Electronic Absorption Spectra and Phosphorescence of Oxygen-Containing Molybdenum(IV) Complexes

Article abstract of DOI:10.1021/ic971186e

Electronic absorption and emission spectra are reported for salts of two oxomolybdenum(IV) cations, [MoOCl-(CN-t-Bu)₄]⁺ and [MoOCl(Ph₂PCH₂CH₂PPh₂) ₂]⁺. and for the new Mo(IV) complex [trans-Mo(OCH₃)₂(CN-t-Bu)₄]²⁺. All three ions show absorption bands (λ_max.abs 550-570 nm; ∈ 45-120 M^-1 cm^-1) attributable to the ¹A₁[(d_xy)²] → ¹E[(d_xy)¹(d_xz,yz)¹] (C_4v) transition, and the last two show weak shoulders in the 700-750 nm range due to the analogous spin-forbidden (¹A₁ → ³E) transition. Phosphorescence (λ_max,em 850-960 nm) occurs in the solid state for all three compounds at both room temperature and 77 K, and for [MoOCl(CN-t-Bu)₄]⁺ in CH₂Cl₂ at room temperature. These are the first phosphorescences recorded for molybdenum(IV) complexes. [MoOCl-(CN-t-Bu)₄](BPh₄) precipitates quickly if NaBPh₄ is added to the Mo(IV) solution prepared from MoCl₅ and tert-butyl isocyanide in CH₃OH. However, if NaPF₆ is used instead, [trans-Mo(OCH₃)₂(CN-t-Bu)₄](PF ₆)₂ (formed by reaction of [MoOCl(CN-t-Bu)₄]⁺ with methanol) crystallizes over a period of ca. 24 h. The crystal structure of [trans-Mo(OCH₃)₂(CN-t-Bu)₄](PF ₆)² has been determined: C₂₂H₄₂F₁₂MoN₄O₂P ₂, monoclinic; space group P2₁/ c; a = 9.1538(8) A?, b = 15.709(2) A?, c = 13.456(2) A?; β= 103.31(1)°; Z = 2; R(F) = 0.063, R_W(F) = 0.056 for 2719 reflections with I > σ(I).

Full text of DOI:10.1021/ic971186e

4258
Inorg. Chem. 1998, 37, 4258-4264
Electronic Absorption Spectra and Phosphorescence of Oxygen-Containing
Molybdenum(IV) Complexes
Ralph A. Isovitsch,† A. Scott Beadle, Frank R. Fronczek, and Andrew W. Maverick*
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804
ReceiVed September 19, 1997
Electronic absorption and emission spectra are reported for salts of two oxomolybdenum(IV) cations, [MoOCl-
(CN-t-Bu)4]+ and [MoOCl(Ph2PCH2CH2PPh2)2]+, and for the new Mo(IV) complex [trans-Mo(OCH3)2(CN-t-
Bu)4]2+. All three ions show absorption bands (λmax,abs 550-570 nm; ꢀ 45-120 M-1 cm-1) attributable to the
1
1A1[(dxy)2] f E[(dxy)1(dxz,yz)1] (C4V) transition, and the last two show weak shoulders in the 700-750 nm range
due to the analogous spin-forbidden (1A1 f 3E) transition. Phosphorescence (λmax,em 850-960 nm) occurs in the
solid state for all three compounds at both room temperature and 77 K, and for [MoOCl(CN-t-Bu)4]+ in CH2Cl2
at room temperature. These are the first phosphorescences recorded for molybdenum(IV) complexes. [MoOCl-
(CN-t-Bu)4](BPh4) precipitates quickly if NaBPh4 is added to the Mo(IV) solution prepared from MoCl5 and
tert-butyl isocyanide in CH3OH. However, if NaPF6 is used instead, [trans-Mo(OCH3)2(CN-t-Bu)4](PF6)2 (formed
by reaction of [MoOCl(CN-t-Bu)4]+ with methanol) crystallizes over a period of ca. 24 h. The crystal structure
of [trans-Mo(OCH3)2(CN-t-Bu)4](PF6)2 has been determined: C22H42F12MoN4O2P2, monoclinic; space group P21/
c; a ) 9.1538(8) Å, b ) 15.709(2) Å, c ) 13.456(2) Å; â ) 103.31(1)°; Z ) 2; R(F) ) 0.063, Rw(F) ) 0.056
for 2719 reflections with I > σ(I).
Introduction
strated for [ReVOCl4(CH3CN)]- and for solids containing
[ReVOCl5]2- 7
We wished to extend this work to oxomolyb-
.
The photochemistry and photophysics of transition-metal
complexes with multiply bonded main-group elements continues
to be a rich area of study. Work in this area began with studies
by Jørgensen and Gray and co-workers of the electronic structure
of the molybdenyl (MoVO3+) ion.1 Since then, the photochemi-
cal and photophysical properties of numerous metal complexes
with multiply bonded C, N, and O atoms have been explored.
Of these, dioxo and nitrido complexes with the d2 configuration
have been studied most often; examples include [ReO2(py)4]+,
[OsNCl4]-, and [ReN(dmpe)2Cl]+.2-4
We have also been interested in monooxo complexes as
chromophores. For example, the oxomolybdenum(V) complex
[MoOCl4(NCCH3)]- fluoresces in solution, 2E[(dxz,yz)1] f 2B2-
[(dxy)1], with a lifetime (110 ns) sufficient for reaction with one-
electron oxidants and reductants.5 We recently explored the
reactivity of oxomolybdenum(V) with phosphines, and we
showed that it oxidizes PEt2Ph and PEtPh2 photochemically to
their oxides.6 Weak phosphorescence has also been demon-
denum(IV) complexes (d2), which might show phosphorescence
as well. Herein we describe our work with the known
oxomolybdenum(IV) complexes [MoOCl(dppe)2]+ (dppe ) Ph2-
PCH2CH2PPh2) and [MoOCl(CN-t-Bu)4]+ and the new dimethox-
omolybdenum(IV) complex [trans-Mo(OCH3)2(CN-t-Bu)4]2+
.
