Probing the influence of local coordination environment on ligand binding in nickel hydrogenase model complexes

Abstract

We report the synthesis and structure of three structurally-related mononuclear S-ligated Ni (I1) complexes, 6-coordinate [ N~( L S~N Z ) ( M~C N ) ]~+ (1; LS3NZ = ( H z N C Z H~S C~H~)~S ) , 6-coordinate [Ni(BATES)Cl]+ (%BATES = ( H~N -~-C~H~S C Z H~) Z S ) , and 5-coordinate Ni(Lss) (3; Ls5 = (-%-C~H~SCZ&)~S), which contain Ni in a S-rich environment possibly resembling that of the active form of nickel hydrogenases. Complex 1 crystallizes in the monoclinic space group P21/n with a = 19.036(2) A, b = 12.022(8) A, c = 24.953(4) A, / 3 = 107.78(1)', and 2 = 4. Complex 2 crystallizes in the space group P i with a = 11.213(1) A, b = 13.482(2) A, c = 14.414(2) A, a = 72.82(1)', @ = 89.862(9)', y = 69.396(9)', and Z = 2. Complex 3 crystallizes in the monoclinic space group P2l/c with u = 10.759(2) A, b = 11.251(3) A, c 14.198(1) A, @ = 90.82(1)', and 2 = 4. Structures 1-3 share a common trithioether (Sapial(C~H4SRcq)2; I & = C2H4NH2 (l), CaH4NH2 (2), or CsHdS-(3)) "backbone" and possess either a "vacant" or a labile coordination site. These structures differ in terms of their molecular charge (2+ (l), 1+ (2), 0 (3)) and terminal ligating atoms. The amine-ligated complexes 1 and 2 have a higher affinity for axial ligands and are more reactive than the thiolate-ligated complex 3. The Ni-bound MeCN solvent molecule of 1 is exchangeable and readily displaced by weaker coordinating solvents, as well as anionic ligands. X-ray structures of the most stable derivatives of 1, [Ni(Ls3~z)(py)]~' (7), [N~(Ls~Nz)(N~)]+ (81, and [Ni(Ls3~2)(Cl)]+ (9), are presented, and structural parameters are compared for [N~(Ls~Nz)(L)]"' (n = 1,2; L = MeCN (l), py (7), N3-(8), C1-(9), and B r (4)). Reaction of 2 with NaBH4 at 4 2 'C affords a thermally unstable species 10, which behaves like a Ni-H in terms of its reactivity. Hydridederivative 10 will hydrogenate olefins (styrene and cyclohexene) and convert CDCl3 to CHDClZ, In contrast to the amine-ligated complexes, thiolate-ligated 3 is fairly unreactive with respect to binding axial ligands. The only ligand examined in this study which was strong enough to induce the spin-state change required in converting 3 to a 6-coordinate structure was pyridine. Implications that this observed decrease in reactivity, upon substitution of terminal amines with aromatic thiolates, has with respect to the probable structure of the Ni-binding site of hydrogenase is also discussed.Research efforts in our group have been directed toward the design and synthesis of mononuclear Ni complexes which contain Ni in a S-rich environment resembling that of the reactive, reduced form of Ni hydrogenases (H2-ases) l4 and which incorporate features designed to promote reactivity. The Ni site in Hz-ases catalyzes H+/D2 exchange, HZ activation and oxidation, or, in some cases, Hz evolution. Although the Ni sites do not appear to be identical for all Hz-ases, all of them are mononuclear, redox t In this paper, boldface numbers denoting chemical species may refer to 0 Abstract published in Aduance ACS Abstracts, October 15, 1...

Structural Comparison of Five-Coordinate Thiolate-Ligated MII = FeII, CoII, NiII, and ZnII Ions Wrapped in a Chiral Helical Ligand

“…The mean M−N distance in the [M II (S R2 N 3 (Pr,Pr)] (M = Fe ( 1 ), Co ( 2 ), Ni ( 3 ), Zn ( 4 ); R = H, Me) structures decreases monotonically (Table ) from 1 to 3 and follows the trend ( 1 > 2 ≫ 3 ) predicted based on ionic radii (with distances in 1 approximately equivalent to those in 4 ). Although the number of structures available for comparison is limited (five-coordinate structures are rare), these distances fall in the usual range (2.071(4)−2.27(3) Å for Fe 2+ ; 2.037(6)−2.212(7) Å for Zn 2+ ; 2.036(4)−2.146(8) Å for Co 2+ ; 2.01(1)−2.131(6) Å for Ni 2+ ). ,− Trends in the mean M−S distance in structures 1 − 4 ( 1 ≥ 4 = 3 > 2 ) do not follow the order predicted on the basis of covalent ( 2 ≥ 1 ≫ 3 > 4 ) or ionic ( 1 ≥ 4 > 2 ≫ 3 ) radii, implying that the nature of the M−SR bonds changes through the series. On the basis of a comparison of these distances (2.35(1) Å in 1 , 2.33(1) Å in 4 , 2.33(3) Å in 3 , and 2.28(1) Å in 2 ) with those predicted from covalent (2.27 Å in 1 , 2.22 Å in 4 , 2.23 Å in 3 , and 2.28 Å in 2 ) or ionic (2.54 Å in 1 , 2.52 Å in 4 , 2.51 Å in 2 , and 2.47 Å in 3 ) radii, it would appear that all of these have significant covalent character (∼60% in 3 and 4 , ∼70% in 1 , and ∼100% in 2 ).…”

A new family of sulfur-rich ligands based on the dmit system: synthesis and metal complexation of 4–4′-covalently bridged bis(2-thioxo-1,3-dithiol-5-thiolato) units

“…14 The Ni-N bond lengths (2.095(5) and 2.109(5) Å) are similar to those found in related nickel thioether complexes with pyridine as additional ligands. [15][16][17] The angle between the least squares planes of the two pyridine rings is 53.89(26)Њ.…”

A Coordination Polymer of Nickel(II) Based on a Pentadentate N, S, and O Donor Ligand

“…Given the ubiquity of this diaxial ligation phenomena, [4][5][6]8,14 it is likely that such is operative in the mechanism of formation of complex 3; this is explored below.…”

“…In soft donor sets such as phosphines and thioethers, 18-electron, organometallic type complexes are known, i.e. HNiP 2 S 2 + , P 2 S 2 NiCN + , and P 2 S 2 NiR + (R = Me, C(O)Me) 2e,3 However, attempts to sequentially add axial ligands to square planar Ni II complexes in N 2 S‘ 2 (S‘ = thioether) mixed donor sets generating square pyramidal complexes have generally been unsuccessful; addition of one axial ligand typically brings about addition of a second one. , Crystal structure data implies an apical interaction between the Ni II center and the Br - counterion for [ Ni - 1 (bicycle)]Br 2 , which finds Ni within the N 2 S‘ 2 plane, and ∠S−Ni−Br and ∠N−Ni−Br averaging 90° 6a. Located almost within bonding distance at 2.95 Å, the Br - is still ca.…”