Studies on monovalent η-butadiene derivatives of niobium and tantalum

Polyhedron Vol. 7, No. 19/20, pp. 18194826, Printed in Great Britain

STUDIES

1988 0

0277-5381/88 S3.00+.00 1988 Pergamon Press

plc

ON MONOVALENT q-BUTADIENE DERIVATIVES OF NIOBIUM AND TANTALUM*

PAUL R. BROWN, MALCOLM Inorganic Chemistry Laboratory,

L. H. GREEN? and PHILIP

M. HARE

South Parks Road, Oxford OX1 3QR, U.K. and

JUDITH Chemical Crystallography

A. BANDY

Laboratory,

9 Parks Road, Oxford OX1 3PD, U.K.

Abstract-The synthesis of [truns-Nb(dmpe)2(q-C4H6)H], llV&3-l-MeC,H&?-C,H6)2], M = Nb or Ta, and ~b(dmpe)(~3-l-MeC3H,)(?j-C4H,)I is described. The crystal structure of [trunsNb(dmpe)2(rj-CqH6)H] has been determined. Variable temperature NMR studies and spin magnetization transfer experiments provide evidence for fluxional processes in these compounds.

Low valent derivatives of niobium and tantalum are rare. The earliest examples of zero-valent derivatives were lM(r,r-arene)2] and [M(dmpe),], M = Nb or Ta, dmpe = bis-1,2(dimethylphosphino)ethane, prepared by metal vapour synthesis techniques. 1,2 Here we describe further studies on the synthesis and chemistry of low valent derivatives of niobium and tantalum. A preliminary communication of part of this work has been published. 3 RESULTS

AND DISCUSSION

atom, P( 1) and P(4) lie on a crystallographic mirror plane such that the pair of dmpe ligands occupy two alternative orientations in the equational plane, i.e. that shown in Fig. 1 and its mirror image, each having half occupancy. This disorder is similar to that observed in [M(dmpe),] (M = Nb and Ta) complexes. ’ The mirror plane also bisects the butadiene ligand. The hydride on niobium was not located by electron density syntheses but it is thought to occupy the vacant coordination site opposite the butadiene ligand (Fig. 2). The “bite” angles of the dmpe ligands in (1) are 79.2 and 78.2”, these are slightly larger than

Treatment of the known compound jNb(qC,H6)(dmpe)2Cl]4 in tetrahydrofuran with an excess of LiA1H4 gave orange crystals of [Nb(vC4H6)(dmpe)&j (1) (uide infra). The analytical, and spectral data for the new compounds described in this work are given in Table 1. The NMR data will be discussed only where interpretation is not straightforward. Large needle crystals of (l), suitable for X-ray diffraction studies were obtained by slow sublimation at 80°C and lo- 5 torr. The crystal structure of (1) is shown in Fig. 1,’ with selected bond lengths and angles given in Table 2. The niobium *Dedicated to Professor Sir Geoffrey Wilkinson, F.R.S. in appreciation of his pioneering contributions to organotransition metal chemistry and, in particular for his inspiring guidance as my Ph.D supervisor (M.L.H.G.). t Author to whom correspondence should be addressed. 1819

4 Cl

3

Fig. 1. Molecular structure of (1) with atom labels.

Studies on monovalent q-butadiene derivatives of niobium and tantalum

1821

Table 2. Selected bond lengths (A) and angles (“) for (1) 2.538(2) 2.569(4) 2.538(4) 2.526(2) C(3Wx4) C(9)--c(lO) C(l3)--c(14) C(14’)-c(14)

1.532(10) 1.518(9) 1.371(10) 1.496(16)

WlW(l3) WlW(l4)

2.391(6) 2.300(6)

wwtl) w)--cQw

v 1J-ww

P(l)--c(3) P(2)--c(4) P(2)--c(5) P(2F(6) P(3_(7) P(3>--c(8) P(3_(9) P(4>-c(lO) P(4W(ll) P(4~(12)

114.8(5)

