Synthesis, redox properties, EPR and ENDOR spectroscopy of the 17-electron cycloheptatrienylmolybdenum complexes MoX(Ph2PCH2CH2PPh2)(η-C 7H7)z+ (z = 1, X = I, Br, Cl, F, NCO, NCS, CN, 13CN, Me, C≡CPh; z = 2, X = NCMe, CNMe, 13CNMe, oxacyclopentylidene)

Article abstract of DOI:10.1039/a907205e

The synthesis of a series of new 18-electron cycloheptatrienylmolybdenum complexes [MoX(dppe)(η-C₇H₇)]^z+ (dppe = Ph₂PCH₂CH₂PPh₂; z = 0, X = NCO, NCS, CN, ¹³CN; z = 1, X = CNMe, ¹³CNMe, oxacyclopentylidene) and [Mo(triphos)(η-C₇H₇)]⁺ (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane) is reported. Cyclic voltammetric investigations reveal that each of these complexes undergoes a reversible one-electron oxidation to the corresponding 17-electron radicals which have been generated by chemical oxidation, isolated and characterised. Additional examples of 17-electron radicals [MoX(dppe)(η-C₇H₇)]^z+ (z = 1, X = I, Br, Cl, F, Me, C≡CPh; z = 2, X = NCMe) were either isolated directly or reinvestigated in this work. Fluid solution, X-band EPR studies at 243 K on the 17-electron radicals [MoX(dppe)(η-C₇H₇)]^z+ (z = 1 or 2) afford well resolved spectra characteristic of molybdenum species split by ligand hyperfine interactions with two equivalent phosphorus atoms, seven equivalent protons of the cycloheptatrienyl ring and an additional splitting from the ligand X (where this contains nuclei with non-zero spin). Calculations to determine contributions to the ground state molecular orbital from (i) the ns orbital of the ligand X (based on the EPR work) and (ii) the dppe phosphorus s and p orbitals (based on ENDOR work) suggest that the unpaired electron occupies an essentially metal-based molecular orbital. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a907205e

View Article Online / Journal Homepage / Table of Contents for this issue
Synthesis, redox properties, EPR and ENDOR spectroscopy
of the 17-electron cycloheptatrienylmolybdenum complexes
[MoX(Ph2PCH2CH2PPh2)(ꢀ-C7H7)]z؉ (z ؍ 1, X ؍ I, Br, Cl, F,
13
᎐
NCO, NCS, CN, CN, Me, C᎐CPh; z ؍ 2, X ؍ NCMe, CNMe,
᎐
13CNMe, oxacyclopentylidene)
Georgina M. Aston,b Samer Badriya,a Robert D. Farley,c Richard W. Grime,a Sarah J. Ledger,a
Frank E. Mabbs,a Eric J. L. McInnes,b Hugh W. Morris,a Allen Ricalton,a
Christopher C. Rowlands,c Kirsten Wagnera and Mark W. Whiteley*a
a Department of Chemistry, University of Manchester, Manchester, UK M13 9PL
b EPSRC CW EPR Service Centre, Department of Chemistry, University of Manchester,
Manchester, UK M13 9PL
c EPSRC ENDOR Service Centre, Department of Chemistry, Cardiff University, PO Box 912,
Cardiff, UK CF1 3TB
Received 7th September 1999, Accepted 12th November 1999
The synthesis of a series of new 18-electron cycloheptatrienylmolybdenum complexes [MoX(dppe)(η-C7H7)]zϩ
(dppe = Ph2PCH2CH2PPh2; z = 0, X = NCO, NCS, CN, 13CN; z = 1, X = CNMe, 13CNMe, oxacyclopentylidene)
and [Mo(triphos)(η-C7H7)]ϩ (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane) is reported. Cyclic voltammetric
investigations reveal that each of these complexes undergoes a reversible one-electron oxidation to the corresponding
17-electron radicals which have been generated by chemical oxidation, isolated and characterised. Additional
examples of 17-electron radicals [MoX(dppe)(η-C7H7)]zϩ (z = 1, X = I, Br, Cl, F, Me, C᎐CPh; z = 2, X = NCMe)
᎐
᎐
were either isolated directly or reinvestigated in this work. Fluid solution, X-band EPR studies at 243 K on the
17-electron radicals [MoX(dppe)(η-C7H7)]zϩ (z = 1 or 2) afford well resolved spectra characteristic of molybdenum
species split by ligand hyperfine interactions with two equivalent phosphorus atoms, seven equivalent protons of the
cycloheptatrienyl ring and an additional splitting from the ligand X (where this contains nuclei with non-zero spin).
Calculations to determine contributions to the ground state molecular orbital from (i) the ns orbital of the ligand X
(based on the EPR work) and (ii) the dppe phosphorus s and p orbitals (based on ENDOR work) suggest that the
unpaired electron occupies an essentially metal-based molecular orbital.
The redox chemistry of complexes based on metal phosphine
auxiliaries such as ML2(η-C5R5) (M = Fe or Ru, L = PPh3,
PMe3 etc., L2 = dppm, dppe; R = H or Me) or Re(NO)(PPh3)-
Results and discussion
Synthetic studies
The complexes [MoX(dppe)(η-C7H7)]zϩ under investigation in
this paper fall into two groups. In type 1 systems the ligand X
(η-C5R5) has evolved from fundamental electrochemical studies
on [MXL2(η-C5R5)] (X = halide, pseudo-halide etc.)1,2 to
detailed electrochemical and spectroscopic investigations on
ligand-bridged, mixed-valence systems.3,4 Our interest in this
area has focused upon the cycloheptatrienyl molybdenum
auxiliary Mo(dppe)(η-C7H7) and, building upon the work of
Green and co-workers,5,6 we have developed the synthesis
and redox chemistry of a range of derivatives [MoX(dppe)-
is formally a uninegatively charged species (halide, NCO, NCS,
᎐
᎐
C᎐N, Me, C᎐CR) giving rise to neutral 18-electron complexes
᎐
᎐
[MoX(dppe)(η-C7H7)]. In type 2 systems, X is a neutral ligand
(NCMe, CNMe, oxacyclopentylidene) resulting in cationic
18-electron complexes [MoX(dppe)(η-C7H7)]ϩ. The majority of
these complexes are accessible directly by reaction of a source
of the ligand XϪ/X with an acetone solution of [Mo(OCMe2)-
(dppe)(η-C7H7)][PF6] (generated by reaction of the toluene
sandwich complex [Mo(η-C6H5Me)(η-C7H7)][PF6] with dppe in
acetone).5,6 The objective of the synthetic work was to obtain a
series of complexes of the type [MoX(dppe)(η-C7H7)]zϩ with a
wide range of ligands X, both as 18-electron species and 17-
electron radical cations, thus promoting investigations on the
effect of X on electrochemical properties and EPR spectral
data.
7
8
9
10
᎐
᎐
(η-C H )], [X = C᎐CPh, Cl, Me, C᎐CFc (Fc = ferrocenyl)].
᎐
᎐
7
7
The distinctive features of the cycloheptatrienylmolybdenum
system which have stimulated our investigations are (i) the ease
of one-electron oxidation to the corresponding 17-electron
radical cations [MoX(dppe)(η-C7H7)]ϩ, (ii) the stability of
these 17-electron radicals and finally (iii) the well resolved
solution EPR spectra obtainable for these complexes. This
paper draws together our investigations on the synthesis and
redox chemistry of an extended series of 18-electron com-
plexes [MoX(dppe)(η-C7H7)]zϩ (z = 0 or 1) and presents the
results of EPR and ENDOR investigations on the 17-electron
radicals [MoX(dppe)(η-C7H7)]zϩ (z = 1 or 2). The EPR and
ENDOR work furnish new data with which to explore the
composition of the SOMO (singly occupied molecular
orbital) in these systems.
Type 1 complexes
Reaction of [Mo(OCMe2)(dppe)(η-C7H7)][PF6] with an excess
of anhydrous LiX (X = I, Br, Cl) in acetone affords directly the
known6 17-electron radical cations [MoX(dppe)(η-C7H7)][PF6]
J. Chem. Soc., Dalton Trans., 1999, 4379–4388
This journal is © The Royal Society of Chemistry 1999
4379

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(dppe)(η-C7H7)][PF6] in acetone–methanol (1:10) and the
carbon-13 labelled analogue [Mo(13CN)(dppe)(η-C7H7)] 9a was
prepared by an identical procedure but employing K13CN as a
reagent. Cyclic voltammetric investigations (see below) revealed
that each of 6a, 8a and 9a undergo reversible one-electron
oxidations to the corresponding 17-electron radical cations
at potentials accessible to the ferrocenium ion as a chemical
oxidant and accordingly the radicals [MoX(dppe)(η-C7H7)]-
[PF6] (X = NCO, 6b; X = CN, 8b) were isolated by respective
one-electron oxidation with [Fe(η-C5H5)2][PF6] in CH2Cl2. The
synthesis and characterisation of the remaining type 1 com-
plexes discussed in this paper, [MoMe(dppe)(η-C7H7)] and
7,9
᎐
[Mo(C᎐CPh)(dppe)(η-C H )] have been described previously.