All three of these compounds phosphoresce in the solid state at
room temperature and at 77 K, and [MoOCl(CN-t-Bu)4]+ also
phosphoresces in solution at room temperature. We assign the
emissions to the 3E[(dxy)1(dxz,yz)1] f 1A1[(dxy)2] (C4V) transition.
As far as we are aware, this work represents the first demonstra-
tion of phosphorescence in molybdenum(IV) complexes. These
complexes thus have potential as new chromophores for energy
conversion and as new reagents for photochemical substrate
transformations. The present complexes are more stable and
they phosphoresce at higher energy and with longer lifetime
than [ReVOCl4(CH3CN)]-.7a Also, the formation of [Mo-
(OCH3)2(CN-t-Bu)4]2+ is an unusual example of the reaction
of an oxo complex with an alcohol to give a dialkoxo species.
† Present address: Division of Science and Mathematics, University of
the Virgin Islands, St. Thomas, VI 00802.
Experimental Section
Materials and Procedures. All reactions and manipulations were
performed using standard drybox techniques. Reagents and anhydrous
solvents were of the highest commercial grade available. 1,2-Bis-
(diphenylphosphino)ethane (dppe) was purchased from Organometallics,
Inc. Benzylviologen dichloride (Sigma) was converted to its tetrafluo-
(1) Jørgensen, C. K. Acta Chem. Scand. 1957, 11, 73-85. Gray, H. B.;
Hare, C. R. Inorg. Chem. 1962, 1, 363-368. Hare, C. R.; Bernal, I.;
Gray, H. B. Inorg. Chem. 1962, 1, 831-835.
(2) Winkler, J. R.; Gray, H. B. Inorg. Chem. 1985, 24, 346-355. Winkler,
J. R.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 1373-1374.
(3) Hopkins, M. D.; Miskowski, V. M.; Gray, H. B. J. Am. Chem. Soc.
1986, 108, 6908-6911 and references therein. Cowman, C. D.;
Trogler, W. C.; Mann, K. R.; Poon, C. K.; Gray, H. B. Inorg. Chem.
1976, 15, 1747-1751.
(4) Neyhart, G. A.; Seward, K. J.; Boaz, J.; Sullivan, B. P. Inorg. Chem.
1991, 30, 4486-4488.
(5) (a) Mohammed, A. K.; Maverick, A. W. Inorg. Chem. 1992, 31, 4441-
4443. (b) For earlier reports of fluorescence from oxomolybdenum-
(V) in the solid state, see: Winkler, J. R.; Gray, H. B. Comments
Inorg. Chem. 1981, 1, 257-263. Winkler, J. R. Ph.D. Dissertation,
California Institute of Technology, 1984.
(6) Isovitsch, R. A.; May, J. G.; Fronczek, F. R.; Maverick, A. W.
Submitted for publication.
(7) (a) Maverick, A. W.; Yao, Q.; Mohammed, A. K.; Henderson, L. J.,
Jr. AdV. Chem. Ser. 1993, 238, 131-146. Yao, Q. Ph.D. Dissertation,
Washington University, 1991. (b) We are aware of only one other
report of phosphorescence from monooxo complexes, namely, the four-
coordinate d4 species [ReIIIOI(RCtCR)2] and [ReIIIO(PPh3)-
(RCtCR)2]+: Reber, C.; Zink, J. I. Inorg. Chem. 1991, 30, 2994-
2996.
S0020-1669(97)01186-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/31/1998

Oxygen-Containing Mo(IV) Complexes
Inorganic Chemistry, Vol. 37, No. 17, 1998 4259
a,b
roborate salt, BV(BF4)2, by metathesis, as described by Mohammed et
al.8 Elemental analysis was performed by MHW Laboratories (Phoenix,
AZ).
Table 1. Crystal Data for [Mo(OCH3)2(CN-t-Bu)4](PF6)2
formula
fw
MoC22H42N4O2P2F12
780.47
Instrumentation. FT-IR spectra were recorded in KCl disks on a
Perkin-Elmer 1760X spectrometer. 1H and 13C NMR spectra were
recorded on a Bruker 300 MHz spectrometer; chemical shifts are
reported in parts per million vs TMS. 31P NMR spectra were recorded
on a Bruker 250 MHz spectrometer operating at 101.2 MHz; chemical
shifts are reported in parts per million vs 85% H3PO4. Cyclic
voltammetry was performed using a PAR model 174 polarographic
analyzer, with degassed CH2Cl2/0.1 M (Bu4N)(O3SCF3) as supporting
electrolyte, Pt working and counter electrodes, and Ag/AgCl (saturated
NaCl(aq)) reference electrode.
Electronic absorption spectra were recorded on an Aviv 14DS
spectrophotometer. Emission spectra (λexc 550 nm) were obtained by
using a Spex Instruments Fluorolog 2 model F112X fluorometer, with
a Hamamatsu R406 PMT; they were corrected for variation in detector
response with wavelength. An Oxford Instruments model DN1704
cryostat, or a Pyrex dewar, was employed for low-temperature
measurements. Lifetime measurements employed a Nd:YAG laser as
excitation source (532 nm, second harmonic), with Schott colored-glass
filters and a Spex 220M monochromator to isolate the emitted light.
The emission signal from the photomultiplier tube (Hamamatsu R406)
was passed to a Tektronix digitizing oscilloscope. Lifetimes were
determined by exponential fits to the digitized phosphorescence decay
curves.
space group
Z
λ/Å
a/Å
b/Å
c/Å
P21/c
2
0.710 73 (Mo KR)
9.1538(8)
15.709(2)
13.456(2)
103.31(1)
1882.9(8)
1.377
5.04
0.9499-0.9998
0.063
â/deg
V/Å3
Fx/g cm-3
µ/cm-1
transm coeff
R(F) (obsd data)b
Rw(F)c
0.056
a In Tables 1-3, estimated standard deviations in the least significant
digits of the values are given in parentheses. b R ) ∑||Fo| - |Fc||/
∑|Fo|; data with I > σ(I). c Rw ) x(∑w(|Fo| - |Fc|)2/∑wFo ); w )
2
4Fo /(σ2(I) + (0.02Fo )2).
