A series of runs using the giao method in gaussian94 and the 3-21G basis set on all atoms reproduced the trends in the variation of the shielding tensor with bond distances reported here. Running the same geometry with 3-21G and 6-311G gave smaller calculated components of the shielding for 6-311G, particularly s 11, but the results were qualitatively similar. The models for O- and N-ligating ligands were chosen as the largest of the models proposed by Newton and Hall [26] to represent porphyrin rings that would comfortably ®t the available resources. These have two conjugated bis-chelating ligands in the plane and are shown in Fig. 1a, [Co{NH(CH)3NH}2(NO)], and Fig. 1b, [Co{NH(CH)3O}2(NO)]. For the S4-ligating systems, our model was a dithiocarbamato complex, Fig. 1c, which is closer to the structure of the S4-ligating complexes whose spectra have been observed. For the N4-ligating models the Co±N(basal) distance was 197.6 pm, close to that in [Co(NO)(TPP)] [27]. For the N2O2 ligating systems, the Co-basal N distance was 195 pm and the Co-basal O distance 183 pm as in [Co(NO)(esal)2]. The Co±S(basal) distance was set as 210 pm. In most of the calculations the oxygen of the nitrosyl group was positioned

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Table 1 Calculated (DFT, HF) 15N shielding parameters as a function of Co 15NO geometry and coligating atoms on cobalt /CoNO (8) s iso (ppm) s 11 (ppm) s 22 (ppm) s 33 (ppm) V (ppm) k

Coligating atoms Method r (pm)

a a(8)

Co±NO CoNvO N4

N2O2

S4

a b c

DFT DFT DFT DFT DFT DFT DFT DFT HF HF HF HF HF HF HF HF HF DFT DFT DFT DFT DFT DFT DFT HF HF DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT

173.8 b 173.8 173.8 173.8 173.8 173.8 173.8 183.3 173.8 173.8 173.8 183.3 183.3 183.3 183.3 183.3 183.3 173 173.8 173.8 173.8 183.3 193 193 177 183.3 172.8 172.8 172.8 172.8 172.8 172.8 172.8 172.8 173.8 c 173.8 177 177 183 183 183 183 193 193 193

101 101 112 112 112 114 116 112 116 116 116 101 101 101 101 101 101 101 112 112 112 101 101 116 101 101 110 110 110 110 110 110 112 115 101 112 101 110 101 110 112 116 110 110 110

120 120 120 130 140 120 120 120 115 120 125 110 115 120 125 130 140 125 120 130 140 130 125 120 130 130 120 130 140 150 160 170 120 130 120 120 120 120 120 120 120 120 120 130 140

Angle between the 1 direction and the NO bond vector. r Co±Nbasal 183 pm. (O over S).

2593 2607 2900 2928 2966 2955 21011 21040 21676 21812 22028 2640 2723 2741 2762 2788 2873 2581 2864 2905 2964 2666 2749 21480 2754 2741 2874 2900 2924 2936 2857 298 2929 21065 2665 2945 2667 2931 2721 21015 21084 21221 21159 21231 21377

21655 21694 22438 22659 23023 22572 22705 22866 24941 25509 26403 21817 22413 22317 22544 22863 24158 21653 22326 22562 22942 21989 22172 24083 22617 22564 22396 22601 22896 23351 24050 24200 22534 23023 21874 22622 21884 22575 22048 22834 23010 23361 23269 23664 24431

2189 2200 2310 2213 231 2334 2360 2297 2322 2184 28 2281 2178 288 43 240 387 2178 2317 2233 283 2113 2206 2375 123 125 2276 2198 249 242 572 1340 2299 2255 2219 2285 2182 2272 2177 2266 2289 2340 2258 2142 89

66 72 48 89 156 40 32 45 235 259 291 176 152 181 215 258 1152 89 50 79 132 102 191 18 232 220 50 99 173 301 906 2565 45 83 98 74 64 52 62 53 49 38 51 113 209

1721 1766 2486 2748 3179 2612 2737 2911 5176 5768 6694 1993 2295 2498 2759 3121 5310 1742 2376 2641 3074 2091 2363 4101 2849 2784 2446 2700 3069 3652 4856 6755 2579 3106 1972 2696 1948 2627 2110 2887 3059 3399 3320 3777 4640

0.70 0.69 0.71 0.78 0.78 0.71 0.71 0.77 0.78 0.85 0.92 0.54 0.71 0.78 0.88 0.99 0.71 0.69 0.69 0.76 0.86 0.79 0.69 0.81 0.92 0.93 0.73 0.78 0.86 0.97 0.87 0.64 0.73 0.78 0.68 0.73 0.75 0.75 0.77 0.78 0.78 0.78 0.81 0.86 0.95

