Organometallics 1996,14, 2951-2960
2951
Photochemical Metal-to-OxoMigrations of Aryl and Alkyl Ligands Seth N. Brown1 and James M. Mayer*s2 Department of Chemistry, P.O. Box 351700,University of Washington, Seattle, Washington 98195-1700 Received February 10, 1995@ Photolysis of the oxo-phenyl complex (HBpzs)ReO(Ph)Cl(HBpz3 = hydrotris( 1-pyrazoly1)borate) in the presence of pyridine or other donor ligands (MeCN, OPMe3) slowly gives paramagnetic rhenium(II1) phenoxide complexes (HBpz3)Re(OPh>(Cl)(L). Phenyl-to-oxo migration is predominantly intramolecular, as indicated by a crossover experiment involving and (HBpz3)Re(160)(C6Ds)(cl). The radical traps, photolysis of (HBpz3)Re(180)(C6H5)(cl) MeCN and PhSH, have little influence on the reaction, further ruling out the involvement of free phenyl radicals. Migration of substituted aryl ligands in (HBpza)ReO(Cl)(Ar)(Ar = p-anisyl, p-phenoxyphenyl, 2,4-dimethylphenyl, 2,5-dimethylphenyl) occurs exclusively with carbon-oxygen bond formation t o the ipso carbon of the aryl ligand. The reactions are therefore simple [1,2]-migrations; they are proposed to take place via nucleophilic attack of the oxo ligand on the aryl group in the excited state. Photochemical ethyl-to-oxo migration also occurs in (HBpz3)ReO(C2HdClto give (HBpz3)Re(OCzH5)Cl(py). But net ethyl migration occurs by a process involving free ethyl radicals, a s indicated by facile trapping by PhSH. Thus two different mechanisms are involved in these processes, the first clear examples of [1,2]-migration of hydrocarbon ligands from a metal to a n oxo group. Migration of an alkyl or aryl ligand t o a coordinated oxo group (eq 1) has been proposed as a way of transferring oxidizing equivalents from a metal complex t o an organic fragment.3 This reaction has never been observed d i r e ~ t l ythough ,~ the reverse reaction, involving alkyl or hydride migration to metal centers, has been documented in a handful of cases.5
plexes and arenes.6 In this paper, the subsequent photochemical aryl-to-oxo migration of these compounds to give aryloxide complexes is described. The preparation and analogous migration reaction of an oxo-ethyl complex is also discussed. Surprisingly, aryl and saturated alkyl ligands undergo the same formal migration by quite different mechanisms. A portion of this work has been previously reported.6a
Results The selective oxidation of organic groups is of importance in a variety of areas of chemistry, and the involvement of organometallic complexes, as in eq 1, is an area of great promise. For instance, the transformation of eq 1, coupled with formation of metal-carbon bonds from hydrocarbons, could be a method of hydrocarbon oxidation with novel selectivity. We have previously reported the photochemical formation of rhenium(V) oxo-aryl complexes from rhenium(V) iodide com@Abstractpublished in Advance ACS Abstracts, May 15, 1995. (1) Current address: Department of Chemistry 164-30, California Institute of Technology,Pasadena, CA 91125. Electronicmail: snbrown@ cco.caltech.edu. (2)Electronic mail:
[email protected]. (3) (a) Sharpless, K. B.; Teranishi, A. Y.; Backvall, J.-E. J . Am. Chem. SOC.1977, 99, 3120-3128. (b) Hentges, S. G.; Sharpless, K. B. J . Am. Chem. SOC. 1980, 102, 4263-4265. (c) Gobel, T.; Sharpless, K. B. Angew. Chem., Int. Ed. Engl. 1993, 32, 1329-1331. (d) Gable, K. P.; Phan, T. N. J . Am. Chem. SOC.1994,116,833-839. (e)Jorgensen, K. A,; Schiett, B. Chem. Reu. 1990,90, 1483-1506. (4) This or similar migrations have been proposed on the basis of indirect evidence, e.g.: (a)Reichle, W. T.; Carrick, W. L. J . Organomet. Chem. 1970, 24, 419-426. (b) Nugent, W. A.; Harlow, R. L. J . Am. Chem. SOC.1980, 102, 1759-1760. (c) Vivanco, M.; Ruiz, J.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993,12, 1802-1810. (5) (a) van Asselt, A,; Burger, B. J.; Gibson, V. C.; Bercaw, J. E. J . Am. Chem. SOC. 1986, 108, 5347-5349. (b) Parkin, G.; Bunel, E.; Burger, B. J.; Trimmer, M. S.; van Asselt, A,; Bercaw, J. E. J. Mol. Catal. 1987, 41, 21-41. (c) Nelson, J. E.; Parkin, G.; Bercaw, J. E. Organometallics 1992, 11, 2181-2189. (d) Tahmassebi, S. K.; Conry, R. R.; Mayer, J. M. J . Am. Chem. SOC.1993,115, 7553-7554.
