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The first-year undergraduate laboratory course in chemistry at the University of Bristol has been changed in an attempt to have students focus on learning practical skills, rather than on their exp...

A mechanochemical strategy for the synthesis of N-heterocyclic carbene complexes is described, in which 1,3-dibenzylimidazole complexes of palladium and platinum are produced in a two-step process by grinding together the reactants with a mortar and pestle. Crystallographic characterization reveals that unlike the solution syntheses, which produce a mixture of products, the solid-state reactions occur under topochemical conditions affording isomerically and polymorphically pure products.

Herein we demonstrate both the importance of Fe(I) in Negishi cross-coupling reactions with arylzinc reagents and the isolation of catalytically competent Fe(I) intermediates. These complexes, [FeX(dpbz)(2)] [X = 4-tolyl (7), Cl (8a), Br (8b); dpbz = 1,2-bis(diphenylphosphino)benzene], were characterized by crystallography and tested for activity in representative reactions. The complexes are low-spin with no significant spin density on the ligands. While complex 8b shows performance consistent with an on-cycle intermediate, it seems that 7 is an off-cycle species.

Two new isostructural two-dimensional (2D) coordination polymers exhibiting spin crossover (SCO) behavior of formulation [Fe(4,4'-bipy)(2)(NCX)(2)]·4CHCl(3) (4,4'-bipy = 4,4'-bipyridine; X = S [1·4CHCl(3)], Se [2·4CHCl(3)]) have been synthesized and characterized, and both undergo cooperative spin transitions (ST). For 1·4CHCl(3) the ST takes place in two steps with critical temperatures of T(c1)(down) = 143.1 K, T(c2)(down) = 91.2 K, T(c1)(up) = 150.7 K, and T(c2)(up) = 112.2 K. 2·4CHCl(3) displays half ST characterized by T(c)(down) = 161.7 K and T(c)(up) = 168.3 K. The average enthalpy and entropy variations and cooperativity parameters associated with the ST have been estimated to be ΔH(1)(av) = 5.18 kJ mol(-1), ΔS(1)(av) = 35 J K(-1) mol(-1), and Γ(1) = 2.8 kJ mol(-1) and ΔH(2)(av) = 3.55 kJ mol(-1), ΔS(2)(av) = 35 J K(-1) mol(-1), and Γ(2) = 2.6 kJ mol(-1) for 1·4CHCl(3), and ΔH(av) = 6.25 kJ mol(-1), ΔS(av) = 38.1 J K(-1) mol(-1), and Γ = 3.2 kJ mol(-1) for 2·4CHCl(3). At T > [T(c1) (1·4CHCl(3)); T(c) (2·4CHCl(3))], both compounds are in the space group P2/c while at T < [T(c1) (1·4CHCl(3)); T(c) (2·4CHCl(3))] they change to the C2/c space group and display an ordered checkerboard-like arrangement of iron(II) sites where the high- and low-spin states coexist at 50%.

Crystalline coordination compounds [MnCl(2)(Hpz)(2)] 3, [CdCl(2)(Hpz)(2)] 5, [MnCl(2)(Him)(2)] 9, and [CdCl(2)(Him)(2)] 13 (Him = imidazole; Hpz = pyrazole) can be synthesized in solid state reactions by grinding together the appropriate metal chloride and 2 equiv of the neutral ligand. Similarly, grinding together the metal chlorides with the ligand hydrochloride salts produces the halometallate salts [H(2)pz][MnCl(3)(OH(2))] 1, [H(2)pz][CdCl(4)] 4, [H(2)im](6)[MnCl(6)][MnCl(4)] 8, and [H(2)im](6)[CdCl(6)][CdCl(4)] 11. In contrast, reacting the metal chloride salt with the ligand in concentrated HCl solution yields a second set of salts [H(2)pz][MnCl(3)] 2, [H(2)im][MnCl(3)(OH(2))(2)] 7, and [H(2)im][CdCl(3)(OH(2))]·H(2)O 12. Compound 5 can be partly dehydrochlorinated by grinding with KOH to form an impure sample of the pyrazolate compound [Cd(pz)(2)] 6, while recrystallizing 9 from ethanol yielded crystals of solvated [Mn(4)Cl(8)(Him)(8)] 10. The crystal structure determinations of 1, 2, 4, 11, and 12 are reported.

