Matrix isolation and theoretical study of the reaction of substituted phosphines with CrCl2O2

Article abstract of DOI:10.1021/jp066215y

The reactions between CrO₂Cl₂ and a series of substituted phosphines have been investigated using matrix isolation infrared spectroscopy. For all of the phosphines except PF₃, twin jet co-deposition of the two reagents into argon matrices at 14 K initially led to the formation of weak bands due to the corresponding phosphine oxide. For all of the phosphines, subsequent irradiation with light of λ > 300 nm led to the growth of a number of intense new bands that have been assigned to the phosphine oxide complexed to CrCl₂O, following an oxygen atom transfer reaction. Gas-phase, merged jet reactions prior to matrix deposition led to a significant yield of the uncomplexed phosphine oxide. Theoretical calculations at the B3LYP/6-311++g(d,2p) level were carried out in support of the experimental work, to support product band assignments and clarify the nature of the molecular complexes.

Full text of DOI:10.1021/jp066215y

13786
J. Phys. Chem. A 2006, 110, 13786-13791
Matrix Isolation and Theoretical Study of the Reaction of Substituted Phosphines with
CrCl2O2
Adam J. Delson and Bruce S. Ault*
Department of Chemistry, UniVersity of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45220
ReceiVed: September 22, 2006; In Final Form: October 24, 2006
The reactions between CrO2Cl2 and a series of substituted phosphines have been investigated using matrix
isolation infrared spectroscopy. For all of the phosphines except PF3, twin jet co-deposition of the two reagents
into argon matrices at 14 K initially led to the formation of weak bands due to the corresponding phosphine
oxide. For all of the phosphines, subsequent irradiation with light of λ > 300 nm led to the growth of a
number of intense new bands that have been assigned to the phosphine oxide complexed to CrCl2O, following
an oxygen atom transfer reaction. Gas-phase, merged jet reactions prior to matrix deposition led to a significant
yield of the uncomplexed phosphine oxide. Theoretical calculations at the B3LYP/6-311++g(d,2p) level
were carried out in support of the experimental work, to support product band assignments and clarify the
nature of the molecular complexes.
Introduction
substituted phosphines to determine the mechanism(s) of
reaction and whether phosphine oxides would formed in this
manner. High-level theoretical calculations were employed to
complement the experimental data.
High-valent transition-metal oxo compounds, including
CrCl2O2, are very strong oxidizing agents and are known to
oxidize a wide range of organic substrates.1,2 Of particular
interest is the selectivity of the oxidation of organic compounds
and the specificity of the product(s) formed, as these metal oxo
compounds are able to transfer an oxygen atom to certain olefins
and hydrocarbons.3-12 In contrast, the reactions of CrCl2O2 with
silanes, phosphines, and arsines are not well studied even though
substituted phosphine oxides are analogs of nerve agents such
as sarin.13,14 Possible reactions include valence expansion and
addition, and bond insertion (such as to form a silanol from the
corresponding silane). Extensive theoretical calculations of the
potential energy surfaces for the reactions of CrCl2O2 with small
substrates have been conducted.10,11,15-17 Finally, while much
of the experimental work has been done in solution, only a few
studies have explored the reactions of CrCl2O2 in the gas phase.
The matrix-isolation technique16-18 was developed to facilitate
the isolation and spectroscopic characterization of reactive
intermediates and may provide access to the study of initial
intermediates in the above reactions. Recent matrix isolation
studies from this laboratory have investigated the thermal and
photochemical reactions of high-valent transition-metal com-
pounds, including CrCl2O2, with a number of small organic and
inorganic substrates. Three classes of reactions have been
observed to date, the first of which is formation of an initial
complex, followed by HCl elimination from the complex and
addition of the organic (or inorganic) fragment to the metal
center to retain a four-coordinate complex.19,20 The second is
oxygen atom transfer from the metal center to the substrate,
leading to an oxidized substrate species, such as through the
oxidation of a carbon-carbon multiple bond to a ketone or
ketene (e.g., the oxidation of ethyne to ketene).21-24 The third
class is the insertion of an oxygen atom into a X-H bond (Xd
C, Si) to form an alcohol).25 When the second or third reactions
occurred as a result of irradiation after deposition, the two
species (e.g., CrCl2O and oxidized substrate) then formed a
relatively strongly bound complex. The aim of the present study
was to investigate the reactions of CrO2Cl2 with a series of
Experimental Details
All of the experiments in this study were conducted on a
conventional matrix isolation apparatus that has been described
previously.26 Chromyl chloride, CrO2Cl2 (Acros Organics), was
introduced into the vacuum system as the vapor above the room-
temperature liquid, after purification by freeze-pump-thaw
cycles at 77 K. PF3 (Ozark-Mahoning) was introduced into the
vacuum manifold from a lecture bottle, while PCl3, PBr3, CH3-
PCl2, and (CH3)2PCl (all Aldrich) were introduced into the
vacuum manifold as the vapor above the room-temperature
liquid and purified by repeated freeze-pump-thaw cycles at
77 K. Argon was used as the matrix gas without further
purification.
