- Email: [email protected]
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,,,,<,,(4/5)), 0.3 (s> 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,
T. Onak,
(c) R.R. DC.
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.
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,,,,<,,(4/5)), 0.3 (s> 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,
T. Onak,
(c) R.R. DC.
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.