MOLECULAR AND CRYSTAL STRUCTURE OF FLUXIONAL BISll ...

Pol.vhdron Vol. IO. No. 2, pp. Printed in Great Britain

179-185.

0277-5387/91 $3.00+.00 (3 1991 Pergamon Press

1991

plc

MOLECULAR

AND CRYSTAL STRUCTURE OF FLUXIONAL BISll-ISOPROPYL-3-METHYL4NALKYL(ARYL)ALDIMINOPYRAZOLE-5-THION]ATO NICKEL(I1) COMPLEXES [pyr(N-CH,)&]Ni, [pyr(N-Ph),SJNi, [pyr(N-t-Bu),SJNi

A. L. NIVOROZHKIN,” L. E. NIVOROZHKIN and V. I. MINKIN Institute of Physical and Organic Chemistry, Rostov University, 344711 Rostov on Don, U.S.S.R.

T. G. TAKHIROV and 0. A. DIACHENKO Department of Institute of Chemical Physics of the Academy of Science of the U.S.S.R., 142432 Chernogolovska, U.S.S.R. (Received 23 April 1990 ; accepted 6 August 1990) Abstract-X-ray crystallographic studies of fluxional bis[ 1-isopropyl-3-methyl-4-Nalkyl(aryl)aldiminopyrazole-5-thionlato nickel(I1) complexes [pyr(N-CH3)2S2]Ni, [pyr(NPh)2S2]Ni and [pyr(N-t-Bu)2S2]Ni show them to possess pseudo-tetrahedral configurations with the values of dihedral angles 0 between chelate planes being 81.9 and 82.4 ; 68.7 ; 96.3” respectively. The values of 8 as well as those of the NNiN’ valence angles have been found to correlate with the energy barriers against intramolecular enantiomerization of tetrahedral nickel configuration R e S.

In nickel(I1) metal chelates possessing the coordination site of type NiN2X2 (X = 0, S, Se), the stereoconfiguration of the central atom is strongly affected by the type of ligating atom X. ligand structure and steric volume of the substituent attached to the coordinated nitrogen atom. ‘**Of these compounds, 1,3-substituted bis[4-N-alkyl(ary1) aldiminopyrazole-5-thion(seleno)]ato nickel(I1) complexes display a significant stabilization of the tetrahedral form both in solution and in the solid state, which has been established by ‘H NMR spectral study, measurements of magnetic moments and by X-ray structure investigation of two of these complexes. 3-6 In solution, the above complexes undergo a rapid interconversion of their enantiomerit forms R Z$ S, with the free activation energy of the inversion process falling within the range of 4&75 kJ mol-‘.4 The magnitude of the energy

barrier increases when going from N-n-alkyl and aryl to N-s-alkyl and t-Bu substituents presumably owing to the steric strain developed by bulky substituents in the transition or intermediate planar structure inherent in the mode of enantiomerization. In the present work a crystallographic study of complexes 1a-c containing aryl and alkyl groups at

R, , R,. R3.Alk.

I RI

*Author to whom correspondence

I RI

Scheme 1.

should be addressed. 179

Ar

A. L. NIVOROZHKIN

180

coordinated nitrogen has been carried out with the aim of clarifying whether a correlation exists between the tetrahedral-to-planar distortion of the metal bond configuration and the energy barriers to the R$S interconversion. The previously reported’ structural data on the complex [pyr(NC6HI ,-cyclo),S,]Ni (Id) are also discussed in this connection. CH,

_N

/R R.CH,(a).Ph(b).t-BU(C).

/ N\5

\ r I-Pr

Y2

C,H,,-cycle

methods and refined by the least-squares technique, with full-matrix anisotropic (Ni, S, N and C atoms) and isotropic (H atoms) approximation using the Cruickshank weighting scheme. The hydrogen atoms in Ib,c were located by difference Fourier synthesis. Selected bond distances are listed in Table 2. Calculations were carried out on a BESM-6 computer using the RENTGEN-75 program package.’ The stereoprojections (Figs l-3) were obtained by the use of the ELLIDS program. lo

