•Reaction of MesCNO (Mes = 2,4,6-trimethylphenyl) with [Pd(IPr)(P(p-tolyl)3)] yields [Pd(IPr)(NCMes)(κ2-O-NC-Mes(–N–C(O)Mes))].•Three major steps are proposed to be involved in the reaction: i. oxidation of the coordinated phosphine ligand to phosphine oxide ii. oxygen atom transfer forming a CO bond from the N–O bond of MesCNO, and iii. cycloaddition of a final MesCNO ligand to yield product.•Addition of two equivalents of NO at low temperature to [Pd(IPr)2(η2-O2)] generates the N-bonded complex trans-[Pd(IPr)2(NO2)2]. Insight into the energetics of this reaction are probed by DFT calculations.•The computed enthalpy of binding of two moles of NO2 to form [Pd(IMe)2(NO2)2] is −112 kcal/mol indicating that its preparation from [Pd(IMe)2], N2 and 2 O2 is thermodynamically favorable by −96 kcal/mol.•Crystal structures of [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))] and trans-[Pd(IPr)2(NO2)2] are reported.Reaction of three equivalents of MesCNO (Mes = 2,4,6-trimethylphenyl) with one equivalent of [Pd(IPr)(P(p-tolyl)3)] in toluene yields the solid complex [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))]. Three major steps are proposed to be involved in the reaction based on spectroscopic studies as well as literature precedents for related cycloadditions: i. oxidation of the coordinated phosphine ligand to phosphine oxide ii. oxygen atom transfer forming a CO bond from the N–O bond of MesCNO, and iii. cycloaddition of a final MesCNO ligand to yield product.Addition of two equivalents of NO at low temperature to the in situ generated peroxide complex [Pd(IPr)2(η2-O2)] generates the N-bonded complex trans-[Pd(IPr)2(NO2)2] in keeping with a literature precedent reported for similar complexes. Insight into the energetics of this reaction are probed by DFT calculations using the truncated ligand complex [Pd(IMe)2]. The computed enthalpy of binding of two moles of NO2 to form [Pd(IMe)2(NO2)2] is −112 kcal/mol indicating that its preparation from [Pd(IMe)2], N2 and 2O2 is thermodynamically favorable by −96 kcal/mol. Crystal structures of [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))] and trans-[Pd(IPr)2(NO2)2] are reported.Download high-res image (64KB)Download full-size image

The reaction of Pt(COD)2 with one equivalent of tri-tert-butylstannane, But3SnH, at room temperature yields Pt(SnBut3)(COD)(H)(3) in quantitative yield. In the presence of excess But3SnH, the reaction goes further, yielding the dinuclear bridging stannylene complex [Pt(SnBut3)(μ-SnBut2)(H)2]2 (4). The dinuclear complex 4 reacts rapidly and reversibly with CO to furnish [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2 (5). Complex 3 reacts with N,N′-di-tert-butylimidazol-2-ylidene, IBut, at room temperature to give the dinuclear bridging hydride complex [Pt(SnBut3)(IBut)(μ-H)]2 (6). Complex 6 reacts with CO, C2H4, and H2 to give the corresponding mononuclear Pt complexes Pt(SnBut3)(IBut)(CO)(H)(7), Pt(SnBut3)(IBut)(C2H4)(H)(8), and Pt(SnBut3)(IBut)(H)3 (9), respectively. The reaction of IBut with the complex Pt(SnBut3)2(CO)2 (10) yielded an abnormal Pt-carbene complex Pt(SnBut3)2(aIBut)(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(SnBut3)(IBut)(H)3 have been performed, for NHC = IMe and R = Me and for NHC = IBut and R = But. The structures of complexes 3–8 and 11 have been determined by X-ray crystallography and are reported.

