Rhenium(VI) benzylidyne complexes, Re(≡C-2,4,6-C6H2Me3)(PPh 3)(H2O)X3 (X = Cl and Br): Crystal structure and spectroscopic properties

Full text of DOI:10.1021/ic970342e

Inorg. Chem. 1997, 36, 6437-6439
6437
300 K. Data were corrected for diamagnetism using Pascal’s constants
(-3.50 × 10-4 and 3.93 × 10-4 emu mol-1 for 2 and 3, respectively).
Cyclic voltammetry was performed with a Princeton Applied Research
(PAR) Model 175 universal programmer and a Model 273 potentiostat
coupled to a Kipp & Zoner X-Y recorder. A conventional two-
compartment cell was used with Ag/AgNO3 (0.1 mol dm-3 in
acetonitrile) as the reference electrode. The supporting electrolyte was
[nBu4N][PF6] (0.1 mol dm-3). Glassy carbon was used as the working
electrode.
Rhenium(VI) Benzylidyne Complexes,
Re(tC-2,4,6-C6H2Me3)(PPh3)(H2O)X3 (X ) Cl
and Br): Crystal Structure and Spectroscopic
Properties
Wen-Mei Xue,† Michael C. W. Chan,†
Thomas C. W. Mak,‡ and Chi-Ming Che*,†
Steady state emission spectra were recorded on a SPEX 1681
Fluorolog-2 series F111AI spectrometer. The emission spectra were
corrected for monochromator and photomultiplier efficiency and for
xenon lamp stability. Emission quantum yields were measured in
deoxygenated solution using the method of Denas and Crosby with
aqueous quinine sulfate as reference.11 Emission lifetimes were
determined with a Quanta Ray DCR-3 Nd-YAG laser (pulse output
355 nm, 8 ns) in deoxygenated solutions. The emission signals were
detected by a Hamamatsu R928 photomultiplier tube and recorded on
a Tektronix Model 2430 digital oscilloscope.
Stern-Volmer quenching experiments were carried out in deoxy-
genated N,N-dimethylformamide solutions of the metal complex in the
presence of the quenchers [Q]. In each case, a linear plot of τ0/τ Versus
[Q] was obtained, from which second-order quenching rate constants,
kq, were deduced according to the equation τ0/τ ) 1 + kqτ0[Q], where
τ0 and τ refer to the emission lifetimes in the absence and presence of
[Q].
Departments of Chemistry, The University of Hong Kong,
Pokfulam Road, Hong Kong, and The Chinese University of
Hong Kong, Shatin, New Territories, Hong Kong
ReceiVed March 21, 1997
Introduction
While the photochemistry of d2 metal complexes containing
metal-ligand multiple bonds has been documented,1-4 studies
on corresponding d1 complexes are rare. The possibility of
producing long-lived excited states from such species was
suggested by Winkler and Gray in detailed spectroscopic work
on the molybdenyl(V) ion,5 and fluorescence from an oxomo-
lybdenum(V) moiety has been investigated by Mohammed and
Maverick.6 However, reports on emissive rhenium(VI) com-
plexes are scarce,7 although the electronic structure of the first
d1 trans-dioxo complex, trans-[ReO2(dmap)4][PF6]2 (dmap )
4-(dimethylamino)pyridine), was presented by Gray et al.8
The rhenium(V) benzylidyne complexes ReV(tCAr′)(CO)2-
Synthesis. ReVI(tCAr′)(PPh3)(H2O)Cl3 (2). [ReV(tCAr′)(PPh3)2-
(CO)(H2O)Cl]ClO4 (1)10 (100 mg, 0.1 mmol) was dissolved in
chloroform (20 cm3), and excess aqueous concentrated HCl (2 cm3, 12
mol dm-3) was added. The yellow solution was refluxed for 5 days to
give a green solution. The organic layer was separated and dried with
MgSO4 and then evaporated to dryness in Vacuo. The crude product
was recrystallized from dichloromethane/petroleum ether (40-60 °C)
to give yellow-green crystals (yield 60 mg, 85%). Anal. Calcd for
C28H28OCl3PRe: C, 47.78; H, 3.98. Found: C, 48.15; H, 3.87. FAB
MS (m/z): 703 (M+), 685 (M+ - H2O), 650 (M+ - H2O - Cl). IR
(cm-1): ν 1436 (RetC).
