Symmetry and geometry considerations of atom transfer: Deoxygenation of (silox)3WNO and R3PO (R = Me, Ph, tBu) by (silox)3M (M = V, NbL (L = PMe3, 4-picoline), Ta; silox = tBu3SiO)

Inorg. Chem. 2003, 42, 6204−6224

Symmetry and Geometry Considerations of Atom Transfer:

Deoxygenation of (silox)₃WNO and R PO (R ) Me, Ph, ^tBu) by (silox)₃M

3

t

(M ) V, NbL (L ) PMe₃, 4-Picoline), Ta; silox ) Bu₃SiO)

,†

Adam S. Veige,^†LeGrande M. Slaughter,^†Emil B. Lobkovsky,^†Peter T. Wolczanski,*

,§

Nikita Matsunaga,^‡Stephen A. Decker,^§and Thomas R. Cundari*

Department of Chemistry & Chemical Biology, Cornell UniVersity, Baker Laboratory,

Ithaca, New York 14853, Department of Chemistry, Long Island UniVersity,

Brooklyn, New York 11201, and Department of Chemistry, UniVersity of North Texas,

Box 305070, Denton, Texas 76203

Received January 13, 2003

Deoxygenations of (silox)₃WNO (12) and R PO (R ) Me, Ph, ^tBu) by M(silox)₃(1-M; M ) V, NbL (L ) PMe₃,

3

4-picoline), Ta; silox ) ^tBu SiO) reflect the consequences of electronic effects enforced by a limiting steric environment.

3

1-Ta rapidly deoxygenated R PO (23 °C; R ) Me (∆G°_rxn(calcd) ) −47 kcal/mol), Ph) but not ^tBu₃PO (85°, >2

3

days), and cyclometalation competed with deoxygenation of 12 to (silox)₃WN (11) and (silox)₃TaO (3-Ta; ∆G°_rxn-

(calcd) ) −100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (silox)₃V-OPR (3-OPR ) with

3

OPR . 1-Nb(4-picoline) (S ) 0) and 1-NbPMe₃(S ) 1) deoxygenated R PO (23 °C; R ) Me (∆G°_rxn(calcd from

3

1-Nb) ) −47 kcal/mol), Ph) rapidly and 12 slowly (∆G°_rxn(calcd) ) −100 kcal/mol), and failed to deoxygenate

^tBu₃PO. Access to a triplet state is critical for substrate (EO) binding, and the S f T barrier of ∼17 kcal/mol

(calcd) hinders deoxygenations by 1-Ta, while 1-V (S ) 1) and 1-Nb (S f T barrier ∼ 2 kcal/mol) are competent.

Once binding occurs, significant mixing with an ¹A excited state derived from population of a σ*-orbital is needed

1

to ensure a low-energy intersystem crossing of the ³A (reactant) and ¹A (product) states. Correlation of a reactant

2

1

σ*-orbital with a product σ-orbital is required, and the greater the degree of bending in the (silox)₃M−O−E angle,

the more mixing energetically lowers the intersystem crossing point. The inability of substrates EO ) 12 and

^tBu₃PO to attain a bent M−O−E due to sterics explains their slow or negligible deoxygenations. Syntheses of

relevant compounds and ramifications of the results are discussed. X-ray structural details are provided for 3-OPMe₃

t

( V−O−P ) 157.61(9)°), 3-OPBu₃( V−O−P ) 180°), 1-NbPMe₃, and (silox)₃ClWO (9).

Introduction

oxo groups, as in cytochrome P450,^2,3various oxotransferase

enzymes, and mimics,^1,4-9from peroxides in haloperoxi-

dases,¹⁰or from bridging oxo units, as exemplified by

Atom transfer is a deceptively simple reaction that is

central to various transformations in inorganic chemistry

involving formal redox changes among reactants. Oxygen-

atom transfers are prominent examples of these inner sphere

processes due to their importance and scope.¹Several

biological oxidations involve O-atom transfer from terminal

(2) (a) Cytochrome P450, Structure, Mechanism and Biochemistry,

2nd ed.; Ortiz de Montellano, P. R., Ed.; Plenum: New York, 1995.

(b) Enemark, J. H.; Young, C. G. AdV. Inorg. Chem. 1993, 40,

1-88.

(3) (a) De Visser, S. P.; Ogliaro, F.; Sharma, P. K.; Shaik, S. J. Am. Chem.

Soc. 2002, 124, 11809-11826. (b) Schoneboom, J. C.; Lin, H.; Reuter,

N.; Thiel, W.; Cohen, S.; Ogliaro, F.; Shaik, S. J. Am. Chem. Soc.

2002, 124, 8142-8151. (c) de Visser, S. P.; Ogliaro, F.; Sharma, P.

K. Shaik, S. Angew. Chem., Int. Ed. 2002, 41, 1947-1950. (d) de

Visser, S. P.; Ogliaro, F.; Harris, N.; Shaik, S. J. Am. Chem. Soc.

2001, 123, 3037-3047. (e) Ogliaro, F.; Harris, N.; Cohen, S.; Filatov,

M.; de Visser, S. P.; Shaik, S. J. Am. Chem. Soc. 2000, 122, 8977-

8989.

* Authors to whom correspondence should be addressed. E-mail:

ptw2@cornell.edu (P.T.W.); tomc@unt.edu (T.R.C.).

^†Cornell University.

^‡Long Island University.

^§University of North Texas.

(1) (a) Shilov, A. E.; Shteinman, A. A. Acc. Chem. Res. 1999, 32, 763-

771. (b) Woo, L. K. Chem. ReV. 1993, 93, 1125-1136. (c) Jorgensen,

K. A. Chem. ReV. 1989, 89, 431-458. (d) Holm, R. H. Coord. Chem.

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6204 Inorganic Chemistry, Vol. 42, No. 20, 2003

Published on Web 08/29/2003

Symmetry and Geometry Considerations of Atom Transfer

methane monoxygenase.^11-14In the synthesis of fine organ-

ics, the utilization of catalytic systems that transfer one^15-17

or two^18-22oxygens is of increasing importance as regio-

and enantioselectivities rise. For inorganic applications, the

placement or removal of an oxo group is often a critical

synthetic procedure, especially in applications to early

transition metal complex synthesis.^23-26Commodity chemi-

cals synthesis also revolves around oxidation processes that

utilize O-atom transfer, including many where dioxygen is

the penultimate source.²⁷

The transfer of an oxygen atom from a metal-oxo

functionality to a substrate, and its microscopic reverse, is a

straightforward oxygenation reaction. While the chemistry

of (silox)₃WNO (12) was being investigated, the identifica-

tion of (silox)₃WN (11) was needed, and deoxygenation of

12 seemed to be a plausible route given Cummins’ related

preparation of a Cr(VI) nitride from its corresponding Cr-

(II) nitrosyl.²⁴During the course of examining the deoxy-

genation of a 12 by (silox)₃M (M ) Nb (1-Nb), Ta (1-Ta);

t

silox ) Bu₃SiO) some unusual observations were made.²⁸

Previously, (silox)₃Ta (1-Ta) was shown to swiftly strip

oxygen atoms from N₂O, NO,²⁹CO₂, CO,³⁰and epoxides³¹

to form (silox)₃TaO (2-Ta) below room temperature, yet no

O-atom transfer from the tungsten nitrosyl was observed at

23 °C. When an appropriate masked version of its niobium

derivative, (silox)₃Nb(η²-N,C-4-picoline) (1-Nb-4-pic),

was examined, the deoxygenation of (silox)₃WNO (12) did

occur, albeit at elevated temperatures. It was surprising that

the tungsten nitrosyl was stable to 1-Ta under ambient

conditions, and was only observed to undergo deoxygenation

at significantly higher temperatures with byproduct forma-

tion.

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Reported herein is a study of the metal dependence on

the deoxygenation of (silox)₃WNO (10) and R₃PO (R ) Me,

Ph, ^tBu) by (silox)₃M (M ) V, Nb, Ta), replete with synthetic

and structural details of the tungsten system and calculational

support pertaining to the thermodynamics of the atom transfer

events. In this investigation, the symmetry requirements of

O-atom transfer and related geometric constraints are re-

vealed. Related interpretations of state selective chemistry

have been proferred by Shaik in the actions of cytochrome

P450,³and Groves in the study of manganese V oxo

porphyrin derivatives.⁸While Theopold did not find spin state

changes to be consequential in O-atom transfer involving

chromium oxo species,³²Nocera has designed photochemi-

cally activated “pac-man” porphyrin systems that take

advantage of the preferred side-on geometry for O-atom

transfer.³³It is clear that the tuning of electronic states^34,35s

whether intentional or serendipitousshas profound implica-

tions on atom transfer events.

(12) Ambundo, E. A.; Friesner, R. A.; Lippard, S. J. J. Am. Chem. Soc.

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1723.

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592. (c) Linde, C.; A° kermark, B.; Norrby, P.-O.; Svensson, M. J. Am.

Chem. Soc. 1999, 121, 5083-5084. (d) Linde, C.; Arnold, M.; Norrby,

P.-O.; A° kermark, B. Angew. Chem., Int. Ed. Engl. 1997, 36, 1723-

1725.

(18) (a) Kolb, H. C.; VanNieuwenzhe, M. S.; Sharpless, K. B. Chem.

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V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 472-

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(19) (a) DelMonte, A. J.; Haller, J.; Houk, K. N.; Sharpless, K. B.;

Singleton, D. A.; Strassner, T.; Thomas, A. A. J. Am. Chem. Soc.

1997, 119, 9907-9908. (b) Norrby, P.-O.; Rasmussen, T.; Haller, J.;

Strassner, T.; Houk, K. N. J. Am. Chem. Soc. 1999, 121, 10186-

10192.

(27) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; Wiley-

Interscience: New York, 1992. (b) Sheldon, R. A.; Kochi, J. K. Metal

Catalyzed Oxidations of Organic Compounds; Academic Press: New

York, 1981.

(28) Veige, A. S.; Slaughter, L. M.; Wolczanski, P. T.; Matsunaga, N.;

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6420.

(29) Veige, A. S.; Kleckley, T. S.; Chamberlin, R. L. M.; Neithamer, D.

R.; Lee, C. E.; Wolczanski, P. T.; Lobkovsky, E. B.; Glassey, W. V.

J. Organomet. Chem. 1999, 591, 194-203.

(30) Neithamer, D. R.; LaPointe, R. E.; Wheeler, R. A.; Richeson, D. S.;

Van Duyne, G. D.; Wolczanski, P. T. J. Am. Chem. Soc. 1989, 111,

9056-9072.

(31) Bonanno, J. B.; Henry, T. P.; Neithamer, D. R.; Wolczanski, P. T.;

Lobkovsky, E. B. J. Am. Chem. Soc. 1996, 118, 5132-5133.

(32) Hess, J. S.; Leelasubcharoen, S.; Rheingold, A. L.; Doren, D. J.;

Theopold, K. H. J. Chem. Soc. 2002, 124, 2454-2455.

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7884-7885.

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Res. 1997, 30, 494-501.

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(21) (a) Chen, K.; Costas, M.; Kim, J. H.; Tipton, A. K.; Que, L. J. Am.

Chem. Soc. 2002, 124, 3026-3035. (b) Chen, K.; Costas, M.; Que,

L. J. Chem. Soc., Dalton Trans. 2002, 672-679. (c) Costas, M.; Que,

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J.; Costas, M.; Chen, K.; Nam, W.; Que, L. Chem. Commun. 2002,

1288-1289.

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123, 7194-7195.

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1995, 117, 6613-6614.

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Inorganic Chemistry, Vol. 42, No. 20, 2003 6205

Veige et al.

Table 1. Selected Characterizations of (silox)₃M Derivatives: ¹H and ¹³C{¹H} NMR Spectral Data (C₆D₆unless Otherwise Noted); µ_effin µ_Bfrom

Evans Measurements; Elemental Analyses (EA)

¹H NMR (δ, assgmt)^a

((H₃C)₃C)₃

1.95

ν_1/2) 26 Hz

1.33

¹³C NMR (δ, assgmt)^b

C(CH₃)₃

EA (C,H,N)

% calcd (% found)

compound

R

C(CH₃)₃

R

µ

_eff(µ_B)

(^tBu₃SiO)₃V

86.25

46.99

31.28

2.8

61.98 (60.17)

11.70 (11.89)

60.59 (60.09)

11.44 (10.89)

59.32 (57.26)

11.49 (11.60)

66.46 (66.25)

9.92 (9.69)

62.94 (59.26)

11.88 (11.33)

57.44 (56.15)

11.13 (11.10)

49.05 (49.07)

9.26 (8.99)

(1-V)

(^tBu₃SiO)₃VO

(2-V)^c

25.08

156.18

145.45

f

(^tBu₃SiO)₃VOPMe₃

(3-OPMe₃)

1.77

d

e

f

39.90

39.95

d

e

2.8

2.7

2.6

2.4

ν_1/2) 352 Hz

(^tBu₃SiO)₃VOPPh₃

(3-OPPh₃)

1.75

ν_1/2) 440 Hz

1.82

(^tBu₃SiO)₃VOP^tBu₃

(3-OP^tBu₃)

ν_1/2) 456 Hz

1.69

(^tBu₃SiO)₃NbPMe₃

(1-NbPMe₃)

34.60 (PMe)

229.75

25.43

88.52

121.00

23.65

83.15

30.89

72.68

22.65

30.80

d

g,h

ν_1/2) 33 Hz

1.35

ν_1/2) 336 Hz

(^tBu₃SiO)₃ClWO (7)

(^tBu₃SiO)₃WO (8)

2.32

1.2

51.10 (49.18)

9.65 (9.36)

ν_1/2) 15 Hz

(^tBu₃SiO)₃ClWNO (9)

(^tBu₃SiO)₃WNO (10)

0.97

ν_1/2) 9 Hz

1.25

50.27 (49.98)

9.49 (9.56)

1.63 (1.58)

51.22 (51.19)

10.36 (9.49)

1.66 (1.52)

i

(^tBu₃SiO)₃WN (11)

1.30

1.27

24.61

23.61

30.98

30.73

21.97 (H)

1.29 (^tBu)

23.52 (SiC)

(^tBu₃SiO)₂HTaOSi^tBu₂-

CMe₂CH₂(12-Ta)ⁱ

24.93 (SiCMe₂)

1.37 (Me₂)

1.89 (br, CH₂)

33.61 (C(CH₃)₃)₂

39.80 (C(CH₃)₂)

97.04 (CH₂)

1.25

11.66, 12.44 (H)

1.25 (^tBu)

23.72

30.49

23.50, 23.57 (SiC)

(^tBu₃SiO)₂HNbOSi^tBu₂-

CMe₂CH₂(12-Nb)^j,k

30.71 (SiCMe₂)

1.37 (Me₂)

2.25, 2.78 (CH₂)

31.95, 32.01 (C(CH₃)₃)₂

39.51, 39.77 (C(CH₃)₂)

85.24, 94.16 (CH₂)

^aReferenced to C₆D₅H at δ 7.15. ^bReferenced to C₆D₆at δ 128.00. ^cWith Cl₃VO serving as an external reference, its ⁵¹V NMR resonance is at δ

-733.7. ^dPMe₃not observed. ^ePPh₃not observed/assigned. ^fP^tBu₃not observed/assigned, and poor solubility hampered data aquisition. ^gReferenced to

C₇D₇H (toluene) methyl proton at δ 2.09. ^hReferenced to C₇D₈(toluene) methyl carbon at δ 24.04. ⁱSee ref 55. ^jAssignments based on correlation with

1

12-Ta. ^kTwo isomers are evident (isomer A:isomer B in H NMR spectrum ∼ 1.8:1).

