Synthesis, structure and hapticity interconversion reactions of pentahapto-bonded cyclooctadienyl complexes of molybdenum

Article abstract of DOI:10.1039/b209975f

The pentahapto-bonded cyclooctadienylmolybdenum complexes [Mo(CO)₂(L₂)(1-3:5,6-η-C₈H₁₁)][B F₄] (L₂ = Ph₂P(CH₂)_nPPh₂, n = 1, (dppm), 1a; n = 2, (dppe), 2a) are synthesised by reaction of [MoBr(CO)₂(L₂)(1-3-η:5,6-C₈H₁₁)] with Ag[BF₄] in CH₂Cl₂; [Mo(CO)₂(dppm)(1-5-η-C₈H₁₁)][BF₄ ], 1b, is obtained from [MoBr(CO)₂(dppm)(1-3-η:4.5-C₈H₁₁)] by an identical procedure. Related syntheses afford [Mo(CO)₂{P(OMe)₃}₂(1-5-η-C₈ H₁₁)][BF₄], 4b, and [Mo(CO)₂(CNBu^t)₂(1-3:5,6-η-C₈ H₁₁)][BF₄], 5a. The complexes [Mo(CO)₂(NCMe)₃(η³-C₈ H₁₁)]⁺ (η³-C₈H₁₁ = 1-3-η:5.6-C₈H₁₁, 1-3-η:4.5-C₈H₁₁) are precursors to [Mo(CO)(CNBu^t)₃(1-3:5,6-η-C₈H₁₁) ][BF₄], 6a, [Mo(CO)(CNBu^t)₃(1-5-η-C₈H₁₁)][BF ₄], 6b and [Mo(CO)₂(norborna-2.5-diene)(1-3:5,6-η-C₈ H₁₁)][BF₄], 7a. The X-ray crystal structures of complexes 1a and 6b have been determined. NMR spectroscopic investigations on the 1-5-η-C₈H₁₁ complexes 1b, 4b and 6b, indicate a high barrier to rotation of the metal group with respect to the cyclooctadienyl ring. Complexes 1a, 1b and 2a undergo ligand addition reactions with accompanying η⁵ → η³ hapticity conversion at the cyclooctadienyl ligand to yield adducts [Mo(CO)₂(L′)(L₂)(η³-C₈ H₁₁)]⁺ (L′ = NCMe or CO, L₂ = dppm or dppe, η³-C₈H₁₁ = 1-3-η:5,6-C₈H₁₁ or 1-3-η:4,5-C₈H₁₁, not all combinations). The facility of these processes is strongly dependent upon the identity of the variables L₂ and C₈H₁₁.

Full text of DOI:10.1039/b209975f

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Synthesis, structure and hapticity interconversion reactions of
pentahapto-bonded cyclooctadienyl complexes of molybdenum†
Dale M. Spencer, Roy L. Beddoes, Madeleine Helliwell and Mark W. Whiteley*
Department of Chemistry, University of Manchester, Manchester, UK M13 9PL.
E-mail: mark.whiteley@man.ac.uk
Received 10th October 2002, Accepted 6th December 2002
First published as an Advance Article on the web 16th January 2003
The pentahapto-bonded cyclooctadienylmolybdenum complexes [Mo(CO)2(L2)(1–3:5,6-η-C8H11)][BF4]
(L2 = Ph2P(CH2)nPPh2, n = 1, (dppm), 1a; n = 2, (dppe), 2a) are synthesised by reaction of [MoBr(CO)2(L2)(1–3-η:5,6-
C8H11)] with Ag[BF4] in CH2Cl2; [Mo(CO)2(dppm)(1–5-η-C8H11)][BF4], 1b, is obtained from [MoBr(CO)2(dppm)-
(1–3-η:4,5-C8H11)] by an identical procedure. Related syntheses afford [Mo(CO)2{P(OMe)3}2(1–5-η-C8H11)][BF4],
4b, and [Mo(CO)2(CNBut)2(1–3:5,6-η-C8H11)][BF4], 5a. The complexes [Mo(CO)2(NCMe)3(η3-C8H11)]ϩ (η3-C8H11
1–3-η:5,6-C8H11, 1–3-η:4,5-C8H11) are precursors to [Mo(CO)(CNBut)3(1–3:5,6-η-C8H11)][BF4], 6a, [Mo(CO)-
=
(CNBut)3(1–5-η-C8H11)][BF4], 6b and [Mo(CO)2(norborna-2,5-diene)(1–3:5,6-η-C8H11)][BF4], 7a. The X-ray crystal
structures of complexes 1a and 6b have been determined. NMR spectroscopic investigations on the 1–5-η-C8H11
complexes 1b, 4b and 6b, indicate a high barrier to rotation of the metal group with respect to the cyclooctadienyl
ring. Complexes 1a, 1b and 2a undergo ligand addition reactions with accompanying η5
η3 hapticity conversion at
the cyclooctadienyl ligand to yield adducts [Mo(CO)2(LЈ)(L2)(η3-C8H11)]ϩ (LЈ = NCMe or CO, L2 = dppm or dppe,
η3-C8H11 = 1–3-η:5,6-C8H11 or 1–3-η:4,5-C8H11, not all combinations). The facility of these processes is strongly
dependent upon the identity of the variables L2 and C8H11.
bridged by methylenes C7 and C8. Further to our preliminary
Introduction
communication,9 we now present a full report on the synthesis,
structure and reactivity of the first examples of complexes of
both 1–3:5,6-η-C8H11 and 1–5-η-C8H11 ligands coordinated to
molybdenum.
Edge-bridged dienyl ligands such as 6,6-dimethylcyclohexa-
dienyl, cycloheptadienyl (1–5-η-C7H9) and cyclooctadienyl
(1–5-η-C8H11), present a special interest in the chemistry of
metal-dienyl complexes.1 In general, such ligands possess
properties intermediate between “closed”, cyclic systems such
as cyclopentadienyl and “open”, acyclic ligands such as penta-
dienyl and 2,4-dimethylpentadienyl. However, even within the
sequence of six-, seven- and eight-membered, edge-bridged
systems, there are some significant differences in structure and
reactivity as demonstrated by a series of investigations on edge-
bridged metallocenes and their ligand adducts.2–5 Clearly, it is
important to understand the role of the edge-bridge and the
impact of its size in terms of effects on structure and reactivity
and our contribution to this objective centres upon our work
with half-sandwich complexes of Mo and W containing the
edge-bridged dienyls C7H9 6–8 and C8H11.9,10 The merits of these
systems are first, the direct comparability of C7H9 and C8H11
ligands with, in each case, only H substituents attached to the
edge-bridge methylene carbons and secondly the ready avail-
ability of data for analogous cyclopentadienyl, indenyl11 and
pentadienyl12–15 complexes for purposes of comparison. The
criteria that we have examined to elucidate the effect of the
additional methylene group in the edge-bridge of the 1–5-η-
C8H11 ligand, include structural and spectroscopic data and,
most definitively, ligand addition reactions of complexes of the
type [Mo(CO)2(L2)(η5-dienyl)]ϩ to give adducts in which the
dienyl ligand is bonded trihapto. The cyclooctadienyl ligand is
commonly encountered in two isomeric forms and, for the pur-
pose of additional comparisons, both types are described in this
paper. The 1–5-η-C8H11 ligand incorporates an authentic, fully
conjugated, η5-dienyl system, edge-bridged by three adjacent
methylene groups and it is this ligand which permits a direct
comparison with our previous studies on cycloheptadienyl
chemistry. The 1–3:5,6-η-C8H11 ligand may also be considered
as pentahapto-bonded but it incorporates isolated allyl and
“ene”units separated by a methylene group (C4) and edge-
Results and discussion
Synthesis of pentahapto-bonded cyclooctadienylmolybdenum
complexes
We have previously described10 the synthesis of a series of
molybdenum complexes containing the trihapto-bonded cyclo-
octadienyl ligands, 1–3-η:5,6-C8H11 and 1–3-η:4,5-C8H11 and
these complexes, [MoBr(CO)2(L2)(η3-C8H11)] (L2 = dppm, dppe,
2,2Ј-bipy, 2CNBut) and [Mo(CO)2(L)3(η3-C8H11)]ϩ (L = NCMe
or CNBut), now serve as the precursors to pentahapto-bonded
cyclooctadienyl systems. Two synthetic strategies have been
explored. First, by analogy with the acyclic pentadienyl chem-
istry developed by Liu and co-workers,13 halide abstraction
from [MoBr(CO)2(L2)(η3-C8H11)] would be expected to afford
cationic products of formulation [Mo(CO)2(L2)(η5-C8H11)]ϩ.
The alternative approach, developed mainly in the chemistry of
the cycloheptadienylmolybdenum system, involves reaction of
the tris-nitrile complex [Mo(CO)2(NCMe)3(η3-C8H11)]ϩ with
ligand(s) L2, resulting in displacement of all three nitrile ligands
and formation of [Mo(CO)2(L2)(η5-C8H11)]ϩ. Both synthetic
routes were successfully employed in the current work but the
application of [Mo(CO)2(NCMe)3(η3-C8H11)]ϩ as a precursor
was limited by the formation of stable mono-nitrile complexes
[Mo(CO)2(NCMe)(L2)(η3-C8H11)]ϩ for some identities of L2.
Our synthetic studies (Scheme 1(a) and (b)) resulted in the iso-
lation of four categories of pentahapto-bonded cyclooctadienyl
system as classified by the supporting ligands (chelate P-donor
ligands, monodentate P-donor ligands, isocyanide ligands and
diene ligands). Microanalytical, IR and mass spectroscopic
data for the new pentahapto-bonded cyclooctadienyl complexes
are given in Table 1 whilst NMR spectroscopic data (1H,
13C{1H} and 31P{1H}) are presented in Table 2.