i
X2(py) (Ar′ ) 2,4,6-C6H2Me3; X ) Cl, O-2,6-C6H3 Pr2) have
been prepared by Williams and Schrock,9 while we recently
described the photoluminescence and photoredox properties of
related derivatives, including [ReV(tCAr′)(PPh3)2(CO)(H2O)-
Cl]ClO4.10 Using the latter as precursor, we now report the
synthesis of ReVI(tCAr′)(PPh3)(H2O)Cl3 and ReVI(tCAr′)-
(PPh3)(H2O)Br3, which are the first examples of emissive d1
complexes containing a metal-carbon multiple bond.
ReVI(tCAr′)(PPh3)(H2O)Br3 (3). A solution of 1 (100 mg, 0.1
mmol) in dried and deoxygenated dibromomethane (50 cm3) was
irradiated with UV-visible light (Model RPR-100 Rayonet photo-
chemical chamber reactor equipped with 16 8-W RPR 3500-Å Hg
lamps) for 30 h at room temperature. The green solution was
evaporated to dryness in Vacuo. The crude product was recrystallized
from dichloromethane/petroleum ether (40-60 °C) to give a pale green
crystalline solid (yield 65 mg, 78%). Anal. Calcd for C28H28OBr3-
PRe: C, 40.13; H, 3.34. Found: C, 40.35; H, 3.35. FAB MS (m/z):
837 (M+). IR (cm-1): ν 1437 (RetC).
Experimental Section
Physical Measurements. Infrared spectra were recorded with KBr
disks on a Nicolet 20FXC FT-IR spectrophotometer. UV-visible
absorption spectra were obtained on a Milton Roy Spectronic 3000
diode array spectrophotometer. Elemental analyses were performed
by Butterworth Laboratories (UK). Mass spectra were obtained on a
Finnigan Mat 95 fast atom bombardment mass spectrometer. Magnetic
moment measurements were performed with solid crystalline samples
using a CAHN 2000 magnetobalance in the temperature range 75-
X-ray Crystallography. A yellow-green prism crystal of 2 with
dimensions 0.10 × 0.20 × 0.30 mm was used for X-ray analysis.
Intensity data were collected using graphite crystal monochromatic Mo
KR radiation (λ ) 0.710 73 Å) on a Rigaku AFC7R diffractometer,
using variable rate ω scan at 294 K. Of the 4883 unique reflections
measured (4° e 2θ e 50°, +h,+k,(l), 3733 were considered observed
(F > 4σ(F)). The structure was solved by direct methods and
subsequently refined by full-matrix least-squares using the Siemens
SHELXTL PLUS (PC Version).12 Final cycle of blocked-cascade least-
squares refinement: maximum and minimum peaks in difference
Fourier map ) +0.88 and -0.95 e Å-3. Crystallographic data: C28H28-
OCl3PRe, fw ) 703.8, monoclinic, P21/n, a ) 11.907(2) Å, b ) 15.750-
(3) Å, c ) 14.763(3) Å, â ) 90.53(3)°, V ) 2768.5(14) Å3, Z ) 4,
† The University of Hong Kong.
‡ The Chinese University of Hong Kong.
(1) (a) Winkler, J. R.; Gray, H. B. Inorg. Chem. 1985, 24, 346. (b) Thorp,
H. H.; Houten, J. V.; Gray, H. B. Inorg. Chem. 1989, 28, 889.
(2) Neyhart, G. A.; Bakir, M.; Boaz, J.; Vining, W. J.; Sullivan, B. P.