᎐
7
7
Type 2 complexes
In these systems, X is a neutral, 2-electron ligand resulting in
18-electron complexes of formulation [MoX(dppe)(η-C7H7)]ϩ
and 17-electron radical dications of formulation [MoX(dppe)-
(η-C7H7)]2ϩ. The principal derivatives for which both members
of the redox pair are isolable are: X = NCMe,8 CNMe and
CCH2CH2CH2O (oxacyclopentylidene). Other derivatives
(X = C=C(Me)But,11 CO6) were investigated by cyclic voltam-
metry but their 17-electron radical dications were not isolable.
The red-pink isocyanide complex [Mo(CNMe)(dppe)-
(η-C7H7)][BF4] 10a was prepared by methylation of the cyanide
complex 8a by reaction with [Me3O][BF4] in CH2Cl2 and the
carbon-13 labelled analogue [Mo(13CNMe)(dppe)(η-C7H7)]-
[BF4] 11a was obtained from 9a by an identical procedure. The
radical dication [Mo(CNMe)(dppe)(η-C7H7)][BF4]2 10b was
successfully isolated following oxidation with [NO][BF4] or
[Fe(η-C5H5){η-C5H4(C(O)CH3)}][BF4] in CH2Cl2 but proved to
be sensitive in solution with respect to reduction back to 18-
electron 10a. The synthesis of the cyclic oxacarbene complex
Scheme 1
(X = I, 1b; Br, 2b; Cl, 3b; see Scheme 1). The direct formation of
the 17-electron systems is unexpected and we cannot exclude
the operation of adventitious oxidants in the reaction mixture.
However the reactions are reproducible and we have consist-
ently observed8 this outcome in the reaction of LiCl with both
[Mo(OCMe2)(dppe)(η-C7H7)][PF6] and [Mo(NCMe)(dppe)-
(η-C7H7)][PF6] although in some other cases, mixtures of
neutral 18-electron and cationic 17-electron complexes have
been isolated as reaction products.11 The neutral 18-electron
complexes [MoX(dppe)(η-C7H7)] (X = I, 1a; Br, 2a; Cl, 3a) have
been reported previously6 and, as we have demonstrated for the
chloride derivative, can be conveniently prepared by cobalto-
cene reduction of the 17-electron radicals in toluene.8 In addi-
tion to the known halide complexes (X = I, Br, Cl), two new
systems were prepared. The novel fluoride derivative [MoF-
(dppe)(η-C7H7)][PF6] 4b was isolated as a green solid from the
reaction of [Mo(OCMe2)(dppe)(η-C7H7)][PF6] with anhydrous
LiF and the ring-substituted complex [MoCl(dppe)(η-C7H6-
Me)][PF6] 5b was prepared starting from [Mo(η-C6H5Me)-
(η-C7H6Me)][PF6].12 Attempts to reduce 4b to its neutral
18-electron counterpart were unsuccessful and reduction of
[Mo(CCH2CH2CH2O)(dppe)(η-C7H7)][PF6] 12a by reaction of
᎐
[Mo(OCMe2)(dppe)(η-C7H7)][PF6] with HC᎐CCH CH OH
᎐
2
2
in refluxing acetone has been described previously.13 Cyclic
voltammetric studies revealed that 12a undergoes a reversible
one-electron oxidation and chemical oxidation with [Fe(η-
C5H5)2][PF6] led to isolation of [Mo(CCH2CH2CH2O)(dppe)-
(η-C7H7)][PF6]2 12b as a yellow-orange solid. Our attempts
to prepare stable, isolable phosphine complexes of the type
[Mo(PR3)(dppe)(η-C7H7)][PF6] were unsuccessful, probably
due to the restrictions of the relatively crowded metal centre
of the Mo(dppe)(η-C7H7) auxiliary.14 However a related tris-
phosphine substituted system [Mo(triphos)(η-C7H7)][PF6] 13a
was realised from the reaction of [Mo(η-C6H5Me)(η-C7H7)]-
[PF6] with 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos)
in refluxing acetone. Complex 13a undergoes a reversible one-
electron oxidation in CH2Cl2 and the radical dication [Mo-
(triphos)(η-C7H7)][PF6]2 13b was successfully isolated from
treatment of 13a with [Fe(η-C5H5)2][PF6] although the reaction
was incomplete due to unfavourable redox potentials (see
below).
ring-substituted 5b was not pursued.
The pseudo-halide ligands X = NCO, NCS, C᎐N were also
᎐
᎐
introduced to the molybdenum centre by direct reaction of XϪ
with [Mo(OCMe2)(dppe)(η-C7H7)][PF6]. Treatment of an acet-
one solution of [Mo(OCMe2)(dppe)(η-C7H7)][PF6] with excess
KNCO resulted in the formation of the neutral 18-electron
cyanate complex [Mo(NCO)(dppe)(η-C7H7)] 6a which was
isolated as a green solid. However an analogous reaction with
excess KNCS gave a product mixture of [Mo(NCS)(dppe)-
(η-C7H7)] 7a and [Mo(NCS)(dppe)(η-C7H7)][PF6] 7b together
with a further unidentified species. As described in the Experi-
mental section, the 17-electron radical 7b was successfully
separated from the product mixture and subsequently reduced
to neutral 7a by reaction with cobaltocene in toluene. The
18-electron cyanide complex [Mo(CN)(dppe)(η-C7H7)] 8a
was isolated as a pink solid following purification by column
chromatography from the reaction of KCN with [Mo(OCMe2)-
Spectroscopic studies (excluding EPR work)
Details of the characterisation of the new complexes described
in this work are presented in Table 1 (microanalytical, IR and
mass spectroscopic data) and Table 2 (1H and 13C{1H} NMR
spectroscopic data). Several of the ligands X include a group
᎐
with an infrared active ν(C᎐N) stretch and two points of inter-
᎐
est arise from the infrared data. First, examination of the pairs
of natural isotopic abundance carbon and carbon-13 labelled
cyanide and isocyanide complexes (8a, 9a) and (10a, 11a) con-
firms the expected shift to lower wavenumber by incorporation
of carbon-13. In the case of the cyanide complexes 8a and 9a,
᎐
ν(C᎐N) for the carbon-13 labelled complex 9a can be predicted
᎐
᎐
to within one wavenumber based on ν(C᎐N) for 8a and the
᎐
application of a reduced mass calculation.
4380
J. Chem. Soc., Dalton Trans., 1999, 4379–4388

View Article Online
Table 1 Microanalytical, infrared and mass spectroscopic data
Analysisa (%)
Infraredb
Ϫ1
Complex
C
H
N
ν(C᎐N)/cm
Mass spectral datac
᎐
᎐
4b [MoF(dppe)(η-C7H7)][PF6]
5b [MoCl(dppe)(η-C7H6Me)][PF6]
6a [Mo(NCO)(dppe)(η-C7H7)]
6b [Mo(NCO)(dppe)(η-C7H7)][PF6]
7a [Mo(NCS)(dppe)(η-C7H7)]
53.0 (52.9) 4.1 (4.1)
52.5 (52.3) 4.3 (4.2)
606 (Mϩ), 587 ([M Ϫ F]ϩ)
637 (Mϩ), 602 ([M Ϫ Cl]ϩ)
65.4 (65.1) 5.2 (4.9) 2.3 (2.2) 2225
53.2 (52.9) 4.0 (4.0) 1.8 (1.8) 2217
63.4 (63.5) 4.8 (4.8) 2.1 (2.2) 2084
52.1 (51.8) 4.2 (3.9) 1.5 (1.8) 2030
66.6 (66.8) 5.3 (5.1) 2.0 (2.3) 2073
53.4 (54.0) 4.0 (4.1) 1.7 (1.9) 2103
66.8 (66.8) 5.4 (5.1) 2.1 (2.3) 2027
59.1 (58.9) 4.8 (4.8) 1.9 (2.0) 2147
52.5 (52.5) 4.3 (4.3) 1.7 (1.8) 2228
58.6 (59.0) 4.7 (4.8) 1.9 (2.0) 2108
629 (Mϩ), 587 ([M Ϫ NCO]ϩ)
629 (Mϩ), 587 ([M Ϫ NCO]ϩ)
645 (Mϩ), 587 ([M Ϫ NCS]ϩ)
645 (Mϩ), 587 ([M Ϫ NCS]ϩ)
613 (Mϩ), 587 ([M Ϫ CN]ϩ)d
613 (Mϩ), 587 ([M Ϫ CN]ϩ)
614 (Mϩ), 587 ([M Ϫ 13CN]ϩ)d
628 (Mϩ), 587 ([M Ϫ CNMe]ϩ)
628 (Mϩ), 587 ([M Ϫ CNMe]ϩ)
629(Mϩ)d
7b [Mo(NCS)(dppe)(η-C7H7)][PF6]
8a [Mo(CN)(dppe)(η-C7H7)]
8b [Mo(CN)(dppe)(η-C7H7)][PF6]
9a [Mo(13CN)(dppe)(η-C7H7)]
10a [Mo(CNMe)(dppe)(η-C7H7)][BF4]
10b [Mo(CNMe)(dppe)(η-C7H7)][BF4]2
11a [Mo(13CNMe)(dppe)(η-C7H7)][BF4]
12b [Mo(CCH2CH2CH2O)(dppe)(η-C7H7)][PF6]2
13a [Mo(triphos)(η-C7H7)][PF6]
13b [Mo(triphos)(η-C7H7)][PF6]2
46.3 (47.0) 4.0 (3.9)
60.2 (60.3) 5.0 (4.8)
52.3 (52.3) 4.7 (4.2)
657 (Mϩ), 587 ([M Ϫ CCH2CH2CH2O]ϩ)
813 (Mϩ)d
406 (M2ϩ
)
d
a Calculated values in parentheses. b Solution spectra in CH2Cl2. c FAB mass spectra unless stated otherwise, m/z values based on 98Mo. d Electrospray
mass spectrum.