79.22( 11) 78.18(9)

c(ll-w>--cw2) c(lww-ww C(l)-P(l>-c(3) c(lO2~P(l)--c(3) c(202)-P( 1)--c(3) c(4)-Pt2~(5) C(4)-P(2-(6) C(5)_P(2>--c(6)

96.9(29) 105.5(23) 97.4(28) 112.7(16) 88.6(22) 96.3(16) 104.1(16) 94.9(13)

103.5(23) 107.0(17) 87.7(14)

C(lO)-P(4)--c(ll) c(lO>-P(4)--c(l2) c(1 l)-P(4)-~2)

114.4( 14) 108.6(15)

observed in [Nb(dmpe),J [74.6(2)“] ; which may be considered more sterically crowded. Also, there may be disorder of the dmpe backbones as previously described,’ but the overlapping dmpe ligands made this difficult to resolve.

100.4( 14) 91.6(19) 104.0(12) 112.3(11) 112.2(11)

Primes denote atoms generated by mirror plane (-x,

Fig. 2. Structure of (1) orthogonal to Fig. 1 showing hydride cavity.

1.837(8) 1.842(10) 1.843(10) 1.876(9) 1.871(9) 1.843(g) 1.838(g) 1.845(g) 1.836(9) 1.859(8) 1.874(9) 1.839(9) 1.837(8)

y, z).

The Nb-P distances range from 2.526 to 2.538 A (cf. 2.526(3) A in [Nb(dmpe),]} and the four phosphorus atoms form an approximate plane. For the butadiene ligand the niobium to terminal-carbon distances are longer than to the internal-carbons (by 0.091 A). The terminal-internal C-C bond is 0.125 8, shorter than that between the two internal carbons indicating the (~~-1: 2,3 : 4) bonding character for the butadiene. This is unusual for early transition metal butadiene complexes such as [Hf(q-C4H,J,(dmpe)16 which tend to adopt the alternative (1,4-1’) or metallacyclo-3-pentene structure, and is indicative of the butadiene ligand in (1) acting more as an electron donor than acceptor. The butadiene ligand is also essentially planar and approximately parallel to the plane defined by the four phosphorus atoms. The variable temperature solution NMR data of (1) indicates that the molecule is fluxional at room temperature but on cooling to -85°C adopts the conformation found in the crystalline state. In particular, the observation of an Nb-H resonance at 6 - 2.1 [J(P-H) = 47 Hz] in the room temperature ‘H NMR spectrum of (1) indicates that the 3’P nuclei are magnetically equivalent. Since the buta-

P. R. BROWN

1822

diene ligand also appears as three 2-hydrogen multiplets at this temperature, it is likely that rapid rotation of the q-butadiene ligand about the niobiumligand axis is occurring at room temperature. On cooling to -85°C the ‘H NMR spectrum becomes markedly different. The hydride signal became a broad, featureless band. The internal hydrogens of the butadiene are observed at 6 3.6 and 4.2, which suggests that butadiene rotation is slow on the NMR timescale at this temperature. However, the resonances due to the terminal vinyllic hydrogens were not readily distinguished because the region 6 O-2 shows several overlapping broad resonances. The two large doublets due to the dmpe methyl hydrogens have also collapsed into this broad signal. This is most likely due to the process of flexing of the methylene backbone in the dmpe ligands being in the regime of intermediate exchange. The variable temperature 31P{‘H} NMR spectrum of (1) (Fig. 3) exhibits several interesting features. At - 106°C the spectrum consists of two broad resonances in the ratio 3 : 1. At - 70°C the two bands coalesce to a single broad resonance

. ..

-90°C

;

80

60

40

20

km

0

Fig. 3. Variable temperature 101.26MHz 3’P{‘H} NMR spectra of (1) in [*H,]-toluene.

et al.