Photolysis of THF solutions of 10-vertex [N(PPh3)2][6,6,6-(CO)3-closo-6,1-FeCB8H9] (1) in the presence of PEt3 results in [N(PPh3)2][6-CO-6,6-(PEt3)2-closo-6,1-FeCB8H9] (2), which readily oxidizes in air to form the neutral and paramagnetic, 17-electron Fe(III) species [6-CO-6,6-(PEt3)2-closo-6,1-FeCB8H9] (3). Substitution of one PEt3 ligand by cyanide occurs on addition of [NBun4][CN] and Me3NO to 3 in CH2Cl2, giving [NBun4][6-CO-6-CN-6-PEt3-closo-6,1-FeCB8H9] (4). When [IrCl(CO)(PPh3)2] and Tl[PF6] (1:1) are added to CH2Cl2 solutions of 4, the neutral, paramagnetic complex [6-{(μ-CN)Ir(CO)(PPh3)2}-6-CO-6-PEt3-closo-6,1-FeCB8H9] (5) is obtained. Results of X-ray diffraction studies carried out on 3 and 5 are presented herein.

The photophysical and electrochemical properties of a platinum(II) diimine complex bearing the bidentate diacetylide ligand tolan-2,2'-diacetylide (tda), Pt(dbbpy)(tda) [dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine] (1), are compared with two reference compounds, Pt(dbbpy)(C[triple bond]CPh)(2) (2) and Pt(dppp)tda [dppp = 1,3-bis(diphenylphosphino)propane] (3), respectively. The X-ray crystal structure of 1 is reported, which illustrates the nearly perfect square planarity exhibited by this metallacycle. Chromophore 2 possesses low-lying charge-transfer excited states analogous to 1, whereas structure 3 lacks such excited states but features a low-lying platinum-perturbed tda intraligand triplet manifold. In CH(2)Cl(2), 1 exhibits a broad emission centered at 562 nm at ambient temperature, similar to 2, but with a higher photoluminescence quantum yield and longer excited-state lifetime. In both instances, the photoluminescence is consistent with triplet-charge-transfer excited-state parentage. The rigidity imposed by the cyclic diacetylide ligand in 1 leads to a reduction in nonradiative decay, which enhances its room-temperature photophysical properties. By comparison, 3 radiates highly structured tda-localized triplet-state phosphorescence at room temperature. The 77 K emission spectrum of 1 in 4:1 EtOH/MeOH becomes structured and is quantitatively similar to that measured for 3 under the same conditions. Because the 77 K spectra are nearly identical, the emissions are assigned as (3)tda in nature, implying that the charge-transfer states are raised in energy, relative to the (3)tda levels in 1 in the low-temperature glass. Nanosecond transient absorption spectrometry and ultrafast difference spectra were determined for 1-3 in CH(2)Cl(2) and DMF at ambient temperature. In 1 and 2, the major absorption transients are consistent with the one-electron reduced complexes, corroborated by reductive spectroelectrochemical measurements performed at room temperature. As 3 does not possess any charge-transfer character, excitation into the pipi* transitions of the tda ligand generated transient absorptions in the relaxed excited state assigned to the ligand-localized triplet state. In all three cases, the excited-state lifetimes measured by transient absorption are similar to those measured by time-resolved photoluminescence, suggesting that the same excited states giving rise to the photoluminescence are responsible for the absorption transients. ESR spectroscopy of the anions 1- and 2- and reductive spectroelectrochemistry of 1 and 2 revealed a LUMO based largely on the pi* orbital of the dbbpy ligand. Time-dependent density functional theory calculations performed on 1-3 both in vacuum and in a CH(2)Cl(2) continuum revealed the molecular orbitals, energies, dipole moments, and oscillator strengths for the various electronic transitions in these molecules. A DeltaSCF-method-derived shift applied to the calculated transition energies in the solvent continuum yielded good agreement between theory and experiment for each molecule in this study.

