Structural consequences of the one-electron reduction of d4 [Mo(CO)2(η-PhC≡CPh)Tp′]+ and the electronic structure of the d5 radicals [M(CO)L(η-MeC≡CMe) Tp′] {L = CO and P(OCH2)3CEt}

Article abstract of DOI:10.1039/b514951g

Reduction of [M(CO)₂(η-RCCR′)Tp′]X {Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate, M = Mo, X = [PF₆] ^-, R = R′ = Ph, C₆H₄OMe-4 or Me; R = Ph, R′ = H; M = W, X = [BF₄]^-, R = R′ = Ph or Me; R = Ph, R′ = H} with [Co(η-C₅H₅)₂] gave paramagnetic [M(CO)₂(η-RCCR′)Tp′], characterised by IR and ESR spectroscopy. X-Ray structural studies on the redox pair [Mo(CO)₂(η-PhCCPh)Tp′] and [Mo(CO)₂(η- PhCCPh)Tp′][PF₆] showed that oxidation is accompanied by a lengthening of the CC bond and shortening of the Mo-C_alkyne bonds, consistent with removal of an electron from an orbital antibonding with respect to the Mo-alkyne bond, and with conversion of the alkyne from a three- to a four-electron donor. Reduction of [Mo(CO)(NCMe)(η-MeCCMe)Tp′][PF ₆] with [Co(η-C₅H₅)₂] in CH ₂Cl₂ gives [MoCl(CO)(η-MeCCMe)Tp′], via nitrile substitution in [Mo(CO)(NCMe)(η-MeCCMe)Tp′], whereas a similar reaction with [M(CO){P(OCH₂)₃CEt}(η-MeCCMe)Tp′] ⁺ (M = Mo or W) gives the phosphite-containing radicals [M(CO){P(OCH₂)₃CEt}(η-MeCCMe)Tp′]. ESR spectroscopic studies and DFT calculations on [M(CO)L(η-MeCCMe)Tp′] {M = Mo or W, L = CO or P(OCH₂)₃CEt} show the SOMO of the neutral d⁵ species (the LUMO of the d⁴ cations) to be largely d_yz in character although much more delocalised in the W complexes. Non-coincidence effects between the g and metal hyperfine matrices in the Mo spectra indicate hybridisation of the metal d-orbitals in the SOMO, consistent with a rotation of the coordinated alkyne about the M-C₂ axis. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b514951g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Structural consequences of the one-electron reduction of d4 [Mo(CO)2(g-
PhC≡CPh)Tpꢀ]+ and the electronic structure of the d5 radicals [M(CO)L(g-
MeC≡CMe)Tpꢀ] {L = CO and P(OCH2)3CEt}†
Christopher J. Adams,a Ian M. Bartlett,a Supakorn Boonyuen,a Neil G. Connelly,*a David J. Harding,a
Owen D. Hayward,a Eric J. L. McInnes,b A. Guy Orpen,a Michael J. Quaylea and Philip H. Rieger‡c
Received 24th October 2005, Accepted 8th February 2006
First published as an Advance Article on the web 2nd March 2006
DOI: 10.1039/b514951g
Reduction of [M(CO)2(g-RC≡CRꢀ)Tpꢀ]X {Tpꢀ = hydrotris(3,5-dimethylpyrazolyl)borate, M = Mo, X =
[PF6]−, R = Rꢀ = Ph, C6H4OMe-4 or Me; R = Ph, Rꢀ = H; M = W, X = [BF4]−, R = Rꢀ = Ph or Me;
R = Ph, Rꢀ = H} with [Co(g-C5H5)2] gave paramagnetic [M(CO)2(g-RC≡CRꢀ)Tpꢀ], characterised by IR
and ESR spectroscopy. X-Ray structural studies on the redox pair [Mo(CO)2(g-PhC≡CPh)Tpꢀ] and
[Mo(CO)2(g-PhC≡CPh)Tpꢀ][PF6] showed that oxidation is accompanied by a lengthening of the C≡C
bond and shortening of the Mo–Calkyne bonds, consistent with removal of an electron from an orbital
antibonding with respect to the Mo–alkyne bond, and with conversion of the alkyne from a three- to a
four-electron donor. Reduction of [Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ][PF6] with [Co(g-C5H5)2] in
CH2Cl2 gives [MoCl(CO)(g-MeC≡CMe)Tpꢀ], via nitrile substitution in [Mo(CO)(NCMe)(g-MeC≡
CMe)Tpꢀ], whereas a similar reaction with [M(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ]+ (M = Mo or
W) gives the phosphite-containing radicals [M(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ]. ESR
spectroscopic studies and DFT calculations on [M(CO)L(g-MeC≡CMe)Tpꢀ] {M = Mo or W, L = CO
or P(OCH2)3CEt} show the SOMO of the neutral d5 species (the LUMO of the d4 cations) to be largely
dyz in character although much more delocalised in the W complexes. Non-coincidence effects between
the g and metal hyperfine matrices in the Mo spectra indicate hybridisation of the metal d-orbitals in
the SOMO, consistent with a rotation of the coordinated alkyne about the M–C2 axis.
further insight into the electronic structure of the paramagnetic
complexes.
Introduction
During our studies of the redox properties of metal–alkyne
complexes1 we have shown that oxidation of the (formal) d6
complex [Cr(CO)2(g-PhC≡CPh)(g-C6Me5H)] to the d5 cation
Results and discussion
[Cr(CO)2(g-PhC≡CPh)(g-C6Me5H)]+ results in a shortening of
The synthesis and reduction of [M(CO)2(g-RC≡CR)Tpꢀ]z (z = 0
the Cr–Calkyne bonds, in accord with electron loss from a
HOMO antibonding with respect to the metal–alkyne bond
and with the conversion of the alkyne from a two- to a three-
and 1)
electron donor.2 Preliminary studies showed3 that oxidation of d5
[Mo(CO)2(g-PhC≡CPh)Tpꢀ] to d4 [Mo(CO)2(g-PhC≡CPh)Tpꢀ]+
{Tpꢀ = hydrotris(3,5-dimethylpyrazolyl)borate} resulted in a sim-
ilar shortening of the Mo–Calkyne bonds as the alkyne was converted
from a three-electron to a four-electron donor. We now give
details of these structural changes, of the reduction of [M(CO)L(g-
RC≡CRꢀ)Tpꢀ]+ {M = Mo or W, L = CO or P(OCH2)3CEt} to
[M(CO)L(g-RC≡CRꢀ)Tpꢀ] {L = CO or P(OCH2)3CEt}, and of
ESR spectroscopic studies and DFT calculations which provide
In order to study their redox properties, the known complexes
[W(CO)2(g-RC≡CRꢀ)Tpꢀ][BF4] (R = Rꢀ = Ph or Me; R =
Ph, Rꢀ = H) were prepared by the published method4 and
the new analogues [Mo(CO)2(g-RC≡CRꢀ)Tpꢀ][PF6] (R = Rꢀ =
Ph, 1+; C6H4OMe-4, 2+ or Me, 3+; R = Ph, Rꢀ = H, 4+) by
the oxidation of [Mo(CO)3Tpꢀ] with [Fe(g-C5H5)2][PF6] in the
presence of the alkyne. The new complexes were characterised
by elemental analysis and IR (Table 1) and NMR spectroscopy,
with assignments of the latter spectra made on the basis of those
for [W(CO)2(g-RC≡CRꢀ)Tpꢀ][BF4].4 The 1H NMR spectra of the
4-MeOC6H4C≡CC6H4OMe-4 and MeC≡CMe complexes 2+ and
3+ each showed two methyl resonances for the alkyne substituents,
and the 13C spectra of the complexes of symmetrical alkynes (i.e.
R = Rꢀ) each showed two peaks for the coordinated carbons
(with chemical shifts in the region associated with four-electron
alkyne ligands5,6). Thus, there is little or no alkyne rotation at
room temperature.
aSchool of Chemistry, University of Bristol, Bristol, BS8 1TS, UK. E-mail:
neil.connelly@bristol.ac.uk; Fax: +44 (0)117 929 0509; Tel: +44 (0)117
928 8162
bSchool of Chemistry, University of Manchester, Manchester, M13 9PL,
UK
cDepartment of Chemistry, Brown University, Rhode Island, RI 02912, USA
1
† Electronic supplementary information (ESI) available: 1H, 13C-{ H}
and 31P NMR spectroscopic data for the alkyne complexes. See DOI:
Cyclic voltammetry shows that the new cationic dicarbonyls 1+–
4+ and [W(CO)2(g-RC≡CRꢀ)Tpꢀ][BF4] (R = Rꢀ = Ph, 5+ or Me, 6+;
10.1039/b514951g
‡ Deceased.
