Cooperative Effects in π1‐Ligand‐Bridged Dinuclear Complexes, XIII[1]. – Unexpected Weak d‐d Interactions in Synfacial Heterodinuclear μ1‐Cyclooctatetraene Complexes of the Type {(CpCr)[(CO)3M′]}μ1‐Cot (M′Fe, Cr, W)
The heterodinuclear complexes {(CpCr)[(CO)3M′]}μ1‐Cot (CotC8H8; M′Fe, Cr, W) (5 – 7) and {(Cp*Cr)[(CO)3Cr]}μ1‐Cot (Cp*C5Me5) (9) are synthesized by the reaction of the corresponding mononuclear Cot complexes CpCr(η61‐Cot) (4) and Cp*Cr(η61‐Cot) (8), respectively, with M′(CO)3(EtCN)3 (M′Cr, W) and with Fe2(CO)9. The CrFe compound 5 is obtained in very low yield only. However, much better yields of 5 are achieved in the reaction of CrCl2 and CrCp2 with (CO)3Fe(η41‐Cot) (10) in the presence of an excess of Zn. X‐ray structure determinations of 5 and 6 reveal synfacial configurations of the CpCr and M′(CO)3 units with a μ1‐η5:3‐ and μ1‐η4:51‐Cot bonding mode for 5 and 6, respectively. The metal – metal distances are 2.9369(13) Å for 5 and 2.81(2) Å for 6 indicating metal – metal single bonds. 5 contains 33 valence electrons (ve) and is paramagnetic with one unpaired electron. A well‐resolved fluid‐solution ESR spectrum of 5 shows a 53Cr and 1H hyperfine structure (hfs). The 1H hfs can be calculated with three different 1H hyperfine coupling constants (1H hfcc) indicative of protons in α1‐position with respect to the paramagnetic metal center: one 1H hfcc of the five Cp protons (1.68 G), one of three (3.32 G) and one of two protons of the Cot ligand (5.04 G). This agrees with the Cot bonding mode of the CpCr unit in the crystalline phase. The ratio of 3:2 protons for the Cot ligand prove the rigidity of the Cot ligand on the ESR time scale. The excellent agreement of the ESR data of the liquid and frozen solution ESR spectra of 5 with the ESR data of the mononuclear complex CpCr(η61‐Cot) leads to the conclusion that the unpaired electron in 5 must predominantly be chromium‐centered. 6, 7, and 9 have 31 ve and are paramagnetic with three unpaired electrons as shown by means of ESR spectroscopy and magnetic measurements. The calculations of the quartet ESR spectra of 6, 7, and 9 reveal zero‐field splitting parameters D ranging from 3.6 to 4.8 cm–1 which are similar to that of the mononuclear quartet compound vanadocene. The small range in the parameters D prove the minor influence of the second metal on going from Cr to W. Hence, the three unpaired electrons are assumed to be mainly localized on the Cr center of the CpCr unit. Cyclic voltammetry studies result in the redoxcascade − 2 ⇄ − 1 ⇄ 0 ⇄ + 1 for 5 and 7 at +20°C and for 6 at –35°C. However, at +20°C 6 and 9 only show one electrochemically reversible reduction wave 0/–1, whereas the oxidation 0/+1 and the second reduction –1/–2 are irreversible. The redox potentials indicate a considerable influence of the metal combinations. On the strength of the ESR spectroscopic results and according to the results of crystal structure determinations, an isolobal relationship is drawn between the (CO)3M′(η8 – ×1‐Cot) fragment and a cyclic organic π1‐ligand wi...