Treatment of red, CH2Cl2 solutions of [MoBr(CO)2(dppm)-
(η3-C8H11)] (η3-C8H11 = 1–3-η:5,6-C8H11 or 1–3-η:4,5-C8H11)
with Ag[BF4] in CH2Cl2 resulted in precipitation of AgBr and
† Electronic supplementary information (ESI) available: 1H and 13C
NMR data for [Mo(CO)2(LЈ)(L2)(η3-C8H11) 8a, 9a 11a, 12a, 3b, 8b, 9b,
10b, 11b, 12b. See http://www.rsc.org/suppdata/dt/b2/b209975f/
D a l t o n T r a n s . , 2 0 0 3 , 6 3 8 – 6 5 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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the respective formation of [Mo(CO)2(dppm)(η5-C8H11)][BF4]
(η5-C8H11 = 1–3:5,6-η-C8H11, 1a; 1–5-η-C8H11, 1b) which were
isolated as orange–pink solids. An identical synthetic procedure
starting from [MoBr(CO)2(dppe)(1–3-η:5,6-C8H11)] afforded
[Mo(CO)2(dppe)(1–3:5,6-η-C8H11)][BF4], 2a which was isolated
as a CH2Cl2 solvate. However, attempts to prepare [Mo(CO)2-
(dppe)(1–5-η-C8H11)][BF4] starting from [MoBr(CO)2(dppe)-
(1–3-η:4,5-C8H11)] were unsuccessful. IR monitoring of the
reaction solution suggested the intermediacy of the required
product (ν(CO)(CH2Cl2): 1988, 1919 cmϪ1) but isolation
attempts resulted in the formation of low yields of [Mo(CO)3-
(dppe)(1–3-η:4,5-C8H11)][BF4] which was identified by com-
parison of spectroscopic data with those of an authentic
sample prepared by an independent route (see later). The reac-
tions of [Mo(CO)2(NCMe)3(η3-C8H11)][BF4] (η3-C8H11 = 1–3-
η:5,6-C8H11 or 1–3-η:4,5-C8H11) with dppm or dppe in CH2Cl2
were also investigated as an alternative route to 1a, 1b and 2a.
The procedure was successful in the case of [Mo(CO)2-
(dppm)(1–3:5,6-η-C8H11)][BF4], 1a, but in all other examples,
the required product was formed as an inseparable mixture with
the corresponding acetonitrile adduct [Mo(CO)2(NCMe)-
(L2)(η3-C8H11)][BF4] (L2 = dppm or dppe) (see later).
In contrast with the chelate phosphine complexes, our
attempts to form pentahapto-bonded cyclooctadienyl com-
plexes starting from the 2,2Ј-bipyridine precursors [MoBr-
(CO)2(bipy)(η3-C8H11)] (η3-C8H11 = 1–3-η:5,6-C8H11 or 1–3-
η:4,5-C8H11) were unsuccessful. We were unable to characterise
satisfactorily the products resulting from reaction of [MoBr-
(CO)2(bipy)(η3-C8H11)] with Ag[BF4] but IR monitoring of
the reaction mixtures revealed ν(CO) data more consistent with
the retention of a trihapto bonding mode for the cycloocta-
dienyl ligand. A possible explanation for this is that the
cyclooctadienylmolybdenum system behaves analogously to
the acyclic pentadienyl complex [MoBr(CO)2(bipy)(η3-C5H7)]
which affords anion-coordinated [Mo(σ-FBF3)(CO)2(bipy)-
(η3-C5H7)] on reaction with Ag[BF4].14 In this respect, the
chemistry of the cyclooctadienyl and cycloheptadienyl molyb-
denum systems shows a clear distinction, with the structure
of the corresponding pentahapto-bonded cycloheptadienyl
complex [Mo(CO)2(bipy)(η5-C7H9)][BF4] established crystallo-
graphically.7
The existence of the cycloheptadienyl complexes [Mo(CO)2-
(L2)(η5-C7H9)][BF4] with monodentate P-donor ligands (L2 =
2PPh3 or 2P(OMe)3) prompted an investigation of the syn-
thesis of analogous cyclooctadienyl complexes. Our initial
approach involved reaction of [Mo(CO)2(NCMe)3(η3-C8H11)]-
[BF4] (η3-C8H11 = 1–3-η:5,6-C8H11 or 1–3-η:4,5-C8H11) with two
equivalents of PPh3 or P(OMe)3 in CH2Cl2, a strategy success-
fully applied in the analogous cycloheptadienylmolybdenum
system. However, with the ligand PPh3, this reaction led to the
formation of mono-substituted products [Mo(CO)2(NCMe)2-
(PPh3)(η3-C8H11)][BF4]; an analytical sample of [Mo(CO)2-
(NCMe)2(PPh3)(1–3-η:4,5-C8H11)][BF4], 3b, was prepared by
reaction of [Mo(CO)2(NCMe)3(1–3-η:4,5-C8H11)][BF4] with
one equivalent of PPh3 in CH2Cl2 at 0 ЊC. Spectroscopic data
(see ESI data, Tables S1 and S2†), particularly the magnitude of
J(P–C) couplings to carbonyl carbons, suggest that 3b adopts a
structure in which PPh3 is located trans to the cyclooctadienyl
ring. Attempts to displace further NCMe ligands with PPh3
were unsuccessful and, when 3b was dissolved in NCMe, the tris
nitrile complex [Mo(CO)2(NCMe)3(1–3-η:4,5-C8H11)][BF4] was
reformed, indicating a relatively weak attachment of the PPh3
ligand. The reaction of the complexes [Mo(CO)2(NCMe)3-
(η3-C8H11)][BF4] with two equivalents of P(OMe)3 also gave
trihapto-bonded cyclooctadienyl products but in this case we
were unable to establish the extent of substitution of NCMe
by P(OMe)3. An alternative strategy for the synthesis of
[Mo(CO)2(L2)(η5-C8H11)][BF4] (η5-C8H11 = 1–3:5,6-η-C8H11 or
1–5-η-C8H11; L2 = 2PPh3 or 2 P(OMe)3) involves initial
formation of [MoBr(CO)2(L2)(η3-C8H11)] followed by halide
Scheme 1 Reagents and conditions: (a): (i) Ag[BF4] in CH2Cl2, 1 h
stirring; (ii) L = CO, Ag[BF4] in CH2Cl2, 20 min stirring; (iii) L =
CNBut, (a) 3 equivalents of CNBut in CH2Cl2, 15 min stirring, remove
solvent and dry in vacuo for 1 h; (b) reflux in CH2Cl2, 1 h; (iv) 5ϩ
equivalents of nbd in CH2Cl2, 24 h stirring. (b): (i) Ag[BF4] in CH2Cl2,
1 h stirring; (ii) L = P(OMe)3, LЈ = CO, (a) 2 equivalents of P(OMe)3 in
CH2Cl2 (Ϫ70 ЊC warmed to 0 ЊC over 30 min stirring), remove solvent
and dry in vacuo for 1 h; (b) Ag[BF4] in CH2Cl2, 30 min; (iii) L = LЈ =
CNBut, (a) 3 equivalents of CNBut in CH2Cl2, 15 min stirring, remove
solvent and dry in vacuo 1 h; (b) reflux in CH2Cl2, 1 h; (iv) 1 equivalent
of PPh3 in CH2Cl2, 30 min stirring at 0 ЊC.
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abstraction. Reaction of [MoBr(CO)2(NCMe)2(η3-C8H11)]
(η3-C8H11 = 1–3-η:5,6-C8H11 or 1–3-η:4,5-C8H11) with two
equivalents of P(OMe)3 in CH2Cl2 at Ϫ60 ЊC followed by slow
warming to ambient temperature resulted in the formation
Table 3 Important bond lengths (Å) and angles (Њ) for complex 1a
Mo–P(1)
Mo–P(2)
Mo–C(2)
Mo–C(3)
Mo–C(6)
Mo–C(7)
Mo–C(8)
Mo–C(9)
Mo–C(10)
C(9)–O(9)
2.484(2)
2.519(2)
2.495(7)
2.576(7)
2.350(7)
2.329(8)
2.482(8)
1.935(8)
2.010(8)
1.157(10)
C(10)–O(10)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(1)
1.138(10)
1.52(1)
1.32(1)
1.50(1)
1.44(1)
1.49(1)
1.42(1)
1.37(1)
1.49(1)
of orange product solutions (IR: ν(CO)(CH2Cl2): η3-C8H11
=
1–3-η:5,6-C8H11, 1956, 1869 cmϪ1; 1–3-η:4,5-C8H11, 1977, 1940,
1854 cmϪ1). These data may suggest the formation of [MoBr-
(CO)2{P(OMe)3}2(η3-C8H11)] but attempts to isolate these
products as solids suitable for characterisation were unsuccess-
ful resulting only in partially decomposed materials. However,
in the case of the 1–3-η:4,5-C8H11 derivative, further in situ
reaction with Ag[BF4] in CH2Cl2 resulted in the formation
of [Mo(CO)2{P(OMe)3}2(1–5-η-C8H11)][BF4], 4b, which was
isolated as a yellow solid following extensive purification.
Attempts to employ an analogous synthesis of the PPh3 deriv-
atives [Mo(CO)2(PPh3)2(η5-C8H11)][BF4] were unsuccessful.
We have previously described isocyanide derivatives of the
cycloheptadienylmolybdenum system [Mo(CO)3Ϫn(CNBut)n-
(η5-C7H9)]ϩ (n = 2 or 3)7 and this suggested that analogous
cyclooctadienylmolybdenum complexes might be accessible,
albeit via different synthetic routes. In principle, the bis-
(isocyanide) complexes [Mo(CO)2(CNBut)2(η5-C8H11)][BF4]
(η5-C8H11 = 1–3:5,6-η-C8H11, 5a, or 1–5-η-C8H11, 5b) can be
obtained via reaction of the appropriate [Mo(CO)2(NCMe)3-
(η3-C8H11)][BF4] derivative with two equivalents of CNBut.
However, the degree of substitution in [Mo(CO)2(NCMe)3-
(η3-C8H11)][BF4] is difficult to control, with tris(isocyanide)
products [Mo(CO)2(CNBut)3(η3-C8H11)][BF4] very readily
formed. Therefore 5a and 5b were more conveniently prepared
by treatment of the halide precursors [MoBr(CO)2(CNBut)2-
(η3-C8H11)] with Ag[BF4] in CH2Cl2. Complex 5a was isolated
as an orange–red solid and fully characterised but the 1–5-η-
P(1)–Mo–P(2)
C(9)–Mo–C(10)
P(1)–Mo–C(9)
P(1)–Mo–C(10)
P(2)–Mo–C(9)
P(2)–Mo–C(10)
Mo–C(9)–O(9)
Mo–C(10)–O(10)
67.95(6)
86.2(3)
92.3(2)
82.5(2)
84.1(2)
148.3(2)
177.1(6)
175.7(7)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(6)
C(5)–C(6)–C(7)
C(6)–C(7)–C(8)
C(7)–C(8)–C(1)
C(8)–C(1)–C(2)
122.5(7)
125.2(8)
120.6(8)
120.6(8)
126.4(7)
128.1(8)
131.3(7)
109.5(7)
with respect to the cyclooctadienyl ring. The molecular
configurations, annotated with crystallographic numbering
schemes, and tables of important bond lengths and angles are
given in: Fig. 1, Table 3, 1a; Fig. 2, Table 4, 6b.