Coord. Chem. ReV. 1991, 111, 27.
(3) (a) Che, C. M.; Lau, T. C.; Lam, H. W.; Poon, C. K. J. Chem. Soc.,
Chem. Commun. 1989, 114. (b) Che, C. M.; Lam, H. W.; Mak, T. C.
W. J. Chem. Soc., Chem. Commun. 1989, 1529. (c) Che, C. M.; Lam,
H. W.; Tong, W. F.; Lai, T. F.; Lau, T. C. J. Chem. Soc., Chem.
Commun. 1989, 1883. (d) Yam, V. W. W.; Che, C. M. Coord. Chem.
ReV. 1990, 97, 93.
(4) Goll, J. G.; Liu, W.; Thorp, H. H. J. Am. Chem. Soc. 1993, 115, 11048.
(5) Winkler, J. R.; Gray, H. B. Comments Inorg. Chem. 1981, 1, 257.
(6) Mohammed, A. K.; Maverick, A. W. Inorg. Chem. 1992, 31, 4441.
(7) Maverick, A. W.; Yao, Q.; Mohammed, A. K; Henderson, L. J., Jr.
PhotosensitiVe Metal-Organic Systems; ACS Advances in Chemistry
Series 238; American Chemical Society: Washington, DC, 1993;
Chapter 7, p 131.
Fcalcd ) 1.689 g cm-3, µ (Mo KR) ) 4.76 mm-1, R ) 0.033, Rw
0.039, GOF ) 1.28.
)
Results and Discussion
The air-stable rhenium(VI) benzylidyne derivatives ReVI-
(tCAr′)(PPh3)(H2O)X3 (X ) Cl (2) and Br (3)) are prepared
from ReV(tCAr′)(PPh3)2(CO)(H2O)Cl]ClO4 (1) by thermal
(8) Brewer, J. C.; Thorp, H. H.; Slagle, K. M.; Brudvig, G. W.; Gray, H.
B. J. Am. Chem. Soc. 1991, 113, 3171.
(9) Williams, D. S.; Schrock, R. R. Organometallics 1994, 13, 2101.
(10) Xue, W. M.; Wang, Y.; Mak, T. C. W.; Che, C. M. J. Chem. Soc.,
Dalton Trans. 1996, 2827.
(11) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(12) SHELXTL-PC User manual; Siemens Analytical Instruments: Madi-
son, WI, 1990.
S0020-1669(97)00342-X CCC: $14.00 © 1997 American Chemical Society

6438 Inorganic Chemistry, Vol. 36, No. 27, 1997
Notes
Table 1. Selected Bond Distances (Å) and Angles (deg) for 2
Re-Cl(1)
Re-Cl(2)
Re-Cl(3)
Re-O(1w)
2.348(2)
2.436(2)
2.351(2)
2.320(4)
Re-P(1)
Re-C(19)
C(19)-C(20)
2.476(2)
1.738(6)
1.442(9)
Cl(1)-Re-Cl(2)
Cl(1)-Re-Cl(3)
Cl(2)-Re-Cl(3)
Cl(1)-Re-O(1w)
Cl(2)-Re-O(1w)
Cl(3)-Re-O(1w)
Cl(1)-Re-P(1)
Cl(2)-Re-P(1)
90.5(1)
Cl(3)-Re-P(1)
O(1w)-Re-P(1)
Cl(1)-Re-C(19)
Cl(2)-Re-C(19)
Cl(3)-Re-C(19)
O(1w)-Re-C(19)
P(1)-Re-C(19)
Re-C(19)-C(20)
88.0(1)
93.6(1)
100.2(2)
91.8(2)
100.9(2)
172.9(2)
93.3(2)
158.8(1)
90.5(1)
78.5(1)
81.3(1)
80.8(1)
89.1(1)
174.9(1)
169.7(5)
Table 2. Photophysical Data for 2 and 3 in Dichloromethane at
Room Temperaturea
UV-vis
ꢀ (dm3
(nm) mol-1 cm-1
emission
Figure 1. Effective magnetic moment (µeff) versus temperature for 2.
