~ Pergamon PII : S0277-5387(97)00271-4

Polyhedron Vol. 17, No. 1, pp. 17 26, 1998 @ 1997 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0277-5387/98 $19.00+0.00

Tris (4-bromo- 1H-pyrazol- 1-yl)borato derivatives of first-row transition and group 12

and 14 metals. X-ray crystal structure of [HB(4- Brpz)3]2Cd. 113Cd solution NMR study of

bislpoly(pyrazolyl)borato] cadmium complexes

Giancar lo G i o i a Lobbia , a* Bruna Bovio,b~ " Carlo Santini , a Patrizio Cecchi , c C laudio Pettinari a and F a b i o Marchett i a

"Dipartimento di Scienze Chimiche, Universit~ di Camerino, Via S. Agostino I, 1-62032 Camerino, Italy

bDipartimento di Chimica Generale, Universifft di Pavia, viale Taramelli 12, 1-27100 Pavia, Italy

~Dipartimento di Agrobiologia ed Agrochimica, Universit~ della Tuscia, Via S. C. De Lellis, 1-01100 Viterbo, Italy

(Received 21 April 1997; accepted 23 June 1997)

Abstract--A series of metal complexes M[HB(4-Brpz)3]2 [HB(4-Brpz)3 = hydridotris(4-bromo-IH-pyrazol-1- yl)borate and M = Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb] were obtained from reaction of K[HB(4-Brpz)3] and the appropriate metal salt in aqueous solution. These air-stable complexes are sparingly soluble in most common solvents ; they have been investigated through elemental analysis, conductivity measurements, IR, UV-vis and, in the case of diamagnetic compounds, ~H and 13C N M R spectroscopy. The X-ray crystal structure of cadmium(II)bis[hydridotris(4-bromo-lH-pyrazol-l-yl)borate] has been resolved. Six-coordinate cadmium is present in a slightly distorted octahedron with the axes not linear [angles 176.8(3), 177.2(3) and 178.7(3)°]. The 'bite' of the ligand, i.e. N - - C d - - N angle, lies in the range 81.6(2)-84.1 (3) ° (weighted average 82.4(4)°). The C d - - N distances vary from 2.298(8) to 2.329(8) A (weighted average 2.317(5) A). The geometry around the B atom is tetrahedral. The N - - B - - N angles range from 105.8(8) to 110.5(7) ° (weighted average 108.5(7) °) ; the B - - N bond distances vary from 1.53(2) to 1.62(1)/k (weighted average 1.58(1)/k). We report and discuss also the It3Cd N M R solution data of several cadmium poly(pyrazol-l-yl)borates). © 1997 Elsevier Science Ltd

Keywords: pyrazolyl borates; group 12 and 14 complexes; cadmium; l J3Cd N M R ; X-ray; boron.

Poly(pyrazol-l-yl)borates are a versatile class of uni- negative nitrogen-donor ligands [1], which form several stable coordination compounds with transition and main-group metal ions [2]. These stable and flexible polydentate ligands are ideally suited for the synthesis of monomeric derivatives in which the coordination sphere about the metal atom can be carefully controlled. Several metal derivatives containing poly- (pyrazol-1-yl)borates have been found to be excellent

* Corresponding author. ] Corresponding author for the X-ray structure analysis,

model compounds in the interpretation of the spec- troscopic features of biological systems [3].

We have recently reported on the interaction between M n transition metal ions and two anionic tridentate chelating donors, namely hydridotris (4-methylpyrazol-l-yl)borate {HB(4-Mepz)3} and tetrakis (4-methylpyrazol- 1-yl)borate {B(4-Mepz)4} [4]. These ligands incorporate respectively three and four pyrazole rings with a methyl substituent in the 4- position which can display its electronic effect on the donor strength without any additional steric influence on the metal site, as has been observed in the case of 3- and 3,5-substituted poly(pyrazolyl)borates, currently employed by other researchers.

17

18

H

I B.°., ~s

B r - ~ [ N ~N,~ ] N ~ Br

[ N ~ Br

Fig. 1. Ligand used in this work.

G. Gioia Lobbia et al.

O

It seemed interesting to replace an electron releasing group such as a methyl with an electron withdrawing one such as Br, to investigate the consequent net elec- tronic effect on the chelating ability of the resulting ligand.

The present account details the synthesis and analytical and spectroscopic characterization of the anionic hydridotris(4-bromopyrazol-l-yl)borates {HB(4-Brpz)3} (Fig. 1) and of its derivatives with first- row transition and post-transition M 2÷ metal ions.

We also report the X-ray crystal structure and the 1~3Cd solution NMR characterization of the cadmium derivative Cd[HB(4-Brpz)3]2 together with a detailed comparison of the structural features of analogous cadmium compounds.

