Studies of six-coordinate adducts all-trans-[M(OAr)2(X)2(L)2] (M = Nb, Ta; X = Cl, Br; L = tertiary phosphine, pyridine)

Article abstract of DOI:10.1016/s0020-1693(00)00195-x

An extensive series of d¹-derivatives of niobium and tantalum [M(OAr)₂(X)₂(L)₂] (M = Nb, Ta; X = Cl, Br; L = tertiary phosphine, pyridine) have been isolated by reduction of the corresponding adducts [M(OAr)₂(X)₃(L)] in the presence of excess L. Crystallographic studies show an all-trans, octahedral arrangement of ligands about the metal center. The effect of the halide, donor ligand and aryloxide substituents upon the structural parameters is discussed. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/s0020-1693(00)00195-x

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Inorganica Chimica Acta 307 (2000) 63–70
Studies of six-coordinate adducts all-trans-[M(OAr)2(X)2(L)2]
(M=Nb, Ta; X=Cl, Br; L=tertiary phosphine, pyridine)
Scott W. Schweiger, Emily E. Freeman, Janet R. Clark, Mark C. Potyen,
Phillip E. Fanwick, Ian P. Rothwell *
1393 Brown Building, Department of Chemistry, Purdue Uni6ersity, West Lafayette, IN 47907-1393, USA
Received 22 May 2000; accepted 26 May 2000
Abstract
An extensive series of d1-derivatives of niobium and tantalum [M(OAr)2(X)2(L)2] (M=Nb, Ta; X=Cl, Br; L=tertiary
phosphine, pyridine) have been isolated by reduction of the corresponding adducts [M(OAr)2(X)3(L)] in the presence of excess L.
Crystallographic studies show an all-trans, octahedral arrangement of ligands about the metal center. The effect of the halide,
donor ligand and aryloxide substituents upon the structural parameters is discussed. © 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Crystal structures; Niobium complexes; Tantalum complexes; Aryloxide complexes
2. Results and discussion
1. Introduction
The organometallic chemistry of niobium and tanta-
lum supported by aryloxide ligation continues to be an
active field of research. Two broad areas of investiga-
tion involve the stoichiometric and catalytic reactivity
of d0-alkyl and hydrido aryloxides [1,2], and the activa-
tion of unsaturated organic substrates by highly reduc-
ing, low valent aryloxides of niobium and tantalum
[3,4]. During the exploration of the utility of the mixed
halo aryloxides [M(OC6H3Pri2-2,6)2Cl3]2 (M=Nb, 1;
Ta, 2), [Ta(OC6H3Pri2-2,6)2Br3]n (3) and [Ta(OC6H2Pri2-
2,6-Br-4)2Cl3]n (4) as precursors for the formation of
metal-hydride derivatives as well as low-valent arylox-
ides, we have isolated a series of six coordinate d1-metal
complexes [M(OAr)2(X)2(L)2] (M=Nb, Ta; X=Cl, Br;
L=py, various tertiary phosphines). We wish to report
here on the solid state structures of these d1-molecules
and compare their structural parameters with various
other aryloxide derivatives of niobium and tantalum as
well as the distorted six-coordinate d0-adducts
[M(OAr)x(Cl)5−x(L)] (M=Nb, Ta; x=2, 3).
2.1. Synthesis and spectroscopic characterization of
compounds
The chloro-bridged, edge-shared bis-octahedral
dimers [M(OC6H3Pr2i -2,6)2Cl3]2 (M=Nb, 1; Ta, 2) have
been previously isolated and fully characterized. The
new, mixed halo, aryloxides [Ta(OC6H3Pri2-2,6)2Br3]n
(3) and [Ta(OC6H2Pr2i -2,6-Br-4)2Cl3]n (4) have also been
obtained directly from the metal halide and 2 equiv. of
the corresponding phenol. The exact molecularity of
these new derivatives has not been determined. Com-
pounds 1 and 2 have been shown to form an extensive
series of adducts [M(OC6H3Pri2-2,6)2Cl3(L)] with a vari-
ety of O1 [5], N2, and P donor ligands [6]. These
1 A series of diethyl–ether complexes of tantalum have been re-
ported by Wigley et al., see Refs. [2a,c], e.g. cis,mer-
[TaC13(OAr)2(OEt2)] and trans,mer-[TaC12(OAr)3(OEt2)] (OAr=2,
6-dimethyl and 2,6-diisopropylphenoxide); a number of tetrahydro-
furan complexes of mixed chloro aryloxides of niobium have been
structurally characterized by Nakamura et al. [5].
