Three-component reaction of aryl Grignard reagents, alkynes, and aryl halides in the presence of 1 mol % of NiCl2 proceeded sequentially through carbomagnesiation of the alkyne followed by cross-coupling of the resulting alkenyl Grignard reagent with aryl halide to give tetrasubstituted alkenes in high yields.

•The first dedicated and comprehensive review on Group 6 metal NHC complexes.•The syntheses, structures, reactivities and functions of ca. 500 complexes are reviewed.•Some emerging trends and applications are summarized.Across the periodic table, Group 6 metals (Cr, Mo, W) play an important role in the development of carbene chemistry. In this review, we highlight the synthetic methodologies, structures, reactivities and functions of about 500 N-heterocyclic carbene (NHC) complexes of these metals reported in the last four decades. Some emerging trends and applications are also summarized.

•Mo/W(0) carbony NHC complexes were synthesized and characterized.•Their catalytic activities towards cyclooctene epoxidation were reported.•The stability of Mo–CNHC bond under oxidative condition was investigated.A series of Mo(0) and W(0) carbonyl N-heterocyclic carbene (NHC) complexes containing monodentate NHC ligands with the formulae of M(CO)4(NHC)2 (NHC = IBz = 1,3-dibenzylimidazol-2-ylidene, M = Mo (1a), W (2a); NHC = InPr = 1,3-dipropylimidazol-2-ylidene, M = Mo (3), W (4a)) and M(CO)5(NHC) (NHC = IBz, M = Mo (1b), W (2b); NHC = InPr, M = W (4b); NHC = IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, M = Mo (5), W (6)) and the complexes containing chelating NHC ligands Mo(CO)4(PyNHC) (PyNHC = 3-methyl-1-picolylimidazol-2-ylidene (7)) and Mo(CO)3(Py2NHC) (Py2NHC = 1,3-dipicolylimidazol-2-ylidene (8)) have been synthesized and fully characterized. They show moderate catalytic activities towards cyclooctene epoxidation in the presence of tert-butyl hydroperoxide (TBHP). The stability of Mo–CNHC bond under oxidative condition has been investigated.Mo/W(0) carbonyl N-Heterocyclic Carbene (NHC) complexes containing monodentate NHC ligands or chelating NHC ligands have been synthesized and characterized. Their catalytic activities towards cyclooctene epoxidation in the presence of tert-butyl hydroperoxide and the stability of Mo–CNHC bond under oxidative condition have been investigated.

A series of new benzothiazolyl-imidazolium salts 1–3 (1-(benzothiazol-2-yl)-3-methylimidazolium chloride (1); 1-(benzothiazol-2-yl)-3-benzylimidazolium chloride (2); 1-(benzothiazol-2-yl)-3-allylimidazolium chloride (3)) have been prepared from nucleophilic substitution of 2-chlorobenzothiazole by imidazoles. They serve as the precursors of hybrid N-heterocyclic carbene (NHC) ligands L1–L3 (L1 = 1-(benzothiazolin-2-yl)-3-methylimidazol-2-ylidene; L2 = 1-(benzothiazolin-2-yl)-3-benzylimidazol-2-ylidene; L3 = 1-(benzothiazolin-2-yl)-3-allylimidazol-2-ylidene). Reactions of 1–3 with Ag2O result in Ag(I) NHC complexes 4–6 [Ag(L)(μ-Cl)]2 (L = L1, 4; L = L2, 5) and Ag(L3)Cl (6), in which L1–L3 act as monodentate carbene ligand with a benzothiazolyl pendant. Subsequent transmetalation of 4–6 with CpMo(CO)3Br (Cp = cyclopentadienyl) and anion exchange reaction with AgX (X = BF4, PF6, or OTf) give complexes [CpMo(CO)2(L)]2[Ag2Br4] (L = L1, 7a; L = L2, 8a; L = L3, 9a) and [CpMo(CO)2(L)][X] (L = L1, X = BF4, 7b; L = L2, X = BF4, 8b; X = PF6, 8c; X = OTf, 8d; L = L3, X = BF4, 9b), whose structures are reported herein. The ligands L1–L3 show their versatility by switching to be N,C-chelating in these Mo(II) complexes. The chelation of the hybrid NHC ligand results in shorter Mo–Ccarbene bonds (2.14–2.16 Å) comparing with known Mo(II) NHC complexes in the literature. These complexes are active toward cyclooctene epoxidation with tert-butyl hydroperoxide (TBHP) affording up to 90% yield of epoxide (for 8b) in 3 h at 55 °C.