Comparison is made with the t t3Cd solution NMR data of several poly(pyrazolyl)borate cadmium com- plexes [4,5]. In fact the H3Cd NMR spectroscopy, based on the fact that cadmium has two important NMR active isotopes with spins = 1/2, has been used

as a 'spin spy' in the study of the properties of proteins containing zinc and other spectroscopically silent met- als [6]. In the last few years Reger et al. have inves- tigated the synthesis and J l3Cd NMR characterization of several Cd[HB(Pz)3]2 [7], [HB(Pz)3]CdL [8] and [HB(Pz)3]CdR [9] (where HB(Pz)3 stands for tris (pyrazolyl)borates in general and L = N3-, N2-, 02- and S2-donors, and R = alkyl and aryl groups), con- taining CdN6, CdNs, CdN4, CdN302, CdN3S2 and CdN3C central cores. The present lt3Cd NMR inves- tigation would be a further support of the actual knowledge of the rules governing H3Cd NMR chemi- cal shift in solution of this kind of complexes.

RESULTS AND DISCUSSION

Synthesis o f the derivatives

Derivatives 1-8 were synthesized in a water solution by the metathetic reaction of K[HB(4-Brpz)3] with the appropriate metal salt in the ratio 2 : 1, as indicated in the following equation :

MX 2 +2K[HB(4-Brpz)3] H:O ,

M[HB(4-Brpz)3]2 + 2KX (1)

1-8

M = Mn, Co, Ni, Zn, Cd or Pb, X = CI M = Fe or Cu, X2 = SO4

All the complexes were isolated as micro crystalline air-stable solids. Analytical data or derivatives 1-8 are summarized in Table 1. Some of them are high-melt-

Table 1. Analytical data of compounds 1-8

Yield M.p. Compoun& No. Color (%) (°C)

Elemental analysis ~

C H N

Mn[HB(4-Brpz)3]2 1 colourless 88 320 dec c CIsHI4B2Nt2Mn Fe[HB(4-Brpz)3]: 2 purple 86 > 320 CIsHt4B2NIzFe Co[HB(4-Brpz)3]2 3 pink-yellow 84 > 320 C~sH~4B~N~2Co Ni[HB(4-Brpz)3]2 4 lilac 84 > 320 CIsH~4B2NI2Ni Cu[HB(4-Brpz)3]2 5 sky-blue 86 320 dec' CIsHL4B2NIzCu Zn[HB(4-Brpz)3]2 6 white 87 308-311 C18HlaB2N 12Zn Cd[HB(4-Brpz)3]2 7 white 83 312-314 CtsHI4B2NI2Cd Pb[HB(4-Brpz) 3]2 8 white 84 266-268 CtsHI4B2NtePb

22.8 1.4 16.5 (22.7) (1.5) (16.7) 22.5 1.4 17.8

(22.6) (1.5) (17.6) 22.5 1.5 17.5

(22.6) (1.5) (17.6) 22.4 1.6 17.4

(22.6) (1.5) (17.6) 22.5 1.5 17.3

(22.5) 0.5) 07.5) 22.2 1.4 17.5

(22.4) (1.5) (17.4) 21.3 1.3 16.3

(21.4) (1.4) (16.6) 19.5 1.3 15.0

(19.5) (1.3) (15.2)

[HB(4-Brpz)3] is hydridotris(4-bromopyrazol-l-yl)borate CgHTBN6. b Calculated values in parentheses. Decomposes without melting.

Poly(pyrazol-l-yl) borates 19

ing solids whereas some others gradually decompose on heating without melting. They are insoluble in water, alcohols and hydrocarbons, and moderately soluble in chlorinated solvents, DMSO, aromatic hydrocarbons and acetone, in which they are not elec- trolytes.

IR data

Selected infrared data of the spectra of derivatives 1-8 in the solid (Nujol mull) state, are shown in Table 2.

In all the compounds 1 ~ a medium absorption in the range 2460-2480 cm-~, due to v(B--H) stretching, has been detected. This band is generally shifted to higher frequencies with respect to that of the pot- assium ligand salt K[HB(4-Brpz)3], and appears as a

single neat peak, whereas in the derivatives of {HB(4- Mepz)3} it was always split into two components [4].

Besides the expected weak absorptions over 3000 cm-~, assignable to heterocyclic v(C--H) stretching, we have found the bands due to 'breathing' of the pyrazole rings at unusually high frequencies, in the range 1650-1700 cm- l ; the same bands in the case of {HB(pz)3} and {HB(4-Mepz)3} derivatives were found in the range 1550-1600 cm -~ [1,4]. This fact could be explained by a stronger polarization on the pseudo-double C- -C and C - - N pz bonds in M [HB(4- Brpz)3]2, caused by the presence of an electron with- drawing group such as Br in 4-position. The bands assignable to v(B--N) vibrations are probably hidden under the typical strong absorptions of Nujol.