Results

2. (silox)₃VO (2-V). (silox)₃V (1-V) was exposed to excess

N₂O in benzene solution until the color became off-white,³⁹

indicating the formation of (silox)₃VO (2-V), which could

be isolated in 72% yield as a powder upon removal of

solvent. 2-V is a diamagnetic compound, and possesses NMR

Synthesis and Characterization. 1. (silox)₃V (1-V).

Treatment of {(Me₃Si)₂N}₃V³⁶with (silox)H in hexanes for

1.5 h afforded purple (silox)₃V (1-V) in 74% yield upon

crystallization from pentane (eq 1). 1-V exhibits a broad

23 °C, 45 min

(silox) VO

(silox)₃V + N₂O (excess)

8

+ N₂(2)

3

{(Me₃Si)₂N}₃V + 3(silox)H ^{23 °C, 1.5 h}8

benzene

2-V

hexanes

spectral characteristics (Table 1) similar to those of (silox)₃MO

(M ) Nb, 2-Nb;²⁹Ta, 2-Ta).³⁰Infrared spectral evidence

for the oxo ligand was not obtained due to overlap with

ligand absorptions.

(silox) V

+ 3(Me₃Si)₂NH (1)

3

1-V

1

signal (ν_1/2) 26 Hz) in its H NMR spectrum at δ 1.95

and broadened ¹³C NMR spectroscopic resonances at δ

86.25 (Me) and 46.99 (SiC) characteristic of the silox group

(Table 1.). An Evans measurement³⁷revealed µ_effto be 2.8

µ_B, which is typical for an S ) 1, d², 3-coordinate vanadium

center.³⁸

3. (silox)₃VOPR₃(3-OPR₃, R ) Me, Ph, ^tBu). Treatment

t

of (silox)₃V (1-V) with R₃PO (R ) Me, Ph, Bu) provided

the crystalline, green adducts (silox)₃V-OPR₃(3-OPR₃; R

) Me (60%), Ph (80%), ^tBu (75%)). Evans measurements³⁷

revealed µ_effto be 2.7(1) µ_Bfor each complex, consistent

23 °C

(silox)₃V-OPR₃

3-OPR₃(R ) Me, Ph, ^tBu)

(silox)₃V + R₃PO

8

(3)

(36) Alyea, E. C.; Bradley, D. C.; Copperthwaite, R. G. J. Chem. Soc.,

Dalton Trans. 1972, 1580-1584.

benzene

(37) (a) Evans, D. F. J. Chem. Soc. 1959, 2003-2005. (b) Orrell, K. G.;

Sik, V. Anal. Chem. 1980, 52, 567-569. (c) Schubert, E. M. J. Chem.

Educ. 1992, 69, 62.

with an S ) 1, d², pseudotetrahedral vanadium.^{38 1}H and

(38) Drago, R. S. Physical Methods for Chemists; Saunders: New York,

1992.

(39) Neithamer, D. R. Ph.D. Thesis, Cornell University, 1989.

6206 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

t

Table 2. Crystallographic Data for (silox)₃V-OPR₃(3-OPR₃; R ) Me, Bu), (silox)₃NbPMe₃(1-NbPMe₃), and (silox)₃ClWO (7)

3-OP^tBu₃

1-NbPMe₃

7

a

3-OPMe₃

formula

fw

C₃₉H₉₀VO₄PSi₃

789.29

C₄₈H₁₀₈VO₄PSi₃

915.52

C₃₉H₉₀NbO₃PSi₃

815.26

C₃₆H₈₁ClO₄Si₃W

881.58

space group

Z

P2₁/n

P31c

C2/c

P2₁

4

2

8

2

a, Å

13.2336(12)

17.4681(17)

21.070(2)

90

16.587(7)

12.883(7)

90

46.322(7)

12.8527(14)

18.580(3)

90

8.7021(1)

21.0660(3)

12.5399(2)

90

b, Å

c, Å

R, deg

â, deg

γ, deg

V, Å³

90.294 (2)

90

4870.7(8)

1.076

0.343

193(2)

0.71073

R₁) 0.0309

wR₂) 0.0906

R₁) 0.0374

wR₂) 0.0984

1.022

90

120

3070(2)

0.991

0.279

173(2)

0.71073

R₁) 0.0735

wR₂) 0.1965

R₁) 0.0817

wR₂) 0.2043

1.074

92.718(11)

90

11050(3)

0.980

0.338

243

0.71073

R₁) 0.0398

wR₂) 0.1153

R₁) 0.0550

wR₂) 0.1302

0.914

105.5070(7)

90

2215.11(5)

1.322

2.781

173(2)

0.71073

R₁) 0.0345

wR₂) 0.0904

R₁) 0.0364

wR₂) 0.0915

1.065

F

_calcd, g cm^-3

µ, mm^-1

temp, K

λ (Å)

R indices [I > 2σ(I)]^b,c

R indices (all data)^b,c

GOF^d

1

2

^aThe asymmetric unit is

/

of the formula unit. ^bR₁) ∑||F_o| - |F_c||/∑|F_o|. ^cwR₂) [∑w(|F_o| - |F_c|)²/∑ωF_o]^1/2

.

^dGOF (all data) ) [∑w(|F_o| -

3

|F_c|)²/(n - p)]^1/2, n ) number of independent reflections, p ) number of parameters.

Table 3. Interatomic Distances (Å) and Angles (deg) Pertaining to

(silox)₃VOPMe₃(3-OPMe₃)

V-O2

V-O1

1.8593(12) V-O3

1.8541(12) V-O4

1.8519(12)

1.9906(13) Si1-O2 1.6344(12) Si2-O3 1.6325(12)

Si3-O4 1.6287(12) O1-P

1.4964(14) P-C1

1.780(2) Si-C_av

1.782(2)

1.936(3)

P-C2

C-C_av

1.784(2)

1.541(6)

P-C3

O1-V-O2 100.16(6) O1-V-O3 104.12(6) O1-V-O4 101.08(6)

O2-V-O3 116.61(6) O2-V-O4 116.92(5) O3-V-O4 114.30(6)

V-O2-Si1 160.34(9) V-O3-Si2 164.48(9) V-O4-Si3 169.93(8)

V-O1-P 157.61(9) O1-P-C1 109.71(10) O1-P-C2 113.06(11)

O1-P-C3 113.10(11) C1-P-C2 107.21(13) C1-P-C3 106.94(14)

C2-P-C3 106.47(13) O-Si-C_av107.5(4) C-Si-C_av111.3(6)

Si-C-C_av111.6(20) C-C-C_av107.2(11)

(157.61(9)°), and the comparative silox angle ( Si-O-V

) 160.34(9)°, 164.48(9)°, 169.93(8)°) is somewhat straighter.

Intersilox repulsions render the SiO-V-OSi angles (114.30-

(6)°, 116.61(6)°, and 116.92(5)°) wider than the SiO-V-

OP angles (100.16(6)°, 101.08(6)°, and 104.12(6)°). The

^tBu₃Si groups are typical for the silox ligand, and the

geometry about the phosphorus is roughly tetrahedral ( C-

P-O_av) 112.0(19)°, C-P-C_av) 106.9(4)°), and perhaps

reflective of the steric influence of the siloxes. Detailed

geometric data is given in Table 3.

Figure 1. Molecular view (40% ellipsoids) of (silox)₃VOPMe₃(3-OPMe₃).

¹³C{¹H} NMR spectral characteristics for 3-OPMe₃and

3-OPPh₃were consistent with paramagnetic species, as was

1

the silox portion of the H NMR spectrum of 3-OP^tBu₃

(Table 1), whose ¹³C{¹H} NMR spectrum was unable to be

1

determined due to solubility difficulties. H NMR spectral

signals for the PR₃fragments either were not observed or

could not be assigned with confidence.

5. Structure of (silox)₃VOP^tBu₃(3-OP ^tBu₃). An X-ray

structure determination of (silox)₃VOP^tBu₃(3-OP^tBu₃) in the

trigonal crystal system (Table 2) revealed a unique silox

group, one P^tBu fragment, and a crystallographically imposed

4. Structure of (silox)₃VOPMe₃(3-OPMe₃). An X-ray

structure determination (Table 2) of (silox)₃VOPMe₃(3-

OPMe₃) revealed near tetrahedral symmetry at the core, as

Figure 1 illustrates. The silox d(V-O) of 1.8593(12), 1.8541-

(12), and 1.8519(12) Å⁴⁰are noticeably different from the

phosphine oxide-vanadium distance of 1.9906(13) Å. The

P-O-V linkage of the datively bonded ligand is clearly bent

t

linear V-O-P linkage (Figure 2.). All the Bu groups are

relatively normal, as the distances in Table 4 reveal, and the

d(V-OSi) of 1.882(6) Å⁴⁰contrasts with the longer dative

interaction of the phosphine oxide (d(V-OP) ) 2.083(9)

Å). In comparison to (silox)₃VOPMe₃(3-OPMe₃), the greater

vanadium-oxygen distances reflect the severe steric require-

ments of the OP^tBu₃ligand; the phosphine oxide-vanadium

interaction is almost 0.1 Å longer. Because of the shorter

vanadium-oxygen distances of the silox ligands, steric

repulsion among them is greater ( SiO-V-OSi ) 114.18-

(14)°) than the silox/OP^tBu₃interactions ( SiO-V-OP )

104.2(2)°). As usual, the Si-O-V angle approaches linearity

(40) (a) Henderson, R. A.; Janas, Z.; Jerzykiewicz, L. B.; Richards, R. L.;

Sobota, P. Inorg. Chim. Acta 1999, 285, 178-183. (b) Henderson, R.

A.; Hughes, D. L.; Janas, Z.; Richards, R. L.; Sobota, P.; Szafert, S.

J. Organomet. Chem. 1998, 554, 195-201. (c) Scott, M. J.; Wilisch,

W. C. A.; Armstrong, W. H. J. Am. Chem. Soc. 1990, 112, 2429-

2430. (d) Wilisch, W. C. A.; Scott, M. J.; Armstrong, W. H. Inorg.

Chem. 1988, 27, 4333-4335. (e) Carrano, C. J.; Mohan, M.; Holmes,

S. M.; de la Rosa, R.; Butler, A.; Charnock, J. M.; Garner, C. D.

Inorg. Chem. 1994, 33, 646-655.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6207

Veige et al.

Figure 3. Molecular view (40% ellipsoids) of (silox)₃NbPMe₃(1-

NbPMe₃).

Figure 2. Molecular view (40% ellipsoids) of (silox)₃VOP^tBu₃(3-OP^t-

Bu₃).

Table 5. Selected Interatomic Distances (Å) and Angles (deg) for

(silox)₃NbPMe₃(1-NbPMe₃)

Table 4. Interatomic Distances (Å) and Angles (deg) Pertaining to

(silox)₃VOP^tBu₃(3-O P^tBu₃)

Nb-P

2.4923(7)

1.9572(17)

1.6345(18)

1.819(4)

Nb-O1

O1-Si1

P-C1

1.9433(17)

1.6300(18)

1.809(3)

Nb-O2

O2-Si2

P-C2

1.9438(16)

1.6292(17)

1.816(4)

Nb-O3

O3-Si3

P-C3

V-O1

V-O2

Si1-C2

C2-C_av

1.882(6)

2.083(9)

1.933(11)

1.550(49)

O1-Si1

O2-P

Si1-C3

C3-C_av

1.642(7)

P-C1

1.867(10)

1.556(36)

1.934(11)

1.531(41)

Si-C_av

1.925(12)

C-C_av

1.540(17)

1.466(10)

1.954(11)

1.504(28)

C1-C_av

Si1-C4

C4-C_av

P-Nb-O1 99.77(7) O1-Nb-O2 115.91(8) Nb-O1-Si1 166.70(13)

P-Nb-O2 103.33(6) O1-Nb-O3 116.97(8) Nb-O2-Si2 169.05(11)

P-Nb-O3 100.41(6) O2-Nb-O3 116.16(7) Nb-O3-Si3 167.07(12)

Nb-P-C1 118.95(11) Nb-P-C2 116.87(13) Nb-P-C3

C1-P-C2 100.96(18) C1-P-C3 101.29(17) C2-P-C3

O-Si-C_av107.3(8)

C-C-C_av107.4(14)

SiO-V-OSi 114.18(14) SiO-V-O2 104.2(2) C-P-C

V-O1-Si 171.8(4) V-O2-P 180 O2-P-C1

O1-Si1-C2 108.2(5)

Si1-C2-C_av112.6(25) Si1-C3-C_av111.2(38) Si1-C4-C_av111.0(38)

C-Si-C_av110.2(3) P-C1-C_av110.1(26) C-C(Si)-C_av107.2(16)

C-C(P)-C_av108.8(6)

110.8(3)

114.01(13)

102.2(2)

111.4(18)

108.1(3)

O1-Si1-C3 108.3(5) O1-Si1-C4 109.7(4)

C-Si-C_av

111.5(6)

Si-C-C_av

(1-NbPMe₃) revealed a flattened tetrahedral geometry that

is nearly pyramidal (Figure 3), with PNbO ) 101.1(19)°

(av) and ONbO ) 116.3(6)° (av). The d(NbO) ) 1.948-

(171.8(4)°), presumably due to the steric constraints around

the first-row metal.

6. (silox)₃NbPMe₃(1-NbPMe₃). Reduction of (silox)₃NbCl₂

(4) with Na/Hg in neat PMe₃afforded purple (silox)₃NbPMe₃

(1-NbPMe₃) in 53% yield upon crystallization from hexanes.