Table 2 1H and selected 13C{1H} NMR data for [MoX(dppe)(η-C7H7)]zϩ (z = 0 or 1) and [Mo(triphos)(η-C7H7)]ϩ a
1H NMR data
13C NMR data
dppe/triphos
Ph
dppe
Ph
Complex
X
C7H7
CH2
X
C7H7
CH2
6ab
7ab
4.79, t, (1.8)
4.82, t, (1.8)
4.84, t, (2.1)
7.71, m, 7.43,
m, 7.29, m
7.69, m, 7.46,
m, 7.29, m
7.73, m, 7.39,
m, 7.28, m
7.70, m, 7.37,
m, 7.27, m
2.06 (br)
87.2
87.3
87.4
87.4
89.1
89.0
138.2–128.4
138.2–128.5
139.9–127.9
139.9–128.1
137.6–127.7
137.1–129.0
25.8, m
25.9, m
26.9, m
26.9, m
27.1, m
27.0, m
2.01 (br)
143.3, NCS
8ab
2.48, m, 2.06 m
2.44, m, 2.04 m
2.42 (br)
157.6, t, {20}, CN
157.8, t, {20}, CN
9ab
4.82, dt, (2.1),
[0.7]
5.01, t, (2.2)
10a
2.50, t, (1.9)
CNCH3
2.40, dt, [3.7],
(1.9) CNCH3
7.47, m, 7.27, m
163.7, (br) CNMe,
29.4, CNMe
164.9, {18} CNMe,
29.1, CNMe
11ac
13ad
5.02
7.53, m, 7.28, m
2.49, m
5.50, q, (2.2)
7.37, m, 7.29,
m, 7.23, m
2.49, m, CH2,
2.05, q (2.2) Me
a 300 MHz 1H NMR spectra, 75 MHz 13C NMR spectra; all signals singlets unless stated otherwise, d = doublet, t = triplet, q = quartet, m = multi-
plet. Coupling constants in Hz indicated in parentheses: ( ) indicates J(P–H), [ ] indicates J(13C–H), { } indicates J(P–C). Chemical shifts downfield
from SiMe4, spectra recorded in CDCl3 unless stated otherwise. b Trace quantity of CoCp2 added to NMR sample. c In CD2Cl2. d In acetone-d6.
31P{1H} NMR (acetone-d6), (122 MHz), δ 20.1
from this and previous work.7,8,10 It is generally accepted that
Table 3 Shift in ν(CN) or ν(CC) (∆ν) resulting from one-electron
oxidation of 18-electron [MoX(dppe)(η-C7H7)]zϩ (z = 0 or 1) to the
᎐
ligands (X᎐Y) such as CO, which act as good π-acceptors
᎐
corresponding 17-electron radicalsa
exhibit an increase in stretching frequency resulting from metal-
based one-electron oxidation processes.15 This may be attrib-
᎐
᎐
᎐
᎐
ν(C᎐C)/(C᎐N)
ν(C᎐C)/(C᎐N)
᎐
᎐
᎐
᎐
uted to a reduced component of π-back donation to the ligand
(cmϪ1
)
(cmϪ1
)
᎐
π* orbitals resulting in strengthening of the (X᎐Y) triple bond.
᎐
18-electron
complex
17-electron
complex
X
z
∆ν (cmϪ1
81
80
30
31
18
Ϫ8
Ϫ13
Ϫ50
Ϫ54
)
Typically in the case of the CO ligand, ν(CO) increases by ca.
100 cmϪ1 (∆ν) following a metal-based one-electron oxidation.
Inspection of Table 3 reveals that both the isocyanide (CNMe)
and cyanide ligands comply with the expectations of operation
as good π-acceptor ligands. By contrast we,7 and more recently,
others,16 have observed that alkynyl ligands exhibit a negative
∆ν following a metal-based one-electron oxidation and, in the
more extreme cases (generally involving an alkynyl bridge link-
CNMe
13CNMe
CN
1
1
0
0
1
0
0
0
0
2147
2108
2073
2027
2261
2225
2045
2052
2084
2228
2188
2103
2058
2279
2217
2032
2002
2030
13CN
NCMeb
NCO
c
᎐
C᎐CPh
᎐
᎐
d
ing two metal centres) the decrease in ν(C᎐C) has been attrib-
᎐
᎐
C᎐CFc
᎐
uted to an allenylidene resonance form17 (M᎐C᎐C᎐Fc in the
NCS
᎐ ᎐ ᎐
case of ferrocenyl systems). It is in this context that the results
a All spectra recorded in CH2Cl2. b Data from ref. 8. c Data from ref. 7.
d Data from ref. 10.
for X = NCO or NCS (complexes 6a, 6b and 7a, 7b) are of
᎐
especial interest. In the case of the NCO ligand, ν(C᎐N) is not
᎐
significantly affected by the redox state of the metal centre
The data in Table 3 present the effect of one-electron oxid-
but by contrast one-electron oxidation of the analogous NCS
Ϫ1
᎐
᎐
᎐
ation upon infrared active ν(C᎐N) or ν(C᎐C) stretches in a
complex 7a results in a large decrease in ν(C᎐N) (Ϫ54 cm ). We
᎐
᎐
᎐
series of redox pairs based on the Mo(dppe)(η-C7H7) auxiliary
attribute this to a significant contribution from the resonance
J. Chem. Soc., Dalton Trans., 1999, 4379–4388
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Table 4 Cyclic voltammetric data for [MoX(dppe)(η-C7H6R)]zϩ and
[Mo(triphos)(η-C7H7)]zϩ
form M᎐Nϩ᎐C᎐S in the oxidised species 7b. Inspection of the
᎐
᎐ ᎐
literature reveals a similar large and negative ∆ν (Ϫ44 cmϪ1) for
the related redox pair [Fe(NCS)(dppe)(η-C5H5)]zϩ (z = 0,
Ϫ1 18
ν(C᎐N) (KBr) 2084 cm ; z = 1, ν(C᎐N) (KBr) 2040 cmϪ1 1).
X
z
Complex
EЊ/V
Ref.
᎐
᎐
᎐
᎐
Hydrogen-1 and 13C{1H} NMR data are presented in Table
2; as we have discussed previously, well resolved NMR data for
easily oxidised type 1 complexes (in the case of this work 6a, 7a,
8a and 9a) are best obtained by the addition of a trace quantity
of the one-electron reductant [Co(η-C5H5)2] to the NMR
sample.19,20 Inspection of Table 2 reveals a fairly standard data
set for the Mo(dppe)(η-C7H7) auxiliary with the cyclohepta-
Me
0
0
1
1
1
1
1
0
1
0
1
0
1
0
1
1
2
1
Ϫ0.37
Ϫ0.15
0.04
9
7
᎐
C᎐CPh
᎐
I
1b
2b
3b
4b
5b
6a
6b
7a
7b
8a
8b
9a
This work
This work
7
Br
Cl
F
0.00
Ϫ0.05
Ϫ0.28
Ϫ0.09
Ϫ0.04
Ϫ0.04
0.11
0.11
0.21
0.20
0.21
0.51
0.70
0.70
0.70
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8
Cl (R = Me)
NCO
NCO
NCS
NCS
CN
1
trienyl ring resonance in the H NMR spectra appearing as a
triplet signal due to a coupling of ca. 2 Hz to the two equivalent
phosphorus atoms of the dppe ligand. The triphos complex 13a
exhibited a quartet pattern for the C7H7 resonance and this,
together with the observation of only a singlet resonance in the
31P{1H} NMR spectrum for the three phosphorus donor atoms
of the triphos ligand supports the formulation of 13a as a com-
plex with a tridentate triphos ligand. The principal features of
interest in the NMR spectra arise from the ligands (X = CN,
CNMe) and the effects of carbon-13 labelling. In the 13C{1H}
NMR spectrum of [Mo(CN)(dppe)(η-C7H7)], 8a, the cyanide
carbon is observed as a weak triplet signal with J(P–C) 20 Hz
and this assignment is definitively confirmed in the 13C{1H}
spectrum of [Mo(13CN)(dppe)(η-C7H7)], 9a for which the
cyanide carbon was observed as a high intensity resonance.
CN
13CN
NCMe
CNMe
CNMe
13CNMe
10a
10b
11a
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CCH2CH2CH2O
CCH2CH2CH2O
᎐
CO
triphos
triphos
1
12a
12b
0.57
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2
1
1
1
2
0.56
1.05
1.23
0.64
0.64
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C᎐C(Me)But
13a
13b
1
A further feature of 9a, evident in the H NMR spectrum and
R = H unless stated otherwise. EЊ values (in V) recorded in CH2Cl2 with
0.2 M [NBun ][BF4] as supporting electrolyte at a carbon working elec-
not observed for 8a, is a small coupling (0.7 Hz) to the C7H7
protons superimposed upon the triplet pattern due to J(P–H)
coupling. We believe that this additional feature may be attrib-
uted to the observation of J(13C–H) coupling. The isocyanide
carbon of [Mo(CNMe)(dppe)(η-C7H7)][BF4], 10a was observ-
able only as a broad resonance in the 13C{1H} NMR spectrum
but again the carbon-13 labelled sample 11a provided well
4
trode relative to SCE and standardised with respect to the ferrocene–
ferrocenium couple as internal calibrant for which EЊ = 0.56 V under
these conditions.