In the far-IR region two main groups of absorp- tions, likely due to various bending modes of vibration of the ligand skeleton, have been detected: the first

Table 2. Selected IR data (Nujot mull) of derivatives 1-8

No. Compound v(C--H)az v(B--H) 1500-1700 >600

K[HB(4-Brpz)3] 3155m 2418m 1694m, 1682m 3142w 1661m, 1514m

1504m

1 Mn[HB(4-Brpz)3]: 3128m 2476m 1689m, 1678sh 3114m 1662m, 1515m

1506w

2 Fe[HB(4-Brpz)3]2 3131m 2488m 1676m, 1654m 1636m, 1519m 1506w

3 Co[HB(4-Brpz)3]2 3128m 2474m 1694m, 1654m 1515m

4 Ni[HB(4-Brpz)3]2 3122m 2484m 1695s, 1654m 1515m

5 Cu[HB(4-Brpz)3]2 3120m 2482m 1696m, 1663m 1636m, 1514m

6 Zn[HB(4-Brpz)3]2 3150m 2460w 1684m, 1652m 1634m, 1515m 1505m

7 Cd[HB(4-Brpz)3]2 3128m 2472m 1684m, 1661m 3114m 1516m

8 Pb[HB(4-Brpz)3]2 3133w 2475sh 1695m, 1678m 2456m 1644m, 1635m

1510m

464w, 448m, 425m 380w, 322m, 304m 280w, 256m, 226w 21 lw, 202w 472m, 444w, 414m 336s, 306m, 289w 280w, 257sh, 245s 225w, 202s 488w, 474m, 428s 417m, 348m, 325w 315m, 304w, 290w 279m, 260m, 247w 227m, 21 lm 476m, 417m, 351m 322m, 295w, 265m 247w, 213m, 203m 477m, 418m, 357m 326m, 301w, 275w 269m, 253w, 231m 478m, 416m, 364m 345m, 312m, 290w 282w, 262m, 245w 234m, 226~, 202m 478m, 457~, 439m 418m, 382m, 376m 313m, 300m, 247m 363w, 340m, 31 lw 303m, 278m, 254m 224m 470m, 412m, 332m 304m, 290w, 247m 226m, 210w 456s, 419w, 409w 399w, 355w, 341m 325m, 302w, 268w 254m, 247m, 224m 203m

20 G. Gioia Lobbia et aL

between 480 and 400 cm ~ and the second between 300 and 200 cm -1. Finally the medium or strong absorptions at ca 380-330 cm-J could be tentatively assigned to v(N--M) stretching frequencies modes [lO].

UV-vis data

The UV-vis spectra of the potassium ligand salt and of derivatives 1-8 have been carried out in chloroform solution and the absorption maxima with their extinc- tion coefficients are reported in Table 3.

In all cases the extinction coefficient of the band at 240 nm is approximately twice of that found for K[HB(4-Brpz)3], in agreement with the 2 : 1 ligand to metal ratio found by elemental analyses (Table 1).

In all the spectra a broad and intense absorption was detected at ca 240 nm, which is due to the pyrazolyc intraligand transition [4,11]. This band is similar to that found in the spectra of {HB(4-Mepz)3} derivatives [4], thus indicating that substitution of an electron releasing group (Me) with an electron withdrawing one (Br) doesn't give rise to appreciable electronic changes on the relative energy levels of HOMO and LUMO in pz chromophores of the ligand.

It is worth noting that the derivatives 1-8 are less soluble in chlorinated solvents with respect to ana- logues {HB(4-Mepz)3} derivatives, and none of the expected d -d transition bands were detected at the concentration employed. Consequently a comparison of the crystal field stabilization energies between {HB(4-Brpz)3}, {nB(4-Mepz)3} [4] and {HBpz3} [11] derivatives is impossible.

In the spectra of Fe, Co and Cu complexes, two

additional bands at 270-290 and 310-340 nm were observed, which should be attributed to 'metal to ligands charge transfer' (MLCT) transitions, based on their relatively high extinction coefficient [11]. The MLCT absorption (e = 11620) found at 336 nm in the spectrum of Fe[HB(4-Brpz)3]2 is particularly intense.

N M R data

The ~H and ~3C NMR spectra have been performed in the case of diamagnetic zinc, cadmium and lead derivatives and the data are reported in Table 4. The choice of the deuterated solvent employed was dic- tated by the solubility of the derivatives, K[HB(4- Brpz)3] being sufficiently soluble only in protic sol- vents such as water and methanol, whereas Zn[HB(4- Brpz)3]2 only in deuterated DMSO.

The presence of only one set of signals in both proton and carbon NMR spectra, over the tem- perature range 220-300 K indicates equivalence of the three pz rings and, as expected, the presence of ligands acting as tridentate not only in the solid state (see crystallographic section) but probably also in solu- tion.

The 3- and 5-H resonances are downfield shifted upon coordination, those of Cd[HB(4-Brpz)3]2 being more deshielded with respect to those of Zn[HB(4- Brpz)3]2, in accordance with a likely stronger donation of the ligand in the former compound, and also with that observed in the other Zn and Cd poly- (pyrazolyl)borates derivatives previously synthesized [4,51.

