Manuel Temprado

Oxygen Binding to [Pd(L)(L')] (L= NHC, L' = NHC or PR3, NHC = N-Heterocyclic Carbene). Synthesis and Structure of a Paramagnetic trans-[Pd(NHC)2(η1-O2)2] Complex

The reactivity of a number of two-coordinate [Pd(L)(L)] (L = N-heterocyclic carbene (NHC) and L = NHC or PR(3)) complexes with O(2) has been examined. Stopped-flow kinetic studies show that O(2) binding to [Pd(IPr)(P(p-tolyl)(3))] to form cis-[Pd(IPr)(P(p-tolyl)(3))(η(2)-O(2))] occurs in a rapid, second-order process. The enthalpy of O(2) binding to the Pd(0) center has been determined by solution calorimetry to be -26.2(1.9) kcal/mol. Extension of this work to the bis-NHC complex [Pd(IPr)(2)], however, did not lead to the formation of the expected diamagnetic complex cis-[Pd(IPr)(2)(η(2)-O(2))] but to paramagnetic trans-[(Pd(IPr)(2)(η(1)-O(2))(2)], which has been fully characterized. Computational studies addressing the energetics of O(2) binding have been performed and provide insight into reactivity changes as steric pressure is increased.

A retro Diels-Alder route to diphosphorus chemistry: molecular precursor synthesis, kinetics of P2 transfer to 1,3-dienes, and detection of P2 by molecular beam mass spectrometry.

The transannular diphosphorus bisanthracene adduct P2A2 (A = anthracene or C14H10) was synthesized from the 7-phosphadibenzonorbornadiene Me2NPA through a synthetic sequence involving chlorophosphine ClPA (28-35%) and the tetracyclic salt [P2A2Cl][AlCl4] (65%) as isolated intermediates. P2A2 was found to transfer P2 efficiently to 1,3-cyclohexadiene (CHD), 1,3-butadiene (BD), and (C2H4)Pt(PPh3)2 to form P2(CHD)2 (>90%), P2(BD)2 (69%), and (P2)[Pt(PPh3)2]2 (47%), respectively, and was characterized by X-ray diffraction as the complex [CpMo(CO)3(P2A2)][BF4]. Experimental and computational thermodynamic activation parameters for the thermolysis of P2A2 in a solution containing different amounts of CHD (0, 4.75, and 182 equiv) have been obtained and suggest that P2A2 thermally transfers P2 to CHD through two competitive routes: (i) an associative pathway in which reactive intermediate [P2A] adds the first molecule of CHD before departure of the second anthracene, and (ii) a dissociative pathway in which [P2A] fragments to P2 and A prior to addition of CHD. Additionally, a molecular beam mass spectrometry study on the thermolysis of solid P2A2 reveals the direct detection of molecular fragments of only P2 and anthracene, thus establishing a link between solution-phase P2-transfer chemistry and production of gas-phase P2 by mild thermal activation of a molecular precursor.

Uptake of one and two molecules of CO2 by the molybdate dianion: a soluble, molecular oxide model system for carbon dioxide fixation

Tetrahedral [MoO4]2− readily binds CO2 at room temperature to produce a robust monocarbonate complex, [MoO3(κ2-CO3)]2−, that does not release CO2 even at modestly elevated temperatures (up to 56 °C in solution and 70 °C in the solid state). In the presence of excess carbon dioxide, a second molecule of CO2 binds to afford a pseudo-octahedral dioxo dicarbonate complex, [MoO2(κ2-CO3)2]2−, the first structurally characterized transition-metal dicarbonate complex derived from CO2. The monocarbonate [MoO3(κ2-CO3)]2− reacts with triethylsilane in acetonitrile under an atmosphere of CO2 to produce formate (69% isolated yield) together with silylated molybdate (quantitative conversion to [MoO3(OSiEt3)]−, 50% isolated yield) after 22 hours at 85 °C. This system thus illustrates both the reversible binding of CO2 by a simple transition-metal oxoanion and the ability of the latter molecular metal oxide to facilitate chemical CO2 reduction.

Thermodynamic and Kinetic Study of Cleavage of the N–O Bond of N-Oxides by a Vanadium(III) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2, 1) from compounds containing N-O bonds with a range of BDEs spanning nearly 100 kcal mol(-1): PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N2O (62) > MesCNO (53) > N2O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis(diisopropyl)phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N2O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N2O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to 1 show that the V-O bond in O≡V(N[t-Bu]Ar)3 is strong (BDE = 154 ± 3 kcal mol(-1)) compared with all the N-O bonds cleaved. In contrast, measurement of the N-O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol(-1)) and that dissociation of dbabhNO to anthracene, N2, and a (3)O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N2O and MesCNO to the bulky complex 1 show a faster rate than in the case of free N2O or MesCNO despite increased steric hindrance of the adducts.

