Synthesis, redox chemistry, and electronic structure of the butadiynyl and hexatriynyl complexes [Mo{(C≡C)nC≡CR}(L 2)(η-C7H7)]z + (n = 1, 2; Z = 0, 1; R = SiMe3, H; L2 = 2,2′-bipyridine, Ph 2PCH2CH2PPh2)

Article abstract of DOI:10.1021/om3005756

Two series of extended carbon chain butadiynyl and hexatriynyl complexes, [Mo{(C≡C)_nC≡CSiMe₃}(bpy)( η-C ₇H₇)] (n = 1, 2; bpy = 2,2′-bipyridine) and [Mo{(C≡C)_nC≡CR}(dppe)(η-C₇H₇)] (n = 1, R = H, SiMe₃; n = 2, R = SiMe₃; dppe = Ph ₂PCH₂CH₂PPh₂), have been prepared and structurally characterized. The redox chemistry of these complexes has been investigated by cyclic voltammetry, and the 17-electron radical cations resulting from one-electron oxidation have been characterized by spectroelectrochemical IR and UV-visible methods and EPR spectroscopy. DFT calculations on the H-terminated model complexes [Mo{(C≡C) _nC≡CH}(L₂)(η-C₇H₇)] ^z+ (L₂ = bpy, dppe) reveal a largely metal-centered HOMO (z = 0) with a modest increase in carbon chain character with increasing chain length. Spin density calculations for the 17-electron radical cations (z = 1) show large coefficients of spin density at the metal center, consistent with the remarkably high stability of the experimental complexes. However, both DFT theoretical and experimental synthetic studies highlight a distinction between the bpy- and dppe-supported systems. The 17-electron complexes [Mo{(C≡C)_nC≡CSiMe₃}(bpy)( η-C ₇H₇)]PF₆ (n = 1, 2) are unique examples of isolable, metal-stabilized butadiynyl and hexatriynyl radicals. In contrast, the dppe radical [Mo(C≡CC≡CSiMe₃)(dppe)( η-C ₇H₇)]⁺ exhibits chain-centered reactivity, consistent with enhanced coefficients of spin density at C_β and C_δ in the model complex [Mo(C≡CC≡CH)(dppe)( η-C₇H₇)]⁺.

Full text of DOI:10.1021/om3005756

Article
pubs.acs.org/Organometallics
Synthesis, Redox Chemistry, and Electronic Structure of the
Butadiynyl and Hexatriynyl Complexes [Mo{(CC)nCCR}(L2)
(η‑C7H7)]z+ (n = 1, 2; z = 0, 1; R = SiMe3, H; L2 = 2,2′-bipyridine,
Ph2PCH2CH2PPh2)
§
§
Hannah N. Roberts,† Neil J. Brown,‡ Ruth Edge, Emma C. Fitzgerald,† Yien T. Ta,† David Collison,
Paul J. Low,*,‡ and Mark W. Whiteley*,†
†School of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
‡Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
§EPSRC National Service for EPR Spectroscopy, School of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
S
* Supporting Information
ABSTRACT: Two series of extended carbon chain butadiynyl and hexatriynyl
complexes, [Mo{(CC)nCCSiMe3}(bpy)(η-C7H7)] (n = 1, 2; bpy = 2,2′-
bipyridine) and [Mo{(CC)nCCR}(dppe)(η-C7H7)] (n = 1, R = H, SiMe3;
n = 2, R = SiMe3; dppe = Ph2PCH2CH2PPh2), have been prepared and
structurally characterized. The redox chemistry of these complexes has been
investigated by cyclic voltammetry, and the 17-electron radical cations resulting
from one-electron oxidation have been characterized by spectroelectrochemical
IR and UV−visible methods and EPR spectroscopy. DFT calculations on the H-
terminated model complexes [Mo{(CC)nCCH}(L2)(η-C7H7)]z+ (L2
=
bpy, dppe) reveal a largely metal-centered HOMO (z = 0) with a modest
increase in carbon chain character with increasing chain length. Spin density
calculations for the 17-electron radical cations (z = 1) show large coefficients of
spin density at the metal center, consistent with the remarkably high stability of
the experimental complexes. However, both DFT theoretical and experimental
synthetic studies highlight a distinction between the bpy- and dppe-supported systems. The 17-electron complexes [Mo{(C
C)nCCSiMe3}(bpy)(η-C7H7)]PF6 (n = 1, 2) are unique examples of isolable, metal-stabilized butadiynyl and hexatriynyl
radicals. In contrast, the dppe radical [Mo(CCCCSiMe3)(dppe)(η-C7H7)]+ exhibits chain-centered reactivity, consistent
with enhanced coefficients of spin density at Cβ and Cδ in the model complex [Mo(CCCCH)(dppe)(η-C7H7)]+.
metal auxiliary, leading to the rapid formation of secondary
products.5 One strategy to promote a more controlled redox
chemistry is the introduction of a sterically demanding
protecting group R at the nonmetal chain terminus, but this
is unlikely to be effective for longer chains (n > 1). An
alternative approach, employed in the current work, is the use
of a supporting metal auxiliary in which the relative
contributions of the metal and carbon chain ligand to the
frontier orbitals can be fine-tuned; in this way reactivity at the
diynyl/polyynyl ligand can be attenuated via a redox orbital
with reduced carbon chain character. In a recent DFT study, we
have shown that alkynyl complexes of the cycloheptatrienyl
molybdenum auxiliary Mo(dppe)(η-C7H7) possess a strongly
INTRODUCTION
■
Monometallic diynyl and polyynyl complexes of the type
M{(CC)nCCR} represent an important class of organo-
metallic compounds containing linear “carbon-rich” ligands.1
However, in contrast with the remarkable success in the
preparation and crystallographic characterization of bimetallic
compounds containing bridging carbon ligands of exceptional
length,2 investigations on monometallic systems featuring
polyynyl ligands are restricted mainly to synthetic studies on
butadiynyl (n = 1) complexes,3 with fewer examples of
hexatriynyl (n = 2) and longer chain species.4
The electronic properties of metal alkynyl complexes MC
CR have been reported in detail for many metal systems,5 but
very little work has been carried out to probe the effect of
extending the chain length on the electronic structure6 and
redox chemistry of these systems.3c,e A key issue, central to a
productive investigation of the redox chemistry of M{(C
C)nCCR} is the moderation of reactivity at the carbon chain
in activated radical complexes; the remote carbons are
unprotected by the immediate coordination sphere of the
2
metal-centered HOMO with significant dz character (the z axis
is directed along the metal ring−centroid vector). As a
consequence, the effectiveness of the interaction between the
metal and filled π orbitals on the alkynyl ligand is symmetry
Received: June 22, 2012
© XXXX American Chemical Society
A
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constrained, manifested experimentally by limited redox-
induced perturbation of the alkynyl ligand and the exceptional
stability of 17-electron radicals of the type [Mo(CCR)-
(dppe)(η-C7H7)]+.7 In the current work, this principle is
developed to examine the interplay of the metal orbitals and
carbon π system in extended chain complexes [Mo{(C
C)nCCR}(L2)(η-C7H7)]z+ (L2 = bpy, dppe; n = 1, 2). The
balance between metal and carbon character in the frontier
orbitals leads to the stabilization of 17-electron radicals, thereby
permitting unprecedented spectroscopic, structural, and re-
activity studies on metal-stabilized butadiynyl and hexatriynyl
radicals. Some aspects of this work on the redox chemistry of
the butadiynyl systems have recently been communicated.8
current work, the reaction sequence developed for the diynyl
complexes 1 and 3 was successfully extended to the hexatriynyl
analogues 5 and 6 (Scheme 1, n = 2). Thus, reaction of a thf
solution of Li{(CC)2CCSiMe3} (prepared from
Me3SiCCCCCCSiMe3 and MeLi/LiBr) with [MoBr-
(bpy)(η-C7H7)] afforded a deep purple-red solution of
[Mo{(CC)2CCSiMe3}(bpy)(η-C7H7)] (5), which was
isolated in moderate yield as a deep purple solid following
purification by chromatography on alumina. The ligand
substitution of bpy by dppe proceeded in refluxing toluene to
give [Mo{(CC)2CCSiMe3}(dppe)(η-C7H7)] (6) as a
deep green solid which was purified by chromatography on
Celite.
The new butadiynyl and hexatriynyl complexes 2, 5, and 6
were characterized by microanalysis, mass spectrometry,
RESULTS AND DISCUSSION
■
1
infrared spectroscopy, and H, 31P{1H} and 13C{1H} NMR
Synthetic Studies. Several general methods are known for
the preparation of metal complexes featuring butadiynyl,
hexatriynyl, and higher polyynyl ligands. These include reaction
of an appropriately ligated metal halide with Li{(CC)nC
CR},3a,e,4d transmetalation reactions,3b,9 and chain modification
and extension protocols in which complexes of the type
M{(CC)nCCH} are elaborated by variations on the
Sonogashira, Hay, and Cadiot-Chodkiewicz cross coupling
reactions.4a,c We have previously described the synthesis of the
butadiynyl complex [Mo(CCCCSiMe3)(bpy)(η-C7H7)]
(1; bpy = 2,2′-bipyridine) by direct reaction of [MoBr(bpy)(η-
C7H7)] with LiCCCCSiMe3 (prepared from Me3SiC
CCCSiMe3 and MeLi/LiBr complex) (Scheme 1, n = 1),10
spectroscopy (see the Experimental Section and Table 1).