The C(3) and C(5) nuclei absorb at approximately the same frequencies observed in Zn[HB(4-Mepz)3]2 and Cd[HB(4-Mepz)3]2 [4], whereas C(4) resonance

Table 3. UV~is data (in CHCI3) of derivatives 1-8

No. Compound A" ~:h C c Assignment a

K[HB(4-Brpz)3] 240 3480 5.7 x l0 4 intraligand l Mn[HB(4-Brpz)3]2 242 8990 3.6 x 10 -4 intraligand 2 Fe[HB(4-Brpz)3]2 242 8980 4.6 x 10 4 intraligand

294sh 4640 MLCT 336 11620 MLCT

3 Co[HB(4-Brpz)3]2 242 11420 3.2 x 10 -4 intraligand 284sh 1390 MLCT 310sh 720 MLCT

4 Ni[HB(4-Brpz)3]2 244 6690 5.1 x 10 4 intraligand 5 Cu[HB(4-Brpz)3]2 242 9220 3.2 x 10 -4 intraligand

268 1330 MLCT 308 645 MLCT

6 Zn[HB(4-Brpz)3]2 240 11100 1.0 x 10 4 intraligand 7 Cd[HB(4-Brpz)3]2 240 8600 0.8 × l0 4 intraligand 8 Pb[HB(4-Brpz)3]2 242 15990 1.1 x 10 -4 intraligand

"A in nm. ~ in (mol/1) i cm-i. 'C in mol/l. d MLCT stands for 'Metal to Ligand Charge Transfer'.

Poly(pyrazol-l-yl)borates

Table 4. tH and ~3C NMR data (in CDCI3) of derivatives 6-8

21

H-3 or H-5

No. Compound Solvent 6

C-3 or C-5

6 C-4

K[HB(4-Brpz)3] CD3OH 7.59 D20 7.64

6 Zn[HB(4-Brpz) 3]2 DMSO 7.56 7 Cd[HB(4-Brpz) 3]2 CDCI3 7.75 8 Pb[HB(4-Brpz) 3]2 CDC13 7.75

7.30 142.8 136.6 93.9 7.32 144.7 138.2 95.6 6.78 142.8 135.5 93.1 7.34 141.2 137.0 93.5 7.32 141.1 136.8 93.6

has been found in the range 93 96 ppm, upfield shifted of ca 20 ppm with respect to C(4) in {HB(4-Mepz)3} analogues [4], due to the different electronic features of the 4-substituents Br and Me.

T h e X - r a y c r y s t a l s t r u c t u r e of Cd[HB(4-Brpz)3]2

The crystal structure of the title compound consists of discrete molecules. The representation of the mol- ecule along the b axis with the numbering scheme drawn with the program O R T E P [12] is given in Fig. 2.

The molecule shows a virtual C3, symmetry although it exhibits no real crystallographic symmetry and all atoms occupy general positions.

Since the differences among chemically equivalent interatomic distances or angles have only relative meaning, their average values are given in Table 5, whereas selected bond distances and angles in the coordination sphere are listed in Table 6.

The packing of the molecules in the crystal is ensured by intermolecular short approaches: Br(3) . . .C(5D) i 3.74(1) A, where i means l + x , y, z; N(1E) . . -Br(1) ii 3.46(1), C(5C). . .Br(5) ii 3.72(1), C(3E) . . .Br ( I ) ii 3.59(1), C(4E) . . .Br(1) ii 3.54(1), C(5E). . .Br(1) ii 3.50 (1), B r ( 2 ) ' " C ( 4 F ) ii 3.58(1), Br(2)- . .C(5F) ii 3.56(1), Br(4) . . .C(4B) ii 3.70(1),

Br3t~/,it 5

Brl ~ ~ ~ B 1

4 N2 Cd N2

5 ~ Br/.

Br5 Fig. 2. ORTEP view of the derivative Cd[HB(4-Brpz)3]2.

Table 5. Average bond distances (/k) and angles (9 with their standard errors" of Cd[HB(4-Brpz)~] 2

N X m CYm ~7~,,

Cd--N 6 2.317 0.005 0.003 B--N 6 1.577 0.012 0.006 N(I)--N(2) 6 1.382 0.010 0.004 N(2)--C(3) 6 1.347 0.007 0.005 C(3)--C(4) 6 1.388 0.011 0.005 C(4)--C(5) 6 1.409 0.010 0.005 C(4)--Br 6 1.843 0.004 0.004 C(5)--N(1) 6 1.375 0.010 0.005 N(2)--Cd--N(2) h 6 82.4 0.4 0.1 N(2)--Cd--N(2') ' 6 97.6 0.9 0.1 N(2)--Cd--N(2') J 3 177.6 0.6 0.2 Cd--N(2)--N(I) 6 118.5 0.7 0.2 Cd--N(2)--C(3) 6 134.6 0.8 0.2 N (2)--N(I)--C(5) 6 111.7 0.2 0.3 N(2)--N(1)--B 6 122.4 0.9 0.3 C(5)--N(I)--B 6 126.0 0.9 0.3 N(I)--N(2)--C(3) 6 106.1 0.4 0.3 N(2)--C(3)--C(4) 6 109.5 0.4 0.3 C(3)--C(4)--C(5) 6 108.2 0.2 0.3 C(3)--C(4)--Br 6 126.3 0.9 0.3 C(5)--C(4)--Br 6 125.6 1.0 0.3 C(4)--C(5)--N(1) 6 104.3 0.3 0.3 N(I)--B--N(1) 6 108.5 0.7 0.3

"All values were calculated according to A. Domenicano, A. Vaciago and C. A. Coulson, Acta Crvst. , 1975, B31, 221.

