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Functional Modeling of CO Dehydrogenase Catalytic Reduction of Methylviologen by COH2O with an N O S-Ligated Nickel Catalyst.

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the fluorenyl system are also evident and are possibly an
indication of the considerable strength of the intramolecular
T1-arene interaction in the gas phase.
Functional Modeling of CO Dehydrogenase:
Catalytic Reduction of Methylviologen by
CO/H,O with an N, 0, S-Ligated Nickel
Catalyst**
Experimen tul Procedure
By Zlzeng Lu,* Corbet White, Arnold L. Rheingold,
and Robert H . Crabtree*
3a: A suspension of 2 (0.28 g. l.Omino1)[5] and TIC1 (0.24g. 1.0mmol) in
hexane ( 1 5 mL) was stirred for 24 h at room temperature. The clear, yellowish
solution was filtered away from a brown precipitate. The filtrate was concentrated under high vacuum giving a sohd or oily residue which was then taken
up in benzene ( 4 mL) and stored at room temperature. Within 24h colorless
crystals of 3a (0.220.46%) had formed, which melt at 123 C (dccomp) and are
soluble in benzene and other arenes. minimally soluble in dichloromethaiie. and
insoluble in hexane. Correct elemental analysis. MS (El. 70 eV): ni 2 476 ( M e .
3%). 370 ( M @ ) C,H,,, 10). 311 ( M e - C,,H,. 14). 205 (TIe, 60). 165
(C,,H,", 100). 106 (C8H,<,@,
9). 91 (C7HYe,14); molecular mass (cryoscopic.
C,H,) 465.
An important class of CO dehydrogenases,['] of which
that from Clostridium thermonceticum (Ct)is the best characterized, catalyzes reaction (a), the reduction of the electron
acceptor methylviologen (MV") with CO. Essential to the
CO
+ H 2 0 + 2 MV2'
$
CO,
+ 2 M V ' + 2H'
(a)
enzyme is nickel. which is believed to be the site of C O
binding.[" Of the two Ni centers, one is close to and probably linked to a Fe4S, cluster. CO reduces the Nil' form of the
protein to an EPR-active Nil form. The EPR signal for this
form (g = 2.028 and 2.074) shows coupling to 61Ni, 57Fe,
and 3C (of the CO) in the isotopically labeled protein, indicating that the unpaired eIectron resides on the Ni, CO carbon, and Fe atoms. EXAFS data suggests the presence of a
N- (or 0-)and S-donor coordination sphere around NLt3]
3a. b: 'H N M R (400 MHz. [D,]toluene, 193 K. TMS. all signals broad).
We
~=0.66(~.3H.CH,).1.48(~.3H:CH,).4.17(~.lH:CH).5.62.5.83.5.99.have therefore been looking for Ni complexes with an
N,O,S ligand environment which show catalytic activity in
6.74 (m. 4 H ; C,H,), 7.11 -7.66 (m. 8 H : arom. H): 'H NMR (400MHz.
[D,]toluene, 298 K. TMS): d = 0.88 (s. 6 H : C(CH&. 3.88 (s, I H: CH). 5.72.
CO dehydrogenase (CODH) reactions. Active catalysts must
5.93 (m, 4 H , J(H.TI) ca. 28 and 21 Hz; C,H,). 6.80-7.33 (m. 8 H : arom. H):
bind the substrate to the metal, yet thiolato ligands tend to
I 3 C (, 1 H ) NMR (100.6 MHr. [DJbenzene. 300 K ) . 6 = 23.0 (C2. C3). 39.2
reduce the number of open sites and catalytic activity by
(0).
60.8 (C9).105.3 (br. s). 107.5 (C5. C8 and C6. C7). 119.7. 120.3 (br s)
forming p-SR bridges. Thioamides are more convenient S(C17. C21 and C14. CIX), 125.9. 126.0 (CIS, C19 and Cl6, C20). 127.1 (br. s)
(C4). 142.9, 147.0 (C10, C13 and C11. CIZ).