Parameters diagnostic of the presence of the carbon chain
ligand in these complexes are the IR-active ν(CC) stretching
absorptions and the Cα to Cζ resonances in the 13C{1H} NMR
spectrum; these key data are summarized in Table 1. Data for
the diynyl complexes 1, 3, and 4 have been reported
previously10 but are included in Table 1 to facilitate direct
comparisons between butadiynyl and hexatriynyl systems.
Table 2 presents reported ν(CC) IR data and 13C{1H}
NMR chemical shifts for carbon chain resonances where data
are available for direct comparison between diynyl and triynyl
systems. In the diynyl/triynyl pairs 1 and 5, and 3 and 6,
substitution of the C4 with the C6 ligand is evident in a number
of trends. In the IR spectra of both series of complexes, the
ν(CC) absorptions fall into two regions: 2150−2090 and
1990−1950 cm−1. However, comparison of analogous diynyl
and triynyl complexes reveals a difference in the intensity of the
ν(CC) absorption in the range 1990−1950 cm−1; in the
diynyl complexes 1−4, this is the weakest ν(CC) band,
whereas in the triynyl complexes 5 and 6, it is the most intense
band. In the 13C{1H} NMR spectra, the diynyl complexes 1−4
exhibit four resonances for the carbon chain Cα−Cδ, with
chemical shifts decreasing along the series to the terminal chain
carbon Cδ, which has the lowest δ value. However, on further
chain extension to hexatriynyl systems, in general, the central
carbons Cγ and Cδ are located to high field of the end of chain
carbons Cε and Cζ (see Table 2). For complex 6, this pattern of
assignment is supported by the observation of phosphorus
coupling to the resonance at 66.2 ppm attributed to Cγ. An
additional feature of chain extension from C4 to C6 is a shift of
Cα to low field by 5−10 ppm; this trend is also observed for the
literature examples presented in Table 2.
Structural Investigations. While there are now many
examples of structurally characterized bimetallic complexes of
general form M{μ-(CC)n}M,2a only a limited series of
structural characterizations of monometallic carbon chain
complexes M{(CC)nCCR} with n > 1 have been
reported,1,4b,13 and examples which enable a direct comparison
of the structural effects of increasing the carbon chain length
against a common metal−ligand fragment are even more
restricted.4a,e,f We have previously reported the X-ray crystal
structures of the butadiynyl complexes 1 and 410 and the 17-
electron radical [1]PF6,8 and in the current work, we describe
the expansion of this series to include 3 and the hexatriynyl
complexes 5 and 6. The molecular structures of 3, 5, and 6
together with atomic numbering schemes are presented in
Figures 1−3, respectively, and key bond lengths and angles are
Scheme 1. Synthetic Routes to [Mo{(CC)nC
a
CR}(L2)(η-C7H7)] Complexes
a
Reagents and conditions: (i) Li{(CC)nCCSiMe3} in thf, n = 1,
2.5 h and n = 2, 4.5 h; (ii) dppe, toluene reflux, 3 h; (iii) nBu4NF/thf,
1.5 h.
and in the current work this methodology was extended to the
substituted bpy derivative [Mo(CCCCSiMe3)(tBu2-bpy)-
t
(η-C7H7)] (2; Bu2-bpy = 4,4′-tBu2-2,2′-bipyridine). Substitu-
tion of the bpy ligand of 1 for dppe gave [Mo(CCC
CSiMe3)(dppe)(η-C7H7)] (3), which was desilylated with
nBu4NF in thf to give [Mo(CCCCH)(dppe)(η-C7H7)]
(4).10
The formation of the hexatriynyl reagent Li{(CC)2C
CSiMe3} from Me3SiCCCCCCSiMe3 and MeLi/LiBr
complex4d,11 is more sensitive to conditions than the
preparation of the analogous butadiynyl system, but in the
B
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a
b
Table 1. IR and 13C{1H} NMR Data for [Mo{(CC)nCCR}(L2)(η-C7H7)]
δ, ppm
complex
IR ν(CC), cm−1
Cα
Cβ
95.1
Cγ
91.6
Cδ
Cε Cζ SiMe3 C7H7
L2 = bipy, dppe
1
2151 (m), 2111 (m), 1991 (w)
142.1
76.7
−0.4
0.2
90.0 149.1, CA; 147.6, CE; 130.8, CC; 121.3, 120.5,
CB, CD
2
3
4
2151 (m), 2110 (m), 1987 (w)
144.3
96.7
92.2
77.4
75.7
59.5
89.0 156.1, CC; 149.5, CA; 148.6, CE; 119.8, 118.1,
CB, CD; 34.9, CMe3; 30.7, CMe3
2153 (w), 2105 (sh), 2090 (m), 142.8, t
105.1, t 93.0, t
(2)
0.6
87.8 26.9, m, PCH2; 140.5−128.1, m, PPh2
2063 (sh), 1976 (w)
(25)
(4)
c
2098 (m), 1956 (w)
138.7, t
(25)
103.6
72.7, t
(4)
87.7 26.8, m, PCH2; 140.5−128.0, m, PPh2
d
5
2131 (s), 2106 (m), 1967 (s)
2121 (m), 2092 (m), 1949 (s)
6
151.9, t
(25)
105.1
66.2, t
(5)
57.0
91.4,
83.5
0.0
88.0 26.7, m, PCH2; 140.2−128.1, m, PPh2
a
b
CH2Cl2 solution spectra. 100 MHz spectra. Abbreviations: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad. Chemical shifts are
downfield from SiMe4, with coupling constants in Hz (given in parentheses). In CD2Cl2 solution unless stated otherwise. Carbon chain carbons
identified as α, β, γ, δ, ε, and correspond to carbons 1−6, respectively, along the carbon chain with Cα (C(1)) attached to the metal center;
ζ
c
d
assignment of bpy carbons follows the lettering shown in Scheme 1. IR: 3301 cm−1 (m), CH. 13C NMR data for complex 5 not obtained due to
poor solution stability.
a
b
Table 2. Comparison of IR and 13C{1H} NMR Data for Selected Diynyl and Triynyl Complexes
δ, ppm
complex
IR ν(CC) cm−1
Cα
Cβ
Cγ
Cδ
Cε
Cζ
ref
[Fe(CCCCH)(CO)2Cp*]
2141
106.4
114.6
120.1
92.8
93.7
71.9
64.5
96.6
69.8
53.5
48.6
66.9
49.4
80.6
12
11
3f
[Fe{(CC)2CCH}(CO)2Cp*]
2158, 2100
70.4
77.9
61.7
92.2
c
[Ru(CCCCSiMe3)(dppe)Cp*]
[Ru{(CC)2CCSiMe3}(dppe)Cp*]
[Re(CCCCSiMe3)(NO)(PPh3)Cp*]
[Re{(CC)2CCSiMe3}(NO)(PPh3) Cp*]
[Re(CCCCH)(NO)(PPh3)Cp*]
[Re{(CC)2CCH}(NO)(PPh3) Cp*]
2171 (w), 2095 (m), 1990, (w)
115.8
94.1
c
2110 (s), 1971 (s)
137.6, t (23)
4c
3c
4a
3c
4a
2118 (m), 2098 (m),
105.8, d (15.9)
113.3, d (15.9)
102.1, d (15.9)
113.6, d (15.4)
112.3
112.8
110.8
111.2
93.2, d (2.7)
66.5, 63.8
d
2132 (m), 2109 (m), 1979 (vs)
92.2, 86.3
e
2115 (s), 1975 (w)
72.4
65.2
61.5
d
2140 (s), 2080 (m), 1970 (m)
63.7, d (2.8)
70.3
c
68.4
a
b
d
IR data in CH2Cl2 solution unless stated otherwise. 2J(P−C) in Hz, given in parentheses. Abbreviations: d = doublet, t = triplet. In Nujol. In thf
e
solution. IR: 3305 cm−1, CH.
Figure 2. Plot of the molecular structure of 5 showing the atom
labeling scheme.
A key structural feature is the contraction of the Mo−Cα
bond length as the carbon chain is elongated. Thus, in the dppe
series of complexes, the Mo−Cα distance of 2.138(2) Å in the
alkynyl complex7 [Mo(CCBut)(dppe)(η-C7H7)] decreases
to 2.117(2) Å in 3 with a further reduction to 2.100(3) Å in the
hexatriynyl derivative 6. These changes in the metal−Cα bond
length are consistent with the results of electronic structure
calculations on the model complexes [Ru{(CC)nCCR}-
(PH3)2Cp] and have been attributed to enhanced Coulombic
attraction arising from greater charge separation Mδ+−{(C
C)nCCR}δ− between the metal and chain fragments with an
increase in chain length.6b In the bpy series, the Mo−Cα bond
length in 1 is slightly longer than for the dppe analogue 3, and
this trend probably extends to the triynyl analogues 5 and 6.