Synthesis and Structure of (HBpz3)Re(OPh)(Cl)(py). When the oxo-phenyl complex (HBpzs)ReO(Ph)C1 (HBpz3 = hydrotris(1-pyrazoly1)borate)is photolyzed in benzene in the presence of a few equivalents of pyridine, NMR signals for the oxo complex slowly decrease in intensity. They are replaced by signals due to a red paramagnetic complex, (HBpz3)Re(OPh)(Cl)(py) (eq 21, which can be separated from unreacted starting
material by column chromatography on silica gel. The phenoxide complex is stable to air for a few weeks in solution and indefinitely in the solid state. The photoreaction takes place in good yield (85% based on converted rhenium) but goes rather slowly (-80% conversion in 10 days) and slows down dramatically as the reaction proceeds, presumably because of light (6)(a) Brown, S. N.; Mayer, J. M. J . Am. Chem. SOC. 1994, 116, 2219-2220. (b) Brown, S. N. Ph.D. Thesis, University of Washington, Seattle, WA, 1994; Chapter 2.
0276-733319512314-2951$09.00/0 0 1995 American Chemical Society
2952 Organometallics, Vol. 14, No. 6, 1995
Brown and Mayer
1i
Sd
-
-
4
4
I 0 I
I
15.0
10.0
r
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.--
7-10.0 -5.0
m
'
I
-10.0
I '
-15.0
I I I
r.
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Figure 1. IH NMR spectrum of (HBpzdRe(OPh)(Cl)(py)in C6D6. An expanded plot of each peak is displayed on the upper part of the figure. Peaks due to C6D5H (6 7.15) and HzO (6 0.40) are marked with asterisks. absorption by the product. Since (HBpzs)ReO(Ph)Clis formed in high yield on photolysis of (HBpzdReO(C1)I in benzene in the presence of pyridine,6 (HBpzdRe(OPh)(Cl)(py) can be prepared in one pot simply by prolonged photolysis of (HBpz3)ReO(Cl)I under these conditions. This represents a net oxidation of benzene to phenoxide. The product is identified as the phenoxide complex (HBpz3)Re(OPh)(Cl)(py)on the basis of spectroscopic, chemical, and structural data. Its mass spectrum shows a parent ion at mlz = 607, and the IR spectrum lacks a metal-oxo stretch but has a strong C-0 stretch at 1258 cm-l absent in the starting material. The lH NMR spectrum (Figure 1) is paramagnetically shifted but extremely sharp (the 2 Hz couplings of the pyrazole protons are easily seen), as expected for a n octahedral rhenium(II1) complex.' There are resonances due to three inequivalent pyrazole groups (three triplets and six doublets), indicating the absence of any symmetry in the molecule. There is also one pyridine ligand (6 -2.32, +16.92, and -0.79 ppm in CsD6) and one phenyl group (6 -11.71, +10.56, and -4.73 ppm) whose resonances are absent if the compound is made from (HBpzs)ReO(CsD&l. (HBpz3)Re(OPh)(Cl)(py)can be synthesized independently by reduction of the rhenium(V) phenoxide complex (HBpzs)ReO(OPh)Cl, formed by treatment of (HBpzdReOC12 with TlOPh. Deoxygenation of rhenium(V) precursors in the presence of an appropriate ligand is a general route to rhenium(II1) complexes,8 as illustrated by the synthesis of (HBpzs)ReCl~(py) (eq 3); (7) (a) Shaw, D.; Randall, E. W. J . Chem. SOC.,Chem. Commun. 1965,82-83. (b) Chatt, J.; Leigh, G. J.; Mingos, D. M. P. J . Chem. SOC.A 1969,1674-1680.( c ) Randall, E.W.; Shaw, D. J . Chem. SOC.A 1969,2867-2872.
the diamagnetic 3-hexyne complex (HBpz3)ReCl2(EtC= CEt) may be prepared analogously.
Deoxygenation of (HBpzdReO(0Ph)Cl is accomplished by treatment with trimethyl phosphite under forcing conditions. In the presence of pyridine, this forms (HBpzdRe(OPh)(Cl)(py),which is spectroscopically identical to that formed by photoinduced migration of the phenyl group in (HBpzs)ReO(Ph)Cl (eq 4).
