Synthesis and structures of niobium(V) complexes stabilized by linear-linked aryloxide trimers

Article abstract of DOI:10.1021/ic025882c

The preparation and characterization of a series of niobium(V) complexes that incorporate the linear-linked aryloxide trimers 2,6-bis(4,6-dimethylsalicyl)-4-tert-butylphenol [H₃(Me-L)] and 2,6-bis(4-methyl-6-tert-butylsalicyl)-4-tert-butylphenol [H₃(^tBu-L)] are described. The chloride complex [Nb(Me-L)Cl₂]₂ (1) was prepared in high yield by reaction of NbCl₅ with H₃(Me-L) in toluene. In contrast, the analogous reaction with H₃(^tBu-L) gave a mixture of [Nb(^tBu-L)Cl₂]₂ (2) and [Nb(de-^tBu-L)Cl₂]₂ (3a). During the formation of 3a, one of tert-butyl groups at the ortho position in the ^tBu-L ligand was lost. When the NbCl₅/H₃(^tBu-L) reaction was carried out in acetonitrile, Nb[H(^tBu-L)]Cl₃(NCMe) (4) was obtained. Heating a solution of 4 in toluene generated 2 and 3a. The isolated complex 4 underwent ligand redistribution in acetonitrile to produce Nb[H(^tBu-L)]₂Cl(NCMe) (5). Treatment of NbCl₅ with Li₃(^tBu-L) in toluene afforded 2. The chloride ligands in 1 and 2 smoothly reacted with 4 equiv of MeMgl and LiS^tBu, resulting in [Nb(R-L)Me₂]₂ [R = Me (6), ^tBu (7)] and Nb(Me-L)(S^tBu)₂ (8), respectively. A number of the above complexes have been characterized by X-ray crystallography. In the structures of 1, 2, and 6, the R-L ligand is bound to the metal center with a U-coordination mode, while an alternative S-conformation is adopted for 3a and 8. Complexes 4 and 5 contain a bidentate H(^tBu-L) diphenoxide-monophenol ligand.

Full text of DOI:10.1021/ic025882c

Inorg. Chem. 2002, 41, 6090−6098
Synthesis and Structures of Niobium(V) Complexes Stabilized by
Linear-Linked Aryloxide Trimers
Tsukasa Matsuo and Hiroyuki Kawaguchi*
Coordination Chemistry Laboratories, Institute for Molecular Science, and CREST,
Japan Science and Technology Corporation (JST), Myodaiji, Okazaki 444-8585, Japan
Received July 17, 2002
The preparation and characterization of a series of niobium(V) complexes that incorporate the linear-linked aryloxide
trimers 2,6-bis(4,6-dimethylsalicyl)-4-tert-butylphenol [H (Me-L)] and 2,6-bis(4-methyl-6-tert-butylsalicyl)-4-tert-
3
t
butylphenol [H (Bu-L)] are described. The chloride complex [Nb(Me-L)Cl2]2 (1) was prepared in high yield by reaction
3
t
t
of NbCl5 with H (Me-L) in toluene. In contrast, the analogous reaction with H (Bu-L) gave a mixture of [Nb(Bu-
3
3
t
L)Cl2]2 (2) and [Nb(de-Bu-L)Cl2]2 (3a). During the formation of 3a, one of tert-butyl groups at the ortho position in
the tBu-L ligand was lost. When the NbCl5/H (Bu-L) reaction was carried out in acetonitrile, Nb[H(Bu-L)]Cl3(NCMe)
t
t
3
(4) was obtained. Heating a solution of 4 in toluene generated 2 and 3a. The isolated complex 4 underwent ligand
t
t
redistribution in acetonitrile to produce Nb[H(Bu-L)]2Cl(NCMe) (5). Treatment of NbCl5 with Li3(Bu-L) in toluene
t
afforded 2. The chloride ligands in 1 and 2 smoothly reacted with 4 equiv of MeMgI and LiSBu, resulting in
[Nb(R-L)Me2]2 [R ) Me (6), tBu (7)] and Nb(Me-L)(SBu)2 (8), respectively. A number of the above complexes have
t
been characterized by X-ray crystallography. In the structures of 1, 2, and 6, the R-L ligand is bound to the metal
center with a U-coordination mode, while an alternative S-conformation is adopted for 3a and 8. Complexes 4 and
t
5 contain a bidentate H(Bu-L) diphenoxide−monophenol ligand.
Introduction
(ArO)nM fragment. However, ligand redistribution reaction
is a common reaction pathway through which coordinatively
unsaturated metal complexes decompose.2 These undesired
ligand redistribution reactions are occasionally a severe
obstacle to synthetic efforts. One of strategies for overcoming
this problem is to use covalently linked ancillary ligands,
thereby limiting ligand mobility and leaving little possibility
to reorganize the molecule. This strategy has been a very
useful concept in coordination chemistry and has allowed
the isolation of transition metal complexes in unstable
oxidation states or in unusual coordination geometries and
many with diverse highly reactive functionalities.3-8
The ability of alkoxide and aryloxide ligands to support
and promote various important organic/inorganic reactions
at metal centers has been known for many years.1 In this
context, they complement the well-studied cyclopentadienyl-
based systems, with the major difference being the greater
reactivity of the former complexes due to their relatively
higher unsaturation and lower coordination numbers for a
* To whom correspondence should be addressed. E-mail: hkawa@
ims.ac.jp. Fax: +81-564-55-5245.
(1) (a) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. J. Am. Chem.
Soc. 1984, 106, 6806-6815. (b) Chisholm, M. H.; Folting, K.;
Hampden-Smith, M. J.; Hammond, C. E. J. Am. Chem. Soc. 1989,
111, 7283-7285. (c) Thorn, M. G.; Hill, J. E.; Waratuke, S. A.;
Johnson, E. S.; Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc.
1997, 119, 8630-8641. (d) Parkin, B. C.; Clark, J. R.; Visciglio, V.
M.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1995, 14, 3002-
3013. (e) Rothwell, I. P. Acc. Chem. Res. 1988, 21, 153-159. (f)
Miller, R. L.; Wolczanski, P. T.; Rheingold, A. L. J. Am. Chem. Soc.
1993, 115, 10422-10423. (g) 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. (h) Wolczanski, P. T.
Polyhedron 1995, 14, 3335-3362. (i) Gray, S. D.; Weller, K. J.; Bruck,
M. A.; Briggs, P. M.; Wigley, D. E. J. Am. Chem. Soc. 1995, 117,
10678-10693. (j) Listemann, M. L.; Schrock, R. R. Organometallics
1985, 4, 74-83. (k) Schrock, R. R.; Listemann, M. L.; Sturgeoff, L.
G. J. Am. Chem. Soc. 1982, 104, 4291-4293. (l) Saito, S.; Yamamoto
H. Chem. Commun. 1997, 1585-1592. (m) Teng, X.; Wada, T.;
Okamoto, S.; Sato, F. Tetrahedron Lett. 2001, 42, 5501-5503.
We set out to investigate linear aryloxide trimers H3(R-
L) as new ancillary ligands, in which aryloxide units are
(2) (a) Minhas, R.; Duchateau, R.; Gambarotta, S.; Bensimon, C. Inorg.
Chem. 1992, 31, 4933-4938. (b) Avens, L. R.; Barnhart, D. M.; Burns,
C. J.; McKee, S. D.; Smith, W. H. Inorg. Chem. 1994, 33, 4245-
4254. (c) McKee, S. D.; Burns, C. J.; Avens, L. R. Inorg. Chem. 1998,
37, 4040-4045. (d) Clark, D. L.; Grumbine, S. K.; Scott, B. L.;
Watkin, J. G. Organometallics 1996, 15, 949-957. (e) No¨th, H.;
Schmidt, M. Organometallics 1995, 14, 4601-4610. (f) Minhas, R.
K.; Edema, J. J. H.; Gambarotta, S.; Meetsma, A. J. Am. Chem. Soc.