Thermodynamic, Kinetic, and Mechanistic Study of Oxygen Atom Transfer from Mesityl Nitrile Oxide to Phosphines and to a Terminal Metal Phosphido Complex

The enthalpies of oxygen atom transfer (OAT) from mesityl nitrile oxide (MesCNO) to Me(3)P, Cy(3)P, Ph(3)P, and the complex (Ar[(t)Bu]N)(3)MoP (Ar = 3,5-C(6)H(3)Me(2)) have been measured by solution calorimetry yielding the following P-O bond dissociation enthalpy estimates in toluene solution (±3 kcal mol(-1)): Me(3)PO [138.5], Cy(3)PO [137.6], Ph(3)PO [132.2], (Ar[(t)Bu]N)(3)MoPO [108.9]. The data for (Ar[(t)Bu]N)(3)MoPO yield an estimate of 60.2 kcal mol(-1) for dissociation of PO from (Ar[(t)Bu]N)(3)MoPO. The mechanism of OAT from MesCNO to R(3)P and (Ar[(t)Bu]N)(3)MoP has been investigated by UV-vis and FTIR kinetic studies as well as computationally. Reactivity of R(3)P and (Ar[(t)Bu]N)(3)MoP with MesCNO is proposed to occur by nucleophilic attack by the lone pair of electrons on the phosphine or phosphide to the electrophilic C atom of MesCNO forming an adduct rather than direct attack at the terminal O. This mechanism is supported by computational studies. In addition, reaction of the N-heterocyclic carbene SIPr (SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene) with MesCNO results in formation of a stable adduct in which the lone pair of the carbene attacks the C atom of MesCNO. The crystal structure of the blue SIPr·MesCNO adduct is reported, and resembles one of the computed structures for attack of the lone pair of electrons of Me(3)P on the C atom of MesCNO. Furthermore, this adduct in which the electrophilic C atom of MesCNO is blocked by coordination to the NHC does not undergo OAT with R(3)P. However, it does undergo rapid OAT with coordinatively unsaturated metal complexes such as (Ar[(t)Bu]N)(3)V since these proceed by attack of the unblocked terminal O site of the SIPr·MesCNO adduct rather than at the blocked C site. OAT from MesCNO to pyridine, tetrahydrothiophene, and (Ar[(t)Bu]N)(3)MoN was found not to proceed in spite of thermochemical favorability.

Two-Step Binding of O2 to a Vanadium(III) Trisanilide Complex To Form a Non-Vanadyl Vanadium(V) Peroxo Complex

Treatment of V(N[(t)Bu]Ar)(3) (1) (Ar = 3,5-Me(2)C(6)H(3)) with O(2) was shown by stopped-flow kinetic studies to result in the rapid formation of (η(1)-O(2))V(N[(t)Bu]Ar)(3) (2) (ΔH(‡) = 3.3 ± 0.2 kcal/mol and ΔS(‡) = -22 ± 1 cal mol(-1) K(-1)), which subsequently isomerizes to (η(2)-O(2))V(N[(t)Bu]Ar)(3) (3) (ΔH(‡) = 10.3 ± 0.9 kcal/mol and ΔS(‡) = -6 ± 4 cal mol(-1) K(-1)). The enthalpy of binding of O(2) to form 3 is -75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of 3 and 1 to form 2 equiv of O≡V(N[(t)Bu]Ar)(3) (4) occurs by initial isomerization of 3 to 2. The results of computational studies of this rearrangement (ΔH = 4.2 kcal/mol; ΔH(‡) = 16 kcal/mol) are in accord with experimental data (ΔH = 4 ± 3 kcal/mol; ΔH(‡) = 14 ± 3 kcal/mol). With the aim of suppressing the formation of 4, the reaction of O(2) with 1 in the presence of (t)BuCN was studied. At -45 °C, the principal products of this reaction are 3 and (t)BuC(═O)N≡V(N[(t)Bu]Ar)(3) (5), in which the bound nitrile has been oxidized. Crystal structures of 3 and 5 are reported.