donors than thiolates, because they seem to be less apt to
form such bridges, yet they bind in the iminothiolate
(R,N =C-S ) form.[41
Received: July 29. 1992 [Z5489IE]
We found that the new complex [{Ni(tmtss))2] (1 a)
German version: Aigeti-. (%f'iii. 1993. 105, 102
3b: A solution of 1 (0.27 g. 1.0 mino1)[5] in toluene (20 mL) was added dropwise to a solution of thallium ethoxide (0.25 g. 1 .0mmol) in toluene (20 mL) at
0 C over 1 h. The reaction mixture was then stirred for 24h at approximately
60 C to give a pale yellow solution abobe an oily sediment. When the solution
was concentrated colorless crystals of 3b formed which were washed with hexane (0.40 g, 8 5 % ) . I n several experiments the crystalliLation lasted days. The
elongated crystals. readily soluble in aromatic solvents and insoluble in hexane.
have a n apparently metastable melting point of 97 C (second heating 105
107 C ) . Correct elemental analysis.
+
[I] P. Jutzi. A h . Orgunoiner. Chein. 1986, 26, 217.
[2] H. Schmidbaur, Angeiv. C/iem. 1985. 97, 893;Ai7grw. CIicni. Inr. Ed. 6 7 y l .
1985. 24. 893; H. Schmidbaur. T. Probst. B. Huber. 0. Steigelmann. G.
Muller. Organo~neraNi~s
1989, 8, 1567.
[3] G. Wulfsberg, R. West. J. An7. Cl7rm. So(. 1972, 94, 6069.
[4] H. Schumann. H. Kucct, A. Dietrich. L. Esser. Chein. 5er. 1990. 123.
1811.
[ S ] J. A. Ewen. R. L. Jones. A. Razavi. J. D. Ferrara. J ,4177. Chcnz. So?. 1988,
110. 6255.
[6] 3 . C,,H,,TI, M, = 475.78: 3a: u = 1132.3(8). h = 94934). (. = 1539.4(6)
pm, /I
= 96.89(8)-, Z = 4, space group P2,;ii. four-circle diffractometer,
Mo,, radiation. 21 I7 independent reflections with F > 2a(F,), empirical
absorption correction, 207 refined parameters, R = 0.053, K, = 0.039:
3b: u = 1689.2(4). b = 971.4(6). c = 998.8(4) pm. Z = 4. space group
Pnu2,. four-circle diffractometer, Mo,, radiation. 1043 independent reflections with F > 2a(F0), empirical absorption correction. 200 refined
parameters. R = 0.052. K, = 0.036. polarity: 7 = 0.80(9). Further details
ofthecrystal structure investigationsmay be obtained from the Fachinformationszentrum Karlsruhe. Gesellschaft fur wissenschaftlich-technische
Information mbH, D-W-7514 Eggenstein-Lcopoldshafen 2 (FRG). on
quoting the depository number CSD-56746. the names of the authors. and
the journal citation.
a) E. Frasson, F. Menegus,C. Panattoni. Nurui-e 1963. IY9, 1087: b) M. B.
Freedman, L. G. Sneddon. J. C. Huffman. J. ,4111. C/imn. Soc. 1977, 99,
5194: c) H. Werner, H. Otto, H. J. Kraus, J Orgunoinet. Cheni. 1986.328,
C57: d ) S. Harvey. C. L. Raston. B. W. Skelton, A. H. White, M. F. Lappert. G. Srivastava. ihfd. 1987. 328. C I : e ) D. M. Schuhert. M. A. Bandman. W, s. Rees, C B. Knoblcr. P. Lu. W. Nam, M. F. Hawthorne.
Orgu~ioinrruNics1990. Y. 7 ; f) P. Jutzi. J. Schnittger. M. B Hursthouse.
Chmi. 5 t r 1991. 124, 1693
F.X . Kohl, R. Dickbreder. P. Ju1zi.G. Miillcr, B. Huber. Chrw7. Ber. 1989,
122. 871
a) H. Schmidbaur, W. Bublak. J. Riede. G. Muller. A n p i . . Ch~n71985. 417.
402: Anyex.. C'hm7. Inr. Ed. Engl. 1985, 24, 414; b) .I.
Beck. .I.
Striihle, Z.
Nuri/i-for,\ch. Secr. B 1986. 41. 1381, c) M. D. Noirot. 0. P. Anderson,
S. H. Strauss. Inory. Chcn?.1987.26, 2216: d) H. Schmidbaur, W. Bublak.
B. Huber. J. Hofmann, G. Muller, Ciiem. BL,r. 1989. 122. 265.
C. Janiak. R. Hoffmann. J. A m Cken?.Soc. 1990. 112. 5924.
~
1
l a , R ' = M e , R'
l b , R'
=
=
H
H,R 2 = Me
(tmtssH, = 2'-hydroxy-4,5'-dimethylacetophenone-4-niethylthiosemicarbazone) catalyzes the CODH reaction shown in
Equation (a) with MV2+ as the electron acceptor. The unusual 0-bridged structure 1 was assigned on the basis of a
crystal structure[51(Fig. 1 ) of the closely related species 1 b,
which is a less active catalyst but crystallizes well.