In the complex [Mo(CO)2(dppm)(1–3:5,6-η-C8H11)][BF4], 1a,
the 1–3:5,6-η-C8H11 ligand is attached to molybdenum through
an η3-allyl system [C(6)–C(8)] (crystallographic numbering
C8H11 derivative 5b (IR, (CH2Cl2): ν(CN) 2182, 2163 cmϪ1
;
ν(CO) 2029, 1964 cmϪ1) could not obtained in a pure form. The
tris(isocyanide) complexes [Mo(CO)(CNBut)3(η5-C8H11)][BF4]
(η5-C8H11 = 1–3:5,6-η-C8H11, 6a, or 1–5-η-C8H11, 6b) were
obtained by carbonyl elimination from the appropriate pre-
cursor derivative of [Mo(CO)2(CNBut)3(η3-C8H11)][BF4].10 In
practice, [Mo(CO)2(CNBut)3(η3-C8H11)][BF4] were generated in
situ from the reaction of [Mo(CO)2(NCMe)3(η3-C8H11)][BF4]
with three equivalents of CNBut and then converted directly
to 6a, 6b. Both carbonyl eliminations are effected by gentle
warming (30–40 ЊC) of [Mo(CO)2(CNBut)3(η3-C8H11)][BF4] in
CH2Cl2 but, as we have noted previously,10 the formation of 6a
will proceed slowly even at ambient temperature. Finally, we
examined the synthesis of the diene complexes [Mo(CO)2-
(η4-nbd)(η5-C8H11)][BF4] (nbd
reaction of [Mo(CO)2(NCMe)3(η3-C8H11)][BF4] (η3-C8H11
= norborna-2,5-diene) via
=
1–3-η:5,6-C8H11 or 1–3-η:4,5-C8H11) with nbd in CH2Cl2; this
method parallels that employed in the synthesis of the
cycloheptadienyl analogue [Mo(CO)2(η4-nbd)(η5-C7H9)][BF4].6
However, in the current work, the reactions were accompanied
by extensive polymerisation of norbornadiene and only one
example, [Mo(CO)2(η4-nbd)(1–3:5,6-η-C8H11)][BF4], 7a, was
isolated successfully.
Fig. 1 Molecular structure of complex 1a; BF4 counter anion omitted.
Structural and spectroscopic characterisation of pentahapto-
bonded cyclooctadienylmolybdenum complexes
The complexes described above are the first examples both of
1–3:5,6-η-C8H11 and 1–5-η-C8H11 ligands coordinated to molyb-
denum and therefore merit a full structural investigation.
Moreover, structurally characterised examples of complexes
containing the 1–3:5,6-η-C8H11 ligand are very scarce, irrespect-
ive of the identity of the metal. Therefore X-ray structural
characterisations were carried out on two representative
complexes, [Mo(CO)2(dppm)(1–3:5,6-η-C8H11)][BF4], 1a, and
[Mo(CO)(CNBut)3(1–5-η-C8H11)][BF4], 6b. The key features of
interest are the ring conformation, the metal to cyclooctadienyl
ligand bond distances and the orientation of the MoL4 unit
Fig. 2 Molecular structure of complex 6b; BF4 counter anion omitted.
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Table 4 Important bond lengths (Å) and angles (Њ) for complex 6b
requirements of the 1–3:5,6-η-C8H11 ligand, especially in cases
where the MoL4 unit incorporates a sterically demanding
ligand such as dppm.
Mo–C(1)
Mo–C(2)
Mo–C(3)
Mo–C(4)
Mo–C(8)
Mo–C(9)
Mo–C(10)
Mo–C(15)
Mo–C(20)
C(9)–O(1)
C(10)–N(1)
2.300(6)
2.307(5)
2.300(5)
2.422(6)
2.359(6)
1.974(6)
2.095(5)
2.108(7)
2.113(5)
1.140(7)
1.145(6)
C(15)–N(2)
C(20)–N(3)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(1)
1.133(8)
1.142(6)
1.391(9)
1.413(9)
1.404(8)
1.494(9)
1.494(10)
1.496(10)
1.490(9)
1.412(9)
The Mo(CO)2(dppm) unit of 1a adopts a distorted square-
based pyramidal arrangement, very similar to that observed
in the related cycloheptadienyl complex [Mo(CO)2(dppm)-
(η5-C7H9)][BF4].6 Thus the two complexes exhibit P–Mo–CO
bond angles comparable to within 5Њ and non-bonded ligand-
to-dienyl plane distances are also very similar (P-ligand donor
atom to dienyl plane distances (Å), 1a: 2.82, 3.66; [Mo(CO)2-
(dppm)(η5-C7H9)][BF4]: 3.04, 3.64; carbonyl ligand donor atom
to dienyl plane distances (Å), 1a: 1.94, 2.96; [Mo(CO)2-
(dppm)(η5-C7H9)][BF4]: 1.92, 2.93). The preferred orientation
of the ML4 unit with respect to the dienyl ligand is a subject of
considerable interest. There are no previous examples of com-
plexes of the type [ML4(1–3:5,6-η-C8H11)]nϩ for comparison
but, in 1a, the P(2)–Mo–C(10) axis roughly bisects the 1–3:5,6-
η-C8H11 ligand along C(1) and the mid-point of C(4)–C(5)
whilst the carbonyl carbon C(9) is located under the allyl
unit, C(6)–C(8). Significantly, P(1) lies under the alkene unit
C(2)–C(3) consistent with sterically induced elongation of the
Mo–C(2) and Mo–C(3) bond lengths.
C(9)–Mo–C(10)
C(9)–Mo–C(15)
C(9)–Mo–C(20)
C(10)–Mo–C(15)
C(10)–Mo–C(20)
C(15)–Mo–C(20)
Mo–C(9)–O(1)
Mo–C(10)–N(1)
Mo–C(15)–N(2)
102.5(2)
78.5(2)
82.7(2)
78.4(2)
80.6(2)
147.9(2)
179.1(5)
174.0(4)
174.5(5)
Mo–C(20)–N(3)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(6)
C(5)–C(6)–C(7)
C(6)–C(7)–C(8)
C(7)–C(8)–C(1)
C(8)–C(1)–C(2)
178.0(5)
129.2(6)
128.2(6)
128.6(5)
117.0(6)
112.9(6)
118.4(6)
125.5(6)
127.6(6)
scheme), and an isolated alkene unit [C(2)–C(3)] and these five
coordinated atoms are approximately coplanar (mean deviation
0.056 Å). The non-bonded methylene carbons [C(1), C(4) and
C(5)] are folded away from the metal centre giving a boat con-
formation of the C8H11 ligand with interplanar angles [C(3)–
C(4)–C(5)–C(6)]–[C(2)–C(3)/C(6)–C(7)–C(8)] 55Њ; [C(2)–C(1)–
C(8)]–[C(2)–C(3)/C(6)–C(7)–C(8)] 54Њ. Similar boat conform-
ations of the 1–3:5,6-η-C8H11 ligand are observed in each of
the reported examples [Ru(1–3:5,6-η-C8H11)(η6-C8H10)][PF6],16
[Ru(1–3:5,6-η-C8H11)(η6-C6H5BX3)] (X = Ph or F),17 [Ru(1,4,7-
trithiacyclononane)(1–3:5,6-η-C8H11)][PF6]18 and [{Ir(µ-Cl)-
In the complex [Mo(CO)(CNBut)3(1–5-η-C8H11)][BF4], 6b,
the cyclooctadienyl ligand is attached to Mo through the dienyl
carbons C(8)–C(1)–C(2)–C(3)–C(4) (crystallographic number-
ing scheme) which are roughly coplanar. In common with
the majority of structurally characterised examples of this
ligand,16,21,22 the 1–5-η-C8H11 ring may be considered as folded
into three planes defined by C(8)–C(1)–C(2)–C(3)–C(4), C(4)–
C(5)–C(7)–C(8) and C(5)–C(6)–C(7) (respective interplanar
angles as defined in ref. 23: α = 133Њ, β = 58Њ measured on the
face of the ligand remote from the Mo centre). The metal to
ring-carbon bonding distances also comply with a general
pattern in which the molybdenum to terminal dienyl carbon
distances Mo–C(4) and Mo–C(8) are elongated by com-
parison with the bonds to C(1)–C(3). The average elongation of
approximately 0.1 Å is very similar to that observed for
[Mo(CO)2L2(η5-C7H9)]ϩ (L2 = dppm,6 bipy7) and [Ru(PMe2-
Ph)3(1–5-η-C8H11)]ϩ.16 However, a more detailed comparison
of the structural parameters of the dienyl ligand in [Mo(CO)2-
(bipy)(η5-C7H9)][BF4] and 6b reveals some significant differ-
ences. An obvious impact of the additional methylene group
in the edge-bridge of the cyclooctadienyl ligand is to increase
the separation between the terminal dienyl carbons. Thus in 6b,
the non-bonded, transannular separations are C(1)–C(3):
2.54(1) Å, C(4)–C(8): 3.15(1) Å. The corresponding distances
for the dienyl unit of the cycloheptadienyl ring in [Mo(CO)2-
(bipy)(η5-C7H9)][BF4]7 are 2.53(1) and 2.92(1) Å, respectively.
A second effect is evident in a folding of the plane of the
η5-dienyl unit. In 6b, the dienyl unit is folded about C(1) and
C(3) with an interplanar angle [C(1)–C(2)–C(3) to C(1)–C(3)–
C(4)–C(8)] of 9.6Њ (towards the high end of previously reported
values4) but, by contrast, the corresponding fold in the
cycloheptadienyl ring of [Mo(CO)2(bipy)(η5-C7H9)][BF4] is just
3.5Њ.