b
λmax
Eem
(nm)
φem
τc
(µs)
complex
)
ReVI(tCAr′)(PPh3)- 315
9650
1260
80
696 7 × 10-4 0.49
(H2O)Cl3 (2)
410
550
658
60
ReVI(tCAr′)(PPh3)- 330
6080
3530
770
180
90
728 6 × 10-3 0.41
(H2O)Br3 (3)
386
435
558
696
a Concentration of complexes is in the range 10-4-10-5 mol dm-3
b Quantum yields ( 20%. c Lifetimes ( 10%.
.
chloroform only for 5 days; thus, HCl is evidently needed. This
transformation sharply contrasts with the reaction of tungsten
and osmium alkylidyne complexes with HCl to yield the
corresponding alkylidene derivatives.15
The paramagnetic nature of 2 and 3 was confirmed by
magnetic measurements. Figure 1 shows the effective magnetic
moment (µeff) of 2 in the temperature range 75-300 K. The
µ
eff values of 2 and 3 at 300 K are 1.96 and 1.90 µB, respectively,
which approaches the theoretical value of 1.73 µB for a d1
system. By comparison, µeff for ReVI(tCMe3)(η5-C5Me5)Cl2
increases from 1.55 to 1.65 µB as the temperature is lowered
from 300 to 10 K.13
Figure 2. Perspective view of Re(tCAr′)(PPh3)(H2O)Cl3 (2).
reaction with HCl in CHCl3 and by photooxidation in CH2Br2,
respectively. The only reported examples of rhenium(VI)
alkylidyne complexes are ReVI(tCMe3)(η5-C5Me5)X2 (X ) Cl,
Br, and I).13 Complex 2 is also formed by photolysis of 1 in
deoxygenated dichloromethane or chloroform with UV-visible
light (λ > 300 nm), but the reaction time is increased (>5 days).
This is in accordance with the greater tendency for CH2Br2 to
be reduced compared to CH2Cl2 or CHCl3.14a
Photooxidation of metal complexes in halogenated solvents
is well established.14 However, the treatment of 1 with HCl in
chloroform to yield the benzylidyne species 2 is unusual. We
have found that the reaction can be performed under aerobic
conditions, so the oxidant is likely to be molecular oxygen and/
or the acid. No reaction is observed after refluxing 1 in
The molecular structure of 2 is shown in Figure 2, and
selected bond distances and angles are presented in Table 1.
The Re-C(19) distance of 1.738(6) Å is typical for a rhenium-
carbon triple bond,16 and the Re-C(19)-C(20) angle (169.7-
(5)°) is almost linear. Besides the rhenium examples already
mentioned,13 there are few reports concerning d1 alkylidyne
t
complexes. The X-ray structure of trans-[MoF(tCCH2 Bu)-
(dppe)2][BF4] (dppe ) 1,2-bis(diphenylphosphino)ethane) was
described by Pombeiro and co-workers,17 while Hopkins and
co-workers synthesized [W(tCPh)(dmpe)2Br][PF6] (dmpe )
1,2-bis(dimethylphosphino)ethane) by oxidation of W(tCPh)-
(dmpe)2Br with [C7H7][PF6].18 In contrast, the dimeric dia-
magnetic complex [W2(tCPMe3)2(PMe3)4Cl4][AlCl4]2 was
prepared by Schrock and co-workers through oxidation of
W(tCH)Cl(PMe3)4 with C2Cl6 and AlCl3.19 The reactive 17-
electron benzylidyne intermediate [Mo(tCPh)(η5-C5H5)(PMe3)2]-
Cl was proposed in the photooxidation of Mo(tCPh)(η5-
(13) Felixberger, J. K.; Kiprof, P.; Herdtweck, E.; Herrmann, W. A.; Jakobi,
R.; Gu¨tlich, P. Angew. Chem., Int. Ed. Engl. 1989, 28, 334.