[Mo(triphos)(η-C7H7)][PF6]2 were investigated as one-electron
reduction processes and again the criteria for diffusion control
and chemical reversibility were fully satisfied. The designation
of the electron-transfer processes observed in the cyclic
voltammetry as one-electron changes is, in many cases, fully
established by the synthetic studies and the isolation and
characterisation of both members of the redox pair for each
separate identity of X.
1
resolved and definitive data. In the H NMR spectra the two
samples 10a and 11a differed principally in the pattern of the
isocyanide methyl resonance. In common with the isomeric
5
acetonitrile derivative,8 both present long range J(P–H) coup-
ling but in addition the carbon-13 labelled sample 11a exhibits
3J(13C–H) coupling between the isocyanide carbon and the
methyl protons.
A compilation of data for the formal reduction potentials EЊ
(vs. SCE) for each of the complexes considered is presented in
Table 4. The halide complexes [MoX(dppe)(η-C7H7)]ϩ repre-
sent only the second example in which reversible electrode
potentials are available for a complete organometallic LnMX
series (X = I, Br, Cl or F). Our results complement the work of
Tilset and co-workers21 on the complexes [FeX(dppe)(η-C5-
Me5)]zϩ (z = 0, X = I, Br, Cl; z = 1, X = F). For both series
[MoX(dppe)(η-C7H7)]zϩ and [FeX(dppe)(η-C5Me5)]zϩ there is a
small but consistent shift of EЊ to more negative values from
X = I to X = Cl, probably associated with the enhanced π-donor
capacity of chloride with respect to iodide. This trend continues
with the fluoride derivatives but here there is a much larger
negative shift in EЊ (ca. 0.2 V) than expected from the trend for
the I, Br and Cl analogues. This discontinuity in the trend has
been attributed to the ability of the fluoride ligand to act as an
exceptionally good donor.21 Comparison of EЊ data for the
chloride complexes 3b and 5b permits an estimation of the
effect of methyl substitution at the cycloheptatrienyl ring and,
in accord with related studies,22 a shift in EЊ to negative poten-
tial by approximately 40 mV for substitution of one hydrogen
by a methyl substituent is observed.
Electrochemical investigations
Our previous synthetic and electrochemical investigations have
established that the Mo(dppe)(η-C7H7) auxiliary supports
reversible one-electron oxidation processes for a wide range of
ligands X. The majority of the 17-electron radicals [MoX(dppe)-
(η-C7H7)]zϩ (z = 1 or 2) are stable, not only on the electro-
chemical timescale, but also on a timescale long enough to
allow chemical generation, isolation and subsequent character-
isation as detailed in the synthetic studies above. We now pres-
ent a compilation of cyclic voltammetric data for an extended
series of complexes including the new systems described in this
work and those reported previously.
The initial scans in the cyclic voltammograms of each of the
᎐
18-electron complexes [MoX(dppe)(η-C H )] (X = Me, C᎐CPh,
᎐
7
7
NCO, NCS, CN), [MoX(dppe)(η-C7H7)]ϩ (X = NCMe, CNMe,
CCH CH CH O, C᎐C(Me)But) and [Mo(triphos)(η-C H )]ϩ in
᎐
2
2
2
7
7
CH2Cl2 are all characterised by reversible one-electron oxid-
ation processes. The electron-transfer processes are diffusion
¹
ox
Ϫ1

²
controlled (ip /ν is constant for scan rates ν = 50–500 mV s )
and chemically reversible with the ratio of cathodic to anodic
currents being unity over the same scan range. The observed
separation between the cathodic and anodic peak potentials
compared well with that measured for the ferrocene–
ferrocenium couple measured under identical conditions. The
17-electron radical cations [MoX(dppe)(η-C7H7)]ϩ (X = I,
Br, Cl, F, NCO, NCS, CN), [MoCl(dppe)(η-C7H6Me)][PF6],
Type 2 complexes for which the formulation of the
18-electron systems is [MoX(dppe)(η-C7H7)]ϩ (X = NCMe,
CNMe, CCH CH CH O, C᎐C(Me)But, CO) are clearly more
᎐
2
2
2
difficult to oxidise to their 17-electron radicals than their type 1
counterparts as evidenced by the EЊ values in Table 4. Neverthe-
8
less both [Mo(NCMe)(dppe)(η-C7H7)][PF6]2 and the carbene
[MoX(dppe)(η-C7H7)]2ϩ (X = CNMe, CCH2CH2CH2O) and
complex [Mo(CCH2CH2CH2O)(dppe)(η-C7H7)][PF6]2 12b
4382
J. Chem. Soc., Dalton Trans., 1999, 4379–4388

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Table 5 Isotropic EPR parameters obtained from the CH2Cl2 fluid solution X-band spectra of [MoX(dppe)(η-C7H6R)]zϩ at 243 Ka
Complex
No.
X
giso
Aiso(Mo)
aiso(31P)
aiso(1H)
aiso(X)
% s-orbital character
Me
2.002
1.995
2.055
2.014
1.991
1.978
1.989
1.985
1.985
1.996
1.996
1.990
38.0
34.8
24.6
22.7
4.7(7)
4.3(7)
6.8 (3, 1H)
—
1.34
—
᎐
C᎐CPh
᎐
1b
2b
3b
4b
5b
6b
7b
8b
9b
Ib
Br
Cl
F
39.0
40.0
41.5
40.0
40.0
38.0
34.0
34.0
41.0
23.2
23.2
22.7
23.2
22.5
22.2
23.2
23.2
22.7
4.2(7)
4.6(7)
4.9(7)
5.0(6) 5.3(3)
4.9(7)
4.3(7)
4.7(7)
4.7(7)
4.5(7)
9.7 (79,81Br)
2.1 (35,37Cl)
28.0 (19F)
0.08
0.10
0.15
Cl (R = Me)
NCO
NCS
12CN
13CN
NCMec
1.5 (14N)
2.2 (14N)
—
0.23
0.34
—
9.4 (13C)
3.5–3.1 (3, 1H)
1.0–0.0 (14N)
—
0.70
10b
11b
12b
13b
12CNMec
13CNMec
oxacarbenec
triphosc
1.997
1.997
1.998
1.998
35.0
35.0
35.5
38.0
22.8
22.8
24.0
23.5
4.3(7)
4.3(7)
4.5(7)
4.5(7)
—
0.45
—
6.0 (13C)
—
d
a R = H, z = 1 unless stated otherwise. All hyperfine splitting constants are in G (1 G = 10Ϫ4 T), the numbers in parentheses for aiso(1H) are the
numbers of equivalent protons. Estimated errors: g values 0.001, Aiso(Mo), 0.5 G, aiso(other), 0.1 G. Oxacarbene = CCH2CH2CH2O, triphos
replaces (dppe ϩ X). b Hyperfine splittings not resolved for X = I. c z = 2. d All phosphorus atoms in the ligand give the same aiso
.
could be obtained by ferrocenium oxidation of the 18-electron
precursors. The EЊ value for the isocyanide derivative [Mo-
(CNMe)(dppe)(η-C7H7)][PF6] 10a is too positive for the radical
dication to be accessible by ferrocenium oxidation but use of
either [NO][BF4] or the acetylferrocenium ion [EЊ (CH2Cl2) 0.81
V under the standard conditions used in this work] effected a
synthesis of the 17-electron radical 10b. Complexes 10a/10b
and 12a/12b were confirmed as redox pairs by complementary
advances were achieved both in the quality of the experimental
data and in its interpretation through the use of variable tem-
perature studies, second derivative spectra and the application
of simulation techniques. Fluid solution spectra in CH2Cl2 were
obtained at 243 K on the radicals 1b to 13b, [MoX(dppe)-
(η-C7H7)]ϩ (X = Me or C᎐CPh) and [Mo(NCMe)(dppe)-
᎐
᎐
(η-C7H7)]2ϩ at X-band frequency as second derivative spectra.
Optimum resolution was in all cases realised at around 243 K
and this temperature was used as a standard for the spectral
investigations. With the exception of the iodo derivative 1b (for
which no hyperfine splittings could be resolved), all the
examples investigated gave well resolved spectra characteristic
of a molybdenum species split by ligand hyperfine interactions.
The spectra of the non ring-substituted dppe complexes were
simulated satisfactorily by incorporating hyperfine splitting
with two equivalent phosphorus atoms, seven equivalent pro-
tons plus an additional hyperfine splitting from the ligand X,
where this contains nuclei with non-zero nuclear spin. The
parameters obtained from spectral simulation are presented in
Table 5 and the experimental and simulated spectra for each of
three examples (X = CN, NCS, F) which illustrate the effect of
increasing the magnitude of aiso(X) on the spectral pattern are
shown in Fig. 1 to 3. The molybdenum hyperfine splittings
given in Table 5 were derived from the wings of the first deriv-
ative spectra and a typical example (X = NCS) together with the
simulation is shown in Fig. 4.