The MV" formed in Eq. (a) was detected by its characteristic dark blue color
= 610 nm). The protons released
were detected from the change in the pH of the medium
during the reaction. Finally, the CO, formed was precipitated as CaCO, after passing the gas mixture through aqueous
Ca(OH),. When the reaction was conducted with CO/
H2'80/MV2+/1a, the v4 and v 5 bands (COf-) of the CaCO,
precipitated were shifted from 872 and 710 cm- to 863 and
substitution of the car664 c m - ' resulting from '80-160
bonate. The complex is selective for CO and does not give
MV" with RNC or H,. Recrystallization of 1a caused no
["I
[**I
Z. Lu. Prof. Dr. R. H. Crabtree
Chemistry Department. Yale
225 Prospect Street. New Haven. CT 06511-8118 (USA)
C. White. Prof. Dr. A. L. Rheingold
Chemistry Department, University of Delaware
This work was supported by the National Institutes of Health
C O reacted with l a in CH,CI, to give a paramagnetic
complex 2, which has an EPR spectrum (Fig. 3, gx.y= 2.29,
g, = 2.05) consistent with a Nil metal center. In the absence
of M V Z + ,Nil' therefore acts as electron acceptor. Complex
2 has so far proved too unstable for a crystal structure to be
obtained.
241
1
2280
Fig. I Molecular structure of 1 b (ORTEP). Selected bond lengths [A] and
angles[ 1: Ni(l)-Ni(2) 2.729(1), Ni(1)-O(1) l.X51(3), Ni(1)-O(2) 1.912(4). Ni(1)1.855(5). Ni(Z)-O(I) 1.898(4), Ni(l)-S(l) 2.129(1), C@)-S(lj 1.744(6),
"1)
C(X)-N(2) 1.298(5): Ni(l)-O(2)-Ni(l j 93.0(2), O(l)-Ni(l)-O(2) 76.4. Dihedral
angle Yi[l )0(1
)0(2)-Ni(2)0(1)0(2). 136.9(2) .
change in rate, and neither NiCI, nor Ni metal mediated the
react ion.
The system was active even in neutral solution, but the
buildup of protons stopped the process under these conditions. The enzyme was also inhibited at low pH.['] When
sodium acetate (20 mM) was used as a proton acceptor, the
reaction proceeded faster. The formation of the methylviologen radical and changes in pH (Fig. 2)[61indicated that the
reaction proceeds steadily at 1.O turnovers per hour a t 25 "C.
40
2780
3280
BIG1
Fig. 3. X band EPR spectrum ( v
2.
= 9.0519
-
I
3780
4280
GHz) o f a frozen solution (5.6 K ) of
The reaction of C N - and 1a gave 3, which has an IR band
at 21 18 cm-' (vCN) appropriate['] for a terminal cyanide
ligand. C N - is more resistant to nucleophilic attack than
CO, and this may be what allowed us to isolate 3. The carbony1 analogue of 3, [Ni"(CO)(tmtss)], could not be prepared o r observed spectroscopically, but may be a reactive
intermediate in the C O reaction.
Na[Ni''(CN)(tmtss)]
3
The characteristic blue color of MV'+ makes the system a
selective sensor for C O when the reaction is conducted under
anaerobic conditions as above. Since MV'+ is rapidly reoxidized by air to M V Z f and the catalyst is not air-sensitive, the
acetate-buffered system can be run aerobically for catalytic
air oxidation of CO to CO, (0.94 turnovers h- ').
30
Experimental Procedure
10
0
0
20
10
30
40
50
60
70
t [miniFig. 2. Release of H i n the reduction of MV"
without (+) catalyst l a . [6].