1993, 115, 6710-6717. (g) Veige, A. S.; Kleckley, T. S.; Chamberlin,
R. M.; Neithamer, D. R.; Lee, C. E.; Wolczanski, P. T.; Lobkovsky,
E. B.; Glassey, W. V. J. Organomet. Chem. 1999, 591, 194-203. (h)
Chisholm, M. H.; Huang, J. H.; Huffman, J. C.; Streib, W. E.; Tiedtke,
D. Polyhedron 1997, 16, 2941-2949.
6090 Inorganic Chemistry, Vol. 41, No. 23, 2002
10.1021/ic025882c CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/19/2002

Niobium Complexes of Aryloxide Trimers
bond of dinitrogen.15 In this article we report the full details
of the synthesis and structures of a variety of niobium
complexes incorporating linear aryloxide trimers.
Chart 1
Experimental Section
General Considerations. All manipulations of air- and moisture-
sensitive materials were performed under an argon atmosphere using
standard Schlenk line techniques. All dried solvents and chemicals
commercially available were used as received without further
purification. Linear aryloxide trimers, H3(Me-L), H3(tBu-L), and
H3(tBu-LMe), were synthesized according to the literature proce-
dures.9d,13,15a MeMgI was prepared from methyl iodide and mag-
nesium in diethyl ether, while tBuSLi was obtained by the reaction
of 2-methyl-2-propanethiol with butyllithium in toluene at 0 °C.
Deuterated chloroform (CDCl3), benzene (C6D6), and toluene (C7D8)
was distilled from calcium hydride or sodium prior to use. Elemental
analyses (C, H, N, and S) were carried out on Yanaco CHN Corder
connected at the ortho positions through methylene linkers
(Chart 1). Although linear aryloxide trimers were being
investigated for their use as building blocks for the synthesis
of macrocyclic compounds,9 there are few examples of metal
complexes with these trimers in coordination chemistry.3,7,10-12
Independent from us, Scott and Hofmeister demonstrated the
synthesis of compounds having linear-linked aryloxide
trimers for lithium, sodium, aluminum, and titanium(IV).13,14
We have recently exploited these trimers in the formation
of tetravalent and trivalent titanium complexes and a reactive
niobium derivative which is found to cleave the NtN triple
1
MT-6 and Leco-CHNS element analyzers. H NMR (500 MHz)
spectra were recorded at room temperature using a JEOL LA-500
1
spectrometer. Chemical shifts (in ppm) for H NMR spectra were
referenced to residual protic solvent peaks.
Synthesis of [Nb(Me-L)Cl2]2 (1). A mixture of NbCl5 (2.47 g,
9.14 mmol) and H3(Me-L) (3.83 g, 9.15 mmol) was suspended in
toluene (40 mL). The resulting red brown solution was refluxed
for 2 h, during which time yellow brown crystals precipitated. After
standing at room temperature for 3 h, this material was collected
by filtration, washed with ether, and dried in vacuo to afford 1 in
88% (4.66 g). 1H NMR (500 MHz, CDCl3): δ 1.18 (s, 18H, tBu),
2.34 (s, 12H, Me), 2.41 (s, 12H, Me), 2.41 (d, J ) 13.5 Hz, 4H,
CH2), 3.41 (d, J ) 13.5 Hz, 4H, CH2), 6.68 (s, 4H, Ar H), 6.98 (s,
4H, Ar H), 7.18 (s, 4H, Ar H). Anal. Calcd for C60H70O6Cl4Nb2‚
C7H8: C, 61.57; H 6.02. Found: C, 61.67; H, 6.11.
Synthesis of [Nb(tBu-L)Cl2]2 (2). A hexane solution of butyl-
lithium (1.57 M, 11.0 mL, 17.2 mmol) was added dropwise to H3-
(tBu-L) (2.90 g, 5.77 mmol) in toluene (80 mL) at 0 °C. The solution
was stirred at room temperature for 1 h, and then NbCl5 (1.56 g,
5.77 mmol) was added to the resulting white suspension. The color
(3) (a) Gielens, E. E. C. G.; Dijkstra, T. W.; Berno, P.; Meetsma, A.;
Hessen, B.; Teuben, J. H. J. Organomet. Chem. 1999, 591, 88-95.
(b) Sernetz, F. G.; Mulhaupt, R.; Fokken, S.; Okuda, J. Macromol-
ecules 1997, 30, 1562-1569. (c) van der Linden, A.; Schaverien, C.
J.; Meijboom, N.; Ganter, C.; Orpen, A. G. J. Am. Chem. Soc. 1995,
117, 3008-3021.
(4) (a) Schaverian, N.; Mejiboom, N.; Orpen, A. G. J. Chem. Soc., Chem.
Commun. 1992, 124-126. (b) Hultzsch, K. C.; Bonitatebus, P. J.;
Jernelius, J.; Schrock, R. R.; Hoveyda, A. H. Organometallics 2001,
20, 4705-4712. (c) Kayal, A.; Ducruet, A. F.; Lee, S. C. Inorg. Chem.
2000, 39, 3696-3704. (d) Kayal, A.; Lee, S. C. Inorg. Chem. 2002,
41, 321-330. (e) Eilerts, N. W.; Heppert, J. A. Polyhedron 1995, 14,
3255-3271. (f) Yoshikawa, N.; Yamada, Y. M. A.; Das, J.; Sasai,
H.; Shibasaki, M. J. Am. Chem. Soc. 1999, 121, 4168-4178.
(5) (a) Arnold, P. L.; Natrajan, L. S.; Hall, J. J.; Bird, S. J.; Wilson, C. J.
Organomet. Chem. 2002, 647, 205-215. (b) Nakayama, Y.; Watanabe,
K.; Ueyama, N.; Nakamura, A.; Harada, A.; Okuda, J. Organometallics
2000, 19, 2498-2503. (c) Takashima, Y.; Nakayama, Y.; Harada, A.
Chem. Lett. 2001, 488-489. (d) Okuda, J.; Masoud, E. Macromol.
Chem. Phys. 1998, 199, 543. (e) Chaudhuri, P.; Hess, M.; Weyher-
mu¨ller, T.; Wieghardt, K. Angew. Chem., Int. Ed. 1999, 38, 1095-
1098. (f) Kru¨ger, H.-J. Angew. Chem., Int. Ed. 1999, 38, 627-631.
(6) (a) Dinger, M. B.; Scott, M. J. Inorg. Chem. 2001, 40, 1029-1036.
(b) Dinger, M. B.; Scott, M. J. Inorg. Chem. 2000, 39, 1238-1254.
(c) Dinger, M. B.; Scott, M. J. Chem. Commun. 1999, 2525-2526.
(7) (a) Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C. J. Am. Chem.
Soc. 1999, 121, 8296-8305. (b) Zanotti-Gerosa, A.; Solari, E.;
Giannini, L.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1998, 120, 437-438. (c) Guillenmot, G.; Solari, E.; Scopelliti,
R.; Floriani, C. Organometallics 2001, 20, 2446-2448. (d) Ozerov,
O. V.; Ladipo, F. T.; Patrick, B. O. J. Am. Chem. Soc. 1999, 121,
7941-7942.
(11) (a) Okuda, J.; Fokken, S.; Kang, H.-C.; Massa, W. Chem. Ber. 1995,
128, 221-227. (b) Higham, L.; Thornton-Pett, M.; Bochmann, M.
Polyhedron 1998, 17, 3047-3051. (c) Mulford, D. R.; Fanwick, P.
E.; Rothwell, I. P. Polyhedron 2000, 19, 35-42. (d) Chisholm, M.
H.; Huang, J.-H.; Huffman, J. C.; Parkin, I. P. Inorg. Chem. 1997,
36, 1642-1651. (e) Corazza, F.; Floriani, C.; Chiesi-Villa, A.; Guastini,
C. Inorg. Chem. 1991, 30, 145-148. (f) Floriani, C.; Corazza, F.;
Lesueur, W.; Chiesivilla, A.; Guastini, C. Angew. Chem., Int. Ed. Engl.
1989, 28, 66-67. (g) Toscano, P. J.; Schermerhorn, E. J.; Dettelbacher,
C.; Macherone, D.; Zubieta, J. J. Chem. Soc., Chem. Commun. 1991,
933-934.
(12) (a) Floriani, C.; Floriani-Moro, R. AdV. Organomet. Chem. 2001, 47,
167-233. (b) Floriani, C. Chem.sEur. J. 1999, 5, 19-23. (c) Wieser,
C.; Dieleman, C. B.; Matt, D. Coord. Chem. ReV. 1997, 165, 93-
161. (d) Corazza, F.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J.
