Novel geometries exhibited by three palladium(II) macrocyclic complexes: crystal and solution structures

ELSEVIER

Inorganica Chimica Acta 246 (1996) 349-360

Novel geometries exhibited by three palladium(I1) macrocyclic complexes: crystal and solution structures B. Chaka, A. McAuleyap*, T.W. Whitcombeb aDepartment of Chemistry, Universiv of Victoria, Victoria, B.C., V8W 386, Canada bDepartment of Chemistry, University of Northern British Columbia, Prince George, B.C., V2N 429, Canada

Abstract The synthesis of three related macrocyclic complexes of palladium(I1) is reported, together with their structural characterization by crystallography and NMR. The crystal structure of [PdLt](CI)(PF6) (Lt = 1,4,7-trithia-11-azacyclotetradecane) (Pi (No. 2), a = 10.127(2) A, b = 12.568(4) A, c = 7.141(2)& a = 87.99(2)“, /l = 9S.S5(2)‘, y = 91.02(2)“, Z= 2, R = 0.0422, R, = 0.0486) exhibits an ‘endo’ coordination of the metal ion. For the related pendant-arm macrocylic ligand, N-(2’-pyridylmethyl)-1,4,7-trhhia-llazacyclotetradecane (h), the solid state structures of the [PdL&l](PF6) (P2t2t2t (No. 19), a = 13.305(3) A, b = 12.413(4) A, c = 14.060(3) A, Z= 4, R = 0.0538, R, = 0.0529) and [PdLJ(BF&0.SH20 (LYu (No. 15) (non-standard setting of the space group Cut), a = 19.045(8) A, b = 16.952(4) A, c = 16.635(6) A, #l = 113.47(3)“, Z= 8, R = 0.0532, R, = 0.0560) compounds exhibit a palladium ion that is ‘chelated’ by the macrocyclic ligand and is only partially coordinated by the macrocyclic donor set. Each of these latter complexes exhibits fluxional NMR spectra. The former complex ion undergoes a simple inner-sphere substitution while the latter exhibits more complex behaviour. Keywords:

Crystal structure; Palladium complexes; Azathia-macrocycle complexes; Pendant-arm macrocycle complexes

1. Introduction The study of complexes containing macrocyclic ligands with pendant arms continues to be an area of interest [l-4]. Much of the driving force for such investigations derives from a variety of factors including the facility of the ligand geometry to adapt to a number of possible coordination environments, the synthesis of polymacrocyclic ions, the formation of multinuclear species, and the potential of such ligand complexes to act as model compounds of biological interest [4-6]. In addition, some of these systems have demonstrated the capacity to act as catalysts for the reduction of small molecules [7]. Although macrocyclic complexes of palladium(I1) are relatively novel [8,9], a flexibility in geometry has been observed, which relates, in part, to the availability of additional donor sites within the ligand. In contrast to the simple square planar ions that are expected for a d8 elec* Corresponding author.

0020-1693/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved PII

SOO20-1693(96)05121-S

tronic configuration, there is evidence not only of five coordinate systems but of axial interactions in S6 donor sets [3,8b,lO]. In the present paper, data are presented indicating distortion of a nominally square planar palladium ion by coordination effects produced by the macrocyclic donor set. While distortions of the palladium geometry are observed in the solid state, such deviations are frequently considered not to be maintained in solution. Rather, the complexes with a surfeit of donor atoms are observed to undergo fluxional behaviour on the NMR time scale, as has been shown in previous observations for macrocyclic complexes of this metal ion and its cogener [2a,9,11]. The observed phenomena are a measure of the lability of the palladium(I1) ion and the flexibility of the pendant-arm macrocyclic ligands [4]. 2. Experimental All chemicals

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B. Chak et al. I Inorganica Chimica Acta 246 (1996) 349-360

2.4. X,4,7-Trithia-11-azacyclotetradecane

(L, )

The tosylated macrocycle, III (3.5 g; 8.63 mmol) was dissolved in dry THF (20 ml) and added dropwise to a stirred suspension of LiAlH4 (4.6 g; 0.121 mmol; 14 equiv.) in dry THF (75 ml) in a three-necked round bottom flask equipped with a reflux condenser, magnetic stirrer, dropping funnel, and nitrogen adapter. The reaction mixture was stirred at reflux under nitrogen for 72 h. After cooling, the excess LiA1H4 was destroyed by dropwise addition of THF/I&O (45 ml; 2:l v/v). The white solid was washed repeatedly with CH& and the extracts combined. Rotary evaporation of the solvent afforded a colourless oil which was used without further purification. ‘H NMR (90 MHz, CDCl,): 6 2.60-6 2.75 (m, C&N, C&S, NH, 17H), 6 1.70 (m, CH2 CH, CH2 , 4H). r3C {‘H} NMR (62.9 MHz, CDC13): 6 46.7 (Cl&N), 6 32.3, 6 31.8 (SCH,CI-I,S), 6 30.2 (CH,S), 6 28.8 (CH,CI$CH,). MS (EI): 25 1 (M+). 2.5. N-(2’-Pyridylmethyl)-1,4,7-trithia-ll-azacyclotetradecane (L2 ) A two-necked round bottom flask equipped with a magnetic stirrer, reflux condenser, and nitrogen inlet was charged with a solution of the macrocycle LI (1.12 g, 4. 5 mmol), triethylamine (1.2g, 11.8 mmol), and 2-(chloroethyl)pyridine hydrochloride (0.73g, 4.4 mmol) in 50 ml of absolute ethanol. The solution was refluxed under nitrogen for 12 h with stirring. After cooling, the orange-red reaction mixture was extracted with CHCl, (3 X 100 ml). The chloroform layer was separated and washed with water (5 X 100 ml), then dried over anhydrous Na2S04. The solvent was removed using a rotary evaporator and the resultant brown residue was purified by column chromatography (silica gel, 30% ethyl acetate/70% dichloromethane). Fractions with an Rf value of 0.2 were collected and removal of the solvent yielded a pale yellow solid. Yield 500 mg, 33%. ‘H NMR (360 MHz, CDC13): 6 8.49, 6 7.62, 6 7.38, 6 7.14 (m, pyridine Hs, 4H) , 6 3.64 (s, CH,-py, 2H) , 6 2.70 (s, SCH, CH2 S, 8H), 6 2.50 (m, CH, N, CH2S, 8H), 6 1.71 (m, CH2CH2CH2, 4H). i3C {‘H) NMR (90.6 MHz, CDC13): 6 159.8 (quat. pyridine C) , 6 148.8, 6 136.2, 6 122.8, 6 121.8 (pyridine Cs), 6 61.7 (CH2-py) , 6 52.8 (CH,N), 6 31.2, 6 30.8 (SCH*CH$), 6 29.3 (CH,S), 6 27.8 (CH2CH2CH2). MS(C1): 406 (M + l), 434 (M + 29).

