Molecular Electrochemistry of Coordination Compounds …
417
2.3 Molybdenum and Tungsten Scorpionates
The 16e Mo and W nitrosyl scorpionates (a common name for tris(pyrazol-1yl)borate complexes), stable in solid and in solution, show well-defined reversible
reduction potentials (and in some cases also reversible 1e oxidation processes),
readily controllable in the enormous range of 2200 mV by selecting suitable coligands [32]. Their catalytic properties in dehalogenation of some chloroalkanes
were detected only recently (vide infra), whereas the closely related oxo-analogues
were utilised in enzyme modelling a long time ago [33, 34].
The redox potential of the nitrosyl, oxo- and oxothio-Mo
II/I and W
II/I pairs in the
catalytically relevant alkoxide scorpionates can be accurately reproduced (to 0.05 V)
by DFT calculations at the IEF-PCM/B3LYP(-D2)/LACV3P+/6-311++G(2d,2p)
level on top of the IEF-PCM/B3LYP/LANL2DZ/6-31G(d,p) or, when more precise geometries are needed, COSMO/B3LYP/def2-TZVPP structures. A triple-ζ
basis set containing a diffuse d function on metal is essential in description of the
low Mo
I or W
I oxidation state. Amongst the computationally investigated examples of this class of complexes are the [Mo(NO)Tp
Me2 (OMe) 2 ] complex (Tp
Me2
hydrotris(3,5-dimethylpyrazol-1-yl)borate), which undergoes reduction at E 1/2
−1.84 V (E
◦
calc −1.82 V)
5 as well as oxidation at +0.91 V [35], and tungsten
analogues [36] depicted in Fig. 1. As can be seen, in all these cases, the calculated potentials match the experimental values very accurately, and ν NO frequency
is reproduced well for the W
II complex. Interestingly, the difference between the
SOMO energies for the Mo and W dimethoxy scorpionates (0.58 eV) agrees nicely
with the difference between their E 1/2 values for reduction (0.56 V). The characteristic feature of these dimethoxy Mo and W complexes is that the reduction is an
evidently slower process with E p 250–122 mV, an effect of a relatively high
inner-sphere reorganisation energy due to the appreciable elongation of the M−O alk
bond (by 0.12 Å) and a decrease in M−O alk −C angle (thus, a lesser degree of pπ O -
dπ Mo charge donation occurs), whereas upon oxidation the structural changes are
more subtle (d M−O alk 0.05 Å) and E p drops to 82–68 mV.
The potentials obtained in the DFT modelling of the nitrosyl Mo and W species
in CH 2 Cl 2 were received with the use of 4.84 V as E
◦
abs (Fc
+ /Fc) (see above), which
deserves a comment. Obviously, this value is not likely to be the absolute potential
of Fc in CH 2 Cl 2 (vide supra), and the remarkable accuracy (to 0.05 V) achieved for
a series of {M
II/I (NO)(Tp
Me2 )}
2+/1+ alkoxides [35–37] is due to a systematic error of
0.3 V of the B3LYP functional (and/or the IEF-PCM solvation) for these complexes.
Intriguingly, it seems that this is not the case for oxo-Mo
V species discussed below,
i.e. where the same methodology as for the nitrosyl species was used but a value of
5.14 V for E abs (Fc) yielded excellent agreement with experiment. In contrast to the
accurate B3LYP results, the non-hybrid functionals BP86 and OLYP underestimate
the reduction potentials for this class of Mo−NO complexes, yielding too cathodic
values by even hundreds of mV.
5 The computed potential given in [35] (−1.80 V) was obtained from calculations done in a slightly
differently contracted basis set (LANL2DZ) for Mo.
417
2.3 Molybdenum and Tungsten Scorpionates
The 16e Mo and W nitrosyl scorpionates (a common name for tris(pyrazol-1yl)borate complexes), stable in solid and in solution, show well-defined reversible
reduction potentials (and in some cases also reversible 1e oxidation processes),
readily controllable in the enormous range of 2200 mV by selecting suitable coligands [32]. Their catalytic properties in dehalogenation of some chloroalkanes
were detected only recently (vide infra), whereas the closely related oxo-analogues
were utilised in enzyme modelling a long time ago [33, 34].
The redox potential of the nitrosyl, oxo- and oxothio-Mo
II/I and W
II/I pairs in the
catalytically relevant alkoxide scorpionates can be accurately reproduced (to 0.05 V)
by DFT calculations at the IEF-PCM/B3LYP(-D2)/LACV3P+/6-311++G(2d,2p)
level on top of the IEF-PCM/B3LYP/LANL2DZ/6-31G(d,p) or, when more precise geometries are needed, COSMO/B3LYP/def2-TZVPP structures. A triple-ζ
basis set containing a diffuse d function on metal is essential in description of the
low Mo
I or W
I oxidation state. Amongst the computationally investigated examples of this class of complexes are the [Mo(NO)Tp
Me2 (OMe) 2 ] complex (Tp
Me2
hydrotris(3,5-dimethylpyrazol-1-yl)borate), which undergoes reduction at E 1/2
−1.84 V (E
◦
calc −1.82 V)
5 as well as oxidation at +0.91 V [35], and tungsten
analogues [36] depicted in Fig. 1. As can be seen, in all these cases, the calculated potentials match the experimental values very accurately, and ν NO frequency
is reproduced well for the W
II complex. Interestingly, the difference between the
SOMO energies for the Mo and W dimethoxy scorpionates (0.58 eV) agrees nicely
with the difference between their E 1/2 values for reduction (0.56 V). The characteristic feature of these dimethoxy Mo and W complexes is that the reduction is an
evidently slower process with E p 250–122 mV, an effect of a relatively high
inner-sphere reorganisation energy due to the appreciable elongation of the M−O alk
bond (by 0.12 Å) and a decrease in M−O alk −C angle (thus, a lesser degree of pπ O -
dπ Mo charge donation occurs), whereas upon oxidation the structural changes are
more subtle (d M−O alk 0.05 Å) and E p drops to 82–68 mV.
The potentials obtained in the DFT modelling of the nitrosyl Mo and W species
in CH 2 Cl 2 were received with the use of 4.84 V as E
◦
abs (Fc
+ /Fc) (see above), which
deserves a comment. Obviously, this value is not likely to be the absolute potential
of Fc in CH 2 Cl 2 (vide supra), and the remarkable accuracy (to 0.05 V) achieved for
a series of {M
II/I (NO)(Tp
Me2 )}
2+/1+ alkoxides [35–37] is due to a systematic error of
0.3 V of the B3LYP functional (and/or the IEF-PCM solvation) for these complexes.
Intriguingly, it seems that this is not the case for oxo-Mo
V species discussed below,
i.e. where the same methodology as for the nitrosyl species was used but a value of
5.14 V for E abs (Fc) yielded excellent agreement with experiment. In contrast to the
accurate B3LYP results, the non-hybrid functionals BP86 and OLYP underestimate
the reduction potentials for this class of Mo−NO complexes, yielding too cathodic
values by even hundreds of mV.
5 The computed potential given in [35] (−1.80 V) was obtained from calculations done in a slightly
differently contracted basis set (LANL2DZ) for Mo.