Chem. Soc., Chem. Commun. 1990, 1083-1084. (e) Acho, J. A.;
Doerrer, L. H.; Lippard, S. J. Inorg. Chem. 1995, 34, 2542-2556. (f)
Olmstead, M. M.; Sigel, G.; Hope, H.; Xu, X.; Power, P. P. J. Am.
Chem. Soc. 1985, 107, 8087-8091. (g) Ozerov, O. V.; Rath, N. P.;
Ladipo, F. T. J. Organomet. Chem. 1999, 586, 223-233. (h) Leverd,
P. C.; Rinaldo, D.; Nierlich, M. Eur. J. Inorg. Chem. 2001, 2021-
2025. (i) Gibson, V. C.; Redshaw, C.; Elsegood, M. R. J. J. Chem.
Soc., Dalton Trans. 2001, 767-769.
(8) (a) Slaughter, L. M.; Wolczanski, P. T. Chem. Commun. 1997, 2109-
2110. (b) Toupance, T.; Dubberley, S. R.; Rees, N. H.; Tyrrell, B. R.;
Mountford, P. Organometallics 2002, 21, 1367-1382.
(9) (a) Dhawan, B.; Gutsche, C. D. J. Org. Chem. 1983, 48, 1536-1539.
(b) Boehmer, V.; Marschollek, F.; Zetta, L. J. Org. Chem. 1987, 52,
3200-3205. (c) No, K.; Kim, J. E.; Kwon, K. M. Tetrahedron Lett.
1995, 36, 8453-8456. (d) Sone, T.; Ohba, Y.; Yamazaki, H. Bull.
Chem. Soc. Jpn. 1989, 62, 1111-1116.
(13) Gordon, B. W. F.; Scott, M. J. Inorg. Chim. Acta 2000, 297, 206-
216.
(14) Appiah, W. O.; DeGreef, A. D.; Razidlo, G. L.; Spessard, S. J.; Pink,
M.; Young, V. G., Jr.; Hofmeister, G. E. Inorg. Chem. 2002, 41, 3656-
3667.
(15) (a) Matsuo, T.; Kawaguchi, H.; Sakai, M. J. Chem. Soc., Dalton Trans.
2002, 2536-2540. (b) Kawaguchi, H.; Matsuo, T. Angew. Chem., Int.
Ed. 2002, 41, 2792-2794.
(10) Methylene-linked polyphenol ligands are mainly divided into 2,2′-
methylenebis(aryloxide) ligands (linked aryloxide dimers)3,11 and cyclic
calixarenes.7,12
Inorganic Chemistry, Vol. 41, No. 23, 2002 6091

Matsuo and Kawaguchi
of the solution immediately turned red. The mixture was refluxed
for 3 h, during which time red microcrystals were formed. After
concentration and standing at room temperature for 16 h, the
solution was removed by decantation. The red crystalline solid was
recrystallized from hot toluene to give 2 as red blocks in 41% yield
(1.67 g). 1H NMR (500 MHz, CDCl3): δ 1.21 (s, 18H, tBu). 1.42
C72H94N2O6ClNb: C, 71.36; H, 7.82; N, 2.31. Found: C, 71.20;
H, 7.66; N, 2.15.
(b) From NbCl5 and H3(tBu-L). To a solution of NbCl5 (0.21
g, 0.78 mmol) in CH3CN (10 mL) was added 2 equiv of H3(tBu-L)
(0.78 g, 1.55 mmol). The solution was stirred at 80 °C for 3 h,
during which time orange microcrystals of 5 precipitated. This
material was collected by filtration, washed with ether, and dried
in vacuo to afford 5 in 91% yield (0.83 g).
Synthesis of [Nb(Me-L)Me2]2 (6). A solution of MeMgI (4.45
mmol) in Et2O (5 mL) was added dropwise to a suspension of 1
(1.29 g, 1.11 mmol) in toluene (80 mL) at -78 °C. The mixture
was allowed to return to room temperature and stirred for a further
4 h. After the reaction mixture was centrifuged to remove an
insoluble material, the dark-yellow supernatant was evaporated to
dryness. The resulting residue was recrystallized from Et2O, yielding
6 as yellow crystals (0.64 g, 44%). 1H NMR (500 MHz, C6D6): δ
-0.42 (s, 6H, Nb-Me), 1.04 (s, 18H, tBu), 1.27 (s, 6H, Nb-Me),
2.20 (s, 24H, the methyl protons of Me-L are overlapped), 3.16 (d,
J ) 13.0 Hz, 4H, CH2), 4.89 (d, J ) 13.0 Hz, 4H, CH2), 6.75 (s,
4H, Ar H), 6.89 (s, 4H, Ar H), 7.00 (s, 4H, Ar H). 13C{1H} NMR
(125.65 MHz, C6D6): δ 34.3, 32.7 (Nb-Me). Anal. Calcd for
C60H74O6Nb2‚2C4H10O: C, 66.66; H, 7.73. Found: C, 66.15; H,
7.56.
t
(s, 36H, Bu), 2.35 (s, 12H, Me), 2.36 (s, 3H, toluene), 3.31 (d, J
) 13.7 Hz, 4H, CH2), 4.97 (d, J ) 13.7 Hz, 4H, CH2), 7.04 (s, 4H,
Ar H), 7.06 (s, 4H, Ar H), 7.17 (m, 7H, Ar H + toluene), 7.25 (m,
2H, toluene). Anal. Calcd for C68H86O6Cl4Nb2‚C7H8: C, 63.47; H
6.68. Found: C, 63.80; H, 6.69.
Synthesis of [Nb(de-tBu-L)Cl2] (3a). (a) From NbCl5 and H3-
(tBu-L). A mixture of NbCl5 (2.83 g, 10.5 mmol) and H3(tBu-L)
(5.23 g, 10.4 mmol) was suspended in toluene (60 mL). After the
reaction mixture was refluxed for 80 min, the solvent was removed
in vacuo to afford a red solid. The residue was redissolved in
toluene, and then the solution was centrifuged to remove a small
amount of 2. Evaporation of the supernatant gave 3a as wine-red
1
microcrystals (4.52 g, 72% yield). H NMR (500 MHz, CDCl3):
t
t
δ 1.10 (s, 18H, Bu), 1.43 (s, 18H, Bu), 2.420 (s, 6H, Me), 2.424
(s, 6H, Me), 3.59 (d, J ) 15.2 Hz, 2H, CH2), 3.62 (d, J ) 15.3 Hz,
2H, CH2), 5.74 (d, J ) 15.3 Hz, 2H, CH2), 5.75 (d, J ) 15.2 Hz,
2H, CH2), 6.71-7.01 (m, 14H, Ar H). Anal. Calcd for C60H70O6-
Cl4Nb2‚C7H8: C, 61.57; H 6.02. Found: C, 61.67; H, 6.11.
(b) From Nb[H(tBu-L)]Cl3(NCMe). A solution of Nb[H(tBu-
L)]Cl3(NCMe) (4) (0.69 g, 0.93 mmol) in toluene (20 mL) was
refluxed for 1 h, during which time red microcrystals of 2 and 3a
precipitated. Evaporation of the solvent afforded a wine-red solid
that was washed with toluene/hexane to give 0.35 g of 3a in 62%
yield.
Synthesis of [Nb(tBu-L)Me2]2 (7). (a) From [Nb(tBu-L)Cl2]2
(2). The same procedure as used for 6 was followed. Reaction of
MeMgI (2.3 mmol) in Et2O (4 mL) with 2 (0.57 g, 0.43 mmol) in
THF (60 mL) afforded 7 as yellow crystals (0.21 g, 39% yield).
1H NMR (500 MHz, C6D6): δ 0.03 (s, 6H, Nb-Me), 1.14 (s, 18H,
t
tBu), 1.39 (s, 36H, Bu), 1.67 (s, 6H, Nb-Me), 2.32 (s, 6H, Me),
3.21 (d, J ) 13.5 Hz, 4H, CH2), 4.63 (d, J ) 13.5 Hz, 4H, CH2),
7.01 (s, 4H, Ar H), 7.04 (s, 4H, Ar H), 7.15 (s, 4H, Ar H). Anal.
