Decomposition of electron-excited molecules of molybdenum and tungsten phosphine hydride and phosphine nitrogen complexes : 111. *Photodehydrogenation, photodenitrogenation, and formation of coordinatively unsaturated species

Article abstract of DOI:10.1007/s11172-008-0044-5

The kinetics and mechanism of photodehydrogenation of the phosphine hydride complexes MH₄L₄ (M = Mo, W; L are phosphine ligands) and the formation of coordinatively unsaturated species M_L4 were studied by the absorbance of

Full text of DOI:10.1007/s11172-008-0044-5

Russian Chemical Bulletin, International Edition, Vol. 57, No. 2, pp. 289—291, February, 2008
289
Decomposition of electronꢀexcited molecules of molybdenum and tungsten
phosphine hydride and phosphine nitrogen complexes
1.* Photodehydrogenation, photodenitrogenation, and formation
of coordinatively unsaturated species
A. P. Pivovarov
Institute of Problem of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (252) 49 676. Eꢀmail: pivovar@cat.icp.ac.ru
The kinetics and mechanism of photodehydrogenation of the phosphine hydride complexes
MH4L4 (M = Mo, W; L are phosphine ligands) and the formation of coordinatively unsaturated
species ML4 were studied by the absorbance of longꢀwavelength bands with λmax at 450—460 nm
appeared in the absorption spectra of the photoproducts. The rate constants of the reactions of
the coordinatively unsaturated M(DPPE)2 species (M = Mo, W; DPPE = Ph2PCH2CH2PPh2)
with molecular nitrogen in benzene were determined (kW = 200 s–1, kMo = 8700 s–1).
Key words: coordination compounds, photochemistry, primary mechanism of photodeꢀ
composition, activation and fixation of small molecules.
It has previously been shown1—7 that the products of
photodecomposition of the molybdenum and tungsten
phosphine hydride complexes are able, under mild condiꢀ
tions, to efficient activation and fixation of various molꢀ
ecules, including rather inert molecules, such as N2, CO,
CO2, H2, alkanes, cycloalkanes, alkenes, and arenes.
Therefore, it was of interest to study the mechanism of
decomposition of electronꢀexcited molecules of the phosꢀ
phine hydride complexes of transition metals MH4L4
(M = Mo, W; L = PHPh2, PMePh2, PEtPh2, PEt2Ph,
PBuPh2, PPrPh2, P(OPri)2Ph, P(OPri)3, 1/2 DPPE
(DPPE = Ph2PCH2CH2PPh2), and others). In the present
work, we studied the kinetics of photodehydrogenation of
the phosphine hydride complexes upon irradiation in deuꢀ
terated benzene (C6D6) and cyclohexane (C6D12) and the
formation of coordinatively unsaturated species from the
Mo and W phosphine hydride (MH4L4) and phosphine
nitrogen (transꢀM(N2)2L4) complexes under the condiꢀ
tions of stationary and pulse irradiation at 77 and 300 K,
respectively.
M(N2)2L4 were irradiated in quartz cells placed in a Dewar
vessel with liquid nitrogen. The electronic absorption spectra in
the visible and nearꢀIR regions were recorded during irradiation.
The pulse irradiation of solutions of the MH4L4 and transꢀ
M(N2)2L4 complexes at 300 K was studied on a laser pulse
photolysis setup9 using an YAG: Nd3+ laser (alumoyttrium garnet
with neodymium) and the harmonic with λ = 354 nm.
Results and Discussion
The irradiation of solutions of the phosphine hydride
MH4L4 complexes at 300 K results in the evolution of H2
into the gas phase over solutions, and free phosphine
ligands L appear in the solution. The quantum yields for
C (mol.%)
120
H2
80
Experimental
40
H—D
Solutions of the MH4L4 complexes in a concentration of
∼10–3 mol L–1 in deuterated solvents were irradiated with the
monochromatic light at ∼300 K in cylindrical quartz cells with
continuous stirring of the solutions. The light intensity was
measured by the ferrioxalate actinometer.8 The gas phase over
the photolyzed solutions was analyzed using a Töpler pump and
a mass spectrometer. Solid solutions of MH4L4 and transꢀ
0
100
200
300
Irradiation time/min
Fig. 1. Photochemical dehydrogenation of a solution of
MoH4(DPPE)2 in C6D6 upon irrdiation with λ = 254 nm,
I0 = 1.5•10–8 Einstein s–1 mL–1, 25 °C.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 282—284, February, 2008.
1066ꢀ5285/08/5702ꢀ0289 © 2008 Springer Science+Business Media, Inc.