(σ-CCF3C(H)CF3)(1–3:5,6-η-C8H11)}2].19
Carbon–carbon
bond lengths within the cyclooctadienyl ring compare reason-
ably well with other examples, with the C(2)–C(3) carbon–
carbon double bond shortest as expected. A comparison of
metal to ring carbon distances for complexes of the 1–3:5,6-η-
C8H11 ligand is presented in Table 5 (ring numbering system
adjusted to that employed in this work). Two features of inter-
est emerge. First, in common with some other reported struc-
tures, there is an elongation of the Mo–C(8) distance leading to
an asymmetric Mo–(η3-allyl) interaction. Secondly, the average
Mo to alkene distance [Mo–C(2), Mo–C(3)] is ca. 0.15 Å longer
than the average Mo to allyl carbon distance and also ca. 0.12 Å
longer than the average molybdenum to alkene carbon distance
in [Mo(CO)4(η4-nbd)] (Mo to alkene carbon distances (Å)
2.401(2), 2.407(2), 2.422(3), 2.434(2)).20 Only for [{Ir(µ-Cl)-
(σ-CCF3C(H)CF3)(1–3:5,6-η-C8H11)}2] is a similar elongation
of the metal to alkene carbon bond distances observed. It has
been suggested that the 1–3:5,6-η-C8H11 ligand is a sterically
demanding group with an estimated ligand cone angle of 196Њ
(cf. 1–5-η-C8H11, 159Њ) and in complexes of the type [RuL3-
(η5-C8H11)]ϩ, sterically demanding ligands L appear to promote
adoption of the 1–5-η-C8H11 form.16 In half-sandwich molyb-
denum complexes of the type [MoL4(η5-C8H11)]ϩ, we have not
observed interchange of 1–3:5,6-η-C8H11 and 1–5-η-C8H11
forms, but suggest that the elongation of metal to alkene
carbon distances is an alternative response to the steric
In common with other complexes of formulation [MLЈ3LЉ-
(η5-dienyl)]nϩ
,
([ReH(PMe2Ph)3(η5-2,4-Me2-pentadienyl)]ϩ,24
[Nb(CO)3(PMe2Ph)(η5-2,4-Me2-pentadienyl)]25 and [V(CO)3-
Table 5 Comparison of metal–ligand bonding distances (Å) in complexes of the 1–3:5,6-η-C8H11 (L) liganda
M–C distance
1a
[Ru(L)(η6-C8H10)]ϩ
[Ru(L)(η6-C6H5BPh3)]
[Ru(L)(9-S-3)]ϩ
[{Ir(µ-Cl)(σ-CCF3C(H)CF3)(L)}2]
M–C(2)
M–C(3)
M–C(6)
M–C(7)
M–C(8)
2.49
2.58
2.35
2.33
2.48
2.31
2.26
2.21
2.20
2.46
2.16
2.18
2.23
2.23
2.69
2.21
2.19
2.23
2.19
2.42
2.35
2.37
2.18
2.13
2.19
a Data from refs. 16–19; atom numbering scheme for the 1–3:5,6-η-C8H11 ligand adjusted to that employed in the current work. 9-S-3 = 1,4,7-
trithiacyclononane.
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Table 6 IR data (ν(CO)/cmϪ1), for [Mo(CO)2(L2)(η5-dienyl)]ϩ a
η5-Dienyl
L2
C7H9
1–5-η-C8H11
1–3:5,6-η-C8H11
dppeb
dppm
2 P(OMe)3
2 CNBut
nbd
2008, 1991 (sh), 1922
2014, 1924
2002, 1931
2030, 1963
2037, 1990
1988, 1919c
1996, 1930
2001, 1929
2029, 1964c
—
1986, 1886
1998, 1897
—
2013, 1919
2033, 1967
a Solution spectra in CH2Cl2. b Comparative data for analogous dienyl systems, v(CO)/cmϪ1, CH2Cl2: closed, cyclic, [Mo(CO)2(dppe)(η-C5H5)]ϩ,
1995, 1928;27 open, acyclic, [Mo(CO)2(dppe)(η-C5H7)]ϩ, 1980, 1930.13 c Complex not isolated.
Table 7 Comparison of 1H and 13C{1H} NMR data for Mo and Ru complexes of 1–3:5,6-η-C8H11 and 1–5-η-C8H11 ligandsa
1H NMR
Complex H1 H2
13C{1H} NMR
C1 C2
H3
H4
H5
H6
H7
H8
C3
C4
C5
C6
C7
C8
6a
A
4b
B
5.11 4.64 4.28 3.36, 3.23 3.36 4.46
3.95 3.93 3.55 2.96, 2.93 3.16 4.55
2.39
2.39, 2.28 83.4 104.7
65.1 24.0 62.1 108.6 32.9 28.2
37.1 20.7 62.0 104.9 35.0 27.0
2.33, 2.27 2.05, 1.80 68.1
81.7
99.5
4.28 4.79 6.54
3.48 4.77 6.27
2.43, 1.84 1.31, 0.65
2.03, 1.72 1.16, 0.34
94.9 110.6
94.4 99.0
28.3 19.3
26.1 19.4
53.2
a Complex A = [Ru(CNBut)3(1–3:5,6-η-C8H11)]ϩ in CDCl3, complex B = [Ru{P(OMe)3}3(1–5-η-C8H11)]ϩ in (CD3)2CO, data from ref. 16; complexes 6a
and 4b in CD2Cl2; numbering as in Scheme 1(a) and (b); in the 1–5-η-C8H11 ligand, pairs of carbons or protons [(2 with 4), (1 with 5) and (6 with 8)]
are equivalent.
(PMe2Ph)(η5-C5H7)]),26 the 1–5-η-C8H11 ring of 6b, orientates
itself above the MLЈ3LЉ fragment such that two of the
metal–ligand bonds lie in the vertical mirror plane bisecting the
dienyl ligand. In 6b, CNBut ligands are placed beneath the
central dienyl carbon C(2) and the centre of the edge bridge
C(6). This locates a CNBut and carbonyl ligand beneath the
bonds C(3)–C(4) and C(1)–C(8), respectively, and thereby
imposes an asymmetry upon the molecular structure in contrast
[Mo(CO)2(dppe)(η5-pentadienyl)]ϩ 13 and [Mo(CO)2(dppe)(η-
C5H5)]ϩ,27 are also included in Table 6 but the differences in
ν(CO) are relatively small and, in contrast to [V(CO)(η5-
dienyl)2],4 no significant trend emerges.
The NMR data in Table 2 are arranged to facilitate com-
parison between complexes of each specific ligand type; thus
complexes of the 1–3:5,6-η-C8H11 ligand (1a, 2a, 5a–7a) are
presented first followed by those of the 1–5-η-C8H11 ligand (1b,
4b, 6b). The spectroscopic numbering scheme is shown in
Scheme 1(a) and (b). In the majority of cases the spectral
assignments were fully elucidated with the aid of [1H–1H]
COSY and [1H–13C] HETCOR experiments. Table 7 presents a
to other reported examples of the type [MLЈ3LЉ(η5-dienyl)]nϩ
.
IR spectroscopic data for [Mo(CO)2(L2)(η5-C8H11)]ϩ
(Table 1) are generally consistent with a cis arrangement of
carbonyl ligands with the exception of the P(OMe)3 derivative
4b and possible exceptions of [Mo(CO)2(CNBut)2(η5-C8H11)]-
[BF4], (η5-C8H11 = 1–3:5,6-η-C8H11 5a, or 1–5-η-C8H11, 5b). In
the case of 4b, the relative intensities of the two ν(CO) bands
are clearly indicative of a trans arrangement of CO ligands. A
similar pattern is observed for 5b (for both ν(CO) and ν(CN))
but we were unable to isolate this complex in a pure form to
permit further investigations. By contrast, the 1–3:5,6-η-C8H11
derivative 5a, exhibited relative intensity patterns for ν(CO) and
ν(CN) bands more typical of a cis ligand arrangement
although, as with [Mo(CO)2(CNBut)2(η5-C7H9)]ϩ,6 there was
some evidence in the 13C{1H} NMR spectrum of 5a for the
presence of two species, albeit with one in very low relative
abundance.
1
comparison of H and 13C{1H} NMR data for [Mo(CO)(CN-
But)3(1–3:5,6-η-C8H11)][BF4] 6a, and [Mo(CO)2{P(OMe)3}2-
(1–5-η-C8H11)][BF4], 4b, with the related cyclooctadienyl-
ruthenium half-sandwich complexes [Ru(CNBut)3(1–3:5,6-η-
C8H11)][PF6] and [Ru{P(OMe)3}3(1–5-η-C8H11)][PF6].16 First, it
is clear that the NMR data for a specific cyclooctadienyl ring
type correlate well, independent of the identity of the attached
metal centre. The exceptions to this are most apparent in the
13C{1H} NMR spectra. For the 1–3:5,6-η-C8H11 ligand, the
terminal allyl carbon C3 is significantly more shielded in the Ru
system and again for the 1–5-η-C8H11 ligand, the terminal
dienyl carbons C1,5 are shielded in the Ru complex by com-
parison with 4b. However comparison of 13C{1H} NMR data
for 4b with that of its cycloheptadienyl analogue [Mo(CO)2-
{P(OMe)3}2(η5-C7H9)][BF4] (see Experimental section) reveals
that the terminal dienyl carbons of the latter complex have an
additional low field shift of ca. 10 ppm.