Matrix samples were deposited in both the twin-jet and
merged-jet modes. In the former, the two gas samples were
deposited from separate nozzles onto the 14 K cold window,
allowing for only a brief mixing time prior to matrix deposition.
These matrices were then irradiated for 2 or more hours with
the H2O/Pyrex filtered output of a 200-W medium-pressure Hg
arc lamp, after which additional spectra were recorded on a
Perkin-Elmer Spectrum 2000 Fourier transform infrared spec-
trometer at 1 cm-1 resolution. In the merged-jet experiments
the two deposition lines were joined with an Ultratorr tee
approximately 90 cm from the cryogenic surface, and the
flowing gas samples were permitted to mix and react during
passage through the merged region.27 This region could be
heated to temperatures as high as 200 °C.
Theoretical calculations were carried out using the Gaussian
03 and 03W suites of programs.28 Density functional calculations
using the B3LYP functional were used to locate energy minima,
determine structures, and calculate vibrational spectra of
potential intermediate species. Final calculations with full
geometry optimization employed the 6-311++G(d,2p) basis set,
10.1021/jp066215y CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/07/2006

Reaction of Substituted Phosphines with CrCl2O2
J. Phys. Chem. A, Vol. 110, No. 51, 2006 13787
Figure 1. Infrared spectra before (lower) and after (upper) irradiation of argon matrices formed by the twin-jet deposition of samples of Ar/PF3
and Ar/CrCl2O2. The band marked with an asterisk denotes the major product bands.
after initial calculations with smaller basis sets were run to
approximately locate energy minima.
bands also maintained a relatively constant intensity ratio with
respect to one another. One of these matrices was also annealed
to 32 K, recooled, and a scan recorded (after initial deposition
and before irradiation). No changes were observed as a result
of annealing.
Results
Prior to any co-deposition experiments, blank experiments
were conducted on each of the reagents used in this study. In
each case the blanks were in good agreement with literature
spectra29-34 and with blanks run previously in this laboratory.
Each blank experiment was then irradiated by the H2O/Pyrex
filtered output of a 200-W Hg arc lamp for 2 h. No changes
were observed in any of these spectra as a result of irradiation.
CrCl2O2 + PF3. In an initial twin jet experiment, a sample
of Ar/CrCl2O2 ) 250 was co-deposited with a sample of Ar/
PF3 ) 500. After 22 h of deposition, no new absorptions were
noted in the spectrum. This matrix was then irradiated for 2 h
with light of λ > 300 nm. Numerous new peaks were seen in
the resulting spectrum, with the most intense features appearing
Experiments were also conducted in which the two reagents
were deposited in the merged-jet mode at similar concentrations.
When the merged region was kept at room temperature, no
reaction products were observed. In two additional experiments,
the merged or reaction region was heated, first to 120 °C and
then to 170 °C. Final spectra in these experiments did not contain
any new product bands.