A simple and efficient catalytic arylmagnesiation of diarylacetylenes and aryl(alkyl)acetylenes is accomplished by NiCl2·6H2O at r.t. in the absence of stabilising ligands. The corresponding tetra-substituted alkenes can be obtained in good yields by subsequent treatment with different electrophiles.

Structurally distinctive dinuclear Ni(II) complexes with furan or thiophene tethered amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized. All complexes are catalytically active towards cross-coupling of aryl/alkyl Grignard reagents with β-H containing alkyl halides at room temperature in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA). The catalytic efficacy of the complexes is dependent on the tether substituent at the central amine. Two species, Ni(II) TMEDA and Mg(II) TMEDA complexes, have been isolated from the catalytic reaction mixtures under different conditions. Some ligand-stabilized Ni(II) and Mg(II) bimetallic species have also been identified in the ESI-MS spectra.

A high-valent molybdenum NHC dioxo complex [CpMoO2(IBz)]2[Mo6O19] (IBz = 1,3-dibenzylimidazol-2-ylidene) has been isolated in its hexamolybdate form and crystallographically elucidated to show a Mo–C(NHC) bond of 2.173(4) Å. (NHC = N-heterocyclic carbene).

Reaction of [Pt(L)(μ-Cl)]2 (L = ppy (2-phenylpyridine) or bzq (benzo[h]quinoline)) with 2-mercaptobenzoxazole (NOSH) and NaOAc in THF at r.t. yields the dinuclear Pt(II) d8–d8 complexes [Pt2L2(μ-NOS-κN,S)2] (L = ppy, 1; L = bzq, 2) and the Pt(III) d7–d7 complexes [Pt2(ppy)2(μ-NOS-κN,S)2(NOS-κS)2] (L = ppy, 3; L = bzq, 4) in one pot. The C,N-cyclometalated ligand is chelating whereas the N,S-donating benzoxazole-2-thiolates doubly bridge the two metal centers. The Pt⋯Pt separations of 3.0204(3) and 2.9726(8) Å in 1 and 2 contract to 2.685(1) Å in 3 and 2.6923(3) Å in 4, respectively, when two S-bound thiolate ligands coordinate trans- to the Pt⋯Pt axis. However, cyclometalation is preserved and there is minimum perturbation of the bridging ligands. Complexes 3 and 4 can be also obtained by oxidative addition of the thiolate ligand. In the presence of NaBH4, 3 and 4 can be reduced to 1 and 2, respectively. At r.t., 1 and 2 exhibit intense orange-red luminescence at 625 nm and 631 nm, respectively. The electrochemical properties of 1–4 have been also discussed.

Structurally distinctive Fe(II) complexes with furan, thiophene and pyridine functionalized amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized. The tether substituent at the central amine plays an active role in determining the coordination mode of the ligand and the metal geometry. All complexes are catalytically active towards cross-coupling of aryl Grignard reagents with primary and secondary alkyl halides with β-hydrogen under ambient conditions. ESI-MS spectra analysis revealed the ligand-stabilised Fe(II) and Mg(II) species.

