Organometallic cobalt(II) and nickel(II) complexes supported by thioether ligation: Unexpected nickel alkylation by the borato ligand phenyltris((tert-butylthio)methyl)borate [1]

Full text of DOI:10.1021/ja002638g

J. Am. Chem. Soc. 2001, 123, 331-332
331
Communications to the Editor
Organometallic Cobalt(II) and Nickel(II) Complexes
Supported by Thioether Ligation: Unexpected Nickel
Alkylation by the Borato Ligand
Phenyltris((tert-butylthio)methyl)borate
Peter J. Schebler, Beaven S. Mandimutsira,
Charles G. Riordan,* Louise M. Liable-Sands,
Christopher D. Incarvito, and Arnold L. Rheingold
Department of Chemistry and Biochemistry
UniVersity of Delaware, Newark, Delaware 19716
ReceiVed July 19, 2000
The organometallic chemistry of cobalt and nickel is well
developed and of great utility in a wide diversity of stoichiometric
and catalytic transformations.1 However, few complexes have
been prepared and structurally authenticated in which the orga-
nometalloid fragment is supported by thioether donors.2 This is
surprising given the importance of this ligand combination in
biological3 and industrial4 catalysis. For example, the NiFe4S4
enzyme acetyl CoA synthase (ACS) catalyzes the reversible
formation of acetyl-CoA from methyl, CO, and CoA groups via
a series of organometalloid intermediates.5 The general scarcity
of such ligand combinations prompted us to utilize the borato
ligands, [PhTtR], in this pursuit. Our recently reported [PhTttBu
]
ligand6 seemed to provide an ideal framework, permitting access
to synthetic precursors of the type [PhTttBu]MCl in which three
thioether substituents coordinate in a facial array.7 Preparative
manipulations using the chloride derivatives were designed to
yield the target Td [PhTttBu]M(R) complexes. The decision to
prepare Td organometallic derivatives is additionally motivated
by the scarcity of these divalent late metal species and, therefore,
the potential to elucidate their novel molecular and electronic
structures and patterns of reactivity.8 Specifically, Td nickel alkyls
are without precedent. Furthermore, the target complexes are
expected to be paramagnetic and of low (<18) electron counts
uncommon properties in organometallic chemistry.
Figure 1. Thermal ellipsoid plots of [PhTttBu]Co(CH3) (A) and [κ2-
PhTttBu]Ni(η2-CH2SBut) (B) at the 30% probability level with hydrogen
atoms not shown. Selected bond distances (Å) for [PhTttBu]Co(CH3): Co-
C, 2.052(3); Co-S1, 2.341(1); Co-S2, 2.341(2); Co-S3, 2.352(2).
Selected bond distances (Å) and bond angles (deg) for [κ2-PhTttBu]Ni(η2-
CH2SBut): Ni-C, 1.939(5); Ni-S1, 2.180(2); Ni-S2, 2.256(2); Ni-
S4, 2.172(2); S1-Ni-S2, 96.22(4); S2-Ni-S4, 106.92(5); S4-Ni-C16,
49.8(2).
Spectroscopic data9 are consistent with the indicated formula
confirmed by X-ray diffraction analysis.10 Paramagnetic [PhTttBu]-
Co(CH3) displays well-resolved, contact-shifted proton NMR
resonances for the tert-butyl and phenyl protons. The Co-CH3
resonance could not be detected and was presumed broadened
into the baseline. The solution magnetic moment of 5.1 µB is in
Reaction of [PhTttBu]CoCl4 with freshly prepared (CH3)2Mg
or CH3Li in THF resulted in a rapid color change from blue to
forest green, Scheme 1. Recrystallization of [PhTttBu]Co(CH3)
from concentrated pentanes produced green crystals in 85% yield.
3
accord with the S ) /2 ground state.11 The molecular structure
(1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry, 2nd ed.; University
Science Books: Mill Valley, CA, 1987.