The kinetics of the reaction of Ph3SnH with excess •Cr(CO)3C5Me5 = •Cr, producing HCr and Ph3Sn–Cr, was studied in toluene solution under 2–3 atm CO pressure in the temperature range of 17–43.5 °C. It was found to obey the rate equation d[Ph3Sn–Cr]/dt = k[Ph3SnH][•Cr] and exhibit a normal kinetic isotope effect (kH/kD = 1.12 ± 0.04). Variable-temperature studies yielded ΔH‡ = 15.7 ± 1.5 kcal/mol and ΔS‡ = −11 ± 5 cal/(mol·K) for the reaction. These data are interpreted in terms of a two-step mechanism involving a thermodynamically uphill hydrogen atom transfer (HAT) producing Ph3Sn• and HCr, followed by rapid trapping of Ph3Sn• by excess •Cr to produce Ph3Sn–Cr. Assuming an overbarrier of 2 ± 1 kcal/mol in the HAT step leads to a derived value of 76.0 ± 3.0 kcal/mol for the Ph3Sn–H bond dissociation enthalpy (BDE) in toluene solution. The reaction enthalpy of Ph3SnH with excess •Cr was measured by reaction calorimetry in toluene solution, and a value of the Sn–Cr BDE in Ph3Sn-Cr of 50.4 ± 3.5 kcal/mol was derived. Qualitative studies of the reactions of other R3SnH compounds with •Cr are described for R = nBu, tBu, and Cy. The dehydrogenation reaction of 2Ph3SnH → H2 + Ph3SnSnPh3 was found to be rapid and quantitative in the presence of catalytic amounts of the complex Pd(IPr)(P(p-tolyl)3). The thermochemistry of this process was also studied in toluene solution using varying amounts of the Pd(0) catalyst. The value of ΔH = −15.8 ± 2.2 kcal/mol yields a value of the Sn–Sn BDE in Ph3SnSnPh3 of 63.8 ± 3.7 kcal/mol. Computational studies of the Sn–H, Sn–Sn, and Sn–Cr BDEs are in good agreement with experimental data and provide additional insight into factors controlling reactivity in these systems. The structures of Ph3Sn–Cr and Cy3Sn–Cr were determined by X-ray crystallography and are reported. Mechanistic aspects of oxidative addition reactions in this system are discussed.

The enthalpy of oxygen atom transfer (OAT) to V[(Me3SiNCH2CH2)3N], 1, forming OV[(Me3SiNCH2CH2)3N], 1–O, and the enthalpies of sulfur atom transfer (SAT) to 1 and V(N[t-Bu]Ar)3, 2 (Ar = 3,5-C6H3Me2), forming the corresponding sulfides SV[(Me3SiNCH2CH2)3N], 1–S, and SV(N[t-Bu]Ar)3, 2–S, have been measured by solution calorimetry in toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) and Ph3SbS as chalcogen atom transfer reagents. The V–O BDE in 1–O is 6.3 ± 3.2 kcal·mol–1 lower than the previously reported value for 2–O and the V–S BDE in 1–S is 3.3 ± 3.1 kcal·mol–1 lower than that in 2–S. These differences are attributed primarily to a weakening of the V–Naxial bond present in complexes of 1 upon oxidation. The rate of reaction of 1 with dbabhNO has been studied by low temperature stopped-flow kinetics. Rate constants for OAT are over 20 times greater than those reported for 2. Adamantyl isonitrile (AdNC) binds rapidly and quantitatively to both 1 and 2 forming high spin adducts of V(III). The enthalpies of ligand addition to 1 and 2 in toluene solution are −19.9 ± 0.6 and −17.1 ± 0.7 kcal·mol–1, respectively. The more exothermic ligand addition to 1 as compared to 2 is opposite to what was observed for OAT and SAT. This is attributed to less weakening of the V–Naxial bond in ligand binding as opposed to chalcogen atom transfer and is in keeping with structural data and computations. The structures of 1, 1–O, 1–S, 1–CNAd, and 2–CNAd have been determined by X-ray crystallography and are reported.