* Corresponding author. Tel.: +1-765-494 7012; fax: +1-765-494
0239.
E-mail address: rothwell@chem.purdue.edu (I.P. Rothwell).
2 A series of quinoline (quin) complexes. cis,mer-[TaC13-
(OC6H3Pri2-2,6)2(quin)] and trans,mer-[TaC12(OC6H3Pr2i -2,6)3(quin)]
have been reported by Wigley et al., see Refs. [3d–f].
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00195-X

64
S.W. Schweiger et al. / Inorganica Chimica Acta 307 (2000) 63–70
Scheme 1.
adducts contain a distorted octahedral geometry in
which the cis ligands are bent towards the donor ligand
(e.g. PMe2Ph adducts 5 and 6, Scheme 1). In the case of
the tantalum phosphine adduct [Ta(OC6H3Pri2-
2,6)2Cl3(PMe2Ph)] (4), reaction with Bun3SnH has been
shown to be a good synthetic route to the seven coordi-
nate hydride derivatives [Ta(OC6H3Pri2-2,6)2(Cl)3−x
-
(H)x(PMe2Ph)2] (x=1, 7; x=2, 8, Scheme 1) [2]. The
di-hydride complex 8 has also been obtained by hy-
drogenolysis of [Ta(OC6H3Pri2-2,6)2(CH2C6H4-4Me)2Cl]
(7) in the presence of PMe2Ph (Scheme 1). The six-coor-
dinate dihydride [Ta(OC6H3But2-2,6)2(Cl)(H)2(PMe2Ph)]
(9) containing the bulkier aryloxide was found to be
even more distorted than the halide adducts both in
solution and in the solid state (Scheme 1) [2,6].
In contrast, addition of Bun3SnH to hydrocarbon
solutions of the niobium adduct [Nb(OC6H3Pri2-
2,6)2Cl3(PMe2Ph)] (3) was found to lead to the evolu-
tion of H2 gas and formation of the pale yellow,
sparingly
soluble
derivative
[Nb(OC6H3Pri2-
2,6)2Cl2(PMe2Ph)2] (10) in high yield (Scheme 2). Large,
well formed crystals of 8 were also obtained by hy-
drogenolysis of the bis(4-methylbenzyl) complex
[Nb(OC6H3Pr2i -2,6)2(CH2C6H4-4Me)2Cl] (11) in the
presence of PMe2Ph (Scheme 2). This product pre-
sumably arises via a ligand redistribution reaction in
solution. During an attempt to carry out the catalytic
hydrogenation of triphenylphosphine by a mixture of
Scheme 2.

S.W. Schweiger et al. / Inorganica Chimica Acta 307 (2000) 63–70
65
hexane. The reaction of
a
benzene solution of
[Nb(OC6H3Pri2-2,6)2Cl3(py)] (17) with Bun3SnH results in
the rapid evolution of H2 and formation of a reaction
mixture from which d1-[Nb(OC6H3Pr2i -2,6)2Cl2(py)2]
(18) can be isolated upon dilution with hexane (Scheme
2).
The sodium amalgam (1 Na per Ta) reduction of 3 or
4 in the presence of excess PMe2Ph led to the formation
of the corresponding derivatives [Ta(OC6H3Pri2-
2,6)2Br2(PMe2Ph)2] (19) and [Ta(OC6H2Pri2-2,6-Br-
4)2Cl2(PMe2Ph)] (20) (Scheme 3). The reaction of 3 or 4
with Bun3SnH (excess) in the presence of PMe2Ph
proved to be more complex. The d1-complexes 19 and
20 were detected in the reaction mixture by EPR.
