Niobium metallaboranes: A novel metallaborane analogue of pentaborane(11)

Cl

Journal of Organometallic

Chemistry, 382 (1990) Cl-C5

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

JOM 20244PC Preliminary communication

Niobium metallaboranes: of pentaborane( 11) *

a novel metallaborane

analogue

Peter D. Grebenik, John B. Leach, Jayne M. Pounds, Chemistry

and Biochemistry

Section,

School of Biological

and Molecular

Sciences,

Oxford

Polytechnic,

Gypsy Lane, Oxford, OX3 OBP (U.K.)

Malcolm L.H. Green and Philip Mountford Inorganic

Chemistry Laborarory, South Parks Road, Oxford

OX1 3QR (U.K.)

(Received May l&h, 1989)

Abstract Treatment of [Nb(+,H,),H,] with pentaborane(9) gives the novel arachnometallaboranes [Nb(q-C,H,),B,H,] (1) and [Nb(q-C,H,)2B3H,] (2). Compound 1 is the first reported base subrogated analogue of the neutral aruchno-pentaborane(l1) and has an apex to niobium hydrogen bridge. Compound 2, for which the X-ray crystal structure has been determined, is an analogue of the neutral arachno-tetraborane(lO).

Aruchno-pentaborane(l1) is exceptional among simple boranes in having a hydrogen atom bridging asymmetrically across the open B(l)B(2)B(5) apex-base face [1,2]. In the three reported metallaborane analogues of pentaborane(ll), [Ir(CO)(PR,),B,H,] (PR, = PMe, and PMe,Ph) [3,4] and [RuH(C,Mq)B,H,] [5], the metal atoms occupy apex positions. Here we report the first characterised example of a basal metal subrogated analogue of arachno-pentaborane(l1). Interestingly, it contains an apex to basal niobium hydrogen bridge. Treatment of [Nb(q-C,H,),H,] with pentaborane(9) at ca. 0” C leads to the formation of [Nb(a-C,H,),B,H,] (1) (Fig. l), [Nb(q-C,H,),B,H,] (2) (Fig. 2), and the previously known [Nb(&H,),BH,] (3) w hl‘ch were separated by fractional crystallisation * *. In solution, at ca. 50” C 1 decomposes to form 2; at room temperature 1 reacts slowly with [Nb(q-C,H,),H,] to form 2 and 3, and with 3 to form 2 along with further unidentified products.

* Dedicated to Prof. G. Wilke on the occasion of his 65th birthday. * * In a typical experiment 2.2 mmol [Nb(+Z,H,),H,] was treated with 20 cm3 BSHg/toluene solution (5 mmol). Crystallisation from 40/60 petroleum ether and toluene gave the following solids: [Nb(q-C,H,),B,H,I (1) (0.49 mmol), tNb(q-CSH,),B3H,I (2) (0.66 mmol), [Nb(q-C&)2BHA (3) (0.35 mmol).

0022-328X/90/$03.50

0 1990 Elsevier Sequoia S.A.

c2

CP = Fig. 1. Proposed

structure

(lp-C&l

of [Nb( yC,H,)ZB4H9]

(1).

Compound 1 has been characterised by elemental analysis and ’ H. ’ ’ B, and ‘“C NMR spectroscopy *. The “B NMR shows the presence of four inequivalent boron positions (Fig. 3). Three of these resonances occur as doublets due to terminal hydrogen couplings. The low frequency of the doublet at S - 55.7 is characteristic of the apex position in a pentaborane(l1) species. The remaining resonance occurs as a triplet 6 - 1.4 and is characteristic of the basal BH, unit in this structure. The ‘H NMR shows four different bridging hydrogen atoms. Selective heteronuclear “B{‘H} decoupling experiments show three of these to occupy bridging positions between the basal niobium (2) and boron atom (3) and between the basal boron atoms (38~4) and (4&j) respectively. The chemical shift of the remaining bridge at 6 -8.5 shows that it is bonded to Nb while selective heteronuclear “B{‘H) and ‘H{“B} decoupling experiments establish coupling to the apex boron atom (I). We have found no evidence of coupling between this Nb(2)--H(l.2)-B(1) proton and the basal boron B(5). The exact location of this apex to base bridging hydrogen remains to be determined. Two cyclopentadienyl resonances are evident in both the ‘H and 13C NMR spectra corresponding to exe- and en&-dispositions for these rings. Compound 2 has been characterised by single crystal X-ray crystallography. The