(d)

lo-d

RESULTS

EXPERIMENTAL The crystal samples have been obtained by slow crystallization from toluene-octane (1 : 1) mixture. Crystal data, details of intensity measurement and refinement for 1a-c are given in Table 1. The unit-cell dimensions were refined on a DRON-1 diffractometer with a monocrystal attachment. The absorption corrections were introduced for Ib,c by Gaussian integration7 using Udelnov’s program. * The structures Is-c were determined by direct

Compound Formula Molecular weight Melting temperature Crystal system

(“C)

a(A)

b (A) c (4 Y (7 Volume (A’) Space group Z D, (g cm- ‘) P (cm- ‘) Crystal dimensions (mm’) Data collection instrument Radiation Min sin Z?jn Max sin !3/1 Scan method Number of observed reflections Reflections used in refinement Final R was determined

AND DISCUSSION

The complexes 1a-c possess pseudo-tetrahedral structures. The values of dihedral angles 8 between the planes of chelate rings, which indicate the degree of the tetrahedral-to-planar distortion (0 = 90” for ideal tetrahedron), are equal to 8 1.9, 82.4 (Ia), 68.7 (Ib) and 96.3 (1~). The value of 8 exceeding 90” as is the case with complex Ic corresponds to the S . . . S trans-planar distorted structure. For the complexes Ia,b (6’ < 90’7 a distortion towards the c&planar geometry occurs. The diagonal twist mechanism of the Rz+S enantiomerization includes reciprocal rotation of

Table 1. Crystal data, details of intensity

“The structure

et al.

measurement

and refinement

Ia

Ib

C 18H 28N 6NiS2 451.3 213 Monoclinic 29.966(8) 12.640(4) 6.105(2) 79X(2) 2274( 1) B2/b 4 1.33 10.8 0.35 x 0.45 x 0.60 DAR-UM MO-K, 0.034 0.067 0x0120

C 28H 32 N 6NiS2 575.4 215 Monoclinic 21.387(6) 17.326(6) 9.375(4) 121.93(4) 2948(4) B2/b 4 1.30 23.3 0.04 x 0.55 x 0.18 DAR-UM Cu-K, 0.055 0.604 uI-u/2w

146812

1584Z> 20(Z)

161312

1426 0.055

1459 0.051

2a(Z)

1260 0.072 for pseudo-unit-cell

[c’ = 2c = 12,210(4) A].

IC

GJ-LoN6N& 535.4 260 Monoclinic 18.095(5) 17.086(5) 9.394(2) 76.64(4) 2826(3) P2,lb 4 1.26 23.9 0.18 x 0.07 x 0.21 RED-4 Cu-K, 0.028 0.499 (lw/2w 3a(1)

Fluxionai nickel(I1) complexes chelate rings about the C(2) axis towards the achiral planar structure. Therefore, a correlation between the values of energy barriers of the tetrahedral inversion and the degree of the tetrahedral-toplanar distortion may, in principle, take place in accordance with the structure co~eiation concept. ’ ‘3’* The preference of c&planar structures

181

for the numerous bis-chelate complexes with the NiN& coordination site has been revealed,‘s2 thus the gradual decrease of @values may be associated with the decrease of the energy barriers to the enantiomerization of complexes I, AGS, as found from ‘I3 dynamic NMR spectral data. Indeed, a comparison of 8 and Gf values (Table 3)

Table 2. Selected bond distances and valence angles of complexes @a-d) Bond

D (A)

Ni-S Ni-N( 1) S-C(l) N(l)-C(3) N(2)-N(3) N(2WXl) N(3tC(5) C(0-W) C(2)-C(3) C(2)-C(5)

Mol. A Mol. B 2.237(2) 2.247(2) 1.975(7) 1.985(7) 1.715(8) 1.726(X) 1.28(l) 1.28(l) 1.38(l) 1,38(l) 1.36(l) 1.36(I) 1.31(l) 1.30(l) 1.41(l) 1.41(l) 1.43(l) 1.43(l) 1.43(l) 1.42(l)

Ni-S Ni-N(1) S-C(l) N(l)--CtJ) N(2)-N(3) N(2)-(W) N(3WXO) C(lWX2) C(2)-C(3) C(2WXlO)

2.229(l) 1.984(3) 1*708(4) I .300(6) 1.380(6) 1.344(6) 1.31l(5) 1.403(5) 1.412(5) 1.419(7)

Angle (Ia)