Reduction of the rhenium(V) phenoxide complex is preferred over the photochemical reaction as a synthetic method for the rhenium(II1)phenoxide despite its lower yield since the internal filtering effects in the latter procedure drastically limit the conversion possible in large-scale preparations. Attempts to synthesize the phenoxide complex by metathesis of a chloride ligand (8)(a) Rouschias, G. Chem. Reu. 1974,74, 531-566. (b) Abrams, M. J.; Davison, A.; Jones, A. G. Inorg. Chim. Acta 1984,82,125-128. ( c ) Conry, R. R.; Mayey, J. M. Inorg. Chem. 1990,29,4862-4867. (d) Paulo, A.; Domingos, A,; Pires de Matos, A.; Santos, I.; Carvalho, M. F. N. N.; Pombeiro, A. J. L. Inorg. Chem. 1994,33,4729-4737. (e) Brown, S. N.; Masui, C. S.; Mayer, J. M.; Schneemeyer, L. F.; Waszczak, J. V. Manuscript in preparation.
Photochemical Metal-to-Oxo Migrations
Organometallics, Vol. 14, No. 6,1995 2953 Table 1. Crystal Data for (HBpzs)RdOPh)(Cl)(py)c13& formula fw cryst size, mm3
CzsHzoDsBC1N7ORe 691.05 0 . j x 0.15 x 0.25
P iT,F "C
P1
a,W b, A C,
A
a, deg
P>deg Y deg 9
Z
v, A3
ecaled,
g
p , cm-1
no. of reflns measd no. of unique reflns no. of obsd reflns no. of refined params
R," R c33 34
Figure 2. ORTEP diagram of (HBpz3)Re(OPh)(Cl)(py). in the rhenium(II1) complex (HBpzs)ReCln(py) with lithium phenoxide or thallium alkoxides have been unsuccessful. The phenoxide group in (HBpz3)Re(OPh)(Cl)(py) may be removed from the rhenium by electrophilic reagents. Concentrated aqueous HC1 immediately releases phenol and forms (HBpzs)ReCla(py)from a benzene solution of the rhenium(II1) phenoxide complex. Trimethylsilyl chloride reacts analogously, but more slowly (32 days, 85 "C),to give the same rhenium product and PhOSiMes (eq 5).
The structure of (HBpzdRe(OPhXCl)(py)was determined by single-crystal X-ray diffraction. An ORTEP diagram is shown in Figure 2, crystallographic details are listed in Table 1,and bond distances and angles are in Table 2. The molecular structure is essentially superimposable with that of (HBp~3)ReC12(py),~ with the phenoxide group substituted for one chloride. The coordination geometry in each case is a regular octahedron with a symmetrically bonded HBpz3 ligand (deviations from octahedral angles r5"), in contrast to the [tris(pyrazolyl)boratolrhenium oxo complexes which invariably show substantial distortions due to the large trans influence of the oxo ligand.1° The pyridine is bound 0.057(8) A farther away from the rhenium than are the pyrazole nitrogens, on average. The pyridine ring and the phenyl ring of the phenoxide are oriented to bisect clefts between cis pyrazoles. This is also true of the phenyl groups in ( H B ~ z ~ ) R ~ Oand ( P ~is ) presumably for steric reasons. (9)(a)Brown, s.N. Ph.D. Thesis, University of Washington,Seattle, WA, 1994; Chapter 3. (b) Reference 8e. (10) (a) Brown, S. N.; Mayer, J. M. Znorg. Chem. 1992,31, 40914100. (b) Reference 8d and references therein.