2
2
Table 2. Selected Interatomic Distances/Å and Angles/deg for
[Mo(OCH3)2(CN-t-Bu)4](PF6)2
a
Mo-O1
Mo-C1
Mo-C6
1.791(3)
2.166(5)
2.173(4)
O1-C11
C1-N1
C6-N2
1.399(7)
1.134(7)
1.130(7)
[MoOCl(dppe)2](BF4).9 [MoOCl(dppe)2]Cl was prepared in metha-
nol solution according to the literature procedure.10 Treatment of this
solution with excess methanolic NaBF4 produced a purple precipitate,
which was recrystallized from methanol to give large, dark purple
crystals. 31P NMR (CD2Cl2): 41.6. 1H NMR (CD3CN): 7.34, 7.15,
7.02 (m, 40H, C6H5), 3.10, 2.83 (m, 8H, CH2). Anal. Calcd for C52H48-
BClF4MoOP4: C, 60.58; H, 4.69; Cl, 3.44. Found: C, 60.72; H, 4.88;
Cl, 3.55.
O1-Mo-C1
O1-Mo-C1′
O1-Mo-C6
O1-Mo-C6′
C1-Mo-C6
C1-Mo-C6′
93.6(2)
86.4(2)
89.8(2)
90.2(2)
86.2(2)
93.8(2)
Mo-O1-C11
Mo-C1-N1
Mo-C6-N2
C1-N1-C2
C6-N2-C7
172.5(4)
170.9(4)
175.1(4)
176.3(5)
174.3(5)
a Primes denote atoms related by the crystallographically imposed
inversion center.
tert-Butyl Isocyanide Complexes. Both of these preparations began
with a solution similar to that used by Novotny and Lippard11 to
synthesize [MoOCl(CN-t-Bu)4](I3). MoCl5 (1.0 g, 4 mmol) was
dissolved in CH3OH (15 mL) to give an emerald green solution. To
this was added tert-butyl isocyanide (1.83 g, 22 mmol) all at once,
causing the color to change to dark red.
CN. The CH3CN solution was filtered and then evaporated to yield
lavender microcrystals of the product. Yield: 1.1 g, 40%. IR: 2213
cm-1 (CN). 1H NMR (CD3CN): 3.82 (s, OCH3), 1.68 (s, C(CH3)3).
13C NMR (CD3CN): 138.9 (CN), 70.9 (OCH3), 62.2, 30.7 (C(CH3)3).
Anal. Calcd for C22H42F12MoN4O2P2: C, 33.86; H, 5.42; N, 7.18.
1
Found: C, 33.71; H, 5.52; N, 7.21. The H NMR integration for this
(a) [MoOCl(CN-t-Bu)4](BPh4). The above Mo-CN-t-Bu mixture
was stirred for 5-10 min, and then methanolic NaBPh4 (4 mmol) was
added. The pale purple product precipitated promptly, and it was
quickly collected, washed with diethyl ether, and dried in a vacuum
desiccator. Yield: 1.0-1.1 g, 35-40%. IR: 2198 (CN), 952 cm-1
(MoO). 1H NMR (CDCl3): 7.10 (m, 20H, C6H5), 1.62 (s, 36H,
C(CH3)3). Anal. Calcd for C44H56BClMoN4O: C, 66.13; H, 7.06; N,
7.01; Cl, 4.44. Found: C, 65.97; H, 7.22; N, 7.04; Cl, 4.63.
In addition to the absorption bands discussed below, samples of this
complex sometimes showed a band at ca. 700 nm. The intensity of
this band sometimes reached ca. 30% that of the 550 nm band,
especially if the solid was allowed to stand in air for some time after
preparation. We were unable to identify this impurity by IR or NMR.
However, we selected samples with the smallest possible impurity
concentrations, prepared without exposure to air, for the measurements
reported here. These samples typically showed an absorbance at 700
nm <2% of that at 550 nm. The extinction coefficient at 550 nm and
the phosphorescence intensity and lifetime were approximately the same
for samples with larger and smaller amounts of the impurity.
(b) [Mo(OCH3)2(CN-t-Bu)4](PF6)2. The red Mo-CN-t-Bu reaction
mixture was stirred for 1 h. Then a solution of NaPF6 (1.84 g, 11
mmol) in 5 mL of CH3OH was added. After continued stirring
overnight, the brick-red reaction mixture had deposited a lavender,
powdery precipitate, which was collected and then dissolved in CH3-
compound was generally poor (ratio C(CH3)3:OCH3 found 6.4-8.9;
calcd 6.0), but we found no evidence of impurities in NMR (1H or
13C); also, IR spectra (νMoO region) showed no evidence for loss of the
methoxo ligands.
X-ray Analysis of [Mo(OCH3)2(CN-t-Bu)4](PF6)2. A crystal suit-
able for X-ray analysis was obtained by slow evaporation of a CH3CN
solution in a drybox. Diffraction data were collected on an Enraf-
Nonius CAD4 diffractometer fitted with a Mo KR source and a graphite
monochromator, using the θ-2θ scan method; see summary in Table
1. Final unit cell constants were determined from the orientations of
25 centered high-angle reflections. The intensities were corrected for
absorption using ψ scan data for five reflections. The MOLEN12 set
of programs was used. Structure solution was by direct methods; all
non-hydrogen atoms were refined anisotropically, and H atoms were
placed in calculated positions. Tables 2 and 3 list selected bond
distances and angles and atomic coordinates. Complete X-ray informa-
tion is available in CIF format via the Internet (see Supporting
Information paragraph).
Phosphorescence Quenching Experiments. Solutions of [MoOCl-
(CN-t-Bu)4](BPh4) (2 mM in acetone or 10 mM in CH3CN) were
degassed by bubbling with N2, and their phosphorescence intensities
measured both with and without the addition of excess BV(BF4)2 (BV2+
) benzylviologen (1,1′-dibenzyl-4,4′-bipyridinium)). Treatment with
the benzylviologen salt usually produced a small amount of precipitate,
which was removed by filtration, and a slight deepening of the purple
color of the solution; however, spectral measurements showed that both
chromophore and quencher remained present in the mixed solutions.
