Richard A. Henderson

Syntheses, properties and Mössbauer studies of cyanamide and cyanoguanidine complexes of iron(II). Crystal structures of trans-[FeH(NCNH2)(Ph2PCH2CH2PPh2)2][BF4] and trans-[Fe(NCNEt2)2(Et2PCH2CH2PEt2)2][BF4]2

Abstract The complexes trans -[FeH(NCR)(dppe) 2 ][BF 4 ] ( 1 ) (R=NH 2 , NMe 2 , NEt 2 or NC(NH 2 ) 2 ; dppe=Ph 2 PCH 2 CH 2 PPh 2 ) and trans -[FeL(NCR)(depe) 2 ]Y n (R=NH 2 , NMe 2 , NEt 2 or NC(NH 2 ) 2 ; depe=Et 2 PCH 2 CH 2 PEt 2 ; Y=BF 4 or BPh 4 ; ( 2 ), L=Br, n =1; ( 3 ), L=NCR, n =2) have been prepared by treatment of trans -[FeHCl(dppe) 2 ] (in THF and in the presence of Tl[BF 4 ]) or trans -[FeBr 2 (depe) 2 ] (in MeOH and in the presence of [NBu 4 ][BF 4 ] or Na[BPh 4 ]), respectively, with the appropriate cyanamide. NMR and Mossbauer spectral, as well as FAB mass spectrometric data are reported. Mossbauer partial isomer shift (PIS) and partial quadrupole splitting (PQS) parameters have been estimated for the cyanamide and dppe ligands and rationalised, with the overall IS and QS, in terms of π- and σ-electronic effects, the cyanamides behaving as more effective σ-donors and weaker π-acceptors than organonitriles. FAB MS fragmentation patterns are also proposed. The crystal structures of 1 (R=NH 2 ) and 3 (R=NEt 2 , Y=BF 4 ) are reported.

Preparation and crystal structures of the mononuclear vanadium phenoxide complexes [VCl(OC6H3Pri2-2,6)2(C4H8O)2] and [VO(OC6H3Pri2-2,6)3]: procatalysts for ethylene polymerisation

Abstract The complexes [VCl(OC 6 H 3 Pr i 2 -2,6) 2 (thf) 2 ] 1 (thf, tetrahydrofuran) and [VO(OC 6 H 3 Pr i 2 -2,6) 3 ] 2 have been prepared and structurally characterised. Compound 1 has a distorted trigonal bipyramidal structure with apical thf ligands [d(V–O phenox ) 1.865(2), d(V–O thf ) 2.120(3) and d(V–Cl) 2.277(2) A]. It reacts with Li[SC 6 H 2 Me 3 -2,4,6] to give [V(SC 6 H 2 Me 3 -2,4,6)(OC 6 H 3 Pr i 2 -2,6) 2 (thf) 2 ]. Compound 2, which has trigonal pyramidal geometry, is disordered about a 2-fold crystallographic axis [d(V=O) 1.564(4), d(V–O av ) 1.761(1) A]. Compounds 1 and 2 are pro-catalysts for the polymerisation of C 2 H 4 .

Protonation and substitution reactions of Fe–S ‘basket’ clusters including extracted FeMo-cofactor of nitrogenase

Abstract In an extension to our earlier work on the mechanisms of reactions of Fe–S-based clusters, we now report studies on the substitution kinetics of structurally more ‘open’ Fe–S clusters. In particular, we concentrate on protonation of the ‘basket’ cluster [Fe 6 S 6 Cl 2 (PEt 3 ) 4 ]. Protonation at a μ n -S can either accelerate or inhibit the rate of substitution of the cluster depending on: the number of protons added and whether substitution is associative or dissociative. Analysis of the kinetics allows us to calculate the p K a values of the protonated clusters and comparison with a variety of other Fe–S-based clusters shows that, irrespective of the structure and charge, the first (p K a  1 =17.9–18.9) is rather insensitive to the cluster ligands whilst the second (p K a  2 =13.6–16.9) is markedly more sensitive. Analogous studies on extracted FeMo-cofactor [MoFe 7 S 9 (R-homocitrate)(NMF) 2 ] (NMF= N -methylformamide) from the nitrogenase of Klebsiella pneumoniae shows the same general protonation/substitution characteristics as synthetic Fe–S-based clusters.