/' cis intraligand. ' cis interligand. ,i trans interligand.

Br (4 ) ' "C(5B) ii 3.47(1) /~, where ii means 3 / 2 - x ,

1/2+y, 1 / 2 - z .

The structure of Cd[HB(4-Brpz)3]2 is a distorted octahedron where the C d - - N distances vary from 2.298(8) to 2.329(8) • with a weighted average of 2.317(5) A, slightly shorter than the values found in other similar complexes given in Table 7. The crystal data, data collection and refinement of the structure are reported in Table 8.

The chelate rings restrain the intraligand N - - C d - - N bond angles in the range 81.6(2)-84.1 (3)°, with a weighted average 82.4(4)°, whereas the cis inter- ligand N - - C d - - N bond angles range from 95.2(3)

22 G. Gioia Lobbia et al.

Table 6. Selected interatomic distances (A) and bond angles (°) with e.s.d.'s in parentheses of Cd[HB(4-Brpz)3]2

Cd--N(2A) 2 .298(8) Cd--N(2D) 2.329(8) Cd--N(2B) 2.312(8) Cd--N (2E) 2.329(6) Cd--N(2C) 2 .312(6) Cd--N(2F) 2.315(7) N(2A)--Cd--N(2B) 8 2 . 3 ( 3 ) N(2B)--Cd--N(2F) 177.2(3) N(2A)--Cd--N(2C) 8 2 . 4 ( 3 ) N(2C)--Cd--N(2D) 98.4(3) N(2A)--Cd--N(2D) 1 7 8 . 7 ( 3 ) N(2C)--Cd--N(2E) 176.8(3) N(2A)--Cd--N(2E) 9 5 . 2 ( 3 ) N(2C)--Cd--N(2F) 100.6(3) N(2A)--Cd--N(2F) 96.6(3) N (2D)--Cd--N(2E) 84.1(3) N(2B)--Cd--N(2C) 8 1 . 8 ( 3 ) N(2D)--Cd--N(2F) 82.2(3) N(2B)--Cd--N(2D) 9 8 . 8 ( 3 ) N(2E)--Cd--N(2F) 81.6(2) N(2B)--Cd--N(2E) 95.9(3)

Table 7. Comparison of relevant structural data of CdN 6 derivatives

Complex

N--Cd--Nav Cd--Nav N--Cd--N (bite)

Cd--N (A) (bite) (deg.) Ref.

A ~

B h

C c

D ~

E ~

F /

G ~

2.348(5) 2.35 82.7(1) 82.7 2.362(8), 2.308(8) 2.34 81.0(3), 82.3(3) 81.9 2.365(8) 82.3(3) 2.314(3), 2.387(3) 2.33 78.7(1), 84.8(1) 80.9 2.284(3) 79.1(1) 2.341(6), 2.282(5) 2.33 83.4(2), 78.9(2) 81.8 2.357(6), 2.355(6) 83.8(3), 83.5(2) 2.293(6), 2.371(6) 78.2(2), 83.2(2) 2.389(6), 2.415(6) 2.38 84.23(2), 85.57(2) 85.2 2.351 (6) 85.76(2) 2.380(11), 2.411(10) 2.38 81.5(4), 81.5(4) 81.4 2.392(11), 2.313(11) 84.3(4), 80.6(4) 2.397(11), 2.369(10) 81.1 (4), 79.6(4) 2.298(8), 2.312(8) 2.32 82.3(3), 82.4(3) 82.4 2.312(6), 2.329(8) 81.8(3), 84.1(3) 2.329(6), 2.315(7) 82.2(3), 81.6(2)

[7b] [7b]

[7b]

[7d]

[7d]

[7d]

This work

"A is Cd[HB(3,5-Mezpz)3]2. b B is Cd[B(3-Mepzh]2. ' C is Cd[B(pz)4]2. a D is Cd[HB(pz)3]2. e E is Cd[HB(3-phpz)3]2. IF is [B(pzh]Cd[HB(3-phpz)3]. g G is Cd[HB(4-Brpz)3]2.

to 100.6(3) °, with weighted average 97.6(9y ~. These angles are comparable with the corresponding angles 82.7(1) and 97.3(1) ° found in Cd[HB(3,5-Me2pz)3]2 [7b]. The trans interligand N - - C d - - N bond angles are in the range 176.8(3)-178.7(3) °, with a weighted average 177.6(6) °. The weighted average intraligand N . . . N non-bonding distances for the donor atoms is 3.05(1) /k whereas the weighted average interligand N . . . N is 3.48(2)/k.