(cur,* = 64 Hz). On further warming to 0°C the resonance broadens further (oljz = 271 Hz), and then apparently separates into two equal components with the separation between the maxima being 190 Hz at room temperature. The doublet appearance of the room temperature spectrum is attributed to partial collapse of the 3’P-g7Nb quadrupole coupling. Differential broadening of the spectral lines of spin-l/2 nuclei coupled to quadrupolar nuclei (g7Nb; Z = 9/2) is observed when the spin-lattice relaxation time (T,) of the latter is comparable to (J)) ’ (where J = quadrupole coupling constant). As the temperature is lowered the T, of quadrupolar nuclei generally decreases sharply and at low temperatures (typically < - 70°C) line-broadening effects on spin-l/2 nuclei due to quadrupolar coupling are removed. The activation energy for the fluxional process leading to coalescence of the 3’P nuclei at -80°C is estimated to be AGS = 36 kJ mall ‘. We assume this process to be rotation of the butadiene ligand. Hydrogen-l spin magnetization transfer experiments on (1) show no evidence for intramolecular hydrogen exchange processes. However, ‘H and *H NMR spectra of the orange crystals formed from mb(r&H,)(dmpe),Cl] and LiAlD4, namely [Nb(n-C,H,)(dmpe),D], (l-[*m) indicate statistical scrambling of deuterium into the terminal exo and endo sites of the butadiene ligand. There was no deuterium incorporation into the internal hydrogen sites. A similar hydrogen scrambling has been observed in the compound [NbH(qC2D,)(dmpe)2].7 We note that the crystal structure of (1) shows that the butadiene and the hydride ligands are mutually trans, and therefore substantial rearrangement must occur to enable intramolecular scrambling. Co-condensation of niobium or tantalum atoms with an excess of buta-1,3-diene gives, after sublimation, highly sensitive purple or orange crystals of [Nb(rZ-l-MeC3H4)(rZ-CqH6)d (2) or Da(n-lMeC,H,)(q-C,H,),] (3), respectively. Compounds (2) and (3) are electron-rich* formally 16-electron compounds. In the light of the recently established occurrence of agostic bonding in other formally 16-electron q-methylallyl compoundsg,” we studied their dynamic NMR spectra to attempt to establish whether agostic bonds were present. The ‘H NMR spectrum of (2) is highly complex and despite extensive double resonance experiments only partial assignment is possible. The spectra clearly corresponded to the presence of two q-butadiene groups and a n- 1-methylallyl group in (2) but do not allow distinction between possible conformers arising from the relative dispositions of the ligands (see Scheme 1). The ‘H NMR spectrum of