RW
goodness of fit
0.710 73 -90 8.874(2) 11.917(2) 13.070(3) 92.58(2) 98.29(1) 105.16(1) 2 1315.2(4) 1.745 47.55 6228 4613 4010 (F > 4uF) 335 0.0195 0.0334 0.0412 1.09
Table 2. Selected Bond Distances (A) and Bond h g l e s (deg) in (HBpzdRe(OPh)(Cl)(py)C& Re-0 Re-N(32) Re -N( 12) Re-N(22) 0-Re-Cl O-Re-N(51) 0-Re-N( 12) O-Re-N(22) O-Re-N(32) N(51)-Re-N(12) N(51)-Re-N(22) N(51)-Re-N(32)
2.004(4) 2.091(5) 2.084(5) 2.093(4) 91.7(1) 86.9(2) 91.1(2) 95.3(2) 177.1(2) 90.0(2) 177.4(2) 94.1(2)
Re-C1 Re -N(5 1) O-C(41) Cl-Re-N(51) C1-Re-N(12) Cl-Re-N(22) Cl-Re-N(32) N( 12)-Re-N(22) N(12)-Re-N(32) N(22)-Re-N(32) Re-O-C(41)
2.391(2) 2.146(4) 1.351(6) 89.9(1) 177.3(1) 91.5(1) 91.1(1) 88.5(2) 86.2(2) 83.6(2) 130.8(4)
The phenoxide group in (HBpz3)Re(OPh)(Cl)(py)is strongly bent (L Re-O-C(41) = 130.8(4)"). The rhenium-oxygen distance of 2.004(4) A is slightly longer than that in the rhenium(II1) oxo phenoxide complex (MeC=CMe)zRe(O)(OPh)(1.966(14)&,I1 in which some Re-0 n bonding was proposed t o be present, but significantly shorter than the distance in the d6-cresolate complex fac-(C0)3(PPh3)2Re(OC~H4Me) (2.143(4) A),12where only n-antibonding interactions are possible. Both of these structures also have small Re-0-C angles (124.5(11)" and 131.5(11)0). The rheniumphenoxide bond appears to be normal, with neither significant n-bonding nor n-antibonding interactions, consistent with the complex's chemistry, which shows that the aryloxide linkage is not especially reactive. Mechanism and Scope of Aryl-to-OxoMigration Reactions. The oxo-phenyl complex (HBpzs)ReO(Ph)= 661 nm, E = 120 C1 is blue due to a d-d band (A, M-l cm-l). However, intense visible light (filtered through a A > 455 nm filter) does not lead to rearrangement. Instead, the photochemically active band appears to be a charge-transfer band in the near W (321 nm, E = 3000 M-l cm-'). Though the complex has absorption bands a t higher energy, these do not appear to contrib~ ~ ute ~ significantly to the reaction, since photolysis of acetonitrile solutions of (HBpzdReO(Ph)Cl in thin(11)Erikson, T. K. G.; Bryan, J. C.; Mayer, J. M. Organometallics 1988, 7,1930-1938. (12) Simpson,R. D.; Bergman, R. G. Organometallics 1993,12,781796.
2954 Organometallics, Vol. 14, No. 6, 1995
Brown and Mayer
walled NMR tubes produces migration products at equal rates in the presence or absence of a thick-walled Pyrex beaker (1 > 300 nm). The charge-transfer band at 321 nm is higher in energy than any associated with the chloride ligand, as the lowest-energy charge-transfer band in (HBpz3)ReOClz is at 259 nm.8b Since the charge-transfer band shifts to lower energy in the anisyl complex (HBpz3)ReO(p-CsH4OMe)Cl(Amax = 356 nm),6 it may be assigned as phenyl n to metal d (rheniumoxo n"). Photochemical phenyl-to-oxo migration does not depend on the presence of pyridine. Photolysis in neat CH3CN or in benzene solutions containing Me3PO produce paramagnetic adducts (HBpzs)Re(OPh)(Cl)(L) (L = CHsCN, Me3PO) analogous to the pyridine complex described above. The C=N stretching frequency drops slightly on coordination to the (HBpzs)Re(OPh)(Cl) fragment (2244 cm-l vs 2255 cm-l in free CH3CN13), possibly indicating some degree of back-bonding by the rhenium(II1) center.14 Dimethyl sulfoxide presumably also binds, but the observed product is the diamagnetic oxo-phenoxide (HBpzs)ReO(OPh)(Cl)due to net oxygen atom transfer from Me2SO (eq 6). The apparent rate of disappearance of (HBpzs)ReO(Ph)Cl is in all cases unaffected by the nature of the added trap.
hv Me,SO
+
Me,S
C6D6
In the absence of added ligands, photolysis of (HBpz3)ReO(Ph)Cl causes the solution to darken and two new paramagnetic compounds to appear, tentatively assigned as the two diastereomers of a n unsymmetric pox0 rhenium(IV) dimer, (HBpzs)Re(OPh)(Cl)@-O)(Ph)(C1)Re(HBpz3)(eq 7).
2
H Observed Calculated
100 i=,
.r(
80
3 60 9)
-2 .r( Y
(d
40
20 0 604
606
608
610 Mass
612
614
Figure 3. Observed and calculated isotope patterns for (HBpz3)Re(OPh)(Cl)(py)formed by photolysis of 44.8% (HBPZ~)R~(~~~)(C~D~)(C~) and 55.2% (HBpzs)Re(*O)(C6H5)(Cl) ("0 = 64.4% lSO). The calculated isotope pattern assumes an entirely intramolecular reaction. Phenyl-to-oxo migration is predominantly intramolecular, as little crossover is observed by FABMS in the products of photolysis of a mixture of (HBpz3)Re(lsO)in the presence (C~Hs)(cl) and (HBpz3)Re(160)(C6D5)(C1) of pyridine (Figure 3). The observed mass spectrum of (HBpz3)Re(OPh)(Cl)(py)is consistent with a n entirely intramolecular migration, though small amounts of crossover (