Table 6 presents a comparison of ν(CO) data for the series
of complexes [Mo(CO)2(L2)(η5-dienyl)]ϩ (dienyl = 1–5-η-C7H9,
1–5-η-C8H11 or 1–3:5,6-η-C8H11) classified by the dienyl
ligand and the supporting ligand(s) L2. Two conclusions can be
drawn concerning the influence of the dienyl ligand. First,
comparison of the authentic η5-dienyl ligands 1–5-η-C7H9 and
1–5-η-C8H11 reveals little difference in ν(CO) data with the
relatively small exception of L2 = dppm, dppe. This suggests that
exchange of 1–5-η-C7H9 for 1–5-η-C8H11 does not have a major
electronic influence upon the Mo centre despite the additional
methylene group in the edge bridge of the cyclooctadienyl
ligand. By contrast, exchange of 1–5-η-C8H11 for 1–3:5,6-η-
C8H11 results in a shift in ν(CO)average to lower wavenumber by
ca. 15–30 cmϪ1 and an increase in the separation of the carb-
onyl bands by ca. 30 cmϪ1. It seems therefore that the 1–3:5,6-η-
C8H11 ligand, with isolated η3-allyl and η2-alkene groups,
acts as a better net electron donor to the Mo centre than the
isomeric 1–5-C8H11 system. For comparison, IR data for
The remainder of the discussion will focus on the complexes
1b, 4b and 6b which contain the 1–5-η-C8H11 ligand. In prin-
ciple, the ligand is bisected by a plane of symmetry and there-
fore, assuming either free rotation of the MoL4 unit or altern-
atively a symmetrical orientation of a static MoL4 unit with
respect to the ring, it might be expected that only five ring
carbon environments would be observed in the 13C{1H} NMR
spectrum. However, this is the case only for 4b. For 1b and 6b, it
is clear that the two sides of the cyclooctadienyl ring are distinct
and eight discrete ring carbon resonances are detected. This
observation can be rationalised by a combination of two
factors: first a high activation barrier to rotation of the MoL4
group and secondly a preferred asymmetric orientation of
the MoL4 unit with respect to the 1–5-η-C8H11 ring. It is well
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established that the 1–5-η-C8H11 ligand presents a relatively
high activation barrier with respect to rotation of an attached
MLn group23 and moreover, in 6b, we have demonstrated crys-
tallographically that the Mo(CO)(CNBut)3 unit is located to
exclude a symmetry plane through the molecule. Therefore, for
6b, retention of the solid state structure in solution could
rationalise the observed NMR data. Furthermore, we suggest
that the same constraints operate for [Mo(CO)2{P(OMe)3}2-
(1–5-η-C8H11)][BF4], 4b. The distinction here is that the two
P(OMe)3 ligands are mutually trans and therefore, assuming
that the MoL4 unit is orientated with respect to the 1–5-η-C8H11
ligand as in 6b, a symmetry plane through the cyclooctadienyl
ligand will be retained without the need for averaging by
rotation of the MoL4 unit. Finally, in 1b, a static, asymmetric
Mo(CO)2(dppm) unit would exclude a symmetry plane through
the molecule. In fact 31P{1H} NMR investigations on 1b con-
firm two discrete phosphorus environments at 25 ЊC in contrast
to the cycloheptadienyl analogue [Mo(CO)2(dppm)(η5-C7H9)]-
[BF4] which exhibits a singlet resonance in the 31P{1H} NMR
spectrum at 25 ЊC; only on cooling to Ϫ20 ЊC are two separate
phosphorus environments observed. Moreover, the 1H and
13C{1H} NMR spectra of [Mo(CO)2(dppm)(η5-C7H9)][BF4] are
broad at ambient temperature but low-temperature spectra are
consistent with an asymmetric structure in which all C7H9 ring
resonances are inequivalent.6 We suggest that the contrast in
the ambient temperature NMR spectra of [Mo(CO)2(dppm)-
(η5-dienyl)][BF4] (dienyl = C7H9 or 1–5-η-C8H11) is a clear
example of the effect of increasing the size of the edge-bridge
of the dienyl ligand upon the activation energy to rotation of
the attached MLn unit and that our results are in accord with
theoretical predictions.23
ꢀ5
ꢀ3 Hapticity interconversion reactions of [Mo(CO)2(L2)-
(ꢀ5-C8H11)]؉ (ꢀ5-C8H11 ؍ 1–3:5,6-ꢀ-C8H11 or 1–5-ꢀ-C8H11)
It is well established that the identity of the dienyl ligand has a
marked effect on the facility of η5
η3 hapticity changes pro-
Scheme 2 Reagents and conditions: (i)/(ii) LЈ = NCMe, stir in NCMe,
1 h; LЈ = CO, stir in CH2Cl2 with passage of CO gas, 11a, 3 h, 0 ЊC; 10b,
25 min, ambient temperature; (iii) dppe in NCMe, 1.5 h stirring;
(iv) LЈ = CO, stir in CH2Cl2 with passage of CO gas, 45 min.
moted by ligand addition at the metal centre. Thus “open”
acyclic pentadienyl ligands undergo hapticity interconversions
much more readily than “closed”, cyclic dienyls such as the
cyclopentadienyl ligand. Edge-bridged dienyls appear to have
properties intermediate between those of the “open” acyclic
and “closed” cyclic systems and the purpose of the current
work was to fit the cyclooctadienyl ligand into this pattern.
The reactions examined are of the general type:
9a; η3-C8H11 = 1–3-η:4,5-C8H11, L2 = dppm, 8b) were fully
formed (as monitored by IR spectroscopy). The complexes
were isolable as stable yellow to orange solids with the
exception of 8a which very readily dissociates NCMe. The
formal acetonitrile adduct of [Mo(CO)2(dppe)(1–5-η-C8H11)]ϩ,
i.e. [Mo(CO)2(NCMe)(dppe)(1–3-η:4,5-C8H11)][BF4], 9b, was
also synthesised by reaction of [Mo(CO)2(NCMe)3(1–3-η:4,5-
C8H11)][BF4] with dppe in acetonitrile. These isolable prod-
ucts present a contrast with the analogous cycloheptadienyl
system; [Mo(CO)2(dppe)(η5-C7H9)]ϩ dissolves in NCMe to
form an equilibrium mixture with [Mo(CO)2(NCMe)(dppe)-
(η5-C7H9)]ϩ and [Mo(CO)2(dppm)(η5-C7H9)]ϩ does not react
with NCMe.6,7
[Mo(CO)2(L2)(η5-C8H11)]ϩ ϩ LЈ
[Mo(CO)2(LЈ)(L2)(η3-C8H11)]ϩ (1)
where LЈ (NCMe, CO or CNBut) promotes hapticity conver-
sion between the η5 and η3 bonding modes of the cycloocta-
dienyl ligand. The principal objectives of our investigations
were:
(i) to compare the facility with which directly analogous
η5-C7H9 and 1–5-η-C8H11 complexes undergo η5
η3 hapticity
The relative ease of formation of the acetonitrile adducts,
8a, 8b and 9a suggested that other ligands, not employed for
conversions, thus allowing an assessment of the effect of an
additional methylene group in the edge bridge,
the cycloheptadienyl analogues, might drive η5
η3 conver-
(ii) to determine the effect of different conjugation patterns
in the cyclooctadienyl ring (1–3:5,6-η-C8H11 vs. 1–5-η-C8H11)
sions in the cyclooctadienyl system. Accordingly, passage of
CO gas through CH2Cl2 solutions of 1b and 2a led to the
respective isolation of the tricarbonyl complexes [Mo(CO)3-
(dppm)(1–3-η:4,5-C8H11)][BF4] 10b and [Mo(CO)3(dppe)(1–3-
η:5,6-C8H11)][BF4] 11a. The complex [Mo(CO)3(dppe)(1–3-
η:4,5-C8H11)][BF4], 11b (the formal CO adduct of [Mo(CO)2-
(dppe)(1–5-η-C8H11)][BF4]) was also prepared by reaction of
9b with CO in CH2Cl2 and identified as the product of the
attempted preparation of [Mo(CO)2(dppe)(1–5-η-C8H11)][BF4]
from [MoBr(CO)2(dppe)(1–3-η:4,5-C8H11)] and Ag[BF4]. By
contrast, complex 1a was unreactive towards CO, a result con-
sistent with the relatively poor stability of 8a (the NCMe
adduct of 1a).
on η5
η3 conversions processes and
(iii) to examine the effect of the supporting ligand(s) L2.
The majority of the useful results in addressing these object-
ives was obtained with the chelate phosphine derivatives
[Mo(CO)2(L2)(η5-C8H11)]ϩ (η5-C8H11 = 1–3:5,6-η-C8H11, L2 =
dppm, 1a, L2 = dppe, 2a; η5-C8H11 = 1–5-η-C8H11, L2 = dppm,
1b) and therefore these reactions, as summarised in Scheme 2,
are discussed first.
When 1a, 2a and 1b were dissolved in NCMe, the respective
mono-acetonitrile adducts [Mo(CO)2(NCMe)(L2)(η3-C8H11)]-
[BF4] (η3-C8H11 = 1–3-η:5,6-C8H11, L2 = dppm, 8a, L2 = dppe,
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Characterisation details for the new complexes [Mo(CO)2-
(LЈ)(L2)(η3-C8H11)]ϩ are given in Table 1 (microanalytical, IR
and mass spectroscopic data) and Tables S1 (1H, 31P{1H} NMR
data) and S2 (13C{1H} NMR data).† We have previously dis-
cussed the important features of the NMR spectra of trihapto-
bonded cyclooctadienylmolybdenum systems10 and therefore
limit this discussion to issues specific to the complexes [Mo-
(CO)2(LЈ)(L2)(η3-C8H11)]ϩ described here. In common with
[Mo(CO)2(NCMe)3(η3-C8H11)][BF4]. The bis(phosphite) com-
plex 4b did react when dissolved in NCMe but a product mix-
ture was formed, probably by partial ligand substitution of
P(OMe)3 by NCMe. Each of the isocyanide complexes 5a, 6a
and 6b reacted with a further equivalent of CNBut. The bis-
(isocyanide) complex 5a yielded the adduct [Mo(CO)2(CNBut)3-
(1–3-η:5,6-C8H11)][BF4], previously obtained by direct reaction
of [Mo(CO)2(NCMe)3(1–3-η:5,6-C8H11)][BF4] with three
equivalents of CNBut.10 This observation contrasts with the
cycloheptadienyl analogue [Mo(CO)2(CNBut)2(η5-C7H9)]ϩ
which reacts with CNBut to yield carbonyl-substituted [Mo-
(CO)(CNBut)3(η5-C7H9)]ϩ directly7 with no explicit evidence
for the intermediacy of [Mo(CO)2(CNBut)3(η3-C7H9)][BF4].
The tris(isocyanide) complexes 6a and 6b react with a fur-
ther equivalent of CNBut to yield the adducts [Mo(CO)-
(CNBut)4(η3-C8H11)][BF4] (η3-C8H11 = 1–3-η:5,6-C8H11, 12a;
1–3-η:4,5-C8H11, 12b) which could also be prepared via
direct reaction of the appropriate derivative of [Mo(CO)2-
(NCMe)3(η3-C8H11)][BF4] with four equivalents of CNBut. We
have previously reported on the related complexes [Mo(CO)-
other systems of the type [Mo(CO)2(LЈ)(L2)(η3-R)]ϩ, (L2
=
chelate P-donor ligand, R = η3-allyl28,29 or η3-dienyl7,13),
spectroscopic data for complexes 8–11 are consistent with a
structure in which one P-donor atom is located trans to the
cyclooctadienyl ligand; 31P{1H} NMR data for 9b and 10b
reveal, in each case, two discrete phosphorus environments.
Moreover, in each group of complexes (LЈ = NCMe or CO) the
pattern of J(P–C) values for the separate carbonyl resonances is
consistent with only one carbonyl (that at low field) arranged
trans to a P-donor atom. This imposes a meridional arrange-
ment of carbonyl ligands in the tricarbonyl complexes 11a, 10b
and 11b. The ambient temperature 1H and 13C NMR spectra of
the acetonitrile adducts 8a,b, 9a,b exhibited some broadened
features which resolved on cooling to Ϫ30 ЊC and in the case of
complexes of the 1–3-η:5,6-C8H11 ligand (8a, 9a), resolved spec-
tra indicated the presence of two isomeric components similar
to our previous observations for [MoBr(CO)2(L2)(η3-C8H11)]
(L2 = dppm, dppe).10 The NCMe adducts 8b, 9b incorporating
the 1–3-η:4,5-C8H11 ligand did not exhibit this phenomenon.