+
by COiH,O with
(0)and
The rate is first order with respect to Ni. Like the enzyme,"]
this system was inhibited both by C N - (inhibited rate:
3 . 0 lo-,
~
h - ' with 2.5 mM C N - ) and by Me1 (0.78 h - '
with 50mM MeI). This appears to be the first complex to
mediate the CODH reaction of Equation (a); stoichiometric
model reactions related to the acetylCoA synthase activity of
the enzyme have been studied by Holm et al.['' The watergas shift reaction, in which CO/H,O gives CO, and H,, is
well known, but not with Ni" as catalyst.'''
tmtssH,: 2-Hydroxy-4',S-dimethylacetophenone(1.64 g. 0.01 mol) readily
condensed with 4-methylthiosemicarbazide (1.05 g. 0.01 mol) in ethanol
(25 mL). Yield 69%. correct C,H,N analysis. ' H N M R (250 MHz, CDCI,):
6 =10.50(s, 1 H.OH).8.59(s, 1 H;NH).7.1X(s, 1 H;C6H,).6.78(s.2H;NH,
C,H,). 3.25 (d. 3 H , CH,). 2 . 3 6 6 , 3 H ; CH,). 2.24(s. 3 H ; CH,), 2.21 (s, 3 H ;
CHJ
I a . To a stirred suspension of tmtssH, (0.5 g, 2 mmol) in ethanol (20 mL) was
slowly added a solution of nickel acetate (0.496 g, 2 mmol) In ethanol (20 mL).
After 2 h at reflux, a brownish precipitate was filtered off, washed with cold
ethanol. and dried in vdcuo. Yield 0.549 g, 89%. correct C,H. analysis. The
complex can he recrystallized from CHZCI,/Et,O. 'H NMR (250 MHL.
[DJDMSO); 6 =7.21 ( s . 1 H : C,H,), 6.55 (s, 2 H ; NH, C,H,). 2.59 (d. 3 H ;
CH,). 2.40 (s, 3 H: CH J. 2.1 1 (5.3 H ; CH,). 2.07 (s. 3 H ; CH,). I b was prepared
analogously.
Received: July 15, 1992
Revised: October 8. 1992 [Z 5468 IE]
German version: A n g w . Chern. 1993, 105. 121
[I] G. Diekert in The Biornorgunic Chemistr! uf"ickt4 (Ed.. J R. Lancaster),
VCH, Weinheim. 1988.
121 H. G. Wood. S. W. Ragsdale, E. Pezacka, Brorhern. In/. 1986, 12, 421; L.
Ljungdahl, Annu. Rev. Microhiol. 1986, 40. 415.
[ 3 ] S. W. Ragsdale, H. G. Wood, T. A. Morton, L. G . Ljungdahl, D. Dervartanian in The Bioinorgunrc Chernistq of Nirkd (Ed.. J. R . Lancaster), VCH,
Weinheim. 1988.
[4] M. Zimmer, G . Schulte. X -L. Luo, R. H . Crabtree, Angel!.. Cliem. 1990.
103. 205: A n p i i . Clwtn. In!. Ed. Ennl. 1991. 30. 193.
[5] Structure analysis: 1b : triclinic, space group Pi. u = 8.220(2), h =
13.100(3),~=13.X64(3)A:x=114.56(3),8=103.59(3),; = 92.07(3) , V =
1304.6(5) A3. 2 = 2. g,, = 1.568 gcm-': 5702 unique reflections. 3637
observed (F z 5n(F)),325 variables, K = 0.0426. R , = 0.0426. Further details are available from the Director of the Cambridge Crystallographic
Data Centre. University Chemical Laboratory. Lensfield Road. GB-Cambridge CB2 IEW ( U K ) on quoting the full journal citation.
[6] Conditions: CO ( 5 m L m i n - ' ) was passed at room temperature (20 C )
through a solution ofCH,CI, and CH,OH (1 : I . 60 mLjcontaining 0.010g
of 1 a, 0.10 g of MV2'. 0.10 g of NaOAc, and 1 .O mL of water. To quantify
the proton release. the pH observed (pH meter) was compared with the pH
changes found for an identical solution titrated with 0.01 hi HCI. Control
reactions were run under identical conditions without 1 a and with tmtssH,
but without NI.
[7] P. Stavropoulos. M. C . Muetterties. M. Carrie, R. H. Holm. J. A m . C/imi.
Soi,. 1991, 113. 8485.
[XI J. W. Reppe. Jirtrus Liehrgs Ann. Cbem. 1953. 582. 121
[9] M. Sato. F. Sato. T. Yoshida. J1 Orgunumi,f.Chctii. 1971, 31. 415.