(8) Å (av) and d(NbP) ) 2.4923(7) Å are normal (e.g., r_cov

-

(Nb) + r_cov(P) ) 1.34 + 1.06 ) 2.40 Å), and the silox

ligands display their usual 3-fold splay about the niobium

center, with NbOSi ) 167.6(13)°.

23 °C, 12 h, PMe₃

(silox) NbPMe

(silox)₃NbCl₂

8

(4)

3

8. (silox)₃W Derivatives. Treatment of (silox)₃WCl (6),

prepared from reduction of (silox)₃WCl₂(5),⁴³with 1 equiv

of N₂O afforded the C_3V(vide infra) oxo derivative

(silox)₃ClWO (7, 60%), and N₂according to Scheme 1. Sharp

resonances were observed in the NMR spectra, and a band

at 920 cm^-1was tentatively assigned to the ν(WO).⁴⁴This

is on the low end of the normal range of 920-1058 cm^-1

for tungsten-oxo complexes,^45,46perhaps suggesting that

the chloride exerts a substantial trans influence. Reduction

of 7 with 1.5 equiv of Na/Hg in THF afforded a purple

solution that gave gray-green (silox)₃WO (8) upon tritura-

tion with hexanes and recrystallizations from pentane

(22%). The W(V) oxo derivative was characterized by broad

Na/Hg, PMe₃

1-NbPMe₃

4

A µ_effof 2.4 µ_Bwas determined via the Evans method,³⁷

consistent with a pseudotetrahedral d²system (S ) 1), and

the observation of the silox ^tBu protons at δ 1.69 (ν_1/2) 33

Hz) and the phosphine methyls as a very broad resonance at

1

δ 34.60 (ν_1/2) 336 Hz) in the H NMR spectrum. In the

¹³C{¹H} NMR spectrum, the tertiary carbons are markedly

affected by the paramagnetism of 1-NbPMe₃and are

observed at δ 229.75. Even the silox methyls shift dramati-

cally to δ 83.15, and no resonance could be confidently

assigned to the PMe₃ligand; no resonance was observed in

the ³¹P{¹H} spectrum either. Niobium(III) is typically found

in dimeric or cluster derivatives,^41,42so this simple coordina-

tion compound is quite unusual.

1

resonances in the H NMR spectrum at δ 2.32 (ν_1/2) 15

(43) For the X-ray crystal structures (e.g., (silox)₃WCl₂(5, tbp, axial Cl’s)

and (silox)₃WCl (6, highly dist sq pl)) and chemistry of silox tungsten

chlorides, see: (a) Majol, A.-R. Ph.D. Thesis, Cornell University, 1999.

(b) Chamberlin, R. L.; Douthwaite, R. E.; Majol, A.-R.; Slaughter, L.

M.; Veige, A. S.; Wolczanski, P. T. Manuscript in preparation.

(44) A DFT calculation on (HO)₃ClWO (7′), which modeled (silox)₃ClWO

(7), revealed a ν(WO) of 945 cm^-1; significant coupling with the axial

Cl or the oxygens was not noted.

7. Structure of (silox)₃NbPMe₃(1-NbPMe₃). An X-ray

structural determination (Table 2, Table 5) of (silox)₃NbPMe₃

(41) Tayebani, M.; Conoci, S.; Feghali, K.; Gambarotta, S.; Yap, G. P. A.

Organometallics 2000, 19, 4568-4574.

(42) (a) Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli,

C.; Chiesi-Villa, A. J. Am. Chem. Soc. 2000, 122, 3652-3670. (b)

Kawaguchi, H.; Matsuo, T. Angew. Chem., Int. Ed. 2002, 41, 2792-

2794.

(45) Parkin, G.; Bercaw, J. E. Polyhedron 1988, 7, 2053-2082.

(46) Nugent, W. A.; Mayer, J. M., Metal-Ligand Multiple Bonds; John

Wiley & Sons: New York, 1988.

6208 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

Scheme 1

Hz) and in the ¹³C{¹H} NMR spectrum at δ 72.68 (CH₃)

and δ 88.52 (CMe₃), and a µ_effof 1.2 µ_B(Evans method). A

ν(WO) of 909 cm^-1was observed in its IR spectrum, and

this tentative assignment is consistent with the expected

lowering of the oxo stretch from that of the W(VI) deriva-

tive, 7, due to occupation of a d-orbital with some π*

character.

Synthesis of the tungsten(II) nitrosyl (silox)₃WNO (10)

was accomplished via addition of Na/Hg to (silox)₃WCl

(6) in the presence of NO. Since (silox)₃ClWNO (9) was

likely to be the true target of reduction, this process was

complicated by the same side reactions described above, and

optimal conditions were found to be 2 equiv of Na/Hg, 2

equiv of NO, and <10 min reaction time in THF. Precipi-

tation from Et₂O afforded 10 in 24% yield as a yellow

Exposure of (silox)₃WCl (6) to 1 equiv of NO in a sealed

NMR tube (C₆D₆) rapidly (<5 min, 23 °C) provided three

products in a rough 2:1:1 ratio: nitrosyl chloride (silox)₃-

ClWNO (9), dichloride 5, and nitrosyl (silox)₃WNO (10).

Over the course of 8 days (23 °C), it is likely that only the

nitrosyl chloride was converted to (silox)₃ClWO (7) and

¹/₂N₂, because the ratio of 7/9:5:10 remained constant.

Formation of the dichloride (5) and W(II) nitrosyl 10 is a

plausible consequence of chlorine-atom transfer between the

nitrosyl chloride (9) and the starting monochloride (6). When

1

powder that contained minimal impurities (<5% by H

NMR analysis). The 1574 cm^-1nitrosyl stretch observed in

the IR spectrum of 10 is indicative of significant back-

bonding from the W(II) center to the NO π*-orbitals. Similar

low ν(NO) values have been observed by Legzdins et al.⁴⁸

for a number of tungsten(II) nitrosyl complexes of the type

Cp*W(NO)(R)(R′) (e.g., R ) Me, R′ ) tolyl, ν(NO) ) 1536

cm^-1).

Abstraction of the oxygen atom from (silox)₃WNO (10)

is a plausible route to (silox)₃WN (11), as Cummins et al.

have shown via the deoxygenation of (ArNR)₃CrNO by

(Mes)₃V(THF) to give (ArNR)₃CrN and (Mes)₃VO.²⁴Sepa-

ration difficulties render this route problematic on a prepara-

tive scale, so 2-methylaziridine was explored as a N-atom

source⁴⁹in order to obtain a pure sample of 11. Treatment

1

6 was treated with only /₂equiv of NO under indentical

conditions, the dichloride (5) and nitrosyl 10 formed in a

1:1 ratio. Given the likelihood that Cl-atom transfer could

disrupt attempts to synthesize pure (silox)₃ClWNO (9),

(silox)₃WCl (6) was exposed to a greater excess (6

equiv) of NO. Unfortunately, this only accelerated the

degradation of 9 to oxo chloride 7, and under a vast excess

of NO, this route provided an alternative means to cleanly

synthesize 7.

Because of these complications, (silox)₃ClWNO (9) could

only be prepared as a mixture. The best results were obtained

when a hexane solution of (silox)₃WCl (6) was exposed to

3 equiv of NO for 5 min at -78 °C, followed by a rapid

removal of excess NO and solvent. A red solid was obtained

consisting of 78% 9, 11% (silox)₃WCl₂(5), and 11%

(silox)₃WNO (10). The IR spectrum of the mixture revealed

a strong band at 1651 cm^-1attributed to the ν(NO) of 9,

and a weaker absorbance at 1574 cm^-1that was assigned to

the nitrosyl stretch of 10.⁴⁷

of (silox)₃WCl (6) with 2 equiv of HNCH₂CHMe in benzene

provided (silox)₃WN (11) in 22% yield as a light pink solid

that contained <5% (silox)₃WCl₂(5). Nitride 11 could be

obtained in reasonable analytical purity upon repeated

recrystallizations from pentane.

(47) Hayton, T. W.; Legzdins, P.; Sharp, W. B. Chem. ReV. 2002, 102,

935-991.

(48) (a) Legzdins, P.; Rettig, S. J.; Ross, K. J.; Weltheer, J. E. J. Am. Chem.

Soc. 1991, 113, 4361-4363. (b) Brouwer, E. B.; Legzdins, P.; Rettig,

S. J.; Ross, K. J. Organometallics 1994, 13, 2088-2091. (c) Debad,

J. D.; Legzdins, P.; Reina, R.; Young, M. A.; Batchelor, R. J.; Einstein,

F. W. B. Organometallics 1994, 13, 4315-4321.

(49) Mindiola, D. J.; Cummins, C. C. Angew Chem., Int. Ed. 1998, 37,

945-947.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6209

Veige et al.

nitride (silox)₃WN (11) was considered as a possible product

in the reactions, hence alternative routes to the complex were

sought.⁵¹

Once (silox)₃WNO (10) was eventually prepared, it was

treated with (silox)₃WCl (6), but under no conditions was

O-atom transfer to give (silox)₃ClWO (7) effected. Since the

transfer of oxygen from Cl₄WO to PMe₃is supposed to be

thermodynamically favorable by ∼12 kcal/mol,⁵²7 was

treated with PMe₃, but upon thermolysis at 100 °C in C₆D₆

for >1 day, no Me₃PO was detected. Failed attempts to

effectively deoxygenate 10 to give nitride (silox)₃WN (11)

prompted a more systematic investigation of O-atom trans-

fers, with (silox)₃M (1-M, M ) V, NbL (L ) 4-pic, PMe₃),

Ta) utilized as potential oxygen-atom acceptors.

2. (silox)₃WNO (10) Deoxygenations. The reactivity of

(silox)₃WNO (10) with (silox)₃M (1-M, M ) V, NbL (L )

4-pic,^29,53,54PMe₃), Ta)³⁰is summarized in Scheme 2. With

vanadium as the O-atom acceptor, deoxygenation of 10 took

place rather slowly at 85 °C in benzene-d₆, with a rough

rate constant of k ∼ 1.4 × 10^-4M^-1s^-1(∆G^q(85 °C) ∼ 27

kcal/mol). (silox)₃Nb(4-pic) (1-Nb-4-pic) also deoxygenated

the nitrosyl of 10 under the same conditions (50% after 4

h), but the reaction slowed noticeably with time, implicating

inhibition by free 4-picoline. With (silox)₃NbPMe₃(1-

NbPMe₃) as the deoxygenating agent, milder conditions (23

°C, benzene-d₆) were employed, but the observations were

similar. Initial deoxygenation products appeared swiftly, but

the reaction then slowed beyond that expected for a simple

second-order process (61% conversion after 2 days; 86%

after 9 days). With 8 equiv of PMe₃present, only 10%

conversion to deoxygenation products was noted after 2 days.

Surprisingly, coordinatively unsaturated (silox)₃Ta (1-Ta)

was the least effective of the three derivatives. 1-Ta, which

deoxygenates small molecules like CO and NO at <0 °C

very rapidly,^29.30mostly (87%) cyclometalated to (silox)₂-

Figure 4. Molecular view (40% ellipsoids) of (silox)₃ClWO (7).

Table 6. Selected Interatomic Distances (Å) and Angles (deg) for

(silox)₃ClWO (7)

W-O4

W-Cl

Si1-O1

Si-C_av

1.723(6)

2.518(2)

1.701(6)

1.91(2)

W-O1

W-O3

Si2-O2

C-C_av

1.830(6)

1.841(4)

1.707(6)

1.54(5)

W-O2

1.840(5)

1.700(5)

Si3-O3

O1-W-O4 95.5(3) O2-W-O4 95.7(3) O3-W-O4 96.5(2)

O1-W-Cl 84.0(2) O2-W-Cl 84.09(19) O3-W-Cl 84.15(19)

O1-W-O2 118.2(3) O1-W-O3 120.4(3) O2-W-O3 118.2(3)

O4-W-Cl 179.3(2) W-O1-Si1 165.7(4) W-O2-Si2 161.5(4)

W-O3-Si3 166.6(5) O-Si-C_av105.6(14) C-Si-C_av113.0(13)

Si-C-C_av112.0(38) C-C-C_av106.3(63)

9. Structure of (silox)₃ClWO. Single-crystal X-ray analy-

sis of (silox)₃ClWO (7) confirmed its C_3V, pseudo-tbp

geometry, with the silox groups occupying equatorial posi-

tions (Figure 4, Table 2, Table 6). The axial oxo ligand has

a somewhat long bond length (vide infra) of 1.723(6) Å,⁴⁶

perhaps influenced by the trans-axial chloride, whose d(W-

Cl) ) 2.5180(19) Å. The tungsten-oxygen bond lengths of

the silox ligands are typical (1.837(6) Å (av)), and the

O(silox)-W-O(oxo) angles are 95.9(5)°, thereby reflecting

the influence of the short, multiply bonded oxo ligand. In

contrast, the long tungsten chloride bond accommodates the

acute O(silox)-W-Cl angles of 84.08(8)°, while the O(si-

lox)-W-O(silox) angles are only subtly perturbed from

trigonal (118.9(13)°).

The faint pink color of the crystal used for the X-ray

analysis belied the W(VI) oxidation state of the oxo chloride.

A ¹H NMR spectrum of the sample from which it was taken

indicated the presence of 0.91% (silox)₃WCl₂(5), a bright

pink W(V) complex. Parkin⁵⁰showed that a molybdenum

site containing oxo and chloride is dominated by the latter.

From his data, it is estimated that the actual d(WO) in (7) is

likely to be 0.02-0.04 Å shorter, which brings the bond

length into a normal or slightly elongated regime.

HTaOSi^tBu₂CMe₂CH₂(12-Ta)⁵⁵with a rough rate constant

of k ∼ 5.0 × 10^-6s^-1(23 °C). About 12% of (silox)₃WN

(11) and the corresponding tantalum oxo, (silox)₃TaO (2-

Ta) were observed upon completion, and an appropriate

amount of tungsten nitrosyl (10) remained. Slightly more

deoxygenation (23%) was observed when the reaction was

conducted at 85 °C, and upon completion (11 h), the NMR

tube was kept in the constant-temperature bath for 65 days

and occasionally monitored by ¹H NMR spectroscopy.

Within experimental error, the ratio of products did not

change over the course of time, hence 12-Ta did not appear

to reversibly generate 1-Ta.

(51) (a) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. Inorg. Chem.

1983, 22, 2903-2906. (b) Chisholm, M. H.; Folting, K.; Lynn, M.

