Coordination Properties of Multidentate Phosphanylborane Ligands in Tungsten Nitrosyl Complexes

The ambiphilic and small-bite-angle diphosphanylborane ligands Ph2PCH(PPh2)CH2B(C8H14) (2) and Ph2PCH2CH[B(C8H14)]PPh2 (3) containing the Lewis acidic 9-boranorbornyl group B(C8H14) have been prepared in one step by regioselective anti-Markovnikov hydroborations of 1,1-bis(diphenylphosphanyl)ethylene and 1,2-bis(diphenylphosphanyl)ethylene with 9-borabicyclo[3.3.1]nonane (9-BBN) under relatively drastic reaction condition. The coordination properties of Ph2PCH2CH2B(C8H14) (1) and the newly prepared 2 and 3 towards the tungsten nitrosyl fragment {WNO}6 have been studied structurally and spectroscopically. Herein, we report the cis and trans tungsten nitrosyl complexes cis-[W(CO)2(1)2(NO)(Cl)] (cis-4a), cis-[W(CO)2(1)2(NO)(H)] (cis-4b), trans-[W(CO)2(1)2(NO)(Cl)] (trans-4a), trans-[W(CO)2(1)2(NO)(H)] (trans-4b), cis-[W(CO)2(2)(NO)(Cl)] (cis-5a), cis-[W(CO)2(2)(NO)(H)] (cis-5b), cis-[W(CO)2(3)(NO)(Cl)] (cis-6a), cis-[W(CO)2(3)(NO)(H)] (cis-6b), which were readily obtained from straightforward ligand-substitution reactions of the tungsten carbonyl nitrosyl precursor [W(NO)(CO)4(ClAlCl3)] in tetrahydrofuran. The crystal structures of the tungsten chloride complexes cis-4a, trans-4a, and cis-5a revealed that the trialkyl boron centers are free pendants in the secondary coordination spheres, whereas in the hydride complexes trans-4b, cis-5b, and cis-6b the hydride ligand trans to the nitrosyl ligand is sigma coordinated to the tethered B(C8H14) group and form three-center, two-electron (3c-2e) W-H-B bonds, which were observed in their crystal structures. The authenticity of the 3c-2e W-H-B bond in the tungsten hydride complex cis-5b was further checked by a DFT structural optimization and natural bond order (NBO) analysis.