CrCl2O2 + PCl3, PBr3. In an analogous manner, twin jet
and merged jet experiments were conducted with CrCl2O2 and
PCl3. In each of the twin jet experiments, weak new absorptions
were noted upon initial deposition (i.e., before irradiation), a
doublet at 590 and 597 cm-1 and a multiplet near 1300 cm-1
,
with the most distinct component at 1302 cm-1. These initial
product bands were unchanged by irradiation for 2 h with light
of λ > 300 nm. However, many new product bands grew in
upon irradiation, the most intense of which were located at 447,
629, 633, 1009, and 1215 cm-1. All of the product bands are
listed in Table 2 and shown in Figure 2. The results were
consistent and reproducible in all of the twin jet experiments,
with band intensities that varied directly with the sample
concentrations.
at 449, 524, 1011, 1042, 1054, 1321 (multiplet), and 1378 cm-1
as shown in Figure 1 and listed in Table 1.
,
This initial experiment was repeated several times, using a
range of sample concentrations for each reagent, along with
different irradiation times. Similar product bands were seen in
all of these experiments, with product band intensities that varied
directly with the concentration of the reagents. The product
TABLE 1: Product Bands and Assignments after
Irradiation of Argon Matrices Containing PF3 and CrCl2O2
TABLE 2: Product Bands and Assignments after Merged
and Twin-Jet Deposition of PCl3 and CrCl2O2 into Argon
Matrices
exptla
calcdb
calcdc
litd
assignment
449
436
F3POsCrCl2O, CrsCl stretch
F3PO, δ (PF3)
F3PO, δ (PF3)
F3POsCrCl2O, δ (PF3)
F3POsCrCl2O, δ (PF3)
F3PO, PsF stretch
F3POsCrCl2O, PsF stretch
F3PO, PsF stretch
F3POsCrCl2O, PsF stretch
F3POsCrCl2O, PsF stretch h
F3POsCrCl2O, CrdO stretch
F3POsCrCl2O, PdO stretch
F3PO, PdO stretch
441
454
473
485
exptl
447d
calcda
calcdb
litc
assignment
436
490
Cl3PO-CrCl2O
Cl3PO-CrCl2O
Cl3PO
489
524
457
515
546e
593e
483
551
585
477
586
815
929
873
990
Cl3PO
861
839
629d
Cl3PO-CrCl2O
Cl3PO-CrCl2O
Cl3PO-CrCl2O
Cl3PO-CrCl2O
Cl3PO
633d
589
990
1046
1011
1321
968
990
1135
1290
1011d
1215d
1302e
1130
1193
1301
1313
1388
1415
a Calculated for the Cl3PO-CrCl2O complex, unscaled. b Calculated
a Twin-jet experiment. b Calculated for the F3PO-CrCl2O complex,
for Cl3PO, unscaled. c From ref 39. d Twin-jet experiment. e Merged
jet.
unscaled. c Calculated for F3PO, unscaled. d From ref 38.

13788 J. Phys. Chem. A, Vol. 110, No. 51, 2006
Delson and Ault
Figure 2. Infrared spectra before (lower) and after (upper) irradiation of argon matrices formed by the twin jet deposition of samples of Ar/PCl3
and Ar/CrCl2O2. The band marked with an asterisk denotes the major product bands.
Figure 3. Infrared spectra before (middle) and after (top) irradiation of argon matrices formed by the twin jet deposition of samples of Ar/PBr3
and Ar/CrCl2O2, compared to a similar matrix prepared by merged jet deposition (bottom).
Several merged jet experiments were conducted at different
concentrations as well, with the merged region held at room
temperature. In each experiment, distinct product bands were
observed, at 546 (m), 590 (s), 597 (s), and 1302 (s) cm-1, along
with a significant reduction in the intensities of the parent bands.
The relative intensities of the product bands remained constant
throughout the merged jet experiments. It is noteworthy that
the more intense bands in these experiments matched exactly
the weak product bands observed in the twin-jet experiments
before irradiation. Since extensive reaction occurred with the
reaction zone at room temperature, experiments with added heat
were not carried out.
The reaction of CrCl2O2 and PBr3 was also explored using
twin and merged jet deposition. In the twin jet experiments, a
weak band was noted at 1279 cm-1 upon initial deposition. This
band was not affected by subsequent irradiation. However,
strong new product bands were observed upon irradiation, at
440, 520, 526, 1008, 1185, and 1193 cm-1. These results were
reproduced in several experiments at different sample concen-
trations. In several merged jet experiments, different product
bands were observed along with a reduction in the intensities
of the parent bands. The most intense were at 490 (doublet)
and 1279 cm-1, this latter band matching the weak product band
observed before irradiation in the twin-jet experiments. Ad-
ditionally, a weaker product band was observed at 558 cm-1
.