L.; Tiedtke, D. B.; Lemoigno, F.; Eisenstein, O. Chem. Eur. J. 1999,

5, 2318-2326. (c) Pollagi, T. P.; Stoner, T. C.; Dallinger, R. F.;

Gilbert, T. M.; Hopkins, M. D. J. Am. Chem. Soc. 1991, 113, 703-

704.

(52) Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571-593.

(53) Covert, K. J.; Neithamer, D. R.; Zonnevylle, M. C.; LaPointe, R. E.;

Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494-2508.

(54) (a) Kleckley, T. S.; Bennett, J. L.; Wolczanski, P. T.; Lobkovsky, E.

B. J. Am. Chem. Soc. 1997, 119, 247-248. (b) Kleckley, T. S. Ph.D.

Thesis, Cornell University, 1998.

Oxygen Atom Transfer Studies. 1. Tungsten. In the

original tungsten experiments regarding (silox)₃WCl (6) and

N₂O or NO, identification of the new products was somewhat

problematic because of the lack of common NMR spectro-

scopic handles, and the minimal chemical shift dispersion

1

pertaining to H NMR signals of the silox ligand. Initially,

(55) Miller, R. L.; Toreki, R.; LaPointe, R. E.; Wolczanski, P. T.; Van

Duyne, G. D.; Roe, D. C. J. Am. Chem. Soc. 1993, 115, 5570-5588.

(50) Parkin, G. Acc. Chem. Res. 1992, 25, 455-460.

6210 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

Scheme 2

The relative unreactivity of 1-Ta toward deoxygenation

of (silox)₃WNO (10) and its proclivity toward binding

N-heterocycles⁵³suggested that it could be used as a

scavenger as indicated in Scheme 3. When 1-Ta was present

along with (silox)₃Nb(4-pic) (1-4-pic) and 10, the formation

of (silox)₃Ta(4-pic) (1-Ta-4-pic) occurred swiftly (<5 min)

and concomitantly with niobium cyclometalation (>90%)

shown that (silox)₃VO (2-V) and PMe₃could be very slowly

converted (∼70%) to (silox)₃VOPMe₃(3-OPMe₃) over the

course of 86 days at 100 °C.^56-60

For niobium, deoxygenation of R₃PO (R ) Me, Ph) by

(silox)₃NbPMe₃(1-NbPMe₃) occurred swiftly (<5 min)

under ambient conditions in benzene-d₆, as did the formation

of (silox)₃NbO (2-Nb) and PMe₃from (silox)₃Nb(4-pic) (1-

Nb-4-pic) and Me₃PO. When the 4-picoline derivative was

used to deoxygenate Ph₃PO, only 50% deoxygenation was

observed at <5 min and at 15 h 85% conversion was

achieved. Apparently the buildup of 4-picoline during the

course of the reaction is enough to inhibit the second-order

to two isomers of (silox)₂HNbOSi^tBu₂CMe₂CH₂(12-Nb) and

deoxygenation (<10%) to (silox)₃NbO (2-Nb) and (silox)₃WN

(11). The cyclometalation products 12-Nb could not be

isolated from this reaction, but their spectral signatures are

t

O-atom transfer process. With Bu₃PO as the substrate,

comparable to that of (silox)₂HTaOSi^tBu₂CMe₂CH₂(12-Ta)⁵⁵

as Table 1 indicates. Furthermore, addition of 4-picoline to

the mixture containing 12-Nb regenerated 1-Nb-4-pic, and

12-Nb slowly deoxygenated 10 over the course of days,

hence cyclometalation appears reversible. The tantalum

scavenging experiment is rationalized as the capture of

4-picoline that dissociates from 1-Nb-4-pic, or the associative

abstraction of 4-picoline by 1-Ta. Transient (silox)₃Nb (1-

Nb) then undergoes predominantly first-order cyclometala-

tion in competition with second-order deoxygenation on a

neither Nb derivative gave products indicative of deoxygen-

t

ation. At 23 °C in the presence of 10 equiv of Bu₃PO,

1-NbPMe₃slowly degraded while the phosphine oxide

remained intact. The 4-picoline “masked Nb(III)” species,

1-Nb-4-pic, can withstand substantial thermolysis, but after

(56) Similar intermediates have recently been identified in transferases:

Smith, P. D.; Millar, A. J.; Young, C. G.; Ghosh, A.; Basu, P. J. Am.

Chem. Soc. 2000, 122, 9298-9299.

(57) (a) Espensen, J. H.; Shan, X.; Wang, Y.; Huang, R.; Lahti, D. W.;

Dixon, J.; Lente, G.; Ellern, A.; Guzei, I. A. Inorg. Chem. 2002, 41,

2583-2591. (b) Wang, Y.; Lente, G.; Espensen, J. H.; Guzei, I. A.

Inorg. Chem. 2002, 41, 1272-1280.

1

time scale too swift to follow by H NMR spectroscopy.

3. R₃PO (R ) Me, Ph, ^tBu) Deoxygenations. As Scheme

4 illustrates, (silox)₃V (1-V) did not effect the deoxygenation

of the phosphine oxides, but instead generated the adducts

(58) Nemyin, v. N.; Divie, S. R.; Mondal, S.; Rubie, N.; Kirk, M. L.;

Somogyi, A.; Basu, P. J. Am. Chem. Soc. 2002, 124, 756-757.

(59) (a) Gangopadhyay, J.; Sengupta, S.; Bhattacharyya, S.; Chakraborty,

I.; Chakravorty, A. Inorg. Chem. 2002, 41, 2616-2622. (b) Bhatta-

charyya, S.; Chakraborty, I.; Dirghangi, B. K. Inorg. Chem. 2001,

40, 286-293.

t

(silox)₃VOPR₃(3-OPR₃; R ) Me, Ph, Bu),⁵⁶which could

be synthesized in high yield as explained above. Thermolysis

of the adducts was ineffective at oxygen removal, and it was

(60) Arias, J.; Newlands, C. R.; Abu-Omar, M. M. Inorg. Chem. 2001,

40, 2185-2192.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6211

Veige et al.

Scheme 3

Scheme 4

6212 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

35 days at 85 °C with ^tBu₃PO present, the complex failed to

induce O-atom transfer. Instead, loss of ¹/₂equiv of 4-picoline

and ring opening of the heterocycle to (silox)₃NbdCH(cis-

CHdCMe)(cis-CHdCH)NdNb(silox)₃(13) was evident.

Although the compound was not isolated, its spectral

signature and the stoichiometry of the reaction left little doubt

as to its composition. Its stereochemistry is tentatively

assigned from comparisons to the related ring-open products

of pyridine, 2-picoline, 3-picoline and the stereoisomers of

(silox)₃NbdCHCHdCHCHdCHNdTa(silox)₃.⁵⁴

by (HO)₃M (1′-M, M ) V, Nb, Ta) shown in eq 5, each

25 °C

(HO) WNO

(HO)₃M

1′-M, M ) V, Nb, Ta

(HO) WN

+

8

+

3

10′

11′

(HO)₃MO

(5)

2′-M

case was determined to be exoergic by a substantial amount.

At arguably the highest level of theory, CCSD(T) (coupled

clusters with single and double excitations and triples added

noniteratively), ∆G°(25 °C, V) ) -66 kcal/mol, ∆G°(Nb)

) -100 kcal/mol, and ∆G°(Ta) ) -100 kcal/mol. Consid-

erably greater variation in vanadium (see Experimental

Section) was evidenced in comparison to Nb and Ta, whose

free energies for eq 5 were remarkably close (standard

deviation less than 15 kcal/mol at all correlated levels).

Calculational aspects of phosphine oxide deoxygenation

were not explored as extensively as the nitrosyl case, and

only Me₃PO was examined, as illustrated in eq 6. The

As Scheme 3 shows, even the use of (silox)₃Ta (1-Ta) as

t

a 4-picoline scavenger did not result in Bu₃PO deoxygen-

ation by (silox)₃Nb(4-pic) (1-Nb-4-pic). With 10 equiv of

^tBu₃PO present at 23 °C in benzene-d₆, abstraction of

4-picoline from 1-Nb-4-pic by 1-Ta afforded (silox)₃Ta(4-

pic) (1-Ta-4-pic) as expected, but the cyclometalation isomers

(silox)₂HNbOSi^tBu₂CMe₂CH₂(12-Nb) were the only nio-

bium-containing products. If formed, it is clear that an

intermediate of the type “(silox)₃NbOPR₃” cannot deoxy-

genate when R ) ^tBu. Note that the structure of (silox)₃VOP^t-

Bu₃(3-OP^tBu₃) shows that a similar niobium adduct is

certainly feasible from a steric standpoint.

As for (silox)₃Ta (1-Ta), the deoxygenation studies in-

volving R₃PO are strikingly similar to the niobium results.

Both Me₃PO and Ph₃PO are deoxygenated rapidly (<2

min) at 23 °C in C₆D₆, but no O-atom transfer from ^tBu₃PO

was noted, even at 85 °C, and cyclization to (silox)₂-

25 °C

(HO)₃M

1′-M, M ) V, Nb, Ta

(HO) MO

Me₃PO +

8 Me₃P +

(6)

3

2′-M

electronic differences between the phosphines of interest were

not deemed to be significant enough to warrant exhaustive

examination. Since Me₃PO was estimated to be more difficult

than Ph₃PO by ∼6 kcal/mol,⁵²and was the simplest of the

three, it was used in the study. For M ) Nb and Ta, the

deoxygenation of Me₃PO was calculated to be exoergic, with

∆G°(Nb) ∼ ∆G°(Ta) ) -45 kcal/mol. For vanadium, eq 6

was calculated to be only -15 kcal/mol exoergic, and the

observed stability of (silox)₃VOPR₃(3-OPR₃, R ) Me, Ph,

^tBu) was of some concern. As a consequence, the formation

of (HO)₃VOPMe₃(3′-OPMe₃) from 1′-V and Me₃PO was

also calculated, as eq 7 reveals, and its ∆G° of -20 kcal/

HTaOSi^tBu₂CMe₂CH₂(12-Ta) was observed instead. Pro-

longed thermolysis (85 °C, 27 days) of the “tuck-in”

t

derivative 12-Ta in the presence of Bu₃PO failed to elicit

any evidence of reversible cyclometalation, and the phos-

phine oxide was untouched.

4. Thermochemical Calculations on Deoxygenation. A

variety of levels of theory were employed in conjunction

with the SBK(d) effective core potential/valence basis sets

on model systems where HO was used in place of the ^tBu₃-

SiO ligand.^61-66For the deoxygenation of (HO)₃WNO (10′)

25 °C

(HO) VOPMe

(HO) V

1′-V

Me₃PO +

8

(7)

3

3′-OPMe₃

mol concurred with experiment; the adduct was the most

stable entity in the vanadium system.

Calculations on the thermochemistry arising from deoxy-

genation of Cl₄WO by PH₃(∼+50 kcal/mol) or PMe₃(∼+40

kcal/mol) were extremely endoergic, in direct conflict with

experimental data.⁵²In addition, a significant degree of

variation in the calculations was experienced when calibration

of other O-atom transfers involving WCl₄and PMe₃was

conducted. As a consequence, the O-atom transfer chemistry

of the tungsten complexessor lack thereofswas set aside

until further experiments and more accurate calculations

could be done.

(61) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem.

1992, 70, 612-630.

(62) Benson, M. T.; Cundari, T. R.; Lutz, M. L.; Sommerer, S. O. In

ReViews in Computational Chemistry; Boyd, D., Lipkowitz, K., Eds.;

Wiley: New York, 1996; Vol. 8, p 145.

(63) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.

(64) Bartlett, R. J.; Stanton, J. F. In ReViews in Computational Chemistry;

Boyd, D. B., Lipkowitz, K. B., Eds.; Wiley: New York, 1994; Vol.

5, pp 65-169.

(65) Frenking, G.; Antes, I.; Bohme, M.; Dapprich, S.; Ehlers, A. W.; Jonas,

V.; Neuhaus, A.; Otto, M.; Stegmann, R.; Veldkamp, A.; Vyboish-

chikov, S. F. In ReViews in Computational Chemistry; Boyd, D.,

Lipkowitz, K., Eds.; Wiley: New York, 1996; Vol. 8, pp 63-144.

(66) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

M. A. Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;

Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,

A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,

V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;

Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.,

Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;

Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;

Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.

L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,

A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;

Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon,

M.; Replogle, E. S.; Pople, J. A. Gaussian, Inc.; Pittsburgh, PA,

1998.

Discussion

Coordination Complexes. The new coordination com-

plexes described herein are relatively unexceptional given

previous studies, but a few derivatives are worth comment.

The Nb(III) derivative, (silox)₃NbPMe₃(1-NbPMe₃), is a rare

monomeric example of this uncommon oxidation state.^41,42

While it is tempting to conclude that its distorted tetrahedral

geometry, which approaches that of a trigonal monopyra-

mid,⁶⁷is a natural consequence of the severe steric constraints

Inorganic Chemistry, Vol. 42, No. 20, 2003 6213

Veige et al.

imposed by the silox groups, the PMe₃ligand actually has a

substantially larger cone angle.⁶⁸An increase in the

O(pπ)fNb(dπ) interactions as the O₃Nb core approaches a

trigonal planar geometry may help offset the phosphine/silox

sterics, which are also somewhat alleviated by the subtle

canting of the Nb-O-Si linkages.

NbL][10]/{(k_-1k_-a+ k_-1k_t)[L]} when (k_-1k_-a+ k_-1k_t)[L] is

consequential, and quite plausibly -d[1-NbL]/dt ) k₁k_ak_t-

[1-NbL][10]/k_-1k_-a[L] if O-atom transfer is rate determining.

As Scheme 3 reveals, when 1-Nb is generated, deoxygenation

of 10 is uncompetitive with cyclometalation, hence eqs 8

and 9 cannot be operational for 1-NbPMe₃. Although this

path is not probable for 1-Nb-4-pic either, it cannot be ruled

out since deoxygenations with 1-Nb-4-pic were conducted

at 85 °C, and 4-pic was shown to convert 12-Nb to the

picoline adduct.