15 10 7 16 12 9 4

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351

Fig. 2. 15N isotropic shielding, with CoNO angle 1208, as a function of CoNyO distance for: B Ð an N4-coligating complex with r CoN 173:8 pm (full line), A Ð an S4-coligating complex with r Co±NO 183 pm (dashed line).

between two basal ligating atoms. This corresponds to the position commonly found in the crystal structures

of cobalt nitrosyl porphyrins and in the optimised structure. Some runs on S4-ligating systems had the oxygen over one sulphur atom. The dependence of the 15N nitrosyl shielding tensor on Co±NO bond distance, CoNyO bond distance and CoNO bond angle was studied by varying each of these separately with the other two ®xed for several values of the ®xed quantities. 3. Results

Fig. 3. 15N isotropic shielding, with CoNO angle 1208, as a function of Co±NO distance for: B Ð an S4-coligating complex with r NO 101 pm, S Ð an S4-coligating complex with r NO 110 pm, O Ð an N2O2-coligating complex with r NO 116 pm.

Variation in CoNyO distance. The calculated 15N NMR shielding results for N4-, N2 O2- and S4-ligating ligands are shown in Table 1. V is the span (s 33 2 s 11) and k the skew 3(s 22 2 s iso)/V . As illustrated in Fig. 2, the isotropic shielding decreases strongly with an increasing CoNyO distance, mainly due to a decrease in s 11. Changes in s 22 and s 33 are smaller with s 22 generally decreasing and s 33 increasing with increasing CoNyO distance, so that the variation of s 11 with distance dominates the changes in the isotropic shift. The variation in span is also dominated by the variation in s 11 but the skew remains constant for several of the systems, for

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Fig. 4. 15N isotropic shielding as a function of CoNO angle for, A Ð an N4-coligating complex with r CoNyO 101 pm and r Co±NO 183 pm (HF), X Ð an S4-coligating complex with r CoNyO 110 pm and r Co±NO 172:8 pm, S Ð an N2O2-coligating complex with r CoNyO 112 pm and r Co±NO 173:8 pm, O Ð an S4-coligating complex with r NO 110 pm and r CoN 193 pm.

example the N4-ligating system with r CoN 173:8 pm and a CoNO angle of 1208. The shielding derivative with respect to the N±O distance is found to be very large and negative, as found with a more limited dataset [28]. Derivatives for complexes with different basal ligands are similar. Variation in Co±NO distance. Calculated values of the 15N shielding tensor with variation of the Co±NO distance are presented in Fig. 3. Most of these results show a decrease in the shielding with an increasing Co±NO distance, except for complexes with CoNyO distances of 101 pm and short Co±NO distances for which the change in Co±NO distance has little effect on the isotropic shielding. The shielding derivative with respect to the Co±NO distance is again large and negative, but much less than with respect to the CoNyO distance. Thus for the dithiocarbamato complexes the derivative with respect to the Co±NO distance is about half that with respect to the CoNyO distance. The derivative increases as the CoNyO distance increases as can be seen in Fig. 3 where we have plotted values for r NO 101 pm, 110 pm and

116 pm. The derivative with respect to Co±NO distance also increases with CoNO bond angle. This can be seen in Table 1 by comparing the values for the S4-ligating complexes with different Co±N distances at angles of 120, 130 and 1408. The variation in isotropic shielding generally follows that of the 11 component. The 22 component was found to vary by a maximum of only 6 ppm in earlier work in which the angle was ®xed at 1278 and the Co±NO distance was varied across a smaller range, 183±193 pm [28]. We ®nd greater variation for small bond distances (r CoN , 183 pm and r N±O 101 pm) and for larger angles (1408), see Table 1. In almost all cases the span and skew increase with the Co±NO distance. Variation in MNO angle. There is a sizeable difference between the nitrogen shielding for complexes with linear CoNO and that for complexes with CoNO angles in the range 110±1408 and this can be used to distinguish the two geometries [29]. For ®xed bond distances, we ®nd a decrease in the shielding with increasing angle up to 1408 stemming from a

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353

Fig. 5. 15N isotropic shielding as a function of (DE 21), in S4-coligating complexes, with variation of: (a) the CONyO distance, (b) the Co±NO distance, and, (c) the CoNO angle, where DE is the energy difference between the n(N) and p p(NO) orbitals.