Although our investigations on the reactions of the penta-
hapto-dienyl complexes 1a, 1b and 2a with ligands LЈ led to the
successful isolation of adducts [Mo(CO)2(LЈ)(L2)(η3-C8H11)]ϩ-
(LЈ = NCMe or CO), they failed to provide a clear distinction
between the effect of the identity of ligands L2 and C8H11 upon
the relative stability of trihapto- and pentahapto- bonding
modes of the dienyl ligand. Further to address this issue,
we examined the ease of reversion of [Mo(CO)2(LЈ)(L2)-
(η3-C8H11)]ϩ to the precursor η5-dienyls. When the acetonitrile
adducts 8a, 8b, 9a, and 9b, were dissolved in CH2Cl2 (ca. 0.05 g
in 5 cm3) and the reaction monitored by IR spectroscopy,
differences in reactivity dependent upon the identity of L2 and
the cyclooctadienyl ligand became clear. Both complexes of
the 1–3-η:5,6-C8H11 ligand (8a, L2 = dppm and 9a, L2 = dppe)
rapidly lost NCMe with reversion to the pentahapto-bonded
dienyls 1a and 2a, respectively. However, whilst conversion of
8a to 1a was complete, the dppe complex 2a was formed as an
inseparable mixture with small quantities of unchanged 9a. The
acetonitrile adducts of complexes of the 1–3-η:4,5-C8H11 ligand
(8b, L2 = dppm and 9b, L2 = dppe) exhibited much greater
stability in CH2Cl2. Monitoring by IR spectroscopy revealed
only slow conversion to the respective pentahapto-bonded
dienyls and moreover, in each case, a substantial proportion of
8b or 9b was retained even after heating the reaction solution.
Further distinction between 1–3-η:5,6-C8H11 and 1–3-η:4,5-
C8H11 ligands is apparent from the carbonyl elimination reac-
tions of the tricarbonyl complexes [Mo(CO)3(dppe)(η3-C8H11)]-
[BF4]. The 1–3-η:5,6-C8H11 complex 11a, rapidly undergoes CO
elimination in CH2Cl2 leading to complete conversion to 2a,
after 30 min at 30 ЊC. By contrast, CH2Cl2 solutions of the 1–3-
η:4,5-C8H11 complex 11b, are unchanged under these condi-
tions although reflux in CH2Cl2 for 2 h resulted in conversion to
unidentified products.
30
7
(CNBut)4(η3-R)]ϩ (R = C7H7 or C7H9 ) and, on the basis of
the four spectroscopically distinct CNBut ligands, it is probable
that 12a and 12b share their asymmetric structure with a CNBut
ligand located trans to the η3-C8H11 ring.
To conclude, we now address the objectives outlined at the
beginning of this section. First, a comparison is made of
analogous complexes of the fully conjugated dienyl ligands,
η5-C7H9 and 1–5-η-C8H11. Based on the reactions of [Mo-
(CO)2(dppm)(η5-dienyl)]ϩ with LЈ (dienyl = η5-C7H9 or 1–5-η-
C8H11; LЈ = NCMe and CO), it is clear that the 1–5-η-C8H11
ligand preferentially enhances reactivity to addition of LЈ with
accompanying change in dienyl ligand hapticity from η5 to
η3. Thus [Mo(CO)2(dppm)(1–5-η-C8H11)]ϩ reacts with both
NCMe and CO to give stable adducts of formulation [Mo(CO)2-
(LЈ)(dppm)(1–3-η:4,5-C8H11)]ϩ but the cycloheptadienyl ana-
logue [Mo(CO)2(dppm)(η5-C7H9)]ϩ does not react with either
NCMe or CO under similar conditions. Other observations
further support the conclusion that the 1–5-η-C8H11 ligand
exhibits a reduced preference for the pentahapto bonding mode
by comparison with the corresponding cycloheptadienyl
system. For example, we have demonstrated that [Mo(CO)2-
(bipy)(η5-C7H9)]ϩ is an isolable complex with a structure estab-
lished by X-ray crystallography but attempts to synthesise
[Mo(CO)2(bipy)(1–5-η-C8H11)]ϩ were unsuccessful, probably
due to formation of a BF4 coordinated system analogous to the
corresponding acyclic pentadienyl system. Secondly, reaction
of [Mo(CO)2(CNBut)2(η5-dienyl)]ϩ (dienyl = η5-C7H9 or 1–5-η-
C8H11) with one equivalent of CNBut affords carbonyl-
substituted[Mo(CO)(CNBut)3(η5-dienyl)]ϩ butonlyinthecyclo-
octadienyl system can the intermediate [Mo(CO)2(CNBut)3-
(η3-dienyl)]ϩ be observed prior to CO elimination. Finally, reac-
tion of [Mo(CO)2(NCMe)3(η3-dienyl)]ϩ with two equivalents of
PPh3 proceeds very differently dependent on the identity of the
dienyl ligand; the cycloheptadienyl complex [Mo(CO)2(PPh3)2-
(η5-C7H9)]ϩ is formed with elimination of all three NCMe
ligands whereas the cyclooctadienyl system undergoes more
limited substitution to yield [Mo(CO)2(NCMe)2(PPh3)(1–3-η-
4,5-C8H11)]ϩ in which the trihapto-bonding mode is retained.
The origin of the contrasting behaviour of η5-C7H9 or 1–5-η-
C8H11 systems is difficult to rationalise simply in terms of
differences in electronic effects and metal–ligand bonding
parameters; as discussed in the previous section, structural and
spectroscopic data are generally comparable except for the
increased barrier to rotation of the MoL4 group in the
cyclooctadienyl system. However, the differences in reactivity
might be accounted for by the relative stabilities of η5 and η3
bonded forms. For both dienyl ligands, the conversion from a
pentahapto- to trihapto-bonded dienyl ligand with accompany-
ing folding of the edge-bridge away from the metal centre
should relieve steric interactions with co-ligands at the
To complete our investigations on the effect of support-
ing ligand L2 on hapticity conversion processes, we examined
the reactions of the remaining three classes of complex,
[Mo(CO)2{P(OMe)3}2(1–5-η-C8H11)][BF4], 4b; the isocyanide
complexes 5a, 6a and 6b, and the diene complex [Mo(CO)2-
(η4-nbd)(1–3:5,6-η-C8H11)][BF4] 7a with NCMe and (in selected
cases) CNBut. Complexes 5a, 6a, 6b and 7a (in common with
their cycloheptadienyl analogues) were unreactive towards
NCMe, an observation consistent with the successful syn-
theses of these materials from the tris(nitrile) precursors
D a l t o n T r a n s . , 2 0 0 3 , 6 3 8 – 6 5 0
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molybdenum centre. However, we believe that the relative
stabilities of pentahapto-bonded 1–5-η-C8H11 and η5-C7H9
ligands differ as follows. First, folding of the dienyl unit in the
1–5-η-C8H11 system together with an increase in the separation
of the terminal dienyl carbons may result in destabilisation by
comparison with analogous η5-C7H9 complexes. Secondly, it is
probable that the enlarged edge-bridge of the 1–5-η-C8H11 will
increase steric interactions with co-ligands, an effect which may
be augmented by the high barrier to rotation of the MoL4 unit
with respect to the 1–5-η-C8H11 ligand.
(η5-C8H11 = 1–3:5,6-η-C8H11 or 1–5-η-C8H11) react with ligands
LЈ (LЈ = NCMe or CO) to give adducts [Mo(CO)2(LЈ)(L2)-
(η3-C8H11)]ϩ (η3-C8H11 = 1–3-η:5,6-C8H11 or 1–3-η:4,5-C8H11)
with accompanying η5
η3 hapticity conversion at the
cyclooctadienyl ligand. Complexes of the 1–5-η-C8H11 ligand
are activated to η5 η3 hapticity conversions by comparison
with exactly analogous complexes of the cycloheptadienyl
system; steric interactions imposed by the additional methylene
group in the edge-bridge of the 1–5-η-C8H11 ligand and differ-
ences in dienyl unit configuration are probably responsible for
this effect. The 1–3:5,6-η-C8H11 ligand is less effective than 1–5-
The second comparison is between analogous complexes of
the two distinct types of cyclooctadienyl ligand, 1–5-η-C8H11
and 1–3:5,6-η-C8H11. We note that reaction of [Mo(CO)2-
(dppm)(η-C8H11)]ϩ (1a and 1b) with CO to give [Mo(CO)3-
(dppm)(η3-C8H11)]ϩ proceeds only for the 1–5-η-C8H11
derivative 1b. Conversely ligand adducts of the type [Mo(CO)2-
(LЈ)(L2)(η3-C8H11)]ϩ revert to pentahapto-bonded forms much
less readily where the product contains the 1–5-η-C8H11 ligand
(compare, for example, the NCMe adducts 8a, 8b and the CO
adducts 11a and 11b). Finally, carbonyl elimination from
[Mo(CO)2(CNBut)3(η3-C8H11)]ϩ to give [Mo(CO)(CNBut)3-
(η5-dienyl)]ϩ (dienyl = 1–3:5,6-η-C8H11, 6a; 1–5-η-C8H11, 6b)
proceeds much more readily in the formation of 6a. The
observation that complexes of the pentahapto-bonded 1–3:5,6-
η-C8H11 ligand are relatively more stable with respect to conver-
sion to η3-bonded adducts is difficult to explain simply on the
basis of steric requirements (estimated ligand cone angles: 1–5-
η-C8H11, 159Њ; 1–3:5,6-η-C8H11, 196Њ)16 although the separation
of the coordinated carbons into separate allyl and ene units
eliminates the need for folding of the dienyl plane and any
resultant effect on stability. Alternatively, ν(CO) data (Table 6)
suggest that the 1–3:5,6-η-C8H11 ligand is a better electron
donor than its 1–5-η-C8H11 counterpart so allowing 1–3:5,6-η-
C8H11 to compete more effectively with ligands LЈ for a
coordination site at the Mo centre.
η-C8H11 in promoting η5
η3 hapticity conversions; this may
be connected with the enhanced donor capacity of the 1–3:5,6-
η-C8H11 ligand and/or the configuration of the coordinated
carbons into two separate groups.
