Visible light enhanced selective reductive elimination of a methylmanganese complex from a heterodinuclear dimethylphenyl(4,4/-di-rerr-butyl- 2,2/-bipyridine)platinum-pentacarbonylmanganese complex

Article abstract of DOI:10.1021/om900319a

Novel visible light induced reductive elimination of a heterodinuclear triorganoplatinum-manganese complex, ('Bu₂bpy)Me₂PhPt- Mn(CO)₅, gives a methylmanganese complex, MnMe(CO)₅, and a methylphenylplatinum compl

Full text of DOI:10.1021/om900319a

3608 Organometallics 2009, 28, 3608–3610
DOI: 10.1021/om900319a
Visible Light Enhanced Selective Reductive Elimination of a
Methylmanganese Complex from a Heterodinuclear Dimethylphenyl-
(4,40-di-tert-butyl-2,20-bipyridine)platinum-Pentacarbonylmanganese
Complex
Sanshiro Komiya,* Sei Ezumi, Nobuyuki Komine, and Masafumi Hirano
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and
Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
Received April 27, 2009
Summary: Novel visible light induced reductive elimination of
a heterodinuclear triorganoplatinum-manganese complex,
(tBu2bpy)Me2PhPt-Mn(CO)5, gives a methylmanganese
complex, MnMe(CO)5, and a methylphenylplatinum com-
plex, PtMePh(tBu2bpy).
C-C bond forming reductive elimination is the most well
studied both experimentally4 and theoretically.5 Other reduc-
tive elimination processes involving hetero elements are also
intrinsically of importance in various catalyses promoted by
transition-metal complexes.6 We previously reported the
reversible alkyl group transfer reaction of heterodinuclear
organotransition-metal complexes,7 which is regarded as
reductive elimination of an organotransition-metal complex
from the dinuclear complex. We wish to report here novel
visible light enhanced reductive elimination of a methylman-
ganese complex under ambient conditions from heterodinuc-
lear triorganoplatinum-manganese complexes.
Use of light energy in controlling the selectivity and activity
of chemical reactions is a highly fascinating method, but it is in
generally very difficult, since irradiation frequently causes
homolytic bond cleavage, dissociation of ligands, etc. or
sometimes even luminescence.1,2 On the other hand, reductive
elimination is one of the most important key steps in organo-
transition-metal chemistry in relation to transition-metal-
mediated organic reactions and catalyses.3 Among them,
A series of (2,20-bipyridine)- or (4,40-di-tert-butyl-2,20-bipyr-
idine)triorganoplatinum-manganese pentacarbonyl com-
plexes were prepared by the simple metathetical reactions of
the corresponding triorgano(nitrato)platinum(IV) species
with sodium pentacarbonylmanganate in THF (eq 1).
*To whom correspondence should be addressed. E-mail: komiya@
cc.tuat.ac.jp.
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In these heterodinuclear complexes, a large R2 group is
always placed trans to Mn, probably due to steric hindrance
between the square-planar Pt coordination plane and the four
equatorial carbonyl ligands at Mn. 1a in THF shows two
absorption bands at 533 (ε = 2900 M-1 cm-1) and 325 nm
(ε = 15000 M-1 cm-1), which are ascribed to MLCT bands
from the HOMO orbital based on the Pt-Mn bond to the
LUMOassignable to twoemptyπ* orbitalsof the bpy ligand.8
(6) (a) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852–860. (b) Hartwig,
J. F. Angew. Chem., Int. Ed. 1998, 37, 2046–2067.
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Sugiura, T.; Komiya, S. Organometallics 1994, 13, 4033–4044.
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(8) DFT calculations of 1a and 1c and their corresponding products,
which were performed by using Spartan 06 Windows from Wavefunc-
tion, Inc. at the B3LYP/LACVP/6-31G* level of theory, reveal that
reductive elimination of the former is endothermic but that of the latter is
exothermic. The calculated wavelengths of two MLCT bands of 1a were
554 and 400 nm. See the Supporting Information.
ꢀ
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´
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r
2009 American Chemical Society
pubs.acs.org/Organometallics
Published on Web 05/28/2009