Calcd for C72H98O6Nb2‚2C7H8: C, 72.25; H, 8.04. Found: C, 71.61;
H, 7.92. Solubility constraints prevent us from acquiring the 13C
NMR spectrum of 7.
(b) From Nb[H(tBu-L)]Cl3(NCMe) (4). Addition of MeMgI
(2.14 mmol) in Et2O (6 mL) to 4 (0.79 g, 1.07 mmol) in toluene
(80 mL) followed by workup similar to the above provided 0.20 g
of 7 in 30% yield.
Synthesis of [Nb(de-tBu-LMe)Cl2] (3b). The same procedure as
used for 3a was followed. Reaction of NbCl5 (1.73 g, 6.40 mmol)
and H3(Me-LMe) (2.97 g, 6.44 mmol) in toluene (60 mL) afforded
3b as red microcrystals in 71% yield (2.57 g). 1H NMR (500 MHz,
CDCl3): δ 1.42 (s, 36H, tBu), 2.34 (s, 6H, Me), 2.42 (s, 12H, Me
x 2), 3.59 (d, J ) 14.9 Hz, 2H, CH2), 3.62 (d, J ) 14.9 Hz, 2H,
CH2), 5.74 (d, J ) 14.9 Hz, 2H, CH2), 5.75 (d, J ) 14.9 Hz, 2H,
CH2), 6.75-7.20 (m, 14H, Ph). Anal. Calcd for C54H58Cl4Nb2O6:
C, 57.36; H 5.17. Found: C, 57.07; H, 5.60.
Synthesis of Nb(Me-L)(StBu)2 (8). Addition of butyllithium
(2.36 mL, 3.71 mmol, 1.57 M in hexane) to a toluene solution of
2-methyl-2-propanethiol (0.42 mL, 3.72 mmol) at 0 °C gave a white
suspension. After the suspension was stirring for 30 min, 1 (1.03
g, 0.89 mmol) was added. The mixture was stirred at 80 °C for 3
h, during which time a red color appeared and LiCl precipitated.
The solution was centrifuged to remove an insoluble material.
Concentration of the supernatant and cooling to -30 °C provided
Synthesis of Nb[H(tBu-L)]Cl3(NCMe) (4). A solution of NbCl5
(0.90 g, 3.33 mmol) in CH3CN (40 mL) was subjected to the
addition of H3(tBu-L) (1.67 g, 3.32 mmol) at room temperature.
The color of the solution immediately changed from yellow to red.
The mixture was refluxed for 2 h, and then solvent was removed
in vacuo to leave a red solid. The residue was dissolved in CH2Cl2
(10 mL) containing CH3CN (1 mL), and hexane (30 mL) was added.
The solution was concentrated to yield 4 as red crystals in 57%
1
7 as orange crystals (0.93 g, 76% yield). H NMR (500 MHz,
1
t
t
CDCl3): δ 1.26 (s, 9H, Bu), 1.64 (s, 18 H, StBu), 2.25 (s, 6H,
(1.41 g). H NMR (500 MHz, CDCl3): δ 1.17 (s, 9H, Bu), 1.37
t
t
(s, 9H, Bu), 1.53 (s, 9H, Bu), 2.21 (s, br, MeCN), 2.25 (s, 3H,
Me), 2.37 (s, 3H, Me), 1.55 (d, J ) 13.9 Hz, 1H, CH2), 4.22 (br,
2H, overlapping CH2 and OH), 4.78 (br, 1H, CH2), 5.29 (d, J )
13.9 Hz, 1H, CH2), 6.77 (s, 2H, Ar H), 7.01 (s, 1H, Ar H), 7.05 (s,
1H, Ar H), 7.18 (s, 1H, Ar H), 7.31 (s, 1H, Ar H). IR (cm-1):
3530 (m, νOH), 2316 (w, νCN), 2289 (w, νCN). Anal. Calcd for
C36H47NO3Cl3Nb: C, 58.35; H, 6.39; N, 1.89. Found: C, 58.02;
H, 6.33; N, 1.62.
Me), 2.56 (s, 6H, Me), 3.3 (br, 2H, CH2), 4.4 (br, 2H, CH2), 6.83
(s, 2H, Ar H), 6.94 (s, 2H, Ar H), 7.04 (s, 2H, Ar H). Anal. Calcd
for C36H49O3S2Nb: C, 62.96; H, 7.19; S, 9.34. Found: C, 62.71;
H, 7.09; S, 8.97.
Analysis of the Fluxionality of 8 in Solution. Complex 8 (15
mg) was dissolved in toluene-d8, and the NMR tube was sealed
under an argon atmosphere. 1H NMR spectra were taken at 9
temperatures ranging from -20 to 80 °C. Line shape analysis of
the methylene region was carried out at each temperature using
the gNMR program.16 The k1 values obtained at given temperature
are as follows: -19.9 °C, 17; -9.7 °C, 42; -0.2 °C, 88; 19.9 °C,
5.6 × 102; 30 °C, 9.5 × 102; 40 °C, 2.1 × 103; 50 °C, 4.0 × 103;
Synthesis of Nb[H(tBu-L)]2Cl(NCMe) (5). (a) From Nb[H(tBu-
L)]Cl3(NCMe). Complex 4 (0.47 g, 0.32 mmol) was dissolved into
CH3CN (60 mL). The solution was allowed to stand at room
temperature for 28 h, yielding 0.12 g of orange needles 5‚CH3CN
(31% based on the ligand). Solubility constraints prevent us from
1
acquiring the H NMR spectrum of 5. IR (cm-1): 3516 (s, νOH),
(16) Budzellar, P. H. M. gNMR for Windows Version 4.1; Cherwell
2280 (w, νCN), 2251 (w, νCN, free CH3CN). Anal. Calcd for
Scientific Ltd.: Oxford, U.K., 1999.
6092 Inorganic Chemistry, Vol. 41, No. 23, 2002

Niobium Complexes of Aryloxide Trimers
t
Table 1. Crystallographic Data for [Nb(R-L)Cl2]2 (R ) Me (1), Bu (2)), [Nb(de-tBu-LMe)2Cl2]2 (3b), Nb[H(tBu-L)]Cl3(NCMe) (4),
Nb[H(tBu-L)]2Cl(NCMe) (5), [Nb(tBu-L)Cl2]2 (6), and Nb(Me-L)(StBu)2 (8)
1‚2CH2Cl2
2‚C7H8
3b‚2C7H8
4
5‚CH3CN
6‚2C4H10
O
8‚1/2C7H8
formula
Mr
T/K
C60H66O6Cl8Nb2
1352.61
193
C75H94O6Cl4Nb2
1419.19
123
C68H74O6Cl4Nb2
1314.95
173
C36H47NO3 Cl3Nb C72H94N2O6ClNb
C68H94O8Nb2
1225.30
173
C39.5H53O3 S2Nb
732.88
173
741.04
173
1211.90
173
cryst size/mm
cryst system
space group
a/Å
b/Å
c/Å
0.30 × 0.20 × 0.02 0.15 × 0.05 × 0.05 0.40 × 0.25 × 0.20 0.35 × 0.15 × 0.10 0.15 × 0.04 × 0.02 0.30 × 0.20 × 0.20 0.25 × 0.25 × 0.20
monoclinic
C2/c (No. 15)
27.4464(13)
16.640(2)
triclinic
P1h (No. 2)
12.621(6)
12.594(4)
14.256(6)
66.44(2)
85.86(2)
64.99(2)
1870(1)
1
triclinic
P1h (No. 2)
10.607(3)
14.566(4)
21.501(6)
85.513(9)
89.343(9)
71.501(5)
3140.2(13)
2
monoclinic
P21/n (No. 14)
18.921(2)
monoclinic
P21/n (No. 14)
13.2637(10)
17.850(1)
monoclinic
P21/n (No. 14)
11.841(4)
monoclinic
P21/n (No. 14)
9.772(2)
9.3838(9)
22.820(3)
15.601(5)
17.967(7)
27.527(6)
14.509(3)
13.7110(4)
29.325(2)
R/deg
â/deg
103.7695(9)
115.228(4)
93.814(4)
91.094(4)
94.385(3)
γ/deg
V/Å3
6081.8(5)
4
1.477
3665.2(7)
4
1.343
5.81
6927.4(8)
4
1.162
2.61
3318.4(18)
2
1.226
3.95
3891.5(13)
4
1.251
4.50
Z
Dc/g cm-3
1.260
4.97
1.391
5.85
µ(Mo KR)/cm-1 7.76
reflcns collcd
unique reflcns
28 867
6956
14 249
8130
428
24 469
13 680
795
29 352
8247
435
55 006
15 854
839
24 368
7504
399
30 321
8879
473
refined params 352
GOF on F2
1.05
1.00
0.084
0.242
1.04
0.039
0.098
1.01
0.067
0.200
1.02
0.072
0.230
1.04
0.036
0.092
1.02
0.044
0.130
R1 [I > 2σ(I)]a 0.060
wR2 (all data)b 0.169
b
2
-
2
2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. wR2 ) [Σ(w(Fo
Fc )2/Σw(Fo )2]1/2
.