decrease in s 11. This is shown in Fig. 4, where it can also be seen that at larger angles, the shielding increases with the angle. s 22 and s 33 increase with increasing angle, offsetting changes in s 11, reducing the changes in isotropic shielding. The interplay of the two trends lead to an increase in isotropic shielding with increasing angle in some cases, as found previously [28]. The span, surprisingly, increases with bond angle. As the observed span for complexes

with linear CoNO groups is much smaller than that for those with bent CoNO groups, this result is unexpected but con®rms the previous work. The skew ®rst increases and then decreases. The decrease at large angles is a result of the increased span. The 1 direction is close to the NO vector. We have included Hartree±Fock results in the Table as these give the orientation of the principal axes accurately, although the actual shielding is less accurate. The

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angle a measures the extent to which the 1 direction deviates from the direction of the NO bond vector. It decreases with increasing bond angle in the range 110±1308, being around 158 for a CoNO angle of 1108, 108 for a CoNO angle of 1208 and 58 for a CoNO angle of 1308, giving an angle of 130 ^ 58 between the Co±NO bond and the 1 axis. Thus the 1 axis in these complexes with strongly bent CoNO groups only lies within a narrow range of directions and the direction is close to the experimentally observed direction of the NO bond and to the direction predicted for the stabilisation of bent MNO groups [10,11]. For complexes with angles in the range 140±1608 the principal axes were not so clearly related to the geometry of the CoNO group. Variation in Co-basal ligand distance In two cases, we investigated the effect of moving the basal ligands closer to the cobalt atom using DFT calculations. For an N4-ligating model, with a CoNyO distance of 101 pm, a Co±NO distance of 173.8 pm and a CoNO angle of 1208, we decreased the Co±N(basal) distance from 197.6 pm to 183 pm and for a S4ligating system, with a CoNyO distance of 110 pm, a Co±NO distance of 172.8 pm and a CoNO angle of 1208, we reduced the Co±S distance from 210 pm to 200 pm. In both cases, this resulted in an increase in the isotropic shielding from 2607 to 2593 and from 2874 to 2782 ppm, respectively. The change was mainly due to an increase in s 11; s 22 and s 33 showing little change. Hartree±Fock calculations con®rmed that there was very little change in the isotropic shielding at short CoNyO distances, but at longer CoNyO distances, the change in s 11 led to a decrease in the shielding with an increase in Co-basal atom distance. Nitrosyl swinging. It is known [29,30] that in several complexes with apical nitrosyl, there is a low barrier to rotation of the NO group, or the crystal structure shows disorder in the O position. Calculations with the O atom positioned directly over a ligating atom rather than between the ligands showed that this made little difference to the isotropic shielding. For example for the S4 model with a CoNyO distance of 110 pm, a Co±NO distance of 172.8 pm and a CoNO angle of 1208, the isotropic shielding is 2890 ppm with the O above one S atom and 2874 ppm with the O atom between two S atoms.

The shielding components were also similar in magnitude. In averaging these components, however, it is necessary to take into account the different directions of the principal axes and this leads to a reduction in the span and skew. For complete statistical averaging over the four sites with O above S for a model complex, using the previously obtained formula [27], with the above geometry we obtain s uu 2818 ppm and s ' 2926 ppm! Change of ligating atom. For complexes with the same CoNO geometry, the nitrogen shielding varied in the order S4 , N4 , N2O2. 4. Discussion The variation of shielding with CoNyO distance is as expected if the excitation energy is the dominant term. The large values of s 11 and hence s iso thus arise primarily from the paramagnetic contribution of the n(N) orbital. This would be expected to be large due to the low energy of the transition n(N) ! p p(NO) which provides a circulation around the NO bond. As the NO bond lengthens, the p bonding weakens lowering the energy of the p p(NO) orbital. Fig. 5 is a plot of the isotropic shielding against the reciprocal of the calculated n±p p energy gap for S4-ligating complexes. As can be seen in Fig. 5a, as the NO distance is changed, the variation in the shielding is directly proportional to the variation in the reciprocal energy, DE 21(p p ±n). The shielding is less sensitive to changes in the Co± N distance, and analysis of the Hartree±Fock results indicates that this may be due to partial cancellation of the change arising from variation in the n±p p energy by changes of opposite sign arising from other transitions. Fig. 5b shows, however, that the variation in the shielding with CoN distance, as for the variation with NO distance, is primarily determined by the change in the n±p p energy gap. The calculated n±p p energy gap increases with increasing angle as the dz 2 ±p p(NO) overlap decreases, but the variation of the isotropic shielding with CoNO angle is only proportional to the reciprocal of this energy difference for angles below 1408. As shown in Fig. 5c, at angles of 150±1708 the shielding increases with increasing angle although the energy gap is still increasing.