351

yellow precipitate formed immediately, but the reaction was allowed to stir for 16 h at room temperature to ensure completeness. The precipitate was filtered, washed with diethyl ether and air dried. Yield of the chloride salt, Pd(Li)C12, was 0.34 g (50%). The salt was dissolved in absolute EtOH and added to a saturated ethanolic solution of NH4PF6. The creamy yellow precipitate was filtered and redissolved in a minimum amount of CH$N. Slow diffusion of diethyl ether into the supernatant yielded X-ray quality yellow crystals. Anal. Calc. for PdCn,H2iNS3ClPF6*H20: C, 21.59; H, 4.17; N, 2.52. Found: C, 21.65; H, 3.91; N, 2.85%. 2.7. [Pd(L,)Cl](PF6) To a round bottom flask charged with a CH2C12 (10 ml) solution of L2 (0.160 g, 0.47 mmol) was added an acetonitrile solution (10 ml) of Pd(PhCN)& (0.142 g; 0.37 mmol). The reaction mixture was stirred at room temperature under a nitrogen atmosphere for 16 h, generating a creamy yellow precipitate and a pale yellow supernatant. The precipitate was collected by suction filtration, washed with diethyl ether and air dried. The yellow solid was dissolved in a methanolic solution of NH4 PF, and warmed for 30 min. The methanol was removed on a rotary evaporator and the solid was dissolved in acetonitrile from which yellow X-ray quality crystals were obtained by slow evaporation. Yield 180 mg, 77%. Anal. Calc. for PdC1t6HZ6N2SC1PF6: C, 30.53; H, 4.16; N, 4.45. Found: C, 30.82; H, 4.11; N, 4.68%. 2.8. [Pd(L2)](BF&0.5(CH,cN) An acetonitrile solution of L2 (O.l74g, 0.5 1 mmol) in a round bottom flask was purged with nitrogen for 10 min. A degassed acetonitrile solution of [Pd(CH3CN),](BF& (0.20 g, 0.45 mmol) was added dropwise. The mixture immediately changed colour to orange-brown but was refluxed under nitrogen for 1.5 h to ensure completeness of the reaction. After cooling, the solution was concentrated to -1 ml. Yield 200 mg, 71%. Anal. Calc. for PdC,6H26N2S3B2Fs.0.5(CH3CN): C, 3 1.75; H, 4.31; N, 5.44. Found: C, 31.71; H, 4.29; N, 5.42%. Slow diffusion of diethyl ether into the acetonitrile solution yielded orange-brown X-ray quality crystals which upon structural analysis were shown to contain 0.5 waters of crystallization replacing the acetonitrile observed in the analytical sample.

2.6. [Pd(L,)If Cl)fPF6) 2.9. Crystallography A solution of [PdCI,(CH3CN)z] in acetonitrile was prepared by dissolving PdCl* (0.25 g; 1.4 mmol) in 40 ml of dry CH$N and refluxing for a short period of time. Trace amounts of unreacted PdC12 were removed by Eltration. The yellow filtrate was added dropwise to an acetonitrile solution of Lt (0.354 g; 1.6 mmol). A pale

The experimental parameters for all three structures are presented in Table 1. The crystals were mounted in glass Lindemann tubes using epoxy resin. A preliminary unit cell was obtained, in each case, by using Weissenberg and precession photography.

352

B. Chak et al. I Inorganica Chimica Acta 246 (1996) 349-360

The [Pd(L,)](Cl)(PF6) crystal was transferred to a Nonius-Enraf CAD4 diffractometer. The unit cell was refined by using 25 pairs of reflections in the range 2646” in 28. The intensity measurements were obtained with a scan speed of 10” min-r while intensity standards were measured every hour. No appreciable change occurred in the standards during the data collection. Absorption correction to the data was made using EMPABS [ 121. The [Pd(L,)C1](PF6) crystal was transferred to a Picker Four-circle diffractometer automated with a PDP ll/lO computer. The unit cell was refined using 17 pairs of reflections in the 28 range, 25-49”. The intensity measurements were obtained by scanning in the 8-20 mode using 200 steps of 0.01” in 28, counting for 0.25 s per step. The background radiation was measured for 25 s before and after each scan. A set of three standard reflections preceded each batch of 50 measurements, with no statistically significant change in intensity observed during the data collection. The absorption correction was made using a locally modified version of an existing procedure [ 131. The [Pd(LZ))(BF&*0.5H,0 crystal was transferred to a Picker four-circle diffractometer automated with a PDP ll/lO computer. The unit cell was refined using 16 pairs of reflections in the 28 range, 20-45”. The intensity measurements were obtained by scanning in the 8-28

mode using 200 steps of 0.01” in 20, counting for 0.25 s per step. The background radiation was measured and a set of three standard reflections were treated as above. The direct-methods package of SHELXS [14] provided the location of the palladium atom for [Pd(LZ)](BF&*0.5Hz0 and [Pd(LZ)C1](PF6), and the lighter atoms were located by sets of Fourier maps and the structures refined by least squares procedures. A Patterson map was utilized to locate the palladium atom for [Pd(L,)](Cl)(PF& with the remaining atoms obtained through least squares refinement. The atomic scattering factors were those included in the SHELXS program together with the Pdf-curve obtained from the usual source [ 151. For all of the complexes, all of the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were neither located nor included in calculated positions in any of the structures. For [Pd(L,)](Cl)(PF,), refinement converged to a final R = 0.0532 (R, 0.056) with a maximum shift/e.s.d. of 0.05 in the final cycle and a maximum residual peak of 0.62 e Am3.For [Pd(L2)C1](PF& refinement converged to a final R = 0.0538 (R, = 0529) with a maximum shift/e.s.d. of 0.01 in the final cycle and a maximum residual peak of 0.70 e Am3. For [Pd(L2)](BF&0.5H,0, refinement converged to a final R = 0.0422 (R, 0.0486)