60 °C, 7.8 × 103; 80 °C, 2.4 × 104. Activation parameters were
determined from Eyring plots of the above data.
ArOM (M ) Li, Na, K), or ArOSiMe3. In this study, we
began to examine the reactions between NbCl5 and H3(R-
L). The course and outcome of the reaction chemistry are
dependent on the identity of the outer aryloxide ring
substituents of the tridentate R-L3- ligands. The reaction of
NbCl5 with 1 equiv of H3(Me-L) in toluene under reflux led
to elimination of HCl and formation of the chloride complex
[Nb(Me-L)Cl2]2 (1) as brown crystals in 88% isolated yield.
Spectroscopic data and combustion analysis of 1 are in
agreement with its formulation. In the 1H NMR spectrum of
1, the methylene proton signals appear as a pair of doublets
along with two methyl and one tert-butyl signals, indicating
the rigid and symmetric conformation of the Me-L ligand.
In addition, 1 was characterized by X-ray structural analysis.
The molecular structure of 1 is shown in Figure 1, and
X-ray Crystallography. Crystallographic data are summarized
in Table 1. X-ray-quality single crystals were obtained from CH2-
Cl2/hexane (for 1‚2CH2Cl2 as brown blocks), toluene (for 2‚C7H8,
3b‚2C7H8, and 8 as red needles, wine-red blocks, and orange blocks,
respectively), CH3CN (for 4 and 5‚CH3CN as red blocks and orange
needles, respectively), and Et2O (for 8 as yellow blocks). Crystals
were immersed in Paraton-N oil on a nylon loop and transferred to
a Rigaku Mercury CCD diffractometer equipped with a Rigaku
GNNP low-temperature device. Data were collected under a cold
nitrogen stream using graphite monochromated Mo KR radiation
(λ ) 0.710 70 Å). Equivalent reflections were merged, and the
images were processed with the CrystalClear (Rigaku) program.17
Corrections for Lorentz-polarization effects and absorption were
performed.
All structures were solved by direct methods and refined on F2
by the full-matrix least-squares method using CrystalStructure
(Rigaku) software package.18 In the case of 1, 2, and 4, methyl
carbons of the para-tert-butyl group of the R-L ligand were
disordered with occupancy factors of 50:50. The crystal solvents
in 1‚2CH2Cl2, 2‚C7H8, and 8‚1/2C7H8 were disordered over two
site positions, and the individual components were resolved and
refined. Anisotropic refinement was applied to all non-hydrogen
atoms except for the distorted atoms. The disordered atoms were
isotropically refined, and no hydrogen atoms on the disordered
atoms were included. The phenolic OH protons for 4 and 5 were
located from Fourier difference syntheses and isotropically refined,
while the other hydrogen atoms were put at calculated positions
with C-H distances of 0.97 Å. Additional crystallographic data
are given in the Supporting Information.
selected bond distances and angles are given in Table 2. The
metal center is in a pseudooctahedral environment consisting
of one Me-L tridentate ligand, one central aryloxide [O(2A)]
of another Me-L ligand, and two mutually cis chlorides. The
tridentate Me-L ligand meridionally coordinates to niobium
in the U-conformation (Scheme 1), which is consistent with
its 1H NMR spectrum. The two Nb(Me-L)Cl2 fragments are
related by a crystallographic inversion center, and the Nb2O2
core is thus required to be planar. A notable feature of the
tridentate Me-L3- ligand in the U-conformation concerns the
narrow Nb-O(2)-C(8) angles of the central aryloxide
[108.2(3)°] relative to the corresponding angles of the outer
aryloxides (average 150.6°), because of the constrained
Results and Discussion
t
Synthesis of [Nb(R-L)Cl2]2 [R ) Me (1), Bu (2)].
Aryloxides complexes of early transition metals are typically
synthesized by reactions of metal chlorides with either ArOH,
(17) (a) CrystalClear Software Package, Rigaku and Molecular Structure
Corp., 1999. (b) Pflugrath, J. W. Acta Crystallogr. 1999, D55, 1718-
1725.
(18) Crystal Structure Analysis Package, Rigaku and Molecular Structure
Corp., 2001.
Inorganic Chemistry, Vol. 41, No. 23, 2002 6093

Matsuo and Kawaguchi
there are four doublets due to the methylene linkers. After
the crude product was washed with toluene/hexane, 3a was
obtained as analytically pure wine-red microcrytals in 72%
yield. Unfortunately, attempts to isolate 2 from the reaction
mixture have yet to be successful. However, the structures
of both 2 and 3a have been determined by X-ray diffrac-
tion.
Figure 1. Molecular structure of [Nb(Me-L)Cl2]2 (1). One set of the
disordered methyl carbons of tert-groups of the Me-L ligands is shown.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[Nb(Me-L)Cl2]2 (1), [Nb(tBu-L)2Cl2]2 (2), and [Nb(Me-L)Me2]2 (6)
1
2
6
The structure of 2 is a cenrtosymmetric dimer analogous
to 1. Selected bond distances and angles are given in the
second column of Table 2. The slight differences observed
between 1 and 2 are due to the nature of the substituents at
ortho positions of the outer aryloxide rings, possibly generat-
ing slightly differing steric and electronic effects. In com-
parison with 1, the Nb-Nb separation is 0.095 Å longer in
2. A corresponding trend is observed in the planar Nb2O2
dimer ring, and the Nb-O(2A) distance of 2 is elongated
relative to that of 1. In contrast, the Nb-O(2) distance of 2
[2.058(4) Å] is slightly shorter than that of 1 [2.097(4) Å].
This is ascribed to the more obtuse O(1)-Nb-O(3) angle
in the linked aryloxide trimer [163.1(2)° in 1 vs. 168.0(2)°
in 2].
Nb-O(1)
Nb-O(2)
Nb-O(3)
Nb-O(2A)
Nb-Nb(A)
Nb-Cl(1)
Nb-Cl(2)
Nb-C(29)
Nb-C(30)
1.874(4)
2.097(4)
1.862(4)
2.151(4)
3.554(1)
2.352(2)
2.327(2)
1.902(4)
2.058(4)
1.904(4)
2.237(4)
3.649(1)
2.365(2)
2.335(2)
1.899(2)
2.061(2)
1.886(2)
2.320(2)
3.7095(9)
2.156(2)
2.189(3)
O(1)-Nb-O(2)
O(2)-Nb-O(3)
O(1)-Nb-O(3)
O(2)-Nb-O(2A)
Nb-O(2)-Nb(A)
Nb-O(1)-C(1)
Nb-O(2)-C(8)
Nb-O(3)-C(15)
Nb(A)-O(2)-C(8)
Cl(1)-Nb-Cl(2)
C(29)-Nb-C(30)
86.9(2)
86.3(2)
91.5(2)
90.7(2)
89.34(6)
90.42(7)
168.42(7)
64.41(6)
115.59(6)
151.2(1)
101.45(12)
155.9(2)
143.0(1)
163.1(2)
66.39(16)
113.61(16)
148.0(4)
108.2(3)
153.1(4)
138.1(3)
96.54(6)
168.0(2)
63.69(16)
116.31(16)
151.6(4)
102.6(3)
153.0(4)
141.1(3)
104.59(6)
In the case of 3a, the disorder problem prevents us from
discussing its metric parameters in detail.19 Thus the Bu-
101.21(11)
t
LMe derivative, which has the methyl group at the para
position of the central aryloxide (Scheme 1), was chosen
for a structural study. Complex [Nb(de-tBu-LMe)Cl2]2 (3b)
was prepared by the analogous reaction of NbCl5 with H3-
(tBu-LMe) in toluene. Complete solution of X-ray diffraction
data for 3b was possible (Figure 2), and the molecular
structure is similar to that of 3a as determined from the
preliminary X-ray analysis. Selected bond distances and
angles are listed in Table 3. The structure of 3b shows a
binuclear geometry with octahedral niobium centers, each
possessing four aryloxides and two mutually trans chlorides.