Figure 1. Plot of the molecular structure of 3 showing the atom
labeling scheme. In this and all subsequent structural figures, thermal
ellipsoids are plotted at the 50% probability level. Hydrogen atoms and
solvent of crystallization have been omitted for clarity.
summarized in Table 3 alongside structural data for 1, [1]PF6,
and 4. With the exception of complexes [1]PF6 and 5, the
structural data are of good precision, and the series of closely
analogous complexes allow a valid comparison of key structural
parameters.
C
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Table 4. Cyclic Voltammetric Data for [Mo{(CC)nC
a
CR}(L2)(η-C7H7)]
compd
E1/2 (V)
1
2
3
4
5
6
−0.66
−0.73
−0.58
−0.58
−0.64
−0.53
b
[Mo(CCBut)(dppe)(η-C7H7)]
−0.82
−0.72
b
[Mo(CCPh)(dppe)(η-C7H7)]
a
All potentials for complexes 1−6 are reported vs FeCp2/[FeCp2]+ =
0.00 V, from 0.1 M nNBu4PF6 /CH2Cl2 solutions at ambient
b
temperature at a Pt working electrode. Data from ref 7.
separation between cathodic and anodic peak potentials
comparable to that determined for the internal ferrocene
standard.
Figure 3. Plot of the molecular structure of 6 showing the atom
labeling scheme.
Descriptions of the electrochemical properties of mono-
metallic butadiynyl and hexatriynyl complexes M{(CC)nC
CR are very limited,3c,e,f and in the majority of cases the redox
processes are chemically irreversible, making attempts to study
the redox products difficult. The current work therefore
provides a rare opportunity to make systematic comparisons
over an extended series (n = 0−2). The E1/2 values for the
reversible one-electron oxidation of [Mo{(CC)nCCR}-
(L2)(η-C7H7)] show a modest dependence upon chain length
(n) and the supporting ligand L2. As the chain length increases,
the general trend is a shift in E1/2 to positive potential,
consistent with a decrease in σ-donor character of the
polyyne.6b Thus, the diynyl complex 3 is harder to oxidize
than related alkynyl complexes by 0.15−0.25 V, although
further chain extension to the hexatriynyl derivative 6 leads to a
much smaller change in E1/2 (0.05 V). A minor shift in E1/2 to
positive potential with an increase in chain length is also
observed for the bpy complexes 1 and 5, with substitution of
dppe by bpy resulting in a small shift in E1/2 to negative
potential by ca. 80 mV, reflecting the strongly electron-
donating character of the bpy ligands. In principle, E1/2 values
For the SiMe3- terminated derivatives some additional
shortening of Mo−Cα may result from hyperconjugation
effects,14 although a comparison of data for 3 with those for
the H-terminated analogue 4 reveals a significant difference
only in the Cγ−Cδ distance. A second feature of note is the
curvature of the carbon chain, which can be expressed by the
parameter ∑(bend) (Table 3). Significant distortion from
linearity of extended sp carbon chains has been reported
previously, analyzed in detail, and attributed to the low bending
force constant associated with the −CC− moiety and
underlying crystal-packing phenomena.4a,15 Therefore, further
discussion of this effect will not be made here other than to
draw attention to the bending of the hexatriynyl chain in 5 and
6, apparent from Figures 2 and 3.
Electrochemistry. The electrochemical response of the
complexes [Mo{(CC)nCCR}(L2)(η-C7H7)] (1−6) was
examined by cyclic voltammetry; the results are presented in
Table 4. Under the conditions given in Table 4, each of the
complexes undergoes a diffusion-controlled, chemically and
electrochemically reversible, one-electron oxidation with the
Table 3. Key Structural Parameters for [Mo{(CC)nCCR}(L2)(η-C7H7)]z+
1
3
4
5
6
[1]PF6
Bond Lengths (Å)
2.114(3)
Mo−Cα
Cα−Cβ
Cβ−Cγ
Cγ−Cδ
Cδ−Cε
Cε−Cζ
Cδ−R/Cζ−R
Mo−N/Mo−P
2.135(2)
1.207(3)
1.382(3)
1.214(3)
2.118(2)
1.227(3)
1.373(3)
1.220(4)
2.106(13)
1.215(8)
1.393(9)
1.194(8)
1.366(9)
1.200(9)
1.854(9)
2.156(4)
2.141(4)
2.100(3)
1.235(4)
1.357(4)
1.220(4)
1.368(4)
1.215(4)
1.841(3)
2.4682(9)
2.4640(9)
2.11(2)
1.24(2)
1.35(2)
1.20(2)
1.223(4)
1.369(4)
1.198(4)
1.834(2)
2.134(2)
2.142(2)
1.854(3)
2.4543(8)
2.4675(9)
1.82(2)
2.19(1)
2.15(2)
2.4726(7)
2.4578(6)
Bond Angles (deg)
174.6(2)
Mo−Cα−Cβ
Cα−Cβ−Cγ
Cβ−Cγ−Cγ
Cγ−Cδ−Cε
Cδ−Cε−Cζ
174.5(2)
175.6(2)
176.4(2)
176.0(2)
171.9(3)
176.6(3)
174.3(16)
178.0(2)
177.0(2)
177.8(12)
177.4(9)
176.6(9)
19
171.5(2)
173.9(3)
176.3(3)
175.6(3)
177.6(3)
172.4(2)
33
171.1(2)
175.6(2)
169.9(2)
175.2(3)
178.8(3)
Cγ−Cδ−R/Cε−Cζ−R
170.3(2)
23
172.6(2)
23
170.2(2)
33
a
∑(bend)
a
∑(bend) is the sum of deviations of angles from 180° along the carbon chain.
D
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are also sensitive to the carbon chain substituent R but, as
observed in other systems,3f exchange of SiMe3 for H in the
butadiynyl complexes 3 and 4 has a very small or negligible
effect on the magnitude of E1/2
.
IR and UV−Visible Spectroelectrochemical Analysis of
[Mo{(CC)nCCR}(L2)(η-C7H7)]z+. The reversibility of the
one-electron-oxidation processes associated with the butadiynyl
and hexatriynyl complexes under investigation suggested that
the corresponding 17-electron radicals might be sufficiently
stable for spectroscopic analysis and, potentially, isolation. In
general, one-electron oxidation of butadiynyl or hexatriynyl
metal complexes leads to highly reactive species, and therefore
the chemical stability of extended carbon chain complexes
based on the Mo(L2)(η-C7H7) auxiliary provides a potential
entry point to some unique chemistry. However, before
probing the synthetic redox chemistry of the 17-electron
radicals [1]+−[6]+, we investigated their characterization by IR,
UV−visible, and EPR spectroscopy using samples generated
either by spectroelectrochemical methods or in situ by addition
of [FeCp2]PF6 to solutions of the 18-electron precursors.
Guided by the results of the cyclic voltammetry, we carried
out an IR spectroelectrochemical investigation on each of the
butadiynyl complexes 1−4 and the hexatriynyl derivatives 5 and
6. With the exception of the unprotected chain complex 4, the
stability of the radical cation (and hence the validity of the
assignment of the spectroscopic data to the redox product) was
established by back-reduction, which resulted in full recovery of
the IR spectrum assigned to the 18-electron precursor.
Spectroelectrochemically generated IR spectra for [1]z+, [3]z+,
and [5]z+ (z = 0, 1) in the ν(CC) region are presented in
parts a−c, respectively, of Figures 4, and a summary of the data
obtained is given in Table 5.
A detailed analysis of redox-induced changes in the ν(CC)
stretching frequencies of metal diynyl and triynyl systems is far
more involved than for alkynyl derivatives because of the
multiple absorptions observed for the extended chain ligands
and is therefore not presented in the current discussion.
However, only small shifts in absorption frequencies were
observed following one-electron oxidation (for example in 1, all
the bands shift to higher wavenumber by just 6−7 cm−1) and
these small changes may reflect the largely metal-centered
redox process imposed by the Mo(L2)(η-C7H7) system.
The UV−visible spectra of the cycloheptatrienyl molybde-
num alkynyl complexes [Mo(CCR)(dppe)(η-C7H7)]z+
exhibit distinctive visible region MLCT/LMCT charge transfer
bands involving the alkynyl ligand. For example, in the case of
[Mo(CCPh)(dppe)(η-C7H7)]z+ (z = 0, 1), the neutral 18-
electron species exhibits a low-intensity MLCT band centered
at 18 900 cm−1, whereas in the 17-electron radical, a strong
LMCT (C2Ph → metal) band is observed around 16 500 cm−1,
giving rise to the intense blue coloration of the complex.7 In the
current investigation, the evidence for the related involvement
of extended chain, diynyl, and triynyl ligands in MLCT and
LMCT processes was examined. In view of the results of the IR
spectroelectrochemical investigations, UV−visible studies were
not made on H-terminated [4]z+ due to the limited stability of
the 17-electron radical. Table 6 summarizes experimental data
for complexes 1, 3, 5, and 6 together with the results of a TD-
DFT treatment (see later) of the H-chain-terminated model
complexes [Mo{(CC)nCCH}(L2)(η-C7H7)]z+ to assist in
the assignment of the experimental spectra. The experimental
spectra for [3]z+, [6]z+, and [5]z+ are illustrated in parts a−c,
respectively, of Figure 5.