The planarity of the six pyrazole rings was derived from the least-squares planes. The probability that the ring is non-planar turned out to be: P > 99% for N(1B)--C(5B), N(1C)--C(5C), N(1D)--C(5D) and N(1F)- -C(5F) ; P = 26.5% for N(1A)--C(5A) and

P = 98.7% for N(1E)--C(5E). Maximum out of plane displacement (/~): C(5B) -0.029(9), C(3C) 0.025(11), C(5D) 0.024(10), C(5E) 0.016(11) and C(3F) -0.029(11). In the pyrzole rings, the rules about the pattern of values for the angles [13] apply: Cd- -N(2) - -C(3) > Cd- -N(2) - -N(1) (weighted average 134.6(8) > 118.5(7)°), N(2)- -N(1)- -C(5) > N(1)- -N(2)- -C(3) (weighted average 111.7(3) > 106.1(4)°), N(2)- -C(3)- -C(4) > C(4)--C(5)--N(1) (weighted average 109.5(4)> 104.3(3)°). A similar trend was observed in [HB(pz)3Cu(PPh3)] [14] and in Zn[HB(4-Mepz)3]2 [4].

The geometry around the B atom is tetrahedral. The B - - N bond distances vary from 1.53(2) to 1.62(1)

Poly(pyrazol-l-yl)borates

/~ with weighted average 1.58(1) /k, longer than the values found in Zn[HB(pz)3]> 1.541(3) [15] and in Zn[HB(4-Mepz)3]> 1.547(6) A [4]. The angles N - - B - - N range from 105.8(8) to 110.5(7) ° with weighted average 108.5(7) °, comparable with the values found in Zn[HB(pz)3]2 108.9(5) ° [15] and in Zn[HB(4-Mepz)3]2 108.7(7) ° [4]. The B(I) . . 'Cd and B(2). • 'Cd distances are 3.37(1) and 3.43(1)/k (weigh- ted average 3.40(3)/k) ; the B(1)-.-Cd. • .B(2) angle is 177.8(3) ° .

It is also of interest to compare the structural data of the coordination environment of the metal of Zn[HB(pz)3]2 (M--N,v 2.15 /~, N - - M - - N a y 85.6 °) [15], Zn[HB(3,5-Me2pz)3]2 (M--Nay 2.17 A, N - - M - - N ~ v 86.4 °) [16] and Zn[HB(4-Mepz)3]2 (M--N,v 2.14/k, N - - M - - N a y 85.5 °) [4] with those of homoleptic Cd[HB(pz)3]2 (M--Nay 2.33 /~, N - - M - - N a ~ 81.8 °) [7d], Cd[HB(3,5-Me2pz)3]2 (M--N,v 2.35 A, N - - M - - N a y 82.7 °) [7b] and our Cd[HB(4-Brpz)3]2 (M--Nav 2.32 A., N - - M - - N a y 82.4°). All the structures are six-coordinate, but there is an average M - - N bond distance increase of 0.18

together with an average N - - M - - N bond angle decrease of 3.53 °, on going from zinc to cadmium derivatives, caused by the larger cadmium atom size, compared to that of the zinc atom [17].

The steric demand of the ligands determine the stab- ility of complexes, in ligand competition and exchange [18]. It is interesting to calculate effective cone angles of ligands from the crystallographic structural data.

23

For the {HB(4-Brpz)3} ligands, the cone vertex was centered on the cadmium atom and the middle of the C d - - N vectors was assumed to be the cone axes. The angle between the cone axes and the vector which touches the van der Waals radii of the hydrogen atoms closest to Cd (r = 1.17 A) is: 100.5 ° for H(3A), 100.2 ° for H(3B) and 100.9 ° for H(3C) for one ligand and : 100.0 ° for H(3D), 98.6 ° for H(3E) and 100.0 ° for H(3F) for the other ligand. These angles correspond to a total average value 0 :201 .0 and 199.1 °, respec- tively. These angles are narrower than the cor- responding angles 212.2 ° found in the complex [HB(pz)3Cu(PPh3)] [14] and 208.0 and 212.0 ° found in Zn[HB(4-Mepz)3]2 [4]. The difference is attributable to the longer C d - - N bond distance 2.317(5) A as compared with the corresponding C u - - N 2.076(6) [14] or Z n - - N 2.135(15) A [4] bond distances.

ll3Cd NMR data

The 113Cd N M R chemical shifts of various bis (polypyrazolyl)borate cadmium derivatives [4,5] have been recorded in CDC13 and acetone-d6 solutions, at concentrations ranging from 0.5 to 1.0 mM, and are given in Table 9.