Studies on monovalent q-butadiene derivatives of niobium and tantalum

1823

cluded quantitative measurements but the data suggests that the rates of exchange between isomers 1 ) and hydrogen migration rates must be comparable. In conclusion, it was not possible to confirm the suspected presence of agostic methyl groups in (2) and (3) due to the complexity of their NMR spectra arising from low symmetry and fluxional behaviour. Treatment of (2) with dmpe gave the bright ii orange oily solid ~(r,r-C,H,Me)(+Z,H,)(dmpe)] (4). The low resolution mass spectrum of (4) showed a parent ion peak at M/e = 352 corresponding to (Z),M=Nb (3). h4= Ta NbC14H29P2. The NMR data may be interpreted Scheme 1. (i) LiAlH4 in THF, room temperature, 85% in terms of a formally 16-electron structure for (4) shown in the Scheme 1. The connectivity of the and (ii) dmpe in pentane, - lO”C, 25%. signals was established by the use of a ‘H--‘H COSY two-dimensional NMR spectrum. This showed the bands assigned to H,, Hb, Hr, Hi, Hj (2) in [2H8]-tetrahydrofuran at room temperature and Hi, (Table 1) comprise a butadiene ligand at and at - 120°C showed no significant differences. Of particular interest was the band in the room chemical shifts typical for their respective environtemperature 13CNMR spectrum of (2) assigned to ments. The vinylic hydrogens of the I-methylallyl group may be assigned to the bands H,, Hd, H, and the l-methyl group, which occurs as a binomial quartet with J(C-H) = 124 Hz. This coupling con- Hk, with the methyl group appearing as a doublet stant is consistent with either a normal methyl at 6 1.06. Doublets assignable to the dmpe methyl group, or, with a fluxional agostic methyl group. ‘O hydrogens are observed at 6 1.850.95 and 0.75, the No resolvable change in the coupling constant is fourth overlapping with the resonance of the methyl observed on cooling to - 85°C. Further, there was of the I-methylallyl group. The fully coupled 13C no evidence for the presence of other conformers of NMR spectrum showed a binomial quartet (2). However, the linewidths of the resonances other (Jcu = 125 Hz) for the l-methyl group. ‘H magthan that of the l-methyl group were very tem- netization transfer experiments on (4) at 45°C perature dependent. On warming from - 85°C the showed no observable transfer. The new reactions and proposed structures for spectrum of (2) changes markedly. The signals the new compounds (lH4) are shown in Scheme 1. became very broad, indeed at - 60°C they flattened into the baseline, and then the spectrum sharpens again as the temperature approaches room temperature. Since no other conformers of (2) were EXPERIMENTAL readily detected at low temperatures, we attribute the dramatic variations in the linewidths to a comAll preparations, manipulations and reactions bination of changes in the molecular symmetry and were carried out under an inert atmosphere of dinithe consequent unpredictable quadrupolar relax- trogen (< 10 ppm oxygen, < 20 ppm water) using ation times of the 97Nb nucleus. standard Schlenk tube and vacuum-line techniques, A spin saturation transfer experiment on (2) at or in a dry box. Dinitrogen was purified by passage 45°C shows that all of the hydrogens on the terminal through a column containing BTS catalyst and 5 8, carbons of all three ligands undergo exchange (AG* molecular sieves. 95 + 5 kJ mol- ‘). This process is expected to proAll solvents were thoroughly deoxygenated ceed via an equilibrium between [Nb(q-C,H,Me)(qbefore use by repeated evacuation followed by C,H&] and the unobserved l&electron hydride admission of dinitrogen. Solvents were pre-dried over activated molecular sieves and then distilled NW?-C,H,),I. The room temperature ‘H NMR spectrum of from potassium (tetrahydrofuran, THF), sodium the tantalum analogue (3) was very similar to that (toluene), sodium-potassium alloy [light petroleum observed for (2). The detailed analysis of the spectra (b.p. 4060°C throughout), diethyl ether] or phoswas complicated by the appearance of a minor low phorus pentoxide (dichloromethane), under an symmetry isomer at low temperatures. Spin satu- inert atmosphere of dinitrogen before use. Methration transfer experiments at 50°C showed that the anol was dried over activated molecular sieves and hydrogens of the methyl group are exchanging with deoxygenated before use. Deuterated solvents for all the terminal hydrogens in both the major and NMR samples were stored in Rotaflo ampoules minor isomers. The complexity of the spectrum pre- over activated molecular sieves and transferred by

1824

P. R. BROWN

vacuum distillation. Celite 545 filtration aid (KochLight) was dried in an oven at 80°C before use. IR spectra were recorded as Nujol mulls between CsI plates on a Perkin-Elmer 1510 FT interferometer. Hydrogen-l NMR spectra were determined at 300 and 500 MHz using Bruker WH-300 and AM-500 spectrometers, respectively. Carbon13 and 31P NMR spectra were determined at 62.89 and 101.26 MHz, respectively, using a Bruker AM250 spectrometer. Spectra were referenced internally using the residual solvent ( ‘H) and solvent ( ’‘C) resonances relative to tetramethylsilane (6 = 0 ppm), or externally using trimethylphosphate p(O)(OMe),] in DzO (“P). All chemical shifts are quoted in 6 (ppm) and coupling constants are in Hertz (Hz). Metal vapour synthesis experiments were carried out using the apparatus described elsewhere. ’ ’