3466 | Dalton Trans., 2006, 3466–3477
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R = Ph, Rꢀ = H, 7+) undergo reversible one-electron reduction in
the potential range 0.0 to −0.5 V (Table 1), with the potential
depending on the alkyne (Eoꢀ = 4-MeOC6H4C≡CC6H4OMe-
4 < MeC≡CMe < PhC≡CPh < PhC≡CH) and the metal;
the tungsten complexes are more difficult to reduce than the
molybdenum analogues by ca. 300 mV. Most complexes also show
a second reduction wave at a potential more negative than the
first by ca. −1.1 to −1.2 V. Interestingly, the reversibility of the
second wave is enhanced in the presence of [Co(g-C5H5)2][BF4]
(added to the electrochemical cell as an internal standard of
potential, reduction potential −0.86 V) suggesting some mediation
of heterogeneous electron transfer to the platinum electrode. This
second reduction would correspond to the formation of the d6
anion [Mo(CO)2(g-RC≡CRꢀ)Tpꢀ]−, isoelectronic with [Cr(CO)2(g-
RC≡CRꢀ)(arene)].2
On the basis of the potentials measured for the first re-
duction process, treatment of [Mo(CO)2(g-RC≡CRꢀ)Tpꢀ][PF6] or
[W(CO)2(g-RC≡CRꢀ)Tpꢀ][BF4] with the one-electron reductant
[Co(g-C5H5)2] gave the air-sensitive neutral complexes [M(CO)2(g-
RC≡CRꢀ)Tpꢀ]. In the case of [Mo(CO)2(g-PhC≡CPh)Tpꢀ] 1, the
only isolable compound in this series, green-black crystals were
formed after reducing [Mo(CO)2(g-PhC≡CPh)Tpꢀ][PF6] 1+[PF6]−
in CH2Cl2, evaporating the dark green solution to dryness,
extracting the black residue into hot n-hexane, and cooling the
extract to −20 ◦C.
The paramagnetic complex 1 was characterised by elemental
analysis (as a 0.5 n-hexane solvate—confirmed by the X-ray
structural analysis, see below) and by IR spectroscopy (Table 2)
which showed carbonyl bands at 1962 and 1876 cm−1 in CH2Cl2,
shifted to lower wavenumber by ca. 100–130 cm−1 from the
values for [Mo(CO)2(g-PhC≡CPh)Tpꢀ][PF6] 1+[PF6]− (2065 and
2003 cm−1). The CV of the neutral complex 1 showed one
oxidation wave and one reduction wave at potentials effectively
identical to those for the two sequential reductions of [Mo(CO)2(g-
PhC≡CPh)Tpꢀ][PF6] 1+[PF6]−.
The analogous compounds [M(CO)2(g-RC≡CRꢀ)Tpꢀ] (M =
Mo; R = Rꢀ = C6H4OMe-4 2, Me 3, R = Ph, Rꢀ = H 4; M =
W; R = Rꢀ = Ph 5 or Me 6, R = Ph, Rꢀ = H 7) were characterised
in solution, by IR (Table 2) and ESR spectroscopy, after generation
by [Co(g-C5H5)2] reduction of the corresponding cations in situ.
As for 1, the IR spectra showed two carbonyl bands shifted to
lower wavenumber on reduction. The ESR spectra, together with
those of the other paramagnetic species in this paper, are described
below.
Complex [Mo(CO)2(g-PhC≡CPh)Tpꢀ] 1 was also structurally
characterised, enabling a comparison to be made of the redox pair
[Mo(CO)2(g-PhC≡CPh)Tpꢀ]z (z = 0 and +1).
The X-ray structures of [Mo(CO)2(g-PhC≡CPh)Tpꢀ]·0.5C6H14,
1·0.5C6H14 and [Mo(CO)2(g-PhC≡CPh)Tpꢀ][PF6] 1+[PF6]−
The structures of [Mo(CO)2(g-PhC≡CPh)Tpꢀ] 1 and [Mo(CO)2(g-
PhC≡CPh)Tpꢀ]+ 1+, previously reported in ref. 3, are generally
similar; that of 1+ is shown in Fig. 1 as a representative example,
and selected bond lengths and angles for both 1 and 1+ are given
in Table 3. In both complexes, the molybdenum atom adopts
a pseudo-octahedral geometry with three fac sites occupied by
the j3-Tpꢀ group and two cis sites by the carbonyl ligands; the
diphenylacetylene completes the coordination sphere.
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Table 2 IR and ESR spectroscopic data for [M(CO)2(g-RC≡CRꢀ)Tpꢀ] (1–7) and [M(CO){P(OCH2)3CEt}(g-RC≡CRꢀ)Tpꢀ] (10 and 11)
ESR isotropic parametersb ESR anisotropic parametersb
Complex IRa/cm−1 m(CO)
M
R
Rꢀ
ꢁgiso
ꢂ
ꢁAisoꢂ/Gc
30.7
g1
g2
g3
gave
1c
2
3
4
5
6
7
1962, 1876
1968, 1887d
1956, 1870
1963, 1881d
1945, 1849
1952, 1864d
1964, 1880
1969, 1890d
1948, 1854
1957, 1866d
1932, 1840
1941, 1856d
1950, 1857
1957, 1870d
—
Mo Ph
Ph
2.007
2.036
2.039e
2.039
2.038
2.098
2.102
2.100
2.009
2.009
2.016
2.010
2.004
2.015
2.006
1.977
1.979
1.979
1.975
1.947
1.955
1.947
2.007
2.009
2.011
2.008
2.016
2.024
2.018
Mo C6H4OMe-4 C6H4OMe-4 2.007
30.1
Mo Me
Mo Ph
Me
H
2.011
2.008f
2.013
2.021
2.016
31.1
31.2
g
W
W
W
W
Ph
Ph
Me
H
Me
Ph
42
50
g
10
11
Me
Me
Me
2.015f
2.001f
(19)
2.126h,i (11) 2.002 (11) 1.951 (11) 2.026(11)
2.027i,j (32) 1.990 (37) 1.944 (36) 1.987 (35)
—
Mo Me
35 (32)
a In CH2Cl2 unless otherwise stated. b In toluene at 290–300 K (isotropic) or 77 K (anisotropic) unless stated otherwise. c Metal hyperfine coupling; 31
P
coupling in parentheses. d In n-hexane. e At 110 K. f At 240 K in CH2Cl2 : thf (1 : 2). g Not resolved. h At 110 K in CH2Cl2 : thf (1 : 2). i 31P coupling in
parentheses. j At 120 K in CH2Cl2 : thf (1 : 2).
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) for [Mo(CO)2(g-
PhC≡CPh)Tpꢀ][PF6] 1+ [PF6]− and [Mo(CO)2(g-PhC≡CPh)Tpꢀ] 1a
1+ [PF6]−
1
Mo(1)–Nave
Mo(1)–C(3)
Mo(1)–C(4)
C(3)–C(4)
Mo(1)–C(1)
Mo(1)–C(2)
C(1)–O(1)
2.207(3)
2.041(4)
2.069(3)
1.334(6)
2.108(4)
2.029(4)
1.114(5)
1.125(5)
141.3(3)
142.6(3)
91.8(2)
2.245(3)
2.136(3)
2.175(3)
1.282(3)
2.000(3)
1.974(3)
1.149(3)
1.154(3)
C(2)–O(2)
C(4)–C(3)–C(14)
C(3)–C(4)–C(5)
C(1)–Mo(1)–C(2)
bc
140.7(2)b
143.7(2)
88.7(1)
22.8, 72.4
22.7, 68.0
a Numbering as in Fig. 1. b Atom numbering for this angle in 1 is C(4)-
C(3)-C(15). c b = the angles between the C≡C and M–C(O) vectors. {The
two angles relate to the two CO groups of the Mo(CO)2 unit.}
The orientation of the alkyne, between the two Mo(CO) groups
in both 1+ and 1, does not change on reduction but is very
different from that in [Cr(CO)2(g-PhC≡CPh)(g-C6Me5H)]z (z =
0, d6 and 1, d5) where the alkyne lies approximately parallel to
the plane of the arene ring.2 In both 1+ and 1 the alkyne aligns
more with one carbonyl ligand than the other of the Mo(CO)2
group, with the angle between the C≡C and M–C(O) vector, b,
ca. 23◦ in both cases. (Note that the observation of a 2 : 1 ratio of
pyrazolyl rings in the NMR spectra of 1+ suggests a facile fluxional
process by which the two carbonyl ligands also become equivalent
in solution, presumably alkyne oscillation but not rotation—see
above.) This angle is somewhat smaller than that in [W(CO)2(g-
PhC≡CMe)Tpꢀ]+ (b = 34.5◦).7
Fig.