The most notable feature in the structure is that the two
ligands adopt an S-conformation and span both metal centers.
The niobium atoms are bridged by the two central aryloxides
of the ligand. This conformation of the tridentate ligands in
3b contrasts with that observed for 1 and 2. The remaining
tert-butyl groups of the de-tBu-LMe ligands in 3b are located
away from each other, resulting in a solid-state molecular
geometry with approximately C2 symmetry. We note that
geometry of the linked ligand. The bridging Nb-O dis-
tances are asymmetric, and the Nb-O(2) distance
[2.097(4) Å] is shorter than the Nb-O(2A) distance
[2.151(4) Å]. As expected, the bridging Nb-O distances
(average 2.124 Å) are longer than the terminal Nb-O
distances (average 1.868 Å). The Nb-Cl(1) distance of
2.352(2) Å is elongated relative to the Nb-Cl(2) distance
of 2.327(2) Å, reflecting the stronger Nb-O(2) interaction
trans to Cl(2) in comparison with the Nb-O(2A) interaction
trans to Cl(1).
When H3(tBu-L) was used instead of H3(Me-L), the
analogous reaction gave a red solution. After removal of
volatiles in vacuo, analysis of the crude products by 1H NMR
spectroscopy showed resonances corresponding to the forma-
tion of [Nb(tBu-L)Cl2]2 (2) and [Nb(de-tBu-L)Cl2]2 (3a) in
1
the intensity ratio of 15:85. According to H NMR spec-
troscopy, we found no rapid conversion of 2 into 3a. The
1H NMR spectrum of 2 is analogous to that of 1 and sug-
gests that these compounds possess similar structures. In the
1H NMR spectrum of 3a, the presence of the asymmetric
de-tBu-L ligand leads to two methyl and two tert-butyl
singlets with relative integrations of 1:1:3:3. Additionally,
(19) Crystal data for 3a: monoclinic, P21/a (No. 14), a ) 16.696(5) Å, b
) 22.044(7) Å, c ) 22.570(7) Å, â ) 103.783(5)°, V ) 8067.7(4)
Å3, Z ) 4.
6094 Inorganic Chemistry, Vol. 41, No. 23, 2002

Niobium Complexes of Aryloxide Trimers
Figure 3. Molecular structure of Nb[H(tBu-L)]Cl3(NCMe) (4). One set
t
of the disordered methyl carbons of para tert-groups of the Bu-L ligands
is shown.
Table 4. Selected Bond Distances (Å) and Angles (deg) for
Nb[H(tBu-L)]Cl3(NCMe) (4)
Figure 2. Molecular structure of [Nb(de-tBu-LMe)Cl2]2 (3b).
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[Nb(de-tBu-LMe)2Cl2]2 (3b)
Nb-O(1)
Nb-O(2)
Nb-N
1.839(3)
1.847(3)
2.274(4)
1.142(6)
87.12(6)
95.7(1)
81.57(11)
95.14(11)
85.80(10)
90.17(11)
93.3(1)
Nb-Cl(1)
Nb-Cl(2)
Nb-Cl(3)
2.386(2)
2.409(1)
2.363(2)
Nb(1)-O(1)
Nb(1)-O(2)
Nb(1)-O(5)
Nb(1)-O(6)
Nb(1)-Cl(1)
Nb(1)-Cl(2)
Nb(1)-Nb(2)
1.852(2)
2.093(2)
2.136(2)
1.839(2)
2.391(1)
2.420(1)
3.4721(3)
Nb(2)-O(2)
Nb(2)-O(3)
Nb(2)-O(4)
Nb(2)-O(5)
Nb(2)-Cl(3)
Nb(2)-Cl(4)
2.133(2)
1.847(2)
1.846(2)
2.118(2)
2.402(1)
2.412(1)
N-C(35)
Cl(1)-Nb-Cl(2)
Cl(1)-Nb-O(1)
Cl(1)-Nb-N
Cl(2)-Nb-O(1)
Cl(2)-Nb-N
Cl(1)-Nb-Cl(3)
Cl(1)-Nb-O(2)
Cl(2)-Nb-Cl(3)
Cl(2)-Nb-O(2)
Cl(3)-Nb-O(1)
Cl(3)-Nb-N
O(1)-Nb-N
Nb-N-C(35)
Nb-O(2)-C(8)
163.94(5)
92.83(11)
87.60(6)
171.48(10)
99.8(1)
82.93(11)
177.1(2)
173.3(4)
161.3(3)
Cl(3)-Nb-O(2)
O(1)-Nb-O(2)
O(2)-Nb-N
O(1)-Nb(1)-O(2)
O(1)-Nb(1)-O(5)
O(1)-Nb(1)-O(6)
O(2)-Nb(1)-O(5)
O(2)-Nb(1)-O(6)
O(5)-Nb(1)-O(6)
Cl(1)-Nb(1)-Cl(2)
Nb(1)-O(1)-C(1)
Nb(1)-O(2)-C(8)
97.01(7)
159.17(7)
105.31(8)
70.13(6)
154.52(7)
91.39(7)
175.33(2)
156.1(2)
O(2)-Nb(2)-O(3)
O(2)-Nb(2)-O(4)
O(2)-Nb(2)-O(5)
O(3)-Nb(2)-O(4)
O(3)-Nb(2)-O(5)
O(4)-Nb(2)-O(5)
Cl(3)-Nb(2)-Cl(4)
Nb(2)-O(2)-C(8)
92.65(7)
157.00(7)
69.72(6)
105.90(7)
153.58(7)
96.55(7)
174.21(2)
126.02(13)
151.1(2)
85.76(13)
158.4(3)
Nb-O(1)-C(1)
t
butylation reaction of the Bu-L ligand. This proved to be
the case, as shown in eq 3.
123.51(13) Nb(2)-O(3)-O(15)
Nb(1)-O(5)-C(35) 126.15(13) Nb(2)-O(4)-O(28)
156.1(2)
Nb(1)-O(6)-C(42)
Nb(1)-O(2)-Nb(2)
155.1(2)
110.46(7)
Nb(2)-O(5)-O(35) 124.40(13)
Nb(1)-O(5)-Nb(2) 109.41(7)
the tBu-L3- ligand cannot coordinate in a manner similar to
those of de-tBu-L and de-tBu-LMe found in 3a,b, because of
steric congestion between the bulky tert-butyl groups. The
terminal and bridging Nb-O distances average 1.846 and
2.120 Å, respectively, which are comparable to the corre-
sponding distances found in 1 and 2. The Nb-Nb separation
in 3b is 3.4721(3) Å, and this is significantly shorter than
those in 1 and 2.
The reaction of NbCl5 with 1 equiv of H3(tBu-L) in
acetonitrile under reflux afforded Nb[H(tBu-L)]Cl3(CH3CN)
(4), which was isolated as dark red crystals in 57% yield by
recrystallization from CH2Cl2/hexane. The 1H NMR spectrum
of the crude reaction mixture showed no evidence for the
formation of 2 or 3a. The formulation of 4 was inferred from
the spectroscopic data and combustion analysis, and the
structural data were obtained by X-ray analysis (Figure 3,
selected bond distances and angles in Table 4). Molecules
of 4 contain a pseudooctahedral niobium(V) center with a
acetonitrile ligand and three meridionally arranged chloride
ligands. The coordination sphere is completed by a bidentate
The reaction between NbCl5 and H3(tBu-L) involves loss
t
of one of the tert-butyl groups of Bu-L3-, resulting in the
formation of 3a. The de-tert-butylation occurred regioselec-
tively at one of the ortho positions of the ligand. The leaving
tert-butyl group is captured by a Cl- nucleophile, because
2-chloro-2-methylpropane was detected in the reaction
mixture by 1H NMR and GC-MS spectra. This retro-Friedel-
Crafts reaction is one example of what is likely to be a
general tendency to lose a tert-butyl group in some manner
in calixarene and phenol derivatives.20 We felt that the retro-
Friedel-Crafts process was promoted by the acidic Nb(V)
metal center and speculated that a less acidic niobium
compound [NbCl5(NCCH3)] might prevent the de-tert-
(20) (a) Takeshita, M.; Nishio, S.; Shinkai, S. J. Org. Chem. 1994, 59,
4032-4034. (b) Kanamathareddy, S.; Gutsche, C. D. J. Org. Chem.
