Mononuclear NiIII alkyl complexes (Alkyl = Me and Et): Relevance to the acetyl-CoA synthase and methyl-CoM reductase

Article abstract of DOI:10.1021/ja102430d

Mononuclear, distorted trigonal bipyramidal [PPN][Ni^III(R)- (P(C₆H₃-3-SiMe₃-2-S)₃)] (R) Me (1); R) =Et (2)) were prepared by reaction of [PPN][Ni_IIICl(P(C ₆H₃-3-SiMe₃-2-S)₃)] and CH ₃MgCl/C₂H₅MgCl, individually. EPR, SQUID studies as well as DFT computations Reveal that the Ni^III in 1 has a low-spin d⁷ electronic configuration in a distorted trigonal bipyramidal ligand field. The Ni-C bond lengths of 1.994(3) a in 1 and 2.015(3) a in 2 are comparable to that in the Ni^III-methyl state of MCR (2.04 a) (Sarangi, R.; Dey, M.; Ragsdale, S. W. Biochemistry 2009, 48, 3146). Under a CO atmosphere, CO triggers homolytic cleavage of the Ni^III-CH₃ bond in 1 to produce Ni ^II-thiolate carbonyl [PPN][Ni^II(CO)(P(C₆H ₃-3-SiMe₃-2-S)₃)] (3). Additionally, protonation of 1 with phenylthiol generates Ni^III-thiolate [PPN][Ni^III(SPh)-(P(C₆H₃-3-SiMe ₃-2-S)₃)] (4).