Figure 3 shows merged and twin-jet spectra in the PdO
stretching region, while product band positions are listed in
Table 3.
CrCl2O2 + CH3PCl2, (CH3)2PCl. Samples of Ar/CrCl2O2
were co-deposited with samples of Ar/CH3PCl2 in several twin-
jet experiments. Weak product bands were observed at 547 and
1306 cm-1 upon initial matrix deposition. After irradiation of
the matrix, these two bands were unchanged. However, distinct
new product bands were noted, at 441, 595, 777, 897, 901, 1008,
1180, and 1200 cm-1. Several of these were moderately intense.
The product bands in these twin jet experiments were reproduc-

Reaction of Substituted Phosphines with CrCl2O2
J. Phys. Chem. A, Vol. 110, No. 51, 2006 13789
TABLE 3: Product Bands and Assignments after Merged
and Twin-Jet Deposition of PBr3 and CrCl2O2 into Argon
Matrices
TABLE 5: Product Bands and Assignments after Merged
and Twin-Jet Deposition of (CH3)PCl and CrCl2O2 into
Argon Matrices
exptl
calcda calcdb
litc
assignment
exptl calcda calcdb litc
assignment
410
443
Br3POsCrCl2O, PsBr stretch
Br3POsCrCl2O, CrsCl stretch
488 Br3PO, PsBr stretch
Br3POsCrCl2O, PsBr stretch
432d
540d
695e
720d
863e
920d
433
511
(CH3)2ClPOsCl2CrO, CrsCl stretch
(CH3)2ClPOsCl2CrO, PsCl stretch
696 (CH3)2ClPO, PsC stretch
(CH3)2ClPOsCl2CrO, PsCl stretch
868 (CH3)2ClPO, CH3 rock
440d
490e
463
671
886
520d
484
495
1127
1156
1277
691
897
526d
Br3POsCrCl2O, PsBr stretch
Br3POsCrCl2O, CrdO stretch
Br3POsCrCl2O, PdO stretch
1008d
1189d
1279e
(CH3)2ClPOsCl2CrO, CH3 rock
(CH3)2ClPOsCl2CrO, CrdO stretch
(CH3)2ClPOsCl2CrO, PdO stretch
1007d 1126
1156d 1153
1249e
1261 Br3PO, PdO stretch
1249 1271 (CH3)2ClPO, PdO stretch
a Calculated for the Br3PO-CrCl2O complex, unscaled. b Calculated
a Calculated for the (CH3)2ClPO-Cl2CrO complex, observed bands
only, unscaled. b Calculated for (CH3)2ClPO, observed bands only,
unscaled. c From ref 34. d Twin-jet experiment. e Merged jet.
for Br3PO, unscaled. c From ref 40. d Twin-jet experiment. e Merged
jet.
TABLE 4: Product Bands and Assignments after Merged
and Twin-Jet Deposition of CH3PCl2 and CrCl2O2 into
Argon Matrices
absent. Product band positions for both twin and merged jet
experiments are listed in Table 5.
exptl calcda calcdb
litc
assignment
Results of Calculations. The evidence presented below
strongly supports oxygen atom transfer from CrCl2O2 to each
of the substituted phosphines. On the basis of prior experience
and the literature, the most likely products are the corresponding
phosphine oxide, CrCl2O, and the complex between the two,
X3PO:CrCl2O. Consequently, theoretical calculations focused
on locating energy minima for each of these species. Density
functional theory calculations were undertaken using the B3LYP
hybrid functional and basis sets as high as 6-311++G(d,2p).
All of these species optimized to energy minima on the potential
energy surface, with all real frequencies. The complexes were
all complexed via η1 coordination, with the CrCl2O species
interacting with a lone pair on the oxygen atom. Thermodynamic
quantities were also determined for all of the species, the zero-
point-corrected ∆E° for the oxygen transfer and complexation
reactions. A representative calculated structure is shown in
Figure 4, the unscaled vibrational frequencies listed in Tables
1-5, and the ∆E° values in Table 6.