Highlights•Enthalpies of ligand substitution are measured for Mo(C5H5)(CO)2(NO).•Phosphines and N-heterocyclic carbenes are stronger ligands and displace CO.•Backbonding to π∗ orbitals is an important part of complex stability.•FTIR studies show shifts to lower wavenumbers of ν-CO and ν-NO.•Structural studies show lengthening of the CO and NO bonds.Enthalpies of ligand substitution for [Mo(η5-C5H5)(CO)2(NO)] producing [Mo(η5-C5H5)Mo(CO)(L)(NO)] have been measured by solution calorimetry at 30 °C in THF for L = P(OMe)3 < PMePh2 < SIPr < PMe2Ph < IPr < PMe < PnBu3 (SIPr = 1,3-bis(2,6-bis(diisopropylphenyl)imidazolinylidene; IPr = 1,3-bis(2,6-bis(diisopropylphenyl)-imidazol-2-ylidene)). The accepting metal fragment [Mo(η5-C5H5)(CO)(NO)] has a vacant site containing strongly π-accepting carbonyl and nitrosyl ligands and this is shown to influence the stability of the product complex. Infrared studies of both νCO and νNO show that metal-to-ligand backbonding increases in the order P(OMe)3 < PMe3 < SIPr < IPr implying that both steric and electronic factors play a role in determining complex stability. The crystal structures of [Mo(η5-C5H5)(CO)(IPr)(NO)] and [Mo(η5-C5H5)(CO)(SIPr)(NO)] are reported.

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 λ5-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(η3-C3H5)Cl(CO)2 (91%), (nppn)NiCl2 (76%), and [(nppn)Ni(η3-2-C3H4Me)][OTf] showing that these ligands provide an attractive preorganized binding pocket for both late and early transition metals.

Pt(IV) complexes trans-Pt(PEt3)2(R)(Br)3 (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe3 analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br2. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination.

Treatment of V(N[tBu]Ar)3 (1) (Ar = 3,5-Me2C6H3) with O2 was shown by stopped-flow kinetic studies to result in the rapid formation of (η1-O2)V(N[tBu]Ar)3 (2) (ΔH⧧ = 3.3 ± 0.2 kcal/mol and ΔS⧧ = −22 ± 1 cal mol–1 K–1), which subsequently isomerizes to (η2-O2)V(N[tBu]Ar)3 (3) (ΔH⧧ = 10.3 ± 0.9 kcal/mol and ΔS⧧ = −6 ± 4 cal mol–1 K–1). The enthalpy of binding of O2 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[tBu]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 O2 with 1 in the presence of tBuCN was studied. At −45 °C, the principal products of this reaction are 3 and tBuC(═O)N≡V(N[tBu]Ar)3 (5), in which the bound nitrile has been oxidized. Crystal structures of 3 and 5 are reported.

The reactivity of a number of two-coordinate [Pd(L)(L′)] (L = N-heterocyclic carbene (NHC) and L′ = NHC or PR3) complexes with O2 has been examined. Stopped-flow kinetic studies show that O2 binding to [Pd(IPr)(P(p-tolyl)3)] to form cis-[Pd(IPr)(P(p-tolyl)3)(η2-O2)] occurs in a rapid, second-order process. The enthalpy of O2 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(η2-O2)] but to paramagnetic trans-[(Pd(IPr)2(η1-O2)2], which has been fully characterized. Computational studies addressing the energetics of O2 binding have been performed and provide insight into reactivity changes as steric pressure is increased.