Aminocarbyne coupling reactions at M(Ph2PCH2CH2PPh2)2(M = Mo or W) sites. Synthesis and properties of the diaminoacetylene complexes trans-[MX(η2-MeHNCCNHMe)-(Ph2PCH2CH2PPh2)2]A (X = F, Cl or ClO4; A = BF4′PF6′HCl2or ClO4) and of their di(aminocarbyne)-type precursors

Treatment of trans-[M(CNMe)2(dppe)2]1(M = Mo or W, dppe = Ph2PCH2CH2PPh2) in CH2Cl2 with HA (A = BF4′ PF6′ Cl or ClO4) gives the diaminoacetylene complexes trans-[MX(η2-MeHNCCNHMe)(dppe)2]A 2(X = F, Cl or ClO4; A = BF4′ PF6′ HCl2 or ClO4). The crystal structure of trans-[MoF(η2-MeHNCCNHMe)(dppe)2][BF4] has been determined. A key intermediate in the reaction, trans-[M(CNHMe)2(dppe)2]A23(A = HCl2 or ClO4), has also been isolated and shown to be best viewed as a ‘iminomethylenium’ species CNHMe. The acetylenic CC triple bond in 2 undergoes base-induced cleavage (e.g., by LiBun) to form the parent complex 1.

The mechanisms of protonation of [M(η5-C5H5)2H2](M = Mo or W)

The reaction between [M(η5-C5H5)2H2](M = Mo or W) and an excess of anhydrous HCl occurs by essentially the same mechanism for the two complexes. Protonation of the tungsten complex involves initial attack at a hydride ligand to generate the spectroscopically detected [W(η5-C5H5)2H(η2H2)]+, which subsequently undergoes an intramolecular oxidative cleavage of the dihydrogen ligand to give the trihydride, [W(η5-C5H5)2H3]+. The kinetic and thermodynamic isotope effects associated with these elementary reactions have been determined. Protonation of the analogous molybdenum complex also involves initial attack at a hydride ligand. However, in this system kinetic and spectroscopic studies indicate that the subsequent cleavage of the dihydrogen ligand to form [Mo(η5-C5H5)2H3]+. occurs via the rapid formation of a detectable binuclear species which is probabaly [{Mo(η5-C5H5)2H2}2(µ-H)]+.

Mechanistic studies on hydrazido(2–)-complexes. Cleavage of the nitrogen–nitrogen bond in the reaction of [Mo{NN(CH2)4CH2}(Ph2PCH2CH2PPh2)2] with acid

The kinetics of the reaction between [Mo{N[graphic omitted]H2}(dppe)2](dppe = Ph2PCH2CH2PPh2) and [NHEt3]BPh4 to yield [graphic omitted]H2 and [MoN(NCR)(dppe)2]BPh4(RCN is the solvent; R = Me, Et, or Ph) has been studied. In general the mechanism of the reactions involves two parallel pathways. Initial rapid protonation of the hydrazido(2–)-ligand and attack of a solvent molecule at the metal yields the spectrophotometrically detected hydrazidium species, trans-[Mo{N[graphic omitted]H2}(NCR)(dppe)2]+. This species can either undergo slow nitrogen–nitrogen cleavage or react via an acid-catalysed pathway involving trans-[Mo{NH[graphic omitted]H2}(NCR)(dppe)2]2+ to yield the products. The factors influencing whether protonation of hydrazido(2–)-ligands results in amine formation (N–N bond cleavage) or hydrazine formation (N–N bond cleavage) are discussed.

Kinetic study of the alkylation of cyanide at [NBu4][trans-Re(CN)2(dppe)2]. Crystal structures of [NBu4][trans-Re(CN)2(dppe)2] and trans-[Re(CN)2(dppe)2]

Abstract The mechanism of alkylation of [NBu 4 ][ trans -Re(CN) 2 (dppe) 2 ] ( 1 ) by alkyl iodides (RI; R=Me, Et or Pr) or EtBr was studied by stopped-flow techniques, which indicate that it involves a fast first alkylation to give trans -[Re(CN)(CNR)(dppe) 2 ] that undergoes a subsequent relatively slow alkylation at the cyano-ligand with a rate constant that decreases with the increase of the carbon chain length of the R group and with the replacement of iodide by bromide in the organohalide. Sodium iodide inhibits the rates of alkylation, probably by forming ion pairs with trans -[Re(CN) 2 (dppe) 2 ] − as confirmed by the formation of the adducts [Re(CN)(CNM)(dppe) 2 ] (M=Li, Na, Tl or Ag) by reaction of 1 with convenient metal salts and by the kinetics of the reaction between MeI and [Re(CN)(CNNa)(dppe) 2 ]. The X-ray molecular structures of [NBu 4 ][ trans -Re(CN) 2 (dppe) 2 ] and trans -[Re(CN) 2 (dppe) 2 ] confirm they have pseudo octahedral geometries and indicate that the former crystallizes in the triclinic P 1 space group with a =17.938(2), b =18.473(3), c =20.061(3) A and the latter in the monoclinic space group P 2 1 / c with a =11.673(2), b =13.302(3), c =17.166(4) A.