(8) Mohammed, A. K.; Fronczek, F. R.; Maverick, A. W. Inorg. Chim.
Acta 1994, 226, 25-31.
(9) Atkinson, L. K.; Mawby, A. H.; Smith, D. C. J. Chem. Soc. D 1970,
1399-1400.
(10) Butcher, A. V.; Chatt, J. J. Chem. Soc. A 1971, 2356-2358.
(11) Novotny, M.; Lippard, S. J. Inorg. Chem. 1974, 13, 828-831.
(12) Fair, C. K. MOLEN: An InteractiVe Structure Solution Procedure;
Enraf-Nonius: Delft, The Netherlands, 1990.

4260 Inorganic Chemistry, Vol. 37, No. 17, 1998
Isovitsch et al.
Table 3. Positional Parameters and Their Estimated Standard
Deviations for the Cation in [Mo(OCH3)2(CN-t-Bu)4](PF6)2
Table 4. Electronic Absorption Spectral Data for Mo(IV) and
Related d2 Complexes
atom
x
y
z
Beq/Å2
ꢀ/M-1
complexa
[MoOCl(dppe)2]+
λ
max/nm cm-1
solvent
ref
Mo
O1
N1
N2
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
0.5
0.5
0.5
3.56(1)
4.15(8)
4.6(1)
4.3(1)
4.1(1)
5.3(1)
8.0(2)
8.1(2)
10.0(2)
4.4(1)
4.4(1)
8.3(2)
8.4(2)
8.0(2)
9.3(2)
0.4429(4)
0.2200(4)
0.6631(5)
0.3088(5)
0.1128(6)
0.0848(8)
-0.0290(7)
0.1863(8)
0.6132(5)
0.7129(6)
0.8559(8)
0.5886(8)
0.7316(8)
0.4174(8)
0.6062(2)
0.4027(3)
0.4822(2)
0.4423(3)
0.3472(4)
0.2738(4)
0.3986(5)
0.3194(5)
0.4902(3)
0.4691(3)
0.4201(5)
0.4191(5)
0.5543(4)
0.6923(4)
0.5214(2)
0.5681(3)
0.7448(3)
0.5432(4)
0.6018(4)
0.5278(5)
0.5969(5)
0.7097(5)
0.6604(3)
0.8549(3)
0.8736(5)
0.8849(5)
0.9042(5)
0.5378(6)
567
75 CH3CN this work
4600
115 CH2Cl2 this work
310 shb
550
[MoOCl(CN-t-Bu)4]+
328
155
[Mo(OCH3)2(CN-t-Bu)4]2+ 558
45 CH3CN this work
1250
pyridine
250 CH2Cl2
2980
4000
40 CH3CN 7a
35
93 CH2Cl2
77
200 CH3CN 27
300 CH3CN 28
1000
285 sh
415
[ReO2(py)4]+
2
4
[ReN(dmpe)2Cl]+
362
257
238
1170
840
493
441
400
370
317
[ReOCl4(NCCH3)]-
[OsNCl4]-
3
[OsN(NH3)4]3+
[OsO2(CN)4]2-
Results and Discussion
[PhCtWCl(CO)2(tmeda)] 448
390 toluene 29a
Synthesis and Characterization. The preparation of the two
oxomolybdenum(IV) complexes10,11 depends on the addition of
dppe or tert-butyl isocyanide to a Mo(V) solution to effect
reduction to molybdenum(IV). All of the complexes studied
here are purple, crystalline materials. They are air-stable in the
solid state, with only [Mo(OCH3)2(CN-t-Bu)4](PF6)2 showing
discoloration after several months. All of the complexes are
stable in aerated solution for several days, except for [MoOCl-
(CN-t-Bu)4](BPh4), which appears to react slowly with air; see
Experimental Section. In contrast, other oxomolybdenum(IV)
complexes, such as [MoOCl2(PR3)3], are considerably more air-
and water-sensitive.13-16 [MoOCl(dppe)2](BF4) is stable in a
variety of solvents, whereas [MoOCl(CN-t-Bu)4](BPh4) is most
stable in CHCl3, CH2Cl2, and acetone, and [Mo(OCH3)2(CN-
t-Bu)4](PF6)2 is most stable in CH3CN.
a py ) pyridine; dmpe ) Me2PCH2CH2PMe2; tmeda ) Me2NCH2-
CH2NMe2. b sh ) shoulder.
-
Novotny and Lippard isolated [MoOCl(CN-t-Bu)4]+ as its I3
salt.11 The strongly colored I3- anion was likely to complicate
a photophysical investigation of the molybdenum(IV) cation,
-
so we attempted to isolate [MoOCl(CN-t-Bu)4]+ as the BPh4
-
and PF6 salts. Isolation of [MoOCl(CN-t-Bu)4](BPh4) was
successful when the reaction mixture was stirred for only a few
minutes after the addition of the tert-butyl isocyanide, and
methanolic NaBPh4 was used to induce immediate precipitation
of the desired product.
When we repeated this procedure with PF6- as the counterion,
no precipitate formed at first; rather, a lavender solid formed
over a period of 24 h. The electronic absorption and emission
spectra of this material are similar to those of [MoOCl(CN-t-
Bu)4](BPh4) (see Tables 4 and 5), but its IR spectrum shows
no absorption in the νMoO region. X-ray analysis identified it
as [trans-Mo(OCH3)2(CN-t-Bu)4](PF6)2 (see ORTEP17 drawing
in Figure 1).
The [Mo(OCH3)2(CN-t-Bu)4]2+ cation is pseudooctahedral
with four equatorial isocyanide ligands and two axial methoxo
groups. The bonds to the equatorial isocyanide ligands are very
similar in length to those in the oxomolybdenum(IV) complex
Figure 1. ORTEP17 drawing of the Mo(IV) cation in [Mo(OCH3)2-
(CN-t-Bu)4](PF6)2, with ellipsoids at the 40% level.