1996, 61, 2511-2516. (c) de Mendoza, J.; Carramolino, M.; Cuevas,
F.; Nieto, P. M.; Prados, P.; Reinhoudt, D. N.; Verboom, W.; Ungaro,
R.; Casnati, A. Synthesis 1994, 47-50. (d) Sartori, G.; Bigi, F.; Maggi,
R.; Porta, C. Tetrahedron Lett. 1994, 35, 7073-7076.
Inorganic Chemistry, Vol. 41, No. 23, 2002 6095

Matsuo and Kawaguchi
H(tBu-L)2- diphenoxide-monophenol ligand, and one outer
phenolic oxygen [O(3)] is not bound to niobium. The Nb-O
and Nb-Cl distances are within the ranges found in 1, 2,
and 3b. The Nb-N distance of 2.274(4) Å is comparable to
that of NbCl5(NCMe) (2.236(4) Å),21 and the binding of the
acetonitrile molecule is linear [Nb-N-C(35) of 173.3(4)°
and N-C(35)-C(36) of 179.8(6)°].
Compound 4 was characterized by 1H NMR and IR
spectroscopy. A characteristic feature of the 1H NMR
spectrum in CDCl3 is the presence of four methylene signals
along with three tert-butyl and two methyl signals, reflecting
the asymmetric coordination of the ancillary H(tBu-L)2-
ligand. The signal of the OH proton accidentally overlaps
with one of methylene signals (δ 4.22), but the IR spectrum
clearly exhibits the OH stretching vibration at 3530 cm-1.
The presence of the acetonitrile molecule is characterized
by the ν(CN) IR absorptions at 2316 and 2289 cm-1, which
are slightly higher than that of free acetonitrile (2251 cm-1).
This small increase in ν(CN) stretching frequency is con-
sistent with η1-coordination of acetonitrile.22
Figure 4. Molecular structure of Nb[H(tBu-L)]2Cl(NCMe) (5).
As expected, the retro-Friedel-Crafts reaction was inhib-
ited by the presence of acetonitrile in the NbCl5/H3(tBu-L)
reaction. However, once isolated, analytically pure 4 is found
to be labile in solution. For example, upon heating a benzene-
d6 solution of 4 at 80 °C in sealed tube, we noticed the
Table 5. Selected Bond Distances (Å) and Angles (deg) for
Nb[H(tBu-L)]2Cl(NCMe) (5)
Nb-O(1)
Nb-O(2)
Nb-O(4)
Nb-O(5)
1.869(4)
1.919(4)
1.898(4)
1.933(4)
Nb-Cl
Nb-N(1)
N1-C(69)
2.480(2)
2.296(5)
1.140(8)
1
formation of 2 and 3a in a 1:4 ratio according to the H
O(1)-Nb-O(2)
O(1)-Nb-O(5)
O(1)-Nb-N(1)
O(2)-Nb-O(5)
O(2)-Nb-N(1)
O(4)-Nb-Cl
94.6(2)
93.2(2)
171.3(2)
168.4(2)
92.2(2)
159.27(12)
86.88(12)
80.4(1)
159.3(4)
153.2(4)
O(1)-Nb-O(4)
O(1)-Nb-Cl
105.6(2)
94.81(13)
91.0(2)
83.86(12)
95.3(2)
79.7(2)
79.3(2)
153.6(5)
147.0(4)
145.5(4)
NMR spectra. On the other hand, upon standing an aceto-
nitrile solution of 4 at room-temperature resulted in the
precipitation of Nb[H(tBu-L)]2Cl(NCMe) (5) as orange
plates, where a ligand redistribution reaction took place. This
complex was alternatively synthesized in 91% yield by
treating NbCl5 with 2 equiv of H3(tBu-L) in CH3CN at 80
°C for 3 h. The combustion analysis of 5 was in agreement
with the proposed formulation, while the IR spectrum
exhibits ν(OH) and ν(CN) bands at 3516 and 2280 cm-1,
respectively. However, we were unable to obtain other
spectroscopic data for 5 due to its insolubility in common
organic solvents. Single crystals of 5 suitable for X-ray
diffraction were obtained from a dilute CH3CN solution of
4 at room temperature.
O(2)-Nb-O(4)
O(2)-Nb-Cl
O(4)-Nb-O(5)
O(4)-Nb-N(1)
O(5)-Nb-N(1)
Nb-N(1)-C(69)
Nb-O(2)-C(8)
Nb-O(5)-C(42)
O(5)-Nb-Cl
Cl-Nb-N(1)
Nb-O(1)-C(1)
Nb-O(4)-C(35)
The molecular structure of 5 is presented in Figure 4, and
selected bond distances and angles are given in Table 5. The
structure shows that two diphenoxide-monophenol ligand
are bound to niobium in a bidentate fashion. Like 4, a
phenolic group of each H(tBu-L) ligand [O(3) and O(6)] acts
as a pendant arm. The 2.63(7) Å O(3)-H‚‚‚Cl and 3.278(5)
Å O(3)‚‚‚Cl distances found in 5 are appropriate for
intramolecular hydrogen bonding, and the O(3)-H‚‚‚Cl angle
is 152(8)°. The geometry at niobium is best described as an
octahedron, and the nitrile and chloride ligands are coordi-
nated to the niobium center cis to each other. The Nb-O
distances range from 1.869(4) to 1.933(4) Å, with a slight
elongation of the Nb-O bonds trans to O [O(2) and O(5)].
The Nb-N and Nb-Cl distances are relatively long com-
pared to those in 4, reflecting the increased number of strong
aryloxide donors. The bent Nb-N(1)-C(69) angle [153.6(5)°]
implies a sterically crowed niobium center in the solid state.
The elimination of hydrochloric acid as a byproduct from
the reaction of NbCl5 with H3(tBu-L) causes the degradation
t
of the Bu-L ligand, so we have sought an alternative
synthetic method for the desired compound 2. Scott et al.
have reported the preparations of solvated lithium salts of
linked aryloxide trimers in THF and Et2O. For our purpose,
(21) Willey, G. R.; Woodman, T. J.; Drew, M. G. B. Polyhedron 1997,
16, 351-353.
(22) Farona, M. F.; Kraus, K. F. Inorg. Chem. 1970, 9, 1700-1704.
6096 Inorganic Chemistry, Vol. 41, No. 23, 2002

Niobium Complexes of Aryloxide Trimers
Figure 5. Molecular structure of [Nb(Me-L)Me2]2 (6).
treatment of H3(tBu-L) with 3 equiv of butyllithium in toluene
afforded unsolvated Li3(tBu-L) as a white powder. The
conversion of H3(tBu-L) to trilithio salts was confirmed by
its IR spectrum, which showed no ν(OH) absorption. The
stoichiometric reaction of Li3(tBu-L) with NbCl5 in toluene
under reflux produced 2 as red crystals. Multigram quantities
of the tBu-L derivative 2 were prepared in 41% isolated yield
through this metathesis.
Figure 6. Molecular structure of Nb(Me-L)(StBu)2 (8).
the solid state and the molecule lies across a crystallographic
inversion center. The structure of 6 is broadly analogous to
that of 1 with the methyl groups replacing the chloride
ligands, although the dimer of 6 appears to possess a
considerably weaker bridging niobium-oxygen interaction
as judged by the Nb-O(2A) distances [2.320(2) Å in 6 but
2.151(4) Å in 1]. This reflects the stronger σ donor property
of the methyl ligand relative to the chloride ligand. The Nb-
Me distances of 2.156(2) and 2.189(3) Å are in agreement
with the values quoted in the literature for niobium-alkyl
bonds.23
Synthesis of Methyl and tert-Butyl Mercaptan Deriva-
tives. The chloride complexes 1 and 2 proved to be versatile
precursors to organometallic and coordination complexes via
halide displacement reactions with MeMgI and LiStBu.