Ni-S(1) N&S(2) Ni-N( I) Ni-N(4) S(l)-W) S(2)--c(l3) N(lPCX3) N(2)---N(3) N(2)-(W) N(3PW) N(4)-C( 15) N(5)--N(6) N(5WW3) N(6t-C(20) C(l>-e(2) C(2)--C(3) C(2)---C(8) C(l3>--c(l4) C(l4)--CXl5) C( 14)---C(20)

2.254(l) 2.253(l) I .984(4) I .995(4) 1.697(4) 1.694(7) I,288(5) 1.372(S) 1.362(6) 1.31lf6) 1.289(6) 1.383(5) 1.34016) 1.30X(7) 1.396(6) 1.42?(6) 1.416(6) 1.416(7) 1.413(8) I .409{6)

* (“)

R = CH, S-Ni-S S-Ni-N( 1) S-Ni-N( 1’) N(l)--Ni-N(1’) Ni-S-C(l) Ni-N(l)-C(3) S-“-W)--c(2) Ccl )+2)--c(3) N(l)--C(3)-C(2)

Mol. A Mol. B 117.5(l) 116.5(l) 100.9(2) 101.8(2) 118.2(2) 117.6(2) 100.9(3) 101.2(3) 10X1(3) 103.6(3) 129.7(6) 128.0(?) 129.5(6) 131.5(7) 130.1(8) 128.6(7) 124.7(9) 126.5(8)

(Ib)R=Ph S-Ni-S S-Ni-N( 1) S-Ni-N( 1’) N(l)---Ni-N(1’) Ni-S--C!( I) Ni-N(I)-C(3) S---W>-c(2) C( 1t-C(2)--c(3) N( 1)---C(3)-C(2)

98.9(l) 100.2(l) 128.6(l) 104.0(l) 105.7(1) 128.5(3) 130.2(4) 128.8(S) 126.2(4)

(Ie) R = t-&i S(1jNi-S(2) S(lj-Ni-N(1) S(1)--Ni-N(4) S(2)---Ni-N( 1) S(2j-Ni-N(4) N( I)---Ni-N(4) Ni-S(I)-C(l) Ni-S(2)--C( 13) Ni-N( 1)---C(3) Ni-N(4 jC( 15) S(l)---W)--c(2) C(l)--~(2~(3) N( 1)--C(3t-c(2) S(2)---W3)-W4) C(l3)---C(l4)-WV N(4~(15~~14)

105.22(7) 102.0(l) 106.7(1) 105*2(l) 102.2(1) 133.0(2) 103.9(2) 104.4(2) I25.8(4) 124.6(4) 131.8(4) 12&l(4) 128.4(S) 130.5(4) 128.5(4) 129.3(5)

A. L. NIVOROZHKIN et al.

182

Table 2-continued Bond

Ni-S( 1) Ni-S(2) Ni-N( 1) Ni-N(4) S(l)--c(l) S(2)---W5) N(1)-C(3) N(2)_N(3) N(2)-C(l) N(3)---C(l0) N(4)--C( 17) N(5)-N(6) N(5)--C(l5) N(6)--C(24) C(l)-C(2) C(2)--c(3) C(2)-C(lO) C(l5)--c(l6) C(l6)-C(l7) C(l6>-c(24)

Angle

D (A) 2.243(l) 2.258(1) 1.975(2) 1.992(2) 1.720(3) 1.724(3) 1.276(4) 1.366(3) 1.357(4) 1.318(4) 1.290(4) 1.375(3) 1.344(4) 1.309(3) 1.389(4) 1.436(4) 1.497(4)

(Id) R = CBH , ,-cycle” S(l)--Ni-S(2) S(l)---Ni-N(1) S(l)--Ni-N(4) S(2)-Ni-N( 1) S(2)---Ni-N(4) N( I)-Ni-N(4) Ni-S(l)-C(1) Ni-S(2)---C( 15) Ni-N( 1)-C(3) Ni-N(4)-C( 17) S(2)-W)--c(2) C(l)--c(2)-C(3) N(l)-~(3>--~(2) S(2)--c(l5)-C(l6) C(l5)--c(l6PC(l7) N(4)-C(l7>-C(l6)