Full text of DOI:10.1021/ja102430d

Published on Web 06/22/2010
Mononuclear NiIII-Alkyl Complexes (Alkyl ) Me and Et): Relevance to the
Acetyl-CoA Synthase and Methyl-CoM Reductase
Chien-Ming Lee,*,† Chien-Hong Chen,‡ Fu-Xing Liao,§ Ching-Han Hu,§ and Gene-Hsiang Lee|
Department of Applied Science, National Taitung UniVersity, Taitung City 95092, Taiwan, School of Applied
Chemistry, Chung Shan Medical UniVersity, Taichung City 40201, Taiwan, Department of Chemistry, National
Changhua UniVersity of Education, Changhua 50058, Taiwan, and Instrumentation Center, National Taiwan
UniVersity, Taipei 10764, Taiwan
Received March 24, 2010; E-mail: cmlee@nttu.edu.tw
Scheme 1
Abstract: Mononuclear, distorted trigonal bipyramidal [PPN][NiIII(R)-
(P(C6H3-3-SiMe3-2-S)3)] (R ) Me (1); R ) Et (2)) were prepared
by reaction of [PPN][NiIIICl(P(C6H3-3-SiMe3-2-S)3)] and CH3MgCl/
C2H5MgCl, individually. EPR, SQUID studies as well as DFT
computations reveal that the NiIII in 1 has a low-spin d7 electronic
configuration in a distorted trigonal bipyramidal ligand field. The
Ni-C bond lengths of 1.994(3) Å in 1 and 2.015(3) Å in 2 are
comparable to that in the NiIII-methyl state of MCR (∼2.04 Å)
(Sarangi, R.; Dey, M.; Ragsdale, S. W. Biochemistry 2009, 48,
3146). Under a CO atmosphere, CO triggers homolytic cleavage
of the NiIII-CH3 bond in 1 to produce NiII-thiolate carbonyl
[PPN][NiII(CO)(P(C6H3-3-SiMe3-2-S)3)] (3). Additionally, proton-
ation of 1 with phenylthiol generates NiIII-thiolate [PPN][NiIII(SPh)-
(P(C6H3-3-SiMe3-2-S)3)] (4).
The chemistry of Ni-alkyl complexes has been actively pursued,
motivated primarily by the development of inexpensive catalysts
NiIII-CH3 intermediate will be generated for the paramagnetic
for selective hydrocarbon activation1 and, in particular, the relevance
of NiII/III-alkyl intermediates (alkyl ) methyl) to the catalytic
chemistry of two enzymes, acetyl-CoA synthase (ACS) and methyl-
coenzyme M reductase (MCR).2,3 ACS is a nickel-based enzyme
responsible for the assembly of CO, CH3, and the coenzyme A
to generate acetyl-CoA;4 MCR is also a nickel-containing
enzyme which catalyzes the conversions of methyl-coenzyme
M (CH3-SCoM) and coenzyme B (CoB-SH) to methane and
heterodisulfide (CoM-S-S-CoB).5 Recently, the crystal struc-
tures of ACS and MCR systems have been elucidated.6,7 The
active sites of two enzymes are associated with nickel ion which
is believed to be selected for biological processes due to its
unique redox and coordination properties. Although several
groups have employed various spectroscopic and theoretical
methods to characterize the intermediates formed during catalytic
reactions, there has been a lot of debate about the catalytic
mechanisms in both ACS and MCR systems.8,9 For instance,
an intense discussion in MCR is whether reactions of the MCRred1
(NiI) state with CH3-SCoM results in the formation of a
mechanism, and it will form the NiII-CH3 intermediate in the
diamagnetic pathway.
Inspired by Liaw’s recent work in the synthesis of a series
mononuclear NiIII-L complexes (L ) OMe, OPh, SePh, SEt, SPh,
and Cl) supported by ligand [P(C6H3-3-R-2-S)3]3- (R ) H, SiMe3)10
and the biological significance, we are interested in the chemistry
of NiIII complexes containing an alkyl group. Herein, we report
the syntheses and structural characterization of two trivalent
alkylnickel complexes [PPN][NiIII(R)(P(C6H3-3-SiMe 3-2-S)3)] [PPN
) bis(triphenylphosphoranylidene)-ammonium; R ) Me, Et]. To
the best of our knowledge, no example of a methyl group bonded
to NiIII has been structurally characterized.
When a CH3CN-THF (1:4 v/v) solution of [NiIII(Cl)(P(C6H3-
3-SiMe3-2-S)3)]s was treated with 1.1 equiv of RMgCl (R ) Me,
Et) at -20 °C, an immediate change in solution from bright green
to dark red-brown occurred (Scheme 1a). The reaction mixture led
to the isolation of five-coordinate [NiIII(R)(P(C6H3-3-SiMe3-2-S)3)]s
(R ) Me for 1, Et for 2) as a dark-red solid after recrystallization
from THF-CH3CN/Et2O (yield: 70% for 1, 50% for 2). Both 1
and 2 are soluble in CH3CN-THF (1:4 v/v) and display thermal
sensitivity in solution. Compared with the spectrum of 1 having
absorption bands at 500 and 825 nm, the bands of 2 coordinated
by a stronger electron-donating ethyl ligand exhibit a blue shift to
490 and 815 nm, respectively (see Supporting Information, Figure
S1). The 1H NMR spectra of 1 and 2 at ambient temperature show
diagnostic signals with phenyl protons well removed from the
diamagnetic region. The proton resonances shifted to downfield at
δ 14.29 (br) and 12.73 (br) and to upfield at -1.86 (br) for 1 (14.94
(br), 12.33 (br), and -0.86 (br) for 2) are assigned to NiIII-bound
9a
•
NiIII-CH3 intermediate (Mechanism I) or a CH3 radical and
NiII-thiolate species (Mechanism II).9c In ACS, the central metal
ion of the active state is in the NiI valence state according to
the paramagnetic proposal,8b but it is in the Ni0 valence state
based on the diamagnetic proposal.8a Therefore, upon reacting
+
with CH3 donated from a corrinoid iron-sulfur protein, the
† National Taitung University.
‡ Chung Shan Medical University.
§ National Changhua University of Education.
| National Taiwan University.
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10.1021/ja102430d  2010 American Chemical Society