Figure 4. IR spectra (ν(CC) region) obtained in a spectroelec-
CSiMe3)(bpy)(η-C7H7)]z+ ([1]z+), (b) [Mo(CCCCSiMe3)-
(dppe)(η-C7H7)]z+ ([3]z+), and (c) [Mo{(CC)2CCSiMe3}-
(bpy)(η-C7H7)]z+ ([5]z+) (z = 0, 1).
n
trochemical cell (CH2Cl2/0.1 M NBu4PF6) of (a) [Mo(CCC
The dppe complexes [3]z+ and [6]z+ are discussed first since
these are free of additional absorptions arising from transitions
to the bpy π* level, characteristic of complexes [1]z+ and [5]z+.
As is evident from Figure 5a, the principal features of interest in
the spectra of [3]z+ are the absorptions at 17 390 and 19 690
cm−1 in the 17-electron radical, assigned with the aid of a DFT
treatment as LMCT bands involving the C4 chain. As the chain
E
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Organometallics
Article
length increases to the hexatriynyl complexes [6]z+, in the 17-
electron radical, the absorption associated with the LMCT
process from the extended carbon chain appears as a broad,
weak feature shifted to higher energy (21 640 cm−1) by
comparison with the butadiynyl analogue.
Table 5. Spectroelectrochemical Infrared Spectroscopic Data
for the Complexes [Mo{(CC)nCCR}(L2)(η-C7H7)]z+ (z
n
= 0, 1) Measured in 0.1 M NBu4PF6/CH2Cl2
ν(CC) (cm−1
)
complex
z = 0
z = 1
The UV−visible spectrum of the 18-electron bpy complex 1
exhibits three strong absorptions in the visible region around
13 000, 19 000, and 22 000 cm−1. These features (which are
also present in the UV−visible spectrum of [MoBr(bpy)(η-
C7H7)]) are attributed to MLCT processes, arising principally
from transitions from the metal to the bpy π* level. In the
visible region of the spectrum of the 17-electron radical [1]+,
the strong MLCT features of the 18-electron complex are
replaced by two weaker absorptions centered around 18 700
and 20 900 cm−1, assigned as LMCT/LLCT transitions from
1
2151 (m), 2111 (m), 1991 (w)
2158 (w), 2117 (w), 1998
(m)
2
3
2151 (m), 2110 (m), 1987 (w)
2158 (w), 2116 (w), 1999
(m)
2153 (w), 2105 (sh), 2090 (m),
2063 (sh), 1976 (w)
2151 (vw), 2110 (vw), 2095
(vw), 1970 (m)
4
5
6
2098 (m), 1956 (w)
1952 (m)
2131 (m), 2106 (m), 1967 (s)
2121 (m), 2092 (m), 1949 (s)
2146 (m), 1965 (vw)
2138 (vw), 2114 (vw), 2103
(vw), 1934 (m)
a
Table 6. UV−Visible Data for Selected Members of the Series [Mo{(CC)nCCR}(L2)(η-C7H7)]z+ (n = 1, 2; z = 0, 1)
z
exptl transition/cm−1 (R = SiMe3) (ε/dm3 mol−1 cm−1
)
calcd transition/cm−1 (R = H) (intensity)
character of transition
[Mo(CCCCR)(dppe)(η-C7H7)]z+ (3)
0
43 480 (20 060)
37 500 (0.087 5)
Mo→dppe
32 790 (15 770)
34 400 (0.057 8)
24 700 (0.046 1)
MoC4(C7H7)→dppe
MoC4(C7H7)→dppe
28 490 (8 000), sh, 23 980 (4 000), sh
17 640 (800), vbr
+1
43 860 (18 700), 36 100 (13 970)
32 900 (0.069 6)
25 100 (0.068 7)
20 000 (0.000 3)
17 300 (0.003 7)
16 800 (0.046 9)
10 300 (0.002 2)
dppe→MoC7H7
C4→MoC7H7
19 690 (2 170), 17 390 (2 790)
C4→MoC7H7
C4→MoC7H7
dppe/C4→MoC7H7
C4→MoC7H7
[Mo{(CC)2CCR}(dppe)(η-C7H7)]z+ (6)
0
43 860 (24 810)
36 400 (0.140 5)
32 750 (0.187)
MoC6→dppe
MoC6→dppe
Mo→dppe
38 290 (18 920)
29 790 (4 520), 26 000 (3 780)
22 090 (890)
23 800 (0.043 3)
17 700 (0.001 6)
16 600 (0.002 3)
30 700 (0.158 1)
23 950 (0.116 7)
14 600 (0.073 4)
10 300 (0.009 8)
Mo→C7H7
MoC6→dppe/C7H7
MoC6→dppe
C6→MoC7H7
C6→Mo
+1
0
43 860 (22 740), 37 590 (16 850)
21 640 (1 120), vbr
C6→Mo
[Mo(CCCCR)(bpy)(η-C7H7)]z+ (1)
40 980 (19 680)
34 840 (14 950)
30 960 (8 490), sh
22 120 (2 920)
18 450 (2 450)
12 870 (3 820)
43 670 (19 960)
35 210 (11 990)
29 150 (4 000), sh
20 920 (1 200)
18 730 (1 010)
44 500 (0.145 5)
36 900 (0.332 1)
24 500 (0.091)
21 900 (0.029 1)
19 200 (0.023 6)
13 800 (0.063 5)
38 500 (0.083)
36 200 (0.092 4)
25 900 (0.060 6)
24000 (0.034 5)
15 600 (0.050 9)
C7H7→bpy
bpy→bpy
C4→bpy
Mo→C7H7
Mo→bpy
Mo→bpy
Mo→bpy
MoC4→bpy
MoC4→bpy
C4→Mo
+1
C4→Mo
[Mo{(CC)2CCR}(bpy)(η-C7H7)]z+ (5)
0
43 600 (24 000) vbr
37 000 (0.303 1)
32 600 (0.383 8)
23 600 (0.083 1)
13 700 (0.064 2)
37 400 (0.207)
C7H7→C6
Mo→C7H7
MoC6→bpy
MoC6→bpy
MoC6→bpy
MoC6→bpy
MoC6→bpy
C6→Mo
33 440 (15 400)
26 870 (8 060), 21 200 (3 050), 18 820 (1 930)
12 690 (2 450)
+1
40 720 (19 840), 33 340 (12 380)
27 600 (5 850)
27 200 (0.085 8)
22 800 (0.080 5)
13 600 (0.080 6)
18 900 (1 150), 16 625 (1 020)
a
n
νmax/ cm−1, (ε/ dm3 mol−1 cm−1) determined by spectroelectrochemical methods in 0.1 M NBu4PF6/CH2Cl2
F
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EPR Investigations. As reported previously, 17-electron
radical cations of the type [MoX(L2)(η-C7H7)]+ (L2 = bpy,
dppe) exhibit isotropic X-band EPR solution spectra with well-
resolved hyperfine couplings to 95/97Mo, 14N/31P, and 1H of the
cycloheptatrienyl ring.16,17 DFT electronic structure calcula-
tions on the alkynyl radicals [Mo(CCR)(dppe)(η-C7H7)]+
reveal that spin density is located principally at the metal center
in a molecular orbital with significant metal dz character.7
2
However, some spin density does extend onto the alkynyl
ligand, as demonstrated experimentally by the observation of
hyperfine coupling to 19F in the 17-electron radical [Mo(C
CC6H4-4-F)(dppe)(η-C7H7)]+.18 Therefore, in the current
work, direct experimental evidence for delocalization of spin
density onto the diynyl or triynyl chain was investigated by
exploiting the advantageous EPR characteristics of the Mo-
(L2)(η-C7H7) system.
Samples of the 17-electron radicals [1]+−[6]+ were
generated by in situ oxidation with [FeCp2]PF6 in CH2Cl2
followed by immediate cooling to 243 K (the optimum
temperature for resolution of isotropic hyperfine couplings in
complexes of this type).16 Although the radical [Mo(CCC
CH)(dppe)(η-C7H7)]+ ([4]+) was difficult to observe in the
ambient-temperature spectroelectrochemical work, its stability
was enhanced by the low-temperature conditions employed for
the EPR studies. A full analysis of the EPR spectral parameters
for [Mo{(CC)nCCR}(L2)(η-C7H7)]+ (confirmed by
simulation) is presented in Table 7, and representative EPR
spectra of [1]+, [3]+, and [4]+ (as second derivatives) are
illustrated in parts a−c, respectively, of Figure 6.
Table 7. EPR Data for the 17-Electron Radicals [Mo{(C
a
C)nCCR}(L2)(η-C7H7)]+
aiso(Mo) aiso(31P)/aiso(14N)
aiso(1H)
4.6 (C7H7)
giso
[1]+
[2]+
[3]+
[4]+
[5]+
[6]+
38
39
32
32
38
31
2.1
1.975
1.972
1.996
1.996
1.974
1.995
not obsd
22.8
not obsd
4.3 (C7H7)
23.0
4.3 (C7H7), 1.9 (C4H)
not obsd
not obsd
22.6
4.1 (C7H7)
a
X-band solution spectra in CH2Cl2 at 243 K; hyperfine couplings in
gauss.
