N NH N HN N N N N Fe [Fe(tmtaa)] H 2 tmtaa LiBu FeCl 2 (thf) 1.5 N N N N Fe C Ph Ph 1 2 [(tmtaa)Fe=CPh 2 ] Ph 2 CN 2 O 2 Ph 2 C=O + [(tmtaa)Fe–O–Fe(tmtaa)] 4 3 C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(22) C(23) C(31) C(32) C(33) C(34) C(35) C(36) C(41) C(42) C(43) C(44) C(45) C(46) Fe N(1) N(2) N(3) N(4) Iron–carbene functionalities supported by a macrocyclic ligand: iron–carbon double bond stabilized by tetramethyldibenzotetraazaannulene Alain Klose, a Euro Solari, a Carlo Floriani,* a Nazzareno Re, b Angiola Chiesi-Villa c and Corrado Rizzoli c a Institut de Chimie Min´ erale et Analytique, BCH, Universit´ e de Lausanne, CH-1015 Lausanne, Switzerland b Dipartimento di Chimica, Universit` a di Perugia, I-06100 Perugia, Italy c Dipartimento di Chimica, Universit` a di Parma, I-43100 Parma, Italy We report an unprecedented entry to the chemistry of the iron–carbene functionality supported by an easily available macrocycle. The metal–carbene unit is a key functionality both in organome- tallic and organic chemistry. 1 It is usually bonded to conven- tional ancillary ligands, with a few remarkable exceptions in metal–porphyrin chemistry, 2–6 and particularly in the case of iron 2 and ruthenium. 2b,3,5 Despite of considerable effort in this field, owing to the relevance of metal–porphyrin carbene derivatives in chemistry and biology, 2,3 some facts are partic- ularly surprising: (i) there is an absence of any modeling approach in macrocycle chemistry, i.e. the use of readily available macrocycles; (ii) the potential of the metal–carbene bonded to a macrocyclic structure is still to be explored; (iii) very few (three, at present) metal–porphyrin carbenes have been structurally characterized. 2a,6 Here, we report a straightforward synthesis of iron–carbenes stabilized by the easily accessible tetramethyldibenzo- tetraazaannulene ligand (H 2 tmtaa) 7 along with their structural characterization in solution and in the solid state. The availability of tetramethyldibenzotetraazaannulene encourages the use of [Fe(tmtaa)] for developing iron–carbene chemistry based on this and other macrocycles. The synthesis was performed by reacting 2 8a with the corresponding diazoalkane (Scheme 1). The reaction of 2 with Ph 2 CN 2 in THF at 230 °C, and then at room temperature, gave high yields of 3 as a green crystalline solid.† Complex 2 showed a very high thermal stability and resistance to hydrolysis. The reaction of 3 with dioxygen gave, almost quantitatively, benzophenone and the well known m-oxo dimer 4. 7a The stability and the nature of the final compound derived from the reaction of 2 with diazoalkanes is strongly dependent on the substituents at the carbene carbon. In the case of PhCHN 2 , the carbene derivative forms only at low tem- perature. It decomposes at room temperature leading to the starting material and trans-stilbene (81%) and cis-stilbene (19%), while in the presence of dioxygen it forms PhCHO and complex 4. Owing to its diamagnetism, 3 has been readily characterized by 1 H and 13 C NMR spectroscopies and structural details have been revealed by X-ray analysis‡ (see Fig. 1). The coordination environment of iron is a distorted tetragonal pyramid, unlike iron–prophyrin 2a or osmium–porphyrin 6b ex- amples, where the metal is always six-coordinate. The metal is displaced by 0.335(1) Å from the N 4 average plane. The Fe– C(23) vector is perpendicular to the N 4 core, the dihedral angle with the normal to the N 4 plane being 1.0(1)°. The carbene plane C(23)C(31)C(41) is almost parallel to the N(2)···N(4) vector, the torsion angles C(31)–C(23)–Fe–N(4) and C(41)–C(23)–Fe– N(2) being 215.5(3) and 215.2(3)°, respectively. The Fe– C(23) bond distance [1.794(3) Å] is particularly short compared to the only available iron–porphyrin [FeNCCl 2 1.83(3) Å] 2a or organometallic derivatives of iron, where it ranges from 1.978(3) to 1.85(3) Å. 9 The question whether the oxidation state of iron in 3 is +2 or +4 can be reasonably answered, in the absence of M¨ ossbauer measurements, by considering the Fe distance from the N 4 plane. A correlation in tmtaa complexes has been established between the out of plane distance of the metal and its d n configuration. 7b The value found in the present case [0.335(1) Å] is expected for low-spin d 6 five-coordinate iron(ii), e.g. 0.29 Å in [Fe(tmtaa)(CO)]. 8c Much longer distances would be found for a d 4 configuration. The extended Scheme 1 Fig. 1 ORTEP drawing of complex 3 (50% probability ellipsoids. Selected interatomic distances (Å) and angles (°): Fe–N(1) 1.921(3), Fe–N(2) 1.926(2), Fe–N(3) 1.911(3), Fe–N(4) 1.920(2), Fe–C(23) 1.794(3), C(23)–C(31) 1.487(5), C(23)–C(41) 1.472(4); N(3)–Fe–N(4) 94.6(1), N(2)–Fe–N(4) 163.3(1), N(2)–Fe–N(3) 82.7(1), N(1)–Fe–N(4) 82.3(1), N(1)–Fe–N(3) 156.4(1), N(1)–Fe–N(2) 93.6(1), Fe–C(23)–C(41) 122.6(2), Fe–C(23)–C(31) 121.7(2), C(31)–C(23)–C(41) 115.7(3). Chem. Commun., 1997 2297 Published on 01 January 1997. Downloaded on 29/10/2014 02:46:48. View Article Online / Journal Homepage / Table of Contents for this issue