The enthalpies of oxygen atom transfer (OAT) from mesityl nitrile oxide (MesCNO) to Me3P, Cy3P, Ph3P, and the complex (Ar[tBu]N)3MoP (Ar = 3,5-C6H3Me2) have been measured by solution calorimetry yielding the following P–O bond dissociation enthalpy estimates in toluene solution (±3 kcal mol–1): Me3PO [138.5], Cy3PO [137.6], Ph3PO [132.2], (Ar[tBu]N)3MoPO [108.9]. The data for (Ar[tBu]N)3MoPO yield an estimate of 60.2 kcal mol–1 for dissociation of PO from (Ar[tBu]N)3MoPO. The mechanism of OAT from MesCNO to R3P and (Ar[tBu]N)3MoP has been investigated by UV–vis and FTIR kinetic studies as well as computationally. Reactivity of R3P and (Ar[tBu]N)3MoP 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 Me3P 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 R3P. However, it does undergo rapid OAT with coordinatively unsaturated metal complexes such as (Ar[tBu]N)3V 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[tBu]N)3MoN was found not to proceed in spite of thermochemical favorability.

Variable temperature equilibrium studies were used to derive thermodynamic data for formation of η1 nitrile complexes with Mo(N[tBu]Ar)3, 1. (1-AdamantylCN = AdCN: ΔHo = −6 ± 2 kcal mol−1, ΔSo = −20 ± 7 cal mol−1 K−1. C6H5CN = PhCN: ΔHo = −14.5 ± 1.5 kcal mol−1, ΔSo = −40 ± 5 cal mol−1 K−1. 2,4,6-(H3C)3C6H2CN = MesCN: ΔHo = −15.4 ± 1.5 kcal mol−1, ΔSo = −52 ± 5 cal mol−1 K−1.) Solution calorimetric studies show that the enthalpy of formation of 1-[η2-NCNMe2] is more exothermic (ΔHo = −22.0 ± 1.0 kcal mol−1). Rate and activation parameters for η1 binding of nitriles were measured by stopped flow kinetic studies (AdCN: ΔHon‡ = 5 ± 1 kcal mol−1, ΔSon‡ = −28 ± 5 cal mol−1 K−1; PhCN: ΔHon‡ = 5.2 ± 0.2 kcal mol−1, ΔSon‡ = −24 ± 1 cal mol−1 K−1; MesCN: ΔHon‡ = 5.0 ± 0.3 kcal mol−1, ΔSon‡ = −26 ± 1 cal mol−1 K−1). Binding of Me2NCN was observed to proceed by reversible formation of an intermediate complex 1-[η1-NCNMe2] which subsequently forms 1-[η2-NCNMe2]: ΔH‡k1 = 6.4 ± 0.4 kcal mol−1, ΔS‡k1 = −18 ± 2 cal mol−1 K−1, and ΔH‡k2 = 11.1 ± 0.2 kcal mol−1, ΔS‡k2 = −7.5 ± 0.8 cal mol−1 K−1. The oxidative addition of PhSSPh to 1-[η1-NCPh] is a rapid second-order process with activation parameters: ΔH‡ = 6.7 ± 0.6 kcal mol−1, ΔS‡ = −27 ± 4 cal mol−1 K−1. The oxidative addition of PhSSPh to 1-[η2-NCNMe2] also followed a second-order rate law but was much slower: ΔH‡ = 12.2 ± 1.5 kcal mol−1 and ΔS‡ = −25.4 ± 5.0 cal mol−1 K−1. The crystal structure of 1-[η1-NC(SPh)NMe2] is reported. Trapping of in situ generated 1-[η1-NCNMe2] by PhSSPh was successful at low temperatures (−80 to −40 °C) as studied by stopped flow methods. If 1-[η1-NCNMe2] is not intercepted before isomerization to 1-[η2-NCNMe2] no oxidative addition occurs at low temperatures. The structures of key intermediates have been studied by density functional theory, confirming partial radical character of the carbon atom in η1-bound nitriles. A complete reaction profile for reversible ligand binding, η1 to η2 isomerization, and oxidative addition of PhSSPh has been assembled and gives a clear picture of ligand reactivity as a function of hapticity in this system.