[MoOCl(CNCH3)4](I3) (average Mo-C 2.16 Å).18 The axial
Mo-OCH3 bonds are longer than terminal MotO bonds in six-
coordinate complexes, which are typically 1.64-1.70 Å in
length.18,19 However, they are shorter than the Mo-OH2 bond
in [MoO(OH2)(CN)4]2- (2.271(14) Å), the Mo-OH distance
in [MoO(OH)(CN)4]3- (2.077(7) Å), and even the Mo-O
distances in [MoO2(CN)4]4- (1.834(9) Å).20 These comparisons
suggest relatively strong Mo-O π bonding in [Mo(OCH3)2-
(CN-t-Bu)4]2+, as does the nearly linear Mo-O-C bond angle.
Strong π donation from the methoxo ligands is favored by the
(13) Carmona, E.; Galino, A.; Guille-Photin, C.; Sanchez, L. Polyhedron
1988, 7, 1767-1771.
(14) Yoon, K.; Parkin, G.; Rheingold, A. L. J. Am. Chem. Soc. 1991, 113,
1437-1438. Yoon, K.; Parkin, G.; Rheingold, A. L. J. Am. Chem.
Soc. 1992, 114, 2210-2218.
(15) Young, C. G.; Enemark, J. H. Inorg. Chem. 1985, 24, 4416-4419.
(16) Boyd, I. W.; Spence, J. T. Inorg. Chem. 1982, 21, 1602-1606.
(17) Johnson, C. K. ORTEP-II: A Fortran Thermal-Ellipsoid Plot Program
for Crystal-Structure Illustrations. Report ORNL-5138; National
Technical Information Service, U.S. Department of Commerce:
Springfield, VA, 1976.
(18) Lam, C. T.; Lewis, D. L.; Lippard, S. J. Inorg. Chem. 1976, 15, 989-
991.
(19) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John Wiley
& Sons: New York, 1988.
(20) Robinson, P. R.; Schlemper E. O.; Murmann, R. K. Inorg. Chem. 1975,
14, 2035-2041. Day, V. W.; Hoard, J. L. J. Am. Chem. Soc. 1968,
90, 3374-3379.

Oxygen-Containing Mo(IV) Complexes
Inorganic Chemistry, Vol. 37, No. 17, 1998 4261
Table 5. Phosphorescence Data for Mo(IV) and Related d2 Complexes
phosphorescence λmax/nm
77 K
complexa
300 K
940
860
medium
ref
[MoOCl(dppe)2]+
960
solid
CH2Cl2
solid
this work
this work
[MoOCl(CN-t-Bu)4]+
ca. 800
800, 854
655
788, 856, ca. 935
780, 870, 930
[Mo(OCH3)2(CN-t-Bu)4]2+
[ReO2(py)4]+
solid
this work
pyridine
CH2Cl2
CH3CN
solid
CH3CN
solid
2
4
7a
3
27
[ReN(dmpe)2Cl]+
[ReOCl4(NCCH3)]-
[OsNCl4]-
508
ca. 1300
710b
[OsN(NH3)4]3+
545
565, 600, 627, 688
[OsO2(CN)4]2-
[PhCtWCl(CO)2(tmeda)]
710
640
CH3CN
toluene
28
29a
a py ) pyridine; dmpe ) Me2PCH2CH2PMe2; tmeda ) Me2NCH2CH2NMe2. b At 5 K.
[MoO2(CN)4]4- + H+ f [MoO(OH)(CN)4]3-
[ReO2L4]+ + RX f [ReO(OR)L4]2+ + X-
[MoOCl(dppe)2]+ + CH3OH + CO32- f
(2)
(3)
overall positive charge of the complex, as well as by the
presence of the equatorial π acceptor ligands CN-t-Bu. Nearly
linear M-O-C linkages are also found in [MoIVO(OCH3)-
(dppe)2][BPh4],21 and in two oxo(alkoxo)rhenium(V) com-
plexes,22 although these all show M-OCH3 distances longer
than that in the present dimethoxomolybdenum(IV) cation. On
the other hand, the tungsten(IV) complex [WCl2(OC6H3-2,6-
Ph2)2(PMe2Ph)2], which has no strong π acceptor ligands, shows
longer W-O distances (1.966(4) Å) and more acute W-O-C
angles (140.0(4)°).23
[MoO(OCH3)(dppe)2]+ + HCO3- + Cl- (4)
As an alternative rationale for production of both [MoOCl-
(CN-t-Bu)4]+ and [Mo(OCH3)2(CN-t-Bu)4]2+ from the same
solution, we considered the possibility that both cations are
formed initially on reaction of Mo(V) with the isocyanide, and
that BPh4- precipitates the former and PF6- the latter. However,
we believe that this is unlikely, because this would not explain
Thus, two different molybdenum(IV)-isocyanide complexes
-
can be produced from the same reaction mixture. When BPh4
is the counterion, the initially formed [MoOCl(CN-t-Bu)4]+ ion
is removed quickly by precipitation. On the other hand,
[MoOCl(CN-t-Bu)4](PF6) is apparently substantially more soluble
in the reaction mixture, and this allows time for reaction of
[MoOCl(CN-t-Bu)4]+ with methanol to give [Mo(OCH3)2(CN-
t-Bu)4]2+, as in eq 1. The dimethoxo complex may be more
why the precipitation with PF6- is slow, while that with BPh4
-
is rapid. Also, the yield of the oxo complex is highest after a
very short reaction time, whereas precipitation of the dimethoxo
complex requires a number of hours; this suggests that eq 1
correctly describes the reaction occurring in these solutions.
We also attempted to carry out the interconversions between
the oxo and dimethoxo species using purified Mo(IV) com-
plexes. For example, we dissolved crystalline [MoOCl(CN-t-
Bu)4](BPh4) in methanol in order to study reaction 1 in the
forward direction. In separate experiments, we added [Mo-
(OCH3)2(CN-t-Bu)4](PF6)2 to methanol/HCl(aq) as a probe for
the reverse reaction. Unfortunately, as mentioned above, neither
of the complexes is stable in methanol for long periods of
time: these solutions decomposed before any interconversion
had occurred in either direction.