Table 1 Crystallographic parameters for [PdLl](BF&,

[PdL$l](PFe),

and [Pdw(BF4)2*0.5H20 [PdL2](BF4)2*0.5H20

Formula Mol. wt. Crystal system Space group Unit cell

PdC,&$J!$ClPFe 537.3 Triclinic pi (No. 2)

PdCfeH26N&ClPFe 629.39 Orthorhombic P212121 (No. 19)

a (A) b (A) c (A) or) B (“) Y (“) V(A3) Z Radiation Filter Temperature

10.127(2) 12.568(4) 7.141(2) 87.99(2) 95.55(2) 91.02(2) 904.0

13.305(3) 12.413(4) 14.060(3) 90.00 90.00 90.00 2322.1 4 MoKcz (0.71069) Graphite 20°C 1.767 1.800 0.20 x 0.17 x 0.60 12.61 0.777-0.817 ABSB02 Picker 0.1-50

D,,, (g cmm3) D ca~c.(g cme3) Dimensions (mm) p (cm-‘) Transmission range Abs. method Diffractometer 26 (“) h,k,l (min): h,k,l (max): Reflections I>& Parameters R. R,

1

MOKU (0.71069) Graphite 20°C 1.959 1.974 0.20 x 0.70 x 0.25 15.14 0,805-l .O EMPABS Nonius 0.1-50 -ll,O, -8 11, 14, 8 3079 2806 (n = 2) 208 0.0422,0.0486

0, 0.0 15, 14, 16 2333 2075 (n = 1) 271 0.0538,0.0529

PdCld-b%.%B2F80o.5

640.6 Monoclinic 12/a (No. 15)

19.045(g) 16.952(4) 16.635(6) 90.00 113.47(3) 90.00 4926.3 8 MoKo (0.71069) Graphite 20°C 1.723 1.727 0.15 x 0.37 x 0.50 10.85 0.680-0.856 ABSBOZ Picker 0.1-45 -2o,o, 0 18, 18, 16 3214 2106 (n = 3) 298 0.0532.0.0560

B. Chak et al. I Inorganica Chimica Acta 246 (1996) 349-360

353

with a maximum shift/e.s.d. of 0.03 in the final cycle and a maximum residual peak of 1.04 e A-3. 2.10. Instrumentation ‘H and lsC NMR spectra were either recorded at 250.1 and 62.9 MHz, respectively, on a Bruker WM250 spectrometer or 360 and 90.6 MHz, respectively, on a Bruker AX360 instrument. In both cases, the spectrometer was locked to the solvent deuterium resonance. 13C spectra were obtained with broad-band (‘noise’) irradiation at appropriate frequencies to decouple the protons. Twodimensional spectra were obtained using standard pulse sequences. ‘H spectra were also obtained on a Perkin Elmer R32 NMR spectrometer operating at a nominal frequency of 90 MHz. All deuterated solvents were used without further purification. Simulated NMR spectra were calculated by using UEAITR [ 161 and NMRPLOT [ 171 on an IBM 3090, utilizing an IBM PS/2 Model 50 as a remote terminal. The dynamic spectra and line-shape analysis were performed using DNMR3 [18]. 3. Results and discussion The synthesis of the ligands is illustrated in Scheme 1. It is a modification of various techniques, primarily the Richmann-Atkins synthesis of macrocyclic ligands [4c]. The cyclization step proceeded in reasonable yields. However, it is necessary to carry out a reductive detosylation to minimize cleavage of the carbon-sulfur bond. The synthesis of the palladium complexes is reasonably facile with apparent colour changes and, presumably, complexation occurring rapidly. The inclusion of one half of an acetonitrile in the molecular formula for the [Pd(L)](BF& complex provides a very close fit to the experimentally obtained analysis. The crystal structure contains one half of a water of solvation, which has metathesized with the acetonitrile found in the analytical sample. The presence of water was confirmed by the density measurement for the crystalline solid.

Fig. 1. ORTEP diagram for the [PdLt12 + cation. Selected bond lengths: Pd-N(1) = 2.088(4); Pd-S(I) = 2.302(l); Pd-S(2) = 2.269(l); PdS(3) =2.309(l) A.

Fig. 2. ORTEP diagram for the [PdL&l]+ cation. Selected bond lengths: Pd-Cl = 2.287(3); Pd-S(1) = 2.281(3); Pd-N(1) = 2.035(S); PdN(2) = 2.093(S) A. The interatomic distance between Pd and S(3) is 4.77 A.

3. I. Solid state structures

ORTEP presentations for the structures of the three complexes investigated in this work are provided in Figs. l-3, together with the atomic labelling scheme. The fractional atomic coordinates and selected bond lengths and angles are provided in Tables 2-6. The macrocyclic ligand in [Pd(L1)]2+ (Fig. 1) adopts an unusual structure in that the atoms of chelating bridges are all below the plane described by the donor set of atoms, except C7 which is 0.20 A above. The palladium atom is not co-planar with the donor set but lies above the plane at a perpendicular distance of 0.11 A (Table 7). The net result is that in the solid state the donor set is coordinated in a fashion similar to a square pyramidal geometry

Fig. 3. ORTEP diagram for the [Pdb I2 +cation. Selected bond lengths: Pd-N (1) = 2.078 (8); Pd-N (2) = 2.067 (8); Pd-S (1) 2.260(3); PdS(2) = 2.308(3) A. The interatomic distance between Pd and S(3) is 3.12 A, indicating an axial interaction.