In a more probable alternative sequence, 10 displaces L

from 1-Nb to afford the precursor complex (silox)₃Nb-ONW-

(silox)₃(1-Nb-10, eq 9), which then transfers oxygen in the

rate-determining step (eq 11) and fragments (eq 12). Here

the rate expression is -d[1-NbL]/dt ) k′₁k_t[1-NbL][10]/(k′_-1-

[L] + k_t), which would simplify to -d[1-NbL]/dt ) k′₁k_t-

[1-NbL][10]/k′_-1[L] when the concentration of [L] is ap-

preciable. Either mechanism reveals an inhibition by [L] as

it accumulates as a byproduct in the deoxygenation. There

is an additional complication in the case of 1-Nb-4-pic

deoxygenation, because the released 4-picoline also binds

reversibly to (silox)₃WNO, which could also be inhibiting

deoxygenation. The binding has been independently con-

firmed.

Both tungsten oxo derivatives prepared during the course

of these studies are unusual. The C_3Vgeometry of (silox)₃-

ClWO (7) is uncommon, in part because it contains an axial

oxo ligand directly opposite another fairly strong π-donor,

the chloride. The elongation of d(W-Cl) by ∼0.15 Å relative

to that of (silox)₃WCl₂(5) is a testament to the trans influence

of the oxo ligand, whose much shorter bond distance permits

it to exert a greater repulsive effect on the silox oxygens,

resulting in a slight cant of the siloxides toward the chloride.⁴⁶

(silox)₃WO (8), which is likely to possess a pseudotetrahedral

C_3Vstructure, is a rare example of a tungsten(V) oxo

derivative; most WO species are either W(VI) or W(IV).

Unfortunately, efforts at crystallographic characterization of

8 or the related (silox)₃TaO (2-Ta) were not successful.

(silox)₃WNO (10) Deoxygenations. 1. Relative Rates.

Even allowing for calculational inaccuracies, there can be

no question that the deoxygenation of (silox)₃WNO (10) by

(silox)₃M (1-M, M ) V, Nb, Ta) is greatly thermodynami-

cally favorable. From the standpoint of a possible linear free

energy relationship, the rates of deoxygenation should follow

the trend Ta ∼ Nb > V. Moreover, simple atom transfers

that are -65 kcal/mol (V) and -100 kcal/mol (Nb, Ta)

exoergic would normally be exceedingly swift.

Because of the involvement of L in the niobium experi-

ments, it is difficult to estimate a rate for comparison to the

vanadium and tantalum cases. For the sake of further

argument, assume the associative path for niobium, with eqs

9,10, and 12 operable and O-atom transfer rate determining.

Using (silox)₃Nb(4-pic) (1-Nb-4-pic), the rough half-life of

4 h (85 °C, [1-Nb-4-pic] ) [10]) provides a crude rate of

k_obs(Nb) ∼ 1.4 × 10^-3M^-1s^-1, which includes a preequi-

librium factor k′₁/k′_-1[L] that is not applicable to the

vanadium or tantalum examples. Since ∼2% of (silox)₃Nb-

ONW(silox)₃(1-Nb-10) would probably be observable by

NMR spectroscopy, the preequilibrium can be factored out

as roughly <0.02, and k_obs(Nb) can be estimated as >7 ×

10^-2M^-1s^-1. Likewise, the half-life of the deoxygenation

of 10 involving (silox)₃NbPMe₃(1-NbPMe₃) at 23 °C led

to an estimate of its rate as ∼2 × 10^-4M^-1s^-1(roughly 1

× 10^-2M^-1s^-1at 85 °C), consistent with the above

arguments. By again factoring out the preequilibrium, a final

estimate of >0.5 M^-1s^-1for 1-NbPMe₃loss is obtained at

85 °C.

Niobium appears to deoxygenate 10 the swiftest, although

the process is complicated by the apparent inhibition by

released ligand, since (silox)₃Nb (1-Nb) is only stable as an

adduct, (silox)₃NbL (1-NbL). Two mechanisms for L (L )

4-picoline, PMe₃) inhibition seem plausible. In the first, L

dissociation occurs to provide 1-Nb, which can either

recapture L or attack 10 and proceed to deoxygenate (eqs 8,

10-12). The rate expression for this scenario is -d[1-NbL]/

k₁

(silox) NbL

(silox) Nb

{

}

+ L

(8)

3

k_-1

1-NbL

1-Nb

(silox) NbL

+

3

1-NbL

k′₁

(silox) WNO

(silox) Nb-ONW(silox)

{

}

₃+ L (9)

3

k′_-1

10

1-Nb-10

For the vanadium case, eqs 10-12 presumably apply, and

k

_obs(85 °C) ) (k_a/k_-a)k_tfrom simplification of the rate

(silox)₃M

1-M, M ) V, Nb, Ta

+

expression k_ak_t[1-V][10]/(k_-a+ k_t), where k_-a. k_t. The rate

is approximately 1.4 × 10^-4M^-1s^-1as determined from

the half-life. The same situation applies for tantalum, but

cyclometalation interferes and is the major pathway when

(silox)₃Ta (1-Ta) and (silox)₃WNO (10) are combined. By

using the cyclometalation rate at 85 °C (∼2.8 × 10^-3s^-1)

and assuming pseudo-first-order conditions for deoxygen-

k_a

(silox) M-ONW(silox)

1-M-10 (precursor)

(silox) WNO

{_k}

(10)

(11)

(12)

3

10

-a

k_t

98

(silox)₃M-ONW(silox)₃

1-M-10, M ) V, Nb, Ta

(silox) MO-NW(silox)

3

2-M-11 (successor)

fast

8

(silox)₃MO-NW(silox)₃

2-M-11, M ) V, Nb, Ta

(silox) MO (silox) WN

+

3

11

2-M

(67) Veige, A. S.; Wolczanski, P. T.; Lobkovsky, E. B. Chem. Commun.

2001, 2734-2735.

(68) (a) Tolman, C. A. Chem. ReV. 1977, 77, 313-348. (b) Bunten, K. A.;

Chen, L.; Fernandez, A. L.; Poe¨, A. J. Coord. Chem. ReV. 2002, 233-

234, 41-51.

dt ) k₁k_ak_t[1-NbL][10]/{(k_-1k_-a+ k_-1k_t)[L] + k_ak_t[10]},

which leads to the simplification -d[1-NbL]/dt ) k₁k_ak_t[1-

6214 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

Figure 5. B3LYP/SBK(d) and BLYP/SBK(d) geometries (brackets) of model complexes (HO)₃M (1′-M, M ) V, Nb, Ta; S ) 0, 1; Å and deg) and their

singlet/triplet crossing points.

ation at early conversion, the crude estimate of the tantalum

k_obsfor deoxygenation is 2 × 10^-2M^-1s^-1.

the d_xzd_yzorbital set. 1-Ta is a ground-state singlet (S ) 0,

1

2

A₁′), and the (d_z) configuration is repulsive toward potential

donors, as has been previously described.⁵³What is the

disposition of niobium?

In summary, the rough relative rates of deoxygenation for

eqs 10 and 11 are Nb (k_rel> 3500) > Ta (150) > V (1),

which is somewhat in line with that postulated on the basis

of a linear free energy relationship. Upon closer inspection,

though, the situation is less satisfying. It is certainly likely

that Nb is substantially greater than the estimation, since

(silox)₃NbL (1-NbL) must lose L to form the precursor

complex (silox)₃Nb-ONW(silox)₃(1-Nb-10). Why then are

the rates of Nb and Ta so different, and what factors are

causing the rates for a single atom transfer with -65 (V) to

-100 (Nb, Ta) kcal/mol driving force to occur so slowly?

2. Electronic States of (silox)₃M (1-M, M ) V, Nb, Ta).

Historically, atom transfer is separated into the three steps

described by eqs 10-12.⁶⁹The first, docking of the substrate

to form the precursor complex, is given by eq 10. O-atom

transfer to give the successor complex is given by eq 11,

and dissociation of the successor species to product fragments

is typically believed to be kinetically inconsequential (eq 12).

Immediately, one reason for slow deoxygenation with

tantalum becomes apparent. Unlike (silox)₃V (1-V), which

forms adducts with a variety of donors, (silox)₃Ta (1-Ta)

does not incur dative bonding. 1-V is a ground-state triplet

In order to elaborate on the potentially consequential states

of (silox)₃M (1-M, M ) V, Nb, Ta), high-level quantum

calculations were performed on both singlet and triplet

configurations of (HO)₃M (1′-M, M ) V, Nb, Ta; S ) 0, 1)

to probe the energetics. Calculations on singlet and triplet

1′-M were performed initially with RHF and ROHF wave

functions,⁷⁰respectively, to probe the different conformations

arising from M-O torsion. As expected, their energetic

differences were very small (<1 kcal/mol), and the lowest

conformations were then submitted to full B3LYP/SBK(d)

and BLYP/SBK(d) geometry optimizations to yield the

singlet-triplet energy differences that are tabulated in the

Experimental Section.^61-66

The geometries of (HO)₃M (1′-M, M ) V, Nb, Ta; S )

0, 1) are illustrated in Figure 5, and some striking differences

are apparent. All of the singlet geometries are trigonals

ignoring HOM linkagessand the metal-oxygen bond dis-

tances are relatively normal. The 1.872 Å distance accorded

d(Ta-O) is slightly short for silox derivatives,^{30,31,53-55,67,71}

but there is a lack of true Ta(III) derivatives for ready

comparison, hence the available data mostly corresponds to

Ta(V) species that are higher coordinate. It is also difficult

1

3

2

(S ) 1), and in its probable (d_z) (d_xzd_yz) configuration ( E′′),

2

the d_zorbital can more readily engage in dative bonding to

afford C_3V³A₂(silox)₃VL (1-VL), with a pair of electrons in

(70) RHF ) restricted Hartree-Fock; ROHF ) restricted open-shell

Hartree-Fock. Young, D. In Computational Chemistry; Wiley: New

York, 2001; pp 20, 227-229.

(69) (a) Lappin, A. G. Redox Mechanisms in Inorganic Chemistry; Ellis

Horwood: New York, 1994. (b) Tobe, M. L.; Burgess, J. Inorganic

Reaction Mechanisms; Addison-Wesley Longman: New York, 1999.

(71) Wolczanski, P. T. Polyhedron 1995, 14, 3335-3362.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6215

Veige et al.

Figure 6. Energetics (kcal/mol, 25 °C) of (HO)₃M (1′-M, M ) V, Nb, Ta) as models for (silox)₃M (M ) V, 1-V; Nb, 1-Nb; Ta, 1-Ta). S and T refer to

the singlet and triplet energies at those optimized geometries. S@T refers to the singlet energy at the optimized triplet geometry, and vice versa. Singlet/

triplet crossing barriers are indicated by the middle solid vertical lines.

to assess what amount of lengthening could be attributable

to steric factors of the silox groups. Nonetheless, the d(Nb-

O) of 1.891 Å is also slightly short, and (HO)₃V (1′-V, S )

0) contains vanadium-oxygen bonds that are 1.771 Å, in

line with the shorter covalent radius of the first-row element.

The geometries of the triplet configurations are irregular.

(HO)₃V (1′-V, S ) 1) exhibits a slight pyramidalization

(∑ O-M-O ) 364°) accompanied by a modest shift

toward a Y-shape, while the triplet geometries of 1′-Nb (S

) 1) and 1′-Ta (S ) 1) distort toward a T-shape (∑ O-

M-O ) 360°). For 1′-V (S ) 1), two angles (123.5° and

124.4°) are larger than the third (115.9°), and the d(V-O)

of 1.81 Å (av) represents a subtle, but real elongation versus

the singlet. The magnitude of the distortions is greater for

the second- and third-row congeners. For 1′-Nb (S ) 1), the

135.6° O-Nb-O angle is much greater than the remaining

set (111.5° and 112.9°), and the Nb-O bond (1.962 Å)

opposite the largest O-Nb-O is longer than the remaining

two (1.925, 1.930 Å), while all are elongated relative to their

singlet partners. In 1′-Ta (S ) 1), the 133.8° O-Ta-O angle

is significantly larger than its 118.8° and 107.6° partners,

the Ta-O bond (1.947 Å) opposite the largest O-Ta-O

is longer than the others (1.928, 1.901 Å), and all are long

relative to the corresponding singlet. Occupation of the

metal-oxygen π*-orbital has the expected consequence of

increasing d(M-O), and in combination with Jahn-Teller

distortions, etc., the geometries of the triplet states are readily

rationalized.

some account of electron correlation, the S-T splitting is

fairly consistent at 15(2) kcal/mol. For (HO)

₃Ta (1′-Ta), a

singlet ground state is indicated at all levels of theory above

Hartree-Fock (∆G_S-T(av) ) -14(1) kcal/mol for all cor-

related levels of theory). For the niobium congener, (HO)₃Nb

(1′-Nb), the singlet-triplet energy difference is considerably

smaller, with ∆G_S-Tranging from 3.9 to -1.4 kcal/mol. The

slight preference for a triplet derives mostly from the

enthalpic and entropic corrections to the singlet state.

Interestingly, the singlet state is preferred (-1.4 kcal/mol)

at the CCSD(T)/SBK(d)//B3LYP/SBK(d) level of theory,

which is arguably the most accurate in these instances.

2

Apparently the d_z/(d_xzd_yz) orbital energy difference for V is

considerably smaller than its pairing energy, while the related

gap for Ta is much larger, leading to its singlet ground state.

For niobium, the orbital energy gap and pairing energy must

be very similar. The second- and third-row species have

greater field strengths and correspondingly greater radial

distribution than the vanadium complex, hence their differ-

ences are easily rationalized.³⁸The difference in behavior

between Nb and Ta is less easily explained, since the

lanthanide contraction leads to similar radii. Natural bond

order calculations on ¹Nb(HO)₃(¹1′-Nb) and ¹Ta(HO)₃(¹1′-

2

Ta) indicate that 5d_z/6s mixing for the latter is greater than

2

the corresponding 4d_z/5s mixing accorded the former. This

is also the case for the gas-phase neutral metals.⁷²Relativistic

effects are an important factor in energetically lowering the

2

more penetrating 6s orbital to improve mixing with 5d_z,

At all levels of theory from Hartree-Fock to CCSD(T),

(HO)₃V (1′-V) is predicted to possess a triplet ground state,

as illustrated in Figure 6. For levels of theory that include

(72) Oxtoby, D. W.; Gillis, H. P.; Nachtrieb, N. H. Principles of Modern

Chemistry, 5th ed.; Thomson Brooks/Cole: U.S.A., 2002; p 552.

6216 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

thereby minimizing its torus, and providing a much larger

d_z/(d_xzd_yz) energy gap than in the niobium case.

nm bands, and the lowest of those is energetically related to

the S ) 1 state of the model complex 1′-Ta.