1
The molecular structure of the cation [Mo(CO)2(g-PhC≡
CPh)Tpꢀ]+ 1+. Hydrogen atoms have been omitted for clarity.
The most significant structural change on reduction is the
˚
lengthening of the mean Mo–Calkyne distance from 2.055 A in
+
˚
1 to 2.155 A in 1. Such a lengthening, also observed when the
d5 complex [Cr(CO)2(g-PhC≡CPh)(g-C6Me5H)]+ is reduced to d6
[Cr(CO)2(g-PhC≡CPh)(g-C6Me5H)],2 results from population of
the LUMO of 1+ [Fig. 2(a)] which is anti-bonding with respect to
the metal–alkyne bond. {This orbital is also bonding with respect
to the alkyne C–C bond so that C(3)-C(4) decreases from 1.334(6)
The preferential alignment of the alkyne with one carbonyl more
than the other can be understood in terms of the nature of the
HOMO of 1+ [Fig. 2(b)], which also leads to the bond Mo(1)–
C(2) being shorter than Mo(1)–C(1) in both 1+ and 1 {1.974(3) cf.
2.000(3) in 1 and 2.029(4) cf. 2.108(4) in 1+} via “a constructive
+
˚
˚
A in 1 to 1.282(3) A in 1.} Effectively, the alkyne changes from
a four- to a three-electron donor as 1+ is reduced to 1. (The mean
+
˚
Mo–Calkyne distance for 1 is in the typical range—2.06 to 2.00 A—
for four-electron molybdenum or tungsten alkyne complexes.6)
3468 | Dalton Trans., 2006, 3466–3477
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more negative potentials of about 0.8 V for the reduction wave.
(Assuming a similar shift for the oxidation wave, the potential
for the oxidation of [Mo(CO)2(g-MeC≡CMe)Tpꢀ][PF6] 3+[PF6]−
would be approximately 1.9 V, outside the accessible potential
range in CH2Cl2.) The irreversibility of the reduction wave for 8+
indicates that electron addition is followed by a rapid chemical
reaction, possibly loss of MeCN.
Chemical reduction of [Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ]-
[PF6] 8+[PF6]− with [Co(g-C5H5)2] in CH2Cl2 resulted in the
formation of a complex showing one IR carbonyl band
at 1922 cm−1, subsequently isolated and characterised as
[MoCl(CO)(g-MeC≡CMe)Tpꢀ].8 The chloro complex shows a
one-electron oxidation wave at 0.61 V, identical to the product
wave observed in the CV of 8+. Hence, even on the timescale
of cyclic voltammetry, [MoCl(CO)(g-MeC≡CMe)Tpꢀ] is formed
from [Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ]+ presumably via the
neutral complex [Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ] (although
this paramagnetic species has not been detected spectroscopically,
cf. the much more stable dicarbonyl analogues 1–7).
Fig. 2 Schematic MO diagram for the pseudo-octahedral complexes
[Mo(CO)L(g-alkyne)Tpꢀ]z. The three fac sites are shown as vacant. Orbital
(a) is the SOMO for the d5 species (z = 0) and the LUMO for the d4
complexes (z = 1); orbital (b) is the HOMO for z = 1. (Adapted from
Fig. 3 in ref. 2.)
The synthesis and electrochemistry of
[M(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ]+
In order to assess further the effects of L on the redox prop-
erties of [M(CO)L(g-RC≡CR)Tpꢀ]+ (cf. L = CO and MeCN
above) the phosphite derivatives [M(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ]X (M = W, X = [BF4]−; M = Mo, X = [PF6]−)
have also been prepared. One related phosphite complex, namely
[W(CO){P(OMe)3}(g-PhC≡CMe)Tpꢀ]+, has been reported4 al-
though the reaction of [W(CO)2(g-MeC≡CMe)Tpꢀ]+ 6+ with
P(OMe)3 in thf yielded an inseparable mixture of prod-
ucts, with IR carbonyl bands at 1876, 1911 and 1947 cm−1,
rather than [W(CO){P(OMe)3}(g-MeC≡CMe)Tpꢀ].+ (Earlier
work showed that the reaction of P(OMe)3 with [W(CO)2(g-
PhC≡CH)Tpꢀ]+ resulted in nucleophilic attack on the metal-
bound alkyne carbon and subsequent loss of Me+ to
interaction between the carbonyl carbon and the proximal alkyne
4
˚
˚
carbon ” {leading to C(2)–C(4) = 2.482(4) A for 1 and 2.462(6) A
1+}.
The synthesis and electrochemistry of
[M(CO)(NCMe)(g-MeC≡CMe)Tpꢀ]+
The tungsten complexes [W(CO)(NCMe)(g-RC≡CR)Tpꢀ][BF4]
(R = Rꢀ = Ph or Me; R = Ph, Rꢀ = H or Me) have been
prepared previously,4 either by halide abstraction from [WI(CO)-
(g-RC≡CR)Tpꢀ] with Ag[BF4] in MeCN or by nitrile substition
of one CO ligand of [W(CO)2(g-RC≡CR)Tpꢀ][BF4]. The new
molybdenum analogue [Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ]-
[PF6] 8+[PF6]− was prepared by the latter route, i.e. by stirring
[Mo(CO)2(g-MeC≡CMe)Tpꢀ][PF6] in MeCN for ca. 5 min. Purifi-
cation using thf–n-hexane then gave the product as a moderately
air-sensitive, pale green 1 : 1 thf solvate which was characterised
by IR (Table 1) and NMR spectroscopy and cyclic voltammetry.
The presence of an asymmetric metal centre in [Mo(CO)-
(NCMe)(g-MeC≡CMe)Tpꢀ]+ is reflected in the 1H and 13C NMR
spectra which are very similar to those of [W(CO)(NCMe)(g-
MeC≡CMe)Tpꢀ][BF4] 9+[BF4]−,4 i.e. all three pyrazolyl rings of
the Tpꢀ ligand are inequivalent. Two sharp 1H singlets, at 2.78 and
3.62 ppm, for the alkyne methyl protons, and two 13C resonances
for the alkyne methyl substituents (21.6 and 23.5 ppm) and metal-
bound carbon atoms (218.9 and 231.4 ppm) are consistent with no
alkyne rotation at room temperature, and with the alkyne acting
as a four-electron donor.6
2
2
ꢀ
=
give the g -vinyl complex [W(CO)2{g -CPh CHPO(OMe)2}Tp ]
[m(CO) = 1880, 1968 cm−1].9 The carbonyl bands at 1876
and 1947 cm−1 may therefore correspond to a similar prod-
2
ꢀ
=
uct, i.e. [W(CO)2{g -CMe CMePO(OMe)2}Tp ]). The reaction
of [Mo(CO)2(g-MeC≡CMe)Tpꢀ][PF6] 3+[PF6]− with P(OMe)3 in
MeCN was also unsuccessful, leading only to the formation of
[Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ]+ 8+, and in CHCl3 a mix-
ture of products was formed, probably including [MoCl(CO)(g-
MeC≡CMe)Tpꢀ].8
However, heating [W(CO)2(g-MeC≡CMe)Tpꢀ][BF4] 6+[BF4]−
with P(OCH2)3CEt {which cannot undergo the rearrangement
observed with P(OMe)3} in thf under reflux for 16 hours afforded
a deep blue solution from which [W(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ][BF4] 10+[BF4]− was isolated as blue crystals on
addition of n-hexane and cooling to −20 ◦C. The reaction appears
to proceed via an associative pathway as IR carbonyl bands at
1866 and 1897 cm−1 are initially observed, consistent with the
formation of [W(CO)2{P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ].+ The
formation of such an intermediate, in which the alkyne would act
as a two-electron donor, would facilitate substitution by an asso-
ciative mechanism, as suggested for the related cyclopentadienyl
complexes [Mo(CO)L(g-RC≡CRꢀ)(g-C5H5)]+ (L = PEt3, PCy3 or
PPh3; R = Rꢀ = Me or C6H5Me-4, R = H, Rꢀ = Ph, But or Pri).10
The CV of [Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ][PF6] 8+[PF6]−
in CH2Cl2 is different from that of [Mo(CO)2(g-MeC≡CMe)Tpꢀ]-
[PF6] 3+[PF6]− in showing a reversible oxidation wave at 1.17 V,
and an irreversible reduction wave at −0.95 V associated with a
reversible product wave at 0.61 V. Thus, substitution of one CO
of 3+ by the better donor MeCN, to give 8+, results in a shift to
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◦
˚
centre. Again, the alkyne methyl resonances, at 2.75 and 3.51 ppm
for 10+ and at 2.83 and 3.46 for 11+, are sharp suggesting that
there is no alkyne rotation at room temperature, in contrast to
fluxional [Mo(CO)(PR3)(g-MeC≡CMe)(g-C5H5)]+ (R = Et, Cy or
Ph) where the energy barrier to alkyne rotation increases as PR3
becomes a better donor.10 Presumably the greater steric bulk of
Table 4 Selected bond lengths (A) and angles ( ) for [W(CO){P(OCH2)3-
CEt}(g-MeC≡CMe)Tpꢀ][BF4]·2CH2Cl2, 10+[BF4]−·2CH2Cl2
W(1)–C(1)
W(1)–C(3)
W(1)–C(4)
P(1)–W(1)–C(1)
P(1)–W(1)–C(3)
P(1)–W(1)–C(4)
C(1)–W(1)–C(3)
ba
1.973(4)
2.057(3)
2.012(3)
85.3(1)
82.0(1)
95.0(1)
71.9(1)
16.9
W(1)–P(1)
C(3)–C(4)
C(1)–O(1)
C(1)–W(1)–C(4)
C(3)–W(1)–C(4)
C(4)–C(3)–C(2)
C(3)–C(4)–C(5)
2.452(1)
1.309(5)
1.159(4)
107.9(1)
37.5(1)
141.9(4)
141.9(3)
1
the Tpꢀ ligand hinders such rotation. The 31P-{ H} NMR spectra
of 10+ and 11+ show peaks at 131.9 and 139.5 ppm respectively,
the former with satellites due to coupling to the 183W nucleus (I =
1/2, 14%), with J(31P183W) = 494 Hz.