Electronic Structure: Absorption Spectra. We chose the
present complexes for initial study in part because of their
relatively high symmetry: all three have approximate C4V
symmetry, as opposed to species such as cis-mer-MoOCl2(PR3)3
(C2V). A qualitative energy diagram for the d orbitals in these
complexes is shown in Scheme 1. With strong axial π donor
ligands, the gap between dxy and dxz,yz is expected to be large,
and a d2 complex should have the ground state 1A1[(dxy)2]. The
[MoOCl(CN-t-Bu)4]+ + 2CH3OH f
[Mo(OCH3)2(CN-t-Bu)4]2+ + H2O + Cl- (1)
stable than the oxo complex under our conditions. Also, the
lower solubility of [Mo(OCH3)2(CN-t-Bu)4](PF6)2 in the reaction
medium, as compared to [MoOCl(CN-t-Bu)4](PF6), may help
to drive reaction 1 in the forward direction.
This reaction can be compared with other routes for the
formation of alkoxo- or hydroxo-metal complexes with the
d2 configuration: proton transfer (an example is shown in eq
2),20 alkylation (eq 3),22 and ligand substitution (eq 4).10 In all
of these reactions, the product contains at least one metal-oxo
bond; in contrast, our reaction in methanol converts one metal-
oxo bond into two metal-alkoxo bonds. Reaction 1 is similar
to the conversion of a ketone to its ketal, in that neither acid
nor base is consumed.24 The relatively strong Mo-O bonds in
the dimethoxo complex (see above) may help to favor its
formation in methanol solution.
3
1
lowest-energy transitions are to the E and E excited states,
1
(21) Adachi, T.; Hughes, D. L.; Ibrahim, S. K.; Okamoto, S.; Pickett, C.
J.; Yabanouchi, N.; Yoshida, T. J. Chem. Soc., Chem. Commun. 1995,
1081-1083.
(22) Ram, M. S.; Skeens-Jones, L. M.; Johnson, C. S.; Zhang, X. L.; Stern,
C.; Yoon, D. I.; Selmarten, D.; Hupp, J. T. J. Am. Chem. Soc. 1995,
117, 1411-1421. Be´langer, S.; Beauchamp, A. L. Inorg. Chem. 1997,
36, 3640-3647.
(23) Coffindaffer, T. W.; Steffy, B. D.; Rothwell, I. P.; Folting, K.;
Huffman, J. C.; Streib, W. E. J. Am. Chem. Soc. 1989, 111, 4742-
4749.
(24) See, for example: March, J. AdVanced Organic Chemistry: Reactions,
Mechanisms, and Structures, 4th Ed.; Wiley: New York, 1992; pp
889-891.
both arising from the (dxy)1(dxz,yz)1 configuration; and the A1
f 1E transition should be readily observable. This scheme has
been used to discuss the spectra of a number of d2 transition
metal oxo and nitrido complexes.2,25,26
The present Mo(IV) complexes all exhibit an absorption in
the visible at approximately 550 nm, as illustrated by the spectra
(25) Thorp, H. H.; Van Houten, J.; Gray, H. B. Inorg. Chem. 1989, 28,
889-892.
(26) Neyhart, G. A.; Bakir, M.; Boaz, J.; Vining, W. J.; Sullivan, B. P.
Coord. Chem. ReV. 1991, 111, 27-32.

4262 Inorganic Chemistry, Vol. 37, No. 17, 1998
Isovitsch et al.
Figure 5. Corrected phosphorescence spectrum of [MoOCl(CN-t-Bu)4]-
[BPh4] in CH2Cl2 at room temperature.
Figure 2. Electronic spectra of [MoOCl(dppe)2][BF4]. Absorption: s,
300 K, CH3CN solution. Corrected phosphorescence (arbitrary units):
O, 300 K, solid; 2, 77 K, solid.
complexes,2-4,7a,27,28 and a closely related tungsten(IV) alkyli-
dyne complex,29 for comparison. Novotny and Lippard noted
an absorption maximum at 547 nm (ꢀ ) 124) for [MoOCl-
(CNCH3)4](PF6) in acetone,11 very close to our values for
[MoOCl(CN-t-Bu)4](BPh4). Dziegielewski observed this ab-
sorption band at 578 nm for [MoOCl(dppe)2]Cl in ethanol.30
A
band with similar position and intensity was also reported by
+ 31
Levason et al. for WIVOCl(dppe)2
.
We attribute these
prominent low-energy bands to the 1A1[(dxy)2] f 1E[(dxy)1-
(dxz,yz)1] transition. The next-higher-energy transitions, begin-
ning at 280-330 nm, are not well resolved in any of the
complexes; their greater intensity indicates that they may have
substantial charge-transfer character.
[MoOCl(dppe)2](BF4) and [Mo(OCH3)2(CN-t-Bu)4](PF6)2
also exhibit weak absorption bands (ꢀ < 10) in the 700-800
1
nm region. These bands are attributable to the A1[(dxy)2] f
Figure 3. Electronic spectra of [MoOCl(CN-t-Bu)4][BPh4]. Absorp-
tion: s, 300 K, CH2Cl2 solution. Corrected phosphorescence (arbitrary
units): O, 300 K, solid; 2, 77 K, solid.
3E[(dxy)1(dxz,yz)1] transition. A similar absorption is also present
in [MoOCl(CN-t-Bu)4](BPh4), but this may be due to small
amounts of an impurity (see Experimental Section) and not
1
3
entirely to the A1 f E transition.
[Mo(OCH3)2(CN-t-Bu)4]2+ and the two oxo complexes have
very similar absorption and emission spectra. This suggests
that the two methoxo groups provide a π-donor environment
nearly identical to that of one oxo group. In contrast, proto-
nation or alkylation of a single oxo group, as in [ReVO2]+
complexes22 or [MoO2(CN)4]4- 32
, typically leads to a red shift
of at least 100 nm in the dxy f dxz,yz absorption band.