B. Chak et al. / Inorganica Chimica Acta 246 (1996) 349-360

354 Table 2

Fractional atomic coordinates for [PdLI](Cl)(PF6) x/a

Atom Wl)

Cl(l) S(1) S(2) S(3) P(l) F(1) F(2) F(3) F(4) F(5) F(6) N(1) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10)

22313(3) 26357(15) 5400( 13) 7401(15) 38451(15) 7675(2) 7691(7) 7217(7) 7329(7) 6161(6) 7938(8) 9159(5) 3615(4) -778(6) -248(6) 1828(9) 3044(8) 4921(6) 5690(6) 4955(6) 3649(6) 2394(6) 1140(6)

25294(3) 42146(12) 36453(11) 15534(11) 14309(12) 1020(l) 1085(4) 975(5) 2241(4) 755(5) -186(4) 1357(7) 3550(4) 3223(5) 2668(5) 1332(8) 867(8) 2489(5) 2984(5) 3114(5) 4643(4) 5272(4) 4876(4)

z/C

27459(5) 73560(19) 16040(18) 42823(24) 42195(24) -843(2) 1349(6) -3042(7) -1023(9) -1008(10) -655(9) -638(10) 1601(6) 3058(9) 4903(9) 6532(11) 6207(13) 5203(9) 3621(10) 1666(9) 2486(8) 1888(8) 2714(8)

u

=l

270(2) 430(5) 340(4) 437(5) 451(5) 43(l) 94(2) 106(3) 1 lO(3) 115(3) 123(3) 134(3) 34(l) 43(2) 45(2) 85(4) 82(3) 45(2) 50(2) 48(2) 39(2) 41(2) 37(2)

Estimated standard deviations are given in parentheses. Coordinates x lo” where n = 5,5,5,4,4,4,4 for Pd, Cl, S, P, F, N, C. Temperature parameters x 10” where n = 4, 4, 4, 3, 3, 3, 3 for Pd, Cl, S, P, F, N, C. Q = the equivalent isotropic temperature parameter. Uq= 1/3Z~illiiai*ai*(ai’ai), T= exp -(8X2Uiso sin20/L2).

without the presence of an axial ligand or interaction. Further evidence for this effect is provided by the fact that the basal angles for Sl-Pd43 (173.6”) and S2-Pd-Nl (173.2’) are intermediate between the 180” angle that might be anticipated for a square planar geometry and the 165” angle that is calculated for a square pyramidal geometry [ 191. In addition, the donor set exhibits a tendency towards a butterfly arrangement which has been observed in a number of other palladium macrocyclic complexes [8d,20,21]. This results in two of the donor atom set (Sl and S3) lying above the mean plane and two atoms (S2 and Nl) below. Although these deviations are not very large, the nature of the coordination geometry is unusual with an effectively square pyramidal geometry being realized. The butterfly distortion constitutes a slight shifting of the atoms towards a trigonal bipyramidal coordination. Despite this displacement of the palladium from the plane of the donor set, the bond lengths between the metal ion and the donor atom set are typical of complexes studied [3], with Pd-Sl and Pd-S3 having closely similar values of 2.302(l) A and 2.309(l) A. The Pd-S2 bond is shortened slightly to 2.269(l) A, owing to the truns influence of the nitrogen donor. The Pd-Nl bond at 2.088 A, is similar to that found in other aza-macrocyclic complexes of Pd(I1) [8a,20-231.

The bond angles for the five- and six-membered chelate rings are also typical of the values that are observed in similar complexes [8a,20-231. Hence, the displacement of the palladium from the coordination plane and the unusual geometry observed for the macrocyclic ligand are not a consequence of large distortions in the nature of the chelating rings. In the [Pd(L2)Cl]+ cation (Fig. 2), the presence of the pyridine ring results in the palladium coordinating in an exo-cyclic or chelating manner, utilizing only two of the atoms in the macrocyclic donor set. The palladium atom is essentially coplanar with the coordinating atoms (Table 7) with deviations of not higher than 0.01 A observed for all five atoms. A similar disposition of the carbon atoms in the chelating rings to that observed in [Pd(L1)12+ is also not obtained. This is, in part, due to the bridging geometry adopted by the macrocyclic ligand in accommodating the metal centre. The six-membered chelating ring does adopt a chair geometry similar to that found in other macrocyclic complexes.

Table 3 Fractional atomic coordinates for [PdL2Cl](PF6) Atom

/a

Pd(l) Cl(l) S(l) S(2)

75391(6) 86400(23) 89373(21) 96448(26) 72210(23) 6301(6) 6445(6) 76779(29) 7522( 16) 6798(9) 7806(11) 8533(12) 8407(10) 6955( 11) 6660(9) 7610(9) 8613(11) 9148(9) 9474(12) 8730(10) 7648(11) 5950(9) 5923(9) 6187(9) 5559(8) 5435(8) 4509(9) 4472(9) 5377(11) 6276(9)

S(3) N(1) N(2) P(1) F(1) F(2) F(3) F(4) F(5) F(6) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(l0) C(11) C(21) C(22) ~(23) ~(24) ~(25)

Z/C

-486(6) -10773(25) 741l(24) 81379(27) -15259(24) -773(7) 817(7) 47254(26) 5132(12) 3878(11) 4268(10) 5552(13) 3879(10) 5627(10) 2ooo(9) 2193(8) 2097(9) 47(9) -1097(12) -2965( 11) -2661(11) -1378(10) -899(10) 283(8) 819(9) -245(8) -626( 11) -1608(10) -2194(9) -1769(9)

84102(5) 92679(22) 77808(21) 57892(24) 52913(22) 8969(6) 7663(6) 34584(30) 2459(10) 3289(11) 4458(8) 3609(15) 3069(10) 3844(11) 7498(9) 6850(8) 7364(10) 6650(8) 6873(9) 5849(12) 6004(13) 5744(7) 6769(8) 6737(7) 8329(8) 8860(7) 9183(9) 9638(8) 9743(9) 9405(7)