a b = the angle between the C≡C and M–C(O) vectors.
The CVs of 10+ and 11+ show reduction waves at −1.01 and
−0.66 V, and oxidation waves at 1.28 and 1.39 V respectively;
all are reversible and diffusion-controlled. The molybdenum
complex, 11+, is more easily reduced than the tungsten analogue
by 0.35 V. The oxidation potential for 11+ is lower than that of
10+, albeit by only 0.11 V, but underlining the trend whereby
the tungsten complex is more electron rich than the molybdenum
complex.
Attempts to prepare the analogous molybdenum complex
[Mo(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ][PF6] similarly in thf
proved unsuccessful. However, the reaction of [Mo(CO)2(g-
MeC≡CMe)Tpꢀ][PF6] 3+[PF6]− with P(OCH2)3CEt in MeCN
under reflux for 10 minutes, and subsequent purifica-
tion using CH2Cl2–n-hexane, gave [Mo(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ][PF6] 11+[PF6]− as a green solid in moderate yield.
The complexes 10+[BF4]− and 11+[PF6]− were characterised
by IR (Table 1) and NMR spectroscopy, cyclic voltammetry
and, in the case of 10+[BF4]−·2CH2Cl2, by X-ray crystallog-
raphy. The structure of the cation [W(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ]+ 10+ is shown in Fig. 3 and important bond
lengths and angles are given in Table 4. The tungsten coordination
sphere is generally similar to that of 1+[PF6]− with the W–
Substitution of the carbonyl ligand in [M(CO)2(g-
MeC≡CMe)Tpꢀ]+ by P(OCH2)3CEt results in the reduction
wave shifting to more negative potentials by ca. 0.5 V, mirroring
the trend observed for the chromium(I) cations, [Cr(CO)L(g-
RC≡CR)(g-C6Me6)]+ {L = CO and P(OR)3}.2 However, the shift
is less than observed for carbonyl substitution by the better donor,
MeCN.
ESR spectroscopy of [M(CO)L(g-RC≡CRꢀ)Tpꢀ] {L = CO or
P(OCH2)3CEt}
˚
Calkyne bond distances {2.057(3) and 2.012(3) A} again consistent
with the alkyne acting as a four-electron donor.6 The alkyne is
aligned approximately parallel to the carbonyl ligand {the angle,
b, between the C≡C and W(CO) vectors is 16.9◦} as in 1 and
1+, (see above) and in other d4 species, e.g. the indenyl complex
As noted above, all of the complexes [M(CO)L(g-
MeC≡CMe)Tpꢀ]+ undergo one-electron reduction to the neutral
molecules [M(CO)L(g-MeC≡CMe)Tpꢀ]. For L = MeCN, the
reduced form is unstable, giving [MCl(CO)(g-MeC≡CMe)Tpꢀ]
in CH2Cl2. However, for L = CO and P(OCH2)3CEt the
paramagnetic species are stable enough to be studied by ESR
spectroscopy.
[Mo(CO)(PEt3)(g-MeC≡CMe)(g -C9H7)]+.10
5
The room temperature, fluid solution ESR spectrum of the iso-
lated complex [Mo(CO)2L(g-PhC≡CPh)Tpꢀ] 1 in toluene is shown
in Fig. 4. The spectra of 2–7 are qualitatively similar (Table 2) and
were obtained in situ by reducing the corresponding diamagnetic
cations with [Co(g-C5H5)2] in toluene or, in the case of 5, by
Fig. 3 The molecular structure of the cation [W(CO){P(OCH2)3CEt}-
(g-MeC≡CMe)Tpꢀ]+ 10+. Hydrogen atoms have been omitted for clarity.
The 1H and 13C-{ H} NMR spectra of 10+ and 11+ are similar
1
to those of [W(CO){P(OMe)3}(g-PhC≡CMe)Tpꢀ]+4 and the nitrile
complexes 8+ and 9+, and are consistent with the asymmetric metal
Fig. 4 Fluid solution ESR spectrum of the isolated complex [Mo(CO)2-
(g-PhC≡CPh)Tpꢀ] 1 in toluene. The asterisk indicates an impurity.
3470 | Dalton Trans., 2006, 3466–3477
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reducing the cation 5+ in CH2Cl2, evaporating the reaction mixture
to dryness, and extracting the paramagnetic product into toluene.
The isotropic spectra of the molybdenum complexes 1–4 consist
of a central line (giso = 2.007–2.011) flanked by 95,97Mo satellites
(I = 5/2, 25.5% combined natural abundance) with Aiso(95,97Mo)
ca. 30 G (Table 2). The tungsten analogues 5–7 show relatively
broad isotropic spectra (giso = 2.016–2.021), with resolved 183W
1
satellites (I = 3/2, 14.3% natural abundance). H coupling was
not observed for the PhC≡CH complexes 4 and 7, in contrast
to the 4.2 G coupling observed previously for [Cr(CO)2(g-
PhC≡CH)(g-C6Me6)]+.2 Spectra of the phosphite-containing
analogues, [W(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] 10 and
[Mo(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] 11, were obtained
by reducing the cations 10+ and 11+ with [Co(g-C5Me5)2] and
[Co(g-C5H5)2] respectively in CH2Cl2 : thf (1 : 2). The isotropic
ESR spectra of 10 and 11 (Fig. 5) show coupling of the unpaired
electron to 31P (I = 1/2, 100%) in addition to the metal nuclei.
Fig.
6
Frozen toluene solution ESR spectra of (a) [Mo(CO)2-
(g-MeC≡CMe)Tpꢀ] 3 and (b) [W(CO)2(g-MeC≡CMe)Tpꢀ] 6 at 77 K. In
each case, the upper spectrum is experimental and the lower spectrum is
the simulation using the parameters described in the text with Gaussian
line widths of W1 = 11, W2 = 8, W3 = 10 G (3) and W1 = W2 = 11, W3 =
10 G (6).
Fig. 5 Fluid solution ESR spectra of (a) [Mo(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ] 11 and (b) [W(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ]
10 in CH2Cl2 : thf (1 : 2). The asterisk indicates an impurity.