Communication
Organometallics, Vol. 28, No. 13, 2009 3609
Figure 2. First-order plots for reductive elimination of 2f in the
dark (O) and under visible light irradiation (b) in benzene at
20 °C ([ 2f ] = 0.300 mM).
Figure 1. Molecular structure of (bpy)Me3Pt-Mn(CO)5 (1a).
All hydrogen atoms are omitted for clarity. Ellipsoids are given
at the 50% probability level.
concentrations, suggesting that the reaction proceeds without
prerequisite prior tBu2bpy and heterolytic Mn anion dissocia-
tions (Figure 2 and Figures S3 and S4 in the Supporting
Information). The first-order rate constant (kobsd = (1.60 (
0.00) ꢀ 10-4 s-1) at 20 °C in benzene is significantly larger
than those of reductive elimination of the corresponding
monomethylplatinum-manganese complex (dppe)MePt-
Mn(CO)5, which gives no methylmanganese complex.7c This
reflects that the electron deficiency at Pt due to high metal
valency favors reductive elimination.4a-4c Although interac-
tion of electron-deficient olefin is known to enhance reductive
elimination, and a significant acceleration effect of acryloni-
trile is observed in (dppe)MePt-Mn(CO)5, no effect of added
acrylonitrile on the rate was observed in the present case. This
may be due to coordinative saturation of the Pt atom, which
disfavors additional coordination of acrylonitrile.
Similarly, the p-anisyl analogue 2g also caused smooth
reductive elimination. Attempted preparation of methyldi-
phenyl- and triphenylplatinum derivatives failed to give
reductive elimination products, suggesting that high steric
congestion around the Pt-Mn bond causes preferential cis
reductive elimination. The trialkylplatinum-manganese
complexes 2a-e are relatively stable, and further heating
of 2a to 70 °C gave a complex mixture of MnMe(CO)5
(37%), PtMe2(tBu2bpy) (47%), PtMe2(CO)2 (29%), and
tBu2bpy (8%). Theoretical calculations8 revealed that reduc-
tive elimination of the dimenthylphenylplatinum complex is
exothermic (ΔE = -11.3 kJ mol-1 for 1b), whereas the
reaction of the trimethylplatinum complex is endothermic
(ΔE = 12.2 kJ mol-1 for 1a), supporting the above facts.
The most notable fact in this reductive elimination of the
M-C bond from the triorganoplatinum-manganese het-
erodinuclear complex is the unexpected acceleration effect of
visible light irradiation on the reductive elimination. When 2f
(0.300 mM) was irradiated by visible light (>500 nm) under
the same conditions, reductive elimination was significantly
enhanced (kobsd = (24.6 ( 0.4) ꢀ 10-4 s-1, ca. 15 times faster
than the reaction in the dark), causing selective formation of
the methylmanganese complex as shown in Figure 2. The
quantum yield for this photoinduced reductive elimination
of 2f estimated as ca. 0.50 using chemical actinometry.14
X-ray structure analysis revealed the molecular structure
of 1a (Figure 1).9 The geometry at the Pt metal is essentially
octahedral, where the Mn(CO)5 moiety and bpy ligand are
placed cis to each other. The Mo moiety possesses a pseudo-
square-pyramidal structure, which is similar to that of the
known Mn(-I) anionic complex [Ph4P][Mn(CO)5].10 The
Pt-Mn bond distance (3.0337(8) A) in 1a is significantly
longer than those observed in other heterodi- or heterotri-
nuclear complexes containing platinum and manganese,11,12
suggesting effective steric repulsion between the Pt and Mn
moieties. In fact, four ligands at Pt and the carbonyl ligands
cis to Pt in Mn are placed in a staggered conformation in
order to minimize their steric congestion.
When the dimethylphenyl(4,40-di-tert-butyl-2,20-bipyri-
dine)platinum-manganese complex 2f was heated in ben-
zene at 20 °C, selective and smooth cis reductive elimination
of the methylmanganese(I) complex 3 took place, giving
PtMePh(tBu2bpy) (4) (eq 2).13
The reaction is first order in heterodinuclear complex
concentration, and the estimated first-order rate constant
kobsd was independent of both the tBu2bpy and [Mn(CO)5]-
(9) Crystallographic data for 1a: C18H17N2O5PtMn, monoclinic,
P21/a, a = 13.574(3) A, b = 10.5780(17) A, c = 13.601(3) A,
β = 103.415(14)°, V = 1899.6(6) A3, Z = 4, Dcalcd = 2.068 g cm-3
,
F000 = 1128.00, μ (MoKR) = 80.226 cm-1, 4523 reflections measured,
4340 independent reflections, R1 = 0.0293 (I > 2.00σ( I )), wR2 =
0.0797 (all reflections).
(10) Seidel, R.; Schnautz, B.; Henkel, G. Angew. Chem., Int. Ed. Engl.
1996, 35, 1710–1712.
(11) Komiya, S.; Muroi, S.; Furuya, M.; Hirano, M. J. Am. Chem.
Soc. 2000, 122, 170–171.
(12) Braunstein, P.; Jud, J.-M. Nouv. J. Chim. 1984, 8, 771–776.
(13) Reductive elimination involving hetero elements from triorga-
noplatinum complexes: (a) Byers, P. K.; Canty, A. J.; Crespo, M.;
Puddephatt, R. J.; Scott, J. D. Organometallics 1988, 7, 1363–1367.
(b) Goldberg, K. I.; Yan, J.; Winter, E. L. J. Am. Chem. Soc. 1994, 116,
1573–1574. (c) Goldberg, K. I.; Yan, J.-Y.; Breitung, E. M. J. Am. Chem.
Soc. 1995, 117, 6889–6896. (d) Williams, B. S.; Holland, A. W.;
Goldberg, K. I. J. Am. Chem. Soc. 1999, 121, 252–253. (e) Williams, B. S.;
Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 2576–2587.
(f ) Pawlikowski, A. V.; Getty, A. D.; Goldberg, K. I. J. Am. Chem. Soc.
2007, 129, 10382–10393.
No significant effect of the added t-Bu2bpy ligand (kobsd
(24.6 ( 0.4) ꢀ 10-4 s-1 for [t-Bu2bpy] = 0 mM, kobsd
=
=
(22.8 ( 0.3) ꢀ 10-4 s-1 for [t-Bu2bpy] = 3.2 mM in benzene)
(14) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993.