2

(HO)₃Nb (1′-Nb) is the most interesting of the three. At

the B3LYP/SBK(d) level of theory the triplet is ∼1 kcal/

mol lower than the singlet state, and the S-T crossing point

is only ∼2 kcal/mol above these states and possesses a

geometry that has an intermediate d(Nb-O) with a modest

T-shaped distortion. As one would expect in this situation,

the S@T and T@S energies are 43 and 44 kcal/mol above

the respective S ) 1 and S ) 0 states. As Figure 6 shows,

these high energies and the low-energy S-T crossing point

are a consequence of two energetically close states with a

modest geometric (i.e., reaction coordinate) difference.

3. Consequences of (HO)₃M (1′-M, M ) V, Nb, Ta)

Electronic States on Deoxygenation. The thermochemistry

for (silox)₃WNO (10) deoxygenation, modeled as (HO)₃WNO

(10′), was broken down into three steps. In eq 13, the energy

The singlet-triplet (S-T) crossing points for (HO)₃M (1′-

M; M ) V, Nb, Ta) were determined at the B3LYP/SBK(d)

level of theory, and their geometries are shown in Figure 5.

The crossing point is the geometry at which the singlet and

triplet states of 1′-M are degenerate, and can be considered

as a “transition state” connecting the two surfaces, assuming

intersystem crossing is probable.^34,35,73,74To complement

these calculations, the energies of the singlet state of 1′-M

at the optimized triplet geometry (S@T), and its triplet state

at the optimized singlet geometry (T@S), were also deter-

mined in order to construct the free energy vs reaction

coordinate diagram illustrated in Figure 6.

For (HO)₃V (1′-V), the S ) 1 state is the ground state

with the singlet state about 15 kcal/mol above it. Interest-

ingly, the triplet state is still ∼7 kcal/mol lower than the S

) 0 state at the optimized geometry of the latter, and the

S-T crossing point is only 2 kcal/mol above the singlet

geometry. The singlet state does not appear to be very

sensitive to geometry, changing only by ∼4 kcal/mol in

smoothly traversing from its optimized geometry to that of

the triplet. The S-T crossing point geometry reflects its

position in the reaction coordinate, with d(V-O) intermediate

between the singlet and triplet values, and a modest angular

change in which the Y-shape of the S ) 1 state transitions

to a T-shape before becoming trigonal at the S ) 0 state.

1

3

required to convert singlet M(OH)₃(¹1′-M) to triplet M-

(OH)₃(³1′-M) is assessed, followed by deoxygenation to give

1

3

singlet nitride W(OH)₃N (11′) and triplet oxo M(OH)₃O

(³2′-M, eq 14), and relaxation of the oxo to its singlet state

(2′-M, eq 15). At the CCSD(T)/SBK(d)//B3LYP/SBK(d)

^{25 °C}8

(13)

¹M(OH)₃

³1′-M, M ) V, Nb, Ta

³M(OH)₃

³1′-M

³M(OH)₃

³1′-M, M ) V, Nb, Ta

(HO)₃WNO

(HO) WN

3

+

^{25 °C}8

+

In the tantalum case, the situation is reversed from

vanadium, and the energy changes are more significant, as

expected for the third-row species. The S-T crossing point

is proximate to the optimized triplet state along the reaction

coordinate and only 3 kcal/mol above it. Bond distances for

the S-T crossing point are within error of the S ) 1 state,

but its T-shaped geometry is more evident, with one angle

of 142° opposed by an oddly long d(Ta-O) of 1.99 Å. The

S-T crossing point lies 17 kcal/mol above the S ) 0 ground

state, and now the singlet state is still ∼7 kcal/mol lower

than the triplet at the optimized geometry of the latter. The

triplet surface is ∼28 kcal/mol above the optimized singlet

conformation, and is reasonably consistent with the spec-

troscopy of (silox)₃Ta (1-Ta).⁵³The lowest energy absorption

band associated with 1-Ta occurs at 928 nm (ꢀ ) 34 cm^-1

10′

11′

³M(OH)₃O

(14)

³2′-M

^{25 °C}8

(15)

³M(OH)₃O

³2′-M, M ) V, Nb, Ta

¹M(OH)₃O

³2′-M

level of theory, the calculations predict that ³1′-Nb and ³1′-

Ta require 1 and 17 kcal/mol of free energy to be formed

from their respective ground state singlets (only at this level

of theory is the niobium ground state predicted to be S )

0). The deoxygenation step occurs as expected from peri-

odicity, with tantalum (-39 kcal/mol) more exoergic than

niobium (-23 kcal/mol), which is more exoergic than

vanadium (-18 kcal/mol). It is interesting to note that the

greater free energy change for tantalum over niobium

compensates for the “activation” of its triplet state, because

the relaxation of the oxo triplets to their ground-state singlets

is -78 kcal/mol for both metals; relaxation to the singlet

state for vanadium oxo is -48 kcal/mol. Together (eqs 13-

15), the deoxygenations are -66 kcal/mol exoergic for V,

and -100 kcal/mol for Nb and Ta, as espoused above.

Docking of the substrate clearly hampers ready deoxy-

genation by (silox)₃Ta (1-Ta), and inhibition by L (L )

PMe₃, 4-pic) is apparent in deoxygenations by (silox)₃NbL

(1-NbL), yet some critical questions remain. Why is deoxy-

genation by (silox)₃V so slow if the reaction is exoergic by

-66 kcal/mol? Why does rebinding of L compete with

deoxygenation by (silox)₃Nb (1-Nb) when the latter event

is favorable by -100 kcal/mol?

1

M^-1, 11900 cm^-1), and was tentatively assigned as a A₁′

f ³E′′ transition corresponding to a₁′²f a₁′¹e′′¹. This band

is accompanied by a similar one at higher energy (612 nm,

16300 cm^-1), and both have shoulders. The lower energy

band corresponds to ∼31 kcal/mol, which is conspicuously

close to the calculated T@S configuration. It is possible that

two triplet states arising from Jahn-Teller distortion of the

formally D_3h³E′′ state are manifested by the 928 and 612

(73) S-T crossing calculations conducted via methods of Harvey, J. N.,

University of Bristol; personal communication. The use of B3LYP

and BLYP functionals does not result in significant changes in the

S-T splitting energies or geometries of M(OH)₃(1′-M, M ) V, Nb,

Ta).

(74) (a) Harvey, J. N. In Spin Forbidden Reactions in Transition Metal

Chemistry; Cundari, T. R., Ed.; Marcel Dekker: Basel, 2001. (b)

Harvey, J. N. J. Am. Chem. Soc. 2000, 122, 12401-12402 and

references therein.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6217

Veige et al.

Figure 7. Orbital correlation diagram for generic L_nM-O-E (MOE) a

L_nMO (MO) + E (E ) substrate); energetics are based on EHMO

calculations of (HO)₃VOPMe₃(3′-OPMe₃), PMe₃, and (HO)₃VO (2′-V).

Reactant (S ) 1) states efficiently intersystem cross with product S ) 0

states when reactant excited states with populated σ*-orbitals are mixed in

effectively; the reduction in symmetry upon bending M-O-E facilitates

this process.

Figure 8. Truncated electronic state diagram (C_3V) for generic L_nM-O-E

(MOE) a L_nMO (MO) + E (E ) substrate) showing only critical state

correlations (solid lines) and correlations of electronic configuration (dashed

lines); energetics are based on EHMO calculations of (HO)₃VOPMe₃(3′-

OPMe₃), PMe₃, and (HO)₃VO (2′-V).

tion. Aside from the energies, the general features will be

the same for niobium, tantalum, and a variety of substrates

(E).

t

R₃PO (R ) Me, Ph, Bu) Deoxygenations. 1. Relative

Rates. As previously noted, phosphine oxide adducts of

(silox)₃V (1-V) are the thermodynamic sinks in that system.

For niobium and tantalum, there exists a curious dichotomy

between substrates R₃PO (R ) Me, Ph) and ^tBu₃PO. While

the former are rapidly deoxygenated by (silox)₃Ta (1-Ta)

and (silox)₃NbL (1-NbL, L ) 4-pic, PMe₃) at 23 °C, with

the subtle exception of inhibition by released 4-picoline,

neither system reacts with ^tBu₃PO, even at elevated temper-

atures. Since the gas-phase deoxygenation of Ph₃PO is

considered to be only 6 kcal/mol easier than that of Me₃-

PO,⁵²it does not seem likely that the electronic difference

upon replacing a Me with a ^tBu would render deoxygenation

rapid in the former, yet nonexistent in the latter! Perhaps

the widely disparate sterics of the two substrates plays a

role.

2. Orbital Correlation Diagram. For a simple visualiza-

tion of the O-atom transfer event, an EHMO calculation,⁷⁵

whose results are indicated in Figure 7, was done on a linear

reactant (HO)₃V-O-PMe₃(3′-OPMe₃) M-O-E) and its

products, oxide (HO)₃VO (2′-V ) MdO) and PMe₃(E). The

first-row metal was chosen because of the relative energies

of the fragments, and the ease and accuracy of the calcula-

The adduct M-O-E features a triplet ground state with

the HOMO being a degenerate set of vanadium-oxygen

π-antibonding orbitals (2e) that are half-occupied. Below are

only two σ-type orbitals that essentially describe the E-O

(1a₁) and M-O (2a₁) single bonds, and the fully occupied

π-bonding orbital 1e. The M-O-E intermediate possesses

one net π- and two σ-interactions. In the products, oxygen

(1a₁) and substrate (3a₁) lone pairs constitute low-lying σ-type

orbitals in addition to the M-O σ bond (2a₁). The metal-

oxygen π-bonding orbital 1e is also fully occupied. The

contradiction between adduct M-O-E, and oxo MdO and

E is readily apparent. In order for the reactant to generate

products in a symmetry-allowed process, both must have the

same number and type of orbitals. The disallowed nature of

the reaction may be overcome if two electrons on the reactant

side are promoted from 2e to 3a₁, or two electrons on the

product side are promoted from 3a₁to 2e, thereby equalizing

the populated orbital types. Promotions of one electron for

both reactant and products can also lead to correlation, but

no matter how the process is viewed, excited states must be

utilized in the transformation.

3. State Correlation Diagrams. Figure 8 illustrates the

roughly energetically ordered electronic states of (HO)₃V-

O-PMe₃(3′-OPMe₃) M-O-E) and (HO)₃VO (2′-V )

MdO) + PMe₃(E) critical to understanding the O-atom

(75) Extended Hu¨ckel molecular orbital (EHMO) calculations were con-

ducted using the HyperChem program: HyperChem(TM), Hypercube,

Inc., 1115 NW 4th St., Gainesville, FL 32601.

6218 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

Figure 9. Truncated electronic state correlation diagram for generic L_nM-O-E (MOE) a L_nMO (MO) + E (E ) substrate) showing how the reduction

in symmetry upon bending M-O-E facilitates atom transfer by reducing the intersystem crossing barrier through mixing.

transfer reaction, including their electron configurations of

origin. While the A₂ground state of M-O-E correlates

with an excited state of MdO possessing triply and singly

σ-character and their admixtures allow the barriers connecting

the ¹A′ and ³A′′ states to be energetically lowered; therefore,

the intersystem crossing point is lowered as well.

3

occupied 1e and 2e orbitals, respectively, its orbital correla-

tion is with a much higher A₂state, hence the symmetry-

4. Steric Factors and Deoxygenation. Deoxygenation of

R₃PO (R ) Me, Ph) by (silox)₃Ta (1-Ta) and (silox)₃NbL

(1-NbL, L ) 4-pic, PMe₃) occurs swiftly because the

substrate is allowed to access the metal center via the side

of the OP linkage, which is the equivalent of accessing the

bent M-O-E adduct addressed above. Attack at the side

of the PO bond, or at oxygen such that M-O-P is

substantially less than 180°, permits P-O cleavage to occur

with facility due to the consequences of lower symmetry

elaborated above. If the substrate is small enough, R₃PO may

also attack empty π-type orbitals on the metal.

3

allowed transformation, i.e., reactant ³A₂f product ³A₂, may

have a substantial energetic barrier depending on how well

those product states mix. The same is true for the product

¹A₁ground state, which correlates with an excited state

derived from the lowest electronic configuration. Its orbital

correlation is with a much higher energy state, thus this

transition also has a potentially large barrier that depends

1

on how well the A₁reactant excited states mix.

It is likely that the transformation from (HO)₃V-O-PMe₃

(3′-OPMe₃) M-O-E) to (HO)₃VO (2′-V ) MdO) +

PMe₃(E) would be adiabatic, because intersystem crossing

in a transition metal based system is not rigorously forbidden

due to a variety of factors (e.g., spin-orbit coupling, low

symmetry, etc.). As a consequence of the state correlations,

the point of intersystem crossing that is necessary for

t

Substrates such as (silox)₃WNO (10) and Bu₃PO are

restricted by sterics in their approach to (silox)₃M (1-M, M

) V, Nb, Ta). The nitrosyl deoxygenation is unusually slow

despite a great thermodynamic impetus because the geometry

for O-atom transfer is required to be nearly linear by steric

factors, and the resulting intersystem crossing point remains

energetically high. Similar conformational factors completely

3

1

conversion of the A₂reactant to A₁products is relatively

high in energy, and deoxygenation can have a significant

barrier.

t

shut down the deoxygenation of Bu₃PO relative to other

pathways even though there is clearly room to bind the

substrate, as the structure of (silox)₃VOP^tBu₃(3-P^tBu₃)

indicates.

Figure 9 reveals how the barrier to deoxygenation is most

easily lowered. Consider the M-O-E angle (θ) to deviate

from 180° (C_3V). The A₁and A₂states become A′ and A′′,

respectively, and various E states split into A′ and A′′ states

as the symmetry is lowered to C_s. The intersystem crossing

5. General Observations. The generality of symmetry

considerations and the impact of electronic states in atom

transfers bears some additional scrutiny. Woo⁷⁶rationalized

differences in reactivity of (por)CrdO/(por)CrCl and (por)-

CrdO/(por)Cr substrates with respect to the stabilities of a

[(por)Cr-O-Cr(por)]ⁿ(n ) 0, +1) intermediate (n ) +1)

and product (n ) 0) based on Hoffmann’s relevant treatment

of iron porphyrins.⁷⁷Jahn-Teller distortions in the mixed-

valence intermediate are proposed to induce degradation and

1

point is thereby lowered for several reasons. A lower A′

state on the left now correlates with the MdO + E ground

3

state on the right, while a lower A′′ state on the right now

correlates with the M-O-E ground state on the left. Greater

mixing is facilitated by the splitting of E states, which are

basically π-type in C_3V, into A′ and A′′ states in C_sthat permit

σ-character to be introduced; correspondingly, π-character

is mixed into states that were previously purely σ. Basically,

as θ decreases, the orbitals lose their distinctive π- or

(76) Woo, L. K.; Goll, J. G.; Berreau, L. M.; Weaving, R. J. Am. Chem.