The spectra obtained are typical of those previously reported
for radicals of the type [MoX(L2)(η-C7H7)]z+ (L2 = bpy, dppe),
with the main difference between the spectral patterns of the
bpy complexes [1]+, [2]+, and [5]+ and those of the dppe
complexes [3]+, [4]+, and [6]+ attributable to the much smaller
hyperfine coupling to 14N than to 31P. In fact, only in the case
of [1]+ was the hyperfine coupling sufficiently well resolved to
extract the aiso(14N) parameter. To probe the delocalization of
spin density onto the carbon chain ligand, two strategies were
investigated. First, the effect of chain elongation on hyperfine
Figure 5. UV−visible spectra obtained in a spectroelectrochemical cell
n
(CH2Cl2/0.1 M NBu4PF6) of (a) [Mo(CCCCSiMe3)(dppe)(η-
C7H7)]z+ ([3]z+), (b) [Mo{(CC)2CCSiMe3}(dppe)(η-C7H7)]z+
([6]z+), and (c) [Mo{(CC)2CCSiMe3}(bpy)(η-C7H7)]z+ ([5]z+)
(z = 0, 1).
1
the carbon chain to the metal center or bpy π* level in accord
with the predictions of the TD-DFT calculations. The pattern
of the visible region spectrum of [1]+ is almost identical with
that of the dppe analogue [3]+, except that the bands are blue-
shifted by ca. 1000 cm−1 in the bpy complex. As the chain
length is increased to the hexatriynyl complexes [5]n+, the key
features observed in the visible region of [1]n+ are retained,
including twin absorptions for the cation [5]+ (16 625, 18 900
cm−1), which are assigned to LMCT/LLCT processes from the
C6 chain to the metal center or bpy ligand.
couplings to 95/97Mo, 14N/31P, and H was examined, with the
expectation that increased delocalization of spin density onto
the carbon chain would correlate with a reduction in hyperfine
1
coupling to 95/97Mo and possibly to 14N/31P and H of the
C7H7 ring. However, any effect of chain length on aiso values is
very small, with data for [6]+ very similar to those of the alkynyl
complex [Mo(CCPh)(dppe)(η-C7H7)]+ (aiso(Mo) = 31 G;
aiso(31P) = 22.6 G; aiso(1H) = 4.3 G; CH2Cl2 solution spectrum,
243 K). The second approach was to observe directly hyperfine
coupling to the terminal H of the diynyl chain of [4]+.
G
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with diethyl ether. The identity of complexes [1]PF6, [2]PF6,
and [5]PF6, as examples of isolable, metal-stabilized butadiynyl
and hexatriynyl radicals, was confirmed by cyclic voltammetry
complementary to that of the relevant 18-electron precursor
and IR and EPR spectra identical with those obtained by
spectroelectrochemistry or by in situ chemical oxidation. In
addition to spectroscopic characterization, the structure of the
butadiynyl radical [1]PF6 has been verified by a single-crystal X-
ray study (see Table 3 for key bond lengths and angles).8
However, although complexes 1 and [1]PF6 comprise a
structurally characterized redox pair, the relatively low precision
of the data for [1]PF6 prevent a meaningful discussion of
redox-induced modifications in molecular geometry.
In contrast to the 2,2′-bipyridine systems, the 17-electron
radicals resulting from one-electron oxidation of the dppe
complexes 3 and 6 were not stable over extended time scales,
even with the SiMe3 protecting group at the carbon chain
terminus. As reported previously,8 reaction of 3 with [FeCp2]-
PF6 proceeds via Cβ−Cβ′ coupling of the 17-electron radical
intermediate [3]+ to give the C4-bridged, dimeric complex
[{Mo(dppe)(η-C7H7)}2{μ-CC(CCSiMe3)C(C
CSiMe3)C}][PF6]2 (Figure 7a). The Cβ−Cβ′ regiospecificity
Figure 6. Fluid solution (CH2Cl2, 243 K), second-derivative X-band
EPR spectra of (a) [Mo(CCCCSiMe3)(bpy)(η-C7H7)]+ ([1]+),
(b) [Mo(CCCCSiMe3)(dppe)(η-C7H7)]+ ([3]+) and (c)
[Mo(CCCCH)(dppe)(η-C7H7)]+ ([4]+) (expansion).
Figure 7. Products of redox-induced butadiynyl chain coupling: (a) 3
and [FeCp2]PF6; (b) [Ru(CCCCSiMe3)Cl(dppe)2] and
[FeCp2]PF6.
of butadiynyl coupling observed for [3]+ contrasts with the
reaction between [Ru(CCCCSiMe3)Cl(dppe)2] and
[FeCp2]PF6, which leads to the formation of a bridged product
(Figure 7b) via a [2 + 2] coupling process centered at Cγ−Cδ.19
Electronic Structure Calculations. The experimental
investigations described above demonstrate that the reactivity
of the 17-electron radicals [Mo{(CC)nCCR}(L2)(η-
C7H7)]+ is consistent with some delocalization of spin density
onto the carbon chain ligand but is moderated by the identity
of R and L2. To rationalize the enhanced stability afforded by
the bpy supporting ligand and the specificity of carbon chain
centered reactivity shown by [Mo(CCCCSiMe3)(dppe)-
(η-C7H7)] ([3]+), a DFT computational study was conducted
using the B3LYP functional with the LANL2DZ basis set for
Mo and 6-31g* for all other atoms. Four extended chain
complexes [Mo{(CC)nCCH}(L2)(η-C7H7)]z+, 1-H, 4-H,
5-H, and 6-H, were investigated (Table 8) together with the
corresponding 17-electron radicals [1-H]+, [4-H]+, [5-H]+, and
[6-H]+. The computational systems are marked “-H” to
differentiate them from the experimental complexes and to
signify the H substituent at the carbon chain terminus. The
Comparison of parts b and c of Figure 6 confirms the presence
of an additional coupling of 1.9 G resulting from the
substitution of SiMe3 in [3]+ with H in [4]+. The observation
of hyperfine coupling to the terminal H of the butadiynyl chain
in [4]+, which was repeatable with independently prepared
samples recorded on separate occasions, provides specific
experimental evidence for the delocalization of unpaired spin
density along the full length of the butadiynyl chain.
Synthetic Redox Chemistry. The spectroelectrochemical
investigations on the bipyridine complexes 1, 2, and 5 indicated
that the 17-electron radical cations might be stable species, and
therefore their isolation was attempted. Reaction of the
butadiynyl complex 1 or 2 with 1 equiv of [FeCp2]PF6 or
AgPF6 in CH2Cl2 at −40 °C resulted in a rapid change from
purple to orange and subsequent isolation of the products
[1]PF6 and [2]PF6 as orange solids following addition of
diethyl ether. Similarly, reaction of the hexatriynyl analogue 5
with 1 equiv of [FeCp2]PF6 in CH2Cl2 at low temperature
resulted in a rapid color change from purple to brown and the
17-electron radical [5]PF6 was isolated following precipitation
H
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Organometallics
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complexes is directed along the axis connecting Mo with the
centroid of the C7H7 ring). In contrast, HOMO-1 features
significant ring and carbon chain contributions with the
combination of a strong metal−ring δ bond and a filled−filled
interaction between dxy and the CC π bond orthogonal to
that involved in the HOMO. The importance of the metal−ring
δ bond is highlighted by the substantial C7H7 ring contribution
(30−40%) to the composition of HOMO-1. This basic
description is then fine tuned by altering the carbon chain
length and L2 support ligand.
Considering first the effect of chain elongation, the members
of the dppe series 7-H (n = 0), 4-H (n = 1), and 6-H (n = 2)
provide a good basis for comparison. The key effect of
increasing chain length is to enhance the contribution of carbon
chain character to the HOMO and HOMO-1 with an
accompanying decrease in the metal and C7H7 character. For
example, comparison of the composition of the HOMO of 7-H
(n = 0) and 6-H (n = 2) reveals a shift in Mo(η-C7H7)/carbon
chain contribution from 78%/17% (7-H) to 61%/36% (6-H).
The composition of the LUMO is also modified with some
increase in carbon chain and Mo(η-C7H7) contribution at the
expense of the dppe ligand. Substitution of dppe by the bpy
support ligand primarily results in enhancement of the L2 ligand
character of the LUMO and HOMO. The LUMO of the
bipyridine complexes 1-H and 5-H is stabilized by ca. 1 eV by
comparison with the dppe analogues 4-H and 6-H, and its
composition is dominated by bpy character (83% (1-H), 84%
(5-H)) independent of carbon chain length. In the HOMO, in
contrast to the dppe complexes, significant L2 character is
retained; this increase in L2 composition is mainly derived from
a decrease in metal character.
Table 8. Numbering System for the Calculated Model
Complexes [Mo{(CC)nCCH}(L2)(η-C7H7)]z+
complex
n
L2
1-H
4-H
5-H
6-H
7-H
1
1
2
2
0
bpy
dppe
bpy
dppe
dppe
known alkynyl complex [Mo(CCH)(dppe)(η-C7H7)]20 was
also examined in the theoretical treatment as [7-H]z+ to
facilitate an extended comparison of the effect of increasing the
length of the carbon chain ligand. These calculations comple-
ment existing computational work carried out on [Fe(C
CCCH)(CO)2Cp] and [Ru{(CC)nCCR}(PH3)2Cp]z+
(z = 0, 1),6 with the distinction that the complexes [Mo{(C
C)nCCR}(L2)(η-C7H7)]z+ represent a discrete category of
electronic structure featuring an electron-rich metal center with
an attenuated interaction between the metal and carbon chain
ligand.