However, the majority of the product was determined
to be the dihydride derivatives [Ta(OC6H3Pri2-
2,6)2(H)2Br(PMe2Ph)2] (21) and [Ta(OC6H2Pri2-2,6-Br-
4)2(H)2Cl(PMe2Ph)] (22) respectively (Scheme 3). One
crystal obtained directly from the reaction mixture and
analyzed by X-ray diffraction was solved as a solid
1
solution of 7% of 19 with 93% 21. In the H NMR
spectrum of bromide, dihydride 21 the Ta-H resonance
appears at l 15.92 ppm, upfield of the value of l 16.55
ppm found for the corresponding chloride
[Ta(OC6H3Pri2-2,6)2(H)2Cl(PMe2Ph)2] [2c]. However,
the p-bromo substituent in 22 has a minimal impact,
causing the hydride signal to shift upfield only slightly
to l 16.45 ppm.
Scheme 3.
[Nb(OC6H3Pri2-2,6)2Cl3]2 (1) and Bun3SnH, dark orange
crystals
of
the
sparingly
soluble
complex
[Nb(OC6H3Pri2-2,6)2Cl2(PPh3)2] (12) were also obtained
(Scheme 2). 31P NMR detected no hydrogenation of
phenyl-phosphine groups within the reaction mixture.
Previously it has been shown by Wigley et al. that
reduction of the adducts [Ta(OC6H3Pri2-2,6)2Cl3(py)]
(13) and [Ta(OC6H3Pr2i -2,6)2Cl3(quin)] (14) by sodium
amalgam (1 Na per Ta) in the presence of pyridine
(py) or quinoline (quin) led to the corresponding d1-
derivatives [Ta(OC6H3Pri2-2,6)2Cl2(py)2] (15) and
[Ta(OC6H3Pri2-2,6)2Cl2(quin)2] (16) respectively (Scheme
2) [3f]. We find that treatment of a benzene solution of
the adduct [Ta(OC6H3Pri2-2,6)2Cl3(py)] (13) with
Bun3SnH in the presence of excess py leads to a mixture
2.2. Structural studies
The compounds 10, 12, 19, 20 (including the benzene
solvate) and isomorphous 15 and 18 have been sub-
jected to X-ray diffraction analysis. Table 1 collects
some important structural parameters, while Tables 2
and 3 contain crystallographic data. The molecular
structures of 10, 15 and 20 (benzene solvate) are shown
in Figs. 1–3 respectively. All of the compounds can be
seen to adopt an all-trans arrangement of the halide,
aryloxide and donor ligands. The bond angles around
the metal centers are very close to those expected for an
octahedral geometry. This is in sharp contrast to the
situation found for the parent d0-adducts cis-mer-
1
of unreacted 13 (detected by H NMR) along with a
small amount of 15 (crystallographically characterized).