The reaction of PR3 (R = Cy, iPr) with 1-adamantyl azide (N3Ad) in benzene results in an equilibrium of the starting material and the phosphazide R3PN3Ad. Thermodynamic and kinetic measurements were taken of the reaction of PiPr3 with N3Ad and yielded ΔH = −18.7 ± 1.0 kcal mol−1, ΔS = −52.5 ± 2.0 cal mol−1 K−1, ΔH⧧ = +12.0 ± 1.0 kcal mol−1, and ΔS⧧ = −25.3 ± 1.2 cal mol−1 K−1. The phosphazides, R3PN3Ad, do not readily lose N2 for R = Cy and iPr; however, the reaction of PMe3 with N3Ad results in a smooth formation of the phosphazene Me3P═NAd with N2 loss. Calorimetric investigations of this reaction led to an estimation of ΔH = −40 ± 3 kcal mol−1 for the loss of N2 from the intermediate phosphazide R3PN3Ad and also yield an estimation of 72 ± 5 kcal mol−1 for the bond dissociation energy of the P═N bond in R3P═NAd. The X-ray crystal structure of Cy3PN3Ad is reported.

The enthalpies of binding of a number of N-donor ligands to the complex Mo(PiPr3)2(CO)3 in toluene have been determined by solution calorimetry and equilibrium measurements. The measured binding enthalpies span a range of ∼10 kcal mol−1: ΔHbinding = −8.8 ± 1.2 (N2−Mo(PiPr3)2(CO)3); −10.3 ± 0.8 (N2); −11.2 ± 0.4 (AdN3 (Ad = 1-adamantyl)); −13.8 ± 0.5 (N2CHSiMe3); −14.9 ± 0.9 (pyrazine = pz); −14.8 ± 0.6 (2,6-Me2pz); −15.5 ± 1.8 (Me2NCN); −16.6 ± 0.4 (CH3CN); −17.0 ± 0.4 (pyridine); −17.5 ± 0.8 ([4-CH3pz][PF6] (in tetrahydrofuran)); −17.6 ± 0.4 (C6H5CN); −18.6 ± 1.8 (N2CHC(═O)OEt); and −19.3 ± 2.5 kcal mol−1 (pz)Mo(PiPr3)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 νCN. Computed structural and spectroscopic parameters for the complexes Mo(PiPr3)2(CO)3L are in good agreement with experimental data. Computed binding enthalpies for Mo(PiPr3)2(CO)3L 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(PMe3)2(CO)3L show a better fit to experimental data than that for Mo(PiPr3)2(CO)3L using a smaller basis set. Crystal structures of Mo(PiPr3)2(CO)3(AdCN), W(PiPr3)2(CO)3(Me2NCN), W(PiPr3)2(CO)3(2,6-F2C6H3CN), W(PiPr3)2(CO)3(2,4,6-Me3C6H2CN), W(PiPr3)2(CO)3(2,6-Me2pz), W(PiPr3)2(CO)3(AdCN), Mo(PiPr3)2(CO)3(AdNC), and W(PiPr3)2(CO)3(AdNC) are reported.

The reaction of 1-adamantyl azide (N3Ad) with [Cr(CO)3Cp]2 under Ar results in conversion of N3Ad to 1-adamantyl isocyanate (OCNAd) and [Cr(CO)2Cp]2. Two reaction intermediates were detected: 3Cr2(CO)5Cp2 and an electron spin resonance active complex proposed to be •Cr(CO)2(O═C═NAd)Cp. Under a CO atmosphere, 3Cr2(CO)5Cp2 regenerates [Cr(CO)3Cp]2 and approximately five turnovers can be achieved. Hydrogenation of N3Ad by HMo(CO)3Cp yields H2NAd, N2, and [Mo(CO)3Cp]2. N3Ad reacts with Mo(CO)3(PiPr3) to form Mo(N3Ad)(CO)3(PiPr3)2, which slowly isomerizes to Mo(κ2-iPr3PN3Ad)(CO)3(PiPr3), the structure of which is reported.