Frozen toluene solutions of 1–7 reveal rhombic g-values with
associated hyperfine satellites. Example spectra, of complexes 3
and 6, are shown in Fig. 6. Complexes 10 and 11 behave similarly
with additional splitting to 31P (Fig. 7).
Simulations. The fluid and frozen solution ESR spectra
were simulated for [Mo(CO)2(g-MeC≡CMe)Tpꢀ] 3, [W(CO)2(g-
MeC≡CMe)Tpꢀ] 6, [W(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ]
10 and [Mo(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] 11, com-
plexes with a common coordinated alkyne (MeC≡CMe), in order
to probe the electronic structure. (Simulations of the spectra of
[Mo(CO)2(g-PhC≡CPh)Tpꢀ] gave very similar parameters to those
of 3, so the nature of the alkyne appears to have little influence.)
The spectra of the tungsten complexes 6 and 10 could be simulated
assuming simple rhombic models (i.e. x = y = z, but the principal
axes of the g and A matrices are coincident with each other and
with the molecular axis system) with the parameters in Table 5.
These two complexes have near axial 183W hyperfine matrices with
‘unique’, large A3-values [Fig. 6(b) and 7(b)]. Note that there
are additional features in the spectrum of 10 that cannot be
reproduced—these are due to an impurity, as is also observed
in the fluid solution spectrum.
Fig. 7 Frozen CH2Cl2 : thf (1 : 2) solution ESR spectra of (a) [Mo(CO)-
{P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] 11 at 120
K and (b) [W(CO)-
{P(OCH2)3CEt}(g-PhC≡CPh)Tpꢀ] 10 at 110 K. In each case, the upper
spectrum is experimental and the lower spectrum is the simulation using
the parameters described in the text with Gaussian line widths of W1
W2 = 9, W3 = 15 G (11) and W1 = 12, W2 = W3 = 14 G (10).
=
Initial attempts to simulate the frozen solution ESR spectra of
the Mo complexes 3 and 11 used a simple rhombic model similar
to that for the W analogues. Although these gave reasonable fits,
there were notable discrepancies in the positions of the hyperfine
transitions (and the line shapes) on the low-field shoulders of g1
and between g2 and g3. In addition, the patterns of the hyperfine
coupling constants obtained (two large values and one small value,
with the “unique” value corresponding to the middle g-value) were
not consistent with the unpaired electron in one of the t2g orbitals.
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Table 5 ESR simulation parameters for [M(CO)L(g-MeC≡CMe)Tpꢀ] {M = Mo, L = CO 3 or P(OCH2)3CEt 11; M = W, L = CO 6 or P(OCH2)3-
CEt 10}
ab
Aiso(P)
A1(P)
A2(P)
A3(P)
a
c
giso
g1
g2
g3
Aiso(M)
A1(M)
A2(M)
A3(M)
3
2.011
2.001
2.021
2.015
2.039
2.027
2.102
2.126
2.016
1.990
2.015
2.002
1.979
1.944
1.955
1.951
31.1
35
+19
+19
−37
−40
+22
+23
−38
−36
+44
+44
−53
−59
35
50
—
—
—
32
—
19
—
25
—
11
—
35
—
11
—
31
—
11
11
6
42
d
10
a A in 10−4 cm−1 b Monoclinic model with twist about g2/A2 axis of ao. c Modelled with P hyperfine axes coincident with g-matrix principal axes. d Not
.
resolved.
For these low spin d5 complexes one would expect one hyperfine
coupling to be significantly larger than the other two and to cor-
respond to the smallest g-value (< ge) (see below), as observed for
the W complexes 6 and 10 (see above). Therefore, the possibility of
non-coincidence effects, and reducing the symmetry to monoclinic
(i.e. one of the g and A axes is coincident, with the other two rotated
through an angle a), was investigated. If the axis of rotation is such
that a numerically large hyperfine value is mixed with a small value,
this can have the effect of producing two apparently numerically
similar values in the observed spectrum. Introducing an angle of
twist a about g2/A2 will thus mix A1 and A3.
A process of trial and error gave the best fit parameters in
Table 5. A significant twist of a = 35◦ is found for [Mo(CO)2(g-
MeC≡CMe)Tpꢀ] 3, with a small value of A1 and a larger value
of A3. Thus, the diagonal A-matrix is now near-axial with a
“unique” large A-value, associated with the smallest g-value, as
expected for low spin d5. This model gives a much better fit to the
experimental spectrum than the simple rhombic model [Fig. 6(a)].
For complex [Mo(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] 11, a
similar process was followed as for 3. Initially, only the g-matrix
and 31P hyperfine matrix were considered. The 31P hyperfine
is near-axial about A2, and the spectra were not sensitive to
non-coincidence between the g and 31P A-matrices. After these
parameters were set the Mo-hyperfine matrix was included; again
a noticeably better fit is found for a monoclinic model [Fig. 7(a),
parameters in Table 5].
more apparent for larger values of the nuclear spin quantum
number I. It is possible that there is non-coincidence in these
systems and that it is simply not detectable in the frozen solution
ESR spectra.
Analysis. For all of the complexes [Mo(CO)2(g-MeC≡
CMe)Tpꢀ] 3, [W(CO)2(g-MeC≡CMe)Tpꢀ] 6, [W(CO){P-
(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] 10 and [Mo(CO){P(OCH2)3
CEt}(g-MeC≡CMe)Tpꢀ] 11 the principal values of the diagonal
metal hyperfine matrices average well to the experimentally
observed isotropic value. Therefore, in each complex A1(M), A2(M)
,
A
3(M) and Aiso(M) have the same sign. The same is also true of the 31
P
couplings in 10 and 11. The absolute signs of the metal hyperfine
components arise from the analysis below.
In the Mo complexes, the coordinated alkyne tends to align more
with one CO than the other (see above). As shown below, DFT
calculations predict that the SOMO in this case is primarily M
dyz based where z is defined by the M–C2 direction, x is along
the M–CO bond co-parallel to the alkyne C≡C bond, and y is
along the M–P or second M–CO axis (Fig. 8).
It should be noted that there are other sets of parameters
that give reasonable simulations of 3 and 11 in a monoclinic
model. For example, a simulation of the spectrum of 11 using
genetic algorithm methods to optimise the parameters,11 gives
the best fit parameters with g1 = 2.012, g2 = 2.029, g3 = 1.949,
A1(Mo) = 26.0 G, A2(Mo) = 40.2 G, A3(Mo) = 36.2 G, A1(P) = 26.2 G,
A2(P) = 37.3 G, A3(P) = 37.5 G, and a monoclinic twist of 35.6o
about g1/A1. The principal g and A(Mo) values are similar to
those obtained from a simple rhombic model (two large and
one small metal hyperfine) highlighting the fact that mixing two
numerically similar A-values has little effect on the observed
spectrum. Moreover, these complexes do not have monoclinic
symmetry—their true symmetry is lower which could in principle
lead to triclinic ESR symmetry (non-coincidence between all three
axes of the g/A-matrices). However, the spectra can be fitted well
with a single twist and do not justify including further fitting
parameters. We therefore restrict ourselves to the simpler model,
preferring the solution using the parameters in Table 5, as the
values are intuitively more sensible. Simulations of the spectra
of the W complexes 6 and 10 proved to be insensitive to non-
coincidence effects, which is not unexpected as such effects are
Fig.
8 The SOMO and coordinate system of [M(CO)L(g-
MeC≡CMe)Tpꢀ].
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If the sole metal contribution to the SOMO is the dyz orbital,
one would expect the system to have rhombic ESR symmetry
with AX the largest component of the hyperfine matrix.12 For a
where R = b2/a2 (i.e. the hybridisation ratio). This give the
diagonal terms:
dyz ground state in a t2g system one would also expect12 one g-
5
ꢂ
ꢃ
ꢃ
ꢃ
4
AX = As − Pd a2 + b2
value (gX) below the free electron value ge and two larger (gY, gZ),
as observed experimentally. Thus, g3 and A3 can be confidently
assigned as gX and AX, respectively, for all the complexes. If a is
the LCAO coefficient of the dyz orbital in the SOMO, one can then
derive a2 from eqn (1):
7
ꢂ
2
AY = As + Pd a2 + b2
7
ꢂ
2
AZ = As + Pd a2 + b2
ꢀ
ꢁ
7
4
2
5
AX − ꢁAꢂ = Pd − a2 + DgX −
(DgY + DgZ)
(1)
and hence,
7
3
42
ꢂ
ꢃ
4
AX − ꢁAꢂ = − Pd a2 + b2
(3)
where ꢁAꢂ = (AX + AY + AZ)/3, and the Dg terms are the shifts of
7
the g-values from ge.