This reaction mixture yielded both dark red crystals of
13 along with purple crystals of 15 when diluted with
Table 1
,
Selected bond distances (A) and angles (°) for all-trans-[M(OAr)2X2L2]
Compound
M
OAr
X
L
M–OAr [M–O–C]
M–X
M–L
10
12
19
20
20·C6H6
18
15
Nb OC6H3Pri2-2,6
Nb OC6H3Pri2-2,6
Ta OC6H3Pri2-2,6
Cl
Cl
PMe2Ph 1.901(2) [177]
PPh3 1.892(7) [174]
2.4552(9)
2.456(3)
2.6004(6)
2.450(1)
2.448(1)
2.4413(8); 2.4464(8)
2.429(1); 2.450(1)
2.6647(8)
2.782(4)
2.659(1)
2.646(1)
2.655(1)
2.256(2); 2.253(2)
2.231(3); 2.231(3)
Br PMe2Ph 1.904(2) [175]
Ta OC6H2Pri2-2,6-Br-4 Cl
Ta OC6H2Pri2-2,6-Br-4 Cl
PMe2Ph 1.909(3) [175]
PMe2Ph 1.893(4) [179]
Nb OC6H3Pri2-2,6
Ta OC6H3Pri2-2,6
Cl
Cl
py
py
1.903(2) [164]; 1.889(2) [164]
1.906(3) [164]; 1.896(3) [164]

66
S.W. Schweiger et al. / Inorganica Chimica Acta 307 (2000) 63–70
Table 2
Crystal data and data collection parameters for 10, 12, 15 and 18
10
12
15
18
Formula
Formula weight
Space group
C40H56Cl2O2P2Nb
794.65
P21/c
C
60H64Cl2O2P2Nb
C34H44Cl2O2N2Ta
764.60
P1 (no. 2)
C34H44Cl2O2N2Nb
676.55
P1 (no. 2)
1042.94
Pbca (no. 61)
Unit cell parameters
,
a (A)
9.4842(7)
10.8233(9)
20.307(2)
90
97.393(7)
90
2067.2(6)
2
1.277
13.342(1)
18.842(2)
20.823(2)
90
90
90
5234(1)
4
11.603(3)
13.318(3)
13.598(3)
69.16(2)
86.49(2)
64.86(2)
1767.8(8)
2
11.649(1)
13.283(1)
13.616(1)
69.328(7)
86.592(9)
64.887(9)
1774.7(4)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
,
V (A)
Z
zcalc (g cm−3
)
1.323
1.436
1.266
Temperature (K)
293
293
293
293
,
Radiation (wavelength, A)
R
Mo Ka (0.71073)
0.037
Mo Ka (0.71073)
0.047
Mo Ka (0.71073)
0.029
Mo Ka (0.71073)
0.033
RW
0.052
0.052
0.033
0.042
Table 3
Crystal data and data collection parameters for 19, 20 and 21
19
20
20·C6H6
0.9321+0.0719
Formula
Formula weight
Space group
C40H56Br2O2P2Ta
971.61
P21/c (no. 14)
C40H54Br2Cl2O2P2Ta
1040.50
P1 (no. 2)
C46H60Br2Cl2O2P2Ta
1118.61
C2/c (no. 15)
C40H57.87Br1.07O2P2Ta
898.78
P21/n (no. 14)
Unit call parameters
,
a (A)
9.3486(7)
10.9854(3)
20.1368(9)
90
97.2516(19)
90
2051.5(2)
2
1.573
9.3866(2)
11.0972(4)
11.1275(4)
77.0487(13)
77.570(2)
81.546(2)
1097.17(8)
1
29.3085(10)
10.7106(4)
17.7393(4)
90
120.215(2)
90
4812.0(6)
4
1.544
9.7230(3)
24.3048(8)
17.6904(5)
90
90.7760(18)
90
4180.1(4)
4
1.428
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
,
V (A)
Z
zcalc (g cm−3
)
1.575
Temperature (K)
173
203
173
173
,
Radiation (wavelength, A)
R
Mo Ka (0.71073)
0.043
Mo Ka (0.71073)
0.035
Mo Ka (0.71073)
0.054
Mo Ka (0.71073)
0.039
RW
0.106
0.084
0.136
0.083
[M(OAr)2(Cl)3(L)] (M=Nb, Ta) [6]. In these com-
pounds there is a distinct distortion away from octahe-
dral geometry with the cis-chloride ligands being bent
towards the single donor ligand. This distortion has
been rationalized in terms of allowing an increase in the
p-bonding between the aryloxide trans- to the L group
and the metal center. The all-trans geometry in the
d1-compounds means that there is nothing to be gained
by any distortion away from a regular octahedral ge-
ometry. The geometry of six-coordinate adducts of the
tetrahalides [MX4L2] has been the focus of many stud-
ies dating back to the work of McCarlet et al. [7] and
Walton et al. [8]. Both cis and trans isomers have been
isolated and structurally characterized, e.g. cis-
[NbCl4(NCMe)2] [9], cis-[TaBr4(PMe2Ph)2] [10], trans-
[TaCl4(py)2] [11], trans-[NbCl4(PEt3)2] and trans-
[NbCl4(PEtPh2)2] [12]. Interestingly in the cis-isomers
there is a distinct bending of the halides towards the
donor ligands. Seven-coordinate tris(phosphine) ad-
ducts and fused, square-antiprismatic dinuclear deriva-
tives of the metal(IV) halides have also been isolated
[13].