Kinetic and Theoretical Studies on the Protonation of [Ni(2-SC6H4N){PhP(CH2CH2PPh2)2}]+: Nitrogen versus Sulfur as the Protonation Site

The complexes [Ni(4-Spy)(triphos)]BPh(4) and [Ni(2-Spy)(triphos)]BPh(4) {triphos = PhP(CH(2)CH(2)PPh(2))(2), 4-Spy = 4-pyridinethiolate, 2-Spy = 2-pyridinethiolate} have been prepared and characterized both spectroscopically and using X-ray crystallography. In both complexes the triphos is a tridentate ligand. However, [Ni(4-Spy)(triphos)](+) comprises a 4-coordinate, square-planar nickel with the 4-Spy ligand bound to the nickel through the sulfur while [Ni(2-Spy)(triphos)](+) contains a 5-coordinate, trigonal-bipyramidal nickel with a bidentate 2-Spy ligand bound to the nickel through both sulfur and nitrogen. The kinetics of the reactions of [Ni(4-Spy)(triphos)](+) and [Ni(2-Spy)(triphos)](+) with lutH(+) (lut = 2,6-dimethylpyridine) in MeCN have been studied using stopped-flow spectrophotometry, and the two complexes show very different reactivities. The reaction of [Ni(4-Spy)(triphos)](+) with lutH(+) is complete within the deadtime of the stopped-flow apparatus (2 ms) and corresponds to protonation of the nitrogen. However, upon mixing [Ni(2-Spy)(triphos)](+) and lutH(+) a reaction is observed (on the seconds time scale) to produce an equilibrium mixture. The mechanistic interpretation of the rate law has been aided by the application of MSINDO semiempirical and ADF calculations. The kinetics and calculations are consistent with the reaction between [Ni(2-Spy)(triphos)](+) and lutH(+) involving initial protonation of the sulfur followed by dissociation of the nitrogen and subsequent transfer of the proton from sulfur to nitrogen. The factors affecting the position of protonation and the coupling of the coordination state of the 2-pyridinethiolate ligand to the site of protonation are discussed.

Protonation of the iron–sulfur core in [Fe4S4X4]2–(X = Cl or Br): chemical precedent for the elementary reaction of the hydrogenases and nitrogenases

Kinetic studies on [Fe4S4X4]2–(X = Cl or Br) show that the substitution of the first halide for thiolate is catalysed by protonation of the Fe4S4 cubane core, this is the first demonstration that these cores will bind protons.

The preparation, structure, and reactions of the mononuclear thiolate–hydride complexes [MoH(SR)(dppe)2](R = bulky alkyl or aryl group, dppe = Ph2PCH2CH2PPh2); X-ray structure determination of cis-[MoH(SC6H2Pri3-2,4,6)(dppe)2]

Reaction of trans-[Mo(N2)2(dppe)2](dppe = Ph2PCH2CH2PPh2) with the bulky thiols RSH (R = Pri, But, C6H2Me3-2,4,6, C6H2Pri3-2,4,6, or C6H2BrPri2-4,2,6) at reflux in tetrahydrofuran (thf) gives the green hydride complexes [MoH(SR)(dppe)2]. The complex cis-[MoH(SC6H2Pri3-2,4,6)(dppe)2]·0.5C6H5Me has a distorted octahedral structure, with distances: Mo–S 2.402(2), Mo–H 1.61(9), Mo–P 2.380(2)–2.465(2)A. The 1H hydride resonances of these compounds occur in the range –3.98 to –4.98 p.p.m. (relative to SiMe4) and show fluxionality at room temperature with respect to the phosphorus atoms except for R = C6H2BrPri2-4,2,6. Other spectroscopic properties of the compounds are used as a basis to discuss the mechanism of their formation. The complex [MoH(SC6H2Pri3-2,4,6)(dppe)2] reacts at 20 °C with CO or ButNC to give the known compounds [Mo(CO)2(dppe)2] and [Mo(ButNC)2(dppe)2], and with PhSH to give trans-[Mo(SPh)2(dppe)2].