Kinetics of reactions of 2,2′-bipyridyltetracarbonylmolybdenum(0)

Summary Kinetic results are presented for reaction of Mo(CO)₄(bipy) with cyanide in several nonaqueous solvents and in DMSO-H₂O mixtures, with methoxide, and with azide; for reaction of Mo(CO)₄(5-NO₂phen) with cyanide and with methoxide; and for reaction of Mo(CO)₄(phen) and of W(CO)₄(bipy) with cyanide. Solvent effects on the reaction of Mo(CO)₄(bipy) with cyanide are dissected into their initial state and transition state components. Here, and in the dependence of the activation volume for this and related reactions on solvent, the important role played by cyanide solvation is apparent. Preliminary investigations on reactions of compounds of this M(CO)₄(diimine) type with tertiary phosphines, diethyldithiocarbamate, and ether peroxides are described.     

Solvent Effects on the piezochromism of molybdenum(IV) and tungsten(IV) anions [M(CN)3O(diimine)]−

Solvatochromism of the [Mo(CN)₃O(phen)]⁻ ion has been studied. Solvent effects on pressure dependence (up to 1.25 kbar) of wavelengths of maximum absorption for charge-transfer bands of several molybdenum(IV) and tungsten(IV) complexes, [M(CN)₃O(diimine)]⁻ (M = Mo or W), are described. The effects of applied pressure are larger for less polar unstructured solvents than for highly polar hydrogen-bonded solvents. This revised version was published online in June 2006 with corrections to the Cover Date.     

Kinetics of nucleophilic attack on coordinated organic moieties : VI. Mechanism of addition of tri-n-butylphosphine to [(C7H7)M(CO)3]BF4 (M = Cr, Mo, W) and related cations

Phosphonium adduct formation via attack of tri-n-butylphosphine on the cations [(C₇H₇)M(CO)₃]⁺ (M = Cr, Mo, W) obeys the rate law, Rate = k [complex] [PBu₃]. The very similar rate constants for the Cr, Mo and W complexes confirm the similar electrophilicities of the tropylium rings in these cations, and also support the view that there is direct addition to the rings. The related complexes [(C₆H₇)Fe(CO)₃]BF₄ and [(C₆H₆)Mn(CO)₃]BF₄ also form adducts with PBu₃, and the quantitative reactivity order [(C₆H₇)Fe(CO)₃]⁺ > [(C₇H₇)Cr(CO)₃]⁺ » [(C₆H₆)Mn(CO)₃]⁺ (160:60:1) has been established.     

Synthesis and characterization of chromium and tungsten tetracarbonyl bound to the bridging ligand 2,3-bis(2-pyridyl)pyrazine (dpp)

The synthesis, electronic absorption, infrared and ¹³C NMR spectra, and electrochemistry of [Cr(CO)₄]dpp and [W(CO)₄]dpp (dpp = 2,3-bis(2-pyridyl)pyrazine) are reported. Electronic absorption spectra in different solvents show solvatochromic behavior and indicate the metal to ligand charge transfer (MLCT) excited state is lower in energy than the ligand field (LF) excited state. Electrochemical results show the [Cr(CO)₄]dpp complex undergoes semi-reversible one electron oxidation, while [W(CO)₄]dpp undergoes irreversible one electron oxidation. ¹³C NMR spectra of both complexes show all carbons in the metal-bound dpp ring shift downfield. The amount of downfield shift for the series [M(CO)₄]dpp (M = Cr, Mo, W) is interpreted as indicating better M dπ → dpp pπ backbonding occurs for Mo and W than for Cr.     

Solvent,ligand, pressure,and temperature effects on charge-transfer spectra of tetracarbonylmolybdenum(0) diimine complexes

Summary The solvent, pressure, and temperature-dependence of the lowest energy metal-to-ligand charge-transfer absorption bands are reported for a series of [Mo(CO)₄(LL)], compounds where LL is a bidentate diimine ligand containing the chelating fragment –N=CRCR=N–, and one terdentate ligand analogue [Mo(CO)₃(LLL)]. The effects of ligand nature on these dependences are discussed, as are their interrelations and their connection with solvent properties such as polarity.     