441d
431
515
554
753
CH3Cl2POsCl2CrO, CrsCl stretch
462
512
738
497 CH3Cl2PO
CH3Cl2POsCl2CrO
552 CH3Cl2PO
CH3Cl2POsCl2CrO
756 CH3Cl2PO
CH3Cl2POsCl2CrO
547e
595d
756e
777d
888e
893e
897d
901d
925
928
893 CH3Cl2PO
898 CH3Cl2PO
941
951
CH3Cl2POsCl2CrO
CH3Cl2POsCl2CrO
CH3Cl2POsCl2CrO
CH3Cl2POsCl2CrO, PdO st.
1008d 1126
1190d 1181
1285e
1281 1302 CH3Cl2PO, PdO stretch
CH3Cl2POsCl2CrO, δ CH3
1358
a Calculated for the CH3Cl2PO-Cl2CrO complex, unscaled. b Cal-
culated for CH3Cl2PO, unscaled. c From ref 34. d Twin-jet experiment.
e Merged jet
ible and had relative intensities that were constant in all of the
several experiments. Merged-jet experiments with a room-
temperature reaction zone and this pair of reagents led to rather
different product bands. Quite intense bands were observed at
547 and 1306 cm-1 along with somewhat less intense bands at
756, 888, and 893 cm-1. Product band positions for both twin
and merged jet experiments are listed in Table 4.
Discussion
The growth of new bands, in some systems following thermal
reaction and in other systems following irradiation, indicates
the formation of products for all of the systems studied here.
Each system will be discussed and products assigned, based on
the type and extent of reaction that was observed.
A similar set of experiments was conducted with CrCl2O2
and (CH3)2PCl as reactants. In the twin-jet experiments, weak
product bands at 432, 540, 720, 863, 920, 1007, 1156, 1260,
and 1309 cm-1 were observed upon initial deposition. In contrast
to the above systems, the majority of these bands grew strongly
upon irradiation, while the bands at 863, 1260, and 1309 cm-1
were unaffected by irradiation. These results were reproducible
in several experiments. To test whether the initial weak product
bands were forming as a result of light from either the infrared
source of the instrument or room lights, a dark experiment was
conducted in which the sample was exposed to radiation only
during the time in which the spectrum was recorded. Identical
results were observed in this experiment.
For the reaction pair CrCl2O2 and PF3, product bands were
not observed upon initial twin-jet co-deposition but strong new
bands were observed after irradiation of these matrices with light
Merged jet experiments were also conducted, leading to an
extended multiplet of product bands between 1260 and 1309
cm-1, along with bands at 695 and 863 cm-1. The 1260-cm-1
band was the most intense component of this multiplet. These
results were reproduced in several experiments, including one
in which the length of the merged region was reduced to 15
cm to shorten the time available for reaction. In all of the merged
jet experiments, the bands of parent (CH3)2PCl were completely
Figure 4. Representation of the Cl2CrO - OPF3 complex, calculated
at the B3LYP/6-311++g(d,2p) level.

13790 J. Phys. Chem. A, Vol. 110, No. 51, 2006
Delson and Ault
TABLE 6: Reaction Energeticsa for the Reactions of
CrCl2O2 with Substituted Phosphines
CrCl2O2 to the phosphine. Oxygen atom transfer in merged jet
deposition has been observed previously for the CrCl2O2 +
(CH3)2SO and H2S systems.24,41 These results are also consistent
with the calculated ∆E0° and ∆G298° values shown in Table 6,
where these values ranged from essentially thermoneutral for
the PCl3 reaction to -37 kcal/mol for the PBr3 reaction. The
observation of the uncomplexed phosphine oxide upon initial
deposition in the twin-jet experiments demonstrates that the
oxygen transfer reaction is very rapid and occurs with a low
activation barrier. The second product of the thermal oxygen
transfer reaction is CrCl2O. Consistent with previous merged-
jet studies, this species was not observed. This is very likely
due to the low volatility of CrCl2O, leading to condensation on
the walls of the deposition line tubing.