3610 Organometallics, Vol. 28, No. 13, 2009
Komiya et al.
or added [Mn(CO)5]- anion (kobsd = (19.4 ( 0.3) ꢀ 10-4 s-1
for [[PPN][Mn(CO)5]] = 0 mM, kobsd = (14.2 ( 0.2) ꢀ
10-4 s-1 for [[PPN][Mn(CO)5]] = 3.4 mM in THF) on the
rate was observed, excluding possible mechanisms involving
prior ligand dissociation and heterolytic Pt-Mn bond clea-
vage processes. Addition of a large excess amount of a
radical scavenger such as 2,6-di-tert-butylphenol (8.1 mM,
27 equiv/2f) or p-methxyphenol (3.00 mM, 10 equiv/2f)
showed no effect on the rate, excluding radical processes.
Substitution of the para position of the phenyl in 2g by an
OMe group also showed no significant effect.
We can speculate on a possible mechanism for this new
photoinduced reaction from the above experimental facts.
Since the irradiation causes electron promotion from the
HOMO (Pt-Mn bond) to the LUMO (bpy),15 the excited
state of the complex involves a triplet Mn(I) cation with a
bpy radical anion ligand (eq 3). Selective reductive elimina-
tion may take place from such an excited state. The origin of
this enhancement effect on reductive elimination by visible
light is not completely understood at present, but the strong
trans effect of the tBu2bpy anion radical ligand in the excited
state could be responsible for it.16
In conclusion, we found the unprecedented visible
light induced reductive elimination of a heterodinuclear
triorganoplatinum-manganese complex. Although many
light-induced reactions of organometallic compounds
are known to cause ligand dissociation, homolytic bond
cleavage, or luminescence, the present results illustrate
the importance of light energy in fundamental organome-
tallic elemental reaction processes, strongly related to
catalyses.
(15) Photoirradiation of 2f with UV light was also performed, show-
ing that both have an enhancement effect on the rate, though visible light
has a stronger effect (kobs = (24.6 ( 0.4) ꢀ 10-4 s-1 under visible light,
(11.9 ( 0.3) ꢀ 10-4 s-1 under UV light, (1.6 ( 0.0) ꢀ 10-4 s-1 in the
dark). Since 2f has two MLCT bands at 546 and 323 nm in benzene
assignable to electron promotion from the HOMO (Pt-Mn bond) to the
first and second LUMOs (bpy), both irradiations would give similar bpy
radical anion species eventually to cause reductive elimination. On the
other hand, irradiation by our visible light having line spectra at 550 and
580 nm of a super-high-pressure 500 W UV lamp equipped with a Y-50
sharp cut filter and HA-50 heat-absorbing filter showed solvent effects.
The MLCT band λmax 550 nm in benzene is sifted to λmax 450 nm in
acetone, and accordingly the rate of reductive elimination under visible
light irradiation is decreased from (24.6 ( 0.4) ꢀ 10-4 s-1 in benzene to
(10.4 ( 0.1) ꢀ 10-4 s-1 in acetone. These results also support that the
enhancement effect of visible light is due to absorption of the MLCT
band.
Acknowledgment. This work was financially suppor-
ted by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and
Technology of Japan.
Supporting Information Available: Text, figures, tables, and a
CIF file giving experimental details, an X-ray crystal structure
analysis of 1a, and details concerning the DFT calculations.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(16) Sakaki, S.; Mizutani, H.; Kase, Y. Inorg. Chem. 1992, 33, 4575–
4581.