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Functional Modeling of Ni Fe Hydrogenases A Nickel Complex in an N O S Environment.

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Functional Modeling of Ni,Fe Hydrogenases:
A Nickel Complex in an N,O,S Environment.**
By Marc Zirnrner*, Gayle Schulte, Xiuo-Liang Luo,
and Robert H . Crabtree*
Hydrogenases containing both nickel and iron"] catalyze
HJD exchange,"' but their physiological role is H, oxidation."] There is circumstantial evidence in favor of a Ni site
for H, binding.'"] Various ESR signals coupled to 61Ni
( I = 5) are found for 61Ni, Fe hydrogenases~2a~C1
but the
oxidation state assignment for some of these (nickel(1) or
(in)) is controversial.[2'C-e1Exposure to 0, deactivates the
enzymes, but they may be reductively activated to give the
'ready' form.[''] Little is known about the ligand environment, but extended X-ray absorption line structure
(EXAFS) data suggest that three[jl to fourr4] sulfurs are
coordinated to Ni, while ESR data only show one."] Very
few Nil or Ni"' S-donor complexes are known;'6, '1 most Ni"'
species have highly noninnocent ligands,I8,I'
dimers,"'. ' '1 or decompose to disulfides.[I2.
some interesting Ni"' thiolates were
We now report an N,O,S-donor Ni" complex that is octahedral in the
solid state but square planar in solution, can be reduced to
an air-stable Nil form, and, like hydrogenase itself, catalyzes
Hz/D exchange. This is the first nickel-containing hydrogenase model system which is functional for isotopic exchange.
The previous ones, the 0 s and Ru porphyrins of Collrnan et
al.," 5al and the Pd(salen) (salen = bis(sa1icylidene)ethylenediamine) systems of Olive et
did not contain physiologically relevant metals.
We screened a large number of S-donor Ni complexes for
activity and find H/D exchange only for [NiL,]CI, (I)
(L = o-C,H,(OH)CH=N-NHCSNH,) which is formed by
refluxing NiCI, with L[163171
in ethanol for two hours and
crystallizes as dark green plates in 81 % yield. An X-ray
crystal structure of 1 . EtOH (Fig. 1) shows a pseudooctahedral dicationic complex.1181Both ligands are in the phenol,
rather than the phenolate form. The magnetic moment in the
solid state is 1.98 B.M. (1.83 x
JT-I), significantly
lower than the spin-only
The Ni-S bonds are short
(2.386(3) A and 2.362(3) 8, vs. 2.45-2.55 8, in typical Ni"
octahedral specie^^'^^]), consistent with significant
R,NQ=C-Se character for the ligand. The long Ni-O(Aryl)
bonds (2.143(6) and 2.091(6) A), are further evidence for
protonated phenols.'' 9b1
In solution the complex is diamagnetic, as shown by the
Evans method[201and the normal 'H NMR spectrum. This
suggests that the complex has lost one or two ligands. Similar
behavior has been observed for Ni" complexes of
thiourea.["] The long Ni-O(Ary1) bond and the position of
the phenol resonances (intensity: 2) almost unshifted from
the free ligand (-OH at 6 = 9.90) suggest that both phenol
groups dissociate (Scheme 1).
Scheme 1. Probable structure of the dication 1 in solution. 3: Proposed catalyt
ic intermediate for hydrogen activation.
Reduction of 1 electrochemically ( E > - 1.05 V, scan
speed = 100 mV s-*; ZJZC = 1.64; BE = 0.098 V, quasi reversible)1221
or with NaBH, (irreversible) produces a Ni'species (2) which is stable in air (ESR) for hours. Nil is unusual
in an S environment and no air-stable examples are known.
