Paramagnetic transition-metal complexes assembled on surfaces are of great interest for potential applications in organic spintronics. The magnetochemical interactions of the spin of the metal centers with both ferromagnetic surfaces and optional axial ligands are yet to be understood. We use a combination of X-ray magnetic circular dichroism (XMCD) and quantum-chemical simulations based on density functional theory (DFT+U) to investigate these metal-organic interfaces with chemically tunable magnetization. The interplay between an optional axial ligand (NO, spin S=1/2 or NH 3 , S=0) and Ni and Co ferromagnetic surfaces affecting the spin of Co(II) tetraphenylporphyrin (d 7 , S=1/2), Fe(II) tetraphenylporphyrin (d 6 , S=1), Mn(II) tetraphenylporphyrin (d 5 , S=5/2) and Mn(II) phthalocyanine (d 5 , S=3/2) is studied. We find that the structural trans effect on the surface rules the molecular spin state as well as the sign and strength of the exchange interaction with the substrate. We refer to this observation as the surface spin-trans effect.

Realization of long-range magnetic order in surface-supported two-dimensional systems has been challenging, mainly due to the competition between fundamental magnetic interactions as the short-range Kondo effect and spin-stabilizing magnetic exchange interactions. Spin-bearing molecules on conducting substrates represent a rich platform to investigate the interplay of these fundamental magnetic interactions. Here we demonstrate the direct observation of long-range ferrimagnetic order emerging in a two-dimensional supramolecular Kondo lattice. The lattice consists of paramagnetic hexadeca-fluorinated iron phthalocyanine (FeFPc) and manganese phthalocyanine (MnPc) molecules co-assembled into a checkerboard pattern on single-crystalline Au(111) substrates. Remarkably, the remanent magnetic moments are oriented in the out-of-plane direction with significant contribution from orbital moments. First-principles calculations reveal that the FeFPc-MnPc antiferromagnetic nearest-neighbour coupling is mediated by the Ruderman–Kittel–Kasuya–Yosida exchange interaction via the Au substrate electronic states. Our findings suggest the use of molecular frameworks to engineer novel low-dimensional magnetically ordered materials and their application in molecular quantum devices.

The presence of an oxygen reconstruction on the Cu(001) surface results in the self-metalation of 5,10,15,20-tetraphenylporphyrin (2HTPP) below room temperature (at ~285 K), in contrast to 2HTPP on the bare Cu(001) substrate, where a temperature of ~450 K is required. This study demonstrates the decisive impact of a surface reconstruction on the redox reaction in the solvent-free ultra-high vacuum environment.

The control of exchange coupling across the molecule−substrate interface is a key feature in molecular spintronics. This Perspective reviews the emerging field of on-surface magnetochemistry, where coordination chemistry is applied to surface-supported metal porphyrins and metal phthalocyanines to control their magnetic properties. The particularities of the surface as a multiatomic ligand or "surface ligand" are introduced. The asymmetry involved in the action of a chemical ligand and a surface ligand on the same planar complexes modifies the well-established "trans effect" to the notion of the "surface-trans effect". As ad-complexes on ferromagnetic substrates are usually exchange-coupled, the magnetochemical implications of the surface-trans effect are of particular interest. The combined action of the different ligands allows for the reproducible control of spin states in on-surface supramolecular architectures and opens up new ways toward building and operating spin systems at interfaces. Notably, spin-switching has been demonstrated to be controlled collectively via the interaction with a ligand (chemical selectivity) and individually via local addressing at the interface.A century ago, Alfred Werner laid the foundation of modern coordination chemistry, an achievement for which he was awarded the Nobel Prize in Chemistry in 1913. The innovative concept of primary and secondary valences in coordination complexes that he introduced has since then developed into oxidation states and coordination numbers and become textbook knowledge for introductory chemistry. 1 Briefly, the transition-metal ion in a given coordination complex with a fixed oxidation state can be either diamagnetic or paramagnetic, depending on the number of the ligands as well as their nature. It is the ligands that are responsible for the splitting of the d orbitals of the metal ion into t 2g and e g levels, which are at the heart of ligand field theory (LFT). 1 Thereby, ligand coordination directly alters the magnetic properties. Among the existing coordination complexes, particularly spin-bearing four-coordinated square-planar porphyrins and phthalocyanines have received much attention during the last six decades. Here, the coordination number of the metal ion and hence its magnetic properties can be easily modified by coordination with external axial ligands leading to five/six-fold coordination complexes. 2 Recently, on-surface coordination chemistry of metal porphyrins and metal phthalocyanines has raised increasing attention. 3−5 In this field of research, coordination chemistry of the planar molecules is studied in the presence of a surface ligand and an optional axial ligand binding to the free on-top site. A central issue is whether the known gas-phase coordination chemistry concepts remain valid for ligand coordination of spin-bearing planar metal−organic molecules assembled on (magnetic) surfaces. Emerging evidence shows that due to the surface−molecule interaction, novel magnetochemical effects arise beyond those known...

A bottom-up approach is introduced to fabricate two-dimensional self-assembled layers of molecular spin-systems containing Mn and Fe ions arranged in a chessboard lattice. We demonstrate that the Mn and Fe spin states can be reversibly operated by their selective response to coordination/decoordination of volatile ligands like ammonia (NH3).