Substituting the values in Table 5 for the tungsten complexes
6 and 10 (in this analysis Y and Z do not need to be assigned)
with Pd = +57.9 × 10−4 cm−1 for W, calculated using Rieger’s
methodology for a 5d5 configuration,13 one obtains a2 = 0.22 and
0.33 for 6 and 10, respectively. Note that sensible answers (positive
values of a2) are only obtained for hyperfine couplings having
negative sign. Thus, the SOMO is only 20–30% localised on the
metal d-orbitals, with the phosphite complex 10 having greater
metal character inthe SOMOwhich presumablyreflects the greater
electron withdrawing nature of the CO ligand cf. the phosphite.
The non-coincidence effects between the principal axes of the
diagonal g and AMo matrices in 3 and 11 must arise from mixing of
d-orbitals in the SOMO. A possible cause of the non-coincidence
effects here could be a rotation of the alkyne about z, which
could mix dyz with dxz in the SOMO. This is supported by the
DFT calculations on 3, 6, 10 and 11, which show that the only
other significant metal contribution to the SOMO is indeed dxz. In
the analysis of the Mo hyperfine couplings, it is assumed that
|SOMO> = a|yz> + b|xz> where a > b. Mixing these two
orbitals requires a rotation about the molecular z-axis, and thus
(vide supra) g2 and A2 are labelled as gZ and AZ. If a > b (i.e. the
dyz orbital is dominant) then the largest component of the metal
hyperfine (A3) is expected to be AX as before (see below). This
leaves g1, A1 as gY, AY. (Note that here X and Y refer to the non-
coincident principal axes of the diagonal g and A-matrices and are
therefore not a consistent axis system—twisting through a about
Z, which is common to both sets of axes and to the molecular
axes, interconverts them.)
Substitution of the data in Table
5 into eqn (3) for
[Mo(CO)2(g-MeC≡CMe)Tpꢀ] 3 and [Mo(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ] 11 (with Pd = −51.9 × 10−4 cm−1 calculated
for Mo with a 4d5 configuration)13 gives the total metal d-orbital
contributions to the SOMO (a2 + b2) = 0.53 and 0.51, respectively.
Including approximate corrections for spin–orbit coupling makes
little difference to these values because of the small experimentally
observed g-shifts. Note that sensible answers are only obtained
for hyperfine couplings having positive sign. Substitution of the
experimental angles of non-coincidence in to eqn (2) gives the
hybridisation ratios b2/a2 = 0.10 and 0.22 for 3 and 11, respectively.
Thus, in 3 the SOMO is 48% dyz and 5% dxz, and in 11 the SOMO
is 42% dyz and 9% dxz. It should be borne in mind that, because of
the ambiguities in the simulation of the spectra of 3 and 11 (see
above), these a2 + b2 values should probably be treated as identical
within the accuracy of the analysis.
The 31P hyperfine coupling for [W(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ] 10 is isotropic within the resolution of the
spectra, meaning that the P 3p orbital contribution to the SOMO
must be very small. However, in 11 the largest component of the
anisotropic hyperfine couplings is A2, which is AZ following the
assignment of the g-values and metal hyperfine couplings and
using the same coordinate system. This implies a 3pz contribution
to the SOMO, which can mix with the Mo 4dyz orbital, and this
is supported by the DFT calculations (see below and Fig. 8). This
contribution can be quantified from:
4
AZ − ꢁAꢂ = Ppc2
In this case the non-zero elements of the metal hyperfine matrix
are expected to be [ignoring spin–orbit coupling terms (g-shifts)
for simplicity]:
5
where c is the LCAO coefficient of the P 3pz in the SOMO and
Pp = +306 × 10−4 cm−1,14 to give c2 = 0.02, i.e. a 2% contribution
to the SOMO of 11.
ꢂ
ꢃ
2
Axx = As + Pd −2a2 + b2
7
DFT calculations on [Mo(CO)2(g-MeC≡CMe)Tpꢀ] 3,
[W(CO)2(g-MeC≡CMe)Tpꢀ] 6, [W(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ] 10 and [Mo(CO){P(OCH2)3CEt}(g-
MeC≡CMe)Tpꢀ] 11
ꢂ
ꢃ
2
Ayy = As + Pd a2 − 2b2
7
ꢂ
ꢃ
2
Azz = As + Pd a2 + b2
7
In order to understand further the results of the ESR anal-
ysis, DFT calculations have been carried out on [Mo(CO)2(g-
MeC≡CMe)Tpꢀ] 3, [W(CO)2(g-MeC≡CMe)Tpꢀ] 6, [W(CO){P-
(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] 10 and [Mo(CO){P(OCH2)3-
CEt}(g-MeC≡CMe)Tpꢀ] 11, with the same coordinate system
as used in the ESR analysis. The geometries of the neutral
compounds were optimised as unrestricted spin-doublets using
2
Axy = As + Pd (3ab)
7
where As is the Fermi contact term. This matrix can be diago-
nalised by rotation about z by an angle a such that: 12
√
−2
R
tan a =
(2)
1 − R
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˚
the M–C2 direction is the mechanism for mixing the dyz and dxz
orbitals in the SOMO.
Table 6 X-Ray crystallographically determined bond lengths (A)
and the angle, b, between the C≡C and W–C(O) vectors for
[W(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ]+ 10+, and calculated values
for both 10+ and 10
One striking feature is that, although the trends are in agree-
ment, the computationally calculated metal spin populations are
considerably larger than those from the ESR spectroscopic data.
The dyz contribution to the SOMO is computed at 57% and
66% for 3 and 11 respectively, compared to values from the ESR
analysis of 48% and 42%. The analogous values for 6 and 10 are
calculated at 47% and 61%, compared to 22% and 33% from the
ESR spectra. Clearly, there is a significant discrepancy between the
two approaches, with either the calculated values being too large
or the ESR values being too small (or, probably more likely, both,
with the real values lying between those from the two approaches).
A comparison of the calculated and crystallographic structures
of 10+ indicates one possible source of error. The computed
metal–phosphite distance is too long (which is recognised to
be a problem when comparing crystallographically determined
structures with gas-phase calculations18) which, since the ligand is
a p-acceptor, might imply that the model system has not included
sufficient p back-donation. If this were also the case for the neutral
compounds, then the computations would indicate too much
unpaired electron density in the dyz orbitals, as seems to be the
case for all compounds. It would also indicate too little electron
density in the phosphorus pz orbital of 10 and 11, leading to a
computed value of about 1% for 11 compared with the value from
the ESR spectrum of about 2%. However, the apparent margin of
disagreement about the phosphorus pz occupancy (approximately
1% between the two methods) is an order of magnitude smaller
than the disagreement over the metal dyz occupancy (24 to 29%),
and it seems unlikely that correcting the former would resolve the
latter. It is worth noting at this point that the calculation of metal
hyperfine coupling constants using conventional basis sets (such
as that used here) is recognised to be problematic (because of the
form at the nucleus of the Gaussian orbitals used).19
W–Xa C≡C W–P
M–CO C≡O b/◦
10+ (X-ray)
1.927
1.944
2.072
1.309 2.452 1.973
1.322 2.505 1.999
1.289 2.378 1.970
1.159 16.9
1.160 17.4
1.173 13.4
10+ (calculated)
10 (calculated)
a X = midpoint of C≡C bond.
B3LYP density functional theory implemented by the programme
Jaguar,15 with the 6–31G* basis set for non-metals and LACV3P16
for the metal atom. This optimised geometry was then used as the
basis for a natural bond orbital (NBO) analysis,17 which gave the
total spin population on each atom and a breakdown of the SOMO
into contributions from each atomic orbital. For the purposes of
comparison with the crystallographic study, the structure of 10+
was calculated using the same methods.
Table 6 compares the crystallographically determined bond
lengths of 10+ with those calculated for 10 and 10+. There is good
agreement between the calculated and structurally determined
metrics of 10+, especially as the calculated structure is of an isolated
gas-phase ion. The calculated changes in the metal–alkyne unit
upon reduction of 10+ to 10 are in agreement with those predicted
by the bonding model from ref. 2 in that the orbital depopulated
upon oxidation is metal–alkyne antibonding.
In all cases, the unpaired electron is located mainly in the
dyz orbital of the metal (again, consistent with the MO scheme
from ref. 2, and in agreement with this being the major metal
contribution to the SOMO from the analysis of the ESR spectra).