Of most interest to this study are the interatomic
distances around the central coordination sphere of the
metal center. The metal–oxygen distances for all com-
pounds span the very narrow range of 1.889(2)–
,
1.909(3) A. These distances are indistinguishable within
experimental error. Previous studies have shown that

S.W. Schweiger et al. / Inorganica Chimica Acta 307 (2000) 63–70
67
Fig. 1. Molecular structure of 10 showing the atomic numbering
scheme.
Fig. 3. Molecular structure of 20 (benzene solvate) showing the
atomic numbering scheme.
the metal–ligand distances for isomorphous niobium
and tantalum compounds are essentially the same.
Hence we can say that the replacement of chloride
ligands in niobium compound 10 by bromide in tanta-
lum compound 19 has minimal effect on the metal
aryloxide bond length. Also the introduction of a p-
bromo-substituent onto the phenoxide nucleus has no
effect upon this parameter. The M–O–Ar angles are all
large, as typically found for early d-block metal arylox-
ides [14]. The metal–chloride distances are also identi-
cal for all new compounds, varying from
slightly longer than those found in the parent d0-ad-
ducts. For example the compound cis-mer-
[Nb(OC6H3Pri2-2,6)2Cl3(PMe2Ph)] has Nb–O and
Nb–Cl distances of 1.82(2), 1.83(2) and 2.366(8)–
,
2.425(7) A respectively. The isomorphous pyridine
adducts cis-mer-[M(OC6H3Pri2-2,6)2Cl3(py)] have corre-
sponding distances of 1.832(3)–1.852(3) and 2.366(2)–
,
2.403(1) A [6]. There appears to be slightly less oxygen
(or chlorine) to metal p-bonding in the d1-compounds,
consistent with other studies which have shown the
dramatic sensitivity of the metal–aryloxide bond to the
electrophilicity of the metal center [4b].
,
2.429(1)–2.456(3) A. The metal–aryloxide and metal–
chloride distances in these d1-derivatives are, however,
Fig. 2. Molecular structure of 15 showing the atomic numbering scheme.

68
S.W. Schweiger et al. / Inorganica Chimica Acta 307 (2000) 63–70
The metal-donor ligand distances are also worthy of
comment. The M–N distances in isomorphous 15 and
(HCl) and a change in color from yellow to deep red.
This solution was stirred for 40 min and the solvent
removed in vacuo to yield the product as a yellow
powder which was washed with hexane and dried. Yield
,
18, 2.231(3)–2.256(2) A, are shorter than the M–N
,
distances of 2.331(4) and 2.315(6) A in isomorphous
cis-mer-[M(OC6H3Pr2i -2,6)2Cl3(py)] (M=Nb, Ta) [6].
This possibly reflects the fact that in the d0-adducts the
pyridine ligand is trans to an aryloxide (trans influence).
88.8g (almost quantitative). H NMR (C6D6, 30°C): l
1
7.17–7.36 (m, 4H, aromatics); 3.75 (m, 2H, CHMe2);
0.967 (d, 9H, CHMe2). Anal. Calc. for TaO2Cl3-
Br2C24H32: C, 36.05; H, 4.03. Found C, 35.94; H,
4.07%.