Bis[bis- 1,2(dimethylphosphino)ethane](q - buta- 1,3 -

diene)hydridoniobium (1) The compound [NbCl(q-C,H,)(dmpe), (100 mg, 0.21 mmol), prepared as described,4 in THF (30 cm’) was treated with a suspension of LiA1H4 (10 mg, 0.26 mmol) in THF (30 cm3) with stirring. No immediate colour change was observed but after stirring for 12 h the initially orange solution darkened. The reaction mixture was filtered and solvent was removed from the filtrate under reduced pressure. The solid residue was extracted with petroleum ether (b.p. 6&8O”C, 70 cm’) giving a yelloworange solution and a large quantity of offwhite powder. The solution was filtered and reduced to 30 cm3. Cooling to -25°C produced orange needle crystals which were isolated and dried in vacua. Yield, 80 mg, 85%. Further purification was achieved by fractional sublimation at lop4 torr and 80°C. After 12 h excellent needle crystals had grown on the sides of the sublimation tube.

[Bis- 1,2(dimethylphosphino)ethane](q-buta- 1,3-diene) &uterioniobium(l-[ZHJ)

et al.

kW (5.9 kV at 320 mA), were co-condensed over 4 h with an excess of the co-reactant (90 cm3 of a mixture of 15% tetrahydrofuran and 85% buta1,3-diene). During the co-condensation, the matrix was orange. On warming, the matrix melted and became purple. After melting the reaction mixture was extracted from the apparatus with cooled tetrahydrofuran (300 cm3 at -78°C). The extract was intensely purple and contained a quantity of unreacted metal. The product mixture was maintained at - 78°C and rapidly transferred to a vacuum line. The volatile components were then removed under reduced pressure with the mixture initially remaining at -78°C. After approximately 30 min the majority of the excess butadiene had been removed and the mixture was allowed to warm slowly whilst the volatile components were still being removed. In this way the polymerization of the excess butadiene in the extract could be kept to a minimum. After approximately 2 h all of the more volatile components had been removed giving an intensely coloured rubbery material. To this solid was added petroleum ether (60-8O”C, 200 cm3) and the mixture left for 3 h at room temperature to slowly extract the organometallic product. A purple extract was filtered through Celite from pale grey polymeric material. The clear filtrate was reduced in volume to ca 50 cm3 by removal of the solvent under reduced pressure. The residue was sublimed onto a liquid dinitrogen cooled probe at 40°C and 1O-4 torr. The deep purple sublimate was extracted with petroleum ether (b.p. 3&4O”C, 50 cm’) and the extract was concentrated (to 10 cm3) and cooled to - 30°C for 12 h giving a purple microcrystalline solid. The supernatant was decanted from the solid which was washed with petroleum ether (b.p. 30-4O”C, 2 x 5 cm3) at - 30°C and dried in vacua. Yield, ca 1 g. It was found (‘H NMR) that the product still contained a small quantity of impurity which could be removed by resublimation in vacua at 40°C.

Bis(q-butadiene)(q-l-methylallyl)tantalum

(3)

Tantalum atoms (ca 2 g, Il. 1 mmol) were generated from a ca 9 g ingot of the metal at a power This was prepared from [NbCl(q-C,H,)(dmpe)d of 1.7 kW (5 kV at 340 mA) and co-condensed over (0.12 g, 0.25 mmol) in a manner identical to that 4 h with an excess of the co-reactant (90 cm3 of a for (1) except using LiAID4. The product was remixture of 15% tetrahyrofuran and 85% buta-1,3crystallized from petroleum ether (b.p. 4@-6O”C, diene). During the co-condensation the matrix was 10 cm3). Yield, 40 mg, 35%. orange. On warming, the matrix melted and remained deep orange. The isolation followed the same procedure as for the niobium analogue Bis(q-butadiene)(q-1-methylallyZ)niobium (2) except that the tantalum compound required a temperature of 70°C for the sublimation. Yield, Niobium atoms (ca 1 g, 10.7 mmol), generated from ca 6 g ingots of the metal at a power of 1.9 ca 0.6 g.