290
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 2, February, 2008
Pivovarov
C (mol.%)
D
1.0
0.8
0.6
0.4
0.2
2
120
4
H2
1
80
40
H—D
3
D2
0
300
400
500
λ/nm
50
100
150
Irradiation time/min
Fig. 3. Absorption spectra of MoH4(PMePh2)4 (C0 = 10–3 mol L–1) in
toluene at 77 K, d = 1 mm: initial spectrum (1), after irradiation
with λ = 254 nm for 1.5 h (2); different spectrum (2 – 1) (3),
and the absorption spectrum of the coordinatively unsaturated
photoproduct formed at 300 K after the pulse irradiation of a
solution of transꢀMo(N2)2(PMePh2)4 (4).
Fig. 2. Photochemical dehydrogenation of a solution of
WH4(PMePh2)4 in C6D12 upon irradiation with λ = 254 nm,
I0 = 1.5•10–8 Einstein s–1 mL–1, 25 °C.
the photoelimination of hydrogen (ϕH ) and phosphine
2
ligands (ϕL) are ∼10–1 and ∼10–3—10–4, respectively; thereꢀ
fore, the main process of [MH4L4]* decomposition is
photodehydrogenation. As shown in Figs 1 and 2, HD and
D2 are formed along with Н2 by the photodehydrogenation
of MH4L4 in deuterated R—D solvents.
It is shown in Fig. 3 that upon the irradiation of a
solution of MoH4(PMePh2)4 at 77 K (λ = 254 nm) the
initial electronic absorption spectrum (curve 1) is transꢀ
formed into the overall spectrum of the photoproduct and
residues of the initial complex (curve 2). The difference
curve 3 (2 – 1) represents the absorption spectrum with
the absorption maximum at 450—460 nm. This longꢀwaveꢀ
length absorption band presumably belongs to the
coordinatively unsaturated species MoH2(PMePh2)4 or
Mo(PMePh2)4.
Note that the longꢀwavelength absorption band, apꢀ
peared after the pulse irradiation of a benzene solution of
transꢀMo(N2)2(DPPE)2, coincides well with the band in the
absorption spectra of the photoproducts formed upon the
photolysis of MoH4(DPPE)2 or transꢀMo(N2)2(PMePh2)2
in a solid solution of toluene at 77 K (cf. curves 3 and 4 in
Fig. 3). This means that the same coordinatively unsaturꢀ
ated ML4 species are formed upon the photolysis of MH4L4
and transꢀM(N2)2L4.
Possible primary photochemical reactions during the
photodehydrogenation of MH4L4 can be the heterolytic
cleavage of the M—H with the formation of H+ or H–,
homolytic cleavage forming atomic hydrogen H•, and
finally, H2 photoelimination via the molecular mechaꢀ
nism. The molecular mechanism seems to be the most
probable. First, in such nonpolar media as benzene or
cyclohexane the heterolytic cleavage of the M—H bond is
improbable. Second, in the present study, no atomic hyꢀ
drogen H• or radical products of its addition to molecules
with unsaturated bonds and no other radical or radical ion
products were observed by the ESR method upon the
photolysis of MH4L4 in solid solutions at 77 K. Third, the
analysis of the kinetic curves presented in Figs 1 and 2
shows that H2 is formed in the primary photochemical
processes and HD and D2 are formed only after an inducꢀ
tion period (∼70—90 min), i.e., in the secondary (dark
and photochemical) reactions. It is most likely that the
coordinatively unsaturated species appeared due to the
primary photochemical reactions can interact with the
deuterated R—D solvents to give the oxidative addition
products, whose irradiation can result in the formation of
HD and D2.
D550•104
–lnD550
1
2
7.5
8
6
4
2
7.0
MH4L4
H2 + MH2L4
6.5
MH2(D)(R)L4
HD
(1)
(2)
0
1
2
3
Time/ms
MH4L4
2 H2 + ML4
MD2R2L4
Fig. 4. Pulse photolysis of a nitrogenꢀsaturated benzene solution
of transꢀW(N2)2(DPPE)2 at 300 K: kinetics of changes in the
absorbance at λ = 550 nm (D550) after a photolyzing pulse (1);
lnD550 (2).
D2

Photolysis of Moꢀ and Wꢀcontaining complexes
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 2, February, 2008
291
600 nm was 8700 s–1. By analogy to the isoelectronic
tungsten complex, this constant is the rate constant for
the addition of the first N2 molecule to the coordinatively
unsaturated Mo(DPPE)2 species. Note that the ratio of
the rate constants for the addition of the first N2 molecule
to the isoelectronic coordinatively unsaturated M(DPPE)2
species for Mo and W is 43.
Thus, the irradiation of the phosphine hydride (MH4L4)
or phosphine nitrogen (transꢀM(N2)2L4) complexes of Mo
and W in the region λ < 350 nm results in the elimination
of two H2 or two N2 molecules, respectively, to form the
primary coordinatively unsaturated ML4 species characꢀ
terized by the longꢀwavelength absorption bands with
D500
–lnD500
7
0.08
6
2
1
0.06
0.04
0.02
5
4
3
2
0.3
0.4
0.5
0.6
Time/ms
λ
max 450—460 nm.
Fig. 5. Pulse photolysis of a nitrogenꢀsaturated benzene solution
of transꢀMo(N2)2(DPPE)2 at 300 K: kinetics of changes in the
absorbance at λ = 500 nm (D500) after a photolyzing pulse (1);
lnD500 (2).
The author is grateful to V. A. Nadtochenko for help
in pulse measurements by a nanosecond laser pulse phoꢀ
tolysis technique.
For the pulse photolysis of transꢀW(N2)2(DPPE)2 in
benzene at room temperature, a longꢀwavelength band at
400—600 nm appears in the absorption spectrum of the
solution after a pulse with λ = 354 nm. In this case, as shown in
Fig. 4, the absorbance at λ = 550 nm decays exponentially
References
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in an N2 atmosphere with the rate constant k = 200 s–1
.
It is known10 that after a photolyzing pulse of the
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THF the absorbance decreases at λ = 300 nm, i.e., in the
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Mo(N2)2(DPPE)2 under N2 at 300 K, the absorbance at
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Received June 27, 2006;
in revised form October 3, 2007