(14) (a) Bock, C. R.; Wrighton, M. S. Inorg. Chem. 1977, 16, 1309. (b)
Miessler, G. L.; Zoebisch, E.; Pignolet, L. H. Inorg. Chem. 1978, 17,
3636. (c) Vogler, A.; Kunkely, H. J. Am. Chem. Soc. 1981, 103, 1559.
(d) Carter, J. D.; Kingsbury, K. B.; Wilde, A.; Schoch, T. K.; Leep,
C. J.; Pham, E. K.; McElwee-White, L. J. Am. Chem. Soc. 1991, 113,
2947. (e) Lindsay, E.; Malkhasian, A. Y. S.; Langford, C. H. Inorg.
Chem. 1994, 33, 944.
(16) Murdzek, J. S.; Schrock, R. R. In Carbyne Complexes; Fischer, H.,
Hoffman, P., Freissl, F. R., Schrock, R. R., Schubert, U., Weiss, K.,
Eds.; VCH Verlagsgesellschaft: Weinheim, 1988; p 147ff.
(17) Lemos, M. A. N. D. A.; Pombeiro, A. J. L.; Hughes, D. L.; Richards,
R. L. J. Organomet. Chem. 1992, 434, C6.
(15) For W: Mayr, A.; Asaro, M. F.; Kjelsberg, M. A.; Lee, K. S.; Van
Engen, D. Organometallics 1987, 6, 432. For Os: Clark, G. R.;
Cochrane, C. M.; Marsden, K.; Roper, W. R.; Wright, L. J. J.
Organomet. Chem. 1986, 315, 211.
(18) Manna, J.; Gilbert, T. M.; Dallinger, R. F.; Geib, S. J.; Hopkins, M.
D. J. Am. Chem. Soc. 1992, 114, 5870.
(19) Holmes, S. J.; Schrock, R. R.; Churchill, M. R.; Wasserman, H. J.
Organometallics 1984, 3, 476.

Notes
Inorganic Chemistry, Vol. 36, No. 27, 1997 6439
significant mixing between the (dxz,dyz) orbitals and benzylidyne
ligand. They denoted the LUMO to be π*(MtCsAr), derived
from the out-of-phase combination dxz(M)-px(C) or dyz(M)-
py(C). Thus, by comparison to previous studies,6,8,18 we
tentatively assign the lowest energy bands in 2 and 3 to the
spin-allowed dxy f [π*(RetCsAr)] transition (we suggest that
this transition contains d-d character and is not purely MLCT).
The halide-to-metal charge-transfer contribution in these transi-
tions is minor in view of the small energy difference between
2 and 3.
Excitation of 2 (Figure 3b) and 3 in CH2Cl2 at 658 and 696
nm, respectively, gives red emission (Table 2). The same
emission spectra are obtained upon excitation at 300-400 nm.
The minor energy difference between the lowest energy absorp-
tion band and the emission implies that this excited state also
originates from the dxy f [π*(RetCsAr)] transition. The
classification of this excited state as metal-centered (d-d) or
MLCT depends on the relative rhenium and carbon parentage
in the π*(RetCsAr) orbital, and this is not resolved in the
present study. However, the small shift in emission energy
between 2 and 3 suggests that there is little halide-to-rhenium-
(VI) LMCT character in the excited state.
Cyclic voltammetry experiments on 2 in dichloromethane
show irreversible reduction and oxidation waves at -1.48(2)
and 1.02(2) V Vs Ag/AgNO3, respectively. Corresponding
waves for 3 appear at -1.59(2) and 0.65(2) V Vs Ag/AgNO3.