“q

290(2) 468(10) 381(9) 543(11) 465(10) 32(3) 31(3) 591(12) 211(10) 138(6) 137(6) 198(10) 135(6) 157(7) 44(4) 44(4) 51(4) 46(4) 64(5) 67(5) g2(6) 43(4) 45(4) 37(3) 42(4) 36(3) 52(4) 51(4) 54(4) 39(4)

Estimated standard deviations are given in parentheses. Coordinates X 10” where n = 5, 5,5,4,5,4,4 for Pd, Cl, S, N, P, F, C. Temperature parameters x lo” where n = 4, 4, 4, 3, 4, 3, 3 for Pd, Cl, S, N, P, F, C. Ueq = the equivalent isotropic temperature parameter. ueq = 1/3~iriuiiai*ai*(ai.ai), T= exp -(8n2Ys” sin20/A2).

B. Chak et al. I Inorganica Chimica Acta 246 (I 996) 349-360 Table 4 Fractional atomic coordinates for [PdL2J(BF4)2*OSH20 Atom

Pd(l)

S(l) S(2) S(3) N(1) N(2) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(l0) C(l1) C(21) C(22) C(23) ~(24) C(25) B(1) B(2) F(1) F(2) F(3) F(4) F(5) F(6) F(7) F(8) O(1)

x/a -2776(S) -67(19) 7741(16) 8354(20) -682(5) -1169(5) -1547(7) -1608(8) -916(8) 361(8) 1029(7) 1577(6) 1481(7) 510(9) -181(9) -883(7) -1781(7) -1385(6) -1765(7) -1401(g) -669(7) -327(6) 1666(10) 6303(9) 996(5) 2222(6) 1662(8) 1771(9) 6452(7) 6567(6) 5514(5) 6611(4) 2297(8)

z/C

32853(5) 45570( 18) 33126(18) 22629(19) 2175(5) 3214(5) 3992(7) 4567(7) 5105(S) 4912(8) 4373(7) 2926(g) 3028(7) 2613(8) 3186(8) 2789(7) 2720(7) 2054(7) 1335(7) 762(7) 906(6) 1622(6) 685(10) 3806( 10) 308(6) 220(7) 1248(8) 956(8) 4484(6) 3198(6) 3783(6) 3819(4) 3664(8)

19608(5) 23767(22) 16007(20) 35004(22) 1462(5) 2369(6) 2457(9) 1751(10) 1972(12) 1579(11) 1671(11) 2564(9) 3422(10) 4333(9) 3%5(8) 3249(8) 1680(9) 1441(7) 1131(7) 824(8) 849(7) 1177(7) 1130(12) 516(10) 848(7) 1471(9) 1609(12) 445(10) l%(7) 223(6) 2W6) 1415(5) 488(12)

%

440(3) 659(14) 606(13) 795(16) 43(3) 56(4) 80(7) 80(7) 96(8) 95(8) 95(8) 76(6) 81(6) 94(7) 85(7) 71(6) 72(6) 52(5) 64(5) 71(6) 62(5) 51(4) 73(8) 71(7) 139(6) 170(7) 250(12) 216(11) 150(7) 151(6) 124(5) 89(4) 212(12)

Nonoyama and Nonoyama [24] have examined a series of Pd(I1) complexes with 1,4,7-triazaalkanes wherein the 12membered ring coordinates in a bidentate fashion while the 13-membered ring is tridentate. It appears that the minimum bridge length that can traverse the truns sites of a square planar palladium is six atoms long, as seen in the present case. In addition, in the [Pd(L2)12+complex ion, the presence of a sulphur atom facilitates the bridging between sites since the C-S bond is longer than the C-C bond, allowing for a greater span of distance. However, the longer Pd-S bond may not be accommodated by this increase in the bridge length and the resulting strain across the ring may be the cause of the deviation from planarity observed (Table 7). The other consequence of this bridging geometry in [Pd(L2)12+, as opposed to the chelating geometry of is that the non-bonding sulphur atom is PW&ll+, proximal to the axial site of the palladium atom and at distance of only 3.120 8. Under such circumstances, the palladium(I1) centre could be viewed as being quasi-five coordinate with the position of the axial sulphur providing the locus for a nascient substitution reaction. In both of the pendant-arm complexes, the apical sulphur atom is readily pre-disposed to interaction with the metal centre, leading to a fluxional molecular structure in solution (vide infra). However, in [Pd(L2)12+more complex mechanistic behaviour is observed. In contrast to other macrocyclic palladium(I1) comTable 5 Selected bond and interatomic distances (A) Atoms

Estimated standard deviations are given in parentheses. Coordinates are X lo” where n = 5,5,4,4,4,4,4 for Pd, S, N, C, B, F, 0. Temperature parameters X 10” where n = 4.4, 3. 3, 3, 3, 3 for Pd. S, N, C, B, F, 0. UW = the equivalent isotropic temperature parameter. Ueq = 1/3~i~iClipi*ai*(ai’ai), T= exp -(8Z2Uiso sin26/A2).

Of principal interest in this complex is the disposition of the apical sulphur atom. The intra-molecular distance between the palladium atom and S3 is 4.772 8, while the inter-molecular distance is 3.302 A. Neither of these distances is within the normal bonding range for Pd-S bonds (2.30-2.50 A) and they are in excess of the distances observed for other axial interactions [3]. The absence of bonding in the solid state is manifested in the presence of the palladium within the plane of the donor set since neither interaction results in a distortion of the palladium coordination geometry. However, the inter-molecular distance does indicate a stacking of the cations within the crystalline lattice. The (Pd(L2)12+ cation (Fig. 3) exhibits a third type of coordination geometry in which the donor set is tetrahedrally distorted from the square planar arrangement observed in [Pd(L,)CI]+. This is in part due to the sixmembered bridge linking N2 and S2, through S3.