W(o) 114.74(4) 100.85(8) 125.71(8) 108.69(8) 100.37(8) 105.6(l) 104.5(1) 104.0(1) 128.1(2) 125.6(2) 131.6(2) 127.2(3) 127.7(3) 129.9(3) 128.7(3) 128.2(3)

1.392(4)

1.417(4) 1.410(4)

a Data from ref. 5.

for the complexes of type I shows that the compounds with alkyl substituents Ia,c,d are consistent with anticipated correlation. For the complex Ib, containing the phenyl substituent, the value of 8 falls appreciably, and is not accompanied by a corresponding change in AGf. Moreover, the cor-

relation of activation energies with steric constants of N-alkyl substituents, Es, ’3 has been observed. It may be assumed that, apart from 0, some other structural parameters reflect the steric strain in Iad. These include the valence angles at the nickel atom. Actually, the unusually high value of the

@P C(6)

Fig. 1. Molecule of complex la (R = CH,). A and B correspond to the molecules of Ia occupying the different positions within the same unit cell. The positions of Ni, C(4), C(7) and C(8) atoms have not been separated.

183

Fluxional nickel(I1) complexes

&H(l4.3)

Fig. 2. Molecule of complex Ib (R = Ph).

H(21.1)

aH(l9.3) 11,-n> H(IO) n”c”’

H(IZ2)fB

t-i117 II

~H(l2.2) @H(l2.3)

Fig. 3. Molecular of complex Ic (R = t-Bu).

A. L. NIVOROZHKIN

184

et al.

Table 3. -Ia (R = CH,) Q (grad) NNiN’ (grad) AC: (kJ mall ‘) E,

Id (R = C,H, ,-cycle)” Ic (R = t-B@

81.9; 82.4 100.9; 101.2 42.7 0.00

u Structural data from ref .5. ' The value obtained after re-examination

85.3 115.0 49.5h -0.79

III (R = Ph)

96.3 133.0 73.7 -1.54

68.7 104.3 45.2

of ‘H NMR spectral patterns observed earlier.4

NNiN’ angle in Ic, 133.0”, is considerably larger than the standard tetrahedral one of 109”. It is similar to that of the bis(N-t-butylsalicylideneiminato) nickel(I1) complex, 128.6”.14 Perhaps, such a bisphenoidal torsion arises by virtue of steric repulsion of t-Bu groups. A decrease in the steric volume of R for Ia,b,d as compared with Ic leads to the decrease in the NNiN’ angle in the order as follows Ic > Id > Ib > Ia. Noteworthy is that the enhancement of the bisphenoidal distortion rigorously follows the rise of enantiomerization barriers. According to the recent consideration by HaalandI based on the VSEPR theory, the increase in valence angles formed by metal-ligand bonds can be provoked by their larger covalency, manifesting itself in the shortening of the bond lengths. The examination of Ni-S and Ni-N bond lengths in Iad shows that Ni-N bonds remain practically unchanged, ca 1.98 A. whilst Ni-S demonstrates a tendency towards shortening, with the decrease of the value of the angle NNiN’. The changes in the Ni--S bond lengths span the range 2.253 (Ic) to 2.229 8, (Ib). The disagreement with what might be expected in view of the above concept can be explained in that the VSEPR approach is not fully extended to transition metal complexes. For all complexes Is-c studied, the chelate rings have nearly planar conformation. The deviations from the plane common to the pyrazole and chelate ring do not exceed 4”. The structure Ib contains a highly shortened intramolecular S. . . S contact, 3.388(2) A, similar to the known reference value of 3.43 &I6 as well as shortened contacts between phenyl substituents, C(9). . . C(9’) 3.241(8) A, C(4). . . C(4’) 3.346(7) A. Phenyl rings, which are rotated by 54” along the C-N axis relative to the chelate ring. form a contact with the CH=N moiety 0.2-0.3 8, less than the conventional value. As seen from the bond lengths in complexes Iad, the electron structure of the coordinated ligand is consistent with the delocalized iminothiolato form. It can be illustrated by the values of the bond

lengths in the SC(l)C(2)C(3)N(l) portion of the chelate ring, which are intermediate between the lengths in the model aminothione moiety S= C(l)-C(2)=C(3)--N(1) for I-isopropyl-3-methyl4-N-cyclohexylenamine-5-pyrazolethione, and iminothiol S-C(l)=C(2)