Synthesis and Structure of the First Homoleptic
Imidotechnetium Complex: [Tc,(NAr),]
(Ar = 2,6-diisopropylphenyl)**
one set of resonances for the aryl groups, suggesting that all
the imido ligands in 1 are chemically equivalent.
X-ray structure analysis of a single crystal of 1, grown by
slow evaporation of a THF/(Me,Si),O solution, reveals an
ethanelike structure (Fig. I).[,] The two Tc atoms are joined
Fig. I . Crystal structure of [Tc,(NAr),] ( I ) . Selected bond angles ['I for 1. not
already mentioned m the text: N-Tc- Tc(A) = 103.6(1). N-Tc-N(A) = 114.6(1).
By Anthony K . BurreN and Jeflrey C. Bryan*
Transition metal complexes containing only imido ligands
are currently known for niobium, rhenium, osmium, and the
Group 6 metals.[' - 3 1 An example especially pertinent to this
work is the complex [Re2(NtBu),(p-NtBu),1, which is
formed by reduction of [Re(NtBu),(OSiMe,)] with sodium
amalgam.[21~Re,(NtBu),(p-NfBu),] is an edge-bridged tetrahedral dimer, similar to all other structurally characterized M,E, complexes (where E stands for dianionic ligdnds
such as NtBu, S, Se, which form multiple bonds to the metal
center)."
We report here the first homoleptic imidotechnetium complex [TcJNAr),] (1, Ar = 2,6-diisopropylphenyl), which adopts an unprecedented "ethanelike" structure.
Tetrdhydrofurdn solutions of [ T c ( N A ~ ) , I ] [react
~ ~ quickly
at room temperature with sodium metal to form [Tc,(NAr),]
( I ) in high yield (Scheme 1 , M = Tc). Complex 1 does not
M = Tc, Re
Ar = 2,6-iPr2C6H,
react with iodine at room temperature to re-form [Tc(NAr),T]; 1 is air-stable in solution and can be purified on a
silica gel column. The 'H N M R spectrum of I shows only
I**]
A,
l,M=Tc
Scheme 1 Synthesis of 1. The analogous Re complex appears to be accessible
by the same route.
[*I
by an unbridged metal-metal bond, and all six imido ligands
are terminally bound, three to each technetium. The Tc-Tc
bond lies on a crystallographic S, axis making all six imido
ligands symmetry equivalent, and requiring a staggered arrangement. If the imido ligands in 1 are counted as dianionic,[jl the Tc atoms can be considered formally to be in the
oxidation state + V I . The Tc-Tc bond length of 2.744(1) 8,
could then only be considered a single bond. consistent with
the formulation of 1 as a d'-d' dimer and the diamagnetic
behavior of the complex. The Tc-Tc bond is longer than
known single bond lengths for Tc"-TcV1 dimers;"] this may
be due to the sterically demanding Ar groups of the imido
hgands. The short Tc-N distances of 1.758(2)
and the
nearly linear Tc-N-C angles of [167.6(3)'] are consistent
with strong multiple bonding between the Tc atom and the
iinido ligands.[jl
All previous examples of structurally characterized M,E,
complexes are edge-bridged tetrahedral dimers with sterically less demanding ligands such as S, Se, and NtBu
(Scheme 2).r2 Simple molecular mechanics calculations[g1
on [Tc,(NAr),] clearly indicate that the observed ethanelike
structure is strongly preferred over the dimeric configuration
of edge-bridged tetrahedra
Dr. J. C. Bryan
Inorganic and Structural Chemistry Group. INC-1. C346
Los Alamos National Laboratory
Los Alamos. NM 87545 (USA)
Dr. A. K. Burrell
Nuclear and Radiochemistry Group. Los Alamos National Laboratory
This work was supported by the Laboratory Directed Research and Development program Los Alamos National Laboratory. We also thank Dr.
David L. Clark for helpful discussions.
E
\M-M
tE\ YE
E// E
''
NE
Scheme 2. MzE6complexes which can be considered edge-bridged tetrahedral
dimers.
This preference has been extensively demonstrated for
M,X, complexes, where X is a monoanionic ligand; they
adopt a dimeric structure of edge-bridged tetrahedra for
nonbulky X ligands and an ethanelike structure for bulky X
hgdnds.['ol Since [Re,(NtBu),(p-NtBu),I adopts an edgebridged tetrahedral dimer structure, we attempted to prepare
[Tc,(NtBu),(pNrBu),], without immediate success. However, preliminary work indicates that reduction of [Re-
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