Initial state and transition state solvation in substitution at the square-planar complexes, [PtCl4]2− and [Pd(Et4dien)Cl]+

Summary Rate constants have been determined for the reaction of [PtCl₄]^2– with cyanide in water and in 20% and 40% (v/v) methanol, and for the reaction of [Pd(Et₄dien)Cl]⁺ with thiourea in water, in 50% methanol, and in 50% DMSO, in all cases at 298.2K. The solubility of K₂PtCl₄ has been determined in water and in 20% and 40% methanol; the solubility of [Pd(Et₄dien)Cl]Cl has been determined in water, in 20%, 40%, 60%, and 80% methanol, and in 40% and 80% DMSO; again in all cases at 298.2K. From these solubilities, Gibbs free energies of transfer for the [PtCl₄]^2– and [Pd(Et₄dien)Cl]⁺ ions have been estimated. From these transfer data, published transfer data for cyanide and thiourea, and these and earlier kinetic results, solvent effects on reactivity have been dissected into their initial state and transition state components for the following four reactions: [PtCl₄]^2– hydrolysis, [PtCl₄]^2– plus cyanide, [Pd(Et₄dien)Cl]⁺ substitution, and [Pd(Et₄-dien)Cl]⁺ plus thiourea. The patterns thus established are discussed, and compared with those previously obtained for some other reactions of transition metal complexes.     

New aspects of metal carbonyl ylide chemistry

Synthesis and properties of N₂H₂, H₂h₄ and NH₃ complexes of molybdenum are described. N₂H₄ and NH₃ react in solution with Mo(CO)₆ under normal conditions yielding Mo(CO)₅H₂H₄ and Mo(CO)₅NH₃ respectively. The oxidation of Mo(CO)₅H₂H₄ with H₂/Cu²⁺ yields μ-N₂H₂[Mo(CO)₅]₂; this complex contains the N₂H₂ ligand as diimine, HN=NH, which is presumed to be the first stage of reduction of the N₂ molecule in N₂ fixation. μ-N₂H₄[Mo(CO)₅]₂ is obtained by low temperature synthesis from Mo(CO)₅THF and N₂H₄; decomposition of this complex leads to the bis-μ-hydrazine complex [N₂H₄]₂[Mo(CO)₄]₂- μ-N₂H₂[Mo(CO)₅]₂ can be converted to μ-N₂H₄[Mo(CO)₅]₂, which is another proof for the (1) oxidation state of nitrogen in the N₂H₂ complex. A comparison of the analogous Cr, Mo and W complexes shows that the molybdenum compounds exhibit the highest reactivity and lowest stability.     

The limiting dissociative mechanism for substitution at thed 6 centres molybdenum(O), iron(II), and cobalt(III): Kinetics of substitution at the pentacyano-4-Cyanopyridineferrate(II) anion and at 4-cyanopyridinemolybdenum(I) pentacarbonyl

Summary Rate constants are reported for reaction of the 4-cyanopyridine complexes [Fe(CN)₅(4CNpy)]^3– and [Mo(CO)₅(4CNpy)] with a variety of incoming ligands, in aqueous methanol (40 vol % MeOH) and in toluene respectively, at 298.2 K (ambient pressure). The dependence of rate constants on the nature and concentration of the incoming ligand is discussed in terms of the operation of the limiting dissociative,D, mechanism for substitution; the operation of this mechanism here, and in analogous pentacyanoferrate(II), pentacarbonylmolybdenum(I), and penta- and tetra-cyanocobaltate(III) complexes is reviewed. The effect of pressure on rate constants for replacement of 4-cyanopyridine in [Mo(CO)₅(4CNpy)], in toluene solution at 298.2 K, indicates an activation volume of +3 cm³ mol^–1.     

[3+2] Fragmentation of a Pentaphosphido Ligand by Cyanide

The activation of white phosphorus (P₄) by transition‐metal complexes has been studied for several decades, but the functionalization and release of the resulting (organo)phosphorus ligands has rarely been achieved. Herein we describe the formation of rare diphosphan‐1‐ide anions from a P₅ ligand by treatment with cyanide. Cobalt diorganopentaphosphido complexes have been synthesized by a stepwise reaction sequence involving a low‐valent diimine cobalt complex, white phosphorus, and diorganochlorophosphanes. The reactions of the complexes with tetraalkylammonium or potassium cyanide afford a cyclotriphosphido cobaltate anion 5 and 1‐cyanodiphosphan‐1‐ide anions [R₂PPCN]^? ( 6‐R ). The molecular structure of a related product 7 suggests a novel reaction mechanism, where coordination of the cyanide anion to the cobalt center induces a ligand rearrangement. This is followed by nucleophilic attack of a second cyanide anion at a phosphorus atom and release of the P₂ fragment.     

Synthesis, spectroscopic and structural characterisation of molybdenum, tungsten and manganese carbonyl complexes of tetrathio- and tetraseleno-ether ligands

A range of complexes of the binucleating tetrathio- and tetraseleno-ether ligands, 1,2,4,5-C₆H₂(CH₂EMe)₄ (E = S, L³ or Se, L⁴) or C(CH₂EMe)₄ (E = S, L⁵ or Se, L⁶) and of bidentate analogues 1,2-C₆H₂(CH₂EMe)₂ (E = S, L¹ or Se = L²) with molybdenum and tungsten carbonyls and manganese carbonyl chloride have been prepared, and characterised by IR and multinuclear NMR (¹H, ¹³C{¹H}, ⁷⁷Se, ⁵⁵Mn, ⁹⁵Mo) spectroscopy and mass spectrometry. Crystal structures are reported for [Mo(CO)₄(L²)], [Mo(CO)₄(L³)], [Mo(CO)₄(μ-L³)Mo(CO)₄], [Mo(CO)₄(L⁴)], [Mn(CO)₃Cl(μ-L³)Mn(CO)₃Cl], [Mo(CO)₄(μ-L⁵)Mo(CO)₄], [Mn(CO)₃Cl(L⁵)] and two forms (containing meso and DL diastereoisomers) of [W(CO)₄(L⁵)].     