(2) (a) Stavropoulos, P.; Carrie´, M.; Muetterties, M. C.; Holm, R. H. J.
Am. Chem. Soc. 1991, 113, 8485-8492. (b) Sellmann, D.; Schillinger, H.;
Knoch, F.; Moll, M. Inorg. Chim. Acta 1992, 198, 351-357.
(3) Ragsdale, S. W.; Riordan, C. G. J. Biol. Inorg. Chem. 1996, 1, 489-
493.
of [PhTttBu]Co(CH3) is depicted in Figure 1. Expectedly, [PhTttBu]-
Co(CH3) is isomorphous with the starting chloride complex. The
methyl group resides on the molecular pseudo 3-fold axis, B-
Co-C, 178.9°. The average Co-S bond length of 2.345 Å is
slightly longer than that for [PhTttBu]CoCl. The Co-C bond
distance is 2.052(3) Å, in the range found for other Td Co(II)
alkyls reported recently.8a,c
(4) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysts in
Catalysis: Science and Technology; Anderson, J. R., Boudart, M., Eds.;
Springer-Verlag: Berlin, 1996.
(5) Ragsdale, S. W.; Riordan, C. G. J. Biol. Inorg. Chem. 1996, 1, 489-
493.
(9) [PhTttBu]Co(CH3): 1H NMR (C6D6) δ 18.9 (br, C6H5), 10.8 (br, C6H5),
9.8 (br, C6H5), 3.2 (br, (CH3)3); UV-vis (CH2Cl2), λmax (ꢀ, M-1 cm-1) 632
(570), 658 (690), 707 (710), 930 (90).
(6) Abbreviations: [PhTttBu], phenyltris((tert-butylthio)methyl)borate; [Tpt-
Bu], hydrotris(3-tert-butylpyrazolyl)borate; [PhTptBu], phenyltris(3-tert-bu-
tylpyrazolyl)borate.
(10) X-ray data: [PhTttBu]Co(CH3), C22H41BCoS3, FW ) 471.47, mono-
clinic, P21/n, green block, a ) 9.7856(2) Å, b ) 21.6045(4) Å, c ) 12.6512-
(2) Å, â ) 99.1316(5)°, V ) 2640.73(8) Å3, Z ) 4, Z′ ) 1, T ) 295(2) K,
GOF ) 1.757, R(F) ) 4.99% for 4125 observed independent reflections (4°
e 2θ e 50°).
(7) Schebler, P.; Riordan, C. G.; Guzei, I.; Rheingold, A. L. Inorg. Chem.
1998, 37, 4754-4755.
(8) Recently, examples of late metal alkyls based on bulky [TpR] ligands
have appeared. (a) Jewson, J. D.; Liable-Sands, L. M.; Yap, G. P. A.;
Rheingold, A. L.; Theopold, K. H. Organometallics 1999, 18, 300-305. (b)
Kisko, J. L.; Hascall, T.; Parkin, G. J. Am. Chem. Soc. 1998, 120, 10561-
10562. (c) Shirasawa, N.; Akita, M.; Hikichi, S.; Moro-oka, Y. Chem.
Commun. 1999, 417-418.
(11) The experimental value exceeds the predicted spin-only moment due
to spin-orbit coupling which is significant for Td Co(II). See: Drago, R. S.
Physical Methods for Chemists, 2nd ed.; Saunders College Publishing: New
York, 1992; p 486.
10.1021/ja002638g CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/21/2000

332 J. Am. Chem. Soc., Vol. 123, No. 2, 2001
Communications to the Editor
Scheme 1
that could also be unstable with respect to borato ligand loss. To
discriminate between the two proposed decomposition routes, the
reaction of [PhTttBu]NiCl with CH3Li was carried out in the
presence of a suitable donor ligand to trap potential intermediates
(Scheme 1). Addition of P(CH3)3 or CO intercepted Ni(I)
complexes of the form [PhTttBu]Ni(L) in high yield as stable,
isolable species. Each has been characterized by X-ray diffrac-
tion.17 In [PhTttBu]Ni(CO) the CO is located on the molecular
pseudo-3-fold axis. The average Ni-S bond distance is 2.24 Å
and Ni-C is 1.754(7) Å with the Ni-C-O angle at 171.0(8)°.