In complexes other than ring-substituted 5b, the proton split-
ting required to simulate the fluid solution X-band spectra is
attributed to seven equivalent protons on the cycloheptatrienyl
ligand. The magnitude of this splitting is in the range 4.2 to
4.9 G and is comparable to that (4.6 G) previously reported19
for [Mo(CNBut)(bipy)(η-C7H7)][PF6]2. There seems to be no
obvious relationship between the magnitude of a(1H) and the
identity of X. To establish further the assignment of hyperfine
coupling to the protons of the cycloheptatrienyl ring, the ring-
substituted derivative 5b was investigated. The only parameters
which would reproduce the experimental spectrum required six
equivalent ring protons plus a separate set of three equivalent
protons. The three equivalent protons are attributed to the ring
methyl substituent and spectral simulation reveals that the aiso
for these is larger than that for the six ring protons. Further
observations of note are the trend in g values along the series
X = I, Br, Cl, F (1b to 4b) and the observation of coupling to
14N for X = NCO (6b), NCS (7b) supporting the view that these
ligands are N-bonded. However the hyperfine coupling to 14N
cyclic voltammetric data. Finally the complexes [Mo{C᎐C(Me)-
᎐
But}(dppe)(η-C7H7)][PF6] and [Mo(CO)(dppe)(η-C7H7)][PF6]
incorporate very efficient π-acceptor vinylidene and carbonyl
ligands and this is reflected in the large and positive EЊ values
(note that the disubstituted vinylidene ligand was selected to
avoid complications arising from redox-induced C–H bond
fission in complexes with mono-substituted vinylidene ligands
of the type C᎐C(H)R).23 The electrochemical data for the vinyl-
᎐
idene complex [Mo{C᎐C(Me)But}(dppe)(η-C H )][PF ] fully
᎐
7
7
6
satisfy the criteria for chemical and electrochemical reversibility
but data for the carbonyl complex [Mo(CO)(dppe)(η-C7H7)]-
c
a
[PF6] reveal a steady decline in the ratio ip /ip as the scan rate
c
is decreased (ip /ipa = 0.9 for a scan rate of 50 mV sϪ1). Attempts
to generate and observe the 17-electron radical derived from
[Mo{C᎐C(Me)But}(dppe)(η-C H )][PF ] were unsuccessful.
᎐
7
7
6
However comparison of EЊ data for the oxacarbene system
12a and the vinylidene analogue [Mo{C᎐C(Me)But}(dppe)-
᎐
(η-C7H7)][PF6] suggests that the vinylidene ligand is a much
more efficient π-acceptor ligand.
Although not strictly a member of the series [MoX(dppe)(η-
C7H7)]ϩ, the triphos complex 13a gives an indication of the
position of complexes of the type [Mo(PR3)(dppe)(η-C7H7)]ϩ
within the redox series. The measured EЊ value for 13a (0.64 V)
is to positive potential of EЊ for [Mo(NCMe)(dppe)(η-C7H7)]ϩ
and also of the FeCp2–FeCp2ϩ couple but oxidation of 13a with
a small excess of the ferrocenium ion did afford low yields of
the 17-electron radical 13b in an incomplete reaction. We esti-
mate that, taking ∆EЊ = Ϫ0.08 V and for equimolar quantities
of reagents, the redox reaction might be expected24 to generate
13b in 20% yield.
EPR and ENDOR Investigations
EPR Spectroscopy. Fluid solution EPR investigations on
selected complexes [MoX(dppe)(η-C7H7)]ϩ (X = I, Br, Cl,6
7
9
2ϩ 8
᎐
C᎐CPh, Me ) and [Mo(NCMe)(dppe)(η-C H )]
have been
᎐
7
7
reported previously. However in the current work, significant
J. Chem. Soc., Dalton Trans., 1999, 4379–4388
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Fig. 1 Fluid solution (CH2Cl2), second derivative X-band spectrum
of [Mo(CN)(dppe)(η-C7H7)][PF6], 8b, ν = 9.633 GHz. (a) Simulated
spectrum using the parameters in Table 5 with Lorentzian lineshape
and peak to peak width 4.0 G; (b) experimental spectrum.
Fig. 3 Fluid solution (CH2Cl2), second derivative X-band spectrum of
[MoF(dppe)(η-C7H7)][PF6], 4b, ν = 9.450 GHz. (a) Simulated spectrum
using the parameters in Table 5 with Lorentzian lineshape and peak to
peak width 5.0 G; (b) experimental spectrum.
Fig. 4 Fluid solution (CH2Cl2), first derivative X-band spectrum of
[Mo(NCS)(dppe)(η-C7H7)][PF6], 7b, ν = 9.399 GHz. (a) Experimental
spectrum; (b) simulated spectrum with the parameters in Table 5 plus
peak to peak Lorentzian linewidth of 3.5 G.
Fig. 2 Fluid solution (CH2Cl2), second derivative X-band spectrum of
[Mo(NCS)(dppe)(η-C7H7)][PF6], 7b, ν = 9.452 GHz. (a) Simulated
spectrum using the parameters in Table 5 with Lorentzian lineshape
and peak to peak width 4.3 G; (b) experimental spectrum.
in the acetonitrile derivative [Mo(NCMe)(dppe)(η-C7H7)]2ϩ
was less clearly defined with satisfactory simulations obtained
for a range of values of aiso(14N) and aiso(1H) (from the aceto-
nitrile methyl group) as detailed in Table 5. In the case of
ligands X with a carbon donor atom possessing zero nuclear
spin [complexes 8b, 10b, 12b and [MoX(dppe)(η-C7H7)]ϩ (X =
The X-band solution spectra of the radicals 1b, 2b, and 3b
may be compared with the related molybdenum systems
[MoX2(dppe)(η-C5H5)] (X = I, Br or Cl).25 In the case of
[MoX2(dppe)(η-C5H5)] there is a corresponding trend in g
values with variation in X and the magnitudes of aiso(31P) are
also similar but typical Aiso(Mo) values are rather smaller than
in the [MoX(dppe)(η-C7H7)]zϩ system. Moreover coupling to
the protons of the cyclopentadienyl ring was not observed and,
only in one case (X = Br), was hyperfine coupling to the ligand
X resolved.
In an attempt to obtain anisotropic parameters, frozen
solution (CH2Cl2–toluene) spectra of the radical com-
plexes [MoX(dppe)(η-C7H7)]zϩ (z = 1 or 2) were obtained at
both X- and Q-band frequencies. All of the X-band spectra
were broad and poorly resolved, mainly due to small g-
anisotropy and unresolved ligand hyperfine splittings and no
attempt was made to interpret these spectra. The Q-band
spectra were also broad but here there was evidence for g-value
anisotropy. In a few cases numerical differentiation to give
second derivative spectra, followed by Fourier transformation
to remove the high frequency noise, afforded useable spectra
which, although not fully resolved, were amenable to spectrum
simulation; Fig. 5 illustrates a representative example. In these
cases (4b to 8b) the g values and anisotropic 31P hyperfine split-
᎐
Me, C᎐CPh)], information derived from hyperfine coupling to
᎐
X is not available (although coupling to the methyl protons of
[MoMe(dppe)(η-C7H7)][PF6] was observed). However our pre-
vious synthetic and spectroscopic studies on alkynyl derivatives
of the Mo(dppe)(η-C7H7) auxiliary suggest some delocalisation
of the unpaired electron onto the ligand X as evidenced by
᎐
the ligand-centred dimerisation of the radical [Mo(C᎐CPh)-
᎐
(dppe)(η-C7H7)]ϩ and spectroscopic studies on the alkynyl-
ϩ
᎐
bridged bimetallic [Mo(C᎐CFc)(dppe)(η-C H )] . To provide a
᎐
7
7
more effective spectroscopic probe for study of these observ-
ations with carbon-bonded X ligands, we synthesised and
investigated the EPR spectra of the carbon-13 labelled com-
plexes [Mo(13CN)(dppe)(η-C7H7)]ϩ and [Mo(13CNMe)(dppe)-
(η-C7H7)]2ϩ. The results in Table 5 reveal a marked difference in
aiso(X) between the cyanide and isocyanide derivatives but even
so the magnitudes of the hyperfine couplings do not support
significant delocalisation of the unpaired electron onto CN or
CNMe ligands.
4384
J. Chem. Soc., Dalton Trans., 1999, 4379–4388

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Table 6 Anisotropic EPR parameters for [MoX(dppe)(η-C7H6R)]ϩ from frozen Q-band spectraa
Complex
g1
g2
g3
a1(31P)
a2(31P)
a3(31P)
〈g〉
〈a(31P)〉
4bb
2.008
2.010
2.006
2.011
2.004
1.988
1.995
1.992
1.995
1.999
1.955
1.986
1.966
1.968
1.993
22.0 (61.9)
22.0 (61.9)
21.5 (60.4)
21.2 (59.7)
25.0 (70.1)
22.0 (61.9)
26.5 (73.9)
23.5 (65.5)
22.5 (62.8)
25.0 (69.9)
25.0 (68.4)
24.5 (68.1)
24.2 (66.6)
24.2 (66.6)
25.0 (69.7)
1.984
1.997
1.988
1.991
1.999
23.0
26.3
23.1
22.6
25.0
5b (R = Me)
6b
7b
8b
a In CH2Cl2–toluene (10:1). Values of a(31P) are in G with values in parentheses in MHz. Estimated errors: g values 0.001, a(31P) 1.0 G.
b In addition a1(19F) = 25.0 G, a2(19F) = 84.0 G, a3(19F) = 10.0 G.