Neither the ligand itself nor [ZnLJSO, reacts with NaBH,,
suggesting that ligand L is not reduced under these conditions. Nickel is the predominant site of reduction as shown
by the ESR spectrum in dimethylformamide (DMF) (Fig. 2,
-0.2 -0.5
Figure 1. Structure of the Ni-salicylaldehyde thiosemicarbazone cation [18]
(selected Ni-L bond lengths [A]: S1, 2.39; Sl', 2.36; N3, 2.02; N3', 2.01; 01,
2.14; 01'. 2.09).
Prof. Dr. R. Crabtree, G. Schulte, X.-L. Luo
Yale Chemistry Dept.
225 Prospect Street. New Haven, CT 06511-8118 (USA)
Prof. M. Zimmer,
Chemistry Department, Connecticut College
New London, CT 06320 (USA)
This research was supported by the National Institutes of Health. We
thank Prof. G. W Brudvig for discussions, and Ms.C. A . Buser and Dr. J.
Bocorv/r for obtaining ESR spectra.
Angew. Chem. h i 1 . Ed. Engl. 30 (1991) N o . 2
Figure 2. Frozen solution (7 K) of X band ESR spectrum (15 = 9.0519 GHz) of
2. Insert: Cyclic Voltammogram of 1 [22].
g = 2.25, 2.12, and 2.06),[231which is consistent with an
axially distorted Ni' species. The reduced forms of the
analogous Ni complexes which lack the phenol groups,[241
VerlugsgeseNschuJt mbH, W-6940 Weinheim, 1991
OS70-0833/9ll0202-0193 X 3 S 0 f ,2510
four coordinate 4, and five coordinate 5 have identical g,
and g,l to 2. UV-vis spectra show that 5 does not dispropor[Ni(C6H,CH=N-NHCSNH2),1C1Z
tionate to 1 and 4. The Nil complexes are therefore fourcoordinate with an N,S, environment.
We looked at DJH exchange with ethanol-OH protons
catalyzed by 1. Exposure to D, (1 atm) gave H/D exchange,
especially in the presence of promoters like HI or HBF,. A
0.1 M solution of 1 in a dimethyl sulfoxide/ethanol mixture
(90:lO) gave 7.5 turnoversf2’’ of H/D exchange after five
minutes at 25 “C and 1 atm pressure of D,. Deuteriation of
the EtOH proton was followed by 2 D NMR by monitoring
the EtOD resonance at 6 = 4.15 versus C,D, as standard.
The catalytic activity disappeared after 7.5 turnovers. Neither 4 nor 5 promotes H/D exchange, so both phenolic OH
groups are required for activity. A possible catalytic intermediate is 3,[261in which one phenol group is bound to the
nickel, and the other forms a hydrogen bond with H,
(Scheme 1). Hydrogen bonding may increase the affinity of
the complex for H, and promote exchange (Scheme 2).
This work reveals some unusual features of Ni chemistry
in an S-donor environment. A stable Nil species is formed
from 1 on reduction. In solution, ligand dissociation leads to
a species which interacts with D, and promotes H/D exchange. Compound 1 is thus a functional model for hydrogenases.
Received: August 15. 1990 [Z4135 I€]
German version: Angeir. Chcin. 103 (1990) 205
CAS Registry numbers:
1, 52637-24-8; 1 . EtOH, 131564-04-0; 2 , 331457-27-7; 4. 51341-69-6; 5.
131457-23-3; L, 5351-90-6; [Ni(C,HsCH=NNHCSNH,)Ja.
[NiL(EtOH)]*@,131457-26-6; hydrogenase, 9035-82-9.
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,cVCH Verlugsgesellsrhujt mbH, W-6940 Wcinheim, 1991
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[IS] Monoclinic, space group PZ,/n (No. 14). a = 14.865(4), b = 9.474(2),
e = 18.619(2) A; p = 110.05(1)’. V = 2463.2(9) A’, Z = 4 for d,,,, =
1.526 g cm-3; 5162 unique reflections were collected on a automated diffractometer (Mo,, radiation, i. = 0.71069 A). The structure was solved
using the Patterson method. A total of 289 parameters were refined to a
findl R = 0.044. R , = 0.046, using 2639 reflections. Further details of the
crystal structure investigation are available on request from the Director
of the Cambridge Crystallographic Data Centre, University Chemical
Laboratory, Lensfield Road. GB-Cambridge CB2lEW (UK). on quoting
the full journal citation.