In many respects, the chemistry of the cyclooctadienyl-
molybdenum system most closely resembles that of analogous
acyclic pentadienyl complexes. The dominant hapticity type of
both cyclooctadienyl ligand types is trihapto. This is in contrast
to the cycloheptadienyl ligand which exhibits a comparative
preference for pentahapto-bonding and is more similar to
related indenyl complexes. Directly analogous complexes of the
η5-C7H9 and 1–5-η-C8H11 differ only by the additional methyl-
ene group in the edge-bridge, yet the contrast in reactivity is
considerable. There is no clear evidence for a major difference in
electronic factors between η5-C7H9 and 1–5-η-C8H11 and there-
fore the impact of the enlarged edge-bridge most probably
originates from increased steric interactions between the edge-
bridge and supporting co-ligands and from differences in the
dienyl unit configuration.
Experimental
General procedures
The preparation, purification and reactions of the complexes
described were carried out under dry nitrogen. All solvents were
dried by standard methods, distilled and deoxygenated before
use. The complexes [MoBr(CO)2(L2)(η3-C8H11)], (L2 = dppm,
dppe, bipy, 2 NCMe or 2 CNBut) and [Mo(CO)2(NCMe)3-
(η3-C8H11)]ϩ (η3-C8H11 = 1–3-η:5,6-C8H11, 1–3-η:4,5-C8H11)
were prepared by published procedures.10 The chemicals dppe,
dppm, P(OMe)3, CNBut, and norborna-2,5-diene were supplied
by Aldrich Chemical Co.; Ag[BF4] was purchased from Lancas-
Finally, as in the analogous cycloheptadienyl complexes, the
supporting ligands of the MoL4 unit significantly affect reactiv-
ity. Based on the reactions of [Mo(CO)2(L2)(1–3:5,6-η-C8H11)]ϩ
(L2 = dppe, dppm, 2CNBut, nbd) with NCMe, the pattern of
reactivity is very similar to that observed for analogous
cycloheptadienyl systems, although in the current work the
comparison is limited to a smaller range of ligands L2. The
activating effect towards η5
η3 hapticity conversion in the
cyclooctadienyl complexes lies in the order dppe > dppm >
CNBut, nbd and it is probable that the large steric requirements
of dppe and dppm ligands contribute to their effectiveness in
1
ter Synthesis. 300 MHz H, 121.5 MHz 31P{1H} and 75 MHz
13C{1H} NMR spectra were recorded on Bruker AC 300 E,
Varian Associates XL 300 or Varian Unity Inova 300 spectro-
meters; 500 MHz 1H and 125 MHz 13C{1H} NMR spectra were
obtained on a Varian Unity 500 instrument. IR spectra were
acquired on a Perkin Elmer FT 1710 spectrometer and mass
spectra using a Kratos Concept 1S instrument. Microanalyses
were by the staff of the Microanalytical Service of the Depart-
ment of Chemistry, University of Manchester.
promoting η5
η3 hapticity conversions. Pertinently, our syn-
thetic studies demonstrate that the combination of dppe and
1–5-η-C8H11 ligands affords an unstable system in the complex
[Mo(CO)2(dppe)(1–5-η-C8H11)]ϩ .
Conclusions
This paper continues our investigations on the synthesis, struc-
ture and reactivity of edge-bridged dienyl ligands coordinated
to molybdenum and tungsten. The first examples of complexes
of the cyclooctadienyl ligands 1–3:5,6-η-C8H11 and 1–5-η-
C8H11, bonded pentahapto to molybdenum are reported. Two
synthetic routes to these complexes were developed involving
either halide abstraction from [MoBr(CO)2(L2)(η3-C8H11)] or
displacement of NCMe from [Mo(CO)2(NCMe)3(η3-C8H11)]ϩ
(η3-C8H11 = 1–3-η:5,6-C8H11 or 1–3-η:4,5-C8H11). The latter
method is similar to that employed for the synthesis of analo-
gous complexes of the C7H9 (cycloheptadienyl) ligand but is
less generally applicable to the cyclooctadienyl system. X-Ray
structural studies on the representative complexes [Mo(CO)2-
(dppm)(1–3:5,6-η-C8H11)][BF4], 1a, and [Mo(CO)(CNBut)3-
(1–5-η-C8H11)]ϩ, 6b, reveal asymmetric structures and, in 1a,
elongation of some metal to cyclooctadienyl ligand bonds.
Selected complexes of the type [Mo(CO)2(L2)(η5-C8H11)]ϩ
Preparations
[Mo(CO)2(dppm)(1–3:5,6-ꢀ-C8H11)][BF4] 1a. Method (a).
[MoBr(CO)2(dppm)(1–3-η:5,6-C8H11)] (0.185 g, 0.26 mmol)
was dissolved in CH2Cl2 (20 cm3) followed by addition of
Ag[BF4] (0.055 g, 0.28 mmol). After stirring for 1 h, the solu-
tion was filtered to remove AgBr and the resulting orange solu-
tion reduced in volume and treated with diethyl ether to precipi-
tate 1a as an orange–pink solid. The product was recrystallised
from CH2Cl2–diethyl ether; yield 0.135 g (71%). The pale
orange complex [Mo(CO)2(dppe)(1–3:5,6-η-C8H11)][BF4].CH2-
Cl2, 2a, was prepared similarly in 73% yield starting from
[MoBr(CO)2(dppe)(1–3-η:5,6-C8H11)] (0.500 g, 0.68 mmol) and
Ag[BF4] (0.149g, 0.76 mmol). Method (b). [Mo(CO)2-
(NCMe)3(1–3-η:5,6-C8H11)][BF4] (0.250 g, 0.53 mmol) was dis-
solved in CH2Cl2 (20 cm3) and dppm (0.205 g, 0.53 mmol)
added to give an orange solution. After 1 h stirring, the solution
D a l t o n T r a n s . , 2 0 0 3 , 6 3 8 – 6 5 0
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was filtered and work up carried out as described in method (a);
yield 0.270 g (69%).
reduced in volume resulting in further precipitation of poly-
meric material. The remaining yellow solution was transferred
to a separate flask and diethyl ether added to precipitate 7a as a
bright yellow solid; yield 0.057 g (53%).
[Mo(CO)2(dppm)(1–5-ꢀ-C8H11)][BF4]
1b.
[MoBr(CO)2-
(dppm)(1–3-η:4,5-C8H11)] (0.142 g, 0.20 mmol) was dissolved in
CH2Cl2 (20 cm3) followed by addition of Ag[BF4] (0.042 g, 0.22
mmol). After stirring for 1 h, the solution was filtered to remove
AgBr and the resulting orange solution reduced in volume and
treated with diethyl ether to precipitate 1b as an orange–pink
solid. The product was recrystallised from CH2Cl2–diethyl
ether; yield 0.112 g (79%).
[Mo(CO)2(NCMe)(dppm)(1–3-ꢀ:5,6-C8H11)][BF4] 8a. The
complex [Mo(CO)2(dppm)(1–3:5,6-η-C8H11)][BF4], 1a (0.233 g,
0.319 mmol) was stirred in NCMe (30 cm3) for 1 h. The result-
ing solution was filtered, reduced in volume and diethyl ether
added to precipitate the crude product. Subsequent recrystallis-
ation from NCMe–diethyl ether gave 8a as an orange–yellow
solid; yield 0.158 g (64%). The complexes [Mo(CO)2(NCMe)-
(dppm)(1–3-η:4–5-C8H11)][BF4] 8b and [Mo(CO)2(NCMe)-
(dppe)(1–3-η:5,6-C8H11)][BF4] 9a were prepared similarly and
isolated as orange and yellow solids, respectively. Complex 8b
was prepared in 57% yield from [Mo(CO)2(dppm)(1–5-η-
C8H11)][BF4] (0.206 g, 0.282 mmol); complex 9a was obtained in
48% yield starting from [Mo(CO)2(dppe)(1–3:5,6-η-C8H11)]-
[BF4].CH2Cl2 (0.201 g, 0.24 mmol).
[Mo(CO)2(NCMe)2(PPh3)(1–3-ꢀ:4,5-C8H11)][BF4] 3b.
A
stirred solution of [Mo(CO)2(NCMe)3(1–3-η:4,5-C8H11)][BF4]
(0.214 g, 0.456 mmol) in CH2Cl2 (20 cm3) at 0 ЊC was treated
with PPh3 (0.120 g, 0.456 mmol). After 30 min the solution was
filtered and the volume reduced to ca. 5 cm3 followed by addi-
tion of NCMe (3 cm3). Subsequent addition of diethyl ether
yielded the product 3b as an orange solid; yield 0.202 g (64%).
[Mo(CO)2{P(OMe)3}2(1–5-ꢀ-C8H11)][BF4] 4b. [MoBr(CO)2-
(NCMe)2(1–3-η:4–5-C8H11)] (0.500 g, 1.19 mmol) was dissolved
in CH2Cl2 (30 cm3) and the reaction solution cooled to Ϫ70 ЊC.
The solution was treated with P(OMe)3 (0.32 g, 2.58 mmol)
then stirred for 30 min as the temperature was increased to 0 ЊC.
The resulting orange solution was evaporated to dryness whilst
retaining the temperature at 0 ЊC and the residue dried in vacuo
for 1 h. Subsequently the residue was redissolved in CH2Cl2
(20 cm3) and immediately treated with Ag[BF4] (0.232 g,
1.19 mmol). After 30 min stirring the resulting green solution
was filtered to remove AgBr, the volume of the solution reduced
and diethyl ether added to precipitate the crude product as an
orange oil. Fractional crystallisation of the crude product from
CH2Cl2–diethyl ether afforded [Mo(CO)2{P(OMe)3}2(1–5-η-
C8H11)][BF4] 4b as a bright yellow solid; yield 0.135 g (19%).
[Mo(CO)2(NCMe)(dppe)(1–3-ꢀ:4,5-C8H11)][BF4] 9b. The
complex [Mo(CO)2(NCMe)3(1–3-η:4,5-C8H11)][BF4] (0.800 g,
1.71 mmol) was dissolved in NCMe (20 cm3) and dppe (0.679 g,
1.71 mmol) added. The resulting orange solution was stirred for
1.5 h then filtered, reduced in volume and the product precipi-
tated as a red oil by addition of diethyl ether. Subsequent
removal of the mother-liquor and stirring the crude product in
neat diethyl ether gave 9b as an orange solid; yield 1.225 g
(92%).
[Mo(CO)3(dppm)(1–3-ꢀ:4,5-C8H11)][BF4] 10b. [Mo(CO)2-
(dppm)(1–5-η-C8H11)][BF4] (0.206 g, 0.282 mmol) was dis-
solved in CH2Cl2 (20 cm3) at room temperature and CO gas
bubbled through the resulting orange solution. After 25 min,
monitoring by IR spectroscopy suggested that the reaction was
complete. The reaction mixture was filtered to give a yellow
solution which was reduced in volume and treated with diethyl
ether to precipitate the crude product. Subsequent recrystallis-
ation from CH2Cl2–diethyl ether gave 10b as a bright yellow
solid, yield 0.142 g (66%).