Table 7 The 31P hyperfine splitting parameters from the frozen solution (10 K) ENDOR spectra and the calculated percentage of 31P s- and
p-orbital charactera
Complex
a1(31P)
a2(31P)
a3(31P)
aiso(31P)b
b
% s
% p
s:p
3b
4b
7b
8b
62.4
59.8
59.3
59.5
62.4
62.5
59.3
63.4
71.6
69.7
67.8
72.0
65.5 (64.6)
64.0 (62.8)
62.1 (61.7)
65.0 (64.8)
3.10
2.85
2.80
3.52
0.54
0.48
0.47
0.49
0.85
0.76
0.76
0.96
1.6
1.6
1.6
2.0
a In CH2Cl2–toluene (10:1). All values of a(31P) are in MHz. b Values in parentheses are from the X-band fluid solution spectra.
ENDOR Spectroscopy. In an attempt to obtain more precise
ligand hyperfine parameters from the frozen solutions,
ENDOR spectra at X-band frequencies were obtained on
selected compounds (3b, 4b, 7b and 8b). The low g-value
anisotropy makes it impossible to perform orientation selection
experiments26,27 and therefore the field positions for the
ENDOR experiments were chosen at the centre of each frozen
solution spectrum. This means that the vast majority of
molecular orientations were selected by the magnetic field. This
in turn resulted in ENDOR spectra in which the observed
features correspond to all principal values of the local hyperfine
splitting tensors.
The main features in the experimental ENDOR spectra were
attributed to 1H and 31P coupling to the unpaired electron. No
signals assignable to 35,37Cl, 19F or 14N, where these elements
were the donor atom of ligand X, were identified. The large
number of protons in each complex, together with the use of
protic solvents, made the interpretation of the proton ENDOR
extremely difficult and this has not been attempted. The results
therefore focus on the 31P hyperfine splitting. A typical 31P
ENDOR spectrum is shown in Fig. 6 and the 31P hyperfine
parameters, obtained directly from the ENDOR spectra, are
presented in Table 7. These anisotropic parameters are similar
to the less accurate values obtained above from simulation of
the frozen solution Q-band EPR spectra (see Table 6). In addi-
tion the average values calculated from the ENDOR results are
comparable to those derived from the fluid solution X-band
EPR spectra (Table 5).
The average values and the anisotropies in g, where they
have been measured, taken together with the values of Aiso(Mo)
are compatible with a metal-based molecular orbital for the
unpaired electron and this is reinforced by the small ligand
hyperfine couplings observed. The anisotropies in g, and the
values, are indicative of an orbital singlet state, with excited
electronic states well separated from the ground state. An
orbital singlet ground state is compatible with the formulations
of the complexes which at best indicate Cs point symmetry.
In this point symmetry all of the degeneracy of the metal
d-orbitals will be removed.
Where they have been measured, there is good agreement
between the values of 〈a(31P)〉 from the frozen solution Q-band
EPR spectra and from the ENDOR spectra and aiso(31P)
obtained from the fluid solution X-band EPR spectra. This
indicates that there are no significant structural changes on
cooling to 10 K. The ENDOR results reveal a small degree
of anisotropy in a(31P), see Table 7. These results enable an
Fig. 5 Frozen solution (CH2Cl2–toluene) Q-band spectra of [Mo-
(NCS)(dppe)(η-C7H7)][PF6] 7b at 120 K, ν = 34.040 GHz. (a) First
derivative spectrum; (b) second derivative of (a) obtained by numerical
differentiation followed by Fourier filtering; (c) simulation of (b) using
the parameters in Table 6 plus a Gaussian lineshape function with peak
to peak linewidths wx = 18.0 G, wy = wz = 20.0 G.
ting constants were obtained. In addition, for 4b it was neces-
sary to include anisotropic 19F hyperfine splitting. Because of
the incomplete resolution, the ligand hyperfine splitting
parameters are subject to relatively large errors of approxi-
mately 1 G. A summary of the observed g values and
typical hyperfine parameters is given in Table 6. Despite the
possible large errors, the derived 31P hyperfine parameters are
comparable with those obtained from ENDOR spectroscopy
(see later and Table 7).
J. Chem. Soc., Dalton Trans., 1999, 4379–4388
4385

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(dppe)(η-C7H7)]zϩ (z = 1 or 2) have been developed starting
from the sandwich complex [Mo(η-C6H5Me)(η-C7H7)][PF6].
The electrochemical, EPR and ENDOR spectroscopic
properties of these complexes have been investigated. Cyclic
voltammetric studies at a carbon electrode reveal reversible
one-electron processes for the 18/17-electron couples with EЊ
values ranging from Ϫ0.37 (X = Me) to 1.23 V (X = CO)
dependent on the identity of X. The X-band, CH2Cl2 solution
EPR spectra of the 17-electron radicals [MoX(dppe)(η-C7-
H7)]zϩ (z = 1 or 2) at 243 K are exceptionally well resolved and
permit determination of hyperfine couplings to the two equiv-
alent phosphorus atoms of the dppe ligand, the seven protons
of the cycloheptatrienyl ring and exceptionally an additional
hyperfine splitting from the ligand X where this contains nuclei
with non-zero nuclear spin. X-Band and Q-band EPR investi-
gations, supplemented by ENDOR studies lead to the conclu-
sion that the unpaired electron in radical complexes of general
formulation [MoX(dppe)(η-C7H7)]zϩ (z = 1 or 2) occupies an
essentially metal-based molecular orbital.
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 [Mo(η-C6H5Me)(η-C7H7)][PF6],30 [Mo-
(η-C6H5Me)(η-C7H6Me)][PF6],12 [MoX(dppe)(η-C7H7)][PF6]
9
7
13
(X = Me, C᎐CPh, CCH CH CH O, C᎐C(Me)But,11 CO6),
᎐
᎐
᎐
2
2
2
8
[Mo(NCMe)(dppe)(η-C7H7)][PF6]2 and [Fe(η-C5H5){η-C5H4-
(C(O)Me)}][BF4]24 were prepared by published procedures. The
chemicals dppe, 1,1,1-tris(diphenylphosphinomethyl)ethane
(triphos), K13CN, [Me3O][BF4] and [NO][BF4] were supplied
by Aldrich Chemical Co. and alumina (Brockmann activity II)
for column chromatography was purchased from Merck. 300
MHz 1H and 75 MHz 13C{1H} NMR spectra were recorded on
Bruker AC 300 E, Varian Associates XL 300 or Varian Unity
Inova 300 spectrometers. Infrared spectra were obtained on a
Perkin-Elmer FT 1710 spectrometer and mass spectra using
Kratos Concept 1S (FAB spectra) or Micromass Platform II
(electrospray spectra) instruments. X- (ca. 9.6 GHz) and Q-
band (ca. 34 GHz) spectra on fluid and frozen solutions were
recorded on a Bruker ESP 300E spectrometer. Spectrum
manipulations were performed using the Bruker software
supplied with the spectrometer. Spectral simulations were per-
formed using in-house software which has been described else-
where.31 ENDOR spectra were recorded on frozen solutions at
10 K using a Bruker ESP 300E ESR/ENDOR spectrometer
fitted with an ESP 360 DICE ENDOR unit and an ENI A300
RF amplifier operating at 8 dB and 12.5 kHz modulation
frequency. Cyclic voltammetric studies were carried out, as
described previously,7 at a carbon working electrode (area 0.28
Fig. 6 ENDOR spectrum of [Mo(NCS)(dppe)(η-C7H7)][PF6], 7b, in
CH2Cl2–toluene at 10 K and ν = 9.465 GHz (a) CW EPR spectrum
indicating the field position for the ENDOR experiment. (b) 31P
ENDOR spectrum (* indicates instrumental artifacts).
estimate of the s- and p-orbital character of phosphorus in the
ground state molecular orbital to be made using the following
equations:
1
–
aiso = 3[a1 ϩ a2 ϩ a3]
1
–
b = 3[a1 Ϫ ¹(a2 ϩ a3)]
¯
²
(where a1 is the a value with the largest magnitude).
% s-orbital character = aiso × 100/a0
% p-orbital character = b × 100/b0
(where a0 = 13306 MHz and b0 = 366.8 MHz, the intrinsic
splittings for an unpaired electron in a 31P s- and p-orbital
respectively28).
cm2) using 0.2 M [NBun ][BF4] as supporting electrolyte in solu-
4
The results, which make no allowance for any dipolar
contribution to the ligand hyperfine splitting from the unpaired
electron on the metal, are given in Table 7. Within the approx-
imations used, the s- and p-orbital character and the ratio of
s:p orbital character from the phosphorus ligand are essentially
independent of the nature of X. Using tabulated values28,29 of
a0(X) and the experimental aiso(X) in Table 5, the ns-orbital
contribution from the ligand X to the ground state molecular
orbital has also been calculated (see Table 5). As for the phos-
phorus ligand, these s-orbital contributions are small, further
supporting the view that the unpaired electron in radical com-
plexes of general formulation [MoX(dppe)(η-C7H7)]zϩ (z = 1 or
2) occupies an essentially metal-based molecular orbital.
tions purged with nitrogen gas. All potentials are referenced
to an aqueous calomel electrode and, under these conditions,
EЊ for the couple [Fe(η-C5H5)2]–[Fe(η-C5H5)2]ϩ is 0.56 V in
CH2Cl2. Microanalyses were conducted by the staff of the
Microanalytical Service of the Department of Chemistry,
University of Manchester.