(i). Closed-Shell Systems [Mo{(CC)nCCH}(L2)(η-C7H7)].
Key metrical parameters from the DFT optimized geometries
of complexes 1-H, 4-H, 5-H, 6-H, and 7-H are summarized in
Table 9. The data for 4-H, 1-H, 5-H, and 6-H are in good
agreement with the crystallographically determined structures
for 4 and the SiMe3-substituted derivatives 1, 5, and 6,
respectively (see Table 3); in particular, the decrease in Mo−Cα
along the dppe series 7-H, 4-H, and 6-H accurately reflects the
trend observed experimentally for the But/SiMe3 analogues in
the sequence [Mo(CCBut)(dppe)(η-C7H7)], 3, and 6. The
energies and composition of the key orbitals (LUMO, HOMO,
and HOMO-1) are set out in Table 10, and Figure 8 illustrates
the plots of these frontier orbitals for 4-H and 5-H.
(ii). 17-Electron Radicals [Mo{(CC)nCCH}(L2)(η-C7H7)]+.
Key bond lengths from the DFT optimized geometries of the
17-electron radicals [1-H]+, [4-H]+, [5-H]+, [6-H]+, and [7-
H]+ are presented in parentheses in Table 9; the principal
effects of one-electron oxidation of these systems are a decrease
in Mo−Cα and a small increase in Mo−P or Mo−N consistent
with a decrease in Mo to L2 back-donation accompanying
reduction in electron density at the metal center. The energies
and composition of the key frontier orbitals (α-LUSO, β-
LUSO, β-HOSO, and α-HOSO) together with plots of these
orbitals for [4-H]+ and [5-H]+ are presented as Supporting
Information. To provide the best correlation between theory
and the experimental reactivity of the radicals [Mo{(C
C)nCCH}(L2)(η-C7H7)]+, spin densities were determined as
summarized in Table 11; plots of unpaired spin density
distribution are shown in Figure 9.
Before the specific effects of chain length and supporting
ligand L2 are discussed in detail, the general features common
to all of the systems investigated should be noted. The results
of the electronic structure calculations for [Mo{(CC)nC
CH}(L2)(η-C7H7)] are consistent with the previous DFT
treatment of the alkynyl complex [Mo(CCPh)(dppe)(η-
C7H7)], where the electronic structure is dominated by metal−
ring, π, and δ interactions, which in turn control the energy
ordering of the metal d orbital manifold.7 For the complexes
[Mo{(CC)nCCH}(L2)(η-C7H7)] the principal character
of the LUMO, HOMO, and HOMO-1 is unchanged
throughout the series. Thus, the LUMO is mostly centered
on the L2 support ligand, dppe or bpy, and, with the exception
of 6-H, possesses negligible carbon chain character. The
2
As illustrated in Figure 9, in all of the systems examined, spin
density along the carbon chain follows an alternating pattern of
α and β spin. The coefficient of spin density decreases markedly
HOMO is strongly metal centered, based on the dz orbital,
which exhibits an antibonding interaction with the filled CC
π orbital of the carbon chain ligand (the z axis in these
Table 9. Selected Bond Lengths (Å) for the Calculated Systems [Mo{(CC)nCCH}(L2)(η-C7H7)]z+ (z = 1 Shown in
Parentheses)
complex
Mo−Cα
Cα−Cβ
Cβ−Cγ
Cγ−Cδ
Cδ−Cε
Cε−Cζ
1-H ([1-H]+)
2.112 (2.059)
4-H ([4-H]+)
2.118 (2.063)
5-H ([5-H]+)
2.105 (2.049)
6-H ([6-H]+)
2.105 (2.050)
7-H ([7-H]+)
2.118 (2.063)
1.240 (1.241)
1.236 (1.239)
1.365 (1.359)
1.218 (1.217)
1.240 (1.241)
1.364 (1.358)
1.219 (1.218)
1.241 (1.245)
1.352 (1.345)
1.230 (1.231)
1.358 (1.353)
1.218 (1.217)
1.245 (1.248)
1.350 (1.344)
1.231 (1.232)
1.357 (1.353)
1.218 (1.217)
Mo−N/Mo−P 2.178, 2.180 (2.238, 2.238) 2.517, 2.525 (2.580, 2.583) 2.182, 2.182 (2.236, 2.236) 2.521, 2.528 (2.580, 2.581) 2.517, 2.525 (2.580, 2.583)
I
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Table 10. Energy and Composition of Frontier Orbitals in the Model Complexes [Mo{(CC)nCCH}(L2)(η-C7H7)]
complex
orbital
energy
% Mo
% C7H7
% L2
% Cα
% Cβ
% Cγ
% Cδ
% Cε
% Cζ
7-H
LUMO
−0.73
−3.96
−4.53
−0.86
−4.05
−4.54
−1.09
−4.16
−4.58
−2.00
−4.15
−4.63
−2.12
−4.25
−4.68
11
72
28
15
62
20
18
56
15
12
50
16
11
46
13
22
6
66
5
0
3
1
14
18
2
HOMO
HOMO-1
LUMO
40
24
6
7
6
4-H
6-H
1-H
5-H
56
4
1
0
1
2
3
1
4
0
1
3
0
2
5
2
10
13
6
HOMO
HOMO-1
LUMO
4
13
16
5
33
26
5
6
10
5
29
4
1
1
1
6
7
HOMO
HOMO-1
LUMO
4
13
14
0
10
12
0
28
4
6
10
0
10
83
18
3
HOMO
HOMO-1
LUMO
4
5
13
16
0
10
15
0
33
4
13
0
84
16
2
0
1
1
0
6
HOMO
HOMO-1
4
5
12
14
9
28
12
13
11
Figure 8. LUMO, HOMO, and HOMO-1 of 4-H and 5-H plotted as an isosurface of 0.04 au.
along the chain, with remoteness from the metal center such
that spin coefficients at Cε and Cζ in the triynyl complexes [5-
H]+ and [6-H]+ are relatively small. As discussed above, the
complexes [Mo{(CC)nCCH}(L2)(η-C7H7)]z+ exhibit a
correlation between an increase in carbon chain length and
enhanced chain character of the HOMO (z = 0) or β-HOSO (z
= 1). However, the analysis presented in Table 11 indicates that
a related correlation does not extend to spin density
distribution. In the bpy complexes [1-H]+ and [5-H]+, the
partition of spin density between the Mo(L2)(η-C7H7) and
(CC)nCCR fragments is almost independent of chain
length, whereas in the dppe series [7-H]+, [4-H]+, and [6-H]+,
the peak in spin delocalization onto the carbon chain ligand is
observed for the butadiynyl complex [4-H]+. The significant
coefficient of spin density at Cδ (0.234) is consistent with the
observation of hyperfine coupling to the terminal H in the EPR
spectrum of the experimental system [4]+.
A key objective of the electronic structure calculations was to
rationalize the results of the synthetic redox chemistry of the
butadiynyl radicals [Mo(CCCCSiMe3)(L2)(η-C7H7)]+
(L2 = bpy, [1]+; L2 = dppe, [3]+). The comparison of
experimental systems [1]+ and [3]+ with the calculated models
([1-H]+ and [4-H]+, respectively) represents an approximation,
because the SiMe3 protecting group of the experimental
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(C7H7) = 4.6 G; [3]+, aiso(Mo) = 32 G, aiso(1H) (C7H7) = 4.3
G) which indicate greater localization of spin density at the
MoL2(η-C7H7) center for the bpy derivative. The localization
of spin density at the metal center in [1-H]+ with a reduced
component on the butadiynyl chain provides some ration-
alization for the remarkable stability of the radical [1]+. The
second challenge is to explain the Cβ specificity of the
dimerization of [3]+ to afford a C4-bridged bis-
(alkynylvinylidene) complex (see Figure 7a). Inspection of
spin density along the butadiynyl chain of [4-H]+ reveals the
largest coefficient at Cβ (0.314). Although the spin density
coefficient at Cγ (0.234) is also substantial, in the experimental
system [3]+, this position is protected by the SiMe3 substituent
which may impede Cδ−Cδ′ radical−radical coupling. Moreover,
the preference for a vertical orientation of vinylidene
substituents in Mo(η-C7H7) complexes21 reduces steric
constraints to Cβ−Cβ′ coupling, whereas the more usual
horizontal arrangement of vinylidene substituents appears to
obstruct Cβ−Cβ′ coupling at sterically crowded metal centers.22
The specificity of Cβ−Cβ′ coupling of [3]+ is therefore probably
a result of the complementary effects of a higher coefficient of
spin density at Cβ, steric protection of the Cδ site, and the
potential to accommodate sterically demanding substituents at
Cβ in the dimeric divinylidene product.23
Table 11. Spin Density Distribution in the Model Complexes
[Mo{(CC)nCCH}(L2)(η-C7H7)]+
[7-H]+
[4-H]+
[6-H]+
[1-H]+
[5-H]+
Mo
Cα
Cβ
Cγ
Cδ
Cε
Cζ
1.166
0.988
1.015
−0.124
0.221
−0.088
0.135
−0.034
0.047
0.16
1.023
0.999
−0.120
0.219
−0.089
0.143
−0.036
0.051
0.17
−0.128
0.288
−0.172
0.314
−0.135
0.247
−0.134
0.234
−0.095
0.166
∑ spin density
0.16
0.24
0.18
a
(carbon chain)
C7H7
−0.256
−0.223
−0.192
−0.251
−0.240
P (dppe), N
(bpy)
−0.030,
−0.024,
−0.020,
−0.019,
−0.023,
−0.032
−0.024
−0.023
−0.019
−0.023
a
Sum of spin density on all carbons along C2 (Cα−Cβ), C4 (Cα−Cδ)
and C6 (Cα−Cζ) ligands.
systems is replaced by H in the calculated models. Nevertheless,
the spin density distribution calculated for [1-H]+ and [4-H]+
should act as a guide to the properties of [1]+ and [3]+.