Kinetics of substitution reactions of transition metal complexes in 1,3-dioxolan-2-one + water and 4-methyl-1,3-dioxolan-2-one + water solvent mixtures

Summary Rate constants are reported and discussed for several substitutions of inorganic complexes in ethylene carbonate (1,3-dioxolan-2-one) + water and in propylene carbonate (4-methyl-1,3-dioxolan-2-one) + water solvent mixtures. The reactions include aquation ofcis- and oftrans-[Co(en)₂Cl₂]⁺, aquation oftrans-[Cr(OH₂)₄Cl₂]⁺, bromide substitution at [Pd(Et₄dien)Cl]⁺, thiourea substitution atcis-[Pt(4-NCpy)₂Cl₂], and aquation and cyanide attack at [Fe(X-phen)₃]²⁺ cations.     

Binuclear 2,2′-bipyrimidine complexes derived from chromium,molybdenum and tungsten carbonyls

Reaction of 2,2′-bipyrimidine (bpym) with [Mo(CO)₄(diene)] gives [Mo(CO)₄(bpym)], which will react with [M(CO)₄(diene)] to form [MoM(CO)₈(bpym)] (M = Cr, Mo, W). The bipyrimidine complexes are characterised by microanalysis and spectroscopy (IR, ¹H and ¹³C NMR, UV/vis). Reduction of [Mo₂(CO)₈(bpym)] produces an anion in which the unpaired electron is localised on the bridging bpym ligand.     

Kinetics of substitution atcis-tetracarbonylbis(4-methylpyridine)-tungsten(0)

Summary Rate constants have been obtained for reactions ofcis-tetracarbonylbis(4-methylpyridine)tungsten(0),cis-[W(CO)₄(4Mepy)₂], with 1, 10-phenanthroline, 2,2-bipyridyl, and related diimine ligands in toluene solution at 298.2K. Effects of varying the nature and concentration of the entering ligand, and of addition of 4-methylpyridine, indicate the operation of a limiting dissociative (D) mechanism. This is confirmed by the establishment of an activation volume of + 8 cm³ mol^–1, in toluene at 298.2K.     

Nitrene Insertion into CC and CH Bonds of Diamide Diimine Ligands Ligated to Chromium and Iron

The impact of redox non‐innocence (RNI) on chemical reactivity is a forefront theme in coordination chemistry. A diamide diimine ligand, [{‐CH?N(1,2‐C₆H₄)NH(2,6‐iPr₂C₆H₃)}₂]ⁿ (n=0 to ?4), (dadi)ⁿ, chelates Cr and Fe to give [(dadi)M] ([ 1 Cr(thf)] and [ 1 Fe]). Calculations show [ 1 Cr(thf)] (and [ 1 Cr]) to have a d⁴ Cr configuration antiferromagnetically coupled to (dadi)^2?*, and [ 1 Fe] to be S=2. Treatment with RN₃ provides products where RN is formally inserted into the C? C bond of the diimine or into a C? H bond of the diimine. Calculations on the process support a mechanism in which a transient imide (imidyl) aziridinates the diimine, which subsequently ring opens.     

Nitrene Insertion into C?C and C?H Bonds of Diamide Diimine Ligands Ligated to Chromium and Iron

The impact of redox non‐innocence (RNI) on chemical reactivity is a forefront theme in coordination chemistry. A diamide diimine ligand, [{‐CHN(1,2‐C₆H₄)NH(2,6‐iPr₂C₆H₃)}₂]ⁿ (n=0 to −4), (dadi)ⁿ, chelates Cr and Fe to give [(dadi)M] ([ 1 Cr(thf)] and [ 1 Fe]). Calculations show [ 1 Cr(thf)] (and [ 1 Cr]) to have a d⁴ Cr configuration antiferromagnetically coupled to (dadi)²⁻*, and [ 1 Fe] to be S=2. Treatment with RN₃ provides products where RN is formally inserted into the C C bond of the diimine or into a C H bond of the diimine. Calculations on the process support a mechanism in which a transient imide (imidyl) aziridinates the diimine, which subsequently ring opens.     