We have thought to employ both deuterated chloro- form and acetone in order to compare the effects of the chemical shift caused by a non-polar solvent and a polar one, respectively. All the absorptions fall in

Table 8. Crystal data, data collection and refinement of the structure of Cd[HB(4-Brpz)3]2

Formula C18HI4B2NI2Br6Cd Formula weight 1011.84 Space group P21/n Color colorless a (A) 12.076(1) b (A) 14.29(1) c (A) 20.870(1) fl (°) 93.511(5) vc (A 3) 3595(4) z 4 Dealed (g cm 3) 1.87 Crystal size (mm) 0.98 × 0.28 × 0.24 /1 (Cu-K~) (cm i) 140.2 Data collection instrument Enraf-Nonius CAD4 Radiation (monochromated) Cu-K~ (2 = 1.5418 A) T (K) 293 Scan mode ~o/20 Data collection range 4 ° < 20 < 70 ° Stds (measured every 300 min) 30-5, 017, 413 No. of unique reflections measured 6761 (h, k, + 1) No. of data with F~o > 3a(Fo 2) 4234 No. of parameter refined 352 R Q 0.047 Rw b 0.050

"R = (Xl IFol-klF~l l)l~IFol.

Rw = {Xw(l&l -- klF~l)=l~.WIFol ~) ~"~.

24 G. Gioia Lobbia et al.

Table 9. I L3Cd NMR data

Compound Acetone-d6 CDC13 Ref.

Cd[B(4-Mepz)4]2 216.0 218.3 [4] Cd[B(3-Mepz)4]2 202.5 [7b] Cd[B(pz),]2 221.1 [7b] Cd[HB(3,5-Me2pz)3]2 insoluble 201.9 [7b] Cd[HB(3-Mepz)3]2 insoluble 200.0 [5] Cd[HB(pz)3]2 196.4 198.3 [7b] Cd[HB(4-Mepz)3]2 193.6 195.9 [4] Cd[HB(4-Brpz)3]2 187.6 193.8 This work

the range typical for six-coordinate species [7,8,9] and, apart from a general increase in shielding (ca 2-6 ppm), which could be ascribed to a secondary effect such as dipolar interactions between solvent molecules and the B--H fragment in tris or with the free lone pair of N donor atom of the fourth pyrazole in tetrakis, the choice of acetone as solvent does not produce any other relevant effect on "3Cd chemical shift for coor- dinatively saturated CdN6 complexes.

The value of Cd[B(4-Mepz)4]2 (218.3 ppm) can be compared to those of Cd[B(pz)4]2 (221.1 ppm) [7b] and of Cd[B(3-Mepz)4]2 (202.5 ppm) [7b], the only other tetrakis cadmium derivatives till now known. It seems that the Me group in 4- and mainly in 3-position of the pyrazole rings produces a shielding with respect to the unsubstitute tetrakis :

{B(3-Mepz)4} > {B(4-Mepz)4} > {B(pz)4}

In the case of tris(pyrazolyl)borate cadmium derivatives more H3Cd data are available [7,8,9]. The following trend of shielding power for Tris (pyrazolyl)borate donors is found :

{HB(4-Brpz)3} > {HB(4-Mepz)3} > {HB(pz)3}

> {HB(3-Mepz)3} > {HB(3,5-Mezpz)3}

It is interesting to note that the chemical shift of cad- mium complexes of {HB(4-Brpz)3}, {HB(4-Mepz)3} and {HB(pz)3} ligands cannot be related to pK, of the pyrazole moiety [19]. If we compare the ~3Cd NMR data of our derivatives with those reported in literature for analogous compounds it can be noted that in the case of derivatives containing donors hav- ing relatively small or no substituents in pz rings, a decrease in Cd--Nav bond distance is accompanied by an increase in shielding.

It has been argued by Reger [7d] that the H3Cd shielding effect depends upon the coordination number and the geometry, primarily the Cd- -N distances. In order to vary the M - - N bond lengths it has been recently demonstrated [7,8,9] that the steric hindrance in 3-position plays a relevant role, while other elec- tronic effects can also be introduced by changing the 5-position of the rings and/or going from a tris (pyrazolyl)borate to a tetrakis one.

With regard to 1198n NMR spectroscopy, the trends

observed are easier to explain [20], whereas in the case of H3Cd, at this stage of the research it is not clear why our ligand {HB(4-Brpz)3} is able to generate a higher shielding of Cd nucleus with respect to {HB(pz)3} and to {HB(4-Mepz)3}, which differ by a H and a Me in 4-position, respectively. Additional X- ray structural data are necessary to further investigate the effect of substitution in 4-position upon t~3Cd NMR chemical shifts here discussed, and several stud- ies are ongoing in our laboratory regarding this point on mixed donor atom combinations and also in pres- ence of 4-substituted neutral bis- and tris (pyrazolyl)methanes [21], which are isoelectronic analogues of the anionic poly(pyrazolyl)borate donors.