Phosphorescence. Solid samples of all three Mo(IV)
complexes phosphoresce at room temperature and at 77 K; see
data in Table 5. The emissions are more intense at 77 K in all
cases. [MoOCl(CN-t-Bu)4]+ also phosphoresces in CH2Cl2,
CHCl3, and acetone solutions (see Figure 5) and in PMMA films
at room temperature (PMMA ) poly(methyl methacrylate)).
However, we could not detect any emission from solutions (CH3-
CN) or films (PMMA or PVA) containing either [MoOCl-
(dppe)2]+ or [Mo(OCH3)2(CN-t-Bu)4]2+ (PVA ) poly(vinyl
Figure 4. Electronic spectra of [Mo(OCH3)2(CN-t-Bu)4][PF6]2. Ab-
sorption: s, 300 K, CH3CN solution. Corrected phosphorescence
(arbitrary units): O, 300 K, solid; 2, 77 K, solid.
Scheme 1. Energy Diagram (C4V) for MtO and Related
Complexes
(27) Lam, H.-W.; Che, C.-M.; Wong, K.-Y. J. Chem. Soc., Dalton Trans.
1992, 1411-1416.
(28) Yam, V. W.-W.; Che, C.-M. Coord. Chem. ReV. 1990, 97, 93-104.
(29) (a) Bocarsly, A. B.; Cameron, R. E.; Rubin, H.-D.; McDermott, G.
A.; Wolff, C. R.; Mayr, A. Inorg. Chem. 1985, 24, 3976-3978. (b)
Phosphorescence has recently been reported from a closely related
Mo(IV) complex, PhCtMo(Cp)(CO)(P{OMe}3): 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, 7769-7775.
(30) Dziegielewski, J. O. Polyhedron 1984, 3, 1131-1134.
(31) Levason, W.; McAuliffe, C. A.; McCullough, F. P., Jr. Inorg. Chem.
1977, 16, 2911-2916.
in Figures 2-4. Maxima and extinction coefficients are listed
in Table 4, with data for some similar Re and Os d2
(32) Lippard, S. J.; Russ, B. J. Inorg. Chem. 1967, 6, 1943-1947.

Oxygen-Containing Mo(IV) Complexes
Inorganic Chemistry, Vol. 37, No. 17, 1998 4263
acetate)). Our assignment of these emissions as phosphores-
cence, 3E[(dxy)1(dxz,yz)1] f 1A1[(dxy)2], is supported by the large
Stokes shifts between the prominent absorption and emission
bands, and by emission lifetime data (see below).
Scheme 2. Modified Latimer Diagram for [MoOCl(CN-t-
Bu)4]+ a
Phosphorescence of [MoOCl(CN-t-Bu)4](BPh4) in the solid
state (Figure 3) and in CH2Cl2 solution (Figure 5) is more intense
than that of the other two complexes. At room temperature,
the phosphorescence spectrum of solid [MoOCl(CN-t-Bu)4]-
(BPh4) exhibits poorly resolved vibrational fine structure, which
becomes more distinct at 77 K, with maxima at 790, 855, and
ca. 935 nm. The first two peaks at 77 K are separated by 960
cm-1, very close to the ground state MotO stretching frequency
of 952 cm-1; the third peak occurs with a similar spacing, but
it is not well enough resolved for accurate measurement.
Structured phosphorescence in nitridorhenium(V) complexes has
been attributed to vibronic transitions;4 detailed analyses of
vibrational fine structure in absorption and emission have also
been carried out on related dioxorhenium(V),2 nitridoosmium-
(VI),3 and oxomolybenum(V)5b complexes.
The phosphorescences of solid [MoOCl(dppe)2](BF4) (Figure
2) and [Mo(OCH3)2(CN-t-Bu)4](PF6)2 (Figure 4) are broad,
showing slight red shifts at 77 K. Especially at 77 K, the spectra
show evidence for multiple maxima attributable to vibrational
fine structure.
Pulsed-laser excitation (Nd:YAG, 532 nm) of the solid Mo-
(IV) complexes also induces phosphorescence. Emission decay
curves were exponential for CH2Cl2 solutions of [MoOCl(CN-
t-Bu)4][BPh4], yielding a lifetime of 2.5 µs ((10%) at room
temperature.
We were unable to measure lifetimes for the solid-state
emissions: the phosphorescence decay curves were nonexpo-
nential, falling off more rapidly at first and then approaching
exponential behavior at longer delay times. Approximate
limiting lifetimes, estimated at the longest delay times, were as
follows: [MoOCl(dppe)2](BF4), 1 µs at room temperature and
1.5 µs at 77 K; [Mo(OCH3)2(CN-t-Bu)4](PF6)2, 2 µs at both
room temperature and 77 K; and [MoOCl(CN-t-Bu)4](BPh4),
ca. 10 µs at room temperature and >30 µs at 77 K. Measure-
ments with [MoOCl(CN-t-Bu)4](BPh4) in PMMA films yielded
approximately the same limiting lifetimes at room temperature
and 77 K as those with the powder.
a Redox potentials are in volts vs Fc/Fc+; excited state energy is in
electronvolts.
be spontaneous, the redox potential for oxidation of the
Mo(IV) excited state (i.e., for the Mo(V)/Mo(IV)* couple) must
be less than that for the A/A- couple. The Mo(V)/Mo(IV)*
potential can be estimated from the ground-state redox potential
and the energy of the excited state with the aid of the modified
Latimer diagram shown in Scheme 2.
The energy of an excited state can be determined from the
overlap of absorption and emission bands, with the aid of high-
resolution spectral measurements at low temperature. However,
in the present case, this information is not available. Instead,
we can estimate the excited-state energy from the position of
the high-energy “tail” of the luminescence band. The phos-
phorescence “tail” in Figure 3 is at ca. 750 nm (ca. 13 300 cm-1
,
or 1.6 eV). Thus, the excited-state redox potential ([MoIVOCl-
(CN-t-Bu)4]+*/[MoVOCl(CN-t-Bu)4]2+) is ca. -0.8 V vs Fc/
Fc+, or ca. -0.35 V vs Ag/AgCl; and the A/A- couple should
be more positive than -0.8 V vs Fc/Fc+. (Since the wave we
observed in cyclic voltammetry is irreversible, the redox
potentials shown in the diagram are estimates; the actual values
may be more negative, i.e., it may be easier to oxidize Mo(IV)
than the diagram indicates.)