355

Distance

(a) [PdLtl(Cl)(PFa) 2.302(l) S(l)-Pd(1) 2.269(l) S(2tPd(l) S(3)-Pd( 1) 2.309(l) N(l)-Pd(1) 2.088(4) 1.829(6) C(l)-S(l) 1.830(5) C(lO)_S(l) 1.827(6) C(2)-S(2) 1.873(8) C(3)-S(2) 1.819(8) C(4)-S(3) (b) [PdhClI(PFe) Cl(l)-Pd(1) S(l)-Pd(1) N( I)-Pd( 1) N(2)-Pd(1) C(l l)-N(2) C(2l)c(ll) S(2),..Pd(l)

2.287(3) 2.281(3) 2.035(8) 2.093(S) 1.506(13) 1.526(15) 4.910

(c) [Pdu(BF4)2.0.5H20 2.260(3) S(l)-Pd(1) S(2)-Pd( 1) 2.308(3) 2.078(8) N(l)-Pd(1) 2.067(8) N(2)-Pd( 1)

Atoms

Distance

C(5)-S(3) C(7)-N(1) C(8)_N(l) W)-c(l) C(4)c(3) C(Stc(5) C(7)-c(6) C(9)-c(8) C(lOkC(9)

1.821(6) 1.468(7) 1.531(7) 1.524(S) 1.415(12) 1.540(9) 1.523(9) 1.527(S) 1.520(8)

C(21)-N(1) C(25)_N(l) C(22)-C(21) C(23)-C(22) C(24)-C(23) C(25)-C(24) S(3)...Pd(l)

1.334(13) 1.381(14) 1.3%(15) 1.377(17) 1.413(18) 1.391(16) 4.772

C(21)-N(1) C(21)-C(ll) C(l l)-N(2) S(3)...Pd(l)

1.341(13) 1.498(15) 1.520(14) 3.120

Estimated standard deviations are given in parentheses.

B. Chak et al. I Inorganica Chimica Acta 246 (1996) 349-360

356

plexes, the proximity of the donor atom to the metal centre does not result in a putative square pyramidal structure. The tetrahedral distortion of the [Pd(L2)12+ cation results in the metal ion position approximating the centroid of the donor atom set. The Pd-S bond lengths are 2.260(3) 8, and 2.308(3) %, and the Pd-N bond lengths are 2.078(8) 8, and 2.067 A, which are comparable to those obtained in the other cations. The shortened Pd-S(1) and slightly longer Pd-N(1) bonds are rrans to one another, reflecting the relative trans influence of the sulphur and nitrogen donors. The bond angles within the five- and sixmembered chelating rings are consistent with the observed geometry in analogous complexes involving this metal ion [ 8a,20-231. The observed variations in the coordination geometries of the three cation complexes in the solid state are replicated in solution. The palladium complex of the parent macrocycle, L,, does not exhibit any indication of fluxional behaviour within the accessible temperature range, consistent with the encapsulation of the metal ion. In contrast, the complexes of L2 display differing forms of fluxionality involving the non-bonding donor atoms of the macrocycle.

Table 6 Selected bond angles (“) Angle

Atoms

Angle

C(5)-S(3)-c(4) C(7)-N( 1)-Pd( 1) C(8)-N(l)-Pd( 1)

C(3)-S(2)-C(2) C(4)-S(3)-Pd(l) C(5)S(3)-Pd(1)

88.6(l) 173.6(l) 87.1(l) 90.4( 1) 173.2(l) 93.2(l) 101.6(2) 99.4(2) 101.8(3) 96.9(2) 96.7(3) 100.6(4) 102.5(3) 96.3(2)

C(8)-N(lkC(7) C(2)-c(l)-S(l) C(l)-c(2)-S(2) C(4)-c(3)-S(2) C(3)-c(4)-S(3) C(6)-c(5)-S(3) C(7)C(6)-c(5) C(6)-C(7)-N(1) C(9)-C(8)-N(1) C(lO)-c(9)-c(8) C(9)-C( IO)-S( 1)

104.8(4) 114.3(4) 111.4(3) 110.9(4) 112.7(4) 106.6(4) 111.4(6) 115.8(5) 108.3(4) 117.7(S) 115.5(S) 112.1(4) 116.1(5) 110.0(4)

(b) [PdL#](PFe) S(l)-Pd(l)-Cl(l) N(l)-Pd(l)-Cl(l) N(l)-Pd(l)-S(1) N(2)-Pd(l)-Cl(l) N(2)-Pd(l)-S(1)

85.5( 1) 93.9(3) 179.2(3) 175.7(3) 98.7(2)

N(2)-Pd(l)-N(1) C(21)-N(l)-Pd(1) C(l I)-N(2)-Pd(l) C(21)&C(ll)-N(2) C(ll)-C(21)-N(1)

81.8(3) 116.0(7) 103.5(6) 112.8(8) 112.8(9)

(c) [PdL2](BF4)2*0.5H20 86.8(l) S(2)-Pd(l)-S(I) 171.7(2) N(l)-Pd(l)-S(1) N(I)-Pd(l)-S(2) 97.9(2) 94.2(3) N(2)-Pd(l)-S(1) 175.6(3) N(2)-Pd(l)-S(2)

N(2)-Pd(l)-N(1) C(21)-N(l)-Pd(1) C(1 I)-N(2)-Pd(1) C(21)-C(ll)-N(2) C(ll)-C(21)-N(1)

81.7(3) 110.9(7) 105.6(7) 107.4(9) 118.8(9)

Atoms

(a) [PdLdtCl)tPFd S(2)-Pd(l)-S(1) S(3)-Pd(l)-S(1) S(3)-Pd(l)-S(2) N(l)-Pd(l)-S(1) N(l)-Pd(l)-S(2) N(l)-Pd(l)-S(3) C(l)-S(l)-Pd(l) C(lO)-S(l)-Pd(1) C(lO)-S(l)-C(I) C(2)S(2)-Pd(1) C(3)-S(2)-Pd( 1)

Estimated standard deviations are given in parentheses.