Soc. 1992, 114, 7411-7415.

(77) Tatsumi, K.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 3328-3341.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6219

Veige et al.

Figure 10. Cofacial bis-porphyrin O-atom transfer system developed by Nocera et al. (ref 33) illustrating likely side-on attack of the FeO by the substrate

SMe₂.

oxygen/chloride self-exchange, while the Cr(III)-O-Cr(III)

species was stable. While it is conceivable that electronic

state changes directly impact the kinetics of µ-oxo formation

and disruption in relation to the O-atom transfers (vide supra),

this aspect of the chemistry was not discussed.

transfer from Cr(V), i.e., Cp/Cp*CrCl₂O plus ethylene,³²the

spin crossing event (Cr(V) f Cr(III)) occurs after initial

C-O bond formation, and it is argued that such a process is

relatively inconsequential, especially since it is likely to occur

with facility. While this combination of experiment and DFT

calculations is compelling, the stepwise transfer of oxygen

to the substrate and the nature of the intermediate Cr(IV)-

More recently, Nocera has utilized µ-O-diiron cofacial bis-

porphyrins to examine O-atom transfer from ferryl groups

generated upon photolysis.³³As Figure 10 illustrates, the

more flexible (DPD)Fe₂O system transferred an O atom to

substrate dimethyl sulfide ∼10³times more efficiently than

the rigid (DPX)Fe₂O analogue. Two factors are likely to be

at play: (1) once the ferryl is generated photochemically

(photochemical activation is likely to be bridge-independent),

the kinked DPD pincer has a smaller recombination rate

constant than the DPX system, and (2) the more open DPD

system permits swifter substrate attack than the more

constrained DPX ligand. With both ligands, there is little

doubt that substrate attack occurs in a side-on fashion,

and the DPD system approaches that of an unbridged

control.

Perhaps the most notable systems in which state selective

chemistry has been proferred are those of cytochrome P450³

and related oxomanganese O-atom transfer agents.^8,9In

the former, Shaik has developed models based on DFT

calculations that suggest that cytochrome P450 is highly

dependent on environment, and that different electronic

states may mediate different reaction paths. In the latter,

Groves attributes differing rates of oxo transfer from

Mn(V)O derivatives as reflecting spin state crossover

effects.

•

OCH₂CH₂species possibly render this example distinct from

the O-atom transfers above.

While not initially considered examples of symmetry and

geometry constraints, certain reactions and observations

warrant closer inspection. The enantioselective epoxidation

of olefins catalyzed by (salen)XMnO derivatives is thought

to occur via a Mn(V) oxo that is sterically “blocked” by 2,4-

tert-butyl phenoxide substituents, forcing the substrate to

approach over a 1,2-diiminocyclohexane backbone that relays

its conformation to the prochiral face of the approaching

olefin (Figure 11a).^15-17While it seems reasonable that the

tert-butyl groups dissuade approach of the substrate from

the phenolic side of the chelate plane, the angle of approach

over the cyclohexane is considerably open, as the figure (θ

) 0-90°) illustrates. If the symmetry and geometry factors

above are also applicable in this case, the approach of the

substrate olefin is constrained to be substantially away from

θ ) 0°. The high enantioselectivities observed by Jacobsen

et al. are more easily rationalized at such severe approach

angles, where the influence of the cyclohexane backbone is

likely to be maximized. In partial corroboration, DFT

calculations on oxotransferase models manifest transition

states with M-O-E angles of ∼130°.⁶

Theopold has argued that spin crossover processes are

generally very swift. In a CO substitution study, no ap-

preciable diminution of rate was observed even though

diamagnetic Tp^iPr,MeCo(CO)₂loses CO to afford paramagnetic

Tp^iPr,MeCo(CO).⁷⁸In another investigation involving O-atom

In another case, Mayer et al. concluded that the lack of

phosphine oxide deoxygenation by WCl₂L₄(L ) PMePh₂,

(78) Detrich, J. L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. J.

Am. Chem. Soc. 1995, 117, 11745-11748.

6220 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

transition states, yet actually occur via bent variants. A

prominent example is recent discovery of Mo(III)-mediated

dinitrogen cleavage.⁸⁰The conversion of 2{^tBu(3,5-

Me₂C₆H₃)N}₃Mo and N₂to 2 equiv of nitride, {^tBu(3,5-

Me₂C₆H₃)N}₃MoN, has been estimated to occur with ∆H°

∼ -86 kcal/mol.⁸¹Despite the favorable thermodynamics,

the enthalpic barrier from [{^tBu(3,5-Me₂C₆H₃)N}₃Mo]₂(µ-

N₂) to the nitride is large (∆H^q) 23.3 (3) kcal/mol), and

DFT calculations suggest that a “kink” in the MoNNMo

linkage is required for dinitrogen scission to take place

(Figure 11c.). The linear dinitrogen complex contains three

filled σ orbitals (the two MoN bonds and the NN bond),

whereas the product nitrides contain four (the MoN and N

lone pairs) pairs of electrons of σ-character. The “kink”, or

deviation of the MoNN angle from linearity, has the same

effect as the bending of the M-O-E angle in O-atom

transfer. The symmetry lowering permits σ/π mixing and

allows triplet/singlet intersystem crossing to occur at a lower

energy, rendering the cleavage allowable.

In a related case,⁸²Brown noted that a nitride hetero-

coupling reaction to give N₂based on the combination

of Os(VI) and Mo(VI) nitrides occurred with far greater

facility than the corresponding self-coupling reactions.⁸³

Since it is likely that the formation of N₂from two Os(VI)

nitrides is more thermodynamically favorable than the

heterocoupling event, it was concluded that the asymmetry

of the reaction, in this case the favorable polarization of the

transition state due to the disparate metals, was playing an

important role.

Even in cases where the symmetry is low enough that

formal electronic state changes are not critical, bond scission

events do not occur in a linear fashion. Parkin⁸⁴has recently

observed the thermal degradation of Cp*₂Mo(N₃)₂to Cp*₂-

MoN(N₃) and has utilized DFT calculations to examine its

energetics. Not only are the Mo-N-N bonds bent through-

out the course of the reaction in response to electronic factors

that reflect the EAN rule, but the N-N cleavage itself occurs

via a bent N-N-N geometry and ∆H^qis appreciable (26.6-

(2) kcal/mol). Additional orbital considerations establish the

best geometry for generation of the azide-nitride product.

It is interesting that the dinitrogen cleavage of Cummins and

this seemingly differrent azide cleavage apparently share a

similar degradation path independent of electronic state

requirements. The results support the contention that linear

bond cleavage events that are not of the donor/acceptor

type are intrinsically disfavored relative to bent transition

states.

Figure 11. Possible examples of symmetry constraints on atom transfer:

(a) Jacobsen’s catalyst imparts enantioselectivity via high θ approach by

substrate olefin; (b) chelation of phosphine/phosphite ligand of Mayer et

al. (ref 79) permits orientation favorable toward OAT; (c) Cummins et al.

(ref 80) show that µ-N₂ligand splits via “kinked” transition state.

PMe₃) was due to a kinetic barrier, because rudimentary

thermodynamic arguments clearly portrayed the process as

favorable.^25,79As Figure 11b shows, deoxygenation does

take place if the substrate is the chelate, Ph₂PCH₂CH₂Ph₂-

PO. The authors originally suggested that chelation was

necessary to overcome the intrinsically weak binding by

phosphine oxides, but isolation of the intermediate WCl₂-

{OPPh₂CH₂CH₂Ph₂P}(PMePh₂)₂revealed that additional

factors contribute to the slowness of the subsequent O-atom

transfer event, which is observed upon heating.⁷¹It is

plausible that the chelate, whose W-O-P angle is pro-

bably near 120°, suffers from the same electronic factors

expressed above. During thermolysis, the W-O-P angle

may approach 90°, thereby allowing the symmetry constraints

to be overcome by the more favorable geometry for atom

transfer.

(80) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,

E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.

Soc. 1996, 118, 8623-8638.

(81) Cherry, J.-P. F.; Johnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.; Cummins,

C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.; Hoff, C.

D.; Haar, C. M.; Nolan, S. P.J. Am. Chem. Soc. 2001, 123, 7271-

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(82) Seymore, S. B.; Brown, S. N. Inorg. Chem. 2002, 41, 462-469.

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R. A.; Parkin, G. J. Am. Chem. Soc. 2001, 123, 10111-10112.

Finally, the interplay of electronic states and symmetry/

geometry critical to the above chemistry is not restricted to

O-atom transfers, but is generally applicable to bond-making

and -breaking events that apparently transpire via linear

(79) Brock, S. L.; Mayer, J. M. Inorg. Chem. 1991, 30, 2138-2143.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6221

Veige et al.

was dried over activated 4 Å molecular sieves, vacuum transferred,

and stored under N_2;toluene-d₈and THF-d₈were dried over sodium

benzophenone ketyl. All glassware was oven-dried, and NMR tubes

were additionally flame-dried under dynamic vacuum. Gaseous

reagents (NO, N₂O (Matheson)) were used as received and passed

over a -78 °C trap when possible. {(Me₃Si)₂N}₃V³⁶and (silox)₃-

NbCl₂were prepared via literature methods, and (silox)₃WCl₂

(5) and (silox)₃WCl (6) were prepared according to previous

work.⁴³

In summary, the effects of differing electronic states on

atom transfers, ligand dynamics,⁸⁵ligand substitution or

loss,^34,35,86-88CH-bond activation, and presumably a variety

of other transition metal reactions, while often difficult to

detect, can be profound.

29

Conclusions

Specific. In the systems studied, the deoxygenations of

(silox)₃WNO (10) and R₃PO (R ) Me, Ph, ^tBu) by (silox)₃M

(1-M, M ) V, Nb, Ta), the linear transfer of an oxygen atom

is forbidden by symmetry, and a significant barrier for the

process is apparent, even for cases with a large thermody-

namic driving force. Degradation of an intermediate

(silox)₃M-O-E (1-MOE) species via deoxygenation is

facilitated by bending the M-O-E angle that enables a

lowering of the barrier via the electronic consequences of

lower symmetry. There is a related symmetry barrier for the

docking of a substrate (EO) to (silox)₃Ta (1-Ta) due to its

singlet ground state. In order to form adducts, the triplet state

of 1-Ta must be accessed, or the EO molecule must approach

the molecule such that the Ta-O-E angle deviates sub-

stantially from 180°.

NMR spectra were obtained using Varian XL-400, INOVA-400,

and Unity-500 spectrometers, and chemical shifts are reported

relative to benzene-d₆(¹H, d 7.15; ¹³C, d 128.39), THF-d₈(¹H, d

3.58; ¹³C, 67.57), toluene-d₈(¹H, d 2.09; ¹³C, 20.40), and cyclo-

hexane-d₁₂(¹H, d 1.38; ¹³C, 24.6). Infrared spectra were recorded

on a Nicolet Impact 410 spectrophotometer interfaced to a Gateway

PC. Magnetic measurements were conducted via the Evans

method.³⁷Elemental analyses were performed by Oneida Research

Services, Whitesboro, NY, or Robertson Microlit Laboratories,

Madison, NJ.

Procedures. 1. (silox)₃V (1-V). A 100 mL flask was charged

with {(Me₃Si)₂N}₃V (1.74 g, 3.27 mmol) and hexanes (20 mL).

An addition funnel, containing 2.5 equiv (use of <3.0 equiv

minimized impurities) of ^tBu₃SiOH (1.77 g, 8.18 mmol) in hexanes

(20 mL), was attached. The ^tBu₃SiOH/hexanes solution was added

dropwise over 15 min and the resulting solution stirred for 1.5 h

before all volatiles were removed. The resulting purple residue was

triturated with pentane (2 × 10 mL), dissolved in pentane (20 mL),

and filtered. The filtrate was reduced to 10 mL, cooled to -78 °C,

and filtered, providing 1-V as a purple powder (1.40 g, 74% yield

A Valence Bond Viewpoint. Pedagogically, the discus-

sions above can be simplified and generalized within a

valence bond framework. In a simple treatment for a linear

O-atom transfer, one is only required to count the number

of filled σ-type orbitals on each side of the equation; if they

are the same, then O-atom transfer is allowed. For example,

where M ) (silox)₃Ta (1-Ta), formation of the adduct

(silox)₃Ta-O-E (e.g., E ) PR₃) is forbidden because the

t

based on Bu₃SiOH).

2. (silox)₃VO (2-V). A 25 mL flask was charged with 1-V (300

mg, 0.43 mmol), a gas bulb was attached, and the system was

evacuated. Benzene (15 mL) was condensed and nitrous oxide (88

Torr) was passed over a dry ice/acetone trap and condensed from

a 108.4 mL gas bulb at 77 K. Upon thawing the purple solution

quickly turned light beige and N₂evolved. The solution was stirred

for 45 min, and all volatiles were removed, leaving 2-V as an off-

white powder (220 mg, 72% yield).

2

pair of electrons in 1-Ta reside in a σ-type orbital, d_z. In

combination with the EO σ-bond and O σ-lone pair, these 6

σ electrons are two more than what resides in the σ-bonds

of linear (silox)₃Ta-O-E. Only when the triplet state is

accessed can 1-Ta bind the substrate. Likewise for O-atom

transfer from (silox)₃M-O-E (S ) 1) to (silox)₃MO + E,

the adduct has 6 π-electrons (2 in a degenerate d_xz/d_yzset; 4

in O pπ-orbitals) and 4 σ-based electrons (the M-O and

O-E bonds), but the products have only 4 π-bonding

electrons (MO π-bonds) and 6 σ-type (lone pairs on O

and E, and the MO σ-bond). This O-atom transfer is

symmetry forbidden unless M-O-E bending allows the

mixing that turns an O pπ-orbital into one of predominantly

σ-character.

3. (silox)₃VOPMe₃(3-OPMe₃). A 10 mL flask was charged with

1-V (150 mg, 0.215 mmol) and Me₃PO (22 mg, 0.237 mmol). The

system was evacuated, and benzene (8 mL) was condensed. Upon

thawing, the solution immediately turned emerald green. After

stirring for 45 min all volatiles were stripped and the resulting green

solid was triturated with pentane (2 × 6 mL). Pentane (3 mL) was

condensed and the insoluble product filtered to obtain 3-OPMe₃as

dark green microcrystals (101 mg, 60% yield).