Activation of the Ge-H bond of Et3GeH in photochemical reaction with molybdenum(0) carbonyl complexes and hydrogermylation of norbornadiene

The germane intermediate σ-complexes, characterized by high-field resonances in the region from −6 to −8 ppm, have been detected during the ¹H NMR spectroscopy monitoring of the photochemical reaction of Et₃GeH with Mo(CO)₆, [Mo(CO)₄(η⁴-cod)], and [Mo(CO)₄(η⁴-nbd)] in the NMR tube. The activation of the Ge-H bond of germane in photochemical reaction of the norbornadiene (nbd) complex [Mo(CO)₄(η⁴-nbd)] has been applied in the hydrogermylation of norbornadiene, which leads to the formation of triethylgermylnorbornene.     

Molybdenum carbonyl complexes of binucleating pyridine-based ligands

Reaction of [Mo(CO)₄(diene)] with 4,4′-bipyridine (44′B), trans-1,2-bis(2-pyridyl)ethene (2-bpe) and trans-1,2-bis(4-pyridyl)-ethene (4-bpe) gives polymeric [Mo(CO)₄(44′B)]_n, mononuclear cis-[Mo(CO)₄(2-bpe)₂] and binuclear [Mo(CO)₄(4-bpe)]₂ respectively. Reaction of the same ligands with [Mo(CO)₄(bpy)] (bpy is 2,2′-bipyridine) produces the bridged binuclear complexes [{Mo(CO)₃(bpy)}₂(44′B)] and [{Mo(CO)₃(bpy)}₂(4-bpe)]. Products are characterised by microanalysis and spectroscopy (IR, ¹H NMR, UV/vis). Reduction of [{Mo(CO)₃(bpy)}₂(44′B)] produces an anion in which the unpaired electron is localised on the chelating bpy ligand.     

Reactivity and solvatochromism of 2,2′-bi(4H-5,6-dihydrothiazine)-tetracarbonylmolybdenum(O)

Summary The dependence of the charge-transfer frequency for [Mo(CO)₄(btz)], btz = 2,2-bi(4H-5,6-dihydrothiazine), on solvent is described, and the solvatochromic behaviour of this compound compared with that of other [Mo(CO)₄(LL)] species, with LL = 2,2-bipyrimidine or 2,2-bipyridine, and of iron(II) analogues [Fe(btz)₂(CN)₂] and [Fe(bipy)₂(CN)₂]. Kinetics of solvolysis (k, H, S) are reported for [Mo(CO)₄(btz)] in methanol, acetonitrile, and dimethyl sulphoxide. These kinetic results are analysed into initial state and transition state contributions. A parallel analysis of the solvatochromic results for [Mo(CO)₄(btz)] into ground state and excited state solvation contributions is compared with similar analyses for the solvatochromic organic compoundsp-nitroanisole and dimethylindoaniline.     

Crown thioethers and the kinetic macrocyclic effect. Solvolysis of molybdenum(0) tricarbonyl complexes of cyclic and acyclic trithioethers

Summary Solvolysis of the macrocyclic trithioether complexes fac-[Mo(CO)₃([9]aneS₃)] ([9]aneS₃ = 1,4,7-trithiacyclononane) and fac-[Mo(CO)₃(ttob)] (ttob = 2,5,8-trithia[9]-o-benzenophane), and the acyclic trithioether complex fac-[Mo(CO)₃(ttn)] (ttn = 2,5,8-trithiononane) in dimethylsulfoxide (DMSO) solution gives the free trithioether in each case. The kinetics of these reactions were studied by ¹H-n.m.r. spectroscopy. In the absence of acid or base, the rate of solvolysis of [Mo(CO)₃([9]aneS₃)] is at least 4.6 × 10⁵ slower than that of [Mo(CO)₃(ttn)]; larger macrocycles give intermediate rates. [Mo(CO)₃-([9]aneS₃) and [Mo(CO)₃(ttob)] undergo stoichiometric reactions with acid and base in DMSO which in all cases lead to more rapid loss of the intact macrocycle than in DMSO alone. The results are discussed in terms of the structure and bonding of likely intermediate complexes.     

Charge relocation in cationic diene complexes: formation of an iminium-substituted allyl complex and its deprotonation to an enamine

The reaction of [Mo(NCME)₂(CO)₂(η⁵-C₉H₇)][BF₄] (1) with 1-dimethylaminocyclohexa-1,3-diene affords the cationic η³-allyl complex [Mo(η³-C₆H₇NMe₂)(CO)₂- (η⁵-C₉H₇)][BF₄] (3), in which the positive charge is located at an exocyclic iminium centre. Addition of Li[N(SiMe₃)₂] to 3 results in deprotonation and the formation of an enamine species [Mo(η³-C₆H₆NMe₂)(CO)₂(η⁵-C₉H₇)] (8), which undergoes stereofacial attack upon treatment with electrophiles.