Table 7 Mean planes for coordinated atoms Atoms

X

Y

Z

P

(a) [PdLII(Cl)@‘F,$ Equation: -0.0375.x - 0.5938~ 0.3545 S(1) 0.4190 S(2) 3.5705 S(3) 3.4712 N(1) Other atoms 2.0134 Pd(1) (b) [PdL$t](PF6) Equation: 0.0167x - 0.681 ly 11.4955 Cl(l) 11.8911 S(1) 8.3832 N(1) 8.5749 N(2) Other atoms 10.0308 Pd(1) 12.8324 S(2) 9.6075 S(3) (c) [PdL2](BF4)2*0.5H20 Equation: -0.2243x + 0.3238~ -1.5874 S(1) 0.4138 S(2) -2.2680 N(1) -3.7948 N(2) Other atoms -1.8278 Pd(1) -0.7281 S(3)

- 0.8037~ + 3.6755 = 0 4.6189 1.1394 2.0540 3.0419 1.8986 2.9973 4.4985 1.1376 3.2439

0.0036 -0.0048 0.0051 -0.0404

1.9506

0.1060

- 0.7320~ + 8.4352 = 0 -1.3372 13.0307 0.9199 10.9398 -0.9596 12.6101 1.0148 10.7739

0.0001 -0.0001 -0.0013 0.0012

-0.0603 10.1016 -1.8941

-0.0114 -4.1895 4.4423

11.8247 8.1396 7.4396

- 0.9192~ + 0.5005 = 0 7.7250 3.6265 5.6155 2.4425 3.6867 2.2302 5.4484 3.6145 5.5692 3.8361

2.9919 5.3412

0.0243 -0.0192 0.1530 -0.2065 -0.0364 -3.0036

3.2. Solution structures The *H and i3C NMR spectra for the [Pd(L,)Cl]+ and [Pd(L2)12+ cations, at -40°C are provided in Figs. 4 and 5. These spectra are viewed as providing the static structures for the complexes. The 13C NMR spectrum for [Pd(L1)12+ exhibits five lines which is consistent with a time-averaging of the crystalline structure around the molecular mirror plane. Hence, for example, C7 and C8 are seen as a single resonance at 6 54.76 ppm. This time-averaging does not require inversion of any of the donor atoms but instead simple torsional rotations around the Pd-N and Pd-S bonds. Hence, it is anticipated that it is a very low energy process. Observation of the ‘H and 13C NMR spectra to -45°C provided no evidence for fluxionality or coalescence of any of the peaks, indicating that the static structure is achieved only at much lower temperatures. Using -45°C as an upper limit for the exchange mechanism and a reasonable chemical shift difference between the static conformations, as depicted in the crystal structure, an activation barrier of =l kcal mol-’ would be obtained. This is reasonable considering that the interconversion of the isomers is through a simple mechanism involving the twisting of torsional angles.

Et. Chak et al. I Inorganica Chimica Acta 246 (1996) 349-360

PPM

PPM

160

x

6

4

2

0

Fig. 4. 1H and 13C NMR spectra of the [PdL$l]+ cation at -40°C in CD$N. The two resonances occurring at 4.2 and 4.7 ppm in the proton spectrum are the AB spin system associated with the methylene protons of the pendant arm (Cl 1, Fig. 2). Integration of the peaks is 1:l but the visual intensity would suggest a 2:l ratio. This is a result of coupling of the resonance at 4.7 ppm into the pyridine ring.

The t3C NMR spectrum of [Pd(L,)Cl]+ at -40°C exhibits 10 lines in the aliphatic region which may be ascribed to the 11 non-equivalent carbons observed in the static structure, with two of the resonances not being resolved. The absence of any other resonances suggests that at low temperatures, under conditions with no exchange, the solution structure of the molecule is equivalent to the solid state cation illustrated in Fig. 2. However, at increasing temperatures the molecule undergoes a fluxional interconversion to the equivalent conformation in which S3 is bound to the palladium atom and Sl is apical (Scheme 2). Similar macrocyclic inner-sphere substitution reactions have been observed for both Pd and Pt complexes [9,11]. That the interconversion proceeds directly to the equivalent conformation and not through an intermediate in which S2 is bound to the palladium is evidenced by the absence of a second low temperature species.

Elucidation of the rate constants and activation parameters for the inner-sphere substitution in complete detail is contingent upon complete simulation of the observed spectra. Unfortunately, despite the use of available 2D NMR techniques, deconvolution of the proton spectrum was not feasible. However, the AB doublet arising from the methylene protons of Cl 1 is distinct in the ‘H NMR spectrum. Any mechanism which involves the exchange of the axial and bound sulphur atoms of the ligand would necessitate a twisting of the bound tertiary nitrogen. By implication, the inversion of the conformation around this nitrogen must also involve the equilibration of the axial and equatorial protons. Thus, it is possible to follow the exchange reaction albeit in an indirect manner. An analysis of this AB exchange was performed using the DNMR3 computer programme [ 181 and from the temperature dependence of the exchange rate constants a value of ~15.5 kcal mol-t was obtained for the substitution reaction. This is comparable to the value obtained in the analogous Pt-thioether exchange mechanisms [ 111. The structure of the molecule, as indicated by the low temperature spectra, is similar to the crystalline structure, which has the apical sulphur atom pre-disposed to coordination to the palladium ion. This structural arrangement compared with intermolecular processes [25]. Although the synthesis of the complex [Pd(L2)]*+ was originally intended to provide a better understanding of the exchange processes in [Pd(L*)Cl)+, by a simplification of the possible

PPM

PPM

Scheme 2.

357

160

8

6

4

2

0

Fig. 5. ‘H and 13C NMR spectra of the [PdL2]*+ cation at -40°C in CD&N. The resonances occurring at ca. 22, 31 and 57 ppm in the carbon spectrum appear to be the result of the low temperature fluxional process.