Addition of 4 equiv of MeMgI in Et2O to the chloride
complexes in toluene at -78 °C followed by standard
workup afforded [Nb(R-L)Me2]2 [R ) Me (6), 44%; tBu (7),
39%] as yellow crystals. The formation of the methyl
complexes was found to be sensitive to the alkylating reagent,
as the analogous reactions with methyllithium resulted in a
mixture of 6 (or 7) and the corresponding solvated lithium
salt of R-L3-.13 Alkylation of 4 with 3 equiv of MeMgI
provided an alternative route to the target methyl complex
7 in 30% yield. These methyl complexes, 6 and 7, were
characterized by NMR spectra and combustion analyses. In
the 1H NMR spectra, the resonance patterns due to the R-L3-
ligand are entirely analogous to those of the parent com-
plexes, 1 and 2. The Nb-CH3 resonances appear as in-
equivalent signals at δ -0.42 and 1.27 for 6 and at δ 0.03
and 1.67 for 7, indicating a rigid conformation of the R-L
ligand in solution. Single crystals of 6 suitable for an X-ray
analysis were grown from a saturated Et2O solution at -30
°C.
We then examined the reaction of the Me-L complex 1
with tBuSLi, anticipating that the bulky thiolate ligand might
affect the coordination mode of the tridentate Me-L ligand.
While the reaction of 1 with 4 equiv of tBuSLi in THF gave
complicated mixtures, Nb(Me-L)(StBu)2 (8) was obtained as
orange crystals in 76% yield when the reaction was carried
out in toluene at 60 °C. Although attempts to prepare mixed
thiolate/alkoxide complexes frequently result in complicated
mixturers,24 8 is robust toward ligand redistribution reactions.
In contrast to the dimeric structures of the Me-L complexes
1 and 6, the structural analysis confirms the monomeric
nature of 8 and the coordination geometry of the complex is
best described as a trigonal bipyramidal (Figure 6, selected
bond distances and angles in Table 6). The ancillary Me-L
ligand assumes an S-shaped conformation and is bound in a
meridional fashion with the outer aryloxide donors [O(1) and
O(3)] occupying axial positions. The greatest deviation from
ideal trigonal-bipyramidal geometry is evident in the axial
O(1)-Nb-O(3) bond angle of 166.40(8)°. The equatorial
sites comprise two thiolate ligands and the central aryloxide
(23) (a) Pugh, S. M.; Tro¨sch, J. M.; Shinner, E. G.; Gade, L. H.; Mountford,
P. Organometallics 2001, 20, 3531-3542. (b) Isoz, S.; Floriani, C.;
Schenk, K.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1996, 15,
337-344. (c) Schweiger, S. W.; Salberg, M. M.; Pulvirenti, A. L.;
Freeman, E. E.; Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc., Dalton
Trans. 2001, 2020-2031. (d) Jaffart, J.; Etienne, M.; Maseras, F.;
McGrady, J. E.; Eisenstein, O. J. Am. Chem. Soc. 2001, 123, 6000-
6013.
Structural analysis revealed the molecular geometry for 6
shown in Figure 5, and selected bond distances and angles
are given in the third column of Table 2. Like 1, the
compound 6 exists as a central-aryloxide-bridged dimer in
(24) (a) Firth, A. V.; Witt, E.; Stephan, D. W. Organometallics 1998, 17,
3716-3722. (b) Chisholm, M. H.; Davidson, E. R.; Huffman, J. C.;
Quinlan, K. B. J. Am. Chem. Soc. 2001, 123, 9652-9664.
Inorganic Chemistry, Vol. 41, No. 23, 2002 6097

Matsuo and Kawaguchi
Table 6. Selected Bond Distances (Å) and Angles (deg) for
Nb(Me-L)(StBu)2 (8)
display two methyl and one tert-butyl singlets associated with
the Me-L ligand, and the two tert-butyl mercaptan groups
are in a chemically equivalent environment. On the other
hand, the methylene protons are seen as two broad signals
(δ 3.66 and 4.89) at 10 °C. On cooling of the sample to
-20 °C, the methylene protons are observed as a pair of
mutually coupled doublets (δ 3.66 and 4.91; J ) 13.4 Hz),
which is consistent with the molecule possessing effective
C2 symmetry with a 2-fold axis (on the NMR time scale)
bisecting the S-Nb-S angle. On warming of the sample to
35 °C, these coalesce into one single peak. Line shape
analysis for methylene resonances of the Me-L ligand gives
the following activation parameters: ∆Hq ) 51.8 kJ mol-1;
∆Sq ) -16.2 J mol-1 K-1.16 Thus, the activation parameters
favor a nondissociative rearrangement for the exchange
process, possibly with a somewhat ordered transition state.
Nb-O(1)
Nb-O(2)
Nb-O(3)
1.901(2)
1.928(2)
1.910(2)
Nb-S(1)
Nb-S(2)
2.357(1)
2.389(1)
S(1)-Nb-S(2)
S(1)-Nb-O(2)
S(2)-Nb-O(1)
S(2)-Nb-O(3)
O(1)-Nb-O(3)
Nb-O(1)-C(9)
Nb-O(2)-C(16)
Nb-O(3)-C(23)
114.21(3)
112.35(6)
87.60(6)
93.67(6)
166.40(8)
163.3(2)
131.2(2)
140.5(2)
S(1)-Nb-O(1)
S(1)-Nb-O(3)
S(2)-Nb-O(2)
O(1)-Nb-O(2)
O(2)-Nb-O(3)
Nb-S(1)-C(1)
Nb-S(2)-C(5)
102.54(6)
89.30(6)
133.11(6)
87.65(8)
81.60(8)
115.88(11)
115.85(11)
group [O(2)], and the equatorial NbOS2 is planar to within
0.07 Å. The equatorial Nb-O(2) bond of 1.928(2) Å is
slightly elongated relative to the axial Nb-O bonds of
1.901(2) and 1.910(2) Å, while the Nb-O(1)-C(9) angle
of 163.3(2)° is larger than those at O(2) and O(3) [131.2(2)
and 140.5(2)°]. The dihedral angle between the NbO3 plane
and the central aryloxide ring is 60.0°. This twist in the
methylene-linked Me-L ligand may be necessary to maintain
a trigonal bipyramidal environment for the Nb center. The
Nb-S distances of 2.357(1) and 2.389(1) Å compare with
the equatorial Nb-S distances in the trigonal bipyramidal
complex (NEt4)[Nb(S)(StBu)4] [ranging from 2.364(6) to
2.407(2) Å].25
Conclusion
In this work, we have provided full details of the
preparation and structures of a series of Nb(V) complexes
that incorporate the acyclic linked aryloxide trimers R-L3-
t
(R ) Me, Bu) and de-tBu-L3-. The chloride complexes 1,
2, and 3a provide useful entry points to new organometallic
and coordination complexes of niobium. The chloride ligands
in 1 and 2 readily reacted with MeMgI and tBuSLi, resulting
in 6-8. The flexible nature of the R-L ligand is revealed by
the crystal structure of the κ2-bound compounds 4 and 5 and
the formation of U-shaped (1, 2, 6, and 7) and S-shaped (3a,b
and 8) compounds. Studies of the reaction chemistry of
transition metal complexes supported by the R-L ligand
systems are underway and will be reported in due course.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (Nos. 14703008 and 13740347)
from the Ministry of Education, Science, Sports, and Culture
of Japan.
Supporting Information Available: Listings of X-ray crystal-
lographic files in CIF format for the complexes of 1, 2, 3b, 4-6,
and 8. This material is available free charge via the Internet at http://
pubs.acs.org.
1
The thiolate derivative 8 is fluxional in solution. The H
NMR spectra in toluene-d8 between -80 °C and 80 °C
(25) Coucouvanis, D.; Al-Ahmand, S.; Kim, C. G.; Koo, S.-M. Inorg. Chem.
1992, 31, 2996-2998.
IC025882C
6098 Inorganic Chemistry, Vol. 41, No. 23, 2002