Synthesis of [Pt(SnBut3)(IBut)(μ-H)]2, a Coordinatively Unsaturated Dinuclear Compound which Fragments upon Addition of Small Molecules to Form Mononuclear Pt–Sn Complexes

The reaction of Pt(COD)2 with one equivalent of tri-tert-butylstannane, Bu(t)3SnH, at room temperature yields Pt(SnBu(t)3)(COD)(H)(3) in quantitative yield. In the presence of excess Bu(t)3SnH, the reaction goes further, yielding the dinuclear bridging stannylene complex [Pt(SnBu(t)3)(μ-SnBu(t)2)(H)2]2 (4). The dinuclear complex 4 reacts rapidly and reversibly with CO to furnish [Pt(SnBu(t)3)(μ-SnBu(t)2)(CO)(H)2]2 (5). Complex 3 reacts with N,N-di-tert-butylimidazol-2-ylidene, IBu(t), at room temperature to give the dinuclear bridging hydride complex [Pt(SnBu(t)3)(IBu(t))(μ-H)]2 (6). Complex 6 reacts with CO, C2H4, and H2 to give the corresponding mononuclear Pt complexes Pt(SnBu(t)3)(IBu(t))(CO)(H)(7), Pt(SnBu(t)3)(IBu(t))(C2H4)(H)(8), and Pt(SnBu(t)3)(IBu(t))(H)3 (9), respectively. The reaction of IBu(t) with the complex Pt(SnBu(t)3)2(CO)2 (10) yielded an abnormal Pt-carbene complex Pt(SnBu(t)3)2(aIBu(t))(CO) (11). DFT computational studies of the dimeric complexes [Pt(SnR3)(NHC)(μ-H)]2, the potentially more reactive monomeric complexes Pt(SnR3)(NHC)(H) and the trihydride species Pt(SnBu(t)3)(IBu(t))(H)3 have been performed, for NHC = IMe and R = Me and for NHC = IBu(t) and R = Bu(t). The structures of complexes 3-8 and 11 have been determined by X-ray crystallography and are reported.

Experimental and Computational Studies of Binding of Dinitrogen, Nitriles, Azides, Diazoalkanes, Pyridine, and Pyrazines to M(PR3)2(CO)3 (M = Mo, W; R = Me,iPr)

The enthalpies of binding of a number of N-donor ligands to the complex Mo(P(i)Pr(3))(2)(CO)(3) in toluene have been determined by solution calorimetry and equilibrium measurements. The measured binding enthalpies span a range of approximately 10 kcal mol(-1): DeltaH(binding) = -8.8 +/- 1.2 (N(2)-Mo(P(i)Pr(3))(2)(CO)(3)); -10.3 +/- 0.8 (N(2)); -11.2 +/- 0.4 (AdN(3) (Ad = 1-adamantyl)); -13.8 +/- 0.5 (N(2)CHSiMe(3)); -14.9 +/- 0.9 (pyrazine = pz); -14.8 +/- 0.6 (2,6-Me(2)pz); -15.5 +/- 1.8 (Me(2)NCN); -16.6 +/- 0.4 (CH(3)CN); -17.0 +/- 0.4 (pyridine); -17.5 +/- 0.8 ([4-CH(3)pz][PF(6)] (in tetrahydrofuran)); -17.6 +/- 0.4 (C(6)H(5)CN); -18.6 +/- 1.8 (N(2)CHC( horizontal lineO)OEt); and -19.3 +/- 2.5 kcal mol(-1) (pz)Mo(P(i)Pr(3))(2)(CO)(3)). The value for the isonitrile AdNC (-29.0 +/- 0.3) is 12.3 kcal mol(-1) more exothermic than that of the nitrile AdCN (-16.7 +/- 0.6 kcal mol(-1)). The enthalpies of binding of a range of arene nitrile ligands were also studied, and remarkably, most nitrile complexes were clustered within a 1 kcal mol(-1) range despite dramatic color changes and variation of nu(CN). Computed structural and spectroscopic parameters for the complexes Mo(P(i)Pr(3))(2)(CO)(3)L are in good agreement with experimental data. Computed binding enthalpies for Mo(P(i)Pr(3))(2)(CO)(3)L exhibit considerable scatter and are generally smaller compared to the experimental values, but relative agreement is reasonable. Computed enthalpies of binding using a larger basis set for Mo(PMe(3))(2)(CO)(3)L show a better fit to experimental data than that for Mo(P(i)Pr(3))(2)(CO)(3)L using a smaller basis set. Crystal structures of Mo(P(i)Pr(3))(2)(CO)(3)(AdCN), W(P(i)Pr(3))(2)(CO)(3)(Me(2)NCN), W(P(i)Pr(3))(2)(CO)(3)(2,6-F(2)C(6)H(3)CN), W(P(i)Pr(3))(2)(CO)(3)(2,4,6-Me(3)C(6)H(2)CN), W(P(i)Pr(3))(2)(CO)(3)(2,6-Me(2)pz), W(P(i)Pr(3))(2)(CO)(3)(AdCN), Mo(P(i)Pr(3))(2)(CO)(3)(AdNC), and W(P(i)Pr(3))(2)(CO)(3)(AdNC) are reported.