Preparations
[MoCl(dppe)(ꢀ-C7H7)][PF6] 3b. An acetone solution of
[Mo(OCMe2)(dppe)(η-C7H7)][PF6] [prepared by overnight
reflux of [Mo(η-C6H5Me)(η-C7H7)][PF6] (0.917 g, 2.16 mmol)
with dppe (0.864 g, 2.17 mmol) in acetone (40 cm3)] was treated
with anhydrous LiCl (0.457 g, 10.75 mmol). The resulting green
solution was refluxed for a further 4 h then the solution was
filtered, and the filtrate reduced to dryness in vacuo. Two suc-
cessive recrystallisations of the residue from CH2Cl2–diethyl
Conclusions
Syntheses of an extensive class of 17-electron radicals [MoX-
4386
J. Chem. Soc., Dalton Trans., 1999, 4379–4388

View Article Online
ether gave 3b as a bright yellow solid; yield 0.517 g (31%)
(Found: C, 52.1; H, 5.4. Calc. for C33H31ClMoP3F6: C, 51.8; H,
4.1%). The yellow bromide derivative 2b was similarly prepared
in 10% yield using LiBr in place of LiCl (Found: C, 49.4; H, 3.9.
Calc. for C33H31BrMoP3F6: C, 48.9; H, 3.8%) and the purple-
red iodide derivative 1b in 35% yield using LiI as the reagent
(Found: C, 47.3; H, 4.0. Calc. for C33H31IMoP3F6: C, 46.2;
H, 3.6%).
presence of two further components of the reaction mixture:
[Mo(NCS)(dppe)(η-C7H7)] 7a (ν(CO) (CH2Cl2) 2084 cmϪ1) and
an unidentified species (ν(CO) (CH2Cl2) 2064 cmϪ1). However
all attempts to isolate pure 7a from the mother-liquors of the
reaction mixture were unsuccessful.
[Mo(NCS)(dppe)(ꢀ-C7H7)] 7a. A mixture of [Mo(NCS)-
(dppe)(η-C7H7)][PF6] (0.140 g, 0.18 mmol) and [Co(η-C5H5)2]
(0.038 g, 0.20 mmol) in toluene (40 cm3) was stirred for 25 min
to give a green solution. The reaction mixture was filtered,
reduced to 2 cm3 volume in vacuo and treated with n-hexane to
precipitate 7a as a yellow-brown solid; yield 0.012 g (10%).
[MoF(dppe)(ꢀ-C7H7)][PF6] 4b. An acetone solution of
[Mo(OCMe2)(dppe)(η-C7H7)][PF6] [prepared by overnight
reflux of [Mo(η-C6H5Me)(η-C7H7)][PF6] (0.772 g, 1.82 mmol)
with dppe (0.729 g, 1.83 mmol) in acetone (40 cm3)] was treated
with anhydrous LiF (0.241 g, 9.27 mmol). The resulting green
solution was refluxed for 1 h then the volume was reduced and
the crude product precipitated by addition of diethyl ether. The
crude product was dissolved in thf (20 cm3), the resulting solu-
tion filtered and the product again precipitated by addition of
diethyl ether. Finally the product was dissolved in acetone
(5 cm3) and precipitated as three fractions by slow addition of
diethyl ether. A pure sample of green [MoF(dppe)(η-C7H7)]-
[PF6] was isolated from the final fraction; yield 0.235 g (17%).
[Mo(CN)(dppe)(ꢀ-C7H7)] 8a. An acetone solution of [Mo-
(OCMe2)(dppe)(η-C7H7)][PF6] [prepared by overnight reflux of
[Mo(η-C6H5Me)(η-C7H7)][PF6] (1.134 g, 2.67 mmol) with dppe
(1.065 g, 2.68 mmol) in acetone (80 cm3)] was reduced in vol-
ume to ca. 10 cm3 and then AnalaR methanol (80 cm3) added.
The resulting solution was immediately treated with KCN
(0.83g, 12.77 mmol) then stirred at room temperature for 20
min and finally gently refluxed for 20 min to give a purple-red
reaction mixture which was reduced to dryness in vacuo. The
residue was extracted with CH2Cl2 (60 cm3), filtered to remove
the majority of KCN, and again evaporated to dryness. The
resulting crude purple-red product was dissolved in CH2Cl2 and
transferred to an alumina–n-hexane chromatography column.
Elution with acetone–n-hexane (1:1) increasing to neat acetone
gave a red-pink band which was collected and evaporated to
dryness. Recrystallisation of the crude product from CH2Cl2–
hexane gave pure [Mo(CN)(dppe)(η-C7H7)] as a pink solid,
yield 0.540 g (33%). A second, minor product of this reaction,
eluted from the column with neat acetone after [Mo(CN)(dppe)-
(η-C7H7)] as an orange-red band, was the cyanide bridged
bimetallic [{Mo(dppe)(η-C7H7)}2(µ-CN)][PF6]. Further details
on the synthesis and properties of this system will be reported
in a subsequent paper. The carbon-13 labelled complex
[Mo(13CN)(dppe)(η-C7H7)] 9a was prepared as a pink solid
(yield 0.165 g, 23%) by an identical procedure to that described
for 8a starting from [Mo(η-C6H5Me)(η-C7H7)][PF6] (0.498 g,
1.17 mmol) and dppe (0.467 g, 1.17 mmol) in acetone (40 cm3)
followed by treatment with K13CN (0.223 g, 3.38 mmol) in
acetone–methanol (1:3).
[MoCl(dppe)(ꢀ-C7H6Me)][PF6] 5b. An acetone solution of
[Mo(OCMe2)(dppe)(η-C7H6Me)][PF6] [prepared by overnight
reflux of [Mo(η-C6H5Me)(η-C7H6Me)][PF6] (0.500 g, 1.14
mmol) with dppe (0.457 g, 1.15 mmol) in acetone (40 cm3)] was
treated with anhydrous LiCl (0.247 g, 5.81 mmol). The resulting
yellow solution was refluxed for a further 1 h then reduced to
dryness in vacuo. The residue was dissolved in CH2Cl2 (50 cm3),
the resulting solution filtered, reduced in volume and diethyl
ether added to precipitate the product as a yellow solid. Further
recrystallisation from acetone–diethyl ether gave a pure sample
of yellow [MoCl(dppe)(η-C7H6Me)][PF6]; yield 0.130 g (15%).
[Mo(NCO)(dppe)(ꢀ-C7H7)] 6a. An acetone solution of
[Mo(OCMe2)(dppe)(η-C7H7)][PF6] [prepared by overnight
reflux of [Mo(η-C6H5Me)(η-C7H7)][PF6] (0.641 g, 1.51 mmol)
with dppe (0.602 g, 1.51 mmol) in acetone (40 cm3)] was treated
with KNCO (1.008 g, 12.44 mmol). The resulting brown-green
solution was refluxed for a further 5 h then reduced to dryness
in vacuo. The residue was dissolved in CH2Cl2 (50 cm3), the
resulting solution filtered, reduced in volume and n-hexane
added to precipitate the product as a green solid. Further
recrystallisation from toluene–n-hexane gave a pure sample of
green [Mo(NCO)(dppe)(η-C7H7)]; yield 0.340 g (36%).
[Mo(CN)(dppe)(ꢀ-C7H7)][PF6] 8b. A mixture of [Mo(CN)-
(dppe)(η-C7H7)] (0.207 g, 0.34 mmol) and [Fe(η-C5H5)2][PF6]
(0.110 g, 0.33 mmol) was stirred for 20 min in CH2Cl2 (20 cm3)
to give a yellow solution. The volume of the reaction mixture
was then reduced in vacuo and diethyl ether added to precipitate
the product as a yellow solid which was washed with diethyl
ether and dried; yield 0.113 g (44%). Dichloromethane solu-
tions of the carbon-13 labelled analogue [Mo(13CN)(dppe)-
(η-C7H7)][PF6] 9b for EPR investigations were prepared simi-
larly starting from 9a and [Fe(η-C5H5)2][PF6]. Owing to the
[Mo(NCO)(dppe)(ꢀ-C7H7)][PF6] 6b. A mixture of [Mo-
(NCO)(dppe)(η-C7H7)] (0.200 g, 0.319 mmol) and [Fe(η-
C5H5)2][PF6] (0.107 g, 0.323 mmol) was stirred for 10 min in
CH2Cl2 (20 cm3) to give a green solution. The volume of the
reaction mixture was then reduced in vacuo and diethyl ether
added to precipitate the crude product which was washed with
toluene and dried. Two subsequent recrystallisations from
acetone–diethyl ether gave pure [Mo(NCO)(dppe)(η-C7H7)]-
[PF6] as a bright yellow solid; yield 0.054 g (22%).
small scale, complex 9b was not isolated but the observed shift
᎐
(∆ν) in the ν(C᎐N) stretching frequency following one-electron
᎐
oxidation, identical to that observed in the formation of 8b
from 8a (see Table 3), points to the successful generation
of 9b.
[Mo(NCS)(dppe)(ꢀ-C7H7)][PF6] 7b. An acetone solution of
[Mo(OCMe2)(dppe)(η-C7H7)][PF6] [prepared by overnight
reflux of [Mo(η-C6H5Me)(η-C7H7)][PF6] (1.01 g, 2.38 mmol)
with dppe (0.945 g, 2.37 mmol) in acetone (40 cm3)] was treated
with KNCS (1.92 g, 19.78 mmol). The resulting brown solution
was refluxed for a further 1 h then reduced to dryness in vacuo.