,
The Ta–N distance is 2.222(7) A in trans-[TaCl4(py)2],
very similar to that found in 15 (18). The Nb–P
,
distance of 2.6647(8) A in 10 is also shorter than the
i
distance of 2.742(8) A found in cis-mer-[Nb(OC6H3Pr2-
3.2.3. [Nb(OC6H3Pri2-2,6)2Cl2(PMe2Ph)2] (10)
,
2,6)2Cl3(PMe2Ph)]. As expected, the less basic PPh3
derivative 12 has a longer Nb–P distance of 2.782(4) A
To a suspension of [Nb2(OC6H3Pri2-2,6)4Cl6] (1) (2.01
g, 1.81 mmol) in benzene (10 ml) was added PMe2Ph
(1.10 g, 7.95 mmol) with stirring. To this stirred reac-
tion mixture was added Bun3SnH (2.0 ml, 7.4 mmol)
dropwise over 10 min. The initial orange solution dark-
ened and gas (H2) was evolved. After complete addition
the mixture was allowed to stand for 24 h yielding the
product as orange crystals, which were washed with
hexane and dried. Yield 2.29 g (79%). Anal. Calc. for
NbO2Cl2P2C40H56: C, 60.46; H, 7.10; Cl, 8.92; P, 7.80.
Found C, 60.71; H, 7.41; Cl, 8.65; P, 7.36%. EPR
(C7H8, 30°C): g=1.9346 (aNb=134 G).
,
compared to 10.
3. Experimental
3.1. General
All operations were carried out under a dry nitrogen
atmosphere using standard Schlenk techniques. The
hydrocarbon solvents were distilled from sodium ben-
zophenone and stored over sodium ribbons under ni-
trogen until use. Reagents were purchased from Aldrich
Chemical Co., Inc. and used without further purifica-
3.2.4. [Nb(OC6H3Pri2-2,6)2Cl2(PPh3)2] (12)
A 50 ml round bottomed flask was charged in a
Dri-Lab with a mixture of PPh3 (2.50 g, 9.5 mmol) and
[Nb2(OC6H3Pri2-2,6)4Cl6] (1) (0.25 g, 0.23 mmol) in
benzene (20 ml). The mixture was ‘activated’ with
Bun3SnH (0.40 ml, 1.5 mmol) and placed in a Parr
model 4561, 300 ml minireactor, pressurized with H2
(1200 psi) and heated to 60°C for 24 h. After cooling
and depressurizing, the flask was removed from the
minireactor within the Dri-Lab. A few golden crystals
of 12 were observed and isolated from the mother
liquor. These were characterized entirely by X-ray crys-
tallography. Analysis of the solution by 1H and 31P
NMR showed no hydrogenation of benzene or PPh3.
tion. The H and 13C NMR spectra were recorded on a
1
Varian Associates Gemini-200 spectrometer and refer-
enced to protio impurities of commercial benzene-d6 or
deuterated chloroform as internal standards. Mass
spectra, elemental analyses and X-ray diffraction stud-
ies were obtained in-house at Purdue University. Gen-
eral operating procedures have been reviewed
elsewhere.
3.2. Synthesis of compounds
3.2.1. [Ta(OC6H3Pri2-2,6)2Br3] (3)
To a stirred solution of [TaBr5] (13.47 g, 23.0 mmol)
in toluene (250 ml), was slowly added 2,6-diisopropy-
lphenol (9.49 ml, 51.0 mmol) via syringe. The solution
became dark orange and was stirred for 24 h. Removal
of solvent and generated HBr under vacuum yielded the
crude product as a brown solid. Layering a saturated
benzene solution of the crude material with hexane
yielded the pure product as a yellow crystalline solid,
which was washed with hexane and dried. Yield 10.75 g
(59.8%). 1H NMR (C6D6, 30°C): l 6.8–7.2 (m, 6H,
aromatics); 3.7 (m, 2H, CHMe2); 1.22 (d, 9H, CHMe2).