Considering first the exceptional stability of the 17-electron bpy
radical [1]+, it is clear that the coefficients of spin density along
the carbon chain in the H-terminated bpy complex [1-H]+ are
relatively small by comparison with the dppe analogue [4-H]+.
This finding is consistent with experimental EPR hyperfine
coupling parameters (Table 7; [1]+, aiso(Mo) = 38 G, aiso(1H)
Figure 9. Plots of spin densities for the model complexes [Mo{(CC)nCCH}(L2)(η-C7H7)]+ shown as an isosurface of 0.04 au.
K
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continued for a further 2 h to give a red-purple solution. The entire
reaction mixture was then transferred to an alumina/n-hexane
chromatography column, and the product was eluted as a purple
band with n-hexane/CH2Cl2 (1/1). The band was collected, the
solvent was removed in vacuo, and the residue was recrystallized from
CH2Cl2/n-hexane to give 2 as a dark purple solid. Yield: 1.12 g (52%).
1H NMR: δ 0.00 (s, 9H, SiMe3), 1.40 (s, 18H, tBu, bpy), 4.85 (s, 7H,
C7H7), 7.17 (d, 2H, HB, bpy), 8.03 (s, 2H, HD, bpy), 9.03 (d, 2H, HA,
bpy). MALDI-MS (m/z): 578, [M]+. Anal. Calcd for C32H40MoN2Si:
C, 66.7; H, 6.9; N, 4.9. Found, C, 66.7; H, 7.1; N, 4.6.
CONCLUSIONS
■
The butadiynyl (n = 1) and hexatriynyl (n = 2) complexes
[Mo{(CC)nCCSiMe3}(bpy)(η-C7H7)] (n = 1, 2; bpy =
2,2′-bipyridine) and [Mo{(CC)nCCR}(dppe)(η-C7H7)]
(n = 1, R = H, SiMe3; n = 2, R = SiMe3; dppe =
Ph2PCH2CH2PPh2) have been prepared by a reaction sequence
starting from [MoBr(bpy)(η-C7H7)] and Li{(CC)nC
CSiMe3}. X-ray structural characterization of selected examples
experimentally establishes a decrease in the metal−Cα distance
as the chain length is increased. Each of the complexes
described exhibits a chemically and electrochemically reversible
one-electron-oxidation process (by cyclic voltammetry), and
the 17-electron radicals [Mo{(CC)nCCR}(L2)(η-C7H7)]+
are sufficiently stable to observe by spectroelectrochemical (IR,
UV−visible) and EPR techniques. The unique electronic and
frontier orbital characteristics of the Mo(L2)(η-C7H7) auxiliary
permit fine tuning of the degree of interaction between the
metal center and carbon chain ligand. Moderation of spin
density located on the carbon chain in the 17-electron radicals
[Mo{(CC)nCCR}(L2)(η-C7H7)]+ confers controlled re-
activity, thus facilitating spectroscopic characterization and
investigations on the further reaction chemistry of these
systems.
Preparation of [Mo{(CC)2CCSiMe3}(bpy)(η-C7H7) (5). A
solution of Me3SiCCCCCSiMe3 (0.35 g, 1.82 mmol) in
stirred, cooled (−78 °C) thf (20 cm3) was treated with MeLi−LiBr
(1.2 cm3 of a 1.5 M solution in diethyl ether), and the reaction mixture
was then warmed to room temperature over a period of 2 h. The
solution of Li{(CC)2CCSiMe3} so prepared was transferred to a
stirred solution of [MoBr(bpy)(η-C7H7)] (0.70 g, 1.65 mmol) in thf
(30 cm3) at −78 °C. The reaction mixture was warmed to room
temperature (30 min), and the reaction continued for a further 4 h at
room temperature to give a red-purple solution. The entire reaction
mixture was then transferred to an alumina/n-hexane chromatography
column, and the product was eluted as a purple band with n-hexane/
CH2Cl2 (1/1). The band was collected, the solvent was removed in
vacuo, and the residue was recrystallized from CH2Cl2/n-hexane to
give 5 as a dark-purple solid. Yield: 0.20 g (25%). 1H NMR (CDCl3):
δ 0.01 (s, 9H, SiMe3), 4.82 (s, 7H, C7H7), 7.01 (m, 2H, HB, bpy), 7.36
(m, 2H, HC, bpy), 7.93 (d, 2H, HD, bpy), 9.05 (d, 2H, HA, bpy).
MALDI-MS (m/z): 490, [M]+. ES(+)-MS (m/z): calcd for
C26H24MoN2Si 490.0778; found 490.0768.
Preparation of [Mo{(CC)2CCSiMe3}(dppe)(η-C7H7)] (6). A
mixture of [Mo{(CC)2CCSiMe3}(bpy)(η-C7H7)] (0.47 g, 0.97
mmol) and dppe (0.58 g, 1.45 mmol) was refluxed in toluene (40
cm3). The initial purple reaction mixture became green-brown, and
after 3 h the solvent was removed. The residue was dissolved in
CH2Cl2 and transferred to a n-hexane/Celite chromatography column.
Elution with n-hexane/CH2Cl2 (10/1) gave a green-brown band,
which was collected and evaporated to dryness. The residue was
extracted with pentane and the extract filtered through Celite and then
evaporated to dryness to give 6 as a deep green solid. Yield: 0.39 g
(55%). 1H NMR: δ 0.00 (s, 9H, SiMe3), 1.91 (m, 2H, CH2), 2.32 (m,
2H, CH2), 4.66 (s, C7H7), 7.60, 7.30, 7.25, 7.15 (m, 20H, Ph). 31P{1H}
NMR: δ 62.2 (s, dppe). HR ES(+)-MS (m/z): calcd for
C42H40MoP2Si 732.1423; found 732.1437. Anal. Calcd for
C42H40MoP2Si: C, 68.9; H, 5.5. Found, C, 68.7; H, 5.6.
EXPERIMENTAL SECTION
■
General Procedures. The preparation, purification, and reactions
of the complexes described were carried out under dry nitrogen using
standard Schlenk techniques. The complexes [MoBr(bpy)(η-C7H7)]
and [MoBr(tBu2-bpy)(η-C7H7)] were prepared by published
procedures.17 The diyne Me3SiCCCCSiMe3 was purchased
from commercial sources, while the triyne Me3SiCCCCC
CSiMe3 was prepared by a published procedure.24 NMR spectra were
recorded from CD2Cl2 solutions unless otherwise stated, containing
trace amounts of CoCp2 to prevent accumulation of MoI species by
aerial oxidation, on Varian Inova 400 and Bruker Avance 400
spectrometers at room temperature and referenced against solvent
resonances (1H, 13C) or external H3PO4 (31P). Infrared spectra were
obtained on Perkin-Elmer FT RX1 and Nicolet Avatar spectrometers.
Mass spectra were recorded using Thermo Quest Finnigan Trace GC/
MS and Thermo Electron Finnigan LTQ FT mass spectrometers.
MALDI mass spectra were recorded using a Micromass/Waters TOF
Spec 2E instrument. Microanalyses were conducted by the staff of the
Microanalytical Services of the School of Chemistry, University of
Manchester. Cyclic voltammograms were recorded (v = 100 mV s−1)
Preparation of [Mo(CCCCSiMe3)(bpy)(η-C7H7)]PF6 ([1]PF6). A
purple solution of [Mo(CCCCSiMe3)(bpy)(η-C7H7)] (1; 0.494
g, 1.06 mmol) in dichloromethane (20 cm3) was cooled to −40 °C
and treated with [FeCp2]PF6 (0.352 g, 1.06 mmol). After the mixture
was stirred for 1 h at −40 °C, the resulting orange-red solution was
warmed to room temperature and then filtered and reduced in volume
to 5 cm3 in vacuo. Addition of diethyl ether resulted in precipitation of
[1]PF6 as an orange-red solid. The crude product was washed with
toluene (2 × 10 cm3) and diethyl ether (1 × 10 cm3) and then
recrystallized from CH2Cl2/diethyl ether. Yield: 0.384 g (59%). HR
ES(+)-MS (m/z): calcd for C24H24MoN2Si 466.0767; found 466.0754.