If A is neutral or anionic, then A- will be an anion, and the
electron-transfer products will have opposite charges, so that
they are likely to remain closely associated after reaction 3
occurs. This will permit extremely rapid back electron transfer
(reaction 4 above), making it difficult to observe A- or Mo-
(V). The chances of observing these electron-transfer products
are best if A is a cation. Among the most popular cationic
oxidative quenchers in photoredox experiments are the violo-
gens, because they are colorless and their one-electron-reduced
forms are deep blue or violet.33 The intense color of the reduced
viologens makes confirmation of an electron-transfer reaction
possible e.g. by spectral measurements in flash photolysis.
Benzylviologen, BV2+, is one of the best oxidants in the
viologen family (E1/2 ) -0.76 V vs Fc+/Fc in CH3CN8), and it
is also soluble in appropriate solvents. Addition of excess BV-
(BF4)2 to solutions of [MoOCl(CN-t-Bu)4](BPh4) in either
acetone or acetonitrile gave solutions that still showed [MoOCl-
(CN-t-Bu)4]+ phosphorescence at approximately its original
intensity. This indicates that BV2+ does not quench the
phosphorescence of [MoOCl(CN-t-Bu)4]+ efficiently. An elec-
tron acceptor with a more positive redox potential would be
more likely to quench the Mo(IV) phosphorescence, but we are
not aware of any cationic acceptors with the right combination
of redox potential and spectral properties appropriate for
detection of electron transfer.34
Redox and Photoredox Properties. Since [MoOCl(CN-t-
Bu)4]+ phosphoresces in solution, we were also interested in
whether it could undergo photoredox reactions. We first
explored the electrochemical properties of the ion. Cyclic
voltammetry experiments with [MoOCl(CN-t-Bu)4](BPh4) in
CH2Cl2 revealed an irreversible oxidation wave at ca. 0.8 V vs
Fc/Fc+ (ca. 1.25 V vs Ag/AgCl), and no other waves within
the stability range of the solvent. The simplest redox process
that is consistent with this electrochemical behavior is one-
electron oxidation of [MoIVOCl(CN-t-Bu)4]+ to [MoVOCl(CN-
t-Bu)4]2+, followed by rapid decomposition of the Mo(V)
product.
If [MoIVOCl(CN-t-Bu)4]+ can be oxidized electrochemically,
its phosphorescent excited state might also react with one-
electron acceptors A. This would result in oxidative quenching
of the excited state, as in reaction 5. In order for reaction 5 to
[MoIVOCl(CN-t-Bu)4]+* + A f
[MoVOCl(CN-t-Bu)4]2+ + A- (5)
Comparison with other d2 and d1 systems. The photo-
physical data for the present compounds may be compared with
those for the other d2 systems in Tables 4 and 5, and with a
representative fluorescent d1 complex, [MoOCl4(CH3CN)]-. All
[MoVOCl(CN-t-Bu)4]2+ + A- f
[MoIVOCl(CN-t-Bu)4]+ + A (6)
(33) Bird, C. L.; Kuhn, A. T. Chem. Soc. ReV. 1981, 10, 49-82.

4264 Inorganic Chemistry, Vol. 37, No. 17, 1998
Isovitsch et al.
of the d2 systems show excited-state lifetimes in the microsecond
range, as compared with 110 ns for [MoOCl4(CH3CN)]-,5a and
less for other fluorescent d1 systems.5a,7a The d2 systems are
expected to show longer lifetimes, since their emission is
phosphorescence, while the d1 systems show only fluorescence.
Likewise, the d2 systems show much larger Stokes shifts
between their prominent absorption and emission bands than
the d1: 6400-7000 cm-1 in our Mo(IV) complexes, and g8000
cm-1 for Re(V) and Os(VI), vs 2800 cm-1 in [MoOCl4-
(CH3CN)]-. This is also not surprising, because the prominent
transitions in absorption and emission are different in d2 but
the same in d1. Finally, the energies of the dxy f dxz,yz transitions
are very similar in the Mo(V) and Mo(IV) monooxo systems,
but substantially larger in the nitrido and dioxo complexes. This
reflects the stronger π bonding of one N or two O atoms as
compared to a single O atom.
Summary
The photophysical characteristics of the known oxomolyb-
denum(IV) complexes [MoOCl(dppe)2](BF4) and [MoOCl(CN-
t-Bu)4](BPh4) and the new dimethoxomolybdenum(IV) complex
[Mo(OCH3)2(CN-t-Bu)4](PF6)2 have been explored. The crystal
structure of [Mo(OCH3)2(CN-t-Bu)4](PF6)2 has also been de-
termined. All of these oxygen-containing d2 molybdenum
complexes phosphoresce in the solid state at room temperature
and at 77 K. [MoOCl(CN-t-Bu)4](BPh4) also phosphoresces
in solution at room temperature. We are now exploring the
scope of phosphorescence in related Mo(IV)-oxo complexes,
as well as their capacity for other photochemical reactions.
Acknowledgment. Support for improvements to the X-ray
Crystallography Facility was provided by Grant LEQSF(1996-
97)-ENH-TR-10, administered by the Louisiana Board of
Regents.
(34) The neutral acceptors chloranil, 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone, and tetracyanoethylene are all significantly stronger oxidants
than BV2+ (Mann, C. K.; Barnes, K. K. Electrochemical Reactions in
Nonaqueous Solutions; Dekker: New York, 1970). However, when
we added these acceptors to solutions of [MoOCl(CN-t-Bu)4]+, the
color of the solutions changed substantially. This suggests that the
more powerful acceptors form electron donor-acceptor charge-transfer
complexes with the Mo(IV) complex, making the study of simple
excited-state electron-transfer reactions impossible.
Supporting Information Available: X-ray data for [Mo(OCH3)2-
(CN-t-Bu)4](PF6)2, in CIF format, is available on the Internet only.
Access information is given on any current masthead page.
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