The residue was dissolved in CH2Cl2 (40 cm3) and the resulting
solution filtered and toluene (40 cm3) added. Slow reduction in
volume in vacuo resulted in the precipitation of a red-purple
solid, identified as 7b, which was isolated by filtration and
further recrystallised from CH2Cl2–diethyl ether; yield 0.224 g
(12%). IR spectroscopy on samples from the brown, CH2Cl2–
toluene mother liquors from which 7b was isolated revealed the
[Mo(CNMe)(dppe)(ꢀ-C7H7)][BF4]
10a.
Addition
of
[Me3O][BF4] (0.081 g, 0.55 mmol) to a stirred solution of
[Mo(CN)(dppe)(η-C7H7)] 8a (0.216 g, 0.35 mmol) in CH2Cl2
resulted in a colour change from purple-red to a paler red-pink
solution over the course of 1 h. After this time the reaction
mixture was evaporated to dryness and the residue recrystal-
lised from CH2Cl2–diethyl ether to give 10a as a red-pink solid;
yield 0.055 g (22%). The carbon-13 labelled analogue [Mo-
(13CNMe)(dppe)(η-C7H7)][BF4] 11a was prepared by an identi-
cal procedure (yield, 0.035 g, 60%) starting from 9a (0.050 g
0.082 mmol) and [Me3O][BF4] (0.018 g, 0.12 mmol).
J. Chem. Soc., Dalton Trans., 1999, 4379–4388
4387

View Article Online
3 N. Le Narvor, L. Toupet and C. Lapinte, J. Am. Chem. Soc., 1995,
117, 7129.
[Mo(CNMe)(dppe)(ꢀ-C7H7)][BF4]2 10b. A solution of [Mo-
(CNMe)(dppe)(η-C7H7)][BF4] (0.242 g, 0.34 mmol) in CH2Cl2
(20 cm3) was treated with [NO][BF4] (0.060 g, 0.51 mmol). After
stirring for 5 min the resulting deep red solution was filtered,
reduced in volume and diethyl ether added to precipitate the
crude product. Recrystallisation from CH2Cl2–diethyl ether
gave [Mo(CNMe)(dppe)(η-C7H7)][BF4]2 as a red solid; yield
0.107 g (39%). Dichloromethane solutions of the carbon-13
labelled analogue [Mo(13CNMe)(dppe)(η-C7H7)][BF4]2 11b for
EPR investigations were prepared similarly starting from 11a
and [Fe(η-C5H5){η-C5H4(C(O)CH3)}][BF4]. Owing to the small
4 M. Brady, W. Weng, Y. Zhou, J. W. Seyler, A. J. Amoroso, A. M.
Arif, M. Böhme, G. Frenking and J. A. Gladysz, J. Am. Chem. Soc.,
1997, 119, 775.
5 E. F. Ashworth, J. C. Green, M. L. H. Green, J. Knight, R. B. A.
Pardy and N. J. Wainwright, J. Chem. Soc., Dalton Trans., 1977,
1693.
6 M. L. H. Green and R. B. A. Pardy, Polyhedron, 1985, 4, 1035.
7 J. S. Adams, C. Bitcon, J. R. Brown, D. Collison, M. Cunningham
and M. W. Whiteley, J. Chem. Soc., Dalton Trans., 1987, 3049.
8 C. Bitcon, R. Breeze, P. F. Miller and M. W. Whiteley, J. Organomet.
Chem., 1989, 364, 181.
9 R. W. Grime and M. W. Whiteley, J. Chem. Soc., Dalton Trans.,
1994, 1671.
10 Z. I. Hussain, M. W. Whiteley and E. J. L. McInnes, J. Organomet.
Chem., 1997, 543, 237.
11 R. L. Beddoes, C. Bitcon, R. W. Grime, A. Ricalton and M. W.
Whiteley, J. Chem. Soc., Dalton Trans., 1995, 2873.
12 A. Roberts and M. W. Whiteley, J. Organomet. Chem., 1993, 458,
131.
13 R. L. Beddoes, R. W. Grime, Z. I. Hussain and M. W. Whiteley,
J. Chem. Soc., Dalton Trans., 1996, 3893.
14 C. E. Davies, I. M. Gardiner, J. C. Green, M. L. H. Green, N. J.
Hazel, P. D. Grebenik, V. S. B. Mtetwa and K. Prout, J. Chem. Soc.,
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scale, 11b was not isolated but the observed shift (∆ν) in the
᎐
ν(C᎐N) stretching frequency following one-electron oxidation,
᎐
identical to that observed in the formation of 10b from 10a
(see Table 3), points to the successful generation of 11b.
[Mo(CCH2CH2CH2O)(dppe)(ꢀ-C7H7)][PF6]2 12b. An orange
solution of [Mo(CCH2CH2CH2O)(dppe)(η-C7H7)][PF6] 12a
(0.310 g, 0.387 mmol) in CH2Cl2 (30 cm3) was treated with
[Fe(η-C5H5)2][PF6] (0.128 g, 0.387 mmol) resulting in an
immediate colour change to yellow-green. After 1 h the solvent
was removed in vacuo and the residue washed with toluene and
dried. Recrystallisation from CH2Cl2–diethyl ether gave 12b as
a yellow-orange solid; yield 0.084 g (23%).
15 N. G. Connelly, M. J. Freeman, A. G. Orpen, A. R. Sheehan,
J. B. Sheridan and D. A. Sweigart, J. Chem. Soc., Dalton Trans.,
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16 N. G. Connelly, M. P. Gamasa, J. Gimeno, C. Lapinte, E. Lastra,
J. P. Maher, N. Le Narvor, A. L. Rieger and P. H. Rieger, J. Chem.
Soc., Dalton Trans., 1993, 2575.
17 M. Sato, H. Shintate, Y. Kawate, M. Sekino, M. Katada and
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18 P. M. Treichel and D. C. Molzahn, Synth. React. Inorg. Met.-Org.
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19 S. P. M. Disley, R. W. Grime, E. J. L. McInnes, D. M. Spencer,
N. Swainston and M. W. Whiteley, J. Organomet. Chem., 1998, 566,
151.
[Mo(triphos)(ꢀ-C7H7)][PF6] 13a. The sandwich complex
[Mo(η-C6H5Me)(η-C7H7)][PF6] (0.500 g, 1.179 mmol) was
refluxed in acetone (40 cm3) for 10 min to give a red solution
which was then cooled and treated with triphos (0.737 g, 1.181
mmol). The reaction mixture was refluxed overnight then
reduced to low volume in vacuo and the crude product precipi-
tated by addition of diethyl ether. Subsequent recrystallisation
from CH2Cl2–diethyl ether gave 13a as a lilac solid; yield 0.889 g
(79%).
20 J. Cambridge, A. Choudhary, J. Friend, R. Garg, G. Hill, Z. I.
Hussain, S. M. Lovett and M. W. Whiteley, J. Organomet. Chem.,
1999, 577, 249.
[Mo(triphos)(ꢀ-C7H7)][PF6]2 13b. A mixture of [Mo(triphos)-
(η-C7H7)][PF6] (0.147 g, 0.15 mmol) and [Fe(η-C5H5)2][PF6]
(0.061 g, 0.18 mmol) was stirred for 30 min in CH2Cl2 (15 cm3)
to give a green-blue solution. The volume of the reaction mix-
ture was then reduced in vacuo resulting in the precipitation of
unreacted [Fe(η-C5H5)2][PF6]. Filtration through Celite gave an
orange solution which was treated with small aliquots of
diethyl ether to the onset of precipitation of 13b as a bright
orange solid. The product was recrystallised from acetone–
diethyl ether; yield 0.043 g (25%).
21 M. Tilset, J.-R. Hamon and P. Hamon, Chem. Commun., 1998,
765.
22 M. M. Sabbatini and E. Cesarotti, Inorg. Chim. Acta, 1977, 24, L9.
23 R. S. Iyer and J. P. Selegue, J. Am. Chem. Soc., 1987, 109, 910.
24 N. G. Connelly and W. E. Geiger, Chem. Rev. 1996, 96, 877.
25 S. T. Krueger, B. E. Owens and R. Poli, Inorg. Chem., 1990, 29, 2001.
26 L. Kevan and P. A. Narayana, Disordered Matrices, in Multiple
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Plenum Press, New York, 1979, p. 229 and refs. therein.
27 F. Gerson, Acc. Chem. Res., 1994, 27, 63.
28 J. A. Weil, J. R. Bolton and J. E. Wertz, Electron Paramagnetic
Resonance, Elementary theory and practical applications, John Wiley
and Sons, New York, 1994 and refs. therein.
29 F. E. Mabbs and D. Collison, Electron Paramagnetic Resonance of d
Transition Metal Compounds, Elsevier, Amsterdam, 1992, pp. 427
and 1214.
30 M. Bochmann, M. Cooke, M. Green, H. P. Kirsch, F. G. A. Stone
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31 F. E. Mabbs and D. Collison, Electron Paramagnetic Resonance
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Acknowledgements
We thank the EPSRC for research studentships (to R. W. G.
and A. R.).
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J. Chem. Soc., Dalton Trans., 1999, 4379–4388