A series of N-heterocyclic carbene (NHC) complexes CpMo(CO)2(NHC)X (NHC = IMe = 1,3-dimethylimidazol-2-ylidene, X = Br, 1; NHC = 1,3-dipropylimidazol-2-ylidene, X = Br, 2; NHC = IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, X = Br, 3; NHC = IBz = 1,3-dibenzylimidazol-2-ylidene, X = Br, 4a, and X = Cl, 4b; NHC = 1-methyl-3-propylimidazol-2-ylidene, X = Br, 5) and [CpMo(CO)2(IMes)(CH3CN)][BF4] (6) have been synthesized and fully characterized. The stability of metal−NHC ligand bonds in these compounds under oxidative conditions has been investigated. The thermally stable Mo(VI) dioxo NHC complex [CpMoO2(IMes)][BF4] (9) has been isolated by the oxidation of the ionic complex 6 by TBHP (tert-butyl hydrogen peroxide). Complex 6 can be applied as a very active (TOFs up to 3400 h−1) and selective olefin epoxidation catalyst. While under oxidative conditions (in the presence of TBHP), compounds 1−5 decompose into imidazolium bromide and imidazolium polyoxomolybdate. The formation of polyoxomolybdate as oxidation products had not been observed in a similar epoxidation catalyzed by Mo(II) and Mo(VI) complexes. DFT studies suggest that the presence of Br− destabilizes the CpMo(VI) oxo NHC carbene species, consistent with the experimental observations.

Structurally distinctive dinuclear Ni(II) complexes with furan or thiophene tethered amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized. All complexes are catalytically active towards cross-coupling of aryl/alkyl Grignard reagents with β-H containing alkyl halides at room temperature in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA). The catalytic efficacy of the complexes is dependent on the tether substituent at the central amine. Two species, Ni(II) TMEDA and Mg(II) TMEDA complexes, have been isolated from the catalytic reaction mixtures under different conditions. Some ligand-stabilized Ni(II) and Mg(II) bimetallic species have also been identified in the ESI-MS spectra.

Reaction of [Pt(L)(μ-Cl)]2 (L = ppy (2-phenylpyridine) or bzq (benzo[h]quinoline)) with 2-mercaptobenzoxazole (NOSH) and NaOAc in THF at r.t. yields the dinuclear Pt(II) d8–d8 complexes [Pt2L2(μ-NOS-κN,S)2] (L = ppy, 1; L = bzq, 2) and the Pt(III) d7–d7 complexes [Pt2(ppy)2(μ-NOS-κN,S)2(NOS-κS)2] (L = ppy, 3; L = bzq, 4) in one pot. The C,N-cyclometalated ligand is chelating whereas the N,S-donating benzoxazole-2-thiolates doubly bridge the two metal centers. The Pt⋯Pt separations of 3.0204(3) and 2.9726(8) Å in 1 and 2 contract to 2.685(1) Å in 3 and 2.6923(3) Å in 4, respectively, when two S-bound thiolate ligands coordinate trans- to the Pt⋯Pt axis. However, cyclometalation is preserved and there is minimum perturbation of the bridging ligands. Complexes 3 and 4 can be also obtained by oxidative addition of the thiolate ligand. In the presence of NaBH4, 3 and 4 can be reduced to 1 and 2, respectively. At r.t., 1 and 2 exhibit intense orange-red luminescence at 625 nm and 631 nm, respectively. The electrochemical properties of 1–4 have been also discussed.

A simple and efficient catalytic arylmagnesiation of diarylacetylenes and aryl(alkyl)acetylenes is accomplished by NiCl2·6H2O at r.t. in the absence of stabilising ligands. The corresponding tetra-substituted alkenes can be obtained in good yields by subsequent treatment with different electrophiles.

Structurally distinctive Fe(II) complexes with furan, thiophene and pyridine functionalized amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized. The tether substituent at the central amine plays an active role in determining the coordination mode of the ligand and the metal geometry. All complexes are catalytically active towards cross-coupling of aryl Grignard reagents with primary and secondary alkyl halides with β-hydrogen under ambient conditions. ESI-MS spectra analysis revealed the ligand-stabilised Fe(II) and Mg(II) species.

A high-valent molybdenum NHC dioxo complex [CpMoO2(IBz)]2[Mo6O19] (IBz = 1,3-dibenzylimidazol-2-ylidene) has been isolated in its hexamolybdate form and crystallographically elucidated to show a Mo–C(NHC) bond of 2.173(4) Å. (NHC = N-heterocyclic carbene).