4. (silox)₃VOPPh₃(3-OPPh₃). A 10 mL flask was charged with

1-V (150 mg, 0.215 mmol) and Ph₃PO (66 mg, 0.24 mmol). The

system was evacuated, and benzene (6 mL) was condensed. Upon

thawing, the solution immediately turned bright green. After stirring

for 30 min all volatiles were stripped and the resulting green solid

was triturated with pentane (3 × 5 mL). Pentane (6 mL) was

condensed and the insoluble product filtered to obtain 3-OPPh₃as

dark green microcrystals (166 mg, 80% yield).

5. (silox)₃VOP^tBu₃(3-OP^tBu₃). A 10 mL flask was charged with

1-V (150 mg, 0.215 mmol) and ^tBu₃PO (22 mg, 0.237 mmol). The

system was evacuated, and benzene (8 mL) was condensed. Upon

thawing, the solution immediately turned emerald green. After

stirring for 45 min all volatiles were stripped and the resulting green

solid was triturated with pentane (2 × 6 mL). Pentane (3 mL) was

condensed and the insoluble product filtered to obtain 3-OP^tBu₃as

dark green microcrystals (79 mg,75% yield).

Experimental Section

General Considerations. All manipulations were performed

using either glovebox or high vacuum line techniques. Hydrocarbon

solvents containing 1-2 mL of added tetraglyme, and ethereal

solvents were distilled under nitrogen from purple benzophenone

ketyl and vacuum transferred from same prior to use. Benzene-d₆

(85) Smith, K. M.; Poli, R.; Legzdins, P. Chem. Eur. J. 1999, 5, 1598-

1608.

(86) Smith, K. M.; Poli, R.; Harvey, J. N. New J. Chem. 2000, 24, 77-80.

(87) Franke, O.; Wiesler, B. E.; Lehnert, N.; Na¨ther, C.; Ksenofontov, V.;

Neuhausen, J.; Tuczek, F. Inorg. Chem. 2002, 41, 3491-3499.

(88) (a) Smith, K. M.; Poli, R.; Harvey, J. N. Chem. Eur. J. 2001, 7, 1679-

1690. (b) Green, J. C.; Harvey, J. N.; Poli, R. J. Chem. Soc., Dalton

Trans. 2002, 1861-1866.

6222 Inorganic Chemistry, Vol. 42, No. 20, 2003

Symmetry and Geometry Considerations of Atom Transfer

6. (silox)₃NbPMe₃(1-NbPMe₃). A 100 mL flask was charged

with (silox)₃NbCl₂(4, 3.0 g, 3.7 mmol) and 2.5 equiv of Na/Hg

(22.5 g, 0.95% Na). The system was evacuated, and trimethylphos-

phine (40 mL) was condensed. Upon thawing, the solution turned

light blue and then bright purple-blue after several hours with

obvious salt formation. The solution was stirred overnight, and all

volatiles were removed. The residue was triturated with hexanes

(3 × 15 mL), dissolved in hexanes (20 mL), and filtered through

Celite (1 in.) and the salt cake was washed repeatedly. The filtrate

was condensed to 10 mL, cooled to -78 °C for 2 h, and filtered to

provide 1-NbPMe₃as a bright purple crystalline solid (1.6 g, 53%

yield).

The final solid was dissolved in hexanes (10 mL) and filtered

through 1 in. of Celite. The salt cake was washed repeatedly and

the filtrate stripped of all volatiles. Diethyl ether (4 mL) was

condensed and the solution was cooled to -78 °C for 2 h and

filtered to provide 10 as a light yellow powder (104 mg, 24%

yield).

11. (silox)₃WN (11). A bomb was charged with (silox)₃WCl (6,

420 mg, 0.49 mmol), and a gas bulb was attached. The system

was evacuated and benzene (10 mL) condensed. 2-Methylaziridine

(66 Torr, 0.990 mmol) was condensed from a 276.5 mL gas bulb

at 77 K. Upon thawing, the solution quickly turned bright blue and

then light pink. The solution was stirred for a total of 15 min, and

then all volatiles were removed. The resulting pink solid was

dissolved in pentane (5 mL), cooled to -78 °C for 2 h, and filtered

to give 4 as a light pink solid (90 mg, 22% yield) with <5%

(silox)₃WCl₂(5) impurity. Analytically pure samples could be

prepared by repeated recrystallizations from pentane.

7. (silox)₃ClWO (7). A solution of (silox)₃WCl (6, 520 mg, 0.601

mmol) in 12 mL of hexane was prepared, and 1.4 equiv of N₂O

was admitted from a gas bulb into the flask at -196 °C. Upon

warming to 25 °C, the solution changed from green to colorless

and gas evolution was apparent. The solution was concentrated to

2.5 mL and cooled to -78 °C to yield 320 mg of faint pink crystals

(60%). An assay by ¹H NMR spectroscopy indicated less than 1%

(silox)₃WCl₂(5). IR (Nujol, cm^-1): 934 (w), 920 (ν(WO), m), 858

(s), 813 (s), 722 (m), 629 (m).

12. NMR Tube Reactions (General). NMR tubes were sealed

to a 14/20 adapter and flame-dried under active vacuum im-

mediately before use. The tubes were charged with reagent and

substrate solids and then evacuated. An appropriate deuterated

solvent was then condensed at 77K and the tube sealed under active

vacuum. If a condensable (77 K) gas was a reagent, it was added

via calibrated gas bulb prior to sealing of the tube. Reactions were

8. (silox)₃WO (8). (silox)₃ClWO (7) (236 mg, 0.268 mmol) was

placed in a flask with 1.5 equiv of 0.5 wt % Na/Hg (1.85 g, 0.402

mmol of Na). THF (10 mL) was added at -78 °C, and the mixture

was allowed to slowly warm. The solution became deep purple as

it reached 25 °C, and stirring was continued for another 30 min.

The solvent was removed, and the residue was triturated with

hexanes (3 × 5 mL), resulting in conversion of the hexane-insoluble

purple solid (possibly [(silox)₃ClWO]Na(THF)_x) to a green-brown

hexane-soluble material that was dissolved in pentane and filtered

through Celite. The solution was concentrated to 1.5 mL, cooled

to -78 °C to precipitate a gray-green solid, and filtered. The product

(8) was collected (50 mg, 22% yield), but a ¹H NMR spectral assay

revealed the presence of 2-3% unidentified impurities. An Evans

method measurement gave µ_eff) 1.24 µ_Bwhen corrected for

diamagnetic impurities. IR (Nujol, cm^-1): 933 (w), 909 (ν(WO),

m), 845 (s), 814 (s), 722 (m), 627 (m).

1

assayed periodically by H NMR spectroscopy.

Physical Studies. 13. Kinetics. Deoxygenations of (silox)₃WNO

(10) by (silox)₃M (1-M, M ) V, NbL (L ) 4-pic, PMe₃), Ta) were

monitored in NMR tubes as described in procedure 12, as were

the reactions of R₃PO (R ) Me, Ph, ^tBu) with (silox)₃M (1-M, M

) V, NbL (L ) 4-pic, PMe₃), Ta). While the reactions were roughly

conducted, the reproducibility of each deoxygenation was checked,

and all thermolyses were conducted in constant temperature ((1

°C) oil baths.

14. Single-Crystal X-ray Diffraction Study of (silox)₃VOPMe₃

(3-OPMe₃). In a drybox, a 10 mL vial was charged with a solution

of 3-OPMe₃(30 mg) in toluene (0.8 mL). A crystal suitable for

X-ray analysis was obtained by cooling the solution for 12 h at

-30 °C. An emerald green crystal (0.4 × 0.3 × 0.2 mm) was

covered in epoxy and placed on the goniometer head of a Siemens

SMART CCD Area Detector system equipped with a fine-focus

molybdenum X-ray tube (λ ) 0.71073 Å). All non-hydrogen atoms

were anisotropically refined, and hydrogen atoms were treated as

idealized contributions.

15. Single-Crystal X-ray Diffraction Study of (silox)₃VOP^tBu₃

(3-OP^tBu₃). A sealed tube containing a solution of 3-OP^tBu₃(50

mg) in cyclohexane (0.8 mL) was slowly cooled from 155 to 25

°C. An emerald green crystal (0.2 × 0.05 × 0.05 mm) was covered

in polyisobutylene and placed on the goniometer head of a Siemens

SMART CCD Area Detector system equipped with a fine-focus

molybdenum X-ray tube (λ ) 0.71073 Å). The initial structure

solution in P6(3)/mmc gave acceptable connectivity, but the

displacements, interatomic distances and an R₁∼ 0.20 were

problematic. Twinning possibilities exist for a simulated possible

space group of P6₃/mmc,⁸⁹and a successful solution involved two

twin fragments related by a 60° rotation along the c-axis in space

group P31c (SHELX command: TWIN -1 0 0 1 1 0 0 0 1).

Refinement then proceeded routinely with all non-hydrogen atoms

anisotropically refined. Hydrogen atoms were treated as idealized

contributions.

9. (silox)₃ClWNO (9, with Byproducts 5 and 10). To a flask

containing 150 mg (0.173 mmol) of 6 was added 10 mL of hexane.

After dissolution, the solution was frozen at -196 °C and 3 equiv

of NO was admitted via a gas bulb. The flask was warmed to -78

°C, and upon thawing the solution turned deep red with some red

precipitate. The mixture was stirred for an additional 5 min at -78

°C, and the excess NO was removed. The solvent was stripped

under vacuum as the reaction mixture warmed slowly to 23 °C.

The resulting red solid (132 mg, roughly 85% material yield, 66%

yield of 9) was observed (¹H NMR spectroscopic assay) to contain

78% 9, 11% 5, and 11% 10. The purity could not be improved by

recrystallization. IR (9, Nujol, cm^-1): 1651 (ν(NO), s), 1015 (w),

1006 (w), 957 (w), 933 (m), 918 (w), 857 (s), 835 (s), 819 (s), 805

(s), 788 (br s), 627 (m).

10. (silox)₃WNO (10). A 25 mL flask was charged with

(silox)₃WCl (6, 445 mg, 0.514 mmol) and 2 equiv of Na/Hg (3.37

g, 0.7% Na). A gas bulb was attached and the system evacuated.

THF (15 mL) was condensed and allowed to thaw to dissolve all

solids. The solution was frozen, and NO (173 Torr) was condensed

from a 109.4 mL gas bulb at 77 K. The solution was rapidly thawed

and turned red after 9 min, and the excess NO was removed. The

solution was stirred for an additional 10 min before all volatiles

were removed, and the resulting orange-brown residue was triturated

with hexanes (3 × 10 mL). Sometimes the crude product contained

as much as 25% unreacted 6 and the above procedure was repeated.

(89) International Tables for Crystallography; Wilson, A. J. C., Ed.;

Kluwer Academic Publishers: Boston, 1995; Vol. C, p 13, Table

1.3.4.2.

Inorganic Chemistry, Vol. 42, No. 20, 2003 6223

Veige et al.

16. Single-Crystal X-ray Diffraction Study of (silox)₃NbPMe₃

(1-NbPMe₃). In a drybox, a 10 mL vial was charged with a solution

of 1-NbPMe₃(30 mg), hexanes (0.5 mL), and (Me₃Si)₂O (0.5 mL).

Crystals suitable for X-ray analysis were obtained by slow

evaporation of the solvent in a refrigerator (-30 °C) over a 1 week

period. A bright blue crystal (0.4 × 0.4 × 0.4 mm) was covered in

epoxy and placed on the goniometer head of a Siemens SMART

CCD Area Detector system equipped with a fine-focus molybdenum

X-ray tube (λ ) 0.71073 Å). All non-hydrogen atoms were

anisotropically refined, and hydrogen atoms were treated as

idealized contributions.

17. Single-Crystal X-ray Diffraction Study of (silox)₃ClWO

(7). Slow evaporation of a hexane solution of 7 yielded faintly

pink rectangular prismatic crystals. One crystal of dimensions

0.4 × 0.3 × 0.2 mm was selected, coated in polyisobutylene,

and placed under a 173 K nitrogen stream on the goniometer

head of a Siemens SMART CCD Area Detector system equipped

with a fine-focus molybdenum X-ray tube (λ ) 0.71073 Å). All

non-hydrogen atoms were refined with anisotropic displacement

parameters, and hydrogen atoms were included at calculated

positions.

Computational Methods. All calculations (Table 7) reported

herein employ the effective core potentials (ECPs) and valence

basis sets (VBSs) of Stevens et al.⁶¹These ECPs were derived

from Dirac-Fock relativistic atomic wave functions and include

all core electrons for main group elements. For transition metals

(TMs) a semi-core approximation (i.e., valence and outermost

core orbitals are explicitly calculated) was utilized. Transition

metal and main group VBSs were valence triple-ú and double-

ú-plus-polarization, respectively. For hydrogens, the -31G

basis set was employed. This ECP/VBS scheme, designated SBK-

(d), has seen wide usage with a variety of wave function types and

has been found reliable for accurate modeling of the structure,

energetics, and spectroscopy of a wide variety of transition metal

species.⁶²

Optimizations were carried out using density functional theory

(DFT) methods, although little difference was found regardless of

the functional employed. Reported calculations employed the

B3LYP hybrid functional⁶³for geometry optimization unless

stated otherwise. Energies of single point coupled cluster calcula-

tions (CCSD(T)) were determined at B3LYP/SBK(d) stationary

points.^64,65Vibrational frequencies were calculated at all B3LYP

stationary points to characterize the obtained points as minima or

transition states. Reported Gibbs free energies include enthalpic

and entropic corrections (using vibrational frequencies calculated

at the B3LYP/SBK(d) level of theory) to 298.15 K and 1 atm. All

calculations were performed with the Gaussian 98 suite of

programs.⁶⁶

Acknowledgment. We thank the National Science Foun-

dation (CHE-9528914, CHE-0212147 (P.T.W.) and CHE-

9983665 (T.R.C.)) for support of this research.

Supporting Information Available: Crystallographic data in

CIF format. This material is available free of charge via the Internet

at http://pubs.acs.org.

IC0300114

6224 Inorganic Chemistry, Vol. 42, No. 20, 2003

Article Doi

Symmetry and geometry considerations of atom transfer: Deoxygenation of (silox)3WNO and R3PO (R = Me, Ph, tBu) by (silox)3M (M = V, NbL (L = PMe3, 4-picoline), Ta; silox = tBu3SiO)

DOI: 10.1021/ic0300114

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Article abstract of DOI:10.1021/ic0300114

Full text of DOI:10.1021/ic0300114

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