[PhTttBu]Ni(L) complexes display sharp, contact-shifted 1H NMR
resonances at room temperature.18 The IR spectrum of [PhTttBu]-
Ni(CO) contains νCO at 1999 cm-1 that shifts appropriately, to
1951 cm-1, for the 13CO derivative. The value reproduces that
observed in the carbonylated form of ACS, 1999 cm-1 19
.
Additionally, Na/Hg reduction of [PhTttBu]NiCl yielded [κ2-
PhTttBu]Ni(η2-CH2SBut) in the absence of trap and [PhTttBu]Ni-
(CO) under CO trapping conditions. Taken together, the obser-
vations point to the reductive mechanism as the most likely route
to the species observed without requiring intermediacy of [PhTttBu]-
Ni(CH3).
In summary, the sterically demanding borato ligand, [PhTttBu],
provides a sulfur-only donor environment capable of stabilizing
Td organocobalt(II) functional groups. Attempts to prepare the
corresponding Td organonickel complex resulted in clean forma-
tion of a novel metallacycle generated by borato ligand alkylation
as confirmed by control experiments. Efforts to trap a meth-
ylnickel intermediate led to isolation of neutral Ni(I) complexes,
[PhTttBu]Ni(L), L ) CO, P(CH3)3. Experiments continue to
attempt interception of [PhTttBu]Ni(CH3).
In contrast to the transformation outlined above for Co, attempts
to prepare an unprecedented Td Ni-CH3 species by reaction of
[PhTttBu]NiCl7 with (CH3)2Mg or CH3Li resulted in production
of an orange-red, diamagnetic complex in moderate (40%) yield,
Scheme 1. [κ2-PhTttBu]Ni(η2-CH2SBut) has been characterized
fully12 and its molecular structure, determined by X-ray diffrac-
tion,13 is contained in Figure 1. The borato ligand is coordinated
in the bidentate mode with the chelate ring in the twist boat
conformation. The Ni-S bond distance for the thioether trans to
the alkyl group is 2.256(2) Å compared with the other, mutually
trans thioethers at Ni-S ) 2.180(2) and 2.172(2) Å. The solid-
state structure shows a slight twist from square planar, 14°, a
consequence of close contact between the phenyl substituent of
the borato ligand and the tert-butyl of the alkyl. Surprisingly,
[κ2-PhTttBu]Ni(η2-CH2SBut) does not react with CO. Commonly,
nickel alkyls react readily with CO to afford the corresponding
acyl derivatives.14,15
Acknowledgment. This work was supported by the National Science
Foundation (CHE-997628 and an NYI award to C.G.R.). We thank Brian
Rhatigan for solving the X-ray structures of [PhTttBu]Ni(PMe3) and
[PhTttBu]Ni(CO).