B. Chuk et al. / Inorganica Chimica Acta 246 (19%) 349-360

358

mechanisms, the iH and [ 131 C NMR spectra obtained are more complicated. Over the full range of temperatures accessible in acetonitrile (-40 to 70”(Z), three separate but related fluxional processes occur for [Pd(L2)12+(For the purposes of discussion, these may be divided up into a low temperature process, occurring below the accessible temperature range of the solvent; a moderate temperature process, with coalescence near 10°C; and a high temperature process occurring above 50°C.) The 13C NMR spectra obtained at -40°C exhibits 26 lines, some of which are broadened significantly as a result of the low temperature fluxional process. (Fig. 5) Both the ‘H and i3C NMR spectra at this temperature indicate the presence of two different magnetic environments for the pyridine moiety suggesting that, in addition to the three fluxional processes, the molecule exists in two unique geometries. Examination of the 2-D NMR spectra of the complex, in particular the iH-13C HetCorr (Fig. 6), confirms that there are two distinct pyridine environments present (Py” and Pyb). The broadening of the peaks for Py”, with the concurrent loss of fine structure resolution, indicates a loss of the structural rigidity of this portion of the molecule and suggests that the pyridine is no longer bound to the palladium ion. In contrast, the ‘H NMR fine structure of species Pyb is analogous to that observed in [Pd(L2)Cl]+ and may be attributed to coupling of the ring protons to the methylene protons of the pendant arm indicating a more rigid structural environment. This assignment is confirmed by simulated spectra, the relative sizes of the AB peaks observed in the -40°C spectra (Fig. 5), and results obtained for similar molecules [3]. In this context, species Pyb is viewed to be a solution analogue of the solid state geometry. The existence of two low temperature forms of the cation and the indication that the pyridine in one of the forms is not structurally rigid and hence, not bound to the

-130

150

c

-JI,

1 PPM PPM

6.8

8.4

8.0

7.6

7.2

Fig. 6. 1H-13C Hetcorrelation spectrum of the [Pw2+ cation at -40°C in CD3CN. The coupling of the pyridine peaks into two subsets of spectra is evidenced by the two peaks occurring at 150.56 and 150.36 ppm and their corresponding proton peaks at 8.85 and 8.28 ppm, respectively.

S,. Sg exchange

Exchange 01 pyridmes

r

I Torsional

equilibrium

J

Scheme 3.

palladium atom, conforms with the illustrated mechanism (Scheme 3). In this mechanistic scheme, Pya is assigned to the complex in which the palladium ion is bound within the macrocyclic cavity while Pyb is considered to involve the ‘chelated’ palladium ion. The low temperature fluxional process is a result of the inhibited torsion of the molecule around the palladiumtertiary nitrogen bond for the macrocyclic bound palladium (Py”). Although the solid state structure of this species is not available, it is presumably analogous to the [PdL212+ cation, which exhibits distinct six-membered chelating rings in the solid state. The absence of observable fluxionality in the NMR spectra for [Pd(L1)12+is due to the significant decrease in steric bulk for the proton relative to the methylpyridyl group. The torsional twisting of the macrocyclic ring around the palladium-nitrogen bond is responsible for the observed broadening of the 13C NMR peaks at -4O”C, which appear to be just coming out of coalescence. Preliminary calculations suggest that the activation barrier for this process is less than 4 kcal/mole. At higher temperatures, the aliphatic portion of i3C NMR spectrum of this complex collapses to six lines which do not undergo further fluxional processes until much higher temperatures. This is consistent with the time averaged imposed symmetry of the molecule. The remaining spectroscopic features are a result of the crystal structure-like molecule (Pyb), which undergoes a fluxional process in the available solvent temperature range. An analysis of the AB sub-spectrum resulting from the methylene protons of the pendant arm provides a value of ca. 12.3 kcal mol-i. Again, this analysis is not complete in that the total spectroscopic analysis was not possible and thus, it makes the assumption that the exchange of the apical ligand must entail and be coincident with the inversion of the tertiary nitrogen. However, the

B. Chak et al. I Inorganica Chimica Acta 246 (I 996) 349-360

359

of the binding of palladium(I1) within macrocyclic cavities. The solid state structural features are, in part, lost in solution due time-averaging but the observed NMR spectra display distinctive features. In particular, the observed spectra for [Pd(L2)12+indicate that pendant-arm macrocycles provide a rich diversity of structural forms. 5. Supplementary

material

A complete list of all bond lengths and angles, interatomic distances, and lists of structure factors is available from the authors upon request. Acknowledgements

PPM

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'

6

'4

'

2

0

Fig. 7. lH and 13C NMR spectra of the [Pdg2+ cation at 50°C in CD3CN. The loss of spectroscopic resolutionin both spectra is indicative of the high temperature interconversion of the two structural forms of the cation.

value obtained is typical of the activation barrier for this type of substitution process. That it is less than the barrier obtained for [Pd(L2)Cl]+ is consistent with the differences observed between the two cations in the solid state. Finally, at higher temperatures (Fig. 7), there is an additional process which results in the complete broadening of both the ‘H and 13C spectra. This is the interconversion of the two structural forms and represents the incorporation and displacement of the palladium atom within the macrocyclic cavity. An estimate of the activation barrier is not possible due to our inability to model the spin systems completely. Line shape analysis suggests a value greater than 18 kcal mol-*. The fluxional processes observed in this molecule are considered to be internally consistent and the value obtained for the activation barrier for the interconversion of the ‘chelated’ compound is certainly consistent with values obtained in this and other studies. The apical disposition of the sulphur atom and the trans-disposed points of attachment would certainly be expected to provide a lower activation barrier for the inner-sphere substitution process observed here. However, the complexity of the solution structure for [Pd(L2 )12+ precludes a more detailed interpretation of the data. 4. Conclusion The solid state and solution structure of the three cations analyzed within this study illustrate the variability

We would like to thank the Natural Sciences and Engineering Council of Canada (NSERC) and the University of Victoria for their continued support of this research. B.C. also acknowledges the receipt of a NSERC postgraduate scholarship. The assistance of Mrs. K. Beveridge is appreciated. References

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