ExperimentalGeneral Remarks. All manipulations were performed in air, except where otherwise noted. The solvents thf and hexane (analytical grade) were freshly distilled from sodium/potassium alloy, dichloromethane was distilled from calcium hydride, the other solvents (acetonitrile, diethylether, acetone) were used as purchased. Deuterated solvents for NMR measurements were distilled from the appropriate drying agents under N 2 immediately prior to use following standard literature methods. 15 Air-sensitive compounds were stored and weighed in a glovebox. The reagents 1,2-dibromoethane, 1,3dibromopropane, 1,4-diiodobutane, 2,6-dimethylaniline, 2,4,6-trimethylaniline, 2,6-diisopropylaniline, triethylorthoformate, sodium tetrafluoroborate, and potassium bis(trimethylsilyl)amide were used as received. 1 H and 13 C NMR spectra were obtained on Bruker Avance AMX 400, 500 or Jeol Eclipse 300 spectrometers. The chemical shifts are given as dimensionless values and are frequency referenced relative to TMS. Coupling constants J are given in hertz (Hz) as positive values regardless of their real individual signs. Abbreviations used: st = septet, br = broad. Mass spectra (MS) and high-resolution mass spectra (HRMS) were obtained in positive electrospray (ES) mode unless otherwise reported, on a Waters Q-TOF micromass spectrometer. 1,3-Bis-(2,4,6-trimethylphenyl)-4,5,6,7-tetrahydro-3H-[1,3]diazepin-1-ium iodide, 7-Mes•HI. The reaction was performed on a 71.0 mmol scale of amidine (19.90 g), 5.00 g of K 2 CO 3 (36.0 mmol) and 22.00 g of 1,4-diiodobutane (71.0 mmol) in 1 L of acetonitrile. The solution was heated under reflux for 5 hours to yield 29.20 g (63.0 mmol, 89%) of white, crystalline material. 1,3-Bis-(2,6-dimethylphenyl)-4,5,6,7-tetrahydro-3H-[1,3]diazepin-1-ium iodide, 7-Xyl•HI. The reaction was performed on a 43.3 mmol scale of amidine (10.93 g), 5.8 mL of 1,4-diiodobutane (13.63 g, 44 mmol), 3.01 g of K 2 CO 3 (22.5 mmol) in 0.5 L of acetonitrile. The solution was heated under reflux for 5 hours to yield 14.85 g (34.2 mmol, 79%) of white, crystalline material. 1,3-Bis-(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-3H-[1,3]diazepin-1-ium iodide, 7-Pr i •HI. The reaction was performed on a 11.0 mmol scale of amidine (4.00 g), 0.78 g of K 2 CO 3 (5.6 mmol), 1.6 mL of 1,4-diiodobutane (3.76 g, 12.1 mmol) in 400 mL of acetonitrile. The solution was heated under reflux for 17 hours to yield 3.87 g (7.1 mmol, 64%) of white, crystalline material. 2,4-Bis-(2,4,6-trimethylphenyl)-4,5-dihydro-1H-benzo[e][1,3]diazepin-2-ium bromide, Xyl7-Mes•HBr. The reaction was performed on a 35.8 mmol scale of amidine (10.03 g), 36.0 mmol of , 'dibromo-o-xylene (9.49 g), 2.49 g of K 2 CO 3 (18.0 mmol) in 0.5 L of acetonitrile. The solution was heated under reflux for 5 hours to yield 12.42 g (26.2 mmol, 73%) of white, crystalline material. 1 H

The synthesis of the novel seven-membered N-heterocyclic carbene (NHC) 1,3-dicyclohexyl-1,3diazepan-2-ylidene (3) and its 5,6-dioxolane derivative 4 is reported and their coordination chemistry with Rh(I), Ir(I), and Pt(0) discussed. The M(cod)(3)Cl, where M ) Rh and Ir, complexes display a high rotation barrier at room temperature about the M-C NHC bond, whereas for the M(CO) 2 ( 3)Cl and Pt-(nbe) 2 (3) complexes rapid rotation of the carbene ligand is observed at ambient temperature. The infrared ν(CO) values of the Rh(I) and Ir(I) derivatives M(CO) 2 (3)Cl give a measure of the donor ability of the new carbene ligands. The crystal structures of the amidinium salts 3‚HPF 6 and 4‚HPF 6 together with those of M(cod)(3)Cl [M ) Rh, Ir], Ir(cod)(4)Br, Ir(CO) 2 (3)Cl, and Pt(nbe) 2 (3) are reported. Both the salts and the coordinated carbene ligands exhibit extremely large NCN angles; for the complexes the angles are in the range 115.5(3)°[Pt(3)] to 122(11)°[Ir(4)].