∆E° (free)b
(kcal/mol)
∆G298° (free)b
(kcal/mol)
∆E° (complexed)c
phosphine
(kcal/mol)
PF3
PCl3
PBr3
CH3PCl2
(CH3)2PCl
-3.1
+ 1.7
-36.6
-6.1
-3.7
+ 0.6
-37.3
-6.6
-17.7
-16.5
-55.6
-24.4
-39.6
-10.6
-11.3
a Calculated at the B3LYP/6-311++g(d,2p) level. b For the reaction
CrCl2O2 + PX3 f Cl2CrO + X3PO. c For the reaction CrCl2O2 + PX3
f X3PO-Cl2CrO.
of λ > 300 nm. Merged-jet deposition did not yield any thermal
reaction product. Identification of the photochemical product
comes from location of the product bands combined with
theoretical calculations. Two of the product bands for this system
were observed at 449 and 1011 cm-1, very near known bands
of Cl2CrO. This species has been observed by several research
groups,25,35-37 and has characteristic absorptions near 1014 and
450 cm-1, with the exact position of these absorptions varying
slightly from system to system, due to complexation of Cl2-
CrO to the oxidized product. The two product bands observed
here (and their counterparts in the photochemical reactions
described below for the other systems in this study) are very
reasonably assigned to Cl2CrO. This assignment indicates that
oxygen atom transfer has occurred.
On the basis of the known chemistry of PF3, addition of an
oxygen atom might well lead to valence expansion, forming
triflurophosphine oxide, F3PO. However, since this product
would be formed in the same matrix cage with Cl2CrO,
interaction will occur leading to complex formation. Thus, the
observed product in this case is likely to be F3PO:ClCr2O. To
examine this possibility, theoretical calculations were carried
out for both “free” PF3O and the F3PO:ClCr2O molecular
complex. The complex was found to be bound by 18 kcal/mol
relative to the separated species, indicating a significant interac-
tion between the species. As shown in Table 1, the PdO stretch
of the complex was calculated to come at 1290 cm-1, close to
the observed band at 1321 cm-1. Further the shift of this mode
upon complexation was calculated to be -98 cm-1, very close
to the shift of -94 cm-1 (gas phase38 for PF3O minus argon
matrix for the complex). Similar shifts were observed upon
complexation for the P-F stretching modes (+61 cm-1
calculated vs +56 cm-1 experimental). All of the observed
product bands can be assigned in a similar manner, supporting
assignment of the product bands observed here to the F3PO:
ClCr2O molecular complex.
The systems PCl3 + CrCl2O2, PBr3 + CrCl2O2, and CH3-
PCl2 + CrCl2O2 had similar reaction characteristics to one
another, and different from those described above for PF3. For
these 3 systems, merged-jet co-deposition led to thermal reaction
products. Also, weak product bands were observed upon initial
twin jet deposition before irradiation and matched the bands
seen in the merged jet experiments. These bands did not grow
upon irradiation of the resulting matrix, while strong new
product bands grew in. The products in the merged-jet experi-
ments, typified by the bands at 490 and 1279 cm-1 in the PBr3
+ CrCl2O2 system all were quite close to the known bands of
the corresponding phosphine oxide. For example, Br3PO absorbs
at 483 and 1287 cm-1 in an argon matrix.39 Given the
experimental differences, the agreement for all of these systems
is excellent, and the products of the merged-jet co-deposition
for each system are identified as the substituted phosphine
oxide34,40 formed through gas-phase oxygen atom transfer from
After irradiation in the twin jet experiments, a different set
of intense product bands was observed for each system, typified
by the bands at 447, 629, 633, 1011, and 1215 cm-1 for the
CrCl2O2 + PCl3 system. The bands at 447 and 1011 cm-1, as
with the PF3 system above, are assigned to Cl2CrO and
demonstrate that oxygen atom transfer has occurred photo-
chemically in the argon matrix. The 629- and 633-cm-1 bands
are assigned to P-Cl stretching modes, shifted from 593 cm-1
for uncomplexed Cl3PO in the merged jet experiment due to
complexation with the Cl2CrO species. The shift from “free”
to “complexed” Cl3PO was calculated to be +34 and +38 cm-1
,
compared to the +36 and +40 cm-1 shifts that were observed.