E.A. Moore, J. Mason / Journal of Molecular Structure 602-603 (2002) 347±356

The variation of s 11 with CoNO angle parallels the variation in the reciprocal of the excitation energy for n(N) ! p p(NO) for angles in the range observed in complexes with bent CoNO groups, for example from 120±1508 for complexes [Co{NH(CH)3NH}2(NO)] with r CoN 183:3 pm and r NO 101 pm. For larger angles, the calculated 11 component still decreases with increasing angle but the 22 and 33 components become more positive and their contributions more important. As to the experimental data that are available, the solution results [12] show no correlation of shielding and CoNybond distance, but the shielding decreases with increasing Co±NO bond distance. A change of 4 pm in the Co±NO distance produces a change of about 100 ppm in the shielding, considerably more than is predicted. The observed shielding shows a tendency to increase with increasing angle but the correlation is not strong. These trends are evident for the tensors which have been measured in the solid state [13]. We can divide the complexes into two groups. One set has large values of the span and skew values of 0.45±0.9. Complexes in this set have large negative 11 components and relatively small 33 components and give the closest matches to the calculated shielding tensors. They include the complexes in which the conformation of the N±O group is ®xed or else disordered over equivalent positions: [Co (TPP)NO] at low temperature, [Co(OEP)NO], [Co(N-Etsalim)2NO]. That the pattern of shielding components for these complexes with ®xed CoNO geometry resembles that of the calculated tensors suggests that complexes in the other set have labile CoNO groups. An averaged spectrum is observed for a crystal with NO groups freely librating on the NMR time-scale. This affects the isotropic shielding very little but the tensor components are averaged reducing the span and skew. This is known to occur in [Co(TPP)NO] where at room temperature, the shielding tensor appears to be that of a complex with linear NO, but on cooling a transition occurs to ®xed positions. We suggest that a partially averaged shielding tensor is observed for e.g. [Co(N-Mesalim)2NO] and [Co(N-Busalim)2NO] because of swinging or spinning of the nitrosyl. We take as an example an N2O2 model complex. We can obtain a calculated value (2989 ppm) close to the observed value

355

(2917 ppm) for the isotropic shift for [Co(ketox)2] for a complex of similar geometry: r NO 112 pm, r CoN 180:6 pm and a CoNO angle of 1278 compared to the experimental of r NO 111:4 pm, r CoN 180:6 pm, CoNO angle 126.38. However the calculated principal components of the shielding tensor are 22785, 2247 and 67 ppm as opposed to the experimental values of 21260, 21007, 2484 ppm. If the oxygen were completely disordered as in [Co(TPP)NO], then the components based on the calculated values would be 21229 and 2899 ppm. The observed values lie between the calculated values for a rigidly ®xed O atom and for completely free swinging of the NO group with the observed 11 and 22 components very close to the averaged components. A third possibility is that the rate of libration is comparable to the NMR time scale. Under these circumstances, extracting the tensor components is problematical. 5. Conclusions Calculated shieldings using DFT methods can now reproduce observed isotropic shieldings, although as noted by Godbout et al. [30], the best match to experimental results comes from model complexes with smaller than observed NO distances. The principal axis system is similar to that of aromatic nitrosyls with the 1 axis close to the CoNyO bond and at about 1308 to the Co±NO bond and the 3 axis at 908 to the CoNO plane. Overall the picture of trends that we gain is that the N shielding is strongly in¯uenced by the CoNyO and Co±NO distances with the CoNO angle and Co distances from the basal ligands having less in¯uence. With increasing CoNyO and Co±NO distances, the isotropic shielding decreases; the derivative of the shielding with respect to the CoNyO distance being about twice that with respect to the Co±NO distance. Our results suggest that the isotropic shielding ®rst decreases and then increases with CoNO bond angle. The 11 component of the shielding tensor decreases with increasing CoNyO distance, increasing Co±NO distance and increasing CoNO angle. With increasing CoNyO bond distance, the 22 and 33 components show little change, but with increasing Co±NO

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distance and CoNO angle these components increase. This increase offsets the decrease in the shielding due to the 11 component, and at large CoNO angles determines the variation of shielding with angle. The span increases in parallel with s 11. The skew is unaffected by increasing CoNyO distance, but increases with increasing Co±NO distance and increases then decreases with increasing CoNO bond angle. For linear CoNO groups, k 1:0 by symmetry. The calculated results lend support to the proposal that the observed variations in the span and skew for complexes with similar isotropic shifts is due to differences in NO lability rather than differences in CoNO geometry.

[14] [15] [16] [17] [18]

Acknowledgements We thank the Engineering and Physical Sciences Research Council of the UK for time on the Convex facility at the University of London Computer Centre, grant no. GR/K66307 and UK computational chemistry working party for time on the super scalar facility Columbus at the Rutherford Appleton Laboratory.

[19]

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