Studies on monovalent q-butadiene derivatives of niobium and tantalum Bis - 1,2(dimethylphosphino)ethane(q - butadiene)(v 1- methylallyl)niobium (4)

-

The compound [Nb(dmpe)(+,H& 1-Me C3H4)] (400 mg, 1.6 mmol) in pentane (60 cm’) at - 10°C was treated with dmpe (0.35 g, 2.3 mmol) in pentane (50 cm’) at - 10°C in a dropwise manner. The initially deep red-purple solution became deep red. The solvent was removed under reduced pressure to give a dark red-orange oil. This was sublimed at lo- 5 torr and 60°C giving an oily orange solid. Attempts to recrystallize this material from pentane at -78°C did not effect any increase in purity, due to extremely high solubility. Yield ca 100 mg. Crystal data

ClbH3,NbP4, M = 448.3, orthorhombic, a = 13.246(2), b = 13.336(2), c = 13.057(9) A, U = 2306.5 A3, space group Cmc2,, Z = 4, D, = 1.29 Mgn- 3, F(ooo) = 936, &Mo-&) = 0.70930, /@IO&) = 7.69 cm- ‘, crystal dimensions 0.9 x 0.15 x 0.16 mm.

1825

the unresolved anisotropic model which was retained. No hydrogens were located. During the latter stages of retiement corrections were made for isotropic extinction” and anomalous dispersion16 and reflections were assigned weights17 according to w = l/E A,T,(X) where n = 3, A, = 5.87, -2.48, and 4.19, T, is the polynomial function and X is the function ]FO;ol/]FO_]. At convergence R = 0.033, R, = 0.035. All calculations were performed on the VAX 1l/750 computer of the Chemical Crystallography Laboratory with the CRYSTALS package.” Atomic scattering factors were taken from International Tables. I9 Atomic coordinates, mean planes, anisotropic thermal vibration parameters and observed and calculated structures factors have been deposited as supplementary material. R = E (IF,1-I&D/~ IFA, R, = {C w(lFO’ol - lf’,j)2/E w IF012}“*. AcknowletlTqementsWe wish to thank British Petroleum plc for a scholarship (to P.M.H.) and the Science and Education Research Council for a grant (to P.R.B.)

Data collection andprocessing

The crystal was sealed under nitrogen in a Lindemann glass capillary and mounted on an EnrafNonius CAD4 diffractometer. Cell dimensions were obtained by least-squares methods from the positions of 25 carefully centred reflections. Data collection used ~28 scan mode with o-scan width (0.9 f0.35 tan 6)” and scan speed range 0.84671” min- ‘. 4988 reflections measured (1 < 8 < 30”), 1823 unique [merging R = 0.017 after Lp and absorption corrections” (max., min. correction 1.07, l.Ol)] giving 1327 observed [I > 3a(Q]. Structure

analysis and refinement

Solution by heavy atom method and direct methods on the difference structure using DIRDIF13 in Cmc2, to yield Nb and P positions. The structure was developed slowly by difference Fourier methods resulting in the location of all carbon atoms. P(1) and P(4) were placed on the mirror plane, with Nb( l), to avoid ill-conditioned matrices. Refinement was by full-matrix least-squares with all non-hydrogen atoms anisotropic. The geometry of the dmpe ligands was restrained l4 to aid convergence. The carbon atoms of these ligands have relatively large thermal vibration parameters indicating there may be disorder as observed in a number of other dmpe complexes. Attempts to resolve this disorder for all carbons, except C(102) : C(202) gave a poorer fit to the observed electron density than

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17. J. R. Carruthers and D. J. Watkin, Acta Cryst. 1979, A35, 698 ; E. Prince, Mathematical Techniques in Crystallography, Springer, New York (1982). 18. D. J. Watkin, J. R. Carruthers and P. W. Betteridge, CRYSTALS User Guide, Chemical Crystallography Laboratory, University of Oxford (1985). 19. Znternational Tables for X-ray Crystallography, Vol. IV, p. 99. Kynoch Press, Birmingham (1974).