A series of new crystal structures of salts containing [PtCl4]2- or [PdCl4]2- with organic cations containing pyridinium and piperidinium groups have been prepared: [4,4‘-H2bpe][PtCl4] (2), [4,4‘-H2bpethane][PtCl4] (3), [HNC5H4CO2H-4]2[PtCl4] (6a), [HNC5H4CONH2-3]2[PtCl4] (8), [H2NC5H9CO2H-4]2[PtCl4] (9), [H2NC5H9CO2H-3]2[PtCl4] (10), [H2NC5H9CO2H-4]Cl (11), [HNC5H4CO2H-4]Cl (12), [4,4‘-H2bpe][PdCl4] (13), [4,4‘-H2bipip][PdCl4] (14a, 14b, 14c), [piperazinium][PdCl4]·2H2O (15), and [HNC5H4CO2H-4]2[PdCl4]·2H2O (16). The molecular recognition motifs formed in these and related structures are identified and discussed. The robustness of these supramolecular synthons and their applicability in synthetic crystallography are analyzed. Cases of structural mimicry of a molecule by a supramolecular dication (1 and 6a), isomorphism (2 and 3; 4 and 14a; 6 and 16), polymorphism (14a−c), and “latent” polymorphism (2 and 13) are identified.

The complex [Mo(CO)(PhC≡CPh) 2 (η 5 -C 5 Me 5 )][BF 4 ] reacts with 3 equiv of CNBu t to yield the mixed alkyne-isocyanide complex [Mo(CNBu t ) 3 (PhC≡CPh)(η 5 -C 5 Me 5 )][BF 4 ], 1. Reaction of 1 with a fourth equivalent of CNBu t generates [Mo{=C(NHBu t )C(Ph)=C(Ph)CN}(CNBu t ) 2 -(η 5 -C 5 Me 5 )][BF 4 ], 2, containing an η 3 -vinylcarbene ligand formed from the coupling of two of the coordinated isocyanide ligands with the diphenylacetylene, with concomitant protonation of one of these isocyanide fragments and dealkylation of the other. Complex 2 may be deprotonated to give [Mo{C(=NBu t )C(Ph)=C(Ph)CN}(CNBut) 2 (η 5 -C 5 Me 5 )], 3. Protonation of 1 with HBF 4 .Et 2 O generates [Mo{=C(Ph)C(Ph)C=NHBu t }(CNBu t ) 2 (η 5 -C 5 Me 5 )][BF 4 ] 2 , 4, by inducing the coupling of a protonated isocyanide ligand with diphenylacetylene, and 4 reacts with CNBu t to give 2. A similar reaction of 4 with P(OMe) 3 generates [Mo{=C(NHBu t )C-(Ph)=C(Ph)CN}(CNBu t ){P(OMe) 3 }(η 5 -C 5 Me 5 )][BF 4 ], 5, involving the same coupling and elimination pattern. The diphenylacetylene and CNBu t ligands of 1 may also be reductively coupled. Thus, treatment of 1 with sodium-mercury amalgam and subsequent protonation with HBF 4 .Et 2 O gives the metallacyclopentatriene-like [Mo{=C(NHBut)C(Ph)=C(Ph)C-(NHBu t )}(CNBu t )(η 5 -C 5 Me 5 ][BF 4 ], 6, or [Mo{=C(NHBu t )C(Ph)C(Ph)CH-NHBu t }(CNBu t )(η 5 -C 5 Me 5 )][BF 4 ], 7, depending upon the solvent. Complex 7 contains an N-protonated η 4 -monoazadiene ligand with a pendant carbene functionality that is also coordinated to the metal. The crystal structures of 1, 2, 3, 6, and 7 have been determined.

Square-planar Pt(II) complexes of the bis(mesitylimino)acenaphthene (mesBIAN) ligand are emissive from a MMLL'CT excited state in a dichloromethane solution at room temperature. Investigation of the nature of the frontier orbitals in these near-IR emitters by a combination of emission spectroscopy, electron paramagnetic resonance spectroscopy, and density functional theory calculations suggests that emission is enabled by the presence of low-lying ligand pi* orbitals on the mesBIAN.