In the uncomplexed phosphine oxide, the P-Cl mode is doubly
degenerate and splits in the complex due to the lower symmetry
of the complex. Likewise, the complexation shift of the PdO
stretch was calculated to be -108 cm-1, while a shift of -87
cm-1 was observed. These large shifts suggest a strong
interaction in the complex. This is supported by the calculated
complexation energies, which ranged from -15 to -19 kcal/
mol for these three complexes. All of these points support
assignment of the product bands produced upon irradiation after
twin jet deposition to the respective phosphine oxide complexed
to Cl2CrO.
The results for the final system, (CH3)2PCl + CrCl2O2, were
in several ways similar, but they differed in one major respect
from the above systems. Merged jet co-deposition led to product
formation, with bands observed at 695, 863, and 1285 cm-1
.
These bands were also seen weakly upon initial twin-jet
deposition, did not grow upon irradiation, and are readily
assigned based on the literature and on theoretical calculations
to the phosphine oxide (CH3)2ClPO (literature bands at 696,
868, and 1271 cm-1 in the gas phase).34 The difference for this
system is that a number of additional weak product bands were
formed upon initial twin-jet deposition. All of these bands grew
more than a factor of 10 upon irradiation with light of λ > 300
nm. Among these were bands at 432 and 1007 cm-1, which
can be assigned to Cl2CrO. This suggests oxygen atom transfer
and complexation of (CH3)2ClPO to Cl2CrO. This is supported
by the agreement of the observed bands with those calculated
for the complex (e.g., PdO stretch calculated at 1153 cm-1 and
observed at 1156 cm-1) and calculated shifts relative to
uncomplexed (CH3)2ClPO (for the PdO stretch, calculated shift
of -96 cm-1 vs the observed shift of -129 cm-1). Thus, the
product bands that were observed upon initial deposition and
grew upon irradiation are assigned to the (CH3)2MeClPO:Cl2-
CrO complex. The obserVation of this complex prior to
irradiation has not been seen preViously and indicates that
oxygen atom transfer and complex formation occurs within the
cryogenic matrix cage (reaction before cage formation would
likely lead to the uncomplexed products). This requires a very

Reaction of Substituted Phosphines with CrCl2O2
J. Phys. Chem. A, Vol. 110, No. 51, 2006 13791
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low activation barrier, as well as the proper orientation of the
two molecules within the matrix cage.
Nature of the Complex. As noted above, theoretical calcula-
tions indicated that all of the phosphine oxide complexes with
Cl2CrO were strongly bound, with ∆E° values for the com-
plexation reaction ranging from -14 to -18 kcal/mol. The site
of coordination was between the chromium atom and the oxygen
atom of the phosphine oxide, specifically interacting with one
of the oxygen lone pairs. The PdOsCr angle was calculated
to range from 136° for the F3POsCl2CrO complex to 150° for
the CH3Cl2POsCl2CrO complex. For each complex, the PdO
bond was lengthened approximately 0.03 Å relative to the free
phosphine oxide. The dihedral angle in the Cl2CrO species (a
measure of the degree of deviation from planarity) ranged from
146 to 150°, indicating substantial flattening out of the species
compared to 120° for this angle in the pseudotetrahedral parent
CrCl2O2 [free or uncomplexed Cl2CrO is calculated to be planar
(dihedral angle of 180°)]. At the same time, the XsPdO angle
in the complexes ranged from 111 to 114°, slightly less than
the range of 114-117° for the corresponding parent compounds.
All of these observations demonstrate that these molecular
complexes are distinct, interesting chemical species, and sug-
gests that the chemistry of the X3PO subunit in the complex is
likely to differ from that of the free X3PO species.
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Conclusions
Merged-jet reactions of a series of substituted phosphines with
CrCl2O2 led to gas-phase oxygen atom transfer and formation
of the corresponding phosphine oxide, for all of the phosphines
except PF3. While this last reaction is calculated to be
exothermic, the activation barrier must be too high for reaction
to occur at temperatures up to 170 °C. Twin-jet deposition of
the same reagents led to the formation of small amount of the
phosphine oxide on initial deposition (except, again, for PF3).
Subsequent irradiation left the phosphine oxide unchanged, and
produced the phosphine oxide:Cl2CrO complex in substantial
yield.
Acknowledgment. The National Science Foundation is
gratefully acknowledged for support of this research through
Grant CHE 02-43731. Dr. Nicola Goldberg is thanked for her
assistance with the initial stages of this research.
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