In an effort to elucidate the mechanism of formation of [κ2-
PhTttBu]Ni(η2-CH2SBut), [PhTttBu]NiCl was reacted with [PhTttBu]-
Tl. This reaction led to isolation of the metallacyclic complex in
greater than 85% yield, suggesting the reaction of [PhTttBu]NiCl
and CH3Li results in liberation of free borato ligand that
subsequently alkylates [PhTttBu]NiCl.16 Generation of the borato
ligand could occur either via formation of an unstable Td [PhTttBu]-
Ni(CH3) which undergoes ligand loss as a decomposition pathway
or via CH3Li reduction of [PhTttBu]NiCl yielding a Ni(I) species
Supporting Information Available: Synthetic details and spectro-
scopic analyses of new compounds and tables of crystal data, structure
solution and refinement, atomic coordinates, and bond lengths and bond
angles for [PhTttBu]Co(CH3), [κ2-PhTttBu]Ni(η2-CH2SBut), [PhTttBu]Ni-
(P(CH3)3), and [PhTttBu]Ni(CO) (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) [κ2-PhTttBu]Ni(η2-CH2SBut): 1H NMR (C6D6) δ 7.91 (d, o-C6H5, 2
H), 7.41 (t, m-C6H5, 2 H), 7.20 (t, p-C6H5, 1 H), 2.40 (d, BCH2, 3 H), 2.28 (d,
BCH2, 3 H), 1.47 (d, NiCH2, 1 H), 1.28 (s, (CH3)3 27 H), 1.04 (d, NiCH2, 1
H), 0.75 (s, (CH3)3 9 H). UV-vis (CH2Cl2), λmax (ꢀ, M-1 cm-1) 465 (210).
(13) X-ray data: [κ2-PhTttBu]Ni(η2-CH2SBut), C26H49BNiS4, FW ) 559.41,
monoclinic, P21/n, orange block, a ) 13.6906(2) Å, b ) 16.4889(2) Å, c )
14.6641(2) Å, â ) 110.574(1)°, V ) 3099.17(6) Å3, Z ) 4, Z′ ) 1, T )
173(2) K, GOF ) 1.889, R(F) ) 6.27% for 4989 observed independent
reflections (4° e 2θ e 50°).
JA002638G
(17) [PhTttBu]Ni(CO): 1H NMR (C6D6) δ 116 (br, BCH2), 14 (br, C6H5),
10 (br, C6H5), 9 (br, C6H5), -1 (br, (CH3)3); 13C NMR (C6D6) δ 250 (br,
CO); IR (KBr) νCO 1999 cm-1; X-ray data for C22H38BNiOS3, FW ) 484.22,
monoclinic, P21/n, yellow plate, a ) 9.595(1) Å, b ) 20.868(3) Å, c ) 12.471-
(2) Å, â ) 99.498(3)°, V ) 2462.8(6) Å3, Z ) 4, Z′ ) 1, T ) 173(2) K, GOF
) 1.438, R(F) ) 8.01% for 4136 observed independent reflections (4° e 2θ
e 50°). [PhTttBu]Ni(P(CH3)3): 1H NMR (C6D6) δ 86 (br, BCH2), 22 (br,
P(CH3)3), 18 (br, C6H5), 11 (br, C6H5), 10 (br, C6H5), -6 (br, (CH3)3); 31P
NMR (C6D6) δ 264; X-ray data for C24H47BNiPS3, FW ) 532.29, orthor-
hombic, P212121, yellow block, a ) 11.7026(6) Å, b ) 14.2156(8) Å, c )
17.703(1) Å, V ) 2945.1(3) Å3, Z ) 4, Z′ ) 1, T ) 173(2) K, GOF ) 0.854,
R(F) ) 4.06% for 6468 observed independent reflections (4° e 2θ e 56°).
(14) Matsunaga, P.; Hillhouse, G. L. Angew. Chem., Int. Ed. Engl. 1994,
33, 1748-1749.
(15) Kubiak, C. P. In ComprehensiVe Organometallic Chemistry; Abel,
E., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1994; Vol. 9,
Section 1.2.
(16) Reaction of the bidentate ligand [Ph2BttBu]Tl with NiCl2 in a 2:1
stoichiometry yielded the analogous metallacycle, [Ph2BttBu]Ni(η2-CH2SBut);
the structure was confirmed by X-ray diffraction analysis. P. Ge unpublished
results.
(18) Consistent with the informative H NMR spectra, [PhTttBu]Ni(L) do
1
not exhibit EPR signals at 77 K.
(19) Kumar, M.; Ragsdale, S. W. J. Am. Chem. Soc. 1992, 114, 8713-
8715.