* Analytical Selected NMR

data for compound 1: Found: C, 43.69; H. 7.01. C,,H,,B,Nb calcd.: C, 43.61: H. 6.95%. NMR data (solvent benzene-d, for compound 1 and toluene-d, for compound 2): ‘H

at 200 MHz

and

“B

NMR

at 64.2 MHz;

chemical

shifts

6 in ppm

and coupling

constants

in

(Hz). Compound 1: ‘H{“B) NMR, 8 5.6 (s, lH(4)). 4.3 (s. 5H(Cpd)), 4.1 (s. 5H(C’ph)), 3.7(~. lH(3)). 3.4 (s. 1 H(5 rxo,endo))) 2.6 (s, 1H(5cxo;endo )), -1.1 (s, lH(1)). -1.4(s, ~H(/L~,~)). -2.5 (s. if-It/.+,)), --X.5 (s, lH(f+)). -10.2(s, ~H(P,.,); “BNMR(J(“B-IH)), S 21.4(d. lB(4). (154)) 6.9(d, lB(3).(127)), - 1.4 (t, lB(5), (119)X -55.7 (d. lB(l), (178)); ‘jC( ‘H} NMR 6 93.5 (s, S(‘(Cp”)). 92.4 (s. 5C(Cph)). 2.7 (s. Compound 2: ‘H(“B) NMR 6 4.5 (s, 5H(p”)), 4.29 (a. 5H(Cpb)). 4.26 (s. lH,,,,,,,,,,,,(4/5)t. H ,x,,,, 2H(1,3)), -1.1 ts, Wp,,,; p,,d)), -12.7 (s. 2H(/+; P>,~ )). “R NMR (J(“B-‘H)) 6 6.0 (t, lB(4). (105)). (s, SC(Cpb)).

-32,s

(d. 2B(1,3),

(111);

riC(‘H}

NMR

S 92.7 (s. K(C$)).

97.3

c3

’

Cl0

Fig. 2. Crystal structure of [Nb(q-CsH5)2B3H8] (2). Hydrogen atoms on the Cp rings have been excluded for clarity. Selected bond distances and angles are from one of the molecules in the asymmetric unit only. Selected bond distances (A) (Cp = q-&H,): Nb(2)-Cp(centroid) 2.047, Nb(Z)-B(1) 2.555(6), Nb(2)-B(3) 2.566(6), B(l)-B(3) 1.727(S), B(l)-B(4) 1.804(9), B(3)-B(4) 1.79(l). Selected bond angles (“): Cp(centroid)-Nb(2)-Cp(centroid) 137.95, B(l)-Nb(2)-B(3) 39.4(2), Nb(2)-B(l)-B(4) 108.1(4), Nb(2)-B(3)-B(4) 108.1(4), B(l)-B(4)-B(3) 61.6(4), dihedral angle between planes B(l).Nb(2),B(3) and B(lj,B(4),B(3) 124.93.

I

I

Fig. 3.

20 “B

1

10

I

0

I

-10

I

I

-20 PPH

NMR at 64.2 MHz of [Nb(q-C,H,),B,H,]

-30

(1).