Anal. Calc. for C24H34Br3O2Ta: C, 37.19; H, 4.42; Br,
30.92. Found: C, 37.96; H, 4.11; Br, 30.11%.
3.2.5. [Ta(OC6H3Pri2-2,6)2Cl2(py)2] (15)
To a suspension of [Ta2(OC6H3Pri2-2,6)4Cl6] (1) (2.31
g, 1.80 mmol) in benzene (20 ml) was added pyridine
(1.0 ml, 12.3 mmol) with stirring. To this stirred reac-
tion mixture was added Bun3SnH (2.0 ml, 7.4 mmol)
dropwise over 10 min. The initial yellow solution dark-
ened. After complete addition the mixture was allowed
to stand for 24 h before the solvent was removed under
vacuum to yield a dark brown tar. Dissolving the crude
material in benzene and layering with pentane yielded a
mixture of yellow crystals of adduct [Ta(OC6H3Pri2-
2,6)2Cl3(py)] (NMR) and dark red crystals of 15 (iden-
tified crystallographically).
3.2.2. [Ta(OC6H2Pri2-2,6-Br-4)2Cl3] (4)
To a suspension of [TaCl5] (39.9 g, 0.11 mol) in
benzene (500 ml) was added HOC6H2Pri2-2,6-Br-4 (61.0
g, 0.237 mol) via syringe resulting in gas evolution
3.2.6. [Nb(OC6H3Pri2-2,6)2Cl2(py)2] (18)
To a suspension of [Nb2(OC6H3Pri2-2,6)4Cl6] (1) (2.00
g, 1.80 mmol) in benzene (20 ml) was added pyridine

S.W. Schweiger et al. / Inorganica Chimica Acta 307 (2000) 63–70
69
(1.0 ml, 12.3 mmol) with stirring. To this stirred reac-
tion mixture was added Bun3SnH (2.0 ml, 7.4 mmol)
dropwise over 10 min. The initial orange solution dark-
ened and gas (H2) was evolved. After complete addition
the mixture was allowed to stand for 24 h before the
solvent was removed under vacuum to yield a dark tar.
Dissolving the crude material in benzene and layering
with pentane yielded a few red crystals of adduct
[Nb(OC6H3Pri2-2,6)2Cl3(py)] (NMR) with the majority
being dark orange crystals of 18. Anal. Calc. for
NbO2Cl2N2C34H44: C, 60.36; H, 6.55; Cl, 10.48; N,
4.14. Found C, 60.46; H, 6.44; Cl, 9.99; N, 4.92%.
ping doublets, CHMe2 and PMe2). Analysis of one
crystal obtained from this procedure by X-ray crystal-
lography showed a disordered mixture of 7% 19 and
93% 21.
3.2.10. [Ta(OC6H2Pri2-2,6-Br-4)2(H)2Cl(PMe2Ph)2]
(22)
To a suspension of [Ta(OC6H2Pri2-2,6-Br-4)2Cl3] (5.0
g, 6.2 mmol) in benzene (25 ml) was added PMe2Ph
(3.11g, 23 mmol) giving a yellow solution. To this was
added Bun3SnH (10.92 g, 36.8 mmol) resulting in a
brown solution, which was stirred overnight. Addition
of hexane led to a pale precipitate which was dried in
vacuo, washed three times with hexane and re-dried to
yield 4.3 g (46%) of a yellow solid which is a mixture of
3.2.7. [Ta(OC6H3Pri2-2,6)2Br2(PMe2Ph)2] (19)
An orange suspension of [Ta(OC6H3-2,6-Pri2)2Br3 (1.0
g, 1.38 mmol) in benzene (25 ml), was added to an
Na/Hg amalgam (0.03 g, 1.38 mmol) upon which the
solution turned dark red. Upon addition of PMe2Ph
(0.587 ml, 4.12 mmol) via syringe the solution turned
dark green. The reaction mixture was stirred overnight,
filtered to remove sodium salts and dried in vacuo to
give the crude product as a green solid (0.48 g, 32%).
Recrystallization from hot benzene afforded a green
crystalline sample that was washed with pentane and
dried in vacuo. Anal. Calc. for C40H56Br2O2P2Ta: C,
49.44; H, 5.81. Found: C, 48.94; H, 5.71%.