Anal. Calcd for C24H24MoN2SiPF6: C, 47.3; H, 3.9; N, 4.6. Found: C,
46.1; H, 3.7; N, 4.3.
n
from 0.1 M NBu4PF6/CH2Cl2 solutions ca. 1 × 10−4 M in analyte
using a three-electrode cell equipped with a Pt working electrode, Pt-
wire counter electrode, and Pt-wire pseudo reference electrode. All
redox potentials are reported with reference to an internal standard of
the ferrocene/ferrocenium couple (FeCp2/[FeCp2]+ = 0.00 V). UV−
visible and IR spectroelectrochemical experiments were performed at
room temperature with an airtight OTTLE cell of Hartl design25
equipped with Pt-minigrid working and counter electrodes, a Ag-wire
reference electrode, and CaF2 windows using either a Nicolet Avatar
spectrometer or a Perkin-Elmer Lambda 900 spectrophotometer. EPR
experiments were conducted on a Bruker BioSPin EMX micro-
spectrometer at X-band (9 GHz); spectra were recorded at 243 K and
are the average of 16 scans. Spectral analysis and simulation was
carried out using Bruker WinEPR software (Bruker Biospin Ltd.).
Preparations. Preparation of [Mo(CCCCSiMe3)(tBu2-bpy)(η-
C7H7)] (2). A solution of Me3SiCCCSiMe3 (1.04 g, 3.57 mmol) in
stirred, cooled (−78 °C) thf (25 cm3) was treated with MeLi−LiBr
(3.6 cm3 of a 1.5 M solution in diethyl ether), and the reaction mixture
was then warmed to room temperature over a period of 2 h. The
solution of LiCCCCSiMe3 so prepared was transferred to a
stirred solution of [MoBr(tBu2-bpy)(η-C7H7)] (2.00 g, 3.74 mmol) in
thf (30 cm3) at −78 °C. The solution was stirred at −78 °C for 30 min
and then warmed to room temperature, and the reaction was
Preparation of [Mo(CCCCSiMe3)(tBu2-bpy)(η-C7H7)]PF6
([2]PF6). A purple solution of [Mo(CCCCSiMe3)(tBu2-bpy)(η-
C7H7)] (2; 0.340 g, 0.59 mmol) in dichloromethane (20 cm3) was
treated with AgPF6 (0.150 g, 0.59 mmol), resulting in a rapid color
change to orange-red. After it was stirred for 30 min, the solution was
filtered and reduced in volume to 5 cm3 in vacuo and diethyl ether was
added, resulting in the precipitation of [2]PF6 as an orange-red solid.
The crude product was washed with toluene (2 × 10 cm3) and diethyl
ether (1 × 10 cm3) and then recrystallized from CH2Cl2/diethyl ether.
Yield: 0.262 g (62%). MALDI-MS (m/z): 578, [M]+. Anal. Calcd for
C32H40MoN2SiPF6: C, 53.3; H, 5.5; N, 3.9. Found: C, 53.2; H, 5.7; N,
3.8.
L
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Table 12. Crystal Data and Refinement Parameters for Complexes 3, 5, and 6
3
5
6
formula
C
43.5H40MoP2Si
C26H15MoN2Si
479.43
C42.5H40ClMoP2Si
772.17
mass (amu)
temp (K)
cryst syst
space group
a (Å)
748.73
100(2)
220(2)
100(2)
monoclinic
P21/c
triclinic
monoclinic
P21/c
P1
̅
8.492(5)
13.161(5)
16.167(5)
17.750(5)
90.00
12.271(5)
18.489(5)
16.679(5)
90.00
b (Å)
10.802(5)
14.077(5)
109.940(5)
96.832(5)
94.964(5)
1194(10); 2
0.613
c (Å)
α (deg)
β (deg)
99.998(5)
90.00
100.584(5)
90.00
γ (deg)
V (Å3); Z
abs coeff (mm−1
θ range (deg)
3719(2); 2
0.501
3720(2); 2
0.572
)
2.78−28.31
3.77−25.02
2.72−28.56
limiting indices (h,k,l)
total no. of rflns
−17 to +17; −21 to +20; −23 to +23
8336
−8 to +10; −12 to +12; −16 to +16
4147
−16 to +16; −24 to +23; −15 to +21
8339
no. of indep rflns, I >
2σ(I)
5896
2573
5837
R1
0.0329
0.0786
0.901
0.0534
0.1137
0.986
0.0364
0.0895
0.878
wR2
data completeness
Preparation of [Mo{(CC)2CCSiMe3}(bpy)(η-C7H7)]PF6 ([5]PF6).
A purple solution of [Mo{(CC)2CCSiMe3}(bpy)(η-C7H7)] (5;
0.45 g, 0.91 mmol) in CH2Cl2 (10 cm3) was cooled to −40 °C and
treated with [FeCp2]PF6 (0.30 g, 0.91 mmol). After the mixture was
stirred for 1 h at −40 °C, the resulting brown solution was warmed to
room temperature and reduced in volume to 5 cm3 in vacuo. Addition
of diethyl ether resulted in precipitation of [5]PF6 as a brown solid.
Yield: 0.41 g (76%). IR (CH2Cl2): ν(CC) 2146 (m), 1965 (vw)
cm−1. MALDI-MS (m/z): 490, [M]+. HR ES+-MS (m/z): calcd for
C26H24N2SiMo 490.0758; found 490.0768.
Electronic Structure Calculations. All calculations were carried
out using the Gaussian 03 package.26 The model geometries discussed
were optimized using B3LYP,27 with no symmetry constraints. A
mixed basis set containing the pseudopotential LANL2DZ28 was used
for the Mo atom and the 6-31g* basis set29 for all other atoms.
Frequency calculations were carried out on the optimized geometries
at the corresponding levels and shown to have no imaginary
frequencies. Molecular orbital and TD-DFT computations were
carried out on optimized geometries at the appropriate level of
theory, and orbital contributions were generated with the aid of
GaussSum.30
generated by inversion. H atoms of the toluene molecule were not
included. The anisotropic displacement parameters of the atoms C3S
and C5S of the toluene molecule were disordered and restrained, since
they tended to become unrealistic. Data for complex 5 were collected
at 220(2) K to avoid crystal fracture which occurred at lower
temperatures. The C6 chain and SiMe3 group of 5 were split and
constrained due to disorder of SiMe3; H atoms of the SiMe3 group
were not included in the refinement due to overlapping groups. In 6,
the asymmetric unit contains 0.5 CH2Cl2 solvent of crystallization and
the Cl atom was split into parts due to disorder. The H atoms of the
CH2Cl2 molecule have been omitted due to disorder.
ASSOCIATED CONTENT
* Supporting Information
■
S
Table of DFT calculated energies and composition of frontier
orbitals (α-LUSO, β-LUSO, β-HOSO and α-HOSO) for [1-
H]+, [4-H]+, [5-H]+, [6-H]+, and [7-H]+, plots of these orbitals
for [4-H]+ and [5-H]+, and CIF files giving crystallographic
data for complexes 3, 5, and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
X-ray Crystal Structures of [Mo(CCCCSiMe3)(dppe)(η-
C7H7)]·0.5(toluene) (3), [Mo{(CC)2CCSiMe3}(bpy)(η-C7H7)]
(5), and [Mo{(CC)2CCSiMe3}(dppe)(η-C7H7)]·0.5CH2Cl2 (6).
The majority of details of the structure analyses carried out on the
complexes 3·0.5(toluene), 5, and 6·0.5CH2Cl2, are given in Table 12.
Single crystals of the complexes were obtained as follows: 3, vapor
diffusion of pentane into a toluene solution of the complex to give
green blocks; 5, vapor diffusion of diethyl ether into a CH2Cl2 solution
of the complex to give brown plates; 6, vapor diffusion of pentane into
a CH2Cl2 solution of the complex to give green blocks. In all cases the
X-ray data were collected on an Oxford Diffraction XCalibur 2
diffractometer using graphite-monochromated Mo Kα (λ = 0.710 73
Å) radiation. Data collection, cell refinement, and data reduction were
carried out with Oxford Diffraction Ltd. CrysAlis CCD and CrysAlis
RED software; SHELXS-9731 was employed for computing the
structure solution and SHELXL-9732 for computing the structure
refinement. An empirical absorption “multiscan” correction was
applied. All structures were solved by direct methods with refinement
by full-matrix least squares based on F2 against all reflections. All non-
hydrogen atoms were refined anisotropically, and hydrogen atoms
were included in calculated positions. The asymmetric unit of 3
incorporates half a disordered molecule of toluene, with the other half
AUTHOR INFORMATION
Corresponding Author
*E-mail: p.j.low@durham.ac.uk (P.J.L.); Mark.Whiteley@
manchester.ac.uk (M.W.W.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EPSRC for support of this work through grants
EP/E025544/1 and EP/E02582X/1. P.J.L. holds an EPSRC
Leadership Fellowship.
REFERENCES
■
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Organometallics
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dx.doi.org/10.1021/om3005756 | Organometallics XXXX, XXX, XXX−XXX