Mechanism and Scope of Phosphinidene Transfer from Dibenzo-7-phosphanorbornadiene Compounds

Dibenzo-7-phosphanorbornadiene compounds, RPA (A = C14H10 or anthracene), are investigated as phosphinidene sources upon thermally induced (70-90 °C) anthracene elimination. Analysis of substituent effects reveals that π-donating dialkylamide groups are paramount to successful phosphinidene transfer; poorer π-donors give reduced or no transfer. Substituent steric bulk is also implicated in successful transfer. Molecular beam mass spectrometry (MBMS) studies of each derivative reveal dialkylamide derivatives to be promising precursors for further gas-phase spectroscopic studies of phosphinidenes; in particular, we present evidence of direct detection of the dimethylamide derivative, [Me2N═P]. Kinetic investigations of iPr2NPA thermolysis in 1,3-cyclohexadiene and/or benzene-d6 are consistent with a model of unimolecular fragmentation to yield free phosphinidene [iPr2N═P] as a transient reactive intermediate. This conclusion is probed by density functional theory (DFT) calculations, which favored a mechanistic model featuring free singlet aminophosphinidenes. The breadth of phosphinidene acceptors is expanded to unsaturated substrates beyond 1,3-dienes to include olefins and alkynes; this provides a new synthetic route to valuable amino-substituted phosphiranes and phosphirenes, respectively. Stereoselective phosphinidene transfer to olefins is consistent with singlet phosphinidene reactivity by analogy with the Skell hypothesis for singlet carbene addition to olefins.

Functionalization Reactions Characteristic of a Robust Bicyclic Diphosphane Framework

The 3,4,8,9-tetramethyl-1,6-diphospha-bicyclo-[4.4.0]deca-3,8-diene (P2(C6H10)2) framework containing a P-P bond has allowed for an unprecedented selectivity toward functionalization of a single phosphorus lone pair with reference to acyclic diphosphane molecules. Functionalization at the second phosphorus atom was found to proceed at a significantly slower rate, thus opening the pathway for obtaining mixed functional groups for a pair of P-P bonded λ(5)-phosphorus atoms. Reactivity with the chalcogen-atom donors MesCNO (Mes = 2,4,6-C6H2Me3) and SSbPh3 has allowed for the selective synthesis of the diphosphane chalcogenides OP2(C6H10)2 (87%), O2P2(C6H10)2 (94%), SP2(C6H10)2 (56%), and S2P2(C6H10)2 (87%). Computational studies indicate that the oxygen-atom transfer reactions involve penta-coordinated phosphorus intermediates that have four-membered {PONC} cycles. The P-E bond dissociation enthalpies in EP2(C6H10)2 were measured via calorimetric studies to be 134.7 ± 2.1 kcal/mol for P-O, and 93 ± 3 kcal/mol for P-S, respectively, in good agreement with the computed values. Additional reactivity with breaking of the P-P bond and formation of diphosphinate O3P2(C6H10)2 was only observed to occur upon heating of dimethylsulfoxide solutions of the precursor. Reactivity of diphosphane P2(C6H10)2 with azides allowed the isolation of monoiminophosphoranes (RN)P2(C6H10)2(R = Mes, CPh3, SiMe3), and treatment with additional MesN3 yielded symmetric and unsymmetric diiminodiphosphoranes (RN)(MesN)P2(C6H10)2 (91% for R = Mes). Metalation reactions with the bulky diiminodiphosphorane ligand (MesN)2P2(C6H10)2 (nppn) allowed for the isolation and characterization of (nppn)Mo(η(3)-C3H5)Cl(CO)2 (91%), (nppn)NiCl2 (76%), and [(nppn)Ni(η(3)-2-C3H4Me)][OTf] showing that these ligands provide an attractive preorganized binding pocket for both late and early transition metals.