EXPERIMENTAL

General laboratory equipment

Concentration was always carried out & vacuo (water aspirator). The samples were dried in vacuo till constant weight (20°C, ca 0.1 Torr). Carbon, hydro- gen, and nitrogen analyses were carried out on a Fisons Instruments EA 1108 CHNS-O, while molec- ular weight determinations were performed by a Knauer membrane osmometer. Infrared spectra were recorded from 4000 to 250 cm ~ on a Perkin-Elmer 2000 System Series FTIR instrument. The electronic spectra were recorded in chloroform solution with an HP 8452A Diode Array Spectrophotometer. ~H-, ~3C- and ~3Cd-NMR spectra were recorded on a Varian VXR-300 spectrometer operating at room tem- perature (299.94 MHz for ~H, 75.05 MHz for ~3C and 66.55 MHz for ~3Cd). Some spectra were also recorded on a Varian Gemini-200 (200 MHz for ~H, 50 MHz for ~3C). The chemical shift is reported in ppm from Me4Si (1H and J3C, calibration from internal deuterium solvent lock) and CdC104 (~3Cd) values. The conductivity of the acetone solutions was measured with a Crison CDTM 522 conductimeter at room temperature.

Synthesis

Potassium[hydridotris(4- bromo- 1H- pyrazol- 1 -yl) borato], K[HB(4-Brpz)3]. A mixture of 4-Br-pyrazole (20 g, 0.136 mol) and KBH4 (1.83 g, 0.034 mol) was heated gradually. Melting with a slight evolution of hydrogen occurred at ca 90°C. The mixture was finally heated at 170°C until the volution of 3 equiv, of hydro- gen per KBH4. The mixture was cooled to room tem- perature and xylene was added with vigorous stirring. The xylene solution was separated by filtration. This treatment was repeated several times to remove unre- acted 4-Br-pyrazole. The remaining solid was washed with hexane-petroleum ether (1 : 1 v/v) to give a white crystalline solid, which was identified as K[HB(4- Brpz)3] by elemental and spectral analyses. M.p. : 287-

Poly(pyrazol-l-yl)borates 25

289°C. Elemental analyses calcd for CgH7BBr3KN6: C, 22:1 ; H, 1.4; N, 17.2; found: C, 22.2; H, 1.5; N, 16.9.

Bis[hydridotris(4 - bromo - 1H-pyrazol - 1 - yl)borate] metal (II), 1-9. These compounds were prepared by adding an aqueous solution (ca 50 cm 3) of 1 mmol of the appropriate metal salt to an aqueous solution (ca 50 cm 3) of potassium hydridotris(4-bromo-lH-pyr- azol-l-yl)borate in a 1 : 2 molar ratio. The compounds 1-8 precipitated immediately : the complexes were fil- tered off, washed with diethyl ether, and dried in u a c u o .

X-ray analysis

Suitable crystals of the complex Cd[HB(4-Brpz)3]2 were obtained by crystallization from dichloro- methane/acetonitrile. A crystal of dimensions 0.24 × 0.28 × 0.98 mm was used for data collection. Accurate unit-cell parameters were determined from a least-squares fit for 25 reflections measured on the Enraf-Nonius CAD4 diffractometer with graphite- monochromated Cu-K~ radiation of the Centro Grandi Strumenti dell'Universit&, Pavia, Italy.

The intensities of 6761 independent reflections in a complete quarter of the reflection sphere were col- lected by the 09/20 scan technique within the angular range 4 < 20 < 70 °. A total 4234 reflections having I > 3a(/) were regarded as observed and used in the structure analysis. The data were corrected for Lor- entz-polarization effects and for absorption according to North et al. [22] with minimum and maximum absorption factors of 0.7150 and 0.9997. An approxi- mate absolute scale factor and a mean thermal par- ameter of 6.54 A 2 were determined by Wilson's method [23]. Crystal data, intensities data collection parameters and structure refinement results are sum- marized in Table 8.

Structure determination and refinement

The structure was solved by Patterson and Fourier methods. The positional parameters of the cadmium atom derived from the three-dimensional Patterson map and the isotropic thermal parameter, were refined in three cycles of least-squares refinement. A sub- sequent three dimensional difference Fourier synthesis revealed all the remaining non-hydrogen atoms. The full-matrix least-squares method refined the positional and first isotropic and later anisotropic thermal par- ameters. The positions of hydrogen atoms were cal- culated from the geometry of the compound and checked in a final difference Fourier map and not refined but included in the calculations in fixed positions, with the same isotropic thermal parameters of their bonded atoms.

The final R was 0.047 for the observed reflections. At all stages of the analysis the observed reflections were given unit weight, since the use of weights

obtained from counting statistics did not lead to better results. The average shift/esd ratio in the final refine- ment cycle was 0.02 both in the positional and in the thermal parameters of the non-hydrogen atoms. The final difference Fourier map showed maximum and minimum Ap values 0.674 and -0.349 e A -3.

All calculations were carried out with the Enraf- Nonius SDP crystallographic computing package [24] and with local programs. Supplementary material--Tables of atomic positional parameters of non-hydrogen atoms and hydrogen atoms, table of anisotropic thermal parameters, tables of bond distances and bond angles involving non- hydrogen atoms and hydrogen atoms, table of plan- arity of molecular regions and a listing of observed and calculated structure factors (total of 40 pages).

Acknowledyements--We gratefully acknowledge the Uni- versities of Camerino and of Pavia, and the Consiglio Nazionale delle Ricerche for financial support.

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