C O M M U N I C A T I O N S
and 2 is much easier to cleave homolytically. To test this hypothesis,
we treat the CH3CN-THF (1:4 v/v) solution of 1 with CO at room
temperature.
The insertion of CO into the NiII-methyl bond in monomeric
or dimeric alkylnickel model complexes producing the NiC(O)Me
species was described in the literature.2c,e,15 In contrast, the reaction
between CO and 1 leads to NiII-thiolate carbonyl [NiII(CO)(P(C6H3-
3-SiMe3-2-S)3)]s (3),10b as shown in Scheme 1b. The lack of
observation of the acylnickel species from the reaction of CO with
1 but isolation of 3 may imply that the formation of the proposed
[(CO)NiIII(CH3)(P(C6H3-3-SiMe3-2-S)3)]s intermediate followed by
Ni-CH3 homolytic cleavage seems preferable to methyl migration
or CO migratory insertion.
The methyl group of 1 can undergo alkyl-for-chalcogenate
exchange with diphenyl dichalcogenide or protonation with
phenylthiol.15e When 1 and (PhE)2 (E ) S or Se) or PhSH are
mixed in solution at ambient temperature, [PPN][NiIII(EPh)(P(C6H3-
3-SiMe3-2-S)3)] (E ) S (4); Se (5)) are isolated in high yield
(Scheme 1c) (Figure S6).10
Figure 1. Plot of the spin density of 1. Orbital and atomic contributions
to the spin densities of 1 are shown in Supporting Information (Table S1).
Selected computed structural data (Å): Ni-CMe 1.949, Ni-Sav 2.329, Ni-P
2.175 (B3LYP/6-31G*).
In summary, we succeeded in the syntheses and structural
characterization of thermally unstable high-valent NiIII-alkyl
species (alkyl ) Me (1), Et (2)) with tetradentate ligand [P(C6H3-
3-SiMe3-2-S)3]3s, although a high redox potential of the NiIII-alkyl
state might be expected to undergo spontaneous reduction and
polymerization. The stability of 1 and 2 might be attributed to the
tunable electron-donating functionalities of ligand [P(C6H3-3-SiMe3-
2-S)3]3s, which shares similar characteristics with the noninnocent
factor of the F430 in MCR.9b Additionally, the results obtained from
this work may lend support to the intermediacy of NiIII-CH3 species
proposed in both ACS and MCR catalytic cycles. We also note
that 1 and 2 are capable of being used in reagents for alkyl transfer.
Detailed investigations are ongoing.
Figure 2. ORTEP drawings of [NiIII(CH3)(P(C6H3-3-SiMe3-2-S)3)]s (a)
and [NiIII(CH2CH3)(P(C6H3-3-SiMe3-2-S)3)]s (b) with thermal ellipsoids
drawn at the 50% probability level. Selected bond distances (Å): Ni-C(1)
1.994(3) for 1; Ni-C(1) 2.015(3) for 2.
ligand [P(C6H3-3-SiMe3-2-S)3]3 s (Figure S2). Attempts to identify
resonances of R hydrogens in 1 and 2 were unsuccessful, presum-
ably due to paramagnetic broadening, but the peak at δ -45.07
(br) is most likely due to ꢀ hydrogens of 2 (Figure S2b).
The 77 K EPR spectrum of 1 exhibits rhombicity with three
principal g values of 2.44, 2.00, and 1.96 (2.44, 1.99, 1.93 for 2,
Figure S3). The average g value of 1 (gav ) 2.13) indicates that
the unpaired electron is primarily associated with the nickel ion.10a
Indeed, the spin density result from density functional theory
computation reveals that the unpaired electron resides predominantly
Acknowledgment. We thank the National Science Council
(Taiwan) for financial support and Prof. Wen-Feng Liaw, Peter
P.-Y. Chen, and Dr. Tsai-Te Lu for helpful discussions.
Supporting Information Available: Crystallographic data in CIF
format and additional figures and experimental details (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
in the dxy and dx -y orbitals of Ni.11 The spin density plot of 1 is
shown in Figure 1 (Figure S4 for 2). According to the figures, the
atomic spin densities are 0.98 on nickel and -0.08 on methyl
carbon, respectively. Similarly, for 2 the atomic spin densities are
0.99 on nickel and -0.09 on ethyl carbon (Table S1). These results
are comparable to that of the EPR-active state of MCR (g⊥ ) 2.10,
g| ) 2.22), in which the unpaired electron is primarily located in the
2
2
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