Fluorescent oxomolybdenum(V) complexes are known to
undergo photoinduced electron-transfer reactions.6 In this work,
we find that the emission lifetimes of 2 and 3 (0.49 and 0.41
µs, respectively) are sufficient for viable photoredox processes.
For example, the emission of 2 is quenched by N,N′-dimethyl-
4,4′-bipyridinium hexafluorophosphate in N,N-dimethylforma-
mide solution with a rate constant of 9.2(2) × 108 mol-1 dm3
Figure 3. UV-visible absorption (a) and emission (b, λex ) 658 nm)
spectra of 2 (5.6 × 10-5 mol dm-3) in CH2Cl2 at room temperature.
C5H5)(P(OMe)3)2 in chloroform or dichloromethane in the
presence of PMe3.14d
s-1
.
The photophysical data for 2 and 3 are summarized in Table
2. The UV-visible absorption spectrum of 2 (Figure 3a)
exhibits a high-energy peak at 315 nm, a low-energy shoulder
in the 410-450-nm region, and two weak bands at 550 and
658 nm, with molar absorptivities less than 100 mol-1 dm3
cm-1. Complex 3 displays similar absorptions in the 400-
750-nm range, but there are more signals at 300-400 nm.
Indeed, the lowest energy bands for 2 and 3 (at 658 and 696
nm, respectively) have absorptivities comparable to that for the
In summary, reports of emissive alkylidyne complexes are
confined to d2 metal centers, e.g., Mo(IV) and W(IV),14d,20,21
Re(V),10 and Os(VI).22 This study demonstrates that d1
congeners also exhibit attractive photochemical properties.
Acknowledgment. We thank Prof. X. Z. You (Nanjing
University, P.R. China) for magnetic moment measurements,
Mr. F. Xue (The Chinese University of Hong Kong) for X-ray
crystal structure determination, and The University of Hong
Kong, Croucher Foundation and Hong Kong Research Grants
Council for financial support.
spin-allowed B2[(dxy)1] f E[(dxz,dyz)1] transitions of the d1
[MoOX4(CH3CN)]- (X ) Cl, Br) complexes, which are thought
to be localized on the MoO3+ unit.6 The electronic structures
of the d1 complexes trans-[ReO2(dmap)4]2+ 8 and [W(tCPh)-
(dmpe)2Br]+ 18 have been discussed, and in both cases the
HOMO was proposed to be the metal dxy orbital (metal-ligand
multiple bond taken as z axis), which is nonbonding with respect
to the oxo and benzylidyne ligands. Extensive spectroscopic
studies1a have revealed that, in the d1 and d2 rhenium oxo
complexes, the lowest energy transition is dxy f (dxz,dyz), the
latter orbitals being antibonding with respect to the metal-oxo
π bonds. However, the situation is different for benzylidyne
complexes: the investigations by Hopkins et al.18 on d1 W(V)
species and by Schanze, McElwee-White, and co-workers20 on
d2 Mo(IV) and W(IV) analogues suggested that there is
2
2
Supporting Information Available: Tables giving crystal data,
atomic coordinates, calculated hydrogen coordinates, anisotropic dis-
placement coefficients, and bond distances and angles for 2 (8 pages).
Ordering information is given on any current masthead page.
IC970342E
(20) Schoch, T. K.; Main, A. D.; Burton, R. D.; Lucia, L. A.; Robinson,
E. A.; Schanze, K. S.; McElwee-White, L. Inorg. Chem. 1996, 35,
7769.
(21) Bocarsly, A. B.; Cameron, R. E.; Rubin, H. D.; McDermott, G. A.;
Wolff, C. R.; Mayr, A. Inorg. Chem. 1985, 24, 3976.
(22) Trammell, S.; Sullivan, B. P.; Hodges, L. M.; Harman, W. D.; Smith,
S. R.; Thorp, H. H. Inorg. Chem. 1995, 34, 2791.