[I91 a) Nickel(i1) thiourea complexes with magnetic moments ranging from 0.6
t o 2.5 B.M. ( 0 . 5 6 - 2 . 3 2 ~
J T - ’ ) have been reported in [19h]; b) L.
Sacconi, F. Mani, A. Bencini in G. Wilkinson, (Ed.): Comprehensive Coordinution Chemisrr!. Pergamon. New York 1987.
[20] D. F. Evans. J. Chem. SOC.19S9. 2003.
[21] C. Furlani. T. Tdrantelli. P. Riccieri, J. Inorg. Nu</. Chem. 33 (1971) 1389.
[22] A solution of I ( 5 mM in 0.1 M n-Bu,N(CIO,)/DMF) was reduced by controlled potential electrolysis using a Pt basket working electrode, a PI wire
auxiliary electrode and an SCE reference. Current integrated for
0.92 e mo1-l of 1. Potential vs. SCE, with E , , , = 0.48 V for Fce/Fc
(Fc = ferrocene) in DMF. N o Ni”/Ni”’ wave was seen.
1231 Frozen D M F solution (7 K). X Band ESR spectrum ( v = 9.0519 GHz.)
[24] 5 was made by refluxing the ligand with NiCI, in EtOH for 2 h. 4 was made
by refluxing 1 equiv of the tridentate ligand with NiCI, as above, to give
[NiL(EtOH)]. followed by refluxing with 1 equiv of the bidentate ligand.
[25] N o HID exchange was observed in a variety of control experiments (L
alone; NiCI, alone; L with He; NiCI, with He and [ZnL,] alone); 2
turnovers were observed in absence of a promotor.
[26] An H, complex is a plausible intermediate because no dicationic first row
metal is sufficiently r[ basic to lead t o oxidative addition of H, and [lr(7,8benzoquinolinate)(PPh,),H(rlZH,)IO is an extemely effective catalyst
[26b] for H / D exchange between EtOH and D,. b) A. Albeniz. D. M.
Heinekey, R. H. Crabtree. unpublished results.
Eight-Coordinate Metal Carbonyls Containing
Only Monodentate Ligands. Syntheses and
Structural Characterization of
[nPr4N),[(Ph3Sn),M(Co),j, M = Zr, Hf **
By John E. Ellis,* Kai-Ming Chi, Anthony4 DiMaio,
Scot1 R . Frerichs, Jason R. Sfenzel, Arnold L. Rheingold,
and Brian S. Haggerlv
Until the recent synthesis of the hexacarbonylmetalates(2-) of zirconiumr’*’I and hafnium,’’] only q5-or q6bonded ligands such as qs-CsH,, qS-C,Me,,[3*41and q61,3,5-tri-tert-b~tylbenzene[~l
and bi- or tridentate ligands
[*] Prof. J. E. Ellis. K.-M. Chi, Dr. A.-J. DiMaio, S. R. Frerichs. J. R. Stenzel
Department of Chemistry, University of Minnesota
Minneapolis, MN 55455 (USA)
Prof. A. L. Rheingold‘”, B. S. Haggerty
Department of Chemistry, University of Delaware
Newark. DE 19716 (USA)
[‘I X-ray structure analyses
[**I Highly Reduced Organometallics, Part 29. This work was supported by
the U.S. National Science Foundation and the Petroleum Research Fund
administered by the American Chemical Society. Part 28: [I].
OS70-0833/91/0202-0194 3 3.S0+.2S/0
Angew. Chem. hi.Ed. Engl. 30 (1991) No. 2
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environment, nickell, hydrogenase, complex, modeling, function
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