1
20 and 22. H NMR (C6D6, 30°C): l 16.45 (m, 2H,
TaH); 6.85–7.3 (m, 14H, aromatics); 4.00 (m, 2H,
CHMe2); 3.89 (m, 2H, CHMe2); 1.53 (m, 12H,
PMe2Ph); 1.11 (d, 6H, CHMe2); 1.00 (d, 6H, CHMe2).
31P NMR (C6D6, 30°C): l −3.0 ppm.
3.2.11. X-ray data collection and reduction
A suitable crystal was mounted on a glass fiber in a
random orientation under a cold stream of dry nitro-
gen. Preliminary examination and final data collection
,
were performed with Mo Ka radiation (u=0.71073 A)
on a Nonius Kappa CCD. Lorentz and polarization
corrections were applied to the data [15]. An empirical
absorption correction using SCALEPACK was applied
[16]. Intensities of equivalent reflections were averaged.
The structure was solved using the structure solution
program PATTY in DIRDIF92 [17]. The remaining atoms
were located in succeeding difference Fourier syntheses.
Hydrogen atoms were included in the refinement but
restrained to ride on the atom to which they are
bonded. The structure was refined in full-matrix least-
3.2.8. [Ta(OC6H2Pr2i -2,6-Br-4)2Cl2(PMe2Ph)2] (20)
An orange suspension of [Ta(OC6H2Pri2-2,6-Br-4)2Cl3
(1.0 g, 1.33 mmol) in benzene (25 ml), was added to an
Na/Hg amalgam (0.03 g, 1.33 mmol) upon which the
solution turned dark red. Upon addition of PMe2Ph
(0.568 ml, 3.99 mmol) via syringe the solution turned
dark green. The reaction mixture was stirred overnight,
filtered to remove sodium salts and dried in vacuo to
give the crude product as a green solid (0.59 g, 43%).
Recrystallization from an ambient benzene solution
layered with pentane was found to lead to crystals of 20
containing no solvent of crystallization. However, re-
crystallization from hot benzene afforded large green
crystals of the benzene solvate. Anal. Calc. for 20·C6H6,
C46H62Cl2Br2O2P2Ta: C, 49.39; H, 5.59. Found: C,
49.33; H, 5.35%.
2
squares where the function minimized was ꢀw(ꢁFoꢁ −
2 2
ꢁFcꢁ ) and the weight
w
is defined as w=
1/[|2(F2o)+(0.0585P)2+1.4064P] where P=(Fo2+
2Fc2)/3. Scattering factors were taken from the Interna-
tional Tables for Crystallography [18]. Refinement was
performed on a AlphaServer 2100 using SHELX-97 [19].
Crystallographic drawings were done using programs
ORTEP [20].
3.2.9. [Ta(OC6H3Pri2-2,6)2(H)2Br(PMe2Ph)2] (21)
To a solution of [Ta(OC6H3Pri2-2,6)2Br3] (3) (0.51 g,
0.66 mmol) in benzene (5 ml) was added PMe2Ph (0.27
ml, 1.91 mmol) and Bun3SnH (0.52 ml, 1.91 mmol) with
stirring. The mixture was then diluted with hexane (15
ml) and allowed to stand. Almost colorless crystals of
product were formed over 24 h. Yield 0.19 g (34%).
Anal. Calc. for TaO2BrP2C40H58: C, 53.76; H, 6.54; Br,
8.94; P, 6.93. Found C, 53.46; H, 6.74; Br, 9.18; P,
4. Supplementary material
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Center, CCDC Nos. 144478 (10), 144479 (12),
144480 (15), 144481 (18), 144482 (19), 144483 (20),
144484 (20·C6H6), and 144485 (mixture of 7% 19 and
93% 21). Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-
1
6.62%. H NMR (C6D6, 30°C): l 15.92 (m, 2H, TaH);
6.9–7.5 (m, 6H, aromatics), 4.25 (septet, 2H, CHMe2),
3.78 (septet, 2H, CHMe2), 0.8–1.8 (series of overlap-

70
S.W. Schweiger et al. / Inorganica Chimica Acta 307 (2000) 63–70
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Acknowledgements
We thank the Department of Energy for financial
support of this research.
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