,

-LO

L

-50

I

-60

c4

crystal structure * showed two independent molecules of 2 in the asymmetric unit. There are no significant differences between the structural parameters of the two molecules. The butterfly geometry. shown for one of the molecuIes in the asymmetric unit (Fig. 2). is well established for arachno-2-metallaborane analogues of tetraborane(l0). The solution ‘H, “B and ‘jC NMR spectra are consistent with the solid state structure, but interestingly, spin saturation transfer experiments show the presence of scrambling processes involving exchange of the terminal protons on B(1,3), one of the exo/endo protons on B(4) and the bridging protons H(1,4) and H(3,4). Similar processes have been observed in compound 1 involving the terminal protons on B(5) and B(4) with the bridging proton H(4.5). Previously reported systems showing bridge/ terminal exchange processes include the [3,3.3,3-(CO),arachno-WB,Ht2]anion [6] and B,H,, [7]. The {Nb(&H,), } fragment can be considered to subrogate for a {BH, } fragment and to contribute three orbitals and three electrons towards the skeletal electron count. Thus, from electron counting procedures, 1 can be considered to be an analogue of the arachno-B, II,, cluster whereas 2 can be considered to be an analogue of the aruchno-B,H,O cluster. Previously reported metalla subrogated analogues of pentaborane(l1) are apex subrogated. In the absence of any obvious steric effects to explain the subrogation position in compound 1. we suggest that the planar spatial disposition of the frontier orbitals of the {Nb( n-C,H, )2 } fra gment favours base subrogation, whereas the pyramidal arrangement of the frontier orbitals of the {Ir(CO)(PR,),} and {RuH(C,Me& f ra g ments is more compatible with apex subrogation.

We thank the SERC for a CASE award to P.M.; Oxford Polytechnic thank the DTJ for award of grant towards the purchase of a Brucker AC200 multinuclear NMR spectrometer.

References 1 (a) T. Onak R.R.

and J.B. Leach,

Rietz,

and L.G. Roseberry * Crystal 24.433(3) crystal

R. Schaeffer

Sot.,

Sneddon,

92 (1970)

Inorg.

Chem.,

Sneddon, J. Am. Chem. Sot., 92 (1970) 3514; and R. Schaeffet, ibid., 95 (1973) 2496. data:

C,,H,,NbB,,

size ca. 0.30~0.25X

applied

using and

0.05 mm.

graphite

structure

3513;

(b) J.B. Leach,

9 (1970)

(d) A.O.

2170;

Clause,

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Moody,

Data

monochromated

solution

and

were collected MO-K,

refinement

(28,,,,,

radiation.

map

and

the cyclopentadienyl

included rings

from

the

Director

Rietz.

T.

were

carried

out using

in the refinement were included

subject

in calculated

to soft restraints. positions.

correction

30.57 observed

reflections

Hydrogen

Full matrix

of

the

Cambridge

Laboratory, Lensfield Rd., Cambridge literature citation for this communication.

CB2

Crystallographic IEW.

Any

request

Data should

CAD:

absorption

Centre,

using SHELXS from a Fourier

atoms

least

of 316 least squares parameters has led to final agreement factors of R = 0.0268 and weights). The atomic coordinates, bond lengths, angles and thermal parameters request

R. Schaeffer R.R.

50 o ) on an Enraf-Nomus An empirical

(I > 3n( I)) from the 4183 independent reflections measured. The structure was solved and routine Fourier methods. Hydrogen atoms on the B,H, fragment were located difference

J. Spielman,

Rietz.

M = 263.6, monoclinic. space group Cr,c, (t 24.684(4), h 8.039(l), lJ 4748 A’, 2=16, DC 1.47 g cm-.“, F(OOO) = 2144, ~(Mo-K,) 9.3 cm-‘,

A, / 101.71(1)“,

diffractometer, was

J. Am. Chem.

and L.G.

squares

attached

to

refinement

R w := 0.0298 (unit are available on

University

be accompanied

Chemical by the full

c5 2 R. Greatrex, N.N. Greenwood, D.W.H. Rankin and H.E. Robertson, Polyhedron, 6 (1987) 1849. 3 S.K. Boocock, M.A. Toft, K.E. Inkrott, L.-Y. Hsu, J.C. Huffman, K. Folting and S.G. Shore, Inorg. Chem., 23 (1984) 3084. 4 J. Bould, N.N. Greenwood and J.D. Kennedy, J. Chem. Sot. Dalton Trans., (1982) 481. 5 M. Bown, N.N. Greenwood and J.D. Kennedy, J. Organomet. Chem., 309 (1986) C67. 6 X.L.R. Fontaine, N.N. Greenwood, J.D. Kennedy, I. Macpherson and M. Thornton-Pett, J. Chem. Sot., Chem. Commun., (1987) 476. 7 R. Schaeffer and L.G. Sneddon, Inorg. Chem., 11 (1972) 3098.