The second largest contribution is from the metal dxz orbital;
no other atomic orbital contains more than 4% of the unpaired
electron. Table 7 lists pertinent values. More electron density
resides on the molybdenum atoms than on the tungsten atoms,
and this is also reflected in the greater amount of spin population
of the phosphorus pz orbital for 10 compared to 11. The total spin
population on the metal atom increases upon replacing a carbonyl
ligand with a phosphite, as would be expected on replacing a
good p-acceptor with a poorer p-acceptor. For the molybdenum
compounds 3 and 11, the ratio of the dxz contribution to that of the
dyz is approximately 0.17, whereas for 6 and 10 it is about 0.10. This
is consistent with the computed values of b {the angle between the
C≡C and M–C(O) vectors}, because it is the breaking of the local
symmetry by a rotation of the alkyne that allows mixing of the two
orbitals, and the molybdenum compounds have larger values of b.
The larger calculated value of b for 11 cf. 3 is also consistent with
the larger angle of non-coincidence (a) found between the g and
Mo hyperfine matrices for 11 if the rotation of the alkyne about
A second possible source of error might be the simplifying
assumptions used in the ESR spectroscopic analysis, in particular
that the metal orbital contribution to the SOMO is either purely
dyz or a dyz/dxz hybrid; the symmetry at the metal is undoubtedly
lower than assumed, and there are therefore contributions to the
SOMO from the remaining d orbitals. Hence, although modelling
the spectra as rhombic or monoclinic gives good fits to the
experimental data, it neglects some small effects.
Conclusions
The dicarbonyls [M(CO)2(g-RC≡CRꢀ)Tpꢀ]+ {M = Mo or W, Tpꢀ =
hydrotris(3,5-dimethylpyrazolyl)borate} undergo one-electron
reduction to paramagnetic [M(CO)2(g-RC≡CRꢀ)Tpꢀ]; structural
studies on the redox pair [Mo(CO)2(g-PhC≡CPh)Tpꢀ]z (z = 0 and
Table 7 NBO calculated spin populations (%) for [M(CO)L(g-MeC≡CMe)Tpꢀ] {M = Mo, L = CO 3 or P(OCH2)3CEt 11; M = W, L = CO 6 or
P(OCH2)3CEt 10}, and b, the angle between the C≡C and M–C(O) vectors
Total metal unpaired electron density
Metal dyz
Metal dxz
dxz/dyz
Phosphorus pz
b/◦
3
0.735
0.580
0.740
0.848
0.569
0.470
0.605
0.657
0.098
0.043
0.065
0.118
0.17
0.09
0.11
0.18
—
—
0.037
0.011
15.7
9.5
13.4
17.7
6
10
11
3474 | Dalton Trans., 2006, 3466–3477
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1) show reduction is accompanied by a lengthening of the Mo–
Calkyne bonds, and a shortening of the C≡C bond, consistent with
conversion of a three- to a four-electron donating alkyne ligand.
Subsitution of one carbonyl ligand of [M(CO)2(g-
RC≡CRꢀ)Tpꢀ]+ gives [M(CO)L(g-RC≡CRꢀ)Tpꢀ]+ {L = MeCN
or P(OCH2)3CEt}, the reduction potentials of which are signifi-
cantly more negative. Chemical reduction of the nitrile complex
[Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ][PF6] results in the forma-
tion of [MoCl(CO)(g-MeC≡CMe)Tpꢀ] in CH2Cl2 whereas more
stable paramagnetic phosphite complexes, [M(CO){P(OCH2)3-
CEt}(g-RC≡CRꢀ)Tpꢀ], have been characterised by ESR spectro-
scopy. The ESR spectroscopic results illustrate that the SOMO
of the neutral, d5 species (the LUMO of the cationic d4 species)
are considerably more delocalised for the tungsten than for the
molybdenum complexes, in agreement with DFT calculations.
Furthermore, they support a decrease in electron density at the
metal on replacement of CO with the poorer p-acceptor phosphite
ligand. The ESR spectra of the Mo species also reveal non-
coincidence effects between the g and metal hyperfine matrices,
a possible mechanism for which is a rotation of the coordinated
alkyne about the M–C2 axis, and consequent mixing of the metal
d-orbitals.
the conditions used, Eoꢀ for the one-electron oxidation of [Fe(g-
C5H5)2] or [Fe(g-C5Me5)2] and the one-electron reduction of [Co(g-
C5H5)2][BF4], added to the test solutions as internal calibrants, is
0.47, −0.08 and −0.86 V respectively in CH2Cl2.
Microanalyses were carried out by the staff of the Microanalysis
Service of the School of Chemistry, University of Bristol.
Syntheses
[Mo(CO)2(g-PhC≡CPh)Tpꢀ][PF6]·CH2Cl2, 1+[PF6]−·CH2Cl2.
To a stirred solution of [Mo(CO)3Tpꢀ] (2.0 g, 4.19 mmol) and
PhC≡CPh (1.50 g, 8.42 mmol) in CH2Cl2 (100 cm3) was added
[Fe(g-C5H5)2][PF6] (1.38 g, 4.17 mmol). The dark brown solution
was heated under reflux for 24 h. The solvent was removed in vacuo
leaving a red solid which was washed thoroughly with diethyl ether
(4 × 50 cm3) to remove [Fe(g-C5H5)2] and PhC≡CPh. The residue
was redissolved in CH2Cl2 (20 cm3) and filtered through Celite.
Addition of diethyl ether (50 cm3) and cooling to −20 ◦C induced
precipitation of a red powder. A second purification using CH2Cl2–
diethyl ether gave the product as the crimson-red crystalline 1 : 1
CH2Cl2 solvate, yield 2.16 g (60%).
The complex [Mo(CO)2(g-4-MeOC6H4C≡CC6H4OMe-4)Tpꢀ]-
[PF6]·0.5CH2Cl2, 2+[PF6]−·0.5CH2Cl2 was prepared similarly in
65% yield, but only required the reaction mixture to be heated
under reflux for 4 h.
Experimental
[Mo(CO)2(g-MeC≡CMe)Tpꢀ][PF6], 3+[PF6]−. To a stirred so-
lution of [Fe(g-C5H5)2][PF6] (0.69 g, 2.08 mmol) and MeC≡CMe
(0.5 cm3, 6.38 mmol) in CH2Cl2 (50 cm3) was added [Mo(CO)3Tpꢀ]
(1.0 g, 2.10 mmol). After 45 min the solvent was removed in vacuo
leaving a green solid which was washed thoroughly with diethyl
ether (3 × 25 cm3) to remove [Fe(g-C5H5)2] and any residual
MeC≡CMe. The residue was redissolved in CH2Cl2 (20 cm3) and
filtered through Celite. Toluene (50 cm3) was added and the volume
of the mixture reduced in vacuo, inducing the precipitation of a
green powder. A second purification using CH2Cl2–diethyl ether
gave the product as a green crystalline solid, yield 0.82 g (60%).
The preparation, purification and reactions of the complexes
described were carried out under an atmosphere of dry nitro-
gen, using dried and deoxygenated solvents purified either by
distillation or by using Anhydrous Engineering double alumina
or alumina/copper catalyst drying columns. Reactions were
monitored by IR spectroscopy where necessary. Unless stated
otherwise complexes were purified by dissolution in CH2Cl2,
filtration of the solution through Celite, addition of n-hexane to
the filtrate, and reduction of the volume of the mixture in vacuo to
induce precipitation.
The compounds [M(CO)3Tpꢀ] (M = Mo, W),20 [W(CO)2(g-
RC≡CRꢀ)Tpꢀ][PF6] (R
=
Rꢀ
=
Me, Ph;
R
=
Ph,
[Mo(CO)2(g-PhC≡CH)Tpꢀ][PF6]·CH2Cl2,
4+[PF6]·CH2Cl2.
Rꢀ = H),4 [WI(CO)3Tpꢀ],4 [Fe(g-C5H5)2][PF6],21 [Fe(g-C5H5)(g-
C5H4COMe)][BF4],21 [Co(g-C5H5)2]22 and [Co(g-C5Me5)2]23 were
prepared by published methods.