[Mo(CO)2(CNBut)2(1–3:5,6-ꢀ-C8H11)][BF4] 5a. Reaction of
[MoBr(CO)2(CNBut)2(1–3-η:5,6-C8H11)] (0.169 g, 0.335 mmol)
with Ag[BF4] (0.072 g, 0.369 mmol) in CH2Cl2 (20 cm3) gave an
orange solution. After 15 min the reaction mixture was filtered,
reduced in volume and diethyl ether added to precipitate an
orange oil. Removal of the mother liquors and subsequent stir-
ring in neat diethyl ether for 30 min gave the product 5a as a
red–orange solid, yield 0.05 g (30%).
[Mo(CO)3(dppe)(1–3-ꢀ:5,6-C8H11)][BF4] 11a. [Mo(CO)2-
(dppe)(1–3:5,6-η-C8H11)][BF4]ؒCH2Cl2 (0.222 g, 0.27 mmol)
was dissolved in CH2Cl2 (20 cm3) at 0 ЊC and CO gas bubbled
through the resulting orange solution. The solution was main-
tained at 0 ЊC over a period of 3 h during which time the colour
slowly changed to yellow. After 3 h the solution was filtered
cold, reduced in volume and cold (0 ЊC) diethyl ether added to
precipitate 11a as a yellow solid; yield 0.142 g (68%).
[Mo(CO)(CNBut)3(1–3:5,6-ꢀ-C8H11)][BF4] 6a. [Mo(CO)2-
(NCMe)3(1–3-η:5,6-C8H11)][BF4] (0.265 g, 0.565 mmol) was
dissolved in CH2Cl2 (20 cm3) and three equivalents of CNBut
(0.141 g, 1.70 mmol) added resulting in a colour change from
yellow to orange. The reaction mixture was stirred at room
temperature for 15 min then solvent was removed in vacuo and
the residue dried under vacuum for 1 h. The residue was redis-
solved in CH2Cl2 (20 cm3) and the solution was refluxed gently
for 1 h then filtered, reduced in volume and diethyl ether added
to precipitate the crude product as an orange oil. Subsequent
stirring in neat diethyl ether for 1 h. gave 6a as an orange–
yellow solid, yield 0.179 g (56%). The complex [Mo(CO)-
(CNBut)3(1–5-η-C8H11)][BF4], 6b was similarly prepared as an
orange–red solid in 71% yield starting from [Mo(CO)2-
(NCMe)3(1–3-η:4–5-C8H11)][BF4] (0.440 g, 0.94 mmol) and
CNBut (0.234 g, 2.82 mmol) in CH2Cl2 (20 cm3).
[Mo(CO)3(dppe)(1–3-ꢀ:4,5-C8H11)][BF4] 11b. The complex
[Mo(CO)2(NCMe)(dppe)(1–3-η:4,5-C8H11)][BF4] (0.158 g,
0.201 mmol) was dissolved in CH2Cl2 at ambient temperature
and CO gas bubbled through the solution. The progress of the
reaction was monitored by IR spectroscopy and after 45 min
the resulting yellow reaction mixture was filtered, reduced in
volume and diethyl ether added to precipitate the crude prod-
uct. Subsequent removal of the mother-liquor and stirring the
crude product in neat diethyl ether gave 11b as a yellow solid;
yield 0.102 g (66%).
[Mo(CO)(CNBut)4(1–3-ꢀ:5,6-C8H11)][BF4] 12a. The complex
[Mo(CO)(CNBut)3(1–3:5,6-η-C8H11)][BF4] (0.096 g, 0.169
mmol) was dissolved in CH2Cl2 (20 cm3) and treated with a
small excess of CNBut (0.017 g, 0.205 mmol). The reaction
mixture was stirred for 2 h, then filtered, reduced in volume and
diethyl ether added to precipitate 12a as an orange oil. The
mother-liquor was removed, and the product washed with di-
ethyl ether then dried in vacuo to give 12a as an orange solid;
yield 0.038 g (35%). The complex [Mo(CO)(CNBut)4(1–3-η:4,5-
[Mo(CO)2(ꢀ4-nbd)(1–3:5,6-ꢀ-C8H11)][BF4] 7a. [Mo(CO)2-
(NCMe)3(1–3-η:5,6-C8H11)][BF4] (0.115 g, 0.245 mmol) was
dissolved in CH2Cl2 (20 cm3) and a five molar excess of
norbornadiene (0.113 g, 1.23 mmol) was added. Strands of
black oily material (assumed to be associated with polymeris-
ation of norbornadiene) rapidly formed in a yellow solution.
Additional norbornadiene was added dropwise and the solu-
tion stirred for 24 h. The reaction mixture was then filtered and
D a l t o n T r a n s . , 2 0 0 3 , 6 3 8 – 6 5 0
648

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Table 8 Crystal and data collection parameters for complexes 1a and 6b
1a
6b
Formula
Mass
Crystal system
Temperature, T /ЊC
C35H33MoO2P2BF4
C24H38MoON3BF4
567.32
Monoclinic
22
730.34
Monoclinic
22
Space group
a/Å
b/Å
c/Å
P21/n (no. 14)
10.182(4)
16.698(4)
19.141(6)
94.86(3)
3243(2)
4
P21/n (no. 14)
13.396(2)
13.653(19)
15.810(2)
94.126(12)
2884(4)
4
β/Њ
Volume, V/Å3
No. of molecules in unit cell, Z
µ/cmϪ1
47.83
5.00
Total data
‘Observed’ data, No
5005
5111
3808a
3774b
R
Rw
0.056
0.069
0.057
0.150
a [I > 3σ(I )], b [I > 2σ(I )].
C8H11)][BF4] 12b was prepared similarly in 18% yield start-
ing from [Mo(CO)(CNBut)3(1–5-η-C8H11)][BF4] (0.172 g, 0.303
mmol) and CNBut (0.030 g, 0.361 mmol) except that the
reaction was complete after 30 min.
were included in the structure factor calculation in idealised
positions (C–H = 0.95 Å).
[Mo(CO)(CNBut)3(1–5-η-C8H11)] 6b–Orange, prismatic
crystals of 6b were grown by vapour diffusion of diethyl ether
into a CH2Cl2 solution of the complex. The unit cell dimensions
were derived from the setting angles of 15 reflections in the
range 20.45 < 2θ < 24.50Њ. The intensities of three represent-
ative reflections were measured after every 150 reflections. Over
the course of data collection the standards decreased by 3.0%
and a linear correction factor was applied to the data to
account for this phenomenon. An empirical absorption correc-
tion based on azimuthal scans was applied which resulted in
transmission factors ranging from 0.95 to 1.00 and the data
were corrected for Lorentz and polarisation effects. The struc-
ture was solved by Patterson methods. All non-hydrogen atoms
except for B(1) were refined anisotropically, B(1) was refined
isotropically. Hydrogen atoms were included in calculated
positions.
[Mo(CO)2{P(OMe)3}2(ꢀ5-C7H9)][BF4]. A stirred solution of
[Mo(CO)2(NCMe)3(η5-C7H9)][BF4]6 (0.272 g, 0.598 mmol) in
CH2Cl2 (20 cm3) was treated with P(OMe)3 (0.160 g, 1.29 mmol)
resulting in an immediate colour change from red–brown to
yellow. The volume of the solution was reduced to ca. 2 cm3 in
vacuo and then the reaction mixture was treated with diethyl
ether. The product, [Mo(CO)2{P(OMe)3}2(1–5-η-C7H9)][BF4]
precipitated as a yellow solid; yield 0.159 g (46%). IR ν(CO)-
(CH2Cl2)/cmϪ1: 2002m, 1931s; microanalysis: found (calc.): C,
30.9 (31.0); H, 4.7 (4.7); mass spectrum (FAB): 495 (Mϩ), 467
([M Ϫ CO]), 371 ([M Ϫ P(OMe)3]ϩ), 341 ([M Ϫ P(OMe)3 Ϫ CO
Ϫ 2H]ϩ), 311 ([M Ϫ P(OMe)3 Ϫ 2CO Ϫ 4H]ϩ); 1H NMR
(CDCl3, 300 MHz), numbering scheme as in ref. 6: 6.25, m,
1H, H5; 5.21, m, J(H4–H3) = J(H6–H7) = 10, 2H, H4,6; 4.73,
m, 2H, H3,7; 3.88, d, J(P–H) 11, 18H, P(OCH3)3; 2.24, m,
CCDC reference numbers 195204 and 195205.
See http://www.rsc.org/suppdata/dt/b2/b209975f/ for crystal-
lographic data in CIF or other electronic format.
2H, 2.06, m, 2H, H1,1Ј,2,2Ј
J(P–C) 34, CO; 101.5, 96.2, 95.2, C3–7; 54.9, m, P(OCH3)3; 34.4,
C1,2
;
13C NMR (CDCl3, 75 MHz): 222.8, t,
.
Acknowledgements
Crystallography
We thank the EPSRC for a research studentship (to D. M. S.).
The majority of details of the structure analyses carried out
on 1a and 6b are given in Table 8. A Rigaku AFC5R diffract-
ometer was employed for both structures utilising either
a Mo-Kα source (λ = 0.71069 Å) (6b) or a Cu-Kα source
(λ = 1.54178 Å) (1a). Neutral atom scattering factors were taken
from ref. 31 and all calculations were performed using the
TEXSAN crystallographic software package,32 with the excep-
tion of the computing structure solution which was carried out
with SIR9233 for 1a and DIRDIF92 PATTY34 for 6b and the
computing structure refinement for 6b (SHELXL-97).35
[Mo(CO)2(dppm)(1–3:5,6-η-C8H11)][BF4] 1a–Orange, pris-
matic crystals of 1a were grown by vapour diffusion of diethyl
ether into a CH2Cl2 solution of the complex. The unit cell
dimensions were derived from the setting angles of 25 reflec-
tions in the range 29.18 < 2θ < 32.98Њ. The intensities of three
representative reflections were measured after every 150 reflec-
tions. Over the course of data collection the standards
decreased by 1.60% and a linear correction factor was applied
to the data to account for this phenomenon. An empirical
absorption correction based on azimuthal scans was applied
which resulted in transmission factors ranging from 0.87 to 1.00
and the data were corrected for Lorentz and polarisation
effects. The structure was solved by direct methods. All non-
hydrogen atoms were refined anisotropically, hydrogen atoms
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