To a stirred solution of [Mo(CO)3Tpꢀ] (1.00 g, 2.10 mmol) and
PhC≡CH (0.5 cm3, 4.51 mmol) in CH2Cl2 (50 cm3) was added
[Fe(g-C5H5)2][PF6] (0.69 g, 2.09 mmol). After 20 min the dark
brown solution was evaporated to dryness in vacuo. The brown
residue was washed thoroughly with diethyl ether (3 × 25 cm3)
to remove [Fe(g-C5H5)2] and residual PhC≡CH. The residue was
redissolved in CH2Cl2 and filtered through Celite. Addition of
diethyl ether (50 cm3) and cooling to −20 ◦C induced precipitation
of a brown powder. A second purification using CH2Cl2–diethyl
ether gave the product as the orange-brown crystalline 1.0 CH2Cl2
solvate, yield 0.65 g (40%).
IR spectra were recorded on a Nicolet 5ZDX FT spectrometer
or a Perkin Elmer Spectrum One FT-IR spectrometer fitted
with a Perkin Elmer ZnSe Universal ATR sampling accessory.
1
Proton, 31P and 13C-{ H} NMR spectra were recorded on JEOL
GX270, GX400 or k300 spectrometers with SiMe4 (1H and 13C)
or H3PO4 (31P) as internal standards. X-Band ESR spectra were
recorded on a Bruker ESP300 spectrometer equipped with a
Bruker variable temperature accessory and a Hewlett-Packard
5350B microwave frequency counter. The field calibration was
checked by measuring the resonance of the diphenylpicrylhydrazyl
(dpph) radical before each series of spectra. Electrochemical
studies were carried out using an EG&G model 273A potentiostat
in conjunction with a three-electrode cell. The auxiliary electrode
was a platinum wire and the working electrode a platinum disc
(1.6 or 2.0 mm diameter). The reference was an aqueous saturated
calomel electrode separated from the test solution by a fine
porosity frit and an agar bridge saturated with KCl. Solutions
were 1.0 × 10−3 mol dm−3 in the test compound and 0.1 mol dm−3
[Mo(CO)2(g-PhC≡CPh)Tpꢀ]·0.5C6H14, 1·0.5C6H14. To
a
stirred solution of [Mo(CO)2(g-PhC≡CPh)Tpꢀ][PF6]·CH2Cl2
(0.80 g, 0.933 mmol) in CH2Cl2 (40 cm3) was added [Co(g-C5H5)2]
(0.176 g, 0.933 mmol). The solution rapidly became dark green
and after 10 min the solvent was removed in vacuo. The black
◦
residue was extracted into hot n-hexane (50 C; 1 × 70 cm3,
2 × 25 cm3) and filtered through Celite. Cooling the extract to
−20 ◦C for 18 h gave the green-black crystalline product as a 0.5
n-hexane solvate, yield 0.32 g (48%) (Found C, 61.0; H, 5.8; N,
12.8. C34H39N6BO2Mo requires C, 60.9; H, 5.9; N, 12.5%).
in [NBun ][PF6] as the supporting electrolyte in CH2Cl2. Under
4
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[Mo(CO)(NCMe)(g-MeC≡CMe)Tpꢀ][PF6]·thf, 8+[PF6]−·thf.
On stirring [Mo(CO)2(g-MeC≡CMe)Tpꢀ][PF6] (400 mg,
0.62 mmol) in MeCN (30 cm3) CO gas was evolved and
the solution rapidly changed colour from yellow-green to pale
blue. After 5 min the solvent was removed in vacuo. The blue
residue was then redissolved in thf and n-hexane was added. On
cooling the mixture to −20 ◦C for 6 h a pale green solid was
isolated as a 1 : 1 thf solvate, yield 345 mg (76%).
Structure determinations of
[Mo(CO)2(g-PhC≡CPh)Tpꢀ]·0.5C6H14, 1·0.5C6H14,
[Mo(CO)2(g-PhC≡CPh)Tpꢀ][PF6], 1+[PF6]− and
[W(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ][BF4]·2CH2Cl2,
10+[BF4]−·2CH2Cl2
Black crystals of 1·0.5 C6H14, red-black crystals of 1+[PF6]−
and blue crystals of 10+[BF4]−·2CH2Cl2 were grown by allow-
ing n-hexane to diffuse into a solution of the complex in
dichloromethane.
[W(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ][BF4], 10+[BF4]−.
To a stirred solution of [W(CO)2(g-MeC≡CMe)Tpꢀ][BF4] (372 mg,
0.55 mmol) in thf (30 cm3) was added P(OCH2)3CEt (98 mg,
0.61 mmol). The mixture was heated under reflux for 16 h and
then evaporated to dryness in vacuo. The resulting blue solid was
redissolved in CH2Cl2 (10 cm3), n-hexane (40 cm3) was added and
Crystal data for 10+[BF4]−·2CH2Cl2. C28H40N6O4B2Cl4F4PW:
M = 978.90, monoclinic, space group P21/c, a = 15.065(2), b =
◦
3
˚
˚
21.148(4), c = 13.192(2) A, b = 114.27(2) , V = 3831.5(9) A ,
T = 173 K, Z = 4, l = 3.397 mm−1, reflections collected = 24687,
independent reflections (Rint) = 8790 (0.0393), final R indices [I >
2r(I)]: R1,wR2 = 0.0292, 0.0646. Crystal data for 1·0.5 C6H14 and
1+[PF6]− are given in ref. 3; CCDC reference codes are RAPCOA
and RAPCOG respectively.
◦
the solution was cooled to −20 C giving, after several days, the
product as a blue, crystalline 1 : 1 CH2Cl2 solvate, yield 116 mg
(26%).
[Mo(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ][PF6]·0.5CH2Cl2,
11+[PF6]·0.5CH2Cl2. The salt [Mo(CO)2(g-MeC≡CMe)Tpꢀ]-
[PF6] (318 mg, 0.49 mmol) was dissolved in MeCN (40 cm3) and
P(OCH2)3CEt (90 mg, 0.55 mmol) added. The mixture was heated
under reflux for 10 min and then evaporated to dryness in vacuo.
The green solid was redissolved in CH2Cl2 (10 cm3), n-hexane
(40 cm3) was added and the volume reduced in vacuo, inducing
precipitation of a green solid. The solid was purified by allowing
n-hexane (50 cm3) to diffuse into a concentrated solution of the
complex in CH2Cl2 (4 cm3) at −20 ◦C. This gave the product as a
green, crystalline 0.5 CH2Cl2 solvate, yield 200 mg (47%).
CCDC reference number 287241.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b514951g
Acknowledgements
We thank the EPSRC (D.J.H. and O.D.H.), the University of
Bristol (I.M.B. and M.J.Q.) and the Royal Thai Government
(S.B.) for Postgraduate Scholarships. We also thank Dr Zbigniew
Sojka (Jagiellonian University, Krakow) for simulating the ESR
spectrum of complex 11 using genetic algorithm methods.
The generation of [M(CO)2(g-RC≡CRꢀ)Tpꢀ] (M = Mo; R = Rꢀ =
Me, C6H4OMe-4, R = Ph, Rꢀ = H; M = W; R = Rꢀ = Me, R =
Ph, Rꢀ = H) and [M(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ]
(M = Mo or W) for ESR spectroscopy
References and notes
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To a stirred suspension of [Mo(CO)2(g-MeC≡CMe)Tpꢀ][PF6]
(10.0 mg, 0.015 mmol) in toluene (2.0 cm3) was added [Co(g-
C5H5)2] (2.0 mg, 0.011 mmol). After 5 min the solution of
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transferred to a deoxygenated ESR tube. The solution was
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Me; R = Ph, Rꢀ = H) were generated similarly. A solution
of [W(CO)2(g-PhC≡CPh)Tpꢀ] (5) was prepared by reducing
[W(CO)2(g-PhC≡CPh)Tpꢀ][BF4] with [Co(g-C5H5)2] in CH2Cl2,
evaporating the resulting mixture to dryness and then extracting
the product into toluene.
Samples of [M(CO){P(OCH2)3CEt}(g-MeC≡CMe)Tpꢀ] (M =
W, 10 or Mo, 11) were prepared by adding the solid reducing
agent ([Co(g-C5Me5)2] and [Co(g-C5H5)2] respectively) to a frozen
solution of 10+[BF4]− or 11+[PF6]− in CH2Cl2 : thf (1 : 2) in an
ESR tube. The tube was transferred to the ESR spectrometer at
low temperature, and then the contents allowed to melt in order
for reduction to occur. Once reaction had occurred, at the lowest
temperature possible, the sample was refrozen and the anisotropic
spectrum recorded. The temperature of the sample was then slowly
increased until an isotropic spectrum was obtained.
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